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 C3h 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 Tr2‐ in {Cryptand[2.2.2](Cs+)}2(Tr2‐) (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 Tr2‐ 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+}{TbIII(TMHD)3⋅Tr}‐⋅1.5 C7H8 (4) and {Cp*2Co+}{MnII(acac)2⋅Tr}‐⋅2.4 C6H4Cl2 (5) (TMHD is 2,2,6,6‐tetramethyl‐3,5‐heptanedionate; acac is acetylacetonate). Antiferromagnetic TbIII–Tr⋅− coupling is observed in 4. The χMT value of 3.32 emu⋅K/mol at 300 K for 5 with high‐spin MnII is lower than expected. That indicates a spin state of S=2 owing to the antiparallel alignment of MnII and Tr⋅− spins. Strong MnII–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. [ABSTRACT FROM AUTHOR]