DFT Study on the Interaction of Tris(benzene-1,2-dithiolato

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DFT Study on the Interaction of Tris(benzene-1,2-dithiolato) Molybdenum Complex with Water. A Hydrolysis Mechanism Involving a Feasible Seven-Coordinate Aquo-Molybdenum Intermediate Lorenzo Fernandez, Francisco Fernando Perez-Pla, Iñaki Tuñón, and Elisa Llopis J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/acs.jpca.6b10233 • Publication Date (Web): 15 Nov 2016 Downloaded from http://pubs.acs.org on November 19, 2016

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The Journal of Physical Chemistry

DFT Study on the Interaction of Tris(Benzene-1,2-Dithiolato) Molybdenum Complex with Water. A Hydrolysis Mechanism Involving a Feasible Seven-Coordinate Aquo-Molybdenum Intermediate. Lorenzo Fernández,∗,† Francisco F. Pérez-Pla,† Iñaki Tuñoń,‡ and Elisa Llopis† †Institut de C´ıencia dels Materials (ICMUV), c/ Catedr´ atico Beltrán 2, 46980, Valencia, Spain ‡ Departamento de Qu´ımica F´ısica, Universitat de València, Dr. Moliner 50, 46100, Burjassot, Valencia, Spain E-mail: [email protected]

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Abstract In the present work, the reactivity of the tris(benzene-1,2-dithiolato) molybdenum complex ([Mo(bdt)3 ]) toward water is studied by means of the Density Functional Theory (DFT). DFT calculations were performed using the M06, B3P86 and B3PW91 hybrid functionals for comparison purposes. The M06 method was employed to elucidate the reaction pathway, relative stability of the intermediate products, nature of the Mo-S bond cleavage and electronic structure of the involved molybdenum species. This functional was also used to study the transference of electrons from the molybdenum center toward the ligands. The reaction pathway confirms that [Mo(bdt)3 ] undergoes hydrolysis, yielding dihydroxo-bis(benzene-1,2-dithiolato) molybdenum complex ([Mo(OH)2 (bdt)2 ]) and benzenedithiol. The reaction takes place through seven transition structures, one of them involving an aquo seven-coordinate molybdenum intermediate stabilized by a lone pair (LP) LPO →LPMo hyperconjugative interaction. This hepta-coordinate species allows understanding the observed oxygen atom exchange between water and tertiary phosphines mediated by these complexes. Calculations also show that [Mo(C2 H4 S2 )3 ] and [Mo(OH)2 (C2 H4 S2 )2 ] have d2 and d0 electronic configuration, and hence an electron pair must be transferred during the course of the hydrolysis. The Frontier Molecular Orbital (FMO) analysis concludes that the electron pair is transferred in the rupture of the second Mo-S bond, from the occupied donating Mo dx2 − y2 orbital to the unoccupied C2 H4 (SH)2 S-C σ ∗ ligand-orbital. This result is supported by the bond dissociation energy calculations, which demonstrate that the neutral dissociation of the second Mo-S bond is energetically the more favorable.

Introduction The chemistry of benzene-1,2-dithiolato complexes of transition metals has been under study long before it was suspected that their related pyranopterin-dithiolene metal complexes were present in molybdenum and tungsten enzymes. Most of these enzymes mediate the formal oxygen atom exchange between various substrates and water 1 via a mechanism involving at 2


