Perovskite-Like Polar Lanthanide Formate ... - ACS Publications

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Perovskite-Like Polar Lanthanide Formate Frameworks of [NH2NH3][Ln(HCOO)4] (Ln = Tb−Lu and Y): Synthesis, Structures, Magnetism, and Anisotropic Thermal Expansion Tian-Meng Zhao, Sa Chen, Ran Shang, Bing-Wu Wang, Zhe-Ming Wang,* and Song Gao* Beijing National Laboratory for Molecular Sciences, State Key Laboratory of Rare Earth Materials Chemistry and Applications, College of Chemistry and Molecular Engineering, Peking University, Beijing 100871, P. R. China S Supporting Information *

ABSTRACT: A series of isostructural hydrazinium lanthanide (Ln) formate framework compounds of [NH2NH3][Ln(HCOO)4] for Ln3+ ions from Tb3+ to Lu3+ and Y3+ have been successfully prepared by utilizing NH2NH3+. The compounds crystallize in orthorhombic polar space group Pca21, with cell parameters at 180 K of a = 18.2526(7)−18.1048(5) Å, b = 6.5815(2)−6.5261(2) Å, c = 7.6362(3)−7.5044(2) Å, and V = 917.33(6)−886.67(4) Å3, showing the effect of lanthanide contraction. The compounds possess polar perovskite-like structures incorporating the hydrazinium cations in the cavities of the NaCl-like framework, in which the Ln3+ ions in a bicapped trigonal prism are connected by anti−anti and syn−anti formate groups. The N−H···Oformate hydrogen-bonding interactions are between the hydrazinium cations and the anionic framework. One anti−anti formate group is frustrated by the competitive N−H···Oformate hydrogen-bonding interactions. It thus twists or flips upon warming, resulting in large anisotropic thermal expansion and negative thermal expansion below 180 K. A comparison with the transition metal and magnesium analogues revealed that the structural compactness, tighter binding of the hydrazinium cation by the framework, and symmetrically better match between the framework and ammonium cation for Ln compounds could inhibit the occurrence of phase transition in the series. The IR spectroscopic, thermal, and magnetic properties are investigated.

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smallest NH4+ to the largest and longest tetraammonium of tptaH44+ [tptaH44+ = H3N(CH2)3NH2(CH2)3NH2(CH2)3NH3] employed to date,11−18,21b,22,23 and the various perovskites of [AH][TM(HCOO)3] (AH = monoammonium) with (412·63) topology are the majority.12−16,21b,22,23c−e Lanthanide (Ln) ions possess higher coordination numbers and more variable coordination geometries compared to the fixed octahedral coordination geometry of TM. Ln-AMFFs thus show more complicated structures and framework topologies.19,20,21a,24,25 However, these lanthanide formate frameworks are better understandable as perovskites, diamonds, or pillared layers. In stoichiometry, Ln-AMFFs have one formate per mono-ammonium more than those in TM-AMFFs. In [AH][TM(HCOO)3],11−13 oscillators, pendulums, and rotators of different AH+ cations and the freezing of their motions during transitions have been observed. AMFFs incorporating polyamonium cations exhibit more complicated, various phase transition patterns, such as multiple-step freezing of the polyammonium motion and unusual alternation of the lattice symmetry, because of the conformational flexibility of polyammonium cations and the subtle coupling or synergy between the ammonium cation and framework.17,18,20 These phase

INTRODUCTION The last few decades have witnessed great development in the research of metal−organic frameworks (MOFs).1−3 While these materials have shown a great variety and richness in their chemistry, it has been recognized, only in recent years, that the materials could possess abundant physics, parallel to conventional materials such as oxides.4−9 Indeed, a rich variety of structural, magnetic, and electric phase transitions and their relevant properties,5 negative thermal expansion (NTE),6 negative linear compressibility (NLC),7 amorphization,8 mechanical properties,9 and so on, along both the temperature and pressure axes, have been documented in MOFs and aroused great interest. In this context, ammonium metal formate frameworks (AMFFs), belonging to a relatively small but unique class of MOFs, are especially attractive and promising.10−25 The combination of the constituent ammonium, metal ion, and formate not only produces various frameworks but also provides the necessary elements and requirements for various phase transitions and the relevant magnetic, electric, and mechanical properties via magnetic coupling, hydrogen bonding, and state alteration of ammonium and/or formate (mainly the order−disorder type) and coupled framework regulation.10 The framework structures are templated by the shape, size, and charge of the ammonium cations. For all AMFFs of 3d metal ions or Mg2+ (TM), a structural hierarchy of (412·63)m(49·66)n (m = 0, 1; n = 0, 1, 2, 3) framework topologies has been established for ammonium cations from the © 2016 American Chemical Society

