Ligand “noninnocence” in coordination complexes vs. kinetic, mechanistic, and selectivity issues in electrochemical catalysis

Significance

Coordination complexes are constantly sought as catalysts in the transformation of small molecules involved in contemporary energy and environmental challenges. Insistent emphasis on the well-established notion of ligand “noninnocence” may blur the essential role of the metal center when kinetic, mechanistic, and selectivity issues are targeted rather than the detailed electronic structure of the starting catalyst molecule. With the help of modern concepts and techniques of molecular electrochemistry, several examples are described in which extremely efficient catalysis takes place at the metal center despite an a priori discouraging large delocalization of charge and electron density on the noninnocent ligand.

Abstract

The world of coordination complexes is currently stimulated by the quest for efficient catalysts for the electrochemical reactions underlying modern energy and environmental challenges. Even in the case of a multielectron−multistep process, catalysis starts with uptake or removal of one electron from the resting state of the catalyst. If this first step is an outer-sphere electron transfer (triggering a “redox catalysis” process), the electron distribution over the metal and the ligand is of minor importance. This is no longer the case with “chemical catalysis,” in which the active catalyst reacts with the substrate in an inner-sphere manner, often involving the transient formation of a catalyst−substrate adduct. The fact that, in most cases, the ligand is “noninnocent,” in the sense that the electron density and charge gained (or removed) from the resting state of the catalyst are shared between the metal and the ligand, has become common-place knowledge over the last half-century. Insistent focus on a large degree of noninnocence of the ligand in the resting state of the catalyst, even robustly validated by spectroscopic techniques, may lead to undermining the essential role of the metal when such essential issues as kinetics, mechanisms, and product selectivity are dealt with. These points are general in scope, but their discussion is eased by adequately documented examples. This is the case for reactions involving metalloporphyrins as well as vitamin B12 derivatives and similar cobalt complexes for which a wealth of experimental data is available.

Catalysis of electrochemical reactions by low-valent (for reductions; high-valent for oxidations) coordination metal complexes is a topic that has recently garnered boosted attention triggered by modern energy and environmental challenges. Considering, for example, the case of reductive processes, the question may arise of whether the electron introduced in the starting complex to produce the catalytic species will “sit” on the metal or on the ligand (or, symmetrically, where is the hole located in case of oxidations), therefore hopefully orienting the follow-up chemistry toward catalysis. This makes us inescapably enter the world of “noninnocent ligands,” a terminology that has flourished after it was realized that, in redox reactions, both the metal and the ligand can host the incoming electron in reductions (or holes in oxidations). An excellent retrospective (1) traces back the origin of this picturesque expression of the rather straightforward notion that the metal and ligand orbitals mix, forming global molecular orbitals. The repetitive recent use of this terminology (2, 3) is thus surprising, noting that the weight of the ligand contribution in these global molecular orbitals has indeed no reason to be negligible, the more so if the ligand has intrinsic redox properties or, simply, electronic withdrawing/donating ability that render it noninnocent. The same is respectively true for the metal. Conventional designation of the oxidation state of a metallic complex by the formal oxidation state 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 the bookkeeping of the number of electrons globally exchanged. In the framework of molecular catalysis of electrochemical reactions, designing selective efficient catalysts and benchmarking a catalyst requires kinetic and mechanistic insights. Relying on electronic distribution in the catalyst resting state to predict reactivity involving the ligand or the metal may be misleading, as shown later on.

There is actually no obvious relationship between ligand noninnocence and catalytic efficiency that could be used, per se, to help benchmark catalysts and design more efficient ones. Pursuing these goals requires gathering kinetic or transient information through application of molecular electrochemistry concepts and techniques. Strategies in this area are enlightened by delineation of two types of catalyzed electrochemical processes (Fig. 1) (4).

Fig. 1.

Contrasting redox and chemical catalysis of an electrochemical reaction (in the case of a reduction). A, substrate; B, products; P/Q, the catalyst couple; QA, the Sabatier adduct or intermediate.

