DOI: 10.1002/chem.201403233

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& Metal–O2 Complexes

On the Reaction Mechanism of the Complete Intermolecular O2 Transfer between Mononuclear Nickel and Manganese Complexes with Macrocyclic Ligands Jhon Zapata-Rivera,[a, d] Rosa Caballol,*[a] Carmen J. Calzado,*[b] Dimitrios G. Liakos,[c] and Frank Neese[c]

Abstract: The recently described intermolecular O2 transfer between the side-on Ni-O2 complex [(12-TMC)Ni-O2] + and the manganese complex [(14-TMC)Mn]2 + , where 12-TMC and 14-TMC are 12- and 14-membered macrocyclic ligands, 12-TMC = 1,4,7,10-tetramethyl-1,4,7,10-tetraazacyclododecane and 14-TMC = 1,4,8,11-tetramethyl-1,4,8,11-tetraazacyclotetradecane, is studied by means of DFT methods. B3LYP calculations including long-range corrections and solvent effects are performed to elucidate the mechanism. The potential energy surfaces (PESs) compatible with different electronic states of the reactants have been analyzed. The calculations confirm a two-step reaction, with a first rate-determining bimolecular step and predict the exothermic charac-

Introduction Reactions between transition-metal complexes and molecular oxygen and the reactivity of the resulting adducts LM-O2 have attracted great interest over the past two decades due to their potential role in synthetic and biochemical catalytic processes.[1–3] Binuclear copper sites in metalloenzymes and metalloproteins, which are able to bind O2, provide many examples of [a] Dr. J. Zapata-Rivera, Prof. R. Caballol Departament de Qumica Fsica i Inorgnica Universitat Rovira i Virgili Marcel·l Domingo, s/n, 43007 Tarragona (Spain) E-mail: [email protected]

ter of the global process. The relative stability of the products and the reverse barrier are in line with the fact that no reverse reaction is experimentally observed. An intermediate with a m-h1:h1-O2 coordination and two transition states are identified on the triplet PES, slightly below the corresponding stationary points of the quintet PES, suggesting an intersystem crossing before the first transition state. The calculated activation parameters and the relative energies of the two transition sates and the products are in very good agreement with the experimental data. The calculations suggest that a superoxide anion is transferred during the reaction.

the Cu2O2 core reactivity. O2 adducts of tyrosinase or catechol oxidase catalyze the oxidation of aromatic substrates by an electrophilic attack onto the aromatic ring, whereas dopamine b-monooxygenase catalyzes CH bond hydroxylation.[1, 4, 5] The advances in synthetical chemistry have also supplied new molecules that mimic the metal sites of natural compounds. The structure and properties of a diversity of complexes with the general structure (LM)n-O2 (n = 1, 2) have been described.[2, 3, 6–12] LM-O2 adducts present usually end-on or side-on coordination modes (see Scheme 1) and can react to give (LM)2–O2 binuclear complexes, or LM-O2-L’M’ when two different metal centers are involved, where the most frequent coordination modes are mh2 :h2-O2 in many Cu complexes,[2, 11] bis(m-O) in Cu,[2, 11] Mn,[13–17]

[b] Dr. C. J. Calzado Departamento de Qumica Fsica Universidad de Sevilla c/Profesor Garca Gonzlez s/n, 41012, Sevilla (Spain) calzado@us,es [c] Dr. D. G. Liakos, Prof. F. Neese Max-Plank-Institut fr Chemische Energie Konversion Stiftstrasse 34–36, 45470 Mlheim (Germany) [d] Dr. J. Zapata-Rivera Present address: Facultad de Ingeniera Corporacin Universidad de la Costa Calle 58 # 55–66, Barranquilla (Colombia) Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/chem.201403233. Chem. Eur. J. 2014, 20, 13296 – 13304

Scheme 1. Metal-O2 coordination modes.

