Sulfonium Cation in the Service of π-Acid Catalysis.

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.

Effect of Internal Ligand Strain on Coordination Behavior of PSP Pincer Ligands.

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.

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.

Synthesis, structural characterization, and bonding analysis of two-coordinate copper(I) and silver(I) complexes of pyrrole-based bis(phosphinimine): new metal-pyrrole ring π-interactions.

The reaction between 2,5-bis(diphenylphosphinomethyl)pyrrole and Me3SiN3 gave the new pyrrole-based bis(phosphinimine) L1H in an excellent yield. L1H reacts with [CuCl(COD)]2, AgBF4, or AgOTf to give the corresponding two-coordinate mononuclear ionic complex formulated as [M{(L1H)-κ2N,N}]+[X]- where M = Cu and Ag; X = [CuCl2], BF4 or OTf. Their single crystal X-ray diffraction studies confirmed the two-coordinate geometry formed by the chelate bonding mode of L1H. These 10-membered metalacycles exhibit planar chirality and were also characterized by spectroscopic methods. In addition, in all three structures, there exists a hitherto unknown π-interaction between the pyrrole ring atoms and metal, represented as η2-(Cα-N) in the copper(i) complex, and η3-(Cα-N-Cα') in the silver(i) complexes. These weak interactions were supported by DFT calculations in terms of their electron densities, non-covalent interaction plots and the decrease in the aromaticity of the pyrrole ring.

Versatility of the bis(iminopyrrolylmethyl)amine ligand: tautomerism, protonation, helical chirality, and the secondary coordination sphere with halogen bonds in the formation of copper(II) and nickel(II) complexes.

The reaction of N,N-di(2,6-bis(isopropyl)phenylimino-pyrrolyl-α-methyl)-N-methylamine H2L1 with copper(i) sources such as CuX (X = Cl (1), Br (2), and I (3)) afforded bis(chelated) ionic copper(ii) complexes of the type [CuL1H]X. A similar type of mononuclear structure was obtained with Cu(NO3)2·(H2O)3. Conversely, binuclear copper(ii) complexes [Cu2(µ-L1)(µ-OOCCH3)(µ-OH)](4) and [Cu2(µ-L1H)(µ-OOCPh)(µ-O)] (5) were obtained from the reaction of Cu(O2CR)2·H2O with H2L1. Notably, these reactions in the presence of a base yielded the neutral copper(ii) complex [CuL1] (6). This product was also obtained from the reaction of complex 2 or 4 with NaOH in methanol. All structures feature a dianionic imino-pyrrole motif and a protonated central amine function except 4. The reaction of H2L1 with NiCl2·DME gave the mononuclear complex [NiCl2(L1H2)], 7. In contrast to this, the reaction of the newly synthesized sterically less encumbered ligand N,N-di(phenylimino-pyrrolyl-α-methyl)-N-methylamine H2L2 with NiCl2·DME gave the binuclear complex [NiCl(L2H2)(HOMe)]2[Cl]2 (8). Both 7 and 8 show the amine-azafulvene ligand form and coordination of the central amine. The reaction of complex 7 with NaHBEt3 yielded a neutral complex [NiL1] (8) containing the imino-pyrrole form. In the molecular structures, interesting secondary coordination spheres incorporating guest molecules such as CHCl3 and MeOH in the crystal lattices and the presence of helical enantiomers were observed and analysed. In one case, CHCl3 was found inside an unusual cage-like structure supported by halogen bonds. Preliminary DFT calculations on the geometry of the nickel complex with H2L1 showed that the pentacoordinated tbp geometry is more stable than the square planar geometry.

Crystal structure and DFT analyses of a pentacoordinated PCP pincer nickel(II) complex.

The reaction of NiCl2 with 1,3-bis[(diphenylphosphanyl)methyl]hexahydropyrimidine in the presence of 2,6-dimethylphenyl isocyanide and KPF6 afforded a new pentacoordinated PCP pincer NiII complex, namely {1,3-bis[(diphenylphosphanyl)methyl]hexahydropyrimidin-2-yl-κN2}(2,6-dimethylphenyl isocyanide-κC)nickel(II) hexafluoridophosphate 0.70-hydrate, [Ni(C9H9N)(C30H30ClN2P2)]PF6·0.7H2O or [NiCl{C(NCH2PPh2)2(CH2)3-κ3P,C,P'}(Xylyl-NC)]PF6·0.7H2O, in very good yield. Its X-ray structure showed a distorted square-pyramidal geometry and the compound does not undergo dissociation in solution, as shown by variable-temperature NMR and UV-Vis studies. Density functional theory (DFT) calculations provided an insight into the bonding; the nickel dsp2-hybridized orbitals form the basal plane and the nearly pure p orbital forms the axial bond. This is consistent with the NBO (natural bond orbital) analysis of analogous nickel(II) complexes.

Nickel(II) and Palladium(II) Complexes Bearing an Unsymmetrical Pyrrole-Based PNN Pincer and Their Norbornene Polymerization Behaviors versus the Symmetrical NNN and PNP Pincers.