Special Issue: New Trends and Applications for Lanthanides Received: April 17, 2016 Published: July 1, 2016 10075

DOI: 10.1021/acs.inorgchem.6b00952 Inorg. Chem. 2016, 55, 10075−10082

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Inorganic Chemistry transitions usually cause dipolar or electric orderings.10 Therefore, many magnetic AMFF compounds show the coexistence or synergy of magnetic and electric orderings,11−14,18 or they are considered as MOF-based multiferroics. Anisotropic thermal expansion (ATE), NTE, or NLC have been documented in [NH4][TM(HCOO)3], some [AH][TM(HCOO)3] perovskites, and one Ln-AMFF of [NH2CHNH2][Er(HCOO)4].11,15,19c Mechanical properties such as stiffness and Young’s modulus have been studied also for several [AH][TM(HCOO)3] perovskites.16 Temperature/pressure-induced phase transitions with rearrangement of metal−formate bonds and/or hydrogen bonds have been observed in [tmenH2][Er(HCOO)4]2 [tmenH22+ = (CH 3 ) 2 NH(CH 2 ) 2 NH(CH 3 ) 2 ] and [CH 3 CH 2 NH 3 ][Cu(HCOO)3].21 Phase transitions and electric/magnetic properties modulated by mixed ammonium cations22 or mixed-metal ions23 have recently been investigated. All of these excellent works demonstrate that the AMFF class is a good and valuable platform for relevant researches. However, these have mainly been achieved for TM-AMFFs, and Ln-AMFFs have not been systematically explored and investigated. To be inspired by the very interesting [NH2NH3][TM(HCOO)3] series showing polymorphism, ferroelectric phase transitions, prominent ATE and NTE, and electric and magnetic ordering12,22 and to continue our research on Ln-AMFFs,19a,20,25 in this work we use hydrazinium (NH2NH3+) to construct the series of isostructral [NH2NH3][Ln(HCOO)4] for Ln3+ ions of Tb3+−Lu3+ and Y3+, and they are named as 1Tb, 2Dy, 3Ho, 4Er, 5Tm, 6Yb, 7Lu, and 8Y. The compounds possess polar perovskite-like structures, with the hydrazinium cations in the cavities of the NaCl-like lanthanide formate framework. Lanthanide contraction leads to the shrinkage of unit cells within the series, and the twist or flip motion of one anti−anti formate group in the framework results in large ATE and NTE below 180 K. The more structural compactness, tighter binding of the ammonium by the framework, and symmetrically better match between the frameworks and ammonium probably reduce the occurrence of phase transitions in Ln-AMFFs compared to those in TM-AMFFs. During the manuscript preparation, 2Dy was reported by Mac̨ zka et al.,24 together with the dysprosium compound incorporating [CH3NH2CH3]+.25 However, the present paper provides not only a systematic research work but

also more insight into the structure−functionality relationships for this Ln-AMFF series.

