Consecutive Cyclometalation by Platinum(II)

The mechanism of the selective formation of the new benzylic mono- and bicyclometalated platinum(II) complexes 2a, b and 3b has been studied. Monitoring the initial sp 3 C−H activation of the ligand DIPPIDH (α 2 -( d i i so p ropyl p hosphino) i so d urene; 1 ) and (1,5-cyclooctadiene)dimethylplatinum(II) ((COD)PtMe 2 ) at room temperature by 31 P{ 1 H} NMR reveals that, after displacement of 1,5-cyclooctadiene, the complex cis -(DIPPIDH)(DIPPID)PtMe ( 2a ) is formed selectively by oxidative addition of a benzylic C−H bond and subsequent reductive elimination of CH 4 . This reaction is controlled by electronic factors. Heating of 2a results in isomerization to the thermodynamically more stable isomer trans -(DIPPIDH)(DIPPID)PtMe ( 2b ) as well as the formation of the double-C−H activated complex trans -(DIPPID) 2 Pt ( 3b ) with liberation of CH 4 in parallel pathways. Continuous heating results in the quantitative formation of the thermally stable 3b . Mechanistically, the second C−H activation proceeds analogously to the first one. The molecular structure of 3b, possessing two six-membered metallacycles, two methylene bridges, and two phosphines in mutually trans positions, was determined by complete single-crystal diffraction studies.