Tetra­methyl anthracene-2,3,6,7-tetra­carboxyl­ate–tetra­methyl 9,10-dihydro-9,10-dioxoanthracene-2,3,6,7-tetra­carboxyl­ate (1/1)

In the title co-crystal, C₂₂H₁₆O₁₀·C₂₂H₁₈O₈, the independent tetra­methyl 9,10-dihydro-9,10-dioxoanthracene-2,3,6,7-tetra­carboxyl­ate, (I), and tetra­methyl anthracene-2,3,6,7-tetra­carboxyl­ate, (II), components occupy separate crystallographic inversion centers. In (II), the dihedral angles between the mean aromatic plane and the two independent carboxyl­ate planes are 41.32 (10) and −38.35 (10)°. The methyl­carboxyl­ate groups of (I) are disordered, with each resolvable into two groups. In the least disordered carboxyl­ate, the apparent angles between the mean aromatic plane and the two partial carboxyl­ate planes [site occupations = 0.510 (3) and 0.490 (3)] are 16.8 (3) and 23.3 (3)°. In the highly disordered group, the apparent angles between the mean aromatic plane and the two partial carboxyl­ate planes [site occupations = 0.510 (3) and 0.490 (3)] are 78.3 (3) and −74.1 (3)°. In addition, this extreme disorder leads to an artificially elongated C(aromatic)—C(carbox­yl) bond.