Axial versus equatorial coordination of thioether sulfur: Mixed ligand copper(II) complexes of 2-pyridyl-<i>N</i>-(2′-methylthiophenyl)-methyleneimine with bidentate diimine ligands

The synthesis, structure and spectral and redox properties of the copper(II) complexes [Cu(pmtpm)Cl₂] (<b>1</b>) and [Cu(pmtpm)₂](ClO₄)₂ (<b>6</b>), where <b>pmtpm</b> is the linear tridentate ligand 2-pyridyl-<i>N</i>-(2′-methylthiophenyl)methyleneimine containing a thioether and two pyridine donors, are described. Also, the mixed ligand complexes [Cu(pmtpm)(diimine)](ClO₄)₂ (<b>2–5</b>), where the diimine is 2,2′-bipyridine (<b>bpy</b>) (</b>2</b>), 1,10-phenanthroline (<b>phen</b>) (<b>3</b>), 2,9-dimethyl-1,10-phenanthroline (<b>2,9-dmp</b>) (<b>4</b>) or dipyrido-[3,2-d:2′,3′-<i>f</i>]-quinoxaline (<b>dpq</b>) (<b>5</b>), have been isolated and studied. The X-ray crystal structures of the complexes <b>1</b>, [Cu(pmtpm)(2,9-dmp)](ClO₄)₂ <b>4</b> and <b>6</b> have been successfully determined. The complex <b>1</b> possesses a trigonal bipyramidal distorted square based pyramidal (TBDSBP) coordination geometry in which three corners of the square plane are occupied by two nitrogens and thioether s of <b>pmtpm</b> ligand and the remaining equatorial and the axial positions by two chloride ions. The complex <b>4</b> contains a CuN₄S chromophore also with a TBDSBP coordination geometry in which two nitrogens and the thioether sulfur of <b>pmtpm</b> ligand occupy three corners of the square plane. One of the two nitrogens of 2,9-dmp ligand is equatorially coordinated and the other axially to copper. On the other hand, the complex <b>6</b> is found to possess a square based pyramidal distorted trigonal bipyramidal (SPDTBP) coordination geometry. The CuN₂S trigonal plane in it is comprised of the pyridine and imine nitrogens and the thioether sulfur of the <b>pmtpm</b> ligand. The pyridine nitrogens of the ligand occupy the axial positions and the second thioether sulfur remains uncoordinated. On long standing in acetonitrile solution the mixed ligand complexes <b>2</b> and <b>3</b> undergo ligand disproportionation to provide the corresponding bis-complexes of bpy and phen, respectively, and <b>6</b>. The electronic and EPR spectral parameters and the positive redox potential of complex <b>4</b> are consistent with the equatorial location of the thioether sulfur in the square-based coordination geometry around copper(II). On the other hand, the higher <i>g</i> and lower <i>A</i> values and lower <i>E</i>_1/2 values for the complexes <b>2</b>, <b>3</b> and <b>5</b> are consistent with the axial coordination of the thioether sulfur. Also, the Cu(II)/Cu(I) redox potentials increase with increase in number of aromatic rings of the diimine ligand. The steric and electronic effects of the bidentate diimine ligands in orienting the thioether coordination to axial or equatorial position are discussed.