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 (T-5(2)) or triplet (T-3(1)) term states can be stabilized as the lowest-energy ligand-field excited state of low-spin octahedral d(6) transition-metal complexes. The T-3(1) 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 Fe-II ion, only the T-5(2) state has ever been documented. Herein we report that 2,6-bis(2-carboxypyridyl)pyridine (dcpp), when bound to Fe-II, presents a sufficiently strong ligand field to Fe-II such that the T-5(2)/T-3(1) crossing point of the d(6) 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.