7i-stacking interactions

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. 

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...

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... 

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. 

Figure 1. Competing association events in solution of hosts and guests with large apolar surfaces capable of forming intra-and-extra-cavity 7i-stacking interactions.

The second 7i-stacking interaction in the 11-TENF complex increases its — AG° by 5 kcal mol-1 over that of the 20-TENF complex (Table 4). A simple approach for predicting how large an increase in stability should be expected for the second... 

The hexanuclear cluster cation 78 is formed from reaction of 79 (L = Cl) with T1PF6 or directly from a solution of 79 (L = tetrahydrothiophene) at room temperature over 48 h, and consists of an unusual almost planar arrangement of metal a toms and strong 7i-7i-stacking interactions between the cations. Interestingly, the structure is maintain in solution.56... 

Spin-casting techniques have also been used to prepare imprinted thin films. Makote and Collinson recently prepared metal oxide thin films imprinted with recognition sites for dopamine [18], The dopamine template was loaded at 4 mole % in a sol with a 10 1 ratio of tetramethoxysilane and phenyltrimethoxysilane. The prepared film had a thickness of ca. 450 nm. CV analysis found that 90% of the templates could be removed by washing the films with pH 7 phosphate buffer. The opened receptor sites offered selective binding for related molecules containing catechol amines, such as dopamine, epinephrine and norepinephrine, as determined by CV. The use of phenyltrimethoxysilane turns out to be an essential ingredient for the gel matrix and is believed to provide some complementary affinity for the catechol amines via hydrophobic and/or 7i-stacking interactions. 

An increase in the number of adjacent bpy pairs introduced in DNA or PNA duplexes led to a systematic decrease of the duplex stability, as did the pairing of bpy against either a natural base or an abasic site (16, 108, 109). Brotschi proposed that incorporation of consecutive bpy pairs in adjacent positions in DNA creates a zipper-like interstrand stacking motif with a stretched phosphate backbone and all bpys having their distal rings involved in 7i-stacking interactions (see schematic representation in Entry 4, Table V) (109). 

These amino acids form hydrophobic (water-repelling) nonpolar regions in proteins. There are three more of this kind with special roles. Phenylalanine and tryptophan have aromatic rings and, though they are still hydrophobic, they can form attractive 7i-stacking interactions with other aromatic molecules. Enzyme-catalysed hydrolysis of proteins often happens next to one of these residues. Proline is very special. It has its amino group inside a ring and has a different shape from all the other amino acids. It appears in proteins where a bend or a twist in the structure is needed. 

In 1984, Yamatera and coworkers showed by H-NMR studies that the complex [Ru(phen)3] (4.4) displays self-association in solution.In aqueous media this species gives rise to NMR spectra which are significantly dependent on concentration and display features consistent with 7i-stacking interactions between cations to form dimers (Figure 4.4). 

Idziak and Benrebouh [46] also used 4-vinylpyridine as functional monomer to imprint 17-a-ethynylestradiol—though in this case the optimal imprinting solvent (due to the hydrophobicity of the template) was toluene. An MIA was developed in toluene, and exhibited excellent selectivity (<0.1% cross-reactivity with 17-p-estradiol). Recognition in this case is considered to be through hydrogen bonds and 7i-stacking interactions. 

The diminished enantioselectivity of this substrate is due to the much smaller steric hindrance of methyl groups to compared to phenyl groups and to the absence of 7i-stacking interactions of 14 with the chiral ligands. Ferrocenyl ligands 16 and biarylic 17 have been used, leading to good yields but low stereoselectivities. 

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