Insights into Activation of Cobalt Pre-Catalysts for C()-H Functionalization.

The activation of readily prepared, air-stable cobalt (II) bis(carboxylate) pre-catalysts for the functionalization of C()-H bonds has been systematically studied. With the pyridine bis(phosphine) chelate, PNP, treatment of with either BPin or HBPin generated cobalt boryl products. With the former, reduction to (PNP)CoBPin was observed while with the latter, oxidation to the cobalt(III) dihydride boryl, -(PNP)Co(H)BPin occurred. The catalytically inactive cobalt complex, Co[PinB(OCBu)]2, accompanied formation of the cobalt-boryl products in both cases. These results demonstrate that the pre-catalyst activation from cobalt(II) bis(carboxylates), although effective and utilizes an air-stable precursor, is less efficient than activation of cobalt(I) alkyl or cobalt(III) dihydride boryl complexes, which are quantitatively converted to the catalytically relevant cobalt(I) boryl. Related cobalt(III) dihydride silyl and cobalt(I) silyl complexes were also synthesized from treatment of -(PNP)Co(H)BPin and (PNP)CoPh with HSi(OEt), respectively. No catalytic silylation of arenes was observed with either complex likely due to the kinetic preference for reversible C-H reductive elimination rather than product- forming C-Si bond formation from cobalt(III). Syntheses of the cobalt(II) bis(carboxylate) and cobalt(I) alkyl of PONOP, a pincer where the methylene spacers have been replaced by oxygen atoms, were unsuccessful due to deleterious P-O bond cleavage of the pincer. Despite their structural similarity, the rich catalytic chemistry of PNP was not translated to PONOP due to the inability to access stable cobalt precursors as a result of ligand decomposition via P-O bond cleavage.