Spodium Bonds: Noncovalent Interactions Involving Group 12 Elements

The term spodium (Sp) bond is proposed to refer to a net attractive interaction between any element of Group 12 and electron-rich atoms (Lewis bases or anions). These noncovalent interactions are markedly different from coordination bonds (antibonding Sp–ligand orbital involved). Evidence is provided for the existence of this interaction by calculations at the RI-MP2/aug-cc-pVTZ level of theory, atoms-in-molecules, and natural bond orbital analyses and by examining solid-state structures in the Cambridge Structure Database.     

Ab initio Study of the Interactions between CO2 and N‐Containing Organic Heterocycles

In the garden of dispersion: High‐accuracy ab initio calculations are performed to determine the nature of the interactions and the most favorable geometries between CO₂ and heteroaromatic molecules containing nitrogen (see figure). Dispersion forces play a key role in the stabilization of the dimer, because correlation effects represent about 50 % of the total interaction energy.

     

Rational Design for Complementary Donor–Acceptor Recognition Pairs Using Self‐Complementary Hydrogen Bonds

An adaptable and efficient molecular recognition pair has been established by taking advantage of the complementary nature of donor–acceptor interactions together with the strength of hydrogen bonds. Such distinct molecular recognition propagates in orthogonal directions to effect extended alternating co‐assembly of two different appended molecular entities. The dimensions of the assembled structures can be tuned by stoichiometric imbalance between the donor and acceptor building blocks. The morphology of the self‐assembled material can be correlated with the ratio of the two building blocks.     

Temperature‐Dependent Electron Shuffle in Molecular Group 13/15 Intermetallic Complexes

Monovalent RAl (R=HC[C(Me)N(2,6‐iPr₂C₆H₃)]₂) reacts with E₂Et₄ (E=Sb, Bi) with insertion into the weak E? E bond and subsequent formation of RAl(EEt₂)₂ (E=Sb 1 ; Bi 2 ). The analogous reactions of RGa with E₂Et₄ yield a temperature‐dependent equilibrium between RGa(EEt₂)₂ (E=Sb 3 ; Bi 4 ) and the starting reagents. RIn does not interact with Sb₂Et₄ under various reaction conditions, but formation of RIn(BiEt₂)₂ ( 5 ) was observed in the reaction with Bi₂Et₄ at low temperature.     

Complementary Hydrogen Bonding Modulates Electronic Properties and Controls Self‐Assembly of Donor/Acceptor Semiconductors

A comprehensive investigation of the complementary H‐bonding‐mediated self‐assembly between dipyrrolo[2,3‐b:3′,2′‐e]pyridine (P2P) electron donors and naphthalenediimide/perylenediimide (NDI/PDI) acceptors is reported. The synthesis of parent P2P and several aryl‐substituted derivatives is described, along with their optical, redox, and single‐crystal packing characteristics. The dual functionality of heteroatoms in the P2P/NDI(PDI) assembly, which act as proton donors/acceptors and also contribute to π‐conjugation, leads to H‐bonding‐induced perturbation of electronic levels. Concentration‐dependent NMR and UV/Vis spectroscopic studies revealed a cooperative effect of H‐bonding and π–π stacking interactions. This H‐bonding‐mediated co‐assembly of donor (D) and acceptor (A) components leads to a new charge‐transfer (CT) absorption that can be controlled throughout the visible range. The electronic interactions between D and A were further investigated by time‐dependent DFT, which provided insights into the nature of the CT transition. Electropolymerization of difuryl‐P2P afforded the first conjugated polymer incorporating H‐bonding recognition units in its main chain.     

