Synthesis, Characterization, and Reactivity of the First Osmium β‑Diketiminato Complexes and Application in Catalysis

The
strongly chelating anionic β-diketiminate ligand has
been employed to formulate complexes involving almost every metal
of the periodic table; however, the heavier metals of the d block
remain relatively unexplored. This paper describes the synthesis and
characterization of the first two osmium β-diketiminato compounds,
including a coordinatively unsaturated cationic complex. In parallel
to the analogous Ru­(II) complexes, the cationic (η6-arene)­osmium­(II) complex demonstrates bifunctional behavior through
[4 + 2] cycloaddition with ethylene, cleavage of dihydrogen under
mild conditions, and protonation/chloride addition with [Et2OH]­Cl. Metal-centered activity in both the Ru­(II) and Os­(II) β-diketiminates
has until now remained elusive, as the cationic Os complex is shown
to readily coordinate an aryl isonitrile. The applicability of Os­(II)
β-diketiminato complexes in catalytic olefin hydrogenation demonstrates
significantly greater activity in terms of conversion and TOF for
a range of substrates, including styrene, cyclohex-1-ene, and 1-methylcyclohex-1-ene.
Moreover, selective hydrogenation of the exocyclic alkenyl group in
limonene was observed, whereas the corresponding isostructural Ru­(II)
complexes are inactive. In contrast, the cationic (η6-arene)­ruthenium­(II) β-diketiminato complex proved more active
for the catalytic dehydrogenation of N,N-dimethylamine borane (Me2NBH3) than the equivalent
Os­(II) species. A detailed DFT study of the Ru­(II) and Os­(II) β-diketiminato
species using charge decomposition analysis (CDA) demonstrates differences
in metal–ligand interactions, which in turn considerably influences
the extent of bifunctional reactivity.