Structural Characterization of Methanol Substituted Lanthanum Halides

Abstract

The first study into the alcohol solvation of lanthanum halide [LaX₃] derivatives as a means to lower the processing temperature for the production of the LaBr₃ scintillators was undertaken using methanol (MeOH). Initially the de-hydration of {[La(µ-Br)(H₂O)₇](Br)₂}₂ (1) was investigated through the simple room temperature dissolution of 1 in MeOH. The mixed solvate monomeric [La(H₂O)₇(MeOH)₂](Br)₃ (2) compound was isolated where the La metal center retains its original 9-coordination through the binding of two additional MeOH solvents but necessitates the transfer of the innersphere Br to the outersphere. In an attempt to in situ dry the reaction mixture of 1 in MeOH over CaH₂, crystals of [Ca(MeOH)₆](Br)₂ (3) were isolated. Compound 1 dissolved in MeOH at reflux temperatures led to the isolation of an unusual arrangement identified as the salt derivative {[LaBr_2.75•5.25(MeOH)]^+0.25 [LaBr_3.25•4.75(MeOH)]^−0.25} (4). The fully substituted species was ultimately isolated through the dissolution of dried LaBr₃ in MeOH forming the 8-coordinated [LaBr₃(MeOH)₅] (5) complex. It was determined that the concentration of the crystallization solution directed the structure isolated (4 concentrated; 5 dilute) The other LaX₃ derivatives were isolated as [(MeOH)₄(Cl)₂La(µ-Cl)]₂ (6) and [La(MeOH)₉](I)₃•MeOH (7). Beryllium Dome XRD analysis indicated that the bulk material for 5 appear to have multiple solvated species, 6 is consistent with the single crystal, and 7 was too broad to elucidate structural aspects. Multinuclear NMR (¹³⁹La) indicated that these compounds do not retain their structure in MeOD. TGA/DTA data revealed that the de-solvation temperatures of the MeOH derivatives 4 – 6 were slightly higher in comparison to their hydrated counterparts.

Introduction

Rare earth halide (LnX₃) scintillators are of interest for the detection of γ-radiation due to the fact that these materials display excellent luminosity and proportionality, have high spectral energy resolution at room temperature, exhibit short decay times, and possess sufficient stopping power.[1] Additionally, the linear energy response that LnX₃ materials display also makes them attractive for a variety of imaging applications, including chemical tomography (CT), positron emission tomography (PET), and single photon emission computed tomography (SPECT).[1–8] As these materials have been studied in depth, it has been reported that in the presence of trace amounts of oxide LaX₃ readily converts to lanthanum oxy halide, which upon hot-pressing yields an opaque material that detracts from the desired scintillator properties. [2, 3, 5, 6, 8–12] Since the standard synthetic route to LnX₃ materials involves the reaction of rare earth oxide with the appropriate ammonium halide, commercially available materials inherently have residual oxide present.[2, 3, 5, 6, 8–12] Therefore, simple, high-yield routes to oxide free LnX₃ materials are of interest.

In the early 1950’s, reports detailing the syntheses and subsequent calorimetric analyses of LnCl₃•n H₂O crystals isolated from the reaction of Ln° in 1 N HCl(aq) were reported.[13, 14] Since that time, no information pertaining to the single crystal structures of the products generated from this or similar (eq 1) methods have been disseminated.[15–19] Recently, we undertook the synthesis and structural characterization of the products isolated from the dissolution of Ln° in (conc)HX (eq 1) and for the first time crystallographically characterized all of the LnX₃• nH₂O (Ln = group 3 and lanthanides; X = Cl, Br, I) products.[20] In agreement with the spurious literature structure reports (typically obtained from the hydration of “anhydrous LaX₃”), [21–36] three structure types were noted for this family of compounds: (i) hydrate dimer [{(H₂O)₇Ln(μ-X)}₂](X)₂ (ii) 2IS/1OS as [LnX₂(H₂O)₈](X) and (iii) 3OS as [Ln(H₂O)_x](X)₃ (where IS = inner sphere, OS = outer sphere (see Figure 1a–c) and Ln = group 3, the lanthanide series cations). Table 1 lists the structures adopted by the LnX₃•nH₂O series, isolated from eq 1.[20] Additional information pertaining to the synthesis and characterization of these compounds is available in the supplementary information. There are variations in hydration and structural crossover points for these products (eq 1) in comparison to the literature reports.[21–36]

$$\begin{array}{ccc}\mathbf{\text{Ln}}\mathbf{°}+\mathbf{(}\mathit{\text{xs}}\mathbf{)}\mathbf{\text{HX}}& \to & {\mathbf{\text{Ln}}\mathbf{(}\mathbf{X}\mathbf{)}}_{\mathbf{3}}•{\mathit{n}\mathbf{H}}_{\mathbf{2}}\mathbf{O}\end{array}$$ (1)

$$\begin{array}{ccc}{\mathbf{\text{Ln}}\mathbf{(}\mathbf{X}\mathbf{)}}_{\mathbf{3}}•{\mathit{n}\mathbf{\text{H}}}_{\mathbf{2}}\mathrm{O}& \underset{{10}^{-3}\text{Torr}}{\overset{\text{Δ}}{\to }}& {\mathbf{\text{LnX}}}_{\mathbf{3}}+{\mathit{n}\mathbf{H}}_{\mathbf{2}}\mathbf{O}\end{array}$$ (2)

$$\mathbf{\text{Ln}}=\mathbf{\text{Sc}},\mathbf{Y},\mathbf{\text{and}}\mathbf{\text{the}}\mathbf{\text{lanthanide}}\mathbf{\text{series}};\mathbf{X}=\mathbf{\text{Cl}},\mathbf{\text{Br}},\mathbf{I}$$

Figure 1.

