Showing papers on "Aquation published in 2017"

Photoaquation Mechanism of Hexacyanoferrate(II) ions: Ultrafast 2D UV and Transient Visible and IR Spectroscopies

Abstract: Ferrous iron(II) hexacyanide in aqueous solutions is known to undergo photoionization and photoaquation reactions depending on the excitation wavelength. To investigate this wavelength dependence, we implemented ultrafast two-dimensional UV transient absorption spectroscopy, covering a range from 280 to 370 nm in both excitation and probing, along with UV pump/visible probe or time-resolved infrared (TRIR) transient absorption spectroscopy and density functional theory (DFT) calculations. As far as photoaquation is concerned, we find that excitation of the molecule leads to ultrafast intramolecular relaxation to the lowest triplet state of the [Fe(CN)6]4– complex, followed by its dissociation into CN– and [Fe(CN)5]3– fragments and partial geminate recombination, all within <0.5 ps. The subsequent time evolution is associated with the [Fe(CN)5]3– fragment going from a triplet square pyramidal geometry, to the lowest triplet trigonal bipyramidal state in 3–4 ps. This is the precursor to aquation, which occu...

Catalysis and Inhibition in the Electrochemical Reduction of CO2 on Platinum in the Presence of Protonated Pyridine. New Insights into Mechanisms and Products.

Abstract: In the framework of modern energy challenges, the reduction of CO2 into fuels calls for electrogenerated low-valent transition metal complexes catalysts designed with considerable ingenuity and sophistication. For this reason, the report that a molecule as simple as protonated pyridine (PyH+) could catalyze the formation of methanol from the reduction of CO2 on a platinum electrode triggered great interest and excitement. Further investigations revealed that no methanol is produced. It appears that CO2 is not really reduced but rather participates, on the basis of its aquation into carbonic acid, in hydrogen evolution. Actually, the situation is not that straightforward, as revealed by scrutinizing what happens at the platinum electrode surface. The present study confirms the lack of methanol formation upon bulk electrolysis of PyH+ solutions at Pt and provides a detailed account of the Faradaic yield for H2 production as a function of the electrode potential, but the main finding is that CO2 reduction is...

Structure and reactivity of [RuII(terpy)(N^N)Cl]Cl complexes: consequences for biological applications

Abstract: The crystal structures of [RuII(terpy)(bipy)Cl]Cl·2H2O and [RuII(terpy)(en)Cl]Cl·3H2O, where terpy = 2,2′:6′,2′′-terpyridine, bipy = 2,2′-bipyridine and en = ethylenediamine, were determined and compared to the structure of the complexes in solution obtained by multi-nuclear NMR spectroscopy in DMSOd-6 as a solvent. In aqueous solution, both chlorido complexes aquate fully to the corresponding aqua complexes, viz. [RuII(terpy)(bipy)(H2O)]2+ and [RuII(terpy)(en)(H2O)]2+, within ca. 2 h and ca. 2 min at 37 °C, respectively. The spontaneous aquation reactions can only be suppressed by chloride concentrations as high as 2 to 4 M, i.e. concentrations much higher than that found in human blood. The corresponding aqua complexes are characterized by pKa values of ca. 10 and 11, respectively, which suggest a more labile coordinated water molecule in the case of the [RuII(terpy)(en)(H2O)]2+ complex. Substitution reactions of the aqua complexes with chloride, cyanide and thiourea show that the [RuII(terpy)(en)(H2O)]2+ complex is 30–60 times more labile than the [RuII(terpy)(bipy)(H2O)]2+ complex at 25 °C. Water exchange reactions for both complexes were studied by 17O-NMR and DFT calculations (B3LYP(CPCM)/def2tzvp//B3LYP/def2svp and ωB97XD(CPCM)/def2tzvp//B3LYP/def2svp). Thermal and pressure activation parameters for the water exchange and ligand substitution reactions support the operation of an associative interchange (Ia) process. The difference in reactivity between these complexes can be accounted for in terms of π-back bonding effects of the terpy and bipy ligands and steric hindrance on the bipy complex. Consequences for eventual biological application of the chlorido complexes are discussed.

Interstrand DNA covalent binding of two dinuclear Ru(ii) complexes. Influence of the extra ring of the bridging ligand on the DNA interaction and cytotoxic activity

