F/Cl-exchange on AlCl(3)-pyridine adducts: synthesis and characterization of trans-difluoro-tetrakis-pyridine-aluminum-chloride, [AlF2(Py)4]+Cl-

Whereas liquid CCl3F reacts with solid AlCl3 exothermically under chlorine-fluorine-exchange already above -20 degrees C, no reaction takes place between CCl3F and the pyridine complexes of AlCl3 (AlCl3.Py, AlCl3.2Py, or AlCl3.3Py) up to 100 degrees C. The desired chlorine by fluorine substitution on the monomer AlCl3-pyridine adducts occurs, however, easily using Me3SiF as fluorinating agent. By reacting AlCl3.3Py with Me3SiF (even up to 10-fold stoichiometric excess) in pyridine as a solvent, only two of the three Cl atoms can be substituted by fluorine, leading in good yield to the new "mixed aluminum halide", AlF2Cl.4Py. Actually, it represents the first example of a stable solid donor-acceptor adduct of an aluminum-III halide with two different halogens of defined stoichiometry. It was characterized by multinuclear solid-state NMR (27Al and 19F), IR spectroscopy, as well as single-crystal structure analysis. The new compound has an ionic solid-state structure with helical trans-octahedral [(Py)4AlF2]+ cations and isolated Cl- anions. The comparison of its 27Al MAS solid-state NMR spectra with those of a compound bearing the analogous [(Py)4AlCl2]+ cation reveals an extreme increase in the quadrupolar coupling constants, from 0.24 MHz in case of the chlorine cation to about 16 MHz in case of the new [(Py)4AlF2]+ cation.     

Aluminum chloride as a solid is not a strong Lewis acid

Aluminum chloride is used extensively as Lewis acid catalyst in a variety of industrial processes, including Friedel-Crafts and Cl/F exchange reactions. There is a common misconception that pure AlCl3 is itself a Lewis acid. In the current study, we use experimental and computational methods to investigate the surface structure and catalytic properties of solid AlCl3. The catalytic activity of AlCl3 for two halide isomerization reactions is studied and compared with different AlF3 phases. It is shown that pure solid AlCl3 does not catalyze these reactions. The (001) surface of crystalline AlCl(3) is the natural cleavage plane and its structure is predicted via first principles calculations. The chlorine ions in the outermost layer of the material mask the Al3+ ions from the external gas phase. Hence, the experimentally found catalytic properties of pure solid AlCl3 are supported by the predicted surface structure of AlCl3.     

19F NMR study of the equilibria and dynamics of the Al3+/F- system

A careful reinvestigation by high-field 19F NMR (470 MHz) spectroscopy has been made of the Al3+/F- system in aqueous solution under carefully controlled conditions of pH, concentration, ionic strength (I), and temperature. The 19F NMR spectra show five distinct signals at 278 K and I = 0.6 M (TMACl) which have been attributed to the complexes AlFi(3-i)+(aq) with i < or = 5. There was no need to invoke AlFi(OH)j(3-i-j)+ mixed complexes in the model under our experimental conditions (pH < or = 6.5), nor was any evidence obtained for the formation of AlF6(3-)(aq) at very high ratios of F-/Al3+. The stepwise equilibrium constants obtained for the complexes by integration of the 19F signals are in good agreement with literature data given the differences in medium and temperature. In I = 0.6 M TMACl at 278 K and in I = 3 M KCl at 298 K the log Ki values are 6.42, 5.41, 3.99, 2.50, and 0.84 (for species i = 1-5) and 6.35, 5.25, and 4.11 (for species i = 1-3), respectively. Disappearance of the 19F NMR signals under certain conditions was shown to be due to precipitation. Certain 19F NMR signals exhibit temperature- and concentration-dependent exchange broadening. Detailed line shape analysis of the spectra and magnetization transfer measurements indicate that the kinetics are dominated by F- exchange rather than complex formation. The detected reactions and their rate constants are AlF2(2+) + *F- reversible AlF*F2+ + F- (k02 = (1.8 +/- 0.3) x 10(6) M-1 s-1), AlF3(0) + *F- reversible AlF2*F0 + F- (k03 = (3.9 +/- 0.9) x 10(6) M-1 s-1), and AlF3(0) + H*F reversible AlF2*F0 + HF (kH03 = (6.6 +/- 0.5) x 10(4) M-1 s-1). The rates of these exchange reactions increase markedly with increasing F- substitution. Thus, the reactions of AlF2+(aq) were too inert to be detected even on the T1 NMR time scale, while some of the reactions of AlF3(0)(aq) were fast, causing large line broadening. The ligand exchange appears to follow an associative interchange mechanism. The cis-trans isomerization of AlF2+(aq), consistent with octahedral geometry for that complex, is slowed sufficiently to be observed at temperatures around 270 K. Difference between the Al3+/F- system and the much studied Al3+/OH- system are briefly commented on.     

