Role of Group 12 Metals in the Reduction of H2O2 by Santi’s Reagent: A Computational Mechanistic Investigation

PhSeZnCl, which is also known as Santi’s reagent, can catalyze the reduction of hydrogen peroxide by thiols with a GPx-like mechanism. In this work, the first step of this catalytic cycle, i.e., the reduction of H2O2 by PhSeZnCl, is investigated in silico using state-of-the-art density functional theory calculations. Then, the role of the metal is evaluated by replacing Zn with its group 12 siblings (Cd and Hg). The thermodynamic and kinetic factors favoring Zn are elucidated. Furthermore, the role of the halogen is considered by replacing Cl with Br in all three metal compounds, and this turns out to be negligible. Finally, the overall GPx-like mechanism of PhSeZnCl and PhSeZnBr is discussed by evaluating the energetics of the mechanistic path leading to the disulfide product.


INTRODUCTION
−4 A similar zinc selenolate was first proposed as an intermediate nucleophilic reagent for the synthesis of allyl halides 5 and α-selenocarbonyl derivatives. 6Then, with a novel protocol, 1 1-ZnCl was isolated as a bench-stable solid; furthermore, PhSeZnBr (1-ZnBr) was also synthesized with a similar protocol, starting from PhSeBr.These compounds have been widely used as nucleophilic selenenylating agents in organoselenium chemistry, like in vinylic substitutions 7 and in ring openings of epoxides and aziridines. 8Other applications of these reagents include the synthesis of selenosteroids, 9,10 the functionalization of polymers, 11 and the synthesis of a fluorescent probe to detect glutathione. 12Finally, 1-ZnCl has become commercially available in 2016, when it was named, for the first time, Santi's reagent. 13−37 As nucleophilic selenenylating reagent, 1-ZnCl was also used as catalyst in the oxidation of thiols to disulfides in the presence of air or peroxides. 38This catalytic activity is efficient with many thiols, including glutathione (GSH); notably, GSH is not oxidized when exposed to air unless 1-ZnCl is added; finally, Santi's reagent also accelerates the oxidation of GSH by hydrogen peroxide (H 2 O 2 ). 38−42 These enzymes catalyze the reduction of hydroperoxides using GSH as cofactor; in particular, in the oxidative step, the selenocysteine present in the active site is oxidized to selenenic acid by the hydroperoxide.Then, in the two-step reductive stage, 2 equiv of GSH restore the native form of the enzyme, passing through a selenosulfide intermediate. 43The proposed mechanism for the GPx-like mechanism of 1-ZnCl 38 is shown in Scheme 1A, and the Zn 2+ Lewis acid plays a significant role.Also in this case, the first step of the catalytic cycle is the reduction of H 2 O 2 by 1-ZnCl leading to water and to a peculiar product, i.e., 2-ZnCl.In analogy with the direct oxidation of organochalcogen substrates to the corresponding oxides, 44 we expected that the oxidation of a chalcogen containing ligand would lead, in this case, to the zinc selenenate-κSe.In contrast, the mechanism proposed by Santi et al. 38 starts with the insertion of oxygen into the Zn−Se bond, forming an oxygen bridge between the metal and the chalcogen and thus resulting in the zinc selenenate-κO.The following steps show close analogies to those of GPx (Scheme 1B).First, GSH binds to the Se atom with the formation of the Se−S bond characterizing selenosulfide 3-ZnCl.Then, a second GSH attacks the sulfur center, leading to the cleavage of the oxidized glutathione (GSSG) and to the regeneration of the catalyst 1-ZnCl.
In this work, we present the results of a computational mechanistic investigation of the reduction of H 2 O 2 by PhSeMX (1-MX in Scheme 2, M = Zn, Cd, Hg; X= Cl, Br, I), elucidating the role of the metal as well as the effect of the halogen.Different scenarios are described: depending on the metal, the halogen, and the environment, we assist to the formation of the selenenate-κSe 2-MX, the selenenate-κO 4-MX, and the addition product 5-MX (Scheme 2).Last, we describe the essential features of the whole catalytic cycle following H 2 O 2 reduction, rationalizing the peculiar role of zinc.

