Ligand Substituents Research Articles

Electronic influence of ligand substituents in the ring-opening polymerization of l-Lactide promoted by OSSO-type zirconium complexes

The zirconium complexes (OSSOCl)ZrOtBu2 (1), (OSSOBr)ZrOtBu2 (2) and (OSSOtBu)ZrOtBu2 (3) bearing OSSO-type ligands (OSSOCl-H = 1,4-dithiabutanedyl-2,2′-bis(4,6-dichlorophenol); OSSOBr-H = 1,4-dithiabutanedyl-2,2′-bis(4,6-dibromophenol); OSSOtBu-H = 1,4-dithiabutanedyl-2,2′-bis(4,6-di-tert-butyl-phenol)) were prepared and characterized. NMR spectroscopy revealed a rigid octahedral coordination geometry with a facial-facial coordination of the [OSSO] ligand to the metal complex. These compounds were active in the ring-opening polymerization of l-lactide exerting an effective control over the polymerization reaction. Activities correlated with the electron-withdrawing character of the ortho and para groups of the OSSO ligands. Density functional theory (DFT) investigations indicated that substitution with increasingly electron-withdrawing groups favors the monomer coordination and determines an earlier, more reactive transition state.

Computational Investigation of Scandium-Based Catalysts for Olefin Hydroaminoalkylation and C–H Addition

Great progress has been achieved in the olefin hydroaminoalkylation by using amines, which is an atom-efficient route for the synthesis of alkylated amine derivatives. However, success in the catalytic olefin hydroaminoalkylation with a simple tertiary amine is hitherto very limited. In this study, density functional theory was applied to investigate the hydroaminoalkylation of olefins with tertiary amines, catalyzed by a series of homoleptic tris(benzyl) scandium complexes. It is found that the catalytic performance can be improved via substitution of electron-withdrawing groups and modifying ligand frameworks to reduce their steric hindrance. In addition, the potential applications of scandium catalysts in the α-C(sp3)–H alkylation of various heteroatom-containing (P, As, O, S, and Se) substrates were explored. The results suggest that alkyl sulfides and selenides are promising substrates to undergo α-C(sp3)–H addition to olefins. Importantly, the effects of ligand backbone and substituent on catalytic ...

The Effect of Electrode Potential on the Conductivity of Polymer Complexes of Nickel with Salen Ligands

The influence of the electrode potential on the electric conductivity of polymer complexes of nickel with salen-type ligands (where salen is N,N′-ethylene-bis(salicylideneimine)) differing by substituents in ligand′s benzene ring is studied in the course of measuring cyclic voltammograms. The highest electron conductivity is observed for the film of poly[N,N′-ethylene-bis(3-methoxysalicylideneiminato) nickel(II)] (poly[Ni(CH3OSalen)]) in 1 M LiPF6 solution in the ethylene carbonate-diethylcarbonate mixture (EC: DEC = 1: 1). Poly[N,N′-ethylene-bis(3-methyl-salicyleneiminato) nickel(II)] (or poly[Ni(CH3Salen)]) was found to have the widest potential range of electronic conductivity. Conditions are selected for synthesizing films from solutions of the corresponding monomers in 1 M LiPF6 EC: DEC electrolyte. The electrode potential intervals suitable for the use of poly[Ni(CH3Salen)] as the buffer interlayer between the aluminum substrate and the cathode mass in lithium-ion batteries for protecting the latter against overcharge are found.

Solution-State Spin Crossover in a Family of [Fe(L)2(CH3CN)2](BF4)2 Complexes

We report herein on five new Fe(II) complexes of general formula [Fe(L)2(NCCH3)2](BF4)2•xCH3CN (L = substituted 2-pyridylimine-based ligands). The influence of proximally located electron withdrawing groups (e.g., NO2, CN, CF3, Cl, Br) bound to coordinated pyridylimine ligands has been studied for the effect on spin crossover in their Fe(II) complexes. Variable-temperature UV-visible spectroscopic studies performed on complexes with more strongly electronegative ligand substituents revealed spin crossover (SCO) in the solution, and thermodynamic parameters associated with the spin crossover were estimated.

A Continuum from Halogen Bonds to Covalent Bonds: Where Do λ3 Iodanes Fit?

