Electronic Structure of Corrole Derivatives: Insights from Molecular

Review pubs.acs.org/CR

Electronic Structure of Corrole Derivatives: Insights from Molecular Structures, Spectroscopy, Electrochemistry, and Quantum Chemical Calculations Abhik Ghosh* Department of Chemistry and Center for Theoretical and Computational Chemistry, UiT−The Arctic University of Norway, 9037 Tromsø, Norway ABSTRACT: Presented herein is a comprehensive account of the electronic structure of corrole derivatives. Our knowledge in this area derives from a broad range of methods, including UV−vis−NIR absorption and MCD spectroscopies, single-crystal X-ray structure determination, vibrational spectroscopy, NMR and EPR spectroscopies, electrochemistry, X-ray absorption spectroscopy, and quantum chemical calculations, the latter including both density functional theory and ab initio multiconfigurational methods. The review is organized according to the Periodic Table, describing free-base and main-group element corrole derivatives, then transition-metal corroles, and finally f-block element corroles. Like porphyrins, corrole derivatives with a redox-inactive coordinated atom follow the Gouterman four-orbital model. A key difference from porphyrins is the much wider prevalence of noninnocent electronic structures as well as full-fledged corrole•2− radicals among corrole derivatives. The most common orbital pathways mediating ligand noninnocence in transition-metal corroles are the metal(dz2)−corrole(“a2u”) interaction (most commonly observed in Mn and Fe corroles) and the metal(dx2−y2)−corrole(a2u) interaction in coinage metal corroles. Less commonly encountered is the metal(dπ)−corrole(“a1u”) interaction, a unique feature of formal d5 metallocorroles. Corrole derivatives exhibit a rich array of optical properties, including substituent-sensitive Soret maxima indicative of ligand noninnocence, strong fluorescence in the case of lighter main-group element complexes, and room-temperature near-IR phosphorescence in the case of several 5d metal complexes. The review concludes with an attempt at identifying gaps in our current knowledge and potential future directions of electronic−structural research on corrole derivatives.

CONTENTS 1. Introduction 1.1. Introductory Remarks 1.2. Methods for Electronic Structure Determination 1.3. Scope and Organization 2. Free-Base and Main-Group Corroles 2.1. Free-Base Corroles 2.1.1. Molecular Structure and Tautomerism 2.1.2. Acid−Base Properties 2.1.3. Photophysical Properties 2.1.4. Electrochemical Properties 2.2. Alkali Metal Corroles 2.3. Group 13 Corroles 2.3.1. Boron Corroles 2.3.2. Aluminum and Gallium Corroles 2.4. Group 14 Corroles 2.4.1. Germanium Corroles 2.4.2. Tin Corroles 2.4.3. Lead Corroles 2.5. Group 15 (Pnictogen) Corroles 2.5.1. Phosphorus Corroles 2.5.2. As, Sb, and Bi Corroles 3. Transition-Metal Corroles 3.1. Group 3 metallocorroles © XXXX American Chemical Society

3.2. Group 4 Metallocorroles (Ti, Zr, Hf) 3.3. Group 5 Corroles (V, Nb, Ta) 3.4. Group 6 Corroles (Cr, Mo, W) 3.4.1. Chromium Corroles 3.4.2. Molybdenum Corroles 3.4.3. Tungsten Corroles 3.5. Group 7 Metallocorroles (Mn, Tc, Re) 3.5.1. Manganese(III) Corroles 3.5.2. MnCl and Mn−Aryl Corroles 3.5.3. Electrochemistry and UV−Vis Spectroelectrochemistry of Mn(III), MnCl, and Mn−Aryl Corroles 3.5.4. Mn Corroles at the Formal +V and VI Oxidation States 3.5.5. Technetium and Rhenium Corroles 3.6. Group 8 Metallocorroles (Fe, Ru, Os) 3.6.1. Iron(III) Corroles 3.6.2. FeCl Corroles 3.6.3. Fe−Aryl Corroles 3.6.4. FeNO Corroles: Cryptic Noninnocence

B B C E F F F G H H J K K L N N P Q R R U W W

W X Y Y AA AB AE AE AG

AG AI AM AN AN AP AT AU

Special Issue: Expanded, Contracted, and Isomeric Porphyrins Received: August 30, 2016

A

DOI: 10.1021/acs.chemrev.6b00590 Chem. Rev. XXXX, XXX, XXX−XXX

Chemical Reviews 3.6.5. μ-Oxo Diiron Corroles: Another Case of Cryptic Noninnocence 3.6.6. Fe(V) and Fe(VI) Corroles? 3.6.7. Ru Corrole Dimers 3.6.8. RuNO Corroles 3.6.9. Osmium Corroles 3.7. Group 9 Metallocorroles 3.7.1. Cobalt Corroles: Structure, Bonding, and Spectroscopy 3.7.2. Co Corroles: Electrochemistry and Electrocatalysis 3.7.3. Rhodium and Iridium Corroles 3.8. Group 10 Metallocorroles (Ni, Pd, Pt) 3.9. Group 11 Metallocorroles (Cu, Ag, Au) 3.9.1. Comparative Structural Chemistry 3.9.2. Singlet−Triplet Gaps and Equilibria 3.9.3. Electronic Absorption and Emission Spectroscopy 3.9.4. Electrochemistry and Spectroelectrochemistry 3.10. Group 10 Metallocorroles 4. f-Block Metallocorroles 5. Overview and Future Directions Author Information Corresponding Author ORCID Notes Biography Acknowledgments Abbreviations References

Review

BS BT BU BW BX BX BX BX BX BX BX BY

synthetic routes have since been reported, including those for β-substituted corroles.7,8 Corroles sustain an extensive coordination chemistry that is quite different from that of porphyrins.9 There are two main reasons for this difference. Unlike porphyrins, which generally act as dianionic ligands, free-base corroles are triprotic acids (Figure 1) and coordinate as formally trianionic ligands.10 Second, as contracted porphyrins, corroles provide tight, sterically constrained environments for coordinated atoms. Both these characteristics frequently lead to formally high-valent transition-metal complexes that are one equivalent oxidized relative to their stable metalloporphyrin congeners. As discussed herein, many of the formally high-valent complexes do not contain true, high-valent metal centers; instead, the metal often has a normal oxidation state, while the corrole is oxidized, with partial to full corrole•2− character.11 Indeed, this phenomenon is so ubiquitous that metallocorroles may be said to provide new paradigms for noninnocent ligands. That said, the large class of main-group element corrole derivatives by and large comprises innocent corrole ligands, thus providing useful references for evaluating the noninnocence of transition-metal corroles.12,13 Theoretical studies14 have long indicated that corrole derivatives with redox-inactive coordinated atoms should follow Gouterman’s four-orbital model.15,16 According to this model, the two HOMOs of an aromatic porphyrinoid ligand are neardegenerate as are the two LUMOs, and these four molecular orbitals (MOs) are well separated energetically from all other occupied and unoccupied MOs. In terms of the related perimeter model,17,18 the two HOMOs and the two LUMOs correspond to the ML = ±4 and ±5 levels of the π-MOs of the 15-atom (C11N4) inner perimeter of the corrole. The Soret and Q bands then

Corrole, the fully aromatic analogue of the corrin ligand found in B12 cofactors, was first synthesized by Johnson and Kay in 1965 via photocyclization of an a,c-biladiene,1 which was itself the end product of a many-step synthesis. For the next three decades, the corrole field remained largely dormant, until Vogel and coworkers in 1994 reported multiple stable, formal Fe(IV) derivatives of octaethylcorrole.2 Formal Ni(III) and Cu(III) corroles were also reported soon thereafter.3 As described in depth at a later stage (sections 3.8 and 3.9), the electronic descriptions of these high-valent complexes have evolved significantly in recent years. In 1999, the Gross4 and Paolesse5 groups independently reported corrole formation in the course of pyrrole−aldehyde condensations,6 providing simple, one-pot access to the meso-triarylcorroles. A number of additional

Figure 2. Frontier MOs of Au[C], a paradigmatic innocent metallocorrole, comprising the Gouterman four-orbital set; [C] = unsubstituted corrole.

AX AY AZ BB BB BD BD BF BH BJ BL BM BN BN

1. INTRODUCTION 1.1. Introductory Remarks

Figure 1. Free-base corrole tautomers and corrole atomic numbering system. B


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Figure 3. Examples of saddling (χ) and ruffling (ψ) dihedrals. Note that for each corrole there are four distinct dihedrals for both saddling and ruffling.

correspond to allowed and forbidden transitions involving these four frontier MOs. The lower symmetry of a metallocorrole relative to a metalloporphyrin of course alleviates some of the forbiddeness of corrole Q bands. The presence of open-shell transition-metal centers can significantly modulate this picture, even though the four-orbital model generally remains a useful conceptual starting point. The majority of stable main-group corroles as well as several 5d metallocorroles exhibit electronic absorption spectra that correspond relatively straightforwardly to the four-orbital model. Figure 2 depicts the frontier MOs for unsubstituted gold(III) corrole (see section 3.9 for a detailed discussion), which we have chosen as an exemplar of an innocent C2v-symmetric metallocorrole. Note that the HOMO (which

Figure 4. Two paradigmatic metal(d)−corrole(π) orbital interactions responsible for ligand noninnocence.

transforms as b1 under C2v) and the HOMO−1 (a2 in C2v) resemble the classic porphyrin a2u and a1u HOMOs in shape; the LUMOs are also qualitatively similar to porphyrin LUMOs. To underscore the interconnections of porphyrin and corrole chemistry, we will generally use the D4h irreducible representations to describe the corrole frontier MOs in this review. In terms of molecular structure, corroles are less diverse than porphyrins.19,20 The archetypal metallocorrole may be said to be a five-coordinate, square-pyramidal complex with a nearly planar to mildly domed corrole macrocycle. Such a geometry is natural for many coordinated atoms that do not easily fit within the constricted N4 cavity. Higher degrees of doming are also observed, most often as a result of the steric demands of a large, coordinated atom. In contrast, ruffled metallocorroles are virtually unknown, and the saddled conformation is exclusively limited to coinage metal corroles (see Figure 3 for a definition of these deformations).21 The general lack of ruffled and saddled corrole derivatives may be attributed to the rigidity of the direct C1−C19 bipyrrole linkage. Ruffling in particular is energetically extremely unfavorable for metallocorroles. DFT calculations on the simple metallocorrole model complex Co[Cor](PH3) (Cor = unsubstituted corrole) have predicted that a ruffling deformation of ψ = 40°, a relatively modest deformation, costs as much as 12 kcal/mol. Even for sterically hindered βoctabromo-meso-triarylcorrole complexes, the corrole macrocycle is generally essentially planar.

