Article pubs.acs.org/IC
Ammonothermal Synthesis, Crystal Structure, and Properties of the Ytterbium(II) and Ytterbium(III) Amides and the First Two Rare-EarthMetal Guanidinates, YbC(NH)3 and Yb(CN3H4)3 Arno L. Görne,† Janine George,† Jan van Leusen,† Gerald Dück,† Philipp Jacobs,† Naveen Kumar Chogondahalli Muniraju,‡ and Richard Dronskowski*,† †
Institute of Inorganic Chemistry, RWTH Aachen University, Landoltweg 1, 52056 Aachen, Germany Jülich Centre for Neutron Science (JCNS), Forschungszentrum Jülich GmbH, Outstation at Spallation Neutron Source (SNS), and Chemical and Engineering Materials Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, United States
‡
S Supporting Information *
ABSTRACT: We report the oxidation-controlled synthesis of the ytterbium amides Yb(NH2)2 and Yb(NH2)3 and the first rare-earthmetal guanidinates YbC(NH)3 and Yb(CN3H4)3 from liquid ammonia. For Yb(NH2)2, we present experimental atomic displacement parameters from powder X-ray diffraction (PXRD) and density functional theory (DFT)-derived hydrogen positions for the first time. For Yb(NH2)3, the indexing proposal based on PXRD arrives at R3̅, a = 6.2477(2) Å, c = 17.132(1) Å, V = 579.15(4) Å3, and Z = 6. The oxidation-controlled synthesis was also applied to make the first rareearth guanidinates, namely, the doubly deprotonated YbC(NH)3 and the singly deprotonated Yb(CN3H4)3. YbC(NH)3 is isostructural with SrC(NH)3, as derived from PXRD (P63/m, a = 5.2596(2) Å, c = 6.6704(2) Å, V = 159.81(1) Å3, and Z = 2). Yb(CN3H4)3 crystallizes in a structure derived from the [ReO3] type, as studied by powder neutron diffraction (Pn3,̅ a = 13.5307(3) Å, V = 2477.22(8) Å3, and Z = 8 at 10 K). Electrostatic and hydrogen-bonding interactions cooperate to stabilize the structure with wide and empty channels. The IR spectra of the guanidinates are compared with DFT-calculated phonon spectra to identify the vibrational modes. SQUID magnetometry shows that Yb(CN3H4)3 is a paramagnet with isolated Yb3+ (4f13) ions. A CONDON 2.0 fit was used to extract all relevant parameters.
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liquid ammonia in steel autoclaves.12 In the ytterbium− ammonia system, the compounds Yb(NH3)614 and double amides NaYb(NH2)4,15,16 M3Yb(NH2)6 (M = Na, K, Rb),17,18 and Cs3Yb2(NH2)919 were also reported. In recent years, our group has studied the strong base guanidine CN3H5 and reported its crystal structure as solved from single-crystal data almost 150 years after guanidine’s first reference in the scientific literature.20,21 Guanidine is a molecular crystal held together by hydrogen bonding, as validated by single-crystal neutron diffraction.21−23 It is soluble in liquid ammonia.24 Building on this work, we have begun to establish a new class of materials, namely, unsubstituted guanidinate salts with deprotonated anions of the strong base guanidine (Figure 1). The first examples are given by the alkali-
INTRODUCTION Liquid ammonia is a solvent that has found widespread use for water-sensitive solutes and reactions. While NH3(l) is still polar, it is more covalent than water and can thus dissolve organic molecules, inorganic salts, and even some metals.1 As a liquid, it was prepared for the first time in 1812 by Bacelli and in 1823 by Faraday.2,3 In the same century, the solubility of certain metals in liquid ammonia was discovered,4,5 and the first reactions in ammonia as a solvent were pioneered.6 Elemental ytterbium metal can also be dissolved in ammonia as discovered in 1956. It forms a typical blue solution, caused by solvated electrons that react over time with the ammonia, forming hydrogen and NH2−.7 The first ytterbium amide was observed in 1968.8 Shortly after, the crystal structure of YbII(NH2)2 and the powder X-ray diffraction (PXRD) pattern of YbIII(NH2)3 were reported.9,10 Interestingly, ytterbium metal is first dissolved as YbII but then undergoes a follow-up reaction with NH2− and precipitates as ytterbium(III) amide, releasing yet another electron.11,12 This reaction is typically fast,11 explaining why Yb(NH2)2 prepared from liquid ammonia also contains a considerable amount of Yb(NH2)3.9 Instead, phasepure but amorphous Yb(NH2)2 was synthesized from ammonia gas.13 Only recently was a reaction reported in which phasepure and moderately crystalline Yb(NH2)2 was obtained from © XXXX American Chemical Society
Figure 1. From left to right: the guanidinium cation CN3H6+, the guanidine CN3H5, the guanidinate anion CN3H4−, and the doubly deprotonated C(NH)32− anion. Received: March 24, 2016
A
DOI: 10.1021/acs.inorgchem.6b00736 Inorg. Chem. XXXX, XXX, XXX−XXX
