Synthesis of 5- and 6-donor schiff base ligands for the selective extraction and transport of selected base metals ions

Ogutu, Hezron Felix Odhiambo (2018-12)

Thesis (PhD)--Stellenbosch University, 2018.

Thesis

ENGLISH ABSTRACT: Schiff base ligands have various advantageous properties that can be employed for use in solvent extraction processes. They are relatively cheap to synthesize, stable, robust and can be modified to accommodate multiple donor groups. Schiff base ligands were therefore synthesized through a Schiff base condensation method using five aldehydes; salicylaldehyde, para-methoxy salicylaldehyde; para-tertbutyl salicylaldehyde, para-nonyl salicylaldehyde and 2-hydroxynaphthylaldehyde with N-(2-aminoethyl)ethane-1,2-diamine, N-(3-aminopropyl)propane-1,3-diamine, N,N-bis(2-aminoethyl)ethane-1,2-diamine to make five pentadentate ligands with ethylalkyl spacers (class A ligands L1-L5). Five pentadentate ligands with propylalkyl spacer groups (class B ligands L6-L10) and five heptadentate tripodal with ethyl alkyl spacers (class C ligands L11-L15) with N-donor groups. 2-(2-Aminoethoxy) ethylamine and 2,2'-[ethane-1,2-diylbis(oxy)]diethanamine was then used to synthesise the pentadentate (class D ligands L16-L20) and hexadentate O-donor ligands (class E ligands L21-L25). All these ligands were obtained in good yield and were fully characterized by FTIR, 1H NMR, and 13C {H}NMR, ESI-MS, and CHN analysis. The IR and NMR studies of the imine peak of the synthesized products show different shifts due to the different substituents. The analysis of the ligands on the IR showed a red shift for methoxy, tertbutyl and nonyl substituents in comparison to the unsubstituted salicylaldehyde. The naphthylaldehyde showed a blue shift in comparison to the unsubstituted salicylaldehyde. The analysis through the 1H and 13C {H} NMR of the imine peak also showed the same trend with the chemical shift indicating an upfield shift of the imine peak and downfield shift due to the naphthylaldehyde. The addition of the alkyl group in the spacer and the change of the different donor group from the N-donor to the O- donor group did not show a significant shift of the imine peaks. The results obtained for the class B ligands with the naphthylaldehyde showed that the ligand exists in two tautomeric forms; the quaternary imine and the hydroxyl form. A study of the 1H NMR of class D ligands shows that these ligands undergo molecular dynamism caused by the symmetry and flexibility of these molecules. One of the class E ligands, ligand, L22 containing the methoxy substituent was observed to undergo photochromic colour change that was attributed to tautomerism which is unusual behavior for Schiff base ligands. The colour change was studied using confocal microscopy showing fluorescence emission of the ligand in solid state with a wide band gap of between 611 nm - 407 nm. Three of the class D ligands were studied using single crystal X-ray diffraction (SXRD). The ligand L18 with the naphthylaldehyde was obtained showing the tautomeric form of the ligand with the proton shift of the OH proton to the imine N-atom with the phenolic oxygen stabilized by the aromatic system. The Schiff base ligand complexes were synthesized and fully characterized using ATIR, UV-Vis and ESI-MS analysis. Five crystal structures of L1-Cu, L1Co, L4-Cu, L17-Cu, L18-Cu, L19-CuPb were also obtained and analyzed using the SXRD. The complexes synthesized from the class A ligands L1-Co was obtained showing an octahedral coordinate complex around the Co metal, while the L1-Cu, L4-Cu, were obtained showing a metal induced hydrolyzed square planar geometry. The L17-Cu, L18-Cu complexes synthesized from class D ligands with the methoxy, and naphthyl groups respectively were observed to form metallocycles with Cu-metal. The geometry of the Cu-centre was observed to be four coordinate, forming a distorted square-planar geometry. L19-CuPb with the tertbutyl substituent was observed to form a bi-metallic Cu-Pb complex with four Cu-metallocyles coordinated to one Pb-metal in octahedral geometry. All the ligands were subjected to solvent extraction studies. The results obtained showed that all the class A and class C ligands, except the nonyl based substituent, showed a high degree of bleeding into the aqueous phase. The ligands with the nonyl-containing substituents for class A and class C were observed to selectively extract 83% and 86% of Cu2+. The ligands that showed high degree of bleeding to the aqueous phase were subsequently used for synergistic solvent extraction studies using three different synergists. Synergist A (palmitic acid) was observed to synergistically promote selectivity of Pb2+, synergist B (dodecylsulfonate) synergistically promoted selectivity of Cu2+ and Pb2+ while synergist C (dodecylbenzylsulfonate) was observed to extract both Cu2+ and Pb2+ slightly since it showedlow selectivity. The ligands that did not show bleeding into the aqueous phase (class B, D and E ligands) were observed to be very selective towards Cu2+ in accordance with the nature of stability of their complexes as shown by the Irving Williams series. The class D and E ligands containing the O-donor group showed excellent extraction and selectivity of Cu2+. The highest extraction was observed for the non-substituted salicylaldehyde based ligand at 87% and 95% for the naphthylaldehyde based ligand for class D and class E ligands respectively. For all the O-donor based ligands, the % extraction of a secondary metal was less than 10% indicating exceptional selectivity of Cu2+. These results were very good and showed that despite the increased organic nature of the alkyl chain substituent, the non-substituted ligand showed the highest extraction of Cu2+. This high selectivity and extraction were correlated to the crystal structure obtained. This correlation showed that the nature of coordination of the ligand to the metal ion has a profound effect on the % extraction of the ligand in solvent extraction.

