Article pubs.acs.org/Organometallics
Insights into the Intermolecular C−H Activations of Hydrocarbons Initiated by Cp*W(NO)(η3‑allyl)(CH2CMe3) Complexes Guillaume P. Lefèvre, Rhett A. Baillie, Diana Fabulyak, and Peter Legzdins* Department of Chemistry, The University of British Columbia, Vancouver, British Columbia, Canada V6T 1Z1 S Supporting Information *
ABSTRACT: Thermolyses of 18e Cp*W(NO)(η3-allyl)(CH2CMe3) compounds (Cp* = η5-C5Me5) result in the intramolecular elimination of CMe4 and the formation of 16e η2-diene and/or η2-allene intermediate complexes that effect a variety of intermolecular C−H activations of hydrocarbons. The outcomes of the reactions of the Cp*W(NO)(η3allyl)(CH2CMe3) compounds with both C(sp3)−H and C(sp2)−H bonds of hydrocarbons are dependent on the natures of the allyl ligands in ways that are not immediately obvious. In an effort to better understand the different selectivities of the various C−H activation processes, we have examined several of these transformations by DFT calculations. The results of these computational investigations have provided several interesting and useful insights into the mechanistic pathways involved. Specifically, they have established that geminal dialkyl substituents on the allyl ligands markedly stabilize the η2-allene intermediate complexes, whereas the absence of such substituents favors the formation of the η2-diene complexes. In the case of the analogous molybdenum systems, the η2-diene intermediate complexes undergo rapid isomerization to the η4-diene complexes and do not effect intermolecular C−H activations. In some instances involving the tungsten complexes, the initially formed η1-hydrocarbyl product (which may or may not be isolable) isomerizes by intramolecular exchange of the newly formed hydrocarbyl ligand with a hydrogen atom on the allyl ligand or undergoes additional C−H activations and is converted to a new hydrido allyl compound. DFT methods indicate that a plausible mechanism for the latter transformation involves a β-hydrogen abstraction from the lateral alkyl chain by the allyl ligand. The rate-determining step of this process is thus the formation of a 16e η2-olefin complex with the olefin originating from the alkyl chain, and this process should be favored by relatively electron-rich Cp*W(NO)(η3-allyl)(n-alkyl) complexes, as is experimentally observed. In all cases of benzene C(sp2)−H activations by the tungsten systems, the η2-allene intermediate complexes exhibit better reactivity than the η2-diene intermediates. However, theoretical considerations indicate that the stereochemical properties of the first-formed Cp*W(NO)(η3-allyl)(Ph) products determine their differing thermal stabilities. If the aryl−allyl coupling product, Cp*W(NO)(η2-allyl-Ph), contains an activatable C−H bond close to the tungsten center, then the thermodynamically favored intramolecular exchange of the phenyl ligand with a hydrogen atom on the allyl ligand occurs. Otherwise, it does not, and the Cp*W(NO)(η3-allyl)(Ph) complexes persist.
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(2) The initially formed η1-hydrocarbyl complex (which may or may not be detectable) isomerizes by intramolecular exchange of the newly formed hydrocarbyl ligand with a hydrogen atom on the allyl ligand. (3) The initially formed η1-hydrocarbyl complex undergoes additional C−H activations and is converted to a new hydrido allyl compound. The outcomes of the reactions of the Cp*W(NO)(η3-allyl)(CH2CMe3) compounds with both C(sp3)−H and C(sp2)−H bonds of hydrocarbons are dependent on the nature of the allyl ligands in ways that are not immediately obvious. In an effort to better understand the different selectivities of the various C−H activation processes, we have examined several of these
INTRODUCTION In recent years we have been exploring the C−H activation chemistry of the family of 18e Cp*W(NO)(η3-allyl)(alkyl) compounds (Cp* = η5-C5Me5). Particularly interesting members of this family are those that contain neopentyl (CH2CMe3) ligands since they eliminate CMe4 intramolecularly when thermolyzed at different temperatures and form 16e intermediate species.1 One type of intermediate is an η2-diene complex, and the other is an η2-allene complex; both effect a variety of C−H bond activations, some of which are unique to these complexes. The intermolecular C−H activation chemistry of these complexes that has been established to date involves the three distinct types of chemical transformations that are summarized in Scheme 1.1 The three types of conversions are as follows: (1) Selective single activation of a terminal C(sp3)−H bond of the hydrocarbon substrate produces an isolable η1hydrocarbyl complex. © 2013 American Chemical Society
Received: August 15, 2013 Published: October 3, 2013 5561
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and/or an η2-allene complex.2−4 However, if both of these intermediates are formed simultaneously, their relative ratio is markedly dependent on the nature of the allyl ligand. Two representative allyl complexes have been investigated in this regard by DFT methods, namely, Cp*W(NO)(η 3 CH2CHCHMe)(CH2CMe3) (1), which upon thermolysis forms exclusively an η2-diene complex, and Cp*W(NO)(η3CH2CHCMe2)(CH2CMe3) (2), which affords both 16e η2allene and η2-diene complexes. The two pathways subjected to computational analysis are illustrated in Scheme 2, which depicts the terminal C(sp3)−H activation of n-pentane by 1 and 2 involving (a) η2-allene intermediate complexes and (b) η2-diene intermediate complexes. The computed free energies (in kJ mol−1) of the various species shown in Scheme 2 are collected in Table 1. It may be noted at this point that the results of these more realistic calculations resemble qualitatively those computed previously for the reactions of the model intermediate complexes CpW(NO)(η 2 -CH 2 CHCHCH 2 ) and CpW(NO)(η 2 -CH2 C CHMe) with propane.4,5 The relatively low activation barrier for the formation of the allene complex from the dimethylallyl precursor 2 (ΔG = 147.8 kJ mol−1, Table 1) is evidently due to the well-known strong stabilization of allenyl systems by geminal dialkyl substituents.6 This fact explains why the 18e PMe3 adduct of the allene complex (2a) is obtained as the major product when 2 is thermolyzed in
Scheme 1
transformations by DFT calculations, and in this article we present the results of these investigations that have provided some interesting and useful insights into the mechanistic pathways involved.
