Synthesis of meso-phenyl-4,6-dipyrrins, preparation of their Cu(II), Ni(II), and Zn(II) chelates, and structural characterization of bis[meso-phenyl-4,6-dipyrrinato]Ni(II)

Article abstract of DOI:10.1139/v96-245

meso-Phenyldipyrromethanes can be oxidized by 2,6-dicyano-3,5-dichloro-para-benzoquinone (DDQ) to the corresponding meso-phenyldipyrrins. As expected, these novel, stable bipyrrolic pigments readily form metal chelates with copper(II), nickel(II), and zinc(II). Their UV-VIS spectra are compared with a series of known alkyl-substituted dipyrrin chelates and, based on the UV-VIS spectral analysis, the dihedral angle between the two ligands in the bis[meso-phenyldipyrrinato]Ni(II) complex was calculated to be 42°. The molecular structure of this complex was determined by X-ray crystallography, essentially confirming the calculation. Crystals of C₃₀H₂₂N₄Ni are orthorhombic, a = 17.156(3), b = 35.217(1), c = 7.886(1) A, Z = 8, space group Fddd. The structure was solved by direct methods and refined by full-matrix least-squares procedures to R = 0.040 and R_w = 0.031 for 1058 reflections with I ≥ 3σ(F²). The central nickel is coordinated in a distorted square-planar fashion by four nitrogens. The pair of the planar dipyrrinato ligands enclose a dihedral angle of 38.5°. This is the lowest angle reported for nickel(II) complexes of this kind. As a result of this, and in sharp contrast to previously described nickel(II) dipyrrin chelates, the central metal is diamagnetic.