AFRIKAANSE OPSOMMING: Schiff-basis ligande besit ‘n aantal voordelige eienskappe wat hulle doeltrefend maak in ekstraksie prosesse. Hulle is goedkoop om te sintetiseer, hulle is stabiel, veerkragtig en kan verander word om veelvuldige skenker groepe te akkomodeer. Schiff-basis ligande was dus gesintetiseer deur die Schiff-basis kondensasie metode met gebruik van vyf verskillende aldehiede: salisielaldehied, para-metoksie salisielaldehied, para-butiel salisielaldehied, para-noniel salisielaldehied en 2-hidroksienaftielaldehied. Die amiene was N-(2-aminoetiel)etaan-1,2-diamien, N-(3-aminopropiel)propaan-1,3-diamien, N,N-bis(2-aminoetiel)etaan-1,2-diamien. Vyf pentadentaat ligande met etielalkiel groepe (klas A ligande L1 – L5) en vyf pentadentaat ligande met propielalkiel groepe (klas B ligande L6 – L10) is gesintetiseer. Vyf heptadentate tripodiel ligande met etiel alkiel groepe (klas C ligande L11 – L15) is ook gesintetiseer. 2-(2-aminoetoksie) etielamien en 2,2’-[etaan-1,2-dielbis(oksie)]dietaanamien is dan gebruik om pentadentaat ligande (klas D ligande L16 – L20) en heksadentaat ligande (klas E ligande L21 – L25) te sintetiseer. Alle ligande se opbrengs was goed en hulle is volledig gekarakteriseer deur middel van FTIR, 1H KMR, en 13C {H}KMR, ESI-MS, en CHN analiese. Die IR en KMR studies van die imien piek van die gesintetiseerde produkte toon verskillende verskuiwings as gevolg van die verskillende substituente. Die analise van die ligande op die IR het 'n rooi verskuiwing vir metoksie-, tert-butiel- en noniel-substituente getoon in vergelyking met die ongesubstitueerde salisielaldehied. Die naftielaldehied het 'n blou verskuiwing vertoon in vergelyking met die ongesubstitueerde salisielaldehied. Die analise deur die 1H en 13C {H} NMR van die imien piek het ook dieselfde tendens getoon met die chemiese verskuiwing wat 'n opwaartse verskuiwing van die imien piek en afwaartse verskuiwing weens die naftielaldehied aandui. Die toevoeging van die alkielgroep in die brug en die verandering van die verskillende donor groep van die N-skenker na die O-donor groep het nie 'n beduidende verskuiwing van die imien pieke getoon nie. Die resultate verkry vir die klas B ligande met die naftielaldehied het getoon dat die ligand in twee tautomeriese vorms bestaan uit die kwaternêre imien en die hidroksielvorm. 'n Studie van die 1H-KMR van klas D ligande toon dat hierdie ligande molekulêre dinamiek ondervind deur simmetrie en die buigsaamheid van hierdie molekules te ondergaan. Een van die klas E ligande, ligand L22 wat die metoksiesubstituent bevat, is waargeneem om fotochromiese kleurverandering te ondergaan wat toegeskryf word aan tautomerisme wat ongewone gedrag vir Schiff-basis ligande is. Die kleurverandering is bestudeer met behulp van konfokale mikroskopie wat fluoresensie-emissie van die ligand in vaste toestand toon met 'n wye band gaping tussen 611 nm - 407 nm. Drie van die klas D ligande is bestudeer met behulp van enkel kristal X-straal diffraksie (SXRD). Die ligand L18 met die naftielaldehied is verkry wat die tautomerise vorm van die ligand toon met die protonverskuiwing