Ring Substituents Research Articles

Modulating the oxygen affinity of porphyrins with metals, ligands, and functional groups: A DFT study.

The interaction between different metals (M), axial ligands (L), and ring substituents (R) in porphyrins was investigated using density functional theory. Different combinations of iron and cobalt as metal centers; imidazole, chlorine, and an n-heterocyclic carbene (NHC) as axial ligands, and unsubstituted, octaethyl-, and tetraphenyl-porphyrins were explored in their low, intermediate, and high-spin states, alongside oxygen affinity. Remarkably, the n-heterocyclic carbene enhanced the affinity of cobalt porphyrins to oxygen, with binding energies on average 4.4 kcal mol-1 higher than FeP with the same ligand, and 0.78 kcal mol-1 higher than FeP with imidazole. The planarity of the iron tetraphenyl porphyrin with imidazole compared to its ruffled cobalt counterpart is noteworthy in both oxy- and deoxy-forms, highlighting imidazole's stabilizing influence on the porphyrin structure, particularly iron porphyrins, alongside imidazole's stabilizing effect on the affinity to O2. Despite the significant non-planarity induced by NHC as an axial ligand -regardless of the metal or ring substituent used-, it did not hinder the affinity of CoP to O2 (14.26 kcal mol-1, on average) as it did with the FeP with NHC (9.88 kcal mol-1, on average). Cobalt porphyrins with n-heterocyclic carbene ligands show promising potential for O2 activation or oxygen transport applications. The results show the complex interactions between the different parts of metalloporphyrins and highlight the capability of tailoring their affinity to O2. It also exemplifies the stabilizing effect of imidazole on the porphyrins, providing a very narrow range of binding energies and smaller differences in their geometries.

Density Functional Theory Rationalization of the Mechanism, Selectivity, and Role of Substituents in Au(I)-Catalyzed Synthesis of Pyrazolines and Dihydropyridines.

A comprehensive theoretical investigation into the gold-catalyzed synthesis of polysubstituted pyrazolines and dihydropyridines from imines and methyl phenylpropiolate was conducted in this study. Three imines with distinct substituents were selected as model reactants. The computational outcomes reveal that four-membered intermediates generated from aza-enyne metathesis significantly affect the reaction selectivity. For nitrogen-centered NHCO2Me substituents (series A), an outward ring opening occurs during the metathesis of the aza-alkyne. This leads to the formation of Z-butadiene intermediates and ultimately to pyrazoline products. Conversely, with an aromatic substituent at the nitrogen site (series B and C), an inward ring opening takes place. This results in E-butadiene intermediates and the synthesis of dihydropyridine derivatives. The dihydropyridine product's configuration is determined by the aromatic ring's substituent. Electron-donating groups tend to directly form 1,4-dihydropyridine through a 6π electrocyclization (series B). In contrast, strong electron-withdrawing substituents initially undergo azayne metathesis, followed by 6π electrocyclization to produce 1,2-dihydropyridine products (series C). Furthermore, the distinctive selectivities were investigated in depth using global reactivity index and distortion/interaction methods. This research may contribute to the design of more effective and selective protocols to access pyrazolines, dihydropyridines, and related compounds.

Tension ring-functionalized bicyclic ammonium ionic liquids as hypergolic fuels with superior energy density

Hypergolic ionic liquids are a new type of liquid propellants, and the energy density is critical to propulsion performance. We propose to combine cage structures with tensile rings to design and synthesize new ionic liquid molecules. Cycloalkane-substituted 1-aza-bicyclo[2.2.2]octane and 1,4-diazabicyclo[2.2.2]octane-like ionic liquids were synthesized using dicyanamide root as an anion, and its structure and properties were determined. The obtained ionic liquids possess higher energy density up to 1.87 kJ·mL−1. The three-membered ring substituent can increase the energy density of ionic liquids by 79.8 % and reduce the ignition delay time by 52.5 % than that of straight-chain alkanes. This work provides an important basis for the design and synthesis of the new type of hypergolic ionic liquids.

