Adsorption of the lanthanum and thorium cryptates at mercury electrode

The adsorption behavior of cryptates of La³⁺ and Th⁴⁺ cations at the Hg/aqueous solution interface is studied by using the electrochemical impedance spectroscopy method. It was shown that the studied complexes have high surface activity at the interface. The lanthanum cryptate adsorption parameters are found using the regression analysis based on the two parallel capacitor model supplemented by the Frumkin adsorption isotherm. The differential capacitance curves calculated by using these parameters agree satisfactorily with the experimental ones. The more complicated adsorption behavior of lanthanum cryptate, as compared with the complexes of lower valence cations, is interpreted with taking into consideration the strong interaction of the La³⁺ cation contained in the [2.2.2.] cryptand intramolecular cavity with the supporting electrolyte anions.     

New 2-methylenepropylene-bridged cryptands with high sodium ion selectivity: A thermodynamic study of complexation

Thermodynamic quantities for the interactions of mono- and tri(2-methylenepropylene)-bridged cryptands, cryptand [3.3.1], cryptand [2.2.2], and 18-crown-6-with Na⁺, K⁺, Rb⁺, and Cs⁺ have been determined by calorimetric titration in an 80:20 (v/v) methanol: water solution at 25°C. Incorporation of the 2-methylenepropylene (–CH₂C(=CH₂)CH₂–) bridge(s) into cryptand [2.2.2] results in a large change in the ligand-cation binding properties. Tri(2-methylenepropylene)-bridged cryptand [2.2.2] (2) shows high selectivity factors for Na⁺ over K⁺ and other alkali cations, while 2-methylenepropylene-bridged cryptand [2.2.2.] (1) selects K⁺ over Na⁺, as does cryptand [2.2.2]. The K⁺/Na⁺ selectivity is reversed with increasing number of 2-methylenepropylene bridges. This observation indicates that increasing the number of 2-methylenepropylene bridges on cryptand [2.2.2] favors complexation of a small cation over a large one. The logK values for the formation of 1 and 2 complexes (except 1-Cs⁺ and 2-Na⁺) decrease as compared with those for the corresponding [2.2.2] complexes. Formation of six-membered chelate ring(s) by the propyleneoxy unit(s) of 1 and 2 with a cation stabilizes the cryptate complexes of the small Na⁺ and destabilizes the complexes of large alkali metal cations. Thermodynamic data indicate that the stabilities of the cryptate complexes studied are dominated mostly by the enthalpy change. In most cases, both stabilization of Na⁺ complexes and destabilization of the complexes of large alkali metal cations by six-membered chelate ring(s) also result from an enthalpic effect. Cryptand [3.3.1] shows a selectivity for K⁺ over Cs⁺, despite its two long CH₂(CH₂OCH₂)₃CH₂ bridges. The [3.1] macroring portion of [3.3.1]may be too small to effectively bind the Cs⁺, resulting in the low stability of the Cs⁺ complex.     

Synthesis of large ring proton cryptate tridecalino [2.2.2] cryptand⊃2hI

Condensation of 8,9:17,18-didecalino-4,13-diaza-18-crown-6 with 1,8-diiodo-4,5-decalino-3,6-dioxaoctane in the presence of sodium carbonate produced proton cryptate, tridecalino[2.2.2]cryptand⊃2HI.     

Effect of Water–Ethanol Solvents on the Protonation Constants of Cryptand[2.2.2]

The protonation constants of cryptand[2.2.2] are determined potentiometrically at 298 K in water–ethanol solvents of variable composition. An increase in the concentration of a solution’s nonaqueous component reduces the equilibrium constants of the reactions of mono- and biprotonated cryptand[2.2.2] formation. The contribution from the resolvation of reagents to the change in the Gibbs energy of the studied reactions is estimated. The reduction in the protonation constants of cryptand[2.2.2] in water–ethanol solvents is mainly due to enhancement of the solvation of protons in water–ethanol mixtures.     

Cationic cryptand complexes of tin(II)

A series of cationic cryptand complexes of tin(II), [Cryptand[2.2.2]SnX][SnX(3)] (10, X = Cl; 11, X = Br; 12, X = I) and [Cryptand[2.2.2]Sn][OTf](2) (13), were synthesized by the addition of cryptand[2.2.2] to a solution of either tin(II) chloride, iodide, or trifluoromethanesulfonate. The complexes could also be synthesized by the addition of the appropriate trimethylsilyl halide (or pseudohalide) reagent to a solution of tin(II) chloride and cryptand[2.2.2]. The complexes were characterized using a variety of techniques including NMR, Raman, and temperature-dependent M?ssbauer spectroscopy, mass spectrometry, and X-ray diffraction.     

