Porphyrin distortion

Olea C, Kuriyan J, Marietta MA (2010) Modulating heme redox potential through protein-induced porphyrin distortion. J Am Chem Soc 132 12794—12795... 

Figure 3-2 Proposed mechanism for the ferrochelatase catalyzed reaction. Porphyrin distortion induced by metal binding leads to a domed transition state with concomitant porphyrin-proton release (Adapted from [41]). FC, ferrochelatase M, substrate metal ion.

Resonance Raman spectra were used to analyse distortions of the porphyrin substrate in enzyme and antibody catalysed ferrochelation reactions. SERS data were used to follow the metallation of 5,10,15,20-tetrakis(l-methyl-4-pyridyl)porphyrin in silver colloids.The resonance Raman spectrum of mesoporphyrin IX bound to ferrochelatase gave evidence of porphyrin distortion due to the binding. ... 

An interesting system for the catalytic aerobic oxidation of benzyl alcohol was developed by Albrecht and co-workers. A porphyrin rhodium(iii) complex [(IMe)2Rh(TPP)]Cl (TPP = meso-tetraphenyl porphyrin) with two apical NHC ligands led to porphyrin distortion and dearomatization. This coordi-natively saturated complex was catalytically active in the oxidation of benzyl alcohol. It was demonstrated that its catalytic activity was imparted by NHC dissociation, as facile cleavage of the Rh-NHC bond was promoted by the strong trans effect of the distal NHC ligand and by the distorted and partially dearomatized porphyrin. 

Methyl-coenzyme M reductase participates in the conversion of CO2 to CH4 and contains 6-coordinate nickel(II) in a highly hydrogenated and highly flexible porphyrin system. This flexibility is believed to allow sufficient distortion of the octahedral ligand field to produce low-spin Ni" (Fig. 27.7) which facilitates the formation of a Ni -CHs intermediate. 

In (87BBPC941), Limbach made a major contribution to understanding crystalline and amorphous environments using compounds that in the gas phase show symmetric double minimum potentials. Several compounds were used to illustrate the periodic (crystal) and random (glass) distortions of the potential surface, among them H2P and porphycene. Tlie seminal paper on the tautomerism of porphyrins in the solid state is that of Limbach... 

Smith (91) reported an X-ray crystal structure of a zinc porphyrin polymer (77, Fig. 32) where, unusually, the coordination bond is between a nitro group and the zinc center. The tetranitroporphyrin is highly substituted, and the resulting steric hindrance causes the macrocycle to be noticeably distorted. Adjacent porphyrin planes in the polymer are almost orthogonal. However, there is no evidence of polymerization in solution, and the nitro-zinc interaction is probably too weak to maintain this structure outside the solid state. 

The consequences of polychlorination of porphyrins on redox properties of complexes has been investigated.1404 The highly chlorinated porphyrin 3-octachloro-/ /c.vo-tetrakis(3,5-dichloro-2,6-dimethoxyphenyl)porphyrin exhibits a substantial anodic shift for reduction of over 0.5 V and a smaller shift for oxidation versus the unchlorinated precursor. Contrastingly, small potential shifts for the octabromo-substituted 5,10,15,20-tetraphenylporphyrinate arise from the dominance of macrocycle ruffling over electronic effects. In the polychloro complex, distortion does not compensate fully for electron-withdrawing effects of the Cl substituents. 

A detailed analysis of Ni11 complexes with mew-substituted porphyrins bearing zero, one, two, or four /-butyl groups revealed that both the out-of-plane and in-plane distortion depend on the perturbation symmetry of the peripheral substituents (number and position of substitutents), and their orientation.1775 These results have implications for understanding the role of nonplanar distortions in the function of metalloproteins containing nonplanar porphyrins.1776... 

Square-planar zinc compounds predominate with these ligand types as would be predicted. This is in contrast to the prevalence of tetrahedral or distorted tetrahedral geometries for four-coordinate species that have been discussed thus far. Zinc porphyrin complexes are frequently used as building blocks in the formation of supramolecular structures. Zinc porphyrins can also act as electron donors and antenna in the formation of photoexcited states. Although the coordination of zinc to the porphyrin shows little variation, the properties of the zinc-coordinated compounds are extremely important and form the most extensively structurally characterized multidentate ligand class in the CSD. The examples presented here reflect only a fraction of these compounds but have been selected as recent and representative examples. Expanded ring porphyrins have also... 

The functionalization of zinc porphyrin complexes has been studied with respect to the variation in properties. The structure and photophysics of octafluorotetraphenylporphyrin zinc complexes were studied.762 Octabromoporphyrin zinc complexes have been synthesized and the effects on the 11 NMR and redox potential of 2,3,7,8,12,13,17,18-octabromo-5,10,15,20-tetraarylporphyrin were observed.763 The chiral nonplanar porphyrin zinc 3,7,8,12,13,17,18-heptabromo-2-(2-methoxyphenyl)-5,10,15,20-tetraphenylporphyrin was synthesized and characterized.764 X-ray structures for cation radical zinc 5,10,15,20-tetra(2,6-dichlorophenyl)porphyrin and the iodinated product that results from reaction with iodine and silver(I) have been reported.765 Molecular mechanics calculations, X-ray structures, and resonance Raman spectroscopy compared the distortion due to zinc and other metal incorporation into meso dialkyl-substituted porphyrins. Zinc disfavors ruffling over doming with the total amount of nonplanar distortion reduced relative to smaller metals.766 Resonance Raman spectroscopy has also been used to study the lowest-energy triplet state of zinc tetraphenylporphyrin.767... 

Besides ruthenium porphyrins (vide supra), several other ruthenium complexes were used as catalysts for asymmetric epoxidation and showed unique features 114,115 though enantioselectivity is moderate, some reactions are stereospecific and treats-olefins are better substrates for the epoxidation than are m-olcfins (Scheme 20).115 Epoxidation of conjugated olefins with the Ru (salen) (37) as catalyst was also found to proceed stereospecifically, with high enantioselectivity under photo-irradiation, irrespective of the olefmic substitution pattern (Scheme 21).116-118 Complex (37) itself is coordinatively saturated and catalytically inactive, but photo-irradiation promotes the dissociation of the apical nitrosyl ligand and makes the complex catalytically active. The wide scope of this epoxidation has been attributed to the unique structure of (37). Its salen ligand adopts a deeply folded and distorted conformation that allows the approach of an olefin of any substitution pattern to the intermediary oxo-Ru species.118 2,6-Dichloropyridine IV-oxide (DCPO) and tetramethylpyrazine /V. V -dioxide68 (TMPO) are oxidants of choice for this epoxidation. 

The puzzle may be explained by the operation of electrostatically induced geometric distortion in both ground and giant CS states of syn,syn-24. Gas phase ab initio MO calculations indicate that there is an electrostatic attraction between the zinc porphyrin and the viologen dication in the ground state of... 

The geometry at cerium is distorted square antiprismatic with a mean stagger of -28°, a value noticeably less than that (42°) observed in the porphyrin complex. The Ce-N distances are in the range 2.411(9)-2.430(9) A, distances that are markedly shorter than those in the porphyrin structure that range between 2.467(3) and 2.483(3) A (Fig. 8). 

Structural Analysis. H2[pz(A4)] A = dispiroketal (190) has been structurally characterized, Fig. 40, and is shown to exhibit a noncrystallographic D2 symmetry the central core is planar to within 0.08 A. The presence of hydrogen atoms on the nitrogen centers distorts the potential D4 symmetry of the non-hydrogen atoms The transannular porphyrin N—N distances differ by-0.15 A (11). 

---