Coordination Chemistry of (Triphos)Fe(0) Ethylene Complexes and Their Application to CO₂ Valorization

Abstract

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.

An Investigation of Allyl‐Substituted Bis(Diphosphine) Iron Complexes: Towards Precursors for Cooperative CO2 Activation

“…Despite our inability to access compounds having boron‐based SCSs, we were nevertheless interested in pursuing reactivity of the generated compounds with CO 2 . Indeed, complexes of the type [Fe(diphosphine) 2 ] are known to mediate the cycloaddition of unsaturated carbon‐carbon bonds (alkenes and alkynes) with CO 2 to provide ferra(II)lactones [17,34–36] . We thus questioned if the cycloferrated compound 5 might serve as a reactive synthon towards CO 2 .…”

“…Indeed, complexes of the type [Fe(diphosphine) 2 ] are known to mediate the cycloaddition of unsaturated carbon-carbon bonds (alkenes and alkynes) with CO 2 to provide ferra(II)lactones. [17,[34][35][36] We thus questioned if the cycloferrated compound 5 might serve as a reactive synthon towards CO 2 . Being unable to successfully isolate compound 4 in pure form, a 3 : 1 mixture of compounds 4 and 5 was heated to generate 5 (> 95 %) and accordingly exposed to 1 atm.…”