Synthesis, magnetism, ¹H NMR and redox activity of dicopper(II) complexes having a discrete {Cu₂(μ-phenoxide)₂}²⁺ unit supported by a non-macrocyclic ligand environment. Crystal structure of [Cu₂(L)₂(OClO₃)₂] [HL = 4-methyl-2,6-bis(pyrazol-1-ylmethyl)phenol]

Abstract

Reaction between 4-methyl-2,6-bis(pyrazol-1-ylmethyl)phenol (HL) or its 3,5-dimethylpyrazole derivative (HL') and Cu(ClO₄)₂·6H₂O afforded [Cu^II₂(L/L')₂(OClO₃)₂] 1 and 2. Complex 1 has been structurally characterized showing that each copper(II) centre is square pyramidal with two bridging phenoxide oxygens and two terminal pyrazole nitrogens in the equatorial plane and a perchlorate oxygen atom axially co-ordinated. Variable-temperature magnetic susceptibility measurements revealed that the dicopper(II) centres are strongly antiferromagnetically coupled [singlet-triplet energy separation, 2J (in cm^-1): -1204 for 1 and -798 for 2]. The complexes exhibit ¹H NMR spectra within δ 0-10 due to their S = 0 ground state. In MeCN solution they exhibit ligand field transitions in the range 14 300-16 600 cm^-1 and phenolate-to-copper(II) charge-transfer transition at ≈22 700 cm^-1. In MeCN solution each complex displays three consecutive irreversible responses (scan rate of 50 mV s^-1) with E_pc values (V vs. SCE) at -0.02, -0.54 and -0.86 (1) and 0.00, -0.42 and -0.80 (2). The first two responses are due to Cu^II-Cu^I and the most cathodic response to Cu^I-Cu⁰ redox processes, respectively.