Synthesis and Thermal Decomposition of Palladacyclopentane Derivatives of the type [Pd(CH2CHRCHRCH2)L2] (R=H or Me). X-Ray Crystal Structure of [Pd(CH2CH2CH2CH2)(bipy)]

A series of new palladacyclopentane derivatives of formula [Pd(CH2)CHRCHRCH2L2] [L2 = (C6H11)2PCH2CH2P(C 6H11)2 (dcpe) or Ph2P(CH2)4PPh2 (dppb), R = H; L2 = Ph2PCH2CH2PPh2 (dppe), 2,2′-bipyridyl (bipy), or Me2NCH2CH2NMe2 (tmen), R = Me] has been prepared. The first X-ray crystal structure determination of a palladacyclopentane derivative is reported: the compound [Pd(CH2CH2CH2CH2)(bipy)] gives crystals belonging to the C2/c space group: a = 16.643(9), b = 11.174(4), c = 7.451(3) Å, β = 116.05(9)°, and Z = 4; R = 0.0340 for 826 observed reflections. The metal co-ordination is square planar and the molecules lie on a two-fold axis. The palladacyclopentane ring shows a half-chair conformation with the two-fold axis running through Pd and the middle of the C(β)-C(β′) bond. A study of the thermal decomposition of the palladacyclopentanes has been carried out: [Pd(CH2CH2CH2CH2)L2] (L = PPh3; L2 = dppe, tmen, bipy, dcpe, or dppb) gives butenes as the major products; cyclobutane (L = PPh3) and ethylene (L2 = dppe or dcpe) are also formed as minor products. By comparing these results with those for the decomposition of some methyl-substituted palladacyclopentanes, it is shown that the presence of ethylene is not attributable to fragmentation of the metallacyclic skeleton, but rather to the rupture of the P-C bonds of the diphosphine ligands. The decomposition of palladacyclopentanes is also induced by Bun 2O·BF3: linear C4 hydrocarbons are formed.