Assembly of Molybdenum/Titanium μ-Oxo Complexes via Radical Alkoxide C−O Cleavage

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Abstract

Three-coordinate Ti(NRAr)3 [R = C(CD3)2(CH3), Ar = C6H3Me2] 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)(OEt2) in the presence of TMEDA. Ti(tBuNPh)3 was prepared similarly in 75% yield by treatment of TiCl3(THF)3 with 3 equiv of Li(tBuNPh)(OEt2) in the presence of TMEDA. Reaction of Ti(NRAr)3 with NMo(OtBu)3 in hydrocarbon solvents at −35 °C generates a thermally unstable intermediate formulated as (tBuO)3Mo[μ-N]Ti(NRAr)3, which readily loses a tert-butyl radical and isomerizes at 25 °C. Kinetics of the latter process were obtained over the temperature range 20−60 °C; the process exhibits clean first-order behavior. The following activation parameters were obtained: ΔH⧧ = 21.4 ± 0.2 kcal mol-1 and ΔS⧧ = −3.7 ± 0.6 cal mol-1 K-1. The oxo-bridged product (tBuO)2(N)Mo[μ-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 15N NMR study. Ti(NRAr)3 (1 equiv) reacts further with (tBuO)2(N)Mo[μ-O]Ti(NRAr)3 to generate a species formulated as a second paramagnetic nitrido-bridged intermediate, (tBuO)2Mo{[μ-O]Ti(NRAr)3}{[μ-N]Ti(NRAr)3}, which at 25 °C loses a tert-butyl radical and isomerizes to give the final product, (tBuO)(N)Mo{[μ-O]Ti(NRAr)3}2, isolated as an orange powder in 91% yield. Characterization of the latter diamagnetic complex included an 15N NMR study. Attempts to displace a third tert-butyl radical by treatment of (tBuO)(N)Mo{[μ-O]Ti(NRAr)3}2 with Ti(NRAr)3 led to no reaction. Treatment of (tBuO)(N)Mo{[μ-O]Ti(NRAr)3}2 with neat methyl iodide led to the isolation of (MeO)(N)Mo{[μ-O]Ti(NRAr)3}2 in 51% yield; 13C and nitrido-15N derivatives of this species were prepared for spectroscopic characterization. O2Mo{[μ-O]Ti(tBuNPh)3}2 was prepared in 59% yield upon treatment of MoO2(OtBu)2 with 2 equiv of Ti(tBuNPh)3 in benzene at 65 °C. Full characterization of O2Mo{[μ-O]Ti(tBuNPh)3}2 included a single-crystal X-ray diffraction study. Previously reported (iPrO)3V[μ-O]Ti(NRAr)3 was oxidized with ferrocenium triflate to give TfOTi(NRAr)3 and OV(OiPr)3. TfOTi(NRAr)3 was prepared independently in 80% yield by treatment of Ti(NRAr)3 with ferrocenium triflate. (iPrO)3V[μ-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 (tBuO)3V[μ-O]Ti(NRAr)3 was prepared as orange-brown needles in 94% yield and was found to be thermally stable. The relatively robust μ-nitrido compound (Me2N)3Mo[μ-N]Ti(tBuNPh)3, which was prepared in 77% isolated yield, showed no decomposition when heated in benzene at 70 °C for 13 h.

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