Nitriles with High Gas-Phase Basicity—Part II Transmission of the Push–Pull Effect through Methylenecyclopropene and Cyclopropenimine Scaffolds Intercalated between Different Electron Donor(s) and the Cyano N-Protonation Site

This work extends our earlier quantum chemical studies on the gas-phase basicity of very strong N-bases to two series of nitriles containing the methylenecyclopropene and cyclopropenimine scaffolds with dissymmetrical substitution by one or two electron-donating substituents such as Me, NR₂, N=C (NR₂)₂, and N=P (NR₂)₃, the last three being strong donors. For a proper prediction of their gas-phase base properties, all potential isomeric phenomena and reasonable potential protonation sites are considered to avoid possible inconsistencies when evaluating the energetic parameters and associated protonation or deprotonation equilibria B + H⁺ = BH⁺. More than 250 new isomeric structures for neutral and protonated forms are analyzed. The stable structures are selected and the favored ones identified. The microscopic (kinetic) gas-phase basicity parameters (PA and GB) corresponding to N sites (cyano and imino in the cyclopropenimine or in the substituents) in each isomer are calculated. The macroscopic (thermodynamic) PAs and GBs, referring to the isomeric mixtures of favored isomers, are also estimated. The total (pushing) substituent effects are analyzed for monosubstituted and disubstituted derivatives containing two identical or two different substituents. Electron delocalization is examined in the two π–π conjugated transmitters, the methylenecyclopropene and cyclopropenimine scaffolds. The aromatic character of the three-membered ring is also discussed.     

Molecular control over vitrimer-like mechanics – tuneable dynamic motifs based on the Hammett equation in polyimine materials

In this work, we demonstrate that fine-grained, quantitative control over macroscopic dynamic material properties can be achieved using the Hammett equation in tuneable dynamic covalent polyimine materials. Via this established physical-organic principle, operating on the molecular level, one can fine-tune and control the dynamic material properties on the macroscopic level, by systematic variation of dynamic covalent bond dynamics through selection of the appropriate substituent of the aromatic imine building blocks. Five tuneable, crosslinked polyimine network materials, derived from dianiline monomers with varying Hammett parameter (σ) were studied by rheology, revealing a distinct correlation between the σ value and a range of corresponding dynamic material properties. Firstly, the linear correlation of the kinetic activation energy (E_a) for the imine exchange to the σ value, enabled us to tune the E_a from 16 to 85 kJ mol⁻¹. Furthermore, the creep behaviour (γ), glass transition (T_g) and the topology freezing transition temperature (T_v), all showed a strong, often linear, dependence on the σ value of the dianiline monomer. These combined results demonstrate for the first time how dynamic material properties can be directly tuned and designed in a quantitative – and therefore predictable – manner through correlations based on the Hammett equation. Moreover, the polyimine materials were found to be strong elastic rubbers (G′ > 1 MPa at room temperature) that were stable up to 300 °C, as confirmed by TGA. Lastly, the dynamic nature of the imine bond enabled not only recycling, but also intrinsic self-healing of the materials over multiple cycles without the need for solvent, catalysts or addition of external chemicals.

Controlling macroscopic material properties of dynamic covalent polyimines via the electronic effect of dianiline monomers based on the Hammett equation.     

Insight into Spodium–π Bonding Characteristics of the MX2⋯π (M = Zn,Cd and Hg; X = Cl,Br and I) Complexes—A Theoretical Study

The spodium–π bonding between MX₂ (M = Zn, Cd, and Hg; X = Cl, Br, and I) acting as a Lewis acid, and C₂H₂/C₂H₄ acting as a Lewis base was studied by ab initio calculations. Two types of structures of cross (T) and parallel (P) forms are obtained. For the T form, the X–M–X axis adopts a cross configuration with the molecular axis of C≡C or C=C, but both of them are parallel in the P form. NCI, AIM, and electron density shifts analyses further, indicating that the spodium–π bonding exists in the binary complexes. Spodium–π bonding exhibits a partially covalent nature characterized with a negative energy density and large interaction energy. With the increase of electronegativity of the substituents on the Lewis acid or its decrease in the Lewis base, the interaction energies increase and vice versa. The spodium–π interaction is dominated by electrostatic interaction in most complexes, whereas dispersion and electrostatic energies are responsible for the stability of the MX₂⋯C₂F₂ complexes. The spodium–π bonding further complements the concept of the spodium bond and provides a wider range of research on the adjustment of the strength of spodium bond.     

