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

Abstract

AbstractNew dimeric nickel(II) and copper(II) complexes [(L)Ni(μ‐L′)]2[ClO4]2 1 and [(L)Cu(μ‐L′)]2[ClO4]2 2, [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) Å] 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) Å] 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)