Relative Stabilities Of Complexes Research Articles

Impact of dielectric constant of solvent on the formation of transition metal-ammine complexes.

The DFT-level computational investigations into Gibbs free energies (ΔG) demonstrate that as the dielectric constant of the solvent increases, the stabilities of [M(NH3 )n ]2+/3+ (n = 4, 6; M = selected 3d transition metals) complexes decrease. However, there is no observed correlation between the stability of the complex and the solvent donor number. Analysis of the charge transfer and Wiberg bond indices indicates a dative-bond character in all the complexes. The solvent effect assessed through solvation energy is determined by the change in the solvent accessible surface area (SASA) and the change in the charge distribution that occurs during complex formation. It has been observed that the SASA and charge transfer are different in the different coordination numbers, resulting in a variation in the solvent effect on complex stability in different solvents. This ultimately leads to a change between the relative stability of complexes with different coordination numbers while increasing the solvent polarity for a few complexes. Moreover, the findings indicate a direct relationship between ΔΔG (∆Gsolvent -∆Ggas ) and ΔEsolv , which enables the computation of ΔG for the compounds in a particular solvent using only ΔGgas and ΔEsolv . This approach is less computationally expensive.

Investigating the Role of Ligand Electronics on Stabilizing Electrocatalytically Relevant Low-Valent Co(I) Intermediates.

Cobalt complexes have shown great promise as electrocatalysts in applications ranging from hydrogen evolution to C-H functionalization. However, the use of such complexes often requires polydentate, bulky ligands to stabilize the catalytically active Co(I) oxidation state from deleterious disproportionation reactions to enable the desired reactivity. Herein, we describe the use of bidentate electronically asymmetric ligands as an alternative approach to stabilizing transient Co(I) species. Using disproportionation rates of electrochemically generated Co(I) complexes as a model for stability, we measured the relative stability of complexes prepared with a series of N, N-bidentate ligands. While the stability of Co(I)Cl complexes demonstrates a correlation with experimentally measured thermodynamic properties, consistent with an outer-sphere electron transfer process, the set of ligated Co(I)Br complexes evaluated was found to be preferentially stabilized by electronically asymmetric ligands, demonstrating an alternative disproportionation mechanism. These results allow a greater understanding of the fundamental processes involved in the disproportionation of organometallic complexes and have allowed the identification of cobalt complexes that show promise for the development of novel electrocatalytic reactions.

Host compounds (+)-(2R,3R)-1,1,4,4-tetraphenylbutane-1,2,3,4-tetraol (TETROL) and (2R,3R)-(−)-2,3-dimethoxy-1,1,4,4-tetraphenylbutane-1,4-diol (DMT) with guests o-, m- and p- toluidine: A comparative investigation

In this work, we have compared the host abilities of closely related compounds (+)-(2R,3R)-1,1,4,4-tetraphenylbutane-1,2,3,4-tetraol (TETROL) and (2R,3R)-(−)-2,3-dimethoxy-1,1,4,4-tetraphenylbutane-1,4-diol (DMT) when these were recrystallized from single and mixed toluidine guests. Significant differences in host behaviour and selectivities were revealed and these were explained by means of single crystal diffraction experiments. Thermal analyses were used to determine the relative complex stabilities, and these data correlated exactly with the host selectivity orders for both TETROL and DMT.

Chelation-driven rearrangement of primary alkyl aminopalladation products to stable trisubstituted alkyl-palladium complexes.

The formation of highly substituted carbon centers using catalysis has been a widely sought after goal, but complexes of highly substituted carbon atoms with transition metals are rare, and the factors that affect the relative stability of complexes with differentially substituted carbon atoms are poorly understood. In this study, a set of equilibrating alkyl-palladium complexes were subtly tuned to form either a primary or trisubstituted alkyl complex as the more thermodynamically favored state, depending on either the substrate or reaction conditions. An X-ray crystal structure of the trisubstituted alkyl-palladium complex is presented and compared with the corresponding primary alkyl complex. The mechanism for rearrangement and the factors that drive the change in stability are discussed.

