Aromatic Carbon–Carbon Bond Cleavage Using Tungsten Complexes

Aug 25, 2011 - The chemistry of the carbon-transition metal double and triple bond: Annual survey covering the year 2011. James W. Herndon. Coordinati...
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Aromatic Carbon Carbon Bond Cleavage Using Tungsten Complexes. A DFT Computational Study Gian Pietro Miscione,*,† Maria Angels Carvajal,‡ and Andrea Bottoni*,† † ‡

Dipartimento di Chimica “G. Ciamician”, Universita di Bologna, via Selmi 2, 40126 Bologna, Italy Departament de Quimica Fisica i Inorganica, Universitat Rovira i Virgili, Marcel.li Domingo, s/n-Campus Sescelades, 43007 Tarragona, Spain

bS Supporting Information ABSTRACT: The mechanism of aromatic C C bond cleavage in quinoxaline (QoxH) catalyzed by the tungsten complexes W(PMe3)4(η2-CH2PMe2)H has been theoretically investigated at the DFT level. The mononuclear species (η2(N,C)Qox)W(PMe3)4H (experimentally isolated and characterized) is directly involved in the formation of the final product [k2(C2)-C6H4(NC)2]W(PMe3)4, where the aromatic C C bond of quinoxaline has been broken. The mechanism requires a double insertion of the metal into two quinoxaline C H bonds, thus affording the “strained” benzyne-type species η2(C2)-C6H4(NCCN)W(PMe3)4H2. After expulsion of H2 the η2(C2) complex leads to the [k2(C2)-C6H4(NC)2]W(PMe3)4 product, thus relieving the structural strain of the small WCC cycle in η2(C2). Also, the theoretical investigation shows the existence of a stable (η4(C2N2)-QoxH)W(PMe3)3H2 complex (an unprecedented example of η4 coordination), in agreement with its experimental observation and characterization by X-ray diffraction.

1. INTRODUCTION Activation and cleavage of unreactive C C bonds is an area of great interest in modern organometallic chemistry and organic synthesis. Given the ubiquity of these bonds, discovering novel strategies for their cleavage can provide new important tools for organic synthesis and enable valuable transformations in the area of hydrocarbons and basic petroleum products.1 11 Two methods are currently available to break a C C bond. One is β-carbon elimination,11,12 where, given a M X C C(γ) system (X = O, N, C, etc.), the migration of the γ-carbon onto the metal results in the cleavage of the C C(γ) bond, the formation of a new M C bond, and the expulsion of an unsaturated CdX group. An alternative strategy consists in the oxidative addition of a metal to a C C bond.1 11 In this case a low-valent metal center directly inserts into C C with increase of the formal oxidation state of the metal and generation of a bis(organyl) metal complex. Unfortunately, the apparently simple oxidative addition of metals to C C bonds is a difficult process. C C single bonds are strong, and aromatic C C bonds are even stronger. Thus, the formation of two relatively weak metal carbon bonds (20 30 kcal mol 1) at the expense of a stable C C bond (about 90 kcal mol 1) is not a thermodynamically favored process. Furthermore, these bonds tend to re-form along easier reverse pathways (the reductive elimination of two carbon substituents bonded to a metal center is the reverse reaction of oxidative addition). Also, a higher kinetic barrier is expected for C C than for C H bonds: the former are less accessible than the latter and, when a metal approaches a hydrocarbon, it is much simpler to interact with the more exposed (and more numerous) C H bonds, r 2011 American Chemical Society

which, under given conditions, are more easily activated and broken than C C bonds.1 3,5 Thus, while the C H bond activation is a well-documented process in the literature,3,5,13 18 examples of effective synthetic strategies leading to metal insertion into C C bonds, in spite of their importance, are much less abundant. To facilitate the cleavage of C C bonds, specific reaction conditions (mainly aimed at increasing the energy of the starting material and/or lowering the energy of the resulting cleaved product) have been employed. For instance, strain relief in the product, starting from ring-strained molecules, can provide the driving force necessary to achieve C C bond activation and cleavage.1 5,7,8 In most of the examples of this type the ringstrain energy of three- or four-membered rings is relieved by the formation of more stable four- and five-membered metallacycles after direct insertion of a metal into a C C bond. Energy gain from aromatization in the product can represent another important driving force for activating C C bonds.3,19 Furthermore, examples are available in the literature where the activation of C C bonds is achieved by vicinal carbonyl groups or chelation assistance.3,5 However, in recent years, significant advances have characterized this research field and a number of examples of new strategies, which do not need particular conditions, have been reported.20 26 A remarkable example of C C bond activation that does not require any particular “assistance” has been recently Received: June 10, 2011 Published: August 25, 2011 4924

