Consequences of the Protonation of the 19-Electron Anion [Co(η5-C5H5)(1,5-C8H12)]

The electrochemical and chemical reduction of CoCp(1,5-COD) (1, Cp = η5-C5H5; COD = cyclooctadiene) has been reinvestigated in THF. Whereas the initially formed monoanion 1 − (E 1/2 = −3.01 V vs Fc, Fc = FeCp2 0/+) has been well-characterized electrochemically and by ESR spectroscopy, the long-term electrolysis product had been previously assigned as the isomerized anion [CoCp(1,3-COD)]−. New magnetic resonance data show that the ultimate product is the 17-electron cyclooctenyl complex CoCp(η3-C8H13) (2, E 1/2 = −2.17 V), formed by the reaction of 1 − with adventitious proton donors. The η1:η3-bonded isomer of 1, CoCp(η1:η3-C8H12) (3), reduces (E 1/2 = −2.97 V) to yield the same product 2 on the cyclic voltammetry (CV) time scale. Deliberate addition of the weak proton donor 2,4,6-trimethylphenol to solutions of 1 facilitates protonation and results in scan-rate- and concentration-dependent formation of 2 on the CV time scale as 1 is reduced in an ECE mechanism. Under these conditions, the reduced 2 − anion undergoes further reaction with proton donors, which results in the formation of another electroactive product, 4, which is assigned the structure [CoCp(η3-C8H13)H] on the basis of NMR evidence and DFT calculations.