In “redox catalysis,” the active form of the catalyst acts as a mere outer-sphere single-electron transfer. In the other—“chemical catalysis”—a catalytic intermediate is formed by an inner-sphere reaction between the electron transfer-generated active form of the catalyst and the substrate, opening a reaction sequence that regenerates the starting form of the catalyst and is kinetically more favorable than a mere outer-sphere electron transfer. This Sabatier intermediate (5) may be simply an adduct formed by addition of the substrate to the active form of the catalyst (QA in Fig. 1) or another type of catalytic intermediate formed with a lower barrier than that of an outer-sphere electron transfer. The identification and the structural characterization of trapped intermediates are ways of unraveling mechanisms, with the possible drawback that the mechanism and kinetics of the catalytic reaction under normal operating conditions would not be the simple extension of what they are under intermediate trapping conditions. A particularly effective way of investigating and operating electron transfer-triggered catalysis is offered by molecular electrochemistry. Control of the electrode potential provides a continuous variation of the concentration of the active catalyst at the electrode, while the current affords straightforward access to the catalytic reaction kinetics. An additional advantage is that these experiments can be carried out in a microelectrolytic context, where the recording of the current (kinetic) vs. electrode potential (thermodynamic) can be carried out with negligible consumption of the substrate by means of a nondestructive technique such as cyclic voltammetry (CV), thus avoiding cumbersome preparative-scale electrolyses.

In redox catalysis, the distribution of the incoming electron in the lowest unoccupied molecular orbit of the active catalyst (for reductions, or of the departing electron in the highest occupied molecular orbital for oxidation) is not of much importance, since the catalyst works globally as an outer-sphere electron donor (or acceptor, respectively). This is not the case with chemical catalysis where the formation of an intermediate involving the active catalyst and the substrate is an essential step in agreement with the general Sabatier principle (5) as illustrated in Fig. 1. The figure also shows that the Sabatier intermediate should not be too stable at the risk of its decomposition becoming the slow step of 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 be eased by the “redox noninnocent” ligand, acting as an “electron reservoir” for the metal. In the absence of a coupled chemical step, the uptake of a second electron is generally more difficult than that of the first (4). The molecular factors that govern the separation of the two ensuing standard potentials are, anyway, properties of the common metal−ligand molecular orbitals, rather than properties depending separately on the metal and the ligand, making irrelevant the “electron reservoir” notion. It should be noted that, in practice, molecular catalysis is not only multielectron processes but also multistep processes, in which electron 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 successive intermediates involved in the global reaction.

Fig. 7.

Catalysis of the electrochemical CO₂-to-CO conversion by the TPPFe^“I/0” couple in DMF + 0.1 M n-Bu₄BF₄ in the presence of various weak Brönsted acids. (Top Left) Cyclic voltammograms of FeTPP recorded in the absence (blue) and presence (red) of CO₂ and PhOH substrates. (Top Right) Correlation between the rate constant of the rate-determining step and the pK_a of the acid. (Bottom) Mechanism.

Metalloporphyrins offer a remarkable illustration of these issues. This will be the object of Catalytic and Noncatalytic Reactions of Electrogenerated Low-Valent Metalloporphyrins, Vitamin B12s, and Similar Cobalt Complexes, where catalytic and noncatalytic reactions will be analyzed. Vitamin B12s—the famous Co(I) “supernucleophile”—raises similar problems, albeit that a recent report concerning a quite similar cobalt complex introduces a very different mechanism invoking “ligand noninnocence” (see Fig. 5). Ligand Noninnocence and Ligand Substitution Effects: Through-Structure and Through-Space Effects explores the relation between ligand noninnocence and ligand substitution effects.

Fig. 5.

Alkylation of a Co^I complex.

Catalytic and Noncatalytic Reactions of Electrogenerated Low-Valent Metalloporphyrins, Vitamin B12s, and Similar Cobalt Complexes

The Fe^{“I”/“0”} porphyrin couple is involved in several catalytic processes, notably catalysis of the CO₂-to-CO electrochemical conversion, where the systematic analysis of ligand substitution has led to the design of the most efficient catalysts of this reaction today (7–9). As detailed later on, the Fe⁰ resonance form is considered, among the mesomeric forms, as the best representation of the active catalyst at the start of the catalytic loop,

$$\left[\mathrm{F}{\mathrm{e}}^{\mathrm{0,2}\mathrm{-}}\left(\mathrm{P}\right)\right]\leftrightarrow \left[\mathrm{F}{\mathrm{e}}^{\mathrm{I,}\mathrm{-}}\left(\mathrm{P}·\mathrm{-}\right(\right]\leftrightarrow \left[\mathrm{F}{\mathrm{e}}^{\mathrm{II}}\left(P:,2\mathrm{-}\right)\right].$$

