Ligand Effects in Carbon-Boron Coupling Processes Mediated by σ-BH Platinum Complexes

The reaction of tri-coordinated boranes (derived from dioxaborolanes and diazaborolanes) with cyclometalated low-electron count platinum complexes [Pt(NHC')(NHC)][BArF] (NHC=(IBuPr)-Bu-t-Pr-i, IMes, IMes*) led, at low temperature, to the formation of the corresponding sigma-BH species. Some of these species have been characterized by X-Ray diffraction methods showing a rare eta(1)-coordination mode. These compounds are thermally unstable and undergo a carbon-boron coupling process whose reversibility depends on the NHC ligand. DFT calculations indicate that the energy barriers required for C-B bond formation events (together with Pt-H bonds) are lower than the competitive reactions leading to C-H bond formation (and Pt-B bonds). However, the C-B coupling products appear to be formed under kinetic control with (IBuPr)-Bu-t-Pr-i ligands, whereas the relative low energy barrier leading to C-H bond formation is sufficiently low to form the thermodynamically more stable platinum boryl complexes at rt. The latter energy barrier is, nevertheless, too high for the systems bearing IMes and IMes* ligands.

Automated summary

The reaction of tri-coordinated boranes with cyclometalated low-electron count platinum complexes forms sigma-BH species with rare eta(1)-coordination mode under low temperature, which are thermally unstable and undergo carbon-boron coupling process under kinetic and thermodynamic control depending on the NHC ligand. The energy barriers for C-B bond formation events are lower than the competitive reactions leading to C-H bond formation, with different outcomes observed for different NHC ligands.