Olefin π-coordination chemistry at low-oxidation-state boron

The coordination and substitution of hydrocarbon substrates is common for transition metals, and has been demonstrated in a handful of cases with molecules based on heavier p-block elements. By contrast, this behaviour is uncommon for the first-row p-block elements, probably due to the generally irreversible nature of their covalent bond formation. Boranes can loosely bind and, in cooperation with strong Lewis donors, covalently bind and functionalize olefins. However, the single-site reversible coordination and functionalization of olefins by first-row p-block elements remains elusive. Here we present a monovalent boron system able to coordinate olefins and mediate their liberation and functionalization, based on stable and well-defined borylene-olefin π complexes. These molecules are shown to be distinct from their conventional covalently bound form-boriranes-instead better resembling borylenes with π-bound olefins. The high π-complex character of these species suggests a rationale for their unique ability to reversibly bind and functionalize olefins.