From the very first times the catalytic or stoichiometric reactivity of doubly reduced Fe^II porphyrin were investigated, until very recently, various spectroscopic techniques have indicated that the Fe^II resonance form appears as largely predominant over the Fe⁰ form (10–21). Taking as an example the reduction of CO₂ to CO, the starting molecular reductant would be the Fe^II resonance form of the doubly reduced Fe^II porphyrin and the final product Fe^IICO. From this perspective, the metal oxidation number seems to be invariant over the course of the reaction, and hence the ligand is not only noninnocent but also “redox active.” However, in this example as well as others, reasons for nevertheless insisting on an Fe⁰-based depiction of the mechanism of these reactions are founded on kinetic and selectivity experimental investigations.

Pursuing the application of the Sabatier principle (Fig. 1), electrophiles more potent than CO₂ are predicted to give rise to noncatalytic reactions in which the Sabatier intermediate is so stable 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 a perfectly characterized n-butyl-Fe^II complex, with no detectable ligand alkylation. This seems difficult to explain by means of a reaction scheme involving an Fe^II complex with no change of the Fe oxidation state along the reaction path,

$$\begin{array}{l}\mathrm{F}{\mathrm{e}}^{\mathrm{0,2}\mathrm{-}}\left(\mathrm{P}\right)+n\text{-BuBr}\to n\text{-BuF}{\mathrm{e}}^{\mathrm{II},\mathrm{-}}\left(\mathrm{P}\right)+\mathrm{B}{\mathrm{r}}^{\mathrm{-}}\\ vs.\\ \mathrm{F}{\mathrm{e}}^{\mathrm{II}}\left(\mathrm{P}:,2\mathrm{-}\right)+n\text{-BuBr}\to n\text{-BuF}{\mathrm{e}}^{\mathrm{II},\mathrm{-}}\left(\mathrm{P}\right)+\mathrm{B}{\mathrm{r}}^{\mathrm{-}}.\end{array}$$

Fe^II porphyrins are indeed known for their appetite for Lewis bases as axial ligands and even more for double-bond-forming ligands such as O, CO, carbenes, etc., but not for electrophiles. This is confirmed by the S_N2 character of the iron alkylation process as a result of a systematic investigation of the reaction kinetics of a series of Fe^“I” and Fe^“0” porphyrins with n-,s-,t-butyl bromides, compared with that of aromatic anion radicals acting as outer-sphere electron donors (Fig. 2) (23).

Fig. 2.

(Left) Rate constant (k) vs. reaction standard free energy ($\mathrm{\Delta }{G}^0$) plot for the reaction of the three butyl bromides (n-, s-, t-, from top to bottom) with the iron porphyrins shown in DMF + 0.1 M Bu₄NBF₄ (solid diamonds). The solid circles represent a series of aromatic anion radicals playing the role of outer-sphere one-electron donors. The full lines represent the fitting of these data points by the quadratic law for the dissociative electron transfer of the three butyl bromides according the Morse curve model of dissociative electron transfer in ref. 4. (Right) Structures of the porphyrins.

Some contribution of the Fe⁰ resonant form ought thus to be present in the initial complex to initiate the dynamic process that leads to its alkylation in which the ligand–metal redox swap is likely to be influenced by the approach of the electrophile reactant to the iron center. It should be noted, in this connection, that the contribution of the Fe^II(P:,2‒) form is likely to be less important in the electrochemical conditions [N,N-dimethylformamide (DMF) + 0.1 M Et₄NClO₄ solutions] than in the spectroscopic studies where the negative charges borne by the porphyrin ring are stabilized by ion pairing with the sodium ions deriving from the reductive reagent used in much less polar solvents (ethers) than DMF. An even more striking example is provided by the catalysis of H₂ evolution from the reduction of Et₃NH⁺ 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 h of electrolysis. By contrast, the complex resulting from the addition of two electrons to TPPCu^II does not catalyze hydrogen evolution under the same conditions despite the fact that the standard potential of the TPPCu^II‒/II2‒ couple (−1.63 V vs. SCE) is almost the same as the standard potential of the TPPFe^{“I”/“0”} couple (−1.60 V vs. 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 to the successive formation of the Fe^II, Fe^“I”−, and Fe^“0”2− complexes. Upon addition of the acid, the first two waves remain unchanged (this is shown in Fig. 3 for wave 1/1’, representing the Fe^{“II”/“I”} couple). Generation of Fe^“0”2− at wave 2/2’ triggers the appearance of a catalytic irreversible wave, noted as 2C in Fig. 3.

