pH-Dependent spectroscopic and luminescent properties, and metal-ion recognition studies of Re(I) complexes containing 2-(2'-pyridyl)benzimidazole and 2-(2'-pyridyl)benzimidazolate

Stereoelectronic Profiling of Neutral and Monoanionic Biimidazoles and Mixed Diimines

14 mono-, di-, and tetranuclear palladium complexes were prepared to study the coordination chemistry of symmetrical and unsymmetrical azole-derived diimines and their anions. The diverse range of complexes obtained highlights the structural and electronic diversities imposed by these ligands. Using the monopalladium species, the electronic properties of selected bidentate ligands were determined, ranked, and compared by ¹³C NMR spectroscopy, extending the scope of the HEP2 (Huynh electronic parameter 2) scale, which can detect even subtle differences. Moreover, the %V_bur (percentage volume buried) values as an estimate for the steric bulk of some ligands were determined using the solid-state molecular structures of their complexes, and a preliminary stereoelectronic map was established.

Base-assisted synthesis of 4-pyridinate gold(I) metallaligands: a study of their use in self-assembly reactions

The synthesis of di- and tritopic gold(i) metallaligands of the type [(Au4-py)2(μ2-diphosphane)] (diphosphane = bis(diphenylphosphanyl)isopropane or dppip (1), 1,2-bis(diphenylphosphanyl)ethane or dppe (2), 1,3-bis(diphenylphosphanyl)propane or dppp (3) and 1,4-bis(diphenylphosphanyl)butane or dppb (4)) and [(Au4-py)3(μ3-triphosphane)] (triphosphane = 1,1,1-tris(diphenylphosphanylmethyl)ethane or triphos (5) and 1,3,5-tris(diphenylphosphanyl)benzene or triphosph (6)) from [(AuCl)2(μ2-diphosphane)] or [(AuCl)3(μ3-triphosphane)] and 4-pyridylboronic acid in the presence of Cs2CO3 has been conducted. Interestingly, when [(AuCl)2(μ2-dppm)] (dppm = bis(diphenylphosphanyl)methane) was used as a starting material, the cyclic tetranuclear gold(i) compound [(Au4-py)2(CH)2{μ2-Au(PPh2)2}2] (I) was obtained instead. All the products have been characterized by IR and multinuclear NMR spectroscopy, mass spectrometry and elemental analysis and in the case of 1, 3, 5 and I by X-ray crystallography, which showed the presence of aurophilic interactions in all of them. The obtained metallaligands have been used as building blocks in self-assembly reactions with cis-blocked palladium or platinum acceptor moieties producing [2 + 2] metallamacrocycles or trigonal bipyramidal (TBP) [2 + 3] metallacages in good yields. The photophysical properties of both the metallaligands and the corresponding assemblies have been investigated.

Anion-Recognition Studies of a Rhenium(I) 4-Mercaptopyridine Compound and Its Ligand-Coupling Products

The reaction of Re(CO)₅Cl with 4-mercaptopyridine (4-PySH) led to the formation of [Re(CO)₃(4-HPyS)₃]Cl (1), showing three hydrogen-bonding donors of 4-PySH ligands as well as a characteristic ligand-to-metal charge-transfer absorption at ca. 380 nm. In this regard, a variety of anions, i.e., CN^-, OAc^-, F^-, Cl^-, Br^-, I^-, PF₆^-, NO₃^-, ClO₄^-, and H₂PO₄^-, were examined to study anion-recognition studies through hydrogen-bonding functionalities. Upon the addition of CN^- to a methanolic solution of complex 1, a remarkable spectral change with an isosbestic point at ca. 314 nm in the absorption spectra was observed, with a binding constant (K_b) calculated to be 24770 M^-1. Moreover, the OAc^- anion also shows a similar trend, but a mild spectral change, with K_b calculated to be 2170 M^-1. Unlike those of CN^- and OAc^-, the addition of F^-, Cl^-, Br^-, and I^- anions causes a less pronounced spectral change with an isosbestic point at ca. 350 nm and K_b calculated to be 2863-750 M^-1. However, almost no spectral change can be observed for other anions (i.e., PF₆^-, NO₃^-, H₂PO₄^-, and ClO₄^-). Interestingly, the molecular loops of [Re(CO)₃Cl(Py₂S₂)]₂ (2; Py₂S₂ = 4,4'-dipyridyl disulfide) and [Re(CO)₃Cl(Py₂S)_0.35(Py₂S₂)_0.65]₂ (3; Py₂S = 4,4'-dipyridyl sulfide) can be isolated and structurally characterized by X-ray diffraction, where those crystals were grown from diethyl ether diffusion into a methanolic solution of complex 1 with [Bu₄N]CN and [Bu₄N]NO₃, respectively. It is noted that such unusual ligand-coupling reactions toward the homoligand and hybrid-ligand loops of complexes 2 and 3 can be achieved at room temperature in this study.