COMPUTATIONAL METHODS
All DFT calculations were carried out with the Amsterdam Density Functional (ADF) program 2019.307 and the Amsterdam Modeling Suite (AMS) 2020.104program. 45,46Zeroth-order regular approximation (ZORA) was employed to include scalar relativistic effects in the calculations, as recommended in the presence of heavy atoms. 47or all the geometry optimizations, the OLYP 48,49 functional was used; this choice is supported by a benchmark study 50 and analogous works on the oxidation of organochalcogenides. 44,51 For all atoms, the TZ2P basis set was used, i.e., a large, uncontracted set of Slater-type orbitals of triple-ζ, augmented with two sets of polarization functions per atom; furthermore, a small frozen core approximation was employed.This level of theory is denoted as ZORA-OLYP/TZ2P. For al fully optimized structures, frequency calculations were performed to extract thermodynamic corrections and assess whether a true minimum was reached.All minima have real frequencies, whereas transition states have one imaginary frequency associated with the normal mode connecting reactants to products.Reaction paths were calculated using the intrinsic reaction coordinate (IRC) method, 52 as implemented in AMS 2020. Imlicit solvation effects (water) have been included, either in the optimization process (level of theory: COSMO-ZORA-OLYP/TZ2P) or in single-point energy calculations (level of theory: COSMO-ZORA-OLYP/TZ2P//ZORA-OLYP/ TZ2P), by means of the conductor-like screening model (COSMO).53 To rationalize the energetics and trends of the oxidations, the activation strain model (ASM) was used, and energy decomposition analysis (EDA) was performed along the reaction coordinate 54−57 using IRC profiles with the program PyFrag. 58 AS is a fragment-based approach that allows to express the total energy at any point along the reaction coordinate (ζ) as the sum of two contributions: where strain (ΔE strain ) is the difference between the energy of the reactants with the structure they have at the investigated point and the energy of the free (undistorted) reactants and interaction (ΔE int ) is the actual chemical interaction energy between the distorted reactants.The latter term can be further split into different chemically meaningful contributions within the EDA scheme: where ΔE elstat (ζ) is the semiclassical electrostatic interaction between the unperturbed electron densities of the distorted fragments and ΔE Pauli (ζ) (Pauli or exchange repulsion) is related to the repulsion between occupied orbitals localized on the two fragments.Finally, ΔE OI (ζ) accounts for all of the occupied-void orbital interactions, such as the HOMO−LUMO interaction.

RESULTS AND DISCUSSION
The oxidation of an organic chalcogenide is reported to lead to the corresponding chalcogenoxide 44 even when a metal− chalcogen bond is involved. 59For dichalcogenides, a direct Depending on the nature of M and X and on the environment, the selenenate-κSe 2-MX, the selenenate-κO 4-MX, and the addition product 5-MX can be formed.conversion between the chalcogenoxide and the corresponding anhydride, via the insertion of O in the chalcogen−chalcogen bond, is also possible although energetically disfavored. 51This chemical behavior can be modified by the presence of a selenium−metal bond; thus, the product of the reduction of H 2 O 2 by 1-ZnCl, i.e., the first step of the catalytic cycle shown