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Scheme 1

described by Sattler and Parkin.26 These authors found a way to cleave an aromatic C C bond in the quinoxaline molecule with the aid of a tungsten complex, which showed a significantly different behavior with respect to that previously observed for molybdenum complexes and quinoxaline.27 In addition to its academic interest, the process discovered for tungsten is particularly valuable, because nitrogen-containing aromatic systems such as quinoxaline are very common in nature: for instance, in fossil fuels. Thus, the work by Sattler and Parkin may inspire the development of novel strategies to be used not only in academic organic synthesis laboratories but also in industrial applications. The starting complex of the reaction reported by Sattler and Parkin is W(PMe3)4(η2-CH2PMe2)H. Experimental studies have demonstrated that this complex is significantly more stable than the corresponding oxidized isomer W(PMe3)5, from which

it forms rapidly by metalation of one methyl C H bond.13,28 This means that tungsten is initially bonded to seven ligands, which makes its coordination sphere rather crowded. The addition of quinoxaline (QoxH) affords two final products which have been characterized by the authors: i.e., (η4(C2N2)-QoxH)W(PMe3)3H2 and [k2(C2)-C6H4(NC)2]W(PMe3)4 (24% and 15% yields, respectively, after 18 h at 90 °C). Even if the dihydride η4(C2N2) complex is an unprecedented example of η4 coordination, the most significant product of this reaction is the k2(C2) complex resulting from the cleavage of one C C aromatic bond of the quinoxaline heterocyclic ring. A similar behavior has been observed when 6-methylquinoxaline (QoxMeH) and 6,7-dimethylquinoxaline (QoxMe2H) were used in place of QoxH. In their paper Sattler and Parkin have proposed a possible reaction mechanism which is schematically represented in 4925

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Figure 1. Schematic representation of the intermediate species found along the first stretch of path 1. Energy values (kcal mol 1) are relative to AL: i.e., the W(PMe3)4(η2-CH2PMe2)H complex (M0) and a noninteracting quinoxaline molecule. Values in brackets are corrected by solvent effects. Bond lengths are in angstroms.

Scheme 1 (in this scheme, based on the scheme reported in ref 26, the structures in parentheses are hypothesized by the authors, while the other ones correspond to isolated species). This mechanistic scheme involves a series of C H bond cleavage reactions instead of the more obvious direct tungsten insertion into the C C bond and entails the formation of a benzyne-type η2(C2)-C6H4(NCCN)W(PMe3)4H2 intermediate. A reductive elimination of H2 and C C bond cleavage would lead to the relevant product [k2(C2)-C6H4(NC)2]W(PMe3)4. The feasibility of this mechanistic hypothesis, as outlined by the authors, is mainly based on two factors: (a) the existence of a ruthenium benzyne-type η2-C6H6Ru(PMe3)4 complex reported by Hartwig and co-workers,29 which is structurally similar to the hypothesized η2(C2)-C6H4(NCCN)W(PMe3)4H2 species, and (b) the isolation and characterization by X-ray diffraction techniques of two intermediates, (k2(N,C)-Qox)W(PMe3)4H and (η2(N,C)Qox)W(PMe3)4H, which originate from the insertion of W into a C H bond of the QoxH molecule and demonstrate that C H cleavage is an easy process in these systems. Although experimental evidence tends to support the above mechanistic scheme, many aspects need to be elucidated. For instance, the mechanism of the initial coordination of QoxH to the metal is not obvious. In the starting complex, tungsten is not easily accessible to such a cumbersome ligand and its co-

