Spontaneous Deprotonation Research Articles

From Metal‐Metal Bonding to Supra‐Metal‐Metal Bonding Directed Self‐Assembly: Supramolecular Architectures of Group 10 and 11 Metals with Ligands from Mono‐ to Poly‐Pyrazoles

AbstractThe supramolecular architectures of pyrazole ligands with polynuclear metal centers have established a wide variety of constructing motifs. This review focuses on the metal‐metal bonding and Supra‐Metal‐Metal Bonding directed self‐assembly of supramolecular architectures of pyrazole ligands ranging from simple mono‐pyrazoles and derivatives to bipyrazoles and polypyrazoles with group 10 and 11 elements. The utility of the constructing motifs (μ‐pyrazolato‐N,N′)2 doubly‐bridged dimetal corners in the metal‐metal‐bonding directed self‐assembly approaches with spontaneous deprotonation from bipyrazole to tripyrazole and tetrapyrazolyl calix[4]arene ligands, metallomacrocyclic and cage‐like complexes have been synthesized. The proton switching programmable self‐assembly strategy using pyrazole‐based bifunctional ligands has been developed by Yu. They have obtained organic homo‐ or hetero‐metallic supramolecular complexes from macrocycles, cages, to one dimensional polymers.

Synthesis, characterization and ROS-mediated antitumor effects of palladium(II) complexes of curcuminoids

Based on the synthesis of curcumin and its derivatives from aromatic aldehydes, a novel series of palladium(II) complexes with curcumin (or its derivatives) and 2,2′-bipyridine have been synthesized through a directed self-assembly approach that involves spontaneous deprotonation of the curcuminoid ligands in H2O/acetone solution. These complexes have been characterized by 1H (13C) NMR, HRMS and elemental analysis. Crystal structure of 3h has been determined by X-ray diffraction analysis. Their cytotoxicity was tested by MTT. The preliminary results showed that complexes 3d, 3f, 3h have significant inhibition on proliferation of three carcinoma cells such as MCF-7, HeLa and A549 cells, which were more active than cisplatin. Further mechanistic studies indicated that the tested complex 3h arrested the cell cycle in the S phase and can disrupted mitochondrial membrane potential and induced tumor cell apoptosis through reactive oxygen species (ROS)-dependent pathway.

Phytic Acid Doped Polyaniline Nanofibers for Enhanced Aqueous Copper(II) Adsorption Capability

This study demonstrates the enhanced Cu2+ adsorption capability of polyaniline nanofibers (PAni NFs) by doping of phytic acid. The PAni NFs were synthesized by radical polymerization process using acidic solutions of hydrochloric and phytic acid, yielding chlorinated (Cl-) and phytic acid-doped (Ph-) PAni NFs. The Ph-PAni NFs showed remarkably higher Cu2+-adsorption efficiency than Cl-PAni NFs, presumably owing to high capacity and/or high ionic affinity of the doped phytic acid in Ph-PAni NFs. The pH-dependent adsorption capability exhibited increasing Cu2+ adsorption trend as increasing aqueous pH because of spontaneous deprotonation of the doped phytic acid in a basic environment. Furthermore, Ph-PAni NFs showed stable, high Cu2+ adsorption capability, irrespective of Co2+ concentration in the bimetallic Cu and Co aqueous solution. Surface morphologies of PAni NFs were investigated using electron microscopy, and molecular structures were identified using X-ray photoemission and Fourier transform infrar...

Carbon dioxide capture in 2-aminoethanol aqueous solution from ab initio molecular dynamics simulations

The reaction of carbon dioxide (CO2) with aqueous 2-aminoethanol (MEA) has been investigated using both blue moon ensemble and metadynamics approaches combined with ab initio molecular dynamics (AIMD) simulations. The AIMD simulations predicted the spontaneous deprotonation of the intermediate compound, MEA zwitterion, and they were used to study two possible routes for subsequent proton transfer reactions: the formation of the protonated MEA and the formation of MEA carbamic acid. The free-energy curve depicted by blue moon ensemble technique supported the favorable deprotonation of MEA zwitterion. The overall free-energy profile showed the favorable formation of the ionic products of MEA carbamate ion and protonated MEA.

