Cyclo[n]carbons Form Strong N → C Dative/Covalent Bonds with Piperidine

Chemistry of Cyclo[18]Carbon (C18): A Review

Carbon-based allotropes are propelling a technological revolution in communication, sensing, and computing, concurrently challenging fundamental theories of the previous century. Nevertheless, the demand for advanced carbon-based materials remains substantial. The crux lies in the efficient and reliable engineering of novel carbon allotrope. Although C18 has undergone theoretical and experimental investigation for an extended period, its preparation and direct observation in the condensed phase occurred only recently through STM/AFM techniques. The distinctive cyclic ring structure and the dual 18-center π delocalization character introduce various uncommon properties to C18, rendering it a subject worthy of in-depth exploration. In this context, this review delves into past developments contributing to the state-of-the-art understanding of C18 and provides insights into how future endeavours can expedite practical applications. Encompassing a broad spectrum, this review comprehensively investigates almost all facets of C18, including geometric characteristics, electron delocalization, bonding nature, aromaticity, reactivity, electronic excitation, UV/Vis spectrum, intermolecular interaction, response to external fields, electron affinity, ionization, and other molecular properties. Moreover, the review also outlines representative strategies for the direct synthesis and characterization of C18 using atom manipulation techniques. Following this, C18-based complexes are summarized, and potential applications in catalysis, electrochemical devices, optoelectronics, and sensing are discussed.

Chalcogen-Bond-Assisted Formation of the N→C Dative Bonds in the Complexes between Chalcogenadiazoles/Chalcogenatriazoles and Fullerene C60

The existence of the N→C dative bonds in the complexes between N-containing molecules and fullerenes have been verified both theoretically and experimentally. However, finding stable N→C dative bonds is still a highly challenging task. In this work, we investigated computationally the N→C dative bonds in the complexes formed by fullerene C60 with 1,2,5-chalcogenadiazoles, 2,1,3-benzochalcogenadiazoles, and 1,2,4,5-chalcogenatriazoles, respectively. It was found that the N→C dative bonds are formed along with the formation of the N-Ch···C (Ch = S, Se, Te) chalcogen bonds. In the gas phase, from S-containing complexes through Se-containing complexes to Te-containing complexes, the intrinsic interaction energies become more and more negative, which indicates that the N-Ch···C chalcogen bonds can facilitate the formation of the N→C dative bonds. The intrinsic interaction energies are compensated by the large deformation energy of fullerene C60. The total interaction energies of Te-containing complexes are negative, while both total interaction energies of the S-containing complexes and Se-containing complexes are positive. This means that the N→C dative bonds in the Te-containing complexes are more easily observed in experiments in comparison with those in the S-containing complexes and Se-containing complexes. This study provides a new theoretical perspective on the experimental observation of the N→C dative bonds in complexes involving fullerenes. Further, the formation of stable N→C dative bonds in the complexes involving fullerenes can significantly change the properties of fullerenes, which will greatly simulate and expand the application range of fullerenes.

The Stability of Hydrogen-Bonded Ion-Pair Complex Unexpectedly Increases with Increasing Solvent Polarity

The generally observed decrease of the electrostatic energy in the complex with increasing solvent polarity has led to the assumption that the stability of the complexes with ion-pair hydrogen bonds decreases with increasing solvent polarity. Besides, the smaller solvent-accessible surface area (SASA) of the complex in comparison with the isolated subsystems results in a smaller solvation energy of the latter, leading to a destabilization of the complex in the solvent compared to the gas phase. In our study, which combines Nuclear Magnetic Resonance, Infrared Spectroscopy experiments, quantum chemical calculations, and molecular dynamics (MD) simulations, we question the general validity of this statement. We demonstrate that the binding free energy of the ion-pair hydrogen-bonded complex between 2-fluoropropionic acid and n-butylamine (CH3CHFCOO-…NH3But+) increases with increased solvent polarity. This phenomenon is rationalized by a substantial charge transfer between the subsystems that constitute the ion-pair hydrogen-bonded complex. This unexpected finding introduces a new perspective to our understanding of solvation dynamics, emphasizing the interplay between solvent polarity and molecular stability within hydrogen-bonded systems.

Mechanistic Inquisition on the Reduction of C17Si(NH2)2 to NH3: A DFT Study

Activation of molecular nitrogen by silicon-substituted cyclo[18]carbon and its ability to produce the C17Si-(NH2)2 derivative, as the precursor of NH3, has been recently reported. This specific acquisition has piqued an interest to investigate the possibility of NH3 formation with further addition of H2 molecules in the gaseous reaction media. The current investigations reported in this article show that two moles of molecular H2 generate two molecules of NH3 and a C17Si-H2 byproduct from its precursor. The catalyst gets restored by an in situ reaction between some unreacted C17Si-N2 and the byproduct in the media. This reaction also produces the next C17Si-(NH)2 adduct, which restarts the catalytic cycle for NH3 production again.