Cooperative B–H bond activation: dual site borane activation by redox active κ2-N,S-chelated complexes

Cooperative dual site activation of boranes by redox-active 1,3-N,S-chelated ruthenium species, mer-[PR₃{κ²-N,S-(L)}₂Ru{κ¹-S-(L)}], (mer-2a: R = Cy, mer-2b: R = Ph; L = NC₇H₄S₂), generated from the aerial oxidation of borate complexes, [PR₃{κ²-N,S-(L)}Ru{κ³-H,S,S′-BH₂(L)₂}] (trans–mer-1a: R = Cy, trans–mer-1b: R = Ph; L = NC₇H₄S₂), has been investigated. Utilizing the rich electronic behaviour of these 1,3-N,S-chelated ruthenium species, we have established that a combination of redox-active ligands and metal–ligand cooperativity has a big influence on the multisite borane activation. For example, treatment of mer-2a–b with BH₃·THF led to the isolation of fac-[PR₃Ru{κ³-H,S,S′-(NH₂BSBH₂N)(S₂C₇H₄)₂}] (fac-3a: R = Cy and fac-3b: R = Ph) that captured boranes at both sites of the κ²-N,S-chelated ruthenacycles. The core structure of fac-3a and fac-3b consists of two five-membered ruthenacycles [RuBNCS] which are fused by one butterfly moiety [RuB₂S]. Analogous fac-3c, [PPh₃Ru{κ³-H,S,S′-(NH₂BSBH₂N)(SC₅H₄)₂}], can also be synthesized from the reaction of BH₃·THF with [PPh₃{κ²-N,S-(SNC₅H₄)}{κ³-H,S,S′-BH₂(SNH₄C₅)₂}Ru], cis–fac-1c. In stark contrast, when mer-2b was treated with BH₂Mes (Mes = 2,4,6-trimethyl phenyl) it led to the formation of trans- and cis-bis(dihydroborate) complexes [{κ³-S,H,H-(NH₂BMes)Ru(S₂C₇H₄)}₂], (trans-4 and cis-4). Both the complexes have two five-membered [Ru–(H)₂–B–NCS] ruthenacycles with κ²-H–H coordination modes. Density functional theory (DFT) calculations suggest that the activation of boranes across the dual Ru–N site is more facile than the Ru–S one.

Redox-active ruthenium complexes supported by hemilabile κ²-N,S-chelated ruthenacycles undergo unusual dual site B–H bond activation through metal–ligand cooperation with free and bulky boranes.     

Can acyclic conformational control be achieved via a sulfur–fluorine gauche effect?

The gauche conformation of the 1,2-difluoroethane motif is known to involve stabilising hyperconjugative interactions between donor (bonding, σ_C–H) and acceptor (antibonding, σ*C–F) orbitals. This model rationalises the generic conformational preference of F–C_β–C_α–X systems (φ_FCCX ≈ 60°), where X is an electron deficient substituent containing a Period 2 atom. Little is known about the corresponding Period 3 systems, such as sulfur and phosphorus, where multiple oxidation states are possible. Conformational analyses of β-fluorosulfides, -sulfoxides and -sulfones are disclosed here, thus extending the scope of the fluorine gauche effect to the 3rd Period (F–C–C–S(O)_n; φ_FCCS ≈ 60°). Synergy between experiment and computation has revealed that the gauche effect is only pronounced in structures bearing an electropositive vicinal sulfur atom (S⁺–O^–, SO₂).     

