Synthesis and characterization of new metal-organic frameworks by the use of 2-pyridyl oximes or alcohols in combination with poly-carboxylic ligands

Abstract

The aim of this project is the synthesis and characterization of novel mixed-ligand coordination polymers and metal-organic frameworks (MOFs) and their further study as magnetic sensors for the adsorption of non-environmental friendly species. For the synthesis of the mixed-ligand MOFs, pyridyl oximes or pyridyl alcohols have been used for the stabilization/formation of inorganic secondary building units (sbus), whereas di-, tri- and tetra- topic carboxylates (1,4- benzene-dicarboxylic acid/terephthalate, 1,3,5- benzene-tricarboxylic acid or trimesic acid, 1,2,4,5-benzenetetracarboxylic acid or pyromellitic acid) have been employed as pillars. The compounds were synthesized under solvothermal conditions. The choice of linkers is crucial for the properties of the synthesized materials. Pyridyl oximes and alcohols are broadly used in the field of Single-Molecule Magnets (SMMs) and have proved to stabilize small and high nuclearity metal complexes with interesting magnetic properties; however, they have not been systematically explored in the field of MOFs. Polycarboxylates were one of the first family of linkers used in the field of MOFs for the formation of stable 2D and 3D networks. Combining these two categories of organic linkers, we targeted the isolation of small or high nuclearity novel secondary inorganic units and MOFs with interesting topologies and magnetic properties. The main drawback of investigating mixed-ligand systems is that there are more than one product (bi-products) that can be isolated during this process before the final coordination polymer or MOF is precipitated. As such, mixed ligand systems are not generally high yielding or thermodynamically stable and for this reason, an in-depth understanding and investigation is required to understand the conditions upon which they precipitate/crystallize and to be able to isolate the intermediate products in absence of the final coordination polymer or MOF. This can be achieved by modifying the synthetic conditions. All synthesized compounds were characterized through single-crystal and powder X-ray crystallography, infrared spectroscopy, thermogravimetric analysis, with magnetic studies obtained for relevant products. The capacity of representative MOFs to encapsulate and detect environmentally hazardous species was also investigated.The investigation of the combination of pyrdine-2-amidoxime (pyaoxH2) with 1,4- benzene-dicarboxylic acid (H2btc) and 1,3,5-benezene-tricarboxylic acid (H3btc) led to isolation of one-dimensional coordination polymers [Zn(bdc)(pyaoxH2)(DMF)]n, {[Ni(bdc)(pyaoxH2)2]·2DMF}n, [Mn(bdc)(pyaoxH2)2]·2DMF}n, [Ni(Hbtc)(pyaoxH2)2]n and a mononuclear complex [Ni(Hbtc)(pyaoxH2)2]n]·2H2O. This system was expanded further with the employment of benzene-1,3,5-tricarboxylic acid and pyridine-2-amidoxime (pyaoxH2) or 2-methyl- pyridyl ketoxime (mpkoH) in ZnII and CuII chemistry. Five novel coordination polymers and MOFs have been isolated and characterized, namely [Zn(H2btc)2(pyaoxH2)2]·2H2O, [Zn(Hbtc)(pyaoxH2)2]n, [Cu(Hbtc)(pyaoxH2)]n, [Cu(Hbtc)(mpKoH)]n and [Cu2(Hbtc)2(mpkoH)2(H2O)2]·4H2O. The Dc magnetic susceptibility of [Cu(Hbtc)(pyaoxH2)]n was carried out and revealed the presence of weak exchange interactions between the metal centers; where the experimental data were fitted with a theoretical model and fittings parameter being J = -0.16(1) cm-1 and g = 2.085(1). The isotropic g value was also confirmed by electronic paramagnetic resonance (EPR) spectroscopy. Also, reactivity studies were performed in presence of metal cations.Expanding further the exploration of this unique combination, by the use of a tetra-topic carboxylate, nine new compounds have been isolated and characterized. Among them, [Zn2(pma)(pyaoxH2)2(H2O)2]n and [Cu4(OH)2(pma)(mpko)2]n are the first MOFs of this combination with the latter displaying a novel 3,4,5,8-c net topology. Coordination polymers were isolated through this combination leading to [Zn2(pma)(pyaoxH2)2]n, [Cu2(pma)(pyaoxH2)2(DMF)2]n, and [Cu2(pma)(mpkoH)2(DMF)2]n. Discrete metal complexes were also isolated through this synthetic investigation leading to [Zn(H2pma)(pyaoxH2)(H2O)2], two isostructural dinuclear complexes [M2(pma)(pyaoxH2)2(H2O)6] where MII = Co, Mn and [Zn2(pma)(mpkoH)2(H2O)4]·2H2O. The [Cu4(OH)2(pma)(mpko)2]n is stable under different chemical conditions and for this reason, it was further studied for the encapsulation of Fe3+ ions. The encapsulation of iron was investigated through a variety of techniques, namely, UV-vis, EDX, and magnetism studies. The MOF itself and the loaded MOF were studied magnetically and it was revealed that this could be a nice example for the further investigation of MOFs as magnetic sensors, as it selectively adsorbs Fe3+.As the combination of pyridyl-oximes and carboxylates was studied in-depth, we continued to investigate the behavior of an analogue family of small rigid ligands, pyridine alcohols. The oximic group (-N-OH) of pyridyl-oximes was replaced by the hydroxyl group (-OH) of pyridine alcohols. This combination was introduced for the first time in the field of coordination polymers and MOFs. The simultaneous use of 2-pyridinemethanol (hmpH) with benzene-1,4-dicarboxylic acid (H2bdc) or 1,3,5-benzenetricarboxylic acid (H3btc) led to the isolation of five new coordination compounds [M2(Hbtc)2(Hhmp)4]·DMF (M=CoII; NiII), [Ni(bdc)(Hhmp)2]n·4H2O, [Zn2(bdc)(hmp)2]n·DMF and [Fe3(bdc)3(Hhmp)2]n. [Zn2(bdc)(hmp)2]n·DMF and [Fe3(bdc)3(Hhmp)2]n are the first examples of metal-organic frameworks bearing hmp- while the Zn-MOF exhibits an unprecedented 4,4,4-c net with point symbol {32.103.11}{32.104}2. Regarding the Fe-MOF, this is based on a trinuclear linear FeII secondary building unit and possesses a 3,4,6T2 topology with point symbol {42.83.10}3{43}2{46.86.103}. This compound was magnetically studied and revealed the existence of antiferromagnetic coupling between the FeII cations with a superexchange value of J = -8.46 cm-1.In order to investigate the impact of an additional hydroxyl group on the identity of the isolated species, the pyridinemethanol was replaced by 2,6-pyridinedimethanol. The combination of 2,6-pyridinedimethanol (H2pdm) with 1,4-benzene dicarboxylic acid (H2bdc), led to three new MOFs [M3(bdc)3(Hpdm)2]n·DMF (where MII = Zn, Mn), [Mn3(bdc)3(H2pdm)2]n, and [Mn3(bdc)3(DMF)4]n and a mononuclear metal complex [Ni(H2pdm)2](bdc), which forms a strong 3D-hydrogen bonding network.Overall, the initial combination of 2-pyridyl oximes or alcohols with polycarboxylates provided access to twelve novel MOFs adopting new topologies, eight coordination polymers, and nine metal complexes. Also, it was proved that the [Cu4(OH)2(pma)(mpko)2]n is suitable for the sensing of Fe3+ with sensing capacity 129.89 mg Fe/g MOF.

