Molecular anions

The experimental and theoretical study of molecular anions has undergone explosive growth over the past 40 years. Advances in techniques used to generate anions in appreciable numbers as well as new ion-storage, ion-optics, and laser spectroscopic tools have been key on the experimental front. Theoretical developments on the electronic structure and molecular dynamics fronts now allow one to achieve higher accuracy and to study electronically metastable states, thus bringing theory in close collaboration with experiment in this field. In this article, many of the experimental and theoretical challenges specific to studying molecular anions are discussed. Results from many research groups on several classes of molecular anions are overviewed, and both literature citations and active (in online html and pdf versions) links to numerous contributing scientists' Web sites are provided. Specific focus is made on the following families of anions: dipole-bound, zwitterion-bound, double-Rydberg, multiply charged, metastable, cluster-based, and biological anions. In discussing each kind of anion, emphasis is placed on the structural, energetic, spectroscopic, and chemical-reactivity characteristics that make these anions novel, interesting, and important.     

Vanadogermanate cluster anions

Three novel vanadogermanate cluster anions have been synthesized by hydrothermal reactions. The cluster anions are derived from the (V(18)O(42)) Keggin cluster shell by substitution of V=O(2+) "caps" by Ge(2)O(OH)(2)(4+) species. In Cs(8)[Ge(4)V(16)O(42)(OH)(4)].4.7H(2)O, 1, (monoclinic, space group C2/c (No. 15), Z = 8, a = 44.513(2) A, b = 12.7632(7) A, c = 22.923(1) A, beta = 101.376(1) degrees ) and (pipH(2))(4)(pipH)(4)[Ge(8)V(14)O(50).(H(2)O)] (pip = C(4)N(2)H(10)), 2 (tetragonal, space group P4(2)/nnm (No. 134), Z = 2, a = 14.9950(7) A, c = 18.408(1) A), two and four VO(2+) caps are replaced, respectively, and each cluster anion encapsulates a water molecule. In K(5)H(8)Ge(8)V(12)SO(52).10H(2)O, 3, (tetragonal, space group I4/m (No. 87), Z = 2, a = 15.573(1) A, c = 10.963(1) A), four VO(2+) caps are replaced by Ge(2)O(OH)(2)(4+) species, and an additional two are omitted. The cluster ion in 3 contains a sulfate anion disordered over two positions. The cluster anions are analogous to the vanadoarsenate anions [V(18)(-)(n)()As(2)(n)()O(42)(X)](m)(-) (X = SO(3), SO(4), Cl; n = 3, 4) previously reported.     

Colorimetry of sulphur anions

Summary The colorimetric method for the determination and evaluation of polythionates above trithionate has been adapted for use in conjunction with sulphur sols. This has been achieved simply by increasing the period of cyanolysis. The method gives good results for the concentration of sulphur. The n values in the formula S _n O ₆ ^2– , which also represents sulphur sols, are not very accurate, but the accuracy might be increased by high precision colorimetry. However, they do serve to distinguish between polythionates and colloidal sulphur.Part III.: Urban, P. J.: Z. analyt. Chem. 180, 110 (1961).     

Radical anions of flavonoids

Several representatives of natural flavonoids and their synthetic nitro‐derivatives have been investigated by polarography and electron paramagnetic resonance (EPR) spectroscopy under electrochemical reduction in acetonitrile, dimethylformamide (DMF), dimethylsulfoxide (DMSO) or 1,2‐dimethoxyethane. All the compounds studied are reduced in the first stage by one‐electron transfer, apart from flavanone, which accepts two electrons simultaneously. However, the primary radical anions were detected by EPR spectroscopy only for 4′‐nitroflavone. It was shown that radical anions of other flavonoids quickly dimerized. The analysis of the temperature dependence of the hyperfine interaction constants and broadening of lines in EPR spectra of 4′‐nitroflavone radical anions has shown that the distribution of spin density is due to both the change of polarity of the medium and rotation of the nitrophenyl moiety. The assignment of hyperfine structure constants for the 4′‐nitroflavone radical anion was confirmed by density functional theory (DFT) calculations. Copyright © 2011 John Wiley & Sons, Ltd.     

Enzymatic determination of anions

Publications devoted to the determination of a number of anions by enzymatic methods are reviewed. The opportunities of utilizing native and immobilized enzymes for the determination of anions in medical, environmental, and food samples are considered.     

Solutions of octacyanometallate anions

Calculation of ligand-ligand repulsion energies shows that there is no potential energy barrier to prevent a metal atom, surrounded by eight equivalent monodentate ligands, changing from dodecahedral to square antiprismatic stereochemistry in solution. Such a change involves a twisting of ligans about the major dodecahedral axis by about 15 in either direction. A continuation of this distortion in the same direction to an alternative dodecahedron and then to an alternative square antiprism also appears possible. These distortions correspond to a continuous movement of ligands about the metal atom, rather than to the usual reciprocatory bending vibrations. The consequences of this hypothesis are examined for the particular case of the octacyanide ions [M(CN)₈] ^n– (M=Mo, W, Re; n=2, 3, 4).

