Spin distribution in copper(I) phosphine complexes of verdazyl radicals

Copper(I) forms mixed ligand coordination compounds with the stable, paramagnetic bipyridine analogue, 1,5-dimethyl-3-(2-pyridyl)-6-oxoverdazyl (pyvd) and a variety of monodentate and bidentate phosphine ligands. These compounds were characterized in solution by titration, UV-vis spectra, ESR spectra and electrospray mass spectrometry. Coordination of the phosphine gives a metal-ligand charge transfer transition in the UV-vis that is red shifted by more electron donating phosphines. ESR indicates that spin density on copper increases, both with strongly electron donating phosphines and with weakly basic phosphite ligands. This can be explained by the presence of two different orbital interactions: with more donating phosphines the interaction is predominantly through the filled copper(I) d-orbitals, but with weakly donating phosphite ligands the interaction is through an empty copper(I) p-orbital. The differing spin transfer mechanisms may have implications in the design of molecular magnetic systems.