Electrochemistry and Spectroelectrochemistry of Cobalt Porphyrins

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

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Electrochemistry and Spectroelectrochemistry of Cobalt Porphyrins with π‑Extending and/or Highly Electron-Withdrawing Pyrrole Substituents. In Situ Electrogeneration of σ‑Bonded Complexes Xiangyi Ke,† 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: A series of cobalt porphyrins with π-extending or highly electron-withdrawing β-pyrrole substituents were investigated as to their electrochemistry, spectroscopic properties, and reactivity after electroreduction or electroxidation in nonaqueous media. Each porphyrin, represented as PorCo (where Por = TPP(NO2)Y2 or TPP(NO2)Y6 and Y = phenyl, phenylethynyl, Br, or CN) was shown to undergo multiple redox reactions involving the conjugated π-ring system or central metal ion which could exist in a Co(III), Co(II), or Co(I) oxidation state under the application of an applied oxidizing or reducing potential. Thermodynamic half-wave potentials for the stepwise conversion between each oxidation state of [PorCo]n (where n ranged from +3 to −3) were measured by cyclic voltammetry and analyzed as a function of the compound structure and properties of the electrochemical solvent. UV−visible spectra were obtained for each oxidized or reduced porphyrin in up to six different oxidation states ranging from [PorCo]3− to [PorCo]3+ and analyzed as a function of the compound structure and utilized electrochemical solvent. Chemically or electrochemically generated Co(I) porphyrins are known to be highly reactive in solutions containing alkyl or aryl halides, and this property was utilized to in situ generate a new series of methyl carbonbonded cobalt(III) porphyrins with the same π-extending or highly electron-withdrawing substituents as the initial Co(II) derivatives. The electrosynthesized carbon-bonded Co(III) porphyrins were then characterized as to their own electrochemical and spectroscopic properties after the addition of one, two, or three electrons in nonaqueous media.

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INTRODUCTION The electrochemistry of numerous synthetic metalloporphyrins has been studied in great detail over the last five decades under a variety of solution conditions.1−3 The specific potentials for oxidation or reduction of a given metalloporphyrin have been shown to systematically vary with changes in the type and oxidation state of the central metal ion, the number, and type of axial ligands and the number and type of substituents on the macrocycle.1−3 In this regard, a considerable effort has been made over the years to not only measure redox potentials of each newly synthesized metalloporphyrin but also to evaluate trends in physiochemical properties for structurally related porphyrins with the aim of “fine-tuning” the compound’s redox reactivity, as well as predicting and tuning absorption spectra of new derivatives in each accessible oxidation state, the end goal of these studies being in many cases to gain an improved ability to chemically or electrochemically generate new porphyrins with enhanced properties for use in a variety of specific applications. Our own work in the field of porphyrin electrochemistry has focused in large part on elucidating the redox properties of transition metal derivatives, with a major emphasis on compounds that can exist in multiple oxidation states under © XXXX American Chemical Society

the application of a controlled oxidizing or reducing potential. One series of porphyrins that satisfies this criteria are the cobalt derivatives, which are generally synthesized as air-stable Co(II) complexes but which can be easily converted between five or six different oxidation states by chemical or electrochemical methodologies. The most often studied of the cobalt porphyrins have been those containing a tetraphenylporphyrin or substituted tetraphenylporphyrin (TPP) skeletal structure. These compounds were shown to undergo facile Co(II)/(III) and Co(II)/ (I) processes in addition to two oxidations at the π-ring system of the macrocycle to give the cobalt(III) π-cation radicals and dications.4−9 However, further reductions of the Co(I) porphyrin to give π-anion radicals and dianions were almost never reported due in part to the difficulty in accessing a doubly reduced cobalt porphyrin within the negative potential limit of most electrochemical solvents (∼−1.9 V vs SCE or less) and in part to the presence of coupled chemical reactions involving the electrogenerated cobalt(I) porphyrin when the reaction was carried out in a chlorinated solvent, such as CH2Cl2. Received: November 8, 2017

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DOI: 10.1021/acs.inorgchem.7b02856 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry Chart 1. Structures of Reference Compounds 1 and 2 and Newly Investigated Porphyrins 3−10

1b and represented as TPP(NO2)Y2Co, (compounds 3−6), while the hexa-β-substituted Br and PE derivatives are shown in Chart 1c and represented as TPP(NO2)Y6Co (compounds 7− 9) where Y = phenyl, phenylethynyl, Br or CN. Also included in Chart 1c by virtue of its related redox behavior, is the earlier described tetracyano derivative TPP(CN)4Co (compound 10)13 which is now investigated in further detail. Each porphyrin in Chart 1 was characterized by cyclic voltammetry and thin-layer spectroelectrochemistry in CH2Cl2, PhCN, and pyridine, with the sites of electron transfer being assigned on the basis of previous data in the literature for related cobalt porphyrins combined with electrochemical reactivity patterns and spectroscopic data of the newly investigated compounds. With the exception of unsubstituted TPPCo 1, all of the singly reduced porphyrins in the current study were found to be relatively stable in CH2Cl2 and could be reversibly converted to their higher reduced forms on the cyclic voltammetry time scale. However, a chemical reaction occurred in solution following the electrogeneration of [PorCo]− in PhCN containing added CH3I, and this gave a stable product which was assigned as an in situ generated σ-bonded Co(III)-methyl porphyrin having the same skeletal structure as the compounds shown in Chart 1. These electrosynthesized porphyrins are also characterized in the present manuscript as to their electrochemistry and spectroelectrochemistry.

It was long believed that a maximum of two electrons could be reversibly added to the π-ring system of a metalloporphyrin,1,2 but we recently demonstrated that porphyrin trianions and tetraanions could be reversibly electrogenerated in nonaqueous media for derivatives containing nonredox active central metal ions and β-pyrrole substituents with highly electron-withdrawing or π-extending properties.10 The presence of these groups on the porphyrin periphery resulted in a significant positive shift of all reduction potentials for derivatives of Cu, Ni, Zn, and 2H, thus enabling the higherreduced forms of the porphyrin to be easily observed within the negative potential limit of the electrochemical solvent and also enabling a facile characterization of the never before seen electroreduction products by thin-layer UV−visible spectroelectrochemistry. The electrochemistry of TPP(NO2)(PE)6Co where PE = phenylethynyl was also reported in the above paper and was shown to exhibit three well-defined one-electron reductions at −0.37, −1.08, and −1.48 V in PhCN, the first of which was spectroscopically monitored. The rich redox behavior and spectroscopic properties for the other reduced and oxidized forms of this porphyrin are now fully investigated in the current manuscript along with a series of structurally related cobalt porphyrins whose electrochemistry was briefly described in CH2Cl211,12 but not characterized in any detail. The β-substituted porphyrins investigated in the current study are shown in Chart 1 along with the reference compounds TPPCo 1 and TPP(NO2)Co 2. The di-βsubstituted Br, PE, and CN derivatives are shown in Chart B

