Spectral, Electrochemical, and ESR Characterization of Manganese

Article Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX

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Spectral, Electrochemical, and ESR Characterization of Manganese Tetraarylporphyrins Containing Four β,β′-Pyrrole Fused Butano and Benzo Groups in Nonaqueous Media Yuanyuan Fang,*,† Liping Wang,† Weijie Xu,† Zhongping Ou,‡ Mingyuan Chen,†,‡ Lei Cong,‡ Wenqian Shan,‡ Xiangyi Ke,‡ and Karl M. Kadish*,‡ †

School of Chemistry and Chemical Engineering, Jiangsu University, Zhenjiang, 212013, China Department of Chemistry, University of Houston, Houston, Texas 77204-5003, United States

‡

Inorg. Chem. Downloaded from pubs.acs.org by IOWA STATE UNIV on 02/05/19. For personal use only.

S Supporting Information *

ABSTRACT: Two series of β,β′-pyrrole butano- and benzosubstituted mangenese(III) tetraarylporphyrins were synthesized and characterized with regard to their spectral and electrochemical properties. The investigated compounds have the general formula butano(Ar) 4PorMnCl and benzo(Ar)4PorMnCl, where Por is the dianion of the porphyrin and Ar is a p-CH3Ph, Ph or p-ClPh group on each of the four meso-positions of the macrocycle. Each manganese(III) butano- or benzoporphyrin was examined in CH2Cl2 and/or pyridine containing 0.1 M tetra-n-butylammonium perchlorate and the data then were compared to that of the parent tetraarylporphyrins having the same meso-substituents. Up to four reductions are observed for each compound, the first being metal-centered to generate a Mn(II) porphyrin, and the second and third being porphyrin ring-centered to give a Mn(II) porphyrin π-anion radical and dianion, respectively. The one-electron reduced manganese porphyrins have an ESR spectrum with signals at g⊥= 5.6−5.8 and g// = 2.0, indicating a mixture of the fourand five-coordinated Mn(II) complexes in a high-spin state (3d5, S = 5/2, I = 5/2). Data from cyclic voltammetry and spectroelectrochemistry both suggest that formation of the porphyrin dianion is followed by a chemical reaction at the electrode surface to give an electroactive phlorin anion. The effects of solvent and porphyrin substituents on ultraviolet−visible light (UVvis) spectra, redox potentials, and electron transfer mechanisms are discussed.

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INTRODUCTION Synthetic manganese porphyrins have been widely studied, with respect to their potential applications in the fields of catalysis and functional materials,1−10 and several meso- and/or β-substituted manganese porphyrins have also been electrochemically examined.11−30 However, to the best of our knowledge, no electrochemical data on manganese(III) β,β′butanoporphyrins has been reported in the literature and only a few manganese(III) β,β′-benzo-substituted tetraarylporphyrins have been characterized to date.31−34 These earlier studies on benzoporphyrins were devoted mainly to oxidations and applications of the compounds in catalysis, and very little is known about their reductive behavior, which we wished to investigate in detail. We previously reported the electrochemical and spectroscopic properties for a series of β,β′-butano and/or β,β′-benzosubstituted iron(III),35 copper(II),36 cobalt(II),37 zinc(II),38 and platinum(II)39 porphyrins and we now turn our attention to manganese(III) derivatives with these two types of macrocycles. The newly examined compounds are shown in Chart 1 and are represented as butano(Ar)4PorMnCl and benzo(Ar)4PorMnCl, where Por is a dianion of the porphyrin and Ar is a p-CH3Ph, Ph, or p-ClPh group on each of the four © XXXX American Chemical Society

Chart 1. Structures of Investigated Manganese(III) Porphyrins

meso-positions of the macrocycle. The structure of the parent porphyrins is also shown in Chart 1. Each neutral, reduced, and/or oxidized form of the porphyrin was examined in CH2Cl2 or pyridine containing 0.1 M tetra-n-butylammonium perchlorate and the effect of the solvent and fused β,β′-group on the ultraviolet−visible light (UV-vis) spectra, redox Received: November 13, 2018

A

DOI: 10.1021/acs.inorgchem.8b03184 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry Scheme 1. Synthetic Routes for Obtaining (a) Butano(Ar)4PorMnIIICl and (b) Benzo(Ar)4PorMnIIICl

potentials, and the electron transfer mechanisms are discussed, with comparisons made to the tetraarylporphyrin derivatives lacking these β,β′-fused groups.

