Cooperative effects in the complexation of anions and Lewis bases by a heteronuclear bifunctional Lewis acid

As shown by the complexation of electron-rich substrates to the heteronuclear bifunctional Lewis acid bis(mu-1,8-naphthalenediyl)(mu-chloride)methyltin-chlorogallium (1), the primary Lewis acidic site of this molecule is the triorganotin chloride moiety whose electron deficiency is enhanced through partial abstraction of its chloride ligand by the neighbouring gallium centre.     

Fluoride ion chelation by a bidentate phosphonium/borane Lewis acid

The phosphonium borane [1-Mes2B-2-MePh2P-(C6H4)]+ ([2]+) has been synthesized as an iodide salt by alkylation of 1-Mes2B-2-Ph2P-(C6H4) with MeI. This novel cationic borane complexes fluoride to afford the corresponding zwitterionic fluoroborate complex 1-FMes2B-2-MePh2P-(C6H4) (2-F) with a binding constant in MeOH exceeding that of 1-Mes2B-4-MePh2P-(C6H4) ([1]+) by at least 4 orders of magnitude. Structural and computational results indicate that the high fluorophilicity of [2]+ arises from both Coulombic and cooperative effects which lead to formation of a B-F-->P interaction with a F-->P distance of 2.666(2) A. These results, which are supported by NBO and AIM analyses, show that the latent phosphorus-centered Lewis acidity of the phosphonium moiety in [2]+ can be exploited to enhance fluoride binding via chelation.     

Fluoride ion complexation by a cationic borane in aqueous solution

The phosphonium borane [p-Mes(2)B(C(6)H(4))PMePh(2)](+) complexes fluoride in water containing 10% methanol with a binding constant of 1.0(+/-0.1) x 10(3) M(-1) to afford the zwitterion p-Mes(2)FB(C(6)H(4))PMePh(2).     

A heteronuclear bidentate Lewis acid as a phosphorescent fluoride sensor

Reaction of dimesityl-1,8-naphthalenediylborate (1) with C6F5HgCl results in the formation of a B/Hg heteronuclear bidentate Lewis acid (2), which complexes fluoride to afford [2-mu2-F]-. Structural and photophysical studies carried out in solution and in the solid state indicate that 2 is a highly selective and sensitive phosphorescent fluoride sensor. The proximity of the two Lewis acidic sites enforced by the 1,8-naphthalenediyl backbone promotes fluoride anion chelation and is, therefore, responsible for the high binding constant. The interplay of conjugative and spin-orbit coupling effects mediated by the boron and mercury atoms, respectively, results in the phosphorescent signaling of fluoride binding. Remarkably, fluoride binding occurs in partially aqueous solutions and results in a drastic change of the phosphorescence observed when the solutions are frozen.     

Cyanide ion complexation by a cationic borane

While we have previously reported that [1-(Mes2B)-8-(Me3NCH2)-C10H6]+ ([2]+) complexes fluoride ions to form [1-(Mes2FB)-8-(Me3NCH2)-C10H6] (2-F), we now show that this cationic borane also complexes cyanide to form [1-(Mes2(NC)B)-8-(Me3NCH2)-C10H6] (2-CN). This reaction also occurs under biphasic conditions (H2O-CHCl3) and may serve to transport cyanide in organic phases. The zwitterionic cyanoborate 2-CN has been fully characterized and its crystal structure determined. UV-vis titration experiments carried out in THF indicate that [2]+ has a higher affinity for fluoride (K > 10(8) M(-1)) than cyanide (K = 8.0 (+/-0.5) x 10(5) M(-1)). Steric effects which impede cyanide binding to the sterically congested boron center of [2]+ are most likely at the origin of this selectivity. Finally, electrochemical studies indicate that [2]+ is significantly more electrophilic than its neutral precursor 1-(Mes2B)-8-(Me2NCH2)-(C10H6) (1). These studies also show that reduction of [2]+ is irreversible, possibly because of elimination of the NMe3 moiety under reductive conditions. In fact, [2]OTf reacts with NaBH4 to afford 1-(Mes2B)-8-(CH3)-(C10H6) (4) which has also been fully characterized.     

