Computational Insights on the Mechanism of H2 Activation at Ir2S2

Article pubs.acs.org/IC

Computational Insights on the Mechanism of H2 Activation at Ir2S2(PPh3)4: A Combination of Multiple Reaction Pathways Involving Facile H Migration Processes Andrés G. Algarra* Departamento de Ciencia de los Materiales e Ingeniería Metalúrgica y Química Inorgánica, Facultad de Ciencias, Universidad de Cádiz, Apartado 40, Puerto Real, 11510 Cádiz, Spain S Supporting Information *

ABSTRACT: The complex Ir2S2(PPh3)4 (1) is known to react with 1 and 2 equivalents of H2 leading to [Ir(H)(PPh3)2]2(μ-S)2 (2) and Ir2(μ-S)(μ-SH)(μ-H)H2(PPh3)4 (4), respectively (Linck, R. C.; Pafford, R. J.; Rauchfuss, T. B. J. Am. Chem. Soc. 2001, 123, 8856−8857). Herein, the results of a thorough computational (DFT) study of these formally homo- and heterolytic H2 activation processes, respectively, are presented. These indicate that 2 is formed in a two-step process whereby the oxidative addition of H2 at a single IrII center of 1 generates an intermediate (A) that rearranges into 2 by means of a facile H migration to the neighboring Ir center. Activation of the second equivalent of H2 most likely occurs at the bridging sulfur ligands of 2 leading to a reaction intermediate (3aa) that features two (μ-SH) ligands. Intermediate 3aa present two isomers that differ only on the stereochemistry of the (μ-SH) ligands, and both of them can further evolve into 4 via H migration from (μ-SH) to bridging (μH). Nevertheless, an alternative mechanism based on the initial isomerization of 2 into A, and followed by H2 coordination and activation steps at a single Ir center cannot be completely ruled out. In general, the results herein show that the mechanisms for the activation of H2 at 1 and 2 involve facile H migration processes, in agreement with the experimentally observed intermetallic hydride exchange in 2 and the exchange between IrH and SH centers in 4, which proceed with computed free energy barriers of ca. 19−23 kcal mol−1.

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INTRODUCTION The activation of the molecular hydrogen constitutes a fundamental chemical transformation1 with major implications in biology,2 industry,3 and more recently also in relation to energy storage.4 There are two mutually exclusive pathways for the cleavage of the H−H σ-bond, heterolytic and homolytic. The process is often catalyzed by transition metal compounds, with the operating system depending on factors such as the following: (a) The oxidation state of the metal centers. Heterolytic H+/ H− activation is common for metals in high oxidation state, whereas those in low oxidation states tend to cleave H2 homolytically through oxidative addition. (b) The nature of the ligands, which can lead to metal− ligand cooperativity effects.5 Hydrogenases represent well-known examples of heterolytic H+/H− activation. Their active sites contain sulfur ligands attached to the metal centers, and nowadays it is accepted that the process involves the formation or breakage of S−H bonds. (c) The presence of metal−metal interactions can also result in cooperative effects,6 that is, additional reaction pathways in which both metals are somehow involved in the activation process. Bimetallic compounds are © XXXX American Chemical Society

common in Nature, and again not only some hydrogenases but also other metalloenzymes feature binuclear active sites. Soluble bimetallic sulfide complexes represent a valuable set of compounds for the understanding of the relative importance of each of these factors and how they interact. Nevertheless, the presence of two metal centers together with potentially active ligands leads to a number of homo- and heterolytic pathways whose study is not only interesting but also relatively complex. Scheme 1 includes a selection of dihydrogen activating soluble bi- and monometallic sulfides useful to exemplify this complexity. Homogeneous Cp2Mo2S4 complexes such as that in Scheme 1a have been largely studied as models to understand the role of molybdenum sulfides in hydrodesulfurization (HDS) catalysts.7 Interestingly, the reactivity of [Cp*Mo]2(μ-S)2(μ-S2) (Cp* = pentamethylcyclopentadienyl) with H2 is sulfur-based and the formation of [Cp*Mo]2(μS)2(μ-SH)2 does not involve direct participation of the molybdenum centers. In fact, recent computational studies have shown that the reaction starts with the homolytic activation of H2 at two adjacent sulfur centers followed by Received: August 3, 2016

A

DOI: 10.1021/acs.inorgchem.6b01888 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry Scheme 1

Scheme 2. Reaction between Ir2S2(PPh3)4 (1) and 2 Equivalents of H2a

a

For simplicity, only the P atom of each PPh3 ligand has been represented.

Table 1. Key Bond Distances (Å) and Angles (deg) in 1, 2, and the Two Isomers of 4a parameter

1 (X-ray)

1 (calcd)

2 (X-ray)

2 (calcd)b

4a (calcd)c

4e (calcd)c

Ir−Ir Ir−P Ir−P′ Ir−S Ir−S′ Ir−S−Ir S−Ir−Ir−S′

2.758 2.26 2.27 2.31/2.27 2.27/2.31 74.0 136.4

2.757 2.28 2.26 2.33/2.29 2.29/2.33 73.4 131.7

2.754 2.27 2.27 2.29/2.36 2.36/2.29 72.6 123.8

2.835 2.29 2.24 2.34/2.37 2.37/2.34 74.0 121.3

2.786 2.27 2.29 2.44/2.51 2.48/2.44 69.1 106.4

2.79 2.27 2.29 2.44/2.51 2.48/2.43 68.6 102.0

a

The notation is based on Scheme 2. The corresponding parameters from the crystal structures of 1 and 2 are also indicated.11 bComputed Ir−H bond distances of 1.60 Å. cComputed Ir−H and Ir−(μ-H) bond distances of 1.61 and 1.78 Å, respectively. Computed S−H bond distance of 1.36 Å.

