Acetylenes and noble metal compounds. Part XII. Reactions of dimethyl acetylenedicarboxylate with palladium(II) chloride and the structure of {[chloro(methoxycarbonyl)(1,2,3,4,5-pentakismethoxycarbonylcyclopenta-2,4-dienyl)-2-MeOCO]methyl}(pentane-2,4-dionato)palladium(II)

Abstract

The structure of the complex [LPd(µ-Cl)₂PdL][I; L =–CCl(CO₂Me)·C₅(CO₂Me)₄C(OMe):O–], obtained from cyclotrimerisation of dimethyl acetylenedicarboxylate with PdCl₂, has been determined spectroscopically and with the aid of X-ray structure analysis of [PdL(acac)](IV; acac = pentane-2,4-dionato). The organic ligand contains pentakis(methoxycarbonyl)cyclopenta-2,4-diene bearing a –C(CO₂Me)(Cl)Pd substituent at C(1). Two further co-ordination sites of the square-planar Pd atom are occupied by acac and the remaining one by the ester carbonyl group [O(2a)] attached to C(2). This interaction forms a non-planar six-membered ring. The bond lengths Pd–C(6)[2·038(14)], Pd–O(2a)[2·052(10)], and the Pd–acac lengths Pd–O(7)[1·995(10)] and Pd–O(9)[2·024(11)Å], are normal. Thermal or aqueous-cyanide decomposition of complex (I) affords hexamethyl mellitate. Analogues of (IV) and (I), such as [XIII; L′=–CBr(CO₂Me)·C₅(CO₂Me)₄C(OMe):O–] and [L″Pd-(µ-Br)₂PdL″][XIIb; L″=–CBr(CO₂Me)·C₅(CO₂Me)₂(CF₃)₂C(OMe):O–]. have been obtained by reaction of the σ-buta-1,3-dienyl complex [Pd{C₄(CO₂Me)₄Br}Br]_n(IX) with dimethyl acetylenedicarboxylate and hexafluorobut-2-yne respectively. Reaction of complex (XIIb) with aqueous cyanide gives 1,2,3,4-tetrakis-(methoxycarbonyl)-5,6-bis (trifluoromethyl)benzene. The structures of some degradation products from complex (I) have been reformulated as 1 -substituted pentakis(methoxycarbonyl)cyclopenta-2,4-dienes C₅(CO₂Me)₅·C(CO₂Me)(Cl)X (X = H, Cl, or Br).