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[MoO2]2+-Mediated Oxygen Atom Transfer via an Unusual Lewis Acid Mechanism Marta Castiñeira Reis,† Marta Marín-Luna,† Carlos Silva López,† and Olalla Nieto Faza*,‡ †

Departamento de Quı ́mica Orgánica, Universidade de Vigo, Lagoas-Marcosende, 36310 Vigo, Spain Departamento de Quı ́mica Orgánica, Universidade de Vigo, Campus As Lagoas, 32004 Ourense, Spain

‡

S Supporting Information *

ABSTRACT: Density functional theory is applied to the study of the oxygen atom transfer reaction from sulfoxide (DMSO) to phosphine (PMe3) catalyzed by the [MoO2]2+ active core. In this work, two fundamentally different roles are explored for this dioxometal complex in the first step of the catalytic cycle: as an oxidizing agent and as a Lewis acid. The latter turns out to be the favored pathway for the oxygen atom transfer. This finding may have more general implications for similar reactions catalyzed by the same [MoO2]2+ core.

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INTRODUCTION The intermolecular transfer of one heteroatom is a fundamental reaction both in chemistry and biology, and it has become the subject of a great number of experimental1−7 and theoretical studies.8−10 Many of these studies are focused on oxygen atom transfer (OAT) reactions, in particular on those catalyzed by transition metals which typically act as atom carriers.3,11 In addition, catalytic OAT reactions are utilized for the epoxidation of different organic substrates in the petroleum and plastic industries12−14 and are involved in the operation of many metalloenzymes.15−18 In this regard, molybdenum is found in many types of enzymes capable of transferring an oxygen atom from or to a substrate, referred to as oxotransferases.19 They can be classified into three different families according to the structure of their active center: xanthine oxidase, sulfito oxidase, and dimethyl sulfoxide reductase.19 They have in common the presence of a catalytic core consisting of a mononuclear molybdenum(VI) center with at least one oxo ligand in the prostetic group: MoOS, MoO2, and MoO(OR), respectively. Synthetic models of these enzymes have been designed to mimic their chemical behavior, although those models are mainly focused on complexes in which molybdenum is hexa- or pentacoordinated.3,8,20 Little is known about the role in oxygen atom transfer reactions of tetracoordinated species such as MoO2Cl2, which has mainly been envisaged as a Lewis acid.12,21−23 Nevertheless, interesting and versatile redox chemistry has been reported with this tetracoordinated core by Sanz et al. They developed a thermal chemoselective deoxygenation of sulfoxides to sulfides catalyzed by MoO2Cl2(DMF)2 using triphenylphosphine as the reducing agent in acetonitrile (Scheme 1).24 In order to understand the structural and electronic aspects that turn the [MoO2]2+ core into an efficient redox catalytic center, we have carried out a detailed theoretical study on the © 2017 American Chemical Society

Scheme 1. Molybdenum-Catalyzed Deoxygenation of DMSO with PPh3a

a

Experimentally, 1.1 equiv of the reducing agent (PPh3 or P(OPh)3)) and 2% of the catalyst were considered. CH3CN was used as solvent, and the experiment revealed a quantitative yield under reflux conditions, even though the reaction can evolve at 20 °C.24

molybdenum(VI)-catalyzed OAT reaction from DMSO to PMe3 (we use the latter as a proxy for the larger PPh3, in order to reduce the computational cost). Particular attention has been paid to the spin state of the species involved in the reaction pathways explored and also to the coordination sphere of the metal. These two parameters are found to be crucial in how they modulate the reaction evolution both in terms of the reaction barriers and of the competitive mechanistic pathways.

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EXPERIMENTAL SECTION

The geometries of all the molecules were fully optimized by using the UB3PW91 functional25,26 with the Def2-SVPD basis set.27 The associated effective core potential (ECP) was employed to describe the Mo atom. The Def2-SVPD is a modern and well-balanced basis set that, in terms of size and polarization level, is comparable with the popular triple-ζ quality 6-311+G(d,p) basis set by Pople.28 The effect of the solvent (CH3CN) was taken into account using the Polarizable Continuum Model (PCM)29 with the default parameters implemented in the Gaussian 09 package.30 The Orca program31 was used to obtain Received: June 16, 2017 Published: August 22, 2017 10570

DOI: 10.1021/acs.inorgchem.7b01529 Inorg. Chem. 2017, 56, 10570−10575

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Inorganic Chemistry the minimum energy cross-points between triplet and singlet potential energy surfaces. In these cases, the conductor-like screening model (COSMO)32 with the same solvent was applied in order to take into account the solvation effect. Harmonic analysis was performed to characterize minima and transition states. The wave function stability was confirmed in all stationary points.33 Bond orders, atomic charges, and Wiberg bond indices were calculated with the NBO3.1 program.34 Effective oxidation states have been calculated with the APOST-3D program.35

