Ethanol Ligands Research Articles

Coordination geometries of bis(4-amino-3-alkyl-1,2,4-triazole-5-thione) complexes of first row transition metals: crystal structures of the cobalt and nickel complexes of 4-amino-3-trifluoromethyl-1,2,4-triazole-5-thione

Bis bidentate complexes of 4-amino-3-trifluoromethyl-1,2,4-triazole-5-thione, SN 4C 3H 3F 3 (afmt, 1) of the divalent ions Co and Ni, have been crystallized by direct combination of the ligand and metal nitrate hydrate salt in ethanol. The structures of the free ligand 1 as well as the complexes were determined by single-crystal X-ray diffraction techniques. The triazole bonds to the metal ion through the amine and thione substituents on the five-membered ring. Two water molecules complete the octahedral coordination sphere. [Co(afmt) 2(H 2O) 2](NO 3) 2 ( 2) and [Ni(afmt) 2(H 2O) 2](NO 3) 2 ( 3) are essentially isostructural with a trans arrangement of all ligands. These structures are compared with those of four recently reported series of 4-amino-3-alkyl-1,2,4-triazole-5-thione complexes (alkyl=H, CH 3, C 2H 5, and C 3H 7) and it is found that the behavior of the CF 3-substituted ligand is most like that of the ethyl analog. Attempts to make complexes with other metals yielded crystals of the acid salt of the thiocarbohydrazide starting material. The crystal structure of thiocarbohydrazidium nitrate, [SN 4CH 7]NO 3 ( 4) has also been determined. The structure consists of layers of cations and anions in which the ammonium protons are hydrogen-bonded to the nitrate ions.

Ruthenium and Rhenium Complexes of Fluorene-Based Bipyridine Ligands: Synthesis, Spectra, and Electrochemistry

Bipyridine ligands featuring fluorene and ferrocene in the conjugation chain were synthesized by Pd(II)/Cu(I)-catalyzed Sonogashira coupling reactions, and their complexation behavior was investigated. The octahedral complexes formed by refluxing Ru(bpy)2Cl2 and Re(CO)5Cl with the ligands in ethanol or benzene were characterized by NMR, electronic spectra, fluorescence spectra, mass spectra, elemental analyses, and electrochemical studies. The bipyridine ligands and the complexes except the ferrocene-containing compounds are luminescent at room temperature. The luminescence in the ruthenium complexes is possibly from a MLCT state, and the fluorescence due to the ligand is completely quenched. When compared to the Ru(bpy)32+, the ruthenium complexes studied in this work exhibit longer lifetimes, which probably stems from the delocalization of charge into the extended fluorene conjugation. The crystal structure of a ferrocenyl fluorene derivative is also reported.

Structure and host–guest properties of the nanoporous diaquatetrakis( p-nitrobenzoato)dicopper(II) framework

[Cu2(p-O2NC6H4CO2)4(H2O)2]·Me2CO has been prepared and characterized by single crystal X-ray diffraction. The Cu–Cu distance is 2.608(1) Å with the four p-nitrobenzoate ligands coordinated in bidentately bridging fashion, such that the copper atoms have a square pyramidal coordination. It possesses a framework structure with continuous channels parallel to the c axis with an opening size of ≈5.6 × 7.9 Å, giving a solvent accessible volume of 22% of the crystal volume. The framework remains intact and crystalline after removal of the included acetone molecule, as determined by X-ray powder diffraction. It is shown, by thermogravimetric measurements, that also benzene and water guest molecules can reversibly be incorporated and released. For comparison, [Cu2(p-O2NC6H4CO2)4(EtOH)2] was prepared and characterized by single crystal X-ray diffraction. Here, the channels are occupied by the axial ethanol ligands.

Anilino Radical Complexes of Cobalt(III) and Manganese(IV) and Comparison with Their Phenoxyl Analogues

The pendent arm macrocyclic aniline ligands 1-(2-amino-3,5-di-tert-butylbenzyl)-4,7-dimethyl-1,4,7-triazacyclononane, H[L1], and 1,4,7-tris(2-amino-3,5-di-tert-butylbenzyl)-1,4,7-triazacyclononane, H3[L2], have been synthesized. The reaction of these ligands in methanol (1:1) and/or ethanol with CoII(ClO4)2·6H2O and potassium di-tert-butylacetylacetonate (1:1) or MnII(acetate)2·4H2O in the presence of air produced the anilido complexes [CoIII(L1)(Bu2acac)](ClO4) (1) and [MnIV(L2)]X (X = ClO4 (2), BPh4 (2‘)), respectively. The reaction of H3[L2] and CuCl2·2H2O in ethanol produced upon addition of NaClO4 blue crystals of [CuII(H3L2)Cl](ClO4) (3) containing two coordinated and one uncoordinated aniline pendent arm. Complexes 1, 2, and 3 have been structurally characterized by X-ray crystallography. Electrochemically, 1 is reversibly oxidized with formation of the paramagnetic anilino radical species [CoIII(L1•)(Bu2acac)]2+ (S = 1/2) whereas the cyclic voltammogram of 2 displays three ligand-centered one-elec...

