Article pubs.acs.org/crystal
Utilization of σ‑Holes on Sulfur and Halogen Atoms for Supramolecular Cation···Anion Interactions in Bilayer Ni(dmit)2 Anion Radical Salts Tetsuro Kusamoto,*,† Hiroshi M. Yamamoto,# and Reizo Kato RIKEN, 2-1 Hirosawa, Wako-shi, Saitama 351-0198, Japan S Supporting Information *
ABSTRACT: We prepared novel Ni(dmit)2 anion radical salts with ethyl-4-halothiazolium cations (Et-4XT, with X denoting the halogen: I, Br, or Cl), (Et-4IT)[Ni(dmit)2]2 (1), (Et-4BrT)[Ni(dmit)2]2 (2), and (Et-4ClT)2[Ni(dmit)2]5 (3). Single-crystal Xray diffraction analysis of 1−3 indicates that, unlike the halogen atoms that have only one σ-hole each, the cations’ sulfur atoms each have two σ-holes that lie approximately along the extensions of the C−S bonds. The presence of the σ-holes is supported by electrostatic potential of the cations calculated based on the density functional theory method. In the crystals of 1−3, these σ-holes interact with lone pairs on the terminal thioketone moieties in the Ni(dmit)2 anion radicals to form electrostatic σ-hole bonds (halogen bonds and chalcogen bonds). This results in supramolecular cation···anion networks. Crystal and electronic structure analyses, and electrical and magnetic measurements reveal that the salts 1 and 2 are isostructural bilayer Mott systems, in which two crystallographically different Mott-insulating anion layers coexist in one crystal. The unusual magnetic properties, including the ferromagnetic anomalies of 1 and 2, are consistent with one of the anion layers forming an antiferromagnetic short-range ordering (SRO) state and the other layer forming a ferromagnetic SRO state. The spin-polarization of the Ni(dmit)2 anion radical was shown to influence significantly the observed ferromagnetic interactions, while the antiferromagnetic interactions resulted from π−π overlapping in the anions. The competition between these two interactions dominates the low-temperature magnetic properties of the present bilayer Mott systems. This study reveals that noncovalent intermolecular interactions mediated by σ-holes are influential in preparing novel crystal and electronic structures and that they have the potential to allow the development of materials with unusual physical properties.
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INTRODUCTION Noncovalent intermolecular interactions play significant roles in determining structures and properties of molecule-based systems. For example, hydrogen bonds in biological molecules induce the secondary and tertiary structures.1a,b π−π overlap interactions between π-conjugated molecules mediate electron transport in conducting molecular crystals.1c The solid-state properties of molecular crystals largely depend on the molecular arrangements in the crystal lattice; thus, the introduction of weak but non-negligible noncovalent intermolecular interactions into the molecular crystal is an attractive strategy to develop new solid state physical phenomena.1d−f Among several noncovalent interactions, the halogen bond is a highly directional noncovalent interaction that operates between an electron-deficient region on the outer side of the halogen, X, in a molecule, R−X, and negative sites on other molecules, such as the lone pair of a Lewis base.2 The electrondeficient region on the halogen atom is called a σ-hole; it is associated with highly positive electrostatic potentials that are greater than those on the other areas of the atomic surface. The noncovalent intermolecular interaction mediated by the σ-hole is called a σ-hole bond.3 © XXXX American Chemical Society
We have been interested in introducing noncovalent interactions such as halogen bonds into a molecular conductor based on the Ni(dmit)2 (dmit = 1,3-dithiole-2-thione-4,5dithiolate) anion radical. 4 − 6 Our study of (alkyldihalopyridinium)[Ni(dmit)2]2 salts has shown that halogen bonds between the halogen atom on the cation and the lone pair of the anion’s terminal thioketone moiety lead to a new type of crystal structure, called a bilayer system.4,5 The bilayer system contains two crystallographically independent (i.e., nonequivalent) Ni(dmit)2 anion layers in one crystal. The bilayer salts form unusual electronic structures, such as the coexistence of metallic and Mott insulating states observed in (methyl-3,5-diiodopyridinium)[Ni(dmit)2]2, which results in interesting magnetic and conducting properties. Our next study on a supramolecular Ni(dmit)2 anion radical salt with Et-4BrT cation (Et-4BrT = ethyl-4-bromothiazolium), (Et-4BrT)[Ni(dmit)2]2 (2), has shown that 2 is a novel bilayer Mott system, in which two crystallographically different Mott-insulating Received: July 8, 2013 Revised: August 16, 2013
A
dx.doi.org/10.1021/cg401028u | Cryst. Growth Des. XXXX, XXX, XXX−XXX
