Herein, we present
the cathodic paths of the Group-7 metal complex
[Re(3,3′-DHBPY)(CO)3Cl] (3,3′-DHBPY = 3,3′-dihydroxy-2,2′-bipyridine)
producing a moderately active catalyst of electrochemical reduction
of CO2 to CO. The combined techniques of cyclic voltammetry
and IR/UV–vis spectroelectrochemistry have revealed significant
differences in the chemistry of the electrochemically reduced parent
complex compared to the previously published Re/4,4′-DHBPY
congener. The initial irreversible cathodic step in weakly coordinating
THF is shifted toward much less negative electrode potentials, reflecting
facile reductive deprotonation of one hydroxyl group and strong intramolecular
hydrogen bonding, O–H···O–. The latter process occurs spontaneously in basic dimethylformamide
where Re/4,4′-DHBPY remains stable. The subsequent reduction
of singly deprotonated [Re(3,3′-DHBPY-H+)(CO)3Cl]− under ambient conditions occurs at
a cathodic potential close to that of the Re/4,4′-DHBPY-H+ derivative. However, for the stabilized 3,3′-DHBPY-H+ ligand, the latter process at the second cathodic wave is
more complex and involves an overall transfer of three electrons.
Rapid potential step electrolysis induces 1e–-reductive
cleavage of the second O–H bond, triggering dissociation of
the Cl– ligand from [Re(3,3′-DHBPY-2H+)(CO)3Cl]2–. The ultimate product
of the second cathodic step in THF was identified as 5-coordinate
[Re(3,3′-DHBPY-2H+)(CO)3]3–, the equivalent of classical 2e–-reduced [Re(BPY)(CO)3]−. Each reductive deprotonation of the
DHBPY ligand results in a redshift of the IR ν(CO) absorption
of the tricarbonyl complexes by ca. 10 cm–1, facilitating
the product assignment based on comparison with the literature data
for corresponding Re/BPY complexes. The Cl– dissociation
from [Re(3,3′-DHBPY-2H+)(CO)3Cl]2– was proven in strongly coordinating butyronitrile.
The latter dianion is stable at 223 K, converting at 258 K to 6-coordinate
[Re(3,3′-DHBPY-2H+)(CO)3(PrCN)]3–. Useful reference data were obtained with substituted parent [Re(3,3′-DHBPY)(CO)3(PrCN)]+ that also smoothly deprotonates by the
initial reduction to [Re(3,3′-DHBPY-H+)(CO)3(PrCN)]. The latter complex ultimately converts at the second
cathodic wave to [Re(3,3′-DHBPY-2H+)(CO)3(PrCN)]3– via a counterintuitive ETC step generating
the 1e– radical of the parent complex, viz., [Re(3,3′-DHBPY)(CO)3(PrCN)]. The same alternative reduction path is also followed
by [Re(3,3′-DHBPY-H+)(CO)3Cl]− at the onset of the second cathodic wave, where the ETC step results
in the intermediate [Re(3,3′-DHBPY)(CO)3Cl]•– further reducible to [Re(3,3′-DHBPY-2H+)(CO)3]3– as the CO2 catalyst.
