Insertion chemistry of iron(II) boryl complexes

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, RCCR', 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, -CCPh). 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, RCN (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.