A ladderlike chain aluminum fluoride ([Al2F8]2-)n with edge-sharing AlF6 octahedra

A new aluminum fluoride, Al(2)F(8).2NC(5)H(6).C(6)H(3)(CO(2)H)(3), was synthesized under mild hydrothermal conditions (200 degrees C, 3 days) in the presence of 1,3,5-benzenetricarboxylic acid (btc) in pyridine/HF (pyr/HF) solvent. Its structure is characterized with single-crystal XRD analysis and high-resolution solid-state NMR. The inorganic framework consists of the corner- and edge-shared connections of AlF(6) octahedra. They are linked via a common edge and form a bioctahedral motif which is trans-connected through the corner-shared fluorine. It results in the formation of an original infinite double file of AlF(6) octahedra running along the a axis. A high-power decoupled MAS (27)Al{(19)F} Hahn echo NMR spectrum allowed us to distinguish the two crystallographic hexacoordinated Al sites. Four unresolved (19)F NMR signals are observed in the MAS spectra to account for the eight crystallographic fluorine atoms. Half of the terminal fluorine sites interact via strong hydrogen bonds with the ammonium groups of the pyridine moieties. The resulting mixed pyridine-fluoroaluminate chains are intercalated by the btc molecules which are hydrogen-bonded to the remaining free terminal fluoride anions through the protonated carboxylic acid function. The (1)H nuclei of both organic molecules are observed in the protonated form.     

Anionic charge is prioritized over geometry in aluminum and magnesium fluoride transition state analogs of phosphoryl transfer enzymes

Phosphoryl transfer reactions are ubiquitous in biology and metal fluoride complexes have played a central role in structural approaches to understanding how they are catalyzed. In particular, numerous structures of AlFx-containing complexes have been reported to be transition state analogs (TSAs). A survey of nucleotide kinases has proposed a correlation between the pH of the crystallization solution and the number of coordinated fluorides in the resulting aluminum fluoride TSA complexes formed. Enzyme ligands crystallized above pH 7.0 were attributed to AlF3, whereas those crystallized at or below pH 7.0 were assigned as AlF4-. We use 19F NMR to show that for beta-phosphoglucomutase from Lactococcus lactis, the pH-switch in fluoride coordination does not derive from an AlF4- moiety converting into AlF3. Instead, AlF4- is progressively replaced by MgF3- as the pH increases. Hence, the enzyme prioritizes anionic charge at the expense of preferred native trigonal geometry over a very broad range of pH. We demonstrate similar behavior for two phosphate transfer enzymes that represent typical biological phosphate transfer catalysts: an amino acid phosphatase, phosphoserine phosphatase from Methanococcus jannaschii and a nucleotide kinase, phosphoglycerate kinase from Geobacillus stearothermophilus. Finally, we establish that at near-physiological ratios of aluminum to magnesium, aluminum can dominate over magnesium in the enzyme-metal fluoride inhibitory TSA complexes, and hence is the more likely origin of some of the physiological effects of fluoride.     

