Layered Iron(III) and Cobalt(II) Phosphonates Decorated by

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DOI: 10.1021/cg100559c

Layered Iron(III) and Cobalt(II) Phosphonates Decorated by Hydrophilic Sulfone Groups: Syntheses, Structures and Magnetic Properties

2010, Vol. 10 3721–3726

Zi-Yi Du,*,† He-Rui Wen,† Cai-Ming Liu,*,‡ Yu-Hui Sun,† Ying-Bing Lu,† and Yong-Rong Xie† †

College of Chemistry and Life Science, Gannan Normal University, Ganzhou, 341000, P. R. China, and Beijing National Laboratory for Molecular Sciences, Center for Molecular Science, Institute of Chemistry, Chinese Academy of Sciences, Beijing, 100190, P. R. China

‡

Received April 27, 2010; Revised Manuscript Received June 24, 2010

ABSTRACT: Hydrothermal reactions of FeIII or CoII ion with dimethyl methylsulfonomethanephosphonate afforded two novel layered transition metal phosphonates containing oxo-bridging hydroxides (H2O or OH-) as coligands, namely, [Fe(L)(OH)(H2O)] (1) and [Co4(L)4(H2O)4] 3 2H2O (2) (H2L = CH3SO2CH2PO3H2). Magnetic property studies reveal that spincanting and metamagnetic behaviors coexist in complex 2 with a Tc of 7.0 K.

Introduction It was well-known that metal phosphonate compounds are most often lamellar in nature, in which the metal ions are bridged by the phosphonate moieties to form an inorganic layer and the remaining organic group of the phosphonate generally dangles in the interlayer region.1 To create metal complexes with unusual magnetic properties, layered metal phosphonates have advantages and disadvantages. The prominent advantage is that phosphonate group could show various coordination modes via its three potential O donors, and thus, many different layered arrangements may be realized by a modification of the ligand or the reaction condition.1,2 The main disadvantage is that magnetic exchange via phosphonate group generally is weak. However, if an oxobridging coligand is also present, then significant superexchange can be expected through such a coligand.3,4 In regard to the widely used hydrothermal reaction system, the O2-, OH-, or H2O may be good candidates as oxo-bridging coligands. In our current studies, we use a phosphonate ligand in which the phosphonate moiety is directly attached by a simple sulfone group, CH3SO2CH2PO3H2 (H2L), as a potential building block to address the above issue. The sulfone group usually exhibits weak coordination ability to many metal ions but is often inclined to participate in hydrogen bonding primarily with metal-bound hydroxide (H2O or OH-).5 Hence, we wish that the decoration of a sulfone group on the phosphonate ligand may promote the presence of a metalbound hydroxide. Additionally, the introduction of hydrophilic sulfone groups into the metal phosphonate also facilitates the formation of crystalline product which is convenient for X-ray structural analysis. So far no metal complexes of sulfone-phosphonate ligands have been structurally characterized, although a number of metal complexes based on sulfonate-phosphonate ligands have been reported by us and others.6,7 Our such research efforts yielded two novel layered metal phosphonates decorated by hydrophilic sulfone groups and with the presence of oxo-bridging coligands, namely, Fe(L)(OH)(H2O)] (1) and

[Co4(L)4(H2O)4] 3 2H2O (2). Herein, we report their syntheses, crystal structures, and magnetic properties. Experimental Section

*To whom correspondence should be addressed. E-mail: ziyidu@gmail. com (Z.-Y. D.); [email protected] (C.-M. L.).

