Metal Complexes Research Articles

Relationship between Transition-Metal Hydride Bond Lengths and Stretching Wavenumbers.

Here it is demonstrated that there is a linear relationship between the terminal 3d metal hydride stretching wavenumber νMH and the metal hydride distance dMH reported to date: νMH ∼ (-1.05dMH + 3.35) × 103 cm-1. That is, upon moving from Group 3 to Group 10 metals in complexes and binary molecules, νMH (cm-1) increases as the metal-hydrogen distance dMH (Å) decreases as determined by X-ray diffraction, molecular spectroscopy, and, in one case, neutron diffraction. This trend contrasts with the relatively constant bond dissociation free energy (BDFE) of the diamagnetic complexes and the usually smaller BDFE (by as much as -20 kcal/mol) of paramagnetic ones, thus showing that there is no correlation between either νMH or its force constant with the BDFE. If a positive charge is added to the complex by metal-ion oxidation, νMH may increase or decrease depending on the coordination number and spin state of the complex.

Controllable Self-Assembly of V═O Metalloradical Complex with Intramolecular Charge Transfer for Enhanced NIR-II Fluorescence Imaging-Guided Photothermal Therapy.

Near-infrared second region (NIR-II) fluorescence imaging provides enhanced tissue penetration, achieving efficient NIR-II fluorescence and photoacoustic imaging (PA)-guided photothermal therapy (PTT) all in one material remains a challenging yet promising approach in cancer treatment. Herein, open-shell V═O metalloradical complex (VONc) is self-assembled into VONc nanospheres (VONc NPs). VONc NPs exhibit light absorption from 300 to 1400 nm, fluorescence spectra ranging from 900 to 1400 nm, and a distinct fluorescence signal even at 1550 nm. Moreover, VONc NPs exhibit outstanding photostability and a higher photothermal conversion efficiency of 46.6% than that of closed-shell zinc naphthalocyanine nanorods (ZnNc NRs). V═O centered metalloradical serves as transient electron-withdrawing groups to facilitate charge transfer (CT), introducing additional nonradiative energy dissipation pathways and enhancing efficient heat generation. In vitro experiments of VONc NPs indicate that a highly effective photothermal action causes harm to both mitochondria and lysosomes, resulting in the death of tumor cells, closed-shell ZnNc NPs exhibit almost no cell killing as contrast. In vivo anti-tumor therapy results of VONc NPs demonstrate excellent NIR-II fluorescence imaging-guided PTT against tumors with a favorable biosafety profile. "Centered metalloradical boosting CT" toward open-shell metal complexes provides significant insight for developing single-material integrated nanosystems for diagnostic and therapeutic applications.

A New Heteroleptic Zn(II) Complex with Schiff Bases Sensitizes Triple-Negative Breast Cancer Cells to Doxorubicin and Paclitaxel

Background/Objectives: Triple-negative breast cancer (TNBC) is the most challenging molecular subtype of breast cancer (BC) in clinical practice, associated with a worse prognosis due to limited treatment strategies and its insensitivity to conventional drugs. Zinc is an important trace element for homeostasis, and its Schiff base metal complexes have shown promise in treating advanced tumors. In this study, four new heteroleptic Zn(II) complexes (1–4) with Schiff bases were synthesized, characterized, and evaluated for their activity in BC cells. Methods: Compounds were synthesized, characterized, and their crystal structures were determined. Biological activity was assessed using MTT, clonogenic, scratch wound healing, caspase 3 and 8 activity, qPCR, and chemosensitization assays. Results: The complexes exhibited cytotoxicity against MCF-7 (luminal BC), MDA-MB-453 (HER2-positive BC), and MDA-MB-231 (TNBC) cell lines, with IC50 values ranging from 0.01 to 20 µM. Complex 4 showed reduced cytotoxicity toward non-tumor cell lines. This, complexation with Zn(II) increased the cytotoxicity of the ligands, a trend not observed for complexes 1–3. Due to its favorable profile, complex 4 was selected for further assays, in which it inhibited colony formation and the cell migration of TNBC cells in a dose-dependent manner. Furthermore, this compound induced cell death independently of caspases, decreasing the activity of caspase 8. Interestingly, complex 4 sensitized TBNC cells to doxorubicin and paclitaxel, possibly modulating the epithelial–mesenchymal transition mechanism, as evidenced by increased CDH1 expression. Conclusions: Results suggest the potential of complex 4 in sensitizing aggressive BC cells to chemotherapy, proving to be a promising alternative in cases of therapeutic failure.

