Free-Base Corrole Anion

Free-base corroles have long been known to be acidic, readily undergoing deprotonation by mild bases and in polar solvents. The conjugate base, however, has not been structurally characterized until now. Presented here is a first crystal structure of a free-base corrole anion, derived from tris(p-cyanophenyl)corrole, as the tetrabuylammonium salt. The low-temperature (100 K) structure reveals localized hydrogens on a pair of opposite pyrrole nitrogens. DFT calculations identify such a structure as the global minimum but also point to two cis tautomers only 4–7 kcal/mol above the ground state. In terms of free energy, however, the cis tautomers are above or essentially flush with the trans-to-cis barrier so the cis tautomers are unlikely to exist or be observed as true intermediates. Thus, the hydrogen bond within each dipyrrin unit on either side of the molecular pseudo-C2 axis through C10 (i.e., between pyrrole rings A and B or between C and D) qualifies as or closely approaches a low-barrier hydrogen bond. Proton migration across the pseudo-C2 axis entails much higher activation energies >20 kcal/mol, reflecting the relative rigidity of the molecule along the C1-C19 pyrrole-pyrrole linkage.


■ INTRODUCTION
The tautomeric structures of free-base porphyrin-type compounds reflect a fascinating interplay of hydrogen bonding, aromaticity, steric effects, and solvation. 1Tautomerism is also critical to applications of the compounds as components of emerging molecular-electronic and memory-storage devices. 2,3−47 Even when presenting the very first synthesis of a corrole, Johnson and Kay noted that it forms a stable anion. 48Subsequently, several researchers observed deprotonation of corroles by mild bases and even simply by polar solvents such as DMF, 49−52 while Gross et al. determined the apparent acidity constant of an amphiphilic corrole. 53Corrole anions were found to exhibit more intense Q bands relative to the neutral, free-base forms. 54−57 A key aspect of free-base corrole anions, however, has remained unclear: their structure (Scheme 1).While there is extensive evidence that free-base corroles exist as a pair of essentially equienergetic tautomers, 58−61 key questions remain about the tautomeric structure of the anions.Is the trans tautomer, the one experimentally observed in this study, the only low-energy tautomer?Do the two equienergetic trans tautomers (AC and BD in Scheme 1) readily interconvert?If so, are cis tautomers (one or more among AB, CD, BC, and AD) involved as intermediates?Might the cis tautomers prove observable or even isolable?Herein, we present the X-ray structure of free-base tris(p-cyanophenyl)corrole anion and a set of DFT calculations that shed light on the these questions.
■ RESULTS AND DISCUSSION Synthesis and Structural Characterization.Free-base 5,10,15-tris(p-cyanophenyl)corrole (also, for simplicity, abbreviated as H 3 Cor) was synthesized via the water−methanol method reported by Gryko and Koszarna, 62 with all spectroscopic data matching those reported earlier. 63The free-base corrole anion, H 2 Cor − , was obtained as the tetrabutylammonium salt, TBA[H 2 Cor], in ∼50% yield by treating the neutral free base with tetrabutylammonium fluoride (TBAF) in dichloromethane followed by crystallization from a mixture of dichloromethane and hexane.The salt was fully characterized by standard spectroscopic techniques, including UV−vis absorption, fluorescence, FT-IR, and 1 H NMR spectroscopies, ESI mass spectrometry, and single-crystal X-ray diffraction analysis (see the Supporting Information).Deep blue crystals of TBA[H 2 Cor] were isolated from a solution of the free-base corrole and TBAF in CH 2 Cl 2 -hexane mixtures upon slow evaporation at room temperature.The crystal structure (Figure 1 and Table S1) reveals several notable features: (a) First, the corrole core is almost strictly planar, unlike that of a neutral corrole, which is invariably strongly puckered as a result of steric repulsion among the three central NH hydrogens.(b) Second, the four central nitrogens are arranged in the form of an isosceles trapezoid in which the longest side (∼3.0 Å) is antipodal to the direct pyrrole-pyrrole C α −C α linkage, while the other three sides are each about 2.6 Å. (c) Third, an ordered trans arrangement of the central NH hydrogens is observed, with each NH hydrogen within hydrogen-bonding distance of two neighboring unprotonated nitrogens.Note that the apparently short N−H bond distances ∼0.9 Å merely reflect the fact that X-ray diffraction measures electron density, not nuclear position, which, for hydrogen, is a key distinction.It is clear nonetheless that each NH group engages in two hydrogen bonds of different lengths, a point that we will revisit below with DFT calculations.The pyrrole rings are designated A−D.The corrole anion tautomers are designated by a two-letter symbol such as AC, which indicates that the NH protons reside on rings A and C. Meso-aryl groups have been omitted for clarity.In the present study, the meso aryl groups do not exert a significant impact on the relative energetics of the tautomers; thus, we shall see that trans tautomers AC and BD (or for that matter cis tautomers AB and CD) are essentially equienergetic The Journal of Organic Chemistry Such a structure results in voids, which are occupied by tetrabutylammonium cations (see the Supporting Information for details).