Schematic structure plots of hydrate species (i) dimer, (ii) 2IS/1OS, and (iii) 3OS. The halides are in orange, the oxides in red, and the lanthanide cations in yellow or green.

Table 1.

Structural arrangements recorded for LnX₃•nH₂O family of compounds isolated from the reaction of Ln^° in (conc) HX.[20]

	Chloride				Bromide				Iodide
Ln	Solv	IS	OS	Nu	Solv	IS	OS	Nu	Solv	IS	OS	Nu
La	7	2(μ)	4	2	7	2(μ)	4	2	9	0	3	1
Ce	7	2(μ)	4	2	7	2(μ)	4	2	9	0	3	1
Pr	7	2(μ)	4	2	7	2(μ)	4	2	9	0	3	1
Nd	6	2	1	1	6	2	1	1	9	0	3	1
Sm	6	2	1	1	6	2	1	1	9	0	3	1
Eu	6	2	1	1	6	2	1	1	9	0	3	1
Gd	6	2	1	1	6	2	1	1	9	0	3	1
Tb	6	2	1	1	6	2	1	1	9	0	3	1
Dy	6	2	1	1	6	2	1	1	9	0	3	1
Ho	6	2	1	1	8	0	3	1	9	0	3	1
Y	6	2	1	1	8	0	3	1	---	---	---	---
Er	6	2	1	1	8	0	3	1	9	0	3	1
Tm	6	2	1	1	8	0	3	1	8	0	3	1
Yb	6	2	1	1	8	0	3	1	8	0	3	1
Lu	6	2	1	1	8	0	3	1	8	0	3	1
Sc	4	2	1	1	7	0	3	1	7	0	3	1

Ln = lanthanide; Solv = number of H₂O bound per metal; IS = inner sphere; OS = outer sphere; Nu = nuclearity; --- structure not solved.

Color Key – different shades indicate slightly different structure types:

Green = (dimer) [Ln(H-X)(H₂O)₇]₂(X)₄;

Brown (2IS/1OS) [LnX₂(H₂O)₆](X) or [ScCl₂(H₂O)₄][Cl•H₂O]

Blue (3OS) [Ln(H₂O)n](X)₃.

Due to the presence of water, the hydrates are obviously unattractive for scintillator applications; however, in contrast to literature reports,[9, 10, 17, 37] it was determined that anhydrous LaBr₃ could be isolated by simply heating [{(H₂O)₇La(μ-Br)}₂](Br)₂ (1, see Figure 1a) under vacuum (eq 2).[20] This gave a simple, reproducible, large-scale route that produced no waste for production of LaBr₃ scintillator materials. However, the higher temperatures required for this processing often led to darkening of the material upon hot-pressing into a ceramic disk. This color change was ultimately associated with Br volatility.[20] Therefore, lower processing routes to anhydrous or ‘dry’ LaBr₃ were sought to minimize the Br loss.

One approach to remedy this problem was to replace the bound waters of 1 with other Lewis bases that were less strongly bound, with a particular emphasis on alcohol (HOR) derivatives.[11, 18, 19, 38–73] The HOR solvents were of interest since it was reasoned they were strong enough to displace the bound water but potentially offered lower de-solvation temperatures. Reports on the structural aspects of Lewis basic functionalized LnX₃ compounds have been extensive, [18, 19] with a substantial number of compounds possessing the fundamental ‘Ln, HOR, 3X ‘[38–73] composition. However, most of the HOR derivatives were either part of a larger organic moiety or had a large organic ligand bound to the metal. Recently Boatner and co-workers reported on the crystal structure of simple MeOH derivative as [(MeOH)₄(Cl)₂Ce(μ–Cl)]₂ (MeOH = CH₃OH = methanol), which was also reported to be the first example of a metal–organic scintillator. [72, 73]

Based on the lack of ROH derivatives of LaX₃ and our interest in dehydrating 1,[20] it became of interest to understand the structural aspects of LaX₃ in MeOH and determine their physical properties. The products isolated through various synthetic attempts included [La(H₂O)₇(MeOH)₂](Br)₃ (2), [Ca(MeOH)₆](Br)₂ (3), {[LaBr_2.75•5.25(MeOH)]^+0.25[LaBr_3.25•4.75(MeOH)]^−0.25} (4) and the methanolation of LnX₃ to yield [LaBr₃(MeOH)₅] (5) [(MeOH)₄(Cl)₂La(μ-Cl)]₂ (6) and [La(MeOH)₉](I)₃•MeOH (7). The synthesis and characterization of these compounds and their dehydration are presented below.

Experimental

All compounds described below were handled with rigorous exclusion of air and water using standard Schlenk line and glove box techniques unless otherwise noted. Analytical data were collected on dried crystalline samples. All solvents were used as received (from Aldrich and Alfa Aesar) without further purification, including: MeOH (anhydrous, Sure/Seal™ bottle), conc. HCl (aq, 37%), conc. HBr (aq, 48%), conc. HI (aq, 55%), CaH₂, and La^°. Compound 1 (Figure 1a) and LaX₃ where X = Br, Cl and I were synthesized according to eq 1 and eq 2.[20] Yields approached quantitative for each reaction investigated.

