Dinuclear copper(II) complexes with the new [Cu 2(μ-OR)(μ-OAc) 2] + (R = alkyl) core: preparation and characterization of [Cu 2(OR)(OAC) 2(bpy) 2] + (R = Me, Et, Pr n) salts

Abstract

Procedures are described for the synthesis of complexes containing the cations [Cu 2(μ-OR)(μ-O 2CMe) 2(bpy) 2] + (R = Me, Et, Pr n). Treatment of Cu 2(O 2CMe) 4(H 2O) 2 with 2 equivs of bpy and 1 equiv. of NaOH in anhydrous EtOH, followed by addition of NBu 4 nPF 6 in MeCN gave highly crystalline blue [Cu 2(OEt)(O 2CMe) 2(bpy) 2]PF 6) ( 1a) in ≈70% yield. The corresponding reaction in dry MeOHMeCN led to green crystals of the methoxo analogue [Cu 2(OMe)(O 2CMe) 2(bpy) 2](PF 6) ( 2a), also in ≈70% yield. Layering of a methanolic solution of 1a with a mixture of Et 2O and hexanes led to clean, high-yield conversion to complex 2a. Complexes 1a, 2a and [Cu 2(OPr n)(O 2CMe) 2(bpy) 2] (PF 6) ( 3) were also prepared from the reactions of the compound [Cu 2(OH)(O 2CMe) 2(bpy) 2](PF 6)·3DMF with an excess of the appropriate alcohol. In order to study the influence of the counterion on the structure of the [Cu 2(OR)(O 2CMe) 2(bpy) 2] + ions, the blue-turquoise complex [Cu 2(OEt)(O 2CMe) 2(bpy) 2](ClO 4) ( 1b) and the corresponding green methoxo-bridged compund [Cu 2(OMe)(O 2CMe) 2(bpy) 2](ClO 4) ( 2b) were obtained in the same manner as complexes 1a and 2a, respectively. The structure of the cations of both complexes determined by X-ray diffraction consisted of triply-bridged pairs of copper(II) atoms. Two of the bridging ligands are syn, syn η 1: η 1: μ 2 acetate groups, the third being the OR − ion; a terminal bpy molecule completes five-coordination at each metal atom. The Cut3Cu separations are 3.093(1) and 3.230(1) Å for 2a and 1a, respectively. In 1a, the metal coordination geometry is best described as square pyramidal; the oxygen atoms [O(30) and O(32)] of one acetate occupy basal positions while the oxygens of the other acetate [O(34) and O(36)] occupy the apical positions of the two square pyramids. The cation of 2a consists of two distorted trigonal bipyramids sharing the methoxo oxygen as a common apex. The solid-state electronic and IR spectra of the complexes are discussed in terms of the nature of the bonding and structures of 1a and 2a. A cyclic voltammetry study of 1a and 2a in MeCN reveals a quasi-reversible reduction at ≈−0.75 V vs ferrocene, and an irreversible reduction at E p = −1.83 V for 1a and −1.63 V for 2a, yielding copper metal, which deposits on the electrode.