DFT Calculations.DFT (OLYP-D3 and B3LYP-D3) calculations with large all-electron STO-TZ2P basis sets afforded a great deal of additional insight into the structure and potential energy surface of H 2 Cor − (Figures 2 and 3 and Table 1).Consistent with the crystal structure (Figure 1), the ground state corresponds to either AC or BD, i.e., the protons are attached to either pyrrole rings A and C or pyrrole rings B and D. The two tautomers are not identical, as a result of the asymmetry created by the meso-phenyl groups, but are equienergetic for all practical purposes.Both tautomers exhibit bifurcated hydrogen bonding, where each central NH interacts with two neighboring unprotonated nitrogens, much as in a freebase porphyrin.Unlike in porphyrin, however, the bifurcated hydrogen bonding here is asymmetric, with each NH group interacting via an ultrashort hydrogen bond (∼1.8 Å) and a somewhat longer hydrogen bond (∼2.4 Å).The shorter hydrogen bonds always involve a pair of pyrrole rings on one side of the molecular pseudo-C 2 axis (such as pyrroles A and B or C and D), whereas the longer hydrogen bonds span a pair of pyrrole rings on opposite sides of the pseudo-C 2 axis (pyrroles A and D or pyrrole B and C).
Figure 2 presents DFT geometrical highlights and energies of the tautomers and trans-to-cis transition states of H 2 Cor − .In terms of electronic energy, the cis tautomers BC and AD are remarkably stable, about 0.15−0.25 eV (3.5−6.0 kcal/mol) above the ground state (Figure 2).Their stability undoubtedly reflects the fact that they both preserve the ultrashort hydrogen bonds between pyrroles A and B and between pyrroles C and D. The low energies of the cis tautomers are somewhat reminiscent of isobacteriochlorin, for which the trans and a cis tautomer are essentially equienergetic and typically found to coexist. 24,27,30In contrast, the cis tautomer of a simple porphyrin is about 7−8 kcal/mol above the trans ground state. 9−69 ) In contrast, cis tautomers AB and CD exhibit much higher energies, about 2/3 of an eV (16 kcal/mol) relative to the trans ground state, reflecting loss of preferred hydrogen bond pathways.
Proton migration appears to be most favorable along the ultrashort hydrogen bonds, i.e., from pyrrole A to B, or vice versa, or from pyrrole C to D, and vice versa.Exceedingly low activation energies (electronic energies) are associated with these migrations, only about a quarter to a third of an eV (6.5− 7.5 kcal/mol) relative to the trans ground state.Again, for comparison, the activation barrier for proton migration is ∼12 kcal/mol for the conjugate base of free-base porphine and ∼16 kcal/mol for neutral free-base porphine.In contrast, much higher activation energies, on the order of an eV (23 kcal/mol), are associated with proton migration between rings A and D and between rings B and C of H 2 Cor − .
Free energy calculations add an interesting twist to the above picture (Figure 3 and Table 1).As it happens, the free energy of the transition state (A-B)C ‡ is lower than that of the cis intermediate BC (see Figure 2 for an explanation of our notation for transition states).In other words, the lowest vibrational state of the cis intermediate is energetically higher than the transition states separating it from the trans tautomers.A similar scenario also applies to the cis intermediate AD whose free energy is almost flush with that of transition state A(C-D) ‡ .Our calculations thus suggest that the cis tautomers are unlikely to be observable as true intermediates.Instead, they are a part of the energy barrier through which one trans form tunnels to the other.In this respect, free-base corrole anion differs from both free-base porphyrin, for which the cis tautomer is a true intermediate on the potential energy surface, 9,12−14 and freebase isobacteriochlorin, for which the trans and one cis tautomer coexist in equilibrium. 24,27,30−73 Such hydrogen bonds have been invoked (albeit controversially 74−76 ) as responsible for reaction rate enhancements in enzymatic catalysis.Recently, low-barrier hydrogen bonds have also been implicated in enzyme cooperativity. 77Against this backdrop, the free-base corrole anion may serve as a well-characterized and readily accessible model system for low-barrier N−H•••N hydrogen bonds (several examples are known involving oxygen 78−80 ).Additional spectroscopic studies are clearly warranted, of which NMR spectroscopy and nitrogen 1s X-ray photoelectron spectroscopy seem particularly promising.The latter technique has shed substantial light on the charge asymmetry of low-barrier hydrogen bonds in porphycenes. 20hotophysical Properties.  2. Like other free-base corroles, H 3 Cor exhibits a strong Soret band at ∼427 nm and three weaker Q bands at 585, 620, and 655 nm.In contrast, TBA[H 2 Cor] exhibits a red-shifted Soret band at 455 nm, along with Q bands at 554, 600, and 653 nm.Upon excitation at 430 nm, the neutral free base exhibits strong emission with a maximum at 676 nm, a quantum yield of