Synthesis and Characterization of Cationic Low‐Valent Gallium Complexes of Cryptand[2.2.2]

The synthesis and characterization of two bimetallic, cationic low‐valent gallium–cryptand[2.2.2] complexes is reported. The reaction of cryptand[2.2.2] with Ga₂Cl₄ gave two different cations, [Ga₃Cl₄(crypt‐222)]⁺ ( 1 ) or [Ga₂Cl₂(crypt‐222)]²⁺ ( 2 ), depending on whether or not trimethylsilyl triflate (Me₃SiOTf) was added as a co‐reagent. Complexes 1 and 2 are the first examples of bimetallic cryptand[2.2.2] complexes, as well as the first low‐valent gallium–cryptand[2.2.2] complexes. Computational methods were used to evaluate the bonding in the gallium cores.     

Adsorption of compounds with the skeleton molecular structure from dimethylsulfoxide solutions on mercury electrode

The adsorption of adamantane (Ad), adamantanol (AdOH), thiocamphor (TC), and a supramolecular complex (cryptate) of sodium ion [Na⁺ ⊂ 2.2.2.] from DMSO solutions on the mercury electrode is studied by the differential capacitance method. In the considered systems, the surfactants exhibit the high surface activity, which manifests itself in different ways depending on the potential scan direction. For AdOH, TC, and [Na⁺ ⊂ 2.2.2.] that have either a dipole moment or an electrostatic charge, it is assumed that the important role is played by the adsorbate-solvent interaction at the interface, which can be the key factor determining the formation of a new adsorption layer structure in the positive potential range. The adsorption behavior of the mentioned group of surfactants radically differs from that of Ad hydrocarbon, namely, the adsorption of the latter is not accompanied by the formation of a new adsorption layer structure. The obtained results suggest that for the adsorption of surfactants from nonaqueous solvents (in contrast to aqueous solutions), the interaction between the adsorbate and the solvent molecules, which under certain condition results in the formation of two-dimensional supramolecular structures at the electrode/solution interface, acquires substantial importance.     

Synthesis of a “Masked” Terminal Zinc Sulfide and Its Reactivity with Brønsted and Lewis Acids

The “masked” terminal Zn sulfide, [K(2.2.2‐cryptand)][^MeLZn(S)] ( 2 ) (^MeL={(2,6‐ⁱPr₂C₆H₃)NC(Me)}₂CH), was isolated via reaction of [^MeLZnSCPh₃] ( 1 ) with 2.3 equivalents of KC₈ in THF, in the presence of 2.2.2‐cryptand, at ?78 °C. Complex 2 reacts readily with PhCCH and N₂O to form [K(2.2.2‐cryptand)][^MeLZn(SH)(CCPh)] ( 4 ) and [K(2.2.2‐cryptand)][^MeLZn(SNNO)] ( 5 ), respectively, displaying both Brønsted and Lewis basicity. In addition, the electronic structure of 2 was examined computationally and compared with the previously reported Ni congener, [K(2.2.2‐cryptand)][^tBuLNi(S)] (^tBuL={(2,6‐ⁱPr₂C₆H₃)NC(^tBu)}₂CH).     

Tetrasilacyclobutadiene ((t)Bu(2)MeSi)(4)Si(4): a new ligand for transition-metal complexes

The anionic complex of [tetrakis(di-tert-butylmethylsilyl) tetrasilacyclobutadiene]dicarbonylcobalt, [(R4Si4)Co(CO)2]-.K+ (R = SiMetBu2) 2-.K+, was synthesized by the reaction of tetrasilacyclobutadiene dianion dipotassium salt [R4Si4]2-.2K+ 1 with an excess of CpCo(CO)2 in THF. X-ray analysis of 2-.[K+(diglyme)2(THF)] showed an almost planar Si4 ring of rectangular shape with an in-plane arrangement of the silyl substituents. 2- was also prepared as a free anion with the [K+[2.2.2]cryptand] counterion by complexation with [2.2.2]cryptand and as a dimer {2-.[K+(THF)3]}2 without complexing reagents in THF. Such a tetrasilacyclobutadiene fragment represents a new type of ligand for Co complexes, being the first example of a cyclobutadiene containing only heavier group 14 elements.     

