Experimental and Theoretical Studies on the Platinum-Mediated Selective C(sp)–Si Bond Cleavage of Alkynylsilanes

A series of cis-alkynyl­(silyl)­platinum­(II)
complexes was prepared via the chemoselective C­(sp)–Si bond
cleavage of alkynylsilanes by a platinum(0) complex ligated with the
P–N hemilabile bidentate ligand. The coordination of the triple
bond to the platinum center triggers selective C­(sp)–Si bond
cleavage. Hammett plots of the 31P­{1H} NMR spectroscopic
properties (δ and J values) reflect an electronic
effect on platinum­(II) complexes; trans substituents of arylethynyl
groups are influenced, but cis-positioned silyl groups are not affected,
as evidenced by 29Si­{1H} NMR. In comparison,
Hammett plots show that C­(sp)–Si bond cleavage rates are accelerated
by electron-rich alkynylsilanes, which is opposite to the ordinary
oxidative addition of aryl halides to transition metals often observed
in catalytic cross-coupling reactions. A DFT calculation reveals that
intermediates and transition states are stabilized by electron-rich
alkynylsilanes and that the five-membered hemilabile P–N ligand
is essential, in which a reactive electron-deficient 14-electron platinum(0)
species is produced via the dissociation of nitrogen, giving rise
to a monodentate phosphine coordination. Electron-rich alkynylsilanes
allow decreased π back-donation from the platinum center to
the ligand, accelerating the dissociation of the more labile nitrogen.
Steric congestion between diisopropylphosphino and silyl groups thermodynamically
disfavors C­(sp)–Si bond cleavage.