Molecular structure, characterization and magnetic properties of novel mixed-valence copper(I,II) α,β-unsaturated carboxylate complexes with triphenylphosphine and methanol ligands

Abstract

Two new mixed-valence tetranuclear copper(I,II) α,β-unsaturated carboxylate complexes Cu 2 ICu 2 IIA 6(PPh 3) 4(CH 3OH) 2 ( 1, A=CH 2CH–COO −; 2, A=CH 2C(CH 3)–COO −) have been synthesized by partial reduction of Cu 2A 4(H 2O) 2 by triphenylphosphine in methanol solution, and characterized by elemental analyses, IR, ESR, electronic reflectance spectroscopies as well as magnetic susceptibility measurements. In complex 2 two copper(II) atoms in a molecule are linked by four carboxylate groups forming a dimeric unit Cu 2 IIA 4, and each copper(II) atom coordinates with a copper(I) moiety through a μ 2-O,O′ carboxylate group. The electronic reflectance spectra in the solid state suggest a square pyramidal coordination environment around the copper(II) atoms. Bond valence calculations further confirm the presence of the mixed-valence system and the oxidation state of the copper atoms. Room temperature X-band ESR spectra of powdered samples and variable-temperature magnetic susceptibility studies indicate the presence of strong antiferromagnetic exchange interactions between two copper(II) atoms, with 2 J=−310 cm −1 for complex 1 and 2 J=−288 cm −1 for complex 2. The formation mechanism of the title complexes and the structural interconversion among related mixed-valence copper(I,II) carboxylate complexes are also discussed.