Unique behaviour of dinitrogen-bridged dimolybdenum complexes bearing pincer ligand towards catalytic formation of ammonia.

It is vital to design effective nitrogen fixation systems that operate under mild conditions, and to this end we recently reported an example of the catalytic formation of ammonia using a dinitrogen-bridged dimolybdenum complex bearing a pincer ligand, where up to twenty three equivalents of ammonia were produced based on the catalyst. Here we study the origin of the catalytic behaviour of the dinitrogen-bridged dimolybdenum complex bearing the pincer ligand with density functional theory calculations, based on stoichiometric and catalytic formation of ammonia from molecular dinitrogen under ambient conditions. Comparison of di- and mono-molybdenum systems shows that the dinitrogen-bridged dimolybdenum core structure plays a critical role in the protonation of the coordinated molecular dinitrogen in the catalytic cycle.

Molybdenum-catalyzed transformation of molecular dinitrogen into silylamine: experimental and DFT study on the remarkable role of ferrocenyldiphosphine ligands.

A molybdenum-dinitrogen complex bearing two ancillary ferrocenyldiphosphine ligands, trans-[Mo(N(2))(2)(depf)(2)] (depf = 1,1'-bis(diethylphosphino)ferrocene), catalyzes the conversion of molecular dinitrogen (N(2)) into silylamine (N(SiMe(3))(3)), which can be readily converted into NH(3) by acid treatment. The conversion has been achieved in the presence of Me(3)SiCl and Na at room temperature with a turnover number (TON) of 226 for the N(SiMe(3))(3) generation for 200 h. This TON is significantly improved relative to those ever reported by Hidai's group for mononuclear molybdenum complexes having monophosphine coligands [J. Am. Chem. Soc.1989, 111, 1939]. Density functional theory (DFT) calculations have been performed to figure out the mechanism of the catalytic N(2) conversion. On the basis of some pieces of experimental information, SiMe(3) radical is assumed to serve as an active species in the catalytic cycle. Calculated results also support that SiMe(3) radical is capable of working as an active species. The formation of five-coordinate intermediates, in which one of the N(2) ligands or one of the Mo-P bonds is dissociated, is essential in an early stage of the N(2) conversion. The SiMe(3) addition to a "hydrazido(2-)" intermediate having the NN(SiMe(3))(2) group will give a "hydrazido(1-)" intermediate having the (Me(3)Si)NN(SiMe(3))(2) group rather than a pair of a nitrido (≡N) intermediate and N(SiMe(3))(3). The N(SiMe(3))(3) generation would not occur at the Mo center but proceed after the (Me(3)Si)NN(SiMe(3))(2) group is released from the Mo center. The flexibility of the Mo-P bond between Mo and depf would play a vital role in the high catalysis of the Mo-Fe complex.