Diorganotin(IV) complexes of imidazole-2-carbaldehyde thiosemicarbazone (H2ImTSC). The crystal and molecular structures of the “free” ligand and of [SnMe2(ImTSC)

The compounds [SnR2(ImTSC)] (R = Me, nBu or Ph; H2ImTSC = imidazole-2-carbaldehyde thiosemicarbazone) were synthesized by treating the appropriate diorganotin(IV) oxide with the ligand in ethanol. The structures of H2ImTSC·0.5H2O and [SnMe2(ImTSC)]·EtOH were determined by X-ray diffractometry. The thiosemicarbazone chain of the former has E,E,E configuration and is almost coplanar with the imidazole ring. A rich network of hydrogen bonds involving all potential donor and acceptor groups of H2ImTSC and the water molecule stabilizes the crystal lattice. The crystal of the dimethyltin(IV) complex is composed of discrete molecules that are likewise linked by hydrogen bonds. The Sn is bound to the two methyl groups and to an N,N,S-tridentate thiosemicarbazonato dianion (ImTSC2−) in a highly distorted trigonal bipyramidal arrangement with the thioamide sulfur and the imidazole nitrogen apical. The Sn–N bond lengths [2.129(5) and 2.174(5) A] are close to the sum of the covalent radii and dominate the metal–ligand interaction. The Sn–S bond length [2.659(2) A] suggests a weak bond. The unusual co-ordination mode of the ImTSC2− anion creates four- and six-membered stannole rings. The EtOH is bound to the complex via hydrogen bonds involving the nitrogen of the imidazole that is not co-ordinated to the tin and the thiosemicarbazone NH2 group. The IR spectra of the nBu and Ph derivatives suggest that they are structurally similar to the methyl complex. 1H, 13C and 119Sn NMR spectroscopy indicates that the structure of the complexes in the solid state persists in CDCl3 solution. For H2ImTSC, 15N NMR data are presented in addition to IR and 1H and 13C NMR data.

Effect of ligands on crystal structures and optical properties of TiO 2 prepared by sol–gel processes

The refractive index and crystal structures of TiO 2 films formed on soda lime glass substrates by sol–gel processes with and without organic ligands were investigated to establish the effect of the ligands on the optical property and crystal structure. The TiO 2 prepared with 2-(2-methoxyethoxy)ethanol (MEE) ligand crystallized to anatase at firing temperatures between 350 and 400°C. The film with 2-metyhl-2,4-pentanediol (MPD) ligand, however, crystallized between 400 and 500°C. The refractive indices of the films with MEE were the highest among the three types of films at all the firing temperatures, but those of the films with MPD were the lowest. The MEE ligand aggregates grains of titania gels and accelerates the crystallization of the film, resulting in the densest film and the highest refractive index. The MPD ligand, however, does not aggregates the titania grains and is still present up to high firing temperature of 500°C, resulting in the lowest refractive index. In this sol–gel process, the MEE acts as a kind of a binder in ceramics extrusion processes.

Some complexes derived from zinc salicylate or 3,5-di-tert-butylsalicylate. The crystal structure of (2,2′-bipyridyl)(methanol)( O-salicylato)( O,O′-salicylato)zinc

The aqua ligands of diaquabis(salicylato)zinc and diaquabis(3,5-di-tert-butylsalicylato)zinc are easily displaced generating related bis-pyridine and 2,2′-bipyridyl complexes. Attempts to recrystallise the tert-butylsalicylate from ethanol resulted in replacement of the aqua ligands by ethanol ligands. The attempted recrystallisation of (2,2′-bipyridyl)bis(salicylato)zinc from methanol led to the isolation of octahedral [Zn(O 2CC 6H 4OH-2) 2(bipy)(MeOH)] in which one salicylato ligand is coordinated in a monodentate fashion and the other in a bidentate chelating manner, both coordination modes involving carboxylate rather than hydroxyl oxygen coordination. The methanol is strongly bonded trans to a nitrogen of the chelating 2,2′-bipyridyl and three further intramolecular six-membered hydrogen bonded rings are present, generated from interactions between the carboxyl and hydroxyl groups of the salicylate ligands and the hydroxyl group of the methanol ligand.

