Novel Bimetallic-Dicyanamide Extended Two- and Three-Dimensional

Jul 31, 2009 - ... Universidad de Granada, 18071-Granada, Spain. ‡ Department of Chemistry, Faculty of Sciences M'hannech, University of Tétouan, Této...
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DOI: 10.1021/cg9003438

Novel Bimetallic-Dicyanamide Extended Two- and Three-Dimensional Networks through [Cu(rac-CTH)]2þ Cation Templation

2009, Vol. 9 4102–4107

Jose Suarez-Varela,† Jose Marı´ a Moreno,*,† Ikram Ben Maimoun,‡ Francesc Lloret,§ Jerzy Mrozinski,^ Raikko Kivekas,# and Enrique Colacio*,† †

Departamento de Quı´mica Inorg anica, Facultad de Ciencias, Universidad de Granada, 18071-Granada, etouan, Spain, ‡Department of Chemistry, Faculty of Sciences M’hannech, University of T anica/Instituto de Ciencia Molecular (ICMol), T etouan, Morocco, §Departament of Quı´mica Inorg ^ Universitat de Valencia, Polı´gono la Coma s/n, 46980 Paterna (Valencia), Spain, Institute of Chemistry, University of Wroclaw, 14 F. Joliot-Curie, 50383 Wroclaw, Poland, and #Department of Chemistry, Laboratory of Inorganic Chemistry, P.O. Box 55, FIN-00014, University of Helsinki, Finland Received March 27, 2009; Revised Manuscript Received July 7, 2009

ABSTRACT: Reaction of [Cu(rac-CTH)]2þ with dicyanamide and Mn2þ or Fe2þ produces three bimetallic extended structures. Complexes 1 and 2, both obtained in the same reaction pot, consist of two different (4,4) two-dimensional (2D) layers, with the [Cu(rac-CTH)(dca)]þ cations noncoordinated and coordinated to the Mn(μ1,5-dca) network for 1 and 2, respectively (rac-CTH=racemic-5,5,7,12,12,14-hexamethyl-1,4,8,11-tetraazacyclotetradecane). Complex 2 shows an unusual monodentate coordination mode through the central amide nitrogen of the dca ligand which is first reported for a polynuclear complex. The structure of 3 consists of a three-dimensional network which can be described as a stacking of layers made of heptanuclear defective cubane units, very similar to those found in 1. The magnetic properties of 3 are indicative of antiferromagnetic coupling with a magnetic phase change occurring at 19 K.

Introduction Crystal engineering of a homo- and heteroleptic extended system based on dicyanamide complexes has attracted much attention not only because of their structural and topological richness but also because of their physicochemical properties (microporosity, molecular sensors, nonlinear optical activity, heterogeneous catalysis, or magnetic properties).1 The former arises from the variability in coordination modes, from monoto pentadentate, that dca can display, as well as the common existence of polymorphism and formation of multiple products in a single reaction.1 Dicyanamide-bridged complexes exhibit interesting magnetic properties, such as magnetic switches, molecule-based magnets, and spin-crossover,2 which have been the driving force of exhaustive research in the past few years. Alternative routes to prepare new systems different from the well-known R-M(dca)2 involve the use of a coligand,1,3 acting in a terminal or bridging mode, or the use of a cation template in [M(dca)3]- and [M(dca)4]networks.4 In this latter strategy, paramagnetic metal complexes can be used as countercations leading to a variety of heterometallic systems.5 Following this line and using [Cu(rac-CTH)]2þ (rac-CTH = racemic-5,5,7,12,12,14-hexamethyl-1,4,8,11-tetraazacyclotetradecane), we have succeeded in obtaining a series of two- and three-dimensional (2D and 3D) polymeric anionic metal-dca complexes containing bidentate μ1,5-dca bridges.4,5b In these compounds, the copper countercations can be coordinated, semicoordinated through H-bonds, or noncoordinated to the extended network. Herein we report the synthesis, structure, and magnetic properties of three new heterometallic complexes obtained by the reaction of [Cu(rac-CTH)]2þ with dca- and *To whom correspondence should be addressed. E-mail: jmoreno@ ugr.es (J.M.M.), [email protected] (E.C.). pubs.acs.org/crystal

