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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
Author(s) -
Christian Gemma J.,
Terrett Richard N. L.,
Stranger Robert,
Cavigliasso Germán,
Yates Brian F.
Publication year - 2009
Publication title -
chemistry – a european journal
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.687
H-Index - 242
eISSN - 1521-3765
pISSN - 0947-6539
DOI - 10.1002/chem.200900539
Subject(s) - endothermic process , chemistry , phosphine , dimer , ligand (biochemistry) , enthalpy , exothermic reaction , crystallography , amide , bond cleavage , medicinal chemistry , catalysis , organic chemistry , thermodynamics , adsorption , biochemistry , physics , receptor
Abstract 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 NN 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 NN 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 NN 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 NN 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.