Effect Of Ligand Structure Research Articles

Investigating the effect of ligand structure on the anticancer properties of several new Co(II) complexes of vitaminB3-based phosphoramides

Nicotinamide, known as Vitamin-B3, has shown promising potential in improving various medical conditions. Carbacylamidophosphates (CAPh) are versatile phosphoramide ligands with a wide range of applications in both biochemistry and chemistry. Herein, to obtain compounds with enhanced anticancer activity and study the effect of the structure on this activity, four new Co(II) complexes of vitaminB3-based CAPh ligands with the formula of CoCl2[3-NC5H4CONHPO(NC5H10)2]2(C1), CoCl2[3-NC5H4CONHPO(NC5H9CH3)2]2(C2), CoCl2[3-NC5H4CONHPO(NC6H12)2]2(C3), and CoCl2[3-NC5H4CONHPO(NC4H10)2]2(C4) were designed and synthesized. FT-IR, UV–Vis, Atomic Absorption (AAS),1H, 13C, and 31PNMR, and Mass spectroscopies beside CHN and Molar conductivity methods were utilized to characterize the synthesized compounds. Using MTT-assay and Flow Cytometry, the anticancer effects of these complexes were studied on three distinct cell lines, including one normal cell line (MCF10A) and two cancer cell lines (MDA-MB-231, MCF-7). Results showed that our ligands could form complexes by coordinating with cobalt, which, not only have a very strong killing effect on cancer cells but also have a higher level of safety for normal cells and are more cost-efficient than Cisplatin. C3 was the most effective complex at inhibiting the growth of MCF-7 and MDA-MB-231 cells which exhibited a remarkable 97.5 % reduction in cancer cell growth and a Selectivity Index up to > 37. This is an impressive 93 and 54 times more selective and safer than commonly used drugs like Cisplatin and Doxorubicin, respectively.Flow Cytometry analysis shows complex-induced breast cancer cell apoptosis.The ligands’ amine structure and ring size can directly impact the complexes’ anticancer effect and safety for normal cells.

Effect of ligand structure and metal center on ethylene oligomerization with N, N′ Schiff base late transition complexes: experiments and calculations

A series of N, N’ bis(pyridine-imine) Schiff base late transition metal (Fe, Co, and Ni) complexes with different ligand structures and catalytic active sites were synthesized. Ethylene oligomerization results showed that bis(pyridine-imine) iron complex had the highest catalytic activity (10.8 × 105 g/mol Fe h), and the catalytic activity of cobalt complex was the lowest (2.5 × 105 g/mol Co h); nickel complex had the highest selectivity toward high carbon olefins (28.6%), while the iron complex had the lowest (0.2%). The structure-activity relationship between the catalyst structure and catalytic properties for ethylene oligomerization was further studied by density functional theory (DFT) calculations. The bis(pyridine-imine) iron complex had the highest energy gap value and the most stable catalyst structure, which was more conducive to oligomerization; and the N–M–N bond of the nickel complex had the smallest bond angle, which was beneficial for chain growth. Based on the Cossee mechanism, a mechanism for ethylene oligomerization catalyzed by bis(pyridine-imine) metal complexes was proposed. To further study the micro reaction process of ethylene oligomerization, a molecular model of pyridine imine nickel complex and its catalytic ethylene oligomerization reaction path were proposed. The structure and energy of the reaction intermediate and transition state were calculated using DFT. The molecular model of the nickel complex had a lower ethylene insertion barrier and β-H elimination of energy barriers, indicating that the molecular model of the pyridine imine nickel complex had ethylene oligomerization active centers.

Homogeneous Non-Metallocene Group 4 Metals Ligated with [N,N] Bidentate Ligand(s) for Olefin Polymerization.