[La(H₂O)₇(MeOH)₂](Br)₃ (2)

On the bench-top, a sample of 1 (1.00 g, 1.02 mmol) in a vial was dissolved in MeOH (20 mL) and stirred for 2–3 h. After this time, the reaction was removed from the stir plate and set aside with the cap loose to allow any volatile material to slowly evaporate, until the X-ray quality crystals of 2 were isolated. ¹³⁹La (56.5 MHz, MeOD) δ = −32.7 ppm.

[Ca(MeOH)₆](Br)₂ (3)

CaH₂ (0.100 g) was added to mixture of LaBr₃ (1.00 g, 2.64 mmol) in MeOH (25 mL) and allowed to stir. The CaH₂ dissolved to generate a clear solution. Upon slow evaporation, crystals of 3 were isolated.

{[LaBr_2.75•5.25(MeOH)]^+0.25[LaBr_3.25•4.75(MeOH)]^−0.25} (4)

On a Schlenk line, a sample of 1 (1.00 g, 1.02 mmol) was dissolved in dry MeOH (25 mL) and heated to reflux temperatures for 2 h. The solution was transferred back to an argon filled glove box where the reaction mixture was allowed to slowly evaporate until X-ray quality crystals of 4 were isolated.

Compounds 5 – 7

The appropriate LaX₃ (X = Cl: 1.00 g, 4.08 mmol; Br: 1.00 g, 2.64 mmol; I: 1.00 g, 1.92 mmol) was dissolved in dry MeOH (∼ 10 mL). After stirring for 10 min, the sample was set aside and the volatile portion was allowed to slowly evaporate until X-ray quality crystals were isolated as: [LaBr₃(MeOH)₅] (5) ¹³⁹La (56.5 MHz, MeOD) δ = −34.7 ppm., [(MeOH)₄(Cl)₂La(μ-Cl)]₂ (6) ¹³⁹La (56.5 MHz, MeOD) δ = −31.2 ppm, and [La(MeOH)₉](I)₃•MeOH (7) ¹³⁹La (56.5 MHz, MeOD) δ = −35.1 ppm.

Characterization

All analytical data were collected on dried crystalline material. Solution ¹³⁹La NMR spectra were obtained on a Bruker DRX400 instrument operating at 56.5 MHz using a 5 mm broadband probe with standard single pulse acquisition conditions. The static wide line ¹³⁹La NMR experiments were performed on a Bruker Avance 400 using 4 mm broadband probe and a solid echo θ-τ-θ-acq sequence using a 1 μs (θ = π/12) pulse length, and a inter-pulse spacing τ = 20 µs with 32K scan averages. Both the solution and solid-state ¹³⁹La NMR spectra were referenced to an external secondary standard of 1 M LaCl₃ (δ = 0.0 ppm). Deconvolutions of line-widths and chemical shifts, along with the simulation of the static second order quadrupolar line shape were performed in software program DMFIT.[74]

Standard powder XRD data were collected using a Siemens D500 diffractometer equipped with a diffracted beam graphite monochromator and scintillation detector using 1° incident and diffracted beam slits (diffractometer radius = 250 mm). Scans were collected from 5 to 60° for 2θ, at 0.04° steps, and 2 sec count time using CuKα radiation at 40 kV 30 mA. Simultaneous thermogravimetric/differential thermal analysis (TGA/DTA) experiments were performed on an STD 2960 under an atmosphere of argon at a ramp-rate of 5 °C/min. to 650 °C.

Information pertaining to the details of the beryllium dome XRD (BeD-XRD) analyses has been previously disseminated;[20, 75, 76] hence, only a short description is presented here. All sample preparation was performed in an argon filled glovebox using a 1 cm quartz disk (zero-background plate), where the sample was pressed into the specimen cavity, leveled to the holder base using a glass slide, and the BeD cover sealed. The BeD holder was carefully loaded into the Siemens D500 diffractometer. For all scans the instrument settings were 40 kV and 30 mA with a: 0.04° step-size, 1 sec count-time, scan range of 5–30° 2θ, 1° divergence and receiving slits; the goniometer radius was 250 mm. Note: Due to the presence of potentially toxic Be°, it is important that only trained personnel, wearing the appropriate personal protective equipment (i.e., rubber gloves) handle the BeD. If poor handling techniques or any other means shatter the BeD-XRD, proper safety clean-up and disposal protocols must be followed.

General X-ray Crystal Structure Information

Single crystals were mounted onto a glass fiber from a pool of Fluorolube™ and immediately placed in a cold N₂ vapor stream, on a Bruker AXS diffractometer employing an incident-beam graphite monochromator, MoKα radiation (λ = 0.7107 Å) and a SMART APEX CCD detector. Lattice determination and data collection were carried out using SMART Version 5.054 software. Data reduction was performed using SAINTPLUS Version 6.01 software and corrected for absorption using the SADABS program within the SAINT software package. Structures were solved by direct methods that yielded the heavy atoms, along with a number of the lighter atoms or by using the PATTERSON method, which yielded the heavy atoms. Subsequent Fourier syntheses yielded the remaining light-atom positions. The hydrogen atoms were fixed in positions of ideal geometry and refined using SHELX software. The final refinement of each compound included anisotropic thermal parameters for all non-hydrogen atoms. Table 2 lists the unit cell parameters for the structurally characterized compounds 2 – 7. All final CIF files were checked using the CheckCIF program (http://www.iucr.org/). Additional information concerning the data collection and final structural solutions can be found in the supplemental information or by accessing CIF files through the Cambridge Crystallographic Data Base. Additional information concerning the data collection and final structural solutions can be found in the CCDC database.

Table 2.

Data Collection parameters for 2 – 7 and S1 - S3.

