Mesoionic C-donor complexes
Normal and abnormal N-heterocylic carbene ligands: Similarities and
differences of different mesoionic C–donor complexes
Martin Albrecht*
School of Chemistry & Chemical Biology, University College Dublin, Belfield, Dublin 4,
Ireland
* Corresponding author: e-mail address: martin.albrecht@ucd.ie
Contents
1 Introduction and General Considerations
2 Ligand Nomenclature: Abnormal or Mesoionic Carbene Complexes?
3 Electronic Considerations
4 Reactivity of Complexes with Sterically Comparable Ligands
4.1 Imidazolylidene Complexes
4.1.1 Structure and electronics of normal vs abnormal mesoionic imidazolylidene
complexes
4.1.2 Comparative evaluation of imidazolylidene complexes in hydrogenation
catalysis and related transformations
4.1.3 Comparative evaluation of imidazolylidene complexes in cross-coupling
catalysis
4.2 N,X-heterocyclic Carbene Complexes
4.3 Triazolylidene Complexes
4.4 Pyridylidene Complexes
4.4.1 Monodentate pyridylidene complexes
4.4.2 Polydentate pyridylidene complexes
4.4.3 Catalytic applications of pyridylidene complexes
5 Conclusions and Outlook
Acknowledgments
References
Keywords
N-heterocyclic carbene – mesoionic – abnormal bonding – catalysis – structures –
triazolylidenes – pyridylidenes – hydrogenation – cross coupling – donor properties
Abstract
Similarities and differences of normal vs abnormal carbene complexes highlighted, with an
emphasis on effects that can be directly attributed to the different bonding mode and which
are thus not imparted by potential stereoelectronic effects such as more or less shielding of the
M–C bond. While generally, the bonding scheme is similar throughout all classes of carbenes,
distinctly different behavior has been noted in some reactivity patterns, in particular when
comparing different bonding modes of imidazolylidenes. This distinction is blurred with
triazolylidenes, and the normal/abnormal nomenclature is not particularly meaningful in these
cases. When considering pyridylidenes, structural differences are noted in the ground state,
while reactivity patterns are not significantly dependent on the metal binding site. Common
denominators for all normal and abnormal heterocyclic carbene complexes include (i) a
strongly mesoionic character of both normal and abnormal bonding modes of the ligand, and
(ii) a negligibly small carbenic character. Accordingly ‘carbene’ is a most debatable name for
this class of ligands, and ‘mesoionic C-donor’ may be more accurate.
1 Introduction and General Considerations
One of the most significant hallmarks of modern organometallic chemistry has undoubtedly
been the discovery of N-heterocyclic carbenes (NHCs) as a versatile class of ligands for
(transition) metals.i While carbenes have long been considered fleeting intermediates due to
their intrinsically high reactivity,ii Bertrand and coworkers demonstrated that through
ingenious substituent variation, these carbenes can be stabilized and spectroscopically
characterized.iii Further taming has been achieved through introduction of a rigid cyclic
structure, while keeping two nitrogen heteroatoms as push-pull stabilizers. Accordingly,
deprotonation of imidazolium salts by Arduengo and coworkers resulted in highly stable
imidazol-2-ylidenes, which were analyzed in the solid state by X-ray diffraction analysis.iv
The carbenic nature of these imidazolylidenes provides access to a new class of formally
neutral C-donor ligands which have a variety of advantages: they are synthetically easily
accessible from (substituted) imidazoles, they are highly modular in terms of substituents on
the nitrogen atoms adjacent to the carbene center (the wingtip groups), and they are stable as
free-base ligands and hence available for standard Lewis acid/base coordination.v Metal
complexation with the free carbene is straightforward and similar to metal coordination with
other neutral ligands such as phosphines, imines, or sulfides. The high stability of a variety of
imidazolium-derived carbenes has allowed these compounds to be stored, and as a
consequence, eventually to be commercially distributed.
The easy accessibility and commercial availability has spurred the exploitation of NHCs.
These ligands have demonstrated a substantial impact in a number of areas in materials
science, for example for the fabrication of new components for light-emitting diodesvi and
molecular electronics,vii for biomedical applications,viii and for supramolecular selfassemblies.ix By far the most impressive advances have been accomplished, however, in the
development of new catalytic systems.x A variety of NHC metal complexes are most
powerful catalysts, with Grubbs’ second-generation olefin metathesis catalyst as a prime
example.xi Likewise, NHC palladium complexes developed by Organ and others display
outstanding performance in cross-coupling catalysisxii and largely surpass the ubiquitous
phosphine analogs in these transformations. Many of these catalyst precursors have been
patented and are commercially available, thus underlining the enormous impact of NHCs in
catalyst discovery and development.
The extraordinary performance of NHC complexes has been generally attributed to two
factors: firstly to the strong M–CNHC bond, which imparts robust bonding of the carbene
ligand and hence renders the NHC a reliable spectator ligand that will be available for
modularization (ligand tuning); and secondly to the high donor ability of the NHC ligand,
which induces high electron density at the metal center as well as a high trans effect, and
consequentially an increased reactivity of substrates bound in this position. It is worth noting
that the electron donor ability of the NHC ligand is directly influenced by the heteroatoms in
the heterocycle. The donor properties can thus be further enhanced when one or both
heteroatoms are displacedxiii or even completely removed from the positions adjacent to the
carbene center.1b,xiv At the same time, such heteroatom engineering markedly decreases the
stability of the free carbene,xv and probably because this free-base stability is a key element
for the popularity of NHCs, this avenue has received less attention than wingtip group
substitution.
The synthetic limitations associated with the low stabilization of N-heterocyclic carbenes with
less than two or with remote heteroatoms have been overcome by different approaches. Thus,
pioneering work by Bertrand and coworkers disclosed the high stability of a variety of free
carbenes with reduced heteroatom stabilization.xvi Crystal structures, melting points, and
coordination properties have been determined for cyclic (amino)(alkyl)carbenes, CAACs, and
acyclic versions with only one heteroatom adjacent to the carbene. xvii Even carbenes that have
no heteroatom adjacent to the carbene center have successfully been stabilized, as for example
in amino-substituted cyclopropylidenes or pyrazol-derived bent allenes,xviii unveiling a high
stability of these free carbenes.xix In addition, new methodologies for the metalation of
heterocyclic carbene precursors have been developed that circumvent the formation of free
carbenes, including examples of direct metallation via C–H bond activation,xx,
transmetalation, oxidative addition of a C–X bondxxi or a C–H bond,xxii or alkylation of a
metal-bound anionic azolyl ligand,xxiii thus yielding a series of NHC complexes without the
need to stabilize or isolate the free carbene.xxiv Formation of imidazol-4-ylidene complexes,xxv
isomeric to Arduengo-type imidazol-2-ylidene complexes, is exemplary for illustrating these
complementary approaches (Fig 1, Scheme 1). Introduction of appropriate substituents on N1,
C2, and N3 of the imidazolium salt and careful deprotonation with KN(SiMe3)2 as a strong
base has enabled the isolation and crystallographic characterization of the free carbene. xxvi
Subsequent coordination of the free carbene to Au(I) gave the NHC gold carbene complex.
Alternatively, treatment of a tetrasubstituted imidazolium salt with Ag2O and an Ir(I)
precursor affords, via a transmetalation reaction but without formation of a free
imidazolylidene, a similar NHC complex.xxvii Metalation is directed to the imidazolium C4/C5
position either through activation of the C4/C5 position,xxviii for example by inserting a halide
for subsequent oxidative addition, or by using imidazolium salts in which the C2 position is
protected (via alkylation or arylation), or sterically shielded (through substitutents at the N1
and N3 position).xxix In a special case, metalation of N,N’-disubstituted imidazolium salts, i.e.
typical precursors for Arduengo-type imidazol-2-ylidene complexes, with either [IrH5(PPh3)2]
or with [OsH6(PiPr3)2] yielded predominantly the C4-bound imidazolylidene metal
complex.xxx
[M]
1
R N
R
[M]
2 3
5 4
3
N R
R N
R
R
4 5
N
2 1
R
R
Figure 1. Normal 2-imidazolylidene complex (left) and abnormal 4-imidazolylidene complex (right) with atom
numbering scheme adopted in this review. Note that formally, atom numbering of abnormal carbene complexes
is dependent on the substituents on N1 and N3. Throughout this text, we ignored that fact and set the metalbound carbon per default as C4.
Scheme 1. Abnormal carbene complex formation according to methodologies also established previously for
normal carbene complexation.
HX2–
H
Dip N
Ph
+
N
BF4–
N
Ph
[IrH5(PPh3)2]
or
[OsH6(PiPr3)2]
Dip
1
KN(SiMe3)2
+
[M]
–
Dip N
Ph
+
[AuCl(SMe2)]
R N
N
Ph
Ag2O
[IrCp*Cl2]2
+
N
3
I–
N
+
N
N
Dip
2
R/H
N
H/R
R
4
[Pd(dba)2]
N
N
I
Br–
+
N
5
iPr
Our own interest in using such abnormal carbenes as ligands for catalytically active metal
centers has been stimulated predominantly by the fact that these ligands should be
extraordinarily strong donors, significantly stronger than their normal analogs and thus
providing access to exceptionally high donor systems that may stabilize high oxidation state
metal centers and that may induce new reactivity patterns for oxidation catalysis. In addition,
the mesoionic character is expected to constitute a charge/hole reservoir that may assist in the
transient storage and release of electrons with obvious implications for redox catalysis.