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least the molybdenum atom in their oxidation states IV and VI. 2 The study of the electronic structure of active sites of molybdoenzymes is a crucial issue in understanding the oxo-transfer activity of these catalysts. Although the detailed study of the catalytic activity in vivo of enzymes is a complicated task, progress made in activesite modelization by quantum-chemical methods is allowing a deeper knowledge of these systems. 3–6 Very often, simulations are based on synthesized model-dithiolene complexes, of which the catalytic activity is known. 7–15 This approach has the advantage of correlating theoretical and experimental data that can be used to explain enzyme activity. Although, nearly all important aspects of metal-dithiolato chemistry have been covered, 16 research into their electronic structure , redox behavior, reactivity toward water and the nature of S-Mo bonds remain matters of special importance to explain the mechanisms of the oxo-transfer activity of these complexes. The progress made in detailing the electronic structure of metal dithiolates has been a vehicle to understanding their unusual properties. Dithiolene ligands often are referred to as non-innocent when coordinated to transition metals. 17–19 The origin of such “noninnocent” behavior, which masks the oxidation state of the metal, is theoretically explained by the charge transfer towards the metal when the ligand is mainly described by its ene-1,2dithiolate dianionic resonance form. 19,20 However, this behavior is not general. In particular, Mo tris(dithiolato) complexes undergo ligand-based oxidation, behaving the dithiolene ligands in a non-innocent manner with the molybdenum atom. In contrast, oxo bis(dithiolene) molybdenum complexes usually exhibit definite oxidation states, behaving the ligands innocently in these complexes. 11 The above mentioned electronic features are related to flexible redox behavior of the highly covalent metal-dithiolato unit. The oxidation of dithiolate via a radical anion was proposed to facilitate the intramolecular metal-ligand redox processes 21 that contribute to the multiple redox states of their metal complexes. 22 Initial developments in this field, 17,23–25 led to the recognition that many complexes with the same metal and ligand may be interrelated

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by facile one-electron oxidation-reduction reactions during which coordination presumably remains intact. Let us turn our attention to the chemistry of tris-dithiolato complexes. In contrast to their well-studied redox behavior, bond-making and bond-breaking reactivity of trisdithiolato complexes has been paid less attention. 26 Perhaps the most amazing aspect of tris-dithiolato chemistry is the reactivity toward water which is not expected to be relevant due to the stability of the Mo-S bond and the electronic structure of tris-dithiolato complexes. However, oxygen exchange between water and phosphines or sulfite mediated by [Mo(C6 H4 S2 )3 ] has been reported. 27,28 The mechanism is supposed to involve initial coordination of water to the molybdenum-dithiolato species. 27 The oxo-transfer activity of neutral tris-dithiolato complexes is difficult to understand based only on the d1 , 21 or more probably d2 , 13 electronic structure of metal center, which would make the postulated initial formation of an hepta-coordinated aquo tris-dithiolato intermediate an unlikely event. 21 However, such a mechanism is relevant from a technical perspective, vg. the water photodecomposition, 29 and, therefore, an understanding of the interactions between substrates and metal-thiolato units in the presence of water gives us the keys for the proposed oxo-transfer mechanism by these catalysts. Clearly, delocalized orbitals in these complexes that involve the metal and multiple sulfur centers must be considered as strong candidates for the binding and activation of water, and/or the stabilization of key intermediates. Other striking aspect of the aqueous tris-dithiolato complex chemistry is the so-called solvent effect exhibited by all [M(C6 H4 S2 )3 ] complexes. 30–32 This effect denotes the drastic change in color due to reduction of neutral complexes to their mono-anionic or even dianionic forms, when they are extracted or dissolved in weakly basic solvents such as acetone, alcohols, dimethylformamide or dimethylsulfoxide. The solvent effect is even more difficult to explain because water or hydroxide ions, which are stable oxidized species, should act as reductors. A feasible explanation of this phenomenon is that dithiolato ligands constitute the electron source for the reduction process when they are activated by water.

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These observations furnish the starting point for examination of the elementary reactions of water/molybdenum tris-dithiolato complexes, and here we report a complete set of theoretical results including full mechanistic details of interaction with water.

