Pancake Bonds Research Articles

Supramolecular Spin Chains via Radical-Radical Contacts Stabilizing Ferromagnetic Interactions Between Heisenberg or Ising-Like Spins.

A new paramagnetic ligand, 4-(2'-4-(2''-furyl)-pyrimidyl)-1,2,3,5-dithiadiazolyl (furylpymDTDA) and three transition metal coordination complexes, M(hfac)2(furylpymDTDA) M=Mn, Co, Ni; hfac=1,1,1,5,5,5-hexafluoroacetylacetonato-), are reported. The solid-state structures are influenced by the geometry of the coordination sphere of the M(II) centers: trigonal (Mn) vs. octahedral (Co and Ni). While the hs-Mn(II) complex exhibits pairwise multi-centre 2-electron bonds (i. e., pancake bonds) between the planar π radical DTDA moieties, the hs-Co(II) and Ni(II) complexes crystallize with close contacts between coordinated furylpymDTDA radical ligands that define linear 1D arrays of molecular units. The magnetic data for the hs-Co(II) and Ni(II) complexes indicate ferromagnetic (FM) interactions between molecular units, apparently mediated by radical-radical contacts along the supramolecular chains. Computational analysis suggests proximity between regions of large α- and β-spin density on neighbouring molecular sites enabling FM exchange, in accordance with the McConnell I mechanism. The magnetic data for the Ni(II) complex are consistent with a Heisenberg spin chain, whereas the hs-Co(II) complex exhibits Ising-like spin chain behaviour and a magnetic phase transition to an FM ordered state at 4.6 K.

Polymorphism of a planar neutral radical: magnetic and optical implications

The synthesis and characterization of a new paramagnetic ligand, 4-( N′-methyl-2′-benzimidazolyl)-1,2,3,5-dithiadiazolyl (MebimDTDA), is presented. MebimDTDA exhibits polymorphism in the solid state. The red microcrystalline form (Form I-red) is diamagnetic below T = 370 K, consistent with typical multi-center two-electron bonding interactions (i.e., pancake bonds) between planar π-radicals. By contrast, the green–black crystalline form (Form II-green) exhibits paramagnetic properties consistent with the observed unusual crystal packing in which some, but not all, of the radicals are arranged in a typical pancake-bonded geometry. Crystallographic analysis, magnetic data, and computational methods are employed to elucidate the structure-property relationship, with an emphasis on the role of crystal packing.

Structure and Bonding in π-Stacked Perylenes: The Impact of Charge on Pancake Bonding.

Perylene (PER) is a prototype of polycyclic aromatic hydrocarbons (PAHs), which play a pivotal role in various functional and electronic materials due to favorable molecule-to-molecule overlaps, which enhance electronic transport. This study provides guidelines regarding the impact of molecular charge on pancake bonding, a form of strong π-stacking interaction. Pancake bonding significantly boosts interaction energies within the monopositive dimer ([(C20H12)2]•+ or PER2+), crucial for stabilizing aggregation and crystal formation. We discovered energetically feasible sliding and rotation pathways within the [(C20H12)2]•+ dimer, connecting different configurations found in the Cambridge Structural Database (CSD). The dimer's charge profoundly influences the pancake bond order (PBO) and the strength and structural preferences of pancake bonding. The most stable configuration is found in the monocationic state (PER2+), featuring a pancake bond order of 1/2 with one-electron multicenter bonding (1e/mc) with similar characteristics for charge -1. Increasing the total charge of the dimer to +2 or -2 leads to an unstable local minimum. Diverse distribution of pancake bonding types present in crystal structures is interpreted with modeling based on dimer computations with varying charges.

Triply Bonded Pancake π-Dimers Stabilized by Tetravalent Actinides.

Aromatic π-stacking is a weakly attractive, noncovalent interaction often found in biological macromolecules and synthetic supramolecular chemistry. The weak nondirectional nature of π-stacking can present challenges in the design of materials owing to their weak, nondirectional nature. However, when aromatic π-systems contain an unpaired electron, stronger attraction involving face-to-face π-orbital overlap is possible, resulting in covalent so-called "pancake" bonds. Two-electron, multicenter single pancake bonds are well known, whereas four-electron double pancake bonds are rare. Higher-order pancake bonds have been predicted, but experimental systems are unknown. Here, we show that six-electron triple pancake bonds can be synthesized by a 3-fold reduction of hexaazatrinaphthylene (HAN) and subsequent stacking of the [HAN]3- triradicals. Our analysis reveals a multicenter covalent triple pancake bond consisting of a σ-orbital and two equivalent π-orbitals. An electrostatic stabilizing role is established for the tetravalent thorium and uranium ions in these systems. We also show that the electronic absorption spectrum of the triple pancake bonds closely matches computational predictions, providing experimental verification of these unique interactions. The discovery of conductivity in thin films of triply bonded π-dimers presents new opportunities for the discovery of single-component molecular conductors and other spin-based molecular materials.

