Proton-coupled electron transfer of macrocyclic ring hydrogenation: The chlorinphlorin

The methyl hangman substitution (Y = CH₃) allows the redox chemistry of the hangman chlorin to be determined in the absence of PCET. CVs of chlorins H₂CX³-Me and H₂CX¹⁰-Me are shown in Fig. 3. Consistent with tetraphenylchlorin (18), the hangman compounds exhibit two reversible waves corresponding to the formation of the mono- and direduced forms of the chlorins, characterized by the standard potentials $E_1^0$ and $E_2^0$ given in Table 1. Similar CVs are obtained with ZnCX³-Me and ZnCX¹⁰-Me (SI Appendix, Fig. S7), although the standard potentials are cathodically shifted by 200 mV for $E_1^0$ and 100 mV for $E_2^0$ (Table 1). Indeed, thin-layer spectroelectrochemistry of ZnCX³-Me (SI Appendix, Fig. S1A) and ZnCX¹⁰-Me (SI Appendix, Fig. S1B) at a potential of –2.00 V vs. Fc⁺/Fc (ferrocene) leads to the formation of the one-electron reduced species, Chl^•– (18); the same Chl^•– spectrum is observed for H₂CX³-Me (SI Appendix, Fig. S1C) and H₂CX¹⁰-Me (SI Appendix, Fig. S1D) in the absence of benzoic acid.

The hangman chlorins H₂CX³-H and H₂CX¹⁰-H exhibit more complicated CVs (Fig. 4) due to the pendant acid functionality. The CVs are consistent with the ECE (E, electron transfer step; C, chemical step) mechanism shown in Scheme 1. Within this framework, the first cathodic wave involves electron transfer (characterized by a standard potential $E_3^0$) to furnish the chlorin anion radical followed by an intramolecular irreversible proton transfer (characterized by a first-order rate constant $k_{PT}$). This subsequent radical is reversibly reduced at a more negative potential ($E_4^0$).

The $E_3^0$ and $E_1^0$ values for the Y = H and Me chlorins are assumed to be similar as the pendant proton should have little effect on the porphyrin frontier orbitals and thus, on the potential of the first cathodic process to produce the radical anion (Table 1). However, this is not the case for the second electron reduction. $E_4^0$ is more positive than $E_2^0$ (200 to 300 mV) (Table 1) and only 80 to 90 mV more negative of $E_1^0$ due to proton transfer from the hangman group, which neutralizes the charge on the chlorin ring. However, in contrast to the classical ECE disproportionation mechanism (19) (where the C step is a bimolecular protonation, an addition of an electrophile to the anion radical, or bond cleavage in the anion radical), the second electron transfer occurs at more negative potential of the first arising from the residual negative charge on the hangman carboxylate. Consequently, the disproportionation reaction (i.e., a bimolecular reaction between the two anion radicals involved in the proton transfer step) is thermodynamically unfavorable, in contrast to the classical mechanism for electrochemical hydrogenation of aromatic hydrocarbons in the presence of an external proton source (20). The intramolecular proton transfer rate constant $k_{PT}$ is obtained from the first irreversible peak potential according towhere proton transfer is assumed to be fast and irreversible, thus corresponding to “pure kinetic” conditions where the initial one-electron reduced Chl^•– does not accumulate because it is at steady state in a very thin diffusion–reaction layer due to mutual compensation between its diffusion from the electrode surface where it is produced and its rapid intramolecular protonation (19). The obtained rate constants (Table 1) are indeed consistent with a large kinetic parameter $\mathit{\lambda }=\frac{RT}{Fv}{k}_{PT}\gg 1$, which is a measure of the competition between the intramolecular protonation and the timescale of the CV, thereby validating this assumption. Qualitatively similar behaviors were observed with ZnCX¹⁰-H and ZnCX³-H, but the internal proton transfers are slower (Table 1).

