Acceptor Number AN Research Articles

A free boratriptycene-type Lewis superacid.

Bicyclic pyrazabole-bridged ferrocenes with BH groups at their bridgehead positions were prepared from [Li(thf)]2[1,1′-fc(BH3)2] and pyrazole or 3,5-dimethylpyrazole in the presence of Me3SiCl (1 or 1Me, respectively; 1,1′-fc = 1,1′-ferrocenylene); Me3SiH and H2 are released as byproducts. Treatment of 1 or 1Me with 1 eq. of the hydride scavenger [Ph3C][B(C6F5)4] afforded the borenium salts [2][B(C6F5)4] (72%) and [2Me][B(C6F5)4] (77%). According to X-ray crystallography, [2Me]+ contains one trigonal-planar borenium cation, the cyclopentadienyl (Cp) rings of the 1,1′-fc fragment remain parallel to each other, but the Cp–B bond vector is bent out of the Cp plane by an unprecedentedly large dip angle α* of 40.6°. The Fe⋯B(sp2) distance is very short (2.365(4) Å) and the 11B NMR signal of the cationic B(sp2) center is remarkably upfield shifted (23.4 ppm), suggesting a direct Fe(3d) → B(2p) donor–acceptor interaction. Although this interpretation is confirmed by quantum-chemical calculations, the coupling between the associated orbitals corresponds to an energy of only 12 kJ mol−1. Accordingly, both the experimental (e.g., Gutmann–Beckett acceptor number AN = 111) and theoretical assessment (e.g., Et3PO and F−-ion affinities) of the Lewis acidity proves that [2]+ is among the strongest boron-based Lewis acids available to date.

Mild C–H activation of activated aromatics with [8,8′-μ-I-3,3′-Co(1,2-C2B9H10)2]: Just mix them

Reactions of the iodonium derivative of cobalt bis(dicarbollide) anion [μ-8,8′-I-3,3′-Co(1,2-C2B9H10)2] with activated aromatics were studied. The reactions with ortho- and meta-xylenes result in mild arene C–H activation under ambient conditions, whereas the reaction with para-xylene requires the presence of PPh3 as soft Lewis base. The obtained aryl derivatives [8-Ar-8′-I-3,3′-Co(1,2-C2B9H10)2]− were found to have the transoid conformation both in the solid state and in solution due to intramolecular hydrogen CHcarb⋯IB bonds and aromatic CHcarb⋯π interactions between the dicarbollide ligands. The Lewis acidity of the quasi-borinium cation formed on the breakage of the iodonium bridge was estimated using Beckett–Gutmann method. According to the Gutmann acceptor number AN = 112 (in CD2Cl2), this quasi-borinium cation belongs to the most strong boron Lewis acids and can be considered as Lewis superacid.

Comparative study of the solvent effect on the spectral properties of aminopolyenic molecules with different acceptor substituents

The spectral and fluorescent properties of a number of ωω′-bis(aminopolyenyl)ketones (ketocyanines) and ωω′-bis(3-dimethylaminopropen-2-yliden)-alkylidenmalononitriles (dinitriles), being analogues of usual ketocyanines, but containing the dicyanomethylene fragment instead of the carbonyl group, were studied. Specific and nonspecific solvation of dinitriles and corresponding ketocyanines was studied on the basis of the solvatochromic shifts of the absorption spectra. The photophysical data obtained for the different molecular structures were correlated with different empirical parameters, which characterize solvation by organic solvents: the Dimroth-Reichardt ET(30) parameter of the solvating power, the acceptor number AN, the donor number DN, and the constant π* characterizing the solvent polarity and polarizability. A linear dependence of the maximums in the absorption spectra on the acceptor number AN, which characterizes specific solvation, was found for the ketocyanines under study. This indicates that the electrophilic solvation plays a key role in ketocyanine solutions. For dinitriles, the linear dependence of the maximums of the absorption spectra was obtained as f(π*, AN, DN). In the latter case, along with the electrophilic solvation, there is a substantial contribution of nonspecific and nucleophilic solvation.