Materials and Instrumentation. Dimethyl methylsulfonomethanephosphonate (CH3SO2CH2PO(OCH3)2) was synthesized using a published procedure.8 All other chemicals were obtained from commercial sources and used without further purification. Elemental analyses were performed on a German Elementary Vario EL III instrument. FT-IR spectra were recorded on a Nicolet 5700 spectrometer using KBr pellets in the range of 4000-400 cm-1. X-ray powder diffraction (XRD) patterns (Cu KR) were collected on a Bruker Advance D8 θ-2θ diffractometer. Thermogravimetric analyses were carried out on a Diamond TG/DTA 6000 unit at a heating rate of 15 C/min under a nitrogen atmosphere. Variable-temperature magnetic susceptibility, zero-field ac magnetic susceptibility, and field dependence of magnetization were measured on a Quantum Design MPMSXL5 (SQUID) magnetometer. Diamagnetic corrections were estimated from Pascal’s constants for all constituent atoms. Synthesis of [Fe(L)(OH)(H2O)] (1). A mixture of FeCl3 3 6H2O (0.33 mmol), CH3SO2CH2PO(OCH3)2 (0.30 mmol), and ethanol (3 mL) in 10 mL distilled water was sealed into a Parr Teflon-lined autoclave (23 mL) and heated at 150 C for 3 days. The final pH value was about 2.5, and yellow plate-shaped crystals of 1 were collected in a ∼66% yield based on Fe. Its purity has also been confirmed by powder XRD (Figure S1 in the Supporting Information). Anal. Calcd for C2H8O7S1P1Fe1 (Mr = 262.96): C 9.14, H 3.07%. Found: C 9.09, H 3.16%. IR data (KBr, cm -1): 3456(s), 3013(m), 2967(m), 2917(m), 2360(m), 1617(m), 1380(s), 1305(m), 1227(m), 1112(vs), 1074(s), 1006(s), 961(m), 835(m), 798(m), 725(m), 529(m), 462(m). Synthesis of [Co4(L)4(H2O)4] 3 2H2O (2). A mixture of CoCO3 (0.28 mmol), CH3SO2CH2PO(OCH3)2 (0.30 mmol), and ethanol (3 mL) in 10 mL distilled water was sealed into a Parr Teflon-lined autoclave (23 mL) and heated at 150 C for 3 days. The final pH value was about 5.5, and purple plate-shaped crystals of 2 were collected in a ∼75% yield based on Co. Its purity has also been confirmed by powder XRD (Figure S2 in the Supporting Information). Anal. Calcd for C8H32O26S4P4Co4 (Mr=1032.18): C 9.31, H 3.12%. Found: C 9.25, H 3.22%. IR data (KBr, cm-1): 3437(s), 2999(m), 2929(m), 2853(m), 2363(m), 2052(m), 1630(m), 1367(m), 1295(s), 1219(m), 1141(vs), 1107(s), 1065(s), 994(m), 954(m), 845(m), 794(m), 757(m), 570(m), 520(m), 462(m). Single-Crystal Structure Determination. Data collection for compounds 1 and 2 were performed on a Smart ApexII CCD diffractometer equipped with a graphite-monochromated Mo KR radiation (λ = 0.71073 A˚). Intensity data for both compounds were collected using j and ω scans at 296 K. The data sets were corrected for

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Table 1. Summary of Crystal Data and Structural Refinements for Compounds 1 and 2 compound 1 2 C8H32O26S4P4Co4 empirical formula C2H8O7S1P1Fe1 formula weight 262.96 1032.18 P21/c space group P21/c a (A˚) 11.935(7) 8.4557(7) b (A˚) 7.395(4) 13.2169(12) c (A˚) 9.756(6) 28.218(3) R (deg) 90 90 β (deg) 111.845(7) 102.063(7) γ (deg) 90 90 799.2(8) 3084.0(5) V (A˚3) Z 4 4 2.185 2.223 Dcalcd (g cm-3) 2.346 2.693 μ (mm-1) 1.047 1.054 GOF on F2 a 0.0587, 0.1436 0.0788, 0.1741 R1, wR2 [I > 2σ(I)] 0.0781, 0.1550 0.1523, 0.1957 R1, wR2 (all data)a P P P P a R1 = ||Fo| - |Fc||/ |Fo|; wR2 = { w[(Fo)2 - (Fc)2]2/ w2 2 1/2 [(Fo) ] } .

Figure 1. ORTEP representation of the selected unit of 1. The thermal ellipsoids are drawn at 50% probability. Symmetry codes for the generated atoms: A. -x, -y, 2 - z; B. x, 1/2 - y, -1/2 þ z; C. -x, 1/2 þ y, 3/2 - z; D. x, 1/2 - y, 1/2 þ z; E. -x, -1/2 þ y, 3/2 - z.

