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This article is published as part of the Dalton Transactions themed issue entitled:
Self-Assembly in Inorganic Chemistry
Guest Editors Paul Kruger and Thorri Gunnlaugsson
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Published in issue 45, 2011 of Dalton Transactions
Image reproduced with permission of Mark Ogden
Articles in the issue include:
PERSPECTIVE:
Metal ion directed self-assembly of sensors for ions, molecules and biomolecules
Jim A. Thomas
Dalton Trans., 2011, DOI: 10.1039/C1DT10876J
ARTICLES:
Self-assembly between dicarboxylate ions and a binuclear europium complex: formation of stable
adducts and heterometallic lanthanide complexes
James A. Tilney, Thomas Just Sørensen, Benjamin P. Burton-Pye and Stephen Faulkner
Dalton Trans., 2011, DOI: 10.1039/C1DT11103E
Structural and metallo selectivity in the assembly of [2 × 2] grid-type metallosupramolecular
species: Mechanisms and kinetic control
Artur R. Stefankiewicz, Jack Harrowfield, Augustin Madalan, Kari Rissanen, Alexandre N. Sobolev
and Jean-Marie Lehn
Dalton Trans., 2011, DOI: 10.1039/C1DT11226K
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Cite this: Dalton Trans., 2011, 40, 12132
PAPER
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Structures, host–guest chemistry and mechanism of stepwise self-assembly of
M4 L6 tetrahedral cage complexes†
Benjamin R. Hall,a Lauren E. Manck,b Ian S. Tidmarsh,a Andrew Stephenson,a Brian F. Taylor,a
Emma J. Blaikie,a Douglas A. Vander Griend*b and Michael D. Ward*a
Received 28th April 2011, Accepted 27th June 2011
DOI: 10.1039/c1dt10781j
The ligand Lbip , containing two bidentate pyrazolyl–pyridine termini separated by a 3,3¢-biphenyl
spacer, has been used to prepare tetrahedral cage complexes of the form [M4 (Lbip )6 ]X8 , in which a
bridging ligand spans each of the six edges of the M4 tetrahedron. Several new examples have been
structurally characterized with a variety of metal cation and different anions in order to examine
interactions between the cationic cage and various anions. Small anions such as BF4 - and NO3 - can
occupy the central cavity where they are anchored by an array of CH ◊ ◊ ◊ F or CH ◊ ◊ ◊ O
hydrogen-bonding interactions with the interior surface of the cage, but larger anions such as
naphthyl-1-sulfonate or tetraphenylborate lie outside the cavity and interact with the external surface of
the cage via CH ◊ ◊ ◊ p interactions or CH ◊ ◊ ◊ O hydrogen bonds. The cages with M = Co and M = Cd
have been examined in detail by NMR spectroscopy. For [Co4 (Lbip )6 ](BF4 )8 the 1 H NMR spectrum is
paramagnetically shifted over the range -85 to +110 ppm, but the spectrum has been completely
assigned by correlation of measured T 1 relaxation times of each peak with Co ◊ ◊ ◊ H distances. 19 F
DOSY measurements on the anions show that at low temperature a [BF4 ] - anion diffuses at a similar
rate to the cage superstructure surrounding it, indicating that it is trapped inside the central cage cavity.
Furthermore, the equilibrium step-by-step self-assembly of the cage superstructure has been elucidated
by detailed modeling of spectroscopic titrations at multiple temperatures of an acetonitrile solution of
Lbip into an acetonitrile solution of Co(BF4 )2 . Six species have been identified: [Co2 Lbip ]4+ , [Co2 (Lbip )2 ]4+ ,
[Co4 (Lbip )6 ]8+ , [Co4 (Lbip )8 ]8+ , [Co2 (Lbip )5 ]4+ , and [Co(Lbip )3 ]2+ . Overall the assembly of the cage is entropy,
and not enthalpy, driven. Once assembled, the cages show remarkable kinetic inertness due to their
mechanically entangled nature: scrambling of metal cations between the sites of pure Co4 and Cd4 cages
to give a statistical mixture of Co4 , Co3 Cd, Co2 Cd2 , CoCd3 and Cd4 cages takes months in solution at
room temperature.
Introduction
Polyhedral coordination cages continue to attract considerable
attention for many reasons.1–3 They often have elegant and
aesthetically appealing highly-symmetric structures that mirror
those seen in areas from mathematics (Platonic and Archimidean
solids) to biology (some viruses); they form by self-assembly from
a large number of components which combine in a specific way
using labile interactions to reach a thermodynamic minimum; and
the large central cavities offer numerous possibilities for host–guest
chemistry.
a
Department of Chemistry, University of Sheffield, Sheffield, UK, S3 7HF.
E-mail: m.d.ward@sheffield.ac.uk
b
Department of Chemistry and Biochemistry, Calvin College, 1726 Knollcrest
circle SE, Grand Rapids, MI, 49546-4403, USA. E-mail: dvg@calvin.edu
† Electronic supplementary information (ESI) available: CCDC reference
numbers 825637–825642. For ESI and crystallographic data in CIF or
other electronic format see DOI: 10.1039/c1dt10781j
12132 | Dalton Trans., 2011, 40, 12132–12145
The smallest cage based on a three-dimensional polyhedral core
is the tetrahedron and there have been many examples reported of
tetrahedral cages which can either have an M4 L4 formulation (if
the ligand L is triply bridging and caps one face of the tetrahedron,
connecting to three metal ions)4 or an M4 L6 formulation (if the
bridging ligand L is doubly bridging and spans one edge of the
tetrahedron connecting a pair of metal ions).5,6 We have reported
several examples of M4 L6 cages (where L = LPh , Lnaph or Lanth ;
Scheme 1)6 based on ligands that contain two chelating bidentate
pyrazolyl-pyridine units combined with octahedral M2+ ions (M =
Co, Ni, Zn, Cd) in which the central cavity is occupied by an
anion such as [BF4 ]- or [ClO4 ]- or (in one case) [SiF6 ]2- .6c In these
cases the central anion is completely encapsulated by the cage and
cannot exchange with external anions on the NMR timescale, and
in fact acts as a template around which the cage assembles.
We have also prepared larger tetrahedral cages of the type
[M4 (Lbip )6 ]X8 based on the more extended bridging ligand Lbip
(Scheme 1).7 These differ from the small cages based on LPh , Lnaph
This journal is © The Royal Society of Chemistry 2011
in the self-assembly process, but also led to the determination
of their molar absorptivity values and most importantly the
thermodynamic values (DG◦ , DH ◦ , DS◦ ) for the assembly steps.
Results and Discussion
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Syntheses and crystallographic studies
Scheme 1
or Lanth in two ways. Firstly the cage superstructure has lower
symmetry, with crystallographic studies revealing a C 3 -symmetric
structure in which the ‘apical’ metal ion has a fac tris-chelate
arrangement of the three pyrazolyl-pyridine ligands whereas the
three ‘basal’ metal ions have a mer tris-chelate arrangement.
Secondly the lengthier bridging ligands result in a more open
cage structure with windows in the centres of the faces that permit
temperature-dependent exchange of the encapsulated anions such
as [BF4 ]- or [PF6 ]- as shown by 19 F NMR studies. The greater
availability of the central cavity to possible guest species makes
these attractive targets for further study.
In this paper we report the results of several further avenues of
investigation into this family of [M4 (Lbip )6 ]X8 cages. New examples
with different metal cations and/or different anions, or mixed
combinations of anions, have been prepared and structurally
characterised; and detailed NMR analysis on a number of the
new compounds has been conducted including the presentation of a fully assigned paramagnetic 1 H NMR spectrum for
[Co4 (Lbip )6 ](BF4 )8 based on a correlation of T 1 measurements with
crystallographic data. We also report a detailed study of the assembly of a cage in solution by UV/Vis spectroscopic titrations in
which unrestricted and equilibrium-restricted factor analysis have
not only allowed for the identification of intermediates involved
This journal is © The Royal Society of Chemistry 2011
Complexes based on a single type of anion. The cage complexes
were simply prepared by solvothermal reaction of a metal salt and
Lbip (2 : 3 ratio; Scheme 1) in methanol followed by slow cooling.