Study of the correlations of the MLCT Vis absorption maxima of 4‐pentacyanoferrate‐ 4′‐arylsubstituted bispyridinium complexes with the Hammett substituent parameters and the solvent polarity parameters E and AN

AbstractIn this work six new 4‐aryl substituted bispyridinium salts have been synthesized using 2,4‐dinitrophenyl‐ bispyridinium chloride as the starting material, which was arenamine exchanged via the Zincke reaction. These aromatic heterocyclic salts were used as ligands, acting as Lewis bases to form a series of 4‐pentacyanoferrate‐4′‐aryl substituted bispyridinium complex salts in high yields. The complex salts were characterized using several spectroscopic techniques including 1H as well as 13C NMR spectroscopy, UV–Vis and FTIR spectrophotometry. These materials present a number of interesting electronic and optical properties. Herein we mainly focused on the electronic absorption of these materials. The visible metal‐to‐ligand‐charge‐transfer (MLCT) band of these materials assigned as dπ(FeII) → π*(L) was proved to be affected by substituent changes (of the benzene ring of the complexes) but mainly by solvent polarity changes. Band shifts of even 4000 cm−1 were induced by small solvent polarity changes (e.g., water to methanol). The Vis absorption spectra were investigated in four protic solvents (water, 2,2,2‐trifluoroethanol, ethylene glycol, and methanol). The substituent effects were then quantified using the Hammett equation which correlates the MLCT absorption wavenumbers with the Hammett constant (σx). Furthermore, two of the most successful solvent polarity parameters (the acceptor number AN and the normalized solvent polarity scale E) were used to quantify the solvent polarity effects on the MLCT absorption wavenumbers. The correlations obtained in all cases proved to be satisfactory. The dominant interaction responsible for the solvent polarity effects proved to be the hydrogen‐bond formation between the cyano groups of the complex salts and the protic solvents. Copyright © 2009 John Wiley & Sons, Ltd.

In-situ investigation of surface reactions on alkali halides by atomic force microscopy

In-situ AFM images have been produced of dissolution processes on NaCl and KBr single crystals under different organic solvents. Using a home-built sample holder stable imaging conditions have been achieved and atomic resolution could be obtained on both materials. Dissolution rates have been determined quantitatively from the AFM images and compared with empirical solvent parameters such as acceptor number AN and ET(30) value. The AFM results are in agreement with the expected behaviour of the liquids. Dissolution rates could be controlled using mixtures of polar and non polar solvents. The results are a basis for further investigations of other systems and technologically important processes such as corrosion and surface modification.

Photosensitized generation of superoxide radical in aprotic solvents: an EPR and spin trapping study.

The UV or visible irradiation of pigments such as curcumin, anthralin, benzanthrone, 1,8-dihydroxyanthraquinone, and rose bengal- or eosine-complexes with cationic surfactants in aerated aprotic solvents, such as benzene, toluene, acetone, n-heptane, cyclohexane, in the presence of 5,5-dimethyl-1-pyrroline-N-oxide (DMPO) generates EPR spectra with hyperfine splitting constants (hfsc's) aN = 12.75 G, a beta H = 10.50 G, a gamma H = 1.26 G in toluene, 12.83 G, 10.64 G, 1.24 G in benzene, 12.75 G, 10.19 G, 1.35 G in acetone and 12.54 G, 10.46 G, 1.38 G in n-heptane and cyclohexane. These spectra are similar to those observed when DMPO reacts with 18-crown-6 ether-solubilized KO2 in the respective solvents and suggests that the photoinduced EPR spectra can be safely assigned to the DMPO/superoxide radical adduct (1). A correlation between the hfsc's of 1 and solvent parameters, the solvent acceptor number AN and the Kosower Z value, has been evaluated in terms of its usefulness for the identification of the DMPO/superoxide adduct in organic media.

