Versatile Routes to Mono- and Bis(alkynyl) Manganese(II) and Manganese(III) Complexes via Manganocenes

With the manganocenes (RC 5 H 4 ) 2 Mn (R = H, Me) as starting materials, paramagnetic d 5 half-sandwich complexes (RC 5 H 4 )Mn(dmpe)(C⋮CR‘) (dmpe = 1,2-bis(dimethylphosphino)ethane; R = Me, R‘ = Ph, 1a; R = Me, R‘ = SiMe 3, 2a; R = H, R‘ = Ph, 1b; R = H, R‘ = SiMe 3, 2b ) have been prepared by the reaction with terminal or SnMe 3 -substituted acetylenes and dmpe. The derivatives 1a and 2a could be isolated, while 1b and 2b have been identified in mixtures with the bis(dmpe)bis(acetylide)manganese species 3 (R‘ = Ph) and 4 (R‘ = SiMe 3 ). The latter compounds are obtained as the sole products, when the manganocenes are reacted with dmpe and the corresponding acetylenes in a 1:2:2 ratio. 1a and 2a can be reversibly oxidized to the corresponding cationic Mn(III) complexes [(MeC 5 H 4 )Mn(dmpe)(C⋮CR‘)][BF 4 ] (R‘ = Ph, [ 1a ] +; R‘ = SiMe 3, [ 2b ] + ). Compounds 1a and 2a act as scavengers of H • radicals in the 1:1:1 reaction mixtures or in the presence of n -Bu 3 SnH or (C 5 Me 5 )Mo(CO) 3 H, forming the vinylidene species (MeC 5 H 4 )Mn(dmpe)( C C(H)R‘) (R‘ = Ph, 5; R‘ = SiMe 3, 6 ). In the absence of a special H • donor, 1a slowly dimerizes to give the binuclear complex 7 . A similar process occurs with [ 1a ] + to form the bis(carbyne) complex 8 . 8 can be reduced to 7 by a reaction with 2 equiv of (MeC 5 H 4 ) 2 Co, and in turn 7 can be oxidized to 8 with 2 equiv of a ferrocenium salt. Equal amounts of 7 and 8 comproportionate to afford the mixed-valence complex [ 7 ] + . If applicable, the new compounds have been characterized by spectroscopic (NMR, EPR, near-IR) and electrochemical measurements, as well as X-ray diffraction studies ( 2a, 5, 7, and 8 ) and DFT calculations.