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Successive Protonation and Methylation of Bridging Imido and Nitrido Ligands at Titanium Complexes
Paula Barriopedro1, Jorge Caballo1, Miguel Mena1,2
1Departamento de Química Orgánica y Química Inorgánica, Universidad de Alcalá, 28805 Alcalá de Henares, Madrid, Spain.
Abstract:
The reactions of nitrido complexes [{Ti(η5-C5Me5)(μ-NH)}3(μ3-N)] (1) and [{Ti(η5-C5Me5)}4(μ3-N)4] (2) with electrophilic reagents ROTf (R = H, Me; OTf = OSO2CF3) in different molar ratios have allowed the structural characterization of a series of titanium intermediates en route to the formation of the ammonium salts [NR4]OTf and [NR4][Ti(η5-C5Me5)(OTf)4]. The treatment of the trinuclear imido-nitrido complex 1 with 5.5 equiv of triflic acid in toluene at room temperature led to the dinuclear complex [Ti2(η5-C5Me5)2(μ-N)(NH3)(μ-O2SOCF3)2(OTf)] (3) and [NH4]OTf. Compound 3, along with the ammonium salts [NMe4]OTf and [NMe4][Ti(η5-C5Me5)(OTf)4] (5), was also obtained in the reaction of 1 with 8 equiv of methyl triflate in toluene at 100 °C. The trinuclear complex [Ti3(η5-C5Me5)3(μ-N)(μ-NH)2(μ-O2SOCF3)(OTf)] (4), an intermediate in the formation of 3, was isolated in the treatment of 1 with 4 equiv of MeOTf, although compound 4 was prepared in better yield by treatment of 1 with Me3SiOTf (2 equiv). Addition of a large excess of MeOTf or HOTf reagents to solutions of 3 resulted in the clean formation of ammonium salts [NR4][Ti(η5-C5Me5)(OTf)4] (R = Me (5), H (6)). Treatment of the tetranuclear nitrido complex [{Ti(η5-C5Me5)}4(μ3-N)4] (2) with 1 equiv of ROTf in toluene afforded the precipitation of the ionic compounds [{Ti(η5-C5Me5)}4(μ3-N)3(μ3-NR)][OTf] (R = H (8), Me (9)), while a large excess of HOTf led to the formation of [{Ti(η5-C5Me5)}4(μ3-N)3(μ3-NH)][Ti(η5-C5Me5)(OTf)4(NH3)] (10) by rupture of a fraction of tetranuclear molecules. Complex 2 reacted with 1 equiv of [M(η5-C5H5)(CO)3H] (M = Mo, Cr) via hydrogenation of one nitrido ligand to give the molecular derivative [{Ti(η5-C5Me5)}4(μ3-N)3(μ3-NH)] (11) and [{M(η5-C5H5)(CO)3}2], while a second 1 equiv of [M(η5-C5H5)(CO)3H] produced the ionic compounds [{Ti(η5-C5Me5)}4(μ3-N)2(μ3-NH)2][M(η5-C5H5)(CO)3] (M = Mo (12), Cr (13)) by protonation of another nitrido group. The X-ray crystal structures of 3-5, 9, 10, and 13 were determined.
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