Oxidatively-induced C(sp)-C(sp) bond formation at a tucked-in iron(iii) complex.

Carbon-carbon (C-C) bond formation is a cornerstone of synthetic chemistry, relying on routes such as transition-metal mediated cross-coupling for the introduction of new carbon-based functionality. For {[M] -C} (M = metal) structural units, studies that offer well-defined relationships between metal oxidation state, hydrocarbon strain, and {[M] -C} bond thermochemistry are thus informative, providing a means to reliably access new product classes. Here, we show that one-electron oxidation of the iron tucked-in complex [(η-CMe[double bond, length as m-dash]CH)Fe(dppe)] (dppe = 1,2-bis(di--propylphosphino)ethane) results in C(sp)-C(sp) bond formation giving unique {Fe} dimers. Freeze-quenched CW X-band EPR spectroscopy allowed for spectroscopic identification of the reactive [(η-CMe[double bond, length as m-dash]CH)Fe(dppe)] intermediate. Density functional theory (DFT) calculations reveal a primarily Fe-centered radical and a weak {[Fe]-C} bond (BDE = 24.5 kcal mol, BDE = 90 kcal mol). For comparison, a structurally analogous Fe(iii) methyl complex was prepared, [CpFe(dppe)(CH)] (Cp = CMe ), where C(sp)-C(sp) coupling was not observed, consistent with a larger calculated BDE value of 47.8 kcal mol. These data are analogized to the simple hydrocarbons ethane and cyclopropane, where a strain-induced BDE decrease of 33 kcal mol is witnessed on cyclization.