Carbon dioxide functionalization is a potentially transformative method to enhance the sustainability of carbon-based commercial chemicals. The reductive functionalization of CO2 via C–C bond forming reactions is one route to deriving structurally diverse products from this renewable resource. Despite considerable advances in catalytic CO2 reduction into smaller molecular fragments, such as CO, there remains relatively few transition metal mediated processes to elaborate CO2 via reductive C–C bond functionalization. An investigation of iron(0) complexes capable of reductive coupling with CO2 to generate acrylate and oxalate are described here. A set of [(depe)2Fe] (depe = 1,2-bis­(diethylphosphino)­ethane) complexes were found to reductively functionalize CO2 with ethylene or a second molecule of CO2 to afford C3 and C2 products, respectively. Several factors influencing the stoichiometric yield of these transformations were examined including a deleterious CO2 disproportionation process which produces (depe)2Fe­(CO) and (depe)2Fe­(CO3). Additionally, an intermediate iron–lactone complex in route to acrylate formation has been structurally characterized.

The coordination chemistry of zerovalent iron complexes bearing triphosphine ligands of the type tBuP(CH2CH2PR2)2 (R = Et, iPr, Cy) was attenuated via ancillary ligand effects. Iron(0) dinitrogen ethylene complexes, [tBuP(CH2CH2PR2)]Fe(C2H4)(N2) (R = Et, iPr, Cy), were prepared via alkali metal reduction of the corresponding iron dichloride precursors and were characterized by single-crystal X-ray diffraction. In complexes bearing iPr- and Cy-substituted triphosphine ligands, reversible dissociation of dinitrogen from [tBuP(CH2CH2PR2)]Fe(C2H4)(N2) was observed, generating paramagnetic four-coordinate iron(0) monoethylene species [tBuP(CH2CH2PR2)]Fe(C2H4) (R = iPr, Cy). Conversely, the Et-substituted [tBuP(CH2CH2PEt2)]Fe(C2H4)(N2) was stable in solution and reacted with additional ethylene to afford a five-coordinate iron(0) bis(ethylene) species, [tBuP(CH2CH2PEt2)]Fe(C2H4)2. Analysis of structural parameters obtained from X-ray diffraction and IR stretching frequencies obtained from the iron dicarbonyl species [tBuP(CH2CH2PR2)]Fe(CO)2 (R = Et, iPr, Cy) suggests that the attenuations in coordination chemistry are likely steric in origin. Examination of CO2 functionalization from [tBuP(CH2CH2PR2)]Fe(C2H4)(N2) (R = Et, iPr, Cy) revealed selective formation of methylmalonic acid upon protonolysis of an insoluble organometallic product, consistent with the coupling of bound ethylene with two molecules of carbon dioxide.