Structure and binding energy of anion-pi and cation-pi complexes: a comparison of MP2, RI-MP2, DFT, and DF-DFT methods

Several complexes of benzene with cations, hexafluorobenzene with anions, 1,3,5-trifluorobenzene with cations and anions, and s-triazine with cations and anions have been evaluated and compared at the MP2 and resolution of the identity MP2 (RI-MP2) levels. The RI-MP2 method is considerably faster than the MP2 and the interaction energies and equilibrium distances are almost identical for both methods. A similar result is found when comparing DFT and density fitting DFT (DF-DFT) levels. Therefore RI-MP2 and DF-DFT methods are well suited for the study of ion-pi interactions.     

Polarizability effects and dispersion interactions in alkene-Br2 pi-complexes

Weakly bound molecular complexes play an important role in chemistry, physics, and biodisciplines. The preequilibrium pi-complexes of various alkenes with bromine have been examined quantitatively, and a direct relationship between association constants (KF) of these pi-complexes and polarizability of the olefins was found. The stability of the Br2-olefin pi complexes is affected by both the donor ionization potential and the polarizability of the olefin, and an equation able to take into account both effects is proposed.     

Theoretical study of interaction of 2,6-dithiopurine with Li+, Na+, K+, Be2+, Mg2+, and Ca2+ metal cations

The geometries of the complexes of Li⁺, Na⁺, K⁺, Be²⁺, Mg²⁺, and Ca²⁺ metal cations with different possible 2,6-dithiopurine anions (DTP) were studied. The complexes were optimized at the B3LYP level and the 6-311++G(d, p) basis set. The interactions of the metal cations at different nucleophilic sites of various possible 2,6-dithiopurine anions were considered. It was revealed that metal cations would interact with 2,6-dithiopurine anions in a bicoordinate manner. In the gas phase, the most preferred position for the interaction of Li⁺, Na⁺, and K⁺ cations is between the N₃ and S₂ sites, while all divalent cations Be²⁺, Mg²⁺, and Ca²⁺ prefer binding between the N₇ and S₆ sites of the corresponding 2,6-dithiopurine. The influence of aqueous solvent on the relative stability of different complexes has been examined using the Tomasi’s polarized continuum model. The basis set superposition error (BSSE) corrected interaction energy was also computed for complexes. The AIM theory has been applied to analyze the properties of the bond critical points (electron densities and their Laplacians) involved in the coordination between 2,6-dithiopurine anions and the metal cations. It was revealed that aqueous solution would have significant effect on the relative stability of complexes obtained by the interaction of 2,6-dithiopurine anions with Mg²⁺ and Ca²⁺ cations. The effect of metal cations on different NH and CS stretching vibrational modes of 2,6-dithiopurine has also been discussed.     

Gas-phase ion/ion reactions of transition metal complex cations with multiply charged oligodeoxynucleotide anions

Multiply deprotonated hexadeoxyadenylate anions, (A6-nH)(n-), where n = 3-5, have been subjected to reaction with a range of divalent transition-metal complex cations in the gas phase. The cations studied included the bis- and tris-1,10-phenanthroline complexes of CuII, FeII, and CoII, as well as the tris-1,10-phenanthroline complex of RuII. In addition, the hexadeoxyadenylate anions were subjected to reaction with the singly charged FeIII and CoIIIN,N'-ethylenebis(salicylideneiminato) complexes. The major competing reaction channels are electron-transfer from the oligodeoxynucleotide anion to the cation, the formation of a complex between the anion and cation, and the incorporation of the transition-metal into the oligodeoxynucleotide. The latter process proceeds via the anion/cation complex and involves displacement of the ligand(s) in the transition-metal complex by the oligodeoxynucleotide. Competition between the various reaction channels is governed by the identity of the transition-metal cation, the coordination environment of the metal complex, and the oligodeoxynucleotide charge state. In the case of the divalent metal phenanthroline complexes, competition between electron-transfer and metal ion incorporation is particularly sensitive to the coordination number of the reagent metal complexes. Both electron-transfer and metal ion incorporation occur to significant extents with the bis-phenanthroline ions, whereas the tris-phenanthroline ions react predominantly by metal ion incorporation. To our knowledge this work reports the first observations of the gas-phase incorporation of multivalent transition-metal cations into oligodeoxynucleotide anions and represents a means for the selective incorporation of transition-metal counter-ions into gaseous oligodeoxynucleotides.     

