Structural systematics in isomorphous binary co-crystal solvates comprising 2,2′-dithiodibenzoic acid, 4-halobenzoic acid and dimethylformamide (1 : 1 : 1), for halide = chloride, bromide and iodide

Abstract

The 1 : 1 : 1 binary co-crystal solvates formulated as 2,2′-dithiodibenzoic acid (DTBA), 4-halobenzoic acid (4-XBA) and dimethylformamide (DMF) for X = Cl (1), Br (2) and I (3) are isomorphous and the supramolecular association in this series has been probed by a wide range of computational chemistry techniques. The common feature of the molecular packing is the formation of robust three-molecule aggregates sustained by a non-symmetric {⋯HOCO}₂ synthon, formed between DTBA and 4-XBA, and a DTBA-hydroxyl-O–H⋯O(carbonyl-DMF) hydrogen bond (with a reciprocating DMF-C–H⋯O(carbonyl-DTBA) contact). Supramolecular tapes are evident and feature DTBA-C–H⋯O(DMF), DTBA-C–H⋯S(DTBA), DMF-O⋯π(DTBA-phenyl) and DMF-C–H⋯π(DTBA-phenyl) contacts. The point-to-point connections between the tapes are of the type π(4-XBA-phenyl)⋯π(DTBA-phenyl), π(4-XBA-phenyl)⋯π(4-XBA-phenyl), DTBA-C–H⋯π(DTBA-phenyl) and DMF-C⋯X tetrel bonding. However, a difference is noted here between 1 and 2, and 3, in that the DTBA-C–H⋯π(DTBA-phenyl) contacts are beyond standard separations, a feature of the packing that is traced directly to the influence of the iodide atom as opposed to the lighter congeners. There are systematic increases in the b and c unit-cell lengths correlating with the size of the halide, and in the case of 3 this expansion diminishes the influence of the DMF-C–H⋯π(DTBA-phenyl) contact as the tapes are pushed apart. This contact along the a-direction in 3 is replaced by DMF-H⋯O(DTBA-carbonyl) and DTBA-C–H⋯S(DTBA) contacts. Additional attractive interactions along the a-axis include C–X⋯quasi-π[{⋯HOCO}₂] bonding interactions which are at a maximum strength in 3 (ca. 5.0 kJ mol⁻¹) and contribute to the shortening of the a-axis in the order 3 < 2 < 1.