CRYSTAL GROWTH & DESIGN
Copper Phosphonates based on 2-Phosphonic-isonicotinate: Structures and Magnetic Properties
2008 VOL. 8, NO. 4 1213–1217
Yi-Fan Yang, Yun-Sheng Ma, Li-Rong Guo, and Li-Min Zheng* State Key Laboratory of Coordination Chemistry, Coordination Chemistry Institute, School of Chemistry and Chemical Engineering, Nanjing UniVersity, Nanjing 210093, P. R. China ReceiVed July 20, 2007; ReVised Manuscript ReceiVed December 13, 2007
ABSTRACT: Hydrothermal reactions of 2-phosphonic isonicotinic acid (pinH3, HOOC-C5H3N-PO3H2) with cupric chloride afford two copper phosphonates: Cu3(pin)2(H2O)2 (1) and Cu(pinH2)2 (2). Compound 1 crystallizes in the monoclinic space group C2/c with a ) 15.913(5) Å, b ) 6.016(2) Å, c ) 18.043(5) Å, β ) 90.518(9)°, and V ) 1727.2(8) Å3. In this structure, dimers of {Cu2O2} are linked by {CuO6} octahedra through O(1W) forming an inorganic chain. The adjacent chains are further connected by {CPO3} tetrahedra to form an undulating inorganic layer. The inorganic layers are pillared by the pyridyl groups of the ligand, resulting in a three-dimensional framework structure. Compound 2 crystallizes in the monoclinic space group C2/c with a ) 19.039(3) Å, b ) 7.185(1) Å, c ) 12.355(2) Å, β ) 116.666(3)°, and V ) 1510.4(4) Å3. It shows a chain structure in which the adjacent {CuN2O4} octahedra are doubly bridged by two phosphonate ligands. Ferromagnetic interactions are found to be mediated between the Cu(II) centers in 1. Ac magnetic susceptibility measurements suggest a long-range ferromagnetic ordering at ca. 1.8 K. For 2, the magnetic behavior is in principle paramagnetic. Introduction Copper phosphonates are a class of interesting hybrid materials because of their rich structures and potential applications in the area of ion exchange, sorption, catalysis, and magnetism etc.1 Usually, copper monophosphonates with formula Cu(RPO3)(H2O) (R ) alkyl, phenyl, vinyl) exhibit layered structures, in which the {CuO5} tetragonal pyramids are linked by {CPO3} tetrahedra into an inorganic layer with the organic groups filling the interlayer spaces.2 By introduction of an additional functional group such as hydroxyl,3 amino,4 carboxylate,5 pyridyl6,7or a second phosphonate group,8 compounds with versatile architectures and interesting properties can be obtained. Previously, we found that 2-pyridylphosphonic acid could react with copper salts, leading to three compounds, namely, Cu(C5H4NPO3H)2 with a discrete dimeric structure and Cu3(OH)2(C5H4NPO3)2 · 2H2O and Cu(C5H4NPO3) with layer structures.9 Weak ferromagnetism is observed for Cu(C5H4NPO3). When 2-pyridylphosphonic acid react with both copper and lanthanide salts, heterometallic phosphonate compounds can be obtained which exhibit chiral open-framework structures.10 In this paper, we present the reactions between copper salt and 2-phosphonic isonicotinic acid (pinH3, HOOC-C5H3N-PO3H2), which contains an additional carboxylate group in 2-pyridylphosphonic acid (Scheme 1). Two new copper phosphonates, Cu3(pin)2(H2O)2 (1) and Cu(pinH2)2 (2), are obtained and their magnetic properties are studied. Experimental Section Materials and Methods. All the starting materials were reagent grade used as purchased. The 2-phosphonic isonicotinic acid (pinH3) was prepared according to the literature.11 Elemental analyses were performed on a PE 240C elemental analyzer. The infrared spectra were recorded on a VECTOR 22 spectrometer with pressed KBr pellets. Thermal analyses were performed in nitrogen with a heating rate of 10 °C/min on a PerkinElmer Pyris 1 TGA instrument. The magnetic susceptibility data were obtained on polycrystalline samples using a * Correspondence E-mail:
[email protected], Fax: +86-25-83314502.
