Reduction of Metal Coordinated N2 to NH3 with H2 by Heterolytic Hydrogen Cleavage induced by External Lewis Bases – a DFT Study

Abstract

AbstractThe synthesis of NH3 from N2 and H2 employing molecular ruthenium pincer complexes as catalysts was investigated by DFT calculations for an hitherto unusual type of outer sphere mechanism in N2 chemistry. The mechanistic approach relies on the heterolytic cleavage of a H2 molecule between the sterically demanding P(tBu)3 as a Lewis base and the dicationic ruthenium pincer complex [Ru(PNP)(N2)(CO)2]2+ [PNP = 2,6‐bis(diisopropylphosphanyl)lutidin] as a Lewis acid. Cleavage of the H2 molecule results in a hydride, which is transferred onto the external N atom of the N2 molecule coordinated at the metal complex, while the proton binds to the base. In a subsequent step the proton is also transferred to the HNN unit. Repetition of these two reaction steps with two more H2 molecules results in the formation of two NH3 molecules. Closed catalytic cycles could be calculated, which proves that this concept is applicable in principle. However, some activation barriers in the middle stages of the catalytic cycle are too high to envision practical realization with this particular acid‐base combination. Low lying intermediates, which occur in late stages of the catalytic cycle, are another challenge as they increase the overall activation barrier significantly. As a result it is suggested that this novel approach of hydrogen transfer onto N2 should be extended to other complexes.