Solvent-induced aggregation through metal...metal/pi...pi interactions: large solvatochromism of luminescent organoplatinum(II) terpyridyl complexes

A dramatic color change and tremendous emission enhancement have been "switched on" upon increasing diethyl ether ratio in acetonitrile or acetone solution of [Pt(tpy)(CC-CCH)]OTf, attributed to the formation of Pt...Pt and pi-pi interactions. Two crystal forms (dark-green and red) of [Pt(tpy)(CC-CCH)]OTf, together with [Pt(tBu3-tpy)(CC-CCH)]OTf, show different crystal-packing modes as revealed by X-ray crystallography.     

Noncovalent interactions between cytosine and SWCNT: curvature dependence of complexes via pi...pi stacking and cooperative CH...pi/NH...pi

Fragments of C24H12, adapted from a variety of armchair [(n,n), (n = 5, 7, and 8)] and zigzag [(m,0) (m = 8, 10, and 12)] single-walled carbon nanotube (SWCNT), are used to model corresponding SWCNTs with different diameters and electronic structures. The parallel binding mainly through pi...pi stacking interaction, as well as the perpendicular binding via cooperative NH...pi and CH...pi between cytosine and the fragments of SWCNT have been extensively investigated with a GGA type of DFT, PW91LYP/6-311++G(d,p). The eclipsed tangential (ET) conformation with respect to the six-membered ring of cytosine and the central ring of SWCNT fragments is less stable than the slipped tangential (ST) conformation for the given fragment; perpendicular conformations with NH2 and CH ends have higher negative binding energy than those with NH and CH ends. At PW91LYP/6-311++G(d,p) level, two tangential complexes are less bound than perpendicular complexes. However, as electron correlation is treated with MP2/6-311G(d,p) for PW91LYP/6-311++G(d,p) optimized complexes, it turns out there is an opposite trend that two tangential complexes become more stable than three perpendicular complexes. This result implies that electron correlation, a primary source to dispersion energy, has more significant contributions to the pi...pi stacking complexes than to the complexes via cooperative NH...pi and CH...pi interactions. In addition, it was found for the first time that binding energies for two tangential complexes become more negative with increasing nanotube diameter, while those for three perpendicular complexes have a weaker dependence on the curvature; i.e., binding energies are slightly less and less negative. The performance of a novel hybrid DFT, MPWB1K, was also discussed.     

Novel [2]catenane structures introducing communication between transition metal centers via pi...pi interactions

[2]Catenane systems containing copper(II) and nickel(II) as metal centers have been self-assembled using tetraazamacrocyclic complexes and benzo-24-crown-8 as building blocks. A variety of methods, including X-ray crystallography, ESI mass spectrometry, (13)C and (1)H NMR, and electrochemistry, were applied to characterize these new face-to-face bismacrocyclic systems. Weak pi...pi interactions introduced by interlocking transition metal complexes with benzocrown moieties were shown to increase the communication (cooperativity) of metal centers. Introduction of the benzocrown increases the stability of the mixed valence state of the macrocyclic complex, which is reflected in high values of conproportionation constants. Moreover, this effect was found to be stronger than that obtained by shortening the length of the spacer between the two tetraazamacrocyclic subunits in the parent bismacrocycles. The extent of communication is larger for the nickel catenane than for the copper one.     

Enhanced pi ... pi interactions in alpha,beta-unsaturated carbonyls

High level ab initio calculations on complexes of benzene with acrolein and ethene reveal that pi ... pi interactions to electron deficient acrolein are remarkably similar to those found in the benzene dimer.     

