Experimental Evidence for a Calix[4]arene-Proton Complex

From extraction experiments and γ-activity measurements, the extraction constant corresponding to the equilibrium H⁺(aq) + 1 ·Na⁺(nb) ⇆ 1 ·H⁺(nb) + Na⁺(aq) taking place in the two-phase water-nitrobenzene system (1 = p-tert-butylcalix[4]arene-tetrakis(N, N-diethylacetamide); aq = aqueous phase, nb = nitrobenzene phase) was evaluated as log K _ex(H⁺, 1 ·Na⁺) = −1.4 ± 0.1. Further, the stability constant of the p-tert-butylcalix[4]arene-tetrakis(N,N-diethylacetamide)-H⁺ complex in water saturated nitrobenzene was calculated for a temperature of 25°C as log β_nb(1 · H⁺) = 8.1 ± 0.1.     

Experimental Evidence for a Calix[4]arene-Proton Complex

Summary. From extraction experiments and γ-activity measurements, the extraction constant corresponding to the equilibrium H⁺(aq) + 1 ·Na⁺(nb) ⇆ 1 ·H⁺(nb) + Na⁺(aq) taking place in the two-phase water-nitrobenzene system (1 = p-tert-butylcalix[4]arene-tetrakis(N, N-diethylacetamide); aq = aqueous phase, nb = nitrobenzene phase) was evaluated as log K _ex(H⁺, 1 ·Na⁺) = −1.4 ± 0.1. Further, the stability constant of the p-tert-butylcalix[4]arene-tetrakis(N,N-diethylacetamide)-H⁺ complex in water saturated nitrobenzene was calculated for a temperature of 25°C as log β_nb(1 · H⁺) = 8.1 ± 0.1.     

Solubility and hydrolysis of lutetium at different [Lu<Superscript>3+</Superscript>]<Subscript>initial</Subscript>

Solubility product (Lu(OH)_3(s)⇆Lu³⁺+3OH⁻) and first hydrolysis (Lu³⁺+H₂O⇆Lu(OH)²⁺+H⁺) constants were determined for an initial lutetium concentration range from 3.72·10⁻⁵ mol·dm⁻³ to 2.09·10⁻³ mol·dm⁻³. Measurements were made in 2 mol·dm⁻³ NaClO₄ ionic strength, under CO₂-free conditions and temperature was controlled at 303 K. Solubility diagrams (pLu_aq vs. pC _H) were determined by means of a radiochemical method using ¹⁷⁷Lu. The pC _H for the beginning of precipitation and solubility product constant were determined from these diagrams and both the first hydrolysis and solubility product constants were calculated by fitting the diagrams to the solubility equation. The pC _H values of precipitation increases inversely to [Lu³⁺]_initial and the values for the first hydrolysis and solubility product constants were log₁₀ β* _Lu,H = −7.92±0.07 and log₁₀ K*_sp,Lu(OH)3 = −23.37±0.14. Individual solubility values for pC _H range between the beginning of precipitation and 8.5 were S _Lu3+ = 3.5·10⁻⁷ mol·dm⁻³, S _Lu(OH)2+ = 6.2·10⁻⁷ mol·dm⁻³, and then total solubility was 9.7·10⁻⁷ mol·dm⁻³.     

A combined experimental and DFT study on the complexation of Mg<Superscript>2+</Superscript> with beauvericin

From extraction experiments and γ-activity measurements, the extraction constant corresponding to the equilibrium Mg²⁺(aq) + 1·Sr²⁺(nb) ⇆ 1·Mg²⁺(nb) + Sr²⁺(aq) taking place in the two-phase water–nitrobenzene system (1 = beauvericin; aq = aqueous phase, nb = nitrobenzene phase) was evaluated as log K _ex (Mg²⁺, 1·Sr²⁺) = 0.0 ± 0.1. Further, the stability constant of the 1·Mg²⁺ complex in nitrobenzene saturated with water was calculated for a temperature of 25 °C as log β_nb (1·Mg²⁺) = 9.1 ± 0.2. By using quantum mechanical DFT calculations, the most probable structures of the non-hydrated 1·Mg²⁺ and hydrated 1·Mg²⁺·3H₂O complex species were predicted.     

