Structural survey of technetium complexes

Technetium Nitrido Complexes of Tetradentate Thiosemicarbazones: Kit-Based Radiolabeling, Characterization, and In Vivo Evaluation

Bis(thiosemicarbazone) and pyridylhydrazone-thiosemicarbazone chelators have demonstrated utility in nuclear medicine. In particular, the 64Cu2+ complexes have been extensively developed for hypoxia imaging and molecular imaging of peptide and protein markers of disease. However, the chemistry and application of bis(thiosemicarbazone) and pyridylhydrazone-thiosemicarbazone chelators in combination with 99mTc, the most widely used radionuclide in nuclear medicine, is underexplored. Herein, a series of bis(thiosemicarbazone) and pyridylhydrazone-thiosemicarbazone chelators were radiolabeled with nitrido-technetium-99m in an optimized one-pot synthesis from [99mTc]TcO4-. Optimization of the radiochemical syntheses allowed for production of the complexes in >90% radiochemical conversion with apparent molar activities of 3.3-5 GBq/μmol. Competition experiments demonstrated the excellent stability of the complexes. The nitrido-technetium-99 complexes were synthesized, and the chemical identities were investigated using mass spectrometry, spectroscopy, and density functional theory calculations. Complexation of nitrido-rhenium(V) was achieved with the N4-dialkylated bis(thiosemicarbazones). Planar imaging and ex vivo biodistribution studies of the five 99mTc complexes were conducted on healthy BALB/c mice to determine in vivo behavior. The lipophilic nature of the complexes resulted in uptake of 1.6-5.7% ID g-1 in the brain at 2 min postinjection and retention of 0.4-1.7% ID g-1 at 15 min postinjection. The stability of the complexes and the biodistribution data demonstrate that these chelators are ideal platforms for future production of radiopharmaceutical candidates.

DFT and TDDFT exploration on the role of pyridyl ligands with copper toward bonding aspects and light harvesting

CONTEXT

Schiff base-containing metal complexes have been the subject of extensive research. In this work, a coordination polymer-derived complex called [Cu(L)] that is solution-stable (L = 2-(2-hydroxybenzylidene-amino)phenol) has been explored theoretically with five different pyridyl-based ligands using DFT/TDDFT in order to understand the structural-functional and electronic transitions of these five complexes. Frontier molecular orbital (FMO) analysis was carried out to assess the reactivity behavior of all five complexes. For the purpose of studying the charge energy distribution over complexes, electrostatic potential maps were also drawn. Furthermore, in order to identify any stabilizing interactions that may be present in the given complexes, an NBO analysis was studied. To learn more about any potential correlations between the properties of these five complexes, a comparative analysis was explored. Our calculations demonstrate that complex 3 having pyridine-4-carboxamide as a ligand has a lower energy gap and a higher negative electrostatic potential which may indicate its higher reactivity and this may be due to the electron withdrawing group (carboxamide). TDDFT results show that the highest light harvesting efficiency (LHE) of all the studied complexes is found in the range of 440-448 nm. Complexes 1, 2, and 4 show the higher light harvesting efficiency as compared to complexes 3 and 5. Our findings are in good accordance with the available experimental data.

METHODS

All DFT computations were performed using the Gaussian16 with unrestricted B3LYP-D2 functional with the basis sets 6-31G(d,p) for O, N, C, and H while LanL2DZ for Cu. The polarized continuum model (PCM) was used for the solvation. The software GaussView6.1 was utilized for the modeling of initial geometries and the plotting of MEP maps. The NBO6.0 program which is incorporated in Gaussian16 was utilized to investigate the bonding nature and stabilization energies of the complexes. The ORCA program was used to simulate the absorption spectra.

Medicinal applications of vanadium complexes with Schiff bases

Many transition metal complexes have been explored for their therapeutic properties after the discovery of cisplatin. Schiff bases have an efficient complexation tendency with the transition metals and several medicinal properties have been reported. However, fewer studies have reported the medicinal utility of vanadium and its Schiff base complexes. This paper provides a comprehensive overview of vanadium complexes with Schiff bases along with their mechanistic insight. Vanadium complexes in + 4 and + 5 oxidation states have exhibited well-defined geometry and found to be thermodynamically stable. The studies have reported the G0/G1 phase cell cycle arrest and decreased delta psi m, inducing mitochondrial membrane depolarization in cancer cell lines along with the alterations in the metabolism of the cancer cells upon dosing with the vanadium complexes. Cancer cell invasion and growth are also found to be markedly reduced by peroxo complexes of vanadium. The studies included in the review paper have been taken from leading indexing databases and focus was laid on recent reports in literature. The biological potential of vanadium complexes of Schiff bases opens new horizons for future interdisciplinary studies and investigation focussed on understanding the biochemistry of these complexes, along with designing new complexes which have better bioavailability, solubility and low or non-toxicity.

