1H NMR Chemical Shifts Research Articles

Novel triazole derivatives possessing imidazole: Synthesis, spectroscopic characterization (FT-IR, NMR, UV–Vis), DFT studies and antibacterial in vitro evaluation

A new triazole compounds derived from N-(3-Aminopropyl)imidazole and (E)-5-((1H-1,2,4-triazol-3-yl)diazenyl)-2‑hydroxy-3-arylbenzaldehyde were obtained. The prepared compounds were characterized through, elemental analysis, MS, FT-IR, UV–Vis, 1H and 13C NMR spectroscopy. The optimized structure geometry and vibrational frequencies of the compounds were performed through DFT approach by the 6–31 G (d,p) level of theory. The 1H and 13C NMR chemical shifts with respect to TMS were calculated using the (GIAO) method in DMSO and compared with the experimental data. TD-DFT approach was used to produce the theoretical UV–Visible spectra in DMSO and gas phase. The frontier molecular orbitals and the molecular electrostatic potential surface (MEP) of triazole molecules were also explored. The compounds were tested for their antibacterial activity. The MIC values showed that the tested compounds indicated good antibacterial activity.

Precisely predicting the 1H and 13C NMR chemical shifts in new types of nerve agents and building spectra database

Following the recent terrorist attacks using Novichok agents and the subsequent decomposition operations, understanding the chemical structures of nerve agents has become important. To mitigate the ever-evolving threat of new variants, the Organization for the Prohibition of Chemical Weapons has updated the list of Schedule 1 substances defined by the Chemical Weapons Convention. However, owing to the several possible structures for each listed substance, obtaining an exhaustive dataset is almost impossible. Therefore, we propose a nuclear magnetic resonance-based prediction method for 1H and 13C NMR chemical shifts of Novichok agents based on conformational and density functional study calculations. Four organophosphorus compounds and five G- and V-type nerve agents were used to evaluate the accuracy of the proposed procedure. Moreover, 1H and 13C NMR prediction results for an additional 83 Novichok candidates were compiled as a database to aid future research and identification. Further, this is the first study to successfully predict the NMR chemical shifts of Novichok agents, with an exceptional agreement between predicted and experimental data. The conclusions enable the prediction of all possible structures of Novichok agents and can serve as a firm foundation for preparation against future terrorist attacks using new variants of nerve agents.

Structural determination and anticholinesterase assay of C-glycosidic ellagitannins from Lawsonia inermis leaves: A study supported by DFT calculations and molecular docking

An ellagitannin monomer, lythracin M (1), and a dimer, lythracin D (2), along with eight known monomers (3−10) were isolated from Lawsonia inermis (Lythraceae) leaves. Lythracin M (1) is a C-glycosidic ellagitannin with a flavogallonyl dilactone moiety that participates in the creation of a γ-lactone ring with the anomeric carbon of the glucose core. Lythracin D (2) was determined as an atropisomer of the reported lythcarin D. These newly discovered structures (1 and 2) were determined by intensive spectroscopic experiments and by comparing DFT-calculated 1H1H coupling, 1H NMR chemical shifts, and ECD data with experimental values. The anti-acetylcholinesterase assay of the compounds 1–10 revealed that the C-1 ellagitannin epimers [casuarinin (7; IC50 = 34 ± 2 nM) and stachyurin (8; IC50 = 56 ± 3 nM)], and the new dimer (2; IC50 = 61 ± 4 nM) possess enzyme inhibitory effects comparable to the reference drug (donepezil, IC50 = 44 ± 3 nM). Molecular docking of compounds 1–10 with AChE identified the free galloyl moiety as an important pharmacophore in the anticholinesterase activity of tannins.

