Homochiral transition-metal camphorate coordination architectures containing “piperazine–pyridine” ligands

Abstract

A total of five homochiral metal–organic coordination polymers (MOCPs) showing high thermal stability have been assembled from the reaction of divalent transition-metal ions (Co²⁺, Zn²⁺, and Cd²⁺), rigid enantiopure D-camphoric acid (D-H₂Cam), and achiral neutral N,N′-bis(pyrid-4-yl)piperazine (bpypip) ligand under hydro(solvo)thermal conditions. Compound [Co₂(D-Cam)₂(bpypip)] (1) features a chiral-layered cobalt–camphorate coordination (4,4)-net, which is pillared by auxiliary linear N-tethering bpypip linkers to yield a homochiral three-dimensional coordination framework with Schläfli symbol of 4⁴·6¹⁰·8. Compounds [M(D-HCam)₂(bpypip)] (2, M = Cd; 3, M = Zn) have nearly identical one-dimensional M–bpypip zigzag coordination-chain structures, in which each metal ion further attaches to two partially deprotonated monoanionic D-HCam terminals, which exhibit two types of partially deprotonating manner in 2 but only one type in 3. Through chain-to-chain C(O)OH⋯O(carboxylate) hydrogen bonds, the zigzag coordination-chain becomes extended to a three-dimensional uninodal 6-connected supramolecular network, which can be considered as a result of a M–D-HCam-based 4-connected supramolecular 6⁵·8-net intersected by M–bpypip-based zigzag coordination-chains. Of particular interest, the supramolecular assembly contains a couple of single-stranded right- (SRH) and left-handed (SLH) supramolecular helices and a couple of double-stranded right- (DRH) and left-handed (DLH) supramolecular helices, and therefore exists as a meso-net in topology, i.e., there is no spatial chirality. Compounds [M₂(OH)(OAc)(D-Cam)(bpypip)] (4, M = Co; 5, M = Zn) have identically homochiral ladder-like chain structures having meso-M₂ units and presenting three kinds of different anions, i.e., hydroxyl, acetate, and D-Cam. The obtained acetate anion may be generated from the hydrolyzation of dimethylacetamide (DMAc) under hydro(solvo)thermal conditions.