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RESULTS AND DISCUSSION Synthesis. Free-base tetrabutanotetraarylporphyrins were synthesized following a procedure in the literature,40 and the

Figure 2. UV-vis spectral changes obtained during the titration of (a) butano(Ph)4PorMnCl 2b and (b) benzo(Ph)4PorMnCl 3b by pyridine in CH2Cl2 (the inset Hill plot used for determination of the number of axially coordinated Py and the reaction equilibrium constant log K).

demetalation with acid40 and then reaction with MnCl2· 4H2O in DMF to generate the final desired product in yields of 64%−79%. UV-vis Spectra. UV-vis spectra of each porphyrin were measured in CH2Cl2 and pyridine containing 0.1 M TBAP. The parent compounds 1a−1c in CH2Cl2 are characterized by two low-intensity bands between 374 and 402 nm, a sharp Soret band at 478−479 nm, and three low-intensity Q bands between 528 and 622 nm (see Figure 1a, as well as Figure S1a in the Supporting Information). Sharp Soret bands at similar wavelengths are also obtained in pyridine (see Figures 1b, as well as Figure S1b in the Supporting Information), but the first set of lower intensity bands are red-shifted while the Q bands are blue-shifted in this solvent, compared to the spectra in CH2Cl2, with the difference suggesting that one pyridine

Figure 1. UV-vis spectra of (ClPh)4PorMnCl (1c), butano(ClPh)4PorMnCl (2c), and benzo(ClPh)4PorMnCl (3c) in (a) CH2Cl2 and (b) pyridine, 0.1 M TBAP.

Mn(III) butanoporphyrins 2a−2c were then obtained via a reaction between butano(Ar)4PorH2 and MnCl2·4H2O in DMF to give the final products with a yield of 46%−73% (Scheme 1a). The benzo(Ar)4MnCl complexes 3a−3c were synthesized via the route shown in Scheme 1b. Attempts to generate the free-base benzoporphyrins by direct oxidation of the corresponding free-base butanoporphyrins failed, and therefore they were obtained by an initial synthesis of the Cu(II) derivatives as described in the literature,36 followed by B


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Inorganic Chemistry

and 2b in CH2Cl2 and pyridine (Figures S2a and S3a in the Supporting Information). A different spectral pattern is seen for the tetrabenzoporphyrins 3a−3c, compared to the tetraarylporphyrins 1a−1c or tetrabutanoporphyrins 2a−2c under the same solution conditions (see Figure 1, as well as Figures S1−S3 in the Supporting Information). The benzoporphyrins are characterized by three strong absorption bands in CH2Cl2, as shown in the bottom spectrum of Figure 1a for benzo(ClPh)4PorMnCl 3c where the bands are located at 394, 516, and 686 nm. The spectrum for this compound in pyridine differs from that in CH2Cl2 (bottom right spectrum of Figure 1b), again because one pyridine molecule is axially coordinated to the central Mn(III) ion of the porphyrin. The pyridine binding reactions of the Mn(III) porphyrins were examined by performing a spectrally monitored titration of 2b and 3b in CH2Cl2, and the number of bound pyridine molecules was determined from diagnostic plots of log((Ai − A0)/(Af − Ai)) vs log[Py]. Examples of the spectral changes obtained during the titration of 2b and 3b are illustrated in Figure 2, where the slope of the Hill plot is 1.0, indicating the addition of a single pyridine molecule and the formation of butano(Ph)4PorMn(Py)Cl and benzo(Ph)4PorMn(Py)Cl as shown in eqs 1 and 2. The measured pyridine binding constants were calculated as logK = 1.02 for 2b and 0.97 for 3b, values which are smaller than the earlier measured log K (= 2.70)29 for (TPP)MnCl, under the same solution conditions. butano(Ar)4 PorMnCl + Py F butano(Ar)4 PorMn(Py)Cl (1)

benzo(Ar)4 PorMnCl + Py F benzo(Ar)4 PorMn(Py)Cl

Figure 3. Cyclic voltammograms showing the reduction of (a) (Ph)4PorMnCl 1b, (b) butano(Ph)4PorMnCl 2b, and (c) benzo(Ph)4PorMnCl 3b in CH2Cl2 containing 0.1 M TBAP at a scan rate of 0.10 V/s.

(2)

