Influence of Water Ligands on Structural Diversity: From a One

Nov 24, 2008 - Christopher E. Anson,† and Annie K. Powell*,† ... D-76128 Karlsruhe, Germany, CNRS, UPR 8641, Centre de Recherche Paul Pascal (CRPP...
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CRYSTAL GROWTH & DESIGN

Influence of Water Ligands on Structural Diversity: From a One-Dimensional Linear Coordination Polymer to Three-Dimensional Ferrimagnetic Diamondoid Metal-Organic Frameworks

2009 VOL. 9, NO. 1 577–585

Sudarshana Mukherjee,† Yanhua Lan,† George E. Kostakis,† Rodolphe Cle´rac,*,‡,§ Christopher E. Anson,† and Annie K. Powell*,† Institute fu¨r Anorganische Chemie der UniVersita¨t Karlsruhe, Engessertsr. Geb. 30.45, D-76128 Karlsruhe, Germany, CNRS, UPR 8641, Centre de Recherche Paul Pascal (CRPP), Equipe “Mate´riaux Mole´culaires Magne´tiques”, 115 aVenue du Dr. Albert Schweitzer, Pessac, F-33600, France, and UniVersite´ de Bordeaux, UPR 8641, Pessac, F-33600, France ReceiVed August 10, 2008; ReVised Manuscript ReceiVed September 11, 2008

ABSTRACT: Four three-dimensional (3D) metal-organic frameworks [Mn3(3-Me-sal)4(py)4]n (1), [Mn3(4-Me-sal)4(py)4(MeOH)]n · n(H2O) (2), [Mn3(5-Me-sal)4(py)4(H2O)2]n · n(MeOH) (3), and [Mn3(3-Me-sal)4(4-Me-py)4]n (4) and the one-dimensional (1D) coordination polymer {[Mn2(4-Me-sal)2(4-Me-py)2(H2O)2(MeOH)2][Mn(4-Me-sal)2(4-Me-py)2]}n (5) have been synthesized, where x-Me-salH2 ) x-methyl salicylic acid (x ) 3, 4, 5), py ) pyridine, and 4-Me-py ) 4-methyl-pyridine. The 3D frameworks of compounds 1-4 can be described as diamondoid networks. Magnetic studies show that weak MnII-MnIII antiferromagnetic interactions (in the range of -0.55 to -0.22 K) mediated by syn-anti carboxylate bridges are present in all compounds. While 5 remains paramagnetic down to 1.8 K, the 3D networks exhibit long-range ferrimagnetic ordering below 7.4 K for 1, 4.6 K for 2, 3.0 K for 3, and 7.7 K for 4. The decrease of the critical temperature reflects the increase of the coordination number around the Mn(II) site from four in 1, five in 2, and six in 3 that lower the bond strength and the magnetic interactions. This result also reinforces the hypothesis that the structures of 1 and 4 are similar as suggested by the X-ray analysis. Introduction In recent years, the use of crystal engineering concepts has produced a variety of metal-organic frameworks (MOFs),1 many of which exhibit unusual and fascinating structures resulting from the combination of individual motifs.2 There is an increasing interest in these species not only for their potential properties as functional solid materials, in catalysis,3 magnetism,4 gas separation,5 and luminescence and chemical sensing,6 but also for their intriguing architectures and topologies such as molecular grids, herringbones, bricks, ladders, rings, boxes and honeycombs. Some structures of minerals, such as diamond,7 quartz,8 NbO,9 PtS,10 CdSO4,11 moganite,12 and SrSi213 have been artificially produced utilizing organic ligands as linkers and metal centers as nodes. One of our research goals is to construct MOFs possessing novel structural motifs and/or giving emphasis to magnetic properties. To our knowledge, complexes of 3d transition metal elements and substituted methyl salicylic acids have received less attention than systems with other aromatic carboxylic acids.14 In this report, we focus on our ambient syntheses in the mixed-ligand systems of {Mn - substituted salicylic acid pyridine}, reporting the synthesis, characterization, and magnetic properties of four novel three-dimensional (3D) MOFs, [Mn3(3Me-sal)4(py)4]n (1), [Mn3(4-Me-sal)4(py)4(MeOH)]n · n(H2O) (2), [Mn3(5-Me-sal)4(py)4(H2O)2]n · n(MeOH) (3), and [Mn3(3-Mesal)4(4-Me-py)4]n (4) and one coordination polymer {[Mn2(4Me-sal)2(4-Me-py)2(H2O)2(MeOH)2][Mn(4-Me-sal)2(4-Mepy)2]}n (5), where x-Me-salH2 ) x-methyl salicylic acid (x ) 3, 4, 5), py ) pyridine, and 4-Me-py ) 4-methyl-pyridine. * To whom correspondence should be addressed. E-mail: powell@ aoc.uni-karlsruhe.de (A.K.P.); [email protected] (R.C.). † Institute fu¨r Anorganische Chemie der Universita¨t Karlsruhe. ‡ CNRS, UPR 8641, Centre de Recherche Paul Pascal (CRPP). § Universite´ de Bordeaux.