Diversity of chain, interwoven and network structures formed by metal complexes with 2,3,5,6-tetrafluoro-1,4-bis(4-pyridyl-sulfenyl)benzene †

The preparations are reported of a range of complexes formed by 2,3,5,6-tetrafluoro-1,4-bis(4-pyridylsulfenyl)benzene (L) with MnII, CoII, NiII, ZnII and CdII, and single crystal X-ray diffraction studies have been made on four of the compounds. In the complex CoLI2·0.5CH2Cl2, the organic ligand bridges tetrahedrally coordinated CoII centres so as to form chains. The compound Cd2L3(NO3)4 has a more complex structure in which seven-coordinate Cd centres are linked by two different types of L bridges so as to form arrays of 90-membered macrocycles comprising sheets of linked alternating enantiomeric helices. In the complexes MnL2(EtOH)2X2 (X = Br or NCS), the bis(pyridyl) ligand utilises only one of its pyridyl units to coordinate directly to the metal atom. In both cases the Mn coordination environment comprises an all-trans octahedral arrangement of two anions, two ethanol ligands and two pyridyl units derived from two different L ligands. However in both compounds the non-coordinated pyridyl units bridge adjacent Mn centres by forming hydrogen bonds to the ethanol ligands. The resulting, complex networks are, however, dramatically different. The isothiocyanate complex forms sheets of partially interpenetrating helices whereas the bromide forms a system of triply interpenetrating (4,4) nets of ‘over 2-under 2’ type. X- and Q-Band EPR spectra of the manganese complexes are reported as are relevant spectroscopic results for the other complexes.

Crystal structures and magnetic properties of the interpenetrating rutile-related compounds M(tcm)2 [M = octahedral, divalent metal; tcm− = tricyanomethanide, C(CN)3−] and the sheet structures of [M(tcm)2(EtOH)2] (M = Co or Ni)

The isomorphous structures of M(tcm)2 [MII = Cr, Mn, Fe, Co, Ni, Cu, Zn, Cd or Hg; tcm– = tricyanomethanide, C(CN)3–] contain two interpenetrating rutile-related networks generated by octahedral six-connecting metal ions and trigonal three-connecting tcm– anions. The detailed variable temperature and variable field magnetic properties of this series of high-spin complexes generally point to the existence of very weak intraframework coupling with no evidence for long range magnetic order or interframework effects. The compound Cr(tcm)2 is the most strongly coupled and displays a field independent maximum in susceptibility at 14.5 K and a J value of –1.6 cm–1 (using a –2JS1·S2 Heisenberg chain model); Co(tcm)2 displays high temperature magnetic moments typical of essentially uncoupled octahedral centres but with a most unusual field dependence in µCo observed below 10 K, probably due to very weak ferromagnetic coupling, and an Msat value in high fields, at 2 K, which is significantly less than that expected for S = 3/2. Doping of M(tcm)2 with another M′ member of the series leads to a crystal structure isomorphous with M(tcm)2 in which the dopant metal M′ occupies the M site in a random fashion. The resultant magnetism is simply intermediate between those of the parent phases. The isomorphous structures of [M(tcm)2(EtOH)2] (MII = Co or Ni) contain pseudo square-grid sheets in which the tcm– ligands are each co-ordinated to two metal ions and act as a kinked bridge. Each metal is co-ordinated to four tcm– anions in an equatorial arrangement and to two axial ethanol ligands. Extensive intrasheet hydrogen bonding exists between the ethanol molecules and the unco-ordinated nitrile of the tcm– bridges.

A Cobalt(III) Chiroporphyrin and Its Amine Adducts. A Potential Chiral NMR Shift Reagent for Amines.

