Metal Complexes In Solution Research Articles

Dealuminated H–Y zeolites generate, stabilize and catalytically insert carbenes from diazocarbonyl compounds

Carbenes are among the most powerful reactants in organic synthesis, with capacity to insert into a variety of otherwise stable bonds, and generate two new bonds in a straightforward manner. However, the intrinsic instability of such carbenes makes them to be catalytically generated, in–situ, from precursors such as diazocarbonyl compounds, and the catalyst, in turn, also controls the subsequent insertion reaction. The catalyst is generally a metal complex in solution, mainly Cu, Ag or Rh, but also others, including protons in rare cases. Here we show that carbenes are generated, stabilized and inserted into C–C, C–H, O–H, N–H, Si–H and O–O bonds after reacting diazocarbonyl compounds with catalytic amounts of metal–free, commercially available dealuminated H–Y zeolites. These results open the way to design carbene–mediated organic reactions on readily available and reusable catalytic solids without involving metals.

Enter MnIV-NHC: A Dark Photooxidant with a Long-Lived Charge-Transfer Excited State.

Detailed photophysical investigation of a Mn(IV)-carbene complex has revealed that excitation into its lowest-energy absorption band (∼500 nm) results in the formation of an energetic ligand-to-metal charge-transfer (LMCT) state with a lifetime of 15 ns. To the best of our knowledge, this is the longest lifetime reported for charge-transfer states of first-row-based transition metal complexes in solution, barring those based on Cu, with a d10 configuration. A so-called superoxidant, Mn(IV)-carbene exhibits an excited state potential typically only harnessed via excited states of reactive organic radical species. Furthermore, the long-lived excited state in this case is found to be a dark doublet, with its transition to the quartet ground state being spin-forbidden, a contrast to most first-row literature examples, and a possible cause of the long lifetime. Showcasing excited state properties which in some cases exceed those of complexes based on precious metals, these findings not only advance the library of earth-abundant photosensitizers but also shed general insight into the photophysics of d3 and related Mn complexes.

Probing Homogeneous Catalysts and Precatalysts in Solution by Exchange-Mediated Overhauser Dynamic Nuclear Polarization NMR.

Triphenylphosphine (PPh3) is a ubiquitous ligand in organometallic chemistry that has been shown to give enhanced 31P NMR signals at high magnetic field via a scalar-dominated Overhauser effect dynamic nuclear polarization (OE DNP). However, PPh3 can only be polarized via DNP in the free form, while the coordinated form is DNP-inactive. Here, we demonstrate the possibility of enhancing the 31P NMR signals of coordinated PPh3 in metal complexes in solution at room temperature by combining Overhauser effect DNP and chemical exchange between the free and coordinated PPh3 forms. With this method, we successfully obtain 31P DNP enhancements of up to 2 orders of magnitude for the PPh3 ligands in Rh(I), Ru(II), Pd(II), and Pt(II) complexes, and we show that the DNP enhancements can be used to determine the activation energy of the ligand exchange reaction.

Thermodynamics of complex formation of silver(I) with substituted pyridines and cyclic amines in non-aqueous solvents

The understanding of the thermodynamic stability and speciation of metal complexes in solution requires access to their enthalpy and entropy of formation. In this work, we specifically focus our investigation on the complexation process of silver(I) ion in acetonitrile (AN) with substituted mono pyridines and cyclic monoamines. The aim of this study is to provide reliable thermodynamic data to obtain insights on metal complex formation, focusing on ligands donor properties and solvation effects. Carefully designed potentiometric and calorimetric experiments allowed to define the species present at different ligand/metal ratios and to obtain the complex formation constants and enthalpies. In general, the enthalpy terms associated with the complex formation are highly exothermic, while the entropy values are always unfavorable. The formation constants of AgLj species for the ligands investigated in AN are compared with those previously obtained in dimethyl sulfoxide (DMSO) and water. The trends in stability constants and enthalpy values are discussed in relation to the pKa data available in the different solvents. Higher pKa values correspond to greater ligand basicity and result in more stable and more enthalpy stabilized complexes.

