Structures and Magnetism of a Series Mn(II) Coordination Polymers

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Structures and Magnetism of a Series Mn(II) Coordination Polymers Containing Pyrazine-Dioxide Derivatives and Different Anions Hao-Ling Sun,† Song Gao,*,† Bao-Qing Ma,† Gang Su,‡ and Stuart R. Batten§

CRYSTAL GROWTH & DESIGN 2005 VOL. 5, NO. 1 269-277

College of Chemistry and Molecular Engineering, State Key Laboratory of Rare Earth Materials Chemistry and Applications, PKU-HKU Joint Laboratory on Rare Earth Materials and Bioinorganic Chemistry, Peking University, Beijing 100871, China, Department of Physics, The Graduate School of the Chinese Academy of Sciences, P. O. Box 3908, Beijing 100039, China, and School of Chemistry, P. O. Box 23, Monash University, Clayton, Victoria 3800, Australia Received November 18, 2003;

Revised Manuscript Received April 20, 2004

ABSTRACT: The reaction of MnCl2‚4H2O and 2,5-dimethylpyrazine-dioxide (2,5-dmpdo) with various anions in H2O affords different complexes, the solid structures of which are controlled by the choice of counteranions. Reaction of MnCl2‚4H2O and 2,5-dmpdo with NH4NCS yields [Mn(NCS)2(2,5-dmpdo)1.5(H2O)2]‚0.5(2,5-dmpdo)‚H2O (1), which contains Mn(II) dimers bridged by 2,5-dmpdo. However, with the replacement of NH4NCS with NaN3 or sodium dicyanamide (Na(dca)) in the above reaction, Mn(N3)2(2,5-dmpdo)(H2O)2 (2) or Mn(dca)2(2,5-dmpdo) (3) was isolated. Compound 2 consists of a one-dimensional zigzag chain constructed by 2,5-dmpdo bridging ligands. Compound 3 is comprised of two-dimensional layers constructed by bridging 2,5-dmpdo and dca ligands. These results unequivocally indicate that the nature of the counteranions, which play different roles in each complex, is the key factor governing the structural topologies of them. While using 2,3-dimethylpyrazine-dioxide (2,3-dmpdo) instead of 2,5-dmpdo, [Mn(dca)2(2,3-dmpdo)]‚H2O (4) was obtained, which contains a CdSO4-like three-dimensional open framework constructed by 2,3-dmpdo and dca, indicating a minor change of the bridging ligand can have great effect on the topology of the crystal structure. Magnetic studies reveal that the four complexes exhibit spin-flop transitions below 2.12 K for 1, 6.14 K for 2, 6.23 K for 3, and 5.37 K for 4, respectively. During the past decade, considerable research effort has been made on the design and synthesis of multidimensional networks on the basis of open shell transition metals in the context of molecule-based magnetic materials.1 Complexes containing azide, cyanide, and dicyanamide have been intensively studied because they are not only good bridging ligands for transfer spin exchange couplings but also are easily combined with other ancillary ligands. So far, a number of one-, two-, and three-dimensional frameworks have already been generated with these ligands and ancillary ligands containing nitrogen donor atoms.2-4 In two recent communications, we reported the 2D M(NCS)2(pzdo)2 (M ) Co, Mn) and the 3D Mn(N3)2(pzdo) (pzdo ) pyrazine-dioxide) complexes in which pzdo serves as a bridging ligand, and the magnetic study reveals that they have antiferromagnetic ordering temperatures which are remarkably higher than analogous ones constructed by pyrazine, indicating that pzdo is a good bridging ligand that can exchange the antiferromagnetic coupling efficiently.5 In addition, the variety of possible bridging geometries for pyrazine-dioxide derivatives5 are quite different from the single mode for the pyrazine derivatives.6 The bridging geometries of the former can be tuned by introducing different substituents into different positions. The different stereoeffects (steric restrictions) of substituents can create different coordination modes * Corresponding author: Song Gao, Telephone: 0086-10-62756320; fax: 0086-10-62751708; e-mail: [email protected]. † Peking University. ‡ The Graduate School of the Chinese Academy of Sciences. § Monash University.

