Facile and Reversible Electrogeneration of Porphyrin Trianions and

Jul 5, 2017 - Facile and Reversible Electrogeneration of Porphyrin Trianions and Tetraanions in Nonaqueous Media ... Department of Chemistry, Indian I...
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Facile and Reversible Electrogeneration of Porphyrin Trianions and Tetraanions in Nonaqueous Media Xiangyi Ke,† Pinky Yadav,‡ Lei Cong,† Ravi Kumar,‡ Muniappan Sankar,*,‡ and Karl M. Kadish*,† †

Department of Chemistry, University of Houston, Houston, Texas 77204-5003, United States Department of Chemistry, Indian Institute of Technology Roorkee, Roorkee 247667, India

‡

S Supporting Information *

ABSTRACT: The first examples for the facile, reversible, and stepwise electrogeneration of triply ring-reduced porphyrin macrocycles are presented. The investigated compounds are represented as MTPP(NO2)(PE)6, MTTP(PE)8, NiTPP(NO2)(Ph)4, and MTPP(CN)4, where TTP and TPP are the dianions of tetratolylporphyrin and tetraphenylporphyrin, respectively, NO2, phenylethynyl (PE), and CN are substituents at the β-pyrrole positions of the macrocycle, and M = CuII, NiII, ZnII, CoII, or 2H. Each porphyrin undergoes three or four reductions within the negative potential limit of the electrochemical solvent. The UV−visible spectra of the first three reduction products were characterized by means of thin-layer UV−vis spectroelectrochemistry, and the generation of multianionic porphyrins is interpreted in terms of extensive stabilization of the LUMOs due to the electron-withdrawing and/or extended π-conjugation of the βsubstituents.

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INTRODUCTION It has long been known that metalloporphyrins can undergo two one-electron additions to the conjugated macrocycle in nonaqueous media.1−5 These processes, which lead to the stepwise generation of π-anion radicals and dianions, have been characterized over the last five decades in a variety of electrochemical solvents6,7 and are mostly reversible when measured by cyclic voltammetry in “clean” and relatively nonreactive electrochemical solvent/supporting electrolyte mixtures with a large enough negative potential window to record both ring-centered reductions. Additional one-electron or multielectron reductions beyond the porphyrin dianion have sometimes been reported for porphyrins with redox inactive central metal ions, but these “extra” processes have generally not been characterized in detail or assigned as to the site of electron transfer due in large part to the fact that they are often irreversible or located at very negative potentials close to the solvent limit. In contrast to the above situation, a number of transition metal porphyrins have been shown to undergo three reversible one-electron reductions within the negative potential limit of the electrochemical solvent, and when this has occurred, a metal-centered redox reaction has most often been invoked, the best examples being derivatives containing Mn(III), Fe(III), or Co(II) central metal ions.6,7 The potentials for reduction of a given metalloporphyrin are known to be directly related to the electron-donating or electron-withdrawing properties of substituents on the macrocycle.6,7 Relatively facile ring-centered reductions will occur for porphyrins containing highly electron-withdrawing substituents such as CN, NO2, Br, Cl, F, or CF3,6−14 while more difficult © 2017 American Chemical Society

reductions will occur for porphyrins with electron-donating CH3 or OCH3 substituents. Most electrochemical studies of porphyrins in nonaqueous media have been carried out in solvents whose negative potential limit extends to −2.0 V vs SCE or more, but this limit was often not reached when characterizing the redox reactivity of porphyrins having highly electron-withdrawing substituents, sometimes because only the first (or first and second) reduction was of interest and other times for the simple reason that no further electrode reactions of the porphyrin macrocycle were expected to occur at potentials beyond formation of the dianion. Thus, a broadbased search for monomeric porphyrins with highly reduced πring systems has, to our knowledge, never been attempted, despite reports more than 25 years ago that zinc and free base tetraphenylporphyrins could be reversibly reduced to the tetraand hexa-anionic forms in dimethylamine at −65 °C.15,16 We recently described the synthesis and characterization of porphyrins containing multiple phenylethynyl (PE) and/or other highly electron-withdrawing substituents at the β-pyrrole positions of the macrocycle.17−19 The electrochemistry of these porphyrins was examined in CH2Cl2 and the potentials recorded for the stepwise formation of π-anion radicals and dianions after the addition of two electrons. However, we were surprised to see that a few of the electrochemically examined porphyrins exhibited an unexpected third reduction in dichloromethane, but this was not examined in detail and we were unable to evaluate the possibility of a previously uncharacterized metal-centered reduction or perhaps a Received: May 18, 2017 Published: July 5, 2017 8527

DOI: 10.1021/acs.inorgchem.7b01262 Inorg. Chem. 2017, 56, 8527−8537

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Inorganic Chemistry reduction at the phenylethenyl substituents in the case of the easily reduced PE substituted porphyrins. This is now done in the current study which involves a comprehensive electrochemical investigation of the earlier synthesized porphyrins under a variety of different solution conditions and provides the first report for the facile and reversible generation of porphyrin trianions and tetraanions where the added electrons are localized on the π-ring system of the macrocycle. The structures of the investigated porphyrins are shown in Chart 1 and represented as MTPP(NO2)(PE)6, MTTP(PE)8, Chart 1. Structures of Investigated Porphyrins with βPhenyl, β-Phenylethynyl (PE), β-Nitro, and β-CN Substituents

Figure 1. Cyclic voltammograms illustrating the reductions of CoTPP, CoTPP(NO2), and CoTPP(NO2)(PE)6 in PhCN, 0.1 M TBAP. Scan rate = 0.1 V/s.

