Thermal degradation of acetate-intercalated hydroxy double and layered hydroxy salts

Two hydroxy double salts (HDSs), zinc copper hydroxy acetate (ZCA) and zinc nickel hydroxy acetate (ZNA), and an analogous layered compound, zinc hydroxy acetate (ZHA), have been prepared by a coprecipitation method. The thermal degradation of these materials was characterized via thermogravimetric analysis (TGA), differential thermal analysis (DTA), and TGA coupled with Fourier transform infrared spectroscopy of gas-phase products, TGA-FTIR. Loss of physisorbed and interlayer H2O was observed between 50 and 150 degrees C for all compounds. Acetic acid, acetone, water, and CO2 were released at high temperatures with relative acetone yields found to be dependent on precursor identity, with very little formed from ZCA compared with ZHA and ZNA. Combined FTIR and XRD analysis of solid residues extracted at different points in the heating profile suggests that ketonization occurs via dissociative adsorption of acetic acid on ZnO surfaces. Nanometer-sized ZnO particles were formed from ZHA, showing slight preferential growth in the ZnO (002) lattice direction, while the presence of a second metal, Ni or Cu, served to retard ZnO crystallite growth at temperatures below 600 degrees C and eliminate preferential growth. ZCA leads to the formation of reduced copper species (metallic copper and Cu2O) when heated to 250 degrees C.     

Formation and stability of enolates of acetamide and acetate anion: an Eigen plot for proton transfer at alpha-carbonyl carbon

Second-order rate constants were determined in D(2)O for deprotonation of acetamide, N,N-dimethylacetamide, and acetate anion by deuterioxide ion and for deprotonation of acetamide by quinuclidine. The values of k(B) = 4.8 x 10(-8) M(-1) s(-1) for deprotonation of acetamide by quinuclidine (pK(BH) = 11.5) and k(BH) = 2-5 x 10(9) M(-1) s(-1) for the encounter-limited reverse protonation of the enolate by protonated quinuclidine give pK(a)(C) = 28.4 for ionization of acetamide as a carbon acid. The limiting value of k(HOH) = 1 x 10(11) s(-1) for protonation of the enolate of acetate anion by solvent water and k(HO) = 3.5 x 10(-9) M(-1) s(-1) for deprotonation of acetate anion by HO(-) give pK(a)(C) approximately 33.5 for acetate anion. The change in the rate-limiting step from chemical proton transfer to solvent reorganization results in a downward break in the slope of the plot of log k(HO) against carbon acid pK(a) for deprotonation of a wide range of neutral alpha-carbonyl carbon acids by hydroxide ion, from -0.40 to -1.0. Good estimates are reported for the stabilization of the carbonyl group relative to the enol tautomer by electron donation from alpha-SEt, alpha-OMe, alpha-NH(2), and alpha-O(-) substituents. The alpha-NH(2) and alpha-OMe groups show similar stabilizing interactions with the carbonyl group, while the interaction of alpha-O(-) is only 3.4 kcal/mol more stabilizing than for alpha-OH. We propose that destabilization of the enolate intermediates of enzymatic reactions results in an increasing recruitment of metal ions by the enzyme to provide electrophilic catalysis of enolate formation.     

Sulfate as a synergistic anion facilitating iron binding by the bacterial transferrin FbpA: the origins and effects of anion promiscuity

