Formation of Three-Dimensional Electronic Networks Using Axially Ligated Metal Phthalocyanines as Stable Neutral Radicals

Abstract

Organic π-radical crystals are potential single-component molecular conductors, as they involve charge carriers. We fabricated new organic π-radical crystals using axially ligated metal phthalocyanine anions ([M^III(Pc)L₂]⁻) as starting materials. Electrochemical oxidation of [M^III(Pc)L₂]⁻ afforded single crystals of organic π-radicals of the type M^III(Pc)Cl₂·THF (M = Co or Fe, THF = tetrahydrofuran), where the π-conjugated macrocyclic phthalocyanine ligand is one-electron oxidized. The X-ray crystal structure analysis revealed that M^III(Pc)Cl₂ formed three-dimensional networks with π-π overlaps. The electrical resistivities of Co^III(Pc)Cl₂·THF and Fe^III(Pc)Cl₂·THF at room temperature along the a-axis were 6 × 10² and 6 × 10³ Ω cm, respectively, and were almost isotropic, meaning that M^III(Pc)Cl₂·THF had three-dimensional electronic systems.

1. Introduction

Ordinary organic radicals are unstable due to their high chemical reactivity, which is caused by the presence of an unpaired electron. However, organic radicals can be considered to have charge carriers, because their highest occupied molecular orbital (HOMO) is singly occupied, leading to a half-filled band in the solid state. Recently, a nonconjugated organic radical crystal has been reported to be conductive, although the conductivity is low in the solid state [1]. This might be due to the low degree of intermolecular overlap for the molecular orbital in which the radical lies. On the other hand, organic π-radicals tend to stack with significant overlap between adjacent molecules, giving rise to conducting pathways. For this reason, organic π-radicals have attracted significant interest from those studying molecular conductors, and it has been reported that despite the expected half-filled band, most organic π-radical crystals behave as Mott insulators because of the larger on-site Coulomb repulsion energy (U) compared to the transfer energy (t) [2,3]. However, external changes such as pressure can control the competition between U and t, leading to drastic phase transitions from the insulating state to metallic or superconducting states being reported for various molecular Mott insulators [3,4]. This means that if a stable neutral π-radical crystal is fabricated, it could be possible to obtain a single-component molecular conductor, which is a hot topic in the study of molecular crystals [5,6,7,8,9].

Axially ligated metal phthalocyanine anions ([M^III(Pc)L₂]⁻) are attractive components for the construction of neutral π-radical crystals as single-component molecular conductors, because the wide π-conjugated system of phthalocyanine (Pc) might suppress U. Inabe expected that the electrochemical oxidation of the π-conjugated macrocyclic ligand in [M^III(Pc)L₂]⁻ would give rise to conducting crystals, while the M^III(Pc)L₂ unit could impart many kinds of dimensionality to the molecular arrangement as the axial ligand induces slipped stacks (Figure 1) [10]. Indeed, it was found that there were two different types of π-π overlaps between the M^III(Pc)L₂ units (Figure 2). Although the π-π overlaps are significantly reduced compared to those seen in M(Pc)X-type conductors having face-to-face stacking (the overlap integral of type A is about 40% of that of M(Pc)X-type conductors) [11], it is enough for electrical conduction. Consequently, a large number of molecular conducting crystals composed of M^III(Pc)L₂ units were fabricated, including π-neutral radical crystals as well as crystals of highly conducting partially oxidized salts, with molecular arrangements showing the formation of one-dimensional, two-dimensional, and also three-dimensional networks [12,13,14]. Although all π-neutral radical crystals composed of M^III(Pc)L₂ are Mott insulators, their resistivities are relatively small.

Herein, we focused on the dimensionality of M^III(Pc)L₂ crystals. It has been reported that the conductivity of M^III(Pc)L₂ radical crystals tends to increase as the dimensionality of the network is increased [14]; the first example of a three-dimensional neutral radical crystal of Co^III(Pc)(CN)₂·2H₂O exhibited a resistivity at room temperature of no more than 10⁰ Ω cm with an activation energy of less than 0.1 eV [10]. However, only a few examples of three-dimensional systems exist at present [14]. We have fabricated a new three-dimensional system consisting of neutral radical crystals of M^III(Pc)Cl₂·THF (M = Co or Fe, THF = tetrahydrofuran). In this paper, the crystal structures and electrical properties of M^III(Pc)Cl₂·THF are reported.

