A novel sulfonated CNN pincer ligand
has been designed to support
CH and O2 activation at a Pt(II) center. The derived cycloplatinated
aqua complex 7 was found to be one of the most active
reported homogeneous Pt catalysts for H/D exchange between studied
arenes (benzene, benzene-d
6, toluene-d
8, p-xylene, and mesitylene)
and 2,2,2-trifluoroethanol (TFE) or 2,2,2-trifluoroethanol-d; the TON for C6D6 as a substrate
is >250 after 48 h at 80 °C. The reaction is very selective;
no benzylic CH bond activation was observed. The per-CH-bond reactivity
diminishes in the series benzene (19) > toluene (p-CH:m-CH:o-CH = 1:0.9:0.2) >
xylene
(2.9) > mesitylene (1.1). The complex 7 reacts slowly
in TFE solutions under ambient light but not in the dark with O2 to selectively produce a Pt(IV) trifluoroethoxo derivative.
The H/D exchange reaction kinetics and results of the DFT study suggest
that complex 7, and not its TFE derivatives, is the major
species responsible for the arene CH bond activation. The reaction
deuterium kinetic isotope effect, k
H/k
D = 1.7, the reaction selectivity, and reaction
kinetics modeling suggest that the CH bond cleavage step is rate-determining.
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