Alkoxide Oxygen Research Articles

Postsynthetically Modified Alkoxide-Exchanged Ni2(OR)2BTDD: Synergistic Interactions of CO2 with Open Metal Sites and Functional Groups.

Postsynthetic modifications (PSMs) of metal-organic frameworks (MOFs) play a crucial role in enhancing material performance through open metal site (OMS) functionalization or ligand exchange. However, a significant challenge persists in preserving open metal sites during ligand exchange, as these sites are inherently bound by incoming ligands. In this study, for the first time, we introduced alkoxides by exchanging bridging chloride in Ni2Cl2BTDD (BTDD=bis (1H-1,2,3,-triazolo [4,5-b],-[4',5'-i]) dibenzo[1,4]dioxin) through PSM. Rietveld refinement of synchrotron X-ray diffraction data indicated that the alkoxide oxygen atom bridges Ni(II) centers while the OMSs of the MOF are preserved. Due to the synergy of the existing OMS and introduced functional group, the alkoxide-exchanged MOFs showed CO2 uptakes superior to the pristine MOF. Remarkably, the tert-butoxide-substituted Ni_T exhibited a nearly threefold and twofold increase in CO2 uptake compared to Ni2Cl2BTDD at 0.15 and 1 bar, respectively, as well as high water stability relative to the other exchanged frameworks. Furthermore, the Grand Canonical Monte Carlo simulations for Ni_T suggested that CO2 interacts with the OMS and the surrounding methyl groups of tert-butoxide groups, which is responsible for the enhanced CO2 capacity. This work provides a facile and unique synthetic strategy for realizing a desirable OMS-incorporating MOF platform through bridging ligand exchange.

Postsynthetically Modified Alkoxide‐Exchanged Ni2(OR)2BTDD: Synergistic Interactions of CO2 with Open Metal Sites and Functional Groups

AbstractPostsynthetic modifications (PSMs) of metal–organic frameworks (MOFs) play a crucial role in enhancing material performance through open metal site (OMS) functionalization or ligand exchange. However, a significant challenge persists in preserving open metal sites during ligand exchange, as these sites are inherently bound by incoming ligands. In this study, for the first time, we introduced alkoxides by exchanging bridging chloride in Ni2Cl2BTDD (BTDD=bis (1H‐1,2,3,–triazolo [4,5‐b],–[4′,5′‐i]) dibenzo[1,4]dioxin) through PSM. Rietveld refinement of synchrotron X‐ray diffraction data indicated that the alkoxide oxygen atom bridges Ni(II) centers while the OMSs of the MOF are preserved. Due to the synergy of the existing OMS and introduced functional group, the alkoxide‐exchanged MOFs showed CO2 uptakes superior to the pristine MOF. Remarkably, the tert‐butoxide‐substituted Ni_T exhibited a nearly threefold and twofold increase in CO2 uptake compared to Ni2Cl2BTDD at 0.15 and 1 bar, respectively, as well as high water stability relative to the other exchanged frameworks. Furthermore, the Grand Canonical Monte Carlo simulations for Ni_T suggested that CO2 interacts with the OMS and the surrounding methyl groups of tert‐butoxide groups, which is responsible for the enhanced CO2 capacity. This work provides a facile and unique synthetic strategy for realizing a desirable OMS‐incorporating MOF platform through bridging ligand exchange.

Synthesis, characterization and X-ray crystal structure of a mononuclear ampyrone based zinc complex

A novel mononuclear zinc complex derived from a pyrazole-based ligand was studied. The structure of the complex was explored using C, H, N, FT-IR, NMR, and UV/Vis studies, thermogravimetric analysis, and single-crystal X-ray crystallography. The single XRD study shows the coordination of the zinc ion to the alkoxide oxygen atom, two chloride ions, and one additional oxygen atom from the ethanol molecule in the complex. The intermolecular interactions in the complex were investigated using Hirshfeld surface analysis and revealed that the H…H is the most important interaction, accounting for 45.9% of the overall crystal packing contribution of the complex.

Synthesis and structural characterization of a new heterometallicmolybdate coordination polymer based on a µ3-bridging amino alcohol

Abstract The reaction of Na2MoO4·2H2O with 2-amino-2-(hydroxymethyl)propane-1,3-diol (LH) in water at room temperature results in the formation of the heterometallic coordination polymer [Mo2O6L2(Na2(H2O)4)]·2H2O 1 (L = 2-amino-3-hydroxy-2-(hydroxymethyl)propan-1-olato). The structure of 1 consists of a neutral (Mo2O6) unit located on an inversion center. The Mo atoms exhibit hexa-coordination and are bonded to two terminal and two bridging oxido ligands, an alkoxide oxygen and the amine N atoms of an anionic ligand L– resulting in the formation of an edge-sharing {Mo2O8N2} bioctahedron. The Na+ cations of a centrosymmetric bis(μ2-aqua)-bridged (Na2(H2O)4)2+ unit are penta-coordinated and bonded to two symmetry related L– ligands via the oxygen atoms of their OH groups. The µ3-bridging tetradentate binding mode of L– results in the formation of a two-dimensional heterometallic coordination polymer. The constituents of 1 viz. (Mo2O6), (L)–, (Na2(H2O)4)2+ and lattice water molecules are interlinked with the aid of three varieties of hydrogen bonding interactions. The corresponding tungstate reported recently has been obtained through a similar synthetic protocol and is isostructural.

