Synthesis, Structure, and Magnetic Properties of a Novel Pillared

Bangalore 560 012, India, Department of Chemistry, UniVersity College, London, United Kingdom, and. Theoretical Sciences Unit, Jawaharlal Nehru Centre...
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J. Phys. Chem. B 2007, 111, 12700-12706

Synthesis, Structure, and Magnetic Properties of a Novel Pillared Layered Iron(III) Arsenate, [4,4′-bpyH2]3[Fe9(H2O)6F3(HAsO4)12(AsO4)2]‚2H2O Vandavasi K. Rao,† Mark A. Green,‡ Swapan K. Pati,*,§ and Srinivasan Natarajan*,† Framework Solids Laboratory, Solid State and Structural Chemistry Unit, Indian Institute of Science, Bangalore 560 012, India, Department of Chemistry, UniVersity College, London, United Kingdom, and Theoretical Sciences Unit, Jawaharlal Nehru Centre for AdVanced Scientific Research, Jakkur P.O., Bangalore 560 064, India ReceiVed: June 26, 2007; In Final Form: August 24, 2007

A three-dimensional iron arsenate [4,4′-bpyH2]3[Fe9(H2O)6F3(HAsO4)12(AsO4)2]‚2H2O, 1, has been synthesized using 4,4′-bipyridine as the templating agent under hydrothermal conditions. The structure is formed by FeO6, FeO3F3 octahedral units connected with HAsO4 and AsO4 units, forming one- and two-dimensional units in which the one-dimensional units act as a pillar. The presence of face-shared Fe-dimer units in the onedimensional unit is noteworthy. Detailed magnetic studies indicate two possible magnetic interactions: antiferromagnetic interactions within the layer and a weak ferromagnetic polarization between the layers at very low temperatures through the Fe-dimer units.

Introduction Open-framework structures based on arsenates are beginning to attract the attention of synthetic chemists as the larger size of As5+ (0.335 Å) compared to P5+ (0.17 Å) may give rise to novel architectures.1 In addition, a comparison of the pKa values of H3PO4 and H3AsO4 also gives an idea about the relative ease of removal of protons. Thus, for H3PO4, pKa1, pKa2, and pKa3 are 2.12, 7.21, and 12.32, respectively, while for H3AsO4 the corresponding values are 2.3, 6.9, and 11.5, respectively. This indicates that the arsenic acid is somewhat weaker than the phosphoric acid. The weak acidity of the arsenic acid, the larger size of As5+, and the associated toxicity of arsenic are some of the reasons for the lack of large number of arsenate-based extended structures. Persistent research over the years gave rise to arsenate frameworks of Al,2 Ga,3 V,4 Zn,5 Mo,6 and Fe.7 It is becoming apparent that the arsenate networks also exhibit considerable variety in their structures. Most of the openframework compounds are prepared employing hydrothermal methods. In some cases, HF was used as the mineralizer, which also becomes part of the structure by bridging the metal centers. This type of connectivity usually results in -M-F-M- infinite one-dimensional chains and facilitates facile magnetic interactions between the metal centers. Of the many arsenate frameworks that have been reported, those belonging to iron are important for their many interesting structures.7 Though most of the iron arsenates have octahedral iron and arsenate tetrahedral units connected through the vertices, there are subtle differences in the mode of connectivity. One of the notable structural features observed in many of the iron arsenates is the exclusive presence of one particular secondary building units (SBUs), originally proposed by Ferey.8 Thus, SBU-4 units are * To whom correspondence should be addressed. E-mail: (S.P.) [email protected] and (S.N.) [email protected]. † Indian Institute of Science. ‡ University College. § Jawaharlal Nehru Centre for Advanced Scientific Research.

