Full paper
Australian Journal of Chemistry
Towards the synthesis of guanidinate- and amidinate-bridged
dimers of Mn and Ni
Francesca A. Stokes,a Lars Kloo,b Philip J. Harford,a Andrew J. Peel,a Robert J. Less,a
Andrew E. H. Wheatley*a and Dominic S. Wright*a
a
Department of Chemistry, University of Cambridge, Lensfield Road, Cambridge, CB2 1EW,
U. K.; Fax: +44 (0)1223 336362; Tel: +44 (0)1223 763966/36333; email:
aehw2@cam.ac.uk, dsw1000@cam.ac.uk
b
Department of Chemistry, Royal Institute of Technology, Teknikringen 36, S-100 44,
Stockholm, Sweden
2
Abstract
Reactions of Cp2M (Cp = cyclopentadienyl, M = Mn, Ni) with lithium amidinates and
guanidinates are reported. The highly oxophilic nature of Mn leads to the isolation of the
interstitial oxide Mn4O(MeN…CH…NMe)6 (4) in preference to the intended paddle-wheel
homo-dimer Mn2(MeN…CH…NMe)4 when employing the sterically uncongested amidinate
[MeN…CH…NMe]– ligand. In contrast, an analogous reaction using Cp2Ni yielded
Ni2(MeN…CH…NMe)4 (5). The use of mono-protic guanidinate ligands also gave contrasting
results for Mn and Ni. In the first case, the highly unusual spirocycle Mn{NC(NMe2)2}4Li23THF (6) was produced in low yield. For M = Ni, use of the [hpp]– (=
1,3,4,6,7,8-hexahydro-2H-pyrimido[1,2-a]pyrimidinate) ligand gives results comparable to
the synthesis of 5, with Ni2(hpp)4 (7) isolated. In contrast to recent data obtained using Cp2Cr,
the guanidinate ligands do not sequester co-formed CpLi. DFT analysis corroborates the view
that the inter-metal distance in each of the reported dinickel paddle-wheel complexes
(2.4846(8) and 2.3753(5) in 5 and 7, respectively) is defined by the geometric parameters of
the bidentate ligands and that inter-metal bonding is not present.
3
Introduction
N,N´-bidentate ligands generated by the deprotonation of amidines and guanidines
(RN(H)CHNR and RN(H)C(NR2)NR, respectively, Scheme 1) have been extensively
employed in conjunction with transition metals, for example rendering paddle-wheel
complexes that have hitherto promised a range of applications in catalysis and materials
science.[1] The ligands themselves have proved capable of stabilising low oxidation state
metals and their versatility has been enhanced by the inclusion of donor atoms that are ideally
situated for bridging, their donor capability and accessibility being easily adapted through
variation of the R groups. Whereas neutral amidines have been employed as ligands in a range
of coordination complexes,[2] in recent years (anionic) amidinates and guanidinates have
become increasingly popular choices for the stabilisation of both main group[3] and transition
metals.[4] Lately, interest in the potential applications of these systems has focused on the
possibility of achieving amidinate-ligated polymerization catalysts.[5] Guanidinate ligands,
meanwhile, are structurally closely related to their amidinate counterparts, notwithstanding
that they have the potential to delocalise, and so stabilise, a negative charge across three
nitrogen atoms. Like those of amidinates, guanidinate metal complexes have been established
for a variety of metal elements, with the ligands exhibiting a range of bonding modes.[6]
Diverse applications in ring-opening polymerization catalysis, the stabilization of low
oxidation state metallocycles, and metal cage formation have emerged.[7] Recently, the
deployment of guanidinate ligands has also focused on the formation of pre-catalysts for the
dimerization of aldehydes.[8]
4
Scheme 1
Our own prior work with metallocenes, long established as means of supplying organically
soluble sources of transition metals to reactions, has focused on their ability to transfer these
metals in their +2 oxidation state without oxidation or reduction of the metal taking place.