NiTPP(NO2)(Ph)4, and MTPP(CN)4, where TTP and TPP are the dianions of tetratolylporphyrin and tetraphenylporphyrin, respectively, NO2, phenylethynyl (PE), and CN are substituents at the β-pyrrole positions of the macrocycle, and M = CuII, NiII, ZnII, CoII, or 2H. Each porphyrin was characterized by thin-layer UV−vis spectroelectrochemistry in addition to cyclic voltammetry in up to four different nonaqueous solvents. Low temperature electrochemical measurements were also carried out in order to minimize the occurrence of coupled chemical reactions following electron transfer and to extend the negative potential limit of the solvent when possible.

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RESULTS AND DISCUSSION Synthesis. MTPP(NO2)PE6 and MTPP(CN)4, where M = 2H, Co(II), Ni(II), Cu(II), and Zn(II), and NiTPP(NO2)(Ph4) were synthesized according to literature methods.17−20 H2TTP(PE)8 was synthesized in 80% yield by Stille coupling of H2TTPBr8 with tributyl(phenylethynyl)stannane in refluxing 1,4-dioxane for 1 h under an inert atmosphere. The Cu(II) and Zn(II) complexes were prepared by refluxing 10 equiv of M(OAc)2·nH2O and the free base porphyrin in a mixture of CHCl3 and MeOH (9:1, v/v), whereas Ni(II) insertion was carried out by refluxing H2TPP(PE)8 in DMF with 10 equiv of Ni(OAc)2·4H2O for 4 h under an inert atmosphere.18 A

Figure 2. Spectral changes during first reduction of (a) CoTPP, (b) CoTPP(NO2), and (c) CoTPP(NO2)(PE)6 in PhCN, 0.1 M TBAP.

detailed spectral characterization of the MTTP(PE)8 derivatives where M = 2H, Ni(II), Cu(II), and Zn(II) is given in the Supporting Information. The optical absorption spectra of MTTP(PE)8 were recorded in CH2Cl2 at 298 K. H2TTP(PE)8 exhibited a characteristic Soret band (B band) at 507 nm and three Q bands (572, 692, and 758 nm) whereas the metal complexes exhibit a Soret band at 498−504 nm and two Q 8528

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Figure 3. Cyclic voltammograms illustrating the reduction of (a) MTPP(NO2)(PE)6 and (b) MTTP(PE)8 in pyridine, 0.1 M TBAP.

Table 1. Half-Wave Potentials (V vs SCE) for Reduction of MTPP(NO2)(PE)6 and MTTP(PE)8 in Nonaqueous Solvents Containing 0.1 M TBAPa Reduction Potentials (E1/2, V vs SCE) Macrocycle

M

MTPP(NO2)PE6

Co

Ni

Cu

Zn

2H

MTTP(PE)8

Ni Cu

2H Zn a

Solv

1st

2nd

3rd

4th

DCM PhCN Py THF DCM PhCN Py THF DCM PhCN Py THF DCM PhCN Py THF DCM PhCN Py THF Py THF DCM Py THF Py THF Py

−0.26 −0.37 −0.60b −0.50b −0.65 −0.66 −0.71 −0.60 −0.70 −0.68 −0.66 −0.68 −0.78 −0.75 −0.72 −0.77 −0.61 −0.59 −0.52 −0.55 −0.82 −0.72 −0.83 −0.81 −0.79 −0.66 −0.66 −0.84

−1.05 −1.08 −1.08 −1.06 −0.88 −0.94 −0.89 −0.88 −0.92 −0.96 −0.90 −0.90 −1.15 −1.04b −0.99 −1.05 −0.72 −0.77 −0.69 −0.68 −1.03 −0.99 −1.08 −1.05 −1.02 −0.84 −0.81 −1.18

−1.52 −1.48 −1.50 −1.67 −1.37 −1.38 −1.35 −1.38 −1.38 −1.39 −1.35 −1.38 −1.50b −1.48b −1.51 −1.52b −1.40 −1.38 −1.39 −1.36 −1.58 −1.59 −1.70 −1.60 −1.63 −1.60 −1.62 −1.76 b

−1.84 −1.86b −1.83b −1.90 −1.79b −1.80b −1.80b −1.88b −1.82b −1.84b −1.84b −1.86b −1.88b −1.80b −1.92b −1.89b −1.88b −1.86b −1.86b −1.86 −1.87 c c −1.95 c −1.96b c

b

Scan rate = 0.1 V/s. Peak potential. cReaction beyond negative potential limit of the solvent.