2 Ligand Nomenclature: Abnormal or Mesoionic Carbene Complexes?
The reaction outcome described above for the reaction of imidazolium salts 3 with osmium
and iridium polyhydrides is surprising since the C2-bound proton is about 6 orders of
magnitude more acidicxxxi and likewise, imidazol-4-ylidene has been calculated to be some 80
kJ mol–1 less stable than imidazol-2-ylidene.xxxii Therefore, the C2–H bond is considered to be
the easiest to be activated. While metalation of the C2 position is indeed more facile with
most metal precursors and has thus been established as the ‘normal’ course of metalation, the
metalation of the C4/5 position is unexpected, which led to the coinage of the term
‘abnormal’ for this carbene bonding mode.xxxiii The terms ‘normal’ and ‘abnormal’ have
initially been used to differentiate between the supposedly more stabilized (normal) C2 and
the less stable (abnormal) C4 and C5 positions, respectively. This pragmatic but scientifically
undefined terminology has been expanded to all types of heterocyclic carbenes.13 Accordingly
a normal carbene refers to a carbene, for which a neutral canonical resonance form can be
drawn, while an abnormal carbene requires the introduction of two opposite charges or a
diradical structure in the carbene-type resonance forms. This definition of an abnormal
carbene also includes for example pyridylidenes and pyrazolylidenes, and it is essentially
identical to the IUPAC definition of mesoionic compounds.xxxiv Hence, a scientifically more
accurate terminology for imidazol-4-ylidenes and other ‘abnormal’ carbenes would be
‘mesoionic’ carbenes.
While the mesoionic descriptor is indeed useful to characterize some of the ‘abnormal’
carbenes, it raises some questions: How should ‘normal’ NHCs be termed? Consequentially,
such ‘normal’ NHCs should be denominated as ‘non-mesoionic’ as a neutral resonance form
exists for these compounds. And what is effectively distinguishing abnormal/mesoionic
carbene ligands from their normal/non-mesoionic homologues? When schematically drawing
NHC metal complexes, we generally refrain from writing a neutral carbene structure even for
normal NHC complexes and instead, it has become widely accepted that the formally carbenic
two electrons are actually delocalized over the N–C–N fragment in imidazolylidenes, thus
adopting an ylidic structure (A, B in Scheme 2). Mesoionic behavior of the N–C–N fragment
in 2-imidazolylidenes may even be necessary for rationalizing some of the observed
reactivity. For example, Fryzuk and coworkers observed the ethylene insertion into a Ni–H
bond trans to a carbene and subsequent migration of the ethyl group to the carbene carbon,
while preserving some of the Ni–CNHC bonding (Scheme 3).xxxv Similar reactivity patterns
have been observed by Danopoulos.xxxvi The reactivity of the metal bound carbon towards
formally cationic alkyl fragments suggests some mesoionic character also in Arduengo-type
imidazol-2-ylidenes, and this is indeed implicitly assumed when schematically drawing the
M–CNHC bond as a single bond. This generally accepted schematic representation, which is
supported by experimental and theoretical data,xxxvii implies that the  electron density is
predominantly delocalized over the N–C–N fragment (viz. a ylidic structure) with only a
relatively small  contribution to the M–C bond (actual carbene form; M = electron-rich
metal center). Unfortunately, these  electrons are often and unduly omitted in schematic
representations, and the sp2-hybridization of the carbenic carbon is not obvious in such
representations.xxxviii The same considerations hold for the free carbene structures. Obviously
the borderline between the two limiting representations—carbene vs ylide— is continuous,
and the issue of whether the ligand is a carbene or not may become semantic at this level.
Taking into account that also normal 2-imidazolylidenes are generally drawn and considered
to be ylidic/mesoionic, the distinction between C2- and C4-imidazolylidenes should probably
be referred to as ‘less mesoionic’ and ‘more mesoionic’ carbenes. This description is probably
most accurate, but evidently lacks practicality. Moreover, this nomenclature becomes
similarly ill-defined (‘more’ vs ‘less’) as the ‘abnormal’ vs ‘normal’ description is.
Scheme 2. Resonance structures of normal carbene complexes, with a prevailing mesoionic/ylidic form.
[M] –
N
–
[M]
+
+
N
N
N
[M]
N
[M]
R N
+
–
[M]
N
N
N
[M]
N R
A
R N
N R
B
Scheme 3. Reactivity of a nickel complex directly emerging from an ylidic behavior.
H
iPr2
iPr2
P Ni P
N
N
6
+
iPr2
P
PiPr2
Et Ni
H2C CH2
N
+
N
7
In a critical review, Crabtree analyzed these terminologies and highlighted benefits and
disadvantages of both terms.xxxix In short, the normal/abnormal nomenclature emphasizes the
similarity between the two classes of compounds, while the introduction of mesoionic
carbenes underlines diverging properties. The use of either one or the other term will thus
depend on how similar or distinct the abnormal/mesoionic carbenes and carbene complexes
are in comparison to their normal/non-mesoionic homologues. The following sections
therefore aim at collating key data and to shed light on these aspects and to identify similar
and different properties imparted by the bonding mode of the heterocyclic carbene.
Some comparison is compromised by steric differences. Specifically, C2-bound
imidazolylidenes generallyxl feature two pseudo-ortho substituents due to alkylation or
arylation of the two heterocyclic nitrogen sites. However, a substantial number of abnormal
C4-bound carbene complexes have been produced from similar N,N-disubstituted
imidazolium salts, thus leaving one of the adjacent positions (C5) unsubstituted. Accordingly,
in a number of cases, structural or reactivity differences can only partially be attributed to
different carbene bonding modes and more/less shielding of the metal center and the metal–
carbon bond needs to be taken into account when comparing the different bonding modes in
these cases.
3 Electronic Considerations
The limited aromaticity of imidazolesxli suggests that the  electron density of the NC(M)N
unit in normal Arduengo-type 2-imidazolylidene complexes is only little overlapping with the
backbone C=C  orbitals (cf A in Scheme 2). Accordingly, the C=C fragment is neutral, while
the NCN unit features either a carbenic, or a ylide-type ground structure. The ylidic
represenation is generally used when representing NHC complexes, viz. M–C single bonds
and  electron density delocalized over the NCN fragment. It should be noted that in the
context of nomenclature, this ylidic representation implies a mesoionic structure, i.e.
resonance-conjugated positive and negative charges on nitrogen and carbon, respectively.
A semi-localized  electron density distribution over the C=C and the NCN fragment is
supported by some oxidation and substitution reactivity studies that indicate olefinic character
of the backbone C=C bond in the free imidazolylidene.xlii Crystal structure analyses typically
reveal a C–C bond length around 1.33 Å (complex 8, Figure 2)xliii which further supports a
localized olefinic bond linking C4 and C5 in these normal carbene complexes. The shortening
of the double bond is a metal-induced effect, as in imidazolium salts including for example 2haloimidazolium salts, this bond is ca. 1.38 Å long and hence in the expected range for
conjugated double bonds. In contrast, recent studies investigating the electronic interactions
of two metal centers across a rigid phenylene-linked dicarbene bridge in complex 9 suggest
facile electron transfer and substantial valence-delocalization in the mixed-valent RuII/RuIII
state.xliv Hence, orbital overlap between the NCN fragment and the backbone C=C bond may
be more substantial than the early studies suggested. One implication of this overall electronic
situation is that the carbenic character of the ligand is fairly minor, which may cast doubts on
the overall description of these formally neutral C-donor ligands as carbenes. (Obviously, the
same critical re-assessment of terminology should be applied to Fischer carbene complexes.)
5+
1.33 Å
N
valence delocalization
N
N
N
Me
Cl Pd
N
N
8
N
Ru
N
N
N
N
N
N
N
Ru
N
N
N
N
9
Figure 2. Complex 8 as an example of 2-imidazolylidene complexes featuring a localized C=C bond, and
complex 9 which facilitates smooth electron transfer between the ruthenium centers.
A similar consideration on the  electron density distribution in mesoionic/abnormal
imidazol-4-ylidene complexes implies a (formal) separation of the cationic and anionic
charges that is much more pronounced than in normal 2-imidazolylidenes. In the latter, the
positive and negative charge are confined in the same ylidic NC(M)N fragment, while in
mesoionic/abnormal imidazol-4-ylidenes, the metal-bonding fragment is the olefinic portion
and thus resembles a vinyl anion-type ligand or a heteroarylium system, while the positive
charge formally resides on the NCN unit, which is therefore reminiscent of an allyl-cation
unit (Scheme 4). Such a separation has indeed been predicted by a theoretical analysis of the
bimetallic complex 10 (Figure 3).xlv The bonding in this complex features a high-lying
occupied molecular orbital centered on the C=C–Pd fragment as donor moiety that interacts
with the LUMO of the [Ag(NCMe)3]+ fragment, thus providing the most relevant contribution
for stabilizing the Pd–Ag bond in complex 10 (Figure 3b). Likewise, most X-ray structure
analyses of C4-metalated imidazolylidenes reveal a longer C=C bond (1.36–1.38 Å),xlvi which
suggests a more delocalized double bond than in imidazolylidenes that bind the metal via C2.
Figure 3. a) Schematic representation of the bimetallic complex 10; b) bonding interaction between the two
metal centers, emphasizing donation of a filled Pd–C–C localized MO to an empty silver 4d orbital.
Furthermore protonation and deuteration experiments on abnormal C4-bound imidazolylidene
rhodium and iridium complexes reveal selective exchanges at the C5 position.xlvii Selective
isotope exchange at C5 has also been observed when C4-bound NHC rhodium complexes
were used as catalysts for silane activation.xlviii This reactivity and selectivity may be
rationalized by a de-/re-aromatization pathway when invoking the charge separation expected
for mesionic compounds with a metalla-allyl anion unit as indicated in for complex 11 (D
exchange; Scheme 4).xlix It is worth noting that such H/D isotope exchange is selectively
occurring at the C5 position, even when the C2-position is unsubstituted (cf formation of
monodeuterated complex 11–D). This outcome re-inforces the pronounced separation of 
electron density with little overlap between the C=C and the NCN units, rather than an
exchange process with the most acidic position. This distinction to imidazol-2-ylidenes is
expected to have significant implications on the stability of the metal-carbon bond in normal
vs abnormal carbene complexes, on the electron density at the metal center, and on the
reactivity of both, the metal and the carbene ligand.
