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Ligand “noninnocence” in coordination complexes vs.kinetic, mechanistic, and selectivity issues inelectrochemical catalysisCyrille Costentina,1,2, Jean-Michel Savéanta,1, and Cédric Tardb,1
aUniversité Paris Diderot, Sorbonne Paris Cité, Laboratoire d’Electrochimie Moléculaire, Unité Mixte de Recherche Université, CNRS 7591, 75205 Paris Cedex13, France; and bLCM, CNRS, Ecole Polytechnique, Université Paris-Saclay, 91128 Palaiseau Cedex, France
Contributed by Jean-Michel Savéant, July 13, 2018 (sent for review June 21, 2018; reviewed by Marc Fontecave and Clifford P. Kubiak)
The world of coordination complexes is currently stimulated bythe quest for efficient catalysts for the electrochemical reactionsunderlying modern energy and environmental challenges. Even inthe case of a multielectron−multistep process, catalysis starts withuptake or removal of one electron from the resting state of thecatalyst. If this first step is an outer-sphere electron transfer (trig-gering a “redox catalysis” process), the electron distribution overthe metal and the ligand is of minor importance. This is no longerthe case with “chemical catalysis,” in which the active catalystreacts with the substrate in an inner-sphere manner, often involv-ing the transient formation of a catalyst−substrate adduct. Thefact that, in most cases, the ligand is “noninnocent,” in the sensethat the electron density and charge gained (or removed) from theresting state of the catalyst are shared between the metal and theligand, has become common-place knowledge over the last half-century. Insistent focus on a large degree of noninnocence of theligand in the resting state of the catalyst, even robustly validatedby spectroscopic techniques, may lead to undermining the essen-tial role of the metal when such essential issues as kinetics, mech-anisms, and product selectivity are dealt with. These points aregeneral in scope, but their discussion is eased by adequately docu-mented examples. This is the case for reactions involving metal-loporphyrins as well as vitamin B12 derivatives and similar cobaltcomplexes for which a wealth of experimental data is available.
electrochemical reactions | catalysis | contemporary energy challenges |ligand/metal noninnocence | cyclic voltammetry
Catalysis of electrochemical reactions by low-valent (for re-ductions; high-valent for oxidations) coordination metal
complexes is a topic that has recently garnered boosted atten-tion triggered by modern energy and environmental chal-lenges. Considering, for example, the case of reductive processes,the question may arise of whether the electron introduced in thestarting complex to produce the catalytic species will “sit” on themetal or on the ligand (or, symmetrically, where is the hole lo-cated in case of oxidations), therefore hopefully orienting thefollow-up chemistry toward catalysis. This makes us inescapablyenter the world of “noninnocent ligands,” a terminology that hasflourished after it was realized that, in redox reactions, both themetal and the ligand can host the incoming electron in reduc-tions (or holes in oxidations). An excellent retrospective (1)traces back the origin of this picturesque expression of the ratherstraightforward notion that the metal and ligand orbitals mix,forming global molecular orbitals. The repetitive recent use ofthis terminology (2, 3) is thus surprising, noting that the weight ofthe ligand contribution in these global molecular orbitals hasindeed no reason to be negligible, the more so if the ligand hasintrinsic redox properties or, simply, electronic withdrawing/do-nating ability that render it noninnocent. The same is re-spectively true for the metal. Conventional designation of theoxidation state of a metallic complex by the formal oxidationstate of metal, the ligand being viewed as perfectly “innocent”and the metal perfectly “guilty,” has obviously no realistic scope
as to where the charge is located but is merely taking care of thebookkeeping of the number of electrons globally exchanged. Inthe framework of molecular catalysis of electrochemical reac-tions, designing selective efficient catalysts and benchmarking acatalyst requires kinetic and mechanistic insights. Relying onelectronic distribution in the catalyst resting state to predict re-activity involving the ligand or the metal may be misleading, asshown later on.There is actually no obvious relationship between ligand non-
innocence and catalytic efficiency that could be used, per se, tohelp benchmark catalysts and design more efficient ones. Pursuingthese goals requires gathering kinetic or transient informationthrough application of molecular electrochemistry concepts andtechniques. Strategies in this area are enlightened by delineationof two types of catalyzed electrochemical processes (Fig. 1) (4).In “redox catalysis,” the active form of the catalyst acts as a