Table 1. Soret Maxima (nm) of Different Tris(p-Xphenyl)corrole Derivatives series Class I Cu[TpXPC] Cu[F8TpXPC] Cu[Br8TpXPC] Cu[(CF3)8TpXPC] Mn[TpXPC]Cl Fe[TpXPC]Cl Fe[F8TpXPC]Cl Fe[TpXPC](NO) {Fe[TpXPC]}2(μ-O) Co[TpXPC](PPh3) Ag[Br8TpXPC] Pt[TpXPC](Ar)(Ar′) Class II H3[TpXPC] H3[F8TpXPC] H3[Br8TpXPC] {(FBOBF)[TpXPC]}− Cr[TpXPC](O) Mn[TpXPC]Ph Fe[TpXPC]Ph Mo[TpXPC](O) Rh[TpXPC](PPh3) Ag[TpXPC] W[TpXPC]2 Re[TpXPC](O) Os[TpXPC](N) Pt[TpXPC](PhCN) (Ar) Au[TpXPC] Au[Br8TpXPC]

H

CH3

OCH3

for details, see

423 402 353 385 383 385 416, 448 443

413 409 439 459 433 411 355 390 386 387 425 453

418 421 453 471 441 421 360 400 389 392 438 460

433 436 468 507 460 426 367 416 375, 410 399 450 475

section 3.9.3 section 3.9.3 section 3.9.3 section 3.9.3 section 3.5.2 section 3.6.2 section 3.6.2 section 3.6.4 section 3.6.5 section 3.7.1 section 3.9.3 section 3.8

417 404 447 421 404 398 384 439 431 423 356 438 441 430

417 403 444 419 403 394 383 438 429 423 357 439 442 426

417 405 445 415 404 389 383 439 430 423 359 440 443 427

421 410 450

441 445 427

section 2.1.3 section 2.1.3 section 2.1.3 section 2.3.1 section 3.4.1 section 3.5.2 section 3.6.3 section 3.4.2 section 3.7.3 section 3.9.3 section 3.4.2 section 3.5.5 section 3.6.9 section 3.8

419 429

418 429

420 430

420 431

section 3.9.3 section 3.9.3

CF3 407 401 436

404 387 385 440 427 423

1.2. Methods for Electronic Structure Determination

Our understanding of the electronic structure of corroles (and related molecules) derives from a wide range of methods, including UV−vis absorption and MCD spectroscopies, singlecrystal X-ray structure determination, vibrational spectroscopy, NMR and EPR spectroscopies, electrochemistry, X-ray absorption spectroscopy, and quantum chemical calculations. The Soret absorption maxima of a set of meso-tris(p-X-phenyl)corrole (TpXPC) complexes provide a simple test of the innocence/ noninnocence of the corrole ring system. Thus, a Soret maximum that redshifts markedly with increasing electron-donating character of X (as for Class I in Table 1) is indicative of noninnocent or corrole•2− character. On the other hand, if the C


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Table 2. Redox Potentials (Volts vs SCE) and Electrochemical HOMO−LUMO Gaps for Selected TPC Derivativesa compounds

E1/2ox1

E1/2red1

corroles with redox-inactive coordinated atoms 0.79 −1.36 Ge[TPC](OCH3) Sn[TPC](Ph) 0.57 −1.47 P[TPC](OH)2 0.72 −1.41 Re[TPC]O 1.01 −1.23 Os[TPC]N 0.91 −1.28 Au[TPC] 0.80 −1.38 corroles with redox-active coordinated atoms Cr[TPC]O 0.92 −0.106 Mo[TPC]O 0.94 −0.54 Mn[TPC] 0.32 −1.46 Mn[TPC]Cl 1.032 0.093 Mn[TPC]Ph 0.77 −0.95 (ir) Fe[TPC]Cl 1.068 0.05 Fe[TPC]NO 0.86 −0.33 Fe[TPC]Ph 0.82 −0.35 (Fe[TPC])2O 0.635 −0.305 Co[TPC](PPh3) 0.51 −0.73 (ir) Cu[TPC] 0.76 −0.20 Ag[TPC] 0.73 −0.86 PtIV[TPC](Ar) 0.63 −0.83 (PhCN) 0.88 0.09 PtIV[TPC•2−](Ar) (Ar′) a

EHOMO−LUMO

section

2.15 2.04 2.13 2.24 2.19 2.18

section 2.4.1 section 2.4.2 section 2.5.1 section 3.5.5 section 3.6.9 section 3.9.4

1.026 1.48 1.78 0.939

0.96 1.59 1.46

section 3.4.1 section 3.4.2 section 3.5.1 section 3.5.2 section 3.5.2 section 3.6.2 section 3.6.4 section 3.6.3 section 3.6.5 section 3.7.2 section 3.9.4 section 3.9.4 section 3.8

0.79

section 3.8

1.018 1.19 1.17 0.94

Table 3. Redox Potentials (Volts vs SCE) and Electrochemical HOMO−LUMO Gaps for Selected TPFPC Derivatives compounds

E1/2ox1

E1/2red1

corroles with redox-inactive coordinated atoms 0.55 −1.58 Al[TPFPC](py)2 Ga[TPFPC](py) 0.70 −1.49 Ga[F8TPFC](py) 1.13 −1.06 Ga[Cl8TPFC](py) 1.16 −0.89 Ga[Br8TPFC](py) 1.11 −0.86 Ge[TPFPC](OH) 1.13 −0.99 Sn[TPFPC]Cl 1.20 −0.94 P[TPFPC](OH)2 1.05 −1.05 Sb[TPFPC](py) 1.00 −1.10 Sb[TPFPC]F2 1.47 −0.64 Au[TPFPC] 1.29 −1.02 corroles with redox-active coordinated atoms Cr[TPFPC]O 1.24 0.11 Cr[TPFPC](NMes) 1.31 −0.47 Mo[TPFC]O 1.32 −0.35 Mn[TPFPC] 0.71 −1.01 Mn[TPFPC](NMes) 1.21 −0.36 Mn[Br8TPFPC] 1.67 0.01 (NMes) Fe[TPFPC]Cl 0.44 −1.01 (ir) Fe[TPFPC](NO) 1.07 0.0 Ru[TPFPC](NO) 0.72 −0.42 {Ru[TPFPC]}2 0.76 −0.48 Co[TPFPC](py)2 0.67 Co[TPFPC](PPh3) 0.70 −0.40 Rh[TPFPC](py)2 0.72 Rh[TPFPC](PPh3) 0.79 Ir[TPFPC](tma)2 0.66 Ir[Br8TPFPC](tma)2 1.19 −1.21 Ir[TPFPC](py)2 0.71 Ir[TPFPC](PPh3) 0.72 Cu[TPFPC] 1.14 0.12 Ag[TPFPC] 1.27 −0.31

ir = irreversible.

Soret maximum is independent of the para substituent X (as for Class II), the corrole is expected to be innocent. This simple optical probe has proven surprisingly powerful; thus, as discussed in sections 3.6, 3.8, and 3.9, Ghosh and co-workers used it in a predictive manner to identify new families of noninnocent metallocorroles, including FeNO (section 3.6.4) and FeOFe (section 3.6.5) corrole derivatives. On the basis of a time-dependent density functional theory (TDDFT) study of copper triarylcorroles,22 the substituentsensitive Soret maxima are currently thought to reflect aryl-tocorrole charge transfer character mixing in with one or more transitions in the Soret envelope. What is remarkable is the regularity with which the Soret maxima of noninnocent TpXPC derivatives redshift with increasing electron-donating character of X; indeed, the redshifts are almost always monotonic with respect to the Hammett substituent constants for X. Given that the Soret band consists of multiple transitions of comparable intensity, the regular substituent-induced shifts of the peak maximum do not really follow from qualitative first principles. For the time being, they are best viewed as an empirical probe of ligand noninnocence that has been reliably applied across many families of metallocorroles. Not surprisingly, NMR and EPR spectroscopies also serve as valuable probes of corrole radical character. NMR spectroscopy has proved particularly useful for iron and copper corroles. For copper corroles (section 3.9.2), temperature-dependent 1H NMR spectroscopy provides an effective probe of the singlet− triplet equilibrium. For iron triarylcorroles (sections 3.6.1−3.6.3), the β- and meso-aryl proton shifts provide a detailed experimental probe of the overall spin density profile. EPR spectroscopy can address such issues as to whether a given radical is metal or corrole centered. In an instructive comparative study of Group 9 M[TPFPC](py)2 complexes (section 3.7),

