¹²⁷I Solid-State NMR Spectroscopy of Organic Periodates Featuring Halogen Bonded IO₄ ^- ⋅ ⋅ ⋅ IO₄ ^- Adducts

Hydroxyanions and oxyanions can overcome electrostatic repulsion between like charges to form supramolecular architectures whose formation is driven by non-covalent interactions such as hydrogen bonds and halogen bonds (HaB). We report here a ¹²⁷I solid-state nuclear magnetic resonance (SSNMR) study of a series of six organic periodates including compounds which feature I ⋅ ⋅ ⋅ O HaB between pairs of IO₄ ^- anions and control samples which do not feature HaB. ¹²⁷I SSNMR spectra of powdered samples acquired under stationary conditions at 9.4, 11.7, and 21.1 T are simulated using an exact diagonalization of the Zeeman-quadrupolar Hamiltonian to provide the isotropic chemical shift, ¹²⁷I nuclear quadrupolar coupling constant (C_Q), and quadrupolar asymmetry parameter for each compound. One of the HaB compounds, 4-(pyrrolidin-1-yl)pyridinium periodate, is characterized by the largest C_Q(¹²⁷I) value measured to date for a periodate anion, 52.70 MHz. Control organic periodates which do not have HaBs have C_Q(¹²⁷I) values that are much lower than those seen in the halogen-bonded systems, thereby easily differentiating between these two sets of compounds. The C_Q(¹²⁷I) values for those compounds featuring only halogen-bonded periodate anions correlate with the shear strain of the anion, which may be attributed to the influence of the HaB on the local geometry. More rigorous correlations between structure and the ¹²⁷I NMR data are confounded by the presence of dynamics in some of the samples.