2. Materials and Methods

2.1. Materials

Co^II(Pc) and Fe^II(Pc) were prepared by refluxing CoCl₂·6H₂O or FeCl₂·4H₂O with four molar equivalents of 1,2-dicyanobenzene in quinoline, and Co^II(Pc) or Fe^II(Pc) (3.0 g, 5.2 mmol) was treated with SOCl₂ (5 mL, 69 mmol) in nitrobenzene (70 mL) at 70 °C for 6 h to produce Co^III(Pc)Cl₂ (yield 3.2 g, 5.0 mmol, 95%) or Fe^III(Pc)Cl₂ (yield 2.1 g, 3.3 mmol, 62%) [15,16]. Then, K[M^III(Pc)(SCN)₂] was prepared according to the literature [16,17], by reacting M^III(Pc)Cl₂ (2.0 g, 3.1 mmol) with KSCN (6.0 g, 62 mmol) in acetone (200 mL) at 25 °C for 48 h (yield 2.1 g, 2.9 mmol, 94%).

Electrochemical oxidation of K[M^III(Pc)(SCN)₂] (30 mg) with two equivalents of tetraethylammonium chloride or tetrabutylammonium chloride was performed under a constant current of 1 μA in a THF:acetonitrile (30 mL, 1:1 v/v) solution at 25 °C using an electrocrystallization cell equipped with a glass frit between the two compartments, and yielded black square-pillar crystals of M^III(Pc)Cl₂·THF on the anode. The typical size of the crystals obtained was between 0.5 × 0.3 × 0.3 mm³ and 1.0 × 0.5 × 0.5 mm³. The obtained radical crystals were stable, and no decomposition was detected in the X-ray crystal structures or electrical resistivity measurements over several months. The Schematic route for the synthesis is shown in Figure 3.

2.2. X-ray Crystal Structure Determination

Crystal data for M(Pc)Cl₂·THF were collected at 298 K using an automated Rigaku SuperNova system (Rigaku, Tokyo, Japan) with monochromated Mo-Kα radiation (λ = 0.71073 Å). The structure was solved by using SIR2019 [18,19], and was refined by a full-matrix least-squares technique with SHELXL-2018/1 [20] using anisotropic and isotropic thermal parameters for non-hydrogen and hydrogen atoms, respectively. Orientational disorder was observed for the solvent site (THF molecule). We assigned C and O atoms according to the bond lengths.

Crystal data for Co(Pc)Cl₂·THF at 298 K: Formula, C₃₆H₂₄Cl₂CoN₈O, Formula weight 714.46, monoclinic, P2₁/n (#14), a = 8.2526(2), b = 15.4184(6) Å, c = 11.8762(4) Å, β = 92.996(3)°, V = 1509.08(9) ų, Z = 2, d_cal = 1.572 g cm⁻³, μ (MoKα) = 0.793 mm⁻¹ (λ = 0.71073 Å), R₁ = 0.0545, wR₂ = 0.1852, GoF = 1.280. CCDC 2013803.

Crystal data for Fe(Pc)Cl₂·THF at 298 K: Formula, C₃₆H₂₄Cl₂FeN₈O, Formula weight 711.38, monoclinic, P2₁/n (#14), a = 8.2062(1), b = 15.4115(1) Å, c = 11.9043(1) Å, β = 93.2238(9)°, V = 1503.15(2) ų, Z = 2, d_cal = 1.572 g cm⁻³, μ (MoKα) = 0.727 mm⁻¹ (λ = 0.71073 Å), R₁ = 0.0435, wR₂ = 0.1797, GoF = 1.503. CCDC 2013804.

CCDC 2013803 and 2013804 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge via http://www.ccdc.cam.ac.uk/conts/retrieving.html (or from the CCDC, 12 Union Road, Cambridge CB2 1EZ, UK; Fax: +44-1223-336033; E-mail: [email protected]).

Extended Hückel calculations were performed using a CAESAR 2 software package (Prime Color Software, Inc., Charlotte, NC, USA) with the atomic parameters determined by the X-ray structure analysis. Default parameters were used.