Stabilizing Cationic Perylene Dimers through Pancake Bonding and Equal Charge Share

Stabilizing Cationic Perylene Dimers through Pancake Bonding and Equal Charge Share

Two Electron Multicenter Bonding (Pancake Bonding) between Electron Donors and Acceptors in Charge-Transfer Compounds ofN,N,N′,N′-Tetramethyl-p-phenylenediamine and (Semi)quinoid Electron Acceptors

Five charge transfer complexes of electron donor N,N,N′,N′-tetramethyl-p-phenylenediamine (TMPD or Wurster’s blue) with quinoid electron acceptors tetrafluoro- (F4Q), tetrachloro- (Cl4Q), tetrabromoquinone (Br4Q), 7,7,8,8-tetracyanoquinodimethane (TCNQ), and 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F4TCNQ) were prepared and studied by X-ray crystallography, impedance spectroscopy, and DFT computations. Mixed donor–acceptor π-stacks were observed in all compounds; in TMPD·F4TCNQ, the moieties formed 2D stacked arrays. All moieties had a partial radical character due to charge transfer. The presence of two-electron multicenter bonding (pancake bonding) was confirmed by quantum chemical computation. In TMPD·F4Q, alternating longer and shorter interplanar distances were consistent with formation of pancake-bonded dimers; in complexes of TMPD with Cl4Q, Br4Q, and TCNQ, the stacks were equidistant implying existence of pancake-bonded polymers.

Reimagining the Wave Function in Density Functional Theory: Exploring Strongly Correlated States in Pancake-Bonded Radical Dimers

Pancake bonding between π-conjugated radicals challenges conventional electronic structure approximations, due to the presence of both dispersion (van der Waals) interactions and "strong" electron correlation. Here we use a reimagined wave function-in-density functional theory (DFT) approach to model pancake bonds. Our generalized self-interaction correction extends DFT's reference system of noninteracting electrons, by introducing electron-electron interactions within an active space. We show that a small variation on our previous derivation recovers a DFT-corrected complete active space method proposed by Pijeau and Hohenstein. Comparison of the two approaches shows that the latter provides reasonable dissociation curves for single bonds and pancake bonds, including excited states inaccessible to conventional linear response time-dependent DFT. The results motivate broader adoption of wavefunction-in-DFT approaches for modeling pancake bonds.

Charge-Transfer Benzoquinone:Catechol Complexes, Their Aging Reactions, High-Pressure Polymorphs and Skew Pancake Bonds

Charge-transfer complexes are the key elements of optoelectronic devices. Quinhydrone (Qh) was the first discovered charge-transfer complex, and for its structure, the π-stacking and pancake bonds were identified. Later, the atom-over-atom criterion was added to the pancake-bond definition. However, this feature is absent in the structures of both Qh polymorphs. Here, we report new charge-transfer complexes of para-benzoquinone (pBq) and catechol (Ct), the only unknown isomers of Qh. Compounds pBq and Ct preferentially co-crystallize to a 1:1 brown complex, α-pBq:Ct, with the molecules alternatively stacked into columns. We have revealed the accelerated aging reaction of α-pBq:Ct, leading to an orange pBq:2Ct complex, the only representative of the 1:2 stoichiometry for the complexes of pBq with isomeric hydroquinone (Hq), resorcinol (Rs), and Ct. On compression to 1.5 GPa, α-pBq:Ct undergoes a transition converting the catechol conformers syn into anti in a markedly darker new phase β-pBq:Ct. This conversion extensively extends the transition hysteresis between phases α- and β-pBq:Ct. At about 2.5 GPa, a radical complex γ-pBq:Ct is formed, which starting from 3.0 GPa gradually transforms into the black amorphous phase δ-pBq:Ct. In phase α-pBq:2Ct, both Ct-conformers syn and anti are present, but high pressure induces a phase transition, eliminating conformers syn in a much darker new phase, β-pBq:2Ct. High pressure reduces the energy band gap of all pBq:Ct and pBq:2Ct phases. The skew geometry of pancake bonds associated with p–p interactions is common for all Qh, pBq:Rs, pBq:Ct, and pBq:2Ct complexes.