■ CONCLUSIONS
In summary, we have presented the first crystal structure of a free-base corrole anion.The structure exhibits localized NH hydrogens on a pair of opposite pyrrole rings, which we may designate as rings A and C or equivalently as B and D. A key feature of the structure is the short-strong N−H•••N hydrogen bond within each dipyrrin moiety on either side of the molecular pseudo-C 2 axis through the C 10 meso-carbon.DFT calculations identify AC and BD as equienergetic global minima with the two cis tautomers AD and BC only 4−7 kcal/mol above the global minima (in terms of electronic energy).In terms of free energy, however, the cis tautomers are just above or essentially flush with the trans-to-cis barrier heights.Accordingly, the hydrogen bonds in free-base corrole anion qualify as or closely approach lowbarrier hydrogen bonds.Proton migration across the molecular pseudo-C 2 axis, on the other hand, entails much higher activation energies of >20 kcal/mol.Given the overall mobility of the low-barrier hydrogen bond system, we find it gratifying that our low-temperature crystal structure has revealed a single, ordered tautomer.

■ EXPERIMENTAL SECTION
General.Unless otherwise mentioned, all chemicals were obtained from Merck.For spectroscopic studies, HPLC-grade solvents were used.Free-base 5,10,15-tris(4-cyanophenyl)corrole was prepared as previously reported. 61Elemental analyses were carried out with a Euro EA elemental analyzer.UV−vis absorption spectra were acquired on a Perkin-Elmer LAMBDA-750 spectrophotometer.Emission spectra were recorded on an Edinburgh FLS920 spectrofluorimeter equipped with a Ge-detector using an optical cell with 1 cm path length.FT-IR spectra were recorded on a Perkin-Elmer spectrophotometer with samples prepared as KBr pellets.NMR spectra were obtained on a Bruker 400 MHz NMR spectrometer.Chemical shifts are expressed in parts per million (ppm) relative to residual acetonitrile (δ = 1.93).Electrospray mass spectra were recorded on a Bruker Micro TOF−QII mass spectrometer.
Crystal Structure Determination.Single crystals of TBA-[H 2 Cor] were grown from the solution of TBA[H 2 Cor] in dichloromethane layered over hexane followed by slow evaporation under atmospheric conditions.X-ray crystallographic data on TBA[H 2 Cor] were collected on a Rigaku Oxford diffractometer (Cu Kα radiation) at 100 K, with all key parameters listed in Table S1.The data were corrected for Lorentz polarization and absorption effects.The structure was solved with intrinsic phasing methods (SHELXT40 82 ) and refined by full matrix least squares on F 2 (SHELXL-201841 83 ).Hydrogen atoms were included in the refinement as riding atoms.Contributions of H atoms for the water molecules were included but were not fixed.Disordered solvent molecules were taken out using the SQUEEZE command in PLATON. 84The corresponding CCDC no of TBA + [H 2 Cor − ] is 2258522.The Journal of Organic Chemistry Scheme 1. Tautomers of Free-Base Corrole and Corrole Anion a

Figure 4
depicts the UV−vis− NIR and fluorescence spectra of free-base H 3 Cor and TBA-[H 2 Cor] in CH 2 Cl 2 , with key numerical data summarized in Table

Figure 2 .
Figure 2. OLYP-D3 and B3LYP-D3 electronic energies (eV, in order) of trans-to-cis transition states and cis intermediates of H 2 Cor − .Selected N-H (black) and N•••N (blue) distances (Å) are shown.A notation such as (A-B)C ‡ is used for transition states, which indicates that a proton is migrating between pyrrole rings A and B, while pyrrole ring C carries a localized proton.

Figure 3 .
Figure 3. OLYP-D3 free energies (eV) of key stationary points on the potential energy surface of H 2 Cor − .Results for trans minima, cis intermediates, and transition states are indicated in blue, green, and red, respectively.
imaginary frequencies (cm −1 ) are indicated for transition states.

a
Emission quantum yields were calculated using coumarin (Φ em = 0.54) as the reference in degassed CH 2 Cl 2 . 80b Emission lifetimes were measured in degassed CH 2 Cl 2 solution.

Table 2 .
Photophysical Data of H 3 Cor and TBA[H 2 Cor] in CH 2 Cl 2 at 298 K