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Full Paper Ni,[18–20] Co,[19, 20] or Fe[9, 21–24] compounds, and m-h1:h1-O2 in a few dinuclear Cu[25–27] and Co[28–30] complexes. Upon O2 binding, hemocyanin, tyrosinase, and catechol oxidase give similar m-h2 :h2O2 side-on Cu-O2-Cu complexes[4, 5, 31] Examples of complexes with different metals are given from the reaction of ironperoxo porphyrins, Porp-FeIII-O2, with a series of CuII-L complexes as chemical models of cytochrome c oxidase. m-h2 :h2-O2 PorpFeIII-O2-CuII-L complexes are generated.[32] Copper and iron are more freFigure 2. Proposed O2 transfer mechanism as determined from kinetic data. quently involved in metalloenzymes, but many adducts of first, second- and third row transition metals have been syncomplexes to FeII complexes[37–39] because the complete O2 thesized and characterized and their reactivity has been invesmolecule is transferred in this case. In acetonitrile and at room tigated in oxidation reactions.[33–35] These biomimetic systems temperature, the reaction is fast and the reverse reaction does are relevant because they provide additional understanding of not occur. A kinetic study was performed at low temperature reaction mechanisms, but also because new synthetic catalysts in acetone to determine the activation parameters. A second with similar oxidizing properties than the natural ones can be order rate law was determined at 50 8C, with first order kinetobtained. Recently, Cho and co-workers[36] have synthesized ics in each of the reactants. The temperature dependence of and characterized a side-on nickel–oxygen adduct undergoing the rate constant between 60 and 30 8C led to an activation an unprecedented reaction in which a complete transfer of O2 enthalpy of DH° = 11.7 kcal mol1 and a significantly negative activation entropy of DS° = 18 cal mol1 K1, which suggests to a manganese complex is observed, in the reaction given in Equation (1). a bimolecular O2 transfer reaction. The global information is compatible with a two-step mechanism, in which the forma½ð12-TMCÞNi-O2 þ þ½ð14-TMCÞMn2þ ! ½ð12-TMCÞNi2þ þ tion of the [L12Ni-O2-MnL14]3 + intermediate is presumed to be ð1Þ the rate-determining step.[36] The final products are [L14Mn-O2] + ½ð14-TMCÞMn-O2 þ and [L12Ni]2. The reaction mechanism schematized in Figure 2 is still hypothetical because the intermediate has not been deThe resulting [(14-TMC)Mn-O2] + complex presents also tected even at low temperature. a side-on coordination between the Mn ion and the O2 moleThe complete O2 transfer reaction instead of the usual forcule. 12-TMC and 14-TMC are 12- and 14-membered macrocymation of a binuclear O2 species seems to be associated to the clic tetradentate ligands, respectively, with 12-TMC = 1,4,7,1014 2+ Mn] complex, because a similar reaction occurs with the [L tetramethyl-1,4,7,10-tetraazacyclododecane and 14-TMC = cobalt analog [L12Co-O2], although this reaction is 1000 times 1,4,8,11-tetramethyl-1,4,8,11-tetraazacyclotetradecane, hereafter [40, 41] slower. L12 and L14, respectively. Both ligands are represented in To the best of our knowledge, no computational study has Figure 1. been performed on this reaction, although the reactant [L12NiThis is the first described O2 transfer reaction between this + O2] was previously studied at the BP86[36] and at the DDCI type of biomimetic complexes. This reaction differs from the levels.[42] We report here a B3LYP study of the involved mechapreviously reported intermolecular oxo transfer from FeIV-oxo nism. We first discuss the electronic states of reactants and products to evaluate the potential energy surfaces (PESs) compatible with them. The stationary points are described and the mechanism is discussed taking into account the experimental data. The detailed analysis of the electronic structure of all species involved in the O2 transfer mechanism will be the object of a forthcoming study.

Computational details Figure 1. L12 (left) and L14 in cis configuration (right). Chem. Eur. J. 2014, 20, 13296 – 13304

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The reaction path calculations were performed with the B3LYP functional,[43–45] that has shown to give good results in similar sys-