Rational approach to endo/exocoordinated heteronuclear macrocyclic network: supramolecular Hg(ll), Cu(ll), and Hg(ll)/Cu(ll) complexes of a NS4 macrocycle

A rationally designed sulfur-rich macrocycle L incorporating one pyridine and one benzo subunit was synthesized and structurally characterized. From the assembly reactions of L with copper(II) nitrate and mercury(II) iodide, an endocyclic monocopper(II) complex and an exocyclic dimercury(II) complex were prepared, respectively. On the basis of these results, the construction of a Hg(II)/Cu(II) heteronuclear 1D network complex with an endo/exocyclic coordination mode was achieved by one-pot assembly reaction of L with mercury(II) iodide in the presence of copper(II) nitrate.     

Gas-phase ion chemistry of GeH(4)/B(2)H(6) mixtures

The gas phase ion-molecule reactions in positively and negatively ionized germane/diborane mixtures have been studied by ion trap mass spectrometry. Reaction sequences and rate constants for the most interesting processes have been determined. In positive ionization, formation of Ge-B bonds exclusively occurs through condensation reactions of B(n)H(m)(+) ions with germane, followed by H(2) or BH(3) loss. No reactions of ions from germane with B(2)H(6) were observed under the experimental conditions used here. In negative ionization, the Ge(n)H(m)(-) (n = 1, 2) ion families react with diborane to yield the Ge(n)B(p)H(q)(-) (p = 1, 2) ions, again via dehydrogenation and BH(3) loss, while diborane anions proved to be unreactive. In both positive and negative ionization, Ge-B ions reach appreciable abundances. The present results afford fundamental information about the intrinsic reactivity of gas-phase ions and provide valuable indications about the first nucleation steps ultimately leading to amorphous Ge and B-doped semiconductor materials by chemical vapor deposition methods.     

Voltammetric study of the redox behaviour of the Hg(II)/Hg(I)/Hg system at a rotating metal-ring/glassy-carbon disc electrode

The reduction of Hg(II) at a glassy-carbon electrode in various electrolytes has been studied by rotating ring-disc voltammetry. Reduction proceeds directly to metallic mercury in a single 2-electron step. However, at the foot of the wave, and only during the first reduction sweep after pretreatment of the electrode surface, a small amount of Hg(I) species is detected at the ring. The appearance of an Hg(I) intermediate is most pronounced in sulphuric acid solution. The reduction of Hg(II) is found to proceed irreversibly and to be of first order. At sufficiently negative potentials the reduction is convective-diffusion controlled. Stripping voltammetric experiments indicate that the dissolution of mercury gives Hg(II) in complexing electrolytes. In non-complexing electrolytes the initially formed Hg(II) reacts with mercury atoms on the electrode surface to give Hg(I). During electrodissolution, two stripping peaks may be observed as a result of underpotential adsorption of mercury on glassy carbon. The difference in peak potential between the adsorption (mono) layer peak and the bulk mercury peak has been related to the difference in work functions of the deposit (mercury) and substrate (carbon). A rotating glassy-carbon electrode has been used for the anodic stripping determination of mercury. When an appropriate amount of a cation such as cadmium(II) or copper(II) is added to the test solution, mercury down to 2 x 10(-9)M (0.4 ng ml ) can be determined in acidified thiocyanate electrolyte with a relative standard deviation of about 22%.     

