Seven Clues to Ligand Noninnocence: The Metallocorrole Paradigm

Article Cite This: Acc. Chem. Res. 2019, 52, 2003−2014

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Seven Clues to Ligand Noninnocence: The Metallocorrole Paradigm Sumit Ganguly and Abhik Ghosh*

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Department of Chemistry and Arctic Center for Sustainable Energy, UiT−The Arctic University of Norway, N-9037 Tromsø, Norway CONSPECTUS: Noninnocent ligands do not allow an unambiguous definition of the oxidation state of a coordinated atom. When coordinated, the ligands also cannot be adequately represented by a classic Lewis structure. A noninnocent system thus harbors oxidizing (holes) or reducing equivalents (electrons) that are delocalized over both the ligand and the coordinated atom. To a certain degree, that is true of all complexes, but the phenomenon is arguably most conspicuous in complexes involving ligands with extended π-systems. The electronic structures of such systems have often been mischaracterized, thereby muddying the chemical literature to the detriment of students and newcomers to the field. In recent years, we have investigated the electronic structures of several metallocorrole families, several of which have turned out to be noninnocent. Our goal here, however, is not to present a systematic account of the different classes of metallocorroles, but rather to focus on seven major tools (in a nod to A. G. CairnsSmith’s Seven Clues to the Origin of Life) that led us to recognize noninnocent behavior and subsequently to characterize the phenomenon in depth. (1) The optical probe: For a series of noninnocent meso-triarylcorrole derivatives with different para substituents X, the Soret maxima are typically exquisitely sensitive to the nature of X, red-shifting with increasing electrondonating character of the group. No such substituent sensitivity is observed for the Soret maxima of innocent triarylcorrole derivatives. (2) Quantum chemistry: Spin-unrestricted density functional theory calculations permit a simple and quick visualization of ligand noninnocence in terms of the spin density profile. Even for an S = 0 complex, the broken-symmetry method often affords a spin density profile that, its fictitious character notwithstanding, helps visualize the intramolecular spin couplings. (3) NMR and EPR spectroscopy: In principle, these two techniques afford experimental probes of the electronic spin density. (4) Structure/X-ray crystallography. Ligand noninnocence in metallocorroles is often reflected in small but distinct skeletal bond length alternations in and around the bipyrrole part of the macrocycle. In addition, for Cu and some Ag corroles, ligand noninnocence manifests itself via a strong saddling of the macrocycle. (5) Vibrational spectroscopy. Unsurprisingly, the aforementioned bond length alternations translate to structure-sensitive vibrational marker bands. (6) Electrochemistry. Noninnocent metallocorroles exhibit characteristically high reduction potentials, but caution should be exercised in turning the logic around. A high reduction potential does not necessarily signify a noninnocent metallocorrole; certain high-valent metal centers also undergo metal-centered reduction at quite high potentials. (7) X-ray absorption spectroscopy (XAS). By focusing on a given element, typically the central atom in a coordination complex, X-ray absorption near-edge spectroscopy (XANES) can provide uniquely detailed local information on oxidation and spin states, ligand field strength, and degree of centrosymmetry. For metallocorroles, some of the most clear-cut distinctions between innocent and noninnocent systems have come from the Kedge XANES of Mn and Fe corroles. For researchers faced with a new, potentially noninnocent system, the take-home message is to employ a good majority (i.e., at least four) of the above methods to arrive at a reliable conclusion vis-à-vis noninnocence.

1. INTRODUCTION

Thus, ligands such as oxo, imido, nitrido, alkyl, aryl, carbenes, and carbynes are commonly more oxidized than their formal oxidation states imply. And yet we typically do not refer to these ligands as noninnocent. There is a certain logical inconsistency here as well as a good justification, which is that oxidation states are by definition purely formal quantities and if we designate most, if not all, ligands as noninnocent, we risk jeopardizing the usefulness of both “oxidation states” and “ligand noninnocence”. Our emphasis here is accordingly on ligands with extended π-systems, specifically corroles,5 which

Over a half-century ago, the Danish chemist C. K. Jørgensen divided ligands into “innocent” and “suspect”.1 A “suspect” or “noninnocent” ligand was defined as one that leaves the oxidation state of the coordinated atom uncertain or debatable. This definition, like many others in chemistry (such as electronegativity and nucleophilicity), is obviously far from rigorous. For ligands that involve extended π-systems, ligand noninnocence typically entails partial oxidation and reduction of the π-system, that is, π-radical character.2,3 The essential phenomenon of charge delocalization across a central atom and its ligands, however, is much more universal and extends to many ligands that do not involve extended π-systems.4 © 2019 American Chemical Society

Received: March 3, 2019 Published: June 21, 2019 2003

DOI: 10.1021/acs.accounts.9b00115 Acc. Chem. Res. 2019, 52, 2003−2014

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Accounts of Chemical Research

are ring-contracted, triprotic analogues of porphyrins and which are increasingly recognized as noninnocent in many of their complexes (Figure 1). Many aspects of corrole derivatives have been recently reviewed, such as their synthesis,6 coordination chemistry,7 electrochemistry,8 and medical applications.9 The subject of ligand noninnocence in metallocorroles, however, has not been systematically reviewed. Our goal here, however, is not to present a systematic review, but rather to highlight seven major tools that we have regularly used to identify and characterize ligand noninnocence in metallocorroles (seven echoing A. G. Cairns-Smith’s Seven Clues to the Origin of Life). The same tools applied in concert should lead to not only a better understanding of other noninnocent systems but also the identification of as yet unknown cases of ligand noninnocence. We thus urge researchers faced with a potential noninnocent system to deploy at least 3−4 techniques, as opposed just one

Figure 1. A partial selection of meso-triarylcorrole derivatives prepared in our laboratory.

Table 1. Soret Maxima (nm) of Different meso-Triarylcorrole (TArC) Derivatives, Including meso-Tris(para-X-phenyl)corrole (X = OMe, Me, H, and CF3) Derivatives M[TArC](L)n

Ar = pOMeP

Cu[TArC] Cu[F8TArC] Cu[Br8TArC] Cu[(CF3)8TArC] Mn[TArC]Cl Fe[TArC]Cl Fe[F8TArC]Cl Fe[TArC](NO) Fe[Br8TArC](NO) {Fe[TArC]}2O Co[TArC](PPh3) Co[Br8TArC](PPh3) Co[TArC](py) Co[Br8TArC](py) Ag[Br8TArC] Pt[TArC](Ar1)(Ar2)

433 436 468 507 460 426 367 416 394 375, 410 399 423 402 392 450 475

H3[TArC] Cr[TArC](O) Mn[TArC]Ph Fe[TArC]Ph Co[TArC](py)2 Co[Br8TArC](py)2 Mo[TArC](O) Mo[TArC]2 Rh[TArC](PPh3) Ag[TArC] W[TArC]2 Ru[TArC]NO Ru[TArC]N {Ru[TArC]}2 Re[TArC](O) Tc[TArC](O) Os[TArC](N) {Os[TArC]}2 Pt[TArC](Ar1)(PhCN) Pt[TArC](Ar1)(py) Au[TArC] Au[Br8TArC]