An AAA–DDD Triply Hydrogen‐Bonded Complex Easily Accessible for Supramolecular Polymers

For a complementary hydrogen‐bonded complex, when every hydrogen‐bond acceptor is on one side and every hydrogen‐bond donor is on the other, all secondary interactions are attractive and the complex is highly stable. AAA–DDD (A=acceptor, D=donor) is considered to be the most stable among triply hydrogen‐bonded sequences. The easily synthesized and further derivatized AAA–DDD system is very desirable for hydrogen‐bonded functional materials. In this case, AAA and DDD, starting from 4‐methoxybenzaldehyde, were synthesized with the Hantzsch pyridine synthesis and Friedländer annulation reaction. The association constant determined by fluorescence titration in chloroform at room temperature is 2.09×10⁷ M ^?1. The AAA and DDD components are not coplanar, but form a V shape in the solid state. Supramolecular polymers based on AAA–DDD triply hydrogen bonded have also been developed. This work may make AAA–DDD triply hydrogen‐bonded sequences easily accessible for stimuli‐responsive materials.     

Polymer Binders Constructed through Dynamic Noncovalent Bonds for High-Capacity Silicon-Based Anodes

Silicon (Si) is a promising candidate for high-capacity anode materials owing to its high theoretical capacity (3579 mAh g⁻¹), low working voltage, and wide natural abundance, although its huge volume variation during charge/discharge processes always results in a short cycling life. Polymer binders play a vital role in improving the cycling performance of Si-based anodes, although traditional polyvinylidene difluoride cannot fulfil the requirements owing to its weak van der Waals forces with the Si surface. Recently, polymer binders constructed by dynamic bonds have been developed, which are reported to allow high-energy-density electrodes with improved electrochemical performance. With dynamic bonds including hydrogen bonding, ionic bonding, and host–guest interactions, these polymer binders possess self-healing capabilities and enhanced mechanical performance, achieving a tremendous advance in addressing the capacity fading of Si-based anodes. In this review, we will summarize the research progress of polymer binders constructed with dynamic bonds, and the challenges for their real applications in advanced Li-ion batteries will also be discussed.     

Comparative Study of Phosphine and N‐Heterocyclic Carbene Stabilized Group 13 Adducts [L(EH3)] and [L2(E2Hn)]

Quantum chemical calculations using density functional theory at the BP86/TZ2P level have been carried out to determine the geometries and stabilities of Group 13 adducts [(PMe₃)(EH₃)] and [(PMe₃)₂(E₂H_n)] (E=B–In; n=4, 2, 0). The optimized geometries exhibit, in most cases, similar features to those of related adducts [(NHC^Me)(EH₃)] and [(NHC^Me)₂(E₂H_n)] with a few exceptions that can be explained by the different donor strengths of the ligands. The calculations show that the carbene ligand L=NHC^Me (:C(NMeCH)₂) is a significantly stronger donor than L=PMe₃. The equilibrium geometries of [L(EH₃)] possess, in all cases, a pyramidal structure, whereas the complexes [L₂(E₂H₄)] always have an antiperiplanar arrangement of the ligands L. The phosphine ligands in [(PMe₃)₂(B₂H₂)], which has C_s symmetry, are in the same plane as the B₂H₂ moiety, whereas the heavier homologues [(PMe₃)₂(E₂H₂)] (E=Al, Ga, In) have C_i symmetry in which the ligands bind side‐on to the E₂H₂ acceptor. This is in contrast to the [(NHC^Me)₂(E₂H₂)] adducts for which the NHC^Me donor always binds in the same plane as E₂H₂ except for the indium complex [(NHC^Me)₂(In₂H₂)], which exhibits side‐on bonding. The boron complexes [L₂(B₂)] (L=PMe₃ and NHC^Me) possess a linear arrangement of the LBBL moiety, which has a B?B triple bond. The heavier homologues [L₂(E₂)] have antiperiplanar arrangements of the LEEL moieties, except for [(PMe₃)₂(In₂)], which has a twisted structure in which the PInInP torsion angle is 123.0°. The structural features of the complexes [L(EH₃)] and [L₂(E₂H_n)] can be explained in terms of donor–acceptor interactions between the donors L and the acceptors EH₃ and E₂H_n, which have been analyzed quantitatively by using the energy decomposition analysis (EDA) method. The calculations predict that the hydrogenation reaction of the dimeric magnesium(I) compound L′MgMgL′ with the complexes [L(EH₃)] is energetically more favorable for L=PMe₃ than for NHC^Me.     

