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The effect on chromatographic behavior, on thin layers of silica gel, of the electronegativity of ligator atoms has been studied for three series of β-diketonato complexes of the type [M(acac) 3− n (phacphac) n ], [M(acac) 3− n (phacphSac) n ], and [M(acac) 3− n (phSacphSac) n ] (where M represents cobalt(III), chromium(III) or ruthenium(III), acac is the 2,4-pentanedionato ion, phacphac is the 1,3-diphenyl-1,3-propanedionato ion, phacphSac is 3-mercapto-1,3-diphenyl-prop-2-en-1-one, phSacphSac is the 3-mercapto-1,3-diphenyl-prop-2-en-1-tion ion, and n = 0–3). For chromatographic separations, ten typical normal-phase mobile phases were used, five mono-component and five two-component. For all the complexes examined it was found that substitution of the donor oxygen atom by a less electronegative sulfur atom resulted in increased mobility of the complex. It was also established that substitution of the acac ligand resulted in increased hR F values. For all the complexes examined a linear dependence was observed between the R M values of the complexes and the number of acac ligands substituted by phSacphSac ligands. On the basis of these results, possible separation mechanisms are discussed.

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The effect of substituents of the β -ketoiminato ligand of 24 copper(II) and nickel(II) complexes on hR F values obtained by TLC on polyacrylonitrile (PAN) has been studied. Chelate ligands were prepared by condensation of diamine (dm) and the corresponding β -diketone. Copper(II) and nickel(II) complexes with chelate ligands containing ethylenediamine (en = 1,2-diaminoethane) or propylene diamine (pn = 1,2-diaminopropane) as the amine part and acetylacetone (acac = 2,4-pentanedionato ion) and/or phenylacetylacetone (phacac = 1-phenyl-1,3-pentanedionato ion), and/or trifluoroacetylacetone (tfacac = 1,1,1-trifluoro-1,3-pentanedionato ion), i.e. phenylacetylacetone as β -diketone, were synthesized. Ten non-aqueous mobile phases (six mono-component and four two-component) and two aqueous mobile phases were used for chromatographic separation. The results revealed that the mobility of the complexes decreased on substitution of the CH 3 group in the β -ketoiminato ligand by CF 3 or C 6 H 5 . The chromatographic behavior of the complexes on PAN was compared with that on other adsorbents. Possible separation mechanisms are discussed.

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The lipophilic character of some biologically active 1-arylpiperazines has been studied. A reversed-phase planar chromatographic procedure was established for determination of log P O-W values ( n -octanol-water partition coefficients) for the compounds. The compounds were chromatographed with five so-called ‘standards’ on RP-18 silica plates with methanol-water and dioxane-water binary systems as mobile phases. n -Octanol-water partition coefficients for five standard compounds determined experimentally by use of the shake-flask method were used to obtain the log P O-W values of the 1-arylpiperazines investigated. Experimentally established log P O-W values were compared with log P values calculated by use of five different software packages.

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The effect of the central ion of tris- β -diketonato complexes of transition metals on their hR F values in thin layer chromatography (TLC) on silica gel has been investigated for 24 Co(III), Cr(III), and Ru(III) complexes of the types [M(acac) 3− n (phacphac) n ] and [M(acac) 3− n (phacphSac) n ], where n = 0–3, and with four complexes of the type [M(acac) 3 ], where M = Sc(III), Y(III), Nd(III), or La(III), acac is the 2,4-pentanedionato ion, phacphac is the 1,3-diphenyl-1,3-propanedionato ion, and phacphSac is the 3-mercapto-1,3-diphenylprop-2-en-1-one ion. One- and two-component non-aqueous mobile phases were used for the chromatographic separations. The dependence of retention data on the length of ionic radius and on En/r i value is discussed. The mobility ( hR F values) of the tris(acetylacetonato) complexes on thin layers of silica gel is directly proportional to the ionic radius of the central ion. On the basis of the results obtained, adsorption was assumed to be the dominant mechanism of separation under the chromatographic conditions used.

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The chromatographic behavior of four mixed tris ( β -diketonato) complexes of the type [Co(acac) 3− n (phacphac) n ] ( n = 0–3), three ( β -ketoiminato)-Cu(II) complexes, two tris (alkylxanthato)-Co(III) complexes, and five bis (alkylxanthato)-Ni(II) complexes has been studied on thin layers of CN-silica gel with fifteen mobile phases, thirteen of which resulted in normal-phase systems. Linear relationships were established between the R M values of tris ( β -diketonato) complexes and the amount [% v/v ] of acetone in hexane-acetone mobile phases, and between the R M values of these complexes and the number of acac ligands substituted by phacphac ligands. The results obtained were compared with previously observed chromatographic behavior of the same substances on thin layers of silica gel and polyacrylonitrile. The corresponding separation mechanisms were also considered. It was assumed that under conditions of normal-phase chromatography with solvents of low polarity the retention behavior of complexes containing aromatic rings in the molecule is mainly determined by donor-acceptor interactions of the cyano groups of the adsorbent with the π -electron systems of the substances being separated.

