Metal-functionalized cavitands are promising platforms for mimicking the chemical environments of hydrophobic pockets in natural metalloenzymes. However, successfully combining the unique supramolecular capabilities of cavitand scaffolds with the high reactivities of transition metal complexes still remains a major challenge. In this study, we present an original cavitand architecture featuring a coordinatively unsaturated Mn(II) center embedded deep within its pore. This metallocavitand was employed to generate a Mn(IV)-oxo species inside a molecular cavity. This elusive intermediate was fully characterized spectroscopically (UV–vis, EPR, X-ray photoelectric spectroscopy (XPS), and HRMS) and, for the first time for a pseudo-octahedral Mn(IV)-oxo species, also by XRD. The experimental data was corroborated by detailed ab initio/time-dependent density functional theory (TDDFT) and natural bond orbital (NBO) calculations, confirming the Mn(IV)-oxo (rather than Mn(III)-oxyl) electronic character of this species. Reactivity and mechanistic studies, including monitoring the decay of this complex in various chlorinated solvents and its reactions with representative substrates, revealed that, despite the steric protection provided by the cavitand scaffold, its Mn(IV)-oxo core remains highly reactive in both H atom abstraction (HAA) and O atom transfer (OAT) reactions. Moreover, this reactivity is subject to a high degree of steric control imposed by the cavitand framework capable of discriminating between potential substrate molecules based on their size and shape. This was further demonstrated by the regioselective oxidation of a bisphosphine substrate, emulating the regioselectivity of natural metalloenzymes.

A bicyclic PSP-pincer ligand featuring a sulfonium cation at the bridgehead position of its rigid triptycene-like scaffold was synthesized and metallated with a bis-cationic Pt(II) center. The resulting tris-cationic Pt(II) complex exhibits excellent photostability and improved catalytic performance compared to its analogue with the previously reported sulfonium pincer ligand.

Thioxanthone (TX) molecules and their derivatives are well-known photoactive compounds. Yet, there exist only a handful of luminescent systems combining TX with transition metals. Recently, we reported a TX-based PSP pincer ligand (L1) that appears as a promising platform for filling this niche. Herein, we demonstrate that with Cu(I) this ligand exclusively assembles into dimeric structures with either di- or polynuclear Cu(I) cores. With cationic Cu(I) precursors, complexes featuring solvent-bridged bis-cationic cores were obtained. These coordinatively unsaturated bimetallic systems showed surprisingly facile activation of the chloroform C–Cl bonds, suggesting a possible metal–metal cooperation. The reaction of L1 with binary Cu(I) halides afforded dimeric complexes with polynuclear [CuX]_n (n = 3 or 4) cores. With X = Br or I, emissive complexes containing stairstep [CuX]₄ clusters were obtained. Emission lifetimes in the microsecond range measured for these complexes were indicative of a triplet emission (phosphorescence), which according to our time-dependent density functional theory study originates from a halide-metal-to-ligand charge transfer between the [CuX]₄ cluster and the TX backbone of L1. Finally, the distinctive polynucleating behavior of L1 toward Cu(I) was also showcased by a comparison to another PSP ligand with a diaryl thioether backbone (L2), which formed only mononuclear pincer-type complexes, lacking any unusual reactivity or photoluminescence.

For a long time, the small group of cationic ligands stood out as obscure systems within the general landscape of coordinative chemistry. However, this situation has started to change rapidly during the last decade, with more and more examples of metal-coordinated cationic species being reported. The growing interest in these systems is not only of purely academic nature, but also driven by accumulating evidence of their high catalytic utility. Overcoming the inherently poor coordinating ability of cationic species often required additional structural stabilization. In numerous cases this was realized by functionalizing them with a pair of chelating side-arms, effectively constructing a pincer-type scaffold. This comprehensive review aims to encompass all cationic ligands possessing such pincer architecture reported to date. Herein every cationic species that has ever been embedded in a pincer framework is described in terms of its electronic structure, followed by an in-depth discussion of its donor/acceptor properties, based on computational studies (DFT) and available experimental data (IR, NMR or CV). We then elaborate on how the positive charge of these ligands affects the spectroscopic and redox properties, as well as the reactivity, of their complexes, compared to those of the structurally related neutral ligands. Among other systems discussed, this review also surveys our own contribution to this field, namely, the introduction of sulfonium-based pincer ligands and their complexes, recently reported by our group.

While still rare, cationic ligands offer much promise as tunable electron-withdrawing ligands for π-acid catalysis. Recently, we introduced pincer-type sulfonium cations into the list of available strongly π-acidic ancillary ligands. However, the M−S bond in sulfonium complexes of these ligands was found highly labile, precluding their catalytic applications. Herein we demonstrate that this obstacle can be overcome by increasing the rigidity of the sulfonium pincer scaffold. X-ray analyses confirm that despite bearing a formal positive charge, the sulfur atom of this newly designed sulfonium ligand maintains its coordination to the Pt(II)-center, while DFT calculations indicate that by doing so it strongly enhances the electrophilic character of the metal. Kinetic studies carried out on three model cycloisomerization reactions prove that such a tris-cationic sulfonium-Pt(II) complex is highly reactive, compared to its thioether-based analogue. This proof-of-concept study presents the first example of employing sulfonium-based ligands in homogeneous catalysis.

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.

Chelating ligands and most specifically pincer ligands, with their characteristic co-planar binding, usually undergo deformations upon coordination, resulting in a significant ligand strain. Such an effect on the properties of the so formed complex has rarely been explored. This study is an attempt to analyze this strain and its contribution to the overall binding energy and coordination behavior of PSP pincer ligands. Hence, we designed a rigid thioxanthone-based PSP pincer ligand (I) and studied the difference in the coordination properties with the more flexible thioxanthene and thioether-based PSP pincer ligands (II and III). Although with one equivalent of Pd(II) precursor, the three ligands exhibited a similar coordination behavior leading to similar κ³-P,S,P pincer complexes, an in-depth computational analysis pointed out the different contributions of the internal strain energy in lowering the binding energy of these complexes. This effect was clearly reflected when we calculated the enthalpy change of these ligand-exchange reactions. As these exchange reactions are enthalpy-driven, these results could also be confirmed experimentally. With two equivalents of Pd(II), the three ligands diverged in their coordination behavior. Specifically, ligands I and III gave each a binuclear complex, with different coordination modes, whereas the pincer complex of ligand II remained unaffected by excess of Pd(II). Our calculations suggest that the driving force for the formation of binuclear Pd(II) complexes is the relief of the internal ligand strain. With Pt(II), only the mononuclear κ³-P,S,P pincer complexes were obtained irrespectively of the amount of the Pt(II) precursor. In these cases, we assume that kinetic inertness of the formed mononuclear pincer Pt(II) complexes prevents binding of an additional Pt(II) nucleus. This study points out the important role of the internal ligand strain in PSP pincer ligand coordination behavior. We believe that our findings can be extended to other pincer ligands systems as well.

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.