Unveiling the dual-state emissive behaviour of 4,6-diarylpyrimidin-2-amines through a plug-and-play approach

We present here a series of 4,6-diarylpyrimidin-2-amine derivatives (5a-j) with tunable optical properties both in the solid and solution states. Our plug-and-play fluorophore design demonstrates that the aryl groups at the 4th and 6th positions of the 2-aminopyrimidine core enable distinct optical characteristics for each derivative. The fluorophore design concept was validated using theoretical and spectroscopic methods. The designed compounds were synthesised in moderate to good yields, and their structures were confirmed via IR, NMR, HRMS, and single-crystal XRD analyses. Optical studies revealed that varying the aryl substituents significantly impacts absorption, emission, and bandgap values in both phases. The absolute quantum yields (ϕF) of the synthesised derivatives ranged from 8.11 to 71.00% in DMF and 5.86–29.43% in thin films, with fluorescence lifetimes (τ) between 0.8 and 1.5 ns in DMF and 0.63–3.16 ns in films, respectively. CIE (Commission Internationale de l’Éclairage) diagrams indicate blue-green emission for 5a-j in the visible spectrum. The electrochemical analysis confirmed that the HOMO/LUMO (Highest Occupied Molecular Orbital/Lowest Unoccupied Molecular Orbital) energy levels can be modulated by altering the substituent rings. These results highlight the dual-state emission properties of 2-aminopyrimidine fluorophores, demonstrating their potential for a wide range of optoelectronic and advanced applications.

Ruthenium-catalyzed synthesis of novel phenoxy(phenyl)-1H-benzo[d]imidazoles: Spectral characterization and DFT studies

The synthesis of a series of novel phenoxy(phenyl)-1H-benzo[d]imidazoles by oxidative cyclization of phenoxybenzaldehyde and diamine in the presence of ruthenium(II)-catalysts is described. The transformation using a catalytic amount of [RuCl2(p-cymene)]2 was found to be highly efficient as the reaction selectively produces monoarylated benzimidazoles in excellent yields (up to 92%). This result signify high efficacy of the catalyst for the selective synthesis of phenoxy(phenyl)-1H-benzo[d]imidazoles (monoarylated benzimidazole-containing diphenyl ether scaffolds). The use of other ruthenium catalyst viz. N^O chelate ruthenium(II)-complex gave monoaryled benzimidazole and 1,2-diarylbenzimidazole as the major and minor products, respectively. On the other hand, the reaction carry out using excess MgSO4/TFA produced mono- and 1, 2-diarylbenzimidazole derivative in almost equal amount or diarylated compounds as the major product. Electron withdrawing substituent in the diphenyl ether ring gave higher yields then electron donating substituents. Theoretical study such as structure optimization, HUMO-LUMO, hardness, softness and electronegativity determination of 2-arylbenzimidazole derivatives have been carried out in DFT/B3LYP/6–311++G(d,p) quantum chemical method and basis set using the Gaussian 09 W-Gaussian-View 5.1 package program. The synthesized compounds were characterized using FTIR, NMR and mass spectral data. The crystal structure of a precursor compound, 2-(2, 4-difluorophenoxy)benzaldehyde, has been determined by a single X-ray crystallography.

O- and N-Difluoromethylation of 2-Pyridones with Chlorodifluoromethane

Difluoromethylation of various 2-pyridones with the available industrial reagent chlorodifluoromethane (Freon-22) has been studied. Some relationships between the ratio of difluoromethylation products and the reaction conditions, as well as the presence of substituents in the pyridine ring, have been found. The possibility of obtaining difluoromethylation products at the nitrogen atom, in some cases with a preparative yield, has been investigated.

Synthesis, characterization and photophysical properties of phthalimide-pyrazole compounds: Effect of the substituent in the pyrazole ring on the crystal structures

Four phthalimide-pyrazole derivative compounds (L1 = H, L2 = NO2, L3 = Br, and L4 = I) were synthesized and fully characterized. Single crystals of each compound were obtained, and their conformational and supramolecular structures were analyzed. The four crystal structures (L1 to L4) are stabilized by different intermolecular interactions and therefore have different characteristics from each other. L1 is dominated by long-range Van der Waals forces and not by short hydrogen bonds. On the other hand, L2 packs into a molecular sheet along the b and c axes. L3 and L4 have a common monoclinic structure (P21/c) and thus allow stabilization of the respective compounds by halogen-halogen interactions (Br–Br, I–I) and σ-hole effects. In all compounds, these interactions, whether Van der Waals forces, π‧‧‧‧π stacking or halogen bonds, are responsible for their packing and molecular stability. Therefore, it was concluded that the substituent on the pyrazole ring is the determining factor in their crystallization. In addition, photophysical and theoretical studies showed that the compounds exhibit emission in the presence of DCM, acetonitrile (MeCN), and DMSO, primarily due to the HOMO-LUMO orbitals in the phthalimide ring.