and can result in different crystal structures. To elucidate the roles of the pzdo ligand and its derivatives in mediating the magnetic interaction and tuning the crystal structure, we synthesized 2,5-dimethylpyrazinedioxide (2,5-dmpdo) and 2,3-dimethylpyrazine-dioxide (2,3-dmpdo), and four complexes of them, [Mn(NCS)2(2,5-dmpdo)1.5(H2O)2]‚0.5(2,5-dmpdo)‚H2O (1), Mn(N3)2(2,5-dmpdo)(H2O)2 (2), Mn(dca)2(2,5-dmpdo) (3), and [Mn(dca)2(2,3-dmpdo)]‚H2O (4) with different anions, and studied their crystal structures and magnetic properties. Experimental Procedures Elemental analyses of carbon, hydrogen, and nitrogen were carried out with an Elementar Vario EL. Variable-temperature magnetic susceptibility, zero-field ac magnetic susceptibility measurements, and field-dependence of magnetization were performed on a Maglab System 2000 magnetometer.7 The experimental susceptibilities were corrected for the diamagnetism of the constituent atoms (Pascal’s tables).8 Synthesis. 2,5-Dimethylpyrazine-dioxide (2,5-dmpdo) and 2,3-dimethylpyrazine-dioxide (2,3-dmpdo) were prepared from 2,5-dimethylpyrazine and 2,3-dimethylpyrazine by the literature method.9 Sodium dicyanamide (Na(dca)) and other chemicals were purchased and used without further purification. [Mn(NCS)2(2,5-dmpdo)1.5(H2O)2]‚0.5(2,5-dmpdo)‚H2O (1). An aqueous solution of MnCl2‚4H2O (0.25 mmol, 10 mL) with aqueous NH4NCS (0.5 mmol, 10 mL) were mixed. After stirring of the sample for 30 min at room temperature 2,5-dmpdo (0.5 mmol) was added. The resulting solution was filtered, and left to stand at room temperature after another 30 min stirring. After two weeks, orange-red column single crystals suitable for structure determination were obtained (yield 70%). Elemental anal. Calcd for 1 C14H22MnN6O7S2: C, 33.26; H, 4.39;

10.1021/cg034223n CCC: $30.25 © 2005 American Chemical Society Published on Web 11/04/2004

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Table 1. Crystal Data and Structure Refinement for 1-4 formula fw λ (Å) T (K) space group a (Å) b (Å) c (Å) R (deg) β (deg) γ (deg) V (Å3) Z Dcalcd (mg/m3) µ (mm-1) GoF F(000) data collected unique data observed data (I > 2σ(I)) Rint R1a [I > 2σ(I)] wR2b [I > 2σ (I)] R1a [all data] wR2b [all data] a

1

2

3

4

C14H22MnN6O7S2 505.44 0.7107 298 P21/n 6.7051(1) 31.4258(6) 10.5128(3) 90 100.9356(6) 90 2174.96(8) 4 1.544 0.847 1.070 1044 23140 4938 2999

C6H12MnN8O4 315.18 0.7107 298 P1 h 5.6867(2) 7.7266(3) 7.7669(3) 66.614(2) 85.828(1) 88.356(3) 312.40(11) 1 1.675 1.083 1.103 161 5913 1395 1221

C10H8MnN8O2 327.18 0.7107 298 P1 h 6.5035(3) 7.2870(3) 7.5566(5) 70.180(2) 80.994(2) 86.808(2) 332.75(3) 1 1.633 1.010 1.046 165 6285 1479 1245

C10H10MnN8O3 345.20 0.7107 298 C2/c 9.3968(3) 11.5836(4) 13.1449(5) 90 97.382(2) 90 1418.95(9) 4 1.625 0.958 1.039 708 12608 1613 1170

0.0738 0.0519 0.1307 0.1081 0.1484

0.0338 0.0379 0.0972 0.0469 0.1033

0.0459 0.0313 0.0648 0.0439 0.0690

0.0690 0.0369 0.0890 0.0648 0.0999

R1 )∑||Fo| - |Fc||/∑|Fo|. b wR2 ) [∑w(Fo2 - Fc2)2]/∑w(Fo2)2]1/2.

