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 d_z² 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.