Electroreduction of carbonylmanganese(I) cations. Mechanism of ligand substitution and hydride formation via manganese(0) intermediates

Article abstract of DOI:10.1021/om00144a023

Hydridomanganese(I) complexes (OC)₃MnL₂H are the principal products from a series of acetonitrile derivatives of carbonylmanganese cations, i.e., (OC)₃Mn(NCMe)_nL_3-n⁺ (I), when the reductions are carried out electrochemically in the presence of added phosphines L. Cyclic voltammetric studies show that the conversion to the hydride can be formulated in three discrete stages. First, the carbonylmanganese cation such as (OC)₃Mn(NCMe)₃⁺ with n = 3 undergoes an electrocatalytic ligand substitution with added L to form the bis(phosphine) complex (OC)₃Mn(NCMe)L₂⁺ via labile 19-electron Mn(0) intermediates as in Scheme V. Second, the substitution product (OC)₃Mn(NCMe)L₂⁺ is reduced to the Mn(0) radical (OC)₃Mn(NCMe)L_2?; which is common to all cationic precursors irrespective of the degree of prior phosphine coordination in the precursor I (i.e., n = 1, 2). Third, the hydridomanganese product is derived by hydrogen atom transfer to the carbonyl ligand of the 19-electron intermediate (OC)₃Mn(NCMe)L_2? to form the formyl complex (OC)₂Mn(NCMe)L₂CHO, followed by the electrocatalytic extrusion of the coordinated ligand (MeCN) as in Scheme III. The third stage is established in the reductive conversion of the cationic (OC)₄Mn(PPh₃)₂⁺ to the hydride (OC)₃Mn(PPh₃)₂H by the observation of the intermediate (OC)₃Mn(PPh₃)₂CHO during cyclic voltammetry. A competition to form the carbonylmanganate (OC)₃MnL₂^- as a byproduct of reduction is also delineated.