Electronic Structure Study of Seven-Coordinate First-Row Transition Metal Complexes Derived from 1,10-Diaza-15-crown-5: A Successful Marriage of Theory with Experiment

A detailed study of the electronic structure of seven-coordinate Mn(II), Co(II), and Ni(II) complexes with the lariat
ether N,N‘-bis(2-aminobenzyl)-1,10-diaza-15-crown-5 (L1) is presented. These complexes represent new examples
of structurally characterized seven-coordinate (pentagonal bipyramidal) complexes for the Mn(II), Co(II), and Ni(II)
ions. The X-ray crystal structures of the Mn(II) and Co(II) complexes show C2 symmetries for the [M(L1)]2+ cations,
whereas the structures of the Ni(II) complexes show a more distorted coordination environment. The magnetic
properties of the Mn(II) complex display a characteristic Curie law, whereas those of the Co(II) and Ni(II) ions show
the occurrence of zero-field splitting of the S = 3/2 and 1 ground states, respectively. Geometry optimizations of
the [M(L1)]2+ systems (M = Mn, Co, or Ni) at the DFT (B3LYP) level of theory provide theoretical structures in
good agreement with the experimental data. Electronic structure calculations predict a similar ordering of the metal-based β spin frontier MO for the Mn(II) and Co(II) complexes. This particular ordering of the frontier MO leads to
a pseudodegenerate ground state for the d8 Ni(II) ion. The distortion of the C2 symmetry in [Ni(L1)]2+ is consistent
with a Jahn−Teller effect that removes this pseudodegeneracy. Our electronic structure calculations predict that
the binding strength of L1 should follow the trend Co(II) ≈ Mn(II) > Ni(II), in agreement with experimental data
obtained from spectrophotometric titrations.