Scheme 1. In Situ Hydrolysis of the Dimethylphosphonate Ester Table 2. Selected Bond Lengths (A˚) for Compounds 1 and 2a 1 Fe(1)-O(6) Fe(1)-O(6)#1 Fe(1)-O(2)#3

1.974(4) 1.976(4) 2.016(4)

Fe(1)-O(1) Fe(1)-O(3)#2 Fe(1)-O(1W)

O(4) 3 3 3 O(1W)#1

hydrogen bond 2.784(6)

1.975(4) 1.997(4) 2.123(4)

Chart 1. Coordination Modes of the L2- Ligand in 1 and 2

2 Co(1)-O(13) Co(1)-O(7) Co(1)-O(3W) Co(2)-O(11)#1 Co(2)-O(6)#1 Co(2)-O(2W) Co(3)-O(2) Co(3)-O(7)#2 Co(3)-O(2W) Co(4)-O(1) Co(4)-O(3W) Co(4)-O(18)#2

2.010(8) 2.089(7) 2.166(7) 1.995(7) 2.124(8) 2.150(7) 2.008(8) 2.131(7) 2.155(7) 2.021(7) 2.130(7) 2.157(8)

Co(1)-O(3) Co(1)-O(16) Co(1)-O(1W) Co(2)-O(18) Co(2)-O(3) Co(2)-O(1W) Co(3)-O(12)#1 Co(3)-O(16)#2 Co(3)-O(4W) Co(4)-O(6)#3 Co(4)-O(12) Co(4)-O(4W)#4

O(1W) 3 3 3 O(6W) O(5W) 3 3 3 O(19)#2 O(6W) 3 3 3 O(9)#1

hydrogen bond 2.670(12) O(4W) 3 3 3 O(5W) 2.919(17) O(5W) 3 3 3 O(15)#2 2.802(16) O(6W) 3 3 3 O(17)#2

2.053(7) 2.135(7) 2.234(8) 2.092(7) 2.129(7) 2.227(8) 2.076(7) 2.141(7) 2.216(8) 2.095(7) 2.139(7) 2.243(7) 2.594(12) 2.945(15) 2.878(11)

a Symmetry codes: For 1: #1 -x, y þ 1/2, -z þ 3/2; #2 x, -y þ 1/2, z - 1/2; #3 -x, -y, -z þ 2. For 2: #1 x þ 1, y, z; #2 -x þ 1, y - 1/2, -z þ 1/2; #3 -x, y - 1/2, -z þ 1/2; #4 x - 1, y, z.

Lorentz and polarization factors as well as for absorption by SADABS program.9 Both structures were solved by the direct method and refined by full-matrix least-squares fitting on F2 by SHELX-97.10 All non-hydrogen atoms except O(18) in 2 were refined with anisotropic thermal parameters. All hydrogen atoms were generated geometrically and refined isotropically. The hydrogen atoms for the water molecules are not included in the refinements. Crystallographic data and structural refinements for 1 and 2 are summarized in Table 1. Important bond lengths are listed in Table 2. More details on the crystallographic studies, as well as atom displacement parameters, are given in the Supporting Information.

Results and Discussion Syntheses. The preparations of compounds 1 and 2 rely on the well-established hydrothermal method, using dimethylphosphonate as starting material. During the course of the hydrothermal treatment, the high temperature and high pressure help the hydrolysis of the phosphonic ester to