Electrochemical Properties and Reactivity Study of [MnV(O)(μ-OR-Lewis Acid)] Cores.

A mononuclear manganese(V) oxo complex of a bis(amidate)bis(alkoxide) ligand, (NMe4)[MnV(HMPAB)(O)] [2; H4HMPAB = 1,2-bis(2-hydroxy-2-methylpropanamido)benzene], was synthesized and structurally characterized. A Mn-Oterm distance of 1.566(4) Å was observed in the solid-state structure of 2, consistent with the Mn≡O formulation. The reaction of redox-inactive metal ions (Mn+ = Li+, Ca2+, Mg2+, Y3+, and Sc3+) with 2 resulted in the formation of 2-Mn+ species, which were characterized by UV-vis, 1H NMR, cyclic voltammetry, and in situ IR spectroscopy. Theoretical calculations suggested that the alkoxide oxygen atoms of the ligand scaffold are energetically most favorable for coordinating the Mn+ ions in 2. Complex 2 revealed one-electron-reduction potential at -0.01 V versus ferrocenium/ferrocene, which shifted anodically upon coordination of Mn+ ions to 2, and such a shift became more prominent with stronger Lewis acids. The oxygen-atom transfer (OAT) reactivities of 2 and 2-Mn+ species with triphenylphosphine were compared, which exhibited a systematic increase of the reaction rate with increasing Lewis acidity of Mn+ ions, and a plot of log k2 versus Lewis acidity of Mn+ ions (ΔE) followed a linear relationship. It was observed that 2-Sc3+ was ca. 3200 times more reactive toward the OAT reaction compared to 2. Hammett analysis of 2 exhibited a V-shaped plot, indicating a change of the reaction mechanism upon going from electron-rich to electron-deficient Ar3P substrates. In contrast, 2-Ca2+ and 2-Sc3+ showed an electrophilic nature toward the OAT reaction, thus demonstrating the role of the Lewis acid in controlling the OAT mechanism. The hydrogen-atom abstraction reaction of 2 and 2-Mn+ adducts with 1-benzyl-1,4-dihydronicotinamide was investigated, and it was observed that the rate of reaction did not vary considerably with the Lewis acidity of Mn+ ions. On the basis of Eyring analysis of 2 and 2-Mn+ adducts, we hypothesized an entropy-controlled hydrogen-atom-transfer reaction for 2-Sc3+, which is different from the reaction mechanism of 2 and 2-Ca2+.

Synthesis and Crystal Structures of New Strontium Complexes with Aminoalkoxy and β-Diketonato Ligands

New heteroleptic strontium complexes were synthesized using substitution reaction of bis(trimethylsilyl)amide of Sr(btsa)2·2DME with aminoalkoxide and β-diketonate ligands. The complexes [Sr(bdmp)(btsa)]2·2THF (1), [Sr(bdeamp)(btsa)]2 (2), [Sr(dadamb)(btsa)]2 (3), [Sr(bdmp)(hfac)]3 (4), [Sr(bdeamp)(hfac)]3 (5), [Sr(dadamb)(hfac)]3 (6), and [Sr3(dadamb)4(tmhd)2] (7) were prepared and characterized by means of various analysis techniques such as Fourier transform infrared, NMR, thermogravimetric analysis, and elemental analysis. Complexes 1–3 were further structurally confirmed by single-crystal X-ray crystallography, and they displayed dimeric structures in which strontium atoms were connected by alkoxide oxygen atoms of the μ2 type. Compound 1 has a trigonal prismatic structure, whereas 2 and 3 have a distorted square pyramidal structure. In complexes 5–7, trimeric structures were obtained with strontium atoms connected by μ3–O bonds of alkoxide oxygen atoms and μ2–O bonds of alkoxide and β-diketonate oxygen atoms. The crystal structures of 5, 6, and 7 showed distorted capped octahedral geometry, while 7 (middle Sr atom) displayed a distorted trigonal prism geometry. Complexes 5–7 displayed ∼70% mass loss in the temperature range from 25 to 315 °C.