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Full Paper tems[2, 46–49] reproducing with high accuracy the geometries of a large set of 3d, 4d, and 5d transition-metal complexes.[50, 51] The basis set superposition error (BSSE) was estimated by using the counterpoise method.[52] After some exploratory calculations with the Ahlrichs-type[53–55] valence double-zeta def2-SVP basis set, the valence triple-zeta basis set def2-TZVP[56] was used for optimization, location, and characterization of stationary points. Relativistic corrections by means of the zero order regular approximation, ZORA,[57] and Grimme[58] dispersion correction were included in the calculations. Both corrections are implemented in the ORCA package.[59] Solvent effects were also considered as a continuous dielectric, by means of the COSMO[60] approach. Geometry optimization and transition-states location were performed with the rational function optimization, RFO,[61, 62] and the partitioned rational function optimization, P-RFO,[61] algorithms, respectively. The starting geometry of [L12Ni-O2] + , [L14Mn-O2] + , and [L12Ni]2 + was generated from the X-ray diffraction structures. The [L12Ni-O2] + and [L12Ni] + 2 structures were described by Cho et al.[36] and the one of [L14Mn-O2] + by Seo et al.[63] Both O2 adducts are side-on coordinated. No reported structural data on [L14Mn]2 + were found and the crystallographic data of [L14Mn-Cl] + were used.[64]

Results and Discussion Preliminary calculations Some preliminary calculations were performed to get a first overview of the reaction path. First, single-point CASSCF calculations were carried out to have a qualitative description of the low-lying spin states and of the compatible reaction paths connecting reactants and products (see the Supporting Information for details). Crystallographic data were used in these calculations.[36, 63, 64] [L12Ni-O2] + is a side-on coordinated Ni-O2 adduct.[36] Its structure belongs to the Cs point group and the ground state is 2A’ as previously described from MR calculations.[42] At the CASSCF(11,7) level, the first excited state is 4A’’, with a relative energy of 6.3 kcal mol1 at this level of calculation. On the other hand, the complex [L14Mn]2 + has C2 symmetry and CASSCF(5,5) gives 6A as the ground state, and 4A as the first excited state, with a relative energy of 56 kcal mol1. According to the spin multiplicity of both complexes, the lowest energy PES has septet or quintet spin multiplicity, both degenerate for non-interacting reactants. At the CASSCF level, a triplet is also found at 6.3 kcal mol1 above the ground state, which corresponds to the interaction between the lowest [L12Ni-O2] + quartet and the ground state of [L14Mn]2 + . On the products side, [L14Mn-O2] + is EPR silent but the 1 H NMR data indicate a high-spin state, with S = 2. The EPR data have not been given for [L12Ni] + 2 but the complex has been proposed as a S = 1 spin system, consistent with a high spin d8 configuration. CASSCF calculations give a 5B ground state for [L14Mn-O2] + , and 3A’ for [L12Ni]2 + . These spin multiplicities are compatible with septet, quintet, and triplet PESs, for the non-interacting products. The reaction can therefore occur in a quintet or septet PES from the reactants in their ground state or in a triplet PES, if the quartet state of the [L12Ni-O2] + complex is stabilized along the reaction coordinate, leading to an intersystem crossing. Therefore, the three possibilities, S = 3, S = 2, and S = 1, have been explored. Chem. Eur. J. 2014, 20, 13296 – 13304