DOI: 10.1021/acs.inorgchem.7b02856 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

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RESULTS AND DISCUSSION Electrochemistry of PorCoII Complexes. The electrochemistry of the reference compound TPPCo 1 has been characterized under numerous solution conditions1−9 and was most often described as having the reactivity pattern shown in Scheme 1, where the first one-electron abstraction generates a

centered reduction at quite negative potentials. The electrogenerated Co(I) porphyrin is moderately stable in electrochemical solvents such as PhCN, DMSO or pyridine,5,8,9 but rapidly reacts with CH2Cl2 when used as a solvent to give a transient carbon-bonded species identified as TPPCo(CH2Cl).14−16 The same type of reaction is seen in a nonchlorinated electrochemical solvent containing [TPPCoI]− and added alkyl halides and this property was previously utilized to electrosynthesize cobalt(III) carbon σ-bonded porphyrins of the form TPPCo(R), which were characterized as to their own electrochemical and physiochemical properties.16,17 As will be described on the following pages, a reaction with CH2Cl2 also occurs after the first one-electron reduction of the currently investigated compounds, but this reaction proceeds more slowly than for [TPPCo]−, thus leading to reversible reductions being observed by cyclic voltammetry but irreversible reductions on the slower time scale of thin-layer spectroelectrochemistry. This result can be accounted for by two resonance forms of the singly reduced porphyrin, one being described as a stable Co(II) porphyrin π-anion radical and the other a reactive Co(I) porphyrin with an unreduced πring system as shown in eq 1. Evidence for this resonance description of the reduction product is provided by the

Scheme 1. Known Electrochemistry of TPPCo 1 in Nonaqueous Media Containing Alkyl Halides

Co(III) porphyrin, followed by two ring-centered one-electrons oxidations, and the first one-electron addition generates a highly reactive Co(I) porphyrin followed by a second ring-

Table 1. Half-Wave or Peak Potentials (V vs SCE) for Redox Reactions of Investigated Co(II) Porphyrins in CH2Cl2, Pyridine, and PhCN, Containing 0.1 M TBAPa oxidation solvent

compound

Σσpb

CH2Cl2

TPPCo 1 TPP(NO)2Co 2 TPP(NO2)(Ph)2Co 3 TPP(NO2)Br2Co 4 TPP(NO2)(PE)2Co 5 TPP(NO2)(CN)2Co 6 TPP(NO2)(Ph)6Co 7 TPP(NO2)Br6Co 8 TPP(NO2)(PE)6Co 9 TPP(CN)4Co 10 TPPCo 1 TPP(NO)2Co 2 TPP(NO2)(Ph)2Co 3 TPP(NO2)Br2Co 4 TPP(NO2)(PE)2Co 5 TPP(NO2)(CN)2Co 6 TPP(NO2)(Ph)6Co 7 TPP(NO2)Br6Co 8 TPP(NO2)(PE)6Co 9 TPP(CN)4Co 10 TPPCo 1 TPP(NO)2Co 2 TPP(NO2)(Ph)2Co 3 TPP(NO2)Br2Co 4 TPP(NO2)(PE)2Co 5 TPP(NO2)(CN)2Co 6 TPP(NO2)(Ph)6Co 7 TPP(NO2)Br6Co 8 TPP(NO2)(PE)6Co 9 TPP(CN)4Co 10

0 0.78 0.76 1.24 1.10 2.10 0.72 2.16 1.74 2.64 0 0.78 0.76 1.24 1.10 2.10 0.72 2.16 1.74 2.64 0 0.78 0.76 1.24 1.10 2.10 0.72 2.16 1.74 2.64

Py

PhCN

4th

1.72 1.69 1.78 1.70

1.90 1.70

reduction

3rd

2nd

1st

1st

2nd

3rd

1.16 1.28 1.25 1.36 1.32 1.59 1.10 1.42 1.28

0.98 1.05 1.02 1.12 1.10 1.27 0.94 1.17 1.07

0.75 0.81 0.79 0.83 0.83 0.96 0.73 0.88 0.87 0.92 −0.12 −0.14 −0.17 −0.08 −0.10 0.04 −0.18 0.12 0.00 0.15 0.58d 0.65d 0.65d 0.73d 0.73d 0.88d 0.58d 0.85d 0.80d 0.88d

−0.83c −0.64 −0.63 −0.50 −0.52 −0.38 −0.60 −0.24 −0.29 −0.25 −1.16d −0.86 −0.83 −0.75 −0.76 −0.65 −0.96d −0.50 −0.58 −0.46 −0.85 −0.66 −0.62 −0.51 −0.55 −0.38 −0.66 −0.29 −0.37 −0.26

−1.28 −1.29 −1.21 −1.21 −1.16 - 1.44d −1.07 −1.05 −0.89 −1.95 −1.32 −1.30 −1.21 −1.16 −1.14 −1.40 −1.10 −1.07 −0.89 −1.97 −1.32 −1.31 −1.22 −1.20 −1.12 −1.38 −1.16 −1.00 −0.89

−1.89d −1.90d −1.84d −1.76d −2.00d −1.90d −1.63d −1.48 −1.80d

1.37 1.41 1.35 1.45 1.44 1.64 1.19 1.50e 1.44

1.19 1.24 1.23 1.32 1.29 1.47 1.05 1.50e 1.31 1.56d

−1.88d −1.88d 1.92d −1.83d −1.96 −2.02d −1.64d −1.46 −1.77 −1.93d −1.93d −1.86d −1.83d −1.90 −1.96c −1.78c −1.48 −1.80d

a Scan rate = 0.1 V/s. bThe σ values are taken from ref 18 and apply to the β-pyrrole substituents of the investigated compounds. cPotentials reported at −40 °C. dPeak potential for a scan rate of 0.1 V/s. eOverlapped broad peak.

