Structural and Coordination Properties of 3,3-Diphenyl-1- (2,4,6-tri-<i>tert</i>-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 C1 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 Cs symmetry with the phosphino lone pair almost coplanar with the PC plane. A very small 2JPP coupling constant for the 2-(trimethylsilyl)-1,3-diphosphapropene was observed by 31P NMR spectroscopy, indicating that the conformation of the C1 symmetry was kept in solution. On coordination of the phosphorus atoms to the carbonyltungsten(0) moiety, the inherently predominant C1 form of the 2-(trimethylsilyl)-1,3-diphosphapropene changed to Cs symmetry. On the other hand, sulfurization of the 2-(trimethylsilyl)-1,3-diphosphapropene afforded the corresponding 3-thioxo-2-(trimethylsilyl)-1,3-diphosphapropene, indicating C1 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.