Radical Behavior of CO₂ versus its Deoxygenation Promoted by Vanadium Aryloxide Complexes: How the Geometry of Intermediate CO₂‐Adducts Determines the Reactivity.

Abstract

The reactivity of carbon dioxide with vanadium(III) aryloxo complexes has been investigated. The formation of either carbon monoxide or incorporation into the ligand system with the ultimate formation of organic ester was observed depending on the overall electron donor ability of the ligand field. DFT calculations were carried out to investigate the proposed mechanism for carbon dioxide coordination and reduction.

Reaction of CO2 with a Vanadium(II) Aryl Oxide: Synergistic Activation of CO2/Oxo Groups towards H‐Atom Radical Abstraction

“…The formation of formate in the other step would imply a linearly end‐on bonded CO 2 moiety, which then undergoes bending, one‐electron transfer, and the abstraction of one H atom from the solvent. This radical behavior of linearly bonded CO 2 would be in line with our previous findings in the chemistry of V III aryl oxides …”

“…Furthermore, by tuning the electronic properties of the metal through judicious ligand replacement, it was possible to switch from the two‐electron transfer to one‐electron reduction, thus entering an unprecedented pattern of reactivity (i.e. radical attack on the ligand system with the incorporation of CO 2 into organic esters) …”

“…The possibility of one‐electron transfer is clearly related to both the reducing power of the metal (i.e. oxidation state) and the nature of the ligand system, which can together extend the lifetime of the intermediate carbon‐centered radical anion . Another critical factor is the coordination mode of CO 2 to the metal center .…”

“…For this purpose, we recently used low‐valent vanadium, a strong reductant, in combination with bulky aryl oxide ligands and investigated its reactivity with CO 2 . While the much‐anticipated deoxygenation to oxo species and CO was still observed, the study also featured the occurrence of the rare end‐on coordination mode.…”

Linear End‐On Coordination Modes of CO2

“…[1][2][3][4][5][6] Besides the well-established reactions with strong nucleophiles (for example,a lkyls,a mides,a nd hydrides) which always resolve in addition-insertion reactions,t he chemical behavior of this exceedingly stable molecule mainly consists of redox transformations. [1][2][3][4][5][6][7][8][9][10][11][12] Thee arly work of Floriani four decades ago has indicated that while at woelectron attack (either cooperative interaction from two metals or monometallic) affords deoxygenation or disproportionation, one-electron donation may allow valuable CÀC forming reactions. [7][8][9][10][11]13] However, the fact that CO 2 requires fairly strong reducing agents makes the conversions within the C1-redox series (CO 2 ,f ormic acid, CO,f ormaldehyde, methanol, methane) generally difficult to control.…”