connected to form one-dimensional structures in [NH3C2H4NH3]0.5[VO(H2O)(AsO4)]4c and [NH3(CH2)2NH(CH2)2NH3][Fe2F4(HAsO4)2].7h The same building unit is also connected together forming two-dimensional structures in [M][(VO)2(AsO4)2] (M ) Ba, Sr)4b and [C10N4H28][FeF(OH)(HAsO4)]4.7g SBU-4 units are connected to form a cluster, [Fe6As8O36(OH)4], which are further linked forming a three-dimensional structure in a mixed-valent iron-arsenate, [C4N2H12][Fe2IIIFeII(AsO4)2(HAsO4)2].7f SBU-5 units have been observed in the onedimensional iron arsenate, [C2N2H10][Fe(HAsO4)2(H2AsO4)]‚ H2O,7b,c which has triply bridged iron centers. The variety and diversity of structures in the family of iron arsenates appears to be limited, which prompted us to study formation of such structures by employing hydrothermal methods using HF as the mineralizer. During the course of this study we have now prepared a new iron arsenate, [4,4′-bpyH2]3[Fe9(H2O)6F3(HAsO4)12(AsO4)2]‚2H2O, 1, exhibiting a pillared layered structure. The structure is formed by one- and twodimensional units in which the one-dimensional units act as a pillar. The pillaring of the layers gives rise to a two-dimensional channel system bound by 14-T (T ) Fe, As) atoms. In this paper, we report the synthesis, structure, and magnetic properties of the compound. Experimental Section Synthesis and Initial Characterization. Synthesis of the title compound was carried out under hydrothermal conditions in a 23-mL PTFE-lined stainless steel acid-digestion vessel in the presence of 4,4′-bipyridine (bpy). In a typical synthesis, 1.249 g of H3AsO4 was dissolved in 4 mL of Millipore water. A 0.197 g amount of iron(II) oxalate dihydrate and 0.701 g of bpy were added under constant stirring. Finally, 0.24 mL of HF (48%) was added, and the mixture was homogenized at room temperature for 30 min. The final mixture with the composition 1FeC2O4·2H2O:8H3AsO4:4bpy:6HF:200H2O was heated at 150 °C for 72 h and 180 °C for 24 h. The initial and final pH

10.1021/jp0749625 CCC: $37.00 © 2007 American Chemical Society Published on Web 10/17/2007

Novel Pillared Layered Iron(III) arsenate TABLE 1: Crystal Data and Structure Refinement Parameters for [4,4′-bpyH2]3[Fe9(H2O)6F3(HAsO4)12(AsO4)2]‚ 2H2O, 1a empirical formula fw cryst syst space group T (K) a (Å) b (Å) c (Å) γ (deg) V (Å3) Z D (calcd/g cm-3) µ (mm-1) λ (Mo KR/Å) θ range (deg) total data collected unique data R indices [I > 2σ(I)] R indices (all data)

C5H9.67As2.33F0.50Fe1.50N1O10.67 522.9 trigonal P-3c1 (no. 165) 293 13.9026(12) 13.9026(12) 23.919(4) 120 4003.7(8) 12 2.602 7.454 0.71073 1.69-25.35 28 514 2461 R1 ) 0.0315, wR2 ) 0.0867 R1 ) 0.0350, wR2 ) 0.0882

a R1 ) Σ||F0| - |Fc||/Σ|F0|; wR2 ) {Σ[w(F02 - Fc2)2]/Σ[w(F02)2]}1/2. w ) 1/[σ2(F0)2 + (aP)2 + bP], P ) [max(F02,0) + 2(Fc)2]/3, where a ) 0.0340 and b ) 25.4415.

of the reaction mixture was ∼2.0, and there was no appreciable change in the pH during the reaction. The resulting light yellow hexagonal crystals were filtered and washed with distilled water and dried at ambient temperature (yield ) ∼75% based on iron salt). Initial characterizations were carried out using powder XRD, EDAX, elemental analysis, IR, and TGA measurements. The powder XRD (Philips, X’pert Pro) studies indicated that the pattern was new; it was entirely consistent with the simulated XRD pattern based on the structure determined using singlecrystal X-ray diffraction (Figure S1). An energy-dispersive X-ray analysis (EDAX) on many single crystals indicated a Fe:As ratio of 2:3, agreeing with the single-crystal structure. Anal Calcd (ThermoFinnigan Flash EA 1112 CHNS analyzer) C, 12.03; H, 1.95; N, 2.81. Found: C, 13.07; H, 2.12; N, 2.91. IR spectroscopic studies (Perkin-Elmer, Spectrum 1000) were carried out with a KBr pellet in the range 400-4000 cm-1. The results indicate characteristic peaks (Figure S2). The various observed bands and their assignments are ν(N-H) ) 3247 cm-1, ν(C-H) ) 3112 cm-1, ν(AsO4) ) 827 cm-1, δ(N-H) ) 1636 cm-1, δ(C-H) ) 1476 cm-1, δ(As-OH) ) 1233 cm-1, δ(C-N) ) 1104 cm-1. Single-Crystal X-ray Diffraction. A suitable single crystal of 1 was used in the intensity data collection using a Bruker AXS Smart Apex CCD diffractometer at 293(2) K. The X-ray generator was operated at 50 kV and 35 mA using Mo KR (λ ) 0.71073 Å) radiation. Data was collected with ω scan width of 0.3°. A total of 606 frames were collected in different settings of φ keeping the sample-to-detector distance fixed at 6.0 cm and the detector position (2θ) fixed at -25°. Pertinent experimental details of the structure determination are given in Table 1. The data was reduced using SAINTPLUS,9 and an empirical absorption correction was applied using the SADABS program.10 The crystal structure was solved and refined using SHELX-9711 present in the WinGx suite of programs (version 1.63.04a).12 The hydrogen atoms of the lattice water molecule were not located. We made use of detailed bond valence sum calculations13 to arrive at the hydrogen positions for the hydroxyl

