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.
The complexes trans-Cp*W(NO)(CHCMe)(H)(L) (Cp* = η-CMe) result from the treatment of Cp*W(NO)(CHCMe) in n-pentane with H (∼1 atm) in the presence of a Lewis base, L. The designation of a particular geometrical isomer as cis or trans indicates the relative positions of the alkyl and hydrido ligands in the base of a four-legged piano-stool molecular structure. The thermal behavior of these complexes is markedly dependent on the nature of L. Some of them can be isolated at ambient temperatures [e.g., L = P(OMe), P(OPh), or P(OCH)CMe]. Others undergo reductive elimination of CMe via trans to cis isomerization to generate the 16e reactive intermediates Cp*W(NO)(L). These intermediates can intramolecularly activate a C-H bond of L to form 18e cis complexes that may convert to the thermodynamically more stable trans isomers [e.g., Cp*W(NO)(PPh) initially forms cis-Cp*W(NO)(H)(κ-PPhCH) that upon being warmed in n-pentane at 80 °C isomerizes to trans-Cp*W(NO)(H)(κ-PPhCH)]. Alternatively, the Cp*W(NO)(L) intermediates can effect the intermolecular activation of a substrate R-H to form trans-Cp*W(NO)(R)(H)(L) complexes [e.g., L = P(OMe) or P(OCH)CMe; R-H = CH or MeSi] probably via their cis isomers. These latter activations are also accompanied by the formation of some Cp*W(NO)(L) disproportionation products. An added complication in the L = P(OMe) system is that thermolysis of trans-Cp*W(NO)(CHCMe)(H)(P(OMe)) results in it undergoing an Arbuzov-like rearrangement and being converted mainly into [Cp*W(NO)(Me)(PO(OMe))], which exists as a mixture of two isomers. All new complexes have been characterized by conventional and spectroscopic methods, and the solid-state molecular structures of most of them have been established by single-crystal X-ray crystallographic analyses.
Reaction of Na[η(5)-C5H4(i)Pr] with W(CO)6 in refluxing THF for 4 days generates a solution of Na[(η(5)-C5H4(i)Pr)W(CO)3] that when treated with N-methyl-N-nitroso-p-toluenesulfonamide at ambient temperatures affords (η(5)-C5H4(i)Pr)W(NO)(CO)2 (1) that is isolable in good yield as an analytically pure orange oil. Treatment of 1 with an equimolar amount of I2 in Et2O at ambient temperatures affords (η(5)-C5H4(i)Pr)W(NO)I2 (2) as a dark brown solid in excellent yield. Sequential treatment at low temperatures of 2 with 0.5 equiv of Mg(CH2CMe3)2 and Mg(CH2CH═CMe2)2 in Et2O produces the alkyl allyl complex, (η(5)-C5H4(i)Pr)W(NO)(CH2CMe3)(η(3)-CH2CHCMe2) (3), as a thermally sensitive yellow liquid. Complex 3 may also be synthesized, albeit in low yield, in one vessel at low temperatures by reacting 1 first with 1 equiv of PCl5 and then with the binary magnesium reagents specified above. Interestingly, similar treatment of 1 in Et2O with PCl5 and only 0.5 equiv of Mg(CH2CH═CMe2)2 results in the formation of the unusual complex (η(5)-C5H4(i)Pr)W(NO)(PCl2CMe2CH═CH2)Cl2 (4), which probably is formed via a metathesis reaction of the binary magnesium reagent with (η(5)-C5H4(i)Pr)W(NO)(PCl3)Cl2. The C-D activation of C6D6 by complex 3 has been investigated and compared to that exhibited by its η(5)-C5Me5, η(5)-C5Me4H, and η(5)-C5Me4(n)Pr analogues. Kinetic analyses of the various activations have established that the presence of the η(5)-C5H4(i)Pr ligand significantly increases the rate of the reaction, an outcome that can be attributed to a combination of steric and electronic factors. In addition, mechanistic studies have established that in solution 3 loses neopentane under ambient conditions to generate exclusively the 16e η(2)-diene intermediate complex (η(5)-C5H4(i)Pr)W(NO)(η(2)-CH2═CMeCH═CH2), which then effects the subsequent C-D activations. This behavior contrasts with that exhibited by the η(5)-C5Me5 analogue of 3 which forms both η(2)-diene and η(2)-allene intermediates upon thermolysis. Sixteen-electron (η(5)-C5H4(i)Pr)W(NO)(η(2)-CH2═CMeCH═CH2) has been isolated as its 18e PMe3 adduct. All new organometallic complexes have been characterized by conventional spectroscopic and analytical methods, and the solid-state molecular structures of two of them have been established by single-crystal X-ray crystallographic analyses.
As illustrated in the accompanying diagram, thermolysis of Cp*W(NO)(CH 2 CMe 3 ) 2 (Cp* = η 5 -C 5 Me 5 ) at 80 °C in neat linear alkanes effects three successive C−H bond activations of the hydrocarbon substrates and forms Cp*W-(NO)(H)(η 3 -allyl) complexes in which the allyl ligands are derived from the alkanes. These allyl hydrido compounds exist in solutions as mixtures of isomers containing monosubstituted (i.e., terminal) or 1,3-disubstituted (i.e., internal) allyl ligands which can have either an endo or exo orientation with the substituent groups being either proximal or distal to the nitrosyl ligand. Due to steric factors the most abundant isomer in all cases has a monosubstituted allyl ligand in the endo orientation with the alkyl end distal to the nitrosyl ligand. In addition, the relative abundance of Cp*W(NO)(H)(η 3 -allyl) isomers having monosubstituted allyl ligands decreases with increasing length of the n-alkane chain. Further thermolysis of the Cp*W(NO)(H)(η 3 -allyl) complexes results in the liberation of alkenes. Whether initiated by Cp*W(NO)(CH 2 CMe 3 ) 2 or independently synthesized Cp*W(NO)(H)(η 3 -allyl) complexes, the n-alkane dehydrogenations generally result in the preferential formation of 1-alkenes. They are stoichiometric, and their outcomes are not significantly affected by varying the experimental conditions employed (e.g., time, temperature, an open system, use of an H 2 acceptor, etc.) or by changing the initial bis(neopentyl) tungsten reactant to its Cp Et (η 5 -C 5 Me 4 Et) and Cp iPr (η 5 -C 5 H 4 i Pr) analogues or to Cp*Mo(NO)(CH 2 CMe 3 ) 2 . The results of DFT calculations are consistent with these dehydrogenations proceeding via 16e Cp*M(NO)(η 2 -alkene) (M = Mo, W) intermediates that are in equilibrium with their more stable 18e Cp*M(NO)(H)(η 3 -allyl) isomers. These intermediates facilitate the allyl ligand exchange reactions depicted in the accompanying diagram by functioning as internal hydrogen acceptors during the dehydrogenation of the linear alkanes. Thermolysis of the final hydrido allyl complexes liberates the desired alkenes.
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