ordination, which should lead to the two isomeric structures (k2(N,C)-Qox)W(PMe3)4H and (η2(N,C)-Qox)W(PMe3)4H, must be accompanied by the removal of one phosphine group and by a reductive elimination, which makes the η2-CH2PMe2 motif disappear. However, it has been demonstrated elsewhere that in transition-metal complexes the replacement of a phosphine by a different ligand usually occurs in two steps rather than in a concerted fashion.13 Thus, it is conceivable that W(PMe3)4(η2-CH2PMe2)H first loses one phosphine and then binds QoxH. In addition to this point, the timing of the reductive elimination is also not obvious: i.e., does it occur before or after the phosphine displacement? A further interesting point of the proposed mechanism concerns the two isomers detected by the authors (η2(N,C) and k2(N,C)), which are supposed to be in equilibrium with a third complex (k1(N)), where only one nitrogen atom of QoxH is bonded to the metal. Within the hypothetical mechanism of Scheme 1 the η2(N,C) isomer represents the key species leading to the final product. This hypothesis is reasonable but further evidence is needed to support it. An additional C H cleavage is necessary to yield the η2-(C2) benzyne-type intermediate from the η2(N,C) complex. The authors suggest that this transformation could involve a k1(C) isomeric species where only one W C bond connects Qox to the metal. However, also a direct conversion of η2(N,C) to the η2(C2) 4926

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Organometallics benzyne type could occur in principle and the experimental evidence does not help to clarify this point. Following Scheme 1 a new benzyne-type η2(C2) complex, structurally similar to the aforementioned ruthenium benzyne complex [(η2-C6H4)Ru(PMe3)4], should form after H2 elimination and, then, the cleavage of one aromatic C C bond in the strained threemembered cycle would lead to the final product. In addition to the open questions arising from this mechanistic scenario, it is not evident how the dominant product (η4-C2N2-Qox)W(PMe3)3H2 originates. The authors observe that, under the employed experimental conditions, this species does not convert into the more interesting product [k2(C2)-C6H4(NC)2]W(PMe3)4 and suggest that the hydrogen molecule, which is liberated after the formation of the second η2(C2) benzyne-type intermediate, could be the source of the two hydride ligands. Since the above mechanistic scenario is grounded on a small amount of experimental evidence that is not conclusive and the authors themselves mention the need of a computational study, we have decided to investigate in detail at the DFT level the potential energy surface of this process. In this paper we discuss the results of this investigation, where we have considered the mechanistic scheme proposed by Sattler and Parkin26 and other possible reaction pathways, such as direct insertion of the metal into the C C bond.

2. COMPUTATIONAL PROCEDURES All DFT computations have been carried out with the Gaussian0330 series of programs using the nonlocal hybrid Becke three-parameter exchange functional B3LYP.31 For all atoms except the metal we have used the DZVP basis, which is a local spin density (LSD)-optimized basis set of double-ζ quality.32 Tungsten has been described by the energyadjusted pseudopotential basis set proposed by Preuss and co-workers33 (denoted as sdd pseudopotentials in the Gaussian03 formalism). We have performed frequency computations to determine the nature of the various critical points. In most cases, the shape of the transition vector provided by the frequency computations clearly indicates which intermediates a given transition state is connected to. In some cases, where the connection between the reaction intermediates was less obvious, IRC computations have been carried out. Furthermore, to roughly evaluate the solvent effects and the reliability of the gas-phase model, the geometry and energy of all critical points have been recomputed with the polarized continuous model method available in Gaussian03 (PCM computations)34, starting from the gas-phase optimized structures: the dielectric constant ε = 2.247 emulating benzene (the solvent used in the experiments) has been chosen. The model system used throughout the paper is formed by the W(PMe3)4(η2-CH2PMe2)H complex reacting with the QoxH molecule. To establish the effect of the methyl substituents on structure and energetics, some critical points have been recomputed using QoxMe2H in place of QoxH.

3. RESULTS AND DISCUSSION A. Breaking the C C Aromatic Bond: The Road To Form the [j2(C2)-C6H4(NC)2]W(PMe3)4 Species. We discuss here all

the possible reaction paths leading to the formation of the [k2(C2)-C6H4(NC)2]W(PMe3)4 complex. The structures of the intermediates located along the various reaction pathways, those of the most important and interesting transition states and a schematic representation of the singlet potential energy surface are given in Figures 1 7.