Inorganic Chemistry
in Scheme 1A, should not be taken for granted without accurate scrutiny.We started our investigation focusing on this reaction in the gas phase; then, the role of the metal center was evaluated by replacing Zn with its siblings of group 12 (Cd and Hg).The effect of the solvation (water) on the mechanism and energetics was evaluated; finally, the role of the halogen was assessed by replacing Cl with Br on the three compounds.
3.1.Role of the Metal.In the gas phase, when the reactants approach, a reactant complex (RC) forms (Figures 1A and 2A), which is weakly stabilized with respect to the free reactants, i.e., 1-ZnCl and H 2 O 2 .A product complex (PC), lying 42.14 kcal mol −1 below RC, is produced crossing a barrier of 14.39 kcal mol −1 (Table 1), in which a water molecule is loosely coordinated to the oxidized product with the oxygen atom inserted between Zn and Se (Figures 1A and 2A).Thus, DFT calculations (level of theory: ZORA-OLYP/TZ2P) fully support the mechanism proposed by Santi et al.: 38 all attempts to find a transition state leading directly to 4-ZnCl (Scheme 2) failed.This latter species, 4-ZnCl, was located on the PES (potential energy surface), but it is destabilized by 15.35 kcal mol −1 with respect to 2-ZnCl.
To analyze the effect of the metal center on the reduction of H 2 O 2 , the same reaction was studied by replacing Zn in 1-ZnCl with its heavier group 12 siblings (Cd and Hg) at the same level of theory (ZORA-OLYP/TZ2P).Cd-and Hgsubstituted reactants (1-CdCl and 1-HgCl) undergo the same reaction with H 2 O 2 , but the mechanism differs from metal to metal.1-CdCl oxidation shows a transition state quite similar to the one found for 1-ZnCl, but the outcome of the reaction is different (Figure 1B).Starting from a much more stabilized RC (−3.49kcal mol −1 ), the reaction proceeds with the attack of the chalcogen on one oxygen of the peroxide, O−O bond cleavage, and binding of the second OH fragment of H 2 O 2 to the metal.Thus, we do not assist in the intramolecular proton transfer leading to the formation and exit of water.As a consequence, the addition product 5-CdCl forms (Scheme 2 and Figure 2A).This species is characterized by two OH groups bonded to Cd and Se, respectively, and is stabilized by 36.22 kcal mol −1 with respect to the RC (Table 1).Thus, the reduction of H 2 O 2 in the presence of Cd instead of Zn implies a higher activation energy (19.67 vs 14.39 kcal mol −1 ) and leads to a different product, which is less stabilized with respect to the corresponding RC.
Finally, H 2 O 2 is reduced by 1-HgCl that is directly oxidized to 4-HgCl without appreciable perturbation of the metal− selenium bond (Scheme 2 and Figure 1C), as previously observed by some of us for the oxidation of analogous Hg compounds. 59Compared to the reduction by 1-ZnCl, a higher activation energy is computed (25.37 vs 14.39 kcal mol −1 ), and the PC is much less stabilized with respect to the RC (−32.50 vs −42.14 kcal mol −1 ).
To better understand the behavior of 1-CdCl, we made the hypothesis that the oxidation of 1-CdCl to 5-CdCl can be considered as the first step for the formation of 4-CdCl.The addition product is a selenenic acid coordinated to a cadmium hydroxy halide; this structure suggests the possibility of a proton transfer from the Se-bonded OH moiety to the vicinal hydroxyl group with the cleavage of a water molecule from the metal center.This second step was confirmed by the presence of a transition state (TS2 in Figure 2A) with a modest energy barrier (3.36 kcal mol −1 with respect to 5-CdCl).Notably, the product complex lies very close to the transition state, i.e., only 0.75 kcal mol −1 below.As final products, a water molecule and the selenenate-κSe 4-CdCl, analogous to 4-HgCl, are formed lying at −26.04 kcal mol −1 with respect to the initial free reagents.
In all cases shown in Figure 1, the reactions end with peroxide O−O bond breaking.Although 4-ZnCl was located at a much higher energy than 2-ZnCl, we searched for the transition state connecting them, and as expected, their conversion requires a rather high activation energy (17.97 kcal mol −1 ).Actually, also 4-CdCl and 4-HgCl might undergo isomerization to the selenenates-κSe 2-CdCl and 2-HgCl, respectively.In fact, we can hypothesize the existence of an additional step leading to a product analogous to 2-ZnCl.We found that, for Cd, this step may occur easily, crossing a moderate energy barrier of 4.58 kcal mol −1 and accompanied by the release of 7.09 kcal mol −1 .However, both 4-CdCl and 2-CdCl are less stable than 5-CdCl (Figure 2A).In contrast, in the case of Hg, the isomerization of 4-HgCl to 2-HgCl seems difficult because of its relatively high activation energy (18.84 kcal mol −1 ) and a slight destabilization of the product by 1.57 kcal mol −1 (Figure 2A).In Figure 3, the structures of the reactants, the transition states, and the corresponding products of these isomerizations are shown.
The evidence that 5-MCl is formed only in the case of Cd prompted us to seek if 5-ZnCl and 5-HgCl exist on the PES.5-ZnCl was found to lie 46.40 kcal mol −1 below 1-ZnCl and, thus, 2.58 kcal mol −1 below the product complex of 2-ZnCl.In contrast, 5-HgCl lies 30.26 kcal mol −1 below 1-HgCl and is less stable than the product complex of 4-HgCl, which is directly formed, by 4.21 kcal mol −1 .However, to draw the correct conclusions about the relative stability of the different oxidation products, thermodynamic corrections must be added.Gibbs free energy values computed in the gas phase at 298 K and 1 atm are reported in Table 2.
The inclusion of entropic effects discloses that any product complexes and the addition products 5-MCl are destabilized on the PES, whereas free products 2-MCl and 4-MCl become the most stable species depending on the metal.In the case of Zn, 2-ZnCl is more stable than 5-ZnCl by 2.15 kcal mol −1 , and 2-CdCl is more stable than 5-CdCl by 3.19 kcal mol −1 .In the case of Hg, 5-HgCl is destabilized with respect to 4-HgCl by 11.89 kcal mol −1 .These results confirm that 5-ZnCl and 5-HgCl can be ruled out from the oxidation mechanism in the presence of these metals.Thus, the predominant species formed in the oxidation of Zn and Cd reactant is the selenenate-κO, whereas in the oxidation of Hg reactant, it is the selenenate-κSe.
The different oxidation paths make the quantitative comparison between the energetics not straightforward, particularly in the case of Cd.Qualitatively, considering the PCs, they all lie at a negative energy with respect to the corresponding RCs, and on going from Zn to Hg, the PCs are less and less stabilized.The same trend is observed for the most stable products formed, i.e., 2-ZnCl, 2-CdCl, and 4-HgCl.The same trends are observed when the Gibbs free energies are considered (Table S1).Thus, we conclude that the reactions become less favored moving from Zn to Hg.Conversely, the 4-MCl electronic as well as Gibbs free energies follow the opposite trend.This explains why 4-MCl is the most stable product only for oxidation of the Hg compound.The activation energies (TS, Table 1 and Table S1) can also be easily rationalized: the values increase along group 12. Hence, 1-ZnCl has the lowest activation energy.
The reduction of H 2 O 2 by 1-MCl (M = Zn, Cd, and Hg) was analyzed in the frame of the activation strain model (see Computational Methods).The chosen reaction coordinate (r.c.) is the O−O distance that changes upon bond breaking because it is the molecular event common to all three different processes.As shown in Figure 4A, an identical strain along the reaction coordinate characterizes the Zn and Cd systems; this stems from the similarity of the structures of their transition states.In contrast, a much lower ΔE strain is computed for the Hg systems because, in this case, the overall molecular structure remains almost unperturbed.Nevertheless, the activation energy decreases from that of 1-HgCl to 1-ZnCl.This evidence reveals the controlling role of ΔE int : the Zn system has the most stabilizing interaction, very close to the Cd one, which decreases (in absolute value) for the Hg system.The relative position of the transition states along the reaction coordinate (Figure 4A) is more difficult to rationalize; the earliest one is found for the Cd system followed by Hg and Zn in close proximity.The absence of a trend reflecting the group sequence is likely due to the differences in the reaction mechanisms.This holds especially true for 1-CdCl: no water molecule is cleaved during the oxidation, and almost no change in the slope of strain/interaction curves is observed.Furthermore, the transition state of the Hg system is a bit earlier than the Zn one because of the significantly lower strain computed for the former along the reaction coordinate.
Because ΔE int is mainly responsible for the activation energy trend, through EDA, the prevailing component can be assessed.Both the electrostatic interaction (Figure 4C) and the orbital interaction (Figure 4D) vary consistently with the interaction energy, whereas Pauli repulsion (Figure 4B) is characterized by a reverse energy order of the curves; i.e., 1-ZnCl is the most destabilized, and 1-HgCl is the least destabilized.The electrostatic interaction is the dominant contribution.
These trends can be rationalized by considering the size (atom radius) of the metal centers.Zn is the smallest metal; thus, its orbital overlap with oxygen is better than that of its heavier siblings.Indeed, the orbital contributions involve both the metal and selenium as acceptors in the case of Zn and Cd, whereas for Hg, only the lone pair of Se is involved (Figure S1).In addition, Pauli repulsion is larger in 1-ZnCl than in 1-CdCl and 1-HgCl; in fact, the repulsion between occupied orbitals is higher in smaller metals.Finally, 1-ZnCl is characterized by the highest electrostatic potential density.1-HgCl represents the opposite extreme, and 1-CdCl has intermediate properties.These outcomes are supported by the electrostatic potential maps (Figure 5).