As shown in Fig. 4, the CVs of the hangman free base compounds exhibit an additional cathodic feature. A reversible wave is observed at a more negative potential of the second electron reduction (e.g., –1.98 and –2.03 V vs. Fc⁺/Fc for H₂CX³-H and H₂CX¹⁰-H, respectively) that is ascribed to further reduction of the chlorin macrocycle ($E_5^0$ in Scheme 1). H₂CX¹⁰-H also exhibits an anodic irreversible wave at –0.75 V vs. Fc⁺/Fc. As demonstrated by a CV where the potential is reversed just negative of the first wave (blue trace in Fig. 4B), this anodic wave corresponds to the oxidation of a species formed at the level of the first wave. We suspect that the radical formed upon the initial reduction and subsequent intramolecular PCET is partially protonated at the carboxylate (either from trace acid or from another molecule of substrate). The resulting neutral radical is then immediately reduced (ECE process), leading to a more stabilized anion whose reoxidation is then observed at ~ –0.75 V vs. Fc⁺/Fc for H₂CX¹⁰-H. This second reduction is in competition with a fast chemical step, as attested by the irreversibility of the reoxidation wave (SI Appendix, Fig. S2).

To investigate the mechanism of electrochemical hydrogenation of chlorins containing a pendant methyl ester group, CVs of H₂CX³-Me and H₂CX¹⁰-Me were recorded with increasing amounts of benzoic acid (pK_a = 20 in acetonitrile) (21). As shown in Fig. 5, the presence of a weak acid significantly perturbs the CVs. At low acid concentrations (Fig. 5A and SI Appendix, Fig. S3B for H₂CX¹⁰-Me), the second reversible wave decreases simultaneously with the increase of the first wave, and a small quasireversible wave appears at an intermediate potential (~ –1.85 V vs. Fc⁺/Fc) (dotted line in Fig. 5C). As the acid concentration is increased (Fig. 5B and SI Appendix, Fig. S3D for H₂CX¹⁰-Me), the first wave slightly shifts anodically by roughly ∂E_p/∂log[AH] ∼ 35 mV (where [AH] is the concentration of benzoic acid), becomes irreversible, and attains a stoichiometry of 2e^–. Similar trends were obtained for H₂CX³-Me, as shown in SI Appendix, Fig. S3 A and C.

To gain insight into the product of the 2e^– reduction in the presence of acid, spectroelectrochemical studies were undertaken. Fig. 6 A and B shows the absorption profile obtained when solutions of H₂CX³-Me and H₂CX¹⁰-Me, respectively, were reduced in the presence of excess benzoic acid (50 mM) at a potential of –1.65 V vs. Fc⁺/Fc. A very broad absorption band centered at λ_max = ~530 nm is observed. This ill-defined absorption profile is reminiscent of the only previous report of spectral data for a chlorinphlorin (10). As in porphyrins (5, 11, 18, 22), electrochemical reduction of the macrocycle results in dearomatization of the conjugated ring system, which should favor protonation of the meso carbon to afford a phlorin intermediate. To unequivocally confirm the formation of a chlorinphlorin, we sought to chemically reduce chlorin in the presence of acid and structurally characterize the resulting compound. A protonation–reduction sequence was achieved by reduction of meso-tetraphenylchlorin with cobaltocene subsequent to protonation using tosylic acid monohydrate. The crystal structure of the chlorinphlorin product is shown in Fig. 7. This represents an elusive example of a crystallographically characterized chlorinphlorin. The structure is distinguished by the tetrahedral geometry of the C(11) methine carbon. As shown in SI Appendix, Fig. S4, meso-tetraphenylchlorin exhibits the same reductive spectroelectrochemical behavior in the presence of acid (shown in Fig. 6) as for the hangman chlorins. Moreover, the ultraviolet-visible (UV-vis) absorption spectrum of the chemically prepared chlorinphlorin (SI Appendix, Fig. S4B) matches the observed spectroelectrochemical spectrum obtained from reduction of the compound (SI Appendix, Fig. S4A).

