Structural and Magnetic Studies on Two-Dimensional Square Planar

DOI: 10.1021/cg101481c

Structural and Magnetic Studies on Two-Dimensional Square Planar Lattice Magnets Composed of Organic Radical Cation Salts (Benzo[1,2-d:4,5-d0 ]Bis[1,3,2]dithiazolyl-2-zolium) 3 X (X = TlBr4, TlI4, and InI4)

2011, Vol. 11 575–582

Wataru Fujita,*,† Kazuyuki Takahashi,‡ and Hayao Kobayashi§ †

Department of Chemistry, Tokyo Metropolitan University, Minami-osawa 1-1, Hachioji, Tokyo 192-0397, Japan, ‡Division of New Materials Science, Institute for Solid State Physics, The University of Tokyo, Kashiwanoha 5-1-5, Kashiwa, Chiba 277-8581, Japan, and § Department of Chemistry, Nihon University, Sakura-josui 3-25-40, Setagaya-ku, Tokyo 156-8550, Japan Received November 9, 2010; Revised Manuscript Received December 22, 2010

ABSTRACT: The magnetostructural correlation was investigated in three organic radical cation salts based on the monocation of benzo[1,2-d:4,5-d0 ]bis[1,3,2]dithiazolyl (BBDTA), which is an S = 1/2 system, and the tetrahedral diamagnetic monoanions, TlBr4(1), TlI4- (2), and InI4- (3), by analysis of their crystal structures and magnetic properties. X-ray diffraction studies showed that these three salts consisted of alternatively stacked layers of the diamagnetic anions layers and the organic cation layers in which BBDTAþ formed a square planar lattice, which is rare in organic radical crystals. 1 exhibited canted antiferromagnetism with an antiferromagnetic interaction in the organic cation layer below 11.6 K, whereas 2 and 3 showed metamagnetism with a ferromagnetic interaction in the organic cation layer below 8.9 and 11.3 K, respectively. Their intra- and interlayer magnetic coupling constants, 2J/ kB and zJ0 /kB, were estimated to be -11.2 and -4.1 K for 1, þ11.4 and -3.7 K for 2, and þ12.8 and -2.3 K for 3, respectively, using a two-dimensional square planar S = 1/2 Heisenberg model with mean field approximation. A consideration of the molecular orbitals showed that the differences in the intralayer magnetic interactions in 1 and, 2 or 3, originate in the molecular orientation of BBDTAþ in the organic cation layer. Overlap relation between the singly occupied molecular orbitals (SOMOs) of the nearest neighbor BBDTAþ pairs was found in 1, whereas an orthogonal relation was found in 2 and 3.

Introduction The magnetic properties of low-dimensional materials have been attractive in the field of solid-state science. In particular, two-dimensional S = 1/2 Heisenberg magnets with a square planar lattice network composed of cuprous oxides have been associated with the origin of high temperature superconductivity1 and have been important to the basic theory of magnetic phase transitions, etc.2 Many organic radicals provide low-dimensional S = 1/2 Heisenberg magnetic networks, especially, one-dimensional magnetic systems in their crystals,3,4 although it is difficult to construct organic radical magnets with a two-dimensional magnetic system. Usually, traditional organic radical molecules have a planar molecular structure with a large π-conjugate system because of energy gain due to delocalization of an unpaired electron on the molecular plane. In crystalline states, organic radicals form a π-stacking structure with a large π-orbital overlap between the radical molecules toward the stacking direction. Therefore, many organic radicals form a one-dimensional magnetic system with strong antiferromagnetic interactions toward the stacking direction and show diamagnetic properties via the spin-Peierls-like transition at low temperature.5 Cyclic thiazyl radicals and related materials exhibit strong intermolecular interactions via the characteristic S 3 3 3 N and/ or S 3 3 3 S contacts between the molecules and easily form multidimensional networks in their solid state, and are interesting for use as building blocks in molecule-based magnetic materials, *To whom correspondence should be addressed. E-mail: fujitaw@ tmu.ac.jp. r 2011 American Chemical Society

