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Reduction of Metal Coordinated N 2 to NH 3 with H 2 by Heterolytic Hydrogen Cleavage induced by External Lewis Bases – a DFT Study
Author(s) -
Hölscher Markus,
Leitner Walter
Publication year - 2015
Publication title -
zeitschrift für anorganische und allgemeine chemie
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.354
H-Index - 66
eISSN - 1521-3749
pISSN - 0044-2313
DOI - 10.1002/zaac.201400337
Subject(s) - heterolysis , pincer movement , chemistry , hydride , lewis acids and bases , catalysis , ruthenium , molecule , frustrated lewis pair , catalytic cycle , steric effects , metal , cleavage (geology) , stereochemistry , pincer ligand , medicinal chemistry , crystallography , organic chemistry , materials science , fracture (geology) , composite material
The synthesis of NH 3 from N 2 and H 2 employing molecular ruthenium pincer complexes as catalysts was investigated by DFT calculations for an hitherto unusual type of outer sphere mechanism in N 2 chemistry. The mechanistic approach relies on the heterolytic cleavage of a H 2 molecule between the sterically demanding P( t Bu) 3 as a Lewis base and the dicationic ruthenium pincer complex [Ru(PNP)(N 2 )(CO) 2 ] 2+ [PNP = 2,6‐bis(diisopropylphosphanyl)lutidin] as a Lewis acid. Cleavage of the H 2 molecule results in a hydride, which is transferred onto the external N atom of the N 2 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 H 2 molecules results in the formation of two NH 3 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 N 2 should be extended to other complexes.