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A second exploratory set of calculations was performed at the B3LYP level, with the valence double-zeta def2-SVP[56] basis set. At the B3LYP gas-phase level, the quartet state of the [L12Ni-O2] + complex was found more stable than the doublet, contrarily to the experimental data. Because the kinetic study was performed in acetone as solvent, the solvent effect was included by means of the COSMO[60] algorithm with e = 20.7. The stability was reversed when adding dispersion and solvent effects, the doublet being more stable than the quartet by 2.3 kcal mol1 with this small basis set (see Table S1 in the Supporting Information for more details). It was concluded that both effects should be included in further calculations. Complexes [L12Ni-O2] + and [L14Mn]2 + interact to produce an hypothetical intermediate, [L12Ni-O2-MnL14]3 + . A guess geometry was generated for this complex, by combining the X-ray structures of [L12Ni-O2] + and [L14Mn-O2] + with a single O2 molecule shared by the two subsystems. Tentative single-point calculations were performed on the [L12Ni-O2-MnL14]3 + complex, to determine the most probable O2 coordination mode with both metallic centers and the relative orientation of both L12Ni and L14Mn fragments in the different spin multiplicities. The mh1:h1 coordination of the O2 molecule was found as the most plausible one, in contrast to the more frequent m-h2 :h2 coordination in (LM)2-O2 binuclear complexes, probably due to the steric repulsion of the axial methyl substituents of both macrocyclic ligands. In a next step, the geometry of [L12Ni-O2MnL14]3 + was completely optimized at the gas-phase level. For S = 1, a minimum was located, with a lower energy than the corresponding S = 2 and S = 3 states. The optimized geometry after adding dispersion and solvent effects with S = 1 gave a minimum, but the energy of this intermediate, 37 kcal mol1, was manifestly incompatible with the experimental data. The calculations of both the separated reactants and the supermolecule including the solvent effect lead to similar results. Because all involved species have positive charge (+ 1 for the adduct, + 2 for the Mn complex, and + 3 for the intermediate) we concluded that the anomalous behavior in solution could be originated by definition of the cavity in the differently charged and shaped species and concluded to add counterions. Perchlorate, ClO4 , is the counterion for both reactants in the experimental conditions. The optimization and characterization calculations are very heavy and we choose the chloride anion as a compromise to face the computational cost, we will come back to this choice in the next section. The geometry of the reactants was only slightly affected by including both solvent effects and counterions, as reported in Table S2 in the Supporting Information. It was also verified (Table S1 in the Supporting Information) that the solvent in this case also ensures the correct ground state of the reactants. Reactants and products All the calculations presented from here were performed with the valence triple-zeta basis set def2-TZVP.[56] The geometries of the neutral reactants and the products were completely reoptimized with this extended basis set and the B3LYP function-

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Full Paper al, including the Grimme correction and solvent effects by means of the COSMO algorithm with e = 20.7. Cl-L12Ni-O2, ClL14Mn-O2, Cl2-L12Ni, and Cl2-L14Mn were calculated starting from X-ray structures and changing the respective counterions by chloride when necessary. The optimized geometries of the reactants and the products, characterized as minima, are represented in Figures 3 and 4 and the main geometrical parameters are reported in Table 1. The ground state for the reactants is the quintet state, resulting from the interaction between the local ground states of the two fragments, that is, the [L12Ni-O2] + doublet and the [L14Mn]2 + sextet states. The excited S = 1 state is only 3.3 kcal mol1 above the ground state, and comes from the interaction between the local excited [L12Ni-O2] + quartet and the ground [L14Mn]2 + sextet states. All the energy values are referred hereafter to the S = 2 reactants ground state. When explicit Cl counterions, dispersion, and solvent effects were included, the relative energy of the products was found to be 7.2 kcal mol1. The predicted exothermicity combined with the reported value of the activation enthalpy of 11.6 kcal mol1 are compatible with the fact that the reverse reaction is not observed. To check the impact of this modeling on the reaction path, the reactants and the products with perchlorate as counterions were also completely optimized, (see Table S4 in the Supporting Information). For all complexes, the optimized distances are comparable. In the [L12Ni]2 + and [L14Mn]2 + complexes, the distance of one of the counterions to the metal is shorter in all cases, as represented in Figure S2 in the Supporting Informa-

Figure 3. Optimized geometry of the reactants Cl-L12Ni-O2 (left) and Cl2L14Mn (right).

Figure 4. Optimized geometry of the products Cl2-L12Ni (left) and Cl-L14MnO2 (right). Chem. Eur. J. 2014, 20, 13296 – 13304

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Table 1. Optimized geometrical parameters of the metal-coordination sphere in the reactants and the products, including explicit counterions and dispersion and solvent effects. Distances in [], angles in [8]. The Cartesian coordinates are reported in Table S3 in the Supporting Information. [L12Ni-O2] + XR[36] this work d(OO)

1.38 1.89 1.88 a(M-O-O) 68.9 2.16 2.16 d(MN) 2.03 2.04

d(MO)

1.37 1.90 1.88 68.7 2.20 2.20 2.05 2.05

[L14Mn]2 + XR[64][a] this work

[L14Mn-O2] + XR[63] this work

[L12Ni]2 + XR[36] this work

– – – – 2.26 2.28 2.23 2.26

1.40 1.88 1.88 68.2 2.32 2.31 2.22 2.22

– – – – 2.05 2.05 2.05 2.05

– – – – 2.33 2.34 2.25 2.25

1.41 1.86 1.86 67.7 2.38 2.38 2.21 2.21

– – – – 2.09 2.11 2.23 2.23

[a] [Cl-L14Mn] + complex.

tion. The relative energy of the products is practically unaffected, 7.5 kcal mol1 for ClO4 versus 7.2 kcal mol1 for Cl . We are therefore confident that the modeling of the counterions by Cl does not affect the main results of the present study.