THERMODYNAMICS OF INTERMOLECULAR INTERACTION REACTIONS OF BIOMOLECULES IN WATER AND WATER-ORGANIC SOLVENTS

The paper summarizes the results of our own research into the formation of molecular complexes of cyclodextrins with biologically active polyphenol compounds in water-organic solvents, chemical equilibria in the solutions of hydrazones and Schiff bases and their metal complexes in the solutions of DNA and proteins. It was found that an increase in the content of the non-aqueous component in the binary solvent leads to a decrease in the stability of molecular complexes of cyclodextrins with polyphenols and a decrease in the exothermicity of their formation reactions. It has been shown that metal complexes in an aqueous buffer solution containing also DNA or protein dissociate, and the released cation and free ligand bind to the biomolecule. For citation: Usacheva T.R., Gamov G.A., Kuranova N.N., Zavalishin M.N., Kabirov D.N., Alister D.A., Grazhdan K.V., Gushchina A.S., Isaeva V.A., Kashina O.V., Kuzmina I.A., Tukumova N.V., Sharnin V.A. Thermodynamics of intermolecular interaction reactions of biomolecules in water and water-organic solvents. ChemChemTech [Izv. Vyssh. Uchebn. Zaved. Khim. Khim. Tekhnol.]. 2023. V. 66. N 7. P. 59-75. DOI: 10.6060/ivkkt.20236607. 6842j.

CH vs. HC-Promiscuous Metal Sponges in Antimicrobial Peptides and Metallophores.

Histidine and cysteine residues, with their imidazole and thiol moieties that deprotonate at approximately physiological pH values, are primary binding sites for Zn(II), Ni(II) and Fe(II) ions and are thus ubiquitous both in peptidic metallophores and in antimicrobial peptides that may use nutritional immunity as a way to limit pathogenicity during infection. We focus on metal complex solution equilibria of model sequences encompassing Cys-His and His-Cys motifs, showing that the position of histidine and cysteine residues in the sequence has a crucial impact on its coordination properties. CH and HC motifs occur as many as 411 times in the antimicrobial peptide database, while similar CC and HH regions are found 348 and 94 times, respectively. Complex stabilities increase in the series Fe(II) < Ni(II) < Zn(II), with Zn(II) complexes dominating at physiological pH, and Ni(II) ones-above pH 9. The stabilities of Zn(II) complexes with Ac-ACHA-NH2 and Ac-AHCA-NH2 are comparable, and a similar tendency is observed for Fe(II), while in the case of Ni(II), the order of Cys and His does matter-complexes in which the metal is anchored on the third Cys (Ac-AHCA-NH2) are thermodynamically stronger than those where Cys is in position two (Ac-ACHA-NH2) at basic pH, at which point amides start to take part in the binding. Cysteine residues are much better Zn(II)-anchoring sites than histidines; Zn(II) clearly prefers the Cys-Cys type of ligands to Cys-His and His-Cys ones. In the case of His- and Cys-containing peptides, non-binding residues may have an impact on the stability of Ni(II) complexes, most likely protecting the central Ni(II) atom from interacting with solvent molecules.

Electrospray ionization developed by Lidija Gall's group

This historical review is dedicated to Prof. Lidija Gall. She was awarded the Thomson medal (2022) for her contribution to the development of electrospray ionization mass spectrometry in the early 1980s in Leningrad, Soviet Union. We describe the rationale and provide historical references that highlight her way to the discovery of a method of solvated ion extraction at atmospheric pressure (SIEAP), known by the Russian abbreviation ЭРИАД (ERIAD), which is the direct analogue of electrospray ionization developed in the West. Even at the early prototype stage, ERIAD was able to determine the intact masses of a broad scope of polar biomolecules including peptides, small proteins, nucleotides, carbohydrates, and prostanglandins. The ERIAD source was directly coupled to microbore liquid chromatography and was applied to analyze protein digests, to characterize the substrate specificity of proteolytic enzymes, to mapping of mutation sites in physiologically important proteins and also to increase the confidence in protein sequence determinations. In inorganic chemistry, ERIAD was successfully applied to analyze the solutions of rare-earth metals and metal complexes. Altogether, a set of experimental studies and a broad scope of applications contributed to better understanding of the ionization mechanism and the methods needed to acquire and interpret ERIAD spectra. The method was first implemented on a magnetic sector mass spectrometer and then was adopted to a variety of mass analyzers and mobility separators in collaboration with Russian mass spectrometry groups.