Crystals suitable for single crystal X-ray diffraction were obtained
following the diffusion of diethyl ether vapour into concentrated
nitromethane solutions of the complexes. The previous crystal
structures that we reported for this series of complexes were
all of Co(II) salts.7 We describe here the structure of the two
Cd(II) complexes [Cd4 (Lbip )6 ]X8 (X- = BF4 - or NO3 - ) with a
particular emphasis on the interactions of the anions with the
cage superstructure.
[Cd4 (Lbip )6 ](BF4 )8 has the same core structure that we have seen
before with one bridging ligand spanning each of the six edges of
the Cd4 tetrahedron, and a counter-ion in the central cavity (Fig. 1,
2; see Table 1 for all crystallographic data). The Cd4 tetrahedron is
slightly irregular with Cd ◊ ◊ ◊ Cd separations vary between 11.69–
12.24 Å. Cd(1) at the apex has a fac (pyridine)3 (pyrazole)3 donor
set, whereas Cd(2), Cd(3) and Cd(4) (around the basal plane) have
a mer arrangement all with the same optical configuration. Cd–N
distances lie in the range 2.27–2.41 Å and are unremarkable. The
complex cage therefore has (non-crystallographic) C 3 symmetry.
The structure is stabilised by inter-ligand aromatic p-stacking
interactions around the periphery of the cage, which is emphasised
in Fig. 1, with arrows indicating the stacks; these stacks are
repeated three times around the cage given the C 3 symmetry [axis
vertically through Cd(1)] such that all phenyl and all but three
of the pyridyl-pyrazole ligands participate in such interactions.
Fig. 1 Structure of part of the complex cation of [Cd4 (Lbip )6 ](BF4 )8 ,
emphasising the topology of the cage super structure and the encapsulated
anion. Cd(1) possesses fac tris-chelate geometry and lies on the pseudo-threefold axis; Cd(2), Cd(3) and Cd(4) have mer tris-chelate geometry.
The aromatic stacking regions are shown with arrows with the direction
of the arrows indicating the donor→acceptor interaction.
Dalton Trans., 2011, 40, 12132–12145 | 12133
Table 1 Crystallographic data for the new compoundsa
Compound
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Formula
Molecular weight
Habit
T/K
Crystal system
Space group
a/Å
b/Å
c/Å
a (◦ )
b (◦ )
g (◦ )
V /Å3
Z
r/g cm-3
m/mm-1
Data, restraints,
parameters
2q max (◦ )
Rint for
independent data
Final R1, wR2b
[Cd4 (Lbip )6 ](BF4 )8 ·
5MeNO2
[Zn4 (Lbip )6 ](BF4 )8 ·
5MeNO2
[Ni4 (Lbip )6 ](BF4 )8 ·
6MeNO2
[Cd4 (Lbip )6 ](NO3 )8 ·
4MeNO2 ·Et2 O
[Co4 (Lbip )6 ](C10 H7 SO3 )2
(BF4 )6 ·2MeCN·2H2 O
[Cd4 (Lbip )6 ](BPh4 )1.5 (BF4 )6.5
C185 H159 B8 Cd4 F32 N41 O10
4260.61
Colourless block
150(2)
Triclinic
P1̄
18.9821(6)
19.2984(7)
30.8148(10)
86.206(2)
86.115(2)
65.108(2)
10207.2(6)
2
1.386
0.505
59359, 2441, 2189
C185 H159 B8 F32 N41 O10 Zn4
4072.49
Colourless block
150(2)
Monoclinic
P21 /n
18.976(5)
36.829(10)
32.327(10)
90
91.42(2)
90
22585(11)
4
1.198
0.505
28763, 2198, 1893
C186 H162 B8 F32 N42 Ni4 O12
4106.90
Purple block
100(2)
Monoclinic
P21 /c
37.1159(12)
37.0889(11)
37.9740(12)
90
118.119(2)
90
46105(2)
8
1.183
0.407
60245, 4449, 3481
C188 H166 Cd4 N48 O33
4075.29
Colourless block
150(2)
Triclinic
P1̄
19.0710(5)
19.3566(5)
30.2041(8)
87.1790(10)
88.0310(10)
64.2780(10)
10031.8(5)
2
1.349
0.498
46147, 404, 2194
C204 H168 B6 Co4 F24 N38 O8 S2
4100.46
Orange block
150(2)
Triclinic
P1̄
18.7977(4)
19.7726(4)
32.5810(7)
82.6170(10)
84.7620(10)
65.0120(10)
10876.0(4)
2
1.252
0.402
27905, 232, 1890
C216 H174 B8 Cd4 F26 N36
4303.99
Colourless needle
150(2)
Cubic
I23
35.7585(7)
35.7585(7)
35.7585(7)
90
90
90
45723.3(16)
8
1.250
0.445
9998, 656, 695
61
0.0359
45
0.2739
45
0.0561
55
0.0322
45
0.0632
45
0.1665
0.1170, 0.3840
0.1947, 0.4722
0.1310, 0.4095
0.0853, 0.2962
0.1239, 0.3857
0.0826, 0.2371
a
Every structure required the use of the SQUEEZE function in PLATON to eliminate regions of diffuse electron density that could not be satisfactorily
modelled; see Experimental section for more details. Accordingly the formulae are necessarily incomplete as badly disordered solvent molecules are
omitted. b The value of R1 is based on ‘observed’ data with I > 2s(I); the value of wR2 is based on F 2 values of all data.
to the ‘basal’ Cd centres being 7.75, 7.77 and 8.02 Å respectively.
This off-centre binding displaced towards Cd(1) allows the anion
to be involved in C–H ◊ ◊ ◊ F hydrogen-bonding interactions8 with
the cage superstructure, all involving inwardly-directed protons
from the methylene CH2 groups: these H ◊ ◊ ◊ F separations lie
within the range 2.5–2.6 Å with the associated C ◊ ◊ ◊ F distances
being 3.04–3.18 Å (Fig. 3). Each face of the cage possesses a large
enough window for the central anion to diffuse in and out of the
structure, which is clear from the space-filling view in Fig. 2.
Fig. 2 A space-filling view of the complex cation of [Cd4 (Lbip )6 ](BF4 )8
with each ligand coloured differently for clarity. The dark green atoms in
the centre are F atoms of the encapsulated BF4 - anion.
The view in Fig. 1 illustrates both a threefold stack involving two
phenyl rings sandwiched either side of a coordinated pyrazolylpyridine group, and pairwise stacks involving one phenyl group
and one pyrazolyl-pyridine group. Within each stack the interplanar separations of the overlapping components lie in the range
of 3.3–3.6 Å.
The encapsulated tetrafluoroborate anion is located off-centre in
the cavity, which is reflected in the Cd ◊ ◊ ◊ B separations which span
the range 5.81–8.02 Å; the shortest Cd ◊ ◊ ◊ B separation involves the
apical Cd(1) ion (5.81 Å), with the other three Cd ◊ ◊ ◊ B separations
12134 | Dalton Trans., 2011, 40, 12132–12145
Fig. 3 Part of the complex cation of [Cd4 (Lbip )6 ](BF4 )8 detailing the array
of CH ◊ ◊ ◊ F hydrogen-bonding interactions involving the central anion and
the methylene groups close to Cd(1).
The crystal structure of [Cd4 (Lbip )6 ](NO3 )8 is similar (Fig. 4)
with
an almost identical cage superstructure; the Cd ◊ ◊ ◊ Cd
separations along the edges are in the range 11.15–12.35 Å and
the nitrate ion in the central cavity again is nearer to the apical
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the ‘base’ of the cavity also have inwardly-directed pyrazolyl H5
protons which form CH ◊ ◊ ◊ O hydrogen bonds to nitrate. The result
is a complex network of CH ◊ ◊ ◊ O interactions involving the nitrate
ion interacting with six methylene and three pyrazolyl CH protons.
We also structurally characterised the cages [Ni4 (Lbip )6 ](BF4 )8
and [Zn4 (Lbip )6 ](BF4 )8 which are exactly analogous to the structure
of [Cd4 (Lbip )6 ](BF4 )8 . The quality of the refinements for these two
structures were rather poor, being sufficient to establish the gross
connectivity but detailed discussion of structural minutiae would
be inappropriate. Details are given in the supporting information.†
It is apparent however that this tetrahedral cage structure can
be formed with metal ions having a wide range of ionic radii,
from Ni(II) (this work) through Co(II)7a to Cd(II) (this work) and
Hg(II).7b
Fig. 4 A view of part of the complex cation of [Cd4 (Lbip )6 ](NO3 )8 with only
three of the six ligands included for clarity looking down the pseudo-C 3 axis
[which passes through Cd(1) which has a fac tris-chelate configuration].