Experimental and Theoretical Investigations of Three-Ring Ester/Azomethine Materials

New three-ring ester/azomethine homologues series, (E)-4-((4-hydroxybenzylidene)amino)phenyl 4-(alkoxy)benzoate In, were prepared and their properties were investigated experimentally and theoretically. FT-IR, NMR, and elemental analyses were used to confirm the chemical structures of the synthesized compounds. The mesomorphic activities of the planned homologues were evaluated using differential scanning calorimetry (DSC) and polarized optical microscopy. All of the homologous examined were found to have non-mesomorphic properties. Theoretical calculations using the density functional theory (DFT) were used to validate the experimental data and determine the most stable conformation of the synthesized compounds. All calculated conformers’ thermal properties, dipole moments, and polarizability were discussed. The results show that the terminal alkoxy chain length affects the thermal parameters of the conformers. The correlations between these parameters’ values and the conformer type were demonstrated. The base component was expected to be in two conformers according to the orientation of the N atom of imine-linkage. DFT calculations revealed the more probable of the two possible conformers, and the incorporation of the alkoxy terminal chain in one position affect its geometrical and mesomerphic characteristics.

Palladium(0) Catalyzed Synthesis of (E)-4-Bromo-N-((3-bromothiophen-2-yl)methylene)-2-methylaniline Derivatives via Suzuki Cross-Coupling Reaction: An Exploration of Their Non-Linear Optical Properties, Reactivity and Structural Features

A series of (E)-4-bromo-N-((3-bromothiophen-2-yl)methylene)-2-methylaniline analogs synthesized in considerable yields through Suzuki cross-coupling reactions. Various electron donating and withdrawing functional moieties were successfully incorporated under the employed reaction conditions. Reaction of 4-bromo-2-methylaniline (1) with 3-bromothiophene-2-carbaldehyde (2b) in the existence of glacial acetic acid, provided (E)-4-bromo-N-((3-bromothiophen-2-yl)methylene)-2-methylaniline (3b) in excellent yield (94%). Suzuki coupling of 3b with different boronic acids in the presence of Pd(PPh₃)₄/K₃PO₄ at 90 °C led to the synthesis of the monosubstituted and bisubstituted products 5a-5d and 6a-6d in moderate yields (33-40% and 31-46%, respectively). Density functional theory (DFT) investigations were performed on different synthesized analogues 5a-5d, 6a-6d to determine their structural characteristics. The calculations provide insight into the frontier molecular orbitals (FMOs) of the imine-based analogues and their molecular electrostatic potential (MESP). Reactivity descriptors like ionization energy (I), electron affinity (A), chemical hardness (ƞ) and index of nucleophilicity have been calculated for the first time for the synthesized molecules.

Synthesis, Conformational Analysis, Infrared, Raman and UV-Visible Spectra of Novel Schiff Bases compiled with DFT Calculations

Objective:

Two novel Schiff bases named, 2-((2-Hydroxybenzylidene)amino)-4,5,6,7- tetrahydrobenzo[b] thiophene-3-carbonitrile (BESB1) and 2-((Furan-2-ylmethylene)amino)-4,5,6, 7-tetrahydro-benzo[b]thiophene-3-carbonitrile (BESB2) were synthesized.

Methods:

The structures were characterized based on CHN elemental analysis, mid-infrared (400– 4000 cm-1), Raman (100-4000 cm-1), 1H NMR, mass and UV-Vis spectroscopic measurements. In addition, quantum mechanical calculations using DFT-B3LYP method at 6-31G(d) basis set were carried out for both Schiff bases. Initially, we have carried out complete geometry optimizations followed by frequency calculations for the proposed conformational isomers; BESB1 (A–E) and BESB2 (F–J) based on the orientations of both CN and OH groups against the azomethine lonepair (NLP) in addition to the 3D assumption.

Results:

The computational outcomes favor conformer A for BESB1 in which the C≡N and OH moieties are cis towards the NLP while conformer G is preferred for BESB2 (the C≡N/furan-O are cis/trans towards the NLP) which was found consistent with the results of relaxed potential energy surface scan. Aided by normal coordinate analysis of the Cartesian coordinate displacements, we have suggested reliable vibrational assignments for all observed IR and Raman bands. Moreover, the electronic absorption spectra for the favored conformers were predicted in DMSO solution using TD-B3LYP/6-31G(d) calculations. Similarly, the 1H NMR chemical shifts were also estimated using GIAO approach implementing PCM including solvent effects (DMSO-d6).

