Predicting reactivity based on simple orbital pictures is routine in organic systems, but remains controversial in inorganic systems, which have more orbitals to allow them to sneak around apparent symmetry-imposed barriers. We are exploring nitrogen atom and carbyne group transfer reactions of simple, mononuclear terminal nitrido and carbyne complexes, such as the Diels-Alder-style cycloaddition of an osmium nitride shown below, to see if these reactions are affected by orbital symmetry considerations. In this case, orbital considerations do predict reactivity, but in other cases, wild and unexpected transformations can take place. For example, osmium nitrides react quite generally with conjugated alkenes to give C=C bond cleavage with net nitrogen atom insertion. Studies of the mechanisms of these reactions and efforts to apply them to organic synthesis are currently in progress.

Catalytic Oxygenation Reactions

Atmospheric oxygen is a cheap, abundant, environmentally friendly oxidant, but it does not usually react selectively with organic molecules. Transition metal catalysis is an obvious and promising solution to this problem, but a general difficulty is that potentially selective oxidants such as metal oxo complexes are often difficult to regenerate with oxygen, while compounds that react with oxygen, such as those of the later transition metals, usually do not form stable terminal oxo groups. To sidestep this, we are investigating late transition metal complexes with unusual (non-octahedral) coordination geometries. The hope here is that the unusual geometries will stabilize terminal oxo complexes while the complexes retain the sensitivity to oxygen typical of late metals. We have succesfully demonstrated this strategy using the only known terminal oxo complex of iridium, oxotris(mesityl)iridium(V). Regeneration of this species by air is rapid and clean over many catalytic cycles, but it reacts only with very electron-rich substrates like triphenylphosphine or triphenylarsine. Analogues that retain the ability to be regenerated by oxygen, but are more reactive toward organics, are under investigation. Another approach that taps the unreactivity of (mes)₃Ir=O toward organics is to use it as a cocatalyst with other oxidants. This approach has been successfully implemented with osmium tetroxide to allow the dihydroxylation of olefins using air and water.