Alex "Puzo" Muci is one of the best chemists I have ever met and I thank him for his advice and support. Father-to-be Chris "Theory" Brandow was at my right side in the computer room throughout the writing of this dissertation, and I wish him and Maggie the best of luck in West Virginia. In my eyes, Jeff Yoder has surpassed Bob Bergman in the art of double labeling experiments.
Joseph Sadighi is the best lab partner one could hope for and I thank him for his. The Bercaw Group has changed a lot during my stay here and I am very grateful to the people who made this last year one of the best: Susan "Aloha" Schofer, Sara. This result implies that dissociative loss of pentafluoropyridine is the rate-limiting step in the CH activation reactions of 1 .
The stoichiometric conversion of alkanes (cyclohexane, ethane) to olefins (cyclohexene, ethylene) is achieved by C-H activation with [(N-N)Pt(CH3)(CF3CH20H)]BF4 (1, N-N is N,N'-bis(3,5) -di-t-butylphenyl)-1,4-diazabutadiene), resulting in the formation of olefin hydride complexes. The first step in the C-H activation reaction is the formation of a platinum(II) alkyl, which undergoes α-hydrogen elimination to form an olefin hydride complex.
Catalytic Alkane Functionalization
Oxidation of Dimethylplatinum(II) Complexes with Dioxygen
The oxidation of hydrocarbons by molecular oxygen is one of the largest commercial applications of transition metal complexes in homogeneous catalysis (see, for example, eq. 1).1 The oxidation follows a radical mechanism, in which the main role of the transition metal catalyst is the decomposition of alkyl hydroperoxides formed by radical processes. Alkane activation at low temperature can be achieved with cationic platinum(II) a-diimine complexes (eq ll).:n The cationic species is generated by protonation of (a-diimine)Pt(CH3)2 with aqueous HBF4 in trifluoroethanol. . The second step of the Shilov cycle (Scheme 1) is the oxidation of a platinum(II) alkyl to a platinum(IV) alkyl with PtCl/-.
Our studies of oxygen reactions with platinum(II) complexes are described in the third chapter of this thesis. The final step of the Šilov cycle is the replacement of platinum (IV) with water or chloride of the SN2 type, which gives an alcohol or an alkyl chloride and regenerates catalytically active platinum (II)?3 species. The proposed mechanism agrees with the results of the study of reaction 1 with C6F5CH3 and C6H6 • The rate of the C-H activation reaction is independent of benzene concentration and no kinetic isotope effect was observed.
The activation of carbon-hydrogen bonds by electrophilic platinum(II) complexes has been extensively studied (equiv. 1).1-5 The individual steps are difficult to study experimentally, since the well-defined alkane a-complex of platinum(II) has eluded isolation. An electrophilic platinum(II) complex, [(tmeda)Pt(CH3)(NC5F5)][BArr4] (1, BArr4 = tetra-3,5-bistrifluoromethylphenyl borate) was synthesized in our group, and its reactions with alkanes are Studies ligand substitutions in 1 and their significance for C-H activation reactions are described here. Incorporation of deuterium into the Pt-CH3 part along with the formation of methane isotopomers (CH3D, CH2D2).
The selective catalytic oxidation of alkanes by aqueous platinum salts was originally reported by Shilov in 1972 (eq. 1).1 The mechanism of this unique catalytic cycle, a subject of extensive mechanistic studies in a number of researches. Analysis of the solutions of (phen)Pt(CH3)2 (3, phen = 1,10- . phenanthroline) and (tmeda)Pt(CH3h (1) in methanol-d4 by 1H NMR spectroscopy revealed that slow deuterium incorporation into Pt-CH3 groups, followed by the formation of methane isotopomers (CH3D, CH2D2 and CHD3) Oxidation of (tmeda)Pt(CH3)2• Exposure of the solution of (tmeda)Pt(CH3)2 in methanol to air led to the formation of several intermediates, observed by 1H NMR and UV-Vis spectroscopies, which finally gave rise to a single product, (tmeda)Pt(OH)(OCH3)(CH3h (4, eq.
The ability of a platinum (II) (N-N) PtX2 complex to react with dioxygen depends not only on its nature. Since the oxidation results-i.e., the formation of a mixt of (tmeda)Pt(OOH)(OCH3)(CH3)2. According to the proposed mechanism, the ratio of platinum(IV) products (hydroperoxy/hydroxy) should depend on the concentrations of (tmeda)Pt(CH3)z and dioxygen.
Structure determination of (tmeda)Pt(OH)(OCH3)(CH)2 • The title complex was crystallized from methanol at -78 OC. Seven of these 18 peaks are close to the platinum atom, including the largest positive peak of 2.98 e-A-3 at a distance of 0.89 A. The 6 most unpleasant reflections all have k = 5 and FcaJc weak and less than. A mixture of olefin hydride 7 and ethyl cation 8 was formed when (phen)PtEt2 was treated with [H(OEt2)z]BArr~ at low temperatures in CD2Cl2• The relative ratio of 7 and 8 changes at different temperatures with 8 being favored . at higher temperature.
The weighted R-factor (wR) and goodness-of-fit (L) are based on F2, conventional R-factors (R) are based on F, with F set to zero for negative F2• The threshold expression for F2 > 2cr( F2 ) is used only for calculating R-factors(gt), etc., and is not relevant for selecting reflections for refinement.
Approaches to Ca taly tic Oxidative Dehydrogenation of Alkanes
One of the main drawbacks of the Shilov system is the use of stoichiometric platinum (IV). Most of the strong oxidants used in the above systems are not selective for alkylplatinum(II) intermediates, instead all platinum(II) is converted to platinum(IV). Here we report our studies on the oxidation of dimethylplatinum(II) complexes with dioxygen.11 We have synthesized the proposed intermediate.
There are only a few examples of the oxidation of platinum (II) complexes with dioxygen. 17)Y It is worth noting that in all cases a tridentate ligand attached to K2 is coordinated to platinum (II). Therefore, the overall reaction stoichiometry for the formation of the hydroxy complex is two equivalents of platinum(II) for one equivalent of dioxygen. Thus, bulky α-diimines with substituents in the ortho positions of the aryl ring do not react with dioxygen.
In both cases, protonation of the dioxygen intermediate is the step that yields the hydroperoxy complex (Eq. 22). This interaction raises the energy of the HOMO of the platinum complex (a-antibonding dz2-based orbital) and facilitates oxidation. Ligands with a "free" arm located in the axial position of the platinum complex should raise the energy of the HOMO.
Studies of the individual steps of the proposed catalytic cycle are summarized in this chapter. With cyclohexane, a mixture of the hydroxy dimer 3 and cyclohexene anhydride 4 formed after a few days at room temperature (eq. 6). One of the olefin compounds has been tentatively identified as the complex of platinum(II) with vinyl ether.
The dihedral angle is the angle between the Pt-N(imine) bond and the C=C bond of the coordinated ethylene. As mentioned above, the ethylene is positioned trans to the pyrrolide moiety perpendicular to the platinum plane. Although only diphosphine ligands were used, there were striking differences in the observed behavior of the compounds.
Based on our studies of the reaction of platinum(II) complexes with dioxygen, we predicted that neutral complexes would be more easily oxidized than their cationic counterparts. The atomic coordinates (x 1 04) and the equivalent isotropic displacement parameters (A2x 1 03) for 1 U(eq) are defined as one-third traces of the orthogonalized tensor Uii. The rest of the molecule bends to accommodate these two positions, as seen above.
