Can [M(H)2(H2)(PXP)] pincer complexes (M=Fe, Ru, Os; X=N, O, S) serve as catalyst lead structures for NH3 synthesis from N2 and H2?

The potential of pincer complexes [M(H)(2)(H(2))(PXP)] (M=Fe, Ru, Os; X=N, O, S) to coordinate, activate, and thus catalyze the reaction of N(2) with classical or nonclassical hydrogen centers present at the metal center, with the aim of forming NH(3) with H(2) as the only other reagent, was explored by means of DF (density functional) calculations. Screening of various complexes for their ability to perform initial hydrogen transfer to coordinated N(2) showed ruthenium pincer complexes to be more promising than the corresponding iron and osmium analogues. The ligand backbone influences the reaction dramatically: the presence of pyridine and thioether groups as backbones in the ligand result in inactive catalysts, whereas ether groups such as gamma-pyran and furan enable the reaction and result in unprecedented low activation barriers (23.7 and 22.1 kcal mol(-1), respectively), low enough to be interesting for practical application. Catalytic cycles were calculated for [Ru(H)(2)(H(2))(POP)] catalysts (POP=2,5-bis(dimethylphosphanylmethyl)furan and 2,6-bis(dimethylphosphanylmethyl)-gamma-pyran). The height of activation barriers for the furan system is somewhat more advantageous. Formation of inactive metal nitrides has not been observed. SCRF calculations were used to introduce solvent (toluene) effects. The Gibbs free energies of activation of the numerous single reaction steps do not change significantly when solvent is included. The reaction steps associated with the formation of the active catalyst from precursors [M(H)(2)(H(2))(PXP)] were also calculated. The otherwise inactive pyridine ligand system allows for the generation of the active catalyst species, whereas the ether ligand systems show activation barriers that could prohibit practical application. Consequently the generation of the active catalyst species needs to be addressed in further studies.