etc.6-11 Recently, we have systematically investigated the crystal structure and magnetic properties of the monocationic dithiazolyl radical (S = 1/2) of benzo[1,2-d:4,5-d0 ]bis[1,3,2]dithiazolyl,12 abbreviated as BBDTAþ (Chart 1), with various anions in search of new magnetic materials with a higher magnetic transition temperature. In particular, the salts of the BBDTAþ cation and some tetrahalogenometallates, MX4-, formed various magnetic phases. We have found ferromagnetic ordering in β-BBDTA 3 GaBr4,13 γ-BBDTA 3 GaCl4,14 and BBDTA 3 FeCl415 below 0.3 K, 7.0 K, and 44.5 K, and the spin-Peierls transition in BBDTA 3 InCl416 and BBDTA 3 InBr417 at 108 and 250 K, respectively. In addition, R-BBDTA 3 GaBr4 gave a two-dimensional square planar magnetic lattice system and showed antiferromagnetic ordering below 15.5 K.18 The variations in magnetic properties of BBDTAþ salts originate from the variation of molecular orientation in their crystals. In the present study, we report the crystal structure and magnetic properties of three two-dimensional magnets, BBDTA 3 X (X = tetrahedral anions; TlBr4 (1), TlI4 (2), InI4 (3)). In these salts, the BBDTAþ cations formed a square planar lattice. An antiferromagnetic phase transition was observed in 1 at 11.6 K, and metamagnetic phase transitions were observed in 2 and 3 at 8.9 and 11.3 K. We discuss the magnetostructural correlation and the magnetic difference in the three salts from the point of view of their molecular orbitals and molecular orientation. Experimental Section Syntheses. The starting reagents of these salts were obtained commercially and used as received. The precursor, BBDTA 3 BF4, was prepared by metathesis of BBDTA 3 FeCl4 3 CH3CN12,19,20 and Published on Web 01/14/2011

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tetra(n-butyl)ammonium tetrafluoroborate.21,22 The final materials, BBDTA 3 X (X=TlBr4 (1), TlI4 (2), and InI4(3)), were obtained by metathesis of BBDTA 3 BF4 and the corresponding tetra (n-butyl)ammonium tetrahalogenometallate in dry acetonitrile. Single crystals were grown in acetonitrile solutions of the corresponding salts at -23 °C. Incidentally, the GaI4 salt could not be obtained because it was unstable in acetonitrile. Single Crystal X-ray Analyses. X-ray diffraction data were collected using graphite-monochromated Mo-KR (λ = 0.71073 A˚) radiation on a RIGAKU Mercury CCD diffractometer at 120 K. All structures were solved by a direct method using the SHELXS-90 program23 and refined by successive differential Fourier syntheses and a full-matrix least-squares procedure using the SHELXL-97 program.24 Anisotropic thermal factors were applied to all nonhydrogen atoms. The crystal parameters of 1-3, and R-BBDTA 3 GaBr4, reported in the previous work,18 are summarized in Table 1. Magnetic Measurements. Magnetic measurements were carried out for a powder sample on a SQUID magnetometer (Quantum Design MPMS XL). The dc magnetic measurements were carried out under 2000 Oe for 1 and 2, and under a 5000 Oe applied field for 3. The field dependence of magnetization of these three materials was measured at 2 K. The ac magnetic measurements were carried out under an applied field of 3 Oe (100 Hz) for the ac field and 0 Oe for the dc field for 1-3. Molecular Orbital Calculations. Molecular orbital calculations were conducted using the Gaussian 09 program.25 The energy of the singlet and triplet states for the nearest neighbor pairs of BBDTAþ in 1-3 was estimated by means of the UB3LYP/6-311Gþþ(d, p) level without geometrical optimization, using the atomic coordinates from the crystal structure analyses.