These computed bond distances match those found in X-ray crystal structures for complexes bearing the MoO(OPR3) fragment and clearly reveal the formation of a phosphine oxide product that maintains only a weak interaction with the metal center.36−38 This process has been previously rationalized as the attack of the phosphorus lone pair onto the π*MoO orbital, with a simultaneous nucleophilic attack of this π bond on the σP−C * orbital.39−42 We, however, could not find any evidence supporting this dual attack mode; the NBO analysis does not show such interactions, and a simple analysis of the bond distances also provides contradicting results. The PC bond distances, for instance, are kept almost constant along the way from reactants to transition state TS1−2 (1.83−1.85 vs 1.85 Å in PMe3), suggesting that the interaction with the σ*P−C orbital is very weak, if there is any. The electron configuration of the Mo(IV) metallic center in 2 (d2) opens the possibility to the coexistence of the singlet and triplet spin states. In fact, the energy difference between these states in 2 is 6.5 kcal/mol, with the triplet more stable. This change in the spin state requires a crossing between the singlet and the triplet potential energy surfaces between TS1−2 and 2. The minimum energy crossing point (MECP) along the seam of both surfaces (CP1−2) was computed. Analysis of some of the most relevant distances in CP1−2, namely, dMo−O = 1.98 Å and dO−P = 1.59 Å, are very similar to those observed in 2, suggesting that this crossing point is located closer to the reaction product 2. The electronic energy is also in agreement with this interpretation (CP1−2 is 13 kcal/mol uphill from 2 and about 24 kcal/mol downhill from the previous transition state TS1−2). The release of OPMe3 in TS2−3 on the triplet energy surface (ΔG⧧ = 20.2 kcal/mol) provides a trigonal planar complex 3 (−5.7 kcal/mol). The formation of a vacant coordination site on the metal complex allows for the coordination of a DMSO molecule leading to 4 via transition state TS3−4, with an energy barrier of 13.3 kcal/mol. In this complex, the Mo−O (2.09 Å) as well as the O−S (1.59 Å) bond distances reveal a very weak coordination of DMSO to the metal center. A Wiberg bond index of 0.07 for the Mo−O(SMe2) bond and the fact that the energy requirements for decomplexation are very low (4.0 kcal/ mol compared to the dissociated state in 3) are in excellent agreement with this description. Finally, the facile release of dimethylsulfide through TS1−4 (ΔG⧧ = 6.2 kcal/mol, dMo−O = 1.89 Å, dO−S = 1.79 Å) allows for the reoxidation of the metal complex and the recovery of the initial Mo(VI) catalyst in an exergonic process (ΔG = −48.8 kcal/mol). A second crossing between the triplet and singlet potential energy surfaces before TS1−4 is observed. The difference in energy between the two spin states in 4 and in TS1−4 is 4.5 and 12.0 kcal/mol, respectively, revealing that the minimum energy crossing point should be close to 4. Analysis of key geometric parameters in CP1−4 also supports this interpretation (dMo−O = 1.96 Å, dO−S = 1.67 Å). A few polyoxomolybdenum compounds have a history of Lewis acid reactivity12,21−23 that is often overlooked when dealing with redox chemistry. When molybdenum is considered as a Lewis acid in the first step of the catalytic cycle, the scope of possible reactions is remarkably broadened, since the metal can be coordinated to any of the Lewis bases participating in the OAT reaction, either PMe3 or DMSO. The two possible sequential coordination alternatives to Mo(VI) should therefore be considered. To the best of our knowledge there are no previous studies in which the Mo atom is regarded as a Lewis

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RESULTS AND DISCUSSION In our first calculations, MoO2Cl2 was selected as the model of the MoO2Cl2(DMF)2 catalyst used in the experiment by Sanz et al., for a number of reasons: On one hand, MoO2Cl2 contains the catalytic active core [MoO2]2+ and is a simple structure that avoids the need to explore both the complex conformational space of the DMF ligands and the possibility of isomers on a trigonal bipyramidal or octahedral structure. On the other hand, taking into account the labile character of dimethylformamide ligands (DMF) and the experimental reaction conditions (80 °C), it is expected that the cleavage of such ligands from the metal atom can proceed easily, leading to the presence of MoO2Cl2 in the reaction medium. A number of mechanisms can be conceived for the molybdenum-catalyzed OAT reaction from DMSO to PMe3. As the molybdenum catalyst can be considered a typical oxidizing agent, we initially explored reaction pathways resembling those proposed for the enzymatic systems (mechanism A, Figure 1).19

Figure 1. Mechanism A: Proposed mechanism for the molybdenumcatalyzed OAT reaction between DMSO and PMe3 considering the reduction of the catalyst as the first step. Computations were performed at the UB3PW91/Def2-SVPD(PCM,CH3CN) theoretical level. Gibbs free energies in kcal/mol (1 atm and 298 K) relative to 1 + PMe3 + DMSO are shown.