Moderately reactive molecules forming stable ionic compounds with superhalogens

An explanation of the ability of selected moderately reactive molecules to form stable systems with superhalogens (AlF(4) and AlCl(4)) is provided on the basis of theoretical considerations supported by ab initio calculations. It is demonstrated that even the molecules possessing high ionization potentials (such as SiO(2), NH(3), CHCl(3), CCl(2)F(2)) should form stable and strongly bound ionic compounds when combined with a properly chosen superhalogen system (acting as an oxidizing agent). The conclusion is supported by providing the structural parameters and interaction energies for the SiO(2)AlF(4), NH(3)AlF(4), CHCl(3)AlF(4), CCl(2)F(2)AlF(4), SiO(2)AlCl(4), and NH(3)AlCl(4) compounds obtained at the CCSD(T)/6-311++G(d,p)//MP2/6-311++G(d,p) level. On the other hand, the AlF(4) and AlCl(4) superhalogens were found to be incapable of reacting with molecules whose ionization potentials (IP) exceed 13 eV (e.g., CO(2), CH(4)). Finally, it is demonstrated that the competition between the electron binding energy of the superhalogen system and the IP of the molecule the superhalogen is combined with is a key factor for predicting the stability of certain species.     

An atomistic MD simulation and pair-distribution-function study of disorder and reactivity of alpha-AlF3 nanoparticles

Cubic nanoparticles of alpha-AlF(3) containing 864 and 2048 atoms were investigated by using molecular dynamics simulations. Significant structural rearrangements of these particles occurred, primarily at the edges and corners of the particles, and 3 and 5 membered (Al-F-)(n) ring structures were observed in addition to the 4-membered rings seen in bulk alpha-AlF(3). These 3 and 5 membered ring structures are, however, present in other metastable forms of AlF(3), which are formed at low temperatures from high surface area precursors. The surfaces of the nanoparticles were very dynamic on the timescale of the MD run, Al-F bonds being continually broken and formed, resulting in the movement of the low coordinate Lewis acid Al sites on the surfaces of the particles. The Lewis acid sites, which represent the catalytically active sites for F/Cl exchange reactions, are largely present at the corners and edges of the particles. The particles show larger rhombohedral distortions than present in the bulk phase and do not undergo a rhombohedral to cubic phase transition at elevated temperatures. The results are compared with pair distribution function (PDF) analysis results from fluorinated gamma-Al(2)O(3), nanoparticles of AlF(3) prepared by plasma routes and alpha- and beta-AlF(3). Broad peaks between 3.3 and 4.5 A in the PDF plots of the fluorinated Al(2)O(3) and the nanoparticles indicate a distribution of Al-F distances arising from Al and F atoms in connected AlF(6) octahedra; this is consistent with the presence of ring structures other than those found in alpha-AlF(3).     

An investigation of the roles of surface aluminum and acid sites in the zeolite MCM-22

Ammonia adsorption studies reveal that the observed Lewis acidity in the zeolite MCM-22 is derived from at least two types of framework aluminum sites (AlF), that is, octahedral AlF and three-coordinate AlF. Comparative ammonia or trimethylphosphine (TMP) adsorption experiments with MCM-22 confirm that octahedral Al species gives rise to the signal at delta(iso) approximately 0 in the 27Al NMR spectrum; this is a superposition of two NMR signals from the different Al species on the water-reconstructed zeolite surface. A sharp resonance assigned to framework Al reversibly transforms on ammonia adsorption to delta(iso)27Al approximately 55 from tetrahedral AlF, while the broad peak is assigned to nonframework aluminum which results from hydrothermal treatment. This study also demonstrates the effectiveness of 27Al magic angle spinning (MAS) and multiple quantum (MQ) MAS NMR spectroscopy as a technique for the study of zeolite reactions.     