The chlorocobalt(III) complex of alphabetaalphabeta-tetramethylchiroporphyrin, CoCl(TMCP), has been prepared as a potential enantioselective host or chiral NMR shift reagent for optically active amines. The X-ray structure of CoCl(OHCH(2)CH(3))(TMCP) shows the six-coordinate cobalt(III) ion at the center of a strongly ruffled porphyrin. The 2-fold-disordered ethanol ligand interacts with the chiroporphyrin host by two C-H.O hydrogen bonds to the carbonyl groups of two ester substituents. Primary amines bind to this diamagnetic cobalt(III) center to form cationic 2:1 complexes in which the (1)H NMR resonances of the axial ligands are shifted upfield of tetramethylsilane by the porphyrin ring current. Coordinated enantiopure 2-alkylamines exhibit NMR signals for the protons of the amine group which are characteristic of their (R or S) absolute configuration. The bis-complexes of the same amines in racemic form exist as three different species, (R,R), (S,S), and (R,S), in 1:1:2 relative ratios. Negligible enantioselection by the chiral host suggests kinetic control of bis(amine) complex formation on cobalt(III). The X-ray structure of the bis((S)-2-butylamine) complex [Co((S)-NH(2)CH(CH(3))CH(2)CH(3))(2)(TMCP)][CoCl(4)](0.5) shows a 2-fold-disordered amine on one face of the porphyrin only. The unique amine on the other face is held within the porphyrin groove by a network of weak interactions including N-H.O and C-H.O hydrogen bonds. With its ability to induce good resolution of axial ligand (1)H NMR resonances and slow dissociation kinetics of its bis-adducts, CoCl(TMCP) may be useful as a chiral NMR shift reagent for conformational studies of chiral amines and as an analytical reagent for the determination of their enantiomer composition.

Synthesis and characterization of bis(1,1,1,5,5,5,-hexafluoropentane-2,4-ionato) strontium and barium adducts with O- and N- ancillary ligands; crystal structure of Sr(HFAC) 2 · (BPY) 2

Complexes of M(hfac) 2·Ln [M = Sr, n = 1, L = tetraglyme, db24c8; n = 2, L = bpy, phen; M = Ba, n = 1, L = tetraglyme, db24c8] have been prepared by in situ reactions of MX 2 (X = OH, OAc) with Hhfac in the presence of the ancillary ligands in aqueous ethanol in high yields and characterized by 1H NMR, IR, mass spectrometry and elemental analysis. A molecular structure of the complex Sr(hfac) 2 · (bpy) 2 has been determined by X-ray single crystal analysis.

Technetium and rhenium complexes of mercaptoacetyl glycine ligands 2. Formation and molecular structure of Re(V) complexes with mercaptoacetyl glycine and mercaptoacetyl glycine ethylester

Reaction of [ReOCl 4] − with mercaptoacetyl glycine (MAG 1) in a 1:1 ratio gives [ReO(MAG 1)Cl] −. Using an excess of the ligand in ethanol, a neutral oxorhenium(V) complex [ReO(MAGEt 1) 2] is formed where the metal is coordinated by S and N of one ligand, and by S and O (amide group) of the other ligand molecule. The carboxylic groups of both mercaptoacetyl glycine ligands have been esterified with the solvent ethanol. Both complexes were characterized by X-ray crystallography.

Synthesis, Characterization, and Magnetic Properties of Fe3O4Thin Films Prepared via a Sol–Gel Method

Different thickness Fe3O4thin films, deposited on quartz substrates, have been directly synthesized via a sol-gel method. Homogeneous sols for preparing Fe3O4thin films were obtained by interacting iron salts and polydentate ligands in ethanol. X-ray diffraction (XRD) and thermogravimetric analysis (TGA) identified the Fe3O4phase and showed that there was no organic residue left in the films. A SQUID magnetometer was used to measure the coercive force of Fe3O4thin films. The obtained coercive force of 30 Oe, measured from the trilayer thin film (275-nm thickness), showed the particles in the films were nanophase Fe3O4. A dramatic increase of coercivity of Fe3O4thin films from 30 to 625 Oe and a demagnetization behavior, as the film thickness decreased from 275 to 120 nm, were observed on these films.

Synthesis and crystal structures of oxo pyridinemethanolate technetium(V) complexes

