CuII2, CuII4 and CuII6 complexes with 3-(2-pyridyl)pyrazolate. Structure, magnetism and core interconversion

Abstract Reactions of stoichiometric amounts of L1(–) (HL1 = 3-(2-pyridyl)pyrazole) with [Cu(H2O)6](ClO4)2, with or without PhCO2–, in MeOH or N,N'-dimethylformamide (dmf), led to the isolation of three copper(II) complexes of varying nuclearity, [CuII2(L1)2(ClO4)2(MeOH)2] (1) , [CuII4(L1)4(O2CPh)2(MeOH)4](ClO4)2•2H2O (2) and [CuII6(L1)6(O2CPh)2(ClO4)2(dmf)4](ClO4)2•2dmf•2H2O (3) . Structural analysis reveals two centrosymmetric four-coordinate {CuII(L1)(ClO4)(MeOH)} units are dipyrazolate-bridged in 1, giving rise to a square-pyramidal (SP; τ = 0.13) coordination to the CuII ion. In 2, two centrosymmetric four-coordinate dipyrazolate-bridged {CuII2(μ-L1)2(MeOH)2}2+ units in two layers are held by two syn-anti PhCO2– bridges, giving rise to CuII centres with a distorted SP geometry (τ = 0.28) geometry. In 3, a three-coordinate dipyrazolate-bridged {CuII2(L1)2}2+ unit is held between two {CuII2(L1)2(ClO4)(dmf)2}}2+ units in two different layers by two syn-anti bridging PhCO2– ligands in a centrosymmetric manner, generating two SP (τ = 0.03 and 0.28) and a square-planar geometry. In 2, two successive layers are held by two and in 3 by one PhCO2– bridge(s). Systematic core-interconversion studies as a function of PhCO2–, solvent (MeOH, dmf) and concentration have been done and the observed results have been rationalized. Variable-temperature magnetic studies (1.9–300 K) for powdered samples of 1–3 revealed strong antiferromagnetic coupling between the two copper(II) centres in the {CuII2(μ-pyrazolato)2}2+ units. For 2 and 3, additional magnetic interactions through PhCO2– bridge(s) between two layers have also been realized. The magnitude of the coupling is discussed in terms of the structural parameters.