The benzo(Ar)4PorMnCl complexes 3a−3c are all characterized by a relatively high intensity Q band, compared to (Ar)4PorMnCl 1a−1c or butano(Ar)4PorMnCl 2a−2c. Similar porphyrin UV-vis spectra characterized by an intense Q band have been reported for other tetrabenzotetraarylporphyrins containing Co(II), Ni(II), Cu(II), Zn(II), or Pt(II) central ions31,36−41 and this is consistent with a significant interaction between the β,β′-fused benzo groups and the macrocycle. In contrast to the large effect of the β,β′-fused groups on the UV-vis spectra, changes in the meso-substituents have only a small effect on the UV-vis spectra, which are almost identical for the porphyrins 1a−1c in the two utilized solvents. The wavelength of the Soret band shifts by only 1−2 nm upon changing from an electron-donating meso-CH3Ph group to an electron-withdrawing meso-ClPh group on the porphyrin. This is also the case for the butano and benzo derivatives, where very similar spectra are seen for the series of porphyrins 2a−2c and 3a−3c under the same solution conditions. Electrochemistry in CH2Cl2. The three types of Mn(III) porphyrins were examined by cyclic voltammetry in CH2Cl2 containing 0.1 M TBAP, and examples of cyclic voltammograms illustrating the reductions are shown in Figure 3 for 1b, 2b, and 3b. In the case of 1b, two irreversible reductions are observed at Epc = −0.36 and −1.60 V, but three reductions are observed for the butanoporphyrin 2b under the same solution conditions. The first reduction is quasi-reversible and located at Epc = −0.80 V and Epa = −0.43 V for a scan rate of 0.10 V/s (Figure 3b). There is also a second reduction at Epc = −1.58 V and a third at E1/2 = −1.73 V. Similar reduction behavior was observed for the other two butano-substituted compounds 2a

Scheme 2. Proposed Mechanism for the First Reduction of Butano(Ar)4PorMnIIICl in CH2Cl2 Containing 0.1 M TBAP (Potentials for Compound 2b)

molecule is axially coordinated with the central Mn(III) ion of the porphyrin, as previously reported in the literature.29 Under the same solution conditions, the butanoporphyrins 2a−2c exhibit spectra that are similar to (Ar)4PorMnCl 1a− 1c, but the peaks of the split Soret band are of similar intensity and only two weak Q bands are observed instead of three. For example, the spectrum of butano(ClPh)4PorMnCl 2c (middle spectrum in Figure 1a) has intense bands at 378 and 492 nm and weaker bands at 587 and 632 nm in CH2Cl2. Virtually the same Soret band maximum (493 nm) is seen in pyridine, but the band at 378 nm for 2c in CH2Cl2 is red-shifted to 396 nm in pyridine and the two Q bands of 2c in CH2Cl2 are slightly blue-shifted in pyridine (middle spectrum in Figure 1b). Similar solvent-dependent differences in spectra are seen for 2a C


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Inorganic Chemistry

Table 1. Half-Wave Potentials (V vs SCE) of Manganese(III) Porphyrins in CH2Cl2 and Pyridine Containing 0.1 M TBAP MnIII/MnII

Ring Oxidation

Ring Reduction

solvent

compound

ox2

ox1

ΔEo2-o1 (V)

Epa (V)

Epc (V)

red1

CH2Cl2

1a 1b 1c

1.44 1.50 1.53

1.13 1.14 1.20

0.31 0.36 0.33

−0.08 −0.10 −0.02

−0.38 −0.36 −0.35

−1.63b −1.60b −1.54b

2a 2b 2c

1.02 1.10 1.13

0.80 0.86 0.94

0.22 0.24 0.19

−0.48 −0.43 −0.38

−0.88 −0.80 −0.76

−1.69b −1.58b −1.52b

3a 3b 3c

1.04 1.06 1.10

0.76 0.78 0.82

0.28 0.28 0.26

pyridine

red2

red3a

−1.83 −1.73 −1.66

E1/2 (V) −0.17 −0.15 −0.07

−1.75b −1.67b −1.58b

1a 1b 1c

−0.27 −0.25 −0.18

−1.35 −1.32 −1.24

−1.89 −1.84 −1.73

2a 2b 2c

−0.49 −0.46 −0.41

−1.56 −1.53 −1.42

−2.00b −1.95b −1.84b

3a 3b 3c

−0.08 −0.05 −0.01

−1.33 −1.32 −1.25

−1.77b −1.75b −1.63b

−1.96 −1.94 −1.84

a Reduction of phlorin anion in solution after the second electron addition to the porphyrin macrocycle. bIrreversible peak potentials at a scan rate of 0.10 V/s.

Figure 5. Cyclic voltammograms showing the reduction of benzo(Ph)4PorMnCl 3b in pyridine containing 0.1 M TBAP at a scan rate of 0.10 V/s.

The nature of the products generated during the second and third reductions of the Mn(III) butanoporphyrins was not examined in further detail, but on the basis of data previously reported for other structurally related Mn(III) tetraarylporphyrins, both reductions can be assigned as involving the porphyrin π-ring system to give Mn(II) porphyrin π-anion radicals and dianions.25,27,28 The benzoporphyrins also undergo two reductions similar to the parent compounds (Ar)4MnIIICl, but the first process is reversible (fast electron transfer) and located at E1/2 = −0.17 V

Figure 4. Cyclic voltammograms showing the reduction of (a) (Ph)4PorMnCl 1b and (b) butano(Ph)4PorMnCl 2b in pyridine containing 0.1 M TBAP at a scan rate of 0.10 V/s.