Facile Synthesis of 5-Aryl-N-(pyrazin-2-yl)thiophene-2-carboxamides via Suzuki Cross-Coupling Reactions,Their Electronic and Nonlinear Optical Properties through DFT Calculations

Synthesis of 5-aryl-N-(pyrazin-2-yl)thiophene-2-carboxamides (4a–4n) by a Suzuki cross-coupling reaction of 5-bromo-N-(pyrazin-2-yl)thiophene-2-carboxamide (3) with various aryl/heteroaryl boronic acids/pinacol esters was observed in this article. The intermediate compound 3 was prepared by condensation of pyrazin-2-amine (1) with 5-bromothiophene-2-carboxylic acid (2) mediated by TiCl₄. The target pyrazine analogs (4a–4n) were confirmed by NMR and mass spectrometry. In DFT calculation of target molecules, several reactivity parameters like FMOs (E_HOMO, E_LUMO), HOMO–LUMO energy gap, electron affinity (A), ionization energy (I), electrophilicity index (ω), chemical softness (σ) and chemical hardness (η) were considered and discussed. Effect of various substituents was observed on values of the HOMO–LUMO energy gap and hyperpolarizability. The p-electronic delocalization extended over pyrazine, benzene and thiophene was examined in studying the NLO behavior. The chemical shifts of ¹H NMR of all the synthesized compounds 4a–4n were calculated and compared with the experimental values.     

Polycyclic aromatic hydrocarbon-substituted push–pull chromophores: an investigation of optoelectronic and nonlinear optical properties using experimental and theoretical approaches

A series of new push–pull chromophores were synthesized in moderate to very high yields (65%–97%) by treating TCNE and TCNQ with alkynes substituted by electron-rich diethylaniline and polycyclic aromatic hydrocarbons. Some of the chromophores exhibit strong intramolecular charge-transfer bands in the near-IR region with λ_max values between 695 and 749 nm. With the help of experimental and theoretical analysis, it is concluded that the trend in λ _max values is affected by PAH substituents sterically, not electronically. Steric constraints led to the increased dihedral angles, reducing conjugation efficiencies. The absorption properties of push-pull compounds have been investigated in solvents possessing different polarities. All chromophores exhibited positive solvatochromism. As an additional proof of efficient charge-transfer in push–pull chromophores, quinoid character (dr) values were predicted using calculated bond lengths. Remarkably, substantial dr values (0.045–0.049) were predicted for donor diethylaniline rings in all compounds. The effects of various polycyclic aromatic hydrocarbons on optical and nonlinear optical properties were also studied by computational methods. Several parameters, such as band gaps, Mulliken electronegativity, chemical hardness and softness, dipole moments, average polarizability, first hyperpolarizability, were predicted for chromophores at the B3LYP/6-31++G(d,p) level of theory. The predicted first hyperpolarizability β_(tot) values vary between 198 to 538 × 10^–30 esu for the reported push–pull chromophores in this study. The highest predicted β_(tot) value in this study is 537.842 × 10^–30 esu, 8150 times larger than the predicted β_(tot) value of benchmark NLO material urea, suggests possible utilization of these chromophores in NLO devices. The charge-transfer character of the synthesized structures was further confirmed by HOMO-LUMO depictions and electrostatic potential maps.     

Molecular insights into chirality transfer from double axially chiral phosphoric acid in a synergistic enantioselective intramolecular amination

In the most general practice of asymmetric catalysis, a chiral catalyst, typically bearing a center or an axis of chirality, is employed as the chiral source for imparting enantiocontrol over the developing product. Given the current interest toward optically pure compounds, various forms of chiral induction enabled by diverse chiral sources as well as the use of multiple catalysts under one-pot conditions have been in focus. In one such promising development, an achiral N-sulfonamide protected 1,6-amino allyl alcohol (NaphSO₂NHCH₂C(Ph)₂CH₂CH CHCH₂OH) was subjected to Tsuji–Trost activation and an intramolecular amination to form important chiral pyrrolidine frameworks. A dual catalytic system comprising Pd(PPh₃)₄ and DAPCy (β-cyclohexyl substituted double axially chiral phosphoric acid derived from two homocoupled BINOL backbones with a dynamic central chiral axis) under mild conditions was reported to offer quantitative conversion with an ee of 95%. Here, we provide molecular insights into the origin of chiral induction by DAPCy, as obtained through a comprehensive density functional theory (SMD_(toluene)/B3LYP-D3/6-31G**,Pd(SDD)) investigation. Two key steps in the mechanism are identified to involve a cooperative mode of activation of the Pd-bound allyl alcohol in the form of a Pd-π-allyl moiety at one end of the substrate, followed by an intramolecular nucleophilic addition of N-sulfonamide from the other end to yield a pyrrolidine derivative bearing an α-vinyl stereogenic center. (S,R,S)-DAPCy is found to steer the dehydroxylation to yield a Pd-π-allyl intermediate with a suitably poised si prochiral face for the nucleophilic addition. In the enantiocontrolled (as well as the turn-over determining step) nucleophilic addition, the chiral catalyst is identified to serve as a chiral phosphate counterion. The chiral induction is facilitated by a series of N–H⋯O, C–H⋯O, C–H⋯π, lone pair (lp)⋯π, O–H⋯O, O–H⋯π, and π⋯π noncovalent interactions, which is noted as more effective in the lower energy C–N bond formation transition state through the si prochiral face of the Pd-π-allyl moiety. These insights into the novel dynamic axially double chiral catalyst could be valuable toward exploiting such modes of stereoinduction.

