Centrosymmetric dimer

The 1-alkoxytitatranes can be synthesized by the reaction of equimolar amounts of tetraalkyl titanates and triethanolamine (105). X-ray crystallographic analysis of the soHd isolated from the reaction of one mole of triethanolamine and one mole of TYZOR TPT confirms the stmcture as a centrosymmetric dimer having a Ti isopropoxy nitrilotriethoxy ratio of 1 1 1. The titanium atoms have achieved a coordination number of six via a rather unsymmetrical titanium—oxygen bridge (106). 

Examples of the C2N3S ring system containing three- or four-coordinate sulfur are also well known. The monohalogenated derivatives 12.2 (E = S, Se), are best prepared by the condensation of imidoyl amidines with SCI2 or SeCU, respectively (Scheme 12.1)." In the case of the selenium derivative, the initial product is heated at 60°C and then at 120°C in order to convert it to 12.2 (E = Se) via (PhC)2(NH)N2SeCl2. In the solid state this intermediate is a weakly associated, centrosymmetric dimer with Se-Cl and Se Cl distances of 2.42 and 3.39 A, respectively. ... 

Figure 11.19 Schematic diagram of the centrosymmetric dimer in [Cii2(NO ).i(py) lpy showing the two bridging nitrato groups each coordinated to the 2 Cii atoms by a single O atom the dimer also has an unsymmetrical bidentate nitrate group on each Cu.

Reaction of Ph2PLi with Pr N = C = NPr in THF proceeds via insertion of the carbodiimide into the Li-P bond, affording the lithium phosphaguanidinate salt Li[Ph2PC(NPr )2] in 72% yield. The preparation and reactivity of this new ligand are summarized in Scheme 217. An X-ray crystal structure analysis of the product obtained after removal of the solvent from the reaction mixture revealed the presence of a mono-solvated, centrosymmetric dimer in the solid state (Figure... 

Sila-procyclidine, (cyclohexyl)phenyl[2-pyrrolidin-l-yl]silanol, may be prepared by hydrolysis of the corresponding methoxysilane (220) and is interesting in that it can form two types of hydrogen-bonded structure depending on whether it is enantiomerically pure or a racemate. In the racemate, the compound forms centrosymmetric dimers of (R)- and (S)-configuration molecules with an 0---N distance of 1.791 A. In the pure (R)-compound, however, the molecules are linked into infinite chains via intermolecular 0-H---N hydrogen bonds (0---N distance 2.792 A) (221), again similar to those in (2-morpholinoethyl)diphenylsilanol shown in Fig. 3. 

Sila-biperidin rac-(SiRS, C2 SR), 35, can be prepared by hydrolysis of the corresponding methoxysilane and behaves in a similar way to sila-procyclidine The exo form has centrosymmetric dimers formed via a pair of intermolecular OH---N hydrogen bonds (0---N distance 2.810 A), while the endo form has intermolecular OH---N hydrogen bonds linking the molecules into infinite chains (222). 

Substitution of the dimethylsilyl group by bis(tert-butyl)-stannyl does not change the structure in solution, e.g. 33 is found to be monomeric. A very interesting dimer is 26. In contrast to the centrosymmetrical dimer of 1 (C-Symmetry), 26 has a twofold axis (C2, see Fig. 9). This special structure may be due to intramolecular Lewis acid-base interactions between the boron and nitrogen atoms 39). Nevertheless,... 

The structure of the 1 1 methanol solvate of olanzapine has been reported, where pairs of olanzapine molecules form a centrosymmetric dimer by means of C—H—-7t interactions [66]. The solvent molecule was linked to the drug substance through O—H-N, N—H O, and C—interactions. In a new polymorph of the 1 1 dioxane solvatomorph of (+)-pinoresinol, the structure was stabilized by O—H O hydrogen bonds between the compound and the solvent [67], Two new polymorphs of 2-cyano-3-[4-(/Y,jV-diethylamino)-phenyl]prop-2-enethioamide and its acetonitrile solvatomorph have been characterized [68], Although crystallization of the title compound was conducted out of a number of solvents, only the acetonitrile solvatomorph could be formed. 

