Promoter effect of chloride ions on the ruthenium-catalyzed hydroesterification of ethylene with methyl formate. Design and evaluation of new poly- and mononuclear catalyst precursors

Article abstract of DOI:10.1021/om00004a027

The catalytic hydroesterification of ethylene with methyl formate to produce methyl propionate is shown to take place in the presence of trinuclear ruthenium carbonyl complexes modified by anionic nucleophiles such as amido (anilinopyridyl), alkoxy (pyridonate), or halide ligands, regarded as potential promoters. The best results (100% conversion, 99% selectivity) are obtained with the mixture Ru₃(CO)₁₂ + [PPN]Cl (1/1) under the following experimental conditions: [HCOOMe]/[cat.] = 345, DMF solvent, P(C₂H₄) = 20 atm (at 25°C), T = 160°C, time = 12 h. The complex [PPN][Ru₃(μ₃-Cl)(CO)₉] (3) resulting from the addition of [PPN]-Cl to Ru₃(CO)₁₂ reacts cleanly with an excess of [PPN]Cl in refluxing THF under a stream of inert gas to produce the unique dianionic species [PPN]₂[Ru₄(μ-Cl)₂(CO)₁₁] (4) (70% yield). The X-ray structure analysis of 4 is reported (triclinic P1, No. 2, a = 18.209(2) A?, b = 18.877-(3) A?, c = 13.895(2) A?, α = 110.70(1)°, β = 108.43(1)°, γ = 87.43(1)°, V = 4226(1) A?³, Z = 2, R = 0.053, R_w = 0.069). The dianionic tetranuclear unit of 4 consists of a basic triangular metal framework Ru₃(CO)₉ one face of which is supported by a spiked Ru(CO)₂Cl₂ fragment involving a 16e metal center exhibiting a square pyramidal geometry. Extended Hu?ckel MO calculations indicate a large HOMO/LUMO gap (1.45 eV). Facile loss of Cl^- from the above dianion is induced by capture of CO, leading to the known butterfly complex [PPN][Ru₄(μ-Cl)(CO)₁₃] (5). Complex 4 also reacts with O₂ at 25°C to provide the new oxo derivative [PPN]₂[Ru₄(μ₄-O)(μ-Cl) ₄(CO)₁₀] (6). The structure of 6 has been determined by X-ray diffraction (triclinic P1, No. 2, a = 13.226(5) A?, b = 25.533(2) A?, c = 12.771(3) A?, α = 92.39(1)°, β = 114.77(3)°, γ = 85.92(2)°, V = 3906(2) A?³, Z = 2, R = 0.038, R_w = 0.045). Its dianionic unit consists of a distorted quadratic antiprism based on two rectangular faces Ru(μ-Cl)₂Ru and containing an encapsulated oxygen atom linked to the four ruthenium centers. The anionic complexes 4-6 also act as catalyst precursors for the hydroesterification reaction. Analysis of the solutions recovered at the end of all catalytic runs indicate the presence of [PPN]₂[Ru₆(C)(CO)₁₆] as the principal metal-containing derivative. The probable mononuclear nature of the active species is suggested. A detailed investigation of the catalytic system based on [PPN][Ru(CO)₃Cl₃] (7) (prepared here in 81% yield by a new one-pot procedure) is reported. The results show that catalysis in the presence of the latter complex takes place readily without an induction period. Furthermore, a comparative evaluation of the four salts 7, [PPN][Ru(CO)₃I₃] (8), [PPN][Ru(CO)₃Cl₂I] (9), and [PPN][Ru(CO)₃-ClI₂] (10) as catalyst precursors reveals that chloride is a better promoter than iodide under the experimental conditions defined above, using DMF (or related amides) as solvent. Catalysis in the presence of 7 (and 3 equiv of NEt₃ as cocatalyst) is complete within 2 h (100% conversion and 99% selectivity; overall turnover frequency = 170 h^-1, corresponding to an initial activity of the order of 700 h^-1). Complex 7 is seen to react readily with DMF at 160°C within 30 min to produce the new complex [PPN][Ru(CO)₂Cl₃(η¹-DMF)] (11), isolated in 60% yield. The X-ray structure of 11 is reported (monoclinic P2₁/c, a = 9.000(2) A?, b = 21.176(2) A?, c = 21.080(1) A?, β = 93.89(8)°, V = 4008.3(9) A?³, Z = 4, R = 0.028, R_w = 0.030). The three chloride ligands adopt a meridional arrangement, whereas the DMF ligand, bound via its oxygen atom, occupies one of the two apical sites.