Simplifying the 3D frameworks of compounds 1-4 shows that they are based on diamondoid networks, in which the MnII center acts as a distorted tetrahedral node, while the MnIII centers form linear 2-fold nodes. Magnetic studies show that in these compounds, the MnII and MnIII magnetic interactions are always of antiferromagnetic nature inducing a ferrimagnetic 3D order in the 3D network compounds. The current work can be considered as a continuation of our research efforts on 3d or 4f MOFs.15 Experimental Section Materials and Methods. All chemicals and solvents used for synthesis were obtained from commercial sources and were used as received, without further purification. All reactions were carried out under aerobic conditions. The elemental analysis (C, H, N) were carried out at the Institu¨te fu¨r Anorganische Chemie at the Universita¨t Karlsruhe (TH) using an Elementar Vario EL analyzer. Fourier transform IR spectra were measured on a Perkin-Elmer Spectrum one spectrometer with samples prepared as KBr discs. X-ray powder diffraction patterns for 4 were measured at room temperature using a Stoe STADI-P diffractometer with a Cu KR radiation. Synthesis of {[Mn2(4-Me-sal)2(4-Me-py)2(H2O)2(MeOH)2][Mn(4-Me-sal)2(4-Me-py)2]}n (5). The same procedure was employed to prepare all complexes and hence only the compound 5 is described here in detail. MnCl2 · 4H2O (79 mg, 0.4 mmol) was added to a methanolic solution (20 mL) of 4-methyl salicylic acid (4-Me-salH2) (101 mg, 0.6 mmol) and 4-Me-py (157 µL, 1.6 mmol) to give a light yellow colored solution. This solution was then stirred for 1 h followed by the addition of triethylamine (100 µL, 0.7 mmol), which gave a black solution after a few minutes. After filtration, slow evaporation of the resulting solution gave shiny black needle shaped crystals after 2 days. The crystals were collected by filtration, washed with cold methanol, and air-dried. Yield: 61 mg, 37% based on Mn. Anal. calcd for C58H64Mn3N4O16 (found): C, 56.27 (56.22); H, 5.21 (5.17); N, 4.53 (4.52). Selected IR data (KBr disk, cm-1): 3417 (b), 2922 (m), 1615 (s), 1563 (s), 1423 (b,s), 1325 (s), 1251 (m), 1170 (s), 1012 (m), 958 (m), 781 (s), 751 (s), 625 (s), 487 (m), 426 (w).

10.1021/cg800879c CCC: $40.75  2009 American Chemical Society Published on Web 11/24/2008

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Table 1. Crystallographic Data and Structure Refinement for Compounds 1-5

formula Mr cryst size [mm] color cryst syst space group T [K] a [Å] b [Å] c [Å] R [deg] β [deg] γ [deg] V [Å3] Z Fcalcd [g cm-3] µ (Mo KR) [mm-1] F(000) reflns collected unique reflns Rint reflns with I > 2σ(I) parameters/restraints GOF on F 2 R1 [I > 2σ(I)] wR2 (all data) largest difference peak/hole [e Å-3] CCDC number

1

2

3

4

5

C52H44Mn3N4O12 1081.73 0.26 × 0.21 × 0.16 black tetragonal P43 100 18.7991(2) 18.7991(2) 13.8136(3) 90 90 90 4881.81(13) 4 1.472 0.832 2220 34044 11063 0.0213 10556 645/1 1.020 0.0352 0.0863 +1.02/-0.29 694596

C53H51Mn3N4O14 1132.80 0.25 × 0.18 × 0.11 black tetragonal I41 100 18.9660(7) 18.9660(7) 13.9262(10) 90 90 90 5009.4(4) 4 1.502 0.817 2336 13860 4687 0.0303 4452 348/2 1.037 0.0582 0.1424 +2.41/-0.46 694597

C53H52Mn3N4O15 1149.81 0.05 × 0.04 × 0.04 black monoclinic P21/c 150 18.888(3) 13.7658(14) 20.054(3) 90 93.780(12) 90 5203.0(13) 4 1.468 0.789 2372 17781 4704 0.1283 2867 693/6 1.038 0.0856 0.1808 +0.71/-0.60 694598