J. Phys. Chem. B, Vol. 111, No. 44, 2007 12701 TABLE 2: Selected Bond Distances Observed in [4,4′-bpyH2]3[Fe9(H2O)6F3(HAsO4)12(AsO4)2]‚2H2O, 1a bond

distance, Å [BVS]b

bond

distance, Å [BVS]b

As(1)-O(1) As(1)-O(3) As(1)-O(2) As(1)-O(4) Σ(As-O) As(2)-O(5) As(2)-O(6) As(2)-O(7) As(2)-O(8) Σ(As-O) As(3)-O(9)#1 As(3)-O(9)#2 As(3)-O(9) As(3)-O(10) Σ(As-O) Fe(1)-O(6)#3 Fe(1)-O(6) Fe(1)-O(6)#4

1.662(4) [1.328] 1.665(4) [1.317] 1.669(4) [1.303] 1.712(4) [1.160] [5.108] 1.669(4) [1.303] 1.675(3) [1.282] 1.682(3) [1.258] 1.700(4) [1.198] [5.041] 1.650(4) [1.371] 1.650(4) [1.371] 1.650(4) [1.371] 1.707(7) [1.176] [5.289] 2.016(3) [0.499] 2.016(3) [0.499] 2.016(3) [0.499]

Fe(1)-O(6)#5 Fe(1)-O(6)#6 Fe(1)-O(6)#7 Σ(Fe-O) Fe(2)-O(9) Fe(2)-O(1) Fe(2)-O(2)#4 Fe(2)-O(3)#8 Fe(2)-O(5) Fe(2)-O(100) Σ(Fe-O) Fe(3)-O(7)#7 Fe(3)-O(7)#6 Fe(3)-O(7) Fe(3)-F(1) Fe(3)-F(1)#6 Fe(3)-F(1)#7 Σ(Fe-O/F)

2.016(3) [0.499] 2.016(3) [0.499] 2.016(3) [0.499] [2.994] 1.949(4) [0.598] 1.961(4) [0.579] 1.990(4) [0.535] 2.000(4) [0.521] 2.003(4) [0.517] 2.139(4) [0.358] [3.108] 1.930(3) [0.629] 1.930(3) [0.629] 1.930(3) [0.629] 2.032(3) [0.385] 2.032(3) [0.385] 2.032(3) [0.385] [3.042]

a Symmetry transformations used to generate equivalent atoms: #1 -x + y, -x+1, z; #2 -y + 1, x - y + 1, z; #3 -x, -y, -z + 1; #4 x - y, x, -z + 1; #5 y, -x + y, -z + 1; #6 -y, x - y, z; #7 -x + y, -x, z. b Values in brackets are the bond valences. Their sum of bond valences (BVS) in bold appears at the end of the list of the distances around every cation.

