Spontaneous resolution of heterometallic complex: cis-[Co(NH3)4(H2O)2][Fe(ox)3]·2H2O – A rare example for labile iron(III) complex

The new conglomerate forming compound of the formula cis-[Co(NH3)4(H2O)2][Fe(ox)3]·2H2O (1) (ox = C2O42−) was synthesized and structurally, spectroscopically and magnetically characterized revealing weak antiferromagnetic couplings. Two forms of (1) crystallizing in hexagonal enantiomorphic space groups, P6522 and P6122, were identified proving that spontaneous resolution of the racemic mixture occurred at the crystallization stage, because chiral (1a) and (1b), respectively, were obtained from the same synthesis using racemic K3[Fe(ox)3]·3H2O substrate. It is rare example of conglomerate formation with labile Fe(III) ion as a coordination center. In packing along z axis for (1a) and (1b) structures we observe pattern with iron and cobalt complexes as well as water molecules arranged in separated helices, left-handed for (1a) and right-handed for (1b). Analysis of the intermolecular contacts formed by the “false block” with inverted chirality placed in the homochiral network revealed steric hindrance as well as disruption of approximately 50% of hydrogen bonds inside a helix and between adjacent helices. This analysis was supported by calculations performed using gaussian-03 run with DFT B3LYP potential using 6–31+G(d,p) basis. They revealed significantly more stable structure for the true block due to disruption of several hydrogen bonds and steric hindrance introduced by the false block. Hence, the helices imposed by chiral [Fe(ox)3]3− blocks are a template for supramolecular structure, which reveals achiral water molecules and Co(III) moieties forming helices of the same handedness as iron blocks. These Fe(III) moieties are trapped inside tightly packed crystal network, which is maintained by the robust hydrogen bond network. In this structure Co(III) blocks transmit interactions between adjacent helices. Analysis of intermolecular interactions network allows also for deeper insight into a nature of light- and/or time-catalyzed degradation as well as spontaneous resolution processes.

The assembly of chiral [Fe(ox)3]3− blocks forming helices (arranged of [Δ-Fe(ox)3]3− and [Λ-Fe(ox)3]3− units in (1b) and (1a), respectively) and acting as a template for the whole crystal network is the pivot point in chiral recognition. The cobalt(III) moieties and water molecules follow pattern imposed by iron(III) units and are responsible for transmission of the helical arrangement across the crystal network.Figure optionsDownload as PowerPoint slide