The development of catalysts has significantly advanced the progress of polyolefin materials. In particular, group 4 (Ti, Zr, Hf) non-metallocene catalysts ligated with [N,N] bidentate ligand(s) have garnered increasing attention in the field of olefin polymerization due to their structurally stability and exceptional polymerization behaviors. Ligands containing nitrogen donors are diverse and at the core of many highly active catalysts. They mainly include amidine, guanidinato, diamine, and various N-heterocyclic ligands, which can be used to obtain a series of new polyolefin materials, such as ultrahigh molecular weight polyethylene (UHWMPE), olefin copolymers (ethylene/norbornene and ethylene/α-olefin) with high incorporations, and high isotactic or syndiotactic polypropylene after coordination with group 4 metals and activation by cocatalysts. Herein, we focus on the advancements and applications of this field over the past two decades, and introduce the catalyst precursors with [N,N] ligand(s), involving the effects of ligand structure, cocatalyst selection, and polymerization conditions on the catalytic activity and polymer properties.

Effect of Ligand Structure on Ethylene Oligomerization over Salicylaldehyde Imine Nickel Complexes: Experiments and Calculations

Salicylaldehyde imine nickel complexes Ni1‐Ni10 containing different electron‐donating and electron‐withdrawing groups on benzene rings were synthesized, using ethylenediamine, salicylaldehyde derivatives with different substituents and nickel chloride hexahydrate as raw materials. Structure characterization, such as elemental analysis, Fourier transform infrared (FT‐IR), nuclear magnetic resonance (NMR), ultraviolet–visible (UV–Vis), and electrospray ionization mass spectrometry (ESI‐MS), confirmed that the structure of synthesized complexes was consistent with the theoretical. Salicylaldehyde imine nickel complexes exhibited good catalytic activity in ethylene oligomerization process, and complexes containing electron‐donating groups on the benzene ring had higher catalytic activity than complexes containing electron‐withdrawing groups on the benzene ring. With the increase of steric hindrance of phenol hydroxyl ortho‐substituents, the catalytic activity decreased, as well as the catalytic selectivity toward olefins with higher carbon number. The relationship between structure and catalytic properties was further studied by density functional theory (DFT) calculations, and the possible mechanism of nickel complexes catalyzing ethylene oligomerization was proposed.

Nickel-Catalyzed Selective Incorporation of Isolated In-Chain Carbonyls into Ethylene/Carbon Monoxide Copolymer Using Metal Carbonyls as a Carbonyl Source

Abstract Non-alternating copolymers of ethylene and carbon monoxide were obtained by nickel-catalyzed polymerization of ethylene in the presence of Fe2(CO)9 as a carbon monoxide source. Higher selectivity to isolated ketone group than the previously reported nickel-catalyzed systems, while having a higher catalytic activity than the palladium catalysts in our previous report has been achieved. Furthermore, various P-stereogenic phosphine-phenolate ligands were developed to investigate the effect of ligand structure on the copolymerization. Finally, the degradability of the resulting polymer was evaluated.

Electrochemical CO2 Reduction to Methane by Cu Complex‐Derived Catalysts in Non‐Aqueous Media

AbstractElectrochemical CO2 reduction using Cu complex catalysts in non‐aqueous media leads exclusively to carbon monoxide or formic acid. This study reports highly selective electrochemical CO2 reduction to methane using a 2,9‐dimethyl‐1,10‐phenanthroline Cu2+ complex in dimethylformamide. XRD reveals the formation of a Cu complex‐derived catalyst during the electrochemical testing, containing Cu and an organic phase when the electrolyte contained tetrabutylammonium cations. Insulating carbonates were preferentially deposited when the electrolyte contained alkali metal cations, leading to low Faradaic efficiency for methane. The dramatic electrolyte dependence is explained by the relative solubility of deposits in dimethylformamide under reductive potentials. The Faradaic efficiency for methane production varied from sample to sample, with underlaying variation in the morphology of the catalyst. Selective methane production using Cu complex‐derived catalysts in non‐aqueous media is intriguing, opening the possibility of combinatorial studies of the effect of ligand structures on product selectivity by involving ligands not soluble in water.

Ligand-Structure Effects on N-Heterocyclic Carbene Rhenium Photo- and Electrocatalysts of CO2 Reduction.