Extraction and DFT study on the complexation of the cesium cation with a hexaarylbenzene-based receptor

From extraction experiments and γ-activity measurements, the extraction constant corresponding to the equilibrium Cs⁺(aq) + A⁻(aq) + 1(nb) ⇆ 1·Cs⁺(nb) + A⁻ (nb) taking part in the two-phase water–nitrobenzene system (A⁻ = picrate, 1 = hexaarylbenzene-based receptor; aq = aqueous phase, nb = nitrobenzene phase) was evaluated as log K _ex (1·Cs⁺, A⁻) = 2.8 ± 0.1. Further, the stability constant of the hexaarylbenzene-based receptor·Cs⁺ complex (abbrev. 1·Cs⁺) in nitrobenzene saturated with water was calculated for a temperature of 25 °C: log β _nb (1·Cs⁺) = 4.7 ± 0.1. By using quantum mechanical DFT calculations, the most probable structure of the 1·Cs⁺ complex species was solved. In this complex having C ₃ symmetry, the cation Cs⁺ synergistically interacts with the polar ethereal oxygen fence and with the central hydrophobic benzene bottom via cation–π interaction. Finally, the calculated binding energy of the resulting complex 1·Cs⁺ is −220.0 kJ/mol, confirming relatively high stability of the considered cationic complex species.     

Experimental and theoretical study on the complexation of the cesium cation with dibenzo-30-crown-10

From extraction experiments and γ-activity measurements, the extraction constant corresponding to the equilibrium Cs⁺ (aq) + A⁻ (aq) + 1(nb) \rightleftarrows \rightleftarrows 1·Cs⁺(nb) + A⁻(nb) taking place in the two-phase water–nitrobenzene system (A⁻ = picrate, 1 = dibenzo-30-crown-10; aq = aqueous phase, nb = nitrobenzene phase) was evaluated as log K _ex (1·Cs⁺, A⁻) = 4.0 ± 0.1. Further, the stability constant of the 1·Cs⁺ complex in nitrobenzene saturated with water was calculated for a temperature of 25 °C: log β _nb (1·Cs⁺) = 5.9 ± 0.1. Finally, by using quantum–mechanical DFT calculations, the most probable structure of the resulting cationic complex species 1·Cs⁺ was derived.     

Extraction and DFT study on the complexation of the cesium cation with dibenzo-21-crown-7

From extraction experiments and γ-activity measurements, the extraction constant corresponding to the equilibrium \textCs ⁺ ( \textaq ) + \textA ^- ( \textaq ) + 1( \textnb )\underset \rightleftharpoons 1·\textCs ⁺ ( \textnb ) + \textA ^- ( \textnb ) {\text{Cs}}^{ + } \left( {\text{aq}} \right) + {\text{A}}^{ - } \left( {\text{aq}} \right) + {\mathbf{1}}\left( {\text{nb}} \right)\underset {} \rightleftharpoons {\mathbf{1}}\cdot{\text{Cs}}^{ + } \left( {\text{nb}} \right) + {\text{A}}^{ - } \left( {\text{nb}} \right) taking place in the two-phase water-nitrobenzene system (A⁻ = picrate, 1 = dibenzo-21-crown-7; aq = aqueous phase, nb = nitrobenzene phase) was evaluated as log K _ex (1·Cs⁺, A⁻) = 4.4 ± 0.1. Further, the stability constant of the 1·Cs⁺ complex in nitrobenzene saturated with water was calculated for a temperature of 25 °C: log β_nb (1·Cs⁺) = 6.3 ± 0.1. Finally, by using quantum mechanical DFT calculations, the most probable structure of the resulting cationic complex species 1·Cs⁺ was solved.     

Kinetics and mechanism of dehydration of kaolinite,muscovite and talc analyzed thermogravimetrically by the third-law method

