Kinetics and mechanism of oxidation of thiourea by a bridging superoxide in the presence of Ellman’s reagent

<p>In aqueous acetate buffer, reaction between the superoxo complex [(dien)(en)Co^III(O₂)Co^III(en)(dien)]⁵⁺ (<b>1</b>) and thiourea (Tu, SC(NH₂)₂) proceeds through a complex kinetic pathway and the change of the absorbance of <b>1</b> with time follows a sigmoidal trace. Only in the presence of Ellman’s reagent (5,5′-dithio-bis-(2-nitrobenzoic acid), DTNB), the reaction becomes quantitative and Tu reduces the bound superoxo group (––) of <b>1</b> to the corresponding peroxo group (––) in [(dien)(en)Co^III(O₂)Co^III(en)(dien)]⁴⁺ (<b>2</b>). In the presence of DTNB, with large excess of Tu over [<b>1</b>], the reaction follows first-order kinetics and the observed rate constant, <i>k</i>_o, increases linearly with the increase in [<b>1</b>], [T_Tu] ([T_Tu] is the analytical concentration of Tu), [H⁺], and the media ionic strength (<i>I</i>). The presence of an intercept in the plot of <i>k</i>_o <i>versus</i> [H⁺] suggests that both tautomeric forms of thiourea, i.e. Tu_Z and its protonated form Tu_ZH⁺, are kinetically reactive reductants. The calculated rate constant values for <i>k</i>₁ (~10²) and <i>k</i>₂ (~10⁴) indicate that Tu_ZH⁺ plays the leading role in the redox reaction. The present work also reveals the prospective involvement of Ellman’s reagent as a radical scavenger.</p

Penicillamine and captopril: mechanistic exploration of defensive actions of thiol drugs against a metal bound-superoxo complex

<p>In acid-media ([H⁺] = 0.01–0.06 M), each of the thiol compounds, D-penicillamine (PEN, L_PH₂) and captopril (CAP, L_CH₂) exist in several proton-dependent forms which can reduce the superoxo complex [(en)(dien)Co^III(O₂)Co^III(en)(dien)]⁵⁺ (<b>1</b>) to the corresponding peroxo [(en)(dien)Co^III(O₂)Co^III(en)(dien)]⁴⁺ (<b>2</b>) or the hydroperoxo complex [(en)(dien)Co^III(OOH)Co^III(en)(dien)]⁵⁺ (<b>3</b>). The observed first-order rate constants, <i>k</i>_o,P and <i>k</i>_o,C for PEN and CAP increase with the increase in [T_PEN] and [T_CAP] (which are the analytical concentrations of the respective thiols) but decrease with the increase in the media-acidity ([H⁺]) and the media ionic strength (<i>I</i>). The protolytic equilibria in aqueous solution allow several potentially reducing forms to coexist for both PEN (L_PH₃⁺, L_PH₂, L_PH⁻, and L_P²⁻) and CAP (L_CH₂, L_CH⁻, L_C²⁻) but the kinetic analyses reveal that the order of reactivity for the species are L_PH₃⁺ ~ L_PH₂ <<< L_PH⁻ and L_CH₂ < L_CH⁻ <<< L_C²⁻, respectively. The predominance and higher reactivities of the anionic species, L_PH⁻ and L_C²⁻ are supported by the negative slopes of the plots of <i>k</i>_o,P or <i>k</i>_o,C <i>versus I</i>. Moreover, a large value of <i>k</i>_H/<i>k</i>_D for PEN suggests an inner-sphere electroprotic reaction pathway while the absence of such effect for CAP strongly supports an outer-sphere electron transfer reaction. These propositions are supported by the structural features of L_PH⁻ and L_C²⁻.</p

Thermodynamic Study of Rhodamine 123-Calf Thymus DNA Interaction: Determination of Calorimetric Enthalpy by Optical Melting Study

In this paper, the interaction of rhodamine123 (R123) with calf thymus DNA has been studied using molecular modeling and other biophysical methods like UV–vis spectroscopy, fluoremetry, optical melting, isothermal titration calorimetry, and circular dichroic studies. Results showed that the binding energy is about −6 to −8 kcal/mol, and the binding process is favored by both negative enthalpy change and positive entropy change. A new method to determine different thermodynamic properties like calorimetric enthalpy and heat capacity change has been introduced in this paper. The obtained data has been crossed-checked by other methods. After dissecting the free-energy contribution, it was observed that the binding was favored by both negative hydrophobic free energy and negative molecular free energy which compensated for the positive free energies due to the conformational change loss of rotational and transitional freedom of the DNA helix

On the Importance of Noncovalent Carbon-Bonding Interactions in the Stabilization of a 1D Co(II) Polymeric Chain as a Precursor of a Novel 2D Coordination Polymer

