Proton reduction catalysis by manganese vinylidene and allenylidene complexes Dmitry A. Valyaev a , Mikhail G. Peterleitner a , Oleg V. Semeikin a , Kamil I. Utegenov a , Nikolai A. Ustynyuk a, * , Alix Sournia-Saquet b , Noe ¨l Lugan b , Guy Lavigne b a A.N. Nesmeyanov Institute of Organoelement Compounds (INEOS), Russian Academy of Sciences, 28 Vavilov Street, 119991 Moscow, Russia b Laboratoire de Chimie de Coordination du CNRS, 205 route de Narbonne, 31077 Toulouse Cedex 4, France Received 18 December 2006; received in revised form 30 January 2007; accepted 30 January 2007 Available online 8 February 2007 Abstract The present paper reports the unprecedented observation of a catalytic electrochemical proton reduction based on metallocumulene complexes. Manganese phenylvinylidene (g 5 -C 5 H 5 )(CO)(PPh 3 )Mn@C@C(H)Ph (1) and diphenylallenylidene (g 5 -C 5 H 5 )(CO) 2 - Mn@C@C@CPh 2 (3) are shown to catalyze the reduction of protons from HBF 4 into dihydrogen in CH 2 Cl 2 or CH 3 CN media at 1.60 and 0.84 V (in CH 3 CN) vs. Fc, respectively. The working potential for 3 (0.84 V vs. Fc in CH 3 CN) 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 [(g 5 -C 5 H 5 )(CO)(PPh 3 )Mn„C–CH 2 Ph]BF 4 ([2]BF 4 ), [(g 5 -C 5 H 5 )(CO) 2 Mn„C–CH@CPh 2 ]BF 4 ([4]BF 4 ) followed by their reduction to the corresponding 19-electron radicals 2 Å , 4 Å , respectively. Both carbyne radicals undergo a rapid homolytic cleavage of the C b –H bond generating an H-radical producing molecular hydrogen with concomitant recovery of the neutral metallocumulenes thereby completing a catalytic cycle. Ó 2007 Elsevier B.V. All rights reserved. Keywords: Proton reduction catalysis; Vinylidene complexes; Allenylidene complexes; Carbyne complexes; Cyclic voltammetry 1. Introduction Hydrogen production by water electrolysis is an impor- tant challenge of both fundamental and economical signif- icance in the quest for alternative energy sources. Though Pt-based catalysts are known to achieve such a reaction, their replacement by cheaper homogeneous transition metal catalysts is becoming highly desirable [1]. To date, only hydrogenases effectively catalyze this process with TOF up to 10 3 –10 4 molecules H 2 s 1 per site [2]. However, the known biomimetic models for the active site of [Fe]- only hydrogenases based on iron dithiolate clusters (l-SCH 2 XCH 2 S-l)Fe 2 (CO) 6 (X = CH 2 [3],O [4], NR [5]) are much less active than the native enzymes and are char- acterized by an excessive overvoltage for hydrogen production. To date, all proposed catalytic cycles for electrochemical proton reduction involve the formation of transition metal hydrido intermediates and their subsequent transformation resulting in dihydrogen elimination (Scheme 1). Depending on the catalyst structure such processes can proceed via various catalytic pathways differing in the sequences of the electrochemical and chemical elemental steps. For complexes containing polydentate nitrogen ligands [6] and [Fe]-only hydrogenase model compounds (l- SCH 2 XCH 2 S-l)Fe 2 (CO) 6 (X = CH 2 , O) [3b,3c,3d,4] of low basicity the reduction step (c) producing anionic com- plexes precedes the protonation step (d) that affords termi- nal hydride species [M]H. The reverse order – protonation (a) preceding reduction (b) – was encountered for [1.1]fer- rocenophane [7] and the anionic iron complex [Fe(CO) 2 (PMe 3 )(l-S(CH 2 ) 3 S-l)Fe(CO) 2 CN]  [3a]. In the latter case 0022-328X/$ - see front matter Ó 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.jorganchem.2007.01.055 * Corresponding author. Tel.: +7 495 135 5064; fax: +7 495 135 5085. E-mail address: ustynyuk@ineos.ac.ru (N.A. Ustynyuk). www.elsevier.com/locate/jorganchem Journal of Organometallic Chemistry 692 (2007) 3207–3211