421 404 387 385 434, 453 446, 462 440 350 427 423 404 419 329, 406 441 413 445 286, 407 427 420 431

Ar = pMeP

Ar = Ph(TPC)

Noninnocent Metallocorroles 418 413 421 409 453 439 471 459 442 433 419 410 360 355 400 390 395 397 389 386 392 387 418 412 393 388 391 392 438 425 460 453 Innocent Metallocorroles 417 417 404 403 389 394 383 383 437, 453 437, 452 445, 461 445, 461 439 438 362 356 430 429 423 423 359 357 404 404 418 418 329, 398 328, 397 440 439 412 410 443 442 287, 407 287, 405 427 426 430 427, 437 420 418 430 429 2004

Ar = pCF3P

Ar = C6F5

407 401 436

406

423 401 353 385 391 383 385 421 386 396 416, 448 443 417 404 398 384 442, 453(sh) 447, 460 439 431 423 356 404 417 328, 397 438 410 441 287, 407 430 427, 438 419 429

442 414 370, 396 378 392 382 376, 408

440

428 421

421 428

ref 16 17 16 18 31,41 13,14 17 14,15 25 14,26 27,28 28 30 30 53 37 16 59 31 13 27,30 30 59 61 14,28 23,52 60 56 56 63 57 55 58 63 37 38 23,51 22,48


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Accounts of Chemical Research or two. A multitechnique approach should greatly reduce the chances of electronic-structural misassignments, which happen all too regularly for noninnocent systems. A word or two may be appropriate on why ligand noninnocence matters. In recent years, many chemists have exploited noninnocent systems as redox catalysts. In many such systems, the ligand acts as an electron reservoir, facilitating reactions that innocent ligands would not.10 Others have criticized a so-called obsession with ligand noninnocence as a potential impediment to catalyst discovery, pointing to the lack of any systematic correlation between ligand noninnocence and catalytic efficiency.11 While we see the point of the latter view, detailed electronic-structural descriptions of catalysts and their reactive intermediates do enhance our overall understanding of catalytic processes and should ultimately lead to the discovery of new catalytic systems. Presented below are brief thumbnail sketches of the seven tools and of the insights they provide into ligand noninnocence, using metallocorroles as our paradigm.

Figure 3. Gouterman frontier MOs of unsubstituted Au corrole.

ligand, that is, one with significant corrole•2− character.7 A Soret maximum that is essentially invariant with respect to X, on the other hand, indicates an innocent corrole, that is, corrole3−.7 Three series of iron corroles (FeCl,12−14 FeNO,15 and FePh13) illustrating this “optical probe” of ligand noninnocence are presented in Figure 2. Additional classic illustrations of the optical probe are provided by Cu corroles,16−18 which are invariably noninnocent (as further discussed below). The optical spectra of archetypal closed-shell porphyrins (say, Zn porphyrins) are described to a good approximation by Gouterman’s four-orbital model, according to which the two HOMOs (a1u and a2u in D4h symmetry) and the two LUMOs (eg) are energetically well-separated from other occupied and unoccupied MOs and the Soret and Q bands result from transitions among these four MOs.19,20 Early on, we found that this model holds for corroles as well.21 Figure 3 depicts the four Gouterman MOs of unsubstituted Au corrole,22,23 which may be viewed as an exemplar of an innocent transition metal corrole. That said, the Soret region is generally more complex in terms of its orbital origin than the Q region; nonGouterman transitions often mix in to various degrees in the Soret envelope. A time-dependent density functional theory (TDDFT) study of Cu[TPC] (TPC = meso-triphenylcorrole) suggests that the substituent-sensitive Soret maxima of noninnocent metallotriarylcorroles originate from aryl-tocorrole•2− character mixing in one or more transitions under the Soret envelope.24 Understandably, an analogous transition is not observed for innocent metallocorroles. Considering its largely empirical character, the optical probe has proven extraordinarily effective in the identification of new classes of noninnocent metallocorroles. Indeed, several wellknown metallocorrole families, such as FeNO,15,25 Fe2(μ-O),26 and Co-PPh327−30 corroles, were first recognized as noninnocent on the basis of meso-substituent effects on their Soret maxima.

2. OPTICAL SPECTRA For meso-tris(para-X-phenyl)corrole (TpXPC) derivatives, optical spectra provide a simple and surprisingly reliable

Figure 2. UV−vis spectra of three series of iron meso-tris(para-Xphenyl)corrole, Fe[TpXPC](L), where L = Cl, NO, and Ph.

3. QUANTUM CHEMISTRY In principle, quantum chemical calculations can provide all observable properties of a given molecule. In this section, however, we will focus specifically on the spin densities of metallocorroles obtained with DFT calculations, since these provide a quick, visualizable probe of the ligand noninnocence. A selection of Fe corroles makes for nice illustrative examples

diagnostic for a noninnocent corrole macrocycle. As shown in Table 1, a Soret maximum that red shifts markedly in response to increasing electron-donating character of the para substituent X is generally indicative of a noninnocent corrole 2005


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Figure 4. Broken-symmetry B3LYP spin density profiles for selected Fe-TPC derivatives. The peripheral phenyl groups have been omitted for clarity.

(Figure 4). Let us first consider the two S = 1 complexes Fe[TPC]Cl and Fe[TPC]Ph.12,13 Observe the substantial amounts of minority or negative spin density on the corrole for Fe[TPC]Cl, but not for Fe[TPC]Ph. The plots immediately suggest that Fe[TPC]Cl is not a true Fe(IV) complex but is better viewed as an S = 3/2Fe(III) center antiferromagnetically coupled to a corrole•2− radical; the much smaller quantities of spin density on the corrole ligand in Fe[TPC]Ph, on the other hand, suggests that the corrole does not have much radical character and that the Fe center accordingly is close to Fe(IV). It is worth noting that, despite the difference in spin state (S = 1 vs 3/2), the spin density profiles of Mn[TPC]Cl12 and Mn[TPC]Ph31 are analogous to those of their Fe congeners. For Fe[TPC](NO)15 and {Fe[TPC]}2(μ-O),26 for which intramolecular spin couplings result in an overall S = 0 state, the so-called broken-symmetry method (which typically works best with a hybrid functional such as B3LYP) may be used to derive spin density profiles. Although these spin densities are not real, they provide a wonderful means of visualizing the intramolecular spin coupling pathways (Figure 4), which may be described as

