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.