Site preference energetics, fluxionality, and intramolecular M-H center dot center dot center dot H-N hydrogen bonding in a dodecahedral transition metal polyhydride

Two successive decoalescence events in the hydride region of the H-1 NMR spectrum of [ReH5(PPh3)(2)(py)] (py = pyridine) are now firmly associated with turnstile and pseudorotation fluxionality mechanisms by eliminating an alternative pairwise mechanism. Ab initio (B3LYP) calculations on ReH5(PH3)(2)L (L = pyridine) have located the transition state for the turnstile mechanism, which proves to be a second dodecahedral tautomer of the starting complex with the pyridine in the normally unfavorable A site. The fluxional process can therefore be considered as an interconversion of two dodecahedral tautomers, and the barrier for the process is identical with the energy difference of the two tautomers. From a comparison in ReH5(PPh3)(2)L (L = 2-(acetylamino)pyridine and 4-(acetylamino)pyridine), it is clear that having a potentially hydrogen-bonding NH group at the ortho or para positions of the pyridine ring causes an acceleration of the fluxionality, as a result of intramolecular Re-H ... H-N hydrogen bonding. The theoretical calculations on ReH5(PH3)(2)L (L = 2-aminopyridine and 4-aminopyridine) show that the experimental barriers are the result of a compromise between two factors: hydrogen bonding, which lowers the barrier for the 2-amino compound, and H ... H repulsion resulting from an excessively close approach of the two H atoms in the transition state, which raises the barrier. This implies that the particular hydrogen-bonding ligands chosen were too rigid for optimal rate acceleration.