EHOMO−LUMO

section

2.13 2.19 2.16 2.05 1.97 2.12 2.14 2.10 2.10 2.11 2.31

section 2.3.2 section 2.3.2 section 2.3.2 section 2.3.2 section 2.3.2 section 2.3.2 section 2.4.2 section 2.5.1 section 2.5.2 section 2.5.2 section 3.9.4

1.13 1.78 1.67 1.72 1.57 1.68

section 3.4.1 section 3.4.1 section 3.4.2 section 3.5.1 section 3.5.4 section 3.6.2

1.45 1.07 1.14 1.24

section 3.6.4 section 3.6.4 section 3.6.7 section 3.6.7 section 3.7.2 section 3.7.2 section 3.7.3 section 3.7.3 section 3.7.3 section 3.7.3 section 3.7.3 section 3.7.3 section 3.9.4 section 3.9.4

1.10

2.40

1.02 1.58

Gray and co-workers used EPR spectroscopy to show that the one-electron-oxidized forms of the Co and Rh complexes were essentially corrole radicals, whereas the oxidized Ir complex appeared best described as Ir(IV). In the same way, Ghosh and co-workers showed that charge-neutral diarylplatinum corroles are best described as full-fledged corrole radicals, i.e., PtIV[corrole•2−]ArAr′ (section 3.8). Ligand noninnocence manifests itself in the structural chemistry of metallocorroles, where the clues range from pronounced and unmistakable to subtle and barely discernible.19 Given the rarity of saddling in metallocorroles, the fact that copper corroles are generally quite strongly saddled (with χ typically >40°) suggests that a specific metal−ligand orbital interaction may be at work. Quantum chemical calculations readily identified such an orbital interaction, viz., a Cu(dx2−y2)− corrole(“a2u”) interaction (Figure 4, see section 3.9 for details), which is symmetry forbidden for a planar corrole but becomes symmetry allowed under saddling. This interaction allows electron density to flow from the a2u corrole π-HOMO into the formally empty Cu dx2−y2 orbital of the “Cu(III)” center; copper corroles thus have substantial CuII−corrole•2− character. For square-pyramidal complexes, the metal(dz2)−corrole(a2u) interaction may also lead to a noninnocent corrole•2− ligand (Figure 4). Such an interaction has been best studied in FeCl D


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manner. Although formally a single-configuration method, spinunrestricted DFT provides an excellent first-order description of spin-coupled systems, such as bridged, multimetal active sites of metalloenzymes, single-molecule magnets, and metalloradicals such as noninnocent metallocorroles. Although the resulting broken-symmetry electron densities (where the up- and downspin electrons are allowed to spatially separate) do not correspond to a pure spin state, Noodleman outlined a protocol called the broken-symmetry method that allows the calculation of the coupling between the individual spin sites.24 Section 3.6.5 provides a worked example of the method applied to μ-oxo diiron corroles. Multiconfigurational ab initio methods provide the most natural and rigorous approach for the theoretical analysis of spin-coupled systems, but they are far more computationally demanding than DFT and have only been applied to a handful of metalloporphyrin and metallocorrole systems. Multiconfigurational methodology, however, is currently advancing rapidly, and it may not be long before such methods are applicable to essentially the full range of noninnocent metalloporphyrinoids. On a final note, given the ubiquitous questions about the oxidation state of the metal in metallocorroles, X-ray absorption spectroscopy (XAS) has been surprisingly absent as an ideal tool for this purpose. Major advances in technology promise that the present state of affairs will change soon.

corroles (section 3.6.2) but is also thought to occur in a number of other metallocorroles (sections 3.5.2, 3.6.4 and 3.6.5). This type of ligand noninnocence also manifests itself in structural effects, specifically via small but characteristic bond length alternations (on the order of 0.02 Å) within and adjacent to the bipyrrole part of the corrole macrocycle. These effects are generally discernible in high-quality X-ray structures and DFToptimized geometries as well as via vibrational spectroscopy. An interesting question concerns whether electrochemical redox potentials (as exemplified by Tables 2 and 3) provide an indication of the innocence or noninnocence of a given metallocorrole. The general answer to this question is no. Thus, for a given corrole ligand, FeCl (best described as FeIII− corrole•2−) and FePh (best described as FeIV−corrole3−) complexes exhibit similar redox potentials (see sections 3.6.2 and 3.6.3). That said, the electrochemical HOMO−LUMO gap, defined as the algebraic difference between the first oxidation and the first reduction potentials, is very useful in this regard. A value of ∼2.2 V, which corresponds to the π−π* energy gap for most closed-shell corrole ring systems, is generally indicative of corrole-centered oxidation and reduction. UV−vis spectroelectrochemistry is another valuable electronic-structural probe. A sharp increase in the intensity of the Soret band is generally associated with restoration of corrole3− character and macrocycle aromaticity. Among the quantum chemical tools applied to metalloporphyrins and metallocorroles, density functional theory (DFT) is by far the most important.23 Unlike conventional wave function methods, DFT assumes a functional dependence of the energy on the electron density and thereby accounts for both exchange and correlation in an implicit and highly efficient

1.3. Scope and Organization

The present review attempts to provide a comprehensive account of the electronic structure of corrole derivatives through the mid-2016. The term “electronic structure” is interpreted broadly, and electronic-structural insights derived with all of the above-mentioned methods, as well as synthetic and reactivity studies, are covered in depth. On the other hand, “corrole

Table 4. Absorption and Emission Maxima (nm) and Fluorescence Lifetimes (ns) and Quantum Yield for Main-Group Corrole Derivatives

a

compounds

solvents

λmax,abs (ε × 10−4)

λmax,em

τ

Φf

section

Al[TPFPC](py)2 Ga[TPFPC](py) Ga[TPFC](py)2 {Ge[TPC]2}O Ge[TPC](OMe) P[TPFPC](OH)2 P[TPFPC](F2) P[TPC](OH)2 P[TPC](OCH3)2

toluene/pyridine toluene pyridine CH2Cl2 CH2Cl2 toluene toluene THF methanol

432 (29.4), 620 (3.7) 424 (28.3), 596 (2.4) 426 (21.4), 610 (1.8) 399 (36.3), 528 (1.34), 567 (2.1), 598 (2.4) 412 (16.0), 522 (0.7), 560 (0.9), 590 (2.1) 394 (3.4), 414 (14.9), 563 (1.2), 588 (2.0) 385 (6.8), 406 (48.7), 558 (2.3), 574 (2.5) 411(5.34), 421 (5.31) 407 (5.41), 514 (5.36)

627, 689 602, 656 606, 620, 660, 680 n.r.a n.r.a 592, 648 577, 631 572, 610, 667 600, 656

6.60 3.04 3.45 30 cm−1 range spanned by β-unbrominated FeNO triarylcorroles. Although it is tempting to attribute the relative constancy of the Soret maxima to a lesser degree of radical character of the electron-deficient octabromocorrole ligands, that is actually not the case. Instead, the lack of substituent effects in these complexes is thought to reflect steric inhibition of resonance: as a result of the steric effects of the β-bromines, the meso aryl groups are essentially orthogonal to the corrole and hence much less able to transmit their electronic effects. Indeed, spin-unrestricted B3LYP calculations yielded exactly the same type of broken-symmetry solutions for Fe[Br8TPC](NO) and Fe[Br 8 TPFPC](NO) as for Fe[TPC](NO), indicative of the same {FeNO}7−(corrole•2−) electronic description.186 Further, both a single-crystal X-ray structure and B3LYP/STO-TZ2P calculations indicated characteristic bond distance alternations for Fe[Br8TPFPC](NO), exactly analogous to those predicted for Fe[TPC](NO). In contrast, no such bond length alternations were observed for Ir[Br8TPFPC](Me3N)2, where the corrole is expected to be innocent (Figures 109 and 110). Electrochemical studies in several laboratories have established that FeNO corroles typically undergo reversible oneelectron oxidation and reduction. DFT calculations strongly suggest that both processes are ligand centered; in other words, the oxidized, neutral, and reduced species are all {FeNO}7, with the corrole best described as corrole−, corrole•2−, and corrole3−, respectively.186 UV−vis spectroelectrochemical studies of a series of FeNO corroles appear to be consistent with these

Figure 110. Skeletal geometries (Å) of Fe[Br8TPFPC](NO) and Ir[Br8TPFPC](Me3N)2: X-ray (left) and B3LYP/STO-TZ2P (Cs, right) distances. Red and blue arrows indicate longer and shorter bonds, respectively. Reproduced from ref 186 according to copyright retained by the author.

assignments, even though the prevailing view of FeNO corroles at the time of publication was one of a simple {FeNO}6 complex.187 IR spectroelectrochemical studies on two FeNO corroles (Figure 111), however, revealed drastic downshifts of ∼170 cm−1 in the νNO’s upon one-electron reduction, which would tend to argue against {FeNO}7 for both the neutral and the anionic states. However, the absence of metal−ligand antiferromagnetic coupling in the reduced {FeNO}7−corrole3− state is expected to result in a strongly bent FeNO unit (as in {FeNO}7 porphyrins); this sharp difference in geometry may well account for the large difference in νNO between the neutral and the anionic states. In an interesting flash photolysis study of Fe[TpXPC](NO) derivatives, Ford and co-workers found that the second-order NO recombination rates, ranging within 1−9 × 109 M−1 s−1 at room temperature, were 3 orders of magnitude higher than that observed for the S = 0 {FeNO}6 porphyrin complex Fe[TPP](NO2)(NO) and more in line with that observed for Fe[TPP](NO).159,174 The explanation appears to be that the S = 3/2 fourcoordinate intermediates FeIII[TpXPC] (S = 3/2) are better AW