Favorable Symmetric Structures of Radiopharmaceutically Important Neutral Cyclen-Based Ligands

Cyclen-based ligands are prominent tools for transferring radioisotopes through the human body. A crucial criterion is the stability of their complexes, which is partly determined by the stabilization of the free ligand in solution. For the assessment of the later property, the favored conformation(s) in the solution must be known. In the present study, the conformational space of four neutral cyclen-based ligands was elucidated by a multi-step procedure: the survey of the conformational space using molecular mechanics (MM) was followed by Density Functional Theory (DFT) calculations on the low-energy conformers and evaluation of the solvent effects. The results revealed several low-energy conformers in aqueous solution. In terms of electronic energies, a significant preference of symmetric structures (C4 or C2—similar to the ligand arrangements in their metal complexes) was obtained. The thermal contributions to the Gibbs free energy (mainly the vibrational ones) tend to decrease this preference by several kJ/mol against non-symmetric structures. Nonetheless, the advantage of compact symmetric structures was confirmed in all the four studied cases.

Ni(II) and Cu(II) Ion Coordination by the Novel (2E,2′E)-N,N′-(2-Hydroxypropane-1,3-diyl)bis[(2-hydroxyimino)propanamide] Ligand in the Solid State and in Aqueous Medium

In the process of a systematic study of the bis-chelate oxime-amide ligands with different polymethylene spacers and their transition metal complexes, two new Ni(II) and Cu(II) complexes of the mhiea2poh ligand have been isolated, and their molecular and crystal structures determined by single crystal X-ray diffraction. Potentiometric titrations were performed on a range of aqueous solutions in both systems, which allowed the identification of various complex species and afforded their stability constants. ESR spectra of samples with optimised concentrations of complexes in Cu(II)–L–H system were recorded. A distinguishing feature of both systems is the dimerization of anionic pseudo-macrocyclic complexes. The latter is caused either by centrosymmetric hydrogen bonding of the hydroxy group on the propane spacer to the oximato oxygen of the opposing unit or by back-to-back π-stacking of planar complex units. ESR measurements indicated likely coupling of Cu-Cu paramagnetic centres in the dimers.

Recent Advances of Metal Complexes in the Treatment of Hepatocellular Carcinoma.

Chemotherapy has long been used in the clinical management of hepatocellular carcinoma (HCC), driving the development of anticancer chemotherapy drugs. Platinum complexes have attracted significant attention and have led to the creation of a series of platinum-based drugs used in diverse cancer treatments, including HCC. However, the clinical use of platinum drugs faces critical challenges due to drug resistance and side effects. Consequently, ongoing efforts have been devoted to the continuous development of new metal complexes with antitumor properties, aiming to serve as effective alternatives for HCC treatment. In this Perspective, we summarize and highlight the progress and relevant mechanisms related to new metal complexes in the treatment of HCC over the past decade. The development of metal complexes has the potential to further expand the scope of chemotherapy applications for HCC.

Boosting the Performance of Iridium Single Atom Catalyst in a Porous Organic Polymer for Glycerol Conversion to Lactic Acid

Single‐atom catalysts (SACs) inherit the merit of both homogeneous and heterogeneous systems with atomically dispersed mononuclear metal centers on the solid supports. Herein, we developed an Ir‐SAC catalyst via the polymerization of an active homogeneous 2‐picolinylhydrazone ligand‐based iridium (Ir) metal complex. Such catalysts provide great stabilization against migration and agglomeration due to the strong covalent C‐C bond linkage of active complexes and the polymer matrix. This Ir‐SAC catalyst shows excellent selectivity towards glycerol to lactic acid conversion with a remarkable recyclability to offer an unprecedentedly high TON of over 104 million under optimized conditions.

