Oxidatively-induced C(sp³)-C(sp³) 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 [(η⁶-C₅Me₄[double bond, length as m-dash]CH₂)Fe(dnppe)] (dnppe = 1,2-bis(di-n-propylphosphino)ethane) results in C(sp³)-C(sp³) bond formation giving unique {Fe₂} dimers. Freeze-quenched CW X-band EPR spectroscopy allowed for spectroscopic identification of the reactive [(η⁶-C₅Me₄[double bond, length as m-dash]CH₂)Fe(dnppe)]⁺ intermediate. Density functional theory (DFT) calculations reveal a primarily Fe-centered radical and a weak {[Fe]-C} bond (BDE_[Fe]-C = 24.5 kcal mol^-1, c.f. BDE_C-C(ethane) = 90 kcal mol^-1). For comparison, a structurally analogous Fe(iii) methyl complex was prepared, [Cp*Fe(dnppe)(CH₃)]⁺ (Cp* = C₅Me₅ ^-), where C(sp³)-C(sp³) coupling was not observed, consistent with a larger calculated BDE_[Fe]-C value of 47.8 kcal mol^-1. These data are analogized to the simple hydrocarbons ethane and cyclopropane, where a strain-induced BDE_C-C decrease of 33 kcal mol^-1 is witnessed on cyclization.