Photoreduction of Pt(IV) Halo-Hydroxo Complexes: Possible Hypohalous Acid Elimination

Concentrated
hydrogen peroxide addition to trans-Pt­(PEt3)2Cl­(R) [1 (R = 9-phenanthryl), 2 (R = 4-trifluoromethylphenyl)] yields hydroxo-hydroperoxo complexes trans-Pt­(PEt3)2(Cl)­(OOH)­(OH)­(R) [5 (R = 9-phenanthryl), 4 (R = 4-trifluoromethylphenyl)],
where the hydroperoxo ligand is trans to R. Complex 5 is unstable and reacts with solvent CH2Cl2 to give trans,cis-Pt­(PEt3)2(Cl)2(OH)­(9-phenanthryl) (3). Treatment of 4 with HCl yields analogous trans,cis-Pt­(PEt3)2(Cl)2(OH)­(4-trifluoromethylphenyl) (6) and
HBr gives trans-Pt­(PEt3)2(Br)­(Cl)­(OH)­(4-trifluoromethylphenyl)
(7), where the Br and 4-trifluoromethylphenyl ligands
are trans. Photolysis of 3 or 6 at 313 or 380 nm causes reduction to trans-Pt­(PEt3)2Cl­(R) (1 or 2, 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 PPh3 to OPPh3 are detected in photolyzed benzene solutions.
Photolysis of 3 or 6 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 7 with TME yields
the bromohydrin (2-bromo-2,3-dimethyl-3-butanol) and 2, consistent with cis-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.