A Discrete N,O,N-Supported Gallium Amido Complex for the Intermolecular Hydroamination of Terminal Alkynes

The diamino-ether ligand {(C₅H₉)­NH-C₆H₄}₂O (<b>1</b>) was found to readily react with 0.5 equiv of Ga₂(NMe₂)₆ via an amine elimination route to afford the N,O,N-supported Ga amido species {η³-<i>N,O,N</i>-((C₅H₉)­N-C₆H₄)₂O}­GaNMe₂ (<b>2</b>) in a reasonable yield (51%). As determined by X-ray crystallography, the four-coordinate Ga center in <b>2</b> adopts an unusual trigonal-monopyramidal geometry. Compound <b>2</b> effectively catalyzes the hydroamination of terminal alkynes (such as 1-hexyne and phenylacetylene) in the presence of primary amines (aniline and butylamine). Kinetic studies on the latter catalytic reactions suggest that these proceed with a first-order rate dependence on alkyne and on species <b>2</b>. In preliminary studies aiming at the isolation of intermediates relevant to the present catalysis, the dimeric Ga complex [{η²-<i>N,N</i>-((C₅H₉)­N-C₆H₄)₂O}­Ga­(μ-NHPh)]₂ (<b>3</b>) was synthesized by an aminolysis reaction between compound <b>2</b> and aniline; its identity was confirmed by X-ray crystallographic analysis

A Discrete N,O,N-Supported Gallium Amido Complex for the Intermolecular Hydroamination of Terminal Alkynes

The diamino-ether ligand {(C₅H₉)­NH-C₆H₄}₂O (<b>1</b>) was found to readily react with 0.5 equiv of Ga₂(NMe₂)₆ via an amine elimination route to afford the N,O,N-supported Ga amido species {η³-<i>N,O,N</i>-((C₅H₉)­N-C₆H₄)₂O}­GaNMe₂ (<b>2</b>) in a reasonable yield (51%). As determined by X-ray crystallography, the four-coordinate Ga center in <b>2</b> adopts an unusual trigonal-monopyramidal geometry. Compound <b>2</b> effectively catalyzes the hydroamination of terminal alkynes (such as 1-hexyne and phenylacetylene) in the presence of primary amines (aniline and butylamine). Kinetic studies on the latter catalytic reactions suggest that these proceed with a first-order rate dependence on alkyne and on species <b>2</b>. In preliminary studies aiming at the isolation of intermediates relevant to the present catalysis, the dimeric Ga complex [{η²-<i>N,N</i>-((C₅H₉)­N-C₆H₄)₂O}­Ga­(μ-NHPh)]₂ (<b>3</b>) was synthesized by an aminolysis reaction between compound <b>2</b> and aniline; its identity was confirmed by X-ray crystallographic analysis

A Discrete N,O,N-Supported Gallium Amido Complex for the Intermolecular Hydroamination of Terminal Alkynes

The diamino-ether ligand {(C₅H₉)­NH-C₆H₄}₂O (<b>1</b>) was found to readily react with 0.5 equiv of Ga₂(NMe₂)₆ via an amine elimination route to afford the N,O,N-supported Ga amido species {η³-<i>N,O,N</i>-((C₅H₉)­N-C₆H₄)₂O}­GaNMe₂ (<b>2</b>) in a reasonable yield (51%). As determined by X-ray crystallography, the four-coordinate Ga center in <b>2</b> adopts an unusual trigonal-monopyramidal geometry. Compound <b>2</b> effectively catalyzes the hydroamination of terminal alkynes (such as 1-hexyne and phenylacetylene) in the presence of primary amines (aniline and butylamine). Kinetic studies on the latter catalytic reactions suggest that these proceed with a first-order rate dependence on alkyne and on species <b>2</b>. In preliminary studies aiming at the isolation of intermediates relevant to the present catalysis, the dimeric Ga complex [{η²-<i>N,N</i>-((C₅H₉)­N-C₆H₄)₂O}­Ga­(μ-NHPh)]₂ (<b>3</b>) was synthesized by an aminolysis reaction between compound <b>2</b> and aniline; its identity was confirmed by X-ray crystallographic analysis

Group 1 and 2 and Early Transition Metal Complexes Bearing N‑Heterocyclic Carbene Ligands: Coordination Chemistry, Reactivity, and Applications

