Halogen-bond interactions: a crystallographic basicity scale towards iodoorganic compounds

Halogen bond (X-bond) interactions involving organic iodine have been investigated using data retrieved from the Cambridge Structural Database (CSD) and Density Functional Theory (DFT) calculations. The analysis of the mean normalised intermolecular distances involving Csp-I, Csp(2)-I and Csp(3)-I X-bond donors first shows that, for interactions with the same acceptor site, the shortest mean distance is always measured for Csp-I X-bond donors. This experimental trend is rationalised through molecular electrostatic potential calculations on the iodine surface, along the sigma-hole of the iodine atom, since Csp-I donors are characterised by the strongest V-S,V-max values, that is to say the more electron poor iodine atoms. In agreement with the trends revealed from the analysis of the I center dot center dot center dot N distances, the X-bond with Csp-I donors appear more linear (mean of 169.5 degrees) than with Csp(2)-I and Csp(3)-I donors, their respective values being close to 164 degrees. From a survey of the geometries of the X-bond contacts observed in the most extended dataset (Csp(2)-I: 1213), a crystallographic order of X-bond acceptor strength has been obtained through a careful consideration of the chemical functions and subfunctions to which the acceptor atom belongs. This order is in good agreement with the one observed in solution on the diiodine basicity scale pK(BI2). An exception to this trend is thioureas, which show unexpected long (weak) S center dot center dot center dot I distances. However, these observations are rationalised from the sulfur behaviour, which acts simultaneously as a X-bond acceptor and hydrogen-bond (H-bond) acceptor in these structures. Finally, an interesting correlation between the pK(BI2) and the I center dot center dot center dot Y normalised intermolecular distances is found for a wide and varied collection of organic bases, since we have been able to delineate 22 families of organic compounds covering more than four pK units on the pK(BI2) scale.