Preparation, characterization and applications of macrocycle-dendrimer conjugates

Wilbers, Derik (2013-12)

Thesis (MSc)--Stellenbosch University, 2013.

Thesis

ENGLISH ABSTRACT: In this thesis we describe various attempts at incorporating macrocycles into dendritic architectures to form macrocycle-dendrimer conjugates with the aim of preparing materials that would exhibit properties that are more than the sum of the constituent parts, in this case macrocycles and dendrimers. A further aim was the synthesis and characterization of metallodendrimers based on such scaffolds and to test these as catalyst precursors in the catalytic oxidation of alcohols. The synthesis of two different types of conjugate systems was attempted; viz. dendrimers functionalized with macrocycles on the peripheries and dendrimers with macrocyclic cores. The synthesis of conjugate systems based on cyclam as the macrocycle was attempted. This required the mono functionalization of cyclam with a linker molecule capable of further reaction with the functional groups at the periphery of commercially available N,N,N,N-tetrakis(3-aminopropyl)-1,4-butanediamine dendrimer. Several approaches were taken in trying to make such conjugate systems but they were not entirely successful. One of the major issues was the final deprotection step, of the Boc-protected cyclam units which proved difficult in our hands. Another approach to prepare the target conjugates involved the use of click chemistry in order to synthesize a dendrimer with an aromatic core and cyclam peripheries. A dendrimer with Boc-protected cyclam peripheries that are bonded through triazole groups to the aromatic core was synthesized. However, subsequent attempts at de-protection of the cyclam functionalities of this conjugate failed to yield the pure de-protected dendrimer. Greater success was achieved with the preparation of a dendrimer with a macrocyclic core. A cyclam cored dendrimer with salicylaldimine peripheries was successfully synthesized and characterized. This dendritic ligand was complexed to Cu(II), Ni(II) and Zn(II) metal ions respectively to form a series of new metallodendrimers. These metallodendrimers were fully characterized using a range of analytical techniques including FT-IR spectroscopy, mass spectrometry, elemental analysis, thermogravimetric analysis, magnetic susceptibility measurements and NMR spectroscopy where appropriate. The Cu(II) and Ni(II) metallodendrimers were tested as catalyst precursors in the catalytic oxidation of benzyl alcohol to benzaldehyde. The catalytic system consisted of the appropriate metallodendrimer, the free radical, 2,2,6,6-tetramethylpiperidinyl- 1-oxyl (TEMPO) and O2 as the oxidant. The reaction parameters, namely the nature of the solvent, catalyst loading, substrate concentration and reaction temperature were sequentially optimized to achieve the best catalytic efficiency. The Cu(II) catalyst precursor exhibited relatively high catalytic activity and achieved TOF’s between 40 and 30 when operating under the optimized conditions, while the Ni(II) catalytic system showed very poor catalytic activity.

AFRIKAANSE OPSOMMING: In hierdie tesis beskryf ons pogings om makroringe in die dendritiese argitektuur te inkorporeer om makroring-dendrimeer gekonjugeerdes te vorm met die hoop dat sulke molekules eienskappe sal toon wat meer is as die somtotaal van die afsonderlike eenhede. ‘n Verdere doel was die sintese en karakterisering van metallodendrimere gebaseer op sulke draers sowel as die toetsing van hierdie molekules as pre-katalisore in die katalitiese oksidasie van alkohole. Pogings tot die sintese van twee verskillende tipes makroring-dendrimeer gekonjugeerdes word beskryf naamlik, dendritiese ligande met makroringe by die buiterand sowel as dendritiese ligande met ‘n makroring as kern word bespreek. Die sintese van makroring-dendrimeer gekonjugeerdes gebasseer op die makroring cyclam word beskryf. Hierdie sintese vereis die gebruik van ‘n monogefunksioneerde cyclam wat ‘n gepaste koppelingsgroep besit. Hierdie koppelingsgroep kan dan verder met funksionele groepe op die oppervlak van die kommersieel beskikbare DAB-dendrimeer reageer. Verskeie pogings is aangewend om sulke gekonjugeerde stelsels te sintetiseer maar hierdie pogings was nie volkome suksesvol nie. ‘n Groot uitdaging was die gebruik en gevolglike latere verwydering van beskermende groepe soos Boc. ‘n Ander benadering het gebruik gemaak van “click” chemie met die doel om ‘n dendrimeer bestaande uit ‘n aromatiese kern en cyclam periferie te vorm. ‘n Dendrimeer met Boc beskermde cyclam eenhede op die buiterand geheg aan ‘n aromatiese kern deur triasool groepe is gesintetiseer. Die verwydering van die beskermende groepe geheg aan die cyclam eenhede was egter weereens ‘n probleem en hierdie metode kon nie die suiwer dendrimeer lewer nie. Groter sukses is behaal met die sintese van ‘n dendrimeer met ‘n cyclam kern en salisielaldimien periferieë. Die dendritiese ligand is vervolgens met metaalsoute van Cu(II), Ni(II) en Zn(II) gereageer om verskeie multikern metaalkomplekse te vorm. Die metaalkomplekse is volledig gekarakteriseer deur verskeie analitiese tegnieke insluitende infrarooi spektroskopie, massa spektrometrie, termografiese analiese, mikroanaliese asook KMR spektroskopie waar moontlik. Die Cu(II) en Ni(II) metaalkomplekse is geëvalueer as pre-katalisatore in die katalitiese oksidasie van alkohole. Hierdie katalitiese sisteem bestaan uit die metaalkompleks, die radikaal TEMPO en molekulêre suurstof. Die invloed van verskeie reaksie- parameters soos die tipe oplosmiddel, die hoeveelheid katalisator, die konsentrasie van die alkohol asook die temperatuur is ondersoek. Gevolglik is die optimale kondisies bepaal om die hoogste opbrengs van bensaldehied te lewer. Die Cu(II) kompleks het ‘n relatief hoë omset van bensielalkohol na bensaldehied getoon met omset frekwensie waardes tussen 30 en 40 onder die optimale kondisies. Die Ni(II) kompleks het egter swak aktiwiteit getoon vir hierdie transformasie.

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