Low-Valent Ruthenium Complexes of the Non-innocent 2,6-Bis(imino)pyridine Ligand

A series of low-valent ruthenium complexes bearing 2,6-bis(imino)pyridyl (“[N3]”) ligands has been synthesized and characterized. Reduction of [N3]RuCl2(C2H4) ([N3xyl] = 2,6-(XylNCMe)2C5H3N, 1a; [N3mes] = 2,6-(MesNCMe)2C5H2N, 1b; [tBu-N3mes] = 2,6-(MesNCMe)2p-tBuC5H2N, 1c) with hydridosilanes in an arene solvent such as toluene yields new 18e− η6-arene complexes [κ2-N3]Ru(η6-MeC6H5), 2a,b,c, in which the [N3] ligand is bidentate and only one imine group is coordinated to the metal. The arene ligand can be displaced with dinitrogen in non-arene solvents to yield the binuclear, four-coordinate, formally Ru(0) complexes {[N3]Ru}2(μ-N2), 3a,b,c. Pyrophoric complex 3c is a rare example of a structurally characterized Ru(0) dinitrogen complex. Treatment of low-valent complexes 2 or 3 with donor ligands generates five-coordinate complexes [N3xyl]RuL1,2 (L1,2 = C2H4, 4a; L1,2 = PMe3, 5a; L1,2 = CO, 6a; L1 = PMe3, L2 = CO, 7a). Complexes 2a, 3c, 5a, 6a, and 7a are diamagnetic and have been structurally characterized by single-crystal X-ray diffraction methods. New six-coordinate Ru(II) complexes [N3xyl]RuCl2(L) (L = PMe3, CO) were also isolated and structurally characterized. The infrared data, observed geometrical parameters, and reactivity patterns of the formally Ru(0) centers suggest varying degrees of electron delocalization to the “non-innocent” bis(imino)pyridyl, but probably not to the extent implied by the valence tautomeric [N3]2−/Ru(II) canonical form. Although the [N3]−/Ru(I) representation may portray the electron distribution more accurately than “Ru(0)”, the inherent odd electron counts on both ligand and metaland requisite antiferromagnetic couplingprovides little in the way of “useful” distinctions or predictive value for the low-valent [N3]Ru(L)2 complexes with strong-field co-ligands such as CO and PMe3. These five-coordinate adducts seem to be adequately described as Ru(0) complexes of the neutral [N3] ligand. However, “non-innocent” valence tautomeric canonical forms such as [N3]−/Ru+ may be more applicable to the four-coordinate dinitrogen complexes {[N3]Ru}2(μ-N2).