Synthesis and Decomposition of Alkyl Complexes of Tungsten(IV) That Contain a [(Me3SiNCH2CH2)3N]3- Ligand

The synthesis of a variety of tungsten alkyl complexes of the type [N 3 N]WR ([N 3 N] 3 - = [(Me 3 SiNCH 2 CH 2 ) 3 N] 3 -; R = Me, Et, Bu, CH 2 Ph, CH 2 SiMe 3, CH 2 CMe 3, cyclopropyl, cyclobutyl, cyclopentyl, cyclohexyl, cyclopentenyl) was attempted by alkylation of [N 3 N]WCl complexes. Only the methyl, phenyl, and cyclopentenyl complexes could be isolated. The susceptibility of the [N 3 N]WR complexes is similar to that of [N 3 N]WCl; the effective moment is depressed as a consequence of a combination of spin−orbit effects and low-symmetry ligand-field components that result in zero-field splitting of the d 2 ground-state spin triplet. No linear alkyl complexes could be observed as a consequence of α,α-dehydrogenation to give molecular hydrogen and alkylidyne complexes, [N 3 N]W⋮CR‘. [N 3 N]W(cyclopropyl) evolves ethylene in a first-order process to give [N 3 N]W⋮CH, while [N 3 N]W(cyclobutyl) is converted into the 1-tungstacyclopentene complex, [N 3 N]W(CHCH 2 CH 2 CH 2 ), as confirmed in an X-ray study. [N 3 N]W(CHCH 2 CH 2 CH 2 ) decomposes readily in a first-order manner to give [N 3 N]W⋮CCH 2 CH 2 CH 3 . An attempt to prepare [N 3 N]W(cyclopentyl) led to formation of [N 3 N]W(cyclopentylidene)(H), as confirmed in an X-ray study. NMR studies suggest that [N 3 N]W(cyclopentylidene)(H) is in equilibrium with [N 3 N]W(cyclopentyl) where Δ H ° = 11.8(6) kcal mol - 1 and Δ S ° = 33(2) eu or K eq ≈ 0.1 at 46 °C. From these and other data it is concluded that the rate constant for α elimination in [N 3 N]M(cyclopentyl) is approximately the same for Mo and W. The [N 3 N]W(cyclopentenyl) complex is unstable, decomposing to give a complex containing a “bent imido” ligand, [(Me 3 SiNCH 2 CH 2 ) 2 (NCH 2 CH 2 )N]W(1-(trimethylsilyl)cyclopentene), as confirmed in an X-ray study.