Group 1 and 2 and Early Transition Metal Complexes Bearing N‑Heterocyclic Carbene Ligands: Coordination Chemistry, Reactivity, and Application

Novel N,O,N-Supported Tetracoordinate Aluminum Complexes for the Highly Controlled and Immortal ROP of Trimethylene Carbonate (TMC) under Mild Conditions: Access to Narrowly Disperse poly-TMC and Derived Copolymers

The novel N,O,N-supported tetracoordinate amidoaluminum complexes {η³(<i>N</i>,<i>O</i>,<i>N</i>}-(C₅H₉)N-C₆H₄}₂OAlNMe₂ (<b>2a</b>, R = C₅H₉; <b>2b</b>, R = C₆H₁₁) have been synthesized and structurally characterized. In the solid state, as determined from X-ray crystallographic studies, complex <b>2a</b> consists of a four-coordinate Al species in which the Al center adopts a distorted-trigonal-monopyramidal (tmp) geometry with the nitrogens of the three amido groups (defining the pyramidal base) being nearly coplanar with Al. Such Al species, when combined with an alcohol source such as benzyl alcohol, effectively polymerize trimethylene carbonate (TMC) at room temperature in a highly controlled manner to yield narrowly disperse poly(trimethylene carbonate), as deduced from polymer characterizations with various kinetic studies. The high degree of molecular chain length control of the present system was further exploited to access narrowly dispersed PEG-functionalized amphiphilic copolymers. The attractive features of the system lie in the combination of an excellent activity, a high level of chain length control, and mild reaction conditions

Neutral and Cationic N‑Heterocyclic Carbene Zirconium and Hafnium Benzyl Complexes: Highly Regioselective Oligomerization of 1‑Hexene with a Preference for Trimer Formation

Various zirconium and hafnium amido, chloro, and benzyl complexes supported by a tridentate N-heterocyclic carbene bis-phenolate dianionic ligand ((OCO)^2–) have been synthesized and structurally characterized. The alcohol elimination reaction of the protio ligand <i>N</i>,<i>N′</i>-bis­(2-hydroxy-3,5-di-<i>tert</i>-butylphenyl)-4,5-dihydroimidazolium chloride (<b>1</b>) and the metal alkoxide precursors M­(O^<i>i</i>Pr)₄(HO^<i>i</i>Pr) (M = Zr, Hf) and a subsequent alkoxide/chloride exchange reaction (upon addition of trimethylsilyl chloride, TMSCl) afforded the corresponding Zr and Hf carbene dichloro complexes as THF adducts: (OCO)­MCl₂(THF) (<b>2a-THF</b>, M = Zr; <b>2b-THF</b>, M = Hf). As determined by single-crystal X-ray crystallographic studies, the molecular structure of the Hf derivative <b>2b-THF</b> confirmed the proposed formulation and the effective formation of a (OCO)­Hf chelate. In the case of Zr, an amine elimination reaction between protio ligand <b>1</b> and Zr­(NMe₂)₄ yielded the corresponding Zr amido THF adduct (OCO)­Zr­(NMe₂)­(Cl)­(THF) (<b>3a-THF</b>) when carried in THF as a solvent, while the Zr–NHMe₂ adduct (OCO)­Zr­(NMe₂)­(NHMe₂)­(THF) (<b>3a-NHMe</b>_<b>2</b>) was isolated using CH₂Cl₂ as the reaction solvent. <b>3a-THF</b> may be readily and quantitatively converted to the dichloro derivative <b>2a-THF</b> upon addition of TMSCl. The toluene elimination reaction of protio ligand <b>1</b> and M­(CH₂Ph)₄ (M = Zr, Hf) followed by a salt metathesis with 1 equiv of PhCH₂MgCl afforded the corresponding Zr and Hf carbene dibenzyl complexes (OCO)­M­(CH₂Ph)₂ (<b>4a</b>, M = Zr; <b>4b</b>, M = Hf), whose solid-state structures were confirmed by X-ray crystallography. <b>4a</b> and <b>4b</b> each feature a five-coordinate metal center with both benzyl moieties binding in a η² fashion. The protonolysis reaction between species <b>4a</b> (or <b>4b</b>) and [HNMe₂Ph]­[B­(C₆F₅)₄] afforded the clean and quantitative formation of the corresponding Zr (or Hf) anilinium benzyl cation <b>5a</b>⁺ (or <b>5b</b>⁺). Remarkably, the cation <b>5a</b>⁺ catalyzes the highly regioselective oligomerization of 1-hexene with a marked preference for trimer formation

