Monomethyl Pt complexes coordinated to hemilabile facially coordinating ligands in aqueous solution: Aerobic oxidation and C-O coupling to form methanol.
MetadataShow full item record
Oxidation of monomethyl PtII to monomethyl PtIV followed by reductive elimination of methanol are key consecutive steps in homogenous catalytic oxidation of methane to methanol. The development of a catalytic system that uses water as the solvent and molecular oxygen as the oxidant, both of which are inexpensive and non-toxic, would be a major advancement. Facially coordinating ligands containing a hemilabile arm have been shown to enhance aerobic oxidation of monomethyl PtII to PtIV without restricting reductive elimination of the methanol from the monomethyl PtIV product. A major focus of this thesis was the study of the reactivity of methyl platinum when coordinated to the tridentate ligand bis(3,5-dimethylpyrazol-1-yl)-acetate (NNO, 1). In water, the dimethyl NNOPtII complex (5) undergoes protonlysis to generate methane and monomethyl NNOPtII complex (6) while the monomethyl NNOPtII product is stable up to 100 °C. In both cases the reaction is shown to be highly pH dependent with protonlysis of the monomethyl NNOPtII occurring at room temperature at pH < 4. The reaction of monomethyl NNOPtII (6) with molecular oxygen produced monomethyl NNOPtIV (10) with concurrent coordination of the hemilabile carboxylate arm in NNO. In addition to oxidation, a rare oxidative methyl transfer reaction was observed producing dimethyl NNOPtIV (11) with an equivalent of demethylated NNOPtII (7). Studies of the reaction show that higher pH and temperature favored oxidative methyl transfer over oxidation. A mechanism is proposed in which the reaction of 6 with O2 produces a hydroperoxide monomethyl NNOPtIV intermediate (I1) that can then react with a second equivalent of 6 resulting in two equivalents of the oxidized monomethyl PtIV product 10. Isomerization of I1 to I2 moves the Pt-CH3 group trans to a good leaving group, the carboxylate arm of NNO. In this position, the platinum bound methyl group is more susceptible to removal by SN2 attack of 6 forming the dimethyl NNOPtIV product (11) with an equivalent of 7. Reductive elimination from the monomethyl PtIV product of oxidation (10) generating methanol and 7 was also observed. Interestingly, at high  the decomposition of 10 was found to be first-order in 10 and 7. A mechanism is proposed in which reductive elimination is faster from a dinuclear monomethyl NNOPtII/NNOPtIV intermediate (I3) formed from 7 and 10. A novel facially coordinating ligand was synthesized in which the hemilabile arm contains an anionic N from a sulfonyl amidate group (NNN, 15). Three different NNN variants were synthesized with a different substituent (R) bound to the hemilabile arm (R = H: (bis(3,5-dimethylpyrazol-1-yl)-N-benzenesulfonylacetamide (18), R = CH3: Bis(3,5-dimethylpyrazol-1-yl)-N-toluenesulfonylacetamide (16), R = Cl: Bis(3,5-dimethylpyrazol-1-yl)-N-p-chlorobenzenesulfonylacetamide (20)). The substituents were chosen based on their Hammett para parameter (CH3: -0.17, H: 0.00, Cl: 0.23) which should in turn effect the electron density at the sulfonyl amidate N. In general, the NNNPt system performed similarly to the NNOPt system in terms of: protonlysis of dimethyl NNNPtII in water, aerobic oxidation of monomethyl NNNPtII to monomethyl NNNPtIV and reductive elimination of methanol from monomethyl NNNPtIV.
- Chemistry