Synthesis, characterization and 59Co NMR study of novel Co(III) complexes with selected N-acyl-N’,N’-dialkylthiourea ligands: an assessment of spontaneous metathesis by multinuclear NMR spectroscopy and rp-HPLC

Barnard, Ilse (2016-12)

Thesis (MSc)--Stellenbosch University, 2018.


ENGLISH ABSTRACT: A number of ligands of the type N,N-dialkyl-N’-acyl(aroyl)thiourea as well as their corresponding complexes with cobalt(III) have been synthesized and characterized by means of various spectroscopic techniques including 1H NMR, 13C NMR, mass spectrometry and infrared spectroscopy. These ligands have been found to form relatively stable complexes of the type fac-[Co(L-S,O)3]. X-ray diffraction results supported the assumption that these complexes favour the facial conformation and coordination of the ligands occurs by means of the bidentate mode via the S and O donor atoms. Considering some of the more favourable properties of 59Co NMR spectroscopy, which include its high receptivity and chemical shift sensitivity, the technique was thus utilized for studying the effect of varying the structure of the acylthiourea ligands on the octahedral-d6 cobalt(III) complexes. By selecting one of the compounds under consideration as an external reference, the extent with which the shielding of the 59Co nucleus is effected by various factors such as concentration, temperature, solvent and ligand structure could be determined. The sensitivity of the 59Co nucleus would also prove to be beneficial for the identification and separation of possible isomers present in the complex. This includes not only various possible E/Z configurational isomers, but also the separation of more than one possible diastereomer present in solution. The possibility of ligand exchange occurring amongst these relatively inert Co(III) complexes was also investigated. A brief study followed on the extent and relative rate of ligand exchange occurring amongst two different homoleptic [Co(Ln-S,O)3] and [Co(Lm-S,O)3] complexes. The reaction was monitored by means of 59Co NMR spectroscopy as well as rp-HPLC. For the ligand exchange reaction study by means of 59Co NMR spectroscopy, the reactivity of two different sets of complexes was compared in a solution of chloroform. The ligand exchange reactions were very slow and both sets of complexes reached equilibrium after only approximately 30 days, at room temperature. The spectra initially gave two peaks corresponding to the two homoleptic complexes initially added in the solution. Two new peaks began to appear, after about 24h, which over time grew in intensity whilst that of the first mentioned complexes decreased in overall intensity. These new peaks were attributed to the formation of the heteroleptic species, and the four peaks in the 59Co NMR spectra was assigned as: [Co(Ln-S,O)3], [Co(Ln-S,O)2(Lm-S,O)], [Co(Ln-S,O)(Lm-S,O)2] and [Co(Lm-S,O)3]. The reaction between two different homoleptic [Co(Ln-S,O)3] and [Co(Lm-S,O)3] complexes were also monitored by means of liquid chromatography in a solution of acetonitrile. The same trend could be seen as for the 59Co NMR investigation, wherein a state of equilibrium would eventually be achieved between the homoleptic complexes initially added in the solution and the newly formed heteroleptic complexes. The relative rate of ligand exchange in acetonitrile on the other hand was reduced by almost half the time compared to the 59Co NMR study in chloroform, with the only differences between subsequent experiments being the reaction medium and concentration used. Unambiguous identification of the various peaks in the chromatogram was done by means of Liquid Chromatography-Mass Spectrometry. In conclusion, it was found by means of 59Co NMR and rp-HPLC that complexes of the type fac- [Co(L-S,O)3] could be well characterized with these techniques. The 59Co NMR chemical shift proved a good and sensitive tool for the study of complexes under investigation, whereby the most significant finding showed the possibility of ligand exchange occurring upon mixing two different homoleptic fac-[Co(L-S,O)3] complexes together in solution. The result was the formation of a mixture of heteroleptic complexes and occurred in a solution of chloroform and acetonitrile respectively. To our knowledge, no such ligand exchange regarding fac-[Co(L-S,O)3] complexes have previously been demonstrated.