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anion layers coexist in one crystal.6 The X-ray crystallographic investigations and density functional theory (DFT) calculations revealed that the sulfur atom in the thiazolium ring of the cation has two σ-holes along the extension of the C−S bonds. In the crystal, the three σ-holes on the Et-4BrT cation (two on the sulfur atom and one on the bromine atom at the 4-position) interact with the lone pairs of the thioketone moieties of the Ni(dmit)2 anion to form chalcogen and halogen bonds. The chalcogen bond indicates a noncovalent interaction mediated via the σ-holes on the sulfur atom.25 These noncovalent anion···cation interactions form a supramolecular network. It is remarkable that the presence of σ-holes on the sulfur atoms were reported by Burling and Goldstein in 1992, who detected electrostatically driven S···O and Se···O interactions in thiazofurin and selenazofurin, both antitumor agents.7 Politzer et al. reported the appearance of the σ-holes on a Group V−VII atom, including sulfur. They suggested the electrostatic nature of the σ-hole bonds.3,8 Given that sulfur has two σ-holes, while halogen atoms have only one, the chalcogen bonds formed by sulfur atoms can provide more diverse and complex molecular arrangements than halogen bonds. However, utilization of the σ-holes on the sulfur for supramolecular assembly or crystal engineering is rarely reported. In this study, we aimed to investigate the utility and generality of the chalcogen and halogen bonds in forming a supramolecular network and focused on a series of Ni(dmit)2 anion radical salts with Et-4XT cations (X = I, Br, Cl; Chart 1).
and conducting and magnetic properties of 1−3 were studied.23 The crystal structure of (Et-4BrT)Br was also investigated to confirm the cation’s σ-hole bond forming ability with a simple Br− anion. The results show that the σ-holes on the sulfur atom in the Et-4XT cations play important roles in determining the crystal structure. The crystal structure is shown to be controlled by tuning the strength of the halogen bond. In 1 and 2, Xcation··· Sanion and Scation···Sanion interactions mediated by the σ-holes afford bilayer Mott systems with unusual magnetic properties: ferromagnetic (FM) and antiferromagnetic (AFM) SRO layers coexist in one crystal lattice.6 Comparison of the magnetic properties and the crystal and electronic structures of the two isostructural bilayer Mott salts (1 and 2) indicates that the magnetic properties of these bilayer Mott systems originate from competition between the FM intermolecular interactions via spin polarization and the AFM interactions via π−π overlap interactions.
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RESULTS AND DISCUSSION Preparation of Et-4XT Cations and σ-Holes on Et-4BrT. Et-4XT cations (X = I, Br, Cl) were synthesized from the appropriate 4-halothiazoles via ethylation reaction. 4-Halothiazoles reacted with Et3O·BF4 to yield (Et-4IT)BF4, (Et4BrT)BF4, and (Et-4ClT)BF4, respectively. A metathesis reaction between (Et-4BrT)BF4 and Me4N·Br in CH3CNEt2O afforded single crystals of (Et-4BrT)Br, the crystal structure of which was analyzed by single-crystal X-ray diffraction (XRD) to confirm the existence of σ-holes in the cation. As shown in Figure 1a, Brcation···Branion contact (3.262(2) Å) is observed in the crystal lattice. The Br− anion is located along the extension of the C−Br bond of the cation, as similarly observed in (Et-2,5-DBrP)Br·H2O.5 This is indicative of a σhole-mediated, noncovalent interaction, i.e., a halogen bond. Interestingly, the observed Scation···Branion distance (3.417(3) Å) is shorter than the sum of the van der Waals radii of S and Br atoms (3.65 Å). The Br− anion lies within the plane of thiazolium heterocycle and is located along the extension of one of the C−S bonds in the cation. A similar S···Nu (Nu:
Chart 1
We prepared new salts (Et-4IT)[Ni(dmit)2]2 (1) and (Et4ClT)2[Ni(dmit)2]5 (3). The crystal and electronic structures
Figure 1. Crystal structure of (Et-4BrT)Br viewed along the a-axis. Dotted lines indicate atomic contacts (Å) between cation and anion (a). View along the b-axis. Cations are depicted as capped sticks and anions as balls (b). Electrostatic potential of Et-4XT cations mapped onto isoelectric density surface (isovalue of 0.005 electrons/Å3) around sulfur atoms (c) and halogen atoms (d). Color bar indicates electrostatic potential. B
