K.C. Mengel, C. Förster, A. Breivogel, K. Mack, J.R. Ochsmann, F. Laquai, V. Ksenofontov and K.
Chemistry - A European Journal 21(2), 704-714, (2015)
Charge transfer, Excited states, Iron, Ligand field splitting, Tridentate ligands
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.