Unsymmetrical pincers have been shown to be better than the corresponding symmetrical pincers in several catalysis reactions. A new unsymmetrical PNN propincer, 2-(3,5-dimethylpyrazolylmethyl)-5-(diphenylphosphinomethyl)pyrrole (1), was synthesized from pyrrole through Mannich bases in a good yield. In addition, the new byproduct 2-(3,5-dimethylpyrazolylmethyl)-5-(dimethylaminomethyl)- N-(hydroxymethyl)pyrrole was also isolated. The reaction of 1 with [PdCl2(PhCN)2] and Et3N in toluene yielded [PdCl{C4H2N-2-(CH2Me2pz)-5-(CH2PPh2)-κ3 P,N,N}] (2). The analogous reaction between 1 and [NiCl2(DME)] or NiX2 (X = Br, I) in the presence of NEt3 in acetonitrile afforded [NiX{C4H2N-2-(CH2Me2pz)-5-(CH2PPh2)-κ3 P,N,N}] (3; X = Cl, Br, I). All complexes were structurally characterized. The norbornene polymerization behaviors of the unsymmetrical pincer complexes 2 and 3 in the presence of MMAO or EtAlCl2 were compared with those of the symmetrical pincer complexes chloro[2,5-bis(3,5-dimethylpyrazolylmethyl)pyrrolido]palladium(II) (NNN), chloro[2,5-bis(diphenylphosphinomethyl)pyrrolido]palladium(II), and chloro[2,5-bis(diphenylphosphinomethyl)pyrrolido]nickel(II) (PNP) at different temperatures. The PNN and NNN complexes exhibited far greater activity on the order of 107 g of PNB/mol/h, with quantitative yields in some cases, in comparison to the PNP pincer palladium and nickel complexes. This trend was also supported by the iPr group substituted PNP nickel and palladium pincer complexes. These polymerization behaviors are explained using steric crowding around the metal atom with the support of NMR studies and suggested that the activity increases as the Npyrazole donor increases. Polymers were characterized by 1H NMR, IR, TGA, and powder XRD methods.

Facile synthesis of Pd(ii) and Ni(ii) pincer carbene complexes by the double C-H bond activation of a new hexahydropyrimidine-based bis(phosphine): catalysis of C-N couplings.

Hexahydropyrimidine-based bis(phosphine), a pro-NHC ligand, was synthesized in one step and excellent yield. It underwent spontaneous double C-H bond activation to give cationic pincer NHC complexes of the type [(PCP)MCl]X (M = Pd, Ni and X = Cl, BF4) in the absence of any external reagents. Their structures were determined by X-ray diffraction methods and the mechanism of formation of palladium carbene complexes as analyzed by DFT calculations showed two transition states. The Pd(ii) carbene complex effectively catalyzes a few C-N cross coupling reactions.

Ultrasonic-assisted synthesis of graphene oxide - fungal hyphae: An efficient and reclaimable adsorbent for chromium(VI) removal from aqueous solution.

In this study, a hybrid film bio-nanocomposite material was developed based on the graphene oxide/fungal hyphae (GO-FH) interaction. The developed GO-FH bio-nanocomposite material was used for the removal of hexavalent chromium from aqueous solution. The GO-FH bio-nanocomposite material was prepared by ultrasonic irradiation technique. The synthesized GO-FH bio-nanocomposite material was characterized by XRD, FT-IR, SEM, TEM and TGA. The adsorption experiments were carried out in batch mode to optimize parameters such as pH, adsorbent dosage, initial Cr(VI) ion concentration, contact time and shaking speed. The results indicated that the adsorption of Cr(VI) onto GO-FH bio-nanocomposite material was pH dependant, with the maximum adsorption capacity of 212.76 mg/g occurred at pH 2.0. The adsorption studies followed, Langmuir isotherm and pseudo second order kinetic model. Findings demonstrates that GO-FH bio-nanocomposite material exhibited excellent regeneration performance.

Preparation of graphene oxide/chitosan/ferrite nanocomposite for Chromium(VI) removal from aqueous solution.

A magnetically modified graphene oxide/chitosan/ferrite (GCF) nanocomposite material was synthesized and exploited for removal of Chromium(VI) from aqueous solution. The GCF nanocomposite material was characterized by powder-X-ray diffraction (powder-XRD), Fourier transform infrared spectroscopy (FT-IR), scanning electron microscope- energy dispersive X-ray (SEM-EDX) analysis, transmission electron microscopy (TEM) thermogravimetric analysis (TGA), UV-vis diffusive reflectance spectra and Brunauer-Emmett-Teller (BET) analysis. The effect of pH, adsorbent dose, contact time and initial Cr(VI) metal ion concentration were studied in batch process. The GCF nanocomposite material showed an adsorption capacity of 270.27 mg g-1 for Cr(VI) at pH 2.0. The adsorption mechanism of GCF adsorbent material was well described by Langmuir isotherm and pseudo second order kinetic model, with a high regression coefficient (<0.99). The results have shown that GCF nanocomposite material can be used as a suitable adsorbent for removal of Cr(VI) from wastewater.