Published on Web 07/31/2009

Mn2þ or Fe2þ: [Cu(rac-CTH)(dca)]2{Mn(μ1,5-dca)6[Mn(dca) (H2O)2]2} 3 6H2O (1), [Mn(μ1,5-dca)4Cu(rac-CTH)] (2), and [Cu(rac-CTH)(dca)]2[Fe3(μ1,5-dca)8(H2O)2] 3 3H2O (3). Experimental Section Materials. The macrocycle rac-CTH was prepared by the method reported in the literature.6 The purity was checked by elemental analysis, IR spectroscopy, and 1H NMR. All other chemicals were purchased from commercial sources and used as received. Synthesis of the Complexes [Cu(rac-CTH)(dca)]2{Mn(μ1,5-dca)6[Mn(dca) (H2O)2]2} 3 6H2O (1) and [Mn(μ1,5-dca)4Cu(rac-CTH)] (2). An aqueous solution (10 mL) of Cu(rac-CTH)(ClO4)2 (0.06 g, 0.1 mmol) was added dropwise to an aqueous solution (10 mL) of Na(dca) (0.05 g, 0.056 mmol) and Mn(NO3)2 3 4H2O (0.04 g, 0.15 mmol). The resulting purple solution was stirred at 50 C and allowed to evaporate at room temperature. Crystals were obtained within a few days which were washed with water and air-dried. It should be noted that in the majority of the syntheses only crystals of 2 were obtained. Crystals of 1 and 2 were separated manually. Moreover, despite numerous attempts not enough sample could be obtained for recording accurate magnetic data for complex 1. Complex 1. Found: C, 36.70; H, 5.49; N, 31.31; Cu, 7.40; Mn, 9.68. Calc. for C52H92Cu2Mn3N38O10: C, 36.72; H, 5.46; N, 31.31, Cu, 7.41, Mn, 9.70%. IR (νmax/cm-1, KBr disk): 3431br (OH), 3201s (NH), 2289s, 2246m, 2230s, 2219w and 2177s (conj. CN). Complex 2 (33.6 mg, 40%). Found: C, 43.36; H, 5.59; N, 33.49, Cu, 9.51, Mn, 8.37. Calc. for C48H72Cu2Mn2N32: C, 43.21; H, 5.44; N, 33.59, Cu, 9.53, Mn, 8.23%. IR (νmax/cm-1, KBr disk): 3255m and 3151s (NH), 2297s, 2242s, 2218m, 2196w and 2173s (conj. CN). Synthesis of the Complex [Cu(rac-CTH)(dca)]2[Fe3(μ1,5-dca)8(H2O)2] 3 3H2O (3). An aqueous solution (20 mL) of Cu(rac-CTH)(ClO4)2 (0.06 g, 0.1 mmol) was added dropwise to an aqueous solution (20 mL) of Na(dca) (0.05 g, 0.056 mmol) and Fe(SO4)2 3 7H2O (0.047 g, 0.17 mmol). The resulting blue (3) solution was stirred at 50 C and left to evaporate at room temperature. Crystals were obtained within a few days which were washed with water and air-dried (36.5 mg, 40%). Found: C, 38.70; H, 4.20; N, 27.42, Cu, 7.56, Fe, 10.20. Calc. for C52H82Cu2Fe3N38O5: C, 39.00; r 2009 American Chemical Society

