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 N MR spectrum of [ReH5(PPh3)(2)(py)] (py = pyridine) are now firmly asso ciated with turnstile and pseudorotation fluxionality mechanisms by el iminating an alternative pairwise mechanism. Ab initio (B3LYP) calcula tions on ReH5(PH3)(2)L (L = pyridine) have located the transition stat e for the turnstile mechanism, which proves to be a second dodecahedra l tautomer of the starting complex with the pyridine in the normally u nfavorable A site. The fluxional process can therefore be considered a s an interconversion of two dodecahedral tautomers, and the barrier fo r the process is identical with the energy difference of the two tauto mers. From a comparison in ReH5(PPh3)(2)L (L = 2-(acetylamino)pyridine and 4-(acetylamino)pyridine), it is clear that having a potentially h ydrogen-bonding NH group at the ortho or para positions of the pyridin e ring causes an acceleration of the fluxionality, as a result of intr amolecular 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 f actors: hydrogen bonding, which lowers the barrier for the 2-amino com pound, and H ... H repulsion resulting from an excessively close appro ach of the two H atoms in the transition state, which raises the barri er. This implies that the particular hydrogen-bonding ligands chosen w ere too rigid for optimal rate acceleration.