The impact of ortho‐substituents on Bonding in Silver(I) and Halogen(I) complexes of 2‐Mono‐ and 2,6‐Disubstituted Pyridines : An In‐depth Experimental and Theoretical Study

The coordination nature of 2-mono- and 2,6-disubstituted pyridines with electron-withdrawing halogen and electron-donating methyl groups for [N–X–N]⁺ (X =I, Br) complexations have been studied using 15N NMR, X-ray crystallography, and Density Functional Theory (DFT) calculations. The 15N NMR chemical shifts reveal iodine(I) and bromine(I) prefer to form complexes with 2-substituted pyridines and only 2,6-dimethylpyridine. The crystalline halogen(I) complexes of 2-substituted pyridines were characterized by using X-ray diffraction analysis, but 2,6-dihalopyridines were unable to form stable crystalline halogen(I) complexes due to the lower nucleophilicity of the pyridinic nitrogen. In contrast, the halogen(I) complexes of 2,6-dimethylpyridine, which has a more basic nitrogen, are characterized by X-crystallography, which complements the 15N NMR studies. DFT calculations reveal that the bond energies for iodine(I) complexes vary between -291 and -351 kJmol-1 and for bromine between -370 and -427 kJmol-1. The bond energies of halogen(I) complexes of 2-halopyridines with more nucleophilic nitrogen are 66 - 76 kJmol-1 larger than those of analogous 2,6-dihalopyridines with less nucleophilic nitrogen. The experimental and DFT results show that the electronic influence of ortho-halogen substituents on pyridinic nitrogen leads to a completely different preference for the coordination bonding of halogen(I) ions, providing new insights into bonding in halogen(I) chemistry.