Sulfonium-Pincer Ligands Flexibility in Pt(II) Complexes

The coordination chemistry of sulfonium cations, dormant since the early eighties, 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 PtCl complex was prepared and characterized by XRD. Although its S-Pt bond was significantly shorter than in its PtMe analog, 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.