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As outlined in the Introduction, the mechanism proposed by Sattler and Parkin (Scheme 1)26 involves the formation of a η2(N,C) complex, which originates from the insertion of W into a C H bond of a quinoxaline molecule. This complex was actually isolated by these authors and its structure characterized by X-ray diffraction techniques. We started the computational investigation of the reaction surface by computing the structure of this adduct, the reaction pathways leading to its formation from the initial complex W(PMe3)4(η2-CH2PMe2)H and a noninteracting quinoxaline (QoxH) molecule (asymptotic limit: AL), and then the route to the [k2(C2)-C6H4(NC)2]W(PMe3)4 species. The transformation affording the η2(N,C) complex involves several reaction steps and different species, which are schematically represented in Figure 1. For each species we have reported the energy relative to the starting reactants (AL) and the most significant geometrical parameters and, for each step, the related activation barrier (the Cartesian coordinates of the structures of all intermediates and transition states, together with their absolute and relative energies, are reported in the Supporting Information). The first step, which requires a high activation barrier of 29.9 kcal mol 1 to be overcome, is a sort of Berry pseudorotation where the positions of the equatorial hydrogen and one equatorial phosphine in W(PMe3)4(η2-CH2PMe2)H are interchanged. In the new complex M1, which is 11.0 kcal mol 1 higher than AL, the hydrogen atom is adjacent to the methylene unit and this makes its transfer to the carbon atom easier in the subsequent step (Ea = 11.2 kcal mol 1), thus affording a new trimethylphosphine group. The resulting complex W(PMe3)5 (M2) has a trigonal-bipyramidal structure and is almost degenerate with M1. In the third step the nitrogen atom of the quinoxaline molecule coordinates the metal atom (Ea = 5.5 kcal mol 1), first forcing the complex to an octahedral structure (M3) and, then (fourth step), back again to the trigonal form M4 (degenerate to the starting reactants AL), after expulsion of a phosphine unit; the activation energy Ea required to form this k1(N) species is 7.7 kcal mol 1. A η2(N,C) complex (M5) forms from M4 after overcoming a barrier of 14.8 kcal mol 1. The corresponding transition state TS5, describing the insertion of the metal into the C H bond ortho to nitrogen, is represented in Figure 2A: the hydrogen moving from carbon to tungsten and the newly forming W C bond (2.12 Å) are evidenced. The new complex M5 is 9.4 kcal mol 1 more stable than the reactants and, by overcoming a further barrier of 21.1 kcal mol 1, can isomerize to an even more stable η2(N,C) complex, M6 (11.4 kcal mol 1 below AL), where the hydrogen is anti to the Qox ring. M6 corresponds to the mononuclear species (η2(N,C)-Qox)W(PMe3)4H. The stability of this species, revealed by our computations, explains the fact that the analogous species obtained from dimethylquinoxaline (i.e., (η2-N,C-QoxMe2)W(PMe3)4H) was observed during the course of the reaction and characterized by Sattler and Parkin.26 We have carried out additional computations on the (η2(N,C)-QoxMe2)W(PMe3)4H complex to evaluate the effect of the methyl groups on the structure and stability of M6. As expected, the effect is very small, the (η2(N,C)QoxMe2)W(PMe3)4H) species being 10.3.kcal mol 1 below the corresponding AL. The authors properly point out that the experimental observation of complexes such as M6 ((η2(N,C)-Qox)W(PMe3)4H or (η2-(N,C)-QoxMe2)W(PMe3)4H)) only provides evidence that the C H bond can be easily cleaved in these systems, but its presence cannot be considered a proof that “this species is directly involved in the C C bond cleavage process”, as outlined 4927

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Figure 2. Schematic representation of some relevant transition structures located on the reaction surface. Energy values (kcal mol 1) are relative to AL. Values in brackets are corrected by solvent effects. Bond lengths are in angstroms and angles in degrees.

