1 research outputs found
Photocatalytic CO<sub>2</sub> Reduction by Trigonal-Bipyramidal Cobalt(II) Polypyridyl Complexes: The Nature of Cobalt(I) and Cobalt(0) Complexes upon Their Reactions with CO<sub>2</sub>, CO, or Proton
The
cobalt complexes Co<sup>II</sup>L1Â(PF<sub>6</sub>)<sub>2</sub> (<b>1</b>; L1 = 2,6-bisÂ[2-(2,2′-bipyridin-6′-yl)Âethyl]Âpyridine)
and Co<sup>II</sup>L2Â(PF<sub>6</sub>)<sub>2</sub> (<b>2</b>;
L2 = 2,6-bisÂ[2-(4-methoxy-2,2′-bipyridin-6′-yl)Âethyl]Âpyridine)
were synthesized and used for photocatalytic CO<sub>2</sub> reduction
in acetonitrile. X-ray structures of complexes <b>1</b> and <b>2</b> reveal distorted trigonal-bipyramidal geometries with all
nitrogen atoms of the ligand coordinated to the CoÂ(II) center, in
contrast to the common six-coordinate cobalt complexes with pentadentate
polypyridine ligands, where a monodentate solvent completes the coordination
sphere. Under electrochemical conditions, the catalytic current for
CO<sub>2</sub> reduction was observed near the CoÂ(I/0) redox couple
for both complexes <b>1</b> and <b>2</b> at <i>E</i><sub>1/2</sub> = −1.77 and −1.85 V versus Ag/AgNO<sub>3</sub> (or −1.86 and −1.94 V vs Fc<sup>+/0</sup>),
respectively. Under photochemical conditions with <b>2</b> as
the catalyst, [RuÂ(bpy)<sub>3</sub>]<sup>2+</sup> as a photosensitizer,
tri-<i>p</i>-tolylamine (TTA) as a reversible quencher,
and triethylamine (TEA) as a sacrificial electron donor, CO and H<sub>2</sub> were produced under visible-light irradiation, despite the
endergonic reduction of CoÂ(I) to Co(0) by the photogenerated [RuÂ(bpy)<sub>3</sub>]<sup>+</sup>. However, bulk electrolysis in a wet CH<sub>3</sub>CN solution resulted in the generation of formate as the major
product, indicating the facile production of Co(0) and [Co–H]<sup><i>n</i>+</sup> (<i>n</i> = 1 and 0) under electrochemical
conditions. The one-electron-reduced complex <b>2</b> reacts
with CO to produce [Co<sup>0</sup>L2Â(CO)] with ν<sub>CO</sub> = 1894 cm<sup>–1</sup> together with [Co<sup>II</sup>L2]<sup>2+</sup> through a disproportionation reaction in acetonitrile, based
on the spectroscopic and electrochemical data. Electrochemistry and
time-resolved UV–vis spectroscopy indicate a slow CO binding
rate with the [Co<sup>I</sup>L2]<sup>+</sup> species, consistent with
density functional theory calculations with CoL1 complexes, which
predict a large structural change from trigonal-bipyramidal to distorted
tetragonal geometry. The reduction of CO<sub>2</sub> is much slower
than the photochemical formation of [RuÂ(bpy)<sub>3</sub>]<sup>+</sup> because of the large structural changes, spin flipping in the cobalt
catalytic intermediates, and an uphill reaction for the reduction
to Co(0) by the photoproduced [RuÂ(bpy)<sub>3</sub>]<sup>+</sup>