Make One of Naught—Switchable Complexes of Novel Pyridine-Based Podands

Abstract

Molecules with switchable properties are of growing interest for sensor technology, high- density data storage and processing. In particular, metal complexes with incompletely filled d- or f-shells exhibit attractive properties for their design.
To gain access to novel smart contrast agents for magnetic-resonance imaging (MRI), dipodal 2-methyl-2-(pyridin-2-yl)propane-1,3-diamine, tripodal 2-[6-(1-amino-2-methyl­propane-2-yl)-pyridin-2-yl]-2-me­thyl­propane-1,3-diamine and tetrapodal 2,2′-(pyridin-2,6-diyl)bis(2-methyl­propane-1,3-diamine) were attached to known derivatives of 1-hydroxy-2(1H)-pyridone (1,2-HOPO) through amide groups. The intermediates and products were characterised comprehensively. Lanthanum(III) model-complexes of the products were insoluble in aqueous media or potentially coordinatively unstable. Efforts to synthesise 1,2-HOPO chelators ether-linked to podands were unsuccessful. 3,6-Substituted 1,2-HOPO derivatives, as necessary to tether pH-sensitive reporting groups, proved to be inaccessible via 3-hydroxy-2(1H)-pyrone-6-carboxylic acid or ring-closing metathesis (RCM).
For application in spintronics, 1-{6-[1,1-di(pyridin-2-yl)ethyl]pyridin-2-yl}-N,N-di­methyl­methan­amine (L) was produced as N₄ chelator for light d-metals. Starting with 2,2′-[1-(6-bromopyridin-2-yl)ethane-1,1-diyl]dipyridine, the versatile mixed amine-pyridine ligand was synthesised via copper-catalysed cyanation, palladium-catalysed hydrogenation and subsequent Eschweiler–Clarke dimethylation. Intermediates, by-products, and products were characterised exhaustively. By addition to metal salts or simple precursors, the complexes [M^IICl₂L] (M = Mn, Fe, Co, Ni, Zn, Ru), [Cu^IIIL(MeCN)]PF₆, [Cu^IIClL]Cl, [Fe^II(CN)₂L], [Fe^II(NCS)₂L], as well as [Fe^IIICl₂L]PF₆ were produced and fully characterised. Comparative crystal-structure analyses revealed the coordination environment to be ideal for low-spin iron(II). [Cu^IL(MeCN)]PF₆ showed signs of catalytic N-demethylation and dehydrogenation activity making it a potential model for peptidylglycine α-hydroxylating monooxygenase (PHM) or copper-containing amine oxidases. The solvatochromism of [Fe^II(CN)₂L], typical of its substance class, was comprehensively investigated. When deposited on TiO₂ particles, the complex induced photoelectrochemical photocurrent switching (PEPS) dependent on potential and wavelength. However, the complex decomposed after oxidation and proved to be poorly adsorbed. [Fe^II(NCS)₂L] showed a reversible, gradual, complete, one-step spin-crossover (SCO) without hysteresis as powder, single crystal and in solution. For single crystals, the transition was correlated with molecular structural features evaluating the thermal-expansion tensor. The complex was deposited on highly oriented pyrolytic graphite (HOPG) in the form of a submonolayer using ultra-high vacuum (UHV). [Fe^II(NCS)₂L] is the first 2D-ordered complex shown to be capable of an SCO in direct contact with a surface, as monitored by X-ray absorption spectroscopy (XAS).

Dr. Dennis Wiedemann

Technical Consultant for Environmental Issues
Chemist · Crystallographer

I hold a doctorate in chemistry and provide advice concerning permits in German immission and water protection law.