| 19746461 |
Christian GJ, Terrett RN, Stranger R, Cavigliasso G, Yates BF: Dinitrogen activation by Fryzuk's [Nb (P (2) N (2))] complex and comparison with the Laplaza-Cummins [Mo{N (R) Ar}(3)] and Schrock [Mo (N (3) N)] systems. Chemistry. 2009 Oct 26;15(42):11373-83.
The reaction profile of N (2) with Fryzuk's [Nb (P (2) N (2))] (P (2) N (2)=PhP (CH (2) SiMe (2) NSiMe (2) CH (2))(2) PPh) complex is explored by density functional calculations on the model [Nb (PH (3))(2)(NH (2))(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 (N (3) N)] (N (3) N=[(RNCH (2) CH (2))(3) N](3-)) systems. When the model system is constrained to reflect the geometry of the P (2) N (2) 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 (P (2) N (2))] system is compared to those calculated for the model Laplaza and Cummins [Mo{N (R) Ar}(3)] and Schrock [Mo (N (3) N)] systems. For both [Mo (N (3) N)] and [Nb (P (2) N (2))], 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 (N (3) N)]. 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. |
1(0,0,0,1) |