Spin state variability in Fe2+ complexes of substituted (2-(pyridin-2-yl)-1,10-phenanthroline) ligands as versatile terpyridine analogues
Fe2+ spincrossover complexes [Fe(L)2]2+ (L = 2-(6-R1-pyridin-2-yl)-1,10-phenanthroline with R1 = 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 T1/2 ranging from 130 K to 600 K depending on the substitution pattern. The use of 1H NMR spectroscopy to elucidate thermodynamics and kinetics of SCO in solution of this series is described in detail. By introduction of an additional pyrazole donor (R1) in ortho-position to the pyridine, the N6 octahedral coordination sphere is expanded to N8 coordination with a trigonal dodecahedral structure. That leads to a strong stabilization of the high spin state and an increased longitudinal relaxation R1 of the proton spin. The larger R1 were ascribed to different electronic structure with non-orbital degenerate quintet ground states and larger energetic separation from the first exited 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 T1/2 and allowed the calculation of the complex structure in experimentally non accessible spin states. Complexes of this series can be oxidized to the Fe3+ complexes in a chemically reversible fashion. Interestingly, the lowest oxidation potential was observed for the N8 coordinated complex.