Polar effects in nitride coupling reactions

The nucleophilic molybdenum nitride (Et2NCS2)(3)MoN (1) reacts with the electrophilic osmium nitride complex TpOsNCl(2) (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)Cl-2 (4). Labeling experiments establish that the N-2 produced comes specifically (>96%) from mixed-metal (molybdenum-osmium) coupling. The major transition-metal-containing product of the reaction is the mu-nitrido complex TpOSCl(2)(mu-N)Mo(S2CNEt2)(3) (3), where the bridging nitride derives primarily (82%) from the osmium nitride 2. The mu-nitrido complex 3 has been characterized crystallographically, and shows a nitride bridge that is very asymmetric (Mo-N = 1.721(3) Angstrom, Os-N = 1.906(3) Angstrom), 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.