Sulfur-based Ligands Research Articles

Rational Design of Regenerable Amino-Functionalized Fluorescent Covalent Organic Framework for the Exclusive Detection of Mercury(II).

Goal-oriented development of novel covalent organic frameworks (COFs) to construct a sensing platform for highly toxic mercury (II, Hg2+) is of tremendous significance. Recently, numerous COFs with sulfur-based ligands were developed for Hg2+ monitoring; however, strong binding of Hg2+ by sulfur makes their regeneration very tough. Herein, we designed and developed an amino-functionalized fluorescent COF (COF-NH2) through facile postmodification for Hg2+ detection in which the π-conjugation skeleton is the signal reader and the nitrogen-based side is the highly selective Hg2+ receptor. More importantly, this nitrogen-based receptor permits the reversible binding of Hg2+. As a sensing platform, the outstanding performance of COF-NH2 for Hg2+ detection was reached with respect to high sensitivity with an ultralow detection of 15.3 nM, real-time response with rapid signal change of 10 s, and facile visualization with significant fluorescence color change. Expectedly, COF-NH2 obtained facile recycling which still shows excellent response performance toward Hg2+ after six cycles based on the reversible interaction between amino groups and Hg2+. Our work not only shows an attractive foreground of fluorescent COF for Hg2+ detection but also emphasizes the easy construction of novel COF materials via the rational introduction of metal ligands for the recognition of other metal ions.

Semiconductive Behavior and Photoconductivity of Uranyl Dithiophosphinate Single Crystal.

Herein, we detail the synthesis, structure, and photoconductivity of the uranyl dithiophosphinate single crystal UO2[S2P(C6H5)2]2(CH3OH)·CH3OH (denoted as U-DPDPP). The formation of bonds between uranyl ions and sulfur-based ligands endows U-DPDPP with a distinct electronic absorption property with a broadband spectrum spanning from 250 to 550 nm, giving rise to a unique semiconductive property. Under X-ray illumination, U-DPDPP displays a distinctive photoconductivity response, with a charge carrier mobility lifetime (μτ) of 2.78 × 10-4 cm2·V-1 achieved, which contradicts the electronic-silence behavior of uranyl nitrate crystal.

Study of cobalamin adducts with cysteine and its oxidized sulfenic, sulfinic and sulfonic derivatives

In this paper, the reactivity of cobalamin towards S-oxidized cysteine derivatives (sulfenic, sulfinic, sulfonic and S-sulfate) is analyzed and compared to the reactivity towards related nitrogen, oxygen and sulfur-based ligands, focusing on the concept of linkage isomerism. UV-Vis spectra complemented by DFT and TD-DFT calculations show that cysteine and its oxidized derivatives do yield adducts, with a preference for binding to the cobalt through the sulfur.

Thiomolybdate Clusters: From Homogeneous Catalysis to Heterogenization and Active Sites.

Thiomolybdates are molecular molybdenum-sulfide clusters formed from Mo centers and sulfur-based ligands. For decades, they have attracted the interest of synthetic chemists due to their unique structures and their relevance in biological systems, e.g., as reactive sites in enzymes. More recently, thiomolybdates are explored from the catalytic point of view and applied as homogeneous and molecular mimics of heterogeneous molybdenum sulfide catalysts. This review summarizes prominent examples of thiomolybdate-based electro- and photocatalysis and provides a comprehensive analysis of their reactivities under homogeneous and heterogenized conditions. Active sites of thiomolybdates relevant for the hydrogen evolution reaction are examined, aiming to shed light on the link between cluster structure and performance. The shift from solution-phase to surface-supported thiomolybdates is discussed with a focus on applications in electrocatalysis and photocatalysis. The outlook highlights current trends and emerging areas of thiomolybdate research, ending with a summary of challenges and key takeaway messages based on the state-of-the-art research.

