Reasons behind the Relative Abundances of Heptacoordinate Complexes along the Late First-Row Transition Metal Series

A series of transition metal complexes
[ML1] (H2L1 = 1,4,10-trioxa-7,13-diazacyclopentadecane-N,N′-diacetic acid, M = Co, Ni,
Cu, or Zn) have been prepared and characterized. The X-ray structures
of the [CoL1] and [CuL1] complexes reveal that
the metal ions are seven-coordinated with a distorted pentagonal bipyramidal
coordination. The five donor atoms of the macrocycle define the pentagonal
plane of the bipyramid, while two oxygen atoms of the carboxylate
groups coordinate apically. The [NiL1] complex presents
a very distorted structure with long Ni–O distances involving
two oxygen atoms of the crown moiety [2.544(3) Å]. This distortion
is related to the Jahn–Teller effect that is expected to operate
in d8 pentagonal bipyramidal complexes. The spectroscopic
characterization of the [ZnL1] and [CuL1] complexes
using NMR and EPR and the theoretical calculation of the 13C NMR shifts and g- and A-tensors
using DFT confirm that these complexes retain the pentagonal bipyramidal
coordination in aqueous solution. The stability trend of the [ML1] complexes (Co2+ > Ni2+ < Cu2+ > Zn2+), which is in contradiction with the
Irving–Williams
order, has been analyzed using DFT calculations (TPSSh functional).
The free energy values calculated in the gas phase for [CoL1]­(g) + [M­(H2O)6]2+(g) → [ML1]­(g) + [Co­(H2O)6]2+(g) (M
= Ni, Cu, Zn) reproduce fairly well the stability trend observed experimentally,
the agreement being improved significantly upon inclusion of solvent
effects. Our results indicate that the pentagonal bipyramidal coordination
is particularly unfavorable for Ni2+, and thus preorganized
ligands that favor this geometry such as L1 are selective
for Co2+ over Ni2+ cations.