Review pubs.acs.org/CR
Electrochemistry of Corroles in Nonaqueous Media Yuanyuan Fang,† Zhongping Ou,*,† 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
‡
ABSTRACT: This review describes the known electrochemistry of corroles in nonaqueous media from 1980 until the present. The outline of the review is grouped according to the periodic table, proceeding from left to right, describing first monomeric free-base derivatives and then transition-metal compounds, followed by main-group corroles, before ending with a brief description of lanthanide and actinide corroles. Many similarities exist between the redox properties of metallocorroles and metalloporphyrins, but there are also many differences due, in part, to the different charges of the two conjugated macrocycles and the noninnocence of the corrole ligand in a variety of compounds. One part of this review will focus on describing redox behavior as a function of metal ion and axial ligands, while another will focus on how changes in structure of the macrocycle are associated with changes in redox behavior. It is hoped that this review will answer the majority of the readers’ questions as to what has been electrochemically observed for corroles in the past while at the same time enabling the reader to utilize data in the literature to predict and “tune” what might be observed in future electrochemical studies of corroles that have yet to be synthesized and characterized.
CONTENTS 1. Introduction 2. Overview of Corrole Electrochemistry 2.1. Utilized Solvents and Supporting Electrolytes 2.2. Meso-Substituent Effects 2.3. β-Pyrrole Substituent Effects 2.3.1. Nitro Corroles 2.3.2. Octabromocorroles and Octabromopentafluorophenylcorroles 3. Free-Base (Metal-Free) Corroles 3.1. Neutral, Mono-Protonated, and Mono-Deprotonated Derivatives 3.2. meso-Nitrophenylcorroles 3.3. HOMO−LUMO Gap 3.3.1. [(Cor)H2]− 3.3.2. (Cor)H3 and [(Cor)H4]+ 3.4. Highly Protonated Corroles [(Cor)H5]2+ and [(Cor)H6]3+ 4. Transition-Metal Corroles 4.1. Group 6 4.1.1. Chromium 4.1.2. Molybdenum 4.1.3. Tungsten 4.2. Group 7 4.2.1. Manganese 4.2.2. Rhenium 4.3. Group 8 4.3.1. Iron 4.3.2. Ruthenium 4.3.3. Osmium 4.4. Group 9 4.4.1. Cobalt 4.4.2. Rhodium 4.4.3. Iridium 4.5. Group 10 4.5.1. Nickel © 2016 American Chemical Society
4.5.2. Platinum 4.6. Group 11 4.6.1. Copper 4.6.2. Silver 4.6.3. Gold 4.7. Group 12 4.7.1. Zinc 5. Main-Group Corroles 5.1. Group 13 5.1.1. Aluminum 5.1.2. Gallium 5.2. Group 14 5.2.1. Germanium 5.2.2. Tin 5.3. Group 15 5.3.1. Phosphorus 5.3.2. Arsenic 5.3.3. Antimony 5.3.4. Bismuth 6. Lanthanide and Actinide Corroles 7. Summary and Brief Overview of Trends in Structure−Reactivity Relationships Author Information Corresponding Authors Notes Biographies Acknowledgments Abbreviations Used References
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Special Issue: Expanded, Contracted, and Isomeric Porphyrins Received: August 12, 2016 Published: December 23, 2016 3377
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1. INTRODUCTION The first synthesis of a corrole was reported in 1965,1 5 years after the macrocycle was introduced by Johnson and Price in 1960 as a tetradehydrocorrin.2 Corroles have been closely identified with porphyrins over the last 50 years (see numbering system of both macrocycles in Chart 1), but the lack of a meso C20 carbon atom in the corrole leads to a smaller cavity than in the case of porphyrins and also to a reduced symmetry. The lack of the C-20 carbon atom also causes the corrole macrocycle to be trivalent as compared to divalent in the case of porphyrins, with steric strain forcing the corrole molecule slightly out of the plane of the four nitrogens.3,4
illustrated in Chart 2 along with structures of other corroles and related porphyrins. The total number of published papers on corroles has increased significantly over the last 2 decades, going from less than 100 papers in 1996 to more than 1300 in 2016, as determined by a simple literature search with SciFinder. Also, according to SciFinder, more than 70 reviews on selected aspects of corroles have been published over the last 2 decades,5−74 although most of these publications would more accurately be classified as minireviews on very narrowly focused aspects of corrole chemistry or properties. Some of the above SciFinder-listed reviews on corroles have included discussions of the compound’s redox reactivity,6−15 but none has been devoted exclusively to the electrochemical properties of corroles as a function of changes in the central metal ion, compound structure, or solution conditions. This is addressed in the current review, which covers the published work on corroles from 1980 to 2016 and summarizes the known electrochemistry of free-base and metallocorroles in nonaqueous media. An understanding of corrole redox reactions and how these reactions compare to the reactions of porphyrins is important to better understand the use of these molecules in the fields of medicine and material science and as essential components in many applications. One part of this review will focus on how changes in the structure (substituents) of the macrocycle can be associated with changes in redox behavior of the corrole for derivatives with a given central metal ion, while another section will involve a description of how corrole electrochemistry for a given series of compounds with the same macrocycle [for example, TPC, OEC, Br8(Ph)3Cor, or Br8(F5Ph)3Cor, the structures of which are shown in Chart 2] and different metal ions will vary with changes in the type and oxidation state of the central metal ion and the presence or absence of axial ligands. When describing the electrochemistry of corroles as a function of the central metal ion, one must always consider the effect of axial coordination and the effect of the solution conditions, i.e., the solvent and supporting electrolyte used for the electro-
Chart 1. Porphyrin and Corrole Skeletal Structures with Numbering System of Macrocycle
The earliest synthesized corroles contained “simple” alkyl substituents at the eight β-pyrrole positions (2, 3, 7, 8, 12, 13, 17, 18) of the trinegatively charged macrocycle or aryl substituents at the three meso positions (5, 10, 15). The β-substituted compounds are best represented by octaethylcorrole (OEC) or octamethylcorrole (OMC) and the meso-substituted derivatives by triphenylcorrole (TPC). More recent studies of corroles have focused on derivatives with eight β-Br substituents and three meso-phenyl groups, Br8(Ph)3CorM, or derivatives with eight βBr groups and three meso-pentafluorophenyl substituents, such as in the case of Br8(F5Ph)3CorM, the structure of which is Chart 2. Structurally Related Corroles and Porphyrins
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Figure 1. Periodic table of metallocorroles. The shaded elements indicate specific corroles that have so far been electrochemically examined. The nine elements in bold and italic indicate corroles that have been synthesized but not electrochemically characterized.
always observed for corroles as compared to porphyrins with the same central metal ion? (vi) What about the ability to actually define the site of electron transfer for corroles with noninnocent macrocyclic ligands? (vii) When does the neutral corrole possess an uncharged macrocycle, and when is the formulation of an uncharged metallocorrole with a formal M(III) and M(IV) central metal ion more accurately given as (Cor+•)M(II) or (Cor+•)M(III), respectively? Does an equilibrium exist between two oxidation state assignments of the uncharged compounds in solution? This point is still open to discussion and resolution.78−91 (viii) How does the HOMO−LUMO gap and the magnitude of β-pyrrole or meso-substituent effects depend upon the innocence or noninnocence of the corrole macrocycle? (ix) What about the number of metal oxidation states that can be accessed for a given metallocorrole as compared to a structurally similar porphyrin? For example, cobalt porphyrins have been characterized with Co(III), Co(II), and Co(I) central metal ions, while cobalt corroles have been described in the literature as derivatives of Co(IV), Co(III), Co(II), and Co(I). (x) How does the chemical reactivity of corroles in different oxidation states compare to porphyrins with the same central metal ion; i.e., how do the metallomacrocycles compare in their ability to act as catalysts for a variety of homogeneous reactions? As indicated above, a detailed review of all corrole electrochemistry has not been published, as is the case with porphyrins,75−77 nor has there been published a detailed analysis of the trends in electrochemical data for corroles under a variety of solution conditions. This is done in the current review, which examines trends in the redox behavior of corroles as a function of macrocycle structure, type of central metal ion, metal oxidation state, type and number of axial ligands, and solution conditions. It is hoped that this review will answer the majority of the readers’ questions as to what has been electrochemically observed in the past while at the same time enabling the reader to utilize data in the literature to predict what might be observed in future electrochemical studies of corroles that have yet to be synthesized while at the same time enabling the “tuning” of redox reactions in the case of selected corroles or a series of corroles.
chemical measurements. In this regard, the following 10 questions should be posed when analyzing electrochemical data for a given metallocorrole, when comparing data between two series of metallocorroles with different macrocyclic structures, i.e., (OEC)M and Br8(F5Ph)3CorM, or when comparing data for a series of corroles and porphyrins with the same central metal ion and β-pyrrole or meso-substituents, for example, (TPC)M and (TPP)M or Br8(F5Ph)3CorM and Br8(F5Ph)4PorM (see structures in Chart 2). Answers to some of the questions are straightforward, while answers to others are not, but whatever the case, the following questions should in many cases be considered when evaluating the redox behavior of a newly synthesized corrole or series of corroles: (i) Do the series of compounds being compared undergo similar redox mechanisms and possess similar sites of electron transfer or do they not? If the mechanisms and sites of electron transfer are similar, how different are the redox potentials? (ii) Do the series of compounds being compared have a similar HOMO−LUMO gap for derivatives with the same metal ion and the same set of β-pyrrole and meso-substituents or are they different? (iii) Should a third oxidation of the trinegatively charged corrole ring be observed under optimum solution conditions? It has long been known that porphyrins will undergo two ringcentered oxidations to give π-cation radicals and dications,75−77 but what about corroles? (iv) What about the number of ring-centered reductions for a given corrole under optimum solution conditions? Porphyrins are known to undergo two ring-centered reductions to give πanion radicals and dianions, but what about corroles? Most corroles are more difficult to reduce than porphyrins due to the higher negative charge on the macrocycle, and a second electron addition to the π-ring system is generally not observed except in the case of free-base derivatives lacking one central proton, [(Cor)H2]−, or metallocorroles with highly electron withdrawing substituents, where all potentials are shifted in a positive direction, thus making additional electron transfers easier to observe within the negative potential limit of the solvent. (v) What about the overall number of metal-centered oxidations and reductions for corroles with redox-active metal centers? Are higher oxidation states of the central metal ion 3379
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Chart 3. Selected Structures for Electrochemically Investigated Corroles
Table 1. Physical Properties and Potential Limit (V vs SCE) of Several Common Solvents Used for Corrole Electrochemistrya
a
solvent
DN122
AN122
εr123
FP123
BP123
dichloromethane (DCM) 1,2-dichloroethane (EtCl2) benzonitrile (PhCN) acetonitrile (MeCN) tetrahydrofuran (THF) N,N′-dimethylformamide (DMF) dimethyl sulfoxide (DMSO) pyridine (py)
0.0 0.0 11.9 14.1 20.0 26.6 29.8 33.1
20.4 16.7 15.5 18.9 8.0 16.0 18.8 14.2
8.93 10.37 25.20 35.94 7.58 36.71 46.68 12.91
−96.7 −35 −13.5 −45.2 −108.5 −61 18.5 −41.6
40 83.4 191.1 81.6 65 153 189 115.5
potential limit (V vs SCE) +1.8 +1.8 +1.7 +1.8 +1.8 +1.6 +0.7 +0.8
−1.9 −1.9 −1.8 −2.0 −2.8 −2.3 −1.9 −1.8
DN = donor number, AN = acceptor number, εr = dielectric constant, FP = freezing point (°C), and BP = boiling point (°C).
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Table 2. List of Selected Substituents and Values of 3σ for pPhenyl-Substituted Triphenylcorrolesa
a
substituent
σp
substituent
σp
NO2 CN COOMe COOH Cl Br F
0.78 0.66 0.45 0.41 0.23 0.23 0.06
Ph H Me C(Me)3 OMe OH OCH2Ph
0.01 0.00 −0.17 −0.20 −0.27 −0.37 −0.41
by studies from the group of Murakami on corroles containing the higher oxidation state Cr(V),94 Mo(V),95−97 or Re(V)96 central metal ions. The earliest electrochemical investigations of corroles, for the most part, involved a characterization of “simple” alkyl β-substituted octamethylcorrole (OMC) or octaethylcorrole (OEC), and derivatives with the same macrocycles were also later electrochemically characterized and described in joint papers from the groups of Vogel and Kadish91,98−104 or from Paolesse and Kadish,105 as the “periodic table of corroles” was being expanded. Many of the corroles later examined as to their electrochemistry involved derivatives containing both β-pyrrole and meso-phenyl substituents, one early example being given by (Me)8(Ph)3CorCo,106 which was synthesized by Paolesse, who has continued to work in the field of corroles for more than 2 decades. Although numerous laboratories now routinely carry out electrochemical studies of corroles, the work of two research groups deserves special mention. One is that of Gross and the other that of Ghosh. The relevant electrochemical contributions of these authors will be cited often in the following pages. The corrole macrocycle possesses a highly conjugated π-ring system, which is an ideal location for the addition or abstraction of electrons. Most neutral corroles can be oxidized by two electrons and reduced by one within the positive and negative potential limits of the electrochemical solvent, although the site of electron transfer is not always clear and all three of the above redox reactions might not always be observed for a given
Values of σ are taken from ref 126.
2. OVERVIEW OF CORROLE ELECTROCHEMISTRY The metal-free (free-base) corrole macrocycle possesses three protons that can be replaced by metal ions in both high and low oxidation states. Numerous transition and main-group metals can be complexed by the −3 charged corrole macrocycle to form a metallocorrole (see the periodic table of corroles in Figure 1) that can process a central metal ion in oxidation states ranging from +1 to +6. Metallocorroles with no axial ligands are fourcoordinate, while those with one or two axial ligands are five- and six-coordinated derivatives, respectively. The first electrochemistry of a corrole was reported in the early 1970s92,93 and involved studies of formal Cu(III),92 Ni(III),92 and Co(III)92,93 derivatives, and this was followed a decade later
Figure 2. Plots of E1/2 or Ep for six redox reactions of (Me)8(XPh)3CorCo(PPh)3 in PhCN, 0.2 M TBAP. Adapted with permission from ref 106. Copyright 1995 American Chemical Society. 3381
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Figure 3. (a) Cyclic voltammograms of (NO2)x(tBuPh)3CorCu (x = 0, 1, 2) in DCM, 0.1 M TBAP and (b) correlation between E1/2 for the first reduction of (NO2)x(tBuPh)3CorCu in PhCN and the Soret band maximum of the neutral compounds in wavenumbers. Adapted with permission from ref 85. Copyright 2011 American Chemical Society.
2.1. Utilized Solvents and Supporting Electrolytes
compound in the same solvent system. The electrogeneration of corroles with a triply oxidized or double reduced macrocycle is also possible, although evidence for these processes is scant and usually not supported by spectroscopic data confirming the site of electron abstraction. Like porphyrins, many metal-centered redox processes have been reported for corroles with electroactive central metal ions, with examples being given for transition-metal derivatives with nickel, copper, iron, cobalt, manganese, silver, gold, platinum, chromium, and ruthenium, as well as with some main-group corroles (see later sections of the review). However, unlike the case of porphyrins, the site of electron transfer is not always clear-cut due to the known noninnocence of the corrole ligand. In addition to the ring- and metal-centered redox reactions of corroles, some electroactive substituents or axial ligands on the corrole might also be oxidized or reduced. Two examples of wellcharacterized corroles with redox-active substituents include derivatives with meso-linked ferrocene107−111 or meso-linked nitrophenyl groups.112−121 Structures for some of these compounds are shown in Chart 3. The meso-linked ferrocene groups on the corrole can be oxidized via overlapping or well-separated one-electron-transfer steps between +0.4 and +0.6 V vs SCE (the saturated caromel electrode, which was used as the reference electrode in this review except where specified), while the meso-linked nitrophenyl groups are reduced over a range of E1/2 values between −0.95 and −1.35 V vs a SCE. In each case, the exact redox potential will depend upon the solution conditions, other substituents on the macrocycle, and the possibility of interaction between multiple equivalent, or nonequivalent, redox active centers on the molecule (see later discussion).
Most electrooxidations of corroles have been carried out in dichloromethane (DCM), acetonitrile (MeCN), or benzonitrile (PhCN). Although the anodic (positive) and cathodic (negative) potential range of these solvents do not extend much above +1.8 and −2.0 V, this is often sufficient to view two and sometimes three one-electron oxidations and at least one reduction of most metallocorroles. Numerous laboratories have utilized DCM as a solvent for characterizing the electrochemistry of corroles, thus allowing for the possibility of low-temperature electrochemical measurements that might enable the slowing down (freezing out) of coupled chemical reactions following electron transfer88,103,105 and, at the same time, allowing for comparisons to be made directly between electrochemical results obtained in this chlorinated solvent and spectroscopic data obtained in other nonbonding solvents such as CCl4 or CHCl3. Tetrahydrofuran (THF) and pyridine (py) have also been used as solvents to examine the redox properties of corroles, both at room temperature and low temperature. The anodic and cathodic potential limits of these two solvents and others that have been used for electrochemical studies of corroles are given in Table 1, which includes data on selected solvent properties. Most electrochemical studies of corroles in nonaqueous media have utilized tetraalkylammonium salts as the supporting electrolytes, in part because of their high solubility in nonaqueous solvents and in part because of their commercial availability at a relatively low cost. The most commonly used supporting electrolyte salt has been tetra-n-butylammonium perchlorate (TBAP), although some electrochemical studies have also utilized tetraalkylammonium salts of BF4− or PF6−. 3382
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Table 3. Effect of β-NO2 Groups on the First Reduction and First Oxidation of Corroles and Porphyrins in Nonaqueous Mediaa first oxidation type of compd β-NO2 corrole
macrocycleb (MePh)3Cor
M H3
H2
(MeOPh)3Cor
(NO2Ph)3Cor (tBuPh)3Cor
Fe(III)
Fe(III) Ag(III) Cu(III)
β-NO2 porphyrin
TPP
2H Cu(II) Zn(II)
TmPP
2H Fe(III) Mn(III)
TdmPP
2H Fe(III) Mn(III)
no. of NO2 0 1 2 0 1 2 0 1 2 0 1 0 1 0 1 2 0 1 0 1 0 1 0 1 0 1 0 1 0 1 0 1 0 1
E1/2 (V) 0.56 0.70 0.83 0.09 0.25 0.41 0.82 1.01 1.18 0.97 1.18 −0.03 0.21 0.71 0.90 1.11 1.03 1.15 0.69 0.78 0.57 0.70 0.95 1.06 1.07 1.14 1.08 1.18 1.02 1.15 1.14 1.23 1.14 1.26
first reduction
ΔE (mV)c
d
140 130 160 160 190 170 210 240 190 210 120 90 130 110 70 100 130 90 120
E1/2 (V) −1.33 −0.87 −0.75 −1.88 −1.34 −1.05 −0.35 −0.16 0.04 −0.14 0.05 −0.79 −0.54 −0.17 0.00 0.20 −1.01 −0.72 −1.64 −1.27 −1.67 −1.29 −1.19 −0.81 −0.27 −0.18 −0.31 −0.15 −1.12 −0.83 −0.28 −0.16 −0.29 −0.10
ΔE (mV)c
ref 138
460 120 138 540 290 120 190 200 120 190 135 250 85 170 200 137 290 137 370 137 380 136 280 136 90 136 160 136 290 136 120 136 190
a
The exact structure of the compounds and the solution conditions are given in the indicated references. The listed potentials are given as V vs SCE. TMPCor and TNPCor are trianions of the trimethoxylphenylcorrole and trinitrophenylcorrole, respectively, and (tBuPh)3Cor is the trianion of tributylphenylcorrole. cΔE1/2 = the potential difference between E1/2 for the reduction or oxidation of the macrocycle with each increase of a nitro group. dPeak potential at scan rate = 0.1 V/s. b
Mann and Barnes124 reported data on the potential range of DCM containing different supporting electrolytes. They indicate that salts of I−, Br−, or Cl− may be used for reductions but are not recommended for oxidations due to the fact that the anions are easily oxidized at potentials less than +1.0 V vs SCE. The anions might also coordinate to the central metal ion of the corroles in their neutral or oxidized forms, thus leading to redox potentials that could be quite different than when the measurements are made in solutions of TBAP.125 The majority of metallocorrole redox reactions involve reversible one-electron transfers (as is also the case for porphyrins), and these potentials are generally reported as halfwave potentials (E1/2) versus a standard reference electrode, which is most often a SCE. Some potentials are reported versus Ag/AgCl, while others are sometimes reported versus the ferrocene/ferrocenium couple (Fc/Fc+). Unless otherwise noted, all potentials mentioned in this review are given vs a SCE.
