Influence of the Lewis Acidity of Gallium Atoms on the Reactivity of a Frustrated Lewis Pair: Experimental and Theoretical Studies

The reactivity of the Ga/P-based frustrated Lewis pair (FLP) Mes₂P–C­[C­(H)–Ph]–Ga^<i>t</i>Bu₂ (<b>3</b>) is influenced by the relatively weak Lewis acidity of its Ga atom and differs significantly from that of the analogous Al compound <b>1</b>. The adduct of <b>3</b> with CO₂ was only detectable at low temperature by NMR spectroscopy. Benzaldehyde was coordinated only via a Ga–O bond; the P atom was not involved. In contrast, a relatively persistent adduct was formed with soft CS₂ to yield a five-membered GaCPCS heterocycle. Dehydrocoupling with H₃B←NHMe₂ afforded the dimeric amidoborane (H₂B–NMe₂)₂, while an adduct with a GaCPBN heterocycle was isolated with the sterically less shielded ammonia–borane H₃B←NH₃. The latter product was unstable in solution and decomposed by H₂ elimination and formation of oligomeric BN compounds. Small quantities of <b>3</b> catalyzed hydrogen transfer from H₃B←NH₃ to an imine. The Lewis acidities of the Al/P- and Ga/P-based FLPs were examined by experiments (Gutmann–Beckett method) and by calculation of the fluoride ion affinity (including the B and In analogues). The Al compound is the strongest Lewis acid; the Ga FLP is significantly weaker but is a stronger F^– acceptor in comparison to the unknown analogues of B and In. These results reflect the different reactivities of these FLPs and may help to develop FLPs with finely adjusted properties

Reactions of an Al–P-Based Frustrated Lewis Pair with Carbonyl Compounds: Dynamic Coordination of Benzaldehyde, Activation of Benzoyl Chloride, and Al–C Bond Cleavage with Benzamide

Treatment of the Al/P-based frustrated Lewis pair (FLP) Mes₂PC­(CHPh)­Al­(CMe₃)₂ (<b>1</b>) with benzaldehyde afforded the adduct <b>2</b> with a five-membered AlCPCO heterocycle. The carbonyl oxygen atom is bound to aluminum and the carbonyl carbon atom to phosphorus. <b>2</b> is dynamic in solution at room temperature, which results in a fast equilibration of the enantiomeric molecules by cleavage of the P–C and fast rotation about the Al–O bond. Benzoyl chloride and <b>1</b> yielded three products (<b>3</b>–<b>5</b>). Quinoid structures were formed by C–Cl bond activation, chlorine abstraction, and loss of aromaticity in the benzoyl phenyl group. Alkylation of the <i>p</i>-C atom by an AlCMe₃ group completed the transformation and resulted concomitantly in the formation of derivatives with Al–Cl bonds. The complexes may be described as a ketene molecule coordinated to FLP <b>1</b>. Benzamide reacted as a proton donor and gave cleavage of the Al–C bond to the vinylic carbon atom of <b>1</b>. An alkenylphosphine, Mes₂PC­(H)C­(H)­CMe₃, and a dinuclear amidate complex with two dialkylaluminum groups bridged by two chelating ligands were isolated

Gallium–Gallium Bonds As Effective Templates for the Generation of Macrocycles and Supramolecular Entities

Treatment of the tetraalkyldigallium(4) compound R₂Ga–GaR₂ [<b>1</b>; R = CH­(SiMe₃)₂] with the highly functionalized acids 3- and 4-carboxyphenylthiourea, 7-azaindole-3-carboxylic acid, and 6-aminonicotinic acid afforded macrocyclic compounds in which two or four Ga–Ga bonds are bridged by the respective number of organic spacer ligands. In each reaction two equivalents of CH₂(SiMe₃)₂ per formula unit of <b>1</b> were released. The Ga–Ga bonds of the products <b>2</b> to <b>5</b> are bridged by a carboxylato group and a chelating ligand containing two nitrogen donor atoms or the sulfur and nitrogen atoms of a thiourea group. The thiourea derivatives afforded different species with four or eight gallium atoms in the heterocycles (<b>2</b> and <b>3</b>) depending on their substitution patterns (1,3- versus 1,4-positions at the benzene rings). Supramolecular aggregates resulted that had up to 12 THF molecules encapsulated in the heterocycles or bonded to the surface of the molecules via hydrogen bonding or orthogonal dipolar interactions

