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 N-2 with Fryzuk's [Nb(P2N2)] (P2N2 = PhP(CH2SiMe2NSiMe2CH2)(2)PPh) 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 N-2 with the three-coordinate Laplaza-Cummins [Mo{N(R)Ar}(3)] and four-coordinate Schrock [Mo(N3N)] (N3N = [(RNCH2CH2)(3)N](3-)) systems. When the model system is constrained to reflect the geometry of the P2N2 macrocycle, the N-N bond cleavage step, via a N-2-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 N-2 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 Nb-II with Mo-III has the opposite effect, resulting in a reduction in the overall reaction exothermicity by over 160 kJ mol(-1). The reaction profile for the model [Nb-(P2N2)] 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.