A new cobalt­(II) coordination polymer <b>2</b> with μ_1,5 dicyanamide (dca) and a bidentate ligand 3,5-dimethyl-1-(2′-pyridyl)­pyrazole (<i>pypz</i>) is prepared in a stepwise manner using the newly synthesized one-dimensional linear Co­(II) coordination polymer <b>1</b> as a precursor. The structural and thermal characterizations elucidate that the more stable complex <b>2</b> shows a two-dimensional layer structural feature. Here, Co­(II) atoms with μ_1,5 dicyanamido bridges are linked by the ligand <i>pypz</i> forming a macrocyclic chain that runs along the crystallographic ‘<i>c</i>’ axis having ‘sql’ (Shubnikov notation) net topology with a 4-connected uninodal node having point symbol {4⁴.6²}. The remarkable noncovalent carbon-bonding contacts detected in the X-ray structure of compound <b>1</b> are analyzed and characterized by density functional theory calculations and the analysis of electron charge density (atoms in molecules)

pH Dependent Formation of Unprecedented Water–Bromide Cluster in the Bromide Salts of PTP Assisted by Anion−π Interactions: Synthesis, Structure, and DFT Study

Two new terpyridine derivatives [PTPH₃]­(Br)₃·3H₂O (<b>1</b>) and [PTPH₃]­(Br₃)­(Br)₂·H₂O (<b>2</b>), (PTP = 4′-(4-pyridyl)-2,2′:6′,2″-terpyridine) were synthesized and characterized by single crystal X-ray diffraction analyses. In <b>1</b>, intricate combination of anion···π/π–π/π···anion interactions generates the supramolecular network while <b>2</b> exhibits a combination of two different anion···π/π–π/π···anion and anion···π···anion assemblies. As anticipated, both of them are interlinked through hydrogen bonds affording molecular networks, but surprisingly, <b>1</b> shows hydrate aggregation and interactions with anions to generate water–bromide cluster blends in the solid lattices. A search of the crystal structure database (CSD) yielded only eight hits presenting an infinite tape in the solid state structure formed by bromide anions and water molecules, none of which had similar distribution of bromide and water as observed in <b>1</b>. Interestingly, in compound <b>1</b>, the bromide anion is involved in the formation of anion–water cluster and also engaged in anion···π interactions, thus generating a unique cluster of water–anion/anion···π/π–π/π···anion/anion–water cluster network. The different networks have been investigated by means of DFT calculations and the interactions characterized using the Bader’s theory of “atoms-in-molecules”

Exploring 3D non-interpenetrated metal–organic framework with malonate-bridged Co(II) coordination polymer: structural elucidation and theoretical study

<p>A Co(II)-based coordination polymer with tetranuclear cobalt(II)-malonate cluster has been easily generated by aqueous medium self-assembly from Cobalt(II) chloride hexahydrate and malonic acid. The structure exhibits a non-interpenetrating, highly undulating two-dimensional (2D) bi-layer network with (4,4) topology. The crystal structure is composed of infinite interdigitated 2D metal–organic bi-layers which extended to an intricate 3D framework through the interbilayer hydrogen bonds. We have studied energetically by means of Density Functional Theory (DFT) calculations the H-bonding interactions that connect the 2D metal–organic bi-layers. The finite theoretical models have been used to compute conventional O‒H∙∙∙O and unconventional C‒H∙∙∙O interactions which plays a key role to build 3D architecture.</p

Experimental and Computational Study of Counterintuitive ClO₄^–···ClO₄^– Interactions and the Interplay between π⁺–π and Anion···π⁺ Interactions

The novel noncovalent interactions between the charged and neutral aromatic rings and with anions are utilized to design the solid-state assembly of triply protonated PTPH₃ (PTP = 4′-(4-pyridyl)-3,2′:6′,3″-terpyridine) with H₂O and three ClO₄^–, which is synthesized and characterized by single-crystal X-ray diffraction analysis. Crystallography reveals that the π⁺–π⁺, π⁺–π, and various anion···π interactions are the major driving forces in the stabilization of the self-assembled structure. In the title complex, a layered assembly is formed through the mutual influence of π⁺–π⁺ and π⁺–π interactions. The anions are interacting with the charged π-acceptors, which are again stabilized through π⁺–π interactions. Therefore, the overall stabilization is governed through π⁺–π/π–π⁺, (π⁺–π⁺)_<i>n</i>, and anion···π⁺/π⁺–π/π–π⁺ networks in the solid state. The interaction energies of the main driving forces observed in the crystal structure have been calculated using density functional theory. In addition, the short O···O contact between ClO₄^– anions has been analyzed in detail both computationally and exploring the Cambridge Structural Database