Solvation Effect.
The introduction of water solvation in the calculations does not modify the reduction mechanism of H 2 O 2 by 1-ZnCl; conversely, in the case of 1-CdCl, changes are foreseen.The mechanisms for all three reagents were studied, including the solvation in the geometry optimization of stationary points.Figure 2B shows the energy profiles connecting the free reactants to the corresponding selenenate-κSe 2-MCl and selenenate-κO 4-MCl in water.The reduction of H 2 O 2 by 1-ZnCl is still a single step process leading to the formation of water and 2-ZnCl; the isomerization of the latter to 4-ZnCl remains energetically disfavored.Differently from the results in the gas phase commented in the previous paragraph, the reduction mechanism of H 2 O 2 by 1-CdCl in water is similar to the Zn one: the reagent is directly oxidized to 2-CdCl, which is slightly more stable than 4-CdCl.Finally, the reduction of H 2 O 2 by 1-HgCl in water retains the same mechanism predicted in the gas phase, i.e., a single step formation of water and 4-HgCl; the isomerization of the latter to 2-HgCl is energetically disfavored.
Table 3 shows the electronic energies (kcal mol −1 ) of the stationary points located on the PESs of these reactions in water (level of theory: COSMO-ZORA-OLYP/TZ2P) relative to the free reactants.The main energy trends found in the gas phase are maintained also in water: the energies of 2-MCl are still more negative in the presence of the lighter metals, whereas the energies of 4-MCl still follow the opposite trend.It is noteworthy that solvation stabilizes both products for the three reactants, but particularly 4-MCl.In the case of Zn, the better stabilization of 4-ZnCl than that of 2-ZnCl is not sufficient to invert the energy order of the two possible products; thus, the isomerization to the selenenate-κSe is still energetically disfavored.Analogous considerations can be drawn in the case of Cd: 2-CdCl is still more stable than 4-CdCl, but the energy difference in water is just 1.17 kcal mol −1 , suggesting that they can both be thermodynamically available.Finally, for reduction of H 2 O 2 by 1-HgCl, the most stable product remains 4-HgCl, and the isomerization to 2-HgCl becomes even more energetically disfavored.It is also noteworthy that in water, the direct oxidation of 1-HgCl to 4-HgCl becomes energetically more favored than the direct oxidation of 1-CdCl to 2-CdCl , due to the different stabilization of the two products.
Kinetically, the reduction of H 2 O 2 by 1-ZnCl is the most favored reaction, whereas the activation energies for 1-CdCl and 1-HgCl in water are higher and comparable.This can be mainly ascribed to the stabilization of the transition state in the case of the Hg reactant, whose geometry is neatly different from those with Zn and Cd, which remain very similar to each other, as in the gas phase.In particular, in the transition state involving Hg, the oxygen atom of the peroxide is close to Se (distances Hg−O and Se−O are 3.8 and 2.3 Å, respectively), whereas in the cases of Zn and Cd, the analogous oxygen is close both to the metal and to Se (distances Zn−O and Cd−O are 2.0 and 2.4 Å, whereas distance Se−O is 2.5 and 2.4 Å for the former and the latter system, respectively).Thus, in the former case, the peroxide is more exposed to the polar environment, explaining the higher level of stabilization of the heaviest metal TS in water.However, because of a concomitant slight stabilization of the Hg reactant complex and destabilization of the Cd reactant complex, the energy barriers relative to these intermediates in the presence of the heaviest metals are no longer equal; thus, the activation energy trend, with respect to the reactant complexes, is restored.
As in the gas phase, Gibbs free energy values (Table S1) show that the product complexes and the addition product 5-  MCl is destabilized on the PES (5-HgCl does not even exist at the employed level of theory).Overall, the predominant species formed in the oxidation of Zn and Cd reactants is the selenenate-κO, whereas in the oxidation of Hg reactant, it is the selenenate-κSe, analogously to the gas phase.