N, 16.64. Found: C, 32.25; H, 4.53; N, 15.89. IR (cm-1, KBr disc): 2069 s, 2023 m. Mn(N3)2(2,5-dmpdo)(H2O)2 (2) and Mn(dca)2(2,5-dmpdo) (3) were synthesized by a method similar to compound 1, using NaN3 for 2 and Na(dca) for 3 instead of NH4NCS (yield 50% for 2, 80% for 3). Elemental anal. Calcd for 2 C6H12MnN8O4: C, 22.85; H, 3.84; N, 35.56. Found: C, 22.85; H, 3.12; N, 35.24. Calcd for 3 C10H8MnN8O2: C, 36.70; H, 2.47; N, 34.26. Found: C, 36.84; H, 2.68; N, 33.78. IR for 2 (cm-1, KBr disc): 2086 s, 2011 m. IR for 3 (cm-1, KBr disc): 2291 ms, 2237 ms, 2175 s. [Mn(dca)2(2,3-dmpdo)]‚H2O (4) was prepared by a method similar to compound 3 in which 2,3-dimethylpyrazine-dioxide (2,3-dmpdo) was used instead of 2,5-dimethylpyrazine-dioxide (2,5-dmpdo) (yield 70%). Elemental anal. Calcd for 4 C10H10MnN8O3: C 34.58; H 3.48; N 32.28; Found: C 34.38; H 3.00; N 31.56. IR (cm-1, KBr disc): 2310 ms, 2239 ms, 2172 s. Crystallographic Data Collection and Structure Determination. An orange-red crystal of 1 (0.15 × 0.10 × 0.10 mm), a deep red crystal of 2 (0.15 × 0.15 × 0.10 mm), an orange red crystal of 3 (0.10 × 0.10 × 0.03 mm), and an orange red crystal of 4 (0.15 × 0.15 × 0.10 mm) were selected for single-crystal diffraction analysis. All the data collections were carried out on a Nonius Kappa CCD diffractometer with graphite-monochromated Mo KR radiation (0.71073 Å) at 293 K. The structures were solved by direct methods and refined by a full-matrix least-squares technique based on F2 using the SHELXL 97 program.10 All non-hydrogen atoms were refined anisotropically, and the hydrogen atoms of water molecules were located from difference electronic Fourier maps. Other hydrogen atoms were placed on the calculated positions, and refined isotropically. The details of crystal data, selected bond lengths, and angles for compounds 1-4 are listed in Tables 1 and 2, respectively.

Result and Discussion Description of the Structures. Reaction of 2,5dmpdo with NH4NCS and MnCl2‚4H2O in aqueous solution gives a binuclear compound, while using NaN3 and Na(dca) instead of NH4NCS produces a 1D zigzag chain or a 2D sheet structure. Substituting 2,3-dmpdo for 2,5-dmpdo with Na(dca) and MnCl2‚4H2O in aqueous solution yields a CdSO4-like 3D structure. The results

Figure 1. ORTEP view of the dinuclear unit of compound 1, together with the atom labeling.

show that the change of anionic ligands or the neutral bridging ligands could have a strong influence on the formation of the final solid structure, even if the change in the neutral ligand is minor. All the selected bond lengths and angles for compounds 1-4 are listed in Table 2. Binuclear Unit. The structure of compound 1 consists of binuclear units. In these units, the manganese (II) ions are bridged by a 2,5-dmpdo ligand in the trans mode. The Mn‚‚‚Mn intramolecular separation is 8.546(1) Å, which is similar to that found in pyrazine dioxide analogoues.5 A perspective view of the molecule is shown in Figure 1. Each Mn(II) ion is coordinated by one oxygen atom from the bridging 2,5-dmpdo molecule, one oxygen atom from the terminal 2,5-dmpdo, two terminal NCS- groups, and two coordinated water molecules in an octahedral environment. The two NCSanions are nearly linear with N-C-S angles of 179.0(4)° and 178.1(4)°. The terminal 2,5-dmpdo ligands not only coordinate with the metal ions but also hydrogen bond with coordinate water molecules from adjacent units to connect the binuclear units into ladders parallel to the ac plane with intermolecular Mn‚‚‚Mn distances of 6.7051(1) Å [O6‚‚‚O1a ) 2.909 Å, O6-H4w‚‚‚O1a ) 167.39°; a ) x + 1, y, z], shown in Figure 2. The ladders are expanded by hydrogen bonds between the terminal 2,5-dmpdo ligands, solvent 2,5dmpdo molecules, and lattice waters into a 3D network [O5‚‚‚O7 ) 2.731 Å, O5-H1w‚‚‚O7 ) 173.25°; O6‚‚‚O3