seminal 1994 study of Fe octaethylcorrole derivatives, Vogel and co-workers noted extreme downfield chemical shifts for the meso protons of Fe[OEC]Cl (OEC = octaethylcorrole) (S = 1), δ 177 (br, 2 H, 5,15-H) and 189 (br, 1 H, 10-H), which are in sharp contrast to the strongly upfield-shifted meso proton resonance of Fe[OEP]Cl (OEP = octaethylporphyrin) (S = 5/ 2).34 The correct interpretation of these results came a few years later from Walker and co-workers, who posited that the meso spin populations of FeCl octaalkylporphyrins and octaalkylcorroles must have opposite signs.35 The overall S = 1 states of the latter could then be explained in terms of antiferromagnetic coupling between an intermediate-spin S = 3/2 Fe(III) center and a corrole•2− radical. 1H NMR studies of Fe[TPC]Cl also provided support for the same picture.12,36 For this complex, the ortho, meta, and para protons of the meso phenyl groups were found to exhibit paramagnetic shifts downfield, upfield, and downfield, respectivelythat are opposite in sign relative to those for Fe[TPP]Cl (TPP = meso-tetraphenylporphyrin), which are upfield, downfield, and upfield, respectively. Again, these results implied an antiferromagnetically coupled FeIII(S = 3/2)−corrole•2− description, with net negative spin density at the corrole meso positions. The results may be contrasted with the much more moderate meso paramagnetic shifts observed for both Fe[OEC](Ph) (δ = 54.5 for 5,15-H and 49.3 for 10-H)34 and Fe[TPC](Ph),13 consistent with a comparatively innocent corrole macrocycle. As described above in section 3, DFT spin density profiles have proved beautifully consistent with the picture afforded by 1H NMR spectroscopy. Unfortunately, unlike iron complexes, which generally yield sharp 1H NMR spectra, most paramagnetic complexes yield broad signals that cannot be readily observed or interpreted. EPR spectroscopy is potentially helpful in such cases. Thus, EPR spectroscopy established that the apparent Pt(V)−diaryl complexes Pt[TpXPC]Ar1Ar2 are actually PtIV−corrole•2− species with full-fledged corrole radicals.37,38 Since the Pt oxidation state in these complexes is not in any doubt whatsoever, these complexes arguably should not be described

Fe[TPC](NO): TPC•2 −( ↓ )Fe III(↑↑↑)NO−(↓↓) {Fe[TPC]}2 (μ‐O): TPC•2 −( ↓ )Fe III(↑↑↑)Fe(↓↓↓)TPC•2 −( ↑ )

Multiconfigurational ab initio calculations provide an alternative, more rigorous approach to the theoretical study of ligand noninnocence. Although not quite synonymous, the inorganic chemists’ term “noninnocent” and the theoreticians’ term “multideterminantal” overlap substantially. Multiconfigurational calculations are far more challenging than DFT, but a good beginning has been made32 and a great deal of interesting results may be expected within the foreseeable future.

4. NMR AND EPR SPECTROSCOPIES 1 H NMR spectroscopy played a major role in establishing ligand noninnocence in FeCl corroles.33 Already in their 2006


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Accounts of Chemical Research as noninnocent; a more appropriate term might be “metalloradicals”. In another interesting application of EPR spectroscopy, Gray and co-workers studied the one-electronoxidized, cationic forms of group 9 M[TPFPC](py) 2 [TPFPC = meso-tris(pentafluorophenyl)corrole] complexes and showed that whereas the Co and Rh species are essentially full-fledged corrole radicals, the cationic Ir species has partial Ir(IV) character.39,40 EPR spectroscopy, applied in isolation, may prove deceptive. A good example is provided by MnCl41 and Mn−OH42 corroles, which exhibit typical Mn(IV)-like S = 3/2 signals. A careful multitechnique approach, including the optical probe, DFT spin density profiles, X-ray structures, and XAS, however, is indicative of a strongly antiferromagnetically coupled MnIII(S = 2)−corrole•2− description for MnCl corroles.31

5. STRUCTURES As far as metallocorroles are concerned, the structural manifestations of ligand noninnocence fall into two major

Figure 6. Thermal ellipsoid plots for Cu (top) and Au (bottom) βoctakis(trifluoromethyl)-meso-triarylcorrole derivatives.

length alternation occurs in Fe[TPC]Cl and Fe[TPC](NO), but not for Fe[TPC]Ph; as mentioned above, the corrole ligand in the last compound is essentially innocent.15,13,35 Both good-quality X-ray structures and optimized DFT geometries nicely capture this structural phenomenon. A second structural manifestation of ligand noninnocence is largely specific to Cu corroles and to a much lesser extent the other coinage metal corroles. Metallocorroles, even highly sterically hindered ones, are generally resistant to nonplanar distortions, except for mild doming. Against this context, even simple Cu corroles are exceptional in being inherently saddled.43 The degree of saddling increases with increasing steric crowding on the corrole periphery.44,45 This phenomenon is dramatically illustrated by a Cu β-octakis(trifluoromethyl)-meso-triarylcorrole,46 which is exceptionally saddled (with adjacent pyrrole rings essentially orthogonal to one another) and the corresponding Au complex, which is planar (Figure 6).47 In the Cu case, saddling is driven by a Cu(3dx2−y2)−corrole(π) orbital interaction, which results in a flow of electron density from the corrole “a2u” HOMO into the empty dx2−y2 orbital of the formal Cu(III) center. In other words, Cu corroles are not true Cu(III) complexes but have significant CuII−corrole•2− character. The much higher energy of the Au 5dx2−y2 orbital (in part a result of relativistic effects)

Figure 5. Skeletal bond distances in (broken-symmetry) B3LYP/ STO-TZP optimized structures of Fe[TPC](L) (L = Cl, NO, Ph) complexes, with the red and blue double-headed arrows indicating bond length alternation.

categories. The more general of the two is a small but distinct skeletal bond length alternation in and around the bipyrrole unit. Such a pattern has been observed for FeCl, FeNO,15 and MnCl31 corroles, among others. Figure 5 shows that this bond 2007


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Figure 7. Experimental (left) and DFT simulated IR spectra of Fe(TPC)(L) complexes. Selected structure-sensitive bands are highlighted. Reproduced with permission from ref 15. Copyright 2015 Royal Society of Chemistry.

Figure 8. Visual depiction of key structure-sensitive vibrational eigenvectors, with corresponding B3LYP and experimental frequencies (cm−1). Reproduced with permission from ref 15. Copyright 2015 Royal Society of Chemistry.

according to the optical probe; on the other hand, the βoctabromo series Ag[Br8TpXPC] is thought to be noninnocent, based on both substituent-sensitive Soret maxima and a strongly saddled crystal structure.53 As discussed below,

renders the analogous interaction much less effective for Au corroles, which understandably are planar.48−51 Silver corroles are of unusual interest in this regard.52 Simple silver triarylcorroles are planar and supposedly “innocent” 2008


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Table 2. First Oxidation and Reduction Potentials (V vs Saturated Calomel Electrode) of Charge-Neutral Corrole Derivatives, M[Cor](L)n OEC M[Cor](L)n

