Structural and conformational trends of silver(i) and iodine(i) complexes of 3-substituted pyridines

To systematically explore the impact of meta-substituents on the halogen bond of [bis(pyridine)iodine(i)]⁺ complexes, five new iodine(i) complexes of 3-substituted pyridines 1-5 with substituents ranging from weakly electron-donating (-CH₃) to strongly electron-withdrawing (-F) have been synthesized. The iodine(i) complexes were prepared from their silver(i) analogs by cation exchange. Crystals of both the silver(i) and iodine(i) complexes were grown by vapor diffusion and analyzed by single-crystal X-ray diffraction. Notably, in each case the silver(i) and iodine(i) complexes were found to crystallize in opposite syn-anti conformations with respect to the 3-substituent. However, DFT calculations indicate no preference for either conformation in any of the complexes, suggesting that the observed phenomenon is coincidental and the result of packing forces. The silver(i) complex of the 2,5-disubstituted pyridine 6 adopted different structures depending on the counterion: it existed as discrete [6-Ag-6]⁺ units in the PF₆ ^- salt, formed dimers of [6-Ag-6]⁺ in the BF₄ ^- salt and assembled into polymeric 6-Ag-PO₂F₂ chains. The latter two counterions resulted from the hydrolysis of PF₆ ^-. Crystallographic studies were augmented with ¹H and ¹H-¹⁵N HMBC NMR studies in solution. This revealed only small differences in ¹⁵N NMR shifts between the different pyridines, indicating that substituents in the 3-position have a diminished ability to influence the halogen bonds in [bis(pyridine)iodine(i)]⁺ complexes. This means that the 3-position can be employed in the design of supramolecular motifs without any concerns relating to the potential weakening of the N-I⁺-N bond.