Ni(Ii) Complexes Of The Redox-Active Bis(2-Aminophenyl)Dipyrrin: Structural, Spectroscopic, And Theoretical Characterization Of Three Members Of An Electron Transfer Series

The sterically hindered bis(2-aminophenyl)dipyrrin ligand (H3L)-L-N was prepared. X-ray diffraction discloses a bifurcated hydrogen bonding network involving the dipyrrin and one aniline ring. The reaction of (H3L)-L-N with one equivalent of nickel(II) in the air produces a paramagnetic neutral complex, which absorbs intensively in the Vis-NIR region. Its electron paramagnetic resonance spectrum displays resonances at g(1) = 2.033, g(2) = 2.008, and g(3) = 1.962 that are reminiscent of an (S = 1/2) system having a predominant organic radical character. Both the structural investigation (X-ray diffraction) and density functional theory calculations on [Ni-II(L-N(center dot))] points to an unprecedented mixed pyrrolyl-anilinyl radical character. The neutral complex [Ni-II(L-N(center dot))] exhibits both a reversible oxidation wave at -0.28 V vs Fc(+)/Fc and a reversible reduction wave at -0.91 V. The anion was found to be highly air-sensitive, but could be prepared by reduction with cobaltocene and structurally characterized. It comprises a Ni(II) ion coordinated to a closed-shell trianionic ligand and hence can be formulated as [Ni-II(L-N)](-). The cation was generated by reacting [Ni-II(L-N(center dot))] with one equivalent of silver hexafluoroantimonate. By X-ray diffraction we established that it contains an oxidized, closed-shell ligand coordinated to a nickel(II) ion. We found that a reliable hallmark for both the oxidation state of the ligand and the extent of delocalization within the series is the bond connecting the dipyrrin and the aniline, which ranges between 1.391 angstrom (cation) and 1.449 angstrom (anion). The cation and anion exhibit a rich Vis-NIR spectrum, despite their nonradical nature. The low energy bands correspond to ligand-based electronic excitations. Hence, the HOMO-LUMO gap is small, and the redox processes in the electron transfer series are exclusively ligand-centered.