Scheme 4. Selective deuterium exchange at the C5 position of abnormally bound carbenes in rhodium
complexes and proposed mechanism for this exchange based on the mesoionic character of the imidazolylidene
ligand and in particular the anionic metalla-allyl configuration.
+
N
Rh
N
N nBu
I
N
nBu
11
+
N
D+
NCMe
NCMe
Rh
N
NCMe
N nBu
H
I
N
D
nBu
11–D
NCMe
via:
M
_
M D
D+
N
N
N
M
+
N
+
–
N
1,3-shift
+
N
M
M
+
_
N
N
D+
M
D
N
N
+
H
–H+
–
N
D
+
N
However, we noted that the reactivity towards acids and bases is not primarily dependent on
the (non)mesoionic nature of the NHC ligand, but much more on the type of N-heterocyclic
structure. Thus, while imidazol-4-ylidene complex 12 undergoes rapid H/D exchange at C5
under acidic conditions (within seconds, Scheme 5), the reactivity under basic conditions is
very slow and several days are required to complete deuteration at this position at pH 14. l
This reactivity pattern contrasts the reactivity of complex 13 with an triazolylidene ligand,
which comprises a nitrogen (N2) instead of a C2–R group in the heterocycle and which thus
belongs to the same abnormal carbene ligand family as imidazol-4-ylidenes. With these
triazolylidenes bound to iridium(III), acidic H/D exchange is completely suppressed (vs
seconds for the imidazolylidene analogue), while the reaction is very fast and complete in less
than one minute under basic conditions to afford 13–D. Hence, even though both complexes
contain a mesoionic/abnormal carbene ligand, the triazolylidene system exhibits
predominantly
electrophilic
reactivity
(deuteration
with
NaOD/D2O),
while
the
imidazolylidene system is nucleophilic (deuteration with D+). While illustrating the diverging
properties of mesoionic/abnormal carbene ligands, these experiments also convey a
remarkable robustness of the Ir–C bond in these complexes.
Scheme 5. Diverging reactivity patterns of related abnormal carbene complexes 12 and 13 featuring an
imidazolylidene and triazolylidene ligand, respectively (reaction times to reach completion in italics)
+
<min
D
N
DCl
Ir
N
+
Cl
N
NaOH
Ir
D2O
N
Cl
2d D
Ir
D2O
N
N
+
N
OH
N
N
1h D3C
12–D
12
12–D4
+
no reaction
DCl
NaOH
Ir
D2O
N
N
Cl
N
N
+
<min
D
Ir
D2O
13
N
N
N
OH
N
13–D
Moreover, the reactivity of the triazolylidene complex with acids may be controlled by the
presence of an additional and unsubstituted nitrogen atom in the heterocycle as compared to
imidazolylidenes, and reversible protonation of this nitrogen would enhance the electrophilic
character of the heterocycle,li thus rendering acid-catalyzed isotope exchange at the C5
position unlikely to occur. Hence, even though both complexes 12 and 13 contain a formally
mesoionic/abnormal ligand, the ligand reactivity is drastically different and may not be based
on common ‘abnormal’ or ‘mesoionic’ origins.
4 Reactivity of complexes with sterically comparable ligands
Only few studies highlight reactivity differences of complexes that contain normal and
abnormal carbene ligands with essentially identical steric requirements, thus enabling to
identify electronic differences of distinct bonding modes. These studies include metal
complexes with imidazolylidenes, thiazolylidenes, triazolylidenes, and pyridylidenes, and
hence cover a broad range of different heterocycles.
4.1 Imidazolylidene complexes
4.1.1 Structure and electronics of normal vs abnormal mesoionic imidazolylidene
complexes. In an effort to compare C4 vs C2 bonding in sterically indifferent complexes, we
have synthesized the palladium complexes 14 and 15 (Fig. 4a).45 The introduction of a
chelating dicarbene ligand was motivated by the fact that NHCs tend to give biscarbene
complexes when using C–H activation or transmetallation protocols,43 which are formed as
mixtures of cis and trans isomers. The complexity due to this mixturelii and the potential of
mutually canceling trans effects of NHCs in the trans isomer are avoided when using a
chelating dicarbene ligand. Depending on the positioning of the methyl groups, the
diimidazolium salts afford after C–H bond activation with [Pd(OAc)2] either normal C2bound or abnormal C4-bound imidazolylidenes. The two complexes 14 and 15 are structural
essentially analogous; differences in reactivity patterns can thus be attributed to the electronic
impact of normal vs abnormal carbene bonding exclusively, while stereoelectronic effects are
negligible.
Figure 4. a) Schematic drawing of the isostructural complexes 14 and 15; b) Superimposition of the crystal
structures of complexes 14 (italic atom labels) and 15 (underlined atom labels).
When comparing the structures obtained from X-ray diffraction analyses, the similarity of the
complexes is obvious even on the microscopic level and bond angles and lengths are almost
identical (Figure 4b). The most diagnostic difference between complexes 14 and 15 pertain to
the metal-halide bond lengths. In the abnormal carbene complex 15, the Pd–Cl bonds are
significantly longer than in the normal analogue 14 (average Pd–Cl 2.404(4) Å vs. 2.357(2)
Å, thus demonstrating a higher trans influence of the C4-bound imidazolylidene compared to
the C2-bound homologue. It is interesting to note that this difference is evanescent when the
corresponding iodide analogues are compared. Presumably, the larger ionic radius of I–
compared to Cl– induces steric congestion due to the presence of methyl groups in pseudoortho position and accordingly, the Pd–I bond length is dictated by the limited accessibility of
the palladium center due to the heterocyclic substituents and not by the trans influence of
ancillary ligands. Interestingly, however, the Pd–CNHC bond lengths are identical in C2- and
C4-bound imidazolylidene complexes, a feature that has been noted also for a broader set of
palladium complexes which include also sterically dissimilar complexes of this class of
ligands.46
Along similar lines, two benzimidazolylidene complexes 16 and 17 featuring identical steric
ligand environment yet a diverging carbene coordination mode have been prepared very
recently.liii While complex 16 contains a normal benzimidazolylidene, complex 17 features an
abnormal coordination mode. In these complexes, no significant trend has been observed in
the trans influence as a consequence of the different carbene bonding mode and the Pd–Npyr
distance are essentially identical within standard deviations in the normal and the abnormal
carbene complex.
F
F
N
N
Cl
N
Pd
N
Cl
Cl
Pd
pyr
pyr
16
17
pyr = pyridine
Cl
Figure 5. Isostructural monoimidazolylidene complexes 16 and 17.
The C4-bound imidazolylidene palladium complexes have significant nucleophilic character.
Thus when complex 15 is treated with a slight excess of medium or strong acids such as
H3PO4 or H2SO4, rapid Pd–CNHC bond cleavage is observed and the imidazolium salt 18 has
been identified spectroscopically, (Scheme 6).45 In contrast, the C2-bound imidazolylidene
complex 14 is completely inert under similar conditions, even at elevated temperatures and
with a significant excess of H2SO4. One plausible rationale for the lower resistance of the
abnormal carbene complex may be based on the mesoionic character and the enhanced vinylanionic character of the ligand. A more pronounced carbanion ligand site obviously increases
the nucleophilicity of the metal-bound carbon and thus its propensity to be protonated.49
Alternatively, the strong donor ability of the C4-bound imidazolylidenes may enhance the
electron density at the metal center, thus increasing its reactivity to protons and other
electrophiles. Such an argumentation relies on an anionic metalla-allyl fragment, thus
including the metal and the vinylic electron density (cf Figure 3b).
While the putative proton adduct of complex 15 has not been detected thus far, computational
work suggests that donation of the metal center is a key interaction in the facile C(sp3)–H
bond activation with rhodium(III) complexes when coordinated 4-imidazolylidenes, while no
such reactivity was observed with C2-bound imidazolylidenes.liv Further support for a metalmediated reaction trajectory in the acidolysis of complex 15 has been obtained when the
Lewis acid is exchanged from H+ to Ag+.45 Thus, reaction with AgBF4 affords the
heterobimetallic complex 10, which features a formally five-coordinate palladium(II) center
(cf Figure 3a). In addition to the expected halide abstraction, the Ag+ is involved in an
interaction with the palladium center. The bimetallic complexes are stable and have been
crystallized repeatedly. The molecular structure reveals a remarkably short AgPd contact of
2.8701(6) Å and thus suggests a strong metal-metal interaction. The silver(I) ion adopts a
distorted tetrahedral geometry with three NCMe ligands and the palladium(II) center
occupying the fourth coordination site. The palladium center is only marginally distorted out
of the original square plane, and the silver ion is not located in an ideal apical position but is
shifted towards the carbene ligands. Computational analyses indicate that the stabilization of
the silver(I) ion occurs predominantly through an occupied molecular orbital that includes the
two heterocyclic carbons C4 and C5 as well as the palladium center, thus supporting the
metalla-allyl type bonding, as well as the bending of the silver ion out of the ideal apical
position towards the carbene ligands. Moreover, the calculations suggest that the frontier
orbitals are indeed characterized by a formal separation of the  electron density in these C4bonding imidazolylidenes, with the NCN fragment and the C–C–Pd moiety separated by a
node (cf Figure 3b). The PdAg interaction is unstable in the presence of specific Lewis
bases, and exposure of complex 10 to DMSO or MeOH cleanly affords the silver-free
solvento complex 19. These solvents are apparently competitive bases to palladium(II) in
complex 10 and thus support a pronounced nucleophilicity of the metal center in these
abnormal dicarbene complexes. No such reactivity has been observed with the normal
dicarbene complex 14, and in the presence of Ag+, the expected halide abstraction proceeds
smoothly and directly yields the normal analogue of complex 19.
Scheme 5. Reactivity of complex 15 towards Lewis acids H+ and Ag+.