The origin of enantiocontrol in an intramolecular amination involving Pd(PPh₃)₄ and a double axially chiral phosphoric acid (DAPCy) dual catalytic system is traced to a more effective series of noncovalent interactions in the lower energy C–N bond formation transition state.     

Synthesis and Photophysical Properties of a Series of Dimeric Indium Quinolinates

A novel class of quinolinol-based dimeric indium complexes (1–6) was synthesized and characterized using ¹H and ¹³C(¹H) NMR spectroscopy and elemental analysis. Compounds 1–6 exhibited typical low-energy absorption bands assignable to quinolinol-centered π–π* charge transfer (CT) transition. The emission spectra of 1–6 exhibited slight bathochromic shifts with increasing solvent polarity (p-xylene < tetrahydrofuran (THF) < dichloromethane (DCM)). The emission bands also showed a gradual redshift, with an increase in the electron-donating effect of substituents at the C5 position of the quinoline groups. The absolute emission quantum yields (Φ_PL) of compounds 1 (11.2% in THF and 17.2% in film) and 4 (17.8% in THF and 36.2% in film) with methyl substituents at the C5 position of the quinoline moieties were higher than those of the indium complexes with other substituents.     

Aromatic Rings as Molecular Determinants for the Molecular Recognition of Protein Kinase Inhibitors

Protein kinases are key enzymes in many signal transduction pathways, and play a crucial role in cellular proliferation, differentiation, and various cell regulatory processes. However, aberrant function of kinases has been associated with cancers and many other diseases. Consequently, competitive inhibition of the ATP binding site of protein kinases has emerged as an effective means of curing these diseases. Over the past three decades, thousands of protein kinase inhibitors (PKIs) with varying molecular frames have been developed. Large-scale data mining of the Protein Data Bank resulted in a database of 2139 non-redundant high-resolution X-ray crystal structures of PKIs bound to protein kinases. This provided us with a unique opportunity to study molecular determinants for the molecular recognition of PKIs. A chemoinformatic analysis of 2139 PKIs resulted in findings that PKIs are “flat” molecules with high aromatic ring counts and low fractions of sp³ carbon. All but one PKI possessed one or more aromatic rings. More importantly, it was found that the average weighted hydrogen bond count is inversely proportional to the number of aromatic rings. Based on this linear relationship, we put forward the exchange rule of hydrogen bonding interactions and non-bonded π-interactions. Specifically, a loss of binding affinity caused by a decrease in hydrogen bonding interactions is compensated by a gain in binding affinity acquired by an increase in aromatic ring-originated non-bonded interactions (i.e., π–π stacking interactions, CH–π interactions, cation–π interactions, etc.), and vice versa. The very existence of this inverse relationship strongly suggests that both hydrogen bonding and aromatic ring-originated non-bonded interactions are responsible for the molecular recognition of PKIs. As an illustration, two representative PKI–kinase complexes were employed to examine the relative importance of different modes of non-bonded interactions for the molecular recognition of PKIs. For this purpose, two FDA-approved PKI drugs, ibrutinib and lenvatinib, were chosen. The binding pockets of both PKIs were thoroughly examined to identify all non-bonded intermolecular interactions. Subsequently, the strengths of interaction energies between ibrutinib and its interacting residues in tyrosine kinase BTK were quantified by means of the double hybrid DFT method B2PLYP. The resulting energetics for the binding of ibrutinib in tyrosine kinase BTK showed that CH–π interactions and π–π stacking interactions between aromatic rings of the drug and hydrophobic residues in its binding pocket dominate the binding interactions. Thus, this work establishes that, in addition to hydrogen bonding, aromatic rings function as important molecular determinants for the molecular recognition of PKIs. In conclusion, our findings support the following pharmacophore model for ATP-competitive kinase inhibitors: a small molecule features a scaffold of one or more aromatic rings which is linked with one or more hydrophilic functional groups. The former has the structural role of acting as a scaffold and the functional role of participating in aromatic ring-originated non-bonded interactions with multiple hydrophobic regions in the ATP binding pocket of kinases. The latter ensure water solubility and form hydrogen bonds with the hinge region and other hydrophilic residues of the ATP binding pocket.     