This was exemplified by the recent report of formation of the VV bonded moieties V2(hpp)4
(hppH = 1,3,4,6,7,8-hexahydro-2H-pyrimido[1,2-a]pyrimidine) in [{V2(hpp)4}2(Cp2Li3)]+[Cp3Li2]– (1, Cp = cyclopentadienyl), which was isolated and characterized
following the reaction of Cp2V with hppLi (Figure 1).[9]
Figure 1 Structure of the [{V2(hpp)4}2(-Cp2Li3)]+ component of 1
Amidinate ligands are well documented as being able to bridge or chelate metal centres,
depending on the steric properties of their R groups.[10] In this context, the sterically
undemanding nature of the R groups in N,N´-dimethylformamidine rendered it ideal for
supporting the formation of multiple bonds between metals by bridging between them whilst
leaving the metals relatively accessible. Accordingly, the 1:2 reaction of Cp2Cr with
MeN(Li)…CH…NMe yielded the amidinate-bridged complex Cr2(MeN…CH…NMe)4 (2,
5
Figure 2 and Scheme 2).[11] Crystallography revealed four amidinate ligands bridging two
chromium centres in a typical paddle-wheel style, the inter-metal distance (1.8722(6) Å) being
rather short.[12] Whilst 2 represents a member of a large family of neutral, quadruply bonded
dichromium compounds of general formula Cr2L4 (L = bidentate ligand), it is noteworthy that
the use of a sterically unhindered amidinate ligand was able to give a remarkably kinetically
unprotected structure in the present case. This offers potential in the trapping of small
molecules, as has been recently exemplified by O2-trapping experiments.[13]
Figure 2
Attempts to replicate the synthesis of 2 by treating chromocene with lithiated 1,3diphenylguanidine yielded the more complex salt of the tetraanion
[Cr2{PhN…C(NH)…NPh}4]4– in which the dichromium core is complexed by both neutral
LiCp and cationic [Li2THF]+ to yield an overall formulation of
Cr2{PhN…C(NH)…NPh}4(Li2THF)4(CpLi)2 3 (Figure 2). Data clearly revealed that the
tetraanionic dichromium core formed an ion-pair with four bis-THF solvated Li+ ions and that
this arrangement then formed a co-complex with two CpLi monomer units, generated as a by-
6
product of reaction (Scheme 2). Taken together, these complexes add to the family of
dichromium systems in which quadruply-bonded CrII centres are bridged by four N,N´bidentate ligands.[12,14] In so doing they contrast with the single previously reported example
of an anionic amidinate-bridged dichromium species. Instead of revealing a [Cr2L4]n– (n = 0,
4) structure this was based on the triply-bridged monoanion [Cr2{(1,2-Me2C6H3N])2CH}3]–
and contained two CrI centres that were formally quintuply bonded to one another.[15]
Scheme 2
In the present work, attempts to extend the synthesis of homo-dimers in which transition
metals are bridged by N,N´-bidentate ligands is extended to Mn and Ni. In particular, the
ability to develop the recently noted[11] fabrication of paddle-wheel complexes in the absence
of significant kinetic protection is demonstrated though the use of amidinate and guanidinate
ligands that lack sterically demanding N-aryl groups.
Results and Discussion
Metallocene reaction with lithium N,N´-dimethylformamidinate
Initial efforts focused on the treatment of the sterically accessible N,N´dimethylformamidine[16] salt MeN(Li)…CH…NMe with the polar metallocene manganocene
(Cp2Mn). This reagent has hitherto described a chemistry somewhat distinct from that of other
3d transition metal metallocenes,[17] adding weak nucleophiles to give manganates,[18,19] and
also being implicated in solvent decomposition.[20] Alternatively, it has proved possible to
7
displace Cp– in the presence of stronger nucleophiles (Nu–) to give monosubstituted dimers of
Cp(Nu)Mn[21] or (Nu)3Mn.[22] It has also formed adducts such as (η1-Cp)(η5Cp)Mn{(BnNHCH2)2}[23] (Bn = benzyl) with weak acids, whilst functioning as a Brønsted
