Modulating reactivity in iridium bis(N-heterocyclic carbene) complexes: influence of ring size on E-H bond activation chemistry.

The changes in the steric and electronic properties of N-heterocyclic carbenes (NHCs) as a function of ring size have a profound effect on the reactivity of their late transition metal complexes. Comparison of closely related complexes featuring either a saturated 5- or 6-membered NHC, reveals that the larger ring is associated with an increased propensity towards intramolecular C-H activation, but with markedly lower reactivity towards external substrates. Thus, systems of the type IrL2(H)2 give rise to contrasting chemical behaviour, primarily reflecting the differing possibilities for secondary stabilization of the metal centre by the N-bound aryl substituents: highly labile Ir(5-Mes)2(H)2 can only be studied by trapping experiments, while Ir(6-Mes)2(H)2 is air and moisture stable, and unreactive towards many external reagents. With appropriate substrates, this heightened reactivity can be exploited, and in situ generated Ir(5-Mes)2(H)2 is capable of intermolecular B-H and N-H activation chemistry. In the case of H3B·NMe2H, this affords a rare opportunity to study amine/aminoborane coordination via single crystal neutron diffraction methods.