Gallium–Gallium Bonds As Effective Templates for the Generation of Macrocycles and Supramolecular Entities

Treatment of the tetraalkyldigallium(4) compound R₂Ga–GaR₂ [<b>1</b>; R = CH­(SiMe₃)₂] with the highly functionalized acids 3- and 4-carboxyphenylthiourea, 7-azaindole-3-carboxylic acid, and 6-aminonicotinic acid afforded macrocyclic compounds in which two or four Ga–Ga bonds are bridged by the respective number of organic spacer ligands. In each reaction two equivalents of CH₂(SiMe₃)₂ per formula unit of <b>1</b> were released. The Ga–Ga bonds of the products <b>2</b> to <b>5</b> are bridged by a carboxylato group and a chelating ligand containing two nitrogen donor atoms or the sulfur and nitrogen atoms of a thiourea group. The thiourea derivatives afforded different species with four or eight gallium atoms in the heterocycles (<b>2</b> and <b>3</b>) depending on their substitution patterns (1,3- versus 1,4-positions at the benzene rings). Supramolecular aggregates resulted that had up to 12 THF molecules encapsulated in the heterocycles or bonded to the surface of the molecules via hydrogen bonding or orthogonal dipolar interactions

Gallium–Gallium Bonds As Effective Templates for the Generation of Macrocycles and Supramolecular Entities

Treatment of the tetraalkyldigallium(4) compound R₂Ga–GaR₂ [<b>1</b>; R = CH­(SiMe₃)₂] with the highly functionalized acids 3- and 4-carboxyphenylthiourea, 7-azaindole-3-carboxylic acid, and 6-aminonicotinic acid afforded macrocyclic compounds in which two or four Ga–Ga bonds are bridged by the respective number of organic spacer ligands. In each reaction two equivalents of CH₂(SiMe₃)₂ per formula unit of <b>1</b> were released. The Ga–Ga bonds of the products <b>2</b> to <b>5</b> are bridged by a carboxylato group and a chelating ligand containing two nitrogen donor atoms or the sulfur and nitrogen atoms of a thiourea group. The thiourea derivatives afforded different species with four or eight gallium atoms in the heterocycles (<b>2</b> and <b>3</b>) depending on their substitution patterns (1,3- versus 1,4-positions at the benzene rings). Supramolecular aggregates resulted that had up to 12 THF molecules encapsulated in the heterocycles or bonded to the surface of the molecules via hydrogen bonding or orthogonal dipolar interactions

Gallium–Gallium Bonds As Effective Templates for the Generation of Macrocycles and Supramolecular Entities

Treatment of the tetraalkyldigallium(4) compound R₂Ga–GaR₂ [<b>1</b>; R = CH­(SiMe₃)₂] with the highly functionalized acids 3- and 4-carboxyphenylthiourea, 7-azaindole-3-carboxylic acid, and 6-aminonicotinic acid afforded macrocyclic compounds in which two or four Ga–Ga bonds are bridged by the respective number of organic spacer ligands. In each reaction two equivalents of CH₂(SiMe₃)₂ per formula unit of <b>1</b> were released. The Ga–Ga bonds of the products <b>2</b> to <b>5</b> are bridged by a carboxylato group and a chelating ligand containing two nitrogen donor atoms or the sulfur and nitrogen atoms of a thiourea group. The thiourea derivatives afforded different species with four or eight gallium atoms in the heterocycles (<b>2</b> and <b>3</b>) depending on their substitution patterns (1,3- versus 1,4-positions at the benzene rings). Supramolecular aggregates resulted that had up to 12 THF molecules encapsulated in the heterocycles or bonded to the surface of the molecules via hydrogen bonding or orthogonal dipolar interactions