A Triply Negatively Charged Nanographene Bilayer with Spin Frustration

AbstractWe report on the largest open‐shell graphenic bilayer and also the first example of triply negatively charged radical π‐dimer. Upon three‐electron reduction, bilayer nanographene fragment molecule (C96H24Ar6)2 (Ar=2,6‐dimethylphenyl) (12) was transformed to a triply negatively charged species 123⋅−, which has been characterized by single‐crystal X‐ray diffraction, electron paramagnetic resonance (EPR) spectroscopy and magnetic properties on a superconducting quantum interference device (SQUID). 123⋅− features a 96‐center‐3‐electron (96c/3e) pancake bond with a doublet ground state, which can be thermally excited to a quartet state. It consists of 34 π‐fused rings with 96 conjugated sp2 carbon atoms. Spin frustration is observed with the frustration parameter f>31.8 at low temperatures in 123⋅−, which indicates graphene upon reduction doping may behave as a quantum spin liquid.

A Triply Negatively Charged Nanographene Bilayer with Spin Frustration.

There is resurgent interest in atomically precise nanographene radical ions composed of fused hexagons. Two monoradical anions can readily form a doubly negatively charged π-dimer, but to our knowledge, no triply negatively charged π-dimer species have been isolated till now. Herein we report the largest open-shell grapheneic bilayer and also the first example of triply negatively charged radical π-dimer. Upon three-electron reduction, bilayer nanographene fragment molecule (C96H24Ar6)2 (Ar = 2,6-dimethylphenyl) (12) was transformed to a triply negatively charged species 123•-, which has been characterized by single-crystal X-ray diffraction, EPR spectroscopy and SQUID measurements. 123•- features a 96-center-3-electron (96c/3e) pancake bond with a doublet ground state, which can be thermally excited to a quartet state. It is the largest open-shell nanographene bilayer species till now constructed with 34 π-fused rings with 96 conjugated sp2 carbon atoms. Spin frustration is observed with the frustration parameter f = |θcw|/TN > 31.8 at low temperatures in 123•-, which indicates graphene upon reduction doping may behave as quantum spin liquids.

Pyrolysis mechanism of composite binder composed of coal tar pitch and phenolic resin for carbon materials

The composite binder consisting of coal tar pitch (CTP) and phenolic resin (PR) has both low viscosity and high char yield, and it has great potential in the preparation of advanced carbon materials. However, the lack of understanding of its synergetic behavior and mechanism hinders the optimization of the preparation process. In this work, the co-pyrolysis of CTP and PR was systematically investigated. The char yield of the composite binder was confirmed to be remarkably higher than the weighted average of the two single binders. Furthermore, the calculated activation energy for the pyrolysis of the composite binder was higher than that of CTP and PR after 400 °C. The interaction and structure evolution mechanisms of composite binder were clarified. Specifically, the ortho-position of the phenoxy carbon in CTP cross-link with the para-position of the phenoxy carbon in PR by the methylene bridge, and the free polycyclic aromatic hydrocarbons stack onto the as-crosslinked polycyclic aromatics driven by the pancake bonds.

Unraveling the Electrical and Magnetic Properties of Layered Conductive Metal‐Organic Framework With Atomic Precision

AbstractThis paper describes structural elucidation of a layered conductive metal‐organic framework (MOF) material Cu3(C6O6)2 by microcrystal electron diffraction with sub‐angstrom precision. This insight enables the first identification of an unusual π‐stacking interaction in a layered MOF material characterized by an extremely short (2.73 Å) close packing of the ligand arising from pancake bonding and ordered water clusters within pores. Band structure analysis suggests semiconductive properties of the MOF, which are likely related to the localized nature of pancake bonds and the formation of a singlet dimer of the ligand. The spin of CuII within the Kagomé arrangement dominates the paramagnetism of the MOF, leading to strong geometrical magnetic frustration.

Unraveling the Electrical and Magnetic Properties of Layered Conductive Metal-Organic Framework With Atomic Precision.

This paper describes structural elucidation of a layered conductive metal-organic framework (MOF) material Cu3 (C6 O6 )2 by microcrystal electron diffraction with sub-angstrom precision. This insight enables the first identification of an unusual π-stacking interaction in a layered MOF material characterized by an extremely short (2.73 Å) close packing of the ligand arising from pancake bonding and ordered water clusters within pores. Band structure analysis suggests semiconductive properties of the MOF, which are likely related to the localized nature of pancake bonds and the formation of a singlet dimer of the ligand. The spin of CuII within the Kagomé arrangement dominates the paramagnetism of the MOF, leading to strong geometrical magnetic frustration.