Highly Oxygenated Triterpenoids and Diterpenoids from Fructus Rubi (Rubus chingii Hu) and Their NF-kappa B Inhibitory Effects

During a phytochemical investigation of the unripe fruits of Rubus chingii Hu (i.e., Fructus Rubi, a traditional Chinese medicine named “Fu-Pen-Zi”), a number of highly oxygenated terpenoids were isolated and characterized. These included nine ursane-type (1, 2, and 4–10), five oleanane-type (3, 11–14), and six cucurbitane-type (15–20) triterpenoids, together with five ent-kaurane-type diterpenoids (21–25). Among them, (4R,5R,8R,9R,10R,14S,17S,18S,19R,20R)-2,19α,23-trihydroxy-3-oxo-urs-1,12-dien-28-oic acid (rubusacid A, 1), (2R*,4S*,5R*,8R*,9R*,10R*,14S*,17S*, 18S*,19R*,20R*)-2α,19α,24-trihydroxy-3-oxo-urs-12-en-28-oic acid (rubusacid B, 2), (5R,8R,9R,10R, 14S,17R,18S,19S)-2,19α-dihydroxy-olean-1,12-dien-28-oic acid (rubusacid C, 3), and (3S,5S,8S,9R, 10S,13R,16R)-3α,16α,17-trihydroxy-ent-kaur-2-one (rubusone, 21) were previously undescribed. Their chemical structures and absolute configurations were elucidated on the basis of spectroscopic data and electronic circular dichroism (ECD) analyses. Compounds 1 and 3 are rare naturally occurring pentacyclic triterpenoids featuring a special α,β-unsaturated keto-enol (diosphenol) unit in ring A. Cucurbitacin B (15), cucurbitacin D (16), and 3α,16α,20(R),25-tetrahydroxy-cucurbita-5,23- dien-2,11,22-trione (17) were found to have remarkable inhibitory effects against NF-κB, with IC₅₀ values of 0.08, 0.61, and 1.60 μM, respectively.     

Picking Out Logic Operations in a Naphthalene β-Diketone Derivative by Using Molecular Encapsulation,Controlled Protonation,and DNA Binding

On–off switching and molecular logic in fluorescent molecules are associated with what chemical inputs can do to the structure and dynamics of these molecules. Herein, we report the structure of a naphthalene derivative, the fashion of its binding to β-cyclodextrin and DNA, and the operation of logic possible using protons, cyclodextrin, and DNA as chemical inputs. The compound crystallizes out in a keto-amine form, with intramolecular N−H⋅⋅⋅O bonding. It shows stepwise formation of 1:1 and 1:2 inclusion complexes with β-cyclodextrin. The aminopentenone substituents are encapsulated by β-cyclodextrin, leaving out the naphthalene rings free. The binding constant of the β-cyclodextrin complex is 512 m⁻¹. The pK_a value of the guest molecule is not greatly affected by the complexation. Dual input logic operations, based on various chemical inputs, lead to the possibility of several molecular logic gates, namely NOR, XOR, NAND, and Buffer. Such chemical inputs on the naphthalene derivative are examples of how variable signal outputs based on binding can be derived, which, in turn, are dependent on the size and shape of the molecule.     