van die OH proton na die imien-N-atoom met die fenoliese suurstof wat deur die aromatiese stelsel gestabiliseer is. Die Schiff-basis komplekse is gesintetiseer en ten volle gekaraktiseer deur gebruik te maak van ATIR, UV-Vis en ESI-MS analise. Vyf kristalstrukture van L1-Cu, L1-Co, L4-Cu, L17-Cu, L18-Cu, L19-CuPb is ook verkry en geanaliseer met behulp van die SXRD. Die komplekse gesintetiseer uit die klas A ligande L1-Co is verkry met 'n oktahedriese koördinaatkompleks rondom die Co-metaal, terwyl die L1-Cu, L4-Cu verkry is wat metaal veroorsaakde hidroliseerde vierkante vlak geometrie toon. Die L17-Cu, L18-Cu komplekse wat gesintetiseer uit onderskeidelik klas D ligande met die metoksiel- en naftielgroepe is waargeneem om metallosiklisse met Cu-metaal te vorm. Die geometrie van die Cu-sentrum is waargeneem om vier koördinaat te wees, wat 'n vervormde vierkantige geometrie vorm. L19-CuPb met die tertbutiel substituent is waargeneem om 'n bi-metaal Cu-Pb kompleks te vorm met vier Cu-metallosiklisse wat gekoordineer is tot een Pb-metaal in oktahedriese geometrie. Al die ligande is onderworpe aan oplosmiddel ekstraksie studies. Die resultate wat verkry is, het getoon dat al die klas A- en klas C-ligande, behalwe die noniel-substituent, 'n hoë graad van loging toon. Die ligande met die noniel-bevattende substituente vir klas A en klas C is waargeneem om selektief 83% en 86% Cu2+ te onttrek. Die ligande wat 'n hoë graad van loging toon, is later gebruik vir sinergistiese oplosmiddel ekstraksie studies met behulp van drie verskillende sinergiste. Synergis A (palmitiensuur) is waargeneem om sinergisties die selektiwiteit van Pb2+ te bevorder, sinergis B (dodesielsulfonaat) synergisties bevorder die selektiwiteit van Cu2+ en Pb2+ terwyl sinergis C (dodesielbenzielsulfonaat) waargeneem is om al die metaalione met beide Cu2+ en Pb2+ effens hoër te onttrek in vergelyking met die ander metaalione. Die ligande wat nie uitloging (klasse B, D en E ligande) toon nie, was baie selektief teenoor Cu2+ in ooreenstemming met die aard van stabiliteit van hul komplekse, soos aangedui deur die Irving Williams-reeks. Die klasse D- en E-ligande wat die O-skenkergroep bevat, het uitstekende ekstraksie en selektiwiteit van Cu2+ getoon. Die hoogste ekstraksie is waargeneem vir die nie-gesubstitueerde salisielaldehiedgebaseerde ligand by 87% en 95% vir die naftielaldehiedgebaseerde ligand van onderskeidelik klas D en klas E ligande. In al hierdie O-donor-gebaseerde ligande was die % ekstraksie van 'n sekondêre metaal minder as 10% wat uitsonderlike selektiwiteit van Cu2+ aandui. Hierdie resultate was baie goed en het getoon dat ondanks die verhoogde organiese aard van die alkielketting substituent, die nie-gesubstitueerde ligand die hoogste ekstraksie van Cu2+ getoon het. Hierdie hoë selektiwiteit en ekstraksie is gekorreleer met die kristalstruktuur wat verkry is. Hierdie korrelasie het getoon dat die aard van koördinasie van die ligand na die metaalioon 'n diepgaande uitwerking op die % ekstraksie van die ligand op die metaalioon het.

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