The intrinsic bonding nature of λ 3 -iodanes was investigated to determine where its hypervalent bonds fit along the spectrum between halogen bonding and covalent bonding. Density functional theory with an augmented Dunning valence triple zeta basis set ( ω B97X-D/aug-cc-pVTZ) coupled with vibrational spectroscopy was utilized to study a diverse set of 34 hypervalent iodine compounds. This level of theory was rationalized by comparing computational and experimental data for a small set of closely-related and well-studied iodine molecules and by a comparison with CCSD(T)/aug-cc-pVTZ results for a subset of the investigated iodine compounds. Axial bonds in λ 3 -iodanes fit between the three-center four-electron bond, as observed for the trihalide species IF 2 − and the covalent FI molecule. The equatorial bonds in λ 3 -iodanes are of a covalent nature. We explored how the equatorial ligand and axial substituents affect the chemical properties of λ 3 -iodanes by analyzing natural bond orbital charges, local vibrational modes, the covalent/electrostatic character, and the three-center four-electron bonding character. In summary, our results show for the first time that there is a smooth transition between halogen bonding → 3c–4e bonding in trihalides → 3c–4e bonding in hypervalent iodine compounds → covalent bonding, opening a manifold of new avenues for the design of hypervalent iodine compounds with specific properties.

Mechanisms Underlying Allosteric Molecular Switches of Metabotropic Glutamate Receptor 5.

The metabotropic glutamate 5 (mGlu5) receptor is a class C G protein-coupled receptor (GPCR) that is implicated in several CNS disorders making it a popular drug discovery target. Years of research have revealed allosteric mGlu5 ligands showing an unexpected complete switch in functional activity despite only small changes in their chemical structure, resulting in positive allosteric modulators (PAM) or negative allosteric modulators (NAM) for the same scaffold. Up to now, the origins of this effect are not understood, causing difficulties in a drug discovery context. In this work, experimental data was gathered and analyzed alongside docking and Molecular Dynamics (MD) calculations for three sets of PAM and NAM pairs. The results consistently show the role of specific interactions formed between ligand substituents and amino acid side chains that block or promote local movements associated with receptor activation. The work provides an explanation for how such small structural changes lead to remarkable differences in functional activity. While this work can greatly help drug discovery programs avoid these switches, it also provides valuable insight into the mechanisms of class C GPCR allosteric activation. Furthermore, the approach shows the value of applying MD to understand functional activity in drug design programs, even for such close structural analogues.

Salen‐Manganese Complexes and their Application in the Ring‐Opening Polymerization of Lactide and ϵ‐Caprolactone

AbstractEfficiency and sustainability are of vital concern to the polymer industry. Ring‐opening polymerizations (ROP) of lactide (LA) and ϵ‐caprolactone (ϵ‐CL) producing biodegradable polymers polylactide (PLA) and polycaprolactone (PCL) were widely researched. Owing to their abundance and non‐toxicity, manganese catalysts are a very good choice. A group of salen manganese (III) complexes were synthesized. Their catalytic activity in the ROP of LA and ϵ‐CL in propylene oxide (PO) was. Under activation of PO, manganese complexes could efficiently mediate the ROP of LA and ϵ‐CL. The molecular weight distributions could be controlled well. The influence of the bridging part and substitutions were fully discussed. Surprisingly, chlorine substituents showed a negative effect on catalytic activity. The analog computation results indicated that the metal center of 7 a did not become more electron‐deficient. By switching the ligand backbone and substituents, the configuration of polylactide (PLA) turned from slight isotactic to slight heterotactic.

Nonclassical Applications of closo-Carborane Anions: From Main Group Chemistry and Catalysis to Energy Storage.

Classically closo-carborane anions, particularly [HCB11H11]- and [HCB9H9]-, and their derivatives have primarily been used as weakly coordinating anions to isolate reactive intermediates, platforms for stoichiometric and catalytic functionalization, counteranions for simple Lewis acid catalysis, and components of materials like liquid crystals. The aim of this article is to educate the reader on the contemporary nonclassical applications of these anions. Specifically, this review will cover new directions in main group catalysis utilized to achieve some of the most challenging catalytic reactions such as C-F, C-H, and C-C functionalizations that are difficult or impossible to realize with transition metals. In addition, the review will cover the utilization of the clusters as dianionic C σ-bound ligands for coordination chemistry, ligand substituents for coordination chemistry and advanced catalyst design, and covalently bound spectator substituents to stabilize radicals. Furthermore, their applications as solution-based and solid-state electrolytes for Li, Na, and Mg batteries will be discussed.

