7i-stacking

Fujita and coworkers have also reported the encapsulation of multimeric porphyrin assemblies in the box-shaped cavities of ternary Pd6 coordination cages. Two types of cofacial porphine dimers A and B could be stabilized (133). In the smaller [Pd6(L14)2(L15)6]12+ cage 29, whose diameter is 10.4 A (Fig. 21), two porphyrin molecules can be stackedo directly on top of each other with an interplane distance of 3.4 A. In the larger cage 30, an additional molecule of L14 is intercalated between the two porphyrin bases. All complexes were found to be water-soluble in contrast to other 7i-stacked porphyrin dimers. The encapsulation... 

Strong intermolecular interactions between active SCO mononuclear building blocks stem from the presence of efficient hydrogen-bonding networks or 7i-7i stacking interactions and have led to abrupt spin transitions [1], sometimes with associated hysteresis [2-4]. Despite the important efforts made by crystal engineers in establishing reliable connections between molecular and supramolecular structures on the basis of intermolecular interactions, the control of such forces is, however, difficult and becomes even more complicated when uncoordinated counter-ions and/or solvent molecules are present in the crystal lattice. 

Most of the reported X-ray studies have been carried out to establish structures or conformations of different compounds. The X-ray of fully conjugated diene compound 20 has been reported showing supramolecular assemblies resulting in a 7i-stacking interaction between two adjacent molecules <2004TL7363>. The other structures reported in Figure 2 have been determined and parameters described in the corresponding papers. 

An NOE study of the intermediate [Pd(ti -PhCHCHCHPh)(Binap)], 82, thought to be involved in the Pd-catalyzed allylic alkylation of a 1,3-diphenylpropene, revealed that two phenyl rings, one from the auxiliary, D, and one from the substrate, F, are forced to take up parallel positions, i.e, they are 7i-stacked, as shown in 83 [103]. Since the ti-stacking is repulsive, and thus selectively weakens one of the two Pd-C(allyl) bonds, the reaction becomes stereoselective. The D and F rings do not show inter-ligand NOEs. 

An interesting question then arises as to why the dynamics of proton transfer for the benzophenone-A, Al-dimethylaniline contact radical IP falls within the nonadiabatic regime whUe that for the napthol photoacids-carboxylic base pairs in water falls in the adiabatic regime given that both systems are intermolecular. For the benzophenone-A, A-dimethylaniline contact radical IP, the presumed structure of the complex is that of a 7i-stacked system that constrains the distance between the two heavy atoms involved in the proton transfer, C and O, to a distance of 3.3A (Scheme 2.10) [20]. Conversely, for the napthol photoacids-carboxylic base pairs no such constraints are imposed so that there can be close approach of the two heavy atoms. The distance associated with the crossover between nonadiabatic and adiabatic proton transfer has yet to be clearly defined and will be system specific. However, from model calculations, distances in excess of 2.5 A appear to lead to the realm of nonadiabatic proton transfer. Thus, a factor determining whether a bimolecular proton-transfer process falls within the adiabatic or nonadiabatic regimes lies in the rate expression Eq. (6) where ([ (R), the distribution function for molecular species with distance, and k(R), the rate constant as a function of distance, determine the mode of transfer. 

Figure 3. Cofacial 7i-stacked columns of alternating [Fe(Cp )2] cations and 2a anions.

The TT-stacked eolumns are detailed in Figure 7B and are eomprised of alternating sets of one 4a anion to two methylpyridinimn eations, and thus all of the cations in the crystal reside within these 7i-stacked columns. In contrast to the anions comprising the edge-to-edge sheets, the anions here are completely planar (<1 deviation). The face-to-face distance between anions and cations is 3.6... 

In contrast to [MePy][4a], its brominated analogue [MePy][4e] exhibits no extended 7i-stacking. Here each anion has one S -S close contact with a neighboring anion and one Ti-stacking interaction with a methylpyridinium cation. The S -S distance is 3.385 A and the face-to-face distance between anion and cation is 3.4 A. ... 

Figure 13. Short contacts between anions in [Et4N][ll] (A) and 7i-stacked columns of 12 (B).

Fig. 10 Crystal structure of the BN/BQ/AN cocrystal material (BN, red BQ, green AN, yellow) prepared by solid-state grinding. Dotted lines indicate 7i-stacking interactions and hydrogen bonded chains...

Scheme 5 Pre-association mechanism involving 7i-stacking in the decarboxylation of MTh. Reprinted with permission from Reference 50. Copyright 2006 American Chemical Society.

Lewis acid-promoted asymmetric conjugate addition of an allylsilane to a series of 8-arylmenthol-derived Af-acyl-2,3-dihydro-4-pyridoncs (122) has been reported to lead to 2-allyl-4-piperidones with moderate to high levels of asymmetric induction the highest levels were attained with Ar = 2-naphthyl. The stereochemical course of the reaction was attributed to 7i-stacking and the method was applied to the asymmetric synthesis of (—)-/V-mcthylconiinc.95... 

In an example of a [3]-catenane formed by this methodology, the compound 7.64 is formed from the coupling of the copper(i) complex formed from the reaction of [Cu(MeCN)4]+ with one equivalent of 7.63 and one equivalent of 7.65 in 58 % yield (Fig. 7-44) The conformation of the [3]-catenane is partially controlled by intramolecular 7i-stacking interactions (Fig. 7-45). A number of other strategies have also been adopted... 

Control over the structure adopted in solution by selective introduction of appropriate anions has been detailed by Schmuck.22 Compound 25 was synthesized and it was observed that in DMSO discrete dimeric units could be observed by ll NMR and ESI mass spectrometry analysis. In the presence of halide anions, no discemable change was noted, implying that there was no effect upon the relative stability of the dimer. Introduction of picrate anions that have the ability to complete with the 7i-stacking... 

The crystals of radical 29, which are diamagnetic by electron spin resonance (ESR), consist of cofacial dimers with four dimers per unit cell linked by two long, albeit unequal (3.053 and 3.309 A), interannular S-S bond <2005IC1837>. This mode of association has not hitherto been observed for 1,2,3-dithiazolyls. Dimers of radical 29 do not form 7i-stacked arrays instead, they adopt the closed packed herringbone arrangement which allows for a clustering of the radical heads so as to maximize S- -S and S- -N contacts. 

The crystal structure of S-Se-N-based heterocycle benzo[2,l- 3,4- ]bis(l,2,2-th aselenazole) 31 allows for more extensive intermolecular Se—Se contacts <2001IC4705>. The crystal structure of the charge-transfer (CT) salt [31]3[C1C>4]2 consists of slipped 7i-stacks based on the triple-decker closed-shell [31]32+ building block. 

The crystal structure of the gallate salt 48 of the selenium-based cation [49]+ has been determined by single crystal X-ray diffraction <2005IC1837>. Summaries of pertinent intramolecular and intermolecular distances are provided in Table 18. The crystal structure of the salt 48 consists of cations [49]+ and anions [GaCU]- oriented so as to allow a pair of close intermolecular Cl—Se contacts. In addition, there is a third contact (Cl(3)—Se(l )) to the anion located above the molecular plane. The cations, which are planar to within 0.021(4)A, adopt a head-to-tail 7i-stacked arrangement running parallel to the jc-direction. The interplanar separation within the head-to-tail pairs is... 

The crystal structure of the 1,3-ditelluretane derivative 27 shows that the compound is planar <2003TL2397>. The crystal packing, down the crystallographic axis b, reveals the formation of columns through 7I-7I stacking interactions. The distance between these molecules in the columns is 6.41 A. 

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