Synthesis of Triruthenium Complexes Containing a Triply Bridging Pyridyl Ligand and Its Transformations to Face-Capping Pyridine and Perpendicularly Coordinated Pyridyl Ligands

Abstract

Unlike the reactions of carbonyl clusters with pyridine leading to the formation of μ-pyridyl complexes, the reaction of the triruthenium pentahydrido complex {Cp*Ru(μ-H)} 3 (μ 3 -H) 2 (Cp* = η 5 -C 5 Me 5 ) ( 1 ) with pyridines provided μ 3 -η 2 (//)-pyridyl complexes, ( Cp*Ru) 3 (μ-H) 4 (μ 3 -η 2 (//)-RC 5 H 3 N) ( 2a, R = H; 2b, R = 4-COOMe; 2c, R = 4-COOEt; 2d, R = 4-Me; 2e, R = 5-Me), in which the molecular plane of the pyridyl group was tilted with respect to the Ru 3 plane. Electron-rich metal centers of the trimetallic core enabled back-donation to the pyridyl group, which caused the additional π-coordination of the C═N bond. The electron-rich metal centers of 2a – 2c also promoted further transformation into face-capping pyridine complexes {Cp*Ru(μ-H)} 3 (μ 3 -η 2:η 2:η 2 -RC 5 H 4 N) ( 3a, R = H; 3b, R = 4-COOMe; 3c, R = 4-COOEt) upon heating. In contrast, the thermolysis of 2d did not afford a face-capping picoline complex because of the poor electron-accepting ability of the picolyl moiety. Instead, the coordinatively unsaturated μ 3 -picolyl complex (Cp*Ru) 3 (μ-H) 2 (μ 3 -η 2 -4-Me-C 5 H 3 N) ( 4d ) was obtained. Owing to its unsaturated nature, 4d can react with γ-picoline to yield 4,4′-dimethyl-2,2′-bipyridine. Although the reaction rate was slow, complex 1 catalyzed the dehydrogenative coupling of 4-substituted pyridines containing an electron-donating group. The protonation of 2a also afforded the coordinatively unsaturated pyridyl complex [(Cp*Ru) 3 (μ-H) 2 (μ 3 -H)(μ 3 -η 2:η 2 (⊥)-C 5 H 4 N)] + ( 5a ), but the coordination mode of the pyridyl group in 5a was completely different from that in 4d . The pyridyl moiety in 5a was coordinated on one of the Ru–Ru bonds in a perpendicular fashion. The methylation of the face-capping pyridine complex 3a, which led to the formation of the N- methyl pyridinium complex [(Cp*Ru) 3 (μ-H) 3 (μ 3 -η 2:η 2:η 2 -C 5 H 5 NMe)] + ( 7b ) was also examined. NMR studies on 7b as well as X-ray diffraction studies suggested enhanced back-donation to the pyridinium moiety because of the localized cationic charge on the nitrogen atom.