Ligand Substituents Research Articles

AIE and ESIPT Schiff base ligands and corresponding Eu(III) complexes: Multiple-color and fluorescence properties

The organic ligands with good coordination ability to Eu(III) ion attracted great attention, but the preparation of multifunctional organic ligands with excellent performance and the potential application was still a challenge in the field of rare earth coordination. In this work, a series of Schiff-base ligands and their corresponding Eu(III) complexes were successfully prepared. The ligands L1−4 exhibited typical AIE properties with various fluorescence emissions and high fluorescence quantum yields in an aggregated state. The luminescence intensity and luminescence lifetime of Eu(III) complexes were investigated by comparing different substituents of ligands. Density functional theory (DFT) calculations suggested that the electronegativity of the ligands L1−4 changed with the change of the substituents on the salicylaldehyde, which led to the phenomenon that the ligands exhibited diverse fluorescence emission from green to orange (maximum emission wavelengths were 525 nm, 546 nm 569 nm and 582 nm, respectively). All target Eu(III) complexes exhibited red luminescence of Eu(III) ions, accompanied by outstanding thermal stability and long luminescence lifetime. The effect of various substituent groups on luminescence intensity and luminescence lifetime of complexes was ordered: EuL3(Phen)(NO3)3 > EuL2(Phen)(NO3)3 > EuL1(Phen)(NO3)3 > EuL4(Phen)(NO3)3. Based on the above results, all ligands and corresponding target Eu(III) complexes had good application prospects in the field of optical materials.

Tuning of Lewis Acidity of Phebox-Al Complexes by Substituents on the Benzene Backbone and Unexpected Photocatalytic Activity for Hydrodebromination of Aryl Bromide

Abstract We revealed the effects of the substituents in ligands on properties of Phebox-aluminum complexes. The substituents at the 4-position of Phebox ligands effectively influenced the Lewis acidity via the conjugation between the vacant orbital on the Al atom and π* orbital of the benzene ring, which is supported by DFT calculation. The catalytic activity was also tuned, and a series of substituted Phebox-Al(ClO4)2 effectively catalyzed hydroboration of benzaldehyde. In addition, an unexpected photocatalytic activity of Phebox-Al complexes was found and applied to the hydrodebromination of a 4-bromo benzoic ester.

Ring size and nothing else matters: unusual regioselectivity of alkyne hydration by NHC gold(I) complexes.

We have investigated the role of ring sizes and substituents in NHC ligands in some (NHC)Au(i) complexes in the hydration of internal alkynes. Despite the fact that using (NHC)Au(i) complexes in the hydration of diarylacetylenes leads to Markovnikov-type products, the precise tuning of ligands allows changing the regioselectivity in arylalkylacetylene hydration to the anti-Markovnikov-type.

Effect of ligand substituents and tuning the spin-state switching in manganese(iii) complexes.

Three mononuclear manganese(iii) complexes based on flexible hexadentate ligands obtained from the condensation of N,N'-bis(3-aminopropyl)ethylenediamine and salicylaldehyde or salicylaldehyde with substitutions at the 5 or 3,5 positions, namely [Mn(X-sal2-323)](BPh4) (X = 5 H, 1; X = 5 Br, 2, and X = 3,5 Br, 3) have been synthesized. The impact of ligand substituents has been studied by variable temperature single-crystal X-ray diffraction analyses, and magnetic, spectroscopic and electrochemical investigations. The complexes have an analogous monocationic MnN4O2 surrounding offered by the flexible hexadentate ligand in a distorted octahedral geometry. Complex 1 remains in the high spin state over the entire temperature range, while complex 2 shows a reversible and complete two-step thermo-induced spin-state switching. An incomplete spin-state switching from a high spin to an intermediate high-spin low-spin (1 : 1) state was observed for complex 3. Single-crystal X-ray structural studies show the presence of three different spin states in 2 during the occurrence of the spin-state switching process. Electrochemical investigations showed that the reduced state of manganese(iii) centers in 3 is easily accessible in comparison to complexes 1 and 2.

Ligand substituent effect on the cytotoxicity activity of two new copper(ii) complexes bearing 8-hydroxyquinoline derivatives: validated by MTT assay and apoptosis in MCF-7 cancer cell line (human breast cancer).

