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 The N₄ ligand 2,2′-bis-(2-pyridylmethylamino)biphenyl (N₄-mpy) in which the sulfurs have been replaced by nitrogens also was prepared.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 E^o′ is +0.77, +0.21 and +0.53 V vs. SCE.