Dynamic processes in hydrido-carbonyl trirhenium clusters containing bridging nitrogen heterocyclic ligands: An NMR investigation

The dynamic processes occurring in the triangular clusters [Re₃(μ-H)₃(μ-pz-κN¹:κN²)(CO)₁₀]⁻ (pz = pyrazolate, 4), [Re₃(μ-H)₂(μ-pydz-κN¹:κN²)(CO)₁₀] ⁻ (pydz = pyridazine, 5) and [Re₃(μ-H)₃(μ-pydz-κN¹:κN²)(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 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 Re₂(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:κC⁶)(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 C_s symmetry in the molecule. The process has been monitored by both ¹H and ¹³C analysis, affording quite similar activation parameters (E_a = 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.