(Invited) Challenges with Quantum Chemistry-Based Screening of Electrochemical Stability of Lithium Battery Electrolytes

Challenges with the quantum chemistry based-screening of electrochemical stability of solvents and salts with potential applications in lithium batteries are discussed. Initial high throughput screening of carbonate and phosphate-based electrolyte solvents provided insight into first and second reduction and oxidation potentials and reorganization energies of these solvents. It has been found that it was important to include a lithium cation in the screening of semifluorinated solvents. Two reduction pathways has been found for lithium complexed with semifluorinated solvents and salts such as lithium bis(fluorosulfonyl)imide (LiFSI): the low rate defluorination reaction occurring at high potentials and fast solvent or anion reduction occurring at significantly lower potentials. A spontaneous deprotonation of carbonate solvents at the surface of the completely de-lithiated LiNi0.5Mn1.5O4 cathode has been found.

Spin-state dynamics of a photochromic iron(II) complex and its immobilization on oxide surfaces via phenol anchors

This work presents a detailed study of the photo-induced spin-state dynamics of the photochromic iron(II) complex 1, where the metal ion is in the field of a tripodal hexa-imine ligand with protolysable phenol groups. The nature of the complex’s ground state has been identified as a spin singlet by 1H NMR and steady-state UV/vis spectroscopies, and its distorted octahedral structure was analyzed via crystal structure determination. Sub-picosecond and nanosecond time-resolved laser flash photolysis experiments identify the long-lived quintet state of 1 as the selective product of photoexcitation in the UV/vis spectral region. Thermal barriers of spin-state interconversion as a function of solvent and added base are derived from temperature-dependent rates of transient decay. Ground-state recovery is found to be significantly affected by the solvent and is strongly enhanced, in particular, by base-driven solvolysis of the ligand’s phenol groups. Partial spontaneous deprotonation of the phenolic hydroxyl groups of 1 seems to prevail on metal oxide surfaces, i.e. on alumina. Composite materials, like 1 at Al2O3, that retain the characteristic spectral features of the parent iron(II) complex can be readily obtained by wet impregnation of hydrous alumina with solutions of 1.

Why Is the Spontaneous Deprotonation of [Cu(uracil)2](2+) Complexes Accompanied by Enolization of the System?

The reaction-force formalism is applied to carry out a detailed analysis of the mechanisms behind the enolization processes undergone by the complexes formed on interaction of uracil dimers with Cu(2+) ions after spontaneous deprotonation of the resulting complexes. These enolization processes apparently involve a single proton transfer (PT) from an NH group to a carbonyl group of the same uracil moiety, which should involve a rather high activation barrier that prevents the process occurring. However, the reaction-force, chemical-potential, and electronic-flux profiles unambiguously indicate that the actual mechanism involves three low-barrier elementary steps, and this explains why enolization of the [Cu(uracil-H)(uracil)](+) complexes is a highly facile, assisted PT process. All of the observed PT processes show a typical profile for both the chemical potential and the electronic flux associated with the bond-breaking and the bond-formation processes.

Enhanced adsorption of aqueous copper(II) ions using dedoped poly-N-phenylglycine nanofibers

We report on adsorption of aqueous Cu2+ using poly N-phenylglycine (PPG) nanofibers (NFs) prepared by radical polymerization process. The carboxylic acid groups of pristine PPG NFs were deprotonated to form carboxylate groups, resulting in hydrogen-dedoped PPG NFs. The Cu2+-adsorption capability of PPG NFs was markedly increased after the deprotonation, presumably due to high ionic attraction feature of carboxylate in dedoped PPG. We further investigated the pH-dependent adsorption capability of PPG NFs and found much higher Cu2+ adsorption capacity under high pH conditions, owing to spontaneous deprotonation by the basic solution. X-ray photoemission confirmed the adsorption of Cu2+ on PPG NFs, and Fourier transform infrared spectroscopic analyses confirmed the reduced density of carboxylic acid by deprotonation. The ability of pristine and dedoped PPG NFs to capture aqueous Cu2+ is discussed. Our method to improve the adsorption capability using surface modification could be readily applied to many other polymeric or organic nanostructures for metal removal.

A QM-MD simulation approach to the analysis of FRET processes in (bio)molecular systems. A case study: complexes of E. coli purine nucleoside phosphorylase and its mutants with formycin A.

