Concerted Ligand Exchange and the Roles of Counter Anions in the Reversible Structural Switching of Crystalline Peptide Metallo-Macrocycles

To understand reversible structural
switching in crystalline materials, we studied the mechanism of reversible
crystal-to-crystal transformation of a tetranuclear NiII macrocycle consisting of artificial β-dipeptides. On the basis
of detailed structural analyses and thermodynamic measurements made
in a comparison of pseudo-isostructural crystals (NO3 and
BF4 salts), we herein discuss how ligand-exchange reactions
take place in the crystal due to changes in water content and temperature.
Observations of the structural transformation of NO3 salt
indicated that a pseudo crystalline phase transformation takes place
through concerted ligand-exchange reactions at the four NiII centers of the macrocycle with hydrogen bond switching. A mechanism
for this ligand exchange was supported by IR spectroscopy. Thermodynamic
measurements suggested that the favorable compensation relationship
of the enthalpy changes due to water uptake and structural changes
are keys to the reversible structural transformation. On the basis
of a comparison with the pseudo-isostructural crystals, it is apparent
that the crystal packing structure and the types of counter anions
are important factors for facilitating reversible ligand exchange
with single crystallinity.