C56H52Mn3N4O12 1192.78

C58H64Mn3N4O16 1237.95 0.18 × 0.14 × 0.09 black triclinic P1j 150 11.8534 (12) 12.5635 (14) 19.530 (2) 82.457 (9) 83.345 (8) 85.861 (8) 2859.0 (5) 2 1.438 0.725 1286 21350 12060 0.0486 7513 780/14 0.979 0.0734 0.1974 +0.77/-1.01 694599

Synthesis of [Mn3(3-Me-sal)4(py)4]n (1). Yield: 65 mg, 45% based on Mn. Anal. Calcd for C52H44Mn3N4O12 (found): C, 57.74 (57.69); H, 4.10 (4.07); N, 5.18 (5.17). Selected IR data (KBr disk, cm-1): 3425 (b), 2923 (m), 1599 (s), 1576 (m), 1520 (s), 1463 (s), 1446 (s), 1423 (s), 1249 (m), 1082 (m), 875 (m), 760 (s), 698 (s), 630 (m), 439 (b,m). Synthesis of [Mn3(4-Me-sal)4(py)4(MeOH)]n · n(H2O) (2). Yield: 59 mg, 39% based on Mn. Anal. Calcd for C53H51Mn3N4O14 (found): C, 56.19 (56.14); H, 4.54 (4.50); N, 4.95 (4.94). Selected IR data (KBr disk,cm-1): 3412 (b), 2922 (m), 1599 (s), 1556 (s), 1445 (s), 1325 (s), 1166 (s), 1035 (m), 957 (m), 778 (s), 749 (m), 699 (m), 625 (s). Synthesis of [Mn3(5-Me-sal)4(py)4(H2O)2]n · n(MeOH) (3). Yield: 35 mg, 45% based on Mn. Anal. Calcd for C53H52Mn3N4O15 (found): C, 55.36 (55.35); H, 4.56 (4.53); N, 4.87 (4.87). Selected IR data (KBr disk, cm-1): 3336 (b), 2967 (m), 1618 (s), 1599 (s), 1561 (s), 1543 (s), 1479 (s), 1443 (s), 1420 (s), 1350 (s), 1316 (s), 1231 (s), 1208 (m), 1145 (m), 1067 (m), 1037 (m), 1007 (m), 844 (m), 810 (m), 755 (m), 699 (s), 626 (m), 538 (m), 416 (m). Synthesis of [Mn3(3-Me-sal)4(4-Me-py)4]n (4). Yield: 52 mg, Anal. Calcd for this compound for C56H52Mn3N4O12 (found): C, 59.11 (59.10); H, 4.61 (4.57); N, 4.92 (4.93). Selected IR data (KBr disk,cm-1): 1618 (s), 1599 (s), 1574 (s), 1537 (s), 1462 (s), 1422 (s), 1324 (s), 1249 (s), 1155 (m), 1082 (s), 1023 (m), 876 (s), 812 (s), 762 (s), 695 (m), 536 (m), 439 (m). Magnetic Measurements. Magnetic susceptibility measurements were obtained using a Quantum Design SQUID MPMS-XL susceptometer. This magnetometer works between 1.8 and 400 K for dc applied fields ranging from -7 to +7 T. Measurements were performed on a polycrystalline sample of 17.8 mg (1), 15.9 mg (2), 11.5 mg (3), 7.4 mg (4), and 15.3 mg (5). ac susceptibility measurements have been measured with an oscillating ac field of 3.5 Oe and ac frequencies ranging from 1 to 1500 Hz. The magnetic data were corrected for the sample holder and the diamagnetic contribution. The samples were first checked for the presence of ferromagnetic impurities by measuring the magnetization as a function of the field at 100 K. For pure paramagnetic or diamagnetic systems, a perfect straight line is expected and is indeed observed for these compounds indicating the absence of any ferromagnetic impurities. X-ray Data Collection and Structure Determination. Data were collected at 150 K on a Stoe IPDS II area detector diffractometer (3, 5) or at 100 K on a Bruker SMART Apex CCD diffractometer (1, 2) using graphite-monochromated Mo–KR radiation. Crystals of 3 are weakly diffracting, and the intensity drops off rapidly at a high angle. No measurable intensity was detected corresponding to higher resolution than 1.05 Å, and the data were truncated at this point. Semiempirical absorption corrections were made using SADABS16a or XPREP in

