Structure-sensitive resonance Raman bands of capped tetraphenylporphyrinatoiron complexes

Abstract

Resonance Raman spectra have been determined for the capped tetraphenylporphyrinatoiron complexes and their base adducts. The five-coordinate mono-base complexes of these porphyrin complexes dislay Raman spectra that are nearly identical to those previous y determined for fivecoordinate iron(II) tetraphenylporphyrin complexes in the marker regions around 1540, 1345, and 370 cm-1. A similar correspondence is found between Raman bands of iron(II) complexes of the capped complexes and those of tetraphenylporphyrin. By contrast, the unusual pseudo-six-coordinate iron(II) complexes more closely resemble high-spin five-coordinate systems in the 1540 and 1345 cm-regions, but in the 380 cm-1 region the resemblance is closer to the low-spin six-coordinate tetraphenylporphyrin complexes. These results provide spectroscopic confirmation for the unique nature of these nseudo-six-coordinate complexesThe capped tetraphenylporphyrins ( Fig. 1) provide an interesting testing ground for the molecular features that control reversible binding of dioxygen (1-4). Recently it was shown that the cap size has a profound influence on the binding of bases and dioxygen (3,4). When the cap is compact, x = 2 in Fig. 1, the iron(II) complex of the 5,10,15,20-[pyrromellitoyl(tetrakis-o-oxyethoxyphenyl)]porphyrinate ion is designated as Fe(Cap). A less sterically hindered iron(II) complex, 5,10,15, 20-[pyrromellitoyl(tetrakis - Fe(HmCap), is formed with the homologous cap porphyrin, for which x = 3 in Fig. 1. Detailed equilibrium measurements reveal that the more hindered Fe(Cap) adds only one ligand, such as 1-methylimidazole (1-MeIm) or pyridine (py), to form five-coordinate complexes. The less sterically hindered Fe(HmCap) is capable of forming pseudo-six-coordinate complexes with these same bases. These pseudo-six-coordinate complexes, such as Fe(HmCap)(1-MeIm)2, are of intermediate electronic spin (S = 1) and add oxygen without ligand displacement to give pseudo-sevencoordinate complexes-e.g., Fe(HmCap)(1-MeIm)2(02). Smaller bases, such as n-propylamine (n-PrNH2) and secbutylamine, add normally to Fe(HmCap) and give the expected diamagnetic bis-base complexes Fe(HmCap)(base)2. Bulky bases, such as t-butylamine and 1,2-dimethylimidazole, form only five-coordinate complexes with both iron(II) capped systems.In the present Raman spectroscopic investigations of these unusual systems our objectives were to extend the correlation of structure-sensitive bands to the capped tetraphenylporphinatoiron(II) complexes and then to investigate the behavior of the structure-sensitive vibrations for the unique complexes of Fe(HmCap) with hindered bases. In addition we were interested in the possibility of obtaining Raman spectroscopic evidence for porphyrin strain or other factors that might explainThe publication costs of this article were defrayed in part by page charge payment. This article must therefore be hereby marked "advertisenent" in accordance with 18 U. S. C. §1734 solely to indicate this fact. the difference in oxygen affinity of the...