Intraligand Transitions Research Articles

Synthesis, structure, and solution chemistry of mixed-ligand oxidovanadium(IV) complexes incorporating ONO donor hydrazone ligands

Four mixed-ligand oxidovanadium(IV) complexes, [VIVO(L1-4)(phen)], 1-4, incorporating tridentate dibasic ONO donor hydrazone ligands (H2L1-4, general abbreviation H2L) derived from the condensation of 2-picolinyl hydrazide with 2-hydroxy acetophenone and its 5-substituted derivatives as primary ligands and 1,10-phenanthroline (phen) as auxiliary ligand have been synthesized in excellent yields. The complexes were characterized by elemental analyses, magnetic susceptibility measurements, and various spectral (e.g. IR, UV–Vis and EPR) studies. The molecular structure of one of the four complexes has been determined by single-crystal X-ray diffractometry. In 1, the hydrazone ligand is meridionally disposed of, coordinating through one enolic-O, one phenolic-O, and one imine-N. The other basal position is occupied by one pyridine-N of the phen moiety. The axial positions are occupied by one oxido-O atom and the other pyridine-N atom of the phen moiety. The complexes are paramagnetic with one unpaired electron (μeff ≈ 1.73 BM), suggesting no interaction with the neighboring molecules and are EPR active. The complexes display two d-d transitions, one near 725 and the other near 840 nm due to d xy → d xz , d yz and d xy → d x 2 − y 2 transitions, respectively. The other d-d transition associated with d xy → d z 2 transition was not observed as it falls in the UV region, which is masked by highly intense intra-ligand transitions. The complexes display one-electron oxidation peaks in the +0.75 to +0.89 V vs. Ag/AgCl region in the CH3CN solution that show a linear relationship with the Hammett parameter (σ) of the substituents in the aryloxy ring.

Integrated Molecular Docking and Experimental Analysis of Ni(II) Proline Dithiocarbamate Cytotoxicity in MCF-7 Breast Cancer Cells.

Chemotherapy is one of the most effective and widely used treatment types for breast cancer. The Ni(II) proline dithiocarbamate (Ni(II)ProDtc) complex has been synthesized as a potential anticancer agent with minimal systemic toxicity. The dithiocarbamate ligand, combined with the amino acid proline, holds promise as a radio chemotherapeutic target agent in tumors. The anticancer activity of a Ni(II) complex compound with a proline dithiocarbamate ligand was tested on the MCF-7 breast cancer cell line as part of a study on essential metal-based therapeutics. Molecular docking studies identified the active sites for the estradiol-estrogen receptor-α protein. The Ni(II)ProDtc complex was synthesized and characterized using melting point analysis, conductivity measurements, UV-Vis spectroscopy, and FT-IR spectroscopy. The cytotoxicity of the complex was evaluated in vitro using the MCF-7 breast cancer cell line. The UV-Vis spectrum at 246 nm indicated the π→π* intraligand transition of the CS2 group, while FT-IR analysis revealed peaks at 364-457 cm-1 corresponding to the bonding between Ni and Sulfur (S) and Oxygen (O) from proline. Further, the UV-Vis spectrum displayed bands at 212 and 676 nm, and FT-IR data at 387-691 cm-1, confirming the coordination of the Ni(II) atoms with sulfur, nitrogen, and oxygen in the isoleucine dithiocarbamate ligand. In vitro, cytotoxicity tests revealed that Ni(II)ProDtc induced cell death in the breast cancer cell line, showing significant morphological changes in MCF-7 cancer cells, with an IC50 value of 315.70 µg/mL. The Ni(II)ProDtc complex was successfully synthesized and demonstrates anticancer activity in MCF-7 breast cancer cells, indicating significant potential as an anticancer agent for breast cancer.

