Spin state variability in Fe2+ complexes of substituted (2-(pyridin-2-yl)-1,10-phenanthroline) ligands as versatile terpyridine analoguesElectronic supplementary information (ESI) available. CCDC 1519515-1519519 and 1538100. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c7dt00422b

Fe 2+ spin crossover complexes [Fe( L ) 2 ] 2+ (L = 2-(6-R 1 -pyridin-2-yl)-1,10-phenanthroline with R 1 = H, methoxy, bromo, -(1 H -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 1 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 1 ) 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 1 of the proton spins. The larger R 1 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 Fe 3+ complexes in a chemically reversible fashion. Interestingly, the lowest oxidation potential was observed for the N8 coordinated complex. Fe 2+ spin crossover complexes [Fe( L ) 2 ] 2+ (L = substituted (pyridin-2-yl)-1,10-phenanthroline) were prepared and SCO properties were investigated in solution and in the solid state by an experiment and in silico .