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Structures and magnetic properties of transition metal complexes of 1,3,5-benzenetricarboxylic acid
Inorganica Chimica Acta 342 (2003) 193 /201 www.elsevier.com/locate/ica
Structures and magnetic properties of transition metal complexes of 1,3,5-benzenetricarboxylic acid Wanru Zhang a, Sandra Bruda b, Christopher P. Landee a,, Judith L. Parent b, Mark M. Turnbull b, b
a Department of Physics, Clark University, 950 Main Street, Worcester, MA 01610, USA Carlson School of Chemistry and Biochemistry, Clark University, 950 Main Street, Worcester, MA 01610, USA
Received 1 March 2002; accepted 23 May 2002
Abstract Treatment of the potassium salt of 1,3,5-benzenetricarboxylic acid with transition metal ions in aqueous solution produced a series of coordination complexes. Reaction of Ni(ClO4)2 ×/6H2O with K3[C6H3(CO2)3] gave complexes 1, Ni3[C6H3(CO2)3]2(H2O)14 ×/ 4H2O. The compound crystallized as trimers with two terminal Ni(H2O)5 units bridged by two benzenetricarboxylate ligands to a central Ni(H2O)4. Magnetic susceptibility data indicate the presence of either weak antiferromagnetic interactions between metal ions, or single ion anisotropy. Reaction of Co(ClO4)2 ×/6H2O with K3[C6H3(CO2)3] gave complexes 2, Co3[C6H3(CO2)3]2(H2O)12. The complexes exists as a pendant chain coordination polymer with both bridging and terminal Co2 ions. Each cobalt center is also coordinated to four water molecules. A systematic linear decrease in effective moment is observed below 100 K. This decrease can be understood entirely in term of crystal field and spin-orbit coupling effects on the individual Co(II) ions and implies magnetic interaction are weak. Magnetic susceptibility data were also obtained for the Co /BTCA complexes 3, Co(BTCA)(py)2 ×/0.67py. These data can also be described as arising solely from single-ion effects due to the non-cubic environment around the Co2 ion with no evidence of the presence of magnetic interactions. Reaction of Mn(ClO4)2 ×/6H2O with K3[C6H3(CO2)3] gave complexes 4, Mn3(C6H3(CO2)3)2 ×/8H2O. Magnetic susceptibility data indicate weak antiferromagnetic interactions and suggest that the complexes have a low dimensional structures. # 2002 Elsevier Science B.V. All rights reserved. Keywords: 1,3,5-Benzenetricarboxylate; Magnetochemistry; S /1 trimer model
1. Introduction As part of our ongoing research in molecular basedmagnets, we are interested in developing systems exhibiting spin frustration [1]. In a spin glass material, frustration occurs due to site disorder, but in a molecular magnet it can arise due to geometrical frustration, via competing interactions between magnetic moments of equal magnitude [2 /4]. Thus, an antiferromagnetic 2D Kagome´ lattice based on equilateral triangles will inherently generate frustration, defined as the inability of the system to simultaneously
Corresponding authors. Present address: Depto. De Quimica Fisica Facultad de Quimica, Universidad de Barcelona, Av. Diagonal 647, 08028 Barcelona, Spain (M.M.T.). Tel.: /34-93-402 1220; fax: /34-93-402 1231
minimize the energies of competing interactions [4]. The ground state of such system will be highly degenerate [5]. This feature will lead to unusual magnetic properties, most likely to occur in the low temperature regime [2]. The synthesis and characterization of compounds showing frustration are very important in order to provide an experimental insight for the theoretical models developed to date [6]. Unfortunately, there are few such compounds reported, the majority of them being inorganic oxides [2]. Among the molecular-based complexes, Awaga et al. [7] reported 2D compounds based on paramagnetic radicals, but in which the antiferromagnetic interaction was established between ferromagnetically coupled radical dimers. Miller et al. [8] and Murray et al. [9] studied the tricyanomethanide complexes Cr[C(CN)3]2 and V[C(CN)3]2 by a variety of magnetic, neutron powder
0020-1693/02/$ - see front matter # 2002 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 0 - 1 6 9 3 ( 0 2 ) 0 1 1 3 9 - 8
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diffraction and specific heat measurements. In these systems though, the equilateral triangular lattice is distorted towards isosceles, relieving the frustration. Our synthetic approach focused on 1,3,5-benzenetricarboxylic acid (BTCAH3) and 1,3,5-benzenetricarboxlyate (BTCA) as a ligand, since its threefold symmetry makes it a very attractive choice for obtaining a (6,3) type structure [10], the framework of a Kagome´ lattice (Fig. 1(a)). However, the unpredictable factor in designing the desired product comes from the fact that each carboxylate group can coordinate in a variety of modes: unidentate (I), m-1,3-bridging (II), m-1,1-bridging (III) or chelating (IV) (Fig. 1(b)). A literature survey shows that the BTCA ligand has attracted a great deal of interest in the synthesis of zeolite-like materials [11 /13] in addition to a smaller number of simple coordination complexes [14]. The geometry of the ligand allows for large size pores in such open metal/organic frameworks. Spectacular results were obtained by Yaghi et al., who synthesized an extremely stable compound showing reversible selective inclusion [12]. Another of his compounds CoC6H3(COOH1/3)3(py)2 ×/2/3py (py /pyridine) [11] is also able to selectively include guest molecules. Its
structures is based on a triangular pattern of metal centers, but no magnetic data were reported. A copper(II) based nanoporous material was reported by Williams et al. [15,16]. Its magnetic properties show cooperative behavior arising from the presence of copper dimers [15]. Suh et al. [17] were able to generate a Kagome´ lattice based on a nickel(II) macrocycle, but the reported anti-ferromagnetic interaction was very weak (J //1 cm 1). Additional work has been done with copper complexes of the related aryl carboxylate ligands phthalate [18], terephthalate [19,20] and benzenetetracarboxylate [19,21]. Both ferromagnetic [18] and antiferromagnetic [19] magnetic interactions have been found. The strength of the magnetic interaction through carboxylate groups and a benzene ring can be very large (/70 cm 1) [19]. We were interested in BTCA due to its propensity to intermediate superexchange interactions between magnetic centers [22,23]. However, there are also disadvantages in using it. First, the distance between two metal centers will be quite large, decreasing the strength of the interaction. Second, the MNDO calculations [24] showed that the HOMO orbitals of BTCA are doubly degenerate and consist mainly of pp-orbitals of the carbon and oxygen atoms. This will lead to incompatibility, or poor overlap between the magnetic orbitals of the metal and ligand, respectively [25]. However, the threefold symmetry of the ligand was sufficient to warrant investigation of its ability to propagate magnetic exchange. Here we report the results of our investigations of nickel, cobalt and manganese complexes of BTCA.
2. Experimental
Fig. 1. (a) Two types of lattices that can be generated by using the BTCA ligand: Kagome´ and triangular; (b) coodination modes for the carboxylate group.
1,3,5-Benzenetricarboxylic acid was purchased from Aldrich Chemical Co. and used without further purification. Metal chlorides and perchlorates were purchased from Aesar and the perchlorates were dried under vacuum prior to use. Infrared spectra were taken on a Perkin/Elmer 467, 1330, or Paragon 500 spectrophotometer in KBr matrix and calibrated against polystyrene (br, broad; s, strong; m, medium). Elemental analyses were performed by the University Instrumentation Center of the University of New Hampshire. Caution: although we have not encountered any difficulties in working with these compounds, the potential for explosion with perchlorate salts is well documented. We have prepared related complexes from both the perchlorate and chloride salts and can detect no difference in the products. The perchlorates were used initially for ease of detection of impurities by IR.