base towards other organic acids.[23,24] In the present context, Cp2Mn[25] was reacted with
MeN(Li)…CH…NMe in THF with the aim of transferring metal ions in the +2 oxidation state
in the manner reported recently for vanadium.[9] The expectation that the bidentate amidinate
ligands would facilitate dimerization also suggested the possibility of observing inter-metal
bonding in the resulting aggregate (Scheme 3). In the event, repeated attempts at reaction
yielded only a single isolable product. Consistent with expectation, spectroscopic analysis of
the product revealed delocalized amidinate anions. However, in spite of efforts to eliminate
both oxygen and moisture from the reaction, X-ray crystallography demonstrated that this
material was in fact the interstitial oxide Mn4O(MeN…CH…NMe)6 (4, Figure 3) and that it
adopted the basic zinc acetate structure.[26] Like the recently reported Mn6O aggregate core
motif,[11] the oxide-containing core of 4 is relatively rare. Tetranuclear oxides of Mn have
been reported in the context of inverse crown chemistry, with the structures of
(TMP)4Mn2M2O (TMP = 2,2,6,6-tetramethylpiperidide, M = Li, Na)[20,27] and
(HMDS)4Mn2Na2O (HMDS = 1,1,3,3-hexamethyldisilazide)[27] all revealing square planar
oxide encapsulation. Meanwhile, although several reports of essentially tetrahedral Mn4O
structures exist based on bridging N,O-ligands or utilising ligands with substantially expanded
bite angles (e.g. NCCN-based) relative to those in 4,[28] the present structure (Mn–O–Mn
107.7(7)-111.6(1) ) is closely analogous only to that of Mn4O(PhN…CH…NPh)6.[29] However,
in that case H2O was considered to represent the oxide source.[29b] In contrast, in the case of 4,
the ability to enhance the yield by one-third (from 39% to 52% wrt Cp2Mn) by the intentional
introduction of dried O2(g) into the reaction mixture suggests that molecular oxygen was
responsible.
8
Scheme 3
Figure 3 Molecular structure of 4 with H atoms omitted for clarity.
Selected bond lengths (Å) and angles (): Mn–O 2.014(2)-2.028(2),
Mn–N 2.104(2)-2.124(2), Mn–O–Mn 107.7(7)-111.6(1), N…C…N
125.9(3)-126.8(3).
A similar reaction to that discussed above was next attempted using nickelocene, Cp2Ni.
N,N´-Dimethylformamidine was monometallated using t-BuLi in THF prior to treatment with
the bis(cyclopentadienyl) reagent. Subsequent recrystallization at –30 C afforded brown
9
crystals which revealed only amidinate ligands by NMR spectroscopy. Single-crystal X-ray
diffraction showed that, in contrast to the partially oxidized product 4 seen for Mn, reaction
with Cp2Ni had successfully afforded the amidinate-bridged paddle-wheel homo-dimer
Ni2(MeN…CH…NMe)4 (5, Scheme 4 and Figure 4) in satisfactory yield (66% wrt Cp2Ni).
Scheme 4
Figure 4 Molecular structure of dinickel complex 5. Selected bond
lengths (Å) and angles (): NiNi 2.4846(8), N–Ni 1.903(3)-1.907(2),
C…N 1.308(3)-1.321(4), N–Ni–N 89.7(1)-90.2(1), N…C…N 124.2(3)124.8(3).
The paddle-wheel complex 5 reveals an inter-metal distance of 2.4846(8) Å and a structural
motif that has been reported before in dinickel complexes incorporating N,N´diarylformamidinates prepared using NiX2 (X = Cl, Br, OAc).[30] However, in contrast to the
prior art the present work generalizes both the ability of Cp2M to react with a lithium
amidinate to yield an NCN-bridged dimer and the ability of metallocene chemistry to extend
the field of paddle-wheel motifs to incorporate kinetically unprotected dimers in which the
10
terminal groups of the amidinate ligands exhibit minimal steric influence. Hence, though the
essential structural motif is replicated, the ligand choice in 5 can be contrasted with that of the
necessarily bulky/protective N,N´-di(4-tolyl)formamidinate[30a] and N,N´-di(3,5xylyl)formamidinate[30b] ligands used previously.
