NMR Crystallographic Journey from Light to Heavy Atoms of Mercury(II)-DOTAM Complexes and Extraction of Related Structural Parameters

Complexes of macrocyclic ligands are routinely used as MRI contrast agents and radionuclide carriers for PET and SPECT diagnostics and radiotherapy. This study explores the structural and electronic environments of two materials containing [Hg(dotam)]²⁺ cations, using an integrated approach combining single-crystal X-ray diffraction (SC-XRD), multinuclear solid-state magnetic resonance (ssNMR) spectroscopy (¹³C, ¹⁵N, ¹⁹⁹Hg), and relativistic density functional theory (DFT) calculations. SC-XRD revealed distinct coordination motifs, including octa- and heptacoordinated [Hg(dotam)]²⁺ cations. Scalar and spin-orbit relativistic DFT computations accurately reproduced ¹³C and ¹⁵N chemical shifts, with a root-mean-square deviation of ∼0.7 ppm for ¹³C and ∼4.8 ppm for ¹⁵N, highlighting the importance of relativistic heavy atom effects. For ¹⁹⁹Hg NMR, relativistic cluster-based methods (ADF/ReSpect) outperformed nonrelativistic approaches. An empirical regression model (<i>χ̅</i>) linked ¹⁹⁹Hg shifts to the coordination number (<i>CN</i>) and averaged donor electronegativity (χ̅) (<i>R</i>² = 0.86), enabling rapid structural inference. The isotropic ¹⁹⁹Hg shift correlates with the charge on the Hg atom, influencing the <i>p</i>-type frontier molecular orbitals and their paramagnetic contributions to NMR shielding. This work highlights the potential of ¹⁹⁹Hg NMR as a structural descriptor and offers a strategy for NMR crystallography involving heavy elements with possible implications for catalysis, ionic liquids, and Hg-based pharmaceuticals.