Cyclopropenium Cations Break the Rules of Attraction to Form Closely Bound Dimers

Cation-Stacking Approach Enabling Interconversion between Bis(xanthylium) and its Reduced Species

Cyclophane-type dications with two units of xanthylium were designed, with the expectation that intramolecular interaction between cation units could induce changes in absorption and redox behavior. The desired dications were synthesized via the macrocyclic diketone as a key intermediate, which was efficiently obtained by a stepwise etherification. X-ray and UV/Vis measurements revealed that the cyclophane-type dications adopt a stacking structure in both the crystal and solution. Due to the intramolecular interaction caused by π-π stacking of the xanthylium units, a considerable blue shift compared to the corresponding monocations and a two-stage one-electron reduction process were observed in the dications. Furthermore, upon electrochemical reduction of dications, the formation of biradicals via radical cation species was demonstrated by UV/Vis spectroscopy with several isosbestic points at both stages. Therefore, the cation-stacking approach is a promising way to provide novel properties due to perturbation of their molecular orbitals and to stabilize the reduced species even though they have open-shell characters.

Crystal structure of N-butyl-2,3-bis-(di-cyclo-hexyl-amino)-cyclo-propeniminium chloride benzene monosolvate

N-Butyl-2,3-bis-(di-cyclo-hexyl-amino)-cyclo-propenimine (1) crystallizes from benzene and hexa-nes in the presence of HCl as a mono-benzene solvate of the hydro-chloride salt, [1H]Cl·C6H6 or C31H54N3 +·Cl-·C6H6, in the P21/n space group. The protonation of 1 results in the generation of an aromatic structure based upon the delocalization of the cyclo-propene double bond around the cyclo-propene ring, giving three inter-mediate C-C bond lengths of ∼1.41 Å, and the delocalization of the imine-type C-N double bond, giving three inter-mediate C-N bond lengths of ∼1.32 Å. Ion-ion and ion-benzene packing inter-actions are described and illustrated.

Unusual Alternating Crystallization-Induced Emission Enhancement Behavior in Nonconjugated ω-Phenylalkyl Tropylium Salts

The alternating physical properties, especially melting points, of α,ω-disubstituted n-alkanes and their parent n-alkanes had been known since Baeyer's report in 1877. There is, however, no general and comprehensive explanation for such a phenomenon. Herein, we report the synthesis and examination of a series of novel ω-phenyl n-alkyl tropylium tetrafluoroborates, which also display alternation in their physicochemical characters. Despite being organic salts, the compounds with odd numbers of carbons in the alkyl bridge exist as room temperature ionic liquids. In stark contrast to this, the analogues with even numbers of carbons in the linker are crystalline solids. These solid nonconjugated molecules exhibit curious photoluminescent properties, which can be attributed to their ability to form through-space charge-transfer complexes to cause crystallization-induced emission enhancement. Most notably, the compound with the highest photoluminescent quantum yield in this series showed an unusual arrangement of carbocationic dimer in the solid state. A combination of XRD analysis and ab initio calculations revealed interesting insights into these systems.

Discrete Oligomers and Polymers of Chloride Monohydrate Can Form in Encapsulated Environments: Structures and Infrared Spectra of [Cl4 (H2 O)4 ]4- and {[Cl(H2 O)]- }∞

A discrete tetrachloride tetrahydrate cluster, [Cl₄ (H₂ O)₄ ]^4- , was obtained with a partially-fluorinated triaminocyclopropenium cation, [C₃ (N(CH₂ CF₃ )₂ )(NEt₂ )(NPr₂ )]⁺ . The cluster consists of a [Cl₂ (H₂ O)₂ ]^2- square with each Cl^- coordinated by another H₂ O bridged to another Cl^- . A 1D polymer of chloride monohydrate, {[Cl(H₂ O)]^- }_∞ , was obtained with [C₃ (N(CH₂ CF₃ )₂ )₂ (NBuMe)]⁺ . The tetrameric and polymeric structures were found to be computationally-unstable in the gas phase which indicates that an encapsulated environment is essential for their isolation. DFT and DFTB calculations were carried out on gas-phase [Cl₄ (H₂ O)₄ ]^4- to assist the infrared assignments. Anharmonically-corrected B3LYP transition frequencies were in close agreement with experiment, but DFTB models were only appropriate for qualitative interpretation. Solid-state DFTB calculations allowed the vibrational modes to be assigned. The results found are consistent with "discrete" chloride hydrates.