Eox

Ered

TPC ΔE

Ru[Cor](N) Tc[Cor](O) Re[Cor](O) Os[Cor](N) Pt[Cor](Ar1) (py) Au[Cor] Cr[Cor](O) Mn[Cor] Mn[Cor]Ph Fe[Cor]Ph Co[Cor](py)2 Mo[Cor]O Mo[Cor]2 Ru[Cor]NO {Ru[Cor]}2 Rh[Cor](PPh3) Ag[Cor] W[Cor]2 {Os[Cor]}2 Pt[Cor](Ar1) (PhCN) Mn[Cor]Cl Fe[Cor]Cl Fe[Cor](NO) {Fe[Cor]}2(μO) Co[Cor](PPh3) Co[Cor](py) Cu[Cor] Ag[Cor] Pt[Cor](Ar1) (Ar2)

Eox 0.88 1.18 0.98 0.91 0.61 0.80

0.36 0.59 0.43

−1.58 −1.15 −0.62

1.94 1.74 1.05

0.70

−0.72

1.42

0.85 0.76 0.61

−0.01 −0.08 −0.41

0.86 0.84 1.02

0.43

−0.34

0.77

0.92 0.32 0.77 0.82 0.19 0.94 0.23 0.64 0.55 0.46 0.73 0.25 0.60 0.63

Ered

TPFPC ΔE

Eox

Ered

Br8TPC ΔE

Eox

Ered

Br8TPFPC ΔE

Eox

Ered

ΔE

Corrole Derivatives with Redox-Inactive Central Atoms −1.32 2.20 −0.91 2.09 −1.26 2.24 −1.28 2.19 −1.49 2.10

56 55 57 58 38

−1.38 2.18 0.83 −1.39 2.22 1.29 −1.02 Innocent Metallocorroles with Redox-Active Metal Centers −0.11 1.03 1.10 0.17 0.93 −1.46 1.78 0.71 −1.01 1.72 1.25 −0.37 −0.95a −0.35 1.17 −0.44a 0.67 0.69 −0.21a −0.54 1.48 1.32 −0.35 1.67 −0.83 1.06 −0.73 1.37 0.72 −0.42 1.14 −0.86 1.41 −1.34a −0.86 1.59 1.16 −0.61 1.77 −1.07 1.32 −1.29 1.89 −0.83 1.46

1.05 1.09 0.86 0.63

Noninnocent Metallocorroles with Corrole•2− Character 0.10 0.95 0.04 1.05 −0.33 1.19 1.07 0.00 1.07 −0.30 0.93

0.54 0.24 0.76

−0.85a −0.31 −0.20

0.96

0.88

0.09

0.79

0.70

−0.40

1.10

1.12

0.21

0.91

0.99 0.71 1.14 1.27

ref

2.31

22,23,51 8,59 8 8,31 8,13 8,30 8,59 61 8,56 63 28 8,23 60 63 37

1.62

8,31 13 8,25 13 −0.38a −0.06 0.12 −0.31

1.02 1.58

1.44

0.64

0.80

8,28 30 8,16 53 37

a

Irreversible peak potential.

(Table 2). Neutral metallocorroles with corrole•2− character may be expected to undergo one-electron reduction at relatively high potentials, which is indeed what is observed for the noninnocent systems discussed so far, including Cu, FeCl, FeNO, and MnCl corroles. Metallocorroles with redoxinactive metal centers, such as TcO,55 RuN,56 ReO,57 OsN,58 and Au23 corroles, on the other hand, undergo one-electron reduction at much more negative potentials and exhibit an electrochemical HOMO−LUMO gap (defined as the algebraic difference between the one-electron oxidation and reduction potentials) of around 2.1 V. These differences are illustrated for coinage metal corroles in Figure 9.53 We may now reformulate our question: does a high reduction potential necessarily imply a noninnocent corrole ligand? For manganese corroles, the answer appears to be “yes”. Thus, the reduction potentials of noninnocent, chargeneutral MnCl corroles hover just above 0.0 V, while those of innocent, charge-neutral MnPh corroles are almost around −1.0 V.31 Iron corroles, in contrast, behave quite differently. Like MnCl corroles, FeCl corroles reduce around 0.0 eV, but unlike MnPh corroles, FePh corroles, in which the corrole is thought to be essentially innocent, reduce around −0.3 V, that

X-ray absorption spectroscopy (XAS) also appears to support such a picture.54

6. VIBRATIONAL SPECTROSCOPY The bond length alternations brought about by ligand noninnocence translate to characteristic oxidation-state-sensitive marker bands. Thus, the IR spectra of the two noninnocent complexes Fe[TPC]Cl and Fe[TPC](NO) exhibit marker bands that are significantly shifted or even entirely absent for Fe[TPC]Ph, which has a comparatively innocent corrole (Figure 7). 15 Figure 8 depicts the corresponding vibrational eigenvectors. A larger-scale effort to map out the main structure and oxidation state marker bands of metallocorroles, however, has yet to be undertaken. 7. ELECTROCHEMISTRY Do redox potentials provide clues as to the innocence or noninnocence of metallocorroles? This turns out to be a tricky question. The oxidation potentials of many simple metallocorroles hover around +1.0 V against the saturated calomel electrode. The reduction potentials on the other hand vary enormously 2009


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increase in aromatic character of the corrole ligand and vice versa. By way of illustration, for Au corroles, in which the metal center is redox-inactive, both one-electron oxidation and reduction result in bleaching of the Soret band, consistent with loss of aromaticity of the corrole. Similar bleaching of the Soret band is also observed for one-electron oxidation of the Cu corroles (CuII−corrole•2− → CuII−corrole−), which is thought to lead to a potentially antiaromatic, doubly oxidized corrole ligand.53 By contrast, one-electron reduction of the Cu corroles results in the appearance of new Soret bands with extinction coefficients that are comparable to or even higher than those of the neutral complexes, indicating that the anionic states are aromatic and may be described as CuII−corrole3−. Space does not permit a discussion of the rich electrochemical properties of metallocorrole sandwich compounds60−62 and of metal−metal bonded corrole dimers,63 and of the implications of these studies vis-à-vis metal−ligand interactions. The interested reader is referred to the relevant references.60−63

8. X-RAY ABSORPTION SPECTROSCOPY There have been only a handful of XAS studies on metallocorroles.13,30,31,54,64 The unique strength of XAS

Figure 9. Cyclic voltammetry for the first reduction and first oxidation of selected metallocorroles in PhCN. Reproduced with permission from ref 53. Copyright 2015 Wiley.