Chelation‐Driven Rearrangement of Primary Alkyl Aminopalladation Products to Stable Trisubstituted Alkyl–Palladium Complexes

AbstractThe formation of highly substituted carbon centers using catalysis has been a widely sought after goal, but complexes of highly substituted carbon atoms with transition metals are rare, and the factors that affect the relative stability of complexes with differentially substituted carbon atoms are poorly understood. In this study, a set of equilibrating alkyl–palladium complexes were subtly tuned to form either a primary or trisubstituted alkyl complex as the more thermodynamically favored state, depending on either the substrate or reaction conditions. An X‐ray crystal structure of the trisubstituted alkyl–palladium complex is presented and compared with the corresponding primary alkyl complex. The mechanism for rearrangement and the factors that drive the change in stability are discussed.

Optimum complexation of uranyl with amidoxime in aqueous solution under different pH levels: density functional theory calculations

This study aims to understand the complexation of uranyl with amidoxime in aqueous solution under different pH levels using density functional theory calculations. The geometries, relative stabilities of complexes, and changes in Gibbs free energies for different complexing reactions were investigated. Amidoximate ions were gradually added to the equatorial plane of uranyl to understand the complexation process. We modelled the effect of pH by varying the number of OH− and H2O ligands accompanying amidoximate ions in the equatorial plane of uranyl. The ŋ2 binding motif was the most favourable form, regardless of the uranyl/amidoximate ratio and the pH level. Compared with low and high pH, neutral pH condition was more beneficial to the complexation of uranyl with amidoxime.

Quantum chemical study of atmospheric aggregates: HCl•HNO3•H2SO4.

HCl, HNO3 and H2SO4 are implicated in atmospheric processes in areas such as polar stratospheric clouds in the stratosphere. Ternary complexes of HCl, HNO3 and H2SO4 were investigated by ab initio calculations at B3LYP level of theory with aug-cc-pVTZ and aug-cc-pVQZ basis sets, taking into account basis set superposition error (BSSE). The results were assessed in terms of structures (five hexagonal cyclic structures and two quasi-pentagonal cyclic structures), inter-monomeric parameters (all ternary complexes built three hydrogen bonds), energetics (seven minima obtained), infrared harmonic vibrational frequencies (red shifting of complexes from monomers), and relative stability of complexes, which were favorable when the temperature decreases under stratospheric conditions, from 298K to 188K, and in concrete, at 210K, 195K and 188K.

Photocatalytic Hydrogen Production Using Models of the Iron–Iron Hydrogenase Active Site Dispersed in Micellar Solution

Iron-thiolate complexes of the type [Fe2 (μ-bdt)(CO)6-x P(OMe3 )x ] (bdt=S2 C6 H4 =benzenedithiolate, x≤2) are simplified models of iron-iron hydrogenase enzymes. Recently, we have shown that these water-insoluble organometallic complexes, when included into micelles formed by sodium dodecyl sulfate (SDS), are good catalysts for the electrochemical production of hydrogen in aqueous solutions at pH<6. We herein report that the all-CO derivative [Fe2 (μ-bdt)(CO)6 ] (1), owing to its comparatively low reduction potential, is also a robust molecular catalyst for visible-light-driven production of H2 in aqueous SDS solutions at pH 10.5. Irradiation at λ=455 nm of a system consisting of complex 1, Eosin Y as a sensitizer, and triethylamine as an electron donor produced up to 0.86 mL of H2 in 4.5 h, corresponding to a turnover number of 117 mol of H2 per mol of catalyst. In the presence of a large excess of sensitizer, the production of H2 lasted for more than 30 h, stressing the relative stability of complex 1 under the photocatalytic conditions used herein. Thermodynamic considerations and UV/Vis spectroscopy experiments suggest that the catalytic cycle begins with the photo-driven reduction of complex 1. The reduced intermediate reacts with a proton source to yield iron hydride. Subsequent reduction and protonation steps produce H2 , regenerating the starting complex. As a result, the iron-thiolate complex 1 is a versatile proton reduction catalyst that can utilize either solar or electrical energy inputs, providing a starting point for the construction of noble metal-free molecular systems for renewable H2 production.

