Sequential halogen bonding with ditopic donors: σ hole evolutions upon halogen bond formation
Abstract
The halogen bonding ability of ditopic halogen bond donors can be evaluated from the maximum value of the molecular surface electrostatic potential, called the σ hole, at the two possibly different halogen atoms. We show here that in N,N'-diodo-dimethylhydantoin (DIH), the halogen bonding (XB) ability of the two nitrogen-bound iodine atoms does not follow this σ-hole rule. The co-crystallization of DIH with a series of para-substituted pyridines, Py-R (R = pyrrolidinyl, NMe2, Me, H, CO2Me, CF3, CN), affords bis-adducts DIH?(Py-R)2 with the most electron-rich pyridines while mono-adducts DIH?(Py-R) are favored with the most electron poor pyridines (R = CO2Me, CF3, CN). Analysis of the structural characteristics of these mono- and bis-adducts, combined with theoretical calculations, demonstrates that the formation of a first N?I???N'Py-R XB deeply modifies the XB ability (and associated σ-hole) of the second uncoordinated iodine atom. Under these conditions, the latter might associate through I???O XB to the carbonyl oxygen atom of a neighboring mono-adduct in the crystal rather than to a second pyridine. These studies show that when working with di-, tri- or tetratopic XB donors, one should always consider the deactivation of the remaining halogen atoms following sequential XB formation