bands. The Soret band of H2TTP(PE)8 exhibits a dramatic red shift of 92 nm as compared to H2TPP, and the Qx(0,0) bands

are shifted by 111 nm, suggesting that each PE group shifts the wavelength by ∼11−14 nm. Interestingly, the core imino 8529

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Figure 4. Controlled potential changes during the first, second, and third reductions of MTPP(NO2)(PE)6 in PhCN, 0.1 M TBAP where M is (a) Cu, (b) Ni, and (c) 2H.

the electron-withdrawing and conjugative effect of the eight PE groups. The 1H NMR spectra of NiTTP(PE)8 and ZnTTP(PE)8 are devoid of NH signals, indicating that the metal ion has been inserted into the porphyrin core. The meso-phenyl proton resonances of the Ni(II) and Zn(II) octa-PE complexes are shifted upfield by 0.03−0.21 ppm as compared to the freebase H2TTP(PE)8. The integrated intensities of the proton resonances for these synthesized β-phenylethynyl substituted tetratolylporphyrins are consistent with the proposed structures. Electrochemistry. Cobalt Porphyrins. A cyclic voltammogram illustrating the first three reversible reductions of CoTPP(NO2)(PE)6 in PhCN is given in Figure 1 along with voltammograms for the related non-PE derivatives CoTPP and CoTPP(NO2). The first reversible one-electron reduction of CoTPP at −0.85 V has previously been assigned as generating a Co(I) porphyrin product,21,22 and the second one-electron reduction, at the solvent edge (E1/2 = −1.98 V), would then correspond to the generation of a Co(I) porphyrin π-anion radical. Similar assignments for the electron transfer site have been made for a large number of cobaltporphyrins with TPP or OEP (octaethylporphyrin) skeletal structures.6,7,23 However, the formation of a cobalt(I) porphyrin after reduction of CoTPP(NO2) is less clear-cut, in part because of the much smaller separation between the two one-electron reductions (0.66 V) as compared to the 1.13 V separation for CoTPP (see Figure 1) and in part because of an earlier assignment of a ringcentered reduction for a structurally related cobalt tetratolylporphyrin, CoTPP(NO2), which was investigated under the same solution conditions.24 The potential separation between the first two reductions of CoTPP(NO2)(PE)6 is also much smaller (0.71 V) than in the case of CoTPP, and thus, the site of the first reduction for this compound is not intuitively

Figure 5. Cyclic voltammograms showing third and fourth reductions of Ni porphyrins in THF, 0.1 M TBAP. The measured half-wave potentials are given in the figure.

protons of H2TTP(PE)8 (−1.35 ppm) in the NMR spectra are shifted downfield as compared to H2TTP (−2.80 ppm) due to 8530

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Figure 6. Cyclic voltammetry of MTPP(CN)4 derivate (M = Ni, Cu, Zn, and 2H) in THF, 0.1 M TBAP.

700−900, as does CoTPP(NO2) (Figure 2b). However, it does not possess a split Soret band, as is the case for all previously characterized Co(I) porphyrins, and the NIR band of [CoTPP(NO2)(PE)6]− is also of higher intensity than the corresponding NIR band of [CoTPP(NO2)]−. These differences between the two spectra thus indicate a greater radical character for singly reduced CoTPP(NO2)(PE)6 than is the case of the non-PE containing porphyrin with a single β-pyrrole nitro group. The second or third reduction of CoTPP(NO2)(PE)6, at E1/2 = −1.08 or −1.48 V, might then involve the conversion of Co(II) to Co(I), or a porphyrin with a triply reduced macrocycle might also be generated. A fourth irreversible reduction of the nitro group is also seen for CoTPP(NO2)(PE)6 at Ep = −1.86 V for a scan rate of 0.1 V/s (Figure S1). MTPP(NO2)(PE)6 and MTTP(PE)8. In order to further elucidate the effect of the electron-withdrawing PE and NO2 groups on the porphyrin redox behavior, two series of compounds with a different number of PE groups and different central metal ions were electrochemically investigated in four different nonaqueous solvents. As described below, three and sometimes four facile one-electron reductions were observed by cyclic voltammetry for MTPP(NO2)(PE)6 and MTTP(PE)8. Examples for the prevailing redox behavior of MTPP(NO2)(PE)6 are given by the cyclic voltammograms in Figure 3a, where the reductions were carried out in pyridine. A mono-, di-, and trianionic porphyrin is reversibly generated for all four

