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, (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, 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.
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- Charge transfer
- Excited states
- Ligand field splitting
- Tridentate ligands
ASJC Scopus subject areas
- Organic Chemistry