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RESULTS AND DISCUSSION C(sp3)−H Activations Initiated by Cp*W(NO)(η3-allyl)(CH2CMe3) Complexes. As noted in the Introduction, the first step during the activation of alkane C(sp3)−H bonds by 18e Cp*W(NO)(η3-allyl)(CH2CMe3) complexes involves the formation of two possible 16e transient species, namely, an η2-diene Scheme 2
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Table 1. Computed Free Energies (in kJ mol−1) of the Various Species Shown in Scheme 2 allene pathway 1-methylallyl precursor 1,2-dimethylallyl precursor diene pathway 1-methylallyl precursor 1,2-dimethylallyl precursor
Cp*W(NO)(η3-allyl) (CH2CMe3)
TS
σ-C−H 18e complex
16e allene complex
TS′
Cp*W(NO)(η3allyl)(C5H11)
0.0 (1) 0.0 (2)
199.1 (TS1a) 147.8 (TS2a)
77.3 (1a′) 62.3 (2a′)
34.1 (1a) 19.4 (2a)
84.7 (TS2a′)
−16.4 (2′)
Cp*W(NO)(η3-allyl) (CH2CMe3)
TS
σ-C−H 18e complex
16e diene complex
TS′
Cp*W(NO)(η3allyl)(C5H11)
0.0 (1) 0.0 (2)
145.1 (TS1b) 168.3 (TS2b)
49.3 (1b′) 39.3 (2b′)
17.5 (1b) 18.8 (2b)
162.2 (TS1b′) 153.9 (TS2b′)
−38.2 (1′) −16.4 (2′)
hydrocarbon solutions in the presence of PMe3.3 In contrast, the lack of stabilization of the intermediate allene complex obtained from the 1-methylallyl precursor (1) (i.e., complex 1a, Table 1) leads to a high activation barrier (199.1 kJ mol−1), which makes the formation of 1a unlikely. Consequently, the role of the allene complex, 1a, in n-pentane C−H activation has not been investigated further. Indeed, only evidence for the diene intermediate (1b) during such activations has been obtained experimentally.2 Consistently, the diene complex, 1b, requires a computed activation barrier of 145.1 kJ mol−1. This barrier can be compared with the formation of the η2-2-methylbutadiene complex, 2b, which requires an activation barrier of 168.3 kJ mol−1, which is probably a manifestation of the greater steric hindrance in 2, which makes the approach of the neopentyl group more difficult (i.e., TS2b is higher in energy than TS2a). Furthermore, it should be noted that the neopentane-releasing step from the σ-complexes 1a′, 1b′, 2a′, and 2b′ that leads to the 16e allene and diene complexes 1a, 1b, 2a, and 2b is in all cases strongly exothermic. In other words, the formation of the latter 16e reactive species is under kinetic control because of this irreversible expulsion of neopentane. Interestingly, the 16e allene intermediate, 2a, exhibits a reactivity that is superior to that exhibited by the isomeric 16e diene complex, 2b, during the n-pentane C(sp3)−H activation step (computed activation barrier for 2a is 65.3 kJ mol−1, whereas that for 2b is 135.1 kJ mol−1). Furthermore, the computed n-pentane activation barrier for the η2-butadiene complex, 1b, is in the same range as that computed for 2b (144.7 kJ mol−1). The overall process leading to the n-pentyl complexes 1′ and 2′ is thermodynamically favored. Kinetic and Thermodynamic Factors that Affect the Reactivity of the η2-Diene Intermediate Complexes. The thermal chemistry of a molybdenum analogue of 1, namely, Cp*Mo(NO)(η3-CH2CHCHMe)(CH2SiMe3), has been previously investigated.5 A computational analysis using Cp*Mo(NO)(η3-CH2CHCHMe)(CH2CMe3) as a model indicates that, similar to 1, the molybdenum complex rapidly forms the η2-diene intermediate, 1b,Mo, at room temperature in n-pentane with a comparable activation barrier (Scheme 3). The computed barrier for the formation of the diene complex is 122.9 kJ mol−1 for Cp*Mo(NO)(η3-CH2CHCHMe)(CH2CMe3) versus 145.1 kJ mol−1 for Cp*W(NO)(η3-CH2CHCHMe)(CH2CMe3) (Table 1). However, the molybdenum η2-diene complex undergoes rapid isomerization to the η4-diene complex, Cp*Mo(NO)(η4-CH2CHCHCH2), and does not effect the C(sp3)−H activation of n-pentane (Scheme 3).5 In contrast, the tungsten η2-diene intermediate complex, Cp*W(NO)(η2-CH2CHCHCH2), persists long enough to effect terminal C(sp3)−H activation of n-pentane and does not form the 18e isomer, Cp*W(NO)(η4-CH2CHCHCH2). Computational investigations also indicate that isomerization of Cp*M(NO)(η 2 -CH 2 CHCHCH 2 ) to Cp*M(NO)(η 4 CH2CHCHCH2) (M = Mo or W) involves a high rotational
Scheme 3
barrier of the diene ligand. However, this rotational barrier is significantly higher than the C−H activation barrier in the case of Cp*W(NO)(η2-CH2CHCHCH2), whereas the opposite is true for Cp*Mo(NO)(η 2-CH2CHCHCH2) (Scheme 4), thus providing a rationale as to why these two isolobal complexes exhibit such different reactivities. An explanation of these differences can be obtained by analyzing the molecular-orbital overlaps involved in the M−η2CC linkages of the Cp*M(NO)(η2-CH2CHCHCH2) (M = Mo, W) complexes (i.e., the bonding and back-bonding electron densities of the bound CC ligands). The key step of the η2 → η4 hapticity shift is that the η2-diene starting complex has to undergo a partial decomplexation of the CC double bond to lead to the transition state depicted in Scheme 4. The relevant molecular orbitals of the starting η2-diene complexes are displayed in Figure 1a, while Figure 1b and c display Walsh correlation diagrams between the molecular orbitals of the starting complexes and the corresponding transition states. In both parts b and c of Figure 1, Ψ2,M is the HOMO and Ψ1,M is the HOMO−3 of complexes 1b and 1b,Mo. HOMO−1 and HOMO−2 correspond to orbitals located only on the metal centers, and they are not involved in M−η2-CC bonding. Consequently, the energies of these latter molecular orbitals remain essentially constant during the η2 → η4 isomerization process. In both the W and Mo cases, decomplexation of the CC bond from the metal is not favored, as indicated by an increase in energy of the two molecular orbitals involved in the metal−CC bonding. Furthermore, the back-bonding stabilization lost by decomplexation of the CC double bond from the metal is higher for the tungsten complex (0.69 eV, Figure 1b) than for the molybdenum analogue (0.63 eV, Figure 1c), a feature that is consistent with the better electron-donating properties of the thirdrow transition metal as compared to the second-row metal. Less important are the changes in the energies of the bonding interactions during the isomerization process (see the fate of the orbitals Ψ1,W and Ψ1,Mo in Figure 1b and c). Experimental results consistent with the theoretical calculations have been observed during the preparation of the 2,2dimethyl analogue, Cp*Mo(NO)(η 3 -CH 2 CHCMe 2 )(CH2CMe3), from the dichloride precursor Cp*Mo(NO)(Cl)2 5563
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Scheme 4
Figure 1. Molecular-orbital overlaps involved in the M−η2-CC linkages of complexes 1b and 1b,Mo.
