The halogen bond I...O, I...N, Br...N as a temperature function

The X...B interaction, or halogen bond, between a halogen atom (X = I, Br) in perfluoro derivatives and a neutral or charged base (B = amines, pyridines, Noxides, iodide etc.), is a useful synthon we have extensively used to produce a variety of supra-molecular structures. The goal of the present study is the temperature dependence of the halogen bond length. Three model systems were investigated, namely I...O in di-(4-pyridyl-N-oxide) 1,4- diiodotetrafluorobenzene (1), I...N in 4,4-dipyridine 1,4-diiodotetrafluorobenzene (2), and Br...N in trans-1,2-(4-pyridyl)ethylene 1,4-dibromotetrafluorobenzene (3). We chose these structures on the basis of their space group and Z at room temperature [P-1 and 1 for all the structures] assuring that no phase transition would happen in the temperature range under investigation. Data at 90, 145, 200 and 291 K were collected for all three structures taking care to minimize the effects of systematic errors (same crystal, collection strategy, instrument, etc.). The cell volume expansion, from 90 K to room temperature, is 3.09%, 5.05% and 4.75% for (1), (2) and (3) respectively. In the same temparature range, there is an I...O contraction from 2.753(2) to 2.723(2) Å in (1), I...N changes from 2.820(2) to 2.768(2) Å in (2) and Br...N shortens from 2.873(2) to 2.814(1) Å in (3). The thermal expansion and the variation of X...B distance is then quite high in (2) and (3), while is rather smaller in (1). We explain this difference with the fact that in (1) an O...H hydrogen bond between di-(4-pyridyl-N-oxide) molecules increases the packing energy and reduces the thermal expansion coefficient.