Hydrogen–deuterium exchange in hydride chemistry: Dihydrogen bonded complexes as key intermediates

The hydrogen–deuterium exchange mechanism is analyzed at the DFT/B3LYP level for main group element and transition metal hydride compounds (decahydro-closo-decaborate anion, [B10H10]2−, and Cp*Mo(CO)(PMe3)2H, where Cp* = η5-C5Me5) using methanol as deuterium source. The effect of bulk methanol was represented by CPCM calculations combined with the representation of discreet alcohol as the trimer, (CH3OD)3. This model helped to solve the problem of calculation of the reaction Gibbs energy in solution. The dihydrogen bond formation is shown being the first reaction step and determines the selectivity of [1,10-B10H8D2]2− formation. The transition state for the H/D exchange resembles the non-classical (η2-H2) complex and is similar to that for metal hydrides protonation. The evolution of the reaction energy profile with the acid strength is analyzed, showing the gradual transition from the proton-hydride (H/D) exchange to complete proton transfer to hydride ligand. The possibility of η2-H2 complex formation and its stability depend on the properties of both metal atom and proton donor.