Redox characteristics of nickel and palladium complexes of the open-chain tetrapyrrole octaethylbilindione: a biliverdin model.

Octaethylbiliverdin, C35H46N4O2, is a linear tetrapyrrole that can exist in coordinated form as the fully reduced trianion (OEB)3-, as the two electron oxidized monoanion, (OEBOx)-, or as the one electron oxidized radical, (OEB.)2-. The three-membered electron-transfer series involving [M(OEB)]n with n = +1, 0, and -1, and M = Ni or Pd has been characterized electrochemically. The most highly oxidized members of these series have been isolated in the form of their diamagnetic triiodide salts, [NiII(OEBOx)]I3 and [PdII(OEBOx)]I3, and characterized spectroscopically and by X-ray diffraction. [NiII(OEBOx)]I3 crystallizes in the orthorhombic space group Pbcn with a = 15.121(3) A, b = 16.777(3) A, and c = 14.628(3) A at 130(2) K with Z = 4. Refinement of 3311 reflections with 209 parameters and no restraints yielded wR2 = 0.136 and R = 0.054 for 2643 reflections with I > 2 sigma(I). The structure involves helical coordination of the linear tetrapyrrole ligand about the nickel with all four nitrogen atoms coordinated to the metal and Ni-N distances of 1.867(5) and 1.879(5) A. The triiodide ion is not coordinated to the nickel but sits over one of the meso carbon atoms of the tetrapyrrole. In the solid state, pairs of [NiII(OEBOx)]+ crystallize about a center of symmetry so that two identical tab/slot hydrogen-bonded arrangements involving the lactam oxygen of one complex and methine and two methylene protons of the adjacent cation. Similar hydrogen-bonded motifs are found in other complexes derived from octaethylbilindione. [PdII(OEBOx)]I3 is isomorphic with the nickel analogue and crystallizes in the orthorhombic space group Pbcn with a = 15.2236(6) A, b = 16.7638(7) A, and c = 14.6289(6) A at 90(2) K with Z = 4. Refinement of 6123 reflections with 209 parameters and no restraints yielded wR2 = 0.094 and R = 0.036 for 4042 reflections with I > 2 sigma(I). The odd electron compound PdII(OEB.) has also been isolated and characterized by single-crystal X-ray diffraction. Black PdII(OEB.) crystallizes in the monoclinic space group I2/a with a = 13.274(3) A, b = 18.655(4) A, c = 14.114(3) A, and beta = 116.00(3) degrees at 140(2) K with Z = 4. Refinement of 2030 reflections with 195 parameters and no restraints yielded wR2 = 0.090 and R = 0.042 for 1715 reflections with I > 2 sigma(I). The structure again involves helical coordination of the linear tetrapyrrole to the palladium through the four pyrrole nitrogen atoms. Reduction of the complex causes a slight elongation of the Pd-N bonds from 1.983(2) and 1.986(2) A in [PdII(OEBOx)]I3 to 2.011(4) and 2.012(4) A in PdII(OEB.).