Bis(amino)cyclopropenium Ion as a Hydrogen-Bond Donor Catalyst for 1,6-Conjugate Addition Reactions

The catalytic application of the bis(amino)cyclopropenium ion has been investigated in conjugate addition reactions. The hydrogen atom, which is attached to the cyclopropene ring of bis(amino)cyclopropenium salts, is moderately acidic and can potentially serve as a hydrogen-bond donor catalyst in some organic transformations. This hypothesis has been successfully realized in the 1,6-conjugate addition reactions of p-quinone methides with various nucleophiles such as indole, 2-naphthol, thiols, phenols, and so forth. The spectroscopic studies (NMR and UV-vis) as well as the deuterium isotope labeling studies clearly revealed that the hydrogen atom (C-H) that is present in the cyclopropene ring of the catalyst is indeed solely responsible for catalyzing these transformations. In addition, these studies also strongly indicate that the C-H hydrogen of the cyclopropene ring activates the carbonyl group of the p-quinone methide through hydrogen bonding.

A Strained Ion Pair Permits Carbon Dioxide Fixation at Atmospheric Pressure by C-H H-Bonding Organocatalysis

The cycloadditions of carbon dioxide into epoxides to afford cyclic carbonates by H-bond donor (HBD) and onium halide (X) cocatalysis have emerged as a key strategy for CO₂ fixation. However, if the HBD is also a halide receptor, the two will quench each other, decreasing the catalytic activity. Here, we propose a strained ion pair tris(alkylamino)cyclopropenium halide (TAC·X), in which TAC repels X. TAC possesses a positively charged cyclopropenium core that makes the vicinal C-H or N-H a nonclassical HBD. The interionic strain within TAC·X makes TAC a more electrophilic HBD, allowing it to activate the oxygen of the epoxide and making X more nucleophilic and better able to attack the methylene carbon of the epoxide. NMR titration spectra and computational studies were employed to probe the mechanism of the cycloaddition of CO₂ to epoxides reactions under the catalysis of TAC·X. The ¹H and ¹³C{¹H}NMR titration spectra of the catalyst with the epoxide substrate unambiguously confirmed H-bonding between TAC and the epoxide. DFT computational studies identified the transition states in the ring-opening of the epoxide (TS1) and in the ring-closure of the cyclic carbonate (TS2).

A Discrete Chloride Monohydrate: A Solid-State Structural and Spectroscopic Characterization

The solid-state structure of a discrete chloride monohydrate species, [Cl(H₂O)]^-, is reported for the first time. It was isolated as a salt of the tris(dipropylamino)cyclopropenium cation and has been structurally characterized by X-ray and neutron diffraction. Infrared (IR), far-infrared, and Raman spectroscopic studies were also carried out. Additionally, the D₂O and HDO isotopomers were investigated. Of the six fundamental vibrational modes, only the out-of-plane bend ν₃ was not observed as it forms an IR- and Raman-inactive local mode phonon.

A Series of Discrete Dichloride Dihydrates: Characterisation and Symmetry Effects

A series of three discrete dichloride dihydrates [Cl₂ (H₂ O)₂ ]^2- have been isolated with different triaminocyclopropenium (TAC) cations and with different crystallographic symmetries. The cluster exhibits D_2h symmetry with the tris(dimethylamino)cyclopropenium cation [C₃ (NMe₂ )₃ ]⁺ , C_2h symmetry with the fluorinated cation [C₃ (N(CH₂ CF₃ )₂ )(NBu₂ )₂ ]⁺ (containing two 2,2,2-trifluoroethyl substituents) and C_2v symmetry with the more fluorinated [C₃ (N(CH₂ CF₃ )₂ )₂ (NBu₂ )]⁺ cation. The effect of symmetry on the infrared spectra of the dichloride ion-pair clusters, as well as deuterated analogues, has been investigated. The D_2h - and C_2h -symmetric clusters each exhibit two stretching bands in the infrared at 3427 and 3368 cm^-1 for D_2h symmetry and 3444 and 3392 cm^-1 for C_2h symmetry, whereas the C_2v -symmetric cluster exhibits three bands at 3475, 3426 and 3373 cm^-1 . Computational studies were carried out on a [Cl₂ (H₂ O)₂ ]^2- cluster with C_2v symmetry to aid the infrared band assignments.

