Metal vs ligand reduction in complexes of 1,3-dimethylalloxazine (DMA) with copper(I), ruthenium(II), and tungsten(VI). Crystal structures of (DMA)WO2Cl2 and (bis(1-methylimidazol-2-yl)ketone)WO2Cl2.

The complexes [(DMA)Cu(PPh3)2](BF4) (1) (DMA = 1,3-dimethylalloxazine), [(DMA)Ru(bpy)2](PF6)2 (2), and (DMA)WO2Cl2 (3) were obtained as O4-N5-chelated species, as evident from an X-ray crystal structure analysis for 3 and from spectroscopy (NMR, IR, and UV-vis spectroelectrochemistry) for 1 and 2. The tungsten(VI) center in 3 has its oxide ligands in a cis/equatorial position and the chloride ligands in a trans/axial position; it also exhibits a relatively short bond to O4 (2.232(3) A) and a very long bond to N5 (2.462(3) A). Comparison with the new structurally characterized compound (BIK)WO2Cl2 (4) (BIK = bis(1-methylimidazol-2-yl)ketone), which has W-N bonds of about 2.30 A, confirms the unusual length of the W-N bond in 3, probably caused by repulsion between one of the oxo ligands and the peri-hydrogen atom (H6) of DMA. One-electron reduction of the complexes occurs reversibly at room temperature in THF (1, 2) or at 198 K in CH2Cl2 (3). EPR spectroscopy reveals that this process is ligand-centered for 1 and 2 but metal-centered for 3. Density functional methods and ab initio methodology are used to illustrate the correspondence in spin distribution between the radical anion pi systems of alloxazine and isoalloxazine ("flavosemiquinone").