mere outer-sphere single-electron transfer. In the other—“chemicalcatalysis”—a catalytic intermediate is formed by an inner-spherereaction between the electron transfer-generated active form ofthe catalyst and the substrate, opening a reaction sequence thatregenerates the starting form of the catalyst and is kinetically morefavorable than a mere outer-sphere electron transfer. This Sabatierintermediate (5) may be simply an adduct formed by addition of thesubstrate to the active form of the catalyst (QA in Fig. 1) or anothertype of catalytic intermediate formed with a lower barrier than thatof an outer-sphere electron transfer. The identification and thestructural characterization of trapped intermediates are ways ofunraveling mechanisms, with the possible drawback that themechanism and kinetics of the catalytic reaction under normal
Significance
Coordination complexes are constantly sought as catalysts inthe transformation of small molecules involved in contempo-rary energy and environmental challenges. Insistent emphasison the well-established notion of ligand “noninnocence” mayblur the essential role of the metal center when kinetic,mechanistic, and selectivity issues are targeted rather than thedetailed electronic structure of the starting catalyst molecule.With the help of modern concepts and techniques of molecularelectrochemistry, several examples are described in which ex-tremely efficient catalysis takes place at the metal center de-spite an a priori discouraging large delocalization of charge andelectron density on the noninnocent ligand.
Author contributions: C.C., J.-M.S., and C.T. designed research, performed research, andwrote the paper.
Reviewers: M.F., Collège de France; and C.P.K., University of California, San Diego.
The authors declare no conflict of interest.
Published under the PNAS license.1To whom correspondence may be addressed. Email: [email protected], [email protected], or [email protected].
2Present address: Department of Chemistry and Chemical Biology, Harvard University,Cambridge, MA 02138.
Published online August 24, 2018.
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operating conditions would not be the simple extension of what theyare under intermediate trapping conditions. A particularly effectiveway of investigating and operating electron transfer-triggered ca-talysis is offered by molecular electrochemistry. Control of theelectrode potential provides a continuous variation of the concen-tration of the active catalyst at the electrode, while the current af-fords straightforward access to the catalytic reaction kinetics. Anadditional advantage is that these experiments can be carried out ina microelectrolytic context, where the recording of the current(kinetic) vs. electrode potential (thermodynamic) can be carriedout with negligible consumption of the substrate by means of anondestructive technique such as cyclic voltammetry (CV), thusavoiding cumbersome preparative-scale electrolyses.In redox catalysis, the distribution of the incoming electron in
the lowest unoccupied molecular orbit of the active catalyst (forreductions, or of the departing electron in the highest occupiedmolecular orbital for oxidation) is not of much importance, sincethe catalyst works globally as an outer-sphere electron donor (oracceptor, respectively). This is not the case with chemical ca-talysis where the formation of an intermediate involving theactive catalyst and the substrate is an essential step in agreementwith the general Sabatier principle (5) as illustrated in Fig. 1. Thefigure also shows that the Sabatier intermediate should not betoo stable at the risk of its decomposition becoming the slow stepof the catalytic process, eventually hampering completely catalysis.In most cases, molecular catalysis is a multielectron process,
hence the idea that the uptake of a second electron would beeased by the “redox noninnocent” ligand, acting as an “electronreservoir” for the metal. In the absence of a coupled chemicalstep, the uptake of a second electron is generally more difficultthan that of the first (4). The molecular factors that govern theseparation of the two ensuing standard potentials are, anyway,properties of the common metal−ligand molecular orbitals,rather than properties depending separately on the metal andthe ligand, making irrelevant the “electron reservoir” notion. Itshould be noted that, in practice, molecular catalysis is not onlymultielectron processes but also multistep processes, in whichelectron transfers (E) are coupled with chemical steps (C) along“EEC”- or, more frequently, “ECE”-type reaction sequences(4, 6). For a typical example, see Fig. 7. The mesomeric metal−ligand relation may well continue to be at work in the successiveintermediates involved in the global reaction.Metalloporphyrins offer a remarkable illustration of these is-
sues. This will be the object of Catalytic and Noncatalytic Reac-tions of Electrogenerated Low-Valent Metalloporphyrins, VitaminB12s, and Similar Cobalt Complexes, where catalytic and non-catalytic reactions will be analyzed. Vitamin B12s—the famousCo(I) “supernucleophile”—raises similar problems, albeit that arecent report concerning a quite similar cobalt complex introduces avery different mechanism invoking “ligand noninnocence” (see Fig.5). Ligand Noninnocence and Ligand Substitution Effects: Through-Structure and Through-Space Effects explores the relation betweenligand noninnocence and ligand substitution effects.