Three novel rhenium N-heterocyclic carbene complexes, [Re]-NHC-1-3 ([Re] = fac-Re(CO)3Br), were synthesized and characterized using a range of spectroscopic techniques. Photophysical, electrochemical and spectroelectrochemical studies were carried out to probe the properties of these organometallic compounds. Re-NHC-1 and Re-NHC-2 bear a phenanthrene backbone on an imidazole (NHC) ring, coordinating to Re by both the carbene C and a pyridyl group attached to one of the imidazole nitrogen atoms. Re-NHC-2 differs from Re-NHC-1 by replacing N-H with an N-benzyl group as the second substituent on imidazole. The replacement of the phenanthrene backbone in Re-NHC-2 with the larger pyrene gives Re-NHC-3. The two-electron electrochemical reductions of Re-NHC-2 and Re-NHC-3 result in the formation of the five-coordinate anions that are capable of electrocatalytic CO2 reduction. These catalysts are formed first at the initial cathodic wave R1, and then, ultimately, via the reduction of Re-Re bound dimer intermediates at the second cathodic wave R2. All three Re-NHC-1-3 complexes are active photocatalysts for the transformation of CO2 to CO, with the most photostable complex, Re-NHC-3, being the most effective for this conversion. Re-NHC-1 and Re-NHC-2 afforded modest CO turnover numbers (TONs), following irradiation at 355 nm, but were inactive at the longer irradiation wavelength of 470 nm. In contrast, Re-NHC-3, when photoexcited at 470 nm, yielded the highest TON in this study, but remained inactive at 355 nm. The luminescence spectrum of Re-NHC-3 is red-shifted compared to those of Re-NHC-1 and Re-NHC-2, and previously reported similar [Re]-NHC complexes. This observation, together with TD-DFT calculations, suggests that the nature of the lowest-energy optical excitation for Re-NHC-3 has π→π*(NHC-pyrene) and dπ(Re)→π*(pyridine) (IL/MLCT) character. The stability and superior photocatalytic performance of Re-NHC-3 are attributed to the extended conjugation of the π-electron system, leading to the beneficial modulation of the strongly electron-donating tendency of the NHC group.

Ligand Effect on Physicochemical Properties of Ionic Liquid.

In the solvent extraction process, the importance of an extractant (or ligand) and a diluent is inferred from their respective physicochemical properties. We have brought together all the recent results reported on the mixture of different extractants dissolved in a well-known ionic liquid diluent: 1-butyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide ([C4 mim][NTf2 ]) in the form of a review and aimed to emphasize the role of ligand polarity and structure on the physicochemical properties of an ionic liquid (IL) diluent. Some of the most important properties such as dynamic viscosity (η), absolute density ( ), energy of activation (Ea ), coefficient of thermal expansion (α), phase separation time (PST), refractive index (n), etc., have been discussed meticulously in the paper. The effect of ligand structure on the aggregation behaviour of IL phase and the physicochemical properties of gamma irradiated solvent phases containing different ligands and their solution with IL phase also have been deliberated in detail.

Sunflower oil as renewable biomass source to develop highly effective oil-soluble catalysts for in-situ combustion of heavy oil

In-situ combustion has received much attention in the last few decades for enhanced heavy oil recovery processes. Although a wide part of research efforts has only focused on the application of different oil soluble catalysts to improve the combustion flame stabilization, the effect of ligand structure and metal–ligand interactions on the performance of heavy oil oxidation in presence of oil soluble catalysts is poorly understood. In this study, sunflower oil as an eco-friendly and cheap source was used to synthesize a new ligand for the preparation of Fe, Ni, Co and Cu-based oil soluble catalysts. The interaction of metals with the ligand was investigated by computational study and the results revealed that Cu-based catalyst was the most effective catalytic system for enhancing heavy oil oxidation process because it possessed the strongest bond energy with the ligand at different temperatures. Subsequently, the impact of oil soluble catalysts on the oxidation of heavy oil was investigated via non-isothermal kinetics and thermodynamic studies using thermogravimetric (TG/DTG) and differential scanning calorimetric analyses (DSC) combined with the isoconversional (Friedman and Kissinger-Akahira-Sunose), and model-based approaches. As anticipated, the kinetic and thermodynamic data demonstrated the efficiency of the used catalysts on the process of heavy oil oxidation process and proved a maximum temperature shift of ∼ 120 °C in high-temperature oxidation zone. To the best of our knowledge, this is the first superior catalyst among investigated oil-soluble catalysts for heavy oil oxidation. The evidence from the obtained results points toward the idea that the choice of the ligand and the associated metal is crucial to achieve the highly active and smart catalytic system for enhanced oil recovery methods.