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2.1. tetradecaaquabis(Benzenetricarboxalato)trinickel(II) tetrahydrate (1)
2.4. octaaquabis(Benzenetricarboxalato)trimanganese(II) (4)
Ni3[C6H3(CO2)3]2(H2O)14 ×/4H2O: 1,3,5-benzenetricarboxylic acid (1.05 g, 4.90 mmol) was combined with KOH (0.85 g, 15 mmol) in 20 ml of water with stirring. Additional solid KOH was added until complete solution was attained. Ni(ClO4)2 ×/6H2O (2.75 g, 7.44 mmol) was dissolved in 15 ml of H2O and added dropwise to the BTCA solution. A green precipitate formed immediately. The mixture was stirred one additional hour and then filtered. The ppt was washed with H2O (3 /20 ml) and acetone (2 /15 ml) and air-dried to give a light green powder, 1.84 g (80%). Crystals suitable for structural determination were grown by slow diffusion of the aqueous reagents through a glass wool plug in the center of a 19 cm column. IR (KBr) */n 3350s br, 1610s, 1540s, 1440s, 1370s, 720m cm 1. Anal. Calc. for C18H42Ni3O3O: C, 23.64; H, 4.63. Found: C, 23.71; H, 4.41%.
Mn3[C6H3(CO2)3]2 ×/8H2O: 1,3,5-Benzenetricarboxylic acid (1.05 g, 5.0 mmol) was combined with KOH (0.88 g, 16 mmol) in 20 ml of water with stirring. Additional solid KOH was added until complete solution was attained. MnCl2 ×/4H2O (1.49 g, 7.53 mmol) was dissolved in 20 ml of H2O and added dropwise to the BTCA solution. A cream colored precipitate formed immediately. The mixture was stirred an additional hour and then filtered. The ppt was washed with H2O (4 /20 ml) and acetone (4 /10 ml) and air-dried to give an offwhite powder, 1.81 g (98%). Combustion analysis suggested non-stoichiometric hydration, so the compound was recrystallized from water (0.100 g/250 ml) to give a tan powder, (93% recovery). IR (KBr) */n 3350s br, 1610s, 1540s, 1440s, 1370s, 720m cm1. Anal. Calc. for C18H22Mn3O20: C, 29.90; H, 3.07. Found: C, 29.81; H, 3.05%. 2.5. Magnetic data collection
2.2. dodecaaquabis(Benzenetricarboxalato)tricobalt(II) (2) Co3[C6H3(CO2)3]2(H2O)12: 1,3,5-Benzenetricarboxylic acid (1.05 g, 5.00 mmol) was combined with KOH (0.85 g, 15 mmol) in 25 ml of water with stirring. Additional solid KOH was added until complete solution was attained. Co(ClO4)2 ×/6H2O (2.74 g, 7.49 mmol) was dissolved in 10 ml of H2O and added dropwise to the BTCA solution. A pink precipitate formed immediately. The mixture was stirred an additional 2 h and then filtered. The ppt was washed with H2O (4 /10 ml) and acetone (4 /10 ml) and air-dried to give a pink powder, 1.74 g (86%). Crystals suitable for structural determination were grown by slow evaporation of the filtrate. IR (KBr) */n 3350s br, 1610s, 1540s, 1440s, 1370s, 720m cm 1. Anal. Calc. for C18H30Co3O24: C, 26.78; H, 3.75. Found: C, 26.66; H, 3.75%.
2.3. bisPyridine(benzenedicarboxylatocarboxylic acid)cobalt(II) pyridine clathrate (3) 1,3,5-Benzenetricarboxlyic acid (0.21 g, 1.0 mmol) was dissolved along with Co(NO3)2 ×/6H2O (0.29 g, 1.0 mmol) in 10 ml of EtOH and placed in a small beaker. This beaker was place in a larger beaker containing 2 ml of pyridine and the outer beaker sealed. After 4 days, deep red hexagonal crystals of 3 were collected by filtration. Physical data and the single crystal X-ray structures agreed with those reported by Yaghi et al. [12].
Powder magnetic susceptibility was determined as a function of temperature between 1.8 and 300 K using a commercial vibrating sample magnetometer (VSM) equipped with a continuous-flow helium cryostat [26] or on a commercial Quantum Design SQUID Magnetometer. Magnetic fields up to 1 (VSM) or 5 T (SQUID) were applied. The data were corrected for diamagnetism and temperature independent paramagnetism.
3. Results and discussion 3.1. Synthesis and structures Reaction of Ni(ClO4)2 ×/6H2O with K3BTCA in aqueous solution gave Ni3[BTCA]2(H2O)14 ×/4H2O (1) in 80/ 90% yield. The infrared spectrum of 1 shows a significant amount of water and the usual bands for the carboxylate ion [27]. The absorptions were too broad to distinguish the nature of coordination from the IR spectrum. Crystals suitable for single crystal X-ray diffraction were by slow diffusion of the reactants. An ORTEP drawing showing the structures of 1 is presented in Fig. 2. The structure has been previously reported [28]. Compound 1 crystallizes as trimers of nickel(II) ions with two terminal Ni(H2O)5 units bridged by two BTCA ligands to a central Ni(H2O)4. Ni1 sits on an inversion center and as such the bond angles for all trans pairs of ligands are 1808. The nickel /water distances range from ˚ and these bonds are slightly 2.066(6) to 2.110(6) A ˚ ) to the longer than the Ni /O distance (2.010(6) A carboxylate oxygen.
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Fig. 2. Molecular structures of 1. Hydrogen atoms and water of hydration have been removed for clarity.
The coordination sphere of the terminal Ni(II) ion (Ni2) is also roughly octahedral with cis bond angles ranging from 85.2(2) to 95.6(2)8. The carboxylate groups are all coplanar with their benzene rings and a Ni /O ˚ is found. The five H2O molecules of distance of 2.025 A the terminal Ni(2) have Ni /O distances ranging from ˚ . In addition, four more H2O 2.022(8) to 2.114(5) A molecules are hydrogen-bonded in the lattice; one each near the water molecules trans to the carboxylates on the terminal Ni(2) ions and one each roughly along a line between the central Ni ion and the terminal Ni ions (not shown in Fig. 2). Reaction of Co(ClO4)2 ×/6H2O with K3BTCA in aqueous solution gave immediate precipitation of Co3[BTCA]2(H2O)12 (2) in 85% yield. The infrared spectrum of 2 is indistinguishable from 1. Crystals suitable for single crystal X-ray diffraction study (elongated bricks) were grown from water by slow evaporation of the filtrate. Cobalt complex 2 exists as extended polymeric chains (Fig. 3). This structures has also been previously reported [13a]). The Co1 ions have four water molecules ˚) in equatorial sites (average Co /O distance /2.119(5) A with the axial sites occupied by oxygens of monodentate carboxylates from two different BTCA anions (Co/O ˚ ), similar to the central Ni in 1. average /2.055(4) A
Fig. 3. Portion of the chain structures of 2. Hydrogen atoms have been removed for clarity.
These bonds are slightly longer than the Co /O bonds found in tetraaquabis(2,6-dimethoxybenzoato)cobalt(II) ˚ ) [29]. The second cobalt ion in the repeat unit, (2.071 A Co2, sits on a site of C2 symmetry with distorted octahedral coordination. The coordination sphere around Co2 contains four water molecules and one chelating carboxylate group. The small bite angle of the bidentate carboxylate (61.48) leads to a rather severe distortion, with the cobalt /water distances ranging from ˚ (trans to the carboxylate) to 2.148(4) A ˚ (cis ). 2.026(6) A The trans O(23) /Co(2) /O(23A) angle is somewhat reduced (176.1(3)8), while cis angles to water molecules are in the range of 84.9(2) /98.7(2)8. The chains can be described as uniform [Co(H2O)42 /BTCA3 /]n chains with additional Co(H2O)42 groups appended to alternating BTCA anions. A second Co(II) complex, previously reported by Yaghi and co-workers [12] was also prepared due to its unique symmetry. Vapor diffusion of pyridine into a solution of Co(NO3)2 and BTCAH3 in ethanol yielded dark red hexagonal crystals of Co(BTCAH)(py)2 ×/2/3py, 3, and X-ray analysis verified that they possessed the same structures as previously reported (see Fig. 4). The compound crystallizes in layers with a single cobalt site. Each Co ion is coordinated to three BTCA anions, two via monodentate carboxylates and one via chelation. There are two distinct BTCA ions in the structures. One coordinates to three Co ions via only monodentate carboxylates, while the other coordinates via only chelating carboxylates. This ensures that there is only one potential magnetic superexchange pathway through any given BTCA ligand and preserves the threefold symmetry of the structures and the potential for the complexes to exhibit magnetic frustration. The layers are separated by coordination of pyridine molecules to the axial positions of the Co ions, ensuring good
Fig. 4. Layer structures of compound 3. The disordered coordinated pyridine molecules are not shown for clarity.