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Zusammenfassung Nach Rechnungen an Komplexen mit acht gleichwertigen, einzÄhligen Liganden zeigt sich, da\ es in Lösung zwischen dodekaedrischer und quadratisch-antiprismatischer Konformation keine Potentialschwelle gibt. Solch ein übergang bedingt eine Verdrehung der Liganden um die Hauptachse des Dodekaeders um 15. Auch der weitere übergang zu einem anderen Dodekaeder bzw. einem anderen quadratischen Antiprisma erscheint möglich. Diese Verzerrung stimmt eher mit einer kontinuierlichen Bewegung der Liganden um das Metallatom, als mit der normalen Knickschwingung überein. Für den Fall der Octacyanoanionen [M(CN)₈] ^n– (M=Mo, W, Re; n=2, 3, 4) wird diese Hypothese überprüft.

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Résumé Le calcul des énergies de répulsion électrostatique ligand-ligand montre qu'il n'y a pas de barrière de potentiel pour empÊcher un atome métallique, entouré de huit ligands monodentés identiques, de passer en solution, d'une conformation dodécaédrique à une conformation antiprismatique carrée. Une telle modification implique une torsion des ligands d'environ 15 de part et d'autre de l'axe dodécaédrique principal. Cette distorsion peut aussi se prolonger dans la mÊme direction vers un autre dodécaèdre ou un autre antiprisme carré. Ces distorsions correspondent à un mouvement continu des ligands autour de l'atome métallique, plutÔt qu'aux vibrations habituelles de déformations réciproques. Les conséquences de cette hypothèse sont examinées pour le cas particulier des ions octocyanures [M(CN)₈] ^n– (M=Mo, W, Re; n=2, 3, 4).

     

Nucleophilicities of nitroalkyl anions

The kinetics of the reactions of eight nitroalkyl anions (nitronate anions) with benzhydrylium ions and quinone methides in DMSO and water were investigated photometrically. The second-order rate constants were found to follow a Ritchie constant selectivity relationship with slightly smaller selectivities than those observed previously for other carbanions and O or N nucleophiles. Evaluation of the kinetic data by the correlation equation log k (20 degrees C) = s(N + E) yields the nucleophilicity parameters (N), which allow a comparison of the nucleophilicities of nitronates with those of other classes of compounds. Although the aliphatic nitronates 1a-c are more nucleophilic than the aromatic representatives 1d-h in DMSO, hydration reduces the nucleophilicities of aliphatic nitronates by a factor of 1 million, which is considerably greater than the reduction of the reactivities of the aromatic nitronates with the consequence that aromatic nitronates are more nucleophilic in water than aliphatic ones. The nucleophilic reactivities of nitronates are only slightly affected by substituent variation in DMSO and even less so in aqueous solution, which is considered to be the reason for the unusual rate equilibrium relationships, the so-called nitroalkane anomaly. Outer-sphere electron transfer does not occur in any of the reactions that were investigated.     

Outer-sphere interactions between anions

The properties of anion associates are reviewed. Experimental electronic absorption spectra of anion-anion and anion-cation pairs were analyzed and compared with calculated data. Stability constants of ion pairs were determined in the framework of the theory of outer-sphere association of ions. Anion association was shown to be a specific case of typical cation-anion association, in which associates of the 2Na⁺,[Coedta]⁻ type play the role of a complex cation. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 5, pp. 833–840, May, 1999.     

Colorimetry of sulphur anions

Summary The colorimetric method of Sörbo for the determination of thiosulphate has been improved in such a way that polythionates do not interfere; it has also been modified for use with larger volumes of solution. The procedure depends on the production of thiocyanate equivalent to the thiosulphate, and determination of the thiocyanate with an excess of ferric iron. By careful control of the p_H using an acetate buffer, the cyanolysis of polythionates and hence their interference is avoided.     

Ultracold water cluster anions

Attachment of free electrons to water clusters embedded in helium droplets leads to water-cluster anions (H2O)n(-) and (D2O)n(-) of size n > or = 2. Small water-cluster anions bind to up to 10 helium atoms, providing compelling evidence for the low temperature of these complexes, but the most abundant species are bare cluster anions. In contrast to previous experiments on bare water clusters, which showed very pronounced magic and anti-magic anion sizes below n = 12, the presently observed size distributions vary much more smoothly, and all sizes are easily observed. Noticeable differences are also observed in the stoichiometry of fragment anions formed upon dissociative electron attachment and the energy dependence of their yield. Spectroscopic characterization of these ultracold water-cluster anions promises to unravel the relevance of metastable configurations in experiments and the nature of the still controversial bonding sites for the excess electron in small water-cluster anions.     

