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.

Successive Protonation and Methylation of Bridging Imido and Nitrido Ligands at Titanium Complexes

“…The first step in the formation of 4 involves the reaction of 1 equiv of Me 3 SiOTf with 1 at room temperature to give the previously reported complex [Ti 3 (η 5 -C 5 Me 5 ) 3 (μ 3 -N)(μ-NH) 2 (μ-NHSiMe 3 )][OTf] (Scheme 1). 28 Subsequent silylation of the NHSiMe 3 amido ligand of this ionic compound with an additional 1 equiv of Me 3 SiOTf at higher temperatures would result in formation of the amine NH(SiMe 3 ) 2 , which is eventually released with concomitant coordination of the triflato groups to give compound 4.…”

“…The trifluoromethanesulfonato ligand that links these two titanium atoms closes a second six-membered [Ti 2 NO 2 S] ring in chair conformation. The Ti–N­(imido) distances (Table ) in 4 range from 1.880(5) to 1.894(5) Å and resemble the values found in the complexes 1 (average 1.930(7) Å) and [Ti 3 (η 5 -C 5 Me 5 ) 3 (μ 3 -N)­(μ-NH)­(μ-NH 2 )­(μ-NR)­(OTf)] (1.831(3)–1.858(3) Å) . However, the Ti–N­(nitrido) distances of 1.791(5) and 1.797(4) Å are clearly shorter and agree with the higher titanium–nitrogen bond order expected for a TiNTi unit .…”

“…The treatment of [{Ti­(η 5 -C 5 Me 5 )­(μ-NH)} 3 (μ 3 -N)] ( 1 ) with 5.5 equiv of triflic acid in toluene at room temperature led to the precipitation of ammonium triflate and a dark orange solution, from which the dinuclear complex [Ti 2 (η 5 -C 5 Me 5 ) 2 (μ-N)­(NH 3 )­(μ-O 2 SOCF 3 ) 2 (OTf)] ( 3 ) was isolated in 81% yield (Scheme ). Reactions of 1 with lower ratios (1/2, 1/3, and 1/4) of HOTf gave complicated mixtures of products, where only compounds [Ti 3 (η 5 -C 5 Me 5 ) 3 (μ 3 -N)­(μ-NH) 2 (μ-NH 2 )­(OTf)] (Scheme ) and 3 were identified by NMR spectroscopy.…”

“…The treatment of [{Ti(η 5 -C 5 Me 5 )(μ-NH)} 3 (μ 3 -N)] (1) with 5.5 equiv of triflic acid in toluene at room temperature led to the precipitation of ammonium triflate and a dark orange solution, from which the dinuclear complex [Ti 2 (η 5 -C 5 Me 5 ) 2 (μ-N)(NH 3 )(μ-O 2 SOCF 3 ) 2 (OTf)] (3) was isolated in 81% yield (Scheme 3). Reactions of 1 with lower ratios (1/2, 1/3, and 1/4) of HOTf gave complicated mixtures of products, where only compounds [Ti 3 (η 5 -C 5 Me 5 ) 3 (μ 3 -N)(μ-NH) 2 (μ-NH 2 )(OTf)] (Scheme 1) 28 Complex 3 was characterized by analytical and spectroscopic methods, as well as by an X-ray crystal structure determination. The IR spectrum (KBr) of 3 shows three bands for the ν NH vibrations, between 3344 and 3183 cm −1 , and one absorption at 1607 cm −1 assignable to the NH 3 bending mode.…”

“…In particular, we reported on the reaction of 1 with 1 equiv of electrophilic reagents ROTf (R = H, Me, Me 3 Si) to generate polynuclear complexes by selective functionalization of the imido groups (Scheme ). , As a logical continuation of this study, herein we describe the reactivity of 1 with higher molar ratios of ROTf reagents and the characterization of titanium nitrido intermediates en route to the formation of the ammonium salts [NR 4 ]­OTf and [NR 4 ]­[Ti­(η 5 -C 5 Me 5 )­(OTf) 4 ].…”