Panchromatic Absorption and Oxidation of an Iron(II) Spin Crossover Complex

In order to expand and exploit the useful properties of d⁶-iron­(II) and d⁵-iron­(III) complexes in potential magnetic, photophysical, or magnetooptical applications, crucial ligand-controlled parameters are the ligand field strength in a given coordination mode and the availability of suitable metal and ligand frontier orbitals for charge-transfer processes. The push–pull ligand 2,6-diguanidylpyridine (dgpy) features low-energy π* orbitals at the pyridine site and strongly electron-donating guanidinyl donors combined with the ability to form six-membered chelate rings for optimal metal–ligand orbital overlap. The electronic ground states of the pseudo-octahedral d⁶- and d⁵-complexes mer-[Fe­(dgpy)₂]²⁺, cis-fac-[Fe­(dgpy)₂]²⁺, and mer-[Fe­(dgpy)₂]³⁺ as well as their charge-transfer (CT) and metal-centered (MC) excited states are probed by variable temperature UV/vis absorption, NMR, EPR, and Mössbauer spectroscopy, magnetic susceptibility measurements at variable temperature as well as quantum chemical calculations.