The ferric binding protein, FbpA, has been demonstrated to facilitate the transport of naked Fe3+ across the periplasmic space of several Gram-negative bacteria. The sequestration of iron by FbpA is facilitated by the presence of a synergistic anion, such as phosphate or sulfate. Here we report the sequestration of Fe3+ by FbpA in the presence of sulfate, at an assumed periplasmic pH of 6.5 to form FeFbpA-SO4 with K'(eff) = 1.7 x 10(16) M(-1) (at 20 degrees C, 50 mM MES, 200 mM KCl). The iron affinity of the FeFbpA-SO4 protein assembly is 2 orders of magnitude lower than when bound with phosphate and is the lowest of any of the FeFbpA-X assemblies yet reported. Iron reduction at the cytosolic membrane receptor may be an essential aspect of the periplasmic iron-transport process, and with an E(1/2) of -158 mV (NHE), FeFbpA-SO4 is the most easily reduced of all FeFbpA-X assemblies yet studied. The variation of FeFbpA-X assembly stability (K'(eff)) and ease of reduction (E(1/2)) with differing synergistic anions X(n-) are correlated over a range of 14 kJ, suggesting that the variations in redox potentials are due to stabilization of Fe3+ in FeFbpA-X by X(n-). Anion promiscuity of FbpA in the diverse composition of the periplasmic space is illustrated by the ex vivo exchange kinetics of FeFbpA-SO4 with phosphate and arsenate, where first-order kinetics with respect to FeFbpA-SO4 (k = 30 s(-1)) are observed at pH 6.5, independent of entering anion concentration and identity. Anion lability and influence on the iron affinity and reduction potential for FeFbpA-X support the hypothesis that synergistic anion exchange may be an important regulator in iron delivery to the cytosol. This structural and thermodynamic analysis of anion binding in FeFbpA-X provides additional insight into anion promiscuity and importance.     

Anion exchange kinetics of nanodimensional layered metal hydroxides: use of isoconversional analysis

Anion exchange reactions of nanodimensional layered metal hydroxide compounds are utilized to create materials with targeted physical and chemical properties and also as a means for controlled release of intercalated anions. The kinetics of this important class of reaction are generally characterized by model-based approaches. In this work, a different approach based on isothermal, isoconversional analysis was utilized to determine effective activation energies with respect to extent of reaction. Two different layered metal hydroxide materials were chosen for reaction with chloride anions, using a temperature range of 30-60 °C. The concentrations of anions released into solution and the changes in polycrystalline solid phases were evaluated using model-based (Avrami-Erofe'ev nucleation-growth model) and model-free (integral isoconversional) methods. The results demonstrate the utility of the isoconversional approach for identifying when fitting to a single model is not appropriate, particularly for characterizing the temperature dependence of the reaction kinetics.     

Dipyrrolyldiketone difluoroboron complexes: novel anion sensors with C-H...X- interactions

1,3-Dipyrrolyl-1,3-propanediones, synthesized from pyrroles and malonyl chloride, form BF2 complexes, which represent a new class of naked-eye sensors for halide and oxoanions. The association mode for the interactions of both the pyrrolyl NH and bridging CH protons with anions was confirmed by 1H NMR chemical shifts in CD2Cl2 and supported by theoretical studies. The binding constants (Ka) were estimated as 8.1x10(4), 2.0x10(3), 3.3x10(2), 1.3x10(4), and 80 M(-1) for F-, Cl-, Br-, H2PO4(-), and HSO4(-) by UV/Vis absorption spectral changes in CH2Cl2. Augmentation of Ka compared with dipyrrolylquinoxaline for H2PO4(-) is much larger than those for other anions. Contrary to other anions, F- quenches the emission almost completely, which was detected by the fluorescence spectrum as well as the naked-eye. In the case of the chloride anion complex, the formation of Cl(-)-bridged 1D networks, in which anion is associated with two BF2 complexes, is observed in the solid state.     

Slow anion exchange,conformational equilibria,and fluorescent sensing in venus flytrap aminopyridinium-based anion hosts

The synthesis, anion binding, and conformational properties of a series of 3-aminopyridinium-based, tripodal, tricationic hosts for anions are described. Slow anion and conformational exchange on the (1)H NMR time scale at low temperature, coupled with NMR titration, results in a high level of understanding of the anion-binding properties of the compounds, particularly with respect to significant conformational change resulting from induced fit complexation. Peak selectivity for halides, particularly Cl(-), is observed. The approach has been extended to dipodal and tripodal podands based on 3-aminopyridinium "arms" containing photoactive anthracenyl moieties. The 1,3,5-tripodal host shows a remarkable selectivity for acetate over other anions, in contrast to the analogous unsubstituted tris(3-aminopyridinium) analogue, despite the fact that low-temperature (1)H NMR experiments reveal a total of four acetate-binding conformations. Photodimerization of anthracene units results in the formation of potential fluorescent anion sensors.     