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EXPERIMENTAL SECTION

Synthesis. All chemicals were commercially available and used as received. The compounds were prepared by similar conventional solution methods. The preparation of 4Er is described as an example. Er(NO3)3·6H2O (0.23 g, 0.50 mmol), HCOOH (0.46 g, 10 mmol), and hydrazine (0.12 g, 2.0 mmol) were dissolved in mixed solvents of methanol and N,N-dimethylformamide [DMF; 30 mL, 2:1 (v/v)]. The solution was sealed and left undisturbed. Plate or block pink crystals were obtained overnight. The crystals were harvested, washed with methanol, and air-dried. The yield was 76%. Elemental analysis results and yields are listed in Table S1 in the Supporting Information (SI). X-ray Crystallography and Physical Characterizations. The single-crystal X-ray diffraction intensity data for 1Tb, 2Dy, 3Ho, 4Er, 6Yb, 7Lu, and 8Y at 180 and 290 K and 5Tm at 100, 180, 290, and 375 K were collected on an SuperNova Dual Atlas CCD diffractometer and the equipped Oxford Cyrostream 700-plus temperature control system, using mirror-monochromated Mo Kα radiation (λ = 0.71073 Å).26 The structure of 4Er was first solved by direct methods and then used as the starting model for other compounds because they are isostructural. All structures were refined by full-matrix least squares on F2 using the SHELX program.27 The H atoms could be located from the difference Fourier synthesis but were added based on the ideal geometries in refinements. The crystallographic data at 180 K are briefly given in Table 1, and the selected molecular geometries are listed in Table 2. Detailed crystallographic data are given in Table S2 and more molecular geometry data in Table S3 in the SI. CCDC 1474526−1474543 contain the supplementary crystallographic data in this work. These data can be obtained free of charge via www.ccdc.cam.ac.uk/conts/retrieving.html. Powder X-ray diffraction (PXRD) data were collected at room temperature for the bulk samples on a Rigaku Dmax 2000 diffractometer using Cu Kα radiation. Elemental analyses of C/H/N were carried out on an Elementar Vario MICRO CUBE analyzer. Fourier transform infrared spectra in the range of 4000−600 cm−1 were recorded for pure samples on a Nicolet iN10 MX spectrometer by a microscopic method. Thermal analyses were performed on a TA SDT Q600 simultaneous thermogravimetric analysis (TGA)−differential scanning calorimetry (DSC) instrument at a rate of 5 °C min−1 in air flow. The accurate DSC measurement for 4Er was performed on a TA Q100 DSC analyzer at a rate of 5 °C min−1 in N2 flow. Magnetic measurements were performed on a Quantum Design MPMSXL5 SQUID system with polycrystalline samples tightly packed and sealed in capsules. Diamagnetic corrections were

Table 1. Brief Crystallographic Data for the Eight Compounds at 180 K formula fw cryst syst space group a, Å b, Å c, Å V, Å3 Z/Dc, g cm−3 μ(Mo Kα), mm−1 F(000) no. of total/uniq reflns. no. obsd [I ≥ 2σ(I)] R1, wR2 [I ≥ 2σ(I)] GOF

1Tb

2Dy

3Ho

4Er

5Tm

6Yb

7Lu

8Y

C4H9TbN2O8 372.05 orthorhombic Pca21 18.2526(7) 6.5815(2) 7.6362(3) 917.33(6) 4/2.694 7.740

C4H9DyN2O8 375.63 orthorhombic Pca21 18.2377(5) 6.5776(2) 7.6098(2) 912.87(4) 4/2.733 8.216

C4H9HoN2O8 378.06 orthorhombic Pca21 18.2209(5) 6.5751(2) 7.5850(2) 908.72(4) 4/2.763 8.738

C4H9ErN2O8 380.39 orthorhombic Pca21 18.1802(5) 6.5630(2) 7.5566(2) 901.63(4) 4/2.802 9.339

C4H9TmN2O8 382.06 orthorhombic Pca21 18.1409(5) 6.5415(2) 7.5452(2) 895.38(4) 4/2.834 9.940

C4H9YbN2O8 386.17 orthorhombic Pca21 18.1272(4) 6.5391(2) 7.5198(2) 891.36(4) 4/2.878 10.524

C4H9LuN2O8 388.10 orthorhombic Pca21 18.1048(5) 6.5261(2) 7.5044(2) 886.67(4) 4/2.907 11.166

C4H9YN2O8 302.04 orthorhombic Pca21 18.2232(5) 6.5790(2) 7.5847(2) 909.33(4) 4/2.206 6.445

