Stereoselective Double-Helicate Assembly from Chiral 2, 2': 6', 2' : 6', 2''-Quaterpyridines and Tetrahedral Metal Centres

摘要

A pair of chiral 2, 2': 6', 2" : 6", 2"'-quaterpyridine ligands 1 and 2 have been prepared and shown to diastereoselectively assemble double-helical coordination compounds. the complexation of 1 or 2 with copper (I) or silver (I) leads to dinuclear double helicates [M_2(L)_2]~(2+) with high diastereoselectivity [diastereomeric excess (d.e.) >= 96%] for the formation of P or M helicates. Circular dichroism (CD) studies of the helicates in solution reveal intense activity in the region of 320-328 nm that is diagnostic of P or M helical chirality. On the basis of the CD spectra it is proposed that the dominant diastereomers with 1 possess P helicity, whilst those with 2 possess M helical chirality. These proposals have been confirmed by solid-state structural determinations of the dominant diastereomers of [Cu_2(1)_2][PF_6]_2 centre dot 2MeCN, [Cu_2(2)_2][PF_6]_2 centre dot 2 MeCN and [Ag_2(1)_2[PF_6]_4 centre dot 0.33H_2O. Redistribution reactions of dicopper (I) and disilver (I) helicates lead to the stereoselective assembly of heterodinuclear helicates, whereas the reaction of [Cu_2(1)_2]~(2+) With [Cu_2(2)_2]~(2+) leads to the formation of the heterochiral, heterostranded species [Cu_2(1)(2)]~(2+).