Introduction

One-dimensional (1D) metal complexes have attracted significant interest as scaffold structures in the design of materials with unique nanoarchitectures and properties.1, 2, 3, 4, 5, 6, 7, 8 These low-dimensional structures typically occur as structural motifs within bulk crystalline materials, and accordingly they have traditionally been studied within the fields of both solid-state inorganic chemistry and physics. The ability to isolate and assemble such linear complexes in solution could potentially lead to a new family of functional molecular wires. We have developed a creative approach to disperse halogen-bridged, mixed-valence platinum complexes in organic media as lipid-packaged nanowires.9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19 These nanowires exhibit supramolecular thermochromism,12, 13, 14 solvatochromism16, 17, 19 and nano assembly into pillared honeycomb stereo architectures.18

On the other hand, 1,2,4-triazoles are known to act as bridging ligands and their linear metal complexes have been actively investigated because of their polymeric structures and magnetic interactions between linearly aligned metal ions.1, 2, 6, 7, 20, 21 For example, a series of oligonuclear compounds based on Mn, Fe, Co, Ni and Zn has been prepared, all of which have been found to undergo antiferromagnetic interactions.22, 23, 24 The most interesting feature of these compounds is their characteristic spin crossover switching between low spin (LS, purple, S=0) and high spin (HS, colorless, S=2).1, 2, 6, 7, 20, 21, 22 In addition, their magnetic properties have rendered these complexes to be obvious candidates for use as information-storage materials. In particular, FeII-1,2,4-triazole complexes have the advantages of ease of synthesis and the ready formation of bridged structures, although they are generally only obtained as powders; thus, the challenge remains to process them into ultrathin films and other delicate structures in preparation for device applications. To date, preliminary studies of the triazole complexes have been limited to the study of bulk powdery samples, and there has been no general methodology developed for the conversion of these materials into nanostructures. One aspect of the development of such complexes, which should ideally be possible, is the tuning of their magnetic properties, based on supramolecular control of the spatial arrangement of the metal ion and triazole ligand, resulting in magnetic cooperation between metal complexes.

Working towards the development of supramolecular triazole complexes, we initially introduced a solvophilic dodecyloxypropyl chain within the ligand (Scheme 1).25, 26 These dodecyloxypropyl chains make it possible to dissolve the metal complex of the lipophilic triazole ligand 1 in organic media. To date, several lipophilic triazole complexes have been reported by supramolecular and coordination chemists.27, 28, 29, 30, 31, 32, 33 However, it is noteworthy that LS complexes of these compounds are destabilized in the solid state or organogels, because van der Waals interactions between the rigid alkyl chains increase the Fe-to-Fe distance and consequently promote conversion to the HS state. In the design of advanced supramolecular systems, it is therefore necessary to not only consider the chemical composition of the triazole ligands but also carefully select the appropriate media, as well as external conditions and stimuli, such as temperature, redox reactions and molecular recognition interactions.

In this review, we focus on the development of flexible lipophilic transition metal-triazole complexes in organic media,34, 35, 36 thin films37 and liquid crystals.38 The flexible ether linkage in triazole ligand 1, for example, enhances the solubility of its metal complexes in organic media and tailors the packing of the alkyl chains.39, 40, 41, 42, 43 Such flexible metal complexes undergo a variety of interactions with organic media, organic molecules and liquid crystals. This paper additionally discusses the coordination structures, magnetic properties and morphological dynamism of these complexes.

Result and Discussion

Self-assembly of lipophilic Co-triazole complexes and resulting thermodynamic and guest-binding properties