Effect of Ligand Structures on Ligand-Protected Gold Clusters: [Au-(p-/m-/o-MBT)]1-8 Clusters.

The controllable preparation of ligand-protected clusters is still an unresolved problem, which may be due to that their formation mechanism is unclear. We propose that the ligand is the key to solve the above problems. Here, by using p-, m-, and o-methylbenzenethiol ligand protected gold clusters as examples, we try to explore the effect of ligand structures on ligand-protected gold clusters. The geometrical structures, relative stabilities and surface properties of small-sized ligand-protected gold clusters [Au-SR]1-8 (SR = p-/m-/o-MBT) have been systematically studied based on the density functional theory. The results show that the ground state structures of [Au-SR]1-8 clusters tend to form closed rings except for [Au-SR]1,2. The different structures of ligand have significant effect on the structures and stabilities of ligand-protected clusters. By analyzing their surface properties and possible growth patterns, it is found that [Au-SR]1,2 clusters serve as the basic building blocks, and the larger clusters can be regarded as the combinations of them. This study provides some insights into the effect of ligands on ligand-protected clusters, which is useful for understanding the formation mechanism of ligand-protected clusters.

Effect of Ligand Structure on the Kinetics of Heavy Oil Oxidation: Toward Biobased Oil-Soluble Catalytic Systems for Enhanced Oil Recovery

Effect of Ligand Structure on the Kinetics of Heavy Oil Oxidation: Toward Biobased Oil-Soluble Catalytic Systems for Enhanced Oil Recovery

The effect of temperature and ligand structure on the solubility of gold nanoparticles

In order to investigate the effect of ligands structure, and temperature on the solubility of the smallest crystallographic gold nanoparticle, Au18S14, quantum mechanical calculations, and molecular dynamics simulations have been performed. Ligands including 2-dimethyl amino Ethan thiol, 4-mercapto benzoic acid, and 3-mercapto-propionic acid have been used to study the effect of ligand type and temperature, and 4-mercapto butyric acid and 5-sulfanyl pentanoic acid ligands have been used in comparison to the 3-mercapto-propionic acid ligand to study the effect of carbon-chain length differences. Furthermore, three different temperatures: 290, 300, and 310 kelvin have been used to study the effect of temperature. The optimized structures of nanoparticles, electronic band-gap, light absorption, dipole moment values, and hardness were obtained using quantum mechanical methods. Further, hydrogen bond number, root mean squared displacement, the moment of inertia, radial distribution functions, mean square displacement, and solvent accessible surface area parameters were studied by molecular dynamic simulation methods. All quantum mechanics analyses indicated that functionalization improves the nanoparticle’s electronic features, solubility, and stability. The studies which have performed with the molecular dynamics simulations indicated that increasing the temperature leads to form more hydrogen bonds between gold nanoparticles and water, making them more soluble. Also, the structures with a longer carbon chain of ligands have stronger interactions with solvent but their mobility in the water can be decreased. From all data of this work, it can be concluded that the 4-mercapto benzoic acid is the ligand that makes more solubility and stability for the mentioned gold nanoparticle.

Copolymerization of Phthalic Anhydride with Epoxides Catalyzed by Amine-Bis(Phenolate) Chromium(III) Complexes.

The effect of ligand structure on the catalytic activity of amine-bis(phenolate) chromium(III) complexes in the ring-opening copolymerization of phthalic anhydride and a series epoxides was studied. Eight complexes differing in the donor-pendant group (R1) and substituents (R2) in phenolate units were examined as catalysts of the model reaction between phthalic anhydride and cyclohexane oxide in toluene. They were used individually or as a part of the binary catalytic systems with nucleophilic co-catalysts. The co-catalyst was selected from the following organic bases: PPh3, DMAP, 1-butylimidazole, or DBU. The binary catalytic systems turned out to be more active than the complexes used individually, and DMAP proved to be the best choice as a co-catalyst. When the molar ratio of [PA]:[epoxide]:[Cr]:[DMAP] = 250:250:1:1 was applied, the most active complex (R1-X = CH2NMe2, R2 = F) allowed to copolymerize phthalic anhydride with differently substituted epoxides (cyclohexene oxide, 4-vinylcyclohexene oxide, styrene oxide, phenyl glycidyl ether, propylene oxide, butylene oxide, and epichlorohydrin) within 240 min at 110 °C. The resulting polyesters were characterized by Mn up to 20.6 kg mol−1 and narrow dispersity, and they did not contain polyether units.