black tetragonal P43 100 19.0290(5) 19.0290(5) 14.7032(8) 90 90 90 5324.1(4) 4

SHELXTL.16b The structures were solved using direct methods, followed by full-matrix least-squares refinement against F2 (all data) using SHELXTL.16b Anisotropic refinement was used for all ordered non-H atoms; organic H atoms were placed in calculated positions, while coordinates of hydroxo or water H atoms were refined. Crystallographic data (excluding structure factors) for the structures in this paper have been deposited with the Cambridge Crystallographic Data Centre as supplementary publication nos. CCDC 694596-694599. http://www.ccdc.cam.ac.uk/cgi-bin/catreq.cgi, e-mail: data_request@ ccdc.cam.ac.uk, or fax: +44 1223 336033. Crystals of 4 are apparently isotypic with those of 1, with a slightly larger unit cell appropriate for the bulkier pyridine ligand. However, full refinement of the structure proved impossible, as a result of what appears to be disorder of the MnII cations. Attempts to solve and refine the structure in lower-symmetry space groups, or to find a superstructure, proved unsuccessful. The structure is, however, clearly closely related to that of 1.

Results and Discussion Synthesis. Compounds 1-5 can be prepared by the reaction of the corresponding salicylate ligands, pyridine coligands and MnCl2 · 4H2O with triethylamine in methanol solution at room temperature. Compounds 1-4 are quite stable even when exposed to air and retain their crystallinity, while compound 5 when exposed to air starts to lose solvent molecules after 4-5 days. Structure Description. X-ray diffraction studies reveal that [Mn3(3-Me-sal)4(py)4]n (1), [Mn3(4-Me-sal)4(py)4(MeOH)]n · n(H2O) (2), [Mn3(5-Me-sal)4(py)4(H2O)2]n · n(MeOH) (3), and [Mn3(3-Me-sal)4(4-Me-py)4]n (4) are 3D MOFs possessing a diamondoid topology, while {[Mn2(4-Me-sal)2(4-Me-py)2(H2O)2(MeOH)2][Mn(4-Me-sal)2(4-Me-py)2]}n (5) is a 1D coordination polymer; the crystal data collection parameters are summarized in Table 1 and the selected bond lengths and angles are listed in Tables S1-S4, Supporting Information. Single-crystal X-ray diffraction study of 1 reveals an infinite 3D network that crystallizes in the tetragonal space group P43; the structure of 1 is shown in Figure 1. The asymmetric unit contains one MnII and two MnIII cations, four 3-Me-sal ligands, and four pyridine ligands. The two MnIII centers Mn1 and Mn2

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Figure 1. The asymmetric unit of 1 (top left) and a packing diagram (top right). Color code: MnIII purple, MnII pink, C black, O red and N blue. (Bottom) A diagram of the diamondoid network motif for compound 1. 4-fold MnII nodes are shown as pink spheres, and 2-fold MnIII nodes are purple.

are each chelated (through phenoxo and carboxylate oxygens) by two different 3-Me-sal ligands (arranged in a trans position) forming an equatorial plane with Mn-O 1.847-2.035 Å, and further ligated by two pyridine nitrogen atoms which define the Jahn-Teller axes, with Mn-N 2.204-2.271 Å. In this way a building unit [MnIII(3-Me-sal)2(py)2]- can be defined. The MnII center Mn3 is coordinated by four carboxylate oxygen atoms from four different bridging 3-Me-sal ligands (O3, O6, O9, O12). The geometry about Mn3 is close to tetrahedral, with Mn3-O bond lengths in the range 2.051-2.095 Å and the O-Mn-O angles 100.22-114.73°, corresponding to a MnII center. Bond valence sum (BVS) analysis confirms the oxidation states of the metals.17 The network can thus be described in terms of the distortedtetrahedral 4-fold MnII nodes being bridged by [MnIII(3-Mesal)2(py)2]- spacers, to give a diamondoid network, which is compressed along the tetragonal 4-fold axis. Alternatively, 4-fold MnII nodes and 2-fold quasi-linear MnIII nodes (in the ratio 1:2) can be considered as linked by 3-methylsalicylate spacers. In either description, the carboxylate groups form syn-anti bridges between MnII and MnIII cations. The four carboxylate bridges about each MnII are crystallographically independent, resulting