Summary The third-law method has been applied to determine the enthalpies, Δ_rH_T⁰, for dehydration reactions of kaolinite, muscovite and talc. The Δ_rH_T⁰values measured in the equimolar (in high vacuum) and isobaric (in the presence of water vapour) modes (980±15, 3710±39 and 2793±34 kJ mol^-1, for kaolinite, muscovite and talc, respectively) practically coincide if to take into account the strong self-cooling effect in vacuum. This fact strongly supports the mechanism of dissociative evaporation of these compounds in accordance with the reactions (primary stages): Al₂O₃·2SiO₂·2H₂O(s)→Al₂O₃(g)↓+2SiO₂(g)↓+2H₂O(g); K₂O·3Al₂O₃·6SiO₂·2H₂O(s) →K₂O(g)↓+3Al₂O₃(g)↓+6SiO₂(g)↓+2H₂O(g) and 3MgO·4SiO₂·H₂O(s) →3MgO(g)↓+4SiO₂(g)↓+H₂O(g). The values of the Eparameter deduced from these data for equimolar and isobaric modes of dehydration are as follows: 196 and 327 kJ mol^-1for kaolinite, 309 and 371 kJ mol^-1for muscovite and 349 and 399 kJ mol^-1for talc. These values are in agreement with quite a few early results reported in the literature in 1960s.     

Additivity Factors in the Binding of the Diethyltin(IV) Cation by Ligands Containing Amino and Carboxylic Groups at Different Ionic Strengths

Complex formation constants were determined potentiometrically (by a ISE-H⁺, glass electrode) in the systems, M²⁺ – L^z – H⁺ [M²⁺ = (C₂H₅)₂Sn²⁺, L^z = malonate, glycinate and ethylenediamine] at t = 25 ^∘C and 0.1 mol-L⁻¹≤ I/ ≤ 1 mol-L⁻¹ in NaCl_aq (0.1 mol-L⁻¹ ≤ I ≤ 0.75 mol-L⁻¹ for the ethylenediamine system). Thermodynamic values of formation constants, at infinite dilution, are [± 95% confidence interval, ^Tβ_pqr refer to the equilibrium, pM²⁺ + qL^z + rH⁺ = M_pL_qH_r^(2+z+r)]: for malonate, log₁₀ ^Tβ₁₁₀ = (5.47 ± 0.10); for glycinate, log₁₀ ^Tβ₁₁₀ = (9.54 ± 0.08), log₁₀ ^Tβ₁₁₁ = (12.97 ± 0.10); and for ethylenediamine, log₁₀ ^Tβ₁₁₀ = (10.47 ± 0.10), log₁₀ ^Tβ₁₂₀ = (16.17 ± 0.12) and log₁₀ ^Tβ₁₁₁ = (15.46 ± 0.10). The dependence on ionic strength of the formation constants was modeled by a simple Debye–Hückel type equation and by the SIT approach. By analyzing the stability of the species in the three different systems we found a simple additivity rule that can be expressed by the relationship: log₁₀ K = 6.46 n_N + 3.96 n_O − 0.60 (n_N²+ n_O²), with a mean deviation, ε(log₁₀ K) = 0.15 (K = equilibrium constant for the interaction of the organometal cation with the unprotonated or protonated ligand, n_N = number of amino groups and n_O = number of carboxylic groups of the ligand(s) involved in the formation reaction of complex species).     

Molecular structure and character of bonding of mono and divalent metal cations (Li+, Na+, K+, Mg2+, Ca2+, Zn2+, and Cu+) with guanosine: AIM and NBO analysis

The B3LYP/6-311++G (d,p) density functional approach was used to study the gas-phase metal affinities of Guanosine (ribonucleoside) for the Li⁺, Na⁺, K⁺, Mg²⁺, Ca²⁺, Zn²⁺, and Cu⁺ cations. In this study we determine coordination geometries, binding strength, absolute metal ion affinities, and free energies for the most stable products. We have also compared the results for Guanosine, with our previously reported results for 2′-Deoxyguanosine. Based on the results, it is obvious that MIA is strongly dependent on the charge-to-size ratio of the cation. Guanosine interacts more strongly with Zn²⁺ than do with Mg²⁺, Ca²⁺, and Cu⁺ and therefore stronger interactions lead to higher MIA. In both free molecules and their complexes, the Syn orientation of the base is stabilized by an intramolecular O5′–H···N3 hydrogen bond and the anti orientation of the base is stabilized by an intramolecular C–H···O hydrogen bond formed between the (C8-H8) and the O5′ atom of the sugar moiety. It is also interesting to mention that linear correlation between calculated MIA values and the atomic numbers (Z) of the metal ions of Li⁺, Na⁺, and K⁺ were found. Furthermore, the influences of metal cationization on the strength of the N-glycosidic bond, torsion angles, angle of pseudorotation (P), and intramolecular C–H···O and O–H···O hydrogen bonds have been studied. Natural bond orbital (NBO) analysis was performed to calculate the charge transfer and natural population analysis of the complexes. Quantum theory of atoms in molecules (QTAIM) was also applied to determine the nature of interactions.     