evident from the potentials of the individual redox reactions or from previous data in the literature. In order to better answer the question as to the site of the first electron transfer in CoTPP(NO2) and CoTPP(NO2)(PE)6, the three porphyrins in Figure 1 were investigated by thin-layer spectroelectrochemistry, and the resulting spectral changes are shown in Figure 2. The final UV−visible spectrum after complete conversion of CoTPP to [CoTPP]− agrees with spectra published in the literature for a Co(I) porphyrin with an unreduced π-ring system, namely a split Soret band of reduced intensity at 364 and 425 nm and the lack of any absorbances between 600 and 900 nm.8 As seen in Figure 2, quite different UV−visible spectra are seen for the singly reduced forms of CoTPP(NO2) and CoTPP(NO2)(PE)6. The electrogenerated [CoTPP(NO2)]− product is characterized by a split Soret band at 376 and 437 nm in addition to a Q-band at 590 nm (Figure 2b), which would suggest Co(I) formation. However, the spectrum of singly reduced [CoTPP(NO2)]− also possesses a broad NIR band at 771 nm, which is an indicator for formation of a porphyrin π-anion radical,8 thus suggesting an equilibrium between two forms of the reduced porphyrin in solution. An equilibrium between two forms of the singly reduced porphyrin (one with a Co(I) center and unreduced ring and the other a Co(II) porphyrin π-anion radical) is also seen for [CoTPP(NO2)(PE)6]−. This spectrum (Figure 2c) has a reduced intensity Soret band and a broad NIR radical marker band at 8531

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Figure 7. Spectral changes during the first, second, and third reductions of (a) CuTPP(CN)4 and ZnTPP(CN)4 in THF, 0.1 M TBAP.

compounds in the MTPP(NO2)(PE)6 series, with E1/2 values for the reversible third reduction being located at E1/2 = −1.35 to −1.51 V. A fourth irreversible reduction of the nitro group is also observed at Ep = −1.80 to −1.92 V (Figure S2 for the Cu and Zn derivatives), and when this occurs, the third oneelectron reduction is no longer reversible on the return positive potential sweep. Remarkably, the Ni, Cu, and 2H derivatives of MTTP(PE)8 exhibit four reversible one-electron reductions at room temperature, a result which has never before been reported for any monomeric porphyrin with a “simple” skeletal structure, such as TPP. Similar redox behavior is seen in all four utilized electrochemical solvents, and a summary of the measured potentials is given in Table 1. Because a Cu(II)/Cu(I) process was assigned to the third reduction of CuTPP(CN)4 in DMF,25 a metal-centered redox process might also be envisioned to occur for the third reduction of the four Cu(II) and Ni(II) derivatives whose cyclic voltammograms are shown in Figure 3. However, metalcentered reactions are not possible for the four structurally related Zn(II) porphyrins with TTP(PE)8 or (TPP)(NO2)(PE)6 macrocycles. A reversible reduction of the nitro group on these compounds is also ruled out for the third electron addition, since this reduction of NO2 is irreversible (See Figures S1 and S2) and located at potentials more negative than −1.35 to −1.51 V in the case of MTPP(NO2)(PE6). Moreover, an NO2 substituent is not even present in the MTTP(PE)8 series

of compounds where four reductions and the formation of a porphyrin tetraanion is observed. A reduction of the PE group is also ruled out in both series of compounds, as verified by measurements of phenylacetylene, which shows a total lack of redox reactivity up to the negative potential limit of the four utilized nonaqueous solvents. As indicated in Table 1, each porphyrin in the MTPP(NO2)(PE)6 series undergoes an irreversible fourth reduction at a peak potential of −1.79 to −1.92 V vs SCE. The irreversibility of this process and the relatively invariant peak potential in all solvents provide further evidence in favor of assigning this process to the nitro group on the macrocycle prior to protonation, as is needed in the eventual conversion of NO2 to NH2 which is known to occur with chemical reductants.26 The UV−visible spectra of each electrogenerated monoanion, dianion, and trianion of MTPP(NO2)(PE)6 and MTTP(PE)8 were measured in PhCN, 0.1 M TBAP, and the spectra recorded during each one-electron transfer under the application of a controlled reducing potential are shown in Figure 4 for MTPP(NO2)(PE)6 and in Figure S3 for MTTP(PE)8. As seen in the two figures, the spectral patterns observed for each electroreduced form of [MTPP(NO2)(PE)6]n− or [MTTP(PE)8]n− are almost independent of the central metal ion or macrocycle but vary with the charge on the compound. For example, the singly reduced [MTPP(NO2)]− derivatives 8532

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Figure 8. Top and side views of the optimized structures of (a) H2TPP, (b) H2TPP(NO2)(PE)6, (c) NiTPP, (d) NiTPP(NO2)(Ph)4, (e) NiTPP(NO2)(PE)6, and (f) NiTPP(CN)4 using the B3LYP functional and LANL2DZ basis set in the gas phase. The meso- and β-substituents are omitted for clarity in the side view.

