Addition of Terminal Alkynes to a Molecular Titanium–Zinc Nitride

Ynamide and Azaalleneyl. Acid-Base Promoted Chelotropic and Spin-State Rearrangements in a Versatile Heterocumulene [(Ad)NCC(tBu)]. Angew Chem Int Ed Engl 2024;63:e202401433. [PMID: 38433099 DOI: 10.1002/anie.202401433] [Citation(s) in RCA: 0] [Impact Index Per Article: 0]

We introduce the heterocumulene ligand [(Ad)NCC(tBu)]- (Ad=1-adamantyl (C10H15), tBu=tert-butyl, (C4H9)), which can adopt two forms, the azaalleneyl and ynamide. This ligand platform can undergo a reversible chelotropic shift using Brønsted acid-base chemistry, which promotes an unprecedented spin-state change of the [VIII] ion. These unique scaffolds are prepared via addition of 1-adamantyl isonitrile (C≡NAd) across the alkylidyne in complexes [(BDI)V≡CtBu(OTf)] (A) (BDI-=ArNC(CH3)CHC(CH3)NAr), Ar=2,6-iPr2C6H3) and [(dBDI)V≡CtBu(OEt2)] (B) (dBDI2-=ArNC(CH3)CHC(CH2)NAr). Complex A reacts with C≡NAd, to generate the high-spin [VIII] complex with a κ1-N-ynamide ligand, [(BDI)V{κ1-N-(Ad)NCC(tBu)}(OTf)] (1). Conversely, B reacts with C≡NAd to generate a low-spin [VIII] diamagnetic complex having a chelated κ2-C,N-azaalleneyl ligand, [(dBDI)V{κ2-N,C-(Ad)NCC(tBu)}] (2). Theoretical studies have been applied to better understand the mechanism of formation of 2 and the electronic reconfiguration upon structural rearrangement by the alteration of ligand denticity between 1 and 2.

Studies on the chemical reduction of polynuclear titanium(IV) nitrido complexes

The redox chemistry of cube-type titanium(IV) nitrido complexes [{Ti4(η5-C5Me5)3(R)}(μ3-N)4] (R = η5-C5Me5 (1), N(SiMe3)2 (2), η5-C5H4SiMe3 (3), and η5-C5H5 (4)) was investigated by electrochemical methods and chemical reactions. Cyclic voltammetry studies indicate that 1-4 undergo a reversible one-electron reduction at ca. -1.8 V vs. ferrocenium/ferrocene. Thus, complex 1 reacts with sodium sand in tetrahydrofuran to produce the highly reactive ionic compound [Na(thf)6][{Ti(η5-C5Me5)}4(μ3-N)4] (5). The treatment of complexes 1-4 in toluene with one equivalent of [K(C5Me5)] in the presence of macrocycles (L) leads to C10Me10 and the formation of more stable derivatives [K(L)][{Ti4(η5-C5Me5)3(R)}(μ3-N)4] (R = η5-C5Me5, L = 18-crown-6 (6), crypt-222 (7); R = N(SiMe3)2, L = 18-crown-6 (8), crypt-222 (9); R = η5-C5H4SiMe3, L = 18-crown-6 (10), crypt-222 (11); R = η5-C5H5, L = crypt-222 (12)). However, the analogous reaction of 4 with [K(C5Me5)] and 18-crown-6 affords [{(18-crown-6)K}2(μ-η5:η5-C5H5)][{Ti4(η5-C5Me5)3(η5-C5H5)}(μ3-N)4] (13) via abstraction of one cyclopentadienide group from a putative intermediate [(18-crown-6)K(μ-η5:η5-C5H5)Ti4(η5-C5Me5)3(μ3-N)4]. In contrast to the cube-type nitrido systems 1-4, the cyclic voltammogram of the trinuclear imido-nitrido titanium(IV) complex [{Ti(η5-C5Me5)(μ-NH)}3(μ3-N)] (14) does not reveal any reversible redox event and 14 readily reacts with [K(C5Me5)] to afford C5Me5H and the diamagnetic derivative [{K(μ4-N)(μ3-NH)2Ti3(η5-C5Me5)3(μ3-N)}2] (15). The treatment of 15 with two equiv. of 18-crown-6 polyethers produces the molecular species [(L)K{(μ3-N)(μ3-NH)2Ti3(η5-C5Me5)3(μ3-N)}] (L = 18-crown-6 (16), dibenzo-18-crown-6 (17)). Complex 17 further reacts with one equiv. of dibenzo-18-crown-6 to yield the ion-separated compound [K(dibenzo-18-crown-6)2][Ti3(η5-C5Me5)3(μ3-N)(μ-N)(μ-NH)2] (18) similar to the ion pair [K(crypt-222)][Ti3(η5-C5Me5)3(μ3-N)(μ-N)(μ-NH)2] (19) obtained in the treatment of 15 with cryptand-222.