Nature of Noncovalent Carbon‐Bonding Interactions Derived from Experimental Charge‐Density Analysis

In an effort to better understand the nature of noncovalent carbon‐bonding interactions, we undertook accurate high‐resolution X‐ray diffraction analysis of single crystals of 1,1,2,2‐tetracyanocyclopropane. We selected this compound to study the fundamental characteristics of carbon‐bonding interactions, because it provides accessible σ holes. The study required extremely accurate experimental diffraction data, because the interaction of interest is weak. The electron‐density distribution around the carbon nuclei, as shown by the experimental maps of the electrophilic bowl defined by a (CN)₂C?C(CN)₂ unit, was assigned as the origin of the interaction. This fact was also evidenced by plotting the Δ²ρ(r) distribution. Taken together, the obtained results clearly indicate that noncovalent carbon bonding can be explained as an interaction between confronted oppositely polarized regions. The interaction is, thus electrophilic–nucleophilic (electrostatic) in nature and unambiguously considered as attractive.     

Supramolecular Hierarchy among Halogen‐Bond Donors

Through a combination of structural chemistry, vibrational spectroscopy, and theory, we have systematically examined the relative structure‐directing importance of a series of ditopic halogen‐bond (XB) donors. The molecular electrostatic potential surfaces of six XB donors were evaluated, which allowed for a charge‐based ranking. Each molecule was then co‐crystallized with 21 XB acceptors and the results have made it possible to map out the supramolecular landscape describing the competition between I/Br–ethynyl donors, perfluorinated I/Br donors, and I/Br–phenyl based donors. The results offer practical guidelines for synthetic crystal engineering driven by robust and directional halogen bonds.     

Coordinative Interactions between Porphyrins and C60, La@C82, and La2@C80

For the first time, a C₆₀ derivative ( 1 ) and two different lanthanum metallofullerene derivatives, La@C₈₂Py ( 2 ) and La₂@C₈₀Py ( 3 ), that feature a pyridyl group as a coordination site for transition‐metal ions have been synthesized and integrated as electron acceptors into coordinative electron‐donor/electron‐acceptor hybrids. Zinc tetraphenylporphyrin ( ZnP ) served as an excited‐state electron donor in this respect. Our investigations, by means of steady‐state and time‐resolved photophysical techniques found that electron transfer governs the excited‐state deactivation in all of these systems, namely 1/ZnP , 2/ZnP , and 3/ZnP , whereas, in the ground state, notable electronic interactions are lacking. Variation of the electron‐accepting fullerene or metallofullerene moieties provides the incentive for fine‐tuning the binding constants, the charge‐separation kinetics, and the charge‐recombination kinetics. To this end, the binding constants, which ranged from log K_assoc=3.94–4.38, are dominated by axial coordination, with minor contributions from the orbital overlap of the curved and planar π systems. The charge‐separation and charge‐recombination kinetics, which are in the order of 10¹⁰ and 10⁸ s^?1, relate to the reduction potential of the fullerene and metallofullerenes, respectively.     

Spectroscopic and Ab Initio Investigation of C−H⋅⋅⋅N Hydrogen‐Bonded Complexes of Fluorophenylacetylenes: Frequency Shifts and Correlations

The C?H???N hydrogen‐bonded complexes of several fluorophenyacetylenes with ammonia and methylamine were characterized by a redshift in the acetylenic C?H stretching vibration of the phenylacetylene moiety. These redshifts were linearly correlated with the stabilization energies calculated at the CCSD(T)/CBS//MP2‐aug‐cc‐pVDZ level. Analysis of various components of the interaction energy indicated that the observed redshifts were weakly correlated with the electrostatic component. The weaker linear correlation between the frequency shifts and the electrostatic component between two data sets can perhaps be attributed to the marginal differences in the Stark tuning rate and zero‐field shifts. The induction and exchange‐repulsion components were linearly correlated. However, the dispersion component depends on the nature of the hydrogen‐bond acceptor and shows a quantum jump when the hydrogen‐bond acceptor is changed from ammonia to methylamine. The observed linear correlation between the redshifts in the C?H stretching frequencies and the total stabilization energies is due to mutual cancellation of deviations from linearity between various components.     

