Enforcing geometrical constraints on metal complexes using biphenyl-based ligands: spontaneous reduction of copper(II) by sulfur-containing ligands

Abstract

Two biphenyl-based N2S2 ligands, 2,2′-bis-(4-methylimidazol-5-yl)methylsulfanyl)biphenyl (N2S2-mim) and 2,2′-bis(2-pyridylmethylsulfanyl)biphenyl (N2S2-mpy), have been synthesized and their complexation with copper(II) precursors studied. In order to assess whether the biphenyl ring is influencing the geometry around the copper atom, the NS ligand 1-methyl-4-(2-pyridylmethylsulfanyl)benzene (NS-mpy) and its copper complexes were prepared. The N4 ligand 2,2′-bis-(2-pyridylmethylamino)biphenyl (N4-mpy) in which the sulfurs have been replaced by nitrogens also was prepared. Treatment of N4-mpy with copper(II) salts led to isolation of copper(II) complexes with the formula [Cu(N4-mpy)]X2 where X = ClO4 or BF4. These complexes were analysed by a combination of elemental analysis, IR spectroscopy, FAB MS and electrochemistry. Likewise, treatment of NS-mpy with copper(II) salts led to isolation of the copper(II) complexes [Cu(NS-mpy)2][ClO4]2 or [Cu(NS-mpy)2][BF4]2. In comparison to the results found for ligands N4-mpy and NS-mpy, treatment of N2S2-mim or N2S2-mpy with [Cu(H2O)6][ClO4]2 or [Cu(H2O)6][BF4]2 in MeOH led to spontaneous reduction to form the copper(I) complexes [Cu(N2S2-mim)]ClO4 [Cu(N2S2-mim)]BF4, [Cu(N2S2-mpy)]ClO4 and [Cu(N2S2-mpy)]BF4. The formulation of these complexes as copper(I) species was confirmed by analytical methods, and for [Cu(N2S2-mpy)]ClO4·MeCN by X-ray crystallography. The copper(I) ion is in a distorted tetrahedral environment ligated by two nitrogens and two thioethers from the ligand. Cyclic voltammetry shows [Cu(N2S2-mpy)]ClO4, [Cu(N4-mpy)][ClO4]2 and [Cu(NS-mpy)2][ClO4]2 undergo quasi-reversible one-electron processes with Eo′ is +0.77, +0.21 and +0.53 V vs. SCE.