Electronic Properties Of Ligands Research Articles

An Adsorptive Membrane Platform for Precision Ion Separation: Membrane Design and First‐Principles Studies

One of the key challenges in separation science is the lack of precise ion separation methods and mechanistic understanding crucial for efficiently recovering critical materials from complex aqueous matrices. Herein, first‐principles electronic structure calculations and in situ Raman spectroscopy are studied to elucidate the factors governing ion discrimination in an adsorptive membrane specifically designed for transition metal ion separation. Density functional theory calculations and in situ Raman data jointly reveal the thermodynamically favorable binding preferences and detailed adsorption mechanisms for competing ions. How membrane binding preferences correlate with the electronic properties of ligands is explored, such as orbital hybridization and electron localization. The findings underscore the importance of the phenolate group in oxime ligands for achieving high selectivity among competing transition metal ions. In‐depth understanding on which specific atomistic site within the microenvironment of metal‐ligand binding pockets governs the ion discrimination behaviors of the host will build a solid foundation to guide the rational design of next‐generation materials for precision separation essential for energy technologies and environment remediation. In tandem, synthetic controllability is demonstrated to transform 3D micrometer‐scale crystals to a 2D crystalline selective layer in membranes, paving the way for more precise and sustainable advances in separation science.

A simply accessible organometallic system to gauge electronic properties of N-heterocyclic carbenes.

The intricate σ and π-bonding of N-heterocyclic carbenes (NHCs) to metals and the need to quantify their electronic properties to rationalize reactivity of complexes have resulted in the creation of numerous methodologies to understand the NHC-metal interaction which are, as we now show, flawed. Our search for a unified, easily accessible system to gauge these fundamental properties has resulted in the discovery of two systems that highlight the flaws present in existing systems and provide a more accurate measure of the NHC ligand electronic properties.

Discovery of high‐affinity amyloid ligands using a ligand‐based virtual screening pipeline

AbstractBackgroundA ligand‐based virtual screening pipeline was developed to discover structurally novel ligands for Aβ(1‐42) fibrils.MethodA database of ligands that bind to Aβ(1‐42) fibrils was used to develop a two‐step ligand‐based virtual screening pipeline. The first step involved developing machine learnings models that use relatively simple chemical descriptors to predict ligand dissociation constants (Kd ). The second step involved constructing 3D models based on field points that describe the surface, shape, and electronic properties of ligands. The combined pipeline was then used to screen 63 million compounds from the ZINC15 database.1 The most promising compounds were experimentally evaluated using binding assays.ResultFor the first chemical descriptor model, a support vector machine model was found to predict the ‐log(Kd /M) of Aβ(1‐42) fibril ligands with a mean absolute error of only 0.41. An ensemble of four 3D models were then developed that each predicted ‐log(Kd /M) with a cross‐validation regression coefficient of 0.48–0.56. The ZINC15 database compounds were screened through the first chemical descriptor model, and the 10,000 compounds with the highest predicted affinities were then screened through the 3D models. The 46 highest‐ranked ligands were then evaluated using Thioflavin T competition assays, resulting in five new Aβ(1‐42) ligands with novel structures that exhibited nanomolar binding.ConclusionA ligand‐based virtual screening pipeline has been developed to discover new fibril‐binding ligands. Using Aβ(1‐42) fibrils as a target, five new structurally diverse ligands were found that exhibit nanomolar binding affinities.1. Sterling and J. J. Irwin, J. Chem. Inf. Model. 2015, 55, 2324‐2337.

Probing Electronic Effects in Tridentate Copper(I) Complexes by CIVP Spectroscopy.

Ligand electronic effects play an important role in catalysis, where small changes to ligand structure can bring about large changes in catalytic activity. Therefore, accurate experimental quantification of ligand electronic properties plays a crucial role in understanding and tuning chemical reactivity. In this work, we used cryogenic ion vibrational predissociation (CIVP) spectroscopy to experimentally quantify electronic effects in terpyridine ligands, as simple model systems, by measuring CIVP spectra of their copper complexes tagged by N2 molecules. We used the N2 stretching vibration as a reporter chromophore to probe electronic effects of the investigated ligands and employed quantum chemical calculations to better understand how different substituents influence the vibrational frequencies of the stretching vibration of the chromophore. Our data show that the electronic character, as well as position and number of substituents, can affect the N≡N vibrational frequency, and that the N≡N bond serves as a sensitive probe for electronic and steric effects.