In order to develop soluble and functional triazole complexes, we have designed the lipophilic triazole ligand 1 (C12OC3Trz, Scheme 1a) and synthesized its CoII complexes.34 When the complex [CoII(1)3]Cl2 is dissolved in chloroform at room temperature, a blue gel-like phase results (Figure 1a). The analysis of such phases via atomic force microscopy and transmission electron microscopy reveals networks of nanofibers with widths 5–30 nm. The blue color is a characteristic of tetrahedral Co(II) (Td, 4A2→4T1(P)). Interestingly, this blue phase transforms into a pale-pink solution upon cooling, with the pink color indicating the formation of fragmented rod-like octahedral (Oh) complexes (Figure 1b). This change is reversible and the blue gel-like phase could be regenerated by heating the solution, which is very different from the behavior of conventional organogels that simply dissolve upon heating.34 This is the first reported observation of the reversible formation of heat-set gel-like networks in organic media to date, where the heat-set transition (from the Oh complex in solution to the Td complex in the gel-like phase) is enthalpically driven.34 The enthalpically driven process indicates the transition from a metastable solution state to a more stable gel-like state, which is ascribed to the ordering process of ligand alkyl chains, possibly to the lamellar structures and interactions between the fibrous aggregates, although the coordination change from Oh to the Td complex is usually entropically driven.34 It is also important to note that the introduction of the flexible ether linkage in the ligand 1 promotes physical gelation and provides higher temperature responsiveness. The incorporation of ether linkages into alkyl chains typically serves to decouple the molecular orientation of different structural modules and often leads to superior self-assembly properties.34

Figure 1
figure 1

Photographs (above), schematic illustrations (middle) and chemical structures (bottom) of heat-set gel-like networks formed from [CoII(1)3]Cl2. (a) A blue gel-like phase at 298 K. (b) A pale-pink solution at 273 K.

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The surface of [CoII(1)3]Cl2 nanofibers in chloroform has a significant number of associated chloride-counter ions, which can serve as acceptors for ionic hydrogen bonding.35 When long-chain alcohols are added to pink [CoII(1)3]Cl2 solutions at 273 K, blue gels are obtained. This change occurs as a result of ionic hydrogen-bond formation between the long-chain alcohols and the chloride ions, which in turn induces closer packing among the alkyl chains (Figure 2). As a consequence, the coordination state of the fragmented Oh complexes is modified to a polymerized Td structure, along with spontaneous bundling of the lamellar nanofiber networks. X-ray diffraction analysis of alcohol-binding xerogels has shown an increase in lamellar long spacing from 35.6 to 44.3 Å, indicating that the alkyl chains adopt a more perpendicular orientation with respect to the coordination structure of main chains. Thus, the binding of alcohol molecules to the surface of [CoII(1)3]Cl2 induces the formation of Td coordination structures, in which ligand alkyl chains and alcohols are aligned in an ordered fashion. The lipophilic [CoII(1)3]Cl2 complex can therefore accommodate guest-alcohol molecules by changing both its coordination structure and the orientation of its alkyl chains. The binding space thus created on the surface of such 1D complexes is referred to as an adaptable molecular cleft.35

Figure 2
figure 2

Schematic illustration of coordination structural changes induced by guest molecules (long-chain alcohols) binding on the surface of [CoII(1)3]Cl2 (a). It occurred as a consequence of ionic hydrogen-bond formation between long-chain alcohols and chloride ions (b).

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Supramolecular control of spin crossover phenomena associated with Fe-triazole complexes in various media

The interactions between alcohols and the surfaces of linear triazole complexes have been applied as a means of controlling the spin crossover phenomenon of the [FeII(1)]3Cl2 complex.37 The FeII complex is a purple powder when in its solid ambient temperature form, which is a characteristic of complexes in their LS state. However, it is transformed into a pale-yellow organogel (its HS state) when dissolved in chloroform.37 Such gels are the result of the formation of nanofiber aggregates, as confirmed by transmission electron microscopy. The casting of such chloroform solutions onto solid substrates produces transparent purple films, where the complex is once again in the LS state. These cast films exhibit sluggish spin crossover (LS⇄HS) phenomena in response to temperature changes, without thermal hysteresis (Figure 3a). In contrast, the co-casting of equimolar quantities of dodecanol or tetradecanol with [FeII(1)]3Cl2 forms composite films, in which alcohol molecules are bound to the complex by ionic hydrogen bonding between the hydroxyl group of the alcohol and the chloride ion, as well as by van der Waals interactions. At room temperature, the cast films consist of regular lamellar structures both before and after doping with alcohol, as determined by estimations from wide-angle X-ray diffraction measurements. Interestingly, binary films made from [FeII(1)]3Cl2 and long-chain alcohols (containing 12 or 14 carbons) exhibit reversible abrupt spin crossover with thermal hysteresis (Figure 3a). The observed bistability of these films is closely related to dynamic structural transformations between lamellar and hexagonal structures, leading to a novel supramolecular strategy for controlling the bistability of spin crossover phenomena (Figure 3b).37