Results and Discussion Crystal Structure. Figure 1 shows the crystal structure of BBDTA 3 TlBr4 (1). It is interesting to note that the TlBr4anion is almost the same size as a GaBr4- anion, and whereas three polymorphs were formed in the salt, BBDTA 3 GaBr4 presented in our previous study,18 only a single phase was isolated in the TlBr4- salt. 1 crystallized in an orthorhombic Pnma space group with half of the cation and the anion crystallographically independent and was isostructural with the alpha phase of the GaBr4- salt. 1 comprised a stack of alternating BBDTAþ assembled layers and TlBr4- anion assembled layers toward the b axis, as shown in Figure 1a. Figure 1b depicts the molecular alignment of the BBDTAþ layer in the ac plane. The molecular long axis and the molecular plane of BBDTAþ were tilted to the ac plane. BBDTAþ did not form a dimer or π-stacking columnar structure. There were short intermolecular S 3 3 3 C interatomic contacts of 3.349(7) and 3.410(7) A˚, shown as broken lines in Figure 1b. The S 3 3 3 C interatomic distances were shorter than 3.50 A˚, Chart 1. The Radical Cation Salt, BBDTAþ

which was the sum of the van der Waals radii of a carbon atom (1.70 A˚) and a sulfur atom (1.80 A˚),26 indicating that there may be a strong magnetic interaction between neighboring BBDTAþ cations in the BBDTAþ layer. On the other hand, there were no short interatomic contacts between BBDTAþ cations of the cation interlayers. The shortest interatomic S 3 3 3 S distance between the BBDTAþ cation layers was 4.327(2) A˚, shown as broken lines in Figure 1a. This value was much larger than the sum of the van der Waals radii. This finding indicates that the interlayer magnetic interaction between BBDTAþ cations is weaker than the intralayer interaction. The TlBr4- layers may play an important role in assuming the two-dimensional magnetic nature in this material. Then, this salt can be regarded as a two-dimensional square planar lattice network, as shown in Figure 1c. Figure 1d presents a space-filling model for the molecular alignment of the BBDTAþ layer in 1 viewed along the b axis. The marked sphere corresponds to a bromide atom of the TlBr4- anion. Each bromide atom was surrounded by four BBDTAþ cations. There were short interatomic S 3 3 3 Br contacts of 3.536(1) and 3.622(1) A˚. The Coulombic interaction between TlBr4- and BBDTAþ may play an important role for the molecular alignment in the BBDTAþ layer. Figure 2 shows the crystal structure of BBDTA 3 TlI4 (2), which was also isolated as just a single phase. 2 crystallized in an orthorhombic Pnma space group with half of the molecule crystallographically independent. 2 also had a layered structure, which was similar to R-BBDTA 3 GaBr418 and 1, as shown in Figure 2a. Figure 2b shows the molecular alignment of BBDTAþ in 2. BBDTAþ also formed a square planar lattice structure. The molecular long axis of BBDTAþ was almost parallel to the ac plane, and the molecular plane of BBDTAþ was perpendicular to the ac plane, in contrast to 1. The short interatomic distances were 3.32(1) A˚ and 3.36(1) A˚ for N 3 3 3 C in the BBDTAþ layer, shown as broken lines in Figure 2b. Figure 2c presents a space filling presentation of the BBDTAþ layer along the b axis in 2. The marked sphere indicates the iodide atom of TlI4-. Each iodide atom was surrounded by four BBDTAþ cations, as well as one of the bromide atoms of 1. Although these salts have the same crystallographic symmetry, the nearest neighbor alignment of BBDTAþ cations in 2 is different from that in 1. It seems that the size of the halogen atom of the counteranions determines the molecular orientations of BBDTAþ. The salt BBDTA 3 InI4 (3) is isostructural with 2 as a single phase (not shown). The molecular alignment of BBDTAþ in 3 was almost the same as that in 2. The short interatomic N 3 3 3 C distances between BBDTAþ cations were 3.276(8) and 3.326(8) A˚, which were shorter than those in 2. Thus, we can regard these three salts 1-3 as two-dimensional square planar magnetic systems with a weak magnetic interaction between the BBDTAþ layers.