Mechanistic aspects of the substrate ionization step in aminium salt catalyzed cyclopropanation

The cyclopropanations of a series of m- and p-substituted trans-β-methylstyrenes (3) by ethyl diazoacetate (4), catalyzed by tris(4-bromophenyl)aminium hexachloroantimonate (1 ^·+) and also by tris(2,4-dibromphenyl) aminium hexachloroantimonate (2 ^·+) have been studied by competition kinetics. For the reactions catalyzed by the milder aminium salt (1 ^·+), the Hammett-Brown ρ values and the fact that the absolute rates are independent of the concentration of 4 establish that ionization to 3 ^·+ is not reversible, but rate-determining. The dependence of the magnitude of ρ upon the absolute concentration of 3 indicates the operation of competing chain and catalytic mechanisms, i.e. the ionization of 3 by both product cation radicals and by the catalyst. The extremely low ρ value observed in the reactions catalyzed by 2 ^·+ indicates the exclusive operation of a relatively unselective chain mechanism. These mechanistic assignments are further supported by the observation of the formation of the same products under electrochemical conditions, in the absence of a chemical catalyst, in closely comparable diastereoisomer ratios and with ρ values which correspond nicely with the ρ values observed for equipotential aminium salt catalysts.     

Electrocatalytic interaction of nano-engineered palladium on carbon nanofibers with hydrogen peroxide and β-NADH

The growing demands for reagentless hydrogen peroxide (H₂O₂) and β-nicotinamide adenine dinucleotide (β-NADH) sensors from food, pharmaceutical, chemical, and biochemical fields have stimulated extensive research interest on nano-engineered Pd. In this paper, Pd/carbon composite nanofibers were prepared by electrodepositing Pd onto electrospun carbon nanofibers to act as a catalyst toward the electrocatalytic redox reactions of H₂O₂ and β-NADH. The morphology of nano-engineered Pd was controlled by selectively adjusting the electrodeposition time and potential. Scanning electron microscopy and transmission electron microscopy results showed that nanocactus- and nanoflower-like Pd depositions were obtained on the surface of carbon nanofibers. Electrocatalytic analysis demonstrated a high electrocatalytic activity of the composite nanofibers for the redox of H₂O₂ and oxidation of β-NADH.     

Experimental Evidence for a Valinomycin – Proton Complex

Summary. From extraction experiments and γ-activity measurements, the extraction constant corresponding to the equilibrium H⁺ (aq) + NaL⁺ (nb) = HL⁺ (nb) + Na⁺ (aq) taking place in the two-phase water-nitrobenzene system (L = valinomycin, aq = aqueous phase, nb = nitrobenzene phase) was evaluated as log K_ex (H⁺, NaL⁺) = −1.1 ± 0.1. Further, the stability constant of the valinomycin-proton complex in nitrobenzene saturated with water was calculated as log β_nb(HL⁺) = 5.3 ± 0.1. Finally, the stability constants of complexes of some univalent cations with valinomycin were summarized and discussed.     

Extraction and <Emphasis Type="Italic">ab initio</Emphasis> calculation studies on the complexation of Ca<Superscript>2+</Superscript> with valinomycin

From extraction experiments and γ-activity measurements, the exchange extraction constant corresponding to the equilibrium Ca²⁺ (aq)+1·Sr²⁺ (nb) ⇆ 1·Ca²⁺ (nb) + Sr²⁺ (aq) taking place in the two-phase water-nitrobenzene system (1 = valinomycin; aq = aqueous phase, nb = nitrobenzene phase) was evaluated as log K _ex (Ca²⁺, 1·Sr²⁺) = 2.4±0.1. Further, the stability constant of the valinomycin-calcium complex (abbrev. 1·Ca²⁺) in nitrobenzene saturated with water was calculated for a temperature of 25 °C: log β _nb (1·Ca²⁺) = 8.3±0.1. By using quantum mechanical DFT calculations, the most probable structure of the 1·Ca²⁺·2H₂O complex species was predicted. In this complex, the “central” Ca²⁺ cation is bound by strong bonds to two oxygen atoms of the respective water molecules and to four ester carbonyl oxygens of the parent valinomycin ligand 1. Finally, the calculated binding energy of the considered complex 1·Ca²⁺·2H₂O is −319.2 kcal/mol, which confirms the relatively high stability of this cationic complex species.     