Calculations DFT studies were performed using the Gaussian 09 package. 33 Geometry of reactants, products and transition structures was fully optimized at DFT level without geometrical restrictions using the M06, 34 B3PW91 35–37 and B3P86 38 hybrid DFT functionals. Geometry optimizations were carried out with the LanL2DZ basis set with an effective core potential (ECP) for molybdenum, 39–41 whereas the standard 6-311G(d) basis set was used for carbon, sulfur, oxygen and hydrogen. The LanL2DZ basis has been used successfully to study other dithiolene molybdenum complexes, resulting in a good agreement between the theoretical and experimental reported electronic structures. 12,13,42 Additional single point calculations were performed using the Def2-TZVP triple zeta basis set with ECP for molybdenum, 43 including f functions, with the aim of checking the influence of the basis size on the energy of reaction intermediates and the electronic structure of Mo(bdt)3 . Harmonic vibrational frequencies were computed to verify the nature of stationary points and obtain the zeropoint energy. Furthermore, each transition structrure was connected to its corresponding minimum on the potential energy surface (PES) by calculating the minimum energy path through the intrinsic reaction coordinate (IRC). 44,45 The geometry of stationary points found at the IRC pathway were optimized to obtain the corresponding structures of reactants and products. Bond energies were calculated as differences between the complexes and the separated constituents using the LanL2DZ basis set. In this context, a study of hyperconjugative interactions has been conducted. Hyperconjugation may be considered as a stabilizing effect arising from the overlap between an occupied orbital with a neighboring properly-oriented

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electron-deficient orbital. This non-covalent bonding-antibonding interaction is quantitatively described in terms of the Natural Bond Orbital approach (NBO). 46 NBO analysis of dithiolene-Mo complexes in combination with DFT calculations have been previously used to asses the relative importance of resonance structures. 19 This study satisfactorily predict their geometries and NMR chemical shifts of the complexes. NBO analysis has been also employed to characterize the electronic nature of dithiolene-Mo complexes and other metal dithiolato systems. 47–52 The electronic structure of molybdenum complexes was established from DFT-M06 calculations with the basis sets previously referenced, molecular orbital (MO) compositions were analyzed using the population analysis 53 as implemented in Chemissian 54 and geometries, MOs and vibrational modes were visualized using the Molden freeware. 55

Results and Discussion Reaction Pathway This part of the study describes the reaction mechanism of [Mo(bdt)3 ] with water. Several density functional methodologies have been applied, in particular M06, B3PW91 and B3P86. The M06 functional takes advantage for organometallics, catalysis and non-covalent bonds. 34,56–60 In fact, DFT methodologies seem to work adequately for transition metals, 12,13,42,61 including molybdenum. 12,13,42,62–65 Figure 1 shows the reaction pathway calculated at M06/LanL2DZ level including the optimized geometries of reactants, products and transition structures. This pathway is always found regardless of the DFT functional used, although the use of bigger basis sets (e.g. Def2TZVP 43 ) tends to lower the energy barriers of the various calculated transition structures. Two molecules of water were considered during calculations based on various hydrolysis DFT-studies, 66,67 which demonstrate that the activation energy diminishes with increasing the water molecules involved. These studies suggest that the minimum number of water 6