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Table 2. Selected Bond Lengths (Å) and Angles (deg) for 1-4a Mn(1)-N(6) Mn(1)-O(5) Mn(1)-O(6) N(6)-Mn(1)-N(5) N(5)-Mn(1)-O(5) N(5)-Mn(1)-O(4) N(6)-Mn(1)-O(6) O(5)-Mn(1)-O(6) N(6)-Mn(1)-O(1) O(5)-Mn(1)-O(1) O(6)-Mn(1)-O(1) N(6)-C(14)-S(2)

1 2.152(3) Mn(1)-N(5) 2.166(3) Mn(1)-O(4) 2.198(2) Mn(1)-O(1) 91.02(13) N(6)-Mn(1)-O(5) 174.68(11) N(6)-Mn(1)-O(4) 86.78(11) O(5)-Mn(1)-O(4) 88.72(11) N(5)-Mn(1)-O(6) 86.50(10) O(4)-Mn(1)-O(6) 90.34(10) N(5)-Mn(1)-O(1) 84.98(9) O(4)-Mn(1)-O(1) 171.33(10) N(5)-C(13)-S(1) 178.1(4)

Mn(1)-O(2) Mn(1)-N(2) Mn(1)-O(1) O(2)-Mn(1)-O(2)a O(2)a-Mn(1)-N(2) O(2)a-Mn(1)-N(2)a O(2)-Mn(1)-O(1) N(2)-Mn(1)-O(1) O(2)-Mn(1)-O(1)a N(2)-Mn(1)-O(1)a O(1)-Mn(1)-O(1)a

2.158(2) 2.187(2) 2.213(2) 180.0 90.03(8) 89.97(8) 88.96(7) 87.47(8) 91.04(7) 92.53(8) 180.0

Mn(1)-N(4)a Mn(1)-O(1) Mn(1)-N(2)c N(4)a-Mn(1)-N(4)b N(4)b-Mn(1)-O(1) N(4)b-Mn(1)-O(1)c N(4)a-Mn(1)-N(2)c O(1)-Mn(1)-N(2)c N(4)a-Mn(1)-N(2) O(1)-Mn(1)-N(2) N(2)c-Mn(1)-N(2)

2.205(2) 2.210(1) 2.215(2) 180.0 88.45(5) 91.55(5) 84.94(6) 89.11(5) 95.06(6) 90.89(5) 180.0

Mn(1)-N(4)a Mn(1)-O(1) Mn(1)-N(2)c N(4)a-Mn(1)-N(4)b N(4)b-Mn(1)-O(1) N(4)b-Mn(1)-O(1)c N(4)a-Mn(1)-N(2)c O(1)-Mn(1)-N(2)c N(4)a-Mn(1)-N(2) O(1)-Mn(1)-N(2) N(2)c-Mn(1)-N(2)

2.176(2) 2.212(2) 2.215(2) 180.0 88.65(7) 91.35(8) 92.33(8) 87.85(8) 87.67(8) 92.15(8) 180.0

2.163(3) 2.196(2) 2.359(2) 94.24(12) 177.75(12) 87.96(9) 94.41(12) 91.92(10) 94.23(11) 89.36(9) 179.0(4)

2i Mn(1)-O(2)a Mn(1)-N(2)a Mn(1)-O(1)a O(2)-Mn(1)-N(2) O(2)-Mn(1)-N(2)a N(2)-Mn(1)-N(2)a O(2)a-Mn(1)-O(1) N(2)a-Mn(1)-O(1) O(2)a-Mn(1)-O(1)a N(2)a-Mn(1)-O(1)a

2.158(2) 2.187(2) 2.213(2) 89.97(8) 90.03(8) 180.0 91.04(7) 92.53(8) 88.96(7) 87.47(8)

3ii Mn(1)-N(4)b Mn(1)-O(1)c Mn(1)-N(2) N(4)a-Mn(1)-O(1) N(4)a-Mn(1)-O(1)c O(1)-Mn(1)-O(1)c N(4)b-Mn(1)-N(2)c O(1)c-Mn(1)-N(2)c N(4)b-Mn(1)-N(2) O(1)c-Mn(1)-N(2)

2.205(2) 2.210(1) 2.215 (2) 91.55(5) 88.45(5) 180.0 95.06(6) 90.89(5) 84.94(6) 89.11(5)

4iii Mn(1)-N(4)b Mn(1)-O(1)c Mn(1)-N(2) N(4)a-Mn(1)-O(1) N(4)a-Mn(1)-O(1)c O(1)-Mn(1)-O(1)c N(4)b-Mn(1)-N(2)c O(1)c-Mn(1)-N(2)c N(4)b-Mn(1)-N(2) O(1)c-Mn(1)-N(2)

2.176(2) 2.212(2) 2.215(2) 91.35(8) 88.65(8) 180.0 87.67(8) 92.15(8) 92.33(8) 87.85(8)

a Symmetry codes (i): a, -x, -y, -z. (ii) a, x, y, z + 1; b, -x, -y, -z - 1; c, -x, -y, -z. (iii) a, -x + 1, -y, -z + 1; b, x - 1/2, y + 1/2, z; c - x + 1/2, -y + 1/2, -z + 1.