Cd(1) [Cd(1) ◊ ◊ ◊ N(24) separation, 5.70 Å] than it is to the other
three [average Cd ◊ ◊ ◊ N(24) separation, 7.83 Å]. Fig. 5 illustrates
the pattern of hydrogen-bonding interactions in which the central
nitrate ion is involved: every nitrate oxygen atom for two CH ◊ ◊ ◊ O
hydrogen bonds with inwardly-directed CH protons from the
methylene spacer groups. All six ligands have one such H atom
directed into the cavity and participating in this H-bonding. In
addition to the three H-bonding interactions involving the three
ligands around Cd(1) (the red, green and blue ligand fragments
in Fig. 5 whose strongest H-bonding interactions are shown as
black dotted lines) the second H atom of each of these methylene
groups also participates in weaker H-bonding interactions with
the nitrate. For example the alternate methylene protons on each
of the blue and green ligand fragments shown in Fig. 5 lie 2.7–
2.8 Å from O(23). In contrast the other three methylene groups
(associated with the grey, yellow and purple-coloured ligands
in Fig. 5) have their second proton directed out of the cavity
away from the nitrate group. However, these three ligands around
Fig. 5 View of the complex cation of [Cd4 (Lbip )6 ](NO3 )8 showing those
ligand fragments that are involved in formation of CH ◊ ◊ ◊ O hydrogen-bonding interactions with the central nitrate anion.
This journal is © The Royal Society of Chemistry 2011
Complexes based on a mixture of two types of anion. To
see if we could generate complexes containing different guest
anions we crystallised samples of [Co4 (Lbip )6 ](BF4 )8 from solutions
also containing an excess of either NaBPh4 or Na+ (naphthyl-1sulfonate)- (this anion is hereafter abbreviated napSO3 - ). In both
cases we obtained crystal structures of mixed-anion cages, but with
a tetrafluoroborate anion retained in the central cavity.
Fig. 6 shows the whole complex cation and the surrounding
napSO3 - anions. The tetrahedral cage and its encapsulated anion
are basically the same as in the example shown in Fig. 1–
3 with Co ◊ ◊ ◊ Co separations in the range 11.33–12.28 Å. The
napSO3 - anions occupy the spaces between cage cations and
interact with their external surfaces in a variety of ways which
are emphasised in Fig. 7. Atom O(2) from one of the sulfonate
groups [containing S(1)] forms a weak CH ◊ ◊ ◊ O hydrogen-bond
with an externally directed methylene proton attached to C(12)
with a C ◊ ◊ ◊ O separation of 3.35 Å. The naphthyl ring also acts
as an acceptor for two CH ◊ ◊ ◊ p interactions involving protons
[H(253) and H(435)] from a different cage cation; these H atoms
lie 2.64 and 2.65 Å from the mean plane of the naphthyl ring. The
second independent napSO3 - anion behaves similarly (Fig. 6, 7).
Atom O(4) from this sulfonate group is involved in an H-bonding
interaction with a pyrazolyl H5 proton attached to C(625) with a
C ◊ ◊ ◊ O separation of 3.22 Å. The same naphthyl group is involved
in CH ◊ ◊ ◊ p interactions both as a donor and an acceptor, with
protons from a nearby pyrazolyl-pyridine group [specifically atoms
H(335) and H(344)] directed towards its p-cloud, and conversely
the naphthyl proton H(17) lying over the face of a cage phenyl ring.
The distances of H(17), H(335) and H(344) from the mean plane
of the aromatic group to which they are directed are 2.65, 3.01
and 2.67 Å respectively. Thus, although this anion does interact in
many ways with the cage cations, these interactions are only with
the external surfaces – possibly because these napSO3 - anions
are too large to be accommodated in the central cavity. Each
cage cation actually interacts with six closely associated napSO3 anions, as shown in Fig. 6. We note that we have observed that
these highly cationic cage complexes form strong ion pairs with
aromatic anions in other cases.9
Fig. 8 shows part of the structure of [Cd4 (Lbip )6 ](BF4 )6.5 (BPh4 )1.5 .
The stoichiometry of this arises from the fact that the cage cation
lies on a threefold axis passing through Cd(1) (the apical Cd
atom) such that the asymmetric unit contains 1.333 metal cations
and two complete ligands, i.e. one third of the cage cation. In
contrast the BPh4 - anion lies with its B atom on a twofold axis
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Fig. 6
Structure of the complex cation of [Co4 (Lbip )6 ](C10 H7 SO3 )2 (BF4 )6 , highlighting the six nearest neighbour 1-naphthyl sulfonate anions.
such that the asymmetric unit contains half of a BPh4 anion. Since three such asymmetric units complete the complex
cation, this generates 1.5 BPh4 - anions per cage cation, i.e. three
half-anions which are all crystallographically equivalent. As in
the previous complex containing some napSO3 - anions, the BPh4 anions in this case do not occupy the central cavity (which is
occupied by a BF4 - anion, Fig. 8, with the usual array of CH ◊ ◊ ◊ F
hydrogen-bonds to internally-directed CH protons) but lie outside
the cage and are involved in a range of CH ◊ ◊ ◊ p interactions
with its external surface. Some of these CH ◊ ◊ ◊ p interactions are
highlighted in Fig. 9: for example H(213) from a pyridyl ring
of the cage lies 2.59 Å from the mean plane of the BPh4 - ring
C(67)→C(72), and H(65) from the BF4 - anion lies 2.95 Å from
the mean plane of pyridyl ring N(211)→C(216) as well as 3.12 Å
from the mean plane of pyridyl ring N(131)→C(136).
Solution phase NMR studies on [Co4 (Lbip )6 ](BF4 )8 and
[Cd4 (Lbip )6 ](BF4 )8
1
Fig. 7 Close-up view of the naphthyl-1-sulfonate anions in the structure
of [Co4 (Lbip )6 ](C10 H7 SO3 )2 (BF4 )6 , detailing the CH ◊ ◊ ◊ p and CH ◊ ◊ ◊ O
interactions with the external surface of the cage.
12136 | Dalton Trans., 2011, 40, 12132–12145
H NMR spectra of diamagnetic and paramagnetic cages.
When self-assembly experiments are performed using using labile
metal ions there is always the possibility of a mixture of species
forming in solution even if only one form is isolated in the
crystalline state: this was described by Lehn as a ‘virtual combinatorial library’ from which one component could be preferentially
isolated by crystallisation or by addition of a templating anion
which perturb the equilibrium in favour of one component.10 We
therefore used 1 H NMR spectroscopy to confirm that the solidstate structure is retained in solution, something which is not
always true for other members of this family of cages.1i
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Fig. 8 Structure of part of the complex cation of [Cd4 (Lbip )6 ](BPh4 )1.5 (BF4 )6.5 , highlighting the six nearest neighbour tetraphenylborate anions.
Fig. 9 Close-up view of one of the tetraphenylborate anions in the
structure of [Cd4 (Lbip )6 ](BPh4 )1.5 (BF4 )6.5 detailing the CH ◊ ◊ ◊ p interactions
with the external surface of the cage.
The 1 H NMR spectrum of [Cd4 (Lbip )6 ](BF4 )8 (Fig. 10) is clearly
consistent with the tetrahedral cage structure being retained in
solution. There should be two ligand environments corresponding
to the three identical ligands spanning the fac-mer edges and three
more ligands spanning the mer-mer edges. Clearly along the former
type of edge both ends of each ligand will be different, leading to
24 inequivalent proton environments (each corresponding to an
intensity of 3H intensity). The symmetry of the ligands spanning
the mer-mer edges is less obvious. It might be expected that each of
these ligands should have two-fold symmetry with the two halves
equivalent, giving 12 resonances each of double intensity (i.e. each
corresponding to 6H) but the chirality and sense of helical twist
of each ligand result in symmetry breaking. Thus this set of three
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ligands also generates 24 independent proton environments. The
total will therefore be 48 inequivalent proton signals of the same
intensity, which is what is seen in the 1 H NMR spectrum. Although
not all of the signals can be resolved due to substantial overlapping
of signals (even at 500 MHz), from a COSY spectrum we can
readily assign four coupled pairs of pyrazole H3 /H4 protons from
the four inequivalent pyrazolyl rings (labelled in Fig. 10 as A, B, C
and D; these doublets are characterised by very small coupling
constants); and the four pairs of diastereotopic CH2 protons
(labelled as a, b and c and d; these are all doublets with much
larger coupling constants). The remaining protons forming the
pyridyl and phenyl rings cannot all be assigned with certainty but
it is clear from the spectrum that the cage structure is retained
in solution. This is also supported by the 113 Cd NMR spectrum
which displays two signals in a ratio of 3 : 1, consistent with the
presence of three meridional and one facially configured Cd(II)
centre which will be in slightly different environments (Fig. 11).
