The divergent effects of strong NHC donation in catalysis

Abstract

Strong σ-donation from NHC ligands (NHC = N-heterocyclic carbene) is shown to have profoundly conflicting consequences for the reactivity of transition-metal catalysts. Such donation is regarded as central to high catalyst activity in many contexts, of which the second-generation Grubbs metathesis catalysts (RuCl₂(NHC)(PCy₃)([double bond, length as m-dash]CHPh), <b>GII</b>) offer an early, prominent example. Less widely recognized is the dramatically inhibiting impact of NHC ligation on initiation of <b>GII</b>, and on re-entry into the catalytic cycle from the resting-state methylidene species RuCl₂(NHC)(PCy₃)([double bond, length as m-dash]CH₂), <b>GIIm</b>. Both <b>GII</b> and the methylidene complexes are activated by dissociation of PCy₃. The impact of NHC donicity on the rate of PCy₃ loss is explored in a comparison of <b><i>s</i>-GIIm</b>, <i>vs.</i> <b><i>u</i>-GIIm</b>, in which the NHC ligand is saturated H₂IMes or unsaturated IMes, respectively. PCy₃ loss is nearly an order of magnitude slower for the IMes derivative (a difference that is replicated, albeit smaller, for the benzylidene precatalysts <b>GII</b>). Proposed as an overlooked contributor to these rate differences is an increase in the Ru-PCy₃ bond strength arising from π-back-donation onto the phosphine ligand. Strong σ-donation from the IMes ligand, coupled with the inability of this unsaturated NHC to participate in significant π-backbonding, amplifies Ru → PCy₃ π-back-donation. The resulting increase in Ru-P bond strength greatly inhibits entry into the active cycle. For <b><i>s</i>-GII</b>, in contrast, the greater π-acceptor capacity of the NHC ligand enables competing Ru → H₂IMes back-donation (as confirmed by NOE experiments, which reveal restricted rotation about the Ru-NHC bond for H₂IMes, but not IMes). Ru → PCy₃ back-donation is thus attenuated in the H₂IMes complexes, accounting for the greater lability of the PCy₃ ligand in <b><i>s</i>-GIIm</b> and <b><i>s</i>-GII</b>. Similarly inhibited initiation is predicted for other metal-NHC catalysts in which a π-acceptor ligand L must be dissociated to permit substrate binding. Conversely, enhanced reactivity can be expected where such L ligands are pure σ-donors. These effects are expected to be particularly dramatic where the NHC ligand has minimal π-acceptor capacity (as in the unsaturated Arduengo carbenes), and in geometries that maximize NHC-M-L orbital interactions.