Stereoselective formation of helical binuclear metal complexes : Synthesis, characterization, and crystal structures of chiral bis-rhenium(I) quaterpyridine complexes

A series of single-stranded helical Re(I) complexes, of formula [Re₂(L)Br₂(CO)₆], were prepared by reacting [Re(CO)₅Br] with chiral quaterpyridines L1−4. By ¹H and ¹³C NMR analysis, all the crude complexes consisted of a pair of stereoisomers. Sterically more demanding ligand L4 induced a higher ratio (80:20) than L1−3 (about 56:44). Stereochemically pure complexes could be obtained by recrystallization in CH₂Cl₂. X-ray crystallographic analysis of single crystals from purified complexes showed a single-stranded helical structure with a bridging ligand wrapped around two distorted octahedral rhenium metals, both of which possessed one bromide ligand and three carbonyl ligands in a fac coordination. The helical core is established by extensive noncovalent electrostatic interactions and chiral information is transmitted from the ligand to the helix through these interactions. Solution behavior was studied by CD spectroscopy, and the strong Cotton effect confirms the integrity of the helical structure in solution. The diastereoselectivity of helicates is proposed to be induced by steric interaction between the bromine atom and the substituent of the bridging ligand.