Resonance Raman Studies of Macrocyclic Complexes. 1. Structural and Electronic Effects in Synthetic Metal(II) Porphyrin Analogues

Resonance Raman spectra are reported for the Mn(II) (s = 5/2), Fe(II) (s = 1), Co(II) (s = 1/2), Ni(11) (s = 0), Cu(II), and Zn(II) complexes of the dianionic, π-delocalized N₄ macrocyclic ligand 5,7,12,14-tetramethyldibenzo[b,i]-[1,4,8,11]tetraazacyclotetradecahexenate (L^2-). The resonance Raman spectrum of H₂L is also discussed. All of the A₁ normal modes of the carbon-nitrogen-metal skeleton of M^IIL are observed among the resonance Raman spectra of the six complexes studied. Vibrational assignments are made for these modes. The assigned vibrations are correlated among the M^IIL complexes, revealing substantial metal-dependent shifts in ligand vibrational frequencies. The vibrations which exhibit large shifts are associated with the porphyrin-like six-membered chelate rings. These frequency shifts are, in some cases, attributable to electronic (as opposed to structural) effects of the d orbital occupancy of the metal ions. Structural effects upon Raman intensity as well as the frequency of ligand modes are observed for Mn^IIL. This complex is distorted due to considerable displacement of the metal ion out of the N₄ coordination plane. Although the peripheral methyl substituents of M^IIL are not part of the chromophoric π system of the ligand, the Raman scattering due to the methyl group vibrations is nevertheless strongly resonance enhanced. It is suggested that this is a hyperconjugative effect. Selective Raman intensity enhancement of either ligand-centered or metal-associated vibrations is observed when laser excitation is chosen within respectively π → π* or chargetransfer electronic transitions of M^IIL. The results suggest (a) that metalloporphyrin frequency shifts may be due to electronic as well as structural effects of the metal ion, (b) that vibrations of peripheral alkyl substituents must be considered in assigning resonance Raman spectra of π-delocalized macrocycles, and (c) that selective Raman intensity enhancement may have utility in assigning metal-ligand modes in natural and synthetic macrocyclic complexes. Raman intensity patterns which have important implications in the theory of the resonance Raman effect are observed, and these are discussed in the following paper.