Scheme 1
Quantum Design MPMS-XL7 SQUID magnetometer. The data were corrected for the diamagnetic contributions of both the sample holder and the compound obtained from Pascal’s constants.12 Synthesis of Cu3(pin)2(H2O)2 (1). A mixture of CuCl2 · 2H2O (0.15 mmol, 0.0256 g), pinH3 (0.1 mmol, 0.0203 g), and H2O (7 cm3) (pH ) 1.77) was stirred briefly and kept in a Teflon-lined autoclave (23 cm3) at 140 °C for 3 days. After slow cooling to room temperature, light blue block-like crystals were obtained as a major phase (>98%) with very small amount of unrecognized impurities. The light blue crystals were manually collected and used for physical measurements. Yield: 60% based on Cu. Elemental analysis (%) calcd for C12H10Cu3N2O12P2: C 23.05, H 1.61, N 4.48. Found: C 22.86, H 1.44, N 4.44. IR (KBr, cm-1): 3429 vs, 1594 vs, 1546 s, 1469 w, 1410 s, 1375 s, 1246 w, 1200 m, 1145 vs, 1103 s, 1078 vs, 1035 m, 973 s, 882 m, 781 w, 760 w, 742 w, 699 m, 588 m, 520 m, 494 m. Thermal analysis shows a one-step weight loss (5.9%) in the temperature range 120–220 °C, in agreement with the calculated value of 5.7% for the removal of two H2O molecules. Synthesis of Cu(pinH2)2 (2). CuCl2 · 2H2O (0.05 mmol, 0.0086 g), pinH3 (0.1 mmol, 0.0203 g), and H2O (7 cm3) were mixed, and the pH of the suspension was adjusted to 1.44 by 1 M hydrochloric acid. Then the mixture was transferred to a 23 cm3 Teflon-lined autoclave and kept at 140 °C for 3 days. After slow cooling to room temperature, deep blue block-like crystals of 2 were obtained as a major phase (>90%) together with a small amount of unrecognized impurities. The deep blue crystals were manually selected and were used for the physical measurements. Yield: 41% based on Cu. Elemental analysis (%) calcd for C12H10CuN2O10P2: C 30.82, H 2.16, N 5.99. Found: C 30.62, H 1.87, N 5.95. IR (KBr, cm-1): 3442 br, 2882 br, 2648 br, 2526 m, 1699 vs, 1610 w, 1552 w, 1473 w, 1431 m, 1387 m, 1294 vs, 1270 s, 1240 m, 1184 vs, 1133 s, 1108 s, 1081 vs, 1034 m, 938 vs, 910 m, 879 m, 856m, 773 m, 728 m, 677 s, 600 vs, 528 m, 503 m, 463 w, 437 m, 412 w.
10.1021/cg700673x CCC: $40.75 2008 American Chemical Society Published on Web 03/05/2008
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Yang et al.
Table 1. Crystallographic Data for 1 and 2 compound
1
2
formula M crystal system space group a, Å b, Å c, Å β, deg V, Å3 Z Dc, g cm-3 F(000) µ(Mo KR), cm-1 GOF on F2 R1, wR2a [I > 2σ(I)] R1, wR2a (all data) (∆F)max, (∆F)min, e Å-3
C12H10Cu3N2O12P2 626.78 monoclinic C2/c 15.913(5) 6.0160(16) 18.043(5) 90.518(9) 1727.2(8) 4 2.410 1236 3.922 1.078 0.0417, 0.0939 0.0504, 0.0971 0.60, -0.64
C12H10CuN2O10P2 467.70 monoclinic C2/c 19.039(3) 7.1853(13) 12.355(2) 116.666(3) 1510.4(4) 4 2.057 940 1.724 1.039 0.0405, 0.0870 0.0461, 0.0896 0.57, -0.41
a
R1 ) ∑||Fo| - |Fc||/∑Fo; wR2 ) [∑w(Fo2 - Fc2)2/∑w(Fo2)2]1/2. Table 2. Selected Bond Lengths (Å) and Angles (deg) for Compound 1a
Cu(1)-O(1) 1.998(3) Cu(1)-O(1B) 1.960(3) Cu(1)-O(3A) 1.934(3) Cu(1)-O(1W) 2.330(3) Cu(1)-N(1) 1.976(3) Cu(2)-O(2) 1.925(3) Cu(2)-O(4D) 1.952(3) Cu(2)-O(1W) 2.688(3) P(1)-O(1) 1.543(3) P(1)-O(2) 1.496(3) P(1)-O(3) 1.506(3) O(1)-Cu(1)-O(3A) 174.58(13) N(1)-Cu(1)-O(3A) 96.30(13) O(1)-Cu(1)-O(1B) 81.16(12) O(1B)-Cu(1)-N(1) 164.60(14) O(1B)-Cu(1)-O(3A) 96.54(12) O(1)-Cu(1)-N(1) 85.30(13) O(3A)-Cu(1)-O(1W) 100.86(12) O(1)-Cu(1)-O(1W) 84.12(12) O(1B)-Cu(1)-O(1W) 90.57(12) N(1)-Cu(1)-O(1W) 95.33(13) O(2)-Cu(2)-O(4D) 91.28(13) O(2)-Cu(2)-O(1W) 90.94(11) O(4D)-Cu(2)-O(1W) 82.44(12) Cu(1)-O(1)-Cu(1B) 98.84(12) Cu(1)-O(1W)-Cu(2) 120.59(12) a Symmetry codes: (A) x, y + 1, z; (B) 1/2 - x, 3/2 - y, 1 - z; (C) -x, 1 - y, 1 - z; (D) -x, y, 1/2 - z; (E) x, 1 - y, 1/2 + z.
