Mechanistic Studies of Alkane Activation by Platinum(II) Complexes

Citation

Stahl, Shannon Scot (1997) Mechanistic Studies of Alkane Activation by Platinum(II) Complexes. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/CCAZ-9234. https://resolver.caltech.edu/CaltechTHESIS:05012018-081029790

Abstract

Currently, there is considerable interest in alkane oxidation reactions catalyzed by transition metal complexes. Chapter 1 reviews many of the recent advances in this field involving electrophilic late transition metals. The C-H activation step appears to dictate both the rate and selectivity of these reactions. Unfortunately, however, very little is known about the mechanism of this step.

In chapter 2, mechanistic studies of the protonolysis of several alkylplatinum(II) complexes [(tmeda)PtMeCl, (tmeda)Pt(CH₂Ph)Cl, (tmeda)PtMe₂, and trans(PEt₃)₂Pt(CH₃)Cl] are described. These reactions model the microscopic reverse of C-H activation by aqueous Pt(II). Kinetics, activation parameters, and isotope effects were determined, and the results support a common mechanistic sequence for all of the reactions: (1) chloride- or solvent-mediated protonation of Pt(II) to generate an alkylhydridoplatinum(IV) intermediate, (2) dissociation of solvent or chloride to generate a cationic, five-coordinate platinum(IV) species, (3) reductive C-H bond formation producing a platinum(II) alkane σ-complex, and (4) loss of alkane either through an associative or dissociative substitution pathway. These studies provide insight into the role of solvent and ancillary ligands in aqueous Pt(II)-mediated C-H activation. The results also support the viability of Pt(II) σ-adducts and alkylhydridoplatinum(IV) intermediates in this reaction.

Chapter 3 describes the preparation and study of Pt(II) H₂-adducts and Pt(IV) dihydride complexes. The species of interest are generated by protonation of hydridoplatinum(II) complexes of the type trans-(PCy₃)₂Pt(H)X [X = SiH₃, H, CH₃, Ph, Cl, Br, I, CN, CF₃SO₃] and [trans-(PCy₃)₂Pt(H)L][Bar^f₄] [L = CO, 4-picoline; Bar^f₄] = B(3,5-C₆H₃(CF₃)₂)₄]. The proton attacks one of three different sites on these complexes (hydride, platinum, or the trans ligand), depending on which ligand is trans to hydride. These studies reveal several factors affecting the stability and reactivity of Pt(II) σ-adducts, which thus have implications for C-H activation by Pt(II).

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