The divergent effects of strong NHC donation in catalysisElectronic supplementary information (ESI) available: Rate profiles for decomposition of u-GIIm and s-GIIm; X-ray crystallographic details; NOESY spectra, and derivation of the [PCy3]-independence of decomposition. CCDC 1400077. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c5sc02592c

• Main Authors: Lummiss, Justin A. M, Higman, Carolyn S, Fyson, Devon L, McDonald, Robert, Fogg, Deryn E
• Format: Article
• Language: English
• ISSN: 2041-6520; 2041-6539
• DOI: 10.1039/c5sc02592c

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 (RuCl 2 (NHC)(PCy 3 )(&z.dbd;CHPh), GII ) offer an early, prominent example. Less widely recognized is the dramatically inhibiting impact of NHC ligation on initiation of GII , and on re-entry into the catalytic cycle from the resting-state methylidene species RuCl 2 (NHC)(PCy 3 )(&z.dbd;CH 2 ), GIIm . Both GII and the methylidene complexes are activated by dissociation of PCy 3 . The impact of NHC donicity on the rate of PCy 3 loss is explored in a comparison of s -GIIm , vs. u -GIIm , in which the NHC ligand is saturated H 2 IMes or unsaturated IMes, respectively. PCy 3 loss is nearly an order of magnitude slower for the IMes derivative (a difference that is replicated, albeit smaller, for the benzylidene precatalysts GII ). Proposed as an overlooked contributor to these rate differences is an increase in the Ru-PCy 3 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 → PCy 3 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 → H 2 IMes back-donation (as confirmed by NOE experiments, which reveal restricted rotation about the Ru-NHC bond for H 2 IMes, but not IMes). Ru → PCy 3 back-donation is thus attenuated in the H 2 IMes complexes, accounting for the greater lability of the PCy 3 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. The inverse relationship between NHC donicity and catalyst initiation.