Abstract
Axially-ligated iron phthalocyanines have been found to be good molecular conductors with giant negative magnetoresistance (GNMR) which originates from a strong intramolecular -d interaction between the metal and phthalocyanine. Ab initio theoretical calculations showed that substitution of ruthenium into the phthalocyanine complex would result in a significant increase in the -d interaction of the system, potentially intensifying GNMR. This paper presents the crystal preparation and X-ray structural characterization of bis(triphenylphosphine)iminium dichloro(phthalocyaninato(2-))ruthenium(III), PNP [(Pc2−)Cl2]. It is observed that [(Pc2−)Cl2] system has a symmetric planar RuPc unit with perpendicular axial ligands which results in a unidirectional and uniform solid-state arrangement, suitable for -d interaction-based molecular conductors with potentially exceptional GNMR.
1. Introduction
Metallophthalocyanine complexes with mono- or diatomic linear diaxial ligands (Scheme 1) are suitable molecular conductors due to their ability to form a slip-stacked solid-state arrangement that permits intermolecular - overlap for electron conduction [1, 2]. Moreover, the existence of strong intramolecular -d interaction in axially ligated iron(III) phthalocyanine ((Pc)L2; where L = CN, Cl, Br) molecular conductors has resulted in anisotropic giant negative magnetoresistance (GNMR) of up to 95% decrease in electrical resistance at 15 Tesla [3].
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Ab initio theoretical calculations using MOLPRO software package [4] performed on the (Pc)L2 system corroborated experimental observation that the strength of GNMR is directly related to the strength of -d interaction in the order of L = CN > Cl > Br. On the electronic structure representation of [(Pc)L2] species, the configuration gives two-fold degenerate ()2()2()1 = ()2()1()2 while the HOMO is a singly occupied molecular orbital of the delocalized -system of the Pc. Electronic structure calculations using two state-averaged complete active space multiconfigurational SCF method (active space orbitals: Pc-, Fe- and ; Stuttgart-Köln ECP + DZ basis) resulted in (orbital energy difference between / and HOMO; intensity of the -d interaction) of 8.5450 eV, 8.3839 eV, and 7.8655 eV for L = Br, Cl, and CN, respectively. Using the same theoretical calculation framework to (Pc)L2, which is electronically isostructural with the (Pc)L2 species, the homologue system resulted in a remarkable increase of around two-fold in the -d interactions across all L2 species (L: CN = 3.7518 eV, Cl = 3.8419 eV, Br = 3.9411 eV). Given that the intensity of the unique intramolecular -d interaction as the origin of the varying anisotropic GNMR in (Pc)L2, thus the importance of the synthesis of ruthenium(III) phthalocyanine with linear axial ligands.
The synthesis of crystalline ruthenium phthalocyanine Ru(Pc) complexes has long been a challenge in phthalocyanine chemistry. Even upon the report of pure Ru(Pc) synthesis more than three decades ago, the ambiguities of its solid-state/materials science still remain as only very few crystal structures of 6-coordinated axially ligated Ru(Pc) complexes have been reported [5, 6]. However, these Ru(Pc) complexes have bulky and/or unsymmetrical axial ligands unsuitable for structure-property correlation studies. To date, only one axially-ligated magnetic Ru3+()-centered Pc crystal has been reported. Yet, this reported (Pc)L2 crystal has unsymmetrical mixed axial cyano and pyridine ligands from an attempted identical di-axial ligation synthesis [7]. Herein, we report the crystal structure of ruthenium(III) phthalocyanine with identical di-axial linear ligands which can form symmetrical octahedral architecture that could be a potential component for magnetotransport material application.
2. Methodology
2.1. Crystallization
Dichloro(phthalocyaninato(1-)) ruthenium(III), (Pc1−)Cl2, was prepared via the method reported by Myers et al. in preparing various (Pc1−)Cl2 through the reaction of (Pc) with thionyl chloride oxidizing agent [8]. Pc (500 mg; 0.81 mmol) synthesized using the procedure of Farrell et al. [9], was suspended in nitrobenzene (10 mL). Thionyl chloride (2 mL; 28 mmol) was subsequently added to the reaction vessel and refluxed at 70°C for 3 hours. A 1 : 10 mole ratio of (Pc1−)Cl2 and bis(triphenylphosphine) iminium chloride (PNPCl) was dissolved in a 1 : 1 : 1 : 1 (volume) dimethylformamide : acetone : ethanol : hexane solvent system. The resulting solution was then left in an evacuated dessicator compartment at 25°C. Bis(triphenylphosphine)iminium dichloro(phthalocyaninato(2-)) ruthenium(III), PNP[(Pc2−)Cl2], crystallized into dark blue crystals after 8 weeks.
