Abstract
The new, formally Mo(II) complexes [Mo(η3-2-R-allyl)(6,6′-dmbipy)(CO)2Cl] (6,6′-dmbipy = 6,6′-dimethyl-2,2′-bipyridine; 2-R-allyl = allyl for R = H, 2-methallyl for R = CH3) and [Mo(η3-2-methallyl)(pTol-bian)(CO)2Cl] (pTol-bian = bis(p-tolylimino)acenaphthene) share, in this rare case, the same structural type. The effect of the anionic π-donor ligand X (Cl– vs NCS–) and the 2-R-allyl substituents on the cathodic behavior was explored. Both ligands play a significant role at all stages of the reduction path. While 2e–-reduced [Mo(η3-allyl)(6,6′-dmbipy)(CO)2]− is inert when it is ECE-generated from [Mo(η3-allyl)(6,6′-dmbipy)(CO)2(NCS)], the Cl– ligand promotes Mo–Mo dimerization by facilitating the nucleophilic attack of [Mo(η3-allyl)(6,6′-dmbipy)(CO)2]− at the parent complex at ambient temperature. The replacement of the allyl ligand by 2-methallyl has a similar effect. The Cl–/2-methallyl ligand assembly destabilizes even primary radical anions of the complex containing the strongly π-accepting pTol-Bian ligand. Under argon, the cathodic paths of [Mo(η3-2-R-allyl)(6,6′-dmbipy)(CO)2Cl] terminate at ambient temperature with 5-coordinate [Mo(6,6′-dmbipy)(CO)3]2– instead of [Mo(η3-2-R-allyl)(6,6′-dmbipy)(CO)2]−, which is stabilized in chilled electrolyte. [Mo(η3-allyl)(6,6′-dmbipy)(CO)2]− catalyzes CO2 reduction only when it is generated at the second cathodic wave of the parent complex, while [Mo(η3-2-methallyl)(6,6′-dmbipy)(CO)2]− is already moderately active at the first cathodic wave. This behavior is fully consistent with absent dimerization under argon on the cyclic voltammetric time scale. The electrocatalytic generation of CO and formate is hampered by the irreversible formation of anionic tricarbonyl complexes replacing reactive [Mo(η3-2-methallyl)(6,6′-dmbipy)(CO)2]2 along the cathodic route.
Highlights
There is a strong interest in the electrocatalytic reduction of CO2 that offers a sustainable route to a variety of valuable chemical feedstocks for organic synthesis or chemical fuel
The original reports have mostly focused on complexes based on rare and precious metals, such as rhenium in [Re(bipy)(CO)3Cl], where the active catalyst is the 2e−-reduced 5-coordinate anion [Re(bipy)(CO)3]−.3−7 The costs associated with such materials directed current research efforts toward Earth-abundant metals, such as Mn. [Mn(bipy)(CO)3]− has only recently been identified as a catalyst in the presence of small amounts of Brønsted acids.8−11 catalysts with impressive performance based on Earth-abundant first-row transition metals, such as Fe, Co and Ni, are widely known,12 much less attention has been paid to the Group 6 metals (Cr, Mo, W)
The ligand pTol-Bian was prepared according to a literature procedure involving the condensation reaction of acenaphthenequinone and 2,6-dimethylaniline
Summary
There is a strong interest in the electrocatalytic reduction of CO2 that offers a sustainable route to a variety of valuable chemical feedstocks for organic synthesis or chemical fuel. Transition-metal complexes have been identified as highly effective catalysts for the 2e− reduction of CO2, allowing one to take advantage of energy-saving proton-coupled pathways.. The original reports have mostly focused on complexes based on rare and precious metals, such as rhenium in [Re(bipy)(CO)3Cl] (bipy = 2,2′-bipyridine), where the active catalyst is the 2e−-reduced 5-coordinate anion [Re(bipy)(CO)3]−.3−7 The costs associated with such materials directed current research efforts toward Earth-abundant metals, such as Mn. [Mn(bipy)(CO)3]− has only recently been identified as a catalyst in the presence of small amounts of Brønsted acids.− catalysts with impressive performance based on Earth-abundant first-row transition metals, such as Fe, Co and Ni, are widely known, much less attention has been paid to the Group 6 metals (Cr, Mo, W).
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