Formation of bis- and tris[2]catenanes via the cross-catenation of Pd(II)- and Pt(II)-linked coordination rings

We have demonstrated the self-assembly of linear oligo[2]catenanes via selective cross-catenation. A Pd(II)-linked double looped molecule 1 was transformed into the cyclic trimer c-(1)3 through the catenation. When 1 was treated with a kinetically inert Pt(II)-linked single looped molecule 2a in aqueous media (1:1.2 DMSO/H2O) at room temperature, linear oligo[2]catenanes of 2a–(1)n–2a (n=1 and 2) were selectively obtained, because the kinetically inert Pt(II)-linked ring 2a is allowed to thread on only kinetically labile Pd(II)-linked ring of 1. The distribution of the oligomers depends on the monomer ratio of 1 to 2a. When the ratio of 1 to 2a was 1:2, bis[2]catenane 3a (2a–1–2a) was quantitatively assembled. When the ratio of 1 to 2a was 1:1, not only 3a but also tris[2]catenane 4 (2a–1–1–2a) was assembled. The ratio of 3a to 4 was carefully determined to be 1:1 by NMR. The lengths of 3a and 4 in an extended conformation were estimated by MD/MM2 simulation to be 3.6 and 5.4 nm, respectively.