The Nephelauxetic Effect Becomes an Important Design Factor for Photoactive First‐Row Transition Metal Complexes

AbstractThe expansion of d‐orbitals as a result of metal‐ligand bond covalence, the so‐called nephelauxetic effect, is a well‐established concept of coordination chemistry, yet its importance for the design of new photoactive complexes based on first‐row transition metals is only beginning to be recognized. Until recently, much focus has been on optimizing the ligand field strength, coordination geometries, and molecular rigidity, but now it becomes evident that the nephelauxetic effect can be a game changer regarding the photophysical properties of 3d metal complexes in solution at room temperature. In CrIII and MnIV complexes with the d3 valence electron configuration, the nephelauxetic effect was exploited to shift the well‐known ruby‐like red luminescence to the near‐infrared spectral region. In FeII and CoIII complexes with the low‐spin d6 electron configuration, charge‐transfer excited states were stabilized with respect to detrimental metal‐centered excited states, to improve their properties and to enhance their application potential. In isoelectronic (3d6) isocyanide complexes of Cr0 and MnI, the nephelauxetic effect is likely at play as well, enabling luminescence and other favorable photoreactivity. This minireview illustrates the broad applicability of the nephelauxetic effect in tailoring the photophysical and photochemical properties of new coordination compounds made from abundant first‐row transition metals.

The Nephelauxetic Effect Becomes an Important Design Factor for Photoactive First-Row Transition Metal Complexes.

The expansion of d-orbitals as a result of metal-ligand bond covalence, the so-called nephelauxetic effect, is a well-established concept of coordination chemistry, yet its importance for the design of new photoactive complexes based on first-row transition metals is only beginning to be recognized. Until recently, much focus has been on optimizing the ligand field strength, coordination geometries, and molecular rigidity, but now it becomes evident that the nephelauxetic effect can be a game changer regarding the photophysical properties of 3d metal complexes in solution at room temperature. In CrIII and MnIV complexes with the d3 valence electron configuration, the nephelauxetic effect was exploited to shift the well-known ruby-like red luminescence to the near-infrared spectral region. In FeII and CoIII complexes with the low-spin d6 electron configuration, charge-transfer excited states were stabilized with respect to detrimental metal-centered excited states, to improve their properties and to enhance their application potential. In isoelectronic (3d6 ) isocyanide complexes of Cr0 and MnI , the nephelauxetic effect is likely at play as well, enabling luminescence and other favorable photoreactivity. This minireview illustrates the broad applicability of the nephelauxetic effect in tailoring the photophysical and photochemical properties of new coordination compounds made from abundant first-row transition metals.

Operando time-resolved soft x-ray absorption spectroscopy for photoexcitation processes of metal complexes in solutions.

Operando time-resolved soft x-ray absorption spectroscopy (TR-SXAS) is an effective method to reveal the photochemical processes of metal complexes in solutions. In this study, we have developed the TR-SXAS measurement system for observing various photochemical reactions in solutions by the combination of laser pump pulses with soft x-ray probe pulses from the synchrotron radiation. For the evaluation of the developed TR-SXAS system, we have measured nitrogen K-edge x-ray absorption spectroscopy (XAS) spectra of aqueous iron phenanthroline solutions during a photoinduced spin transition process. The decay process of the high spin state to the low spin state in the iron complex has been obtained from the ligand side by N K-edge XAS, and the time constant is close to that obtained from the central metal side by time-resolved Fe K-edge XAS in the previous studies.

Kinetics of ligand exchange in solution: a quantitative mass spectrometry approach.

Complex speciation and exchange kinetics of labile ligands are critical parameters for understanding the reactivity of metal complexes in solution. We present a novel approach to determine ligand exchange parameters based on electrospray ionization mass spectrometry (ESI-MS). The introduction of isotopically labelled ligands to a solution of metal host and unlabelled ligands allows the quantitative investigation of the solution-phase equilibria. Furthermore, ion mobility separation can target individual isomers, such as ligands bound at specific sites. As a proof of concept, we investigate the solution equilibria of labile pyridine ligands coordinated in the cavity of macrocyclic porphyrin cage complexes bearing diamagnetic or paramagnetic metal centres. The effects of solvent, porphyrin coordination sphere, transition metal, and counterion on ligand dissociation are discussed. Rate constants and activation parameters for ligand dissociation in the solution can be derived from our ESI-MS approach, thereby providing mechanistic insights that are not easily obtained from traditional solution-phase techniques.

Structures of Hydrated Metal Ions in Solid State and Aqueous Solution

This review article summarizes the reported crystallographically determined structures of compounds containing a hydrated metal ion and the reported structures of hydrated metal ions in aqueous solution. A short overview of the methods available to study structures of metal complexes in solution is given.

Entropy and Enthalpy Effects on Metal Complex Formation in Non-Aqueous Solvents: The Case of Silver(I) and Monoamines.

