Seven-Coordination versus Six-Coordination in Divalent First-Row Transition-Metal Complexes Derived from 1,10-Diaza-15-crown-5

The complexes of the heptadentate receptor N,N‘-bis(benzimidazol-2-ylmethyl)-1,10-diaza-15-crown-5 (L2) with MnII,
CoII, NiII, CuII, and ZnII are reported. The X-ray crystal structures of the ZnII and NiII complexes show that whereas
the ZnII ion is seven-coordinated in a (distorted) pentagonal-bipyramidal coordination environment, the NiII ion is
only six-coordinated in a distorted octahedral coordination environment. Theoretical calculations on the [M(L2)]2+
systems (M = Mn, Co, Ni, Cu, or Zn) performed at the density functional theory (DFT; B3LYP) level have been
used to obtain information about the structure and electronic properties of these complexes, as well as to rationalize
their preferences for a pentagonal-bipyramidal or an octahedral coordination. We have found that for the MnII, CoII,
CuII, and ZnII complexes, geometry optimizations lead systematically to pentagonal-bipyramidal coordination
environments around the metal ions. However, for the NiII complex, two minimum-energy conformations were obtained,
with the metal ion being in octahedral (o-[Ni(L2)]2+) or pentagonal-bipyramidal (pb-[Ni(L2)]2+) coordination. The
stabilization of the octahedral geometry in the NiII complex can be considered as the result of the Jahn−Teller
effect operating in pentagonal-bipyramidal geometry, which in an extreme case leads to an octahedral coordination.
Spectrophotometric titrations carried out in dimethyl sulfoxide (DMSO) and CH3CN/DMSO (9:1) solutions indicate
the following stability sequence for the complexes of L2:  CoII ≈ NiII > ZnII ≫ MnII. The variations in the geometry
and stability of the complexes may be rationalized in terms of the different occupations of the frontier molecular
orbitals along the first-row transition-metal series. Finally, a time-dependent DFT approach was used to investigate
the absorption spectrum of the [Cu(L2)]2+ complex based on the optimized geometries at the B3LYP level, also
confirming a pentagonal-bipyramidal coordination in solution for this compound.