Thermal and Photochemical Ring-Bromination in Naphthyl‑, Naphthdiyl‑, and Dicarboximideperyl-Platinum Complexes

Brominated polycyclic aromatic compounds are important synthons, but their synthesis can be difficult. Herein, we report that Pt­(IV) centers σ-bonded to naphthalene and a dicarboximideperylene activate the ring systems to selective thermal and photochemical bromination. Thus, <i>trans</i>-Pt­(PEt₃)₂(Br)₃­(4-bromo-1-naphthyl) and Br₂ give <i>trans</i>-Pt­(PEt₃)₂(Br)₃­(7,4-dibromo-1-naphthyl). Introduction of a second Pt­(IV) center is achieved by double oxidative addition of 1,4-dibromonaphthalene to 2Pt­(PEt₃)₄. Bromination of [<i>trans</i>-Pt­(PEt₃)₂Br]₂­(1,4-naphthdiyl) yields [<i>trans</i>-Pt­(PEt₃)₂(Br)₃]₂­(1,4-naphthdiyl), which further brominates on the ring to give [<i>trans</i>-Pt­(PEt₃)₂(Br)₃]₂­(6,7-dibromo-1,4-naphthdiyl). Photoreduction of the Pt­(IV) centers with 1-hexene gives first mixed-valent [<i>trans</i>-Pt­(PEt₃)₂(Br)₃]­[<i>trans</i>-Pt­(PEt₃)₂(Br)]­(6,7-dibromo-1,4-naphthdiyl) and then [<i>trans</i>-Pt­(PEt₃)₂Br]₂­(6,7-dibromo-1,4-naphthdiyl). Photoreduction of <i>trans</i>-Pt­(PEt₃)₂(Br)₃(PMI) (PMI = <i>N</i>-(2,5-di<i>-tert</i>-butylphenyl)­perylen-3-yl-9,10-dicarboximide) without 1-hexene slowly gives ring-bromination at the PMI 12-position. HOTf treatment cleaves the Pt–PMI bond to give 12-bromo-<i>N</i>-(2,5-di<i>-tert</i>-butylphenyl)­perylene-9,10-dicarboximide. The reaction chemistry indicates that the Pt­(IV) center is equivalent to a bulky, electron-donating group for the naphthalene and PMI ring systems

Photoreduction of Pt(IV) Chloro Complexes: Substrate Chlorination by a Triplet Excited State

The Pt­(IV) complexes <i>trans</i>-Pt­(PEt₃)₂­(Cl)₃(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(PEt₃)₂(R)­(Cl) <b>1</b> with Cl₂(g) or PhICl₂. Mixed bromo–chloro complexes <i>trans,trans</i>-Pt­(PEt₃)₂(Cl)₂­(Br)­(R) (R = 9-phenanthryl, 4-trifluoromethylphenyl), <i>trans,cis</i>-Pt­(PEt₃)₂(Cl)₂(Br)­(4-trifluoromethylphenyl), <i>trans,trans</i>-Pt­(PEt₃)₂(Br)₂­(Cl)­(R) (R = 9-phenanthryl), and <i>trans,cis</i>-Pt­(PEt₃)₂(Br)₂(Cl)­(4-trifluoromethylphenyl) were obtained by halide exchange or by oxidative addition of Br₂ to <b>1</b> or Cl₂ to <i>trans</i>-Pt­(PEt₃)_2­(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(PEt₃)₂­(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 ³LMCT excited states. Emission from a triplet is observed in glassy 2-methyltetrahydrofuran at 77 K where photoreductive elimination is markedly slowed

Photoreduction of Pt(IV) Chloro Complexes: Substrate Chlorination by a Triplet Excited State

The Pt­(IV) complexes <i>trans</i>-Pt­(PEt₃)₂­(Cl)₃(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(PEt₃)₂(R)­(Cl) <b>1</b> with Cl₂(g) or PhICl₂. Mixed bromo–chloro complexes <i>trans,trans</i>-Pt­(PEt₃)₂(Cl)₂­(Br)­(R) (R = 9-phenanthryl, 4-trifluoromethylphenyl), <i>trans,cis</i>-Pt­(PEt₃)₂(Cl)₂(Br)­(4-trifluoromethylphenyl), <i>trans,trans</i>-Pt­(PEt₃)₂(Br)₂­(Cl)­(R) (R = 9-phenanthryl), and <i>trans,cis</i>-Pt­(PEt₃)₂(Br)₂(Cl)­(4-trifluoromethylphenyl) were obtained by halide exchange or by oxidative addition of Br₂ to <b>1</b> or Cl₂ to <i>trans</i>-Pt­(PEt₃)_2­(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(PEt₃)₂­(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 ³LMCT excited states. Emission from a triplet is observed in glassy 2-methyltetrahydrofuran at 77 K where photoreductive elimination is markedly slowed

High Quantum Yield Molecular Bromine Photoelimination from Mononuclear Platinum(IV) Complexes

Pt­(IV) complexes <i>trans</i>-Pt­(PEt₃)₂(R)­(Br)₃ (R = Br, aryl and polycyclic aromatic fragments) photoeliminate molecular bromine with quantum yields as high as 82%. Photoelimination occurs both in the solid state and in solution. Calorimetry measurements and DFT calculations (PMe₃ analogs) indicate endothermic and endergonic photoeliminations with free energies from 2 to 22 kcal/mol of Br₂. Solution trapping experiments with high concentrations of 2,3-dimethyl-2-butene suggest a radical-like excited state precursor to bromine elimination

High Quantum Yield Molecular Bromine Photoelimination from Mononuclear Platinum(IV) Complexes

Pt­(IV) complexes <i>trans</i>-Pt­(PEt₃)₂(R)­(Br)₃ (R = Br, aryl and polycyclic aromatic fragments) photoeliminate molecular bromine with quantum yields as high as 82%. Photoelimination occurs both in the solid state and in solution. Calorimetry measurements and DFT calculations (PMe₃ analogs) indicate endothermic and endergonic photoeliminations with free energies from 2 to 22 kcal/mol of Br₂. Solution trapping experiments with high concentrations of 2,3-dimethyl-2-butene suggest a radical-like excited state precursor to bromine elimination