Synthesis and characterisation of macrocyclic ligands with hydroxyalkyl and thiol pendant arms tethered on 1,5,9-triazacyclododecane and their complex formation chemistry
Thesis (MSc (Chemistry and Polymer Science))--University of Stellenbosch, 2010.
ENGLISH ABSTRACT: This investigation comprises the synthesis and characterisation of new macrocyclic ligands with pendant arms appended to the nitrogen donor atoms of 1,5,9-triazacyclododecane (12aneN3) and their coordination to various transition metal ions. The five macrocyclic ligands, 1,5,9-tris[(2S)-2-hydroxypropyl]-1,5,9-triazacyclododecane (THPTACD), 1,5,9- tris(2-hydroxy-2-methylpropyl)-1,5,9-triazacyclododecane (THMPTACD), 1,5,9-tris[(2S)-2- hydroxy-2-phenylethyl]-1,5,9-triazacyclododecane (THPETACD), 1,5,9-tris[(2S)-2- hydroxybutyl]-1,5,9-triazacyclododecane (THBTACD) and 1,5,9-tris(2-mercaptopropyl)- 1,5,9-triazacyclododecane (TMPTACD) were prepared by addition of pendant arms that contain alcohol or thiol end groups to a preformed 12aneN3 macrocycle. The 12aneN3 was synthesised from simple starting materials using 1,3-propanediol and bis(3-aminopropyl)- amine. The macrocycles with oxygen donor atoms on the pendant arms were prepared from the corresponding epoxides whereas for the one that contained sulphur donor atoms, propylene sulphide was used. The reaction progress was followed by 13C and 1H NMR spectroscopy and the final macrocyclic ligands were further analysed by mass spectrometry and in some instances, elemental analysis was also performed. Protonation constants of the free ligands were determined using potentiometric titrations at 25 oC and the ionic strength was kept constant at 0.1000 mol dm-3 using NaNO3. The log K1 values were 11.47, 10.96 and 10.47 for THPTACD, THBTACD and THMPTACD, respectively, whereas the corresponding values of 5.81, 6.02 and 5.94 were obtained for log K2. THPTACD is the most basic mainly due to less steric hindrance whereas THMPTACD is the least basic owing to high steric hindrance to both solvation and formation of strong hydrogen bonds of the protonated species to solvent molecules during the solvation step in a Born Haber-type cycle of the complete process. Protonated THBTACD is the most basic of the three mono-protonated ligands, a result that may be explained in terms of better correlation between inductive and steric effects. The second protonation constant is mostly influenced by inductive effects unlike the first protonation constant which is mostly determined by steric effects. The third protonation constant could not be established because of lack of sensitivity of the glass electrode in very high acidic medium. The complex stability constants of Co(II), Zn(II), Cd(II) and Pb(II) cations were similarly determined using potentiometric titrations at 25 ¡ÆC in 0.1000 mol dm-3 NaNO3. Log K values with THPTACD are 15.45, 21.22, 14.03 and 16.11 for Co(II), Zn(II), Cd(II) and Pb(II), respectively. THBPTACD has corresponding values of 13.93, 20.02, 13.55 and 15.01, whereas for THMPTACD the values of 14.63, 18.08, 12.91 and 14.36 log units were obtained. The Zn(II) 1:1 complexes are the most stable and those of Cd(II) the least stable. A crystal structure determination of [Zn(THPTACD)]2+ shows that optimal interaction between the Zn(II) metal ion and the donor atoms with their short Zn(II)-N bond lengths occurs. The short Zn(II)-N distances indicate that the metal ion is situated very close to the macrocyclic hole. On the other hand, each half of the hydrogen-bonded dimeric molecular structure of [Cd2(THPTACD)2]4+ has long Cd(II)-N bond lengths. Although metal nitrates, perchlorates and acetates were used in attempted crystal structure determinations, only metal nitrates formed suitable crystals. THPTACD complexes with Co(II), Mn(II), Ni(II), Cu(II), Zn(II) and Cd(II) were subjected to such determinations. Each central metal ion in these complexes is six coordinate to the three N atoms of the parent macrocyclic ring on one plane and the three O atoms of the pendant hydroxypropyl arms forming another plane on the other face of the metal ion. The geometry of all six molecular structures is pseudo octahedral with the Cu(II) complexes being the most twisted towards a trigonal prismatic arrangement. The change towards trigonal prismatic can be attributed to packing forces overriding octahedral crystal field stabilisation effects. The overall chirality of the isomorphic complexes of Zn(II), Co(II), Mn(II), Ni(II) and Cu(II) with THPTACD is [¥Ë((2¥ë.)¥ä.¥ä)] whereas the overall chirality of each half of the dimeric [Cd2(THPTACD)2]4+ complex is [¥Ë(¥ë.(2¥ä.)¥ä)]. The Cu(II) complex with THPETACD has the same overall chirality as the Cu(II) complex with THPTACD but is less twisted towards trigonal prismatic geometry. Both Cu(II) complexes exhibited strong evidence of Jahn-Teller tetragonal distortion in the solid state with tetragonality parameter values of 0.87 and 0.81, respectively. The structure of a new di-¥ì-chloro bridged binuclear complex of Cd(II)-12aneN3 was also determined. The molecule contains an inversion centre coinciding with the crystallographic centre of symmetry. Finally, the molecular structure of the protonated 1,5-bis[(2S)-2- hydroxybutyl]-1,5,9-triazacyclododecane shows that the oxygen donor atoms of the two pendant arms are pre-organised for meridional coordination. The hydrogen bond network in this structure emphasises the important role that such weak interactions play in stabilising the proton even in solution during determination of protonation constants in triazamacrocycles with pendant arms carrying oxygen donor atoms.