Spectroscopic studies of novel porphyrin-copper(II) and zinc(II) complexes that share the pinch-porphyrin family structure of iron(III) complex models of peroxidases

Six novel pinch-porphyrin complexes [(picdien)(protoporphyrinate dimethyl ester)]copper(II) (7), [(picdien)(mesoporphyrinate dimethyl ester)]copper(II) (8) and [(picdien)(deuteroporphyrinate dimethyl ester)]copper(II) (9), [(picdien)(protoporphyrinate dimethyl ester)]zinc(II) (13), [(picdien)(mesoporphyrinate dimethyl ester)]zinc(II) (14) and [(picdien)(deuteroporphyrinate dimethyl ester)]zinc(II) (15), were prepared from the corresponding free copper(II)-porphyrins (4-6), and zinc(II)-porphyrins (10-12) and picdien (N-(3H-imidazol-4-ylmethyl)-N′-{2-[(3H-imidazol-4-ylmethyl)-amino]-ethyl}- ethane-2,3-diamine). Spectroscopic studies show that complexes (7-9) and (13-15) have the pinch-porphyrin type structure previously found in iron(III) complex models of peroxidases. Complexes (7-9), were characterized by u.v.-vis., m.c.d., and e.s.r. spectroscopy. E.s.r. spectra of the copper parent compounds (4-6) at ca. 10^-2-10^-4 concentrations were typical of copper(II)-dimers. The addition of the picdien ligand broke up the dimers as detected by e.s.r. Compounds (7-9) are predominantly monomeric at ca. 10^-3 M concentration. The presence of picdien in (7-9) distorts the porphyrin internal portion of the plane so as to make these four internal nitrogen atoms, coordinated to copper(II), e.s.r-distinguishable. MO and ligand field theories were used to characterize and to evaluate the directional covalence parameters of compounds (7-9). A non-fully axial, out-of-the-porphyrin-plane bonding was found for (7-9), similar to the bonding of the pinch-porphyrins-iron(III). However the in-plane distortion produced by the presence of the picdien ligand on copper(II) is significantly larger than in pinch-porphyrin-iron(III). The n.m.r. data show that the porphyrin-zinc(II) is the less strained and has the weakest bonded structure. The coordination number of the pinch-porphyrin with iron(III), copper(II) and zinc(II), is in all cases six.