Molybdenum complexes with citrate revisited. A mononuclear [MoVOCl4(H2O)]− ion as a new synthetic entry

Products of the reaction of a mononuclear molybdenum(V) compound, (PyH)5[MoVOCl4(H2O)]3Cl2, with citric acid (IUPAC name: 2-hydroxypropane-1,2,3-tricarboxylic acid, abbreviated as H4cit) were characterized by spectroscopic techniques and X-ray structure analysis. When the supply of oxygen was limited, molybdenum retained the initial oxidation state and a tetranuclear Mo(V) compound formed, (MeNC5H5)4[MoV4O8(cit)2]·2H2O (1) (MeNC5H5+ = N-methylpyridinium cation). Otherwise, oxidation of metal to +6 state took place resulting in {(C6H5)4P}[MoVIO2Cl(H2cit)] (3), (PyH)2[MoVIO2(H2cit)2]·Py (4), (PyH)4[{MoVIO2(Hcit)}2(µ2-O)]·2Py (5), (PyH)4[MoVI4O11(Hcit)2]·2H2O·1/2CH3OH (6) and (PyH)6[MoVI4O11(Hcit)2]Cl2·H2O·3/2CH3OH (7) (Py = pyridine, C5H5N; PyH+ = pyridinium cation, C5H5NH+). In one instance, (PyH)2[MoVOCl2(Hcit)]0.39[MoVIO2Cl(Hcit)]0.61·CH3CN (2), a compound that is a solid mixture of Mo(V) and Mo(VI) species was obtained. In all complexes, the citrate coordinated to molybdenum in a chelating manner with at least two oxygen atoms that belonged to deprotonated OH and α-carboxyl groups. In {MoVIO2}2+ species, the two trans positions of the Mo=O moieties were occupied with carboxylate oxygens. In order to assess stabilities of different orientations of citrate ligands with respect to Mo=O, the isomeric forms of [MoVIO2Cl(H2cit)]− and [MoVIO2(H2cit)2]2−, the anions of 3 and 4, were subjected to DFT calculations. Results reveal that the relative energies of isomers are close to one another. A particular orientation of citrate does not seem to have a determining impact. As the case of the [MoVIO2(H2cit)2]2− isomers, which gain stability by intramolecular hydrogen bonds, shows, other factors are at work. For the metal–metal bonded {MoV2O4}2+ compounds, comparison with the known complexes suggests that a concept of a preferential orientation of citrate, i.e., with the alkoxide oxygen situated at the site trans to Mo=O, is not valid.

Diversity of Coordination Modes in a Flexible Ditopic Ligand Containing 2-Pyridyl, Carbonyl and Hydrazone Functionalities: Mononuclear and Dinuclear Cobalt(III) Complexes, and Tetranuclear Copper(II) and Nickel(II) Clusters

Syntheses, crystal structures and characterization are reported for four new complexes [Cu4Br2(L)4]Br2 (1), [Ni4(NO3)2(L)4(H2O)](NO3)2 (2), [Co2(L)3](ClO4)3 (3) and [Co(L)2](ClO4) (4), where L− is the monoanion of the ditopic ligand N′-(1-(pyridin-2-yl)ethylidene)pyridine-2-carbohydrazide (LH) built on a picolinoyl hydrazone core fragment, and possessing a bidentate and a tridentate coordination pocket. The tetranuclear cation of 1·0.8H2O·MeOH is a strictly planar, rectangular [2 × 2] grid. Two 2.21011 L− ligands bridge adjacent CuII atoms on the short sides of the rectangle through their alkoxide oxygen atoms, and two 2.11111 ligands bridge adjacent CuII atoms on the long sides of the rectangle through their diazine groups; two metal ions are 5-coordinate and two are 6-coordinate. The tetranuclear cation of 2·0.2H2O·3EtOH is a square [2 × 2] grid. The two 6-coordinate NiII atoms of each side of the square are bridged by the alkoxide O atom of one 2.21011 L− ligand. The dinuclear cation of 3·0.8H2O·1.3MeOH contains two low-spin octahedral CoIII ions bridged by three 2.01111 L− ligands forming a pseudo triple helicate. In the mononuclear cation [Co(L)2]+ of complex 4, the low-spin octahedral CoIII center is coordinated by two tridentate chelating, meridional 1.10011 ligands. The crystal structures of the complexes are stabilized by a variety of π–π stacking and/or H-bonding interactions. Compounds 2, 3 and 4 are the first structurally characterized nickel and cobalt complexes of any form (neutral or anionic) of LH. The 2.01111 and 1.10011 coordination modes of L−, observed in the structures of complexes 3 and 4, have been crystallographically established for the first time in coordination complexes containing this anionic ligand. Variable-temperature, solid-state dc magnetic susceptibility and variable-field magnetization studies at 1.8 K were carried out on complexes 1 and 2. Antiferromagnetic metal ion···metal ion exchange interactions are present in both complexes. The study reveals that the cation of 1 can be considered as a practically isolated pair of strongly antiferromagnetically coupled (through the diazine group of L−) dinulear units. The susceptibility data for 2 were fit to a single-J model for an S = 1 cyclic tetramer. The values of the J parameters have been rationalized in terms of known magnetostructural correlations. Spectral data (infrared (IR), ultraviolet/visible (UV/VIS), 1H nuclear magnetic resonance (NMR) for the diamagnetic complexes) are also discussed in the light of the structural features of 1–4 and the coordination modes of the organic and inorganic ligands that are present in the complexes. The combined work demonstrates the ligating flexibility of L−, and its usefulness in the synthesis of complexes with interesting structures and properties.