704 13589/2091

708 13542/2081

712 13200/2070

716 13434/2057

720 13686/2209

724 14052/2194

728 13446/2185

600 13989/2072

1965

2029

1999

1973

2126

2100

2078

1971

0.0195, 0.0419

0.0148, 0.0358

0.0185, 0.0411

0.0193, 0.0423

0.0187, 0.0412

0.0202, 0.0458

0.0199, 0.0441

0.0189, 0.0410

1.086

1.062

1.083

1.057

1.069

1.049

1.055

1.065

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6.5893 × 2 6.5373 × 2 6.5488 × 2

6.5790 × 2

RESULTS AND DISCUSSION Synthesis and PXRD, IR, and Thermal Properties. The eight Ln-AMFF compounds were prepared by a mild solution method in which the mixed methanol−DMF solvents were used. The yields were satisfactory, and the phase purity of the bulk samples and the isomorphism were confirmed by PXRD (Figure S1 in the SI). This synthetic approach has been proven to be effective and easily employed for preparing other Ln-AMFFs with the ammonium cations from the small NH4+ to large diammonium cations, such as tmenH22+.19a,20,25 The present work, running from Tb to Lu and Y, indicates that the synthetic approach is probably suitable for preparing Ln-AMFFs for heavier Ln3+ and Y3+; however, the preparation of the compounds for Ln3+ before Tb3+ by this synthetic route has still not been successful. The IR spectra of the eight compounds are very similar (Figure S2a in the SI). This is expected for the fact that they are isostructural. The IR absorption bands and their assignments are given in Table S4 in the SI, and they are characteristic of NH2NH3+ and HCOO−.12a,24,29 However, more splits in the bands (ν5, ν2, and ν3) relevant to the formate group than those observed in [NH2NH3][TM(HCOO)3] indicated the existence of different formate groups (anti−anti and syn−anti) in the structures (see later). The combined TGA−DSC runs up to 800 °C are given in Figure S2b in the SI. These materials basically showed two steps in their thermal decomposition procedure: first the loss of NH2NH2·HCOOH and then pyrolysis of the intermediate Ln(HCOO)3 phase. Taking 4Er as representative (Figure 1),

6.5605 × 2

6.5766 × 2

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6.5826 × 2

6.5919 × 2

6.6038 × 2

6.6137 × 2

6.6168 × 2

6.6449 × 2

6.5730 × 2

6.5261 × 2 6.5391 × 2 6.5415 × 2

5.9632 × 2

6.5751 × 2 6.5776 × 2 6.5815 × 2 Ln···Lnanti−anti

6.5630 × 2

6.4929 × 2

5.9352 × 2 5.9355 × 2

6.5144 × 2 6.5407 × 2

5.9313 × 2 5.9524 × 2 5.9461 × 2 5.9396 × 2 Ln···Lnsyn−anti

6.5524 × 2

3.070(6)−3.221(5), 141−151 3.081(4)−3.224(4), 145−152 3.079(6)−3.238(5), 146−151 N2−H···OHCOO (NH2 end)

5.9472 × 2

3.074(3)−3.223(3), 145−152 3.057(6)−3.201(5), 142−156 3.068(5)−3.213(6), 143−156 3.074(6)−3.207(5), 135−150

2.791(5)−3.263(7), 124.7−156.8 2.800(4)−3.346(5), 125.8−159.9 2.797(7)−3.458(8), 124.2−159.9

3.062(6)−3.213(5), 143−154

124.8(2)−127.8(2)

2.791(3)−3.230(3), 125.8−162.1 2.796(6)−3.252(6), 126.7−161.5 2.800(6)−3.236(6), 126.7−161.2 2.792(5)−3.225(6), 126.5−161.5

127.4(2)−159.7(2)

N1−H···OHCOO (NH3+ end)

2.788(6)−3.224(6), 126.6−160.5

123.5(7)−127.2(4)

127.2(3)−163.0(4) 126.9(3)−161.2(4)

124.3(4)−128.0(4) 124.8(4)−128.5(4)

127.7(3)−158.4(3) 127.2(3)−159.3(3) 127.3(3)−158.2(3)

125.2(4)−128.1(4)

127.6(2)−154.5(3)

124.9(6)−128.4(3)

127.3(3)−151.2(4)

125.2(4)−128.1(5)

Ln−O−C

123.7(7)−127.5(4)

estimated using Pascal constants (−112 × 10−6 cm3 mol−1 for 1Tb−6Yb)28 and background correction by experimental measurement on sample holders.

