Complexes of DOTAM with d¹⁰ Divalent Metal Ions: X-ray Diffraction and NMR Studies in Solution and the Solid State

A detailed understanding of coordination asymmetry and metal-ligand bonding strength in macrocyclic complexes remains challenging, as subtle electronic differences often escape detection by conventional structural methods. Complexes of the tetrakis(acetamide) cyclen derivative dotam with diamagnetic d¹⁰ metal ions Zn(II), Cd(II), and Hg(II) were investigated in the solid state and in solution by single-crystal X-ray diffraction, multinuclear solid-state and solution NMR spectroscopy, and density functional theory calculations. The crystal structures reveal a progression from hexacoordinated Zn(II), with only two coordinated pendant arms, to octacoordinated Cd(II) and Hg(II) complexes with distorted square-antiprismatic geometries. Solid-state ¹³C and ¹⁵N MAS NMR spectra agree well with the crystallographic models and GIPAW-DFT calculations. A key finding is the resolved splitting of amide carbonyl ¹³C resonances, attributed to residual ¹³C-¹⁴N dipolar coupling, which sensitively reports metal-induced perturbation of coordinated amide groups not evident from X-ray diffraction alone. Variable-temperature ¹³C NMR spectroscopy in aqueous solution revealed dynamic interconversion processes with activation parameters differing systematically from those of the corresponding DOTA complexes. Uniformly negative activation entropies support a common transition state consistent with transient octacoordination. Together, these results show that replacing acetate with acetamide pendant arms strongly affects both local coordination properties and solution dynamics.