Ozonolysis of vinyl compounds,CH2=CH-X,in aqueous solution--the chemistries of the ensuing formyl compounds and hydroperoxides

Reactions of ozone with some vinyl compounds of the general structure CH2=CH-X were studied in aqueous solution. Rate constants (in brackets, unit: dm3 mol-1 s-1) were determined: acrylonitrile (670), vinyl acetate (1.6 x 10(5)), vinylsulfonic acid (anion, 8.3 x 10(3)), vinyl phenylsulfonate (ca. 200), vinyl diethylphosphonate (3.3 x 10(3)), vinylphosphonic acid (acid, 1 x 10(4); mono-anion, 2.7 x 10(4); di-anion, 1 x 10(5)), vinyl bromide (1 x 10(4)). The main pathway leads to the formation of HOOCH2OH and HC(O)X. As measured by stopped flow with conductometric detection, the latter one may undergo rapid hydrolysis by water, e.g. HC(O)CN (3 s-1). Other HC(O)X hydrolyse much slower, e.g. HC(O)PO3(Et)2 (7 x 10(-3) s-1) and HC(O)P(OH)O2- (too slow to be measured). The OH(-)-induced hydrolyses range from ca. 5 dm3 mol-1 s-1 [HC(O)PO(3)2-] to 3.8 x 10(5) dm3 mol-1 s-1 [HC(O)CN]. HC(O)Br mainly decomposes rapidly (too fast for the determination of the rate) into CO and Br- plus H+, and the competing hydrolysis is of minor importance (3.7%). The slow hydrolysis of HC(O)PO(3)2- at pH 10.2, where HOOCH2OH is rapidly decomposed into CH2O plus H2O2, allows an H2O2-induced decomposition (k = 260 dm3 mol-1 s-1) to take place. Formate and phosphate are the final products.     

Selective anion sensing by a tris-amide CTV derivative: 1H NMR titration, self-assembled monolayers, and impedance spectroscopy

A hydrogen-bond forming tris(amide) receptor based on cyclotriveratrylene (CTV) was prepared. Self-assembled monolayers (SAMs) of the receptor were formed on gold surfaces. Desorption experiments show a surface coverage of 2.26 x 10(-10) mol/cm(2). (1)H NMR and UV measurements confirm that the receptor exhibits the highest affinity for acetate ions among the anions studied. Electrochemical impedance was used to investigate anion sensing by the SAMs and proved to be an efficient and convenient technique for detecting anions in aqueous solutions. Upon binding acetate anions, the monolayer-modified gold electrodes show a drastic increase of the R(ct) values when Fe(CN)(6)(3-/4-) is used as the redox probe. When the probe was changed to a positively charged one, Ru(NH3)(6)(3+/2+), the R(ct) values decreased monotonically as the acetate concentration was increased, thus confirming the accumulation of negative surface charge upon anion binding. H(2)PO(4-) shows some interference when sensing AcO-. Other monovalent anions such as Cl-, Br-, NO3(-) and HSO4(-) do not bind to the CTV receptor either in solution or on the surfaces.     