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DOI: 10.1021/acs.inorgchem.7b02856 Inorg. Chem. XXXX, XXX, XXX−XXX

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Figure 1. Cyclic voltammograms of (a) reference compounds 1 and 2, (b) di- and hexa-Br-substituted porphyrins 4 and 8, and (c) di- and hexa-PEsubstituted porphyrins 5 and 9 in CH2Cl2 containing 0.1 M TBAP. The reduction of TPPCo 1 was measured at −40 °C.

reaction with the solvent is slowed down.) At the same time, much smaller shifts of 60−120 mV are seen in E1/2 values for the three reversible one-electron oxidations of TPP(NO2)Co 2 as compared to TPPCo. A reversible second reduction of TPP(NO2)Co 2 is located at E1/2 = −1.28 V in CH2Cl2, but this process is not observed for TPPCo up to the negative potential limit of the solvent. Positive potential shifts in potential are also observed when comparing cyclic voltammograms of the di- and hexa-Br substituted compounds 4 and 8 (Figure 1b) with that of compound 2, which contains only a single β-NO2 group (Figure 1a). The addition of two or six β-Br groups to the macrocycle of 2 leads, respectively, to stepwise shifts of 140 and 260 mV for the first reduction (from E1/2 = −0.64 to −0.50 and then to −0.24 V), while much smaller positive potential shifts of 70 and 140 mV are seen for the second one-electron

electrochemical and spectroelectrochemical data described later in the manuscript. [PorCoI]− ⇌ [Por •CoII]−

(1)

Prior to carrying out the spectroelectrochemical measurements, each porphyrin in Chart 1 was initially characterized by cyclic voltammetry in CH2Cl2, PhCN, and pyridine. A summary of the measured half-wave or peak potentials is given in Table 1 and examples of cyclic voltammograms for six of the investigated porphyrins in CH2Cl2 are illustrated in Figure 1. As seen in Figure 1a, the presence of one NO2 group on a βpyrrole position of the TPPCo macrocycle leads to a 190 mV positive shift in potential as compared to E1/2 for the first reduction of the TPPCo reference compound 1 which is located at −0.83 V in CH2Cl2 at −40 °C. (This voltammogram is shown at low temperature, where the rate of the chemical D

DOI: 10.1021/acs.inorgchem.7b02856 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

Figure 2. Cyclic voltammograms of selected cobalt porphyrins in pyridine containing 0.1 M TBAP.

reduction which shifts for E1/2 = −1.28 V for 2, to −1.21 for 4 and then to −1.07 V for 8 under the same solution conditions. Again, smaller potential shifts are observed for the three oxidations of the dibromo- and hexabromoporphyrins as compared to the parent compound 2, where the measured E1/2 values for electron abstraction from compounds 2, 4, and 8 differ by only 20−60 mV as seen in the Figure 2a. The electron-withdrawing properties of a Br substituent is greater than that of a PE substituent18 but this is not reflected in the first two reduction potentials of TPP(NO2)Br2Co 4 and TPP(NO2)(PE)2Co 5, where almost identical E1/2 values of −0.51 ± 0.01 and −1.21 V are obtained as seen in Figure 1. Quite similar reduction potentials are also observed for the first two one-electron additions to compounds 8 and 9 which contain one NO2 and six β-Br or six β-PE groups, respectively. However, the PE groups also possess π-extending properties11 not seen for the Br substituents, and this leads to the occurrence of a third well-defined one-electron reduction, which is located at −1.48 V for TPP(NO2)(PE)6Co 9 in CH2Cl2. Several points should be noted with respect to the electrochemistry of compounds 2−10 in CH2Cl2, the most important of which is the fact that first one-electron reduction is reversible in every case, with no evidence in the cyclic voltammograms for a coupled chemical reaction following electron transfer as occurs for TPPCo 1. This might at first suggest the lack of a cobalt(I) reduction product in the case of 2 through 10, since there is no apparent reaction with the CH2Cl2 solvent on the cyclic voltammetry time scale; however, a chemical reaction does occur with CH2Cl2 on the slower spectroelectrochemical time scale and a chemical reaction also occurs after electron transfer when carrying out the reduction of these porphyrins in PhCN containing added CH3I (see later discussion). These facts are both consistent with an equilibrium as shown in eq 1 and also with a significant amount of the Co(II) π-anion radical being generated in the electron transfer.

Despite the reduced reactivity of the singly reduced porphyrins 2−10 in the presence of CH3I, one strong piece of evidence supporting Co(I) formation in the first electroreduction is given by the very positive potential for this oneelectron addition as compared to related metalloporphyrins possessing the same macrocycle and a redox inactive central metal ion. For example, the first one-electron reduction of TPP(NO2)(PE)6Co 9 is located at −0.29 V in CH2Cl2 (Figure 1) and this potential can be compared to the E1/2 values of −0.78 V for the first one-electron reduction of TPP(NO2)(PE)6Zn where the site of electron transfer can only involve the π-ring system of the macrocycle.10 The copper and nickel TPP(NO2)(PE)6M derivatives are also reduced at more negative potentials in the first step10 than the related cobalt porphyrin, these values being reported as E1/2 = −0.65 V for M = NiII and −0.70 V for M = CuII. Finally, it should be noted that the second reversible reduction of compound 9 is separated from the first process by 760 mV in CH2Cl2 (see Table 1), a value much too large for formation of a π-anion radical and dianion, and thus implying formation of a metalcentered reduction in the first one-electron addition. Similar separations of 640−830 mV are also seen between the first two one-electron reductions of all other investigated porphyrins in the current study except for TPPCo (whose reduction is beyond the negative limit on the solvent). By way of comparison, the earlier characterized TPP(NO2)(PE)6M, where M = 2H, Cu, Ni, and Zn show much smaller separations in E1/2 of 110−370 mV between the first two reductions,10 these values of ΔE1/2 being in the range of reported potential separations for reactions involving the formation of porphyrin π-anion radicals and dianions of compounds with TPP and substituted TPP skeletal structures.1−3 The observed shifts in E1/2 for the electrode reactions of compounds 2−10 upon changing the electrochemical solvent from CH2Cl2 to pyridine are in each case consistent with a CoII/I process upon reduction where Co(I) does not contain E

DOI: 10.1021/acs.inorgchem.7b02856 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

Figure 3. Spectral changes observed during the first one-electron reduction of the investigated cobalt porphyrins in pyridine containing 0.1 M TBAP.

