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(μ-H)3(μ-pz-κN1:κN 2)(CO)10]- (pz = pyrazolate, 4), [Re 3(μ-H)2(μ-pydz-κN1:κN 2)(CO)10] - (pydz = pyridazine, 5) and [Re 3(μ-H)3(μ-pydz-κN1:κN 2)(CO)10] (6), have been investigated by 1H and 13C NMR. In the pyrazolate derivative 4 the exchange (k ≈ 1 s-1 at 320 K) between the two carbonyls in the trans-diaxial apical positions has been recognized, and its activation parameters, in C 2D2Cl4, have been determined (Ea = 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(μ-pz) basal fragment, a process analogous to that previously observed in the unsaturated dianion [Re3(μ-H)3(CO) 10] 2- (2) and in the monoanion [Re3(μ-H) 3(μ-NC5H4-κN1:κC 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 (π- or σ-complexes). The ΔG#312K value here measured in acetone for the σ-complex 4 (77 kJ mol-1) is very similar to that previously determined for the other σ-complex 1 (ΔG#305K = 76 kJ mol-1) and significantly higher than the values measured for the π-complex 2 (ΔG#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 Cs symmetry in the molecule. The process has been monitored by both 1H and 13C analysis, affording quite similar activation parameters (Ea = 44(1) and 45(1) kJ mol -1, respectively, in THF-d8), that did not significantly change in CD2Cl2 solution, in agreement with an intramolecular process.