in four different MnIII-MnII distances, at 4.612, 5.025, 5.054, and 5.381 Å. Topological analysis of this framework with TOPOS software reveals a heterogeneous 4-fold diamond-type network and the total Schla¨fli symbol is [66].18 Each MnII node is linked to four other MnII nodes via MnIII 2-fold nodes, so that each MnII participates in six six-membered {MnII}6 rings, with distances between the MnII nodes of 9.615 and 10.415 Å. A single-crystal X-ray diffraction study of 2 reveals an infinite 3D network that crystallizes in the tetragonal space group, I41 (Figure 2). The structure is closely related to that of 1, but with additional symmetry elements resulting from the lattice centring. The asymmetric unit contains half a MnII cation (situated on a crystallographic 2-fold axis), one MnIII, two 4-Me-sal ligands, two pyridine molecules, and half a methanol ligand, which is disordered about the same crystal 2-fold axis. In a manner analogous to that seen in 1, Mn1 forms [MnIII(4-Me-sal)2(py)2]building blocks, which each form two syn-anti carboxylate bridges to MnII centers, Mn2. Carboxylate oxygens from four such bridges coordinate to Mn2, but in contrast to 1, Mn2 is additionally coordinated by a methanol ligand, the oxygen atom of which, O7, is disordered either side of a crystal 2-fold axis. This additional ligand causes a distortion of the tetrahedral

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Figure 2. The asymmetric unit of 2 (left) and a packing diagram (right). Color code: MnIII purple, MnII pink, C black, O red, and N blue.

Figure 3. The asymmetric unit of 3 (right) and a packing diagram (left). Color code: MnIII purple, MnII pink, C black, O red, and N blue.

geometry about Mn2, with two carboxylate oxygens, O3 and O3′, pushed apart to give an angle O3-Mn2-O3′ of 146.5°, while the other two, O6 and O6′ are pushed together, with O6-Mn2-O6′ of 96.5°. The Mn2-O3 and Mn2-O6 bond lengths are 2.037 and 2.119 Å, respectively, while Mn2-O7 is longer, at 2.277 Å. The additional coordination from the methanol ligand has, as would be expected, weakened the Mn2-O(carboxylate) bonding. If we neglect the methanol ligand, the 3D structure of 2 is very closely related to that of 1. Topological analysis of this framework with TOPOS software18 reveals a homogeneous 4-fold non-interpenetrating diamond-type network and the total Schla¨fli symbol is again [66]. In contrast to 1, the MnII · · · MnII distances are now all identical as a result of the higher crystal symmetry, at 10.102 Å. Complex 3 crystallizes in the monoclinic space group, P21/ c, as an infinite 3D network that is related to those in 1 and 2 (Figure 3). Consideration of the unit cell parameters (Table 1) shows that the unit cell is derived from that of 1 by the loss of the tetragonal 4-fold symmetry. The tetragonal 4-fold c-axis has

now become the monoclinic b-axis; the a- and c-axes have similar but different lengths, with the angle between them no longer 90°. The crystal structure of 3 can therefore be regarded as derived from that of 1 by a shear deformation. However, the structure of 3 is nonetheless built up in a similar manner to those of 1 and 2. The asymmetric unit contains one MnII and one and two half MnIII cations (the half cations lying on inversion centers), four 5-Me-sal ligands, four pyridine ligands, two water ligands, and one methanol molecule of solvation; the latter will not be further discussed. The three crystallographically independent MnIII cations Mn1, Mn2, and Mn3 form [MnIII(5Me-sal)2(py)2]- linking units, in the same way as in 1 and 2, and these again form syn-anti carboxylate bridges to the MnII centers, Mn4. The latter has a rather regular octahedral geometry. Two water ligands are coordinated in a cis configuration, while the remaining four sites are filled by carboxylate oxygen atoms from four different [MnIII(5-Me-sal)2(py)2]- units. The two Mn4-OH2 bond lengths are 2.205 and 2.242 Å, while the four Mn4-O (carboxylate) distances are now in the range 2.164-2.192 Å. The trend observed in 1 and 2 continues; the

Influence of Water Ligands on Structural Diversity

Figure 4. Powder pattern for compounds 1-4; the black line is 1, red is 2, blue is 3, and green is 4.