groups; the hydrogen positions were located in the difference Fourier map, and for the final refinement the hydrogen atoms were placed in geometrically ideal positions and refined using the riding mode. The last cycles of refinements included atomic positions, anisotropic thermal parameters for all non-hydrogen atoms, and isotropic thermal parameters for all hydrogen atoms. Full-matrix least-squares structure refinement against |F2| was carried out using the SHELXL package of programs.11,12 The selected bond distances are given in Table 2. Results and Discussion The asymmetric unit of 1 contains 25 non-hydrogen atoms of which three arsenic and iron atoms are crystallographically independent. Of these Fe(1), Fe(3), and As(3) atoms occupy special positions with a site multiplicity of 0.16, 0.33, and 0.33 for Fe(1), Fe(3), and As(3), respectively. Iron atoms Fe(1) and Fe(2) are octahedrally coordinated with six oxygen-atom neighbors with an average bond distance (Fe-O) of 2.016 Å for Fe(1) and 2.007 Å for Fe(2), whereas the Fe(3) atom is octahedrally coordinated with three oxygen and three fluorine atoms with an average distance (Fe(3)-O/F) of 1.981 Å, respectively. The O/F-Fe-O/F bond angles are in the range of74.98-180°.TheFe(1)atomsformsixFe-O-Asbonds(av.136.0°), Fe(2) forms five Fe-O-As bonds (av. 138.1°) and one terminal Fe-OH2 bond, and Fe(3) forms thee Fe-O-As bonds (av. 127.3°). All arsenic atoms make three Fe-O-As linkages and possess one As-O terminal bond with the As-O distances in the range of 1.650(4)-1.712(4) Å and the O-As-O bond angles in the range of 103.20(15)-115.6(2)°. The structure of 1 consists of a network of FeO6, FeO5(H2O), HAsO4, and AsO4 polyhedral units connected through their vertices giving rise to a layer of the formula [Fe3(H2O)3(HAsO4)3(AsO4)]2, encompassing circular 12-membered rings as shown in Figure 1a. This layer is topologically identical to that seen in many layered aluminophosphates based upon the Al3P4O163- ion,14 with which it shares the same space group, P-3c1 (note that the layered aluminophosphates with Al3P4O163stoichiometry are based entirely on tetrahedral nets14). The 12membered rings are surrounded by a series of four-membered

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Figure 1. (a) View of the iron arsenate layer of 1. (b) View of the one-dimensional chain connecting the layers. Oxygen atoms are omitted for clarity. (c) View of the two successive layers showing the T atom (T ) Fe, As) connectivity. The one-dimensional chains connecting these layers are not shown.

rings. In one set of rings the arsenic atoms are part of the wall of the 12-membered ring, while in the other a As(3)O4 group caps a six-membered ring and alternates above and below the plane of the 12-membered ring. Identical layer structure has also been seen in the iron phosphate-oxalate, [NH3(CH2)2NH3]1.5[Fe3(PO4)(HPO4)3(C2O4)1.5]‚xH2O (x ) 1.5-2.0),15a and the iron arsenate-oxalate, [NH3(CH2)CH(NH3)CH3]3[Fe6(AsO4)2(HAsO4)6(C2O4)3].15b In the present structure, however, we find that the Fe(1)O6 octahedron sits in the middle of the 12-membered aperture and is bonded with six HAs(2)O4 units. The HAs(2)O4 units also link with Fe(3)O3F3 octahedra, which forms an unusual face-shared dimer. Thus, the connectivity between the Fe(1)O6, Fe(3)O3F3, and HAs(2)O4 units forms a one-dimensional chain, [FeF(HAsO4)2]2- (Figure 1b), which keeps the two layers apart and intact, giving rise to a new threedimensional structure. The view of two successive layers is shown in Figure 1c (Figure S4). Though the layers are arranged in such a way to superimpose one over the other, the sequence of arrangement follows a ABAB... type. The connectivity

between the one- and two-dimensional units resembles the pillared layered structure commonly observed in clays (Figure 2a).16 The resulting three-dimensional structure possesses two large channels bound by 14-T atoms (T ) Fe, As). The organic amine molecule, the protonated 4,4′-bipyridinium cations, occupies the intersection of these channels (Figure 2a). An identical structure has been observed, recently, in an indium phosphate, [C10N2H9]3[In9(HPO4)14(H2O)6F3]‚(H3O)‚(H2O)2.17 The present compound appears to have a close structural relationship with the iron arsenate-oxalate, [NH3(CH2)CH(NH3)CH3]3[Fe6(AsO4)2(HAsO4)6(C2O4)3],15b as both the structures possess the same layers (Figure 2). In the iron arsenateoxalate the layers are arranged in a AAAA... fashion and connected through the oxalate group forming the threedimensional structure. The 12-membered aperture within the layers is left undisturbed, resulting in a uniform 12-membered one-dimensional channel in which the organic amine molecule, 1,2-diaminopropane, resides. In the present compound the layers