With the chlorinphlorin established as the PCET product of chlorin hydrogenation, the CVs of chlorin in the presence of exogenous acid are in accordance with the ECEC mechanism shown in Scheme 2, where each C step is a proton transfer from the benzoic acid in solution to the chlorin ring. For simplicity, H₂CX³-Me and H₂CX¹⁰-Me are represented by Chl in the mechanism. We propose that the first protonation occurs at nitrogen, characterized by the rate constant $k_{H1}$, and may be facilitated by hydrogen bonding with benzoic acid. From the position of the irreversible first wave in the presence of benzoic acid, the first protonation rate constant $k_{H1}$ (Table 2) can be evaluated using (19)

The neutral radical [Chl(H)^•] obtained after the first EC sequence may be further reduced at the electrode, followed by a second proton transfer ($k_{H2}$) to furnish the meso C–H at the C(11) carbon of Fig. 7. Due to dearomatization caused by the first reduction, the second reduction is easier so that $E_{h1}^0\gg E_1^0$. The small quasireversible wave observed at ∼–1.85 V vs. Fc⁺/Fc results from the reduction of the hydrogenated chlorin, Chl(H₂), formed after the second protonation. Consistent with this observation, a chemically synthesized tetraphenyl derivative shows a cathodic wave at –2.0 V vs. Fc⁺/Fc; the shift of 150 mV may be the result of substituent effects. The low current associated with the Chl(H₂) reduction indicates that the second proton transfer is slow, as confirmed by two successive CV scans; the quasireversible wave attributed to the reduction of Chl(H₂) grows on the timescale of the second scan (SI Appendix, Fig. S5). The corresponding rate constant, $k_{H2}$ = 10 M⁻¹ s⁻¹, for this proton transfer (Table 2) is obtained from digital simulations of the CV of H₂CX¹⁰-Me (Fig. 5D) and of H₂CX³-Me (SI Appendix, Fig. S6) in the presence of benzoic acid. The anion Chl(H)^– is only partially stable on the timescale of the CV due to its slow protonation; its reoxidation (characterized by a standard potential $E_{h1}^0$) is assigned to the anodic wave at ∼–1.10 V vs. Fc⁺/Fc.

The reduction of ZnCX³-Me (SI Appendix, Fig. S7 A and B) and ZnCX¹⁰-Me (SI Appendix, Fig. S7 C and D) in the presence of benzoic acid is also proton dependent in a fashion similar to H₂CX³-Me (SI Appendix, Fig. S3 A and C) and H₂CX¹⁰-Me (SI Appendix, Fig. S3 B and D). Using Eq. 2, the $k_{H1}$ listed in Table 2 are obtained. However, in contrast to the behavior of the free bases, reductive spectroelectrochemistry of ZnCX³-Me (Fig. 6C) or ZnCX¹⁰-Me (Fig. 6D) at –1.70 V vs. Fc⁺/Fc in the presence of excess benzoic acid (50 mM) generates absorption spectra that are characteristic of isobacteriochlorins (23, 24). The coordination of zinc within the chlorin cavity precludes the formation of the initial N–H bond and subsequent formation of a chlorinphlorin.

The presence of a proximate proton perturbs the hydrogenation of the chlorin macrocycle. Fig. 8 shows the CVs of H₂CX¹⁰-H in the presence of benzoic acid; the CVs of H₂CX³-H in the presence of benzoic acid are shown in SI Appendix, Fig. S8. The effect of the proton in the hanging group is immediately obvious from the CV in the absence of benzoic acid (Figs. 4B and 8, red trace). The first reduction wave is irreversible, and moreover, the current of the second reduction wave is reduced. Per the specific site of protonation in the ECE mechanism of Scheme 1, the radical anion is neutralized by the transfer of a proton from the hanging group to the nitrogen of the macrocyclic ring to form the N–H bond. Comparing the rate constant of the first protonation ($k_{H1}$ in Table 2) with the rate constant of the internal proton transfer from the hangman group ($k_{PT}$ in Table 1) leads to the conclusion that the equivalent apparent acid concentration corresponding to the hangman group ($C_{\mathit{app}} = k_{PT}/k_{H1}$) is small at ∼2 mM. With the addition of acid, even as low as <2 mM, an increase in the intensity of the first wave was observed along with a concomitant decrease in the subsequent reversible wave associated with the second reduction process.