type of substituent, its specific location on the macrocycle, the central metal ion, the type and number of axial ligands, the solvent, and, most importantly, the site of electron transfer. Table 2 gives examples of substituents along with literature values of the substituent constants, σp, for electron-donating or electronwithdrawing groups that have been added to the para-position of the meso-phenyl rings on a corrole macrocycle.126 The half-wave potentials of corrole electrode reactions will often vary systematically with changes in the electron-donating or electron-withdrawing substituents on the macrocycle, and values of E1/2 for each redox reaction of a given compound with different substituents have often been analyzed using the electrochemical linear free-energy equation given by eq 1,127 E1/2(X) = E1/2(H) + ∑ σρ
(1)
where E1/2(H) is the half-wave potential for the unsubstituted corrole, E1/2(X) the half-wave potential for the corrole with a specific electron-donating or electron-withdrawing substituent, σ is the Hammett128 or Taft126,129−131 substituent constant, and ρ is the reaction constant, given in volts. The larger the value of ρ,
2.2. Meso-Substituent Effects
The effect of electron-donating or electron-withdrawing substituents on corrole redox potentials will depend on the 3383
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our knowledge, the cobalt octamethyltriphenylcorroles examined in this early study remain the only metallocorroles to date to show such behavior. 2.3. β-Pyrrole Substituent Effects
2.3.1. Nitro Corroles. The addition of one or two nitro substituents to the β-pyrrole positions of a metallocorrole will lead to significant positive shifts in potential for all redox reactions of the compounds. Examples of this have been given for corroles with Ag(III),135 Fe(III),120 Cu,85 and Ge85 central metal ions. The magnitude of the shift in reduction potentials ranges from about 200 mV per each added β-NO2 substituent to a high of 500−800 mV, the largest potential shift being in the case of the second electron addition to the macrocycle.85,120,135 Large positive shifts in E1/2 are also seen upon oxidation of the nitrocorroles, an example of which is shown in Figure 3, which illustrates cyclic voltammograms of three related copper corroles, (NO2)x(tBuPh)3CorCu, where x = 0, 1, or 2.85 As seen in the figure, E1/2 for the first reduction is shifted stepwise from −0.17 to 0.00 V and then to +0.20 V as the number of NO2 groups on β-pyrrole position of the corrole is increased from 0 to 1 and then from 1 to 2. A plot of E1/2 vs the number of NO2 groups on the compound is linear (Figure 3b) and has a slope of 185 mV (ΔE1/2/ΔNO2 groups). A plot of the Soret band maximum vs the number of NO2 groups is also linear, as shown in the figure. The overall positive shift in potential upon going from the first reduction of (tBuPh)3CorCu at −0.17 V to the first reduction of the dinitro corrole at +0.20 V is 370 mV, as seen in Figure 3a. This compares to an overall positive shift of 400 mV for the first oxidation and 300 mV for the second, a trend not seen for porphyrins, for which the shift of potentials due to β-pyrrole substitution is generally much larger for reduction than for oxidation of the same compound.120,136 The largest positive shift in E1/2 upon nitro group addition to the copper corroles occurs in Figure 3 for the second reduction, where the difference in E1/2 amounts to about 600 mV upon going from (tBuPh)3CorCu to (NO2)(tBuPh)3CorCu and 230 mV upon going from (NO2)(tBuPh)3CorCu to (NO2)2(tBuPh)3CorCu, an overall positive shift of 830 mV as compared to the parent compound. The second reduction of (tBuPh)3CorCu is not shown in this figure and is located at about −1.95 V when measured at low temperature in DCM or THF. The E1/2 for the second reduction of (Ph)3CorCu was measured as −1.94 V in DCM at −60 °C.88 A third reduction of (tBuPh)3CorCu is not observed within the potential limit of the PhCN solvent, but it is significant to point out that the ΔE1/2 between the second and third reductions of (NO 2 )(tBuPh)3CorCu and (NO2)2(tBuPh)3CorCu ranges from 330 to just under 400 mV, values quite similar to those for ringcentered reactions of porphyrins,136,137 for which the redox processes involve formation of a π-anion radical and dianion. As described in the following pages, the separation in reduction potential between E1/2 values for the two ring-centered reductions of the two nitrocorroles is also similar to the separation in potential seen for the same redox reactions of [(Cor)H2]−. The especially large effect of β-nitro substitution on corrole redox potentials is not limited to the copper derivatives but is also observed for iron,120 free-base,138 and silver135 nitrocorroles, as shown by the data in Table 3, which summarizes the measured potential shifts for related β-substituted porphyrins and corroles under similar solution conditions.
Figure 4. Cyclic voltammograms of (a) Br8(F5Ph)3CorCu, (b) (F5Ph)3CorCu in THF with 0.4 M TBAP, and (c) (Ph)3CorCu in THF with 0.1 M TBAP. The data in parts a and b of the figure are taken from ref 88.
the larger the effect of the substituents on the examined electron transfer reactions. One early example of linear free-energy relationships used to analyze the oxidations and reductions of structurally related corroles in PhCN is given in Figure 2 for derivatives of (Me)8(XPh)3CorCo(PPh3), where X is the electron-donating or electron-withdrawing substituent on each of the three mesophenyl groups of the compound.106 The examined octamethyltriphenylcorroles undergo four one-electron oxidations and two one-electron reductions within the potential limit of the utilized solvent. As seen in the figure, the potentials for each redox reaction vary linearly as a function of the specific electrondonating or electron-withdrawing substituent on the three mesophenyl rings. As expected, easier oxidations and harder reductions are observed for the corroles with electron-donating groups such as OMe and Me, while harder oxidations and easier reductions occur for derivatives with electron-withdrawing groups such as Cl or F. One key take-home point from Figure 2 is that a linear relationship exists between E1/2 and 3σ for each of the six observed redox reactions of the corroles, indicating that the same electron transfer mechanism occurs for each individual redox process throughout the series of related compounds. Another take-home point is that different values of ρ are observed for the different redox processes, these values ranging from a low of 30 mV for the second one-electron reduction to a high of 92 mV for the first one-electron addition. These values of ρ are within the range of values reported for porphyrins containing the same types of substituted meso-phenyl groups.75 They are also within the range of ρ values observed in later published electrochemical studies of numerous other series of corroles having substituents on the three meso-phenyl rings.78,83,114,119,132−134 One last point needs to be made with respect to the data in Figure 2. This is the fact that all eight examined cobalt corroles in this series of compounds exhibited four reversible one-electron oxidations, as shown in the illustrated voltammogram. This seemed perfectly acceptable at the time of this publication (1995), since it was thought that all metallocorroles would exhibit three ring-centered oxidations of the −3 charged macrocycle in addition to any metal-centered redox reactions, in this case a proposed Co(III) to Co(IV) process. However, to 3384
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Additional details on the redox reactions of brominated metallocorroles will be given in the relevant sections of this review that describe the behavior of compounds having specific central metal ions, but several things should now be pointed out. One is that the effect of Br substituents is not greater for reduction than for oxidation in the case of all metallocorroles. Another is that the effect of adding five electron-withdrawing F groups to each meso-phenyl group of a triphenylcorrole also results in a significant positive shift of all potentials as compared to that of the parent compound. The magnitude of this substituent effect resulting from the three F5Ph groups will vary somewhat with changes in metal ion and solution conditions, but in almost all cases the redox potentials for reduction and oxidation of a given Br8(F5Ph)3CorM derivative will be shifted positively by 800−900 mV with respect to the parent compound lacking both F5Ph and Br substituents. This is shown graphically in Figure 4 for three related corroles containing a copper center and also shown in a summary table of selected potentials for oxidation and reduction of different (Ph)3CorM, (F5Ph)3CorM, and Br8(F5Ph)CorM derivatives. The parent compound in Figure 4, (Ph)3CorCu, is reduced at −0.11 and −1.95 V in THF while Br8(F5Ph)3CorCu exhibits one-electron-transfer reactions at +0.75 and −1.03 V under the same solution conditions.88 The substantial change in potential between the easiest and hardest to reduce corrole may or may not be associated with a change in electronic configuration and electron transfer site but one can more easily examine this possibility for those redox reactions of the parent corrole which might otherwise be too negative in potential to access the low oxidation state form of the compound using standard chemical or electrochemical methodologies. Another thing to point out in Figure 4 is the fact that the synthesized Br8(F5Ph)3CorCu derivative, once added to solution, will immediately be converted to its singly reduced [Br8(F5Ph)3CorCu]− form. This makes no difference in the thermodynamic potential for the process at 0.75 V, but one is actually measuring an oxidation in this case, perhaps Cu(II) to Cu(III), or maybe the reaction involes the conversion of a Cu(II) corrole to it π-cation radical form, [Br8(F5Ph)3Cor•)CuII]+. Under this latter condition, the HOMO−LUMO gap of the corrole actually in solution, [Br8(F5Ph)3Cor)CuII]−, would be given by the difference in E1/2 between the first reduction at −1.03 V and first oxidation at 0.75 V. This gap of 1.78 V for [Br8(F5Ph)3CorCuII]− is not significantly different from the measured HOMO−LUMO gap of 1.84 V for [(Ph)3CorCu]− in THF (Figure 4c) or 1.88 and 1.95 V for the Br8(F5Ph)3Cor derivatives of Al(III) and Ga(III) when measured in MeCN.139 A further analysis of how the HOMO−LUMO gap and individual redox potentials vary with changes in the corrole central metal ion and meso or β-pyrrole substituents is given in later sections of this review.
Additional details on iron, silver, and copper nitrocorrole redox reactions are given in later sections of this review. However, it should be noted that the magnitude of the substituent effect (positive shift in potential) upon adding one NO2 group to the β-pyrrole position of the Ag(III) corrole is almost the same for the first oxidation and first reduction. Both electrode reactions shift positively by 240−250 mV, mirroring what is seen for the two oxidations of the copper and iron complexes. 2.3.2. Octabromocorroles and Octabromopentafluorophenylcorroles. The effect of bromination on the electrochemistry, frontier orbitals, and spectroscopy of metallocorroles was nicely described in a minireview by Gross and co-workers.139 It was shown that the addition of eight electron-withdrawing Br groups to the β-pyrrole positions of a corrole, either (Ph)3CorM or (F5Ph)3CorM, will result in a positive shift of all oxidation and reduction potentials for the resulting Br8(Ph)3CorM and Br8(F5Ph)3CorM derivatives, facilitating access to the electroreduced anionic forms of the compounds and making more difficult an electrogeneration of the corrole in its higher oxidation states.78,81,88,140−146 A similar shift of potential was also observed by Ghosh and co-workers upon going from (Ph)3CorM to F8(Ph)3CorM, as demonstrated for derivatives with Cu and Fe central metal ions.147 The magnitude of the shift in potential as compared to that of the parent compound without Br or F5Ph groups will vary with the type of central metal ion, the type of substituents at the three meso-positions of the macrocycle, the properties of the solvent, and, as always, the site of electron transfer. In some cases, the substituent effect of the electron-withdrawing Br (or F) groups on redox potentials of the metallocorroles will be larger by several hundred millivolts for reductions than for oxidations, thus resulting in a smaller HOMO−LUMO gap for the brominated (or fluorinated) corroles than for the parent compounds. This is especially true for main-group metallocorroles with non-redoxactive central metal ions. By way of example, (F5Ph)3CorGa(py) is reduced and oxidized at −1.55 and 0.68 V vs Ag/AgCl in MeCN (a HOMO−LUMO gap of 2.23 V),139 and these potentials shift to −0.81 and 1.14 V for reduction and oxidation of the related octabromocorrole, giving a HOMO−LUMO gap of 1.95 V for Br8(F5Ph)3CorGa(py), under the same solution conditions.139 The eight Br groups produce a 740 mV shift in potential toward an easier reduction and a 460 mV shift toward a harder oxidation, thus resulting in a 280 mV decrease in the HOMO−LUMO gap for the octabromo derivatives. A similar trend in the relative substituent effect for reduction and oxidation is observed when comparing structurally related octabromoporphyrins and nonbrominated porphyrins, although the effect of the electron-withdrawing Br substituents is much smaller for reduction and oxidation of most metalloporphyrins as compared to redox reactions involving oxidation or reduction of the related metallocorroles. Again the HOMO−LUMO gap is smaller for octabromoporphyrins than for the parent porphyrins lacking the β-Br groups, but depending upon the metal ion, the magnitude of the difference in gap between the brominated and nonbrominated porphyrins may or may not be larger than that for the corroles. For example, the HOMO−LUMO gap decreases from 2.32 V for (F5Ph)4PorZn when measured in DCM to 2.05 V for Br8(F5Ph)4PorZn in the same solvent.146 This 270 mV decrease in gap upon bromination of the Zn porphyrins is virtually identical to the 280 mV decrease in gap for the Ga corroles described above.
3. FREE-BASE (METAL-FREE) CORROLES 3.1. Neutral, Mono-Protonated, and Mono-Deprotonated Derivatives
Numerous transition-metal and main-group metallocorroles have been electrochemically characterized in nonaqueous media (see periodic table of corroles in Figure 1), but relatively few studies have concentrated on the unmetalated free-base corroles. This is due in large part to the fact that most electrode reactions of free-base corroles are irreversible and involve the gain or loss of the protons after electron transfer. Protons may 3385
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Scheme 1. Conversion between (Cor)H3, [(Cor)H2]−, and [(Cor)H4]+
Scheme 2. Proposed Electron Transfer Mechanism for (Cor)H3 in Pyridine148
Figure 6. Cyclic voltammograms illustrating the oxidation of CorH3 in PhCN, 0.1 M TBAP. The identity of the process is given in ref 148. Adapted with permission from ref 148. Copyright 2006 American Chemical Society.
Scheme 3. Proposed Electron Transfer Mechanism for Oxidation of (Cor)H3 in PhCN148
Figure 5. Cyclic voltammograms of ((CF3)2Ph)3CorH3 in (a) PhCN and (b) py, 0.1 M TBAP. Scan rate = 0.1 V/s. Adapted with permission from ref 148. Copyright 2006 American Chemical Society.
also be lost from (Cor)H3 before electron transfer, depending upon the basicity of the solvent in which the measurement is being carried out. The protonated corrole is represented as [(Cor)H4]+ and the deprotonated one as [(Cor)H2]−. All three forms of the metal-free corrole are electroactive, and this often results in complicated redox behavior involving multiple redox processes and multiple coupled chemical reactions from several forms of the corrole in solution.148 The conversion between (Cor)H3 and [(Cor)H2]− or [(Cor)H4]+ is schematically shown in Scheme 1 and is highly dependent upon solvent.125,148−153 The most facile reaction involves the loss of one proton from (Cor)H3, for which the measured K values are on the order of 104, as shown by titrations with different organic bases in DCM or PhCN.138,149,150 For example, in pyridine or DMF, there is often a complete conversion of (Cor)H3 to its deprotonated [(Cor)H2]− form, and the electrochemical reactions are then as described in Scheme 2, where CorH3 is converted to its [(Cor)H2]− deprotonated form in pyridine solutions prior to oxidation or reduction at the electrode surface.148 A similar conversion of (Cor)H3 to [(Cor)H2]− was observed for 11 different corroles in pyridine,148 each of which could be reduced in two reversible and well-defined one-electron-transfer steps to give [(Cor)H2]2− and
Figure 7. Cyclic voltammogram of (NO2Ph)3CorH3 in PhCN containing 0.1 M TBAP.158
[(Cor)H2]3−, respectively. The anionic [(Cor)H2]− corrole formed in pyridine solutions could also be oxidized by one electron to its neutral (•Cor)H2 form. This redox reaction was 3386
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Scheme 4. Four-Electron Reduction of (NO2Ph)3CorH3 in PhCN Containing 0.1 M TBAP
Table 4. Half-Wave Potentials for the First Reduction and First Oxidation and the HOMO−LUMO Gap for Mono-Deprotonated Free-Base Corroles in PhCN REa
corrole macrocycle −
[(p-MePh)3CorH2] [(CF3Ph)3CorH2]− [(Cl2Ph)2(py)CorH2]− [(p-CNPh) (Cl2Ph)2CorH2]− [(m-CNPh)3CorH2]− [(py)2(F2Ph)CorH2]− [((CF3)2Ph)3CorH2]− [((F4N3)Ph)3CorH2]− [(F5Ph)3CorH2]− [(F5Ph)3CorH2]− [(Cl2Ph)(F5Ph)2CorH2]− [(MePh)(F5Ph)2CorH2]− [(Me3Ph)(F5Ph)2CorH2]− [(NO2Ph)(F5Ph)2CorH2]− [((MeO)3Ph)(F5Ph)2CorH2]− [(Nap)(F5Ph)2CorH2]−
SCE
Fc/Fc+
1st ox.
1st red.
H−L gapb(V)
ref
−0.08 0.09 0.09 0.10 0.11 0.14 0.21 0.30 0.31 −0.16 −0.24 −0.26 −0.22 −0.36 −0.27 −0.18
−1.91 −1.71 −1.86 −1.85 −1.72 −1.67c −1.60 −1.82 −1.64 −2.12 −2.38c −2.32c −2.00 −2.34c −2.29c −2.24c
1.83 1.80 1.95 1.95 1.83 1.81 1.81 2.12 1.95 1.96 2.14 2.06 1.78 1.98 2.02 2.06
148 148 148 148 148 148 148 148 148 121 121 121 121 121 121 121
RE = reference electrode. bH−L gap = the HOMO−LUMO gap measured as ΔE1/2 between the first reduction and first oxidation. cIrreversible reaction.
a
Scheme 5. Protonation of Mono- and Dipyridylcorroles, Where Ar = 2,5-diFPh for Monopyridyl and Ar = 2,5-diClPh for Dipyridyl Free-Base Corroles150
reported to occur at E1/2 values between −0.08 and +0.31 V for a series of meso-substituted corroles, the exact potential depending upon other substituents on the macrocycle.121,148 The loss of one proton from free-base corroles will also occur after a one-electron reduction of the neutral compound in PhCN or DCM.121,138,148,150,154,155 Under these conditions, the initial electron transfer proceeds as shown in eq 2, after which the anionic [(Cor)H2]− product generated in solution then undergoes two single-electron reductions at the macrocyclic πring system to give [(Cor)H2]2− and [(Cor)H2]3−, respectively (eqs 3 and 4). +e
−1/2H 2
(Cor)H3 ⇌ [(Cor)H3]− ⎯⎯⎯⎯⎯⎯⎯→ [(Cor)H 2]−
+e
[(Cor)H 2]− ⇌ [(Cor)H 2]2 −
(3)
+e
[(Cor)H 2]2 − ⇌ [(Cor)H 2]3 −
(4)
Cyclic voltammograms that illustrate the electrochemical behavior of ((CF3)2Ph)3CorH3 in PhCN and py are shown in Figure 5, where each redox process is identified in the figure. As evident from the mechanism in Scheme 2, none of the four redox processes in Figure 5b are associated with the actual triprotic corrole added to the pyridine solution. The electroactive species in solution is [(Cor)H2]−, which undergoes two one-electron reductions at −1.52 and −1.86 V and a one-electron oxidation at 0.27 V. The process at Ep = −0.64 V in Figure 5b is associated with reduction of protonated pyridine.
(2) 3387
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numbers of protons and/or oxidation states were spectroscopically identified148 and are represented as (Cor)H3, (·Cor)H2, [(Cor)H2]−, [(·Cor)H2]2−, [(Cor)H4]+, and [(·Cor)H4]2+. 3.2. meso-Nitrophenylcorroles
Nitrophenylcorroles have been studied as potential agents against human cytomegalovirus infection for their antiviral activity.156 They have been electrochemically investigated, as part of a triple-decker compound,157 as monomeric free-base derivatives,158 and as monomeric metallocorroles containing Fe(NO),120 Co(III),119 Ag(III),158 or Mn(III)158 central metal ions. In each case, the redox properties of the nitrophenylcorroles can be approximately described as an overlapping of the nitrobenzene redox reactions (E1/2 = −1.08 V vs SCE in PhCN)119 with that of the structurally related corrole that lacks a redox-active nitrophenyl group. An example of this is shown by the cyclic voltammogram in Figure 7 for (NO2Ph)3CorH3 in benzonitrile. The first reduction is irreversible and leads to formation of [(NO2Ph)3CorH2]− at Ep = −0.92 V. This electrochemical EC mechanism148 is followed by an overall three-electron reduction of the three nitrophenyl groups at E1/2 = −1.18 V to give [(NO2Ph)3CorH2]4−, after which reversal of the potential scan at −1.8 V then generates the neutral (NO2Ph)3CorH2 radical at an E1/2 = 0.11 V. The mechanism for loss of one proton from (NO2Ph)3CorH3 after the first one-electron reduction is the same as described above for other free-base corroles,121,148 with the main difference in behavior being that three additional electrons can be added to the compound to give a stable four-electron-reduced species, as schematically shown in Scheme 4. It is important to point out that an overlapping of potentials for the three one-electron reductions of the meso-nitrophenyl groups is not observed for all derivatives. This is described in other sections of this review dealing with specific metal complexes (see section 4.3.1, for example), some of which exhibit (i) three wellseparated one-electron reductions of the three meso-nitrophenyl groups, (ii) two overlapping reversible one-electron transfers followed by a third reversible electron transfer, or (iii) a single reversible electron transfer followed by two overlapping oneelectron-transfer steps. The exact sequence of steps will, in each case, depend upon the specific corrole β-pyrrole substituents as well as upon the presence of other electron-donating or electronwithdrawing substituents on the three meso-nitrophenyl rings of the macrocycle.120,158
Figure 8. Illustration of the HOMO−LUMO gap (V) for different protonated forms of the metal-free corrole in PhCN containing 0.1 M TBAP. Adapted with permission from ref 148. Copyright 2006 American Chemical Society.
Figure 9. Plot of reduction potentials vs the overall charge on a variety of deprotonated, neutral, and protonated corroles. The identity of the corrole macrocycles is given in ref 150. Adapted with permission from ref 150. Copyright 2007 American Chemical Society.