Binding of Eukaryotic Initiation Factor (eIF) 4F to Barley Yellow Dwarf Virus (BYDV) 3′ UTR Correlates Well With Protein Synthesis Efficiency

BYDV belonging to the luteoviridae family is an economically important plant virus lacking a 5′ cap and 3′poly (A) tail and is translated by cap‐independent translation (CIT) mechanism. The BYDV 3′ Untranslated Region (UTR) translation enhancer element (BTE) stimulates CIT initiation at the 5′upstream AUG via base‐pairing (kissing stem loop interaction) to a complementary loop, 5′UTR SL‐D. 3′ BTE recruits eIFs and transfers them to the 5′ end of the viral RNA where translation initiates.Wild type BTE and BTE mutants with in‐vitro translation efficiencies from 5–164 % were studied. eIF4F binding is the first committed step in cap‐dependent protein synthesis. The role of eIF4F in viral protein synthesis is not fully understood.eIF4F binding to BTE and BTE mutants gave equilibrium binding constant values (Kds) which are a measure of relative complex stability. We show: 1) translation efficiencies correlated well with eIF4F binding with one exception; 2) Kinetics, higher activation energy and interaction of other eIFs contribute; 3) BTE and BTE mutants‐eIF4F interaction is both enthalpically (52–90%) and entropically favorable with more enthalpic contribution to ΔG° at 25°C. The stability measurements and kinetics determine the nature of interactions and provides unique information on the sequential assembly of the initiation complex and it's binding to viral mRNA.Grant Support: NSF MCB 1157632

Influence of Spacer–Receptor Interactions on the Stability of Bivalent Ligand–Receptor Complexes

Experiments show that a ligand-receptor complex formed by binding a bivalent ligand (D) in which the two ligating units are joined covalently by a flexible polymeric spacer (S) can be orders of magnitude more stable than the corresponding complex formed with monomeric ligands. Although molecular models rationalizing this "enhancement effect" have been proffered, they ignore spacer-receptor (S-R) interactions, which can substantially influence the relative stability of complexes. Here, the results of a computational study designed to assess the impact of S-R interactions in the prototypic bivalent complex are presented and compared to results of experiments. The S-R interactions mimicking general features of biological systems are modeled by contoured R surfaces with hills (or depressions) at the binding sites. In the fictitious limit of vanishing S-R interactions, the enhancement is pronounced, as observed in experiments. For strictly repulsive S-R interactions (hard R surface), the enhancement vanishes, or even reverses. This is particularly the case if the R surface is convex (i.e., rising between the binding sites), while the enhancement is only moderately reduced if the R surface is concave. Alternatively, a weak S-R attraction close to the R surface can increase the enhancement. It is concluded that large enhancement should be observed only if both features are present: a concave R surface plus a weak S-R attraction. The latter occurs for spacer material such as polyethylene glycol (PEG), which is weakly hydrophobic and thus attracted by protein surfaces. It is shown that the enhancement of bivalent binding can be characterized by a single key parameter, which may also provide guidelines for the design of multivalent complexes with large enhancement effect.

Determination of the relative ligand‐binding strengths in heteroleptic IrIIIcomplexes by ESI‐Q‐TOF tandem mass spectrometry

An electrospray ionization quadrupole time-of-flight mass spectrometer has been utilized to investigate the relative ligand-binding strengths in a series of heteroleptic-charged iridium(III) complexes of the general formula [(C^N)(2) Ir(III) (S-tpy)](PF(6) ) by using variable collision energies. Collision-induced dissociation experiments were performed in order to study the stability of the Ir(III) complexes that are, for instance, suitable phosphors in light-emitting electrochemical cells. The ratio of signal intensities belonging to the fragment and the undissociated complex depends on the collision energy applied for the tandem mass spectra (MS/MS) analysis. By defining the threshold collision energy and the point of complete complex dissociation, it is possible to estimate the relative complex stabilities depending on the nature of the coordinated ligands [i.e. type of cyclometalating ligand (C^N), substituents on the S-shaped terpyridine (S-tpy)]. The collision energy values differed as a function of the coordination sphere of the Ir(III) centers.

