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.
Obuch et al. (Thu,) studied this question.