NMR and DFT analysis of [Re2H2(CO)9]:: evidence of an η2-H2 intermediate in a new type of fast mutual exchange between terminal and bridging hydrides

Protonation of the anion [Re2H(CO)9]-(1) with a strong acid at 193 K affords the neutral complex [Re2H2(CO)9] (2), that in THF above 253 K irreversibly loses H2 to give [Re2(CO)9(THF)], previously obtained by room-temperature protonation of 1. Treatment of 2 with NEt4OH restores the starting anion 1. Variable temperature 1H and 13C NMR spectra as well as T1 measurements agree with the formulation of 2 as a classical [Re2H(μ-H)(CO)9] complex, in which two dynamic processes takes place. The "windshield-wiper motion" observed in several related complexes equalizes the two carbonyls trans to the hydrides (Ea = 44(1) kJ mol-1), while another much faster process equalizes bridging and terminal hydrides already at 172 K. The variable temperature behavior of the 1H transverse relaxation times revealed also proton exchange between 2, water, and the parent anion 1 (due to the acidity of 2), but such a process is too slow to account for the fast hydrides exchange in 2. The nature of the latter process has been investigated both experimentally and theoretically. Kinetic data, obtained by the analysis of the variable temperature 1H spectra (Ea = 24,5(5) kJ mol-1), revealed a small normal kinetic isotope effect (ca. 1.5). The 2H chemical shift of the fully deuterated isotopomer 2-d2 was found isochronous with 2, thus ruling out the presence of a significant concentration of a nonclassical [Re2(η2-H2)(CO)9]tautomer, in fast exchange with the classical dihydride. Density functional theory (DFT) calculations, carried out at the B3LYP level, confirmed the formulation of [Re2H2(CO)9] as a classical complex. However, when DFT was used to obtain a detailed description of the dynamic behavior of 2 in solution, a new type of hydride fast exchange emerged, involving the nonclassical tautomer as a relatively high energy (12.7 kJ mol-1) intermediate. Isotopic perturbation of the equilibrium by partial deuteration of 2 indicated the preference of deuterium for the bridging sites, with ΔH° = -475(4) J mol-1 and ΔS° = -0.80(2) J K-1 mol-1. The same preference was observed in the anion [Re2H(μ-H)Cl(CO)8]-.