Oxidatively-induced C(sp3)–C(sp3) bond formation at a tucked-in iron(iii) complex

Abstract

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]ⁿ⁺–C} (M = metal) structural units, studies that offer well-defined relationships between metal oxidation state, hydrocarbon strain, and {[M]ⁿ⁺–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-C₅Me₄CH₂)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-C₅Me₄CH₂)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.