Synthesis, structure and thermal investigation of a new volatile iridium (I) complex with cyclooctadiene and methoxy-substituted β-diketonate

The presence of donor groups in volatile metal compounds is interesting both in the thermochemical aspect and as the possibility to form hetero-metallic precursors by a reaction with an acceptor-capable component. Following this trend, the present work deals with synthesis and detailed structural and thermochemical investigation of a first iridium volatile complex with donor-atom-functionalized β-diketonate ligand, namely [Ir(cod)(zis)] (cod = cyclooctadiene-1,5, zis = 2-methoxy-2,6,6-trimethylheptanedionato-3,5). The compound has been characterized by elemental analysis, IR- and NMR-spectroscopy. According to single-crystal X-ray diffraction, the crystal structure of the complex is formed by layered-packed isolated molecules. Within the molecules, the coordination site IrO2C′2 (C′ is the center of C=C bond of the cod-ligand) is implemented to form distorted planar square metal coordination environment with Ir–O and Ir–C′ distances being (2.034–2.046) and (1.965–1.978) A, respectively. TG–DTA study shows that the compound is characterized by extremely low melting point (378 K) and a high thermal stability during evaporation. Then, the temperature dependencies of saturated vapor pressures over both the solid and liquid compounds have been measured by the flow (transpiration) method at (353–376) K and (381–403) K, respectively, giving the molar enthalpy and entropy of sublimation and evaporation processes. In addition, the comparison of the structural and thermal data with the ones for the related [Ir(cod)(L)] complexes containing symmetric alkyl terminal substituents in β-diketonate ligand L has been performed and, thereby, the donor group influence on the characteristics of this type of volatile compounds has been revealed.

Nitro-functionalized metal-organic frameworks with catalase mimic properties for glutathione detection.

In the present study, four copper-based metal-organic frameworks were facilely prepared to study the effects of the substituent groups of the ligand on the enzyme-like properties of these frameworks. It was found that all of them were capable of catalyzing the reaction between 3,3',5,5'-tetramethylbenzidine (TMB) and H2O2; this demonstrated their enzyme-like properties. Moreover, the enzyme-like catalytic properties of all the Cu-MOFs obtained herein were studied in detail. Interestingly, the results indicate that the four Cu-MOFs should be addressed as different enzyme mimics although the chemical structures of their ligands are quite similar. Among these four MOFs, the Cu-MOF with the -NO2 group (Cu-MOF (ii)) exhibits highest catalytic activity at neutral pH, which would be beneficial for its application in real biological samples. Moreover, its catalytic activity should be ascribed to the generation of oxygen induced by Cu-MOF (ii)-catalyzed H2O2 decomposition; this accordingly indicates that this MOF should be called a catalase mimic. Under optimized conditions, the Cu-MOF (ii) was applied to develop a (GSH) colorimetric assay for glutathione (GSH). In addition, GSH detection in serum was performed, and satisfactory results were obtained. Therefore, a simple, sensitive and selective colorimetric assay based on the MOF catalase mimic for the detection of GSH was developed.

Effect of ligand substituents on nickel and copper [N4] complexes: electronic and redox behavior, and reactivity towards protons

The ligand substituents have a major effect on the redox potentials, catalytic efficiency and robustness of the complexes in HER.

Cyaphide-alkynyl complexes: metal-ligand conjugation and the influence of remote substituents.

A homologous series of novel trans-cyaphide-alkynyl complexes, viz. trans-[Ru(dppe)2(C[triple bond, length as m-dash]P)(C[triple bond, length as m-dash]CC6H4R-p)] (R = Me, H, F, CO2Me, NO2) is prepared and comprehensively characterised, alongside their parent phosphaalkyne-complex cations trans-[Ru(dppe)2(η1-P[triple bond, length as m-dash]CSiMe3)(C[triple bond, length as m-dash]CC6H4R-p)]+. Structural data for trans-[Ru(dppe)2(C[triple bond, length as m-dash]P)(C[triple bond, length as m-dash]CC6H4R-p)] (R = Me, F) and trans-[Ru(dppe)2(η1-P[triple bond, length as m-dash]CSiMe3)(C[triple bond, length as m-dash]CC6H4R-p)]+ (R = F, CO2Me) are described, along with that for the previously reported trans-[Ru(dppe)2(C[triple bond, length as m-dash]P)(C[triple bond, length as m-dash]CCO2Me)]. NMR spectroscopic data indicate significant influence of the remote aromatic substituent over the properties of the cyaphide ligand, in line with the Hammett parameter (σp), suggesting appreciable 'communication' along the through-conjugate chain. Cyclic voltammety shows irreversible oxidative behaviour, at more anodic Epa than in the respective alkynyl-chloride complexes, though apparently moderated by the remote substituent.

Structure and bonding in reduced boron and aluminium complexes with formazanate ligands.