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Figure 111. Thin-layer IR spectral changes during the first one-electron reduction of two dinitro-substituted FeNO corroles (whose structures are shown) in CH2Cl2, 0.1 M TBAP at the indicated applied potentials. Adapted with permission from ref 187. Copyright 2012 American Chemical Society.

poised to bind NO than FeIII[TPP](NO2), which is believed to be a low-spin S = 1/2 species. Indeed, water-soluble iron(III) corroles with labile axial ligands were found suitable for NO sensing down to the ppm level.188 3.6.5. μ-Oxo Diiron Corroles: Another Case of Cryptic Noninnocence. The first μ-oxo diiron corrole {Fe[OEC]}2O was reported by Vogel and co-workers in their seminal 1994 paper,2 and several additional examples have since been reported by Gross and Ghosh.72,127 On the basis of the Mössbauer parameters of {Fe[OEC]}2O (δ 0.02 mm·s−1, ΔEQ 2.35 mm· s−1),3 Fe2(μ-O) corroles have been thought of as low-spin Fe(IV) complexes akin to heme protein Compound I and II intermediates. In sharp contrast to fleeting enzymatic Fe(IV)− oxo intermediates,189−191 however, the Fe2(μ-O) corroles are remarkably stable, allowing storage and manipulations such as column chromatography and single-crystal X-ray crystallography under relatively ordinary conditions. According to a recent study, ligand noninnocence involving the corrole macrocycle plays a significant role in determining the stability of these complexes.192 Early studies on {Fe[TpXPC]}2O derivatives showed that the Soret maximum redshifts modestly across the series X = CF3 (383 nm), H (386 nm), and Me (389 nm).127 While revisiting these compounds, Ghosh and co-workers synthesized a fourth member of the series, X = OMe, and found it to exhibit a strongly perturbed optical spectrum (Figure 112).192 Thus, although the Soret maximum of this complex (375 nm) turned out to be blueshifted relative to those of the other three complexes, a prominent shoulder at ∼410 nm appeared to be a plausible

candidate for the LLCT transition characteristic of noninnocent metallocorroles. A reexamination of the published X-ray structure of {Fe[TPFPC]}2O also revealed clear evidence of bond length alternations in and adjacent to the bipyrrole part of the molecule (Figure 113). A detailed DFT study of {Fe[TPC]}2O was accordingly undertaken.192 The broken-symmetry B3LYP/ZORA-STOTZP spin density profile clearly indicated the following intramolecularly spin-coupled description (Figure 114)

Figure 113. Bond distance alternations in μ-oxo diiron corrole structures: (a) upper and lower values refer to the two symmetrydistinct corrole rings in the crystallographic structure; (b) upper and lower values refer to average B3LYP and BP86 values, both obtained with STO-TZ2P basis sets. Reproduced from ref 192 pursuant to rights retained by the author.

Figure 112. Electronic absorption of {Fe[TpXPC]}2(μ-O) in dichloromethane. Reproduced from ref 192 pursuant to rights retained by the author. AX


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Figure 114. Broken-symmetry B3LYP/STO-TZP spin density of {Fe[TPC]}2O (contour = 0.0035 e·Å−3). Phenyl groups have been omitted for clarity.

MS = 0: corrole•2 −( ↓ ) − Fe III( ↑ ↑ ↑ ) − Fe III( ↓ ↓ ↓ ) − corrole•2 −( ↑ )

Importantly, the pure functional BP86 also broke spin symmetry and provided an essentially identical spin density profile. Both B3LYP and BP86 geometry optimizations also reproduced the expected bond length alternations (Figure 113). Finally, in a rather stringent test of the broken-symmetry DFT approach, a B3LYP/STO-TZP vibrational analysis of {Fe[TPC]}2O accurately reproduced the experimentally observed Fe−O−Fe asymmetric stretching frequency (829 cm−1) as well as the 18O isotope effect (−38 cm−1, Figure 115). The intramolecular spin couplings of {Fe[TPC]}2O were analyzed following the Heisenberg−Dirac−van-Vleck (HDvV) spin Hamiltonian H = JFe − corrole (SFe·Scorrole) + JFe − Fe ′(SFe·SFe ′) + JFe ′− corrole ′(SFe ′·Scorrole′)

(1)

The analysis required calculations on the following MS = 2 and 4 states, which fortunately converged,

Figure 115. Experimental and simulated (BS-B3LYP/STO-TZP) IR spectra of {Fe[TPC]}2O, with the Fe−O−Fe asymmetric stretch marked in color. Reproduced from ref 192 pursuant to rights retained by the author.

MS = 2: corrole•2 −( ↓ ) − Fe III( ↑ ↑ ↑ ) − Fe III( ↑ ↑ ↑ ) − corrole•2 −( ↓ ) MS = 4: corrole•2 −( ↑ ) − Fe III( ↑ ↑ ↑ ) − Fe III( ↑ ↑ ↑ )

B3LYP/STO-TZ2P calculations yielded the following vertical HDvV coupling constants:

− corrole•2 −( ↑ )

JFe − corrole = JFe ′− corrole ′ = 0.355 eV (2860 cm−1)

The energies of the three MS states, given by eqs 2−4,

and

E(MS = 0) = −3/4JFe − corrole − 9/4 Fe − Fe ′ − 3/4JFe ′− corrole ′

JFe − Fe ′ = 0.068 eV (548 cm−1)

(2)

E(MS = 2)

The rather high value of JFe−corrole agreed qualitatively with that calculated for Fe[TPC]Cl with the same method (1639 cm−1) and may also be expected on qualitative grounds: an electron in an Fe dz2 orbital, which is expected to be half-occupied for intermediate-spin Fe(III), is ideally situated to strongly couple with a corrole a2u radical. The JFe−Fe′ value is also in qualitative accord with that observed for other μ-oxodiiron(III) complexes, such as [FeIII2(μ-O)(HBpz3)2]2+ (242 cm−1)193 and [FeIII2(μO)(Me3TACN)2]4+ (238 cm−1).194 3.6.6. Fe(V) and Fe(VI) Corroles? There are intriguing indications that Fe(V) and even Fe(VI) corroles might exist as reactive intermediates.195 Newcomb reported the formation of a highly reactive, putative Fe(V)−oxo intermediate with a sharp

= −3/4JFe − corrole + 9/4JFe − Fe ′ − 3/4JFe ′− corrole ′ (3)

E(MS = 4) = 3/4JFe − corrole + 9/4JFe − Fe ′ + 3/4JFe ′− corrole ′ (4)

allowed the evaluation of the two unique coupling constants JFe − corrole = JFe ′− corrole ′ = 1/3[E(MS = 4) − E(MS = 2)] (5)

JFe − Fe ′ = 2/9[E(MS = 2) − E(MS = 0)]

(6) AY


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Soret band upon flash photolysis of Fe[TPFPC](OX) species in CH3CN, where X = NO2 or ClO2 (i.e., axial nitrate or chlorate complexes) (Figure 116).196 The same intermediate could also be generated by photodisproportionation of {Fe[TPFPC]}2(O).197 In the absence of an added substrate, the intermediate exhibited rapid pseudo-first-order decay with a rate constant of 200 s−1. It reacted rapidly with cyclooctene and ethylbenzene with second-order rate constants some 3 orders of magnitude higher than those for Mn[TPFPC](O). The Fe(V) formulation of the species is based largely on its sharp Soret absorption and high reactivity and appears plausible but is still not proven. Pertinent in this regard are several reports of authentic Fe(V)−oxo intermediates, most notably with TAMLtype ligands.198−200 An older DFT study of the model complexes Fe[C](O) and Fe[Cz](O) by Wasbotten and Ghosh addressed the relative energetics of true Fe(V)−oxo versus FeIV(O)−corrole•2− states.201 For the corrole derivative Fe[C](O), the calculations predicted three nearly equienergetic low-energy states, including two S = 1/2 Fe(V) states with dxy2(dxz − dyz)1 and dxy2(dxz + dyz)1 configurations (using the notation of section 3.6.1) and an S = 3/ 2 FeIV(O)−corrole•2− state. For the corrolazine derivative Fe[C](O), on the other hand, the calculations indicated the two alternative dπ1 Fe(V) states as the lowest energy states with the S = 3/2 FeIV(O)−corrole•2− state some 0.5 eV higher in energy. These results are seemingly at variance with EPR, Mössbauer, and DFT studies on putative Fe[TBP8Cz](O)202 and Fe[TBP8Cz](NTs)203 by Goldberg and co-workers indicating an antiferromagnetically coupled FeIV−corrolazine•2− formulation for these species. It appears that the field would benefit significantly from a comprehensive theoretical reinvestigation of Fe(V) corrole and corrolazine intermediates. Simkhovich and Gross reported that Fe corroles exemplified by Fe[TPFPC](Cl), Fe[TPFPC](NO), and Fe[TPFPC](Et2O)2 as well as certain TDCPC analogues are potent catalysts for olefin aziridination with PhINTs as the nitrene source.204 Of the various catalysts investigated, Fe[TPFPC](Cl) was found to exhibit the unique ability to effect aziridination with cheap, commercially available chloramine T as the nitrogen source. In

Figure 117. UV−vis spectra H3[Et6Me2C] (light trace) and {Ru[Et6Me2C]}2 (dark trace) in dichloromethane. (Inset) X-ray structure of {Ru[Et6Me2C]}2. Redrawn with permission based on ref 208. Copyright 2002 Elsevier.

