Biphenyl-strapped diphenylporphyrins: Synthesis and spectroscopic characterization of a series of porphyrins with ether-linked straps. Preliminary CO binding properties of their iron(II) derivatives

The synthesis of a series of meso-5,15-diphenylporphyrins strapped by 2,2'-disubstituted biphenyl units (6H(2), 7H(2), 8H(2)) is described. The biphenyl straps used are linked via their 3,3'-positions by CH2O groups to ortho-positions of the phenyl rings of the 5,15-diphenylporphyrins. The straps of these porphyrins have their 2,2'-positions occupied by H (6H(2)), Me (7H(2)), and OMe (8H(2)). The electronic spectral properties of these strapped porphyrins are given, and their conformational properties, which have been studied by H-1 NMR spectroscopy, are described. The synthesis and spectroscopic characterization of several iron(II) and iron(III) derivatives of these porphyrins with different axial bases are also presented. The X-ray structure of the chloroiron(III) derivative of the diphenylporphyrin dianion 6 has been determined. Violet crystals of composition 6-FeCl . CH3OH . CH2-Cl-2 have been obtained by slow evaporation of CH2Cl2/MeOH solutions of 6-FeCl. These crystals belong to the monoclinic system, space group P2(1)/c, with a = 10.164(3) Angstrom, b = 28.977(9) Angstrom, c = 14.283(4) Angstrom, beta = 106.22(2)degrees and Z = 4. The iron atom of 6-FeCl is five-coordinate, high-spin. The axial chloride ligand lies opposite to the strap. The average Fe-N-p bond distance is 2.046(6) Angstrom, and the Fe-CI bond length equals 2.234(2) Angstrom. The porphyrin is slightly domed and ruffled with the iron atom lying at 0.45(1) Angstrom out of the 4N(p) mean-plane and 0.49(1) Angstrom out of the 24-atom core mean-plane of the porphyrin ring. Ferrous carbonyl adducts of these porphyrins have been obtained in various solvents in the presence or absence of nitrogenous bases. Their structures have been studied by H-1 NMR and IR spectroscopy. Two CO stretching vibrations differing by 20 cm(-1) occur in the IR spectra of 6-Fe(CO)(B) (B = py, ImH, NMeIm). The possible origin of these two vco bands is discussed.