AFRIKAANSE OPSOMMING: Verskeie ligande van die soort N,N-dialkyl-N’-acyl(aroyl)thioureas sowel as hul ooreenstemmende komplekse met kobalt(III) was gesintetiseer en gekarakteriseer deur middel van verskeie spektroskopiese tegnieke insluitend 1H KMR, 13C KMR, massa spektrometrie en infrarooi spektroskopie. Hierdie ligande was gevind om relatiewe stabiele komplekse te vorm van die soort fac-[Co(L-S,O)3]. Die resultate van X-straaldiffraksie-analise het die idee ondersteun dat hierdie komplekse die „facial’ konformasie begunstig en dat koördinasie van hierdie ligande vind plaas deur middel van die „bidentate‟ wyse deur die S- en O-donoratome. Aangesien van die meer gunstige eienskappe van 59Co KMR spektroskopie, wat insluit sy höe ontvanklikheid en chemiese verskuiwing sensitiwiteit, was die tegniek dus aangewend om die effek van die struktuur te wissel van die „acylthiourea‟ ligande op die oktahedriese-d6 kobalt(III) komplekse te bestudeer. Deur een van die komplekse onder oorweging te gebruik as ʼn eksterne verwysing, kon daar vasgetel word tot watter mate die „shielding‟ van die 59Co kern geaffekteer word deur verskeie faktore te bestudeer insluitend konsentrasie, temperatuur, oplosmiddel en ligand struktuur. Die sensitiwiteit van die 59Co kern was ook voordelig in die identifisering en skeiding van moontlike isomere teenwoordig in die kompleks. Dit sluit nie net verskeie moontlike E/Z konfigurasie isomere in nie, maar ook die skeiding van meer as een moontlike diastereomeer teenwoordig in die oplossing. Die moontlikheid van ligand uitreiling wat voorkom onder hierdie kobalt(III) komplekse was ook ondersoek. ʼn Vlugtige studie volg, gebaseer op die mate en relatiewe tempo van ligand uitreiling wat plaasvind tussen twee verskillende „homoleptic‟ [Co(Ln-S,O)3] en [Co(Lm-S,O)3] komplekse. Die reaksie was gemonitor deur middel van 59Co KMR spektroskopie sowel as rp- HPLC. Vir die ligand uitreiling studie deur middel van 59Co KMR spektroskopie, was die reaktiwiteit van twee verskillende stelle komplekse vergelyk in ʼn oplossing van chloroform. Die ligand uitreiling reaksies was baie stadig en albei stelle komplekse het slegs na ongeveer 30 dae ewewig bereik, in kamer temperatuur. Die spektra het aanvanklik twee pieke gewys wat ooreengestem het met die twee „homoleptic‟ komplekse wat aan die begin bymekaar gevoeg is in dieselfde oplossing. Later het twee nuwe pieke verskyn, na ongeveer 24h, wat met tyd in intensiteit gegroei het terwyl die eersgenoemde komplekse in intensiteit afgeneem het. Hierdie nuwe pieke was toegeskryf aan die vorming van die „heteroleptic‟ spesies, en die vier pieke in die 59Co KMR spektra was toegewys as: [Co(Ln-S,O)3], [Co(Ln-S,O)2(Lm-S,O)], [Co(Ln-S,O)(Lm-S,O)2] en [Co(Lm-S,O)3]. Die reaksie tussen twee verskillende „homoleptic‟ [Co(Ln-S,O)3] en [Co(Lm-S,O)3] komplekse was ook deur middel van vloeistof kromotografie ondersoek in ʼn oplossing van asetonitril. Dieselfde tendens was gesien as vir die 59Co KMR studie, waar die ewewigs toestand uiteindelik bereik word tussen die oorspronklike „homoleptic‟ komplekse bygevoeg in dieselfde oplossing en die nuut gevormde „heteroleptic‟ komplekse. Die relatiewe tempo van die ligand uitreiling in asetonitril aan die ander kant was verminder met amper helfte die tyd as vir die 59Co KMR studie in chloroform, met die enigste verskil tussen daaropvolgende eksperimente die reaksie medium en konsentrasie wat gebruik is. Ondubbelsinnige identifikasie van die verskeie pieke in die chromatogramme was gedoen deur middel van vloeistof kromotografie-massa spektrometrie (LC-MS). Om af te sluit, dit was ontdek deur middel van 59Co KMR en rp-HPLC dat komplekse van die tipe fac-[Co(L-S,O)3] goed gekarakteriseer kan word met hierdie tegnieke. Die 59Co KMR chemiese verskuiwing was bewys as ʼn goeie en sensitiewe gereedskap vir die studie van komplekse wat hier ondersoek is, waarby die mees betekenisvolle bevinding die moontlikheid van ligand uitreiling getoon het wanneer twee verskillende „homoleptic‟ fac-[Co(L-S,O)3] komplekse bygevoeg word in dieselfde oplossing. Die gevolg van hierdie reaksie was die vormasie van ʼn mengsel van „heteroleptic‟ komplekse en vind plaas in ʼn oplossing van cholorform en asetonitril onderskeidelik. Tot ons wete, is geen ligand uitreiling reaksie met betrekking tot fac-[Co(L-S,O)3] komplekse voorheen gedemonstreer nie.

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