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[Ni(dmit)2]− and [Ni(dmit)2]0 (Table 2).10 This result suggests that both anions are in the same oxidation state, namely, [Ni(dmit)2]−0.5. In the crystal, a Brcation···Sanion halogen bond is detected between the cation and the anion A, which is shorter than the sum of the van der Waals radii of S and Br atoms (3.65 Å). Two short Scation···Sanion distances are observed between the cation and the anion B, in which the Sanion is located almost along the extension of two C−S bonds in the cation. The lone pair of the thioketonic terminal sulfur atom in the Ni(dmit)2 anion is oriented toward the σ-hole on the cation,11 indicating that these Scation···Sanion contacts occur through electrostatically driven chalcogen bonds. It should be emphasized that, based on these results, the sulfur atom can form two chalcogen bonds along the extension of the two C−S bonds. These results are in good agreement with the calculated electrostatic potentials of the cations shown in Figure 1c, in which the sulfur atom has two regions of high electrostatic potential. The salts 1 and 2 show three kinds of σ-hole bonds: one Brcation···Sanion halogen bond and two Scation···Sanion chalcogen bonds. We note that Hcation··· Sanion hydrogen bond (2.962(1) Å) is found in the crystal lattice, which would be electrostatically driven as indicated by the highly positive electrostatic potential on the atomic surface of the hydrogen atom. These noncovalent cation···anion interactions enable a nonsymmetrical steric environment around the cation, which eventually affords a bilayer structure as discussed below. The two anions, A and B, independently construct layered structures (layers A and B) parallel to the ac plane (Figure 2b). The anions exhibit a “solid-crossing” column structure, in which the two anion layers are different in terms of the anion stacking direction. This structural motif is commonly observed in Pd(dmit)2 anion radical salts.12 The anions stack along the aaxis in layer A and along the a+c direction in layer B. The anion layers are separated from each other by cation layers along the b-axis. Several intermolecular S···S contacts are observed within each layer, while no contacts are detected between the layers A and B. The overlap integrals among the lowest unoccupied molecular orbitals (LUMOs) of Ni(dmit)2 suggest that, similar to 2, the anions are strongly dimerized in layer A; however, the anions are weakly dimerized, and moderate interactions emerge along the diagonal direction (p, r) in layer B (Table 3, Figure 2c,d). These results reveal that 1 is a new bilayer system. Figure 3 shows electronic band structures calculated for each layer based on the tight-binding approximation. In both layers, the LUMO band consists of upper and lower branches separated by an energy gap that is due to the dimerization of the anions. Layer A forms a narrow conduction band with a twodimensional (2D) Fermi surface, while layer B constructs a 2D Fermi surface with a wider bandwidth. In both layers, the LUMO bands should each be 1/4-filled by considering the [Ni(dmit)2]−0.5 charged state of the anions; thus, the Fermi level (εF) is located across the lower band. Consequently, an effectively half-filled state is realized. These results indicate that (Et-4IT)[Ni(dmit)2]2 is a bilayer Mott system, which was confirmed by measurement of the electrical and magnetic properties (vide infra). The layer A in 1 forms a 2D Fermi surface, while that in 2 forms a one-dimensional (1D) surface. The overlap integrals b and r are much larger in 1 than in 2, affording a different dimensionality of the Fermi surface in layer A. It is noteworthy that, in both 1 and 2, layer B forms a 2D electronic structure, because the Ni(dmit)2 anion radicals tend
nucleophile) interacting motif is found in crystals of inorganic sulfides, where the line of S···Nu contact exists approximately along the extension of the bond to the sulfur.9 These results suggest that the σ-hole exists on the sulfur atom in the cation and that they are capable of constructing anion···cation chalcogen bonds.24 DFT calculations of the electrostatic potentials of the cations also support the existence of the σholes on the sulfur atoms (Figure 1c,d). The figures show areas of highly positive electrostatic potential (colored blue) located approximately along the extensions of the two C−S bonds. They are consistent with the positions of σ-holes expected from the crystal structure analysis. These areas extend onto the atomic surfaces of proximal hydrogens and so cause cooperative S···anion and H···anion interactions, which are discussed later. Other areas of highly positive electrostatic potential are found on the halogen atoms; they can behave as halogen bond donor sites. The potential increases with the halogens’ increasing size, indicating that the halogen bond capability is ranked Cl < Br < I. This tendency is consistent with that observed in CF3X (X = Cl, Br, I).2 Crystal and Band Structures of (Et-4IT)[Ni(dmit)2]2 (1). The crystal structure of (Et-4IT)[Ni(dmit)2]2 (1) is shown in Figure 2. 1 is isostructural with (Et-4BrT)[Ni(dmit)2]2 (2).6
Figure 2. Crystal structure of (Et-4IT)[Ni(dmit)2]2 (1): Crystallographically independent cation and anions. Dotted lines indicate cation···anion atomic contacts (Å) (a). View along the c-axis (b). Anion arrangement viewed along the molecular long axis of anion A (c) and anion B (d). a, b, p, q, r in (c) and (d) indicate the overlap integrals listed in Table 3.