Figure 118. Molecular orbital interaction diagram for {Ru[Cor]}2 under D4h symmetry. Reproduced with permission from ref 210. Copyright 2003 Wiley-VCH Verlag GmBH & Co.

addition, Gross and co-workers used Fe[TPFPC](Cl) to effect hydroxylation and olefin epoxidation (with PhIO as the oxygen source),205 sulfoxidation (with H2O2),206 olefin cyclopropanation.72,205 Thus far, there is little concrete information about the nature of the active group-transfer agents involved in these reactions. Thus, a key question for the future concerns whether the processes involve discrete, formal Fe(V) or Fe(VI) intermediates.207 3.6.7. Ru Corrole Dimers. The insertion of 4d and 5d metals into corroles is often a capricious affair, and the synthesis of Ru corroles nicely illustrates the difficulties involved. Early attempts at Ru insertion into β-octamethylcorrole by Boschi and coworkers with RuCl3 or Ru3(CO)12 in refluxing DMF resulted in Ru(II) octamethylporphyrin complexes. Guilard, Kadish, and coworkers successfully accomplished Ru insertion into two different β-octaalkylcorroles via brief interaction with [Ru(COD)Cl2]2 and triethylamine in refluxing 2-methoxyethanol.208 The triethylamine is necessary to capture the HCl produced in the reaction, which otherwise protonates the freebase corrole and stops the metal insertion. Formation of the product, a diamagnetic Ru−corrole dimer, is accompanied by the appearance of a sharply blueshifted Soret band (λmax 336 and 393 nm), as shown in Figure 117 for {Ru[Et6Me2C]}2.209 The same

Figure 116. Proposed reaction scheme for the photogeneration and decay of putative FeV[TPFPC](O). Reproduced with permission from ref 196. Copyright 2005 American Chemical Society. AZ


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Figure 119. Cyclic voltammogram of {Ru[Et6Me2C]}2 in CH2Cl2 with 0.1 M TBAP at −75 °C. Redrawn with permission based on ref 208. Copyright 2002 Elsevier.

Figure 121. EPR spectra of the {Ru[Et6Me2C]}2+ cation at different temperatures. Reproduced with permission based on ref 208. Copyright 2002 Elsevier.

of intermediate-spin Ru(III) centers with a dxy2dxz1dyz1dz21 configuration (Figure 118). For comparison, {Ru[OEP]}2 and {Ru[TPP]}2+ exhibit significantly longer Ru−Ru distances of 2.41 and 2.30 Å, consistent with bond orders of 2 and 2.5, respectively. As shown in Figure 119 for {Ru[Et6Me2C]}2, Ru−corrole dimers exhibit rich electrochemistry with five redox couples under standard cyclic voltammetry conditions.208 Thin-layer spectroelectrochemical measurements revealed only minor changes for the first two oxidations and the first reduction, indicating that these involve only the Ru−Ru unit (Figure 120). The third oxidation, a two-electron process, on the other hand, results in prominent new features at 367 and 486 nm, suggesting corrole-centered oxidation. The {Ru[Et6Me2C]}2+ cation obtained by AgClO4 oxidation of the neutral dimer was found to exhibit an isotropic EPR signal down to 100 K, but a rhombic

Figure 120. UV−vis spectroelectrochemistry of {Ru[Et6Me2C]}2 in CH2Cl2 with 0.2 M TBAP. Reproduced with permission based on ref 208. Copyright 2002 Elsevier.

synthesis method has also been applied to the synthesis of other Ru corrole derivatives, including {Ru[TPFPC]}2.210 The X-ray structures of {Ru[Et6Me2C]}2 and {Ru[TPFPC]}2 reveal structurally quite similar Ru−corrole cores, with Ru−Ru distances of 2.17−2.18 Å and a tetragonal, as opposed to squareantiprismatic, arrangement of the two corroles.209,210 Each Ru− corrole unit is somewhat domed, with Ru−N4 displacements of ∼0.5 Å and the N4 plane ≈ 0.3 Å above the mean corrole C19 plane. The short Ru−Ru distance is consistent with a RuIII2 unit with a σ2π4δ2δ*2 configuration, i.e., a bond order of 3, which may be thought of as arising from antiferromagnetic coupling of a pair BA


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Figure 122. Synthesis of RuNO corroles.

g-value pattern emerged at 77 K with g = 2.049, 1.991, and 1.928. The latter is similar to the EPR signature of other Ru2(III,IV) complexes, thus clearly indicating metal-centered one-electron oxidation for {Ru[Et6Me2C]}2, Figure 121. Cyclic voltammetry of {Ru[TPFPC]}2 in CH3CN also revealed a similarly rich electrochemistry, with five different redox couples but with redox potentials about 0.6−0.7 V upshifted relative to {Ru[Et6Me2C]}2 (Figure 119). 3.6.8. RuNO Corroles. Given that a Ru−corrole dimer is a thermodynamic sink of Ru−corrole interactions, the synthesis of monomeric, axially ligated Ru corroles is somewhat of challenge. By trapping a putative monomeric Ru corrole intermediate in the reaction mixture with NO, Gross and co-workers managed to isolate monomeric RuNO corroles, as shown in Figure 122. Use of H3[TPFPC] led to both {Ru[TPFPC]}2 (39%) and Ru[TPFPC](NO) (30%), but the more sterically hindered H3[TDCPC] led to only Ru[TDCPC](NO) (35−40%).210 The two RuNO corroles Ru[TPFPC](NO) and Ru[TDCPC](NO) exhibit IR νNO’s of 1790 and 1783 cm−1, which are marginally higher than those of structurally related FeNO corroles. In pyridine solution, the putative pyridine adducts Ru[TPFPC](NO)(py) and Ru[TDCPC](NO)(py) exhibit νNO’s of 1835 and 1827 cm−1, reflecting a significant trans effect, which are somewhat lower than that observed for {Ru[TPP](NO)(py)}+ (1879 cm−1).210 While it may be tempting to interpret these results in terms of a noninnocent {RuNO}7−corrole•2− electronic structure analogous to the one proposed for FeNO corroles, we at present lack the necessary additional evidence to either confirm or reject such a hypothesis. Both Ru[TPFPC](NO) and Ru[TDCPC](NO) exhibit one oxidation and two reduction waves in their cyclic voltammograms (Figure 123). Both complexes exhibit significantly lower oxidation potentials and significantly higher reduction potentials and hence much lower HOMO−LUMO gaps than innocent corrole derivatives with redox-inactive coordinated atoms. Gross and co-workers reasonably interpreted these observations as indicative of metal-centered oxidation and reduction processes.210 3.6.9. Osmium Corroles. Refluxing free-base corroles with Os3(CO)12 in a high-boiling solvent under an inert atmosphere led to recovery of the unchanged corroles after 12−24 h. The great stability of square-pyramidal OsVIN complexes suggested that addition of a metal azide to the reaction mixture might prove fruitful, which indeed turned out to be the case.211 The optimum solvent proved to be 1:2 DEGME/glycol and led to yields of ∼50% (Figure 124). The optical and electrochemical properties of OsVIN corroles are very similar to those of their ReVO counterparts (see Tables 1 and 2 for representative data on the TPC complex) and

Figure 123. Cyclic voltammograms of RuNO corroles in CH3CN with 0.05 M TBAP. Reproduced with permission from ref 210. Copyright 2003 Wiley-VCH Verlag GmBH & Co.

indicative of an innocent corrole. The three structurally characterized OsVIN corroles all exhibit OsN distances of 1.643−1.648 Å and domed corrole macrocycles with Os−Ncorrole distances of 1.990 ± 0.015 Å and Os−N4 displacements of ∼0.6 Å, i.e., structural parameters very similar to those of ReVO corroles. Resonance Raman measurements on the complexes identified at least two 15N-sensitive OsN vibrations, which for unlabeled Os[TPC](N) occur at approximately 1099 and 1079 cm−1 (Figure 125). According to preliminary DFT calculations, these vibrations may be viewed as symmetric and antisymmetric combinations of the OsN and Os−Ncorrole stretching modes, a coupling presumably mediated by macrocycle doming.211 Like certain other 5d metallocorroles, the OsN corroles (three examples) exhibit near-IR phosphorescence at room temperBB


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Figure 124. Synthetic route to OsN corroles. Reproduced from ref 211 pursuant to rights retained by this author.

1000 μs). The shortest phosphorescence lifetime is exhibited by Os[TpOMePC](N)] and the longest by [Os{TpCF3PC](N)], which correlates well with the HOMO−LUMO gaps of the dyes (as measured by the emission and Q-band maxima) and their phosphorescence quantum yields. The solution quantum yields are moderate but significantly higher than those of both Ir and Au corroles. The phosphorescence of the complexes is almost completely quenched by molecular oxygen in organic solvents, which makes them potentially suitable for application as oxygen-sensing materials. Oxygen sensors were accordingly constructed by immobilizing the OsN corroles in polystyrene. All three complexes exhibited similar bimolecular quenching constants kq = KSV/τ, where KSV is the Stern−Volmer constant and τ is the

ature (Figure 125 and Table 10).212 Observe that the emission maxima (like the Q maxima but unlike the Soret maxima) redshift somewhat with increasing electron-donating character of the para substituent X, i.e., Os[TpCF3PC](N) < Os[TPC](N) < Os[TpOMePC](N), indicating a decreasing HOMO−LUMO gap along the series, Figure 126. Importantly, all complexes can be excited with bright 590 and 605 nm LEDs, which is important for practical applications. The phosphorescence lifetimes for the three complexes examined are rather long (>100 μs) and intermediate between those of Pt(II) (τ ≈ 50−70 μs) and Pd(II) porphyrins (τ ≈ 300−

Figure 125. Resonance Raman spectra (λex = 413.1 nm) of Os[TPC](N) in CH2Cl2: (a) unlabeled, (b) 50%15N-labeled on Os N, (c) 14N−15N difference spectrum, (d) solvent spectrum. Reproduced from ref 211 pursuant to rights retained by the author.