In this study, we have examined the effect of ligand substituent on the structure–cytotoxicity relationships of the MCF-7 cancer cell line (human breast cancer), by two copper(ii) complexes {[Cu(qmbn)(Hqmba)(q)]·NO3·2H2O} (1) and {[Cu(Hqmba)2(q)]·NO3·2H2O} (2) (where, qmbn = 2-(quinolin-8-yloxy)(methyl) benzonitrile (L1); Hqmba = 2-((quinolin-8-yloxy)methyl)benzoic acid (L2) and q = quinolin-8-olate). The structural analysis reveals that both the complexes exhibit distorted octahedral (CuN3O3) configuration which is further corroborated by density functional theory (DFT) calculations. The cytotoxicity impact of ligands (L1 and L2) and complexes (1 and 2) was screened against the MCF-7 cell line (human breast cancer). The MTT assay uptake indicated that the presence of –COOH functionality in complex 2 leads to higher cytotoxicity (lower IC50) than that observed for complex 1 containing a –CN group. This could be due to the strong H-bonding forming propensity of the carboxylic acids. Incubation of MCF-7 cancer cells with IC50 concentrations of 1 and 2 promoted cellular detachments via nuclear condensation and membrane destabilization followed by apoptosis as a result of metal-assisted generation of reactive oxygen species. Flow cytometry analysis showed that 1 and 2 might prompt early apoptosis in MCF-7 cells as the maximum percentage of cells appeared in the LR quadrant. Furthermore, mRNA expression analysis confirmed that both the complexes induced apoptosis in MCF-7 cells. Comparative mRNA expression analysis of complexes with their respective ligands also confirmed the enhanced apoptotic behavior of complexes. Furthermore, molecular docking studies of the complexes have also been performed with the active site of EGFR kinase receptors (major target for any cancer causing agent) due to similar analogues with FDA-approved EGFR inhibitors in order to rationalize its promising cytotoxicity activity.

Evaluation of substituent bioactivity and anion impact of linear and T-shaped silver(i) pyridinyl complexes as potential antiproliferative, antioxidant, antimicrobial agents and DNA- and BSA-binders

The impact of ligand substituents and anion variation on the bio-activity of pyridinyl Ag(I) complexes was evaluated. The complexes showed potential therapeutic ability with notable anticancer, antioxidant, and antimicrobial activities.

Influence of substituents in 1,10-phenanthroline on the structural and photophysical properties of W(CO)4(1,10-phenanthroline-type) complexes

In this work, methyl and phenyl substituents were introduced in the 2,9- and 4,7-positions of the phen ligand in order to study its impact on the structural and photophysical properties of [W(CO)4(phen-type)] complexes [phen-type = 4,7-dimethyl-1,10-phenanthroline (4,7-DMPhen); 4,7-diphenyl-1,10-phenanthroline (BPhen), and 2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline (BCP)]. Crystallographic analysis of these complexes allows to investigate the correlation between the geometrical parameters with their photophysical properties measured by steady-state and time-resolved spectroscopies. The (OC)eq–W–(CO)eq angle is the most significantly influenced geometrical parameter by the substituents, showing values of 92.5°, 90.2°, and 81.2° for [W(CO)4(4,7-DMPhen)], [W(CO)4(BPhen)], and [W(CO)4(BCP)], respectively. The smallest angle observed for [W(CO)4(BCP)] is attributed to a steric repulsion exerted by the methyl groups to the equatorial carbonyls. Remarkably, a smaller OCeq–W–COeq angle favors the High-Energy (HE) emission band, as evidenced in the photoluminescence (PL) spectra of [W(CO)4(BCP)]. Conversely, a greater angle favors the Low-Energy (LE) emission band, as observed in the PL spectra of [W(CO)4(4,7-DMPhen)] and [W(CO)4(BPhen)]. The PL lifetime of the LE emission band becomes shorter when decreasing the (OC)eq–W–(CO)eq angle. Furthermore, the absorption feature was affected by the 2,9-dimethyl substituents, showing greater contribution from the high-energy shoulder on the visible absorption band. This study shows that the perturbation of the (OC)eq–W–(CO)eq angle induced by the substituents in the phen ligand influences on the photophysical properties of [W(CO)4(phen-type)] complexes.

