Exploring coordination chemistry and reactivity of formazanate ligands

Een groot aantal industriële chemische processen maakt gebruik van katalysatoren op basis van dure edelmetalen (goud, platina, iridium etc.), die in staat zijn bindingen in moleculen te maken/breken door 2 elektronen op te nemen of af te staan. Tegenwoordig wordt er veel onderzoek gedaan naar zogenaamde ‘redox-actieve’ liganden, simpele organische verbindingen die deze rol kunnen overnemen, waardoor het mogelijk wordt goedkopere, meer duurzame alternatieven te ontwikkelen bijvoorbeeld met metalen zoals magnesium, calcium, zink of ijzer. In dit proefschrift worden de eigenschappen van het redox-actieve ligand ‘formazanaat’ beschreven. We bestuderen de synthese, coördinatiechemie en toepassing van formazanaat liganden in complexen met verschillende goedkope metalen. Formazanaat complexen met magnesium en calcium zijn getest als katalysatoren voor de polymerisatie van lactide naar biologisch afbreekbare plastics. In een andere toepassing laten we zien dat deze formazanaat-moleculen kunnen worden omgezet naar nieuwe, stabiele moleculaire schakelaars. De mogelijkheid om de eigenschappen van een molecuul te schakelen met licht is interessant in verschillende gebieden, zoals biomedische toepassingen en informatieopslag. Daarnaast heeft dit onderzoek een nieuw type formazanaat-ijzer moleculen opgeleverd waarvan de eigenschappen (magnetisme, reactiviteit) gestuurd kunnen worden door verandering in temperatuur. Deze nieuwe toepassingen zijn toe te schrijven aan de ongebruikelijke (elektronische) eigenschappen van het formazanaat ligand. Het onderzoek laat nieuwe manieren zien om de eigenschappen/reactiviteit te controleren van moleculen die gebaseerd zijn op goedkope, duurzame alternatieven voor edelmetalen.Many reactions in industrial processes require expensive metals as catalysts, such as gold, platinum, iridium, etc., which take or give two electrons in order to break/make chemical bonds. However, nowadays there are extensive researches on so-called 'redox-active' ligands because these can store electrons and do the job of these expensive metals, using instead cheap and abundant metals such as magnesium, calcium, zinc or iron.In this thesis we explore the properties of the redox-active ligand 'formazanate'. We study the synthesis, coordination chemistry and application of several formazanate ligands in complexes with different earth-abundant metals. With formazanate complexes of magnesium and calcium, we examined their properties as catalysts for polymerization of lactide to produce biodegradable plastics. In another application, we show how formazanate-containing molecules can be used to synthesize new, stable light-switchable molecules. The ability to change molecular properties with light is interesting in various fields, including biomedical applications and information storage. In addition, in this research we have developed molecules with formazanate ligands bound to an iron center for which the properties (magnetism, reactivity) can be switched by varying temperature. These newly developed applications derive from the unusual (electronic) properties of the formazanate ligand. The research suggests novel ways to control the properties/reactivity of molecules containing cheap, abundant alternatives to the noble metals

Three-Coordinate Zinc Methyl Complexes with Sterically Demanding Formazanate Ligands

A series of heteroleptic three-coordinate mono(formazanate)zinc methyl complexes were synthesized, and the influence of the ligand on the structure as well as redox and optical properties of these complexes was investigated. The heteroleptic mono(formazanate)zinc methyl complexes were found to show ligand redistribution in solution, reminiscent of the Schlenk equilibrium, to generate an equilibrium mixture containing the corresponding homoleptic complexes as well. Monitoring the approach to equilibrium by NMR spectroscopy in benzene-d(6) allowed determination of the forward and backward rate constants. A correlation was found between the steric environment around the zinc center and equilibrium concentration of (formazanate)zinc methyl compounds, whereas the kinetics for approach to equilibrium are also dependent on the electronic properties

Spin-Crossover in a Pseudo-tetrahedral Bis(formazanate) Iron Complex

Spin-crossover in a pseudo-tetrahedral bis(formazanate) iron(II) complex (1) is described. Structural, magnetic, and spectroscopic analyses indicate that this compound undergoes thermal switching between an S=0 and an S=2 state, which is very rare in four-coordinate complexes. The transition to the high-spin state is accompanied by an increase in Fe-N bond lengths and a concomitant contraction of intraligand N-N bonds. The latter suggests that stabilization of the low-spin state is due to the π-acceptor properties of the ligand. One-electron reduction of 1 leads to the formation of the corresponding anion, which contains a low-spin (S=1/2) Fe(I) center. The findings are rationalized by electronic structure calculations using density functional theory

Perfluoroaryl‐elemental sulfur SNAr chemistry in covalent triazine frameworks with high sulfur contents for lithium–sulfur batteries

