Planar and nonplanar conformations of (meso-tetraphenylporphinato)nickel(II) in solution as inferred from solution and solid-state Raman spectroscopy

The coexistence in solution of at least two conformers of (meso-tetraphenylporphinato)nickel(II) [Ni(TPP)] is inferred from solution and single-crystal resonance Raman spectra obtained at different temperatures (170 - 297 K) and excitation wavelengths (413.1 and 457.9 nm). The shapes of the structure-sensitive Raman lines v(8) and v(2) are clearly asymmetric and change with temperature. These broad lines can be decomposed into at least two sublines, a low-frequency (LF) and a high-frequency (HF) component. In contrast, the corresponding single-crystal Raman lines of the nonplanar structure of Ni(TPP) in the crystal are narrow and symmetric. For the line v(2), the broad LF subline results from nonplanar conformers and the narrow HF subline arises from a more planar conformer. This assignment is consistent with the observation that the LF subline of v(2) is more enhanced upon changing the excitation wavelength from 413.1 to 457.9 nm. The selective resonance enhancement is caused by the red shifts of the UV-visible absorption bands and Raman excitation profiles of the nonplanar form. The frequency assignment for the sublines of vs is reversed from that of v(2) (i.e., the HF subline of v(8) arises from nonplanar conformers and the LF subline results from a more planar macrocycle). This assignment is based on subline broadness and enhancement behavior using an excitation wavelength located on the red side of the B band. The assignments of the sublines to the nonplanar conformer are also in agreement with the Raman spectra of single crystals in which Ni(TPP) is known from X-ray crystallography to have a predominantly ruffled nonplanar conformation. Specifically, the frequencies of the sublines of v(8) and v(2) that are assigned to the nonplanar form in solution closely match the frequencies of Ni(TPP) in the single crystal. We propose two thermodynamic models for the interpretation of the temperature dependence of the intensity ratios of the sublines. A two-state model assumes one planar and one nonplanar conformer in solution. From the slopes in the van't Hoff plots, the nonplanar conformer is energetically favored by about 1.8 kJ mol(-1) in this model. Since molecular mechanics calculations predict three conformers of Ni(TPP) in solution, we also consider a three-state model. The three structures are two nonplanar structures of purely ruffled (ruf) and purely saddled (sad) macrocyclic distortions and a planar conformer. In the calculations, the ruf conformation is the lowest-energy structure with the planar and sad conformers having almost equal higher energies. Assuming this relationship between the energies, but allowing the actual energy separation to vary, the three-state analysis gives the similar result that the ruf conformation is stabilized by about 2.3 kJ mol(-1) with respect to the planar and sad conformers.