two additional noncarboxylate oxygens coordinated to Mn4 have weakened the MnII-O (carboxylate) bonding still further, with the mean of the four bond lengths increasing in the series 1 (four-coordinate MnII, 2.067 Å), 2 (five-coordinate MnII, 2.078 Å), to 3 (six-coordinate MnII, 2.173 Å). Topological analysis of this framework again reveals a heterogeneous 4-fold diamondoid network with a total Schla¨fli symbol [66]. This diamondoid network is flattened compared to those in 1 and 2, as might be expected given the two cis-orientated aqua ligands on Mn4. Of the four MnII · · · MnII internodal distances, two are equivalent at 10.031 Å, while the other two are 10.995 and 11.363 Å. The three networks are, however, still closely related, and give the opportunity to see how the change in MnII-O (carboxylate) bonding might affect the magnetic properties of the networks. Compound 4, like 1, crystallizes in the space group P43, with a similar, but slightly larger unit cell (Table 1). However, the structure could not be satisfactorily refined, with what appeared to be disorder of Mn3. Consideration of the structure of 1 indicates that the steric requirements of the 4-methyl substituent on the pyridine ligand in 4 is likely to force the 4-methylpyridine ligands to bend up or down. Since these can be considered as occupying separate “compartments” in the lattice, it is assumed that the deviation of these ligands may not be subject to longrange order, and that this results in the refinement problems. However, powder XRD measurements support what could be observed from the single-crystal measurement, indicating a structural motif similar to the 3D diamondoid networks seen in 1, 2, and 3, and thus a molecular formula [Mn3(3-Me-sal)4(4Me-py)4] which is supported by the microanalytical data. Compound 5 exhibits 1D linear chains that are a polymer of the monocationic [MnIII(4-Me-sal)2(4-Me-py)2MnII(H2O)2(MeOH)2]+ building block, which interact via monoanionic mononuclear complexes [MnIII(4-Me-sal)2(4-Me-py)2]- through hydrogen bonds and π-stacking interactions to form a 3D supramolecular network. As shown in Figure 5, the asymmetric unit contains one MnIII and two half MnII cations (the latter two on inversion centers), which form the polymer, and two further half MnIII which build the centrosymmetric mononuclear counteranions. The cationic 1D chain is thus mixed-valence, with alternating MnIII and MnII centers. The MnIII center, Mn1, forms [MnIII(4-Me-sal)2(4-Me-py)2]- units analogous to those

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found in 1-3, and these form syn-anticarboxylate bridges to the MnII centers, Mn2 and Mn3. According to the crystal symmetry, each of Mn2 and Mn3 is coordinated by two carboxylate oxygens in a strictly trans arrangement, with additional coordination from two aqua and two methanol ligands forming an equatorial plane. The mean MnIII · · · MnII separation is 4.888 Å. The MnIII · · · MnII · · · MnIII angles are 180° by symmetry, while the MnII · · · MnIII · · · MnII angle is 174.8°, so the polymer is very close to linear; the chains run parallel to the c axis. These 1D chains are linked via hydrogen bonds from the aqua and methanol ligands on the MnII centers to the noncoordinated salicylate oxygens of the mononuclear counteranionic complexes, forming 2D supramolecular layers parallel to {1 0 0}. The sheets are further reinforced by π-π stacking between aromatic rings and by C-H-π interactions, resulting in the supramolecular structure shown in Figure 6. The shortest intermolecular C · · · C distance between two parallel aromatic rings is 3.432 Å, which corresponds to a strong π-π stacking interaction.19 In structures 1, 2, and 3, the diamondoid network, which requires coordination of four carboxylate oxygens to each MnII, could be maintained even with coordination of one or two additional methanol or aqua ligands to the MnII centers, since this can be accommodated up to an octahedral environment for MnII. Coordination of four water and methanol ligands to each MnII, as in 5, makes it impossible to accommodate four carboxylates as well, and the resultant coordination of only two carboxylates leads to the linear polymer rather than the 3D network. Magnetic Properties. Magnetic measurements were performed on all compounds reported in this paper, but since compounds 1, 2, 3, and 4 possess roughly the same magnetic properties, only compounds 1 and 5 will be described in detail. The temperature dependence of susceptibility for compound 1 is shown in Figure 7. At room temperature, the χT product is 9.6 cm3 K mol-1 (Figure 7a) that is in agreement with the expected value (10.4 cm3 K mol-1) for the presence of one MnII metal ions and two MnIII ions. Upon cooling, the χT product at 1000 Oe steadily decreases to reach a minimum of 4.4 cm3 K mol-1 around 10 K and increases rapidly to reach a maximum of 37.5 cm3 K mol-1 at 6 K and then abruptly drops down to 14.0 cm3 K mol-1 at 1.8 K. This behavior is indicative of dominant antiferromagnetic interactions between MnII and MnIII metal ions through the two different but very similar synanti carboxylate bridges. Therefore, the ground-state of the system should correspond to a ferrimagnetic arrangement of spins. The plots of χ vs T (Figure 7a, inset) at different fields indicate that close to 8 K the χT values start to deviate from the normal paramagnetic behavior and strongly suggest, as expected, the appearance of a magnetically ordered phase with a spontaneous magnetization. The magnetization as a function of field at low temperatures has been measured up to 7 T (Figure 7b). At low fields, as usual for a magnet, there is an abrupt increase of the magnetization up to 1.4 µB at 250 Oe. When the field is further increased, the magnetization gradually reaches 3.8 µB at 7 T without saturation and with a weak sigmoidal shape. It is worth noting that already above 5 T, the magnetization is above the expected value of 3 µB for a ferrimagnetic system with two S ) 2 Mn(III) and one S ) 5/2 Mn(II) spin units. Therefore, the Mn(II)-Mn(III) antiferromagnetic interactions are clearly overcome by the applied dc field and the magnetization is thus expected to saturate at 13 µB well above 7 T. However, for compound 3, the plot of dM/dH vs H (Figure S4, Supporting Information) displays a maximum, that is, a

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Figure 5. Molecular structure of {[Mn2(4-Me-sal)2(4-Me-py)2(H2O)2(MeOH)2][Mn(4-Me-sal)2(4-Me-py)2]}n (5). Color code: MnIII purple, MnII pink, C black, O red, and N blue. All C-H hydrogens are omitted for clarity.