3.3. HOMO−LUMO Gap
3.3.1. [(Cor)H2]−. A summary of the measured potentials is given in Table 4 for the first oxidation and first reduction of different [(Cor)H2]− compounds in PhCN.121,148 The measured potentials from the study of Kadish and co-workers148 are reported vs SCE, while the potentials measured by Osuka and coworkers121 are reported vs the Fc/Fc+ couple, which is located at approximately 0.5 V vs SCE. Thus, assuming a correction of 0.5 V between the two reference electrodes, the potentials for the first oxidation and first reduction of [(F5Ph)3CorH2]− in the study of Kadish and coworkers would be approximately −0.19 and −2.14 V vs Fc/Fc+, values almost identical to the −0.16 and −2.12 V reported for the same reactions in the study by Osuka and co-workers121 and also summarized in Table 4. More importantly, the absolute difference between the first oxidation and the first reduction (the HOMO−LUMO gap) of [(F5Ph)3CorH2]− is 1.95 V in the study by Kadish and co-workers148 and 1.96 V in the study by the group of Osuka121 (see Table 4).
“Complicated” redox behavior is also seen upon oxidation of free-base corroles in nonaqueous media, in which [(Cor)H4]+ is known to be generated in solution. This was first observed by Gisselbrecht et al.151 upon oxidation of (OEC)H3 in DCM. The mechanism for formation of [(Cor)H4]+ upon oxidation was examined in detail by Kadish and co-workers in 2006148 for a series of meso-substituted corroles and then by Osuka and coworkers121 in 2015, who carried out the oxidations in PhCN. Examples of cyclic voltammograms taken from the study of Kadish and co-workers148 are reproduced in Figure 6, and an overall mechanism for oxidation and reduction of 11 mesosubstituted free-base corroles in PhCN is given in Scheme 3, where the average HOMO−LUMO gap of the anionic diprotic corroles ranged from 1.78 to 2.14 V, depending upon the specific compound. In summary, the electrochemistry of free-base corroles will vary as a function of both the structure of the macrocycle and the solvent basicity. Six forms of metal-free corroles with different 3388
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Scheme 6. Proposed Mechanism for the Redox Reactions of (Ar)3CorMnIII and (Ar)3CorMnIVCl in DCM and MeCN Containing 0.1 M TBAP174
3.3.2. (Cor)H3 and [(Cor)H4]+. The electrochemical HOMO−LUMO gap for (Cor)H3 and [(Cor)H4]+ was also determined for a number of free-base triarylcorroles.148 Because the redox processes are irreversible, the ΔE1/2 values should not be considered as a true thermodynamical measurement of the HOMO−LUMO gap in solution. Nonetheless, examples of cyclic voltammograms showing the first reduction and first oxidation of a metal-free corrole in PhCN for three different protonated forms are given in Figure 8. Among the 11 meso-substituted free-base corroles examined in the study of Kadish and co-workers,148 the largest measured HOMO−LUMO gap (ΔE) is for [(Cor)H2]−, which averages 1.86 ± 0.06 for five nonsterically hindered corroles and 2.02 ± 0.06 V for six sterically hindered corroles that have bulky substituents at the meso-positions of the macrocycle. Slightly smaller average HOMO−LUMO gaps of 1.72 ± 0.02 and 1.84 ± 0.07 V were determined for the two series of neutral (Cor)H3 derivatives (nonsterically hindered and sterically hindered) in PhCN, while significantly decreased values of ΔE1/2 (1.55 V) were observed for the fully protonated [(Cor)H4]+ forms of the corrole. It was suggested that the sterically hindered corroles may have a more distorted macrocycle and are generally more difficult to oxidize and more difficult to reduce than the nonsterically hindered derivatives.148
Scheme 7. Proposed Mechanism for the Metal-Centered Reaction of Mn(III) and Mn(IV) Corroles in Pyridine Containing 0.1 M TBAPa 174
a
The initial compounds added to solution are shown in bold.
3.4. Highly Protonated Corroles [(Cor)H5]2+ and [(Cor)H6]3+
Free-base corroles with one or two meso-pyridyl substituents can be stepwise protonated in nonaqueous media by adding TFA to solution, and this leads to formation of [(Cor)H4]+, [(Cor)H5]2+, and [(Cor)H6]3+, as shown in Scheme 5, where the first proton addition(s) involve the meso-linked pyridyl groups and the last addition is at the core nitrogen of the macrocycle.150 The measured reduction potentials of the protonated corroles in Scheme 5 will vary with the overall charge of the molecule, and this is illustrated in Figure 9, which plots E1/2 for the first oneelectron addition vs the charge on the overall complex, which varies from −1 in the case of [(Cor)H2]− to +3 in the case of fully protonated [(Cor)H6]3+. The type and number of substituents on the meso-phenyl groups of the meso-pyridyl corrole will influence the exact potentials for a given compound, but when considering all of the data, one can conclude that each unit increase in positive charge on the corrole will result in an approximate 350 mV shift in potential toward an easier reduction.150 Figure 10. Cyclic voltammograms of (Mes2Ph)CorMnIII in PhCN containing (a) 0.1 M TBAP, (b) 0.1 M TBAP and 1.0 M TBACl, (c) 0.1 M OAc−, and (d) 0.1 M TBAP with added OH−. Adapted with permission from ref 125. Copyright 2008 American Chemical Society.
4. TRANSITION-METAL CORROLES 4.1. Group 6
4.1.1. Chromium. Air-stable chrominum corroles with three different central metal oxidation states have been character3389
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Chart 4. Electrochemically Examined Fe(III) and Fe(IV) Octaethylcorroles102,103
Table 5. Half-Wave Potential (V vs SCE) and Number of Electrons Transferredb by Bulk Electrolysis (shown in parentheses) per Mole of (OEC)M (M = Cu, Ni, Co, Mn) in DCM, 0.1 M TBAPa oxidation M Cu Ni Co Mnc
fourth
1.17 (1.0)
reduction
third
second
first
first
1.14 (1.0) 1.41 (1.0) 0.94 (1.0) 1.56 (1.0)
0.57 (0.5) 0.47 (0.5) 0.57 (0.5) 0.93 (1.0)
0.43 (0.5) 0.21 (0.5) 0.11 (0.5) 0.36 (1.0)
−0.34 (1.0) −0.20 (1.0) −0.30 (1.0) −1.58 (1.0)
Adapted with permission from ref 91. Copyright 1998 American Chemical Society. bCoulometric values given ±10%. cE1/2 values and coulometric data measured in PhCN. a
containing 0.1 M TBAP.163 The exact redox potentials varied with the substituent, and rather normal meso-substituent effects were obtained as in the case of most previously examined mesosubstituted porphyrins and corroles. The (oxo)chromium 5,10,15-tris(pentafluorophenyl)corrole, (F5Ph)3CorCrV(O), also undergoes a reversible oxidation and reduction in DCM, 0.1 M TBAPF6.162 The results of spectroelectrochemistry and ESR measurements indicated that (F5Ph)3CorCrV(O) is reversibly converted to its π-cation radical form [(F5Ph)3CorCrV(O)]+• upon oxidation at E1/2 = 1.24 V vs Ag/AgCl, while a reduction at 0.11 V in DCM was assigned as a metal-centered process to generate a Cr(IV) corrole after the reversible addition of one electron.162 The synthesized chromium nitridocorrole was assigned as possessing a Cr(V) center and a negative charge on the macrocycle, i.e., [(F5Ph)3CorCrV(N)]−, along with an associated salophene cation.161 Because of the single negative charge on the nitrido complex, it is harder to reduce (E1/2 = −0.03 V) and easier to oxidize (E1/2 = 0.56 V) than the neutral oxocorrole having the same (F5Ph)3Cor macrocycle (E1/2 = 0.17 for reduction and 1.10 V for oxidation) when examined under the same MeCN solution conditions. The difference in reduction potentials between the nitrido and oxo complexes in MeCN amounts to 200 mV, while a 540 mV difference in potential is seen upon oxidation. The
Scheme 8. Proposed Reduction Mechanism of (OEC)FeIVCl in PhCN Containing 0.1 M TBAP103
Scheme 9. Proposed Reduction Mechanism of (OEC)FeIII(py) in DCM Containing 0.1 M TBAP103
ized,144,159−162 examples being given by derivatives with CrVI(N), CrV(O), and CrIII(L)2 metal centers, where L = py or PPh3.162 (Ar)3CorCrV(O), where Ar is a CH3OPh, CH3Ph, Ph, or CF3Ph substituent on each meso-position of the corrole, was shown to display a reversible oxidation at E1/2 = 0.85−1.05 V and a reduction at E1/2 = −0.05 to −0.14 V vs SCE in DCM
Chart 5. Comparison of NO2 Substitution at the β-Pyrrole or meso-Phenyl Rings of an Iron Nitrosyl Corrole120
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The imido ligated chromium complexes, (F5Ph)3CorCrV(NMes) and (F5Ph)3CorCrV(NAr), where Mes = 2,4,6-(Me)3Ph and Ar = 2,4,6-Cl3Ph, also undergo a macrocycle-centered oxidation and metal-centered reduction to give the Cr(V) corrole π-cation radical and Cr(IV) forms of the compound, respectively.144 The Cr(V)/Cr(IV) reductions occur at E1/2 = −0.13 and −0.47 V for (F5Ph)3CorCrV(NAr) and (F5Ph)3CorCrV(NMes), respectively. The 340 mV easier reduction for the corrole possessing a 2,4,6-Cl3PhN axial ligand is consistent with the site of electron transfer being metalcentered. The potentials of the NMes and NAr corroles are also shifted negatively by 240−580 mV as compared to the same metal-centered redox process of (F5Ph)3CorCrV(O) (E1/2 = 0.11 V), thus reflecting the greater π-donating strength of the imido axial ligand.144 Three chrominum thienyl-containing meso-A2B-corroles were examined in MeCN containing 0.1 M TBAP.160 These compounds are represented as (A)2(B)CorCrV(O), where A = F5Ph and B = 2-thienyl, 3-thienyl, or 3-thianaphthyl. Similar to (F5Ph)3CorCrV(O), these corroles undergo two reversible oneelectron transfers, a macrocycle-based oxidation to give the corrole π-cation radical and a metal-based reduction that was assigned as a Cr(V)/Cr(IV) process. The reduction and oxidation potentials of the thienyl-containing corroles in this case are positively shifted by ∼140 and ∼200 mV as compared to (F5Ph)3CorCrV(O). Chiral meso-ABC chrominum(V) corroles (where A = F5Ph, B = BrPh, and C = 2-thianaphthyl group, C8H5S) were also examined in MeCN containing 0.1 M TBAP.164 The potential of the metal-centered Cr(V)/Cr(IV) process (E1/2 = 0.0 V vs Ag/ AgCl) is shifted negatively by 110 mV as compared to the same redox reaction of (F5Ph)3CorCrV(O), while the macrocyclebased oxidation of the chiral corrole (E1/2 = 1.25 V) is shifted negatively by 250 mV. These results are consistent with the stronger electron-withdrawing properties of the F5Ph groups as compared to BrPh or C8H5S.164 It might also be noted that the significantly smaller substituent effect observed for reduction as compared to oxidation is similar to what is seen when comparing the effect of the overall compound charge on redox reactions of the neutral corrole derivatives of CrV(O) and the anionic complex of CrV(N). 4.1.2. Molybdenum. The first electrochemistry of a molybdenum corrole was reported in 1981.95 One reduction and two oxidations were observed for (Me4Et4)CorMoV(O) in DCM containing 0.1 M TBAP. The one-electron reduction and one-electron oxidation were reversible and located at E1/2 = −0.72 and 0.70 V, respectively.95 On the basis of analysis of the UV−vis spectroelectrochemical and ESR data, it was suggested that both processes of the corrole are metal-centered and lead to the formation of Mo(VI) and Mo(IV) derivatives under the given solution conditions.95 No corrole macrocycle-centered reductions were reported in this study, which included a characterization of the related oxomolybdenum(V) porphyrins (TPP)Mo(O)X, where X = OMe−, OAc−, or Cl−. Twenty-five years later, two (oxo)molybdenum triarylcorroles containing meso-F5Ph or -Cl2Ph substituents were characterized by Gross and co-workers and a different site of electron transfer was proposed for the oxidation.165 (F5Ph)3CorMo(O) and (Cl2Ph)3CorMo(O) were reduced at E1/2 = −0.35 and −0.49 V, respectively, both vs an Ag/AgCl reference electrode in DCM. The reductions were assigned as metal-centered to give a Mo(IV) corrole, but oxidation of the F5Ph derivative, located at E1/2 = 1.32 V vs Ag/AgCl, was proposed to occur at the corrole
Figure 11. Cyclic voltammograms of (a) (NO2Ph)3CorFe(NO) and (b) 3,17-(NO2)2-(NO2Ph)3CorFe(NO) in DCM containing 0.1 M TBAP. Adapted with permission from ref 120. Copyright 2012 American Chemical Society.
Figure 12. Cyclic voltammogram of [(Et6Me2)CorRu]2 in (a) DCM and (b) pyridine containing 0.1 M TBAP. Adapted with permission from ref 194. Copyright 2002 Elsevier.
smaller effect of the corrole overall charge on the reduction potential is consistent with a metal-centered electron addition and a ring-centered electron abstraction. However, at the same time, the potential shift of 540 mV upon oxidation, as compared to the neutral oxocorrole, is much larger than expected and would suggest a highly localized negative charge on the macrocycle that is quite close to the site of electron abstraction. 3391
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Scheme 10. Proposed Oxidation/Reduction Mechanism of Ru(III) Dimers in DCM Containing 0.1 M TBAP194
Table 6. Productsa Identified upon the First Oxidation of Cobalt Corroles in Different Solvents Containing 0.1 M TBAPb compound
DCM (0.0)c
PhCN (11.9)
THF (20.0)
pyridine (33.1)
(Me6Et2)CorCo (Et)8CorCo (Me2Et2Ph4)CorCo (Me4Ph(p-MeOPh)4)CorCo (Me4Ph(m-MeOPh)4)CorCo (Me4Ph5)CorCo (Me4Ph7)CorCo
D D D D D D D
D D D M M M M
D N/A M M M M M
M M M M M M M
D = π−π radical cation dimer. M = monomer cation radical. N/A = no data available. bAdapted with permission from reference 215. Copyright 2003 American Chemical Society. cDonor number of the solvent in parentheses taken from ref 122. a
4.1.3. Tungsten. The electrochemical properties of three tungsten biscorroles, represented as [(XPh)3Cor]2W, where X = CF3, F, or CH3, were recently reported by Ghosh and coworkers,167 but no monomeric tungsten corroles have been reported to date. Each W biscorrole undergoes two reversible oxidations and two reductions in CH2Cl2, 0.1 M TBAP. The first oxidation was located at E1/2 = 0.21−0.45 V vs SCE, while the first reduction was at E1/2 = −0.88 to −1.11 V, the exact value depending upon the specific meso-phenyl ring substituent. The potential difference between the first reduction and first oxidation (the HOMO−LUMO gap) was about 1.3 V, a value smaller than the HOMO−LUMO gap of ∼2.2 V reported for monomeric metallocorroles, which undergo ring-centered oxidations and reductions. This could indicate a metal-centered contribution to the first reduction and/or first oxidation of the biscorrole, or it could be related to the increased number of πring systems in the molecule.167
Figure 13. Continuous-scan cyclic voltammograms of (a) (OEC)Co, (b) (OEC)Ni, and (c) (OEC)Cu in DCM, 0.1 M TBAP. Scan rate = 0.1 V/s. Adapted with permission from ref 91. Copyright 1998 American Chemical Society.
Scheme 11. Oxidation of (OEC)M in DCM, Where M = Co, Ni, or Cu91
macrocycle upon the basis of analysis of the redox potentials, the UV−vis spectrum of the oxidized species, and ESR data.165 The apparently different sites of oxidation for (Me4Et4)CorMo(O) and (F5Ph)3CorMo(O) could be related to the large differences in basicity between the two macrocycles and the significant difference in potentials for oxidation of the two corroles (1.32 V in one case165 and 0.70 V in another95). In a later study, a singly reduced oxomolybdenum(III) corrole, [(F5Ph)3CorMo(O)]−, was generated by chemical reduction and then spectroscopically examined by Gross and co-workers.166 An electrochemical generation of the same MoIII(O) species has not (yet) been reported, but the overall data in the literature is consistent with molybdenum corroles being synthesized or electrogenerated in four different metal oxidation states, +6, +5, +4, and +3.
4.2. Group 7
4.2.1. Manganese. Almost two dozen papers (refs 12, 26, 91, 98, 116−118, 125, 139, 140, 144, 147, and 168−182) have been published on the electrochemistry of manganese corroles since the initial electrochemical characterization of (OEC)Mn in 1998.91 Some of the published papers have elucidated the redox properties of air stable compounds with a Mn(III) center (refs 26, 91, 98, 116−118, 125, 139, 140, and 168−179), while others have involved characterization of synthesized corroles with Mn(IV) (refs 12, 26, 98, 140, 147, 174, 176, 177, and 180−182) or Mn(V) (refs 26, 144, and 170) central metal ions. Most of the elucidated redox processes have been oxidations, in large part because of the quite negative potential needed to access the 3392
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Chart 6. Structures of Electrochemically Examined Cobalt Corroles (R = OMe)215
Figure 15. Cyclic voltammograms of (a) (Me6Et2Ph)PorCo, (b) (PCO)Co2, and (c) (Me4Ph5)CorCo in PhCN, 0.1 M TBAP. Adapted with permission from ref 227. Copyright 2002 American Chemical Society.
Scheme 13. Proposed Reduction Mechanism of (OMC)RhIII(PPh3) in PhCN Containing 0.1 M TBAP105
Chart 7. Structures of β-Substituted Monomeric and Dimeric Ni Isocorroles234
Figure 14. Cyclic voltammograms of (F5Ph)Mes2CorCo in (a) DCM, (b) THF, (c) PhCN, and (d) pyridine containing 0.1 M TBAP. Adapted with permission from ref 201. Copyright 2008 American Chemical Society.
Scheme 12. Proposed Reduction and Reoxidation Mechanism of (Ar)3CorCoIII(PPh3) in DCM and DMF133
singly reduced form of the compounds. A brief summary of some of these papers is given in the following pages. As mentioned above, the first detailed electrochemistry of a manganese corrole was published in 1998 and involved a study of (OEC)Mn, the redox behavior of which was compared to derivatives of (OEC)Co, (OEC)Cu, and (OEC)Ni under the same solution conditions.91 The formal metal oxidation state of the central metal ion in (OEC)Mn is Mn(III), but an earlier ESR 3393
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Chart 9. Structures of Investigated Cu Corroles88
Chart 8. Schematic Structures of Investigated Pt Corroles of the Types (a) (Ar)3CorPtIV(R)(PhCN) and (b) (Ar)3Cor+•PtIV(R)(R′)235
and 1H NMR characterization of this compound had assigned the corrole as having a Mn(II) center and a positively charged macrocycle, i.e., (OMC+•)MnII, when in the presence of nitrogenous bases.183 An electronic configuration of (OMC+•)MnII should facilitate dimerization of the neutral or oxidized compound via π−π bonding, but (OEC)Mn does not dimerize in the electrochemically utilized solvents, as is the case upon oxidation of (OEC)Cu91 and (OEC)Ni,91 the latter of which had earlier been described as a Ni(II) corrole π-cation radical,184 i.e., (OEC+•)NiII. Thus, the electronic configuration assignment of (OEC+•)MnII for the neutral corrole is inconsistent with the proposed site of oxidation, which was given as metal-centered to generate a Mn(IV) corrole product (see the discussion below), as well as with the lack of dimerization of (OEC)Mn in nonaqueous media. The monomeric (OEC)Mn in PhCN was characterized by a reversible one-electron reduction at −1.58 V and two welldefined one-electron oxidations at 0.36 and 0.93 V vs SCE.91 The singly oxidized corrole was assigned as [(OEC)MnIV]+ based on ESR characterization of the first oxidation product. A third irreversible oxidation of (OEC)Mn was also observed at Ep = 1.56 V in this solvent, and thus, the last two oxidations would be associated with formation of a Mn(IV) corrole π-cation radical and dication, respectively. No oxidation state assignment was made for singly reduced [(OEC)Mn]− in the initial study, but a later more detailed electrochemical investigation of this compound by Ou et al.98 provided indirect evidence for a macrocycle-centered reduction, as opposed to a Mn(III)/Mn(II) process, which is well-
Figure 16. Cyclic voltammograms showing the reduction of (Ph)3CorCu at −60 °C and β-NO2(Ph)3CorCu in DCM containing 0.1 M TBAP (ring red = ring reduction).
characterized and relatively facile for porphyrins75,77 and related metallomacrocycles. In the follow-up study by Ou and co-workers, it was pointed out that (OEC)MnIII and (OEC)MnIII(py) undergo reductions at virtually identical E1/2 values of −1.66 and −1.65 V in pyridine, and the same two compounds are also reduced at E1/2 = −1.58 V and Epc = −1.66 V, respectively, in PhCN containing 0.1 M TBAP.98 These reduction potentials are substantially more negative than potentials for the proposed metal-centered redox reaction of (OEC)FeIII(py) in DCM (−1.04 V) or pyridine (−0.94 V),103 but they are virtually identical with E1/2 values for the ring-centered reductions of (OEC)AsIII (−1.67 V) and (OEC)SbIII (−1.66 V) in PhCN.100 This also suggests that a Mn(II) corrole is not electrogenerated upon reduction but rather that a Mn(III) corrole π-anion radical is formed after a oneelectron addition in nonaqueous media. In the initial study of (OEC)Mn by Kadish, Vogel, and their co-workers, it was stated that the first oxidation of (OEC)Mn
Table 7. Half-Wave Potentials (V vs SCE) for (Ar)3CorPtIV(R)(PhCN) and (Ar)3Cor+•PtIV(R)(R′) in DCM Containing 0.1 M TBAP, where Ar = p-XPh (scan rate = 0.1 V/s)235 oxidation
reduction
formal charge on metal
abbrev compd notation
X
2nd
1st
1st
Pt(IV)
(Ar)3CorPtIV(R) (PhCN)
CF3 H Me OMe CF3 H Me OMe
1.40 1.29 1.25 1.14
0.72 0.63 0.57 0.53 0.97 0.88 0.79 0.74
−0.83 −0.84 −0.85 0.21 0.09 0.03 0.01
Pt(V)
(Ar)3Cor+·PtIV(R)(R′)
3394
2nd
HOMO−LUMO gap (V)
−0.77 −0.82 −0.86 −0.87
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Scheme 14. Proposed Reduction Mechanism of (a) β-NO2-Substituted Copper Triarylcorrole and (b) the Related Non-NitroSubstituted Copper Corrole in DCM Containing 0.1 M TBAPa
a
Potentials are listed for reactions of (MePh)3CorCu and β-NO2(MePh)3CorCu.