Compound	2	3	4
chemical formula	C2H22Br3LaO9	C6H24Br2CaO6	C10H36Br6La2O10
formula weight	568.84	392.15	1075.68
temp (K)	173(2)	173(2)	173(2)
space group	Orthorhombic, Pbca	Hexagonal, P-3	Monoclinic, Pc
a (Å)	12.6540(13)	8.3860(12)	10.1803(16)
b (Å)	12.9686(13)	8.3860(12)	14.794(2)
c (Å)	19.864(2)	6.8655(19)	10.6387(17)
β (deg)			106.532(2)
γ (deg)		120
V (Å3)	3259.7(6)	418.13(14)	1536.0(4)
Z	8	1	2
D calcd (Mg/m3)	2.318	1.557	2.326
μ,(Mo, Kα) (mm−1)	9.996	5.158	10.581
R1a (%) (all data)	1.58 (1.94)	3.99 (4.26)	2.37 (2.57)
	3.33 (3.44)	10.27 (10.38)	4.94 (5.21)
wR2b (%) (all data)
Compound	5	6	7
chemical formula	C5H15Br3LaO5	C8H32Cl6La2O8	C10H40I3LaO10
formula weight	533.81	746.86	840.03
temp (K)	173(2)	173(2)	173(2)
space group	Orthorhombic, Pna2(1)	Monoclinic, P2(1)/c	Monoclinic, P2(1)/c
a (Å)	12.3556(14)	8.750(5)	15.941(3)
b (Å)	9.9655(11)	18.635(11)	8.5737(14)
c (Å)	12.5633(14)	8.278(5)	20.140(3)
β (deg)		108.958(8)	93.625(2)
V (Å3)	1546.9(3)	1276.5(13)	2747.0(8)
Z	4	2	4
Dcalcd(Mg/m3)	2.292	1.943	2.031
μ,(Mo, Kα) (mm−1)	10.506	3.954	4.959
R1a (%) (all data)	4.58 (4.64)	1.73 (1.75)	1.87 (2.10)
wR2b (%) (all data)	11.35 (11.39)	4.35 (4.37)	4.54 (5.52)
Compound	S1	S2	S3
chemical formula	C12H24Br3O3Sc	C16H32Br3NdO4	C20H20Br3N4Nd
formula weight	501.00	672.39	700.34
temp (K)	181(2)	173(2)	173(2)
space group	Orthorhombic, Pbcn	Triclinic, P-1	Orthorhombic, Pbca
a (Å)	8.783(3)	8.2662(13)	16.998(2)
b (Å)	13.884(5)	9.3601(15)	17.515(2)
c (Å)	14.540(5)	15.485(2)	32.617(5)
α(deg)		79.185(2)
β (deg)		87.106(2)
γ (deg)		74.956(2)
V (Å3)	1773.1(11)	1136.5(3)	9711(2)
Z	4	2	16
Dcalcd(Mg/m3)	1.877	1.965	1.916
μ,(Mo, Kα) (mm−1)	7.176	7.573	7.088
R1a (%) (all data)	0.0967 (0.0986)	0.0463 (0.0505)	0.0386 (0.0866)
wR2b (%) (all data)	0.3862 (0.3895)	0.1264 (0.1282)	0.0619 (0.0796)

Results and Discussion

A survey of the crystallographically characterized species[18, 19] available with any Ln cation, 3 Br atoms, and any solvent (not water) was undertaken in order to identify potentially useful solvents that would solubilize LaBr₃ as well as displace the waters of 1.[20] Fewer than 30 species met this criteria,[55, 56, 77–87] with the majority being monomers solvated by either THF[78, 81, 88] or DME[78, 80, 81]. Additionally, we have isolated a number of solvated LnX₃ species that add to this family, including: [ScBr₃(THF)₃] (S1), [NdBr₃(solv)₄] solv = THF (S2) and py (S3). Data collection parameters for S1 – S3 can be found in Table 2, the supplementary information, and the CCDC database. Of interest for this study were the two compounds directly bound by an ROH, identified as [NdBr₃(THF)₂(PrⁱOH)₂] [55] and [SmBr₃(PrⁱOH)₄] [56] where PrⁱOH = (CH₃)₂(H)COH.

For the LnBr₃(solv)_x species, THF has been the most reported solvate crystallized to date; however, in our hands, LaBr₃ displayed only limited solubility in THF at room temperature. Alcohols came to the forefront based on a series of solubility experiments with MeOH in particular raising the most interest since LaX₃ readily dissolved in this solvent at room temperature. Structural reports of [LnX₃(MeOH)_x] where the MeOH is directly bound to the Ln metal center have been limited to the Cl [40–46, 72, 73] derivatives only. [18, 19] Due to the void in structurally characterized ROH derivatives of LaX₃, a study concerning the coordination and desolvation behavior of 1 with MeOH was undertaken. It is of note that standard analytical characterization methods (i.e., FTIR, ¹H and ¹³C NMR, elemental analysis) for these compounds were of limited utility in identifying these compounds. Of critical importance was single crystal X-ray diffraction and ¹³⁹La NMR and these results are presented below.

