Mechanochemical Production of Halogen-Bonded Solids Featuring P=O•••I-C Motifs and Characterization via X-ray Diffraction, Solid-State Multinuclear Magnetic Resonance, and Density Functional Theory

A series of phosphine oxide-iodofluorobenzene cocrystals featuring relatively strongly halogen-bonded P=O center dot center dot center dot I-C motifs has been prepared mechanochemically and characterized by P-31 and O-17 solid-state nuclear magnetic resonance (SSNMR) spectroscopy and single-crystal X-ray diffraction. Direct insights were obtained into the correlation between several NMR observables (chemical shift tensors, electric field gradient tensors, and J(P-31, O-17) coupling) and the molecular and electronic structure of the halogen bonds. The P-31 isotropic chemical shift and span, the O-17 quadrupolar coupling constant and asymmetry parameter, as well as J(P-31, O-17) coupling can be strong indicators of the presence of a halogen bond. Density functional theory calculations using periodic boundary conditions have revealed several notable trends. For example, the oxygen isotropic chemical shift and J(P-31, O-17) values were found to correlate linearly with the strength of the halogen bond. An additional natural localized molecular orbital (NLMO) investigation shows that the three main contributions to J(P-31, O-17) coupling are the P=O bonding orbitals, the oxygen lone pair orbital, and oxygen core orbitals, and that they all together determine the overall trend observed experimentally. Further NLMO analysis revealed a linear correlation between the contribution of the oxygen p(z), lone pair orbital to the quadrupolar coupling constant and the strength of the halogen bond. This work, encompassing the first O-17 SSNMR studies of halogen bonds, demonstrates how NMR observables provide a useful means to characterize halogen bonds and also improve our understanding of the correlation between the electronic structure of the halogen bond and NMR properties.