Figure 2. The ladder formed by the hydrogen bonding between the dinuclear units in 1.

) 2.774 Å, O6-H3w‚‚‚O3 ) 164.45°; O5‚‚‚O2a ) 2.851 Å, O5-H2w‚‚‚O2a ) 171.06°; O7‚‚‚O2b ) 2.821 Å, O7-

Figure 3. The three-dimensional network constructed by the hydrogen bonding between ladders, uncoordinated 2,5-dmpdo, and lattice water in 1.

Figure 4. The 2D sheet formed by the hydrogen bonding between the zigzag chains of 2, together with the atom labeling.

H5w‚‚‚O2b ) 167.96°; a ) x + 1/2, -y + 1/2, z - 1/2, b ) x - 1/2, -y + 1/2, z - 1/2], shown in Figure 3. Although compound 1 is a binuclear complex, the efficient and rich hydrogen bonds generated through 2,5-dmpdo molecules make it an extended threedimensional structure, which is another feature of this type of ligand.11 One-Dimensional Polymer. As shown in Figure 4, the structure of compound 2 consists of zigzag chains in which the Mn(II) ions are bridged by trans 2,5-dmpdo with an intramolecular Mn‚‚‚Mn distance of 8.508(4) Å. Each Mn(II) ion is octahedrally coordinated by two nitrogen atoms from two terminal azides, two coordination water molecules at the equatorial sites [Mn-O ) 2.158(2) Å, Mn-N ) 2.187(2) Å], and two oxygen atoms from the bridging 2,5-dmpdo ligands [Mn-O ) 2.213(2) Å]. The two N3- anions are nearly linear with an N-N-N angle of 177.4(3)°. The azide molecules not only act as a terminal ligand but also form hydrogen bonds with coordinated water molecules from the adjacent chains, extending the 1D chain to a 2D layer parallel to the bc plane with the intermolecular Mn‚‚‚Mn distance of 7.767(2) Å [O2‚‚‚N4a ) 2.771 Å, O2-H2w‚‚‚N4a ) 173.91°; a ) x, y, z + 1]. The layer is further extended into a 3D network by hydrogen bonds

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Figure 5. The three-dimensional network constructed by the hydrogen bonding between the 2D sheets of 2.

Figure 6. View of the two-dimensional structure of 3, together with the atom labeling.

between water molecules and azide ligands from adjacent layers [O2‚‚‚N2a ) 2.802 Å, O2-H1w‚‚‚N2a ) 165.02°; a ) -x + 1, -y, -z], shown in Figure 5. Two-Dimensional Layer. X-ray crystal structure analysis reveals that compound 3 is comprised of twodimensional (4,4) sheets, in which Mn(II) ions are bridged by double µ1,5-dca in the direction of the c axis and trans-2,5-dmpdo along the [0, 1, 1] direction with the Mn‚‚‚Mn distance of 7.5566(5) Å for µ1,5-dca and 8.5359(4) Å for µ2,5-dmpdo, shown in Figure 6. Both the Mn‚‚‚Mn separations bridged by µ2,5-dmpdo and by µ1,5dca are comparable with those found in compounds 1 and 2, and other related structures.2-4 The structure of compound 3 is similar to that of β-M(dca)2(pyrazine), except the trans-2,5-dmpdo bridge has a “Z”-shape, leading to an off-setting of the M(dca)2 chains in the nets, whereas the pyrazine bridge is linear and the M(dca)2 chains are perfectly aligned.4h,4i All the Mn(II) ions are located in an inversion center, and octahedrally coordinated via two oxygen atoms from two bridging 2,5dmpdo molecules [Mn-O ) 2.210(1) Å] in the axial positions and four nitrile nitrogen atoms from bridging dca ligands in the equatorial positions [Mn-N ) 2.215 (2), 2.176(2) Å]. The 2D layer is further extended by weak hydrogen bonding between the amide nitrogen

