From homoleptic to heteroleptic double stranded copper(I) helicates: The role of self-recognition in self-assembly processes

The ligands 2,9-bis[(6-methyl-2,2'-bipyridin-6'- yl)methyleneoxymethylenyl]-1,10-phenanthroline (6), 6'',6'''-bis[(6-methyl- 2,2'-bipyridin-6'-yl)methyleneoxymethylenyl)]-2'',2'''-bipyridine (2), 5,5'- bis[(6-methyl-2,2'-bipyridin-6'yl)methyleneoxymethylenyl]-2,2'-bithiophene (7), and 6,6'-bis[(6-methyl-2,2'bipyridin-6'-yl)methyleneoxymethylenyl]-2,2'- biphenyl (8) and their respective homo- and heteroleptic double-stranded copper(I) complexes were prepared and characterized in order to estimate the importance of self-recognition in the self-assembly processes of double- stranded copper complexes. The homoleptic double-stranded copper complexes of 2, 6, 7, and 8 were characterized by NMR, FAB-MS, and electrochemistry. It was found that 6 and 2 each form a single double-stranded helicate having the structure of [(L)₂Cu₃]³⁺ (L = 2 or 6), 7 forms two double-stranded [(7)₂Cu₃]³⁺ complexes, and 8 results in a mixture of at least two [(8)₂Cu₂]²⁺ complexes. The potential shift, ΔE°, of the Cu⁺/Cu²⁺ redox process of these complexes reflects the binding affinity of the different binding sites to the copper cation. The electrochemical data show that the central units have a higher affinity to Cu⁺ as compared to the off- center binding sites. NMR was used to determine the actual complex composition obtained from different mixtures of 2, 6, or 7 with Cu⁺. Interestingly, we have found that, although 6, 2, and 7 each form homoleptic double-stranded complexes, no heteroleptic double-stranded copper complexes were formed from the mixtures of 7 with either 6 or 2. However, when mixtures of 6 and 2 are used, helicate distributions seem to follow simple statistics. These results are discussed in terms of the relative importance of self- recognition in the self-assembly of double-stranded helicates.