Chiral tether-mediated stabilization and helix-sense control of complementary metallo-double helices

Abstract

A series of novel Pt^II-linked double helices were prepared by inter- or intrastrand ligand-exchange reactions of the complementary duplexes composed of chiral or achiral amidine dimer and achiral carboxylic acid dimer strands joined by trans-Pt^II–acetylide complexes with PPh₃ ligands using chiral and achiral chelating diphosphines. The structure and stability of the Pt^II-linked double helices were highly dependent on the diphosphine structures. An interstrand ligand exchange took place with chiral and achiral 1,3-diphosphine-based ligands, resulting in trans-Pt^II-bridged double helices, whose helical structures were quite stable even in dimethyl sulfoxide (DMSO) due to the interstrand cross-link, whereas a 1,2-diphosphine-based ligand produced non-cross-linked cis-Pt^II-linked duplexes, resulting from an intrastrand ligand-exchange that readily dissociated into single strands in DMSO. When enantiopure 1,3-diphosphine-based ligands were used, the resulting trans-Pt^II-bridged double helices adopted a preferred-handed helical sense biased by the chirality of the bridged diphosphines. Interestingly, the interstrand ligand exchange with racemic 1,3-diphosphine toward an optically-active Pt^II-linked duplex, composed of chiral amidine and achiral carboxylic acid strands, was found to proceed in a diastereoselective manner, thus forming complete homochiral trans-Pt^II-bridged double helices via a unique chiral self-sorting.