Figure 6. A view in a [1 0 0] direction of the unit cell of the 3D supramolecular architecture in 5. The atoms of the 1D cationic chain are presented in orange color, the atoms of the mononuclear compound are in blue, and manganese atoms are in red.

characteristic field that corresponds to an inflection point on the M vs H plot, at 216420 Oe. This behavior is typically seen when the magnetic field overcomes antiferromagnetic interactions allowing for the parallel alignment of the spins. This observation indicates that the interaction in compound 3 should

be weaker than that in compounds 1 and 2, which is consistent with the magnitude of exchange interactions deduced using mean field theory (see below). The M vs H data reveals at 2 K the existence of a hysteresis effect with a small coercive field (about 60 Oe) (Figure 7b,

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Figure 7. (left) Temperature dependence of χT product of 1 at 1000 Oe (with χ ) M/H) and inset: temperature dependence of the susceptibility below 20 K at 1000 Oe. (right) Field dependence of the magnetization from 1.8 to 8 K; hysteresis loop at 2 K (inset).

Figure 8. Temperature dependence of the ac susceptibility of 1 from 100 to 1000 Hz in zero dc field; in phase component: χ′ (left) and out-of-phase component: χ′′ (right).

inset). In support of the presence of a magnetic order, the inphase ac magnetic susceptibility (χ′) exhibits a peak located around 7 K and the out-of-phase ac susceptibility (χ′′) deviates from zero at 7.4 K (Figure 8). This behavior was found to be almost perfectly frequency independent indicating the presence of a true 3D ferrimagnetic order below 7.4 K. As mentioned above, the other 3D compounds, 2, 3, and 4, show very similar magnetic behavior to 1 (Figures S1-S6, Supporting Information). However, it is worth mentioning that the susceptibility starts to deviate from the normal paramagnetic behavior close to 5 K, 3 K, and 7.5 K for 2, 3, and 4 respectively, which is consistent with the ac susceptibility measurements. For these compounds, the out-of-phase ac susceptibility (χ′′) deviates from zero at 4.6 K (2), 3.0 K (3), and 7.7 K (4) as observed at 7.4 K for 1. In addition, the 7 T magnetization is higher in 2 and 3 at 5.5 µB and 10.9 µB, respectively, than in 1 and 4, indicating that the applied magnetic field overcomes the magnetic interactions more easily. This implies that these antiferromagnetic interactions are weaker in 2 and 3 than in 1 and 4. It is worth noting that the small hysteresis effect observed at low temperature in 1 (60 Oe) is also observed in the three other compounds 2 (30 Oe), 3 (100 Oe), and 4 (250 Oe). From the mean field theory and the experimental critical temperature, it is possible to evaluate the average MnII-MnIII antiferromagnetic interaction (J) present in compounds 1, 2, 3, and 4. Employing the same approach as used by Kahn20a and Ohkoshi et al.,20b,c the expression of the critical temperature (TC) can be obtained as a function of J: Tc ) 4|J|35/(3kB) (see Supporting Information). Therefore, considering a TC of about 7.4 K (1), 4.6 K (2), 3.0 K (3), and 7.7 K (4), J/kB is estimated at -0.54 K (1), -0.34 K (2), -0.22 K (3), and -0.56 K (4), respectively. This average J parameter is comparable with

Figure 9. Temperature dependence of the χT product for 5 at 1000 Oe; black circles are experimental data and the red solid line is the best fit obtained with the model described in the text.

similar syn-anti carboxylate bridged MnIII-MnII interactions reported in the literature21 and also the value obtained for compound 5 discussed below. Magnetic properties of compound 5 have also been studied as shown in Figure 9 by the plot of χT vs T. The χT value at room temperature is 10.6 cm3 K mol-1 and upon cooling, the χT product at 1000 Oe steadily decreases till 50 K (10.2 cm3 K mol-1) before decreasing rapidly down to 3.75 cm3 K mol-1 at 1.8 K. While compounds 1-4 exhibit a long-range ferrimagnetic ordering at low temperatures, this compound stays paramagnetic down to 1.8 K. Its thermal behavior is indicative of dominant antiferromagnetic interactions within the onedimensional network of MnII and MnIII metal ions. The experimental data have been fitted to a Curie-Weiss law above 1.8 K leading to a Curie constant (C) of 10.8 cm3 K mol-1 and a Weiss constant (θ) of -3.7 K. The Curie constant is in