Dihydro‐1H‐Pyrazoles as Donor Blocks in Donor–Acceptor Chromophores for Electro‐Optics: A DFT Study of Hyperpolaizability and Electronic Excitations

ABSTRACTA diverse set of promising donors for donor–acceptor chromophores based on dihydro‐1H‐pyrazole (pyrazoline) for use in electro‐optics was investigated using DFT at M06‐2X/aug‐cc‐pVDZ level of theory. These calculations showed that it is possible to achieve a molecular hyperpolarizability of up to 1700*10−30 esu (up to three times higher compared to conventional diethylaniline donors) for a simple tricyanofuran‐based acceptor by carefully tuning the donor structure. It was shown that the molecular hyperpolarizability is mainly affected by the substituents in the aryl rings in positions 3 and 1 of the pyrazoline cycle, while the substituents of the aryl ring in position 5 and the pyrazoline ring itself can be varied without significant effects on the hyperpolarizability. For one of the compounds, a detailed study of the lowest energy electronic excitation was performed using the TD‐DFT, confirming the role of the pyrazoline ring as a secondary donor.

Investigation of Apoptotic and Anticancer Effects of 2-substituted Benzothiazoles in Breast Cancer Cell Lines: EGFR Modulation and Mechanistic Insights.

Benzothiazole derivatives, a class of heterocyclic compounds, exhibited diverse biological activities influenced by substituents in the thiazole ring. This study aimed to synthesize these compounds with two functional groups to investigate their potential as anticancer agents, particularly against breast cancer. While previous research demonstrated the efficacy of 2-substituted benzothiazoles against glioma and cervical and pancreatic cancer cells, there is a gap in studies targeting breast cancer. The synthesized compounds were tested in vitro using MCF-7, MDA-MB-231, and MCF-10A cell lines, with Doxorubicin as the positive control. Various assays were conducted, including Annexin V/PI, cell cycle analysis, wound healing, and measurement of mitochondrial membrane potential. Protein expression of EGFR and transcription levels of apoptosis-related genes (Bax and Bcl-xL) and cancer progression-related genes (JAK, STAT3, ERK, AKT, mTOR) were analyzed. Additionally, the balance between antioxidants and oxidants was evaluated by measuring TAS and TOS levels. Our findings revealed that benzothiazole compounds significantly inhibited breast cancer cell growth by reducing cell motility, disrupting mitochondrial membrane potential, and inducing cell cycle arrest in the sub-G1 phase. These compounds increased reactive oxygen species accumulation, leading to cell death. Furthermore, they decreased EGFR protein levels, increased Bax gene transcription, and downregulated the expression of genes such as JAK, STAT3, ERK, AKT, and mTOR. In conclusion, benzothiazole derivatives exhibited potent inhibitory effects on breast cancer in vitro by promoting apoptosis, downregulating EGFR activity, and modulating key signaling pathways, including JAK/STAT, ERK/MAPK, and PI3K/Akt/mTOR. These results highlighted the potential of benzothiazole derivatives as novel therapeutic agents for breast cancer treatment.

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.