Should a franck-condon or a curve-crossing picture be applied to ion-target collisional activation? A study of keV CO<Stack><Subscript>2</Subscript><Superscript>+·</Superscript></Stack>/He collisions by emission spectroscopy

Collision-induced photon emissions (CIE) were observed for keV CO₂^+·/He collisions from 190 to 1020 nm. The emissions were assigned to the Δν=0 band of the CO₂^+· B ²Σ_u⁺ → X ²Π_g electronic transition and the Δν = +3, +2, +1, 0, −1, −2, −3 vibrational transition progression in the CO₂^+· A ²Π_u → X ²Π_g electronic transition. The other peaks arise from the emissions of excited O^· fragment atoms and the target gas. The relative intensities of the CO₂^+· and O^· emissions are independent of the ion translational energy above 3 keV, supporting the curve-crossing mechanism for collisional excitation. Investigation of the relative intensities within the A ²Π_u → X ²Π_g emission of CO₂^+· indicates that the vibrational distribution is well described by the Franck-Condon principle at high collision energy, a consequence of short collision time but not necessarily an indication of vertical transitions. Below 3 keV ion translational energy, vibrational excitation in the A ²Π_u electronic state was observed. The observation is consistent with the explanation that the reaction occurs at small impact parameters, in which short-range, repulsive interactions between the projectile and the target result in direct translational-vibrational excitation.     

Picoline as ligand with antimony trichloride and triiodide adducts

Solid adducts SbX₃·L-pic (X=Cl, I and L=α-, β- and γ-picolines) were synthesized and characterized by elemental analysis, ¹H and ¹³C NMR, IR spectroscopy and thermal analysis. The infrared spectroscopy and the magnetic resonance for ¹H and ¹³C nuclei of these compounds suggest that the ligands coordinate through nitrogen atom. Kinetic studies were accomplished by means of thermogravimetric data, through isothermal and non-isothermal techniques. The best adjusting models for adducts thermal decomposition were R¹ for isothermal and R₁ and R₂ for the non-isothermal methods. The energy of activation values obtained by isothermal method indicate the following orders of thermal stability for adducts: i) SbCl₃·α-pic>SbCl₃·β-pic>SbCl₃·γ-pic and ii) SbI₃·β-pic>SbI₃·γ-pic>SbI₃·α-pic. The activation energy values obtained by non-isothermal were higher than those from isothermal methods, showing the order of stability:iii) SbCl₃·α-pic<SbCl₃·β-pic<SbCl₃·γ-pic and iv) SbI₃·β-pic>SbI₃·α-pic=SbI_3··γ-pic. These obtained data through R₁ model presented the kinetic compensation effect for trichloride adducts, which could be associated to both isothermal and non-isothermal processes. This revised version was published online in August 2006 with corrections to the Cover Date.     

Polarization effect in radical cations and H-complexes

The effect of substituents X on the ionization potentials IP (process DX + hν ? D^+·X + e) and shifts in vibration frequencies Δν of ν(OH) in the IR spectra of phenol complexes PhO-H + DX ? PhO^δ?-H…D^δ+X for nine series of DX molecules were studied. On compiling with three conditions (a constant donor center D; the electron density donation only from D and not from X; a constant sampling size within each series) it was possible to compare the polarization effect in D^+·X and D^δ+X. In the radical cations D^+·X the polarization effect is on the average 2.2 times larger than in the systems D^δ+X. The systems D^+·X and D^δ+X are virtually indistinguishable with respect to the external delocalization of the positive charge.     

