Structural reinvestigation of the photoluminescent complex [NdCl₂(H₂O)₆]Cl

The structure of the photoluminescent compound hexa­aqua­di­chloridoneodymium(III) chloride has been redetermined from single-crystal X-ray diffraction data at 100 K, with the aim of providing an accurate structural model for the bulk crystalline material. The crystal structure may be described as a network of [NdCl₂(H₂O)₆]⁺ cations with distorted square-anti­prism geometry around the Nd^III centre. The Nd^III cation and the nonbonded Cl⁻ anion are both located on twofold symmetry axes. The crystal packing consists of three different neodymium pairs linked by a three-dimensional network of O—H...Cl inter­molecular inter­actions. The pair distribution function (PDF) calculated from the experimentally determined structure is used for the discussion of the local structure.

S1. Introduction

Luminescent rare-earth (RE) materials have been extensively studied over recent decades for both fundamental and application purposes. These compounds possess narrow emission lines and are very promising for a variety of photonic applications, such as IR lasers, as a source for optical communication networks or as light-emitting diodes, and they are very suitable for photovoltaic devices (Slooff et al., 2001; Kuriki et al., 2002; Klink et al., 2000; Li et al., 2009; Reeves et al., 2009). In this respect, the Er³⁺- and Nd³⁺-based materials are among the most investigated systems and have attracted major inter­est owing to their photoluminescence behaviour. Recently, many theoretical and experimental studies have been devoted to Nd³⁺-based materials, aiming to define the relevant parameters which control the luminescent properties (Mussot et al., 2009; Serqueira et al., 2011; Pivin et al., 2005; Steveler et al., 2011). It was found that the spectroscopic and photonic properties of Nd³⁺-based complexes are strongly affected by the coordination of the RE sites (Ebendorff-Heidepriem et al., 1995; Tanabe et al., 1992). Although the characterization of their optical properties has been performed thoroughly and is quite well documented (Pisarska, 2008; Sokolov et al., 2003), reports on their structural behaviour are still rather scarce and quite disparate.

The synthesis and X-ray single-crystal structural studies of the neodymium hy­droxy­chloride complexes have been described by various workers (Bukin, 1972; Dem'yanets & Emel'yanova, 1969; Habenschuss & Spedding, 1980). Recently, Zehnder et al. (2010) drew a correlation between the structural and spectroscopic properties of the [Nd(OH)₂Cl] complex, showing that the highly linked three-dimensional network seems to contribute to the remarkable inertness of this material in near neutral aqueous environments. This could be one of the key properties for the potential application of actinide bis-hy­droxy­chlorides as nuclear waste forms.

The title compound was originally determined by Habenschuss & Spedding (1980) as isostructural with the [LuCl₃.6H₂O] complex (Habenschuss & Spedding, 1979). Nevertheless, the 12 H atoms (six of them symmetrically independent) were not identified, which prevents a clear inter­pretation of the crystal packing, so no detailed structural discussion was presented. A more complete crystallographic analysis of this compound is therefore of great inter­est to obtain better insights into the structural properties of the title compound.

S2. Experimental

S2.1. Synthesis and crystallization

Crystals of [NdCl₂(H₂O)₆].Cl were obtained by slow evaporation of an aqueous solution [Concentration?] of [NdCl₃(H₂O)₆] powder (99.9% purity, Fluka AG) kept at room temperature. After a few weeks, small colourless single crystals appeared.

S2.2. Refinement

Crystal data, data collection and structure refinement details are summarized in Table 1. The electron density of the H atoms was clearly identified in the Fourier difference map, and their atomic coordinates and isotropic displacement parameters were refined with O—H distances restrained to 0.84 (2) Å.

S3. Results and discussion

Fig. 1 shows the structure of the basic molecular fragment, [NdCl₂(H₂O)₆].Cl, of the title compound, along with the numbering scheme. Selected bond lengths and angles are listed in Table 2. Detailed examination of the coordination of the Nd^III cation reveals a distorted square-anti­prism geometry, where each Nd^III cation is located on a twofold axis and eight-fold coordinated by six water molecules and two inner-sphere Cl atoms (Fig. 1 and Table 2), the twofold symmetry axis bis­ecting the Cl1—Nd—Cl1 and OiW—Nd—OiW (i = 1, 2 or 3) angles. The Nd1—O bond lengths range from 2.4371 (12) to 2.4653 (11) Å with an average value of 2.453 (14) Å and are significantly shorter than Nd1—Cl1 [2.8180 (4) Å].