−1.18, and −1.83 V when the scan is reversed at −2.0 V; a fourth irreversible process is also seen at Ep = −2.10 V when the negative potential scan is extended to −2.6 V vs SCE. The latter voltammogram is shown in Figure S4b and is similar in shape to cyclic voltammograms of the MTPP(NO2)(PE)6 derivatives which undergo three reversible one-electron reductions followed by an irreversible process at more negative potentials (see Figure S2 and Table S1). Like in the case of MTPP(NO2)(PE)6, the fourth irreversible reduction of NiTPP(NO2)(Ph)4 is assigned as an electron addition to the nitro-substituent. As seen in Figure 5, the reversible third reduction of the nickel porphyrin is shifted positively in potential from −1.83 V in the case of NiTPP(NO2)(Ph)4 to −1.59 V for NiTTP(PE)8 and −1.38 V for NiTPP(NO2)(PE)6. The direction of the shift in E1/2 is consistent with the increased electron-withdrawing properties of the PE substituents. The difference in E1/2 values between the octa-PE derivative, NiTTP(PE) 8, and the mononitro hexa-PE porphyrin, NiTPP(NO2)(PE)6, amounts to 290 mV for the first reduction (E1/2= −0.89 and −0.60 V), 300 mV for the second (E1/2 = −1.18 and −0.88 V), and 450 mV for the third (E1/2 = −1.83 and −1.38 V). Thus, the effect of the PE substituents on the third reduction is substantially greater than that on the first and second one-electron additions. The easiest to reduce porphyrin by one or two electrons in Figure 5 is NiTPP(CN)4, and one might therefore predict a much easier third electron addition for this compound on the basis of the facile first two reductions and the strong electronwithdrawing effects of the four β-CN groups. However, this is not the case, as seen in the figure where the reversible third reduction of NiTPP(CN)4 actually occurs at −1.86 V, a value virtually identical to the measured E1/2 for the third reversible reduction of NiTPP(NO2)(Ph)4 (−1.83 V) and NiTTP(PE)8 (−1.87 V).

are characterized by a new low intensity band at 553 to 558 nm and a broad NIR band at 777 to 805 nm (Figure 4). A similar spectral pattern is also seen for the singly reduced [MTTP(PE)8]− compounds, which have a new low intensity band at 565−572 nm and two bands in the NIR region of the spectrum. The addition of a second electron to the porphyrins in the two series then leads to a decrease in intensity for the absorptions at 400−600 nm and the appearance of new strong NIR bands at 800−1100 nm. The NIR bands then decrease in intensity after the addition of a third electron, and the triply reduced porphyrins are all characterized by new bands at 650− 950 nm as shown in Figure 4 and S3. Some differences do exist in the individual spectra, but for the most part, all of the spectra in a given oxidation state are similar to each other, indicating the lack of a metal-centered redox process in the case of the copper or nickel derivatives in the TPP(NO2)(PE)6 or TTP(PE)8 series. Nickel Porphyrins with Different Skeletal Structures. Additional measurements were also made on a series of Ni porphyrins in THF, and examples of the obtained cyclic voltammograms are illustrated in Figure 5 for compounds with five different types of TPP skeletal structures. In the case of NiTPP(Ph)4, two reversible one-electron reductions are obtained at −1.23 and −1.56 V vs SCE, values of E1/2 which are almost identical to those of the parent NiTPP compound when reduced under the same solvent conditions (−1.20 and −1.66 V; see Table S1). These two ring-centered reactions of NiTPP and NiTPP(Ph)4 are then followed by a third multielectron transfer process which is irreversible at RT and remains irreversible at all temperatures down to −60 °C, as shown in Figure S4a, where Ep = −2.60 and −2.44 V for NiTPP(Ph)4 and NiTPP, respectively, at a scan rate of 0.1 V/s. In contrast to the above behavior, NiTPP(NO2)(Ph)4 undergoes three reversible one-electron reductions at −0.89, 8533

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Figure 9. Electronic density distribution of the frontier molecular orbitals (FMOs) of NiTPP, NiTPP(NO2)(Ph)4, NiTPP(NO2)(PE)6, and NiTPP(CN)4.

reduction of MTPP(NO2)(PE)6 and MTTP(PE)8 indicate that a conversion of the doubly reduced porphyrin to its quadruply reduced form occurs via two one-electron transfers in pyridine, and the same is seen for NiTTP(PE)8 in THF (Figure 5). The four one-electron transfers are extremely well defined in THF at room temperature (Figure 5), with virtually identical separations in E1/2 values of 0.27 and 0.28 V, respectively, between the first and second reductions at E1/2 = −0.72 and −0.99 V and the third and fourth reductions at E1/2 = −1.59 and −1.87 V. Two additional reductions are also observed for NiTTP(PE)8 in THF at −60 °C (Figure S4c), one at Ep = −2.13 V and the other at E1/2 = −2.38 V, but it is not clear at this time if these two reactions actually correspond to formation of porphyrin penta- and hexaanions or if the irreversible process at −2.13 V is due to an unknown side reaction and the reversible reduction at −2.38 V is due to generation of the hexaanion. What is clear, however, is that the reversible additions of a third and fourth electron to the Ni, Cu, and 2H forms of MTTP(PE)8 all occur at virtually the same potentials of −1.59 to −1.62 V and −1.87 to −1.96 V in THF (Table 1) and this is also the case for the third reduction of the Ni, Cu, Zn, and 2H forms of MTPP(NO2)(PE)6 in pyridine (Figure 3),

Figure 10. Molecular orbital energies of NiTPP, NiTPP(NO2)(Ph)4, NiTPP(NO2)(PE)6, and NiTPP(CN)4 based on DFT studies.

