Experimental investigation of halogen-bond hardsoft acid-base complementarity

The halogen bond (XB) is a topical noncovalent interaction of rapidly increasing importance. The XB employs a 'soft' donor atom in comparison to the 'hard' proton of the hydrogen bond (HB). This difference has led to the hypothesis that XBs can form more favorable interactions with 'soft' bases than HBs. While computational studies have supported this suggestion, solution and solid-state data are lacking. Here, XB soft-soft complementarity is investigated with a bidentate receptor that shows similar associations with neutral carbonyls and heavy chalcogen analogs. The solution speciation and XB soft-soft complementarity is supported by four crystal structures containing neutral and anionic soft Lewis bases.