Total Spontaneous Resolution of Chiral Covalent Networks from Stereochemically Labile Metal Complexes

Abstract

Stereochemically labile copper and zinc complexes with the N,N'-dimethylethylenediamine ligand (dmeda) have been shown to be promising precursors for the total spontaneous resolution of chiral covalent networks. (N,N')-[Cu(NO3)2(dmeda)]infinity crystallises as a conglomerate and yields either enantiopure (R,R)-1 or enantiopure (S,S)-1. A mixed-valence copper(I/II) complex, [{Cu(II)Br2(dmeda)}3(Cu(I)Br)2]infinity (2), which crystallises as a pair of interpenetrating chiral (10,3)-a nets, is formed from CuBr, CuBr2 and dmeda. One net contains ligands with solely (R,R) configuration and exhibits helices with (P) configuration while the other has solely (S,S)-dmeda ligands and gives rise to a net in which the helices have (M) configuration. The whole crystalline arrangement is racemic, because the interpenetrating chiral nets are of opposite handedness. With zinc chloride (R,S)-[ZnCl(dmeda)2]2[ZnCl4] (3) is obtained, which is a network structure, although not chiral. Total spontaneous resolution of stereochemically labile metal complexes formed from achiral or racemic building blocks is suggested as a viable route for the preparation of covalent chiral networks. Once the absolute structure of the compound has been determined by X-ray crystallography, a quantitative determination of the enantiomeric excess of the bulk product can be undertaken by means of solid-state CD spectroscopy.

The use of X‐ray crystallography to determine absolute configuration

“…For compounds that racemize rapidly in solution, solid-state CD in a KBr disk may be applied to the bulk compound and perhaps even to a powdered single crystal. [23][24][25] One may expect vibrational CD, either IR or Raman, to be used increasingly in the future. Enantioselective chromatography (EC): This sensitive technique is applicable to microgram quantities and provides estimates of the enantiomeric excess.…”

Non-racemic (scalemic) planar-chiral five-membered metallacycles: routes, means, and pitfalls in their synthesis and characterization

“…For compounds that racemize rapidly in solution, solid-state CD in a KBr disk may be applied to the bulk compound and perhaps even to a powdered single crystal. 87 One may expect vibrational CD, either IR or Raman, to be used increasingly in the future.…”

Synthesis, structures and properties of two new chiral rare earth-organic frameworks constructed by l/d-tartaric acid

“…First of all, it is also the most straightforward and reliable way to employ chiral bridging ligands to connect metal ions or secondary building units [22][23][24][25], compounds formed by enaniomerically pure L/D-tartrate are illustrative of this approach [26][27][28][29]. The other two strategies is to use chiral templates that do not participate in the structure but finally forming chiral frameworks [30,31], and to use achiral ligands but crystallizing in chiral space groups [32][33][34].…”