Sulfonium cations as versatile strongly π-acidic ligands

More than a century old, sulfonium cations are still intriguing species in the landscape of organic chemistry. On one hand they have found broad applications in organic synthesis and materials science, but on the other hand, while isoelectronic to the ubiquitous tertiary phosphine ligands, their own coordination chemistry has been neglected for the last three decades. Here we report the synthesis and full characterization of the first Rh(i) and Pt(ii) complexes of sulfonium. Moreover, for the first time, coordination of an aromatic sulfonium has been established. A thorough computational analysis of the exceptionally short S–Rh bonds obtained attests to the strongly π-accepting nature of sulfonium cations and places them among the best π-acceptor ligands available today. Our calculations also show that embedding within a pincer framework enhances their π-acidity even further. Therefore, in addition to the stability and modularity that these frameworks offer, our pincer complexes might open the way for sulfonium cations to become powerful tools in π-acid catalysis.


Introduction
Rethinking the coordination chemistry of main group elements has oen led to breakthroughs in metal-based homogeneous catalysis. For instance, extending the chemistry of B, Al, Ga, Sn, and Bi gave birth to the concept of s-acceptor (aka Z-type) ligands. 1 Peters, 2 Lu 3 and others 4 have used complexes of these ligands for such fundamentally important processes as N 2 xation, CO 2 reduction, and H 2 activation.
The electron-withdrawing nature of Z-type ligands also offered new opportunities for p-acid catalysis, as demonstrated by Inagaki with borane-based pincer ligands, 5 and Gabbai with ligands based on antimony, 6 and carbenium cations. 7 On the other hand, a signicant advance in p-acid catalysis was achieved by Alcarazo by stretching the p-acceptor properties of phosphine 8 and arsine 9 to the extreme through the introduction of positively charged substituents.
While seeking to unravel new facets of main group chemistry, the coordination properties of another main-group species, sulfonium cations, have been greatly overlooked. Yet, sulfonium salts are at the forefront of fundamental and applied research due to their countless applications as precursors for sulfur ylides, 10 alkyl and aryl group sources in cross-coupling reactions, 11 photoacids, 12 and many others. 13 Compared to isoelectronic and isostructural tertiary phosphines, sulfonium cations have their lone pair stabilized by their positive charge, while their low-lying S-C s*-orbitals become available for accepting electron density. Therefore, together with sulfoxonium, they have attracted attention as non-metal Lewis acids 14 and have been utilized as such for catalysis and anion sensing. 15 However, while tertiary phosphines are perhaps the most iconic family of ligands, only three crystallographically characterized sulfonium complexes of Mo(0) and Mn(I) were reported decades ago (Chart 1a), where these ligands exhibited strongly p-acidic character. 16 Yet, no sulfonium complexes relevant to catalysis have ever been reported, even though formation of transient metal-coordinated Chart 1 Previously reported sulfonium complexes (a) compared to the pincer type sulfonium complexes presented in the work (b). sulfonium intermediates during Pd catalyzed cross-coupling reactions of sulfonium salts has been suggested. 11a Here we report the rst synthesis and characterization of a series of complexes of both aliphatic and aromatic sulfonium cations with Rh(I) and Pt(II), two representatives of the Pt metal group, 17 which lies at the core of today's homogeneous catalysis (Chart 1b). Our in-depth theoretical analysis of sulfoniummetal interaction demonstrated it to be dominated by p-back bonding. This strongly p-acidic character is further enhanced by the pincer frameworks, which also provide our complexes with structural robustness and modularity, both properties of pivotal importance in catalysis. 18