584 Crystal Growth & Design, Vol. 9, No. 1, 2009

agreement with the expected value of 10.375 cm3 K mol-1 expected for one MnII metal ion and two MnIII ions at high temperatures. On the other hand, the negative θ further confirms the presence of antiferromagnetic interactions between spin carriers. According to the structure, the magnetic properties of this compound were modeled using a regular chain model (one J value through syn-anti carboxylate bridge between MnII and MnIII) of alternating s ) 2 and S ) 5/2 spins in addition to a Curie contribution for the mononuclear MnIII counterion. Using the approach developed by Fisher et al.22 for a Heisenberg chain of ferromagnetically coupled classical spins and the Hamiltonian N ˆ ) -2J ∑ Si · si, with si ) 2 and Si ) 5/2, the fit of the H i)1

experimental data above 5 K gives gav ) 2.04(1) and J/kB ) -0.55(1) K (Figure 9). As expected for a syn-anti carboxylate bridge between MnII and MnIII and as already seen for 1-4, the interactions are antiferromagnetic and weak.21 It is worth noting that the magnetic anisotropy of the MnIII metal ions has been neglected in this approach and is included phenomenologically in the exchange parameter that is hence probably slightly overestimated. Conclusion Four novel 3D diamondoid MOFs [Mn3(3-Me-sal)4(py)4]n (1), [Mn3(4-Me-sal)4(py)4(MeOH)]n · n(H2O) (2), [Mn3(5-Me-sal)4(py)4(H2O)2]n · n(MeOH) (3), and [Mn3(3-Me-sal)4(4-Me-py)4]n (4) and a 1D coordination polymer {[Mn2(4-Me-sal)2(4-Mepy)2(H2O)2(MeOH)2][Mn(4-Me-sal)2(4-Me-py)2]}n (5), have been obtained by room temperature synthesis using the mixed ligand systems of Mn-substituted salicylic acid-pyridine. To our knowledge, these compounds are the first MOFs produced by this mixed ligand system substituted methylsalicylicilatepyridine. Compounds 1, 2, 3, and 5 have been successfully characterized by X-ray crystallography, whereas powder XRD measurements indicate that compound 4 is isostructural to compounds 1, 2, and 3, possessing a 3D diamond topology. In all these compounds, the structure is reflected by the coordination environment of MnII. When MnII is 4 or 5 coordinated, a 3D diamond framework is constructed, while for coordination number 6, the arrangement of the ligands can induce a change in the structural motif of the final compound; cis arrangement of the two water molecules in compound 3 results in 3D diamondoid framework, while a trans arrangement in 5 induces the construction of 1D coordination polymer. As seen in these 3D frameworks, the modification of the coordination environment of MnII affects the shape of the diamondoid network that is formed. Magnetic studies show that weak antiferromagnetic interactions are always present between MnII and MnIII metal ions through syn-anti carboxylate bridges. The 3D networks exhibit long-range ferrimagnetic ordering at 7.4 K (1), 4.6 K (2), 3.0 K (3), and 7.7 K (4). The decrease of the critical temperature is, as expected, in line with the increase of the coordination number around the MnII site from four in 1, five in 2, and six in 3 that lower the bond strength and also the magnetic interactions. This result also reinforces the hypothesis that the structures of 1 and 4 are very close (probably a coordination sphere of 4 for the MnII in 4) as suggested by the X-ray analysis. While more studies of these systems are in progress, our current efforts are focused on the application of this mixed ligand synthetic strategy to other metal ions and on the removal of the coordinating water molecules by using anhydrous metal salt precursors. Acknowledgment. The authors are grateful to the European network MAGMANet (NMP3-CT-2005-515767), the EC-TMR

Mukherjee et al.

Network QuEMolNa (MRTN-CT-2003-504880), the University of Bordeaux, the CNRS, the Re´gion Aquitaine, the DFG (CFN and SPP-1137) for financial support. Supporting Information Available: X-ray crystallographic files in CIF format for the structure determination of compounds 1, 2, 3, and 5. Figures S1-S7 and Tables S1-S4 with selected bond distances (Å) and bond angles (°). Magnetic data and mean-field theory for compounds 2, 3, and 4. This information is available free of charge via the Internet at http://pubs.acs.org.

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