A DFT study on MOFs with rhodium-coordinated organic phosphine linkers modified by functional groups for catalytic hydroformylation of 1-butene

The organic ligands containing different heteroatoms (N/O/P) to coordinate Rh-active metals were found active for the hydroformylation of 1-butene, and P-containing ligands were superior in catalytic activity and n/i selectivity. In this work, a model of MOF Rh-PCM-18 containing phosphine ligands, similar to the conventional rhodium-phosphine homogeneous catalyst, was constructed, then the phosphine ligands were modified by inserting functional groups to change the electronic properties of the ligands, which might improve the catalytic performance. Five different models were obtained and their structures and electronic properties were calculated. The adsorption energies of the reactant species and the reaction paths on these models were investigated. The results suggested that the enhancement of the electron-donating ability of the phosphine ligand increased the adsorption of CO and H and promoted the oxidative addition of H2 at the metal site, but caused a higher activation energy barrier of the first step. On the other hand, appropriate reduction in the electron-donating ability of the phosphine ligand weakened the adsorption of CO and H and led to a decrease in the energy barrier of the first two steps, which improved the catalytic performance. This work explored the influence of the electronic properties of organic phosphine ligands on the coordinated active metal catalyst for 1-butene hydroformylation, which provided a reference for the selection and modification of ligands when using MOFs to design heterogeneous catalysts for hydroformylation.

Phenylimido complexes of rhenium: fluorine substituents provide protection, reactivity, and solubility.

Reactions of [Re(NPhF)Cl3(PPh3)2] ({NPhF}2- = p-fluorophenylimide) with a variety of alkyl and aryl isocyanides have been studied. Different reactivity patterns and products have been obtained depending on the steric and electronic properties of the individual ligands. This involves the formation of 1 : 1 and 1 : 2 exchange products of Re(V) with the general formulae mer-[Re(NPhF)Cl3(PPh3)(isocyanide)] and cis- or trans-[Re(NPhF)Cl3(isocyanide)2]. The stability of the obtained products is correlated with the substitution pattern of the isocyanide ligands. The products have been studied by single-crystal X-ray diffraction and spectroscopic methods, including IR and multinuclear NMR spectroscopy as well as mass spectrometry. The use of partially fluorinated starting materials and ligands allows the modulation of the solubilities of the starting materials and the products as well as the monitoring of the reactions by means of 19F NMR. The attachment of the CF3 or F substituent on the isocyanides gives control over the steric bulk and the electronic properties of the ligands and, thus, their reactivity.

Oxygenation of copper(I) complexes containing fluorine tagged tripodal tetradentate chelates: significant ligand electronic effects

Copper-dioxygen (O2) interactions are of great importance in biological and chemical transformations involving reversible dioxygen binding, activation, or reduction. In this report, we describe O2-reactions with the mononuclear copper(I) complexes containing two new analogues of the known nitrogen-containing tetradentate tripodal chelate, tris[(2-pyridyl)methyl]amine (TMPA). In both derivatives, one electron-rich and one electron-deficient, fluorine atoms are attached to the ligand framework, allowing for the use of 19F-NMR spectroscopy to probe the oxygenation process. Variations of ligand electronic properties are manifested in the electrochemical behavior of copper complexes and their reactivities toward O2. Our NMR spectroscopic studies, along with variable-temperature electronic absorption measurements, revealed that the copper(I) complexes reversibly react with O2 to form the corresponding 1:1 copper-O2 (i.e., end-on superoxo) intermediates which can further react reversibly with second equivalents of copper(I) complexes to form the related dinuclear 2:1 copper-O2 (i.e., trans-peroxo) adducts. However, considerable differences exist in detail at various temperatures, depending on the chelate. All three end-on superoxo and trans-peroxo species described here possess similar spectroscopic features, although small but significant shifts in the energy of their signature bands were observed, suggesting that the variation in the chelates directly affects the electronic properties of the copper-O2 cores.

Metal incorporated aminothiazole-derived compounds: synthesis, density function theory analysis, in vitro antibacterial and antioxidant evaluation.