L
L
[PdCl4]2–
H H
N +
+
N
N
Ag
Cl
Cl
HCl
N
N
18
N
MeOH
Pd
N
N
N
15
N
10
2+
L
L
L
L
AgBF4
Pd
N
3+
L
Pd
N
N
N
N
N
19
L = NCMe
An interesting aspect of these reactivity studies is associated with the fact that palladium(II)
centers are generally considered to be electrophilic, a key property to use these complexes as
palladium(0)
precursors
in
cross-coupling
and
related
catalysis.
The
C4-bound
imidazolylidene ligands impart a sufficiently high electron density to the palladium center that
this generic electrophilic character is inverted and the metal center is basic in complexes 10
and 15. The higher density has been quantified directly by X-ray photoelectron spectroscopy
(XPS), revealing a 0.5–0.6 eV (ca. 48–58 kJ mol–1) higher kinetic energy for the metal d
electrons in the imidazol-4-ylidene complex 15 than in the C2-bound analogue 14. These
results are supported by 31P NMR data of the sterically dissimilar monocarbene complexes 20
and 21, which reveal a 1.7 ppm upfield shift of the phosphine resonance in the 4imidazolylidene isomer as a consequence of an larger paramagnetic contribution to the
shielding constant (Figure 6).lv Likely, such enhanced electron density will be attractive for
the development of new catalytic processes, in particular when high oxidation-state
intermediates need to become accessible.
+
+
N
N
N
N
N
Ph3P
Pd
PPh3
Ph3P
Pd
PPh3
Br
N
N
N
Br
Br
N
Pd
Pd
N
N
Br
N
I
I
20
21
22
23
P = 18.76
P = 20.48
C = 180.1
C = 181.9
Ph
Figure 6. Isostructural normal and abnormal carbene complexes 20 and 21 containing a 31P NMR probe, and
carbene complexes 22 and 23 containing a benzimidazolylidene ligand as 13C NMR probe.
Coneptually related, Huynh and coworkers have developed a method based on the
13
C NMR
chemical shift of the carbenic carbon of a benzimidazolylidene spectator ligand in complexes
of the type trans-[PdBr2(benzimid)L] (Figure 6).lvi Due to the trans arrangement of the
benzimidazolylidene and an incoming ligand L, the C resonance provides a sensitive probe
for the donor properties of the incoming ligand. Comparison of complexes with L as a normal
vs abnormal imidazolylidene revealed a significant 1.8 ppm deshielding of the diagnostic
benzimidazolylidene carbon resonance in the abnormal carbene complex 23, in good
agreement with a higher electron density at the metal and a stronger trans influence of C4bound imidazolylidenes as deduced from structural and spectroscopic analyses. A limitation
of these comparative NMR spectroscopic studies relates to the fact that the C4-bound
imidazolylidene lacks a substituents in pseudo-ortho position at C5, while the C2-bound
imidazolylidene bears substituents both at N1 and N3. This different substitution pattern is
expected to restrict rotation (or even wagging) about the M–C bond, and may thus affect M–
CNHC orbital overlap. While the data are in good agreement with previous studies on donor
abilities of these imidazolylidenes, stereoelectronic consequences cannot be excluded and
caution is needed to avoid oversimplified comparisons.
Infrared spectroscopy of iridium or rhodium carbonyl complexes [MCl(CO)2(L)] have
emerged as by far most popular method to assess ligand donor properties in carbene
complexes (L = carbene, M = RhI or IrI).lvii This method is particularly susceptible to
stereoelectronic perturbation, as the pseudo-ortho substituents of the carbene directly interfere
with cis-coordinated ligands in a square-planar metal complex, and hence affect the formally
symmetric and asymmetric CO stretch vibration energies in the complex.lviii Two complexes
with identical impact on the first metal coordination sphere are the iridium species 24 and 25
(Figure 7). Comparison of the pertinent CO bands by IR spectroscopy reveals a 16 cm–1
energy difference with the normal carbene species absorbing at higher energy. The stretch
vibration energies have been correlated through linear regression with the more broadly
applicable Tolman electronic parameters (TEPs).lix Accordingly, the TEP of the 4imidazolylidene ligand is 2039 cm–1 vs 2050 cm–1 for the sterically similar 2-imidazolylidene
ligand. For comparison, this difference is about the TEP difference between 2imidazolylidenes and basic bulky phosphines such as PiPr3 and thus underlines the
exceptionally strong donor ability of 4-imidazolylidenes.
Ph
N
N iPr
Ph N
Cl
Ir
CO
CO
Cl
Ir
CO
CO
24
av(CO) = 2019
N iPr
Ph
25
cm–1
av(CO) = 2003 cm–1
Figure 7. Normal and abnormal carbene complexes 24 and 25 with identical first coordination sphere and CO
ligands as probes for determining Tolman electronic parameters for imidazolylidene ligands.
The higher nucleophilicity of 4-imidazolylidenes compared to C2-imidazolylidenes has also
been exploited in metal-free transformations. Bertrand and coworkers have successfully
demonstrated a selective functional group transfer in C2-substituted imidazolium salts.26,lx
Upon deprotonation at the C4/C5 position, functional entities at the C2 position such as
phosphenium and carbonyl groups are transferred to the C4/C5 position, thus leading
eventually to a normal 2-imidazolylidene salt which is functionalized at the olefinic
backbone. The transfer has been rationalized by the transient formation of an abnormal
carbene in the presence of a base, and subsequent intermolecular substitution of the C2-bound
carbene for a C4-bound carbene at phosphorus or the carbonyl group, respectively, thus
releasing a normal imidazol-2-ylidene. Principally, this reaction is reversible and therefore,
the observed products lend further support to an enhanced nucleophilicity and lower stability
of the abnormal carbene than the normal analogue, even though steric effects may in this case
also contribute to the observed selectivity.
As a direct implication of the enhanced electron density at the metal center due to abnormal
carbene bonding, redox processes are expected to be significantly altered when moving from
C2- to C4-bound imidazolylidene complexes. This reactivity has been probed by exposing
complexes 14 and 15 to an atmosphere of chlorine in order to facilitate oxidative additions
and the formation of palladium(IV) species.45 While complex 14 with 2-imidazolylidene
ligands is inert under these conditions and is recovered unmodified, complex 15 reacts
instantaneously. However, the imidazolium salt 26 has been isolated from this reaction rather
than the expected high-valent palladium species (Scheme 6). Formation of this salt has been
rationalized by a sequential oxidative addition and subsequent reductive elimination the
chloroimidazolium cation. An alternative mechanism involving Cl–Cl bond polarization over
the Pd–CNHC bond and subsequent heterolytic bond cleavage in a metathesis-type reaction
appears less probable in particular when considering that related 4-imidazolylidene complexes
that lack a substituents at C5 position do not lead to reductive CNHC–Cl bond formation.
Instead, reductive elimination of the two carbene ligands affords the strained tricyclic dication
27 due to reductive CNHC–CNHC bond formation, even though the Pd–CNHC bond is less
shielded in this complex. The selective homocoupling has been attributed to the increased
steric flexibility of the ligand in the absence of the C5-bound methyl group, which facilitates
conrotatory twisting en route to the C–C bond formation. In contrast, this rotation is restricted
in C5-substituted analogues and as a consequence, reductive C–Cl elimination is the prevalent
pathway. This selectivity difference in reductive eliminations emphasizes the relevance of
steric and stereoelectronic factors when comparing normal and abnormal carbene complexes,
and in particular the importance of the substituents ortho to the metal-bound carbenic site.
Scheme 6. Oxidative addition reactions with different abnormal NHC metal complexes
[PdCl4]2–
Cl Cl
iPr N
+
+
N
N
26
Cl
Cl
Cl2
N iPr
M = Pd
R = Me
R
iPr N
reductive C–Cl
elimination
R
M
N
N
N iPr
M = Pt
R=H
Cl2
[PdCl4]2–
Cl2
M = Pd
R=H
iPr
reductive C–C
elimination
N +
+
N
N
iPr
N
27
Cl
Cl
Cl
Pt
iPr N
N Cl N
N iPr
28
Stabilization of the surmised high-valent intermediate has been accomplished in related
platinum chemistry. In agreement with the generically higher stability of platinum in the +IV
oxidation state as compared to palladium, the platinum(II) dicarbene complexes readily
undergo oxidative addition and the corresponding platinum(IV) species 28 has been
characterized spectroscopically and also by X-ray diffraction analyses (Scheme 6).lxi
The higher propensity of abnormal carbenes to undergo reductive cleavage may hint to a
reduced stability of the M–CNHC bond in 4-imidazolylidenes when compared to their C2bound homologues. Such a trends has also been deduced in a complementary study by Cavell
and co-workers.22 In pioneering work, they have probed reductive eliminations from the
mixed biscarbene platinum(II) hydride 11 in the presence of an alkene (Scheme 7). Selective
elimination the 4-imidazolylidene ligand and formation of the C2-substituted imidzaolium
salt has been observed uon formation of the platinum(0) complex 12. No reductive
elimination of the normally bound 2-imidazolylidene ligand has been detected under these
conditions. Again the steric inequivalence may play a critical role in promoting selective
elimination processes, in particular owing to the fact that the carbenic site of the C2-bound
imidazolylidene is shielded by two bulky mesityl groups while the metal-bound carbon of the
4-imidazolylidene is much more exposed. Despite these steric considerations, this work is in
line with a notion that abnormal imidazolylidenes are more prone to reductive elimination
than normal imidazolylidenes.
Scheme 7. Selective reductive elimination from a mixed normal / abnormal biscarbene palladium complex.
Mes
N
Mes
Br
N
Pt
N
H
Mes
29
N
R'
R
R
N
Br–
R'
N
Pt
N
R'
Mes
30
N
R
31
4.1.2 Comparative evaluation of imidazolylidene complexes in hydrogenation catalysis
and related transformations. The fact that mesoionic/abnormal carbene ligands increase the
electron density at the metal center suggests that catalytic processes should be accelerated
which involve an oxidative addition in the rate-limiting step. Thus, oxidative addition of R–Cl
in cross-coupling chemistry or H2 addition in hydrogenation should theoretically be facilitated
by metal complexes containing abnormal carbenes. However, the observed propensity for
reductive carbene elimination may compromise the reliability of 4-imidazolylidenes as
spectator ligands of a catalytically active metal center and may facilitate reductive elimination
processes.