Simultaneous interactions of anions and cations with cyclohexane and adamantane: aliphatic cyclic hydrocarbons as charge insulators

With ab initio MP2 computational methods, a theoretical study has been carried out to characterize the interaction between aliphatic cyclic hydrocarbons, as models of molecular hydrocarbon monolayers, with cations (Li(+), Na(+), and K(+)), anions (F(-), Cl(-), and Br(-)), and both simultaneously in opposite faces of the hydrocarbons. In addition, the energetic barrier for the cation crossing through the hydrocarbon ring has been calculated. The hydrocarbons chosen for this study are cyclohexane (C(6)H(12)) and adamantane (C(10)H(16)). The energies obtained for the M(+):hydrocarbon:X(-) complexes indicate positive cooperativity in the cases where the hydrocarbon is cyclohexane while diminutive effects are found in the adamantane complexes. The density functional theory-symmetry adapted perturbation theory analysis of the interaction energies shows that the most important term in the complexes with cations is the induction, while in the complexes with anion and with cations and anions simultaneously the most important term is the repulsion-exchange one. The electron density of the complexes has been analyzed using the atoms in molecules methodology and provides some insight to the electron transfer within the complexes.     

Dications of l-histidine and l-ornithine in layers of copper(II) and cobalt(II) dipicolinato anions

Cobalt(II) and copper(II) dipicolinato complexes having l-histidine and l-ornithine dications are synthesized in water and are characterized by various spectroscopic techniques. X-ray studies reveal that the l-histidine dications are strongly hydrogen bonded and intercalated in the layered structures of dipicolinato complex anions with repeated unit in 1:1 ratio. Whereas l-ornithine dications form hydrogen bonded repeated units of cations and anions in 2:2 ratio. The cations are held in the layered structures formed by the association of the amino acid cations and metal dipicolinato anions supported by water through strong hydrogen-bond interactions. The complexes thus formed are optically active and exhibit specific rotation in the range of +4 to +12°.     

Ternary inclusion complexes of γ-cyclodextrin with sodium 1-pyrenesulfonate and cationic and anionic organic compounds having an alkyl chain in aqueous solution

The interactions of an organic anion, 1-pyrenesulfonate (PS), with γ-cyclodextrin (γ-CD) and organic cations such as quaternary ammonium compounds (or organic anions such as 1-decanesulfonate) have been examined by means of absorption and fluorescence spectroscopy. At a low concentration of PS, γ-CD forms a 1:1 inclusion complex with PS. PS forms organic cation–organic anion complexes with quaternary ammonium compounds. The organic cation–organic anion complexes of PS form ternary inclusion complexes with γ-CD. Equilibrium constants for the formation of these complexes have been evaluated from the fluorescence intensity change. As an alkyl chain in the quaternary ammonium compound is lengthened, the equilibrium constant for the formation of the ternary inclusion complex is increased. Although PS does not form complexes with organic anions such as 1-decanesulfonate, the organic anions are bound to the γ-CD cavity accommodating a PS molecule to form ternary inclusion complexes. However, the equilibrium constants for the organic anions (1-decanesulfonate etc.) are significantly less than those for the quaternary ammonium compounds. The small equilibrium constants for the organic anions can be ascribed to the electrostatic repulsion between PS and the organic anions.     

Theoretical study of interaction of urate with li(+), na(+), k(+), be(2+), mg(2+), and ca(2+) metal cations

The geometries and energetics of complexes of Li(+), Na(+), K(+), Be(2+), Mg(2+), and Ca(2+)metal cations with different possible uric acid anions (urate) were studied. The complexes were optimized at the B3LYP level and the 6-311++G(d,p) basis set. Complexes of urate with Mg(2+), and Ca(2+)metal cations were also optimized at the MP2/6-31+G(d) level. Single point energy calculations were performed at the MP2/6-311++G(d,p) level. The interactions of the metal cations at different nucleophilic sites of various possible urate were considered. It was revealed that metal cations would interact with urate in a bi-coordinate manner. In the gas phase, the most preferred position for the interaction of Li(+), Na(+), and K(+) cations is between the N(3) and O(2) sites, while all divalent cations Be(2+), Mg(2+), and Ca(2+) prefer binding between the N(7) and O(6) sites of the corresponding urate. The influence of aqueous solvent on the relative stability of different complexes has been examined using the Tomasi's polarized continuum model. The basis set superposition error (BSSE) corrected interaction energy was also computed for complexes. The AIM theory has been applied to analyze the properties of the bond critical points (electron densities and their Laplacians) involved in the coordination between urate and the metal cations. It was revealed that aqueous solvation would have significant effect on the relative stability of complexes obtained by the interaction of urate with Mg(2+) and Ca(2+)cations. Consequently, several complexes were found to exist in the water solution. The effect of metal cations on different NH and CO stretching vibrational modes of uric acid has also been discussed.     