The present study advocates the combined experimental and computational study of metal-based aminothiazole-derived Schiff base ligands. The structure and electronic properties of ligands have been experimentally studied by spectroscopic methods (UV-Vis, FT-IR, 1H-NMR and 13C-NMR), mass spectrometry, elemental analysis and theoretically by density function theory (DFT). Computational calculations employing the B3LYP/6-31 + G(d,p) functional of DFT were executed to explore the optimized geometrical structures of ligands along with geometric parameters, molecular electrostatic potential (MEP) surfaces and frontier molecular orbital (FMO) energies. Global reactivity parameters estimated from FMO energy gaps signified the bioactive nature of ligands. The synthesized ligands were used for chelation with 3d-transition metals [VO(IV), Cr(III), Fe(II), Co(II), Ni(II), Cu(II) and Zn(II)] in 1 : 2 (metal : ligand) molar ratio. The spectral and magnetic results confirmed the formation of octahedral geometry around all the divalent and trivalent metal centres, whereas the tetravalent vanadyl centres were confirmed to have square-pyramidal geometry. All the as-synthesized compounds were investigated for in vitro antibacterial potential against two Gram-negative (Salmonella typhimurium and Escherichia coli) and two Gram-positive (Bacillus subtilis and Staphylococcus aureus) bacteria. Antibacterial assay results displayed pronounced activity, and their activity is comparable to that of a standard drug (streptomycin). The antioxidant potential of these compounds was assessed by employing diphenyl picryl hydrazide radical scavenging activity. The results displayed that all the metal chelates have exhibited more bioactivity in contrast with free ligands. The chelation was the main reason for their enhanced bioactivity. These results indicated that the thiazole metal-based compounds could be exploited as antioxidant and antimicrobial candidates.

Determination of Stereoelectronic Properties of NHC Ligands via Ion Pairing and Fluorescence Spectroscopy

AbstractWe present an experimental method, which provides information on the steric and electronic properties of ligands in metal complexes. This approach is based on the equilibrium of ion‐pairing of a cationic [(NHC)Ir(cod)]+ and a fluorescent bodipy‐sulfonate (bdpSO3−). The close ion‐pair of [(NHC)Ir(bdpSO3)(cod)] in toluene solution is weakly fluorescent, but dissociates into solvent‐separated ion pairs upon exposure to slightly more polar solvents. This spatial separation leads to a very pronounced increase of the fluorescence. The separation into distinct ions depends on the polarity of the solvent, but more importantly also on the stereoelectronic properties of the NHC ligand. 26 different NHC ligands with varying steric and electronic properties were probed in 1,2‐dichloroethane solvent. The electronic properties of the ligands were examined via the established descriptors (redox potential and ν(CO)). The systematic variation of NHC donation enables the deconvolution of electronic and steric contributions of the respective ligand.

Efficient palladium (II) electrocatalysts with thiophene anchored pyridinium amidates for CO2 reduction

Palladium(II) complexes namely [Pd(LMe)2(Cl)2] (5), [Pd(LEt)2(Cl)2] (6), [Pd(LBu)(Cl)2] (7) and [Pd(LBz)(Cl)2] (8) were synthesized as electrocatalysts towards CO2 reduction into CO, H2O acting as proton source in presence of TFA. These catalysts were prepared by substitution of benzonitrile from bis(benzonitrile)palladium(II)chloride with proligands; [HLMe][OTf] (1), [HLEt][I] (2), [HLBu][Br] (3) and [HLBz][Cl] (4) correspondingly. All the eight new compounds were successfully characterized with spectroscopic and single crystal X-ray diffraction techniques. It was found out that both steric and electronic properties of ligands concomitantly affected the catalytic properties of the catalysts. The activity of these catalysts decreases in the following order: 6 >7 >5 >8. 6 displayed maximum TOF of 337 s−1 and rate constant (kcat) of 1870 M−1s−1 along with 73 % Faradaic efficiency to produce CO from CO2 in the presence of trifluoracetic acid. However, the lowest over potential (η) of −340 mV was shown by 7. All the catalysts were robust with no major current fluctuations were observed during controlled potential electrolysis for more than 3600 s.

Probing Electronic Properties of Triazolylidenes through Mesoionic Selones, Triazolium Salts, and Ir-Carbonyl-Triazolylidene Complexes

Understanding and quantifying the electronic properties of ligands is important both from a fundamental point of view and also for their applications in various branches of chemistry. In this contr...

Analyses of the Structural and Electronic Properties of NHCs with Bicyclic Architectures

Quantification of the steric and electronic properties of ligands is an important task for the development of highly effective chemical transformations when using a metal complex as a catalyst. In ...

