Spin state variability in Fe²⁺ complexes of substituted (2-(pyridin-2-yl)-1,10-phenanthroline) ligands as versatile...

Fe²⁺ spin crossover 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 T_1/2 ranging from 130 K to 600 K depending on the substitution pattern. The use of ¹H 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 (R¹) 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 R₁ of the proton spins. The larger R₁ 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_1/2 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.