Vibrational Spectra of 5,10,15,20-Tetraphenylporphyrin (H2,TPP) and Platinum(II) 5,10,15,20-Tetra(phenyl/pentafluorophenyl) porphyrins (PtTPP and PtTF5PP)

Synthesis of two platinum complexes, 5,10,15,20-tetraphenylporphyrin (PtTPP) and 5, 10,15,20-petyluorotetraphenylporphyrin (PtTF 5 PP), is described. IR spectra of the complexes and metal-free 5,10,15, 20-tetraphenylporphyrin (H2TPP) were obtained in KBr tablets. Structural parameters of conformers were optimized using DF77/B3LYP/cc-pVTZ (C,N,H,F), aug-cc-pVTZ-PP, ECP (Pt), and their vibration frequencies were calculated. Conformers differ mainly in angles of rotation of substituents at meso-position relative to macrohetemcyclic backbone. For H2TPP, the most stable conformer possesses C-2 nu symmetry, for PITPP -D-2d, and for PtTF5PP- D 4th. Energy difference between conformers of each molecule does not exceed 1.1 kJ/mol. Experimental and theoretical IR spectra are presented, their comparison determines the scaling factor (0.977) for theoretical vibration frequencies of all three molecules in the range of 500-1700 cm(-1). Interpretation of experimental IR spectra has been performed. For all frequencies of normal vibrations, the potential enelgv distribution along the internal vibrational coordinates is analyzed. The spectra of the three compounds in the range of 2800-3400 cm(-1). differ from each other. The IR spectrum of the H2TPP compound contains a band at 3370 cm(-1), which belongs to the stretching vibration nu(N-H), while the IRS of the PtTF5PP compound lacks a band at 2925 cm(-1) belonging to the v(C pb -H) vibrations ofphenyl fragments. The frequencies v(N-H), nu(C-b-H) and nu(C-ph-H) correlate with the values of corresponding internuclear distances. The shorter the bond length, the higher the force constant of stretching of this bond and the higher the frequency of the stretching vibration. Introduction of platinum atom into coordination cavity changes the relative intensity of bands in region of 500-1700 cm(-1) and decreases their number, which is associated with an increase in the symmetry of PtTPP and PITF5PP molecules as compared to H2TPP. Most of vibrational modes in the region of 500-800 cm(-1) are mixed and represent a linear combination of several internal coordinates. They refer to nonplanar out-of-plane vibrations of macroheterocyclic backbone, as well as to nonplanar deformations of carbon skeleton in the -C6H5 and -C6F5 substituents. Frequencies in the range 900-1600 cm(-1) mainly refer to stretching vibrations of C-a-C-b, C-ph-C-ph, C-a-C-m, C-a-N, C-F bonds, as well as to plane and non-planar vibrations associated with a change in bond angles of C-a-C-b-H in the backbone and C-ph-C-ph-H angles in the -C6H5 substituents. The calculated IR spectra in the low-frequency region up to 500 cm(-1) reflect the structural nonrigidity of molecules and correspond to vibrations at which the macrocyclic backbone of the dome, raffling, and wave type is distorted, as well as distortions occurring in plane of the backbone. The same region contains the frequencies of torsional vibrations of-C6H5 and -C6F5 substituents. Difference in the IR spectra is considered and the characteristic frequencies of three compounds are determined A distinctive feature of IR spectrum of PtTF5PP is the presence of intense absorptions, which belong to C-F stretching vibrations of different symmetry, and which are absent in IR spectra of H2TPP and PtTPP. H2TPP spectrum contains a band at 883 cm(-1), which is associated with a combination of vibrational coordinates r(C-al-C-b1(2)) and r(C-a2-C-b2) in two nonequivalent fragments. This band is absent in the IR spectrum of platinwn complexes. The relationship between the geometric parameters and frequencies of stretching vibrations of H2TPP, PtTPP, and PtTF5PP molecules is shown.