Novel Sterically Crowded and Conformationally Constrained α-Aminophosphonates with a Near-Neutral pKa as Highly Accurate 31P NMR pH Probes. Application to Subtle pH Gradients Determination in Dictyostelium discoideum Cells

In order to discover new ³¹P NMR markers for probing subtle pH changes (<0.2 pH unit) in biological environments, fifteen new conformationally constrained or sterically hindered α-aminophosphonates derived from diethyl(2-methylpyrrolidin-2-yl)phosphonate were synthesized and tested for their pH reporting and cytotoxic properties in vitro. All compounds showed near-neutral pK_as (ranging 6.28–6.97), chemical shifts not overlapping those of phosphorus metabolites, and spectroscopic sensitivities (i.e., chemical shifts variation Δδ_ab between the acidic and basic forms) ranging from 9.2–10.7 ppm, being fourfold larger than conventional endogenous markers such as inorganic phosphate. X-ray crystallographic studies combined with predictive empirical relationships and ab initio calculations addressed the inductive and stereochemical effects of substituents linked to the protonated amine function. Satisfactory correlations were established between pK_as and both the 2D structure and pyramidalization at phosphorus, showing that steric crowding around the phosphorus is crucial for modulating Δδ_ab. Finally, the hit ³¹P NMR pH probe 1b bearing an unsubstituted 1,3,2-dioxaphosphorinane ring, which is moderately lipophilic, nontoxic on A549 and NHLF cells, and showing pK_a = 6.45 with Δδ_ab = 10.64 ppm, allowed the first clear-cut evidence of trans-sarcolemmal pH gradients in normoxic Dictyostelium discoideum cells with an accuracy of <0.05 pH units.     

Ir6In32S21, a polar,metal-rich semiconducting subchalcogenide

Subchalcogenides are uncommon, and their chemical bonding results from an interplay between metal–metal and metal–chalcogenide interactions. Herein, we present Ir₆In₃₂S₂₁, a novel semiconducting subchalcogenide compound that crystallizes in a new structure type in the polar P31m space group, with unit cell parameters a = 13.9378(12) Å, c = 8.2316(8) Å, α = β = 90°, γ = 120°. The compound has a large band gap of 1.48(2) eV, and photoemission and Kelvin probe measurements corroborate this semiconducting behavior with a valence band maximum (VBM) of −4.95(5) eV, conduction band minimum of −3.47(5) eV, and a photoresponse shift of the Fermi level by ∼0.2 eV in the presence of white light. X-ray absorption spectroscopy shows absorption edges for In and Ir do not indicate clear oxidation states, suggesting that the numerous coordination environments of Ir₆In₃₂S₂₁ make such assignments ambiguous. Electronic structure calculations confirm the semiconducting character with a nearly direct band gap, and electron localization function (ELF) analysis suggests that the origin of the gap is the result of electron transfer from the In atoms to the S 3p and Ir 5d orbitals. DFT calculations indicate that the average hole effective masses near the VBM (1.19m_e) are substantially smaller than the average electron masses near the CBM (2.51m_e), an unusual feature for most semiconductors. The crystal and electronic structure of Ir₆In₃₂S₂₁, along with spectroscopic data, suggest that it is neither a true intermetallic nor a classical semiconductor, but somewhere in between those two extremes.

Subchalcogenides are uncommon, and their chemical bonding results from an interplay between metal–metal and metal–chalcogenide interactions.     

