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RESEARCH PRODUCT

Bis‐μ‐pyrazolate‐Bridged Dinickel(II) and Dicopper(II) Complexes: An Example of Stereoelectronic Preference of Metal Ions and Stabilization of Mixed‐Valence Ni III Ni II Species

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New dimeric nickel(II) and copper(II) complexes [(L)Ni(μ-L′)]2[ClO4]21 and [(L)Cu(μ-L′)]2[ClO4]22, [2-[3-(2′-pyridyl)pyrazol-1-ylmethyl]pyridine (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 {Ni2(μ-L′)2}2+ core [Ni···Ni separation: 4.0765(10) A] in which each distorted octahedral NiII 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 CuII ions, each terminally coordinated by adopting only a bidentate coordination mode of L, in which the –CH2py arm remains uncoordinated. A similar bridging [Cu···Cu separation: 3.9382(24) A] as in 1·MeCN is observed. Thus stereoelectronic preferences of NiII and CuII 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···π interaction [pyridyl C–H (4-position) of pyridylpyrazole unit of L and π cloud of pyridine of bridging deprotonated L′ unit]. In 2·2MeCN, however, π···π 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 E1/2 = 1.36 V (ΔEp = 110 mV) and 1.79 V (ΔEp = 120 mV) vs. SCE, due to NiIIINiII/NiII2 and NiIII2/NiIIINiII redox processes, respectively. Constant potential electrolysis at 1.50 V vs. SCE leads to the generation of dark green mixed-valence NiIIINiII 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, Epc = –0.28 V and –0.50 V vs. SCE).(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2007)