Catalytic and Noncatalytic Reactions of ElectrogeneratedLow-Valent Metalloporphyrins, Vitamin B12s, and SimilarCobalt ComplexesThe Fe“I”/“0” porphyrin couple is involved in several catalyticprocesses, notably catalysis of the CO2-to-CO electrochemicalconversion, where the systematic analysis of ligand substitutionhas led to the design of the most efficient catalysts of this re-action today (7–9). As detailed later on, the Fe0 resonance formis considered, among the mesomeric forms, as the best repre-sentation of the active catalyst at the start of the catalytic loop,
�Fe0,2−ðPÞ�↔ �
FeI,−ðP·−Þ�↔ �FeIIðP :; 2− Þ�.
From the very first times the catalytic or stoichiometric reactivityof doubly reduced FeII porphyrin were investigated, until veryrecently, various spectroscopic techniques have indicated that theFeII resonance form appears as largely predominant over theFe0 form (10–21). Taking as an example the reduction of CO2 toCO, the starting molecular reductant would be the FeII reso-nance form of the doubly reduced FeII porphyrin and the finalproduct FeIICO. From this perspective, the metal oxidationnumber seems to be invariant over the course of the reaction,and hence the ligand is not only noninnocent but also “redoxactive.” However, in this example as well as others, reasons fornevertheless insisting on an Fe0-based depiction of the mech-anism of these reactions are founded on kinetic and selectivityexperimental investigations.Pursuing the application of the Sabatier principle (Fig. 1),
electrophiles more potent than CO2 are predicted to give rise tononcatalytic reactions in which the Sabatier intermediate is sostable that it is itself the reaction product (blue curve in Fig. 1).This is indeed the case with n-BuBr as the electrophile (22),where its reaction with Fe“0” porphyrins directly produces aperfectly characterized n-butyl-FeII complex, with no detectableligand alkylation. This seems difficult to explain by means of a
redox catalysis
no catalysis
two-stepchemical catalysis
reaction coordinate
potentialenergy
QA
QA
Q + A
P + B
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BP
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electrode solution direct electrochemical reaction
redox catalyzed electrochemical reaction
A
P
Q
chemically catalyzed electrochemical reaction
QA (or CI)
B Product (selectivity)
products (no selectivity)
products (no selectivity)
-e
-e
-e
Fig. 1. Contrasting redox and chemical catalysis of an electrochemical re-action (in the case of a reduction). A, substrate; B, products; P/Q, the catalystcouple; QA, the Sabatier adduct or intermediate.
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Fe“0”TPP
Fe“0” (diC3Ph)2TPP
Fe“0”ETIOP
Fe“I”OEPFe“0”F20PP
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Fe“I”OEP
Fe“0”F20PP
Fe“0”TPPFe“0”ETIOP
Fe“0”F20PP
0(eV)GΔ
Fig. 2. (Left) Rate constant (k) vs. reaction standard free energy (ΔG0) plot forthe reaction of the three butyl bromides (n-, s-, t-, from top to bottom) with theiron porphyrins shown in DMF + 0.1M Bu4NBF4 (solid diamonds). The solid circlesrepresent a series of aromatic anion radicals playing the role of outer-sphereone-electron donors. The full lines represent the fitting of these data points bythe quadratic law for the dissociative electron transfer of the three butyl bro-mides according the Morse curve model of dissociative electron transfer in ref. 4.(Right) Structures of the porphyrins.
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reaction scheme involving an FeII complex with no change of theFe oxidation state along the reaction path,
Fe0,2−ðPÞ+ n-BuBr→ n-BuFeII,−ðPÞ+Br−
vs.FeIIðP : , 2− Þ+ n-BuBr→ n-BuFeII,−ðPÞ+Br−.
