An Effective Route to Dinuclear Niobium and Tantalum Imido Complexes

Thermal treatment of the trichloro complexes [MCl₃(NR)­py₂] (R = <i>t</i>Bu, Xyl; M = Nb, Ta) (Xyl = 2,6-Me₂C₆H₃) under vacuum affords the dinuclear imido species [MCl₂(μ-Cl)­(NR)­py]₂ (R = <i>t</i>Bu, Xyl; M = Nb <b>1</b>, <b>3</b>; Ta <b>2</b>, <b>4</b>) with loss of pyridine. Complexes <b>1</b>–<b>4</b> can be easily transformed to the mononuclear starting materials [MCl₃(NR)­py₂] (R = <i>t</i>Bu, Xyl; M = Nb, Ta) upon reaction with pyridine. While reactions of compounds <b>1</b> and <b>2</b> with a series of alkylating reagents render the mononuclear peralkylated imido complexes [MR₃(N<i>t</i>Bu)] (R = Me, CH₂Ph, CH₂CMe₃, CH₂CMePh, CH₂SiMe₃), the analogous treatment with allylmagnesium chloride results in the formation of the dinuclear niobium­(IV) derivative [(N<i>t</i>Bu)­(η³-C₃H₅)­M­(μ-C₃H₅)­(μ-Cl)₂M­(N<i>t</i>Bu)­py₂] (<b>5</b>). Additionally, the treatment of the starting materials <b>1</b> and <b>2</b> with the organosilicon reductant 1,4-bis­(trimethylsilyl)-1,4-diaza-2,5-cyclohexadiene yields the pyridyl-bridged dinuclear derivatives [M₂Cl₂(μ-Cl)₂(N<i>t</i>Bu)₂py₂]₂(μ-NC₄H₄N)₂ (M = Nb <b>6</b>, Ta <b>7</b>). Controlled hydrolysis reaction of <b>1</b> and <b>2</b> affords the oxo chlorido-bridged products [MCl­(μ-Cl)­(N<i>t</i>Bu)­py]₂(μ-O) (M = Nb <b>8</b>, Ta <b>9</b>) in a quantitative way, while the treatment of these latter with one more equivalent of pyridine led to complexes [MCl₂(N<i>t</i>Bu)­py₂]₂(μ-O) (M = Nb <b>10</b>, Ta <b>11</b>). Structural study of these dinuclear imido derivatives has been also performed by X-ray crystallography

C–H Activation on an Oxo-Bridged Dititanium Complex: From Alkyl to μ‑Alkylidene Functionalities

Thermal treatment of the dinuclear compound [{Ti­(η⁵-C₅Me₅)­(CH₂SiMe₃)₂}₂­(μ-O)] (<b>1</b>) provides the formation of the metallacycle derivatives [Ti₂(η⁵-C₅Me₅)₂­(μ-CH₂­SiMe₂CH₂)­(CH₂­SiMe₃)₂­(μ-O)] (<b>2</b>) and [Ti₂(η⁵-C₅Me₅)₂­(μ-CH₂­SiMe₂CH₂)₂­(μ-O)] (<b>3</b>) and the μ-alkylidene complex [Ti₂(η⁵-C₅Me₅)₂­(μ-CH₂­SiMe₂CH₂)­(μ-CHSiMe₃)­(μ-O)] (<b>4</b>) by sequential carbon–hydrogen activation processes. The reaction of <b>3</b> with <i>tert</i>-butylisocyanide, in 1:1 and 1:2 ratios, leads to the η²-iminoacyl complexes [Ti₂(η⁵-C₅Me₅)₂­(μ-<i>t</i>BuNC­CH₂­SiMe₂CH₂)­(μ-CH₂­SiMe₂CH₂)­(μ-O)] (<b>5</b>) and [Ti₂(η⁵-C₅Me₅)₂­(μ-<i>t</i>BuNC­CH₂­SiMe₂CH₂)₂­(μ-O)] (<b>6</b>), respectively. The molecular structures of complexes <b>3</b>, <b>4</b>, <b>5</b>, and <b>6</b> have been determined by single-crystal X-ray diffraction analyses

C–H Activation on an Oxo-Bridged Dititanium Complex: From Alkyl to μ‑Alkylidene Functionalities