Nitrile anions: solvent-dependent cyclizations

Extensive cyclizations in hydrocarbon and polar solvents demonstrate a profound solvent sensitivity for intramolecular nitrile anion alkylations. S(N)i cyclizations enforce very precise steric constraints in the transition state, allowing correlation of the cyclization stereochemistry with the orbital orientation of the nitrile anion. Collectively the cyclizations suggest a continuum of nitrile anion transition states, varying from planar to fully pyramidal, that selectively cyclize to cis- and trans-decalins, respectively.     

Ambident reactivities of pyridone anions

The kinetics of the reactions of the ambident 2- and 4-pyridone anions with benzhydrylium ions (diarylcarbenium ions) and structurally related Michael acceptors have been studied in DMSO, CH(3)CN, and water. No significant changes of the rate constants were found when the counterion was varied (Li(+), K(+), NBu(4)(+)) or the solvent was changed from DMSO to CH(3)CN, whereas a large decrease of nucleophilicity was observed in aqueous solution. The second-order rate constants (log k(2)) correlated linearly with the electrophilicity parameters E of the electrophiles according to the correlation log k(2) = s(N + E) (Angew. Chem., Int. Ed. Engl. 1994, 33, 938-957), allowing us to determine the nucleophilicity parameters N and s for the pyridone anions. The reactions of the 2- and 4-pyridone anions with stabilized amino-substituted benzhydrylium ions and Michael acceptors are reversible and yield the thermodynamically more stable N-substituted pyridones exclusively. In contrast, highly reactive benzhydrylium ions (4,4'-dimethylbenzhydrylium ion), which react with diffusion control, give mixtures arising from N- and O-attack with the 2-pyridone anion and only O-substituted products with the 4-pyridone anion. For some reactions, rate and equilibrium constants were determined in DMSO, which showed that the 2-pyridone anion is a 2-4 times stronger nucleophile, but a 100 times stronger Lewis base than the 4-pyridone anion. Quantum chemical calculations at MP2/6-311+G(2d,p) level of theory showed that N-attack is thermodynamically favored over O-attack, but the attack at oxygen is intrinsically favored. Marcus theory was employed to develop a consistent scheme which rationalizes the manifold of regioselectivities previously reported for the reactions of these anions with electrophiles. In particular, Kornblum's rationalization of the silver ion effect, one of the main pillars of the hard and soft acid/base concept of ambident reactivity, has been revised. Ag(+) does not reverse the regioselectivity of the attack at the 2-pyridone anion by increasing the positive charge of the electrophile but by blocking the nitrogen atom of the 2-pyridone anion.     

Amide based receptors for anions

This review article illustrates the contribution of amide based receptors to the development of anion complexing agents. Amides are incorporated into a wide variety of systems that can be divided into two broad categories; organic and inorganic. The first section is separated into cyclic and acyclic systems built on a solely organic framework. The second section is comprised of metal containing systems such as metallocenes, [Ru(bipy)₃]²⁺ based complexes, porphyrins and other metallo-based receptors. Where appropriate, the results of solution binding studies and sensing outputs are summarized.     

Photochemical reactions of allylphenoxide anions

Photochemical behaviour of sodium allylphenoxides 1a–3a was compared with that of corresponding neutral allylphenols 1b–3b, and a striking difference was found in reactivity as well as in course of the reaction. Remarkably higher photoreactivity of allylphenoxide anions, especially that of o-allylphenoxide 1a, is ascribed to the efficiency of electron transfer. The characteristic reaction products derived only from the anions were as follows: (a) intramolecular cyclization products, indanols 9, from 2a, (b) rearranged reduction products 10 from both 2a and 3a, and (c) degradative methanol-adduct 11 from 3a. Methanol-adducts 7 and photo-reduction products 6 were formed in either case. In addition cyclization products with the participation of O atom, 4 and 5, were obtained from 1a and 1b. Occurrence of the di-π-methane rearrangement was restricted to the reaction of neutral species 1b–3b. A mechanism based on electron transfer in the excited state is suggested for the course of the photoreaction.     

Resonance states of atomic anions

We study destabilization of an atom in its ground state with decrease of its nuclear charge. By analytic continuation from bound to resonance states, we obtain complex energies of unstable atomic anions with nuclear charge that is less than the minimum “critical” charge necessary to bind N electrons. We use an extrapolating scheme with a simple model potential for the electron, which is loosely bound outside the atomic core. Results for O^2? and S^2? are in good agreement with earlier estimates. Alternatively, we use the Hylleraas basis variational technique with three complex nonlinear parameters to find accurately the energy of two‐electron atoms as the nuclear charge decreases. Results are used to check the less accurate one‐electron model. © 2001 John Wiley & Sons, Inc. Int J Quant Chem 82: 255–261, 2001