NO2 bond cleavage by MoL3 complexes

Abstract

The cleavage of one N–O bond in NO₂ by two equivalents of Mo(NRAr)₃ has been shown to occur to form molybdenum oxide and nitrosyl complexes. The mechanism and electronic rearrangement of this reaction was investigated using density functional theory, using both a model Mo(NH₂)₃ system and the full [N(^tBu)(3,5-dimethylphenyl)] experimental ligand. For the model ligand, several possible modes of coordination for the resulting complex were observed, along with isomerisation and bond breaking pathways. The lowest barrier for direct bond cleavage was found to be via the singlet η²-N,O complex (7 kJ mol⁻¹). Formation of a bimetallic species was also possible, giving an overall decrease in energy and a lower barrier for reaction (3 kJ mol⁻¹). Results for the full ligand showed similar trends in energies for both isomerisation between the different isomers, and for the mononuclear bond cleavage. The lowest calculated barrier for cleavage was only 21 kJ mol⁻¹via the triplet η¹-O isomer, with a strong thermodynamic driving force to the final products of the doublet metal oxide and a molecule of NO. Formation of the full ligand dinuclear complex was not accompanied by an equivalent decrease in energy seen with the model ligand. Direct bond cleavage via an η¹-O complex is thus the likely mechanism for the experimental reaction that occurs at ambient temperature and pressure. Unlike the other known reactions between MoL₃ complexes and small molecules, the second equivalent of the metal does not appear to be necessary, but instead irreversibly binds to the released nitric oxide.