Conductometric and microcalorimetric analysis of the alkaline-earth/alkali-metal ion exchange onto polyacrylic acid

The specificity of the exchange between divalent (Di²⁺ = Ca²⁺ or Ba²⁺) and monovalent (M⁺ = Li⁺, Na⁺ or K⁺) ions onto a polyacrylic chain is examined using conductometric and microcalorimetric techniques. Assuming the formation of a bidentate complex between the Di²⁺ and the carboxylate groups, the conductometric data give the exchange ratio (Di²⁺/M⁺) and the speciation of the acrylic groups. No significant difference is observed between the three alkali-metal ions for a given Di²⁺ ion. Comparisons between Ca²⁺ and Ba²⁺ show a stronger hydrophobicity of the former as it precipitates at a complexation ratio r = 0.33 versus r = 0.45 for the Ba²⁺ salt. Microcalorimetric data show that all Di²⁺/M⁺ exchange energies are positive and depend significantly on the type of cations. The largest displacement energy (the more positive) is found for the binding of Ca²⁺ with sodium polyacrylate (8.13 kJ · mol⁻¹) and the smallest for Ba²⁺ with lithium polyacrylate (1.88 kJ · mol⁻¹). The interpretation of the data leads to the conclusion that specificity of the Di²⁺ binding originates in the dehydration phenomenon and specificity between the three alkali-metal ions is due to the decrease in the electrostatic bond strength with an increase in the ionic radii. The Di²⁺/M⁺ exchange is entropically driven. Received: 28 January 1999 Accepted in revised form: 7 April 1999     

Formation of cation–radical anion pairs derived from carboxybenzophenone–tetrabutylammonium salts. Pulse radiolysis studies

Pulse radiolysis of acetonitrile solutions of tetra-n-butyl ammonium salts of 2- and 4-carboxybenzophenones [BP-COO⁻···N⁺(C₄H₉)₄] were performed in order to generate directly the reduced forms of the benzophenone moieties within pre-formed ion pairs. In earlier studies on photochemical electron transfer reactions, ion pairs containing a tetraalkyl ammonium cation and a benzophenone radical anion were formed in an electron transfer to the triplet BP from a quencher consisting of a tetraalkyl ammonium salt of (phenylthio)acetic acid. In the current work, the [BP^•−COO⁻···N⁺(C₄H₉)₄] ion pairs were formed by direct reduction of the salts without the complication of a third moiety, i.e., the (phenylthio)acetic anion. The spectra and kinetic parameters of the radiolytically-reduced salts were compared to the behavior of reduced forms of the 2- and 4-COOH substituted benzophenones. The results from the pulse radiolysis and photochemistry were compared and explained in terms of the different structures of the ion pairs.     

Potentiometric and Multinuclear Magnetic Resonance Study of the Solution Equilibria Between Aluminium(III) Ion and L-Aspartic Acid

Solution equilibria between aluminium(III) ion and L-aspartic acid were studied by potentiometric, ²⁷Al, ¹³C, and ¹H NMR measurements. Glass electrode equilibrium potentiometric studies were performed on solutions with ligand to metal concentration ratios 1:1, 3:1, and 5:1 with the total metal concentration ranging from 0.5 to 5.0 mmol/dm³ in 0.1 mol/dm³ LiCl ionic medium, at 298 K. The pH of the solutions was varied from ca. 2.0 to 5.0. The non-linear least squares treatment of the data performed with the aid of the Hyperquad program, indicated the formation of the following complexes with the respective stability constants log β_p,q,r given in parenthesis (p, q, r are stoichiometric indices for metal, ligand, and proton, respectively): Al(HAsp)²⁺ (log β_1,1,1 = 11.90 ± 0.02); Al(Asp)⁺ (log β_1,1,0 = 7.90 ± 0.03); Al(OH)Asp⁰ (log β_1,1,−1 = 3.32 ± 0.04); Al(OH)₂Asp⁻ (log β_1,1−2 = −1.74 ± 0.08), and Al₂(OH) Asp³⁺ (log β_2,1,−1 = 6.30 ± 0.04). ²⁷Al NMR spectra of Al³⁺ + aspartic acid solutions (pH 3.85) indicate that sharp symmetric resonance at δ∼10 ppm can be assigned to (1, 1, 0) complex. This resonance increases in intensity and slightly broadens upon further increasing the pH. In Al(Asp)⁺ complex the aspartate is bound tridentately to aluminum. The ¹H and ¹³C NMR spectra of aluminium + aspartic acid solutions at pH 2.5 and 3.0 indicate that β-methylene group undergoes the most pronounced changes upon coordination of aluminum as well as α-carboxylate group in ¹³C NMR spectrum. Thus, in Al(HAsp)²⁺ which is the main complex in this pH interval the aspartic acid acts as a bidentate ligand with –COO⁻ and –NH₂ donors closing a five-membered ring.