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

Two new mixed-valence tetranuclear copper(I,II) ol,a-unsaturated carboxylate complexes Cu(2)(I)Cu(2)(II)A(6)(PPh3)(4)( CH3OH)(2) (1, A = CH2=CH-COO-; 2, A = CH2=C(CH3)-COO- ) have been synthesized by partial reduction of Cu(2)A(4)(H2O)(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)(II)A(4), and each copper(II) atom coordinates with a copper(I) moiety through a mu(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 2J= - 310 cm(-1) for complex 1 and 2J= - 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. (C) 2000 Elsevier Science Ltd All rights reserved.