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potassium carbonate was recovered. Sublimation for 10 days at 55 °C gave guanidine as a colorless, phase-pure powder [yield 78% in comparison to guanidinium in (CN3H6)2CO3]. To obtain XRD-pure YbC(NH)3, 34.8 mg of ytterbium metal (0.201 mmol) and 23.7 mg of guanidine (0.401 mmol) were weighed in a steel autoclave equipped with a glass inset. A total of 20 cm3 of solid, dried ammonia was condensed into the reaction vessel before heating to 50 °C over the course of 2.5 days. The product was a redto-violet microcrystalline powder (26% yield with respect to ytterbium). In a typical reaction, Yb(CN3H4)3 was obtained by reacting 125.0 mg of ytterbium metal (0.722 mmol) with 129.0 mg of guanidine (2.184 mmol) in 15 cm3 of solid, dried ammonia in a steel autoclave at 70 °C for 17 days. Yb(CN3H4)3 was recovered as a colorless, microcrystalline powder (75−90% yield with respect to ytterbium). PXRD. For PXRD, the samples were sealed in 0.3 mm glass capillaries and measured with a STOE STADI P diffractometer with monochromatic Cu Kα1 radiation and a position-sensitive detector. The measurement ranges were 5−134° in 2θ with a step size of 0.010° for Yb(NH2)2, 10−125° with a step size of 0.015° for YbC(NH)3, and 6−134° with a step size of 0.010° for Yb(CN3H4)3. Additional measurements were performed with a STOE STADI MP diffractometer with monochromatic Mo Kα1 radiation and a Mythen detector. The measurement range was 3−83° in 2θ with a step size of 0.015° for both Yb(NH2)3 and Yb(CN3H4)3. For ytterbium(II) amide, Yb(NH2)2, Rietveld refinements were performed with the Jana2006 suite39 using the reported anatase structure9 as the starting model. The hydrogen atoms were located by DFT calculations with an accuracy comparable to neutron diffraction experiments (see below).40 The powder pattern of ytterbium(III) amide, Yb(NH2)3, was indexed in a trigonal cell with the DICVOL04 code,41 as provided in the WINXPOW suite. PXRD measurements showed that the ytterbium(II) guanidinate YbC(NH)3 crystallizes isostructurally to SrC(NH)3.30 Rietveld refinements were performed with Jana2006. DFT calculations were used to locate the hydrogen atoms. Complementary powder neutron diffraction experiments of the ytterbium(III) guanidinate Yb(CN3H4)3 were performed using the time-of-flight powder diffractometer POWGEN located at the SNS at Oak Ridge National Laboratory. High-resolution data at 10, 100, 200, and 300 K were collected at bank 3 with center wavelength 1.333 Å (d range = 0.45−6.2 Å) for an accurate description of the atomic displacement parameters and at bank 4 with center wavelength 2.665 Å (d range =1.15−10.5 Å) at 10 and 300 K to observe the reflections with the highest d values. Details of the structural solution and refinement with the FullProf suite42 can be found in the Supporting Information (SI). Further details may be obtained from Fachinformationszentrum Karlsruhe, Eggenstein-Leopoldshafen, Germany, upon quoting the deposition number CSD 431050 for Yb(NH2)2, CSD 431051 for YbC(NH)3, and CSD 431052 as well as CSD 431053 for Yb(CN3H4)3 at 10 and 300 K, respectively. Magnetometry. The magnetic properties of Yb(CN3H4)3 were measured with a SQUID magnetometer (Quantum Design MPMS5XL). The polycrystalline sample was compacted and immobilized into cylindrical poly(tetrafluoroethylene) capsules. Measurements included field- and temperature-dependent molar magnetic susceptibilities (0.05−5.0 T and 2.0−290 K) and determination of the molar magnetization as a function of the applied field at 2.0 K. The data were corrected for diamagnetic contributions of the sample holder and compound (Pascal’s constants, χdia = −2.19 × 10−9 m3 mol−1) and were analyzed by adopting the “full” model Hamiltonian and using the computational framework CONDON 2.0.43,44 IR Spectroscopy. A Bruker Alpha Fourier transform IR spectrometer placed in an argon-filled glovebox and equipped with an ATR Platinum Diamond sample holder with a measurement range of 4000−400 cm−1 was employed to measure the IR spectra of YbC(NH)3 and Yb(CN3H4)3. The results were compared to DFTbased phonon calculations, i.e., lattice vibrations that may or may not
metal guanidinates, 1:1 salts of the alkali metal A+, and the guanidinate anion CN3H4−.25−29 More recently, the first alkaline-earth-metal guanidinate, SrC(NH)3, was introduced, containing a doubly deprotonated C(NH)32− unit, the nitrogen analogue of the carbonate anion.30 For comparison, the substituted guanidinates, derived from the neutral formula HR1NC(NR2)(NR3R4) with organic substituents, are a firmly established class of coordinationchemistry materials with an increasing number of applications. Their metal complexes show (molecular) magnetism,31,32 luminescence,33 and catalytic activity.34 Rare-earth-metal guanidinate complexes of ytterbium catalyze, for example, the hydrophosphonylation of aldehydes,35 or they form as intermediate species in the catalyzed synthesis of substituted guanidinates.36 A magnetic study on transition-metal guanidinate complexes could prove superexchange via the guanidinate ligands.37 The unsubstituted guanidinates can