Chloride ion Cl2 is located on a twofold axis, at a much greater distance of 5.0638 (2) and 5.0651 (4)Å from the Nd^III cation, without any evidence for a direct Nd1···Cl2 inter­action. These inter­atomic distances are clearly visible in the calculated atomic pair distribution function (PDF) diagram (Fig. 2), since peaks in the PDF pattern correspond to distances between atomic pairs and so describe the atomic arrangement in a material (Egami & Billinge, 2003; Billinge, 2007). In Fig. 2 we illustrate the calculated PDF up to 10 Å obtained from the crystal structure described herein (solid line) and that from the previously reported structure (Habenschuss & Spedding, 1980) (open circles). The highest peaks in the G(r) function correspond to atomic pairs between the strongest atomic scatterers in this complex, the Nd^III cations, as well as the most abundant pairs (six water molecules). Moreover, examination of the short-scale region in the PDF diagram gives information on the first few coordination spheres and therefore yields extensive information about the local atomic arrangement in the vicinity of the Nd^III sites. The PDF peak at 2.45 Å corresponds to the Nd1···O bonds, whereas the peak at 2.82 Å is present due to both the Nd1···Cl1 pairs and the intra­prism H₂O···H₂O inter­actions. The peaks at 3.15 and 4.18 Å originate from the intra­molecular Cl1···H₂O inter­actions and the intense G(r) peak near 5.1 Å can be assigned to the Nd···Cl2 inter­actions, together with a significant number of next-nearest Nd···O pairs. As can be seen in Fig. 2, the calculated PDFs are quite similar, exhibiting almost the same features. However, appreciable differences must be noted between the PDF obtained from the reinvestigated structural properties (solid line) and that from the previously reported structure (open circles). At first, a small peak near 1 Å, definitely corresponding to the O—H bond, is only observed in the PDF profile obtained from the results reported in this work, as all 12 H atoms were successfully located in a Fourier difference map in our study. Furthermore, we note that the structural parameters reported here are more precise than those communicated previously, due to the different experimental conditions (low temperature, experimental data-collection setup).

The crystal structure of [NdCl₂(H₂O)₆].Cl is made up of three different neodymium pairs linked by O—H···Cl1 inter­actions ranging from 3.1339 (13) to 3.1617 (13) Å (Fig. 3 and Table 3). These pairs are also characterized by significantly different Nd1···Nd1 distances, ranging from 6.3739 (3) to 6.7491 (3) Å, which are clearly visible in the PDF diagram (Fig. 2) in the 6.13–7.26 Å region.

Chloride ion Cl2 is in contact with six neighbouring [NdCl₂(H₂O)₆]⁺ entities, four in the ab plane and two almost directed along the c crystallographic axis. Anions Cl1 and Cl2 are both involved in the highly linked three-dimensional network of inter­molecular inter­actions connecting the different neodymium pairs (Table 3). It is also noteworthy that the O—H···Cl2 inter­actions are only slightly longer [from 3.1703 (12) to 3.2333 (13) Å] than the O—H···Cl1 inter­actions and that the shortest Cl1···Cl2 inter­action is 4.1821 (5) Å.

Related literature

For related literature, see: Billinge (2007); Bukin (1972); Dem'yanets & Emel'yanova (1969); Ebendorff-Heidepriem, Seeber & Ehrt (1995); Egami & Billinge (2003); Habenschuss & Spedding (1979, 1980); Klink et al. (2000); Kuriki et al. (2002); Li et al. (2009); Mussot et al. (2009); Pisarska (2008); Pivin et al. (2005); Reeves et al. (2009); Serqueira et al. (2011); Slooff et al. (2001); Sokolov et al. (2003); Steveler et al. (2011); Tanabe et al. (1992); Zehnder et al. (2010).

Computing details

Data collection: CrysAlis PRO (Oxford Diffraction, 2009); cell refinement: CrysAlis PRO (Oxford Diffraction, 2009); data reduction: CrysAlis PRO (Oxford Diffraction, 2009); program(s) used to solve structure: SHELXS97 (Sheldrick, 2008); program(s) used to refine structure: SHELXL97 (Sheldrick, 2008) and WinGX (Farrugia, 2012); molecular graphics: ORTEPIII (Burnett & Johnson, 1996) and ORTEP-3 for Windows (Farrugia, 2012); software used to prepare material for publication: enCIFer (Allen et al., 2004).

Figures

Fig. 1. A projection of the title compound, showing the atom-labelling scheme. Displacement ellipsoids are drawn at the 50% probability level. [Symmetry code: (i) -x + 3/2, y, -z + 3/2].

