Ligand Substituents Research Articles

Recent Advances in Metal-Mediated Stereoselective Ring-Opening Polymerization of Functional Cyclic Esters towards Well-Defined Poly(hydroxy acid)s: From Stereoselectivity to Sequence-Control.

Poly(hydroxy acid)s are a family of biocompatible and (bio)degradable polyesters with various outcomes in different domains of application. To date, poly(hydroxy acid)s are best prepared by ring-opening polymerization (ROP) of the corresponding cyclic esters. Using racemic chiral monomers featuring side-chain groups enables to access, providing a stereoselective catalyst/initiator system is implemented, stereoregular functional polymers, thereby improving their physico-chemical properties, and ultimately, widening their range of uses. Here, we highlight a few important advances in metal-mediated stereoselective ROP of cyclic esters towards the synthesis of (functional) stereoregular poly(hydroxy acid)s that have recently been disclosed, emphasizing on (functional) β- and γ-lactones, diolide and O-carboxyanhydride (OCA) monomers and yttrium-based catalysis. Fine-tuning of the substituents flanked on the catalyst ligand enables reaching poly(hydroxy acid)s with syndiotactic and also isotactic microstructures. The stereocontrol mechanisms at work and their probable origin, relying on steric but also electronic factors imparted in particular by the ligand substituents, are discussed. Taking advantage of such stereoselective ROPs, original copoly(hydroxy acid)s with gradient or alternated patterns then become accessible from the use of mixtures of chemically different, oppositely configured enantiopure monomers.

Selective Ethylene Dimerization into 2-Butenes Using Homogeneous and Supported Nickel(II) 2-Iminopyridine Catalysts

In this work, we report a series of novel nickel(II) dibromide and dichloride complexes with 2-iminopyridine and 2-iminoquinoline ligands bearing electron-withdrawing substituents (F, Cl, CF3), that have demonstrated high ethylene dimerization activity [up to 19.2 × 106 g of oligomers·(mol Ni)−1 h−1] in the presence of MAO or Et2AlCl, affording predominantly a mixture of 1-butene and cis- and trans-2-butene (C4 selectivity varies from 89 to 100%; 2-butenes selectivity in the C4 fraction approaches 96%). The effects of ligand substituents and the cocatalyst nature on the activity and selectivity of the nickel(II) complexes in ethylene dimerization have been established. Several nickel complexes were supported on silica-alumina and the resulting heterogeneous catalysts were probed towards ethylene dimerization. These catalysts also afford 2-butenes, with the activity and C4 selectivity being comparable to that of homogeneous catalysts.

Ligand-Dependent Luminescence Properties of Lanthanide-Titanium Oxo Clusters.

A series of lanthanide-titanium oxo clusters (LnTOCs), Ln2Ti8-Ac, Ln2Ti8-p-Toluic, and Ln2Ti8-Anthra (Ln = Eu and Tb), were prepared based on acetic acid (HAc), p-toluic acid (Hp-Toluic), and anthracene-9-carboxylic acid (HAnthra). Crystal structural analysis showed that these clusters possess the same metal topology framework, in which eight Ti4+ ions form a cube and two Ln3+ ions are located on the opposite faces of the cube. The luminescence investigation discovered that the Eu2Ti8-Ac displays the highest quantum yields with 15.6%, and the conjugation effect of ligand substituents can lower the triplet state energy of ligands, thus regulating the luminescence quantum yield of the Ln2Ti8 clusters. These results suggest that the triplet excited-state energy of the ligands should match well with the energy levels of Ln3+ to enhance the luminescence.

Self-Healing Heterometallic Supramolecular Polymers Constructed by Hierarchical Assembly of Triply Orthogonal Interactions with Tunable Photophysical Properties.

