High-frequency EPR and structural data as complementary information on stable radical complexes containing the semi-reduced azo function

Abstract

Dinuclear metal compounds [(L n M)(μ-BL − )(ML n )] k involving radical anions of bridging ligands BL such as 2,2′-azobispyridine or 3,6-bis(2-pyridyl)-1,2,4,5-tetrazine and coordinated inorganic or organometallic complex fragments ML n containing copper(I), rhenium(I), ruthenium(II), osmium(II), rhodium(III) or iridium(III) have been investigated in frozen solutions using high field EPR spectroscopy at W band (95 GHz) or higher frequencies. The generally small but still rather variable anisotropy of the g factor shows typical patterns reflecting the specific metal/ligand interactions, the competition between bridging and ancillary ligands for spin density, and the spin–orbit coupling constants of the metals. The EPR data are thus shown to specify the structural data as obtained from X-ray crystallography for a standard series of diphosphinocopper(I) complexes involving the radical ligands indicated. The (RNNR) − group can be placed next to the related superoxide (OO) − as a paramagnetic ligand with specific geometrical and electronic structure.