Synthesis and electrochemical investigation of chromium(0) ferrocenyl-substituted carbene complexes

Abstract

The series [(CO)5CrC(R)Fc], (1, R=OEt; 2, R=NHnBu) as well as [(OC)5CrC(R)Fc′(R)CCr(CO)5], (3, R=OEt; 4, R=NHnPr) of mono- and biscarbene chromium(0) complexes with Fc=FeII(C5H5)(C5H4) and Fc′=FeII(C5H4)2, were synthesized and characterized spectroscopically, electrochemically and computationally. Electrochemical studies on the new aminocarbene complexes revealed that 2 and 4 are oxidized (and reduced) at lower potential than the EtO derivatives 1 and 3 while the biscarbene complexes 3 and 4 provided evidence for interaction between different redox sites, including the Cr-centers. The Cr0/I couples are electrochemically reversible with the difference in formal potentials ΔE°′ for the consecutive oxidations of the chromium carbene entities being 151 and 105mV in 3 and 4, respectively. Computational and electrochemical results were mutually consistent in showing unambiguously that the Cr(0) center is oxidized before the ferrocenyl group in the carbene complexes 2, 3 and 4. 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 oxidized before Cr(0) oxidation. The ethoxycarbenes 1 and 3 also showed a carbene double bond reduction to an anion radical, −CrC, 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.