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isolation of the layers (only the N atoms of these molecules are shown in Fig. 4). In addition, pyridine molecules intercalate into the structures via p-stacking between the aromatic rings of the BTCA ions of adjacent layers (see Fig. 4). Reaction of MnCl2 ×/4H2O with K3BTCA in aqueous solution gave immediate precipitation of Mn3[BTCA]2 ×/ 8H2O (3) in 85% yield. Repeated recrystallization failed to produce crystals suitable for X-ray study. CHN analysis and magnetic measurements confirmed the stoichiometry.
4. Magnetic properties [Ni3(BTCA)2(H2O)14] ×/4H2O 1: the magnetic susceptibility of a polycrystalline sample of the nickel complex has been measured from 2 to 300 K. A plot of 1/xM versus T is linear in the range 30 /200 K. The Curie/ Weiss constant u determined within this temperature range is near zero within experimental error (see Fig. 5). This small u value indicates that the interactions between ions are very weak. The value of the Curie constant C /1.15(3) cm3 K mol 1 implies a g-factor near 2.16, normal for the Ni(II) ion in an octahedral environment. The powder susceptibility data for compound 1 are plotted as the product xMT versus temperature in Fig. 6. In the high temperature region between 30 and 300 K, the product of xMT is fairly constant, at approximately 3.5 cm3 K mol 1 per mol of trimer. The value of xMT starts to decrease at 30 K until it reaches a value of 1.29 cm3 K mol 1 at approximately 2 K. This decrease in the value of xMT can have several origins. Antiferromagnetic interactions between the nickel ions can align their moments antiparallel, causing a decrease in the product xMT . There are two possible kinds of magnetic interactions within the trimer: a magnetic interaction between
Fig. 6. Plot of xmolT vs. T for compound 1. The solid line is the best fit to the trimer model (see text).
the terminal Ni ions and the central Ni ion, J , and a magnetic interaction between the two terminal Ni ions, J ?. Alternatively, Ni(II) is well known to exhibit singleion anisotropy, in which the ms /9/1 sublevels are split away from the ms /0 sublevel. This splitting introduces an energy gap D and leads to a decrease in the xMT product at low temperature as the fraction of the nickel ions in the excited magnetic states decreases with temperature. Of course, both antiferromagnetic exchange as well as single-ion effects can occur in the same compound. The full Hamiltonian for the isolated trimer in terms of J, J? is written as: H 2J(S1 S2 S2 S3 )2J?S1 S3 D(S21z S22z S23z )
where S1 and S3 are the terminal Ni ions, and S2 is the central Ni ion. We first analyzed the susceptibility in terms of a model of non-interacting spin (J /J ?/0) in the presence of a single-ion anisotropy (assumed to be the same for all three nickel ions). The Hamiltonian is that given in Eq. (1) with J and J? held equal to zero. A positive value of the parameter D means the non-magnetic ms / 0 state lies below the ms /9/1 states. The derivation of the average magnetic susceptibility per mole of trimers of this Hamiltonian has been given elsewhere [30]. It is written as: xM
Fig. 5. Plot of l/xmol vs. T for compound 1 with a fit to the Curie / Weiss law (solid line).
(1)
2Ng2 m2b 2x1 2ex x1 ex kT 1 2ex
(2)
where x /D /kT . The data were fit to Eq. (2), allowing both D and g to vary. The best fit parameters were found to be D /k /10.5(1) K, g /2.15(1). Similar values for D have been reported previously [30,31]. A second model postulated an anti-ferromagnetic interaction between nickel ions in the trimer assuming zero single-ion anisotropy. The Hamiltonian used was a
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simplified form of that given in Eq. (1) where now D is held equal to zero. The expression for the susceptibility of a mole of the trimers by this model [32] is written as:
xM
region predicts a zero-temperature value near 4.8 (cm3 K mol 1), or 1.6 (cm3 K mol1) mol1 of cobalt ions. The isolated Co2(d7) ion in an octahedral crystal
Nmb g2 (6e2J=kT 6eJ?=kT 30e4J2J?=kT 6e4J6J?=kT 30eJ?=kT 84e6J6J?=kT ) 3kT(3e2J=kT e2J?2J=kT 3eJ?=kT 5e4J2J?=kT 3e4J6J?=kT 5eJ?=kT 7e6J6J?=kT )
The data for the nickel complex were fit to Eq. (3) with J? /0 ( linear trimer model). Both the exchange constant J and the g -factor were allowed to vary. The best fit parameters were found for a magnetic interaction strength J /k //0.88(2) K and g /2.22(1). This small value of J is consistent with the Curie /Weiss u value near zero. We also attempted to fit the experimental data with the theoretical calculations of the magnetic susceptibility using the full Hamiltonian (Eq. 1). However, since the exchange coupling constant between the center and terminal nickel ions is very weak, the J and J ? values are highly correlated with the value of the single ion anisotropy. No unique set of J, J ?, D , and g values was found [32]. A more precise determination of the set of magnetic parameters must await the availability of the larger single crystals. [Co3(BTCA)2(H2O)12] (2): the powder magnetic susceptibility data for 2 has been measured between 6 and 300 K. A plot of the product xMT versus temperature is shown in Fig. 7. As this product is related to the effective moment, it is clear the effective moment begins to decrease below 150 K and continues decreasing to a minimum of 5.0 (cm3 K mol 1) at 6 K. The value of xMT between 150 and 300 K is constant at approximately 8.6 (cm3 K mol 1), or a value of 2.87(1) (cm3 K mol 1) per Co ion, as expected for a spin /3/2 ion with gave /2.47. Extrapolating the low temperature linear
Fig. 7. Temperature dependence of the xMT product for 2. Note that there are three cobalt ions per mol of 2.
(3)
field possesses a 4T1 ground state. At higher temperatures (greater than 100 K) the ion can be thought of as a nearly isotropic S /3/2 entity. However, as the result of the tetragonal crystal field distortion, the 4T1g state lies an energy D below a 4Eg excited state. Spin-orbit coupling (l /180 cm 1) divides these two states into a set of six Kramer doublets with an Seff /1/2 doublet state at low energy. At high temperature, the xMT product of the individual ion can range from 2.5 to 3.5 (cm3 K mol 1), depending on the ratio of D /jl j. For lower relative temperature (kT /jlj B/0.5), the xMT product decreases linearly with temperature, reaching a value near 1.7 (cm3 K mol1) as T 0/0 [25]. If any magnetic interactions between cobalt ions are present, their effect will be to cause the xMT product to deviate from its purely linear behavior at low temperatures. It is possible to understand the behavior of the cobalt compound in light of the above discussion without invoking any magnetic interactions. Both cobalt sites (Fig. 4) correspond to octahedral sites with orthorhombic distortions. The cis O /Co /O angles are near 908 for Co1 and near 608 for Co2. The experimental zero temperature limit of xMT , 1.6 cm3 K mol 1 of cobalt ions, is very close to the theoretical value, and the predicted linear growth of xMT is seen between 6 and 70 K. The high temperature limit near 2.9 cm3 K mol 1 of cobalt ions also lies in the middle of the theoretical range. If any magnetic interactions are present in compound 2, they must be small and will only manifest themselves at very low temperature [25,30]. Preliminary measurements of the powder susceptibility of complexes 2 at very low temperature (0.05/4.0 K) have been carried out in the laboratory of Professor D. Reich, Johns Hopkins University. They show that 2 does not order for T /0.05 K. The xMT product remains linear even down to the lowest achieved temperature, confirming the hypothesis of very weak interactions. [Co(BTCAH)(py)2] ×/0.67py (3). The susceptibility of 3 has been measured between 2 and 200 K. The susceptibility increases steadily upon cooling, reaching a maximum value of 0.88 emu mol 1 at 2.0 K: no maximum is observed. The behavior of the xMT product as a function of temperature (presented in Fig. 8) is very
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Fig. 10. A plot of xmolT vs. T for compound 4. The solid line represents the fit to the 2D-Heisenberg model.