Here, we present a method for the building of new bicyclic heterometallic cross-linked supramolecular polymers by hierarchical unification of three types of orthogonal noncovalent interactions, including platinum(II)-pyridine coordination-driven self-assembly, zinc-terpyridine complex, and host-guest interactions. The platinum-pyridine coordination provides the primary driving force to form discrete rhomboidal metallacycles. The assembly does not interfere with the zinc-terpyridine complexes, which link the discrete metallacycles into linear supramolecular polymers, and the conjugation length is extended upon the formation of the zinc-terpyridine complexes, which red-shifts the absorption and emission spectra. Finally, host-guest interactions via bis-ammonium salt binding to the benzo-21-crown-7 (B21C7) groups on the platinum acceptors afford the cross-linked supramolecular polymers. By continuous increase of the concentration of the supramolecular polymer to a relatively high level, supramolecular polymer gel is obtained, which exhibits self-healing properties and reversible gel-sol transitions stimulated by various external stimuli, including temperature, K+, and cyclen. Moreover, the photophysical properties of the supramolecular polymers could be effectively tuned by varying the substituents of the precursor ligands.

In situ formed Cr(III) based silicon-bridged PNS systems for selective ethylene tri-/tetramerization

In-situ formed and activated chromium(III) catalysts based on silicon-bridged PNS (Si-PNS) ligands of the form Ph2PN(R1)Si(CH3)2S(R2) have been investigated for selective ethylene tri-/tetramerization. Ligand substituents at the backbone N and S atoms have been realized to have a marked impact on the product selectivity in these systems. Ligands having the substituents such as cyclohexyl (in ligand 3), ortho-tolyl (in ligand 5) or ethyl (in ligand 6) at backbone S, and a low steric bulk moiety (isopropyl) at backbone N in these ligands favor tetramerization of ethylene and respectively deliver 75%, 76% and 66% C8 selectivity. Ligands having higher steric bulk substituents like 2,6-diisoprpylphenyl (in ligand 1) or cyclopentyl (in ligand 2) at backbone N, and a cyclohexyl moiety at backbone S in these ligands promote trimerization of ethylene and produce C6 fraction. While the ortho-tolyl moiety at backbone S and a 2,6-diisopropylphenyl substituent at backbone N completely diminish the C6 and C8 selectivity in ligand 4 based system and produce higher oligomers. DFT calculations revealed that the sterically relaxed ligand 3 provide a facile and energetically favorable route for C8 formation, while sterically encumbered substituents tend to block the formation of C6 and C8 fractions in ligand 4 based system.

Phenanthroline-Based Molecular Switches for Prospective Chemical Grafting: A Synthetic Strategy and Its Application to Spin-Crossover Complexes.

1,10-Phenanthroline represents a well-known versatile ligand system finding many applications in chemistry, biology, and material science. The properties and thus the use of these molecules are determined by coordinating metal ions and ligand substituents. Advanced ligand systems that, for instance, feature simultaneously an integrated photochrome and a surface anchoring group require the introduction of several differing substituents and the synthesis of asymmetric derivatives. In spite of a long history of the ligand system-and to our great surprise-a general synthetic approach allowing the introduction of differing substituents at positions (3,8) and (5,6) of 1,10-phenanthroline is not known. Here, we present a general approach for the synthesis of such phenanthrolines. The approach is used to integrate a diarylethene photochrome into a functionalized phenanthroline and thus to synthesize a novel photoswitchable phenanthroline and a corresponding spin-crossover molecular photoswitch. The functionality of both the ligand and its iron(II) complex at room temperature has been demonstrated. The importance of this work for chemical grafting of molecular switches based on phenanthrolines is emphasized.

Promising Catalytic Processes of Dicyclopentadiene Metathesis Polymerization to Obtain a New Class of Polymeric and Polymer–Composite Materials

A lineup of highly efficient ruthenium catalysts with N-containing substituents in the benzylidene ligand is developed that differ in the rate of initiation of dicyclopentadiene metathesis polymerization. The choice of the optimal catalyst makes it possible to control the lifetime of the monomer–catalyst reaction mixture in a wide range, which makes the use of various technologies effective for the production of polymer and composite materials. The new catalysts have been used to design a series of innovative polymer and polymer-composite materials with a polymer matrix based on a finished monomer, dicyclopentadiene, which is a raw material of the petrochemical industry. The polymeric materials are distinguished by a unique combination of high mechanical properties and good thermal stability in a wide temperature range (from –60 to +320°C), which allows their use in various climatic zones. Composites with a matrix based on polydicyclopentadiene are not inferior to commercially available counterparts based on epoxy and polyester resins and even surpass them in a number of parameters.