Table 3. XANES Data for Selected Series Metalloporphyrins and Metallocorroles complex

Figure 10. Cyclic voltammograms (V vs SCE) of selected Mn and Fe triphenylcorrole derivatives in dichloromethane containing 0.1 M TBAP.

is, slightly below FeCl corroles.12,13 In other words, a relatively high reduction potential does not necessarily imply a noninnocent corrole, even though the converse appears to be a valid generalization. Another interesting difference between FePh and MnPh corroles is that the former undergo reversible one-electron reduction, but the latter reduce irreversibly, presumably with cleavage of the Mn−Ph bond (Figure 10).31 Chromium(V)−oxo corroles afford yet another example of innocent metallocorroles that undergo reduction at relatively high potentials.59 The key role of controlled-potential UV−vis spectroelectrochemistry in this connection cannot be overstated. The method provides a direct window into the electronic-structural changes accompanying a given redox process. In particular, a redox process accompanied by an intensification of the UV−vis spectra (especially the Soret band) is generally indicative of an

metal preedge

Mn[TpCF3PC] (py)n Mn[TpCF3PC]Cl Mn[TpCF3PC]Ph

Mn K

Fe[TPC]Cl Fe[TPC](NO) {Fe[TPC]}2O Fe[TPC]Ph

Fe Fe Fe Fe

Co[TPP](py)Cl Co[TPC](py)2 Co[TPC](PPh3)

Co K Co K Co K

Cu[TPP] Cu[TPC] Cu[Br8TPP] Cu[Br8TPC] Cu[(CF3)8TPP] Cu[(CF3)8TPC]

Cu Cu Cu Cu Cu Cu

K K K K K K

Ag[TPP] Ag[TPC] Ag[Br8TPP] Ag[Br8TPC]

Ag Ag Ag Ag

L3 L3 L3 L3

Mn K Mn K K K K K

IWAE (eV) Manganese 6540.6 6540.8 6541.4 Iron 7113.2 7113.5 7114.0 7113.8 Cobalt 7710.2 7709.9 7709.7 Coppera 8978.9, 8979.7, 8978.7, 8979.3, 8978.7, 8979.3, Silver 3349.7 3352.1 3349.7 3351.4

relative intensity (au)

ref

1.0

31

2.1 5.3 1.37 1.55 3.02 3.61

13

28

8979.9 8980.9 8979.5 8980.6 8979.7 8980.5

64

13 24 9 21

54

The lower and higher energy peaks have been assigned to a Cu 1s → 3dx2−y2 transition and a Cu 1s → corrole/porphyrin π* transition, respectively. a

centers around its element-specific character and its ability to interrogate local geometric and electronic structure at a given atom in a molecule or material. For our purposes, X-ray absorption near-edge spectra (XANES) are the most pertinent. For K-edge XANES of first-row transition metals, the rising 2010


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series of geometrically similar complexes, the 1s → 3d transition gains intensity via two major mechanisms, via an increase in the number of 3d holes and via symmetry-allowed 3d−4p mixing. The higher-intensity of the K pre-edge features of MnPh and FePh corroles, relative to their MnCl and FeCl analogues, reflects both these factors. Not only do the phenyl complexes have a higher number of 3d holes (because of the essentially tetravalent nature of the metal centers), their 3dz2based LUMOs entail strong mixing with the 4pz orbital. Gratifyingly, these qualitative interpretations are robustly supported by TDDFT calculations. XANES measurements on analogous sets of coinage metal porphyrins and corroles (M[TPP], M[TPC], M[Br8TPP], M[Br8TPC] (M = Cu, Ag)) have also led to significant insights.54,64 For example, although the metal centers in Cu corroles are clearly more oxidized relative to Cu porphyrins, they are clearly also less oxidized than authentic Cu(III) species, consistent with substantial CuII−corrole•2− character. Another interesting observation is that the Cu/Ag pre-edges of the M[Br8TPC] species occur at lower energies relative to M[TPC], suggesting that the metal centers in the former are less oxidized, which, in turn suggests that brominated corrole ligands are more noninnocent (i.e., have higher radical character) than the unbrominated ligands. Furthermore, Table 3 shows that the Ag L3 pre-edge of Ag[Br8TPC] is also less intense than that of Ag[TPC], consistent with a higher effective d electron count for the former. These results support significant AgII−corrole•2− character for Ag[Br8TPC] (and much less for Ag[TPC]), as also suspected on the basis of optical, structural, and DFT evidence.53 Given the multiplicity of factors that affect both the rising edge and pre-edge, XANES may not be reliable for oxidation and spin state assignments for highly covalent systems, as we recently found for Co[TPC](PPh3).28 The Co K rising edge also proved substantially red-shifted (by a margin of ∼2.0 eV) relative to the authentic Co(III) complexes Co[TPC](py)2

Figure 11. Normalized Fe K-edge XANES data for Fe[TPC]Cl (black), Fe[TPC]NO (green), {Fe[TPC]}2O (red), and Fe[TPC]Ph (purple). The inset displays the pre-edge region (top) and the corresponding first derivative (bottom).

edge, which reflects 1s → 4p transitions, often provides an indication of the charge or electrostatic potential at the atom in question. However, the position of the rising edge is also affected by other factors such as multiple scattering, spin state, element−ligand distances, etc. The pre-edge region, which arises from electric-dipole-forbidden, quadrupole-allowed 1s → 3d transitions, often proves more informative, shedding light on ligand field strength, d electron count and spin state, and degree of centrosymmetry. Table 3 presents a summary of the XANES results that we have assembled in collaboration with Dr. Ritimukta Sarangi of the Stanford Synchrotron Radiation Lightsource. Some of the most elegant applications of XANES to metallocorroles have come from experiments on squarepyramidal Mn31 and Fe13 corroles (Figure 11). For a given

Table 4. Summary of the Techniques That Have Provided Key Evidence for the Determination of the Innocent/Noninnocent Character of Selected Metallocorrole Familiesa compound class Mn[Cor]Cl Mn[Cor]Ph Fe[Cor]Cl Fe[Cor]Ph Fe[Cor](NO) {Fe[Cor]}2(μO) Co[Cor](PPh3) Co[Cor](py) Co[Cor](py)2 Cu[Cor] Ag[Cor]b Au[Cor] Pt[Cor](Ar) (py) Pt[Cor](Ar1) (Ar2)

UV−vis substituent effects

DFT spin densities

NMR paramagnetic shifts

X X X X X X

X X X X X X

− − X X − −

− − − − − −

X X X X X X X

X X − X − − −

− − − X − − −

X

X

−

vibrational spectra

electrochemistry

XAS

X X X X X X

− − X X X X

X X X X X X

X X X X X X

noninnocent innocent noninnocent innocent noninnocent noninnocent

− − − − − − −

X − X X X X X

− − − − − − −

X X X X X X X

X − − X X − −

noninnocent noninnocent innocent noninnocent innocent innocent innocent

X

X

−

X

−

full corrole radical

EPR structure

conclusion regarding ligand noninnocence

An X indicates that the technique in question has played a pivotal role in establishing or ruling out ligand noninnocence. A “−” indicates the lack of such a pivotal role. A “−” does not indicate that the property in question (such as an NMR or EPR spectrum) is not observable but only that it has not played a major role with respect to the innocence/noninnocence question. bAs discussed earlier in the Account, simple Ag corroles are innocent; Ag octabromocorroles are thought be substantially noninnocent. a