Figure 126. Electronic absorption and phosphorescence spectra of three Os[TpXPC](N) derivatives. Reproduced from ref 212 pursuant to copyright retained by the author.

Table 10. Absorption and Emission Maxima (nm), Extinction Coefficients (M−1 cm−1), and Phosphorescence Lifetimes (μs) Quantum Yields (%) of Selected 5d Metallocorroles compounds

solvents

Os[TpCF3PC](N) Os[TPC](N) Os[TpOMePC](N) Ir[TPFC](tma)2 Ir[TPFC](Py)2 Ir[Br8TPFC](tma)2 Ir[TpCNPC](4,4′-bipy)2 Au[Br8TPFC] Au[TpCF3PC] Au[TpFPC] Au[TPC] Au[TpMePC] Au[TpOMePC]

toluene toluene toluene toluene toluene toluene THF toluene toluene toluene toluene toluene toluene

λmax,abs (ε × 10−4) 444 (8.02), 554 (1.07), 593 (1.73) 444(9.32), 555(1.28), 595(2.27) 447 (8.31), 558 (1.18), 601 (2.14) 390, 412, 448 (sh), 572, 638 406, 422, 458 (sh), 580, 646 388(4.21), 423 (5.98), 466 (4.33), 564 (1.71), 608 (3.77) 428 (11.10), 543 (1.10), 580 (3.68) 424 (5.05), 564 (4.31), 576 (4.38) 422 (5.66), 561 (4.94), 577 (4.98) 423 (5.20), 562 (4.44), 577 (4.55) 422 (5.71), 562 (4.95), 580 (5.08) 425 (5.24), 562 (4.45), 584 (4.62) BC

λmax,em

τ

Φ

ref

779 784 795 788 792 795 880 769 788 788 794 800 804

150 128 110 0.22 4.91 1.19

1.3 1.3 0.90 0.033 1.2 0.39

195 98 91 86 83 76

0.3 0.19 0.18 0.18 0.24 0.16

212 212 212 240 240 240 241 251 268 268 268 268 268


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multiple low-energy, potentially thermally accessible states for four-coordinate Co(III) corroles. Moreover, both pyridine and triphenylphosphine axial ligands of Co corroles are labile.217 Addition of pyridine to Co−PPh3 corrole complexes in dichloromethane ultimately results in complete conversion to Co−bispyridine derivatives. For several Co−corrole monopyridine adducts (with mixed alkyl and phenyl substituents) in dichloromethane, Guilard, Kadish, and coworkers determined log K1 (where K1 is the formation constant) values of 4.5 ± 0.8. For the formation of bispyridine adducts from monopyridine adducts, log K2 is only ∼2.0.224 Thus, in the absence of added pyridine, Co−bispyridine corroles in solution largely dissociate to the five-coordinate monopyridine complex. NMR and DFT studies strongly suggest that the latter are paramagnetic, most likely Co(II) corrole radicals. Attempted isolation of the monopyridine complex Co[TPFPC](py) via column chromatography of the corresponding bispyridine complex with a noncoordinating solvent as eluent indeed proved successful, but attempted crystallization of the monopyridine complex from the eluate led to an unexpected 3,3′-linked biscorrole, which was isolated and structurally characterized as the Co2(py)4 complex, upon addition of additional pyridine.217 Formation of the 3,3′-linked dimer provides strong evidence of corrole radical character in the five-coordinate complex Co[TPFPC](py). Four-coordinate cobalt corroles also bind a single carbon monoxide molecule, with equilibrium constants similar to those measured for pyridine binding (i.e., log K ≈ 4.5 ± 0.8).224 The coordinated CO exhibits νCO values of 2040−2080 cm−1, substantially above those exhibited by five-coordinate heme− CO complexes such as Fe[TPP](CO) (1969−1972 cm−1 in toluene), indicating significantly weaker backbonding and stronger CO bonding. Accordingly, a significant amount of effort has been dedicated toward exploiting Co corroles as key components of carbon monoxide sensors.225,226 A recent B3LYP-D3/STO-TZ2P study of Co[C](CO) has indicated a CO binding energy of 0.66 eV (based on electronic energies) and a Co−C(O) distance of 1.827 Å.227 Importantly, pure functionals indicate a substantially shorter Co−C(O) distance of ∼1.70 Å. Both B3LYP and pure functionals agree in predicting an S = 0 ground state for this species. The B3LYP-D3 calculations, however, yield a broken-symmetry (MS = 0), antiferromagnetically coupled CoII(dz21)−corrole•2− description for the lowest energy solution, with the corresponding ferromagnetically coupled (MS = 1) solution only ∼0.3 eV higher in energy. The

phosphorescence decay time. All sensors exhibited excellent photostability, a key consideration for practical applications. The OsN corroles also sensitized triplet−triplet annihilationbased upconversion with perylene and Solvent Green 5 as annihilators.212 The orange Q absorption of the complexes is a particularly attractive feature of these complexes in view of the limited number of sensitizers absorbing in this part of the spectrum. The upconversion process with orange light (595 nm) excitation may be summarized as S(S0) + hν → S*(T1) S*(T1) + A(S0) → S(S0) + A*(T1)

A*(T1) + A*(T1) → A*(S1) + A(S0)

A*(S1) → A(S0) + hν′

The upconverted fluorescence quantum yields of the OsN corroles were found to be about 1/4 to 1/3 that of the commonly used sensitizer Pt[TPTBP].212 3.7. Group 9 Metallocorroles

Cobalt corroles, the first metallocorroles to be synthesized, were first reported by A. W. Johnson and co-workers in the form of a set of Co octaalkylcorrole triphenylphosphine complexes.213 Rhodium corroles were also synthesized early on, with key reports on Rh[OMC](PPh3) by the Boschi and Kadish groups.214 Iridium corroles (see section 3.7.3) are of more recent provenance, and their deliberate synthesis by Gross, Gray, and co-workers set the stage for subsequent developments in the now-vibrant field of heavy element corroles. 3.7.1. Cobalt Corroles: Structure, Bonding, and Spectroscopy. Cobalt corroles are best known in the form of diamagnetic triphenylphosphine- and bis(pyridine)-ligated complexes.215−220 Both classes of compounds exhibit short equatorial Co−N distances of ∼1.89 Å, indicating an optimum fit with the corrole ligand. The Co−PPh3 complexes exhibit Co−P distances of ∼2.21 Å and Co−N4 displacements averaging ∼0.27 Å. The Co−bispyridine complexes exhibit Co−Npy distances of ∼1.99 Å. These geometry parameters are all indicative of a classic low-spin Co(III) description for these complexes. Yet, as discussed below, a careful reading of the literature strongly suggests the existence of additional, low-energy open-shell states for many of these complexes. The electronic structure of four-coordinate Co corroles are somewhat unclear, not least because no X-ray structures have been reported for such species. NMR, EPR, and SQUID measurements on Co[TPFPC] and other four-coordinate Co corroles indicate that four-coordinate Co corroles are paramagnetic, as expected for square-planar d6 complexes.221 Quantum chemical calculations on Co[C] also clearly indicate a triplet ground state. The exact nature of the ground state, however, is open to question. Both early BP86222 calculations and ab initio multiconfiguration reference perturbation theory calculations suggest an intermediate-spin S = 1 Co(III) ground state with a dxy2dz22dπ1dπ′1 electronic configuration.223 Pierloot and co-workers, the authors of the latter calculations, did, however, emphasize the difficulty of selecting a suitable active space for Co[C] and of accurately estimating the energies of CoII−corrole•2− states. On the other hand, B3LYP/STO-TZP calculations, carried out specifically for this contribution, suggest a dz21 Co(II) a2u radical ground state for Co[TPC] and Co[TPC](py). Together, the various calculations indicate