The unit cell contains two crystallographically independent Ni(dmit)2]2 anions, A and B, and one crystallographically independent Et-4IT cation. Each anion forms a planar molecular structure, in which the Ni atom adopts a squareplanar coordination geometry. The two anions display similar bond lengths to each other and are between those of C
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Table 1. Crystal Data and Refinement Parameters for 1−3 and (Et-4BrT)Br empirical formula Fw crystal dimension (mm) T (K) crystal system lattice parameters
space group Z value λ (Å) μ (Mo Kα) (mm−1) D(calc) (g/cm3) R1 WR2 goodness of fit
(Et-4BrT)Br
(Et-4IT)[Ni(dmit)2]2 (1)
(Et-4BrT)[Ni(dmit)2]2 (2)
(Et-4ClT)2[Ni(dmit)2]5 (3)
C5H7Br2NS 272.99 0.2 × 0.03 × 0.01 293 orthorhombic a = 6.932(5) Å b = 10.526(7) Å c = 11.729(8) Å α = 90° β = 90° γ = 90° V = 856(1) Å3 Pmcn 4 0.7107 9.634 2.119 0.0315 0.0535 0.815
C17H7INNi2S21 1142.82 0.2 × 0.1 × 0.07 293 triclinic a = 8.458(2) Å b = 32.333(7) Å c = 6.573(2) Å α = 93.159(3)° β = 105.191(3)° γ = 90.361(3)° V = 1731.7(7) Å3 P1̅ 2 0.7107 3.264 2.192 0.0336 0.0798 1.032
C17H7BrNNi2S21 1095.83 0.25 × 0.1 × 0.05 293 triclinic a = 8.4145(2) Å b = 32.407(1) Å c = 6.5610(1) Å α = 93.383(5)° β = 106.376(4)° γ = 89.798(6)° V = 1713.36(9) Å3 P1̅ 2 0.7107 3.564 2.124 0.0318 0.0694 0.985
C20H7ClNNi2.5S26 1277.05 0.3 × 0.1 × 0.03 293 triclinic a = 17.171(3) Å b = 19.413(3) Å c = 6.577(1) Å α = 104.896(6)° β = 96.941(7)° γ = 75.535(7)° V = 2047.8(6) Å3 P1̅ 1 0.7107 2.558 2.071 0.0591 0.1297 1.013
Table 2. Average Bond Lengths (Å) in Ni(dmit)2 Anion in Each Salta
complex
anion
a
b
c
d
e
f
1
A B A B A B C
2.159 2.157 2.163 2.159 2.159 2.153 2.163 2.143 2.206
1.699 1.699 1.698 1.703 1.698 1.691 1.701 1.69 1.742
1.382 1.381 1.380 1.371 1.375 1.384 1.381 1.393 1.377
1.738 1.739 1.745 1.743 1.743 1.741 1.746 1.735 1.737
1.728 1.725 1.730 1.728 1.729 1.736 1.733 1.739 1.726
1.646 1.652 1.648 1.653 1.638 1.635 1.645 1.635 2.052
2 3
Ni(dmit)2010b Ni(dmit)2−110a a
Figure 3. Band structures and Fermi surfaces of layers A (a) and B (b) in 1.
