Reinvestigation of the Pd-catalyzed bis(silylation) of alkynes with 1,1,2,2-tetramethyl-1,2-bis(phenylthiomethyl)disilane: Unexpected formation of the eight-membered siloxane–chelate complex cis-[PdCl2{(PhSCH2SiMe2)2O}

Abstract

The bis(silylated) alkenes Z-(PhSCH2)Me2SiC(H)C(Fc)SiMe2(CH2SPh) (2) and Z-(PhSCH2)Me2SiC(H)C(bipheny)SiMe2(CH2SPh) (3) have been prepared by Pd-catalyzed double silylation of ethynylferrocene and 4-ethynyl-1,1′-biphenyl in the presence of 1,1,2,2-tetramethyl-1,2-bis(phenylthiomethyl)disilane (1). A reinvestigation on the interaction of 1 with [PdCl2(PhCN)2] in technical-grade CH2Cl2 as solvent revealed competition between reduction to elemental palladium (due to oxidative addition of the Si–Si bond across Pd(II) and subsequent reductive elimination) and formation of an unusual eight-membered chelate complex cis-[PdCl2{(PhSCH2SiMe2)2O}] (4), which is fluxional in solution. Based on GC–MS analyses revealing both formation of chlorosilane ClSiMe2CH2SPh and the disiloxane (PhSCH2SiMe2)2O, residual water contained in CH2Cl2 is supposed to be the source for the oxygen atom of the siloxane function. The mechanism of the formation of 4 is discussed. The molecular structures of 1 and 4 were determined by single-crystal X-ray diffraction analyses. The energies of several conformational isomers of 4 were computed by means of DFT calculations, the coexistence of a species featuring a weak dative Pd–O interaction due to a hemi-labile coordination of ligand 1 was evidenced.