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molecules already causing facile proton transfer is equal to two. The energy of reactants, products and transition structures are given in Table 1 for the M06 functional and in tables S1 and S2 in the supporting information for B3P86 and B3PW91 methods. The hydrolysis of [Mo(bdt)3 ] leads to the formation of a dihydroxo-bis(benzene-1,2dithiolato) molybdenum complex ([Mo(OH)2 (bdt)2 ]) and benzenedithiol through a mono oxo bis(benzene-1,2-dithiolato) molybdenum intermediate ([MoO(bdt)2 ]), as shown in Figure 1. Similar species are described in the literature. In particular, Boyde et al. 68 characterized two [MoO(bdt)2 ]n – anions by X-Ray crystallography with formal Mo(IV) and Mo(V) oxidation states and Tenderholt et al. 12 characterized the anionic [MoO(bdt)2 ] – , using sulfur K-edge X-ray absorption spectroscopy (XAS) and DFT calculations. The pathway shown in Figure 1 displays seven transition structures (TS1-TS7). The hydrolysis starts after a nucleophilic attack of water on the Mo atom of 1 (TS1). The metallic center expands its valence forming a seven-coordinate species 2, that is formulated as an aquo-trisdithiolato complex ([Mo(S2 C6 H4 )3 (H2 O)]) in the literature. 27 This expansion is not particularly surprising and is reported by several authors. 11,69–71 The second step consists of a proton transfer from the bonded H2 O to a sulfur atom (TS2) forming the structure 3. Thence, the rupture of the first Mo-SH bond (TS3) occurs at the protonated benzenedithiolato ligand, forming the species 4. In the third step, a proton is transferred from the axial OH bonded to molybdenum of structure 4 to the second water molecule, leading to the formation of a H3 O+ . Simultaneously, the H+ of H3 O+ species is transferred to the bonded sulfur of the benzenedithiolato ligand, forming species 5 through a concerted transition structure (TS4). In the next step (TS5), the second Mo-SH bond from the former benzenedithiolato ligand breaks, forming mono oxo-bis(benzene-1,2-dithiolato) molybdenum complex 6 ([MoO(bdt)2 ]), benzenedithiol and a molecule of water. A second nucleophilic attack of water (TS6) on the molybdenum atom then occurs, in which the metal coordinates a water molecule forming structure 7 and expanding its coordination sphere. Finally, the last step is followed by the intramolecular proton transfer from the aquo ligand towards the

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LanL2DZ Def2TZVP

LanL2DZ Def2TZVP

Basis set LanL2DZ Def2TZVP

1 −3302.00753(0) −3302.73570(0) TS3 −3301.97039(23.30) −3302.70466(19.48) 6 −3301.97267(21.87) −3302.71472(13.17)

TS1 −3301.99197(9.76) −3302.72130(9.04) 4 −3301.97844(18.25) −3302.71346(13.96) TS6 −3301.95875(30.61) −3302.69784(23.76)

2 −3302.00317(2.74) −3302.72792(4.89) TS4 −3301.95882(30.56) −3302.69841(23.40) 7 −3301.95884(30.55) −3302.69824(23.51)

TS2 −3301.96101(29.19) −3302.69279(26.93) 5 −3301.96746(25.14) −3302.70892(16.80) TS7 −3301.9302(48.49) −3302.66720(42.98)

3 −3301.97371(21.22) −3302.70753(17.68) TS5 −3301.96509(26.63) −3302.70670(18.20) 8 −3301.94808(37.30) −3302.68627(31.03)

Table 1: Zero-point Corrected M06 Energies(in Hartrees) and their Differences (in kcal/mol, Parentheses) Relative to Reactants for the Hydrolysis of [Mo(bdt)3 ] Calculated Using the LanL2DZ and Def2TZVP Basis Sets.

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hydrolysis through a neutral bond rupture.

NBO Study on the Stability of Reaction Intermediates Natural bond orbital (NBO) studies were performed for species 2-6 in Figure 1 to provide a deeper insight into the role of electronic delocalization in the hydrolysis pathway. The relevant NBO interactions are collected in Table SP5 in the supporting information. NBO calculations on structure 2 indicate that interaction between the lone pair orbitals of the oxygen aquo-ligand and the vicinal molybdenum atom (LPO →LPMo ) is about 101.60 kcal/mol, i.e., relatively strong. The hyperconjugative interaction related to the σ ∗Mo−S vicinal anti-bonding orbital with the molybdenum lone pair orbital (σ ∗Mo−S →LPMo ) is more intense with an energy of 283.72 kcal/mol, and also exists identical interactions between the two vicinal bdt-ligand σ ∗C−C anti-bonding orbitals (σ ∗C−C → σ ∗C−C ), the NBO delocalization energies of which are 106.45 and 102.16 kcal/mol. These results clearly state that the relative intense LPO →LPMo hyperconjugative interaction plays an important role in stabilizing the species 2 , enabling the formation of a seven-coordinate molybdenum compound from which the hydrolysis starts. A similar interaction is reported in the base-catalyzed hydrolysis of tetraethylorthosilicate, in which, after the attack of an hydroxide anion on the silicon atom, a stable valence-expanded five-coordinated silicon intermediate is formed. 67

