Saddling up copper-new twists on a metallo-wheel

Article abstract of DOI:10.1039/c3dt53267d

A unique octanuclear copper(ii) cluster with a saddle-shaped structural topology has been prepared from a large, flexible polydentate ligand comprising a 4,4′-bipyridine linker bearing four pendant pyrazolate heterocycles.

Full text of DOI:10.1039/c3dt53267d

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Saddling up copper – new twists on a
metallo-wheel†
Niloofar Zarrabi,a John J. Hayward,a William Cleggb and Melanie Pilkington*a
Cite this: Dalton Trans., 2014, 43,
2352
Received 19th November 2013,
Accepted 12th December 2013
DOI: 10.1039/c3dt53267d
www.rsc.org/dalton
A unique octanuclear copper(II) cluster with a saddle-shaped struc- domains and their coordination with metal ions for the self-
tural topology has been prepared from a large, flexible polydentate assembly of magnetic chains, clusters and networks. To date
ligand comprising a 4,4’-bipyridine linker bearing four pendant we have focused our efforts on systems which ligate metals
pyrazolate heterocycles.
within a 4,4′-bipyridine-containing [N3O2] macrocyclic frame-
work6a as well as exploiting the coordination chemistry and
reactivity of 3,3′-diamino-2,2′-bipyridine6b for the prepara-
tion of dicarboxamide6c and reactive Schiff-base bis-imine
ligands.6d In this paper we report the synthesis of the first
example of a new family of 4,4′-bipyridine tetracarboxamide
ligands bearing four amide-bridged pendant heterocycles and
its ability to construct a high-nuclearity copper(II) cluster.
4,4′-Bipyridyl-2,2′,6,6′-tetracarboxylic acid7 was readily con-
verted to the tetramide by first preparing the chloroformyl
derivative that was then refluxed with the 3-amino-5-t-butyl
pyrazole heterocycle in toluene overnight. t-Butyl groups were
introduced to improve the solubility of the ligand in common
organic solvents. Ligand L was characterised by multinuclear
NMR, CHN microanalysis, FAB MS and X-ray diffraction.†
The crystal structure revealed that the pyrazole nitrogen
atoms point into the molecular cleft (as illustrated in Fig. 1),
affording two binding domains for the ligand. The pendant
pyrazole groups are known to form diverse coordination
Metal clusters, rings, wheels or cages are ubiquitous to many
areas of chemistry and biochemistry including main group,
organometallic, molecular magnetism, metallo-supramolecu-
lar and bioinorganic chemistry.1 Particular interest in poly-
nuclear copper clusters is fuelled by the characterisation of
several unusual copper-containing metalloenzymes.2 Cyclic
metal arrays containing large numbers of spin-coupled para-
magnetic metal centres are frequently synthesised from multi-
component systems in an uncontrolled “serendipitous”
process that employs simple organic molecules and/or brid-
ging ligands.3 A more controlled approach that excludes
extraneous bridging fragments can be adopted when designed
polytopic ligands are employed, as then a self-assembly
process proceeds through an algorithm defined by the coordi-
nation preferences of the metal ion and the steric information
contained within the ligand structure.4,5 Cyclic metal arrays
and clusters assembled via this second approach have yet to
offer the high nuclearities and magnetic phenomena associ-
ated with those formed from simpler bridging ligands.5 This is
in part due to the considerable synthetic effort required to syn-
thesise families of polytopic ligands containing both rigid and
flexible bridging groups.
Our research programme is focused towards the synthesis
of multitopic ligands with two or more discrete binding
aDepartment of Chemistry, Brock University, 500 Glenridge Avenue, St Catharines,
ON, L2S 3A1 Canada. E-mail: mpilkington@brocku.ca;
Tel: +1 (905) 688 5550 Ext. 3403
bSchool of Chemistry, Bedson Building, Newcastle University, Newcastle upon Tyne,
NE1 7RU, UK. Tel: +44 (0)191 208 6649
†Electronic supplementary information (ESI) available: Full details of the
synthesis of L and complex 1; crystallographic data for L and 1 in cif format;
frozen solution EPR spectra of 1 in MeOH; UV-vis spectra of 1 in MeOH; mag-
netic susceptibility data for 1. CCDC 969763 and 969764. For ESI and crystallo-
graphic data in CIF or other electronic format see DOI: 10.1039/c3dt53267d
Fig. 1 Crystallographically determined structure of tetramide ligand, L;
two H2O and DMF molecules and the H atoms of the t-butyl groups are
omitted for clarity.