Synthesis of MeOH Derivatives

Bromide

After stirring on the bench-top for 12 h, the reaction mixture of 1 in MeOH was set aside with the cap loose to allow any volatile material to slowly evaporate. The resultant X-ray quality crystals proved to be 2. As can be observed in Figure 2, the original dimer of 1 (Figure 1a) was disrupted upon introduction of MeOH, forming a 3OS monomer. For 2, the La metal center maintains its 9 coordination through the binding of the original 7 H₂O molecules with 2 additional MeOH solvent molecules. Due to the increased steric bulk of the MeOH in comparison to the waters, coupled with the increased number of electron donating oxygen atoms around the La metal center, all of the Br atoms are forced to the outersphere. Since the 3OS structure (Figure 1c) of LnBr₃ has previously been observed only for the smaller heavier Ln anions or for the LnI₃•nH₂O derivatives, an appropriate structural model is not readily available. [20–36] The Ln-O distances of 2 (av. 2.55 Å for H₂O and av. 2.53 Å MeOH) were found to be in agreement with 4 and 5 (see Table 2) and the hydrates (av. 2.55 Å for LaBr₃•n H₂O),[20] but significantly longer than the literature alcohol derivatives (av. 2.46 Å[55, 56]).

Figure 2.

Structure plot of 2. Thermal ellipsoids are drawn at 30 % level.

Several attempts to employ in situ drying agents to remove the water ligands of 1 upon dissolution in MeOH were investigated. In one of these studies, the crystal isolated from the dissolution of 1 in MeOH in the presence of CaH₂ (the drying agent employed) proved to be the Ca derivative 3, not the desired La species. For 3, the Ca atom is octahedrally bound by MeOH with 2 outersphere Br atoms present in the lattice. Since no CaX₂•nMeOH species have been previously presented, [18, 19] it is reported here and shown in Figure 3. The alkaline earth was apparently more halophilic than the lanthanide with a calculated by-product being 2/3 “LaH₃”. All other attempts to isolate hydrate free LnX₃ species at room temperature through drying agents were not successful.

Figure 3.

Structure plot of 3. Thermal ellipsoids are drawn at 30 % level. The missing mirror reflected Br is not shown.

Therefore, higher temperature processes were investigated, without the drying agent additives. Compound 1 was heated in MeOH at reflux temperatures for 2h under an argon atmosphere. After cooling to room temperature and vacuum distillation of the majority of the reaction’s volatile component, X-ray quality crystals were isolated that proved to be 4 (Figure 4). The bound waters were successfully replaced by MeOH but an unusual salt structure was formed instead of the simple coordination species previously reported for ROH derivatives. [18, 19, 40–46, 55, 56, 72, 73] In 4, the 9-coordinated La cation of the first molecule in the unit cell was modeled with 2.75 Br atoms and 5.25 MeOH solvent molecules for an overall +0.25 charge. One of the Br sites (Br7) was partially occupied by an oxygen atom from a MeOH ligand and was refined for the Br:O ratio at 0.75:0.25. A partially occupied methyl carbon atom was also observed coordinated to the disordered O site and was constrained to have the same 0.25 occupancy. The other 9-coordinated La cation in the unit cell was modeled to have 3.25 Br atoms and 4.75 MeOH solvent molecules for an overall −0.25 charge. Again, a coordinated MeOH (O3) was found to partially occupy one of the Br sites at a Br:O ratio of 0.25:0.75. An additional methyl carbon (C3) was located near O3 atom position and constrained to have the same 0.75 occupancy.

Figure 4.

Structure plot of 4. Thermal ellipsoids are drawn at 30 % level.

The metrical data for 2 and 4 are listed in Table 3. Due to the substantial differences in the two structures, the Ln-solv distances and angles are the only comparable metrical data. The fully solvated La cation of 2 has substantially longer Ln-solv distances than that of the salt 4. This must be a reflection of the electron withdrawing strength of the bound halides in 4 and the increased electron donation by the increased number of bound ROH and H₂O molecules in 2.

Table 3.

Metrical data for 2, 4 – 7

cmpd	Ln-X   (Å)	Ln---X   (Å)	Ln-(μX)   (Å)	Ln-solv   (Å)	X-La-X   (deg)	X-La-solv   (deg)	solv-La-solv     (deg)
2	------	5.21 – 14.18	-------	2.55 (H2O) 2.53 (MeOH)	-------	-------	66.6 – 142.3
4	2.98	------	-------	2.46	86.3 −145.3	82.4–145.8	82.4 – 145.8
5	3.00	------	-------	2.56	102.9 −146.4	68.5 −146.7	68.1 – 142.1
6	2.86	------	2.865	2.53	78.9−141.6	69.2−145.7	71.7–142.8
7	------	5.28–13.16	-------	2.55 av	-------	-------	66.0−139.4

Due to the surprising complexity of 4, it was of interest to determine the coordination behavior of anhydrous LaBr₃[20] in MeOH. X-ray quality crystals were successfully grown by slow evaporation of the reaction mixture under an argon atmosphere. The 8-coordinated La metal center of 5 (see Figure 5) is best considered distorted square antiprism using 3IS Br atoms and 5 MeOH solvent molecules to fill the coordination sites. The formation of the 3IS structure of 5 versus the 3OS of 2 must be due to the steric bulk of the MeOH preventing the binding of additional solvent ligands that would electronically satisfy the La; thereby, forcing the Br anions to the outersphere. The only other structures of ROH/Ln/3 Br derivatives reported[18, 19] are the previously discussed monomeric PrⁱOH derivatives[55, 56]. For 5 the Ln-Br and Ln-ROH distances (3.00 and 2.53 Å), respectively, are elongated when compared to the [NdBr₃(THF)₂(PrⁱOH)₂][55] (2.85 and 2.46 Å) and [SmBr₃(PrⁱOH)₄][56] (2.83 and 2.46 Å). This variation is most likely a result of the additional electron donation by the 5 MeOH ligands in comparison to the 4 Lewis bases on the other compounds.

