Strong and symmetric halogen bonding in coordination compounds of carboxylic acid derivatives; the crystal structure of (3,6-dithiaoctanedioato-S,S′-(3,6-dithiaoctanedioic acid-S,S′)-copper(I) redetermined at 173 K

Abstract 3,6-dithianoctanedioic acid forms a Cu(I) compound in which electrical neutrality is achieved by elaborate hydrogen bonding and sharing on protons. The title compund crystallizes in the monoclinic space group P2/n with Z = 2. Unit-cell parameters are a = 11.625(2), b = 7.664(1), c = 9.874(2) A, β = 95.16°, D m = 1.80(2), D c = 1.83 g cm −3 . The structure was solved by means of standard direct methods and refined with full-matrix least-squares techniques to an R-value of 0.026 (R w = 0.042). The Cu(I) ion is tetrahedrally coordinated by four thioether S-atoms (CuS = 2.29–2.33 A). The molecules are linked by very strong hydrogen bonds between non-coordinating carboxylate groups in such a way that the average number of acidic hydrogens per molecule is three. One of these hydrogens lies on a twofold axis and forms a short symmetrical hydrogen bond, with a OO distance of 2.441(2) A. Unusual features in the infrared spectrum of this compund can be interpreted on the basis of the observed crystal structure.