activation of H2. Activation of the first equivalent of H2 generates Ir2H2(PPh3)4 (2) and causes a change in the oxidation state of each Ir center from IrII to IrIII. In contrast to this step, which takes place with no detectable intermediates when monitored by 1H NMR spectroscopy, the activation of the second equivalent of H2 at 2 is formally heterolytic and occurs with formation of a short-lived intermediate (3) that subsequently isomerizes into the final product 4. Unfortunately, the transient nature of 3 has so far hampered its characterization, having only been observed by NMR spectroscopy. The understanding of these H2 activation processes is indeed not straightforward as the possible pathways for each step are complicated by the hydride mobility often observed in hydrogenases12 and other sulfido clusters,8,13 which means that the observed species are not necessarily the direct products of H2 activation but can also originate from H migrations at previously formed intermediates. In line with this, NMR data indicates that intermetallic hydride exchange in 2 and exchange

the isomerization of the resulting species into the observed product.8 In contrast, the dicationic rhodium complex [Rh2(μS)2(triphos)2]2+ is known to reversibly activate 2 equivalents of H2 via consecutive additions across Rh−S bonds.9 Although monometallic, the third example included in Scheme 1c illustrates another possible H2 activation pathway available for metal sulfide complexes. Thus, Cp*2Ti(S)py (py = pyridine) reacts reversibly with H2, and NMR experiments indicate that the process involves a transient η2-H2 intermediate.10 In this work, we have concentrated our efforts on the understanding of the mechanistic details of the processes included in Scheme 2, that is, the reaction of Ir2S2(PPh3)4 (1) with 2 equivalents of H2 to generate Ir2(μ-S)(μ-SH)(μH)H2(PPh3)4 (4).11 Although these were described in 2001, their mechanistic details remain relatively unknown and only general comments based on those experiments have been formulated. The formation of 4 occurs in toluene in a multistep process that formally implies both homolytic and heterolytic B


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Inorganic Chemistry

Figure 1. HOMO − 3, HOMO, and LUMO orbitals of 1 (isovalue 0.05). For simplicity, only the ipso-C atoms of the PPh3 ligands have been drawn.

levels of theory typically resulting in Ir−Ir distances of ca. 3.0 Å (see Table S1). A possible explanation could then be that the observed Ir−Ir interaction partly results from the geometric constraints imposed by the bridging sulfur ligands, together with the dispersion interactions between the two monomeric moieties. As indicated in Scheme 2, further addition of 1 equivalent of H2 at compound 2 results in the formation of 4. This compound appears in solution as a mixture of two diastereoisomers of similar stability that only differ in the relative position of the H atom at the (μ-SH) ligand. This can be located in two different orientations within the molecule, that is, in equatorial or axial orientation with respect to the Ir2S2 core, and these are indicated in Scheme 3 as 4e and 4a,

between IrH and SH centers in 4 proceed with relatively low activation barriers.11 Computational calculations are, undoubtedly, extremely helpful in mechanistic studies. In this article, we summarize the results of a thorough density functional theory (DFT) study of the possible pathways for the formation of 2 and 4 via the reaction of 1 with 1 and 2 equivalents of H2. These are arranged in four main sections: the structures of the stable species 1, 2, and 4 are described initially. Second and third sections deal with the two H2 activation processes leading to species 2 and 4. The exchange between IrH and SH centers in 4 is analyzed in the last section.

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RESULTS AND DISCUSSION Molecular Structures of 1, 2, and 4. The computational study was initiated by optimizing the geometries of compounds 1, 2, and 4. The X-ray structures11 of compounds 1 and 2 were used to test a number of density functionals and basis set systems (see Tables S1 and S2 in the Supporting Information). Notably, inclusion of dispersion effects was found to be critical to obtain structures in good agreement with the experimental data. The BP86-D3 functional, in combination with triple-ζ basis sets, was selected for the study (see Computational Details and the Supporting Information). The main geometrical parameters for the structures of 1, 2, and 4 at this level of theory are given in Table 1. Compound 1 presents a butterfly shaped Ir2S2 core that is characterized by a S−Ir−Ir−S dihedral angle of 131.3° (cf. 136.4° in the X-ray structure). Each Ir center is in a formal oxidation state of II and features an IrP2S2 distorted tetrahedral coordination environment if the Ir−Ir interaction is disregarded. In fact, 1 is diamagnetic, and this has been explained based on the formation of a IrII−IrII bond, evidenced by an Ir− Ir distance of 2.76 Å. In addition, the bridging sulfur ligands are expected to improve efficient delocalization, and indeed analysis of its frontier molecular orbitals shows that they are Ir2S2 centered. Specifically, the HOMO and LUMO of 1, included in Figure 1, are mainly Ir−S antibonding, whereas the HOMO − 3 evidences the bonding interaction between the metal centers. Homolytic activation of 1 equivalent of H2 at compound 1 leads to the formation of 2. This process does not result in major changes in the butterfly shaped Ir2S2 core of 2, with the most notable difference occurring in terms of the S−Ir−Ir−S dihedral angle, which now becomes 121.3° (cf. 123.8° in the Xray structure). Here each IrIII center features a distorted square pyramidal geometry with one of the PPh3 ligands (labeled as P′ in Scheme 2) in the axial position, if again the Ir−Ir interaction is disregarded. An interesting feature of 2 relates nonetheless to its Ir−Ir distance (2.754 Å in the X-ray structure), which is practically the same as in compound 1. This is relatively surprising as it would be expected for this bond to break upon addition of H2. In this regard it is worth noting that optimization of 2 only resulted in Ir−Ir distances in reasonable agreement with the X-ray structure when dispersion effects were used in combination with triple-ζ basis sets, with lower

Scheme 3. Isomerisation between 4a and 4e via TS4a/4ea

a Values in parentheses correspond to free energies (kcal mol−1). For simplicity, only the P atom of each PPh3 ligand has been represented. Key parameters (Å, deg) for TS4a/4e: Ir−Ir′ = 2.93; Ir−S = 2.48; Ir′−S = 3.40; S−H = 1.37; S′−Ir−Ir = 49.6; S−Ir−Ir = 77.5.