Probing the effects of heterogeneity on delocalized pi...pi interaction energies

Dimers composed of benzene (Bz), 1,3,5-triazine (Tz), cyanogen (Cy) and diacetylene (Di) are used to examine the effects of heterogeneity at the molecular level and at the cluster level on pi...pi stacking energies. The MP2 complete basis set (CBS) limits for the interaction energies (E(int)) of these model systems were determined with extrapolation techniques designed for correlation consistent basis sets. CCSD(T) calculations were used to correct for higher-order correlation effects (deltaE(CCSD)(T)(MP2)) which were as large as +2.81 kcal mol(-1). The introduction of nitrogen atoms into the parallel-slipped dimers of the aforementioned molecules causes significant changes to E(int). The CCSD(T)/CBS E(int) for Di-Cy is -2.47 kcal mol(-1) which is substantially larger than either Cy-Cy (-1.69 kcal mol(-1)) or Di-Di (-1.42 kcal mol(-1)). Similarly, the heteroaromatic Bz-Tz dimer has an E(int) of -3.75 kcal mol(-1) which is much larger than either Tz-Tz (-3.03 kcal mol(-1)) or Bz-Bz (-2.78 kcal mol(-1)). Symmetry-adapted perturbation theory calculations reveal a correlation between the electrostatic component of E(int) and the large increase in the interaction energy for the mixed dimers. However, all components (exchange, induction, dispersion) must be considered to rationalize the observed trend. Another significant conclusion of this work is that basis-set superposition error has a negligible impact on the popular deltaE(CCSD)(T)(MP2) correction, which indicates that counterpoise corrections are not necessary when computing higher-order correlation effects on E(int). Spin-component-scaled MP2 (SCS-MP2 and SCSN-MP2) calculations with a correlation-consistent triple-zeta basis set reproduce the trends in the interaction energies despite overestimating the CCSD(T)/CBS E(int) of Bz-Tz by 20-30%.     

Halogen bonding and pi...pi stacking control reactivity in the solid state

The halogen bonding and the pi...pi stacking interactions induce the noncovalent self-assembly of modules into photoreactive supramolecular architecture. The pi...pi interaction pre-organizes the template, and the halogen bonding aligns the olefins to conform to the topochemical principle for photoreaction. The UV irradiation of the crystal resulted in a cyclization product with quantitative yield and stereospecificity.     

Multicoefficient extrapolated density functional theory studies of pi...pi interactions: the benzene dimer

We report tests of new (2005) and established (1999-2003) multilevel methods against essentially converged benchmark results for nonbonded interactions in benzene dimers. We found that the newly developed multicoefficient extrapolated density functional theory (DFT) methods (which combine DFT with correlated wave function methods) give better performance than multilevel methods such as G3SX, G3SX(MP3), and CBS-QB3 that are based purely on wave function theory (WFT); furthermore, they have a lower computational cost. We conclude that our empirical approach for combining WFT methods with DFT methods is a very efficient and effective way for describing not only covalent interactions (as shown previously) but also nonbonded interactions.     

Coordination molecular hats binding acetonitrile via C-H...pi interactions

The [M(3)L(3)](3+) metallo-macrocycles with syn, syn, syn conformation, self-assembled from crown ether functionalized 1,10-phenanthroline Pd(II) nitrates with naphthanoimidazole, bind a solvent CH(3)CN within the naphthanoimidazolate-built cavity through C-H...pi interactions in the crystalline state.     

Computational characterization and modeling of buckyball tweezers: density functional study of concave-convex pi...pi interactions

The geometries and binding energies of a recent buckyball tweezers (C(60)H(28)) and its supramolecular complexes are investigated using recently developed density functionals (M06-L and M06-2X) that include an accurate treatment of medium-range correlation energy. The pincer part of the tweezers, corannulene, has a strong attractive interaction with C(60). However, due to the entropy penalty, the calculated gas-phase free energy of association of the C(60)@corannulene supramolecule is positive 3.5 kcal mol(-1); and this entropy penalty explains why it is difficult to observe C(60)@corannulene supramolecule experimentally. By using a pi-extended tetrathiafulvalene (TTF), in particular 9,10-bis(1,3-dithiol-2-ylidene)-9,10-dihydroanthracene (TTFAQ or C(20)H(10)S(4)), as the pincer part, we modeled a new buckyball tweezers. The geometries and binding energies of the new buckyball tweezers and its supramolecular complexes are also calculated. Due to fact that the attractive interaction between TTFAQ and C(60) is weaker than that between corannulene and C(60), the gas-phase binding free energy in the C(60)@C(60)H (32)S(8) supramolecular complex is smaller than that in the C(60)@C(60)H(28) supramolecule. We also discuss solvent effects.     