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Table 1. Crystal Data and Structure Refinement for [Cu(rac-CTH)(dca)]2{Mn(μ1,5-dca)6[Mn(dca) (H2O)2]2} 3 6H2O (1), [Mn(μ1,5-dca)4Cu(rac-CTH)] (2), and [Cu(rac-CTH)(dca)]2[Fe3(μ1,5-dca)8(H2O)2] 3 3H2O (3) 1 2 3 empirical formula formula weight space groupa a/A˚ b/A˚ c/A˚ β/ V/A˚3 Z Fcalc/g cm-3 μ/mm-1 R(int) R(F)b [I > 2σ(I)] Rw(F)c [I > 2σ(I)] GOF a

C52H92Cu2Mn3N38O10 1701.54 C2/c 31.4115(19) 11.5964(7) 23.1449(15) 112.0030(10) 7816.7(8) 4 1.446 1.082 0.0532 0.0563 0.1263 0.904

C48H72Cu2Mn2N32 1334.34 P21 8.7720(7) 12.5373(11) 14.2431(12) 99.244(2) 1546.1(2) 1 1.433 1.140 0.0291 0.0566 0.1114 0.987

C52H82Cu2Fe3N38O5 1614.19 Pca21 29.846(17) 21.310(8) 12.090(8) 90 7689(7) 4 1.394 1.165 0.1200 0.1109 0.2737 1.007

Flack parameter for 2 = 0.0557(0.0206). b R = Σ||Fo| - |Fc||/Σ|Fo|. c Rw = [Σw(Fo2 - Fc2)2/Σw(Fo2)2]1/2.

H, 4.53; N, 27.98, Cu, 7.93, Fe, 10.46%. IR (νmax/cm-1, KBr disk): 3589m and 3437s (OH), 3230s and 3185s (NH), 2263s, 2220s, 2209w, 2185m, 2151s and 2127s (conj. CN). Single Crystal X-ray Diffraction. Intensity data of compounds 1-3 were collected with a Bruker SMART CCD diffractometer at 298(2) K for 1 and 2 and 294(2) K for 3. Reflections with negative intensities were not collected. In each case, Mo KR radiation (λ= 0.71073 A˚, graphite monochromator) was used and the scan type was ω/2θ. The data were corrected for Lorentz-polarization effects, and empirical corrections for absorption were carried out using the SADABS program. The structures were solved by direct methods combined with subsequent Fourier analysis. In compound 1 two of the dca ligands have the Namide disordered while in 2 there is only one. In 1 and 2 all non-hydrogen atoms were refined anisotropically. Hydrogen atoms were placed on calculated positions (riding model) except water hydrogen atoms for 3 which were omitted. Several crystals of 3 were tested, all of them showing very poor reflection power and statistics of reflections, in view of the fact that only metal atoms were refined anisotropically. Crystallographic data are listed in Table 1. CCDC reference numbers are 694344-694346 for complexes 1-3, respectively.

Results and Discussion Crystal Structures. The structure of 1 (Figure 1a) consists of 2D {Mn(μ1,5-dca)6[Mn(dca) (H2O)2]2}2- anionic layers, [Cu(rac-CTH)(dca)]þ cations located in the cavities of the layers and crystal water molecules, all of them involved in an intricate network of hydrogen bonds. Within the layers, there are two kinds of Mn(II) atoms, Mn(1) and Mn(2), with MnN4O2 and MnN6 coordination environments, respectively, the nitrogen atoms belonging to nitrile groups of μ1,5-dicyanamide ligands and oxygen atoms of water molecules. The Mn(1) atom, with a distorted octahedral geometry, is coordinated to three μ1,5-dicyanamide bridging ligands, one terminal dicyanamide ligand and two molecules of water. The three former are arranged in a fac configuration around Mn(1) and consequently the water molecules are placed in cis positions. The Mn(1)-N and Mn(1)O distances are in the ranges 2.186(4)-2.214(4) A˚ and 2.224(3)-2.238(3) A˚, respectively, whereas cis and trans bond angles range from 84.89(15) to 95.79(16) and from 170.59(16) to 178.39(13), respectively. The Mn(2) atom, which lies on a center of symmetry, exhibits a minimally distorted octahedral geometry with very close Mn-N distances of 2.204(4), 2.211(4), and 2.223(4) A˚, cis N-Mn-N angles in the range 88.76(18)-91.24(19) and crystallographically imposed trans angles of 180.

