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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 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 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 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 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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Experimental R M values for two series of β-diketonato complexes, M(acac) 3- n (phacphac) n and M(acac) 3- n (phSacphSac) n , where M represents cobalt(III) or chromium(III), have been determined by reversed-phase thin-layer chromatography on RP-18 silica with binary tetrahydrofuran-water, acetonitrile-water, and acetone-water mobile phases. R M 0 values were determined for the different mobile phase mixtures by linear extrapolation to 0% ( v/v ) organic modifier. Lipophilicity C 0 was calculated as the ratio of the intercept and slope values. Chromatographically obtained lipophilicity was correlated with calculated C log P values.

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The chromatographic behavior of some phenolic compounds has been studied with the objective of understanding the role of cyano groups in the processes of chromatographic separation on thin layers of CN-silica gel. For this purpose separations were performed on thin layers of CN-silica gel, unmodified silica gel, and polyacrylonitrile adsorbent (PANS) using fourteen mobile phases, nine of which resulted in normal-phase systems. On the basis of the results obtained the effect of compound structure (i.e. properties of ortho substituents of the compounds separated) on their retention is discussed. The dependence of the order of retention of the phenols on the properties of the adsorbent was also considered. Plots of retention data ( R M against R M ) determined by use of different chromatographic systems were used to characterize the systems used. The results enabled hypotheses to be formulated about possible mechanisms involved in the separations of the phenols on these adsorbents.

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JPC - Journal of Planar Chromatography - Modern TLC
Authors: Mirjana Aleksić, Jadranka Odović, Dušanka Milojković-Opsenica, and Živoslav Tešić

The chromatographic behavior of five myorelaxant drugs has been studied under the conditions used for salting-out thin-layer chromatography (SOTLC) on cellulose and alumina. For this purpose, aqueous ammonium sulfate solutions of different concentration were used as mobile phases. It was established that hR F values always decreased in parallel to increasing salt concentrations. When cellulose was used as adsorbent, a linear relationship was observed between the R M values and the ammonium sulfate content of the mobile phase. Regression data of the plots obtained were used to determine the lipophilicity parameters R M 0 and C 0 . Lipophilicity determined in this way correlated with calculated log P values.

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An overview of the adsorption characteristics of 3-cyanopropylsiloxane-bonded silica gel as a specific, moderately polar sorbent which can be used for separation of different classes of organic and inorganic substances in both normal- and reversed-phase chromatography is presented. The most important separation mechanisms that govern the retention on this sorbent are underlined. In addition, some recently published applications of 3-cyanopropylsiloxane-bonded silica gel as a sorbent in thin-layer chromatography are listed.

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