Access to the enthalpy and entropy of the formation of metal complexes in solution is essential for understanding the factors determining their thermodynamic stability and speciation. As a case study, in this report we systematically examine the complexation of silver(I) in acetonitrile (AN) with the following monoamines: n-propylamine (n-pr), n-butylamine (n-but), hexylamine (hexyl), diethylamine (di-et), dipropylamine (di-pr), dibutylamine (di-but), triethylamine (tri-et) and tripropylamine (tri-pr). The study shows that the complex stabilities are quite independent of the length of the substitution chain on the N atom and demonstrates that, in general, the overall enthalpy terms associated with the complex formation are strongly exothermic, whereas the entropy values oppose the complex formations. In addition, we examined the similarity of the formation constants of AgL complexes of the primary monoamines in AN, dimethylsulfoxide (DMSO) and water, which were unexpected on the basis of the difference between the donor properties of solvents.

Resolving Femtosecond Solvent Reorganization Dynamics in an Iron Complex by Nonadiabatic Dynamics Simulations.

The ultrafast dynamical response of solute–solvent interactions plays a key role in transition metal complexes, where charge transfer states are ubiquitous. Nonetheless, there exist very few excited-state simulations of transition metal complexes in solution. Here, we carry out a nonadiabatic dynamics study of the iron complex [Fe(CN)4(bpy)]2– (bpy = 2,2′-bipyridine) in explicit aqueous solution. Implicit solvation models were found inadequate for reproducing the strong solvatochromism in the absorption spectra. Instead, direct solute–solvent interactions, in the form of hydrogen bonds, are responsible for the large observed solvatochromic shift and the general dynamical behavior of the complex in water. The simulations reveal an overall intersystem crossing time scale of 0.21 ± 0.01 ps and a strong reliance of this process on nuclear motion. A charge transfer character analysis shows a branched decay mechanism from the initially excited singlet metal-to-ligand charge transfer (1MLCT) states to triplet states of 3MLCT and metal-centered (3MC) character. We also find that solvent reorganization after excitation is ultrafast, on the order of 50 fs around the cyanides and slower around the bpy ligand. In contrast, the nuclear vibrational dynamics, in the form of Fe–ligand bond changes, takes place on slightly longer time scales. We demonstrate that the surprisingly fast solvent reorganizing should be observable in time-resolved X-ray solution scattering experiments, as simulated signals show strong contributions from the solute–solvent scattering cross term. Altogether, the simulations paint a comprehensive picture of the coupled and concurrent electronic, nuclear, and solvent dynamics and interactions in the first hundreds of femtoseconds after excitation.

Silver(I) and gold(I) complexes of multidentate ligands based on functionalized pyridine

The new ligands 2,6-bis[{2′-(N,N-dialkylaminomethyl)phenylselanyl}methyl]pyridine with the general formula 2,6-[{2′-(R2NCH2)C6H4}SeCH2]2C5H3N [R = Me (L1), Et (L2)], based on pyridine functionalized with organoselenium groups, were obtained by reacting 2-(R2NCH2)C6H4SeNa with 2,6–bis(bromomethyl)pyridine in 2:1 molar ratio. The coordination behaviour of the potentially multidentate ligands was investigated towards gold(I) and silver(I) in reactions with AgOTf (OTf = CF3SO3), AgPF6 and (C6F5)Au(tht) (tht = tetrahydrothiophene), when the metal complexes [AgL]X [X = OTf, L = L1 (1), L2 (2); X = PF6, L = L1 (3), L2 (4)], [(C6F5Au)2L1] (5) and [(C6F5)AuL2] (6), were isolated. Multinuclear NMR spectroscopy (1H, 13C, 19F, 31P, 77Se, as appropriate) and mass spectrometry were employed to investigate the two ligands and the metal complexes in solution. The single crystal X-ray diffraction studies revealed a N,Se,N,Se,N-monometallic pentaconnective (κ2Se1,Se2-κ3N1,N2,N3) coordination behaviour of L1 in 1, a Se,N- bimetallic biconnective (bridging) [μ(Au-Au)(1κ1N1-2κ1Se2) coordination behaviour of L1 in 5 and a Se,N,Se-monometallic triconnective (pincer like, κ2Se1,Se2-κN1) pattern of L2 in 6, while in the free ligands the nitrogen atoms of the two pendant arms are intramolecularly coordinated to selenium. In solid state, both 5 and 6 display metallophilic Au‧‧‧Au interactions.

213Bi]Bi3+/[111In]In3+-neunpa-cycMSH: Theranostic Radiopharmaceutical Targeting Melanoma─Structural, Radiochemical, and Biological Evaluation.

