Cobalt Boryl Complexes: Synthesis, Characterization, and Reactivity Relevant to Catalytic Alkyne Hydroboration

The synthesis and structure of several pincer-ligated cobalt boryl complexes is reported. These compounds include the cobalt(II) species, [Co(BE 2 )( Cy PNP)] (E 2 = pinacolato (pin) or catecholato (cat); Cy PNP = anion of bis(dicyclohexylphosphinomethyl)pyrrole), as well as the formally cobalt(III) species, [Co(H)(BE 2 )( Cy PNP)]. A rare cobalt diboryl complex has also been isolated through reaction of [Co(N 2 )( Cy PNP)] with B 2 cat 2 . The reactivities of the new boryl species are investigated in migratory insertion reactions of unsaturated carbon substrates to investigate likely steps in hydroboration catalysis. The cobalt(II) compounds are found to readily insert both alkynes and allenes resulting in new organometallic species. Catalytic trials with the cobalt complexes demonstrate good activity for alkyne hydroboration, although mechanistic studies implicate a pathway involving a Co(I)/Co(III) cycle as opposed to direct involvement of Co(II) boryl species. The crystallographic structures of several of the new cobalt boryl compounds are reported.