Metallocene reaction with lithium guanidinate reagents
Reaction of Cp2Mn with the lithium salt of monoprotic guanidine HNC(NMe2)2 gave only a
rather low yield (5% wrt Cp2Mn) of crystalline material that analyzed as Mn{NC(NMe2)2}4Li23THF 6 (Scheme 5).[31] In participating in the formation of this highly
unusual spirocycle, the guanidinate ligand showed the ability the bridge between two metal
centres in the fashion demonstrated by 1,1,3,3-tetraethylguanidinate in the heteroleptic dimer
of Mn{-NC(NEt2)2}N(SiMe3)2.[32] However, moving to heterometallic scenarios, the present
spirocycle bears comparison only with alkyl and aryl manganates of the type Mn{R}4Li22TMEDA (TMEDA = N,N,N´,N´-tetramethylethylenediamine),[33] imidomanganate
Mn{-N(t-Bu)}4Li22DME (DME = dimethoxyethane)[34] and more recently reported Mn{NC(t-Bu)2}4Li22THF.[35]
Scheme 5
11
In the case of Cp2Ni, having established that reaction with lithium amidinates could give a
homo-dimer, we next sought to study the analogous reaction with a lithium guanidinate. In the
only previous example we attempted of this type of reaction the novel dichromium tetraanion
in 3 was formed. In that case, two of the three N-centres in each of the four guanidinate
ligands participated in N–Cr bonding whilst the remaining four N-centres resided in the
cluster periphery and stabilized either Li+ or CpLi. While the first of these actions was
imparted by the need for charge balance, the second was not. We were therefore interested to
see whether CpLi would be sequestered during the attempted synthesis of Ni2(hpp)4 to give
Ni2(hpp)4nCpLi or whether the pure paddle-wheel would be obtained. With this in mind,
hppH in THF was monometallated using n-BuLi and Cp2Ni was then added. Subsequent
recrystallization at –30 C afforded purple crystals. NMR spectroscopy suggested the absence
of CpLi, revealing instead the presence only of [hpp]– ligands and thus pointing to the
formation of Ni2(hpp)4 (7) in a modest yield of 15% wrt Cp2Ni. Single crystal X-ray
diffraction confirmed this, revealing a dimeric complex with a 2.3753(5) Å inter-metal
distance, shorter than that seen in 5 (Figure 6 and Scheme 6).
Scheme 6
12
Figure 5 Molecular structure of 7 with H-atoms omitted. Selected bond
lengths (Å) and angles (): NiNi 2.3753(5), N–Ni 1.890(2)-1.916(2),
…
C NNi 1.319(4)-1.335(3), N–Ni–N 88.40(7)-91.36(1), N…C…N 119.6(2)120.2(2).
The isolation and characterisation of 7 contrasts with the recent report of dichromium
tetraanion 3 both in terms of charge and the action of the ligands. In the case of 3, the
formation of a quadruple bond between Cr centres, rendering each 16e–, drove the formation
of a stable tetraanion whose charge was offset by the inclusion of alkali metal counterions. In
contrast, 7 incorporates a ligand set (hpp) that is incapable of formal dimetallation and this
leads to a neutral MII dimer. Related to this, in 3 two guanidinate ligands utilised their third Ncentre to stabilise two [Li2THF]+ ions. However, it is notable that the remaining two ligands
also had active third N-centres; in the absence of the need for further charge equalization these
sequestered half of the CpLi formed as a by-product of dimer formation (Scheme 2). In
contrast, in the present case, 7 is isolable as a pure material in which the final N-centre in each
guanidinate ligand is inactive (Scheme 6) with co-prepared CpLi remaining in solution.
13
Theoretical modelling of dinickel complexes
Prior examples of dimers fabricated using Cp2M (M = V 1, Cr 2 and 3) have yielded multiply
bonded dimetallic frameworks. In 1-3, MII centres are best viewed as being 14 or 16e– and of
participating in triple (1) or quadruple (2 and 3) intermetal bonds. In 5 and 7 the inter-metal
distances are 2.4846(8) and 2.3753(5) Å, respectively. However, since it has been suggested
that each d8 NiII centre should exhibit a diamagnetic square planar geometry such that intermetal bonding in a notional Ni2 moiety is precluded by the filling of both bonding and
antibonding ,  and  orbitals,[36] the structure of 5 was interrogated theoretically[31] using
the Gaussian09 package.[37] Calculations were performed using the D4 point group, although
the true geometrical ground state would have a lower symmetry than this due to the tendency
of the methyl groups to rotate. The inter-metal distance was calculated to be 2.512 Å with a
Natural Bond Order (NBO) of 0.056 indicating that there is no significant direct Ni-Ni
bonding in the electronic ground state of the model compound (Figure 6). As mentioned
above, considering that NiII is a d8 system in a square-planar coordination defined by the
ligands, no Ni-Ni bonding is expected. Of course, excitation, oxidation or reduction of the
compound will most likely mainly involve the orbitals centred on the Ni atoms and may thus
change the overall bonding scheme.
