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

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
D4h symmetry. In this Work, small deviations from D4h 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.