Evidence of a Donor–Acceptor (Ir–H)→SiR₃Interaction in a Trapped Ir(III) Silane Catalytic Intermediate

Abstract

The ionic iridacycle [(2-phenylenepyridine-κN,κC)IrCp*(NCMe)][BArF24] ([2][BArF24]) displays a remarkable capability to catalyze the O-dehydrosilylation of alcohols at room temperature (0.4 × 103 < TON < 103, 8 × 103 < TOFi < 1.9 × 105 h–1 for primary alcohols) that is explained by its exothermic reaction with Et3SiH, which affords the new cationic hydrido-Ir(III)-silylium species [3][BArF24]. Isothermal calorimetric titration (ITC) indicates that the reaction of [2][BArF24] with Et3SiH requires 3 equiv of the latter and releases an enthalpy of −46 kcal/mol in chlorobenzene. Density functional theory (DFT) calculations indicate that the thermochemistry of this reaction is largely dominated by the concomitant bis-hydrosilylation of the released MeCN ligand. Attempts to produce [3][BF4] and [3][OTf] salts resulted in the formation of a known neutral hydrido-iridium(III) complex, i.e. 4, and the release of Et3SiF and Et3SiOTf, respectively. In both cases formation of the cationic μ-hydrido-bridged bis-iridacyclic complexes [5][BF4] and [5][OTf], respectively, was observed. The structure of [5][OTf] was established by X-ray diffraction analysis. Conversion of [3][BArF24] into 4 upon reaction with either 4-N,N-dimethylaminopyridine or [nBu4][OTf] indicates that the Ir center holds a +III formal oxidation state and that the Et3Si+ moiety behaves as a Z-type ligand according to Green’s formalism. [3][BArF24], which was trapped and structurally characterized and its electronic structure investigated by state-of-the-art DFT methods (DFT-D, EDA, ETS-NOCV, QTAIM, ELF, NCI plots and NBO), displays the features of a cohesive hydridoiridium(III)→silylium donor–acceptor complex. This study suggests that the fate of [3]+ in the O-dehydrosilylation of alcohols is conditioned by the nature of the associated counteranion and by the absence of Lewis base in the medium capable of irreversibly capturing the silylium species.