We have also taken this opportunity to assign fully the 1 H
NMR spectrum of [Co4 (Lbip )6 ](BF4 )8 . Although it is paramagnetic,
complexes of high-spin Co(II) lend themselves well to analysis
by 1 H NMR spectroscopy as the signals remain relatively sharp
whilst being spread out enough to minimise the overlap of
aromatic signals that is normal for the diamagnetic complexes
(cf. Fig. 10).6c,11 The 1 H NMR spectrum of [Co4 (Lbip )6 ](BF4 )8 was
mentioned in a previous paper7a but was incomplete: six broad
signals outside of the initial sweep width were missed and no
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Fig. 10 500 MHz 1 H NMR spectrum of [Cd4 (Lbip )6 ](BF4 )8 in CD3 CN
showing the 48 inequivalent signals required by the crystal structure which
shows two independent ligand environments with no internal symmetry for
either. Partial assignments (diastereotopic pairs of CH2 protons, a/b/c/d;
pairs of adjacent protons on pyrazolyl rings, A/B/C D-1 ) were based on a
COSY spectrum.
Fig. 11 113 Cd NMR spectrum of [Cd4 (Lbip )6 ](BF4 )8 in CD3 CN showing
two peaks of intensity 3 : 1 arising from the different Cd(II) environments
(mer and fac tris-chelates, respectively) present in the complex.
assignment of the spectrum was attempted. The complete spectrum is shown in Fig. 12 and shows all 48 independent signals in
the range -85 to +110 ppm, as required by the C 3 symmetry of the
complex which results in two independent ligand environments,
each displaying 24 environments. It is quite clear that some of the
signals are much broader and of correspondingly lower intensity
than others, because they have shorter T 1 relaxation times. Peak
width is inversely proportional to T 1 for each proton, and T 1 is
in turn related to the distance of the relevant proton from the
paramagnetic Co(II) centres. In fact T 1 -1 for a given proton is
proportional to R (rij -6 ), where rij are the pairwise distances between
a particular hydrogen atom and each of the four Co(II) ions.
This relationship provides a mechanism for almost completely
assigning the 1 H NMR spectrum of the complex, by correlating
T 1 values for the separate peaks with [R (rij -6 )]-1 values for H ◊ ◊ ◊ Co
separations obtained from crystallographic data.11a
12138 | Dalton Trans., 2011, 40, 12132–12145
The 1 H NMR assignments for [Co4 (Lbip )6 ](BF4 )8 made on this
basis are given in Table 2 (and are labelled in Fig. 12), which lists the
[R (rij -6 )]-1 values for each of the independent proton environments,
normalised such that the relevant value of the smallest [R (rij -6 )]-1
is scaled to be exactly numerically equal to the smallest T 1 value.
The agreement between the normalised [R (rij -6 )]-1 values (based
on crystallographic data) and the measured T 1 values is excellent
and allows nearly complete assignment of the spectrum. There
are just six exceptions whereby three pairs of signals cannot be
conclusively assigned due to identical [R (rij -6 )]-1 values from the
crystal structure, or identical T 1 values in the NMR spectrum, so
there is some ambiguity in these pairs. For example the signal
at 81.4 ppm could be assigned to be pyridyl H6 of either the
mer or mer* terminus of the mer-mer ligand (in Fig. 2 the mer
termini are indicated by open bonds, the mer* termini by solid
bonds). Apart from this the assignment is complete. The four
broadest 1 H NMR signals clearly correspond to the four pyridyl
H6 proton environments which accordingly have the shortest T 1
values (1.6–2.1 ms) and the shortest distances to Co(II) ions within
the range 3.19–3.23 Å. Also broad and with short T 1 values are the
methylene (CH2 ) protons which are directed into the cavity and
lie ca. 3.5 Å from the Co(II) centres. Conversely the most intense
signals (and therefore the narrowest with the longest T 1 values)
are associated with the phenyl spacers which are the most remote
protons from the Co(II) ions.
NMR studies on the encapsulated guest anions. Fast exchange
of the encapsulated BF4 - anion in [Cd4 (Lbip )6 ](BF4 )8 with ‘free’
external anions can be observed by variable-temperature 19 F
NMR spectroscopy. At room temperature a single broad 19 F
signal is observed at -150.8 ppm which, on cooling down to
233 K, separates into two sharp and well resolved signals at
-150.1 and -143.7 ppm with integrals in the expected 7 : 1
(respectively) ratio (Fig. 13a). Thus, the anion-based host/guest
chemistry of this complex shows behaviour in between that of
the smaller cages in the anion is completely trapped,6 and the
much larger ones where anion exchange through large cavities
in the faces is facile and cannot therefore be frozen out by NMR
spectroscopy.1d It is interesting to compare this with the analogous
19
F NMR spectra recorded for the paramagnetic but isostructural
cage [Co4 (Lbip )6 ](BF4 )8 (Fig. 13b).7a The two complexes show
a similar freezing-out of the anion exchange at 233 K, with
the more intense signal for the free BF4 - anion in about the
same position in each case. However the smaller signal for the
encapsulated anion is at -169 ppm in the Co(II) complex rather
than -143.7 ppm in the Cd(II) complex, as a consequence of the
paramagnetic environment of the bound anion inside the Co4
cage.
We have used diffusion-ordered (DOSY) NMR spectroscopy
on the cages [M4 (Lbip )6 ](BF4 )8 (M = Co, Zn or Cd) and their
guest anions to see if the encapsulated tetrafluoroborate anions
have similar diffusion constants to the surrounding cages at low
temperature, which would imply that the anion is held in the
cage cavity and not free to escape, and therefore diffuses through
solution at the same rate as the host cage. The observation
of separate 19 F resonances at 233 K for the free and trapped
anions in the Co(II) and Cd(II) cages (Fig. 13) implies that there
is no exchange between them on the NMR timescale at that
temperature. DOSY measurements at 233 K using the more
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Fig. 12 400 MHz 1 H NMR spectrum of [Co4 (Lbip )6 ](BF4 )8 in CD3 CN showing 48 inequivalent signals in the range -85 to +110 ppm. Signals corresponding
to the ligand spanning the fac(prefix f )-mer(prefix m ) edges are indicated in bold, with the mer-mer ligands in standard text (labels m and m* differentiate
between the two unique ends of the ligand). Two signals labelled with a dot between 0 and 5 ppm are traces of protonated solvents.
intense 19 F signal for the ‘free’ tetrafluoroborate anion for each
of the three cages gave D values in the range 1.2(2)–1.4(2) 10-9 m2
s-1 (see Table 3). Measurements using the less intense 19 F signal
for the bound tetrafluoroborate anion gave smaller D values in
the range 0.8(2)–1.0(2) 10-9 m2 s-1 , reduced by approximately one
third compared to the free anions in these complexes. Crucially,
the latter values are very similar to the values of 0.9(2)–1.0(2)
10-9 m2 s-1 obtained for the cage superstructures (based on the
average of seven independent measurements, each made from
a different 1 H resonance) at the same temperature. This good
agreement confirms that the encapsulated anions are trapped in
the central cavities at 233 K, with no exchange with external anions
on the NMR timescale. Although the differences between the
diffusion coefficients for free/bound BF4 - anions are not much
larger than the experimental uncertainty for each individual cage
examined, averaging the measurements for three independent but
isostructural cages reduces the error and does show a clear and
statistically significant difference.
We also checked the 1 H NMR behaviour of the mixedand
anion
complexes
[Co4 (Lbip )6 ](BF4 )6 (napSO3 )2
bip
[Cd4 (L )6 ](BF4 )6.5 (BPh4 )1.5 . In both cases the signals associated
with the aromatic anions were barely shifted from their usual
positions as shown by simple salts such as NaBPh4 , and
accordingly there is no evidence in solution for uptake of these
anions into the cavities, just as there was not in the solid state,
presumably on size grounds.
