Iron-hydride and iron-boryl complexes supported by a pyrrole-based pincer ligand, tBuPNP (PNP = anion of 2,5bis(di-tert-butylphosphinomethyl)pyrrole), were employed for a detailed mechanistic study on the hydroboration of internal alkynes. Several novel complexes were isolated and fully characterized, including iron-vinyl and iron-boryl species, which represent likely intermediates in the catalytic hydroboration pathway. In addition, the products of alkyne insertion into the Fe-B bond have been isolated and structurally characterized. Mechanistic studies of the hydroboration reaction favor a pathway involving an active iron-hydride species, [FeH(tBuPNP)], which readily inserts alkyne and undergoes subsequent reaction with hydroborane to generate product. The iron-boryl species, [Fe(BR2)(tBuPNP)] (R2 = pin or cat), was found to be chemically competent, although its use in catalysis entailed an induction period whereby the iron-hydride species was generated. Stoichiometric reactions and kinetic experiments were performed to paint a fuller picture of the mechanism of alkyne hydroboration, including pathways for catalyst deactivation and the influence of substrate bulk on catalytic efficacy.