Synthesis, spectral characterization, DFT and in silico ADME studies of the novel pyrido[1,2-a]benzimidazoles and pyrazolo[3,4-b]pyridines

The chemical behavior of 3-substituted-6,8-dimethylchromones 1-3 was studied towards two selected nucleophiles namely 1H-benzimidazol-2-ylacetonitrile (R1) and 5-amino-2,4-dihydro-3H-pyrazol-3-one (R2). A diversity of pyrido[1,2-a] benzimidazoles and pyrazolo[3,4-b]pyridines was efficiently synthesized through different transformation reactions; depending on the functional group present at C-3 position of the chromone moiety. The two nucleophiles reacted with acrylonitrile 1 through γ-pyrone ring opening at C-2 position followed by recyclization, while compounds 2 and 3 reacted with the two nucleophiles through addition onto the exocyclic double bonds followed by cycloaddition onto the nitrile functions. All theoretical computations were calculated by using Density Functional Theory (DFT) B3LYP method with the help of 6-311G(d,p) basis set. In theoretical studies, the global chemical activity descriptors (FMOs, electron affinity, ionization potential, hardness, softness, etc.) and MEP were calculated to predict the important information related with the stability and reactivity of the prepared molecules. Compound 7 which obtained through ring opening and recyclization of compound 1 by reagent R2 is the more stable product. Fukui functions (FFs) for the starting substrates 1-3 were determined by using the same level of theory. With the aid of local based descriptors in the form of condensed Fukui functions, the sites available for nucleophilic attack for compound 1 were C10 then at C8, and for compounds 2 and 3 were C14 then at C26. The electrons transfer occurs from the nucleophilic reagents (R1 and R2) to the starting substrates 1-3 as deduced from electrophilicity-based charge transfer study (ECT > 0). To explain the nonlinear optical (NLO) properties of the synthesized compounds, the dipole moment, polarizability, and first hyperpolarizability values (in the range 0.84 × 10−30 - 3.85 × 10−30 esu) have been calculated and compared with urea as a reference material. In addition, the 1H NMR chemical shifts of prepared compounds were simulated by GIAO manner and compared with experimental chemical shifts results. The “drug-likeness” and ADME appeared in good agreement with the Lipinski, Ghose and Veber rules, indicating the ability of the prepared compounds to be predictively orally active and bioavailable.

Experimental Spectroscopic (FT-IR, 1H and 13C NMR, ESI-MS, UV) Analysis, Single Crystal X-Ray, Computational, Hirshfeld Analysis, and Molecular Docking of 2-Amino-N-Cyclopropyl-5-Heptylthiophene-3-Carboxamide and Its Derivatives

Amino-N-cyclopropyl-5-heptylthiophene-3-carboxamide (ACPHTC) and its functional derivatives were synthesized and analyzed via single-crystal X-ray diffraction at 296 K, 1H,13C NMR, UV–Vis, FT-IR, and ESI-MS spectral analysis. ACPHTC crystallizes in a tetragonal crystal system, space group I-4 with Z = 16 and the following unit cell dimensions: a = 19.2919(4) Å, b = 19.2919(4) Å, c = 17.1515(5) Å. Using HF and DFT, B3LYP methods with the 6-311++G(d,p) basis established by the GAUSSIAN 16 program, the optimized molecular geometric parametersof ACPHTC and its derivatives were computed. The ACPHTC molecule hyper conjugative interactions are revealed by the NBO analysis, which has been carried out. Electrophilic and nucleophilic sites have been identified throughMEP. The first-order hyperpolarizability, dipole moment, and polarizability of the ACPHTC were taken into account when analyzing its NLO behavior. Additionally, the energies of ACPHTC HOMO and LUMO were found. The TD-DFT has been used to determine the UV-Vis spectra and electronic features such as frontier orbitals and band gap energies. The GIAO method was used to determine the 1H and 13C NMR chemical shifts of ACPHTC molecule and compare them to experimental results. Fingerprint plots,Hirshfeld surface are used to evaluate the properties of intermolecular interactions. Finally, docking studies were used to examine the binding affinity,of ACPHTC and its derivatives.