Exploring Curious Covalent Bonding: Raman Identification and Thermodynamics of Perpendicular and Parallel Pancake Bonding (Pimers) of Ethyl Viologen Radical Cation Dimers.

Viologen radical cations can dimerize in solutions, and the resulting "pimers" were predicted to assemble into parallel and perpendicular conformers by density functional theory (DFT) calculations. Using resonance Raman, we could identify both perpendicular and parallel forms of ethyl viologen dimers. The distinction between the two forms was accomplished by studying the formation of a host-guest complex with γ-cyclodextrin. The dimer's perpendicular form was excluded due to the host cavity size, and γ-cyclodextrin addition caused a decrease in peak intensities at 1171, 1511, and 1602 cm-1 that could be assigned to the perpendicular form. DFT modeling of the vibrational spectra under preresonance conditions allowed us to assign the remaining vibrational modes for the parallel and perpendicular forms. Using variable-temperature UV-vis, the bond dissociation energy (ΔH) for this pancake-bonded dimer was measured as 13.1 ± 0.2 kcal/mol. This type of covalent pancake bonding is a challenge to properly describe using DFT methods. Previously, B97D was found to best describe the ΔG of this dimerization (Angew. Chem. 2017, 129, 9563-9567), but this method underestimates the ΔH by 6 kcal/mol. Of the 11 functionals tested, we found that B3LYP with Grimme's D3 dispersion effect can best reproduce the ΔH. Energy decomposition analysis of the bonding energy showed that solvation effects were the most important contributor-polar solvents are needed to overcome the Coulomb repulsion between the two positively charged monomers. Dispersion effects are second in importance and appear larger than the favorable orbital interaction obtained by singly occupied molecular orbital (SOMO)-SOMO orbital overlap. This study brings forth important insights into the curious cases of covalent bonding between two π-delocalized radicals.

Unexpected Charge Effects Strengthen π-Stacking Pancake Bonding.

Phenalenyls (PLYs) are important synthons in many functional and electronic materials, which often display favorable molecule-to-molecule overlap for electron or hole transport. They also serve as a prototype for π-stacking pancake bonding based on two-electron multicenter bonding (2e/mc). Unexpected near-doubling of the binding energy is obtained for the positively charged PLY2+ dimer with an effect similar to that seen for the positively charged olympicenyl (OPY) radical dimer. This charge effect is reversed for the perfluorinated (PF) dimers, and the negatively charged perfluorinated (PF) dimers PF–PLY2– and PF-OPY2– become strongly bound. Long-range interactions reflect these differences. Also surprising is that in this case the pancake bonding corresponds to single-electron (1e/mc) or a three-electron (3e/mc) multicenter bonding in contrast to the 2e/mc bonding that occurs for the neutral radical dimers. The strong preference for a large intermolecular overlap is maintained in these charged dimers. Importantly, the preference for π-bonding in the charged dimers compared to σ-bonding is strongly enhanced relative to the neutral PLY dimers.

Energy decomposition analysis based on broken symmetry unrestricted density functional theory.

In this paper, the generalized Kohn-Sham energy decomposition analysis (GKS-EDA) scheme is extended to molecular interactions in open shell singlet states, which is a challenge for many popular EDA methods due to the multireference character. Based on broken symmetry (BS) unrestricted density functional theory with a spin projection approximation, the extension scheme, named GKS-EDA(BS) in this paper, divides the total interaction energy into electrostatic, exchange-repulsion, polarization, correlation, and dispersion terms. Test examples include the pancake bond in the phenalenyl dimer, the ligand interactions in the Fe(ii)-porphyrin complexes, and the radical interactions in dehydrogenated guanine-cytosine base pairs and show that GKS-EDA(BS) is a practical EDA tool for open shell singlet systems.

Stabilization of Pancake Bonding in (TCNQ)2 .- Dimers in the Radical-Anionic Salt (N-CH3-2-NH2-5Cl-Py)(TCNQ)(CH3CN) Solvate and Antiferromagnetism Induction.

We report a new antiferromagnetic radical‐anion salt (RAS) formed from 7,7,8,8‐tetracyanquinonedimethane (TCNQ) anion and 2‐amino‐5‐chloro‐pyridine cation with the composition of (N−CH3−2‐NH2−5Cl−Py)(TCNQ)(CH3CN). The crystallographic data indicates the formation of (TCNQ)2 .− radical‐anion π‐dimers in the synthesized RAS. Unrestricted density functional theory calculations show that the formed π‐dimers characterize with strong π‐stacking “pancake” interactions, resulting in high electronic coupling, enabling efficient charge transfer properties, but π‐dimers cannot be stable in the isolated conditions as a result of strong Coulomb repulsions. In a crystal, where (TCNQ)2 .− π‐dimers bound in the endless chainlets via supramolecular bonds with (N−CH3−2‐NH2−5‐Cl−Py)+ cations, the repulsion forces are screened, allowing for specific parallel π‐stacking interactions and stable radical‐anion dimers formation. Measurements of magnetic susceptibility and magnetization confirm antiferromagnetic properties of RAS, what is in line with the higher stability of ground singlet state of the radical‐anion pair, calculated by means of the DFT. Therefore, the reported radical‐anion (N−CH3−2‐NH2−5Cl−Py)(TCNQ)(CH3CN) solvate has promising applications in novel magnetics with supramolecular structures.

