Design and synthesis of hybrid organic -inorganic molecules for magnetic materials
The focus of this work was the synthesis and coordination of t-butyl nitroxide radicals with various paramagnetic transition metal dications to investigate the magnetic interactions in the products. The crystal structure of each coordination complex was studied to elucidate possible structural pathways for magnetic exchange.
A silicon centered tetraradical, tetrakis(N-oxyl-2,2,6,6-tetramethylpiperidin-4-oxyl)silane, was synthesized and coordinated to various metal ions to yield one-dimensional coordination polymers joined through nitroxide-metal-nitroxide (3-spin) units. The Mn(II) complex showed strong antiferromagnetic (AFM) coupling within the 3-spin unit, while the Cu(II) complex showed modest ferromagnetic (FM) coupling within the unit. Both complexes showed weak AFM interactions between 3-spin units.
The new radical 5-(3-[N-tert-butyl-N-aminoxyl]phenyl)pyrimidine was synthesized and coordinated to metal dications, and formed both cyclic 2:2 dimers and extended network complexes that were isolated as stable crystalline solids. The Mn(II) dimer showed strong AFM exchange within the dimer and weak FM exchange between dimer units, based on magnetic analysis by a rectangular four-spin model. A linear Curie-Weiss model for the Co(II) dimer showed strong AFM exchange within these dimer units. The Cu(II) dimer exhibited crystallographic Jahn-Teller distortion accompanied by reversible solid-state thermochromic effects that correlate qualitatively with changes in magnetic behavior with temperature.
5-(3-[N-tert-Butyl-N-aminoxyl]phenyl)pyrimidine formed zero- to three-dimensional complexes with Cu(hfac)2. Most of these were 3:2 Cu:radical complexes with different magnetic behaviors. Two-dimensional complex 25 shows a slight ferromagnetic rise, then antiferromagnetic downturn in the χT(T) plot; it also exhibits multistep behavior in the M( H) plot, with a magnetic moment higher than any comparable complex in this series. Two-dimensional complex 26 undergoes a reversible crystal lattice change from orthorhombic to monoclinic, and shows strong AFM coupling between metal-radical units, with additional AFM coupling between units. Three-dimensional complex 27 shows multi-stage magnetic M(H) behavior. Jahn-Teller distortion of Cu-radical units was noted in complexes 26 and 27.
3-(N-tert-butyl-N-aminoxyl)benzoic acid was isolated as a stable crystalline solid that forms one-dimensional chains of hydrogen bonded dimers. The radical shows antiferromagnetic exchange behavior that could be roughly fit to a Bleaney-Bowers spin pairing model. The radical adsorbs strongly but reversibly to silica based materials from nonpolar solvents.