Synthesis, characterization and computational studies of the triangle shape complex of silver nitrate with 2-amino-1,3,4-thiadiazole

A new mononuclear salt-like silver(I) complex [AgL3]NO3 (Complex 1), which was readily fabricated from 2-amino-1,3,4-thiadiazole (L), is reported. Complex 1 was analyzed by elemental analysis, FT-IR and UV–vis spectroscopy, and single crystal X-ray diffraction. Intermolecular contacts for the cation [AgL3]+ in the crystal structure of Complex 1 were examined using the Hirshfeld surface analysis. The DFT-based calculations were additionally applied to reveal electronic features of Complex 1. The molecular structure of Complex 1 exhibits a mononuclear planar cation [AgL3]+ of the triangle shape with three ligands L being monocoordinated through thiadiazole the nitrogen atom, located at the third position. Two cations [AgL3]+, which, in turn, are stabilized by three NH⋯N hydrogen bonds, are stacked through a rich variety of noncovalent interactions formed by the thiadiazole π-system, yielding a supramolecular dimer {[AgL3]2}2+. These dimers are linked through Ag⋯Ag interactions with the formation of a 1D supramolecular cationic pillar {[AgL3]n}n+. A solution of Complex 1 in EtOH absorbs in the UV region due to intraligand transitions within L. Theoretical calculations revealed a strong electrophilic nature of the optimized structure of Complex 1.

Methyl- versus ethyl-directed formation of square-planar Cobalt(II) and octahedral Cobalt(III) chelates of the in situ generated Schiff bases

Two mononuclear complexes [Co(LI)2] (1) and [Co(LII)3] (2) (HLI = N-cyclohexyl-3-methoxysalicylideneimine, HLII = N-cyclohexyl-3-ethoxysalicylideneimine) are reported. Although both compounds were obtained using the same synthetic procedure, 1 contains Co(II), while 2 is a Co(III) complex. The crystal structure of 1 was found to be the same as reported recently in the literature, while the crystal structure of 2 was established herein for the first time. The ligands in 1 are trans-1,5-N,O-coordinated, yielding a square planar CoN2O2 coordination core, while complex 2 adopts a meridinal isomeric form with both Δ and Λ configurations (Δ-mer-2 and Λ-mer-2). Optical properties of 1 and 2 were studied by the means of UV–vis spectroscopy in CH2Cl2 and MeOH. Both compounds absorb, mainly, in the UV region due to intraligand transitions and a ligand-to-metal charge transfer. The absorption spectra of both complexes also revealed two low intense bands at about 550 and 650 nm, which were attributed to d–d transitions. Molecular docking was applied to probe 1, Δ-mer-2 and Λ-mer-2 toward a series of the SARS-CoV-2 proteins. Both 1 and 2 are active against all the applied proteins; however, 1 is significantly more efficient with the docking scores being either comparable to or even higher of those of the initial ligands. The best binding affinity for 1 was established for PLpro, Mpro and Nsp3_range 207–379-MES. For both isomers of 2, the best binding scores were revealed with Nsp16_GTA site, PLpro, and Nsp3_range 207–379-MES (for Δ-mer-2) and Nsp16_SAM site (for Λ-mer-2). Ligand efficiency scores for the initial ligands as well as complexes 1 and 2 inside the binding sites of the applied proteins were also calculated. Of all complexes, the ligand efficiency scores for complexes of 1 with PLpro and Mpro are within the recommended ranges for a Hit, and the obtained ligand efficiency scores for complexes PLpro–1 and Mpro–1 are superior to those for the initial ligands.

Spectroscopy of End-On Copper(II) Superoxido Complexes: A Wave Function-Based Analysis.

Wave function methods are employed to analyze the ground and low-lying excited states of bipyramid trigonal copper(II) superoxido complexes, up to their characteristic ligand to metal charge transfer band. Several multireference methods have been combined to provide new insights into the interpretation of their experimental absorption spectra. We show that the intraligand transition on the dioxygen leads to a dark state. Among the results, we shall highlight the finding of doubly excited states in the region of the d-d transitions and the subtle interplay between Cu(I) and Cu(II) in the ground and excited states. Some of these findings could be obtained only with multireference methods.

