Spectra and Synthetic-Time-Scale Substitution Reactions of Electrochemically Produced [Cr(CO)3(η6-arene)]+ Complexes

The anodic electrochemical behavior of the piano-stool complexes Cr(CO)(3)(eta(6)-arene), arene benzene (1) or (3-benzyloxy)- 17-hydroxyestra-,3,5(10)-triene (2), has been studied in dichloromethane containing either [B(C6F5)(4)](-) (TFAB) or [B(C6H3(CF3)(2))(4)](-) (BArF24) as the supporting electrolyte anion. Contrary to the behavior of these systems in electrolytes containing the smaller, more traditional, anions such as [PF6](-) or [ClO4]-, the radical cations 1(+) and 2(+) are stable on a bulk electrolysis time scale (30 min or more), allowing systematic study of their spectroscopic and chemical characteristics. IR and ESR spectroscopy confirm that the oxidations of 1 and 2 are highly metal-based. The radical cations were shown to undergo rapid substitution of a single CO by 1313113 (in the case of 1(+)) or P(OR)(3), R = Me or Ph (in the case of 2(+)). Cathodic rereduction of the oxidized substitution products gave high in situ yields of the neutral substituted complexes. This overall "electrochemically switched" ligand substitution process, in which the reactivity of the metal carbonyl complex is switched on and off according to the applied potential, provides an alternative synthetic route to Cr(CO)(2)L(eta(6)-arene) complexes. Approximate 100 mg quantities of Cr(CO)(2)-(PPh3)(eta(6)-C6H6) were prepared by the electrochemical switch method.