Rhenium nitrosyl complexes for hydrogenations and hydrosilylations

The tris(acetonitrile)dibromonitrosylrhenium(I) compound (1a) was obtained by reduction of the paramagnetic [NMe4](2)[Re(NO)Br-5] salt with Zn in MeCN. Subsequent reaction of 1a with THF produced the THF derivative [Re(NO)(THF)(MeCN)(2)Br-2] (1b). Reaction of 1b with PiPr(3), Pcy(3), or P(p-tolyl)(3) yielded bis(acetonitrile)-cis-dibromo(nitrosyl)-trans-bis(phosphine)rfienium complexes (R = iPr 2a, cy 2b, p-tolyl 2c). Treatment of 2a,b with excess NaBH4 produced the known borohydride complexes [Re(H)(eta(2)-BH4)(NO)(PR3)(2)] (R = iPr 3a, cy 3b). Replacement of the BH3 moiety of 3a,b in THF by ethylene (I bar) produced the dihydride complexes [Re(H)(2)(eta(2)-C2H4)(NO)(PR3)(2)] (4) (R = iPr a, cy b). Protonation of 4a,b with HBF4 center dot OEt2 afforded H-2 and the monohydrido tetrafluoroborato species [Re(H)(NO)(PR3)(2)(eta(2)-C2H4)(BF4)] (R = iPr 5a, cy 5b). X-ray diffraction studies were carried out on 1a, 2b,c, and 5b. Complexes 4a,b are catalytically active in olefin, imine, and ketone hydrogenations and in olefin and ketone hydrosilylations, as well as in the scrambling of H-2/D-2 to give HD under mild conditions

Highly efficient large bite angle diphosphine substituted molybdenum catalyst for hydrosilylation

Treatment of the complex Mo(NO)Cl3(NCMe)2 with the large bite angle diphosphine, 2,2′-bis(diphenylphosphino)diphenylether (DPEphos) afforded the dinuclear species [Mo(NO)(P∩P)Cl2]2[μCl]2 (P∩P = DPEphos = (Ph2PC6H4)2O (1). 1 could be reduced in the presence of Zn and MeCN to the cationic complex [Mo(NO)(P∩P)(NCMe)3]+[Zn2Cl6]2–1/2 (2). In a metathetical reaction the [Zn2Cl6]2–1/2 counteranion was replaced with NaBArF4 (BArF4 = [B{3,5-(CF3)2C6H3}4]) to obtain the [BArF4]− salt [Mo(NO)(P∩P)(NCMe)3]+[BArF4]− (3). 3 was found to catalyze hydrosilylations of various para substituted benzaldehydes, cyclohexanecarboxaldehyde, 2-thiophenecarboxaldehyde, and 2-furfural at 120 °C. A screening of silanes revealed primary and secondary aromatic silanes to be most effective in the catalytic hydrosilylation with 3. Also ketones could be hydrosilylated at room temperature using 3 and PhMeSiH2. A maximum turnover frequency (TOF) of 3.2 × 104 h–1 at 120 °C and a TOF of 4400 h–1 was obtained at room temperature for the hydrosilylation of 4-methoxyacetophenone using PhMeSiH2 in the presence of 3. Kinetic studies revealed the reaction rate to be first order with respect to the catalyst and silane concentrations and zero order with respect to the substrate concentrations. A Hammett study for various para substituted acetophenones showed linear correlations with negative ρ values of −1.14 at 120 °C and −3.18 at room temperature