Abstract:
A series of ferrocenyl and thienyl mono- and biscarbene chromium(0) complexes 1–6 were synthesised.
The complexes were characterised both spectroscopically and electrochemically, and the single crystal
X-ray structure of 3 was determined. Electrochemical measurements in CH2Cl2 revealed that the carbene
double bond of 1–6 is reduced to an anion radical, −Cr–C˙ at formal reduction potentials <−1.7 V vs.
FcH/FcH+. A computational study on 1, 3 and 4 (B3LYP/def2-SVP level) is consistent with electrochemical
results in showing that electrochemically generated chromium(I) species may be further electrochemically
irreversibly oxidised to chromium(II) at Epa > 0.95 V. The reactivity towards follow-up chemical reactions of
the anodically produced Cr(II) species is much higher than the reactivity of the cathodically produced
radical anions as the latter was still observably reoxidised to the parent CrvC species at fast scan rates.
The ferrocenyl group is oxidised electrochemically reversibly to ferrocenium at larger potentials than the
electrochemically reversible oxidation of the Cr(0) centre to Cr(I). That all redox centres in 1–6 are involved
in one-electron transfer steps was confirmed by comparing the ferrocenyl voltammetric wave with those
of the other redox centres in linear sweep voltammetric experiments. The ferrocenyl group was electrochemically
shown to stabilise the CrvC centre almost as much as the NHBu, and much more than the
ethoxy and thienyl groups.