Neutral and Cationic N‑Heterocyclic Carbene Zirconium and Hafnium Benzyl Complexes: Highly Regioselective Oligomerization of 1‑Hexene with a Preference for Trimer Formation

Various zirconium and hafnium amido, chloro, and benzyl complexes supported by a tridentate N-heterocyclic carbene bis-phenolate dianionic ligand ((OCO)^2–) have been synthesized and structurally characterized. The alcohol elimination reaction of the protio ligand <i>N</i>,<i>N′</i>-bis­(2-hydroxy-3,5-di-<i>tert</i>-butylphenyl)-4,5-dihydroimidazolium chloride (<b>1</b>) and the metal alkoxide precursors M­(O^<i>i</i>Pr)₄(HO^<i>i</i>Pr) (M = Zr, Hf) and a subsequent alkoxide/chloride exchange reaction (upon addition of trimethylsilyl chloride, TMSCl) afforded the corresponding Zr and Hf carbene dichloro complexes as THF adducts: (OCO)­MCl₂(THF) (<b>2a-THF</b>, M = Zr; <b>2b-THF</b>, M = Hf). As determined by single-crystal X-ray crystallographic studies, the molecular structure of the Hf derivative <b>2b-THF</b> confirmed the proposed formulation and the effective formation of a (OCO)­Hf chelate. In the case of Zr, an amine elimination reaction between protio ligand <b>1</b> and Zr­(NMe₂)₄ yielded the corresponding Zr amido THF adduct (OCO)­Zr­(NMe₂)­(Cl)­(THF) (<b>3a-THF</b>) when carried in THF as a solvent, while the Zr–NHMe₂ adduct (OCO)­Zr­(NMe₂)­(NHMe₂)­(THF) (<b>3a-NHMe</b>_<b>2</b>) was isolated using CH₂Cl₂ as the reaction solvent. <b>3a-THF</b> may be readily and quantitatively converted to the dichloro derivative <b>2a-THF</b> upon addition of TMSCl. The toluene elimination reaction of protio ligand <b>1</b> and M­(CH₂Ph)₄ (M = Zr, Hf) followed by a salt metathesis with 1 equiv of PhCH₂MgCl afforded the corresponding Zr and Hf carbene dibenzyl complexes (OCO)­M­(CH₂Ph)₂ (<b>4a</b>, M = Zr; <b>4b</b>, M = Hf), whose solid-state structures were confirmed by X-ray crystallography. <b>4a</b> and <b>4b</b> each feature a five-coordinate metal center with both benzyl moieties binding in a η² fashion. The protonolysis reaction between species <b>4a</b> (or <b>4b</b>) and [HNMe₂Ph]­[B­(C₆F₅)₄] afforded the clean and quantitative formation of the corresponding Zr (or Hf) anilinium benzyl cation <b>5a</b>⁺ (or <b>5b</b>⁺). Remarkably, the cation <b>5a</b>⁺ catalyzes the highly regioselective oligomerization of 1-hexene with a marked preference for trimer formation

Deprotonation of Al₂Me₆ by Sterically Bulky NHCs: Scope, Rationale through DFT Studies, and Application in the Methylenation of Carbonyl Substrates

