A 1:2 co-crystal of 2,2′-thiodi­benzoic acid and tri­phenyl­phosphane oxide: crystal structure, Hirshfeld surface analysis and computational study

The asymmetric unit of the title co-crystal, 2,2′-thiodi­benzoic acid–tri­phenyl­phosphane oxide (1/2), C(14)H(10)O(4)S·2C(18)H(15)OP, comprises two mol­ecules of 2,2′-thiodi­benzoic acid [TDBA; systematic name: 2-[(2-carb­oxy­phen­yl)sulfan­yl]benzoic acid] and four mol­ecules of tri­phenyl­phosphane oxide [TPPO; systematic name: (di­phenyl­phosphor­yl)benzene]. The two TDBA mol­ecules are twisted about their di­sulfide bonds and exhibit dihedral angles of 74.40 (5) and 72.58 (5)° between the planes through the two SC(6)H(4) residues. The carb­oxy­lic acid groups are tilted out of the planes of the rings to which they are attached forming a range of CO(2)/C(6) dihedral angles of 19.87 (6)–60.43 (8)°. Minor conformational changes are exhibited in the TPPO mol­ecules with the range of dihedral angles between phenyl rings being −2.1 (1) to −62.8 (1)°. In the mol­ecular packing, each TDBA acid mol­ecule bridges two TPPO mol­ecules via hy­droxy-O—H⋯O(oxide) hydrogen bonds to form two three-mol­ecule aggregates. These are connected into a three-dimensional architecture by TPPO-C—H⋯O(oxide, carbon­yl) and TDBA-C—H⋯(oxide, carbon­yl) inter­actions. The importance of H⋯H, O⋯H/H⋯O and C⋯H/H⋯C contacts to the calculated Hirshfeld surfaces has been demonstrated. In terms of individual mol­ecules, O⋯H/H⋯O contacts are more important for the TDBA (ca 28%) than for the TPPO mol­ecules (ca 13%), as expected from the chemical composition of these species. Computational chemistry indicates the four independent hy­droxy-O—H⋯O(oxide) hydrogen bonds in the crystal impart about the same energy (ca 52 kJ mol(−1)), with DTBA-phenyl-C—H⋯O(oxide) inter­actions being next most stabilizing (ca 40 kJ mol(−1)).