The Electronic Structure of Molybdenum Complexes The calculation results presented in the current section concern the electronic structure and oxidation state of [Mo(bdt)3 ], [MoO(bdt)2 ] and [Mo(OH)2 (bdt)2 ] complexes. Nonetheless, the valence orbitals of the ligands are described briefly before discussing such issues. A benzenedithiolato (bdt) ligand has four filled valence MOs derived from four sulfur-p orbitals. The two sulfur-p orbitals out of the S−C−C−S plane form a symmetric (+) and antisymmetric (-) combination denoted by π + and π − , which have bonding and anti-bonding interactions with the C−C double bond. The two in-plane sulfur-p orbitals, nearly perpendicular to the 12


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the Mo-exz and eyz orbitals have σ interactions with the three bdt orbitals composed of the bonding combination of three σ − bdt orbitals. Table 2: Molecular Orbital Composition of [Mo(bdt)3 ] Near the HOMO-LUMO gap.† MO/Mo(bdt)3 e2 (xz, yz) e1 (x2 − y 2 , xy) Π+ AB z2

Mo d 38(33) 50(48) 27(26) 52(52)

Mo s 0(0) 0(0) 1(0) 0(0)

Mo p 0(0) 2(2) 0(0) 0(0)

Atomic orbital charcter in % Sp S s/d 31(28) 2(2) 41(43) 1(1) 47(47) 0(1) 16(17) 6(7)

C/H 29(37) 6(6) 25(26) 26(24)

† Calculations Performed at M06 Level. In parenthesis are given pure functional BP86 results on the M06 optimized geometry.

Table 2 shows the composition of the highest occupied MO of [Mo(bdt)3 ] complex. The molybdenum dz2 is the lowest in energy MO, having 52% Mo-d, 16% sulfur-p and 26% C/H character. The ΠAB+ orbital is empty, whereas the Mo-dz2 orbital is occupied (52% Mo d, 16% S p, and 26% C/H character). Therefore, the Mo center is represented by a d2 electronic configuration with two holes (h) on the tris(dithiolene) framework (d2 [L3 ]2h ), which implies that [Mo(bdt)3 ] complexes are better described as Mo(IV) than Mo(VI) species. The electron configuration d2 [L3 ]2h concurs with that obtained by Tenderholt et al. in previous BP86 calculations carried out using the 6-311G(d) and LanL2DZ basis sets, and experimentally corroborated by S K-edge XAS spectroscopy. 42 For comparison purposes, MOs were re-calculated with the BP86 functional employing the M06 optimized geometries, because many electronic configurations of dithiolato transition-metal complexes have been successfully elucidated using the aforementioned methodology. 12,13,42,73–76 The results shown in Table 2 (in parenthesis) confirm that MO compositions at M06 and BP86 levels are very similar. Finally, an additional single point calculation (SP) has been performed in combination with the M06 functional and the Def2-TZVP basis set to check the basis-size influence on energy and composition of MOs. In general, Def2-TZVP MO energies are close to those obtained with the LanL2DZ. For instance, the HOMO (Mo dz2 ) energies of Mo(bdt)3 calculated 14


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at M06/LanL2DZ and M06/Def2-TZVP levels are −5.686 and −5.605 5 eV respectively, i.e. the use of the Def2-TZVP basis introduces a difference of 0.08 eV only. In connection with the MO composition, the metallic character of the HOMO is slightly reduced with the use of Def2-TZVP, as shown in Table 3. However,the MO-metallic character do not only rely on the basis size, as it is proven by the MO composition collected in Table 3. Table 3: Comparison of the Mo dz2 Metallic Character (in %) for Mo(bdt)3 Calculated Using the M06/B3P86 DFT Functionals and LanL2DZ/Def2-TZVP Basis Sets. M06/LanL2DZ 52

