Molybdenum Nitrosyl Complexes and Their Application in Catalytic Imine Hydrogenation Reactions

Abstract

AbstractThe reaction between [Mo(CO)4(NO)(ClAlCl3)] and the sterically hindered diphosphanes (P∩P) 1,3‐bis(diisopropylphosphanyl)propane (dippp, a), 1,2‐bis(diisopropylphosphanyl)ethane (dippe, b), 1,1′‐bis(diisopropylphosphanyl)ferrocene (dippf, c) and 1,2‐bis(dicyclohexylphosphanyl)ethane (dcype, d) produced the chlorides [Mo(P∩P)(CO)2(NO)Cl] (1a–1d), which were transformed into the corresponding hydrides [Mo(P∩P)(CO)2(NO)H] (2a–2d) by reaction with LiBH4 in Et3N at room temperature. The molybdenum–THF complex [Mo(dippp)(CO)2(NO)(THF)][BArF4] [3a; ArF = 3,5‐(CF3)2C6H3], obtained by the reaction of 2a with [H(Et2O)2][BArF4], was exemplarily tested in the hydrogenation of the imine PhCH=N(α‐naphthyl). Replacement of the [BArF4]– counterion by the more stable [B(C6F5)4]– anion greatly increased the catalytic activity. The use of in situ mixtures of the hydrides 2a–2d and [H(Et2O)2][B(C6F5)4] improved the hydrogenation activity. The hydride 2b in combination with [H(Et2O)2][B(C6F5)4] exhibited the highest TOF value of 123 h–1 in the reduction of PhCH=N(α‐naphthyl). The hydrogenation of the imines PhCH=NPh, p‐ClC6H4CH=NPh, p‐ClC6H4CH=N‐p‐C6H4Cl, PhCH=NCH(Ph)2 and PhCH=NMes showed TOF values of 34, 74, 41, 18 and 84 h–1 at room temperatureand a H2 pressure of 30 bar. A mechanism for the ionic hydrogenation with “proton‐before‐hydride transfer” is anticipated.