Reactions of co-ordinated ligands. Part 54. Synthesis of (1,2,3-η)-trans-butadienyl complexes by deprotonation or desilylation of cationic molybdenum 1,3-diene complexes; formation of η4-vinylketene complexes and crystal structures of [Mo{(1,2,3-η)-trans-CH2CHCCH2}(CO)2(η-C5Me5)], [Mo{η3-CH2CC(Me)CH2CO}(CO)2(η-C5Me5)] and [Mol{η4-CH2CHC(Me)CO}(CO)(η-C5Me5)]

Abstract

One of the products of the reaction of the base Li[N(SiMe₃)₂] with [Mo(η⁴-C₄H₆)(CO)₂(η-C₅Me₅)][BF₄] is the exo-orientated (1,2,3-η)-trans-butadienyl complex [Mo{(1,2,3-η)-trans-CH₂CHCCH₂}(CO)₂(η-C₅Me₅)]. A single-crystal X-ray diffraction study of the latter has revealed that the two adjacent π systems, allyl and CCH₂ within the butadienyl fragment are orthogonal to each other. This complex can be formed selectively and in high yield by reaction of [NBuⁿ₄]F with the cation [Mo{η⁴-CH₂CHC(SiEt₃)CH₂}(CO)₂(η-C₅Me₅)][BF₄]. The desilylation reaction has also been used to transform [Mo{η⁴-syn-CH(Ph)CHC(SiMe₃)CH₂}(CO)₂(η-C₅Me₅)][BF₄] into a mixture of exo- and endo-[Mo{(1,2,3-η)-trans-syn-CH(Ph)CHCCH₂}(CO)₂(η-C₅Me₅)]. An alternative synthetic route to (1,2,3-η)-trans-butadienyl complexes was explored involving reaction of Li[Mo(CO)₃(η-C₅Me₅)] with CH₂CC(Me)CH₂Cl. In the event an unusual cyclisation reaction occurred leading to the formation of the X-ray crystallographically identified complex [Mo{η³-CH₂[graphic omitted]O}(CO)₂(η-C₅Me₅)]. Extended Hückel molecular orbital calculations on [Mo{(1,2,3-η)-trans-CH₂CHCCH₂}(CO)₂(η-C₅Me₅)] using the complete set of crystallographic coordinates shows that reaction with protons should occur on the end carbon of the butadienyl ligand to form a cationic η³-vinylcarbene complex. However, protonation with CF₃SO₃H affords a vinylketene complex [Mo(OSO₂CF₃){η⁴-CH₂CHC(Me)CO}(CO)(η-C₅Me₅)] the structural identity of which was established by an X-ray crystallographic study of the analogue [Mol{η⁴-CH₂CHC(Me)CO}(CO)(η-C₅Me₅)] obtained from the triflate complex via metathesis. The mechanisms of these reactions are discussed.