C−N Bond Formation on Addition of Aryl Carbanions to the Electrophilic Nitrido Ligand in TpOs(N)Cl2
journal contributionposted on 23.01.2001, 00:00 by Thomas J. Crevier, Brian K. Bennett, Jake D. Soper, Julie A. Bowman, Ahmad Dehestani, David A. Hrovat, Scott Lovell, Werner Kaminsky, James M. Mayer
The osmium(VI) nitrido complex TpOs(N)Cl2 (1) has been prepared from K[Os(N)O3] and KTp in aqueous ethanolic HCl. It reacts rapidly with PhMgCl and related reagents with transfer of a phenyl group to the nitrido ligand. This forms Os(IV) metalla-analido complexes, which are readily protonated to give the analido complex TpOs(NHPh)Cl2 (4). The nitrido-phenyl derivatives TpOs(N)PhCl and TpOs(N)Ph2 react more slowly with PhMgCl and are not competent intermediates for the reaction of 1 with PhMgCl. Reactions of 1 with alkyl- and arylboranes similarly result in transfer of one organic group to nitrogen, leading to isolable borylamido complexes such as TpOs[N(Ph)(BPh2)]Cl2 (11). This is an unprecedented insertion of a nitrido ligand into a boron−carbon bond. Hydrolysis of 11 gives 4. Mechanistic studies suggest that both the Grignard and borane reactions proceed by initial weak coordination of Mg or B to the nitrido ligand, followed by migration of the carbanion to nitrogen. The hydrocarbyl group does not go to osmium and then move to nitrogenthere is no change in the atoms bound to the osmium during the reactions. It is suggested that there may be a general preference for nucleophiles to add directly to the metal−ligand multiple bond rather than binding to the metal first and migrating. Ab initio calculations show that the unusual reactivity of 1 results from its accessible LUMO and LUMO + 1, which are the Os⋮N π* orbitals. The bonding in 1 and its reactivity with organoboranes are reminiscent of CO.