Conclusion:

Proper interpretations of the observed electronic transition, chemical shifts, IR and Raman bands were presented in this study.

Strategies for the Molecular Imaging of Amyloid and the Value of a Multimodal Approach

Protein aggregation has been widely implicated in neurodegenerative diseases such as Alzheimer's disease, frontotemporal dementia, Parkinson's disease, and Huntington disease, as well as in systemic amyloidoses and conditions associated with localized amyloid deposits, such as type-II diabetes. The pressing need for a better understanding of the factors governing protein assembly has driven research for the development of molecular sensors for amyloidogenic proteins. To date, a number of sensors have been developed that report on the presence of protein aggregates utilizing various modalities, and their utility demonstrated for imaging protein aggregation in vitro and in vivo. Analysis of these sensors highlights the various advantages and disadvantages of the different imaging modalities and makes clear that multimodal sensors with properties amenable to more than one imaging technique need to be developed. This critical review highlights the key molecular scaffolds reported for molecular imaging modalities such as fluorescence, positron emission tomography, single photon emission computed tomography, and magnetic resonance imaging and includes discussion of the advantages and disadvantages of each modality, and future directions for the design of amyloid sensors. We also discuss the recent efforts focused on the design and development of multimodal sensors and the value of cross-validation across multiple modalities.

Nitrido Technetium-99 m Core in Radiopharmaceutical Applications: Four Decades of Research

The knowledge on element 43 (Tc) of the periodic table, built over the years through the contributions given by the close relationship between chemistry and nuclear medicine, allowed the development of new and increasingly effective radiopharmaceuticals useful both as perfusion and target specific imaging agents for SPECT (single photon emission tomography). Among the manifold Tc-compounds, Tc(V) nitrido complexes played a relevant role in the search for new technetium-99m radiopharmaceuticals, providing efficient labeling procedures that can be conveniently exploited for the design and synthesis of agents, also incorporating small organic molecules or peptides having defined structural features. With this work, we present an overview of four decades of research on the chemistry and on the nuclear medicine applications of Tc(V) nitrido complexes.

Metal complexes driven from Schiff bases and semicarbazones for biomedical and allied applications: a review

Schiff bases are versatile organic compounds which are widely used and synthesized by condensation reaction of different amino compound with aldehydes or ketones known as imine. Schiff base ligands are considered as privileged ligands as they are simply synthesized by condensation. They show broad range of application in medicine, pharmacy, coordination chemistry, biological activities, industries, food packages, dyes, and polymer and also used as an O2 detector. Semicarbazone is an imine derivative which is derived from condensation of semicarbazide and suitable aldehyde and ketone. Imine ligand-containing transition metal complexes such as copper, zinc, and cadmium have shown to be excellent precursors for synthesis of metal or metal chalcogenide nanoparticles. In recent years, the researchers have attracted enormous attention toward Schiff bases, semicarbazones, thiosemicarbazones, and their metal complexes owing to numerous applications in pharmacology such as antiviral, antifungal, antimicrobial, antimalarial, antituberculosis, anticancer, anti-HIV, catalytic application in oxidation of organic compounds, and nanotechnology. In this review, we summarize the synthesis, structural, biological, and catalytic application of Schiff bases as well as their metal complexes.

Role of Pyridine Nitrogen in Palladium-Catalyzed Imine Hydrolysis: A Case Study of (E)-1-(3-bromothiophen-2-yl)-N-(4-methylpyridin-2-yl)methanimine

In the present study, 4-methylpyridin-2-amine was reacted with 3-bromothiophene-2-carbaldehyde and the Schiff base (E)-1-(3-bromothiophen-2-yl)-N-(4-methylpyridin-2-yl)methanimine was obtained in a 79% yield. Coupling of the Schiff base with aryl/het-aryl boronic acids under Suzuki coupling reaction conditions, using Pd(PPh₃)₄ as catalyst, yielded products with the hydrolysis of the imine linkages (5a–5k, 6a–6h) in good to moderate yields. To gain mechanistic insight into the transition metal-catalyzed hydrolysis of the compounds, density functional theory (DFT) calculations were performed. The theoretical calculations strongly supported the experiment and provided an insight into the transition metal-catalyzed hydrolysis of imines.

