NMR Crystallographic Investigations of Group 14 σ-Hole Interactions: Tetrel Bonds

Abstract

The concept of noncovalent bonding has evolved over the last number of years to include a very interesting class of interactions that is analogous to hydrogen bonding, called σ-hole interactions. These result from the depletion of electrostatic charge on the opposite end of a covalent bond between an electron-withdrawing substituent and a bond donor atom, which resides in groups 14-17 of the periodic table. One of these interactions is the tetrel bond (TB), whereby the bond donor is a group 14 element (T=C, Si, Ge, Sn, Pb). This thesis's primary goal is to explore the solid-state NMR parameters arising from the formation of tetrel bonds. To this end, combined density functional theory (DFT) and experimental multinuclear solid-state NMR spectroscopic investigations are carried out on complexes featuring carbon, Pb(II) and tin tetrel bonds. Firstly, solid-state NMR and computational approaches are used to examine a series of cocrystals formed from either caffeine or theophylline and several other small organic acceptor molecules. It is shown that the NMR response due to tetrel bond formation is detectible, but it can be hidden by other effects, including those of crystal packing. Careful analysis of NMR data alongside DFT calculations can reveal that the weak tetrel bond in these sorts of complexes increases the ¹³C chemical shift by 3-5 ppm. Next, a study of five Pb(II) centres hemidirectionally coordinated by isonicotinoyl hydrazone ligands demonstrates that the ²⁰⁷Pb NMR response is highly sensitive to the Pb(II) coordination environment. The NMR data indicate that a tetrel bond can induce an NMR response corresponding to a coordination environment between hemidirectional and holodirectional character. Finally, a series of organotin chloride donor molecules complexed with N-oxides and carboxylates, which feature short and linear tetrel bonds, are subjected to magic angle spinning (MAS) NMR experiments. The recorded data gives rise to a correlation between the tetrel bond length and both the experimental chemical shift and the ¹J(¹¹⁹Sn-³⁵Cl) coupling. Throughout this thesis, it is demonstrated that the isotropic chemical shift, the principal components of the chemical shift tensor, and indirect spin-spin coupling can be used to probe and gain insights into the electronic environment at the tetrel bond. More importantly, this work is fundamental to rationalize NMR data while refining crystal structure data in NMR crystallographic approaches for compounds featuring tetrel bonds.