Group 6 Metal Complexes as Electrocatalysts of CO2 Reduction: Strong Substituent Control of the Reduction Path
of [Mo(η3‑allyl)(CO)2(x,x′‑dimethyl-2,2′-bipyridine)(NCS)]
(x = 4–6)
Posted on 2018-12-07 - 19:21
A series
of complexes [Mo(η3-allyl)(CO)2)(x,x′-dmbipy)(NCS)] (dmbipy
= dimethyl-2,2′-bipyridine; x = 4–6)
have been synthesized and their electrochemical reduction investigated
using combined cyclic voltammetry (CV) and variable-temperature spectroelectrochemistry
(IR/UV-vis SEC) in tetrahydrofuran (THF) and butyronitrile (PrCN),
at gold and platinum electrodes. The experimental results, strongly
supported by density functional theory (DFT) calculations, indicate
that the general cathodic path of these Group 6 organometallic complexes
is closely related to that of the intensively studied class of Mn
tricarbonyl α-diimine complexes, which, themselves, have recently
been identified as important smart materials for catalytic CO2 reduction. The dimethyl substitution on the 2,2′-bipyridine
ligand backbone has presented new insights into this emerging class
of catalysts. For the first time, the 2e– reduced
5-coordinate anions [Mo(η3-allyl)(CO)2)(x,x′-dmbipy)]− were directly observed with infrared spectroelectrochemistry (IR
SEC). The role of steric and electronic effects in determining the reduction-induced
reactivity was also investigated. For the 6,6′-dmbipy, the
primary 1e– reduced radical anions exert unusual
stability, radically changing the follow-up cathodic path. The 5-coordinate
anion [Mo(η3-allyl)(CO)2)(6,6′-dmbipy)]− remains stable at low temperature in strongly coordinating
butyronitrile and does not undergo dimerization at elevated temperature,
in sharp contrast to reactive [Mo(η3-allyl)(CO)2)(4,4′-dmbipy)]− that tends to dimerize
in a reaction with the parent complex. The complex with the 5,5′-dmbipy
ligand combines both types of reactivity. Under aprotic conditions,
the different properties of [Mo(η3-allyl)(CO)2)(x,x′-dmbipy)]− are also reflected in their reactivity toward CO2. Preliminary CV and IR SEC results reveal differences in
the strength of CO2 coordination at the free axial position.
Catalytic waves attributed to the generation of the 5-coordinate anions
were observed using CV, but only a modest catalytic performance toward
the production of formate was demonstrated by IR SEC. For 6,6′-dmbipy,
a stronger catalytic effect was observed for the Au cathode, compared
to Pt.
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Taylor, James
O.; Veenstra, Florentine L. P.; Chippindale, Ann M.; Calhorda, Maria José; Hartl, František (2018). Group 6 Metal Complexes as Electrocatalysts of CO2 Reduction: Strong Substituent Control of the Reduction Path
of [Mo(η3‑allyl)(CO)2(x,x′‑dimethyl-2,2′-bipyridine)(NCS)]
(x = 4–6). ACS Publications. Collection. https://doi.org/10.1021/acs.organomet.8b00676
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AUTHORS (5)
JT
James
O. Taylor
FV
Florentine L. P. Veenstra
AC
Ann M. Chippindale
MC
Maria José Calhorda
FH
František Hartl