The sterically bulky NHCs 1,3-di-<i>tert</i>-butylimidazol-2-ylidene (I^<i>t</i>Bu), 1,3-di-<i>tert</i>-butylimidazolin-2-ylidene (S^<i>t</i>Bu), and 1,3-di-<i>tert-</i>butyl-3,4,5,6-tetrahydripyrimidin-2-ylidene (C6-^<i>t</i>Bu) were found to readily react with excess Al₂Me₆ at room temperature to form salts <b>2</b>, <b>3</b>, and <b>4</b>, respectively, consisting of the polynuclear Me₃Al­(μ³-CH₂)­(AlMe₂)₂(μ²-CH₃)⁻ anion associated with either the cation I^<i>t</i>Bu-H⁺, S^<i>t</i>Bu-H⁺, or C6-^<i>t</i>Bu-H⁺. Such a reaction involving the deprotonation of an Al₂Me₆ moiety by a NHC does not proceed with less sterically hindered NHCs such as 1,3-bis­(2,6-diisopropylphenyl)­imidazol-2-ylidene (IDipp) and 1,3-bis­(2,4,6-trimethylphenyl)­imidazol-2-ylidene (IMes), for which only the classical Lewis pair adducts (NHC)­AlMe₃ were isolated. In line with experimental and density functional theory (DFT) calculations data, such reactivity thus appears to be driven by steric frustration, resulting in the destabilization of the corresponding (NHC)­AlMe₃ adducts, then more prone to dissociate and hence allowing Al₂Me₆ activation/deprotonation. The DFT-estimated profile of the reaction of model adduct (I^<i>t</i>Bu)­AlMe₃ (<b>I</b>) with Al₂Me₆ agrees with a ready adduct dissociation at low energy cost, with a subsequent deprotonation of Al₂Me₆ by the NHC fragment to afford model salt <b>II</b> (isostructural to salt <b>2</b>). Anion Me₃Al­(μ³-CH₂)­(AlMe₂)₂­(μ²-CH₃)⁻ behaves as an efficient CH₂^2– group transfer agent with the methylenation of aldehydes and ketones to afford the corresponding methylene organics in good conversions. Bonding analysis of the latter anion agrees with an enhanced nucleophilicity of the Al-<i>CH</i>₂ moiety (compared to the Al-<i>Me</i> groups), in line with the observed reactivity. DFT calculations also allowed a detailed bonding description of the pentacoordinate methylene carbon in the anion Me₃Al­(μ³-CH₂)­(AlMe₂)₂(μ²-CH₃)⁻

NHC Bis-Phenolate Aluminum Chelates: Synthesis, Structure, and Use in Lactide and Trimethylene Carbonate Polymerization

A novel family of Al­(III) complexes supported by a tridentate, dianionic N-heterocyclic carbene bis-phenolate ligand ((OCO)^2–) was prepared via various synthetic routes, and the derived compounds were all structurally characterized. The methane elimination reaction of the protio ligand <i><i>N,N</i></i>′-bis­(2-hydroxy-3,5-di-<i>tert</i>-butylphenyl)-4,5-dihydroimidazolium chloride (<b>1</b>·H₃Cl) with AlMe₃ quantitatively led to the formation of the bis-phenolate imidazolinium Al zwitterion (<b>1</b>·H)­Al­(Me)­(Cl) (<b>2</b>), whose formulation was established by X-ray diffraction studies. The deprotonation of species <b>2</b> with 1 equiv of lithium diisopropylamide (LDA) proceeded with the elimination of LiCl to afford the Al-NHC methyl derivative [(OCO)­AlMe]₂ (<b>3</b>), which was isolated as a dimer, as confirmed by X-ray diffraction studies. Alternatively, compound <b>3</b> may be accessed via a salt metathesis route involving the reaction of the NHC bis-phenolate Li salt <b>1</b>·Li₂, generated in situ via reaction of <b>1</b>·H₃Cl with 3 equiv of ^<i>n</i>BuLi (−40 °C, THF), with 1 equiv of MeAlCl₂. The serendipitous hydrolysis of compound <b>3</b> allowed the X-ray characterization of the Al-oxo dinuclear species [(OCO)­Al-O-Al-(OCO)] (<b>3</b>′), in which both Al­(III) centers adopt a distorted-trigonal-monopyramidal geometry. The reaction of the salt <b>1</b>·H₃Cl with Al­(O<i>i</i>Pr)₃ afforded the corresponding bis-phenolate imidazolinium Al zwitterion (<b>1</b>·H)­Al­(O<i>i</i>Pr)­(Cl) (<b>4</b>), which incorporates a four-coordinate tetrahedral Al center effectively κ²<i>O,O</i>-chelated by the two phenolate moieties of the OCO^2– ligand. Compound <b>4</b> may be readily converted to the Al-NHC alkoxide derivative [(OCO)­AlO<i>i</i>Pr]₂ (<b>5</b>) upon reaction with 1 equiv of LDA. Alternatively, the alcoholysis of the Al-NHC methyl species <b>3</b> with <i>i</i>PrOH also permitted access to the derived Al alkoxide <b>5</b> and proceeds via the formation of the kinetic product (<b>1</b>·H)­Al­(O<i>i</i>Pr)­(Me) (<b>6</b>) that may readily eliminate methane upon heating to produce species <b>5</b>. The Al alkoxide species <b>5</b> was shown to efficiently polymerize <i>rac</i>-lactide and trimethylene carbonate in a highly controlled manner for the production of narrow disperse materials. The observed catalytic performances are in the range of the majority of those for group 13 metal based ROP catalysts developed thus far, and all data support the noninvolvement of the NHC moiety in these polymerization reactions

Synthesis and Structural Characterization of Various N,O,N-Chelated Aluminum and Gallium Complexes for the Efficient ROP of Cyclic Esters and Carbonates: How Do Aluminum and Gallium Derivatives Compare ?

