Evidence of a Donor-Acceptor (Ir-H)→SiR3 Interaction in a Trapped Ir(III) Silane Catalytic Intermediate.

The ionic iridacycle [(2-phenylenepyridine-$ąppa$N,$ąppa$C)IrCp*(NCMe)][BArF24] ([2][BArF24]) displays a remarkable capability to catalyze the O-dehydrosilylation of alcs. at room temp. (0.4 × 103 {\textless} TON {\textless} 103, 8 × 103 {\textless} TOFi {\textless} 1.9 × 105 h-1 for primary alcs.) that is explained by its exothermic reaction with Et3SiH, which affords the new cationic hydrido-Ir(III)-silylium species [3][BArF24]. Isothermal calorimetric titrn. (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. D. functional theory (DFT) calcns. indicate that the thermochem. 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, resp. In both cases formation of the cationic $μ$-hydrido-bridged bis-iridacyclic complexes [5][BF4] and [5][OTf], resp., was obsd. The structure of [5][OTf] was established by x-ray diffraction anal. 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 oxidn. 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 alcs. is conditioned by the nature of the assocd. counteranion and by the absence of Lewis base in the medium capable of irreversibly capturing the silylium species. [on SciFinder(R)]


Soumis le 12 avril 2018