Halogen oxidation and halogen photoelimination chemistry of a platinum-rhodium heterobimetallic core

The heterobimetallic complexes, PtRh(tfepma) ₂(CN ^tBu)X ₃ (X = Cl, Br), are assembled by the treatment of Pt(cod)X ₂ (cod =1,5-cyclooctadiene) with {Rh(cod)X} ₂, in the presence of tert-butylisonitrile (CN ^tBu) and tfepma (tfepma = bis(trifluoroethoxyl)phosphinomethylamine). The neutral complexes contain Pt-Rh single bonds with metal-metal separations of 2.6360(3) and 2.6503(7) Å between the square planar Pt and octahedral Rh centers for the Cl and Br complexes, respectively. Oxidation of the XPt ^IRh ^IIX ₂ cores with suitable halide sources (PhICl ₂ or Br ₂) furnishes PtRh(tfepma) ₂(CN ^tBu)X ₅, which preserves a Pt-Rh bond. For the chloride system, the initial oxidation product orients the platinum-bound chlorides in a meridional geometry, which slowly transforms to a facial arrangement in pentane solution as verified by X-ray crystal analysis. Irradiation of the mer- or fac-Cl ₃Pt ^IIIRh ^IICl ₂ isomers with visible light in the presence of olefin promotes the photoelimination of halogen and regeneration of the reduced ClPt ^IRh ^IICl ₂ core. In addition to exhibiting photochemistry similar to that of the chloride system, the oxidized bromide cores undergo thermal reduction chemistry in the presence of olefin with zeroth-order olefin dependence. Owing to an extremely high photoreaction quantum yield for the fac-ClPt ^IRh ^IICl ₂ isomer, details of the X ₂ photoelimination have been captured by transient absorption spectroscopy. We now report the first direct observation of the photointermediate that precedes halogen reductive elimination. The intermediate is generated promptly upon excitation (<8 ns), and halogen is eliminated from it with a rate constant of 3.6 × 10 ⁴ s ^-1. As M-X photoactivation and elimination is the critical step in HX splitting, these results establish a new guidepost for the design of HX splitting cycles for solar energy storage.