Preparation of σ- and π-Benzyl Type Derivatives of Molybdenum

The reaction of -LiM₀(CO)₂Cp with α-chloromethylnaphthalene gives a yellow, crystalline σ-benzyl type derivative of molybdenum (1). Irridation of (1) in n-hexane gives a red π-benzyl type derivative (2). Both complexes (1) and (2) are stable in air.     

Local orbitals for the study of the π→π* excitation in polyenes

Localized orbitals have recently been employed in large ab initio calculations, but their use has generally been restricted to ground‐state problems. In this work, we analyze the molecular orbitals of the excited states, optimized with a recently proposed local procedure. This method produces local orbitals of the CAS–SCF type, which permits its application to the study of excited states. In particular, we focus on the π→π* triplet excited state in polyenes, calculated using a 2/2 CAS space which includes two electrons in one π and one π* orbitals. In small polyenes, these two singly occupied active orbitals are delocalized all along the molecule. The extent of the delocalization is analyzed by studying polyenes of increasing size. Different polyenes have been studied, going from C₁₄H₁₆ to the C₇₀H₇₂ polyene. The relation of the π→π* excitation with the cation and anion systems is also discussed. © 2004 Wiley Periodicals, Inc. Int J Quantum Chem, 2005     

The tropolone–isobutylamine complex: a hydrogen‐bonded troponoid without dominant π–π interactions

Tropolone long has served as a model system for unraveling the ubiquitous phenomena of proton transfer and hydrogen bonding. This molecule, which juxtaposes ketonic, hydroxylic, and aromatic functionalities in a framework of minimal complexity, also has provided a versatile platform for investigating the synergism among competing intermolecular forces, including those generated by hydrogen bonding and aryl coupling. Small members of the troponoid family typically produce crystals that are stabilized strongly by pervasive π–π, C—H…π, or ion–π interactions. The organic salt (TrOH·iBA) formed by a facile proton‐transfer reaction between tropolone (TrOH) and isobutylamine (iBA), namely isobutylammonium 7‐oxocyclohepta‐1,3,5‐trien‐1‐olate, C₄H₁₂N⁺·C₇H₅O₂⁻, has been investigated by X‐ray crystallography, with complementary quantum‐chemical and statistical‐database analyses serving to elucidate the nature of attendant intermolecular interactions and their synergistic effects upon lattice‐packing phenomena. The crystal structure deduced from low‐temperature diffraction measurements displays extensive hydrogen‐bonding networks, yet shows little evidence of the aryl forces (viz. π–π, C—H…π, and ion–π interactions) that typically dominate this class of compounds. Density functional calculations performed with and without the imposition of periodic boundary conditions (the latter entailing isolated subunits) documented the specificity and directionality of noncovalent interactions occurring between the proton‐donating and proton‐accepting sites of TrOH and iBA, as well as the absence of aromatic coupling mediated by the seven‐membered ring of TrOH. A statistical comparison of the structural parameters extracted for key hydrogen‐bond linkages to those reported for 44 previously known crystals that support similar binding motifs revealed TrOH·iBA to possess the shortest donor–acceptor distances of any troponoid‐based complex, combined with unambiguous signatures of enhanced proton‐delocalization processes that putatively stabilize the corresponding crystalline lattice and facilitate its surprisingly rapid formation under ambient conditions.     

Facile Fabrication of Solid‐state Electrochemiluminescence Sensor via Non‐covalent π‐π Stacking and Covalent Bonding on Graphite Electrode

Herein, a facile and efficient method was developed for fabrication of solid‐state electrochemiluminescence (ECL) sensor via non‐covalent π‐π stacking and covalent bonding on the graphite electrode (GE) surface. The electrode was firstly modified with 1‐aminopyrene via π‐π stacking between GE surface and the pyrene moiety. Thereafter a stable and efficient solid‐state ECL sensor was fabricated by covalent immobilization of ruthenium(II) onto the GE surface via amidation reaction between the 1‐aminopyrene and bis(2,2′‐bipyridyl)(4‐methyl‐4′‐carboxypropyl‐2,2′‐bipyridyl) ruthenium(II) bishexafluorophosphate. The sensor has been investigated using tripropylamine and tetracycline as representative analytes, and low detection limits of 0.7 nM and 3.5 nM (S/N=3) were reached, respectively.     

