Chemistry of metal-bound anion radicals. A family of mono- and bis(azopyridine) chelates of bivalent ruthenium.

TLDR: Metal-mediated magnetic interaction makes the EPR-silent diradical 4 strongly antiferromagnetic with J = -299 cm-1 and the monoradicals 4+, 5, and 6 uniformly display a strong EPR signal near g = 2.00.

Abstract: The reaction of the dihydride [RuII(H)2(CO)(PPh3)3], 3, with excess azo-2,2‘-bipyridine (abp) in boiling dry benzene has afforded the diradical bischelate [RuII(abp•-)2(CO)(PPh3)], 4, and the hydridic monochelate monoradical [RuII(abp•-)(H)(CO)(PPh3)2], 5. A similar reaction between 3 and 2-(p-chlorophenylazo)pyridine (Clpap) did not yield a bischelate, but the hydridic monoradical [RuII(Clpap•-)(H)(CO)(PPh3)2], 6, has been isolated. Upon treatment of 4−6 with NH4PF6 in a wet dichloromethane−acetonitrile medium, the one-electron-oxidized salts 4+PF6-, 5+PF6-, and 6+PF6- are isolated, H+ being the oxidizing agent. The X-ray structures of 4+PF6-·CH2Cl2, 5+PF6-·H2O, and 6+PF6- have been determined. In the monoradical 4+ the azo N−N bond lengths in the two chelate rings are 1.284(6) and 1.336(6) A, showing that the radical electron is localized in the latter ring. The half-filled extended Huckel HOMO is indeed found to be so localized, and it has a large azo character. Complexes 4−6 display radical redox coup...