Agostic interaction and intramolecular proton transfer from the protonation of dihydrogen ortho metalated ruthenium complexes

Protonation of the ortho-metalated ruthenium complexes RuH(H-2)(X)((PPr3)-Pr-i)(2) [X = 2-phenylpyridine (ph-py) (1), benzoquinoline (bq) (2)] and RuH(CO)(ph-py)((PPr3)-Pr-i)(2) (3) with [H(OEt2)(2)](+)[BAr'(4)](-) (BAr'(4) = [(3,5-(CF3)(2)C6H3)(4)B]) under H-2 atmosphere yields the corresponding cationic hydrido dihydrogen ruthenium complexes [RuH(H-2)(H-X)((PPr3)-Pr-i)(2)][BAr'(4)] [X = phenylpyridine (ph-py) (1-H); benzoquinoline (bq) (2-H)] and the carbonyl complex [RuH(CO)(H-phpy)((PPr3)-Pr-i)(2)][BAr'(4)] (3-H). The complexes accommodate an agostic C-H interaction characterized by NMR and in the case of 1-H by x-ray diffraction. Fluxional processes involve the hydride and dihydrogen ligands in 1-H and 2-H and the rotation of the phenyl ring displaying the agostic interaction in 1-H and 3-H. NMR studies (lineshape analysis of the temperature-dependent NMR spectra) and density functional theory calculations are used to understand these processes. Under vacuum, one equivalent of dihydrogen can be removed from 1-H and 2-H leading to the formation of the corresponding cationic ortho-metalated complexes [Ru(H-2)(THF)(X)((PPr3)-Pr-i)(2)](+) [X = ph-py (1-THF), bq (2-THF)]. The reaction is fully reversible. Density functional theory calculations and NMR data give information about the reversible mechanism of C-H activation in these ortho-metalated ruthenium complexes. Our study highlights the subtle interplay between key ligands such as hydrides, sigma-dihydrogen, and agostic bonds, in C-H activation processes.