Reaction path and energy profile To start with a reasonable guess geometry of the [L12Ni-O2MnL14]Cl3 intermediate, molecular dynamics calculations were performed by means of the Amber99[65] force field included in the GROMACS[66] package. At this level, the previous geometry of the [L12Ni-O2-MnL14]3 + cation was kept fixed and the only degrees of freedom were those of the chloride ions. For more details, see the Supporting Information. Three regions with high density of probability for the Cl ions were found, the starting geometry for the Cl3-L12Ni-O2-MnL14 model was generated and then completely optimized at the B3LYP level including acetone as solvent, with S = 1, according to the lowest state indicated by the preliminary calculations. The converged stationary point was characterized as a minimum and it was concluded that the structure was the searched intermediate I. Figure 5 shows the optimized structure of the intermediate for S = 1. The geometrical parameters of the Ni-O2-Mn core are reported in Table 2. To the best of our knowledge, no m-h1:h1 Ni-O2-Mn complex has to date been isolated, the few examples containing this coordination correspond to dicopper,[25–27] dicobalt,[28–30] and

Figure 5. Optimized geometry of the intermediate.

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Full Paper tivation energy of 9.7 kcal mol1 for S = 1. A stationary point was also characterized for S = 2 without relevant differences in the geometry with respect to TS1 and an energy of 11.8 kcal mol1 with respect to the reactants. The geometry of TS1 is represented in Figure 6 top, the most relevant parameters are

Table 2. Optimized geometrical parameters of the Ni-O2-Mn core in the stationary points of the reaction coordinate (S = 1), including explicit counterions and dispersion and solvent effects. Distances in [], angles in [8].

d(NiO) d(MnO) d(OO) d(NiMn) a (Ni-O-O) a (Mn-O-O) Ni-O-O-Mn

Reactants

TS1

I

TS2

Products

1.89 – 1.37 – 68.7 – –

1.94 3.05 1.32 5.56 100.2 119.9 182.2

2.01 2.09 1.33 4.73 105.6 117.9 183.9

2.11 2.08 1.33 4.92 118.3 113.6 174.3

– 1.86 1.41 – – 67.7 –

diiron[67] complexes. The NiO and MnO bond lengths in the intermediate, 2.01 and 2.09 , respectively, are found longer than the MO bond lengths in these Cu and Co compounds, which are in the range 1.87–1.95 . Accordingly, the OO distance is found shorter, 1.33 , compared to the Cu and Co complexes, ranging from 1.43 to 1.5 . The calculated OO distance and the corresponding stretching frequency n˜ = 1160 cm1 at the B3LYP level, although probably slightly overestimated because a scaling factor of 0.97 is recommended within this method and this basis set quality,[68] suggest a superoxide character for the intermediate, when comparing with the accepted ranges. This is in line with the correlation proposed by Cramer and co-workers[69] that gives a Mayer bond order of 1.3, associated to superoxide forms. The relative energy of the intermediate with respect to the reactants is 0.8 kcal mol1 for S = 1, 4.3 kcal mol1 for S = 2, and 6.2 kcal mol1 for S = 3, without noticeable geometry variations. These energy values are in accordance with a two-step mechanism, where the reactants interact to give the intermediate I through TS1 and then intermediate I evolves to products through TS2. The calculated relative energy of the intermediate is compatible with a small experimental activation energy and this result is a priori satisfactory. The location of the transition states of the two steps, TS1 for the conversion of the reactants to the intermediate and TS2 for conversion of the intermediate to the products, was then undertaken. To help in TS1 location, a scan for S = 1 was performed, from the intermediate to the reactants, taking the MnO distance as the scanning coordinate. The starting point was the optimized geometry of the intermediate, including the counterions, assuming that the transition-state structure will be closer to the intermediate than to reactants, according to Hammond–Leffler postulate.[70, 71] For each value of the reaction coordinate, all the remaining degrees of freedom were relaxed. A maximum was found for a MnO distance of 3.1 . The corresponding geometry was used as starting point to obtain the optimized structure of TS1 by means of the P-RFO[61] gradient minimization algorithm. A single negative eigenvalue of the Hessian confirmed that the stationary point was a transition state. The corresponding normal mode involves simultaneously the Ni-O-O angle deformation and the MnO distance shortening, in agreement with the breaking of a NiO bond and the formation of a MnO bond. The energy of TS1 indicates an acChem. Eur. J. 2014, 20, 13296 – 13304

