EXAFS Studies of Ni^II, Ni¹, and Ni^I—CO Tetraazamacrocycles and the Crystal Structure of (5,7,7,12,14,14-Hexamethyl-1,4,8,11-tetraazacyclotetradeca-4,1 l-diene)nickel(I) Perchlorate

Nickel(II) complexes of tetraazamacrocycles undergo one-electron reduction to produce either a nickel(I) complex or a nickel(II) anion radical. Both reduced species react with CO. The nature of the parent Ni(II) complexes, the reduced species, and the Ni^I—CO complexes were studied in CH₃CN by means of EXAFS and UV-vis spectroscopy to characterize structural differences as a function of oxidation state and axial ligation of the metal in solution. The EXAFS results reveal that the reduction of Ni(II) to Ni(I) results not only in an expansion of the macrocycle core (0.1 Å change in Ni-N bond distance) but also a distortion. On the other hand, the Ni(II) to Ni(II) anion radical reduction leaves the geometry around the nickel atom unchanged. The anion radical of Ni(II) tetraene⁺ (NiL₃⁺, L₃ = 2,3,9,10-tetramethyl-l,4,8,l 1-tetraazacyclotetradeca-1,3,8,10-tetraene) dimerizes in solution forming diamagnetic adducts. The monomer—dimer equilibrium constant was determined to be K₁ = (5.5 ± 1.0) X 10⁴ M⁻¹ from the electronic spectra. EXAFS data on CO adducts of Ni^IL₁, -L₂, and −L₃ (L₁ = 5,7,7,12,14,14-hexamethyl-1,4,8,1 l-tetraazacyclotetradeca-4,11-diene, L₂ = 5,7,7,12,12,14-hexamethyl-1,4,8,1 l-tetraazacyclotetradeca-4,14-diene) clearly indicate that these are five-coordinate complexes with a short Ni—C bond. Both Ni-N_imine and Ni—N_amine distances in the CO adducts of the Ni(I) complexes increase quite dramatically compared to those in the parent Ni(II) and Ni(I) complexes. The structure of the title nickel(I) complex has been determined from single-crystal X-ray diffraction data collected by using Mo Kα radiation. Crystallographic data are as follows: space group P2/n with a = 15.717 (6) Å, b = 8.196 (2) Å, c = 16.049 (6) Å, β = 100.67 (3)°, V = 2031 (2) ų, Z = 4. The two square-planar nickel atoms in the asymmetric unit are situated on crystallographic inversion centers. The Ni−N_imine distances are 1.988 (7) and 1.979 (7) Å and Ni—N_amine distances are 2.063 (6) and 2.068 (6) Å, which are in good agreement with the EXAFS data.