M06/Def2-TZVP (SP) 42

B3P86/LanL2DZ 48

B3PW91/LanL2DZ 41

BP86/LanL2DZ 54

The second analyzed compound was the mono oxo-bis(benzene-1,2-dithiolato) molybdenum complex. Only M06 calculations were performed this time, since the BP86/M06 comparative study on the MO composition demonstrated that M06 and BP86 methods give close results. The MO composition is shown in Table 4. Fig.2 (middle) displays the six calculated MOs in increasing energy order, namely, Π+∗ AB < dx2 −y2 < dyz < dxz < dxy < dz2 , where the symbol Π+∗ AB denotes a new ligand-based MO composed of the inter-ligand antibonding combination of two π + orbitals (bdt2 ) and one oxygen-pz orbital. The Mo-dz2 orbital has σ interactions with ligands (Obdt2 ), which are composed of the bonding combination of the two π + bdt2 and one oxygen-pz orbitals. The Mo-dxy has σ interactions with bdt2 , composed of the bonding combination of the two σ − bdt2 orbitals. The Mo-dxz has π interactions with ligands (Obdt2 ), composed of the bonding combination of the two σ − bdt2 and one oxygen-px orbital. The Mo-dyz has π interactions with ligands (Obdt2 ), composed of the bonding combination of the one σ − and one π − bdt2 and one oxygen-py orbital. Finally, the Mo-dx2 −y2 have π interactions with ligands (Obdt2 ), which are composed of the bonding combination of the two π + bdt2 and one oxygen-px orbital. MO composition of Π+∗ AB and d-type MOs shown in Table 4 suggests that the complex is a molybdenum (VI) species, having d0 electronic configuration with no holes on the Obdt2 15



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Table 4: Molecular Orbital Composition of [MoO(bdt)2 ] and [Mo(OH)2 (bdt)2 ]z Near the HOMO-LUMO gap.† Atomic orbital character in % Mo s Mo p S p S s/d O p O s/d

[MoO(bdt)2 ]

Mo d

C/H

z2 xy xz yz x2 − y 2 Π+∗ AB

42 38 57 55 39 6

1 0 0 0 0 0

12 0 0 0 0 1

18 34 20 16 37 38

2 2 1 1 1 0

19 0 20 21 1 5

1 0 0 0 0 0

5 26 2 7 22 50

[Mo(OH)2 (bdt)2 ] yz xy xz z2 x2 − y 2 Π+∗ AB

43 42 56 48 44 4

0 0 0 0 1 0

6 0 1 3 0 3

21 31 23 29 36 30

1 2 1 1 0 1

11 1 14 9 5 8

0 0 0 0 0 0

18 22 5 10 13 54

[Mo(OH)2 (bdt)2 ] – x2 − y 2 Π+∗ AB

74 5

0 2

0 2

6 56

2 2

8 15

1 0

9 18

[Mo(OH)2 (bdt)2 ]2 – x2 − y 2 Π+∗ AB

82 6

1 1

0 2

3 62

3 2

3 17

0 0

8 10

† Calculations performed at M06 level; z = 0, −1, −2

framework (Π+∗ AB ). The molecular orbital energy diagram of the [Mo(OH)2 (bdt)2 ] product is displayed in Fig. 2 (right), and the MO composition is shown in Table 4. The relative sequence of energy is this time: Π+∗ AB < dx2 −y2 < dz2 < dxz < dxy < dyz . This ligand-based MO is composed of the inter-ligand anti-bonding combination of two π + orbitals (bdt2 ) and two oxygen-pz orbitals of each of hydroxo-groups. The electronic configuration is d0 , as with the [MoO(bdt)2 ] intermediate.