Figure 5.

Structure plot of 5. Thermal ellipsoids are drawn at 30 % level.

Due to the similar make up of 4 and 5, it was of interest to elucidate the conditions that yielded the particular structure. From a sealed capillary tube containing a concentrated solution of LaBr₃ dissolved in MeOH, crystals that proved to be 4 were isolated. This led us to investigate the role that concentration played in the final structure observed. Crystals of 4 were also successfully isolated from a super saturated solution of LaBr₃ n MeOH (i.e., the reaction mixture had to be heated to generate a clear solution). In contrast, a very diluted solution of 1 refluxed in MeOH that was allowed to slowly evaporate yielded crystals of 5. From these reactions, compounds 4 and 5 were isolated from the same reaction mixtures that had previously yielded the other structure. Therefore, rapid crystal growth at high concentrations appears to lead to the disordered salt 4 and longer slower growth yields the monomeric neutral species 5.

Chloride

Based on the significant changes noted for the Br derivatives, in comparison to literature compounds[18–20, 55, 56], it became of interest to understand how the other halide structures would be affected by MeOH substitution. A search of the structure literature concerning compounds with any Ln cation, 3 Cl or I anions, and any solvent (not water) was again undertaken. [18, 19]. In contrast to the small number of compounds found for the ‘Ln/3Br/solvate’ structure search, more than 230 solvated compounds were available that contained three Cl atoms for each Ln center. [18, 19] The majority of these compounds were found to be monomers, using pyridine n-oxide, polyamines, crown ethers, phosphine oxides, pyrazolyl borates, dimethoxyethane, acetonitrile, ROHs, and other simple organic solvents. Of the ROH derivatives bound to the Ln cation, only eight compounds were with MeOH [40–46, 72, 73] and four with ethanol (EtOH)[47–50]; however, these structures were isolated in the presence of other co-ligands such as crown ethers[44, 45, 47, 48] or polypyridyl[40–43, 49]. The one exception was the previously discussed [Ce(μ-Cl)Cl₂(MeOH)₄]₂.[72, 73] Three Ln compounds bound by PrⁱOH, [(PrⁱOH)₃(Cl)₂Ln(μ-Cl)]₂ where Ln = La[51], Ce[52], Nd[53] (similar to the Yb/THF [89] species), and one n-butanol bound Tb salt [TbCl₅(HO(CH₂)₄](HNC₅H4-Me-2)[54] have also been disseminated.

Due to the limited information on MeOH systems of the LaX₃ congeners, a dried sample of LaCl₃ that had been prepared according to eq 1[20] was dissolved in MeOH. After stirring for 24 h, X-ray quality crystals were grown over an extended period of time by slow evaporation of the volatile portion of the reaction mixture. The Cl derivative 6 was solved as the dinuclear species (see Figure 6) where each 8-coordinated La possesses 3IS Cl atoms (two terminal and one bridging) and four MeOH solvent molecules filling the rest of the coordination sphere. This is a similar construct as predicted more than 40 years ago by Smith et al.[90] and observed for the congener [(MeOH)₄(Cl)₂Ce(μ-Cl)]₂,[72, 73] as well as the PrⁱOH derivatives: [(PrⁱOH)₃(Cl)₂Ln(μ-Cl)]₂ where Ln = La[51], Ce[52], Nd[53]. The La-ROH (2.53 Å), La-Cl(2.86 Å), and La(μ-Cl) (2.87 Å) distances of the chloride derivative 6 were consistent with the [[(MeOH)₄(Cl)₂Ce(μ-Cl)]₂[72, 73] when the cation size is taken into account but varied in comparison to [(PrⁱOH)₃(Cl)₂La(μ-Cl)]₂ (av La: -ROH 2.52 Å; -Cl 2.78 Å; -μ-Cl 2.96Å). [51] The (μ-Cl)-La-(μ-Cl) and La-(μ-Cl)-La bond angles (78.9 and 141.6° for 6 versus 72.0 and 108.0° for [(PrⁱOH)₃(Cl)₂La(μ-Cl)]₂[51]) were not found to be in agreement, which was attributed to the additional MeOH around the 8-coordinated La of 6 versus the 7-coordinated La of the PrⁱOH structures.[51–53]

Figure 6.

Structure plot of 6. Thermal ellipsoids are drawn at 30 % level.

Iodide

For the reported structures[18, 19] that possess one Ln, three I, and non-aqueous solvate, more than 55 structures met the desired criteria.[38–40, 83, 91–116] While these complexes ranged in nuclearity, the majority were monomers adopting a 2IS/1OS arrangement [(L)_xLnI₂](I)[38, 40, 91–96, 106, 111, 117] (where L = neutral ligand or organic solvents such as: PrⁱOH,[38] THF,[106, 117] and py[38, 106]) or the 3IS arrangement that utilizes combinations of bulky neutral ligands,[83, 95, 107, 110, 114, 118] THF,[98, 100, 102, 113] or PrⁱOH [39] to fill the coordination sphere. However, no structure reports of the LaI₃•MeOH derivatives have been previously reported.