The product is a centrosymmetric "dimer of dimers" each half... 

Internal amino64 or imino65 67 chelate functions can achieve this type of selenenyl iodide stabilisation. 2-(4,4-Dimethyl-2-oxazolinyl)benzeneselenenyl iodide66 exhibits weak iratermolecular Se- -I contacts (372.5 pm) leading to a centrosymmetric dimer with longer N -> Se bonds (213.3 pm) and shorter Se-I bonds (277.7 pm) [Figure 19(a)] compared to the strictly monomeric 4-ethyl-substituted isomer (Se-N 207.4).65 The structure is related to the dimer of the T-shaped cyano derivative [Figure 19(b)].68... 

X-ray studies confirm Bi2(ju,-OR)2 centrosymmetric dimers 4 in the solid state (24). Terminal alkoxides are generally bound more tightly than bridging units [terminal Bi-0 2.064(7) A bridging Bi-0 2.188(7) and 2.688(7) A], The bismuth oxide alkoxide cluster compounds [Bi6(p,3-07)(p,3-OC6F5) Bi(OC6F6)4 3(sol)2] [sol]2 (sol = thf or toluene) were also isolated as minor products (25). Both structures contain the unusual octahedral I Bi, (/x3-07 X/x3-OCeF.-) I" cores 5, which... 

It is well-known, that in most carboxylic acid crystal structures, the conformation of the carboxyl group is synplanar and that the most frequent and dominant interlink is the syn-syn centrosymmetric dimer synthon, II. This synthon is found in nearly a third of all crystalline carboxylic acids, with or without any other functionality being present in the molecule, signifying robustness.1211 Against such a background, the crystal structures of 1,4-cubane-... 

In the course of investigation of potential antitumor compounds, the triazolobenzodithiazine derivative 37 seemed of special interest (Scheme 4). As no unambiguous proof could be provided by spectroscopic data (cf. Section 11.19.6.) X-Ray analysis of the crystalline product has been carried out <2003BMC1259>. The results revealed that in crystals the molecules formed centrosymmetric dimers by the help of weak NH- N hydrogen bonds. Because of the conformation of the central six-membered ring, the tricyclic molecule had a butterfly shape. 

An asymmetric photosynthesis may be performed inside a crystal of -cinnamide grown in the presence of E-cinnamic acid and considered in terms of the analysis presented before on the reduction of crystal symmetry (Section IV-J). We envisage the reaction as follows The amide molecules are interlinked by NH O hydrogen bonds along the b axis to form a ribbon motif. Ribbons that are related to one another across a center of inversion are enantiomeric and are labeled / and d (or / and d ) (Figure 39). Molecules of -cinnamic acid will be occluded into the d ribbon preferentially from the +b side of the crystal and into the / ribbon from the — b side. It is well documented that E-cinnamide photodimerizes in the solid state to yield the centrosymmetric dimer tnixillamide. Such a reaction takes place between close-packed amide molecules of two enantiomeric ribbons, d and lord and / (95). It has also been established that solid solutions yield the mixed dimers (Ila) and (lib) (Figure 39) (96). Therefore, we expect preferential formation of the chiral dimer 11a at the + b end of the crystal and of the enantiomeric dimer lib at the —b end of the crystal. Preliminary experimental results are in accordance with this model (97). 

It should be noted that there are special features of the 9-substituted anthracenes that may introduce complications. The materials dimerize also in solution, yielding exclusively the centrosymmetric dimers, which are analogous to 87. Further, the few 9,9 -disubstituted dianthracenes that have been prepared are very unstable. Thus the head-to-head approach of two monomers is apparently an energetically unfavorable process. This is not so for the head-to-tail approach, and it is therefore not surprising that structures in which there are overlapped monomers related by centers of symmetry yield topochemical dimers. 

The first and thus far only indasilsesquioxane derivative was synthesized in our laboratory. The compound [Cy7Si70i2lnMe(0H)]2 (55) was prepared in high yield (84%) by reacting trisilanol 3 with trimethylindium in toluene solution at 20 °C (Scheme 18). The molecular structure of 55 has been determined by X-ray diffraction. Self-assembly under formation of a centrosymmetric dimer occurs through... 

---