Insights into the Intermolecular C–H Activations of Hydrocarbons Initiated by Cp*W(NO)(η³-allyl)(CH₂CMe₃) Complexes

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)­(η3-allyl)­(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.

Effects of the η5-C5H4iPr Ligand on the Properties Exhibited by Its Tungsten Nitrosyl Complexes

“…The nitrosyl ligands are linear, and the W−P bond lengths are similar to those extant in analogous four-legged piano-stool molecules. 13,14 As expected, the W−Cl bonds in 4a and 4b trans to the σ-donating PCl 2 CMe 2 CHCH 2 ligands are longer than the W−Cl linkages trans to the π-accepting nitrosyl ligands.…”

“…12 Formation of the η 2diene intermediate via loss of neopentane has a calculated activation barrier of 168. kJ mol −1 that is higher in energy than the activation barrier for the alternative allene pathway (147.8 kJ mol −1 ). 14 Consistently, the 16e (η 5 -C 5 Me 5 )W(NO)(η 2 -CH 2 CCMe 2 ) complex is the dominant intermediate for subsequent C−H activation reactions initiated by the η 5 -C 5 Me 5 system, and it has also been isolated as its 18e PMe 3 adduct. 12 Results of the trapping reaction involving complex 3 (Scheme 7) as well as the labeling studies with benzene-d 6 suggest that the η 5 Complexes 7 and 8 can be prepared from their dicarbonyl nitrosyl precursors (Table 1) via an initial reaction with PCl 5 to form the corresponding dichloro nitrosyl complexes which then undergo sequential salt metatheses reactions with Mg-(CH 2 CMe 3 ) 2 and Mg(CH 2 CHCMe 2 ) 2 to yield the desired products as orange solids in moderate yield, e.g., Scheme 8.…”

Cationic and Neutral Cp*M(NO)(κ2-Ph2PCH2CH2PPh2) Complexes of Molybdenum and Tungsten: Lewis-Acid-Induced Intramolecular C–H Activation

“…In contrast to 9 , heating 10 at 80 °C in CD 2 Cl 2 affords a mixture of products, there being no evidence for the formation of a W–H bond. The reason for this difference between the Mo and W complexes is not immediately obvious, but previous studies with other compounds based on the Cp*M­(NO) scaffold have shown that molybdenum complexes have a tendency to react at significantly greater rates than the corresponding tungsten compounds . Thus, while orthometalation of a phenyl substituent from Cp*M­(NO→B­(C 6 F 5 ) 3 )­(κ 2 -Ph 2 P­CH 2 ­CH 2 ­PPh 2 ) proceeds rapidly for M = Mo, the same transformation for M = W likely proceeds at a slow enough rate to permit various unwanted side reactions to occur.…”

“…The reason for this difference between the Mo and W complexes is not immediately obvious, but previous studies with other compounds based on the Cp*M(NO) scaffold have shown that molybdenum complexes have a tendency to react at significantly greater rates than the corresponding tungsten compounds. 24 Ph 2 PCH 2 PPh 2 )] 2+ dication is shown in Figure 4; its metrical parameters are similar to those of the related Ph 2 PCH 2 CH 2 PPh 2 complexes 1 and 2 (vide supra). Interestingly, this is the only complex of this series to show exchange of the original PhCN ligand for the MeCN-d 3 solvent, despite identical recrystallization conditions being employed for all complexes.…”

Thermal Chemistry of Cp*W(NO)(H)(η3-allyl) Complexes

“…Theoretical calculations concerning the mechanism of this reaction suggest that the single C−H activation of n-pentane by Cp*W(NO)(CH 2 CMe 3 )(η 3 -CH 2 CHCMe 2 ) does indeed occur. 7 However, the initial product formed, namely, Cp*W(NO)(n-C 5 H 11 )(η 3 -CH 2 CHCMe 2 ), is unstable under the experimental conditions employed and converts to the isomers of 4a and 4b through successive, multiple C−H activations. It is therefore likely that complexes 1−3 also effect the selective, single terminal C−H activation of linear alkanes, but the resulting products are thermally unstable and undergo two additional successive C−H activations to form the final allyl hydride complexes.…”