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motifs,8 and can potentially bind two or more metal centres in equatorial positions are occupied by the oxygen of an amide
close proximity, thereby facilitating the formation of clusters unit and the two chloride ions. In the case of Cu(2) and Cu(4),
(often of high nuclearity),8f,g and/or frameworks through judi- the geometries are closer to distorted square pyramidal; the
cious choice of metal ions. Slow diffusion of a methanolic two pyrazole nitrogens are trans to each other in the basal
solution of CuCl2·2H2O into a chloroform solution of L at plane, as are an amide oxygen and one chloride, and the
room temperature afforded green plates suitable for X-ray dif- coordination sphere is completed with the remaining chloride
fraction. Crystallographic studies revealed an octanuclear in the axial position. At each copper centre different Cu–Cl
cluster of stoichiometry [Cu8L4Cl16].†
bond lengths are observed; for the Cu(2) and Cu(4) centres,
The coordination complex crystallises in the triclinic space the pseudo-axial bonds are longer than the equatorial bonds
ˉ
group P1 with one unique cluster in the asymmetric unit.† The by approximately 1.9 Å. A similar trend is observed for the
cluster has approximate S4 symmetry and is comprised of four copper centres with distorted trigonal bipyramidal geometry,
ligands and eight copper centres (Fig. 2a). Unlike a number of but the differences are generally shorter (∼1.5 Å). In order for
previously reported heterocyclic amido complexes9 the nitro- the ligand to chelate all four of its pendant arms to the copper
gen atoms of the amide groups remain protonated and L centres, all four 4,4′-bipyridine backbones are substantially
coordinates the metal centres as a neutral ligand. As a twisted when compared with the geometry adopted by the
consequence, it does not chelate via its tridentate binding native ligand, with torsion angles in the range 41.2°–43.5°
pocket, but instead each polydentate ligand binds four copper between the mean planes of their pyridyl rings.
centres, two in a chelate fashion and two in a monodentate
The structure of 1 consists of a non-planar saddle-like loop
manner to form a unique octanuclear complex with saddle- comprised of eight Cu(II) centres. Although there are examples
shaped geometry. The C–N and C–O bond lengths of the of metal complexes that adopt a saddle-type geometry,11 this
amides are in the range 1.328(6)–1.362(8) Å and 1.211(7)– complex is unique, being the first reported octanuclear copper
1.245(5) Å respectively, consistent with a conjugated amide. cluster with a saddle-shaped or closed sinusoidal confor-
The pentacoordinate geometries around the copper centres are mation (Fig. 2b). The Cu⋯Cu distances around the saddle rim
best interpreted as intermediate between trigonal bipyramidal are in the range 7.462(1) Å–8.695(1) Å, and the distances
and square-based pyramidal; the distortion parameter, τ,10 has between the apical metal centres are 13.818(1) Å and 13.135(1)
calculated values in the range 0.34–0.8 (τ = (θ1 − θ2)/60; in Å for Cu(2)⋯Cu(6) and Cu(4)⋯Cu(8) respectively. For all eight
which the largest angles in the coordination sphere are desig- copper centres, one chloride ion is involved in two hydrogen
nated θ1 and θ2 and τ = 0 for square pyramidal and τ = 1 for tri- bonding interactions with neighbouring amide hydrogen
gonal bipyramidal). Whilst Cu(2) and Cu(4) have values of 0.34 atoms with H⋯Cl contacts in the range 2.33–2.49 Å [see ESI†].
and 0.43, the remaining copper atoms have τ > 0.5 and are The intermolecular Cu⋯Cu distances are long, the shortest
best described as distorted trigonal bipyramidal. For these being 10.324(1) Å between two neighbouring Cu(5) centres,
ions, the axial positions are occupied by two pyrazole nitro- but shorter than the intramolecular apical Cu⋯Cu separation.
gens from two different ligand molecules, while the pseudo- The CHN elemental analysis of 1 is consistent with the
Fig. 2 (a) View of the octanuclear cluster [Cu8L4Cl16] 1 along its approximate S4 axis with the four independent ligands shown as different colours.