Ligand-dictated planar versus bent nickel(II) chelates of the Schiff bases: Synthesis, crystal structures, optical properties, DFT and molecular docking studies

Two nickel(II) coordination compounds [Ni(LI)2] (1) and [Ni(LII)2] (2) (HLI = N-cyclohexyl-3-methoxysalicylideneimine, HLII = N-cyclohexyl-3-ethoxysalicylideneimine) were synthesized with the aim to reveal and compare the influence of the electronically and structurally similar methyl (Me) and ethyl (Et) funcionalities in a close proximity to the chelate backbone of the corresponding parent ligand. It was established that a minor difference in the structure of the parent ligand (Me in LI vs. Et in LII) leads to a crucial difference in the geometry of a complex molecule: a planar structure of 1 with two cycloalkane functionalities directed on the opposite sides of a molecule, and a bent structure of 2 with two cycloalkane functionalities directed on the convex side of a molecule. The coordination core of both complexes is a NiN2O2 planar square, formed due to two trans-coordinated ligands LI and LII, respectively. Solutions of the title compounds in CH2Cl2, MeOH and EtOH absorb up to about 475 nm due to intraligand transitions and LMCT. The DFT-based calculations were also applied to shed light on the structural and optical features of 1 and 2. In silico prediction using a molecular docking approach showed that both complexes can potentially inhibit a number of the SARS-CoV-2 proteins. The so-called ligand efficiency scores for complex Nsp14_N7-MTase–1 are within the recommended ranges for a drug, while the same scores for complexes Nsp14_N7-MTase–2, PLpro–1 and Mpro–1 fall in the ranges for a Hit.

Cd(II) and Zn(II) complexes with 2-mercaptopyridine: Synthesis, crystal structure, Hirshfeld surface analysis, luminescent properties, aggregation behaviours, current-voltage characteristic and antibacterial assay

Two transition metal complexes viz. [Cd(2-mcpH)4](NO3)2 (1) and [Zn(2-mcpH)4](NO3)2 (2) were synthesized by the reaction of cadmium(II) nitrate tetrahydrate or zinc(II) nitrate hexahydrate with 2-mercaptopyridine (2-mcpH) in 1:4 M ratio. These complexes have been fully characterized by UV–Vis, IR, NMR–spectroscopy and Single crystal X-ray analysis. Luminescent properties of complex 2 were also investigated. The emission seen in complex 2 is accredited due to the π⋯π* intra-ligand transitions. The structural architecture of both complexes manifested as monomeric tetra-coordinated M−centers, each adeptly coordinated by four 2-mcpH ligands. These arrangements effectively neutralized the complex charge, employing two nitrate groups. Concurrently, Hirshfeld surfaces analysis showed maximum interactions of 25.6%, 26.6% for H⋯H, and 25.0%, 25.1% for O⋯H contacts in complexes 1 and 2, respectively. In current–voltage (I-V) characteristic studies, it has been observed that both complexes showed Ohmic behavior. FESEM studies give convincing visual facts of the aggregation behaviour of two complexes. To appraise their potential, the ligand and complexes were subjected to antibacterial s. Complex 1 showed the most potent antibacterial effect, particularly against Gram-negative bacteria, while complex 2 exhibited a moderate effect against both Gram-negative and Gram-positive bacteria.

Syntheses, structures, photophysical properties and electroluminescent applications of dinuclear Copper(I) halide complexes containing 9-carbazolyl-substituted 1,2-Bis(diphenylphosphino)benzene

The studies on the photophysical properties of the copper(I) halide complexes based on rigid diphosphine ligands have attracted increasing attention. Here, six copper(I) halide complexes containing 9-carbazolyl-substituted 1,2-bis(diphenylphosphino)benzene, Cu2X2 (L1)2 and Cu2X2 (L2)2 [L1 = 1,2-bis(9-carbazolyl)-4,5-bis(diphenylphosphino)benzene, X = I (1), Br (2), Cl (3); L2 = 1-(9-carbazolyl)-3,4-bis(diphenylphosphino)benzene, X = I (4), Br (5), Cl (6)], were synthesized and characterized. In these complexes, two copper(I) centers are bridged by two halogen ligands to form a dinuclear structure with a four membered Cu2X2 ring. They exhibited efficient yellow green to yellowish orange delayed fluorescence in the powder state at ambient temperature, with peak wavelengths located between 548 and 602 nm (τ = 1.2–2.3 μs; Φ = 0.09–0.53). The introduction of one or two 9-carbazolyl substituents into the diphosphine resulted in the red shifting emission, shorter decay time and higher thermal stability of the complexes. The emission of the complexes originates from (metal + halide)-to-ligand and intraligand charge transitions. Organic light-emitting devices based on complex 1 exhibit yellow green luminescence with maximum external quantum efficiency of 12.0 % and current efficiency of 32.64 cd A−1.