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FMO Study on the Molybdenum-Ligand Electronic Transfer The electronic configuration of [Mo(bdt)3 ] ( d2 [L3 ]2h ) and [Mo(OH)2 (bdt)2 ] (d0 [(OH)2 L2 ]0h ) suggests that a pair of electrons must be redistributed between the Mo center and the leaving ligand in the course of hydrolysis. For this reason, the frontier MOs (FMOs) methodology 77,78 was applied to the transition structures related to Mo-S bond ruptures, in order to gain additional insight into the hydrolysis mechanism. FMO theory is focused on the highest occupied (HOMO) and lowest unoccupied (LUMO) molecular orbitals. The contribution percentage of the occupied donating FMO (HOMO) and the unoccupied accepting FMO (LUMO), estimated from the Mulliken population analysis, allow for the calculation of the changes in their electronic densities along the reaction coordinate. The HOMO and LUMO of each fragment nearly always dominate the bonding features, because strong interactions between orbitals require the orbital overlap to be large, but also the energy separation to be small. In this context, FMO theory has been successfully applied to the analogous oxygen exchange between dimethylsulfoxide (DMSO) and a Mo(IV) bis-dithiolene, in which the oxygen of DMSO is transferred to the complex, and simultaneously an electron pair is transferred from the Mo-dz2 orbital to the sulfur-p orbital of DMSO, forming dimethylsulfide. 13 Calculation results concerning the analysis of the FMOs around the transition structrues involved in the breaking of the two Mo-S dithiolato bonds are given in figures 3 and 4. The first change in the course of hydrolysis appears near TS3, where the rupture of the first Mo-S bond occurs, but there is still no separation of fragments. The second point of interest appears afterward TS5, where transformation into [MoO(bdt)2 ] begins. The occupied donating FMOs experience a drop in their Mo d character from 50% to 41% in moving from conformation 3 to 4 through TS3, which is correlated with a decrease in the C6 H4 S(SH)(Hbdt) character from 18% to 11%. Concerning the unoccupied accepting FMOs, the changes are notable in the Hbdt character in the S1-C1 σ ∗ orbital, in which a large reduction occurs, beginning at 88% and ending at 40%, when moving from 3 to 4 conformations. However, the biggest drop takes place after TS3, as assess the study of 17




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through four structures describing the MO composition of FMOs. The first is the reactant complex 5 just before the cleavage of the second Mo-S bond occurs (TS5). The structure with a Mo-S distance equal to 3.64 ˚ A, found between TS5 and 6, shows almost equal contributions to the FMOs from molybdenum and benzenedithiol based-orbitals. The intermediate structure along the Mo-S transfer coordinate was assigned to a “halfway” point, 13 which was derived systematically increasing the Mo-S distance for the optimized geometry of TS5 ( 3.08˚ A) to 4.26˚ A in the final product. The structure 5 in Fig. 4 has a Mo dx2 −y2 orbital identical to that of the [MoO(bdt)2 ] in Fig.2. The occupied donating FMO has 11% H2 bdt character and 43% Mo-d character, mainly from the Mo dx2 −y2 orbital, while the accepting FMO has 15% Mo-d and 60% H2 bdt character, the last contribution coming primarily from the S2-C2 σ ∗ orbital. As with the transition structrure TS5, the Mo-d character has slightly decreased to 42% in the donating FMO, which correlates to the drop to 11% in the H2 bdt contribution. On the other hand, the accepting FMO increases the Mo-d character to 29%, whilst the H2 bdt character decreases to 7%. As with the rupture of the first Mo-S bond, a large decrease in the contribution of H2 bdt from 60% to 7% occurs for the accepting FMO, when moving from conformation 5 to TS5. Again, one of the bdt ligands has greatly increased its contribution with regard to the others fragments, which rises up from 13% to 40%. In structure 5, the sum of the contributions from the ligands (except H2 bdt) is only 23%, whereas it is 60% in TS5. The analysis of the predominant ligand indicates the sulfur-p and carbon-p characters are 7% and 27% respectively, while the less-dominant bdt ligand exhibits only 8% and 1% of sulfur-p and carbon-p character. It is worth noting that the energy of the unoccupied S2-C2 σ ∗ orbital simultaneously falls with the elongation of the Mo-S bond, enhancing FMOs overlap. Once the “halfway” point is reached at 3.64 ˚ A, the donating and accepting FMOs have 4% and and 2% of H2 bdt and 39% and 40% of Mo-d characters. Finally, the donating FMO has 1% H2 bdt and 38% Mo d characters, while the accepting FMO has 41% Mo d and 0% H2 bdt