Abstract: In this work, we report experimental and computational evidence for the intercalation into the DNA base-pairs of the free quinones quinizarin (Q) and naphthazarin (N) and the interstrand covalent binding of their p-cymene di-ruthenium(ii) complexes (Cl2Ru2X, with X = N, Q bridging ligands). The intercalation extent for the N complex was larger than that for Q, which is in good agreement with the higher relative contour length and melting temperature for the same CX/CDNA ratio and with the computational mean stacking distances between the ligand and the nearest base-pair (3.34 A and 3.19 A) for N and Q, respectively. However, the apparent binding constant of Q/DNA, two orders higher than that of N/DNA, indicates that the thermal stability of the X/DNA complex is more related to the degree of intercalation than to the magnitude of the binding constant. Cl2Ru2X complexes undergo aquation, forming the aqua-derivatives [(H2O)2Ru2X]2+. These can further bind covalently to DNA via interstrand crosslinking, through both Ru centres and two N7 sites of consecutive guanines, to give (DNA1,2)Ru2X complexes, by a mechanism similar to that of cisplatin. To the best of our knowledge, this type of interaction with dinuclear Ru(ii) complexes has not been reported hitherto. The experimental and computational results reveal that the number of rings of the aromatic moiety and the covalent binding to DNA play a key role in the behaviour of the quinones and their Ru(ii) derivatives. The cytotoxicity of the ligands and the corresponding Ru(ii) complexes was evaluated in MCF-7, A2780, A2780cis tumour cells and in the healthy cell line MRC-5. The cytotoxic activity was notable for N and negligible for Q. The IC50 values and the resistance (RF) and selectivity (SF) factors show that the Cl2Ru2N complex is the most promising among the four studied anticancer drugs.

Exploring the Hydrolytic Behavior of the Platinum(IV) Complexes with Axial Acetato Ligands.

Abstract: Platinum(IV) complexes are generally thought to be kinetically inert, and are expected to be stable enough to resist premature aquation before entering the cancer cells. Nevertheless, in this work, complex 2 with axial acetato ligands can hydrolyze relatively quickly under biologically relevant conditions with a half-life of 91.7 min, resulting in the loss of the equatorial chlorido ligand. Further study indicated that the fast hydrolysis of complex 2 may be attributed to the strong σ-donor ability of N-isopropyl-1R,2R-diaminocyclohexane, and an increasing σ-donor ability of the amine group can promote the hydrolysis rate of the corresponding platinum(IV) complex. The experiment results were proven by the corresponding DFT calculation. Our study can help to re-evaluate the aqueous properties of the platinum(IV) complexes with axial acetate, which may be less inert to hydrolysis than expected under biologically relevant conditions.

The Determination of Absolute Values of Entropies of Hydration [ΔSabs0(H+)h]$[\Delta S_{abs}^0{({H^ + })_h}]$ and Aquation [ΔSabs0(H+)aq]$[\Delta S_{abs}^0{({H^ + })_{aq}}]$ and The Thermodynamics of Proton in Solutions

Abstract: Abstract Absolute entropy value of H+ ion i.e. ΔSaq0(H+)=− 22.2 JK−1mol−1 $\Delta {\rm{S}}_{{\rm{aq}}}^0({{\rm{H}}^ + }) = - \;22.2{\rm{ }}J{K^{ - 1}}{\rm{mo}}{{\rm{l}}^{ - 1}}$ in aqueous solution, a fundamental parameter of importance was determined using a number of extrathermodynamic assumptions of doubtful validity. The value can in no way be regarded to be absolute or correct and needs reassessment. However, no value of the entropy change due to hydration ΔSh0(H+) $\Delta {\rm{S}}_{\rm{h}}^0({{\rm{H}}^ + })$ was available. Absolute values for entropy of hydration [ΔSabs0(H+)h] $[\Delta {\rm{S}}_{{\rm{abs}}}^0{({{\rm{H}}^ + })_{\rm{h}}}]$ (entropy change for the transfer of H+ ion from gaseous (g) state to H+ ion in aqueous solution) or entropy of aquation [ΔSabs0(H+)aq] $[\Delta {\rm{S}}_{{\rm{abs}}}^0{({{\rm{H}}^ + })_{{\rm{aq}}}}]$ (entropy change for transfer of H(g) to aqueous Hion+) ${\rm{H}}_{{\rm{ion}}}^ + )$ of H+ ion can only be calculated if the related absolute values of Gibbs energy or enthalpy changes of H+ ion i.e. [ΔGabs0(H+)h or aq $[\Delta {\rm{G}}_{{\rm{abs}}}^0{({{\rm{H}}^ + })_{{\text{h or aq}}}}$ and ΔHabs0(H+)h or aq] $\Delta {\rm{H}}_{{\rm{abs}}}^0{({{\rm{H}}^ + })_{{\text{h or aq}}}}]$ are known. Critical analysis of the methods used for evaluation of thermodynamics of H+ ion was made. Analysis of the methods showed that the methods had limitations due to defective use of Born equation and ionic additivity principle. Reference electrolyte method using TATB (tetraphenyl arsonium tetraphenyl borate, Ph4AsBPh4), Halliwell and Nyburg’s method and Noyes method or modified Noyes method of Lahiri do not give entropy values. Cluster-ion approximation method (used by Coe and co-workers) gives ΔHabs0(H+)h $\Delta {\rm{H}}_{{\rm{abs}}}^0{({{\rm{H}}^ + })_{\rm{h}}}$ and ΔGabs0(H+)h $\Delta {\rm{G}}_{{\rm{abs}}}^0{({{\rm{H}}^ + })_{\rm{h}}}$ and hence ΔSabs0(H+)h=− 153.0 JK−1mol−1. ΔSabs0(H+)aq $\Delta {\rm{S}}_{{\rm{abs}}}^0{({{\rm{H}}^ + })_{\rm{h}}} = - \;153.0{\rm{ J}}{{\rm{K}}^{ - 1}}{\rm{mo}}{{\rm{l}}^{ - 1}}.\;\Delta {\rm{S}}_{{\rm{abs}}}^0{({{\rm{H}}^ + })_{{\rm{aq}}}}$ is obtained by coupling ΔSabs0(H+)h $\Delta {\rm{S}}_{{\rm{abs}}}^0{({{\rm{H}}^ + })_{\rm{h}}}$ with ΔSabs0(H+)g $\Delta {\rm{S}}_{{\rm{abs}}}^0{({{\rm{H}}^ + })_{\rm{g}}}$ [entropy of gaseous H+ ion calculated using Sackur-Tetrode equation], comes out to be –44.2 JK−1mol−1. However, ΔSabs0(H+)h $\Delta {\rm{S}}_{{\rm{abs}}}^0{({{\rm{H}}^ + })_{\rm{h}}}$ and ΔSabs0(H+)aq $\Delta {\rm{S}}_{{\rm{abs}}}^0{({{\rm{H}}^ + })_{{\rm{aq}}}}$ determined by Lahiri and co-workers are –50.0 JK−1mol−1 and 20.0 JK−1mol−1. The values can be regarded to be accurate and reliable. Some comments on the surface potential of water towards ΔGh or aq0(H+) $\Delta {\rm{G}}_{{\text{h or aq}}}^0({{\rm{H}}^ + })$ and error ranges on the energetics of H+ and other ions are given. No attempt was made to determine entropy of hydration or aquation from theoretical calculations.