We had initially anticipated that the reduction of NiTTP(PE)8 at low temperature in THF might be characterized by a conversion of the porphyrin dianion to its tetraanionic and hexaanionic forms, as earlier demonstrated by Heinze, Müllen, and co-workers for structurally related Zn(II) and free base porphyrins which lacked the electron-withdrawing NO2 and PE substituents.15,16 The voltammetric data in Figure 3 for the 8534

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(deg) of the optimized geometries are listed in Table S3. These β-functionalized Ni(II) porphyrins and H2TPP(NO2)(PE)6 exhibit a nonplanar saddle-shaped conformation whereas NiTPP exhibits a ruffled conformation (Figure 8) of the porphyrin core, which is evident from the mean saddling dihedral angle of 10.56−49.95° (i.e., the dihedral between two nearest Cα−Cβ vectors from adjacent pyrrole rings), average displacement of β-carbons (ΔCβ) 0.141−1.162 Å, and 24-core atoms (Δ24) 0.060−0.578 Å from the porphyrin mean plane. Notably, H2TPP(NO2)(PE)6 exhibits the most saddled structure with the highest dihedral angle of 49.95° among the examined β-functionalized porphyrins, whereas H2TPP exhibits a planar conformation of the porphyrin core. The order of nonplanarity based on ΔCβ and Δ24 follows the expected trend of NiTPP(NO2)(PE)6 > NiTPP(NO2)(Ph)4 > H2TPP(NO2)(PE)6 > NiTPP(CN)4 > NiTPP > H2TPP. In general, the mixed substituted porphyrins having a nitro substituent show longer Cβ−Cβ and Cβ′−Cβ′ (1.368− 1.417 Å) bond lengths with marginal increments in the Cβ− Cα−Cm bond angle with decrease in N−Cα−Cm. Pictorial representations of the frontier molecular orbitals (FMOs) and their energy level diagram of the β-functionalized porphyrins are shown in Figures 9, 10, and S6. The HOMO has a contribution from the porphyrin core whereas the LUMO has a contribution from the β-substituents as well as the porphyrin core (Figure 9). The HOMOs of H2TPP(NO2)(PE)6 are marginally altered whereas the LUMOs are highly stabilized with respect to H2TPP (ΔELUMO = 0.767 eV, ΔELUMO+1 = 0.633 eV, ΔELUMO+2 = 1.59 eV, and ΔELUMO+3 = 1.61 eV) as shown in Figure 10. Notably, the LUMO+2 and LUMO+3 levels are extensively stabilized, which leads to formation of porphyrin trianions and tetraanions within the potential limit of the solvent employed. A series of Ni(II) porphyrins was also studied, and the effect of stabilization of energy levels is shown in Figure 10. The HOMOs of NiTPP(NO2)(PE)6 are moderately stabilized (around 0.16−0.20 eV), whereas the HOMOs of NiTPP(CN)4 are highly stabilized by 0.86−1.04 eV as compared to NiTPP. NiTPP(CN)4 possesses highly stabilized LUMO energy levels as compared to NiTPP(NO2)(Ph)4, NiTPP(NO2)(PE)6, or NiTPP. Notably, the LUMO+2 and LUMO+3 of NiTPP(CN)4 are closely spaced and hence only three reductions are observed for NiTPP(CN)4. The trend in the HOMO−LUMO gap calculated from the DFT studies is as follows: NiTPP > NiTPP(NO2)(Ph)4 > NiTPP(NO2)(PE)6 > NiTPP(CN)4, and is in good agreement with both the electronic spectral data17,19 and the redox potentials. These preliminary DFT results clearly specify that the highly stabilized LUMO+2 and LUMO+3 orbitals are responsible for the multireduction of NiTPP(NO2)(PE)6 (Figure 10). Also, the extended π-conjugation of the PE groups in MTPP(NO2)(PE)6 and MTTP(PE)8 can stabilize the electrogenerated anions as compared to MTPP(CN)4 and MTPP(NO2)(Ph)4. It is expected that all of the highly electron deficient porphyrins may stabilize the LUMO levels, which, in turn, may form multianionic porphyrins (Figure S6).