Redox and complexation chemistry of the Cr(VI)/Cr(V)-D-galacturonic acid system

The oxidation of d-galacturonic acid by Cr(VI) yields the aldaric acid and Cr(III) as final products when a 30-times or higher excess of the uronic acid over Cr(VI) is used. The redox reaction involves the formation of intermediate Cr(IV) and Cr(V) species, with Cr(VI) and the two intermediate species reacting with galacturonic acid at comparable rates. The rate of disappearance of Cr(VI), Cr(IV) and Cr(V) depends on pH and [substrate], and the slow reaction step of the Cr(VI) to Cr(III) conversion depends on the reaction conditions. The EPR spectra show that five-coordinate oxo-Cr(V) bischelates are formed at pH < or = 5 with the uronic acid bound to Cr(V) through the carboxylate and the alpha-OH group of the furanose form or the ring oxygen of the pyranose form. Six-coordinated oxo-Cr(V) monochelates are observed as minor species in addition to the major five-coordinated oxo-Cr(V) bischelates only for galacturonic acid : Cr(VI) < or =10 : 1, in 0.25-0.50 M HClO(4). At pH 7.5 the EPR spectra show the formation of a Cr(V) complex where the vic-diol groups of Galur participate in the bonding to Cr(V). At pH 3-5 the Galur-Cr(V) species grow and decay over short periods in a similar way to that observed for [Cr(O)(alpha-hydroxy acid)(2)](-). The lack of chelation at any vic-diolate group of Galur when pH < or = 5 differentiates its ability to stabilise Cr(V) from that of neutral saccharides that form very stable oxo-Cr(V)(diolato)(2) species at pH > 1.     

On the complexation of copper (II) ion with 2-hydroxybenzamide

The complexation of the Cu2+ ion with 2-Hydroxybenzamide (salicylamide, HL) has been studied, at 25 degrees C, by potentiometric measurements with a glass electrode in NaCIO4 media for ionic strength ranging from 0.5 to 3 mol/dm3. The data are consistent with the formation of the complexes CuH(-1)(HL)+, CuH(-2)(HL)2, Cu2H(-2)(HL)2(2+) and CuH(-2)(HL). The minor species, Cu2H(-2)(HL)2(2+) and CuH(-2)(HL), amount to at least 20% of the total copper. Elaboration of the data according to the Specific Interaction Theory yields the constants valid in the infinite dilution reference state: [formulas: see text] and the interaction coefficients (kg/mol) of complex species with medium ions: b(L-,Na+) = 0.11 +/- 0.03; b(CuH(-1)(HL)+,NaClO4) = 0.17 +/- 0.05; b(CuH(-2)(HL)2,NaClO4) = 0.11 +/- 0.05; b(Cu2H(-2)(HL)2(2+),NaClO4) = 0.2(7) +/- 0.1; b(CuH(-2)(HL),NaClO4) = -0.0(3) +/- 0.1.     

Mild synthesis of disaccharidic 2,3-enopyranosyl cyanides and 2-C-2-deoxy pyranosyl cyanides with Hg(CN)(2)/HgBr(2)/TMSCN

Lewis acid-catalyzed dimerization of mono- and disaccharidic per-O-acetylated glycals gave di- and tetrasaccharidic O-acetylated C-glycosides, respectively. 2,3-Enopyranosyl cyanides were obtained from per-O-acetylated glycals by a new, mild anomeric S(N)'-acetoxy displacement with Hg(CN)(2)/HgBr(2)/TMSCN. Per-O-acetylated 2-C-2-deoxy-pyranoses were converted into pyranosyl cyanides by the same reagent. An unprecedented acetic acid elimination from dimers with D-galacto- and L-fuco-configurations accompanied the S(N)-displacement under those conditions. A new set of (1)H NMR coupling constants for 2,3-enopyranosyl systems was used for configurational assignment of complicated tetrasaccharide mimics.     