Effect of the Halogen.
The bromo-derivative of 1-ZnCl was also synthesized. 1 Thus, to assess the role of the halogen, the reduction of H 2 O 2 was studied in the gas phase for 1-MBr too.The presence of Br does not affect the mechanism: 1-ZnBr is directly oxidized to 2-ZnBr with an activation energy of 14.01 kcal mol −1 , releasing 41.56 kcal mol −1 ; 4-ZnBr is destabilized by 15.16 kcal mol −1 with respect to 2-ZnBr.Conversely, the reduction mechanism of H 2 O 2 by 1-CdBr in the gas phase changes: water and 2-CdBr are directly formed in a single step, similarly to what we observe for 1-CdCl in water.Furthermore, 2-CdBr remains more stable than 4-CdBr; thus, isomerization is energetically disfavored.Finally, also the reduction mechanism of H 2 O 2 by 1-HgBr is not affected by the presence of a different halogen; water and 4-HgBr are formed in a single step, and like for the Cl derivative, 2-HgBr lies at higher energy on the PES.The energy profiles of the reduction of H 2 O 2 by 1-MBr in the gas phase are reported in Figure S2A, and the data are shown in Table 4.
Overall, the presence of Br does not modify the trends on the reaction and activation energies stated for 1-MCl.1-ZnBr still has the lowest energy barrier and the most favored energetics for the oxidation to 2-ZnBr, whereas the 1-HgBr case lies at the opposite extreme: the highest energy barrier and the most favored energetics are computed for a process leading to the formation of 4-HgBr.We can conclude that the replacement of the halogen has a negligible effect even on the relative stationary points' energies; indeed, the energy values in Tables 1 and 4 show no significant differences.
Notably, the reduction mechanism of H 2 O 2 by 1-MBr does not include the formation of any 5-MBr intermediate, not even for Cd.However, these three compounds were studied for completeness.5-ZnBr and 5-CdBr lie at −46.42 and −39.87 kcal mol −1 with respect to the free reactants, respectively.Hence, they are the most stable products on the basis of electronic energies.Conversely, 5-HgBr is not the most stable compound if compared to the product complex of 4-HgBr (−30.35vs −34.56 kcal mol −1 ).
When thermodynamic corrections are added and Gibbs free energies are considered (Table S2), this picture changes, and 5-MBr is in all cases destabilized on the PES; thus, 2-ZnBr becomes more stable than 5-ZnBr by 2.20 kcal mol −1 , and 2-CdBr becomes more stable than 5-CdBr by 4.67 kcal mol −1 .Lastly, 5-HgBr, which was not the most stable compound even without including thermodynamic corrections, is further destabilized with respect to 4-HgBr by 12.30 kcal mol −1 ; notably, the same observations hold true for 5-MCl for each different metal.
To further investigate the reduction of H 2 O 2 by 1-MBr, ASM/EDA was performed using the IRC geometries computed along the chosen reaction coordinate (O−O distance).As expected, the results (Figure S3) are fully consistent with the analysis on 1-MCl.Equal strain contributions are obtained for 1-ZnBr and 1-CdBr until the transition state, whereas the strain of 1-HgBr is significantly lower.The activation energy trend is still reproduced by the interaction contributions and particularly by the electrostatic and orbital interactions.It is remarkable that the oxidations of 1-CdX show no difference in the analysis despite leading to different products.This can be explained because the two reactions proceed with the same mechanism until they reach a similar transition state; then, 1-CdCl evolves to 5-CdCl, whereas 1-CdBr evolves directly to 2-CdBr.
Finally, the solvation effects were also considered for the reduction of H 2 O 2 by 1-MBr.In this case, the presence of water does not modify the mechanisms for any reactant, which remains a single step leading to the formation of water and 2-ZnBr, 2-CdBr, or 4-HgBr, respectively.Hence, the mechanism was studied by performing single-point energy calculations in water using the geometry optimized in the gas phase (level of theory: COSMO-ZORA-OLYP/TZ2P//ZORA-OLYP/ TZ2P).The energy profiles of these reactions are reported in Figure S2B.Table 5 shows the electronic energies (kcal mol −1 ) of the stationary points in water.Similarly to the reduction of H 2 O 2 by 1-MCl, the solvation stabilizes 4-MBr more than 2-MBr, leading to the same outcomes: for Zn, 2-ZnBr remains more stable; for Cd, 2-CdBr is slightly more stable; finally, for Hg, 4-HgBr becomes much more stable than 2-HgBr.Still, when the solvent is included, the direct oxidation to 2-CdBr became slightly less energetically favored than the direct oxidation to 4-HgBr.
The activation energy trend in the gas phase is lost in water: the lowest barrier is still found in the presence of Zn, but the reduction of H 2 O 2 by 1-CdBr is the kinetically most difficult case.Differently from the reduction of H 2 O 2 by 1-MCl in water, the trends are maintained also when the energy barriers are computed with respect to the reactant complexes.The loss of a regular kinetic trend along the group is still ascribed to the greater stabilization of the transition state in water in the case of Hg.Indeed, the presence of Cl or Br does not significantly alter the geometry structures of reactants and transition states.Gibbs free energies (Table S3) show analogous thermodynamic and kinetic data, but the energy difference between 2-CdBr and 4-CdBr is close to zero; i.e., they are isoenergetic.
Lastly, the halogen series was completed by the inclusion of iodine in the analyzed systems (Table S4).No halogen effect  was found because the iododerivatives show no significant difference in either the mechanism or the energetics when compared to the reduction of H 2 O 2 by 1-MBr.

