Does a Reinforced Kinetic Macrocyclic Effect Exist? The Demetallation in Strong Acid of Copper(II) Complexes with Open and Cyclic Tetramines Containing a Piperazine Fragment

The demetallation in acidic
solution of the CuII complexes with
open-chain and cyclic tetramines containing
a piperazine unit (2 and 3) has
been investigated in terms of its kinetic
aspects and compared with the behaviour
of unsubstituted counterparts (tetramines
1 and 4). The presence of the
piperazine fragment slows demetallation
of the open-chain-ligand complex
owing to the activation barrier associated
with the conformational change
from boat-to-half-boat; however, it
does not affect the demetallation of the
macrocyclic complex, which involves
the spontaneous boat-to-twist conformational
change. Thus, lateral reinforcement
of a cyclam-like ligand does
not add any further contribution to the
typical inertness in demetallation of
macrocyclic complexes.