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Figure 2. (a) Structure of [4,4′-bpyH2]3[Fe9(H2O)6F3(HAsO4)12(AsO4)2]‚2H2O, 1, showing the T-atom (T ) Fe, As) connectivity. Note that the bipyridinium molecules are in the middle of the channels. (b) Connectivity between the layers in the iron arsenate-oxalate, [NH3(CH2)CH(NH3)CH3]3[Fe6(AsO4)2(HAsO4)6(C2O4)3].

are arranged in a ABAB... fashion (Figure 1c) and the 12membered aperture is filled by one-dimensional chains, resulting in a new three-dimensional structure with two independent channel systems (Figure 2a). The interesting structural features prompted us to make a detailed investigation of the magnetic behavior employing a SQUID magnetometer (Quantum Design) in the temperature range 2-300 K. The temperature-dependent magnetic studies show quite a few interesting characteristics (Figure 3). As can be noted, both the ZFC and FC susceptibility data appear to follow each other down to 36 K, below which the ZFC data shows a decrease down to 22 K while the FC data shows a

monotonic increase in χ. However, as T is reduced below 22 K, the χ values under ZFC conditions start to show an increase. The high-temperature data (T ) 100-300 K) was fitted to the Curie-Weiss behavior, which gave a value of θ ) -45.5 K (inset of Figure 3a). The negative θ value indicates an average antiferromagnetic interaction in the system. For a qualitative understanding of the magnetic structure we fitted the data with a dimer model with Heisenberg spin exchange coupling, H ) JS1‚S2, where S1 and S2 correspond to two S ) 5/2 spins.18 The dimer model, however, could fit only down to T ) 40 K, with a fit giving a J value comparable to that obtained by the CurieWeiss fit for θ. Since both the dimer model and the Curie-

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Figure 3. (a) Thermal variation of χM at H ) 100 G. Inset shows 1/χ vs T. (b) Spin density plot for S ) 0 and S ) 5 for the Fe dimer in the compound. Note the spin polarizations in S ) 5 due to the mixing of Fe orbitals. (c) M vs H study of 1. (Inset) Expanded region at low fields.

Weiss equations are smooth functions of T and valid only at high temperature, there must be some additional interactions governing the magnetization fluctuations at low temperature as seen in Figure 3. A closer look at the structure suggests antiferromagnetic interactions within the layers through wellknown superexchange mechanisms. The value of θ thus gives an idea regarding the effective interactions between the magnetic centers within the two-dimensional layers. It may be noted that the layers are connected by a one-dimensional chain that possesses a face-shared Fe-dimer. To understand whether the exchange processes in the Fedimer in the interlayer spacing is ferromagnetic or not, we retrieved the dimer structure from the crystal data and with appropriate substitution for charge balancing, carried out broken symmetry density-functional theory calculations19 within the Gaussian 03 set of codes to ascertain the magnetic couplings.20 The spin densities are shown in Figure 3b for the lowest spin (S ) 0) and highest spin (S ) 5) dimer configurations. The UB3LYP level energy for the S ) 5 state is found to be more stable than the singlet state. In fact, an effective exchange interaction of about -0.014 eV can be estimated by fitting the

