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

Abstract Truxenone (Tr) with high C 3h symmetry forms crystalline radical anion and dianion salts. The {Bu 3 MeP + }(Tr⋅ − ) ( 1 ) and {Cp* 2 Co + )}(Tr⋅ − )⋅2 C 6 H 4 Cl 2 ( 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* 2 Co + }{Tb III (TMHD) 3 ⋅Tr} ‐ ⋅1.5 C 7 H 8 ( 4 ) and {Cp* 2 Co + }{Mn II (acac) 2 ⋅Tr} ‐ ⋅2.4 C 6 H 4 Cl 2 ( 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 M T 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.