bound py and a CoII/III process upon oxidation, where the Co(III) form of the porphyrin binds py more strongly than does the Co(II) form. In this regard it should be noted that pyridine has been shown to bind moderately strong to TPPCoII (log K1 = 2.9 in CH2Cl2),9 extremely strongly to [TPPCoIII]+ (log β2 = 15.6 in CH2Cl2)9 and not at all for [TPPCoI]−. The redox reactions of the TPPCo complex in pyridine were earlier demonstrated to occur as shown in eqs 2−4,8 where E1/2 values for the first oxidation and first reduction in pyridine are both shifted negatively with respect to the reversible half wave potentials in CH2Cl2, and little to no change is seen in the measured E1/2 values for the second reduction (eq 3) where the reactants and products are both four-coordinate in each solvent. PorCoII(Py) + e− = [PorCoI]− + Py

[PorCoI]− + e− = [PorCoI]2 −

(3)

PorCoII(Py) + Py = [PorCoIII(Py)2 ]+ + e−

(4)

The measured shifts in potentials for 2−10 upon changing the solvent from CH2Cl2 (Figure 1) to pyridine (Figure 2) can also be accounted for by the redox processes shown in eq 2−4. The reversible E1/2 values for the first reduction of 2−10 (eq 2) are shifted negatively by 220−290 mV in pyridine as compared to the corresponding one-electron reductions of the corresponding uncomplexed porphyrins in CH2Cl2, while a negative shift of 720−910 mV is seen for the first one-electron oxidation of the same nine compounds in pyridine (eq 4) as compared to CH2Cl2. Consistent with the lack of solvent binding to the singly and doubly reduced porphyrins, the measured E1/2 values for the second one-electron addition to each compound in the

(2) F

DOI: 10.1021/acs.inorgchem.7b02856 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

dependence of the UV−vis spectrum on the ionic strength of the solution. Spectra of Singly Reduced Porphyrins. Reversible spectral changes were obtained during the first one electron reduction of each porphyrin in the three solvents and examples of the time-resolved UV−vis spectra in pyridine are given, respectively, on the left and right-hand sides of Figure 3 for the di- and hexa-β-substituted derivatives containing Br (4 and 8), Ph (3 and 7), or PE (5 and 9) substituents. Also shown in this figure are the spectral changes obtained during reduction of the reference compounds 1 and 2. Similar UV−visible spectra were obtained for each singly reduced porphyrin in PhCN (Figure S4) as in pyridine (Figure 3), consistent with the lack of solvent binding to the one-electron reduction product in these two solvents. With the exception of [TPPCo]−, UV−visible spectra of the investigated singly reduced porphyrins are consistent in each case with an equilibrium between two forms of the reduction product, one a Co(I) porphyrin with an unreduced macrocycle and the other a Co(II) porphyrin π-anion radical as shown in eq 1. The spectrum of the singly reduced TPPCo reference compound 1 in pyridine (upper right spectrum in Figure 3) exhibits a split Soret band at 364 and 409 nm, a single Q band at 534 nm and no other absorptions between 600 and 1000 nm. This spectrum is consistent with previously published UV− visible spectra of [TPPCo]−, where [TPPCo]− is assigned as a Co(I) porphyrin product,6,20 having a split Soret band, a single Q band and no other absorptions between 600 and 1000 nm. This spectral pattern for [TPPCoI]− contrasts with the spectrum of the singly reduced reference compound 2 where [TPP(NO2)Co]− is characterized by both a split Soret band at 375 and 443 nm, indicating Co(I) formation, and a broad NIR band centered at 789 nm, indicating formation of a porphyrin π-anion radical. The spectrum of [TPP(NO2)Co]− also has a strong Q-band absorbance at 590 nm which is not seen in the case of singly reduced [TPPCo]−, but does appear in spectra of the singly reduced compounds 3, 4, and 5 as shown in Figure 3. These latter three reduced porphyrins, as well as singly reduced 7 and 8, are also characterized by spectra having features of both a metal-centered and a ring-centered reaction, that is, a split Soret band which can be assigned to a Co(I) porphyrin reduction product along with a broad NIR band which can be assigned to the porphyrin π-cation radical. A split Soret band is not observed for [TPP(NO2)PE6Co]− 9 (Figure 3, bottom right) but this band is significantly decreased in intensity as compared to the Soret band of the neutral porphyrin before reduction, as often occurs upon formation of a porphyrin πanion radical. The singly reduced compound 9 also has an intense near IR band at 790 nm as shown in Figure 3 and this is a second diagnostic criteria of π-anion radical formation. Similar spectral features are seen for the singly reduced porphyrins 1−9 in PhCN as described later in the manuscript and also illustrated in Figure S4 for selected derivatives. Spectra of Singly Oxidized Porphyrins. Two general types of UV−visible spectra are obtained upon oxidation of compounds 1−10 in the three solvents. One is assigned to a “pure” Co(III) porphyrin with an unoxidized macrocycle and the other to a Co(III) porphyrin having some π-cation radical character. The one-electron oxidation of cobalt porphyrins with OEP or TPP skeletal structures have almost always been assigned as generating a Co(III) porphyrin with an intact π-ring system1,2 but evidence for the formation of Co(II) π-cation

current study (eq 3) are virtually identical in pyridine and CH2Cl2 and almost the same values of E1/2 are also observed for this electrode reaction in PhCN (see Table 1), again consistent with the lack of solvent binding to the singly or doubly reduced forms of the cobalt porphyrins. A third or fourth reduction is also observed for compounds 2−10 in the three utilized electrochemical solvents. The first three reductions of 6, 9, and 10 are reversible and involve a single electron addition (see Figure 2 and Table 1) and these porphyrins also exhibit a fourth irreversible (and sometimes multielectron) reduction as shown in Figure S1 for TPP(NO2)(PE)6Co 9 in PhCN. This last irreversible reduction is located on the β-NO2 group of the porphyrin and involves the addition of one or more protons following electron transfer. This reaction was not examined in further detail but under acidic solution conditions should lead to the ultimate conversion of NO2 to NH2 under appropriate applied potential.19 An irreversible electroreduction of the β-nitro group was earlier shown10 to occur on the NO2 group of TPP(NO2)(PE)6M, where M = CuII or ZnII and the same irreversible electrode reaction occurs for all of the β-nitrophenyl porphyrins examined in the current study when the negative potential scan was swept to >−2.0 V vs SCE. This reaction involves in each case a combination of proton and multielectron addition and was not investigated in further detail. Spectra of Neutral Compounds. UV−visible spectra of the neutral porphyrins 2−9 in CH2Cl2 were earlier reported and analyzed as a function of the type and number of β-pyrrole substituents on the compounds, which also influence the associated planarity of the porphyrin macrocycle.11 The spectra of the neutral compounds 16,14,20 and 1013 have also been reported when dissolved in solutions of CH2Cl2, pyridine, or other commonly used electrochemical solvents. Our interest in the current study was not to reanalyze spectra of the neutral porphyrins 2−9 since this is already discussed in the literature,11 but rather to monitor those changes which occur upon the stepwise addition or abstraction of electrons during oxidation or reduction, thus obtaining spectra which were not previously reported or, in many cases, not even accessible until the recent synthesis of easy to reduce porphyrin macrocycles containing a combination of π-extending and highly electronwithdrawing pyrrole substituents. The measured UV−vis spectra of the examined compounds in CH2Cl2, PhCN, and pyridine are shown in Figures S2 (compounds 1−6) and S3 (7−10), and a summary of the spectral data for all ten porphyrins in Chart 1 is given in Table S1. As earlier described in the literature, an increase in the number of electron-withdrawing substituents on the β-pyrrole positions of the neutral porphyrin leads to an increasing redshift in both the Soret and Q bands as the macrocycle becomes more nonplanar.10,11 A red shift of the Soret and Q bands is also generally observed for compounds 1−10 upon going from the nonbonding solvent CH2Cl2 to the coordinating solvents, pyridine and PhCN, where similar UV−visible spectra are obtained, as seen by the data in Figures S2 and S3 and Table S1. The spectrum of each neutral porphyrin in the thin-layer spectroelectrochemical cell containing the solvent and 0.1 M TBAP as supporting electrolyte was generally the same as the spectrum in the absence of the electrolyte needed to carry out the electrochemical experiment. This suggests both the lack of axial binding by the ClO4− anion to the investigated compounds in their neutral form and also the lack of a G

