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

Abstract

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).