Phenylcarbamoylated β‐CD: π‐Acidic and π‐basic chiral selectors for HPLC

Two types of chiral stationary phases for HPLC based on π‐acidic or π‐basic perphenylcarbamoylated β‐CDs were synthesized. The relative structural features of the two effective chiral selectors are discussed and compared in both normal‐phase and RP modes. In addition, the nature and concentration of alcoholic modifiers were varied for optimal separation in normal phase and the structural variation of the analytes was also examined. The results showed that hydrogen bonding, steric effect and π‐acidic–π‐basic interaction contributed greatly to enantioseparation. Upon comparison, some of the differences in the separation behavior of the two types of chiral stationary phases might be due to the π‐acidic or π‐basic phenylcarbamate groups.     

Construction of Hetero‐Four‐Layered Tripalladium(II) Cyclophanes by Transannular π⋅⋅⋅π Interactions

A synthetic strategy for the generation of new molecular species utilizing a provision of nature is presented. Nano‐dimensional (23(2)×21(1)×16(1) ų) hetero‐four‐layered trimetallacyclophanes were constructed by proof‐of‐concept experiments that utilize a suitable combination of π???π interactions between the central aromatic rings, tailor‐made short/long spacer tridentate donors, and the combined helicity. The behavior of the unprecedented four‐layered metallacyclophane system offers a landmark in the development of new molecular systems.     

Interplay between Metal⋅⋅⋅π Interactions and Hydrogen Bonds: Some Unusual Synergetic Effects of Coinage Metals and Substituents

The ternary systems of C₂H₄ (C₂H₂ or C₆H₆)‐MCN‐HF (M=Cu, Ag, Au) and the respective binary systems were investigated to study the interplay between metal???π interactions and hydrogen bonds. The metal???π interactions in C₂H₄‐MCN become stronger with the irregular order Ag<Cu<Au, while the hydrogen bonds in MCN‐HF become weaker following the same order. The metal???π interactions are weakened as the H atoms in the π system are replaced with electron‐withdrawing groups and enhanced by electron‐donating groups. Type 1 of these ternary systems, in which MCN acts as Lewis base and acid simultaneously, is more stable than type 2, in which C₂H₄ acts as a double Lewis base. Negative cooperativity is present in type 2 ternary systems with a weakening of the metal???π interactions and the hydrogen bonds. Positive cooperativity is found in type 1 ternary systems with an enhancement of the metal???π interactions and the hydrogen bonds, except for C₂(CN)₄‐AuCN‐HF‐1. The weaker metal???π interaction in C₆H₆‐AuCN has a greater enhancing effect on the hydrogen bond in AuCN‐HF than those in C₂H₄‐AuCN and C₂H₂‐AuCN. These synergetic effects were analyzed with the natural bond orbital and energy decomposition.     

Ab initio and AIM studies on typical π‐type and pseudo‐π‐type halogen bonds: Comparison with hydrogen bonds

Series of typical π‐type and pseudo‐π‐type halogen‐bonded complexes B ··· ClY and B ··· BrY and hydrogen‐bonded complex B ··· HY (B = C₂H₄, C₂H₂, and C₃H₆; Y = F, Cl, and Br) have been investigated using the MP2/aug‐cc‐pVDZ method. A striking parallelism was found in the geometries, vibrational frequencies, binding energies, and topological properties between B ··· XY and B ··· HY (X = Cl and Br). It has been found that the lengths of the weak bond d(X ··· π)/d(H ··· π), the frequencies of the weak bond ν(X ··· π)/ν(H ··· π), the frequency shifts Δν(X? Y)/Δν(H? Y), the electron densities at the bond critical point of the weak bonds ρ_c(X ··· π)/ρ_c(H ··· π), and the electron density changes Δρ_c(X? Y)/Δρ_c(H? Y) could be used as measures of the strengths of typical π‐type and pseudo‐π‐type halogen/hydrogen bonds. The typical π‐type and pseudo‐π‐type halogen bond and hydrogen bond are noncovalent interactions. For the same Y, the halogen bond strengths are in the order B ··· ClY < B ··· BrY. For the same X, the halogen bond strength decreases according to the sequence F > Cl > Br that is in agreement with the hydrogen bond strengths B ··· HF > B ··· HCl > B ··· HBr. All of these typical π‐type and pseudo‐π‐type hydrogen‐bonded and halogen‐bonded complexes have the “conflict‐type” structure. Contour maps of the Laplacian of π electron density indicate that the formation of B ··· XY halogen‐bonded complex and B ··· HY hydrogen‐bonded complex is very similar. Charge transfer is observed from B to XY/HY and both the dipolar polarization and the volume of the halogen atom or hydrogen atom decrease on B ··· XY/B ··· HY complex formation. © 2010 Wiley Periodicals, Inc. Int J Quantum Chem, 2011     