DOI: 10.1021/acs.inorgchem.7b02856 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

Figure 4. Spectral changes observed during oxidation of compounds 4, 8, and 5 in (a) CH2Cl2, (b) PhCN, or (c) pyridine.

Table 2. UV−Visible Spectra Data for the Singly Reduced and Singly Oxidized Cobalt Porphyrins in CH2Cl2, Pyridine, and PhCN Containing 0.1 M TBAPa solvent

a

compound

macrocycle

1 2 3 4 5 6 7 8 9 10 1 2 3 4 5 6 7 8 9 10

[TPPCo]− [TPP(NO2)Co]− [TPP(NO2)(Ph)2Co]− [TPP(NO2)Br2Co]− [TPP(NO2)(PE)2Co]− [TPP(NO2)(CN)2Co]− [TPP(NO2)(Ph)6Co]− [TPP(NO2)Br6Co]− [TPP(NO2)(PE)6Co]− [TPP(CN)4Co]− [TPPCo]+ [TPP(NO2)Co]+ [TPP(NO2)(Ph)2Co]+ [TPP(NO2)Br2Co]+ [TPP(NO2)(PE)2Co]+ [TPP(NO2)(CN)2Co]+ [TPP(NO2)(Ph)6Co]+ [TPP(NO2)Br6Co]+ [TPP(NO2)(PE)6Co]+ [TPP(CN)4Co]+

CH2Cl2

pyridine

383, 377, 447, 434, 454, 384, 493, 438,

436, 533, 598 432, 596, 750 616, 784 575, 609, 698 581, 776 456, 586, 720 585, 620, 789 578, 630, 717

451, 450, 462, 452, 466, 474, 498, 435,

563, 564, 581, 575, 583, 597, 616, 478,

604, 610, 628 633 641, 651, 676 630,

858 889

818 926 726

364, 375, 381, 379, 390, 381, 390, 387, 503, 444, 436, 447, 459, 456, 465, 462, 480, 476, 502, 471,

414, 534 443, 590, 789 450, 602, 787 443, 598, 756 455, 619, 783 442, 577, 926 464, 612, 915 467, 543, 588, 711 583, 628, 790 490, 592, 845, 970 552, 592 567, 613 580, 628 578, 633 586, 637 590, 651 598, 659 595, 651 620, 676 660, 683

PhCN 365, 378, 383, 380, 387, 382, 396, 389, 504, 368, 436, 447, 458, 456, 465, 461, 474, 476, 502, 471,

442, 531 443, 590, 453, 603, 447, 598, 457, 618, 442, 579, 470, 612, 470, 587, 582, 798 446, 588, 552, 595 566, 614 575, 624 578, 623 582, 630 580, 644 591, 649 593, 642 614, 669 658, 685

767 798 759 782 714 767 687 735

The most intense (Soret) bands are given in boldface text.

[PorCoIII]+ ⇌ [Por •CoII]+

radicals has also been given when the oxidations were carried out in rigorously dry CH2Cl2.21,22 The results on compounds 1−10 in the current study suggest the generation of singly oxidized cobalt porphyrins with pure Co(III) character in the two coordinating electrochemical solvents (PhCN and Py) but dual Co(III) and π-cation radical character in CH2Cl2 or, more likely, an equilibrium between two forms of the singly oxidized complex as shown in eq 5.

(5)

Evidence for the above is shown by the spectroelectrochemical data in Figure 4 for oxidation of compounds 4, 8, and 5 in the three electrochemical solvents. The spectra of singly oxidized TPP(NO2)Br2Co 4 and TPP(NO2)Br6Co 8 in CH2Cl2 (Figure 4a) are characterized by a reduced intensity Soret band and a broad NIR absorbance between 700 and 1000 nm, both features being characteristic of a porphyrin π-cation radical.23,24 This contrasts with spectra of the same singly H

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Figure 5. Cyclic voltammograms of 6.5 × 10−4 M TPP(NO2)(Ph)2Co 3 in PhCN containing 0.1 M TBAP with (a) no CH3I, (b) after addition of 1.0 equiv of CH3I, and (c) after addition of 100.0 equiv CH3I. Scan rate = 0.1 V/s.

