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

Pablo Ríos,Francisco José Fernández‐De‐Córdova,Agustí Lledós,Natalia Curado,Javier Borge,Salvador Conejero

doi:10.1002/ejic.202100428

Abstract

AbstractThe reaction of tri‐coordinated boranes (derived from dioxaborolanes and diazaborolanes) with cyclometalated low‐electron count platinum complexes [Pt(NHC’)(NHC)][BArF] (NHC=ItBuiPr, IMes, IMes*) led, at low temperature, to the formation of the corresponding σ‐BH species. Some of these species have been characterized by X‐Ray diffraction methods showing a rare η1‐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 ItBuiPr 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.

Highlights

The interaction of tri-coordinated boranes, HBR2, with transition metal complexes to form σ-BH species is a key step to induce the cleavage of the BÀ H bond on the way to form new BÀ X bonds (X = B, C, N).[1] σ-borane complexes are a rather well established family of compounds,[1k,2] considerably more limited in number in comparison with the parent dihydrogen and σ-silane derivatives.[3]
In most of the cases, the isolated σ-BH species are only involved in processes of cleavage of the BÀ H bond and/or exchange of the H atom with hydrides in metal compounds.[1k,2,4] this type of compounds can be engaged as intermediates in the formation of carbon-boron bonds, in particular in hydroboration processes,[1k,5] in the
In this article we describe the reactivity of boranes with different sterereo-electronic properties (HBpin, HBcat, and substituted benzodiazaborolanes, with benzodiazaborolanes being more sterically demanding and having a less acidic boron atom and HBcat being the less sterically hindered and more acidic) with low-electron count cyclometalated Pt(II) complexes bearing bulky N-heterocyclic carbene ligands IMes, IMes* or less sterically congested ItBuiPr ligands

Summary

Introduction

Part of the “RSEQ-GEQO Prize Winners” Special Collection. borylation of alkanes as described by Hartwig et al.[1j,6] or in other processes involving at some point the reaction of a borane and an alkyl complex.[7]. Borylation of alkanes as described by Hartwig et al.[1j,6] or in other processes involving at some point the reaction of a borane and an alkyl complex.[7] In this sense, it is of particular relevance to understand the factors determining the formation of a carbon-boron bond when a metal-alkyl complex and a σBH borane are part of the same molecule. In this chemical situation, two possible scenarios can be foreseen: either the molecule evolves towards the formation of a metal-boryl (MÀ B) complex and an alkane (CÀ H) or to a metal-hydride (MÀ H) with concomitant formation of a carbon-boron bond (CÀ B) as exemplified in Scheme 1 for a σ-CAM (Complex Assisted Metathesis)[8] mechanism. In this article we describe the reactivity of boranes with different sterereo-electronic properties (HBpin, HBcat, and substituted benzodiazaborolanes, with benzodiazaborolanes being more sterically demanding and having a less acidic boron atom and HBcat being the less sterically hindered and more acidic) with low-electron count cyclometalated Pt(II) complexes bearing bulky N-heterocyclic carbene ligands IMes, IMes* or less sterically congested ItBuiPr ligands

Results and Discussion

Conclusion

Conflict of Interest