Fe spin crossover complexes [Fe(L)] (L = 2-(6-R-pyridin-2-yl)-1,10-phenanthroline with R = H, methoxy, bromo, -(1H-pyrazol-1-yl) or L = 2-(3-methoxy-pyridin-2-yl)-1,10-phenanthroline) were prepared. These air stable and durable complexes show SCO behaviour with very different transition temperatures T ranging from 130 K to 600 K depending on the substitution pattern. The use of H NMR spectroscopy to elucidate the thermodynamics and kinetics of SCO in a solution of this series is described in detail. By introduction of an additional pyrazole donor (R) in the ortho-position to the pyridine, the N6 octahedral coordination sphere is expanded to N8 coordination with a trigonal dodecahedral structure. This leads to a strong stabilization of the high spin state and an increased longitudinal relaxation R of the proton spins. The larger R values were ascribed to different electronic structures with non-orbital degenerate quintet ground states and a larger energetic separation from the first excited state. These results are also supported by Mössbauer spectroscopy. The N8 coordination sphere stabilizes the complex in the high spin state and no indication for SCO was found. DFT calculations confirmed the experimentally obtained order of T and allowed the calculation of the complex structure in experimentally non-accessible spin states. Complexes of this series can be oxidized to the Fe complexes in a chemically reversible fashion. Interestingly, the lowest oxidation potential was observed for the N8 coordinated complex.
Condensation and subsequent reduction of 2,2′‐diaminobiphenyls 5 with 6′‐ and 5′‐substituted 6‐carbaldehyde‐2,2′‐bipyridines 4 yielded N,N′‐bis(2,2′‐bipyridine‐6‐ylmethyl)‐2,2′‐biphenyl‐enediamines 7, which were employed as hexadendate ligands with N6 donor sets in the synthesis of dicationic [Fe2+(7‐κ6N)] complexes 8. Dependent on the substitution pattern the respective complexes are found in the HS state (8b and 8c) or show SCO behaviour. By means of temperature‐dependent susceptibility measurements, usingEvans' method, the thermodynamic parameters ΔH, ΔS and T1/2 for the SCO have been determined. T1/2 as well as ΔH are remarkably susceptible to substitution next to the central C–C bond of the biphenyl bridge.
The novel hexadentate nitrogen based ligand N,N'-bis-(2-(1H-pyrazol-1-yl)pyridine-6-ylmethyl)-2,2'-biphenylenediamine (3) was synthesized and used for the preparation of iron Spin Crossover (SCO) complexes [Fe(3)][BF4]2 (4) and [Fe(3)][ClO4]2 (5), which differ only by the respective counter ion. These complex salts show different spin transition temperatures T(1/2) (135 and 157 K, respectively). This effect was studied by the investigation of the solid state structure of different low- and high-spin isomers. All complexes of this series show closely related crystal packing regardless of the counter ion, metal (Zn/Fe) and spin state. The isomer exhibiting the lower transition temperature (4) was also investigated in respect to its photomagnetic behaviour. The LIESST process could be monitored for this complex, but no reverse-LIESST was observed. The relaxation of the photo-induced state occurs at ca. 80 K, showing a complex, three-state relaxation mechanism.
The novel biphenyl‐based ligand N,N′,N″,N″′‐tetrakis(2,2′‐bipyridin‐6‐ylmethyl)‐4,4′‐dimethyl‐1,1′‐biphenyl‐2,2′,6,6′‐tetramine (6) with 12 nitrogen donor atoms has been synthesized. Ligand 6 is able to host two metal ions in an octahedral coordination sphere. This was shown by the synthesis of the dinuclear iron(II) complex 5 ([Fe2(6)]4+). Furthermore, the corresponding mononuclear iron(II) complex 4 ([Fe(6)]2+), where one coordination site is free, was obtained. Both FeII complexes show spin‐crossover behaviour, which was shown by 1H NMR spectroscopy and Evans' measurements. A comparison of the two complexes gives evidence of a sterically induced cooperative behaviour in the dinuclear complex 5.
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