be prepared by dissolving guanidine together with the corresponding metal in liquid ammonia. Ytterbium is the only metal with different readily available oxidation states from liquid ammonia solutions, making it an interesting reagent to expand the number of known guanidinates. Here, we present an oxidation-controlled synthesis of ytterbium salts in liquid ammonia to achieve the phase-pure amides Yb(NH2)2 and Yb(NH2)3 and to report additional structural parameters obtained from PXRD. We also used this approach for preparing the first two rare-earth-metal-unsubstituted guanidinates, with their crystal structures solved from PXRD and powder neutron diffraction: YbC(NH)3 and Yb(CN3H4)3. They differ in the oxidation states of ytterbium, 2+ and 3+, and in the deprotonation state of the guanidinate: the latter is of the CN3H4− type, while the former is doubly deprotonated with a C(NH)32− unit. Their IR spectra are interpreted with the help of density functional theory (DFT) calculations targeted at the density of phonon states (DPS). Finally, Yb(CN3H4)3 is studied with SQUID magnetometry.
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METHODS
Syntheses. The highly moisture-sensitive compounds were handled in an argon-filled glovebox to prevent degradation. Reactants were used as obtained from the manufacturer. Yb(NH2)2 and Yb(NH2)3 were prepared by placing 51.2 and 302.2 mg of ytterbium (0.296 and 1.746 mmol; chips, 99.9% REO, Strem) with 10 and 15 cm3 of solid, dried ammonia (Linde, 99.999%, without further purification), respectively, in stainless steel autoclaves. The autoclaves sized about 75 cm3 in reaction volume were constructed from corrosion-resistant steel type 1.4571 and a copper ring as a sealing gasket. A detailed description of the autoclaves is available.38 To control the oxidation state of ytterbium, a glass inset covering most of the autoclave wall and a relatively short reaction time of 3 days at 50 °C was used to synthesize Yb(NH2)2; by doing so, this minimized the available metal surface. For Yb(NH2)3, no glass inset was used and the reaction was allowed to proceed for 8 days at 50 °C. Then, the autoclaves were cooled to room temperature. The remaining ammonia was slowly released, and the autoclaves were evacuated and opened to obtain Yb(NH2)2 as a red powder (56% yield with respect to ytterbium) and Yb(NH2)3 as a light-brown powder (65−95% yield with respect to ytterbium). The guanidine CN3H5 was prepared in a one-pot synthesis by reacting 3045.5 mg of (CN3H6)2CO3 (16.904 mmol; Aldrich, 99%) with 1295.3 mg of potassium metal (33.130 mmol; Alfa, 99.95%) together with 20 cm3 of solid, dried ammonia in a stainless steel autoclave. After heating to 50 °C for 3 days, the ammonia was released from the autoclave, and a mechanical mixture of guanidine and B
DOI: 10.1021/acs.inorgchem.6b00736 Inorg. Chem. XXXX, XXX, XXX−XXX
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Inorganic Chemistry be IR-active. By this method, the specific vibrations of the measured bands were identified. Calculations. All DFT calculations were carried out with the Vienna Ab initio Simulation Package.45−48 The PBE49 exchangecorrelation functional with the D350 dispersion correction and the PAW51,52 potential were applied. For all electronic structure calculations, a convergence criterion of at least 10−5 eV was used. To locate the hydrogen positions with the help of DFT,40 a starting model was generated by adding hydrogen atoms from isostructural compounds to the experimental structures. Then, only the hydrogen atoms were optimized until the Hellmann−Feynman forces were smaller than 5 × 10−3 eV Å−1. For the ytterbium(II) compounds Yb(NH2)2 and YbC(NH)3, the 4f electrons were treated as part of the atomic core. All k-point meshes were energetically converged. The DPS were derived using the direct method,53 as implemented in Phonopy.54,55 For calculation of the electronic structure of all supercells, at least 1 k point was used. All q-point meshes for calculation of the DPS were converged. The convergence criterion for the structure optimization of YbC(NH)3 was 10−7 eV. Starting from this optimized structure, 4 × 4 × 4-supercells were calculated. Calculations of the electronic structure of Yb(CN3H4)3 including all 4f electrons for Yb did not converge. Unfortunately, no ytterbium(III) pseudopotential treating the 4f electrons as part of the atomic core was available. To, nonetheless, gain insight into the guanidinate vibrations in the crystal, we computationally substituted ytterbium(III) with the smaller and lighter scandium(III) because, from a chemical point of view, the lattice vibrations of Yb(CN3H4)3 and a hypothetical Sc(CN3H4)3 are expected to be very similar because of being dominated by the anion. Considering the large size of the unit cell, the standard 1 × 1 × 1 cell turned out to be adequate for the calculation.
