Synthesis and Decomposition of Alkyl Complexes of Molybdenum(IV) That Contain a [(Me3SiNCH2CH2)3N]3- Ligand. Direct Detection of α-Elimination Processes That Are More than Six Orders of Magnitude Faster than β-Elimination Processes
1997-12-10T00:00:00Z (GMT) by
A variety of paramagnetic molybdenum complexes, [N3N]MoR ([N3N]3- = [(Me3SiNCH2CH2)3N]3-; R = Me, Et, Bu, CH2Ph, CH2SiMe3, CH2CMe3, cyclopropyl, cyclobutyl, cyclopentyl, cyclohexyl, cyclopentenyl, phenyl), have been prepared from [N3N]MoCl. The several that have been examined all follow Curie−Weiss S = 1 behavior with a magnetic moment in the solid state between 2.4 and 2.9 μΒ down to 50 K. Below ∼50 K the effective moments undergo a sharp decrease as a consequence of what are proposed to be a combination of spin−orbit coupling and zero field splitting effects. NMR spectra are temperature dependent as a consequence of “locking” of the backbone into one C3-symmetric conformation and as a consequence of Curie−Weiss behavior. The cyclopentyl and cyclohexyl complexes show another type of temperature-dependent fluxional behavior that can be ascribed to a rapid and reversible α-elimination process. For the cyclopentyl complex the rate constant for α-elimination is ∼103 s-1 at room temperature, while the rate constant for α-elimination for the cyclohexyl complex is estimated to be ∼200 s-1 at room temperature. An isotope effect for α-elimination for the cyclohexyl complex was found to be ∼3 at 337 K. Several of the alkyl complexes decompose between 50 and 120 °C. Of the complexes that contain linear alkyls, only [N3N]Mo(CH2CMe3) decomposes cleanly (but slowly) by α,α-dehydrogenation to give [N3N]Mo⋮CCMe3. [N3N]MoMe is by far the most stable of the alkyl complexes; no [N3N]Mo⋮CH can be detected upon attempted thermolysis at 120 °C. Other decompositions of linear alkyl complexes are complicated by competing reactions, including β-hydride elimination. β-Hydride elimination (to give [N3N]MoH) is the sole mode of decomposition of the cyclopentyl and cyclohexyl complexes; the former decomposes at a rate calculated to be approximately 10× that of the latter at 298 K. β-Hydride elimination in [N3N]Mo(cyclopentyl) to give (unobservable) [N3N]Mo(cyclopentene)(H) has been shown to be 6−7 orders of magnitude slower than α-hydride elimination to give (unobservable) [N3N]Mo(cyclopentylidene)(H). [N3N]Mo(cyclopropyl) evolves ethylene in a first-order process upon being heated to give [N3N]Mo⋮CH, while [N3N]Mo(cyclobutyl) is converted into [N3N]Mo⋮CCH2CH2CH3. [N3N]MoH decomposes slowly and reversibly at 100 °C to yield molecular hydrogen and [(Me3SiNCH2CH2)2NCH2CH2SiMe2CH2]Mo ([bitN3N]Mo). X-ray structures of [N3N]Mo(triflate), [N3N]MoMe, [N3N]Mo(cyclohexyl), and [bitN3N]Mo show that the degree of twist of the TMS groups away from an “upright” position correlates with the size of the ligand in the apical pocket and that steric congestion in the cyclohexyl complex is significantly greater than in the methyl complex. Relief of steric strain in the ground state in molecules of this general type to give a less crowded alkylidene hydride intermediate is proposed to be an important feature of the high rate of α-elimination relative to β-elimination in several circumstances.