Activation of H–H, HO–H, C(sp ²)–H, C(sp ³)–H, and RO–H Bonds by Transition-Metal Frustrated Lewis Pairs Based on M/N (M = Rh, Ir) Couples

Abstract

Reaction of the dimers [(Cp*MCl) ₂(μ-Cl) ₂] (Cp* = η ⁵-C ₅Me ₅) with Ph ₂PCH ₂CH ₂NC(NH( p-Tolyl)) ₂ ( H ₂L ) in the presence of NaSbF ₆ affords the chlorido complexes [Cp*MCl(κ ² N, P- H ₂L )][SbF ₆] (M = Rh, 1; Ir, 2). Upon treatment with aqueous NaOH, solutions of 1 and 2 yield the corresponding complexes [Cp*M(κ ³ N, N′, P- HL)][SbF ₆] (M = Rh, 3; Ir, 4) in which the ligand HL presents a fac κ ³ N, N′, P coordination mode. Treatment of THF solutions of complexes 3 and 4 with hydrogen gas, at room temperature, results in the formation of the metal hydrido-complexes [Cp*MH(κ ² N, P- H ₂L )][SbF ₆] (M = Rh, 5; Ir, 6) in which the N( p-Tolyl) group has been protonated. Complexes 3 and 4 react with deuterated water in a reversible fashion resulting in the gradual deuteration of the Cp* group. Heating at 383 K THF/H ₂O solutions of the complexes 3 and 4 affords the orthometalated complexes [Cp*M(κ ³ C, N, P- H ₂L _-H )][SbF ₆] [M = Rh, 7; Ir, 8, H ₂L _-H = Ph ₂PCH ₂CH ₂NC(NH( p-Tolyl))(NH(4-C ₆H ₃Me))], respectively. At 333 K, complexes 3 and 4 react in THF with methanol, primary alcohols, or 2-propanol giving the metal-hydrido complexes 5 and 6, respectively. The reaction involves the acceptorless dehydrogenation of the alcohols at a relatively low temperature, without the assistance of an external base. The new complexes have been characterized by the usual analytical and spectroscopic methods including the X-ray diffraction determination of the crystal structures of complexes 1– 5, 7, and 8. Notably, the chlorido complexes 1 and 2 crystallize both as enantiopure conglomerates and as racemates. Reaction mechanisms are proposed based on stoichiometric reactions, nuclear magnetic resonance studies, and X-ray crystallography as well as density functional theory calculations.

Abstract

In solution, masked transition-metal frustrated Lewis pairs (TMFLPs) give rise to the corresponding TMFLP species which activate dihydrogen, water, and alcohols following FLP reaction pathways. When D ₂O or alcohols with deuterated OH groups were employed, H/D exchange at the Cp* ligand (involving C(sp ³)−H activation) was observed. C(sp ²)−H bond activation involving orthometalation of the p-Tolyl ring was also observed.