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states, and on diagonal elements of one-electron energy. Coord. Chem. Rev. 1966, 1, 164−178. (2) Kaim, W.; Schwederski, B. Non-innocent ligands in bioinorganic chemistry − An overview. Coord. Chem. Rev. 2010, 254, 1580−1588. (3) Eisenberg, R.; Gray, H. B. Noninnocence in Metal Complexes: A Dithiolene Dawn. Inorg. Chem. 2011, 50, 9741−9751. (4) Hoffmann, R.; Alvarez, S.; Mealli, C.; Falceto, A.; Cahill, T. J.; Zeng, T.; Manca, G. From Widely Accepted Concepts in Coordination Chemistry to Inverted Ligand Fields. Chem. Rev. 2016, 116, 8173−8192. (5) Thomas, K. E.; Alemayehu, A. B.; Conradie, J.; Beavers, C. M.; Ghosh, A. The Structural Chemistry of Metallocorroles: Combined XRay Crystallography and Quantum Chemistry Studies Afford Unique Insights. Acc. Chem. Res. 2012, 45, 1203−1214. (6) Orłowski, R.; Gryko, D.; Gryko, D. T. Synthesis of Corroles and Their Heteroanalogs. Chem. Rev. 2017, 117, 3102−3137. (7) Ghosh, A. Electronic Structure of Corrole Derivatives: Insights from Molecular Structures, Spectroscopy, Electrochemistry, and Quantum Chemical Calculations. Chem. Rev. 2017, 117, 3798−3881. (8) Fang, Y.; Ou, Z.; Kadish, K. M. Electrochemistry of Corroles in Nonaqueous Media. Chem. Rev. 2017, 117, 3377−3419. (9) Teo, R. D.; Hwang, J. Y.; Termini, J.; Gross, Z.; Gray, H. B. Fighting Cancer with Corroles. Chem. Rev. 2017, 117, 2711−2729. (10) Lyaskovskyy, V.; de Bruin, B. Redox Non-Innocent Ligands: Versatile New Tools to Control Catalytic Reactions. ACS Catal. 2012, 2, 270−279. (11) Costentin, C.; Savéant, J.-M.; Tard, C. Ligand “noninnocence” in coordination complexes vs. kinetic, mechanistic, and selectivity issues in electrochemical catalysis. Proc. Natl. Acad. Sci. U. S. A. 2018, 115, 9104−9109. (12) Steene, E.; Wondimagegn, T.; Ghosh, A. Electrochemical and Electronic Absorption Spectroscopic Studies of Substituent Effects in Iron(IV) and Manganese(IV) Corroles. Do the Compounds Feature High-Valent Metal Centers or Noninnocent Corrole Ligands? Implications for Peroxidase Compound I and II Intermediates. J. Phys. Chem. B 2001, 105, 11406−11413; Erratum: J. Phys. Chem. B 2002, 106, 5312. (13) Ganguly, S.; Giles, L. J.; Thomas, K. E.; Sarangi, R.; Ghosh, A. Ligand Noninnocence in Iron Corroles: Insights from Optical and Xray Absorption Spectroscopies and Electrochemical Redox Potentials. Chem. - Eur. J. 2017, 23, 15098−15106. (14) Simkhovich, L.; Mahammed, A.; Goldberg, I.; Gross, Z. Synthesis and Characterization of Germanium, Tin, Phosphorus, Iron, and Rhodium Complexes of Tris(pentafluorophenyl)corrole, and the Utilization of the Iron and Rhodium Corroles as Cyclopropanation Catalysts. Chem. - Eur. J. 2001, 7, 1041−1055. (15) Vazquez-Lima, H.; Norheim, H. K.; Einrem, R. F.; Ghosh, A. Cryptic Noninnocence: FeNO Corroles in a New Light. Dalton Trans 2015, 44, 10146−10151. (16) Wasbotten, I. H.; Wondimagegn, T.; Ghosh, A. Electronic Absorption, Resonance Raman, and Electrochemical Studies of Planar and Saddled Copper(III) meso-Triarylcorroles. Highly SubstituentSensitive Soret Bands as a Distinctive Feature of High-Valent Transition Metal Corroles. J. Am. Chem. Soc. 2002, 124, 8104−8116. (17) Steene, E.; Dey, A.; Ghosh, A. β-Octafluorocorroles. J. Am. Chem. Soc. 2003, 125, 16300−16309. (18) Thomas, K. E.; Wasbotten, I. H.; Ghosh, A. Copper βOctakis(Trifluoromethyl)Corroles: New Paradigms for Ligand Substituent Effects in Transition Metal Complexes. Inorg. Chem. 2008, 47, 10469−10478. (19) Gouterman, M.; Wagniére, G. H.; Snyder, L. C. Spectra of Porphyrins. Part II. Four-Orbital Model. J. Mol. Spectrosc. 1963, 11, 108−115. (20) Gouterman, M. Optical Spectra and Electronic Structure of Porphyrins and Related Rings. In The Porphyrins; Dolphin, D., Ed.; Academic Press: New York, 1978; Vol. III, Part A, pp 1−165. (21) Ghosh, A.; Wondimagegn, T.; Parusel, A. B. J. Electronic Structure of Gallium, Copper, and Nickel Complexes of Corrole.

and Co[TPP](py)Cl. However, neither the Co K pre-edge region nor Co Kβ XES measurements revealed much of a difference among the three compounds, a good demonstration that these measurements too have their limitations.

9. CONCLUSION In conclusion, Table 4 provides an overview of the techniques that have played a pivotal role in establishing (or ruling out) ligand noninnocence in various metallocorrole families. Clearly, some of the techniques, such as optical spectroscopy, DFT calculations, and X-ray structure determinations, have proven more useful than others.65 Electrochemical studies are also widely applicable, but as emphasized above, the results need to be interpreted with care. XAS (and other X-ray spectroscopies), already very useful, are likely to be deployed much more widely, partly reflecting the availability of newgeneration synchrotrons and spectroscopic hardware. Our final take-home lesson is simple: When faced with a new system, apply multiple techniques, preferably at least three to four, to establish and rigorously characterize ligand noninnocence.

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AUTHOR INFORMATION

ORCID

Sumit Ganguly: 0000-0003-0561-3917 Abhik Ghosh: 0000-0003-1161-6364 Notes

The authors declare no competing financial interest. Biographies Sumit Ganguly (b. 1986) did his B.Sc. (Hons.) and M.Sc. in chemistry at the University of Calcutta. He recently completed his PhD at UiT−The Arctic University of Norway on the subject of ligand noninnocence in metallocorroles under the supervision of Prof. Abhik Ghosh. This Account is based to a significant extent on his Ph.D. thesis. Abhik Ghosh (b. 1964) is a professor of chemistry at UiT−The Arctic University of Norway. He did his B.Sc. (Hons.) studies at Jadavpur University, Kolkata, and his Ph.D. work at the University of Minnesota, the latter under the tutelage of Professor Paul G. Gassman. Over the years, he has been a Senior Fellow at the San Diego Supercomputer Center (1997−2004) and a Visiting Professor at The University of Auckland, New Zealand (2006−2016). His research interests lie at the intersection of bioinorganic, materials, and theoretical chemistry. In recent years, he has played a significant role in developing the field of 5d metallocorroles, a unique class of “sizemismatched” transition metal complexes. With former student Steffen Berg, he wrote the textbook Arrow Pushing in Inorganic Chemistry: A Logical Approach to the Chemistry of the Main Group Elements, which won the 2014 PROSE Award for Best Textbook in the Physical Sciences and Mathematics.