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Figure 6. Geometry of the transition states TS1 (top) and TS2 (bottom).

reported in Table 2. For S = 3, the transition state was not located with the def2-TVZP basis set. In a determination with the def2-SVP basis set including solvent effect, dispersion, and counterions, an adiabatic energy difference of 7.3 kcal mol1 between the characterized S = 1 and S = 3 TS1 was found. All these results suggest that the S = 3 TS1 must be too high in energy to be compatible with the experimental data, and it seems unlikely that the reaction takes place following the S = 3 PES. The second transition state TS2 connects the intermediate and the products in the S = 1 PES, and was located after scanning the NiOMn coordinate, where OMn is the oxygen atom closer to Mn. The structure, which is shown in Figure 6 bottom, with a relative energy of 3.9 kcal mol1 with respect to the reactants and only 3.1 kcal mol1 above the intermediate, was characterized as a transition sate. For S = 2, the optimized relative energy of TS2 is 5.0 kcal mol1. As expected, the relative energy is significantly lower than that of TS1, both for S = 1 and S = 2. The most relevant geometrical parameters are reported in Table 2. According to the above-described results for TS1, the S = 3 TS2 was not explored. An interesting point is the fact that the S = 1 and S = 2 PESs are very close in energy in all the stationary points investigated. In order to estimate the BSSE and its influence on all the relative energies, the counterpoise procedure[52] has been applied to the described stationary points. The BSSE appears to be independent of the total spin of the fragments. The BSSE and the corrected energy, DEc = DEBSSE, are reported in

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Full Paper Table 3. Optimized relative energies referred to the ground state reactants, DE, BSSE estimates, and corrected relative energies, DEc, all in [kcal mol1], of the relevant stationary points, TS1, I, and TS2. The products, P, are free of BSSE.

total spin

TS1

I

S=1 S=2

9.7 11.8

0.8 4.3

DE TS2 3.9 5.0

P

TS1

BSSE I

TS2

TS1

DEc I

TS2

7.2

3.6 3.6

6.7 6.6

6.9 6.9

13.3 15.4

7.5 10.9

10.8 11.9

Table 3. The counterpoise correction enhances the relative energies but is relatively small on the energy barrier, with corrected values of 13.3 and 15.4 kcal mol1 for S = 1 and S = 2, respectively. The effect on the relative energies of intermediate and of TS2 is larger, which can be easily attributed to a more compact geometry with shorter distances in the region of the Ni-O2-Mn core. In order to compare the reaction barrier with the activation enthalpy extracted from kinetic data by Cho et al.,[36] DH° = 11.7 kcal mol1, the zero-point energies (ZPE) of the reactants, TS1, and TS2 as well as the thermal contributions were estimated at 223 K. The corresponding values at the B3LYP level are reported in Table 4. The ZPE increases the energies of TS1 and TS2 by 1.0 and 1.7 kcal mol1, respectively, and the thermal effects slightly moderate this increase to 0.6 and 1.3 kcal mol1. The calculated enthalpies of TS1 and TS2 are 10.3 and 5.2 kcal mol1, respectively, and become 13.9 and 12.1 kcal mol1 after adding the BSSE correction, for S = 1. On the other hand, the electronic energies are not free of spin contamination of higher septet and/or quintet states, supposed to be larger for S = 1 than for S = 2, and all the calculated energy values are probably overestimated because of this mixing with higher spin solutions. Taking into account that the S = 1 and S = 2 energies of TS1 only differ by 2.1 kcal mol1, both are in the range of the experimental activation enthalpy. The activation entropy is also in excellent agreement with the reported value, DS° = 18 cal K1 mol1. The results confirm a two-step mechanism, where the formation of the intermediate is the rate-determining step. From the described results, the corresponding two-step reaction profile is represented in Figure 7 where the BSSE corrected energies from Table 3 have been used. The ground state of the reactants is a quintet, resulting from the antiferromagnetic interaction of the [L12Ni-O2] + doublet and the [L14Mn]2 + sextet. The reaction follows the quintet PES to give the binuclear [L12Ni-O2-MnL14]3 + complex. In TS1, the S = 1 and S =