in ref 26. To elucidate this point, we have investigated its possible connection to the most significant product of the reaction: i.e., the [k2(C2)-C6H4(NC)2]W(PMe3)4 species resulting from the breaking of the C C aromatic bond. We have demonstrated the existence of a reaction pathway leading to k2(C2), and we have reported a schematic representation of the structures located along it in Figure 3. Our computations show that, by overcoming an energy barrier of 13.9 kcal mol 1, M6 can transform to the k1(C) complex (M7, 1.6 kcal mol 1 below AL), where only one carbon atom of the Qox ring is bonded to the metal. A further insertion of the metal into one quinoxaline C H bond requires an energy barrier of 17.1 kcal mol 1. The corresponding transition state TS8 is depicted in Figure 2B: the new W C and W H bonds appear already formed (1.75 and 2.14 Å, respectively),

even if the hydrogen still strongly interacts with the carbon atom (H 3 3 3 C distance 1.67 Å). The insertion affords a η2(C2) complex (M8) which is 16.9 kcal mol 1 higher that AL and where two hydrogen atoms are now bonded to tungsten. M8 can easily isomerize (a low activation barrier of 3.5 kcal mol 1 is required: TS9) to a new more stable η2(C2) complex (M9, only 1.7 kcal mol 1 above AL), where the two W-bonded hydrogen atoms are now opposite to the Qox ring. The two W H bond lengths are 1.75 and 1.76 Å, and the H 3 3 3 H distance is 1.85 Å; thus, the two hydrogen atoms are weakly interacting in M9 and anticipate the formation of a hydrogen molecule. In the subsequent step a new high-energy η2(C2) complex (M10, 33.7 kcal mol 1 above AL) forms after expulsion of the incipient hydrogen molecule from the metal coordination sphere. The expulsion is 4928

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Figure 3. Schematic representation of the intermediate species found along the second stretch of path 1 connecting (η2(N,C)-Qox)W(PMe3)4H (M6) to the product complex [k2(C2)-C6H4(NC)2]W(PMe3)4 (M11). Energy values (kcal mol 1) are relative to AL. Values in brackets are corrected by solvent effects. Bond lengths are in angstroms and angles in degrees.

characterized by a large activation barrier (32.1 kcal mol 1), and the corresponding transition state TS10, where a hydrogen molecule is weakly interacting with the metal (W H distances are approximately 4 Å), is shown in Figure 2C. The high energy of M10 is not surprising, because on passing from M9 to M10, the strain energy of the three-membered ring is approximately unchanged but two W H bonds are broken (the W H bond strength is in the range 55 65 kcal mol 1)35,36 and the incipient H H bond of the leaving hydrogen molecules becomes completed. It is worth outlining that M10 is the highest energy intermediate along the whole reaction pathway that must be surmounted to reach the product region. In the final step, the C C aromatic bond easily breaks (the activation barrier—see transition state TS11 in Figure 2D—is only 0.8 kcal mol 1) to form the final [k2(C2)-C6H4(NC)2]W(PMe3)4 species (M11), only 3.3 kcal mol 1 above AL. The low activation barrier and the stability of the product are easily explained by the relief of strain energy associated with the breaking of the three-membered WCC ring. The geometry at nitrogen is significantly bent, as outlined by Sattler and Parkin.26 The computed CNC angle is 128.7°, which compares very well with the experimental values determined for the isolated k2(C2) complexes that are in the range 125 128°. The computed W C distance (1.98 Å) also agrees satisfactorily with the experimental value of 1.93 Å, thus providing further evidence for the multiple character of the W C bond.

The whole path leading from the starting reactants M0 to the final product complex M11 is schematically represented in Figure 4 (path 1), where the black circles correspond to the reaction intermediates located in the computational investigation of the potential energy surface. In the same figure we have shown the existence of other possible pathways (paths 2 and 3) leading to the final k2(C2) complex. For the various reaction channels, to highlight the bottlenecks encountered in the course of the reaction, we have indicated (in brackets) the highest energy transition structures (the energy values are relative to AL). The structures of the intermediates found along paths 2 and 3 are schematically represented in Figures 5 and 6, respectively. Path 2 is a side reaction channel (M0 f M12 f M13 f M5) that allows the experimentally observed complex η2(N,C) to be reached. However, this alternative path is less favored, since it requires surmounting a rather high transition state (TS14, 40.1 kcal mol 1 above AL, Ea = 33.9 kcal mol 1).37 Alternatively, a “gate” on the surface connects M13 to M4, but even in this case the transition state is rather high in energy (TS15, 36.6 kcal mol 1 above AL, Ea = 30.4 kcal mol 1). Path 3 (M0 f M1 f M2 f M14 f M15 f M16 f M8) can represent a valid alternative reaction path. In this case we apparently avoid the deep “hole” of the surface corresponding to the experimentally observed intermediate M6 (η2(N,C) complex). However, this is not in contrast with the experimental 4929