Ligand design and synthesis
Obviously, coordination of the sulfonium cation is hindered by an electrostatic repulsion between its positive charge and that of a metal center (even if partial). So far, the preparation of sulfonium complexes has been achieved by alkylation of the corresponding sulde complexes. We adopted here a more systematic approach, where the aliphatic or aromatic sulfonium moieties were incorporated within pincer frameworks (I and II, respectively in Chart 2), bearing chelating phosphine arms. A similar strategy was used earlier by Gandelman to achieve coordination of the nitrenium cation. 19 We designed aliphatic and aromatic sulfonium ligands with NMR active nuclei in the vicinity of sulfur, namely methylene protons in I and a uorine atom in II (Chart 2), that would allow detecting the formation of an S-M bond in solution, by tracing their chemical shis and magnetic coupling to NMR-active metal centers, 103 Rh and 195 Pt.
Both sulfonium pincer ligands were prepared by alkylation or arylation of the corresponding bis-phosphine sulde ligands 20 with the phosphines protected as borane adducts or phosphine oxides in aliphatic and aromatic systems, respectively (Scheme 1), resulting aer deprotection in ligands 4a [OTf] and 4b [OTf]. To obtain XRD structures of sulfonium ligands ( Fig. 1) or their complexes ( Fig. 3 and 4, vide infra) the triate counterions were in some cases exchanged for tetraphenylborate or hexauorophosphate.

Synthesis and characterization of the Rh(I)-sulfonium complexes
The coordinative behavior of the aliphatic sulfonium ligand 4a [OTf] towards Rh(I) was tested by reacting it with [RhCl(COE) 2 ] 2 (Scheme 2). A full conversion to a symmetric Rh(I) complex was evident by 31 P NMR, as the chemical shi moved from a singlet at À18.2 ppm to a doublet at +46.6 ppm ( 1 J Rh-P ¼ 127.8 Hz).
In the 1 H NMR spectrum, signicant downeld shis of all aliphatic signals are observed (Fig. 2). Each of the methylene protons signals a and b divides upon coordination into two (a* and b* pairs, respectively), indicating the formation of a rigid structure with no rotation around C-C bonds. Furthermore, an additional splitting of 1.3 Hz appears in the quartet assigned to the ethyl tail methylene protons (c*). By means of 1 H-103 Rh HMBC ( Fig. S3 †), this splitting has been attributed to a throughbond 3 J Rh-H interaction. The latter is only possible if sulfonium is coordinated to the Rh center.
Encouraged by these results, we then turned to the aromatic ligand 4b[OTf] (Scheme 2). Here also, a full conversion of the ligand to a symmetric Rh(I) complex 5b[OTf] was evident from the 31 P NMR spectrum, where the chemical shi changed from a singlet at À13.0 ppm to a doublet of doublets at +48.7 ppm ( 1 J Rh-P ¼ 126.0 Hz; 5 J F-P ¼ 6.0 Hz). Interestingly, the 31 P-19 F Chart 2 Design of sulfonium-based pincer ligands. interaction unobservable in the spectrum of the free ligand became noticeable aer coordination, perhaps due to the additional rigidity of the formed complex.
The 19 F NMR spectrum of 5b [OTf] showed only a small downeld shi compared to the free ligand (À104.1 vs. À105.3 ppm, respectively) and no additional splitting by 103 Rh could be identied. Likewise, no 19 F-103 Rh interactions could be detected by HMBC, hence in this case, metal coordination to the aromatic sulfonium moiety could not be validated by NMR alone.
Nevertheless, the irrefutable evidence of sulfonium-Rh bonding in both systems was provided by XRD. Both complexes 5a[BPh 4 ] and 5b[PF 6 ] exhibited a slightly distorted squareplanar geometry around the metal (with a s parameter of 0.1, Table 1), typical of d 8 complexes ( Fig. 3a and b, respectively). Notably, the sulfonium-Rh(I) bond lengths of 2.126(2) and 2.112(1) A observed in 5a[BPh 4 ] and 5b[PF 6 ], respectively, are among the shortest reported S-Rh bonds (Table 1). These are signicantly shorter than in Rh(I) complexes with suldes (>2.24 A) and even with sulfoxides (typically, 2.159-2.291 A). 21 In fact, shorter Rh(I)-S bonds (2.069-2.100 A) were only observed with the strongest p-acceptor ligands: SO 2 22 and the related N-sulnylaniline. 23 These exceptionally short S-Rh bonds in 5a [BPh 4 ] and 5b[PF 6 ] cannot be explained solely by the grip of the pincer framework. Indeed, in both the analogous aliphatic sulfoxide pincer complex 8 that we prepared for comparison (Fig. S17 †) and the reported aromatic ones, 24 the Rh-S bonds are still longer than in their sulfonium counterparts (2.135 and 2.134 A, respectively).
Undoubtedly, these structures not only broaden the very limited pool of known sulfonium complexes but also proved for the rst time the coordinating ability of an aromatic sulfonium cation. It is noteworthy, that unlike the a-cationic suldes, which undergo oxidative addition with electron rich metals, 25 the sulfonium complexes 5a[OTf] and 5b[OTf] remained stable as solids and in solutions.

