Tetra-methyl anthracene-2,3,6,7-tetra-carboxyl-ate-tetra-methyl 9,10-dihydro-9,10-dioxoanthracene-2,3,6,7-tetra-carboxyl-ate (1/1).

In the title co-crystal, C(22)H(16)O(10)·C(22)H(18)O(8), the independent tetra-methyl 9,10-dihydro-9,10-dioxoanthracene-2,3,6,7-tetra-carboxyl-ate, (I), and tetra-methyl anthracene-2,3,6,7-tetra-carboxyl-ate, (II), components occupy separate crystallographic inversion centers. In (II), the dihedral angles between the mean aromatic plane and the two independent carboxyl-ate planes are 41.32 (10) and -38.35 (10)°. The methyl-carboxyl-ate groups of (I) are disordered, with each resolvable into two groups. In the least disordered carboxyl-ate, the apparent angles between the mean aromatic plane and the two partial carboxyl-ate 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 carboxyl-ate 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(carbox-yl) bond.