strongly indicating the lack of a metal-centered redox process in each case. Reactions of MTPP(CN)4 where M = Ni, Cu, Zn, and 2H. In order to further elucidate the effect of the CN substituents in facilitating formation of the porphyrin trianion and to also rule out the possibility of a metal-centered reaction in NiTPP(CN)4, an additional set of experiments were carried out on a series of tetracyanoporphyrins containing NiII, CuII, ZnII, and 2H. The Ni(II) tetracyanoporphyrin has not previously been characterized by its redox properties while the electrochemistry of the MTPP(CN)4 derivatives with Cu25, Zn27, or 2H28 centers has been reported in the literature. However, an accurate and meaningful comparison of the redox behaviors for the different derivatives having this type of macrocycle is not possible due to the fact that the earlier measurements were carried out by different laboratories who utilized different solvents, different electrode materials, and different electrochemical techniques. The solvent selected for our comparative study of the four TPP(CN)4 derivatives was THF containing 0.1 M TBAP. Under these solution conditions, all four investigated MTPP(CN)4 derivatives undergo three reversible one-electron reductions, as shown in Figure 6. This behavior parallels what is seen for the earlier described porphyrins in the MTPP(NO2)(PE)6 and MTTP(PE)8 series of compounds (Table 1 and Figure 3), suggesting, again, the formation of a triply reduced porphyrin macrocycle in each case for the reasons outlined earlier in the manuscript. Again, the three electroreductions of the tetracyanoporphyrins were investigated by thin-layer spectroelectrochemistry, and examples of the resulting spectra are shown in Figure 7 and Figure S5. Like in the case of electroreduced [MTPP(NO2)(PE)6]n− and [MTTP(PE)8]n−, the UV−visible spectral patterns for the first two reduced forms of [MTPP(CN)4]n− are quite similar to each other and independent of the central metal ion. This is seen for the CuII and ZnII derivatives in Figure 7 and the NiII and 2H species in Figure S5, where the singly reduced porphyrins are characterized by broad absorptions from 700 to 1000 nm and an intense NIR band located between 933 and 966 nm. The NIR bands of the singly reduced tetracyanoporphyrins disappear after the addition of a second electron to generate [MTPP(CN)4]2−, which also has a spectral pattern almost independent of the central metal ion. This similarity in spectra between the doubly reduced Cu and Zn porphyrins in Figure 7 suggests the lack of a metal-centered process after the addition of a second electron to the Cu porphyrin. A similar spectrum is also seen for doubly reduced [NiTPP(CN)4]2− and H2TPP(CN)4]2− (Figure S5). Although slightly different UV−visible spectra are seen after the addition of a third electron to the four tetracyanoporphyrins, the earlier described electrochemical data is self-consistent in indicating the formation of a triply reduced porphyrin trianion in each case. DFT Studies. The ground state geometries of β-functionalized porphyrins were optimized using the B3LYP functional and LANL2DZ basis set in the gas phase to explore the influence of β-substituents and conformational features of the porphyrin macrocycle. The fully optimized geometries of H2TPP, H2TPP(NO2)(PE)6, NiTPP, NiTPP(NO2)(Ph)4, NiTPP(CN)4, and NiTPP(NO2)(PE)6 are shown in Figure 8, and the selected bond angles (deg), bond lengths (Å), mean plane deviation parameters (Å), and saddling dihedral angles

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EXPERIMENTAL SECTION

Chemicals. Unless otherwise noted, all chemicals and starting materials were obtained commercially from Sigma-Aldrich and used without further purification. 1,4-Dioxane was dried over a Na wire with benzophenone ketyl radical as the indicator and freshly distilled prior to use. Dichloromethane (CH2Cl2, anhydrous, ≥99.8%, EMD

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C112H68N4Zn: C, 87.87; H, 4.59; N, 3.70. Found: C, 87.92; H, 4.63; N, 3.59.

