The role of alkane coordination in C-H bond cleavage at a Pt(II) center.
The rates of C H bond activation for various alkanes by [(N-N)Pt(Me)(TFEd(3))](+) (N N = Ar N C(Me) C(Me) N Ar; Ar = 3,5-di-tert-butylphenyl; TFE-d(3) = CF(3)CD(2)OD) were studied. Both linear and cyclic alkanes give the corresponding alkene-hydride cation [(N-N)Pt(H)(alkene)](+) via (i) rate determining alkane coordination to form a C H sigma complex, (ii) oxidative cleavage of the coordinated C H bond to give a platinum(IV) alkyl-methyl-hydride intermediate, (iii) reductive coupling to generate a methane sigma complex, (iv) dissociation of methane, and (v) beta-H elimination to form the observed product. Second-order rate constants for cycloalkane activation (C(n)H(2n)), are proportional to the size of the ring (k approximately n). For cyclohexane, the deuterium kinetic isotope effect (k(H)/k(D)) of 1.28 (5) is consistent with the proposed rate determining alkane coordination to form a C H sigma complex. Statistical scrambling of the five hydrogens of the Pt-methyl and the coordinated methylene unit, via rapid, reversible steps ii and iii, and interchange of geminal C H bonds of the methane and cyclohexane C H sigma adducts, is observed before loss of methane.
Chen
George S
GS
The Arnold and Mabel Beckman Laboratories of Chemical Synthesis, California Institute of Technology, 1200 East California Avenue, Mail Code 127-72, Pasadena, CA 91125, USA.
Labinger
Jay A
JA
Bercaw
John E
JE