Synthesis, crystal structure and spectroscopic properties of a new type of pentanuclear zinc(II) complex

The article describes synthesis, crystal structure, ESI-MS and steady state and time-resolved fluorescence spectrum of a pentanuclear zinc(II) compound of composition [ZnII5(HL)2(μ3-OH)2(NO3)4] (1), where H3L is the [1 + 2] condensation product of 4-ethyl-2,6-diformylphenol and 2-amino-2-methyl-1-propanol. The phenol moiety and one of the two alcohol moieties of a H3L get deprotonated and therefore two ONO compartments of [HL]2− in 1 are inequivalent (O(alkoxide)N(imine)O(phenoxide) and O(alcohol)N(imine)O(phenoxide)). Each of the two compartments of each [HL]2− in 1 is occupied by a ZnII ion, to form a ZnII2 fragment. Two such fragments are linked by two μ3-hydroxide ligands to form a dimer-of-dinuclear type ZnII4 fragment. Each of the two alkoxide oxygen atoms bridges one ZnII in the ZnII4 fragment and a fifth ZnII ion (dangling metal ion) to form the pentanuclear system. The structure reveals that it is a new type of pentanuclear system among the related compounds. Notably, the pentanuclear moiety is retained in solution as evidenced from conductance measurement. The title compound exhibits fluorescence behaviour with the emission band at 447 nm on excitation at 384 nm. The quantum yield is 0.05. TCSPC reveals a monoexponential decay in 1. ESI-MS of 1 reveal the appearance of signals of positive ions corresponding to ZnII5, ZnII2 and ZnII metallic systems as well as to H3L ligand system.

Fe6 clusters of tripodal alcohol ligands: Synthesis, structures and magnetostructural properties

The synthesis, crystal structures, cyclic voltammetric and magnetic characterization of (NEt3H)2[FeIII6Cl6(µ6-O)(thme)4]·3(H2O) (1) (H3thme = trishydroxymethyl ethane) and (NEt3H)2[FeIII6Cl6(µ6-O)(thmp)4]·5.5(H2O) (2) (H3thmp = trishydroxymethyl propane) are reported. Both complexes crystallized in the orthorhombic system and have the space group Pca21. The metallic cores of the complexes comprise of the [(μ6-O)Fe6(μ-O)12] unit. The ligands thme3− and thmp3− adopt the same η2, η2, η2, µ3-bridging mode of the alkoxide oxygen atoms in 1 and 2. Electrochemical studies are consistent with formation of stable quasi-reversible one electron FeII/III redox couples in solution. The fascinating features of the non-covalent supramolecular contacts have been explored and corroborated theoretically by Hirshfeld surface analysis along with electrostatic potential (ESP), deformation density (DD) and ab initio calculations. The interplay of the interactions, (i.e., H⋯O, Cl⋯H and other contacts) give rise to the consolidation of discrete units of 1 and 2 into supramolecular architectures. Variable-temperature and field (H) solid-state direct and alternating current magnetic susceptibility measurements were done on samples of 1 and 2 in the temperature range 1.8–300 K. Analysis of the data confirmed the presence of dominant antiferromagnetic interactions, leading to an S = 0 ground state, and no out of phase ac magnetic susceptibility signals were observed for either cluster, ruling out single-molecule magnet behaviour.

How Does CO2 React with Styrene Oxide in Co-MOF-74 and Mg-MOF-74? Catalytic Mechanisms Proposed by QM/MM Calculations

We report a QM/MM computational study of the cycloaddition between carbon dioxide (CO2) and styrene oxide in two different metal–organic frameworks (MOFs), Co-MOF-74 and Mg-MOF-74, to obtain atomic-level insights into the catalytic mechanism. The results suggest that both reactions begin by forming Lewis acid–base pairs between the epoxide and an open metal site and between CO2 and a phenolate moiety in the linker. Consequently, higher electrophilic and nucleophilic reactivities are conferred on the epoxide and CO2 (CO2(A)), respectively, thereby facilitating the initial ring-opening of the epoxide moiety. The ring-opening process is followed by the adsorption of a second CO2 molecule (CO2(B)), which is necessary for the subsequent ring closure to occur. In the case of Co-MOF-74, the binding of CO2(B) to the alkoxide oxygen increases the flexibility of the substrate moiety, enabling the cyclization pathway in which CO2(A) is incorporated into the final product. In contrast, the carbonate intermediate in t...

Ligand Effect on the Single-Molecule Magnetism of Tetranuclear Co(II) Cubane.