Acidity and basicity scales for polar solvents

Acidity and basicity scales for polar solvents are examined and compared with the mean spherical approximation parameters derived from the Gibbs energy of solvation of the alkali metal and halide ions in 18 polar solvents, both protic and aprotic. On the basis of an analysis of data for the Gibbs energy of transfer of 1-1 electrolytes and standard redox potentials for simple reactions in a variety of these solvents, it is concluded that the most appropriate scales are the acceptor number AN and donor number DN introduced by Gutmann. The extrathermodynamic assumption for separating cationic and anionic contributions to thermodynamic quantities for electrolytes based on equal solvation of the tetraphenylarsonium and tetraphenylborate ions is reexamined

Solvent‐ and salt‐induced coil–helix transition of alkali metal salts of poly(L‐glutamic acid) in aqueous organic solvents

AbstractThe coil–helix transition has been studied for alkali metal salts of poly (L‐glutamic acid) (PLG), i.e., PLGLi, ‐Na, ‐K, and ‐Cs, in aqueous organic solvent systems. Dependence of the transition on the solvent composition has been qualitatively discussed in terms of the solvent dielectric constant D, Gutmann's acceptor number AN, and water activity aw. The helix formation induced by addition of alkali chlorides has also been studied. The sharpness of the transition has been interpreted as a measure of reduction of electrostatic energy of helical PLG through contact ion‐pair formation between a counterion and carboxyl anion.

13C NMR Study of the Interaction of N,N‐Dimethylamides and N,N‐Dimethylamidines with the Nitroxide Radical

Abstract13C NMR contact shifts δ induced by the 2,2,6,6‐tetramethylpiperidine nitroxide radical (TMPN) were measured for N,N‐dimethylacetamide (DMA) and N,N‐dimethylformamide (DMF) in several solvents and for 5 different N,N‐dimethylamidines in acetone‐d6. The shifts δ were straight linear functions of the TMPN concentration CR and were dependent on the electron acceptor properties of the solvents, thus proving the electron donor‐acceptor nature of the interactions between the radical and the other components of a solution. It was found that the radical induced contact shift enables one to detect differences in the electron acceptor properties of methyl groups within a diamagnetic molecule. In particular the different linear relationships between ln(dδ/dCR) and the solvent acceptor number AN was obtained for each methyl group of DMA. In all the compounds studied the N‐methyl group in the conformation antiperiplanar to the double bond (C = O in amides, C = N in amidines) appeared to be a better electron acceptor site than the N‐methyl group in the synperiplanar conformation. On the basis of dδ/dCR values some conjugation between the aromatic ring and C = N bond π‐electrons in arylacetamidines and in arylformamidines is suggested.

IR‐spektroskopische Untersuchung zur Lösungsmittelwechselwirkung von 1‐Benzylidenpyrazolid‐3‐on‐betainen

Investigation of Solvent‐Solute Interactions of 1‐Benzylidenepyrazolidone (3) ‐betaines by I.R. SpectroscopyThe infrared spectra of eight 1‐benzylidenepyrazolidone(3)‐betaines (1–7 and 3 m) have been measured in 22 solvents of different polarity. The carbonyl band of these compounds indicates specific and non‐specific solvent‐solute interactions, which are qualitatively characterized by the electrophilic properties of the solvent (E‐value, acceptor number AN). Intermolecular hydrogen bonds have been detected in protic solvents. The thermodynamic quantities of the associate equilibrium, K,ΔRH,ΔRS were determined in chloroform and deuterochloroform, respectively, by the i.r. intensity method.

Thermochromism and photochromism of aryl-substituted acyclic azines VII: solvent effect on photochemical E-Z isomerization

In the E-Z isomerization of benzophenone-9-anthraldazine and benzophenone-9-acridinealdazine there is a significant relation between φ E → Z and the solvent acceptor number AN, whereas a significant relation between φ Z → E and AN could not be detected. These results are interpreted in terms of the effect of solvent polarity on the energy levels of the E-Z isomers of the azine.

