Diboron Porphyrins: The Raman Signature of the In-Plane Tetragonal Elongation of the Macrocycle.

We describe an unusual in-plane type of porphyrin core distortion, tetragonal elongation (TE), observed experimentally in diboron porphyrins. The vibrational spectra of several of these complexes exhibit shifts that we have assigned to this TE distortion by comparing experimental spectra with DFT computational findings. The influence of TE in porphyrin systems was isolated using DFT analysis of the well-known model compounds Ni(II)porphine and Zn(II)porphine, with the macrocycle ring constrained to eliminate the influence of out-of-plane (OOP) distortions. A significant down-shift in frequencies was observed for porphyrin normal vibrational modes, particularly the in-plane A1g /B1g modes that are dominated by contributions from stretching and bending of Cα -Cm coordinates. In contrast, TE had little effect on the v(Pyrhalfring ) and δ(Pyrdef ) modes, though the lowered symmetry of the system resulted in significant splitting of the B2u and B3u modes. The impact of the TE distortion upon the diboron porphyrin vibrational spectrum was probed experimentally using Raman spectroscopy of B2 O2 (BCl3 )2 (TTP), B2 OF2 (TTP), and B2 OPhOH2 (TTP) (TTP = 5,10,15,20-(tetra- p-tolyl)porphyrin). Comparing the experimentally obtained spectral signatures to the computational findings allowed us to assign the large shifts observed for the v2 and v3 modes to the TE distortion in diboron porphyrins.