NICKEL(II) COMPLEXES OF THE OCTAETHYLOXOPHLORIN DIANION AND OCTAETHYLOXOPHLORIN RADICAL DIANION

The nickel(II) complex of the octaethyloxophlorin dianion, Ni(II)(OEPOH), dissolves in pyridine to form red, air-sensitive solutions that contain diamagnetic, and therefore planar, four-coordinate Ni(II). Crystals of Ni(II)(OEPOH) form from pyridine in the tetragonal space group I4(1)/a with a = 14.780(9) angstrom and c = 13.903(14) angstrom at 130 K with Z = 4. Refinement of 558 reflections and 102 parameters yielded R = 0.057 and R(w) = 0.077. The complex is isomorphic with the tetragonal form of nickel(II) octaethylporphyrin. The meso hydroxyl groups are disordered over four equally populated sites, and the oxophlorin core shows a severely ruffled distortion. The Ni-N distance is 1.921(6) angstrom. Oxidation of Ni(II)(OEPOH) in pyridine with dioxygen or diiodine yields brown solutions of the paramagnetic radical (PY)2Ni(OEPO.), which can be isolated in solid form and shows an isotropic EPR resonance at g = 2.006 in frozen pyridine at -90-degrees-C. The magnetic moment of the radical in pyridine (2.5(2) mu(B)) can be explained with strong antiferromagnetic coupling between the ligand radical and the nickel ion. Crystallization of the radical with exposure to air yields red parallelepipeds that form in the triclinic space group P1BAR with a = 9.848(2) angstrom, b = 10.216(2) angstrom, c = 10.492(2) angstrom, a = 80.34(3)-degrees, beta = 89.82(3)-degrees, gamma = 66.36(3)-degrees at 130 K with Z = 1. Refinement of 2318 reflections and 259 parameters yielded R = 0.036 and R(w2) = 0.068. The nickel is six-coordinate, and the porphyrin is nearly planar. The Ni-N(oxophlorin) distances (2.062(2) and 2.063(2) angstrom) and the longer Ni-N(py) distance (2.226(2) angstrom) are indicative of the presence of high-spin (S = 1) Ni(II) in the solid. The meso oxygen is unequally disordered over four sites. Refinement. of the oxygen atom occupancy led to the discovery that the solid contained additional oxygen above the one oxygen required for (PY)2Ni(II)(OEPO'). Consequently, the crystals are formulated as containing both (PY)2Ni(II)(OEPO') and (py)2Ni(II)(OEPO2) where OEPO2 represents a mixture of the cis- and trans-isomers of octaethyldioxoporphodimethene. In separate chemical experiments, the conversion of the nickel oxophlorin radical by dioxygen into a mixture of Ni(II)(cis-OEPO2) and Ni(II)(trans-OEPO2) has been confirmed.