Protonation-Induced Hyperporphyrin Spectra of meso-Aminophenylcorroles

Abstract

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.

1. 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(p-hydroxyphenyl)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 Mⁿ⁺–corrole^3– are actually better described as M^(n–1)+–corrole^·2– and exhibit substituent-sensitive Soret bands with substantial aryl-to-corrole^·2– charge-transfer character.¹⁵⁻¹⁸ Examples of such noninnocent metallocorroles include MnCl,¹⁹ FeCl,²⁰⁻²³ FeNO,²³⁻²⁵ Co,²⁶⁻²⁸ and Cu²⁹⁻³⁴ 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^VO,³⁵ Os^VIN,³⁶ Pt,^37,38 and Au³⁹⁻⁴¹ corroles all exhibit redshifted Soret bands and sharp, split Q bands. Little, however, has been documented vis-à-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.

2. Results

Spectrophotometric titrations were carried out on approximately 0.03 mM solutions of tris(o⁴⁴/m⁴⁵/p⁴⁶-aminophenyl)corrole (H₃[o/m/p-TAPC]) and triphenylcorrole (H₃[TPC])³¹ (Chart 1) in dimethyl sulfoxide (DMSO) with methanesulfonic acid (MSA) in DMSO (with concentrations ranging from about 1 mM to pure MSA) as titrant (Figures 1–4). Even sub-equivalent amounts of MSA led to substantial spectral changes, consistent with neutralization of the anionic CorH₂^– 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). On the whole, it was clear that neutralization of the anionic states results in a weakening of both the Soret and Q bands.

Chart 1. Compounds Studied in this Work.

Figure 1.

Spectral changes for p-H₃[TAPC] in DMSO as a function of added equivalents of MSA. The three panels approximately correspond to the following transformations: (a) CorH₂^– → CorH₃, (b) CorH₃ → CorH₄⁺, and (c) CorH₄⁺ → CorH₅²⁺.

Figure 4.

Spectral changes for H₃[TPC] in DMSO as a function of added equivalents of MSA. The two panels approximately correspond to the following transformations: (a) CorH₂^– → CorH₃ and (b) CorH₃ → CorH₄⁺.

Table 1. UV–vis Absorption Maxima of Different Protonation States of the Free-Base Corroles Studied.

	CorH2–		CorH3		CorH4+		CorH52+
compound	Soret	Q	Soret	Q	Soret	Q	Soret	Q
H3[p-TAPC]	430a	655a	429a,b	526a, 637a	454a	547, 622, 764a	430a, 458	687a
H3[m-TAPC]	427a, 449	643a	416a,c	572a, 614, 646	428a, 460	690a	431a	684a
H3[o-TAPC]d	425a	578, 632a	414a,c	518, 566a, 604, 638	422a	676a	424a	655a
H3[TPC]	427a, 448	641a	415a,c	567a, 615, 648	427a, 458	685a

^aThe strongest peak in each set is marked with an asterisk.

^bIn acetone.⁴⁵

^cIn dichloromethane.⁴⁶

^dMixture of atropisomers.⁴⁴

Figure 2.

Spectral changes for m-H₃[TAPC] in DMSO as a function of added equivalents of MSA. The three panels approximately correspond to the following transformations: (a) CorH₂^– → CorH₃, (b) CorH₃ → CorH₄⁺, and (c) CorH₄⁺ → CorH₅²⁺.

Further addition of MSA resulted in dramatic redshifts and intensification of the Q bands. For H₃[p-TAPC] (Figure 1), the Q band shifted from the mid-600s to ∼764 nm, i.e., into the near-infrared, with the addition of a few equivalents of MSA. For H₃[o-TAPC] (Figure 3), the Q bands at 575 and 610 nm disappeared and a strong Q band grew at 676 nm, albeit with the addition of larger quantities of MSA (a couple of hundred equivalents). Qualitatively similar changes were also observed for H₃[TPC] (Figure 4), with disappearance of the Q bands at 585 and 618 nm and appearance of a strong Q band at 685 nm. The final spectra were strongly suggestive of hyper character, attributable at least in part to phenyl-to-corrole charge transfer in the H₄[p-TAPC]⁺ and H₄[TPC]⁺ cations. The formation of these monocations was also accompanied by a slight weakening of the Soret band.

Figure 3.

Spectral changes for o-H₃[TAPC] in DMSO as a function of added equivalents of MSA. The three panels approximately correspond to the following transformations: (a) CorH₂^– → CorH₃, (b) CorH₃ → CorH₄⁺, and (c) CorH₄⁺ → CorH₅²⁺.

Addition of a large excess (i.e., thousands of equivalents) of MSA to H₃[o/m/p-TAPC] solutions led to further changes, consistent with the formation of H₅[o/m/p-TAPC]²⁺ dications. The spectral changes are arguably most dramatic for H₃[p-TAPC] (Figure 1) where the Q band blueshifts dramatically from 764 to 687 nm, while a new blue-shifted Soret feature grows at 430 nm. Understandably, H₃[TPC] (Figure 4), which lacks peripheral amino groups, did not evince any indication of dication formation under the experimental conditions. We also could not discern whether tri- and tetracationic states of H₃[o/m/p-TAPC] formed under the conditions of the experiments.

The dramatic spectral changes associated with the formation of CorH₄⁺ species allowed us to qualitatively estimate the relative basicities of the four corroles in terms of the apparent pK_a-app’s of the CorH₄⁺ 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 both H₃[p-TAPC] and H₃[m-TAPC], 4.5 ± 0.1 for H₃[o-TAPC], and 4.1 ± 0.1 for H₃[TPC]. 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.

3. Discussion

The spectral changes accompanying the formation of CorH₄⁺ species are reminiscent of those accompanying the formation of centrally diprotonated tetraarylporphyrins, in particular tetrakis(p-aminophenyl)porphyrin (H₂[p-TAPP]). The redshift of the Q band accompanying the generation of H₄[p-TAPP]²⁺, however, is larger than that accompanying the generation of H₄[p-TAPC]⁺. Thus, the Q band at approx. 637 nm for H₂[p-TAPP] redshifts to approx. 811 nm for H₄[p-TAPP]²⁺.⁷⁻⁹ For H₃[p-TAPC], the Q band shifts from 669 nm for the neutral species to 764 nm for p-H₄[p-TAPC]⁺. The lower spectral shift in the latter case may reflect the lower positive charge of H₄[p-TAPC]⁺ relative to H₄[p-TAPP]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 protonation results in only a modest increase in nonplanarity. For H₂[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 co-workers 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 H₄[p-TAPC]⁺ seems ascribable to the three quinonoid resonance forms shown in Scheme 1, whereas the comparatively blue-shifted Q band of the H₄[p-TAPC]²⁺ dication seems ascribable to only two quinonoid resonance forms.

Scheme 1. Principal Resonance Structures of the Mono- and Diprotonated Forms of H₃[p-TAPC].

4. Conclusions

UV–vis spectrophotometric titration of the ortho, meta, and para isomers of H₃[TAPC] and H₃[TPC] was carried out in DMSO with methanesulfonic acid (MSA). For all the compounds, monoprotonation led to hyperporphyrin spectra with strongly red-shifted and intense Q bands. The effect was especially dramatic for H₃[p-TAPC] for which the Q band was found to red-shift 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 formulated in terms of quinonoid resonance forms.

5. Experimental Section

The ortho, meta, and para isomers of H₃[TAPC] and H₃[TPC] were all freshly prepared as previously described and yielded ¹H 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 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.

The authors declare no competing financial interest.