Thermal treatment of the dinuclear compound [{Ti­(η⁵-C₅Me₅)­(CH₂SiMe₃)₂}₂­(μ-O)] (<b>1</b>) provides the formation of the metallacycle derivatives [Ti₂(η⁵-C₅Me₅)₂­(μ-CH₂­SiMe₂CH₂)­(CH₂­SiMe₃)₂­(μ-O)] (<b>2</b>) and [Ti₂(η⁵-C₅Me₅)₂­(μ-CH₂­SiMe₂CH₂)₂­(μ-O)] (<b>3</b>) and the μ-alkylidene complex [Ti₂(η⁵-C₅Me₅)₂­(μ-CH₂­SiMe₂CH₂)­(μ-CHSiMe₃)­(μ-O)] (<b>4</b>) by sequential carbon–hydrogen activation processes. The reaction of <b>3</b> with <i>tert</i>-butylisocyanide, in 1:1 and 1:2 ratios, leads to the η²-iminoacyl complexes [Ti₂(η⁵-C₅Me₅)₂­(μ-<i>t</i>BuNC­CH₂­SiMe₂CH₂)­(μ-CH₂­SiMe₂CH₂)­(μ-O)] (<b>5</b>) and [Ti₂(η⁵-C₅Me₅)₂­(μ-<i>t</i>BuNC­CH₂­SiMe₂CH₂)₂­(μ-O)] (<b>6</b>), respectively. The molecular structures of complexes <b>3</b>, <b>4</b>, <b>5</b>, and <b>6</b> have been determined by single-crystal X-ray diffraction analyses

Systematic Approach for the Construction of Niobium and Tantalum Sulfide Clusters

Treatment of the imido complexes [MCl₃(NR)­py₂] (R = ^<i>t</i>Bu, 2,6-Me₂C₆H₃; M = Nb <b>1</b>, <b>3</b>; Ta <b>2</b>, <b>4</b>) (Xyl = 2,6-Me₂C₆H₃) with (Me₃Si)₂S in a 1:1 ratio afforded the new cube-type sulfide clusters [MCl­(NR)­py­(μ₃-S)]₄ (R = ^<i>t</i>Bu, 2,6-Me₂C₆H₃; M = Nb <b>5</b>, <b>7</b>; Ta <b>6</b>, <b>8</b>) with loss of Me₃SiCl. Reactions of <b>5</b> and <b>6</b> with cyclopentadienyllithium in 1:4 ratio resulted in the rupture of the coordinative M–S bonds and the replacement of a pyridine molecule and a chlorine atom by an η⁵-cyclopentadienyl group in each metal center, affording the compounds [M­(η⁵-C₅H₅)­(N^<i>t</i>Bu)­(μ-S)]₄ (M = Nb <b>9</b>, Ta <b>10</b>). These processes may develop through formation of the complexes [M₄(η⁵-C₅H₅)₂(μ-Cl)­(N^<i>t</i>Bu)₄py₂(μ₃-S)₂­(μ-S)₂]­(C₅H₅) (M = Nb <b>11</b>, Ta <b>12</b>), also obtained by reaction of <b>5</b> and <b>6</b> with cyclopentadienyllithium in 1:3 ratio. As further evidence, <b>11</b> and <b>12</b> led to complexes <b>9</b> and <b>10</b> by treatment with one more equivalent of the lithium reagent. The structural study of these metal sulfide clusters has been also performed by X-ray crystallography

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(η⁵-C₅Me₅)­(μ-O)}₃(μ₃-N)] (<b>1</b>). Reaction of <b>1</b> with XylNC (Xyl = 2,6-Me₂C₆H₃) in a 1:3 molar ratio at room temperature leads to compound [{Ti­(η⁵-C₅Me₅)­(μ-O)}₃­(μ-XylNCCNXyl)­(NCNXyl)] (<b>2</b>), 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 <b>2</b> gives [{Ti­(η⁵-C₅Me₅)­(μ-O)}₃­(XylNCNXyl)­(CN)] (<b>3</b>) where the heterocumulene [XylNCCNXyl] moiety and the carbodiimido [NCNXyl] fragment in <b>2</b> have undergone net transformations. Similarly, <i>tert</i>-butyl isocyanide (<i>t</i>BuNC) reacts with the starting material <b>1</b> under mild conditions to give the paramagnetic derivative [{Ti₃(η⁵-C₅Me₅)₃­(μ-O)₃­(NCN<i>t</i>Bu)}₂­(μ-CN)₂] (<b>4</b>). However, compound <b>1</b> provides the oxo ketimide derivatives [{Ti₃(η⁵-C₅Me₅)₃­(μ-O)₄}­(NCRPh)] [R = Ph (<b>5</b>), <i>p</i>-Me­(C₆H₄) (<b>6</b>), <i>o</i>-Me­(C₆H₄) (<b>7</b>)] upon reaction with benzophenone, <i>p</i>-methylbenzophenone, and <i>o</i>-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 <b>2</b>–<b>4</b>, <b>6</b>, and <b>7</b> 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(η⁵-C₅H₅)­(μ-O)}₃­(μ₃-N)] (<b>1H</b>). 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 <b>1H</b>, 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

Co-complexation of Lithium Gallates on the Titanium Molecular Oxide {[Ti(η⁵‑C₅Me₅)(μ-O)}₃(μ₃‑CH)]