Beyond Oxidation.
According to Scheme 1A, after the reduction of H 2 O 2 by 1-ZnCl, some other steps are necessary to complete the catalytic cycle.The first one is the reaction with glutathione (GSH) leading to a selenosulfide, which is a reasonable product if the hydration equilibrium of 2-ZnCl is considered (Scheme 3).According to this equilibrium, the phenyl selenenic acid 6 can be involved in the catalytic cycle.The labile nature of selenenic acids is well-known in the literature; 60−64 indeed, it can readily react with thiols, like GSH, and selenols with the formation of a chalcogen− chalcogen bond, as in the GPx cycle 39,40 (Scheme 1B).Hence, the hydration equilibria in the presence of the different metals (Zn, Cd, Hg) and halogens (Cl, Br) have been studied, starting from the most stable products for each metal compound, i.e., 2-ZnX, 2-CdX, and 4-HgX.
Electronic reaction energies, computed in both the gas phase and water, are shown in Table 6.Regardless of the halogen and the environment, the reaction energies of 2-ZnX are close to zero; therefore, 6 can actually form in equilibrium with 2-ZnCl.When going to Cd and Hg, the reaction energies become more positive; the formation of 6 can still occur in a small percentage in the equilibria of 2-CdX.Conversely, when 4-HgX is considered, the hydration becomes neatly disfavored, especially when the water environment is included.This is consistent with the previous results because 4-MX is largely stabilized in water and, thus, their reactivity decreases.Therefore, the formation of compound 6 seems to be limited to the hydration of compounds 2-ZnX and 2-CdX.Gibbs free reaction energies (Table S5) also support this conclusion because the hydration of 2-ZnX and that of 2-CdX are both exergonic (close to zero for Cd), whereas the reaction is endergonic for 4-HgX.Although a small quantity of 6 is sufficient to proceed in the catalytic cycle, from now on, only the reactivity of 1-ZnX will be discussed because they were successfully synthesized and they are associated with the smallest activation barriers for the first step of the cycle, i.e., the hydroperoxide reduction.
Although significant, the formation of 6 is not sufficient to explain the catalytic activity of 1-ZnCl that, as reported, is not just a precursor of a selenenic acid; 38 indeed, the Zn center, as a Lewis acid, may play an active role.After its formation, 6 can react with GSH (which can be modeled as methyl thiol, MeSH, as chosen in previous studies 65−69 ) forming a selenosulfide; then, a second molecule of GSH/MeSH can in principle attack either the S or the Se center, but only the first event leads to the recovery of the initial catalyst.This is the most delicate step for designing efficient low-molecular-weight GPx mimic molecules, 70 and in this framework, the action of the Zn center can be important to drive the nucleophilic attack.
In the last part of the GPx-like mechanism of Santi's reagent, different paths can be foreseen.The postulated coordination of the Zn center to the Se atom of the selenosulfide 8 leads to 7-ZnCl, and an alkaline environment is generated (analogously to the second step in Scheme 1A).In these conditions, MeSH can be deprotonated, thus increasing the nucleophilicity of its sulfur center and thus its reactivity. 71Hence, the thiolate can react with 7-ZnCl either at the S site (path a), leading to the recovery of the catalyst 1-ZnCl and closing the cycle, or at the Se site (path b), leading again to selenosulfide 8 (Scheme 4).The same products of path b can also be obtained if the attack occurs at the Zn center because the nucleophile is the same as the previous step, i.e., glutathione anion.The energetics of these competitive reactions were investigated in silico modeling GS − as a methyl thiolate.
Electronic reaction energies were computed in water (level of theory: COSMO-ZORA-OLYP/TZ2P).For paths (a) and (b), they are −40.19 and −39.06 kcal mol −1 , respectively.Thus, path (a) seems to be energetically slightly more favored.Similar results were obtained by replacing the halogen, i.e., Br instead of Cl: the reaction energies become −39.72(a) and −38.49kcal mol −1 (b), respectively.Moreover, if the Zn center had no active role in this step of the catalysis, reaction (a) would lead to a phenyl selenolate PhSe − with ΔE= −7.11 kcal mol −1 .Thus, the Zn center may indeed play a thermodynamic role as demonstrated by the more negative reaction energy in the presence of the metal.Furthermore, the presence of a Zn 2+ ion might also have a kinetic effect from both an electronic and steric point of view; the reduced electrophilicity of Se and the increased steric hindrance around this atom may disfavor path Scheme 3. Hydration Equilibrium of 2-ZnCl with the Formation of Phenyl Selenenic Acid 6 and ZnOHCl  (b).These aspects require a thorough systematic analysis that is out of the purpose of this work.