Figure 3
figure 3

(a) Temperature dependence of the magnetic susceptibility of [FeII(1)3]Cl2/CnOH samples (n=12 and 14). (b) Schematic illustration of the supramolecular unit structures in the cast films. [FeII(1)3]Cl2/C14OH display the dynamic structural transformation between lamellar (at 298 K) and hexagonal structures (at 373 K).

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In order to derive functional systems through imparting spin-based functionality to guest molecules, we have demonstrated the formation of liquid-crystal gels from linear FeII-1,2,4-triazole complexes and nematic liquid crystals.38 Both JC–1041XX and JD–1002XX are employed as the liquid crystals, because they display nematic liquid-crystal phases over a wide temperature range (T/K of phase transitions: K 291.1 N 365.2 I (JC–1041XX) and K 276.8 N 347.9 I (JD–1002XX), Scheme 2). The purple color of the resulting gels indicates that the [FeII(1)]3Cl2 complex adopts the LS configuration in either JC–1041XX or JD–1002XX, in contrast to the HS gels formed in chloroform.38 At elevated temperatures, the macroscopically homogeneous gel structure is preserved, although the color changes from purple to pale yellow. The observed color changes are thermally reversible, as shown by the temperature dependence of reflectance spectra. In addition, the temperature dependence of the magnetic susceptibility demonstrates that the liquid-crystal gel composed of [FeII(1)]3Cl2 and JC–1041XX exhibits spin crossover at elevated temperatures with the appearance of thermal hysteresis. The spin crossover temperature in the heating cycle (for LS→HS, Tsc↑) is approximately 334 K, which is higher than that observed in the cooling cycle (for HS→LS, Tsc↓, 324 K, Figure 4a). Similarly, a combination of [FeII(1)]3Cl2 and JD–1002XX exhibits higher spin crossover temperatures during heating as opposed to cooling (Tsc↑ at 324 K and Tsc↓ at 319 K). The binary [FeII(1)]3Cl2/liquid-crystal composites therefore exhibit sluggish spin crossover transitions with thermal hysteresis within a higher range of temperatures as compared with pure [FeII(1)]3Cl2, for which both Tsc↑ and Tsc↓ are at 300 K.37 To date, the behavior of 1D Fe(II) complexes of 4-substituted 1,2,4-triazoles and their spin crossover characteristics have been studied solely for the solid state or as organogels.27, 28, 29, 30, 31, 32, 33 In these forms, however, LS complexes are destabilized, because van der Waals interactions between the rigid alkyl chains lead to increased Fe-to-Fe distances and hence promoted conversion to the HS state. In contrast, the system formed from the combination of lipophilic FeII complexes and hydrophobic liquid crystals has several advantages because of the considerable effects of the liquid-crystal environment on either solvophobic compaction of the N–Fe coordination bonds or destabilization of the HS complex (Figure 4b).38 Advantages of composites between liquid crystal and organic (and/or polymeric) molecules have been reported because of their unique interaction, which arise from their bicontinuous phase-separation structures.44, 45 These bicontinuous structures are formed when organic components exhibit substantial miscibility with liquid crystals. Molecular assemblies of functional low-molecular-weight gelators have been also shown to give liquid-crystal gels.46, 47 Therefore, in liquid-crystal hybrids, the functional components, such as lipophilic traizole complexes, will take on a specific role in the design and development of intelligent soft materials by imparting spin-based functionalities and solvophobic interaction.

Figure 4
figure 4

(a) Temperature dependence of the magnetic susceptibility of [FeII(1)3]Cl2/JC–1041XX liquid-crystal gels. Inner photographs are of [FeII(1)3]Cl2/JC–1041XX liquid-crystal gels at 293 K (left) and 363 K (right). (b) Schematic illustration of [FeII(1)3]Cl2/liquid crystals.