DFT calculations of 1H- and 13C-NMR chemical shifts of 3-methyl-1-phenyl-4-(phenyldiazenyl)-1H-pyrazol-5-amine in solution

Geometries of the 3-methyl-1-phenyl-4-(phenyldiazenyl)-1H-pyrazol-5-amine azo-dye compound and its tautomer were optimized using B3LYP and M06-2X functionals in coupling with TZVP and 6–311 + G(d,p) basis sets. The 1H- and 13C-NMR chemical shifts of all species were predicted using 13 density functional theory (DFT) approaches in coupling with TZVP and 6–311 + G(d,p) basis sets at the different optimized geometries by applying the using GIAO method using the eight geometries. The selected functionals are characterized by having different amount of Hartree–Fock exchange. The selected DFT methods were B3LYP, M06-2X, BP86, B97XD, TPSSTPSS, PBE1PBE, CAM-B3LYP, wB97XD, LSDA, HSEH1PBE, PW91PW91, LC-WPBE, and B3PW91. The results obtained were compared with the available experimental data using different statistical descriptors such as root mean square error (RMSE) and maximum absolute error (MAE). Results revealed that the prediction of the 1H-NMR chemical shifts has more significant dependence on the applied geometry than that of the prediction of the 13C-NMR chemical shifts. Among all the examined functionals, B97D and TPSSTPSS functionals were found to be the most accurate ones, while the M06-2X functional is the least accurate one. Results also revealed that the prediction of NMR chemical shifts using TZVP basis sets results is more accurate results than 6–311 + G(2d,p) basis set.

Structural and spectroscopic details of polysaccharide–bile acid composites from molecular dynamics simulations

Interactions of a prototypical bile acid (cholic acid, ‘Ch’) and its corresponding sodium salt (sodium cholate, ‘NaCh’) with a standard dietary β-glucan (β-G), bearing β-D-glucopyranose units having mixed 1-4/1-3 glycosidic linkages are studied using molecular dynamics simulation and density functional theory (DFT) calculations. Self-aggregation of the biliary components and their interaction with fifteen strands of the decameric mixed linkage β-glucan is elucidated by estimating varieties of physical properties like the coordination number, moment of inertia and shape anisotropy of the biggest cluster formed at different time instants. Small angle scattering profiles indicate formation of compact spheroidal aggregates. The simulated results of small angle scattering and 1H NMR chemical shifts are compared to spectroscopic data, wherever available. Density functional theory calculations and estimation of the 1H NMR chemical shifts of Ch-protons lying close to the β-G chains reveal change in chemical shift values from that in absence of the polysaccharide. Hydrogen bonding and non-bonding interactions, primarily short range van der Waals interactions and some extent of inter-molecular charge transfer are found to play significant role in stabilizing the complex soft assemblies of bile acid aggregates and β-G. Communicated by Ramaswamy H. Sarma

AlCl3-Catalyzed Cascade Reactions of 1,2,3-Trimethoxybenzene and Adipoyl Chloride: Spectroscopic Investigations and Density Functional Theory Studies.

The reaction of 1,2,3-trimethoxybenzene with adipoyl chloride in the presence of AlCl3 gave two isomeric cyclopentene derivatives, 1,6-bis(2,3,4-trimethoxyphenyl)hexane-1,6-dione, and two demethylation products of aryl methyl ethers. The cyclopentene derivatives including unconjugated or conjugated enones are products formed in a cascade reaction resulting from first the Friedel-Crafts acylation reaction and then aldol condensation. All compounds were optimized by density functional theory calculated using two functional levels, B3LYP and M06-2X, with the 6-311+G(d,p) basis set. The structural properties were established, natural bond orbital analysis of donor-acceptor interactions was carried out, and charges on the atoms and quantum chemical reactivity identifiers were determined to compare the strength of the intramolecular hydrogen bonds formed and their stabilities. To compare the experimental 1H and 13C NMR chemical shifts with the calculated values, NMR chemical shift calculations were carried out using the gauge-invariant atomic orbital method.

Poly(N-isopropyl acrylamide-co-nbutyl methacrylate)의 분자구조, 진동, 스펙트럼 조사 및 양자화학 DFT 계산

A new copolymer poly(N-isopropylacrylamide-co-nbutylmethacrylate) was synthesized by free radical solution polymerization process using 2,2′-Azobis(2-methylpropionitrile) as an initiator in 1,4-dioxane at 70 °C. The structure of the copolymer was characterized by Fourier Transform Infrared Spectrometer (FTIR), the proton nuclear magnetic resonance (1H NMR), and the ultraviolet-visible spectroscopy (UV-vis) method. Theoretically, density functional theory (DFT) (B3LYP) was utilized to identify the molecular geometry and then examine the vibrational frequencies and electrical characteristics of this molecule, with the 6-31G basis set in the ground state. In dichloromethane solution, UV-vis spectra of the molecule were calculated using the Time-Dependent DFT method. 1H NMR chemical shifts have been calculated with GIAO approximation, and the calculated results show good agreement with experimental chemical shifts.