Formation constants of zinc(II) complexes with Semi-Xylenol Orange

A potentiometric and spectrophotometric investigation on the formation of zinc(II) complexes with Semi-Xylenol Orange (SXO or H(4)L) is reported. In an aqueous solution (mu = 0.1), three 1:1 complex species, MH(2)L, MHL(-), ML(2-), and a 1:2 complex, ML(6-)(2), seem to exist. In a strongly alkaline medium (above pH 12.5) the complexes may dissociate to give zinc hydroxide and L(4-). The formation of a hydroxy complex is not observed. The absorption maxima are at 445 nm (MH(2)L), 466 nm (MHL(-)) and 561 nm (ML(2-)), the molar absorptivities being 2.34 x 10(4), 2.42 x 10(4) and 3.14 x 10(4) 1.mole(-1) .cm(-1) respectively. The formation constants are (at 25 +/- 0.1 degrees ) log K(M)(ML) = 11.84, log K(M)(MHL) = 7.13, log K(M)(MH(2)L) = 2.70, log K(M)(ML(2)) = 16.60.     

A supramolecular receptor of diatomic molecules (O2, CO, NO) in aqueous solution

A per-O-methylated beta-cyclodextrin dimer, Py2CD, was conveniently prepared via two steps: the Williamson reaction of 3,5-bis(bromomethyl)pyridine and beta-cyclodextrin (beta-CD) yielding 2A,2'A-O-[3,5-pyridinediylbis(methylene)bis-beta-cyclodextrin (bisCD) followed by the O-methylation of all the hydroxy groups of the bisCD. Py2CD formed a very stable 1:1 complex (Fe(III)PCD) with [5,10,15,20-tetrakis(p-sulfonatophenyl)porphinato]iron(III) (Fe(III)TPPS) in aqueous solution. Fe(III)PCD was reduced with Na2S2O4 to afford the Fe (II)TPPS/Py2CD complex (Fe(II)PCD). Dioxygen was bound to Fe(II)PCD, the P(1/2)(O2) values being 42.4 +/- 1.6 and 176 +/- 3 Torr at 3 and 25 degrees C, respectively. The k(on)(O2) and k(off)(O2) values for the dioxygen binding were determined to be 1.3 x 10(7) M(-1) s(-1) and 3.8 x 10(3) s(-1), respectively, at 25 degrees C. Although the dioxygen adduct was not very stable (K(O2) = k(on)(O2)/k(off)(O2) = 3.4 x 10(3) M(-1)), no autoxidation of the dioxygen adduct of Fe(II)PCD to Fe(III)PCD was observed. These results suggest that the encapsulation of Fe (II)TPPS by Py2CD strictly inhibits not only the extrusion of dioxygen from the cyclodextrin cage but also the penetration of a water molecule into the cage. The carbon monoxide affinity of Fe(II)PCD was much higher than the dioxygen affinity; the P(1/2)(CO), k(on)(CO), k(off)(CO), and K(CO) values being (1.6 +/- 0.2) x 10(-2) Torr, 2.4 x 10(6) M(-1) s(-1), 4.8 x 10(-2) s(-1), and 5.0 x 10(7) M(-1), respectively, at 25 degrees C. Fe(II)PCD also bound nitric oxide. The rate of the dissociation of NO from (NO)Fe(II)PCD ((5.58 +/- 0.42) x 10(-5) s(-1)) was in good agreement with the maximum rate ((5.12 +/- 0.18) x 10(-5) s(-1)) of the oxidation of (NO)Fe(II)PCD to Fe(III)PCD and NO3(-), suggesting that the autoxidation of (NO)Fe(II)PCD proceeds through the ligand exchange between NO and O2 followed by the rapid reaction of (O2)Fe(II)PCD with released NO, affording Fe(II)PCD and the NO3(-) anion inside the cyclodextrin cage.     