and 2c in CH2Cl2 (see Figure S4b), and the mechanism for the first reduction of these compounds is proposed as shown in Scheme 2. D


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Inorganic Chemistry

Scheme 3. Proposed Mechanism for the Reduction of Manganese(III) Butanoporphyrins and Benzoporphyrins in Pyridine Containing 0.1 M TBAP (Structure and Potentials for 3b)

for 3a, E1/2 = −0.15 V for 3b, and E1/2 = −0.07 V for 3c in CH2Cl2 containing 0.1 M TBAP (see Figures 3c, as well as Figure S4c in the Supporting Information). The fact that reversible reductions are seen for the Mn(III)/Mn(II) process of the benzoporphyrins suggests that the axially bound Cl− on the neutral porphyrin probably does not rapidly dissociate after the first one-electron addition to the central Mn(III) ion under the given solution conditions. A second irreversible reduction of compound 3b is located at Epc = −1.67 V (Figure 3c). The higher peak current seen for this process and the lack of a reverse peak on the return scan suggests a multielectron transfer, followed by one or more chemical reactions. Two reversible one-electron oxidations are observed for both the butanoporphyrins 2a−2c and the benzoporphyrins 3a−3c in CH2Cl2 containing 0.1 M TBAP. Examples of cyclic voltammograms for these porphyrins are given in Figure S5 in the Supporting Information, while the measured half-wave potentials are summarized in Table 1. The first oxidation of the butanoporphyrins is located at E1/2 = 0.80 to 0.94 V and the second at E1/2 = 1.12 to 1.13 V. In the case of the benzoporphyrins, the E1/2 for the first oxidation ranges from 0.76 V to 0.82 V, while the potential for the second process varies from E1/2 = 1.04−1.10 V in CH2Cl2. The potential difference between the two oxidations is slightly larger for the examined benzoporphyrins than for the related butanoporphyrins characterized under the given solution conditions. Electrochemistry in Pyridine. It is known that manganese porphyrins can easily coordinate one pyridine molecule to give [(Por)MnIII(py)]+, which may have a significant effect on the electrochemical behavior compared to (Por)MnIIICl.42,43 As seen in Figure 4a, three reversible oneelectron reductions were observed for (Ph)4PorMnCl 1b in pyridine rather than only two irreversible reductions observed in CH2Cl2 for the same compound (see Figure 3a). Three oneelectron reductions were also observed for the butanoporphyrins, but only the first two of these are reversible in pyridine containing 0.1 M TBAP. For example, the first and second reversible reductions are located at E1/2 = −0.46 V and −1.53

V for 2b (Figure 4b), leading to formation of butano(Ph)4PorMnII(py) and [butano(Ph)4PorMnII(py)]−, respectively, as shown in eqs 3 and 4. [butano(Ph)4 PorMn III(py)]+ + e F butano(Ph)4 PorMn II(py)

(3)

butano(Ph)4 PorMn II(py) + e F [butano(Ph)4 PorMn II(py)]−

(4)

The third reduction of 2b is irreversible and located at Epc = −1.95 V to give [butano(Ph)4PorMnII(py)]2− at the surface of the electrode. However, two reoxidation peaks were observed at Epa = −0.20 and −0.95 V when the potential scan was reversed at −2.10 V, thus indicating a chemical reaction coupled with this electron transfer process. Similar reduction behavior was also seen for the butano derivatives 2a and 2c (Figure S6 in the Supporting Information), indicating that all three of the investigated butanoporphyrins undergo the same electroreduction processes in pyridine. Figure 5 illustrates cyclic voltammograms for reduction of the benzoporphyrin 3b in pyridine. Four reductions are observed for this compound as compared to three reductions for butano(Ar)4MnIIICl and other previously characterized manganese(III) porphyrins under the same solution conditions.20,22,43 The first two reductions of 3b are reversible and assigned to the formation of Mn(II) porphyrin and Mn(II) porphyrin π-anion radical as described for the butanoporphyrins in eqs 3 and 4. The third irreversible reduction of 3b at Epc = −1.75 V is coupled with two reoxidations on the return scan at Epa = −0.29 and −0.60 V for a scan rate = 0.10 V/s. The electrochemical behavior for the first three reductions of benzoporphyrins 3a and 3c is similar to that of 3b (see Figure S7 in the Supporting Information) and the results for all these porphyrins are consistent with a chemical reaction following generation of the porphyrin dianion. The further reversible E


