C−O Coupling of LPt^IVMe(OH)X Complexes in Water (X = ¹⁸OH, OH, OMe; L = di(2-pyridyl)methane sulfonate)

The C−O reductive elimination from LPtIVMe(OH)2 (1, Me = CH3; 1-d3, Me = CD3; L = di(2-pyridyl)methanesulfonate) leading to methanol, dimethyl ether, and LPtII(OH2)2BF4 (3) was studied in
acidic solutions (60 and 120 mM HBF4) in H2O and H218O at 80 °C. In 18O-labeled water a mixture of
two isotopologous methanols, Me18OH and Me16OH, formed in 1:1 to 5:1 ratios. At a given acidity and
a ∼10% conversion of 1-d3 the Me18OH/Me16OH ratio was inversely proportional to the concentration
of the complex 1-d3 (29−120 mM). ESI-MS study showed that a slow 18O/16O exchange in hydroxo
ligands of complexes 1 and 1-d3 occurred that led to higher Me18OH/Me16OH ratios by the end of the
reaction. Similarly, a mono-18O-labeled complex, 1-18O, reacted in H216O in the presence of HBF4 to
form a mixture of Me16OH and Me18OH. 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:  LPtIVMe(OH)(OMe) (4), sym-LPtIVMe(OMe)2 (5), and isomeric
dinuclear heterovalent cationic complexes [LPtIVMe(μ-OH)2PtIIL]BF4 (cis- and trans-6). It was shown
that an isomer of 4, methoxo platinum(IV) complex 13, hydrolyzed in acidic H218O solution to produce
Me16OH 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
Me16OH from 1 in H218O 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·H+ (complex 2) or 6,
leading to non-18O-labeled intermediates 4 or 5, respectively. Subsequent hydrolysis of these intermediates
in H218O solution leads to PtIV−16OMe bond cleavage and formation of Me16OH. 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 PtIV 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.