Fig. 8. Temperature dependence of the xMT product for 3. Note that there is one cobalt ion per mol of 3.
K mol1 at 2 K. This drop in xMT is again indicative of anti-ferromagnetic interactions. For high spin Mn(II) d5, the ground state is 6A1 with the high spin of S /5/2. The g values are isotropic and nearly 2.0. Zero-field splitting is usually small, on the order of 102 K, and the ground state is well isolated from the higher energy levels [30]. In the absence of crystals, no structural information is available. Therefore, the data have been fit to a variety of magnetic models in an attempt to obtain structural information. We use the standard spin-Hamiltonian for magnetic exchange: X H 2J Si Sj i;j
Fig. 9. A plot of 1/xmolT vs. T for compound 4. The solid line represents the fit to the Weiss law.
similar to that of 2 (Fig. 7). In both figures the xMT product per cobalt ion approaches a value of 2.9 emu K mol 1 at high temperatures and demonstrates linear behavior below 70, with non-zero intercepts as the temperature is reduced to zero: 1.84 emu K mol1 for 3 and 1.67 emu K mol 1 for 2. Mn3[BTCA]2 ×/8H2O (4): the magnetic susceptibility of a powder sample of complex 4 was measured over the range 2/300 K. A plot of 1/xM versus temperature is linear with an extrapolated Curie /Weiss u value of /15 K (see Fig. 9), indicating antiferromagnetic interactions. Between 100 and 290 K, the product of xMT is a constant at approximately 13.1 cm3 K mol 1, or a value of 4.3(1) cm3-K mole 1 of Mn(II) ions as expected for a spin /5/2 ion with g /2.00 (Fig. 10). Below 100 K, the value of xMT begins to drop until it reaches 2.19 cm3
where the summation is taken for the nearest neighbors. Compared with the analyses of complexes 1, 2 and 3, the data for 4 are straightforward. The decrease of the xMT product in the low temperature region arises from the antiferromagnetic interaction between the manganese ions. An antiferromagnetic dimer model gave poor correlation with the data set. However, use of either a one-dimensional Heisenberg chain model or a twodimensional Heisenberg layer model gave equally good agreement with a calculated interaction strength J /k / /0.833(8) K and C /4.40(1) cm3 K mol 1 for the onedimensional model, or J /k //0.503(5) K, C /4.45(1) cm3 K mol 1 for the two-dimensional layer model (Fig. 10). This suggests that the manganese complexes exists in a low dimensional structures. These results agree well with the work of Gutschke et al. on the anhydrous manganese compound Mn3BTCA2 [14c]. The hydrothermal synthesis reported generates a complexes with two types of linkages between manganese atoms: one with single oxygen bridges (Mn /O /Mn) and a second through the carboxylates (Mn /O /C /O /Mn). It is clear from the other structures discussed here (1 /3) that in the presence of auxiliary ligands (H2O, py), only linkages of the latter type occur. Thus we may directly compare our
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estimated value for the antiferromagnetic exchange in 4 (//0.5 to /0.8 K) with the value reported for the inter-trimer interactions in Mn3BTCA2 of //0.74 K [14c]. The good agreement between the values supports our belief that the interactions in 4 occur only through carboxylate bridges and that the majority of the water molecules are coordinated to the Mn ions.
with BTCA in refluxing aqueous solution results in a cobalt complexes with lower hydration, which shows unusual behavior with very strong magnetic interactions. Low field susceptibility studies show a xMT product of up to 350 cm3 K mol 1 at T near 16 K [33]. Attempts to determine the structure of this material are in progress.
5. Conclusions
Acknowledgements
The ligand BTCA may coordinate in a variety of ways to 3-d transition metal ions. The coordination may be monodentate, bidentate, or tridentate and thus the BTCA may bind metal ions such as Ni(II), Co(II) and Mn(II) to form a variety of complexes structures. The nickel compound 1 crystallized as trimers with two terminal Ni(H2O)5 units bridged by two benzenetricarboxylate ligands to a central Ni(H2O)4. The cobalt complexes 2 exists as extended polymeric chains with two distinct cobalt coordination sites, while cobalt complexes 3 has only one metal site, but two potential exchange pathways. Magnetic susceptibility data for the nickel complexes, 1, indicate either a weak antiferromagnetic interaction between metal ions, or single ion anisotropy. Testing of the model presented for the linear trimer with both exchange interactions and single-ion anisotropy will require a trimer of S /1 ions with stronger interactions. Data for the cobalt compounds, 2 and 3, show crystal field and spin-orbit coupling effects on the individual Co(II) ions and imply at most a very weak magnetic interaction between the metal ions. The manganese complexes, 4, shows evidence of antiferromagnetic interactions below 50 K and a low-dimensional magnetic structures. Previous work shows the magnetic interaction can be very strong (J/k //100 K) with carboxylate groups through a benzene ring, if the carboxylate groups are coplanar with each other, with the benzene ring, and with the magnetic orbital of the metal ions [18 /21]. The magnetic exchange strengths are all weak (J /k //1.0 K or smaller) through the BTCA group for all the compounds discussed here. This difference could arise from several sources. First, the metal /oxygen distances are slightly longer than those observed in either the phthalate [18] or terephthalate [19] complexes ranging ˚ in 1 and 2.026 to 2.140 A ˚ in 2. For from 2.010 to 2.110 A copper compounds with strong interactions, the Cu /O ˚ (1.972(4) A ˚ [19] and distances are all less than 2 A ˚ [18]). Secondly, the hydration of the metal 1.927(5) A ions prevents chelation by the carboxylate ligands (except for Co2 in 2). Attempted dehydration of the products to reduce the number of coordinated water molecules did not change the magnetic behavior of the complexes significantly. However, reaction of Co(II)
This work was supported by the National Science Foundation (DMR-9006470 and DMR-9803813). We wish to thank Dr. Dan Reich (Johns Hopkins) for the low temperature magnetic data on cobalt compound 2 and Calin Galeriu for assistance in the collection of magnetic data for compound 4. We are also indebted to Professor Helmut Haendler (New Hampshire) for helpful discussions regarding the structure of complex 2. Professor Haendler was an outstanding scientist and we mourn his passing.
