OH-π and halogen-π interactions as driving forces in the crystal organisations of tri-bromo and tri-iodo trityl alcohols

The trityl alcohols bearing three bromine or three iodine atoms at the para-positions of the aromatic units, have been known for more than a hundred years. In our case these compounds have been synthesized in one-pot sequence starting from the 1,4-dihalogenobenzenes via mono-lithiation and the successive reaction with diethylcarbonate. The compounds have been crystallized from different solvent mixtures leading to one structure of bromo-(A) and three structures of iodo trityl alcohols (B-D). The inclusion of dichloromethane (C) or benzene (D) in the crystalline lattices has been observed. In all cases the OH-pi and halogen-pi (and in one case the halogen-halogen and CH-O weak) contacts play crucial role in the crystal organisation. The common motif of the packing is the anti parallel arrangement of the trityl alcohol molecules into infinite chains. There is the gradual change in the intermolecular interactions going from the structures A and B, where only OH-pi contacts between anti-parallel molecules are observed, to C with OH-pi bifurcated contacts between anti-parallel molecules and further to the structure D, where practically no OH-pi contacts are observed any longer. However in the case of D the splitting of the chain with anti-parallel molecules into two anti-parallel chains with parallelly assembled molecules takes place. These OH-pi chains interact further with each other either via halogen-pi or/and halogen-halogen, CH-O weak interactions. The shortest contacts found are 3.429 angstrom (1/A, Br-pi) and 3.486 angstrom (2/D, I-pi) while the longest contacts are 3.558 angstrom (1/A, Br-pi) and 3.668 angstrom (2/B, I-pi). No short contacts between the included solvent molecules and triiodo trityl alcohol within the structures C and D, have been observed. The quantum chemical calculations (DFT on the M05-2X/6-311G(d,p) level) gave the following interaction energies (kcal mol(-1)) between two neighbouring molecules of one "OH-pi chain'': -7.96 (A), -6.60 (B), -6.43 (C) and -6.45 (D), while the interaction energies via the halogen-pi contacts vary from -2.17 to -3.58 kcal mol(-1) (per one contact).