Synthesis, characterization and biological studies of rhenium, technetium-99m and rhenium-188 pentapeptides

A pentapeptide macrocyclic ligand, KYCAR (lysyl-tyrosyl-cystyl-alanyl-arginine), has been designed as a potential chelating ligand for SPECT imaging and therapeutic in vivo agents. This study shows the synthesis and characterization of KYCAR complexes containing nonradioactive rhenium, ^99mTc, or ¹⁸⁸Re. The metal complexes were also biologically evaluated to determine in vivo distribution in healthy mice. The overall goals of this project were (1) to synthesize the Tc/Re pentapeptide complexes, (2) to identify spectroscopic methods for characterization of syn versus anti rhenium peptide complexes, (3) to analyze the ex vivo stability, and (4) to assess the biological properties of the [^99mTc]TcO-KYCAR and [¹⁸⁸Re]ReO-KYCAR complexes in vivo. Details on these efforts are provided below.

METHODS

^NatRe/^99mTc/¹⁸⁸ReO-KYCAR complexes were synthesized, and macroscopic species were characterized via HPLC, IR, NMR, and CD. These characterization data were compared to the crystallographic data of ReO-KYC to assist in the assignment of diastereomers and to aid in the determination of the structure of the complex.

RESULTS

The radiometal complexes were synthesized with high purity (>95%). HPLC, IR, NMR and CD data on the macroscopic ^natReO-KYCAR complexes confirm the successful complexation as well as the presence of two diastereomers in syn and anticonformations. Tracer level complexes show favorable stabilities ex vivo for 2+ h.

CONCLUSION

Macroscopic metal complexes form diastereomers with the KYCAR ligand; however, this phenomenon is not readily observed on the tracer level due to the rapid interconversion. It was determined through pK_a measurements that the macroscopic ^natReO-KYCAR complex is 0 at physiological pH. The [^99mTc]TcO-KYCAR is stable in vitro while the [¹⁸⁸Re]ReO-KYCAR shows 50% decomposition in PBS and serum. Biologically, the tracer level complexes clear through the hepatobiliary pathway. Some decomposition of both tracers is evident by uptake in the thyroid and stomach.

A luminescent dicyanidonitridotechnetium(v) core with tridentate ligand coordination sites

A novel, luminescent technetium complex, [TcN(CN)2bpa] (bpa = bis-(2-pyridylmethyl)amine), with tridentate ligand coordination sites was synthesized and characterized. Photoemission with a maximum wavelength at 666 nm was observed in the solid-state at 296 K.

Facile synthesis of N- (4-bromophenyl)-1- (3-bromothiophen-2-yl)methanimine derivatives via Suzuki cross-coupling reaction: their characterization and DFT studies

A variety of imine derivatives have been synthesized via Suzuki cross coupling of N-(4-bromophenyl)-1-(3-bromothiophen-2-yl)methanimine with various arylboronic acids in moderate to good yields (58–72%). A wide range of electron donating and withdrawing functional groups were well tolerated in reaction conditions. To explore the structural properties, Density functional theory (DFT) investigations on all synthesized molecules (3a–3i) were performed. Conceptual DFT reactivity descriptors and molecular electrostatic potential analyses were performed by using B3LYP/6-31G(d,p) method to explore the reactivity and reacting sites of all derivatives (3a–3i).

Fluorescence Behavior of Schiff Base-N, N'-bis(salicylidene) Trans 1, 2-Diaminocyclohexane in Proteinous and Micellar Environments

Fluorescence properties of N, N'-bis(salicylidene) trans 1, 2-diaminocyclohexane (H ₂ L) is used to probe the anionic (SDS), cationic (CTAB) and nonionic (TX-100) micelles as well as in serum albumins (BSA and HSA) and chicken egg white lysozyme (LYZ) by steady state and picosecond time-resolved fluorescence spectroscopy. The fluorescence band intensity was found to increase with concomitant blue-shift with gradual addition of different surfactants. All the experimental results suggest that the probe molecule resides in the micelle-water interface rather than going into the micellar core. However, the penetration is more towards the micellar hydrocarbon core in nonionic surfactant (TX-100) while comparing with ionic surfactants (SDS and CTAB). Several mean microscopic properties such as critical micelle concentration, polarity parameters and binding constant were calculated in presence of different surfactants. The decrease in nonradiative decay rate constants in micellar environments indicates restricted motion of the probe inside the micellar nanocages with increasing fluorescence emission intensity and quantum yields. Further in this work, we also investigated the interaction behavior of the probe with different proteins at low concentrations under physiological conditions (pH = 7.4). Stern-Volmer analysis of the tryptophan (Trp) fluorescence quenching data in presence of probe reveals Stern-Volmer constant (K_sv) as well as bimolecular quenching rate constant (K_q). The binding constant as well as the number of binding sites of the probe with proteins were also monitored and found to be 1:1 stoichiometry ratio.

