Masters Degrees (Chemistry and Polymer Science)

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Browsing Masters Degrees (Chemistry and Polymer Science) by Author "Barnard, B. F. (Bernardus Francis)"

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    The synthesis, stability and structures of two novel macrocyclic ligands and their complexes
    (Stellenbosch : Stellenbosch University, 2008-03) Barnard, B. F. (Bernardus Francis); Luckay, Robert C.; Stellenbosch University. Faculty of Science. Dept. of Chemistry and Polymer Science.
    ENGLISH ABSTRACT: This study comprises the synthesis and full characterization of two novel pendant-arm donor macrocyclic ligands. The stability and structure of the complexes of these two ligands with a series of metals ions [Mn(II), Co(II), Zn(II), Cd(II), Pb(II)], was subsequently investigated. The two parent macrocyclic ligands, 1,4,7-triazacyclodecane ([10]-ane-N3) and 1,4,8- triazacycloundecane ([11]-ane-N3) were synthesized by means of the "direct synthesis" method using tosylates as protecting groups. Pendant arms were then added to the parent molecules to create hexadentate ligands. The two macrocyclic ligands are asymmetric because of the different lengths of the carbon bridges between the N-donor atoms of the rings. This feature gives the ligands the possibility to form both five- and six-membered rings when coordinated to metal centers. The ligands were fully characterized by means of NMR, mass spectrometry and elementalanalysis. Melting points were also determined. These two novel (triaza macrocyclic) ligands now complete the series between 9-ane- N3 [with its 2-(S)-hydroxypropyl pendant arms] and 12-ane-N3 [with its 2-(S)- hydroxypropyl pendant arms]. Protonation data of the ligands were determined using potentiometric titrations. The respective protonation constants for both ligands in 0.1000 mol dm-3 NaNO3 are: THTD log H 1 K = 9.176 and log H 2 K = 4.20 THTUD log H 1 K = 11.32 and log H 2 K = 5.87 A third protonation constant for both ligands (THTD and THTUD) was observed, but the use of potentiometric methods could not produce reliable values at very low pH values. stability constants of the new ligands with a series of metal ions were determined using 0.1000 mol dm-3 NaNO3 as ionic medium. The log(K) values at 25°C with THTD are: Co(II) 22.93 Zn(II) 14.82 Cd(II) 19.38 Pb(II) 15.47 The log(K) values at 25°C with THTUD are: Co(II) 17.52 Zn(II) 16.43 Cd(II) 18.05 Pb(II) 14.63 Very stable complexes were obtained with the larger Cd(II) ion when compared to other similar ligands. Crystal structures of some of the metal complexes were determined by X-ray crystallography. Metal perchlorates were used in the preparation of the metal complexes with THTD and THTUD, and various methods were utilized for the crystallization process. The general formula for these complexes is [M(L)]2+·2(ClO4)- where M=metal ion and L= neutral ligand. The bond lengths between the nitrogen atom and the metal ion, and the oxygen atom and the metal ion are very much the same in the respective crystal structures. This means that the metal ion lies almost halfway between the nitrogen and the oxygen atoms. The Mn(II)-THTD complex featured both a distorted octahedral and distorted trigonal prismatic configuration in the unit cell. Co(II)-THTUD contains three molecules per unit cell all having a distorted octahedral configuration. Zn(II)-THTUD crystallizes with six molecules per unit cell.