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

摘要

Two biphenyl-based N_2S_2 ligands, 2, 2'-bis-(4-methylimidazol-5-yl)methylsulfanylbiphenyl (N_2S_2-mim) and 2, 2'-bis(2-pyridylmethylsulfanyl)biphenyl (N_2S_2-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-pyridylmethylsufanyl)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 The N_4 ligand 2, 2'-bis-(2-pyridylmethylamino)biphenyl (N_4-mpy) in which the sulfurs have been replaced by nitrogens also was prepared.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 formula [Cu(N_4)-mpy]X_2 where X = ClO_4 or BF_4. 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][ClO_4]_2 or [Cu(NS-mpy)_2][BF_4]_2. In comparison to the results found for ligands N_4-mpy and NS-mpy, treatment of N_2S_2-mim or N_2S_2-mpy with [Cu(H_2O)_6][ClO_4]_2 or [Cu(H_2O)_6][BF_4]_2 in MeOH led to spontaneous reduction to form the copper (I) complexes [Cu(N_2S_2-mim)]ClO_4 [Cu(N_2S_2-mim)]BF_4, [Cu(N_2S_2-mpy)]ClO_4 and [Cu(N_2S_2-mpy)]BF_4. The formulation of these complexes as copper (I) species was confirmed by analytical methods, and for [Cu(N_2S_2-mpy)]ClO_4 centre dot 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(N_2S_2-mpy)]ClO_4, [Cu(N_4-mpy)][ClO_4]_2 and [Cu(NS-mpy)_2][ClO_4]_2 undergo quasi-reversible one-electron processes with E deg' is +0.77, +0.21 and +0.53 V vs. SCE.