Theoretical studies of azapentalenes. Part 4: Theoretical study of the properties of 3a,6a-diazapentalene

A theoretical study of the properties of the isolated 3a,6a-diazapentalene by means of DFT, B3LYP/6-311++G(d,p) and ab initio methods, MP2/6-311++G(d,p), has been carried out. In addition, the complexes formed with hydrogen bond donor, acceptors, cations, and anions have been studied and analyzed. Ring opening into 1,5-diazocine as well as basicity and acidity properties of 3a,6a-diazapentalene have been explored. Their ability to form HB complexes and the complexes formed with anions and cations have been studied.     

Precipitation and characterization of uniform submicrometer lead titanate particles. Part II: Reaction mechanism

The chemical mechanism of the precipitation of lead titanium peroxohydroxide particles in a solution of nitrilotriacetate (NTA) complexes of lead and titanium peroxo-hydroxide is envisioned as the interaction between cations of lead hydroxide and anions of polymeric titanium peroxo-hydroxide.     

Complexes between divalent metals and carboxylic acids: Semiempirical study

The possibility to predict the stability constants of the complexes of magnesium, zinc, cadmium, and lead cations with anions of different carboxylic acids by PM3 calculations was investigated. Linear correlation between calculated complex-formation reaction enthalpies and experimentally measured stability constants were obtained for complexes of each metal, while the overall correlation was not satisfactory. © 1997 John Wiley & Sons, Inc. Int J Quant Chem 62: 653–658, 1997     

Organocuprate conjugate addition: structural features of diastereomeric and supramolecular pi-intermediates

In the reaction pathway of conjugate additions with organocuprate reagents, Cu(I) pi-complexes and Cu(III) sigma-complexes have been identified as central, NMR-detectable intermediate species. However, no experimental evidence for the structures of pi-intermediates with extensive chiral enones or the principal aggregation level and aggregate structure of pi-complexes in diethyl ether has been available so far. Furthermore, the structural characteristics of pi-complexes which are essential for their high reactivities and diastereoselectivities have not yet been rationalized experimentally. Therefore, the pi-intermediates of 4,4a,5,6,7,8-hexahydro-4a-methyl-naphthalen-2(3H)-one and Me2CuLi or Me2CuLi x LiX (X = I, CN) in diethyl ether are investigated in detail. For the first time, the formation of two intermediate cuprate enone pi-complexes on both sides of the double bond is observed. In addition, the conformation of the enone adopted in the major beta-face pi-complex rationalizes the exclusive syn addition observed in the synthetic product. For the investigation of the aggregation level and structure, a NMR screening of pi-complexes with Me2CuLi x LiX (X = I, CN) and three achiral enones is performed, which simplifies the spectra by the generation of enantiotopic pi-complexes. Thus, NMR diffusion experiments on cuprate intermediates and the detection of scalar couplings across copper without isotope labeling are possible for the first time. Extensive NMR studies, including those of cyclohexanone complexes, show that, in principle, salt-free dimethylcuprate is able to complex the carbonyl group. However, in the presence of salt, the carbonyl-complexing aggregates are composed of salt and cuprate moieties. These mixed aggregates cause the formation of large supramolecular pi-intermediate structures which control their reactivity. The pi-complexing cuprate units show a bent geometry as a general structural feature that is unaffected by the presence or kind of salt and the type of enone. Thus, the high diastereoselectivity and the reactivity of organocuprate 1,4-addition reactions are for the first time rationalized on the basis of structural characteristics of selected pi-intermediates.     

Structural and thermodynamic aspects of complexation of a calix[4]resorcinarene with diverse cations in water-organic media

Solvation of a calix[4]resorcinarene and its anions in water-organic media of varying composition was characterized by solubility measurements, acid-base titration and X-ray structural analysis. Methanol, 2-propanol, and DMSO capable of developing calix[4]resorcinarene solvated complexes of varying structure and stability were used as the organic component. It was revealed that the influence of the organic component of water-organic solvents on the stability of guest-host complexes formed by some organic and complex cations with calix[4]resorcinarene anions depended on the structure of the complexes.     

Outer-sphere electron transfer in aqueous solutions of lithium hexacyanoferrates

Electron transfer between the hexacyanoferrate anions in the presence of the Li⁺ cations in aqueous solutions was studied by spectrophotometry. The results obtained were compared with the data for the K⁺- and Me₄N⁺-containing systems. The proposed explanation of the observed cationic catalysis effect of the thermal electron transfer in homogeneous and heterogeneous systems is based on the analysis of electron-transfer distances (viz., contact distances between the anions) in the activated complexes.     