2.3. Measurements

Electrical resistivity measurements along the a-axis of M^III(Pc)Cl₂·THF single crystals were performed by the standard four-probe method. The two-probe method was also adopted for the measurement with the current parallel and perpendicular to the a-axis. The electrical leads, which were gold wires with diameters of 25 μm, were attached to the crystal face using gold paste (Figure 4). The data are typical, as the resistivity and activation energies varied slightly depending on the individual crystal examined. However, we confirmed the reproducibility of the data by examining several crystals of both materials.

3. Results

Co^III(Pc)Cl₂·THF and Fe^III(Pc)Cl₂·THF were found to have a monoclinic unit cell with the P2₁/n space group, and were isostructural to each other (

Table 1. Crystal data of Co^III(Pc)Cl₂·THF and Fe^III(Pc)Cl₂·THF.

	CoIII(Pc)Cl2·THF	FeIII(Pc)Cl2·THF
Space Group	P21/n	P21/n
a (Å)	8.2526(2)	8.2062(1)
b (Å)	15.4184(6)	15.4115(1)
c (Å)	11.8762(4)	11.9043(1)
β(°)	92.996(3)	92.2238(9)
Volume (Å3)	1509.08(9)	1503.15(2)
Temperature (K)	298	298
CCDC	2013803	2013804

). Similar isostructural natures have been reported for M(Pc)L₂-based conductors [21,22,23], because the rigid geometry of the Pc framework is insensitive to the central ion. Figure 5 shows the crystal structure of Co^III(Pc)Cl₂·THF. Because the oxidation potentials of Co^III and Fe^III are higher than that of Pc^2– [10,24], the Pc ligand is oxidized to Pc^•–, meaning M^III(Pc)Cl₂·THF is a neutral π-radical. The M^III(Pc)Cl₂ units form one-dimensional stacks along the a-axis via interactions between two peripheral benzene rings per unit, with interplanar distances of 3.49 and 3.54 Å (type A), and also form additional stacks along the [11$\overline{1}$] and [1$\overline{11}$] directions with one peripheral benzene ring per unit (type B) with interplanar distances of 3.43 and 3.44 Å, respectively, resulting in the formation of three-dimensional networks. To evaluate the effectiveness of the stacking, the overlap integrals between the HOMOs of adjacent Pcs along each stacking direction were calculated by the extended Hückel method based on the structural data (

Table 2. Overlap integrals between HOMOs of adjacent Pcs in M^III(Pc)Cl₂·THF.

		[100] with Type A	[11$\overline{\mathbf{1}}$] with Type B	[1$\overline{\mathbf{11}}$] with Type B
CoIII(Pc)Cl2·THF	Overlap Integral	2.0 × 10−3	0.9 × 10−3	0.9 × 10−3
	Interplanar Distance (Å)	3.49, 3.54	3.43	3.44
FeIII(Pc)Cl2·THF	Overlap Integral	2.3 × 10−3	0.9 × 10−3	0.9 × 10−3
	Interplanar Distance (Å)	3.49, 3.54	3.42	3.43

). Because the intermolecular overlap integral is usually regarded as being proportional to transfer energy, the estimation of the overlap integrals can be a useful index for investigating the anisotropies of molecular stacks [25], and it was found that the overlap integrals along the a-axis of type A stacks were almost twice those seen in other stacks.

Figure 6 shows the temperature dependences of the electrical resistivities along the a-axes of Co^III(Pc)Cl₂·THF and Fe^III(Pc)Cl₂·THF. The electrical resistivities of Co^III(Pc)Cl₂·THF and Fe^III(Pc)Cl₂·THF at room temperature were 6.3 × 10² and 6.1 × 10³ Ω cm, respectively, and both systems showed semi-conducting behavior, with activation energies of 0.20 eV for Co^III(Pc)Cl₂·THF and 0.24 eV for Fe^III(Pc)Cl₂·THF.

Figure 7 shows the temperature dependences of the electrical resistivities of Co^III(Pc)Cl₂·THF and Fe^III(Pc)Cl₂·THF along the a-axis (//a) and perpendicular to the a-axis (⊥a), as measured by the two-probe method. In both systems, the resistivity along the a-axis was smaller than that perpendicular to the a-axis, reflecting the difference in the overlap integrals; however, the current-direction dependence of the electrical resistivity was less than one order of magnitude. The activation energies of Co^III(Pc)Cl₂·THF and Fe^III(Pc)Cl₂·THF were almost independent of the direction of conduction.

