Halogen-bond geometry: a crystallographic database investigation of dihalogen complexes

X-ray crystal structures of 141 halogen-bonded complexes Y—X...B formed between homo- and heteronuclear dihalogens Cl₂, Br₂, I₂, IBr and ICl with O, S, Se, N, P and As Lewis bases show remarkable and constant geometrical features. The metrics of the halogen bond found in the gas phase for simple complexes [Legon (1999a). Angew Chem. Int. Ed. Eng. 38, 2686–2714] is supported (i) in the solid state, (ii) for new Lewis acids (I₂ and IBr), (iii) for new basic centers (Se, As and =N—) and (iv) for more complicated bases. The Y—X...B arrangement is more linear than the corresponding Y—H...B hydrogen bond and the axis of the Y—X molecule lies in the plane of the B lone pair(s), with a preference for the putative lone-pair direction within that plane. However, exceptions to this lone-pair rule are found for sterically hindered thiocarbonyl and selenocarbonyl bases. A bond-order model of the halogen bond correctly predicts the observed correlation between the shortening of the X...B distance and the lengthening, Δd(Y—X), of the Y—X bond. The expectation that the solid-state geometric parameters d(X...B) and Δd(Y—X) reflect the strength of the interaction is supported by their significant relationships with the solution thermodynamic parameters of Lewis acidity and basicity strength, such as the Gibbs energy of 1:1 complexation of Lewis bases with diiodine. This analysis of halogen-bonded complexes in the solid state reinforces the similarities already known to exist between hydrogen and halogen bonding.