Table 3. Selected Bond Lengths (Å) and Angles (deg) for Compound 2a Cu(1)-O(1) Cu(1)-N(1) Cu(1)-O(4A) O(1)-Cu(1)-O(4A) N(1)-Cu(1)-O(4A)
1.938(2) 1.976(3) 2.634(2) 83.88(9) 79.60(9)
P(1)-O(1) P(1)-O(2) P(1)-O(3) O(1)-Cu(1)-N(1) N(1)-Cu(1)-O(4B)
1.504(2) 1.552(3) 1.491(2) 87.31(11) 100.40(9)
a Symmetry codes: (A) x, 1 + y, z; (B) 1/2 - x, 1/2 - y, -z; (C) 1/2 - x, 3/2 - y, -z.
Crystallographic Studies. Single crystals with dimensions 0.34 × 0.22 × 0.22 mm3 for 1 and 0.30 × 0.20 × 0.20 mm3 for 2 were selected for indexing and intensity data collection on a Bruker SMART APEX CCD diffractometer using graphite monochromatized Mo KR radiation (λ ) 0.71073 Å) at room temperature. The data were collected in the θ range 2.26–25.99° for 1 and 2.39–25.99° for 2 using a narrow frame method with scan widths of 0.30° in ω and an exposure time of 5s/ frame. Numbers of observed and unique reflections are 4475 and 1693 (Rint ) 0.0427) for 1 and 3916 and 1482 (Rint ) 0.0305) for 2, respectively. The data were integrated using the Siemens SAINT program,13 with the intensities corrected for Lorentz factor, polarization, air absorption, and absorption due to variation in the path length through the detector faceplate. Absorption corrections were applied. The structures were solved by direct methods and refined on F2 by full matrix least-squares using SHELXTL.14 All the non-hydrogen atoms were located from the Fourier maps and were refined anisotropically. The H atoms of the pyridyl group were placed in calculated positions (C-H ) 0.93 Å). The H atoms in H2O were found from the Fourier difference maps and refined isotropically. Crystallographic and refinement details of 1 and 2 are listed in Table 1. Selected bond lengths and angles are given in Tables 2 and 3 for 1 and 2, respectively.
Figure 1. Building unit of 1 with the atomic labeling scheme (30% probability). All H atoms on the pyridyl group are omitted for clarity.
