C-O coupling of LPtIVMe(OH)X complexes in water (X=18OH, OH, OMe; L = di(2-pyridyl)methane sulfonate)

The C-O reductive elimination from (LPtMe)-Me-IV(OH)(2) (1, Me = CH3; 1-d(3), Me = CD3; L = di(2-pyridyl)methanesulfonate) leading to methanol, dimethyl ether, and LPtII(OH2)(2)BF4 (3) was studied in acidic solutions (60 and 120 mM HBF4) in H2O and (H2O)-O-18 at 80 degrees C. In O-18-labeled water a mixture of two isotopologous methanols, (MeOH)-O-18 and (MeOH)-O-16, formed in 1:1 to 5:1 ratios. At a given acidity and a similar to 10% conversion of 1-d(3) the (MeOH)-O-18/(MeOH)-O-16 ratio was inversely proportional to the concentration of the complex 1-d(3) (29-120 mM). ESI-MS study showed that a slow O-18/O-16 exchange in hydroxo ligands of complexes 1 and 1-d(3) occurred that led to higher (MeOH)-O-18/(MeOH)-O-16 ratios by the end of the reaction. Similarly, a mono-O-18-labeled complex, 1-O-18, reacted in (H2O)-O-16 in the presence of HBF4 to form a mixture of (MeOH)-O-16 and (MeOH)-O-18. A number of intermediates of C-O elimination from 1 in acidic aqueous solution were identified, prepared independently, and characterized by NMR, X-ray diffraction, and elemental analysis: (LPtMe)-Me-IV(OH)(OMe) (4), sym-(LPtMe)-Me-IV(OMe)(2) (5), and isomeric dinuclear heterovalent cationic complexes [(LPtMe)-Me-IV(mu-OH)(2)(PtL)-L-II]BF4 (cis- and trans-6). It was shown that an isomer of 4, methoxo platinum(IV) complex 13, hydrolyzed in acidic (H2O)-O-18 solution to produce (MeOH)-O-16 isotopologue exclusively. Kinetic studies established that C-O elimination from 1 was first order in 1; it was catalyzed by acids and by one of the reaction products, complex 3. In the latter case reversible formation of intermediates 6 occurred. A suggested reaction mechanism for the formation of (MeOH)-O-16 from 1 in (H2O)-O-18 solutions involves a bimolecular nucleophilic attack of a hydroxo ligand of 1 or 4 at the electrophilic carbon atom of the methyl group in the cationic species 1 center dot H+ (complex 2) or 6, leading to non-O-18-labeled intermediates 4 or 5, respectively. Subsequent hydrolysis of these intermediates in (H2O)-O-18 solution leads to Pt-IV-(OMe)-O-16 bond cleavage and formation of (MeOH)-O-16. Similarly, a bimolecular nucleophilic attack of a methoxo ligand of 4 or 5 at the electrophilic carbon atom of complex 2 or 6 leads to intermediate dimethyl ether Pt-IV complexes 7 and/or 8. These intermediates are responsible for the formation of dimethyl ether. The C-O coupling of 4 leading to dimethyl ether, methanol, and transient complex 5 was studied in neutral and acidic aqueous solutions. The Me2O/MeOH ratio was found to increase with decreasing [H+] and increasing concentration of 4.