Scheme 15. Proposed HOMO−LUMO Gap for Cu(II) Corroles
Scheme 16. Reduction Mechanism for Silver Corroles in Pyridine135
Table 8. Half-Wave Potentials (V vs SCE) for (p-XPh)3CorM (M = Cu78 and Ag84) in DCM Containing 0.1 M TBAP (scan rate = 0.1 V/s) first oxidation
first reduction
X
Cu
Ag
Cu
Ag
CF3 H Me MeO
0.89 0.76 0.70 0.65
0.91 0.73 0.69 0.66
−0.08 −0.20 −0.23 −0.24
−0.78 −0.86 −0.88 −0.91
Scheme 17. Proposed Electron Transfer Mechanism for Cu and Ag Corroles
Scheme 18. Proposed Electron Transfer Mechanism for Cu, Ag, and Au Corroles
Figure 17. Cyclic voltammograms of (MePh)3CorAgIII in pyridine containing 0.1 M TBAP. The electrode reactions involving free AgI and Ag0 are indicated by an asterisk. Adapted with permission from ref 135. Copyright 2009 American Chemical Society.
“unambiguously leads to a Mn(IV) complex as ascertained by the ESR spectrum”.91 Assignments of a Mn(IV) oxidation state were also made in a follow-up study of (OEC)Mn by Ou and coworkers,98 as well as in a number of later studies from other laboratories involving a characterization of manganese corroles with different skeletal structures (see refs 12, 26, 98, 140, 147, 174, 176, 177, and 180−182). However, if the examined (OEC)
Mn derivative were actually present in solution as a Mn(II) πcation radical, i.e., (OEC+•)MnII, then the reductions and oxidations in nonaqueous media might both involve the 3395
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−0.23 and 0.36 V)104 under the same solution conditions. This could speak toward the lack of a metal-centered reaction for (OEC)Mn(Ph)98 and the occurrence of this type of reaction in the case of the analogous Fe and Co derivatives. However, it should also be noted that the ΔE1/2 between the reduction and oxidation processes (the HOMO−LUMO gap) of (OEC)Sn(Ph) is 2.21 V in DCM (see later discussion), and here it is clear that no metal-centered reactions are observed for this maingroup compound.101 Finally, note should be taken of the magnitude of ΔE1/2 between the reversible first and second oxidations of (OEC)M(Ph) as compared to separations in potential between the same electrode reactions of the four-coordinate (OEC)M or fivecoordinate (OEC)M(Ph) derivatives, where M = CoIII, FeIII or SnIV. The measured separation in E1/2 between the first two oxidations of (OEC)Mn(Ph)98 is 560 mV, a value that is virtually identical to the ΔE1/2 of 570 mV for (OEC)Mn98 and 600 mV for (OEC)Sn(Ph)101 in PhCN, but significantly smaller than the ΔE1/2 between the first two reversible oxidations of (OEC)Fe(Ph) (730 mV) or (OEC)Co(Ph) (870 mV) under similar solution conditions.98 This may or may not be related to differences in the site of electron transfer in the different compounds. The effect of solvent and central metal oxidation state on the redox potentials of manganese corroles in DCM, MeCN, and pyridine was investigated in 2014 by the Kadish and Ou groups for two related series of compounds with meso-triarylcorrole macrocycles.174 The synthesized compounds were represented as (Ar)3CorMnIII and (Ar)3CorMnIVCl, where Ar represents pClPh, p-FPh, Ph, or p-MePh. Each neutral Mn(III) corrole exists as a four-coordinate complex in DCM or MeCN and as a five-coordinate species in pyridine. The (Ar)3CorMnIII derivatives undergo two oxidations in these solvents, the first of which generating Mn(IV) corrole and the second a Mn(IV) π-cation radical. The corroles also undergo a single reduction to give what was postulated to be a Mn(II) corrole,174 although this assignment has never been spectroscopically confirmed. The initial (Ar)3CorMnIVCl derivatives also exhibited one oxidation in the two nonaqueous solvents to generate a Mn(IV) π-cation radical, while Mn(III) forms of the corroles were said to be stepwise formed after electroreduction. The combined mechanism that was proposed for oxidation and reduction of the two series of Mn(III) and Mn(IV) corroles is given in Scheme 6, where the synthesized electroactive compounds added to solution were (Ar)3MnIII or (Ar)3MnIVCl.170 Somewhat different electrochemical behavior is seen for the above Mn triarylcorroles in pyridine, where five-coordinate complexes are formed in solutions containing 0.1 M TBAP. Each corrole exhibits reversible well-defined reductions at −1.30 to −1.37 V, but more complicated behavior is seen upon oxidation. The proposed oxidation and reduction mechanism for the Mn(III) and Mn(IV) triarylcorroles in pyridine is given in
Table 9. Half-Wave Potentials (V vs SCE) for Br8(p-XPh)3CorM (M = Cu,78 Ag,81 and Au188) in DCM Containing 0.1 M TBAP (scan rate = 0.1 V/s) first oxidation
first reduction
X
Cu
Ag
Au
Cu
Ag
Au
CF3 H Me MeO
1.24 1.14 1.12 1.10
1.34 1.27 1.25 1.21
1.41 1.29 1.27 1.25
0.25 0.12 0.07 0.04
−0.20 −0.31 −0.35 −0.37
−0.90 −1.02 −1.00 −1.09
conjugated macrocycle and lead to an initial formation of [(OEC)MnII]− and [(OEC2+)MnII]+ prior to generation of a final Mn(III) and Mn(IV) corrole, respectively. No evidence has been presented for these types of reactions, but it is important to point out that the absolute difference in potentials between the reversible first reduction and first oxidation of (OEC)Mn at −1.58 and +0.36 V in PhCN is equal to 1.94 V, a separation almost identical to the HOMO−LUMO gaps of many free-base and copper corroles that are described in other sections of this review. As indicated above, the first one-electron-oxidation product of (OEC)MnIII was assigned as containing a Mn(IV) ion, and the same assignment was given in a follow-up study of this compound by Ou et. al,98 who reported the effect of axial ligation on the proposed Mn(III)/(IV) and Mn(IV)/(III) redox reactions of (OEC)Mn, (OEC)MnCl, and (OEC)Mn(Ph). The Mn carbon-bonded compound has no parallel in the area of porphyrins, where σ-bonded manganese derivatives are unknown. The fact that it is found in the corrole series would be consistent with a Mn(IV) oxidation state if it were not for the possible presence of a noninnocent OEC ligand in a number of transition-metal corroles having this macrocycle, most notably OEC derivatives with Fe,7,185 Cu,91 and Ni184 metal centers. The electrochemically examined (OEC)Mn(Ph) exhibits two reversible oxidations at 0.59 and 1.15 V in PhCN. There is also a quasi-reversible reduction at Ep = −1.15 V and a reversible reduction at E1/2 = −1.89 V. The first reduction is followed by a chemical reaction that involves a cleavage of the metal−carbon bond, as indicated by UV−visible spectra of the singly reduced compound that has bands at 392, 466, and 592 nm as compared to 392, 470, and 590 nm for neutral unreduced (OEC)Mn under the same solution conditions.98 However, no new reduction peaks were seen in the cyclic voltammograms of (OEC)Mn(Ph), which could be associated with electrogenerated (OEC)Mn. This suggests that (OEC)Mn is formed on the spectroelectrochemistry time scale but not on the CV time scale, where an uncharacterized reduction product is observed. With respect to the cyclic voltammogram of (OEC)Mn(Ph) in PhCN,98 it is also important to point out that the ΔE1/2 of 1.74 V between the first reduction at −1.15 V and the first oxidation at 0.59 V is much larger than the ΔE1/2 of 1.08 V for (OEC)Fe(Ph) (E1/2 = −0.61 and 0.47 V)103 or 0.59 V for (OEC)Co(Ph) (E1/2 =
Table 10. First Reduction and First Oxidation Potentials (V vs SCE) of (p-XPh)3CorM and Br8(p-XPh)3CorM (M = Cu,78 Ag,81 and Au188) in DCM Containing 0.1 M TBAP (scan rate = 0.1 V/s) first oxidation potential
first reduction potential
M
(Ph)3CorM
Br8(Ph)3CorM
ΔE1/2 (mV)
(Ph)3CorM
Br8(Ph)3CorM
ΔE1/2 (mV)
Cu Ag Au
0.76 0.73 0.80
1.14 1.27 1.29
420 540 490
−0.20 −0.86 −1.38
0.12 −0.31 −1.02
320 550 360
3396
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for copper18,84,143 and several other transition-metal corroles.7,12,63,81,84,134,142,147,187 The first reduction of the investigated oxorhenium(V) corroles in DCM ranged from −1.16 to −1.29 V, while the first oxidation ranged from 0.93 to 1.10 V vs SCE. The electrochemical HOMO−LUMO gap for these compounds averaged 2.24 ± 0.02 V. As described later in this review, similar HOMO−LUMO gaps (2.20 ± 0.10 V) are seen for OEC derivatives of PV and other group 15 metallocorroles with MIII metal ions (section 5.3) as well as for (Ph)3CorAu and Br8(Ph)3CorAu, where the electrochemically measured gaps in DCM were reported to be 2.18 and 2.31 V, respectively (section 4.6.3).188
Scheme 7, where (Ar)3CorMnIII(py) (shown in bold) was said to be initially oxidized to [(Ar)3CorMnIV(py)]+ at a potential of 0.45−0.53 V, the exact value depending upon the specific mesosubstituents. This electrochemical reaction was then followed by the rapid addition of a second pyridine molecule to give [(Ar)3CorMnIV(py)2]+, which was rereduced at 0.30−0.38 V on the reverse scan to give a transient six-coordinate (Ar)3CorMnIII(py)2 species prior to loss of one pyridine axial ligand and re-formation of the original five-coordinate complex. The synthesized chloride-bound Mn(IV) corroles were irreversibly reduced at peak potentials of 0.22−0.32 V in pyridine to give [(Ar)3CorMnIIICl]−, but the anionic product was unstable in pyridine and underwent replacement of the axially bound Cl− by a py molecule, leading to formation of neutral (Ar)3CorMnIII(py), which was reoxdized at 0.42−0.49 V, prior to undergoing a ligand exchange reaction (an electrochemical EC mechanism) to re-form the initial Mn(IV) corrole, as shown in Scheme 7.174 Axial coordination of anions to the Mn(IV), Mn(III), and Mn(II) forms of the corrole were shown to affect the redox potentials. The anions Cl−, OAc−, CN−, OH−, and SCN− were found to stepwise bind to the neutral corrole to give the five- and six-coordinate complexes in nonaqueous media.125 In each case, the corroles with one or two bound anionic axial ligands exhibited an easier oxidation and a harder reduction as compared to the uncomplexed four-coordinate species. This is shown by the cyclic voltammograms illustrated in Figure 10 for (Mes2Ph)CorMn in PhCN containing 0.1 M ClO4− and 0.1 M OAc−, OH−, or Cl−. A series of homobimetallic manganese cofacial porphyrin− corrole dyads were also investigated as to their electrochemistry, spectroelectrochemistry, and ligand-binding properties in nonaqueous media.173 The investigated compounds are represented as (PMes2CorY)Mn2Cl, where P is a dianion of the porphyrin macrocycle, Mes2Cor is a trianion of the corrole containing one meso-phenyl and two meso-mesityl groups, and Y is a spacer such as 9,9-dimethylxanthene, anthracene, dibenzofuran, or diphenyl ether. The Mn(III) porphyrin part of the dyad undergoes two one-electron reductions in pyridine and benzonitrile, the first of which involves a Mn(III)/Mn(II) process, while the second electron addition was proposed to involve the conjugated π-ring system of the macrocycle. The Mn(III) corrole part of the dyads also exhibits two redox processes, one proposed to involve a Mn(III)/Mn(II) process and the other Mn(III)/Mn(IV) under the same solution conditions. The redox behavior of these dyads was examined with respect to the effect of solvent, type and size of the spacer separating the two macrocycles, and the axial ligand. An acetate ion bound within the cavity of the dyads in PhCN solutions of TBAOAc was proposed to remain bound to the corrole and porphyrin parts of the molecule after the stepwise addition or abstraction of electrons under the application of a preselected reducing or oxidizing potential. An intermolecular chloride ion exchange was also reported to occur between the porphyrin and the corrole after the Mn(III)/Mn(IV) oxidation process of the corrole in the dyads that initially contained a Mn(III) porphyrin with an axially bound chloride ion.173 Additional details on these reactions can be found in the original publication.173 4.2.2. Rhenium. The electrochemistry of oxorhenium corroles of the form (Ar)3CorReV(O) has been reported where Ar = CF3Ph, FPh, Ph, MePh, or MeOPh.186 The compounds were assigned as having an innocent (corrole)3− macrocycle, as opposed to a noninnocent (corrole)2− macrocycle as in the case
4.3. Group 8
4.3.1. Iron. The electrochemistry of iron corroles has attracted much attention over the last 2 decades (see refs 102, 103, 115, 120, 134, 140, 142, 147, 177, 180, and 189−192) due in part to their similarity to iron porphyrins, in part to their rich electrochemistry, and in part to their wide range of potential applications. Some reports on iron corrole electrochemistry have been limited to describing only the first reduction and first oxidation of the compounds, while others have demonstrated that iron corroles can undergo multiple reductions and oxidations in nonaqueous media, some of which involve the central metal ion and some of which involve the macrocycle. The exact number of redox reactions exhibited by a given iron corrole will depend upon the number and type of meso- and β-pyrrole substituents (see refs 102, 120, 134, 142, 147, 177, and 189), the type and number of axial ligands (see refs 102, 103, 115, 120, 134, 140, 142, 147, 180, and 189−191), the oxidation state of the central metal ion in the initial compound (see refs 102, 103, 115, 120, 134, 140, 142, 147, 180, and 189−191), and the solution conditions (see refs 102, 103, 180). Five-coordinate iron octaethylcorroles have been characterized as Fe(IV) or Fe(III) complexes with NO, Ph, Cl−, or pyridine axial ligands,103 examples being given in papers by Kadish, Vogel, and their co-workers for the octaethylcorrole derivatives comprising (OEC)FeIVCl, (OEC)FeIII(py), and (OEC)FeIV(Ph) in one paper103 and [(OEC)Fe(NO)] and [(OEC)Fe(NO)]+ in the other.102 Structures of the examined iron corroles are shown in Chart 4. (OEC)FeIVCl was shown to undergo one reversible reduction at E1/2 = −0.07 V and one irreversible reduction at Epc = −1.45 V in PhCN, 0.1 M TBAP, the irreversibility being due to a chemical reaction following electron transfer (Cl− dissociation from the electrogenerated iron(III) corrole as shown in Scheme 8). Once generated, the four-coordinate Fe(II) corrole could be reoxidized back to its four-coordinate Fe(III) form at Ep = −0.62 V for a scan rate of 0.1 V/s, and this irreversible one-electron oxidation was then followed by a rapid reassociation of the chloride axial ligand to give back the initial compound. Under the same solution conditions, the Fe(IV) σ-bonded corrole (OEC)FeIV(Ph) undergoes two reversible one-electron reductions, at E1/2 = −0.61 and −1.98 V, and three oxidations at 0.47, 1.20, and 1.73 V, the first two of which are reversible.103 The measured HOMO−LUMO gap is 1.08 V. One irreversible reduction is observed for (OEC)FeIII(py) in DCM. This process is located at Epc = −1.04 V vs SCE for a scan rate of 0.1 V/s. The final product of (OEC)FeIII(py) reduction in DCM was assigned as [(OEC)FeII]−, which is reoxidized back to the initial five-coordinate form of the corrole at −0.60 V on the reverse scan, as shown by the proposed mechanism in Scheme 9.103 The measured peak potential for the irreversible reoxidation 3397
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of [(OEC)FeII]− to (OEC)FeIII is essentially the same in DCM and PhCN, which would be expected if both oxidation states of the corrole lacked a bound axial ligand (as shown in Schemes 8 and 9). The fact that the FeIII/FeII reaction of four-coordinate OEC is shifted from −0.60/−0.62 V in DCM or PhCN to −1.04 V for (OEC)Fe(py) in DCM and −0.94 V in pyridine is consistent with the strong binding of pyridine to the Fe(III) center of the corrole and an electrode reaction involving the reduction of fivecoordinate (OEC)Fe(py) to five-coordinate [(OEC)Fe(py)]− prior to loss of the axial ligand after electroreduction.103 (OEC)FeIII(NO) is characterized by two one-electron reductions and three one-electron oxidations in PhCN or DCM.102 Both reductions are reversible and located at E1/2 = −0.41 and 1.92 V in PhCN. The first two oxidations of (OEC)Fe(NO) are also reversible and located at E1/2 = 0.61 and 1.14 V. Easier reductions (E1/2 = −0.35 and −1.74 V) and harder oxidations are observed for a related iron triarylcorrole nitrosyl, (p-MeOPh)3CorFe(NO),120 but the oxidations of this compound in PhCN are overlapped in potential (E1/2 = 0.82 V) and shifted positively by 210 mV as compared to the iron OEC derivative with a nitrosyl axial ligand. It has long been known that the difference in half-wave potentials between ring-centered electrode reactions of octaethylporphyrins (OEP) and tetraphenylporphyrins (TPP) having the same metal ion and axial ligands ranges from 200 to 250 mV, with harder (more positive potential) oxidations and easier (more positive potential) reductions occurring for the TPP derivatives.75,77 To our knowledge, a similar comparison has not been made between potentials for ring-centered reactions of metallocorroles with OEC and TPC macrocyclic ligands. However, the positive shifts in potential of 180 and 210 mV for the second reduction and the first oxidation of the TPC corrole (as compared to OEC) are both within the range of shifts observed for similar metalloporphyrins with OEP and TPP macrocycles, which might suggest a ring-centered process in both series of nitrosyl corroles. At the same time, the smaller potential difference of 60 mV between E1/2 values for the first reduction of the OEC and TPC nitrosyl derivatives (−0.41 and −0.35, respectively) might be taken as indirect evidence for a metalcentered redox process in both cases, i.e., the conversion of an Fe(III) nitrosyl corrole to its Fe(II) form. Further insight into the redox behavior of structurally similar iron corroles and porphyrins is given by a comparison of oxidation potentials for two series of five-coordinate compounds, one with a single nitrosyl axial ligand and the other with a single σ-bonded phenyl axial ligand. This comparison involves (OEC)FeIII(NO) and (OEP)FeII(NO) on the one hand and (OEC)FeIV(Ph) and (OEP)FeIII(Ph) on the other. The relevant electrode reactions are given by eqs 5 and 6 for the iron nitrosyl compounds and eqs 7 and 8 for the Fe−carbon bonded derivatives.