Borane-induced dehydration of silica and the ensuing water-catalyzed grafting of B(C6F5)3 to give a supported, single-site Lewis acid, ≡SiOB(C6F5)2

A supported, single-site Lewis acid, ≡SiOB(C(6)F(5))(2), was prepared by water-catalyzed grafting of B(C(6)F(5))(3) onto the surface of amorphous silica, and its subsequent use as a cocatalyst for heterogeneous olefin polymerization was explored. Although B(C(6)F(5))(3) has been reported to be unreactive toward silica in the absence of a Br?nsted base, we find that it can be grafted even at room temperature, albeit slowly. The mechanism was investigated by (1)H and (19)F NMR, in both the solution and solid states. In the presence of a trace amount of H(2)O, either added intentionally or formed in situ by borane-induced dehydration of silanol pairs, the adduct (C(6)F(5))(3)B·OH(2) hydrolyzes to afford C(6)F(5)H and (C(6)F(5))(2)BOH. The latter reacts with the surface hydroxyl groups of silica to yield ≡SiOB(C(6)F(5))(2) sites and regenerate H(2)O. When B(C(6)F(5))(3) is present in excess, the resulting grafted boranes appear to be completely dry, due to the eventual formation of [(C(6)F(5))(2)B](2)O. The immobilized, tri-coordinate Lewis acid sites were characterized by solid-state (11)B and (19)F NMR, IR, elemental analysis, and C(5)H(5)N-TPD. Their ability to activate two molecular C(2)H(4) polymerization catalysts, Cp(2)ZrMe(2) and an (α-iminocarboxamidato)nickel(II) complex, was explored.     

Four- and five-coordinate aluminum ketiminate complexes: synthesis,characterization, and ring-opening polymerization

A series of aluminum complexes featuring with the ketiminate ligand, OCMeCHCMeNHAr (Ar = 2,6-(i)Pr(2)C(6)H(3), 1), have been prepared and characterized spectroscopically and structurally. Reactions of 1 with trialkylaluminum in 1:1 or 1:2 molar ratio generate four- and five-coordinated aluminum complexes (OCMeCHCMeNAr)AlR(2) (R = Me (2); R = Et (3)) and (OCMeCHCMeNAr)(2)AlR (R = Me (4); R = Et (5)) in high yields. Similarly, reaction of AlCl(3) with 1 or 2 equiv of the lithiated 1 in toluene afforded bis(ketiminate) aluminum chloride complex, (OCMeCHCMeNAr)(2)AlCl (6) or (OCMeCHCMeNAr)AlCl(2) (7). Surprisingly, reacting 6 with 1 equiv of AgBF(4) in methylene/acetonitrile mix-solvents generates (OCMeCHCMeNAr)(2)AlF (8) in moderate yield. The structures of complexes 2-6 and 8 have been determined by X-ray crystallography. Complexes 2 and 3 both exhibit tetrahedron structures with the aluminum atom surrounded by oxygen and nitrogen atoms of chelating ketiminate and two alkyl groups. The mono- and bis-ketiminate aluminum complexes 2-5 have shown moderate activity toward the ring-opening polymerization of epsilon-caprolactone.     

N- versus P-coordination in bis(amino)cyclodiphosph(III)azane complexes of aluminum