Perturbing the structure of the 2-norbornyl cation through C-H...N and C-H...pi interactions

Quantum chemical studies on complexes of the 2-norbornyl cation with ammonia and benzene are described. These calculations demonstrate that the delocalized, bridged, nonclassical geometry that is usually favored for this carbocation can be perturbed significantly toward the normally less favorable classical geometry through appropriately oriented noncovalent interactions. Since such cations have been proposed as intermediates in enzyme and antibody catalyzed reactions, these results have implications for the nature of the cationic species that may be generated in the presence of electron-rich amino acid side chains of the sort that may be present in the active sites of biocatalysts.     

Reliable structures and energetics for two new delocalized pi...pi prototypes: cyanogen dimer and diacetylene dimer

Two new prototype delocalized pi[dot dot dot]pi complexes are introduced: the dimers of cyanogen, (N[triple bond]C-C[triple bond]N)(2), and diacetylene, (HC[triple bond]C-C[triple bond]CH)(2). These dimers have properties similar to larger delocalized pi...pi systems such as benzene dimer but are small enough that they can be probed in far greater detail with high accuracy electronic structure methods. Parallel-slipped and T-shaped structures of both cyanogen dimer and diacetylene dimer have been optimized with 15 different procedures. The effects of basis set size, theoretical method, counterpoise correction, and the rigid monomer approximation on the structure and energetics of each dimer have been examined. MP2 and CCSD(T) optimized geometries for all four dimer structures are reported, as well as estimates of the CCSD(T) complete basis set (CBS) interaction energy for every optimized geometry. The data reported here suggest that future optimizations of delocalized pi[dot dot dot]pi clusters should be carried out with basis sets of triple-zeta quality. Larger basis sets and the expensive counterpoise correction to the molecular geometry are not necessary. The rigid monomer approximation has very little effect on structure and energetics of these dimers and may be used without consequence. Due to a consistent cancellation of errors, optimization with the MP2 method leads to CCSD(T)/CBS interaction energies that are within 0.2 kcal mol(-1) of those for structures optimized with the CCSD(T) method. Future studies that aim to resolve structures separated by a few tenths of a kcal mol(-1) should consider the effects of optimization with the CCSD(T) method.     

Oxa-bicyclocalixarenes: a new cage for anions via C-H...anion hydrogen bonds and anion...pi interactions

Density functional theory calculations indicate that the cage molecule 4 can trap F- in the gas phase (-80.5 kcal/mol) as well as in CH2Cl2 (-14.7 kcal/mol) via strong C-H...F- hydrogen bonds and pi...F- interaction.     

Interplay of phenyl-perfluorophenyl stacking,C-H...F,C-F...pi and F...F interactions in some crystalline aromatic azines

Analysis of phenyl-perfluorophenyl stacking synthon, C-H...F, C-F...pi interactions, and F...F tetramer in three closely related azine crystal structures shows the dominance of Ar-ArF synthon while other interactions are turned on/off depending on the H/F stoichiometry in the molecule.     

Fluorine... and pi...alkali metal interactions control in the stereoselective amide enolate alkylation with fluorinated oxazolidines (Fox) as a chiral auxiliary: an experimental and theoretical study

The alpha-alkylation of amide enolates by using a pseudo-C(2) symmetry trans 4-phenyl-2-trifluoromethyloxazolidine (trans-Fox) as a chiral auxiliary occurs with an extremely high diastereoselectivity (>99 % de). The origin of this excellent stereocontrol was investigated by an experimental and theoretical (DFT) study. With this trans chiral auxiliary, both F...metal and pi...metal interactions compete to give the same diastereomer through Re face alkylation of the enolate. A 5.5 kcal mol(-1) energy difference found between the Re face and the Si face attack transition states is consistent with the complete diastereoselectivity that has been experimentally achieved. On the other hand, in the case of the cis chiral auxiliary (cis-Fox) the competition between the F...metal and pi...metal interactions is unfavourable to the diastereoselectivity. In this case, the Re face and the Si face attack transition states were found to be nearly isoenergetic (0.3 kcal mol(-1) difference), which is in good agreement with the very low diastereoselectivity observed.     