A clear dependence on the ligand has been observed for the magnetic properties of a closely related series of Co(II) cubane structures, viz. [Co4(mbm or bm)4(ROH)4Br4] (1-MeOH, 1-EtOH, 2-MeOH, and 2-EtOH, where 1 = [Co4(mbm)4Br4], 2 = [Co4(bm)4Br4], bm = (1H-benzo[d]imidazol-2-yl)methanolate. and mbm = 1-Me-bm.) The [Co4(OR)4] cubane core consists of an octahedral CoII center chelated by the alkoxide oxygen and imidazole nitrogen atoms from monoanionic bm or mbm and coordinated by methanol/alcohol and bromine. Interestingly, electrospray ionization mass spectrometry (ESI-MS) indicates that 1-MeOH and 2-MeOH are unstable in methanol and transformed to the butterfly [Co4L6]2+ but that 1-EtOH and 2-EtOH are stable in ethanol. Their magnetic susceptibilities suggest ferromagnetic coupling between the nearest cobalt centers to give a theoretical S = 4 × 3/2 ground state with considerable magneto-crystalline behavior. The packing and intermolecular interactions appear to influence the geometry of the cubes and thus the anisotropy of cobalt, which leads to different blocking temperatures (TB). Consequently, the compounds with mbm, 1-MeOH and 1-EtOH, exhibit TB > 2 K as shown by the relaxation of magnetization in zero applied dc field where the barriers Ueff/kB are respectively 27 and 21 K and relaxation times are τ0 = 1.3 × 10-9 and 9.7 × 10-9 s. However, the compounds with bm, 2-MeOH and 2-EtOH, remain paramagnetic above 2 K and do not show nonlinear response of the ac susceptibilities. These findings reaffirm the subtle dependence of single-molecule magnetism on coordination geometry and intermolecular interaction.

Synthesis, characterization and single crystal structures of chiral Schiff base and its tetranuclear palladium complex with Pd[sbnd]O[sbnd]Pd bridging and Pd[sbnd]Pd bonds

A new chiral Schiff base ligand 2-{N-[(2S)-(1-hydroxy-3-phenylpropan-2-yl]ethanimidoyl} phenol ((S)H2L) was obtained by acid catalyzed condensation of (2S)-(−)-2-amino-3-phenyl-1-propanol with 2′-hydroxyacetophenone. The palladium complex was prepared by treating a solution of (S)H2L in acetone with a solution of Na2PdCl4 in water in 1:1 M ratio. The new ligand and its complex were characterized by FT-IR, 1H, 13C{1H} NMR spectroscopy, polarimetry and elemental analysis and their molecular structures were determined by single crystal X-ray diffraction. Both the compounds crystallizes in monoclinic system in the space group P21. There exists an intra [OH⋯N (1.62(5) Å)] and intermolecular [OH⋯O (1.53(5) Å) and CH⋯O (2.59 Å)] hydrogen bonding and secondary interactions in the crystal of (S)H2L. The structure of the palladium complex was found very interesting wherein the ligand coordinated to metal center as tridentate dianionic (O−, N, O−) fashion, (S)-L, resulting in a tetranuclear palladium cluster, [Pd4((S)-L)4]. In these supramolecular structures phenolate oxygen coordinated to Pd(II) ion as PdO terminal bonds [1.934(12) - 1.977(11) Å] and the alkoxide oxygen coordinated as PdOPd bridging bonds [1.993(11) - 2.012(12) Å]. The PdN bond lengths found were in the range of 1.949(13) to 1.919(12) Å. There exists two asymmetric tetranuclear complex molecules in its crystal lattice. There exists very strong metal-metal bond interaction, Pd(2)Pd(3) [3.0410(18) Å] and Pd(6)Pd(7) [3.0517(19) Å] respectively in the two asymmetric units.

The magnetic exchange interaction in bi- and tetranuclear copper(II) complexes with the bis-azomethine of 1,3-diaminopropanol-2 and 4-hydroxy-3-formylcoumarin with an azide exogenous bridge

By the reaction of Cu(II) perchlorate with the condensation product of 1,3-diamino-2-propanol and 4-hydroxy-3-formylcoumarin (H3L) in the presence of sodium azide, a tetranuclear copper(II) complex with the composition [Cu2L(μ2-1,1-N3)]2 (I) was obtained. On crystallization from a pyridine solution, the crystalline adduct [Cu2L(μ2-N3)(py)2] (II) (py=pyridine) was obtained and further characterized by XRD. The copper ions in (II) are coordinated by ONO’NO donor atoms of the triply deprotonated pentadentate ligand and the azide anion is coordinated in an asymmetric µ2-1,1 fashion, favoring a “roof”-like conformation of the molecule. The pyridine molecules are coordinated to each copper center, completing the distorted square-pyramidal coordination environment, but in a different way – for one copper ion the solvent molecule populates an apical position, for the other – a position in the basal plane. The tetranuclear complex is formed by linking two binuclear [Cu2L] units by two bridging μ2–1,1–N3 groups in a head-to-tail fashion. The temperature dependence of the magnetic susceptibility was studied for both I and II samples, showing a medium strength overall antiferromagnetic exchange for I and a weak ferromagnetic one with 2J=+5cm−1 for II. Two major superexchange pathways in I – through an alkoxide oxygen atom of the polydenate ligand and through the bridging azide-anion are characterized by opposite magnetic interaction characters – antiferromagnetic (2JO=−328cm−1) and ferromagnetic (2JN=+132cm−1), correspondingly. Quantum-chemical modeling using the broken symmetry approach was performed for II and possible binuclear isomers of I, differing in the azide-anion coordination mode and conformation of the polydentate ligand, and for two possible tetranuclear dimers.

