Sulfonium-Pincer Ligands Flexibility in Pt(II) Complexes

Abstract

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 z 2 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 pincersulfonium−Pt(II) complexes in π-acid catalysis.

Sulfonium Cation in the Service of π‐Acid Catalysis**

“…The 1 J Pt-P of 2037 Hz exhibited by the tris-cationic sulfonium complex [6](BF 4 ) 3 is significantly smaller than that of [5](OTf) 2 (Table 1, entry 3), as well as that of other bis-cationic sulfonium Pt(II) complexes we reported earlier (Table S3, entries 1-3). [9] It is also lower in comparison to the values reported for the Pt(II) and Pt(0) complexes of other cationic and bis-cationic pincer ligands (Table S3, entries 4-8). [16,17] The highly electrophilic character of Pt in [6](BF 4 ) 3 was further supported by the NBO calculations (ωB97X-D3/ Def2-TZVP) we performed on the optimized model analogs of the Pt complexes mentioned above (where t Bu and Ph groups were replaced by H atoms for simplicity).…”

“…In our earlier study, [9b] we showed that flexible PSP‐based sulfonium pincer ligands I and II were found to undergo an easy interconversion between a trans ‐κ 3 ‐PSP and a cis ‐κ 2 ‐PP mode that was always accompanied by a trans‐ to‐ cis isomerization of the phosphines (Scheme 2a). Therefore, maintaining the trans geometry seems to be essential in preserving the Pt−S coordination.…”

“…Computational analysis of our systems attested that embedding sulfonium cations in a pincer framework significantly enhances their inherent π‐acidity and places them among the best π‐acceptor ligands available today. Yet, our preliminary attempts to use them in π‐acid catalysis were unsuccessful due to the hemilability of these sulfonium pincer ligands which we disclosed in a subsequent study [9b] . Indeed, in their Pt(II) complexes, the sulfonium‐metal bond easily dissociates precluding the formation of a potentially catalytically active sulfonium‐Pt(II) core.…”