Single μ1,5-dicyanamide bridges connect each Mn(2) atom to six Mn(1) atoms and each Mn(1) atom to three Mn(2) atoms, giving rise to an anionic CdI2 type layer structure made of fused Mn(2)3Mn(1)4 heptanuclear defective cubane units with Mn1 3 3 3 Mn2 distances of 8.591(3), 8.692(3), and 8.563(3) A˚ (Figure 1). In the CdI2 description, Mn(2) and Mn(1) would occupy the Cd and I positions, respectively. This structure is first described here for dca bridging ligands but already described for CN bridging groups.8 Template [Cu(rac-CTH)(dca)]þ cations are located into the cubane cavities adopting a charge compensating and space-filling role in the material. The Cu(II) atom exhibits a distorted square-pyramidal coordination geometry with τ=0.15 (τ=0 for an ideal square-pyramid and τ=1 for an ideal trigonalbipyramid). Four short bonds of about 2.0 A˚ are formed in equatorial positions with the nitrogen atoms of the macrocycle, whereas the nitrile nitrogen atom of a nonbridging dicyanamide ligand occupies the axial position at a longer distance of 2.236(4) A˚. When viewed down the b axis, sheets are built of quasisquare tubes (Figure 1c), with the Mn(1) and Mn(2) atoms occupying the corners and the dicyanamide bridging ligands the sides of the square. Because of the existence of centers of symmetry at the barycenter of the squares and at the Mn(2) atoms, the same type of Mn(II) atoms, Mn(1) or Mn(2), are lined at diagonally opposite edges of the tube. The Mn(1) 3 3 3 Mn(1) and Mn(2) 3 3 3 Mn(2) distances across the diagonals are 11.438(1) and 12.944(1) A˚, respectively. Each tube shares the Mn(1) containing edges with two neighboring tubes to form 2D sheets with a unique topology. The square channels of the sheets are mainly occupied by a part of the backbone of the macrocyclic ligands (Figure 1c). To avoid steric contacts and to maximize hydrogen bonding interactions, each sheet is shifted with respect to the adjacent sheets to give rise to an ABA repeat pattern of layers, with shortest interlayers Mn 3 3 Mn distances of 5.905(1) A˚ for Mn(1) 3 3 3 Mn(1)I (I = x þ 1/2, y þ 1/2, z þ 1). Coordinated water molecules and the noncoordinated end of the terminal dicyanamide ligands bonded to Mn(1) and Cu(1) atoms are oriented outward from the sheets to form a complicated network of hydrogen bonds with symmetrically related atoms of adjacent sheets and with the noncoordinated water molecules that occupy the interlayer space. In the reaction of Cu(rac-CTH)2þ with Mn2þ and dcacomplex 2 represents the major product. As in complex 1, complex 2 also shows a bidimensional structure, but with

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Figure 1. (a) The Mn(2)3Mn(1)4 heptanuclear defective cubane containing the [Cu(rac-CTH)(dca)]þ cation for 1; (b) view of a layer from “a” (rods represent μ1,5-dca); (c) view of the packing from “b” showing the square channels partially occupied by the cations (only two [Cu(rac-CTH)(dca)]þ are shown for simplicity).