Fig. 3.

CV of TPPFe^IIICl (A, 0.96 mM; B, 0.65 mM) in DMF + 0.1 M Et₄NClO₄ at a mercury electrode in the presence of Et₃NHCl (A, 1.6 mM; B, 7.1 mM). Scan rate is 0.1 V/s. Temperature is 25 °C. Reprinted with permission from ref. 24. Copyright 1996 American Chemical Society.

Noteworthy also is the presence, at more negative potentials, of a small but distinct reversible wave (3/3′), which features the reversible hydride FeH^II−/I2− couple. At low acid/catalyst concentration ratios, the catalytic wave occurs at a more positive potential than the Fe^{“I”/“0”} couple which still gives rise to a reversible wave (Fig. 3A). Upon raising the acid/catalyst concentration ratio, the catalytic wave increases in height and shifts in the negative direction, thus merging with the 2/2’ wave, while the reversibility disappears. This behavior is typical of a “total” catalysis situation where the catalytic reaction is so fast that the current is controlled by the diffusion of the substrate to the electrode surface (4). Very similar results were obtained with CHF₂CO₂H as the acid.

The preceding comparison of the different behaviors of the reduced state of the iron and copper complexes is a good illustration of a more general trend. Accumulation of electron density and negative charge on the ligand indeed quite often leads to its saturation (hydrogenation, carboxylation, alkylation, etc., eventually followed by demetallation) and thus to the deactivation of the catalyst. This is current practice, even if these events—being considered as failures in strategy—are not often reported and analyzed in detail (25).

What happens when the metal is varied while keeping the same ligand can further be illustrated with catalysis by metalloporphyrins of the electrochemical reduction of 1,2-dibromocyclohexane into the corresponding olefin (Fig. 4) (26).

Fig. 4.

Catalysis of the electrochemical reduction of 1,2-dibromocyclohexane (Top Right) into the corresponding olefin (Top Left) in DMF + 0.1 M Bu₄NBF₄ by anion radicals of aromatic hydrocarbons (solid circles) and by the redox couples obtained by one-electron reversible reduction of ETIOP porphyrins (see Fig. 2 for the definition of the ETIOP ring) of Co^II, Fe^II, Ni^II, Zn^II, and Cu^II and the free base (H2) (solid squares), from the data in ref. 26. The full line represents the fitting of these data points by the quadratic law for the dissociative electron transfer by outer-sphere one-electron donors according to the Morse curve model in ref. 4. (Bottom) Catalysis of the electrochemical reduction of 1,2-dibromocyclohexane (OlX₂). For redox catalysis, the key step is the dissociative electron transfer step in which the aromatic anion radicals, but, also, the one-electron reduced Zn^II and Cu^II porphyrins and free base play the role of outer-sphere electron donors, D^•‒. The two metals are perfectly innocent, and the ligand is entirely guilty. For chemical catalysis, the key step is the halonium abstraction by the noninnocent metal at the +I oxidation degree as for Co, Fe, and Ni, with ^‒XM^“III”+L as the Sabatier catalytic intermediate.

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 investigations of other vicinal 1,2-dihalohalides (27) showed that two types of mechanisms take place, as summarized in Fig. 4.

Another interesting example of alkylation, reminiscent of what we have previously discussed with Fe^“0” porphyrins, is provided by the system shown in Fig. 5 (28). The oxidative addition of the alkylation reagent RX on the anionic cobalt complex shown in Fig. 5 follows an S_N2 mechanism. According to ref. 28, Co would remain perfectly innocent at the oxidation degree III, bearing a +1 charge and (very unlikely for an Co^III complex) no axial ligand (Fig. 5, Top). This full “innocence” of the metal would be maintained all along the course of the reaction, which seems in contradiction to the fact that alkylation takes place solely at the metal and not at any atom of the ligand, as should be the case, at least in part, if the whole electron density and negative 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, the reactant is best formulated by the same type of resonance hybrids as 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 reason, traditionally represented by its Co^I resonance form (Fig. 6). It is also a weak Brönsted base, in which, unlike the Co^II or Co^III B₁₂, the cobalt is not coordinated to the endogenous benzimidazole axial ligand (gray tint part of Fig. 6) (30).

