Sulfonium-Pincer Ligands Flexibility in Pt(II) Complexes

The coordination chemistry of sulfonium cations, dormant
since
the early nineties, was recently revived when we reported the synthesis
and characterization of the first Rh­(I) and Pt­(II) pincer-sulfonium
complexes. With the Pt­(II) complexes, we had noticed the hemilability
of our sulfonium-pincer ligands further explored here. This hemilability
led to mononuclear bidentate complexes with both the aromatic and
aliphatic sulfonium ligands. With the latter, due to its flexibility,
dimeric structures of two different kinds were also allowed. The more
rigid aromatic backbone adopted only a mononuclear bidentate mode,
leading to a dynamic equilibrium between two asymmetric geometries.
Computational study of this process predicted a local energy minimum
for a pincer-sulfonium–PtCl complex. However, the activation
energy of its formation, as a possible intermediate, was found to
be too high and indeed was not observed experimentally. Nevertheless,
such a PtCl complex was prepared and characterized by XRD. Although
its S–Pt bond was significantly shorter than in its PtMe analogue,
the former was easily dissociated in coordinating solvents. It seems
that lowering the dz2 orbital
in this complex by strong π back-donation renders the Pt­(II)
nucleus more susceptible to nucleophilic attacks. This comprehensive
study should lay the ground for future applications of pincer-sulfonium–Pt­(II)
complexes in π-acid catalysis.