Oxo Tc(V) complexes with 2-(hydroxymethyl)pyridine and 2,6-di(hydroxymethyl)pyridine were synthesized from the reaction of n-Bu 4[Tc(O)X 4] (X = Cl or Br) with the ligand in methanol or ethanol. The three compounds Tc(O)(OCH 2PyCH 2OH) 2Cl ( 1). Tc(O) (OCH 2PyCH 2OH) 2Br ( 2) and Tc(O) (OCH 2Py) 2Cl ( 3) (Py = pyridine) were studied by crystallographic methods. Crystal 1 is triclinic, P 1, a = 7.479(2), b = 8.043(2), c = 14.940(4) A ̊ , α = 93.66(2), β = 102.16(2), γ = 117.18(2)°, Z = 2 and R = 0.050 . The bromo analogue 2 is also triclinic, P 1, a = 7.416(4), b = 8.115(4), c = 15.151(10) A ̊ , α = 94.36(5), β = 102.09(5), γ = 116.77(4)°, Z = 2 and R = 0.060 . Compound 3 crystallizes in the monoclinic P2 1/ c space group with a = 12.861(4), b = 7.111(2), c = 14.623(6) A ̊ , β = 92.86(3)°, Z = 4 and R = 0.043 . The geometry around the Tc atom is a very distorted octahedron with the oxo and halide ligands in cis position to each other. Crystal 3 is disordered on two positions. The two organic ligands are deprotonated and the disubstituted molecule 2,6-di(hydroxymethyl)pyridine in 1 and 2 is bidentate with one free -CH 2OH group.

Cadmium(II) Chloride Adduct with an Olefinic Double Betaine: catena-Poly[tetraaquadiethanoltris[dichlorocadmium(II)] cis-bis(4-dimethylamino-1-pyridinio)butenedioate hydrate

The structure of a cadmium(II) chloride adduct with an olefinic double betaine, {systematic name: catena-poly[[(diaquacadmium)-di-μ-chloro-(diethanolcadmium)-di-μ-chloro-(diaquacadmium)-di-μ-chloro] cis-bis(4-di-methylamino-1-pyridinio)butenedioate monohydrate]}, [(CdCl2)3(C2H6O)2(H2O)4]n.n(C18H20N4O4).nH2O, is based on an infinite zigzag chain consisting of a series of corner-sharing (CdCl2) quadrilaterals with aqua or ethanol ligands around each CdII atom in a CdCl4O2 octahedral environment. The olefinic double betaine has no association with the metal atoms, being bound to the aqua and ethanol ligands in the chain and the lattice water molecule by hydrogen bonding.

The formation of nido [7,8-(PR 2) 2-7,8-C 2B 9H 10] − from closo 1,2−(PR 2) 2−1,2−C 2B 10H 10 ( [formula omitted]): a process enhanced by complexation

To gain insight into the mutual influence of an electron-rich element directly connected to the carborane cage, the coordination behavior of phosphorus atoms directly bonded to the cage has been studied. The reactivity of bisphosphines [7,8−(PR 2) 2−7,8−C 2B 9H 10] − ( R = Ph, Et, iPr or OEt ) has been investigated. These new chelating ligands exhibit some similarities to the much used 1,2-diphenylphosphinoethane (diphos), such as the spacing between the two phosphorus atoms, and their five-membered ring chelating ability, but there are notable differences, and dissimilar ligating properties are expected. The differences are mainly due to the incorporated carborane cluster and can be summarized as follows: (1) the ligand is anionic, (2) the phosphorus atoms lie in an eclipsed conformation and (3) B(3)−H coordination to the metal is possible. To study the partial degradation of the closo clusters to the nido clusters enhanced by complexation, several metal complexes were treated with the closo ligands in ethanol. The crystal structures of [AuCl 2 {7,8−(P( iPr 2) 2−7,8−C 2B 9H 10}] and [RuCl 2{7,8−(P( iPr 2) 2−7,8−C 2B 9H 10} 2] have been determined.

Synthesis and properties of ruthenium and palladium iodide complexes of phosphorus, arsenic and antimony ligands

Iodoruthenium (II) complexes trans-[Ru(L-L)2I2] (L-L=Ph2PCH2CH2PPh2, cis-Ph2PCHCHPPh2, Ph2AsCH2CH2AsPh2, o-C6H4-(AsMe2)2), trans-[Ru(SbPh3)4I2] and [Ru(PPh3)3I2] have been made by reaction of [Ru(H2O)6] [BF4]2 with the ligand and NaI in aqueous ethanol. Oxidation with conc. HNO3-HBF4 yielded trans-[Ru(L-L)2I2]BF4. The complexes were characterised by spectroscopy (IR, UV-Vis, 31P NMR) and cyclic voltammetry, and their properties compared with chloro- and bromo-analogues. Unsuccessful attempts to prepare palladium (IV) iodo complexes [Pd(L-L)2I2][BF4]2 are described. The crystal structures of [Ru(Ph2AsCH2CH2AsPh2)2I2]BF4 Pd∗o-C6H4(PMe2)2∗2Cl2][BF4]2 are reported.

