Structure and isomerization comparison of Zn(ii), Cd(ii) and Hg(ii) perchlorate complexes of 2,6-bis([(2-pyridyl-methyl)amino]methyl)pyridine

The divalent zinc triad perchlorate coordination chemistry of 2,6-bis([(2-pyridyl-methyl)amino]methyl)pyridine (L) was investigated by X-ray crystallography and solution state ¹H NMR. New complexes [HgL(ClO₄)₂] (1) and [CdL(ClO₄)₂] (2) were isolated as bicapped distorted square pyramidal racemates, contrasting with the approximate trigonal bipyramidal structure of [ZnL](ClO ₄)₂ (3). Although rapid inter- and intramolecular exchange is common for simple complexes of zinc triad metal ions, conditions for slow intramolecular isomerization on both the δ and J_HH time scales were established for 1, 2 and 3. Trends in geminal ¹H coupling suggested that an asymmetric structure was favored for all three metal ions at or below 40 °C. Contributions of a symmetric structure to solution equilibria were both temperature- and metal ion-dependent. Spectral trends were consistent with interconversion of a pair of enantiomeric square pyramidal ligand conformers through a symmetric trigonal bipyramidal ligand conformer by M-N bond cleavage and nitrogen inversion. Racemization was slower than the coupling constant time scale up to 40 °C for all complexes. Differential stabilization of specific small ligand conformations in solution has potential toxicological significance.