Proton reduction catalysis by manganese vinylidene and allenylidene complexes

The present paper reports the unprecedented observation of a catalytic electrochemical proton reduction based on metallocumulene complexes. Manganese phenylvinylidene (η5-C5H5)(CO)(PPh3)Mndouble bond; length as m-dashCdouble bond; length as m-dashC(H)Ph (1) and diphenylallenylidene (η5-C5H5)(CO)2Mndouble bond; length as m-dashCdouble bond; length as m-dashCdouble bond; length as m-dashCPh2 (3) are shown to catalyze the reduction of protons from HBF4 into dihydrogen in CH2Cl2 or CH3CN media at −1.60 and −0.84 V (in CH3CN) vs. Fc, respectively. The working potential for 3 (−0.84 V vs. Fc in CH3CN) is the lowest reported to date for protonic acids reduction in non-aqueous media. The similar catalytic cycles disclosed here include the protonation of 1, 3 into the carbyne cations [(η5-C5H5)(CO)(PPh3)Mntriple bond; length of mdashC–CH2Ph]BF4 ([2]BF4), [(η5-C5H5)(CO)2Mntriple bond; length of mdashC–CHdouble bond; length as m-dashCPh2]BF4 ([4]BF4) followed by their reduction to the corresponding 19-electron radicals 2radical dot, 4radical dot, respectively. Both carbyne radicals undergo a rapid homolytic cleavage of the Cβ–H bond generating an H-radical producing molecular hydrogen with concomitant recovery of the neutral metallocumulenes thereby completing a catalytic cycle.