DFT investigation of the potential of [H-M{(NHCH2CH2)3X}] catalysts (M = Mo, Ru, Os; X = N, P) for the reduction of N2 to NH3 by H2

DFT calculations of model complexes [H-M{(NHCH2CH2)(3)-X}] (M = Mo, Ru, Os; X = N, P) were carried out to investigate the catalytic potential of these complexes towards the reduction of N-2 to NH3 using only H-2 as the reducing agent. Closed catalytic cycles were calculated for all three metal hydrides. The calculations showed that [H-Mo{(NHCH2CH2)(3)-N)], 1-Mo, is not an appropriate catalyst due to very high activation barriers for several steps of the reaction (Delta H-max(double dagger)=69.1 kcal/mol). Much lower activation barriers were found for the Ru and Os catalysts [H-Ru{(NHCH2CH2)(3)N)}, 1-Ru, and [H-Os{(NHCH2CH2)(3)P}], 1-Os(P). With Delta(double dagger)(max)/Delta G double dagger(max)=29.3/35.7 kcal/mol, 1-Os(P) shows potential for future theoretical work. QM/MM calculations were used to investigate some properties of the Ru system empolying the hexaisopropylterphenyl ligand (HIPT). The results indicate that an HIPT substituent at the ligand core has a benefical influence on some reaction steps by lowering the activation barriers, while for other steps the activation energies increase. ((c) Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2006)