Vibrational and DFT Studies and Anticancer Activity of Novel Pd(II) and Pt(II) Complexes with Chloro Derivatives of 7-Azaindole-3-Carbaldehyde

This study investigates the structural, vibrational, and biological properties of novel palladium(II) and platinum(II) complexes with 5-chloro-7-azaindole-3-carbaldehyde (5ClL) and 4-chloro-7-azaindole-3-carbaldehyde (4ClL) ligands. Infrared and Raman spectroscopy, combined with DFT (ωB97X-D) calculations, provided valuable information about metal–ligand interactions, the cis or trans conformation of the aldehyde group in the ligands, and the presence of trans isomers in the metal complexes obtained in the solid state. In vitro tests were used to evaluate the antiproliferative activity of the novel complexes against several cancer cell lines, including ovarian cancer (A2780), cisplatin-resistant ovarian cancer (A2780cis), colon cancer (HT-29), and triple-negative breast cancer (MDA-MB-231), as well as normal mouse fibroblasts (BALB/3T3). The platinum complex, trans-[PtCl2(5ClL)2], exhibited superior activity against A2780cis (IC50 = 4.96 ± 0.49 µM) and MDA-MB-231 (IC50 = 4.83 ± 0.38 µM) compared to cisplatin, while the palladium complexes (trans-[PdCl2(4ClL)2] and trans-[PdCl2(5ClL)2]) demonstrated enhanced selectivity with reduced toxicity to normal fibroblasts (IC50 = 11.29 ± 6.65 µM and 14.98 ± 5.59 µM, respectively).

Exfoliated Hydrotalcite-Transition Metal Complex Composite for Eco-Friendly and Efficient Catalytic Degradation of 4-Nitrophenol

Exfoliated Hydrotalcite-Transition Metal Complex Composite for Eco-Friendly and Efficient Catalytic Degradation of 4-Nitrophenol

Decoding the Catalytic Potential of Dinuclear 1st-Row Transition Metal Complexes for Proton Reduction and Water Oxidation.

The growing interest in renewable energy sources has led to a significant focus on artificial photosynthesis as a means of converting solar energy into lucrative and energy-dense carbonaceous fuels. First-row transition metals have thus been brought to light in the search for efficient and high-performance homogenous molecule catalysts that can accelerate energy transformation and reduce overpotentials during the catalytic process. Their dinuclear complexes have opportunities to enhance the efficiency and stability of these molecular catalysts, primarily for the hydrogen evolution reaction (HER) and water oxidation reaction (WOR). Recently, our group improved the catalytic activity, efficiencies, and stability of dinuclear molecular catalysts, particularly toward HER. Although one dinuclear complex has been tested for WOR, it demonstrated activity as water oxidation precatalysts. First-row transition metals are a great option for sustainable catalysis because they are readily available, reasonably priced, and have multifaceted coordination chemistry. Examples of these metals are cobalt, copper, and manganese. The structure-catalytic performance relationships of this first-row transition metal-based dinuclear catalysts are noteworthily interpreted in this account, providing avenues for optimizing their performance and advancing the development of sustainable and effective energy conversion technologies.

Structural complexity of glyphosate and aminomethylphosphonate metal complexes.

Small differences in the structure and subsequent reactivity of glyphosate complexes can have a highly consequential impact due to the enormous quantities of glyphosate used globally. The gas phase metal speciation of glyphosate and its abundant metabolite, aminomethylphosphonic acid (AMPA), were determined using cross-platform electrospray ionisation ion mobility mass spectrometry. Monomeric [M + L - H]+ complexes, and both larger, and/or higher order clusters formed with divalent metals (M = Mg2+, Ca2+, Sr2+, Ba2+, Mn2+, Co2+, Cu2+, and Zn2+; and L = glyphosate and AMPA). Complexation occurred at more than one ligand donor site for [M + L - H]+, resulting in multidentate complexes. The type of complex depended on M, with central positions maximizing the interactions of the M with donor sites of the L preferred. The isomers were separated by ion mobility and experimental collisional cross sections (N2CCSexp) were derived for all isolated species. An energy threshold DFT approach located the structural families and potential lowest energy forms; these were found to be consistent with confirmed condensed phase (reported crystal structures) and gas phase structures (via infrared multiple photon dissociation, IRMPD). Theoretical nitrogen collisional cross sections (N2CCScalc) of these confirmed structures tended to underestimate the N2CCSexp for both [M + glyphosate - H]+ and [M + AMPA - H]+ complexes. Underestimation ranged between 1-20%, and was not uniform between species. By comparison, helium collisional cross sections (HeCCSexp and HeCCScalc) were in better agreement (within 1-3%). These findings suggest further refinements are needed to collisional cross section modelling for metal containing species, in particular for nitrogen drift gas.