Taking into account the reaction pathways described in oxotransferases, the initial step should involve the reduction of the Mo(VI) center (see Figure 1). The association of a PMe3 molecule to the Mo(VI) catalyst 1 via TS1−2 (ΔG⧧ = 24.2 kcal/ mol) leads to the formation of the thermodynamically more stable phosphoryl complex 2 (ΔG = −12.9 kcal/mol) where the metal has been reduced to Mo(IV). Through this step, a PO bond is formed (dP−O = 1.57 Å), and the MoO bond reduces its multiplicity from that of a weak double bond (dMo−O = 1.68 Å and a Wiberg index of 1.41) to significantly less than single bond (dMo−O = 2.04 Å, featuring a Wiberg index of 0.51). 10571

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Inorganic Chemistry acid in the deoxygenation of sulfoxides. Here we will discuss only the most favorable mechanism, which is depicted in Figure 2. The remaining alternative routes explored are compiled in

Finally, the reaction evolves from 6 through a low energy TS6−7 (ΔG⧧ = 2.2 kcal/mol) with the release of dimethylsulfide. This step concludes with the formation of the most thermodynamically stable compound involved in this mechanism, the reoxidized complex 7 (ΔG = −52.3 kcal/mol). The departure of the OPMe3 ligand through TS1−7 (ΔG⧧ = 17.6 kcal/mol) allows for the recovery of the initial catalyst. Although geometry and NBO bond indices strongly suggest that the molybdenum center has been reduced from Mo(VI) to Mo(IV) on 6 (upon association of the phosphine, the Mo−O distance is dramatically increased from 1.7 to 2.1 Å when going from 5 to 6), a mechanism might be proposed for this reaction where the oxygen transfer from the DMSO to the metal would occur in concert with the oxygen transfer from the metal to the phosphine, leaving molybdenum in 6 still as a Mo(VI) or even in an intermediate oxidation state, depending on the nature of the Mo−O bonds. In order to confirm this, the effective oxidation state of molybdenum has been computed for structure 6 (and also 1 and 3 for comparison) using the method developed by Ramos-Córdoba et al.45 which uses the occupation of effective atomic orbitals (calculated with the TFVC method46) to derive the electronic configuration of the fragments within a molecular system. In structures 1 and 3, the calculated oxidation state of the molybdenum center is VI and IV, as expected, with very high reliability indexes (89−90%). In the case of 6, the oxidation state is unambiguously calculated to be IV (reliability index 96%). As such, we can reliably describe mechanism B as a Mo(VI)/Mo(IV) cycle, as we did for mechanism A, with the main difference between them being the order of the catalyst reduction and DMSO coordination events. It is worth noting that transformations 5 → 6 and 6 → 7 could also occur through two consecutive steps involving initially an attack of PMe3 onto the Mo center, leading to an octahedral complex (10 (4.9 kcal/mol), see the SI) which could then undergo a pseudoreductive elimination of OPMe3 and the loss of Me2S on the way to complex 7. This could be considered as a direct transfer of the oxygen from DMSO to phosphine, favored by coordination of both fragments to the metal center (this process is clearly disfavored against the steps in mechanism B, but it lowers the barrier with respect to the noncatalyzed OAT by 18.1 kcal/mol). In this two-step sequence OPMe3 does not fully dissociate, but is maintained in the coordination sphere of the Mo atom via a dative interaction. A different reductive elimination step can also be proposed starting from 10, now involving the oxygen on one of the oxo ligands. The corresponding transition structure (TS10−6) leads to complex 6, reconnecting with the main catalytic cycle (see the SI). These alternative processes that could account for the 5 → 7 transformation are less plausible than the one proposed in Figure 2 since the transition states for the pseudoreductive eliminations lie about 32−34 kcal/mol above the reference, relative free energy values that are considerably higher than those corresponding to any of the other steps involved in the 5 → 6 → 7 path. Another alternative in this molybdenum-as-a-Lewis-acid mechanism (mechanism B) would involve the reverse sequence for the addition of the Lewis bases to the MoO2Cl2 core (first PMe3 onto the Mo center to provide 9, and then DMSO to provide 10). However, as we have already discussed, the barriers involved in the evolution of 10 are considerably higher in energy than those in the other mechanisms, rendering this alternative pathway uncompetitive (see SI). The pentacoordi-

