Rubidium pentaaqua(l-serine)cobalt(II) hexahydrogenhexamolybdocobaltate(III) l-serine monosolvate decahydrate

The Co2+ ion in the title compound, Rb[Co(C3H7NO3)(H2O)5][H6CoMo6O24]·C3H7NO3·10H2O, is coordinated by five water molecules and one O-monodentate l-serine ligand in a slightly distorted octahedral geometry. The Rb+ ion is irregularly coordinated by nine O atoms. In the crystal, the [H6CoIIIMo6O24]3− polyanions are stacked along the b-axis direction, mediated by bridging Rb—O bonds. N—H⋯O and O—H⋯O hydrogen bonds are observed involving the l-serine molecules.


Comment
Continuous interests in chiral polyoxometalates (POMs) are endowed owing to their prominent molecular applications such as nonlinear optics, magneto optical effect, and circular polarized luminescence (Hasenknopf et al., 2008;Du et al., 2013). Two synthetic procedures for chiral POMs have been delivered to date. One is the way based on the connection of chiral organic ligands and achiral inorganic POMs possible to propagate the local chirality to POM framework, which have been extensively studied by the groups of Hill, Kortz, Pope, Wang, and Yamase for a long time (Fang et al., 2005;Kortz et al., 2002;Sadakane et al., 2001;Tan et al., 2007, Inoue & Yamase, 1995 (Co(II) complex), Rb + , one water molecule, and free L-Ser, and the other water molecules with noncentrosymmetric space group P2 1 2 1 2 1 provided the title compound, of which the absolute configuration was supported by Flack parameter 0.025 (7). The pK a (2.2 and 9.2) of L-Ser and the acidic condition (pH 3.2) in the synthetic procedure suggest that all L-Ser molecules are zwitterionic. In fact, the carboxylate group in L-Ser coordinating to Co 2+ and that in free L-Ser have similar C-O distances (1.250 (7), 1.264 (7) Å and 1.247 (7), 1.257 (7) Å) due to their resonance state (Lide, 2007 (Perloff, 1970;Honda et al., 2007;Yang et al., 2013). Figure 2 shows the crystal structure viewed along the a axis. The two kinds of polyanion (A-type and B-type) with different orientation in the ac plane are alternately connected along the crystallographic b axis by Rb + -bridging of terminal O atoms in the CoMo6 polyanion, forming the b-axially stacked polyanion unit. The polyanion units are aligned along the c axis with a help of hydrogen bonding (O···O < 3.3 Å) relating with some waters of crystallization and aqua ligands in Co(II) complex, of which the unit distance is the half of c axis value. The both distance of A-type···A-type and B-type···B-type are 11.6 Å; (Co···Co distance), which corresponds to the b axis value.

Experimental
An aqueous solution containing the Na salt of CoMo6, L-Ser, and CoCl 2 ·6H 2 O with a molar ratio of 1:4:2 was acidified to pH = 3.2 by conc HCl and then boiled for 1 h. After cooling to room temperature, 5 eq. of RbClO 4 to CoMo6 was added into the solution. Viridian thin platelets of the title compound were obtained at 4 °C being stood for several days.

Refinement
Fragile thin platelet crystals of the title compound gave a medium absorption corrections (transmission factor ranging from 12.3 to 50.0%), resulting in a somewhat high residual electron densities around Mo1, Mo2, Mo4, and Mo6.
Generally, a complete convergence of structure refinement with residual density less than 1 e Å -3 in difference Fourier synthesis indicates the success of structural analysis. However, for the compounds containing many heavy atoms, since the anisotropy of electron cloud around the heavy atoms makes difficult to conduct the ellipse approximation, the relative large residual densities cannot help being remaining. In the structural analysis of polyoxometalate consisting of a variety of heavy atoms such as Mo, V, and W, the relatively high residual densities have been frequently observed for the crystals with poor absorption corrections and crystallinity. In fact, after some elements were allocated to the pointed out densities, the refinements resulted in the divergence. Therefore, the pointed out high residual densities for Mo1, Mo2, Mo4, and Mo6 are not responsible for the incompleteness of structural analysis but the intrinsic crystal qualities such as morphology and crystallinity. All the atoms except for H atoms were refined anisotropically, and the H atoms were isotropically. The H atoms in L-Ser molecules were located in the calculated position, and the remaining H atoms were not included in the refinements. The residual density around O31 may be H atom. Due to the undesirable absorption correction, it is conceivable that high residual density corresponding to H atom is remained. In the polyoxometalate bearing many heavy atoms, the electron density for H atom is hard to be found. In order to escape from the divergence and lowering the accuracy of refinement caused by intentional assignment of H atom to remarkable low electron density, for polyoxometalate compounds, the H atoms in water crystallization are not included in refinements. Therefore, the water of crystallization O atoms (O37 to O45) are refined without H atoms.

Computing details
Data The asymmetric unit of the title compound with displacement ellipsoids drawn at the 50% probability level. White, red, blue, green, moss green and yellowish brown ellipsoids denote Mo, O, Co, Rb, C, and N atoms, respectively. The H atoms protonated to the CoMo6 polyanion are omitted for clarity.