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

The complexes <i>trans</i>-Cp*W­(NO)­(CH₂CMe₃)­(H)­(L) (Cp* = η⁵-C₅Me₅) result from the treatment of Cp*W­(NO)­(CH₂CMe₃)₂ in <i>n</i>-pentane with H₂ (∼1 atm) in the presence of a Lewis base, L. The designation of a particular geometrical isomer as <i>cis</i> or <i>trans</i> 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 <i>trans</i> to <i>cis</i> 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 <i>cis</i> complexes that may convert to the thermodynamically more stable <i>trans</i> isomers [e.g., Cp*W­(NO)­(PPh₃) initially forms <i>cis</i>-Cp*W­(NO)­(H)­(κ²-PPh₂­C₆H₄) that upon being warmed in <i>n</i>-pentane at 80 °C isomerizes to <i>trans</i>-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 <i>trans</i>-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 <i>cis</i> 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 <i>trans</i>-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