This journal is © The Royal Society of Chemistry 2011
Self-assembly of [Co4 (Lbip )6 ](BF4 )8 in solution followed by
spectroscopic titrations
A complete cage such as [Co4 (Lbip )6 ](BF4 )8 consists of ten components (not including the anions, central or otherwise) which
require twenty-four metal ligand bonds to hold them together in
the correct geometry. The assembly of such a cage in solution must
be a complicated process which proceeds via many intermediates.
To understand how these ten components assemble UV/Vis
spectroscopic titrations have been performed at 283 K, 295 K,
and 308 K during which 3 equivalents of the ligand Lbip were
added stepwise to 10 mM solutions of Co(BF4 )2 in MeCN. For
each titration the result is a set of about 50 absorbance curves
(425–700 nm) that can be modelled to ascertain the number
of spectroscopically and stoichoimetrically distinct intermediate
species, the molar absorptivity curve for each, and the standard
free energy, DG◦ , for the chemical reactions between them.12
Unrestricted factor analysis, a purely mathematical technique
for analyzing the additive structure of data matrices,13 consistently
shows that the dataset for each spectroscopic titration experiment
is comprised of 6 to 8 additive factors, which tend to appear
sequentially as the titration progresses. This last fact becomes
apparent when unrestricted factor analysis is done on subsets of
the data such as the first 10 solutions for example.14
Of much greater interest is the equilibrium-restricted factor
analysis that models the additive factors in the data as definite
Dalton Trans., 2011, 40, 12132–12145 | 12139
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Table 2
12)
1
H NMR assignment for [Co4 (Lbip )6 ](BF4 )8 in MeCN (see Fig.
d (ppm)
Measured
T 1 /ms
Normalised
[R (rij -6 )]-1 a
Assignment
81.4
109.0
-39.5
-84.1
-50.6
-72.2
45.5
29.6
28.3
36.4
23.3
95.5
95.7
36.2
-28.1
-18.1
-10.1
-17.2
67.3
65.1
59.7
46.3
83.9
55.3
52.1
57.4
72.7
-2.6
44.5
-4.1
48.3
94.0
53.2
-13.3
38.2
13.6
16.0
16.4
21.7
14.8
9.6
6.8
1.4
18.3
0.2
12.8
11.0
11.1
1.6
1.8
2.1
2.1
2.7
2.8
5.1
7.5
7.9
9.6
11.0
12.5
12.5
13.7
15.9
16.0
16.3
18.6
21.2
21.6
22.1
23.1
23.2
23.5
23.6
24.6
24.9
25.0
25.2
26.0
26.6
27.5
29.1
33.0
35.0
56.7
57.6
59.5
60.9
61.2
64.3
67.5
68.7
72.7
118.7
142.0
161.0
162.9
1.6
1.6
1.7
1.7
2.1
2.5
2.6
2.7
7.2
7.4
7.4
8.4
11.4
14.5
14.7
15.0
16.3
16.5
21.2
21.7
22.4
23.4
23.9
24.2
24.9
25.1
25.3
25.4
26.4
27.4
27.8
28.0
28.4
28.5
28.7
31.0
31.6
48.1
51.5
52.1
55.8
56.4
59.1
59.3
61.2
63.7
85.0
121.3
mer- or mer*-Pyridyl H6
mer- or mer*-Pyridyl H6
fac- or mer-Pyridyl H6
fac- or mer-Pyridyl H6
mer-CH2
fac-CH2
mer-CH2
mer*-CH2
fac-Phenyl H4
mer-Phenyl H4 or H6
mer-Phenyl H4 or H6
mer-Phenyl H2 or mer*-H4
mer-Phenyl H2 or mer*-H4
fac-CH2
mer-CH2
mer*-CH2
mer-Pyridyl H3
mer-CH2
fac-Pyrazolyl H4
mer*-Pyrazolyl H4
mer*-Pyridyl H3
mer-Pyrazolyl H4
mer-Pyridyl H3
fac-Pyridyl H3
mer-Pyrazolyl H4
mer-Phenyl H4
fac-Pyrazolyl H5
mer*-Pyrazolyl H5
mer-Pyridyl H5
mer*-Pyridyl H5
mer*-Phenyl H2
fac-Phenyl H2
mer-Pyridyl H5
mer-Phenyl H2
fac-Pyridyl H5
mer-Pyrazolyl H5
mer-Pyrazolyl H5
mer*-Phenyl H5
fac-Phenyl H5
mer-Pyridyl H4
mer*-Pyridyl H4
mer-Phenyl H5
mer-Pyridyl H4
fac-Pyridyl H4
mer-Phenyl H6
mer*-Phenyl H6
mer-Phenyl H5
fac-Phenyl H6
a
Calculated from crystallographic data (ref. 7a) on the basis of four
different H ◊ ◊ ◊ Co separations of each type of proton in nm (hence in units
of nm4 ) and then normalised against the signal at 81.4 ppm such that this
value was numerically identical to the observed relaxation time of 1.6 ms.
Ideally the numbers in columns 1 and 2 should be identical; the discrepancies arise because of the uncertainties in crystallographic measurements of
bond distances and also the uncertainty in T 1 measurements. Given that
the measured T 1 values arise from signals spread out over nearly 200 ppm,
the general agreement is excellent.
chemical species that must adhere to the thermodynamic laws
of chemical equilibria. The best model for the data at 295 K
and 308 K was found using seven factors which correspond
to uncomplexed Co2+ , [Co2 Lbip ]4+ , [Co4 (Lbip )4 ]8+ , [Co4 (Lbip )6 ]8+
(the tetrahedron), [Co2 (Lbip )4 ]4+ , [Co2 (Lbip )5 ]4+ , and [Co(Lbip )3 ]2+ ;
however, if [Co2 (Lbip )2 ]4+ is used in place of [Co4 (Lbip )4 ]8+ , the model
is nearly as good. The best model for the data at 283 K was identical
12140 | Dalton Trans., 2011, 40, 12132–12145
Table 3 19 F chemical shift values and DOSY measurements for the series
[M4 (Lbip )6 ](BF4 )8 (where M = Co, Zn or Cd) in MeCN
Free (d (ppm))
D/10-9 m2 s-1 )
Trapped (d (ppm))
D/10-9 m2 s-1 )
[Co4 L6 ](BF4 )8
[Zn4 L6 ](BF4 )8
[Cd4 L6 ](BF4 )8
-151.8
1.4(2)
-169.2
1.0(2)
-150.3
1.4(2)
-145.0
0.9(2)
-150.1
1.2(2)
-143.7
0.8(2)
Fig. 13 Variable temperature 400 MHz 19 F NMR spectra of (a)
[Cd4 (Lbip )6 ](BF4 )8 and (b) [Co4 (Lbip )6 ](BF4 )8 (ref. 7a) in CD3 CN illustrating
fast exchange of free and encapsulated [BF4 ]- anions at ambient temperature but slow (on the NMR timescale) exchange at low temperature with
separate signals for free and bound [BF4 ]- .
except that [Co4 (Lbip )8 ]8+ was decidedly better than [Co2 (Lbip )4 ]4+ .
Fig. 14 depicts the stepwise formation of the tetrahedral cage,
and Table 4 contains the optimized free energy values for the best
model of the data at each of the three temperatures.
