Beyond the Nucleophilic Role of Metal–Boryl Complexes in Borylation Reactions

Transition metal–boryl complexes play an important role as reactive intermediates in catalytic borylation processes and have enjoyed tremendous exploration. Over the years, it has been well established that a boryl ligand possesses strong nucleophilicity in copper(I)–boryl complexes. Thus, the role played by boron’s “empty” p orbital has often been overlooked and d-electrons’ effect from the metal is even more seldom mentioned, despite the fact that they in theory should also have influence on the reactivity of a metal–boryl complex. In this work, the reactivity of metal–boryl complexes has been systematically studied with the aid of density functional theory calculations. We have revealed that when the metal is more electronegative, the nucleophilic feature of boryl is diminished and will no longer be active, whereas when the boryl is not substituted with π-donating groups, the “empty” p orbital of boron will be active and can serve as the electrophilic site. In addition, when the metal center changes from relatively inert d¹⁰ to d⁸, the d electrons can also play the role of a nucleophile. With a much more comprehensive understanding of metal–boryl complexes, this work will benefit the future catalyst design related to transition-metal-catalyzed borylation reactions.