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 CH 2 Cl 2 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 E pa > 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.
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New examples of the very rare class
of iridium(I) Fischer carbene complexes (FCCs) are reported from the
facile transmetalation from group 6 FCCs. Postcomplexation modification
of either the carbene ligand or the ancillary coligands results in
a tunable IrI metal center, for unprecedented application
as a (pre)catalyst in a benchmark transfer hydrogenation reaction.
The introduction of an aminocarbene ligand with a pendant N-donor
moiety capable of hemilabile coordination yielded the best catalytic
results with turnover frequencies reaching 445 h–1 and requiring 0.1 mol % catalyst and 0.5 mol % base loading, respectively.
The series [(CO) Electrochemical experiments on 1 were also consistent with this redox sequence of events, but differently, our calculations suggest that for 1, the ferrocenyl group may be oxidised before Cr (0) oxidation. The ethoxycarbenes 1 and 3 also showed a carbene double bond reduction to an anion 2 radical, Cr=C , while the new aminocarbene 4 was the only derivative to show irreversible Cr(I) oxidation to Cr(II) within the potential window of the solvent.
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