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

摘要

Reaction of CuX2 (X~-=Cl~-, Br , NO_3~-), NaOH, and 3{5}-tert-butyl-pyrazole (Hpz~(tBu)) in a 1:1:2 molar ratio in MeOH at 293 K for three days affords [{Cu_3(Hpz~(tBu))_6((mu_3-X)(mu_3-OH)_3)_2Cu]X_6 (X-=Cl~-, 1; X =Br , 2; X~-=NO_3~- , 3) in moderate yields. These compounds contain a centrosymmetric, vertex-sharing double-cubanecore, surrounded by a belt of six hydrogen-bonded X~- ions. For 1 and 2, the ring of guest anions has near C_3 symmetry, that is slightly distorted owing to the axis of Jahn-Teller elongation at the central Cu ion. For 3 only, the NO_3 guest ions are crystallographically disordered, reflecting their poor complimentarity with complex host. Asimilar reaction employing CuF_2 yields(4), whose structure contains a cyclic hexacopper core with approximate C_(2v) symmetry. Finally, an analogous reaction using Cu(NCS)2 gives a mixture of trans-[Cu(NCS)_2(Hpz~(tBu))_2] (5) and [Cu_2(NCS)_2(mu-pz~(tBu))_2(mu-Hpz~(tBu))(Hpz~(tBu))_2] (6). The latter compound contains a Hpz~(tBu) ligand bridging the two Cu ions in an unusual kappa,mu-coordination mode. The variable temperature magnetic properties of 1-3 show antiferromagnetic behavior, leading to a S = l/2ground 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=l/2 spin system and exhibit hyperfine coupling to three ~(63.65)Cu nuclei. Unusually, the coupling is manifest as an eightline 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.