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The versatility of lithium reagents in synthetic organometallic chemistry: Their differing reactions with [(CpMo)-Mo-*(NO)(CH2SiMe3)(2)]

TitleThe versatility of lithium reagents in synthetic organometallic chemistry: Their differing reactions with [(CpMo)-Mo-*(NO)(CH2SiMe3)(2)]
Publication TypeJournal Article
Year of Publication1997
AuthorsLegzdins, P, Sayers, SF
JournalChemistry-a European Journal
Volume3
Pagination1579-1587
Date PublishedOct
Type of ArticleArticle
ISBN Number0947-6539
KeywordsCOMPLEXES, H BOND ACTIVATION, LIGAND, lithium, LIVING POLYMERIZATION, {ALKYLIDENE COMPLEXES
Abstract

[Cp*Mo(NO)(CH2SiMe3)(2)] exhibits three principal types of reaction with the various lithium reagents investigated during this study, namely: regioselective deprotonation, reduction, and addition. Deprotonation of the reactant, achieved by treatment with lithium amide reagents, leads ultimately to the formation of the alkylidene ’’ate’’ complex [Cp*Mo(NO)(CH2SiMe3)(=CHSiMe3)](2)[Li-2(thf)(3)] (1). While LiN(SiMe3)(2) effects this conversion directly with no detectable intermediates, reaction with 1 equiv of LDA in THF for 15 min deprotonates the Cp* ligand to form the lithium salt of the ’’tucked-in’’ ate complex [(eta(5),eta(1)C(5)Me(4)CH(2))Mo(NO)(CH2SiMe3)(2)][Li(thf)(3)] (2) in 40% isolated yield, Complex 2 slowly converts to the thermodynamically more stable 1 when left as a THF or C6D6 solution at ambient temperature fbr 48 h. Reaction of the dialkyl starting material with either tBuLi or PhLi leads to the production of the alkylidene complex 1 in irreproducible yields (10-50% NMR; not isolable). A kinetic analysis of the reaction of [Cp*Mo(NO)(CH2SiMe3)(2)] with LiN(SiMe3)(2) indicated that the reaction was first-order in both the lithium and molybdenum reagents, and the activation parameters of Delta H-double dagger = 7,3 +/- 1.0 kcal mol(-1) and Delta S (double dagger) = -34 +/- 3 e.u. suggest an associative process. Treatment of the neutral dialkyl with 1 equiv of LiPPh2, in THF results in a one-electron reduction and production of {[Cp*Mo(NO)(CH2SiMe3)(2)][Li(thf)]}(2) (3). If left in solution, the 17e(-) dialkyl anion 3 is converted to the 18e(-) alkylidene anion 1 by the Ph2P-PPh2 coproduct, which effects the requisite hydrogen-atom abstraction. Finally, addition of a sterically undemanding alkyllithium reagent such as MeLi to the 16e(-) dialkyl reactant leads to the formation of the 18e(-) trialkyl anionic complex [Cp*Mo(NO)(CH2SiMe3)(2)(Me)][Li(thf)(3)] (4). Warming of 4 in a C6D6 solution results in loss of methane and production of 1. In most cases, the chemistry exhibited by the Mo system is duplicated by the analogous W congener, [Cp*W(NO)(CH2SiMe3)(2)]. The solid-state molecular structures of complexes 1 and 4 have been established by single-crystal X-ray crystallographic analyses.

URL<Go to ISI>://A1997YA85600004