Halogen Oxidation and Halogen Photoelimination Chemistry of a Platinum-Rhodium Heterobimetallic Core

The heterobimetallic complexes, PtRh(tfepma)2((CNBu)-Bu-t)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 ((CNBu)-Bu-t) 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) angstrom between the square planar Pt and octahedral Rh centers for the Cl and Br complexes, respectively. Oxidation of the (XPtRhX2)-Rh-I-X-II cores with suitable halide sources (PhICl2 or Br-2) furnishes PtRh(tfepma)(2)((CNBu)-Bu-t)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-(Cl3PtRhCl2)-Rh-III-Cl-II isomers with visible light in the presence of olefin promotes the photoelimination of halogen and regeneration of the reduced (ClPtRhCl2)-Rh-I-Cl-II 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-(ClPtRhCl2)-Rh-I-Cl-II 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 X 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 FIX splitting cycles for solar energy storage.