Solid-state chelation of metal ions by ethylenediaminetetraacetate intercalated in a layered double hydroxide

The solid-state chelation of transition metal ions (Co(2+), Ni(2+), and Cu(2+)) from aqueous solutions into the lithium aluminum layered double hydroxide ([LiAl(2)(OH)(6)]Cl x 0.5H(2)O or LDH) which has been pre-intercalated with EDTA (ethylenediaminetetraacetate) ligand has been investigated. The intercalated metal cations form [M(edta)](2)(-) complexes between the LDH layers as indicated by elemental analysis, powder X-ray diffraction, and IR and UV-vis spectroscopies. If metal chloride or nitrate salts are used in the reaction with the LDH then co-intercalation of either the Cl(-) or NO(3)(-) anions is observed. In the case of metal acetate salts the cations intercalate without the accompanying anion. This can be explained by the different intercalation selectivity of the anions in relation to the LDH. In the latter case the introduction of the positive charge into LDH structure was compensated for by the release from the solid of the equivalent quantity of lithium and hydrogen cations. Time-resolved in-situ X-ray diffraction measurements have revealed that the chelation/intercalation reactions proceed very quickly. The rate of the reaction found for nickel acetate depends on concentration as approximately k[Ni(Ac)(2)](3).     

Monocarbaborane anion chemistry. [COOH], [CH2OH] and [CHO] units as functional groups on ten-vertex monocarbaborane anionic compounds

B(10)H(14) reacts with para-C(6)H(4)(CHO)(COOH) in aqueous KOH solution to give the [nido-6-CB(9)H(11)-6-(C(6)H(4)-para-COOH)](-) anion 1, which undergoes cage closure with iodine in alkaline solution to give the [closo-2-CB(9)H(9)-2-(C(6)H(4)-para-COOH)](-) anion 2. Upon heating, anion 2 rearranges to form the [closo-1-CB(9)H(9)-1-(C(6)H(4)-para-COOH)](-) anion 3. Similarly, B(10)H(14) with glyoxylic acid OHCCOOH in aqueous KOH gives the [arachno-6-CB(9)H(13)-6-(COOH)](-) anion 4, which undergoes cage closure with iodine in alkaline solution to give the [closo-2-CB(9)H(9)-2-(COOH)](-) anion 5. Upon heating, anion 5 rearranges to give the [closo-1-CB(9)H(9)-1-(COOH)](-) anion 6. Reduction of the [COOH] anions 3 and 6 with diisobutylaluminium hydride gives the [CH(2)OH] hydroxy anions [closo-1-CB(9)H(9)-1-(C(6)H(4)-para-CH(2)OH)](-) and [closo-1-CB(9)H(9)-1-(CH(2)OH)](-) 8 respectively. The [closo-1-CB(9)H(9)-1-(C(6)H(4)-para-CH(2)OH)](-) anion 7 can also be made via isomerisation of the [closo-2-CB(9)H(9)-2-(C(6)H(4)-para-CH(2)OH)](-) anion 9, in turn obtained from the [nido-6-CB(9)H(11)-6-(C(6)H(4)-para-CH(2)OH)](-) anion 10, which is obtained from the reaction of B(10)H(14) with terephthaldicarboxaldehyde, C(6)H(4)-para-(CHO)(2), in aqueous KOH solution. Oxidation of the hydroxy anions 7 and 8 with pyridinium dichromate gives the aldehydic [closo-1-CB(9)H(9)-1-(C(6)H(4)-para-CHO)](-) anion 11 and the aldehydic [closo-1-CB(9)H(9)-1-(CHO)](-) anion 12 respectively, characterised as their 2,4-dinitrophenylhydrazone derivatives, the [closo-1-CB(9)H(9)-1-C(6)H(4)-para-CH=N-NHC(6)H(3)(NO(2))(2)](-) anion 13 and the [closo-1-CB(9)H(9)-1-CH=N-NHC(6)H(3)(NO(2))(2)](-) anion respectively.     

