The Interaction of π-Orbitals in Barrelene

The photoelectron spectrum (PE. spectrum) of barrelene (bicyclo[2.2.2]octatriene, 4 ) is recorded and the first four bands are correlated with orbitals obtained with the MINDO/2-SCF procedure. The structural changes accompagnying the ionisation process 4 → 4 ⁺ are qualitatively derived from the features of the top-occupied a′₂ (π) MO of 4 , which shows complete σ-π separation. The vibrational pattern of the corresponding PE. band 1. as well as complete energy-minimisation of the geometries of 4 and 4 ⁺ support the conclusion that 4 is a rather strained molecule. The interaction of the three π? bonds in 4 are discussed in terms of ‘through-space’ and ‘through-bond’ interaction with lower lying σ-orbitals. It is found that the latter is far from being negligible.     

Enhanced Aerogen–π Interaction by a Cation–π Force

The interaction between a noble gas atom and an aromatic π‐electron system, which mainly originates from the London dispersion force, is very weak and has not attracted enough attention yet. Herein, we reported a type of notably enhanced aerogen–π interaction between cation–π systems and noble gas atoms. The binding strength of a divalent cation–π system with a xenon atom is comparable to a moderate hydrogen bond (up to ca. 7 kcal mol^?1), whereas krypton and argon atoms produce slightly weaker interactions. Energy‐decomposition analysis reveals that the induction interaction is responsible for the stabilization of divalent cation–π?Xe species besides the dispersion interaction. Our results might be helpful to increase the understanding of some unsolved mysteries of aerogens.     

Tricarbonyl(η5‐formylcyclopentadienyl)manganese(I) and tricarbonyl(η5‐formylcyclopentadienyl)rhenium(I) containing short π(CO)...π(CO) and π(CO)...π interactions

The structures of tricarbonyl(formylcyclopentadienyl)manganese(I), [Mn(C₆H₅O)(CO)₃], (I), and tricarbonyl(formylcyclopentadienyl)rhenium(I), [Re(C₆H₅O)(CO)₃], (II), were determined at 100 K. Compounds (I) and (II) both possess a carbonyl group in a trans position relative to the substituted C atom of the cyclopentadienyl ring, while the other two carbonyl groups are in almost eclipsed positions relative to their attached C atoms. Analysis of the intermolecular contacts reveals that the molecules in both compounds form stacks due to short attractive π(CO)...π(CO) and π(CO)...π interactions, along the crystallographic c axis for (I) and along the [201] direction for (II). Symmetry‐related stacks are bound to each other by weak intermolecular C—H...O hydrogen bonds, leading to the formation of the three‐dimensional network.     

π–π Dimerization of a mono(pyridyl–imine) platinum(II) chelate

The cation of the title complex salt, chlorido{2,2‐dimethyl‐N‐[(E)‐1‐(pyridin‐2‐yl)ethylidene]propane‐1,3‐diamine}platinum(II) tetrafluoridoborate, [PtCl(C₁₂H₁₉N₃)]BF₄, exhibits a nominally square‐planar Pt^II ion coordinated to a chloride ion [Pt—Cl = 2.3046 (9) Å] and three unique N‐atom types, viz. pyridine, imine and amine, of the tridentate Schiff base ligand formed by the 1:1 condensation of 1‐(pyridin‐2‐yl)ethanone and 2,2‐dimethylpropane‐1,3‐diamine. The cations are π‐stacked in inversion‐related pairs (dimers), with a mean plane separation of 3.426 Å, an intradimer Pt...Pt separation of 5.0785 (6) Å and a lateral shift of 3.676 Å. The centroid (Cg) of the pyridine ring is positioned approximately over the Pt^II ion of the neighbouring cation (Pt...Cg = 3.503 Å).     

η4‐HBCC‐σ,π‐Borataallyl Complexes of Ruthenium Comprising an Agostic Interaction

Thermolysis of [Cp*Ru(PPh₂(CH₂)PPh₂)BH₂(L₂)] 1 (Cp*=η⁵‐C₅Me₅; L=C₇H₄NS₂), with terminal alkynes led to the formation of η⁴‐σ,π‐borataallyl complexes [Cp*Ru(μ‐H)B{R‐C=CH₂}(L)₂] ( 2 a – c ) and η²‐vinylborane complexes [Cp*Ru(R‐C=CH₂)BH(L)₂] ( 3 a – c ) ( 2 a , 3 a : R=Ph; 2 b , 3 b : R=COOCH₃; 2 c , 3 c : R=p‐CH₃‐C₆H₄; L=C₇H₄NS₂) through hydroboration reaction. Ruthenium and the HBCC unit of the vinylborane moiety in 2 a – c are linked by a unique η⁴‐interaction. Conversions of 1 into 3 a – c proceed through the formation of intermediates 2 a – c . Furthermore, in an attempt to expand the library of these novel complexes, chemistry of σ‐borane complex [Cp*RuCO(μ‐H)BH₂L] 4 (L=C₇H₄NS₂) was investigated with both internal and terminal alkynes. Interestingly, under photolytic conditions, 4 reacts with methyl propiolate to generate the η⁴‐σ,π‐borataallyl complexes [Cp*Ru(μ‐H)BH{R‐C=CH₂}(L)] 5 and [Cp*Ru(μ‐H)BH{HC=CH‐R}(L)] 6 (R=COOCH₃; L=C₇H₄NS₂) by Markovnikov and anti‐Markovnikov hydroboration. In an extension, photolysis of 4 in the presence of dimethyl acetylenedicarboxylate yielded η⁴‐σ,π‐borataallyl complex [Cp*Ru(μ‐H)BH{R‐C=CH‐R}(L)] 7 (R=COOCH₃; L=C₇H₄NS₂). An agostic interaction was also found to be present in 2 a – c and 5 – 7 , which is rare among the borataallyl complexes. All the new compounds have been characterized in solution by IR, ¹H, ¹¹B, ¹³C NMR spectroscopy, mass spectrometry and the structural types were unequivocally established by crystallographic analysis of 2 b , 3 a – c and 5 – 7 . DFT calculations were performed to evaluate possible bonding and electronic structures of the new compounds.     

