Polar Effects in Nitride Coupling Reactions

The nucleophilic molybdenum nitride (Et2NCS2)3MoN (1) reacts with the electrophilic osmium nitride complex TpOsNCl2 (2, Tp = hydrotris(1-pyrazolyl)borate) to produce molecular nitrogen. Reaction of 1 at the nitride is accompanied by a substantial amount of reaction at a sulfur atom of the dithiocarbamate ligand, forming the osmium thionitrosyl complex TpOs(NS)Cl2 (4). Labeling experiments establish that the N2 produced comes specifically (>96%) from mixed-metal (molybdenum−osmium) coupling. The major transition-metal-containing product of the reaction is the μ-nitrido complex TpOsCl2(μ-N)Mo(S2CNEt2)3 (3), where the bridging nitride derives primarily (82%) from the osmium nitride 2. The μ-nitrido complex 3 has been characterized crystallographically, and shows a nitride bridge that is very asymmetric (Mo−N = 1.721(3) Å, Os−N = 1.906(3) Å), with less multiple bonding toward osmium and more toward molybdenum. Heterometallic coupling is much faster than either homometallic coupling reaction, in particular the osmium−osmium coupling, despite the greater oxidizing power of osmium over molybdenum. The origin and implications of this kinetic effect on nitride coupling and dinitrogen cleavage are discussed.