14
Figure 6 Structure of Ni2(MeN…CH…NMe) 5 optimized at
B3LYP level. At 2.512 Å, the calculated inter-metal
distance is extended relative to that of 2.4846(8) Å in 5.
The corresponding NBO is 0.056.
Conclusions
In summary, reactions of Cp2M (M = Mn, Ni) with lithium amidinates and guanidinates are
reported. In the case of Cp2Mn reactions proved difficult to control and oxidation yielded
tetrahedral 4 when using a lithium amidinate substrate, whilst deployment of a lithium
guanidinate led only to the isolation of heterometallic spirocycle 6 in poor yield. In contrast,
for the use of Cp2Ni both lithium amidinates and guanidinates yielded dimeric paddle-wheel
complexes of the type Ni2L4 (L = MeN…CH…NMe 5, hpp 7). Unlike in other recent examples
of this chemistry, the dimetallic cores of these structures were suggested by DFT calculations
to lack inter-metal bonding. However, in common with our other recent work in this field,[9,11]
the isolation of 5 and 7 reinforce the view that the use of Cp2M represents a convenient and
general route by which to fabricate kinetically unprotected paddle-wheel dimers, the
accessibility of the metals within which has the potential to lead to applications in materials
chemistry.
15
Experimental
General Experimental Procedures
Reactions and manipulations were carried out under an inert atmosphere either of dry nitrogen
or dry argon, using double manifold and glove-box techniques. THF was distilled off sodiumpotassium amalgam immediately prior to use. Normal- and tertiary-butyllithium (n-BuLi and
t-BuLi) were purchased from Sigma Aldrich. Cp2M (M = Mn, Ni)[25] and N,N´dimethylformamidinate[16] were prepared using modified literature routes. Other reagents were
purchased and used without further purification. NMR data were collected on a Bruker
Avance 500 BB ATM (500.05 MHz for 1H, 125.78 MHz for 13C) FT NMR spectrometer.
Spectra were obtained at 298 K and chemical shifts are internally referenced to the deuterated
solvent and calculated relative to SiMe4. Chemical shifts are expressed in δ ppm. The
following abbreviations are used for NMR spectra: s = singlet, t = triplet. NMR solvents were
stored in a glove-box over freshly prepared sodium or potassium mirrors.
Mn4O(MeN…CH…NMe)6 4
a) N,N´-Dimethylformamidine (0.32 mL, 4.5 mmol) was stirred in THF (5 mL). t-BuLi (2.7
mL, 1.7 M in hexanes, 4.5 mmol) was syringed into this solution dropwise under Ar at –78
C. The resulting yellow suspension was allowed to warm to room temperature to give a white
suspension. This was cooled to –78 C and a suspension of Cp2Mn (278 mg, 1.5 mmol) in
THF (5 mL) was added dropwise by cannula. The resulting solution was left to warm to room
temperature before being stirred overnight. It was heated to reflux for 10 minutes and then
filtered in vacuo to give a bright pink solution. Storage at –30 C gave colourless crystals of 4
(97 mg, 39 % wrt Cp2Mn), m. p. 228-230 C. δH (500 MHz, d8-THF) 5.32 (s, 1H, N2CH),
1.22 (s, 6H, Me). δC (125 MHz, d8-THF) 130.3 (N2CH), 29.2 (Me). Satisfactory elemental
analysis could not be obtained after multiple attempts. X-ray crystal data: C18H42Mn4N12O, M
16
= 662.40, monoclinic, space group C2/c, a = 19.629(4), b = 11.044(2), c = 27.210(5) Å,  =
93.36(3) , V = 5889(2) Å3, Z = 8, calcd = 1.494 g cm–3, Mo-K radiation,  = 0.71073 Å,  =
1.709 mm–1, T = 180(2)K. 22895 data (5188 unique, Rint = 0.0371,  < 25.02 ) were
collected. wR2 = 0.0921, conventional R = 0.0301 on F values of 4404 reflections with F2 >
2(F2), S = 1.029, 316 parameters. Residual electron density extrema 0.282 eÅ–3.
b) N,N´-Dimethylformamidine (0.32 mL, 4.5 mmol) in THF (5 mL) was reacted with t-BuLi
(2.7 mL, 1.7 M in hexanes, 4.5 mmol) and Cp2Mn (278 mg, 1.5 mmol) as for route a). After
reaction the mixture was cooled to –78 C and the resulting solution treated with O2 (dried
over P4O10) for 5 secs. before being allowed to warm to room temperature. It was then stirred
overnight, heated to reflux for 10 minutes and filtered in vacuo. Storage at –30 C gave
colourless crystals which analysed by m. p., NMR spectroscopy and X-ray diffraction as 4
(128 mg, 52 % wrt Cp2Mn).