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Organometallics evidence, since the stable M6 species can easily form from M8 (see Figure 4), by overcoming two low activation barriers (1.8 kcal mol 1 for M8 f M7 and 0.9 kcal mol 1 for M7 f

Figure 4. Two-dimensional schematic representation showing all possible reaction pathways leading from the starting reactants (i.e., W(PMe3)4(η2-CH2PMe2)H and quinoxaline) to the final [k2(C2)C6H4(NC)2]W(PMe3)4 species. The blue and black arrows identify the two preferred paths: path 1 (Figures 1 and 3) and path 3 (Figure 6), respectively, while the dashed arrows indicate the less favored path (path 2, Figure 5). Energy values (kcal mol 1) are relative to AL (M0). Values in brackets are corrected by solvent effects.

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M6). An interesting aspect of path 3 (which shares the first three critical points M0, M1, and M2 with path 1) concerns the approach of the quinoxaline to the metal. The first interaction occurs between tungsten and a ring hydrogen and leads to the intermediate M14: this interaction is rather strong (the W...H distance is 1.90 Å) and causes a significant lengthening of the C H bond, which becomes 1.18 Å. Insertion of W into the C H bond gives the k1(C) intermediate M15 (Ea = 6.6 kcal mol 1). The corresponding transition state (TS17) is shown in Figure 2E. The easy expulsion of a phosphine (Ea = 4.1 kcal mol 1) affords the M16 intermediate, where a significant interaction between a ring hydrogen and the metal (W 3 3 3 H distance 2.57 Å) is evident. This interaction finally develops toward the insertion of the metal into the C H bond, thus forming the η2(C2) species (M8) previously discussed (Ea = 19.0 kcal mol 1). A representation of the transition structure for W insertion into C H (TS21) is given in Figure 2F. In the figure, in addition to the migrating hydrogen, the newly forming W C bond (2.18 Å) is evidenced. Furthermore, a side path connecting M14 to M16 through the formation of the alternative complex M17 has been revealed by our computations. This complex has interesting structural features: two hydrogen atoms bonded to adjacent ring carbons simultaneously interact with the metal, as shown in Figure 6. The interaction is rather strong (the two W 3 3 3 H distances are 1.93 and 1.96 Å) and causes a weakening of the two C H bonds, the C H bond lengths being 1.15 and 1.14 Å, respectively. However, this side path is less favored, since a high-energy transition state is involved: in particular, an activation energy of 13.8 kcal mol 1 (TS19) is required for the transformation M14 f M17. B. The (j2(N,C)-Qox)W(PMe3)4H Species: An Isomer of (η2(N,C)-Qox)W(PMe3)4H. In addition to (η2(N,C)-Qox)W(PMe3)4H (M6 in Figure 1), another mononuclear isomeric species, i.e. (k2(N,C)-Qox)W(PMe3)4H, has been experimentally observed. More precisely, our computations have shown the existence of two distinct k2(N,C) complexes (M18 and M19 in Figure 7A), which are isomers differing in the orientation of the equatorial hydrogen atom: in one case (M18) the H atom is adjacent to the equatorially bonded Qox and in the other case (M19) it is placed on the opposite side. M18, which can form rather easily from M4 by overcoming a barrier of 13.1 kcal mol 1, is 1.7 kcal mol 1 below AL. The transition state leading to its

Figure 5. Schematic representation of the intermediate species found along path 2. Energy values (kcal mol 1) are relative to AL. Bond lengths are in angstroms. Values in brackets are corrected by solvent effects. 4930

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Figure 6. Schematic representation of the intermediate species found along path 3. Energy values (kcal mol 1) are relative to AL. Bond lengths are in angstroms.