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[25] O. Kahn, Molecular Magnetism, VCH Publishers, Inc, New York, 1993. [26] C.P. Landee, R.E. Greeney, A.C. Lamas, Rev. Sci. Instrum. 58 (1987) 1957. [27] K. Nakamoto, Infrared and Raman Spectra of Inorganic and Coordination Compounds, third ed., Wiley, New York, 1978. [28] A. Michaelides, S. Skoulika, V. Kiritsis, C. Raptopoulou, A. Terzis, J. Chem. Res. (1997) 204. [29] L.S. Erre, G. Micera, F. Cariate, G. Ciani, A. Sironi, H. Kozlowski, J. Baranowski, J. Chem. Soc., Dalton Trans. (1988) 363. [30] R.I. Carlin, Magnetochemistry, Springer, Berlin, 1986, p. 24. [31] C.P. Landee, D.M. Mudgett, B.M. Foxman, Inorg. Chim. Acta 186 (1991) 45. [32] W. Zhang, Ph.D. dissertation, Clark University, Worcester, MA, 1994. [33] (a) C.P. Landee, C.M. Wynn, A.S. Albrecht, W. Zhang, G.B. Vunni, J.L. Parent, C. Navas, M.M. Turnbull, J. Appl. Phys. 75 (1994) 5535; (b) C.M. Wynn, A.S. Albrecht, C.P. Landee, M.M. Turnbull, B. Dodrill, J. Solid State Chem. 159 (2001) 379.
Structures and magnetic properties of transition metal complexes of 1,3,5-benzenetricarboxylic acid Wanru Zhang a, Sandra Bruda b, Christopher P. Landee a,, Judith L. Parent b, Mark M. Turnbull b, b
a Department of Physics, Clark University, 950 Main Street, Worcester, MA 01610, USA Carlson School of Chemistry and Biochemistry, Clark University, 950 Main Street, Worcester, MA 01610, USA
Received 1 March 2002; accepted 23 May 2002
Abstract Treatment of the potassium salt of 1,3,5-benzenetricarboxylic acid with transition metal ions in aqueous solution produced a series of coordination complexes. Reaction of Ni(ClO4)2 ×/6H2O with K3[C6H3(CO2)3] gave complexes 1, Ni3[C6H3(CO2)3]2(H2O)14 ×/ 4H2O. The compound crystallized as trimers with two terminal Ni(H2O)5 units bridged by two benzenetricarboxylate ligands to a central Ni(H2O)4. Magnetic susceptibility data indicate the presence of either weak antiferromagnetic interactions between metal ions, or single ion anisotropy. Reaction of Co(ClO4)2 ×/6H2O with K3[C6H3(CO2)3] gave complexes 2, Co3[C6H3(CO2)3]2(H2O)12. The complexes exists as a pendant chain coordination polymer with both bridging and terminal Co2 ions. Each cobalt center is also coordinated to four water molecules. A systematic linear decrease in effective moment is observed below 100 K. This decrease can be understood entirely in term of crystal field and spin-orbit coupling effects on the individual Co(II) ions and implies magnetic interaction are weak. Magnetic susceptibility data were also obtained for the Co /BTCA complexes 3, Co(BTCA)(py)2 ×/0.67py. These data can also be described as arising solely from single-ion effects due to the non-cubic environment around the Co2 ion with no evidence of the presence of magnetic interactions. Reaction of Mn(ClO4)2 ×/6H2O with K3[C6H3(CO2)3] gave complexes 4, Mn3(C6H3(CO2)3)2 ×/8H2O. Magnetic susceptibility data indicate weak antiferromagnetic interactions and suggest that the complexes have a low dimensional structures. # 2002 Elsevier Science B.V. All rights reserved. Keywords: 1,3,5-Benzenetricarboxylate; Magnetochemistry; S /1 trimer model
1. Introduction As part of our ongoing research in molecular basedmagnets, we are interested in developing systems exhibiting spin frustration [1]. In a spin glass material, frustration occurs due to site disorder, but in a molecular magnet it can arise due to geometrical frustration, via competing interactions between magnetic moments of equal magnitude [2 /4]. Thus, an antiferromagnetic 2D Kagome´ lattice based on equilateral triangles will inherently generate frustration, defined as the inability of the system to simultaneously
Corresponding authors. Present address: Depto. De Quimica Fisica Facultad de Quimica, Universidad de Barcelona, Av. Diagonal 647, 08028 Barcelona, Spain (M.M.T.). Tel.: /34-93-402 1220; fax: /34-93-402 1231
minimize the energies of competing interactions [4]. The ground state of such system will be highly degenerate [5]. This feature will lead to unusual magnetic properties, most likely to occur in the low temperature regime [2]. The synthesis and characterization of compounds showing frustration are very important in order to provide an experimental insight for the theoretical models developed to date [6]. Unfortunately, there are few such compounds reported, the majority of them being inorganic oxides [2]. Among the molecular-based complexes, Awaga et al. [7] reported 2D compounds based on paramagnetic radicals, but in which the antiferromagnetic interaction was established between ferromagnetically coupled radical dimers. Miller et al. [8] and Murray et al. [9] studied the tricyanomethanide complexes Cr[C(CN)3]2 and V[C(CN)3]2 by a variety of magnetic, neutron powder
0020-1693/02/$ - see front matter # 2002 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 0 - 1 6 9 3 ( 0 2 ) 0 1 1 3 9 - 8
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diffraction and specific heat measurements. In these systems though, the equilateral triangular lattice is distorted towards isosceles, relieving the frustration. Our synthetic approach focused on 1,3,5-benzenetricarboxylic acid (BTCAH3) and 1,3,5-benzenetricarboxlyate (BTCA) as a ligand, since its threefold symmetry makes it a very attractive choice for obtaining a (6,3) type structure [10], the framework of a Kagome´ lattice (Fig. 1(a)). However, the unpredictable factor in designing the desired product comes from the fact that each carboxylate group can coordinate in a variety of modes: unidentate (I), m-1,3-bridging (II), m-1,1-bridging (III) or chelating (IV) (Fig. 1(b)). A literature survey shows that the BTCA ligand has attracted a great deal of interest in the synthesis of zeolite-like materials [11 /13] in addition to a smaller number of simple coordination complexes [14]. The geometry of the ligand allows for large size pores in such open metal/organic frameworks. Spectacular results were obtained by Yaghi et al., who synthesized an extremely stable compound showing reversible selective inclusion [12]. Another of his compounds CoC6H3(COOH1/3)3(py)2 ×/2/3py (py /pyridine) [11] is also able to selectively include guest molecules. Its
structures is based on a triangular pattern of metal centers, but no magnetic data were reported. A copper(II) based nanoporous material was reported by Williams et al. [15,16]. Its magnetic properties show cooperative behavior arising from the presence of copper dimers [15]. Suh et al. [17] were able to generate a Kagome´ lattice based on a nickel(II) macrocycle, but the reported anti-ferromagnetic interaction was very weak (J //1 cm 1). Additional work has been done with copper complexes of the related aryl carboxylate ligands phthalate [18], terephthalate [19,20] and benzenetetracarboxylate [19,21]. Both ferromagnetic [18] and antiferromagnetic [19] magnetic interactions have been found. The strength of the magnetic interaction through carboxylate groups and a benzene ring can be very large (/70 cm 1) [19]. We were interested in BTCA due to its propensity to intermediate superexchange interactions between magnetic centers [22,23]. However, there are also disadvantages in using it. First, the distance between two metal centers will be quite large, decreasing the strength of the interaction. Second, the MNDO calculations [24] showed that the HOMO orbitals of BTCA are doubly degenerate and consist mainly of pp-orbitals of the carbon and oxygen atoms. This will lead to incompatibility, or poor overlap between the magnetic orbitals of the metal and ligand, respectively [25]. However, the threefold symmetry of the ligand was sufficient to warrant investigation of its ability to propagate magnetic exchange. Here we report the results of our investigations of nickel, cobalt and manganese complexes of BTCA.
2. Experimental
Fig. 1. (a) Two types of lattices that can be generated by using the BTCA ligand: Kagome´ and triangular; (b) coodination modes for the carboxylate group.