Scheme 5
In conclusion, it is evidently the high affinity between tungsten and the butadiene CC bond in the η2-diene complexes that is at the origin of the strong kinetic barrier inhibiting their conversion to the η4 isomer.7 Selectivity of the C(sp3)−H Activations. Our previous experimental investigations have shown that the fate of the initial Cp*W(NO)(η3-allyl)(n-alkyl) complex obtained by the thermolysis of Cp*W(NO)(η3-allyl)(CH2CMe3) in the corresponding
(Scheme 5): the former spontaneously loses neopentane to afford the transient η 2 -diene complex, Cp*Mo(NO)(η2-CH2CHCMeCH2), which finally converts to the 18-electron η4-isomer, Cp*Mo(NO)(η4-CH2CHCMeCH2) (η4-2b,Mo). The two resulting η4-isomers have been characterized in solution by 1H and 13C NMR spectroscopies and in the solid state by a single-crystal X-ray diffraction analysis (Figure 2a and b). 5564
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n-alkane is strikingly dependent on the nature of the allyl ligand. As summarized in Table 2, in several cases this initial complex thermally converts into a hydrido tungsten compound in which the original allyl ligand has been lost and the lateral alkyl chain has undergone multiple C−H activations, leading to a new η3-allyl ligand (Scheme 1). DFT methods indicate that a plausible mechanism for this transformation is the one depicted in Scheme 6 for Cp*W(NO)(η3-CH2CHCMe2)(n-C5H11) (2′), and it involves a β-hydrogen abstraction from the lateral alkyl chain by the allyl ligand. This process is evidently facilitated by the ability of Cp*W(NO)(η3-allyl)(n-alkyl) complexes to form agostic bonds that result in η1-allyl complexation and a more easily activated alkyl C(sp3)−H bond. Indeed, the Cβ−H bond is weakened in the agostically bonded η1-2′ complex, its Cβ-H bond length being 1.15 Å as opposed to 1.10 Å in 2′. As we have reported previously, the final hydrido complex, 3ethylallyl, is obtained as a mixture of isomers in 58% isolated yield.8 As shown in Scheme 6, the rate-determining step of this process is the first step, namely, the formation of the 16e intermediate alkene complex, 2c, from the 18e 2′. Formally, this transformation results in a decrease in the electron richness of the metal’s coordination sphere. Consequently, Cp*W(NO)(η3allyl)(n-alkyl) complexes that bear an electron-rich allyl ligand should undergo β-hydrogen abstraction more readily and be more prone to undergo overall multiple C(sp3)−H activations. An assessment concerning the electron richness of Cp*W(NO)(η3-allyl)(n-alkyl) complexes can be made by measuring the stretching frequency of the NO bond since the extremely strong π-acidic character of the nitrosyl ligand makes it the “electronic reservoir” of the coordination sphere. For instance, the HOMO of Cp*W(NO)(η3-CH2CHCHMe)(n-C5H11) (1′) is depicted in Figure 3, and it clearly shows that the main part of the electron density in this molecular orbital is located on the nitrosyl ligand. It has been found experimentally that Cp*W(NO)(η3-allyl)(n-alkyl) complexes exhibiting a low NO-stretching frequency tend to undergo multiple C−H activations more readily (Table 2).8 Low NO-stretching frequencies correspond to more electronrich complexes; that is, the more electron-rich the metal, the more back-bonding from it to the NO ligand and the lower the νNO. Theoretical modeling attempting to correlate the computed charge borne by the NO ligand in several Cp*W(NO)(η3allyl)(n-alkyl) complexes with the computed activation barrier for the previously described β-H abstraction step has been performed. As shown in Figure 4, the more electron-rich the NO ligand, the smaller the β-abstraction reaction barrier (and consequently the easier the overall multiple C−H activation processes), as expected. Interestingly, the dependence of the β-abstraction barrier on the electron density on the NO ligand is not linear. In this connection it is important to note that the case of the Cp*W(NO)(η3-CH2CHCHPh)(CH2C6H11) complex must be considered apart from the others. Specifically, β-hydrogen abstraction undergone by this complex leads to a new complex exhibiting an exocyclic CC double bond (Table 2). Consequently, there is an additional steric contribution due to the allylic strain inherent to such systems that leads to a β-abstraction barrier higher than expected. This model also accounts for the reactivity observed experimentally. For instance, complex 2′ has been computed to be the more reactive in the β-hydrogen abstraction process, and it does indeed undergo a rapid and complete conversion to the corresponding multiply C−H activated hydrido product with concomitant loss of the original allyl ligand (Scheme 6). On the
Figure 2. (a) Solid-state molecular structure of η4-2b,Mo (major isomer) with 50% probability thermal ellipsoids shown. Selected bond lengths (Å) and angles (deg): Mo(1)−C(11) = 2.379(5), Mo(1)− C(12) = 2.232(7), Mo(1)−C(13) = 2.270(7), Mo(1)−C(14) = 2.250(7), Mo(1)−N(1) = 1.7762(19), N(1)−O(1) = 1.209(2), C(11)−C(12) = 1.430(6), C(12)−C(13) = 1.415(9), C(13)-C14) = 1.410(7), C(13)−C(15) = 1.520(14), Mo(1)−N(1)−O(1) = 174.38(18), C(11)−C(12)−C(13) = 121.1(6), C(12)−C(13)− C(14) = 115.1(6), C(12)−C(13)−C(15) = 122.6(5). (b) Solid-state molecular structure of η4-2b,Mo (minor isomer) with 50% probability thermal ellipsoids shown. Selected bond lengths (Å) and angles (deg): Mo(1)−C(11b) = 2.387(8), Mo(1)−C(12b) = 2.214(11), Mo(1)− C(13b) = 2.217(12), Mo(1)−C(14b) = 2.214(8), C(11b)−C(12b) = 1.410(9), C(12b)−C(13b) = 1.442(10), C(13b)−C14b) = 1.399(13), C(13b)−C(15b) = 1.53(2), C(11b)−C(12b)−C(13b) = 122.0(10), C(12b)−C(13b)−C(14b) = 116.0(10), C(12b)−C(13b)−C(15b) = 119.8(9). 5565
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Table 2. Correlation between Nitrosyl-Stretching Frequencies and the Charges Borne by the NO Ligands with the Ability of Some Cp*W(NO)(η3-allyl)(n-alkyl) Complexes to Undergo Multiple C−H Activationsa
(*) Charge borne by the NO ligand in Cp*W(NO)(η3-CH2CHCMe2)(n-C5H11) (2′) resulting from thermolysis of 2 in n-pentane. (**) βHydrogen abstraction computed from the Cp*W(NO)(η3-CH2CHCMe2)(n-C5H11) complex.
a
Scheme 6
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Scheme 7
Table 3. Computed Free Energies (in kJ mol−1) of Intermediates Displayed in Scheme 7 Referenced to the Free Energies of the Starting Complexes 1 and 2
Figure 3. Computed HOMO of complex 1′ (isovalue contour: 0.02; energy = −5.55 eV).