Recent Insights to Prepare High-Quality Perovskite Nanocrystals via “Green” and Ecofriendly Solvents and Capping Agents

The synthesis of halide perovskite nanocrystals (PNCs) with mesmerizing photophysical properties has allowed for the fast development of efficient optoelectronic and photovoltaic devices, as well as making them ideal photocatalysts for solar-driven chemical reactions. However, the use of traditional oleic acid/oleylamine with low binding energy and the introduction of some phosphine- and sulfur-based ligands generate the emergence of highly defective PNCs with poor stability, fast quenching of their PL features, and increase in the toxicity of the final perovskite product. In this review, we will show the use of prominent “green” and ecofriendly solvents and capping ligands with the capability to enhance the quality of the PNCs by suppressing structural defects. By introducing promising ecofriendly agents such as biogenic species and ligands extracted from natural sources, it is possible to favor the radiative recombination dynamics into the perovskite, being beneficial to enhance the device performance. Novel passivation alternatives or synthetic routes are highlighted in this contribution, giving a deeper understanding of the control of surface chemistry in PNCs through ligand engineering to prolong the stability of the nanocrystals.

Through the Looking Glass: Using the Lens of [SNS]-Pincer Ligands to Examine First-Row Metal Bifunctional Catalysts.

ConspectusHomogeneous catalysis is at the forefront of global efforts to innovate the synthesis of fine chemicals and achieve carbon-neutrality in energy applications. For decades, the push toward sustainable catalysis has focused on the development of first-row transition metal catalysts to supplant widespread use of precious metals. Metal-ligand cooperativity is an effective strategy to yield high-performing first-row metal molecular catalysts. Despite remarkable progress, state of the art catalysts often employ phosphorus-based ligands which are air-sensitive, potentially toxic, and on occasion offset the cost-savings of the metal. Thus, the development of simple and economical ligands composed of biomimetic donors should be a key focus that cannot be overlooked in the pursuit of sustainable catalyst candidates. This is an Account of our group's efforts to develop first-row transition metal complexes which use [SNS]-pincer ligands for bifunctional catalysis. We have synthesized two potentially tridentate ligands, one bearing an amido and two thioether donors [(SMeNSMe), L1] and one which includes thiolate, imine, and thioether donors [(SNSMe), L2], and used them as platforms upon which to explore the reaction pathways of first-row metals. The [SNS] ligand, L1, leads to formation of high-spin paramagnetic metal complexes of the type M(L1)2 in which the 6-membered ring thioether donor is hemilabile (M = Mn, Fe, Co). This allows Mn(L1)2 to function as a carbonyl hydroboration catalyst that operates by a novel hydride-free, inner-sphere reaction pathway. Exploring the reactivity of L2 with Fe and Ni revealed unique coordination chemistry and a variety of mono-, di-, tri-, and tetranuclear complexes enabled by bridging thiolates. Further studies showed L2 undergoes selective Caryl-S bond cleavage upon coordination to a metal with electron-rich phosphine donors, yielding a new (CNS)2- pincer ligand. The analogous reaction with L1 afforded a new (CNSMe)- pincer ligand via both Caryl-S and benzylic C-H bond cleavage. In an attempt to prepare Fe(L2)2, we obtained instead an Fe(N2S3) complex in which imine C-C bond formation affords a potentially hexadentate redox-active ligand. The Fe(N2S3) complex is a selective catalyst for hydroboration of aldehydes and appears to operate through a complicated mechanism. In contrast, a mechanistic study of Mn(L2)(CO)3-photocatalyzed dihydroboration of nitriles indicated that both the flexibility of the κ3-SNSMe ligand (fac- vs mer-coordination) and ability of Mn to undergo a spin-state change are required to access low energy barriers for this transformation. To effectively compare the reactivity of the thiolate vs amido donor, we prepared two Cu complexes, Cu(L1)(IPr) and Cu(L2)(IPr) [IPr = 1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene], showing that, while both served as carbonyl hydroboration catalysts, only the amido complex was an effective catalyst for carbonyl hydrosilylation. In addition, complexes of the type Zn(L1)2, Zn(L2)2, and Zn(L1)(L2), were also effective for catalytic carbonyl hydroboration. While Zn(L1)(L2) was most active, catalyst speciation studies showed that each undergoes bifunctional catalyst activation to form a Zn bis(alkoxide) catalyst. Overall, our observations using [SNS] ligands with first-row transition metals show how the absence of traditional phosphine donors leads to different fundamental reactivity. Furthermore, this Account demonstrates the gap of knowledge which exists in understanding the reactivity of sulfur-based ligands to promote more widespread adoption of sustainable ligands.

