Investigation of thiazyl radical – metalloporphyrin complexes

Van Laeren, Laura Jane (2017-03)

Thesis (PhD)--Stellenbosch University, 2017.

Thesis

ENGLISH ABSTRACT: The aim of this project was to investigate the formation of novel dithiadiazolyl-metalloporphyrin complexes with the goal of forming complexes with interesting magnetic, conductive or redox behaviour. The crystal structure of one such dithiadiazolyl radical – metalloporphyrin complex, containing 4-(4′-pyridyl)-1,2,3,5-dithiadiazolyl and cobalt (II) tetraphenylporphyrin, was determined through the use of single crystal X-ray diffraction. The complex crystallizes as a coordination polymer, with each dithiadiazolyl radical bridging two cobalt porphyrins via a dithiadiazolyl sulfur atom and the pyridyl nitrogen. The sulfur-sulfur bond remains intact and is unusually long; it is proposed that this is as a result of delocalisation of the unpaired electron from the cobalt ion to the dithiadiazolyl radical, resulting in partial oxidation of the Co(II) centre with associated partial reduction of the dithiadiazolyl radical to a dithiadiazolide anion. This is only the third example of an isolated dithiadiazolyl anion. In solution, initial coordination occurs through the dithiadiazolyl sulfur atom. Following dissolution, the nitrogen-bound species predominates. Ligand-metalloporphyrin association was studied in solution through the use of ultraviolet-visible spectroscopy and a logβ value of 4.19 ± 0.05 in dichloromethane at 25 °C was determined. The interaction between a range of eight dithiadiazolyl radicals and three different cobalt porphyrins was subsequently studied spectroscopically. It was found that changing the substituents on the dithiadiazolyl radical has a limited effect on the strength of coordination, while changing the substituents on the porphyrin has a far greater effect. When electron-donating substituents were added, logβ values were higher than for the parent porphyrin while electron-withdrawing substituents resulted in lower logβ values with respect to any of the dithiadiazolyl ligands. Electrostatic potential maps were generated to provide further insight into the electron density across the eight dithiadiazolyl radicals and confirmed that the substituents had a negligible effect on the coordinating sulfur atoms, rationalising the spectrophotometric observations. Coordination of dithiadiazolyl radicals to cobalt, copper, iron and zinc porphyrins was reported, with negligible interaction with magnesium. Coordination to copper and iron was weaker than for cobalt, while coordination to zinc was stronger. The choice of metal affects the magnetic behaviour of the complexes. Coordination to a paramagnetic metal centre, such as cobalt, copper and iron, results in a diamagnetic complex, while coordination to the diamagnetic zinc leads to paramagnetic complexes. These zinc complexes associate at high concentration, leading to loss of paramagnetism. Finally, coordination of dithiadiazolyl radicals to haematin, an iron-containing biological metalloporphyrin, was investigated in order to illustrate a potential application of these dithiadiazolyl complexes to the understanding of haem chemistry and the treatment of malaria. The association of these ligands to haematin (logβ values of 3.07 ± 0.15 and 2.88 ± 0.07) in dichloromethane is weaker than for chloroquine (logβ value of 5.31 ± 0.04), an antimalarial drug known to interact with haematin. This study provided valuable insight into the formation of dithiadiazolyl radical – metalloporphyrin complexes and lays important groundwork for future work in this field.

AFRIKAANSE OPSOMMING: Die mikpunt van hierdie projek was die vorming van unieke ditiadiazolyl-metalloporfyrien komplekse, met die doelwit om komplekse met interessante magnetiese, geleidende of redoks-gedrag te skep. Die struktuur van een so `n ditiadiazolyl radikale-metalloporfyrien kompleks, met 4-(4'-piridyl) -1,2,3,5- ditiadiazolyl en kobalt (II) tetrafenielporphyrin, was bepaal deur die gebruik van enkelkristal Xstraaldiffraksie. Dié kompleks kristalliseer as `n koördinasie-polimeer, met elke ditiadiazolyl wat twee kobalt porfiriene via `n dithiadiazolyl swael-atoom en die piridylstikstof atom oorbrug. Die swael-swael verbinding bly ongeskonde en is buitengewoon lank; dit word voorgestel dat hierdie as gevolg van delokalisasie van die ongepaarde elektron van die kobaltioon om die ditiadiazolyl radikale is, wat lei tot gedeeltelike oksidasie van die Co(II)-sentrum, wat gepaard gaan met die gedeeltelike reduksie van die ditiadiazolyl radikale tot `n ditiadiazolate anioon. Hierdié is slegs die derde voorbeeld van `n geïsoleerde ditiadiazolyl anioon. Ligand-metalloporfyrine assossiasie is in oplossing bestudeer deur middel van die gebruik van ultraviolet-sigbare spektroskopie, en `n logβ waarde van 4,19 ± 0,05 in dichloormetaan by 25 °C is vasgestel. Die interaksie tussen 'n verskeidenheid van agt ditiadiazolyl radikale en drie verskillende kobalt-porfiriene is daarna spektroskopies bestudeer. Daar is bevind dat die verandering van die substituente op die ditiadiazolylradikale `n beperkte invloed het op die sterkte van die koördinasie, terwyl die verandering van die substituente op die porfirien `n veel groter effek het. Wanneer elektron-skenkende substituente bygevoeg is, word logβ-waardes hoër as vir die oorspronklike porfirien, terwyl elektron-onttrekkende substituente lei tot laer logβ-waardes, met betrekking tot enige van die ditiadiazolyl ligande. Elektrostatiesepotensiaal-kaarte is opgestel om verdere insig in die elektrondigtheid van die agt dithiadiazolyl-radikale te verskaf. Dit het bevestig dat die substituente `n geringe uitwerking op die koördinering swael-atome het en dit het die spektrofotometriese waarnemings gerasionaliseer. Die koördinasie van ditiadiazolyl radikale tot kobalt, koper-, yster- en sink-porfiriene is aangemeld, met weglaatbare interaksie met magnesium. Die koördinasie aan koper en yster was swakker as dié tot kobalt, terwyl koördinasie tot sink sterker was. Die keuse van metaal affekteer die magnetiese gedrag van die komplekse. Koördinasie tot `n paramagnetiese metaalsentrum, soos kobalt, koper en yster, lei tot `n diamagnetiese kompleks, terwyl koördinasie met die diamagneties sink lei tot paramagnetiese komplekse. Hierdie sink-komplekse assosieer teen 'n hoë konsentrasie, en dit lei tot die verlies van paramagnetisme. Ten slotte, koördinasie van dithiadiazolyl-radikale aan haematin, `n biologiese metalloporfyrien wat yster bevat, is waargeneem. Die assosiasie van hierdie ligande aan haematin (logβ waardes van 3,07 ± 0,15 en 2,88 ± 0,07) in dichloormetaan is swakker as vir chlorokien (logβ waarde van 5,31 ± 0,04), `n bekende malariadwelm. Hierdie studie het waardevolle insig bekendgemaak in die vorming van komplekse met dithiadiazolylradikale en metalloporfyrien en lê `n belangrik grondslag vir toekomstige werk in hierdie veld.

Please refer to this item in SUNScholar by using the following persistent URL: http://hdl.handle.net/10019.1/101402
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