Dynamic Processes in Hydrido-Carbonyl Trirhenium Clusters Containing Bridging Nitrogen Heterocyclic Ligands: an NMR Investigation

The dynamic processes occurring in the triangular clusters [Re-3(mu-H)(3)(mu-pz-kappa N-1:kappa N-2)(CO)(10)](-) (pz = pyrazolate, 4), [Re-3(mu-H)(2)(mu-pydz-kappa N-1:kappa N-2)(CO)(10)](-) (pydz = pyridazine, 5) and [Re-3(mu-H)(3)(mu-pydz-kappa N-1:kappa N-2)(CO)(10)] (6), have been investigated by H-1 and C-13 NMR. In the pyrazolate derivative 4 the exchange (k approximate to 1 s(-1) at 320 K) between the two carbonyls in the trans-diaxial apical positions has been recognized, and its activation parameters, in C2D2Cl4, have been determined (E-a = 68(3) kJ mol(-1)). The exchange has been attributed to the rotation of the apical H2Re(CO)(4) fragment with respect to the Re-2(mu-pz) basal fragment, a process analogous to that previously observed in the unsaturated dianion [Re-3(mu-H)(3)(CO)(10)](2-) (2) and in the monoanion [Re-3(mu-H)(3)(mu-NC5H4-kappa N-1:kappa C-6)(CO)(10)](-) (1), containing a bridging orthometallated pyridine ligand. The vertex rotation was not observed in the pyridazine derivatives 5 and 6. An explanation for this different behaviour is presented, based on the view of the fluxional clusters 1, 2 and 4 as adducts between the apical and basal moieties (pi-or sigma-complexes). The Delta G(312K)(#) value here measured in acetone for the sigma-complex 4 (77 kJ mol(-1)) is very similar to that previously determined for the other sigma-complex 1 (Delta G(305K)(#) = 76 kJ mol(-1)) and significantly higher than the values measured for the pi-complex 2 (Delta G(260K)(#) = 60 kJ mol(-1)). The di-hydrido derivative 5 shows a different much faster dynamic process, namely the hopping of one hydride between the two lateral edges, affording a pseudo C-s symmetry in the molecule. The process has been monitored by both H-1 and C-13 analysis, affording quite similar activation parameters (E-a = 44(1) and 45(1) kJ mol(-1), respectively, in THF-d(8)), that did not significantly change in CD2Cl2 solution, in agreement with an intramolecular process. (C) 2011 Elsevier B. V. All rights reserved.