produce phosphonic acid in situ (Scheme 1). It is believed that the in situ slow formation of phosphonic acid facilitates the growth of single crystals.11 Also it is observed that the addition of small amount of ethanol in the aqueous solution helps isolating the crystals of these metal phosphonates. Structure of [Fe(L)(OH)(H2O)] (1). Compound 1 crystallizes in the monoclinic P21/c space group and it features a 2D layered structure. The asymmetric unit contains one Fe3þ ion, one L2- anion, one hydroxyl anion, and one aqua ligand. The þ3 oxidation state of the iron ion was confirmed by the bond valence sum (BVS) analysis (3.07 for Fe(1) ion).12 The Fe(1) ion has a distorted octahedral environment (Figure 1); three of its six coordination sites are filled by three phosphonate O atoms from three L2- anions and the remaining three sites are occupied by two equivalent hydroxyl anions and one aqua ligand. The Fe-O (1.974(4)-2.123(4) A˚) distances are similar to those in other reported iron(III) phosphonates.3 There are three different ligands in 1, including one monodentate aqua ligand, one bidentate hydroxyl ligand, and one tridentate L2- ligand. The aqua ligand serves as a terminal group, while the hydroxyl anion and the L2- ligand bridge with two and three Fe3þ ions, respectively. All three phosphonate O atoms in the L2- ligand are involved in metal coordination but the sulfone group remains free (Chart 1a). The interconnection of Fe3þ ions by the hydroxyl anions and the L2- ligands leads to the formation of a novel 2D layer structure down the Æ101æ axis, in which 1D chains of corner-shared FeO6 octahedra along the b-axis are further linked to two neighboring chains via the P(1) atoms (Figure 2). Such layer features three types of

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corner-shared ring motifs: (a) a six-member R2,3(6) ring comprising two Fe3þ ions, one hydroxyl anion, and two O atoms from one phosphonate group; (b) an eight-member R2,4(8) ring constructing from two Fe3þ ions and four O atoms from two phosphonate groups; (c) a twelve-member R4,6(12) ring composed of four Fe3þ ions, two hydroxyl anions, and four O atoms from two phosphonate groups. The Fe3þ ions in these rings are linked through the Fe-O-Fe or Fe-O-P-O-Fe modes that mainly define the Fe 3 3 3 Fe edges. The shortest Fe 3 3 3 Fe edge via the Fe-O-Fe mode is 3.699(2) A˚, while all the other Fe 3 3 3 Fe edges are longer than 5.0 A˚. The layers in 1 are assembled into a three-dimensional structure via van der Waals forces. Although no hydrogen bonds exist between two adjacent layers, the uncoordinated sulfone groups dangling on both sides of the layer are stabilized by their hydrogen bonding with the aqua ligands (Table 2, Figure 3). Structure of [Co4(L)4(H2O)4] 3 2H2O (2). Compound 2 also crystallizes in the monoclinic P21/c space group, but it features another type of 2D layered structure. The asymmetric unit

Figure 2. View of the layered structure of 1 down the Æ101æ axis. The uncoordinated CH3SO2CH2- groups of the phosphonate ligands have been omitted for clarity. The FeO6 octahedra are shaded in orange. P and O atoms are represented by purple and red circles, respectively.

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contains four Co2þ cations, four L2- anions, four aqua ligands, and two lattice water molecules. All the Co2þ ions are octahedrally coordinated by four phosphonate O atoms from four L2- ligands and two aqua ligands (Figure 4). The Co-O (1.995(7)-2.243(7) A˚) distances are similar to those in other reported cobalt(II) phosphonates.4 The four unique L2- ligands in compound 2 adopt two types of coordination modes and all of them tetradentately bridge with four Co2þ ions via their phosphonate groups (Chart 1b and c). The difference is that one O atom in P1O3 or P3O3 acts as μ2-O, while two O atoms in P2O3 or P4O3 function as μ2-O. In addition, all the four aqua ligands also function as μ2-O. The interconnection of Co2þ ions by the L2- ligands and the aqua ligands leads to the formation of a compact layer structure down the c-axis, in which 1D zigzag chains of edge-shared CoO6 octahedra along the a-axis are further corner-shared with two neighboring chains along the b-axis (Figure 5). The Co2þ ions are linked through the Co-O-Co and Co-O-P-O-Co modes which mainly define the Co 3 3 3 Co edges. The shortest Co 3 3 3 Co edges via the double Co-O-Co modes range from 3.206(2) to 3.262(2) A˚, while the somewhat longer Co 3 3 3 Co edges via the Co-O-Co and Co-O-P-O-Co modes are about 3.7 A˚. The layers in compound 2 are also assembled into a threedimensional structure via van der Waals forces, with the uncoordinated sulfone groups dangling on both sides of each layer. There exist no hydrogen bonds between two adjacent layers, but within each layer, a large number of hydrogen bonds are formed among the uncoordinated phosphonate/sulfone O atoms, aqua ligands, and lattice water molecules (Table 2, Figure 6). Magnetic Property Studies. The temperature dependence of χT and χ values of 1 is displayed in Figure 7. The χT value at 300 K (2.12 emu 3 K 3 mol-1) is significantly smaller than the theoretical spin-only value of 4.375 emu 3 K 3 mol-1 for one highspin Fe3þ ion (S = 5/2, g = 2), and decreases almost linearly with lowering temperature to 0.02 emu 3 K 3 mol-1 at 2 K. A broad maximum of the corresponding χ is observed at 170 K. These behaviors suggest a strong antiferromagnetic interaction. Similar trend was also observed in a HCOObridged Fe3þ cluster complex {[Fe8O3(tea)(teaH)3(HCOO)6]8(HCOO)12}(ClO4)12 3 3CH3OH 3 36H2O [tea=N(CH2CH2O)33and teaH=N(CH2CH2O)2-(CH2CH2OH)2-] reported quite recently.13 The field dependence of magnetization measured

