Bis(terdentate) Pyrazole/Pyridine Ligands: Synthesis, Crystal Structures and Magnetic Properties of Bridged Binuclear and Tetranuclear Copper(II) Complexes

A new binucleating bis(terdentate) ligand, 3,5‐[3‐bis(2‐pyridyl)pyrazole‐1‐ylmethyl]pyrazole (HL2), was synthesized. Reaction of the deprotonated ligand L2 with hydrated CuII salts gives (μ‐pyrazolato)(μ‐hydroxido)‐bridged binuclear and tetranuclear complexes [L2Cu2(μ‐OH)(ClO4)(MeCN)](ClO4) (2), [L2Cu2(dmf)2(μ3‐OH)]2(ClO4)4·4dmf (3·4dmf) and [L2L′Cu2](ClO4)2 [4; HL′ = 3‐(2‐pyridyl)pyrazole]. In these complexes, both μ‐OH and μ3‐OH bridges were observed. This contrasts the situation for a dicopper(II) complex of the related bis(terdentate) ligand 3,5‐bis[6(2,2′‐dipyridyl)]pyrazole (HL1), {L1Cu2(OMe)(MeOH)[κ1‐O‐(NO3)]}{[Cu2(NO3)2(μ‐OMe)2]}0.5·MeOH (1·MeOH), where the shorter and more rigid ligand side arms enforce a larger Cu···Cu separation and the formation of a MeO–HOMe moiety within the bimetallic pocket. Molecular structures of all complexes were elucidated by X‐ray crystallography. Variable‐temperature magnetic susceptibility measurements (295–2 K) for powdered samples of complexes 2–4 reveal strong antiferromagnetic coupling between two copper centres. The magnitude of the coupling is discussed in view of the structural features. During the preparation of complex 4, partial ligand hydrolysis was also observed, but this strongly depends on the reaction conditions.(© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2009)