One-pot synthesis of 5,10,15-triphenylcorrole has been achieved by reaction of benzaldehyde with an excess of pyrrole; the triphenylphosphinocobalt complex of 5,10,15-triphenylcorrole has been structurally characterized using X-ray crystallography

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The influence of substituents with increasing steric demands on the structure of nickel(II) 5,15-disubstituted porphyrins has been investigated with X-ray crystallography, UV−visible absorption spectroscopy, resonance Raman spectroscopy, molecular energy optimization calculations, and INDO/s molecular orbital calculations. Nickel 5,15-diphenylporphyrin is predicted by molecular mechanics calculations to be a mixture of planar and nonplanar conformers. All of the nickel dialkylporphyrins (where the alkyl group is propyl, isopropyl, and tert-butyl) are calculated to be in a predominantly gabled (gab) conformation resulting from an αα orientation of the substituents with respect to the macrocycle. This nonplanar gab distortion is made up of a linear combination of distortions along the lowest-frequency out-of-plane macrocycle normal coordinates of A2u (doming) and B1u (ruffling) symmetry types. A higher energy stable αβ conformer is also predicted for dialkylporphyrins, and its nonplanar structure can be represented as an equal combination of distortions along the x- and y-components of the lowest-frequency Eg (waving) normal coordinate. The nonplanar structures calculated by using molecular mechanics have been structurally decomposed into the displacements along the lowest-frequency normal coordinate of each symmetry type, and the contributions of each type to the total distortion in the calculated structures agree well with contributions obtained from structural decompositions of the available X-ray crystal structures. The predicted gab distortion is confirmed most convincingly by the X-ray crystal structure of [5,15-di-tert-butylporphinato]nickel(II) which is found to be in a gab αα conformation. Finally, INDO/s calculations show that the red shifts in the absorption spectra of the nickel disubstituted porphyrins are caused by the increasing nonplanarity resulting from increasing steric crowding within the series.

The photophysical properties and their temperature dependence are reported for the sterically encumbered nonplanar zinc and free base 2,3,5,7,8,10,12,13,15,17,18,20-dodecaphenylporphyrins (ZnDPP and H2DPP), and 2,3,7,8,12,13,17,18-octaethyl-5,10,15,20-tetraphenylporphyrins (ZnOETPP and H2OETPP), and the zinc complex of 5,10,15,20-tetra-tert-butylporphyrin (ZnT(t-Bu)P). Compared to planar 5,10,15,20-tetraphenylporphyrins (ZnTPP and H2TPP), the above compounds exhibit reduced lifetimes of the lowest excited singlet state, reduced fluorescence yields, and large shifts between their absorption and emission maxima at room temperature. ZnT(t-Bu)P, which is known to adopt a ruffled conformation, displays dramatically altered photophysical properties including a 7 ps 1(π,π*) lifetime compared to one of ∼2 ns for ZnTPP at 296 K. Equally noteworthy is the return of the ZnT(t-Bu)P singlet lifetime to a “normal” value of 2.5 ns at 78 K. An analogous temperature dependence has been observed previously for the free base analog H2T(t-Bu)P. The other porphyrins investigated, with different modes of nonplanarity, display smaller temperature variations but also tend toward more normal properties at low temperatures. A more extreme case of perturbation to the tetrapyrrole electronic structure is found in zinc 2,3,5,5‘,7,8,12,18-octamethyl-13,17-bis(3-methoxy-3-oxopropyl)isoporphyrin perchlorate, a porphyrin tautomer with an interrupted π system. This zinc isoporphyrin also exhibits a short excited state lifetime of 130 ps at 296 K, which again increases to 0.8 ns at 78 K. The results for the various nonplanar porphyrins and for the isoporphyrin in several solvents indicate that the principal cause of the altered excited state lifetimes is the ability of the molecules to traverse multiple conformational surfaces in the excited state. These surfaces appear to be separated by only small energy barriers that vary with the types of conformational distortions and their concomitant perturbations of the electronic structures of the chromophores.

The effects of ruffling on the axial ligation properties of a series of nickel(II) tetra(alkyl)porphyrins have been investigated with UV-visible absorption spectroscopy, resonance Raman spectroscopy, X-ray crystallography, classical molecular mechanics calculations, and normal-coordinate structural decomposition analysis. For the modestly nonplanar porphyrins, porphyrin ruffling is found to cause a decrease in binding affinity for pyrrolidine and piperidine, mainly caused by a decrease in the binding constant for addition of the first axial ligand; ligand binding is completely inhibited for the more nonplanar porphyrins. The lowered affinity, resulting from the large energies required to expand the core and flatten the porphyrin to accommodate the large high-spin nickel(II) ion, has implications for nickel porphyrin-based molecular devices and the function of heme proteins and methyl-coenzyme M reductase.

