Ultrafast and Ultraslow Oxygen Atom Transfer Reactions between Late Metal Centers

Oxotrimesityliridium(V), (mes)3IrO (mes = 2,4,6-trimethylphenyl), and trimesityliridium(III),
(mes)3Ir, undergo extremely rapid degenerate intermetal oxygen atom transfer at room temperature. At
low temperatures, the two complexes conproportionate to form (mes)3IrOIr(mes)3, the 2,6-dimethylphenyl
analogue of which has been characterized crystallographically. Variable-temperature NMR measurements
of the rate of dissociation of the μ-oxo dimer combined with measurements of the conproportionation
equilibrium by low-temperature optical spectroscopy indicate that oxygen atom exchange between iridium(V) and iridium(III) occurs with a rate constant, extrapolated to 20 °C, of 5 × 107 M-1 s-1. The oxotris(imido)osmium(VIII) complex (ArN)3OsO (Ar = 2,6-diisopropylphenyl) also undergoes degenerate
intermetal atom transfer to its deoxy partner, (ArN)3Os. However, despite the fact that its metal−oxygen
bond strength and reactivity toward triphenylphosphine are nearly identical to those of (mes)3IrO, the
osmium complex (ArN)3OsO transfers its oxygen atom 12 orders of magnitude more slowly to (ArN)3Os
than (mes)3IrO does to (mes)3Ir (kOsOs = 1.8 × 10-5 M-1 s-1 at 20 °C). Iridium−osmium cross-exchange
takes place at an intermediate rate, in quantitative agreement with a Marcus-type cross relation. The
enormous difference between the iridium−iridium and osmium−osmium exchange rates can be rationalized
by an analogue of the inner-sphere reorganization energy. Both Ir(III) and Ir(V) are pyramidal and can
form pyramidal iridium(IV) with little energetic cost in an orbitally allowed linear approach. Conversely,
pyramidalization of the planar tris(imido)osmium(VI) fragment requires placing a pair of electrons in an
antibonding orbital. The unique propensity of (mes)3IrO to undergo intermetal oxygen atom transfer allows
it to serve as an activator of dioxygen in cocatalyzed oxidations, for example, acting with osmium tetroxide
to catalyze the aerobic dihydroxylation of monosubstituted olefins and selective oxidation of allyl and benzyl
alcohols.