Oxidatively-induced C(sp 3 )-C(sp 3 ) 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] 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 [(η 6 -C 5 Me 4 [double bond, length as m-dash]CH 2 )Fe(d n ppe)] (d n ppe = 1,2-bis(di- n -propylphosphino)ethane) results in C(sp 3 )-C(sp 3 ) bond formation giving unique {Fe 2 } dimers. Freeze-quenched CW X-band EPR spectroscopy allowed for spectroscopic identification of the reactive [(η 6 -C 5 Me 4 [double bond, length as m-dash]CH 2 )Fe(d n ppe)] + 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(d n ppe)(CH 3 )] + (Cp* = C 5 Me 5 - ), where C(sp 3 )-C(sp 3 ) 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.