The Dichotomy of Mn–H Bond Cleavage and Kinetic Hydricity of Tricarbonyl Manganese Hydride Complexes.

Acid-base characteristics (acidity, pKa, and hydricity, ΔG°_H− or k_H−) of metal hydride complexes could be a helpful value for forecasting their activity in various catalytic reactions. Polarity of the M–H bond may change radically at the stage of formation of a non-covalent adduct with an acidic/basic partner. This stage is responsible for subsequent hydrogen ion (hydride or proton) transfer. Here, the reaction of tricarbonyl manganese hydrides mer,trans–[L₂Mn(CO)₃H] (1; L = P(OPh)₃, 2; L = PPh₃) and fac–[(L–L′)Mn(CO)₃H] (3, L–L′ = Ph₂PCH₂PPh₂ (dppm); 4, L–L′ = Ph₂PCH₂–NHC) with organic bases and Lewis acid (B(C₆F₅)₃) was explored by spectroscopic (IR, NMR) methods to find the conditions for the Mn–H bond repolarization. Complex 1, bearing phosphite ligands, features acidic properties (pKa 21.3) but can serve also as a hydride donor (ΔG^≠_298K = 19.8 kcal/mol). Complex 3 with pronounced hydride character can be deprotonated with KHMDS at the CH₂–bridge position in THF and at the Mn–H position in MeCN. The kinetic hydricity of manganese complexes 1–4 increases in the order mer,trans–[(P(OPh)₃)₂Mn(CO)₃H] (1) < mer,trans–[(PPh₃)₂Mn(CO)₃H] (2) ≈ fac–[(dppm)Mn(CO)₃H] (3) < fac–[(Ph₂PCH₂NHC)Mn(CO)₃H] (4), corresponding to the gain of the phosphorus ligand electron-donor properties. [ABSTRACT FROM AUTHOR]