High-resolution crystal structure of the double nitrate hydrate [La(NO3)6]2[Ni(H2O)6]3·6H2O

Very large high-quality crystals of a new member of the family of double nitrates, namely, [La(NO3)6]2[Ni(H2O)6]3·6H2O, were crystallized in large amounts. The structure was determined via single-crystal X-ray diffraction to high resolution. Extensive structural information, including hydrogen-bonding details, was obtained at the same time.


Chemical context
Double nitrates, which contain two different metal cations and nitrate anions, have applications in various fields.They exhibit unique solubility properties, crystalline structures and special magnetic properties, and act as excellent precursors for the synthesis of mixed oxides.For example, double nitrates of Zn and Cu have been used to produce a mixed Cu and Zn oxide that exhibits high catalytic activity in reactions such as the water gas shift reaction to produce CO 2 and H 2 from CO and H 2 O (Smith et al., 2010), as well as selective CO 2 hydrogenation into methanol (Zhong et al., 2020).Rare earth (RE) transition-metal (TM) double nitrates with the general formula RE 2 TM 3 (NO 3 ) 12 •24H 2 O attracted much attention in the 1960s, in which the RE is a trivalent cation with an atomic number lower than that of Ho and the TM is a divalent cation, including Mg 2+ , Mn 2+ , Fe 2+ , Co 2+ , Ni 2+ , Cu 2+ and Zn 2+ (Hellwege & Hellwege, 1953;Brochard & Hellwege, 1953;Buckmaster et al., 1968).It should be noted that when RE has a +4 oxidation state, double nitrates are isomorphic with the triclinic MgTh(NO 3 ) 6 (H 2 O) 8 salt (S ˇc ´avnic ˇar & Prodic ´, 1965).Cerium(III) magnesium and cerium(III) zinc double nitrates have been used extensively in nuclear orientation experiments because very low temperatures can be obtained by adiabatic demagnetization of the salt (Culvahouse, 1961).Their properties make them suitable magnetic thermometers (Thornley, 1963).Like ruby single crystals (Cr 3+ :Al 2 O 3 ), a Ce-doped lanthanum magnesium double nitrate is an ideal medium to study phonon avalanche, a delayed and sudden relaxation of paramagnetic ions by the emission of phonons (Mims & Taylor, 1969).Heat capacity and susceptibility measurements suggested that Mn, Ni, Co and Cu lanthanum double nitrates show antiferromagnetic transitions below 0.5 K (Mess et al., 1967(Mess et al., , 1968)).The lack of high-quality crystalline structures of these salts limits the profound understanding of the magnetic properties, as well as their theoretical investigation.We report herein on the growth of centimeter-large crystals of [La- 1) and the crystal structure determined by single-crystal X-ray diffraction.

Structural commentary
Similar to those found in the corresponding magnesium double salt, the title compound is made up of two types of ions, [Ni(H 2 O) 6 ] 2+ and [La(NO 3 ) 6 ] 3À , which are linked together by hydrogen bonds with water molecules in the structure.The La atom on the threefold axis is coordinated by 12 O atoms from six nitrate groups to form a slightly distorted icosahedron.The La-O distances range from 2.6339 (8) to 2.7012 (8) A ˚, which are comparable to those found in La 2 Mg 3 (NO 3 ) 12 •24H 2 O determined by neutron diffraction (Anderson et al., 1977).As depicted in Fig. 2, the structure includes two crystallographically independent positions for Ni 2+ .Three water H7a-O7-H7b molecules and three water H8a-O8-H8b molecules surround Ni1, resulting in a distorted [Ni(H 2 O) 6 ] 2+ octahedron with C3 symmetry.In contrast, the Ni2-containing [Ni(H 2 O) 6 ] 2+ octahedron is highly symmetric, as Ni2 is situated in a site with 3 symmetry.The Ni-O bond lengths in both octahedra vary from 2.0471 (8) to 2.0531 (8) A ˚, similar to those found in [Ni(H 2 O) 6 ](NO 3 ) 2 (Breternitz et al., 2015).
As illustrated in Fig. 3, each [La(NO 3 ) 6 ] 3À icosahedron is surrounded by three Ni1-containing [Ni(H 2 O) 6 ] 2+ clusters, and each Ni1-containing octahedron is surrounded by three icosahedra.These two interpenetrating honeycomb networks are arranged in a layer parallel to the ab plane.In this layer, the icosahedra are linked to the [Ni(H 2 O) 6 ] 2+ clusters through strong O7-H7A� � �O4, O9-H9A� � �O1 and O10-H10A� � �O9 hydrogen bonds.The six water H9A-O9-H9B molecules per unit cell do not participate in the coordination of either La or Ni.Two successive layers are separated by Ni2-containing [Ni(H 2 O) 6 ] 2+ clusters, which bridge the layers between them via O10-H10A� � �O9 hydrogen bonds.The complex hydrogen-bonding network between the clusters is shown in Fig. 2 and the actual data for the hydrogen bonds are given in Table 1.The network of bonded clusters form sheets that are stacked perpendicular to the c axis (Fig. 4).The sheets are held together by van der Waals forces in the [La(NO