More-Is-Better Strategy for Constructing Homoligand Polypyridyl Ruthenium Complexes as Photosensitizers for Infrared Two-Photon Photodynamic Therapy

Photodynamic therapy (PDT) uses a combination of photosensitizers (PSs), light sources, and reactive oxygen species (ROS) to damage only the desired target and keep normal tissues from being hurt. The dark cytotoxicity (chemotoxicity) of PSs, leading to whole-body damage in the absence of irradiation, is a major limiting factor in PDT. How to simultaneously increase ROS generation and decrease dark cytotoxicity is an essential challenge that must be resolved in PS research. In this study, a series of homoligand polypyridyl ruthenium complexes (HPRCs) containing three singlet oxygen (1O2)-generating ligands (L) in a single molecule ([Ru(L)3]2+) have been constructed. Compared to the heteroligand complexes [Ru(bpy)2(L)]2+ where bpy is 2,2'-bipyridine, the 1O2 quantum yield under infrared two-photon irradiation and the DNA photocleavage effect of the HPRCs are significantly enhanced with two more ligands L. The intraligand triplet excited states transition played an important role in the activation of oxygen. The HPRCs target the mitochondria but not the nuclei, generating 1O2 intracellularly under irradiation of visible or infrared light. Ru1 exhibits high phototoxicity and low dark cytotoxicity toward human malignant melanoma cells in vitro. Moreover, HPRCs have minimal cytotoxicity to human normal liver cells, suggesting their potential as antitumor PDT reagents with more security. This study may provide inspiration for the structural design of potent PS for PDT.

Diving into the optoelectronic properties of Cu(II) and Zn(II) curcumin complexes: a DFT and wavefunction benchmark.

Curcumin is a popular food additive around the world whose medicinal properties have been known since ancient times. The literature has recently highlighted several biological properties, but besides the health-related usages, its natural yellowish color may also be helpful for light-harvesting applications. This research aims to close a knowledge gap regarding the photophysical description of curcumin and its metallic complexes. We conducted benchmark experiments comparing NEVPT calculations with several DFT functionals (B3LYP, M06-L, M06-2X, CAM-B3LYP, and ωB97X-D) for describing the UV spectra of curcumin and its metallo-derivative, curcumin-copper(II). Once we determined the most suitable functional, we performed tests with different basis sets and conditions, such as solvation and redox state, to identify their impact on excited state properties. These results are also reported for the curcumin-zinc(II) derivative. We found that the accuracy of DFT functionals depends strongly on the nature of curcumin's excitations. Intra-ligand transitions dominate the absorption spectra of the complexes. Curcumin absorption is marginally affected by solvation and chelation, but when combined with redox processes, they may result in significant modifications. This is because copper cation changes its coordination geometry in response to redox conditions, changing the spectrum. We found that, compared to a NEVPT reference, B3LYP is the best functional for a general description of the compounds, despite not being appropriate for charge transfer transitions. M06-L was the best for LMCT transitions. However, compared with NEVPT2 and PNO-LCCSD(T)-F12 results, no functional achieved acceptable accuracy for MLCT transitions.

Synthesis, structure determination, NBO analysis and vibrational/electronic spectroscopic study of Iron(II) Bis(diethyldithiocarbamate) [Fe(DDTC)2