Experimental and Computational Evidence for "Double Pancake Bonds": The Role of Dispersion-Corrected DFT Methods in Strongly Dimerized 5-Aryl-1λ2,3λ2-dithia-2,4,6-triazines.

Crystal structures are reported for bicyclic 3-CF3C6H4CN5S3 and monocyclic 3-CF3C6H4CN3S2, the latter of which is strongly dimerized in a cis-cofacial geometry [3-CF3C6H4CN3S2]2. The title compounds have previously been characterized in solution by NMR, displaying spectra that are consistent with the structure of [3-CF3C6H4CN3S2]2 in the crystal with anti-oriented CF3 substituents. The interannular binding was investigated using density functional theory (DFT) methods. However, the DFT-optimized geometry spreads the aryl rings too far apart (centroid–centroid distances of ≥4.353 Å versus experimental distance of 3.850 Å). Significant improvements are obtained with dispersion-corrected DFT functionals B3LYP-D3, B3LYP-D3BJ, M062X, and APFD using the 6-311+G(2d,p) basis set. However, all of these overbind the aryl rings with centroid–centroid distances of 3.612, 3.570, 3.526, and 3.511 Å, respectively. After selecting B3LYP-D3BJ/6-311+G(2d,p) as the best method, five alternative dimer geometries were tested, and all were found to be binding; however, anti cofacial-4 (matching the structure in the solid state) is the most stable. Computed energies of the remainder are as follows: +7.0 kJ mol–1 (syn-cofacial-5), +26.7 kJ mol–1 (anti-cofacial-64), +27.0 kJ mol–1 (syn-cofacial-150), +102.0 kJ mol–1 (S,S-antarafacial), and +103.7 kJ mol–1 (S,N-antarafacial), where the suffixes are torsional angles around the CN3S2 thiazyl ring centroids. The binding in the four most stable cofacial dimers may be described by “double pancake bonding”.

Magnetic Bistability in Crystalline Organic Radicals: The Interplay of H-bonding, Pancake Bonding, and Electrostatics in 4-(2'-Benzimidazolyl)-1,2,3,5-dithiadiazolyl.

The neutral radical 4-(2'-benzimidazolyl)-1,2,3,5-dithiadiazolyl (HbimDTDA) exhibits a first order phase transition around 270 K without symmetry breaking, preserving its orthorhombic Pbca space group between 340 and 100 K. Associated with this reversible single-crystal-to-single-crystal phase transition, thermal hysteresis of the magnetic susceptibility is observed. The low temperature (LT) phase is diamagnetic owing to pancake bonding between the π-radicals. In the paramagnetic high temperature (HT) phase, the pancake bonds are broken, and new electrostatic contacts are apparent. As a result of the dense 3D network of supramolecular contacts, which includes H-bonds, the HbimDTDA system provides the first example of magnetic bistability for a DTDA radical.

Constructing Stable π-Dimers: Two Parallel Pancake π-π Bonds.

Stable phenalenyl (PLY) radical π-dimers still play an important role for new multifunctional materials owing to their intriguing molecular structure and unusual pancake π-π bonding mode. Herein, we design a new biphenalenyl biradicaloid (1) consisting of two PLYs and one benzene ring linkage tethered by single bonds, which presents an open-shell character. Further, three π-dimers (a1, b1, and c1) combined with 1 and conventional biphenalenyl biradicaloid (a, b, and c), which are capable of forming two staggered PLY dimers, exhibiting a short interlayer distance between the monomers. Interestingly, the analysis of the frontier molecular orbital reveals that two bonding orbitals, namely, the two highest occupied molecular orbitals (HOMO and HOMO-1) are doubly occupied. The results reveal that three π-dimers display two parallel pancake bonds. Moreover, they have small diradical and tetraradical characters, large interaction energies, and strong aromaticity, which indicate the stability of these π-dimers. The present work creates a new class of strong pancake bonding and might be utilized in devising an array of materials.