Table 1. Crystallographic Data for 1-3, and R-BBDTA 3 GaBr4 crystal system space group a/A˚ b/A˚ c/A˚ V/A˚3 Z R1 wR2

1

2

3

R-BBDTA 3 GaBr418

orthorhombic Pnma 10.0243(10) 16.3867(18) 9.0352(10) 1484.2(3) 4 0.0402 0.0750




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Figure 1. Crystal structure of BBDTA 3 TlBr4 (1). (a) Layered structure composed of the radical cation BBDTAþ and the counteranion TlBr4-, (b) molecular alignment of the organic radical cation layer, (c) a two-dimensional square planar lattice, (d) a space filling model of the organic layer and Br atoms of the counteranion.

Magnetic Properties. Figure 3 shows the temperature dependence of the paramagnetic susceptibility, χp, of 1-3. The magnetic behavior of 1 was quite different from that of 2 and 3, as shown in Figure 3a. The maximum values of χp in 2 and 3 were 0.202 emu mol-1 and 0.175 emu mol-1 respectively, which were more than 20 times as large as the maximum value, 0.008 emu mol-1 obtained for 1. The salt 1 showed magnetic behavior similar to RBBDTA 3 GaBr4.18 The inset of Figure 3(a) shows that the plot of χp vs T exhibited a broad maximum around 16 K, presumably as a result of a short-range magnetic order associated with the two-dimensional structural character. At 11.8 K, the paramagnetic susceptibility increased rather sharply. This behavior is ascribed to antiferromagnetic phase transitions with slight canting from antiparallel spin alignment at the stated temperature. A similar magnetic anomaly has been reported in a canted antiferromagnet, (n-C3H7NH3)2MnCl4,27 with a two-dimensional square planar lattice. From Figure 3a, it can be seen that the χp value of 2 and 3 showed an initial increase, followed by a decrease with temperature, and maintained an almost constant value below 7.6 and 11.3 K, respectively, which marked the point at which these materials showed the

maximum χp value. Below these temperatures, the χp values were almost constant. There was no divergence in the χp value. The observed temperature dependences and anomalies are characteristic of an antiferromagnetic phase transition.28 Figure 3b shows the temperature dependence of the product of χp and T in these three salts. In 1, the χpT value decreased, decreasing with temperature. This finding indicates that there are antiferromagnetic interactions between BBDTAþ cations in the organic cation layers. The anomaly observed in the region of 10 K, as shown in the inset of Figure 3b, was related to antiferromagnetic phase transition. In contrast to 1, the χpT value in 2 and 3, increased, as the temperature was decreased with temperature until about 9 and 12 K, respectively, and then, decreased below these temperatures. This suggests that there is a ferromagnetic interaction in the BBDTAþ cation layers and a weak antiferromagnetic interaction between the layers. The analytical expression for the paramagnetic susceptibility of the isolated two-dimensional square lattice model is given as χ2D ¼

X C ðRn =2n n!Þyn ½1 þ T ng1

ð1Þ

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Figure 2. Crystal structure of BBDTA 3 TlI4 (2): (a) layered structure composed of BBDTAþ and the counteranion TlI4-, (b) molecular alignment of the organic radical cation layer, (c) a space filling model of the organic layer and I atoms of the counteranion.

where C is the Curie constant (fixed as 0.375 emu K mol-1), y = J/(kBT), J is half of the intermolecular exchange coupling constant, n is a positive integer number, and the coefficients an are as follows: a1 = 4, a2 = 16, a3 = 64, a4 = 416, a5 = 4544, a6 = 23488, a7 = -207616, a8 = 4205056, a9 = 198295552, and a10 = -2574439424.29 The interlayer magnetic interaction in the paramagnetic susceptibility, χp, of these salts then can be deduced by using a simple mean field approximation:28 χp ¼

