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
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.