Relative stability of 2-methyl-1,3,2-dioxaborinane complexes with alkoxy anions

Cyclic esters of boric acids , in particular, 1,3,2-dioxaborinanes, are known to belong to weak Lewis acids and to be capable of forming associates with bases [1– 7]. The strength of these molecular complexes, for instance, adducts with ammonia, is low [8, 9]. However their formation in the solution should significantly affect the conformational properties of the initial molecule changing the configuration of the boron atom and the parameters of the conformational equilibrium. This circumstance opens wide opportunities for the computer simulation of the mechanisms of the interaction between the boric acid esters and various solvents. On the other hand, the importance of the mentioned conformational effects should grow with the growing stability of the associate which in its turn depends on the base strength. In this study with the use of ab initio approximation RHF/6-31+G(d) in the framework of the software package HyperChem [10] the relative stability of complexes between 2-methyl-1,3,2-dioxaborinane (I) with alkoxy anions (associates A) was investigated for the first time. O B O O B O CH3 RO _ OR

Relative stability of complexes of six-carbon-rings with variable numbers of double bonds: DFT and ab initio results

Protein–ligand interactions, including enzyme–drug interactions, often depend on a variety of non-covalent bonds. Ring–ring interactions, though generally weaker than other non-covalent interactions, are important both in the binding of ligands to proteins, and also in the specific 3-D orientation of the ligand molecule in the protein binding site. We calculated the counterpoise-corrected interaction energies and relative stabilities of complexes of six-membered carbon rings using MP2/aug-cc-pvdz. These two-ring complexes, each with three, two, one, or no double bonds per ring, serve as models for the above mentioned protein–ligand binding. Using MP2/aug-cc-pvdz as a standard, we also evaluated the accuracy of MP2 with smaller basis sets. Further we compared Hartree–Fock and several DFT methods with MP2 in order to see if these less resource-intensive methods could achieve and accuracy comparable to MP2. We find that smaller basis sets within the MP2 theory reproduce accurate trends and relative energies of complexes compared to aug-cc-pvdz, and the HCTH407 DFT method can reproduce accurate relative energies for most of the complexes studied here. We also compute HCTH407-level polarizabilities of the rings and examine the relationship between these values and the interaction energies.

Influence of the [2Fe]H Subcluster Environment on the Properties of Key Intermediates in the Catalytic Cycle of [FeFe] Hydrogenases: Hints for the Rational Design of Synthetic Catalysts

Nature's recipe: A theoretical study analyzes how the environment of the [FeFe] hydrogenase's catalytic cofactor affects its chemical properties, particularly the relative stability of complexes with bridging and terminal hydride ligands (see picture; Fe teal, S yellow, C green, N blue, O red, H gray). The results help to elucidate key rules for the design of bioinspired synthetic catalysts for H(2) production.

Ab Initio Study of Structural, Electronic, and Magnetic Properties of Vn(C60)m Complexes

We have studied structural, electronic, and magnetic properties of transition-metal-fullerene complexes V(n)(C(60))(m), (n, m) = (1, 1), (1, 2), (2, 3), (3, 4), (4, 4), by means of a density functional theory method. We have examined relative stabilities of complexes with different V-C(60) binding sites (V atoms are bound to either pentagonal or hexagonal rings of C(60)) and with different stacking configurations (linear or nonlinear). The linearly stacked sandwichlike complexes with V atoms binding to hexagonal rings of C(60) are the most stable for (n, m) = (n, n + 1), although nonlinearly stacked configurations can be energetically competitive. For (n, m) = (1, 1), the V atom tends to bind to a pentagonal ring of the C(60) molecule. For (n, m) = (4, 4), a riceball-like structure is found to be the most stable. Except for (n, m) = (1, 1), the lowest-energy structures of the complexes are generally in their lowest spin states.