Fig. 6.

Vitamin B12s.

It results from the preceding discussion that, even if the main reaction of vitamin B12s takes place at the cobalt, there is no reason that the corrin ligand should be totally innocent. Indeed, theoretical calculations attempting to reproduce spectrochemical results indicate that there is a substantial, albeit not total, contribution of 67% of the d⁸ Co^I configuration with a 23% contribution of Co^II–noninnocent corrin ring (31). However, these figures should be taken with extreme caution, according to the author himself. They are certainly not an obstacle for vitamin B12s to be a vigorous nucleophile at cobalt. The same is true, for the same reasons, with the Fig. 5 cobalt complexes.

Coming back to the catalysis of the CO₂-to-CO electrochemical conversion by the TPPFe^“I/0” couple, it should be recalled that the noninnocence of the porphyrin ligand, with the predominant contribution of the Fe^II(P:,2‒) resonant form, appeared, at first, as a discouraging obstacle to an efficient catalyst. The turnover number was desperately low, to the benefit of the irreversible saturation of the porphyrin (7). Faradaic efficiencies get considerably better when Lewis or Brönsted acids are added as cosubstrates, to the point of reaching 100% (8). The very fact that addition of proton donors draws the reaction toward CO₂ catalytic reduction rather than toward irreversible saturation of the porphyrin ring is a further indication that the catalytic chemistry takes place at the iron center despite the likely modest contribution of the Fe^0,2‒(P) resonant form at the start of the reaction.

On this basis, and with the help of a systematic CV investigation, the mechanism depicted in Fig. 7 could be established (8). The formation of a, Fe‒CO₂ adduct as the first step of the catalytic process is confirmed by low-temperature resonance Raman (32). Furthermore, in the same vein, installing the acid functionalities inside the catalyst molecule in positions favoring H-bond stabilization and protonation of the initial Fe‒CO₂ adduct considerably improves the catalytic efficiency, according to the mechanism summarized in Fig. 8 (33).

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 substituents in ortho, ortho’ of their phenyl groups in the presence of PhOH.

Ligand Noninnocence and Ligand Substitution Effects: Through-Structure and Through-Space Effects

As seen in the preceding sections, ligand noninnocence amounts to the metal−ligand mesomeric relation involving common molecular orbitals. Rather than repetitive invocation of ligand noninnocence, it therefore appears more fruitful to take advantage of this situation to investigate and rationalize the effects of introducing substituents in the ligand. Substituents are expected to change the standard potential of the catalyst couple along with the electron density and charge on the metal, and hence the catalytic efficiency. These through-structure effects are simply the result of the combination of the orbitals of the noninnocent ligand and of the noninnocent metal to form a common molecular orbital, which passes on to the metal the electronic donating or withdrawing effect of the substituents. If catalysis is of the redox type, such variation of substituents will mostly result in a change of the standard potential of the catalyst outer-sphere redox couple, with a moderate effect on the associated intrinsic barrier.

In the case of chemical catalysis, for a reduction, introduction of an electron-withdrawing substituent leads to a favorable decrease of the overpotential due to a positive shift of the catalyst standard potential and, at the same time, an unfavorable decrease of the turnover frequency (i.e., the apparent pseudo-first-order rate constant of homogeneous reduction of the substrate by the reduced form of the catalyst, the rate constant k_cat), due to the decrease of the electron density on the metal atom. These effects are reversed for electron-donating substituents, and the case of an oxidation is strictly symmetrical.

An ideal situation would be that substitution would both shift $E_{cat}^0$ toward positive values and increase the overall catalytic rate constant, k_cat (for reductions; the opposite for oxidations), in other words, an optimization in terms of both overpotential and kinetics. In fact, it appears that, as far as through-structure substituent effects are concerned, the two effects play in opposite directions.