Results and Discussion Syntheses. Compounds 1 and 2 were initially synthesized through hydrothermal reactions of CuCl2 and pinH3 at 140 °C when the molar ratio of Cu/pinH3 is 3:1 (pH ) 1.64) and 1:3 (pH ) 1.85), respectively. In order to investigate the influence of pH on the final products, the reactions were carried out systematically under similar hydrothermal conditions with different pH by fixing the Cu/pinH3 molar ratio of 3:2 for 1 and 1:2 for 2, respectively, according to the molecular formulas obtained from single crystal structural analyses. When the molar ratio of Cu/pinH3 is 3:2, compound 1 can be obtained in the pH range of 1.64–5.66 as a major phase (>98%), with the presence of very small amount of impurities which could be visually identified and manually discarded (Figure S1, Supporting Information). When the pH is lower than 1.64, the amount of compound 2 increases. When the pH is 1.25, a significant amount of compound 2 appears, as suggested by the XRD measurements (Figure S2, Supporting Information). If the molar ratio of Cu:pinH3 is 1:2, compound 2 can be prepared as a major phase when pH is lower than 1.85. The best pH is 1.06 – 1.44, at which the product becomes almost pure (>90%) (Figure S3, Supporting Information). A higher pH (1.85–3.65) facilitates the formation of compound 1 (Figure S4, Supporting Information). When pH g 3.86, a solution would be formed. Compounds 1 and 2 can also be obtained by replacing CuCl2 by CuSO4 or Cu(NO3)2. Description of Structure 1. Compound 1 crystallizes in monoclinic space group C2/c. The asymmetric unit consists of 1.5 Cu atom, one pin3- ligand and one H2O (Figure 1). There are two crystallographically different Cu atoms. The Cu(1) atom has a distorted tetragonal pyramidal geometry. Four basal positions are filled with N(1), O(1), O(1B) and O(3A) atoms from three equivalent pin3- ligands. The Cu(1)-O and Cu(1)-N bond lengths are 1.934(3)–1.998(3) Å and 1.976(3) Å. The axial site is filled with aqua oxygen O(1W) with an elongated Cu(1)-O(1W) distance 2.330(3) Å. The Cu(2) atom locates at an inversion center and has a distorted octahedral coordination environment. Four oxygen atoms O(2), O(2C), O(4D), O(4E) from four equivalent pin3- ligands occupy the equatorial sites with the Cu(2)-O bond distances in the range of 1.925(3)–1.976(3) Å. The axial positions are filled with two aqua oxygen atoms O(1W) and O(1WB) with elongated Cu(2)-O(1W) distances [2.688(3) Å]. Each pin3- serves as a pentadentate ligand. The
Copper Phosphonates Based on 2-Phosphonic Isonicotinate
Figure 2. One inorganic layer of structure 1 viewed approximately along the c-axis.
Figure 3. Crystal packing of structure 1 viewed along the b-axis. All H atoms are omitted for clarity.
O(1) atom acts as a µ3-O and links two equivalent Cu(1) atoms into a {Cu2O2} dimer. The Cu(1) · · · Cu(1B) distance over the µ3-O(1) is 3.006(1) Å. The Cu(1)-O(1)-Cu(1B) bond angle is 98.84(12)°. The dimers are bridged by {CuO6} octahedra through O(1W) and O-P-O units, forming a chain running along the a-axis. These chains are further connected by the O-P-O units, thus leading to a two-dimensional inorganic layer containing 4-, 6-, 8-, and 12-membered rings (Figure 2). The Cu(1) · · · Cu(2) distance across the µ-O(1W) bridge is 4.362(1) Å. The Cu(1) · · · Cu(1) and Cu(1) · · · Cu(2) distances across the O-P-O units are 5.039(1) and 4.655(1) Å, respectively. The inorganic layers are linked by the pyridyl groups of the ligand, resulting in a pillared layered structure (Figure 3). The structure of compound 1 is significantly different from the layer compounds Cu3(OH)2(C5H4NPO3)2 · 2H2O and Cu(C5H4NPO3) where 2-pyridylphosphonate is involved.9 In the former, edge-sharing {Cu(1)O4N} square pyramids and {Cu(2)O4} planes are found to form an infinite chain with composition {Cu3(µ-OH)2(µ-O)4}. Neighboring chains are linked by the phosphonate groups of the 2-pyridylphosphonate ligands, resulting in inorganic layers containing 4-, 8-, and 12membered rings. In the latter, the {Cu(1)O4} and {Cu(2)O2N2} planes are each corner-shared with the {CPO3} tetrahedra, forming an inorganic layer containing 8- and 16-membered rings. Apparently, the additional carboxylate group plays a key role in forming structure 1. Description of Structure 2. Compound 2 crystallizes in monoclinic space group C2/c. The asymmetric unit consists of
Crystal Growth & Design, Vol. 8, No. 4, 2008 1215
Figure 4. Building unit of structure 2 with the atomic labeling scheme (30% probability). All H atoms on the pyridyl group are omitted for clarity.