−e
(OEP)Fe III(Ph) HoooooI [(OEP)Fe IV (Ph)]+ 0.48 V −e
HoooooI [(OEP)Fe IV (Ph)]2 +
1.30 V
Remarkably, almost identical potentials are observed for the first oxidation in each series, i.e., 0.60 or 0.61 V for electron abstraction from (OEC)FeIII(NO) and (OEP)FeII(NO) and 0.47 or 0.48 V for electron abstraction from (OEC)FeIV(Ph) and (OEP)FeIII(Ph). This similarity in E1/2 values can be rationalized by the presence of a noninnocent corrole macrocycle and assignment of the redox active forms of the corrole as (OEC•+)FeII(NO) and (OEC•+)FeIII(Ph), respectively. Under these conditions, the first oxidation in each series would be metal-centered, Fe(II)/Fe(III) in the case of the two nitrosyl compounds (eqs 5 and 6) and Fe(III)/Fe(IV) in the case of the σ-bonded derivatives (eqs 7 and 8). The two structurally similar macrocycles, OEC+• and OEP, would have identical charges, and thus, one might expect similar oxidation potentials, as is observed. As is the case for other metallocorroles, easier reductions and harder oxidations are observed for iron triarylcorroles containing electron-withdrawing meso substituents, while a reverse trend is seen for corroles with electron-donating meso substituents. For example, the first reduction and first oxidation of (pCF3Ph)3CorFeCl are located at E1/2 = 0.19 and 1.18 V, respectively, in DCM, 0.1 M TBAP, and these potentials are positively shifted by 110−140 mV as compared to potentials for the first reduction and first oxidation of (Ph)3CorFeCl. Similar trends in potential shifts are seen when a comparison is made between a series of (Ar)3CorFe(NO) complexes with different substituents (Ar = F5Ph, CF3Ph, Ph, MePh, or MeOPh).142 The effect of adding NO2 substituents at the β-pyrrole positions of a metallocorrole is described in an earlier section of this review (section 2.3.1). As seen in Chart 5, a 190 mV positive shift in E1/2 for the first reduction and first oxidation is seen upon addition of one nitro group at a β-pyrrole position of the illustrated iron triarylcorrole, and almost the same shifts in potential are seen when adding one NO2 group to each of the three meso-phenyl rings of the corrole in place of the three OMe substituents.120 This result indicates that the substituent effect of a single nitro group at the β-pyrrole of the corrole is similar to that for addition of three nitro groups to the meso-phenyl rings of the compound. The reduction of corroles with meso-PhNO2 substituents involves not only the central metal ion and conjugated macrocycle but also the electroactive PhNO2 groups on the meso-phenyl positions of the compound (see earlier discussion on free-base corroles with meso-nitrophenyl substituents). One example of this behavior is given by the cyclic voltammogram of (NO2Ph)3CorFe(NO) in DCM containing 0.1 M TBAP (Figure 11a). A one-electron reduction of the mesoPhNO2 groups occurs at E1/2 = −1.08 V, and as expected, the peak current for this process is approximately 3 times that of the preceding Fe(III)/Fe(II) reaction at −0.14 V vs SCE. This result is consistent with a one-electron addition in the first reduction and three overlapping one-electron additions in the second, where the overall reduction processes is described by eqs 9 and 10.
−e
(OEC)Fe III(NO) HoooooI [(OEC)Fe(NO)]+ 0.61 V
(5)
−e
(OEP)Fe III(NO) HoooooI [(OEP)Fe III(NO)]+ 0.60 V
(6)
−e
(OEC)Fe IV (Ph) HoooooI [(OEC)Fe IV (Ph)]+ 0.47 V −e
HoooooI [(OEC)Fe (Ph)]
1.20 V
IV
(8)
+e
(NO2 Ph)3 CorFe III(NO) HoooooooI [(NO2 Ph)3 CorFe II(NO)]−
2+
−0.14 V
(7)
(9) 3398
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larger than the ΔE1/2 of 1.05 V between the first reduction and first oxidation of (OEC)Fe(Ph) in DCM.103 Comparisons were not made between the redox behavior of [(Ar)3CorFe]2O and μ-oxo dimers of other metallomacrocycles, but it should be pointed out that obtaining facile reductions of the corrole μ-oxo dimers is quite different than what is seen for related μ-oxo dimers of porphycenes or porphyrins,193 where the reductions are relatively difficult, presumably because of different sites of electron transfer (and the innocence of the macrocyclic ligand). 4.3.2. Ruthenium. Two metal−metal bonded Ru(II) dimers were investigated by Kadish, Guilard, and their co-workers in 2002.194 The investigated compounds exhibited three oxidations, the first two of which involved reversible one-electron abstractions located at E1/2 = 0.14 and 0.55 V for [(Et6Me2)CorRuIII]2 and at E1/2 = 0.13 and 0.55 V for [(Me6Et2)CorRuIII]2. The third oxidation of the metal−metal-bonded dimers is located at almost identical half-wave potentials of 1.14 and 1.13 V vs SCE in DCM containing 0.1 M TBAP, and these involve two overlapping one-electron-transfer steps, as evidenced by a doubling of the peak current as compared to the other process for oxidation or reduction of the dimer. As seen in Figure 12a, only a single reversible one-electron reduction is observed for [(Et6Me2)CorRu]2 in DCM at room temperature, but two well-defined reductions are seen in this solvent at −75 °C. Two one-electron reductions are also seen in pyridine at room temperature (again providing evidence for the need to characterize “difficult” reductions in multiple solvents). UV−vis spectroelectrochemical and ESR data of the diruthenium dimers suggest that the first two one-electron oxidations, as well as the first and second one-electron reductions, all involve the metal center, as shown in Scheme 10. The overall two-electron oxidation at E1/2 = 1.14 V vs SCE was proposed to be macrocycle-centered, leading to formation of a Ru2(IV,IV) species that has two singly oxidized corrole rings.194 Two monomeric Ru(III) triarylcorroles were electrochemically characterized by Gross and co-workers,195 who also synthesized a metal−metal-bonded dimer of (F5Ph)3CorRuIII. The monomeric (F5Ph)3CorRuIII(NO) and (Cl2Ph)3CorRuIII(NO) complexes exhibited two reversible reductions and one reversible oxidation in MeCN. Each reduction involved a one-electron transfer at E1/2 = −0.42 and −0.50 V vs Ag/AgCl, respectively. The two oxidations also involved a one-electron abstraction and occurred at E1/2 = 0.72 and 0.57 V, respectively. The first reduction and first oxidation were both assigned to metal-centered redox reactions.195 The [(F5Ph)3CorRuIII]2 dimers synthesized by Gross and coworkers195 exhibited two reversible reductions at E1/2 = −0.48 and −1.21 V and three reversible oxidations at E1/2 = 0.76, 1.04, and 1.45 V vs Ag/AgCl in MeCN. Half-wave potentials for the first oxidation and two reductions of the dimer were similar to E1/2 values of the (F5Ph)3CorRuIII(NO) monomer, suggesting to the authors that the Ru−Ru and the Ru−NO bonds exerted a similar electronic effect on the metal. The first one-electron reduction and first one-electron oxidation of the dimer with meso-F5Ph substituents were both proposed to occur at the dimetal center, and these processes were located at half-wave potentials of −0.48 and 0.76 V, respectively.195 The reversible first oxidation potential of [(F5Ph)3CorRuIII]2 at E1/2 = 0.76 V is not only almost identical to the first oxidation potential of (F5Ph)3CorRuIII(NO) at 0.72 V but it is also almost identical to the E1/2 for the first oxidation of three other mononuclear (F5Ph)3CorM derivatives that lacked an NO axial
[(NO2 Ph)3 CorFe II(NO)− +3e
HoooooooI [(NO2 Ph)3 CorFe II(NO)]4 −
(10)
−0.18 V
II
−
This addition of 3e to [(NO2Ph)3CorFe (NO)] as shown in eq 10 is consistent with a lack of any interaction between the three meso-PhNO2 groups of the corrole, each of which is reduced by one electron at the same half-wave potential of −1.08 V vs SCE. In contrast to the above, three well-separated one-electron reductions of PhNO2 are observed for the iron nitrosyl corroles in Chart 5 containing three meso-PhNO2 substituents and one or two β-NO2 groups. This electrochemical behavior is illustrated in Figure 11b for 3,17-(NO2)2-(NO2Ph)3CorFe(NO), where the three reversible PhNO2 reductions are located at E1/2 = −0.78, −1.05, and −1.21 V in DCM. Thus, the first four reductions of this corrole can be described by eqs 11−14.120 (NO2 )2 (NO2 Ph)3 CorFe III(NO) +e
HoooooI [(NO2 )2 (NO2 Ph)3 CorFe II(NO)]− 0.27 V
(11)
[(NO2 )2 (NO2 Ph)3 CorFe III(NO)]− +e
HoooooooI [(NO2 )2 (NO2 Ph)3 CorFe II(NO)]2 − −0.78 V
(12)
[(NO2 )2 (NO2 Ph)3 CorFe II(NO)]2 − +e
HoooooooI [(NO2 )2 (NO2 Ph)3 CorFe II(NO)]3 − −1.05 V
(13)
[(NO2 )2 (NO2 Ph)3 CorFe II(NO)]3 − +e
HoooooooI [(NO2 )2 (NO2 Ph)3 CorFe II(NO)]4 − −1.21 V
(14)
The electrochemistry of three iron μ-oxo dimers of mesosubstituted corroles were also examined in DCM, 0.1 M TBAP, with one goal of this study being to elucidate substituent effects of the meso-phenyl groups on the redox potentials while also confirming the noninnocence of the corrole ligand.134 The examined compounds, represented as [(Ar)3CorFe]2O, were shown to undergo three one-electron oxidations and two oneelectron reductions, all of which were relatively facile. For example, peak potentials for two reductions of the unsubstituted parent [(Ph)3CorFe]2O dimer were located at −0.323 and −0.748 V, while the three oxidations were located at Epc values of 0.635, 1.018, and 1.298 V vs SCE in DCM. The measured potentials were examined in terms of linear free energy relationships between E1/2 and the sum of the meso-phenyl substituent constants (see eq 1), giving slopes of ΔE1/2/ΔΣσ not so different than what was observed for two series of monomeric Mn and Fe meso-substituted corroles also examined in the same study.134 Although not mentioned in the paper by Ghosh and coworkers,134 the difference in potential between the average of the two one-electron reductions of the dimers and the dimer’s first oxidation potential is almost exactly the same as the potential difference between the first reduction and first oxidation of the corresponding (Ar)3CorFeCl monomers having the same mesophenyl substituents, in this case, p-Me, p-H, or p-CF3. The measured ΔE1/2 values between the oxidation and reduction processes were close to 1.16 V in each case, which is slightly 3399
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ligand, namely, complexes with M = MnIII (0.71 V),139 RhIII(py)2 (0.72 V),196 and IrIII(PPh3)2 (0.72 V).196 This might at first be explained by an oxidation at the corrole macrocycle for all five compounds, but it should be noted that metal-centered processes were assigned for four of the five corroles, the one exception being for M = Rh(py)2.196 4.3.3. Osmium. Several nitrido osmium(VI) corroles were electrochemically characterized by Ghosh and co-workers.197 These compounds, represented as (Ar)3CorOs(N), where Ar = p-CF3Ph, Ph, p-MePh, or p-MeOPh, exhibited two reversible oxidations and one reduction in DCM containing 0.1 M TBAP. The first oxidation was located at E1/2 values between 0.83 and 1.02 V vs SCE, while the first reduction was located at a half-wave potentials between −1.19 and −1.33 V. The electrochemical HOMO−LUMO gap of these Os(VI) complexes ranged from 2.15 to 2.21 V and was equal to 2.19 V in the case of (Ph)3CorOs(N).197 This separation is similar to the gap reported for a number of triphenylcorroles, examples being given for derivatives of (Ph)3CorM, where M = ReIV(O) (2.24 V),186 AuIII (2.19 V),188 and AlIII(Py)2 (2.13 V).139
that (OEC)Co(Ph)104 and (OEC)Fe(NO),102 as well as other OEC derivatives, form π−π dimers in the solid state.6 The structurally characterized cobalt−carbon-bonded (OEC)Co(Ph) complex was represented as a resonance hybrid between (OEC)CoIV(Ph), which has a formal cobalt(IV) oxidation state, and (OEC+•)CoIII(Ph), which has properties of a Co(III) π-cation radical.104 Whatever its electronic configuration, the electrochemistry of (OEC)Co(Ph) is well-defined in nonaqueous media and characterized by a reversible oneelectron reduction at −0.23 V and two reversible one-electron oxidations at 0.36 and 1.23 V vs SCE in PhCN. A third irreversible oxidation is also seen at Epa = 1.77 V for a scan rate of 0.1 V/s. The products of the one-electron reduction and oneelectron oxidation of (OEC)Co(Ph) were assigned as [(OEC)CoIII(Ph)]− and [(OEC)Co]+ClO4−, respectively, the latter of which was structurally characterized.104 Although not pointed out in the original publications, the similarity in potentials for the oxidation and reduction of (OEC)Co(Ph) and (OEC)Co in PhCN is clearly evident. The first one-electron oxidation of (OEC)Co(Ph) is located at E1/2 = 0.36 V in PhCN104 which is almost exactly the 0.34 V average of the two half-reactions for dimeric (OEC)Co in the same solvent.91 At the same time, the E1/2 of −0.23 V for reduction of (OEC)Co(Ph) in PhCN is not so different than the E1/2 of −0.30 V for reduction of (OEC)Co in PhCN (see Table 5), and this strongly suggests the same site of electron transfer in both compounds, namely, the OEC+• macrocycle in (OEC+•)CoIII(Ph) and the OEC+• macrocycle in (OEC+•)CoII, if such an electronic configuration were to be given in the latter case. In this regard, it should also be noted that the reversible first reduction potential of (OEC)Co(Ph) (−0.23 V) compares well with the measured E1/2 for the first reduction of (OEC)Cu (−0.34 V) and (OEC)Ni (−0.20 V) in PhCN (see Table 5), again suggesting a similar site of electron addition to the OEC+• radical form of the neutral corrole in all four compounds. The corrole ligand also appears to be the site of oxidation in the case of (OEC+•)Co(Ph). Evidence for this assignment is given by the fact that NMR data for structurally characterized [(OEC)Co(Ph)] + ClO 4 − was interpreted in terms of a compound having a doubly oxidized corrole ligand and a lowspin Co(III) center,104 again suggesting that the one-electron oxidation and one-electron reduction of (OEC)Co(Ph) involves, in both cases, the corrole ligand. Finally, it should also be noted that the in situ-generated products of the one-electron oxidation and one-electron reduction of (OEC)Co(Ph) are quite stable in PhCN and DCM and it is difficult to add or abstract another electron. For example, the abstraction of a second electron from [(OEC)Co(Ph)]+ does not occur until E1/2 = 1.23 V vs SCE (870 mV positive of the first oxidation) and no reductions are seen between the half-wave potential for the first reduction of the neutral corrole at −0.23 V and the negative potential limit of the DCM solvent (∼2.0 V). An example of how the solvent and macrocycle substituents will affect dimerization of cobalt corroles under the electrochemical conditions is given in Table 6 for derivatives with different substituents on the meso- and/or β-pyrrole positions of the macrocycle.91,215,217 The structures of the examined compounds are shown in Chart 6.215 In this table, D and M represent the formation of a dimer or a monomer oxidation product, respectively. Dimers were invariably formed in DCM, while only monomers are seen in pyridine. Behavior between these two extremes is seen in PhCN and THF,
4.4. Group 9
4.4.1. Cobalt. Cobalt complexes have been one of the most studied groups of metallocorroles, in part because of their rich redox chemistry and in part because they have shown the possibility for use as catalysts in a variety of reactions,116,119,198−215 the most important of which are oxygen reduction119,198−213,216 and water oxidation.199,214 The electrooxidation of cobalt corroles in nonaqueous media can lead to monomeric or dimeric products, depending upon the electrochemical solvent, the specific substituents on the corrole macrocycle, and the presence or absence of an axial ligand. A good example of how dimerization can be detected in the electrochemistry is given in Figure 13, which illustrates cyclic voltammograms of (OEC)Co, (OEC)Ni, and (OEC)Cu in DCM.91 Five reversible redox processes are observed for (OEC)Co in DCM, three of which involve the addition of one electron per (OEC)Co unit (at E1/2 = −0.30, 0.94, and 1.17 V) and two of which are characterized by the global abstraction of 0.5 electrons in each step as measured coulometrically in PhCN.91 These latter two processes are located at E1/2 = 0.11 and 0.57 V (see Figure 13) and are associated with the oxidation of dimeric [(OEC)Co]2, which is stepwise converted to [(OEC)Co]2+ and then to [(OEC)Co]22+ as shown in Scheme 11, where M = Co, Cu, or Ni. The product of the first oxidation of [(OEC)Co]2 was assigned as [(OEC)Co]2+ on the basis of its ESR spectrum.91 The strength of the interaction between two equivalent redox centers of the (OEC)M dimers can be related to the potential difference (ΔEox) between the two relevant redox reactions involving the half-oxidized and fully oxidized dimer. The stronger the interaction, the larger will be the separation. For example, in Figure 13, the ΔE1/2 of 460 mV between the two one-electron oxidations of (OEC)Co at 0.11 and 0.57 V is relatively large, and this indicates a strong π−π interaction between the two corrole units. Smaller separations are seen between the first two oxidations of (OEC)Ni (260 mV) and (OEC)Cu (140 mV) (see Table 5 for a summary of potentials), and this would correspond to a weaker interaction between the two redox active centers on the molecule. [(OEC)Co]2 was postulated to be a π−π dimer in solutions of DCM or PhCN. This assignment was based in part on the fact 3400
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corrole [(Ar)3CorCoII]− (an electrochemical EC mechanism). The four-coordinate Co(II) species might then be further reduced at more negative potentials (depending upon the specific substituents and solvent), or it can be irreversiblely reoxidized via another electrochemical EC type mechanism to give a transient (Ar)3CorCoIII species followed by re-formation of the neutral five-coordinate Co(III) corrole with a coordinated triphenylphosphine axial ligand. The peak potentials for reduction of the triarylcorroles were shown to be solvent-dependent and ranged from −0.08 to −0.17 V.133 The second one-electron reduction of (Ar)3CorCoIII(PPh3) is reversible in DMF but becomes irreversible in DCM, due to the fact that the electrogenerated corrole reacts with the DCM solvent.106,133 A similar reaction also occurs for porphyrins and has often been used as indirect evidence for the formation of a Co(I) compound, as opposed to a Co(II) π-anion radical.228 Several cofacial cobalt biscorroles (BCY)Co2, which are linked by an anthracene (A), biphenylene (B), dibenzofuran (O), dibenzothiophene (S), or 9,9-dimethylxanthene (X) group, have been examined in DCM, PhCN, or pyridine containing 0.1 M TBAP as the supporting electrolyte.225,227 A series of face-to-face porphyrin−corrole dyads were also examined in nonaqueous media.169,203,218,222,227 The dyads are represented as (PCY)Co2, (PCY)H2Co, and (PCY)MIIIClCoIVCl, where PC is the porphyrin−corrole, M is FeIII or MnIII metal ion, and Y = anthracene (A), biphenylene (B), dibenzofuran (O), dibenzothiophene (S), or 9,9-dimethylxanthene (X) bridge. Among the investigated biscobalt porphyrin−corrole dyads, the (PCO)Co2 derivative was isolated in a stable form where the porphyrin contained Co(II) and the corrole Co(III). The electrochemistry of (PCO)Co2 in PhCN is illustrated in Figure 15, which also includes cyclic voltammograms of the corresponding monoporphyrin and monocorrole as comparisons. As seen in Figure 15b, eight well-defined oxidation and reduction processes are observed for the mixed macrocycle dyad. The processes located at E1/2 = 0.96, 0.39, and −1.02 V were unambiguously assigned to occur at the porphyrin part of the molecule, while the processes at E1/2 = 0.87, 0.47, and −0.19 V were assigned to occur at the corrole part of the dyad. The process at 1.25 V was assigned as involving two overlapping oxidations, one belonging to the porphyrin and the other to the corrole.227 Many porphyrin−corrole dyads have also been investigated as to their electrochemistry by the Kadish/Guilard group169,198,200,203,218,222,223,227 and the sites of electron transfer elucidated. Details on the various proposed mechanisms can be found in the original publications. 4.4.2. Rhodium. The first oxidation of (F5Ph)3CorRhIII(py)2 and (F5Ph)3CorRhIII(PPh3)2 was examined by Gross, Gray, and their co-workers in a 2009 paper with comparisons made to CoIII and IrIII derivatives having the same macrocycle and axial ligands.196 The six-coordinate triphenylphosphine adduct (F5Ph)3CorRhIII(PPh3)2 is oxidized at E1/2 = +0.79 vs Ag/ AgCl in DCM, while the bis-ligated pyridine complex, (F5Ph)3CorRhIII(py)2, undergoes an oxidation at E1/2 = +0.72 V under the same solution conditions. Each process involves the abstraction of one electron at E1/2 values midway between halfwave potentials for oxidation of the analogous CoIII(L)2 and IrIII(L)2 derivatives. The product of the one-electron oxidations was evaluated as being metal-centered in the case of the pentafluorophenyl cobalt and iridium corroles but not in the case
where the formation of a dimer or monomer varied with the specific substituent on the macrocycle.215 (Me6Et2)CorCo, (Me2Et2Ph4)CorCo, and (OEC)Co lack a phenyl group on the 10-meso-position of the macrocycle, and only these compounds form dimers upon oxidation in PhCN.215 The occurrence of dimerization is related to the solvation properties of the solvent as defined by the Gutmann solvent donor number.122 The formation of dimers occurs more easily in solvents having lower donor numbers (e.g., DCM and PhCN), while little or no dimerization occurs in solvents with higher donor numbers (e.g., THF and py). Again, this can be explained by the formation of five- and six-coordinated complexes, which prevent the possibility of stacking. Dimerization also does not occur for the five-coordinate (OMC)Co(PPh3) complex, which exhibits three reversible oneelectron oxidations in PhCN containing 0.1 M TBAP.105 Presumably, dimerization is prevented because the PPh3 axial ligand hinders or weakens formation of a π−π interaction between the two macrocycles. A later study of cobalt triphenylphosphine corroles containing p-OMe, p-Me, p-Cl, m-Cl, o-Cl, m-F, o-F, or H substituents also confirmed the lack of dimerization for these five-coordinate cobalt corroles under the same solution conditions.106 The effect of solvent on redox potentials of cobalt corroles was also examined for meso-substituted cobalt triarylcorroles.133,201 The four-coordinate cobalt corroles with a formal Co(III) oxidation state generally undergo a facile reduction to generate the Co(II) form of the compound and a following reduction at more negative potentials may or may not generate the formal Co(I) corrole complex. Most cobalt corroles undergo multiple one-electron oxidations (see refs 93, 105−107, 116, 133, 139, 169, 189, 196, 201, 211, 212, 215, and 217−227) the exact number of which will depend upon the positive potential limit of the nonaqueous solvent. Examples of cyclic voltammograms for one of the examined corroles, (F5Ph)Mes2CorCo, in four solvents, are shown in Figure 14.201 As seen in this figure, the first oneelectron reduction of (F5Ph)Mes2CorCo is reversible in DCM, THF, and PhCN, but it is irreversible and shifted toward more negative potentials in pyridine. The reversible processes in DCM, THF, and PhCN are consistent with the lack of a ligand exchange upon reduction, while the irreversible first reduction and reoxidation in pyridine can be interpreted in terms of two different redox processes, one for conversion of five-coordinate CoIII(py) to four-coordinate CoII and the other for reoxidation of unligated CoII to CoIII(py) on the reverse scan. The shift in reduction potential for (F5Ph)Mes2CorCo from E1/2 = −0.03 V in DCM to Epc = −1.10 V in pyridine is due to the Co(III) form of the corrole having an extremely large pyridine binding constant that shifts the measured potentials for the Co(III)/Co(II) process. A ligand dissociation reaction follows this electron transfer and this leads to the reaction irreversibility.201 Many five-coordinate meso-substituted cobalt triarylcorroles have been characterized as undergoing two reductions, the first of which is irreversible or quasi-reversible due to a chemical reaction following electron transfer, this chemical reaction being a dissociation of the axial ligand as described above and shown in Scheme 12 for the case of (Ar)3CorCo(PPh3).133 The first electron addition in Scheme 12 involves the conversion of (Ar)3CorCoIII(PPh3) to [(Ar)3CorCoII(PPh3)]−, and this is followed by a rapid dissociation of the axially bound PPh3 ligand to generate the anionic four-coordinate Co(II) 3401
