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'-dipyri-dine-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, highspin state (⁵T₂, (t_2g)⁴(e_g×)²) and a low-spin ground state (¹A₁, (t_2g)⁶(e_g×)⁰). The intermediate triplet spin state (³T₁, (t_2g)⁵(e_g×)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, Möβbauer 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.