Ni Oxidation State and Ligand Saturation Impact on the Capability of Octaazamacrocyclic Complexes to Bind and Reduce CO(2).

Two 15-membered octaazamacrocyclic nickel(II) complexes are investigated by theoretical methods to shed light on their affinity forwards binding and reducing CO(2). In the first complex (1)[Ni(II)L](0), the octaazamacrocyclic ligand is grossly unsaturated (π-conjugated), while in the second (1)[Ni(II)LH](2+) one, the macrocycle is saturated with hydrogens. One and two-electron reductions are described using Mulliken population analysis, quantum theory of atoms in molecules, localized orbitals, and domain averaged fermi holes, including the characterization of the Ni-C(CO2) bond and the oxidation state of the central Ni atom. It was found that in the [NiLH] complex, the central atom is reduced to Ni(0) and/or Ni(I) and is thus able to bind CO(2) via a single σ bond. In addition, the two-electron reduced (3)[NiL](2-) species also shows an affinity forwards CO(2).