Table 4. Relative energy of TS1, TS2, and the products, including ZPE correction, enthalpy in [kcal mol1] and entropy in [cal mol1 K1], at 223 K, for S = 1.

this work

TS1 TS2 P

experimental activation parameters[36]

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DE

DE+D(ZPE)

DH

DS

9.7 3.9 7.2

10.7 5.6 5.6

10.3 5.2 6.0 11.72

17.1 16.7 0.5 18.16

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2 energies are very close and compatible with the activation experimental data, although the triplet is slightly more stable. The quintet and triplet PESs may cross before reaching TS1 and the reaction can follow either the quintet or the triplet reaction coordinate, with 13.3 and 15.4 kcal mol1 relative energies for S = 1 and S = 2, respectively. Let us briefly analyze this possible crossing. B3LYP including dispersion and solvent effects gives for re-

Figure 7. Reaction energy profile, including the BSSE correction. Gray line: S = 1, black line: S = 2. Black dots = Ni, dark gray dots = Mn, light gray dots = O.

actants an S = 1 energy of 3.3 kcal mol1 above the S = 2 ground state. To reach the TS1 structure, a deformation of the Ni-O2 side-on geometry must occur to bind the terminal O to the Mn and evolve thus to the m-h1:h1 Ni-O2-Mn coordination. As discussed in Ref. [42], [L12-Ni-O2] + deformation to an end-on coordination entails the relative stabilization of the quartet in front of the doublet state, which is in line with a possible intersystem crossing of the S = 2 and S = 1 surfaces along the reaction coordinate. An additional argument consistent with this interpretation can be extracted from CASSCF(11,7) calculations of both the doublet and the quartet states of [L12Ni-O2] + , at the geometry adopted by this fragment in TS1, with an open Ni-O-O angle enabling the formation of the OMn bond. The quartet is found 0.5 kcal mol1 more stable than the doublet state. We conclude that the intersystem crossing would be governed by the stabilization of the quartet in front of the doublet state of the L12Ni-O2 moiety when moving along the reaction path, probably before TS1. Its feasibility depends on the spin–orbit coupling in the crossing zone. From TS1, the reaction evolves to the intermediate in either of the two states, with 7.5 and 10.9 kcal mol1 relative energies. The second transition state, TS2, is only 3.3 kcal mol1 above the intermediate (1.0 kcal mol1 on the S = 2 PES) and leads to the products, more stable than the reactants by 7.2 kcal mol1. The global calculated profile, with or without crossing, is compatible with the experimental data.

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Full Paper Regarding the reverse reaction, the calculations provide some indications supporting the fact that the reverse reaction is not observed. The global reverse barrier goes up to 19.9 kcal mol1. This value corresponds to the enthalpy difference between the products and TS1, including the BSSE corrections. The activation enthalpy of the first step, the formation of the intermediate once the TS2 barrier is overpassed, is 18.1 kcal mol1. The direct and reverse activation enthalpies give an estimation of 2.1  104 for the ratio between the direct and the reverse rate constants, kdirect/kreverse, at 223 K. On the other hand, the free energy of the reaction is of 6.1 kcal mol1, which favors the formation of the products, with an equilibrium constant of about K = 106. All these results support the fact that the reverse reaction has not been experimentally observed. A preliminary understanding of the electronic structure of the stationary points on the S = 1 PES is given by the natural orbitals and their occupations, although a detailed analysis would require the use of multiconfigurational methods.[42, 72–74] The most relevant B3LYP natural orbitals for the intermediate are given In Table S5 in the Supporting information, very close to those of the transition states. The occupation numbers of these orbitals are also reported for the three stationary points. The nature and occupation of these orbitals are similar for the three points. The two orbitals involving O2 are shown in Figure 8. They correspond to the bonding and antibonding