At higher concentrations of acid (from 2 to 20 mM), the first wave shifts anodically and becomes a two-electron wave. The cathodic wave of H₂CX¹⁰-H is ~125 mV more positive vs. that of H₂CX¹⁰-Me in the presence of 20 mM benzoic acid. This shift rules out a stepwise electron-transfer/proton-transfer (ET/PT) process because application of Eq. 2 would imply a proton transfer rate constant $k_{H1}$ at least four orders of magnitude larger than the value obtained with the H₂CX¹⁰-Me. Carboxylic acid groups over a porphyrin macrocycle have been shown to preorganize molecules within the cleft via hydrogen bonding (25). Accordingly, as shown in Scheme 3, we propose that, due to hydrogen bonding, the pendant carboxylic acid group and imine N-atom facilitate the prepositioning of benzoic acid. The hydrogen bonding network supports a concerted PCET, thus circumventing the anion radical intermediate. We note that the rate constant (k_CPET) and the standard potentials ($E_{\mathit{CPET}}^0$) cannot be determined independently because the peak potential depends on both k_CPET and $E_{\mathit{CPET}}^0$:where α is the transfer coefficient [α ∼ 0.5 evaluated from the wave width: E_p/2 – E_p = 1.857(RT/αF) with E_p/2 – E_p ∼ 100 mV] (19). The shift of the wave upon the addition of benzoic acid is much more pronounced with the pendant acid functionality than for the chlorins with the ester group (∂E_p/∂log[AH] ∼ 150 vs. 35 mV, respectively), consistent with a CPET mechanism for the former; PCET reduction for the pendant acid derivative is more facile, and this is also in line with the predicted shift for a concerted process $\frac{\partial E_p}{\partial \text{log}\left[\text{AH}\right]}=\frac{RT\text{ln}10}{\alpha F}\approx 120 \text{mV}/\text{decade}$ (Eq. 3) as opposed to $\frac{\partial E_p}{\partial \text{log}\left[\text{AH}\right]}=\frac{RT\text{ln}10}{2F}\approx 30 \text{mV}/\text{decade}$ for a stepwise process (Eq. 2). Note that, in the absence of acid, a stepwise ET/PT process (Scheme 1) prevails owing to a prohibitively long proton transfer distance. The structure of related Ni hangman porphyrin (26) shows a distance of 3.6 Å between the carboxylic acid proton and the nearest pyrrolic nitrogen, with the proton engaging in hydrogen bonding with the xanthene oxygen. This rather long distance between donor and acceptor, along with the high degree of deviation from linearity in the donor–proton–acceptor angle (106°) between the O–H–N groups, likely leads to a prohibitive kinetic barrier for concerted PCET. With the shortening of the proton tunneling distance upon binding acid within the cleft, the mechanism shifts from ET/PT to CPET.

Interestingly, the metallated chlorins, ZnCX³-H and ZnCX¹⁰-H, do not exhibit the same CV features as the corresponding free base chlorins H₂CX³-H and H₂CX¹⁰-H; at high concentrations of exogenous benzoic acid, no anodic shift of the first two-electron irreversible wave is observed when compared with ZnCX³-Me and ZnCX¹⁰-Me, and there is no substantial shift of the wave with increasing acid concentration (SI Appendix, Fig. S9), indicating that the ZnCX³-H and ZnCX¹⁰-H electrochemical hydrogenations follow a stepwise pathway akin to Scheme 2 to produce the isobacteriochlorin (Fig. 6 C and D) as opposed to the chlorinphlorin.

Acetonitrile was purified and dried through a neutral alumina column under argon. The supporting electrolyte, n-Bu₄NPF₆, was purchased from Sigma Aldrich (>99%), recrystallized from a water–ethanol mixture, and dried. All other chemicals were used as received. Chlorins and hangman chlorins were prepared according to previously reported procedures (7). CVs were collected using a CH Instruments potentiostat. Chlorins were dissolved in a solution containing n-Bu₄NPF₆ (0.1 M) as the supporting electrolyte. A three-electrode cell configuration was used with a platinum wire counterelectrode, a nonaqueous Ag⁺/Ag reference electrode, and a glassy carbon working electrode that was meticulously polished before each measurement. The polishing procedure was performed on felt using different diamond pastes of different sizes (15, 6, 3, and 1 μm). The electrode was briefly sonicated in ethanol and dried under a stream of compressed air before use. The working electrodes were repolished with 1-μm diamond paste in between CV measurements. Ohmic drop was compensated for using the positive feedback compensation implemented in the instrument. All potentials are reported vs. the Fc⁺/Fc redox couple. Thin-layer spectroelectrochemistry data were recorded in an N₂-filled glove box on an Ocean Optics spectrometer with a 0.5-mm path-length quartz cell, a Pt mesh working electrode, a Pt wire counterelectrode, and a nonaqueous Ag⁺/Ag reference electrode, all of which were purchased from BioAnalytical Systems. UV-vis-NIR (near-infrared) spectra were recorded at room temperature in quartz cuvettes on a Varian Cary 5000 UV-vis-NIR spectrophotometer. Simulations were performed using the DigiElch program (31) using the mechanism in SI Appendix, Scheme S1; results are summarized in SI Appendix, Table S1.

