Helical complexes of chiral quaterpyridines - Mononuclear Cu^II and dinuclear Cu^I complexes

Two series of chiral helical copper-quaterpyridine complexes were prepared by reacting chiral ligands L1-4 with Cu^I and Cu^II precursors. The Cu^II complexes, [Cu(L)(H₂O)(ClO ₄)₂] or [Cu(L)(ClO₄)₂], were studied by ESI-MS, elemental analysis, and CD spectroscopy, and the Cu^I complexes, [Cu₂(L)₂](PF₆)₂, were examined by NMR, ESI-MS, elemental analysis, and CD spectroscopy. The crystal structures of [Cu(L1)(H₂O)(ClO₄)](ClO₄) and [Cu(L3)(ClO₄)₂] were determined by X-ray diffraction. They are monomeric, distorted octahedral complexes with the quaterpyridine ligand coordinated at the four equatorial positions and one or two perchlorates at the apical position. The crystal structure of [Cu(L1m)(H₂O)(ClO ₄)](ClO₄), which has an achiral quaterpyridine ligand L1m, the meso form of L1, was also determined. Comparison between [Cu(L1)(H ₂O)(ClO₄)](ClO₄) and [Cu(L1m)(H ₂O)(ClO₄)](ClO₄) shows small differences between the two structures. CD analyses of mononuclear Cu^II complexes of sterically demanding L2-4 indicate that these structures require a helical twist in the ligand. The crystal structure of [Cu₂(L3) ₂](PF₆)₂ shows that it is a two-metal two-ligand double-stranded helical structure. Solution NMR studies of the dinuclear Cu^I complexes show that each sample consists of two diastereomers with moderate to high diastereoselectivity (52-99 %). With L1m, head-to-head and head-to-tail configurational isomers were formed in an almost 1:1 ratio. Heterostranded helicates could also be formed with two different chiral ligands but selectivity favors the homostranded one. DFT calculations on the homostranded helicates yield evidence that interactions between the chiral groups of the ligand strands in the helicate are not solely responsible for determining the helical chirality of the Cu^I complexes.