Molecular structure, as well as optical and magnetic properties, of coordination complexes comprising the organic dye flavanthrone with transition metals (MnII, CrIII, CrIV) and lanthanides (DyIII, GdIII)

The reduction of the organic dye flavanthrone (Vat Yellow 1) to the dianion state in the presence of metal-containing compounds yields coordination complexes in which two metal fragments coordinate to flavanthrone dianions to form short M − O bonds. Flavanthrone dianion coordinates to two manganese(II) acetylacetonate units in {flavanthrone⋅[MnII(acac)2]2}2− (1) with an average Mn–O bond length of 2.048(3) Å. Two dysprosium(III) or gadolinium(III) 2,2,6,6-tetramethyl-3,5-heptanedionate (TMHD) units form longer M − O bonds in {cryptand(Cs+)}2{flavanthrone⋅[DyIII(TMHD)3]2}2− (2) and {cryptand(K+)}2{flavanthrone⋅[GdIII(TMHD)3]2}2−⋅2C6H4Cl2⋅ 2C6H14 (3), with bond lengths of 2.192(2) and 2.241(4) Å, respectively. Furthermore, the utilization of decamethylchromocene as a reductant yields the unusual neutral {(flavanthrone2−)⋅[Cp*CrIII(acac)]2} comprising acac anion (4) and {(flavanthrone2−)⋅[Cp*CrIVCl2]2} comprising two chloride anions (5). Both have zwitter-ionic nature in which negative charge of flavanthrone dianions is compensated by chromium countercations. They coordinate to two chromium-containing units with short Cr–O bonds of 1.937(2) and 1.962(5) Å, respectively. The formation of flavanthrone dianions in 1–5 is confirmed by their optical spectra, the disappearance of the bands corresponding to the C=O vibrations in the infrared spectra of the complexes, and the elongation of the carbon–oxygen bonds during the formation of 1–5. The formation of binuclear, paramagnetic species with the flavanthrone dianions induces weak antiferromagnetic couplings between the paramagnetic centers because the flavanthrone dianions are diamagnetic. Stronger couplings with exchange parameters of −0.44 and −0.55 cm−1 were observed for the chromium ions bearing the shortest M − O bonds for 5 and 4, respectively. Conversely, MnII and lanthanides (GdIII, DyIII) comprising longer M − O bonds exhibit smaller J values (close to zero in 3 or equal to −0.026 and −0.05 cm−1 in 2 and 1, respectively). The complexes comprising MnII, GdIII, and CrIII display electron paramagnetic resonance (EPR) signals. The signal from MnII is approximated considering that the exchange interaction between the two MnII ions is −0.05 cm−1. Well resolved zero-field splitting (ZFS) is displayed; it facilitates the determination of the positive ZFS parameter, D, of +0.83 cm−1 and the unusually high E value, +0.62 cm−1. The spectrum of 3 comprising two GdIII ions containing 12 lines was approximated with D and E values of −0.083 and −0.021 cm−1, respectively. The odd-spin CrIII (S = 3/2) display a broad, asymmetric EPR signal at g = 3.77 without ZFS, whereas the even-spin CrIV is EPR-silent. Taken together, this study reveals that the dianions of organic dyes represent promising bidentate ligands for preparing coordination complexes with paramagnetic d- and f-block metal ions.