Luminescent Rhenium(I)tricarbonyl Complexes Containing Different Pyrazoles and Their Successive Deprotonation Products: CO2 Reduction Electrocatalysts

Cationic fac-[Re(CO)₃(pz*H)(pypzH)]OTf (pz*H = pyrazole, pzH; 3,5-dimethylpyrazole, dmpzH; indazole, indzH; 3-(2-pyridyl)pyrazole, pypzH) were obtained from fac-[ReBr(CO)₃(pypzH)] by halide abstraction with AgOTf and subsequent addition of the corresponding pyrazole. Successive deprotonation with Na₂CO₃ and NaOH gave neutral fac-[Re(CO)₃(pz*H)(pypz)] and anionic Na{fac-[Re(CO)₃(pz*)(pypz)]} complexes, respectively. Cationic fac-[Re(CO)₃(pz*H)(pypzH)]OTf, neutral complexes fac-[Re(CO)₃(pz*H)(pypz)], and fac-[Re(CO)₃(pypz)₂Na] were subjected to photophysical and electrochemical studies. They exhibit phosphorescent decays from a prevalently ³MLCT excited state with quantum yields (Φ) in the range between 0.03 and 0.58 and long lifetimes (τ from 220 to 869 ns). The electrochemical behavior in Ar atmosphere of cationic and neutral complexes indicates that the oxidation processes assigned to Re^I → Re^II occurs at lower potentials for the neutral complex compared to cationic complex. The reduction processes occur at the ligands and do not depend on the charge of the complexes. The electrochemical behavior in CO₂ saturated media is consistent with CO₂ electrocatalyzed reduction, where the values of the catalytic activity [i_cat(CO₂)/i_cat(Ar)] ranged from 2.7 to 11.5 (compared to 8.1 for fac-[Re(CO)₃Cl(bipy)] studied as a reference). Controlled potential electrolysis for the pyrazole cationic (3a) and neutral (4a) complexes after 1 h affords CO in faraday yields of 61 and 89%, respectively. These values are higher for indazole complexes and may be related to the acidity of the coordinated pyrazole.

Crystal structure of fac-aqua-[(E)-4-(benzo[d]thia-zol-2-yl)-N-(pyridin-2-yl-methyl-idene)aniline-κ2 N,N']tricarbonylrhenium(I) hexa-fluorido-phosphate methanol monosolvate

In the title compound, fac-[Re(C19H13N3S)(CO)3(H2O)]PF6·CH3OH, the coordination environment of the ReI atom is octa-hedral with a C3N2O coordination set. In this mol-ecule, the N,N' bidentate ligand, (E)-4-(benzo[d]thia-zol-2-yl)-N-(pyridin-2-yl-methyl-idene)aniline, and the monodentate aqua ligand occupy the three available coordination sites of the [Re(CO)3]+ core, generating a '2 + 1' mixed-ligand complex. In this complex, the Re-C bonds of the carbonyl ligands trans to the coordinating N,N' atoms of the bidentate ligand are longer than the Re-C bond of the carbonyl group trans to the aqua ligand, in accordance with the intensity of their trans effects. The complex is positively charged with PF6 - as the counter-ion. In the structure, the complexes form dimers through π-π inter-molecular inter-actions. O-H⋯O and O-H⋯N hydrogen bonds lead to the formation of stacks parallel to the a axis, which further extend into layers parallel to (01). Through O-H⋯F hydrogen bonds between the complexes and the PF6 -counter-anions, a three-dimensional network is established.

