Metal fragment rotation in triangular metal clusters: H-1, H-2, and C-13 NMR studies of the fluxional behavior of the anion {[Re-3(mu-H)(3)(mu-eta(2)-NC5H4)(CO)(10)](-)}

Variable-temperature C-13 NMR spectroscopy, 1D SPT, and 2D EXSY experiments have been used to investigate the fluxionality of the triangular cluster anion {[Re-3(mu-H)(3)(mu-eta(2)-NC5H4)(CO)(10)](-). The (CO)-C-13 resonances have been assigned through H-1-C-13 2D HMQC and C-13 2D DQF-COSY experiments and through the measurement of the (CO)-C-13 longitudinal relaxation times. The exchange pattern observed in the C-13 2D EXSY experiments proved the occurrence of a synchronous axial-axial and equatorial-equatorial carbonyl exchange on the apical metal. The rate constants at 303 and 313 K compare well with those previously measured, through H-1 NMR spectroscopy, for the exchange of the two hydrides bridging the lateral edges of the cluster. The rigid rotation of the whole apical [H2Re(CO)(4)] moiety around an axis passing through the midpoint of the basal bimetallic fragment is in agreement with these results. A deuterium kinetic isotope effect k(H)/k(D) = 1.45(24) has been measured through H-1 and H-2 2D EXSY experiments, supporting the involvement of the hydrides in the rate-determining step of the exchange process.}