Transition metal and organometallic anion complexation agents

Introduction of Ferrocene as a Facilitator for the Construction of Supramolecular Polymers

Proper monomer design is the key to enhancing the strength of noncovalent interactions between the molecules toward the efficient formation of supramolecular polymers (SPs). We have designed and synthesized 1,n'-disubstituted ferrocene-azobenzene-long alkyl chains, Fc(CONH-Azo-TDP)₂ , to afford SPs with a high probability. The design exploits the ''molecular ball-bearing'' property of the ferrocene core, which allows two azobenzene arms to rotate in the planes of cyclopentadienyl rings, generating the most suitable molecular conformation required for SP formation. This ferrocene monomer formed a supergel consisting of SPs supported by strong intermolecular (H-bonding and π-π stacking) interactions and higher enthalpy gain than the reference molecules, where the central ferrocene core was replaced by flexible aliphatic as well as rigid benzene linkers. The molecular conformation involved in SPs, the strength of noncovalent interactions, and the process of supramolecular polymerization were investigated through NMR, UV-Vis, XRD and TEM studies. The results demonstrate that ferrocene may act as a good modulator for constructing efficient SPs.

Forever young: the first seventy years of ferrocene

The discovery of ferrocene, [Fe(η5-C5H5)2], seventy years ago has significantly influenced chemical research and provided a key impetus for establishing and rapidly expanding organometallic chemistry, which has continued at a...

Computational Discovery of Transition-metal Complexes: From High-throughput Screening to Machine Learning

Transition-metal complexes are attractive targets for the design of catalysts and functional materials. The behavior of the metal-organic bond, while very tunable for achieving target properties, is challenging to predict and necessitates searching a wide and complex space to identify needles in haystacks for target applications. This review will focus on the techniques that make high-throughput search of transition-metal chemical space feasible for the discovery of complexes with desirable properties. The review will cover the development, promise, and limitations of "traditional" computational chemistry (i.e., force field, semiempirical, and density functional theory methods) as it pertains to data generation for inorganic molecular discovery. The review will also discuss the opportunities and limitations in leveraging experimental data sources. We will focus on how advances in statistical modeling, artificial intelligence, multiobjective optimization, and automation accelerate discovery of lead compounds and design rules. The overall objective of this review is to showcase how bringing together advances from diverse areas of computational chemistry and computer science have enabled the rapid uncovering of structure-property relationships in transition-metal chemistry. We aim to highlight how unique considerations in motifs of metal-organic bonding (e.g., variable spin and oxidation state, and bonding strength/nature) set them and their discovery apart from more commonly considered organic molecules. We will also highlight how uncertainty and relative data scarcity in transition-metal chemistry motivate specific developments in machine learning representations, model training, and in computational chemistry. Finally, we will conclude with an outlook of areas of opportunity for the accelerated discovery of transition-metal complexes.

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.

Supersensitive Detection of Anions in Pure Organic and Aqueous Media by Amino Acid Conjugated Ellman's Reagent

The last few decades witnessed a remarkable advancement in the field of molecular anion receptors. A variety of anion binding motifs have been discovered, and large number of designer molecular anion receptors with high selectivity are being reported. However, anion detection in an aqueous medium is still a formidable challenge as evident from only a miniscule of synthetic systems available in the literature. We, herein, report 5,5'-dithio-bis(2-nitrobenzoic acid) (Ellman's reagent) appended with amino acids as supersensitive anion sensors that can detect F^- and H₂PO₄^- ions in both aqueous as well as organic media. Interestingly, the sensors showed a dual response to anions, viz., chromogenic response in organic medium and electrochemical response in aqueous solutions. Various spectroscopic techniques such as UV-vis and ¹H NMR are used to investigate the binding studies in acetonitrile, whereas electrochemical methods such as cyclic voltammetry (CV) and differential pulse voltammetry (DPV) are employed to explore the anion binding in water. The host-guest complex stoichiometry and binding constants are calculated using the BindFit software. The geometry of host-guest complex has been optimized by the density functional theory (DFT) method. These molecules are versatile sensors since these function in both water and acetonitrile with extremely low limit of detection (LOD) up to 0.07 fM and limit of quantification (LOQ) up to 0.23 fM. To our knowledge, the present system is the first example of a sensor that can detect the lowest concentration of anions in water quantitatively. The minimalistic design strategy presented here opens up the innumerable possibilities for designing dual anion sensors in a one fell swoop.

