Protonation-Induced Hyperporphyrin Spectra of meso-Aminophenylcorroles.

UV–vis spectrophotometric titrations have been carried out on meso-tris(o/m/p-aminophenyl)corrole (H3[o/m/p-TAPC]) and meso-triphenylcorrole (H3[TPC]) in dimethyl sulfoxide with methanesulfonic acid (MSA). Monoprotonation was found to result in hyperporphyrin spectra characterized by new, red-shifted, and intense Q bands. The effect was particularly dramatic for H3[p-TAPC] for which the Q band red-shifted from ∼637 nm for the neutral species to 764 nm in the near-IR for H4[p-TAPC]+. Upon further protonation, the Q band was found to blue-shift back to 687 nm. A simple explanation of the phenomena has been offered in terms of quinonoid resonance forms.


INTRODUCTION
The electronic spectra of porphyrins were classified by Gouterman and co-workers as normal, hypso, and hyper. 1,2 Normal spectra are observed for free-base and many nontransition element derivatives of simple porphyrins such as tetraphenyl-or octaethylporphyrin and are characterized by the classic Soret and Q bands as well as by an N band in the near-UV. Hypsoporphyrins exhibit blue-shifted Soret and Q bands, while hyperporphyrins exhibit extra bands relative to normal porphyrins at wavelengths above 300 nm. Unlike normal spectra, which are dominated by porphyrin π → π* transitions, hyper spectra also involve additional types of transitions, notably charge transfer (CT) transitions. Heme-thiolate proteins and their model compounds provide many examples of hyperporphyrins. 3,4 Diprotonated tetraarylporphyrins provide another important class of hyperporphyrins; the spectra of these species exhibit additional bands attributed to aryl-to-porphyrin CT transitions. Protonated meso-aminophenylporphyrins provide particularly vivid examples of such spectra. 5−12 An entirely analogous effect is also observed for meso-tetrakis(phydroxyphenyl)porphyrin in alkaline media where the spectra exhibit extra bands due to phenolate-to-porphyrin CT transitions. 13,14 Hyper spectra are also well-established for metallocorroles. Indeed, many metallotriarylcorroles formally described as M n+ − corrole 3− are actually better described as M (n−1)+ −corrole ·2− and exhibit substituent-sensitive Soret bands with substantial aryl-tocorrole ·2− charge-transfer character. 15−18 Examples of such noninnocent metallocorroles include MnCl, 19 FeCl, 20−23 FeNO, 23−25 Co, 26−28 and Cu 29−34 corroles. Although the Soret bands of innocent metallotriarylcorroles do not exhibit the same kind of substituent sensitivity as their noninnocent counterparts, many exhibit overall hyper-type spectra, reflecting corrole(π)-to-metal(d) transitions. Many families of 5d metallocorroles recently reported from our laboratory exhibit such spectra. Thus, Re V O, 35 Os VI N, 36 Pt, 37,38 and Au 39−41 corroles all exhibit redshifted Soret bands and sharp, split Q bands. Little, however, has been documented vis-a-vis the potential hyper character of protonated free-base triarylcorroles, 42,43 in particular meso-aminophenylcorroles. Herein, we show that these systems, upon protonation, exhibit dramatically redshifted Q bands and thus spectra that are aptly described as hyper.  (Figures 1−4). Even subequivalent amounts of MSA led to substantial spectral changes, consistent with neutralization of the anionic CorH 2 − state that is thought to be present in substantial amounts in DMSO solutions. 47 Interestingly, although we could identify peaks that are reasonably attributable to the anions, the broad peaks that were generated in the Q region could not be definitively assigned to a single species such as the neutral corrole (Table 1).

Spectrophotometric
On the whole, it was clear that neutralization of the anionic states results in a weakening of both the Soret and Q bands. The dramatic spectral changes associated with the formation of CorH 4 + species allowed us to qualitatively estimate the relative basicities of the four corroles in terms of the apparent pK a-app 's of the CorH 4 + species. In this approach, used earlier by Wamser and co-workers for aminophenylporphyrins, 9 pK a-app simply equals the negative logarithm of the analytical concentration of MSA at the half-equivalence point, which was estimated from spectral changes at multiple wavelengths. Using this approach, we estimated pK a-app values of 5.2 ± 0.1 for . In other words, the first two compounds are somewhat more basic than the latter two compounds (by just under a factor of 10), potentially reflecting steric inhibition of resonance interactions for the ortho isomer.

DISCUSSION
The spectral changes accompanying the formation of  2+ . Alternatively, or additionally, the lower spectral shift for corrole protonation may be related to the fact that a smaller geometrical change is involved; free-base corroles are already strongly nonplanar and proto- ACS Omega http://pubs.acs.org/journal/acsodf Article nation results in only a modest increase in nonplanarity. For H 2 [p-TAPP], in contrast, protonation of two central nitrogens alters the macrocycle conformation from planar to strongly saddled. 48−50 It would be of great interest to simulate the above spectral shifts by quantum chemical means and thereby dissect the contributions of different factors such as charge transfer, conformation, and substituents on the meso-aryl groups. Such calculations, however, involve considerable challenges largely because charge transfer transitions have long been a weakness for time-dependent density functional theory methods; 51−53 a recent CAM-B3LYP and CC2 study of tetraphenylthiaporphyrin, tetraphenylporphyrin N-oxide, and their protonation, however, have yielded promising results and may point to a way forward. 54 Meanwhile, as discussed by Wamser and coworkers for porphyrins, 9 simple consideration of resonance forms may provide a qualitative explanation of some of the observed spectral shifts. Thus, the strongly redshifted Q band of H 4 [p-TAPC] + seems ascribable to the three quinonoid resonance forms shown in Scheme 1, whereas the comparatively blue-shifted Q band of the H 4 [p-TAPC] 2+ dication seems ascribable to only two quinonoid resonance forms.

EXPERIMENTAL SECTION
The ortho, meta, and para isomers of H 3

[TAPC] and H 3 [TPC]
were all freshly prepared as previously described and yielded 1 H NMR and mass spectroscopic data in accord with the literature. 44−46 UV−vis spectrophotometric titrations were carried out on an HP 8453 spectrophotometer using solutions of methanesulfonic acid in anhydrous DMSO. Corrole solutions were prepared from anhydrous DMSO and purged with argon prior to use. Titrations were performed in a cuvette with an initial corrole solution of 400 μL. Acid additions were performed using a micropipette in gradual increments from 2 to 20 μL, depending on the acid concentration. After each addition, the ACS Omega http://pubs.acs.org/journal/acsodf Article solution was stirred with a small stir bar and allowed to settle for 3 min before the spectrum was recorded. All titrations were repeated several times on different batches of freshly made corrole.