The Effect of Chemical Structure of OEG Ligand Shells with Quaternary Ammonium Moiety on the Colloidal Stabilization, Cellular Uptake and Photothermal Stability of Gold Nanorods.

PurposePlasmonic photothermal cancer therapy by gold nanorods (GNRs) emerges as a promising tool for cancer treatment. The goal of this study was to design cationic oligoethylene glycol (OEG) compounds varying in hydrophobicity and molecular electrostatic potential as ligand shells of GNRs. Three series of ligands with different length of OEG chain (ethylene glycol units = 3, 4, 5) and variants of quaternary ammonium salts (QAS) as terminal functional group were synthesized and compared to a prototypical quaternary ammonium ligand with alkyl chain – (16-mercaptohexadecyl)trimethylammonium bromide (MTAB).MethodsStep-by-step research approach starting with the preparation of compounds characterized by NMR and HRMS spectra, GNRs ligand exchange evaluation through characterization of cytotoxicity and GNRs cellular uptake was used. A method quantifying the reshaping of GNRs was applied to determine the effect of ligand structure on the heat transport from GNRs under fs-laser irradiation.ResultsFourteen out of 18 synthesized OEG compounds successfully stabilized GNRs in the water. The colloidal stability of prepared GNRs in the cell culture medium decreased with the number of OEG units. In contrast, the cellular uptake of OEG+GNRs by HeLa cells increased with the length of OEG chain while the structure of the QAS group showed a minor role. Compared to MTAB, more hydrophilic OEG compounds exhibited nearly two order of magnitude lower cytotoxicity in free state and provided efficient cellular uptake of GNRs close to the level of MTAB. Regarding photothermal properties, OEG compounds evoked the photothermal reshaping of GNRs at lower peak fluence (14.8 mJ/cm2) of femtosecond laser irradiation than the alkanethiol MTAB.ConclusionOEG+GNRs appear to be optimal for clinical applications with systemic administration of NPs not-requiring irradiation at high laser intensity such as drug delivery and photothermal therapy inducing apoptosis.

Lipophilicity of Cationic Ligands Promotes Irreversible Adsorption of Nanoparticles to Lipid Bilayers.

A mechanistic understanding of the influence of the surface properties of engineered nanomaterials on their interactions with cells is essential for designing materials for applications such as bioimaging and drug delivery as well as for assessing nanomaterial safety. Ligand-coated gold nanoparticles have been widely investigated because their highly tunable surface properties enable investigations into the effect of ligand functionalization on interactions with biological systems. Lipophilic ligands have been linked to adverse biological outcomes through membrane disruption, but the relationship between ligand lipophilicity and membrane interactions is not well understood. Here, we use a library of cationic ligands coated on 2 nm gold nanoparticles to probe the impact of ligand end group lipophilicity on interactions with supported phosphatidylcholine lipid bilayers as a model for cytoplasmic membranes. Nanoparticle adsorption to and desorption from the model membranes were investigated by quartz crystal microbalance with dissipation monitoring. We find that nanoparticle adsorption to model membranes increases with ligand lipophilicity. The effects of ligand structure on gold nanoparticle attachment were further analyzed using atomistic molecular dynamics simulations, which showed that the increase in ligand lipophilicity promotes ligand intercalation into the lipid bilayer. Together, the experimental and simulation results could be described by a two-state model that accounts for the initial attachment and subsequent conversion to a quasi-irreversibly bound state. We find that only nanoparticles coated with the most lipophilic ligands in our nanoparticle library undergo conversion to the quasi-irreversible state. We propose that the initial attachment is governed by interaction between the ligands and phospholipid tail groups, whereas conversion into the quasi-irreversibly bound state reflects ligand intercalation between phospholipid tail groups and eventual lipid extraction from the bilayer. The systematic variation of ligand lipophilicity enabled us to demonstrate that the lipophilicity of cationic ligands correlates with nanoparticle-bilayer adsorption and suggested that changing the nonpolar ligand R group promotes a mechanism of ligand intercalation into the bilayer associated with irreversible adsorption.