Aerogen Bonding Interaction: A New Supramolecular Force?

We report evidence of the favorable noncovalent interaction between a covalently bonded atom of Group 18 (known as noble gases or aerogens) and a negative site, for example, a lone pair of a Lewis base or an anion. It involves a region of positive electrostatic potential (σ‐hole), therefore it is a totally new and unexplored σ‐hole‐based interaction, namely aerogen bonding. We demonstrate for the first time the existence of σ‐hole regions in aerogen derivatives by means of high‐level ab initio calculations. In addition, several crystal structures retrieved from the Cambridge Structural Database (CSD) give reliability to the calculations. Energetically, aerogen bonds are comparable to hydrogen bonds and other σ‐hole‐based interactions but less directional. They are expected to be important in xenon chemistry.     

Conceptual Quantum Chemical Analysis of Bonding and Noncovalent Interactions in the Formation of Frustrated Lewis Pairs

The contributions of covalent and noncovalent interactions to the formation of classical adducts of bulky Lewis acids and bases and frustrated Lewis pairs (FLPs) were scrutinized by using various conceptual quantum chemical techniques. Significantly negative complexation energies were calculated for fourteen investigated Lewis pairs containing bases and acids with substituents of various sizes. A Ziegler–Rauk‐type energy decomposition analysis confirmed that two types of Lewis pairs can be distinguished on the basis of the nature of the primary interactions between reactants; dative‐bond formation and concomitant charge transfer from the Lewis base to the acid is the dominant and most stabilizing factor in the formation of Lewis acid–base adducts, whereas weak interactions are the main thermodynamic driving force (>50 %) for FLPs. Moreover, the ease and extent of structural deformation of the monomers appears to be a key component in the formation of the former type of Lewis pairs. A Natural Orbital for Chemical Valence (NOCV) analysis, which was used to visualize and quantify the charge transfer between the base and the acid, clearly showed the importance and lack of this type of interaction for adducts and FLPs, respectively. The Noncovalent Interaction (NCI) method revealed several kinds of weak interactions between the acid and base components, such as dispersion, π–π stacking, C?H ??? π interaction, weak hydrogen bonding, halogen bonding, and weak acid–base interactions, whereas the Quantum Theory of Atoms in Molecules (QTAIM) provided further conceptual insight into strong acid–base interactions.     

Linear Chains of Magnetic Ions Stacked with Variable Distance: Ferromagnetic Ordering with a Curie Temperature above 20 K

We have studied the magnetic properties of the SURMOF‐2 series of metal–organic frameworks (MOFs). Contrary to bulk MOF‐2 crystals, where Cu²⁺ ions form paddlewheels and are antiferromagnetically coupled, in this case the Cu²⁺ ions are connected via carboxylate groups in a zipper‐like fashion. This unusual coupling of the spin ions within the resulting one‐dimensional chains is found to stabilize a low‐temperature, ferromagnetic (FM) phase. In contrast to other ordered 1D systems, no strong magnetic fields are needed to induce the ferromagnetism. The magnetic coupling constants describing the interaction between the individual metal ions have been determined in SQUID experiments. They are fully consistent with the results of ab initio DFT electronic structure calculations. The theoretical results allow the unusual magnetic behavior of this exotic, yet easy‐to‐fabricate, material to be described in a detailed fashion.     

A Charge‐Transfer‐Induced Self‐Healing Supramolecular Hydrogel

In this study, a dual‐component charge‐transfer (CT)‐induced supramolecular hydrogel was fabricated using pyrene‐tailored pyridinium (PYP) and 2,4,7‐trinitrofluorenone (TNF) as the electron donor and acceptor, respectively. Its thermal stability and mechanical property have been modulated effectively by altering the concentration or molar ratio of PYP and TNF. Moreover, this CT hydrogel exhibited a distinct injectable self‐healing property that could be utilized to create desired patterns on substrates. Such property holds potential for this CT hydrogel in fields like three‐dimensional printing and surface coating.