amide-unit octahedra; the amides bridge three ytterbium atoms (Figure 2). The thermal displacement factors, as derived from
Figure 2. Crystal structures of Yb(NH2)2 (I41/amd, left) and YbC(NH)3 (P63/m, right).
Rietveld refinement (Figure 3, left, and Table 1), and the hydrogen positions from DFT are given in Table 2; the latter are expected to have a precision comparable to that of neutron diffraction.40 Let us move on to ytterbium(III) amide, Yb(NH2)3, whose diffraction pattern9 and IR measurements12 have been reported in the past but whose crystal structure is still unknown. Here, the phase is obtained in high crystallinity but with some broad bumps between 20 and 30° of unknown origin. Yb(NH2)3 may be successfully indexed using a trigonal cell with a = 6.2477(2) Å, c = 17.132(1) Å, and V = 579.15(4) Å3, corresponding to Z = 6. The systematic extinctions suggest space group R3̅. This cell has a crystallographic density of 3.80 g cm−3, in perfect agreement with the experimentally measured value of 3.79 g cm−3.9 Not too surprisingly, one may come up with a structural model including the ytterbium and nitrogen positions, but further (neutron) experiments are needed (and underway) for full structure determination. Crystal Structure of YbC(NH)3. YbC(NH)3 crystallizes isostructurally to SrC(NH)330 in space group P63/m, as derived from PXRD (Figure 3, right, and Table 1). Ytterbium is coordinated antiprismatically by six imine nitrogen atoms at 2.541(3) Å (Figure 2, right), leading to a bond valence sum of 1.8, in accordance with the expected charge of 2+.57−59 Each nitrogen atom bridges two ytterbium atoms, while the complete trinacria-shaped guanidinate unit is coordinated in a trigonal prism. The hydrogen atoms could not be detected with PXRD. For the SrC(NH)3 structure, a split position for the hydrogen atoms below and above the CN3 plane was observed from neutron diffraction data, and they also indicated a disordered occupation of these split positions even at low temperatures. Because of the lack of neutron data in the case of YbC(NH)3, DFT calculations40 had to be used for positioning the hydrogen atoms, and they place the hydrogen atoms in the CN3 plane (Table 3). Without experimental confirmation, the theoretical hydrogen positions for YbC(NH)3 are to be taken with a pinch of salt. The unit-cell volume of YbC(NH)3 is approximately 10% smaller compared to SrC(NH)3, in line with the smaller ionic radius of Yb2+ than Sr2+.30,60 The volume increment of the C(NH)32− unit is calculated as 40.1 cm3 mol−1, in agreement with the 41.6 cm3 mol−1 value found in SrC(NH)3.61 The C−N bonds are somewhat elongated with 1.373(5) Å (Figure 4, left) compared to 1.3528(4) Å in SrC(NH)3. This value falls between those of a single or double C−N bond and agrees with a bond order of 11/3, as discussed before.30 In contrast, the Yb−
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RESULTS AND DISCUSSION Oxidation Control. To obtain phase-pure compounds of ytterbium(II) from liquid ammonia solutions, oxidation control is mandatory to avoid the follow-up reaction with NH2− to ytterbium(III). Thus, a low concentration of NH2− should be favorable. The reaction of the solvated electrons with ammonia is typically slow but can be accelerated by increasing the temperature and especially by using a catalyst, for example, platinum or iron oxide.56 Because we suspected that our steel autoclaves also act as reaction catalysts, we therefore employed glass insets that covered the autoclave wall during the synthesis of the ytterbium amides. Normally, at temperatures of 50 °C or higher and reaction times of several days, we obtained phasepure Yb(NH2)3 in our autoclaves. With a glass inset, we found about 20% Yb(NH2)2 even at 70 °C and a reaction time of 10 days. With the glass inset, short reaction times at 50 °C, and moderate amounts of NH3, we then prepared phase-pure Yb(NH2)2 in excellent crystallinity. Adjusting these parameters thus allowed for an effective oxidation control in liquid ammonia. We also synthesized the ytterbium guanidinates by adopting the described oxidation control. There are only two, very distinct crystalline phases: ytterbium(II) forms a doubly deprotonated salt, YbC(NH)3, in contrast to ytterbium(III), which forms a singly deprotonated salt of the formula Yb(CN3H4)3. Reactions with differing reactant ratios or oxidation control targeting the inappropriate oxidation state resulted in mixtures of the two salts or amorphous compounds. We will address the amides and guanidinates in the following, starting with their crystal structures, individually and in comparison, and then moving on to their properties. Crystal Structures of the Ytterbium Amides. Ytterbium(II) amide, Yb(NH2)2, crystallizes isostructurally to calcium, strontium, and europium amide in the tetragonal anatase structure type.9 The ytterbium atoms are contained in distorted C
DOI: 10.1021/acs.inorgchem.6b00736 Inorg. Chem. XXXX, XXX, XXX−XXX
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Figure 3. Rietveld refinement of the PXRD patterns of Yb(NH2)2 (left) and YbC(NH)3 (right).
