Substituent-Induced Perturbation Symmetries and Distortions of meso-tert-Butylporphyrins

Abstract

The out-of-plane and in-plane distortions of a series of nickel(II) meso-substituted porphyrins with 0, 1, 2, or 4 tert-butyl groups [nickel(II) porphine (NiP), nickel(II) mono-tert-butylporphyrin (NiMtBuP), nickel(II) di-tert-butylporphyrin (NiDtBuP), and nickel(II) tetra-tert-butylporphyrin (NiTtBuP)] are investigated using molecular mechanics (MM) calculations, X-ray crystallography, UV-visible absorption spectroscopy, and resonance Raman spectroscopy. MM calculations are used to predict the stable conformations for this series of porphyrins. The out-of-plane distortions are then analyzed in terms of displacements along the normal coordinates of the porphyrin macrocycle using a new normal-coordinate structural decomposition method. As expected, the distortions are found to occur primarily along the lowest-frequency normal coordinate of each symmetry type and the distortions could be adequately simulated using only the lowest-frequency normal coordinates as a basis (the minimal basis). However, the distortions could be simulated significantly more accurately by extending the minimal basis by including the second-lowest-frequency normal coordinate of all symmetries. Using the extended basis is most important for the in-plane distortions. Detailed analysis of the types of distortion revealed that both the out-of-plane and the in-plane distortions depend on the perturbation symmetry of the peripheral substituents. The symmetry primarily depends on the pattern of substitution (number and positions of substituents) and the orientations of substituents. Often the perturbation symmetry can be predicted for a given porphyrin simply from the possible orientations of the substituents. Then, the main type(s) of symmetric deformation occurring for each possible molecular symmetry can be readily predicted from a D(4)(h)() correlation table. The stable conformers predicted by MM for the series of tert-butyl-substituted porphyrins confirm this simple but informative approach. Experimental verification of the calculated contributions of the symmetric deformations is provided by normal-coordinate structural decomposition of the available X-ray crystal structures of NiP, NiMtBuP, and NiDtBuP. The solid-state results are also supported by the resonance Raman and UV-visible absorption spectroscopic characterization of the porphyrins in solutions. The X-ray crystal structure of NiMtBuP is reported here for the first time.

Synthesis of Mono‐ and Disubstituted Porphyrins: A‐ and 5,10‐A2‐Type Systems

“…They were either obtained directly by functionalization of porphine or were isolated as byproducts from condensation reactions in yields of 1 %. Examples are 5-tert-butylporphyrin (14 a, 0.3 %), [11] formylation (no yield), [12a] nitration, [12b] or bromination [12c] of porphine 1, or condensation reactions of pyrrole (27) [13a] or dipyrromethane [13b] and the corresponding aldehyde. Due to the low yields their characterization is often incomplete.…”

“…Looking for an alternative to bilane we focused on using pyrrole-2-carbaldehyde (11) as one building block. Hans Fischer, in one of the earliest syntheses, originally used the aldehyde for preparing porphine.…”

“…This is most likely due to the steric bulk of the tert-butyl group. This group induces a high degree of ruffling in the porphyrin [11,22] and this effect is exacerbated if the steric strain is located in two neighboring quadrants of the macrocycle, [23] presumably leading to an instable intermediate. [24] Mono-and disubstitution of porphyrin by nucleophilic-substitution reactions: Although the described condensation reactions present a practical way of synthesizing 5-and 5,10-substituted porphyrins, reactions that proceed with conversion of~90 % of the starting material into a black, polymeric material are hardly satisfactory from an organic synthetic viewpoint.…”

Coherence Spectroscopy Investigations of the Low-Frequency Vibrations of Heme: Effects of Protein-Specific Perturbations

“…As shown in Figure 10, linear combinations of the main deformation modes (saddling, ruffling, doming, inverse doming, waving x , waving y , and propellering) are able to describe the nonplanar heme structures in proteins under investigation here, as well as most of the published X-ray structural data on heme proteins and synthetic metalloporphyrins. 34,[59][60][61][62] For example, an examination of the frequencies observed in the coherence spectra of the ferric samples of Mb and HRP (Table 1 and Table 2) results in an almost one-to-one correspondence (within ±5 cm −1 ), with differences appearing primarily in the relative amplitudes of the specific modes. This behavior supports the idea that symmetry-breaking deformations of the heme are induced to various degrees that are specific to each protein.…”

Supramolecular Structure of Extrinsically Chiral Porphyrin Hetero‐Assemblies and Achiral Analogues

“…[28][29][30] The main non-planar deformations are the ruffling, a twisting around the M-N p bond, such as in CuTPP and PdTPP, [31] the saddling, i.e the displacement of the pyrrole rings alternately above and below the mean porphyrin plane, for example in CuT4, [32] the doming often observed in five-coordinate complexes, due to the axial ligand such as in MImCoTPP, [33] and the waving, where one pair of opposing pyrrole rings is displaced up and down, while the other pair is twisted in the same direction along their M-N p bond, such as in NiTMeP. [34] Several parameters influence the porphyrin assembling.…”