Copper(II) complexes containing N′-aromatic group substituted N,N′,N-bis((3,5-dimethyl-1H-pyrazol-1-yl)methyl)amines: Synthesis, structures, polymerization of methyl methacrylate and ring opening polymerization of rac-lactide

A series of Cu(II) complexes, [LnCuCl2] (Ln = LA–LF), supported by N′-aromatic-group-substituted N,N-bis((3,5-dimethyl-1H-pyrazol-1-yl)methylamine ligands have been synthesized. Variations of substituents at the ortho position of the aniline moiety influenced the solid-state structures of these complexes. The X-ray structures of [LnCuCl2] (Ln = LA–LC and LF) revealed that ligands are coordinated in a N,N,N-tridentate fashion to Cu(II) center and adopted a distorted square-pyramidal geometry, while [LDCuCl2] with N,N-bidentate coordination adopts distorted tetrahedral geometry. These complexes were capable of polymerizing methyl methacrylate (MMA) in the presence of modified methylaluminoxane (MMAO), with [LFCuCl2] displaying the highest activity (2.81 × 104 g PMMA (mol Cu)−1h−1). Regardless of ligand architecture, syndio-enriched PMMAs have been furnished with a slightly broader polydispersity index (PDI). Additionally, the in situ generated dimethyl derivatives, polymerized rac-LA and furnished PLA with mediocre heterotacticities at room temperature. Importantly, the catalytic efficiencies of the Cu(II) complexes studied have been found to be influenced by the steric and electronic properties of ligand.

Investigations into the ligand steric and electronic effects of Ru-catalyzed C–H bond arylation directed by 8-aminoquinoline as a bidentate-directing group

In this study, we report an investigation into the steric (cone angle, θ) and electronic properties of ligands in Ru-catalyzed C–H arylation of aromatic benzamides bearing 8-aminoquinoline as an N,N’-bidentate-directing group. The study employs [RuCl2( p-cymene)]2 as a precatalyst, and a ligand, under study, as a cocatalyst. Various electronically and sterically different monodentate and bidentate phosphine ligands were examined. Other ligands such as phosphites and amines were also tested. The study reveals that while bidentate phosphines, phosphites, and aryl and alkyl amines were found to be ineffective, monodentate triarylphosphines represented by triphenylphosphine were found to be the most effective ligands in the Ru-catalyzed C–H arylation under the conditions specified. In addition, the study reveals that there is a correlation between the steric effects, cone angle (θ) and the reaction efficiency. Thus, for symmetrical phosphine ligands, as the cone angle increases, the yield of the CH arylation product gradually decreased. Moreover, the electronic properties of triarylphosphine ligands influenced the reaction as demonstrated by the decreased ability of electron-poor ligands to promote the reaction. The study also reveals a correlation between the electronic parameter, υCO, of the triarylphosphine ligand and the reaction efficiency. As the carbonyl stretching frequency increases, the reaction yield gradually decreased.

Inhibitor design for 3‐hydroxy‐3‐methyl‐glutaryl‐CoA reductase enzyme; molecular docking and determination of molecular and electronic properties of ligands by density functional theory method

AbstractDesigning new inhibitors having less side effects is a need which also could reduce cholesterol levels. To fulfill this aim, we have carried out a molecular docking study toward 3‐hydroxy‐3‐methyl‐glutaryl‐CoA (HMG‐CoA) reductase. A set of designed structural derivatives of statin drugs, eight ligands which are used as HIV‐1 integrase inhibitor candidates, a set of terpenoids, and ligands downloaded from Zinc15 database were docked to HMG‐CoA reductase enzyme which contains atorvastatin in crystal structure. The analysis of docking studies revealed that statin derivative ligands are more appropriate for inhibition of HMG‐CoA reductase. To define the contribution of the molecular properties to the binding of ligands to enzyme structure; the highest occupied molecular orbitals‐lowest unoccupied molecular orbitals, hardness, electronegativity, and chemical potential properties of ligands have best score in their sets calculated by quantum mechanical tools.

Photoswitchable azobenzene functionalized anthraquinone and benzimidazole Ru(II)-p-cymene organometallic complexes

A series of five new Ru(II) (η6-p-cymene) organometallic complexes with the incorporation of modified azobenzimidazole and anthraquinone based ligands were synthesized and found to be an excellent photoswitching property in different solvents. The push-pull effect affects the lifetime of the complexes during photoisomerization processes. Upon irradiation at 254 nm UV light, these complexes efficiently undergo the trans ↔ cis isomerization, which is dependant on solvent polarity as well as substituent pattern. In DCM (dichloromethane), two states stable isomers have been found, but in a higher polar solvent (CH3CN), the fast conversion of trans isomer to the cis isomer takes place. However, the cis isomer proceeds slower thermal isomerization to its original trans isomer in dark condition at room temperature with a higher half-life of the complexes. The photoisomerization study has been monitored by using the UV–vis absorption and 1H NMR studies, which indicates that the successful trans ↔ cis isomerization takes place. The coordination of the ruthenium ion successfully alters the electronic properties of ligands. The excellent photoswitching properties and higher half-life of these organometallic complexes are exciting candidates for their applications in catalytic, photonic devices, and medicinal purposes.