some significant differences: the formulas [Mn(μ1,5-dca)4Cu(rac-CTH)], the corrugated shape of the layers, and the linkage of the [μ1,5-dcaCu(rac-CTH)] unit to the Mn2þ ion. Within the layers, all Mn(II) atoms are equivalent with an octahedral MnN6 environment, and all nitrogen atoms belong to dca ligands. From these six dca ligands, five show a common μ1,5-bridging mode and one an unusual monodentate coordination mode through the central amide nitrogen (Figure 2a). This coordination mode, to the best of our knowledge, has been previously described only four times, always in mononuclear complexes.9 This rare monodentate Namide-coordination mode seems to be related with the formation of strong hydrogen bonds involving one or two of the Nnitrile atoms. In 2 both Nnitrile atoms are involved in hydrogen bonds: N(35) 3 3 3 H-N(12)II (II =-x, y - 1/2, 1 - z) 2.946(1) A˚ and 177.61(1); N(31) 3 3 3 H-N(1) 3.126(3) A˚ and 169.79(1), similar to those described for this coordination mode.9 The skeleton of the corrugated sheets (Figure 2b) could be described as formed by the Mn atoms and μ1,5-dca bridging ligands. Since these four dca’s are not in the same plane, which would give rise to

Figure 2. (a) The asymmetric unit of 2; (b) view of a corrugated layer from “c” (rods represent μ1,5-dca); (c) view of the packing from “a”.

a planar square (4,4) 2D sheet, but in a nearly sawhorse (C2v) geometry, the layers adopt a corrugated arrangement of quasi-squares (4,4) with Mn 3 3 3 Mn edge distances of 8.701(1) and 8.772(1) A˚ and diagonal distances of 11.419(9) and 13.225(8) A˚. The remaining [μ1,5-dcaCu(racCTH)] and η1-dca moieties are placed in cis positions, favored by the intramolecular previously described H-bond, and located outward from the layers. The Cu(II) atom exhibits an almost perfect square-pyramidal coordination geometry with τ = 0.06 in which four equatorial bonds between 2.002(3) and 2.037(4) A˚ are formed with rac-CTH while dicyanamide ligand occupies the axial position at a longer distance of 2.219(5) A˚. The layers stack along the c axis in a AAA fashion with a Mn 3 3 3 MnIII (III = x, y, z þ 1) interlayer separation of 14.243(1) A˚, so the outward groups located on a layer fit

