Crystal structure of a samarium(III) nitrate chain cross-linked by a bis-carbamoylmethylphosphine oxide ligand

The crystal structure of the title compound consists of a polymeric chain of SmIII cations and nitrate anions, cross-linked in two dimensions with an organic ligand.


Chemical context
The carbamoylmethylphosphine oxide (CMPO) moiety has been well studied as a chelating group for lanthanides and actinides. To this end, this bidentate phosphoryl/carbonyl moiety is a component of the TRUEX process for the treatment of nuclear waste (Siddall, 1963;Horwitz et al., 1985). A handful of ligands bearing CMPO groups linked through triand tetrapodal caps have been reported in the literature in an attempt to increase the binding strength and selectivity toward f-elements (Arnaud-Neu et al., 1996;Peters et al., 2002;Sharova et al., 2012;Sartain et al., 2014). The title compound, [Sm 2 (NO 3 ) 6 (C 14 H 30 N 2 O 8 P 2 )(H 2 O)], is another representative.

Structural commentary
The asymmetric unit of the title compound contains two Sm III ions, one nine-coordinate and one 12-coordinate, two halves of the di-CMPO ligand tetraethyl [(ethane-1,2-diyl)bis(azanediyl)bis(2-oxoethane-2,1-diyl)]diphosphonate, six nitrate anions ISSN 1600-5368 and one coordinating water molecule (Fig. 1) The large displacement parameters of the methyl group (C5) are likely due to large thermal motion of this terminal group (see Refinement section for more discussion on the treatment of this disorder).
The Sm III metal cations are bridged through shared bisbidentate nitrate anions (N3 and N4), forming a corrugated chain (Fig. 2,bottom) parallel to the a axis. In this figure, bridging bis-bidentate nitrate ions are shown in pink, while nitrate ions bound only to the 12-coordinate Sm III cation are shown in purple. The nine-coordinate Sm III ions of the metal chain are also linked by the organic ligand. The organic ligand lies on an inversion center, lies along the c axis, and cross-links the metal chains (Fig. 2, top). This cross-linking results in sheets that extend parallel to the ac plane (Fig. 3).

Supramolecular features
The lanthanide-organic polymer is reinforced through two separate hydrogen-bonding motifs (Table 1). In the corrugated chain, each H atom (H27A and H27B) of the water molecule bound to Sm2 forms a hydrogen bond with an O atom of a nitrate group on Sm1 (Fig. 2, bottom). In the formation of the cross-linked sheets, the amide NH groups (H1 and H2) form hydrogen bonds with O atoms of two separate nitrate groups bound to Sm1 (Fig. 3). These interactions likely act to rigidify both the Sm III chain and the crosslinked organometallic sheets.
These metal-organic sheets are stacked along the b axis using only van der Waals forces (Fig. 4). No intermolecular hydrogen bonds or shared chelating groups are found between the sheets in this third dimension.

Figure 3
Sheets formed by the cross-linking of the Sm III chains with the di-CMPO organic ligands (viewed down the b axis). Hydrogen bonds between the amide NH groups and metal bound nitrate anions are shown as dashed lines.

Database survey
While numerous polymeric structures of lanthanide-organic compounds can be found in the Cambridge Structural Database (CSD; Version 5.35, last update February 2014; Allen, 2002), one interesting feature of this structure is the bidentate bridging of two lanthanides by one shared nitrate group (Fig. 2 (Albrecht et al., 2005;Hashimoto et al., 2000) with this bidentate bridging motif have been deposited with the CSD.

Synthesis and crystallization
The CMPO ligand was prepared following a reported procedure (Hamadouchi et al., 1999), using ethylenediamine in place of methylamine. This compound was isolated as a white solid. The title metal-ligand coordination polymer was prepared by dissolving the ligand in a minimum amount of acetonitrile. To this solution were added 2 molar equivalents of samarium(III) nitrate hexahydrate as a solution in acetonitrile. The mixture was stirred at room temperature overnight and concentrated under reduced pressure to give an off-white solid. Crystals suitable for X-ray diffraction were grown from vapor diffusion of toluene into a solution of the 2:1 Sm IIIligand complex in acetonitrile.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 2. H atoms were placed in calculated positions and constrained to ride on their parent atoms, with U iso (H) = 1.2U eq (C,N) for methylene and amino groups, and 1.5U eq (C,O) for methyl and water groups. C-H distances were restrained to 0.98 Å for methyl and 0.99 Å for methylene H atoms, N-H distances to 0.88 Å and O-H distances to 0.89 Å . One of the methyl groups on the organic ligand (C5) has relatively large displacement ellipsoids that we attribute to large thermal motion of this terminal group. Attempts to model this disorder by assigning two atom locations for C5 or the entire ethoxy group were unsuccessful. The O3-C4 and C4-C5 bond lengths were constrained using DFIX instructions in SHELXL (Sheldrick, 2008)    Computer programs: APEX2 and SAINT (Bruker, 2012), SHELXS97 and SHELXL97 (Sheldrick, 2008) and OLEX2 (Dolomanov et al., 2009).