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

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.

Cleavage of Dinitrogen from Forming Gas by a Titanium Molecular System under Ambient Conditions

Simple exposure of a hexane solution of [TiCp*Me₃ ] (Cp*=η⁵ -C₅ Me₅ ) to an atmosphere of commercially available and inexpensive forming gas (H₂ /N₂ mixture, 13.5-16.5 % of H₂ ) at room temperature leads to the methylidene-methylidyne-nitrido cube-type complex [(TiCp*)₄ (μ₃ -CH)(μ₃ -CH₂ )(μ₃ -N)₂ ] via dinitrogen cleavage. This paramagnetic compound reacts with [D₁ ]chloroform to give the titanium(IV) methylidyne-nitrido species [(TiCp*)₄ (μ₃ -CH)₂ (μ₃ -N)₂ ], whereas its one-electron oxidation with AgOTf or [Fe(η⁵ -C₅ H₅ )₂ ](OTf) (OTf=O₃ SCF₃ ) yields the diamagnetic ionic derivative [(TiCp*)₄ (μ₃ -CH)(μ₃ -CH₂ )(μ₃ -N)₂ ](OTf). The μ₃ -nitrido ligands of the methylidyne-nitrido cubane complex can be protonated with [LutH](OTf) (Lut=2,6-lutidine) or hydrogenated with NH₃ ⋅BH₃ to afford μ₃ -NH imido moieties.

Carbon-nitrogen bond construction and carbon-oxygen double bond cleavage on a molecular titanium oxonitride: a combined experimental and computational study

New carbon-nitrogen bonds were formed on addition of isocyanide and ketone reagents to the oxonitride species [{Ti(η(5)-C5Me5)(μ-O)}3(μ3-N)] (1). Reaction of 1 with XylNC (Xyl = 2,6-Me2C6H3) in a 1:3 molar ratio at room temperature leads to compound [{Ti(η(5)-C5Me5)(μ-O)}3(μ-XylNCCNXyl)(NCNXyl)] (2), after the addition of the nitrido group to one coordinated isocyanide and the carbon-carbon coupling of the other two isocyanide molecules have taken place. Thermolysis of 2 gives [{Ti(η(5)-C5Me5)(μ-O)}3(XylNCNXyl)(CN)] (3) where the heterocumulene [XylNCCNXyl] moiety and the carbodiimido [NCNXyl] fragment in 2 have undergone net transformations. Similarly, tert-butyl isocyanide (tBuNC) reacts with the starting material 1 under mild conditions to give the paramagnetic derivative [{Ti3(η(5)-C5Me5)3(μ-O)3(NCNtBu)}2(μ-CN)2] (4). However, compound 1 provides the oxo ketimide derivatives [{Ti3(η(5)-C5Me5)3(μ-O)4}(NCRPh)] [R = Ph (5), p-Me(C6H4) (6), o-Me(C6H4) (7)] upon reaction with benzophenone, p-methylbenzophenone, and o-methylbenzophenone, respectively. In these reactions, the carbon-oxygen double bond is completely ruptured, leading to the formation of a carbon-nitrogen and two metal-oxygen bonds. The molecular structures of complexes 2-4, 6, and 7 were determined by single-crystal X-ray diffraction analyses. Density functional theory calculations were performed on the incorporation of isocyanides and ketones to the model complex [{Ti(η(5)-C5H5)(μ-O)}3(μ3-N)] (1H). The mechanism involves the coordination of the substrates to one of the titanium metal centers, followed by an isomerization to place those substrates cis with respect to the apical nitrogen of 1H, where carbon-nitrogen bond formation occurs with a low-energy barrier. In the case of aryl isocyanides, the resulting complex incorporates additional isocyanide molecules leading to a carbon-carbon coupling. With ketones, the high oxophilicity of titanium promotes the unusual total cleavage of the carbon-oxygen double bond.

Reactivity with electrophiles of imido groups supported on trinuclear titanium systems

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.

Electrophilic attack on trinuclear titanium imido-nitrido systems

Alkylation of [{Ti(η(5)-C(5)Me(5))(μ-NH)}(3)(μ(3)-N)] with MeOTf occurs at the imido ligands to produce the methylamido derivative [Ti(3)(η(5)-C(5)Me(5))(3)(μ(3)-N)(μ-NH)(2)(μ-NHMe)(OTf)] which readily rearranges to form the methylimido complex [Ti(3)(η(5)-C(5)Me(5))(3)(μ(3)-N)(μ-NH)(μ-NH(2))(μ-NMe)(OTf)].