Figure 3. View of the structure of 1 down the b-axis. The FeO6 octahedra and CPO3 tetrahedra are shaded in orange and purple, respectively. S, O, and C atoms are represented by yellow, red, and black circles, respectively. Hydrogen bonds are represented by dashed lines.

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Figure 4. ORTEP representation of the selected unit of 2. The thermal ellipsoids are drawn at 50% probability. The uncoordinated CH3SO2CH2- groups of the phosphonate ligands and the lattice water molecules have been omitted for clarity. Symmetry codes for the generated atoms: A. 1 þ x, y, z; B. 1 - x, -1/2 þ y, 1/2 - z; C. -x, -1/2 þ y, 1/2 - z; D. -1 þ x, y, z; E. -x, 1/2 þ y, 1/2 - z; F. 1 - x, 1/2 þ y, 1/2 - z.

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Figure 6. View of the structure of 2 down the a-axis. The CPO3 and C2SO2 tetrahedra are shaded in purple and yellow, respectively. Co and O atoms are represented by cyan and red circles, respectively. Hydrogen bonds are represented by dashed lines.

Figure 7. Plots of χT and χ versus T of 1 measured under 1000 Oe.

Figure 5. View of the layered structure of 2 down the c-axis. The uncoordinated CH3SO2CH2- groups of the phosphonate ligands have been omitted for clarity. The CoO6 octahedra and Co atoms are shaded in cyan. P and O atoms are represented by purple and red circles, respectively.

at 2 K (Figure S3 in the Supporting Information) shows that the magnetization increases gradually with the applied field and reaches a value of 0.07 Nβ 3 mol-1 at 50 kOe, which is far from the expected saturated magnetization value 5.0 Nβ 3 mol-1; this feature further confirms the classical aniferromagnetic interaction in 1. The dc magnetic susceptibility (χ) of 2 was measured under a 1000 Oe applied field over the temperature range of 2-300 K, and is plotted as χT versus T in Figure 8. The value of χT at room temperature of 2 is 12.93 emum 3 K 3 mol-1, a little larger than that expected for four magnetically isolated cobalt(II) ions (11.72 emu 3 K 3 mol-1 supposed g = 2.5). The χT product decreases slowly upon cooling, suggesting an antiferromagnetic interaction. χT reaches a minimum 10.14 emu 3 K 3 mol-1 at 10 K, then increases to a maximum at 7 K, with a value of

Figure 8. Plot of χT versus T of 2 measured under 1000 Oe, the solid line represent the best theoretical fitting. Inset: Plots of FC and ZFC susceptibility for 2.

