Electrochemical Oxidation of [Dibenzo-18-crown-6-K][Cr(CO)₅F] and Cr(CO)₆: Characterization of [Cr(CO)₆]₊ and Its Substitution Reactions with Fluoride and Trifluoroacetate

Electrochemical oxidation of the carbonyl fluoride complex [dibenzo-18-crown-6-K][Cr(CO)₅F] has been studied in dichloromethane, acetone, and acetonitrile to establish whether electrochemical synthesis of relatively rare higher oxidation state carbonyl fluoride complexes may be achieved. Thermodynamically, Cr(CO)₅F is more stable than Cr(CO)₅X (X = Cl, Br, I) but kinetically more reactive. At -70 °C, the formally chromium I complex Cr(CO)₅F is moderately stable on the synthetic time scale in dichloromethane, while the formally chromium(II) species [Cr(CO)₅F]₊can be observed on the electrochemical time scale at the same temperature. ESR data indicate that Cr(CO)₅F decomposes to the very reactive 17-electron species [Cr(CO)₆]₊which is only moderately stable in dichloromethane at -70 °C. [Cr(CO)₆]₊can also be identified as a product of controlled potential electrolysis of Cr(CO)₆in dichloromethane at -80 °C by a well-defined ESR spectrum consisting of a strong line from the ₅₂Cr (I = 0) nucleus and four weaker lines from the less abundant ₅₃Cr nucleus (I = ₃/₂). The data contrast to some literature reports suggesting that Cr(CO)₆cannot be oxidized in dichloromethane prior to the solvent limit and to data in acetonitrile where a broad ESR signal with a comparatively low g value has been reported. Oxidation of Cr(CO)₆in the presence of fluoride does not lead to formation of Cr(CO)₅F or [Cr(CO)₅F]₊. Instead, evidence for formation of a bridged complex of the kind Cr₂(CO)₁₀(M-F) has been obtained. A reinvestigation of oxidation of Cr(CO)₆in the presence of trifluoroacetate demonstrates that the previously noted report of irreversibility cannot be explained by ligand substitution and concomitant slow electron transfer. Rather, the reversible one-electron oxidation step is transformed into a chemically irreversible two-electron oxidation process involving complete loss of carbon monoxide. On the basis of the above data, prospects for electrochemical methods of synthesis of the relatively rare carbonyl fluoride complexes are discussed and successfully applied to the synthesis of Mo(CO)₂(dpe)₂F by electrochemical oxidation of cis-Mo(CO)₂(dpe)₂in the presence of fluoride (dpe = 1,2-bis(diphenylphosphino)ethane).