Predicting FRET pathways in proteins using computer simulation techniques is very important for reliable interpretation of experimental data. A novel and relatively simple methodology has been developed and applied to purine nucleoside phosphorylase (PNP) complexed with a fluorescent ligand — formycin A (FA). FRET occurs between an excited Tyr residue (D*) and FA (A). This study aims to interpret experimental data that, among others, suggests the absence of FRET for the PNPF159A mutant in complex with FA, based on novel theoretical methodology. MD simulations for the protein molecule containing D*, and complexed with A, are carried out. Interactions of D* with its molecular environment are accounted by including changes of the ESP charges in S1, compared to S0, and computed at the SCF-CI level. FRET probability W F depends on the inverse six-power of the D*-A distance, R da. The orientational factor 0 < k2 < 4 between D* and A is computed and included in the analysis. Finally W F is time-averaged over the MD trajectories resulting in its mean value. The red-shift of the tyrosinate anion emission and thus lack of spectral overlap integral and thermal energy dissipation are the reasons for the FRET absence in the studied mutants at pH 7 and above. The presence of the tyrosinate anion results in a competitive energy dissipation channel and red-shifted emission, thus in consequence in the absence of FRET. These studies also indicate an important role of the phenyl ring of Phe159 for FRET in the wild-type PNP, which does not exist in the Ala159 mutant, and for the effective association of PNP with FA. In a more general context, our observations point out very interesting and biologically important properties of the tyrosine residue in its excited state, which may undergo spontaneous deprotonation in the biomolecular systems, resulting further in unexpected physical and/or biological phenomena. Until now, this observation has not been widely discussed in the literature.

Tuning Supramolecular Chiral Architecture of Molecular Corners from Achiral Dipalladium(II) and Diplatinum(II) Complexes with Achiral Anthracyl Pyrazole Ligand

A novel kind of coordination molecular corners [M2L2]([1=(bpy)Pd]2L2, 2=[(dmbpy)Pd]2L2, 3=[(phen)Pd]2L2,4=[(ppy)Pt]2L2, where bpy=2,2′-bipyridine, dmbpy=4,4′-dimethyl-2,2′-bipyridine, phen=1,10-phenanthroline, ppy=2-phenylpyridine) was synthesized through synergistic metal-metal bonding interaction and spontaneous deprotonation from the pyrazole linkers 4-(4-(anthracen-9-yl)phenyl)-3,5-dimethyl-1H-pyrazole(L). Three complexes 1~3 were characterized by single crystal X-ray diffraction,1H and13 C NMR, ESI-MS and fluorescence spectroscopy. The neutral organometallic corner [(ppy)Pt]2L24 was also determined by1 H and13C NMR, MALDITOF-MS and fluorescence spectroscopy. These achiral molecular corners assemble from different achiral dimetallic centers with the achiral ligand HL in quite different crystal structures, in particular, 3 with a [(phen)Pd]2assembling center crystallizes into a supramolecular chiral architecture.

4,4',5,5'-Tetracarboxy-2,2'-bipyridine Ru(II) sensitizers for dye-sensitized solar cells.

Two Ru(II) sensitizers TCR-1 and TCR-2 bearing four carboxy anchoring groups were prepared using 4,4',5,5'-tetraethoxycarbonyl-2,2'-bipyridine chelate and 4-(5-hexylthien-2-yl)-2-(3-trifluoromethyl-1H-pyrazol-5-yl)pyridine and 6-t-butyl-1-(3-trifluoromethyl-1H-pyrazol-5-yl)isoquinoline, respectively. Dissolution of these sensitizers in DMF solution afforded a light green solution up to 10(-5) M, for which their color gradually turned red upon further dilution and deposition on the surface of a TiO2 photoanode due to the spontaneous deprotonation of carboxylic acid groups. These sensitizers were characterized using electrochemical means and structural analysis time-dependent density functional theory (TDDFT) simulation and were also subjected to actual device fabrication. The as-fabricated DSC devices showed overall efficiencies η = 6.16% and 6.23% versus their 4,4'-dicarboxy counterparts TFRS-2 and TFRS-52 with higher efficiencies of 7.57% and 8.09%, using electrolyte with 0.2 M LiI additive. Their inferior efficiencies are possibly caused by the combination of blue-shifted absorption on TiO2, inadequate dye loading, and the perpendicularly oriented central carboxy groups.

