Resonance Raman spectra and structure of meso-mono substituted metalloporphyrins

Resonance Raman (RR) spectra of meso-monosubstituted derivatives of metallo -octaalkylporphyrins were measured in solution and in solid form and compar ed with those of the analogous unsubstituted complexes. For planar complexe s: (2,5,8,12,18-pentamethyl-3,7,13,17-tetraethyl-porphyrinato)zinc(II), 1; (2,8,12,18-tetramethyl-3,7,13,17-tetraethyl-5-phenylporphyrinato)zinc(II), 2; (2,3,7,8,12,13,17,18-octaethyl-5-methyl-porphyrinato)copper(II). 3, appr eciably large downshifts (9-13 cm(-1)) of nu(2) mode involving mainly Cbeta Cbeta stretching vibrations were detected. This fact was explained by steri c interactions of meso-substituents with adjacent beta-ethyl groups resulti ng in the increase of beta-ethyl substituents hyperconjugation with the a(1 u) orbital antibonding for the CbetaCbeta bonds. RR studies also indicate t he absence of any significant electronic and kinematic effects of meso-subs titution. In the case of (2,3,7,8,12,13,17,18-octaethyl-5-methyl-porphyrina to)Ni(II) (4), (2,5,8,12,18-pentamethyl-3,7,13,17-tetraethyl-porphyrinato)N i(II) (5), and (3,5,7,12, 17-pentamethyl-2,8,12,17-tetraethylporphyrinato)N i(II) (6) complexes the substitution with a single methyl group at one of t he methine-bridge carbon leads to the nonplanar distortion with a ruffled t ype structure. The degree of nonplanarity is modulated by the kind and orde ring of alkyl groups on the pyrrole rings and increases in the series of po rphyrins 6 --> 5 --> 4. The RR spectra of all meso-methylsubstituted Ni-por phyrins in solution show the evidence of the conformational heterogeneity b ecause of the presence of conformers with various but dose degree of the ma crocycle distortion. (C) 1999 Elsevier Science B.V. All rights reserved.