O−C−O

1.441(3)

67.18(6)−147.75(6) 67.0(1)−148.0(1) 67.08(9)−147.9(1) 67.4(1)−146.7(1) O−Ln−O

66.9(1)−148.0(1) 67.1(1)−147.7(1) 67.18(7)−147.13(8)

67.22(9)−147.5(1)

1.218(3)−1.266(3) 1.222(6)−1.267(6)

1.442(6) 1.445(5)

1.223(5)−1.270(5) 1.220(5)−1.259(5)

1.437(5)

1.222(6)−1.271(5)

1.443(5)

1.216(5)−1.274(5)

1.439(5)

1.210(6)−1.269(6)

1.442(6)

C−O

N−N

1.206(4)−1.267(4)

1.434(4)

2.272(2)−2.412(2)

8Y 7Lu

2.227(3)−2.382(3) 2.237(3)−2.389(3)

6Yb 5Tm

2.251(3)−2.394(3) 2.260(3)−2.407(3)

4Er 3Ho

2.271(3)−2.417(3)

2Dy

2.286(2)−2.418(2)

1Tb

2.300(3)−2.419(3) Ln−O

Table 2. Selected Bond Distances (Å), Bond Angles (deg), Geometries of the N−H···O Hydrogen Bonds between the NH2NH3+ Cation and Anionic Framework (N···O Distances, Å, and N−H···O Angles, deg), and Ln···Ln Distances (Å) via Formate Bridge in the Structures of the Eight Compounds at 180 K

Inorganic Chemistry

Figure 1. Combined TGA−DSC runs for 4Er. Inset: DSC run for 4Er.

the first endothermic procedure started from 140 °C and had two substeps. The whole weight loss of 18.9% corresponded to the departure of one NH2NH2·HCOOH per formula with the calculated value of 20.5%, and the energy acquirement was 64 kJmol−1. The intermediate Er(HCOO)3 further decomposed around 370 °C, starting from its melting,19a evidenced by first an energy acquirement of 10 kJ mol−1 and then an energy release of 140 kJ mol−1 for pyrolysis. The final residue at 800 °C was 52.2%, in agreement with the calculated value of 50.3% based on Er2O3. The thermal stability and decomposition behavior of 4Er are similar to those of other reported [AH][Er(HCOO)4] compounds19a,20 except the more stable ones containing formamidinium or guanidinium. Other members within the present series showed very similar thermal behaviors 10077


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Inorganic Chemistry (Figure S2b), and they are slightly more thermally stable than the perovskite [NH2NH3][TM(HCOO)3].12a The DSC measurement for 4Er in the temperature range of −70 to +80 °C (Figure 1, inset) revealed the absence of phase transition. Magnetic Properties. The basic static magnetic properties of the six magnetic members from 1Tb to 6Yb were investigated (Figure 2 and Table S5 in the SI). The compounds are