Chromophore-Functionalized Phenanthro-diimine Ligands and Their Re(I) Complexes

A series of diimine ligands has been designed on the basis of 2-pyridyl-1 H-phenanthro[9,10- d]imidazole (L1, L2). Coupling the basic motif of L1 with anthracene-containing fragments affords the bichromophore compounds L3-L5, of which L4 and L5 adopt a donor-acceptor architecture. The latter allows intramolecular charge transfer with intense absorption bands in the visible spectrum (lowest λabs 464 nm (ε = 1.2 × 104 M-1 cm-1) and 490 nm (ε = 5.2 × 104 M-1 cm-1) in CH2Cl2 for L4 and L5, respectively). L1-L5 show strong fluorescence in a fluid medium (Φem = 22-92%, λem 370-602 nm in CH2Cl2); discernible emission solvatochromism is observed for L4 and L5. In addition, the presence of pyridyl (L1-L5) and dimethylaminophenyl (L5) groups enables reversible alteration of their optical properties by means of protonation. Ligands L1-L5 were used to synthesize the corresponding [Re(CO)3X(diimine)] (X = Cl, 1-5; X = CN, 1-CN) complexes. 1 and 2 exhibit unusual dual emission of singlet and triplet parentage, which originate from independently populated 1ππ* and 3MLCT excited states. In contrast to the majority of the reported Re(I) carbonyl luminophores, complexes 3-5 display moderately intense ligand-based fluorescence from an anthracene-containing secondary chromophore and complete quenching of emission from the 3MLCT state presumably due to the triplet-triplet energy transfer (3MLCT → 3ILCT).

In Vitro Anticancer Activity and in Vivo Biodistribution of Rhenium(I) Tricarbonyl Aqua Complexes

Seven rhenium(I) complexes of the general formula fac-[Re(CO)3(NN)(OH2)]+ where NN = 2,2'-bipyridine (8), 4,4'-dimethyl-2,2'-bipyridine (9), 4,4'-dimethoxy-2,2'-bipyridine (10), dimethyl 2,2'-bipyridine-4,4'-dicarboxylate (11), 1,10-phenanthroline (12), 2,9-dimethyl-1,10-phenanthroline (13), or 4,7-diphenyl-1,10-phenanthroline (14), were synthesized and characterized by 1H NMR spectroscopy, IR spectroscopy, mass spectrometry, and X-ray crystallography. With the exception of 11, all complexes exhibited 50% growth inhibitory concentration (IC50) values that were less than 20 μM in HeLa cells, indicating that these compounds represent a new potential class of anticancer agents. Complexes 9, 10, and 13 were as effective in cisplatin-resistant cells as wild-type cells, signifying that they circumvent cisplatin resistance. The mechanism of action of the most potent complex, 13, was explored further by leveraging its intrinsic luminescence properties to determine its intracellular localization. These studies indicated that 13 induces cytoplasmic vacuolization that is lysosomal in nature. Additional in vitro assays indicated that 13 induces cell death without causing an increase in intracellular reactive oxygen species or depolarization of the mitochondrial membrane potential. Further studies revealed that the mode of cell death does not fall into one of the canonical categories such as apoptosis, necrosis, paraptosis, and autophagy, suggesting that a novel mode of action may be operative for this class of rhenium compounds. The in vivo biodistribution and metabolism of complex 13 and its 99mTc analogue 13* were also evaluated in naı̈ve mice. Complexes 13 and 13* exhibited comparable biodistribution profiles with both hepatic and renal excretion. High-performance liquid chromatography inductively coupled plasma mass-spectrometry (HPLC-ICP-MS) analysis of mouse blood plasma and urine postadministration showed considerable metabolic stability of 13, rendering this potent complex suitable for in vivo applications. These studies have shown the biological properties of this class of compounds and demonstrated their potential as promising theranostic anticancer agents that can circumvent cisplatin resistance.

Linking ReI and PtII Chromophores with Aminopyridines: A Simple Route to Achieve a Complicated Photophysical Behavior

The bifunctional aminopyridine ligands H₂ N-(CH₂ )_n -4-C₅ H₄ N (n=0, L1; 1, L2; 2, L3) have been utilized for the preparation of the rhenium complexes [Re(phen)(CO)₃ (L1-L3)]⁺ (1-3; phen=phenanthroline). Complexes 2 and 3 with NH₂ -coordinated L2 and L3, respectively, were coupled with cycloplatinated motifs {Pt(ppy)Cl} and {Pt(dpyb)}⁺ (ppy=2-phenylpyridine, dpyb=dipyridylbenzene) to give the bimetallic species [Re(phen)(CO)₃ (μ-L2/L3)Pt(ppy)Cl]⁺ (4, 6) and [Re(phen)(CO)₃ (μ-L2/L3)Pt(dpyb)]²⁺ (5, 7). In solution, complexes 4 and 6 show ³ MLCT {Re}-based emission at 298 K, which changes to the ³ IL(ppy) state at 77 K. The photophysical properties of compounds 5 and 7 display a pronounced concentration dependence, presumably due to the formation of bimolecular aggregates. Analysis of the spectroscopic data, combined with TD-DFT simulations, suggest that unconventional heteroleptic {Re(phen)}⋅⋅⋅{Pt(dpyb)} π-π stacking operates as the driving force for ground-state association. The latter, together with intra- and intermolecular energy-transfer processes, determines the appearance of multiple emission bands and results in nonlinear relaxation kinetics of the excited states.