Mixed-ligand lanthanide complexes supported by ditopic bis(imino-methyl)-phenol/calix[4]arene macrocycles: synthesis, structures, and luminescence properties of [Ln2(L2)(MeOH)2] (Ln = La, Eu, Tb, Yb)

The lanthanide binding ability of a macrocyclic ligand H₆L² comprising two bis(iminomethyl)phenol and two calix[4]arene units has been studied. H₆L² is a ditopic ligand which provides dinuclear neutral complexes of composition [Ln₂(L²)(MeOH)₂] (Ln = La (1), Eu (2), Tb (3), and Yb (4)) in very good yield. X-ray crystal structure analyses for 2 and 3 show that (L²)^6- accommodates two seven coordinated lanthanide ions in a distorted monocapped trigonal prismatic/octahedral coordination environment. UV-vis spectroscopic titrations performed with La³⁺, Eu³⁺, Tb³⁺ and Yb³⁺ ions in mixed MeOH/CH₂Cl₂ solution (I = 0.01 M NBu₄PF₆) reveal that a 2 : 1 (metal : ligand) stoichiometry is present in solution, with log K₁₁ and K₂₁ values ranging from 5.25 to 6.64. The ratio α = K₁₁/K₂₁ of the stepwise formation constants for the mononuclear (L² + M = ML², log K₁₁) and the dinuclear complexes (ML² + M = M₂L², log K₂₁) was found to be invariably smaller than unity indicating that the binding of the first Ln³⁺ ion augments the binding of the second Ln³⁺ ion. The present complexes are less luminescent than other seven-coordinated Eu and Tb complexes, which can be traced to vibrational relaxation of excited Eu^III and Tb^III states by the coligated MeOH and H₂O molecules and/or low-lying ligand-to-metal charge-transfer (LMCT) states.

Polymeric Planar Microcavities Doped with a Europium Complex

Organo-metallic europium complex tetrakis (dibenzoyl methide) triethylammonium (EuD4TEA) shows a sharp emission spectrum, which makes it interesting for photonic applications. In this work, we embedded it into all-polymeric planar microcavities and investigated the effect of the photonic environment on its emission spectrum. To this end, submicron-sized EuD4TEA crystals were loaded into a blend of polystyrene and carboxylic terminated polystyrene matrix, which served to stabilize the emitter in the polymer and to make the composite processable. The new composite was then casted by spin-coating as a defect layer in a polymeric planar microcavity. Spectroscopic studies demonstrate that fine spectral tuning of the cavity mode on the sharp organometal luminescence is possible and produces spectral redistribution of the fluorophore emission, along with a remarkable cavity quality factor.

A simple benzimidazole styryl-based colorimetric chemosensor for dual sensing application

A new colorimetric styryl-benzimidazole based receptor to recognize more than one analyte trans-2-[4'-(dimethylamino)styryl]benzimidazole) (L1) has been synthesized and fully characterized by ¹³C and ¹H NMR, elemental analysis, UV-vis spectroscopy, and HRMS. Investigation of sensing ability of receptor L1 was carried out in presence of multiple anions (Br^-, CN^-, Cl^-, ClO₄^-, F^-, HSO₄^-, PF₆^-, NO₃^-, S^2-, OH^-, AcO^- and H₂PO₄^-) and cations (Cu²⁺, Cr³⁺, Al³⁺, Mg²⁺, Cd²⁺, Ni²⁺, Fe³⁺, K⁺, Fe²⁺, Mn²⁺, Ag⁺, Hg²⁺, Ca²⁺, Co²⁺, Pb²⁺, Na⁺, and Zn²⁺) by using UV-vis spectroscopy. Receptor L1 showed colorimetric response towards only for HSO₄^- ion. Receptor L1-HSO₄^- interaction confirmed with the help of ¹H NMR titration. Among various cations, L1 selectively sense the Cu²⁺ and Al³⁺ with the drastic colour change from yellow to green and dark yellow respectively. The stoichiometric binding ratio of L1 with HSO₄^-, Cu²⁺, and Al³⁺ found to be 1:1 by jobs method and HRMS data proved the complex formation between L1 and Cu²⁺/Al³⁺ with very low detection limit. In addition to explore practical applicability of L1, paper strips have been made and used to detect HSO₄^- and Cu²⁺ ions, respectively, up to 10 ppm level.

Dinuclear helicate and tetranuclear cage assembly using appropriately designed ditopic triazole-azine ligands

Architecture, helicate or cage, is controlled by choice of meta vs. para phenylene linker in new, robust, ditopic triazole-pyrimidine ligands.

Dinuclear lanthanide complexes supported by a hybrid salicylaldiminato/calix[4]arene-ligand: synthesis, structure, and magnetic and luminescence properties of (HNEt3)[Ln2(HL)(L)] (Ln = SmIII, EuIII, GdIII, TbIII)

Salicylaldimine/calix[4]arenes support dinuclear, triply bridged, luminescent, lanthanide complexes.

Recent Trends in Metallopolymer Design: Redox-Controlled Surfaces, Porous Membranes, and Switchable Optical Materials Using Ferrocene-Containing Polymers

Metallopolymers with metal functionalities are a unique class of functional materials. Their redox-mediated optoelectronic and catalytic switching capabilities, their outstanding structure formation and separation capabilities have been reported recently. Within this Minireview, the scope and limitations of intriguing ferrocene-containing systems will be discussed. In the first section recent advances in metallopolymer design will be given leading to a plethora of novel metallopolymer architectures. Discussed synthetic pathways comprise controlled and living polymerization protocols as well as surface immobilization strategies. In the following sections, we focus on recent advances and new applications for side-chain and main-chain ferrocene-containing polymers as (i) remote-switchable materials, (ii) smart surfaces, (iii) redox-responsive membranes, and some recent trends in (iv) photonic structures and (v) other optical applications.