This work deals with the synthesis, computational molecular modeling, and vibrational/electronic spectroscopic analysis of the coordination complex [Fe(DDTC)2]. The complex was synthesized according to the guidelines provided by the graphical method. The optimization of the molecular structure was performed using Density Functional Theory with the exchange functional B3LYP and basis set 6–311G(d,p). The experimental FT-IR and Raman spectra of the complex were recorded in the range of 4000–0 cm−1 to correlate them with the calculated spectra. Most of the DFT calculated frequencies agreed with the experimental results, and complete vibrational assignments (including metal-ligand) were made. To investigate the internal electronic mobility of the complex, we performed natural bond orbital analysis (NBO), which provided information regarding intramolecular charge transfer interactions resulting from the overlapping of bonding and antibonding orbitals. The NBO analysis also provided information about the electronic distribution between the HOMO and LUMO orbitals due to charge transfers. We found that the NBO analysis provided the main charge transfers between donor and acceptor atoms, and the HOMO-LUMO energy gap revealed that the complex has high kinetic stability. We also correlated the calculated and experimental UV–Vis spectra to investigate the configurations of several excited states of the complex that involve intra-ligand transitions. The charge transfer and d-d (Laporte Forbidden) bands were assigned. The results also corroborate the existence of several Ligand to Metal Charge Transfer (LMCT) and Metal to Ligand Charge Transfer (MLCT) transitions, as well as d-d transitions. We found that the experimental vibrational spectra agreed with the theoretical ones.

Selective Luminescent Chemosensing of Chloride Based on a Cyclometalated Platinum(II) Complex in Water: Crystal Structures, Spectroscopic Studies, Extraction, and Bioimaging

Selective anion sensing by luminescent chemosensors capable of operating in aqueous conditions is a central field of modern supramolecular chemistry that impacts analytical and biological chemistry. A cationic cyclometalated [Pt(N^C^N)NCCH3]OTf complex, 1 [N^C^N = 1,3-bis(1-(p-tolyl)-benzimidazol-2'-yl)benzene, OTf = triflate], was prepared, structurally described by single-crystal X-ray diffraction and studied in-depth as a luminescent chemosensor for anions in aqueous phase and solid state. A series of related neutral [Pt(N^C^N)X] complexes (X = Cl, 2; CN, 3 and I, 4) were formed readily upon treatment of 1 with the respective NaX salt in aqueous media and were described structurally by X-ray diffraction. Complex 1 is hydrostable with phosphorescent green emission originated by intraligand transitions, and [dyz(Pt) → π*(N^C^N)] charge transfer transitions, as evidenced by TD-DFT calculations and lifetime. Additions of halides, pseudohalides, oxyanions, and dicarboxylates to a neutral aqueous solution of 1 modified its green emission intensity with a pronounced affinity (K = 1.5 × 105 M-1) and turn-on signal toward Cl- within the micromolar concentration range. Pt complex 1 is two orders of magnitude more selective for Cl- than the other halides, CN- and basic oxyanions. Such Cl- affinity for a metal-based chemosensor in aqueous media is still rare. On the basis of X-ray crystallographic analysis and multiple spectroscopic tools (NMR, UV-vis, luminescence, MS, lifetimes) the origin of this selectivity hinges on the cooperative three-point recognition involving one coordination bond (Pt-Cl) and two convergent short C-H···Cl- contacts. This strong affinity and efficient optical response can be utilized in quantitative Cl- sensing in real samples and solid-liquid extractions. Additionally, chloro-Pt complex, 2 may be relevant to bioimaging as a marker for cell nuclei, as revealed by its emission within living cells and intracellular distribution by confocal microscopic studies. These results demonstrate the usefulness of the new water-stable luminescent Pt-N^C^N complexes as effective analytical tools in anion sensing and extraction agents.

Investigation of the N^C Ligand Effects on Emission Characteristics in a Series of Bis-Metalated [Ir(N^C)2(N^N)]+ Complexes.

A series of bis-metalated phosphorescent [(N^C)2Ir(bipyridine)]+ complexes with systematic variations in the structure and electronic characteristics of the N^C ligands were synthesized and characterized by using elemental analysis, mass spectrometry, NMR spectroscopy and X-ray crystallography. Investigation of the complexes' spectroscopic properties together with DFT and TD DFT calculations revealed that metal-to-ligand charge transfer (MLCT) and intraligand (LC) transition play key roles in the generation of emissive triplet states. According to the results of theoretical studies, the 3LC excited state is more accurate to consider as an intraligand charge transfer process (ILCT) between N- and C-coordinated moieties of the N^C chelate. This hypothesis is completely in line with the trends observed in the experimental absorption and emission spectra, which display systematic bathochromic shifts upon insertion of electron-withdrawing substituents into the N-coordinated fragment. An analogous shift is induced by expansion of the aromatic system of the C-coordinated fragment and insertion of polarizable sulfur atoms into the aromatic rings. These experimental and theoretical findings extend the knowledge of the nature of photophysical processes in complexes of this type and provide useful instruments for fine-tuning of their emissive characteristics.