Interest in how nonplanar conformational distortions affect the properties of tetrapyrroles, in particular their biological function, has led to the synthesis of sterically crowded and highly nonplanar model systems such as 1-4. 1 More than 30 crystal structures have now been reported for this group of dodecasubstituted porphyrins, sufficient to allow some conclusions to be drawn regarding the conformational preferences and flexibility of these novel macrocycles. Generally, the crystallographic data reveals a strong preference for a saddle conformation, with or without a small amount of ruffle distortion (see Figure 1). However, in contrast to regular porphyrins (e.g., NiOEP, of which there are three known crystalline forms exhibiting both planar and ruffled conformations), 2 dodecasubstituted porphyrins have not so far been demonstrated to possess a high degree of conformational flexibility. To determine if this is a general feature of dodecasubstituted porphyrins, we have undertaken an investigation of the conformational properties of the dodecaarylporphyrins 4-6. We show herein that crystal structures of these porphyrins provide ample evidence of the significant conformational flexibility that can be observed for dodecasubstituted porphyrins; the structures we report include the first highly ruffled dodecasubstituted porphyrin, the first examples of multiple conformations in a single dodecasubstituted porphyrin, and the first "wave" core structure of a dodecasubstituted porphyrin, which is also the most nonplanar wave structure reported to date for any porphyrin. Figure 2a-d shows the crystal structures of the nickel(II) complexes of porphyrins 4-6. The crystal structure of 4 (M ) Ni) 3,4a (Figure 2a) demonstrates a well-defined ruffled core conformation, a deformation mode not previously observed in the sterically crowded dodecasubstituted porphyrin series. A maximum displacement of 0.855 (7) Å (for C meso ) and an average displacement of 0.430(7) Å of the core atoms from the porphyrin mean plane (based on a least-squares plane calculated for the 24 core atoms) were observed, making this one of the most ruffled porphyrins ever reported. The pyrrole rings exhibit twist angles of 22.81(16)°(with respect to the porphyrin mean plane) and Ni-N distances of 1.909(5) Å. In contrast, the closely related porphyrin 5 (M ) Ni) 4b (Figure 2b) crystallizes in a well-defined saddle conformation; in this case, a maximum displacement of 1.014(3) Å (for C ), and an average displacement of 0.542(4) Å for the core atoms from the porphyrin mean plane was observed. Pyrrole tilt angles (relative to the porphyrin mean plane) of 22.77(9)°and Ni-N distances of 1.911(5) Å were observed. Interestingly, for 6 (M ) Ni, 1 and 2) 4c,d two significantly different crystalline forms (Figure 2c,d) were apparent; these represent the first documented occurrence of this phenomenon in a dodecasubstituted porphyrin. The two crystalline forms were found to have different proportions of ruffle and saddle distortions with one form possessing a more ru...

Transient absorption measurements with subpicosecond resolution have been performed on nickel(II) dodecaphenylporphyrin (NiDPP), a highly nonplanar tetrapyrrole. Following photoexcitation, NiDPP deactivates by the pathway (π,π*) f (d,d) f ground state, the route proposed previously for planar analogues. However, the (π,π*) state has now been spectrally and kinetically resolved, and evidence is presented that the deactivation proceeds mainly in the singlet manifold. The lifetime of the 1 (π,π*) state of NiDPP is about 0.7 ps in all solvents investigated and increases only slightly as the temperature is reduced. The (d,d) state exhibits complex spectral evolution over the following 20 ps or more, which is interpreted in terms of vibrational relaxation and cooling, together with changes in the conformation of the porphyrin macrocycle. The ligandfield excited state decays with a lifetime of about 120 ps in toluene, but this decay time, like the vibrational and conformational dynamics in the (d,d) state, slows considerably in mineral oil and at low temperature. The solvent dependence of the ground-state absorption spectrum combined with the dependence of the excitedstate kinetics on detection wavelength, viscosity, and temperature suggests that NiDPP has a number of accessible conformers and that the energies of these conformers and the barriers between them differ with the electronic state. Such conformers probably differ in the type and degree of nonplanar distortion (e.g., saddle or ruffled) and in the orientations and solvent interactions of the phenyl rings. The results presented here for NiDPP, together with previous time-resolved data on metal-free nonplanar porphyrins, suggest that key properties of nonplanar tetrapyrroles include ready access to multiple conformers and a propensity for photoinduced conformational changes. Collectively, the results of spectroscopic studies on nonplanar porphyrins suggest that static and dynamic functional properties of tetrapyrrole cofactors in vivo may be strongly modulated by the steric constraints imposed by a protein matrix.

With the aim of better understanding the electronic and structural factors which govern electron-transfer processes in porphyrins, the electrochemistry of 29 nickel(II) porphyrins has been examined in dichloromethane containing either 0.1 M tetra-n-butylammonium perchlorate (TBAP) or tetra-n-butylammonium hexafluorophosphate (TBAPF(6)) as supporting electrolyte. Half-wave potentials for the first oxidation and first reduction are only weakly dependent on the supporting electrolyte, but E(1/2) for the second oxidation varies considerably with the type of supporting electrolyte. E(1/2) values for the first reduction to give a porphyrin pi-anion radical are effected in large part by the electronic properties of the porphyrin macrocycle substituents, while half-wave potentials for the first oxidation to give a pi-cation radical are affected by the substituents as well as by nonplanar deformations of the porphyrin macrocycle. The potential difference between the first and second oxidations (Delta/Ox(2) - Ox(1)/) is highly variable among the 29 investigated compounds and ranges from 0 mV (two overlapped oxidations) to 460 mV depending on the macrocycle substituents and the anion of the supporting electrolyte. The magnitude of Delta/Ox(2) - Ox(1)/ is generally smaller for compounds with very electron-withdrawing substituents and when TBAP is used as the supporting electrolyte. This behavior is best explained in terms of differences in the binding strengths of anions from the supporting electrolyte (ClO(4)(-) or PF(6)(-)) to the doubly oxidized species. A closer analysis suggests two factors which are important in modulating Delta/Ox(2) - Ox(1)/ and thus the binding affinity of the anion to the porphyrin dication. One is the type of pi-cation radical (a proxy for the charge distribution in the dication), and the other is the conformation of the porphyrin macrocycle (either planar or nonplanar). These findings imply that the redox behavior of porphyrins can be selectively tuned to display separate or overlapped oxidation processes.