1H and 19F NMR chemical shifts for hydrogen bond strength determination: Correlations between experimental and computed values

1H and 19F NMR methods based on chemical shift measurements of different hydrogen bond donors used for quantifying hydrogen bond strength were analyzed and compared. The extracted values from these different methods are shown to be highly correlated with each other and with several experimental and ab initio computed quantities characterizing hydrogen bond formation. The titration method based on 19F NMR spectroscopy was performed for detecting and quantifying the formation of very weak hydrogen bond complexes such as those involving fluorine atoms as hydrogen bond acceptors. This approach represents a powerful and reliable method for studying and characterizing these complexes that, although weak, are very relevant.

Structural insights, spectral, flourescence, Z-scan, C-H…O/N-H…O hydrogen bonding and AIM, RDG, ELF, LOL, FUKUI analysis, NLO activity of N-2(Methoxy phenyl) acetamide

Quantum chemical calculations of geometries and vibrational wavenumbers of N-2(methoxy phenyl)acetamide(N2MPA) in the ground state were carried out by using density functional theory (DFT/B3LYP) method with 6-31G (d,p) basis set. The scaled harmonic vibrational frequencies have been compared with experimental FT-IR spectra. Theoretical vibrational spectra of the title compound were interpreted by means of potential energy distributions (PEDs) using MOLVIB program. The experimental 1H and 13C NMR chemical shifts were investigated and then compared with computed NMR data using the gauge – invariant atomic orbital (GIAO) method. The optical nature of the grown crystal was evaluated by using the UV-Vis optical transmittance spectrum. The lower wavelength absorption range and various optical constant for instance, energy band gap, absorption and extinction coefficient, refractive index, susceptibility and Urbach energy values were evaluated. The colourless crystal of N2MPA was grown by slow-evaporation technique. The nonlinear optical (NLO) properties of N2MPA have been investigated using Z-scan technique. The emission behavior of N2MPA was analyzed by fluorescence spectral analysis. Thermal stability of N2MPA was studied by TG-DTA analysis. The HOMO and LUMO energy levels are constructed and the corresponding frontier energy gaps are determined to realize the charge transfer within the molecule. The molecular electrostatic potential(MESP), natural bond orbital(NBO) analysis and topological analysis such as electron localization function(ELF), reduced density gradient(RDG), localized orbital locator(LOL), atoms in molecule(AIM) and FUKUI function analysis have been used to evaluate the intermolecular interaction, especially the hydrogen bonds. Fingerprint plots of Hirshfeld surfaces were used to locate and analyze the percentage of hydrogen bonding interactions.

Structural elucidation of the O-antigen polysaccharide from Escherichia coli O125ac and biosynthetic aspects thereof.

Enteropathogenic Escherichia coli O125, the cause of infectious diarrheal disease, is comprised of two serogroups, viz., O125ab and O125ac, which display the aggregative adherence pattern with epithelial cells. Herein, the structure of the O-antigen polysaccharide from E. coli O125ac:H6 has been elucidated. Sugar analysis revealed the presence of fucose, mannose, galactose and N-acetyl-galactosamine as major components. Unassigned 1H and 13C NMR data from one- and two-dimensional NMR experiments of the O125ac O-antigen in conjunction with sugar components were used as input to the CASPER program, which can determine polysaccharide structure in a fully automated way, and resulted in the following branched pentasaccharide structure of the repeating unit: →4)[β-d-Galp-(1→3)]-β-d-GalpNAc-(1→2)-α-d-Manp-(1→3)-α-l-Fucp-(1→3)-α-d-GalpNAc-(1→, where the side chain is denoted by square brackets. The proposed O-antigen structure was confirmed by 1H and 13C NMR chemical shift assignments and determination of interresidue connectivities. Based on this structure, that of the O125ab O-antigen, which consists of hexasaccharide repeating units with an additional glucosyl group, was possible to establish in a semi-automated fashion by CASPER. The putative existence of gnu and gne in the gene clusters of the O125 serogroups is manifested by N-acetyl-d-galactosamine residues as the initial sugar residue of the biological repeating unit as well as within the repeating unit. The close similarity between O-antigen structures is consistent with the presence of two subgroups in the E. coli O125 serogroup.