Copper(II) Chelates of Schiff Bases Enriched with Aliphatic Fragments: Synthesis, Crystal Structure, In Silico Studies of ADMET Properties and a Potency against a Series of SARS-CoV-2 Proteins.

We report two complexes [Cu(LI)2] (1) and [Cu(LII)2] (2) (HLI = N-cyclohexyl-3-methoxysalicylideneimine, HLII = N-cyclohexyl-3-ethoxysalicylideneimine). The ligands in both complexes are trans-1,5-N,O-coordinated, yielding a square planar CuN2O2 coordination core. The molecule of 1 is planar with two cyclohexyl groups oriented to the opposite sites of the planar part of a molecule, while the molecule of 2 is significantly bent with two cyclohexyl groups oriented to the same convex site of a molecule. It was established that both complexes in MeOH absorb in the UV region due to intraligand transitions and LMCT. Furthermore, the UV-vis spectra of both complexes revealed two low intense shoulders in the visible region at about 460 and 520 nm, which were attributed to d-d transitions. Both complexes were predicted to belong to a fourth class of toxicity with the negative BBB property and positive gastrointestinal absorption property. According to the molecular docking analysis results, both complexes are active against all the applied SARS-CoV-2 proteins with the best binding affinity with Nsp 14 (N7-MTase), PLpro and Mpro. The obtained docking scores of complexes are either comparable to or even higher than those of the initial ligands. Complex 1 was found to be more efficient upon interaction with the applied proteins in comparison to complex 2. Ligand efficiency scores for the initial ligands, 1 and 2 were also revealed.

Isocyanide Ligands Promote Ligand-to-Metal Charge Transfer Excited States in a Rhenium(II) Complex.

A metal-to-ligand charge transfer with mixed intraligand character is observed for the rhenium hexakisarylisocyanide complex [Re(CNAr)6]PF6 (CNAr = 2,6-dimethylphenylisocyanide, λmax = 300 nm). Upon oxidation to [Re(CNAr)6](PF6)2, the dominant low energy optical transition is a ligand-to-metal charge transfer (LMCT) mixed with intraligand transitions (λmax = 650 nm). TD-DFT was used to identify the participating ligand-based orbitals in the LMCT transition, revealing that the majority of the donor orbital is based on the aryl ring (85%) as opposed to the CN bond (14%). For both [Re(CNAr)6]+ and [Re(CNAr)6]2+, structural characterization by X-ray diffraction reveals deviations from Oh geometry at the central Re ion, with larger reduction in symmetry observed for Re(II). For [Re(CNAr)6]+, these structural changes lead to a broadening of the strong ν(C≡N) stretch (2065 cm-1), as the degeneracy of the T1u IR-active mode is broken. Furthermore, a shoulder is observed for this ν(C≡N) stretch, resulting from deviation of the C-N-Ar bond from linearity. By contrast, [Re(CNAr)6]2+ has two weak bands in the ν(C≡N) region (2065 and 2121 cm-1). DFT calculations indicate that reduction of symmetry at the central rhenium ion manifests in the decrease in intensity and the observed split of the ν(C≡N) band. Stability of both complexes are limited by light-induced decomposition where Re(I) dissociates a isocyanide ligand upon irradiation and Re(II) absorbance decays under ambient light. These data provide new insights to the electronic structure of [Re(CNAr)6]2+, enhancing our understanding of LMCT excited states and the versatility of isocyanide ligands.

Rational design of efficient photosensitizers based on cyclometalated iridium(III) complexes with 2-arylbenzimidazole and aromatic 1,3-diketone ligands.