Carbonyl Activation by Borane Lewis Acid Complexation: Transition States of H2 Splitting at the Activated Carbonyl Carbon Atom in a Lewis Basic Solvent and the Proton-Transfer Dynamics of the Boroalkoxide Intermediate.

By using transition-state (TS) calculations, we examined how Lewis acid (LA) complexation activates carbonyl compounds in the context of hydrogenation of carbonyl compounds by H2 in Lewis basic (ethereal) solvents containing borane LAs of the type (C6 F5 )3 B. According to our calculations, LA complexation does not activate a ketone sufficiently enough for the direct addition of H2 to the O=C unsaturated bond; but, calculations indicate a possibly facile heterolytic cleavage of H2 at the activated and thus sufficiently Lewis acidic carbonyl carbon atom with the assistance of the Lewis basic solvent (i.e., 1,4-dioxane or THF). For the solvent-assisted H2 splitting at the carbonyl carbon atom of (C6 F5 )3 B adducts with different ketones, a number of TSs are computed and the obtained results are related to insights from experiment. By using the Born-Oppenheimer molecular dynamics with the DFT for electronic structure calculations, the evolution of the (C6 F5 )3 B-alkoxide ionic intermediate and the proton transfer to the alkoxide oxygen atom were investigated. The results indicate a plausible hydrogenation mechanism with a LA, that is, (C6 F5 )3 B, as a catalyst, namely, 1) the step of H2 cleavage that involves a Lewis basic solvent molecule plus the carbonyl carbon atom of thermodynamically stable and experimentally identifiable (C6 F5 )3 B-ketone adducts in which (C6 F5 )3 B is the "Lewis acid promoter", 2) the transfer of the solvent-bound proton to the oxygen atom of the (C6 F5 )3 B-alkoxide intermediate giving the (C6 F5 )3 B-alcohol adduct, and 3) the SN 2-style displacement of the alcohol by a ketone or a Lewis basic solvent molecule.

Binuclear zinc(II) complexes of N(4)-substituted bis(thiosemicarbazone) ligands incorporating hydroxyl group and their non-hydroxyl analogues

We have reported the synthesis and physical and chemical studies of the new zinc complexes of N(4)-substituted bis(thiosemicarbazones). The complexes characterized by spectroscopic methods. The crystal structures of the compounds [Zn2L2(CH3COO)]·EtOH and [Zn2L4(CH3COO)]·MeOH is determined. Bis(thiosemicarbazones) involving a pendant OH group having SNONS donor sequences can hold two metal ions in close proximity. The coordination sphere has been formed by the imine nitrogen and sulfur atom, and the remaining positions, in a square-based pyramid, has been occupied by three oxygen atoms derived of pendant OH, ether group and acetate ligand. Acetate ligand and alkoxide oxygen are bridged between two metal centers. Whereas, their non-hydroxyl analogues are dimer. In dimer complexes, two ligands are tetradentate coordinated via N2S2 donor atoms. The presence of bulky groups on the nitrogens of thiosemicarbazide enforce an s-trans conformation for the H2L1 and H2L3 ligands in [ZnL1]2·MeOH and [ZnL3]2·MeOH. Fluorescence studies suggest that the [ZnL1]2·MeOH complex can be utilized as a fluorescent imaging agent.

H-bonding directed formation of 1D-single chains, 2D-sheets, and 3D structures in magnetically coupled tetranuclear nickel(II) complexes with incomplete double cubane core

