Structural and Coordination Properties of 3,3-Diphenyl-1- (2,4,6-tri-tert-butylphenyl)-2-(trimethylsilyl)-1,3-diphosphapropene Derivatives Showing Interesting Conformational Features

( E )-3,3-Diphenyl-1-(2,4,6-tri- tert -butylphenyl)-2-(trimethylsilyl)-1,3-diphosphapropene was prepared from (2-(2,4,6-tri- tert -butylphenyl)-1-(trimethylsilyl)-2-phosphaethenyl)lithium and chlorodiphenylphosphine, and the properties involving conformational aspects, coordination to metals, and sulfurization were investigated. The structure of 2-(trimethylsilyl)-1,3-diphosphapropene shows C 1 symmetry with the phosphino lone pair almost perpendicular to the P C π-system, whereas the 2-chloro- and 2-methyl-1,3-diphosphapropene derivatives show nearly C s symmetry with the phosphino lone pair almost coplanar with the P C plane. A very small 2 J PP coupling constant for the 2-(trimethylsilyl)-1,3-diphosphapropene was observed by 31 P NMR spectroscopy, indicating that the conformation of the C 1 symmetry was kept in solution. On coordination of the phosphorus atoms to the carbonyltungsten(0) moiety, the inherently predominant C 1 form of the 2-(trimethylsilyl)-1,3-diphosphapropene changed to C s symmetry. On the other hand, sulfurization of the 2-(trimethylsilyl)-1,3-diphosphapropene afforded the corresponding 3-thioxo-2-(trimethylsilyl)-1,3-diphosphapropene, indicating C 1 symmetry with the P S bond almost perpendicular to the P C moiety. The 2-(trimethylsilyl)-1,3-diphosphapropene acts as a P2 chelate ligand of a dichloropalladium(II) complex, which showed moderate catalytic activity in the Sonogashira cross-coupling reaction. The structure of the dichloropalladium(II) complex bearing the ligated 2-(trimethylsilyl)-1,3-diphosphapropene was analyzed by X-ray crystallography, indicating structure effects on the properties.