Theoretical Investigation of 1H and 13C NMR Spectra of Diethanol Amine Dithiocarbamate RAFT Agent

In this study, quantum chemical studies of diethanolamine dithiocarbamate (DADC) RAFT compound were carried out. For this purpose, the composite was optimized by using the DFT / B3LYP method 6-311G and the B3PW91 method SDD basis set. By using the optimized structure obtained according to the GIAO method, 1H and 13C NMR chemical shift values in the gas phase were calculated. According to the results obtained, it was seen that the theoretical data were coherent with the experimental data. In addition, in the theoretical part of the study, the FT-IR frequency values of the compound synthesized by using the same methods and basic set were compared experimentally and theoretically. In addition, the structure details and analysis of molecules, electronic properties such as HOMO and LUMO energies, molecular electrostatic potential (MEP) and thermodynamic properties have been performed. The electric dipole moment (μ) and the initial hyperpolarizability (β) values of the studied molecule were calculated by using ab initio quantum mechanical calculations. In addition, the calculated results show that the (DADC) molecule can have nonlinear optical (NLO) behavior with nonzero values. Neutral Band Orbital (NBO) analysis has been calculated with DFT / B3PW91/SDD basis set.

1H, 13C, 15N resonance assignment of the enzyme KdgF from Bacteroides eggerthii

To fully utilize carbohydrates from seaweed biomass, the degradation of the family of polysaccharides known as alginates must be understood. A step in the degradation of alginate is the conversion of 4,5-unsaturated monouronates to 4-deoxy-L-erythro-5-hexoseulose catalysed by the enzyme KdgF. In this study BeKdgF from Bacteroides eggerthii from the human gut microbiota has been produced isotopically labelled in Escherichia coli. Here the 1H, 13C, and 15N NMR chemical shift assignment for BeKdgF is reported.

Isolation of cyclohumulanoids from Daedaleopsis tricolor and their biosynthesis based on in silico simulations

Two protoilludanes tricoprotoilludanone (1) and tricoilludanlactone (2), a cerapicane tricocerapicanol D (3), an illudane tricoilludin D (4), and a ceratopicane tricoceratopicanone A (5) were isolated from Daedaleopsis tricolor along with a spiroaxane tricospiroaxane (6). Density functional theory (DFT)-based 1H and 13C NMR chemical shift studies verified the hypothesized structures. The absolute configurations of 2, 3, 5, and 6 were established by the ECD spectral comparisons with those based on DFT calculations. Although the absolute configurations of 1 and 4 were not directly discussed with the ECD spectra, structural resemblance to other compounds suggested the (11R)-configuration for 1 and 4. The authors hypothesized that protonated a 6,7-α-epoxyprotoilludene biosynthesizes a variety of illudane, cerapicane, and ceratopicane frameworks. These were successfully simulated in silico.