1,3,5-Benzenetricarboxylic acid was purchased from Aldrich Chemical Co. and used without further purification. Metal chlorides and perchlorates were purchased from Aesar and the perchlorates were dried under vacuum prior to use. Infrared spectra were taken on a Perkin/Elmer 467, 1330, or Paragon 500 spectrophotometer in KBr matrix and calibrated against polystyrene (br, broad; s, strong; m, medium). Elemental analyses were performed by the University Instrumentation Center of the University of New Hampshire. Caution: although we have not encountered any difficulties in working with these compounds, the potential for explosion with perchlorate salts is well documented. We have prepared related complexes from both the perchlorate and chloride salts and can detect no difference in the products. The perchlorates were used initially for ease of detection of impurities by IR.
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2.1. tetradecaaquabis(Benzenetricarboxalato)trinickel(II) tetrahydrate (1)
2.4. octaaquabis(Benzenetricarboxalato)trimanganese(II) (4)
Ni3[C6H3(CO2)3]2(H2O)14 ×/4H2O: 1,3,5-benzenetricarboxylic acid (1.05 g, 4.90 mmol) was combined with KOH (0.85 g, 15 mmol) in 20 ml of water with stirring. Additional solid KOH was added until complete solution was attained. Ni(ClO4)2 ×/6H2O (2.75 g, 7.44 mmol) was dissolved in 15 ml of H2O and added dropwise to the BTCA solution. A green precipitate formed immediately. The mixture was stirred one additional hour and then filtered. The ppt was washed with H2O (3 /20 ml) and acetone (2 /15 ml) and air-dried to give a light green powder, 1.84 g (80%). Crystals suitable for structural determination were grown by slow diffusion of the aqueous reagents through a glass wool plug in the center of a 19 cm column. IR (KBr) */n 3350s br, 1610s, 1540s, 1440s, 1370s, 720m cm 1. Anal. Calc. for C18H42Ni3O3O: C, 23.64; H, 4.63. Found: C, 23.71; H, 4.41%.
Mn3[C6H3(CO2)3]2 ×/8H2O: 1,3,5-Benzenetricarboxylic acid (1.05 g, 5.0 mmol) was combined with KOH (0.88 g, 16 mmol) in 20 ml of water with stirring. Additional solid KOH was added until complete solution was attained. MnCl2 ×/4H2O (1.49 g, 7.53 mmol) was dissolved in 20 ml of H2O and added dropwise to the BTCA solution. A cream colored precipitate formed immediately. The mixture was stirred an additional hour and then filtered. The ppt was washed with H2O (4 /20 ml) and acetone (4 /10 ml) and air-dried to give an offwhite powder, 1.81 g (98%). Combustion analysis suggested non-stoichiometric hydration, so the compound was recrystallized from water (0.100 g/250 ml) to give a tan powder, (93% recovery). IR (KBr) */n 3350s br, 1610s, 1540s, 1440s, 1370s, 720m cm1. Anal. Calc. for C18H22Mn3O20: C, 29.90; H, 3.07. Found: C, 29.81; H, 3.05%. 2.5. Magnetic data collection
2.2. dodecaaquabis(Benzenetricarboxalato)tricobalt(II) (2) Co3[C6H3(CO2)3]2(H2O)12: 1,3,5-Benzenetricarboxylic acid (1.05 g, 5.00 mmol) was combined with KOH (0.85 g, 15 mmol) in 25 ml of water with stirring. Additional solid KOH was added until complete solution was attained. Co(ClO4)2 ×/6H2O (2.74 g, 7.49 mmol) was dissolved in 10 ml of H2O and added dropwise to the BTCA solution. A pink precipitate formed immediately. The mixture was stirred an additional 2 h and then filtered. The ppt was washed with H2O (4 /10 ml) and acetone (4 /10 ml) and air-dried to give a pink powder, 1.74 g (86%). Crystals suitable for structural determination were grown by slow evaporation of the filtrate. IR (KBr) */n 3350s br, 1610s, 1540s, 1440s, 1370s, 720m cm 1. Anal. Calc. for C18H30Co3O24: C, 26.78; H, 3.75. Found: C, 26.66; H, 3.75%.
2.3. bisPyridine(benzenedicarboxylatocarboxylic acid)cobalt(II) pyridine clathrate (3) 1,3,5-Benzenetricarboxlyic acid (0.21 g, 1.0 mmol) was dissolved along with Co(NO3)2 ×/6H2O (0.29 g, 1.0 mmol) in 10 ml of EtOH and placed in a small beaker. This beaker was place in a larger beaker containing 2 ml of pyridine and the outer beaker sealed. After 4 days, deep red hexagonal crystals of 3 were collected by filtration. Physical data and the single crystal X-ray structures agreed with those reported by Yaghi et al. [12].
Powder magnetic susceptibility was determined as a function of temperature between 1.8 and 300 K using a commercial vibrating sample magnetometer (VSM) equipped with a continuous-flow helium cryostat [26] or on a commercial Quantum Design SQUID Magnetometer. Magnetic fields up to 1 (VSM) or 5 T (SQUID) were applied. The data were corrected for diamagnetism and temperature independent paramagnetism.
3. Results and discussion 3.1. Synthesis and structures Reaction of Ni(ClO4)2 ×/6H2O with K3BTCA in aqueous solution gave Ni3[BTCA]2(H2O)14 ×/4H2O (1) in 80/ 90% yield. The infrared spectrum of 1 shows a significant amount of water and the usual bands for the carboxylate ion [27]. The absorptions were too broad to distinguish the nature of coordination from the IR spectrum. Crystals suitable for single crystal X-ray diffraction were by slow diffusion of the reactants. An ORTEP drawing showing the structures of 1 is presented in Fig. 2. The structure has been previously reported [28]. Compound 1 crystallizes as trimers of nickel(II) ions with two terminal Ni(H2O)5 units bridged by two BTCA ligands to a central Ni(H2O)4. Ni1 sits on an inversion center and as such the bond angles for all trans pairs of ligands are 1808. The nickel /water distances range from ˚ and these bonds are slightly 2.066(6) to 2.110(6) A ˚ ) to the longer than the Ni /O distance (2.010(6) A carboxylate oxygen.
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Fig. 2. Molecular structures of 1. Hydrogen atoms and water of hydration have been removed for clarity.
The coordination sphere of the terminal Ni(II) ion (Ni2) is also roughly octahedral with cis bond angles ranging from 85.2(2) to 95.6(2)8. The carboxylate groups are all coplanar with their benzene rings and a Ni /O ˚ is found. The five H2O molecules of distance of 2.025 A the terminal Ni(2) have Ni /O distances ranging from ˚ . In addition, four more H2O 2.022(8) to 2.114(5) A molecules are hydrogen-bonded in the lattice; one each near the water molecules trans to the carboxylates on the terminal Ni(2) ions and one each roughly along a line between the central Ni ion and the terminal Ni ions (not shown in Fig. 2). Reaction of Co(ClO4)2 ×/6H2O with K3BTCA in aqueous solution gave immediate precipitation of Co3[BTCA]2(H2O)12 (2) in 85% yield. The infrared spectrum of 2 is indistinguishable from 1. Crystals suitable for single crystal X-ray diffraction study (elongated bricks) were grown from water by slow evaporation of the filtrate. Cobalt complex 2 exists as extended polymeric chains (Fig. 3). This structures has also been previously reported [13a]). The Co1 ions have four water molecules ˚) in equatorial sites (average Co /O distance /2.119(5) A with the axial sites occupied by oxygens of monodentate carboxylates from two different BTCA anions (Co/O ˚ ), similar to the central Ni in 1. average /2.055(4) A
Fig. 3. Portion of the chain structures of 2. Hydrogen atoms have been removed for clarity.
