Tuning of the Properties of Transition-Metal Bispidine Complexes by Variation of the Basicity of the Aromatic Donor Groups

Bispidines (3,7-diazabicyclo[3.3.1]­nonanes)
as very rigid and highly preorganized ligands find broad application
in the field of coordination chemistry, and the redox potentials of
their transition-metal complexes are of importance in oxidation reactions
by high-valent iron complexes, aziridination catalyzed by copper complexes,
and imaging by 64Cu positron emission tomography tracers.
Here, we show that the redox potentials and stability constants of
the copper­(II) complexes of 15 tetradentate bispidines can be varied
by substitution of the pyridine rings (variation of the redox potential
over ca. 450 mV and of the complex stability over approximately 10
log units). It is also shown that these variations are predictable
by the pKa values of the pyridine groups
as well as by the Hammett parameters of the substituents, and the
density functional theory based energy decomposition analysis also
allows one to accurately predict the redox potentials and concomitant
complex stability. It is shown that the main contribution emerges
from the electrostatic interaction energy, and the partial charges
of the pyridine donor groups therefore also correlate with the redox
potentials.