Reactivity with Electrophiles of Imido Groups Supported on Trinuclear Titanium Systems

Abstract

Several trinuclear titanium complexes bearing amido μ-NHR, imido μ-NR, and nitrido μn-N ligands have been prepared by reaction of [{Ti(η(5)-C5Me5)(μ-NH)}3(μ3-N)] (1) with 1 equiv of electrophilic reagents ROTf (R = H, Me, SiMe3; OTf = OSO2CF3). Treatment of 1 with triflic acid or methyl triflate in toluene at room temperature affords the precipitation of compounds [Ti3(η(5)-C5Me5)3(μ3-N)(μ-NH)2(μ-NH2)(OTf)] (2) or [Ti3(η(5)-C5Me5)3(μ3-N)(μ-NH)(μ-NH2)(μ-NMe)(OTf)] (3). Complexes 2 and 3 exhibit a fluxional behavior in solution consisting of proton exchange between μ-NH2 and μ-NH groups, assisted by the triflato ligand, as could be inferred from a dynamic NMR spectroscopy study. Monitoring by NMR spectroscopy the reaction course of 1 with MeOTf allows the characterization of the methylamido intermediate [Ti3(η(5)-C5Me5)3(μ3-N)(μ-NH)2(μ-NHMe)(OTf)] (4), which readily rearranges to give 3 by a proton migration from the NHMe amido group to the NH imido ligands. The treatment of 1 with 1 equiv of Me3SiOTf produces the stable ionic complex [Ti3(η(5)-C5Me5)3(μ3-N)(μ-NH)2(μ-NHSiMe3)][OTf] (5) with a disposition of the nitrogen ligands similar to that of 4. Complex 5 reacts with 1 equiv of [K{N(SiMe3)2}] at room temperature to give [Ti3(η(5)-C5Me5)3(μ3-N)(μ-N)(μ-NH)(μ-NHSiMe3)] (6), which at 85 °C rearranges to the trimethylsilylimido derivative [Ti3(η(5)-C5Me5)3(μ3-N)(μ-NH)2(μ-NSiMe3)] (7). Treatment of 7 with [K{N(SiMe3)2}] affords the potassium derivative [K{(μ3-N)(μ3-NH)(μ3-NSiMe3)Ti3(η(5)-C5Me5)3(μ3-N)}] (8), which upon addition of 18-crown-6 leads to the ion pair [K(18-crown-6)][Ti3(η(5)-C5Me5)3(μ3-N)(μ-N)(μ-NH)(μ-NSiMe3)] (9). The X-ray crystal structures of 2, 5, 6, and 8 have been determined.