respectively.14 The main geometrical parameters for these isomers are very similar (see Table 1), with each Ir center featuring a distorted octahedral environment in which (μ-H) and PPh3 ligands occupy axial positions. Their interconversion takes place with a computed free energy barrier of 13.8 kcal mol−1 (TS4a/4e, Scheme 3), indicative of a facile process at room temperature. The process requires nonetheless the cleavage of the Ir′−S bond (3.40 Å in TS4a/4e) to allow for the rotation of the (μ-SH) ligand,15 being worth noting that similar transition states have been computed for the analogous process on the binuclear compound [Cp*Mo]2(μ-S)2(μ-SH)2 (Cp*= cyclopentadienyl).8 DFT Studies on the Mechanism of the Reaction between 1 and H2 To Form 2. Experimentally, the reaction between 1 and H2 takes place in toluene at room temperature without formation of NMR-detectable intermediates.11 Combining this information with the fact that 2 features one hydride ligand on each Ir center, this could be a priori indicative of a concerted [2 + 2] addition across the Ir−Ir bond (pathway a, Scheme 4). Despite multiple attempts, it has nonetheless been impossible to locate a transition state structure accounting for such mechanism. This agrees with early theoretical calculations indicating its symmetry-forbidden nature.16 Frontier molecular orbital (FMO) arguments support this conclusion, as inspection of the HOMO and LUMO of 1 in Figure 1 shows C


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Inorganic Chemistry Scheme 4. Possible Mechanisms for the Activation of H2 at Compound 1a

a Values in parentheses correspond to free energies (kcal mol−1) quoted relative to 1 + H2. For simplicity, only the P atom of each PPh3 ligand has been represented.

Figure 2. DFT optimized structures of TS1/A (left) and A (right). For simplicity, only the ipso-C atoms of the PPh3 ligands have been drawn. Key distances (Å) for TS1/A: Ir(1)−Ir(2) = 2.79; Ir(1)−H(1) = 2.53; Ir(1)−H(2) = 2.03; H(1)−H(2) = 0.79; Ir(1)−S(1) = 2.31; Ir(1)−S(2) = 2.35; Ir(2)−S(1) = 2.29; Ir(2)−S(2) = 2.33. Key distances (Å) for A: Ir(1)−Ir(2) = 2.73; Ir(1)−H(1) = 1.59; Ir(1)−H(2) = 1.74; Ir(2)−H(2) = 1.82; Ir(1)−S(1) = 2.52; Ir(1)−S(2) = 2.43; Ir(2)−S(1) = 2.32; Ir(2)−S(2) = 2.34.

Figure 3. DFT optimized structures of TS1/C (left) and TSA/ex (right). For simplicity, only the ipso-C atoms of the PPh3 ligands have been drawn. Key distances (Å) for TS1/C: Ir(1)−Ir(2) = 2.738; S(1)−H(1) = S(2)−H(2) = 1.69; H(1)−H(2) = 1.05. Key distances (Å) for TSA/ex: Ir(1)−Ir(2) = 2.76; Ir(1)−H(1) = 1.74; Ir(1)−H(2) = 1.74; H(1)−H(2) = 0.88.

Structurally, this is accompanied by an increase in both Ir(1)−S bond distances of ca. 0.03 Å, all this highlighting the different behavior of the metal centers during the process. Notably, TS1/A does not result in the formation of a formal IrIV(1)− IrII(2) dimer with both H ligands bound to Ir(1). Instead, one of these hydrides adopts a bridging position to generate the intermediate labeled as A (see Figure 2), with the process being roughly thermoneutral (ΔG = 0.4 kcal mol−1). The asymmetry of this structure, with Ir(1) and Ir(2) centers featuring distorted octahedral and square pyramidal coordination environments, respectively, is again evidenced by their Mulliken charges (0.00 and 0.145, respectively) or the differences in terms of Ir−S bond distances, with values ca. 0.1−0.2 Å larger for Ir(1). Rearrangement of A into the more stable 2 (G = −7.0 kcal

that the approach of H2 through the plane formed by the two metal centers and the C2 axis of 1 would not result in significant orbital overlapping with H2 σ* and σ orbitals, respectively. Alternatively, oxidative addition at one Ir center followed by H migration (pathway b, Scheme 4) has been proposed for the H2 activation at other Ir2S2 complexes such as [Ir(μS−tBu)(CO)(P(OR)3)]2 (R = tBu, Me),17 and more recently for other dinuclear iridium compounds.6c,18 In the present case, the transition state TS1/A (see Figure 2) accounts for the formation of intermediate A with free energy barrier of 16.8 kcal mol−1. The Mulliken charges of Ir(1) and Ir(2) centers in TS1/A, with values of 0.21 and 0.09, respectively, are indicative of a rearrangement of the electron density within the cluster fragment (i.e., compound 1) in order to react with H2. D


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Inorganic Chemistry Scheme 5. Possible Mechanisms for the Activation of H2 at a Single Ir Center in Compound 2a

Values in parentheses correspond to free energies (kcal mol−1) quoted relative to 2 + H2. For simplicity, only the P atom of each PPh3 ligand has been represented. a

