Synthesis and crystal structure of cerium(IV) bis(phosphite)

The title substance of cerium(IV) bis(phosphite), Ce(HPO3)2, crystallizes in a trigonal cell and is composed of CeO6 octahedra and phosphite tetrahedra.


Chemical context
Phosphonates are commonly employed within the petroleum industry as antioxidants. Interactions between these antioxidants and possible metal impurities could potentially have unintended consequences in the processing of petroleum products. We are currently studying these interactions by exploring the crystalline materials formed via solvothermal syntheses of lanthanides with phosphorous acid.
Phosphorous acid (H 3 PO 3 ) is a powerful reducing agent and is exceedingly water soluble. The anion HPO 3 2À has many different names in the literature, including (but not limited to) phosphite, phosphonate, phosphorus(III) oxoanion, oxophosphate(III) and hydridotrioxidophosphate(2-). According to IUPAC, when the hydrogen atom is directly bonded to the phosphorus atom, it is to be named phosphonate; whereas when the anion tautomerizes to the PO 2 (OH) 2À anion, it is named as phosphite. However, the latter ion is rarely identified in the solid state. While IUPAC prefers HPO 3 2À to be named phosphonate, this name is also used for organophosphorus compounds with the general formula R-PO(OH) 2 or R-PO(OR) 2 , where R = alkyl or aryl groups. To eliminate any confusion with organophosphorus compounds and to be consistent with the recent literature, we will herein refer to the HPO 3 2À anion as phosphite. The phosphite anion has many structural similarities to both phosphates and organophosphonates. In the phosphite anion, a hydrogen atom has replaced one of the oxygen atoms, which would be found in phosphate; also, the phosphite anion contains no P-C bonds that are found in phosphonates. Phosphite itself can be used as a precursor for making a vast array of phosphonates. In phosphite, the central phosphorus atom is P III instead of the more air-stable P V , which provides the opportunity for redox chemistry. This ability to act as a reducing agent has led to several mixed-valent uranium compounds Villa, Marr et al., 2013;. Moving towards the lanthanides, lanthanide phosphite compounds have been synthesized in one of two main ways: hydrothermally (Cross et al., 2012;Ewald et al., 2003Ewald et al., , 2005Foulon et al., 1993aFoulon et al., ,b,1995Loukili et al., 1988Loukili et al., , 1991Tijani et al., 1988;Xiong et al., 2006Xiong et al., , 2009Zhang et al., 1992) and phosphite flux reactions (Zakharova et al., 2003). We have expanded this chemistry by exploring solvothermal syntheses of lanthanide phosphites. Herein we will discuss the crystal structure of the title compound, a new cerium(IV) bis(phosphite).

Structural commentary
The title compound Ce(HPO 3 ) 2 crystallizes in the space group P3m1. The smallest repeating unit contains one cerium(IV), one oxygen, one phosphorus(III) and one hydrogen atom. This simple structure contains slightly distorted octahedrally coordinated cerium(IV) cations (site symmetry 3m.; Fig. 1), which are linked together by corner-sharing phosphite ligands. These phosphite ligands have a slightly distorted tetrahedral configuration (point group symmetry 3m.). Each phosphorus(III) atom in the phosphite ligand is bonded to three oxygen atoms, comprising the bottom of the tetrahedron, and one hydrogen atom. The sheets of Ce(HPO 3 ) 2 , which are located in the ab plane, contain alternating up-down phosphite ligands around the cerium(IV) metal cation, as indicated by the different directions of the hydrogen atoms (Figs. 2, 3). These sheets are layered down the c axis, where each cerium(IV) atom resides directly below the cerium above it at a distance of 5.6099 (3)  The sheet-like arrangement of the polyhedra parallel to the ab plane in the crystal structure of Ce(HPO 3 ) 2 . The unit cell is shown with blue dashed lines. This polyhedral representation contains the same color scheme as Fig. 1.

Figure 1
The coordination sphere of the cerium(IV) atom with atoms of the asymmetric unit labelled. Displacement ellipsoids are drawn at the 50% probability level. Bond lengths and angles can be found in Table 1. the c axis; between the layers, the Ce-Ce-Ce angle is 180.0 (Fig. 3). This structure is closely related to a known zirconium(IV) bis(phosphite), Zr(HPO 3 ) 2 , which was isolated via a very different synthesis route involving refluxing in concentrated phosphorous acid and using HF to precipitate the desired compound (Millini et al., 1993). The structure was elucidated from powder X-ray diffraction data and has many similarities to the title structure shown above ( Table 1). The main difference between the two is the metal-oxygen bond length, where the Ce-O bond length is unsurprisingly slightly longer.
The zirconium coordination appears to be a bit closer to being octahedral than the cerium, but they are nearly the same after comparing the error of refinement.

Synthesis and crystallization
Cerium(IV) bis(phosphite) was synthesized solvothermally in acetonitrile (CH 3 CN). A stock solution of 1.00 M H 3 PO 3 was prepared in acetonitrile. 0.0572 grams of ceric ammonium nitrate, (NH 4 ) 2 Ce(NO 3 ) 6 , was placed into a PTFE liner along with 2 ml of the 1.00 M phosphorous acid solution, yielding a solution 0.0522 M ceric ammonium nitrate. This gives an approximate molar ratio of cerium(IV) to phosphite of 1:20. After the cerium was completely dissolved, the PTFE liner was capped and sealed inside of a stainless steel autoclave. This was then placed into a programmable box furnace and heated to 363 K over a period of 30 minutes, held at 363 K for four h and then cooled for 975 minutes down to 298 K (or a rate of 4 K per hour).
The resulting mixture was washed with cold water and then placed into a plastic petri dish. The excess water was removed and the crystals were dispersed with acetonitrile. The large, hexagonal prisms of the title compound were light yellow in color. Many suitable crystals were present. A large crystal was isolated in immersion oil and broken perpendicular to the hexagonal face to yield a clean crystal for single-crystal X-ray diffraction.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 2. The hydrogen-atom position was placed as a riding atom on the phosphorus position. The maximum electron density peaks are 0.330 e À Å À3 (located between P1 and O1 at 0.686 and 0.838 Å , respectively), 0.320 e À Å À3 (located adjacent to O1 at 0.608 Å ) and 0.250 e Å À3 (located adjacent to Ce1 at 0.692 Å ), which lead to nothing reasonable. All other maximum density peaks are under 0.2 e À Å À3 . The minimum electron density is a very minimal at À0.609 e À Å À3 . Two electron density holes of about the same magnitude reside around the phosphorus atom. The stacking of the cerium(IV) phosphite sheets are shown in a projection along the a axis. Again, the unit cell is shown with blue dashed lines and the color scheme is the same as the above images.