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 (χ̅) linked 199Hg shifts to the coordination number (CN) and averaged donor electronegativity (χ̅) (R2 = 0.86), enabling rapid structural inference.
The isotropic 199Hg shift correlates with the charge on
the Hg atom, influencing the p-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.