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Inorganic Chemistry

The butanoporphyrins are all harder to reduce than the benzoporphyrins, with the difference in potential amounting to ∼200 mV in the case of the two ring-centered redox processes, the second of which occurs just at the negative potential limit of the pyridine solvent, thus preventing measurements of the phlorin anion reduction potentials. Nonetheless, based on the overall electrochemical behavior of the investigated porphyrins in the two series, it can be proposed that a phlorin anion is also generated from a chemical reaction of the Mn(II) butanoporphyrin dianions electrogenerated in solutions of pyridine. The mechanism for electroreduction of the porphyrins in these two series is then proposed to occur as shown in Scheme 3. The illustrated structure in Scheme 3 is for the benzoporphyrin 3b, which shows a well-defined fourth reduction at −1.94 V in this solvent (see Figure 5). The same mechanism occurs for the butanoporphyrins 2a−2c but the last reduction occurs beyond the potential range of the electrochemical solvent and cannot be observed. Spectroelectrochemistry. Figure 6 shows the spectral changes that occurred for (Ph)4PorMnCl (1b), butano(Ph)4PorMnCl (2b), and benzo(Ph)4PorMnCl 3b during the first one-electron reduction in CH2Cl2 containing 0.1 M TBAP. The spectra of the singly reduced Mn(III) porphyrins 1b, 2b, and 3b are characterized by an intense Soret band at 442, 451, and 482 nm and two weak visible bands between 560 and 660 nm. The one-electron reductions in Figure 6 are assigned as metal-centered electron transfers to generate Mn(II) derivatives with uncharged porphyrin rings under the given solution conditions.20,22 These species each have an intense unsplit Soret band located at 442 nm for 1b, 451 nm for 2b, and 482 nm for 3b. Figure 7 illustrates the spectral changes of (ClPh)4PorMnCl 1c and butano(ClPh)4PorMnCl 2c obtained during controlled potential reductions in pyridine containing 0.1 M TBAP. Similar spectral changes are seen for these two compounds upon the first two reductions, indicating that they follow the same electron transfer mechanism, as shown by eqs 3 and 4 under the given experimental conditions. As seen in this figure, the initial Soret bands at 373 and 477 nm for 1c, at 396 and 493 nm for 2c both decrease in intensity while a sharp Soret band grows in at 445 nm (1c) or 493 nm (2c), indicating the formation of a Mn(II) porphyrin with an intact π-ring system. Upon the second reduction at −1.60 V, the Soret band of the Mn(II) porphyrin decreases in intensity, and a broad, weak Q band appears at 768 nm for 1c and 780 nm for 2c. This result is consistent with the formation of a Mn(II) porphyrin π-anion radical in pyridine. However, different spectral changes are seen between 1c and 2c upon the third reduction. In this process, the Soret and the Q bands decrease substantially, indicating the formation of a porphyrin dianion, but this is not the case for 2c where the product of the reduction has a split Soret band at 415 and 490 nm and a Q band at 811 nm as seen in Figure 7. The reduced butanoporphyrins 2a and 2b also have a Q band at 800−802 nm after the third reduction in pyridine (Figure S8 in the Supporting Information), suggesting the formation of a butanophlorin anion. This conclusion can be reached by comparison with the spectra of phlorin anions electrogenerated from other doubly reduced free-base porphyrins44,45 and metalloporphyrins containing redox-inactive central metal ions such as Zn(II), Ni(II), Cu(II), Pd(II), and Cd(II) under similar solution conditions.38,44−46

Figure 6. Thin-layer UV-vis spectral changes of (a) (Ph)4PorMnCl (1b), (b) butano(Ph)4PorMnCl (2b), and (c) benzo(Ph)4PorMnCl (3b) during the first controlled potential reduction in CH2Cl2 containing 0.1 M TBAP.

one-electron addition at −1.94 V for 3b is then assigned to reduction of a phlorin anion generated at the electrode surface. It should be noted that the generation of a phlorin anion has been reported in the literature for free-base porphyrins as well as for metalloporphyrins containing electrochemically inactive central metal ions such as Zn(II), Ni(II), Cu(II), Pd(II), or Cd(II).38,44−46 In summary, the Mn(III) butanoporphyrins and benzoporphyrins are both reduced to the Mn(II) porphyrin dianion in pyridine and this dianion then undergoes a chemical reaction to give a phlorin anion which can be reoxidized back to the initial porphyrin on the return scan or further reduced by one electron, as shown in Figure 5 for compound 3b, where E1/2 = −1.94 V and in Table 1 where E1/2 = −1.96 V for 3a and −1.84 V for 3c under the same experimental conditions. F


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Inorganic Chemistry

Figure 7. Cyclic voltammograms and UV-vis spectral changes of (a) (ClPh)4PorMnCl 1c and (b) butano(ClPh)4PorMnCl 2c during controlled potential reductions in pyridine containing 0.1 M TBAP.