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ACKNOWLEDGMENTS We acknowledge the Research Council of Norway for longterm support, most recently via grant no. 262229. We are deeply grateful to the many collaborators who contributed to this work over the years.

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REFERENCES

(1) Jørgensen, C. K. Differences between the four halide ligands, and discussion remarks on trigonal-bipyramidal complexes, on oxidation 2012


Article

Accounts of Chemical Research High-Valent Transition Metal Centers Versus Noninnocent Ligands. J. Am. Chem. Soc. 2000, 122, 5100−5104. (22) Alemayehu, A. B.; Ghosh, A. Gold Corroles. J. Porphyrins Phthalocyanines 2011, 15, 106−110. (23) Thomas, K. E.; Alemayehu, A. B.; Conradie, J.; Beavers, C. M.; Ghosh, A. Synthesis and Molecular Structure of Gold Triarylcorroles. Inorg. Chem. 2011, 50, 12844−12851. (24) Alemayehu, A. B.; Conradie, J.; Ghosh, A. A First TDDFT Study of Metallocorrole Electronic Spectra: Copper meso-Triarylcorroles Exhibit Hyper Spectra. Eur. J. Inorg. Chem. 2011, 1857−1864. (25) Norheim, H.-K.; Capar, J.; Einrem, R. F.; Gagnon, K. J.; Beavers, C. M.; Vazquez-Lima, H.; Ghosh, A. Ligand Noninnocence in FeNO Corroles: Insights from β-Octabromocorrole Complexes. Dalton Trans. 2016, 45, 681−689. (26) Ganguly, S.; Vazquez-Lima, H.; Ghosh, A. Wolves in Sheep’s Clothing: μ-Oxo-Diiron Corroles Revisited. Chem. - Eur. J. 2016, 22, 10336−10340. (27) Mahammed, A.; Giladi, I.; Goldberg, I.; Gross, Z. Synthesis and Structural Characterization of a Novel Covalently-Bound Corrole Dimer. Chem. - Eur. J. 2001, 7, 4259−4265. (28) Ganguly, S.; Renz, D.; Giles, L. J.; Gagnon, K. J.; McCormick, L. J.; Conradie, J.; Sarangi, R.; Ghosh, A. Cobalt- and RhodiumCorrole-Triphenylphosphine Complexes Revisited: the Question of a Noninnocent Corrole. Inorg. Chem. 2017, 56, 14788−14800. (29) It is worth mentioning in this connection that dissociation of pyridine from innocent six-coordinate CoIII(py)2 corroles results in noninnocent five-coordinate CoII(py)-corrole•2− species in solution.30 (30) Ganguly, S.; Conradie, J.; Bendix, J.; Gagnon, K. J.; McCormick, L. J.; Ghosh, A. Electronic Structure of Cobalt− Corrole−Pyridine Complexes: Noninnocent Five-Coordinate Co(II) Corrole−Radical States. J. Phys. Chem. A 2017, 121, 9589−9598. (31) Ganguly, S.; McCormick, L. J.; Conradie, J.; Gagnon, K. J.; Sarangi, R.; Ghosh, A. Electronic Structure of Manganese Corroles Revisited: X-ray Structures, Optical and X-ray Absorption Spectroscopies, and Electrochemistry as Probes of Ligand Noninnocence. Inorg. Chem. 2018, 57, 9656−9669. (32) Roos, B. O.; Veryazov, V.; Conradie, J.; Taylor, P. R.; Ghosh, A. Not Innocent: Verdict from Ab Initio Multiconfigurational SecondOrder Perturbation Theory on the Electronic Structure of Chloroiron Corrole. J. Phys. Chem. B 2008, 112, 14099−14102. (33) For a review, see: Walker, F. A.; Licoccia, S.; Paolesse, R. Iron Corrolates: Unambiguous Chloroiron(III) (Corrolate)•2−−π-Cation Radicals. J. Inorg. Biochem. 2006, 100, 810−837. (34) Vogel, E.; Will, S.; Tilling, A. S.; Neumann, L.; Lex, J.; Bill, E.; Trautwein, A. X.; Wieghardt, K. Metallocorroles with Formally Tetravalent Iron. Angew. Chem., Int. Ed. Engl. 1994, 33, 731−735. (35) Zakharieva, O.; Schünemann, V.; Gerdan, M.; Licoccia, S.; Cai, S.; Walker, F. A.; Trautwein, A. X. Is the Corrolate Macrocycle Innocent or Noninnocent? Magnetic Susceptibility, Mössbauer, 1H NMR, and DFT Investigations of Chloro- and Phenyliron Corrolates. J. Am. Chem. Soc. 2002, 124, 6636−6648. (36) Cai, S.; Licoccia, S.; D’Ottavi, C.; Paolesse, R.; Nardis, S.; Bulach, V.; Zimmer, B.; Shokhireva, T. Kh.; Walker, F. A. Chloroiron meso-Triphenylcorrolates: Electronic Ground State and Spin Delocalization. Inorg. Chim. Acta 2002, 339C, 171−178. (37) Alemayehu, A. B.; Vazquez-Lima, H.; Beavers, C. M.; Gagnon, K. J.; Bendix, J.; Ghosh, A. Platinum Corroles. Chem. Commun. 2014, 50, 11093−11096. (38) For a series of innocent Pt(IV) corroles, see: Alemayehu, A. B.; McCormick, L. J.; Gagnon, K. J.; Borisov, S. M.; Ghosh, A. Stable Platinum(IV) Corroles: Synthesis, Molecular Structure, and RoomTemperature Near-IR Phosphorescence. ACS Omega 2018, 3, 9360− 9368. (39) Dong, S. S.; Nielsen, R. J.; Palmer, J. H.; Gray, H. B.; Gross, Z.; Dasgupta, S.; Goddard, W. A. Electronic Structures of Group 9 Metallocorroles with Axial Ammines. Inorg. Chem. 2011, 50, 764− 770.