A judicious selection of substituents in cyclometalating 2-arylbenzimidazoles and an ancillary aromatic 1,3-diketone enabled the creation of heteroleptic iridium(III) complexes demonstrating strong light absorption up to 500 nm (ε ≈ 10 000-12 000 M-1 cm-1). The complexes, which were studied by various spectroscopic techniques, single-crystal X-ray diffraction and cyclic voltammetry, displayed tunable absorption maxima depending on the nature of substituents and their positions. The experimental study was corroborated by quantum chemical calculations, which showed an increased contribution of intraligand charge transfer transitions to the visible light absorption in the case of complexes containing electron-withdrawing substituents in the ligands. Despite being of high intensity, some of these transitions are responsible for the formation of the excited states located at large distances from the 'anchoring' fragment incorporated in the ancillary ligand. In turn, incorporation of electron-donating substituents at the para-position to the Ir-C bonds increases the number of excited states located on the ancillary ligand. The destabilization of the HOMO, which is caused by the increase in the electron-donating ability of the substituents in the metalated rings, translated into negative shifts of the Ir4+/Ir3+ redox potential, affecting, in some cases, the degree of electrochemical reversibility of the complexes. Several complexes having strong light-harvesting characteristics and undergoing reversible oxidation in the appropriate potential range were used for coating the TiO2 photoanodes, which reached an efficiency of 2.15% upon irradiation with the standard AM 1.5 spectrum.

Vesicles, fibres, films and crystals: A low-molecular-weight-gelator [Au(6-thioguanosine)2]Cl which exhibits a co-operative anion-induced transition from vesicles to a fibrous metallo-hydrogel.

We describe a simple coordination compound of Au(I) and 6-thioguanosine, [Au(6-tGH)2]Cl, that has a rich self-assembly chemistry. In aqueous solution, the discrete complex assembles into a supramolecular fibre and forms a luminescent hydrogel at concentrations above about 1 mM. Below this concentration, the macromolecular structure is a vesicle. Through appropriate control of the solvent polarity, the gel can be turned into a lamellar film or crystallised. The molecular structure of [Au(6-tGH)2]Cl was determined using single crystal X-ray diffraction, which showed bis-6-thioguanosine linearly coordinated through the thione moiety to a central Au(I) ion. In the vesicles, the photoluminescence spectrum shows a broad, weak band at 550 nm owing to aurophilic interactions. Co-operative self-assembly from vesicle to fibre is made possible through halogen hydrogen bonding interactions and the aurophilic interactions are lost, resulting in a strong photoluminescence band at 490 nm with vibronic structure typical of an intraligand transition. The vesicle-fibre transition is also revealed by a large increase of ellipticity in the circular dichroism spectrum with a prominent peak near 390 nm owing to the helical structure of the fibres. Atomic force microscopy shows that at the same time as fibres form, the sample gels. Imaging near the vesicle-fibre transition shows that the fibres form between vesicles and a mechanism for the transition based on vesicle collisions is proposed.

Structures and Luminescent Properties of Rare-Earth Metal–Organic Framework Series with Thieno[3,2b]thiophene-2,5-dicarboxylate

Four new rare-earth metal–organic frameworks containing thieno[3,2b]thiophene-2,5-dicarboxylate (ttdc2−) with general formulae [M2(DMF)4(ttdc)3] (M3+ = Y3+ for 1, La3+ for 2, Tb3+ for 3) and [M2(H2O)2(ttdc)3] (M3+ = Lu3+ for 4) were synthesized. Their crystal structures were determined by performing a single-crystal X-ray diffraction analysis. Coordination polymers 1–3 are based on the binuclear metal-carboxylate building units with the formulae {M2(DMF)4(OOCR)6}. The six-connected blocks in 1–3 form a three-dimensional network with the primitive cubic (pcu) topology. Coordination framework 4 is based on chains comprised by stretched pseudo-binuclear metal-carboxylate building units. The chains are interconnected in four directions with ttdc2− linkers forming the 3D framework. The luminescent properties were studied for the synthesized frameworks in the solid state. All the coordination frameworks show a broad blue emission band (λex = 380 nm) typical for intra-ligand electron transitions. The sensing properties of 3 dispersions in solutions were investigated in detail and the luminescent response (quenching) was discovered in the presence of cinnamaldehyde and quinoline in diluted solutions at concentrations of as low as 4 × 10−1 vol.% and 4 × 10−2 vol.% (~3 × 10−3 M), respectively.