atom of the dca ligands and a carbon atom of the 2,5dmpdo ligands in adjacent sheets to a 3D network [N3‚‚‚C3a ) 3.387(3) Å, C3a-H3a‚‚‚N3 ) 163.31°; a ) -x + 1, -y, -z - 1], shown in Figure 7. Three-Dimensional CdSO4-like Network. The labeled diagram for compound 4 is shown in Figure 8. The coordination environment of each Mn(II) ion is similar to that found in compound 3 except 2,3-dmpdo occupies the place of 2,5-dmpdo. However, the substitution of 2,3-dmpdo for 2,5-dmpdo changes the crystal structure from a 2D layer to a 3D network. Mn(II) ions connected by the double dca bridges form 1D chains with a Mn‚‚‚Mn separation of 7.4579(2) Å, and the chains are arranged nearly perpendicularly and further connected by bridging 2,3-dmpdo ligands (Mn‚‚‚Mn separation of 8.5561(3) Å) to produce a 3D network, as shown in Figure 9a. Each Mn(II) ion of 4 provides 4-connected node and the 4-connected Mn(II) ions are expanded to produce a typical CdSO4-like net (Figure 9b). The solvent water molecule lies in the lattice forming weak hydrogen bonding with O1 atom from 2,3-dmpdo [O2‚‚‚O1 ) 3.042(2) Å, O2-H1w‚‚‚O1 ) 175.94°; a ) -x, y, -z + 1/2]. Bridging Modes of Pyrazine-Dioxide Derivatives. The bridging mode of pyrazine is linear, whereas at least two kinds of connection modes (cis or trans) of pyrazine-dioxide were expected. Furthermore, the bridging mode of pyrazine-dioxide can be modified by introducing different substituents at different positions of the pyrazine ring. For the first three compounds reported here (1, 2, and 3), containing 2,5-dmpdo, the torsion angle Mn1-O-O-Mn1a (a ) -x + 1, -y, -z + 1 for 1, -x, -y + 1, -z - 1 for 2 and -x, -y + 1, -z - 1 for 3) is 180°. However, in compound 4 the torsion angle Mn1-O-O-Mn1a (a ) -x + 1, y, 1/2 - z) is 54.71(5)°, closer to cis. This difference in the torsion angle comes from the different position of the methyl group. For 2,5dmpdo, a centrosymmetric ligand, the methyl groups lie in the 2- and 5-positions. However, 2,3-dmpdo is a noncentrosymmetric ligand, and the methyl groups lie in the 2- and 3-positions. The different steric restrictions

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Figure 7. A three-dimensional network constructed by the weak hydrogen bonding between the 2D layers of 3.

Figure 8. ORTEP drawing of 4, together with the atoms labeling.

determine the different Mn1-O-O-Mn1a torsion angles and thus the different crystal structures between 3 and 4. Magnetic Properties. From the crystal structures, one may expect antiferromagnetic coupling between Mn(II) ions in all four compounds due to the bridging pyrazine-dioxide derivatives. The magnetic study on these compounds does indeed reveal antiferromagnetic coupling with the antiferromagnetic ordering temperature of 2.12 K for 1, 6.14 K for 2, 6.23 K for 3, and 5.37 K for 4, which were determined from the sharp peak in d(χMT)/dT. The field dependence of magnetization measured at 1.8 K suggests that a spin-flop transition may occur for each compound below TN. Compound 1. The variable-temperature magnetic susceptibility χM for a collection of crystals of 1 in the temperature range of 2-300 K was measured in a field of 5 kOe, and the results are shown in Figure 10. With the decrease of temperature, χM increases slowly, reaching a maximum value of 0.2 cm3 mol-1 at ca. 6.62 K and then decreases, indicating antiferromagnetic cou-

Figure 9. (a) The three-dimensional structure of 4. Mn atoms are purple, carbon atoms are green, nitrogen atoms are blue, and oxygen atoms are red; hydrogen atoms and guest water molecules are omitted for clarity. (b) The CdSO4-like net of 4. Only the Mn atoms and the connections between them are shown. The orientation of the diagram is the same as for panel (a), i.e., the Mn(2,3-dmpdo) chains run in the horizontal direction across the page.

pling between the Mn(II) ions. The χMT value (4.310 cm3 mol-1 K) at room temperature is consistent with the spin-only value of 4.317 cm3 mol-1 K for one Mn(II) ion. Lines’ model12 for a quadratic layer was used to estimate the J value between Mn(II) ions, and the best fitting of the magnetic data above 6.62 K gives the following results: J ) -0.590(3) cm-1, zJ′ ) -0.03(2) cm-1, g ) 2.017(2) with R ) 5.2 × 10-5 {R ) ∑[(χM)obs - (χM)calcd]2/

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Figure 10. Temperature dependence of χM for 1 (inset: d(χMT)/dT vs T for 1).