Table 1. Crystallographic Data and Refinement Details for Yb(NH2)2, YbC(NH)3, and Yb(CN3H4)3 formula fw (g mol−1) cryst syst space group
Yb(NH2)2 205.09 tetragonal I41/amd (No. 141) 298 5.1960(1) 10.4129(4) 281.13(1) 4 4.7487
YbC(NH)3 230.10 hexagonal P63/m (No. 176) 298 5.2596(2) 6.6704(2) 159.81(1) 2 4.7803
Yb(CN3H4)3 347.23 cubic Pn3̅ (No. 201, origin 2) 10 13.5307(3) =a 2477.22(8) 8 1.862
temperature (K) a (Å) c (Å) V (Å3) Z cryst density (g cm−3) radiation no. of reflns no. of restraints/ constraints no. of refined param Rp, Rwpa
Cu Kα1 79 0
Cu Kα1 95 0
neutron TOF 935 + 140 4/2
17 5.8, 7.3
22 1.8, 2.4
RBragg, RFb
8.3, 4.5
8.4, 5.5
107 bank bank bank bank
a b
Rp =
∑ |y(obs) − y(calc)| ∑ y(obs)
RBragg =
∑ |Iobs − Icalc| ∑ |Iobs|
∑ w[y(obs) − y(calc)]2
× 100; R wp =
× 100; RF =
3: 4: 3: 4:
∑ wy(obs)2
∑ |Fobs| − |Fcalc| ∑ Fobs
1.9, 2.8, 5.5, 4.1,
Figure 4. Sketch of the guanidinate trinacria unit in YbC(NH)3 (left, hydrogen atoms from DFT with arbitrary size) and ORTEP plot of the singly deprotonated guanidinate unit in Yb(CN3H4)3 (right, neutron diffraction at 10 K). Thermal ellipsoids are drawn at 75% probability.
N bond distance is shorter by 40% and only comprises 2.541(5) Å. The N−H distances are computed at a very typical 1.024 Å, and the N−H···N distances are very long for hydrogen bonding with 2.516(2) Å. In SrC(NH)3, no hydrogen bonding was observed, and this probably also holds for YbC(NH)3. The crystal structure is related to the high-temperature potassium carbonate polymorph α-K2CO3 in space group P63/ mmc.62,63 In α-K2CO3, the trigonal prisms not occupied by guanidinate/carbonate molecules are filled with additional potassium atoms on the Wyckoff position 2d. For K2CO3, a number of lower-symmetry polymorphs are found where the carbonate units are rotated out of the plane.62,64 For the strontium guanidinate, no structural transition could be observed between room temperature and 15 K,30 possibly due to steric hindrance. YbCO3 is isostructural to SrCO3,65 which has a structure distinctly different from those of SrC(NH)3 and YbC(NH)3, as discussed in detail in ref 30. Crystal Structure of Yb(CN3H4)3. Yb(CN3H4)3 crystallizes in the cubic space group Pn3̅ with eight formula units per cell (Figure 5 and Table 1). While two ytterbium atoms occupy the high-symmetry 4b and 4c positions, the atoms of the guanidinate molecule are found in the general position 24h (Figure 4, right, and Table 4). Both ytterbium atoms are coordinated in an almost perfect octahedron made up of six different guanidinate units via the imine groups with crystallographically different but chemically equal bond lengths of 2.333(8) and 2.341(8) Å (Figure 6, all values for 10 K). The two Yb−N bond lengths for Yb1 and Yb2 differ slightly, leading to marginally different bond valence sums of 2.9 and 2.8,59 respectively, in close agreement with the expected charge of 3+.
1.1 1.8 2.8 4.3
× 100
× 100
Table 2. Atomic Positions and Displacement Parameters of Yb(NH2)2 in Space Group I41/amda
a
Uiso/*Ueq (Å2)
atom
Wyckoff position
x
y
z
Yb N H (DFT)
4a 8e 16h
0 0 0.1567
0 0 0
0 0.248(1) 0.3079
0.0093(6)* 0.019(3)
The hydrogen position was determined from DFT.