mol−1) takes place with a barrier of 8.3 kcal mol−1 via TSA/2 (see Figure S1). The formation of A thus represents the ratedetermining step of pathway b, which is compatible with the experimental nonobservation of A given that its rearrangement into 2 is expected to be much faster than its formation. Addition of H2 across a metal−sulfur bond leading to species Bc (pathway c, Scheme 4) has been demonstrated for other dinuclear M2S2 complexes such as [Rh2(μ-S)2(triphos)2]2+ (triphos = CH3C(CH2PPh2)3),9a and therefore calculations aiming at characterizing this mechanisms have also been carried out. Interestingly, these showed that Bc can only be formed via isomerization of A. The process features a free energy barrier of 15.0 kcal mol−1 (TSA/Bc, Figure S1), with Bc presenting roughly the same stability as A. Given that TSA/Bc is higher in energy than TSA/2, which directly leads to the formation of the observed product 2, it seems unlikely that Bc is formed under the experimental conditions. Finally, an additional possible interaction between 1 and H2 to form the bis-hydrosulfido compound C is labeled as pathway d. Although the concerted formation of two μ-SH ligands has been evidenced for some dinuclear molybdenum−sulfide complexes such as [Cp*Mo]2(μ-S)2(μ-S2) (Cp* = pentamethylcyclopentadienyl),8 in the present case this seems unlike due to both the computed endergonic nature of the process (ΔGr = 21.8 kcal mol−1) and its relatively large activation barrier (TS1/C, G = 41.1 kcal mol−1, see Figure 3). The activation strain model (ASM) analysis19 can be used at this point to explain the different behavior of these two systems. Within this framework, ΔE# is divided into ΔE#strain and ΔE#int (see eq 1), with ΔE#strain being the energy required for the reactants to adopt their geometries at the TS structure and ΔE#int the energy resulting from the interactions between them. For TS1/C values of ΔE# = 31.8, ΔE#strain(1) = 17.6, ΔE#strain(H2) = 20.4, and ΔE#int = −6.1 kcal/mol−1 are obtained. In contrast with the [Cp*Mo]2(μS)2(μ-S2) system, these values indicate that the energy required to deform both reactants (i.e., 38.0 kcal mol−1) up to the TS1/C geometry is the origin of the relatively large computed barrier,

obtained despite the 6.1 kcal mol−1 stabilizing interaction between them (see Figure S4). # # # ΔE # = ΔEstrain (1) + ΔEstrain (H 2) + ΔE int

(1)

Taken together, the computations in this section show that although formation of 2 is formally a homolytic process, it does not take place in a concerted manner (pathway a). Instead, it involves the initial oxidative addition of H2 at a single IrII center to form A, which rapidly undergoes H migration to the adjacent metal center (pathway b) yielding 2. The mechanism bears similarities with that proposed by Nocera et al. to explain the activation of H2 at the mixed-valence IrII−Ir0 complex Ir2(tfepma)3Cl2 (tfepma = bis(bis(trifluoroethoxy)phosphino)methylamine).6c In such case, the reaction starts with the formation of a dihydrogen complex at the IrII center, and in a second step, this intermediate evolves into the observed (IrI− H)(IrIII−H)(tfepma)3Cl2 dimer via a transition state that implies both the activation of the H−H bond and the migration of one of these H ligands to the adjacent Ir center. Therefore, this concerted activation/migration mechanism does not require the formation of intermediates with (μ-H) ligands similar to A in the present Ir2S2 system. Despite the fact that intermediate A has not been observed by NMR spectroscopy, additional support in favor of pathway b comes from the rapid intermetallic hydride exchange observed for 2 during such experiments. In this sense, the calculations show that exchange between hydride ligands is not possible directly from species 2 but it requires the formation of intermediate A. The process takes place at this compound with a free energy barrier of only 12.5 kcal mol−1 via transition state TSA/ex, whose structure resembles that of a dihydrogen complex (d(H(1)−H(2)) = 0.89 Å, see Figure 3). Note that an analogous mechanism has been proposed to explain the H exchange on the diiron complex [Fe2(H)(μ-H)(pdt)(CO)(dppv)2] (pdt = propanedithiolate; dppv = cis-1,2-bis(diphenylphosphino)ethylene).20 E


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Inorganic Chemistry

Figure 4. DFT optimized structures of D (left) and Bt (right). For simplicity, only the ipso-C atoms of the PPh3 ligands have been drawn. Key distances (Å) for D: Ir(1)−Ir(2) = 3.626; Ir(1)−H(1) = 1.57; Ir(2)−H(2) = 1.62; Ir(1)−H(3) = 1.70; Ir(1)−H(4) = 1.66; H(3)−H(4) = 1.01. Key distances (Å) for Bt: Ir(1)−Ir(2) = 2.891; Ir(1)−H(1) = 1.61; S(1)−H(2) = 1.36.

Scheme 6. Activation of H2 at the (μ-S) Groups of 2a


dihedral angle of 163.1°) is in line with this observation.22 Theoretically, this species can readily rearrange into its butterfly shaped isomer E via TSD/E (G = 17.6 kcal mol−1). Additionally, H2 coordination at the vacant site of Ir(2) in A is also expected to generate species E. Unfortunately, despite significant computational efforts it has been impossible to locate the transition state associated with this process. Instead, its free energy barrier has been estimated as 21.0 kcal mol−1 from the saddle-point obtained from a two-dimensional scan using d(H(3)−H(4)) and d(Ir(2)−H(3)) as variables (see Supporting Information). The process thus features an estimated barrier ca. 5 kcal mol−1 lower than that via TS2/D. Interestingly, the relative barriers of these two H2 coordination processes are in agreement with general trends observed for dinuclear Ir complexes, whose reluctance to H2 addition has been associated with their rigidity, which prevents the structural changes required to generate the empty orbital necessary for the coordination of H2.23 Nevertheless, in the present case the interconversion between 2 and A is relatively facile, thus facilitating the reaction between the latter species and H2. Once the dihydrogen−dihydride intermediate E is formed, the actual

DFT Studies on the Mechanism of the Reaction between 2 and H2 To Form 4. As indicated above, this reaction entails the transient formation of intermediate 3 (see Scheme 2), which has only been observed by 1H NMR spectroscopy.11 The resulting nonconcerted nature of the process leads to a mechanistic analysis significantly more complex than that in the previous section, additionally hampered by the number of isomeric structures that clusters featuring (μ-SH) ligands such as 4 can adopt. H2 activation pathways analogous to those in Scheme 4 have been used as starting point for this computational analysis. Thus, similarly to the reaction between 1 and H2, it has been impossible to locate a transition state for the addition of H2 across the Ir−Ir bond in 2. In contrast, interaction of H2 with a single IrIII center can occur in principle at both compound 2 and the previous intermediate A (see Scheme 5). The former process features an activation barrier of 26.0 kcal mol−1 (TS2/D), with the resulting dihydrogen complex D being 17.1 kcal mol−1 less stable than the separated reactants (see Figure 4). Note that diamondshaped geometries of dinuclear iridium compounds are quite rare,21 and the relatively low stability of D (Ir(1)−Ir(2) nonbonding distance of 3.626 Å and S(1)−Ir(1)−Ir(2)−S(2) F


