Cycloaddition Reactions of Terminal Alkynes and Phosphaalkynes with an Isolable 5-Coordinate β-Diiminate Platinum(IV) Dihydrosilyl Complex

Combining (Cl-nacnac)Pt(H)(1-pentene) (Cl-nacnac = N - p -Cl-phenyl-β-diiminate) and HSiR 3 (SiR 3 = SiPh 3, SiEt 3, SiHPh 2 ) leads to rapid displacement of 1-pentene; oxidative addition of the Si−H bond produces a stable 5-coordinate Pt(IV) silyl dihydride complex which has been isolated and characterized crystallographically. The 5-coordinate silyl dihydride platinum(IV) complex undergoes a cycloaddition reaction with either acetylene or phosphaalkyne to form a vinyl or phosphaalkenyl Pt(IV) product, respectively. One acetylenic carbon adds to the central methine carbon of the Cl-nacnac ligand and the other alkyne carbon or phosphorus coordinates to the vacant site of the d 6 metal. Phosphaalkynes are polarized such that the carbon is at the negative end of the dipole; however we observe a final product that reflects a reversal of this normal polarity. The phosphorus is nucleophilic even after the cycloaddition reaction is complete, and its lone pair can be methylated by MeOTf (OTf = trifluoromethanesulfonate) or protonated by addition of HBF 4 ·Et 2 O.