A study on the inhibition of dihydrofolate reductase (DHFR) from Escherichia coli by gold(i) phosphane compounds. X-ray crystal structures of (4,5-dichloro-1H-imidazolate-1-yl)-triphenylphosphane-gold(i) and (4,5-dicyano-1H-imidazolate-1-yl)-triphenylphosphane-gold(i)

An unprecedented study on the inhibitory activities of a class of phosphane gold(i) complexes on E. coli dihydrofolate reductase (DHFR) is reported. The gold(i) complexes considered in this work consist of azolate or chloride ligands and phosphane as co-ligands. The ligands have been functionalized with polar groups (-COOH, -COO(-), NO2, Cl, CN) to obtain better solubility in polar media. Neutral, anionic and cationic gold(i) complexes have been tested as DHFR inhibitors by means of a continuous direct spectrophotometric method. X-ray structural characterizations were performed on ((triphenylphosphine)-gold(i)-(4,5-dicyanoimidazolyl-1H-1yl) and on the analog (triphenylphosphine)-gold(i)-(4,5-dichloroimidazolyl-1H-1yl). The inhibition constants obtained from the enzyme tests range from 20 μM to 63 nM (auranofin) and are conducive to promoting these compounds as potential DHFR inhibitors.

A pair of dinuclear Re(i) enantiomers: synthesis, crystal structures, chiroptical and ferroelectric properties

A pair of dinuclear Re(i) enantiomers have been successfully synthesized, representing the first example of polynuclear Re(i) complexes with ferroelectric properties and possessing a multifunctional feature combining chirality, NLO-activity and ferroelectricity within one molecule.

Two different one-dimensional supramolecular chains formed from the reaction of 2-[1-(pyridin-4-ylmethyl)-1H-benzimidazol-2-yl]quinoline with two different precursors, Co(NO3)2 and CoCl2

Two different one-dimensional supramolecular chains with Co(II) cations have been synthesized based on the semi-rigid ligand 2-[1-(pyridin-4-ylmethyl)-1H-benzimidazol-2-yl]quinoline (L), obtained by condensation of 2-(1H-benzimidazol-2-yl)quinoline and 4-(chloromethyl)pyridine hydrochloride. Starting from different Co(II) salts, two new compounds have been obtained, viz. catena-poly[[[dinitratocobalt(II)]-μ-2-[1-(pyridin-4-ylmethyl)-1H-benzimidazol-2-yl]quinoline] dichloromethane monosolvate acetonitrile monosolvate], {[Co(NO3)2(C22H16N4)]·CH2Cl2·CH3CN}n, (I) and catena-poly[[[dichloridocobalt(II)]-μ-2-[1-(pyridin-4-ylmethyl)-1H-benzimidazol-2-yl]quinoline] methanol disolvate], {[CoCl2(C22H16N4)]·2CH3OH}n, (II). In (I), the Co(II) centres lie in a distorted octahedral [CoN3O3] coordination environment. {Co(NO3)2L}n units form one-dimensional helical chains, where the L ligand has different directions of twist. The helical chains stack together via interchain π-π interactions to form a two-dimensional sheet, and another type of π-π interaction further connects neighbouring sheets into a three-dimensional framework with hexagonal channels, in which the acetonitrile molecules and disordered dichloromethane molecules are located. In (II), the Co(II) centres lie in a distorted trigonal-bipyramidal [CoCl2N3] coordination environment. {CoCl2L}n units form one-dimensional chains. The chains interact via C-H···π and C-H···Cl interactions. The result is that two-dimensional sheets are generated, which are further linked into a three-dimensional framework via interlayer C-H···Cl interactions. When viewed down the crystallographic b axis, the methanol solvent molecules are located in an orderly manner in wave-like channels.