Cooperation of σ–π and σ*–π* Conjugation in the UV/Vis and Fluorescence Spectra of 9,10-Disilylanthracene

In 1996, we reported that silyl groups of 9,10-disilylanthracenes significantly affect the UV/Vis and fluorescence spectra. Although the results indicate that the silyl groups have strong electronic effects on anthracene, the details of the mechanisms responsible for this have not yet been clarified. This article describes the analysis of the UV/Vis and fluorescence spectra of 9,10-bis(diisopropylsilyl)anthracene by theoretical calculations. This study reveals that π conjugation of anthracene is extended by cooperation of σ–π and σ*–π* conjugation between the silyl groups and anthracene. This effect increases the transition moment of the π–π* transition of anthracene. As a result, the molecular extinction coefficient of the ¹L_a band and the fluorescence quantum yield are increased.     

The peptide NCbz‐Val‐Tyr‐OMe and aromatic π–π interactions

The peptide N‐benzyloxycarbonyl‐L‐valyl‐L‐tyrosine methyl ester or NCbz‐Val‐Tyr‐OMe (where NCbz is N‐benzyloxycarbonyl and OMe indicates the methyl ester), C₂₃H₂₈N₂O₆, has an extended backbone conformation. The aromatic rings of the Tyr residue and the NCbz group are involved in various attractive intra‐ and intermolecular aromatic π–π interactions which stabilize the conformation and packing in the crystal structure, in addition to N—H...O and O—H...O hydrogen bonds. The aromatic π–π interactions include parallel‐displaced, perpendicular T‐shaped, perpendicular L‐shaped and inclined orientations.     

The Influence of Oxygen Atoms on Conformation and π–π Stacking of Ladder‐Type Donor‐Based Polymers and Their Photovoltaic Properties

A novel ladder‐type donor pyran‐bridged indacenodithiophene (IDTP) is developed by introducing two oxygen atoms into indacenodithiophene unit. IDTP possesses a twisted backbone and leads to facially asymmetric arrangement of side chains, resulting in enhanced local π–π stacking of according polymer poly[(5,5,11,11‐tetrakis(4‐octylphenyl)‐5,11‐dihydrothieno[2′,3′:5,6]pyrano[3,4‐g]thieno[3,2‐c]isochromene)‐alt‐4,7‐(5‐fluoro‐2,1,3‐benzothiadiazole)] (PIDTP)‐FBT, which shows extended absorption range. Moreover, oxygen atoms render deeper highest occupied molecular orbital (HOMO) levels of poly[indacenodithiophene‐alt‐4,7‐(5‐fluoro‐2,1,3‐benzothiadiazole)] (PIDTP)‐FBT compared with PIDT‐FBT, therefore bringing a higher open‐circuit voltage (V _oc).     

Photophysical properties of σ–π‐conjugated alternating oligothienylene–oligosilylene polymers

UV‐visible absorption and fluorescence properties of three series of σ–π‐conjugated polymers (copolymers of alternative oligothienylene and oligosilylene units) have been studied in dioxane solution. The energies of the absorption maximum, fluorescence maximum, and the 0–0 transition are found to be linearly dependent on the reciprocal of the number of thiophene rings in the repeating unit of the polymer chain, but almost independent of the silicon atom number. The σ–π‐conjugation in the polymers results in red shift in the absorption and fluorescence maxima, higher fluorescence quantum yields, and longer fluorescence lifetimes of the polymers, with respect to their corresponding analogous α‐oligothiophenes. © 1999 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 37: 1873–1880, 1999     

Intramolecular π–π Interactions in Flexibly Linked Partially Fluorinated Bisarenes in the Gas Phase

Three compounds with phenyl and pentafluorophenyl rings bridged by (CH₂)₃ and (CH₂)₂SiMe₂ units were synthesized by hydrosilylation and C−C coupling reactions. Their solid‐state structures are dominated by intermolecular π stacking interactions, primarily leading to dimeric or chain‐type aggregates. Analysis of free molecules in the gas phase by electron diffraction revealed the most abundant conformer to be significantly stabilized by intramolecular π–π interactions. For the silicon compounds, structures characterized by σ–π interactions between methyl and pentafluorophenyl groups are second lowest in energy and cannot be excluded completely by the gas electron diffraction experiments. C₆H₅(CH₂)₃C₆F₅, in contrast, is present as a single conformer. The gas‐phase structures served as a reference for the evaluation of a series of (dispersion‐corrected) quantum‐chemical calculations.     