Superacidity and spectral signatures of hydroxyl groups in zeolites

The hydroxyls, Brønsted acid sites (BAS) and silanols, provide key contributions in the global acidity of zeolites and have significant impact on their properties and applications. In this work, we present the acidity of BAS and silanols in zeolites depending on their configurations in zeolite nanoparticles. The acidity was evaluated based on the deprotonation energy (DPE) calculated by the density functional method and compared to experimental spectra. The calculated DPE and available experimental data for acidity of small molecules in a gas phase allowed us to position the hydroxyl groups in zeolites into the general scale of gas phase acidity for the first time. The simulated deprotonation enthalpies for the bridging hydroxyls are in the range 1113–1187 kJ/mol while for silanols they vary in larger range 1186–1376 kJ/mol. Compared to gas phase acids, these values imply that the Brønsted acid sites fall in the range of superacids while silanols cover wide range from strong acids to superacids. The high gas phase acidy of the zeolite hydroxyls may be explained with the flexibility of the zeolite framework that efficiently accommodates the negative charge of deprotonated center via structural relaxation, electron density redistribution or formation of hydrogen bonds. Nanosized zeolite in proton form (HZSM-5) was used as a model system, and the proximities between bridging hydroxyls and 27Al centers was estimated by 1H{27Al} REAPDOR MAS NMR technique. A linear correlation between the 1H NMR chemical shifts and stretching O–H vibrational frequencies of the BAS was found similar to the silanol groups. However, no correlation between the deprotonation energy and the spectral characteristics of the corresponding hydroxyl (BAS and silanols) was observed. Thus, the acidity of the hydroxyls cannot be estimated based on the spectral characteristics, which accounts mainly for the formation and strength of hydrogen bonds.

Structural determination of new metabolites from the sea snail Turbo chrysostomus by NMR and DFT calculation

Two new trisaccharides, chrysostosides A and B (1‒2), and a new natural product, chrysostorine (3), together with six known compounds (4‒9) were isolated from the sea snail Turbo chrysostomus. Their structures were elucidated based on interpretation of NMR and HRESIMS data and by comparison of the spectral data with those reported in the literature. The relative configurations of 1 and 2, and the structure of 3 were deduced from 2D NMR experiments and quantum chemical calculations, including the computed JH,H coupling constants, 1H and 13C NMR chemical shifts with DP4+ predictions. All isolated compounds were evaluated for their antimicrobial activity. Among them, compound 4 showed moderate activity against Enterococcus faecalis and Staphylococcus aureus with MIC value of 32 μg/mL.

Structural, vibrational spectroscopy, molecular docking, DFT studies and antibacterial activity of (E)-N1-(3-chlorobenzylidene)benzene-1,4-diamine

In this work, (E)-N1-(3-chlorobenzylidene)benzene-1,4-diamine (CBD) compound was synthesized with good yield. The spectral studies were recorded by FT-IR, FT-Raman, NMR and UV-Vis to determine structural parameters. The geometrical parameters were optimized using DFT calculations at 6-311++G(d,p) basis set. The calculated structural parameters of the molecule were in line with the experimental data. The molecular orbitals of the compound were investigated through highest occupied molecular orbital (HOMO), lowest unoccupied molecular orbital (LUMO) analysis. The hyper conjugative interaction energy E(2) along with donor, acceptor electron densities (EDs) were determined by natural bond orbital (NBO) analysis. The molecular electrostatic potential (MEP), mulliken atomic charges, non-linear optical (NLO) properties and potential energy surface (PES) scan were also calculated. The 1H and 13C NMR chemical shifts calculated using Gauge invariant atomic orbital (GIAO) method were compared with the experimental NMR chemical shifts. Thermogravimetry (TG) and Differential Scanning Calorimetry (DSC) were carried out to characterise the thermal behaviour and stability of CBD molecule. In addition, PreADMET tool was also used to estimate ADME and Toxicity of CBD compound. The compound screened against four pathogens two gram positive and two gram negative had shown good anti-bacterial behaviour. The molecular docking studies executed against anti-bacterial target topoisomerase DNA gyrase enzyme (2XCT) emphasized good binding behaviour over the standard drug. Communicated by Ramaswamy H. Sarma

Does It Bind? A Method to Determine the Affinity of Calcium and Magnesium Ions for Polymers Using 1H NMR Spectroscopy.