Recent Catalytic Routes to 3-Azabicyclo[3.1.0]hexane Derivatives

The review summarizes recent achievements in the synthesis of 3-azabicyclo[3.1.0]hexanes (3-ABH). These heterocyclic systems are ubiquitous in the structure of molecules that can act on a variety of biological targets and are actively used in drug design. Classification and analysis of the modern approaches to 3-ABHs based on the use of transition metal complexes are carried out, and the mechanisms of the key processes are considered. The presented reactions include the assembly of 3-ABH by annulation of three- or five-membered rings as well as numerous one-step syntheses from acyclic precursors via tandem cyclizations.

DFT Study of Coordination of N-Monochlorosubstituation Derivatives of Biguanide

Biguanides [HN=C(NR1R2)-NH-C(NR3R4)=NH] constitute an important family of molecules used as drugs in the treatment of diabetes. It is known that some transition metal complexes with given organic compounds deeply increase both therapeutic actions of the metal and the organic compound. The chlorine atom, a preferred substitute in pharmacy, improves the effectiveness of drugs and reduces their side effects. The mesmeric +M effect of the chlorine atom would make it possible to enhance the nucleophilic character of any N-chlorosubstituted nitrogen. In this work a theoretical study of coordination of N-chlorine derivatives of biguanide is carried out mainly at the DFT/B3LYP/6-31G (d, p) level. The Gaussian 09 & 03, HyperChem softwares and the DCENT-QSAR program are used. The coordinating possibility of each ligand were studied analyzing some coordinating indicators as: interatomic bond lengths, atomic charges, molecular electrostatic potentials, frontier orbital structures and electrophilic superdelocalizability. Then, the formation of its complex with zinc (II) has been calculated. The results of the calculations revealed the imine nitrogen atoms as the most favorable coordination sites. Bioactive complexes of these ligands with Zn (II) are modelled. Testing the bioavailability of obtained complexes according to the Lipinski rules of 5, we noticed that they can be considered as drug-candidates.

Synthesis, characterization and biological study of 2-acetylbenzothiophene 4-cyclohexyl-3-thiosemicarbazone ligand and its transition metal complexes

Synthesis, characterization and biological study of 2-acetylbenzothiophene 4-cyclohexyl-3-thiosemicarbazone ligand and its transition metal complexes

Enantiospecific Synthesis of Planar Chiral Rhodium and Iridium Cyclopentadienyl Complexes: Enabling Streamlined and Computer-Guided Access to Highly Selective Catalysts for Asymmetric C-H Functionalizations.

Chiral cyclopentadienyl (CpX) metal complexes are frequently used in asymmetric catalysis by virtue of their high reactivity and selectivity. Planar-chiral-only rhodium and iridium cyclopentadienyl complexes are particularly promising due to unrestricted chemical space for CpX ligand design while retaining structural simplicity. However, they are currently still niche because of a lack of efficient synthetic strategies that avoid lengthy chiral auxiliary routes or chiral preparatory HPLC resolution of the complexes. To streamline access to such planar-chiral-only CpX-metal complexes, we designed a straightforward, highly enantiospecific, point-to-planar chirality transfer complexation via facially selective concerted-metalation-deprotonation between metal-carboxylate precursor [M(olefin)2OAc]2 and a chiral cyclopentadiene. This entirely avoids the typical stereoablative complexation of an achiral cyclopentadienyl anion that detrimentally yields a racemate. Exploiting the described enantiospecific complexation protocol and a simple divergent synthetic route to suitable chiral cyclopentadienes, we generated a structurally diverse library of new planar chiral Cp-Rh(I), Cp-Ir(I), Cp-Rh(III), and Cp-Ir(III) complexes. Moreover, the enantiospecific complexation step can be concatenated with a preceding Au-catalyzed cyclization in an efficient one-pot process that likely involves an elaborate point-to-axial-to-point-to-planar chirality transfer. Guided by computational selectivity predictions, the structure of a CpX-Rh complex in our library was tuned to optimize reactivity and selectivity in the asymmetric C-H functionalization of a benzamide with various challenging alkenes. With an optimized CpX-Rh complex in hand, we showcased its excellent catalytic performance and high selectivity for refractory alkene substrates that reacted in poor selectivity with previous CpX-Rh catalysts.

