Phenylene spacer

The dication 212+ composed of two methylium units connected to a p-phenylene spacer would be a candidate for new Wurster type violene-cyanine hybrid (Figure 12) (15). The reaction of four molar amounts of azulene 6b with terephthalaldehyde yielded the hydro precursor. Synthesis of the dication 212+ was accomplished by hydride abstraction with DDQ in almost quantitative yield. The dication 212+ was expected to show destabilization, but instead it exhibited high thermodynamic stability just like the corresponding monocation 3b+. 

Figure 12. Wurster type violene-cyanine hybrid composed of two methylium units connected via a p-phenylene spacer.

Furthermore, C-H - - O hydrogen bonds are formed between some of the polyether oxygen atoms and the a-bipyridinium protons. A second type of hydrogen bonding interaction, C-H- -77, is observed between the 1,4-dioxybenzene protons and thep-phenylene spacers. In most systems it is very difficult to measure their individual strength and importance in the assembly process. However, it is usually assumed that rr-stacking between complementary aromatic species is the main supramolecular interaction in these systems. 

A Dramatic Improvement - Use of 1,3-Phenylene Spacer Between the Coordinating Units... 

In 1997 Pu reported a new type of main chain chiral polymer derived from BINOLs [24]. Polymer 16 catalyzed enantioselective ethylation using diethylzinc to give secondary alcohols in up to 94% ee. It is noteworthy that 16 is a derivative of chiral BINOL but the addition of Ti(IV) is unnecessary unlike other reported chiral monomeric diols. In 1998, Pu reported that polymer 17, which has a phenylene spacer between two BINOL moieties, results in better ees of up to 98% [24]. 

PBs featuring para-phenylene spacers have also been prepared. Marder reported the synthesis of derivative 16a (related to the ethynyl-bridged compound 5a) by bromine-lithium-boron exchange... 

C. Ambiphilic ligands featuring ortho-phenylene spacers... 

Inherently, the orf/zo-phenylene spacer induces only a weak energy difference between the closed and open forms, but the ground-state structure of all MPB 7b-d was found to be well-defined, with or without P-B interaction. A different situation was observed for the related DPB 8a and TPB 9.24 The corresponding nB NMR signals (<5 = 43.1 ppm for 8a and 50.1 ppm for 9) are in between those of triarylboranes ( 70 ppm) and tetracoordinate derivatives thereof ( 0 ppm), suggesting a rapid equilibrium in solution between the open and closed forms (Figure 12). This hypothesis was further corroborated by low-temperature NMR experiments and DFT calculations. The minima associated with the open and closed forms were found to be almost isoenergetic (AG < 3 kcal/mol at 25 °C). 

This [2]catenane is composed of a jt-electron-deficient tetracationic cyclophane interlocked with a Jt-electron-rich macrocyclic polyether. In addition to a mechanical bond, [jt Jt] stacking interactions between the complementary aromatic units, [C-H---0] hydrogen bonds between the a-bipyridinium hydrogen atoms and the poly-ether oxygen atoms, and [C-H---Jt] interactions between the 1,4-dioxybenzene hydrogen atoms and the p-phenylene spacers in the tetracationic cyclophane hold the two macrocyclic components together and control their relative movements in solution. As a result of the asymmetry of the tetracationic cyclophane, two transla-... 

N., Belser, P., Sonar, P., Grimsdale, A. C., Mtillen, K., Lutz, M., Spek, A.L. and de Cola, L. (2005) Rodlike bimetallic ruthenium and osmium complexes bridged by phenylene spacers ... 

The low yield achieved for the first dicopper knotted 86-membered macrocycle was improved significantly by changing the length of the linker connecting the two chelates and the long functionalised linker used in the cyclisation step [31-33]. The best yields for the cyclisation reaction were around 8%. The use of a 1,3-phenylene spacer between the coordinating units caused a great improvement [34]. 

Os and Ru bis(terpyridine) complexes have been covalently linked (Figure 13) and their electrochemical, absorption spectra, photochemical and photophysical properties have been studied. The strong electronic interaction decreases as the one or two phenylene spacers are added but remains large enough to allow fast electron transfer. ... 

As an aside, it is noted that the latter may not be a hard and fast rule, at least when other ligand types are present. Sauvage, Balzani et alP used NMR data as well as absorption and fluorescence results to demonstrate the protonation-driven formation of a double-helical structure involving two ligand strands derived from a pair of 2-/ -anisyl-l,10-phenanthroline units linked in their 9-positions by a 1,3-phenylene spacer. In this case, the first protonation step does not simply represent mono-protonation of one of the phenanthroline units but rather appears to involve a co-operative interaction with a pair phenanthroline units (on different strands). 

A comparison between dyad (18) and dyad (21) demonstrates that the phenylene group is a better conducting spacer (see Section 3.1.3 for a caveat on this colloquial terminology, however) as compared to methylene despite the longer metal metal distance, the reaction is faster across the phenylene spacer (k = 3.0 x 10 s )... 

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