Hydrogenation catalysis has been probed initially with palladium(II) complexes 19 and its
normal bisimidazol-2-ylidene analogue 32 as catalyst precursors (Scheme 8). The C4-bonding
mode indeed reveals significantly higher conversion of alkenes to alkanes under one
atmosphere of H2 than the C2-bound analogue. While the higher activity may be attributed to
more facile H2 oxidative addition when the palladium center is electron-rich,lxii mechanistic
investigations involving mercury poisoning and in particular dynamic light scattering (DLS)
experiments provide strong evidence for the formation of a heterogeneous phase in the
submicron range (rh 100–500 nm).lxiii An experiment involving filtration at specific time
intervals and combined analysis of the catalytic activity by DLS and gas chromatography
indicates that catalytic turnover is correlated with particle formation. In parallel, NMR
analysis of reactions under substoichiometric conditions (substrate/catalyst ratio 3:1) indicates
the gradual formation of an imidazolium salt that contains a protonated C4 site. This
reactivity pattern is highly similar to the reductive C–Cl bond formation observed for these
complexes in the presence of Cl2, and thus suggests rapid elimination of the ligand. Instead of
the generation of [PdCl4]2– as observed with chlorine (cf formation of 26 and 27), a related
palladium hydride species is expected to form under H2 atmosphere, which will obviously
aggregate to colloidal and catalytically competent palladium(0). Accordingly, the higher
catalytic activity emerging from complex 19 is indeed a consequence of the higher electron
density at palladium due to the C4-bonding of the imidazolylidene, though the effect is only
exploited for catalyst activation, and not for catalytic conversions. Based on this model, the 2imidazolylidene ligand imparts insufficient electron density for the palladium to undergo
oxidative addition and catalyst activation.
Scheme 8. Catalytic olefin hydrogenation with isosteric bisimidazolylidene palladium complexes.
cat.
H2 (1 atm)
cat:
2+
L
L
Pd
N
N
Pd
N
N
N
19
active, via Pd0 colloids
2+
L
L
N
N
N
32
inactive
Similar reductive elimination processes and the formation of a colloidal metal layer have been
observed when using abnormal dicarbene rhodium complexes.47 Hydrogenation of styrene as
model substrate is only observed if relatively forcing conditions are used (elevated
temperature, high H2 pressure), and these conditions have been demonstrated to lead to rapid
carbene ligand loss and colloidal rhodium as presumed catalytically active species.
Transfer hydrogenation reactivity has been evaluated comparatively with C2- and C4-bound
imidazolylidenes. The mutually related biscarbene rhodium(III) complexes 33 and 34,
prepared by NaOAc-assisted double C–H bond activation from RhCl3 and the corresponding
bisimidazolium salt are catalytic activity in the ketone hydrogenation using iPrOH as
dihydrogen source (Scheme 9).lxiv Conversions are only high when the carbene is bound
through the C4 position, while the C2-bound analogues are essentially inactive. The
comparison is thwarted to some extent by the different ortho shielding in the two complexes.
Such differences can influence C–H bond activation substantially by inhibiting or facilitating
the rotation of the NHC ligand54 (see also C–C vs C–Cl reductive eliminations discussed
above). When considering the remote position of the heterocyclic substituents, it is not
surprising that the substitution pattern on the heterocyclic nitrogen atoms does not influence
the catalytic activity. In contrast, electronic differences are more relevant as and high electron
density of at the metal center is beneficial. In line with this model, an exchange of the iodide
spectator ligands for chlorides in complex 34 increases the catalytic activity by a factor of
three, and the turnover frequency at 50% conversion (TOF50) increases from 100 to 300 h–1.
Kinetics of this transfer hydrogenation are unsurprising and reveal regular conversion curves,
thus hinting to a molecular mechanism rather than a ligand dissociation.
Scheme 9. Catalytic transfer hydrogenation with reladed bisimidazolylidene rhodium(III) complexes.
O
O
cat.
iPrOH, KOH
cat (1 mol%):
+
nBu
N I
N
N
NCMe
Rh
N I
nBu
NCMe
33
17% (2 h)
iPr
N
N
N
N
iPr
+
X
NCMe
Rh
NCMe
X
34
53/91% (0.5/2 h); X = I
73/97% (0.5/2 h); X = Cl
The ruthenium complexes 35 and 36 containing a monodentate normal and abnormal
imidazolylidene ligand, respectively, have been investigated as catalysts in a related hydrogen
transfer reaction that lead overall to the -alkylation of alcohols (Scheme 10).lxv This
hydrogen borrowing cascade reaction proceeds significantly better and with higher selectivity
when abnormal carbene complex 36 is used than with the C2-bound imidazolylidene complex
precursor. While once more, the sterics in the first coordination sphere of ruthenium are
different in complexes 35 and 36 and prevent an unambiguous attribution of the distinct
reactivity patterns to the mesoionic/abnormal coordination mode, the overall trend agrees with
related transfer hydrogenation data (see above).
Scheme 10. Catalytic in -alkylation of alcohols with imidazolylidene ruthenium(II) complexes.
OH
OH
O
cat.
R +
BuOH, KOH
R
cat (1 mol%):
Ru
N
Ru
Cl
N
Cl
N
Cl
Cl
N
35
36
>95%
90:10
yield (22 h)
selectivity (a/k)
60%
78:22
Catalytic hydrosilylation constitutes an alternative to direct or transfer hydrogenation and has
been evaluated in a comparative manner with the imidazolium salts 37 and 38 and [Pt(nbe)3]
(nbe = norbornene) (Scheme 11).lxvi Catalyst formation is surmised in situ as styrene is
hydrosilylated in the presence of a silane in good yields, whereas ketones are only moderately
converted. The presence of the methyl group on the imidazolium salt 38 and hence
presumably the abnormal carbene bonding mode has a pronounced influence on the product
selectivity. While the catalyst derived from [Pt(nbe)3] and imidazolium salt 37 affords the
branched 1-phenethyl silane I as the major product (82% selectivity), the catalytic mixture
originating from imidazolium salt 38 affords predominantly the substitution product II
resulting from dehyrogenative hydrosilylation (83% selectivity). The different selectivity in
C–Si bond formation involving predominantly either the styrene  or  position and the
propensity for dehydrogenation with the C2-methylated imidazolium salt 38 point to
fundamentally different modes of action.
Scheme 11. Catalytic hydrosilylation of styrene with in situ generated platinum catalysts derived from different
imidazolium salts.
SiEt3
[Pt(nbe)3]
37 or 38
SiEt3
HSiEt3
I
ligand precursor:
II
Br–
Mes
N
+
N
37
82:8:10
III
Br–
NEt2 Mes
N
+
N
NEt2
38
selectivity (I/II/III) 5:83:12
Some of the reactivity difference may be a consequence of the different surmised
coordination modes of the imidazolium salts. Compound 38 with a C2-methylated
heterocycle is pre-arranged for metal coordination at the olefinic site to form a 4imidazolylidene complex, while the 1,3-disubstituted salt 37 contains an acidic C2-bound
proton and is thus expected to produce a normal 2-imidazolylidene complex. While both
strategies have been validated in numerous cases, a number of examples have been reported
in which the C2-position, albeit unsubstituted, is sterically too congested to be available for
coordination and hence spontaneous C4/5 bonding takes place instead.29 Likewise, a C2bound methyl group has been reported to be an unreliable protecting group and both, C–C
bond cleavage as well as exocyclic C–H bond activation has been observed instead of the
anticipated C4-metalation.27 This caveat warrants the actual isolation of the carbenic complex
as precursor as it reduces some of the difficulties in identifying the catalytically active
species.
A remarkable feature has been noted in the conversion of silanes with the preformed
dicarbene rhodium(III) complexes 33, 34, and the mixed normal/abnormal dicarbene system
11 (Scheme 12).48 Rather than providing the expected hydrosilylated products, these
complexes catalyze the oxidation of silanes to silanols and siloxanes, depending on the water
content of the solvent. When using dihydrosilanes, R2SiH2, polymeric material is formed and
hence, this methodology may be useful for the production of polysiloxanes without the use of
highly reactive silyl triflates or chlorides as starting material and without the need for a base
additive to deactivate any strong acid side products. With alcohols as substrates, the selective
protection of this functional group is accomplished, and hence, these complexes provide a
clean access to silylethers under acid- and base-free conditions. No significant discrepancy in
catalytic activity has been noted upon changing the carbene coordination mode in these
complexes. However, when a deuterated silane such as DSiEt3 is used as the silylating agent,
selective deuterium incorporation into the heterocyclic C5 position has been noted, and only
in the abnormal heterocyclic carbene moieties. Thus, complex 11, which features both a 4imidazolylidene ligand and a 2-imidazolylidene ligand, affords the monodeuterated complex
11–D, while complex 34 undergoes a double H/D exchange and yields complex 34–D2. The
monodeuteration of complex 11 is intriguing since this complex still features a relatively
acidic C2-bound proton in the 4-imidazolylidene ligand moiety, which is completely inert
towards isotope exchange reactions. Similar reactivity patterns have been observed when
exposing these complexes to CD3COOD (see above). Tentatively, this isotope exchange
points to a pronounced mesoionic character and partially negative charges localized on the C5
atom in the 4-imidazolylidene ligand, but not in the C2-bound anologue. This model hence
suggests that the Si–D bond is activated cooperatively by the mesoionic ligand and the Lewis-
acidic metal center. Such cooperativity has precedents,lxvii and it may become an important
feature together with the exceptional donor ability when designing catalyst precursors based
on 4-imidazolylidenes.
Scheme 12. Catalytic silylation of alcohols with imidazolylidene rhodium(III) complexes.
R OH
cat.