Quasiliving Carbocationic Polymerization. XI. An Interpretation of Solvent Effects by Donor and Acceptor Numbers

Abstract Counteranion/solvent interactions (counteranion solvation) profoundly influence each and every elementary step of carbocationic polymerizations and are just as important as the commonly emphasized cation/solvent interactions (cation solvation). Counteranion solvation and carbocation solvation have been characterized by Gutmann' s acceptor number AN and donor number DN, respectively. Analysis of earlier data leads to the conclusion that the effect of monomer concentration on the rate, molecular weight, and molecular weight distribution obtained in cationic olefin polymerizations in “polar” solvents are in fact due to subtle changes in solvent concentration. Indeed, olefin monomers behave as “nonpolar” solvents and by changing the monomer concentration the character of the medium may profoundly change. It is further concluded that quasiliving polymerizations cannot be achieved in batch operations because the conditions that prevail in the initial charge, although possibly suitable for quasiliving polymerizations, must continuously change with the diminishing monomer concentration, i.e., by continuously changing the solvent character of the system. In contrast, in continuous systems initial conditions in the charge suitable for the attainment of living or quasiliving conditions can be maintained even for long periods of time by continuously replenishing the consumed monomer. By the use of these concepts, heretofore unexplained observations made in the course of quasiliving polymerization studies have been accounted for and, beyond this, new insight into solvation phenomena in cationic polymerizations is generated.

Characterization of Alcohol Solvents by the Empirical Polarity Parameters AN, Z, and ET(30)

Abstract The acceptor number AN and the polarity parameters Z and ET(30) were determined for a series of alcohols applied as media in the study of kinetic solvent effects. The alcohols thus characterized are methanol, ethanol, 1-propanol, 1-butanol, 2-butanol, 2-methyl-2-butanol, 3-pentanol, 2-chloroethanol, 2-methoxyethanol, 2-phenylethanol, 2-cyanoethanol, benzyl alcohol, 3-ethyl-3-pentanol, 2,4-dimethyl-3-pentanol, and 3-ethyl-2,4-dimetliyl-3-pentanol. In addition, AN was determined for 1,2-dichloroethane and Z for the binary solvent mixtures methanol/2-methyl-2-butanol, ethanol/2,2,2-trifluoroethanol. and methanol/pyridine. The data obtained are correlated and the parameters AN, Z, and ET(30) are critically compared.

Solvent effects on VIb metal carbonyls oxidation and reduction potentials

The electrochemical behaviour of M(CO) 6, {M 2(CO) 10} 2−, {M 2H(CO) 10 −} and M(CO) 5S (M = Cr, Mo, W) has been studied [1] on mercury or platinum electrodes in several solvents S (acetonitrile, AN; propylene carbonate, PC; Acetone, A; N,N-dimethylformamide, DMF and dimethylsulfoxide, DMSO) characterized by a wide range of donor numbers [2]. The experimental study has been focused on the following main factors: 1. In a given solvent, the effect of the metal onc the reduction (E cath 1 2 , from Cr(CO) 6 to W(CO) 6. 2. For each of the three M(CO) 6 complexes, the solvent effects on E cath 1 2 . 3. For a given metal carbonyl, in a given solvent, the effect on E cath 1 2 of substituting the ligand CO by solvent molecule(s). The main results are the following: 1. In a given solvent, changing the metal M in M(CO) 6 induces a positive shift of E cath 1 2 , from Cr(CO) 6 to W(CO) 6, all other parameters being held constant. Also, the first monoelectronic oxidation of M(CO) 6 requires increasing energy from Cr(CO) 6 to W(CO) 6. These results may qualitatively be ascribed to an increasing metal to ligand π back-donation, from Cr(CO) 6 to W(CO) 6. 2. For a given metal M, the reduction of M(CO) 6 is facilitated as the donor number DN of the solvent increases. On the contrary, the reduction potential does not parallel the acceptor number AN of the solvent. 3. Gradual substitution of CO by CH 3CN in W(CO) 6 facilitates the oxidation and makes the reduction more difficult (except for W(CO) 5(CH 3CN). This is fully consistent with the fact that CH 3CN is a weaker π acceptor than CO. In conclusion, it is possible to rationalize these effects in terms of metal-to-carbon π back-bonding whose reinforcement results in a facilitated reduction and in a hindered oxidation, from Cr(CO) 6 to W(CO) 6. Also, the influence of the donor number of the solvent upon the E cath 1 2 supports the reduction scheme previously proposed [1] for M(CO) 6, where the stable species, M(CO) 5S, results from the coordination of M(CO) 5 • to the solvent S. In such a reaction indeed, a strengthened coordination of S on the metal is expected as DN of S increases, the consequence of which is to facilitate the reduction process, as observed experimentally.