Fig. 2. Comparison of the PDF [G(r) function] calculated from the crystal structure obtained in this work (solid line) and that determined from the previously reported structure (open circles) (Habenschuss & Spedding, 1980).

Fig. 3. A packing diagram for [NdCl₂(H₂O)₆].Cl, in the ac plane, showing neodymium pairs linked by the three-dimensional network of O—H···Cl intermolecular interactions (dotted lines).

(Hexaaquadichlorido)neodymium(III) chloride

Crystal data

[NdCl2(H2O)6]·Cl	F(000) = 342
Mr = 358.69	Dx = 2.357 Mg m^−3
Monoclinic, P2/n	Mo Kα radiation, λ = 0.71073 Å
Hall symbol: -P 2yac	Cell parameters from 8593 reflections
a = 7.9850 (3) Å	θ = 3.1–32.9°
b = 6.5561 (2) Å	µ = 5.91 mm^−1
c = 9.6727 (4) Å	T = 100 K
β = 93.655 (4)°	Plate, colourless
V = 505.34 (3) Å^3	0.06 × 0.04 × 0.01 mm
Z = 2

Data collection

Oxford SuperNova Dual diffractometer, Cu at zero, with an Atlas detector	1793 independent reflections
Radiation source: SuperNova (Mo) X-ray source	1710 reflections with I > 2σ(I)
Mirror monochromator	Rint = 0.032
Detector resolution: 10.4508 pixels mm^-1	θmax = 32.9°, θmin = 3.1°
ω scans	h = −12→12
Absorption correction: analytical [CrysAlis PRO (Oxford Diffraction, 2009); analytical numerical absorption correction using a multifaceted crystal model based on expressions derived by Clark & Reid (1995)]	k = −9→9
Tmin = 0.712, Tmax = 0.943	l = −14→14
8593 measured reflections

Refinement

Refinement on F^2	Primary atom site location: structure-invariant direct methods
Least-squares matrix: full	Secondary atom site location: difference Fourier map
R[F^2 > 2σ(F^2)] = 0.016	Hydrogen site location: difference Fourier map
wR(F^2) = 0.030	All H-atom parameters refined
S = 1.03	w = 1/[σ^2(Fo^2) + (0.0097P)^2 + 0.0698P] where P = (Fo^2 + 2Fc^2)/3
1793 reflections	(Δ/σ)max < 0.001
71 parameters	Δρmax = 0.57 e Å^−3
6 restraints	Δρmin = −0.57 e Å^−3

Special details

Geometry. All s.u.'s (except the s.u. in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell s.u.'s are taken into account individually in the estimation of s.u.'s in distances, angles and torsion angles; correlations between s.u.'s in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell s.u.'s is used for estimating s.u.'s involving l.s. planes.

Refinement. Refinement of F² against ALL reflections. The weighted R-factor wR and goodness of fit S are based on F², conventional R-factors R are based on F, with F set to zero for negative F². The threshold expression of F² > 2σ(F²) is used only for calculating R-factors(gt) etc. and is not relevant to the choice of reflections for refinement. R-factors based on F² are statistically about twice as large as those based on F, and R-factors based on ALL data will be even larger.

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Ų)

	x	y	z	Uiso*/Ueq
Nd1	0.7500	0.647838 (16)	0.7500	0.00558 (4)
Cl1	0.75958 (5)	0.32866 (5)	0.55554 (4)	0.01015 (7)
Cl2	0.7500	0.87674 (8)	1.2500	0.01062 (10)
O1W	0.91551 (15)	0.92584 (18)	0.85559 (13)	0.0112 (2)
O2W	0.60535 (15)	0.80024 (18)	0.94150 (13)	0.0113 (2)
O3W	0.45290 (14)	0.54572 (18)	0.71823 (13)	0.0115 (2)
H11	1.003 (2)	0.963 (4)	0.825 (2)	0.034 (7)*
H12	0.873 (3)	1.030 (3)	0.880 (2)	0.031 (6)*
H21	0.512 (2)	0.762 (4)	0.953 (2)	0.028 (6)*
H22	0.645 (3)	0.833 (3)	1.0179 (18)	0.029 (7)*
H31	0.420 (3)	0.431 (3)	0.736 (2)	0.029 (6)*
H32	0.398 (3)	0.581 (4)	0.651 (2)	0.031 (7)*

Atomic displacement parameters (Ų)