In an N₂-filled glove box, meso-tetraphenylchlorin (30.0 mg, 0.0486 mmol) was dissolved in 5 mL of dichloromethane (DCM), and tosylic acid monohydrate (9.2 mg, 0.049 mmol) was added as a solid. The reaction mixture was stirred for 2 h at room temperature, and cobaltocene (9.2 mg, 0.049 mmol) was subsequently added dropwise as a solution in DCM (∼2 mL). The resulting violet solution was stirred for 15 min. This procedure was repeated with a second equivalent of tosylic acid monohydrate (9.2 mg, 0.049 mmol) and cobaltocene (9.2 mg, 0.049 mmol). The solvent was then concentrated in vacuo, and the product was purified under an N₂ atmosphere by preparatory thin-layer chromatography on silica with DCM as the mobile phase. The titular compound was isolated as a violet powder (23 mg, 77%). Crystals for X-ray crystallography were grown by vapor diffusion of hexanes into a pyridine solution of the compound. ¹H and ¹³C NMR spectra are shown in SI Appendix, Fig. S10: ¹H NMR (400 MHz, CD₂Cl₂): δ 7.63 (dd, J = 7.8, 1.7 Hz, 2H), 7.50 to 7.33 (m, 13H), 7.30 (br, 1H), 7.20 to 7.09 (m, 3H), 7.00 (d, J = 5.2 Hz, 2H), 6.90 (d, J = 7.9 Hz, 2H), 6.62 (t, J = 2.6 Hz, 1H), 6.60 to 6.54 (m, 3H), 6.51 (t, J = 3.1 Hz, 1H), 6.29 (s, 1H), 6.01 (t, J = 2.8 Hz, 1H) 2.84 to 2.55 (m, 4H); ¹³C NMR (101 MHz, CD₂Cl₂): δ 152.1, 151.4, 142.6, 141.4, 140.6, 140.3, 135.8, 134.1, 133.0, 132.5, 131.3, 131.2, 129.2, 128.81, 128.78, 128.6, 128.3, 127.61, 127.60, 127.3, 126.5, 125.4, 124.3, 120.9, 119.7, 116.5, 115.4, 112.0, 111.0, 109.3, 106.4, 44.6, 36.4, 30.4; high-resolution mass spectrometry (electrospray ionization) mass-to-charge ratio: calculated for [M + H]⁺ (M = C₄₄H₃₄N₄) 619.2856, found 619.2843.

X-ray diffraction data were collected at the Advanced Photon Source at the Argonne National Laboratory on a Huber three-circle goniometer with free-κ using a Pilatus 1M CdTe Pixel Array Detector and an Oxford Cryosystems cryostat operating at 100 K. A synchrotron X-ray source with a wavelength of 0.41328 Å was used. The crystal was mounted on a cryoloop using Paratone oil. Data were integrated using SAINT, and multiscan absorption correction was applied using SADABS. The structure was solved by intrinsic phasing using SHELXT (APEX3 program suite 2016) and refined against F² on all data by full matrix least squares with SHELXL. All atoms were located in the difference-Fourier maps, and all nonhydrogen atoms were refined anisotropically. The crystal structure with the difference-Fourier map (green) used to locate hydrogen atoms bound to pyrrolic nitrogen atoms is shown in SI Appendix, Fig. S11. Crystal data and structure refinement for meso-tetraphenylchlorinphlorin are listed in SI Appendix, Table S2.