With the emergence of [225Ac]Ac3+ as a therapeutic radionuclide for targeted α therapy (TAT), access to clinical quantities of the potent, short-lived α-emitter [213Bi]Bi3+ (t1/2 = 45.6 min) will increase over the next decade. With this in mind, the nonadentate chelator, H4neunpa-NH2, has been investigated as a ligand for chelation of [213Bi]Bi3+ in combination with [111In]In3+ as a suitable radionuclidic pair for TAT and single photon emission computed tomography (SPECT) diagnostics. Nuclear magnetic resonance (NMR) spectroscopy was utilized to assess the coordination characteristics of H4neunpa-NH2 on complexation of [natBi]Bi3+, while the solid-state structure of [natBi][Bi(neunpa-NH3)] was characterized via X-ray diffraction (XRD) studies, and density functional theory (DFT) calculations were performed to elucidate the conformational geometries of the metal complex in solution. H4neunpa-NH2 exhibited fast complexation kinetics with [213Bi]Bi3+ at RT achieving quantitative radiolabeling within 5 min at 10-8 M ligand concentration, which was accompanied by the formation of a kinetically inert complex. Two bioconjugates incorporating the melanocortin 1 receptor (MC1R) targeting peptide Nle-CycMSHhex were synthesized featuring two different covalent linkers for in vivo evaluation with [213Bi]Bi3+ and [111In]In3+. High molar activities of 7.47 and 21.0 GBq/μmol were achieved for each of the bioconjugates with [213Bi]Bi3+. SPECT/CT scans of the [111In]In3+-labeled tracer showed accumulation in the tumor over time, which was accompanied by high liver uptake and clearance via the hepatic pathway due to the high lipophilicity of the covalent linker. In vivo biodistribution studies in C57Bl/6J mice bearing B16-F10 tumor xenografts showed good tumor uptake (5.91% ID/g) at 1 h post-administration with [213Bi][Bi(neunpa-Ph-Pip-Nle-CycMSHhex)]. This study demonstrates H4neunpa-NH2 to be an effective chelating ligand for [213Bi]Bi3+ and [111In]In3+, with promising characteristics for further development toward theranostic applications.

Evaluation of Enantioselective Ligand Exchange Equilibrium of Organic Metal Complex in Solution by Electrospray Ionization Mass Spectrometry

Evaluation of Enantioselective Ligand Exchange Equilibrium of Organic Metal Complex in Solution by Electrospray Ionization Mass Spectrometry

Reaction of Polymerisation Initiators with Metal Salts and Metal Complexes in Solutions. Interaction of Potassium Monopersulphate with Copper(II) Salts and Copper(II)/Hydroquinone Couple

Reaction of Polymerisation Initiators with Metal Salts and Metal Complexes in Solutions. Interaction of Potassium Monopersulphate with Copper(II) Salts and Copper(II)/Hydroquinone Couple

The Reactivity of the Metal Oxide‐Water and Mineral‐Water Interfaces – An Inorganic/Coordination Viewpoint

AbstractThe reactivity of the metal oxide‐water interface and that of the mineral‐water interface can be understood from the viewpoint of coordination chemistry, comparing the reactivity of a metal ion belonging to the surface of a solid with the reactivity of the same metal ion dissolved in an aqueous solution. Ligand substitution reactions are key reactions for metal complexes in solution and metal complexes at the surface. At the surface, these processes lead to the formation of adsorbed species, which can be identified by different surface spectroscopic techniques. Surface reactions, such as proton adsorption‐desorption and anion and cation adsorption‐desorption, are easily understood in this context. With the aid of kinetic measurements performed by ATR‐FTIR, it is shown that adsorption‐desorption reactions should take place through dissociative (D), associative (A), or interchange mechanisms (Id, Ia), as they also occur in solution.

Dosimetric characterization of a Nickel complex for routine use in radiation processing

In this paper, the dosimetric characteristics of a colored metal complex solution were investigated. Nickel nitrate hexahydrate and 1,5-diphenylcarbazone were made as a liquid solution chemical dosimeter at three concentrations with an inexpensive and simple synthesis for use in gamma irradiation in the range of 20–1000 Gy. The maximum absorbance was observed spectrophotometrically at a maximum wavelength of 530 nm. The paper presents the dosimeter response, radiation chemical yield (G(x)), stability, and repeatability. The results showed that this solution is suitable for use in routine dosimetry in the 20–1000 Gy range.