Trends in the stability of covalent dative bonds with variable solvent polarity depend on the charge transfer in the Lewis electron-pair system

In general, the stability of neutral complexes with dative bonds increases as the polarity of the solvent increases. This is based on the fact that the dipole moment of the complex increases as the charge transferred from the donor to the acceptor increases. As a result, the solvation energy of the complex becomes greater than that of subsystems, causing an increase in the stabilization energy with increasing solvent polarity. Our research confirms this assumption, but only when the charge transfer is sufficiently large. If it is below a certain threshold, the increase in the complex's dipole moment is insufficient to result in a higher solvation energy than subsystems. Thus, the magnitude of the charge transfer in the Lewis electron-pair system determines the stability trends of dative bonds with varying solvent polarity. We used molecular dynamics (MD) simulations based on an explicit solvent model, which is considered more reliable, to verify the results obtained with a continuous solvent model.

Trapping of Small Molecules within Single or Double Cyclo[18]carbon Rings

The encapsulation of a set of small molecules, H₂, CO, CO₂, SO₂, and SO₃, by a circular C₁₈ ring is investigated by quantum calculations. These ligands lie near the center of the ring but, with the exception of H₂, are disposed roughly perpendicular to the ring plane. Their binding energies with the C₁₈ vary from 1.5 kcal/mol for H₂ up to 5.7 kcal/mol for SO₂, and the bonding is dominated by dispersive interactions spread over the entire ring. The binding of these ligands on the outside of the ring is weaker but allows the opportunity for each to bond covalently with the ring. A pair of C₁₈ units lie parallel to one another. This pair can bind each of these ligands in the area between them with only small perturbations of the double ring geometry. The binding energies of these ligands to this double ring configuration are amplified by some 50% compared to the single ring systems. The presented data concerning the trapping of small molecules may have larger implications regarding hydrogen storage or air pollution reduction.

Activation and Conversion of Molecular Nitrogen to the Precursor of Ammonia on Silicon Substituted Cyclo[18]Carbon: a DFT Design

Recent synthesis of sp-hybridized cyclo[18]carbon allotrope has attracted immense curiosity. Since then, a generous amount of theoretical studies concerning aromaticity, adsorption, and spectra of the molecule have been performed. However, very few stuides have been carried out concerning its reactivities and catalytic behaviour. In this article, a DFT-based inquisition has been reported regarding the reactivity of Si substituted cyclo[18]carbon molecule towards molecular N₂ . Results show that the Si substituted derivative is effective in producing adducts with molecular nitrogen. Charge calculations and IRC trapping methods indicate that only the Si center of C₁₇ Si and its (HOMO-1) level participate in N₂ addition. The N-adduct so formed, is then found to spontaneously react with molecular H₂ . The addition of two H₂ molecules to the activated nitrogen molecule to give respective amine derivatives have also been studied. The successful generation of the precursor of NH₃ by C₁₇ Si lays a clear emphasis on its potentiality.

Intermolecular interactions between cyclo[18]carbon and XCN (X = H, F, Cl, Br, I): a theoretical study

In this article, the intermolecular interactions of cyclo[18]carbon with XCN (X = H, F, Cl, Br, I) were investigated in detail by quantum chemistry calculations and wavefunction analyses. The electrostatic potential and van der Waals potential of cyclo[18]carbon were examined, then the structures of the complexes, the interaction energies of the intermolecular interactions were studied. Quantum theory of atoms in molecules analysis was performed to help understand the specific interactions. The XCN molecules can insert into the cyclo[18]carbon ring, and ClCN, BrCN, and ICN could also bind with cyclo[18]carbon from outside. Charge transfer in the inner complex is more prominent than that of the outer complex. Plots of electron density difference revealed that electron density shift was significantly different when the X atom changed. The main driving force for molecular binding is dispersion attraction, which is disclosed by interaction region indicator analysis and energy decomposition calculations.

Odd-Number Cyclo[n]Carbons Sustaining Alternating Aromaticity

Cyclo[n]carbons (n = 5, 7, 9, ..., 29) composed from an odd number of carbon atoms are studied computationally at density functional theory (DFT) and ab initio complete active space self-consistent field (CASSCF) levels of theory to get insight into their electronic structure and aromaticity. DFT calculations predict a strongly delocalized carbene structure of the cyclo[n]carbons and an aromatic character for all of them. In contrast, calculations at the CASSCF level yield geometrically bent and electronically localized carbene structures leading to an alternating double aromaticity of the odd-number cyclo[n]carbons. CASSCF calculations yield a singlet electronic ground state for the studied cyclo[n]carbons except for C₂₅, whereas at the DFT level the energy difference between the lowest singlet and triplet states depends on the employed functional. The BHandHLYP functional predicts a triplet ground state of the larger odd-number cyclo[n]carbons starting from n = 13. Current-density calculations at the BHandHLYP level using the CASSCF-optimized molecular structures show that there is a through-space delocalization in the cyclo[n]carbons. The current density avoids the carbene carbon atom, leading to an alternating double aromaticity of the odd-number cyclo[n]carbons satisfying the antiaromatic [4k+1] and aromatic [4k+3] rules. C₁₁, C₁₅, and C₁₉ are aromatic and can be prioritized in future synthesis. We predict a bond-shift phenomenon for the triplet state of the cyclo[n]carbons leading to resonance structures that have different reactivity toward dimerization.

