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

Abstract

The electrochemical and chemical reduction of CoCp(1,5-COD) (1, Cp = η 5 -C 5 H 5 ; COD = cyclooctadiene) has been reinvestigated in THF. Whereas the initially formed monoanion 1 -(E 1/2 = -3.01 V vs Fc, Fc = FeCp 2 0/þ ) 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(η 3 -C 8 H 13 ) (2, E 1/2 = -2.17 V), formed by the reaction of 1with adventitious proton donors. The η 1 :η 3 -bonded isomer of 1, CoCp(η 1 :η 3 -C 8 H 12 ) (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 2anion undergoes further reaction with proton donors, which results in the formation of another electroactive product, 4, which is assigned the structure [CoCp(η 3 -C 8 H 13 )H] on the basis of NMR evidence and DFT calculations.