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Inorganic Chemistry

evaluated by comparison of the ASM energies. For TS2/3aa, ΔE# = 7.3, ΔE#strain(2) = 11.3, ΔE#strain(H2) = 8.5, and ΔE#int = −12.5 kcal/mol−1. Indeed, these values show that the deformation required for the reactants (i.e., 2 and H2) to reach TS2/3aa is only 19.8 kcal mol−1, that is, 18.2 kcal mol−1 lower than that for TS1/c. Moreover, they also show that in TS2/3aa the two reactants interact more strongly than in TS1/c, in this case the additional stabilization reaching 6.4 kcal mol−1. Frontier molecular orbital arguments can again be used to explain this stabilization from a qualitative viewpoint, as the analysis of the HOMO and LUMO orbitals of 2 and H2 at the TS2/3aa geometry clearly supports the symmetry-allowed nature of the two possible HOMO−LUMO interactions. As it can be seen in Figure 5, a significant contribution of both HOMO and LUMO of 2 at TS2/3aa is located at the bridging sulfur centers. These orbitals have the right symmetry to interact with the σ* and σ orbitals of H2, respectively. All in all, this shows that the formation of 3aa is a feasible process. As shown in Scheme 6, 3aa can subsequently rearrange into Fa via TS3aa/Fa. This process features a barrier of 18.3 kcal mol−1 (TS3aa/Fa) and accounts for two concerted H migration processes, that is, one from a (μ-SH) ligand to terminal hydride and the other from terminal to bridging hydride. The calculations indicate nonetheless that Fa is 7.8 kcal mol−1 less stable than 3aa, and therefore such rearrangement is thermodynamically disfavored. Besides the formation of Fa, either one or both axial H atoms at the (μ-SH) ligands in 3aa can also adopt equatorial positions, thus leading to 3ae and 3ee, respectively. The three isomers of 3 present stabilities within the range of ca. 5 kcal mol−1, and the activation barriers for their formation are computed to be 20.2 kcal mol−1 (TS3aa/3ae, see Figure 6) and 22.3 kcal mol−1 (TS3ae/3ee). Interestingly, the presence of (μ-SH) ligands with the H atom in the equatorial position of these conformers allows for a different type of H migration, as in both cases such equatorial H atom can move into the bridging position between the two iridium centers in a process that involves the cleavage of the S−H bond. Accordingly, the structure of TS3ae/4a, included in Figure 6 (see Figure S2 for TS3ee/4e), shows an increase in the S(2)− H(4) bond length from 1.36 Å in 3ae up to 1.56 Å, whereas a nonbonding distance of 2.84 Å is found in 4a. This type of rearrangement has, to our knowledge, never been described before, and it results in the formation of 4a and 4e with free energy barriers of 20.3 and 22.6 kcal mol−1, respectively. It is worth noting that 4e can also be formed via TS4e/4a with a barrier of 13.8 kcal mol−1 (see Scheme 3), thus leading to an alternative reaction pathway for the conversion of 4a into 4e without participation of 3ee. This is in fact less energy demanding than that via TS3ee/4e and therefore expected to represent the actual mechanism for the interconversion between 4a and 4e. The mechanism included in Scheme 6 represents a likely pathway for the reaction between 2 and H2. It hinges on the formation of the bis-hydrosulfido intermediate 3aa, which results from the homolytic activation of H2 at the two (μ-S) ligands and also links the pathways that lead to Fa, 3ae, and 3ee, with the latter two species able to further isomerize into the final products 4a and 4e, respectively. At this point, it is worth highlighting that, based on NMR experiments, a structure analogous to Fa has been suggested as the intermediate formed during the reaction of 2 with H2.11 Conversely, the present computational results indicate that, even if the formation of Fa shows the lowest possible barrier

H−H bond activation can take place readily via TSE/4e (G = 12.8 kcal mol−1), resulting in the formation of 4e. An additional possibility for the interaction of H2 with a single Ir center relies on the isomerization of 2 into Bt via TS2/Bt (ΔG = 19.6 kcal mol−1). The process involves the migration of H(2) onto S(1) and can be formally viewed as the protonation of S(1) coupled with the reduction of Ir(2). Bt is computed to be only 4.0 kcal mol−1 less stable than 2 and, as shown in Figure 4, features Ir(2) in a square planar coordination geometry if the Ir−Ir interaction is disregarded. Notably, H2 can interact with this unsaturated Ir(II) center to generate 4e in a single step via TSBt/4e (see Figure S2). In spite of the fact that the process shows a barrier of 25.0 kcal mol−1, that is, 4.0 kcal mol−1 larger than the formation of species E, it is interesting to highlight the structural similarities between TSBt/4e and TS1/A. These two transition states feature the approaching H2 molecule in a relatively similar position with respect to the cluster core (differences in Ir−H distances smaller than 0.1 Å), in agreement with the IrII···H2 interaction taking place in both cases and the structural similarities between 1 and Bt regarding the Ir center interacting with H2. Besides the pathways involving the interaction of H2 at a single Ir center (Scheme 5), the formation of 4 could also take place via the homolytic activation of H2 at the bridging sulfur ligands of 2, that is, analogous to pathway d in Scheme 4 for the parent compound 1. This results in the formation of 3aa (see Scheme 6), where the notation “aa” refers to the axial position of the H atoms at the (μ-S) ligands. The process is exergonic by 10.7 kcal mol−1 and features an activation barrier of 16.5 kcal mol−1 (TS2/3aa, Figure 5). Interestingly, although both in TS2/3aa and TS1/c (see Figure 3) the dihydrogen molecule approaches the cluster roughly in the same direction, the two TSs are clearly different based on S−H and H−H bond lengths of 1.90 and 0.93 Å in TS2/3aa, respectively (cf. 1.69 and 1.05 Å in TS1/c). Such structural differences are evidently associated with the lower barrier for TS2/3aa, and this can be quantitatively