The optimization of the standard free energy values for the data
at 295 K gave the molar absorptivity values and the concentration
profiles depicted in Fig. 15. The concentration profile shows that
each complex was formed in significant amounts. As expected,
at 1.5 equivalents of ligand added, there is no free cobalt(II)
remaining in solution and the [Co4 L6 ]8+ tetrahedron is the dominant complex in solution. The molar absorptivity values of each
complex were calculated and even those with very similar spectra
such as the [Co2 L5 ]4+ and the [CoL3 ]2+ complexes were resolvable
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Table 4 Optimal thermochemical models for spectroscopic data of the titration of cobalt(II) with Lbip at various temperatures
Reaction
DG◦ /kJ mol-1 283 K
m = 4, n = 4
DG◦ /kJ mol-1 295 K
m = 4, n = 2
DG◦ /kJ mol-1 308 K
m = 4, n = 2
DH ◦ /kJ mol-1
m = 3, n = 3
DS◦ /J K-1 mol-1
m = 3, n = 3
Co2+ + 12 L — 12 [Co2 L]4+
1
[Co2 L]4+ + 12 L — 1/m[Com Lm ]2m+
2
1/m[Com Lm ]2m+ + 12 L — 12 [Co4 L6 ]8+
1
[Co4 L6 ]8+ + 12 L — 1/n[Con L2n ]2n+
4
1/n[Con L2n ]2n+ + 12 L — 12 [Co2 L5 ]4+
1
[Co2 L5 ]4+ + 12 L — [CoL3 ]2+
2
-147(5)
-92.7(2)
-87.1(2)
-84.8(1)
-45.4(1)
-43.0(1)
-154.6(1)
-148.6(1)
-136.2(2)
-49.8(1)
-49.6(1);
-31.3(2);
-154(3)
-155(2)
-147(2)
-60.0(5)
-64.5(6)
-6.2(5)
-17(2)
640(30)
630(20)
-344
92(2)
-428(8)
456(8)
2620(90)
2560(70)
-942(80)
483(8)
-1360(30)
Fig. 14 Schematic representation of the stepwise formation and deconstruction of the [Co4 (Lbip )6 ]8+ tetrahedron in solution as ligand is added to
metal cation; the number of ligands per metal cation is shown along the
bottom.
Fig. 15 (a) Molar absorptivity and (b) concentration profiles for the
optimal thermochemical model for the self-assembly of [Co4 (Lbip )6 ](BF4 )8
and its subsequent disassembly as Lbip is added stepwise to a dry acetonitrile
solution of Co(BF4 )2 at 295 K.
from one other. On a per ligand basis the peak molar absorptivity
values are 26.3, 14.7, 10.6, 8.59, 7.26, and 6.46 M-1 cm-1 going
from the [Co2 L]4+ to the [CoL3 ]2+ complex. The model accounts
for 97.72% of the measured absorbance data, which is virtually all
the signal since the expected noise for absorbance data of roughly
0.1 units is about 2% for this particular spectrometer.
Obviously, there is no single best model for modelling all three
data sets because the sets of complexes most strongly evidenced
in each dataset are not identical. The differences arise between
complexes with the same empirical formula. The balance between
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such complexes is certainly a delicate one in solution. All things
being equal, the smaller versions are expected to dominate at
all temperatures owing to entropic factors;15 however, not all
things are equal here. Geometric strain may well destabilize
[Co2 (Lbip )4 ]4+ even as the coulombic repulsions of the cobalt(II)
cations destabilize the larger analogues. It is difficult to anticipate
which of these enthalpic factors wins out. Regardless, it is not
surprising that the smaller complex seems to dominate at higher
temperatures.
It is important to note that while such modelling does distinguish between complexes with the same empirical formula, it is
not completely decisive. Such complexes are only distinguishable
within the model by their relative equilibrium balances with
other complexes and not directly by their stoichiometry or
molar absorptivity.16 The competition between ligand strain and
charge separation seems to be most relevant for the 1 : 2 metal :
ligand stoichiometry and to a lesser extent the 1 : 1 case. Future
studies are necessary to differentiate between such comparable
complexes in an equilibrium solution. The configurations of other
stoichiometries are not ambiguous.
In order to best compare the free energy values of the reactions
at the three different temperatures, each dataset has also been
modelled using a common set of complexes: Co2+ , [Co2 Lbip ]4+ ,
[Co3 (Lbip )3 ]6+ , [Co4 (Lbip )6 ]8+ , [Co3 (Lbip )6 ]6+ , [Co2 (Lbip )5 ]4+ , and
[Co(Lbip )3 ]2+ . Notice that the stoichoimetries of the 3rd and 5th
complexes in the list represent intermediate values between those
discussed earlier. While these exact complexes may indeed exist
in solution, this list of complexes is chosen because it works best
for all three datasets simultaneously even though it is not the
best for any one dataset. The free energy values for these fits are
used to estimate enthalpy and entropy changes for each reaction
(Table 4). Details of all the models can be found in the supporting
information.†
In the first three steps shown in Table 4b, Lbip ligands are
replacing solvent molecules in the coordination sphere of the
Co(II) ions. The fact that the standard free energy values tend to
decrease in subsequent assembly steps 1–3 is typical for stepwise
coordination steps and presumably enhanced due to the coulombic
interactions between Co(II) cations that coordinate to the same Lbip
molecule. Both the overall enthalpy and entropy for these three
assembly steps are decidedly positive, indicating that the assembly
is clearly entropy driven.
In the last three steps, there is no net increase in cobalt/Lbip
linkages, as additional ligand simply eliminates the need for
multiple cobalt(II) cations to be coordinated by the same ligand
and the cage falls apart in favour of isolated species. The end
result is individual cations coordinated to three separate ligands
each. Overall, these disassembly steps are enthalpy driven as
Dalton Trans., 2011, 40, 12132–12145 | 12141
charge is allowed to separate without any net loss of ligand/metal
coordinative bonds. Consequently, if the cage was assembled by
adding cobalt(II) into a solution of ligand, the assembly would
also be entropy driven.
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Exchange of metal cations between intact homometallic cages
Whilst the analysis above shows clearly that mixtures of Co(II)
cations and Lbip ligands form an equilibrium mixture whose
components are labile enough to interconvert on the timescale
of the experiment, pure M4 L6 tetrahedral cages – in the absence
of additional metal ion or ligand equivalents to facilitate their
breakup – can be surprisingly kinetically inert. Raymond has
demonstrated that in an M4 L6 tetrahedral cage based on kinetically
labile Ga(III) ions, the mechanical stiffness and interlocking
associated with the structure resulted in an optically pure sample of
the cage being configurationally stable for months, even at elevated
temperatures.17 Similarly Fujita recently demonstrated in a Pd12 (m–
L)24 pseudo-spherical cage complex that ligand exchange occurred
on a timescale about 105 times longer than for mononuclear Pd(II)
complexes, due to the remarkable kinetic inertness of the assembly
once formation was complete because of the cooperativity of
48 Pd ◊ ◊ ◊ pyridyl interactions.18 We also showed recently that exchange of ligands between a pair of isostructural M4 L6 tetrahedral
cage likewise proceeded on an extremely slow timescale despite the
use of (nominally labile) ions in the cage.1d
To investigate this phenomenon with the Lbip -based tetrahedral
cages we performed a simple experiment in which equimolar
amounts of [Co4 (Lbip )6 ](BF4 )8 and [Cd4 (Lbip )6 ](BF4 )8 were combined and the composition of the mixture was followed by
electrospray mass spectrometry. This pair of cages was chosen
as their crystal structures have both been determined and show
essentially identical cage structures (apart from variations in M–
N bond distances), and the difference in atomic mass between
Co and Cd makes following the equilibration in solution by mass
spectrometry straightforward. A 1 : 1 mixture of these two cages
complexes (as X-ray quality single crystals) was prepared at a
concentration of ca. 2.5 mM in MeCN and the ES mass spectrum
was monitored at regular intervals. As usual the mass spectrum
showed several peaks for each component associated with successive loss of anions to give the ion series {M4 (Lbip )6 (BF4 )8-n }n+ .
The spectra in Fig. 16 are from the region of the spectrum
where n = 4, i.e. loss of four of the eight anions generating
the species {M4 (Lbip )6 (BF4 )4 }4+ . Immediately after mixing the
spectrum obtained in Fig. 16a was obtained, showing principally
the starting Co4 and Cd4 cages centred at m/z 848 and 902
respectively. The peak cluster at m/z 902 is actually a superposition
of two signals with different isotopic spacings (0.25 and 0.5 mass
units, corresponding to 4+ and 2+ species respectively). The
expected signal from {Cd4 (Lbip )6 (BF4 )4 }4+ is the weaker of the
two, and the signal in this region is dominated by a component
having the same m/z value but a charge of +2, i.e. the fragment
{Cd2 (Lbip )3 (BF4 )2 }2+ ; these are labelled on the figure.
In addition we see (even a very short period after mixing)
evidence for scrambling of metal ions between the cages to give
Co3 Cd and CoCd3 species centres at m/z 862 and 889 respectively;
the 0.25 mass unit spacing confirms that these arise from intactcage {M4 (Lbip )6 (BF4 )4 }4+ with M4 = Co3 Cd or CoCd3 . Finally at
m/z 875 we see a superposition of peaks for the tetranuclear
12142 | Dalton Trans., 2011, 40, 12132–12145
Fig. 16 Electrospray mass spectra of a 1 : 1 mixture of [Co4 (Lbip )6 ](BF4 )8
and [Cd4 (Lbip )6 ](BF4 )8 in MeCN: (a) immediately after mixing; (b) five
months later after the mixture had attained equilibrium.
