Crystal structures of the penta- and hexahydrate of thulium nitrate

The title compounds represent the most hydrated thulium nitrates known so far. Both structures consists of molecular [Tm(NO3)3(H2O)4] complexes and additional water molecules, which are interconnected by medium-strong to weak hydrogen bonds.


Chemical context
The nitrates of the rare earth metals have long been used to separate and purify these elements. For example, when thulium was discovered (Cleve, 1897), fusion of nitrates was already used to separate the element from the erbiumcontaining earth, and a hydrate of thulium nitrate in substance was already described more than 100 years ago with four equivalents of water of crystallization and of highly hygroscopic nature (James, 1911). Later, among others, double nitrates like Mg 3 Ln 2 (NO 3 ) 12 Á24H 2 O and (NH 4 ) 2 Ln(NO 3 ) 5 Á-4H 2 O (Ln = rare earth element) were used to separate the elements by means of fractional crystallization (Prandtl, 1938). Also, when more sophisticated separation procedures such as chromatographic methods and solvent extraction were developed (Bock, 1950), there was still considerable interest in these complex nitrates because of their high solubility even in organic solvents. Numerous structural investigations have been reported for this family, not least for the hydrated compounds (Wickleder, 2002).
Considering the structural information for the maximally hydrated rare earth nitrates, a general tendency of a decreasing amount of water with increasing atomic number is obvious: for the lighter homologues La-Nd and Sm-Tb, the hexahydrates are found as maximally hydrated compounds for the nitrates (La: Eriksson et al., 1980;Ce: Milinski et al., 1980;Pr: Decadt et al., 2012;Nd: Rogers et al., 1983;Sm: Kawashima et al., 2000;Eu: Stumpf & Bolte, 2001;Gd: Taha et al., 2012;Tb: Moret et al., 1990) while for the heavier elements Dy-Er and Yb, only pentahydrates have been reported (Ho: Rincke et al., 2017;other: Junk et al., 1999). Confirming this trend, the highest hydrate of Lu nitrate is the tetrahydrate (Junk et al., 1999), and for Tm the trihydrate exhibits the highest number of water molecules reported so far (Riess, 2012). ISSN 2056-9890 In the present research communication, the new penta-and hexahydrates of Tm(NO 3 ) 3 are reported. While the pentahydrate of Tm(NO 3 ) 3 fills the gap within the known compounds containing Er and Yb, the hexahydrate indeed represents the highest hydrated nitrate including Tm and shifts the border of known stable compounds notably to heavier rare earth elements.

Structural commentary
Tm(NO 3 ) 3 Á5H 2 O crystallizes in the Y(NO 3 ) 3 Á5H 2 O type of structure (Eriksson, 1982) in space group P1 with all atoms at general positions. The structure consists of isolated molecular [Tm(NO 3 ) 3 (H 2 O) 4 ] complexes and one additional free water molecule per formula unit (Fig. 1). The nitrate anions act as bidentate ligands so the Tm III atom is tenfold coordinated. The nitrate ions form an equatorial belt separating one aqua ligand from the other three, and are slightly inclined in the same sense and form a propeller-like shape. The nitrate anions coordinate asymmetrically at one shorter [2.3980 (17) (4) and 111 (4) . The structural entities, i.e. the molecular [Tm(NO 3 ) 3 (H 2 O) 4 ] complexes and H 2 O molecules, are interconnected by almost linear hydrogen bonds (see Fig. 2) of medium-strong to weak strength. In detail, eight of ten independent H atoms form hydrogen bonds shorter than 2.30 Å with O-HÁ Á ÁO angles greater than 163 , while atoms H2 and H10 are part of bifurcated and slightly longer hydrogen bonds (Table 1).

Figure 4
Crystal structure of Tm(NO 3 ) 3 Á6H 2 O in a view along [100]. Hydrogen bonds are shown as dotted lines up to an OÁ Á ÁH distance of 2.5 Å . Anisotropic displacement ellipsoids of non-H atoms are drawn at the 50% probability level.

Figure 5
Structural details to emphasize the molecular pseudo-symmetry in the title compounds: (a) a pseudo-threefold rotation axis in the molecular complex present in Tm(NO 3 ) 3 Á5H 2 O; (b) a pseudo-mirror plane in the molecular complex present in Tm(NO 3 ) 3 Á6H 2 O. Anisotropic displacement ellipsoids of non-H atoms are drawn at the 50% probability level.

Figure 3
Asymmetric unit of Tm(NO 3 ) 3 Á6H 2 O with the atom-numbering scheme. Anisotropic displacement ellipsoids for non-H atoms are drawn at the 50% probability level.
pseudo-symmetries, with the higher symmetry violated at a molecular level and in the first coordination sphere, and incompatible with the space-group symmetry.

Database survey
The crystal structure of anhydrous Tm(NO 3 ) 3 was determined quite recently (Heinrichs, 2013), and one hydrated phase has been reported so far, i.e. the trihydrate (Riess, 2012). In addition, basic oxo-hydroxo-nitrate hydrates are known with Tm (Giester et al., 2009). The thulium nitrate pentahydrate adopts the Y(NO 3 ) 3 Á5H 2 O type of structure (Eriksson, 1982;Klein, 2020), and is isotypic with the respective Eu (Ribá r et al., 1986), Gd (Stockhause & Meyer, 1997), Dy, Er, Yb (Junk et al., 1999) and Ho compounds (Rincke et al., 2017). Tm(NO 3 ) 3 Á6H 2 O is isotypic with the nitrate hexahydrates of Y , Pr (Rumanova et al., 1964;Fuller & Jacobsen, 1976;Decadt et al., 2012), Nd (Rogers et al., 1983;Shi & Wang, 1991), Sm (Shi & Wang, 1990;Kawashima et al., 2000), Eu (Stumpf & Bolte, 2001;Ananyev et al., 2016), Gd (Ma et al., 1991;Taha et al., 2012) and Tb (Moret et al., 1990). [Tm(NO 3 ). From saturated solutions, crystals with sizes up to the millimetre range were grown at room temperature within one day. Single crystals were removed, cleansed from the mother liquor and placed on a microscope slide in air. For the single-crystal data collection, crystals were immersed into perfluoroalkyl ether, which also acts as glue on a glass tip during the measurement. The remaining crystals were carefully ground to measure an X-ray powder pattern that, according to a comparison with the pattern simulated from the single-crystal structure determination, showed exclusively reflections of the pentahydrate. The crystals are hygroscopic and usually deliquesce within hours under ambient conditions depending on air humidity. Rapid re-crystallization within minutes can be induced by scratching on the glass slide. Surprisingly, from one recrystallization the hexahydrate [Tm(NO 3 ) 3 (H 2 O) 4 ]Á2H 2 O was obtained. All investigated crystals from this batch revealed the unit cell of the hexahydrate, so the crystallization seemed to result in a pure product in this case as well. Optically indistinguishable, the crystals of the hexahydrate showed the same deliquescence behaviour. It has not been possible to determine the exact conditions required to obtain the hexahydrate so far. According to EDX measurements, the crystals contain Tm as the only element heavier than oxygen.

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.

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å 2 )
x y z U iso */U eq