Figure 5. (a) DFT optimized structure of TS2/3aa. Key distances (Å): Ir(1)−Ir(2) = 2.757; Ir(1)−H(1) = Ir(2)−H(2) = 1.60; S(2)−H(4) = S(1)−H(3) = 1.90; H(3)−H(4) = 0.93. (b) Schematic representation of the two main symmetry-allowed orbital interactions between 2 and H2 at TS2/3aa (isovalue = 0.05). For simplicity, only the ipso-C atoms of the PPh3 ligands have been drawn. G


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Inorganic Chemistry

Figure 6. DFT-optimized structures of TS3aa/3ae (left) and TS3ae/4a (right). For simplicity, only the ipso-C atoms of the PPh3 ligands have been drawn. Key distances (Å) for TS3aa/3ae: Ir(1)−Ir(2) = 3.734; Ir(1)−S(1) = 2.48; Ir(1)−S(2) = 2.40; Ir(2)−S(1) = 2.43; Ir(2)−S(2) = 2.54. Key distances (Å) for TS3ae/4a: Ir(1)−Ir(2) = 2.766; Ir(1)−S(2) = 2.52; Ir(2)−S(2) = 2.57; Ir(1)−H(4) = 1.88; Ir(2)−H(4) = 1.99; S(2)−H(4) = 1.56.

Scheme 7. Isomerization of Fa into 4a via the Formation of Diamond-Shaped Structuresa


Figure 7. DFT-optimized structures of G (left) and TSG/G′ (right). For simplicity, only the ipso-C atoms of the PPh3 ligands have been drawn. Key parameters (Å,deg) for G: Ir(1)−Ir(2) = 3.65; Ir(1)−H(1) = 1.61; Ir(1)−H(4) = 1.63; Ir(2)−H(2) = 1.59; H(4)−Ir(1)−Ir(2)−H(2) = 150.1. Key parameters (Å,deg) for TSG/G′: Ir(1)−Ir(2) = 3.65; Ir(1)−H(1) = 1.60; Ir(1)−H(4) = 1.63; Ir(2)−H(2) = 1.59; H(4)−Ir(1)−Ir(2)−H(2) = 239.2.

dihedral angle of 169.1° and a nonbonding Ir−Ir distance of 3.65 Å. Energetically, Fa and G present roughly the same stability, being both ca. 8 kcal mol−1 less stable than 3aa. Pseudorotation of the phosphine and hydride ligands bound to Ir(2) in G can potentially lead to the isomers labeled in Scheme 7 as G′ and G″ (12.0 and 18.2 kcal mol−1 less stable than 3aa, respectively).22 The latter species G″ features the hydride ligand bound to Ir(2) in the axial position opposite to that on G, and its diamond-to-butterfly (i.e., closing) rearrangement produces 4a via TSG″/4a. Isomerization of 3a into 4a is therefore possible through the series of rearrangements 3a → G → G′ → G″ → 4a. The transition states TSG/G′ and TSG′/G″ account for the pseudorotation of the phosphine and hydride ligands that result in G′ and G″, respectively. For instance, TSG/G′ (see Figure 7) shows that rotation of the ligands at Ir(2) leads to a

from 3aa, the process is unlikely due to its endergonic nature unless Fa (or one of its isomeric structures that only differ in the stereochemistry of the hydrosulfido ligand, see Scheme S1) can isomerize into 4a or 4e via an alternative mechanism with an overall barrier lower than 20.3 kcal mol−1 (TS3aa/3ae). The most likely mechanism for such isomerization consists of the cleavage of one Ir−(μ-H) bond followed by pseudorotation at the resulting five-coordinate Ir center.24 The results of its computational evaluation are summarized in Scheme 7. The “opening” of the cluster core in Fa via TSFa/G generates species G (see Figure 7), which features Ir(1) in a pseudo-octahedral environment with hydride ligands in equatorial and axial positions, whereas Ir(2) shows a square planar geometry with an additional hydride in an axial position. Additionally, the diamond-shaped geometry of G is evidenced by a S−Ir−Ir−S H


Article

Inorganic Chemistry H(4)−Ir(1)−Ir(2)−H(2) dihedral angle of 239.2°, that is, approximately 90° larger than that in G. Similar results are found in the case of TSG′/G″.25 With a barrier of 26.7 kcal mol−1 when 3aa is taken as zero in relative free energy, the isomerization of G into G′ (TSG/G′) is computed to be the most energy demanding step of the process. The value is however higher than that via 3ae, and therefore the formation of Fa does not seem likely based on these computations. All in all, these computational results show that there are two alternative mechanisms for the formation of 4 with energy barriers that differ by less than 1 kcal mol−1. As shown in Scheme 6, species 2 can directly interact with H2 to generate 4a by a mechanism of the type 2 + H2 → 3aa → 3ae → 4a. This mechanism features an overall barrier of 20.3 kcal mol−1 and implies the formation of 3aa as a reaction intermediate. Alternatively, the presence of H2 can displace the equilibrium between 2 and A toward the latter (Scheme 5). Intermediate A could then coordinate H2 leading to the dihydrogen−dihydride complex E in the most energy demanding step (21.0 kcal mol−1), and this would be followed by the facile E → 4e step whereby the actual H−H bond activation takes place. Given the roughly similar barriers computed for these two mechanisms and the fact that both suggest the formation of different intermediates, it seems impossible at this point to discard any of these mechanistic possibilities based only on the computations above. 1H NMR experiments so far have shown that this intermediate appears in solution as a mixture of isomers with nonequivalent SH and IrH units. This gives support to the mechanism in Scheme 6, which involves the equilibrium between 3aa and 3ae (and probably also 3ee), whereas the mechanism via A (Scheme 5) does not imply the formation of intermediates with SH ligands. Nevertheless, this is an assumption based only on the presence or absence of NMR signals for an unidentified compound and therefore cannot be considered conclusive. Full description of the NMR-observed intermediate is, evidently, a piece of data that would help toward the understanding of this reaction mechanism. Although the structural characterization of this species is difficult not only because it exists in solution as a mixture of isomers but also because they appear in relatively small concentrations, it could be interesting to study the reaction using state-of-the-art NMR techniques. Thus, in addition to its analysis by means of onedimensional 1H and 31P NMR spectroscopy, it could be checked whether diffusion-ordered NMR spectroscopy (DOSY) allows separation of the signals resulting from its different isomers. Moreover, NMR experiments that make use of the nuclear Overhauser effect (NOE) could be also tested, as well as those employing D2 instead of H2. DFT Studies on the H Exchange in 4. Labeling studies have actually been used already to show that the products from the addition of H2 to [Ir(D)(PPh3)2]2(μ-S)2 (2-d2) and of D2 to 2 are indistinguishable, thus indicating that exchanges between Ir−H and S−H atoms in 4 are relatively facile processes. Specifically, three different exchange processes can be hypothesized. As included in Scheme 8, these are (a) exchange between S−H and (μ-H), (b) Ir−H and (μ-H), and (c) S−H and Ir−H. Based on the computational results shown above it is already possible to explain some of these exchange processes. Thus, that between S−H and (μ-H) atoms (exchange a) can take place via the transient formation of 3ee, as once this C2symmetric species is formed both S−H atoms can migrate to the (μ-H) position to regenerate 4e with the same probability.