The binding of calcium and magnesium ions (M2+) by polymers and other macromolecules in aqueous solution is ubiquitous across chemistry and biology. At present, it is difficult to assess the binding affinity of macromolecules for M2+ without recourse to potentiometric titrations and/or isothermal titration calorimetry. Both of these techniques require specialized equipment, and the measurements can be difficult to perform and interpret. Here, we present a new method based on 1H NMR chemical shift imaging (CSI) that enables the binding affinity of polymers to be assessed in a single experiment on standard high-field NMR equipment. In our method, M2+ acetate salt is weighed into a standard 5 mm NMR tube and a solution of polymer layered on top. Dissolution and diffusion of the salt carry the M2+ and acetate ions up through the solution. The concentrations of acetate, [Ac], and free (unbound) M2+, [M2+]f, are measured at different positions along the sample by CSI. Binding of M2+ to the polymer reduces [M2+]f and hinders the upward diffusion of M2+. A discrepancy is thus observed between [Ac] and [M2+]f from which the binding affinity of the polymer can be assessed. For systems which form insoluble complexes with M2+, such as sodium polyacrylate or carboxylate-functionalized nanocellulose (CNC), we can determine the concentration of M2+ at which the polymer will precipitate. We can also predict [M2+]f when a solution of polymer is mixed homogeneously with M2+ salt. We assess the binding properties of sodium polyacrylate, alginate, polystyrene sulfonate, CNC, polyethyleneimine, ethylenediamenetetraacetic acid, and maleate.

Tuning the Ground- and Excited-State Redox Potentials of Octahedral Hexanuclear Rhenium(III) Complexes by the Combination of Terminal Halide and N-Heteroaromatic Ligands.

The present study reports that the ground- and excited-state Re6(23e)/Re6(24e) redox potentials of an octahedral hexanuclear rhenium(III) complex can be controlled by systematically changing the number and type of the N-heteroaromatic ligand (L) and the number of chloride ions at the six terminal positions. Photoirradiation of [Re6(μ3-S)8Cl6]4– with an excess amount of L afforded a mono-L-substituted hexanuclear rhenium(III) complex, [Re6(μ3-S)8Cl5(L)]3– (L = 4-dimethylaminopyridine (dmap), 3,5-lutidine (lut), 4-methylpyridine (mpy), pyridine (py), 4,4′-bipyridine (bpy), 4-cyanopyridine (cpy), and pyrazine (pz)). The bis- and tris-lut-substituted complexes, trans- and cis-[Re6(μ3-S)8Cl4(lut)2]2– and mer-[Re6(μ3-S)8Cl3(lut)3]−, were synthesized by the reaction of [Re6(μ3-S)8Cl6]3– with an excess amount of lut in refluxed N,N-dimethylformamide. The mono-L-substituted complexes showed one-electron redox processes assignable to E1/2[Re6(23e)/Re6(24e)] = 0.49–0.58 V versus Ag/AgCl. The ground-state oxidation potentials were linearly correlated with the pKa of the N-heteroaromatic ligand [pKa(L)], the 1H NMR chemical shift of the ortho proton on the coordinating ligand, and the Hammett constant (σ) of the pyridyl-ligand substituent. The series of [Re6(μ3-S)8X6–n(L)n]n−4 complexes (n = 0, X = Cl, Br, I, or NCS; n = 1–3, X = Cl) showed a linear correlation with the sum of the Lever electrochemical parameters at the six terminal ligands (ΣEL). The cyclic voltammograms of the mono-L-substituted complexes (L = bpy, cpy, and pz) showed one-electron redox waves assignable to E1/2(L0/L–) = −1.28 to −1.48 V versus Ag/AgCl. Two types of photoluminescences were observed for the complexes, originating from the cluster core-centered excited triplet state (3CC) for L = dmap, lut, mpy, and py and from the metal-to-ligand charge-transfer excited triplet state (3MLCT) for L = bpy, cpy, and pz. The complexes with the 3CC character exhibited emission features and photophysical properties similar to those of ordinary hexanuclear rhenium complexes. The emission maximum wavelength of the complexes with 3MLCT shifted to the longer wavelength in the order L = 4-phenylpyridine (ppy), bpy, pz, and cpy, which agreed with the difference between E1/2[Re6(23e)/Re6(24e)] and E1/2(L0/L–). The calculated oxidation potential of the excited hexanuclear rhenium complex with the 3CC character was linearly correlated with pKa(L), σ, and ΣEL. The ground- and excited-state oxidation potentials were finely tuned by the combination of halide and L ligands at the terminal positions.