Complexes of d- and f-metal Ions with a Redox-Active and Highly Symmetric Truxenone Ligand: Effect of Cations and Coordination on Distortions of Radical Anions and Singlet-Triplet Transitions in Dianions

Truxenone (Tr) with high C_3h symmetry forms crystalline radical anion and dianion salts. The {Bu₃MeP⁺}(Tr⋅⁻) (1) and {Cp*₂Co⁺)}(Tr⋅⁻)⋅2 C₆H₄Cl₂ (2) show intense low-energy absorption up to 2000 nm. Calculations predict Jahn–Teller distortions for Tr⋅⁻, which are enhanced by the asymmetric approach of cations to Tr⋅⁻, leading to C=O bond length alternation and EPR signal asymmetry in 2. Two cesium ions form short contacts of 3.07–3.45 Å with oxygen atoms of Tr^2- in {Cryptand[2.2.2](Cs⁺)}₂(Tr^2-) (3), linking the dianion into a 1D polymer. Calculations suggest a triplet state for the dianion, but the unsymmetric approach of Cs⁺ to the oxygen atoms of Tr^2- stabilizes the singlet ground state. The triplet state is populated above 100 K. An estimated singlet–triplet energy gap is 344 ± 7 K. Radical anions of Tr⋅⁻ coordinate with one d- or f-metal, forming {Cp*₂Co⁺}{Tb^III(TMHD)₃⋅Tr}^-⋅1.5 C₇H₈ (4) and {Cp*₂Co⁺}{Mn^II(acac)₂⋅Tr}^-⋅2.4 C₆H₄Cl₂ (5) (TMHD is 2,2,6,6-tetramethyl-3,5-heptanedionate; acac is acetylacetonate). Antiferromagnetic Tb^III–Tr⋅⁻ coupling is observed in 4. The χ_MT value of 3.32 emu⋅K/mol at 300 K for 5 with high-spin Mn^II is lower than expected. That indicates a spin state of S=2 owing to the antiparallel alignment of Mn^II and Tr⋅⁻ spins. Strong Mn^II–Tr⋅⁻ coupling arises from the spin density localization on the oxygen coordinated to Mn(II). Complexes with metals demonstrate low-energy absorption with maxima at 1504–1740 nm.