formation (TS22) is depicted in Figure 2G. The isomer M19 is significantly more stable (7.6 kcal mol 1 below AL) and its energy is close to that of the previously discussed species (η2(N,C)-Qox)W(PMe3)4H (M6, 11.4 kcal mol 1 lower than AL). The calculated energy values of these two mononuclear species (M6 and M19) are in agreement with the fact that they have been both isolated and characterized. C. Formation of the (η4(C2N2)-QoxH)W(PMe3)3H2 Complex. As outlined in the Introduction, an additional species has been isolated in significant amount (24% yield) in the final product mixture. Even if this species does not convert to the more interesting [k2(C2)-C6H4(NC)2]W(PMe3)4 product under the reaction conditions, it represents a peculiar example of η4 binding of an aromatic six-membered ring to a metal. To assess the intrinsic coherence of our computations, we have determined the structure of this η4 complex, which is schematically represented in Figure 7B (M24). As found by Sattler and Parkin, who have determined its structure by X-ray diffraction,26 the two W H bonds lie in a plane which is orthogonal to the QoxH plane and on the opposite side with respect to quinoxaline. Interestingly, this species is rather stable (21.4 kcal mol 1

below AL), which explains its significant amount in the final products. Since the η4 complex isolated by Sattler and Parkin26 was obtained using 6,7-dimethylquinoxaline, we have recomputed the structure of M24 with the quinoxaline ring bearing two methyl groups. As expected, the effect of methyl substituents is very small and the new structure turns out to be 20.5 kcal mol 1 below the corresponding AL. Also, only negligible geometry changes have been detected (see the Supporting Information). This finding demonstrates once again that the replacement of the two methyl groups with two hydrogen atoms represents a satisfactory approximation. Furthermore, we have explored the potential surface to determine the reaction pathway leading to the η4 species. We have found that complex M2 can easily bind a hydrogen molecule (the activation energy Ea for this process is only 0.9 kcal mol 1), affording a new stable species (M20) which is 12.8 kcal mol 1 below the AL. The expulsion of one phosphine group affords a less stable complex (M21) which is 2.1 kcal mol 1 above AL. M21 can eliminate a further phosphine and form M22, where only three phosphine groups and two hydrogen atoms are 4931

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Figure 7. Schematic representations of (A) the two isomers found for the intermediate species (k2(N,C)-Qox)W(PMe3)4H and (B) the intermediate species located along the path leading to the (η4-C2N2-Qox)W(PMe3)3H2 complex. Energy values (kcal mol 1) are relative to AL. Values in brackets are corrected by solvent effects. Bond lengths are in angstroms and angles in degrees.

bonded to the metal. There is now enough space around tungsten to bind a quinoxaline molecule after overcoming a barrier of 8.9 kcal mol 1. In the resulting complex M23, which is rather stable (17.0 kcal mol 1 below the AL), QoxH is coordinated to the metal in a η2 manner, involving a nitrogen atom and the adjacent carbon. A barrier of 7.3 kcal mol 1 is required for the final rearrangement leading to the η4 species. D. The Solvent Effect: Geometry and Energetics. The structure and energy of all critical points located on the reaction surface have been recomputed in the presence of solvent effects, and the complete set of data (energy values and Cartesian coordinates) is reported in the Supporting Information. To illustrate the results of our PCM computations, we have restricted our discussion to the intermediates that have been experimentally isolated and characterized: i.e., the starting complex W(PMe3)4(η2-CH2PMe2)H (M0 in Figure 1), the mononuclear species (η2(N,C)-Qox)W(PMe3)4H (M6 in Figure 1), the product [k2(C2)-C6H4(NC)2]W(PMe3)4 (M11 in Figure 3), the (k2(N,C)-Qox)W(PMe3)4H species (M19 in Figure 7A), and

the (η4(C2N2)-QoxH)W(PMe3)3H2 complex (M24 in Figure 7B). For these points we have reported in brackets the values of the optimized parameters obtained in the presence of solvent effects and the corresponding energy. Furthermore, in Figure 4 for the highest energy transition states the corresponding energy relative to the AL and the energy barrier are given in both the gas phase and solvent. Inspection of the PCM results clearly indicates that the solvent effects on the structural parameters and relative energies of the various critical points (and, of course, on activation barriers) are almost negligible. The same negligible effect of solvent has been found for all remaining critical points (see Table S1 in the Supporting Information). This finding clearly indicates that the gas-phase model is adequate to describe the reactivity of these organometallic species.

