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

Bradley J. Carra, Steven M. Berry, Robert D. Pike, Deborah C. Bebout

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Abstract

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 JHH 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.

Original languageEnglish (US)
Pages (from-to)14424-14431
Number of pages8
JournalDalton Transactions
Volume42
Issue number40
DOIs
StatePublished - Oct 28 2013

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