Self-Aggregation Tendency of All Species Involved in the Catalytic Cycle of Bifunctional Transfer Hydrogenation

The self-aggregation tendency of [RuX(N,N)(eta(6)-p-cymene)] [N,N = amino amidate, X = Cl (1) and H (2)] and [Ru(N,N)(eta(6)-p-cymene)] [N,N = amido amidate, 3] in various solvents was investigated by diffusion NMR spectroscopy. The proper evaluation of the molecular hydrodynamic volume of the 1-3 monomeric species allowed understanding that 1 and 2 are mainly present as monomers in isopropanol-d(8) at concentrations below the millimolar level. Dimers start to become relevant at concentrations over ca. 10 mM [Delta G(0)(aggregation) = -2.2 kcal mol(-1)]. The self-aggregation tendency of I and 2 in CDCl3 is marked [Delta G(0)(aggregation) = -3.4 kcal mol(-1)] and much higher than that of 3. In toluene-d(8), 3 and I readily form dimers and higher aggregates, respectively, even at millimolar concentrations. The structures and energetics of 1-1, 2-2, and 3-3 dimers were investigated by ONIOM(B3PW91/HF) calculations. It was found that the main interaction at the origin of the dimerization process is the establishment of an intermolecular H-bond between one N-H on one monomer and the oxygen of the SO2 moiety of the other. In 3, the amido group is less acidic and less spatially available for H-bonding than in 1 and 2, which explains the reduced tendency to form dimers.