The coordination geometry of each unique Cu(II) centre is highlighted by capped sticks. (b) View of the cluster highlighting the saddle shape of the
Cu8 core and the coordination geometries of the eight independent Cu(II) ions. In both cases hydrogen atoms bonded to carbon are omitted for
clarity. Disordered solvent was removed from the model by applying the SQUEEZE command in PLATON.† The t-butyl groups are disordered (only
one orientation is shown).
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stoichiometry [Cu8L4Cl16]·CHCl3·7MeOH·5H2O. The IR spec- for assistance in simulating these data. We thank
trum confirms the presence of amide N–H bonds, and two Prof. M. Murugesu (University of Ottawa) for magnetic data.
amide CvO stretches at 1694 and 1640 cm−1 can be assigned This work was financially supported by NSERC-DG, CFI, CRC,
to the uncoordinated and Cu2+-coordinated amide carbonyl Ministry of Ontario (ERA) and Brock University.
groups respectively. FAB mass spectrometry of the cluster
exhibited only fragmentation peaks; however, several of these
were multi-metallic, suggesting that the cluster remains at
least partially intact in solution.
Notes and references
The UV-Vis spectrum of 1 in DCM–MeOH† displays a
maximum at λ = 261 nm (ε = 81 000 M−1 cm−1) consistent with
π to π* transitions of the bipyridine ligands; a broad absorp-
tion band at λ = 797 nm (ε = 854 M−1 cm−1) is assigned to d–d
transitions. Five-coordinate copper complexes typically show
d–d absorptions in the range 588–769 nm when possessing a
square pyramidal geometry, while trigonal bipyramidal com-
plexes show absorption bands at longer wavelengths, typically
685–952 nm.12 The EPR spectrum of 1 in frozen methanol
solution at 77 K† revealed a near axial spectrum (g1 = 2.224,
g2 = 2.065 and g3 = 2.035). Whilst the high-field component
revealed some evidence for hyperfine coupling, the features
were not well resolved. Conversely the low-field features
revealed a coupling of ca. 80 G consistent with hyperfine coup-
ling to copper (both 63Cu and 65Cu have I = 3/2). The g-values
give R = 0.1875 [according to the calculation R = (g2 − g1)/(g3 −
g2)],12a reflecting a predominantly dx2–y2 ground state; this is
consistent with a movement from trigonal bipyramidal coordi-
nation towards square pyramidal geometry around the Cu(II)
centres in MeOH solution. The magnetism of the Cu8 cluster
was examined in the temperature range 300–1.8 K. Magnetic
susceptibility data reveal 1 obeys the Curie–Weiss law with C =
3.59 cm3 K mol−1 and θ = −0.74 K. The Curie constant is close
to the value of 3.31 cm3 K mol−1 expected for eight indepen-
dent Cu(II) ions (S = 1/2, g = 2.1) with the negative Weiss con-
stant indicating the presence of very weak antiferromagnetic
interactions consistent with the absence of an efficient
through-bond super-exchange pathway. The field dependence
of the magnetisation at 1.8 K revealed a saturation magnetisa-
tion at 7 T of 7.61 μB, slightly lower than the value of 8.4 μB
expected for eight S = 1/2 ions with g = 2.1. [A discussion of the
dipolar intra- and inter-cluster exchange and magnetic model-
ling as a ring of eight S = 1/2 ions is available as ESI.†]
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tetracarboxamide ligand L comprising a 4,4′-bipyridine back-
bone and pendant pyrazole heterocycles. The resultant octa-
nuclear metallocycle formed with CuCl2 is a rare example of a
polynuclear cyclic array comprising more than six Cu(II) ions
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