Abstract
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-[L(2)Mn(CO)(3)H] (1; L = P(OPh)(3), 2; L = PPh(3)) and fac-[(L-L')Mn(CO)(3)H] (3, L-L' = Ph(2)PCH(2)PPh(2) (dppm); 4, L-L' = Ph(2)PCH(2)-NHC) with organic bases and Lewis acid (B(C(6)F(5))(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 CH(2)-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))(2)Mn(CO)(3)H] (1) < mer,trans-[(PPh(3))(2)Mn(CO)(3)H] (2) ≈ fac-[(dppm)Mn(CO)(3)H] (3) < fac-[(Ph(2)PCH(2)NHC)Mn(CO)(3)H] (4), corresponding to the gain of the phosphorus ligand electron-donor properties.