Ab Initio Study of [n.n]Paracyclophane (n = 2, 3) Complexes with Cations: Unprecedented Through-Space Substituent Effects

What Is the Main Feature Distinguishing the Through-Space Interactions in Cyclophanes from Their Aliphatic Analogues?

Classical cyclophanes with two benzene rings have been compared with cyclophanes with one benzene ring replaced with an aliphatic part and aliphatic compounds, which are cyclophane analogues. Analysis of geometry, atomic charges, and aromatic and steric energy and investigation of intramolecular noncovalent interactions and charge mobility show that there is no special feature that distinguishes the classical cyclophanes from aliphatic analogues, so the definition of cyclophanes can be extended to other compounds.

Interplay between σ Holes, Anion···H-C, and Cation-π Interactions in Dibromo[2,2]paracyclophane Complexes

Theoretical calculations were performed to investigate the interplay between σ-hole, anion-HC and cation-π interactions in the complexes of dibromo[2,2]paracyclophane (DBr[2,2]PCP) with alkali (Li⁺, Na⁺, K⁺), alkaline earth metal cations (Be²⁺, Mg²⁺, and Ca²⁺), and halogen anions (F^-, Cl^-, and Br^-) using the wave function (MP2) and density functional theory (M06-2X and B3LYP) methods with the 6-311++G(d,p) basis set. The study reveals that DBr[2,2]PCP behaves as amphoteric molecule with a predominance of basic character. It prefers to interact with hard cations and hard anions such as Be²⁺ and F^- through cation-π and anion···HC interactions, respectively. Substitution of Br by F and Cl atoms in DBr[2,2]PCP decreases slightly the interaction energies of DX[2,2]PCP-halogen complexes (X = F, Cl, and Br) by 2.0 and 0.3 kcal/mol (M06-2X), respectively. The anion-HC interactions in DBr[2,2]PCP complexes are ∼10 kcal/mol stronger (B3LYP; ∼15 kcal/mol at M06-2X and 7 kcal/mol at MP2) than the σ-hole interactions.

Substituent effect on inter-ring interaction in paracyclophanes

The theoretical calculations, namely multipole-derived charge analysis, quantum theory of atom in molecules, and non-bonding interaction (NCI), were performed for [2.2]paracyclophanes, [2.2]paracyclophane-7,9-dienes, and [3.3]paracyclophanes optimized at B3LYP/6-311++G** level, including dispersion correction. The substituent effect of the electron donor N(Me)2 and electron acceptor NO2 group and the influence of the length of bridges joining the aromatic ring on aromatic ring interaction energy (AIE) and strain energy were discussed. The local and electrostatic character of the substituent effect in paracyclophanes was shown. The presence of the weak orbital through-space C···C interaction between the [3.3]paracyclophane ring and weak CH···O hydrogen bonds between the substituents in the different rings was shown.

Interlocked benzenes in triangular π-architectures: anchoring groups dictate ion binding and transmission

Graphynes and graphenylenes – potential materials for lithium ion batteries.

Aromaticity and Through-Space Interaction between Aromatic Rings in [2.2]Paracyclophanes

The HOMA index calculated for [2.2]paracyclophanes in the solid state reveals a slight decrease of aromaticity. Interactions between aromatic rings of [2.2]paracyclophane have been investigated using AIM and NCI analysis in both crystal and optimized [2.2]paracyclophane structures. AIM analysis showed that the C···C bond path between the two aromatic rings is present only in few [2.2]paracyclophanes. The NCI approach visualized the dispersion and repulsive interactions between the aromatic rings of every [2.2]paracyclophane. Combination of AIM and the NCI approach is necessary for determining and identifying nonbonded interactions in [2.2]paracyclophanes.

A (U)MP2(full) and (U)CCSD(T) theoretical investigation into the substituent effects on the intermolecular T-shaped F-H...π interactions between HF and LBBL (L = -H, : CO, :NN, -Cl, -CN and -NC)

The substituent effects on the intermolecular T-shaped F-H...π interactions are investigated between HF and LBBL (L = -H, : CO, :NN, -Cl, -CN and -NC) using the (U)MP2(full) and (U)CCSD(T) methods with the 6-311++G(2 d,p) basis set. The B ≡ B triple-bond contraction is found in the complexes with lone-pair-electron donors while the B = B double-bond is lengthened in the systems with the single-electron substituents upon complexation. The T-shaped F-H...π interaction energies follow the order of ClB = BCl...HF>HB = BH...HF>NNB ≡ BNN...HF>OCB ≡ BCO...HF>CNB = BNC...HF>NCB = BCN...HF. The electron-donating substituents : CO and :NN increases electron density of the B ≡ B triple bond by the delocalization interaction E ((2)) π ((CO/NN) → Lp(B)) while the electron-withdrawing substituents -CN and -NC decrease electron density of the B = B double bond by means of the π-π conjugative effect. The analyses of the APT atomic charge, "truncated" model, natural bond orbital (NBO), atoms in molecules (AIM) and electron density shifts reveal the nature of the substituent effect and explain the origin of the B ≡ B bond contraction.