Zinc– and Copper–Salicyaldimine complexes: Simultaneous observation of both metal–ligand coordination and weak CH⋯N contact about a single N-donor and the transmetallation reactions

Two salicyaldimine ligands, HsalCl-3-py and HsalOMe-3-py, functionalized with the pyridyl group on the imine side and their Zn and Cu complexes with a general formula [ML2], where M ​= ​Zn, L ​= ​salCl-3-py (1), salOMe-3-py (2); M ​= ​Cu, L ​= ​salCl-3-py (3), salOMe-3-py (4), have been synthesized and characterized by nuclear magnetic resonance (NMR) spectroscopy, mass spectroscopy (MS), infrared (IR) spectroscopy, elemental analysis (EA), scanning electron microscope (SEM) and further by single-crystal X-ray diffraction analysis and X-ray powder diffraction (XRPD). The crystal structures of Zn– and Cu–salicyaldimine complexes exhibited that the M(II) center is coordinated by two N,O-chelated salicyaldimine ligands in a square-planar geometry, and two further pyridyl donating groups from two neighboring molecules at the axial positions, resulted in self-aggregation to form a two-dimensional (4,4)-sheet. Interestingly, apart from the strong M–N(pyridyl) coordination bond, the weak CH⋯N(pyridyl) hydrogen bonding interactions are simultaneously formed between the pyridyl N-donor and one pyridyl hydrogen of the equatorial salicyaldimine ligand. The electronic absorption spectra of the Zn– and Cu–salicyaldimine complexes in methanol solutions showed the appearance of a metal-to-ligand charge transfer absorption other than the intraligand charge transfer transitions after the complexation of azomethine (–CN–) with M(II) center. On the other hand, salicyaldimine ligands and Cu–salicyaldimine complexes showed no or weak fluorescence emissions. By contrast, Zn–salicyaldimine complexes emitted intense cyan and green light fluorescence in both solid-state and methanol solutions. Noteworthy, transmetallation of Zn–salicyaldimine complexes with Cu2+ ions would be achieved, producing corresponding Cu–salicyaldimine complexes. This implies the stronger binding ability of Cu2+ than that of Zn2+ to salicyaldimine ligands.

Synthesis, Characterization, and the N Atom Transfer Reactivity of a Nitridochromium(V) Complex Stabilized by a Corrolato Ligand.

Metal complexes bearing nitrido ligands (M≡N) are at the forefront of current scientific research due to their resemblances with the metal complexes involved in the nitrogen fixation reactions. An oxo(corrolato)chromium(V) complex was used as a precursor complex for the facile synthesis of a new nitrido(corrolato)chromium(V) complex. The nitrido(corrolato)chromium(V) complex was characterized by various spectroscopic techniques. Density functional theory (DFT) calculations were performed on the nitrido(corrolato)chromium(V) complex to assign the vibrational and electronic transitions of this complex. The chromium–nitrogen (nitrido) bond distance obtained in the DFT-optimized structure is 1.530 Å and matches well with the earlier reported authentic Cr≡N bond distances obtained from the single-crystal X-ray diffraction data. This nitrido(corrolato)chromium(V) compound exhibited a sharp Soret band at 438 nm and a Q band at 608 nm. DFT calculations deliver that the origin of the bands at 438 and 608 nm is due to the intraligand charge transfer transitions. The nitrido(corrolato)chromium(V) complex showed one reversible oxidation and one reversible reduction couple at +0.53 and −0.06 V, respectively, vs the Ag/AgCl reference electrode. The simulation of the electron paramagnetic resonance data of the nitrido(corrolato)chromium(V) compound provided the following parameters: giso = 1.987, A53Cr = 26 G, and A14N = 2.71 G. From all these analyses, we can conclude that the electronic configuration in the native state of nitrido(corrolato)chromium(V) can be best described as [(cor3–)CrV(N3–)]−. Reactions of nitrido(corrolato)chromium(V) with the chloro(porphyrinato)chromium(III) complex resulted in a complete intermetal N atom transfer reaction between chromium corrole and chromium porphyrin complexes. A second-order rate constant of 4.29 ± 0.10 M–1 s–1 was obtained for this reaction. It was also proposed that this reaction proceeds via a bimetallic μ-nitrido intermediate.