Molecular structure of the methoxy-iron(III) derivative of 5,15-( o,o′(2-methyl-2′-hydroxy-3,3′-diamidobiphenyl)-diphenyl)-porphyrin and CO binding properties of iron(II)-pyridine complexes of 2,2′-substituted biphenyl strapped porphyrins

Abstract

The molecular structure of the methoxy-iron(III) derivative |(Me,OH)-Fe(OMe)| of the strapped porphyrin (5,15-( o,o′(2-methyl-2′-hydroxy-3,3′-diamidobiphenyl)-diphenyl)-porphyrin ( 1(Me,OH)) is reported. The 2-methyl-2′-hydroxy-substituted biphenyl unit and the porphyrin ring are covalently bound together by two amido-groups linking the 3,3′-positions of the biphenyl strap to two ortho-positions of the two meso-phenyl rings belonging to the 5,15-diphenylporphyrin. The iron(III) center present in | 1(Me,OH)-Fe(OMe)| is five-coordinate, high-spin ( S = 5/2). It is bonded to the four porphyrinato-nitrogens and the oxygen atom of the methoxy group. The mean-value of the four Fe-N p distances is 2.060(4) Å. The Fe-O bond length with the axial methoxy ligand is 1.867(3) Å. The porphyrin core is both domed and ruffled. The doming is small, whereas the ruffling is more pronounced. The dihedral angle of the two phenyl moieties belonging to the biphenyl strap lies close to 105°. The orientation of the entire strap lying over the porphyrin core is probably mainly determined by a strong hydrogen bond of 2.531(5) Å linking the 2′-hydroxy group of the strap and the methoxy-oxygen bonded to the iron atom. This oxygen atom of the 2′-hydroxy strap substituent is also hydrogen bonded to the NH (N50) moiety of an amido group linking covalently the strap to the porphyrin. The binding of carbon monoxide to five-coordinate iron(II)-pyridine complexes of such porphyrins | 1(Me,OH)-Fe(py)| and | 2(Me,OH)-Fe(py)| (py = pyridine) has been explored. These carbonyl adducts | 1(Me,OH)-Fe IICO(py)| and | 2(Me,OH)-Fe IICO(py)| display, respectively, in THF and in KBr pellets, two ν( 12CO) stretching vibrations at 1966, 1944 and 1965 and 1938 cm −1, respectively. The occurrence of two ν( 12CO) stretching vibrations indicates the presence of two Fe-C-O conformers in both compounds. However, these two carbonyl adducts enriched with 13CO display only a single 13CO NMR signal lying at 208.1 and 208.0 ppm, respectively. This result indicates that the two Fe-C-O conformers present in | 1(Me,OH)-Fe II(CO)(py)| and | 2(Me,OH)-Fe IICO(py)| interconvert more rapidly than the NMR time scale. Moreover, an almost linear correlation is observed between the ν( 12CO) stretching vibrations and the 13C chemical shifts of the 13CO NMR signals in a series of iron(II)—carbonyl adducts in which the second axial ligand of iron is pyridine, | 1(Me,OH)-Fe( 13CO)(py)| and | 2(X,Y)-Fe( 13CO)(py)| [(X,Y) = (Me,OH), (Me,OMe), (H,H), (Me,Me), (OMe,OMe) and (NO 2,NO 2)]. In these carbonyl adducts, the ν( 12CO) stretching vibrations vary between 1938 and 1987 cm −1 and the chemical shifts δ( 13C) of the 13C NMR signals range from 202.5 to 208.1 ppm (versus TMS). Values of δ( 13C) as large or larger than 208 ppm are observed in myoglobins and hemoglobins of various origins and in peroxidases in which the second axial ligand of iron is a histidine. Although an involvement of a hydrogen bond between a distal histidine residue and the coordinated CO molecule in the large chemical shifts of the 13CO NMR signals observed in these hemeproteins has been excluded, our model compounds, in which the axial ligand trans to CO is held constant, seem to indicate that the polarity of the 2′-OH biphenyl substituent of the porphyrin strap plays a role in the large chemical shifts of the 13CO NMR signals observed in | 1(Me,OH)-Fe( 13CO)(py)| and | 2(Me,OH)-Fe( 13CO)(py)|.