electron addition generates I− and PorCoIII(CH3), both of which are electroactive as shown by the cyclic voltammograms in Figure 5 for TPP(NO2)(Ph)2 3 in PhCN before and after adding CH3I to solution. Compound 3 undergoes two reversible one-electron reductions at E1/2 = −0.62 and −1.31 V in PhCN, 0.1 M TBAP as shown in Figure 5a. However, after addition of 1.0 equiv of CH3I to solution, the first reduction becomes irreversible and has a shape consistent with a chemical reaction following an initial one-electron reduction (an electrochemical EC mechanism).25 The cyclic voltammogram under this condition is shown in Figure 5b and is characterized by an initial irreversible one-electron reduction at Ep = −0.65 V followed by two reversible reductions at E1/2 = −0.93 and −1.30 V. These latter two redox processes are associated with reversible electron additions to the chemically generated porphyrin product of the chemical reaction which is assigned as the methyl-bonded species TPP(NO2)(Ph)2Co(CH3). The first two reductions of this in situ generated porphyrin involve one electron additions as written in eqs 6 and 7 where Por = TPP(NO2)(Ph)2 and the cobalt oxidation state is assigned as +3 both before and after the first two electron transfers, consistent with electron addition to the conjugated macrocycle in each step. There is also a new oxidation peak located at Ep= +0.02 V on the return potential sweep as shown in Figure 5b. This electrode reaction is assigned to the oxidation of I− in solution, the iodide ion being formed as a product in the chemical reaction between the singly reduced porphyrin 3 and CH3I.16

oxidized porphyrins when the measurements were carried out in PhCN (Figure 4b) or pyridine (Figure 4c). Under these latter two solution conditions, the Soret band absorptions are shifted to the red but are more intense rather than reduced in intensity after the abstraction of one electron. More importantly, the singly oxidized porphyrins 4 and 8 exhibit no absorption bands between 700 and 1000 nm, clearly indicating the lack of a π-cation radical character for [TPP(NO2)Br2Co]+ or [TPP(NO2)Br6Co]+ in these coordinating solvents. A similar trend in solvent effect is seen for singly oxidized 2, 3, 7, and 9 where the spectra are characterized by a diagnostic π-cation radical NIR absorption in CH2Cl2 but not in PhCN or pyridine, and the Soret band is significantly decreased in intensity in CH2Cl2 (Figures S5) but increased in PhCN and pyridine (Figures S6 and S7) as compared to the Soret band of the neutral porphyrin under the same solution conditions. In contrast to the above, only Co(III) character is seen in the spectra of [TPP(NO 2 )(PE) 2 Co] + 5 and [TPP(NO 2 )(CN)2Co]+ 6, independent of the solvent. The measured spectral changes for these two compounds under the application of a controlled oxidizing potential are illustrated in Figures 4 and S5−S7, and a summary of data for all other porphyrins in the three electrochemical solvents is given in Table 2. Electrosynthesis and Electrochemistry of CoIII(CH3) Derivatives. As indicated earlier in the manuscript, each investigated porphyrin in Chart 1 undergoes a reversible oneelectron reduction in PhCN containing 0.1 M TBAP but the singly reduced porphyrin undergoes a chemical reaction with the solvent when the electrochemistry is carried out in CH2Cl2. The reaction with CH2Cl2 is shown in Scheme 1 for TPPCo and generates as final products both iodide ion and TPPCoIII(CH2Cl) as earlier described in the literature.15 A similar reaction also occurs between the singly reduced cobalt porphyrins 2−10 in PhCN when CH3I, was added to solution prior to carrying out the electrochemical measurements. This homogeneous chemical reaction following the

PorCo(CH3) + e− ⇌ [Por•Co(CH3)]−

(6)

[Por •Co(CH3)]− + e− ⇌ [PorCo(CH3)]2 −

(7)

A third reversible one-electron reduction of the in situ generated TPP(NO2)(Ph)2Co(CH3) product can also be seen at E1/2 = −1.45 V, when the concentration of added CH3I is increased to 100 equiv. The cyclic voltammograms obtained I

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namely, n = 0 for the initial compounds before reduction and n = −1, −2, or −3 after the reversible addition of one, two or three electrons. As seen in the table, the neutral PorCo(CH3) complexes are consistently more difficult to reduce in the first step than the structurally related neutral Co(II) porphyrins having the same β-pyrrole substituents. The largest difference in potential is between the TPPCo derivatives with and without a CH3 axial ligand and the smallest is between the two TPP(CN4)Co porphyrins, one with bound CH3 and the other without. For example, the first reduction of TPPCo (compound 1) is located at E1/2 = −0.85 in PhCN, while TPPCo(CH3) is reduced at E1/2 = −1.35 V, a separation of 0.50 V. In contrast, the first one-electron reductions of TPP(CN)4 10 and TPP(CN)4Co(CH3) are located at E1/2 = −0.26 and −0.42 V, respectively, a ΔE1/2 of 0.16 V. The same trend in case of first reduction potential is seen for compounds 2−9, where the separation between E1/2 values averages 0.30 V. This result contrasts with the second one-electron reduction of each related porphyrin in the two series, where the values of ΔE1/2 range from 0.00 for 6 and 10 to −0.10 V for 7 in PhCN and averages 0.04 V for the nine porphyrins in the two series as shown in Table 3. A third one-electron reduction was earlier reported for TPP(CN)4Co 10 and is located at E1/2 = −1.72 to −1.78 in numerous electrochemical solvents,13 but this process could not be characterized in PhCN because of an overlapping of the porphyrin electrode reaction with currents for the direct reduction of CH3I in this solvent, a reaction which occurs at −1.65 V in the absence of the porphyrin. An overlapping of the porphyrin and CH3I reductions also prevented measurements for the third reduction of TPP(NO2)(CN)2Co(CH3) 6 in PhCN. Thus, neither compound 1 nor the two β-cyano methyl porphyrins could be characterized in their trianionic form. All of the other carbon-bonded porphyrins were shown to exhibit a third one-electron reduction at E1/2 values between −1.23 and −1.55 V vs SCE in PhCN (see Table 3). There is no obvious trend between the type or number of βpyrrole substituents and the difference in E1/2 between the first, second or third one-electron additions to the related derivatives of PorCo and PorCo(CH3). However, a linear relationship is seen between the measured E1/2 values and the sum of the Hammett substituent constants. This correlation is shown in Figure S10 for the three reductions of PorCoII and PorCoIII(CH3) in PhCN. Earlier studies of cobalt-σ-bonded porphyrins with TPP macrocycles characterized the first two reductions as involving electron additions to the porphyrin macrocycle as shown in eqs 6 and 7, and this assignment of electron transfer site is also given in the current manuscript based on electrochemical diagnostic criteria of potential separations and spectroelectrochemical data described on the following pages. As seen in Table 3 and related Figures 5, 6, and S9, the separation in E1/2 between the first and second one-electron reductions of PorCo(CH3) ranged from 0.29 to 0.38 V for compounds 2−5, 7, 8, and 9 and averages 0.30 V, consistent with the formation of a porphyrin π-anion radical and dianion. The separation between E1/2 for the second and third reductions of the same series of structurally related porphyrins ranges from 0.12 V in the case of compound 8 to 0.42 V in the case of compound 9 and averages 0.22 V for the seven porphyrins which undergo three one-electron reductions. These average values of ΔE 1/2 between the reversible reductions of PorCoIII(CH3) are shown in Scheme 2, which also includes

under these conditions are illustrated in Figure 5c, where the third one electron reduction is given by eq 8. [PorCo(CH3)]2 − + e− ⇌ [PorCo(CH3)]3 −

