Spontaneous enantiomorphism in poly-phased alkaline salts of tris(oxalato)ferrate(III): crystal structure of cubic NaRb5[Fe(C2O4)3]2

The phenomenon of spontaneous resolution of enantiomers occurs during the crystallization of the sodium and rubidium double salts of the transition metal complex tris(oxalato)ferrate(III).


Chemical context
Chirality is the structural property by which a molecule or ion cannot be superposed upon its mirror image through translation and proper rotation operations. This concept along with the related ones of chiral crystal structures and space groups is discussed by Flack (2003). Chirality is at the core (among other research areas) of the not yet understood origin of the biomolecular asymmetry of life (Meierhenrich, 2008), enantioselective chemical reactions (Knowles, 2001;Noyori, 2001;Sharpless, 2001), biological activity of pharmaceuticals (Nguyen et al., 2006) and in the design of multifunctional solid-state materials endowed with optical activity and longrange magnetic order (Coronado et al., 2003) and also in the understanding of the physical properties of chiral liquid crystals and their tailoring for applications in opto-electronic devices (Goodby, 1998;Coles, 1998).
While attempting to crystallize the rubidium salt of the tris(oxalato)ferrate(III) transition metal complex, one of the preparations segregated into a poly-phased crystal system. It contained the intended Rb 3 [Fe(C 2 O 4 ) 3 ]Á3H 2 O compound (monoclinic P2 1 /c), which turned out to be isotypic to the reported potassium salt (Junk, 2005;Piro et al., 2016), and the triclinic (P1) Rb(C 2 O 4 H)(C 2 O 4 H 2 )Á2H 2 O salt (Kherfi et al., 2010), which is isotypic to the ammonium analogue (Jarzembska et al., 2014). A third phase consisted of large green crystals of a new cubic (P4 3 32) NaRb 5 [Fe(C 2 O 4 ) 3 ] 2 salt. Interestingly, the isotypic counterpart of this salt where rubi- ISSN 2056-9890 dium is replaced by potassium has been reported by Wartchow (1997) to appear in a mixture with crystals of the monoclinic K 3 [Fe(C 2 O 4 ) 3 ]Á3H 2 O salt, hence confirming the tendency of potassium and rubidium alkaline ions to form isotypic crystal analogues (Piro et al., 2016). Curiously, in a previous work, Henneicke & Wartchow (1997) reported the chiral counterpart of the cubic NaK 5 [Fe(C 2 O 4 ) 3 ] 2 salt, which crystallizes in the space group P4 1 32. This prompted us to search for the chiral rubidium analogue in the very same batch as the singlecrystals that solved in the space group P4 3 32 NaRb 5 -[Fe(C 2 O 4 ) 3 ] 2 . By chance, we picked a single crystal and submitted it to X-ray diffraction scrutiny to find that it now belonged to the chiral space group P4 1 32. This strongly suggests that the NaM 5 [Fe(C 2 O 4 ) 3 ] 2 (M = K, Rb) crystal samples could be racemic conglomerates generated by spontaneous resolution, a rare event discovered by Louis Pasteur in 1848 (Pasteur, 1848a,b) in a famous experiment in which he hand-sorted the chirally resolved crystals of sodium ammonium tartrate tetrahydrate on the basis of their observed morphology and then examined their respective solutions with a polarimeter to find opposite rotations of the plane of light polarization (Flack, 2009). Recently, we found that the phenomenon could also have occurred in isotypic [M(Lap) 2 ] n (M = Cd, Mn; HLap = 2-hydroxy-3-(3-methyl-2-butenyl)-1,4naphtoquinone, C 15 H 14 O 3 ) complexes whose enantiomers crystallize in the tetragonal and enantiomorphic space groups P4 3 2 1 2 and P4 1 2 1 2 (Farfá n et al., 2015).  (Farrugia, 2012) drawing of the P4 3 32 enantiomer of the title compound. Bond lengths and angles around iron(III) and within the oxalate dianion are listed in Table 1 and contact distances around the alkali ions are shown in Table 2. All metal ions are at crystallographic special positions while the oxalate anion is on a general position. The iron(III) ion is on a threefold axis, C 3 point group symmetry (Wyckoft c site), in an octahedral environment (FeO 6 core). It is coordinated to three, symmetry-related, oxalate anions acting as bidentate ligands through the oxygen atoms of their opposite carboxylic groups in a propeller-like conformation and along one electron pair lobe on each oxygen ligand. The FeO 6 bond geometry and metrics are consistent with the oxalate being a weak-field ligand that gives rise to the highspin (S = 5/2) electronic ground state exhibited by the complex, as probed by magnetic susceptibility (Delgado et al., 2002) and ESR spectroscopy (Collison & Powell, 1990) (7) O22-C2-C1 121.1 (6) O11-Fe-O11 i 88.7 (2) O21-C2-C1 113.6 (5) O11-Fe-O21 i 106.2 (2) O11-Fe-O21 ii 160.9 (2) Symmetry codes: (i) Àz + 1 2 , Àx + 1, y À 1 2 ; (ii) Ày + 1, z + 1 2 , Àx + 1 2 .