allene pathway 1,2-dimethylallyl precursor diene pathway 1-methylallyl precursor 1,2-dimethylallyl precursor
16e complex 19.4 (2a)
TS 65.4 (TS2a,Ph)
Cp*W(NO)(η3-allyl) (Ph) −53.4 (2Ph)
16e complex
TS
Cp*W(NO)(η3-allyl)(Ph)
17.5 (1b)
139.4 (TS1b,Ph)
−58.7 (1Ph)
18.8 (2b)
130.4 (TS2b,Ph)
−53.5 (2Ph)
The complex Cp*W(NO)(η3-CH2CHCHMe)(Ph) (1Ph), resulting from the benzene activation by the η2-diene intermediate complex 1b (generated as depicted in Scheme 2b), is obtained via an activation barrier of 121.9 kJ mol−1 from the latter (Scheme 7b). This value is slightly higher than the similar barrier for the activation of benzene by complex 2b (111.6 kJ mol−1, Scheme 7b). However, in all cases of arene C(sp2)−H activations, the η2-allene intermediate complex, Cp*W(NO)(η2-CH2CCMe2) (2a), exhibits cleaner reactivity than the two η2-diene intermediates, Cp*W(NO)(η 2 -CH 2 CHCHCH 2 ) (1 b ) and Cp*W(NO)(η2-CH2CHCMeCH2) (2b). Thus, as shown in Table 3, the activation barrier for C(sp2)−H activation with Cp*W(NO)(η2-CH2CCMe2) is 46.0 kJ mol−1, much smaller than the activation barriers for 1b and 2b (121.9 and 111.6 kJ mol−1, respectively). Taking into account the steps leading to the η2-allene and η2-diene intermediate complexes from Cp*W(NO)(η3-CH2CHCMe2)(CH2CMe3) (Scheme 2), the pathway involving the allene intermediate appears to be more favored. Nevertheless, both pathways depicted in Scheme 7 lead to the formation of Cp*W(NO)(η3-CH2CHCMe2)(Ph) (2Ph) via strongly exothermic processes (Table 3). Selectivity of the C(sp2)−H Activation Processes: Avoiding the Aryl−Allyl Coupling. The two complexes resulting from the C(sp2)−H activation of benzene, namely, Cp*W(NO)(η3-CH2CHCHMe)(Ph) (1Ph) and Cp*W(NO)(η3-CH2CHCMe2)(Ph) (2Ph), exhibit opposite thermal stabilities. Complex 2Ph is stable and isolable under ambient conditions,3 but complex 1Ph undergoes subsequent intramolecular isomerization that results in a hydrido complex after
Figure 4. Computed β-H abstraction barrier as a function of the computed charge borne by the NO ligands.
other hand, the higher intrinsic stability of the Cp*W(NO)(η3-CH2CHCHPh)(n-C5H11) complex afforded by its high β-hydrogen abstraction barrier (the highest of the complexes investigated in Table 2) permits its isolation and characterization. The theoretical correlation between the electron-richness of the coordination spheres in Cp*W(NO)(η3-allyl)(n-alkyl) complexes and their ability to undergo further multiple C−H activations of the alkyl ligand can thus be utilized both as a powerful tool for predicting the intrinsic stability of such complexes and, further, for their potential utility as intermediates during the multistep functionalizations of alkanes. C(sp2)−H Activations Initiated by Cp*W(NO)(η3-allyl)(CH2CMe3) Complexes. Complexes 1 and 2 have also been used for the activation of arene C(sp2)−H bonds.3,8,10 Interestingly, the overall barriers computed for the C−H activation of benzene by these complexes, involving the formation of the 16e η2-allene and η2diene intermediate species 2a, 1b, and 2b (Scheme 7 and Table 3), are lower than the barriers for the corresponding C(sp3)−H activations depicted in Scheme 2. 5567
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that are distal to the nitrosyl group, which prevents any subsequent aryl−allyl coupling followed by an irreversible C−H activation leading to a hydride, as occurs with Cp*W(NO)(η3-CH2CHCHMe)(Ph) (Scheme 9). The formation of the thermally unstable alkene complex 2cPh has been confirmed by the characterization of its 18-electron PMe3 adduct (Scheme 10). Thus, thermolysis of 2Ph in C6D6 and excess PMe3 at 70 °C for 9 d affords Cp*W(NO)(PMe3)(η2CH2CH-CMe2Ph), which exists in three isomeric forms differing in the orientation of the bound η2-olefin. The solution molecular structures of the three isomers have been established by selective 1H NOE NMR spectroscopy. Only the major isomer 2cPh-PMe3-1 has been observed in the solid state, and its solid-state molecular structure has been established by single-crystal X-ray diffraction (Figure 5). The C(14)−C(15) bond is longer than a typical C−C double bond, due to π back-donation from the tungsten center. The solid-state molecular structure confirms the coupling of the phenyl ligand to the tertiary C(16) of the allyl ligand. Furthermore, Figure 6 shows 1H and 1H{31P} NMR spectra of 2cPh-PMe3-1 in C6D6 that clearly exhibit the coupling of the P atom to the olefin H atoms. The existence of an equilibrium between 2c and 2cPh has been further substantiated through the trapping of 2c with the bulkier Lewis base triphenylphosphine. The 18e PPh3 adduct Cp*W(NO)(PPh3)(η2-CH2CH-CMe2Ph) (2cPh-PPh3) forms in a manner similar to that of the isomers of 2cPh-PMe3, but in a much lower yield. The complex 2cPh-PPh3 has been characterized by NMR spectroscopy, but attempts to grow crystals suitable for a single-crystal X-ray diffraction analysis have so far been unsuccessful. Attempts to trap 2c with pyridine as a Lewis base did not lead to the formation of the expected 18e adduct, Cp*W(NO)(NC5H5)(η2-CH2CH-CMe2Ph).