χ2D 2zJ 0 χ 1- 2 2 g μΒ N 2D

ð2Þ

where zJ0 , g, μB, and N are the interlayer magnetic interaction parameter regarded as a mean field, the g factor, the Bohr magneton, and Avogadro’s number, respectively. On the basis of eqs 1 and 2, the magnetic parameters, 2J/kB = -11.2 K and zJ0 /kB = -4.1 K for 1, 2J/kB = þ11.4 K and zJ0 /kB = -3.7 K for 2, and, 2J/kB = þ12.8 K and zJ0 /kB = -2.3 K for 3, were estimated by curve fitting, as shown by the solid curves in Figure 3b. Figure 4 depicts the field dependence of magnetization of 1-3 at 2 K. As shown in Figure 4a, magnetization of 1 gradually increased with increasing magnetic field until 10 kOe. An anomaly was observed near 12 kOe. The magnetization

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Figure 3. Temperature dependence of the dc susceptibility of 1-3: (a) the χp vs T plots. (Inset) the magnified χp vs T plot of 1, (b) the χpT vs T plots. (Inset) the magnified χp vs T plot of 1.

behaviors of 1 were very similar to that of R-BBDTA 3 GaBr4.18 We interpret this field dependence as follows: spin canting in 1 may induce a large magnetic moment in the BBDTAþ layer and the weak antiparallel coupling between the organic layers cancel the magnetic moments. It seems that the antiparallel alignment of the magnetic moments between the organic layers is broken up at an external magnetic field of around 12 kOe. The magnetization due to spin canting in the BBDTAþ layer was 31 erg Oe-1 mol-1, as estimated by extrapolating the magnetization above 20 kOe. The canting angle from the antiparallel alignment of spins was ca. 0.32°, which was consistent with those of other organic weak ferromagnets.30,31 Figure 4b shows the field dependence of magnetization of 2 and 3 at 2 K. The M-H curves were quite different from 1. The magnetization value increased rapidly when the magnetic field was weak, and then, saturated until about 50 kOe. The magnetization value in both salts was approximately 5500 erg Oe-1 mol-1. This value is extremely close to the theoretical value of 5585 erg Oe-1 mol-1 for the ferromagnetic ordered state of 1 mol of the S = 1/2 (g = 2.00) spin.28 It seems that 2 and 3 show an antiferromagnetic-to-ferromagnetic phase transition under low field. This behavior is characteristic of a metamagnet.32 In order to investigate the magnetic behavior of the three salts under low magnetic field, the ac susceptibilities were measured around the temperature at which the magnetic


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Figure 4. Field dependence of magnetization at 2 K for (a) 1, (b) 2 and 3.

anomaly was observed in the dc measurements. Figure 5 shows the temperature dependence of the real part of the ac susceptibilities for 1-3. The value of the real part, χ0 , in 1 was much smaller than those of 2 and 3, as well as the results of the dc measurements. The inset of Figure 5 shows that an anomaly in the temperature dependence of the real part of the ac susceptibilities of 1 near 11.6 K while cusps were observed at 8.9 and 11.3 K in 2 and 3, respectively. The magnetic phase transitions in 1-3 are thought to occur at these temperatures. In these salts, the imaginary parts χ00 showed no remarkable anomaly around the stated temperatures (not shown). This means that these salts have no spontaneous magnetization. Thus, we concluded that the three salts 1-3 showed canted antiferromagnetic behavior below 11.6 K, and metamagnetism below 8.9 K and below 11.3 K, respectively. Two-dimensional square planar S = 1/2 Heisenberg magnetic materials have been found in some magnetic materials, in particular, inorganic materials and metal complexes. Because of the aforementioned difficulty in constructing organic radical magnets with a two-dimensional magnetic system, only a few examples of two-dimensional square planar S = 1/2 Heisenberg magnetic materials have been reported in organic radical crystals. McManus et al. found that the methylbenzodithiazolyl, MBDTA, had a strong intermolecular antiferromagnetic interaction of 2J/kB = -144 K for the square planar Heisenberg model and showed no magnetic phase transition until 1.8 K.33 In our last study, we have found that R-BBDTA 3 GaBr4, which was

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Figure 5. Temperature dependence of the real part of the ac susceptibilities in 1-3. The inset shows temperature dependence of the real part of the ac susceptibilities of 1.

