Reactions of Copper(II) Salts with 3{5}-tert-Butylpyrazole: Double-Cubane Complexes with Bound Exogenous Anions, and a Novel Pyrazole Coordination Mode

• Published in: Chemistry : a European journal Vol. 10; no. 7; pp. 1827 - 1837
• Main Authors: Liu, Xiaoming, McAllister, Judith A., de Miranda, Marcelo P., McInnes, Eric J. L., Kilner, Colin A., Halcrow, Malcolm A.
• Format: Journal Article
• Language: English
• Published: Weinheim WILEY-VCH Verlag 02.04.2004; WILEY‐VCH Verlag
• Subjects: cluster compounds; copper; hydrogen bonds; N ligands
• ISSN: 0947-6539; 1521-3765
• DOI: 10.1002/chem.200305613

Reaction of CuX2 (X−Cl−, Br−, NO3−), NaOH, and 3{5}‐tert‐butylpyrazole (HpztBu) in a 1:1:2 molar ratio in MeOH at 293 K for three days affords [{Cu3(HpztBu)6(μ3‐X)(μ3‐OH)3}2Cu]X6 (X−Cl−, 1; X−Br−, 2; X−NO3−, 3) in moderate yields. These compounds contain a centrosymmetric, vertex‐sharing double‐cubane [{Cu3(HpztBu)6(μ3‐X)(μ3‐OH)3}2Cu]6+ core, surrounded by a belt of six hydrogen‐bonded X− ions. For 1 and 2, the ring of guest anions has near C3 symmetry, that is slightly distorted owing to the axis of Jahn–Teller elongation at the central Cu ion. For 3 only, the NO3− guest ions are crystallographically disordered, reflecting their poor complimentarity with complex host. A similar reaction employing CuF2 yields [{Cu3(HpztBu)4(μ‐pztBu)2(μ‐F)2(μ3‐F)}2]F2 (4), whose structure contains a cyclic hexacopper core with approximate C2v symmetry. Finally, an analogous reaction using Cu(NCS)2 gives a mixture of trans‐[Cu(NCS)2(HpztBu)2] (5) and [Cu2(NCS)2(μ‐pztBu)2(μ‐HpztBu)(HpztBu)2] (6). The latter compound contains a HpztBu ligand bridging the two Cu ions in an unusual κ1,μ‐coordination mode. The variable temperature magnetic properties of 1–3 show antiferromagnetic behavior, leading to a S=${{ 1/2 }}$ ground state in which the seven copper(II) ions are associated into three mutually independent distinct spin systems. In confirmation of this interpretation, Q‐band EPR spectra of solid 1 and 2 at 5 K also demonstrate a S=${{ 1/2 }}$ spin system and exhibit hyperfine coupling to three 63,65Cu nuclei. Unusually, the coupling is manifest as an eight‐line splitting of the parallel feature, rather than the usual 10 lines. This has been rationalized by a spin‐projection calculation, and results from the relative magnitudes of coupling to the three Cu nuclei. UV/Vis and mass spectrometric data show that 1–4 decompose to lower nuclearity species in solution. A double‐cubane heptacopper core surrounded by a belt of six X− ions is present in the compounds [{Cu3(HpztBu)6(μ3‐X)(μ3‐OH)3}2Cu]X6 (X−Cl−, Br−, or NO3− (see structure)). These are associated with the cluster core through a combination of very long axial Cu⋅⋅⋅X interactions, hydrogen bonding and hydrophobic interactions. Spin frustration results in the seven Cu spins coalescing into three mutually independent spin systems in their magnetic ground state.