(40) Palmer, J. H.; Lancaster, K. M. Molecular Redox: Revisiting the Electronic Structures of the Group 9 Metallocorroles. Inorg. Chem. 2012, 51, 12473−12482. (41) Golubkov, G.; Bendix, J.; Gray, H. B.; Mahammed, A.; Goldberg, I.; DiBilio, A. J.; Gross, Z. High- Valent Manganese Corroles and the First Perhalogenated Metallocorrole Catalyst. Angew. Chem., Int. Ed. 2001, 40, 2132−2134. (42) Zaragoza, J. P. T.; Siegler, M. A.; Goldberg, D. P. A Reactive Manganese(IV)−Hydroxide Complex: A Missing Intermediate in Hydrogen Atom Transfer by High-Valent Metal-Oxo Porphyrinoid Compounds. J. Am. Chem. Soc. 2018, 140, 4380−4390. (43) Alemayehu, A. B.; Gonzalez, E.; Hansen, L. K.; Ghosh, A. Copper Corroles Are Inherently Saddled. Inorg. Chem. 2009, 48, 7794−7799. (44) Alemayehu, A. B.; Hansen, L. K.; Ghosh, A. Nonplanar, Noninnocent, and Chiral: A Strongly Saddled Metallocorrole. Inorg. Chem. 2010, 49, 7608−7610. (45) Berg, S.; Thomas, K. E.; Beavers, C. M.; Ghosh, A. Undecaphenylcorroles. Inorg. Chem. 2012, 51, 9911−9916. (46) Thomas, K. E.; Conradie, J.; Hansen, L. K.; Ghosh, A. A Metallocorrole with Orthogonal Pyrrole Rings. Eur. J. Inorg. Chem. 2011, 2011, 1865−1870. (47) Thomas, K. E.; Beavers, C. M.; Ghosh, A. Molecular Structure of a Gold β-Octakis(trifluoromethyl)-meso-triarylcorrole: An 85° Difference in Saddling Dihedral Relative to Copper. Mol. Phys. 2012, 110, 2439−2444. (48) Rabinovich, E.; Goldberg, I.; Gross, Z. Gold(I) and Gold(III) Corroles. Chem. - Eur. J. 2011, 17, 12294−12301. (49) Capar, J.; Zonneveld, J.; Berg, S.; Isaksson, J.; Gagnon, K. J.; Thomas, K. E.; Ghosh, A. Demetalation of Copper Undecaarylcorroles: Molecular Structures of a Free-Base Undecaarylisocorrole and a Gold undecaarylcorrole. J. Inorg. Biochem. 2016, 162, 146−153. (50) Thomas, K. E.; Gagnon, K. J.; McCormick, L. J.; Ghosh, A. Molecular structure of gold 2,3,7,8,12,13,17,18-octabromo-5,10,15tris(4”-pentafluorosulfanylphenyl)corrole: Potential insights into the insolubility of gold octabromocorroles. J. Porphyrins Phthalocyanines 2018, 22, 596−601. (51) Sudhakar, K.; Mizrahi, A.; Kosa, M.; Fridman, N.; Tumanskii, B.; Saphier, M.; Gross, Z. Effect of Selective CF3 Substitution on the Physical and Chemical Properties of Gold Corroles. Angew. Chem., Int. Ed. 2017, 56, 9837−9841. (52) Stefanelli, M.; Shen, J.; Zhu, W.; Mastroianni, M.; Mandoj, F.; Nardis, S.; Ou, Z.; Kadish, K. M.; Fronczek, F. R.; Smith, K. M.; Paolesse, R. Demetalation of Silver(III) Corrolates. Inorg. Chem. 2009, 48, 6879−6887. (53) Thomas, K. E.; Vazquez-Lima, H.; Fang, Y.; Song, Y.; Gagnon, K. J.; Beavers, C. M.; Kadish, K. M.; Ghosh, A. Ligand Noninnocence in Coinage Metal Corroles: A Silver Knife-Edge. Chem. - Eur. J. 2015, 21, 16839−16847. (54) Sarangi, R.; Giles, L. J.; Thomas, K. E.; Ghosh, A. Ligand Noninnocence in Silver Corroles: A XANES Investigation. Eur. J. Inorg. Chem. 2016, 2016, 3225−3227. (55) Einrem, R. F.; Braband, H.; Fox, T.; Vazquez-Lima, H.; Alberto, R.; Ghosh, A. Synthesis and Molecular Structure of 99Tc Corroles. Chem. - Eur. J. 2016, 22, 18747−18751. (56) Alemayehu, A. B.; Vazquez-Lima, H.; Gagnon, K. J.; Ghosh, A. Stepwise Deoxygenation of Nitrite as a Route to Two Families of Ruthenium Corroles: Group 8 Periodic Trends and Relativistic Effects. Inorg. Chem. 2017, 56, 5285−5294. (57) Einrem, R. F.; Gagnon, K. J.; Alemayehu, A. B.; Ghosh, A. Metal-Ligand Misfits: Facile Access to Rhenium-Oxo Corroles by Oxidative Metalation. Chem. - Eur. J. 2016, 22, 517−520. (58) Alemayehu, A. B.; Gagnon, K. J.; Terner, J.; Ghosh, A. Oxidative Metalation as a Route to Size-Mismatched Macrocyclic Complexes: Osmium Corroles. Angew. Chem., Int. Ed. 2014, 53, 14411−14414. (59) Johansen, I.; Norheim, H.-K.; Larsen, S.; Alemayehu, A. B.; Conradie, J.; Ghosh, A. Substituent effects on metallocorrole spectra: 2013


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Accounts of Chemical Research insights from chromium-oxo and molybdenum-oxo triarylcorroles. J. Porphyrins Phthalocyanines 2011, 15, 1335−1344. (60) Alemayehu, A. B.; Vazquez-Lima, H.; Gagnon, K. J.; Ghosh, A. Tungsten Biscorroles: New Chiral Sandwich Compounds. Chem. Eur. J. 2016, 22, 6914−6920. (61) Alemayehu, A.; Vazquez-Lima, H.; McCormick, L. J.; Ghosh, A. Relativistic effects in metallocorroles: comparison of molybdenum and tungsten biscorroles. Chem. Commun. 2017, 53, 5830−5833. (62) Schies, C.; Alemayehu, A. B.; Vazquez-Lima, H.; Thomas, K. E.; Bruhn, T.; Bringmann, G.; Ghosh, A. Metallocorroles as inherently chiral chromophores: resolution and electronic circular dichroism spectroscopy of a tungsten biscorrole. Chem. Commun. 2017, 53, 6121−6124. (63) Alemayehu, A. B.; McCormick, L. J.; Vazquez-Lima, H.; Ghosh, A. Relativistic Effects on a Metal−Metal Bond: Osmium Corrole Dimers. Inorg. Chem. 2019, 58, 2798−2806. (64) Lim, H.; Thomas, K. E.; Hedman, B.; Hodgson, K. O.; Ghosh, A.; Solomon, E. I. X-ray Absorption Spectroscopy as a Probe of Ligand Noninnocence in Metallocorroles: The Case of Copper Corroles. Inorg. Chem. 2019, 58, 6722−6730. (65) Certain other measurements such as fluorescence and phosphorescence quantum yields may be correlated with the closedshell character (and hence innocence) of certain metallocorroles. The systems in question, namely, certain main-group and 5d element corroles, however, are generally not of particular interest from the standpoint of ligand noninnocence. As for all iron compounds, Mössbauer spectroscopy is an important tool for the characterization of iron corroles but has played a relatively limited role in delineating the issue of ligand noninnocence. For additional details, the interested reader may consult our comprehensive review on corrole derivatives7 and references cited therein.

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Seven Clues to Ligand Noninnocence: The Metallocorrole Paradigm

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