The role of the nitro group on the formation of intramolecular hydrogen bond in the methyl esters of 1-vinyl-nitro-pyrazolecarboxylic acids

All reactions involving the nitrogen atom in 3(5)-substituted pyrazoles inevitably lead to a mixture of N-substituted 3- and 5- isomers due to tautomerism. The position of substituent can affect the behavior of N-vinyl isomers, in particular, the ability of some functional groups to form =CH…O or =CH…N weak hydrogen bonding type interaction with α-hydrogen of vinyl group due to steric reasons. Moreover, the additional substituents in the pyrazole ring can further influence the possibility and nature of specific intramolecular interactions. Here we have shown the effect of the nitro group on the formation of hydrogen bonds in methyl esters of 1-vinyl-4-nitro-3(5)- and 1-vinyl-5-nitro-3(5)-pyrazole carboxylic acids. A comprehensive study, including synthesis, quantum chemical calculations, NMR and XRD analysis, allowed to conclude that, contrary to expected, in the methyl ester of 1-vinyl-4-nitro-5-pyrazolecarboxylic acid, the formation of a planar conformation is unfavorable, preventing formation of a classical hydrogen bond. On the other hand, there is a hydrogen bond between the nitro group and vinyl group in 1-vinyl-5-nitro-pyrazolecarboxylic acids.

Synthesis and Properties of Highly Tilted Antiferroelectric Liquid Crystalline (R) Enantiomers.

This work reports the synthesis method and various properties of four rod-like antiferroelectric (R) laterally substituted enantiomers, with or without fluorine atoms used as substituents in the benzene ring. The influence of fluorine substitution on the mesophase temperature range was determined. The synthesized compounds are three-ring rod-like smectics with a chiral center based on (R)-(-)-2-octanol. Their chemical and optical purity was checked using high-performance liquid chromatography (HPLC). Two newly synthesized enantiomers and three previously reported (R) enantiomers were used to formulate two antiferroelectric mixtures. The mesomorphic behavior was characterized by polarizing optical microscopy, differential scanning calorimetry, and X-ray diffraction (XRD). The helical pitch and tilt angle measurements were done using the selective light reflection phenomenon and the electro-optical method, respectively. All the enantiomers exhibit a wide temperature range of the antiferroelectric phase, with a high tilt angle. Furthermore, the enantiomer with lateral fluorine substitution in the ortho position has a very long helical pitch (more than 2.0 µm), relatively low enthalpy of melting point, and a tilt angle close to 45 degrees. The designed (R) enantiomers can be useful for formulating eutectic mixtures for further use in various devices, including photonics and optoelectronics.

Synthesis of kanamycin-azole hybrids and investigation of their antifungal activities

The World Health Organization (WHO) recognizes Candida albicans and Cryptococcus neoformans as the critical priority fungal pathogens for which therapeutic solutions are needed. Azole-based antifungal agents, including triazoles, diazoles, and thiazoles, are widely used in the treatments for fungal infections. In light of past successes in the transformation of antibacterial kanamycin into antifungal derivatives via chemical modifications, a new library of kanamycin-azole hybrids was synthesized and tested against a panel of azole-resistant and susceptible Candida and Cryptococcus strains. Structure activity relationship (SAR) studies revealed pivotal roles for antifungal activity of the azole ring (imidazole vs triazole) and halogen substituents on the benzene ring (F vs Cl). Most notably, hybrids 13, 14 and 15 were active against resistant C. albicans, C. tropicalis and C. neoformans strains and non-toxic towards mammalian cells. Mode of action investigations using fluorogenic dyes, (SYTOXTM) showed the fungal active compounds could permeabilize fungal membranes even at ¼ MICs. These findings reveal novel azole-based antifungals that could offer new therapeutic options for candidiasis and cryptococcosis.

RP-HPLC Study of the Complexation of Acetamiprid, Imidacloprid and Carboxin Guest Molecules With Thiophosphorylated Calix [6] Arene in Dimethylformamide-Acetonitrile-Water Mobile Phase.

Reversed-phase high-performance liquid chromatography was used to study the host-guest complexation of thiophosphacalixarenes (TPC 1-3) with acetamiprid, imidacloprid and carboxin as guests. The Onyx Monolithic C 18 column (150 × 0.1mm) and Phenomenex® were used with UV detection at 254nm. The mobile phase consisted of dimethylformamide-acetonitrile-water. The association constant of the 1:1 guest-host complexes was determined from the relationship between the retention factor of the guest and concentration of the thiophosphacalix [6] arene host in the mobile phase. The stability constants are influenced by size, nature, position and quantity of substituents in the benzene ring of the calixarene skeleton. The detection limits for TPC 1, TPC 2 and TPC 3 are 0.95, 1.20 and 1.40μg/mL, while the quantification limits are 2.85, 3.60 and 4.20μg/mL for TPC 1, TPC 2 and TPC 3, respectively.