Scheme 8
a transfer of the aryl group onto the allyl group.8,10 Two regioisomers of the hydrido complex have been characterized (Scheme 8), confirming that two isomers of complex 1Ph coexisted before the aryl−allyl coupling occurred.8,10 The first isomer, 1Ph,prox, has the methyl group of the allyl ligand proximal to the nitrosyl ligand, and the second isomer, 1Ph,dis, has the methyl group in a distal position. It is worth noting at this point that only the distal isomer of complex 2Ph has been detected in solution, likely because of steric factors.3b The stereochemical properties of complexes Cp*W(NO)(η3CH2CHCHMe)(Ph) (1Ph) and Cp*W(NO)(η3CH2CHCMe2)(Ph) (2Ph) are at the origin of their differing thermal stabilities. Thus, the aryl−allyl coupling product, Cp*W(NO)(η2-CH2CHCMe2Ph), obtained from complex 2Ph (Scheme 9b) does not bear any activatable C−H bonds close to the tungsten because of the distal positions of the methyl and phenyl groups with respect to the NO ligand. Consequently, 2Ph is the more thermodynamically stable isomer, and the aryl−allyl coupled product 2cPh is not isolable. On the other hand, both proximal and distal isomers of the alkene complex, i.e., 1cPh,prox and 1cPh,dis, obtained from the proximal and distal isomers of Cp*W(NO)(η3-CH2CHCHMe)(Ph) (1Ph) (Scheme 9a), contain a C−H bond close to the tungsten center. Hence, a strongly thermally and kinetically favored C−H activation can occur, and it leads to the two stable hydrido complexes experimentally observed8,10 (1dPh,prox and 1dPh,dis), which are stabilized by 53.1 and 26.0 kJ mol−1, respectively, compared to the starting complexes (see Table 4). Consequently, in the complex Cp*W(NO)(η3-CH2CHCMe2)(Ph), it is the position of the two methyl groups of the allyl ligand
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EPILOGUE The results of these computational investigations have provided several interesting and useful insights into the mechanistic pathways involved during the intermolecular C−H activations of hydrocarbons initiated by Cp*W(NO)(η3-allyl)(CH2CMe3) complexes. Specifically, they have established that geminal
Scheme 9
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Table 4. Computed Free Energies (in kJ mol−1) of the Species Depicted in Scheme 9 1-methylallyl precursor proximal isomer 1-methylallyl precursor distal isomer 1,2-dimethylallyl precursor distal isomer
Cp*W(NO)(η3-allyl)(Ph)
TS
Cp*W(NO)(η2-alkene)(Ph)
TS′
Cp*W(NO)(η3-allyl)(H)
0.0 (1Ph,prox)
104.5 (TS1Ph,prox)
16.0 (1cPh,prox)
65.7 (TS1Ph,prox′)
−53.1 (1dPh,prox)
0.0 (1Ph,dis)
123.6 (TS1Ph,dis)
24.2 (1cPh,dis)
60.8 (TS1Ph,dis′)
−26.0 (1dPh,dis)
0.0 (2Ph)
124.5 (TS2Ph)
30.0 (2cPh)
n.a.
n.a.
Scheme 10
Figure 5. Solid-state molecular structure of 2cPh-PMe3-1 with 50% probability thermal ellipsoids shown. Selected bond lengths (Å) and angles (deg): W(1)−C(14) = 2.184(4), W(1)−C(15) = 2.288(4), W(1)−P(1) = 2.4485(10), W(1)−N(1) = 1.780(3), N(1)−O(1) = 1.228(4), C(14)−C(15) = 1.452(5), C(14)−H(14a) = 1.00(5), C(14)−H(14b) = 1.07(5), C(15)−H(15) = 0.98(5), W(1)−N(1)−O(1) = 168.6(3), C(14)−C(15)−C(16) = 118.0(3), P(1)−W(1)−N(1) = 83.08(10), N(1)−W(1)−C(14) = 93.41(14), C(14)−W(1)−C(15) = 37.81(14), C(15)−W(1)−P(1) = 83.78(10), P(1)−W(1)−C(14) = 120.73(11), N(1)−W(1)−C(15) = 97.38(14).
is facilitated by a lower rotational barrier for the η2-diene ligand in the molybdenum compounds. In some instances involving the tungsten complexes, the initially formed η1-hydrocarbyl product (which may or may not be isolable) isomerizes by intramolecular exchange of the newly formed hydrocarbyl ligand with a hydrogen atom on the allyl ligand or undergoes additional C−H activations and is converted to a new hydrido allyl compound. DFT methods
dialkyl substituents on the allyl ligands markedly stabilize the η2allene intermediate complexes, whereas the absence of such substituents favors the formation of the η2-diene complexes. In the case of the analogous molybdenum systems, the η2-diene intermediate complexes undergo isomerization to the η4-diene complexes and do not effect intermolecular C−H activations. Computational investigations suggest that this latter isomerization 5569
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both of which were sealed by Kontes greaseless stopcocks. Solvents such as pentane, diethyl ether (Et2O), benzene, and tetrahydrofuran (THF) were dried over sodium/benzophenone ketyl and freshly distilled prior to use; all other solvents were dried according to standard procedures.11 All binary magnesium reagents used were prepared from the corresponding Grignard reagents.12 The following complexes were prepared according to the published procedures: Cp*Mo(NO)Cl2,13 Cp*W(NO)Cl 2 , 13 Cp*W(NO)(Ph)(η 3 -CH 2 CHCMe 2 ) (2 Ph ). 3a Pentamethylcyclopentadiene was obtained from Boulder Scientific Company. All other chemicals and reagents were ordered from commercial suppliers and were used as received. Unless otherwise specified, all IR samples were prepared as Nujol mulls sandwiched between NaCl plates, and their spectra were recorded on a Thermo Nicolet model 4700 FT-IR spectrometer. Except where it has been noted all NMR spectra were recorded at room temperature on Bruker AV-300 and AV-400 (direct and indirect probes) instruments, all chemical shifts are reported in ppm, and coupling constants are reported in Hz. 1H NMR spectra were referenced to the residual protio isotopomer present in C6D6 (7.16 ppm). 13C NMR spectra were referenced to C6D6 (128.39 ppm). 31P NMR spectra were externally referenced to 85% H3PO4. For the characterization of most complexes two-dimensional NMR experiments, {1H−1H} COSY, {1H−13C} HSQC, and {1H−13C} HMBC, were performed to correlate and assign 1 H and 13C NMR signals and establish atom connectivity; 1H NOE NMR spectroscopy was also used for characterization of some complexes. Low- and high-resolution mass spectra (EI, 70 eV) were recorded by Mr. Marshall Lapawa of the UBC mass spectrometry facility using a Kratos MS-50 spectrometer, and elemental analyses were performed by Mr. Derek Smith of the UBC microanalytical facility. X-ray crystallographic data collection, solution, and refinement were performed at the UBC X-ray crystallography facility. Synthesis of Cp*Mo(NO)(η4-CH2CMe-CHMe2) (η4-2b,Mo). In a glovebox, a glass Schlenk flask was charged with Cp*Mo(NO)Cl2 (0.33 g, 0.99 mmol) and a magnetic stir bar. After being connected to a double manifold, the contents of the flask were cooled in a dry ice/ acetone bath (−78 °C), and THF (ca. 25 mL) was added by cannula. A second Schlenk flask was charged in the glovebox with Mg(CH2CMe3)2 (titer: 145 g/mol, 0.15 g, 1.0 mmol) and a magnetic stir bar. The second flask was charged with THF (ca. 50 mL), and its contents were cannulated into the first Schlenk flask. Following the addition, the Schlenk flask was removed from the bath, and its contents were stirred at room temperature for 0.5 h to obtain a dark purple mixture. The solvent was then removed in vacuo to isolate a dark purple residue. A third Schlenk flask was charged in the glovebox with Mg(CH2CHCMe2)2 (titer: 130 g/mol, 0.12 g, 0.92 mmol) and a magnetic stir bar. Et2O (ca. 10 mL) was then added by cannula, and the Schlenk flask was cooled to −78 °C in a dry ice/acetone bath. The contents of the first flask were transferred to the third flask in Et2O (ca. 2 × 50 mL) by cannula, and then the third flask was removed from the bath and its contents were stirred at room temperature for 1 h to afford a yellow mixture. This mixture was then transferred to the top of a basic alumina column (3 × 8 cm). A pale yellow band was collected with Et2O as a yellow eluate. The solvent was removed from the eluate under reduced pressure to obtain a yellow-brown solid (0.198 g, 65% yield). Crystals suitable for single-crystal X-ray diffraction were obtained by recrystallization of this solid from Et2O at −33 °C. Two isomers of Cp*Mo(NO)(η4-CH2 CMe-CHMe2) (η4-2b,Mo) occur both in the solid state and in solutions; the two isomers are detectable in a 54:46 ratio by 1H NMR spectroscopy. Characterization Data for η4-2b,Mo (major isomer, 54%). IR (cm−1): 1605 (s, νNO). MS (LREI, m/z, probe temp 150 °C): 331 [M+, 98 Mo]. MS (HREI, m/z, 92Mo): calcd 325.08477, found 325.08463. Anal. Calcd for C15H23NOMo: C, 54.71; H, 7.04; N, 4.25. Found: C, 53.52; H, 6.91; N, 3.96. 1H NMR (400 MHz, C6D6): δ 0.93 (s, 3H, CMeCH2), 1.67 (s, 15H, C5Me5), 1.75 (dd, 3JHH = 13.3, 2JHH = 3.3, 1H, CHCH2), 2.52 (d, 2JHH = 1.6, 1H, CMeCH2), 3.04 (d, 2JHH = 1.6, 1H, CMeCH2), 3.28 (dd, 3JHH = 13.3, 6.2, 1H, CH CH2), 3.41 (dd, 3JHH = 6.2, 2JHH = 3.3, 1H, CHCH2). 13C NMR (100 MHz, C6D6): δ 11.0 (C5Me5), 19.1 (CMe), 59.9 (CH2), 60.0 (CH2), 91.4 (CH), 106.3 (C5Me5), 106.8 (CMe).