(8)

The rate of the homogeneous chemical reaction between the singly reduced porphyrin 3 in PhCN and the CH3I added to solution was shown to depend on both the concentration of methyl iodide and the temperature. The chemical reaction proceeds too slowly to be observed on the cyclic voltammetric time scale at −40 °C in PhCN containing 1.0 equiv CH3I (bottom CV in Figure 5b), but when 100.0 equiv CH3I have been added to solution, the reaction proceeds rapidly even at low temperature, and three well-defined reductions of the electrosynthesized cobalt(III)−carbon bonded species are seen in addition to an oxidation of I− on the return potential sweep (Figure 5c). Each singly reduced porphyrin in Chart 1 reacted with CH3I in a similar manner and eight of the ten electrosynthesized cobalt methyl-porphyrins were shown to exhibit three oneelectron reductions within the negative potential range of the PhCN solvent as illustrated in Figures 5, 6, S8, and S9. A

Figure 6. Cyclic voltammograms of (a) TPP(NO2)Co 2 and (b) TPP(NO2)Br2Co 4 in PhCN, 0.1 M TBAP before and after addition of excess CH3I to solution. The three electrode reactions within the box correspond to reductions of the electrogenerated PorCo(CH3) complexes in solution.

summary of the measured E1/2 values for the electrosynthesized carbon-bonded porphyrins is given in Table 3 which also includes potentials for reduction of the initial Co(II) derivatives in PhCN not containing CH3I. The data in this table enables a direct comparison of potentials between each redox reaction of the [PorCo]n− and [PorCo(CH3)]n− derivatives having the same overall charge, J

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Table 3. Comparison of Half-Wave Potentials for First and Second Reductions of PorCoII and PorCoIII(CH3) in PhCN, Containing 0.1 M TBAP

a

Measured in pyridine (see Figure 2c).

Scheme 2. Proposed Mechanism for Reduction of PorCo in PhCN

additional indirect evidence for a ring-centered electron transfer in the two structurally related compounds. Spectroelectrochemical Monitoring of PorCo(CH3) Formation and Spectra of Electroreduction Products. The spectral changes that occurred during the initial formation and subsequent reduction of the in situ generated cobalt methyl porphyrins were examined by thin-layer UV−visible spectroelectrochemistry to elucidate both the UV−visible spectra of the electroreduced compounds and to evaluate the site of electron transfer. Examples of the time-resolved spectral changes as a function of the applied potential are illustrated in Figure 7 for TPP(NO2)(Br)6Co 8 and TPP(NO2)PE6Co 9 in PhCN with and without added CH3I. The spectrum of the singly reduced compound 8 is characterized by a split Soret band at 389 and 470 nm and two Q bands at 587 and 687 nm, while the spectrum of the singly reduced compound 9 has bands at 504, 582, 623, and 798 nm. Both spectra are similar to the spectra obtained in pyridine (Figure 3), with more Co(I) character being seen in compound 8 than compound 9. Quite different UV−visible spectra are obtained for reduction of the same two porphyrins in PhCN containing excess CH3I.

the corresponding average separation in potentials between the three related redox reactions of PorCoII under the same solution conditions. A question which remains unanswered is the site of electron transfer in the third reduction of the electrosynthesized PorCo(CH3) derivatives. This electrode reaction is given by eq 8 and can involve either generation of a Co(II) porphyrin dianion where the third electron is added to the metal center or generation of a Co(III) porphyrin trianion, where the third electron is added to the conjugated macrocycle. The existence of porphyrin trianions was recently reported for a number of transition metal complexes with E1/2 values for the third reduction ranging from −1.35 to −1.52 V for TPP(NO2)(PE6)M derivatives with M = ZnII, CuII, NiII, and 2H. This is the same range of potentials measured for the third oneelectron reduction of the electrosynthesized PorCo(CH3) complexes, suggesting formation of a porphyrin trianion in both series of compounds. More importantly, the half wave potential for the third reduction of TPP(NO2)(PE)6Co(CH3) in PhCN (−1.44 V) is quite similar to the reversible E1/2 for the third reduction of TPP(NO2)(PE)6Zn in pyridine, providing K

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Figure 7. UV−visible spectral changes obtained during controlled potential reductions of (a) TPP(NO2)(Br)6Co 8 and (b) TPP(NO2)(PE)6Co 9 in PhCN containing 0.1 M TBAP and excess CH3I.

The singly reduced compound 8 under these solution conditions has a sharp Soret band at 455 nm and a single Q band at 572 nm and is assigned as TPP(NO2)Br6Co(CH3). The singly reduced compound 9 under the same solution conditions is characterized by a sharp Soret band at 490 nm and a single Q-band at 602 nm and is assigned to TPP(NO2)PE6Co(CH3). There is almost no difference in the intensity of the absorption bands between PorCoII and PorCoIII(CH3) but there is a 6−8 nm blue shift of the Soret and Q bands upon formation of the methyl derivative. More significant, however, is the red shift in the absorption band upon changing from 6 Br substituents to 6 PE substituents on the porphyrins. The magnitude of this shift in wavelength amounts to 35 nm for the PorCo(CH3) derivatives (455 nm for 8 vs 490 nm for 9) and is similar to the 33 nm difference between the Soret bands of the same two PorCoII compounds prior to reduction (463 nm for 8 vs 496 nm for 9). The large red shift in absorption bands for the cobalt hexa-PE derivative 9 as compared to compound 8 is due to the π-extending properties of the phenylacetylene

substituents, a property not found in the Br substituents, which have only electron-withdrawing and not π-extending properties. The applied potential for electrosynthesis of the two carbonbonded porphyrins was −0.55 V, a value sufficient to carry out the first reduction of PorCoII but not to further reduce the electrogenerated products whose half wave potentials are −0.66 and −0.72 V, respectively, (see Table 3). The applied potential in the thin-layer cell was then switched to −0.90 V (compound 8) or −0.85 V (compound 9) which led to generation of the singly reduced methyl porphyrins in the thin layer cell. These spectra of [PorCo(CH3)]− are shown in the lower part of Figure 7 and are both characteristic of a porphyrin π-anion radical, namely a spectrum having a significantly reduced Soret band intensity and a broad absorbance in the NIR region of the spectrum. Additional examples of the spectral changes during the first reduction of the other methyl porphyrins in PhCN containing CH3I are shown in Figures S11 and S12 and a summary of the data for all of the electrogenerated PorCo(CH3) complexes is given in Table S2. L

