Molybdenum nitrosyl complexes and their application in catalytic imine hydrogenation reactions

Abstract

The reaction between Mo(CO)(4)(NO)(ClAlCl(3))] and the sterically hindered diphosphanes (P boolean AND 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 boolean AND P)(CO)(2)(NO)Cl] (1a-1d), which were transformed into the corresponding hydrides Mo(P boolean AND 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(4)(F)] 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(4)(F)], was exemplarily tested in the hydrogenation of the imine PhCH= N(alpha-naphthyl). Replacement of the BAr(4)(F)](-) 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(Et2O) 2] B(C6F5) 4] exhibited the highest TOF value of 123 h(-1) in the reduction of PhCH= N(alpha-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 temperature and a H(2) pressure of 30 bar. A mechanism for the ionic hydrogenation with ``proton-before-hydride transfer'' is anticipated.