Harnessing of Cooperative Cu···H Interactions for Luminescent Low‐Coordinate Copper(I) Complexes towards Stable OLEDs

Although two‐coordinate Cu(I) complexes are highly promising low‐cost emitters for organic light‐emitting diodes (OLEDs), the exposed metal center in the linear coordination geometry makes them suffer from poor stability. Herein, we described a strategy to develop stable carbene‐Cu‐amide complexes through installing intramolecular noncovalent Cu···H interactions. The employment of 13H‐dibenzo[a,i]carbazole (DBC) as the amide ligand leads to short Cu···H distances in addition to the Cu–N coordination bond. The resultant Cu(I) complexes exhibit yellow thermally activated delayed fluorescence with photoluminescence quantum yields of up to 86% and radiative decay rate constants on the order of 106 s−1. Comparing with the analogues without Cu···H interactions, the pincer complexes have significantly improved stability. The vacuum‐deposited OLEDs show high‐performance electroluminescence with maximum external quantum efficiencies up to 29.5% and extremely small roll‐offs of only 3.5% at 10,000 cd m‐2. Remarkably, the operational lifetimes (LT90) were determined to be up to 68 h with an initial luminance of 3000 cd m‐2. This work proves a feasible design of robust low‐coordinate metal complexes by leveraging secondary coordination interactions, which helps to overcome the long‐standing stability problem.

Electrochemical CO2 Fixation and Release Cycle Featuring a Trinuclear Zinc Complex for Direct Air Capture

AbstractCO2 capture technology can mitigate greenhouse gas emissions and global warming. CO2 capture driven by electrochemical reactions is attractive because the operation is carried out at normal temperature and pressure and involves a simple input system using electrical energy. Although promising metal complexes with high CO2 fixation performance have been reported, there are few studies on systems that combine electrochemical reactions and metal complexes. Here, we demonstrated stable CO2 fixation‐release cycles using an electrochemical system with trinuclear Zn(II) complex (Zn3L) as the CO2 fixative and an ionic liquid as a supporting electrolyte for the stable operation. This system showed a faster CO2 fixation rate than that of an aqueous alkaline solution at the same concentration. Continuous release and refixation of CO2 were achieved by decomposition and reconstruction of the complex structure induced by H+ and OH− supplied from a bipolar membrane equipped in the electrolytic cell. The CO2 fixation‐release cycle was demonstrated even for dilute CO2 (450 ppm) in air, where the CO2 capture rate reached approximately 46 % of CO2 contained in the air under an air flow condition of 200 mL ⋅ min−1. This case, combining electrochemical drive and metal complexes, will provide a new option for CO2 capture technology.

Local Wave Function Embedding: Correlation Regions in PNO-LCCSD(T)-F12 Calculations.

Many chemical reactions affect only a rather small number of bonds, leaving the largest part of the chemical and geometrical structure of the molecules nearly unchanged. In this work we extended the previously proposed region method [J. Chem. Phys. 128, 144106 (2008)] to PNO-LCCSD(T)-F12. Using this method, we investigate whether accurate reaction energies for larger systems can be obtained by correlating only the electrons in a region of localized molecular orbitals close to the reaction center at high-level (PNO-LCCSD(T)-F12). The remainder is either treated at lower level (PNO-LMP2-F12) or left uncorrelated (Hartree-Fock frozen core). It is demonstrated that indeed the computed reaction energies converge rather quickly with the size of the correlation regions toward the results of the full calculations. Typically, 2-3 bonds from the reacting atoms need to be included to reproduce the results of the full calculations to within ±0.2 kcal/mol. We also computed spin-state energy differences in a large transition metal complex, where a factor of 15 in computation time could be saved, still yielding a result that is within ±0.1 kcal/mol of the one obtained in a full PNO-LCCSD(T)-F12 calculation.

Novel heterocyclic Schiff base ligand and its metal complexes: Synthesis, characterization, molecular docking and DFT

Novel heterocyclic Schiff base ligand and its metal complexes: Synthesis, characterization, molecular docking and DFT

Corrosion Inhibition Efficiency of Metal Complex of Carboxymethylchitosan in 3.5% NaCl

Corrosion Inhibition Efficiency of Metal Complex of Carboxymethylchitosan in 3.5% NaCl