4. Discussions

Despite the isostructural nature of the two compounds, the electrical resistivity of Fe^III(Pc)Cl₂·THF was almost one order of magnitude higher than that of Co^III(Pc)Cl₂·THF, and the activation energy was also higher. Similar enhancements of the electrical resistivity and activation energy have been reported for molecular conductors based on M^III(Pc)L₂ [21,22,23], and could be attributable to the different magnetic moments of the central metal ions. Low-spin Fe^III has a magnetic moment of S = 1/2, while low-spin Co^III has no magnetic moment (S = 0), and it has been reported that the magnetic moment enhances the electrical resistivity [26].

As shown in Figure 7, the electrical resistivity of M^III(Pc)Cl₂·THF depended on the current direction; however, the difference was quite small. In one- or two-dimensional systems, the resistivity along the stacking direction(s) was two orders of magnitude smaller than that perpendicular to the stacking direction(s) [27]. Therefore, the almost isotropic nature observed in the resistivity measurements in this study demonstrates that M^III(Pc)Cl₂·THF had a three-dimensional electronic system. However, the electrical resistivity of Co^III(Pc)Cl₂·THF at room temperature (6.3 × 10² Ω cm) was two orders of magnitude higher than that of Co^III(Pc)(CN)₂·2H₂O, which was the first example of a three-dimensional neutral π-radical crystal of the type M^III(Pc)L₂ [10]. The activation energy of M^III(Pc)Cl₂·THF was also higher than that of Co^III(Pc)(CN)₂·2H₂O. Therefore, we calculated the overlap integrals of Co^III(Pc)(CN)₂·2H₂O based on the reported crystal data, and compared the results with the overlap integrals of Co^III(Pc)Cl₂·THF (

Table 3. Comparison of the overlap integrals between similar three-dimensional systems of Co^III(Pc)Cl₂·THF and Co^III(Pc)(CN)₂·2H₂O.

	with Type A	with Type B
CoIII(Pc)Cl2·THF	2.0 × 10−3	0.9 × 10−3
CoIII(Pc)(CN)2·H2O	5.7 × 10−3	1.7 × 10−3

). Although the molecular arrangement of M^III(Pc)Cl₂ in M^III(Pc)Cl₂·THF appeared to be similar to that of Co^III(Pc)(CN)₂ in Co^III(Pc)(CN)₂·2H₂O, the overlap integrals of Co^III(Pc)(CN)₂·2H₂O were quite different from those of M^III(Pc)Cl₂·THF. The overlap integral along the c-axis, where Co^III(Pc)(CN)₂ formed a type A one-dimensional stack, was 5.7 × 10⁻³, while the overlap integrals along the [112] and [1$\overline{1}$2] directions, where Co^III(Pc)(CN)₂ formed type B stacks, were 1.7 × 10⁻³. These values were more than twice those obtained for Co^III(Pc)Cl₂·THF. These discrepancies could be the cause of the difference in the electrical properties, meaning that slight changes in molecular arrangements could lead to drastic changes in the electrical properties in the neutral π-radical crystals composed of M^III(Pc)L₂.

5. Conclusions

We succeeded in fabricating single crystals of stable organic π-radicals of the type M^III(Pc)Cl₂·THF (M = Co or Fe, THF = tetrahydrofuran). The overlap integrals between the HOMOs of adjacent Pcs were calculated based on the atomic parameters determined by the X-ray crystal structural analyses, and it was revealed that M^III(Pc)Cl₂ formed three-dimensional networks. Furthermore, the electrical resistivities measured were almost independent of the applied current direction, meaning that M^III(Pc)Cl₂·THF had three-dimensional electronic systems, which are rarely observed in the study of molecular conductors. The resistivities of Co^III(Pc)Cl₂·THF and Fe^III(Pc)Cl₂·THF at room temperature were 6 × 10² and 6 × 10³ Ω cm, respectively. These are similar to or significantly smaller than those reported for neutral radical conductors showing pressure-induced metallization [6,8], indicating that further study of three-dimensional Mott insulators composed of M^III(Pc)L₂ under the application of pressure could allow for phase transitions from insulating to metallic or superconducting states. Furthermore, it was revealed that a slight change in molecular arrangement led to drastic changes in the electrical properties. As M^III(Pc)L₂ shows various stacking types depending on the fabrication conditions, there is a strong possibility that new single-component molecular conductors composed of M^III(Pc)L₂ could be produced.