Homochiral triple helicate

Scheme 5 Formation and schematic representation of homochiral triple helicate (A,A)-[FezL 3] 48 and (A,A)-[Fe2L2 3] 49.

Scheme 6 Formation and schematic representation of homochiral triple-helical hexaanions...

Spontaneous self assembly of a dinuclear triple helical complex is observed with linked bis-[4,5]-pineno-2,2 -bipyridines. Studies by electrospray mass spectrometry, CD and NMR determined that the major species in solution was a complex of Zn L = 2 3 stoichiometry with a triple helical structure and an enantiomerically pure homochiral configuration at the metal centers. The preference for the formation of one of the possible stereoisomers over the other is of interest.265 Another binuclear triple helical complex is formed from zinc addition to bis[5-(l-methyl-2-(6-methyl-2 -pyridyl)benzimidazolyl)]methane. Spectrophotometric titrations with a zinc solution... 

From CD and NMR measurements, these were assigned triple-helical structures in which enantiomerically pure, homochiral configurations occur at the metal centres. In effect, the chirality associated with the pinene fragments appears to direct the assembly process such that only one isomer of the resultant dinuclear... 

An X-ray crystallographic analysis revealed that 64 is a 2 -metallacryptand ( = triple helicate) (Figure 29). Each of the two iron centers is octahedrally surrounded by six oxygen atoms. In the chiral, racemic complex 64, both iron centers are identically coordinated. Therefore, the 2 -metallacryptand is either a (A,A)- c or A,A)-fac triple helicate. The crystals obtained are composed of the homochiral 2 -metallacryptand 64. The donating power within the interior of the complex is insufficient for the complexation of alkali cations. Above all, the three phenyl hydrogen atoms in 64 are directed inward towards the empty cavity in the center. 

Surprisingly little work has been carried out on the resolution of homochiral helicates into the two enantiomers. Self-resolution upon crystallization has been observed for two homonuclear triple helicates [37,38], but there seem to be only two well-authenticated cases of enantiomeric resolution, both using antimonyl tartrate the complex [ 02(9)3] " , a dinuclear triple helix [39], and a trinuclear double helical complex of iron(ll) [Fe3(19)2] " with a tm-terpyridyl ligand [40]. The circular dichroism spectrum of [ 02(9)3] " is shown in Figure 13. 

Figure 10.82 (a) Electron microscope images of materials formed by PLP + ULU (right-handed helix), (b) PDP + UDU (left-handed) and (c) PMP -I- UMU (nonhelical), (d) Schematic representation of the model for the triple-helical organisation of three self-assembled homochiral polymer strands (PTP, UTU)n (T = L or d not m). PU hydrogen-bonded pairs on the same strand are represented by circles of the same shading. Alkyl chains stick ont of the side of the cylinder. (Reproduced with permission from J.-M. Lehn, Supramolecular Chemistry, VCH Weinheim, 1995.)... 

More recently Hahn and coworkers reported the synthesis of several homochiral triple-stranded helicates of Ti(IV) using bis bidentate (benzene-o-dithiolate) ligands which are the thiol version of Raymond tetradentate ligands. 

Williams and coworkers reported the synthesis of homochiral triple-stranded helicate of Co(II) (5.15) with a relatively rigid bis(bidentate) ligand 13 flanked by a methylene group (Scheme 5.6). The latter can be used as a probe group in H-NMR spectroscopy to determine whether a homochiral or a heterochiral arrangement has been obtained. 

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