Figure 6. Expansion of the 1H (top, blue) and 1H{31P} (bottom, purple) NMR spectra (2.83 to 2.18 ppm; 0.47 to 0.31 ppm) of complex 2cPhPMe3-1 in C6D6 (400 MHz) displaying the resonances due to H(c) [δ 0.35 (ddd, 3JHH = 10.7, 3JPH = 1.7, 2JHH = 4.5, 1H, CHCH2)], H(b) [δ 2.26 (ddd, 3JHH = 10.6, 3JPH = 2.3, 2JHH = 4.5, 1H, CHCH2)], and H(a) [δ 2.70 (ddd, 3JHH = 10.7, 10.6, 3JPH = 8.0, 1H, CHCH2)]. The 2.54 ppm signal is the CHCH2 resonance for the minor isomer, 2cPhPMe3-3: δ 2.54 (ddd, 3JHH = 9.6, 6.8, 3JPH = 6.9, 1H, CHCH2).
indicate that a plausible mechanism for the latter transformation involves a β-hydrogen abstraction from the lateral alkyl chain by the allyl ligand. The rate-determining step of this process is thus the formation of a 16e η2-olefin complex with the olefin originating from the alkyl chain, and this process should be favored by relatively electron-rich Cp*W(NO)(η3-allyl)(n-alkyl) complexes, as is experimentally observed. In all cases of benzene C(sp2)−H activations by the tungsten systems, the η2-allene intermediate complexes exhibit lower barriers than the η2-diene intermediates. However, theoretical considerations indicate that the stereochemical properties of the first-formed Cp*W(NO)(η3-allyl)(Ph) products determine their differing thermal stabilities. If the aryl− allyl coupling product Cp*W(NO)(η2-allyl-Ph) contains an activatable C−H bond close to the tungsten center, then the thermodynamically favored intramolecular exchange of the phenyl ligand with a hydrogen atom on the allyl ligand occurs. Otherwise, it does not, and the Cp*W(NO)(η3-allyl)(Ph) complexes persist. The results of these DFT calculations not only provide a better understanding of the currently known C−H activation chemistry initiated by the Cp*W(NO)(η3-allyl)(η1-hydrocarbyl) complexes but also suggest some useful guidelines for the synthesis of other members of this family of compounds for the specific activation and functionalization of the C−H bonds of more complex organic molecules. Such studies are currently in progress.
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EXPERIMENTAL SECTION
General Methods. All reactions and subsequent manipulations involving organometallic reagents were performed under anhydrous and anaerobic conditions except where noted. All inert gases were purified by passing them through a column containing MnO and then through a column of activated 4 Å molecular sieves. High-vacuum and inert atmosphere techniques were performed either using double-manifold Schlenk lines or in Innovative Technologies LabMaster 100 and MS-130 BG dual-station gloveboxes equipped with freezers maintained at −33 °C. Preparative scale reactions were performed with Schlenk or round-bottom flasks; reactions were performed in thick-walled glass reaction bombs (larger scale) or J. Young NMR tubes (smaller scale), 5570
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Characterization Data for η4-2b,Mo (minor isomer, 46%). 1H NMR (400 MHz, C6D6): δ 1.07 (d, 2JHH = 2.7, 1H, CMeCH2), 1.36 (dd, 3JHH = 14.3, 6.7, 1H, CHCH2), 1.63 (s, 15H, C5Me5), 1.64 (s, 3H, CMeCH2), 2.51 (dd, 3JHH = 6.7, 2JHH = 2.8, 1H, CHCH2), 3.17 (dd, 3 JHH = 14.3, 2JHH = 2.8, 1H, CHCH2), 3.35 (d, 2JHH = 2.7, 1H, CMe CH2). 13C NMR (100 MHz, C6D6): δ 10.9 (C5Me5), 19.4 (CMe), 58.3 (CH2), 62.7 (CH2), 88.6 (CH), 106.0 (C5Me5), 115.5 (CMe). Reaction of Cp*W(NO)(C6H5)(η3-CH2CHCMe2) with PMe3. In a glovebox, a glass Schlenk flask was charged with Cp*W(NO)(C6H5)(η3-CH2CHCMe2) (0.156 g, 0.315 mmol), benzene-d6 (ca. 5 mL), and an excess of PMe3 (ca. 1 mL) to give an orange mixture. The Schlenk flask was sealed with a Kontes greaseless stopcock, and its contents were heated at 70 °C for 9 d to produce a dark yellow-green mixture. The PMe3 was removed by gentle application of vacuum, and the mixture was then analyzed directly by 1H and 31P NMR spectroscopy to reveal three new organometallic species. These complexes were identified as isomers of Cp*W(NO)(PMe3)(η3-CH2CH-CMe2Ph) (2cPh-PMe3) in a 64:25:11 ratio. The solvent was removed from the final mixture in vacuo, and recrystallization of the residue from 1:1 pentane/Et2O afforded yellow crystals (0.134 g, 74%) suitable for a single-crystal X-ray diffraction analysis. Characterization Data for Cp*W(NO)(PMe 3 )(η 2 -CH 2 CHCMe2Ph) (2cPh-PMe3-1) (64%). IR (cm−1): 1541 (s, νNO). MS (LREI, m/z, probe temperature 120 °C): 571 [M+, 184W], 495 [M+ − PMe3, 184 W], 425 [M+ − H2CCH-CMe2Ph, 184W]. MS (HREI, m/z, 182W): calcd 569.21692, found 569.21733. Anal. Calcd for C24H38NOPW: C, 50.45; H, 6.70; N, 2.45. Found: C, 50.07; H, 6.68; N, 2.37. 