Co2 Cd2 cage (0.25 mass unit spacing, very weak) and the fragment
{CdCo(Lbip )3 (BF4 )2 }2+ (0.5 mass unit spacing, more intense).
The low intensity of the peaks associated with the mixed-metal
tetranuclear cages indicates that the rearrangement of metal ions
between cages is far from complete.
This spectrum evolved slowly over a period of weeks, finally
reaching equilibrium – with no further detectable changes – after
150 days. This spectrum is shown in Fig. 16b and shows a nearperfect binomial distribution of Co4 , Co3 Cd, Co2 Cd2 , CoCd3 and
Cd4 species which is what would be expected if the metal sites
are independent of one another and a statistical distribution
of products is obtained. All of these signals have a spacing
of 0.25 units between components of the isotope cluster and
therefore correspond to complete {M4 (Lbip )6 (BF4 )4 }4+ cages with
no fragment peaks obscuring the main signals. The larger number
of isotope components associated with Cd compared to Co means
that the total signal intensity for the Cd-rich species is distributed
over a larger number of components, so the signals at higher m/z
values are broader but less intense.
For exchange of metal ions to occur requires a metal ion to
be liberated fully from a cage, with all six M–N bonds broken.
Whilst stepwise exchange of bidentate ligand fragments can be
fast at high-spin Co(II),1d in the M4 L6 cages dissociation of a
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bidentate pyrazolyl-pyridine unit does not result in its loss from
the complex as the ligand is still anchored at the other end and
is involved in p-stacking with other ligands. Thus a dissociated
pyrazolyl-pyridine bidentate unit will be available for rapid reattachment and it is therefore easy to see why loss of a metal ion –
involving simultaneous cleavage of three bidentate units which are
all anchored at their alternate termini – should be so slow.
Recrystallisation of this mixture after equilibration afforded a
batch of X-ray quality crystals. These were pale pink in colour [and
so contained at least one Co(II) centre, as the pure Cd4 complex
is colourless]. We hoped that it would be possible to distinguish
between the Co(II) and Cd(II) centres in the cage if they were
localised in specific positions, but it was clear from the refinement
that the mixture of metal ions was disordered over all metal sites
which refined best as a fractional mixture of Co(II) and Cd(II)
giving a final composition of approximately Co3 Cd. Clearly any
structural differences associated with replacing Co(II) by Cd(II) are
so small that a combination of metal disorder over the four sites,
and possibly co-crystallisation of components containing different
metal ion combinations, prevents unambiguous crystallographic
identification of a specific Cox Cdy isomer.
Conclusions
In this paper we have reported a series of structural, spectroscopic
and self-assembly studies on a family of isostructural M4 L6
tetrahedral cages. The main conclusions are as follows:
(i) A family of isostructural cages forms using a wide range
of metal cations [Co(II), Ni(II), Zn(II), Cd(II)] and with different
anions; the cage structures are clearly retained in solution on the
basis of NMR studies. Full analysis of the 1 H NMR spectrum of
a Co(II) cage has been possible by correlation of NMR t1 values
for all signals with crystallographically-derived H ◊ ◊ ◊ Co distances.
Small anions such as BF4 - and NO3 - are accommodated in the
irregular central cavity and form a network of hydrogen-bonding
interactions with inwardly-directed CH protons from methylene
and pyrazolyl groups. Larger anions such as naphthyl-1-sulfonate
or tetraphenylborate are not accommodated in the central cavity
but interact with the external surfaces of the cage principally
via CH ◊ ◊ ◊ p interactions and (for naphthyl-1-sulfonate) CH ◊ ◊ ◊ O
hydrogen-bonds.
(ii) Encapsulated BF4 - anions are in exchange with external
anions at room temperature but the exchange can be completely
frozen out in 19 F NMR spectra by 233 K at which point separate
signals for internal and external anions are observed in a 1 : 7 ratio.
Diffusion coefficients for the encapsulated anions (from 19 F DOSY
measurements) are the same as those of the cage superstructures
(from 1 H DOSY measurements) and significantly less than those
of external un-bound anions, indicating that the cages and their
guest anions behave as a single rigid unit at that temperature.
(iii) The solution-phase self-assembly of the tetrahedral cage is
truly an equilibrium event, driven not by enthalpy, but by entropy
as solvent molecules are freed, new coordinative bonds are formed,
and charge is concentrated. Furthermore, the geometrically designed system dominates to the near exclusion of other complexes
when the stoichiometry between metal and ligand is right. When
the stoichiometry is not 2 : 3, additional oligomeric species exist
in equilibrium with each other, indicating that the driving forces
that facilitate the formation of the tetrahedral cage also lead to
This journal is © The Royal Society of Chemistry 2011
intermediate structures rather than just the target cage molecules
with unreacted pieces. Equilibrium-restricted factor analysis is a
powerful tool for elucidating the assembly process and solution
chemistry of self-assembling, three dimensional supramolecules as
the ensemble of interacting chemical species can be characterized
without the need for chemical isolation.
(iv) A mixture of pure Co4 and Cd4 cages shows a remarkable
degree of kinetic inertness for the fully-formed cages, with mass
spectrometric analysis showing that scrambling of metal cations
between the sites to give a mixture of Co4 , Co3 Cd, Co2 Cd2 , CoCd3
and Cd4 cages takes months in solution at room temperature.
Experimental
Materials and methods
The ligand Lbip was prepared as previously described.7 All other
chemicals were purchased from Aldrich and were used as received.
The following instruments were used for routine spectroscopic
measurements: ES mass spectra, a Walters LCT instrument; NMR
spectra, Bruker AV1-250, AV3-400 or DRX-500 spectrometers.
The 19 F and 113 Cd NMR measurements were carried out on the
AV3-400 instrument, at frequencies of 375.6 MHz and 88.76 MHz
respectively. For the DOSY measurements (on the AV3-400) the
probe gradient power was calibrated using a standard 0.3 M GdCl3
solution in D2 O containing 1% H2 O and 0.1% CH3 OH.
Synthesis of complexes
The complexes [M4 (Lbip )6 ](BF4 )8 (where M = Co, Ni, Zn or Cd) and
[Cd4 (Lbip )6 ](NO3 )8 were prepared solvothermally; the method here
given for [Cd4 (Lbip )6 ](BF4 )8 is typical. A Teflon-lined autoclave was
charged with Cd(BF4 ). H2 O (0.022 g, 0.07 mmol), Lbip (0.05 g,
0.11 mmol) and methanol (9 cm3 ). Heating to 100 ◦ C for
12 h followed by slow cooling to room temperature yielded a
white powder. This was separated and consecutively washed with
methanol and then chloroform before being dried in vacuo. X-ray
quality crystals were obtained on the diffusion of diethyl ether into
a concentrated nitromethane solution of the complex. All samples
for elemental analysis were dried thoroughly but proved to be
hygroscopic, gaining weight when exposed to air; consequently all
C, H, N analytical data are consistent with the presence of several
water molecules per complex molecule.
Data for [Cd4 (Lbip )6 ](BF4 )8 : yield 98%. ESMS: m/z
1891.1, {[Cd4 (Lbip )6 ](BF4 )6 }2+ ; 1231.7, {[Cd4 (Lbip )6 ](BF4 )5 }3+ ;
902.3, {[Cd4 (Lbip )6 ](BF4 )4 }4+ ; 704.2, {[Cd4 (Lbip )6 ](BF4 )3 }5+ . NMR
spectra: see main text. Anal. Found: C, 54.0; H, 3.7; N, 12.2%.
Required for [Cd4 (Lbip )6 ](BF4 )8 ·5H2 O: C, 53.4; H, 3.8; N, 12.5%.
Data for [Zn4 (Lbip )6 ](BF4 )8 : yield 96%. ESMS: m/z 1769.9,
{[Zn4 (Lbip )6 ](BF4 )6 }2+ ; 1168.8, {[Zn4 (Lbip )6 ](BF4 )5 }3+ ; 854.8,
{[Zn4 (Lbip )6 ](BF4 )4 }4+ ; 666.9, {[Zn4 (Lbip )6 ](BF4 )3 }5+ . Anal. Found:
C, 54.6; H, 4.0; N, 12.4%. Required for [Zn4 (Lbip )6 ](BF4 )8 ·9H2 O:
C, 55.0; H, 4.2; N, 12.8%.