Synthesis and Crystal Structure of a [Mo8O26]4– Cluster Derivative with 4‐MePyH+. First β‐Octamolybdate Derivative with π–π Stacking

Dioxobis(pyridine‐2‐thiolate‐N, S)molybdenum(VI) (MoO₂(Py‐S)₂), reacts with of 4‐methylpyridine (4‐MePy) in acetonitrile, by slow diffusion, to afford the title compound. This has been characterized by elemental analysis, IR and ¹H NMR spectroscopy. The X‐ray single crystal structure of the complex is described. Structural studies reveal that the molecular structure consists of a β‐Mo₈O₂₆ polyanion with eight MoO₆ distorted edge‐shared octahedra with short terminal Mo–O bonds (1.692–1.714 Å), bonds of intermediate length (1.887–1.999 Å) and long bonds (2.150–2.473 Å). Two different types of hydrogen bonds have been found: N–H···O (2.800–3.075 Å) and C–H···O (3.095–3.316 Å). The presence of π–π stacking interactions and strong hydrogen bonds are presumably responsible for the special disposition of the pyridinic rings around the polyanion cluster.     

The Bright Future of Unconventional σ/π‐Hole Interactions

Non‐covalent interactions play a crucial role in (supramolecular) chemistry and much of biology. Supramolecular forces can indeed determine the structure and function of a host–guest system. Many sensors, for example, rely on reversible bonding with the analyte. Natural machineries also often have a significant non‐covalent component (e.g. protein folding, recognition) and rational interference in such ‘living’ devices can have pharmacological implications. For the rational design/tweaking of supramolecular systems it is helpful to know what supramolecular synthons are available and to understand the forces that make these synthons stick to one another. In this review we focus on σ‐hole and π‐hole interactions. A σ‐ or π‐hole can be seen as positive electrostatic potential on unpopulated σ* or π⁽*⁾ orbitals, which are thus capable of interacting with some electron dense region. A σ‐hole is typically located along the vector of a covalent bond such as X?H or X?Hlg (X=any atom, Hlg=halogen), which are respectively known as hydrogen and halogen bond donors. Only recently it has become clear that σ‐holes can also be found along a covalent bond with chalcogen (X?Ch), pnictogen (X?Pn) and tetrel (X?Tr) atoms. Interactions with these synthons are named chalcogen, pnigtogen and tetrel interactions. A π‐hole is typically located perpendicular to the molecular framework of diatomic π‐systems such as carbonyls, or conjugated π‐systems such as hexafluorobenzene. Anion–π and lone‐pair–π interactions are examples of named π‐hole interactions between conjugated π‐systems and anions or lone‐pair electrons respectively. While the above nomenclature indicates the distinct chemical identity of the supramolecular synthon acting as Lewis acid, it is worth stressing that the underlying physics is very similar. This implies that interactions that are now not so well‐established might turn out to be equally useful as conventional hydrogen and halogen bonds. In summary, we describe the physical nature of σ‐ and π‐hole interactions, present a selection of inquiries that utilise σ‐ and π‐holes, and give an overview of analyses of structural databases (CSD/PDB) that demonstrate how prevalent these interactions already are in solid‐state structures.     

Comparative studies on group III σ‐hole and π‐hole interactions

The σ‐hole of M₂H₆ (M = Al, Ga, In) and π‐hole of MH₃ (M = Al, Ga, In) were discovered and analyzed, the bimolecular complexes M₂H₆···NH₃ and MH₃···N₂P₂F₄ (M = Al, Ga, In) were constructed to carry out comparative studies on the group III σ‐hole interactions and π‐hole interactions. The two types of interactions are all partial‐covalent interactions; the π‐hole interactions are stronger than σ‐hole interactions. The electrostatic energy is the largest contribution for forming the σ‐hole and π‐hole interaction, the polarization energy is also an important factor to form the M···N interaction. The electrostatic energy contributions to the interaction energy of the σ‐hole interactions are somewhat greater than those of the π‐hole interactions. However, the polarization contributions for the π‐hole interactions are somewhat greater than those for the σ‐hole interactions. © 2016 Wiley Periodicals, Inc.