1H NMR (400 MHz, C6D6): δ 0.35 (ddd, 3JHH = 10.7, 3JPH = 1.7, 2JHH = 4.5, 1H, CHCH2), 0.86 (s, 3H, CMe2Ph), 1.11 (d, 2JHH = 8.4, 9H, WPMe3), 1.31 (s, 3H, CMe2Ph), 1.74 (s, 15H, C5Me5), 2.26 (ddd, 3JHH = 10.6, 3 JPH = 2.3, 2JHH = 4.5, 1H, CHCH2), 2.70 (ddd, 3JHH = 10.7, 10.6, 3 JPH = 8.0, 1H, CHCH2), 7.10 (t, 3JHH = 7.2, 1H, aryl H), 7.25 (t, 3 JHH = 7.6, 2H, aryl H), 7.51 (d, 3JHH = 7.6, 2H, aryl H). 13C NMR (100 MHz, C6D6): δ 11.3 (C5Me5), 18.7 (d, 1JPC = 29.9, WPMe3), 26.4 (CHCH2), 31.2 (CMe2Ph), 36.2 (CMe2Ph), 42.1 (d, 3JPC = 2.8, CMe2Ph), 58.2 (CHCH2), 104.5 (C5Me5), 125.6 (aryl C), 127.4 (aryl C), 128.4 (aryl C), 155.9 (ipso C). 31P NMR (162 MHz, C6D6): δ −17.3 (s, 1JWP = 360.7, PMe3). Sel NOE (400 MHz, C6D6): δ irrad. at 1.11, NOE at 0.86, 1.31, 1.74, 2.70, 7.10, 7.25, 7.51. Characterization Data for Cp*W(NO)(PMe 3 )(η 2 -CH 2 CHCMe2Ph) (2cPh-PMe3-2) (25%). 1H NMR (400 MHz, C6D6): δ −0.21 (ddd, 3JHH = 10.3, 3JPH = 4.5, 2JHH = 5.6, 1H, CHCH2), 1.17 (d, 2JHH = 8.4, 9H, WPMe3), 1.52 (obs, 1H, CHCH2), 1.53 (obs, 1H, CH CH2), 1.54 (s, 15H, C5Me5), 1.58 (s, 3H, CMe2Ph), 1.59 (s, 3H, CMe2Ph), 7.17 (obs, 1H, aryl H), 7.38 (t, 3JHH = 7.6, 2H, aryl H), 7.95 (d, 3 JHH = 7.6, 2H, aryl H). 13C NMR (100 MHz, C6D6): δ 10.6 (C5Me5), 16.2 (d, 1JPC = 32.2, WPMe3), 28.3 (d, 2JPC, CHCH2), 28.6 (CMe2Ph), 30.7 (CMe2Ph), 44.9 (CMe2Ph), 57.2 (CHCH2), 103.8 (C5Me5), 125.5 (aryl C), 127.5 (aryl C), 128.7 (aryl C), 155.6 (ipso C). 31P NMR (162 MHz, C6D6): δ −9.56 (s, 1JWP = 355.7, PMe3). Sel NOE (400 MHz, C6D6): δ irrad. at 1.17, NOE at −0.21. Selected Signals for Cp*W(NO)(PMe3)(η2-CH2CH-CMe2Ph) (2cPhPMe3-3) (11%). 1H NMR (400 MHz, C6D6): δ 1.27 (d, 2JHH = 8.4, 9H, WPMe3), 1.67 (s, 15H, C5Me5), 2.54 (ddd, 3JHH = 9.6, 6.8, 3JPH = 6.9, 1H, CHCH2). 13C NMR (100 MHz, C6D6): δ 10.3 (C5Me5), 17.5 (d, 1 JPC = 30.3, WPMe3), 104.7 (C5Me5). 31P NMR (162 MHz, C6D6): δ −14.4 (s, 1JWP = 364.6, PMe3). Sel NOE (400 MHz, C6D6): δ irrad. at 1.27, NOE at 2.54. Reaction of Cp*W(NO)(η3-CH2CHMe2)(Ph) with PPh3. In a glovebox, a glass Schlenk flask was charged with Cp*W(NO)(C6H5)(η3-CH2CHCMe2) (0.170 g, 0.343 mmol), benzene-d6 (ca. 5 mL), and a slight excess of PPh3 (0.089 g, 0.34 mmol) to give a yellow-orange mixture. The Schlenk flask was sealed with a Kontes greaseless stopcock, and its contents were heated for 4 d in an ethylene glycol bath maintained at 70 °C to obtain a dark brown mixture. The deuterated solvent was removed by gentle application of vacuum, leaving behind a dark brown residue. The residue was dissolved in a minimal amount of Et2O and transferred to the top of a basic alumina column (0.5 × 5 cm). A pale yellow band was developed and eluted with 1:1 pentane/Et2O. The solvents were removed from the eluate in vacuo to obtain a yellow
solid (0.021 g, 8% yield). The organometallic complex Cp*W(NO)(PPh3)(η2-CH2CH-CMe2Ph) (2cPh-PPh3) was identified by 1H, 13C, and 31P NMR spectroscopy. Characterization Data for Cp*W(NO)(PPh 3 )(η 2 -CH 2 CHCMe2Ph) (2cPh-PPh3). IR (cm−1): 1542 (s, νNO). MS (LREI, m/z, probe temperature 150 °C): 611 [M+ − H2CCH-CMe2Ph, 184W]. 1H NMR (400 MHz, C6D6): δ 0.42 (dd, 3JPH = 13.7 Hz, 2JHH = 5.9 Hz, 1H, CH2CH), 1.06 (s, 3H, CMe2Ph), 1.47 (s, 15H, C5Me5), 1.53 (s, 3H, CMe2Ph), 1.93 (dd, 3JPH = 13.7 Hz, 2JHH = 5.9 Hz, 1H, CH2CH), 1.99 (m, CH2CH), 6.85−7.15 (m, 8H, PPh3), 7.13 (t, 3JHH = 7.2 Hz, 2H, CMe2Ph), 7.33 (d, 3JHH = 7.2 Hz, 2H, CMe2Ph), 7.36−8.18 (m, 7H, PPh3), 7.86 (m, 1H, CMe2Ph). 13C NMR (100 MHz, C6D6): δ 10.6 (s, C5Me5), 28.2 (s, CMe2Ph), 29.0 (s, CMe2Ph), 32.8 (d, 2JPC= 11.0 Hz, CH2CH), 44.9 (s, CMe2Ph), 59.5 (s, CH2CH), 104.3 (s, C5Me5), 125.4 (s, 2C, CMe2Ph), 127.1 (s, 1C, CMe2Ph), 128.5 (s, 2C, CMe2Ph), 129.2 (d, 2JPC = 6.4 Hz, aryl PPh3), 134.5 (d, 3JPC = 19.8 Hz, aryl PPh3), 156.4 (s, ipso C). 31P NMR (162 MHz, C6D6): δ 35.3 (s, PPh3, 1JPW = 353.7 Hz). Computational Methods. Density functional theory14a was applied to determine the structural and energetic features of the various organometallic complexes described in this article. All theoretical calculations were performed using Gaussian09.14b The 6-31+G(d) basis set14c,d was used for all atoms (C, H, O, N) except W and Mo, which were treated using the Stuttgart pseudopotential and associated basis set.14e The hybrid exchange correlation functional PBE0 was also used.14f It mixes 25% of Hartree−Fock exchange into the gradientcorrected PBE exchange and correlation functional14g and yields accurate and reliable thermochemistry data for reactions involving transition-metal compounds.14h All structures were calculated without any geometrical constraints, and all stationary points were characterized as minima or transition states by frequency calculations (i.e., only one negative frequency for a transition state, no negative frequencies for minima). Solvent effects (specifically n-pentane and benzene) were included in all