LUMO of Ni(dmit)2 is shown.
intermolecular overlap integral14 and contribute to the formation of the 2D electronic structure. These results indicate that the introduction of supramolecular cation···anion interactions can lead to new types of anion arrangements and resultant electronic structures. Crystal and Band Structures of (Et-4ClT)2[Ni(dmit)2]5 (3). The crystal structure of salt 3 is shown in Figure 4. The unit cell contains five Ni(dmit)2 anions and two Et-4ClT cations. Two anions (A and C), half of anion B, and one Et4ClT cation are crystallographically independent. The cation is
to form 1D electronic structures associated with weak side-byside interactions due to the b2g symmetry of the LUMO.13 An exception is the unconventional molecular stacking known as “spanning overlap” that provides the 2D electronic structure.12a Layer B shows another example of 2D system formation based on a novel side-by-side arrangement derived from Scation···Sanion and Hcation···Sanion interactions. For interaction p and r in Figure 2d, two adjacent anions are arranged so as to reduce cancellation of the interatomic overlap integrals. These arrangements yield moderate values of the side-by-side
Table 3. Overlap Integrals (×10−3) between LUMOs of Ni(dmit)2a (Et-4IT)[Ni(dmit)2]2 (1) (Et-4BrT)[Ni(dmit)2]2 (2) (Et-4ClT)2[Ni(dmit)2]5 (3) a
layer
a
b
A B A B
−17.7 10.7 −17.1 10.6 0
1.52 2.50 0.15 3.96 0.94
c
p
q
r
s
t
u
−16.5
−0.58 −2.03 −0.05 −2.63 0
0.42 0.49 0.41 0.46 0.41
1.56 −5.78 1.88 −5.50 −1.90
0.48
−1.84
0.31
See also Figures 2 and 4. D
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chalcogen bond between the cation and the anion C, in which Sanion is located approximately along the extension of the C−S bond in the cation. The lone pair of the thioketone moiety in the anion is oriented toward the σ-hole on the cation, as observed in 1 and 2. Only one of the two σ-holes on the sulfur atom participates in the forming of a chalcogen bond, whereas both σ-holes form Scation···Sanion chalcogen bonds in 1 and 2. This difference in σ-hole bonding manner causes 3 to show different molecular packing in the crystal. The anions A, B, and C are stacked along the a-axis with an -A-A-C-B-C- manner. The formation of a C-B-C trimer is indicated by the large value of the overlap integral c (Table 3). The anion stacks are arranged within the ac-plane to form an anion layer. Weak interstack overlap integrals along the c-axis cause the electronic structure to be 1D rather than 2D. The anion layers are separated by the cation layer along the b-axis, and no short interlayer Sanion···Sanion contacts are detected. These results reveal that 3 has only one kind of anion layer in the crystal, making it a monolayer system. Considering that there are two electrons on the five Ni(dmit)2 units, forming a 1/5-filled state, and that there is 5-fold periodicity of the anion stacking, 3 is expected to be a band insulator. The band insulating nature is confirmed by the calculated band structure (Figure S4, Supporting Information). The bond lengths in the three anions do not show large differences, which indicates that they have the same formal charge. Characterization of the σ-Hole Bonds. The crystal structures of 1−3 and (Et-4BrT)Br provide information about the characters of Xcation···Sanion halogen bonds (X: halogen) and Scation···Sanion (Scation···Br−) chalcogen bonds. As listed in Table 4, Xcation···Sa3 distance (dXS) decreases in the order dIS < dBrS < dClS, in contrast with the order of the van der Waals radii (rX): rCl < rBr < rI. 1 shows a value of dIS that is 16% shorter than the sum of the radii rI and rS; the corresponding reductions (ΔdXS) in salts 2 and 3 are 12% and 9%, respectively. ΔdBrBr in (Et-4BrT)Br is 11%, similar to dBrS in 2. These differences suggest that the strengths of the halogen bonds are ranked Cl < Br < I, consistent with previous reports.2 The C−X···S angle (α) is similar in all four salts, although 3 shows a slightly different value, probably due to the weaker halogen bonding through the Cl atom. These results clearly indicate the highly directional character of the halogen bonds. High directionality of the Scation···Sanion and Scation···Branion chalcogen bonds was also elucidated, where the C−Scation··· Sanion (or C−Scation···Branion for (Et-4BrT)Br) bond angles β1 and β2 are similar in each salt. Salts 1 and 2 show two kinds of Scation···Sanion chalcogen bonds (Sc···Sa1 and Sc···Sa2, with bond lengths dSS1 and dSS2, respectively). Interestingly, the Sc···Sa1 contact cooperates with the Hc1···Sa1 hydrogen bond to construct multipoint cation··· anion supramolecular interactions. As shown in Figures 1c and S2, one σ-hole on the sulfur is hybridized with the atomic surface of the neighboring hydrogen (Hc1 in Table 4) to yield widely extended areas of highly positive electrostatic potential. This situation enables an electrostatic multipoint interaction. Meanwhile, Hc2···Sa2 contact is not detected. As a result, the lengths and angles dSS1 and β1 slightly differ from dSS2 and β2. 3 does not show the Hc1···Sa1 contact and displays Cc···Sa1 contact (3.44(1) Å); Cc indicates a carbon atom next to the Hc1 in the cation, as shown in Table 4. Similarly, the Cc···Branion contact (3.535(7) Å) is detected in (Et-4BrT)Br. These results show that the sulfur-mediated chalcogen bond is directionalit
Figure 4. Crystal structure of (Et-4ClT)2[Ni(dmit)2]5 (3): Crystallographically independent cation and anions. Dotted lines indicate short contacts (Å) (a). View along the a-axis (b), and anion arrangement viewed along the molecular long axis (c). a, b, c, p, q, r, s, t, u in (c) indicate the overlap integrals listed in Table 3.
disordered at two sites with a 3:1 occupancy ratio (Figure S1, Supporting Information); the major one is displayed in Figure 4. In the crystal, Clcation···Sanion atomic contact indicative of a halogen bond is detected between the cation and the anion A. The halogen bond capacity of the Cl atom is known to be weaker than that of Br and I atoms. The same trend is found in the halogen atoms of the Et-4XT cations, in which the σ-hole on the Cl atom shows less positive electrostatic potentials than those on the Br and I atoms of the other cations (Figure 1d). In these Et-4XT salts, the positive charge on the cation enhances the halogen bond donating ability. In addition to the halogen bond, one short Scation···Sanion contact is assignable to a E
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Table 4. Geometrical Parameters (Å for Bond Lengths and Degree for Angles) of Halogen Bonds and σ-Hole Bondsa
a
complex
dXS(BrBr)
rS + rX
ΔdXS(BrBr)
α
dSS1(SBr)
ΔdSS1(SBr)
β1
dHS(HBr)
dSS2
ΔdSS2
β2
1 2 3 (Et-4BrT)Br
3.161(1) 3.2219(9) 3.233(3) 3.262(2)
3.78 3.65 3.55 3.70
−0.16 −0.12 −0.09 −0.11
170.7(1) 171.30(9) 159.6(4) 175.5(2)
3.511(2) 3.487(1) 3.371(5) 3.417(3)
−0.02 −0.03 −0.06 −0.06
166.4(1) 167.1(1) 157.5(7) 170.7(2)
2.962(1) 2.9778(8)
3.451(2) 3.460(1)
0.04 0.04
158.0(2) 155.7(1)
ΔdXS(BrBr) = dXS(BrBr)/(rS + rX) − 1; ΔdSS1(SBr) = dSS1(SBr)/3.60 − 1; ΔdSS2 = dSS2/3.60 − 1.