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character for the structure 6, in which the bond rupture was accomplished at 4.26˚ A. These results suggest that a substantial part of the electron density around TS5 is transferred from the Mo dx2 −y2 to the S2-C2 σ ∗ orbital. In addition, the elongation of the Mo-S bond stabilizes the S2-C2 σ ∗ orbital, enhancing the overlap between the Mo dx2 −y2 and S2C2 σ ∗ orbitals, which causes the electron pair transfer from molybdenum to sulfur. In other words, the Mo dx2 −y2 HOMO transfers an electron pair to an unoccupied H2 bdt S2-C2 σ ∗ orbital in the course of the second Mo-S bond rupture.

Conclusions The hydrolysis mechanism of the [Mo(bdt)3 ] complex has been studied employing the M06, B3P86 and B3PW91 DFT functionals. The reaction pathway passes through seven transition structrues, leading to [Mo(OH)2 (bdt)2 ] and benzenedithiol as the final products of reaction. The pathway postulates an aquo-seven-coordinated molybdenum complex as a feasible intermediate, which is stabilized by a LPO →LPMo hyperconjugative interaction. The presence of this intermediate provides a better understanding of the oxygen atom exchange between water and tertiary phosphines mediated by this species. The hydrolysis mechanism deduced from the DFT studies (PES, NBO, BDE and electronic calculations) is coherent with the global process shown below, Mo(bdt)3 + 2 H2 O −−→ Mo(OH)2 (bdt)2 + H2 bdt The M06-DFT study confirms that the electronic structure of [Mo(bdt)3 ] and [Mo(OH)2 (bdt)2 ] complexes are d2 [L3 ]2h and d0 [OL2 ]0h , hence the formal oxidation state of the species are IV and VI respectively. The FMO analysis, conducted around the transition structures involved into the cleavages of the two Mo-S bonds, allows concluding that a pair of electrons of the occupied-donating Mo dx2 − y 2 orbital is transferred throughout the course of the rupture of the second Mo-S bond to a unoccupied H2 bdt S2-C2 σ∗ ligand orbital. The bond dissociation energy of this step confirms that the heterolytic Mo-S bond cleavage is energetically 21



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the more favorable. Finally, the mechanism is coherent with the observation of the so-called solvent effect. The [Mo(bdt)3 (H2 O)] intermediate, in absence of appropriate oxo-aceptors, undergoes hydrolysis forming [Mo(OH)2 (bdt)2 ] and benzenedithiol, which in equilibrium with benzenedithiolate anion,

− + H2 bdt + H2 O − ↽− −⇀ − Hbdt + H3 O

This species reduces the remaining [Mo(bdt)3 ] to [Mo(bdt)3 ] – very fast, Mo(bdt)3 + Hbdt− −−→ [Mo(bdt)3 ]− + Hbdt·

giving rise to the characteristic color of the anionic forms of the neutral species observed when trisdithiolato complexes are dissolved in wet polar solvents.

Acknowledgement This research was supported by the Ministerio de Educación y Ciencia (MAT2015-64139C4-2-R and CTQ2015-66223-C2).

Supporting Information Available Results for DFT B3P86 and B3PW91 functionals are given in the supporting information. This material is available free of charge via the Internet at http://pubs.acs.org/.

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DFT Study on the Interaction of Tris(benzene-1,2-dithiolato

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