Reactions of 2,6-diformyl-4-methylphenol (DFMF) with 1-amino-2-propanol (AP) and 2-amino-1,3-propanediol (APD) in the presence of nickel(II) salts, NiX2 (X=CH3CO2−, NO3−, Cl−, ClO4−, BF4−)/sodium azide (NaN3), and triethylamine (TEA), in one pot self-assemble giving tetranuclear coordination complexes of nickel(II) with an incomplete double cubane structural core: [Ni4(H2L−1)2(μ-CH3CO2)2(μ3-X)2(μ-1,1-N3)2]·2CH3OH {X=1,1,1-N3 (1A) and OCH3 (1B)}, [Ni4(H2L−1)2(μ-1,1-N3)2(N3)2(μ3-OCH3)2 (H2O)2] (2), [Ni4(H2L−1)2(μ3-1,1,1-N3)2(μ-1,1-N3)2(CH3OH)2Cl2]·4CH3OH (3), [Ni4(H4L1−1)2(μ3-1,1,1-N3)2(μ-1,1-N3)2(CH3OH)2(OH)2]·2H2O (4), and [Ni4(HL2−2)2(APD−1)2][ClO4]2·2H2O·2CH3OH (5). In 1–3, H2L−1, in 4, H4L1−1, and in 5, HL22− represent a singly deprotonated chiral, potentially pentadentate trianionic double Schiff base ligand, H3L; a singly deprotonated potentially heptadentate pentaanionic double Schiff base ligand, H5L1; and a doubly deprotonated potentially tetradentate trianionic single Schiff base ligand, H3L2 respectively. In 1A, 3, and 4 two dinickel (Ni2) units are connected through two doubly bridging end-on (EO) μ-azido and two triply bridging EO μ3-azido bridges and in 2, though two EO μ-azido and two μ3-1,1,1-methoxy bridges generating tetranuclear units. In each dinuclear unit two nickel centers are bridged through one μ-phenolate oxygen from H2L−1, one μ-CH3CO2−, and one EO μ3-azido nitrogen (1) and through one μ-phenolate oxygen from H2L−1/H4L1−1 and one μ3-methoxy oxygen (2)/one μ3-azido nitrogen (3 and 4). In 5, two nickel(II) ions in the dinuclear unit are bridged though phenoxide oxygen and alkoxide oxygen bridges and two dinuclear units are interconnected via six symmetry related alkoxide bridges forming tetranickel units. In 1–4 neutral tetranuclear [Ni4] units and in 5, cationic tetranickel [Ni4]2+ units are interconnected by a symmetrical network of H-bonding between coordinated/uncoordinated hydroxido groups in the side arms of the double Schiff-base ligands, coordinated hydroxide ion/chloride ion, and uncoordinated methanol/water molecules producing H-bonding stabilized 2D-sheet structures (1, 2, 4), a 1D-single chains (3), and a 3D structure (5). Bulk magnetization measurements on 1–4 demonstrate that the magnetic interactions are completely dominated by an overall ferromagnetic coupling occurring between Ni(II) ions with coupling constants (J1, J2, and J3) ranging from 13.19 to 22.10, (−9.05) to (1.74), and 14.79 to 25.59cm−1 respectively (in the Ĥ=-2J1(Ŝ1Ŝ3+Ŝ2Ŝ4)-2J2(Ŝ1Ŝ4+Ŝ2Ŝ3)-2J3(Ŝ1Ŝ2)+D(ŜZ12+ŜZ22+ŜZ32+ŜZ42) expression). [Display omitted]

PH sensitive functionalized graphene oxide as a carrier for delivering gemcitabine: A computational approach

Atomistic Molecular Dynamics (MD) simulations of both the physisorption process and the hydration behavior of the aggregate consisting of gemcitabine (GEM) - poly(acrylic acid) (PAA) - graphene oxide (GO) are performed in explicit solvent medium at two different pH values. The different ionization of pH sensitive groups characterizing the aggregate influences the energy of physisorption and the hydration shell around the complex. The decrease of negative charge density, occurring at acid pH, facilitates the physisorption process between PAA and GO. On the other hand, at the same pH value, a weaker interaction between GEM and PAA is observed. MD simulations show a partial physisorption of the drug from the polymer at pH=4.0. pH change affects also morphology of GO sheet which is more extended at basic pH value due to intrasheet electrostatic interactions. Radial distribution function (RDF) indicates that carboxylate oxygens of PAA and alkoxide oxygens of GO strongly attract dipolar water molecules, thus affecting the hydration shell around the complex in comparison with unionized GO and bulk water. Finally, an analysis based on H-bond correlation function clearly shows that: (a) hydrogen bond between both carboxylate and alkoxide oxygen atoms are longer lived than those observed in unionized GO and bulk water; (b) RDF decays slower for PAA (f=0.8) with respect to ionized GO, hence indicating that COO− groups bind water molecules stronger than R-O−.

Interesting copper(ii)-assisted transformations of 2-acetylpyridine and 2-benzoylpyridine.