ESR Characterization. Although Mn(III) porphyrins are paramagnetic with a high spin d4 configration, they are “ESR silent” at conventional fields and frequencies due to short spin−lattice relaxation times and large zero-field splittings.47−50 The currently investigated manganese(III) butanoand benzoporphyrins also showed no ESR signal under the given experimental conditions. However, the Mn(II) butanoand benzoporphyrins prepared by chemical reduction exhibit typical Mn(II) ESR signals over a wide range of magnetic fields. The ESR data measured for the currently examined Mn(II) porphyrins are summarized in Table 2 while examples of the ESR spectra are given in Figure 10 for chemically reduced (Ph)4PorMnCl 1a, butao(Ph)4PorMnCl 2b and benzo(Ph)4PorMnCl 3a. In each case, the spectra are comprised of two resonances, one located at g⊥ = 5.6−5.8 and the other at g// = 2.0, with a six line hyperfine splitting from the 55Mn nucleus (I = 5/2), giving the characteristic of a high-spin (3d5, S = 5/2) configuration of Mn(II) porphyrins.49−52 It is known that the ligand binding to the Mn(II) porphyrin has a significant effect on the ESR properties of the compound.

Four well-defined sets of spectral changes were obtained for benzo(ClPh)4PorMnIIICl 3c in pyridine during stepwise application of an applied potential in a thin-layer cell (Figure 8). The spectral changes of 3c obtained during the first two one-electron reductions indicate the formation of a Mn(II) porphyrin with an uncharged macrocycle and a Mn(II) porphyrin π-anion radical (Figures 8a and 8b, respectively). The product obtained after the second ring-centered reduction of 3c at −1.70 V has a split Soret band at 480 and 525 nm and a Q band at 843 nm. This spectrum is also assigned as that of a phlorin anion formed via a chemical reaction of the type shown in Scheme 3. Spectra of the butano- and benzo-substituted Mn(III) porphyrins were also monitored in CH2Cl2 containing 0.1 M TBAP after controlled potential oxidation. Examples of the spectral changes obtained during the two stepwise one-electron oxidations are shown in Figure 9 for butano(Ph)4PorMnCl (2b) and benzo(Ph)4PorMnCl (3b), and the data are consistent with the generation of porphyrin π-cation radicals and dications under the given solution conditions. G


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Inorganic Chemistry

Figure 8. Cyclic voltammogram and thin-layer UV-vis spectral changes of benzo(ClPh)4PorMnCl 3c during the controlled potential reductions at (a) −0.60 V, (b) −1.45 V, (c) −1.70 V, and (d) −1.90 V in pyridine containing 0.1 M TBAP.

Figure 9. Thin-layer UV-vis spectral changes of (a) butano(Ph)4PorMnCl (2b) and (b) benzo(Ph)4PorMnCl (3b) during controlled potential oxidations in CH2Cl2 containing 0.1 M TBAP.

The four-coordinate (TMP)MnII (TMP = tetramesitylporphyrin) only has a distinct resonance at g// ≈ 2.0,51 while fivecoordinate imidazole-bound Mn(II) porphyrins have a strong resonance at g⊥ ≈ 5.9 and a weak signal at g// ≈ 2.0.49−54

It should be noted that a relatively strong resonance is seen at g// ≈ 2.0 for (Ar)4PorMnCl 1a−1c, butao(Ar)4PorMnCl 2a−2c, and benzo(Ar)4PorMnCl 3a−3c, suggesting that both four- and five-coordinate forms of the Mn(II) porphyrin might H


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Inorganic Chemistry Table 2. ESR Data of Manganese(II) Porphyrins compound

g⊥

g//

ref

(CH3Ph)4PorMnCl (1a) (Ph)4PorMnCl ( 1b) (ClPh)4PorMnCl (1c) (Ph)4PorMnX (X = Cl, Br or I) (TMP)Mn K(TMP)MnCN (Ph)4PorMn(Py) (TpivPP)Mn(1-MeIm) (TpivPP)Mn(2-MeIm) butano(CH3Ph)4PorMnCl 2a butano(Ph)4PorMnCl 2b butano(ClPh)4PorMnCl 2c benzo(CH3Ph)4PorMnCl 3a benzo(Ph)4PorMnCl 3b benzo(ClPh)4PorMnCl 3c

5.6 5.6 5.6 6.0

2.0 2.0 2.0 2.0 2.0 2.0 2.0 1.98 2.00 2.0 2.0 2.0 2.0 2.0 2.0

this work this work this work 48, 49 50 50 52, 53 51 51 this work this work this work this work this work this work

5.95 5.96 5.94 5.95 5.6 5.7 5.6 5.8 5.7 5.8

Figure 11. Cyclic voltammograms of (a) butano(Ph)4PorMnCl (2b), (b) (Ph)4PorMnCl (1b), and (c) benzo(Ph)4PorMnCl (3b) in pyridine containing 0.1 M TBAP at a scan rate of 0.10 V/s.