Figure 127. UV−vis spectra of Co[TpXPC](PPh3) complexes taken in CH2Cl2. BD


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character. B3LYP/STO-TZP calculations on Co[TPC](PPh3) also yielded a broken-symmetry MS = 0 solution as the lowest energy solution, with the optimized geometry exhibiting bond length alternations characteristic of corrole a2u radicals (Figure 128). Consistent with this picture of a noninnocent CoII− corrole•2−-like ground state, the calculations also predicted an S = 1 ferromagnetically coupled CoII−corrole•2− excited state only 0.1−0.2 eV above the ground state. Among other interesting aspects of the electronic absorption spectra of cobalt corroles may be mentioned the appearance of a strong Q-like band in the 550−600 nm region upon CO or bispyridine ligation (four-coordinate Co corroles do not absorb in this region). Examples of these spectroscopic features are illustrated for the nonasubstituted Co[Me4Ph5C] complex in Figure 129. We tentatively suggest that this feature has significant MLCT character. We await detailed quantum chemical studies that might shed light on these interesting properties. There are several reports of fairly stable Co[Cor](Ph) and Co[Cor]Cl derivatives, which are formally at the Co(IV) oxidation level. Vogel, Kadish, and co-workers reported the first such complex, Co[OEC](Ph).231 An X-ray structure analysis revealed Co−N distances averaging 1.856 Å, a Co−C distance of 1.937 Å, and a small Co−N4 displacement of 0.185 Å (Figure 130). In the crystal, the complex was found to exist as a π−π dimer with an interplanar separation of 3.50 Å and a lateral shift of 3.58 Å. The complex was found to exhibit a well-resolved EPR

same study also suggests that Co(III) corroles as well as Mn(III) and Fe(III) corroles might bind nitroxyl (HNO) and thereby provide novel probes for this critical biological signaling molecule.227−229 Worth mentioning in this connection is the ability of cobalt corroles to bind a variety of inorganic anions with a strong selectivity for nitrite. In a medically relevant application, optical nitrite sensors based on cobalt corroles have been shown to detect NO emission (NO) from NO-releasing polymers containing S-nitroso-N-acetyl-DL-penicillamine.230 In unpublished work in our laboratory (Ganguly, S.; Ghosh, A. Unpublished work) we observed that the Soret maxima of Co[TpXPC](PPh3) derivatives redshift substantially with increasing electron-donating character of the para substituent X (Figure 127), suggesting a certain amount of corrole a2u radical

Figure 128. B3LYP/STO-TZP broken-symmetry MS = 0 spin density plot for Co[TPC](PPh3).

Figure 130. (a, b) Alternative views of the X-ray structure of Co[OEC](Ph). (c) EPR spectrum of the complex in frozen CH2Cl2 at 77 K. (Inset) BLYP/STO-TZP spin density plot. Reproduced with permission from ref 232. Copyright 1996 American Chemical Society.

Figure 129. UV−vis spectra of Co[Me4Ph5C] under different conditions. Reproduced with permission from ref 224. Copyright 2001 American Chemical Society. BE


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3.7.2. Co Corroles: Electrochemistry and Electrocatalysis. Cobalt corroles exhibit a rich array of electrochemical and electrocatalytic properties. In this discussion, we have chosen Co[TPC](PPh3) (Figure 133),234 Co[OMTPC](PPh3) (Figure 134),235 and Co[Me4Ph5C] (Figure 135)224 as exemplars to illustrate the key redox processes. All three complexes exhibit low electrochemical HOMO−LUMO gaps, 1.24, 1.24, and ∼0.6 V, respectively, clearly indicating that the metal is redox-active. Another notable point is that the Co−PPh3 complexes undergo

spectrum at 77 K in frozen CH2Cl2 or THF (Figure 130). In CH2Cl2, the spectrum is rhombic, with g1 = 2.12, g2 = 2.01, and g3 = 1.94. These g values are similar to those observed for electrogenerated {Co[OMTPC](PPh3)}+ (which are 2.14, 2.00, and 1.89 in frozen PhCN at 120 K). An eight-line hyperfine splitting due to the 59Co nucleus (I = 7/2) was observed in both the low- (A1 = 16 G) and high-field (A3 = 18 G) parts of the spectrum. Nearly identical g and A values were also observed in frozen THF solutions at 77 K. The observed A values are rather small relative to that calculated for a free Co(IV) ion as well as certain other putative Co(IV) complexes, potentially indicating that only a fraction of the spin density is localized on the cobalt. A detailed pulse EPR and ENDOR study of key 13C- and 2Hlabeled isotopologues of the same complex allowed the evaluation of the g and A matrices in the molecular frame.232 The g principal values were found to be g1 = 1.9670, g2 = 2.1122, and g3 = 2.0043, with the g1 and g2 axes pointing toward the corrole nitrogens and the g3 axis perpendicular to the corrole plane. The principal values of the 59Co hyperfine matrix ACo were A1Co = 72, A2Co = 8, and A3Co = 10 MHz, with the A3 and g3 axes parallel to each other and the A1 axis rotated by 45° from the g1 axis so that it is directed toward the meso proton at C10. Relatively large 1H ENDOR couplings with the ethyl protons were observed, indicating significant spin density at the β positions of the corroles. These results are consistent with a dπ (dxz + dyz) unpaired electron that is also substantially delocalized to the corrole a1u HOMO. Indeed, a BLYP/STO-TZP calculation on the complex yielded exactly such a spin density, distributed 35:65 between the Co and the corrole (Figure 130). Note that we already encountered such an electronic description for low-spin Fe(III) corroles (see section 3.6.1), where the dπ SOMO is similarly locked in place via an interaction with the corrole a1u HOMO. Another putative “Co(IV)” complex Co[TPFPC](Cl) was obtained by Nocera and co-workers simply in the course of attempted crystallization of Co[TPFPC] via slow evaporation of a dichloromethane solution.221 An X-ray structure of Co[TPFPC](Cl) revealed relatively unremarkable Co−N (1.89− 1.92 Å) and Co−Cl (2.243 Å) distances and a remarkably high Co−N4 displacement of 0.413 Å. An EPR spectrum in frozen THF at 18 K yielded g and A values crudely similar to those found for Co[OEC](Ph) (Figure 131). Yet the very different Co−N4 displacements in Co[TPFPC](Cl) and Co[OEC](Ph) almost certainly imply substantially different electronic structures for the two molecules. A B3LYP/6-311G calculation on the former yielded a spin density consistent with an intermediate-spin Co(III) center with a d xy 2 (d xz − d yz ) 2 (d xz + d yz ) 1 d z2 1 configuration antiferromagnetically coupled to a corrole a2u radical.221 The stereochemical requirement of an effective dz2− a2u orbital interaction then provides a logical explanation for the high Co−N4 displacement. Perplexingly, an X-ray structure of a formally Co(IV)−Cl corrole, which is part of a cofacial biphenylene-linked porphyrin−corrole assembly, exhibits a Co−N4 displacement of only 0.19 Å (Figure 132),233 which clearly calls for a theoretical explanation. Another structurally characterized, formally high-valent Co corrole is the perchlorate salt {Co[OEC](Ph)}(ClO4).231 The compound exhibited a diamagnetic NMR spectrum, and the Xray structure exhibited pronounced bond length alternation within and around the bipyrrole part of the molecule. These characteristics were thought to be well explained in terms of a CoIIIPh−corrole− electronic description, where the doubly oxidized corrole monoanion is antiaromatic.

Figure 131. (a) X-ray structure and (b) EPR spectrum (black) in frozen THF at 18 K. Simulated spectrum (gray) was generated using g1 = 2.073, g2 = 2.000, g3 = 1.977, ACo1 = 58.51 MHz, ACo2 = 40.23 MHz, ACo3 = 29.28 MHz, and a Lorentzian line width of 0.0018 mT. Adapted with permission from ref 221. Copyright 2011 American Chemical Society.

Figure 132. (a) X-ray structure and (b) EPR spectrum (toluene, 77 K) of a cofacial CoCl−corrole−FeCl−porphyrin assembly. Adapted with permission from ref 233. Copyright 2005 American Chemical Society. BF


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Figure 134. EPR spectra of (a) electroreduced and (b) electrooxidized Co[OMTPC](PPh3) in frozen (77 K) PhCN containing 0.2 M TBAP. Reproduced with permission from ref 235. Copyright 1995 American Chemical Society.

2.002, and g3 = −1.89, along with the lack of resolved hyperfine splitting due to 59Co, suggests a corrole radical with partial Co(IV) character, possibly analogous to that described above for Co[OEC](Ph). Further, a splitting of the central line at g = 2.002 (with a coupling constant of ∼10 G) was interpreted as resulting from hyperfine interaction with the 31P nucleus of the PPh3. The four-coordinate complex Co[Me4Ph5C] exhibits a much narrower HOMO−LUMO gap than the Co−PPh3 complexes, essentially reflecting a greatly elevated Co(III)/Co(II) reduction potential (Figure 135).224 Unsurprisingly, the Co(II)/Co(I) reduction potential of Co[Me4Ph5C] is about the same as that observed for Co−PPh3 complexes, indicating prior dissociation of PPh3 for the latter complexes. Another curious feature of the cyclic voltammogram of Co[Me4Ph5C] in CH2Cl2 is that the first oxidation appears to split into two half-oxidations with smaller peak heights relative to the regular features. These have been attributed to dimer formation, as illustrated in Figure 135b; similar behavior has also been observed for Co[OEC] in CH2Cl2 and PhCN. Cobalt corroles have been extensively studied as electrocatalysts for O2 and proton reduction. These applications have been recently reviewed by Nocera and co-workers and accordingly are not discussed here.236 In general, the use of electron-deficient corroles, such as meso-p-nitrophenyl-substituted corroles and Co[X8TPFPC] derivatives (X = H, F, Cl, Br), have led to better electrocatalysts. For proton reduction, Gross, Dey, and co-workers provided strong evidence that the Co(I) corrole dianion is the active proton-reducing species.69 The putative protonated intermediate, {Co[Cor](H)}−, however, has not been detected, presumably because it undergoes very fast protonolysis to generate {CoII[Cor]}− and H2. For O2 reduction, the key challenge is to manage the proton delivery so that twoelectron reduction to H2O2 or four-electron reduction to water occurs selectively. A variety of superstructured porphyrin and corrole electrocatalysts have been synthesized for this purpose; these are summarized in the aforementioned review by Nocera and co-workers.236

