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 (alpha(2)-(diisopropylphosphino)isodurene; 1) and (1,5-cyclooctadiene)dimethylplatinum(II) ((COD)PtMe(2)) at room temperature by P-31{H-1} 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 CH4. 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 CH4 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.