Rhodium Complexes with a Pyridine-2-yloxy-silyl-Based N,Si-Ligand: Bonding Situation and Activity as Alkene Hydrogenation Catalysts

Rh(III) complexes
[Rh(H)(X)(κ2-NSitBu2OPy)(L)] (X = Cl, L = PCy3, 2a; PHtBu2, 2b;
X = OTf, L = PCy3, 3a; PHtBu2, 3b) (NSitBu2OPy = 4-methylpyridin-2-yloxy-ditertbutylsilyl) have been
prepared and characterized by means of elemental analysis and nuclear
magnetic resonance (NMR) spectroscopy. The solid-state structures
of complexes 2a, 2b, and 3a have been determined by X-ray diffraction studies. Computational
analyses of the bonding situation of these species evidence the electron-sharing
nature of the Rh–Si bond and the significant role of the electrostatic
component in the interaction between the transition metal fragment
[Rh(H)(PR3)(X)]• and the [NSitBu2OPy]• ligand. In addition, a
comparative study of the activity of 2a, 2b, 3a, 3b, and related iridium species as
catalysts for the hydrogenation of olefins has been performed. The
best catalytic results have been obtained when using the Rh(III) species 3a, with triflate and PCy3 ligands, as catalyst.
Computational density functional theory studies show that the formation
of the alkane is thermodynamically favored and that the rate-limiting
step corresponds to the hydrogen activation, which takes place via
a σ-complex-assisted metathesis mechanism.