Multiple C–H Activations of Linear Alkanes by Various (η5‑Cyclopentadienyl)W(NO)(CH2CMe3)2 Complexes

As illustrated in the accompanying diagram, thermolysis of Cp*W­(NO)­(CH2CMe3)2 (Cp* = η5-C5Me5) at 80 °C in neat linear alkanes effects three successive C–H bond activations of the hydrocarbon substrates and forms Cp*W­(NO)­(H)­(η3-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)­(η3-allyl) isomers having monosubstituted allyl ligands decreases with increasing length of the n-alkane chain. Further thermolysis of the Cp*W­(NO)­(H)­(η3-allyl) complexes results in the liberation of alkenes. Whether initiated by Cp*W­(NO)­(CH2CMe3)2 or independently synthesized Cp*W­(NO)­(H)­(η3-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 H2 acceptor, etc.) or by changing the initial bis­(neopentyl) tungsten reactant to its CpEt (η5-C5Me4Et) and CpiPr (η5-C5H4iPr) analogues or to Cp*Mo­(NO)­(CH2CMe3)2. The results of DFT calculations are consistent with these dehydrogenations proceeding via 16e Cp*M­(NO)­(η2-alkene) (M = Mo, W) intermediates that are in equilibrium with their more stable 18e Cp*M­(NO)­(H)­(η3-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.