Electronic Structure and Magnetic Properties of Mixed-Ligand Cobalt Complexes Containing Organogermanium Triangulenes

Computational modeling of cobalt(II) diketonate adducts with o-benzoquinones modified with organogermanium triangular cycles of different size has been performed using the density functional theory method. Energy characteristics of the isomers of the investigated compounds have been found to be determined by the substituents in terminal diketone ligands. Paramagnetism of all the states of the studied molecules has been predicted. The systems, magnetic properties of which can be switched as a result of intramolecular electron transfer between the cobalt ion and the ligand, have been revealed.

Cyclopentadienyl Removal Assisted Nuclearity Expansion in Thermolabile Titanium and Zirconium Organophosphates Sourced from Metallocene Dichlorides

Reaction of titanocene dichloride [Cp2TiCl2] (Cp = C5H5) with (tBuO)2PO2H (di-tert-butylphosphate, dtbpH) in THF at room temperature results in the formation of mononuclear titanophosphate [Cp2Ti(dtbp)2].H2O (1), whereas the reaction of potassium salt of dtbpH (dtbpK) with [Cp2TiCl2] in THF yields tetranuclear cluster [Cp4Ti4(µ-O)3(µ-dtbp)2(dtbp)2(µ-mtbpH)2].2C6H5CH3 (2) (mtbpH = (tBuO)PO3H). Change of the reaction medium to acetonitrile for the reaction dtbpK with [Cp2TiCl2] leads to the isolation of novel hexanuclear titanophosphate cluster [Ti6(µ-O)6(µ-OH)3(µ-dtbp)9] (3), which is free of cyclopentadienyl ligands. Compound 1, when dissolved in CH3CN and allowed to stand under aerobic conditions, converts to either 2 or 3, depending on the dilution (water content in the medium) and the duration of crystallization. While lesser water content and shorter crystallization periods produce 2 as orange crystals, longer exposure of the solution results in the complete hydrolysis of Cp-Ti linkages and produce colorless, Cp-free hexanuclear cluster 3. In contrary, the reaction of either dtbpH or its potassium salt with zirconocene dichloride exclusively leads to the formation of [CpZr(µ-OH)(µ-dtbp)(dtbp)]2 (4) as the only product. Compounds 1-4 have been characterized by analytical and spectroscopic methods. Their molecular structures have further been established by single-crystal X-ray diffraction studies. Thermal decomposition of 1 reveals its thermolabile nature with the initial loss of alkyl substituents of the organophosphate ligands through β-hydride elimination followed by the loss of C5H5 groups to form an organic-free crystalline titanium phosphate phases which sinter finally to titanium pyrophosphate TiP2O7 at 800 ˚C.

Quintuple-Decker Heteroleptic Phthalocyanine Heterometallic Samarium-Cadmium Complexes. Synthesis, Crystal Structure, Electrochemical Behavior, and Spectroscopic Investigation.

A one-pot synthesis methodology was employed for obtaining diverse quintuple-decker phthalocyanine heterometallic lanthanide-cadmium complexes. By using the reaction of a double-decker homoleptic/heteroleptic phthalocyanine samarium compound with metal-free phthalocyanine and cadmium acetate in 1,2,4-trichlorobenzene at 200 °C, two novel quintuple-decker heteroleptic phthalocyanine heterometallic samarium-cadmium compounds, {(Pc)Sm(Pc)Cd(Pc*)Cd(Pc)Sm(Pc)} (1) and {(Pc)Sm(Pc*)Cd(Pc*)Cd(Pc*)Sm(Pc)} (2), together with one homoleptic phthalocyanine species, {(Pc*)Sm(Pc*)Cd(Pc*)Cd(Pc*)Sm(Pc*)} (3), were successfully fabricated, where H2Pc and H2Pc* represent unsubstituted phthalocyanine and 2,3,9,10,16,17,23,24-octakis(n-pentyloxy)phthalocyanine, respectively. Their quintuple-decker structures have been disclosed by various spectroscopic techniques and single-crystal X-ray diffraction. In addition, valence tautomerization of these three quintuple-decker complexes has been achieved by the addition of phenoxathiin hexachloroantimonate, giving three oxidized forms including one-, two-, and three-electron oxidation products. From 1 to 3 with the same oxidation state, the increased number of n-pentyloxy substituents of phthalocyanine ligands induces the blue shift of electronic absorption in the IR region due to the increased gap associated with the introduction of electron-donating substituents. In particular, the electronic absorption spectra of one- and two-electron oxidation products for 1 exhibit a rare band in the middle-IR region around 3000 nm, being one of the farthest electronic transitions captured by UV-vis spectroscopy. The three-electron oxidation product of 1 displays two bands at 2231 and 2740 nm, respectively. These data are well confirmed by IR spectroscopic data and theoretical calculation results.