FeII porphyrins are indeed known for their appetite for Lewisbases as axial ligands and even more for double-bond-formingligands such as O, CO, carbenes, etc., but not for electrophiles.This is confirmed by the SN2 character of the iron alkylationprocess as a result of a systematic investigation of the reactionkinetics of a series of Fe“I” and Fe“0” porphyrins with n-,s-,t-butylbromides, compared with that of aromatic anion radicals actingas outer-sphere electron donors (Fig. 2) (23).Some contribution of the Fe0 resonant form ought thus to
be present in the initial complex to initiate the dynamic pro-cess that leads to its alkylation in which the ligand–metal re-dox swap is likely to be influenced by the approach of theelectrophile reactant to the iron center. It should be noted, inthis connection, that the contribution of the FeII(P:,2‒) formis likely to be less important in the electrochemical conditions[N,N-dimethylformamide (DMF) + 0.1 M Et4NClO4 solu-tions] than in the spectroscopic studies where the negative chargesborne by the porphyrin ring are stabilized by ion pairing with thesodium ions deriving from the reductive reagent used in much lesspolar solvents (ethers) than DMF. An even more striking exampleis provided by the catalysis of H2 evolution from the reduction ofEt3NH
+ by the Fe“I”/“0” tetraphenylporphyrin (TPP) couple in DMF(24). The faradaic efficiency of dihydrogen formation is 100%,with no degradation of the porphyrin catalyst after more than 1 hof electrolysis. By contrast, the complex resulting from the additionof two electrons to TPPCuII does not catalyze hydrogen evolutionunder the same conditions despite the fact that the standard po-tential of the TPPCuII‒/II2‒ couple (−1.63 V vs. SCE) is almost thesame as the standard potential of the TPPFe“I”/“0” couple (−1.60 Vvs. SCE) (24). Instead, addition of the acid leads to a 3e− + 3H+
hydrogenation of the ring.In the absence of the acid, the iron porphyrin exhibits three
successive reversible one-electron CV waves corresponding tothe successive formation of the FeII, Fe“I”−, and Fe“0”2− com-plexes. Upon addition of the acid, the first two waves remainunchanged (this is shown in Fig. 3 for wave 1/1’, representing theFe“II”/“I” couple). Generation of Fe“0”2− at wave 2/2’ triggers theappearance of a catalytic irreversible wave, noted as 2C in Fig. 3.Noteworthy also is the presence, at more negative potentials,
of a small but distinct reversible wave (3/3′), which features thereversible hydride FeHII−/I2− couple. At low acid/catalyst con-centration ratios, the catalytic wave occurs at a more positive
potential than the Fe“I”/“0” couple which still gives rise to a re-versible wave (Fig. 3A). Upon raising the acid/catalyst concen-tration ratio, the catalytic wave increases in height and shifts inthe negative direction, thus merging with the 2/2’ wave, while thereversibility disappears. This behavior is typical of a “total” ca-talysis situation where the catalytic reaction is so fast that thecurrent is controlled by the diffusion of the substrate to theelectrode surface (4). Very similar results were obtained withCHF2CO2H as the acid.The preceding comparison of the different behaviors of the
reduced state of the iron and copper complexes is a good illus-tration of a more general trend. Accumulation of electron den-sity and negative charge on the ligand indeed quite often leads toits saturation (hydrogenation, carboxylation, alkylation, etc.,eventually followed by demetallation) and thus to the de-activation of the catalyst. This is current practice, even if theseevents—being considered as failures in strategy—are not oftenreported and analyzed in detail (25).What happens when the metal is varied while keeping the
same ligand can further be illustrated with catalysis by metal-loporphyrins of the electrochemical reduction of 1,2-dibromo-cyclohexane into the corresponding olefin (Fig. 4) (26).It clearly appears (Fig. 4) that the Zn and Cu porphyrins
behave as outer-sphere electron donors, as does the free base,whereas the Co, Fe, and Ni data points stand well above the outer-sphere line. Additional electrochemical and stereochemical
Fig. 3. CV of TPPFeIIICl (A, 0.96 mM; B, 0.65 mM) in DMF + 0.1 M Et4NClO4 ata mercury electrode in the presence of Et3NHCl (A, 1.6 mM; B, 7.1 mM). Scanrate is 0.1 V/s. Temperature is 25 °C. Reprinted with permission from ref. 24.Copyright 1996 American Chemical Society.