The stability of covalent dative bond significantly increases with increasing solvent polarity

It is generally expected that a solvent has only marginal effect on the stability of a covalent bond. In this work, we present a combined computational and experimental study showing a surprising stabilization of the covalent/dative bond in Me₃NBH₃ complex with increasing solvent polarity. The results show that for a given complex, its stability correlates with the strength of the bond. Notably, the trends in calculated changes of binding (free) energies, observed with increasing solvent polarity, match the differences in the solvation energies (ΔE^solv) of the complex and isolated fragments. Furthermore, the studies performed on the set of the dative complexes, with different atoms involved in the bond, show a linear correlation between the changes of binding free energies and ΔE^solv. The observed data indicate that the ionic part of the combined ionic-covalent character of the bond is responsible for the stabilizing effects of solvents.

Non covalent complexes are often considerably destabilized in the solvent. Here the authors combine vibrational Raman and NMR spectroscopy with a coupled-cluster computational investigation to show that the solvent polarity enhance the complex stability of a Me₃NBH₃ complex.

Heavy-Atom Tunneling in the Covalent/Dative Bond Complexation of Cyclo[18]carbon-Piperidine

Recent quantum chemical computations demonstrated the electron-acceptance behavior of this highly reactive cyclo[18]carbon (C₁₈) ring with piperidine (pip). The C₁₈–pip complexation exhibited a double-well potential along the N–C reaction coordinate, forming a van der Waals (vdW) adduct and a more stable, strong covalent/dative bond (DB) complex by overcoming a low activation barrier. By means of direct dynamical computations using canonical variational transition state theory (CVT), including the small-curvature tunneling (SCT), we show the conspicuous role of heavy atom quantum mechanical tunneling (QMT) in the transformation of vdW to DB complex in the solvent phase near absolute zero. Below 50 K, the reaction is entirely driven by QMT, while at 30 K, the QMT rate is too rapid (k_T ∼ 0.02 s^–1), corresponding to a half-life time of 38 s, indicating that the vdW adduct will have a fleeting existence. We also explored the QMT rates of other cyclo[n]carbon–pip systems. This study sheds light on the decisive role of QMT in the covalent/DB formation of the C₁₈–pip complex at cryogenic temperatures.

P-Doped graphene-C60 nanocomposite: a donor-acceptor complex with a P-C dative bond

Phosphorous-doped graphene can form a covalent dative bond with the electron acceptor, C₆₀ molecule. On the other hand, C₆₀ on graphene and N-doped graphene surfaces can only form vdW complexes. State-of-the-art quantum-chemical techniques have been used to characterise the nature of the P-C dative bond. A considerable amount of charge transfer from the P-Gr surface to C₆₀ has been observed. This complex formation may enable enhancement in the optical limiting response with potential application in energy harvesting. The stability of the P-C dative bond has been assessed using DFT-D molecular dynamics simulations at 300 K for 10 ps.

Bonding Character, Electron Delocalization, and Aromaticity of Cyclo[18]Carbon (C18 ) Precursors, C18 -(CO)n (n=6, 4, and 2): Focusing on the Effect of Carbonyl (-CO) Groups

The bonding character, electron delocalization, and aromaticity of the cyclo[18]carbon (C₁₈ ) precursors, C₁₈ -(CO)_n (n=6, 4, and 2), have been studied by combining quantum chemical calculations and various electronic wavefunction analyses with different physical bases. It was found that C₁₈ -(CO)_n (n=6, 4, and 2) molecules exhibit alternating long and short C-C bonds, and have out-of-plane and in-plane dual π systems (π^out and πⁱⁿ ) perpendicular to each other, which are consistent with the relevant characteristics of C₁₈ . However, the presence of carbonyl (-CO) groups significantly reduced the global electron conjugation of C₁₈ -(CO)_n (n=6, 4, and 2) compared to C₁₈ . Specifically, the -CO group largely breaks the extensive delocalization of πⁱⁿ system, and the π^out system is also affected by it but to a much lesser extent; as a consequence, C₁₈ -(CO)_n (n=6, 4, and 2) with larger n shows weaker overall aromaticity. Mostly because of the decreased but still apparent π^out electron delocalization in the C₁₈ -(CO)_n (n=6, 4, and 2), a notable diatropic induced ring current under the action of external magnetic field is observed, demonstrating the clear aromatic characteristic in the molecules. The correlation between C₁₈ -(CO)_n (n=6, 4, and 2) and C₁₈ in terms of the gradual elimination of -CO from the precursors showed that the direct elimination of two CO molecules in C₁₈ -(CO)_n (n=6, 4, and 2) has a synergistic mechanism, but it is kinetically infeasible under normal conditions due to the high energy barrier.