Organic Selenocyanates as Halide Receptors: From Chelation to One-Dimensional Systems

Organic selenocyanates were recently identified as strong chalcogen bond donors. They also play an important role in biochemistry. Here, we show that 1,2-bis(selenocyanatomethyl)benzene (1) and 1,2,4,5-tetrakis(selenocyanatomethyl)-benzene (2) crystallize from dimethylformamide (DMF) to afford solvates where two ortho-SeCN moieties act as a chelate toward the carbonyl oxygen atom of DMF through strong Se center dot center dot center dot O chalcogen bonds. This result led us to explore their ability to also chelate halide anions (Cl-, Br-) in solution as well as in the solid state, an important issue in view of applications in crystal engineering or organocatalysis. NMR titration experiments provide an association constant between 1 and Cl- of 148 M-1. We also observed the recurrent formation of cocrystal salts from the association of the ChB donors 1 and 2 with the onium salts Ph4PCl, Ph4PBr, and Bu4NCl. We demonstrate that not only mu(2)-halide but also mu(4)-halide structures can be stabilized though ChB interactions, leading to the formation of complex polymeric anionic networks. Continuous shape measure calculations of these mu(4)-halide structures demonstrate that seesaw symmetry best describes the mu(4)-Br- bromide structures, while the smaller chloride anions tend to favor a close-to-tetrahedral mu(4)-Cl- organization, which is also confirmed by density functional theory calculations. Electrostatic surface potential calculations further demonstrate the efficiency of this chelating ortho-bis(selenocyanatomethyl) motif in 1 and 2, with V-s,V-max values reaching 50 kcal mol(-1), to be compared with the simplest benzyl selenocyanate (36.4 kcal mol(-1)) or the reference halogen bond donor F5C6-I (35.7 kcal mol(-1)) in the same conditions.