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0 0- (V)cat targE E
FeCo Ni
Cu
Zn
H2
Fig. 4. Catalysis of the electrochemical reduction of 1,2-dibromocyclohex-ane (Top Right) into the corresponding olefin (Top Left) in DMF + 0.1 MBu4NBF4 by anion radicals of aromatic hydrocarbons (solid circles) and by theredox couples obtained by one-electron reversible reduction of ETIOP por-phyrins (see Fig. 2 for the definition of the ETIOP ring) of CoII, FeII, NiII, ZnII,and CuII and the free base (H2) (solid squares), from the data in ref. 26. Thefull line represents the fitting of these data points by the quadratic law forthe dissociative electron transfer by outer-sphere one-electron donorsaccording to the Morse curve model in ref. 4. (Bottom) Catalysis of the elec-trochemical reduction of 1,2-dibromocyclohexane (OlX2). For redox catalysis,the key step is the dissociative electron transfer step in which the aromaticanion radicals, but, also, the one-electron reduced ZnII and CuII porphyrins andfree base play the role of outer-sphere electron donors, D•‒. The two metalsare perfectly innocent, and the ligand is entirely guilty. For chemical catalysis,the key step is the halonium abstraction by the noninnocent metal at the +Ioxidation degree as for Co, Fe, and Ni, with ‒XM“III”+L as the Sabatier catalyticintermediate.
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investigations of other vicinal 1,2-dihalohalides (27) showedthat two types of mechanisms take place, as summarized inFig. 4.Another interesting example of alkylation, reminiscent of what
we have previously discussed with Fe“0” porphyrins, is providedby the system shown in Fig. 5 (28). The oxidative addition of thealkylation reagent RX on the anionic cobalt complex shown inFig. 5 follows an SN2 mechanism. According to ref. 28, Cowould remain perfectly innocent at the oxidation degree III,bearing a +1 charge and (very unlikely for an CoIII complex) noaxial ligand (Fig. 5, Top). This full “innocence” of the metalwould be maintained all along the course of the reaction, whichseems in contradiction to the fact that alkylation takes placesolely at the metal and not at any atom of the ligand, as shouldbe the case, at least in part, if the whole electron density andnegative charge were borne by the ligand.This is very similar to the previous discussion of the reaction of
Fe“0” porphyrin with alkyl halides. For the same reasons, thereactant is best formulated by the same type of resonance hybridsas shown above.The cobalt complex in Fig. 5 and its alkylation reactions are
reminiscent of vitamin B12s (Fig. 6). This is a potent nucleophile,giving rise to organocobalt derivatives (29, 30), and, for this rea-son, traditionally represented by its CoI resonance form (Fig. 6). Itis also a weak Brönsted base, in which, unlike the CoII or CoIII B12,the cobalt is not coordinated to the endogenous benzimidazoleaxial ligand (gray tint part of Fig. 6) (30).It results from the preceding discussion that, even if the main
reaction of vitamin B12s takes place at the cobalt, there is noreason that the corrin ligand should be totally innocent. Indeed,theoretical calculations attempting to reproduce spectrochemicalresults indicate that there is a substantial, albeit not total, con-tribution of 67% of the d8 CoI configuration with a 23% con-tribution of CoII–noninnocent corrin ring (31). However, thesefigures should be taken with extreme caution, according to theauthor himself. They are certainly not an obstacle for vitaminB12s to be a vigorous nucleophile at cobalt. The same is true, forthe same reasons, with the Fig. 5 cobalt complexes.Coming back to the catalysis of the CO2-to-CO electro-
chemical conversion by the TPPFe“I/0” couple, it should berecalled that the noninnocence of the porphyrin ligand, with thepredominant contribution of the FeII(P:,2‒) resonant form,appeared, at first, as a discouraging obstacle to an efficient cat-alyst. The turnover number was desperately low, to the benefit ofthe irreversible saturation of the porphyrin (7). Faradaic effi-ciencies get considerably better when Lewis or Brönsted acidsare added as cosubstrates, to the point of reaching 100% (8). The
very fact that addition of proton donors draws the reaction to-ward CO2 catalytic reduction rather than toward irreversiblesaturation of the porphyrin ring is a further indication that thecatalytic chemistry takes place at the iron center despite the likelymodest contribution of the Fe0,2‒(P) resonant form at the start ofthe reaction.On this basis, and with the help of a systematic CV in-
vestigation, the mechanism depicted in Fig. 7 could be estab-lished (8). The formation of a, Fe‒CO2 adduct as the first step ofthe catalytic process is confirmed by low-temperature resonanceRaman (32). Furthermore, in the same vein, installing the acidfunctionalities inside the catalyst molecule in positions favoringH-bond stabilization and protonation of the initial Fe‒CO2 ad-duct considerably improves the catalytic efficiency, according tothe mechanism summarized in Fig. 8 (33).