Donor–acceptor–acceptor-type near-infrared fluorophores that contain dithienophosphole oxide and boryl groups: effect of the boryl group on the nonradiative decay

The use of donor–π–acceptor (D–π–A) skeletons is an effective strategy for the design of fluorophores with red-shifted emission. In particular, the use of amino and boryl moieties as the electron-donating and -accepting groups, respectively, can produce dyes that exhibit high fluorescence and solvatochromism. Herein, we introduce a dithienophosphole P-oxide scaffold as an acceptor–spacer to produce a boryl- and amino-substituted donor–acceptor–acceptor (D–A–A) π-system. The thus obtained fluorophores exhibit emission in the near-infrared (NIR) region, while maintaining high fluorescence quantum yields even in polar solvents (e.g. λ_em = 704 nm and Φ_F = 0.69 in CH₃CN). A comparison of these compounds with their formyl- or cyano-substituted counterparts demonstrated the importance of the boryl group for generating intense emission. The differences among these electron-accepting substituents were examined in detail using theoretical calculations, which revealed the crucial role of the boryl group in lowering the nonradiative decay rate constant by decreasing the non-adiabatic coupling in the internal conversion process. The D–A–A framework was further fine-tuned to improve the photostability. One of these D–A–A dyes was successfully used in bioimaging to visualize the blood vessels of Japanese medaka larvae and mouse brain.

Combination of electron-accepting diarylboryl terminal groups and dithienophosphole oxide spacers with electron-donating triarylamine moieties produces donor–acceptor–acceptor type π-systems, which exhibit emissions in the near-infrared region.     

Control of dominant conduction orbitals by peripheral substituents in paddle-wheel diruthenium alkynyl molecular junctions

Control of charge carriers that transport through the molecular junctions is essential for thermoelectric materials. In general, the charge carrier depends on the dominant conduction orbitals and is dominantly determined by the terminal anchor groups. The present study discloses the synthesis, physical properties in solution, and single-molecule conductance of paddle-wheel diruthenium complexes 1R having diarylformamidinato supporting ligands (DArF: p-R-C₆H₄-NCHN-C₆H₄-R-p) and two axial thioanisylethynyl conducting anchor groups, revealing unique substituent effects with respect to the conduction orbitals. The complexes 1R with a few different aryl substituents (R = OMe, H, Cl, and CF₃) were fully characterized by spectroscopic and crystallographic analyses. The single-molecule conductance determined by the scanning tunneling microscope break junction (STM-BJ) technique was in the 10⁻⁵ to 10⁻⁴G₀ region, and the order of conductance was 1OMe > 1CF3 ≫ 1H ∼ 1Cl, which was not consistent with the Hammett substituent constants σ of R. Cyclic voltammetry revealed the narrow HOMO–LUMO gaps of 1R originating from the diruthenium motif, as further supported by the DFT study. The DFT-NEGF analysis of this unique result revealed that the dominant conductance routes changed from HOMO conductance (for 1OMe) to LUMO conductance (for 1CF3). The drastic change in the conductance properties originates from the intrinsic narrow HOMO–LUMO gaps.

Dominant conduction orbitals of paddle-wheel organodiruthenium complexes can be facilely controlled by the substituents embedded in the amidinato ligands.     

Synthesis of unstrained Criegee intermediates: inverse α-effect and other protective stereoelectronic forces can stop Baeyer–Villiger rearrangement of γ-hydroperoxy-γ-peroxylactones

How far can we push the limits in removing stereoelectronic protection from an unstable intermediate? We address this question by exploring the interplay between the primary and secondary stereoelectronic effects in the Baeyer–Villiger (BV) rearrangement by experimental and computational studies of γ-OR-substituted γ-peroxylactones, the previously elusive non-strained Criegee intermediates (CI). These new cyclic peroxides were synthesized by the peroxidation of γ-ketoesters followed by in situ cyclization using a BF₃·Et₂O/H₂O₂ system. Although the primary effect (alignment of the migrating C–R_m bond with the breaking O–O bond) is active in the 6-membered ring, weakening of the secondary effect (donation from the OR lone pair to the breaking C–R_m bond) provides sufficient kinetic stabilization to allow the formation and isolation of stable γ-hydroperoxy-γ-peroxylactones with a methyl-substituent in the C6-position. Furthermore, supplementary protection is also provided by reactant stabilization originating from two new stereoelectronic factors, both identified and quantified for the first time in the present work. First, an unexpected boat preference in the γ-hydroperoxy-γ-peroxylactones weakens the primary stereoelectronic effects and introduces a ∼2 kcal mol⁻¹ Curtin–Hammett penalty for reacquiring the more reactive chair conformation. Second, activation of the secondary stereoelectronic effect in the TS comes with a ∼2–3 kcal mol⁻¹ penalty for giving up the exo-anomeric stabilization in the 6-membered Criegee intermediate. Together, the three new stereoelectronic factors (inverse α-effect, misalignment of reacting bonds in the boat conformation, and the exo-anomeric effect) illustrate the richness of stereoelectronic patterns in peroxide chemistry and provide experimentally significant kinetic stabilization to this new class of bisperoxides. Furthermore, mild reduction of γ-hydroperoxy-γ-peroxylactone with Ph₃P produced an isolable γ-hydroxy-γ-peroxylactone, the first example of a structurally unencumbered CI where neither the primary nor the secondary stereoelectronic effect are impeded. Although this compound is relatively unstable, it does not undergo the BV reaction and instead follows a new mode of reactivity for the CI – a ring-opening process.