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Figure 12. Temperature dependence of χM for 2 (inset: d(χMT)/dT vs T for 2).

Figure 11. Field dependence of magnetization and ac magnetic susceptibility for 1 at 1.8 K.

Figure 13. Field dependence of magnetization at 1.8 K for 1 (inset: ac magnetic susceptibility vs field).

∑ (χM)obs2}. The J value is a bit smaller than that found in ref 5. The field dependence of magnetization for 1 at 1.8 K is not exactly a straight line expected for an antiferromagnet. Further an ac susceptibility study at 1.8 K reveals that there is a shoulder around 25-30 kOe (Figure 11), which may be due to a spin-flop transition that was observed in ref 5. The magnetization below TN increases slowly with increasing magnetic field due to the antiferromagnetic ordering, and then quickly increases in the transition from an antiferromagnetic ordering to a spin-flop state at a transition field of 2530 kOe. The magnetization at 70 kOe is 3.01 Nβ, which is much smaller than the saturation value of 5 Nβ for one Mn(II) ion, also suggesting an antiferromagnetic state. Compound 2. A plot of χM versus T is shown in Figure 12, where χM is the susceptibility per Mn(II) unit. There is also a maximum of 0.137 cm3 mol-1 at 9.80 K in the plot, which indicates an antiferromagnetic interaction between the metal ions. The χMT value at room temperature is 4.714 cm3 mol-1 K, which is somewhat higher than the value of 4.317 cm3 mol-1 K expected for a spin-only Mn(II) ion. The magnetic data above 9.80 K are well fitted by using Lines’ model12 for a quadratic layer, giving the results of J ) -0.854(5) cm-1, zJ′ )

-0.19(2) cm-1, g ) 2.064(2) with R ) 5.8 × 10-5 {R ) ∑[(χM)obs - (χM)calcd]2/∑ (χM)obs2. The exchange constant J is comparable with that found in ref 5. The field dependence of the magnetization at 1.8 K is bent, unlike a straight line for an antiferromagnet. The field dependence of ac susceptibility at 1.8 K shows a peak at about 26.5 kOe, indicating that a spin-flop transition might exist. The slow increase in magnetization suggests an antiferromagnetic state, while the rapid increase in magnetization may be due to a spin-flop state at a transition field of about 26.5 kOe (Figure 13). The magnetization at 70 kOe is 1.90Nβ, which is much smaller than the saturation value of 5.0 Nβ for a Mn(II) ion, also suggesting antiferromagnetic ordering. Compound 3. Although compound 2 comprises a 1D chain, the efficient hydrogen bonding connects the chains to produce a 2D layer, which is just the same as that found in compound 3. The magnetic behavior of 3 should resemble that of compound 2. The shape of the χM versus T plot, the maximum value of 0.147 cm3 mol-1, the maximum value temperature of 9.89 K, the magnetic ordering temperature of 6.23 K, and the room temperature χMT value of 4.648 cm3 mol-1 K are nearly all the same as those of compound 2, as shown in Figure 14. Fitting magnetic data above 9.89 K by using Lines’ model12 gives the results: J ) -0.781(6) cm-1, zJ′ )

Mn(II) Complexes of Pyrazine-Dioxide Derivatives

Figure 14. Temperature dependence of χM in an applied field of 5 kOe for 3 (inset: d(χMT)/dT vs T for 3).

Figure 15. Field dependence of magnetization and ac magnetic susceptibility for 3 at 1.8 K.