A hydrazine-based thiocarbamide probe for colorimetric and turn-on fluorometric detection of PO43− and AsO33− in semi-aqueous medium

20% of the population in West Bengal, India is at risk to fatal levels of arsenic. Easy detection of As(iii) is demonstrated by the naked eye using a hydrazine-based thiocarbamide reagent. Limit of detection is 15 nM (AsO₃³⁻) at pH 8–12.

A pyrene-benzimidazole composed effective fluoride sensor: potential mimicking of a Boolean logic gate

A highly selective fluoride sensor based on a pyrene benzimidazole unit was developed and studied for recyclable memory function.

A novel ferrocenyl–naphthalimide as a multichannel probe for the detection of Cu(ii) and Hg(ii) in aqueous media and living cells

The first reported probe that has been used for the bifunctional fluorescent monitoring Cu²⁺ and Hg²⁺ ions in living cells.

Highly sensitive and selective detection of Pd2+ ions using a ferrocene–rhodamine conjugate triple channel receptor in aqueous medium and living cells

A novel ferrocene–rhodamine conjugate receptor has been developed for efficient triple-channel detection of Pd²⁺ in aqueous medium and living cells.

Optical Sensing of Anions via Supramolecular Recognition with Biimidazole Complexes

Phosphorescent metal complexes with peripheral N-H donor functionalities have attracted great attention as potential molecular sensing units for anionic species lately. In this contribution we discuss the development and potential of anion recognition and sensing features of recent examples of luminescent 2,2'-biimidazole complexes of ruthenium(II), iridium(III), osmium(II) and cobalt(III). The general dependency of photophysical features in these complexes regarding the acid-base chemistry of the peripheral N-H functionalities will be outlined as a basic requirement for optical ion recognition. Systematic strategies for the tuning and specific improvement by synthetic means will be discussed regarding recent reports. With respect to their distinct photophysical features, different transition metals are considered individually to demonstrate particular trends regarding ligand modification within the respective groups. In summary, this review elucidates the current state-of-the-art and future potential of the versatile class of 2,2'-biimidazole based sensor chromophores.

Ferrocene-Containing Acylhydrazone Receptors: Synthesis, Structures and Electrochemical Anion Recognition Characteristics

Ferrocene-containing redox receptors bearing the 2-(quinolin-8-yloxy)acetohydrazide (qa) moiety on one arm, [Fe(Cpqa)Cp], or two arms, [Fe(Cpqa)2], have been synthesised and the X-ray crystal structure of [Fe(Cpqa)2] determined. [Fe(Cpqa)2] can sense H2PO−4 both electrochemically and selectively even in the presence of a large excess of Cl− and ClO−4.

A neutral halogen bonding macrocyclic anion receptor based on a pseudocyclopeptide with three 5-iodo-1,2,3-triazole subunits

The converging arrangement of iodine atoms along a confined cavity causes a cyclic pseudopeptide with three 5-iodo-1,2,3-triazole subunits to interact with halides, in particular with chloride, in 2.5 vol% water/DMSO.

Insights into bonding interactions and excitation energies of 3d-4f mixed lanthanide transition metal macrocyclic complexes

In this contribution, a computational study of equatorial bound tetranuclear macrocycle (butylene linked) [LnZn(HOM^Bu)]³⁺ (Ln = La³⁺, Ce³⁺) complexes was carried out. Here, the electronic structure, bonding interaction and excitation energies were studied within the relativistic density functional theory framework. From the electronic structure analysis, the frontier molecular orbitals (FMOs) were strongly localized in the d-orbitals of the Zn centers and the f-orbitals of the lanthanide ions. Besides, the inner MOs were found to exhibit a π-character from the organic part of the macrocyclic chain. EDA-NOCV was used as a tool for evaluating the bonding interaction, taking the trinuclear metallomacrocycle (ZnHOM^Bu) and the lanthanide center as fragments. This analysis showed that the interaction between these fragments was slightly covalent; with this covalency being the result of a charge transfer from the metallomacrocyclic ring to the lanthanide. This phenomenon was observed in the deformation density channels obtained from the EDA-NOCV study; in which π- and σ-charge transfer was observed. Finally, the TD-DFT study of the excitation energies evidenced three sets of bands: the first set with the highest intensity represented the ligand to metal charge transfer bands; the second set could be attributed to the 3d-4f electronic transitions between the metal centers; and the third set represented the f-f bands found for the open-shell cerium complex. This class of complexes accomplishes the "antenna effect" principle, which states that highly absorptive transition-metal (TM) complexes can be used to enhance the luminescence of poorly emissive systems, and are introduced in this study as self-sensitizer bimetallic d-f systems with potential applications in near infra-red (NIR) technologies.