Assembly of molybdenum/titanium mu-oxo complexes via radical alkoxide C-O cleavage

Three-coordinate Ti(NRAr)(3) [R = C(CD3)(2)(CH3), Ar = C(6)H(3)Me(2)] was prepared in 73% yield by sodium amalgam reduction of ClTi(NRAr)(3) and in 83% yield upon treatment of TiCl3(THF)(3) with 3 equiv of Li(NRAr)-(OEt(2)) in the presence of TMEDA. Ti((t)BuNPh)(3) was prepared similarly in 75% yield by treatment of TiCl3(THF)(3) with 3 equiv of Li((t)BuNPh)(OE(2)) in the presence of TMEDA. Reaction of Ti(NRAr)(3) with NMo(O(t)Bu)(3) in hydrocarbon solvents at -35 degrees C generates a thermally unstable intermediate formulated as ((t)BuO)(3)Mo[mu-N]Ti-(NRAr)(3), which readily loses a tert-butyl radical and isomerizes at 25 degrees C. Kinetics of the latter process were obtained over the temperature range 20-60 degrees C; the process exhibits clean first-order behavior. The following activation parameters were obtained: Delta H-double dagger = 21.4 +/- 0.2 kcal mol(-1) and Delta S-double dagger = -3.7 +/- 0.6 cal mol(-1) K-1. The ore-bridged product ((t)BuO)(2)(N)Mo[mu-O]Ti(NRAr)(3) was isolated in 83% yield from this reaction. Full characterization of the latter diamagnetic complex included an X-ray crystal structure and an N-15 NMR study. Ti(NRAr)(3) (1 equiv) reacts further with ((t)BuO)(2)(N)Mo[mu-O]Ti(NRAr)(3) to generate a species formulated as a second paramagnetic nitrido-bridged intermediate, ((t)BuO)(2)Mo{[mu-O]Ti(NRAr)(3)}{mu-N]Ti(NRAr)(3)}, which at 25 degrees C loses a tert-butyl radical and isomerizes to give the final product, ((t)BuO)(N)Mo{[mu-O]Ti(NRAr)(3)}(2), isolated as an orange powder in 91% yield. Characterization of the latter diamagnetic complex included an N-15 NMR study. Attempts to displace a third tert-butyl radical by treatment of ((t)BuO)(N)Mo{[mu-O]Ti(NRAr)(3)}(2) with Ti(NRAr)(3) led to no reaction. Treatment of ((t)BuO)(N)Mo-{[mu-O]Ti(NRAr)(3)}(2) with neat methyl iodide led to the isolation of (MeO)(N)Mo{[mu-O]Ti(NRAr)(3)}(2) in 51% yield; C-13 and nitrido-N-15 derivatives of this species were prepared for spectroscopic characterization. O2Mo{[mu-O]Ti-((t)BuNPh)(3)}(2) was prepared in 59% yield upon treatment of MoO2(O(t)Bu)(2) with 2 equiv of Ti((t)BuNPh)(3) in benzene at 65 degrees C, Full characterization of O2Mo{[mu-O]Ti((t)BuNPh)(3)}(2) included a single-crystal X-ray diffraction study. Previously reported ((PrO)-Pr-i)(3)V[mu-O]Ti(NRAr)(3) was oxidized with ferrocenium triflate to give TfOTi(NRAr)(3) and OV((OPr)-Pr-i)(3). TfOTi(NRAr)(3) was prepared independently in 80% yield by treatment of Ti(NRAr)(3) with ferrocenium triflate. ((PrO)-Pr-i)(3)V[mu-O]Ti(NRAr)(3) is stable in the presence of methyl iodide. ITi(NRAr)(3) was prepared independently by treatment of Ti(NRAr)(3) with the stoichiometric amount of iodine, Paramagnetic ((t)BuO)(3)V[mu-O]Ti(NRAr)(3) was prepared as orange-brown needles in 94% yield and was found to be thermally stable. The relatively robust mu-nitrido compound (Me(2)N)(3)Mo[mu-N]Ti((t)BuNPh)(3), which was prepared in 77% isolated yield, showed no decomposition when heated in benzene at 70 degrees C for 13 h.