a structure similar to 1, showed an antiferromagnetic phase transition at 15.5 K.18 Two-dimensional square lattice magnets with a ferromagnetic interaction are very rare. To the best of our knowledge, 2 and 3 are the first examples in organic magnetic materials. In addition, organic radicals showing metamagnetism are equally rare.34-39 The previously reported organic radical metamagnets had ferromagnetic chain networks and weak interchain antiferromagnetic interactions in their crystals, with their transition temperatures below 5 K. The new metamagnets 2 and 3 have unique characterstics; their transition temperatures are higher than those of traditional organic radicals and there are twodimensional ferromagnetic networks in their crystals. Molecular Orbital Calculations. There was an antiferromagnetic interaction between the nearest neighbor BBDTAþ cations in 1, whereas a ferromagnetic interaction was observed in 2 and 3. In order to investigate the origin of the differences in their intermolecular magnetic interactions, we performed molecular orbital calculations for these salts. The J value of the magnetic coupling constant 2J between neighboring magnetic molecules could be calculated according to the following equation,40 J ¼

E - HS E HS - LS < S 2 > LS

ð3Þ

where xE and x< S2 > denote the total energy and total anglar momentum of the spin state x, respectively. Using the atomic coordinates from the crystal data of 1-3, the 2J value was estimated to be -1.5 K for 1, þ96 K for 2, and þ104 K for 3. The results roughly corresponded to the value of the magnetic coupling constants from the magnetic measurements of 1-3. The relationship between the magnetic orbital and the molecular orientation of BBDTAþ in the crystals was also investigated. In general, intermolecular magnetic interactions in organic radical crystals are governed by the overlap integrals between the magnetic orbitals of the neighboring radical species.41 An orthogonal relation between neighboring singly occupied molecular orbitals (SOMO) induces a

ferromagnetic interaction, while an overlap between them produces an antiferromagnetic interaction. Figure 6 shows the schematic representation of the SOMO of BBDTAþ and the molecular orientation in neighboring BBDTAþ in the BBDTAþ layers in 1-3. The magnetic orbital of BBDTAþ distributed on the molecular plane, as shown in Figure 6a. The SOMO lobes are positioned on the four sulfur atoms, two nitrogen atoms and the two central carbon atoms, whereas nodes are present on the four carbon atoms bonded to the sulfur atoms. Figure 6b shows the schematic representation of the molecular orientation of BBDTAþ cations in 1. The nitrogen atom of a BBDTAþ cation was close to the central carbon atom of the other BBDTAþ cation. In this situation, the lobe of the SOMO on the nitrogen atom may overlap with that on the central carbon atom, and then an antiferromagnetic interaction may work between the two radical cations. Figure 6c shows the schematic representation of the molecular orientation of BBDTAþ cations in 2 or 3. The molecular orientation in 2 or 3 was different from that in 1. The nitrogen atom of one BBDTAþ cation is in close proximity to the two carbon atoms bonded to the sulfur atoms of the other BBDTAþ cation. In this molecular orientation, the lobe of the SOMO on the nitrogen atom is close to the nodes of the SOMO on the two carbon atoms. As a result, the overlap integral in the region where the product of these SOMOs is positive cancels that in the region where the product is negative. A similar molecular orientation has been observed in other organic radical crystals which exhibited a ferromagnetic interaction.14,42-45 The minimal overlap between the SOMOs is likely to be responsible for the ferromagnetic coupling between the BBDTAþ cations. Finally, we summarized the nearest neighbor magnetic coupling constants, transition temperatures, types of magnetic networks, and magnetic ground states of BBDTAþ salts including various diamagnetic tetrahalogenometallate anions in Table 2. It is interesting that a single organic radical cation shows such a range of variation in the magnetic phenomena in its salts. The molecular orientation and alignment of BBDTAþ, and magnetic properties in its salts, are largely dependent on the counteranions. The nearest neighbor magnetic interactions between BBDTAþ cations in these salts had values ranging from þ31 to -1300 K. The magnetic properties of the salts of BBDTAþ may be easily controlled by judicious combination with the appropriate anion, and attractive magnetic materials which the approximate the theoretical models may be produced. However, the intermolecular ferromagnetic interactions in the BBDTAþ salts are much weaker than the intermolecular antiferromagnetic interactions. This means that the orthogonal relation of SOMOs between BBDTAþ cations does not give rise to strong intermolecular magnetic interactions, and it is difficult to realize the ferromagnetic ordered state which overcomes thermal fluctuation of spins at higher temperatures in BBDTAþ salts. Rather, BBDTAþ may be useful as a building block in high Tc-ferrimagnetic materials utilizing very strong antiferromagnetic interactions.15,46 Summary In this paper, we reported the crystal structure and magnetic properties of three radical cation salts, BBDTA 3 TlBr4 (1), BBDTA 3 TlI4 (2), and BBDTA 3 InI4 (3). In the three salts, the BBDTAþ cations formed a square planar lattice-like arrangement, and canted antiferromagnetism was observed below 11.6 K for 1, whereas 2 and 3 exhibited metamagnetism