Kinetic studies on the complexation of uranium(VI) with chromotropic acid and chromotrope-2B

Kinetic studies on the complexation of U^VI by chromotropic acid, LH₄ (1) and chromotrope-2B, L'H₄, (2) in aqueous solution (pH=3.8–4.5) have been carried out, using stopped-flow spectrophotometry, in the presence of excess U^VI to maintain pseudo-first order conditions and formation of 1∶1 complexes only. Under the experimental conditions complex formation appears to be quantitative for (1), but with (2), which is a much weaker acid, an equilibrium mixture of reactants and the product complex is formed. The kinetic data suggest concurrent reactions of UO ₂ ²⁺ and UO₂(OH)⁺ with a partly deprotonated ligand, LH³⁻ and L'H ₂ ²⁻ . The reactions appear to involve fast unidentate binding of the ligand followed by rate-determining ring closure. Activation parameters corresponding to each of the rate constants have been evaluated, and plausible mechanisms are discussed. TMC 2566     

Fluoride ion detection by 8-hydroxyquinoline-Zr(IV)-EDTA complex

A simple fluorescent detection based on the ligand exchange mechanism is proposed for the fluoride ion in aqueous media. This procedure is based on the exchange of 8-hydroxyquinoline (oxine) coordinated to Zr(IV) by fluoride ion without interference from other common anions. The ternary complex of oxine with [Zr(H(2)O)(2)EDTA].2H(2)O formed by replacing two water molecules in aqueous solution provides a sensitive signalling system for fluoride ion in the concentration range from 6 x 10(-7)M to 8 x 10(-4)M. The green fluorescence (lambda(max)=532 nm) exhibited by the complex upon excitation at 247 nm decreases in intensity with fluoride addition with a detection limit of 12 ppb. The complexation reaction between oxine and Zr(IV)-EDTA and the ligand exchange reaction with fluoride ion has been investigated by UV-vis and fluorescence spectroscopies combined with the PM3 semi-empirical quantum chemical calculations. Job's method of continuous variation and the molar ratio method ascertain a 1:1 stoichiometry composition of the chelate in aqueous media.     

Interpolymer complexation and thermal behaviour of poly(styrene-co-maleic acid)/poly(vinyl pyrrolidone) mixtures

Polymer complexation between poly(styrene-co-maleic acid), (SMA28) and (SMA50) containing 28 and 50 mol% of maleic acid and poly(vinyl pyrrolidone) (PVP), has been investigated by differential scanning calorimeter (DSC), Fourier-transform infrared spectroscopy (FTIR) and thermogravimetric analysis (TGA). All results showed that the ideal complex composition of SMA28/PVP and SMA50/PVP leads, respectively, to 2:1 and 1:1 mole ratio of interacting components.For the investigated systems, the T_g versus composition curve does not follow any of the usual proposed models for polymer blends. Withal, a new model proposed by Cowie et al. is used to fit the T_g data and it is found to reproduce the experimental results more closely. According to n and q obtained values, it seems reasonable to conclude that the inter-associated hydrogen bonds dominate in SMA28/PVP (2:1) complexes. This effect is corroborated by the FTIR study as evidenced by the high displacement of the specific bands and ionic interactions have been clearly identified. Finally, a thermogravimetric study shows that ionic interactions increase the thermal stability of these complexes.     

Thermodynamic parameters of reactions of Cd2+ ion complexation with L-aspartic acid in aqueous solutions

Calorimetric enthalpies of complexation of L-aspartic acid (H₂Asp) with Cd²⁺ ion at 298.15 K and ionic strengths of 0.5, 0.75, and 1.0 (KNO₃) were determined. The thermodynamic parameters of complex formation reactions (for CdAsp and CdAsp ₂ ^2? complexes) at zero and nonzero ionic strength were calculated.     

Synthesis of B/Ga and B/In heteronuclear bidentate Lewis acids: formation of intramolecular pi-arene-gallium(III) and -indium(III) complexes