Investigation of the substitution reaction pathways ofantidiabetic chromium(III) supplements

Abstract: Chromium(III) complexes are regularly included in nutritional supplements and are reported to lower blood glucose levels in type 2 diabetics. However, questions have been raised about potential toxicity. Understanding the reactivity of these complexes is key to understanding their behaviour in physiological environments. Therefore, the central aim of this work is to explore the speciation of these complexes under different conditions using both computational and experimental methods. The computational studies involved a detailed mechanistic investigation of the substitution reaction pathways of several chromium(III) nutritional supplements (e.g. chromium(III) chloride, glycinato-chromium(III) [Cr(gly)x(H2O)6-2x](3-x)+ where x = 1 ‒3) and their conjugate bases and chromium(III) picolinate (Cr(pic)3) using state-of-the art computational chemistry techniques. A number of mechanistic aquation pathways were investigated including associative interchange (Ia), dissociative interchange (Id), associative (A) and dissociative (D), to predict the overall activation enthalpies of these processes. Our initial computational investigations have focussed on the mechanistic pathways for aquation of chromium(III) chloride under acidic (stomach) and mild (duodenum and blood) conditions. DFT calculations reveal that aquation of CrCl3 is slow in an acidic environment and this limits speciation of the complex. However, at mild pH, conjugate base complexes easily form, leading to a significant number of species that may be involved in halide-water exchange pathways. The activation enthalpies for aquation of tris-glycinatochromium(III) and its conjugate base at mild and physiological pHs via the dissociative (D) mechanism are in consistent agreement with the experimental results. Aquation of [Cr(pic)3]0 initially proceeds via a dissociative ring-opening step to form the [Cr(pic)2(H2O)(picH2)]+intermediate in acidic media. The reaction may then proceed via two possible pathways involving either stepwise (two-step) or concerted (one-step) mechanisms. Throughout these studies solvent calculations were performed using the polarizable continuum model (PCM). However, the inclusion of explicit solvation was also found to be important for many processes. The experimental component of the study involved the synthesis of chromium(III) nutritional supplements and characterization (elemental analysis, atomic absorption spectroscopy, NMR, HPLC, ATR-FTIR, Raman, and ESI−MS spectroscopy). Investigation of solvent and pH effects using UV−Vis monitoring (pH ~3.0 to ~8.5) and electron paramagnetic resonance (in both solid and frozen state) complemented the theoretical studies of the speciation and interaction of Cr(III) metal with amino acid ligands. The electrochemical properties of [Cr(gly)3]0 were also investigated for the first time using cyclic voltammetry (CV) and it was found that this complex can undergo reversible electron transfer, which may play a role in its biological activity.

Kinetic studies of ion-pairing effects on the aquation of chloropentaammine cobalt(III) complex in different dicarboxylate media at 30% of tert-butanol

Abstract: The study of the kinetic aquation of chloropentaammine cobalt(III) ion in the presence of different types of dicarboxylate solutions (Malonate, malate, tartarate and succinate), in mixed solvent media of water with tert -butanol (30%, v : v ) is investigated spectrophoto-metrically at different temperatures (30-60 °C) in the light of the effects of ion-pairing on reaction rates and mechanism. Comparison of the k ip (Rate constant of ion-pairing) values with respect of different buffers (Malonate, malate, tartarate and succinate) at 30% of tert -butanol is introduced. Examination of the linear free energy relationship (LFER) at the mentioned conditions will lead to diagnosethe mechanism. The free energy of activation Δ G * ip is more or less linearly varied among the studied dicarboxylate ion-pairing ligands indicating the presence of compensation effect between Δ H * ip and Δ S * ip .