Zeolite-Y entrapped metallo-pyrazolone complexes as heterogeneous catalysts: Synthesis, catalytic aptitude and computational investigation

Transition metal [M = VO(IV) and/or Co(II)] complexes with Schiff base ligand (Z)-3-methyl-1-phenyl-4-(2,2,2-trifluoro-1-(2-hydroxyphenyl)imino)ethyl)-1H-pyrazol-5-ol (H2L) have been entrapped in the super cages of zeolite-Y by Flexible Ligand Method. These nanohybrid materials have been characterized by preferential physico-chemical techniques such as ICP-OES, elemental analyses, (FT-IR, 1H and 13C-NMR and electronic) spectral studies, BET, scanning electron micrographs (SEMs), AAS, X-ray diffraction patterns (XRD) and thermogravimetric analysis. The density functional theory calculations are performed to find optimized structures together with the bond angles, bond lengths, dihedral angles and electronic properties of ligand and neat complexes. The catalytic competence of zeolite-Y entrapped metallo-pyrazole complexes was examined by the oxidation of olefins viz. limonene, cyclohexene, styrene, and α-pinene using H2O2 as an oxidant. So as to ensure the shielding effect of the nanohybrid over the active center on the catalytic properties, the performance of the entrapped complexes (heterogeneous system) was weighing up against the neat complexes (homogeneous system). The effect of experimental variables (such as solvents, mole ratio of substrate and oxidant, the amount of catalyst and reaction time) with their probable justification on the conversion of limonene was discussed. Under the optimized reaction conditions, [VO(L)·H2O]-Y was found to be potential candidate, achieving 87.44%, 90.01%, 82.01%, and 85.44% conversions of limonene, cyclohexene, styrene, and α-pinene oxidation reactions, respectively.

ChemInform Abstract: Polyaromatic N‐Heterocyclic Carbene Ligands and π‐Stacking. Catalytic Consequences

AbstractReview: [40 refs.

Origin of fast catalysis in allylic amination reactions catalyzed by Pd-Ti heterobimetallic complexes.

Experiments and density functional calculations were used to quantify the impact of the Pd-Ti interaction in the cationic heterobimetallic Cl2Ti(N(t)BuPPh2)2Pd(η(3)-methallyl) catalyst 1 used for allylic aminations. The catalytic significance of the Pd-Ti interaction was evaluated computationally by examining the catalytic cycle for catalyst 1 with a conformation where the Pd-Ti interaction is intact versus one where the Pd-Ti interaction is severed. Studies were also performed on the relative reactivity of the cationic monometallic (CH2)2(N(t)BuPPh2)2Pd(η(3)-methallyl) catalyst 2 where the Ti from catalyst 1 was replaced by an ethylene group. These computational and experimental studies revealed that the Pd-Ti interaction lowers the activation barrier for turnover-limiting amine reductive addition and accelerates catalysis up to 10(5). The Pd-Ti distance in 1 is the result of the N(t)Bu groups enforcing a boat conformation that brings the two metals into close proximity, especially in the transition state. The turnover frequency of classic Pd π allyl complexes was compared to that of 1 to determine the impact of P-Pd-P coordination angle and ligand electronic properties on catalysis. These experiments identified that cationic (PPh3)2Pd(η(3)-CH2C(CH3)CH2) catalyst 3 performs similarly to 1 for allylic aminations with diethylamine. However, computations and experiment reveal that the apparent similarity in reactivity is due to very fast reaction kinetics. The higher reactivity of 1 versus 3 was confirmed in the reaction of methallyl chloride and 2,2,6,6-tetramethylpiperidine (TMP). Overall, experiments and calculations demonstrate that the Pd-Ti interaction induces and is responsible for significantly lower barriers and faster catalysis for allylic aminations.

ChemInform Abstract: Substituent Effects on the Electronic Properties of Complexes with Dipyridophenazine and Triazole Ligands: Electronically Connected and Disconnected Ligands

AbstractReview: 73 refs.