Thermal Chemistry of Cp*W(NO)(CH₂CMe₃)(H)(L) Complexes (L = Lewis Base)

The complexes trans-Cp*W(NO)(CH₂CMe₃)(H)(L) (Cp* = η⁵-C₅Me₅) result from the treatment of Cp*W(NO)(CH₂CMe₃)₂ in n-pentane with H₂ (∼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)₃, P(OPh)₃, or P(OCH₂)₃CMe]. Others undergo reductive elimination of CMe₄ 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)(PPh₃) initially forms cis-Cp*W(NO)(H)(κ²-PPh₂C₆H₄) that upon being warmed in n-pentane at 80 °C isomerizes to trans-Cp*W(NO)(H)(κ²-PPh₂C₆H₄)]. 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)₃ or P(OCH₂)₃CMe; R-H = C₆H₆ or Me₄Si] probably via their cis isomers. These latter activations are also accompanied by the formation of some Cp*W(NO)(L)₂ disproportionation products. An added complication in the L = P(OMe)₃ system is that thermolysis of trans-Cp*W(NO)(CH₂CMe₃)(H)(P(OMe)₃) results in it undergoing an Arbuzov-like rearrangement and being converted mainly into [Cp*W(NO)(Me)(PO(OMe)₂)]₂, 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.