Synthesis and characterization of the Pt(II)-sulfonium complexes
Having shown that stable complexes of sulfonium cations with the neutral RhCl fragment can be obtained, we wondered whether, similarly to cationic nitrenium 19b and arenium 26 pincer ligands, our frameworks could also induce bonding between these cations and a net positively charged metal    H NMR signals at 1.20 and 1.56 ppm, corresponding to single methyls, appeared as triplets indicating magnetic equivalence of the two phosphines, which is only possible in a mutual trans-orientation ( Fig. S1 and S2 †). Moreover, the signals of the aliphatic protons in 7a[BF 4 ] 2 followed a pattern similar to that of 5a[OTf] (Fig. 2), suggesting an analogous structure (Fig. S1 †). To further study sulfonium-Pt interaction in solution we applied 1 H-195 Pt HMBC, once again focusing on magnetic interaction between Pt and the methylene protons of the ethyl tail (Fig. S4 †) (Fig. S5 †), even though both complexes exhibited nearly identical chemical shis in 19 F NMR (À102.3 and À102.5 ppm, respectively). The former showed no 19 F-195 Pt correlation, while the latter revealed a prominent cross-peak with a coupling constant of 3.3 Hz, supporting the presence of a sulfonium-Pt bond.

Theoretical analysis of metal-sulfonium bonding and the inuence of the pincer framework
The exceptionally short metal-sulfonium bonds observed in our Rh complexes prompted us to undertake a computational investigation by DFT. To gain a proper insight, we applied the energy decomposition analysis 27 combined with the natural orbitals for chemical valence theory (EDA-NOCV) which provides a quantitative description of L-M bonding in a visual and chemically intuitive manner. 28,29 In this method the overall interaction energy (DE int ) between two molecular fragments (e.g. the sulfonium ligand and the rest of the complex) is assessed by means of EDA; then NOCV is applied to extract the total orbital interaction contribution (DE orb ) and decompose it into individual constituents (DE orb(n) ) according to their orbital symmetry. Each such constituent is then represented by a deformation density plot (Dr (n) ) that visualizes the redistribution of charge upon combination of the two molecular fragments.
First, we considered the Rh-S bonding interactions in the model monodentate aliphatic and aromatic sulfonium complexes 10a and 10b and compared them with analogous complexes of neutral phosphines, suldes and sulfoxides, as well as with a few representative cationic ligands. By inspecting the deformation density plots of the most signicant orbital interactions (DE orb(n) ), we could identify a single s-symmetric interaction that has a clear L / M donation character, and two p-symmetric ones (perpendicular and parallel to the coordination plane) corresponding to the M / L back-donation (see representative deformation density maps of 10a in Fig. 5a and for other maps see Tables S22 and S23 †). Interestingly, in the only reported pincer complex of the isoelectronic telluronium cation the s interaction is in an opposite direction, i.e., it has a M / L character, thus classifying telluronium as a Z-type ligand. 30 This difference in s-bonding characteristics between sulfonium and telluronium can be rationalized by the so-called inert-pair effect, 31 which in this case reects the difference in energy of the 3s electrons of sulfonium compared to the 5s electrons in telluronium. In the latter the energy of this lone pair is too low to play any role in the bonding to the metal; this can only occur thanks to the donation from the metal's d orbital to the s* orbitals of telluronium. Therefore, while isoelectronic, sulfonium and telluronium systems are not isolobal.
As evident from Table 2 in terms of their BDEs and sdonation, sulfonium cations are nearly similar to suldes and sulfoxides. However, sulfonium cations are signicantly stronger p-acceptors, with p-back-bonding interaction being predominant. This is quite unusual and not the case even for the strongly p-acidic peruorinated phosphines (in complexes 14a-c), where similarly to common phosphines (in 13a and 13b), s-donation still prevails. This predominance of p-backdonation over s-donation appears specic only to cationic ligands considered here. Compared to the latter, the p-acidity of sulfonium stands between that of N-heterocyclic nitrenium ([NHN] + , in 15a) and N-heterocyclic phosphenium ([NHP] + , in 15b), and is comparable to Alcarazo's tris-cationic phosphine PR 3+ (in 15c). 