A good illustration is provided by catalysis of the electrochemical CO₂-to-CO conversion by Fe^{“I”/“0”} porphyrin couples. Fig. 9 shows a linear free energy relationship between the log of the global catalytic rate constant TOF_max = k_cat = K₁K₂k₃ (Fig. 7) and the standard potential of the catalyst couple taken as thermodynamical index of the correlation (black straight line in Fig. 9), with a global correlation coefficient of 2. Dissection of this global effect along each step allows a better understanding that what makes that an advantage in terms of overpotential is compensated by a disadvantage in terms of TOF_max.

Fig. 9.

Through-structure and through-space substituent effects in the catalysis of the electrochemical CO₂-to-CO conversion by the Fe^{“I”/“0”} porphyrin couples in DMF + 0.1 M n-Bu₄BF₄. Shown is linear free energy correlation between the overall catalytic reaction and the standard potential of the catalyst couple as appears for the through-structure substituent effects (FeTPP, FeF5TPP, FeF10TPP, FeF10TPP) (36). Through-space electrostatic effects departing from the correlation appear with charged substituents (Fe-o-TMA, Fe-p-TMA, Fe-p-SULF) (34).

Most of the effect of an electron-donating substituent, which renders $E_{cat}^0$ more negative, indeed results from an increase of the nucleophilicity of the Fe⁰ center and of the Brönsted basicity of the oxygens in the initial iron‒CO₂ adduct—two effects that increase the overall catalytic rate constant. These are reverted for an electron-withdrawing substituent. A more ambitious quest is to overcome these limitations with substituents that do not exclusively exert their influence through the electronic structure of the metal complex. Such is the case with charged substituents that may stabilize key intermediates through electrostatic interactions, while still having a through-structure effect. Typical examples are provided by catalysis of the CO₂-to-CO electrochemical conversion by TPPFe^I/0 substituted by four (positively charged) trimethylammonio groups in ortho position or para position or by four (negatively charged) sulfonic groups in para position (Fig. 9), as results from the relevant electrostatic stabilization and destabilization of the negatively charged ${}^{2-}\text{F}{\text{e}}^{\text{0}}{\text{CO}}_2\leftrightarrow {\text{Fe}}^{\text{I}}{\text{CO}}_2^{•-}$ primary intermediate. This effect is small in both cases because of the large distance between the para position and the charge on the initial intermediate. It becomes very large when the trimethylammonio groups are placed in the ortho position, giving rise to the most efficient catalyst of the CO₂-to-CO electrochemical conversion at present (34). Another type of initial adduct stabilization is at work in the catalysis of the CO₂-to-CO electrochemical conversion by the two Fe^I/0 porphyrins bearing acid functionalities as discussed previously (Fig. 8).

Conclusion

As realized for a long time, ligands in coordination complexes are noninnocent in the sense that the metal and the ligand orbital mix to form common molecular orbitals. These are able to host or expel electrons. That the additional electron or hole thus created is delocalized over the metal and the ligand, making further reactions involve one or the other of these parts of the molecule, or both competitively, is a truism. However, the question is relevant to the contemporary efforts in the field of energy and environment, which requires the design of efficient catalysts for the activation of small molecules. The relative location of an incoming electron or hole is not a crucial one in the case of redox catalysis in which the rate-determining step is an outer-sphere electron transfer between the active form of the catalyst and the substrate. The situation is quite different with chemical catalytic processes in which inner-sphere electron transfers or formation of transient intermediates associating the substrate and the active catalyst are operating. Involvement of the metal gives rise to a richer palette of possibilities in terms of efficiency and specificity. The drawback of recent insistence on noninnocence of ligands is that it may miss the really important endeavors in the design of efficient catalysts. More-reasonable approaches consist in gathering and analyzing more experimental kinetic data by use of all of the resources of modern molecular electrochemistry. Among these, catalysts’ benchmarking by means of catalytic Tafel plots is an essential tool in the quest for more-efficient catalysts. They are profitably applied to investigate through-structure and through-space effects of systematic variations of the ligand while keeping the same metal and vice versa (35). Further progress in analysis of such mechanistic subtleties is expected to result from the extension of kinetic studies. This concerns experiments as well as clues from quantum chemical computations. In the latter case, an urgent task should be not only to analyze the ground state of the active form of the catalyst but also to follow the kinetics of its reaction with the substrate and watch the changes of the electron distribution along the reaction coordinate. On the experimental side, many kinetic data can be accessed by drawing on existing studies, but more in-depth investigations might be advisable, such as kinetic isotope effects and temperature-dependent experiments.