Electronic Structure and Magnetic Properties of o-Benzoquinone Iron Complexes with Tetraazamacrocyclic Ligands

Structure, energy characteristics, and magnetic properties of o-benzoquinone iron complexes with tetradentate macrocyclic nitrogen containing bases (pyridinophane and tris(2-pyridylmethyl)amine) are studied with a DFT (UTPSSh/6-311++G(d,p)) quantum chemical method. Electron withdrawing substituents in the redox-active ligand stabilize its dianionic form but do not significantly affect the energy difference between the electromers. Strong ferromagnetic exchange interactions are predicted in the isomers of complexes containing a high-spin divalent iron ion and the semiquinone form of o-benzoquinone. According to the calculations, all studied compounds can undergo thermally induced spin crossover transitions.

A New Class of Pyridine-Amide Containing Ti and Zr Based Catalysts for Olefin Polymerization: Influence of Ligand Substituents

Two tridendate pyridine-amide ligands in di-deprotonated form have been used to synthesize new titanium and zirconium complexes. The molecular structure of pentafluoro containing Titanium and Zirconium complexes were determined by DFT calculations. These new metal complexes have been further evaluated for ethylene polymerization activity in the presence of co-catalyst methylaluminoxane (MAO) under atmospheric pressure, producing polyethylenes with moderate molecular weights and unimodal molecular weight distribution. Remarkably, zirconium complexes show higher activity than titanium complexes under the same polymerization condition. The catalytic activity of all the complexes were found to be temperature dependent and reach their highest efficiency at 70 °C. The influence of the nature of the ligands and reaction parameters upon the catalytic activities has been studied. Additionally, the pentafluoro containing Zirconium and Titanium complexes were further evaluated for the copolymerization of ethylene and propylene and exhibited good catalytic activity. A new class of titanium and zirconium based metal complexes were synthesised from pyridine-amide containing ligands. The synthesised complexes have been evaluated for ethylene polymerization in the presence of co-catalyst methylaluminoxane (MAO) under atmospheric pressure. Interestingly, zirconium complexes show high catalytic activities than titanium complexes under the same polymerization condition.

Tuning the Thiolen: Al(III) and Fe(III) Thiolen Complexes for the Isoselective ROP of rac-Lactide

A series of five iron and aluminum complexes bearing {ONSO} imine thiobis(phenolate) ligands have been prepared and applied to the ring-opening polymerization (ROP) of rac-lactide. Fe(1)Cl produced polylactide with a very strong isotactic bias (Pm = 0.79–0.89) and well-defined melting temperatures (Tm = 154–181 °C). The polymers have been characterized by a combination of 1H{1H} NMR, 13C{1H} NMR, gel permeation chromatography, thermogravimetric analysis, differential scanning calorimetry, and powder X-ray diffraction. Fe(1)Cl has also been shown to activate CO2, at atmospheric pressure and concentrations, to form a carbonato-bridged dimer. Fe(2–5)Cl and Al(1–5)Me were also active for lactide ROP demonstrating good-molecular-weight control (Đ = 1.04–1.12) and moderate isotactic preference (Pm = 0.56–0.72), with polymerization outcome correlating with ligand substituents.

Aqueous Phase Sensing of Fe3+ and Ascorbic Acid by a Metal-Organic Framework and Its Implication in the Construction of Multiple Logic Gates.

A new HfIV -based metal-organic framework with UiO-66 topology was synthesized via a one-step solvothermal method by using 3-methyl-4-phenylthieno[2,3-b]thiophene-2,5-dicarboxylic acid (H2 MPTDC) as a ligand. The MOF material showed a high stability in a broad pH range (from pH 2 to pH 12) in an aqueous medium. The presence of hydrophobic methyl and phenyl substituents in the carboxylic acid ligand and strong Hf-O bond play crucial roles in its stability. The new MOF material was systematically characterized by various techniques such as XRPD, N2 sorption, thermogravimetric analyses and FT-IR spectroscopy. The photophysical properties of the MOF material were also examined by steady-state and time-resolved fluorescence studies. It was observed that the blue fluorescence of the MOF material was selectively quenched in the presence of Fe3+ ion in pure aqueous medium. A mechanistic study disclosed that quenching occurs via a strong inner filter effect (IFE) arising from Fe3+ ion in aqueous medium. Interestingly, the fluorescence of the MOF material can be recovered by elimination of the IFE of Fe3+ ion via reduction of Fe3+ ion by ascorbic acid (AA). Based on the fluorescence recovery by AA, a MOF based on-off-on probe was developed for the sensing of Fe3+ ion and AA in aqueous medium. Inspired by this reversible sensing event, we demonstrate basic (NOT, OR, YES, INHIBIT and IMP) and higher integrated logic operations utilizing this fluorescent MOF. This MOF-based logic systems could be potentially used for next-generation logic-gate based analytical applications as well as for the detection and discrimination of targeted molecules in various complex domains.

