The divergent effects of strong NHC donation in catalysis. Issue 12 (16th October 2015)

• Record Type: Journal Article
• Authors: Lummiss, Justin A. M.; Higman, Carolyn S.; Fyson, Devon L.; McDonald, Robert; Fogg, Deryn E.
• Is Part Of: Chemical science. Volume 6:Issue 12(2015:Dec.)
• Page Start: 6739
• Page End: 6746
• Publication Date: 2015-10-16
• Subjects: Chemistry -- Periodicals; 540.5
• DOI: 10.1039/c5sc02592c
• Languages: English
• ISSNs: 2041-6520

Abstract

Abstract : The inverse relationship between NHC donicity and catalyst initiation. 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 )(CHPh), GII ) offer an early, prominent example. Less widely recognized is the dramatically inhibiting impact of NHC ligation on initiation ofGII, and on re-entry into the catalytic cycle from the resting-state methylidene species RuCl2 (NHC)(PCy3 )(CH2 ), GIIm . BothGII 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 s -GIIm, vs. u -GIIm, in which the NHC ligand is saturated H2 IMes 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 precatalystsGII ). 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 s -GII, in contrast, the greater π-acceptor capacity of the NHC ligand enables competing Ru → H2 IMes back-donation (as confirmed by NOE experiments, which reveal restricted rotation about the Ru–NHC bond for H2 IMes, but not IMes). Ru → PCy3 back-donation is thus attenuated in the H2 IMes complexes, accounting for the greater lability of the PCy3 ligand in s -GIIm and s -GII . 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.