H–SiPhMe2
R O SiPhMe2
cat (1 mol%):
+
nBu
N I
N
N
NCMe
Rh
N I
nBu
NCMe
33
73/92% (0.5/2 h)
iPr
N
N
N
N
iPr
+
+
I
NCMe
Rh
NCMe
I
34
78/96% (0.5/2 h)
N
N
Rh
N nBu
NCMe
NCMe
I
N
nBu
11
88/>99% (0.5/2 h)
4.1.3 Comparative evaluation of imidazolylidene complexes in cross-coupling catalysis.
Several studies have reported on the catalytic activity of abnormal imidazolylidene palladium
complexes in Mizoroki-Heck and in Suzuki-Miyaura coupling reactions, evidently inspired by
the huge success of normal 2-imidazolylidene palladium complexes such as Organ’s PEPPSI
system in this type of transformations.12 Only very few of these studies have been
comparative between 2- and 4-imidazolylidenes, and while the focus is on these systems,
related studies are also discussed.
One motivation for exploring strong donor carbenes in cross-coupling catalysis relies on some
limitations in converting hydrocarbon chlorides owing to their low reactivity in oxidative
additions. Enhancing the nucleophilicity of palladium(0) by coordination to a strong donor
may, in principle, alleviate some of these restrictions and allow the energy of this rate-limiting
step to be lowered. Hence, this approach has the potential to facilitate the conversion of thus
far unreactive substrates.
An early study has evaluated C2- vs C4-bound imidazolylidenes in the mixed
normal/abnormal dicarbene palladium complex 39 (Figure 8). When compared to the
homoleptic complex 40, which features two C2-bound carbenes, Lebel, Nolan and coworkers
recorded a much higher catalytic activity of the mixed species both in the Mizoroki-Heck
coupling of bromobenzene with butyl acrylate, as well as in the Suzuki-Miyaura coupling of
aryl chloride with arylboronic acid.lxviii An in situ protocol involving mesityl-substituted
imidazolium salt, [Pd(OAc)2], and a base induces catalytic activity that is comparable to that
of the preformed complex 39, suggesting a similar catalytically active species from both
procedures. It is worth noting, though, that the latter conditions are identical to those used to
synthesize the catalytically less competent homoleptic biscarbene complex 40. While the
catalytic competence of complex 39 is intriguing, the catalytically active species is elusive. It
is generally accepted that the active cross-coupling species is a [Pd0(L)] species and that one
of the two ligands of the palladium(II) precursor will serve as a ‘throw-away’ ligand,
illustrated e.g. by the pyridine in the PEPPSI system. Accordingly, one of the two ligands in
complex 39 most presumably dissociates en route to the formation of the catalytically active
species. While the strong donation of the 4-imidazolylidene may surrogate a stronger bonding
and hence easier departure of the C2-bound imidazolylidene, the reductive elimination studies
(cf above) suggest a lower stability of the C4-bound ligand. Hence, it remains speculative
whether the higher activity of complex 39 compared to 39 is due less stable bonding and
hence easier elimination of the 4-imidazolylidene ligand, or due to the higher electron density
at palladium in a putative [Pd(4-imidazolylidene)] species.
Mes
Mes
N
Cl
Pd
Mes N
Cl
N
N
Mes
Mes
N
N
Cl
Pd
Cl
N
Mes
Mes
39
40
N
Mes
Figure 8. Palladium complexes featuring normal and abnormal IMes bonding with activity in Suzuki-Miyaura
and Mizoroki-Heck cross-coupling catalysis.
When considering the above mechanistic implications, it is not surprising that biscarbene
complexes such as complexes 14 or 15 or their solvento analogues do not show any
significant catalytic activity in cross-coupling reactions. The strong donor ability of the two
carbene units presumably inhibits the reduction of the palladium to the zero-valent state, and
the rigid chelation does not allow for stabilizing lower coordination number geometries. A
complex closely related to 39 with 2,6-iPr-C6H3 substituents instead of mesityl groups
performs considerably worse than both preformed 2-imidazolylidene complexes as well as in
situ generated catalytic species from mixing [Pd(OAc)2] and the corresponding imidazolium
salt, when used as catalyst precursors in intramolecular arylations.lxix This work contrasts with
a recent study using a very bulky 4-imidazolylidene palladium complex 41,lxx derived from
Bertrand’s persistent abnormal carbene.26 This ligand undergoes cyclometalation and forms a
bidentate species when complexed to palladium. While run under basic conditions, which
favor cyclometalation,lxxi cross-coupling with this chelating palladium system has been
reported to work smoothly at room temperature even when using deactivated aryl chlorides as
substrates (Scheme 13). The activity is remarkable when considering the unfavorable
implications of chelate structures in cross-coupling catalysis.
Scheme 13. Catalytic Suzuki-Miyaura cross-coupling with complex 41.
R
R
+
cat.
(HO)2B
NaOMe
Cl
cat (0.73 mol%):
R
Dip
N
Ph
N
Dip
Pd
Cl
2
CF3
H
Me
OMe
time/h
2
3
4
8
yield
98%
97%
98%
98%
41
In several studies, elevated temperatures have been used to induce cross-coupling catalysis.
The N-dimethylated complexes 20 and 21 (cf Figure 6), which impose the same steric effects
on the palladium center reveal very diverging catalytic activity when using arylbromides as
substrates at room temperature.55 While the C4-bound imidazolylidene complex 21 shows
appreciable activity (80% conversion after 19 h at RT), the C2-bound analogue is essentially
inactive (conversion <3%) under these conditions. When using chloroacetophenone as the
substrate, heating of the reaction mixture is required to induce activity. At 140 °C, the C2bound system is slightly better performing than the C4-bound imidazolylidene complex (79%
vs. 67% conversion after 2 h). The enhanced activity emerging from complex 20 may be a
direct consequence of the forcing reaction conditions, which may facilitate carbene
dissociation. Such complex degradation and the formation of (potentially carbene-free)
nanoparticles has been demonstrated in triazolylidene palladium chemistry under much milder
conditions,lxxii and may thus be a direct risk for reactions performed with carbene palladium
complexes at elevated temperatures. Along similar lines, interpretation of the catalytic activity
of the benzimidazolylidene palladium complexes 16 and 17 (cf Figure 5) requires caution.53
Due to the high temperature (140 °C), homogeneous catalysis may be compromised and
hence, it may be speculative to attribute the slightly higher activity of complex 17 solely to
the stronger electron-donating effect of the abnormal carbene ligand. In fact, this ligand may
be dissociating during the catalyst activation stage and may play only a role in facilitating the
formation of the catalytically active species, but not in the turnover-relevant catalytic cycle.
4.2 N,X-heterocyclic Carbene Complexes
In contrast to the vast investigation of abnormal imidazolylidenes, studies on related
complexes comprising an oxygen or sulfur atom in the heterocyclic structure are very rare. To
the best of our knowledge, only two reports on (is)oxazoline-derived abnormal carbene
complexes have been published,55,lxxiii and due to the lack of analogous normal systems, a
comparison of structure and reactivity is idle.
In elegant work, Raubenheimer and coworkers have designed a set of brominated thiazolium
salts which, after oxidative addition to [Pd(PPh3)4] gives access to all possible carbene
complexes derived from this heterocyclic structure: the normal C2-bound thiazolylidene
complex 42, and the abnormal thiazolylidene complexes 43 and 44 which are bound through
the C4 and C5 position, respectively (Scheme 14).lxxiv Similar oxidative addition to
[Ni(PPh3)4] affords the corresponding 3d metal complexes, though only the analogues of
complexes 42 and 43 have been reported. While complexes 43 and 44 share the abnormal
bonding mode, complexes 42 and 43 feature an identical steric environment with the N–CH3
group in pseudo-ortho position with respect to the carbenic site. While structural data have
not been determined for all complexes of this series, spectroscopic data reveal some
differences. Specifically, palladium complexation induces a downfield shift of the carbenic
13
C NMR resonance by about 40 ppm upon formation of complexes 42 and 43, though only a
comparably small 20 ppm shift for the sterically dissimilar complex 44. This triade has been
evaluated in Suzuki-Miyaura cross coupling catalysis of activated aryl bromides.38 At 70 ºC
and under inert reaction conditions, the same orders of catalytic activity has been noted for all
three complexes with a slight decrease in activity in the order 42 > 43 > 44. For example after
5 h, conversions have been determined as 63%, 51%, and 40%. Obviously, these conversions
are substantially lower than competitive NHC palladium complexes, presumably due to a lack
of stability of the low-valent palladium intermediate in the absence of steric protection. No
significant distinction of normal vs abnormal bonding mode can be deduced from these
studies and the reactivity patterns are largely similar for all three complexes. Modification of
the substituent at the heterocyclic nitrogen may provide further insights into the effects of the
(ab)normal binding mode on the catalytic activity of the coordinated metal center. Due to
their steric similarity, variations of complexes 42 and 43 may be particularly attractive for
such comparative investigations.
Scheme 14. Palladium-catalyzed Suzuki-Miyaura cross-coupling with all three isomers of thiazolylidene.
Scheme 14. Palladium-catalyzed Suzuki-Miyaura cross-coupling with all three isomers of thiazolylidene.
O
O
+
cat.
(HO)2B
NaOMe
Cl
cat (0.1 mol%):
PPh3
N
Pd
OTf
N
Br
S
S
yield (5/24 h)
PPh3 OTf
PPh3 OTf
Pd
N
Br
Pd
Br
S
PPh3
42
PPh3
43
63/81%
51/72%
PPh3
44
40/65%
A series of abnormal S,S-heterocyclic carbene systems has recently been disclosed by a
unique synthetic protocol involving an alkynyl dithiocarbamate, 45, as a masked carbene
(Scheme 15). In the presence of an electrophile, this compound spontaneously cyclizes and
affords a carbene due to the nucleophilic triple bond and the electrophilic carbamate
carbon.lxxv The dynamic properties of the carbene ligand may, however, provide interesting
opportunities for catalytic processes and for the stabilization of crucial intermediates through
different ligand coordination modes.