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4.5. Group 10
of the rhodium corroles with the same macrocycle, for which a radical oxidation product was proposed on the basis of its ESR spectrum.196 A much earlier (1992) study by Kadish et al. elucidated redox properties of (OMC)Rh(PPh 3 ), with comparisons to (OMC)Co(PPh3) under the same solution conditions.105 The behavior of the two corroles were similar upon oxidation but not upon reduction, where the singly reduced [(OMC)RhII]− was shown to rapidly dimerize after electrogeneration. The mechanism proposed for dimerization of this radical species is given in Scheme 13105 and has similarities to the published mechanism for dimerization of electrogenerated Rh(II) porphyrins.229,230 A slowing down of dimerization for the electrogenerated RhII corrole was achieved in THF at low temperature (−70 °C), and when this occurred, (OMC)RhIII(PPh3) was reversibly converted to [(OMC)RhI(PPh3)]2− via two stepwise one-electron reductions. Dissociation of the PPh3 ligand occurred after electrogeneration of [(OMC)RhI(PPh3)]2−, and the resulting [(OMC)RhI]2− was then reoxidized via two one-electrontransfer steps on the return scan to give back the original RhIII corrole, as shown in the lower pathway of Scheme 13b.105 4.4.3. Iridium. Iridium(III) corroles have been electrochemically examined as derivatives with (F5Ph)3Cor141,196,231,232 and Br8(F5Ph)3Cor141 macrocycles. The compounds are extremely electron-rich, and only the bis-ligated Br8(F5Ph)3CorIr(NMe3)2 was shown to undergo a reduction, which was assigned as electron addition to the corrole macrocycle and located at E1/2 = −1.21 V vs SCE in DCM. An oxidation of the same compound was observed at E1/2 = 1.19 V and tentatively assigned as an Ir(III)/Ir(IV) process.141 Easier oxidations are observed for iridium corroles lacking the eight β-Br substituents. (F5Ph)3CorIr(NMe3)2 exhibits two oneelectron oxidations at 0.66 and 1.28 V, with the first electron abstraction being tentatively assigned as involving an Ir(III)/ Ir(IV) process.141 The bis-NH3 derivative with the same macrocycle also undergoes two one-electron oxidations at 0.53 and 1.13 V vs SCE, with the product of the first oxidation being assigned as a mixture of an Ir(IV) corrole and an Ir(III) corrole πcation radical.231 A comparison among the redox behavior of Ir, Co, and Rh corroles was presented by Gross, Gray, and their co-workers,196 who examined the bis-py and mono-PPh3 derivatives of (F 5 Ph) 3 CorIr I I I in DCM. (F 5 Ph) 3 CorIr I I I (py) 2 and (F5Ph)3CorIrIII(PPh3) were reversibly oxidized by one electron at E1/2 = 0.71 and 0.72 V vs Ag/AgCl in DCM. These values were very close to the oxidation potentials of the Co(III) and Rh(III) corroles with the same macrocycles and axial ligands [for example, E1/2 = 0.67 V for CoIII(Py)2 and 0.72 V for RhIII(py)2], but different ESR spectra were observed for the singly oxidized products.196 The ESR spectra of the singly oxidized cobalt and rhodium corroles having two pyridine axial ligands were characterized by a narrow, single-band absorption attributed to a corrole-based radical, but such a spectrum was not obtained for the singly oxidized iridium corrole, which exhibited a highly rhombic spectrum, thus suggesting to the authors the presence of an Ir(IV) corrole oxidation product in DCM.196 Finally, it should be noted that the basicity of the axial ligands strongly effects the potential for the first oxidation of the IrIII corroles, as demonstrated for a series (F 5 Ph) 3 CorIr(L)2 complexes with different substituted pyridine ligands, with the values of E1/2 ranging from 0.035 to 0.305 V vs Fc/Fc+.232
4.5.1. Nickel. (OEC)Ni undergoes three oxidations at 0.21, 0.47, 1.41 V and one reduction at −0.20 V in DCM containing 0.1 M TBAP.91 Its electrochemical behavior is similar to that of (OEC)Co, as described in an earlier section of this review. On the basis of its ESR spectrum, the initial (OEC)Ni was assigned as a Ni(II) π-cation radical, after which oxidation [of (OEC•+)NiII] at Eapp = 0.36 V resulted in the formation of [(OEC)Ni]2+, as ascertained by the coulometrically measured abstraction of 0.5 electrons per molecule of the neutral compound. This oxidation of the half-dimer was accompanied by an approximate 50% decrease in intensity of the ESR signal. A further one-electron oxidation of singly oxidized [(OEC)Ni]2+ at Eapp = 0.70 V led to [(OEC)Ni]22+ and the complete disappearance of the ESR signal, again confirming the existence of oxidized Ni corrole dimers under the given solution conditions.91 A nickel corrole dimer was also observed for electroxidized (MePh)3CorNi in DCM, 0.1 M TBAP.233 The compound in solution was characterized by three oxidations at 0.61, 0.87, and 1.57 V vs SCE and a one-electron reduction at 0.02 V. The first reduction was assigned to a Ni(III)/Ni(II) process and the oxidations as involving reactions of a dimer. Two nickel mesoaryl-substituted isocorroles were also investigated in this same study.233 The (MePh)3-5-isoCorNi and (MePh)3-10-isoCorNi derivatives were shown to undergo two stepwise oxidations to give π-cation radicals and dications in DCM, with the most stable products being obtained in the case of the 10-substituted derivatives. The same isocorroles could also be reduced by one or two electrons, but the initial one-electron-addition products were unstable and underwent a rapid chemical reaction to give a reduced corrole or a corrole-like product that could be reoxidized to the corresponding (MePh)3CorNi under the application of a controlled positive potential. The prevailing series of reactions illustrating an isocorrole-to-corrole conversion upon reduction and reoxidation was monitored by both electrochemistry and thin-layer spectroelectrochemistry.233 An electrochemical study of linked and nonlinked Ni isocorroles with β-substituted alkyl groups (see Chart 7) was earlier carried out by Vogel and co-workers in PhCN.234 The monomeric compound, (Me)4(Et)4-10-isoCorNi, exhibited two reversible reductions at −1.72 and −2.32 V vs Fc/Fc+, two reversible oxidations at −0.14 and 0.37 V, and a HOMO− LUMO gap of 1.58 V. The linked Ni isocorrole dimer, [(Me)4(Et)4-10-isoCorNi]2, exhibited two reductions (at −1.59 and −1.97 V vs Fc/Fc+) and four oxidations (at −0.23, 0.16, 0.77, and 0.86 V). The HOMO−LUMO gap of the linked dimer was given as 1.36 V. 4.5.2. Platinum. The published electrochemistry of platinum corroles is limited to a single communication by Ghosh and coworkers.235 The structures of the examined compounds are shown in Chart 8 and represented here as (Ar)3CorPtIV(R) (PhCN) and (Ar)3Cor +•PtIV(R)(R′), where Ar = p-XPh, R = mCNPh, and R′ = p-MePh. The series of compounds in Chart 8a possessed one carbon-bonded PhCN axial ligand and one noncarbon-bonded PhCN axial ligand and formally contains Pt(IV). The series of corroles in Chart 8b possessed two carbon-bonded axial ligands and formally contains Pt(V), although the electronic configuration of the compound was assigned as a Pt(IV) π-cation radical.235 The electrochemical data is consistent with the above assignments of electronic configuration. The four investigated Pt(IV) corrole π-cation radicals, (Ar)3Cor+•PtIV(R)(R′), all 3402
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triarycorroles with one or two β-NO2 groups as compared to the non-nitro-substituted compounds.85 Generally, only one or two reductions can be observed for the non-nitro-substituted corroles in DCM and PhCN, but three reversible one-electron reductions are seen for the copper β-nitro-substituted corroles under the same solution conditions (see Figure 3). Another example for how copper nitrocorroles differ from copper corroles without β-NO2 substituents is given in Figure 16, which compares the redox behavior of (Ph)3CorCu and NO2(Ph)3CorCu in DCM containing 0.1 M TBAP. (Ph)3CorCu undergoes two reductions at −0.16 and −1.94 V when measured at −60 °C in DCM, but three reductions are seen for βNO2(Ph)3CorCu at E1/2 = 0.0 and −1.40 and Ep = −1.80 V under the same solution conditions. The first reduction leads to formation of a Cu(II) corrole for both compounds, but the halfwave potential of this process is positively shifted by 160 mV upon going from (Ph)3CorCu to β-NO2(Ph)3CorCu, while the second reduction is shifted by 540 mV. This magnitude of positive potential shift seems too large to be accounted for by a single electron-withdrawing nitro substituent, and one possible explanation of this result is that the second electron transfer site is different in the two series of compounds. For example, the second reduction of the nitrocorrole might generate a corrole with a copper(I) central metal ion and an unreduced macrocycle, as compared to the corrole lacking a β-nitro substituent, where the second reduction is proposed to generate a Cu(II) π-anion radical, as graphically illustrated in Scheme 14. Further evidence in support of different sites of second electron transfer for the nitrocorroles and the non-nitrocorroles is given by thin-layer UV−vis spectroelectrochemistry, which was carried in DCM, pyridine, and DMF containing 0.1 M TBAP.85 The final absorption spectrum obtained after the first oneelectron reduction of the nitrocorrole resembles a spectrum reported in the literature for the structurally related nonnitrocorrole, and these spectra were assigned as species containing both a Cu(II) central metal ion and an unreduced π-ring system.85 However, the spectra for the doubly reduced nitrocorroles are inconsistent with formation of a Cu(II) corrole π-anion radical, suggesting a possible Cu(I) reduction product with an unreduced π ring system. An early electrochemical study of (OEC)Cu reported ESR spectra for the one-electron-reduction and one-electronoxidation products of the corrole in DCM.91 Surprisingly, a Cu(II) ESR spectrum was obtained for both the singly reduced and singly oxidized forms of the corrole, thus strongly suggesting the presence of Cu(II) in the initial neutral compound and an oxidation state assignment of (OEC+•)CuII prior to reduction. A CuII oxidation state and a [(Ar)3Cor+•]CuII formulation has also been proposed for unreduced triarylcorrole derivatives,18,84,86 which would be converted to (Ar)3CorCuII upon an initial one-electron addition. The proposed [(Ar)3Cor+•]CuII/(Ar)3CorCuII electron transfer reaction is reversible and is followed by a second reversible one-electron reduction to give what would be a Cu(II) corrole π-anion radical at more negative potentials.88 If this is the case, both redox processes would involve the corrole macrocycle, and the HOMO−LUMO gap of [(Ar)3CorCuII]− would be given by the difference in potential between the ring-centered oxidation of [(Ar)3CorCuII]− and the ring-centered reduction of the same compound, as shown in Scheme 15. The ΔE1/2 between the two reactions of the copper corroles shown in Scheme 15 averages 1.85 V in THF,88 as compared to an average HOMO−LUMO gap of 1.89 ± 0.11 V for the
undergo a facile one-electron reduction at potentials between 0.21 and 0.01 V vs SCE (see E1/2 values in Table 7), and this is followed by a second one-electron reduction at potentials between −0.77 and −0.87 V vs SCE. The first electron addition involves reduction of the corrole ligand, Cor+•, and occurs at a potential where similar redox processes are observed for several metallocorroles with noninnocent macrocyclic ligands. The second reduction of (Ar)3Cor+•PtIV(R)(R′) at −0.77 to −0.87 V may involve electron addition to the neutral corrole ligand, or it could involve a reduction of the central metal ion. This has yet to be investigated. However, as can be seen from the potentials in Table 7, the E1/2 values for the first reduction of the four corroles with formal Pt(IV) oxidation states are located at almost identical half-wave potentials as the second reduction of the neutral corroles having a formal Pt(V) oxidation state and the same meso substituents on the mcrocyclic ligand. The corroles in Chart 8a with a formal Pt(IV) center also exhibit two one-electron oxidations, the first of which is located at E1/2 values between 0.53 and 0.72 V, the exact potential depending upon the specific meso substituents on the macrocycle (see Table 7). These potentials are 210−250 mV more negative than E 1 / 2 for the first oxidation of the (Ar)3Cor+•PtIV(R)(R′) derivatives, this difference in potential being consistent with the difference of one positive charge on the corrole macrocycle of the compounds in each series. Finally, as seen in Table 7, the HOMO−LUMO gap of (Ar)3CorPtIV(R)(PhCN) ranges from 1.38 to 1.46 V, and this value can be compared to a HOMO−LUMO gap of 2.18−2.33 V for a similar series of Au(III) corroles possessing the same meso substituents.84 This might suggest a metal-centered reaction in the case of the Pt(IV) corroles and a ring-centered reaction in the case of the Au(III) derivatives. 4.6. Group 11
4.6.1. Copper. Numerous copper corroles have been characterized as to their electrochemical properties in nonaqueous media (see refs 78, 80−83, 85−91, 107, 108, 112, 143, 147, 153, 160, 176−178, 181, 182, 187, 190, 219, and 235−243). The examined compounds include numerous β-pyrrolesubstituted corroles, meso-substituted triarylcorroles, β,β′benzotriarylcorroles,238 and bis-copper corrole dyads.89,181,190 As discussed in earlier sections of this review (see sections 2.3 and 4.4), up to three reductions and four oxidations can be observed for copper corroles, with the site of electron transfer and possible dimerization of the oxidation product being dependent upon the solvent and specific electron-donating or electron-withdrawing substituents on the macrocycle. The singly reduced copper corroles contain a Cu(II) center, but the electronic configuration of the initial neutral species added to solution has been described in many cases as a Cu(II) π-cation radical or as an equilibrium mixture of a Cu(III) corrole and a Cu(II) cation radical, as shown in eq 15. (Cor)Cu III ⇌ (Cor +•)Cu II
(15)
The electrooxidation of (OEC)Cu in DCM indicates dimer formation at room temperature (see cyclic voltammogram in Figure 13).91 Dimerization of the oxidized corrole is also observed at low temperature for some meso-substituted copper corroles (see structures of examined compounds in Chart 9) having relatively strong electron-donating groups on the macrocycle.88 As described earlier for (NO2)x(tBuPh)3CorCu, where x = 0, 1, or 2, quite different redox properties are observed for copper 3403
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[(Cor)H2]− compounds listed in Table 4 (section 3.3.1), where reversible redox processes are obtained in each case. Similar ΔE1/2 values (and HOMO−LUMO gaps) of 1.78, 1.91, and 1.84 V are also seen when examining the related redox reactions of Br8(F5Ph)3CorCu, (F5Ph)3CorCu, and (Ph)3CorCu (Figure 4) (section 2.3.2).88 The later two HOMO−LUMO gaps can be compared to a ΔE1/2 of 1.95 V for [(F5Ph)3CorH2]− and 1.83 V for [(p-MePh)3CorH2]−, both of which contain the same macrocycle as for the copper corroles. Values of experimentally measured HOMO−LUMO gaps for numerous metallocorroles are presented in several sections of this review, but the similarity in the ΔE1/2 values between the monoanionic Cu(II) corroles and the monoanionic 2H complexes of the same macrocycle is striking. A Cu(IV) corrole has recently been postulated as a possible intermediate in oxidation catalysis,80 but additional compounds should be examined in the future to support this assignment of the higher oxidation state in the corrole. 4.6.2. Silver. Several studies have examined the electrochemistry of silver corroles that contain an Ag(III) ion in their neutral form.81,84,86,135,153,244 Two reversible oxidations and one reversible reduction can be observed in nonaqueous media. The addition of Br groups to the β-pyrrole positions of a silver triarylcorrole leads to a harder oxidation and an easier reduction as compared to those of the parent compound. For example, the first oxidation of Br8(Ph)3CorAg is shifted positively by 470 mV as compared to (Ph)3 CorAg under the same solution conditions,81 while the first reduction is positively shifted by 580 mV as compared to the nonbrominated compound. The combined results of thin-layer UV−vis spectroelectrochemistry and ESR data indicate that a stable Ag(III) π-cation radical is generated after the first one-electron abstraction, while a relatively stable Ag(II) corrole is the product of the first oneelectron reduction in DCM, PhCN, or pyridine.81,135,244 The second reduction is also metal-centered to give a Ag(I) corrole, but the electroreduced species is not stable in this oxidation state and demetalation occurs. An example of demetalation is shown by the cyclic voltammograms in Figure 17 for (MePh)3CorAg in pyridine.81,135 The same corrole demetalation product is obtained after the first and second one-electron additions to the Ag(III) corrole, as shown in Scheme 16, where the neutral corrole is represented as (Cor)AgIII and the final corrole reduction product in solution as [(Cor)H2]−, which could be further reduced by one electron to give [(•Cor)H2]2− or oxidized by one electron to give (•Cor)H2, as described in an earlier section of this review. Redox reactions involving Ag(I) and Ag(0) were also detected in the cyclic voltammogram of (MePh)3CorAg, and these processes are indicated by asterisks in Figure 17.135 In a comparison study of (Ar)3CorCu and (Ar)3CorAg by Ghosh and co-workers,86 it was reported that potentials for the first oxidation of the Cu and Ag corroles in the two series were virtually identical, but this was not the case for reduction, where electron addition to the silver corroles was more difficult by almost 700 mV, as shown by the measured potentials that are given in Table 8.86 The difference in redox behavior between the two series of corroles can be explained by the mechanism shown in Scheme 17, where a one-electron oxidation of the initial copper and silver corroles both involve electron abstraction from the uncharged (Ar)3Cor macrocycle of the M(III) derivatives. In contrast, there are two different sites of reduction, one of which involves the Ag(III) center of the corrole having a neutral macrocycle and the other the positively charged macrocycle of
the corrole, which is assigned as a Cu(II) π-cation radical. Both reductions give the same [(Ar)3CorMII]− one-electron-reduction product, as shown in the scheme. 4.6.3. Gold. Several Au(III) triarylcorroles have been electrochemically examined in nonaqueous media.84,111,145,187,188 Two reversible one-electron oxidations and two reversible one-electron reductions have been reported to occur in DCM containing 0.1 M TBAP.111 The first oxidation potentials of the Au(III) corroles are almost the same as the Cu(III) and Ag(III) analogues with the same macrocycle, but much more negative potentials are seen for the first reduction of the Au(III) derivatives as compared to Cu(III) and Ag(III) corroles under the same solution conditions.84 For example, the first reversible one-electron reduction occurs at E1/2 = −0.20 V for (Ph)3CorCu, at −0.86 V for (Ph)3CorAg, and at −1.38 V for (Ph)3CorAu in DCM, 0.1 M TBAP.84 The first one-electron addition to the Ag(III) corroles was assigned as a metal-centered process to give a Ag(II) corrole, as discussed above. Thus, the significant differences between the first reduction potentials of the Cu(III), Ag(III), and Au(III) corroles with the same macrocyclic structure indicates a different site of electron addition in the three series of metallocorroles. Spectroelectrochemistry, ESR measurements and DFT calculations confirmed that a corrole-ring-centered reduction had occurred for Au(III) corrole, leading to formation of a Au(III) πanion radical in nonaqueous media.84,111 A gold(I) corrole, Br8(F5Ph)3CorAgI(PPh3), was synthesized by Gross and co-workers145 and was shown to undergo what was described as a “semi-reversible” redox process at 1.14 V. No reductions were observed, and further oxidation led to decomposition of the corrole. Oxidation and reduction potentials were also compared for corroles in the series of (Ar)3CorM and Br8(Ar)3CorM compounds having Cu, Ag, and Au centers.188 It was shown that the gold corroles exhibited similar oxidation potentials as the copper and silver derivatives with the same macrocyclic ligands, but the Au(III) corroles were much harder to be reduced then the related Cu and Ag compounds due to the different sites of electron transfer and the different generated reduction products, in this case, an Au(III) π-anion radical for the singly reduced gold corroles as compared to one-electron-reduction products having Ag(II) and Cu(II) metal centers in the case of the related Ag(III) and Cu(III) corroles. Similar trends in redox behavior are observed for the (Ar)3CorM and Br8(Ar)3CorM derivatives with Cu, Ag, or Au central metal ions, and the proposed oxidation/reduction mechanisms are shown in Scheme 18 for the octabromotriarylcorroles. The measured potentials of the investigated corroles are listed in Table 9 and are consistent with the same site of electro-oxidation for compounds in each series but with three different sites of electronreduction for the different metal complexes, as indicated in the scheme. Finally, it should be pointed out that the E1/2 for reduction of Br8(Ph)3CorAuIII is located at −1.02 V vs SCE while an E1/2 of −1.38 V was measured for reduction of (Ph)3CorAuIII, a difference of 360 mV. By way of comparison, the difference in E1/2 between the reduction of (Ph)3CorCu and Br8(Ph)3CorCu is 320 mV in DCM (E1/2 = 0.20 and −0.12 V) as compared to a 550 mV difference in potentials for reduction of the two Ag(III) corroles (E1/2 = −0.31 and −0.86 V) (see Table 10). Further analysis of the Br substituent effect as a function of specific central metal ion is given in a later section of the review. 3404
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Further analyses of the β-Br substituent effects for oxidation and reduction of a given metallocorrole or series of metallocorroles are given in related sections of this review dealing with specific central metal ions, but three things should now be pointed out. The first is that the positive shift of potential upon addition of eight Br groups to (Ph)3CorAu is greater for oxidation than for reduction (ΔE1/2 = 490 vs 360 mV), unlike what is seen for corroles with main-group central metal ions and meso-F5Ph substituents, such as in the case of derivatives with Al(III) and Ga(III) metal ions, where the shift upon oxidation is 450−460 mV as compared to 700−740 mV for reduction.139 The second thing that should be pointed out is that the HOMO− LUMO gap is larger for Br8(Ph)3CorAu (2.31 V) than for (Ph)3CorAu (2.18 V) in DCM rather than smaller, as discussed by Gross and co-workers when considering corroles with other metal ions.139 This third is that the positive shift in E1/2 upon adding eight Br groups to (Ph)3CorCu and Br8(Ph)3CorCu is smaller for reduction than for the related Ag or Au compounds, which may or may not be related to the electronic configuration of the corrole ligand in the copper complex.