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perfectly in the intervalley separation of the upper layer (Figure 2c). The crystal structure of complex 3 consists of an anionic 3D network of formulas [Fe3(dca)8(H2O)2]2-, [Cu(racCTH)(dca)]þ cations and crystal water molecules. The Fe-μ1,5-dca network is formed by three kinds of Fe(II) ions: one FeN6 type and two FeN5O type (N=nitrile nitrogen of dca and O = water molecule), all of them showing an octahedral geometry. The Fe(1) is connected to six Fe centers, three Fe(2) and three Fe(3) in a fac mode. The Fe(2) and Fe(3) belonging to FeN5O are connected to three Fe(1) ions in a fac fashion, to two Fe(2) or Fe(3) in cis position and a water molecule (O(1) and O(2), respectively). Completing the structure are three crystal water molecules and two nonequivalent [Cu(rac-CTH)(dca)]þ cations, these latter playing templating and charge compensating roles. The Cu(II) atoms show a square-pyramidal geometry (equatorial Cu-Nmacrocycle between 1.987(9) and 2.070(10) A˚) distorted toward trigonal-bipyramid (τ = 0.31 and 0.35 for Cu(1) and Cu(2), respectively) with the dca ligand coordinated at the axial position (Cu-N = 2.268(9) and 2.217(10) A˚ for Cu(1) and Cu(2), respectively). The noncoordinated nitrile nitrogen atom of the dca ligands are involved in relatively strong hydrogen bonds with water molecules (N(50) 3 3 3 O(3) = 2.782(1) A˚, O(3) 3 3 3 O(2) = 2.669(1) A˚ and N(45) 3 3 3 O(1) = 2.721(1) A˚), linking these two [Cu(rac-CTH)(dca)]þ cations to the FeN5O centers with a minimum Fe 3 3 3 Cu distance of 7.008(3) for Fe(2) and Cu(2). The 3D network could be described as a stacking of pseudosheets along the a axis which are covalently bonded to each other. The layer in the bc plane consists of a bidimensional packing of heptanuclear defective cubanes (Figure 3a), an almost identical pattern to that previously described for complex 1. This arrangement arises from the fac isomerism existing in the structure: the Fe(1) connect to three Fe(2) and three Fe(3) in a fac fashion as do both the Fe(2) and Fe(3) with Fe(1). The upper (or lower) layer is related by a symmetry plane and displaced along the c axis so that the Fe(2) or Fe(3) of the upper layer is just centered between two Fe(2) or Fe(3) of the lower layer to which it is bonded (Figure 3b). Thus, a stacking of the type ABA yields the tridimensional network. When the network is viewed from the c axis (Figure 3b) two kinds of channels can be distinguished perpendicular to the ab plane: quasi-squares (Fe1-Fe2-Fe1-Fe2 and Fe1-Fe3-Fe1-Fe3) corresponding to the faces of the above-mentioned defective cubanes of the layers (Fe1 3 3 3 Fe1= 11.999(4) and 12.689(4) A˚, Fe2 3 3 3 Fe2= 11.770(4) A˚ and Fe3 3 3 3 Fe3= 11.369(5) A˚) and two types of six membered channels between the AB layers: elongated (Fe1 3 3 3 Fe1= 14.720(7) and Fe2 3 3 3 Fe3= 13.368(4) A˚) and flattened (Fe1 3 3 3 Fe1= 17.501(9) and Fe2 3 3 3 Fe3= 10.901(3) A˚). The [Cu(racCTH)(dca)]þ cations and the water molecules are placed in these hexagonal channels: the Cu(2)-dma 3 3 3 O(3) 3 3 3 O(1)Fe(3) in the elongated ones and the Cu(1)-dma 3 3 3 O(2)Fe(2) in the flattened ones. Besides the unusual monodentate Namide-coordination mode of dca in 2, the rest of the dca ligands in complexes 1-3 show bond lengths and angles similar to those already reported.1 It has been pointed out that Mn-N-C angles in Mn-μ1,5-dca-Mn linkages deviate significantly from linear arrangement.10 We have analyzed the M-μ1,5-dca-M (M = Mn and Fe) moieties included in the Cambridge Structural

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Figure 3. (a) Atomic labeling scheme for the 2D network (water molecules and [Cu(rac-CTH)(dca)]þ cations omitted for clarity); (b) a view of the pseudolayer formed for defective cubanes from “a” for 3; (c) view of the 3D structure from “c”, showing the square and hexagonal channels (rods represent μ1,5-dca in both figures).

Figure 4. Plot of the average value of both M-N-C angles in single or double μ1,5-dca bridged complexes of Mn and Fe found in the Cambridge Structural Database.

Database,11 with the results shown in Figure 4. At first sight there is a large range of values: from 142.48 to 178.71 for Mn and from 143.23 to 177.41 for Fe; however, when we

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Figure 5. Temperature dependence of χMT and field dependence of M (inset) for 2.