32 With the cationic [PtMe] + fragment the calculations conrmed that the monodentate sulfonium complexes 16a and 16b (Table 2) are kinetically stable, despite the electrostatic repulsion between the positively charged metal fragment and   the sulfonium ligand responsible for the calculated positive BDE values. The obtained density plots of the model Pt complexes 16a and 16b were comparable in shape with those of the Rh complexes 10a and 10b (Fig. S18 †), with prominent sand p-symmetric interactions. As expected for a positively charged metal center, the contribution of the p back-bonding in these model Pt complexes is signicantly weaker than in their RhCl counterparts, yet still not negligible.
The inuence of the pincer framework on bonding in both the Rh complexes 17a and 17b and their Pt analogues 18a and 18b is quite pronounced. As evident from Table 3, one can see that in both complexes the geometry deformations imposed by the pincer ligands strengthen the p back-donation within the complexes, so that the overall p/s ratio signicantly increases. Remarkably, in the case of the Pt complexes 18a and 18b p backbonding even becomes comparable to the s-donation, in spite of the positive charge on the metal center.
These changes in bonding character can be rationalized by comparing the geometries of the pincer complexes relative to the monodentate ones. The following discussion of the aliphatic and aromatic Rh complexes, as displayed in Fig. 6a, b and S19, † respectively, is also applicable to the Pt systems.
In the aliphatic sulfonium pincer systems (both 5a[BPh 4 ] and its model analog 17a), the average P-Rh-S angles are $15 smaller than in the optimized monodentate complex 10a (Fig. 6a). Such a decrease essentially pushes the phosphine lone pairs closer to those of sulfonium, increasing repulsive interactions between them. Thus, the sulfonium lone-pair is pushed away from the metal, which results in weakening the s-donation in pincer complexes (Table 4, column 2). At the same time, this angle reduction also causes a stronger repulsion between the lone pairs of the phosphines and the lled d xy orbital of the metal, shiing electron density closer to the adjacent s*-orbital of the sulfonium (Fig. 6b). An enhanced in-plane p-backdonation is thus induced (Table 4, column 3).
In addition, the pincer framework also distorts the otherwise nearly planar coordination environment around the metal, pushing the two phosphines out of the coordination plane (Fig. 6b). This in turn results in repulsive interactions with the lled d xz orbital, similarly strengthening the interaction with the perpendicular s*-orbital of the sulfonium (Fig. 6c). Therefore, p-back-donation in the perpendicular plane increases as well (Table 4, column 4).
Overall, the EDA-NOCV data clearly points out that geometric distortion imposed by the pincer framework not only preserves the unique characteristics of sulfonium cations as weak sdonors and potent p-acceptors, but also enhances them. For comparison, an analogous attempt to incorporate a phosphenium moiety within a pincer framework resulted in a full charge transfer from the metal to the ligand, transforming it into a phosphide. 33

Conclusions
To summarize, in this paper we have consolidated the status of sulfonium cations among the family of rare cationic ligands   demonstrating for the rst time that their coordination chemistry can be extended to the Pt group metals. We also prepared the very rst examples of metal-coordinated aromatic sulfonium cations. These unusual compounds might represent stable analogs of possible transient intermediates forming during Pd-catalyzed cross-coupling of sulfonium salts. Our calculations suggested that sulfonium cations are among the best p-acceptors available. Moreover, the pincer frameworks which offer additional robustness also intensify this propensity. These scaffolds might therefore be the key to transform sulfonium complexes from a chemical curiosity into potential p-acid catalysts, the applications of which are currently studied in our lab.

Data availability
Experimental procedures, NMR spectra and computational details are given in ESI. †

Author contributions
Y. T. conceived the project, R. L., N. B., and O. C. performed the experiments, V. S. performed the XRD data renement and DFT calculations, and V. S., Y. T., and F. T. wrote the paper.

Conflicts of interest
There are no conicts of interest.