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

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

The still modest yield (8%) obtained with the hexamethylene linker being far from satisfactory, we explored various other possibilities. Among them a 1,3-phe-nylene spacer, already used with success by others in the construction of helical complexes [99-101], seemed very appealing owing to its expected chemical robustness (necessary to survive the harsh cyclization conditions) and ease of introduction following our methodology [95]. 

The diphenolic bis-chelate 31 was obtained in 55% yield by reacting 2-(p-ani-syl)-l,10-phenanthroline [97] with 1,3-di-lithiobenzene in tetrahydrofuran, followed by hydrolysis, Mn02 oxidation and subsequent demethylation under usual conditions (HCl-pyridine at 210°C for 3 h [97]) 

The structure of 32 appears nicely wound and is therefore well adapted to the formation of a knot by connecting the appropriate ends of the strands. It is noteworthy that 32 is a rigid and compact edifice, with a much shorter Cu - Cu distance than in previously synthesized knots (4.76 A instead of 6.3 or 7 A) [93, 98]. These factors are, of course, very favorable to the successful continuation of the synthesis. Indeed, reaction of the tetraphenolic double helix (whose structure is 

Analogous to the previously synthesized dicopper(I) knots [97], Cu2(K-84)p+ gives characteristic 1H NMR and FABMS spectroscopy data [102], In particular, some of the aromatic protons of the bis-chelates are strongly shielded, which indicates that Cu2(K-84)p is compact and that its helical core is geometrically very similar to that observed in 32. This was fully confirmed by subsequent X-ray structure determination. 

---