Scheme 15. Synthesis of abnormal S-heterocyclic carbene adducts.
iPr
iPr
R2N
S
iPr
S
NR2
S
+
iPr
iPr
S
–
iPr
45
NR2
iPr S
iPr
S
iPr
E
46 E = e.g. CO2
4.3 Triazolylidene Complexes
The chemistry of 1,2,3-triazolylidene complexes has been recently reviewed,lxxvi and this
section thus predominantly concentrates on normal vs abnormal coordination modes. This
type of triazolylidenes offer a unique set for comparing the different carbene bonding modes
because the position of the heteroatoms in the heterocyclic carbene structure do not need to be
altered when moving from a normal to an abnormal coordination mode. The only difference
pertains to the site of N-substitution (N2 vs N3), which should have only marginal
stereoelectronic effects on the metal and the first coordination site (Figure 9).
[M]
[M]
5
1
4
N
3
2
N N
"normal"
1
5
4
N
3
2
N N
"abnormal"
1,2,3-triazolylidene
Figure 9. Generic representation of normal and abnormal triazolylidenes and atom numbering scheme.
While access to the abnormal 1,2,3-triazolylidenelxxvii and to its metal complexes is
straightforwardlxxviii due to regioselective alkylation of the 1,4- or 1,5-substituted triazole
selectively at the N3 position,lxxix substitution of the N2 site requires a different synthetic
approach and involves.lxxx The most compelling comparison of the normal vs abnormal
ligands is available through IR spectroscopic analyses of the iridium carbonyl complexes
[IrCl(L)(CO)2]. Sterically, the two ligands in complexes 47 and 48 are identical and thus any
stereoelectronic effects of the carbenic ligand on the CO reporter groups is expected to be
identical (Figure 10). Despite the different coordination mode, the recorded CO absorption
bands have essentially the same frequency, ave(CO) = 2018 cm–1 and 2012 cm–1 for complex
47,80 and complex 48, respectively.78 Hence in this pair of complexes, no difference between
normal and abnormal carbene bonding mode is detectable. This is not unexpected when
considering the minute changes upon relocating the methyl group between N2 and N3. Also
structural differences are marginal when considering abnormal triazolylidene iridium79 and
gold complexeslxxxi compared to their normal analogues.80,lxxxii
Ph
N N
N
Cl
Ir
N N
N
CO
CO
47
av(CO) = 2017 cm–1
Cl
Ir
CO
CO
48
av(CO) = 2021 cm–1
Figure 10. Normal and abnormal carbene complexes 47 and 48 with almost identical first coordination sphere
and CO ligands as probes for determining Tolman electronic parameters of triazolylidene ligands.
As an immediate conclusion from these preliminary comparisons, the abnormal vs normal
nomenclature in these 1,2,3-triazolylidene complexes overemphasizes the differences, while
the term mesoionic presumably describes both complexes with alkylated N2 and those with
alkylated N3 position equally well. A difference in nomenclature seems unwarranted in this
case. Also, the abnormal bonding mode of triazolylidenes imparts about the same electron
density on the iridium center as the normal bonding mode of imidazolylidenes (cf Figure 7),
indicating that the number and location of heteratoms may be much more relevant than the
abnormal vs normal bonding. Such a conclusion is also corroborated by the distinct reactivity
patterns of related iridium(III) complexes 12 and 13 (cf Scheme 5), which indicate different
H/D exchange propensity, and nucleophilic vs electrophilic behavior.
A different reativity pattern of triazolylidenes vs normal 2-imidazoylidenes has also been
noted in a mixed
Even though 1,2,3-triazolylidene complexes such as 48 feature formally abnormal ligands,
and should thus be included in the same ligand class as 4-imidazolylidenes, the properties of
the two ligands are substantially different. For example, the metal-free triazolylidene is
unstable if one of the nitrogen atoms contains a methyl substituents and rapid CH3-transfer to
the carbenic site has been noted.77,lxxxiii This reactivity pattern is reminiscent of the functional
group transfer noted for C2-substituted 4-imidazolylidenes (see above), however now
involving N3–C rather than C2–X bond cleavage. No such demethylation has been observed
with imidazolylidenes in their metal-free form. Obviously, the methyl transfer is not sterically
promoted and it is thus likely that the group transfer is a consequence of the high
nucleophilicity.
While 4-imidazolylidene complexes are generally unstable in the presence of strong acids and
rapidly yield the demetallated imidazolium salt, the triazolylidene complexes are substantially
more resistant to protonolysis. For example, strong HOTf is required to cleave a
triazolylidene ligand from a modified Grubbs’ catalyst to give a metathesis-active
species.lxxxiv
4.4 Pyridylidene Complexes
4.4.1 Monodentate pyridylidene complexes. Several studies have compared normal vs
abnormal carbene complexes derived from pyridinium salts. While typically, the pyridyl
nitrogen is alkylated in these pyridylidene ligands in order to fix the carbenic bonding mode,
this substitution exerts some front strain (F-strain)lxxxv and distinguishes sterically 2pyridylidene ligands from their 3- and 4-pyridylidene analogues. Of note, only the 3pyridylidene coordination mode produces a formally abnormal carbene complex, while
neutral resonance structures for 4-pyridylidenes exist. Both 3- and 4-pyridylidene ligands lack
a direct heteratom stabilization and have thus also been termed remote NHCs.
An early investigation avoided this F-strain by exploring N-protonated pyridylidene
complexes, i.e. neutral homologues of phenyl complexes. The complexes 50–52 have been
prepared from the corresponding anionic pyridyl complexes 49 by protonation of the pyridyl
nitrogen (Scheme 16).lxxxvi While these studies have concentrated predominantly on the
impact of the metal unit [MBr(PEt3)2]+ on the basicity of the nitrogen lone pair (M =
palladium, platinum), systematic evaluation of the pKa values indicates distinct properties of
the pyridylidene ligand. Thus, the acidity of the pyridylidene ligand decreases in the order 3pyridylidene > 4-pyridylidene >> 2-pyridylidene. This sequence is in agreement with an
stronger mesoionic contribution and hence a N-localized positive charge for the abnormal 3pyridylidene complex, while a larger relevance of a carbenic resonance structure may favor
proton binding to form the 4- and in particular the 2-pyridylidene complexes. The relevance
of carbenic resonance structures has been deduced from a series of spectroscopic
investigations. Thus, a high-frequency shift of the M–Br stretch vibration upon protonation
has been attributed to a reduced trans influence of the pyridylidene ligand as compared to the
anionic pyridyl ligand. In NMR spectroscopy, a shift of the C resonance to lower field has
been observed and both the Pt–C and the Pt–P coupling constants are lower in the
pyridylidene complex than in the pyridyl precursor. In addition, 1H NMR studies indicate a
hindered rotation about the M–C bond in the 2-pyridylidene complexes, which is not
observed in the pyridyl precursors and hence may point to a higher bond order in the
pyridylidene complexes, in agreement with carbenic bonding.
Scheme 16. Synthesis of pyridylidene palladium complexes with a protonated nitrogen.
N
Et3P
Pd
PEt3
Br
49
H+
+
H
N
+
NH
+
NH
Et3P
Pd
PEt3 Et3P
Pd
PEt3
Et3P
Pd
Br
Br
Br
50
51
52
PEt3
Interestingly, the palladium complexes are more acidic than the platinum complexes,lxxxvii
which suggests less propensity of palladium(II) than platinum(II) to form carbenic -backbonding interactions with the pyridylidene ligand. An opposite behavior is expected when
comparing the potential overlap of 2p/4d vs 2p/5d  orbital overlap and hence may have a
different origin. It is worth considering that related gold(I) complexes have a much higher
affinity to form carbenic interactions than the corresponding silver(I) complexes,lxxxviii in parts
due to relativistic effects.lxxxix
Similar distinction of normal vs abnormal pyridylidene complexes has been noted in
rhodium(III) complexes 53–55 (Scheme 17).xc These complexes have been successfully
prepared by decarbonylation of pyridine aldehydes and subsequent N-alkylation. The
elegance of this method relies on two aspects: firstly, all three pyridylidene isomers are easily
accessible, and secondly, the decarbonylation directly installs a carbonyl ligand as a reporter
group on the rhodium center. IR-spectroscopic analysis of the CO stretch vibration of the
pyridylidene complexes reveals a significantly lower frequency for the 3-pyridylidene
complex 54, while the C2- and C4-bound isomers 53 and 55 both display an absorption at
higher energy. While these differences may or may not suggest a more carbenic and hence
neutral ligand structure for the normal 2- and 4-pyridylidene units, the data unambiguously
establish stronger donor properties for the C3-pyridylidene ligand. This behavior is in
agreement with a more pronounced mesoionic structure and an enhanced anionic character of
the metal-bound carbon. Stronger electrophilicity of the C3 position is also in line with the
partial electron density distribution in pyridine heterocycles and reflects, for example, the
difficulties associated with meta-functionalization of pyridines.
Scheme 17. Synthesis of pyridylidene rhodium(III) complexes.
+
Rh
N
Me
CO
53
N
1.
Rh
DMSO
Me
Me 2. MeI
+
CHO
Rh
N
Me
CO
54
+
Rh
N
Me
CO
55
A comprehensive structural and NMR spectroscopic investigation on palladium and nickel
complexes of the general formula [MCl(PPh3)2(pyridylidene)]+ strongly supports these
conclusions.xci Thus, X-ray diffraction analysis of the nickel complexes 56a–58a indicates a
pronounced double bond localization in the normal pyridylidene complexes 56a and 58a that
is reminiscent of the bonding situation in pyridones, while the 3-pyridylidene isomer 57a
features bond length alterations that do not follow simple valence bond resonance theory.