2.2. X-Ray Crystal Structure Determination
A blue block crystal of PNP [(Pc2−)Cl2] (Formula: C68H46N9Cl2RuP2) having approximate dimensions of 0.15 × 0.10 × 0.05 mm was mounted on a glass fiber. All measurements were made on a Rigaku RAXIS RAPID imaging plate area detector with graphite monochromated Mo-Kα radiation. Indexing was performed from 3 oscillations that were exposed for 90 seconds. The crystal-to-detector distance was 127.40 mm. The data were collected at a temperature of 123 K to a maximum 2θ value of 49.0°. A total of 44 oscillation images were collected. A sweep of data was done using ω scans from 130.0 to 190.0° in 5.0° step, at ° and °. The exposure rate was 150.0 [sec/°]. A second sweep was performed using ω scans from 0.0 to 160.0° in 5.0° step, at ° and °. The exposure rate was 150.0 [sec/°]. The crystal-to-detector distance was 127.40 mm. Readout was performed in the 0.100 mm pixel mode. All post measurement data processing was performed using the CrystalStructure crystallographic software package [10].
3. Results and Discussion
The low solubility of (Pc)Cl2 can be a cause of deterrent for the compound to be used as a precursor in synthesizing PNP[(Pc)Cl2] salt crystal. However, the difficulty can be overcome by a delicate mixture of 1 : 1 : 1 : 1 dimethylformamide : acetone : ethanol : hexane crystallization solvent which produced the title compound.
In Figure 1, it can be observed that PNP[(Pc)Cl2] units form ordered solid-state arrangement. Particularly, the anion component of the title compound, [(Pc)Cl2]−, affords unidirectional orientation. The crystallographic parameters of PNP[Ru(Pc)Cl2] are listed in Table 1. The crystal structure of PNP[Ru(Pc)Cl2] is seen to be isostructural with its Fe homologue, PNP[(Pc)Cl2], which also has a triclinic () crystal system [3].
At the molecular level (Figure 2), the regularity is brought about by the planarity of the RuPc and the linearity of the di-axial chloro ligands which give it a uniform octahedral architecture, that is, the central Ru3+ is aligned with the planarity of the Pc moiety which is manifested by the bond lengths, as well as the bond angles between the central Ru3+ and its adjacent nitrogen atoms being nearly equal (Tables 2 and 3). Furthermore, there is a linear 180° bond angle between the two axial chloro ligands which are perpendicular (90° ± 1.6) with respect to the central metal (Table 3), making [(Pc)Cl2]− suitable for slip-stacked intermolecular arrangement, with the cation bis(triphenylphosphine)iminium (PNP) serving as effective space-filler in the crystal system.
The resulting unidirectional and ordered orientation of [(Pc)Cl2]− units is mainly attributed to the steric influence of small and linear axial ligands of the fully conjugated planar Pc from which electrical and magnetic property manifestations can be designed and modulated based on its bulkiness for corresponding intermolecular - overlap variations [11], as well as on the chemical properties founded on the ligand field energy [3] of the axial ligands.
4. Conclusion
The synthesis of the crystalline PNP[(Pc)Cl2] revealed an ordered octahedral structural architecture of the Ru(Pc)Cl2 moiety. The regularity of the structure, coupled with the steric influence of the linear axial ligands, could effectively result in a slip-stacked arrangement capable of intermolecular - orbital overlap for electron conduction. Furthermore, PNP[(Pc)Cl2] is found to be isomorphous with its Fe homologue, thus opening prospects for the solid-state synthesis of other possible Fe(Pc)L2 homologue species of ruthenium. The resulting Ru(Pc)L2 is expected to have stronger -d interactions than its Fe counterparts that could result in molecular conductors with exceptional GNMR.
Appendix
CCDC 864862 contains the supplementary crystallographic data for this paper. These data can be obtained free of charge from The Cambridge Crystallographic Data Centre via http://www.ccdc.cam.ac.uk/data_request/cif.
Acknowledgments
This work was supported by the Hokkaido University Global Center of Excellence (GCOE) Program in chemistry and materials science (2007-2012) funded by the Ministry of Education, Culture, Sports, Science, and Technology (MEXT) of the Japan Government.