that incorporating multiple acentric centers within the lattice by using simple acentric but achiral building blocks, in the present work NH2NH3+, HCOO−, and Ln3+, can lead to the acentric/ polar/chiral solids. Taking the structure of 4Er at 180 K as the representative (Figure 3), the perovskite-like structure possesses a NaCl-like erbium formate framework10,19a,20,25 incorporating the hydrazinium cations in the framework cavities (Figure 3a,b). The unique Er3+ ion is 8-fold-coordinated by two syn−anti and six anti−anti HCOO− groups, and the coordination environment is a distorted bicapped trigonal prism (Figure 3c). The Er−O bond lengths range from 2.260(3) to 2.407(3) Å and the O−Er−O angles from 66.9(1) to 148.0(1)° (Table 2), all comparable to those observed in known Er-AMFFs.19,20,24,25 Each Er3+ links to eight neighboring Er3+ ions via formate bridges, to form the NaCl-like network composed of rhombohedral units, and the unit has the two neighboring grids diagonally crossed by one anti−anti HCOO−. It is worth pointing out that the formate-crossed grids link and extend straight along the b axis but zigzag along the c axis. The formate-bridged Ln···Ln distances are 5.947 Å via syn−anti HCOO− and 6.552−6.573 Å via anti−anti HCOO−. Topologically, the framework is a uninodal net in the Schäfli symbol of 36·415·57,19a with Er3+ nodes and HCOO− connections. Other descriptions of the structure are possible;24 however, the perovskite-like one should be the most easily understandable and could describe relevant known Ln-AMFFs incorporating mono- and diammonium cations.10a,19,20,25 In the rhombohedral framework cavity (Figure 3d), the NH2NH3+ cation locates toward one acute corner opposite to the two formate-crossed faces and the inside-pointing formates and has its long axis approximately toward the b direction. Both the NH3+ and NH2 ends form N−H···O hydrogen bonds (six for NH3+ and three for NH2) with the anionic framework, strong for the former and relatively weaker for the latter (see Table 2). Such hydrogen-bonding interactions have been found in many other AMFFs.10 However, the N−H···O hydrogen bonds in the present series seemly stronger than those in [NH2NH3][TM(HCOO)3]12 because of the shorter N···O distances observed. From Tb to Lu, the cell constants decrease roughly linearly, with reductions (Tb to Lu, at 180 K) of ca. 0.15 Å in a, b, and c and 30 Å3 in V (Figure 4a and Table 1). The interatomic distances showed similar systematic decreases (Tables 2 and S3a in the SI). The shrinkage is clearly caused by lanthanide contraction.31 For c, the reduction in the percentage is 2 times higher than that for a and b, indicating the softness along the c axis. The structures of 5Tm determined at 100, 180, 290, and 375 K (Tables S2b and S3b in the SI) revealed no structural phase transition in the temperature range. However, the material exhibited ATE behavior (Figure 4b). From 100 to 180 K, a and b expand 1% but c contracts 1%. Therefore, there are positive thermal expansion (PTE) along the a and b axes but NTE along the c axis. The coefficients of thermal expansion (CTEs) are large or “colossal”,6,19c +100 (M K)−1 (in the a or b direction) and −120 (M K)−1 (in the c direction). From 180 to 375 K, all directions showed PTE, with CTEs of 50 (M K)−1 in the a direction but smaller, 10 (M K)−1, in the b and c directions. The close inspection on the structures at 100 and 180 K (and above) revealed the mechanism, as shown in Figure 5, for two neighboring rhombohedral units. At 100 K, the front, middle vertical anti−anti formate group (O3−C2−O4, highlighted by cyan C) has its molecular plane ca. 50° inclined with respect to the b axis, and the C/O atoms have small, less

Figure 2. Magnetism of 1Tb−6Yb: (a) plots of χT versus T under 10 kOe; (b) plots of isothermal magnetization at 2 K.

basically paramagnetic. At 300 K, the χT values were 11.39 (1Tb), 14.40 (2Dy), 14.11 (3Ho), 11.73 (4Er), 7.24 (5Tm), and 2.50 (6Yb) cm3 K mol−1, respectively, close to the expected values for individual free Ln3+ ions.29,30 Upon cooling, the χT values of 1Tb−5Tm decreased slowly from 300 to 50 K and then more rapidly and reached the individual lowest values at 2 K, indicating the progressive depopulation of the Stark levels and/or possible weak antiferromagnetic interaction between Ln3+ ions in the low-temperature region.30 The χT values of 6Yb decreased more significantly in the high-temperature region compared to 1Tb−5Tm. The high-temperature susceptibility data followed the Curie−Weiss law, and the Curie constants (C) and Weiss temperatures (θ) are given in Table S5 in the SI. For 1Tb−5Tm, the θ values are small and negative. For 6Yb, however, θ is large, −49.6 K, corresponding to the more significant decrease of the χT values above 50 K. This indicates the possible large splittings of the ground-state 2F7/2 of Yb3+ by the crystal field in 6Yb. The isothermal magnetization of the materials at 2 K (Figure 2b) shows lower magnetization at high field than the expected values (gJ × J values in Table S5 in the SI) for the free Ln3+ ions, revealing that the effect of depopulation of the Stark levels should be dominant. Crystal Structures, Relevance to Lanthanide Contraction and ATE, and Comparison to [NH2NH3][TM(HCOO)3]. The eight structures are isostructural, belonging to the orthorhombic polar space group Pca21 (Tables 1 and 2 and S1−S3 in the SI). These solids with noncentrosymmetric and polar structures extend our previously proposed principle19a,25 10078