Synthesis and characterization of rhenium(iii) complexes with (Ph2PCH2CH2)2NR diphosphinoamine ligands

The synthesis and characterization of a new series of neutral, six-coordinated compounds [Re^IIIX₃(PNPR)], where X is Cl or Br and PNPR is a diphosphinoamine having the general formula (Ph₂PCH₂CH₂)₂NR (R = H, CH₃, CH₂CH₃, CH₂CH₂CH₃, CH₂CH₂CH₂CH₃ and CH₂CH₂OCH₃) are reported. Stable [Re^IIIX₃(PNPR)] complexes were synthesized, in variable yields, starting from precursors where the metal was in different oxidation states (iii and v), by ligand-exchange and/or redox-substitution reactions. The compounds were characterized by elemental analysis, proton NMR spectroscopy, cyclic voltammetry, UV/vis spectroscopy, positive-ion electrospray ionization mass spectrometry (ESI(+)-MS) and X-ray diffraction analysis. Although the formulation of the complexes allows either meridional or facial isomers, the latter arrangement was prevalent both in the solid and solution states. Only [ReCl₃(PNPH)] showed a meridional configuration both in solution and in the crystalline state. [ReBr₃(PNPme)] prefers the meridional configuration in the crystalline state and the facial one in solution. While ESI(+)-MS and voltammetric data seem to indicate some dependency from the nature of the alkyl substituent at the nitrogen, the available structural data of the complexes show only slight differences both for angles and bond lengths upon change of the alkyl chain tethered to the nitrogen.

Syntheses and characterization of monobasic tridentate Cu(ii) Schiff-base complexes for efficient oxidation of 3,5-di-tert-butylcatechol and oxidative bromination of organic substrates

Enhanced bifunctional catalytic activities are shown by monobasic tridentate Cu(ii) Schiff-base complexes.

Unique mononuclear MnIIcomplexes of an end-off compartmental Schiff base ligand: experimental and theoretical studies on their bio-relevant catalytic promiscuity

Mn^IISchiff base complexes have been experimentally and theoretically studied for their catecholase and phenoxazinone synthase activity.

Exploration of unconventional π–hole and C–H⋯H–C types of supramolecular interactions in a trinuclear Cd(ii) and a heteronuclear Cd(ii)–Ni(ii) complex and experimental evidence for preferential site selection of the ligand by 3d and 4d metal ions

We report the synthesis of a trinuclear cadmium(ii) (1) and a di(phenoxido)-bridged dinuclear cadmium(ii)–nickel(ii) (2) complex derived from a bicompartmental (N₂O₄) Schiff base ligand.

Fluorescence properties of Schiff base - N,N'-bis(salicylidene) - 1,2-Phenylenediamine in presence of bile acid host

Fluorescence properties of Schiff base - N,N'-bis(salicylidene) - 1,2-phenylenediamine (LH2) is used to study the micelles formed by aggregation of different important bile acids like cholic acid, deoxycholic acid, chenodeoxycholic acid and glycocholic acid by steady state and picosecond time-resolved fluorescence spectroscopy. The fluorescence band intensity was found out to increase with concomitant red shift with gradual addition of different bile acids. Binding constant of the probe with different bile acids as well as critical micelle concentration was obtained from the variation of fluorescence intensity on increasing concentration of bile acids in the medium. The increase in fluorescence quantum yields, fluorescence decay times and substantial decrease in nonradiative decay rate constants in bile acids micellar environment points to the restricted motion of the fluorophore inside the micellar subdomains.

Metal complexes designed to bind to amyloid-β for the diagnosis and treatment of Alzheimer's disease

Alzheimer's disease is the most common form of age-related neurodegenerative dementia. The disease is characterised by the presence of plaques in the cerebral cortex. The major constituent of these plaques is aggregated amyloid-β peptide. This review focuses on the molecular aspects of metal complexes designed to bind to amyloid-β. The development of radioactive metal-based complexes of copper and technetium designed as diagnostic imaging agents to detect amyloid burden in the brain is discussed. Separate sections of the review discuss the use of luminescent metal complexes to act as non-conventional probes of amyloid formation and recent research into the use of metal complexes as inhibitors of amyloid formation and toxicity.