These bonds are slightly longer than the Co /O bonds found in tetraaquabis(2,6-dimethoxybenzoato)cobalt(II) ˚ ) [29]. The second cobalt ion in the repeat unit, (2.071 A Co2, sits on a site of C2 symmetry with distorted octahedral coordination. The coordination sphere around Co2 contains four water molecules and one chelating carboxylate group. The small bite angle of the bidentate carboxylate (61.48) leads to a rather severe distortion, with the cobalt /water distances ranging from ˚ (trans to the carboxylate) to 2.148(4) A ˚ (cis ). 2.026(6) A The trans O(23) /Co(2) /O(23A) angle is somewhat reduced (176.1(3)8), while cis angles to water molecules are in the range of 84.9(2) /98.7(2)8. The chains can be described as uniform [Co(H2O)42 /BTCA3 /]n chains with additional Co(H2O)42 groups appended to alternating BTCA anions. A second Co(II) complex, previously reported by Yaghi and co-workers [12] was also prepared due to its unique symmetry. Vapor diffusion of pyridine into a solution of Co(NO3)2 and BTCAH3 in ethanol yielded dark red hexagonal crystals of Co(BTCAH)(py)2 ×/2/3py, 3, and X-ray analysis verified that they possessed the same structures as previously reported (see Fig. 4). The compound crystallizes in layers with a single cobalt site. Each Co ion is coordinated to three BTCA anions, two via monodentate carboxylates and one via chelation. There are two distinct BTCA ions in the structures. One coordinates to three Co ions via only monodentate carboxylates, while the other coordinates via only chelating carboxylates. This ensures that there is only one potential magnetic superexchange pathway through any given BTCA ligand and preserves the threefold symmetry of the structures and the potential for the complexes to exhibit magnetic frustration. The layers are separated by coordination of pyridine molecules to the axial positions of the Co ions, ensuring good
Fig. 4. Layer structures of compound 3. The disordered coordinated pyridine molecules are not shown for clarity.
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isolation of the layers (only the N atoms of these molecules are shown in Fig. 4). In addition, pyridine molecules intercalate into the structures via p-stacking between the aromatic rings of the BTCA ions of adjacent layers (see Fig. 4). Reaction of MnCl2 ×/4H2O with K3BTCA in aqueous solution gave immediate precipitation of Mn3[BTCA]2 ×/ 8H2O (3) in 85% yield. Repeated recrystallization failed to produce crystals suitable for X-ray study. CHN analysis and magnetic measurements confirmed the stoichiometry.
4. Magnetic properties [Ni3(BTCA)2(H2O)14] ×/4H2O 1: the magnetic susceptibility of a polycrystalline sample of the nickel complex has been measured from 2 to 300 K. A plot of 1/xM versus T is linear in the range 30 /200 K. The Curie/ Weiss constant u determined within this temperature range is near zero within experimental error (see Fig. 5). This small u value indicates that the interactions between ions are very weak. The value of the Curie constant C /1.15(3) cm3 K mol 1 implies a g-factor near 2.16, normal for the Ni(II) ion in an octahedral environment. The powder susceptibility data for compound 1 are plotted as the product xMT versus temperature in Fig. 6. In the high temperature region between 30 and 300 K, the product of xMT is fairly constant, at approximately 3.5 cm3 K mol 1 per mol of trimer. The value of xMT starts to decrease at 30 K until it reaches a value of 1.29 cm3 K mol 1 at approximately 2 K. This decrease in the value of xMT can have several origins. Antiferromagnetic interactions between the nickel ions can align their moments antiparallel, causing a decrease in the product xMT . There are two possible kinds of magnetic interactions within the trimer: a magnetic interaction between
Fig. 6. Plot of xmolT vs. T for compound 1. The solid line is the best fit to the trimer model (see text).
the terminal Ni ions and the central Ni ion, J , and a magnetic interaction between the two terminal Ni ions, J ?. Alternatively, Ni(II) is well known to exhibit singleion anisotropy, in which the ms /9/1 sublevels are split away from the ms /0 sublevel. This splitting introduces an energy gap D and leads to a decrease in the xMT product at low temperature as the fraction of the nickel ions in the excited magnetic states decreases with temperature. Of course, both antiferromagnetic exchange as well as single-ion effects can occur in the same compound. The full Hamiltonian for the isolated trimer in terms of J, J? is written as: H 2J(S1 S2 S2 S3 )2J?S1 S3 D(S21z S22z S23z )
where S1 and S3 are the terminal Ni ions, and S2 is the central Ni ion. We first analyzed the susceptibility in terms of a model of non-interacting spin (J /J ?/0) in the presence of a single-ion anisotropy (assumed to be the same for all three nickel ions). The Hamiltonian is that given in Eq. (1) with J and J? held equal to zero. A positive value of the parameter D means the non-magnetic ms / 0 state lies below the ms /9/1 states. The derivation of the average magnetic susceptibility per mole of trimers of this Hamiltonian has been given elsewhere [30]. It is written as: xM
Fig. 5. Plot of l/xmol vs. T for compound 1 with a fit to the Curie / Weiss law (solid line).
(1)
2Ng2 m2b 2x1 2ex x1 ex kT 1 2ex
(2)
where x /D /kT . The data were fit to Eq. (2), allowing both D and g to vary. The best fit parameters were found to be D /k /10.5(1) K, g /2.15(1). Similar values for D have been reported previously [30,31]. A second model postulated an anti-ferromagnetic interaction between nickel ions in the trimer assuming zero single-ion anisotropy. The Hamiltonian used was a
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simplified form of that given in Eq. (1) where now D is held equal to zero. The expression for the susceptibility of a mole of the trimers by this model [32] is written as:
xM
region predicts a zero-temperature value near 4.8 (cm3 K mol 1), or 1.6 (cm3 K mol1) mol1 of cobalt ions. The isolated Co2(d7) ion in an octahedral crystal
Nmb g2 (6e2J=kT 6eJ?=kT 30e4J2J?=kT 6e4J6J?=kT 30eJ?=kT 84e6J6J?=kT ) 3kT(3e2J=kT e2J?2J=kT 3eJ?=kT 5e4J2J?=kT 3e4J6J?=kT 5eJ?=kT 7e6J6J?=kT )
The data for the nickel complex were fit to Eq. (3) with J? /0 ( linear trimer model). Both the exchange constant J and the g -factor were allowed to vary. The best fit parameters were found for a magnetic interaction strength J /k //0.88(2) K and g /2.22(1). This small value of J is consistent with the Curie /Weiss u value near zero. We also attempted to fit the experimental data with the theoretical calculations of the magnetic susceptibility using the full Hamiltonian (Eq. 1). However, since the exchange coupling constant between the center and terminal nickel ions is very weak, the J and J ? values are highly correlated with the value of the single ion anisotropy. No unique set of J, J ?, D , and g values was found [32]. A more precise determination of the set of magnetic parameters must await the availability of the larger single crystals. [Co3(BTCA)2(H2O)12] (2): the powder magnetic susceptibility data for 2 has been measured between 6 and 300 K. A plot of the product xMT versus temperature is shown in Fig. 7. As this product is related to the effective moment, it is clear the effective moment begins to decrease below 150 K and continues decreasing to a minimum of 5.0 (cm3 K mol 1) at 6 K. The value of xMT between 150 and 300 K is constant at approximately 8.6 (cm3 K mol 1), or a value of 2.87(1) (cm3 K mol 1) per Co ion, as expected for a spin /3/2 ion with gave /2.47. Extrapolating the low temperature linear
Fig. 7. Temperature dependence of the xMT product for 2. Note that there are three cobalt ions per mol of 2.