Theoretical study of interaction of 2,6-dithiopurine with Li+, Na+, K+, Be2+, Mg2+, and Ca2+ metal cations

The geometries of the complexes of Li+, Na+, K+, Be2+, Mg2+, and Ca2+ metal cations with different possible 2,6-dithiopurine anions (DTP) were studied. The complexes were optimized at the B3LYP level and the 6-311++G(d, p) basis set. The interactions of the metal cations at different nucleophilic sites of various possible 2,6-dithiopurine anions were considered. It was revealed that metal cations would interact with 2,6-dithiopurine anions in a bicoordinate manner. In the gas phase, the most preferred position for the interaction of Li+, Na+, and K+ cations is between the N3 and S2 sites, while all divalent cations Be2+, Mg2+, and Ca2+ prefer binding between the N7 and S6 sites of the corresponding 2,6-dithiopurine. The influence of aqueous solvent on the relative stability of different complexes has been examined using the Tomasi’s polarized continuum model. The basis set superposition error (BSSE) corrected interaction energy was also computed for complexes. The AIM theory has been applied to analyze the properties of the bond critical points (electron densities and their Laplacians) involved in the coordination between 2,6-dithiopurine anions and the metal cations. It was revealed that aqueous solution would have significant effect on the relative stability of complexes obtained by the interaction of 2,6-dithiopurine anions with Mg2+ and Ca2+ cations. The effect of metal cations on different NH and CS stretching vibrational modes of 2,6-dithiopurine has also been discussed.

Electrospray-Ionization Mass Spectrometry for Screening the Specificity and Stability of Single-Stranded-DNA Templated Self-Assemblies

Supramolecular complexes consisting of a single-stranded oligothymine (dTn) as the host template and an array of guest molecules equipped with a complementary diaminotriazine hydrogen-bonding unit have been studied with electrospray-ionization mass spectrometry (ESI-MS). In this hybrid construct, a supramolecular stack of guest molecules is hydrogen bonded to dTn. By changing the hydrogen-bonding motif of the DNA host template or the guest molecules, selective hydrogen bonding was proven. We were able to detect single-stranded-DNA (ssDNA)-guest complexes for strands with lengths of up to 20 bases, in which the highest complex mass detected was 15 kDa; these complexes constitute 20-component self-assembled objects. Gas-phase breakdown experiments on single- and multiple-guest-DNA assemblies gave qualitative information on the fragmentation pathways and the relative complex stabilities. We found that the guest molecules are removed from the template one by one in a highly controlled way. The stabilities of the complexes depend mainly on the molecular weight of the guest molecules, a fact suggesting that the complexes collapse in the gas phase. By mixing two different guests with the ssDNA template, a multicomponent dynamic library can be created. Our results demonstrate that ESI-MS is a powerful tool to analyze supramolecular ssDNA complexes in great detail.

Speciation analysis of selected metals and determination of their total contents in paired serum and cerebrospinal fluid samples: An approach to investigate the permeability of the human blood-cerebrospinal fluid-barrier