Efficient Production of 4’-Hydroxydihydrochalcones Using Non-Conventional Yeast Strains

The quest for novel therapeutic agents has rekindled interest in natural products, particularly those derived from biotransformation processes. Dihydrochalcones, a class of plant secondary metabolites, exhibit a range of pharmacological properties. Chalcone and dihydrochalcone compounds with the characteristic 4’-hydroxy substitution are present in ‘dragon’s blood’ resin, known for its traditional medicinal uses and complex composition, making the isolation of these compounds challenging. This study investigates the efficient production of 4′-hydroxydihydrochalcones using non-conventional yeast strains. We evaluated the biotransformation efficiency of various 4′-hydroxychalcone substrates utilizing yeast strains such as Yarrowia lipolytica KCh 71, Saccharomyces cerevisiae KCh 464, Rhodotorula rubra KCh 4 and KCh 82, and Rhodotorula glutinis KCh 242. Our findings revealed that Yarrowia lipolytica KCh 71, Rhodotorula rubra KCh 4 and KCh 82, and Rhodotorula glutinis KCh 242 exhibited the highest conversion efficiencies, exceeding 98% within one hour for most substrates. The position of methoxy substituents in the chalcone ring significantly influenced hydrogenation efficiency. Moreover, we observed isomerization of trans-4′-hydroxy-2-methoxychalcone to its cis isomer, catalyzed by light exposure. This study underscores the potential of using yeast strains for the sustainable and efficient production of dihydrochalcones, providing a foundation for developing new therapeutic agents and nutraceuticals.

Effects of Ring Functionalization in Anthracene-Based Cyclophanes on the Binding Properties Toward Nucleotides and DNA.

Supramolecular recognition of nucleobases and short sequences is an emerging research field focusing on possible applications to treat many diseases. Controlling the affinity and selectivity of synthetic receptors to target desired nucleotides or short sequences is a highly challenging task. Herein, we elucidate the effect of substituents in the phenyl ring of the anthracene-benzene azacyclophane on the recognition of nucleoside triphosphates (NTPs) and double-stranded DNA. We show that introducing phenyl rings increases the affinity for NTPs 10-fold and implements groove and intercalation binding modes with double-stranded DNA. NMR studies and molecular modeling calculations support the ability of cyclophanes to encapsulate nucleobases as part of nucleotides.

Insight into efficient degradation of pentacyclic and hexacyclic sulfonamide antibiotics by synthetic trivalent copper: Performance and mechanism

High valent metal species, including Mn(III), Fe(IV) and Cu(III), have been identified as key intermediates in the degradation of pollutants in many advanced oxidation processes. However, unlike Mn(III) and Fe(IV), the current exploration of the reaction activity and selective oxidation mechanism of Cu(III) towards pollutants with different structures is still quite limited. Herein, the copper(III) periodate was synthesized to investigate the reactivity towards six sulfonamide antibiotics (SAs) including typical two pentacyclic structures (sulfamethoxazole (SMX) and sulfathiazole (STZ)) and four hexacyclic structures (sulfadiazine (SDZ), sulfamerazine (SMR), sulfamonomethoxine (SMM) and sulfapyridine (SPD)). The results indicated that all SAs almost completely removed by Cu(III) system after 10 min with the molar ratio of approximately 3:1 (Cu(III):SAs) and Cu(III) direct oxidation played the most important role. SAs with 6-ring substituents were more readily degraded by Cu(III) than SAs with 5-ring substituents, and the presence of electron-rich group such as -CH3 and -S in ring substituent increased the reactivity towards Cu(III). The introduction of coexisting anions (Cl−, SO42− and HCO3−) hardly affected the degradation of SAs by Cu(III) oxidation, while the addition of HA to some extent inhibited SAs degradation. The solution pH greatly affected the degradation of SAs by Cu(III) and the removal efficiencies of SAs roughly followed the rule of neutral > acidic > alkaline. The degradation mechanism of SAs with 5-ring and 6-ring substituents in Cu(III) system mainly included amino nitration, self-coupling, hydroxylation, S-N cleavage in SAs with 5-ring substituents and SO2 extrusion in SAs with 6-ring substituents. Although the real water matrix inhibited the degradation of SAs to varying degrees, Cu(III) still played a satisfactory performance on SAs degradation especially for electron-rich structure.