Ni2(MeN…CH…NMe)4 5
N,N´-Dimethylformamidine (0.32 mL, 4.5 mmol) was stirred in THF (5 mL). t-BuLi (2.7 mL,
1.7 M in hexanes, 4.5 mmol) was syringed into this solution dropwise under Ar at –78 C.
The yellow suspension was left to warm to room temperature to give a white solution. This
was cooled to –78 C and a suspension of Cp2Ni (283 mg, 1.5 mmol) in THF (5 mL) was
added dropwise by cannula. The resulting solution was allowed to warm to room temperature
and was then stirred overnight. It was heated to reflux for 10 minutes and then filtered in
vacuo and stored at –30 C. This gave brown crystals of 5 (200 mg, 66% wrt Cp2Ni), m. p.
>360 C. δH (500 MHz, d8-THF) 5.74 (s, 1H, N2CH) 2.79 (s, 6H, Me). δC (125 MHz, d8-THF)
164.4 (N2CH), 40.5 (Me). Satisfactory elemental analysis could not be obtained after multiple
attempts. X-ray crystal data: C12H28Ni2N8, M = 401.84, monoclinic, space group P21/c, a =
17
8.4174(7), b = 7.3216(3), c = 14.7973(4) Å,  = 106.898(2) , V = 872.57(8) Å3, Z = 2, calcd =
1.529 g cm–3, Mo-K radiation,  = 0.71073 Å,  = 2.167 mm–1, T = 180(2)K. 22026 data
(1987 unique, Rint = 0.078,  < 27.48 ) were collected. wR2 = 0.0848, conventional R =
0.0376 on F values of 1795 reflections with F2 > 2(F2), S = 1.131, 105 parameters. Residual
electron density extrema 0.398 eÅ–3.
Ni2(hpp)4 7
The guanidine reagent hppH (150 mg, 1.10 mmol) was stirred in THF (5 mL). n-BuLi (0.7
mL, 1.6 M in hexanes, 1.10 mmol) was syringed into this solution dropwise under N2 at –78
C, and the reaction mixture allowed to return to room temperature. The resultant white
suspension was then added dropwise to a suspension of Cp2Ni (100 mg, 0.55 mmol) in THF
(5 mL) at –78 C. The system was allowed to warm to room temperature before being stirred
overnight and then filtered warm and stored at –30 C. Dark purple crystals of 7 deposited (28
mg, 15% wrt Cp2Ni), m. p. dec. from 228 C. δH (500 MHz, d8-THF) 3.32 (t, 4H, NCH2), 2.63
(t, 4H, NCH2), 1.56 (tt, 4H, CH2). δC (125 MHz, d8-THF) 155.0 (CN3), 49.3 (NCH2), 46.7
(NCH2), 25.6 (CH2). Anal. Calc. for C14H24N6Ni: C 50.1, H 7.2, N 24.3. Found: C 50.2, H
7.2, N 25.0%. X-ray crystal data: C28H48N12Ni2, M = 620.70, triclinic, space group P–1, a =
9.6812(6), b = 11.1600(7), c = 14.355(2) Å, V = 1477.9(2) Å3, Z = 2, calcd = 1.506 g cm–3,
Mo-K radiation,  = 0.71073 Å,  = 1.316 mm–1, T = 180(2)K. 14243 data (5841 unique, Rint
= 0.0325,  < 27.60 ) were collected. wR2 = 0.1037, conventional R = 0.0399 on F values of
4763 reflections with F2 > 2(F2), S = 1.042, 379 parameters. Residual electron density
extrema 0.692 eÅ–3.
18
X-ray Crystallography
Data were collected using the ‘oil drop technique’[38] to mount crystals on a Nonius KappaCCD diffractometer equipped with an Oxford Cryostream low-temperature device. Structures
were solved by direct methods.[39] Refinement, based on F2, was by full-matrix least
squares.[40] Non-hydrogen atoms were typically refined anisotropically and a riding model
with idealized geometry was employed for the refinement of H-atoms. For 5, crystals grew as
non-merohedral twins. The use of TwinRotMat[41] enabled determination of the twinning law
and the generation of an HKLF5 file for subsequent refinements.