Data for [Ni4 (Lbip )6 ](BF4 )8 : yield 94%. ESMS: m/z 1782.0,
{[Ni4 (Lbip )6 ](BF4 )6 }2+ ; 1159.4, {[Ni4 (Lbip )6 ](BF4 )5 }3+ ; 848.0,
{[Ni4 (Lbip )6 ](BF4 )4 }4+ ; 660.8, {[Ni4 (Lbip )6 ](BF4 )3 }5+ ; 536.2,
{[Ni4 (Lbip )6 ](BF4 )2 }6+ ; 447.3, {[Ni4 (Lbip )6 ](BF4 )}7+ . Anal. Found:
C, 56.3; H, 3.8; N, 12.9%. Required for [Ni4 (Lbip )6 ](BF4 )8 ·5H2 O:
C, 56.4; H, 4.1; N, 13.2%.
Dalton Trans., 2011, 40, 12132–12145 | 12143
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Data for [Cd4 (Lbip )6 ](NO3 )8 : yield 47%. ESMS: m/z
1190.3, {[Cd4 (Lbip )6 ](NO3 )5 }3+ ; 877.6, {[Cd4 (Lbip )6 ](NO3 )4 }4+ .
Anal. Found: C, 53.1; H, 4.4; N, 14.7%. Required for
[Cd4 (Lbip )6 ](NO3 )8 ·16H2 O: C, 53.4; H, 4.4; N, 15.2%.
The counter-ion exchange experiments were conducted as
outlined for [Co4 (Lbip )6 ](BF4 )6 (C10 H7 SO3 )2 . An excess of sodium
naphthalene-1-sulphonate (0.01 g, 0.04 mmol) in MeCN (1 cm3 )
was added to solution of [Co4 (Lbip )6 ](BF4 )8 (0.01 g, 0.0026 mmol)
also in MeCN (1 cm3 ). Diethyl ether was diffused into the resulting
mixture to yield X-ray quality orange crystals.
Data for [Co4 (Lbip )6 ](C10 H7 SO3 )2 (BF4 )6 : yield 26%. ESMS:
m/z 1844, {[Co4 (Lbip )6 ](C10 H7 SO3 )(BF4 )5 }2+ ; 1200, {[Co4 (Lbip )6 ](BF4 )4 (C10 H7 SO3 )}3+ ; 879, {[Co4 (Lbip )6 ](BF4 )3 (C10 H7 SO3 )}4+ .
Anal. Found: C, 57.5; H, 4.6; N, 11.1%. Required for
[Co4 (Lbip )6 ](C10 H7 SO4 )2 (BF4 )6 ·12H2 O: C, 57.4; H, 4.4; N,
11.2%.
Data for [Cd4 (Lbip )6 ](BPh4 )1.5 (BF4 )6.5 : yield 33%. ESMS:
m/z 1309,{[Cd4 (Lbip )6 ](BPh4 )(BF4 )4 }3+ ; 960, {[Cd4 (Lbip )6 ](BPh4 )(BF4 )3 }4+ . Anal. Found: C, 60.0; H, 4.5; N, 12.3%.
Required for [Cd4 (Lbip )6 ](BPh4 )1.5 (BF4 )6.5 ·3H2 O: C, 59.5; H, 4.1;
N, 12.5%.
be over-analysed, but the gross structures of the cages and their
guests are perfectly clear.
UV/Vis spectroscopic titrations
Approximately 0.25 cm3 of dry 0.1 M Co(BF4 )2 solution was
measured out by weight into a quartz, screw-top cuvette. The
cuvette was sealed throughout the course of the titration. Using
a microsyringe, 4.33 cm3 of a cobalt(II)/Lbip solution was added
stepwise until the ratio of Lbip to Co(II) ions was about three.
Although not necessary, the presence of cobalt(II) in the titrant
solution keeps the concentration of cobalt(II) constant throughout
the titration. After each addition of titrant solution, the mass of
the cuvette with the solution was then recorded. The solution was
then manually shaken and allowed to chemically and thermally
equilibrate for three minutes. The absorbance spectrum of the
solution was then measured every nanometre from 425 to 700 nm
with a Cary 100 UV/Vis spectrophotometer using a baseline of
the solvent acetonitrile, a beam width of 2.0 nm, a path length of
1.0 cm, and a scan rate of 600 nm min-1 . The temperature of each
titration was controlled using the accompanying Cary circulator.
Mathematical modelling of spectroscopic data
X-ray crystallography
Data for [Co4 (Lbip )6 ](C10 H7 SO4 )2 (BF4 )6 ·2MeCN·2H2 O were collected at the EPSRC National Crystallography Service at the
University of Southampton on a Bruker APEX 2 diffractometer
using graphite-monochromated Mo-Ka X-radiation (0.71073 Å)
from a rotating anode source. The remaining data were collected
at the University of Sheffield on a Bruker APEX 2 diffractometer
using graphite monochromated Mo-Ka X-radiation (0.71073 Å)
from conventional sealed-tube sources. After integration of the raw
data, and before merging, an empirical absorption correction was
applied (SADABS)19 based on comparison of multiple symmetryequivalent measurements. The structures were solved by direct
methods and refined by full-matrix least squares on weighted F 2
values for all reflections using the SHELX suite of programs.20
Crystallographic data are collected in Table 1.
Crystals of the cage complexes generally scattered weakly due
to the extensive disorder of anions and solvent molecules meaning
that high-angle data were absent and relatively low 2q limits
were used for data collections (see Table 1). In all six structures
it was not possible to clearly locate all of the tetrafluoroborate
anions from the data due to extensive disorder associated with
both those anions and solvent molecules; this resulted in areas
of diffuse electron density that could not sensibly be modeled.
The SQUEEZE function of PLATON21 was used in every case
to eliminate these areas of electron density from the refinement,
as a consequence of which disordered solvent molecules have
been omitted from the formulae and formula weights listed in
Table 1. More details of this, and the disorder of the anions and
solvent molecules that were located, are included in detail in the
individual CIFs. In all cases the structural data described are the
best that could be obtained from several attempts using long data
collections at low temperatures, and in once case the rotating
anode source at the EPSRC National Crystallography Service.
Uncertainties in bond distances/angles for these structures are
accordingly higher than usual, and structural minutiae should not
12144 | Dalton Trans., 2011, 40, 12132–12145
The program ‘Sivvu’ was used to model the all of the data
simultaneously for a single temperature. The details of program
can be found in the Supporting Information† or on the web.22
Sivvu searches for the standard free energy values that result in the
smallest root-mean-square residual of the measured and calculated
absorbance values. In the process, the molar absorptivity values
are determined by least squares analysis of the over-expressed
system of linear equations, which are each an implementation
of the Beer–Lambert law, one for each absorbance datum. The
problem is conveniently expressed using matrix algebra, which
was first detailed by Wallace.23
All equilibrium concentrations are calculated on the basis of
activity, the different computational models of which may affect
the exact values of the standard free energy of the reactions, and
the units of absorptivity. For this work, the molar absorptivity
values are reported in units of absorbance·cm-1 M-1 .
The standard deviations of the standard free energy values are
calculated by re-optimizing on 40 different subsets of the data
with a random 50% of the wavelengths ignored, and subsequently
quantifying the standard deviation in the set of optimized standard
free energy values. The enthalpy and entropy values along with
their standard deviations are calculated using an Arrhenius-type
plot based on the standard free energy values directly from the
re-optimized subsets.
The volumes of analyte and titrant solution at 283 and 308 K
were adjusted based on the density of acetonitrile at these
temperatures.
Acknowledgements
We thank EPSRC for financial support and for funding the
UK National Crystallography Service where the diffraction data
for crystals of [Co4 (Lbip )6 ](C10 H7 SO4 )2 (BF4 )6 ·2MeCN·2H2 O were
measured. DVG and LEM thank the Donors of the American
Chemical Society Petroleum Research Fund, and the National
Science Foundation for the support of the research that led to the
This journal is © The Royal Society of Chemistry 2011
thermodynamic models of spectroscopic data. We also thank Mr
Harry Adams for assistance with the X-ray crystallography.
7
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