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

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(μ3-H)2 (Cp* = η5-C5Me5) (1) with pyridines provided μ3-η2(//)-pyridyl complexes, (Cp*Ru)3(μ-H)4(μ3-η2(//)-RC5H3N) (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 Ru3 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(μ3-η2:η2:η2-RC5H4N) (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 μ3-picolyl complex (Cp*Ru)3(μ-H)2(μ3-η2-4-Me-C5H3N) (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(μ3-H)­(μ3-η2:η2(⊥)-C5H4N)]+ (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 (μ3-η2:η2:η2-C5H5NMe)]+ (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.