SYNTHESIS AND DECOMPOSITION OF ALKYL COMPLEXES OF MOLYBDENUM(IV) THATCONTAIN A [(ME3SINCH2CH2)(3)N](3-) LIGAND - DIRECT-DETECTION OF ALPHA-ELIMINATION PROCESSES THAT ARE MORE THAN 6 ORDERS OF MAGNITUDE FASTERTHAN BETA-ELIMINATION PROCESSES

A variety of paramagnetic molybdenum complexes, [N3N]MoR ([N3N](3-) = [(Me3SiNCH2CH2)(3)N](3-); R = Me, Et, Bu, CH2Ph, CH2SiMe3, CH2CMe3, cy clopropyl, cyclobutyl, cyclopentyl, cyclohexyl, cyclopentenyl, phenyl) , have been prepared from [N3N]MoCl. The several that have been examin ed all follow Curie-Weiss S = 1 behavior with a magnetic moment in the solid state between 2.4 and 2.9 mu(B) down to 50 K. Below similar to 50 K the effective moments undergo a sharp decrease as a consequence o f what are proposed to be a combination of spin-orbit coupling and zer o field splitting effects. NMR spectra are temperature dependent as a consequence of ''locking'' of the backbone into one C-3-symmetric conf ormation and as a consequence of Curie-Weiss behavior. The cyclopentyl and cyclohexyl complexes show another type of temperature-dependent f luxional behavior that can be ascribed to a rapid and reversible alpha -elimination process. For the cyclopentyl complex the rate constant fo r alpha-elimination is similar to 10(3) s(-1) at room temperature, whi le the rate constant for alpha-elimination for the cyclohexyl complex is estimated to be similar to 200 s(-1) at room temperature. An isotop e effect for alpha-elimination for the cyclohexyl complex was found to be similar to 3 at 337 K, Several of the alkyl complexes decompose be tween 50 and 120 degrees C. Of the complexes that contain linear alkyl s, only [N3N]Mo(CH2CMe3) decomposes cleanly (but slowly) by alpha,alph a-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 atte mpted thermolysis at 120 degrees C, Other decompositions of linear alk yl complexes are complicated by competing reactions, including beta-hy dride elimination, beta-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 10x t hat of the latter at 298 K. beta-Hydride elimination in [N3N]Mo(cyclop entyl) to give (unobservable) [N3N]Mo(cyclopentene)(H) has been shown to be 6-7 orders of magnitude slower than alpha-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=CH2CH2 CH3. [N3N]MoH decomposes slowly and reversibly at 100 degrees C to yie ld molecular-hydrogen and [(Me3SiNCH2CH2)(2)NCH2CH2SiMe2CH2]Mo ([bitN( 3)N]Mo). X-ray structures of [N3N]Mo(triflate), [N3N]MoMe, [N3N]Mo(cyc lohexyl), and [bitN(3)N]Mo show that the degree of twist of the TMS gr oups away from an ''upright'' position correlates with the size of the ligand in the apical pocket and that steric congestion in the cyclohe xyl complex is significantly greater than in the methyl complex. Relie f of steric strain in the ground state in molecules of this general ty pe to give a less crowded alkylidene hydride intermediate is proposed to be an important feature of the high rate of alpha-elimination relat ive to beta-elimination in several circumstances.