De-hydrated LaI₃ (eq 1) [20] was dissolved in MeOH and X-ray quality crystals grown as noted above. The crystal solved as 7 (see Figure 7) in a 3OS arrangement for the halides, 9 MeOH solvent molecules bound to the La metal center, and an additional MeOH located in the unit cell lattice. The coordination around the metal center is in direct contrast with the literature reported ROH derivatives [LnI₃(PrⁱOH)₄][39] (Ln = Ce, La) and [LuI₂(PrⁱOH)₄](I)[38]. None of these compounds are reasonable models and as a result the La-ROH distances of 7 (av 2.55 Å) were found to be longer in comparison: 2.238 – 2.271 Å (Lu),[38] 2.492 – 2.524 Å (La) , and 2.478 – 2.496Å (Ce). [38, 39] A better structural model is [La(H₂O)₉](I)₃[20] where the Ln-H₂O (av 2.54 Å) and Ln---I distances (range 5.13 to 6.81 Å) were found to be in line with those noted for 7 (Ln ---I = 5.28 Å).

Figure 7.

Structure plot of 7. Thermal ellipsoids are drawn at 30 % level.

Bulk Powder Characterization

The limitations of standard analytical methodologies in supplying useful data concerning the bulk LnX₃(MeOH)_x products led us to investigate alternative characterization techniques such as Beryllium dome PXRD (BeD XRD) which is a useful method for characterizing air-sensitive species.[20, 75, 76] The BeD-XRD patterns were obtained for 4 – 7 (Figure 8) and were compared to the simulated powder X-ray patterns generated from the solved single crystal structure. The patterns obtained for 6 (Figure 8a) were found to be consistent with the theoretical patterns. Variability of the observed peak intensities results from texture effects and large grains present in the bulk powder versus the assumed randomize powders for the calculated patterns. For 5 and 7 the PXRD patterns observed were not consistent with the simulated pattern. For 5 (Figure 8b) in addition to the expected pattern, the experimental data also reveals some additional peaks, which have been tentatively assigned to partially desolvated species of 5 (i.e., −1 MeOH, −2 MeOH, etc). The patterns noted for 7 were broad and indistinct and no direct comparison can be made.

Figure 8.

BeD-XRD patterns (calculated and experimental) of (a) 6 and (b) 5.

Since the BeD-XRD data yielded limited information on the characterization of the bulk powder, solid-state ¹³⁹La NMR experiments were undertaken and spectra obtained for LaBr₃,[20] 1,[20] 2, and 5 (Figure 9). The ¹³⁹La nucleus has a 7/2 spin nucleus with considerable quadrupolar interaction and possesses a quadrupolar coupling constant (QCC) that is predicted to be between 15 and 30 MHz. These properties lead to broad, second order, quadrupolar line shapes, which are very sensitive to the local bonding symmetry surround the La nucleus.

Figure 9.

Solid-state ¹³⁹La NMR data for: (i) LaBr₃, (ii) 1, (iii) 2, (iv) 5 , (v) 6, (vi) 7.

For LaBr₃, simulation (Figure 9i) of the quadrupolar lineshape gave QCC = 17.4 MHz, an asymmetry parameter of η = 0 and an isotropic chemical shift of δ_iso = 418 ppm.[20] This is consistent with previous literature reports for LaBr₃ (QCC = 17 MHz, η = 0 and δ_iso = 400 ppm).[119] A chemical shielding anisotropy (CSA) of ∼ 100 ppm has also been reported[119] but makes only a minor impact on the NMR spectrum at this moderate magnetic field strength. The symmetric electric-field gradient (EFG) tensor (η = 0) reflects the symmetry around the La that resides on a single crystallographic site with C₃ symmetry and is equivalent to that observed in LaCl₃.[119] The chemical shift for LaBr₃ (δ_iso = +418 ppm) is less shielded than that reported for LaCl₃ (δ_iso = +305 ppm) or LaF₃ (δ_iso = −135 ppm) and reflects a strong inverse-halogen dependence within these La-trihalides.

The addition of water within these compounds modifies the local structure resulting in the removal of the local La nucleus symmetry. For example, LaCl₃•6H₂O shows a dramatic increase in both the quadrupolar coupling constant and asymmetry of the ¹³⁹La EFG tensor, QCC = 23.7 MHz, with η = 0.40, in comparison to LaCl₃, QCC = 15.3 to 15.5 MHz, η = 0.00. [119, 120] For compound 1 (Figure 9ii) the ¹³⁹La NMR data gave QCC = 25.3 MHz, η = 0.39 with δ_iso = +315 ppm reflecting the loss of local symmetry to the La environment. The reduction in the chemical shift of 1 compared to the observed chemical shift of LaBr₃ (δ_iso = +418) reflects the incorporation of oxygen into the coordination environment. This also demonstrates that the La is not fully coordinated by oxygen atoms alone, which would have a chemical shift between +200 and −200 ppm.[121] While the general breadth of the observed ¹³⁹La spectrum is consistent with incorporation of water into the La coordination environment, the “irregular spikes” observed in the spectrum argues that a range of La environments exist.[20] Similar spikes have been reported for the LaI₃ material and in that case were attributed to samples that formed fibrous mats and did not reflect true powder samples.[119]

In contrast to the spectra of 1, the mixed solvate species 2 yielded a much narrower ¹³⁹La NMR spectrum (Figure 9iii) with QCC = 11.8 MHz, η = 0.39 and δ_iso = +16.9 ppm. The reduction in the QCC is consistent with the very similar La-O bond distances in the coordinating H₂O (2.55 Å) and MeOH (2.53 Å) ligands (See Table 2). The dramatic reduction in the chemical shift reflects the increased shielding around the La nuclei, and also demonstrates that the Br nuclei are not directly bonded to the La as observed in the reported structure (Table 2 and Figure 2). For compounds with only oxygen-bearing ligands a correlation between La coordination number (CN) and the isotropic chemical shift has been proposed.[121] Based on this correlation the δ_iso = +16.9 ppm corresponds to a CN = 9 – 10, consistent with the structure (Figure 2).

