Ligand noninnocence in FeNO corroles: insights from ß-octabromocorrole complexes.

The first FeNO octabromocorroles have been synthesized including four ß-octabromo-meso-tris(p-X-phenyl)corrole derivatives Fe[Br8TpXPC](NO) (X = CF3, H, CH3, OCH3) and the ß-octabromo-meso-tris(pentafluorophenyl)corrole complex, Fe[Br8TPFPC](NO). The last complex, which proved amenable to single-crystal X-ray structure determination, exhibits the geometry parameters: Fe-N(O) 1.643(8) Å, N-O 1.158(9) Å, and a FeNO angle of 176.4(6)°. The more electron-deficient complexes exhibit increased instability with respect to NO loss and also higher infrared NO stretching frequencies (νNO). Interestingly, DFT calculations and IR marker bands indicate a noninnocent {FeNO}(7)-(corrole˙(2-)) formulation for all FeNO corroles, both ß-H8 and ß-Br8, with essentially the same degree of corrole radical character. Instead, an electron-deficient corrole appears to exert a field effect resulting in reduced Fe-to-NO backdonation, which accounts for both the increased instability with respect to NO loss and the higher νNO's.

Improved syntheses of ß-octabromo-meso-triarylcorrole derivatives.

In spite of significant applications as starting materials for a variety of metallocorrole derivatives, free-base ß-octabromo-meso-triarylcorroles continue to be viewed as inaccessible. The reasons range from the need for tedious column-chromatographic purification to limitations of the reductive demetallation protocol for selected systems. Here we report that column chromatography may be entirely avoided for a number of ß-octabromo-meso-tris(p-X-phenyl)corrole derivatives, where X=CF3, NO2, F, H, CH3, and OCH3; instead, analytically pure products may be obtained by recrystallization from chloroform/methanol. In addition, we have presented an optimized synthesis of the heretofore inaccessible, sterically hindered ligand ß-octabromo-meso-tris(2,6-dichlorophenyl)corrole, H3[Br8TDCPC], via reductive demetallation of the corresponding Mn(III) complex. With our earlier report of ß-octabromo-meso-tris(pentafluorophenyl)corrole, H3[Br8TPFPC], a comprehensive set of optimized synthetic protocols are thus in place for a good number of ß-octabromo-meso-triarylcorrole ligands. Furthermore, we have illustrated the use of these ligands by synthesizing the iron complexes Fe[Br8TDCPC]Cl and Fe[Br8TDCPC](py)2, of which the latter lent itself to single-crystal X-ray structure determination.

Molecular structures of free-base corroles: nonplanarity, chirality, and enantiomerization.

The molecular structures of free-base corroles are illustrative of a variety of bonded and nonbonded interactions including aromaticity, intra- as well as intermolecular hydrogen bonding, steric interactions among multiple NH hydrogens within a congested central cavity, and the effects of peripheral substituents. Against this backdrop, an X-ray structure of 2,3,7,8,12,13,17,18-octabromo-5,10,15-tris(pentafluorophenyl)corrole, H3[Br8TPFPCor], corresponding to a specific tautomer, has been found to exhibit the strongest nonplanar distortions observed to date for any free-base corrole structure. Two adjacent N-protonated pyrrole rings are tilted with respect to each other by approximately 97.7°, while the remainder of the molecule is comparatively planar. Dispersion-corrected DFT calculations were undertaken to investigate to what extent the strong nonplanar distortions can be attributed to steric effects of the peripheral substituents. For meso-triphenylcorrole, DFT calculations revealed nonplanar distortions that are only marginally less pronounced than those found for H3(Br8TPFPCor). A survey of X-ray structures of sterically unhindered corroles also uncovered additional examples of rather strong nonplanar distortions. Detailed potential energy calculations as a function of different saddling dihedrals also emphasized the softness of the distortions. Because of nonplanar distortions, free-base corrole structures are chiral. For H3[Br8TPFPCor], DFT calculations led to an estimate of 15 kcal/mol (0.67 eV) as the activation barrier for enantiomerization of the free-base structures, which is significantly higher than the barrier for NH tautomerism calculated for this molecule, about 5 kcal/mol (0.2 eV). In summary, steric crowding of the internal NH hydrogens appears to provide the main driving force for nonplanar distortions of meso-triarylcorroles; the presence of additional ß-substituents adds marginally to this impetus.