Dinitrogen Activation by Fryzuk's [Nb(P2N2] Complex and Comparison with the Laplaza-Cummins [Mo{N(R)Ar}3] and Schrock [Mo(N3N] Systems

The reaction profile of N2 with Fryzuk's [Nb(P2N 2)] (P2N2 = PhP(CH2SiMe 2NSiMe2CH2)2PPh) complex is explored by density functional calculations on the model [Nb(PH3) 2(NH2)2] system. The effects of ligand constraints, coordination number, metal and ligand donor atom on the reaction energetics are examined and compared to the analogous reactions of N2 with the three-coordinate LaplazaCummins [Mo{N(R)Ar}3] and four-coordinate Schrock [Mo(N3N)] (N3N = [(RNCH 2CH2)3N]3-) systems. When the model system is constrained to reflect the geometry of the P2N2 macrocycle, the N-N bond cleavage step, via a N2-bridged dimer intermediate, is calculated to be endothermic by 345 kJ mol-1. In comparison, formation of the single-N-bridged species is calculated to be exothermic by 119 kJ mol-1, and consequently is the thermodynamically favoured product, in agreement with experiment. The orientation of the amide and phosphine ligands has a significant effect on the overall reaction enthalpy and also the N-N bond cleavage step. When the ligand constraints are relaxed, the overall reaction enthalpy increases by 240 kJ mol-1, but the N2 cleavage step remains endothermic by 35 kJ mol-1. Changing the phosphine ligands to amine donors has a dramatic effect, increasing the overall reaction exothermicity by 190 kJ mol-1 and that of the N-N bond cleavage step by 85 kJ mol-1, making it a favourable process. Replacing NbIIwith MoII has the opposite effect, resulting in a reduction in the overall reaction exothermicity by over 16OkJ mol-1. The reaction profile for the model [Nb(P2N 2)] system is compared to those calculated for the model Laplaza and Cummins [Mo{N(R)Ar}3] and Schrock [Mo(N3N)] systems. For both [Mo(N3N)] and [Nb(P2N2)], the intermediate dimer is calculated to lie lower in energy than the products, although the final N-N cleavage step is much less endothermic for [Mo(N3N)], In contrast, every step of the reaction is favourable and the overall exothermicity is greatest for [Mo{N(R)Ar}3], and therefore this system is predicted to be most suitable for dinitrogen cleavage. © 2009 Wiley-VCH Verlag GmbH & Co. KGaA.