Reversible anion exchange and sensing in large porous materials built from 4,4'-bipyridine via pi...pi and H-bonding interactions

Two unusual d10 compounds, [Zn2(bipy)3(H2O)8(ClO4)2(paba)2].2(bipy).4H2O (1) and [Cd2(bipy)4(H2O)6(ClO4)2(paba)2].(bipy).5H2O (2) (bipy = 4,4'-bipyridine, paba = p-aminobenzoate), were obtained from reaction of the metal salts, bipy and paba in an EtOH/H2O mixture. The bipy ligands in the two compounds exhibit two new modes of coordinating to transition metal ions, resulting in the formation of large porous frameworks. Immersion of single crystals of 1 in an aqueous solution of NH4PF6 results in the formation of its hexafluorophosphate derivative 3 as shown by single crystal diffraction. Immersion of crystals of 3 in NaClO4 regenerates 1. Furthermore, compound 1 also shows interesting anion sensing properties in an EtOH/H2O mixture.     

In situ cryocrystallization of diphenyl ether: C-H...pi mediated polymorphic forms

Polymorphism in diphenyl ether has been identified during in situ crystallization via single-crystal X-ray diffraction. Only weak inter- and intramolecular C-H...pi interactions control the packing of the molecules in both crystal forms monoclinic centrosymmetric (P21/n) in form I and orthorhombic noncentrosymmetric (P212121) in form II.     

Design of a magnetic bistability molecular system constructed by H-bonding and pi...pi-stacking interactions

Crystal structures and magnetic properties were determined for two novel polymorphs of the complex [H2DABCO][Ni(mnt)2] [(H2DABCO)2+ = diprotonated 1,4-diazabicyclo[2.2.2]octane; mnt2- = maleonitriledithiolate]. For each polymorph, anions form a layered structure in which two kinds of dimers were observed. The adjacent anionic sheets are held together by cations via H-bonding interactions between protons of cations and CN groups of anions. Two polymorphs possess spin bistability; namely, upon cooling, a magnetic transition happens at around 120 K with about 1 K hysteresis on heating for the alpha phase and at 112 K with about 10 K hysteresis for the beta phase. Above the transition, the magnetic behaviors of two polymorphs can be approximately interpreted by a singlet-triplet model of an antiferromagnetically coupled S = 1/2 dimer, which is supported by the crystal structures and spin dimer analyses based on extended Hückel molecular orbital calculations.     

C-H...pi interactions in the low-temperature crystal structures of alpha,omega-unsaturated linear hydrocarbons

The low-temperature crystal structures of 1,7-octadiene (C8H14), 1,9-decadiene (C10H18), 1,7-octadiyne (C8H10) and 1,9-decadiyne (C10H14), determined following in situ crystal growth from the liquid, reveal the role of C-H...pi interactions in alpha,omega-unsaturated linear hydrocarbons in the solid state.     

Coordination complexes exhibiting anion...pi interactions: synthesis, structure, and theoretical studies

The polydentate ligand 2,4,6-tris(dipyridin-2-ylamino)-1,3,5-triazine (dpyatriz) in combination with the Cu(ClO 4) 2/CuX 2 salt mixtures (X (-) = Cl (-), Br (-), or N 3 (-)) leads to the formation of molecular coordination aggregates with formulas [Cu 3Cl 3(dpyatriz) 2](ClO 4) 3 ( 2), [Cu 3Br 3(dpyatriz) 2](ClO 4) 3 ( 3), and [Cu 4(N 3) 4(dpyatriz) 2(DMF) 4(ClO 4) 2](ClO 4) 2 ( 4). These complexes consist of two dpyatriz ligands bridged via coordination to Cu (II) and disposed either face-to-face in an eclipsed manner ( 2 and 3) or parallel and mutually shifted in one direction. The copper ions complete their coordination positions with Cl (-) ( 2), Br (-) ( 3), or N 3 (-), ClO 4 (-), and N, N-dimethylformamide (DMF) ( 4) ligands. All complexes crystallize together with noncoordinate ClO 4 (-) groups that display anion...pi interactions with the triazine rings. These interactions have been studied by means of high level ab initio calculations and the MIPp partition scheme. These calculations have proven the ClO 4 (-)...[C 3N 3] interactions to be favorable and have revealed a synergistic effect from the combined occurrence of pi-pi stacking of triazine rings and the interaction of these moieties with perchlorate ions, as observed in the experimental systems.