Self-Assembly of Aminocyclopropenium Salts: En Route to Deltic Ionic Liquid Crystals

Aminocyclopropenium ions have raised much attention as organocatalysts and redox active polymers. However, the self-assembly of amphiphilic aminocyclopropenium ions remains challenging. The first deltic ionic liquid crystals based on aminocyclopropenium ions have been developed. Differential scanning calorimetry, polarizing optical microscopy and X-ray diffraction provided insight into the unique self-assembly and nanosegregation of these liquid crystals. While the combination of small headgroups with linear p-alkoxyphenyl units led to bilayer-type smectic mesophases, wedge-shaped units resulted in columnar mesophases. Upon increasing the size and polyphilicity of the aminocyclopropenium headgroup, a lamellar phase was formed.

A Discrete Dichloride Tetrahydrate Trapped by a Cyclopropenium Cation: Structure and Spectroscopic Properties

A discrete dichloride tetrahydrate cluster, [Cl₂ (H₂ O)₄ ]^2- , was obtained as a salt of the bis(diphenylamino)diethylamino cyclopropenium cation [C₃ (NPh₂ )₂ (NEt₂ )]⁺ and characterized by single-crystal X-ray diffraction and infrared spectroscopy. This chloride-chloride ion-pair cluster consists of a [Cl₂ (H₂ O)₂ ]^2- square with opposite edges bridged by water molecules to give a chair-like structure of the non-hydrogen atoms. The solid-state structure is essentially the same as the calculated gas-phase structure. Infrared spectra were also collected on the deuterium analogue [Cl₂ (D₂ O)₄ ]^2- . Computational studies were carried out on gas-phase [Cl₂ (H₂ O)₄ ]^2- to confirm the infrared band assignments in the solid state. The structure and infrared spectrum are consistent with the discrete nature of the cluster.

The Binary Iodine-Chlorine Octahalide Series [In Cl8-n ]2- (n=3, 3.6, 4)

The octanuclear iodine-chlorine interhalides [I₄ Cl₄ ]^2- and [I₃ Cl₅ ]^2- were prepared in two steps. Firstly, addition of ICl to the triaminocyclopropenium chloride salt [C₃ (NEt₂ )₃ ]Cl forms the trihalide ICl₂ ^- salt, secondly, addition of half an equivalent of I₂ or ICl, respectively, gave the desired products upon crystallization at low temperature. The non-stoichiometric octahalide [I_3.6 Cl_4.4 ]^2- was obtained after heating a CH₂ Cl₂ solution of the ICl₂ ^- salt to reflux for 2 hours followed crystallization. [I₄ Cl₄ ]^2- is best described as two ICl₂ ^- anions bridged by I₂ , whereas [I₃ Cl₅ ]^2- is best described as an [I₂ Cl₃ ]^- pentahalide with a weak halogen bond to an ICl₂ ^- trihalide. The octahalides were characterized by X-ray crystallography, computational studies, Raman and Far-IR spectroscopies, as well as by TGA and melting point.

Rational Synthesis, Structures and Properties of the Ionic Liquid Binary Iodine-Bromine Octahalide Series [In Br8-n ]2- (n=0, 2, 3, 4)

A series of octanuclear iodine-bromine interhalides [I_n Br_8-n ]^2- (n=0, 2, 3, 4) were prepared systematically in two steps. Firstly, addition of a dihalogen (Br₂ or IBr) to the triaminocyclopropenium bromide salt [C₃ (NEt₂ )₃ ]Br forms the corresponding trihalide salt with Br₃ ^- or IBr₂ ^- anions, respectively. Secondly, addition to Br₃ ^- of half an equivalent of Br₂ gives the octabromine polyhalide [Br₈ ]^2- , whereas addition to IBr₂ ^- of half an equivalent of Br₂ , IBr or I₂ gives the corresponding interhalides: [I₂ Br₆ ]^2- , [I₃ Br₅ ]^2- , and [I₄ Br₄ ]^2- , respectively. The four octahalides were characterized by X-ray crystallography, computational studies, Raman and Far-IR spectroscopies, as well as by TGA and melting point. All of the salts were found to be ionic liquids.