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Figure 6. Schematic representation of SOMO in BBDTAþ (a) and the molecular orientation of neighboring BBDTAþ in 1 (b) and in 2 or 3 (c). Table 2. Nearest Neighbor Magnetic Coupling Constants, Magnetic Transition Temperatures, Molecular Arrangements, and Magnetic Properties for the Radical Cation Salts, BBDTA 3 X X

2J/kB (K)

GaCl4 3 CH3CN γ-GaCl4 R-GaBr4 β-GaBr4 γ-GaBr4 InCl4 InBr4 1 2 3

∼ -1300 þ31 -36 þ4.5, -198 -60 -277 -220 -11.2 þ11.4 þ12.8

Tc (K) 7.0 15.5 0.4 108 K 250 K 11.6 8.9 11.3

below 8.9 and 11.3 K, respectively. Two-dimensional square lattice structures and metamagnetism are very rare in organic radical materials. The nearest neighbor intermolecular magnetic interactions in these salts were related to overlap or orthogonal relation between the SOMOs. In the course of our studies, we have found that BBDTAþ showed magnetic diversity such as diamagnetism, paramagnetism, the spinPeierls transition, antiferromagnetism, ferromagnetism, and ferrimagnetism in its salts, with various counteranions, in addition to metamagnetism reported in this paper. Thus, BBDTAþ is highly attractive as a building block for molecule-based magnetic materials. Acknowledgment. We thank Hirokazu Sakamoto for magnetic measurements and Takashi Kawakami for molecular orbital calculations. This study was supported by the Naito Foundation, by the Foundation for Promotion of Material Science and Technology of Japan, and by Grants-in-Aid for Scientific Research (No. 21550138) from the Ministry of Education, Culture, Sports, Science and Technology of Japan. Supporting Information Available: Crystallographic data of 1-3, atomic coordinates, computational conditions and energies for molecular orbital calculations, and complete ref 25. These materials are available free of charge via Internet at http://pubs.acs.org.

References (1) Anderson, P. W. Science 1987, 235, 1196. (2) De Jongh, L. J., Ed. Magnetic Properties of Layered Transition Metal Compounds; Kluwer Academic Publishers: New York, 1990. (3) Miller, J. S. Ed. Extended Linear Chain Compounds; Plenum Press: New York, 1983; Vol. 3. (4) Keller, H. J., Ed. Chemistry and Physics of One-Dimensional Metals; Plenum Press: New York, 1977. (5) Bray, J. W.; Hart, H. R., Jr.; Interrante, L. V.; Jacobs, I. S.; Kasper, J. S.; Watkins, G. D.; Wee, S. H. Phys. Rev. Lett. 1975, 35, 744. (6) (a) Wolmersh€ auser, G.; Schnauber, M.; Wilhelm, T. J. Chem. Soc., Chem. Commun. 1984, 573. (b) Wolmersh€auser, G.; Johann, R. Angew. Chem., Int. Ed. 1989, 28, 920.

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Structural and Magnetic Studies on Two-Dimensional Square Planar

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