Kinetics and mechanism of the oxidation of hydroxylamine by a {Mn3O4}4+ core in aqueous acidic media

In this work we report the kinetics of oxidation of hydroxylamine by a trinuclear Mn(IV) oxidant, [Mn(3)(μ-O)(4)(phen)(4)(H(2)O)(2)](4+) (1, phen = 1,10-phenanthroline), in aqueous solution over a pH range 2.0-4.0. The trinuclear Mn(IV) species (1) deprotonates in aqueous solution at physiological pH: 1 ⇌ 2 + H(+); pK(1) = 4.00 (± 0.15) at 25.0 °C, I = 1.0 (M) NaNO(3). Both 1 and 2 are reactive oxidants reacting with the conjugate acid of hydroxylamine, viz. NH(3)OH(+) where the deprotonated oxidant 2 reacts faster. This finding is in contrast to a common observation and belief that protonated oxidants react quicker than their deprotonated analogues. Mn(IV)(3) to Mn(II) transition in the present reaction proceeds through the intervention of a spectrally detected mixed-valent Mn(III)Mn(IV) dimer that quickly collapses to Mn(II). The rate of the reaction was found to be lowered in D(2)O-enriched media in comparison to that in pure H(2)O media. An initial one electron one proton transfer to Mn(IV)(3) (electroprotic; 1e, 1H(+)) could be mechanistically conceived as the rate step. We also demonstrate by means of high level DFT studies that, among the two sets of Mn(IV) atoms in the trinuclear oxidant, the unique one that is coordinated with two phen ligands and two oxo-bridges is reduced to Mn(III) at the rate step. This is explained based on energetic and spin density calculations. Moreover, this result agrees with the charge distribution on the Mn atoms of the trinuclear complex.

The role of the ethynyl substituent on the π-π stacking affinity of benzene: a theoretical study

Herein, we report a high-level theoretical study (SCS-RI-MP2(full)/aug-cc-pVTZ) examining the stacking affinity of 1,3,5-triethynylbenzene. The stacking properties of this compound are compared to those of benzene and 1,3,5-trifluorobenzene. The results indicate that the ethynyl substituent improves the stacking affinity of the arene, since the binding energies for the stacked ethynyl-substituted arene dimers are higher than those of both benzene and the fluoro-substituted arene. This interesting behaviour has been studied by examining the energetics, geometries and electron charge density features of the complexes. A query in the Cambridge Structural Database returned several X-ray crystal structures containing π-π stacking interactions of 1,3,5-triethynylaryls that strongly agree with the theoretical results.

Substituent effects in cation/pi interactions and electrostatic potentials above the centers of substituted benzenes are due primarily to through-space effects of the substituents

Substituent effects in cation/pi interactions have been examined using the M05-2X DFT functional and CCSD(T) paired with triple-zeta-quality basis sets. In contrast to popular, intuitive models, trends in substituent effects are explained primarily in terms of direct through-space interactions with the substituents. While there is some scatter in the data, which is attributed to pi polarization, the trend in substituent effects in cation/pi interactions is captured by an additive model in which the substituent is isolated from the aryl ring. Similarly, changes in the electrostatic potential at a point above the center of a substituted benzene arise largely from through-space effects of the substituents; pi polarization is not the dominant underlying cause.

Supramolecular assembly of Mg(II) complexes directed by associative lone pair-pi/pi-pi/pi-anion-pi/pi-lone pair interactions

Two Mg(II) malonate complexes with protonated 2-aminopyridine and protonated 2-amino-4-picoline as counterions, namely, (C(5)H(7)N(2))(4)[Mg(C(3)H(2)O(4))(2)(H(2)O)(2)](ClO(4))(2) (1) and (C(6)H(8)N(2)H)(2)[Mg(C(3)H(2)O(4))(2)(H(2)O)(2)] x 4 H(2)O (2) [C(5)H(7)N(2) = protonated 2-aminopyridine, C(3)H(4)O(4) = malonic acid, C(6)H(8)N(2)H = protonated 2-amino-4-picoline], have been synthesized from purely aqueous media, and their crystal structures have been determined by single-crystal X-ray diffraction. The role of lone pair...pi interactions in stabilizing the self-assembly process appears to be of great importance in both complexes. Additional weak forces like anion...pi and noncovalent O...O interactions are also found to be operating in 1. A rare combination of lone pair...pi and anion...pi interactions in 1, of the type lone pair...pi/pi...pi/pi...anion...pi/pi...lone pair, is observed, and this unusual supramolecular network is fully described here. An attempt to prepare an analogous complex with 2-amino-4-picoline resulted in 2, which is isomorphous with our recently reported transition-metal complexes of the type (C(6)H(8)N(2)H)(2)[M(C(3)H(2)O(4))(2)(H(2)O)(2)] x 4 H(2)O (M = Ni/Co/Mn). A high-level DFT-D study (RI-B97-D/TZVP) has been used to characterize the different noncovalent interactions present in the solid state. We have also analyzed some crystal fragments to examine energetically some important assemblies that drive the crystal packing. Finally, we have studied the influence of magnesium on some hydrogen-bonding interactions.