3‑Picoline Mediated Self-Assembly of M(II)–Malonate Complexes (M = Ni/Co/Mn/Mg/Zn/Cu) Assisted by Various Weak Forces Involving Lone Pair−π, π–π, and Anion···π–Hole Interactions

Five M­(II)–malonate complexes having a common formula (C₆H₉N₂)₄[M^II(C₃H₂O₄)₂(H₂O)₂]­(PF₆)_2.(H₂O)₂ (<b>1</b>–<b>5</b>) [where C₆H₉N₂ = protonated 3-picoline, M­(II) = Ni/Co/Mn/Mg/Zn, C₃H₄O₄ = malonic acid, and PF₆^– = hexafluorophospahte], have been synthesized and their crystal structures have been determined. Complexes <b>1</b>–<b>5</b> were found to be isostructural and protonated 3-picoline has primarily mediated the self-assembly process. Role of a discrete water dimer in complexes <b>1</b>–<b>5</b> was also studied. Weaker π–interactions have also played crucial role in stabilizing 1D chain constructed by discrete [M^II(C₃H₂O₄)₂(H₂O)₂] units. An additional copper complex namely, (C₆H₉N₂)₄[Cu­(C₃H₂O₄)₂]­(PF₆)₂ (<b>6</b>) has been synthesized from the same reagents and was found to have a completely different structure from the others. Structures of all the complexes are fully described and compared here. Moreover, the lone pair−π and π–π noncovalent interactions have been analyzed by means of DFT calculations, mainly focusing our attention to the influence of the coordinating metal on the strength of the interactions and the interplay between hydrogen bonding and π-interactions. We also present here Hirshfeld surface analysis to investigate the close intermolecular contacts

Anion Induced Formation of Supramolecular Associations Involving Lone pair−π and Anion−π Interactions in Co(II) Malonate Complexes: Experimental Observations, Hirshfeld Surface Analyses and DFT Studies

Three Co­(II)–malonate complexes, namely, (C₅H₇N₂)₄[Co­(C₃H₂O₄)₂(H₂O)₂]­(NO₃)₂ (<b>1</b>), (C₅H₇N₂)₄[Co­(C₃H₂O₄)₂(H₂O)₂]­(ClO₄)₂ (<b>2</b>), and (C₅H₇N₂)₄[Co­(C₃H₂O₄)₂(H₂O)₂]­(PF₆)₂ (<b>3</b>) [C₅H₇N₂ = protonated 2-aminopyridine, C₃H₄O₄ = malonic acid, NO₃^– = nitrate, ClO₄^– = perchlorate, PF₆^– = hexafluorophosphate], have been synthesized from purely aqueous media, and their crystal structures have been determined by single crystal X-ray diffraction. A thorough analysis of Hirshfeld surfaces and fingerprint plots facilitates a comparison of intermolecular interactions in <b>1</b>–<b>3</b>, which are crucial in building supramolecular architectures. When these complexes are structurally compared with their previously reported analogous Ni­(II) or Mg­(II) compounds, a very interesting feature regarding the role of counteranions has emerged. This phenomenon can be best described as anion-induced formation of extended supramolecular networks of the type lone pair−π/π–π/π–anion−π/π–lone pair and lone pair−π/π–π/π–anion involving various weak forces like lone pair−π, π–π, and anion−π interactions. The strength of these π contacts has been estimated using DFT calculations (M06/6-31+G*), and the formation energy of the supramolecular networks has been also evaluated. The influence of the anion (NO₃^–, ClO₄^–, and PF₆^–) on the total interaction energy of the assembly is also studied

Melamine-mediated self-assembly of a Cu(II)–methylmalonate complex assisted by <i>π</i>⁺–<i>π</i>⁺ and anti-electrostatic H-bonding interactions

<p>A Cu(II)-methylmalonate complex, (C₃H₇N₆)₄[Cu(II)(C₄H₄O₄)₂](H₂O)₄Cl₂ (<b>1</b>) (where C₃H₇N₆ = protonated melamine, C₄H₄O₄ = methylmalonic acid), has been synthesized from aqueous media and its crystal structure was determined by single-crystal X-ray diffraction. The anionic Cu(II)-methylmalonate complex mediated formation of interesting supramolecular assemblies in the solid state by means of ionic interactions with protonated melamine. Moreover, other forces such as antielectrostatic H-bonding and <i>π</i>⁺–<i>π</i>⁺ interactions also play a crucial role in defining the final 3-D architecture of <b>1</b>. An interesting stacking among protonated melamine molecules is studied by DFT calculations. Lattice water molecules and chlorides form various hydrogen bonds to take part in the self-assembly processes.</p