The exceptionally broad ¹³⁹La NMR spectra of 5 and 6 do not allow for a unique determination of the EFG tensor values but the very large widths do reflect a loss of symmetry around the La nucleus. These spectra are consistent with structures that the possesses 3 coordinating halide atoms and a number of coordinating oxygen nuclei from MeOH. Therefore, it is apparent that the solid state spectra are consistent with the solid state structures of 5 and 6. For 7, the sharp singlet is consistent with the 3OS structure where the nuclei is symmetrically coordinated by MeOH and no broadening is noted since the halides are located in the outersphere.

Solution Characterization

Further characterization of 2, 5 – 7 in MeOD was undertaken using NMR spectroscopic investigations. Due to the limited information that could be garnered by standard NMR nuclei (i.e., ¹H, ¹³C), solution ¹³⁹La NMR spectra were collected (see Figure 10). The similarity of the observed chemical shifts (δ_iso from −31 to −35 ppm) coupled with the large change from the δ_iso measured by solid state ¹³⁹La NMR for 2 and 5 (vide infra), clearly demonstrates that the structures are not retained in solution. The range of δ_iso is consistent with oxygen only coordinating ligands where each La has a CN ∼ 10. This chemical shift (-34 to – 40 ppm) has been previously attributed to the fully hydrated La sites within zeolites.[120, 122] Based on these data, it is reasoned that in MeOH, compounds 2, 5 – 7 adopt a 3OS geometry with the nuclei fully solvated by O bearing solvents (i.e., MeOH or H₂O).

Figure 10.

Solution (MeOD) ¹³⁹La NMR data for (i) 2 (5¹³⁹La = −32.7), (ii) 5 (5¹³⁹La = −34.7), (iii) 6 (5¹³⁹La = −31.2), (iv) 7 (5¹³⁹La = −35.1).

Thermal Decomposition

Since there were significant structural variations noted between the hydrated species (Figure 1) and the methanolated compounds 5 – 7 (Figure 5 – Figure 7), the thermal desolvation temperatures of these MeOH derivatives were investigated. A review of the TGA/DTA data on the dimer LnX₃•nH₂O (X = Cl and Br) species, revealed that there were at least three weight loss steps accompanied by three endotherms initiating/ending at 75/175 °C and 100/225 °C, respectively. In contrast, the I derivative (3OS - Figure 1c) showed only one weight loss and a fairly flat thermal event over this temperature range. [20]

Under identical conditions noted for the hydrates, the TGA/DTA spectra of 5 – 7 were obtained (see Figure 11 i – iii, respectively). The TGA weight loss for full MeOH removal for 5, 6, and 7 were calculated to be 34.3, 30.0 and 38.1 %, respectively and are consistent with the experimentally obtained weight losses achieved at 250 °C. Additional weight losses noted in the TGA spectra at higher temperatures are associated with halide volatility, as observed for the hydrate system. [20] For 5 (Figure 11i), the de-solvation of the 3IS Br adduct initiates at 100 °C and is complete by 200 °C, following multiple weight loss steps and numerous endotherms. Not unexpected, this pattern is similar to the spectrum obtained for the iso-structural [(H₂O)₆Br₂Ln(μ-Br)]₂.[20] The overall spectrum of the Cl derivative, 6 is also nearly identical to its iso-structural hydrate spectrum, with multiple weight loss steps initiated between 75 to 180 °C coupled to three major and one minor endotherm. For 7, three distinct weight losses with three separate endotherms were noted for the two weight loss steps. This is surprisingly different from the iso-structural 3OS hydrate spectrum. Combined, the de-methanolation appears complete at ∼200 for 5 and 6 but 155 °C for 7, which are slightly elevated in comparison to the hydrate species.

Figure 11.

TGA/DTA spectrum of (i) 5, (ii) 6, (iii) 7.

Summary and Conclusion

Attempts to dehydrate 1 through room temperature dissolution in MeOH led to the crystallographic characterization of 2, a 3OS anion species with the La metal center coordinated by a mixture of H₂O and MeOH solvate ligands. An unusual salt derivative (4) was isolated from a heated reaction mixture of 1 in MeOH. Each La metal center is 8-coordinated using MeOH and Br atoms but the distribution of the Br atoms is not equal between the two metal centers. In contrast, the first structural derivatives of LaX₃ solvated by MeOH were synthesized from the dissolution of the anhydrous LaX₃ in MeOH, which have been identified as 8-coordinated species: 5, a monomer using five MeOH and 3IS bromide anions; 6, a dimer with full IS chloride anions (four terminal and two bridging) and MeOH solvate ligands; 7, a monomer with 3OS iodide anions and 8 MeOH ligands. Solution structures were determined by ¹³⁹La NMR studies to be 3OS for 5 – 7. While structural changes were produced by the solvent exchange, in comparison to the water derivatives, the thermal decomposition of the MeOH derivatives (5 – 7) appeared to be very similar or require higher temperatures than the hydrates. Alternative ROH and non-ROH based solvent systems are being explored as a means to generate LaX₃ at lower processing temperatures.

Appendix A. Supplementary data

CCDC (714429 – 714434 and 730384 – 730386) contains the supplementary crystallographic data for 2 – 7 and S1 – S3, respectively. These data can be obtained free of charge via http://www.ccdc.cam.ac.uk/conts/retrieving.html, or from the Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: (+44) 1223-336-033; or e-mail: deposit@ccdc.cam.ac.uk.