Neurodegenerative diseases like Alzheimer's disease and Parkinson's disease are gaining increasing relevance in our aging society. However, the complex multifactorial mechanisms of these diseases are not sufficiently understood yet. Several studies indicate that metal ions play an important role in the promotion of these diseases. Consequently, the transport pathways of metals and their species to the brain are of special interest. Following oral or inhalative uptake metals are absorbed and distributed via the blood stream in the body. Transport into the brain requires crossing of the neural barriers. Our study focuses on the investigation of the permeability of the blood-cerebrospinal fluid (CSF)-barrier for selected metals (Mn, Fe, Cu, Zn, Mg and Ca). For the first time paired human serum and CSF samples obtained from a neurological department were characterised for total metal concentrations and metal species. For CSF few data are available in the literature on total metal contents and applications of element speciation analysis in CSF samples are rare. In our study mean CSF/serum ratios ( n = 29) were 0.7 for Mn, 0.02 for Fe, 0.02 for Cu, 0.03 for Zn, 1.3 for Mg and 0.5 for Ca. Size exclusion chromatography (SEC) online with inductively coupled plasma mass spectrometry was further developed for the size characterisation of the metal species in CSF and serum with limits of detection of 0.4 μg L −1 for Fe, 0.01 μg L −1 for Mn, 0.2 μg L −1 for Cu, 0.2 μg L −1 for Zn, 0.6 μg L −1 for Mg and 3.8 μg L −1 for Ca in the eluate from the HPLC column. Apart from Mn the application of this technique has not been published for metal speciation in CSF, yet. In the case of some Mn species it turned out that methanol, which was contained in the mobile phase of a SEC method previously published from our group on qualitative characterisation of Mn species, was interfering with the quantification. The modified method developed in this work (with NaCl but without methanol in the mobile phase; use of internal standard) allowed reliable quantification. The results clearly indicate changes in the metal species pattern due to different permeation behaviour at the blood-CSF-barrier. As part of the method validation the relative stability of complexes of albumin, transferrin and citrate with Mn, Fe, Cu and Zn was investigated.

DFT Study for p-tert-Butylcalix[4]arene Crown Ether Bridged at the Lower Rim with Pyridyl Unit Complexed with Potassium Ion

Stable molecular conformations were calculated for the p-tert-butylcalix[4]arene crown ether bridged at the lower rim with pyridyl unit (1) in the various conformers and their potassium-ion complexes. The structures of three distinct conformations have been optimized using DFT B3LYP/6-31G(d,p) method. Relative stability of free host 1 is in following order: cone (most stable) > partial-cone > 1,3-alternate conformer. For two different kinds of complexation mode, the potassium cation in the crown-ether moiety (cr) has much better complexation efficiency than in the benzene-rings (bz) pocket for all three kinds of conformation of host molecule 1. The relative stability of complex (1+K+) in the cr-binding mode is in following order: partial-cone (most stable) ~ cone > 1,3-alternate conformer.

Theoretical Study of Interaction of Urate with Li+, Na+, K+, Be2+, Mg2+, and Ca2+Metal Cations

The geometries and energetics of complexes of Li(+), Na(+), K(+), Be(2+), Mg(2+), and Ca(2+)metal cations with different possible uric acid anions (urate) were studied. The complexes were optimized at the B3LYP level and the 6-311++G(d,p) basis set. Complexes of urate with Mg(2+), and Ca(2+)metal cations were also optimized at the MP2/6-31+G(d) level. Single point energy calculations were performed at the MP2/6-311++G(d,p) level. The interactions of the metal cations at different nucleophilic sites of various possible urate were considered. It was revealed that metal cations would interact with urate in a bi-coordinate manner. In the gas phase, the most preferred position for the interaction of Li(+), Na(+), and K(+) cations is between the N(3) and O(2) sites, while all divalent cations Be(2+), Mg(2+), and Ca(2+) prefer binding between the N(7) and O(6) sites of the corresponding urate. The influence of aqueous solvent on the relative stability of different complexes has been examined using the Tomasi's polarized continuum model. The basis set superposition error (BSSE) corrected interaction energy was also computed for complexes. The AIM theory has been applied to analyze the properties of the bond critical points (electron densities and their Laplacians) involved in the coordination between urate and the metal cations. It was revealed that aqueous solvation would have significant effect on the relative stability of complexes obtained by the interaction of urate with Mg(2+) and Ca(2+)cations. Consequently, several complexes were found to exist in the water solution. The effect of metal cations on different NH and CO stretching vibrational modes of uric acid has also been discussed.