Phosphinous Acid-Phosphinito Tetra-Icosahedral Au52 Nanoclusters for Electrocatalytic Oxygen Reduction.

While the formation of superatomic nanoclusters by the three-dimensional assembly of icosahedral units was predicted in 1987, the synthesis and structural determination of such clusters have proven to be incredibly challenging. Herein, we employ a mixed-ligand strategy to prepare phosphinous acid-phosphinito gold nanocluster Au52(HOPPh2)8(OPPh2)4(TBBT)16 with a tetra-icosahedral kernel. Unlike expected, each icosahedral Au13 unit shares one vertex gold atom with two adjacent units, resulting in a "puckered" ring shape with a nuclearity of 48 in the kernel. The phosphinous acid-phosphinito ligand set, which consists of two phosphinous acids and one phosphinito motif, has strong intramolecular hydrogen bonds; the π-π stacking interactions between the phosphorus- and sulfur-based ligands provide additional stabilization to the kernel. Highly stable Au52(HOPPh2)8(OPPh2)4(TBBT)16 serves as an effective electrocatalyst in the oxygen reduction reaction. Density functional theory calculations suggest that the phosphinous acid-phosphinito ligands provide the most active sites in the electrochemical catalysis, with O* formation being the rate-determining step.

Design of some novel dithio π-conjugated ligand-based metal complexes and their semiconductor behavior

Transition metal complexes with sulfur-based ligands are the area of interest for scientists working for semiconducting molecular materials. These complexes have been reported for their interesting electronic architecture and can be used in the manufacture of various kinds of electronic devices including diodes, transistors and integrated circuits. The nitrophenol thiourea benzaldehyde-based metal complexes with various transition metals have been reported for their electrical behavior. The present paper involves synthesis, spectroscopic characterization and variable temperature solid-state conductivity measurement. The complexes show semiconducting nature.

Self-assembly of N-heterocyclic carbenes on Au(111)

Although the self-assembly of organic ligands on gold has been dominated by sulfur-based ligands for decades, a new ligand class, N-heterocyclic carbenes (NHCs), has appeared as an interesting alternative. However, fundamental questions surrounding self-assembly of this new ligand remain unanswered. Herein, we describe the effect of NHC structure, surface coverage, and substrate temperature on mobility, thermal stability, NHC surface geometry, and self-assembly. Analysis of NHC adsorption and self-assembly by scanning tunneling microscopy and density functional theory have revealed the importance of NHC-surface interactions and attractive NHC-NHC interactions on NHC monolayer structures. A remarkable way these interactions manifest is the need for a threshold NHC surface coverage to produce upright, adatom-mediated adsorption motifs with low surface diffusion. NHC wingtip structure is also critical, with primary substituents leading to the formation of flat-lying NHC2Au complexes, which have high mobility when isolated, but self-assemble into stable ordered lattices at higher surface concentrations. These and other studies of NHC surface chemistry will be crucial for the success of these next-generation monolayers.