Intramolecular π–π stacking in diaquabis(2‐hydroxybenzoato‐κO)bis(1,10‐phenanthroline‐κ2N,N′)strontium(II)

In the title compound, [Sr(C₇H₅O₃)₂(C₁₂H₈N₂)₂(H₂O)₂], the Sr^II ion is located on a twofold rotation axis and assumes a distorted square‐antiprism SrN₄O₄ coordination geometry, formed by two phenanthroline (phen) ligands, two 2‐hydroxybenzoate anions and two water molecules. Within the mononuclear complex molecule, intramolecular π–π stacking is observed between nearly parallel coordinated phen ligands, while normal intermolecular π–π stacking occurs between parallel phen ligands of adjacent complex molecules. Classic O—H...O and weak C—H...O hydrogen bonding helps to stabilize the crystal structure.     

Influence of the Li⋅⋅⋅π Interaction on the H/X⋅⋅⋅π Interactions in HOLi⋅⋅⋅C6H6⋅⋅⋅HOX/XOH (X=F,Cl, Br,I) Complexes

The influences of the Li???π interaction of C₆H₆???LiOH on the H???π interaction of C₆H₆???HOX (X=F, Cl, Br, I) and the X???π interaction of C₆H₆???XOH (X=Cl, Br, I) are investigated by means of full electronic second‐order Møller–Plesset perturbation theory calculations and “quantum theory of atoms in molecules” (QTAIM) studies. The binding energies, binding distances, infrared vibrational frequencies, and electron densities at the bond critical points (BCPs) of the hydrogen bonds and halogen bonds prove that the addition of the Li???π interaction to benzene weakens the H???π and X???π interactions. The influences of the Li???π interaction on H???π interactions are greater than those on X???π interactions; the influences of the H???π interactions on the Li???π interaction are greater than X???π interactions on Li???π interaction. The greater the influence of Li???π interaction on H/X???π interactions, the greater the influences of H/X???π interactions on Li???π interaction. QTAIM studies show that the intermolecular interactions of C₆H₆???HOX and C₆H₆???XOH are mainly of the π type. The electron densities at the BCPs of hydrogen bonds and halogen bonds decrease on going from bimolecular complexes to termolecular complexes, and the π‐electron densities at the BCPs show the same pattern. Natural bond orbital analyses show that the Li???π interaction reduces electron transfer from C₆H₆ to HOX and XOH.     

Hydrogen‐bonding and π–π stacking interactions in aquachloridobis(1,10‐phenanthroline)cobalt(II) chloride dichloridobis(1,10‐phenanthroline)cobalt(II) hexahydrate

The title compound, [CoCl(C₁₂H₈N₂)₂(H₂O)]Cl·[CoCl₂(C₁₂H₈N₂)₂]·6H₂O, is the first example of a new 1:1 cocrystal of the octahedral [CoCl₂(phen)₂] and [CoCl(phen)₂(H₂O)]⁺·Cl⁻ complexes (phen is 1,10‐phenanthroline). The latter form heterochiral dimers held by strong π–π stacking interactions via their phenathroline ligands, which confirms that π stacking is an important and reliable synthon in supramolecular design. In addition, the crystal structure is networked by H₂O...H₂O, H₂O...Cl⁻ and H₂O...Cl hydrogen bonds, which interconnect the different units of the cobalt complexes.     

λ3σ3P and λ5σ5P derivatives of (E)-1,2-difluoro-2-(pentafluoro-λ6-sulfanyl)ethylene and (Z)-1,2,3,3,3-pentafluoropropylene

For the first time, the (E)-1,2-difluoro-2-(pentafluoro-λ⁶-sulfanyl)ethenyl group has been bonded to λ³σ³ phosphorus using a Grignard reagent. Similar phosphorus derivatives containing the (Z)-1,2,3,3,3-pentafluoropropenyl moiety were also synthesized for comparison. In three cases, hexafluoroacetone was added to form 4,4,5,5-tetrakis(trifluoromethyl) 1,3,2λ⁵σ⁵-dioxaphospholanes. © 1997 John Wiley & Sons, Inc. Heteroatom Chem 8: 467–471, 1997     

Theoretical determination of the ionization potentials of the 1σ and 2σ electrons of CO(Σ)

Large-basis-set calculations of near Hartree-Fock accuracy were performed on CO⁺(1σ-hole ²Σ⁺) and CO⁺)2σ-hole, ²Σ⁺); correlation energies for these systems and for CO were calculated using an atoms-in-molecule approach, relativistic energies and vibrational structure corrections were also considered. The results are: IP(CO, 1σ) = 542.4 (542.57) eV, IP(CO,2σ) = 297.0 (296.24) cV, D_c(CO, ¹Σ⁺) = 10.8 (11.1) Ev, D₃(CO⁺, 1σ, ²Σ⁺) = 11.9 eV, D_e(CO⁺, 2σ, ²Σ⁺) = 9.1 eV, where IP and D_e stand respectively for ionization potential and dissociation energy, and where the numbers in parentheses refer to the most recent experimental values. The electron transfers resulting from the ionization of inner-shell electrons are discussed. Finally a quantitative correlation is developed correlating absolute chemical shifts to charge densities. Agreement between the calculated values and those derived from the correlation is quite satisfactory.     

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.