Figure 2. Mechanism B: Proposed mechanism for the molybdenumcatalyzed OAT reaction between DMSO and PMe3 when Mo acts as Lewis acid, calculated at the UB3PW91/Def2-SVPD(PCM,CH3CN) level. Gibbs free energies in kcal/mol (1 atm and 298 K) relative to 1 + PMe3 + DMSO are shown.

the Supporting Information. It is worth noting that we have optimized most of the structures depicted in those figures both on the singlet and triplet potential energy surfaces. When the latter is not possible, a single point calculation of the triplet electronic configuration on the singlet geometry has been carried out, to confirm the most favorable spin state along the mechanism. When Mo acts as a Lewis acid, the most favorable catalytic cycle starts with the coordination of a DMSO molecule to the metal center, leading to the distorted trigonal bipyramidal complex 5 (ΔG = 0.3 kcal/mol). This inner-sphere process arranges the chlorine atoms trans to each other and the oxygen atoms in the equatorial plane. This isomer was the only one considered due to its higher stability according to previous reports.43 The distance between the metal and the oxygen atom from the DMSO is 2.12 Å, suggesting again a weak interaction between these two fragments. A slight elongation of the S−O bond (1.58 vs 1.52 Å in the free DMSO) is also in agreement with this interpretation. The energy barrier of the 1 → 5 conversion is only 12.0 kcal/mol. At this point, the attack of a PMe3 molecule to one of the oxo ligands promotes the reduction of the Mo center and yields complex 6 (ΔG = −12.5 kcal/mol). As a consequence of this reduction, the interaction between the DMSO fragment and the metal is strengthened, which can be observed both in terms of geometric parameters (dMo−O decreases in 6 with respect to that in complex 5 from 2.12 to 2.06 Å) and also from the perspective of Wiberg bond indices (0.51 in 5 to 0.59 in 6). Additionally, in complex 6 the DMSO and OPMe3 fragments form an angle of 147°, suggesting that a trans effect due to back-donation may be established between the metal and the phosphorus or the sulfur atoms. All of these effects acting in a collaborative fashion should help weaken the S−O bond and thus may prepare the complex for the cleavage of dimethylsulfide.44 10572

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Inorganic Chemistry

experimental reaction conditions (80 °C).24 According to a Maxwell−Boltzmann distribution, more than 99% of the reacting molecules should follow the Lewis acid route under these thermal conditions. Additionally, the Lewis acid mechanism (path B) operates not only with the advantage of lower energy requirements but also with avoidance of the surface hopping between the singlet and triplet surfaces on mechanism A, which could further disfavor the latter route. The behavior of the metal in cycles A and B is clearly different, both in terms of the spin state and of the mechanism. This seems to depend on the oxidation state and on the geometry of the ligands around the metal center. Thus, Mo(IV) compounds in a tetrahedral arrangement seem to favor the triplet state whereas structures with higher coordination numbers (trigonal bipyramidal or octahedral geometries) show a decreasing preference for it.

nated intermediate 9, resulting from the incorporation of PMe3 to the coordination sphere of the MoO2Cl2 core, could also evolve to the formation of the trimethylphosphine oxide complex 2, providing an alternative to TS1−2 in the innersphere path, but again, the relative free energy of the corresponding transition structure (29.1 kcal/mol) makes this path uncompetitive. Up until this point, we have considered MoO2Cl2 to be the catalytically active species in these cycles. It is well-known, however, that dioxomolybdenum compounds are usually stable as hexacoordinated species, incorporating solvent molecules (such as DMF or DMSO itself) in their coordination sphere. As a result, we have explored the possibility of MoO2Cl2(DMSO)2 as a catalyst. This choice of ligand, instead of the straightforward use of dimethylformamide (used as solvent and also present as a ligand in the molybdenum catalyst used in the experimental work, MoO2Cl2(DMF)2), is convenient in that it connects all the cycles already studied. MoO2Cl2(DMSO) as a catalyst has already been described as a part of the Lewis acid mechanism, for example, and DMSO behaves similarly to DMF in Mo(VI)-catalyzed redox chemistry.47,48 The initial coordination of a DMSO molecule to the MoO2Cl2 core results in the formation of a trigonal bipyramid (5). An additional coordination site is available (see Figure 3),