Cu(II) complexes derived from N-carboxymethyl and N-carboxyethyl amino acids as catalysts for asymmetric oxidative coupling of 2-naphthol

The synthesis, characterization and catalytic performance of chiral Cu(II) complexes derived from N-carboxymethylated and N-carboxyethylated amino acids is reported. The ligand precursors are prepared by single step N-alkylation of the sodium salts of the appropriate chiral amino acid with either sodium chloroacetate or sodium 3-chloropropionate in water. The Cu(II) complexes are obtained upon reaction of Cu(CH3COO)2 with the aqueous or alcoholic suspension of the suitable ligand under vigorous stirring or ultrasound irradiation at room temperature. The Cu(II) compounds are characterised by EPR, UV–vis, circular dichroism and ESI-MS. The molecular structures of two of the prepared complexes are also obtained by single-crystal X-ray diffraction analysis. The catalytic activity of the complexes in the asymmetric oxidative coupling of 2-naphthol is described. All compounds exhibit moderate activity, selectivity and enantioselectivity in ethanol/water mixtures, under aerobic conditions and using potassium iodide as additive. The yields of 1,1′-bi-2-naphthol (BINOL) reached 50% under the optimal conditions, while enantiomeric excesses reached ca. 48%. The effect of variables such as ligand substituents, solvent, temperature and additives on the catalytic activity is also described. In the absence of a base, the complexes only show catalytic activity in the presence of alkali metal iodide such as KI. Details of the oxidative coupling mechanism are studied using spectroscopic and electrochemical methodologies.

(Arylimido)vanadium(V)-Alkylidene Complexes as Catalysts for Ring-opening Metathesis Polymerization (ROMP) of Cyclic Olefins: Ligand Design for Exhibiting the High Activity

(Imido)vanadium(V)-alkylidene complexes of type V(CHSiMe3)(NR)(OR′)(PMe3)2 [R = Ad, C6H5, 2,6-Me2C6H3, 2,6-Cl2C6H3; R′ = 2,6-Me2C6H3, 2,6-iPr2C6H3, 2,6-F2CeH3, C6F5, C6Cl5] exhibited from moderate to remarkable catalytic activities for ring-opening metathesis polymerization (ROMP) of norbornene (NBE). The catalytic activities were affected by the ligand substituents, and V(CHSiMe3)(N-2,6-Cl2C6H3)(OC6X5)(PMe3)2 (X = F, Cl) demonstrated the exceptionally high catalytic activities for ROMP of NBE. The complexes polymerized cycloheptene (CHPE) and cis-cyclooctene (COE), and ROMP of COE by the OC6Cl5 analogue proceeded in a living manner even at 80 °C, and the activity increased with increasing the temperature up to 120 °C. Highly active catalysts for ROMP of cyclic olefins (NBE, cyclopentene, and CHPE) can be generated in situ by premixing isolated V(CHSiMe3)(NC6F5)(O-2,6-iPr2C6H3)(PMe3)2 with 1.0 equiv. of C6F5OH or C6Cl5OH via immediate phenoxy exchange; the activity was affected by the kind of phenol added [TOF in the ROMPs of NBE: 4.62 × 104 min−1 (upon addition of C6F5OH) versus 37.3 min−1 (none)].

