Synthesis and Characterization of a High-Symmetry Ferrous Polypyridyl Complex: Approaching the 5T2/3T1 Crossing Point for FeII

Electronic structure theory predicts that, depending on the strength of the ligand field, either the quintet (5T2) or triplet (3T1) term states can be stabilized as the lowest-energy ligand-field excited state of low-spin octahedral d6 transition-metal complexes. The 3T1 state is anticipated for second- and third-row metal complexes and has been established for certain first-row compounds such as [Co(CN)6]3–, but in the case of the widely studied FeII ion, only the 5T2 state has ever been documented. Herein we report that 2,6-bis(2-carboxypyridyl)pyridine (dcpp), when bound to FeII, presents a sufficiently strong ligand field to FeII such that the 5T2/3T1 crossing point of the d6 configuration is approached if not exceeded. The electrochemical and photophysical properties of [Fe(dcpp)2]2+, in addition to being of fundamental interest, may also have important implications for solar energy conversion strategies that seek to utilize earth-abundant components.