Molybdenum Nitrosyl Complexes and Their Application in Catalytic Imine Hydrogenation Reactions

The reaction between [Mo(CO) 4 (NO)(ClAlCl 3 )] 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 LiBH 4 in Et 3 N at room temperature. The molybdenum–THF complex [Mo(dippp)(CO) 2 (NO)(THF)][BAr F 4 ] [ 3a ; Ar F = 3,5‐(CF 3 ) 2 C 6 H 3 ], obtained by the reaction of 2a with [H(Et 2 O) 2 ][BAr F 4 ], was exemplarily tested in the hydrogenation of the imine PhCH=N(α‐naphthyl). Replacement of the [BAr F 4 ] – counterion by the more stable [B(C 6 F 5 ) 4 ] – anion greatly increased the catalytic activity. The use of in situ mixtures of the hydrides 2a – 2d and [H(Et 2 O) 2 ][B(C 6 F 5 ) 4 ] improved the hydrogenation activity. The hydride 2b in combination with [H(Et 2 O) 2 ][B(C 6 F 5 ) 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 ‐ClC 6 H 4 CH=NPh, p ‐ClC 6 H 4 CH=N‐ p ‐C 6 H 4 Cl, PhCH=NCH(Ph) 2 and PhCH=NMes showed TOF values of 34, 74, 41, 18 and 84 h –1 at room temperatureand a H 2 pressure of 30 bar. A mechanism for the ionic hydrogenation with “proton‐before‐hydride transfer” is anticipated.