exist in solution with an equilibrium between a chloride bound and none chloride complexed form of the porphyrin as shown in Scheme 4. Effect of Butano and Benzo Substituents. We have previously demonstrated that the β,β′-tetrabutano and tetrabenzo groups will have a significant effect on the reduction and oxidation potentials of copper, iron, and cobalt tetraarylporphyrin derivatives.36−38 This is also the case for the currently investigated butanoporphyrin and benzoporphyrin manganese(III) complexes. To illustrate this point, cyclic voltammograms showing the first two reversible reductions of (Ph)4PorMnCl (1b), butano(Ph)4PorMnCl (2b), and benzo(Ph)4PorMnCl) (3b) in pyridine are given in Figure 11. Both reductions of 2b are negatively shifted from E1/2 values for the same electrode reactions of 1b by 210 mV, indicating that Mn(III) butanoporphyrins are harder to reduce than the nonβ,β′-substituted derivatives. This shift in E1/2 values is due to the effect of the electron-donating butano groups. No shift in potential is seen for the second reduction of the benzoporphyrin 3b, but the first is shifted by 200 mV compared to that of 1b. Similar trends are seen when comparing E1/2 values for compounds 3a and 1a or 3c and 1c in pyridine, indicating that benzo groups have a large effect on the first reduction, which involves the Mn(III)/Mn(II) process but not the second, which involves the porphyrin macrocycles.

Figure 10. ESR spectra of the chemically reduced (a) (CH3Ph)4PorMnCl (1a), (b) butano(Ph)4PorMnCl (2b), and (c) benzo(CH3Ph)4PorMnCl (3a) in THF measured at 110 K.

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CONCLUSION Two series of β,β′-pyrrole butano- and benzo-substituted mangenese(III) tetraarylporphyrins were synthesized and characterized with regard to their electrochemical and spectroelectrochemical properties. Up to four stepwise reductions can be observed for Mn(III) butanoporphyrins and benzoporphyrins, depending on the solvent utilized. The first reduction of each examined porphyrin is metal-centered to generate a Mn(II) porphyrin in CH2Cl2 and pyridine. On the basis of cyclic voltammetric and thin-layer spectroelectrochem-

Scheme 4. Chemical Equilibrium between Four- and FiveCoordinate Manganese(II) Porphyrins in Solution

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Article

Inorganic Chemistry

Butano(CH3Ph)4PorMnCl (2a). Yield: 21.0 mg, 70%. UV-vis (CH2Cl2), λmax = 379, 492, 587, 630 nm. MS (MALDI-TOF). Calcd for C64H60MnN4: m/z 940.10. Observed: m/z 939.42. Butano(Ph)4PorMnCl (2b). Yield: 12.0 mg, 46%. UV-vis (CH2Cl2), λmax = 377, 490, 586, 629 nm. MS (MALDI-TOF). Calcd for C60H52MnN4: m/z 883.86. Observed: m/z 883.36. Butano(ClPh)4PorMnCl (2c). Yield: 22.0 mg, 73%. UV-vis (CH2Cl2), λmax = 378, 492, 587, 632 nm. MS (MALDI-TOF). Calcd for C60H48Cl4MnN4: m/z 1021.78. Observed: m/z 1021.20. Synthesis of Benzo(Ar)4PorMnCl..40,56 Benzo(Ar)4PorH2 (0.04 mmol) and excess MnCl2·4H2O (99 mg, 0.5 mmol) was heated to reflux in DMF at 150 °C under nitrogen for 2 h. The mixture was washed by H2O and extracted by CH2Cl2. After being dried by Na2SO4, the solvent was removed under reduced pressure and then passed through an alumina column, using CH2Cl2 as the eluent. The red fraction was collected and evaporated to dryness. Benzo(CH3Ph)4PorMnCl (3a). Yield: 16.0 mg, 64%. UV-vis (CH2Cl2), λmax = 395, 518, 641, 682 nm. MS (MALDI-TOF). Calcd for C64H44MnN4: m/z 924.02. Observed: m/z 923.29. Benzo(Ph)4PorMnCl (3b). Yield: 3.9 mg, 70%. UV-vis (CH2Cl2), λmax = 394, 518, 641, 684 nm. MS (MALDI-TOF). Calcd for C60H36MnN4: m/z 867.91. Observed: m/z 867.23. Benzo(ClPh) 4PorMnCl (3c). Yield: 17.3 mg, 79%; UV-vis (CH2Cl2), λmax = 394, 516, 642, 686 nm. MS (MALDI-TOF). Calcd for C60H32Cl4MnN4: m/z 1005.68. Observed: m/z 1005.07.

ical measurements of the Mn(III) butanoporphyrins and benzoporphyrins in pyridine, formation of a phlorin anion was observed after the 3-electron addition to the butanoporphyrin and benzoporphyrin containing the redox-active Mn(III) central metal ion. The β,β′-butano- and β,β′-benzo-substituents of the manganese(III) porphyrins and the solvent have significant effects on the UV-vis spectra, redox potentials, and the electron transfer mechanism.