Aluminum and Gallium Hydrazides as Active Lewis Pairs: Cooperative C–H Bond Activation with H–CC–Ph and Pentafluorobenzene

Hydroalumination or hydrogallation of a sterically encumbered hydrazone, H₁₀C₅N–NC­(C₉H₁₄) (NC₅H₁₀ = piperidine, C­(C₉H₁₄) = 2-adamantdiyl), afforded hydrazides that, depending on the steric shielding by the substituents at the metal atoms, had different molecular structures. While both di­(<i>tert</i>-butyl)metal derivatives (<b>1a</b>, <b>1b</b>) are monomeric in the solid state with highly strained MN₂ heterocycles (M = Al, Ga), the dimethylmetal compounds (<b>1c</b>, <b>1d</b>) are dimeric with M₂N₂ heterocycles and exocyclic N–N bonds. The latter compounds are highly dynamic in solution. <b>1d</b> crystallized as a mixture of <i>cis</i>- and <i>trans</i>-isomers as detected by crystal structure determinations. These compounds react as active Lewis pairs by their specific donor–acceptor functionality and are able to activate C–H bonds of moderately acidic substrates. Reaction of <b>1a</b> (M = Al) with H–CC–C₆H₅ afforded by C–H bond activation and release of H–CMe₃ trialkynyl compound <b>4</b>, in which three alkynyl groups and a neutral hydrazine ligand are bound to Al. <b>1b</b> (M = Ga) gave only the known dimeric monoalkynyl derivative [(Me₃C)₂Ga–CC–C₆H₅]₂ (<b>5b</b>). The sterically less shielded dimethyl compounds <b>1c</b> and <b>1d</b> similarly yielded trialkynylmetal compounds by methane and hydrogen elimination. In this case a hydrazone ligand is coordinated to the metal atoms. <b>1d</b> reacted with pentafluorobenzene in an unprecedented reaction to yield a diaryl-methylgallium compound with the metal atom bound to two electron-withdrawing groups and a hydrazone ligand completing the coordination sphere of Ga

Crystalline Complexes of Pyr_12O1TFSI-Based Ionic Liquid Electrolytes

This study examines the formation of previously unreported crystalline phases of <i>N</i>-methoxyethyl-<i>N</i>-methylpyrrolidinium bis­(trifluoromethanesulfonyl)­imide (Pyr_12O1TFSI). The melting point of pristine Pyr_12O1TFSI, determined by conductivity measurements, is between −20 and −17.5 °C. Formation of this crystalline phase is difficult and only occurs under specific conditions. Pyr_12O1TFSI readily forms 1:1 phases with both NaTFSI and Mg­(TFSI)_2. The results of single crystal structure determinations are presented. The Na⁺ crystalline phase provides clear evidence that the Pyr_12O1⁺ cation can coordinate some metal ions, but this coordinative interaction does not occur with all metal cations, e.g., Mg²⁺, and in all states of matter, e.g., Na⁺-IL solutions. The TFSI^– ions are found in two different aggregate solvates in the Pyr_12O1TFSI:NaTFSI 1:1 phase and in contact ion pair and aggregate solvates in the Pyr_12O1TFSI:Mg­(TFSI)₂ 1:1 phase. The Pyr_12O1TFSI:Mg­(TFSI)₂ crystalline phase gives insight into the local structure of the liquid electrolyte, where it is likely that a maximum of approximately 30% of the total TFSI^– can likely be coordinated in a bridging geometry, and the rest are in a bidentate coordination geometry. This ratio is determined from both the crystal structure and the Raman spectroscopy results