Molecular Dynamics Simulations of Proposed Intermediates in the CO2 + Aqueous Amine Reaction.

Ab initio molecular dynamics simulations of up to 210 ps have been performed on various aqueous intermediates postulated in the CO2 + amine reaction, important for CO2 capture. Observations of spontaneous deprotonation of aqueous carbamate zwitterions R1R2NHCOO(±) by bulk water (instead of additional amine, or via umbrella sampling) are reported apparently for the first time. Carbamic acid structures R1R2NCOOH were observed in some simulations, arising from zwitterions not via classical 1,3-H-shifts but via Grotthuss-style multiple-H(+) transfer pathways that involve bulk H2O and require carbamate anions R1R2NCOO(-) as an intermediate stage along the way. H(+)-bridging complexes, including not only Zundel ions [water·H(+)·water](+) but neutral carbamate complexes [carbamate(-)·H(+)·water], were observed in simulation. These results should assist efforts in improving underlying mechanisms for kinetic modeling.

Tetrapyrazolyl-calix[4]arene-based supramolecular architectures from [Pd16L4] macrocycle to three-dimensional microporous channels

By employing di-palladium complex [(bpy)2Pd2(NO3)2](NO3)2 (where bpy=2, 2′-bipyridine) as corners and tetrapyrazolate calix[4]arene-based ligand (L4−) as linkers, a novel cavity-containing high-positively-charged polypyrazolate-bridged metallo-macrocycle [Pd16L4]16+·16NO3 with Pd(II)⋯Pd(II) dimetal-coordination motifs has been synthesized by a directed coordination driven self-assembly with spontaneous deprotonation of the 1H-bipyrazole ligands in aqueous solution. The counter ions NO3− were trapped within the amphiphilic 3-D microporous channels in the solid state.

Coexistence of molecular Pd6 triangle and Pd8 square self-assembled from naphthyl-dipyrazole with di-palladium motifs

Two novel nanosized bipyrazolate-bridged metallo-macrocycles with dipalladium(II,II) corners, {[(bpy)Pd]8L4}(NO3)8 and {[(bpy)Pd]6L3}(NO3)6 [L = 1, 4-di(3′,5′-dimethyl-1H-pyrazolyl-4yl)naphthalene] have been synthesized through a directed self-assembly approach that involves spontaneous deprotonation of the 1H-bipyrazolyl ligand in aqueous solution. These crown-shaped complexes with deep cavities potentially served as the host to solvent molecules and anions. Such metallo-macrocyclic structures have been characterized by single-crystal X-ray diffraction analysis, elemental analysis, 1H and 13CNMR, high resolution electrospray ionization mass spectrometry, UV–visible spectroscopy, and luminescence spectroscopy.

Insight into Hydrazinium Nitrates, Azides, Dicyanamide, and 5-Azidotetrazolate Ionic Materials from Simulations and Experiments

A transferrable, polarizable, quantum chemistry (QC) based force field has been developed for hydrazinium (N(2)H(5)(+)), monomethylhydrazinium ((CH(3))N(2)H(4)(+)), and dimethylhydrazinium ((CH(3))(2)N(2)H(3)(+)) cations in combination with the nitrate (NO(3)(-)), azide (N(3)(-)), dicyanamide (N(CN)(2)(-)), and 5-azidotetrazolate (CN(7)(-)) anions. Inclusion of the off-atom charge center to represent a lone pair on the hydrazinium-based cations significantly improved the electrostatic potential description around cations and led to overall a more accurate prediction of ionic crystal cell parameters in molecular dynamics (MD) simulations. Seven different ionic systems have been investigated: [N(2)H(5)][NO(3)], [(CH(3))N(2)H(4)][NO(3)], [(CH(3))(2)N(2)H(3)][NO(3)], [N(2)H(5)][CN(7)], [(CH(3))N(2)H(4)][N(3)], [(CH(3))(2)N(2)H(3)][N(3)], [N(2)H(5)][N(CN)(2)]. For all but [(CH(3))(2)N(2)H(3)][NO(3)] and [N(2)H(5)][N(CN)(2)], QC calculations of a single, gas-phase ion pair predicts spontaneous deprotonation of the cation. Crystal lattice parameters obtained from MD simulations for these seven ionic crystals were compared with the previously published experimental data as well as the crystal structure of [N(2)H(5)][N(CN)(2)] determined in this work from X-ray data. In general, MD simulations predicted crystal lattice vectors/angles (volumes) within a 5% (3%) absolute margin of error from experiments, with outlying volume deviations of 5-6.6% for three crystals [(CH(3))N(2)H(4)][N(3)], [N(2)H(5)][NO(3)], and [(CH(3))N(2)H(4)][NO(3)] with combinations of particularly small anions and/or cations. Structural comparisons between ionic materials in the liquid and crystalline states are made, including the observation of two crystalline systems where the crystalline state induces conformational changes in the methylated hydrazinium cations between the gas-phase and liquid states. Elastic constants and estimated shear and bulk moduli were extracted from MD simulations for all seven ionic crystals and correlated with the structural motifs of ion interactions in the crystals.

