Organic chemistry of dinuclear metal centres. Part 2. Combination of dimethyl acetylenedicarboxylate with carbon monoxide at a ditungsten centre: crystal structure of [W2(CO)4{µ-η2:η2-C(O)C2(CO2Me)2}(η-C5H5)2]

Abstract

Under u.v. radiation dimethyl acetylenedicarboxylate reacts with [W₂(CO)₆(η-C₅H₅)₂] to produce [W₂(CO)₄{µ-η² :η²-C(O)C₂(CO₂Me)₂}(η-C₅H₅)₂] in 55% yield. An X-ray diffraction study has revealed that the molecule contains-a unique bridging ligand composed of linked alkyne and CO. Crystals are triclinic, space group P, with two molecules in a unit cell of dimensions a= 9.419(5), b= 8.089(2), c= 14.561(4)Å, α= 91.16(2), β= 81.96(4), and γ= 106.81(3)° The structure was solved by heavy-atom methods and refined by least squares to R 0.033 for 5 045 diffractometer-measured reflection intensities. The molecule contains a ditungsten unit, one atom of which is part of a metallocyclobutenone ring whose ethylenic bond is η²-co-ordinated to the other tungsten. The compound is fluxional, n.m.r. spectra identifying a process (ΔG^‡ 77 kJ mol^–1) which appears to involve an oscillation of the bridging unit with retention of the alkyne–carbonyl link. Cleavage of this link is evident in the thermal and photochemical instability of the complex with respect to CO liberation and formation of [W₂(CO)₄(µ-MeO₂CC₂CO₂Me)(η-C₅H₅)₂]. The latter is the major product after prolonged irradiation of [W₂(CO)₆(η-C₅H₅)₂] with MeO₂CC₂CO₂Me, but also formed is [W₂(CO)₂{µ-C₄(CO₂Me)₄CO}(η-C₅H₅)₂], containing a bridging unit of two alkyne molecules linked via CO. Protonation of [W₂(CO)₄{µ-η²:η²-C(O)C₂(CO₂Me)₂}(η-C₅H₅)₂] with HBF₄ occurs at exocyclic ketonic oxygen to give [W₂(CO)₄{µ-C(OH)C₂(CO₂Me)₂}(η-C₅H₅)₂][BF₄] quantitatively. The µ-alkyne complex [W₂(CO)₄(µ-MeO₂CC₂CO₂Me)(ηC₅H₅)₂] reacts in boiling octane with MeO₂CC₂CO₂Me to produce [W₂{µ-C₈(CO₂Me)₈(η-C₅H₅)₂], containing four linked molecules of alkyne bridging the ditungsten centre.