Evaluation of 5-hydroxy-1,4-naphthoquinone-cobalt(III) complexes for hypoxia-activated drug delivery

Three novel Py2N2-cobalt(III) complexes with the 5-hydroxy-1,4-naphthoquinone nuclei (NQ) were evaluated as potential hypoxia-activated anticancer prodrugs. The complexes were synthesized and fully characterized by IR and UV–Visible spectroscopies, ESI mass spectrometry and CHN elemental analysis. Structural information was obtained from density functional theory (DFT) calculations. Cyclic voltammetry analysis in acetonitrile indicates that the ligand substituents (H, CH3 and p-tolylthio) do not have a relevant effect on the Co3+/Co2+ redox potential. Reactions with ascorbic acid in phosphate buffers were performed to simulate redox activation of the complexes in biological media. Fast and irreversible dissociation of the NQ ligands was observed for all complexes upon Co3+/Co2+ reduction. Cytotoxic activity of complexes 1 and 3 was evaluated in tumor cells (HT-29 and HCT-116) under hypoxic and normoxic conditions.

Crossover from Antiferromagnetic to Ferromagnetic Exchange Coupling in a New Family of Bis-(μ-phenoxido)dicopper(II) Complexes: A Comprehensive Magneto-Structural Correlation by Experimental and Theoretical Study.

Five neutral bis(μ-phenoxido)dicopper(II) complexes, [Cu2(LMe,Me,Me)2] (1), [Cu2(LMe,Me,Et)2]·CH2Cl2 (2), [Cu2(Li-Pr,i-Pr,i-Pr)2]·2H2O (3), [Cu2(Lt-Bu,Me,i-Pr)2] (4), and [Cu2(Lt-Bu,t-Bu,i-Pr)2]·H2O (5) have been synthesized and characterized by single-crystal X-ray diffraction analyses, magnetic studies, and density functional theory (DFT) calculations, in which the ligands [H2LMe,Me,Me = N,N-bis(2-hydroxy-3,5-dimethylbenzyl)-N′,N′-dimethylethylene-1,2-diamine, H2LMe,Me,Et = N,N-bis(2-hydroxy-3,5-dimethylbenzyl)-N′,N′-dimethylethylene-1,2-diamine, H2Li-Pr,i-Pr,i-Pr = N,N-bis(2-hydroxy-3,5-diisopropylbenzyl)-N′,N′-diisopropylethylene-1,2-diamine, H2Lt-Bu,Me,i-Pr = N,N-bis(2-hydroxy-3-tert-butyl-5-methylbenzyl)-N′,N′-diisopropylethylene-1,2-diamine, and H2Lt-Bu,t-Bu,i-Pr = N,N-bis(2-hydroxy-3,5-di-tert-butylbenzyl)-N′,N′-diisopropylethylene-1,2-diamine] contain the same [O,N,N,O]-donor atoms combination but differ in substituents at phenol rings and at an amino nitrogen atom. The effect of these remote substituents on the nature of exchange coupling interactions (ferromagnetic vs antiferromagnetic) between the copper(II) ions has been investigated. The average Cu–O–Cu angle, Cu–O–Cu–O torsion angle, and Cu···Cu separation in 1–5 are varied systematically by these remote ligand substituents in the range 98.6–83.3°, 26.0–46.5°, and 2.982–2.633 Å, respectively. As a result, the intramolecular spin–spin coupling in these complexes are changing gradually from a strong antiferromagnetic (J = −395 cm–1, where Ĥ = −JŜ1Ŝ2) to a moderate ferromagnetic (J = +53.2 cm–1) regime. The crossover angle at which the magnetic interaction changes from antiferromagnetic to ferromagnetic (J = 0) is determined to be ca. 87° for this series of dicopper(II) complexes. DFT calculations support the experimentally determined crossover angle and disclose various magneto–structural correlations in the series 1–5.

Homo- and Copolymerization of Ethylene with Norbornene Catalyzed by Vanadium(III) Phosphine Complexes.

Herein, we report the homo- and co-polymerization of ethylene (E) with norbornene (NB) catalyzed by vanadium(III) phosphine complexes of the type VCl3(PMenPh3-n)2 [n = 2 (1a), 1 (1b)] and VCl3(PR3)2 [R = phenyl (Ph, 1c), cyclohexyl (Cy, 1d), tert-butyl (tBu, 1e)]. In the presence of Et2AlCl and Cl3CCOOEt (ETA), 1a–1e exhibit good activities for the polymerization of ethylene, affording linear, semicrystalline PEs with a melting temperature of approximately 130 °C. Mainly alternating copolymers with high comonomer incorporation were obtained in the E/NB copolymerization. A relationship was found between the electronic and steric properties of the phosphine ligands and the catalytic performance. Overall, the presence of electron-withdrawing ligand substituents increases the productivity, complexes with aryl phosphine (weaker σ–donor character) exhibiting a higher (co)polymerization initiation rate than those with alkyl phosphines (stronger σ–donor character). Steric effects also seem to play a key role since 1d and 1e, having large size phosphines (PCy3 θ = 170° and PtBu3 θ = 182°, respectively) are more active than 1a (PMe2Ph θ = 122°). In this case, the larger size of PtBu3 and PCy3 likely compensates for their higher donor strength compared to PMe2Ph.