Chemicals Inc.) for electrochemistry and pyridine (Py, biotech. grade, ≥99%, Sigma-Aldrich) for electrochemistry were used as received. Benzonitrile (PhCN, reagent Plus, 99%) for electrochemistry was purchased from Sigma-Aldrich and freshly distilled over P2O5 before use. Tetrahydrofuran (THF, for HPLC, ≥99.9%) for electrochemistry was purchased from Sigma-Aldrich and freshly distilled using a Solvent System PS-MD-5-13-495 from Innovative Technology. Tetra-nbutylammonium perchlorate (TBAP) was purchased from SigmaAldrich. A silica gel (100−200 mesh), used for column chromatography, was purchased from Rankem and used as received. Instrumentation. Cyclic voltammetry was carried out using an EG&G Princeton Applied Research (PAR) 173 potentiostat coupled to an EG&GPAR Model 175 Universal Programmer. Current−voltage curves were recorded on an EG&G PAR R-0151 X-Y recorder. A homemade three-electrode cell was used for cyclic voltammetric measurements and consisted of a glassy carbon working electrode, a platinum counter electrode, and a homemade saturated calomel reference electrode (SCE). The SCE was separated from the bulk of the solution by a fritted bridge of low porosity, which contained the solvent/supporting electrolyte mixture. 1H NMR spectra were recorded on Bruker AVANCE 500 MHz and JEOL ECX 400 MHz spectrometers using CDCl3 as a solvent. MALDI-TOF-MS spectra were measured using a Bruker UltrafleXtreme-TN MALDI-TOF/TOF spectrometer using HABA as a matrix. Ground state geometry optimization of the β-functionalized porphyrins was carried out using the B3LYP functional and LANL2DZ basis set in the gas phase. Synthesis of Porphyrins. MTPP(NO2)(PE)617 and MTPP(CN)4,20 where M = 2H, Co(II), Ni(II), Cu(II), and Zn(II); NiTPP(NO2)(Ph)4;19 and MTPP(PE)8 (M = 2H, Ni(II), Cu(II), and Zn(II))18 were synthesized according to literature methods. 2,3,7,8,12,13,17,18-Octaphenylethynyl-meso-tetratolylporphyrins, (H2TTP(PE)8). 2,3,7,8,12,13,17,18-Octabromo-meso-tetratolylporphyrin, H2TTPBr829 (0.165 mg, 0.126 mmol) and Pd(PPh3)4 (0.029 g, 0.025 mmol) were dissolved in 50 mL of distilled 1,4-dioxane and purged with argon gas for 15 min. Twenty equiv of tributyl(phenylethynyl)stannane (0.88 mL, 2.53 mmol) in 8 mL of degassed dioxane was added to the mixture and heated to 80 °C for 1 h under an argon atmosphere. At the end of this period, the solvent was removed by rotary evaporation. The crude porphyrin was dissolved in CHCl3 and purified on a silica gel column using a CHCl3/hexane mixture (7:3, v/v) and 100% CHCl3 as eluent. H2TTP(PE)8 was recrystallized from a CHCl3/CH3OH mixture (1:3, v/v). The yield was found to be 80% (0.149 g, 0.101 mmol). UV/vis (CH2Cl2): λmax (nm) (log ε) 507 (5.18), 592 (4.23), 672 (3.73), 758 (3.32); 1H NMR (400 MHz, CDCl3) δ (ppm): 8.27 (d, 8H, J = 8 Hz, meso-o-phenyl-H), 7.49 (d, 8H, J = 8 Hz, meso-m-phenyl-H), 7.26−7.29 (m, 40H, βpyrrole o, m and p-PE-H), 2.36 (s, 12H, CH3−H), −1.35 (s, 2H, N− H). MALDI-TOFM (m/z): found 1472.88 [M + H]+, calcd 1472.57. Anal. Calcd for C112H70N4: C, 91.40; H, 4.79; N, 3.81. Found: C, 91.55; H, 4.60; N, 3.72. The Ni(II), Cu(II) and Zn(II) complexes of H2TTP(PE)8 were synthesized according to methods reported in the literature.18 NiTTP(PE)8. Yield = 82%; UV/vis (CH2Cl2): λmax (nm) (log ε) 498 (5.32), 603 (4.21), 647 (3.98); 1H NMR (400 MHz, CDCl3) δ (ppm) 8.06 (d, 8H, J = 8 Hz, meso-o-phenyl-H), 7.42 (d, 8H, J = 8 Hz, mesom-phenyl-H), 7.26- 7.24 (m, 40H, β-pyrrole-o, m- and p-PE-H); 2.33 (s, 12H, CH3−H); MALDI-TOF-MS (m/z): found 1528.60 [M + H]+, calcd 1528.46. Anal. Calcd for C112H68N4Ni: C, 88.01; H, 4.48; N, 3.67. Found: C, 88.20; H, 4.57; N, 3.81. CuTTP(PE)8. Yield = 92%; UV/vis (CH2Cl2): λmax (nm) (log ε) 502 (4.80), 618 (3.98); MALDI-TOF-MS (m/z): found 1534.03 [M + H]+, calcd 1534.32. Anal. Calcd for C112H68N4Cu: C, 87.73; H, 4.47; N, 3.65. Found: C, 87.88; H, 4.58; N, 3.77. ZnTTP(PE)8. Yield = 84%; UV/vis (CH2Cl2): λmax (nm) (log ε) 504 (4.76), 616 (3,71); 1H NMR (400 MHz, CDCl3) δ (ppm) 8.15 (d, 8H, J = 8 Hz, meso-o-phenyl-H), 7.66 (q, 8H, meso-m and p-phenylH),7.41(d, 16H, J = 8 Hz, β-pyrrole-o-PE-H), 7.29−7.26 (m, 24H, βpyrrole-m-and p-PE-H), 2.27 (s, 12H, CH3−H); MALDI-TOF-MS (m/z): found 1535.04 [M + H]+, calcd 1535.15. Anal. Calcd for

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CONCLUSION In summary, we report the first facile, reversible, and stepwise formation of tri- and tetraanionic porphyrins. Remarkably, MTTP(PE)8 exhibits four reversible stepwise one-electron ring reductions at room temperature. NiTTP(PE)8 and CuTTP(PE)8 might also undergo six reductions to give the porphyrin penta- and hexaanions in THF at −60 °C. The mono-, di-, and trianionic porphyrins were characterized by thin-layer spectroelectrochemistry in THF or PhCN. The extensive stabilization of the LUMO energy levels is responsible for the formation of tri- and tetraanions in these highly electron deficient π-expanded porphyrins as supported by DFT studies.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.7b01262. DFT calculation details and CV data tables (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Muniappan Sankar: 0000-0001-6667-3759 Karl M. Kadish: 0000-0003-4586-6732 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The support of the Robert A. Welch Foundation is gratefully acknowledged (KMK, Grant E-680). M.S. sincerely thanks Science and Engineering Research Board (SB/FT/CS-015/ 2012), Council of Scientific and Industrial Research (01(2694)/12/EMR-II), and Board of Research in Nuclear Sciences (2012/37C/61/BRNS), India, for financial support. P.Y. and R.K. thank Council of Scientific and Industrial Research and Ministry of Human Resource Development, India. for Senior Research Fellowship.

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REFERENCES

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