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Supramolecular control of the nanofiber morphology of Fe-triazole complexes via external stimuli

The structural dynamism of FeII-triazole complex nanofibers in response to temperature and guest-molecule interactions has inspired us to develop a technique to allow electrochemical regulation of their nanostructures. As described above, [FeII(1)3]Cl2 forms pale-yellow gel-like networks in chloroform, the color of which is a characteristic of the HS state.37 Interestingly, [FeII(1)3]Cl2 is found to be electrochemically active, and reversible reduction and oxidation current peaks are observed at +0.11 and 0.18 V (vs normal hydrogen electrode (NHE)), respectively, attributed to the FeII/FeIII transition (redox potential: 0.04 V).36 In addition, when the chloroform gel of [FeII(1)3]Cl2 is subjected to an applied electric potential of +0.70 V (vs Normal Hydrogen Electrode (NHE)), absorption by the gel at both 330 and 350 nm increases over time. This spectral change is electrochemically reversible and is accompanied by dissolution of the pale-yellow gel based on [FeII(1)3]Cl2 to a light-brown dispersion containing FeIII species.36

Transmission electron microscopy imagery shows that [FeII(1)3]Cl2 in chloroform, below an electrical potential of 0.30 V, exists as a series of abundant networks of fibrous nanoassemblies (with widths 3–4 nm, Figure 5a). The observed width of 3–4 nm corresponds to the bimolecular length of 1 (ca. 2.2 nm estimated by the Corey-Pauling-Koltun (CPK) model), indicating that [FeII(1)3]Cl2 is dispersed as molecular wires. Alternately, when an electric potential of +0.7 V is applied for 2 h, the number of fibrous nanoassemblies decreases and numerous nanocrystals with lengths in the range 500–1500 nm and widths 30–50 nm are observed (Figure 5b). This observed morphological change is consistent with the dissolution of the gel-like phase upon oxidation of [FeII(1)3]Cl2 to the FeIII complex. It is thus evident that lipophilic FeII-triazole complexes in chloroform tend to form nanowires, which show reversible and electrochemically controllable self-assembly. The oxidation of polymeric FeII-triazole complexes are accompanied by the coordination of chloride ions to the FeIII center, which entails dissociation of the linear coordination structures and the subsequent formation of nanocrystals (Figure 5c).36

Figure 5
figure 5

(a, b) Transmission electron micrographs of [FeII(1)3]Cl2. These samples have been kept in chloroform under the applied potential of (a) −0.3 or (b) +0.7 V vs NHE for 12 h. (c) Schematic illustration of electrochemically controlled self-assembly of [FeII(1)3]Cl2 in chloroform (left, reduction and right, oxidation).

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Conclusion

In this review, novel research concerning lipophilic transition metal-triazole complexes has been described. In particular, this summary covers the solution chemistry, spin crossover characteristics and structural transformations of triazole complexes modified with an ether linkage. Studies of such complexes demonstrate that the soluble nanowire complexes formed in various media or as a result of interaction with guest molecules exhibit characteristics and properties very different from those found in the bulk crystals. The structural dynamism of FeII-triazole complexes has been shown to offer a mean of investigating the electrochemical regulation of such species, and as such, there has been a growing interest in their photochemical characteristics48, 49 and the spin conversion50 via the self-assembly of these complexes. These findings provide a basis for the potential design of flexible nanometal complexes, where the structure and properties of coordination chains are affected by their interactions with external stimuli. Moreover, the self-assembly characteristics of these nanometal complexes may be exploited in order to exert dynamic control over their physical properties. The dynamism of these systems should allow the development of novel nanomaterials exhibiting synergistic properties, enabling the fabrication of supramolecular devices and machines.

scheme 1

(a) Chemical Structure of [MII(1)3]Cl2 (M: Fe and Co). (b) Schematic illustration of modification of [MII(1)3]Cl2 with flexible alkyl chains.

scheme 2

Chemical structure of liquid crystals, JC-1041XX and JD–1002XX.