Diastereoselective Assembly of Helicates Incorporating a Hexadentate Chiral Scaffold

The condensation of enantiopure (1R,2R)-(-)-1,2-diaminocyclohexane with two equivalents of 2,2'-bipyridine-6-carbaldehyde followed by reduction with NaBH4 allows the facile synthesis of the enantiopure hexadentate ligand (R,R)-3. Each tridentate metal-binding domain in (R,R)-3 exhibits a degree of flexibility arising from inversion at the amine nitrogen centre and rotation about N-C and C-C single bonds. This leads to the formation of either M-[M{(R,R)-3}](2+) or P-[M{(R,R)-3}](2+). The solid-state structures of the copper(II), iron(II) and zinc(II) complexes of (R,R)-3 all show a preference for M-[M{(R,R)-3}](2+). In solution, [Fe{(R,R)-3}](2+) exists predominantly as one diastereoisomer (assumed to be the M-form). The preference for the M- over P-form is rationalized in terms of a favourable anti configuration of the NH and cyclohexane-1,2-diyl CH hydrogen atoms in the M-form, and an unfavourable syn-configuration in the P-form. When (R,R)-4, the Schiff base analogue of (R,R)-3, combines with zinc(II) or silver(I), [2+2] double helicates with M-chirality assemble in the solid state. With (S,S)-4, iron(II) also assembles into a dinuclear, double helicate with M-handedness. Complete stereoselectivity is observed in solution with NMR spectroscopic data indicating the presence of one diastereoisomer. The preference for a dinuclear over mononuclear helicate can be traced to a decrease in ligand flexibility on going from 3 (saturated backbone) to 4 (unsaturated imine backbone).