Photoreduction
of Pt(IV) Chloro Complexes: Substrate
Chlorination by a Triplet Excited State
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Abstract
The
Pt(IV) complexes <i>trans</i>-Pt(PEt<sub>3</sub>)<sub>2</sub>(Cl)<sub>3</sub>(R) <b>2</b> (R = Cl, Ph, 9-phenanthryl,
2-trifluoromethylphenyl, 4-trifluoromethylphenyl, 3-perylenyl) were
prepared by chlorination of the Pt(II) complexes <i>trans</i>-Pt(PEt<sub>3</sub>)<sub>2</sub>(R)(Cl) <b>1</b> with Cl<sub>2</sub>(g) or PhICl<sub>2</sub>. Mixed bromo–chloro complexes <i>trans,trans</i>-Pt(PEt<sub>3</sub>)<sub>2</sub>(Cl)<sub>2</sub>(Br)(R) (R = 9-phenanthryl, 4-trifluoromethylphenyl), <i>trans,cis</i>-Pt(PEt<sub>3</sub>)<sub>2</sub>(Cl)<sub>2</sub>(Br)(4-trifluoromethylphenyl), <i>trans,trans</i>-Pt(PEt<sub>3</sub>)<sub>2</sub>(Br)<sub>2</sub>(Cl)(R) (R
= 9-phenanthryl), and <i>trans,cis</i>-Pt(PEt<sub>3</sub>)<sub>2</sub>(Br)<sub>2</sub>(Cl)(4-trifluoromethylphenyl)
were obtained by halide exchange or by oxidative addition of Br<sub>2</sub> to <b>1</b> or Cl<sub>2</sub> to <i>trans</i>-Pt(PEt<sub>3</sub>)<sub>2</sub>(R)(Br). Except for <b>2</b> (R = Ph, 4-trifluoromethylphenyl), all of the Pt(IV) complexes are
photosensitive to UV light and undergo net halogen reductive elimination
to give Pt(II) products, <i>trans</i>-Pt(PEt<sub>3</sub>)<sub>2</sub>(R)(X) (X = Cl, Br). Chlorine trapping experiments
with alkenes indicate a reductive-elimination mechanism that does
not involve molecular chlorine and is sensitive to steric effects
at the Pt center. DFT calculations suggest a radical pathway involving <sup>3</sup>LMCT excited states. Emission from a triplet is observed in
glassy 2-methyltetrahydrofuran at 77 K where photoreductive elimination
is markedly slowed