Scheme 8. Possible H Exchange Processes at Ir2(μ-S)(μSH)(μ-H)H2(PPh3)4

The activation barrier associated with this exchange is therefore given by TS3ee/4e, reaching a value of 21.1 kcal mol−1. Regarding the exchange between Ir−H and (μ-H) atoms (exchange b), it can take place both from 4a and 4e by means of the transition states TS4a/ex (ΔG = 19.2 kcal mol−1) and TS4e/ex (ΔG = 21.2 kcal mol−1), included in Figure S3. These transition states resemble TSA/ex (see Figure 3), implying the transient formation of a dihydrogen complex at one of the iridium centers. Finally, exchange between S−H and Ir−H requires the formation of the dihydrogen−dihydride complex E (see Scheme 5), which is formed via TSE/4e and thus results in a free energy barrier of 23.3 kcal mol−1. All in all, the data in this section agree with the NMR experiments,11 showing that the free energy barriers for the H exchange processes at 4 are all within the range of 19−23 kcal mol−1. This bears similarities with the results for [FeFe]hydrogenase models12 and other sulfido clusters,8,13 all supportive of highly mobile H atoms at metal−sulfur clusters.

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CONCLUSIONS Ir2S2(PPh3)4 (1) is known to react with 1 and 2 equivalents of H2 leading to [Ir(H)(PPh3)2]2(μ-S)2 (2) and Ir2(μ-S)(μSH)(μ-H)H2(PPh3)4 (4). Herein, computational (DFT) methods have been employed to get insights into the mechanistic details of these H2 activation processes, with the results representing a beautiful example of the range of pathways by which metal−sulfur dinuclear clusters can activate H2. The analysis of the possible pathways for the formation of 2 indicates that the process involves the initial oxidative addition of H2 at a single IrII center to form an intermediate (A), which rapidly undergoes H migration to the adjacent iridium center yielding 2. The reaction between 2 and H2 is, mechanistically, more complex than the previous one. Most likely, the process starts with the activation of H2 at the bridging sulfur ligands of 2, leading to an intermediate (3) that features two (μ-SH) ligands. Compound 3 can adopt three isomeric structures (labeled as 3aa, 3ae, and 3ee) that only differ in the stereochemistry of these ligands, and two of these can further evolve into 4 via H migration from (μ-SH) to bridging (μ-H). Nevertheless, the computational results herein indicate the existence of an alternative mechanism that cannot be completely ruled out based on the available data. This implies the initial isomerization of 2 into A, followed by H 2 coordination and activation steps at a single Ir center. Finally, NMR experiments have shown that the exchanges between Ir−H and S−H atoms in 4 are facile processes. Three possible exchange processes have been computationally analyzed, that is, those between S−H and (μ-H), Ir−H and (μ-H), and S−H and Ir−H atoms. In agreement with the experiments, they all feature affordable free energy barriers I


Article

Inorganic Chemistry between 19 and 23 kcal mol−1, all in all supporting the idea of highly mobile H atoms at metal−sulfur clusters.