47.65 emu 3 K 3 mol-1 and then falls again, which may arise from zero-field splitting, Zeeman effects, or weak interlayer interactions. The data above 50 K obey the Curie-Weiss law, with C = 13.23 emu 3 K 3 mol-1 and Θ = -6.08 K. The small negative Θ value confirms the overall weak antiferromagnetic properties. The divergence of the zero-field (ZFC) and filed-cooled (FC) susceptibilities below about 7 K suggests an irreversible behavior of long-range magnetic ordering (Figure 8, inset). 0 (T) and Ac susceptibility measurements indicate that both χac

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The first step started at 166 C and completed at 265 C, which corresponds to the release of 1.5 water molecules (one aqua ligand plus 0.5 H2O from one hydroxyl anion). The observed weight loss of 10.3% is equal to the calculated value (10.3%). The second step started at 314 C and completed at 484 C, which corresponds to the decomposition of the sulfone-phosphonate ligand. The total weight loss at 800 C is 43.6% and the final residuals are not characterized. TGA curve of 2 also exhibits two main steps of weight losses. The first step started at 120 C and completed at 236 C, which corresponds to the release of 6 water molecules. The observed weight loss of 10.7% is very close to the calculated value (10.5%). The second step started at 408 C and completed at 585 C, which corresponds to the decomposition of the sulfone-phosphonate ligand. The total weight loss at 800 C is 42.0% and the final residuals are not characterized. Figure 9. Ac magnetic susceptibility as a function of temperature for 2 at different frequencies and Hac = 2.5 Oe, Hdc = 0 Oe.

Conclusions In summary, the hydrothermal syntheses, crystal structures and magnetic properties of two novel layered metal phosphonates based on a sulfone-phosphonate ligand, namely, [Fe(L)(OH)(H2O)] (1) and [Co4(L)4(H2O)4] 3 2H2O (2) (H2L=CH3SO2CH2PO3H2), have been reported. In both compounds, the presence of the oxo-bridging hydroxide coligand (H2O or OH-) facilitates the magnetic superexchange of the metal centers. The results of our studies indicate that the attachment of a hydrophilic sulfone group to the phosphonate ligand promotes the presence of an oxo-bridging hydroxide coligand, which may be a new synthetic route for creating layered metal phosphonates with unusual magnetic properties. Further research work will be extended to use such method for the syntheses of other layered metal phosphonates featuring novel magnetic properties.

Figure 10. Hysteresis loop at 1.9 K for 2. Inset: M-H curve at 2 K for 2. 00 χac (T) are frequency independent (Figure 9), excluding any glassy or superparamagnetic behaviors. The real part of the ac 0 ) has a maximum at ∼7.0 K accommagnetic susceptibility (χac 00 , suggesting that Tc of 2 is panied by the occurrence of nonzero χac about 7.0 K. Isothermal magnetization experiments performed at 1.9 K exhibit a hysteresis with the large coercive field of 570 Oe and the remnant magnetization (Mr) of 1.82 emu 3 Oe 3 mol-1 (Figure 10). The M-H curve of 2 measured at 1.9 K shows a sigmoidal shape (Figure 10, inset), suggesting the presence of metamagnetic behavior.14 The critical field values are about 301 and 22600 G and correspond to the field at which a maximum in the dM/dH value is reached (Figure S4 in the Supporting Information). The M value at 5 T is only 7.0 Nβ, far from the saturation value of 15 Nβ expected for a [CoII4] system (supposed g=2.5), which is consistent with the weak ferromagnetism owing to spin canting. The canting angle can then be calculated15 as R=tan-1(Mr/Ms)=6.9, where Ms=15 Nβ. A similar trend in which spin-canting and metamagnetic behavior coexist was also observed in a recently reported threedimensional cobalt(II) complex [{Co4(μ-H2O)2(3-pyca)8}0.94{Co5(μ3-OH)2(3-pyca)8}0.06] (3-pycaH=trans-3-pyridylacrylic acid).16 TGA Studies. TGA curve of 1 exhibits two main steps of weight losses (Figure S5 in the Supporting Information).

Acknowledgment. This work was supported by National Natural Science Foundation of China (No. 20671093), the Major State Basic Research Development Program of China (Grant 2006CB932101), the NSF of Jiangxi Province (Grant 2008GQH0013) and the NSF of Jiangxi Provincial Education Department (Grant GJJ09317). Supporting Information Available: X-ray crystallographic files in CIF format, XRD patterns, and TGA curves for both compounds and M-H curve at 2 K for 1, as well as dM/dH vs H plot for 2. This material is available free of charge via the Internet at http://pubs. acs.org.

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