Insertion chemistry of iron(ii) boryl complexes

Abstract

Iron(II) boryl complexes of the pyrrole-based pincer ligand, ^CyPNP (^CyPNP = anion of 2,5-bis(dicyclohexylphophinomethyl)pyrrole) have been synthesized and their insertion reactivity interrogated. Compounds of the type [Fe(BE)(^CyPNP)] (E = pinacholato or catecholato) can be generated by treatment of the precursors, [Fe(OPh)(py)(^CyPNP)] or [FeMe(^CyPNP)], with B₂E₂. The boryl complexes are meta stable, but permit additional reactivity with several unsaturated substrates. Reaction with alkynes, RCCR′, leads to rapid insertion into the Fe–B bond to generate stable vinyl boronate complexes of the type [Fe(C{R}C{R′}BE)(^CyPNP)] (R, R′ = H, Me, Ph, –CCPh). Each of the compounds is five-coordinate in the solid state by virtue of coordination of one of the oxygen atoms of the boronate ester. Similar reaction with nitriles, RCN (R = Ph, Me), results in facile de-cyanation to produce the correpsonding hydrocarbon complexes, [FeR(^CyPNP)]. In the case of the bulky nitrile 1-AdCN, the insertion intermediate, [Fe(C{Ad}NBpin)(^CyPNP)], has been isolated and structurally characterized. Treatment of the boryl complexes with styrene derivatives results in initial insertion to give an alkylboronate complex followed by either β-H elimination or protonation to give the products of C–H borylation and hydroboration, respectively.