Table 3. Atomic Positions and Displacement Parameters of YbC(NH)3 in Space Group P63/ma atom
Wyckoff position
Yb C N H (DFT)
2b 2c 6h 6h
a
x 0 /3 0.3174(4) 0.5294
1
y 0 /3 0.398(1) 0.4391
2
z 0 /4 1 /4 1 /4
1
Uiso/*Ueq (Å2) 0.0063(4)* 0.011(5) 0.002(3)
The hydrogen position was determined from DFT.
D
DOI: 10.1021/acs.inorgchem.6b00736 Inorg. Chem. XXXX, XXX, XXX−XXX
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Figure 5. Rietveld refinement of powder neutron diffraction data of Yb(CN3H4)3 using data from bank 3 (left) and bank 4 (right).
The guanidinate unit bridges two ytterbium atoms, leaving them particularly isolated; the shortest Yb−Yb distance is 6.7653(2) Å. This structure corresponds to the ReO3 structure type with a complex anion. The ReO3 structure type can be derived from the perovskite structure type (CaTiO3) by removing the A-site cation, leaving empty channels in all three dimensions. Here, these are enlarged owing to the complex anions. We want to point out that no significant scattering density has been observed in the channels. The guanidinate molecules are tilted between the ytterbium atoms, and their amine groups are pointing into empty channels. This can easily be understood when analyzing the hydrogen-bonding network: each hydrogen atom of the amine group forms a bond to a neighboring guanidinate’s imino nitrogen (Figure 6, right). Thus, they build a three-dimensional network with N−H···N bond lengths of 2.13(1) and 2.14(1) Å and angles of 159.3(9) and 157.5(9)°. The tilting of the guanidinate between the ytterbium atoms seems to improve the hydrogen bonding. The hydrogen atoms of the imine groups adopt the syn conformation, which was calculated to be more stable in the gas phase.26 Temperature-dependent PXRD measurements confirm that no phase change occurs in the temperature range from 10 K to at least 393 K. The lattice parameter and thus the unit-cell volume increase from 13.5307(3) to 13.601(2) Å and from 2477.2(1) to 2516.1(6) Å3, respectively. This corresponds to a volume increase of 1.6% and a linear thermal expansion coefficient of α = 18(4) × 10−6 K−1 between 100 and 373 K (Figure S2). Starting at 433 K, Yb(CN3H4)3 decomposes very
Table 4. Atomic Positions and Displacement Parameters of Yb(CN3H4)3 in Space Group Pn3̅ at 10 K atom
Wyckoff position
x
Yb1 Yb2 C1 N1 N2 N3 H1 H2 H3 H4
4b 4c 24h 24h 24h 24h 24h 24h 24h 24h
0 0 0.5264(2) 0.5346(6) 0.5380(6) 0.5007(2) 0.540(1) 0.556(1) 0.524(1) 0.502(1)
y
z
Uiso/*Ueq (Å2)
0 /2 0.4943(3) 0.4509(4) 0.4514(5) 0.5926(2) 0.3758(6) 0.3790(6) 0.6302(9) 0.6286(9)
0 0 0.251(1) 0.1616(6) 0.3384(4) 0.2491(5) 0.171(1) 0.332(1) 0.1868(7) 0.3146(7)
0.0011(3)* 0.0011(3)* 0.0070(1)* 0.011(4)* 0.010(4)* 0.015(1)* 0.026(4) 0.020(3) 0.022(3) 0.019(3)
1
Figure 6. Crystal structure of Yb(CN3H4)3 (left) along [001], emphasizing the empty channels. For the full cell, refer to Figure S1. Three-dimensional hydrogen-bonding network (right) in Yb(CN3H4)3.