1‐Decene oligomerization by new complexes bearing diamine‐diphenolates ligands: Effect of ligand structure

A series of new titanium (IV) complexes bearing tetradentate diamine‐diphenolates, homologous to [ONNO] salan type ligands: [Ti{2,2′‐(OC6H2–4,6‐tBu2)2NHC3H6NH}(Cl)2] (Cat1), [Ti{2,2′‐(OC6H2–4,6‐tBu2)2NHC4H8NH}(Cl)2] (Cat2), and [Ti{2,2′‐(OC6H2–4,6‐tBu2)2NHC6H4NH}(Cl)2] (Cat3) were synthesized and fully characterized by elemental analysis and NMR spectroscopy. All three synthesized catalysts, after activation with ([Ph3C][B(C6F5)4]) as a cocatalyst, exhibited high activities in 1‐decene oligomerization experiments. Gel permeation chromatography (GPC) analysis showed that molecular weight of the obtained oligomers had a close relationship with employed ligand structures, so that by increasing length of the aliphatic linkage between nitrogen donor atoms, PDI (Mw/Mn) shifted to the higher values. 1‐decene oligomers obtained by theCat3exhibited highest isotactic value (92% tacticity). Kinetic calculations allowed us to formulate the elementary reaction steps in oligomerization process of the 1‐decene. At the viscosity characterization,KV100amounts of 2.9, 3.9, and 4.1 cSt were determined for synthesized oligomers withCat1,Cat2, andCat3, respectively. High catalyst activity, capability of producing PAOs withKV100< 6, less toxicity and corrosiveness of the catalytic system (compared to commercialized process based on the hazardous BF3catalyst), and simplicity of the preparation procedure are the merits of the new reported catalysts.

Effect of ligand structures on oxygen absorbability and viscosity of metal-containing ionic liquids

Metal-containing ionic liquids (MCILs) composed of a cobalt(II) Schiff base complex and ionic-liquid-based axial ligands (ligand ILs) are potential O2 absorbents. To determine the design criteria of MCILs with both highly selective O2 absorbability and low viscosity, the relationship between the physicochemical properties and chemical structure of the MCILs was investigated. The measurement of the amount of O2 absorbed in MCILs with various ligand ILs indicated that O2 reactivity is determined by the electron density of the Co atom of the MCILs. The electron density of the Co atom could be controlled by the σ electron donation ability of the ligand ILs. Moreover, the viscosity of the MCILs was strongly affected by the interaction among the MCIL molecules caused by the π electron system. This interaction was weakened by the equatorially coordinating Schiff base and the ligand ILs within the chemical structure. Therefore, to develop MCILs with both high O2 reactivity and low viscosity, suppression of the interaction caused by the π electron system without decreasing the electron density of the Co atom is important.

Extraction of various metal ions by open-chain crown ether bridged diphosphates in supercritical carbon dioxide

Abstract Various toxic metal ions were successfully removed from solid matrix into supercritical CO2 (scCO2) by open-chain crown ether bridged diphosphates at 313.15 K and 20 MPa, these diphosphates with different ester side chains and different length of ethylene oxide bridge group are highly soluble in supercritical CO2. The extraction efficiency (E%) of heavy metals is between 55 and 89%. Mulliken charge distribution of ligand’s P=O coordination group was calculated to indicate the stability of metal complex. The ligand structure effects and the rationale for different selectivity were discussed. In addition, binding property of these diphosphates towards the alkaline earth metals was further studied following the same extraction procedures. Alkaline earth metal ions Ca2+, Sr2+ and Ba2+ were extracted with E% at 49–74%, 50–73% and 16–64%, respectively. DFT calculations were performed to investigate the interaction energy of the complexes and the correlation with the E% was discussed.

Effect of Ligand Structure on the Electron Density and Activity of Iridium Catalysts for the Borylation of Alkanes.