Decreased Binding of Asialoglycoproteins to Hepatocytes from Ethanol-fed Rats: CONSEQUENCE OF BOTH IMPAIRED SYNTHESIS AND INACTIVATION OF THE ASIALOGLYCOPROTEIN RECEPTOR

Chronic ethanol administration alters the process of receptor-mediated endocytosis in isolated rat hepatocytes. Using the asialoglycoprotein receptor (ASGP-R) as a model, we have previously shown decreased binding of asialoglycoproteins to this receptor after as early as 1 week of ethanol administration. In the present study, we further analyzed the mechanism(s) responsible for this impairment by determining the ligand and antibody binding characteristics of the ASGP-R in rats fed ethanol over a 5-week time course. The results presented here demonstrate that ethanol treatment for 4 days significantly impaired total ligand binding without affecting antibody binding. Ethanol administration for a longer period of 1-2 weeks resulted in intermediate impairments in both ligand and antibody binding. After 5 weeks of ethanol exposure, ligand and antibody binding were equally lowered. In contrast to total cellular receptor binding, surface binding of both ligand and antibody were decreased over the entire time course of ethanol administration. Our data indicate that the ASGP-R is initially inactivated during the time course of ethanol exposure and that a redistribution of surface receptors to intracellular compartments occurs. Northern blot analysis showed that there was a significant decrease in receptor mRNA content in the 5-week chronically fed animals but not in the animals fed for 1 week. In addition, after 5 weeks of ethanol feeding, biosynthetic labeling of the ASGP-R was decreased in the ethanol cells, indicating impaired synthesis of the ASGP-R. In summary, an early inactivation of the ASGP-R occurs during ethanol exposure followed by an actual decrease in protein and mRNA content for the receptor.

Preparation and Properties of TiO<sub>2</sub> Films by Complexing Agent-Assisted Sol-Gel Method

The TiO2 films were prepared on glass or silicon wafer substrates by sol-gel method using glycols or their ether derivatives as ligands, directed toward control of optical properties. The effect of ligands on the indices and crystal phases of TiO2 films was investigated. On the glass substrates, the refractive indices of TiO2 films depended on the ligands used, although all the films were fired under the same conditions. The 2-(2-methoxyethoxy) ethanol (MEE) ligand gave the most high refractive indices among the ligands used. On the other hand, the 2-methyl-2, 4-pentanediol (MPO) ligand gave the most low refractive indices when the films were fired under 510°C. The change in refractive indices is discussed in connection with crystal phases of TiO2 films measured by X-ray diffraction analysis. In the TiO2 film prepared using MEE as the ligand, the anatase crystal appeared at low temperature (400°C). It gave high refractive indices. In the TiO2 film prepared using MPO as the ligand, amorphous TiO2 appeared under 510°C and gave low refractive indices. The ligands influence size of titanium oxide sols and TiO2 crystal phases, and change the refractive indices of the films. In other words, the ligands can control the refractive indices of TiO2 films on glass substrates. The crystal phases of TiO2 films on silicon wafer also depended on the ligand used. The MPO and triethylene glycol monomethyl ether (TME) ligand accelated the transformation from anatase to rutile form. In contrast MEE ligand inhibited the change to rutile form even at 900°C. Thus, anatase or rutile in the TiO2 film on the silicon wafer can be selectively formed by the selection of ligands used in sol-gel process.

Solution thermodynamics of amino acid–18-crown-6 and amino acid–cryptand 222 complexes in methanol and ethanol. Linear enthalpy–entropy compensation effect

Stability constants, free energies, enthalpies and entropies of complexation of amino acids with synthetic macrocyclic ligands (18-crown-6 and cryptand 222) in methanol and ethanol are reported. No significant variations are found in the free energies of complexation of the various amino acids with these two ligands as a result of a remarkable enthalpy–entropy compensation effect. Free energies of transfer of amino acids and recently isolated amino acid–18-crown-6 and amino acid–cryptand 222 complexes have been mostly obtained from solution thermodynamic data of the free and complexed amino acids in the appropriate solvents. Analysis of transfer-free-energy data indicates that in the amino acid–macrocyclic complex, most of the amino acid molecule is fully exposed to the reaction medium. Previous suggestions on the linear enthalpy–entropy relationship for complexation reactions involving crown ethers and cryptands with metal and organic cations in different reaction media are discussed. The implications of the results obtained for amino acid–macrocyclic complexation reactions to processes of biological importance are considered.