Figure 7. IR transmittance spectra (black) of YbC(NH)3 (left) and Yb(CN3H4)3 (right) compared with those of calculated DPS (red). The wavenumbers (cm−1) of major bands are included, and phonons are not necessarily IR-active. E
DOI: 10.1021/acs.inorgchem.6b00736 Inorg. Chem. XXXX, XXX, XXX−XXX
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differing C−N bond lengths for the imine and amine groups. This is most notable for the ν(C−N) and δ(N−H) vibrations between 1652 and 1475 cm−1, here showing four overlapping bands instead of only one. Furthermore, the ν(N−H) vibrations are split into a higher energy band at 3306 and a lower energy band at 3191 cm−1. These are induced by nonhydrogen-bonded and hydrogen-bonded N−H groups, respectively, i.e., the imine and amine groups.66 For YbC(NH)3, only one band below 3300 cm−1 was observed, indicating weak or no hydrogen bonding. IR measurements are thus a fast and efficient way to identify and distinguish between low-symmetry CN3H4− and C3-symmetric C(NH)32− units. Magnetic measurements of Yb(CN3H4)3 reveal paramagnetic behavior with significant deviations from expectation for different applied magnetic fields at elevated temperatures (T > 150 K; Figure S3). Such behavior is characteristic of small ferri- or ferromagnetic impurities in the sample. In earlier experiments, we observed that liquid ammonia leads to some corrosion of the steel autoclaves, as indicated by microscopic, dark particles in the product phase. Energy-dispersive X-ray measurements verified that these have the same composition as the autoclaves.67 We therefore assume our autoclaves to be the source of these impurities. Field-dependent measurements of the molar magnetic susceptibility χm allow one to correct for this kind of impurity by applying for each temperature the formula below.68−70
noticeably and irreversibly to an almost amorphous phase. This process was not finished after heating to 513 K over the course of several days, indicating an unusually large thermal stability for a guanidinate. Related IR measurements show vibrations around 2000 cm−1, typical for NCN2− or HNCN− units.66 This is in line with the general decomposition of guanidinates toward (hydrogen) cyanamides by eliminating ammonia.26,30 The differences between YbC(NH)3 and Yb(CN3H4)3 are profound: the crystal packing in YbC(NH)3 is governed by electrostatic interaction only, leading to a densely packed salt similar to α-K2CO3 with ρ = 4.7803 g cm−3. In contrast, in Yb(CN3H4)3, hydrogen bonding plays an important role, and the rigid, large anion leads to a porous structure with isolated metal atoms similar to typical metal−organic frameworks (MOFs). At room temperature, its density is only ρ = 1.841 g cm−3. Nonetheless, the phase crystallizes in a simple AB3 structure type. Clearly, electrostatic interactions and hydrogen bonding work hand in hand to stabilize this structure. IR Measurements and Magnetic Properties. IR transmittance spectroscopy of YbC(NH)3 reveals a simple spectrum with only five major bands (Figure 7 and Table 5). Theoretical Table 5. IR Bands of YbC(NH)3 and Yb(CN3H4)3 Assigned Using Vibrations from DPS Calculations vibration
YbC(NH)3 (cm−1)
Yb(CN3H4)3 (cm−1)
ν(N−H) ν(C−N), δ(N−H) ν(C−N), δ(N−H) ν(C−N), δ(N−H) C inversion by the CN3 plane ν(C−N), δ(N−H)
3295 1445 1171
3306, 3191 1652−1475 1243, 1143 994 770 648, 520
780 564
χg (H ) = χg (∞) +
σs H
For this formula, the magnetization (Figure 8, inset) must be a linear function of the field. Therefore, we include data of fields of up to 1 T to rule out errors caused by saturation of Yb(CN3H4)3. Extrapolations to infinitely high fields yield the corrected values χm(∞) through multiplication of χg(∞) by the molar mass of Yb(CN3H4)3. The corrected magnetic data are depicted in Figure 8 as μeff versus T, Mm versus B, and χm−1 versus T curves. The value μeff = 4.30 μB at 290 K is slightly smaller than the expected 4.54 μB for the saturated thermal population of all substates of the freeion ground state 2F7/2.70 With decreasing temperature, μeff monotonically decreases to 2.32 μB at 2.0 K. This is a
phonon calculations match extraordinarily well to all bands. A comparison to its archetype SrC(NH)3 already demonstrated excellent agreement and a shift for all bands for YbC(NH)3 by about 10 wavenumbers to lower energies.30 The IR spectrum of Yb(CN3H4)3 shows a larger number of bands and a general agreement with theoretical phonon calculations, allowing for the fact that the latter are not necessarily IR-active (Figure 7 and Table 5). Most of the bands are similar to the ones of YbC(NH)3 but split owing to the
Figure 8. Left: Effective magnetic moment of Yb(CN3H4)3 in Bohr magnetons as a function of the temperature. Inset: Field dependence of the molar magnetization at 2.0 K. Right: Inverse molar magnetic susceptibility as a function of the temperature: data corrected for ferromagnetic impurities (open circles) and a least-squares fit (solid lines). F
DOI: 10.1021/acs.inorgchem.6b00736 Inorg. Chem. XXXX, XXX, XXX−XXX
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compound is a densely packed salt with ρ = 4.7803 g cm−3. In contrast, Yb(CN3H4)3 crystallizes in a ReO3-derived structure type with the complex CN3H4− unit, leading to large channels. The MOF-like compound has a low density of ρ = 1.841 g cm−3 at room temperature owing to the interplay of electrostatic and hydrogen-bonding interactions. IR measurements are shown to be an efficient method to identify and distinguish between low-symmetry CN3H4− and C3-symmetric C(NH)32− units. The magnetic properties of Yb(CN3H4)3 were quantitatively determined by a least-squares fit. Yb(CN3H4)3 consists of isolated Yb3+ ions with an 4f13 electron configuration in a slightly distorted octahedral ligand field, resulting in nonCurie−Weiss paramagnetism.