Protecting stereoelectronic effects prevent Baeyer–Villiger rearrangement and stabilize γ-OX-γ-peroxylactones (X = H, OH), the previously elusive non-strained Criegee intermediates.     

Relative Stability of Boron Planar Clusters in Diatomic Molecular Model

In the recently introduced phenomenological diatomic molecular model imagining the clusters as certain constructions of pair interatomic chemical bonds, there are estimated specific (per atom) binding energies of small all-boron planar clusters B_n, n = 1–15, in neutral single-anionic and single-cationic charge states. The theoretically obtained hierarchy of their relative stability/formation probability correlates not only with results of previous calculations, but also with available experimental mass-spectra of boron planar clusters generated in process of evaporation/ablation of boron-rich materials. Some overestimation in binding energies that are characteristic of the diatomic approach could be related to differences in approximations made during previous calculations, as well as measurement errors of these energies. According to the diatomic molecular model, equilibrium binding energies per B atom and B–B bond lengths are expected within ranges 0.37–6.26 eV and 1.58–1.65 Å, respectively.     

Reducing the internal reorganization energy via symmetry controlled π-electron delocalization

The magnitude of the reorganization energy is closely related to the nonradiative relaxation rate, which affects the photoemission quantum efficiency, particularly for the emission with a lower energy gap toward the near IR (NIR) region. In this study, we explore the relationship between the reorganization energy and the molecular geometry, and hence the transition density by computational methods using two popular models of NIR luminescent materials: (1) linearly conjugated cyanine dyes and (2) electron donor–acceptor (D–A) composites with various degrees of charge transfer (CT) character. We find that in some cases, reorganization energies can be significantly reduced to 50% despite slight structural modifications. Detailed analyses indicate that the reflection symmetry plays an important role in linear cyanine systems. As for electron donor–acceptor systems, both the donor strength and the substitution position affect the relative magnitude of reorganization energies. If CT is dominant and creates large spatial separation between HOMO and LUMO density distributions, the reorganization energy is effectively increased due to the large electron density variation between S₀ and S₁ states. Mixing a certain degree of local excitation (LE) with CT in the S₁ state reduces the reorganization energy. The principles proposed in this study are also translated into various pathways of canonically equivalent π-conjugation resonances to represent intramolecular π-delocalization, the concept of which may be applicable, in a facile manner, to improve the emission efficiency especially in the NIR region.

The reorganization energies may be significantly reduced by molecular symmetry effect.     

Green Strategies for the Preparation of Enantiomeric 5–8-Membered Carbocyclic β-Amino Acid Derivatives through CALB-Catalyzed Hydrolysis

Candida antarctica lipase B-catalyzed hydrolysis of carbocyclic 5–8-membered cis β-amino esters was carried out in green organic media, under solvent-free and ball-milling conditions. In accordance with the high enantioselectivity factor (E > 200) observed in organic media, the preparative-scale resolutions of β-amino esters were performed in tBuOMe at 65 °C. The unreacted β-amino ester enantiomers (1R,2S) and product β-amino acid enantiomers (1S,2R) were obtained with modest to excellent enantiomeric excess (ee) values (ee_s > 62% and ee_p > 96%) and in good chemical yields (>25%) in one or two steps. The enantiomers were easily separated by organic solvent/H₂O extraction.