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Inorganic Chemistry

Figure 3. Structure of 4Er. (a) Topological view of the NaCl-like erbium formate framework. Spheres are erbium nodes, and sticks are formate connections. Yellow sticks are diagonal formate connections (see the text). (b) Ball-and-stick view of the erbium formate framework with the NH2NH3+ cations in the framework cavities. (c) Local environment around one Er3+ and its eight neighboring Er3+ ions, viewed from the top of the bicapped trigonal prism of Er3+. The capped Er−O bonds are in black. (d) Zoomed view of one framework cavity with the NH2NH3+ cations. Atomic scheme: Er, blue violet; H, white; C, black; diagonally crossed HCOO− bridges, yellow; N, green. In part d, the green thin lines are N−H···O hydrogen bonds.

3.681 Å and N1···O3 3.977 Å. Therefore, the NH3+ end, in fact, forms five N−H···O hydrogen bonds at 100 K. At 180 K, this formate twists, and the molecular plane is nearly parallel to the b axis. This is also allowed for the orientation of the NH2NH3+ cation, with its long axis approximately toward the b axis. The C/O atoms at 180 K show strongly anisotropic thermal ellipsoids, and the large principle displacements are roughly toward the c direction. The twisting or flipping motion of the formate group leads to the formation of an extra weak N−H···O hydrogen bond, N1···O4 3.225 Å; thus, there are six N−H···O hydrogen bonds for NH3+ end, as described before for 4Er. Other N−H···O hydrogen bonds become weaker more or less. This observation indicates that this formate locates at a frustrated position, by competitive N−H···O hydrogen-bonding interactions to the NH3+ ends of the NH2NH3+ cations on both sides. The twist also results in a significant change in the C2−O3−Tm bond angle, from 139.4° to 158.5°. It is clear that such twisting or flipping motions of the formate group and its symmetric equivalents, the back middle vertical one and the left top and right bottom horizontal pairs, or such transverse vibrations6 assisted by hydrogen bonds, lead to NTE in the c direction and, consequently contribute PTE in the a and b directions, in the low-temperature region. The mechanism can be considered as a wine-rack type.7c,19c Above 180 K, such transverse vibrations can compensate for the normal thermal expansion in c somewhat but enhance it in a, both without straight Tm−formate−Tm linkages. In the b direction, the Tm−formate− Tm linkages in triangle arrays exist, and they are rigid. Therefore, larger PTE in the a direction but small PTE in the b and c directions were observed in the high-temperature region. The mechanism of ammonium−framework coupling for ATE and NTE

Figure 4. (a) Normalized cell parameters versus radius of Ln3+ at 180 K. (b) Normalized cell parameters versus T for 5Tm.

anisotropic thermal ellipsoids as other formate groups. Another feature is that the NH3+ end of NH2NH3+ in the left cavity is quite far from this formate group, with two N···O distances of N1···O4 10079


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Inorganic Chemistry

of symmetry,34 the lanthanide formate frameworks are usually low symmetric because of the variable and asymmetric coordination geometries of Ln3+. The frameworks thus easily match and adapt the ammonium cations of lower symmetries usually. Instead, the high-symmetric octahedral coordination geometry of TM intrinsically tends to the formation of a TM-formate framework of higher symmetries and thus mismatches the low-symmetric ammonium units usually used, resulting in disordered arrangements of ammonium and possible state alternations, as have been observed for many TM-AMFFs. Therefore, phase transitions are popular for TM-AMFFs10,34 but probably not so common for Ln-AMFFs. Further more systematic and extensive studies, especially on Ln-AMFFs, will be needed to confirm this point. Finally, the polar structures of the present Ln-AMFFs imply that they are possibly ferroelectric,5b,10,19a,24 and the polar phases will probably remain in the temperature region below thermal decomposition. The materials should be of further interest for ferroelectrics even multiferroics, given the existence of magnetic Ln3+ ions of large and anisotropic magnetic moments.

