T.J. Crevier, B.K. Bennett, J.D. Soper, J.A. Bowman, A. Dehestani, D. Hrovat, S. Lovell, W. Kaminsky, J.M. Mayer: C-N bond formation on addition of aryl carbanions to the electrophilic nitrido ligand TpOs(N)Cl2. Am Chem. Soc. 123 (2001) 1059-1071

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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 also gives 4. Mechanistic studies suggest a mechanism for both the Grignard and borane reactions of 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 nitrogen -- there 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 acsessible LUMO and LUMO+1, which are the Os(triple-bond)N (pi)* orbitals. The bonding in 1 and its reactivity with organoborates are reminiscent of CO.


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