Application of synthesized 2,5-bis(allyloxy)terephthalohydrazide functionalized covalent organic framework material as a fluorescence probe for selective detection of mercury (II)

The mercury ion Hg2+ is a highly toxic environmental contaminant that threatens both the ecosystem and human health. As such, in recent years, significant effort has been invested in developing novel strategies for ultrasensitive detection of Hg2+. Herein, we leveraged the inherent benefits of covalent organic frameworks (COFs) to design a fluorescent COF with the ability to detect Hg2+, then synthesized a 2,5-bis(allyloxy)terephthalohydrazide functionalized COF material, called BATHz-Bt, for selective Hg2+ recognition. Unlike other methods that rely on sulfur-based ligands, BATHz-Bt is comprised of a π-conjugated framework that serves as the signal transducer; and compact, evenly distributed allyloxy groups that function as the Hg2+ acceptor. In addition to the benefits provided by a stable, COF structure, the material is also re-usable, making it reliable, cost effective, and eco-friendly. Furthermore BATHz-Bt showed good detection performance, as evidenced by its high sensitivity, good selectivity, excellent pH stability, and reusability. Moreover, X-ray photoelectron spectroscopy (XPS) confirmed the presence of a robust and selective interaction between the BATHz-Bt allyloxy groups and Hg2+. Thus, while this study demonstrates the application potential of fluorescent COFs for Hg2+ detection, more importantly, it also provides a new, simple approach to environmental monitoring using functionalized COFs.

Exploration of the Interaction Strength at the Interface of Anionic Chalcogen Anchors and Gold (111)-Based Nanomaterials.

Nowadays, the use of sulfur-based ligands to modify gold-based materials has become a common trend. Here, we present a theoretical exploration of the modulation of the chalcogenides-gold interaction strength, using sulfur, selenium, and tellurium as anchor atoms. To characterize the chalcogenide-gold interaction, we designed a nanocluster of 42 gold atoms (Au42) to model a gold surface (111) and a series of 60 functionalized phenyl-chalcogenolate ligands to determine the ability of electron-donor and -withdrawing groups to modulate the interaction. The analysis of the interaction was performed by using energy decomposition analysis (EDA), non-covalent interactions index (NCI), and natural population analysis (NPA) to describe the charge transfer processes and to determine data correlation analyses. The results revealed that the magnitudes of the interaction energies increase following the order S < Se < Te, where this interaction strength can be augmented by electron-donor groups, under the donor-acceptor character the chalcogen–gold interaction. We also found that the functionalization in meta position leads to better control of the interaction strength than the ortho substitution due to the steric and inductive effects involved when functionalized in this position.

PH Dependent Reversible Formation of a Binuclear Ni2 Metal-Center Within a Peptide Scaffold.

A disulfide-bridged peptide containing two Ni2+ binding sites based on the nickel superoxide dismutase protein, {Ni2(SODmds)}, has been prepared. At physiological pH (7.4) it was found that the metal sites are mononuclear with a square planar NOS2 coordination environment with the two sulfur-based ligands derived from cysteinate residues, the nitrogen ligand derived from the amide backbone and a water ligand. Furthermore, S K-edge X-ray absorption spectroscopy indicated that the two cysteinate sulfur atoms ligated to nickel are each protonated. Elevation of the pH to 9.6 results in the deprotonation of the cysteinate sulfur atoms, and yields a binuclear, cysteinate bridged Ni22+ center with each nickel contained in a distorted square planar geometry. At both pH = 7.4 and 9.6 the nickel sites are moderately air sensitive, yielding intractable oxidation products. However, at pH = 9.6 {Ni2(SODmds)} reacts with O2 at an ~3.5-fold faster rate than at pH = 7.4. Electronic structure calculations indicate the reduced reactivity at pH = 7.4 is a result of a reduction in S(3p) character and deactivation of the nucleophilic frontier molecular orbitals upon cysteinate sulfur protonation.