Group 13 complexes of the type [(PhNNC(p-tol)NNPh)ZPh2]2- (Z = B, Al) containing a highly reduced, trianionic formazanate-derived ligand were studied and the differences in the structure, bonding and reactivity between the B and Al compounds were investigated. The increased ionic character in the bonding of the Al complex is evident from the enhanced charge delocalization onto the peripheral ligand substituents (N-Ph) via the π-framework, as shown by the rotation barrier around the N-C(Ph) bond. The electron-rich nature of these compounds allows facile benzylation at the ligand, and the structures of the products were analysed by X-ray crystallography. The products are inorganic analogues of 1-alkylated 1,2,3,4-tetrahydro-1,2,4,5-tetrazines ('leucoverdazyls'). The six-membered heterocyclic cores of the B and Al compounds are shown to be different, having envelope- and boat-type conformations, respectively. Homolysis of the N-C(benzyl) bond in these compounds was studied by NMR spectroscopy under conditions that trap the organic radical as TEMPO-Bn. Analysis of the reaction kinetics affords activation parameters that approximate the N-C(benzyl) bond strength. The ionic Al compound has one of the weakest N-C bonds reported so far in this type of inorganic leucoverdazyl analogues.

Iron(III) Salalen Complexes for the Polymerisation of Lactide

Herein, we report the preparation and characterisation of iron(III) salalen complexes, with variation of ligand substituents and backbone investigated. Six new complexes were prepared and characterised by elemental analysis, mass spectrometry and X‐ray crystallography. These complexes have been applied for the ring opening polymerisation (ROP) of rac‐lactide in propylene oxide. Fe(1)Cl was found to have a moderate isotactic preference (Pm = 0.75–0.80) and demonstrated good molecular weight control in solution (Đ = 1.02–1.18). Fe(2–7)Cl were also active for ROP and activities could be related to ligand structure.

Iron(III)–salen ion catalyzed s-oxidation of l-cysteine and s-alkyl-l-cysteines by H2O2: Spectral, kinetic and electrochemical study

The H2O2 oxidation of l-cysteine and s-alkyl-l-cysteines (s-met-l-cys, s-et-l-cys & s-pro-l-cys) catalyzed by iron(III)–salen (salen = N,N′-bis(salicylidene)ethylenediaminato) complexes in aqueous CH3CN proceeds through Michaelis–Menten kinetics. The rate constant (k) values correlate well with Hammett σ constants, which gives the positive reaction constant (ρ = 1.5–1.9) value. The CV of oxoiron(IV)-salen ion shows a clear oxidation peak at 1.28 V in 0.1 M phosphate buffer (PB) solution using 0.1 M tertiary butyl ammonium perchlorate (TBAP) as supporting electrolyte at 266 K. The rate of the reaction is highly sensitive to the length of the alkyl chains present in the l-cysteines, pH and solvent composition of the medium. The calculated binding constant values (Kf) in the range of 117–613 M−1, indicate that iron(III)–salen complexes carrying electron donating substituents in the salen ligand have higher binding constant values compared to those carrying electron withdrawing substituents. Product analysis shows the conversion of l-cys to its disulfide and s-alkyl-l-cys to the corresponding sulfoxides. Based on the spectral and kinetic data the plausible mechanism has been proposed.

Influence of ligand substituents of unbridged metallocene complexes on stability of their active centers in ethylene polymerization

Four unbridged metallocene catalyst precursors, bis(2,4,7-trimethylindenyl)zirconium dichloride (Cat.1), bis(2,4,6-trimethylindenyl)zirconium dichloride (Cat.2), bis(2,4,5,6-tetramethylindenyl)zirconium dichloride (Cat.3), and bis(2-methyl-4,6-diisopropylindenyl)zirconium dichloride (Cat.4), were synthesized. Cat.4 was found thermodynamically unstable. The correlations between alkyl substituents of the indenyl ligands and olefin polymerization with the metallocene complexes Cat.1, Cat.2 and Cat.3 were investigated via kinetic study on ethylene polymerization. Change of number of active centers ([C⁎]/[Zr]) was determined by quench-labeling the propagation chains with 2-thiophenecarbonyl chloride. According to the kinetic behaviors, kinetic stability of the metallocene complexes was found to follow the order of Cat.1 > Cat.2 > Cat.3. The relationship between ligand substitution mode and kinetic stability of the metallocene complex were discussed.