Fluorine-containing ruthenium-based olefin metathesis catalysts

The review summarizes literature data on the methods for the introduction of fluorine atoms and fluoralkyl groups into different ligands to construct metathesis-active ruthenium carbene complexes. It also analyzes the influence of fluorinated ligands on the catalytic activity of the complexes. The choice, structure and positions of fluorinated substituents in NHC ligands are generally dictated by the desire to increase the electrophilicity of the ruthenium atom due to the electron-withdrawing effect of fluorine atoms and fluoroalkyl groups, resulting, as a rule, in an increase in the activity of the ruthenium complex. In catalysts with unsymmetrical fluorine-containing NHC ligands, there is a possibility of additional Ru–F coordination, making the complexes much more stable and, consequently, more active. The presence of fluorine in chelating alkylidene ligands provides an increase in the catalyst initiation rate due to a weakening of the ruthenium – heteroatom bond. Besides, the introduction of polyfluoroalkyl groups into ligands solves the problem of catalyst recovery using fluorous biphasic systems for reuse.The bibliography includes 172 references.

Oxidative Addition of a Mechanically Entrapped C(sp)-C(sp) Bond to a Rhodium(I) Pincer Complex.

By use of a macrocyclic phosphinite pincer ligand and bulky substrate substituents, we demonstrate how the mechanical bond can be leveraged to promote the oxidative addition of an interlocked 1,3‐diyne to a rhodium(I) center. The resulting rhodium(III) bis(alkynyl) product can be trapped out by reaction with carbon monoxide or intercepted through irreversible reaction with dihydrogen, resulting in selective hydrogenolysis of the C−C σ‐bond.

Oxidative Addition of a Mechanically Entrapped C(sp)–C(sp) Bond to a Rhodium(I) Pincer Complex

AbstractBy use of a macrocyclic phosphinite pincer ligand and bulky substrate substituents, we demonstrate how the mechanical bond can be leveraged to promote the oxidative addition of an interlocked 1,3‐diyne to a rhodium(I) center. The resulting rhodium(III) bis(alkynyl) product can be trapped out by reaction with carbon monoxide or intercepted through irreversible reaction with dihydrogen, resulting in selective hydrogenolysis of the C−C σ‐bond.

Tetradentate N^N°N^N-type luminophores for Pt(II) complexes: Synthesis, photophysical and quantum-chemical investigation

Herein we report on dianionic tetradentate, pyridin-azole-based ligands for luminescent Pt(II) complexes. To improve their solubility in common organic solvents, a hexyl-chain was inserted in the periphery of the luminophoric ligand and different substituents were introduced at the 1H-pyrazole moieties including 4-hexylphenyl- (L1), adamantyl- (L2) and 3,5-dimethoxyphenyl (L3) substituents. For the new ligand precursors L1 and L2, the corresponding Pt(II) complexes Pt1 and Pt2 were synthesized and characterized by exact mass spectrometry as well as by 1H NMR. L3 was also successfully synthesized and isolated, but due to the limited solubility of the resulting coordination compound, the purification and structural characterization of the corresponding Pt(II) complex Pt3 was not achieved. The absorption and steady-state as well as time-resolved photoluminescence spectra of Pt1 and Pt2 were investigated and interpreted employing TD–DFT. We found that the S1 states for Pt1 and Pt2 can be described mainly as single electron HOMO→LUMO excitations with a 1MP-ILCT character comprising the diazole units and the pyridine rings. At room temperature, Pt1 and Pt2 show phosphorescence in the green region of the electromagnetic spectrum peaking at 506 nm and 497 nm with photoluminescence quantum yields reaching 17% and 25%, respectively. DFT calculations show that the T1 states of Pt1 and Pt2 can be mainly described as monoelectronic HOMO→LUMO excitations with predominant 3MP-ILCT character.