bandwidth than layer A.16 These conductivity studies support the bilayer Mott insulating character of 1 and 2, where the charge carriers are mainly localized on each anion dimer [Ni(dmit)2]2−. The differences between the conducting properties of 1 and 2 reflect the layer B of 1 showing weaker anion−anion interactions than that of 2 and are explained by negative chemical pressure effect. Since the van der Waals radius of I is larger than that of Br, the introduction of the I atom in 1 expands the crystal lattice. The lattice expansion, which is confirmed by the increase of crystal volume (Table 1), is accompanied by elongation of the anion···anion distance, which eventually decreases the anion−anion interaction. Indeed, the layer B of salt 2 shows a wider conduction band than that of salt 1 (WB = 0.17 eV vs 0.15 eV) and a smaller energy gap (Eg = 0.08 eV vs 0.09 eV) (Table S1, Supporting Information). The temperature dependence of ρ is semiconducting in 3, with σRT = 0.3 S·cm−1 and Ea = 70 meV. This result is in agreement with the band insulating character of 3 indicated by the crystal and electronic structures. Magnetic Properties of 1, 2, and 3. The static magnetic susceptibilities of the three salts were measured using a superconducting quantum interference device (SQUID) magnetometer (Figure 6). The high-temperature χT behavior of the bilayer salts 1 and 2 can be described by the Curie−Weiss Law to afford Curie constants (C) of 0.397 and 0.372 emu·K·mol−1, respectively, and Weiss temperatures (θ) of −15.6 and −18.0 K, respectively. The obtained C values and room temperature magnetic susceptibilities (χrt) of 1.20 × 10−3 emu·mol−1 for 1 and 1.31 × 10−3 emu·mol−1 for 2 indicate that one spin of S = 1/2 is localized on each [Ni(dmit)2]2− unit. These results provide evidence of the bilayer Mott insulating state of the two salts. The negative θ values suggest dominant AFM interactions between the spins, which are stronger in 2 than in 1. The difference between the θ values is due to the negative chemical pressure effect in 1, as discussed above. In 2, the χT behavior changes from AFM to FM at 30 K.6 In 1, the change is at a lower temperature (20 K). The FM anomaly is apparently weaker in 1 than in 2. Different FM characteristics in the two salts are also observed in plots of magnetization (M) with respect to the magnetic field (H) shown in Figure 6d. At lower magnetic field strengths, M increases rapidly with increasing H, with 2 showing greater increases than 1, and both salts increase
follows the extension of the C−S bondand that the geometry is slightly affected by the environment around the sulfur atom, such as the neighboring hydrogen and carbon atoms. Politzer and co-workers theoretically predicted such a multicontacting mode of the chalcogen bond via sulfur atom in thiazole derivatives, 8 which is indeed observed in the present compounds.15 The experimental results regarding the σ-hole bond confirm that a sulfur atom can hold two σ-holes.3 This is in contrast with halogen atoms, which each have only one σ-hole. Thus, by utilizing the two σ-holes on the sulfur as chalcogen bond donors, a more diverse and complex supramolecular assembly can be achieved. These results indicate that chalcogen bonds can be a powerful tool in crystal engineering. Salts 1−3 are representative examples in that the combination of two kinds of σ-hole bonds (Xcation···Sanion halogen bonds and Scation···Sanion chalcogen bonds) construct a novel anion arrangement that leads to unusual electronic structures and physical properties. Electrical Transport Properties of 1, 2, and 3. The electrical resistivity parallel to the anion layers was measured for each of the salts 1, 2, and 3 using the standard four-probe method (Figure 5).
Figure 5. Temperature dependence of resistivity ρ for (Et-4IT)[Ni(dmit)2]2 (1) (red circles), (Et-4BrT)[Ni(dmit)2]2 (2) (blue triangles), and (Et-4ClT)2[Ni(dmit)2]5 (3) (black diamonds).
The isostructural salts 1 and 2 display semiconducting behavior. Their resistivities (ρ) increase with decreasing temperature, and their room-temperature electrical conductivities (σRT) are 2 and 20 S·cm−1, respectively. The activation energy, Ea, in the high temperature region (>160 K) of 1 (52 meV) is much larger than that of 2 (5 meV). The observed electrical properties of 1 and 2 are mainly due to anion layer B, which constructs a smaller energy gap and wider conduction F
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Origin of Ferromagnetic Behavior in Layer A of 1 and 2. We suggest that the spin-polarization of the Ni(dmit)2 anion radical is crucially important to the FM behavior in layer A of 1 and 2. This anion radical is spin-polarized as shown in Figure 7a. Positive spin density is located on almost all the component
Figure 6. Temperature-dependent χT at 1 T of 1 (a) and 2 (b), magnetic field-dependent χT (c) and magnetization curves at 2 K of 1 (red) and 2 (blue) (d). Dotted line in (d) represents half of a Brillouin function for S = 1/2 (BS=1/2). Figure 7. Spin density distribution of Ni(dmit)2− anion (a) and intermolecular short distances (