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carried out at room temperature. Low temperature CV measurements were made by immersing the cell in an appropriate dry ice/acetone mixture. Synthesis of Porphyrins. TPP(NO2)Co 2, TPP(NO2)X2Co (X = Ph 3, Br 4, PE 5, and CN 6) and TPP(NO2)X6Co (X = Ph 7, Br 8, PE 9) were synthesized according to methods reported in the literature.11 The CoTPP(CN)4 10 was also synthesized as described in the literature.26

In summary, we have reported the electrochemistry, spectroelectrochemistry and chemical reactivity for a series of cobalt nitroporphyrins with π-extending and/or highly electron-withdrawing β-pyrrole substituents. The UV−visible spectra of the singly oxidized compounds were characterized in both bonding and nonbonding nonaqueous media and were interpreted in terms of a solvent and/or substituent dependent equilibrium between two forms of the electrogenerated cationic porphyrin, one having properties of a “pure” Co(III) derivative with an intact π-system, and the other that of a Co(II) porphyrin π-cation radical, where the electron had been abstracted from the conjugated macrocycle. UV−visible spectra of the singly reduced porphyrins were also measured in bonding and nonbonding nonaqueous media and were consistent with a substituent (but not solvent) dependent equilibrium involving two different electroreduction products, one which was characterized as a “pure” Co(I) porphyrin with an unreduced macrocycle and the other a Co(II) porphyrin π anion-radical, where the added electron resided on the conjugated macrocycle. The degree of Co(I) character in the spectrum of a given electrosynthesized porphyrin varied with the specific β-pyrrole substituents on the macrocycle, but independent of the specific oxidation state assignment, a reaction was observed to occur between the electrogenerated monoanionic porphyrin and the methyl iodide added to solution. This leads in each case to the electrosynthesis of a novel carbon-bonded methyl porphyrin whose electrochemistry and spectroelectrochemistry was also characterized in the present study. Early characterizations of electrosynthesized cobalt carbonbonded porphyrins had in the past been limited to the reactions between [TPPCoI]− and different alkyl iodides, and thus nothing was previously known about how the redox behavior of a given PorCo(R) complex would vary with changes in the structure of the porphyrin macrocycle. Answers to this question are now provided in the current manuscript which has reported the first detailed characterization of [PorCo(R)]n‑ where n varies from 0 to −3 and Por is one of several different porphyrins with highly electron withdrawing or π-extending βpyrrole substituents.

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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.7b02856. UV data, CV data tables, and spectroelectrochemical data details (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (K.M.K.). *E-mail: [email protected] (M.S.). 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 (KMK, Grant E-680) is gratefully acknowledged. M.S. sincerely thanks the Science and Engineering Research Board (EMR/2016/4016) for financial support. R.K. thanks the Ministry of Human Resource Development, India, for a Senior Research Fellowship

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REFERENCES

(1) Kadish, K. M. The Electrochemistry of Metalloporphyrins in Nonaqueous Media; John Wiley & Sons, Inc., 1986; Vol. 34; pp 435−605. (2) Kadish, K. M.; Van Caemelbecke, E.; Royal, G. Electrochemistry of Metalloporphyrins in Non-Aqueous Media. In The Porphyrin Handbook; Kadish, K. M., Smith, K. M., Guilard, R., Ed.; Academic Press: New York, 2000; Vol. 8, pp 1−144. (3) Kadish, K. M.; Royal, G.; Van Caemelbecke, E.; Gueletti, L. Metalloporphyrins in Nonaqueous Media: Database of Redox Potentials. In The Porphyrin Handbook; Kadish, K. M., Smith, K. M., Guilard, R., Eds.; Academic Press: 2000; Vol. 9, pp 1−219. (4) Wolberg, A.; Manassen, J. Electrochemical and electron paramagnetic resonance studies of metalloporphyrins and their electrochemical oxidation products. J. Am. Chem. Soc. 1970, 92, 2982−2991. (5) Truxillo, L. A.; Davis, D. G. Electrochemistry of cobalt tetraphenylporphyrin in aprotic media. Anal. Chem. 1975, 47, 2260− 2267. (6) D’Souza, F.; Villard, A.; Van Caemelbecke, E.; Franzen, M.; Boschi, T.; Tagliatesta, P.; Kadish, K. M. Electrochemical and Spectroelectrochemical Behavior of Cobalt(III), Cobalt(II), and Cobalt(I) Complexes of meso-Tetraphenylporphyrinate Bearing Bromides on the β-Pyrrole Positions. Inorg. Chem. 1993, 32, 4042− 4048. (7) Mu, X. H.; Kadish, K. M. Oxidative Electrochemistry of Cobalt Tetraphenylporphyrin under a CO Atmosphere. Interaction between Carbon Monoxide and Electrogenerated [(TPP)Co]+ in Nonbonding Media. Inorg. Chem. 1989, 28, 3743−3747. (8) Walker, F. A.; Beroiz, D.; Kadish, K. M. Electronic Effects in Transition Metal Porphyrins. 2. The Sensitivity of Redox and Ligand

EXPERIMENTAL SECTION

Chemicals. Unless otherwise noted, all chemicals and solvents were obtained from Sigma-Aldrich and used without further purification. Dichloromethane (CH2Cl2, anhydrous, ≥99.8%, EMD Chemicals, Inc.) and pyridine (Py, HPLC grade, ≥99%, SigmaAldrich) 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. Tetra-nbutylammonium perchlorate (TBAP) was purchased from SigmaAldrich. Methyl iodide (CH3I, reagent Plus, 99.5%) was purchased from Sigma-Aldrich and kept in the refrigerator until used. 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. A thin-layer spectroelectrochemical cell set was purchased from Pine Research, Co., with platinum working electrode and SCE reference electrode. Unless otherwise noted, all cyclic voltammetric (CV) experiments were M

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(25) Nicholson, R. S.; Shain, I. Theory of Stationary Electrode Polarography for a Chemical Reaction Coupled between Two Charge Transfers. Anal. Chem. 1965, 37, 178−190. (26) Callot, H. J. Bromination of m-Tetraphenylporphine. Preparation of Alkyl Derivatives and Polycyanoporphines. Bull. Soc. Chim. Fr. 1974, 7−8 (Part 2), 1492−1496.

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