Ligand Noninnocence and Ligand Substitution Effects:Through-Structure and Through-Space EffectsAs seen in the preceding sections, ligand noninnocence amountsto the metal−ligand mesomeric relation involving common mo-lecular orbitals. Rather than repetitive invocation of ligand non-innocence, it therefore appears more fruitful to take advantageof this situation to investigate and rationalize the effects of in-troducing substituents in the ligand. Substituents are expectedto change the standard potential of the catalyst couple alongwith the electron density and charge on the metal, and hencethe catalytic efficiency. These through-structure effects aresimply the result of the combination of the orbitals of the non-innocent ligand and of the noninnocent metal to form a commonmolecular orbital, which passes on to the metal the electronicdonating or withdrawing effect of the substituents. If catalysisis of the redox type, such variation of substituents will mostlyresult in a change of the standard potential of the catalyst outer-sphere redox couple, with a moderate effect on the associatedintrinsic barrier.In the case of chemical catalysis, for a reduction, introduction
of an electron-withdrawing substituent leads to a favorable de-crease of the overpotential due to a positive shift of the catalyststandard potential and, at the same time, an unfavorable de-crease of the turnover frequency (i.e., the apparent pseudo-first-order rate constant of homogeneous reduction of the substrateby the reduced form of the catalyst, the rate constant kcat), due tothe decrease of the electron density on the metal atom. These
Fig. 5. Alkylation of a CoI complex.
Fig. 6. Vitamin B12s.
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effects are reversed for electron-donating substituents, and thecase of an oxidation is strictly symmetrical.An ideal situation would be that substitution would both shift
E0cat toward positive values and increase the overall catalytic rate
constant, kcat (for reductions; the opposite for oxidations), in otherwords, an optimization in terms of both overpotential and kinetics.In fact, it appears that, as far as through-structure substituent ef-fects are concerned, the two effects play in opposite directions.A good illustration is provided by catalysis of the electro-
chemical CO2-to-CO conversion by Fe“I”/“0” porphyrin couples.Fig. 9 shows a linear free energy relationship between the log ofthe global catalytic rate constant TOFmax = kcat = K1K2k3 (Fig. 7)and the standard potential of the catalyst couple taken as thermo-dynamical index of the correlation (black straight line in Fig. 9),with a global correlation coefficient of 2. Dissection of this globaleffect along each step allows a better understanding that whatmakes that an advantage in terms of overpotential is compensatedby a disadvantage in terms of TOFmax.Most of the effect of an electron-donating substituent, which
renders E0cat more negative, indeed results from an increase of
the nucleophilicity of the Fe0 center and of the Brönsted basicityof the oxygens in the initial iron‒CO2 adduct—two effects thatincrease the overall catalytic rate constant. These are revertedfor an electron-withdrawing substituent. A more ambitious questis to overcome these limitations with substituents that do notexclusively exert their influence through the electronic structureof the metal complex. Such is the case with charged substituentsthat may stabilize key intermediates through electrostatic inter-actions, while still having a through-structure effect. Typical exam-ples are provided by catalysis of the CO2-to-CO electrochemicalconversion by TPPFeI/0 substituted by four (positively charged)trimethylammonio groups in ortho position or para position or byfour (negatively charged) sulfonic groups in para position (Fig. 9),as results from the relevant electrostatic stabilization and de-stabilization of the negatively charged 2−Fe0CO2 ↔FeICO•−
2 pri-mary intermediate. This effect is small in both cases because of the
Fig. 7. Catalysis of the electrochemical CO2-to-CO conversion by theTPPFe“I/0” couple in DMF + 0.1 M n-Bu4BF4 in the presence of various weakBrönsted acids. (Top Left) Cyclic voltammograms of FeTPP recorded in theabsence (blue) and presence (red) of CO2 and PhOH substrates. (Top Right)Correlation between the rate constant of the rate-determining step and thepKa of the acid. (Bottom) Mechanism.