Amide and lithium aryloxide gallates [Li⁺{RGaPh₃}⁻] (R = NMe₂, O-2,6-Me₂C₆H₃) react with the μ₃-alkylidyne oxoderivative ligand [{Ti­(η⁵-C₅Me₅)­(μ-O)}₃(μ₃-CH)] (<b>1</b>) to afford the gallium–lithium–titanium cubane complexes [{Ph₃Ga­(μ-R)­Li}­{Ti­(η⁵-C₅Me₅)­(μ-O)}₃(μ₃-CH)] [R = NMe₂ (<b>3</b>), O-2,6-Me₂C₆H₃ (<b>4</b>)]. The same complexes can be obtained by treatment of the [Ph₃Ga­(μ₃-O)₃{Ti­(η⁵-C₅Me₅)}₃(μ₃-CH)] (<b>2</b>) adduct with the corresponding lithium amide or aryloxide, respectively. Complex <b>3</b> evolves with formation of <b>5</b> as a solvent-separated ion pair constituted by the lithium dicubane cationic species [Li­{(μ₃-O)₃Ti₃(η⁵-C₅Me₅)₃(μ₃-CH)}₂]⁺ together with the anionic [(GaPh₃)₂(μ-NMe₂)]⁻ unit. On the other hand, the reaction of <b>1</b> with Li­(<i>p</i>-MeC₆H₄) and GaPh₃ leads to the complex [Li­{(μ₃-O)₃Ti₃(η⁵-C₅Me₅)₃(μ₃-CH)}₂]­[GaLi­(<i>p</i>-MeC₆H₄)₂Ph₃] (<b>6</b>). X-ray diffraction studies were performed on <b>1</b>, <b>2</b>, <b>4</b>, and <b>5</b>, while trials to obtain crystals of <b>6</b> led to characterization of [Li­{(μ₃-O)₃Ti₃(η⁵-C₅Me₅)₃(μ₃-CH)}₂]­[PhLi­(μ-C₆H₅)₂Ga­(<i>p</i>-MeC₆H₄)­Ph] <b>6a</b>

Reactivity of Tuck-over Titanium Oxo Complexes with Isocyanides

The reactivity of the “tuck-over” species [Ti₂(η⁵-C₅Me₅)­(CH₂Ph)₃­(μ-η⁵-C₅Me₄CH₂-κ<i>C</i>)­(μ-O)] (<b>1</b>) and [Ti₂(η⁵-C₅Me₅)­(CH₂CMe₃)­(μ-η⁵-C₅Me₄CH₂-κ<i>C</i>)­(μ-CH₂CMe₂CH₂)­(μ-O)] (<b>2</b>) toward isocyanides has been examined both synthetically and theoretically. Treatment of <b>1</b> with the isocyanides RNC, R = Me₃SiCH₂, 2,6-Me₂C₆H₃, <i>t</i>Bu, <i>i</i>Pr, leads to a series of η²-iminoacyl species (<b>3</b>–<b>6</b>) where the molecule of isocyanide inserts into one of the terminal metal–alkyl bonds. The analogous reaction of the “tuck-over” metallacycle species <b>2</b> with 2,6-Me₂C₆H₃NC and <i>t</i>BuNC results in the initial insertion of one isocyanide into the terminal Ti–alkyl bond to form the iminoacyl complexes <b>7</b> and <b>8</b>, followed by a second insertion into the metallacycle moiety to generate <b>9</b>, in the case of <i>tert</i>-butylisocyanide. DFT calculations support the selective reactivity observed experimentally with a kinetic and thermodynamic preference for RNC insertion on the terminal alkyl groups bound to both metallic centers over the alternative insertion on the “tuck-over” ligand

Reactivity of Tuck-over Titanium Oxo Complexes with Isocyanides

The reactivity of the “tuck-over” species [Ti₂(η⁵-C₅Me₅)­(CH₂Ph)₃­(μ-η⁵-C₅Me₄CH₂-κ<i>C</i>)­(μ-O)] (<b>1</b>) and [Ti₂(η⁵-C₅Me₅)­(CH₂CMe₃)­(μ-η⁵-C₅Me₄CH₂-κ<i>C</i>)­(μ-CH₂CMe₂CH₂)­(μ-O)] (<b>2</b>) toward isocyanides has been examined both synthetically and theoretically. Treatment of <b>1</b> with the isocyanides RNC, R = Me₃SiCH₂, 2,6-Me₂C₆H₃, <i>t</i>Bu, <i>i</i>Pr, leads to a series of η²-iminoacyl species (<b>3</b>–<b>6</b>) where the molecule of isocyanide inserts into one of the terminal metal–alkyl bonds. The analogous reaction of the “tuck-over” metallacycle species <b>2</b> with 2,6-Me₂C₆H₃NC and <i>t</i>BuNC results in the initial insertion of one isocyanide into the terminal Ti–alkyl bond to form the iminoacyl complexes <b>7</b> and <b>8</b>, followed by a second insertion into the metallacycle moiety to generate <b>9</b>, in the case of <i>tert</i>-butylisocyanide. DFT calculations support the selective reactivity observed experimentally with a kinetic and thermodynamic preference for RNC insertion on the terminal alkyl groups bound to both metallic centers over the alternative insertion on the “tuck-over” ligand