The reaction of the bis(amino)cyclodiphosph(III)azane, cis-{(tBuNH)(2)(PNtBu)(2)}, with AlMe(3), AlClMe(2), AlCl(2)Me, and AlCl(3) is reported. The less Lewis acidic compound AlMe(3) forms the adduct cis-[(tBuNH)(2)(PNtBu){P.(AlMe(3))NtBu}] (1), in which the aluminum atom is exclusively coordinated to one phosphorus atom. At elevated temperatures AlMe(3) undergoes migratory exchange between the two phosphorus atoms, but no methane elimination is observed. By using the more Lewis acidic compound AlClMe(2) the P-coordinated compound cis-[(tBuNH)(2)(PNtBu){P(AlClMe(2))NtBu}] (2) can be obtained at low temperatures. Compound 2 rearranges irreversibly to a product in which the AlClMe(2) group is coordinated by one exo-cyclic nitrogen atom. A concomitant 1,2-H shift from this nitrogen atom onto the phosphorus atom is observed. The N-coordinated rearrangement product slowly decomposes via a P-N bond cleavage in solution. Reaction of the even more Lewis acidic compounds AlCl(2)Me and AlCl(3) finally led to stable adducts, cis-[(tBuNH)(PNtBu)(tBuNAlCl(2)Me){P(H)NtBu}] (3), and cis-[(tBuNH)(PNtBu)(tBuNAlCl(3)){P(H)NtBu}] (4), in which the aluminum atoms are N-coordinated by a tBuN=PH unit.     

Synthesis and luminescence of zinc and europium alpha-thiophene carboxylate polymer

The zinc and europium alpha-thiophene carboxylate polymer with very strong red luminescence was prepared by rheological phase reaction method from zinc acetate, europium oxide and alpha-thiophenecarboxylic acid. Molecular weight, thermal analyses and X-ray diffraction pattern were measured. 1H NMR, 13C NMR, IR, XPS, UV, excitation and emission spectra were studied. The polymer with amorphous structure and 7.565x10(5) g/mol weight-average molecular exhibited excellent solubility in common organic solvents and better thermal stability under 270 degrees C in air. The quantum yield of the polymer in acetone was 0.6 relative to quinoline in 0.05 mol/l H(2)SO(4) solution. The energy of the pi,pi(*) excited state of C(4)H(3)SCO(2)(-) can be transferred to Eu(3+) ion resulting in emission from the 5D(0)-->(7)F(j) of Eu(3+) ion through the polymeric chains. Zn(2+) can effectively enhance the luminescence of Eu(3+) in alpha-thiophenecarboxylate polymer.     

Direct solid-state NMR spectroscopic evidence for the NH4AlF4 crystalline phase derived from zeolite HY dealuminated with ammonium hexafluorosilicate

Multinuclear (1)H, (19)F, and (27)Al MAS (magic angle spinning) and corresponding 2D HETCOR (heteronuclear correlation) NMR spectroscopy, in combination with powder XRD measurements, provide the direct evidence for the NH(4)AlF(4) crystalline phase, which was formed from zeolite HY dealuminated with an aqueous (NH(4))(2)SiF(6) solution at 80 degrees C. The NH(4)AlF(4) crystalline phase exhibits a characteristic second-order quadrupolar-induced (27)Al NMR line shape spreading from 0 to -90 ppm (in a magnetic field of 11.7 T) and two (19)F resonances at -151 and -166 ppm in the (19)F NMR spectrum. An( 27)Al quadrupolar coupling constant (C(Q)) of 9.5 MHz and an asymmetry parameter (eta) of 0.1 were identified, for the first time, for the NH(4)AlF(4) crystalline phase observed. On the basis of the (19)F{(27)Al} TRAPDOR (transfer population in double resonance) NMR results, the (19)F resonances at -151 and -166 ppm are therefore assigned to (19)F spins associated with the fluorines in the terminal Al-F and the bridging Al-F-Al groups, respectively.     

A HF Loaded Lewis-Acidic Aluminium Chlorofluoride for Hydrofluorination Reactions

The very strong Lewis acid aluminium chlorofluoride (ACF) was loaded with anhydrous HF. The interaction between the surface of the catalyst and HF was investigated using a variety of characterization methods, which revealed the formation of polyfluorides. Moreover, the reactivity of the HF-loaded ACF towards the hydrofluorination of alkynes was studied.     