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CONCLUSIONS We have performed a thorough computational study of the oxygen transfer reaction between DMSO and a phosphine, catalyzed by a dioxomolybdenum(VI) complex. Two main paths that we have named mechanism A and mechanism B have been characterized, together with alternatives to key steps, and the equivalent transformations on a catalyst with a different number of ligands on its coordination sphere. In nature, molybdoenzymes are capable of catalyzing OATs through a very straightforward process in which the metal exerts the role of an oxidizing agent. Accordingly, a mechanistic pathway (mechanism A) in which the MoO2Cl2 core initiates a catalytic cycle by oxidation of a phosphine through a direct oxygen transfer from the catalyst to the phosphorus atom is the most common proposal to explain this chemistry. As a consequence of this atom transfer the metal changes its oxidation state from Mo(VI) to Mo(IV). Then, this species is reoxidized to Mo(VI) in the presence of a substrate susceptible of acting as co-oxidant, such as DMSO. These changes in oxidation state of the metal are accompanied by changes in the spin state of the system (from 1S to 3T and back to 1S) that have been characterized through the computation of the minimum energy crossing points along the seams between the two surfaces. While molybdenum can exhibit a purely redox behavior, it can also behave as a Lewis acid. When exploring this alternative, we have found not only an energetically more feasible mechanism, but also a mechanism in which there are no spin crossing events along the reaction pathway for the catalytic cycle. In this alternative path the metal behaves in a first step as a Lewis acid coordinating a DMSO molecule, and it later undergoes a reduction from Mo(VI) to Mo(IV) through the transfer of an oxygen atom to the phosphine. The subsequent rearrangement of this structure concludes with the release of a dimethylsulfide molecule and the concomitant reoxidation of the metal center from Mo(IV) to Mo(VI). The energy difference that separates the rate-determining steps of the mechanisms presented for this OAT strongly suggests than only the Lewis acid route (mechanism B) should be operating under the experimental conditions.

Figure 3. Equilibria involved in the coordination of DMSO to the MoO2Cl2 catalyst. Relative energies for the minima, and activation barriers for the transition structures, computed at the UB3PW91/ Def2-SVPD(PCM,CH3CN) level are provided in kcal/mol.

so a second ligand could be incorporated to the metal coordination sphere to complete an octahedral geometry. We have found that, as expected, the coordination of a second DMSO molecule results in an equilibrium process (the barriers for the ligand association/dissociation processes are quite affordable) in which the noncoordinated tetrahedral system is almost degenerate with the trigonal bipyramid and the second ligand addition is disfavored by 2.3 kcal/mol. Despite that, we also modeled the reduction of the octahedral Mo(VI) system 8 with PMe3, and we found that it is energetically more costly (ΔG⧧ = 24.4 kcal/mol) than the reduction of 5 (see the SI for a detailed description of this mechanism). This 5 kcal/mol energy difference in terms of the activation barrier for the reduction step supports that MoO2Cl2 is the most reactive catalyst. Our computations reveal that the reduction step of the molybdenum complex with PMe3 is the rate-determining step in both mechanisms (as shown in Figures 1 and 2). The high energetic cost of this step could be related to the partial breakage of a double MoO bond to form a single Mo OPMe3 bond. The energy difference computed for these key steps is 4.8 kcal/mol, favoring the one associated with mechanism B, involving MoO2Cl2 acting as a Lewis acid (Figure 2). This suggests that the classical redox catalytic cycle (mechanism A) should not be competitive under the

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.7b01529. 10573

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Complete schemes with all the alternative paths studied (with the corresponding relative free energies); a small benchmark of functionals with different amounts of exact exchange, tables with geometric and NBO and QTAIM bonding parameters; and Cartesian coordinates, electronic energies, and the number of imaginary frequencies for all the structures reported in this work (PDF)

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone: +34 988368888. ORCID

Carlos Silva López: 0000-0003-4955-9844 Olalla Nieto Faza: 0000-0001-8754-1341 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors would like to thank Prof. Pedro Salvador for his help in calculating the effective oxidation numbers of 1, 3, and 6. The authors thank the Centro de Supercomputación de Galicia (CESGA) for the allocation of computational resources. M.C.R. is thankful to the Ministerio de Ministerio de Educación Cultura y Deporte of the Spain Goverment for her FPU fellowship. M.M.-L. thanks the Xunta de Galicia for a postdoctoral research contract (ED481B 2016/166-0). This work has been funded by the Ministerio de Economia,́ Industria y Competitividad (Grant CTQ2016-75023-C2-2P), and the Xunta de Galicia (Grant EM2014/040).

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DOI: 10.1021/acs.inorgchem.7b01529 Inorg. Chem. 2017, 56, 10570−10575

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DOI: 10.1021/acs.inorgchem.7b01529 Inorg. Chem. 2017, 56, 10570−10575