A 39K NMR study of potassium solution in tetrahydrofuran containing cryptand[2.2.2]

By means of ³⁹K NMR spectroscopy the presence of potassium anions and complexed potassium cations in blue potassium solutions in THF containing cryptand[2.2.2] was evidenced. Spin-lattice and spin-spin relaxation was studied in the temperature range 178–238 K. The comparison of relaxation behaviour of the investigated system with that of potassium solutions containing 18-crown-6 or tetraglyme instead of cryptand[2.2.2] revealed the major influence of the complexing agent on interactions of K⁺ with its counterion.     

Macrocyclic Polyethers as Enolate Activators in Homogeneous and Heterogeneous Systems

Abstract

The ability of macrocyclic polyethers to activate enolates has been studied in the alkylation of de-oxybenzoin (1) with butyl derivatives nBuY (Y = Br, I, OMes) catalyzed by crown ether PHDB18C6 (7) or cryptand [2.2.2,C10] (8) under phase-transfer catalysis (PTC) and homogeneous (chlorobenzene) conditions. The enolate reactivity is mainly determined by the ligand (cryptand > crown ether) and solvent (increasing with the polarity, in the order: toluene < chlorobenzene < 1,2dichlorobenzene). Regioselectivity of the reaction is also remarkably affected by ligand and alkylating agent.     

Synthesis and alkali metal complexation studies of novel cage-functionalized cryptands

The synthesis of novel cage-functionalized cryptands 1–5 containing adamantane-, 2-oxaadamantane- or noradamantane-moiety [i.e., 1,3-diethyladamantano[2.2.0]cryptand (1), 1,3-diethoxyadamantano[2.2.2]cryptand (2), 1,3-di[(ethyloxy)methyl]adamantano[2.2.2]-cryptand (3), 1,3-di[(ethyloxy)methyl]-2-oxaadamantano[2.2.3]cryptand (4), and 1,2-diethyloxynoradamantano[2.2.2]cryptand (5)] and their alkali metal binding properties are reported. The results obtained by extraction experiments showed that all the cryptands displayed lower extraction capabilities than the parent [2.2.2]cryptand. However, cryptands 1 and 2 showed much higher selectivity toward K⁺ than the reference [2.2.2]cryptand. When the third bridge is enlarged by two additional CH₂-groups as well as by two oxygen atoms, as in cryptands 3 and 4, the complexational abilities for bigger cations (K⁺, Rb⁺ and Cs⁺) are enhanced. Cryptand 5 displayed very good extraction capabilities of all cations, but showed practically no selectivity towards any of the alkali metal cation. The experimental findings are corroborated by calculation studies consisting of force field based conformational search using Monte Carlo method followed by investigation of the stabilities of the complexes of cryptands with Na⁺ and K⁺ metal ions in chloroform by means of quantum chemical calculations at the density functional theory level.     

Extraction of potassium p-nitrophenoxide with macrocyclic crown ethers and cryptands from aqueous medium into diverse organic solvents: a systematic evaluation

A systematic study has been made of the extraction of potassium p-nitrophenoxide from aqueous medium into a number of organic solvents that are immiscible or partly miscible with water, in the presence of several macrocyclic crown ether and cryptand complexing agents. The efficiency of extraction varies extremely widely with the nature of the ligand and the solvent. For some solvent systems, DC-18-C-6 is more efficient than [2.2.2] cryptand as an extradant. The extraction values, however, provide only limited insight into the fundamental reasons behind the observed results. Hence equilibria involved have been considered and the results analysed in terms of the equilibrium constants. The microscopic and macroscopic properties of these systems are discussed.     

Capture of di-protonated [2.2.2]cryptand in the cavity of two p-sulfonated calixarenes as part of 2-D bi-layer lanthanide coordination polymers

Treating p-sulfonatocalix[4]arene with lanthanide ions, Ln3+ (Ln = Ce, Nd, Sm and Eu), in the presence of [2.2.2]cryptand results in a 2-D bi-layer coordination polymer with axially elongated diprotonated cryptand in the cavity of two p-sulfonatocalix[4]arenes.     