Synthesis, characterization, and binding affinity of hydrosulfide complexes of synthetic iron(II) porphyrinates

The binding and reactivity of the hydrosulfide ion (HS−) to iron(II) porphyrinates has been examined for several synthetic meso-tetraphenylporphine (TPP) derivatives. In all cases, HS− coordinates to the iron centers in a 1:1 stoichiometry with formation constants (Kf) that reflect the electronic characteristics of the porphyrinate ligands. In the case of the F8TPP ligand (F8TPP=dianion of 5,10,15,20-tetrakis(2,6-difluorophenyl)porphine), an intermediate complex proposed as the hydrosulfide bridged dimer, (Bu4N)[Fe2(μ-SH)(F8TPP)2], was identified by NMR spectroscopy en route to formation of (Bu4N)[Fe(SH)(F8TPP)]. A robust procedure is reported for the synthesis and isolation of the parent hydrosulfide adduct, (Bu4N)[Fe(SH)(TPP)], which has permitted a detailed examination of its spectroscopy and chemical reactivity. Electrochemical measurements demonstrate that [Fe(SH)(TPP)]− is oxidized reversibly at a potential of −0.832V (vs ferrocene/ferrocenium) consistent with other iron porphyrinates containing sulfur-based ligands. Despite this fact, chemical oxidation of (Bu4N)[Fe(SH)(TPP)] with ferrocenium tetrafluoroborate produced only [Fe(TPP)] indicating that the putative iron(III) hydrosulfide adduct, [Fe(SH)(TPP)], decomposes rapidly. Treatment of (Bu4N)[Fe(SH)(TPP)] with other biologically relevant molecules such as NO and 1,2-dimethylimidazole resulted in simple displacement of the HS− ligand as governed by the relative Kf values of the added ligands. The solid-state structure of one hydrosulfide adduct, (Bu4N)[Fe(SH)(F8TPP)], was determined by X-ray crystallography and found to display the expected five-coordinate geometry about iron with an Fe-S distance of 2.323(1) Å. The relevance of the hydrosulfide chemistry with synthetic iron porphyrinates is discussed in terms of the possible reactivity for H2S and its derivatives at heme sites in biology.

The Sulfur–FluorineGaucheEffect in Coinage-Metal Complexes: Augmenting Conformational Equilibria by Complexation

Controlling the rotation about unhindered C(sp3)–C(sp3) bonds by simple structural changes has obvious benefits in molecular design. While the avoidance of nonbonding interactions remains one of the cornerstones of acyclic conformational control, stabilizing stereoelectronic effects have the added benefit that conformer populations can be fine-tuned by augmenting or diminishing the central interaction. Strategies may include adjusting the oxidation state of a substituent or reversible formation of a complex to modulate MO levels. In the case of the sulfur–fluorine gauche effect, the propensity of the S–C–C–F motif to adopt a synclinal arrangement (ΦFCCS = 60°), the conformer population distribution of the three dominant rotamers partitioned by 120° can be biased by oxidation of the S atom. Motivated by the importance of sulfur-based ligands in main structural chemistry, the sulfur–fluorine gauche effect was translated to an organometallic paradigm as a potential tool to achieve structural preorganization. This would allow the influence of coinage-metal complexation on conformer population to be initially assessed. The synthesis and characterization of a model gold(I) and silver(I) metal complex featuring a ligand system containing a freely rotatable SCCF motif is disclosed. In both complexes, the title stereoelectronic effect manifests itself in the expected conformation, with the synclinal-endo conformer being preferred. This was corroborated by X-ray crystallography and DFT analysis, and the molar fraction of rotamers was extrapolated from a detailed solution-phase NMR spectroscopic analysis. Complexation was found to reinforce the sulfur–fluorine gauche effect.