Figure 1
View of NaRb 5 [Fe(C 2 O 4 ) 3 ] 2 showing the atom labels and displacement ellipsoids at the 50% probability level. For clarity, only the minimum number of oxygen ligands around each metal ion has been labelled. The rest of the environmental oxygen atoms are generated through the symmetry operations of the corresponding point groups: C 3 (Fe), C 2 (Rb1), C 3 (Rb2) and D 3 (Na). Iron-oxalate bonds are indicated by double lines and alkali metal-oxygen contacts by single lines. Symmetry codes: et al., 1970;Sato & Tominaga, 1979;Ladriere, 1992) and in the alkali (Na, Rb, Cs) family of tris(oxalato)ferrate(III) salts (Piro et al., 2016). The planes of the carboxylic -COO À groups of the oxalate ligand are slightly tilted from each other, by 12 (1) around the C-C -bond. As expected, the C-O bond lengths involving the coordinated-to-metal oxygen atoms are significantly longer [1.286 (7) and 1.283 (7) Å ] than the ones corresponding to the uncoordinated oxalate oxygen atoms [both equal to 1.211 (7) Å ].
There are two independent rubidium ions, one (Rb1) lying on a twofold axis, C 2 point group symmetry (d site) in an eightfold coordination with neighbouring oxalate oxygen atoms, the other one (Rb2) on a threefold axis, C 3 point group (c site) in a sixfold coordination. The sodium ion is at a site of D 3 point group symmetry (a site) in a trigonal-antiprismatic NaO 6 coordination with one oxygen atom of six neighbouring, symmetry-related, oxalate ions.
When dealing with octahedral Fe(C 2 O 4 ) 3 tris-chelated metal complexes, it is customary to describe its chirality employing Ãand Á-descriptors (Meierhenrich, 2008). It turns out that the enantiomeric complexes correlate with the corresponding chiral space groups, as indicated in Fig. 2.
The possibility of controlling the crystal chirality and therefore obtaining enhanced optical activity of functional materials has been discussed (Gruselle et al., 2006). To this purpose, two general synthetic routes have been developed to reach optically active coordination compounds, namely either by enantioselective synthesis using enantiopure chiral species, which yields enantiopure samples (Knof & von Zelewsky, 1999) or by spontaneous resolution upon crystallization from a racemate, which yields a conglomerate (Pé rez-García & Amabilino, 2002). As explained above, the chiral NaRb 5 -[Fe(C 2 O 4 ) 3 ] 2 crystals were obtained through the phenomena of spontaneous resolution from a racemic solution of [Fe(C 2 O 4 ) 3 ] 3À complex ions into a racemic conglomerate. This is presumably followed by a structural inductive effect by these chiral molecular ions on the alkali metal ions through shared oxalate ligands. In fact, not only is the Fe III ion a 'stereogenic centre' in the Fe(C 2 O 4 ) 3 tris-chelated metal complex, but so also are the sodium and one (Rb2) of the rubidium ions. These metal ions are in a distorted octahedral environment coordinated to six oxalate anions, acting as monodentate ligand through one of their oxygen atoms and resembling a six-bladed propeller-like conformation. From the structural data, it turns out that the chirality of this local arrangement around the alkaline ions is coincident with the one of the [Fe(C 2 O 4 ) 3 ] 3À inductor and therefore the chiral crystals reported here can be more conveniently described as Ã-NaÃ-Rb 2 Rb 3 [Ã-Fe(C 2 O 4 ) 3 ] 2 (P4 3 32) and Á-NaÁ-Rb 2 Rb 3 [Á-Fe(C 2 O 4 ) 3 ] 2 (P4 1 32). However, no definitive chirality can be unambiguously assigned to the other independent rubidium (Rb1) ion which is in an eightfold polyhedral coordination.

Database survey
The formation of racemic conglomerates of single crystals, adequate for structural X-ray diffraction, generated by spontaneous resolution is an infrequent phenomenon. In fact, a search of the Cambridge Structural Database (Groom et al., 2016) invoking the term 'spontaneous resolution' showed seventeen entries, and another one using as a target 'chiral crystals' produced a further four hits. Among them there were reported the chiral to each other (
(a) At a site of C 2 point group symmetry; (b) at a C 3 site; (c) at a D 3 site.

Refinement details
Crystal data, data collection procedure and structure refinement results are summarized in Table 3. The structure was solved by intrinsic phasing with SHELXT (Sheldrick, 2015a). The stereoisomers were determined through refinement of the Flack absolute structure parameter. This is the fractional contribution to the diffraction pattern due to the molecule racemic twin and for the correct enantiomeric crystal it should be zero to within experimental error.  Computer programs: CrysAlis PRO (Agilent, 2014;Rigaku OD, 2015), SHELXT (Sheldrick, 2015a), SHELXL2014 (Sheldrick, 2015b) and ORTEP-3 for Windows (Farrugia, 2012 (Rigaku OD, 2015) for P4132. Cell refinement:

Sodium pentarubidium bis[tris(oxalato)ferrate(III)] (P4332)
Crystal data Special details Geometry. All esds (except the esd in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell esds are taken into account individually in the estimation of esds in distances, angles and torsion angles; correlations between esds in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell esds is used for estimating esds involving l.s. planes.

Special details
Geometry. All esds (except the esd in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell esds are taken into account individually in the estimation of esds in distances, angles and torsion angles; correlations between esds in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell esds is used for estimating esds involving l.s. planes.