	U^11	U^22	U^33	U^12	U^13	U^23
Nd1	0.00511 (6)	0.00594 (5)	0.00569 (6)	0.000	0.00049 (4)	0.000
Cl1	0.01099 (17)	0.00977 (16)	0.00978 (17)	−0.00071 (12)	0.00144 (14)	−0.00203 (13)
Cl2	0.0090 (2)	0.0122 (2)	0.0107 (2)	0.000	0.00065 (19)	0.000
O1W	0.0088 (5)	0.0099 (5)	0.0153 (6)	−0.0016 (4)	0.0027 (5)	−0.0043 (5)
O2W	0.0100 (6)	0.0140 (5)	0.0102 (6)	−0.0019 (4)	0.0031 (5)	−0.0028 (5)
O3W	0.0086 (5)	0.0118 (6)	0.0139 (6)	−0.0016 (4)	−0.0018 (5)	0.0023 (5)

Geometric parameters (Å, º)

Nd1—O1W	2.4371 (12)	Nd1—Cl1^i	2.8180 (4)
Nd1—O1W^i	2.4371 (11)	O1W—H11	0.812 (16)
Nd1—O2W	2.4569 (12)	O1W—H12	0.808 (16)
Nd1—O2W^i	2.4569 (12)	O2W—H21	0.800 (15)
Nd1—O3W	2.4653 (11)	O2W—H22	0.813 (16)
Nd1—O3W^i	2.4653 (11)	O3W—H31	0.819 (16)
Nd1—Cl1	2.8180 (4)	O3W—H32	0.793 (16)
O1W—Nd1—O1W^i	83.19 (6)	O3W^i—Nd1—Cl1	79.32 (3)
O1W—Nd1—O2W	69.41 (4)	O3W—Nd1—Cl1	77.41 (3)
O1W^i—Nd1—O2W	75.13 (4)	O1W—Nd1—Cl1^i	108.22 (3)
O1W—Nd1—O2W^i	75.13 (4)	O1W^i—Nd1—Cl1^i	142.94 (3)
O1W^i—Nd1—O2W^i	69.41 (4)	O2W—Nd1—Cl1^i	76.39 (3)
O2W—Nd1—O2W^i	132.01 (6)	O2W^i—Nd1—Cl1^i	147.07 (3)
O1W—Nd1—O3W^i	70.06 (4)	O3W^i—Nd1—Cl1^i	77.41 (3)
O1W^i—Nd1—O3W^i	138.34 (4)	O3W—Nd1—Cl1^i	79.32 (3)
O2W—Nd1—O3W^i	120.76 (4)	Cl1—Nd1—Cl1^i	84.099 (16)
O2W^i—Nd1—O3W^i	73.11 (4)	Nd1—O1W—H11	121.9 (17)
O1W—Nd1—O3W	138.34 (4)	Nd1—O1W—H12	121.8 (17)
O1W^i—Nd1—O3W	70.06 (4)	H11—O1W—H12	104 (2)
O2W—Nd1—O3W	73.11 (4)	Nd1—O2W—H21	117.4 (17)
O2W^i—Nd1—O3W	120.76 (4)	Nd1—O2W—H22	128.3 (17)
O3W^i—Nd1—O3W	148.49 (6)	H21—O2W—H22	105 (2)
O1W—Nd1—Cl1	142.94 (3)	Nd1—O3W—H31	123.1 (16)
O1W^i—Nd1—Cl1	108.22 (3)	Nd1—O3W—H32	120.0 (18)
O2W—Nd1—Cl1	147.07 (3)	H31—O3W—H32	106 (2)
O2W^i—Nd1—Cl1	76.39 (3)

Symmetry code: (i) −x+3/2, y, −z+3/2.

Hydrogen-bond geometry (Å, º)

D—H···A	D—H	H···A	D···A	D—H···A
O1W—H11···Cl2^ii	0.81 (2)	2.39 (1)	3.194 (1)	171 (2)
O1W—H12···Cl1^iii	0.81 (2)	2.33 (1)	3.134 (1)	177 (3)
O2W—H21···Cl1^iv	0.80 (2)	2.38 (1)	3.153 (1)	163 (1)
O2W—H22···Cl2	0.81 (2)	2.36 (1)	3.170 (1)	171 (2)
O3W—H31···Cl2^v	0.82 (2)	2.44 (1)	3.233 (1)	163 (2)
O3W—H32···Cl1^vi	0.79 (2)	2.37 (1)	3.162 (1)	176 (2)

Symmetry codes: (ii) −x+2, −y+2, −z+2; (iii) −x+3/2, y+1, −z+3/2; (iv) x−1/2, −y+1, z+1/2; (v) −x+1, −y+1, −z+2; (vi) −x+1, −y+1, −z+1.