Crystallographic data
Crystallographic data (excluding structure factors) have been deposited with the Cambridge
Crystallographic Data Centre as supplementary publications CCDC-XXX – YYY. These data
can be obtained free of charge via http://www.ccdc.cam.ac.uk/conts/retrieving.html, or on
application to CCDC, 12 Union Road, Cambridge CB2 1EZ, UK (fax: +44 1223 336033; email: deposit@ccdc.cam.ac.uk).
Electronic Supplementary Information
The synthesis and characterization of 6 and details of DFT calculations and atomic
coordinates are available electronically.
Acknowledgements
Thanks go to the UK EPSRC for studentships (grant EP/P504856/1; F.A.S., P.J.H., A.J.P.)
and the Leverhulme Trust (R.J.L., D.S.W.). The authors also wish to thank Professor Paul
Raithby (University of Bath) for help with crystallography.
19
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23
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24
Electronic Supplementary Information
Australian Journal of Chemistry
Towards the synthesis of guanidinate- and amidinate-bridged
dimers of Mn and Ni
Francesca A. Stokes,a Lars Kloo,b Philip J. Harford,a Andrew J. Peel,a Robert J. Less,a
Andrew E. H. Wheatley*a and Dominic S. Wright*a
a
Department of Chemistry, University of Cambridge, Lensfield Road, Cambridge, CB2 1EW,
U. K.; Fax: +44 (0)1223 336362; Tel: +44 (0)1223 763966/36333; email:
aehw2@cam.ac.uk, dsw1000@cam.ac.uk
b
Department of Chemistry, Royal Institute of Technology, Teknikringen 36, S-100 44,
Stockholm, Sweden.; email: larsa@kth.se
Synthesis and Characterization of Mn{-NC(NMe2)2}4Li23THF 6
1,1,3,3-Tetramethylguanidine (0.48 mL, 4.5 mmol) was stirred in THF (5 mL). t-BuLi (2.7
mL, 1.7 M in hexanes, 4.5 mmol) was syringed into this solution dropwise under Ar at –78
C, and the resulting yellow suspension allowed to warm to room temperature to give a white
suspension. This was then cooled again to –78 C and a suspension of Cp2Mn (278 mg, 1.5
mmol) in THF (5 mL) was syringed in dropwise by cannula. The resulting solution was
allowed to warm to room temperature before being stirred overnight and then filtered in vacuo
and stored at –30 C. This afforded brown crystals of 6 in low yield (50 mg, 5% wrt
guanidine), m. p. >360 ºC. δH (500 MHz, d8-THF) 2.63 (s, 3H, NMe). δC (125 MHz, d8-THF)
103.4 (CN2), 37.5 (Me). Satisfactory elemental analysis could not be obtained after multiple
attempts. X-ray crystal data: C32H72Li2MnN12O3, M = 741.84, tetragonal, space group P42bc,
a = b = 25.5067(2), c = 13.2394(2) Å, V = 8613.44(16) Å3, Z = 8, calcd = 1.144 g cm–3, MoK radiation,  = 0.71073 Å,  = 0.350 mm–1, T = 150(2)K. 104719 data (12577 unique, Rint =
25
0.0440,  < 30.84 ) were collected. wR2 = 0.1175, conventional R = 0.0475 on F values of
9680 reflections with F2 > 2(F2), S = 1.029, 467 parameters. Residual electron density
extrema 0.554 eÅ–3.
Figure S1 Molecular structure of guanidinate-ligated spirocycle 6. Selected
bond lengths (Å) and angles ():N–Mn(1) 2.0943(18)-2.1189(18), N–Li(1)
2.019(4)-2.036(5), N–Li(2) 1.927(5)-1.941(5), O–Li(1) 2.046(4)-2.056(4),
O–Li(2) 1.942(5), C…NMn 1.253(3)-1.260(3), N(41)/(61)–Mn(1)–
N(51)/(71) 88.75(7)-91.83(7), N–Li–N 95.8(2)-99.9(2), Mn–N–Li
84.28(15)-86.35(13).