Figure 18. Cyclic voltammograms of (a) (OEC)SnIV(Ph), (b) (OEC)SnIVCl, and (c) (OEP)SnIVCl2 in DCM containing 0.1 M TBAP at a scan rate of 0.1 V/s. Adapted with permission from ref 101. Copyright 1998 American Chemical Society.
4.7. Group 12
4.7.1. Zinc. A zinc octamethylcorrole with the formula [(OMC)Zn]−(PyH)+ was synthesized by Paolesse et al. in 1990,245 and two triarylcorroles, N21-benzyl(F5Ph)3CorZnII(py) and N21-picolyl-(F5Ph)3CorZnII, were reported by Gross and co-workers in 2002.246 No electrochemical data were reported for these compounds, but in 2006, a doubly linked bis-zinc corrole, [(F5Ph)3CorZn]2, was electrochemically examined.247 This compound undergoes two reversible oxidations at E1/2 = 0.20 and 0.38 V and two reversible reductions at E1/2 = −0.19 and −0.34 V vs Fc/Fc+ in DCM containing TBAPF6. There is also one irreversible reduction at Epc = −1.90 V. The gap between the first oxidation and first reduction was only 0.40 V. This result, when combined with the ESR data, indicates that the bis-zinc corrole has a biradical character, and the authors suggested a nonbonding character for the frontier orbital of this compound.247
5. MAIN-GROUP CORROLES 5.1. Group 13
5.1.1. Aluminum. The electrochemistry of aluminum corroles was reported by Gross and co-workers in 2011.139 The reduction and oxidation is in each case macrocycle-centered and leads to the formation of π-anion and π-cation radicals. (F5Ph)3CorAl(py)2 undergoes a reversible one-electron oxidation at E1/2 = 0.55 V vs Ag/AgCl and a reversible one-electron reduction at E1/2 = −1.58 V in acetonitrile containing 0.1 M TBAP. The difference in potential between these two processes corresponds to an experimentally measured HOMO−LUMO gap of 2.13 V. Br8(F5Ph)3CorAl(py)2 also undergoes a reversible one-electron oxidation at 1.00 V and a reversible one-electron reduction at −0.88 V vs Ag/AgCl. The difference between these two reactions is 1.88 V, which is 250 mV smaller than the HOMO−LUMO gap for the nonbrominated Al(III) derivative measured under the same solution conditions. The oxidation potential of Br8(F5Ph)3Al(py)2 is shifted by 450 mV and the reduction by 700 mV as compared to (F5Ph)3CorAl(py)2 under the same solution conditions, and it is this difference in substituent effect for the two redox reactions that leads to the smaller gap of the octabromo derivatives.139 In an earlier section of this review, it was pointed out that the difference in potential between the first reduction of (Ph)3CorM
Figure 19. Cyclic voltammograms illustrating the first reduction and first oxidation of (a) (OEC)P(Me)2, (b) (OEC)P(Ph)2, (c) (OEC)P(H)2, and (d) (OEC)PO in PhCN, 0.1 M TBAP. Reprinted with permission from ref 99. Copyright 2000 American Chemical Society.
and the first reduction of Br8(Ph)3CorM (ΔE1/2) ranged from 320 to 550 mV for derivatives of Cu, Ag, and Au (see Table 10). This contrasts with a much larger separation between E1/2 values for the first reduction of (F 5 Ph) 3 CorAl(py) 2 and Br8(F5Ph)3Al(py)2. A similar 740 mV separation is also seen between E1/2 for reduction of (F5Ph)3CorGa(py) and reduction of Br8(F5Ph)3Ga(py) (see the next section). The most important factors affecting the different redox behavior of the Al or Ga corroles as compared to corroles with other central metal ions, such as Cu, Ag, or Au, has yet to be determinated and may be related to the presence or absence of a non-redox-active metal ion, the presence or absence of a non-transition-metal ion, or by the three F5Ph groups, which might influence the site of electron transfer. Further experiments are needed to answer these questions. 3405
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Figure 22. Cyclic voltammograms of (a) (OEC)AsIII and (b, c) [(OEC)AsV(Me)]+ClO4− in PhCN containing 0.1 M TBAP at a scan rate of 0.1 V/s. Adapted with permission from ref 100. Copyright 2000 American Chemical Society.
Figure 20. Plot of half-wave potentials for the first reduction and first oxidation of main-group octaethylcorroles vs the partical charge of the central ion. Reprinted with permission from ref 99. Copyright 2000 American Chemical Society.
Scheme 19. Proposed Redox Mechanism of (a) (OEC)AsIII and (b) [(OEC)AsV(Me)]+ClO4− in PhCN Containing 0.1 M TBAP100
compared to 2.23 V for (F5Ph)CorGa(py), which lacks the β-Br substituents.139 The difference in E 1/2 values between reduction of (F5Ph)3CorGa(py) and Br8(F5Ph)3CorGa(py) amounts to 740 mV, while only a 460 mV difference in potential is seen upon oxidation, as described for octabromocorroles in an earlier section of this review (see section 2.3.2). As indicated above, the 740 mV shift in E1/2 for the Ga(III) complex with eight Br and three F5Ph substituents groups, as compared to the parent compound without these substituents, is similar to what is seen for the same macrocycles having an Al(III) metal ion. Finally, it should be noted that the oxidation and reduction of a (F5Ph)3CorGa(L)2 complex linked with BODIPY at two βpyrrole positions of the macrocycle have also been reported.241
Figure 21. Plot of half-wave potentials for the first reduction and first oxidation of main-group octaethylporphyrins vs the partical charge of the central ion. Reprinted with permission from ref 99. Copyright 2000 American Chemical Society.
5.1.2. Gallium. Several gallium corroles have been examined as to their electrochemistry,139,241 but only the first reduction and first oxidation potentials have been reported in the literature. The one-electron oxidation of Br8(F5Ph)3CorGa(py) occurs at E1/2 = 1.14 V vs Ag/AgCl in acetonitrile,139 which is 460 mV more positive than the one-electron oxidation of (F5Ph)3CorGa(py) (E1/2 = 0.68 V). The first reduction of Br8(F5Ph)3CorGa(py) is located at E1/2 = −0.81 V, while E1/2 for the first reduction of (F5Ph)3CorGa(py) was measured as −1.55 V. Thus, the measured HOMO−LUMO gap for the gallium corrole with eight β-Br and three meso-F5Ph substituents is 1.95 V, as
5.2. Group 14
5.2.1. Germanium. (F5Ph)3GeIV(OH) undergoes one reduction at E1/2 = −0.99 V and one oxidation at E1/2 = 1.13 V 3406
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Kadish, Ou, their and co-workers in 1998.101 Cyclic voltammograms of these corroles are illustrated in Figure 18 along with a voltammogram of the tin(IV) porphyrin (OEP)SnIVCl2. The Sn−carbon-bonded (OEC)SnIV(Ph) complex undergoes a single reversible one-electron reduction at E1/2 = −1.74 V vs SCE in DCM containing 0.1 M TBAP. (OEC)SnIVCl also undergoes one reduction, but this process is irreversible and located at Epc = −1.46 V vs SCE for a scan rate of 0.1 V/s. The first reduction of the chloride-bound corrole remains irreversible at all temparatures down to −60 °C and at a higher scan rate of 2.5 V/s. The electroreduction behavior of both Sn(IV) corroles differs from that of (OEP)SnIVCl2 in that the two corroles undergo only a single electron addition within the solvent potential limit, while the tin(IV) porphyrin undergoes two oneelectron reductions, as shown in Figure 18. UV−vis spectra of the singly reduced (OEC)SnIV(Ph) and (OEP)SnIVCl2 were shown to be similar to each other and also resemble spectra obtained for the product of the first ringcentered reduction of other tin(IV) porphyrins. This suggested to the authors that a one-electron addition to (OEC)SnIV(Ph) leads to formation of a tin(IV) π-anion radical.101 The spectrum obtained after the first controlled potential reduction of (OEC)SnIVCl differs from that of electroreduced (OEC)SnIV(Ph) and (OEC)SnIVCl2, but it is similar to the spectrum of tin(II) porphyrins,249,250 thus suggesting the possible formation of a tin(II) corrole in the thin-layer cell. As seen in Figure 18, three reversible to quasi-reversible oneelectron oxidations were observed for the two examined tin(IV) corroles in DCM, 0.1 M TBAP. The first oxidation leads to formation of tin(IV) corrole π-cation radicals, i.e., [(OEC)SnIV(Ph)]+• and [(OEC)SnIVCl]+•. Because a metal-centered oxidation is not possible for the tin(IV) compounds, the stepwise abstraction of two additional electrons can only occur at the conjugated macrocycle, leading to formation of a Sn(IV) corrole dication and trication.101 Again, the measured HOMO−LUMO gaps for the Sn(IV) corroles are close to values for other main-group corroles not having a redox-active metal center or a noninnocent corrole macrocycle. The measured separations between the first oxidation and first reduction of the Sn(IV) corroles are 2.21 V for (OEC)Sn(Ph),101 2.13 V for (OEC)SnCl,101 and 2.14 V for (F5Ph)3CorSnCl in DCM.191
Figure 23. Cyclic voltammograms of (a) [(OEC)PV(Me)]+ClO4− reduction product after bulk electrolysis at −1.30 V in a solution of PhCN and 0.2 M TBAP and of (b) (OEC)AsIII and (c) (OEC)SbIII in PhCN, 0.1 M TBAP. Reprinted with permission from ref 99. Copyright 2000 American Chemical Society.
Scheme 20. Proposed Oxidation Mechanism of (OEC)SbIII in PhCN Containing 0.1 M TBAP100
vs SCE in DCM, giving a HOMO−LUMO gap of 2.12 V.191 Similar HOMO−LUMO gaps of 2.12 ± 0.02 V were measured for (F5Ph)3SnIVCl and (F5Ph)3PV(OH)2 under the same solution conditions, clearly suggesting that in all cases only the corrole macrocycle is involved in the electron transfer processes. Although (F5Ph)3GeIV(OH) was reported to undergo only one reduction and one oxidation,191 three reductions and two oxidations were seen for (Ph)3GeIV(OH) in DCM containing 0.1 M TBAP. The oxidations were located at 0.57 and 1.15 V, while the reported reductions were located at −0.65, −1.08, and −1.86 V vs SCE.248 Giribabu et. al synthesized a Ge(IV) corrole that was linked with a free base or a zinc tetraarylporphyrin.248 Three or four oxidations and four reductions could be observed for these dyads. Analysis of the data revealed that redox potentials of the dyads were in the same range as potentials for the individual monomeric corrole and porphyrin components, indicating the absence of an electronic interaction between the linked macrocyclic units of the dyad. 5.2.2. Tin. The only electrochemically examined tin(IV) triarylcorrole is (F5Ph)3SnIVCl, which undergoes one oxidation at E1/2 = 1.20 V and one reduction at E1/2 = −0.94 V vs SCE, a separation of 2.14 V. Both redox processes were assigned as corrole-centered to give π-cation and π-anion radicals in DCM containing 0.1 M TBAP.191 The measured gap between the two processes is similar to that of other main-group corroles with (F5Ph)3Cor or (Ph)3Cor macrocycles (see the later discussion). The electrochemical properties of two tin(IV) octaethylcorroles, (OEC)SnIV(Ph) and (OEC)SnIVCl, were reported by
5.3. Group 15
5.3.1. Phosphorus. Five phosphorus(V) octaethylcorroles with different axial ligands were examined by Kadish, Ou, and their co-workers in PhCN containing 0.1 M TBAP.99 A summary of the measured potentials in PhCN is given in Table 11 and examples of cyclic voltammograms which illustrate the first reduction and first oxidation for four of the compounds are given in Figure 19. (OEC)PV(O) and (OEC)PV(H)2 undergo a single reversible reduction at E1/2 = −1.58 and −1.67 V vs SCE, respectively, both of which were assigned as leading to a phosphorus(V) corrole πanion radical, on the basis of analysis of the UV−vis spectroelectrochemical data. The first oxidation of (OEC)PV(O) is reversible and located at E1/2 = 0.80 V, while (OEC)PV(H)2 is irreversibly oxidized at Epa = 0.44 V in PhCN. Three oxidations and one reduction were observed for (OEC)PV(Me)2 and (OEC)PV(Ph)2 in PhCN. ESR and spectroelectrochemical data obtained for both corroles were in agreement with the formation of π-anion and π-cation radicals 3407
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after the first one-electron reduction and the first one-electron oxidation, respectively.99 Different electrochemical behavior was reported for [(OEC)PV(Me)]+ClO4−, which exhibits one reversible oxidation at E1/2 = 1.11 V and two reductions at Ep = −1.16 V and E1/2 = −1.71 V in PhCN, 0.1 M TBAP. The first irreversible reduction at Epc = −1.16 V led to a P(III) product that could be oxidized at E1/2 = 0.48 V and reduced at −1.71 V, a separation of 2.19 V. The first oxidation of [(OEC)PV(Me)]+ClO4− involves two overlapping one-electron-transfer steps at 1.11 V at room temperature, but two well-separated oxidations could be detected when the electrochemical measurements were carried out at a lower temperature of 0 °C under the same solution conditions.99 The electrochemically measured HOMO−LUMO gap (ΔE1/2) for the investigated P(V) octaethylcorroles with five different axial ligands ranged from 2.11 to 2.38 V in PhCN,99 and a similar separation of 2.10 V was reported by Gross and coworkers for (F5Ph)3CorPV(OH)2 in DCM.191 Almost the same HOMO−LUMO gaps were measured for (F 5 Ph) 3 CorGeIV(OH) (2.13 V) and (F5Ph)3CorSnIVCl (2.14 V) in DCM. An analysis of the HOMO−LUMO gap was presented by Kadish et al. for the five electrochemically characterized (OEC)PV derivatives,99 and this data was compared to that of the main-group corroles, all which possessed an average gap of 2.20 ± 0.10 V, as shown in Figure 20. A similar analysis was carried out for main-group porphyrins, for which the average HOMO−LUMO gap was 2.32 ± 0.09 V, as shown in Figure 21.99 To our knowledge, this was the first published data to demonstrate a similarity between the HOMO−LUMO gaps of porphyrins and corroles. Much smaller HOMO−LUMO gaps of 1.48−1.50 V are seen for the three octalkoxy-substituted phosphorus(V) triazatetrabenzocorroles, (BuO)8(TBC)PV(OMe)2, [(BuO) 8 (TBC)P V (OH)] + OH − , and [(BuO) 8 Cl 8 (TBC)PV(OH)]+OH−. This potential difference between the first oxidation and first reduction is related to the expanded π-system of the TBC macrocycle and agrees well with the average ΔE1/2 of 1.50 V for many phthalocyanines.251 5.3.2. Arsenic. Only two arsenic corroles, (OEC)AsIII and [(OEC)AsV(Me)]+ClO4−, have been electrochemically examined in nonaqueous media.100 (OEC)AsIII undergoes a single reversible one-electron reduction at E1/2 = −1.67 V in PhCN containing 0.1 M TBAP, leading to formation of an As(III) πanion radical. The first oxidation occurs at E1/2 = 0.50 V and is followed by a chemical reaction to give a product assigned as [(OEC)AsIV]+ on the basis of ESR measurements. The chemical reaction following formation of the singly oxidized species is slow, and the ΔE1/2 between the reversible first reduction and reversible first oxidation is 2.17 V, as shown by the cyclic voltammogram in Figure 22a. A second irreversible oxidation of (OEC)AsIII is located at Epa = 1.09 V. This electrode reaction is followed by a reversible oneelectron transfer at E1/2 = 1.17 V and a quasi-reversible process at E1/2 = 1.48 V vs SCE. The UV−vis spectrum obtained after the second oxidation of (OEC)AsIII is similar to the spectrum of [(OEC)AsV(Me)]+ClO4−. These results thus suggest that the two-electron-oxidation product of (OEC)AsIII is an As(V) corrole. The two oxidations at E1/2 = 1.17 and 1.48 V were therefore assigned to the stepwise formation of an arsenic(V) corrole π-cation radical and dication, as shown in Scheme 19a.100 The As(V) corrole [(OEC)AsV(Me)]+ClO4− undergoes oxidation at 1.16 V and two reductions at Epc = −1.01 V and
E1/2 = −1.61 V in PhCN, 0.1 M TBAP.100 The one-electron oxidation of the As(V) corrole leads to an As(V) corrole π-cation radical, while the first reduction of the same compound, at Epc = −1.01 V, involves an irreversible one-electron transfer and results in formation of a product that can be reversibly oxidized at E1/2 = 0.48 V (as shown in Figure 22b,c). The proposed mechanism for conversion of [(OEC)AsV(Me)]+ClO4− to (OEC)AsIII involves the initial formation of an As(V) corrole π-anion radical, [(OEC)AsV(Me)]−•, which then rapidly undergoes an internal electron transfer and dissociation of a CH3 radical, with formation of (OEC)AsIII, as shown in Scheme 19b.100 Although [(OEC)AsIV]+ is shown in Scheme 19a,b as being formed in the one-electron oxidation of (OEC)AsIII, the initial product of (OEC)As electrooxidation may actually be an AsIII corrole π-cation radical, as suggested by the similarity in cyclic voltammograms for (OEC)AsIII, (OEC)SbIII, and (OEC)PIII, the last of which was in situ generated. Cyclic voltammograms of these three corroles are illustrated in Figure 23. 5.3.3. Antimony. Two Sb(V) and one Sb(III) triarylcorrole, (F5Ph)3CorSbV(O), (F5Ph)3CorSbV(F)2, and (F5Ph)3CorSbIII(py), were electrochemically examined in DCM containing 0.1 M TBAP.165 Both Sb(V) derivatives undergo one reversible reduction and one reversible oxidation. The SbV(O) corrole is reduced at −0.77 V and oxidized at 1.10 V vs SCE, a difference of 1.87 V. This compares to (F5Ph)3CorSbV(F)2, which is reduced at −0.64 V and oxidized at 1.47 V, thus giving a HOMO−LUMO gap of 2.11 V. Reversible one-electron reductions and oxidations were also recorded for (F5Ph)3CorSbIII(py) at E1/2 = −1.10 and 1.10 V under the same solution conditions, thus giving a HOMO− LUMO gap of 2.10 V.165 A similar HOMO−LUMO gap of 2.13 V was earlier reported for (OEC)SbIII in PhCN, 0.1 M TBAP.100 The reduction is located at E1/2 = −1.66 V and the oxidation at E1/2 = 0.47 V. UV− vis spectroelectrochemistry, combined with ESR measurements, suggested that the final product of the first one-electron oxidation was represented as [(OEC)SbIV]+, but the HOMO− LUMO gap of 2.13 V might suggest formation of an initial SbIII πcation radical on the cyclic voltammetry time scale prior to formation of the spectroscopically characterized product SbIV product. Whatever the site of electron transfer, it is interesting to note that virtually identical HOMO−LUMO gaps of 2.10− 2.13 V were measured for the antimony corroles, which existed as stable compounds in two different metal oxidation states (+3 and +5) and with two quite different macrocyclic ligands, OEC and (F5Ph)Cor. This similarity is even more remarkable when considering that the measurements were also made in two different electrochemical solvents (DCM and PhCN). A second and third oxidation of (OEC)SbIII were located at E 1/2 = 1.03 V and E pa = 1.36 V. Thin-layer UV−vis spectroelectrochemical data obtained during controlled potential oxidation indicated that the process at 1.03 V corresponded to the formation of [(OEC•)SbIV]2+, which was said to slowly convert to [(OEC)SbV]2+. The third oxidation at 1.36 V would therefore correspond to the formation of [(OEC)SbIV]3+, as shown by the proposed reactions given in Scheme 20.100 5.3.4. Bismuth. (OEC)BiIII is reversibly oxidized at 0.24 V and irreversibly reduced at Epc = −1.77 V vs SCE in PhCN containing 0.1 M TBAP. A reoxidation peak is seen at Epa = −1.22 V after the first reduction and assigned as involving a (Me4Ph5)CorH3 demetalation product, which is reduced at Epa = −1.32 V under the same experimental conditions.100 3408
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The first reversible oxidation of (OEC)BiIII at E1/2 = 0.24 V was assigned as leading to formation of a Bi(III) corrole π-cation radical on the basis of spectroelectrochemical data. Assuming an approximate E1/2 of −1.74 V (for the irreversible reduction at Ep = −1.77 V) and using the E1/2 of 0.24 V for the reversible oxidation of (OEC)Bi would give an approximate HOMO− LUMO gap of 1.98 V. The second oxidation of (OEC)BiIII initially led to formation of [(OEC)BiIII]2+, but this Bi(III) dication slowly converted to the Bi(IV) corrole in solution. A third irreversible oxidation of the corrole was also observed at Epa = 1.47 V in PhCN.100 A HOMO−LUMO gap of 1.94 V was recorded for (F5Ph)3CorBi and this value compares well to the approximate gap of 1.98 V for (OEC)Bi.252 The first reversible reduction of (F5Ph)3CorBiIII at E1/2 = −1.28 V was assigned to formation of a Bi(III) π-anion radical, while the two reversible oxidations at E1/2 = 0.66 and 1.07 V were also assigned as macrocycle-centered processes to give the corrole π-cation radical and dication in MeCN containing 0.1 M TBAPF6.252 Similar electrochemical behavior was observed for a series of bismuth(III) A2B-type triarylcorroles that were characterized by two reversible one-electron oxidations, at E1/2 = 0.45−0.53 and E1/2 = 0.94−0.98 V, and one reduction between −1.20 and −1.38 V vs SCE in MeCN containing 0.1 M TBAPF6.253 The measured HOMO−LUMO gaps of the meso-substituted Bi(III) triarylcorroles ranged from 1.70 to 1.94 V.253 These values are smaller than the average gap of about 2.20 ± 0.1 V for other group 15 metallocorroles, but they fit with measured values for the abovediscussed Bi corroles with OEC or (F5Ph)3Cor macrocycles.