Figure 2
The molecular structure of [La(NO 3 ) 6 ] 2 [Ni(H 2 O) 6 ] 3 •6H 2 O, with displacement ellipsoids for all non-H atoms drawn at the 50% probability level.H atoms are represented by small spheres of arbitrary radius.Hatom labels have been omitted for clarity.The colour scheme for the different elements can be found in the legend.The viewing direction is slightly tilted from the c axis, in order to prevent overlap between atoms.
Hydrogen bonds are indicated with thin dotted lines.

Database survey
No record of the same compound was found in the Crystal

Synthesis and crystallization
Lanthanum(III) oxide was dissolved in dilute HNO 3 with a concentration of 1 mol l À 1 (1 M) and nickel(II) oxide in dilute HNO 3 with a concentration of 0.5 mol l À 1 (0.5 M).In order to dissolve the nickel(II) oxide in the dilute HNO 3 , the solution was heated at 423 K over a period of 12 h until the nickel(II) oxide completely dissolved and a green transparent solution was obtained.Lanthanum(III) oxide solution (0.2 l) was first mixed with nickel(II) oxide solution (1.2 l) and then 1 mol of citric acid was added to the mixture under vigorous stirring until complete dissolution.The solution was transferred to a fume hood for slow evaporation.After 30 d, green hexagonal plate-shaped crystals formed with different sizes, the maximum dimension being 2 cm.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 2. H atoms on O atoms were first located in a difference Fourier map and then refined isotropically in riding mode, with U iso (H) values of 1.5U eq of the    (Sheldrick, 2015a), SHELXL2018 (Sheldrick, 2015b), DIAMOND (Brandenburg, 2006) and publCIF (Westrip, 2010).
parent O atoms.The O-H distance was refined against the residual peaks, without further constraint.

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

Figure 1
Figure 1 Left: large green crystals of [La(NO 3 ) 6 ] 2 [Ni(H 2 O) 6 ] 3 •6H 2 O with a pseudo-hexagonal shape placed on a scale paper.The distance between two thick lines is 1 cm.Right: powder X-ray diffraction patterns of [La(NO 3 ) 6 ] 2 [Ni(H 2 O) 6 ] 3 •6H 2 O.The PDF5 (ID = 00-049-1235) pattern available in the database is shown in green.The PXRD pattern calculated from the structure refined from single-crystal data and the experimental pattern measured on a Panalytical Empyrean diffractometer with Cu K� 1 radiation (� = 1.540596A ˚) are shown in blue and red, respectively.In the inset, the high-intensity diffraction peaks absent in the PDF5 pattern are highlighted with purple rectangles.
lography Open Database (COD) or the Inorganic Crystal Structure Database (ICSD).It is listed only once in the Powder Diffraction File (PDF) 2024 version, entry 00-049-1235, without any atomic positions provided.The powder X-ray diffraction (PXRD) pattern available in this database deviates significantly from both the theoretical pattern simulated from the structure refined via single-crystal XRD data and the experimental pattern recorded with powder obtained by crushing a few [La(NO 3 ) 6 ] 2 [Ni(H 2 O) 6 ] 3 •6H 2 O single crystals.Notably, peaks below 10 � , as well as those in the 22-23 � region, are missing in the pattern found in PDF-00-49-1235 (Fig. 1).

Figure 3
Figure 3 The [La(NO 3 ) 6 ] 2 [Ni(H 2 O) 6 ] 3 •6H 2 O structure represented along the c axis.The colour scheme for the different elements of the structure can be found in the figure inset.[Ni(H 2 O) 6 ] 2+ octahedra and [La(NO 3 ) 6 ] 3À icosahedra are plotted as pink and green front-opening polyhedron, respectively.The unit-cell edges are plotted with blue lines.

Figure 4
Figure 4The [La(NO 3 ) 6 ] 2 [Ni(H 2 O) 6 ] 3 •6H 2 O structure represented along the a axis.The colour scheme for the different elements of the structure can be found in the inset.[Ni(H 2 O) 6 ] 2+ octahedra and [La(NO 3 ) 6 ] 3À icosahedra are plotted as pink and green polyhedra, respectively.The unit-cell edges are plotted with blue lines.

Table 2
Experimental details.