Highly active, low-valence molybdenum- and tungsten-amide catalysts for bifunctional imine-hydrogenation reactions

Abstract

The reactions of [M(NO)(CO)4(ClAlCl3)] (M=Mo, W) with (iPr2PCH2CH2)2NH, (PNHP) at 90 °C afforded [M(NO)(CO)(PNHP)Cl] complexes (M=Mo, 1 a; W, 1 b). The treatment of compound 1 a with KOtBu as a base at room temperature yielded the alkoxide complex [Mo(NO)(CO)(PNHP)(OtBu)] (2 a). In contrast, with the amide base Na[N(SiMe3)2], the PNHP ligand moieties in compounds 1 a and 1 b could be deprotonated at room temperature, thereby inducing dehydrochlorination into amido complexes [M(NO)(CO)(PNP)] (M=Mo, 3 a; W, 3 b; PNP=(iPr2PCH2CH2)2N)). Compounds 3 a and 3 b have pseudo-trigonal-bipyramidal geometries, in which the amido nitrogen atom is in the equatorial plane. At room temperature, compounds 3 a and 3 b were capable of adding dihydrogen, with heterolytic splitting, thereby forming pairs of isomeric amine-hydride complexes [Mo(NO)(CO)H(PNHP)] (4 a(cis) and 4 a(trans)) and [W(NO)(CO)H(PNHP)] (4 b(cis) and 4 b(trans); cis and trans correspond to the position of the H and NO groups). H2 approaches the Mo/W[DOUBLE BOND]N bond in compounds 3 a,b from either the CO-ligand side or from the NO-ligand side. Compounds 4 a(cis) and 4 a(trans) were only found to be stable under a H2 atmosphere and could not be isolated. At 140 °C and 60 bar H2, compounds 3 a and 3 b catalyzed the hydrogenation of imines, thereby showing maximum turnover frequencies (TOFs) of 2912 and 1120 h−1, respectively, for the hydrogenation of N-(4-methoxybenzylidene)aniline. A Hammett plot for various para-substituted imines revealed linear correlations with a negative slope of −3.69 for para substitution on the benzylidene side and a positive slope of 0.68 for para substitution on the aniline side. Kinetics analysis revealed the initial rate of the hydrogenation reactions to be first order in c(cat.) and zeroth order in c(imine). Deuterium kinetic isotope effect (DKIE) experiments furnished a low kH/kD value (1.28), which supported a Noyori-type metal–ligand bifunctional mechanism with H2 addition as the rate-limiting step.