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Seven pairs of cis-trans isomers of bis-steroidal tetraoxanes have been examined by both normal-phase (NP) and reversed-phase (RP) planar chromatography. Unmodified silica gel was used with ethyl acetate-toluene and ethyl acetate-petroleum ether as mobile phases in typical normal-phase systems. CN-silica with the mobile phases methanol-water and acetone-water and RP-18 silica with water-organic modifier (methanol, acetone, or dioxane) mobile phases were used as reversed-phase systems. For the RP systems a good linear correlation was established between R M values and amount ([%] ν/ν ) of organic modifier in the mobile phase (usually r > 0.99). It was found that under both NP and RP conditions cis isomers were more weakly retained than the corresponding trans isomers. The only exception to this was the chromatographic behavior of C(24) methyl esters. It was established that increasing the polarity of substituents at C(24) and C(24a) led to stronger retention in NP systems, i.e. weaker retention in RP systems. Highly selective separation was achieved in all the chromatographic systems investigated. Possible separation mechanisms are discussed on the basis of the results obtained.

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The retention behavior of 29 biologically active compounds, mixed 1,2,4,5-tetraoxanes, has been investigated by thin-layer chromatography on unmodified silica, CN-silica, and RP-18-silica. The binary mobile phases ethyl acetate-petroleum ether and ethyl acetate-toluene were used under normal-phase conditions and water-organic modifier (methanol, acetone, dioxane) under reversed-phase conditions. For normal-phase conditions R F values were plotted against the amount [%, v/v ] of more polar component in the mobile phase. For reversed-phase conditions R M values were calculated and plotted against the amount [%, v/v ] of organic modifier in the mobile phase. On the basis of the results obtained the effects of the structures of the compounds on their retention were studied. Retention mechanisms are discussed for all the separations.

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JPC - Journal of Planar Chromatography - Modern TLC
Authors:
Sandra Gaica
,
Dejan Opsenica
,
Bogdan Šolaja
,
Živoslav Tešić
, and
Dušanka Milojković-Opsenica

The thin-layer chromatographic behavior of cholic acid and fourteen of its synthetic derivatives has been studied in six reversed-phase and three normal-phase systems. Four significantly different adsorbents were used — RP-18-silica, CN-silica, polyacrylonitrile adsorbent, and unmodified silica gel. Reversed-phase chromatography was performed with water-organic modifier (methanol, dioxane, or acetone) binary mobile phases of widely variable composition. An approximately linear relationship was obtained between R M values and the amount of organic modifier in the mobile phases investigated. The effect of mobile-phase composition on retention has been considered and the selectivity of the chromatographic systems is also discussed. Separation mechanisms are proposed on the basis of the results obtained. A positive solvation effect was proposed as a predominant factor determining retention under normal-phase conditions whereas hydrophobic interactions of the substances with non-polar parts of the adsorbents were assumed to be predominant under reversed-phase conditions.

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Two-dimensional (2D) TLC of eleven phenols has been performed on octadecyl-silica. The most efficient systems were selected by analysis of retention data obtained by one-dimensional chromatography using aqueous and non-aqueous mobile phases. Correlation graphs were plotted to illustrate the separation in the chromatographic systems examined. Complete separation of the phenols was achieved by using an aqueous mobile phase in the first dimension and a non-aqueous mobile phase in the second dimension. Statistical methods (principal-components analysis and cluster analysis) were used for better characterization of the TLC systems.

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The chromatographic behavior of twelve neutral, mixed cobalt(III) complexes of the uns-cis -edda-type has been investigated in six planar chromatographic systems. Four different stationary phases - unmodified silica gel, CN-silica, cellulose, and alumina - were combined with water-organic solvent (methanol or acetone) binary mobile phases. The effect of the water content of the mobile phases on retention of the complexes was investigation systematically. On the basis of the results obtained, possible separation mechanisms were considered. Hydrophilic-interaction chromatography was assumed to be the mechanism determining separation under normal-phase conditions, i.e. use of mobile phases containing small amounts of water. Reversed-phase chromatography occurs when water-rich mobile phases are used.

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