Fluoride Ionic Liquids in Salts of Ethylmethylimidazolium and Substituted Cyclopropenium Cation Families

A series of solvent-stabilized ionic liquid fluorides were prepared, [EMIM]F.nCH3COOH n = 1. 0, 1.6, 2.1, 2.4, and 3.2, either via exchange from the chloride salt using KF or AgF, or by neutralization of the hydroxide salt using HF. Azeotrope drying was used to remove water. Their viscosity, conductivity and density properties were determined. A diethanol solvate of the triaminocyclopropenium salt [C3(NPr2)3]F was found to be stable and its viscosity, conductivity and density properties were also determined. The monoethanol solvate, however, was found to be unstable with trace water present. Intramolecular stabilization of fluoride was achieved by using OH functionalized cations: [C3(NEt2)2N(CH2CH2OH)2]F, [C3(N(CH2CH2OH)2)3]F, and [Me3NCH2CH2OH]F.H2O, The first of these is an ionic liquid at ambient temperature and has a TGA mass loss onset at 175°C, indicating a useful range of liquid state stability.

Opposite-charge repulsive cation and anion pair cooperative organocatalysis in ring-opening polymerization

A new strained ion pair catalysis was proposed in ring-opening polymerization.

Synthesis and physical properties of tris(dialkylamino)cyclopropenium dicyanamide ionic liquids

The synthesis and properties of 16 tris(dialkylamino)cyclopropenium (TDAC) cations with the dicyanamide (DCA) anion, [N(CN)₂]⁻, are described. D_3h- and C_3h-symmetric cations ([C₃(NR₂)₃]DCA (R = Me, Et, Pr, Bu, Pent, Hex, Dec) and [C₃(NRMe)₃]DCA (R = Bu, St), respectively) were synthesised by reaction of C₃Cl₅H with the corresponding amine. Reaction of the alkoxydiaminocyclopropenium salt [C₃(NEt₂)₂(OMe)]⁺ with amines led to a series of C_2v-symmetric salts [C₃(NEt₂)₂(NR₂)]DCA (R = Me, Bu, Hex) and two C_s-symmetric salts and [C₃(NEt₂)₂(NRMe)]DCA (R = Me, Bu). Similarly, [C₃(NMe₂)₂(OMe)]⁺, was used to prepare the C_s-symmetric salts [C₃(NMe₂)₂(NRMe)]DCA (R = Pr, Bu). In addition to characterisation by NMR, mass spectrometry and microanalysis, the salts were characterised by DSC, TGA, density, viscosity, conductivity and miscibility/solubility studies. Comparisons have been made with similar series of bistriflimide (NTf₂⁻) salts that have been previously reported to see whether the same trends are observed with a different anion.

An investigation of symmetry (D_3h, C_3h, C_2v and C_s) and alkyl chain length (6–60 alkyl carbon atoms) effects on the physical properties of peralkylated triaminocyclopropenium dicyanamide salts.

(15)N NMR Spectroscopy, X-ray and Neutron Diffraction, Quantum-Chemical Calculations, and UV/vis-Spectrophotometric Titrations as Complementary Techniques for the Analysis of Pyridine-Supported Bicyclic Guanidine Superbases

Pyridine substituted with one and two bicyclic guanidine groups has been studied as a potential source of superbases. 2-{hpp}C5H4N (I) and 2,6-{hpp}2C5H3N (II) (hppH = 1,3,4,6,7,8-hexahydro-2H-pyrimido[1,2-a]pyrimidine) were protonated using [HNEt3][BPh4] to afford [I-H][BPh4] (1a), [II-H][BPh4] (2), and [II-H2][BPh4]2 (3). Solution-state (1)H and (15)N NMR spectroscopy shows a symmetrical cation in 2, indicating a facile proton-exchange process in solution. Solid-state (15)N NMR data differentiates between the two groups, indicating a mixed guanidine/guanidinium. X-ray diffraction data are consistent with protonation at the imine nitrogen, confirmed for 1a by single-crystal neutron diffraction. The crystal structure of 1a shows association of two [I-H](+) cations within a cage of [BPh4](-) anions. Computational analysis performed in the gas phase and in MeCN solution shows that the free energy barrier to transfer a proton between imino centers in [II-H](+) is 1 order of magnitude lower in MeCN than in the gas phase. The results provide evidence that linking hpp groups with the pyridyl group stabilizes the protonation center, thereby increasing the intrinsic basicity in the gas phase, while the bulk prevents efficient cation solvation, resulting in diminished pKa(MeCN) values. Spectrophotometrically measured pKa values are in excellent agreement with calculated values and confirm that I and II are superbases in solution.

Synthesis, characterization and properties of amino acid ionic liquids derived from the triaminocyclopropenium cation

Amino acid ionic liquids based on the triaminocyclopropenium cation form a variety of structural types depending on the amino acid. Dications were obtained with Lys, His, Arg, and Cys.