The novel Al and Ga coordination compounds <b>4</b>, <b>5</b>, <b>7</b>, and <b>8</b> of the type (NON)­AlX (NON^2– = {RNC₆H₄}₂O^2–; <b>4</b>, R = Cy, X = Me; <b>5</b>, R = C₅H₉, X = OCH₂Ph; <b>7</b>, R = C₆F₅, X = Me) and (NON)­GaX (NON^2– = {R₂NHC₆H₄}₂O^2–; <b>8</b>, R = C₆F₅, X = Me) have been synthesized via, in the case of <b>4</b>, <b>7</b>, and <b>8</b>, a methane elimination reaction between the corresponding protio ligands (NON)­H₂ and MMe₃ precursors (M = Al, Ga). The Al alkoxide derivative <b>5</b> was prepared via an alcoholysis reaction between κ³-<i>N</i>,<i>O</i>,<i>N</i>-{(C₅H₉)­NC₆H₄}₂O}₂AlNMe₂ and PhCH₂OH. The tetracoordinate Al–THF adduct κ²-<i>N</i>,<i>N′</i>-{(C₅H₉)­NC₆H₄}₂O}₂Al­(Me)­(THF) (<b>6</b>) was prepared via a methane elimination reaction between the protio ligand TfNONH₂ (Tf = CF₃SO₂, <b>1c</b>) and AlMe₃ in a CH₂Cl₂/THF solvent mixture. As determined from X-ray studies, complexes <b>4</b>, <b>7</b>, and <b>8</b> are monomeric in the solid state and feature a central tetracoordinate metal center effectively κ³-<i>N,O,N′</i> chelated and adopting a trigonal-monopyramidal geometry. The presence of Ga···F­(<i>o</i>-C₆F₅) contacts in the solid-state structure of compound <b>8</b> are likely to reflect the Lewis acidity of the Ga center. The Al alkoxide derivative [κ³-<i>N</i>,<i>O</i>,<i>N</i>-{(C₅H₉)­NC₆H₄}₂O}₂AlOCH₂Ph]₂ (<b>5</b>) was isolated as a dimer containing two five-coordinate (trigonal-pyramidal) Al centers and retains its dimeric structure in solution, while the Al–THF adduct <b>6</b> crystallizes in a monomeric form with an Al center in a tetrahedral geometry. The catalytic performances of the Al and Ga species <b>4</b>, <b>5</b>, <b>7</b>, and <b>8</b> as ROP initiators of <i>rac</i>-lactide (<i>rac</i>-LA), ε-caprolactone (ε-CL), and trimethylene carbonate (TMC) were estimated, and most of them efficiently mediate the controlled and immortal ROP of these three monomers in the presence of BnOH, such an alcohol acting as an effective chain transfer agent. Of particular interest, the Ga amido species κ³-<i>N</i>,<i>O</i>,<i>N</i>-{(C₅H₉)­NC₆H₄}₂O}₂Ga-NMe₂ (<b>3</b>) outperforms its Al counterpart κ³-<i>N</i>,<i>O</i>,<i>N</i>-{(C₅H₉)­NC₆H₄}₂O}₂AlNMe₂ (<b>2</b>) in the ROP of <i>rac</i>-LA, whether in terms of control or activity, to afford isotactically enriched PLA (<i>P</i>_m = 0.7). In contrast, all the Al derivatives are more efficient catalysts for the polymerization of ε-CL or TMC than the Ga analogues. For the ROP of TMC initiated by the Al and Ga complexes <b>2</b>–<b>5</b>, an increased Lewis acidity of the metal center is clearly beneficial to both the activity and the ROP control. Notably, the C₆F₅N Ga species <b>8</b> was found to be inactive in the ROP of <i>rac</i>-LA, ε-CL, and TMC, which may be related to both electronic (C–F···Ga interactions) and steric factors