consider the average value of both M-N-C angles in metal ions linked by single or double μ1,5-dca bridges a clear tendency can be outlined: the average value for double M-(μ1,5-dca)2-M bridges is clearly smaller (159.06 and 159.97 for Mn and Fe, respectively) than the corresponding to single dca bridge (166.28 and 168.60 for Mn and Fe, respectively). The greater dispersion of the data for single M-(μ1,5-dca)-M is consistent with a greater variety of the structures compared with those having double dca bridges. In our complexes, the values roughly follows this trend; thus, the average M-N-C angles values for M-(μ1,5-dca)-M are 165.56, 164.03, and 165.10 for 1-3, respectively. Furthermore, the values for 1 and 3 are very similar as are their packing of heptanuclear defective cubanes. All this suggest that the values of the M-N-C angles depend more on the type of bridge, single or double, than on other factors such as the structural geometry or nature of the metal centers. Nevertheless, more examples than the 24 analyzed (16 for Mn12 and 8 for Fe12g,12i,12l,13) are needed in order to confirm this hypothesis. Magnetic Data. The temperature dependence, between 2 and 300 K, of the χMT product of 2 is shown in Figure 5. The value of 4.76 cm3 mol-1 K agrees well with that expected for isolated Cu2þ (S=1/2) and Mn2þ (S=5/2) ions with g=2 of 4.75 cm3 mol-1 K. Upon cooling, the χMT values slightly decrease until a temperature of 50 K is reached and then decreases more rapidly to a value of 2 cm3 mol-1 K at 2 K. This behavior is indicative, as expected in view of the structure,14 of weak antiferromagnetic coupling between Mn(II) ions through the μ1,5-dca bridges combined with very small single-ion zero-field splitting effects.12j,12l,15 Taking into account that dca coordinates Cu(II) in axial position where the electronic density of the unpaired electron is expected to be, if any, very weak, magnetic data were fit to the Lines’16 model for a 2D classical square lattice Heisenberg (H = 2JS1S2) antiferromagnet, including a Curie term to account for the copper(II) contribution. The best fit values were J=-0.136(1) cm-1 with g=2.004(1) and C fixed to 0.375. This small J value is similar to those observed for other 2D Mn(II) complexes containing either double or double and single five-atom dicyanamide bridges.14,15 The M vs H plot at 2 K (see inset Figure 5) continuously increases to reach a value of 5.65 Nβ at the maximum applied field value of 5 T, which is only slightly lower than the expected saturation value of Ms=Ngβ(SMn þ SCu)=6. The magnetic properties of 3 in the form of χMT vs T plot are shown in Figure 6 (top). At room temperature in a field of 0.25 T, the χMT has a value of 9.24 cm3 mol-1 K, which is in

Figure 6. (Top) Temperature dependence of χMT and 1/χM; (bottom) field dependence of M for 3.

agreement with the expected value of 9.38 for isolated metal ions, three Fe2þ (S=2) and a Cu2þ (S=1/2) with gCu=gFe= 2. The magnetic susceptibility in the range 100-300 K obeys the Curie-Weiss law with a Curie constant C = 9.33 cm3 mol-1 K and θ= -27 K. Upon cooling, χMT values slightly decrease to 8.15 cm3 mol-1 K at 20 K, and then exhibits a sharp increase to a maximum value of 11.5 cm3 mol-1 K at 15 K before decreasing to reach a minimum value of 4.32 cm3 mol-1 K at 2 K. The low temperature magnetic behavior could be indicative of the presence of a magnetically ordered state with a net magnetization below 19 K, while the decrease in χMT below 19 K might be due to saturation effects. The magnetic hysteresis loop for 3 shows values of coercitive field and remanent magnetization (Mr) of 60 G and 0.016 Nβ, respectively, which are typical of a very soft magnet (Figure 6, bottom). The isothermal magnetization for MnCu unit at 2 K attains a value of 6.4 Nβ at 5 T, which is significantly smaller than the theoretical saturation value (Ms) of 14 Nβ. All of the above magnetic properties are compatible with a weak ferromagnetism at low temperature caused by noncompensated antiferromagnetic spin-canting. However, this behavior could also be due to an impurity of Fe(dca)2 having a Tn of 19 K.17 To clarify this fact we have carried out seven new syntheses and checked all the crystals used in the magnetic data collection. In six of these, the magnetic behavior is identical to that described above but in one the maximum at 15 K is missing by which it is likely attributed to an impurity of Fe(dca)2.

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

Acknowledgment. This work was supported by the MEC (Spain) (Projects CTQ2005/0935 and CTQ2008/02269/ BQU), the Junta de Andalucı´ a (FQM-195) and the Universidad de Granada. Supporting Information Available: X-ray crystallographic data in CIF format for 1-3. This information is available free of charge via the Internet at http://pubs.acs.org.

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