terized by Gryko, Arnold, and their co-workers in 2013,254 but no electrochemical data were reported for these compounds. In contrast, triple-decker compounds containing a corrole macrocycle have been successfully synthesized and examined as to their electrochemistry in different nonaqueous solvents. These compounds are represented as Eu2[Pc(OC4H9)8]2[(Ph)n(NO2Ph)3‑nCor] (n = 0−3) and M2[Pc(R)8]2[(ClPh)3Cor], where M is Y(III), Pr(III), Nd(III), Sm(III), Eu(III), Gd(III), or Tb(III) and R = H, OC4H9, OC5H11, or OC8H17. Several of the complexes were structurally characterized and showed the corrole to be in the middle of the sandwich, with a phthalocyanine macrocycle at each extreme. Each triple-decker complex undergoes eight reversible or “quasi-reversible” one-electron oxidations and reductions with E1/2 values being linearly related to the ionic radius of the central metal ions. All of the redox reactions are attributed to the successive removal or addition of electrons from the ligand-based orbitals of the compounds.132,157,255 Similar europium tripledecker complexes containing nitrophenyl groups on the mesopositions of the corrole macrocycle undergo even more reductions because the NO2Ph groups themselves can also be reduced under the given solution conditions. As seen in Figure 24, up to five reductions are observed for Eu2[Pc(OC4H9)8]2[(Ph) n(NO2Ph)3−nCor] (n = 0−3) in PhCN containing 0.1 M TBAP.157 The three NO2Ph groups on the meso-positions of the corrole are reduced at different halfwave potentials ranging from E1/2 = −1.19 to −1.33 V vs SCE, indicating an interaction between these redox active centers across the molecule. Half-wave potentials for the five one-electron oxidations and the first one-electron reduction of the triple-decker compounds containing nitro-substituted corroles varied with the number of electron-withdrawing NO2Ph groups on the compounds. Increasing the number of NO2Ph groups on the corrole leads to a progressive positive shift in reversible potentials for the first one-electron reduction and the five one-electron oxidations of the triple-decker compounds, with the magnitude of the shift being dependent upon both the solvent and the specific electron transfer reaction.157 There is also a splitting of the redox reactions for the NO2Ph groups in the triple-decker compounds, as described earlier for monomeric corroles containing similar NO2Ph substituents. The first examples of actinide complexes incorporating corrole ligands were reported by Arnold and co-workers in 2013.256 The thorium(IV) and uranium(IV) corroles were synthesized as μCl2 dimers, [(Mes)2(MeOPh)Cor]2Th2(μCl)2(DME)2 and [(Mes)2(MeOPh)Cor]2U2(μCl)2(DME)2, where DME = dimethoxyethane. These dimers undergo multiple ligand-based oxidations in MeCN containing 0.1 M TBAPF6. The electrochemistry of the dimeric uranium(IV) corrole is more complex. Five oxidations were observed in MeCN in addition to a single reduction. The oxidations of U(IV) corrole are positively shifted as compared to those of the Th(IV) corrole. This is consistent with the larger electronegativity of uranium, as well as the greater distortion of the corrole ring.256
6. LANTHANIDE AND ACTINIDE CORROLES Unlike the large amount of research activity involving corroles containing transition-metal ions, very little is known about the rare-earth complexes with a corrole macrocycle ligand. Derivatives of (Mes)2(MeOPh)CorM, where M = La·4.5DME, Tb·4DME, or Gd·TACNMe3, were synthesized and charac-
7. SUMMARY AND BRIEF OVERVIEW OF TRENDS IN STRUCTURE−REACTIVITY RELATIONSHIPS In writing this review, we have examined the known redox behavior for hundreds of different metallocorroles under various solution conditions. Our initial goal was to summarize the electrochemistry of these metallomacrocycles as a function of the central metal ion, macrocyclic substituents, axial ligands, and
Figure 24. Cyclic voltammograms of corrole−phthalocyanine rare-earth triple-decker complexes in PhCN containing 0.1 M TBAP. Reductions of the meso-NO2Ph substituents are indicated by processes within the boxed area. Adapted with permission from ref 157. Copyright 2015 American Chemical Society. 3409
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Table 11. Half-Wave Potentials of Phosphorous Corroles (V vs SCE) in PhCN, 0.1 M TBAPa oxidation compound (OEC)PO (OEC)P(H)2 (OEC)P(Me)2 (OEC)P(Ph)2 [(OEC)P(Me)]+ClO4−
third b
1.81 1.81b 1.79b,c 1.63b 1.81b
reduction first
first
0.80 0.44b 0.44 0.56 1.11d
−1.58 −1.67 −1.75 −1.71 −1.16e
second b
1.17 1.10b 1.12 1.30b 1.11d
a
b
second
−1.71
c
Reprinted with permission from ref 99. Copyright 2000 American Chemical Society. Epa, at scan rate of 100 mV/s. A small peak can also be seen at Epa = 1.46 V. dThe first two oxidations are overlapped in potential, giving directly a doubly oxidized P(V) corrole after the global abstraction of two electrons. eEpc, at scan rate of 100 mV/s. A reversible reoxidation process can also be seen at E1/2 = 0.48 V after the first reduction.
for others, as seen from the data in Tables 12 and 13 and also described in earlier sections of this review.
solvent conditions and then to elucidate trends in structure− reactivity relationships that might be used along with “electrochemical diagnostic criteria” for the future predicting and tuning of redox properties of yet to be synthesized corroles in their monomeric form or as an essential component in an array or dyad designed for a specific function. The outline of the review is grouped according to the periodic table, and this might at first suggest the possibility of observing similar redox behavior for a given family of corroles having the same macrocycle and formal metal oxidation state, for example, derivatives of Fe, Ru, and Os in group 8; Co, Rh, and Ir in group 9; or Cu, Ag, and Au in group 11. Unfortunately, this turns out not to be the case for the transition-metal corroles, as seen in Table 12 and also discussed in the text, although a number of similarities do exist in the case of the main-group complexes having the same metal oxidation state in the synthesized or electrosynthesized compound (see Table 13 and relevant text for the metallocorroles in group 13, 14, or 15). Other comparisons of the published corrole electrochemistry data are also possible. These might involve derivatives with the same formal metal oxidation state of +4 or +5 and the same nonlabile axial ligand(s), examples being given by derivatives with carbon-bonded phenyl groups such as in the case of (OEC)Mn(Ph), (OEC)Fe(Ph), (OEC)Co(Ph), and (OEC)Sn(Ph) or perhaps oxo-corroles, of which several examples have been described. Another good comparison of the existing corrole electrochemistry data should involve a series of derivatives having the same central metal ion and the same or similar set of axial ligands but with systematically varied macrocyclic structures of the type shown in Chart 2. We know from the porphyrin literature that the ease of reduction and difficulty of oxidation for a given metalloporphyrin is directly related to the basicity of the macrocycle and follows the order Br8(F5Ph)4Por < (F5Ph)4Por < Br8(Ph)4Por < (Ph)4Por < (Et)8Por, with the overall shift of E1/2 for reduction amounting to more than 1.0 V in many cases.75 A similar correlation between potentials and structure is seen for the metallocorroles, as shown by the data in Table 14, which compares E1/2 values for the first one-electron oxidation of selected porphyrins and corroles under similar solution conditions. We also know from the porphyrin literature that the HOMO− LUMO gap for a given series of metalloporphyrins is related to substituents on the macrocycle, averaging close to 2.20 V for many derivatives of (Et)8Por, (Ph)4Por, and (F5Ph)4Por but decreasing in magnitude for derivatives of Br8(F5Ph)4Por and Br8(Ph)4Por.75,76 A similar decrease in the electrochemically measured HOMO−LUMO gap is also observed for some of the examined metallocorroles with eight β-Br substituents but not
AUTHOR INFORMATION Corresponding Authors
*Z.O. e-mail:
[email protected]. *K.M.K. e-mail:
[email protected]. Notes
The authors declare no competing financial interest. Biographies Yuanyuan Fang is an associate professor at Jiangsu University. She received her Ph.D. in 2014 from the University of Houston under the supervision of Prof. Karl M. Kadish. Since 2008, she has been working with Profs. Zhongping Ou and Karl Kadish on the electrochemistry and spectroelectrochemistry of porphyrins, corroles, and related macrocycles and has published over 40 research papers on this topic. Zhongping Ou is a professor of chemistry at Jiangsu University and is currently a visiting professor at the University of Houston. He received his Ph.D. from the University of Houston in 2000 and has published over 180 research papers to date. He has examined the electrochemistry of multiple compounds for many years with an emphasis on porphyrins and corroles. His most recent research has focused on the electrochemistry of expanded porphyrins and N-confused porphyrins and on the use of metalloporphyrins and metallocorroles as catalysts for the electroreduction of molecular oxygen. Karl M. Kadish is a Hugh Roy and Lillie Cranz Cullen University Professor at the University of Houston and held the title Distinguished University Professor of Chemistry from 2003 to 2010. He received his B.S. degree from the University of Michigan in 1967 and his Ph.D. from Penn State University in 1970. He was a postdoctoral fellow at the University of New Orleans in 1970/71 and a chargé de recherche at the University of Paris VI in 1971/72. Before joining the University of Houston in 1976, he was on the faculty of California State University, Fullerton. In 1981, he held a Fulbright research fellowship and was a visiting professor of chemistry at the Université Louis Pasteur in Strasbourg, and in 1985, was appointed as visiting professor of chemistry at the Université de Dijon in France. He was reappointed to the Dijon position a number of times between 1986 and 2009. He received a research fellowship from the Japan Society for the Promotion of Science (JSPS) in 1994 and during the 2001/02 academic year held appointments as visiting professor in the chemistry department at California Institute of Technology and the University of Sydney. In 2003, he was awarded a Docteur Honoris Causa from the Université de Bourgogne in Dijon, France, and in July 2012, he was presented the Hans-Fischer Lifetime Award in Porphyrin Chemistry. His research interests are in analytical chemistry, porphyrin chemistry, chemistry and electrochemistry of compounds having biological interest, redox 3410
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a
3411
M
Cr (O) CrV(N) CrV(NMes) CrV(NAr) MoV(O) MnIII MnIII(py) MnIVCl Mn(Ph) ReV(O) FeIV(Ph) FeIII FeIII(NO) FeIII(py) FeIVCl RuIII(NO) OsVI(N) CoIII CoIII(PPh3) CoIV(Ph) CoIII(py)2 RhIII(PPh3)2 RhIII(py)2 IrIII(tma)2 IrIII(NMe3)2 IrIII(Py)2 Ir(PPh3)2 PtIV(R)PhCN PtIV(R)(R′) NiIII CuIII AgIII AuIII
V
1.4295 1.9491 1.9898 0.8698 1.8498 1.05103 11
1.02 1.25103 0.84103
0.4191 0.59104
0.4191 0.7791
−0.62 −0.41 −1.04 −0.08
−0.30 −0.23
−0.20 −0.34
0.43
0.61 0.21 0.76
0.11
0.36
0.21 0.43
ΔE
−0.72 −1.58 −1.66 −0.01 −1.15
1st red.
0.70 0.36 0.32 0.85 0.59
1st ox.
b
1.58174
−1.46 (−1.23)
0.12 1.01
0.76 0.75 0.80
0.63 0.88 −0.20 −0.87 −1.38
−0.83 0.09
(−0.73)
197
0.9678 1.62 2.18188
1.46235 0.97235
1.13119
2.19
−1.28
0.91 0.40
1.19
−0.33
0.86
142
2.24186
1.48163 1.58174
1.03
163
ΔE
−0.54 (−1.46)
−0.11
1st red.
b
0.94 0.32
0.92
1st ox.
(Ph)3Cor
1.12 1.16
0.53 0.71196 0.72196
196
0.67196 0.79196 0.72196
0.21 −0.61
0.24 (−0.40)
−0.42
0.72 0.89 0.70
0.00
0.17 −0.03 −0.47 −0.13 −0.35 −1.01
1st red.
1.07
1.10 0.56 1.31 1.29 1.32 0.71
1st ox.
(F5Ph)3Cor
0.9188 1.77
0.65201 1.10224
1.14195
1.07
191
0.93 0.59161 1.78144 1.42144 1.67165 2.23139
161
ΔE b
1.14 1.27 1.29
0.98
0.99
1.25
1st ox.
0.12 −0.31 −1.02
0.36
(−0.03)
−0.37
1st red.
Br8(Ph)3Cor
Details on the exact solution conditions are given in the text and/or original references. bPotential difference between the first oxidation and first reduction.
6 6 6 6 6 7 7 7 7 7 8 8 8 8 8 8 8 9 9 9 9 9 9 9 9 9 9 9 9 10 11 11 11
group
(Et)8Cor
Table 12. Summary of First Oxidation and First Reduction for Selected Transition-Metal Corrolesa
1.0278 1.5881 2.31188
0.62139
1.02139
1.62139
ΔE b
1.44
1.19
1st ox.
0.64
-
−1.21
1st red.
Br8(F5Ph)3Cor
0.8088
-
2.40141
ΔEb
Chemical Reviews Review
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Table 13. Summary of First Oxidation and First Reduction (V) for Selected Main-Group Corrolesa (Et)8Cor group
M
1st ox.
AlIII(py)2 GaIII(py) GeIV(OH) SnIVCl SnIV(Ph) PV(OH)2 PV(O) PV(H)2 PV(Me) AsIII AsV(Me) SbV(O) SbVF2 SbIII(py) SbIII BiIII
13 13 14 14 14 15 15 15 15 15 15 15 15 15 15 15
(Ph)3Cor ΔEb
1st red.
1st ox.
0.57 0.67 0.47
(−1.46) −1.74
2.13101 2.21101
0.80 0.44 1.11 0.50 1.16
−1.58 −1.67 −1.16 −1.67 −1.01
2.3899 2.2199 2.2799 2.17100 2.17100
−1.66 (−1.77)
0.47 0.24
1st red.
−0.65
(F5Ph)3Cor ΔEb
1.22248
2.13100 2.01100
Br8(Ph)3Cor ΔEb
1st ox.
1st red.
0.55 0.68 1.13 1.20
−1.58 −1.55 −0.99 −0.94
2.13 2.23139 2.12191 2.14191
1.05
−1.05
2.10191
1.10 1.47 1.00
−0.77 −0.64 −1.10
1.87165 2.11165 2.10165
0.66
−1.28
a
139
1st ox.
1st red.
Br8(F5Ph)3Cor ΔEb
1st ox.
1st red.
ΔEb
1.00 1.14
−0.88 −0.81
1.88139 1.95139
1.94252 b
Details on the exact solution conditions are given in the text and/or original references. Potential difference (HOMO−LUMO gap) between the first oxidation and first reduction.
ABBREVIATIONS USED 2,3,7,8,12,13,17,18-octabromo-5,10,15-tripenBr8(F5Ph)3Cor tafluorophenylcorrole Br8(F5Ph)4Por 2,3,7,8,12,13,17,18-octabromo-5,10,15, 20-tetrapentafluorophenylporphyrin Br8(Ph)3Cor 2,3,7,8,12,13,17,18-octabromo-5,10,15-triphenylcorrole Br8(Ph)4Por 2,3,7,8,12,13,17,18-octabromo-5,10,15, 20-tetraphenylporphyrin Cor general corrole macrocycle DCM dichloromethane DMF N,N-dimethylformamide (Et)8Cor 2,3,7,8,12,13,17,18-octaethylcorrole (Et)8Por 2,3,7,8,12,13,17,18-octaethylporphyrin (Me)8Cor 2,3,7,8,12,13,17,18-octamethylcorrole HOMO highest occupied molecular orbital LUMO lowest unoccupied molecular orbital OEC 2,3,7,8,12,13,17,18-octaethylcorrole OEP 2,3,7,8,12,13,17,18-octaethylporphyrin OMC 2,3,7,8,12,13,17,18-octamethylcorrole (Ph)3Cor 5,10,15-triphenylcorrole PhCN benzonitrile (Ph)4Por 5,10,15,20-tetraphenylporphyrin Por general porphyrin macrocycle Py pyridine SCE saturated calomel electrode TBC tetrabenzocorrole THF tetrahydrofuran TPC 5,10,15-triphenylcorrole TPP 5,10,15,20-tetraphenylporphyrin
Table 14. Comparison of Half-Wave Potentials for the First Oxidation (V vs SCE) of Metallocorroles and Metalloporphyrins with Similar Structuresa corrole macrocycle M
III
(Et)8Cor
(Ph)3Cor
0.36 0.61 0.11 0.43
0.32 0.86
MnIII FeIII(NO) CoIII CuIII AgIII
(F5Ph)3Cor
Br8Cor
0.71 1.07 0.89 0.98 0.76 1.12 1.14 0.75 1.16 1.27 porphyrin macrocycle75
Br8(F5Ph)3Cor 1.25
1.44
MII
(Et)8Por
(Ph)4Por
(F5Ph)4Por
Br8Por
Br8(F5Ph)4Por
NiII CuII ZnII
0.92 0.84 0.63
1.04 1.07 0.82
1.41 1.51 1.37
1.19 1.18 0.96
1.50 1.57
a
See discussion in text and Tables 12 and 13 for literature references to the original data. reactions of transition-metal complexes, redox reactions of dinuclear metal−metal bonded complexes, spectroelectrochemistry, and fullerene chemistry. He has published more than 600 research papers and edited more than 75 books while directing a research group, which, in total, has numbered over 125 different graduate students and postdoctoral associates. Dr. Kadish is Editor-in-Chief of the Journal of Porphyrins and Phthalocyanines and also serves as President of the Society of Porphyrins and Phthalocyanines, a position he has held continuously since June 2000.
ACKNOWLEDGMENTS We thank Yang Song, Jialiang Zhu, Xiaoqin Jiang, Xiangyi Ke, Wenqian Shan, and Lei Cong for help with the collection of data for the preparation of the manuscript. Support from the National Natural Science Foundation of China (21501070), Jiangsu University Foundation (05JDG051, 15JDG131), and the Robert A. Welch Foundation (K.M.K., E-680) are gratefully acknowledged.
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