A Heteroleptic Push-Pull Substituted Iron(II) Bis(tridentate) Complex with Low-Energy Charge-Transfer States

A heteroleptic iron(II) complex [Fe(dcpp)(ddpd)]2+ with a strongly electron‐withdrawing ligand (dcpp, 2,6‐bis(2‐carboxypyridyl)pyridine) and a strongly electron‐donating tridentate tripyridine ligand (ddpd, N,N′‐dimethyl‐N,N′‐dipyridine‐2‐yl‐pyridine‐2,6‐diamine) is reported. Both ligands form six‐membered chelate rings with the iron center, inducing a strong ligand field. This results in a high‐energy, high‐spin state (5T2, (t2g)4(eg*)2) and a low‐spin ground state (1A1, (t2g)6(eg*)0). The intermediate triplet spin state (3T1, (t2g)5(eg*)1) is suggested to be between these states on the basis of the rapid dynamics after photoexcitation. The low‐energy π* orbitals of dcpp allow low‐energy MLCT absorption plus additional low‐energy LL′CT absorptions from ddpd to dcpp. The directional charge‐transfer character is probed by electrochemical and optical analyses, Mößbauer spectroscopy, and EPR spectroscopy of the adjacent redox states [Fe(dcpp)(ddpd)]3+ and [Fe(dcpp)(ddpd)]+, augmented by density functional calculations. The combined effect of push–pull substitution and the strong ligand field paves the way for long‐lived charge‐transfer states in iron(II) complexes. Iron blue dyes: The novel photostable low‐spin FeII complex 22+ (see figure) with two push–pull substituted oligopyridine ligands features highly directional MLCT and LL′CT absorptions extending into the near‐infrared region. Electrochemistry, EPR, and Mößbauer spectroscopy, as well as DFT calculations, confirm iron‐based oxidation and ligand‐centered reduction processes. Relaxation to the ground state after CT excitation seems to circumvent the usual path of FeII complexes via the high‐spin state.