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

The reaction of the dihydride [Ru^II(H)₂(CO)(PPh₃)₃], 3, with excess azo-2,2‘-bipyridine (abp) in boiling dry benzene has afforded the diradical bischelate [Ru^II(abp^•-)₂(CO)(PPh₃)], 4, and the hydridic monochelate monoradical [Ru^II(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 [Ru^II(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) 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 4−6 display radical redox couples with E_1/2 in the range −0.5 to +0.10 V vs SCE. The E_1/2 values qualitatively correlate with corresponding ν_CO 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.