Figure 8. Relevant B3LYP natural orbitals in the intermediate.

combination of the O2 px* and Mn 3dxz orbitals, with a negligible contribution of the Ni 3dxz orbital. A third orbital corresponding to the antibonding combination of the 3dz2 metal and O2 pz* orbitals has a slight weight on the O2 molecule. The corresponding bonding combination is doubly occupied and largely localized on the O2 p*z orbital (orbitals 7 and 9, respectively, in Table S5 in the Supporting Information). Then, the electronic structure of the intermediate and the transition states can be described as the result of the single occupation of one O2 p*, two Ni 3d, and five Mn 3d orbitals. The remaining Ni 3d orbitals are doubly occupied. Consequently, the O2 molecule has a marked superoxide nature in the intermediate and the transition states, and the reaction can be explained as the migration of the superoxide (O2 ) unit between the metals along the reaction coordinate. The OO distance is also similar to other complexes proposed as superoxides. As shown in Table 5, the superoxide character is also reflected in the oxygen Mulliken spin populations of the three structures, which range from 0.94 to 1.12, compatible with a migrating O2 species. On the other hand, the local Ni 3d8 and Mn 3d5 Chem. Eur. J. 2014, 20, 13296 – 13304

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Table 5. Mulliken spin populations of Ni, Mn, and O in TS1, I, and TS2, for S = 1.

atom

TS1

O O Ni Mn

0.56 0.56 1.56 4.91

Spin population I 0.55 0.44 1.59 4.86

TS2 0.44 0.50 1.61 4.86

configurations suggest a MII oxidation state for both metal centers in the three considered structures.

Conclusion The study of the O2 transfer reaction between [(12-TMC)NiO2]Cl and [(14-TMC)Mn]Cl2, where chloride substitutes the original perchlorate counterions, has been performed at the B3LYP level including Grimme corrections and solvent effects. The S = 1 and S = 2 PESs have been analyzed and, excepting for the reactants, the former has been found more stable, by 1–3 kcal mol1, whereas the S = 3 PES is higher in energy. In both spin multiplicities, two transition states and one intermediate have been characterized, confirming the two-step reaction mechanism proposed from the kinetic study at low temperature. For S = 1, the relative energy of the transition state connecting the reactants and the intermediate is 13.3 kcal mol1, higher by 2.5 kcal mol1 than the second transition state, leading to the products. ZPE and thermal effects only increase the barrier by 0.6 kcal mol1 giving an activation enthalpy of 13.9 kcal mol1. This result as well as the calculated activation entropy are in excellent agreement with the kinetic study indicating a first rate-determining bimolecular step. The intermediate presents a m-h1:h1 Ni-O2-Mn coordination and can easily evolve to the products because the energy of the second transition state is only 3.3 kcal mol1 higher, with an enthalpy of 12.1 kcal mol1 above the ground state of the reactants. This explains the different behavior of this reaction, where the intermediate is not detected, with other similar reactions, in which the binuclear product can be isolated. The global enthalpy barrier for the reverse reaction, 19.9 kcal mol1, is also in agreement with the unobserved reverse process. The reactants have a quintet ground state resulting from a doublet for the Ni-O2 adduct and a sextet for the Mn complex, but the Ni-O2 quartet is low in energy and so is the reactants S = 1 PES, which is stabilized by the opening of the Ni-OO angle from the original side-on coordination. An intersystem crossing can thus occur before the first transition state and, the reaction can then evolve on the S = 1 PES. However, for S = 2 the TS1 energy is only 2 kcal mol1 higher, the energy profile is almost parallel, and the evolution following the S = 2 PES cannot be excluded. The reaction implies the transfer of a superoxide anion, O2 from the side-on [(12-TMC)Ni-O2] + adduct to give [(14TMC)Mn-O2] + . Both the reactant and the product have been classified in the literature as MIII–peroxides, although their

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Full Paper structural parameters are intermediate between those of MIII– peroxides and the MII–superoxides. Multiconfigurational calculations are in progress to accurately investigate the electronic structure of the relevant species involved in this reaction.

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Received: April 24, 2014 Published online on September 1, 2014

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