Identification of Noncompetitive Protein-Ligand Interactions for Structural Optimization.

For efficient structure-guided drug design, it is important to have an excellent understanding of the quality of interactions between the target receptor and bound ligands. Identification and characterization of poor intermolecular contacts offers the possibility to focus design efforts directly on ligand regions with suboptimal molecular recognition. To enable a more straightforward identification of these in a structural model, we use a suitably enhanced version of our previously introduced statistical ratio of frequencies (RF) approach. This allows us to highlight protein-ligand interactions and geometries that occur much less often in the Protein Data Bank than would be expected from the exposed surface areas of the interacting atoms. We provide a comprehensive overview of such noncompetitive interactions and geometries for a set of common ligand substituents. Through retrospective case studies on congeneric series and single-point mutations for several pharmaceutical targets, we illustrate how knowledge of noncompetitive interactions could be exploited in the drug design process.

Vapochromism and vapoluminescence of platinum(II) 3,8-bis-(3-hydroxy-3-methylbut-1-yn-1-yl)-phenanthroline organometallic complexes with bis-arylethynyl derivatives

A new series of platinum(II) organometallic complexes with 3,8-bis-(3-hydroxy-3-methylbut-1-yn-1-yl)-1,10-phenanthroline (3,8-Phen-C(CH3)2OH) and nine respective arylethynyl ligands with different substituents (Pt(3,8-Phen-C(CH3)2OH)(Ph-R2)2: R2 = H (1OH), 2-F (2OH), 3-F (3OH), 4-F (4OH), 4-Me (5OH), 4-CF3 (6OH), 4-t-Bu (7OH), 3,5-di-CF3 (8OH), 3,5-di-t-Bu (9OH)) were synthesized and their luminescences in solution and solid state were characterized by photoluminescence spectroscopy. The luminescence of each of the new complexes, 1OH – 9OH, in solution was assigned to the phosphorescence from the mixed transition of 3MLCT/3LLCT (LLCT = ligand-to-ligand charge transfer), revealing that two series had very similar emission spectra: Pt(3,8-PhenTMS)(-Ph-R)21TMS – 9TMS and Pt(3,8-PhenH)(Ph-R)21H – 9H in solution. In solid state, the emission spectra of three series of platinum organometallic complexes, 1OH – 9OH, 1H – 9H, and 1TMS – 9TMS, were observed over a wide range (500–1100 nm) because the phosphorescence of each of these complexes was assigned to the emission from the 3MLCT/3LLCT mixed transition and/or the transition of a metal−metal-to-ligand charge transfer, called 3MMLCT.Results of vapochromism and vapoluminescence experiments with 13 species of volatile organic compounds (VOCs) for three series of the 27 above-mentioned platinum complexes suggested that the vapochromism of the present complex systems was induced by sterically hindered substituents of the arylethynyl ligands as tert-butyl substituents, and that the expansion of the space among the molecules in solid state is important for selective and multicolor detection with vapochromism and vapoluminescence for VOCs. Especially, the vapoluminescence experiment with VOCs for 9TMS, which was composed of a combination of the bulkiest ligands, 3,8-bis-(trimethylsilyl)ethynyl-1,10-phenanthroline and 3,5-di-t-butyl-phenylehtynyl ligands, among the present 27 complexes showed multicolor changes from yellow to dark brown, in contrast with respective VOCs.

Role of alkyl substituents in the structure and luminescence properties of discrete terbium(III)-lithium(I) Β-Diketonates

Functionalized perfluoroalkyl lithium β-diketonates LiL react with terbium (III) salts in methanol to give heterobimetallic complexes of general formula [(TbL3)(LiL)(MeOH)]. The length of both fluoroalkyl and hydrocarbon substituents in ligand was found to affect the crystal packing of metal complexes. Photoluminescence of heterobimetallic β-diketonates in the solid state is reported.