4. CONCLUSIONS In this paper we have carried out a computational DFT investigation of the mechanism of the aromatic C C bond 4932

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Organometallics cleavage in quinoxaline catalyzed by tungsten complexes W(PMe3)4(η2-CH2PMe2)H. Our results confirms on the whole the mechanistic hypothesis of Sattler and Parkin26 and can be summarized as follows. (i) We have demonstrated the existence of the (η2(N,C)-Qox)W(PMe3)4H complex, originating from the insertion of W into a C H bond of the quinoxaline molecule. This complex is rather stable (11.4 kcal mol 1 lower than reactants) and corresponds to the analogous mononuclear species obtained from 6,7-dimethylquinoxaline (i.e., (η2(N,C)-QoxMe2)W(PMe3)4H) which was isolated and characterized by Sattler and Parkin.26 The reaction pathway (path 1) leading to η2(N,C) is a sequence of several steps and involves a k1(N)-type intermediate, as suggested by Sattler and Parkin in their hypothetical mechanistic scheme26 (see Scheme 1). (ii) We have demonstrated that the above η2(N,C) complex is directly involved in the formation of the final product [k2(C2)-C6H4(NC)2]W(PMe3)4, where the aromatic C C bond has been broken. To accomplish the C C cleavage, the η2(N,C) intermediate undergoes a second insertion of the metal into a C H bond, affording first a η2(C2) complex and, then, the k2(C2) product, after expulsion of an H2 molecule from the metal coordination sphere. The η2(C2) complex M10, characterized by the presence of the strained three-membered cycle WCC, represents the highest energy intermediate along the whole pathway: it lies 33.7 kcal mol 1 above the starting reactants (AL). (iii) Two additional reaction paths (path 2 and path 3) can lead to the final k2(C2) product. Path 2 connects reactants to the experimentally observed complex (η2(N,C)-Qox)W(PMe3)4H but is less favored than path 1, since it requires overcoming the high-energy transition state TS14 (40.1 kcal mol 1 above the AL) or, alternatively, transition state TS15 (36.6 kcal mol 1 above AL). Path 3, a side channel which stems from M2, can be competitive with respect to path 1 because the involved transition states have similar energies. This path is characterized by an unusual approach of the quinoxaline molecule to the metal. We have observed a strong interaction between the metal and a ring hydrogen, followed by insertion of W into the C H bond. After expulsion of one PMe3 ligand and the insertion of the metal into a second C H bond, this pathway leads to the [k2(C2)C6H4(NC)2]W(PMe3)4 product but again involves the formation of the “strained” η2(C2) (M10) species. (iv) The “trick” of the synthesis proposed by Sattler and Parkin seems to lie in its capacity of creating a strained intermediate: i.e., the high-energy η2(C2) complex characterized by the three-membered WCC cycle. To relieve this structural strain, it becomes advantageous to the system to break the aromatic C C bond and form a larger unstrained seven-membered cycle. (v) Our computations have shown the existence on the surface of the (k2(N,C)-Qox)W(PMe3)4H and (η4(C2N2)Qox)W(PMe3)3H2 complexes, which have been experimentally observed and characterized by X-ray diffraction. These complexes are rather stable (7.6 and 21.4 kcal mol 1 below the AL), which explains why they could be isolated in significant amounts. The reaction path leading to the η4(C2N2) species originates from

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the W(PMe3)5 trigonal-bipyramidal complex M2 and requires first the coordination of an hydrogen molecule and, then, the expulsion of two phosphine ligands. (vi) We have not found any reaction paths leading to the [k2(C2)-C6H4(NC)2]W(PMe3)4 product via a direct insertion of the metal into the aromatic C C bond. (vii) A comparison between gas-phase and PCM (implicit solvent) computations clearly indicates that the solvent effects on structures and relative energies are in all cases almost negligible and demonstrates the reliability of the gas-phase model.

’ ASSOCIATED CONTENT

bS

Supporting Information. Tables giving energy values and Cartesian coordinates. This material is available free of charge via the Internet at http://pubs.acs.org.

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