Bis-μ-pyrazolate-bridged dinickel(II) and dicopper(II) complexes:: An example of stereoelectronic preference of metal ions and stabilization of mixed-valence NiIIINiII species

New dimeric nickel(II) and copper(II) complexes [(L)Ni(mu-L')](2)[ClO4](2) 1 and [(L)Cu(mu-L')](2)[ClO4](2) 2, [2-[3-(2'-pyridyl)pyrazol-1-ylmethyljpyridine (L) and 3-(2-pyridyl)pyrazole (HL')] have been synthesized, structurally characterized, and their absorption, magnetic, EPR and redox properties investigated. The crystal structure of 1-MeCN reveals a planar [Ni-2(mu-L')(2))(2+) core [Ni center dot center dot Ni separation: 4.0765(10) angstrom] in which each distorted octahedral Ni ions is terminally coordinated by a tridentate ligand L and bridged by two HL' units, in their deprotonated form. The structural analysis of 2-2MeCN reveals two five-coordinate Cull ions, each terminally coordinated by adopting only a bidentate coordination mode of L, in which the -CH(2)py arm remains uncoordinated. A similar bridging [Cu center dot center dot center dot. Cu separation: 3.9382(24) angstrom] as in 1 center dot MeCN is observed. Thus stereo electronic preferences of Ni-II and Cu-II are clearly revealed. Closer inspection of crystal packing diagram of 1-MeCN reveals the formation of 2-D network structure assembled solely via C-H... 71 interaction [pyridyl C-H (4-position) of pyridylpyrazole unit of L and 71 cloud of pyridine of bridging deprotonated L' unit]. In 2 center dot 2MeCN, however, pi center dot center dot pi interactions between uncoordinated pyridyl arms are observed. Variable -temperature magnetic studies on both complexes indicate the existence of strong antiferromagnetic coupling between the two metal ions (singlet-triplet energy separation, J = -20 cm(-1) for 1 and -200 cm(-1) for 2). When investigated by cyclic voltammetry complex 1 displays two quasi-reversible electron-transfer reactions at E-1/2 = 1.36 V (Delta(EP) = 110 mV) and 1.79 V (Delta E-p = 120 mV) vs. SCE, due to Ni-III Ni-II/Ni-2(II) and Ni-2(III)/Ni-III Ni-II redox processes, respectively. Constant potential electrolysis at 1.50 V vs. SCE leads to the generation of dark green mixed-valence (NiNiII)-Ni-III species, which is stable enough to be subjected to characterization by UV/Vis and EPR spectroscopy. Compound 2 exhibits only irreversible reductions (cathodic peak potential, Ep. = -0.28 V and -0.50 V vs. SCE). ((C) Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2007)