Relevantly, the observed bond length alteration is metal-induced and is not observed in the
ligand precursor chloropyridinium salt. Moreover, the trans influence is equal for all the three
pyridylidene isomers and M–Cl bond lengths are identical within the 3 range. Spectroscopic
analysis reveals that upon metallation, the palladium-bound carbon experiences a 40 ppm
downfield shift in C2- and C4-bound pyridylidene complexes, while bonding via C3 only
produces a 30 ppm shift. This shift difference supports a larger carbenic contribution in the
normal pyridylidene complexes. In addition, the deshielding effect exerted by nickel(II) is
stronger than that of palladium(II) (C 5–10 ppm), which tentatively points towards
enhanced Ni–C -back-bonding and thus to a larger double bond character in the nickel
complexes 56a–58a compared to the palladium(II) analogues 56b–58b. Computational
investigations using energy decomposition analysis suggest that the Pd–C bond is decreasing
in the order 4-pyridylidene > 3-pyridylidene > 2-pyridylidene, while the carbenic vs
mesoionic assignments from X-ray and NMR analyses would rather predict the C3-bound
isomer to feature the weakest bond. In an intramolecular competition experiment using 2,4dichloropyridinium, oxidative addition of the C4–Cl bond takes place selectively, which
supports the notion that the C4 position in pyridinium salts is the most electrophilic site.
+
+
+
N
N
N
Ph3P
M
PPh3 Ph3P
Cl
M
PPh3 Ph3P
Cl
57
56
M
PPh3
Cl
58
a M = Ni
b M = Pd
Figure 11. N-Methylated pyridylidene nickel and palladium complexes 56–58.
In a different set-up, Carmona and coworkers explored pyridylidene formation via a formal
tautomerization process from 2-substituted pyridines at an iridium(III) complex containing a
tris(pyrazol)borate ligand.xcii In contrast to the oxidative addition experiments described
above, formation of the 6-pyridylidene complex 59 is observed as the exclusive product
(Scheme 18). Potentially, these diverging reactivity patterns may be associated with the
reversible C–H activation in the iridium complexes, while oxidative addition is presumed to
be irreversible and hence kinetically controlled. In agreement with such a model, deuterium
labeling experiments reveal that iridium-mediated C–H activation involves all aromatic
positions of the heterocycle.xciii Most likely, however, the tautomerization is a stepwise
process involving first C–H activation followed by Npyr-protonation, and hence the stability of
the formed pyridylidene may have only a subordinate role in controlling product selectivity.
Scheme 18. Formal pyridine to pyridylidene tautomerization at iridium(III).
+
+
TpMe2
Ir
N2
N
TpMe2
R
Ir
Ph
Ph
NH
R
Ph
Ph
59
H
B
IrTpMe2
=
N
N
N
N
N
N
Ir
4.4.2 Polydentate pyridylidene complexes
All types of bipyridine synthons have been used for the formation of chelating pyridinepyridylidene complexes, including 2,1’-pyridine-pyridinium salts, 2,2’- 2,3’- and 2,4’bipyridine-derived precursors.xciv While this variability may provide a useful ligand series for
comparative analyses, each ligand precursor has been complexed with different metals, thus
preventing a meaningful comparison. As an exception, ruthenium complexes containing two
regularly N,N-bidentate bound bipyridyl ligands and a C,N-bidentate pyridyl-pyridylidene
ligand have been spectroscopically characterized (60, 61, Figure 12).xcv A nearly 40 nm
hybsochromic shift has been noted when changing the abnormal 3-pyridylidene coordination
mode to the normal remote C4-bound isomer. Similarly, the ruthenium complexes 62 and 63
have been prepared containing tridente terpyridine-derived ligand set featuring either a
2,2’:6’,4’’ connectivity pattern or a 2,2’:6’,3’’ linkage (Figure 13). xcvi While the former
complex contains a formally neutral pyridylidene ligand, the 3’’-linked pyridylidene species
is abnormally/mesoionically bound. The ruthenium(II)/(III) oxidation potential changes as a
consequence of the different pyridylidene bonding mode and is 50 mV more anodic in
complex 62 containing the C4-bound pyridylidene donor. This behavior has been postulated
to originate from a comparably low electron donation of the 4-pyridylidene ligand due to
partial -back-bonding, hence supporting a carbenic resonance structure, while the more
pronounced mesoionic character in the 3-pyridylidene complex 63 increases the carbanionic
character of the 3-pyridylidene donor site. While this reasoning may indeed point to different
carbenic or mesoionic character, the lower electrophilicity of the meta carbons in pyridine
heterocycles and hence purely inductive arguments may equally explain these changes in
redox behavior, without implication of any enhanced carbenic resonance structure. In
agreement with a reasoning based on inductive properties, it is worth noting that
pyridylidene-amides, another class of mesoionic compounds that can serve as ligands, induce
a substantially larger redox shift of 150 mV upon changing from the formally neutral
resonance form to the mesoionic form.xcvii This comparison suggests that the carbenic
resonance form in 62 may not be very pronounced.
2+
2+
N
N
Ru(bpy)2
Ru(bpy)2
N
N
60
61
Figure 12. Analogues of [Ru(bpy)3]2+ featuring different pyridylidene bonding modes.
2+
2+
N
N
N
N
MeS
N Ru
N
N
MeS
N
62
N Ru
N
N
N
63
Figure 13. Redox-active pyridylidene analogues of [Ru(terpy) 2]2+).
4.4.3 Catalytic applications of pyridylidene complexes. Only few comparative studies have
targeted the reactivity differences that are imposed by distinct pyridylidene bonding modes.
Complexes 56b–58b (cf Figure 11) have been used in Suzuki-Myiaura cross coupling
catalysis.91 In model reactions using 4-bromo-acetophenone as substrate, all three complexes
show essentially identical activity up to about 50% conversion. At later stages of the reaction,
diverging behavior has been observed. While the 3-pyridylidene species displays slightly
slower conversion rates than the 2- and 4-pyridylidene species, the 2-pyridylidene complex
deactivates and reaches a maximum conversion of 78%, lower than the 3-pyridylidene (82%)
and the 4-pyridylidene analogue (86%). Even though the differences are relatively small,
these studies suggest that the reduction of the inductive effect exerted by the nitrogen atom
has some influence on the activity of the catalyst. Interestingly, the final conversions correlate
with the computed bond strength Eint from energy decomposition analysis.
5. Conclusions and Outlook
From the above illustrated comparative investigations it becomes clear that there is no
specific trend in the properties and reactivities of abnormal&mesoionic carbenes when
compared to normal carbenes. While in some cases such as in the imidazolylidenes, the
stronger donor properties as well as a presumably lower M–C bond stability imply different
behavior of normal and abnormal structures, the triazolylidene family does not show any
significant distinction. The pyridylidene complexes in turn display specific ground state
properties as a function of normal vs abnormal bonding of the pyridylidene ligand, though in
terms of reactivity, no compelling evidence has been provided that supports different behavior
and abnormal and normal pyridylidene complexes have similar reactivity profiles.
In fact, many of the diverging reactivity patterns can be readily rationalized by consideration
of the electron density at the metal-bound carbon, and HOMO considerations. Thus in
imidazolylidenes, the comparably lower electron donor properties of 2-imidazolylidenes is a
result of the ylidic character of the N1–C2–N3 fragment, which formally comprises both the
positive and the negative charge. In contrast, charge separation is more pronounced in 4imidazolylidenes since the C4=C5 unit is now formally negatively charged while the positive
charge by and large still resides in the N1–C2–N3 fragment, thus providing a more
pronounced mesoionic structure. In other words, conjugation of the negative and positive
formal charges over five atoms (as in 4-imidazolylidenes) leads to a larger separation and a
higher polarity than separation just over three atoms (as in 2-imidazolylidenes). When
applying these considerations to 1,2,3-triazolylidenes, it is evident that the substitution pattern
on the nitrogen atoms does not change much on the overall mesoionic character, as the
positive charge is predominantly located on the N1–N2–N3 fragment, irrespective of whether
N2 or N3 is substituted.
A general conclusion emerging from these considerations is that when bound to a transition
metal, all these NHC systems display only very little—if not insignificant—carbenic
character.xcviii Obviously this statement does not hold for the metal-free ligands, which show
at some properties of carbenes (such as the disubstituted formally hexavalent carbon). In the
metal complexes, however, a reactivity pattern based on ylidic/mesoionic properties is largely
prevailing, both for normal and abnormal ligand systems. Hence, the term ‘carbene’ appears
to be unwarranted and suggests an over-emphasis of a minor contribution. Instead, a term like
‘dative C-donor’ or perhaps more accurately ‘mesoionic C-donor’ may be more appropriate to
be used for this broad ligand class, with the understanding that a mesoionic ligand is formally
neutral. While the coinage of the term ‘N-heterocyclic carbene’ is most logical in the
historical context and certainly after isolation of the first stable free carbenes in the bulk,3,4 the
exciting 25 years of exciting organometallic NHC chemistry have provided substantial
evidence that a change in nomenclature would be justified. (A similar call would be made for
Fischer carbene systems, which in most cases have only minor carbenic resonance form
contributions.) Further work and discussion will be needed to identify advantages and
disadvantages of the ‘mesoionic C-donor’ terminology and Crabtree may be correct in that
“The carbene concept is far too firmly established to be abandoned at this late stage”.39
Irrespective of the—ultimately semantic—nomenclature aspects, mesoionic C-donors with
large charge separation such as 4-imidazolylidenes and 1,2,3-triazolylidenes constitute a
unique set of ligands at the interface of classical carbanionic ligands and neutral carbene
donors. This interface offers great prospects for oxidations and oxidative couplings, for
stabilizing potentially high-valent transition states, and through metal-ligand cooperativity,
for facilitating proton-coupled electron transfer processes.
Acknowledgements
I thank first and foremost all my coworkers, past and present, for their dedication and
commitment and for the many surprising results, which continue to provide new vistas in a
most exciting research area. Our work has been financially supported over recent years by the
ERC (StG 208561, CoG 615653), the Irish Research Council, Science Foundation Ireland, the
Swiss National Science Foundation, and Johnson Matthey.
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