-0.19(3) cm-1, g ) 2.052(2) with R ) 7.1 × 10-5 {R ) ∑[(χM)obs - (χM)calcd]2/∑ (χM)obs2, which is nearly equal to that found in compound 2. The transition field of 13 kOe is much lower than that found in compound 2, which may come from the different interlayer interaction (Figure 15). Although the interchain hydrogen bonding and the weak hydrogen bonding between the layers contribute to the magnetic ordering, the magnetic ordering possesses an obvious low-dimensional character, as the TN/T(χmax) values of 0.627 for 2 and 0.630 for 3 are quite low. Compound 4. The variable-temperature magnetic susceptibility χM in the temperature range of 2-300 K was measured under a field of 10 kOe, as shown in Figure 16. When the temperature decreases, the susceptibility increases slowly at first; while below 50 K χM increases quickly, reaches a maximum of 0.233 cm3 mol-1 at 5.721 K and then goes down, giving a sharp peak which is an indication of a stronger antiferromagnetic interaction between Mn(II) ions. The d(χMT)/dT-T plot gives a λ-shape-like curve, which is similar to that found in 3D antiferromagnets. The ratio of TN/T(χmax) is 0.90, which is further evidence for 3D antiferromagnetic ordering. The temperature dependence of ac magnetic susceptibility presented in Figure 17 shows that the real part of ac magnetic susceptibility (χ′) has

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Figure 16. Temperature dependence of χM for 4 (inset: d(χMT)/dT vs T for 4).

Figure 17. Real (χ′) and imaginary (χ′′) ac magnetic susceptibilities of 4 as a function of temperature for different frequencies (111, 199, 355, 633, and 1111 Hz).

a shoulder at ca. 5.99 K for frequencies of 199, 355, and 633 Hz, and the out-of-phase of ac magnetic susceptibility (χ′′) is negligibly small, which is another indication of three-dimensional antiferromagnetic ordering below 5.99 K. The magnetization M versus H at 1.8 K (Figure 18) shows a pronounced sigmoid shape, which suggests the existence of a spin-flop transition. The field dependence of ac magnetic susceptibility measured at 1.8 K shows a peak in the real part at ca. 18.5 kOe, also suggesting the existence of a spin-flop transition (inset of Figure 18). Considering the magnetic ordering temperatures TN, of 2.12 K for 1, 6.14 K for 2, 6.23 K for 3, and 5.37 K for 4, it is not difficult to understand from the structure information that the magnetic ordering temperatures for 2 and 3 are higher than that for 1. Note also, however, that the TN for 4 is abnormally lower than those for 2 and 3, despite 4 having a three-dimensional structure, while 2 and 3 only have two-dimensional layers. If we look closer at the crystal structures, we may find a clue to understanding this. Although 4 is a 3D compound while 2 and 3 are 2D, the number of the nearest neighbor Mn(II) ions in the compounds 2 and 3 is six, compared to four in compound 4, because of the interlayer hydrogen bonding or the weak hydrogen bonding. On the basis of a mean-field theory, more

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(2)

(3)

Figure 18. Field dependence of magnetization at 1.8 K for 4 (inset: ac magnetic susceptibility vs field).

magnetic neighbors prefer to a higher magnetic ordering temperature in 2 and 3. This series of complexes demonstrate the efficient magnetic contribution of the hydrogen bonding or weak hydrogen bonding. Conclusions A series of Mn(II) complexes 1-4 with the 2,5-dmpdo or 2,3-dmpdo ligands have been prepared and structurally characterized. They present binuclear unit, 1D, 2D, and 3D structural motifs. This study clearly indicates that the anionic ligand and/or the coordination ability/ mode of the neutral ligand can play an important role in the design and synthesis of coordination polymers. The magnetic study reveals that all of these compounds exhibit antiferromagnetic ordering and spin-flop transitions below the corresponding magnetic ordering temperatures owing to both the antiferromagnetic interaction translated by the bridging ligands of 2,5dmpdo or 2,3-dmpdo and the low anisotropy of Mn(II) ions. The comparable ordering temperature of 2 (6.14 K) and 3 (6.23 K) proves that not only the bridging ligand but also the efficient hydrogen bonding has great effect on the magnetic behavior. The antiferromagnetic ordering of 1, 2, and 3 obviously shows low-dimensional character due to their crystal structure, while compound 4 is a 3D antiferromagnet. Acknowledgment. This work was supported by the National Science Fund for Distinguished Young Scholars (20125104, 20221101, and 20391000), the Research Fund for the Doctoral Program of Higher Education (20010001020), the Excellent Young Teachers Fund of MOE, P. R. C., and the Australian Research Council. Supporting Information Available: An X-ray crystallographic file for 1-4, in CIF format. This material is available free of charge via the Internet at http://pubs.acs.org.

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