Enantioselective Benzylic Hydroxylation of Arylalkanes with H2O2 in Fluorinated Alcohols in the Presence of Chiral Mn Aminopyridine Complexes

AbstractA series of chiral bioinspired Mn‐aminopyridine complexes of the type [L*MnII(OTf)2] (where L* is 2,2′‐bipyrrolidine derived ligand, bearing trifluoroalkoxy and alkyl substituents) have been tested as catalysts in benzylic C−H hydroxylation of arylalkanes with H2O2 in fluorinated ethanols media. In 2,2,2‐trifuoroethanol, the yield of the target ethylbenzene oxidation product, chiral 1‐phenylethanol, reaches 45 %, which is much better than in the common solvent CH3CN (5‐6 %). The selectivity for 1‐phenylethanol formation increases in the following order: CH3CN<2‐fluoroethanol<2,2‐difluoroethanol<2,2,2‐trifuoroethanol, while 2,2‐difluoroethanol ensures the highest asymmetric induction in this series, affording chiral benzylic alcohols with up to 89 % ee. In trifluoroethanol, the observed primary kH/kD value of 2.3 has been measured for the oxidation of 1‐phenylethanol/α‐D‐1‐phenylethanol, which is similar to that in CH3CN (2.2). At the same time, depending on the solvent, CH3CN or 2,2,2‐trifuoroethanol, the oxidations of 1‐phenylethanol demonstrates drastically different linear free‐energy relationships; possible effect of the hydrogen‐bond donor (HBD) nature of CF3CH2OH is discussed in this context. Noticeably, it has been shown that by switching the absolute chirality ((S,S)− or (R,R)−) of the catalyst, the oxidation of complex substrate of natural origin, estrone acetate, can be diverted to predominant formation of either the tertiary C9‐alcohol or of the C6‐ketone, respectively.

Manganese-Based Catalysts with Varying Ligand Substituents for the Electrochemical Reduction of CO2 to CO

A series of manganese complexes were synthesized with a variety of ligands and ligand substituents. These complexes were then studied using ultraviolet–visible spectroscopy, cyclic voltammetry, density functional theory calculations, and bulk electrolysis. The UV–vis, cyclic voltammetry, and calculation data show that the bipyridine π* level is modulated by the incorporation of different substituents on the bipyridine and through this interaction moderates the observed catalytic activity of the complex toward CO2 reduction. The calculations were correlated to the experimental UV–vis data and cyclic voltammetry data to demonstrate the relationship among these data, and a Hammett plot showed a good correlation between the substituent identity and the MLCT wavelength from UV–vis (R2 = 0.96). When aliphatic substituents were placed on the 4,4′-positions of the bipyridine, the location of the bpy π* was not significantly altered. However, when more electron withdrawing substituents were placed on the 4,4′-posi...

Controlling Reversible Expansion of Li2O2 Formation and Decomposition by Modifying Electrolyte in Li-O2 Batteries

Summary The aprotic lithium-oxygen (Li-O2) battery has attracted worldwide attention because of its ultrahigh theoretical energy density. However, its practical application is critically hindered by cathode passivation, large polarization, and severe parasitic reactions. Here, we demonstrate an originally designed Ru(II) polypyridyl complex (RuPC) though which the reversible expansion of Li2O2 formation and decomposition can be achieved in Li-O2 batteries. Experimental and theoretical results revealed that the RuPC can not only expand the formation of Li2O2 in electrolyte but also suppress the reactivity of LiO2 intermediate during discharge, thus alleviating the cathode passivation and parasitic reactions significantly. In addition, an initial delithiation pathway can be achieved when charging in turn; thus, the Li2O2 products can be decomposed reversibly with a low overpotential. Consequently, the RuPC-catalyzed Li-O2 batteries exhibited a high discharge capacity, a low charge overpotential, and an ultralong cycle life. This work provides an alternative way of designing the soluble organic catalysts for metal-O2 batteries.

Functionalized MIL-68(In) for the photocatalytic treatment of Cr(VI)-containing simulation wastewater: Electronic effects of ligand substitution

A family of functionalized MIL-68(In) have been successfully synthesized and applied to photoreduction of Cr(VI) aqueous under simulated sunlight (280 nm < λ < 980 nm) irradiation. Meanwhile, we have studied the effect of modification with electron-donating or electron-withdrawing groups (H2BDC, H2BDC-NH2, H2BDC-NO2 and H2BDC-Br) on photocatalytic performance of MIL-68(In)-X (X = H, NH2, NO2, Br). Further experimental results demonstrated that the photoactivities of the obtained MIL-68(In)-X have been greatly affected by the electronic effect of ligand substituent. That is, the reaction rates obtained by functionalized MIL-68(In) have been linearly associated with the Hammett coefficients of different substituents. More significantly, MIL-68(In)-NH2 has also exhibited outstanding performance in a co-existed model system (Cr(VI)/dyes), which represents a first example to use the In-based MOFs as bifunctional photocatalysts. It is hope that this work not only represents the structure-property relationships on photocatalysis in MOFs, but also promotes further interest in design and construction of more efficient MOFs-based photocatalysts for environmental remediation.