structure-optimization and frequency calculations using the PCM implicit solvation model of Tomasi et al. as implemented in the Gaussian code.14i,j No explicit solvent modeling was required because of the extremely low coordinating properties of the hydrocarbon solvents involved in the modeled reactions. Since all tungsten and molybdenum complexes investigated were diamagnetic, all corresponding structures were consequently optimized in the singlet spin state. All charges were computed using an NBO partition analysis.14k Moreover, all computed energies were corrected for basis-set superposition errors using the counterpoise algorithm,14l and connections between intermediates and several key transition states were checked using the IRC algorithm.14m
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ASSOCIATED CONTENT
S Supporting Information *
Text, tables, and CIF files providing full details of the crystallographic analyses of complexes η4-2b,Mo and 2cPhPMe3-1. Full ref 14b and optimized structure and final thermochemical parameters of the various organometallic complexes subjected to DFT calculations. This material is available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS We are grateful to The Dow Chemical Company for continuing financial support of this work and to our Dow colleagues, Drs. Devon C. Rosenfeld, Brandon A. Rodriguez, and Robert D. Froese, for assistance and helpful discussions. This research has been enabled by the use of WestGrid computing resources, which are funded in part by the Canada Foundation for 5571
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Innovation, Alberta Innovation, Science BC Advanced Education, and the participating research institutions. WestGrid equipment is provided by IBM, Hewlett-Packard, and SGI.
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REFERENCES
(1) Baillie, R. A.; Legzdins, P. Acc. Chem. Res. 2013, submitted for publication. (2) Tsang, J. Y. K.; Buschhaus, M. S. A.; Graham, P. M.; Semiao, C. J.; Semproni, S. P.; Kim, S. J.; Legzdins, P. J. Am. Chem. Soc. 2008, 130, 3652. (3) (a) Ng, S. H. K.; Adams, C. S.; Legzdins, P. J. Am. Chem. Soc. 2002, 124, 9380. (b) Ng, S. H. K.; Adams, C. S.; Hayton, T. W.; Legzdins, P.; Patrick, B. O. J. Am. Chem. Soc. 2003, 125, 15210. (4) Baillie, R. A.; Man, R. W. Y.; Shree, M. V.; Chow, C.; Thibault, M. E.; McNeil, W. S.; Legzdins, P. Organometallics 2011, 30, 6201. (5) Tran, T.; Chow, C.; Zimmerman, A. C.; Thibault, M. E.; McNeil, W. S.; Legzdins, P. Organometallics 2011, 30, 738. (6) Furet, P.; Matcha, R. L.; Fuchs, R. J. Phys. Chem. 1986, 90, 5571. (7) In the case of the molybdenum intermediate complexes, this η2 → η4 diene isomerization can be avoided by choosing an initial molybdenum complex whose allyl ligand can only lead to an η2-allene intermediate complex upon thermolysis. For example, single C(sp3)−H activation of mesitylene has been achieved by using the complex Cp*Mo(NO)(η3-CH2CHCHPh)(CH2CMe3).4 (8) Baillie, R. A.; Tran, T.; Lalonde, K. M.; Tsang, J. Y. K.; Thibault, M. E.; Patrick, B. O.; Legzdins, P. Organometallics 2012, 31, 1055. (9) Chow, C.; Patrick, B. O.; Legzdins, P. Organometallics 2012, 31, 7453. (10) Baillie, R. A.; Tran, T.; Thibault, M. E.; Legzdins, P. J. Am. Chem. Soc. 2010, 132, 15160. (11) Armarego, W. L. F.; Chai, C. L. L. Purification of Laboratory Chemicals, 5th ed.; Elsevier: Amsterdam, 2003. (12) Dryden, N. H.; Legzdins, P.; Rettig, S. J.; Veltheer, J. E. Organometallics 1992, 11, 2583. (13) Dryden, N. H.; Legzdins, P.; Batchelor, R. J.; Einstein, F. W. B. Organometallics 1991, 10, 2077. (14) (a) Parr, R. G.; Yang, W. Density-Functional Theory of Atoms and Molecules; Oxford University Press: Oxford, U.K., 1989. (b) Frisch, M. J.; et al. Gaussian 09, revision C.01; Gaussian, Inc.: Wallingford, CT, 2010. (c) Krishnan, R.; Binkley, J. S.; Seeger, R.; Pople, J. A. J. Chem. Phys. 1980, 72, 650. (d) Clark, T.; Chandrasekhar, J.; Spitznagel, G. W.; Schleyer, P. v. R. J. Comput. Chem. 1983, 4, 294. (e) Szentpaly, L. v.; Fuentealba, P.; Preuss, H.; Stoll, H. Chem. Phys. Lett. 1982, 93, 555. (f) Adamo, C.; Barone, V. J. Chem. Phys. 1999, 110, 6158. (g) Perdew, J. P.; Burke, K.; Ernzerhof, M. Phys. Rev. Lett. 1996, 77, 3865. (h) Coquet, R.; Tada, M.; Iwasawa, Y. Phys. Chem. Chem. Phys. 2007, 9, 6040. (i) Miertus, S.; Scrocco, E.; Tomasi. J. Chem. Phys. 1981, 55, 117. (j) Scalmani, G.; Frisch, M. J.; Mennucci, B.; Tomasi, J.; Cammi, R.; Barone, V. J. Chem. Phys. 2006, 124, 094107. (k) Foster, J. P.; Weinhold, F. J. Am. Chem. Soc. 1980, 102, 7211. (l) Boys, S. F.; Bernardi, F. Mol. Phys. 1970, 19, 553. (m) Hratchian, H. P.; Schlegel, H. B. J. Chem. Phys. 2004, 120, 9918.
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