characteristic behavior of most molecular or isolated lanthanide atoms. For isolated lanthanide atoms, such behavior is caused by thermal depopulation of the substates. The corresponding energy spectrum is modified in the presence of exchange interactions between multiple atoms, changing the μeff versus T curve particularly at low temperatures. For Yb(CN3H4)3, the μeff versus T curve reveals either minor antiferromagnetic or the total absence of exchange interactions that are generally very small (|J| ≤ 1 cm−1)71 for lanthanide−lanthanide systems. Curie−Weiss paramagnetism may be readily excluded for Yb(CN3H4)3 from the shape of the inverse molar magnetic susceptibility as a function of the temperature because Curie− Weiss paramagnetism is represented by a straight line,70 but various deviations from that are observed. For temperatures below 100 K, χm−1 versus T is characterized by a distinct curvature, and the subsequent quasi-linear domain for T > 100 K indicates minor undulation. At 2.0 K, the molar magnetization is linear in B for fields up to ca. 1 T. At higher fields, the curve hints at saturation, although the maximum value of a single ytterbium(III) ion (4NAμB) is not reached. We cannot distinguish between isolated or interacting atoms solely based on the Mm versus B curve. Using the computational framework CONDON 2.0,43,44 a semiempirical “full” model was fitted to the complete corrected data set. In a first attempt, we assumed exact Oh ligand-field symmetry, yielding a poor goodness-of-fit SQ = 4.9% that cannot be exclusively explained by the error introduced by the correction for ferromagnetic impurities. On the basis of the crystal structure, we allowed for a minor distortion of the ligand field: Introducing additionally a (small) ligand-field parameter B20 but leaving the other parameters fixed at the cubic ratios B44/B40 = (5/14)1/2 and B64/B60 = −(7/2)1/2 resulted in an approximate but acceptable description of the magnetic properties. The least-squares fit of SQ = 2.1% yielded the parameters given in Table S3, and the corresponding calculated data are shown as solid lines in Figure 8. The parameters represent a slightly distorted octahedral ligand field that splits, considering electron−electron interactions and spin−orbit coupling, the free ground state 2F7/2 into Kramer’s doublets over a range of ca. 720 cm−1 (Figure S4), and the excited state 2 F5/2 over 10240−10760 cm−1. The splitting of the energy levels is in the expected range for Yb3+ but somewhat larger than that for the halides,72 showing that the guanidinate units are stronger ligands. Our results match closely to earlier measurements of Yb3+ compounds in octahedral ligand-field environments73 compared to lower-symmetric ones.74,75 In a last step, we tested for the presence of a potential Heisenberg exchange interaction. Because the related leastsquares fit negligibly improved the goodness of fit and the corresponding interaction parameter J is estimated to be 0.00 ± 0.02 cm−1, we conclude that the ytterbium(III) ions are well isolated, in harmony with the structural features.
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.6b00736. Additional information and figures on structural solution and refinement and on the magnetic characterization of Yb(CN3H4)3 and the complete intensity data in the form of structure factors of all compounds (PDF) X-ray crystallographic data in CIF format (CIF) X-ray crystallographic data in CIF format (CIF) X-ray crystallographic data in CIF format (CIF) X-ray crystallographic data in CIF format (CIF) X-ray crystallographic data in CIF format (CIF) X-ray crystallographic data in CIF format (CIF) X-ray crystallographic data in CIF format (CIF) X-ray crystallographic data in CIF format (CIF)
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Author Contributions
The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS We thank Paul Müller for PXRD measurements, Christina Houben for SQUID magnetometry measurements, and Melanie Kirkham for assistance with the neutron diffraction experiments at POWGEN. The authors gratefully acknowledge the financial support provided by JCNS to perform the neutron scattering measurements at the SNS, Oak Ridge, TN. Part of the research conducted at the SNS was sponsored by the Scientific User Facilities Division, Office of Basic Energy Sciences, U.S. Department of Energy. Moreover, J.G. thanks the Fonds der Chemischen Industrie for a scholarship. DFT calculations were performed with computing resources thankfully granted by JARA-HPC from RWTH Aachen University under Project JARA0069.
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CONCLUSION In summary, we developed methods to control the oxidation state of ytterbium in liquid ammonia, using the ytterbium amides as a test case. Thanks to the improved crystallinity, we reported additional structural data for Yb(NH2)2 and suggested a trigonal metric for Yb(NH2)3. We also used these methods to synthesize phase-pure samples of the guanidinates YbC(NH)3 and Yb(CN3H4)3. YbC(NH)3 crystallizes isostructurally to SrC(NH)3 with doubly deprotonated C(NH)32− units. The
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