O στόχος της διατριβής είναι η σύνθεση και χαρακτηρισμός νέων πολυμερών ένταξης από μικτά συστήματα υποκαταστατών και η περαιτέρω μελέτη τους ως μαγνητικοί σένσορες για την απορρόφηση τοξικών ειδών. Για τη σύνθεση τους χρησιμοποιήθηκαν 2-πύριδυλ οξίμες ή πύριδυλ-αλκοόλες προκειμένου να σχηματιστεί/σταθεροποιηθεί η ανόργανη δομική υπομονάδα, καθώς και πολυτοπικοί καρβοξυλικοί υποκαταστάτες (δι-, τρι- και τέτρα- τοπικοί) (1,4-δικαρβοξυλικό βενζόλιο/τερεφθαλικό οξύ, 1,3,5-τρικαρβοξυλικό βενζόλιο/τριμεσικό οξύ, 1,2,4,5-τετρακαρβοξυλικό βενζόλιο/πυρομεταλλικό οξύ. Όλες οι ενώσεις παρείχθησαν υπο σολβο-θερμικές συνθήκες. Η επιλογή των υποκαταστατών είναι σημαντική καθώς επηρεάζει τις ιδιότητες των παραχθέντων ουσιών. Οι πυριδυλ-οξίμες και αλκοόλες έχουν ευρέως χρησιμοποιηθεί στο πεδίο των Μονομοριακών Μαγνητών και έχει αποδειχθεί ότι έχουν την δυνατότητα σταθεροποίησης μικρών και μεγάλων μεταλλικών πλειάδων, οι οποίες παρουσιάζουν ενδιαφέρουσες μαγνητικές ιδιότητες. Παρ' όλα αυτά δεν έχουν συστηματικά χρησιμοποιηθεί και εξερευνηθεί στο πεδίο των Metal-Organic Frameworks (MOFs). Oι πολυκαρβοξυλικοί υποκαταστάτες είναι από τους πρώτους στο πεδίο των MOFs για την παραγωγή πολυμέρων δύο ή τριων διαστάσεων. Συνδυαζόντας αυτές τις δύο κατηγορίες οργανικών υποκαταστατών στοχεύουμε τη σχεδιασμένη απομόνωση μικρών ή μεγάλων ανόργανων υπομονάδων, προκειμένου να παρασκευαστούν MOFs με ενδιαφέρουσες τοπολογίες και μαγνητικές ιδιότητες. Το κύριο μειονέκτημα εξερεύνησης συστημάτων μικτών-υποκαταστατών είναι ότι υπάρχουν περισσότερα από ένα κύρια προϊόντα (παραπροϊόντα) τα οποία μπορούν να απομονωθούν κατά τη διαδικασία σύνθεσης πριν το τελικό πολυμερές ένταξης/ΜΟF. Αυτού του είδους τα συστήματα δεν είναι γενικά υψηλής απόδοσης ή θερμοδυναμικά σταθερά και γι' αυτό το λόγο απαιτείται η βαθύτερη κατανόηση και εξερεύνηση τους προκειμένου να κατανοηθούν οι συνθήκες κατά τις οποίες κρυσταλλώνουν/λαταβυθίζονται κύρια προϊόντα απουσία των παραπροϊόντων. Αυτό επιτυγχάνεται με αλλαγή των πειραματικών συνθηκών. Όλες οι παραγόμενες ενώσεις έχουν χαρακτηριστεί με κρυσταλλογραφία ακτίνων-Χ, ακρινογραφία κόνεως, φασματοσκοπία υπεριώδους, μελέτη μάζας και μαγνητισμό. Επίσης μελετήθηκη απορρόφηση των τοξικών για το περιβάλλον ουσιών.