An in-depth study of iridium catalysts for the borylation of alkyl C-H bonds is reported. Although the borylation of aryl C-H bonds can be catalyzed by iridium complexes containing phen or bpy ligands at mild temperatures and with limiting arene, the borylation of alkyl C-H bonds remains underdeveloped. We prepared a library of phenanthrolines that contain varying substitution patterns. The corresponding phen-Ir trisboryl carbon monoxide complexes were synthesized to determine the electron-donating ability of these ligands, and the initial rates for the borylation of the C-H bonds in THF and diethoxyethane β to oxygen catalyzed by Ir complexes containing these ligands were measured. For some subsets of these ligands, the donor ability correlated positively with the rate of C-H borylation catalyzed by the complexes containing ligands within a given subset. However, across subsets, ligands possessing similar donor properties to one another form catalysts for the borylation of alkyl C-H bonds with widely varying activity. This phenomenon was investigated computationally, and it was discovered that the stabilizing interactions between the phenanthroline ligand and the boryl ligands attached to Ir in the transition state for C-H oxidative addition could account for the differences in the activity of the catalysts that possess similar electron densities at Ir. The effect of these interactions on the borylation of secondary alkyl C-H bonds is larger than it is on the borylation of primary alkyl C-H bonds.

Effect of Ligand Structures of Copper Redox Shuttles on Photovoltaic Performance of Dye-Sensitized Solar Cells.

In recent years, copper(I/II) complexes have emerged as alternative redox shuttles in dye-sensitized solar cells (DSSCs), exhibiting more positive redox potential than iodine- and cobalt-based redox shuttles. In particular, copper(I/II) complexes with 1,10-phenanthroline- or 2,2'-bipyridyl-based ligands attained moderate to high power conversion efficiencies (6-11%) with a high open-circuit voltage (VOC) over 1.0 V due to the positive potentials. Although copper(I/II) complexes with 1,10-phenanthroline-based ligands with 2,9-substituents have been developed, the effect of their ligand structures on the photovoltaic performance of DSSCs have not been fully addressed due to limited synthetic access to 1,10-phenanthroline derivatives. In this study, we designed and synthesized a series of copper(I/II) complexes with 1,10-phenanthroline ligands with different substituents at the 2,9-positions: bis(2-n-butyl-1,10-phenanthroline)copper(I/II) ([Cu(bp)2]1+/2+), bis(2-ethyl-9-methyl-1,10-phenanthroline)copper(I/II) ([Cu(emp)2]1+/2+), bis(2,9-diethyl-1,10-phenanthroline)copper(I/II) ([Cu(dep)2]1+/2+), and bis(2,9-diphenyl-1,10-phenanthroline)copper(I/II) ([Cu(dpp)2]1+/2+). The more positive redox potentials of [Cu(emp)2]1+/2+ and [Cu(dep)2]1+/2+, compared to that of bis(2,9-dimethyl-1,10-phenanthroline)copper(I/II) ([Cu(dmp)2]1+/2+), originate from the larger steric hindrance of the ethyl group instead of the methyl group, whereas the redox potential of [Cu(bp)2]1+/2+ is significantly shifted to the negative direction because of the lower steric hindrance of the 2-monosubstituted 1,10-phenanthroline ligands. The efficiency of the DSSC with [Cu(bp)2]1+/2+ (5.90%) is almost comparable to the DSSC with [Cu(dmp)2]1+/2+ (6.29%). In contrast, the DSSCs with [Cu(emp)2]1+/2+ (3.25%), [Cu(dep)2]1+/2+ (2.56%), and [Cu(dpp)2]1+/2+ (2.21%) exhibited lower efficiencies than those with [Cu(dmp)2]1+/2+ and [Cu(bp)2]1+/2+. The difference can be rationalized by the electron collection efficiencies. Considering the similar photovoltaic properties of the DSSCs with [Cu(bp)2]1+/2+ and [Cu(dmp)2]1+/2+, the use of copper(I/II) complexes with 2-monosubstituted 1,10-phenanthroline ligands as the redox shuttle may be useful to improve the short-circuit current density while retaining the rather high VOC value when dyes with a smaller bandgap (i.e., better light harvesting) are developed.