Synthesis and crystal structure of (2.2.2-cryptand)(nitrato-O,O’)lead(II) nitrate monohydrate

A new complex, (2.2.2-cryptand)(nitrato-O,O’)lead(II) nitrate monohydrate [Pb(NO₃)(Crypt-222)]⁺ · NO ₃ ^? · H₂O, is synthesized and characterized by X-ray crystallography. The structure of the complex (space group Pbca, a = 14.196 Å, b = 14.001 Å, c = 26.745 Å, Z = 8) is solved by direct methods and refined by the full-matrix least-squares method in the anisotropic approximation to R = 0.073 for 3474 unique reflections (CAD4 automated diffractometer, λMOK _α). The structure contains the host-guest complex cation [Pb(NO₃)(Crypt-222)]⁺. The Pb²⁺ cation is located in the cavity of the 2.2.2-cryptand ligand and is coordinated by all eight heteroatoms (6O + 2N) of the cryptand ligand and two O atoms of the NO ₃ ^? ligand. The coordination polyhedron of the Pb²⁺ cation (ten-coordinate) is a strongly distorted hexagonal bipyramid with a base of four O atoms and two N atoms of the cryptand ligand and with two bifurcated axial vertices at two O atoms of the nitrate ligand and two O atoms of the cryptand ligand. The disordered NO ₃ ^? anion and the water molecule are linked by hydrogen bonds.     

Hydroxymethyl 18-crown-6 and hydroxymethyl [2.2.2]cryptand: versatile derivatives for binding the two polyethers to lipophilic chains and to polymer matrices

The presence of the hydroxymethyl group allows facile functionalisation of 18-crown-6 and of [2.2.2]cryptand. A variety of lipophilic and polymer-supported macrocyclic polyethers is thus available. They are efficient and easily recyclable phase-transfer catalysts.     

Isothermal Titration Calorimetry Study of a Bistable Supramolecular System: Reversible Complexation of Cryptand[2.2.2] with Potassium Ions

Isothermal titration calorimetry (ITC) is used to investigate the thermodynamics of the complexation of potassium ions by 1,10‐diaza‐4,7,13,16,21,24‐hexaoxabicyclo[8.8.8]hexacosane (cryptand[2.2.2]) in aqueous solution. By changing the pH of the solution it was possible to trigger the reversible complexation/decomplexation of the cryptand in consecutive in situ experiments and to assess for the first time the use of ITC to monitor the thermodynamics of a bistable system.     

Adsorption phenomena on a mercury electrode in aqueous cryptand[2.2.2] solutions by the background of tetraalkylammonium salts

The electrochemical behavior of cryptand[2.2.2] (Cry) is studied on a mercury electrode in aqueous solutions of tetraalkylammonium tetrafluoroborates (Me₄N⁺, Et₄N⁺, and Bu₄N⁺). Cryptand [2.2.2] is shown to exhibit high surface activity in Me₄ NBF₄ nd Et₄NBF₄ solutions. Based on the model of two parallel capacitors supplemented by the Frumkin adsorption isotherm, the adsorption parameters of Cry by the background of Me₄BNF₄ were calculated using the regression analysis methods. The calculated dependences of the differential capacitance on the potential adequately agree with experimental curves. The adsorption characteristics of Cry in the studied solutions are compared with those in MgSO₄ solutions. By the background of Bu₄NBF₄, Cry molecules and Bu₄N⁺ cations exhibit very close surface activity and form a mixed adsorption layer.     

Macrocyclic Polyethers as Enolate Activators in Homogeneous and Heterogeneous Systems

Abstract

The ability of macrocyclic polyethers to activate enolates has been studied in the alkylation of deoxybenzoin (1) with butyl derivatives nBuY (Y = Br, I, OMes) catalyzed by crown ether PHDB18C6 (7) or cryptand [2.2.2, C₁₀] (8) under phase-transfer catalysis (PTC) and homogeneous (chlorobenzene) conditions. The enolate reactivity is mainly determined by the ligand (cryptand>crown ether) and solvent (increasing with the polarity, in the order: toluene<chloroben-zene<1,2dichlorobenzene). Regioselectivity of the reaction is also remarkably affected by ligand and alkylating agent.     

An unusual structure of the hydrated sodium chloride complex of cryptand [2.2.2]

The solid state structure of the Na[2.2.2]C1 · 3H₂O complex has the sodium ion displaced towards one of the cryptand nitrogens and the chloride and water molecules associated by hydrogen bonds to form a pseudo cube with two chloride ions at opposite corners of the cube and water oxygens at the other six corners.