Photoreduction of Pt(IV) Halo-Hydroxo Complexes: Possible
Hypohalous Acid Elimination
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Abstract
Concentrated
hydrogen peroxide addition to <i>trans-</i>Pt(PEt<sub>3</sub>)<sub>2</sub>Cl(R) [<b>1</b> (R = 9-phenanthryl), <b>2</b> (R = 4-trifluoromethylphenyl)] yields hydroxo-hydroperoxo complexes <i>trans-</i>Pt(PEt<sub>3</sub>)<sub>2</sub>(Cl)(OOH)(OH)(R) [<b>5</b> (R = 9-phenanthryl), <b>4</b> (R = 4-trifluoromethylphenyl)],
where the hydroperoxo ligand is <i>trans</i> to R. Complex <b>5</b> is unstable and reacts with solvent CH<sub>2</sub>Cl<sub>2</sub> to give <i>trans</i>,<i>cis</i>-Pt(PEt<sub>3</sub>)<sub>2</sub>(Cl)<sub>2</sub>(OH)(9-phenanthryl) (<b>3</b>). Treatment of <b>4</b> with HCl yields analogous <i>trans</i>,<i>cis</i>-Pt(PEt<sub>3</sub>)<sub>2</sub>(Cl)<sub>2</sub>(OH)(4-trifluoromethylphenyl) (<b>6</b>) and
HBr gives <i>trans</i>-Pt(PEt<sub>3</sub>)<sub>2</sub>(Br)(Cl)(OH)(4-trifluoromethylphenyl)
(<b>7</b>), where the Br and 4-trifluoromethylphenyl ligands
are <i>trans</i>. Photolysis of <b>3</b> or <b>6</b> at 313 or 380 nm causes reduction to <i>trans-</i>Pt(PEt<sub>3</sub>)<sub>2</sub>Cl(R) (<b>1</b> or <b>2</b>, respectively). Expected coproduct HOCl is not detected, but authentic
solutions of HOCl are shown to decompose under the reaction conditions.
Chlorobenzene and other unidentified products that oxidize PPh<sub>3</sub> to OPPh<sub>3</sub> are detected in photolyzed benzene solutions.
Photolysis of <b>3</b> or <b>6</b> in the presence of
2,3-dimethyl-2-butene (TME) yields the chlorohydrin (2-chloro-2,3-dimethyl-3-butanol),
3-chloro-2,3-dimethyl-1-butene, and acetone, all expected products
from HOCl trapping, but additional oxidation products are also observed.
Photolysis of mixed chloro-bromo complex <b>7</b> with TME yields
the bromohydrin (2-bromo-2,3-dimethyl-3-butanol) and <b>2</b>, consistent with <i>cis</i>-elimination of HOBr. Computational
results (TDDFT and DFT) and photochemistry of related complexes suggest
a dissociative triplet excited state reaction pathway and that HOCl
elimination may occur by an incipient hydroxo radical abstraction
of an adjacent halogen atom, but a pathway involving hydroxo radical
reaction with solvent or TME to generate a carbon-based radical followed
by halogen abstraction from Pt cannot be eliminated