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

Two biphenyl-based N2S2 ligands, 2,2'-bis-(4-methylimidazol-5-yl)methylsulf anyl)biphenyl (N2S2-mim) and 2,2'-bis(2-pyridylmethylsulfanyl)biphenyl (N2S 2-mpy), have been synthesized and their complexation with copper(II) precur sors studied. In order to assess whether the biphenyl ring is influencing t he geometry around the copper atom, the NS ligand 1-methyl-4-(2-pyridylmeth ylsulfanyl)benzene (NS-mpy) and its copper complexes were prepared. The N-4 ligand 2,2'-bis-(2-pyridylmethylamino)biphenyl (N-4-mpy) in which the sulf urs have been replaced by nitrogens also was prepared. Treatment of N-4-mpy with copper(II) salts led to isolation of copper(II) complexes with the fo rmula [Cu(N-4-mpy)]X-2 where X=ClO4 or BF4. These complexes were analysed b y a combination of elemental analysis, IR spectroscopy, FAB MS and electroc hemistry. Likewise, treatment of NS-mpy with copper(II) salts led to isolat ion 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 N-4-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) comp lexes [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 conf irmed by analytical methods, and for [Cu(N2S2-mpy)]ClO4. MeCN by X-ray crys tallography. The copper(I) ion is in a distorted tetrahedral environment li gated by two nitrogens and two thioethers from the ligand. Cyclic voltammet ry shows [Cu(N2S2-mpy)]ClO4, [Cu(N-4-mpy)][ClO4](2) and [Cu(NS-mpy)(2)][ClO 4](2) undergo quasi-reversible one-electron processes with E-o' is +0.77, 0.21 and +0.53 V vs. SCE.