0.5 Cu atom and one pinH2- ligand (Figure 4). The Cu(1) atom locates at an inversion center and has a distorted octahedral coordination environment. The six positions are occupied by N(1), N(1C), O(1), O(1C), O(4A) and O(4B) atoms from four equivalent ligands. The Cu-O bond distances are 1.938(2)–2.633(2) Å. The pinH2- ligand acts as a bridging tridentate ligand, using one pyridyl nitrogen [N(1)], one phosphonate oxygen [O(1)] and one carboxylate oxygen atom [O(4)]. The phosphonate oxygen atom O(2) and carboxylate oxygen atom O(5) are protonated [P(1)-O(2) ) 1.552(3) Å, C(6)-O(5) ) 1.303(4) Å]. The remaining one phosphonate oxygen atom O(3) is pendant [P(1)-O(3) ) 1.491(2) Å]. The {CuN2O4} octahedra are linked by two equivalent pinH2- ligands, forming a chain along the b-axis (Figure 5). The Cu · · · Cu distance across the ligand is 7.185(1) Å. These chains are stacked in the lattice and are stabilized via hydrogen bond interactions [O(5) · · · O(3) 2.640(4) Å, ∠O(5)-H · · · O(3) 176.0°, O(2) · · · O(3) 2.590(4) Å, ∠O(2)-H · · · O(3) 164.0°] (Figure 6). Magnetic Properties. The temperature-dependent molar magnetic susceptibilities of 1 and 2 were measured at 2 kOe in the temperature range 2–300 K. Figure 7 shows the χM and χMT versus T plots for compound 1. The room temperature χMT is 1.25 cm3 K mol-1, close to the theoretical value (1.36 cm3 K mol-1) for three Cu(II) ions with g ) 2.2. On cooling, the χMT increases gradually and then sharply below 25 K, indicating that ferromagnetic couplings are mediated between the Cu(II) ions. According to the structure of compound 1, the magnetic exchanges between the Cu(II) ions may be propagated through the µ3-O(P), µ2-OH2 bridges within the chain and through the O-P-O units within or between the chains. Compared to the µ-O bridge, the O-P-O pathway usually plays a negligible role in propagating the magnetic exchanges. Therefore, the system could be simplified as a chain of -{Cu(1)-Cu(2)-Cu(1)}n- with weak interchain interactions (Scheme 2). SupposethattherepeatingunitofthechainisCu(1)-Cu(2)-Cu(1) trimer, the theoretical equation for the temperature dependent magnetic susceptibility of a copper trimer (χt) can be derived as12 χt )
Ng2β2 1 + e2J2⁄kT + 10e3J2⁄kT 4kT 1 + e2J2⁄kT + 2e3J2⁄kT
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Figure 5. Chain structure of compound 2.
Figure 7. χM and χMT plots for compound 1.
Scheme 2 Figure 6. Crystal packing of compound 2 viewed along the b-axis.
where J1 is the coupling constant of Cu(1)-Cu(1), J2 is the coupling constant of Cu(1)-Cu(2), N, g, β, and k have their usual meanings. Take St as the effective spin of the copper trimer unit, the expression for this trimer unit can also be derived as below15 Ng2β2 S (S + 1) 3kT t t Therefore, the susceptibility data of the chain could be analyzed by Fisher’s expression for a uniform chain, with the classical spins scaled to a real quantum spin St:12,16 χt )
χchain )
Ng2β2 1 + u S (S + 1) 3kT 1 - u t t
with u ) coth(J1St(St + 1) ⁄ (kT)) - kT ⁄ (J1St(St + 1)) Finally χchain )
Ng2β2 1 + u 1+u S (S + 1) ) χ) 3kT 1 - u t t 1-u t 2 2 Ng β 1 + u 1 + e2J2⁄kT + 10e3J2⁄kT 4kT 1 - u 1 + e2J2⁄kT + 2e3J2⁄kT χchain χM ) 1 - (2zJ ′ /Ng2β2)χchain
where zJ′ accounts for the intermolecular interaction. A good fit, shown as the solid lines in Figure 7, is obtained with parameters g ) 2.21, J1 ) 2.58 cm-1, J2 ) 1.15 cm-1, and zJ′ ) -0.06 cm-1. The saturation magnetization at 70 kOe (3.2 Nβ) is close to the value of 3.3 Nβ anticipated for a spin value of S ) 3/2 with g ) 2.2, which corresponds to the parallel
arrangement of the magnetic moments of three Cu(II) ions in the trimer unit (Figure 8). Temperature dependent Ac susceptibility data were also recorded under Hac ) 5 Oe and frequency 10 Hz (Figure 9). Although no clear peak appears down to 1.8 K, it is found that the real-phase signal (χ′) approaches to a maximum at 1.8 K. Meanwhile the out-of-phase signal (χ′′) increases with decreasing temperature below 2.0 K. Apparently, a long-range magnetic ordering could occur around or below 1.8 K. The significant ferromagnetic interactions in compound 1 can be related to its structure. It is well-known that for planar hydroxy-bridged copper(II) dimers, the magnetic interaction between the copper(II) ions would be antiferromagnetic when the Cu-O-Cu angle (R) is >97.5° and ferromagnetic when R is