Theoretical Modelling of 5
Ni2(MeN…CH…NMe) 5 was investigated by quantum chemical methods at Hartree-Fock
(HF), density-functional (BLYP) and hybrid density-functional (B3PW91, B3LYP) levels
using the Gaussian09 program package and the Natural Bond Order module implemented in
the package.[S1] Carbon, hydrogen and nitrogen were represented by 6-311G basis sets
including an additional diffuse and a polarizing function, and nickel by an effective-core
potential basis set of MDF10-type with an (8s7p6d1f)/[6s5p3d1f] valence space.[S2,S3]
Cartesian coordinates of the geometrically optimized structure of 5 at B3LYP level are as
follows:
Element x/Å
y/Å
z/Å
1.255812
Ni
0.000000
0.000000
Ni
0.000000
0.000000 -1.255812
N
-0.000042
1.931671
1.171236
26
N
0.000042
1.931671 -1.171236
C
0.000000
2.533327
0.000000
H
0.000000
3.632540
0.000000
C
-0.000372
2.768005
2.364575
H
-0.889639
3.410080
2.426675
H
0.888309
3.410899
2.426572
H
-0.000029
2.133853
3.252699
C
0.000372
2.768005 -2.364575
H
0.889639
3.410080 -2.426675
H
0.000029
2.133853 -3.252699
H
-0.888309
3.410899 -2.426572
N
0.000042 -1.931671
N
-0.000042 -1.931671 -1.171236
C
0.000000 -2.533327
0.000000
H
0.000000 -3.632540
0.000000
C
0.000372 -2.768005
2.364575
H
0.889639 -3.410080
2.426675
H
0.000029 -2.133853
3.252699
H
-0.888309 -3.410899
2.426572
C
-0.000372 -2.768005 -2.364575
H
-0.889639 -3.410080 -2.426675
H
0.888309 -3.410899 -2.426572
H
-0.000029 -2.133853 -3.252699
N
-1.931671 -0.000042
N
-1.931671
0.000042 -1.171236
C
-2.533327
0.000000
0.000000
H
-3.632540
0.000000
0.000000
C
-2.768005 -0.000372
2.364575
H
-3.410080 -0.889639
2.426675
H
-3.410899 0.888309
2.426572
H
-2.133853 -0.000029
3.252699
C
-2.768005
0.000372 -2.364575
H
-3.410080
0.889639 -2.426675
1.171236
1.171236
27
H
-2.133853
0.000029 -3.252699
H
-3.410899 -0.888309 -2.426572
N
1.931671
N
1.931671 -0.000042 -1.171236
C
2.533327
0.000000
0.000000
H
3.632540
0.000000
0.000000
C
2.768005
0.000372
2.364575
H
3.410080
0.889639
2.426675
H
2.133853
0.000029
3.252699
H
3.410899 -0.888309
2.426572
C
2.768005 -0.000372 -2.364575
H
3.410080 -0.889639 -2.426675
H
3.410899
H
2.133853 -0.000029 -3.252699
0.000042
1.171236
0.888309 -2.426572
References
[S1]
Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.;
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Scalmani, G.; Rega, N.; Petersson, G. A.; Nakatsuji, H.; Hada, M.; Ehara, M.; Toyota,
K.; Fukuda, R.; Hasegawa, J.; Ishida, M.; Nakajima, T.; Honda, Y.; Kitao, O.; Nakai,
H.; Klene, M.; Li, X.; Knox, J. E.; Hratchian, H. P.; Cross, J. B.; Bakken, V.; Adamo,
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R.; Pomelli, C.; Ochterski, J. W.; Ayala, P. Y.; Morokuma, K.; Voth, G. A.; Salvador,
P.; Dannenberg, J. J.; Zakrzewski, V. G.; Dapprich, S.; Daniels, A. D.; Strain, M. C.;
Farkas, O.; Malick, D. K.; Rabuck, A. D.; Raghavachari, K.; Foresman, J. B.; Ortiz, J.
V.; Cui, Q.; Baboul, A. G.; Clifford, S.; Cioslowski, J.; Stefanov, B. B.; Liu, G.;
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28
Graphical Abstract
The reaction of monolithiated amidines and guanidines with Cp2M (Cp = cyclopentadienyl; M
= Mn, Ni) yields a variety of new organometallic complexes; for the use of Cp2Ni, kinetically
unprotected paddle-wheel homo-dimers of formulation Ni2L4 (L = amidinate, guanidinate) are
obtained.