Thermodynamics of axial ligand addition and spectroscopic trends of a series of symmetrical and unsymmetrical derivatives of tetraphenylporphinatozinc(II) and -iron(III)

Abstract

The electronic spectra and equilibrium constants for addition of 3-picoline to a series of symmetrically and unsymmetrically phenyl-substituted ZnTPP derivatives have been measured. It is found that the α band energy varies slightly nonlinearly, yet systematically, with the sum of the Hammett sigma constants of the substituents within the series ( p-Cl) x( p-NEt 2) yTPPZn(II) and related complexes, while the smaller variation in the β band appears to be linear. The log of the intensity ratio of the α and β bands, log A β/A α, however, varies linearly with the band energies of both the α and β bands for both 4- to 5-coordinate complexes of unsymmetrical ZnTPP derivatives. Likewise, log K eq for 3-picoline addition to this series of ZnTPPs varies linearly with the sum of the Hammett sigma constants for all complexes investigated, irrespective of the symmetry of placement of phenyl substituents. Thus the electronic effects of unsymmetrically placed substituents are averaged by the metal Zn to yield a Lewis acid strength toward 3-picoline which is dependent only on the sum of the electronic effects and not on the identity of the substituents or the symmetry of their distribution. In contrast to the results for Zn(II), the same series of TPPFe(III) complexes do not exhibit spectroscopic trends independent of the symmetry of placement of the substituents in either the high-spin chloroiron(III) or low-spin bis-N-methylimidazoleiron(III) forms. Likewise, logβ 2 for N-methylimidazole addition does not vary linearly with the sum of the Hammett sigma constants for unsymmetrically substituted TPPFe(III) derivatives. Rather, unsymmetrically substituted TPPFe(III) derivatives deviate from the linear relationship shown by the symmetrical complexes [1] in either a negative direction (log β 2 smaller expected, based on Σσ), as is found for p-Cl, p-NEt 2 mixed substituent complexes, or in a positive direction (log β 2 larger than expected on the basis of Σσ), as is found for p-NO 2, p-H or m-NO 2, m-CH 3, or m- or p-NHCOCH 3, H mixed substituent combinations. The reason for the difference in thermodynamic and electronic spectral behavior of the unsymmetrically phenyl substituted TPP derivatives of Zn(II) and Fe(III) must be due to the difference in electron configuration of the two metals (d 10 vs. d 5), and the change in spin state of Fe(III) upon bis-N-methylimidazole complex formation. In the case of Fe(III), the unsymmetrical electron configuration of the low-spin Fe(III) product apparently makes the metal sensitive to both the symmetry and the nature of the substituents, probably due to extensive mixing of metal and porphyrin π-symmetry orbitals (the d xz, d yz, e-symmetry metal orbitals are unsymmetrically filled). The results imply that the nature and pattern of porphyrin substituents in naturally-occurring heme proteins ( i.e., cytochromes b, c, a, hemoglobin, etc.) are carefully chosen to maximize the stability of metal-ligand bonds, in addition to controlling other physical properties.