CONCLUSIONS
To sum up, our study paves the way for the complete understanding of the GPx-like mechanism of 1-ZnX, focusing on the reduction of H 2 O 2 , which is the first and most peculiar step of the catalytic cycle.Furthermore, we have elucidated the role of the central metal by replacing Zn with its siblings in group 12, i.e., Cd and Hg.The reduction of H 2 O 2 by 1-ZnX was found to be favored both thermodynamically and kinetically; this is attributed to more favorable electrostatic and orbital interactions quantified in the framework of the ASM/EDA approaches.Moreover, the role of the halogen was investigated by systematically replacing Cl with Br; the reduction of H 2 O 2 by the three metal compounds shows features very similar to those of both halogens.Finally, the inclusion of an implicit solvation has two main effects: (i) In water, the selenenate-κO 4-MX is much more stabilized than the selenolate-κSe 2-MX.However, this effect does not modify the main outcomes of the reduction of H 2 O 2 by the three metal compounds.(ii) When 1-HgX is involved in the reaction, the transition state is particularly stabilized, making the energy barrier of the process comparable to or even lower than the one associated with 1-CdX.
For what concerns the last part of the GPx-like mechanism of Santi's reagent, further studies are needed to quantitatively assess the kinetic effect of the Zn center considering all mechanistic possibilities because, as above-described, these reactions can occur at the S, Se, and Zn sites.Particularly, in this last case, multiple mechanisms should be considered, i.e., associative, dissociative, and interchange.