Anion exchange reaction potentials as approximate estimates of the relative thermodynamic stabilities of Mg/Al layered double hydroxides containing different anions

Coatings of hydrotalcite-like nitrate-intercalated Mg/Al layered double hydroxides are electrochemically deposited on a Pt electrode by electrogeneration of base by reduction of a mixed metal nitrate aqueous solution. As-prepared coatings are stable to workup and function as rugged electrodes. The voltammetric response generated by anion exchange of intercalated nitrate for dissolved anions from solution under equilibrium conditions is employed to estimate the thermodynamic stabilities of the Mg/Al layered double hydroxides comprising different anions relative to the nitrate-containing phase. Among monovalent anions, the most stable is the fluoride-containing LDH (ΔG° = -48.7 kJ mol(-1)) relative to the nitrate-containing LDH. The stability in aqueous phase decreases as F(-) > Cl(-) > Br(-) > NO(2)(-) > NO(3)(-), whereas, among divalent anions, SO(4)(2-) (ΔG° = -8.7 kJ mol(-1)) > CO(3)(2-) (ΔG° = 14.3 kJ mol(-1)). The results of monovalent ions match well with the Miyata series, whereas the divalent anion series is at variance with the commonly held belief that carbonate-LDHs are more stable than sulfate-LDHs.     

Delamination of surfactant-intercalated brucite-like hydroxy salts of cobalt and copper and solvothermal decomposition of the resultant colloidal dispersions

Surfactant anion intercalated hydroxy salts of copper and cobalt of the formula M(OH)2- x (surf)x.mH2O [M = Cu, Co; surf = dodecyl sulfate, dodecyl benzene sulfonate, and x = 0.5 for Cu and 0.67 for Co] delaminate readily in 1-butanol to give translucent colloidal dispersions that are stable for months. The extent of delamination and the colloidal dispersion observed in these solids is higher than what had been observed for layered double hydroxides. The dispersions yield the corresponding nanoparticulate oxides on solvothermal decomposition. While the copper hydroxy salt forms approximately 300 nm dendrimer-like CuO nanostructures comprising nanorods of approximately 10 nm diameter, the cobalt analogue forms approximately 20 nm superparamagnetic particles of Co3O4.     

Molten salts bearing anion receptor

A series of organoboron molten salts prepared by hydroboration of allyl imidazolium type molten salts with various hydroborating reagents such as monobromoborane dimethyl sulfide complex, 9-borabicyclo[3.3.1]nonane (9-BBN) and mesitylborane, and subsequent anion exchange reaction exhibited selective cation transporting property with ionic conductivity of 7.79 x 10(-5) - 6.25 x 10(-6) S cm(-1) at 323 K.     

Direct determination of rate constants for coupling between aromatic radical anions and alkyl and benzyl radicals by laser-flash photolysis

Coupling rates between the radicals methyl, n-, sec-, tert-butyl and benzyl (R.) and the aromatic radical anions of 1,4-dicyanonaphthalene, 9,10-dicyanoanthracene and fluorenone (A-.) have been obtained using a new laser-flash photolysis method. The radicals R. and the radical anions A-. were generated by a photoinduced electron transfer reaction between the aromatic compound A and the alkyl or benzyl triphenylborate anion RB(Ph)3-. For the first time the rate constants of the coupling reaction between methyl and benzyl radicals with aromatic radical anions have been obtained. For all the measured coupling rate constants an average value of k1 = 1.9 x 10(9) M-1 s-1 was found with a relatively small variation in the coupling rates (0.8-2.9 x 10(9) M-1 s-1). The results demonstrate that the coupling rate k1 is insensitive to changes in the steric and electronic properties of the radicals and the structure and standard potentials of the aromatic radical anions.     

Zn6[MeN(CH2CO2)(CH2PO3)](6)(Zn)]4- anion: the first example of the oxo-bridged Zn6 octahedron with a centered Zn(II) cation

Hydrothermal reactions of N-(phosphonomethyl)-N-methylglycine, MeN(CH(2)CO(2)H)(CH(2)PO(3)H(2)) (H(3)L), with zinc(II) acetate resulted in the formation of a novel zinc carboxylate-phosphonate, [Zn(6)L(6)(Zn)][Zn(H(2)O)(6)](2) x 22H(2)O (1). The structure of 1 contains a heptanuclear zinc phosphonate cluster anion, [Zn(6)L(6)(Zn)](4-), in which seven zinc(II) cations form an unusual Zn(6)(Zn) centered octahedron with six of its Zn(3) triangle faces each further capped by a phosphonate group. The Zn(II) cations of the Zn(6) octahedron are five-coordinated whereas the centered Zn(II) cation is octahedrally coordinated. Packing of these cluster anions creates micropores occupied by the hydrated zinc(II) cations as well as lattice water molecules. The structural skeleton of 1 is retained after the removal of the lattice water molecules.     