Rao et al. energy values with a Heisenberg Hamiltonian for a dimer of S ) 5/2 spins. Since the interaction within the intervening Fedimer structure is weakly ferromagnetic, it is expected that its characteristics would appear in magnetization processes as T f 0 (at very low temperatures). Since the low-temperature behavior of ZFC susceptibility shows some signature of magnetic polarization, we carried out magnetization measurement at low T. In Figure 3c the M vs H studies at T ) 2.5 K are shown. Interestingly, the present compound appears to show a small hysteresis at low field strengths. This behavior is certainly due to the intervening Fedimer, which links the layers. Thus, the higher dimensional couplings show magnetic polarizations at low temperature with explicit presence of ferromagnetic alignment within the interlayer spacings. For a magnetic system possessing a variety of possible interactions the temperature plays a crucial role in probing their magnitudes if they are energetically far apart. This is true in the compound of the present study. Due to the ferromagnetic coupling within the face-shared Fe-dimer the magnetization processes align the interlayer spins at low temperature. Such a variation of the magnetic data with temperature signifies the structure-dominated exchange pathways which if properly tuned can be of immense value in appropriately designing multifeatured magnetic systems for advanced applications. In most of the previously reported iron arsenates we found only simple antiferromagnetic behavior. It is worthwhile to compare the magnetic behavior with the closest related structure, the iron arsenate-oxalate, [NH3(CH2)CH(NH3)CH3]3[Fe6(AsO4)2(HAsO4)6(C2O4)3].15b In [NH3(CH2)CH(NH3)CH3]3[Fe6(AsO4)2(HAsO4)6(C2O4)3] the temperature variation of the magnetic susceptibility clearly indicated an antiferromagnetic behavior with C ) 4.51 emu/mol and θ ) -59 K. The presence of the Fe-dimer units connecting the layers in the present compound gives rise to magnetic polarizations and a weak ferromagnetic behavior. Thus, it appears the connecting unit (oxalate vs Fe-dimer) between the layers is important in the outcome of the final magnetic behavior of the solid. Thermogravimetric analysis (TGA) was carried out (MettlerToledo, TG850) in an atmosphere of flowing air (flow rate ) 50 mL/min) in the temperature range 30-850 °C (heating rate ) 5 °C/min). The results indicate weight losses up to 550 °C (obsd. 47.5%) (Figure 4a), which may correspond to loss of lattice water, amine, coordinated water, part of the arsenic, and fluorine (calcd 44.0%). The final calcined product corresponds to the crystalline phase FeAsO4 (ICDD-00-021-0910) by PXRD. In order to understand the various phases formed during decomposition of 1, we carried out ex-situ heating of the sample in the temperature range 150-850 °C and followed the progress by PXRD. The results are shown in Figure 4b. As can be noted, the sample appears to lose crystallinity at ca. 300 °C, at which point it is likely that the lattice water and amine molecules were removed. On heating further the sample becomes amorphous and continues till ca. 600 °C. The sample starts to crystallize when heated at temperatures >600 °C. The PXRD lines for this phase now correspond to the crystalline condensed iron arsenate FeAsO4 [ICDD-00-021-0910]. On heating further, the sample appears to become more crystalline as the XRD lines become sharper. This study clearly indicates that the thermal stability of the present compound is quite limited and forms an iron arsenate crystalline phase via an amorphous intermediate. Similar behavior has also been observed during decomposition of the iron arsenate-oxalate [NH3(CH2)CH(NH3)CH3]3[Fe6(AsO4)2(HAsO4)6(C2O4)3].15b

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Figure 4. (a) Thermogravimetric analysis (TGA) of 1. (b) Variable-temperature XRD studies on 1 (temperature values are given as °C).

Conclusions

References and Notes

The synthesis, structure, and magnetic studies on a threedimensional iron arsenate have been accomplished. Formation of a well-known layer structure pillared by a one-dimensional chain is noteworthy and interesting. Formation of a face-shared Fe-dimer is rare in framework compounds. Magnetic investigations indicate antiferromagnetic interactions within the layers, and the presence of face-shared Fe-dimer within the interlayer region gives rise to ferromagnetic polarizations at low temperatures.

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Acknowledgment. S.N. thanks the Department of Science and Technology (DST), Government of India, for the award of research grants. S.N. also thanks DST for the RAMANNA fellowship. V.K.R. thanks the Council of Scientific and Industrial Research (CSIR), Government of India, for the award of a research fellowship. S.K.P. thanks DST and CSIR, Government of India, for research grants. Supporting Information Available: Powder X-ray pattern of (a) simulated and (b) experimental of 1 (Figure S1); IR spectra of 1 (Figure S2); view of the three-dimensional structure along the c axis (Figure S3); view of the two successive layers (Figure S4). This material is available free of charge via the Internet at http://pubs.acs.org. Crystallographic data (excluding structure factors) has been deposited with CCDC-296812. The data can be obtained free of charge from the Cambridge Crystallographic Data Centre (CCDC) via www.ccdc.cam.ac.uk/ data_request/cif.

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