Synthesis and characterisation of macrocyclic ligands with hydroxyalkyl and thiol pendant arms tethered on 1,5,9-triazacyclododecane and their complex formation chemistry

[摘要] ENGLISH ABSTRACT: This investigation comprises the synthesis and characterisation of new macrocyclic ligandswith pendant arms appended to the nitrogen donor atoms of 1,5,9-triazacyclododecane(12aneN3) and their coordination to various transition metal ions. The five macrocyclicligands, 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 thatcontain alcohol or thiol end groups to a preformed 12aneN3 macrocycle. The 12aneN3 wassynthesised 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 fromthe corresponding epoxides whereas for the one that contained sulphur donor atoms,propylene sulphide was used. The reaction progress was followed by 13C and 1H NMRspectroscopy and the final macrocyclic ligands were further analysed by mass spectrometryand in some instances, elemental analysis was also performed.Protonation constants of the free ligands were determined using potentiometric titrations at 25oC and the ionic strength was kept constant at 0.1000 mol dm-3 using NaNO3. The log K1values 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 logK2. THPTACD is the most basic mainly due to less steric hindrance whereas THMPTACD isthe least basic owing to high steric hindrance to both solvation and formation of stronghydrogen bonds of the protonated species to solvent molecules during the solvation step in aBorn Haber-type cycle of the complete process. Protonated THBTACD is the most basic ofthe three mono-protonated ligands, a result that may be explained in terms of bettercorrelation between inductive and steric effects. The second protonation constant is mostlyinfluenced by inductive effects unlike the first protonation constant which is mostlydetermined by steric effects. The third protonation constant could not be established becauseof 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 similarlydetermined using potentiometric titrations at 25 ¡ÆC in 0.1000 mol dm-3 NaNO3. Log K valueswith 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 wereobtained. 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 interactionbetween the Zn(II) metal ion and the donor atoms with their short Zn(II)-N bond lengthsoccurs. The short Zn(II)-N distances indicate that the metal ion is situated very close to themacrocyclic hole. On the other hand, each half of the hydrogen-bonded dimeric molecularstructure of [Cd2(THPTACD)2]4+ has long Cd(II)-N bond lengths.Although metal nitrates, perchlorates and acetates were used in attempted crystal structuredeterminations, only metal nitrates formed suitable crystals. THPTACD complexes withCo(II), Mn(II), Ni(II), Cu(II), Zn(II) and Cd(II) were subjected to such determinations. Eachcentral metal ion in these complexes is six coordinate to the three N atoms of the parentmacrocyclic ring on one plane and the three O atoms of the pendant hydroxypropyl armsforming another plane on the other face of the metal ion. The geometry of all six molecularstructures is pseudo octahedral with the Cu(II) complexes being the most twisted towards atrigonal prismatic arrangement. The change towards trigonal prismatic can be attributed topacking forces overriding octahedral crystal field stabilisation effects. The overall chirality ofthe 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 overallchirality as the Cu(II) complex with THPTACD but is less twisted towards trigonal prismaticgeometry. Both Cu(II) complexes exhibited strong evidence of Jahn-Teller tetragonaldistortion 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 alsodetermined. The molecule contains an inversion centre coinciding with the crystallographiccentre 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 twopendant arms are pre-organised for meridional coordination. The hydrogen bond network inthis structure emphasises the important role that such weak interactions play in stabilising the proton even in solution during determination of protonation constants in triazamacrocycleswith pendant arms carrying oxygen donor atoms.

[发布机构] Stellenbosch University