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 (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 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 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.

A dynamic NMR investigation has shown that triangular rhenium clusters containing a nitrogen heterocyclic ligand bound onto a Re3(μ-H)3(CO)10 scaffold can behave either as rigid bodies or as dynamic adducts. Mutual rotation of the vertex with respect to the base of the triangle occurs with pyrazolate or pyridinate ligand, but not with pyridazine.Figure optionsDownload as PowerPoint slideHighlights
► A triangular Re3(μ-H)3 scaffold can behave either as a rigid body or a dynamic adduct.
► Dynamic 13C NMR reveals mutual rotation of monometallic and bimetallic fragments.
► The structural lability depends on the nature of the bridging heterocyclic ligand.