Layered copper hydroxide n-alkylsulfonate salts: synthesis, characterization, and magnetic behaviors in relation to the basal spacing

A series of hybrid inorganic-organic copper(II) hydroxy n-alkylsulfonate with a triangular lattice, Cu(2)(OH)(3)(C(n)H(2)(n)(+1)SO(3)) (n = 6, 8, 10), are prepared by anion exchange, starting from copper hydroxy nitrate Cu(2)(OH)(3)NO(3). These compounds show a layered structure as determined by X-ray diffraction, with interlayer distances of 14.3-34.8 A in alternation with interdigitated bilayer packing. Magnetic properties have been investigated by means of dc and ac measurements. All the compounds show similar metamagnet behaviors, with a Neel temperature of about 11 K. A subtle difference in the ac magnetic susceptibility among the compounds is understood by the existence of hydrogen bonding between the sulfonate headgroup and the hydroxide anion. A detailed molecular structure of the alkyl chains incorporated to the inorganic copper hydroxide layer is also discussed from the FTIR data.     

Thermodynamics and kinetics of the nickel(II)-salicylhydroxamic acid system. Phenol rotation induced by metal ion binding

The kinetics and the equilibria of Ni(II) binding to p-hydroxybenzohydroxamic acid (PHBHA) and salicylhydroxamic acid (SHA) have been investigated in an aqueous solution at 25 degrees C and I=0.2 M by the stopped-flow method. Two reaction paths involving metal binding to the neutral acid and to its anion have been observed. Concerning PHBHA, the rate constants of the forward and reverse steps are k1=(1.9+/-0.1)x10(3) M-1 s-1 and k-1=(1.1+/-0.1)x10(2) s-1 for the step involving the undissociated PHBHA and k2=(3.2+/-0.2)x10(4) M-1 s-1 and k-2=1.2+/-0.2 s-1 for the step involving the anion. Concerning SHA, the analogous rate constants are k1=(2.6+/-0.1)x10(3) M-1 s-1, k-1=(1.3+/-0.1)x10(3) s-1, k2=(5.4+/-0.2)x10(3) M-1 s-1, and k-2=6.3+/-0.5 s-1. These values indicate that metal binding to the anions of the two acids concurs with the Eigen-Wilkins mechanism and that the phenol oxygen is not involved in the chelation. Moreover, a slow effect was observed in the SHA-Ni(II) system, which has been put down to rotation of the benzene ring around the C-C bond. Quantum mechanical calculations at the B3LYP/lanL2DZ level reveal that the phenol group in the most stable form of the Ni(II) chelate is in trans position relative to the carbonyl oxygen, contrary to the free SHA structure, where the phenol and carbonyl oxygen atoms both have cis configuration. These results bear out the idea that the complex formation is coupled with phenol rotation around the C-C bond.     

Construction of a functional layered solid using the tetrakis(imidazolyl)borate coordinating anion

The coordination polymer Pb[B(Im)(4)](NO(3)), constructed by using tetrakis(imidazolyl)borate and lead(II) nitrate solutions, is a layered material with the metal centers facing the interlayer spacing. As in naturally occurring layered minerals, this compound can readily undergo anion exchange in the solid state with retention of crystallinity. We examined stoichiometric exchange of (15)N-nitrate for nitrate and iodide for nitrate by (15)N and (207)Pb SSNMR and confirmed retention of crystallinity by IR and powder XRD diffraction.