Friedel-Crafts sulfonylation in 1-butyl-3-methylimidazolium chloroaluminate ionic liquids.

1-Butyl-3-methylimidazolium chloroaluminate ionic liquids have been employed as an unconventional reaction media and as Lewis acid catalyst for Friedel-Crafts sulfonylation reaction of benzene and substituted benzenes with 4-methyl benzenesulfonyl chloride. The substrates exhibited enhanced reactivity, furnishing almost quantitative yields of diaryl sulfones, under ambient conditions. Studies concerning the effect of Lewis acidity of the ionic liquid on the initial extent of conversion of this reaction has been carried out. (27)Al NMR spectroscopy has been exploited as a tool to investigate the mechanistic details of the reaction. (27)Al NMR spectral studies show the predominance of [Al(2)Cl(7)](-) species in [bmim]Cl-AlCl(3), N = 0.67, acidic ionic liquid in the presence of 4-methyl benzenesulfonyl chloride, and after the reaction with the aromatic hydrocarbon, [AlCl(4)](-) species predominates. This change in speciation of aluminum can be attributed to the interaction of the Lewis acidic species [Al(2)Cl(7)](-) of the ionic liquid with the formed HCl during the sulfonylation reaction, which is evidenced by the control experiment. Preliminary investigations on Friedel-Crafts acylation further substantiate the argument.     

Coupling sol-gel synthesis and microwave-assisted techniques: a new route from amorphous to crystalline high-surface-area aluminium fluoride

A non-aqueous sol-gel Al-based fluoride has been subjected to the microwave solvothermal process. The final material depends on the temperature heat treatment used. Three types of material have been prepared: 1) for low temperature heat treatment (90 degrees C) X-ray amorphous alkoxy fluoride was obtained; 2) for the highest temperature used (200 degrees C) the metastable form beta-AlF3 was obtained with a very large surface area of 125 m2 g(-1). The mechanism of the amorphous=crystalline transformation has been rationalised by the occurrence of a decomposition reaction of the gel fluoride induced by the microwave irradiation. 3) Finally, at intermediate temperature (180 degrees C) a multi-component material mixture exhibiting a huge surface area of 525 m2 g(-1) has been obtained and further investigated after mild post-treatment fluorination using F2 gas. The resulting aluminium-based fluoride still possesses a high-surface-area of 330 m2 g(-1). HRTEM revealed that the solid is built from large particles (50 nm) identified as alpha-AlF3, and small ones (10 nm), relative to an unidentified phase. This new high-surface-area material exhibits strong Lewis acidity as revealed by pyridine adsorption and catalytic tests. By comparison with other materials, it has been shown that whatever the composition/structure of the Al-based fluoride materials, the number of strong Lewis acid sites is related to the surface area, highlighting the role of surface reconstruction occurring on a nanoscopic scale on the formation of the strongest Lewis acid sites.     

An axially chiral, electron-deficient borane: synthesis, coordination chemistry, Lewis acidity, and reactivity

An axially chiral dihydroborepine with a binaphthyl backbone and a C(6)F(5) substituent at the boron atom was prepared by transmetalation from the corresponding tin precursor. This novel motif was structurally characterized by X-ray diffraction analysis as its THF and its PhCN Lewis acid/base complex. (1)H NMR measurements at variable temperatures of the former adduct revealed a remarkable dynamic behavior in solution. Several more Lewis pairs with oxygen, nitrogen, carbon, and phosphorus σ-donors were synthesized and analyzed by multinuclear NMR spectroscopy. The determination of the borane's Lewis acidity with the Gutmann-Beckett method attests its substantial Lewis acidity [85% with Et(3) PO as well as 74% with Ph(3) PO relative to the parent B(C(6)F(5))(3)]. Representative examples of Si-H bond activation (carbonyl reduction and dehydrogenative Si-O coupling) are included, demonstrating the chemical stability and the synthetic potential of the new chiral boron-based Lewis acid.