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

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 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.

Fine-Tuning the Energy Barrier for Metal-Mediated Dinitrogen N≡N Bond Cleavage

“…Upon sodium metal reduction in dimethoxyethane (DME) solution, rearrangement of a diniobium end-on-bridged μ-η 1 :η 1 -N 2 moiety to a diniobium side-on-bridged μ-η 2 :η 2 -N 2 group that is stabilized by secondary sodium–nitrogen bonding interactions within a structurally characterized reduced product has been reported by Floriani et al Fryzuk et al have also previously proposed that thermal conversion of a diniobium μ-η 1 :η 1 -N 2 complex to a product arising from NN bond cleavage and formal intramolecular insertion of a metal nitrido group into a niobium–phosphorus bond of the supporting ligand framework proceeds via a diniobium side-on-bridged μ-η 2 :η 2 -N 2 intermediate. Although additional experimental support for such an intermediate or the proposed bimetallic μ-η 1 :η 1 -N 2 → μ-η 2 :η 2 -N 2 thermal rearrangement step was not provided, a computational investigation of the relative thermodynamic stabilities of simplified structural models for μ-η 1 :η 1 -N 2 vs μ-η 2 :η 2 -N 2 coordination appeared to put this hypothesis on firmer theoretical ground . In our own studies, we have also previously stated that the most likely mechanism for thermal conversion of 1a to 3a involves an intramolecular μ-η 1 :η 1 -N 2 to μ-η 2 :η 2 -N 2 structural isomerization that occurs prior to NN bond cleavage according to the upper pathway of Scheme . , The basis for this conjecture rested on structural data obtained for the set of related group 4 bimetallic dinitrogen derivatives, {(η 5 -C 5 Me 4 R′)M[N(R′′)C(R)N(R′′)]} 2 (μ-η 2 :η 2 -N 2 ) (M = Zr and Hf) ( I ), which display exceedingly large d (NN) values that increase as the magnitude of nonbonded steric interactions within the supporting ligand environment decrease as shown in Chart .…”

NO2bond cleavage by MoL3complexes

“…9 The reaction with nitrogen dioxide was shown to be especially facile, with complete deoxygenation occurring rapidly under mild conditions. 9 The structure and electronic state of the active complex and the mechanism of bond cleavage for several of these known observed reactions has been investigated by theoretical studies, [10][11][12][13][14][15][16][17][18][19][20][21][22][23][24][25][26][27] however these do not provide a detailed understanding of the reaction between MoL 3 and NO 2 . We therefore aimed to further examine and rationalize this reaction, which results in the molybdenum oxide and nitrosyl complexes shown in Fig.…”

Dinitrogen metal complexes with a strongly activated N–N bond: a computational investigation of [(Cy2N)3Nb-(μ-NN)-Nb(NCy2)3] and related [Nb-(μ-NN)-Nb] systems

“…Since the discovery of [(NH 3 ) 5 Ru(NN)] 2+ , 1 the coordination chemistry of molecular nitrogen has been the subject of extensive experimental [2][3][4][5][6] and computational research, [7][8][9][10][11][12][13][14][15][16][17][18][19][20] most notably due to the fact that the synthesis and characterization of metal-dinitrogen complexes are of relevance to both the biological and industrial processes involved in nitrogen fixation, [2][3][4] as a consequence of the presence of transition metal catalytic sites in the nitrogenase enzymes 21 and the Haber-Bosch method, 22 respectively.…”