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Ethylene Insertion in Tp‘Pt(Ph)(η2-CH2CH2) and C−H Activation of Ethylbenzene to Form a Platinum(IV) ortho-Metalated Phenethyl Complex

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journal contribution
posted on 2006-09-11, 00:00 authored by Margaret G. MacDonald, Cynthia Norris Kostelansky, Peter S. White, Joseph L. Templeton
Heating Tp‘Pt(Ph)(η2-CH2CH2) (5) in benzene forms an ortho-metalated phenethyl hydrido platinum(IV) complex, (7). Presumably this net reaction reflects phenyl migration to ethylene to give the insertion product, Tp‘PtCH2CH2Ph, as an unsaturated intermediate. Intramolecular C−H activation of an ortho phenyl proton from this intermediate would produce the metallacyle product. Low-temperature protonation of the phenyl ethylene complex 5 results in the formation of a cationic η2-ethylene phenyl complex, [κ2-(HTp‘)Pt(C6H5)(η2-CH2CH2)][BAr‘4] (6). Low-temperature protonation of the ortho-metalated phenethyl complex 7 followed by addition of acetonitrile leads to reductive coupling of the Pt−H and the alkyl methylene group to give a cationic Pt(II) 2-ethylphenyl complex, [κ2-(HTp‘)Pt(C6H4-2-CH2CH3)(NCCH3)][BAr‘4] (9). Complex 7 has also been isolated as the sole product of gentle heating of Tp‘Pt(Me)2(H) (2) with the Lewis acid B(C6F5)3 in ethylbenzene. Additional analogous Pt(IV) metallacycles, (11a/11b) and (13), have also been synthesized by this route. Note that heating of the dihydride reagent Tp‘Pt(Me)(H)2 (3) with B(C6F5)3 in either ethylbenzene or 2-propylbenzene gave only Tp‘Pt(Ar)(H)2 (14 and 15) products. Attempts to isolate an η2-propylene phenyl complex were unsuccessful; formation of the 1,2-insertion product, (10), and 2,1-insertion products, 11a and 11b, resulted. An in situ attempt to isolate the cationic [κ2-(HTp‘)Pt(C6H5)(η2-CH2CH(CH3))][BF4] resulted in the isolation of the Pt(II) phenyl aqua complex [κ2-(HTp‘)Pt(C6H5)(OH2)][BF4] (12) instead.

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