American Chemical Society
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Chemistry of Metal-Bound Anion Radicals. A Family of Mono- and Bis(azopyridine) Chelates of Bivalent Ruthenium

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posted on 2000-08-25, 00:00 authored by Maya Shivakumar, Kausikisankar Pramanik, Indranil Bhattacharyya, Animesh Chakravorty
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 46 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) Å, showing that the radical electron is localized in the latter ring. The half-filled extended Hückel HOMO is indeed found to be so localized, and it has a large azo character. Complexes 46 display radical redox couples with E1/2 in the range −0.5 to +0.10 V vs SCE. The E1/2 values qualitatively correlate with corresponding νCO values (1900−2000 cm-1). The monoradicals (S = 1/2) 4+, 5, and 6 uniformly display a strong EPR signal near g = 2.00. Metal-mediated magnetic interaction makes the EPR-silent diradical 4 strongly antiferromagnetic with J = −299 cm-1. Crystal data are as follows:  (4+PF6-·CH2Cl2, C40H33Cl2F6N8OP2Ru) monoclinic, space group P21/c (no. 14), a = 14.174(6) Å, b = 16.451(4) Å, c = 18.381(4) Å, β = 98.00(3)°, Z = 4; (5+PF6-·H2O, C47H41F6N4O2P3Ru) monoclinic, space group P21/n (no. 14), a = 9.433(2) Å, b = 38.914(17) Å, c = 13.084(3) Å, β = 103.47(2)°, Z = 4; (6+PF6- , C48H39ClF6N3OP3Ru) monoclinic, space group P21/n (no. 14), a = 10.496(5) Å, b = 22.389(8) Å, c = 19.720(6) Å, β = 90.53(3)°, Z = 4.