An Al/P-Based Frustrated Lewis Pair as an Efficient Ambiphilic Ligand: Coordination of Boron Trihalides, Rearrangement, and Formation of HBX₂ Complexes (X = Br, I)

The Al/P-based frustrated Lewis pair (FLP) Mes₂PC­(CHPh)Al­(CMe₃)₂ (<b>1</b>) reacted with boron halides BX₃ (X = F, Cl, Br, I) as an ambiphilic ligand to form complexes (<b>2</b>–<b>5</b>) in which the boron atoms were coordinated to phosphorus and one of the halogen atoms to aluminum. Nonplanar five-membered heterocycles resulted that had five different ring atoms (AlCPBX). The distance of the bridging halogen atoms to the AlCPB plane increased steadily with the radius of the halogen atoms. Only the BF₃ adduct showed a dynamic behavior in solution at room temperature with equivalent <i>tert</i>-butyl or mesityl groups in the NMR spectra, while in other cases, the rigid conformation led to the magnetic inequivalence of the substituents at Al and P with well-resolved signals for each group. The BBr₃ and BI₃ complexes underwent in solution at room temperature a spontaneous stereoselective rearrangement with the concomitant release of isobutene. The obtained products, Mes₂P(μ-CCHPh)­(μ-HBX₂)AlX­(CMe₃) (<b>6</b> and <b>7</b>) may be viewed as unique adducts of a modified new Al/P-based FLP, Mes₂P­C­(CHPh)­AlX­(CMe₃) (X = Br, I), with dihalogenboranes, HBX₂. The trapped boranes are either completely unknown (X = I) or unstable in the free form. Quantum–chemical calculations suggest an ionic rearrangement mechanism via the formation of a borenium cation, β-hydride elimination, and hydride transfer. The bromine migration from boron to aluminum corresponds to a formal suprafacial 1,3-sigmatropic rearrangement

Cooperative Ge–N Bond Activation in Hydrogallation Products of Alkynyl(diethylamino)germanes (Et₂N)_<i>n</i>Ge(CC^<i>t</i>Bu)_4–<i>n</i>

Treatment of the alkynyl­(diethylamino)­germanes Et₂NGe­(CC^<i>t</i>Bu)₃ (<b>1</b>) and (Et₂N)₂Ge­(CC^<i>t</i>Bu)₂ (<b>2</b>) with dialkylelement hydrides ^<i>t</i>Bu₂MH (M = Al, Ga) afforded in high yields the hydrometalation products (^<i>t</i>BuCC)₂(Et₂N)­Ge­[C­(M^<i>t</i>Bu₂)C­(H)^<i>t</i>Bu] (<b>3</b>), (^<i>t</i>BuCC)­(Et₂N)­Ge­[C­(M^<i>t</i>Bu₂)C­(H)^<i>t</i>Bu]₂ (<b>4</b>) and (^<i>t</i>BuCC)­(Et₂N)₂Ge­[C­(Ga^<i>t</i>Bu₂)C­(H)^<i>t</i>Bu] (<b>6</b>). The Lewis acidic aluminum and gallium atoms showed a close contact to the nitrogen atoms of the amino groups attached to germanium, which resulted in relatively long Ge–N bonds and short Al–N or Ga–N distances. The structures of these molecules and the strengths of the interactions were investigated by dispersion-corrected density functional theory. This activation of the Ge–N bonds caused an unprecedented reactivity of compounds <b>4b</b> and <b>6</b>. <b>4b</b> reacted with PhCCH under mild conditions and elimination of HNEt₂ to give the mixed dialkynyl compound (^<i>t</i>BuCC)­(PhCC)­Ge­[C­(Ga^<i>t</i>Bu₂)C­(H)^<i>t</i>Bu]₂ (<b>5</b>), while facile insertion of RNCX into a Ge–N bond of <b>6</b> led to the formation of the six-membered Ge–C–Ga–X–C–N heterocycles <b>7</b> (R = Ph, Et; X = O, S)