Diastereoselective Assembly of Helicates Incorporating a Hexadentate Chiral Scaffold

The condensation of enantiopure (1 R ,2 R )‐(–)‐1,2‐diaminocyclohexane with two equivalents of 2,2′‐bipyridine‐6‐carbaldehyde followed by reduction with NaBH 4 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).