Mechanistic Studies of Alkyne Hydroboration by a Well-Defined Iron Pincer Complex: Direct Comparison of Metal-Hydride and Metal-Boryl Reactivity

Abstract

Iron-hydride and iron-boryl complexes supported by a pyrrole-based pincer ligand, tBuPNP (PNP = anion of 2,5-bis­(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.

Insertion chemistry of iron(ii) boryl complexes

“…These results contrast those of the related vinylboronate, [Fe(C{Me}C{Me}Bpin)( t Bu PNP)], which was found to undergo reaction with HBpin to produce a diborated alkene and [FeH( t Bu PNP)]. 31 We believe the inability of the Cy PNP system to engage in comparable chemistry is due to the Fe–O bonding interaction ( vide supra ), which is absent in the t Bu PNP system. Donation of the oxygen lone pairs to iron effectively competes with incoming HBpin shutting down reactivity.…”

“…We began by testing the propensity for alkyne insertion based on the success of similar reactions with the t Bu PNP system. 31 Treatment of 3a or 3b prepared from either 1·py or 2 with 2-butyne resulted in a rapid colour change from purple to bright yellow. NMR spectroscopy confirmed the formation of new S = 1 iron complexes in each case consistent with the insertion products 4a , b displayed in Scheme 2.…”

“…29,30 Our laboratory recently examined the catalytic mechanism of alkyne hydroboration using an iron pincer system based on the pyrrole-based ligand tBu PNP displayed in Chart 1. 31 This work built upon methodology first reported by Nishibayashi and coworkers who posited a possible role for four-coordinate iron boryls. 28 In the course of our studies on alkyne hydroboration we observed facile insertion of alkynes into the Fe-B bond.…”

“…Prior work with the tBu PNP ligand established the ability of iron-phenoxide precursors to serve as entry points to stable four-coordinate boryl complexes of iron(II). 31,32 We therefore targeted the previously reported phenoxide complex, [Fe(OPh)(py)( Cy PNP)] (1•py), 33 as a suitable starting point for synthesis of new metal boryls. Treatment of 1•py with B 2 pin 2 or B 2 cat 2 ( pin = pinacolato, cat = catecholato) led to smooth formation of purple solutions of the corresponding boryl species (3, eqn (1)) as judged by 1 H NMR spectroscopy.…”

“…Upon introduction of a CO atmosphere, an immediate colour change from purple to red was observed. 1 H NMR analysis showed formation of a single low-spin, diamagnetic complex with C S symmetry displaying a single 31 P NMR shift at δ 82.13. Complementary infrared analysis demonstrated several signals attributable to metal-bound CO groups at 2042,2018,1993,1963,1934, and 1856 cm −1 .…”