Intermetallic Cooperation in C–H Activation Involving Transient Titanium-Alkylidene Species: A Synthetic and Mechanistic Study

Remote carbon–hydrogen activation on titanium dinuclear complexes [{Ti­(η⁵-C₅Me₅)­R₂}₂(μ-O)] [R = CH₂SiMe₃ <b>2</b>, CH₂CMe₃ <b>3</b>, and CH₂Ph <b>5</b>) have been examined both synthetically and theoretically. While the thermal treatment of the oxoderivative [{Ti­(η⁵-C₅Me₅)­(CH₂SiMe₃)₂}₂(μ-O)] (<b>2</b>) led to a series of metallacycle complexes (<b>2a</b>–<b>c</b>) by sequential carbon–hydrogen activation processes, [{Ti­(η⁵-C₅Me₅)­(CH₂CMe₃)₂}₂(μ-O)] (<b>3</b>) gave rise to the formation of the metallacycle tuck-over species [Ti₂(η⁵-C₅Me₅)­(μ-η⁵-C₅Me₄CH₂-κC)­(CH₂CMe₃)­(μ-CH₂CMe₂CH₂)­(μ-O)] (<b>4</b>), as result of hydrogen abstraction from a η⁵-C₅Me₅ ligand. However, the thermolysis of the tetrabenzyl complex [{Ti­(η⁵-C₅Me₅)­(CH₂Ph)₂}₂(μ-O)] (<b>5</b>) yielded the derivative [Ti₂(η⁵-C₅Me₅)­(μ-η⁵-C₅Me₄CH₂-κC)­(CH₂Ph)₃(μ-O)] (<b>6</b>) that only exhibits tuck-over η⁵-C₅Me₅ metalation. DFT calculations show that the mechanism involves a first α-hydrogen abstraction to generate a transient titanium alkylidene, which enables it to activate β- and γ-C­(sp³)-H bonds on the adjacent titanium center. The calculations also establish a reactivity order for the different type of γ-H abstractions, trimethylsilyl > neopentyl ≌ benzyl, allowing us to explain the observed selectivity

Intermetallic Cooperation in C–H Activation Involving Transient Titanium-Alkylidene Species: A Synthetic and Mechanistic Study

Remote carbon–hydrogen activation on titanium dinuclear complexes [{Ti­(η⁵-C₅Me₅)­R₂}₂(μ-O)] [R = CH₂SiMe₃ <b>2</b>, CH₂CMe₃ <b>3</b>, and CH₂Ph <b>5</b>) have been examined both synthetically and theoretically. While the thermal treatment of the oxoderivative [{Ti­(η⁵-C₅Me₅)­(CH₂SiMe₃)₂}₂(μ-O)] (<b>2</b>) led to a series of metallacycle complexes (<b>2a</b>–<b>c</b>) by sequential carbon–hydrogen activation processes, [{Ti­(η⁵-C₅Me₅)­(CH₂CMe₃)₂}₂(μ-O)] (<b>3</b>) gave rise to the formation of the metallacycle tuck-over species [Ti₂(η⁵-C₅Me₅)­(μ-η⁵-C₅Me₄CH₂-κC)­(CH₂CMe₃)­(μ-CH₂CMe₂CH₂)­(μ-O)] (<b>4</b>), as result of hydrogen abstraction from a η⁵-C₅Me₅ ligand. However, the thermolysis of the tetrabenzyl complex [{Ti­(η⁵-C₅Me₅)­(CH₂Ph)₂}₂(μ-O)] (<b>5</b>) yielded the derivative [Ti₂(η⁵-C₅Me₅)­(μ-η⁵-C₅Me₄CH₂-κC)­(CH₂Ph)₃(μ-O)] (<b>6</b>) that only exhibits tuck-over η⁵-C₅Me₅ metalation. DFT calculations show that the mechanism involves a first α-hydrogen abstraction to generate a transient titanium alkylidene, which enables it to activate β- and γ-C­(sp³)-H bonds on the adjacent titanium center. The calculations also establish a reactivity order for the different type of γ-H abstractions, trimethylsilyl > neopentyl ≌ benzyl, allowing us to explain the observed selectivity