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COMPUTATIONAL DETAILS

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ASSOCIATED CONTENT

Activation of Small Molecules; Tolman, W. B., Ed.; Wiley-VCH Verlag GmbH & Co. KGaA: Weinheim, Germany, 2006; pp 121−158. (2) (a) Xu, T.; Chen, D.; Hu, X. Hydrogen-activating models of hydrogenases. Coord. Chem. Rev. 2015, 303, 32−41. (b) Simmons, T. R.; Berggren, G.; Bacchi, M.; Fontecave, M.; Artero, V. Mimicking hydrogenases: From biomimetics to artificial enzymes. Coord. Chem. Rev. 2014, 270−271, 127−150. (c) Lubitz, W.; Ogata, H.; Rudiger, O.; Reijerse, E. Hydrogenases. Chem. Rev. 2014, 114, 4081−148. (d) Barton, B. E.; Olsen, M. T.; Rauchfuss, T. B. Artificial hydrogenases. Curr. Opin. Biotechnol. 2010, 21, 292−7. (3) (a) Topsøe, H.; Clausen, B. S.; Massoth, F. E. Hydrotreating Catalysis. In Catalysis Science and Technology, Anderson, J. R., Boudart, M., Eds. Springer: Berlin, Heidelberg, 1996; pp 1−269. (b) Angelici, R. J. Hydrodesulfurization & Hydrodenitrogenation. In Encyclopedia of Inorganic Chemistry; King, R. B., Ed.; John Wiley & Sons, Ltd: Chichester, U.K.; 2006. (4) Advances in Hydrogen Production, Storage and Distribution, 1st ed.; Woodhead Publishing: Cambridge, U.K., 2014. (5) Trincado, M.; Grützmacher, H. Cooperating Ligands in Catalysis. In Cooperative Catalysis; Peters, R., Ed.; Wiley-VCH Verlag GmbH & Co. KGaA: Weinheim, Germany, 2015; pp 67−110. (6) (a) Catalysis by Di- and Polynuclear Metal Cluster Complexes; Adams, R. A., Cotton, F. A., Eds.; Wiley-VCH: Weinheim, Germany, 1998. (b) Park, J.; Hong, S. Cooperative bimetallic catalysis in asymmetric transformations. Chem. Soc. Rev. 2012, 41, 6931−6943. (c) Gray, T. G.; Veige, A. S.; Nocera, D. G. Cooperative Bimetallic Reactivity: Hydrogen Activation in Two-Electron Mixed-Valence Compounds. J. Am. Chem. Soc. 2004, 126, 9760−9768. (7) DuBois, M. R. Catalytic applications of transition-metal complexes with sulfide ligands. Chem. Rev. 1989, 89, 1−9. (8) Algarra, A. G. Computational Insights into the Mechanisms of H2 Activation and H 2 /D2 Isotope Exchange by Dimolybdenum Tetrasulfide Complexes. Eur. J. Inorg. Chem. 2016, 2016, 1886−1894. (9) (a) Ienco, A.; Calhorda, M. J.; Reinhold, J.; Reineri, F.; Bianchini, C.; Peruzzini, M.; Vizza, F.; Mealli, C. Activation of Molecular Hydrogen over a Binuclear Complex with Rh2S2 Core: DFT Calculations and NMR Mechanistic Studies. J. Am. Chem. Soc. 2004, 126, 11954−11965. (b) Bianchini, C.; Mealli, C.; Meli, A.; Sabat, M. Reversible double addition of hydrogen on a bis(μ-sulfido) binuclear rhodium complex. Inorg. Chem. 1986, 25, 4617−4618. (10) Sweeney, Z. K.; Polse, J. L.; Bergman, R. G.; Andersen, R. A. Dihydrogen Activation by Titanium Sulfide Complexes. Organometallics 1999, 18, 5502−5510. (11) Linck, R. C.; Pafford, R. J.; Rauchfuss, T. B. Heterolytic and Homolytic Activation of Dihydrogen at an Unusual Iridium (II) Sulfide. J. Am. Chem. Soc. 2001, 123, 8856−8857. (12) Zampella, G.; Fantucci, P.; De Gioia, L. DFT characterization of the reaction pathways for terminal- to μ-hydride isomerisation in synthetic models of the [FeFe]-hydrogenase active site. Chem. Commun. 2009, 46, 8824−8826. (13) (a) Jiménez, M. V.; Lahoz, F. J.; Lukesova, L.; Miranda, J. R.; Modrego, F. J.; Nguyen, D. H.; Oro, L. A.; Perez-Torrente, J. J. Hydride mobility in trinuclear sulfido clusters with the core [Rh3(μH)(μ3-S)2]: molecular models for hydrogen migration on metal sulfide hydrotreating catalysts. Chem. - Eur. J. 2011, 17, 8115−8128. (b) McGrady, J. E.; Gracia, J. Catalytic hydrogenolysis of alkyl halides by sulfido-bridged molybdenum clusters: A density functional study. J. Organomet. Chem. 2005, 690, 5206−5214. (14) Note that throughout the study a number of species featuring similar isomerism will appear. The same notation has been employed in all cases. (15) Attempts to obtain transition state geometries with unaltered Ir2S2 core structures were also attempted by performing scan calculations in which the coordinates of these four atoms were fixed. Nevertheless, SCF convergence problems precluded the optimization of geometries in which S and the three atoms to which is bound, that is, H, Ir, and Ir′, are coplanar (i.e., those with S−Ir−Ir′−H dihedral angles near zero).

All DFT calculations were run with Gaussian 09 (revision D.01).26 Gas phase geometry optimizations were carried out with an ultrafine integration grid (99 radial shells and 590 angular points per shell) and without any symmetry constraint. Based on the X-ray structures of 1 and 2, optimizations with a number of density functionals and basis set systems were initially carried out to determine the most adequate level of theory. The results of these, included in the Supporting Information, showed that dispersion-corrected functionals, together with the triple-ζ basis sets on the Ir, P, and S centers, are necessary to obtain structures in good agreement with the experimental data. Thus, the BP86-D3 functional,27 which includes Grimme’s D3 dispersion corrections, has been employed throughout the study. Ir, P, S, and H atoms were modeled using the Ahlrichs polarized basis set def2TZVP,28 whereas the Pople style 6-31G basis set was used for C atoms (see Supporting Information). All stationary points were characterized through analytical frequency calculations as either minima (all positive eigenvalues) or transition states (one negative eigenvalue). Intrinsic reaction coordinate (IRC) calculations and subsequent geometry optimizations were used to confirm the minima linked by each transition state. Frequency calculations also provided free energy values in the gas phase, computed at 298.15 K and 1 atm. Corrections for the effects of solvent (toluene, ε = 2.3741) were run at the same level of theory using the PCM approach.29 Unless otherwise stated, energies reported in the text correspond to gas phase free energies subsequently corrected for the solvent effects. S Supporting Information *

(PDF) The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/ acs.inorgchem.6b01888. Graphical representations of additional selected structures, results of the basis set and density functional tests based on the X-ray structures of 1 and 2, Cartesian coordinates of all computed structures, and associated thermodynamic data (PDF)

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AUTHOR INFORMATION

Corresponding Author

* E-mail: [email protected]. ORCID

Andrés G. Algarra: 0000-0002-5062-2858 Notes

The author declares no competing financial interest.

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ACKNOWLEDGMENTS Prof. Thomas B. Rauchfuss is acknowledged for helpful discussions. The anonymous referees are also acknowledged for valuable comments and suggestions. Financial support from the Ministerio de Economiá y Competitividad and FEDER funds from the European Union (Grants CTQ2015-65707-C22-P and CTQ2015-71470-REDT) are acknowledged. The University of Cadiz is acknowledged for the provision of computing resources.

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REFERENCES

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Computational Insights on the Mechanism of H2 Activation at Ir2S2

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