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CONCLUSION In conclusion, a series of isostructural [NH2NH3][Ln(HCOO)4] for Tb−Lu and Y have been successfully prepared by utilizing NH2NH3+. The compounds possess polar perovskite-like structures incorporating the hydrazinium cations in the framework cavities, and the Ln3+ ions in a bicapped trigonal prism are connected by anti−anti and syn−anti formate groups. The shrinkage of the unit cell is caused by lanthanide contraction within the series. Large ATE with NTE were observed below 180 K, originating from the twisting or flipping motion of one anti−anti formate group in a frustrated situation raised by the competitive N−H···O hydrogen-bonding interactions. Compared with the TM analogues, the lanthnide structures are more compact, the lanthanide formate frameworks bind the ammonium much more tightly via stronger N−H···O hydrogen bonding, and the frameworks match the ammonium better in the viewpoint of symmetry. Therefore, these Ln-AMFFs show no phase transition, different from the TM analogues. However, the polar structures and magnetic activity render them possible ferroelectric even multiferroic materials, and they merit further investigation.

Figure 5. Two neighboring cavities presented in the thermal ellipsoids (at 90% probability) of non-H atoms in the structures of 5Tm at 100 and 180 K, in the same viewing direction down b. The color scheme is the same as that in Figure 3, plus cyan is C for the twisting anti−anti formate group and the symmetric equivalents (see the text). A deepblue curve arrow at 100 K indicates the twisting of the formate, and “A” labels the existence of the weak N−H···O hydrogen bonds at 180 K but their absence at 100 K.

has been documented for several AMFFs,11,12a,15a and these are quite different from those of other MOF systems.6 The lanthanide compounds within the series seemingly showed no structural phase transition, opposite to [NH2NH3][TM(HCOO)3].12 This could be due to several reasons. First, it is noted that the cavity volumes32a of 34−36 Å3 (data around 180 K here and below) for lanthanide compounds are significantly smaller than the van der Waals size32b of 44 Å3 of NH2NH3+ and also smaller than the cavity volumes of 38−52 Å3 of the TM analogues. The packing coefficients,32a,33 0.816−0.837, of the lanthanide structures are higher compared with the values of 0.710−0.750 for the TM compounds.12a These indicate the denseness and compactness of the lanthanide structures and, therefore, less space for the motion and state alternation of the cation. It is worth pointing out that [tmenH2][Er(HCOO)4]2,20,21a the only known Ln-AMFF showing phase transition to date, also has a lower packing coefficient of 0.753 (at 160 K, ordered phase), although no systematic investigation for phase transition has been conducted for other Ln-AMFFs.19,20,24,25 Second, for lanthanide compounds, the addition of one more HCOO− per NH2NH3+ increases the number of formate groups close to the cation and the N−H···O hydrogen bonds, that is, six neighboring HCOO− and nine N−H···O hydrogen bonds per NH2NH3+ for lanthanide compounds but three neighboring HCOO− and seven or eight N−H···O hydrogen bonds per NH2NH3+ for [NH2NH3][TM(HCOO)3],12 and the N−H···O hydrogen bonds enhance somewhat for lanthanide compounds. NH2NH3+ in lanthanide structures is thus more tightly bound. Third, from the viewpoint

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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.6b00952. Tables S1−S5 and Figures S1 and S2 (PDF) CIF file of crystallography data for the structures in this work (CIF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. Fax: 86-10-62751708. *E-mail: [email protected]. Fax: 86-10-62751708. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the NSFC (Grants 21171010, 91422302, 21321001, 21290170, and 21290171) and the National Basic Research Program of China (Grant 2013CB933401). 10080


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