Privileged Scaffold Hybridization in the Design of Carbonic Anhydrase Inhibitors.

Human Carbonic Anhydrases (hCA) are enzymes that contribute to cancer's development and progression. Isoforms IX and XII have been identified as potential anticancer targets, and, more specifically, hCA IX is overexpressed in hypoxic tumor cells, where it plays an important role in reprogramming the metabolism. With the aim to find new inhibitors towards IX and XII isoforms, the hybridization of the privileged scaffolds isatin, dihydrothiazole, and benzenesulfonamide was investigated in order to explore how it may affect the activity and selectivity of the hCA isoforms. In this respect, a series of isatin thiazolidinone hybrids have been designed and synthesized and their biological activity and selectivity on hCA I, hCA II, hCA IX, and hCA XII explored. The new compounds exhibited promising inhibitory activity results on isoforms IX and XII in the nanomolar range, which has highlighted the importance of substituents in the isatin ring and in position 3 and 5 of thiazolidinone. In particular, compound 5g was the most active toward hCA IX, while 5f was the most potent inhibitor of hCA XII within the series. When both potency and selectivity were considered, compound 5f appeared as one of the most promising. Additionally, our investigations were supported by molecular docking experiments, which have highlighted the putative binding poses of the most promising compound.

Kinetics of uptake, translocation, and metabolism of organophosphate esters in japonica rice (Oryza sativa L.): Hydroponic experiment combined with model

Hydroponics combined with fugacity model was employed to investigate the kinetics of uptake, accumulation, and metabolism of organophosphate esters (OPEs) by japonica rice. The time-dependent process for uptake and accumulation of 5 OPEs and their diester-metabolites in both rice root and shoot fitted well with the pseudo-first-order kinetic model. The peak OPE accumulations in rice root and shoot were significantly positively or negatively correlated with their octanol-water partition coefficient (logKow) respectively, but not for their apparent accumulation rates. Root concentration factors (RCFs) and root-to-shoot translocation factors (TFs) of OPEs were found to be positively and negatively correlated with their logKow, respectively. Triphenyl phosphate with benzene ring substituents showed the highest RCF, but the lowest TF, because of its high potential for root adsorption due to the π electron-rich structures. Sterilized root exudates can hinder the root adsorption and absorption of OPEs from solution probably through competitive adsorption of OPEs with root surface. The first-hand transport and metabolism rates were also obtained by generating these rates to fit the dynamic fugacity model with the measurement values. The simulation indicated that the kinetics of OPE accumulation in rice plants may be controlled by multiple processes and physicochemical properties besides Kow.

Iridium Complexes with Bidentate Pyridinylidene/N-Amidate Ligands for Transfer Hydrogenation Catalysis.

A series of iridium pentamethylcyclopentadienyl (Cp*) complexes, [Cp*Ir(κ2-RLp/m)Cl], that contain the strongly coordinating bidentate ligands RLp/m were synthesized. The donor groups of the bidentate ligands were an N-amidate and either a para-pyridinylidene remote N-heterocyclic carbene (RLp) or a meta-pyridinylidene remote N-heterocyclic carbene (RLm). For each type of bidentate ligand, a set of iridium complexes was synthesized, which differed only according to the substituents (R) on the phenyl ring associated with the amidate group. The iridium complexes were all fully characterized and molecular structures were obtained by single-crystal X-ray diffraction studies for representative examples. The complexes were found to be good precatalysts in iso-propanol for the transfer hydrogenation of benzaldehyde to give benzyl alcohol. The catalytic activity correlated with the Hammett σm/p parameters of the phenyl ring substituents, with more electron-donating substituents leading to increased catalytic activity. In all cases, the meta-pyridinylidene complexes, [Cp*Ir(κ2-RLm)Cl], performed better than the corresponding para analogues, [Cp*Ir(κ2-RLp)Cl].