Figure 133. Selected electrochemical results for Co[TPC](PPh3): (a) Cyclic voltammogram in dichloromethane containing 0.1 M TBAP, (b) UV−vis spectral changes upon one-electron reduction, and (c) UV−vis spectra of the formal Co(III), Co(II), and Co(I) states. Adapted with permission from ref 234. Copyright 2014 Elsevier.

an irreversible reduction, which may be explained by invoking dissociation of PPh3 from the initially formed {Co[Cor](PPh3)}− anion. Evidence for this proposal comes from an early study of Co corroles by Boschi, Kadish, and co-workers, where the authors showed that bulk electroreduction of Co[OMTPC](PPh3) in PhCN results in an EPR spectrum with partially resolved hyperfine splitting due to 59Co but no additional hyperfine splitting due to 31P, indicating dissociation of PPh3.235 The g values (gl = 3.349, g2 = 2.187, and g3 = 1.903) and ACo values resolved in the low-field part of the spectrum (A1Co = 181 × 10−4 cm−1 and A2Co = 76 × 10−4 cm−1) also agreed with other Co(II) corrole species in the literature. Electroreduction of Co−PPh3 corroles generally leads to a significant redshift of the Soret maximum, as illustrated in Figure 133c for Co[TPC](PPh3).234 Interestingly, further reduction of the putative {CoII[TPC]}− anion leads to small spectral shifts and a sharpening of both the Soret and the Q bands, suggesting formation of an S = 0 {CoI[TPC]}− anion. Gross, Dey, and coworkers reported DFT calculations indicating a square-planar d8 Co(I) formulation for the {Co[C]}2− dianion and have also confirmed the diamagnetism of the {Co[Cl8TPFPC]}2−via 19F NMR spectroscopy.69 The nature of {Co[Cor](PPh3)}+ cations is less clearly established. Thus, the EPR spectrum of electrooxidized Co[OMTPC](PPh3) in PhCN at 77 K exhibits a central line at g = 2.002, which is similar to the g values exhibited by Co(III) porphyrin cation−radicals, but also two additional broad features with a separation of 400 G on either side of the central line.235 Increasing the temperature resulted in a lowering of the intensity of the broad bands, but a fully isotropic radical signal could not be obtained. Thus, the overall rhombic signal with gl = −2.14, g2 = BG


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Figure 135. (a) Cyclic voltammograms of Co[Me4Ph5C] in different solvents containing 0.1 M TBAP, and (b) mechanistic picture of the oxidative processes. Reproduced with permission from ref 224. Copyright 2001 American Chemical Society.

3.7.3. Rhodium and Iridium Corroles. Rhodium214,215 and iridium237 are both inserted into corroles via a variant of the carbonyl method, in which a free-base corrole is refluxed with a low-valent organometallic reagent. For both metals the reagent of choice is the [M(cod)Cl]2 (M = Rh, Ir) dimer. In one case of Ir insertion, the initially formed Ir(I)−TPFPC−cod complex was oxidized with trimethylamine (tma) N-oxide to generate the Ir(III) complex Ir[TPFPC](tma)2. For the synthesis of the fivecoordinate PPh3 complexes, however, no added oxidant (other than air) is required. Interestingly, Ir[TPFPC](tma)2 underwent smooth octabromination with elemental bromine, yielding Ir[Br8TPFPC](tma)2.237 According to available X-ray structures,214,215 five-coordinate Rh−PPh3 complexes generally exhibit planar to slightly domed corroles with modest Rh−N4 displacements of ∼0.25 ± 0.02 Å and six-coordinate Rh and Ir complexes, including the sterically hindered Ir[Br8TPFPC](tma)2 (Figure 136), exhibit planar metal−corrole units. Both Rh and Ir corroles exhibit similar M− N distances, 1.96 ± 0.02 Å, regardless of five- versus sixcoordination. These M−N distances, which are almost 0.1 Å

longer than those found for Co−N corroles, provided an early illustration of the size-mismatched nature of many 4d and 5d metallocorroles. Unlike cobalt corroles, all isolated Rh and Ir corroles are thought to be true M(III) complexes with no indication of a noninnocent corrole. According to ongoing research in our laboratory, the Soret bands of Rh[TpXPC](PPh3) exhibit none of the substituent sensitivity observed for the analogous Co complexes. That said, as shown in Figure 136, the UV−vis spectra of isoelectronic sets of Group 9 metallocorroles exhibit remarkable diversity, with major spectral shifts and differences in intensity between Rh and Ir complexes.238 Although the spectra have not been theoretically interpreted, the mere fact that the Rh and Ir spectra differ so much is not surprising. Relativistic effects are expected to result in significant destabilization of the Ir 5d orbitals relative to the Rh 4d, which in turn should engender substantial differences in M(d)−corrole(π) interactions between the two elements. Two aspects of the spectra are particularly deserving of further study, especially by quantum chemical means. First, the majority of the spectra exhibit distinctive split BH


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Figure 136. Electronic absorption spectra (a) of Group 9 TPFPC complexes and (b) of selected complexes as a function of added pyridine or PPh3. (Inset) X-ray structure of Ir[TPFPC](py)2. Adapted with permission from ref 238. Copyright 2009 American Chemical Society.

Soret bands. Second, the bispyridine and bis(PPh3) complexes exhibit unusually strong Q bands. Indeed, in the case of the sixcoordinate Rh[TPFPC] derivatives, the strongest Q band is onehalf to three-quarters as intense as the main Soret band.238 Although the oxidation potentials of all M[TPFPC](PPh3) (0.72−0.83 V) and M[TPFPC](py)2 (0.67−0.71 V) complexes are similar, the one-electron-oxidized products exhibit significantly different EPR spectra. Thus, whereas chemically oxidized Co[TPFPC](py)2 and Rh[TPFPC](py)2 exhibit unambiguous g ≈ 2 radical signals, oxidized Ir[TPFPC](tma)2 (gzz = 2.489, gyy = 2.010, gxx = 1.884) and Ir[TPFPC](py)2 (gzz = 2.401, gyy = 2.000, gxx = 1.937) exhibit highly rhombic spectra, similar to low-spin d5 iron porphyrins and Fe[TPFPC](py)2, thus implying at least partial Ir(IV) character (Figure 137).238 A nonrelativistic DFT (B3LYP) study of the Group 9 metallocorrole oxidation provided a fair reproduction of the observed oxidation potentials but failed to yield an electronic-structural description consistent with the above EPR spectra.239 Ongoing scalar-relativistic calculations of the oxidized states of Group 9 metallocorroles in our laboratory suggest a rather subtle picture, where metalversus corrole-centered oxidation depends not only on the axial ligand(s) but also very significantly on the electron richness or otherwise of the corrole ligand. Interestingly, cyclic voltammetry of Ir[TPFPC](tma)2 in CH2Cl2 did not reveal a reduction potential within the electrochemical window of the solvent, suggesting the corrole

Figure 137. EPR spectra of chemically oxidized (a) Co[TPFPC])(py)2, (b) Rh[TPFPC])(py)2, (c) Ir[TPFPC])(py)2, and (d) Ir[TPFPC](tma)2 in frozen CH2Cl2. Reproduced with permission from ref 238. Copyright 2009 American Chemical Society.

ligand in this complex is unusually electron rich, while the oxidation potential was located at 0.66 V. For the much more electron-deficient Ir[Br8TPFPC](tma)2, both an oxidation (1.19 V) and a reduction potential (−1.21 V) could be located, which BI


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suggesting significant MLCT character in the lowest triplet state. A 10 nm blueshift of the emission maxima at 77 K (relative to room temperature) has also been interpreted as a rigidochromic shift suggestive of charge transfer character.

translates to an electrochemical HOMO−LUMO gap of 2.4 V.237 The high value of the gap strongly suggests corrolecentered oxidation and reduction for this complex, again underscoring that the issue of metalversus ligand-centered oxidation depends a great deal on the corrole ligand. Another notable point is the large effect of β-octabromination on the oxidation potential of Ir[TPFPC](tma)2, which may be contrasted with much smaller effects generally observed for metallotetraarylporphyrins. The difference is related to the fact that β-octabromination of metallotetraarylporphyrins is invariably accompanied by strong saddling, which cancels out much of the stabilizing effect of β-octabromo substitution on the HOMO. To our knowledge, the reductive electrochemistry of Rh− TPFPC derivatives has not been described. An early study of Rh[OMC](PPh3) and Co[OMC](PPh3) by Kadish, Boschi, and co-workers still provides much of our current knowledge of the reductive behavior of Rh corroles.214 Like Co[OMC](PPh3) and other Co−PPh3 corroles, Rh[OMC](PPh3) undergoes an irreversible reduction owing to dissociation of the PPh3. The unique feature of Rh is that the {RhII[OMC]}− forms a metal− metal-bonded dimer. The authors found that the dimerization could be slowed in THF at −70 °C, but a two-electron reduction occurred at −1.39 V under these conditions, leading to a putative {RhI[OMC]}2− dianion. A reversible oxidation under these conditions at 0.23 V then translates to a HOMO−LUMO gap of 1.62 V. Under the same conditions, Co[OMC](PPh3) exhibited an oxidation potential of 0.30 V, a reduction potential of −0.72 V, and a second reduction potential of −1.84 V, implying a much smaller HOMO−LUMO gap of 1.02 V. In other words, relative to the M(III) starting materials, the divalent Rh(II) state is thermodynamically significantly more unstable than Co(II). On a final note, like OsN and Au corroles, Ir corroles exhibit weak near-IR phosphorescence with quantum yields of

Electronic Structure of Corrole Derivatives: Insights from Molecular

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