Impact of ligand substituents on the crystal structures, optical and conducting properties of phenylmercury(II) β-oxodithioester complexes

New phenylmercury(II) complexes 1–5 with functionalized β-oxodithioester ligands, [PhHg(II) (β-oxodithioester)], β-oxodithioester = methyl-3-hydroxy-3-(4-pyridyl)-2-propenedithioate L1 1, methyl-3-hydroxy-3-(p-chlorophenyl)-2-propenedithioate L2 2, methyl-3-hydroxy-3-(naphthyl)-2-propenedithioate L3 3, methyl-3-hydroxy-3-(9-anthracenyl)-2-propenedithioate L4 4 and methyl-3-hydroxy-3-(p-fluorophenyl)-2-propenedithioate L5 5, were synthesized and characterized by elemental (C, H, N) analysis, IR, UV-Vis., 1H and 13C{1H} NMR spectroscopy. Their structures have been investigated by single crystal X-ray diffraction. Linear two-coordinate geometry about the Hg atom, via ipso-C of the phenyl group and by S15 atom of the β-oxodithioester ligands is revealed in all complexes. Intramolecular Hg···O bonding at 2.622(10)–2.813(6) Å is present in all the complexes. In 1–3, the asymmetric unit contains a single discrete molecule whereas complexes 4 and 5 contain two. Except for 4 having bulky anthracene substituent, intriguing supramolecular networks are sustained through intermolecular metal assisted Hg···S and Hg···N and Cl···Cl, Cl···π, S···S, C–H···S, C–H···F, C–H···π interactions in these complexes. In 1 the pyridyl N on 4-position on the substituent is involved in Hg···N bonding interactions on the neighboring molecule at 2.938(7) Å generating a 2-D net-like polymeric structure. All the complexes showed bright green luminescent emissions both in solution and solid phase. The complexes (σrt values = 10−3–10−6 S cm−1) show semiconducting characteristics with Ea values = 0.14–0.64 eV. The electronic and steric properties of the substituents on the dithioester unit significantly influence their structures and properties.

Experiments and Direct Dynamics Simulations That Probe η2-Arene/Aryl Hydride Equilibria of Tungsten Benzene Complexes.

Key steps in the functionalization of an unactivated arene often involve its dihaptocoordination by a transition metal followed by insertion into the C-H bond. However, rarely are the η2-arene and aryl hydride species in measurable equilibrium. In this study, the benzene/phenyl hydride equilibrium is explored for the {WTp(NO)(PBu3)} (Bu = n-butyl; Tp = trispyrazoylborate) system as a function of temperature, solvent, ancillary ligand, and arene substituent. Both face-flip and ring-walk isomerizations are identified through spin-saturation exchange measurements, which both appear to operate through scission of a C-H bond. The effect of either an electron-donating or electron-withdrawing substituent is to increase the stability of both arene and aryl hydride isomers. Crystal structures, electrochemical measurements, and extensive NMR data further support these findings. Static density functional theory calculations of the benzene-to-phenyl hydride landscape suggest a single linear sequence for this transformation involving a sigma complex and oxidative cleavage transition state. Static DFT calculations also identified an η2-coordinated benzene complex in which the arene is held more loosely than in the ground state, primarily through dispersion forces. Although a single reaction pathway was identified by static calculations, quasiclassical direct dynamics simulations identified a network of several reaction pathways connecting the η2-benzene and phenyl hydride isomers, due to the relatively flat energy landscape.

Aromatic β‐Diketone as a Novel Anchoring Ligand in Iridium(III) Complexes for Dye‐Sensitized Solar Cells

A novel β‐diketone ligand bearing a thiophene moiety and an anchoring Ph‐COOCH3 unit has been designed, prepared and used in synthesis of cyclometalated benzimidazole‐based iridium(III) complexes for application in dye‐sensitized solar cells (DSSC). The replacement of traditional 4,4'‐dicarboxy‐2,2'‐bipyridine by this aromatic β‐diketone results in excellent tuning the redox potentials and excited state properties of these IrIII complexes, one of which exhibits good efficiency when used as a dye in DSSC. All the complexes demonstrate reversible redox behavior, with oxidation potentials (Ir4+/Ir3+) strongly depending on the electron‐donor/withdrawing nature of the substituents in the cyclometalated ligands. Surprisingly, the latter has just a little effect on their luminescence spectra, in which structured emission bands are observed. Time‐resolved spectroscopic studies in combination with DFT calculations show that the complexes emit light by mixed 3MLCT–3LC excited states predominantly composed of the diketone ligand‐centered triplet state.