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

The heterobimetallic complexes, PtRh­(tfepma)2(CNtBu)­X3 (X = Cl, Br), are
assembled by the treatment of Pt­(cod)­X2 (cod =1,5-cyclooctadiene)
with {Rh­(cod)­X}2, in the presence of tert-butylisonitrile (CNtBu) 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 XPtIRhIIX2 cores with suitable
halide sources (PhICl2 or Br2) furnishes PtRh­(tfepma)2(CNtBu)­X5, 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-Cl3PtIIIRhIICl2 isomers with visible
light in the presence of olefin promotes the photoelimination of halogen
and regeneration of the reduced ClPtIRhIICl2 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-ClPtIRhIICl2 isomer, details of the X2 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 × 104 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.