Reaction of 1,8-dilithionaphthalene x TMEDA (TMEDA=tetramethylethylenediamine) with dimesitylboron fluoride in diethyl ether followed by treatment with THF affords the Li(THF)(4) salt of dimesityl-1,8-naphthalenediylborate (1). Compound 1-Li(THF)(4) undergoes a ring-opening reaction with trimethyltin chloride to afford high yields of 1-(dimesitylboron)-8-(trimethyltin)naphthalenediyl (2), a compound in which a boryl and a stannyl moiety coexist at the peri positions of a naphthalene core. Compound 2 has been characterized by multinuclear NMR spectroscopy, elemental analysis, and X-ray single-crystal analysis which reveal the existence of a sterically crowded structure. Compound 2 is a useful starting material for the synthesis of other heteronuclear naphthalene derivatives. Thus, the transmetalation reaction of 2 with GaCl(3) or InCl(3) in THF results in high yields of 1-(dichlorogallium)-8-(dimesitylboron)naphthalenediyl (3) and 1-(dichloroindium)-8-(dimesitylboron)naphthalenediyl (4), respectively. These unprecedented boron/gallium and boron/indium heteronuclear bidentate Lewis acids have been characterized by multinuclear NMR spectroscopy, elemental analysis, and X-ray single-crystal analysis. Owing to the presence of a short contact between the ipso-carbon atom of a mesityl group and the heavy Group 13 element (C(ipso)bondGa 2.279(4) A for 3, C(ipso)bondIn 2.442(6) A for 4), compounds 3 and 4 are best described as intramolecular pi-arene complexes and are the first examples of such complexes. As shown by (1)H and (13)C NMR spectroscopy, this pi interaction subsists in solution.     

Surface chemistry of RuO(2)/IrO(2)/TiO(2) mixed-oxide electrodes: secondary ion mass spectrometric study of the changes induced by electrochemical treatment

The IrO(2)/RuO(2)/TiO(2) ternary system is well known for its electrocatalytic activity towards oxygen- and chlorine-evolution reactions. Electrochemical processing induces noticeable chemical and morphological modifications on these electrodes, depending on the noble metal oxide content. In this work, cathodic/anodic polarization and the oxygen-evolution reaction were studied in order to evaluate the electrocatalytic activity at various noble metal oxide percentages. The best performing electrode (30 mol% noble metal oxides) was analyzed before and after electrochemical tests by means of secondary ion mass spectrometry (SIMS) in order to determine the chemical composition modification which occurred on the surface and in deeper regions of the mixed-oxide film.     

Complexation of methylparathion and bis(2‐hydroxyethyl)sulfide by the tridentate lewis acid [(o‐C6F4Hg)3]

The interaction of trimeric perflu‐ oro‐ortho‐phenylene mercury ( 1 ) with bis(2‐hydroxy‐ ethyl)sulfide (S((CH₂)₂OH)₂) in dichloromethane and methylparathion (SP(OMe)₂(p‐C₆H₄NO₂)) in 1,2‐dichloroethane leads to the crystallization of [ 1 ⋅ (S((CH₂)₂OH)₂)] and [ 1 ⋅ (μ₃‐SP(OMe)₂(p‐C₆H₄‐ NO₂))₂], respectively. These two adducts have been characterized by elemental analysis and single crystal X‐ray diffraction. The structure of [ 1 ⋅ S((CH₂)₂OH)₂] shows that the bis(2‐hydroxyethyl)sulfide molecule interacts with the mercury centers of 1 by formation of a Hg–S interaction of 3.138(4) Å. Association of the two components is further strengthened by the coordination of one of the oxygen atoms of the bis(2‐hydroxyethyl)sulfide molecule. This oxygen atom interacts simultaneously with three mercury centers of 1 with Hg–O distances ranging from 2.889(8) to 3.142(9) Å. In the lattice, molecules of [ 1 ⋅ (S((CH₂)₂OH)₂)] associate with compact cofacial dimers with Hg–Hg metallophilic contacts of 3.794 Å and 4.076 Å. The structure of [ 1 ⋅ (μ₃‐SP(OMe)₂(p‐C₆H₄NO₂))₂] is that of a 2:1 complex in which two molecules of methylparathion are triply coordinated via their sulfur atom to the mercury centers of 1 on either side of the molecular plane. The Hg–S contacts fall within the range of 3.278 and 3.651 Å. © 2005 Wiley Periodicals, Inc. 16:292–297, 2005; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/hc.20125