Simultaneous separation of inorganic anions and metal-citrate complexes on a zwitterionic stationary phase with on-column complexation

The retention and separation selectivity of inorganic anions and on-column derivatised negatively charged citrate or oxalate metal complexes on reversed-phase stationary phases dynamically coated with N-(dodecyl-N,N-dimethylammonio)undecanoate (DDMAU) has been investigated. The retention mechanism for the metal-citrate complexes was predominantly anion exchange, although the amphoteric/zwitterionic nature of the stationary phase coating undoubtedly also contributed to the unusual separation selectivity shown. A mixture of 10 inorganic anions and metal cations was achieved using a 20 cm monolithic DDMAU modified column and a 1 mM citrate eluent, pH 4.0, flow rate equal to 0.8 mL/min. Selectivity was found to be strongly pH dependent, allowing additional scope for manipulation of solute retention, and thus application to complex samples. This is illustrated with the analysis of an acidic mine drainage sample with a range of inorganic anions and transition metal cations, varying significantly in their concentrations levels.     

Theoretical characterization of dihydrogen adducts with halide anions

The interaction between a hydrogen molecule and the halide anions F(-), Cl(-), Br(-), and I(-) has been studied at different levels of theory and with different basis sets. The most stable configurations of the complexes have a linear geometry, while the t-shaped complexes are saddle points on the potential energy surface, opposite to what is observed for alkali cations. An electrostatic analysis conducted on the resulting adducts has highlighted the predominance of the electrostatic term in the complexation energy and, in particular, of the quadrupole- and dipole-polarizability dependent contributions. Another striking difference with respect to the positive ions, is the fact that although the binding energies have similar values (ranging between 25 and 3 kJ /mol for F(-) and I(-), respectively), the vibrational shift of the nu(H-H) and in general the perturbation of the hydrogen molecule in complexes are much greater in the complexes with anions (Delta nu(H-H) ranges between -720 and -65 cm(-1)). Another difference with respect to the interaction with cations is a larger charge transfer from the anion to the hydrogen molecule. The Delta nu is the result of the cooperative role of the electrostatics and of the charge transfer in the interaction. The correlation between binding energies and vibrational shift is far from linear, contrary to what is observed for cation complexes, in accordance with the higher polarizability and dynamic polarizability of the molecule along the molecular axis. The observed correlation may be valuable in the interpretation of spectra and thermodynamic properties of adsorbed H(2) in storage materials.     

Electrochemistry of Polypyrrole Films in Aqueous Solutions: The Character of the Bond between the Anion and the Polymer Matrix

Mechanisms of the formation and doping of polypyrrole films and their behavior in aqueous media are studied by voltammetry, chronoamperometry, and other methods combined with IR spectroscopy and the tracer technique. The last two methods reveal that pyrrole molecules in acid media form complexes with protons and anions, and that a strong interaction occurs in a polypyrrole film between links of polypyrrole chains and anions, leading to a change in their structure. The obtained data are used to propose a mechanism for the synthesis of polypyrrole films, which includes the discharge of a complex of protonated pyrrole with the anion yielding radical cations, which induce the growth and development of polypyrrole chains. It is stressed that the incorporation of anions into a film (doping) occurs in the course of its formation during the discharge and partial destruction of the pyrrole complexes with the anion.     

Coordination compounds of electron-deficient boron cluster anions BnH n2? (n = 6, 10, 12)

This survey concerns the coordination ability of B _n H _n ²⁻ (n = 6, 10, 12) boron cluster anions and their derivatives in complex formation. Boron cluster anions form four types of compounds: salts of organic cations and alkali-metal cations, including Cat₂B _n H _n , where specific interactions can be observed between a cation Cat and a boron cluster anion; salts of protonated anions CatB₆H₇ and CatB₁₀H₁₁, analogues of Cat[MB _n H _n ] complexes, where an extra hydrogen atom appears bound with the BBB face of a boron polyhedron and performs as a hard acceptor; metal complexes with outer-sphere boron cluster anions where specific ligand-ligand interactions may be observed between a boron cluster anion and an inner-sphere ligand; and true metal complexes with boron cluster anions that enter the inner coordination sphere. The last case characterizes closo-hydroborate anions as polydentate ligands whose denticity can vary widely under the effect of substituents or other ligands in the complex.     

Anionic Magnesium and Calcium Hydrides: Transforming CO into Unsaturated Disilyl Ethers

The synthesis, characterisation and reactivity of two isostructural anionic magnesium and calcium complexes is reported. By X-ray and neutron diffraction techniques, the anionic hydrides are shown to exist as dimers, held together by a range of interactions between the two anions and two bridging potassium cations. Unlike the vast proportion of previously reported dimeric group 2 hydrides, which have hydrides that bridge two group 2 centres, here the hydrides are shown to be “terminal”, but stabilised by interactions with the potassium cations. Both anionic hydrides were found to insert and couple CO under mild reaction conditions to give the corresponding group 2 cis-ethenediolate complexes. These cis-ethenediolate complexes were found to undergo salt elimination reactions with silyl chlorides, allowing access to small unsaturated disilyl ethers with a high percentage of their mass originating from the C₁ source CO.