Redox Properties of Tanaka’s Water Oxidation Catalyst: Redox Noninnocent Ligands Dominate the Electronic Structure and Reactivity

[Ru(2)(OH)(2)(3,6-(t)Bu(2)Q)(2)(btpyan)](2+) ((t)Bu(2)Q, 3,6-di-tert-butyl-1,2-benzoquinone; btpyan, 1,8-bis(2,2':6',2''-terpyridyl)anthracene) is one of a handful of structurally well-defined homogeneous catalysts that can electrocatalytically oxidize water at room temperature. Unfortunately, the exact composition and the chemical properties of the redox intermediates leading to the catalytically competent species remains poorly resolved. On the basis of the UV-vis spectra the catalyst was previously speculated to lose two protons spontaneously to form an intermediate containing the key O-O bond in water. We evaluated this mechanistic scenario computationally and found that the associated pK(a) values are in the range of 21, much too high to justify spontaneous deprotonation under experimental conditions of pH = 4. In later work, the O-O bond formation was speculated to occur after removal of two protons and two electrons. Extensive exploration of the various oxidation and protonation states that the diruthenium complex may access during catalyst activation reveals surprisingly complex electronic structure patterns in several redox intermediates: the quinone and tpy ligands become redox noninnocent, i.e., they participate actively in the electron transfer processes by temporarily storing redox equivalents. On the basis of this new insight into the electronic structure we propose a novel alternative explanation of the spectroscopic observations reported previously and characterize the electronic structure of the key intermediates in detail. Finally, the redox potential for the first two-electron oxidation is evaluated based on our proposed intermediates and predicted to be 0.411 V, which compares well with the experimentally observed broad two-electron wave at ∼0.32 V.

Energy Levels and Redox Properties of Aqueous Mn2+/3+ from Photoemission Spectroscopy and Density Functional Molecular Dynamics Simulation

Energy-resolved photoemission spectroscopy and density functional molecular dynamics simulations are combined to construct an energy level diagram for the Mn(2+/3+) redox reaction in aqueous solution. Two peaks centered at 8.88 and 10.26 eV electron binding energies can be assigned to the Mn2+ hexa-aquo complex with a peak area ratio of 2:2.83. Using the notation of crystal field theory, the peak at lower energies can be interpreted as arising from ionization from the e(g) levels (highest occupied molecular orbital, HOMO), and the peak at higher energies are from ionization of the t(2g) levels. The difference corresponds to the average crystal field splitting, 1.38 eV. From the position of the HOMO level and the absolute redox potential, an experimental value for the reorganization free energy of the aqueous Mn3+ hexa-aquo complex is estimated to be 2.98 eV. Density functional molecular dynamics simulations can reproduce the experimental vertical ionization energy, redox free energy, and reorganization free energies fairly well, provided that the absolute potential shift in periodic boundary conditions, finite size effects, and inaccuracies of the exchange correlation functional are taken into account. Most strikingly, in the simulations, we observe spontaneous and reversible deprotonation of the aqueous Mn3+ hexa-aquo complex to form MnOH(H2O)5(2+) + H+, in line with the low experimental pKa value of this ion. The interconversion between protonation states leads to interesting redox phenomena for aqueous Mn3+, culminating in a bimodal thermal distribution of the electron affinity.

