Dinitrogen Activation by Fryzuks [NbACHTUNGTRENUNG(P2N2)] Complex and Comparison with the Laplaza–Cummins [Mo{N(R)Ar}3] and Schrock ACHTUNGTRENUNG[Mo(N3N)] Systems

摘要

The reaction profile of N2 with Fryzuks [NbACHTUNGTRENUNG(P2N2)] (P2N2= PhP(CH2SiMe2NSiMe2CH2)2PPh) complex is explored by density functional calculations on the model [NbACHTUNGTRENUNG(PH3)2- ACHTUNGTRENUNG(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 Laplaza- Cummins [Mo{N(R)Ar}3] and four-coordinate Schrock [MoACHTUNGTRENUNG(N3N)] (N3N= [(RNCH2CH2)3N]3) systems. When the model system is constrained to reflect the geometry of the P2N2 macrocycle, the NN bond cleavage step, via a N2-bridged dimer intermediate, is calculated to be endothermic by 345 kJmol1. In comparison, formation of the single-N-bridged species is calculated to be exothermic by 119 kJmol1, and consequently is the thermodynam- ACHTUNGTRENUNGicACHTUNGTRENUNGally 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 NN bond cleavage step. When the ligand constraints are relaxed, the overall reaction enthalpy increases by 240 kJmol1, but the N2 cleavage step remains endothermic by 35 kJmol1. Changing the phosphine ligands to amine donors has a dramatic effect, increasing the overall reaction exothermicity by 190 kJmol1 and that of the NN bond cleavage step by 85 kJmol1, making it a favourable process. Replacing NbII with MoIII has the opposite effect, resulting in a reduction in the overall reaction exothermicity by over 160 kJmol1. The reaction profile for the model [Nb- ACHTUNGTRENUNG(P2N2)] system is compared to those calculated for the model Laplaza and Cummins [Mo{N(R)Ar}3] and Schrock [MoACHTUNGTRENUNG(N3N)] systems. For both [Mo- ACHTUNGTRENUNG(N3N)] and [NbACHTUNGTRENUNG(P2N2)], the intermediate dimer is calculated to lie lower in energy than the products, although the final NN cleavage step is much less endothermic for [MoACHTUNGTRENUNG(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.