The reactions of various copper(ii) sources with 2-acetylpyridine, (py)(me)CO, and 2-benzoylpyridine, (py)(ph)CO, under strongly basic conditions have been studied and novel ligand transformations have been discovered. Reaction of Cu(ClO4)2·6H2O and (py)(me)CO in the presence of NBu4(n)OMe (1 : 1 : 1) in CHCl3 gave a mixture of [Cu2Cl2(HLA)2](ClO4)2 (1) and [Cu2Cl2(LB)2(ClO4)2] (2), where HLA is 3-hydroxy-1,3-di(pyridin-2-yl)-butane-1-one and LB is the zwitterionic-type ligand 3-hydroxy-1-methyl-3-(pyridin-2-yl)-3H-indolizin-4-ium. The ligand HLA is formed through an aldol reaction-type mechanism, while the formation of LB takes place via an intramolecular nucleophilic attack of the remote 2-pyridyl nitrogen atom on the positive carbonyl carbon of HLA, after the transformation of the latter through deprotonation and dehydration. The Cu(II) ions in 1 are bridged by two 2.1111 HLA ligands resulting in a long Cu(II)Cu(II) distance (5.338 Å); the metal ions in 2 are triply bridged by the alkoxide oxygen atoms of the two 2.21 LB ligands and one 2.1100 perchlorato group. The absence of α-hydrogens in (py)(ph)CO leads the reactivity of this ligand in the presence of Cu(II) to different pathways. The Cu(ClO4)2·6H2O/(py)(ph)CO/NBu4(n)OMe reaction mixture in MeOH/H2O (25 : 1 v/v) gave the dinuclear cationic complex [Cu2{(py)(ph)CO}2(LC)2](ClO4)2 (3), where LC(-) is the anion of (methoxy)(phenyl)(pyridin-2-yl)methanol formed in situ via the nucleophilic addition of MeO(-) to the carbonyl carbon of (py)(ph)CO upon Cu(II) coordination. The Cu(II) ions in the cation are doubly bridged by the deprotonated oxygen atoms of the two LC(-) ligands. Replacement of Cu(ClO4)2·6H2O with Cu(NO3)2·3H2O and NBu4(n)OMe with NMe4OH and the decrease of the H2O concentration in the above reaction system yielded the tetranuclear coordination cluster [Cu4(OMe)2(NO3)4{(py)(ph)CO}2(LC)2] (4). The Cu(II) centres in this complex define a parallelogram. Two parallel sides of the parallelogram are each supported by deprotonated oxygen atoms belonging to a 2.21 LC(-) ligand and a 2.2 MeO(-) group. The metal ions that define each of the other two sides are singly bridged by an oxygen atom of a 2.210 nitrato group. No bridging exists between the Cu(II) ions that define the two diagonals of the parallelogram. Replacement of MeOH with EtOH in the reaction system that gave 4 resulted in the dinuclear complex [Cu2(NO3)2(LD)2)(EtOH)] (5), LD(-) being the anion of (ethoxy)(phenyl)(pyridin-2-yl)methanol. The Cu(II) ions are doubly bridged by the alkoxide oxygen atoms of the two 2.21 LD(-) ligands. The 1 : 1 : 1 Cu(NO3)2·3H2O/(py)(ph)CO/NMe4OH reaction system in CH3NO2 gave the dinuclear complex [Cu2(NO3)2(LE)2] (6), where LE(-) is the anion of 2-nitro-1-phenyl-1-(pyridin-2-yl)ethanol. The OH(-) ion abstracts one of the methyl hydrogens of CH3NO2, and once the carbanion (-):CH2NO2 is formed it attacks the positive (δ+) carbonyl carbon of (py)(ph)CO; as the carbanion forms the new C-C bond, the π electrons of the carbonyl group of the original ligand are transferred completely to oxygen forming the alkoxide-type ligand LE(-). The Cu(II) ions are doubly bridged by the alkoxide oxygen atoms of the two 2.21 LE(-) ligands. Simplified mechanistic views of the Cu(II)-assisted formation of the transformed ligands are proposed. Dc magnetic susceptibility studies in the 2-300 K range for the representative complexes 3-6 reveal the presence of very strong antiferromagnetic Cu(II)Cu(II) exchange interactions in the dinuclear complexes 3, 5, and 6 and within the dimeric {Cu2(OMe)(NO3){(py)(ph)CO}(LC)}(+) subunits of 4. The strong antiferromagnetic coupling is discussed in terms of the large Cu-O-Cu angles (101.0-102.9°) in the dinuclear, planar {Cu2O2} units/subunits of 3-6.

Alkoxide coordination of iron(iii) protoporphyrin IX by antimalarial quinoline methanols: a key interaction observed in the solid-state and solution

The quinoline methanol antimalarial drug mefloquine is a structural analogue of the Cinchona alkaloids, quinine and quinidine. We have elucidated the single crystal X-ray diffraction structure of the complexes formed between racemic erythro mefloquine and ferriprotoporphyrin IX (Fe(iii)PPIX) and show that alkoxide coordination is a key interaction in the solid-state. Mass spectrometry confirms the existence of coordination complexes of quinine, quinidine and mefloquine to Fe(iii)PPIX in acetonitrile. The length of the iron(iii)-O bond in the quinine and quinidine complexes as determined by Extended X-ray Absorption Fine Structure (EXAFS) spectroscopy unequivocally confirms that coordination of the quinoline methanol compounds to Fe(iii)PPIX occurs in non-aqueous aprotic solution via their benzylic alkoxide functional group. UV-visible spectrophotometric titrations of the low-spin bis-pyridyl-Fe(iii)PPIX complex with each of the quinoline methanol compounds results in the displacement of a single pyridine molecule and subsequent formation of a six-coordinate pyridine-Fe(iii)PPIX-drug complex. We propose that formation of the drug-Fe(iii)PPIX coordination complexes is favoured in a non-aqueous environment, such as that found in lipid bodies or membranes in the malaria parasite, and that their existence may contribute to the mechanism of haemozoin inhibition or other toxicity effects that lead ultimately to parasite death. In either case, coordination is a key interaction to be considered in the design of novel antimalarial drug candidates.