Moisture stable heteroleptic titanium (IV) complexes derived from 8-hydroxyquinoline: synthesis, antibacterial, and antifungal studies

Systematic efforts have been made for the synthesis of moisture stable derivatives of titanium (IV) with both nitrogen- and sulfur-based ligands. New heteroleptic titanium (IV) complexes derived from 8-hydroxyquinoline (HQ) of the types [(Q2)Ti{((OiPr)2-n(L1OR1)n-1)(L1OR)}] (3a–3c), [(Q2)Ti(OiPr)2-n(L2)] (3d,3g), [(Q2)Ti(L1OR)(L2)] (3e, 3f, 3h, 3i) were prepared by treating the precursor [(Q2)Ti(OiPr)2] (2) with different alkoxyalkanols, 2-hydroxypyridine (HPy), and 2-hetroaryl methyl ketone oxime in different mole ratios using anhydrous toluene as solvent (where, n = 1–2, HQ = 8-HQ, iPr = isopropyl, L1 = O–CH2–CH2–, R = CH3, C2H5, R1 = C2H5, and L2H = 2-HPy, HONC(Me)th-2). Stability study of all these complexes were performed by UV–Visible (UV–Vis) spectroscopy at room temperature and was interpreted on the basis of absorption band arose due to ligand-to-metal charge transfer. All these complexes have good stability against hydrolysis and the shift observed in the absorption band was negligible even after the duration of 72 h persistence in 9:1 mixture of dimethyl sulfoxide and water. All the derivatives were characterized by various physicochemical techniques such as NMR (1H, 13C), Elemental Analysis, Mass, Thermogravimetric Analysis (TGA), FT-IR, and UV–Vis spectroscopy. Monomeric nature of all these derivatives was confirmed by mass spectra. TGA describes the multistage decomposition of complexes at 900 °C. Elemental analyses results are in agreement with the theoretical values. Other spectral data (NMR, FTIR, and UV–Vis) propose the attachment of titanium and ligands in hexa-coordinated fashion. Further, these were subjected for antibacterial and antifungal evaluation against a panel of microorganisms. Complex 3i has shown better activity compared to all its analogs of this series and it was more potent than the standard drugs gentamicin and fluconazole with minimum inhibitory concentrations of 0.017 µM against the Escherichia coli and 0.035 µM for Aspergillus niger, respectively.

Towards a quantitative understanding of palladium metal scavenger performance: an electronic structure calculation approach

Dispersion corrected density functional theory (DFT-D) has been applied to understand the performance of several palladium metal scavengers. Nine different sulfur-based ligands and three different palladium metal sets have been investigated in detail. Based on a thorough analysis of the thermodynamic binding parameters ΔH, ΔG and ΔS, we have identified the best binding modes for all scavenger ligands. Bis-monodentate coordination is favoured over chelation in ΔH and ΔG values for most of the scavenger ligands. Special attention has been paid to the ligand strain energies, which account for the structural changes of the ligands upon complexation indicating that small (5-membered) chelates are considerably less favourable than expected. Some ligands can use their longest chain (>7-atoms) to yield trans chelates, which ligands with shorter chains (≤6-atoms) are unable to form. A secondary amino nitrogen (RR'NH) is found to be the best donor with highest binding enthalpy for Pd(ii) metal systems. In terms of the strength of the initial binding interactions, -SMe > -SH; capping thiols (-SH) as thioethers (-SMe) is therefore suggested to be an effective strategy in scavenger design. These observations mark the beginning of a knowledge base of the full range of possible interactions, leading to the construction of a sulfur ligand database for the design of scavenger systems.

Studies of Iron(III) Porphyrinates Containing Silanethiolate Ligands

The chemistry of several iron(III) porphyrinates containing silanethiolate ligands is described. The complexes are prepared by protonolysis reactions of silanethiols with the iron(III) precursors, [Fe(OMe)(TPP)] and [Fe(OH)(H2O)(TMP)] (TPP = dianion of meso-tetraphenylporphine; TMP = dianion of meso-tetramesitylporphine). Each of the compounds has been fully characterized in solution and the solid state. The stability of the silanethiolate complexes versus other iron(III) porphyrinate complexes containing sulfur-based ligands allows for an examination of their reactivity with several biologically relevant small molecules including H2S, NO, and 1-methylimidazole. Electrochemically, the silanethiolate complexes display a quasi-reversible one-electron oxidation event at potentials higher than that observed for an analogous arenethiolate complex. The behavior of these complexes versus other sulfur-ligated iron(III) porphyrinates is discussed.