−FeICO2•− Sabatier intermediate
i
E
a) to stabilize the CO2 Sabatier intermediate by means of H-bonding
b) to provide a high local concentration of proton donor
The pending O-Hs serve two purposes:
Fig. 8. Catalysis of the electrochemical CO2-to-CO conversion in DMF +0.1 M n-Bu4BF4 by the Fe“I/0” couple of two TPPs bearing phenol substituentsin ortho, ortho’ of their phenyl groups in the presence of PhOH.
0
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Fig. 9. Through-structure and through-space substituent effects in the ca-talysis of the electrochemical CO2-to-CO conversion by the Fe“I”/“0” porphyrincouples in DMF + 0.1 M n-Bu4BF4. Shown is linear free energy correlationbetween the overall catalytic reaction and the standard potential of thecatalyst couple as appears for the through-structure substituent effects(FeTPP, FeF5TPP, FeF10TPP, FeF10TPP) (36). Through-space electrostatic ef-fects departing from the correlation appear with charged substituents (Fe-o-TMA, Fe-p-TMA, Fe-p-SULF) (34).
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large distance between the para position and the charge on theinitial intermediate. It becomes very large when the trimethy-lammonio groups are placed in the ortho position, giving rise tothe most efficient catalyst of the CO2-to-CO electrochemicalconversion at present (34). Another type of initial adduct stabili-zation is at work in the catalysis of the CO2-to-CO electrochemicalconversion by the two FeI/0 porphyrins bearing acid functionalitiesas discussed previously (Fig. 8).
ConclusionAs realized for a long time, ligands in coordination complexesare noninnocent in the sense that the metal and the ligand or-bital mix to form common molecular orbitals. These are able tohost or expel electrons. That the additional electron or hole thuscreated is delocalized over the metal and the ligand, makingfurther reactions involve one or the other of these parts of themolecule, or both competitively, is a truism. However, thequestion is relevant to the contemporary efforts in the field ofenergy and environment, which requires the design of efficientcatalysts for the activation of small molecules. The relative lo-cation of an incoming electron or hole is not a crucial one in thecase of redox catalysis in which the rate-determining step is anouter-sphere electron transfer between the active form of thecatalyst and the substrate. The situation is quite different withchemical catalytic processes in which inner-sphere electron
transfers or formation of transient intermediates associating thesubstrate and the active catalyst are operating. Involvement ofthe metal gives rise to a richer palette of possibilities in terms ofefficiency and specificity. The drawback of recent insistence onnoninnocence of ligands is that it may miss the really importantendeavors in the design of efficient catalysts. More-reasonableapproaches consist in gathering and analyzing more experi-mental kinetic data by use of all of the resources of modernmolecular electrochemistry. Among these, catalysts’ bench-marking by means of catalytic Tafel plots is an essential tool inthe quest for more-efficient catalysts. They are profitably ap-plied to investigate through-structure and through-space effectsof systematic variations of the ligand while keeping the samemetal and vice versa (35). Further progress in analysis of suchmechanistic subtleties is expected to result from the extensionof kinetic studies. This concerns experiments as well as cluesfrom quantum chemical computations. In the latter case, anurgent task should be not only to analyze the ground state ofthe active form of the catalyst but also to follow the kinetics ofits reaction with the substrate and watch the changes of theelectron distribution along the reaction coordinate. On theexperimental side, many kinetic data can be accessed by drawingon existing studies, but more in-depth investigations might be ad-visable, such as kinetic isotope effects and temperature-dependentexperiments.
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