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)]

Abstract

A series of new palladacyclopentane derivatives of formula [[graphic omitted]H₂)L₂][L₂=(C₆H₁₁)₂PCH₂CH₂P(C₆H₁₁)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 [[graphic omitted]H₂)(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: [[graphic omitted]H₂)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 Buⁿ₂O·BF₃: linear C₄ hydrocarbons are formed.