Ruthenium Polyhydrido Clusters Having a Bridging Alkylzinc Group, [(η5-C5Me5)Ru(μ-H)]3(μ3-ZnR)n(μ3-H)2-n and [(η5-C5Me5)Ru]2(μ-ZnR)n(μ-H)4-n (R = Me and Et; n = 1 and 2)

Novel triruthenium polyhydrido complexes having triply bridging alkylzinc ligands, {Cp‘Ru(μ-H)}3(μ3-ZnR)(μ3-H) (3a: R = Me, 3b: R = Et) and {Cp‘Ru(μ-H)}3(μ3-ZnEt)2 (4a: R = Me, 4b: R = Et), are obtained by the reaction of {Cp‘Ru(μ-H)}3(μ3-H)2 (2) with a stoichiometric amount of the corresponding dialkylzinc ZnR2. X-ray diffraction studies of 3b and 4a display the structural identity of those that have a triply bridging alkylzinc ligand. The X-ray studies as well as 1H NMR spectroscopy reveal the existence of bonding interactions between the μ3-zinc and the Ru-bound hydrido ligands in 3 and 4. In the variable-temperature 1H NMR study of 3b, activation parameters, ΔS⧧ and ΔH⧧, for the coordination-site exchange process among the hydrido ligands were estimated at −1.5 ± 1.3 cal mol-1 K-1 and 15.0 ± 0.4 kcal mol-1, respectively. The reaction of a diruthenium tetrahydrido complex, (Cp‘Ru)2(μ-H)4 (1), with an equimolar amount of dialkylzinc yields (Cp‘Ru)2(μ-ZnR)(μ-H)3 (5a: R = Me, 5b: R = Et). Di-μ-alkylzinc complexes, (Cp‘Ru)2(μ-ZnR)2(μ-H)2 (6a: R = Me, 6b: R = Et), are formed in the reaction of 1 with twice the molar amount of dialkylzinc. The crossover reaction between 5b and 1 equiv of dimethylzinc proved the cis-geometry of the bridging alkylzinc ligands in 6. The variable-temperature 1H NMR experiment of 5 demonstrated fluxionality of the hydrido ligands, and the activation parameters were estimated at ΔS⧧ = −5.8 ± 2.0 cal mol-1 K-1 and ΔH⧧ = 11.0 ± 0.5 kcal mol-1 for 5a and ΔS⧧ = −12.4 ± 1.8 cal mol-1 K-1 and ΔH⧧ = 9.6 ± 0.5 kcal mol-1 for 5b by the line-shape analysis.