Crystal structure of Pb3(IO4(OH)2)2

The basic building units of the hydrous periodate Pb3(IO4(OH)2)2 are three Pb2+ cations and two IO4(OH)2 3− anions. The octahedral anions are arranged in a distorted hexagonal rod packing, with the cations (each with a coordination number of eight) located in between.


Chemical context
Lead and mercury can both exist in different oxidation states and each of the two elements exhibits a peculiar crystal chemistry. In the case of Pb 2+ -containing compounds, the crystal chemistry is mainly dominated by the stereoactive 6s 2 lone-pair of lead (Holloway & Melnik, 1997), whereas Hg 2+ -containing compounds show a strong preference for a linear coordination of mercury (Breitinger, 2004). In this respect, it appears surprising that for some Pb 2+ -and Hg 2+containing compounds an isotypic relationship exists, e.g. for PbAs 2 O 6 (Losilla et al., 1995) and HgAs 2 O 6 (Mormann & Jeitschko, 2000b;Weil, 2000), or for the mineral descloizite PbZn(VO 4 )OH (Hawthorne & Faggiani, 1979) and the synthetic phase HgZn(AsO 4 )OH (Weil, 2004). With this in mind, it seemed interesting to study the relation between phases in the systems Hg II -I VII -O-H and Pb II -I VII -O-H. Whereas in the system Hg II -I VII -O-H two compounds have been structurally characterized, viz. Hg 3 (IO 4 (OH) 2 ) 2 (Mormann & Jeitschko, 2000a) and Hg(IO 3 (OH) 3 ) (Mormann & Jeitschko, 2001), a phase in the system Pb II -I VII -O-H has not yet been structurally determined, although several lead(II) periodate phases have been reported to exist. Willard & Thompson (1934) claimed to have obtained only one phase with composition Pb 3 H 4 (IO 6 ) 2 in the system Pb II -I VII -O-H. However, Drá tovský & Matě jčková (1965a,b) reported the existence of three phases with composition Pb 3 (IO 5 ) 2 ÁH 2 O, Pb 2 I 2 O 9 Á3H 2 O and Pb 4 I 2 O 11 Á5H 2 O in this system. To shed some light on the conflicting composition of the Pb:I 3:2 phase [Pb 3 H 4 (IO 6 ) 2 versus Pb 3 (IO 5 ) 2 ÁH 2 O with a lower water content], the synthetic procedure described by Willard & Thompson (1934) was repeated for crystal growth of this lead periodate. The current structure determination of the obtained crystals showed the composition Pb 3 H 4 (IO 6 ) 2 as reported by Willard & Thompson (1934) to be correct. In a more reasonable crystal-chemical sense, the formula of these crystals should be rewritten as Pb 3 (IO 4 (OH) 2 ) 2 .

Structural commentary
Three Pb 2+ cations and two IO 4 (OH) 2 3À octahedra are present in the asymmetric unit. The anions form a slightly distorted hexagonal rod packing with the rods extending parallel to [021]. Cations and anions are linked through common oxygen atoms into a framework structure (Fig. 1).
Each of the Pb 2+ cations exhibits a coordination number of eight if Pb-O interactions less than 3.1 Å are considered to be relevant. The resulting [PbO 8 ] polyhedra are considerably distorted [Pb-O distances range from 2.433 (7) to 3.099 (8) Å ]. The stereochemical activity of the electron lone pairs in each of the polyhedra appears not to be very pronounced (Fig. 2).
Results of bond-valence calculations (Brown, 2002), using the parameters of Brese & O'Keeffe (1991) for I-O bonds and of Krivovichev & Brown (2001)  Atom O11 has also a low bond-valence sum, explainable by its role as a twofold acceptor atom of medium-strength hydrogen-bonding interactions ( Table 2) that additionally stabilize the packing of the structure (Fig. 1).
However, they are isopointal and show the same type of arrangement in terms of the crystal packing. In the mercury compound, the IO 4 (OH) 2 3À octahedra are likewise hexagonally packed in rods (Fig. 3). The cations are situated in between this arrangement which is further consolidated by O-HÁ Á ÁO hydrogen bonding.

Synthesis and crystallization
The preparation conditions described by Willard & Thompson (1934) were modified slightly. Instead of using NaIO 4 as the periodate source, periodic acid was employed.
1.25 g Pb(NO 3 ) 2 was dissolved in 25 ml water, acidified with a few drops of concentrated nitric acid and heated until boiling. Then the periodic acid solution (0.85 g in 25 ml water) was slowly added to the lead solution. The addition of the first portion of the periodic acid solution (ca. 3-4 ml) resulted in an off-white precipitate near the drop point that redissolved under stirring. After further addition, the precipitate remained and changed the colour in the still boiling solution from off-white to yellow-orange within half an hour. After filtration of the precipitate, a few colourless crystals of the title compound formed in the mother liquor on cooling. X-ray powder diffraction data of the polycrystalline precipitate are in very good agreement with simulated data based on the refinement of Pb 3 (IO 4 (OH) 2 ) 2 .

Refinement
All investigated crystals were twinned by non-merohedry. Intensity data of the measured crystal could be indexed to belong to two domains, with a refined twin domain ratio of 0.73 (1):0.27 (1). Reflections originating from the minor component as well as overlapping reflections of the two domains (less than 10% of all measured reflections) were separated and excluded. The H atoms of the IO 4 (OH) 2 octahedra could not be located from difference maps and were therefore not considered in the final model. The O atoms were refined with isotropic displacement parameters. The remaining maximum and minimum electron densities are found 0.73 and 0.68 Å , respectively, from atom Pb2. Structure data were finally standardized with STRUCTURE-TIDY (Gelato & Parthé, 1987). It should be noted that the intensity statistics point to a pronounced C-centred basis cell (space group C2/c with lattice parameters of a ' 14.16, b ' 9.21, c ' 8.97 Å , ' 117.4 ) with weak superstructure reflections violating the C-centering. Acta Cryst. (2014). E70, 14-17 research communications Computer programs: SMART (Bruker, 2008), SAINT-Plus (Bruker, 2008), SHELXS97 and SHELXL97 (Sheldrick, 2008), ATOMS for Windows (Dowty, 2006) and publCIF (Westrip, 2010).

Special details
Geometry. All e.s.d.'s (except the e.s.d. in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell e.s.d.'s are taken into account individually in the estimation of e.s.d.'s in distances, angles and torsion angles; correlations between e.s.d.'s in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell e.s.d.'s is used for estimating e.s.d.'s involving l.s. planes. Refinement. Refinement of F 2 against ALL reflections. The weighted R-factor wR and goodness of fit S are based on F 2 , conventional R-factors R are based on F, with F set to zero for negative F 2 . The threshold expression of F 2 > σ(F 2 ) is used only for calculating R-factors(gt) etc. and is not relevant to the choice of reflections for refinement. R-factors based on F 2 are statistically about twice as large as those based on F, and R-factors based on ALL data will be even larger.  (8)