(3)
field possesses a 4T1 ground state. At higher temperatures (greater than 100 K) the ion can be thought of as a nearly isotropic S /3/2 entity. However, as the result of the tetragonal crystal field distortion, the 4T1g state lies an energy D below a 4Eg excited state. Spin-orbit coupling (l /180 cm 1) divides these two states into a set of six Kramer doublets with an Seff /1/2 doublet state at low energy. At high temperature, the xMT product of the individual ion can range from 2.5 to 3.5 (cm3 K mol 1), depending on the ratio of D /jl j. For lower relative temperature (kT /jlj B/0.5), the xMT product decreases linearly with temperature, reaching a value near 1.7 (cm3 K mol1) as T 0/0 [25]. If any magnetic interactions between cobalt ions are present, their effect will be to cause the xMT product to deviate from its purely linear behavior at low temperatures. It is possible to understand the behavior of the cobalt compound in light of the above discussion without invoking any magnetic interactions. Both cobalt sites (Fig. 4) correspond to octahedral sites with orthorhombic distortions. The cis O /Co /O angles are near 908 for Co1 and near 608 for Co2. The experimental zero temperature limit of xMT , 1.6 cm3 K mol 1 of cobalt ions, is very close to the theoretical value, and the predicted linear growth of xMT is seen between 6 and 70 K. The high temperature limit near 2.9 cm3 K mol 1 of cobalt ions also lies in the middle of the theoretical range. If any magnetic interactions are present in compound 2, they must be small and will only manifest themselves at very low temperature [25,30]. Preliminary measurements of the powder susceptibility of complexes 2 at very low temperature (0.05/4.0 K) have been carried out in the laboratory of Professor D. Reich, Johns Hopkins University. They show that 2 does not order for T /0.05 K. The xMT product remains linear even down to the lowest achieved temperature, confirming the hypothesis of very weak interactions. [Co(BTCAH)(py)2] ×/0.67py (3). The susceptibility of 3 has been measured between 2 and 200 K. The susceptibility increases steadily upon cooling, reaching a maximum value of 0.88 emu mol 1 at 2.0 K: no maximum is observed. The behavior of the xMT product as a function of temperature (presented in Fig. 8) is very
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199
Fig. 10. A plot of xmolT vs. T for compound 4. The solid line represents the fit to the 2D-Heisenberg model.
Fig. 8. Temperature dependence of the xMT product for 3. Note that there is one cobalt ion per mol of 3.
K mol1 at 2 K. This drop in xMT is again indicative of anti-ferromagnetic interactions. For high spin Mn(II) d5, the ground state is 6A1 with the high spin of S /5/2. The g values are isotropic and nearly 2.0. Zero-field splitting is usually small, on the order of 102 K, and the ground state is well isolated from the higher energy levels [30]. In the absence of crystals, no structural information is available. Therefore, the data have been fit to a variety of magnetic models in an attempt to obtain structural information. We use the standard spin-Hamiltonian for magnetic exchange: X H 2J Si Sj i;j
Fig. 9. A plot of 1/xmolT vs. T for compound 4. The solid line represents the fit to the Weiss law.
similar to that of 2 (Fig. 7). In both figures the xMT product per cobalt ion approaches a value of 2.9 emu K mol 1 at high temperatures and demonstrates linear behavior below 70, with non-zero intercepts as the temperature is reduced to zero: 1.84 emu K mol1 for 3 and 1.67 emu K mol 1 for 2. Mn3[BTCA]2 ×/8H2O (4): the magnetic susceptibility of a powder sample of complex 4 was measured over the range 2/300 K. A plot of 1/xM versus temperature is linear with an extrapolated Curie /Weiss u value of /15 K (see Fig. 9), indicating antiferromagnetic interactions. Between 100 and 290 K, the product of xMT is a constant at approximately 13.1 cm3 K mol 1, or a value of 4.3(1) cm3-K mole 1 of Mn(II) ions as expected for a spin /5/2 ion with g /2.00 (Fig. 10). Below 100 K, the value of xMT begins to drop until it reaches 2.19 cm3
where the summation is taken for the nearest neighbors. Compared with the analyses of complexes 1, 2 and 3, the data for 4 are straightforward. The decrease of the xMT product in the low temperature region arises from the antiferromagnetic interaction between the manganese ions. An antiferromagnetic dimer model gave poor correlation with the data set. However, use of either a one-dimensional Heisenberg chain model or a twodimensional Heisenberg layer model gave equally good agreement with a calculated interaction strength J /k / /0.833(8) K and C /4.40(1) cm3 K mol 1 for the onedimensional model, or J /k //0.503(5) K, C /4.45(1) cm3 K mol 1 for the two-dimensional layer model (Fig. 10). This suggests that the manganese complexes exists in a low dimensional structures. These results agree well with the work of Gutschke et al. on the anhydrous manganese compound Mn3BTCA2 [14c]. The hydrothermal synthesis reported generates a complexes with two types of linkages between manganese atoms: one with single oxygen bridges (Mn /O /Mn) and a second through the carboxylates (Mn /O /C /O /Mn). It is clear from the other structures discussed here (1 /3) that in the presence of auxiliary ligands (H2O, py), only linkages of the latter type occur. Thus we may directly compare our
200
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estimated value for the antiferromagnetic exchange in 4 (//0.5 to /0.8 K) with the value reported for the inter-trimer interactions in Mn3BTCA2 of //0.74 K [14c]. The good agreement between the values supports our belief that the interactions in 4 occur only through carboxylate bridges and that the majority of the water molecules are coordinated to the Mn ions.
with BTCA in refluxing aqueous solution results in a cobalt complexes with lower hydration, which shows unusual behavior with very strong magnetic interactions. Low field susceptibility studies show a xMT product of up to 350 cm3 K mol 1 at T near 16 K [33]. Attempts to determine the structure of this material are in progress.
5. Conclusions
Acknowledgements
The ligand BTCA may coordinate in a variety of ways to 3-d transition metal ions. The coordination may be monodentate, bidentate, or tridentate and thus the BTCA may bind metal ions such as Ni(II), Co(II) and Mn(II) to form a variety of complexes structures. The nickel compound 1 crystallized as trimers with two terminal Ni(H2O)5 units bridged by two benzenetricarboxylate ligands to a central Ni(H2O)4. The cobalt complexes 2 exists as extended polymeric chains with two distinct cobalt coordination sites, while cobalt complexes 3 has only one metal site, but two potential exchange pathways. Magnetic susceptibility data for the nickel complexes, 1, indicate either a weak antiferromagnetic interaction between metal ions, or single ion anisotropy. Testing of the model presented for the linear trimer with both exchange interactions and single-ion anisotropy will require a trimer of S /1 ions with stronger interactions. Data for the cobalt compounds, 2 and 3, show crystal field and spin-orbit coupling effects on the individual Co(II) ions and imply at most a very weak magnetic interaction between the metal ions. The manganese complexes, 4, shows evidence of antiferromagnetic interactions below 50 K and a low-dimensional magnetic structures. Previous work shows the magnetic interaction can be very strong (J/k //100 K) with carboxylate groups through a benzene ring, if the carboxylate groups are coplanar with each other, with the benzene ring, and with the magnetic orbital of the metal ions [18 /21]. The magnetic exchange strengths are all weak (J /k //1.0 K or smaller) through the BTCA group for all the compounds discussed here. This difference could arise from several sources. First, the metal /oxygen distances are slightly longer than those observed in either the phthalate [18] or terephthalate [19] complexes ranging ˚ in 1 and 2.026 to 2.140 A ˚ in 2. For from 2.010 to 2.110 A copper compounds with strong interactions, the Cu /O ˚ (1.972(4) A ˚ [19] and distances are all less than 2 A ˚ [18]). Secondly, the hydration of the metal 1.927(5) A ions prevents chelation by the carboxylate ligands (except for Co2 in 2). Attempted dehydration of the products to reduce the number of coordinated water molecules did not change the magnetic behavior of the complexes significantly. However, reaction of Co(II)
This work was supported by the National Science Foundation (DMR-9006470 and DMR-9803813). We wish to thank Dr. Dan Reich (Johns Hopkins) for the low temperature magnetic data on cobalt compound 2 and Calin Galeriu for assistance in the collection of magnetic data for compound 4. We are also indebted to Professor Helmut Haendler (New Hampshire) for helpful discussions regarding the structure of complex 2. Professor Haendler was an outstanding scientist and we mourn his passing.
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