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)(TFE-d₃)]⁺ (N-N = Ar-N=C(Me)-C(Me)=N-Ar; Ar = 3,5-di-tert-butylphenyl; TFE-d₃ = CF₃CD₂OD) 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 σ 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 σ complex, (iV) dissociation of methane, and (v) β-H elimination to form the observed product. Second-order rate constants for cycloalkane activation (C _nH_2n), are proportional to the size of the ring (k ∼ 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 σ 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 σ adducts, is observed before loss of methane.