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EXPERIMENTAL SECTION

Instrumentation. Cyclic voltammetry was performed at 298 K, using an EG&G Princeton Applied Research (PAR) 173 potentiostat/ galvanostat and a CHI-730C Electrochemical Workstation. A homemade three-electrode cell was used for all electrochemical measurements. The three-electrode system used in each case consisted of a glassy carbon working electrode. A platinum wire served as the auxiliary electrode and a saturated calomel electrode (SCE) as the reference electrode, which was separated from the bulk of the solution by means of a salt bridge of low porosity, which contained the solvent/supporting electrolyte mixture. All experiments were performed at room temperature. Thin-layer UV-vis spectroelectrochemical experiments were performed with a home-built thin-layer cell with a light transparent platinum net working electrode or a screen-printed working electrode (Pane Instruments). Potentials were applied and monitored with an EG&G PAR Model 173 potentiostat. Time-resolved UV-vis spectra were recorded with a diode array spectrophotometer (HewlettPackard, Model 8453). High-purity N2 was used to deoxygenate the solution and was kept over the solution throughout each electrochemical and spectroelectrochemical experiment. MALDI-TOF mass spectra were taken on a Bruker BIFLEX III ultrahigh resolution instrument using alpha-cyano-4-hydroxycinnamic acid as the matrix. ESR spectra were obtained at 110 K with a Bruker 300 with 100 kHz modulation. Mn(II) porphyrins were prepared through chemical reductions of the corresponding Mn(III) porphyrins by sodium borohydride (NaBH4) in tetrahydrofuran at room temperature.50 Chemicals. Dichloromethane (CH2Cl2, ≥ 99.8%) was purchased from EMD Chemicals, Inc. and pyridine (Py, ≥99.9%) was purchased from Sigma−Aldrich and used as received. Tetra-n-butylammonium perchlorate (TBAP, ≥99.0%) was purchased from Sigma−Aldrich Co. and used as received. Reagents and solvents (Sigma−Aldrich, Fluka, or Sinopharm Chemical Reagent Co.) for synthesis and purification were of analytical grade and used as received. Synthesis of (Ar)4PorMnCl.55 (Ar)4PorH2 (0.075 mmol) and excess MnCl2·4H2O (0.75 mmol) was dissolved into DMF and the mixture was refluxed at 150 °C under nitrogen for 1 h. The mixture then was washed by water and extracted by CH2Cl2 and then dried by Na2SO4. The solvent was removed under reduced pressure and then chromatographed on a silica gel column using CH2Cl2 as the eluent, and the brown fraction was collected and evaporated to dryness. (CH3Ph)4PorMnCl (1a). Yield: 49.2 mg, 87%. UV-vis (CH2Cl2), λmax = 376, 402, 479, 530, 585, 622 nm. MS (MALDI-TOF). Calcd for C48H36MnN4: m/z 723.23. Observed: m/z 723.43. (Ph)4PorMnCl (1b). Yield: 46.9 mg, 89%. UV−vis (CH2Cl2), λmax = 375, 401, 478, 529, 583, 619 nm. MS (MALDI-TOF). Calcd for C44H28MnN4: m/z 667.17. Observed: m/z 667.34. (ClPh)4PorMnCl (1c). Yield: 56.6 mg, 90%. UV−vis (CH2Cl2), λmax = 374, 400, 478, 528, 583, 618 nm. MS (MALDI-TOF). Calcd for C44H24Cl4MnN4: m/z 803.01. Observed: m/z 805.23. Synthesis of Butano(Ar)4PorMnCl..40,56 Butano(Ar)4PorH2 (0.03 mmol) and excess MnCl2·4H2O (59 mg, 0.3 mmol) were dissolved into DMF and the mixture was refluxed at 150 °C under nitrogen for 2 h. The crude product then was washed by H2O and extracted by CH2Cl2 and the organic layer was dried by Na2SO4. The solvent was removed under reduced pressure and then chromatographed on a neutral alumina column using CH2Cl2 as the eluent. The brown fraction was collected and evaporated to dryness.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.8b03184.

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UV-vis spectra, spectral changes and cyclic voltammograms (PDF)

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (Y. Fang). *E-mail: [email protected] (K. M. Kadish). ORCID

Karl M. Kadish: 0000-0003-4586-6732 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We gratefully acknowledge supports from the National Natural Science Foundation of China (Grant No. 21501070), Jiangsu University Foundation (Grant No. 15JDG131), China Postdoctoral Science Foundation (Grant No. 2017M611707), and the Robert A. Welch Foundation (KMK−Grant No. E-680).

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Spectral, Electrochemical, and ESR Characterization of Manganese

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