Comparing the Halogen Bond to the Hydrogen Bond by Solid-State NMR Spectroscopy: Anion Coordinated Dimers from 2- and...

Halogen bonding is an increasingly important tool in crystal engineering, and measuring its influence on the local chemical and electronic environment is necessary to fully understand this interaction. Here, we present a systematic crystallographic and solid-state NMR study of self-complementary halogen-bonded frameworks built from the halide salts (HCl, HBr, HI, HI₃ ) of 2-iodoethynylpyridine and 3-iodoethynylpyridine. A series of single crystal X-ray structures reveals the formation of discrete charged dimers in the solid state, directed by simultaneous X^- ⋅⋅⋅H-N⁺ hydrogen bonds and C-I⋅⋅⋅X^- halogen bonds (X=Cl, Br, I). Each compound was studied using multinuclear solid-state magnetic resonance spectroscopy, observing ¹ H to investigate the hydrogen bonds and ¹³ C, ³⁵ Cl, and ^79/81 Br to investigate the halogen bonds. A natural localized molecular orbital analysis was employed to help interpret the experimental results. ¹ H SSNMR spectroscopy reveals a decrease in the chemical shift of the proton participating in the hydrogen bond as the halogen increases in size, whereas the ¹³ C SSNMR reveals an increased ¹³ C chemical shift of the C-I carbon for C-I⋅⋅⋅X^- relative to C-I⋅⋅⋅N halogen bonds. Additionally, ³⁵ Cl and ^79/81 Br SSNMR, along with computational results, have allowed us to compare the C-I⋅⋅⋅X^- halogen bond involving each halide in terms of NMR observables. Due to the isostructural nature of these compounds, they are ideal cases for experimentally assessing the impact of different halogen bond acceptors on the solid-state NMR response.