In order to address the challenges associated with lithium–sulfur batteries with high energy densities, various approaches, including advanced designs of sulfur composites, electrolyte engineering, and functional separators, are lately introduced. However, most approaches are effective for sulfur cathodes with limited sulfur contents, i.e., <80 wt%, imposing a significant barrier in realizing high energy densities in practical cell settings. Here, elemental sulfur-mediated synthesis of a perfluorinated covalent triazine framework (CTF) and its simultaneous chemical impregnation with elemental sulfur via SNAr chemistry are demonstrated. SNAr chemistry facilitates the dehalogenation and nucleophilic addition reactions of perfluoroaryl units with nucleophilic sulfur chains, achieving a high sulfur content of 86 wt% in the resulting CTF. The given sulfur-impregnated CTF, named SF-CTF, exhibits a specific capacity of 1138.2 mAh g−1 at 0.05C, initial Coulombic efficiency of 93.1%, and capacity retention of 81.6% after 300 cycles, by utilizing homogeneously distributed sulfur within the micropores and nitrogen atoms of triazine units offering high binding affinity toward lithium polysulfides

An Octanuclear Metallosupramolecular Cage Designed To Exhibit Spin-Crossover Behavior.

By employing the subcomponent self-assembly approach utilizing 5,10,15,20-tetrakis(4-aminophenyl)porphyrin or its zinc(II) complex, 1H-4-imidazolecarbaldehyde, and either zinc(II) or iron(II) salts, we were able to prepare O-symmetric cages having a confined volume of ca. 1300 Å3 . The use of iron(II) salts yielded coordination cages in the high-spin state at room temperature, manifesting spin-crossover in solution at low temperatures, whereas corresponding zinc(II) salts led to the corresponding diamagnetic analogues. The new cages were characterized by synchrotron X-ray crystallography, high-resolution mass spectrometry, and NMR, Mössbauer, IR, and UV/Vis spectroscopy. The cage structures and UV/Vis spectra were independently confirmed by state-of-the-art DFT calculations. A remarkably high-spin-stabilizing effect through encapsulation of C70 was observed. The spin-transition temperature T1/2 is lowered by 20 K in the host-guest complex

Reversible ligand-centered reduction in low- coordinate iron formazanate complexes

Coordination of redox‐active ligands to metals is a compelling strategy for making reduced complexes more accessible. In this work, we explore the use of redox‐active formazanate ligands in low‐coordinate iron chemistry. Reduction of an iron(II) precursor occurs at milder potentials than analogous non‐redox‐active β‐diketiminate complexes, and the reduced three‐coordinate formazanate‐iron compound is characterized in detail. Structural, spectroscopic, and computational analysis show that the formazanate ligand undergoes reversible ligand‐centered reduction to form a formazanate radical dianion in the reduced species. The less negative reduction potential of the reduced low‐coordinate iron formazanate complex leads to distinctive reactivity with formation of a new N−I bond that is not seen with the β‐diketiminate analogue. Thus, the storage of an electron on the supporting ligand changes the redox potential and enhances certain reactivity

Alkali metal salts of formazanate ligands:diverse coordination modes as a result of the nitrogen-rich [NNCNN] ligand backbone

Alkali metal salts of redox-active formazanate ligands were prepared, and their structures in the solid-state and in solution are determined. The nitrogen-rich [NNCNN] backbone of formazanates results in a varied coordination chemistry, with both the internal and terminal nitrogen atoms available for bonding with the alkali metal. The potassium salt K[PhNNC(p-tol)NNPh]center dot 2THF (1-K) is dimeric in the solid state and even in THF solution, as a result of the K atom bridging via interaction with a terminal N atom and the aromatic ring of a second unit. Conversely, for the compounds Na[MesNNC(CN)NNMes]center dot 2THF (2-Na) and Na[PhNNC(Bu-t)NNPh] (3-Na) polymeric and hexameric structures are found in the solid state respectively. The preference for binding the alkali metal through internal N atoms (1-K and 2-Na) to give a 4-membered chelate, or via internal/external N atoms (5-membered chelate in 3-Na), contrasts with the 6-membered chelate mode observed in our recently reported formazanate zinc complexes

CCDC 1020940: Experimental Crystal Structure Determination

Related Article: Raquel Travieso-Puente, Mu-Chieh Chang, Edwin Otten|2014|Dalton Trans.|43|18035|doi:10.1039/C4DT02578D,An entry from the Cambridge Structural Database, the world’s repository for small molecule crystal structures. The entry contains experimental data from a crystal diffraction study. The deposited dataset for this entry is freely available from the CCDC and typically includes 3D coordinates, cell parameters, space group, experimental conditions and quality measures.