Proton reduction catalysis by manganese vinylidene and allenylidene complexes

Abstract

The present paper reports the unprecedented observation of a catalytic electrochemical proton reduction based on metallocumulene complexes. Manganese phenylvinylidene (η 5-C 5H 5)(CO)(PPh 3)Mn C C(H)Ph ( 1) and diphenylallenylidene (η 5-C 5H 5)(CO) 2Mn C C CPh 2 ( 3) are shown to catalyze the reduction of protons from HBF 4 into dihydrogen in CH 2Cl 2 or CH 3CN media at −1.60 and −0.84 V (in CH 3CN) vs. Fc, respectively. The working potential for 3 (−0.84 V vs. Fc in CH 3CN) 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 [(η 5-C 5H 5)(CO)(PPh 3)Mn C–CH 2Ph]BF 4 ([ 2]BF 4), [(η 5-C 5H 5)(CO) 2Mn C–CH CPh 2]BF 4 ([ 4]BF 4) followed by their reduction to the corresponding 19-electron radicals 2 , 4 , respectively. Both carbyne radicals undergo a rapid homolytic cleavage of the C β–H bond generating an H-radical producing molecular hydrogen with concomitant recovery of the neutral metallocumulenes thereby completing a catalytic cycle.