Quantum Chemistry-Based Analysis of the Vibrational Spectra of Five-Coordinate Metalloporphyrins [M(TPP)Cl]

• Published in: Inorganic chemistry Vol. 45; no. 7; pp. 2835 - 2856
• Main Authors: Paulat, Florian, Praneeth, V. K. K, Näther, Christian, Lehnert, Nicolai
• Format: Journal Article
• Language: English
• Published: United States American Chemical Society 03.04.2006
• Subjects: Metals, Heavy - chemistry; Models, Chemical; Models, Molecular; Organometallic Compounds - chemical synthesis; Organometallic Compounds - chemistry; Quantum Theory; Reference Standards; Sensitivity and Specificity; Spectroscopy, Fourier Transform Infrared - methods; Spectroscopy, Fourier Transform Infrared - standards; Spectrum Analysis, Raman - methods; Spectrum Analysis, Raman - standards; Vibration
• ISSN: 0020-1669; 1520-510X
• DOI: 10.1021/ic0510866

Vibrational properties of the five-coordinate porphyrin complexes [M(TPP)(Cl)] (M = Fe, Mn, Co) are analyzed in detail. For [Fe(TPP)(Cl)] (1), a complete vibrational data set is obtained, including nonresonance (NR) Raman, and resonance Raman (RR) spectra at multiple excitation wavelengths as well as IR spectra. These data are completely assigned using density functional (DFT) calculations and polarization measurements. Compared to earlier works, a number of bands are reassigned in this one. These include the important, structure-sensitive band at 390 cm-1, which is reassigned here to the totally symmetric νbreathing(Fe−N) vibration for complex 1. This is in agreement with the assignments for [Ni(TPP)]. In general, the assignments are on the basis of an idealized [M(TPP)]+ core with D 4 h symmetry. In this Work, small deviations from D 4 h are observed in the vibrational spectra and analyzed in detail. On the basis of the assignments of the vibrational spectra of 1, [Mn(TPP)(Cl)] (2), and diamagnetic [Co(TPP)(Cl)] (3), eight metal-sensitive bands are identified. Two of them correspond to the ν(M−N) stretching modes with B1g and Eu symmetries and are assigned here for the first time. The shifts of the metal sensitive modes are interpreted on the basis of differences in the porphyrin C−C, C−N, and M−N distances. Besides the porphyrin core vibrations, the M−Cl stretching modes also show strong metal sensitivity. The strength of the M−Cl bond in 1−3 is further investigated. From normal coordinate analysis (NCA), force constants of 1.796 (Fe), 0.932 (Mn), and 1.717 (Co) mdyn/Å are obtained for 1−3, respectively. The weakness of the Mn−Cl bond is attributed to the fact that it only corresponds to half a σ bond. Finally, RR spectroscopy is used to gain detailed insight into the nature of the electronically excited states. This relates to the mechanism of resonance enhancement and the actual nature of the enhanced vibrations. It is of importance that anomalous polarized bands (A2g vibrations), which are diagnostic for vibronic mixing, are especially useful for this purpose.