Cu8(μ4-H){S2P(OEt)2}6](PF6): A Novel Catalytic Hydride-Centered Copper Cluster for Azide-Alkyne Cycloaddtion

A hydride-centered dithiophosphate cluster [Cu8(μ4-H){S2P(OEt)2}6](PF6) (1)] previously developed by us was applied as a new catalyst to the 1,3-dipolar cycloaddition of organic azides and alkynes for preparing substituted trizoles. With the required catalyst loading as low as 0.4 mol%, the reactions of terminal alkynes with BnN3 all proceeded smoothly at ambient temperature in CH3CN to exclusively produce 1,4-triazoles in good yields. For these reactions, it is assumed that the formation of the requisite copper acetylide intermediate is facilitated by the abstraction of the terminal hydrogen of alkynes by the hydride released from the central of the cluster. With only few examples being documented in literatures, the reactions of a range of internal alkynes have also been realized under the catalysis of 1 (0.8 mol%) in DMF at elevated temperature, to yield 1,4,5-trisubstituted triazoles in moderate to high yields. Our study has provided a preliminary insight into the effect of sulfur-based ligands on the activity of copper ion.

Seven-coordinate iron(II) complexes of sulfur-based N3S2-macrocyclic ligands: synthesis, properties, and crystal structure

The reaction of pyridine-2,6-dicarbaldehyde or 2,6-diacetylpyridine with 1,2-bis(o-aminophenylthio)ethane (1) in acetonitrile in the presence of stoichiometric amounts of iron(II) perchlorate gave the complexes [(pyN3S2)FeII(ClO4)2] (4) and [(pyN3Me2S2)FeII(ClO4)2] (5) of the 15-membered N3S2 macrocyclic ligands, pyN3S2 = {6,7-dihydro-15,19-nitrilobenzo(e,p)(1,4,7,15)dithiadiazacyclo-heptadecine-N,N′,N″,S,S′} and pyN3Me2S2 = {6,7-dihydro-16,18-dimethyl-15,19-nitrilobenzo(e,p)(1,4,7,15)dithiadiazacyclo-heptadecine-N,N′,N″,S,S′}, respectively. Physical measurements led to the conclusion that these complexes contained seven-coordinate iron(II) and a single-crystal X-ray examination of 4 confirmed this. Coordination of the Fe(II) center in 4 is best described as distorted pentagonal-bipyramidal with the three nitrogen atoms and two sulfur donors of the macrocycle defining the pentagonal plane and the perchlorate ions occupying axial positions. Room temperature (293 K) magnetic moments of 4 and 5 (μ eff = 4.9 and 4.7 B.M., respectively) are close to the value predicted for high-spin d6 systems.

New Carbon- and Sulfur-based Ligands in Catalysis

Homogeneous catalysis is a field of research that has gained central importance in both organic and inorganic chemistry and the use of well-defined ligand systems in the synthesis of transition metal complexes has had an enormous impact on the development of such catalysts. Neutral, two-electron donor ligands based on phosphorous and nitrogen have been tremendously successful as ancillary entities for late-transition metal (LTM) catalysts, whereas ligands based on anionic nitrogen, oxygen and the cyclopentadienyl motif (Cp(-)) have propelled early-transition metal (ETM) catalysis forward. We believe that expanding the ligand families capable of acting as successful entities in metal-mediated reactivity and catalysis is crucial for future discoveries in this field. Research in our group therefore tries to identify new non-chiral and chiral ligands for late-transition metal chemistry that are based on neutral, two-electron carbon and sulfur donor atoms. In particular, we have until now focused on the development of modular, monodentate N-heterocyclic carbene ligands (NHCs) that can serve as a basis for the development of chiral ligand frameworks for the application to asymmetric catalytic transformations. In the second major research project developed over the last six years, we have started an investigation on the use of chelating sulfoxide-based ligands in asymmetric late transition-metal based catalysis.