Molybdenum Triamidoamine Systems. Reactions Involving Dihydrogen Relevant to Catalytic Reduction of Dinitrogen

Abstract

[HIPTN[subscript 3]N]Mo(N[subscript 2]) (MoN[subscript 2]) ([HIPTN[subscript 3]N][superscript 3−] = [(HIPTNCH2CH2)3N]3− where HIPT = 3,5-(2,4,6-i-Pr[subscript 3]C[subscript 6]H[subscript 2])[subscript 2]C[subscript 6]H[subscript 3]) reacts with dihydrogen slowly (days) at 22 °C to yield [HIPTN[subscript 3]N]MoH[subscript 2] (MoH[subscript 2]), a compound whose properties are most consistent with it being a dihydrogen complex of Mo(III). The intermediate in the slow reaction between MoN[subscript 2] and H[subscript 2] is proposed to be [HIPTN[subscript 3]N]Mo (Mo). In contrast, MoN[subscript 2], MoNH[subscript 3], and MoH[subscript 2] are interconverted rapidly in the presence of H[subscript 2], N[subscript 2], and NH[subscript 3], and MoH[subscript 2] is the lowest energy of the three Mo compounds. Catalytic runs with MoH[subscript 2] as a catalyst suggest that it is competent for reduction of N[subscript 2] with protons and electrons under standard conditions. [HIPTN[subscript 3]N]MoH[subscript 2] reacts rapidly with HD to yield a mixture of [HIPTN[subscript 3]N]MoH[subscript 2], [HIPTN[subscript 3]N]MoD[subscript 2], and [HIPTN[subscript 3]N]MoHD, and rapidly catalyzes H/D exchange between H[subscript 2] and D[subscript 2]. MoH[subscript 2] reacts readily with ethylene, PMe[subscript 3], and CO to yield monoadducts. Reduction of dinitrogen to ammonia in the presence of 32 equiv of added hydrogen (vs Mo) is not catalytic, consistent with dihydrogen being an inhibitor of dinitrogen reduction.