Towards a Generalized Synthetic Strategy for Variable Sized Enantiopure M4L4 Helicates

Abstract

The reliable and predictable synthesis of enantiopure coordination cages is an important step towards the realization of discrete cages capable of enantioselective discrimination. We have built upon our initial report of a lantern-type helical cage in attempts to expand the synthesis into a general approach. The use of a longer, flexible diacid ligand results in the anticipated cage [Cu4(L1)4(solvent)4] with a similar helical pitch to that previously observed and a cavity approximately 30% larger. Using a shorter, more rigid ligand gave rise to a strained, conjoined cage-type complex when using DABCO as an internal bridging ligand, [{Co4(L2)4(DABCO)(OH2)x}2 (DABCO)]. The expected paddlewheel motif only forms for one of the Co2 units within each cage, with the other end adopting a “partial paddlewheel” with aqua ligands completing the coordination sphere of the externally facing metal ion. The generic approach of using chiral diacids to construct lantern-type cages is partially borne out, with it being apparent that flexibility in the core group is an essential structural feature.