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, highspin state (T-5(2), (t(2g))(4)(e(g)*)(2)) and a low-spin ground state ((1)A(1), (t(2g))(6)(e(g)*)(0)). The intermediate triplet spin state (T-3(1), (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 pi* or- bitals 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, Mossbauer 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.