Self-assembly of bowl-like trinuclear metallo-macrocycles

By employing different diimine complexes and a naphthanoimidazolate (L(1)) or benzoimidazolate (L(2)) anion ligand as linker, a series of trimetallo-macrocycles have been synthesized through a directed self-assembly approach that involves spontaneous deprotonation of the ligands in aqueous solution. Some compounds, namely, [M(3)L(3)](NO(3))(3) (M = (18-crown-6-phen)Pt, 1 or 7; (15-crown-5-phen)Pt, 2; (benzo-24-crown-8-phen)Pt, 8; (benzo-24-crown-8-phen)Pd, 9; (15-crown-5-phen)Pd, 4 or 11; (18-crown-6-phen)Pd, 3 or 10; (dmbpy)Pd, 6 or 14; (bpy)Pd, 5 or 13; (bpy)Pt, 12; (N4-Phen)Pd, 15) were synthesized. In all of these compounds, the L(1)or L(2) anion ligands are in a syn, syn, syn orientation which result in a bowl-like cavity that can serve as a host to bind the methyl of a solvent CH(3)CN within the naphthanoimidazolate or benzoimidazolate-built cavity through C-H...pi hydrogen bonds in the crystal state. The structures are characterized by elemental analysis, (1)H NMR, ESI-MS, and in the cases of 4a, 11, 13, and 14a by single-crystal X-ray diffraction analysis.

Solution Self-Assembly, Spontaneous Deprotonation, and Crystal Structures of Bipyrazolate-Bridged Metallomacrocycles with Dimetal Centers

A series of nanosized cavity-containing bipyrazolate-bridged metallomacrocycles with dimetal centers, namely, {[(bpy)M]8L4}(NO3)8 [L=3,3',5,5'-tetramethyl-4,4'-bipyrazolyl, Pd, Pt; 1,4-bis-4'-(3',5'-dimethyl)-pyrazolylbenzene), Pd; and 1,4-bis-4'-(3',5'-dimethyl)-pyrazolylbiphenyl, Pd], {[(phen)M]8L4}(NO3)8 [L=3,3',5,5'-tetramethyl-4,4'-bipyrazolyl, Pd, Pt; 1,4-bis-4'-(3',5'-dimethyl)-pyrazolylbenzene, Pd; and 1,4-bis-4'-(3',5'-dimethyl)-pyrazolylbiphenyl, Pd], {[(bpy)Pd]6L3}(NO3)6 [L=1,4-bis-4'-(3',5'-dimethyl)-pyrazolylbenzene,], {[(phen)Pd]6L3}(NO3)6 [L=1,4-bis-4'-(3',5'-dimethyl)-pyrazolylbenzene,], {[(bpy)Pd]4L2}(NO3)4 [L=1,3-bis-4'-(3',5'-dimethyl)-pyrazolylbenzene, and 1,2-bis-4'-(3',5'-dimethyl)-pyrazolylbenzene,], and {[(phen)Pd]4L2}(NO3)4 [L=1,3-bis-4'-(3',5'-dimethyl)-pyrazolylbenzene, and 1,2-bis-4'-(3',5'-dimethyl)-pyrazolylbenzene,] (where bpy=2,2'-bipyridine and phen=1,10-phenanthroline) have been synthesized through a directed self-assembly approach that involves spontaneous deprotonation of the 1H-bipyrazolyl ligands in aqueous solution. These complexes, with weak Pd(II)...Pd(II) or Pt(II)...Pt(II) interactions, have been characterized by elemental analysis, 1H and 13C NMR, cold-spray ionization or electrospray ionization mass spectrometry, UV-visible spectroscopy, and luminescence spectroscopy. Complexes and have also been characterized by single-crystal X-ray diffraction analysis.

Novel pyrazolate-bridged dinuclear Pd(II) diimine complexes that bind inorganic anions

Two pyrazolate-bridged dinuclear Pd(II) diimine complexes {[(phen)Pd] 2L 2}(NO 3) 2 ( 1) and {[(bpy)Pd] 2L 2}(NO 3) 2 ( 2) have been synthesized through metal–metal interaction and spontaneous deprotonation from 1H-3,5-diphenylpyrazole (HL) in aqueous solution. These complexes show Pd(II)⋯Pd(II) interactions and anion binding have been characterized by elemental analyses, 1H and 13C NMR, ESI-MS, UV–Vis and luminescent spectroscopy, and single crystal X-ray diffraction methods.