Figure 2 .
Figure2.Energy profiles (kcal mol −1 ) relative to the free reactants for 1-ZnCl (blue), 1-CdCl (red), and 1-HgCl (green) oxidation and isomerization in (A) gas phase and (B) water.For the isomerization, the energy of a single water molecule was added to the stationary points for consistency.Level of theory: (COSMO)-ZORA-OLYP/TZ2P.

Figure 4 .
Figure 4. (A) Activation strain model of 1-ZnCl (blue lines), 1-CdCl (red lines), and 1-HgCl (green lines) oxidations: energy profiles (solid lines), strain contributions (dashed lines), and interaction contributions (dash-dotted lines).Energy decomposition analysis: (B) Pauli repulsion, (C) electrostatic interaction, and (D) orbital interaction.The position of the transition states is indicated by filled dots.The reaction coordinate is defined as r.c.= (d O−O -d O−O 0 ), where d O−O 0 represents the O−O bond length in the reactant complex of each reaction.Level of theory: ZORA-OLYP/TZ2P.

Scheme 4 .
Scheme 4. Competitive Reactions between 7-ZnCl and MeS −a 40heme 1. (A) GPx-like Mechanism of PhSeZnCl according to the Work of Santi et al.; 38 (B) Catalytic Cycle of Glutathione Peroxidase (GPx) according to the Description of Flohéet al.40

Table 1 .
Electronic Energies (kcal mol −1 ) Relative to the Free Reactants for the Reduction of H 2 O 2 by 1-MCl in the Gas Phase a Activation energies relative to reactant complexes are given in parentheses.Level of theory: ZORA-OLYP/TZ2P.b PC in this case is formed after the second TS with a small activation energy of 3.36 kcal mol −1 . a

Table 2 .
Gibbs Free Energies (kcal mol −1 ) Relative to the Free Reactants for the Reduction of H 2 O 2 by 1-MCl in the Gas Phase a Level of theory: ZORA-OLYP/TZ2P.b PC in this case is formed after the second transition state. a

Table 3 .
Electronic Energies (kcal mol −1 ) Relative to the Free Reactants for the Reduction of H 2 O 2 by 1-MCl in Water a a Activation energies relative to reactant complexes are given in parentheses.Level of theory: COSMO-ZORA-OLYP/TZ2P.

Table 4 .
Electronic Energies (kcal mol −1 ) Relative to the Free Reactants for the Reduction of H 2 O 2 by 1-MBr in the Gas Phase a

Table 5 .
Electronic Energies (kcal mol −1 ) Relative to the Free Reactants for the Reduction of H 2 O 2 by 1-MBr in Water a