The Equatorial Ligand Effect on the Properties and Reactivity of Iron(V) Oxo Intermediates.

High-valent metal oxo oxidants are common catalytic-cycle intermediates in enzymes and known to be highly reactive. To understand which features of these oxidants affect their reactivity, a series of biomimetic iron(V) oxo oxidants with peripherally substituted biuret-modified tetraamido macrocyclic ligands were synthesized and characterized. Major shifts in the UV/Vis absorption as a result of replacing a group in the equatorial plane of the iron(V) oxo species were found. Further characterization by EPR spectroscopy, ESI-MS, and resonance Raman spectroscopy revealed differences in structure and the electronic configuration of these complexes. A systematic reactivity study with a range of substrates was performed and showed that the reactions are affected by electron-withdrawing substituents in the equatorial ligand, which enhance the reaction rate by almost 1016 orders of magnitude. Thus, the long-range electrostatic perturbations have a major influence on the rate constant. Finally, computational studies identified the various electronic contributions to the rate-determining reaction step and explained how the equatorial ligand periphery affects the properties of the oxidant.

A Facile Enantioselective Alkynylation of Chromones

AbstractThe first catalytic enantioselective alkynylation of chromones is reported. In this process, chromones are silylated to form silyloxybenzopyrylium ions that lead to silyl enol ethers after Cu‐catalyzed alkyne addition using StackPhos as a ligand. The outcome of the reaction is impacted by distal ligand substituents with differing electronic character and it was found that successful reactions could be achieved with different ligand congeners by using different solvents. This sequence enables access to different products by protonation or further functionalization, thus increasing complexity in a divergent manner. The transformation is high yielding over a broad scope to provide a variety of useful chromanones in high enantioselectivity.

A Facile Enantioselective Alkynylation of Chromones.

The first catalytic enantioselective alkynylation of chromones is reported. In this process, chromones are silylated to form silyloxybenzopyrylium ions that lead to silyl enol ethers after Cu-catalyzed alkyne addition using StackPhos as a ligand. The outcome of the reaction is impacted by distal ligand substituents with differing electronic character and it was found that successful reactions could be achieved with different ligand congeners by using different solvents. This sequence enables access to different products by protonation or further functionalization, thus increasing complexity in a divergent manner. The transformation is high yielding over a broad scope to provide a variety of useful chromanones in high enantioselectivity.

Heterogeneous molecular catalysts for electrocatalytic CO2 reduction

This review provides an overview of the literature regarding heterogeneous molecular catalysts for electrochemical CO2 reduction (ECR). Fundamental aspects of the science, including aggregation, electrochemical rate laws, and electrode-catalyst electronic coupling, are discussed to provide a solid foundation on which to design experiments and interpret results. Mechanistic aspects of ECR are presented based on electrokinetic and spectroscopic measurements as well as density functional theory (DFT) calculations. Consensus is improving for electrokinetic measurements, but the redox state of the metal center under reaction conditions and DFT reaction pathways lack agreement in the literature. Concerning the tunable aspects of the molecular catalyst, the impacts of the metal center, ligand substituents, and electrode support on the activity and selectivity toward ECR are presented with an emphasis on those studies that controlled for aggregation and minimized mass-transport limitations. Extended three-dimensional (3D) structures such as polymers, metal-organic frameworks (MOFs), and covalent-organic frameworks (COFs) are discussed as highly tunable architectures that begin to mimic the catalytic pockets of enzyme active sites. To achieve the full potential of these catalysts, design principles must emerge based on a combination of deconvoluting measurements to extract intrinsic catalyst properties and more reliable theoretical calculations to predict reaction pathways.