Redox Activity as a Powerful Strategy to Tune Magnetic and/or Conducting Properties in Benzoquinone-Based Metal-Organic Frameworks

Multifunctional molecular materials have attracted material scientists for several years as they are promising materials for the future generation of electronic devices. Careful selection of their molecular building blocks allows for the combination and/or even interplay of different physical properties in the same crystal lattice. Incorporation of redox activity in these networks is one of the most appealing and recent synthetic strategies used to enhance magnetic and/or conducting and/or optical properties. Quinone derivatives are excellent redox-active linkers, widely used for various applications such as electrode materials, flow batteries, pseudo-capacitors, etc. Quinones undergo a reversible two-electron redox reaction to form hydroquinone dianions via intermediate semiquinone radical formation. Moreover, the possibility to functionalize the six-membered ring of the quinone by various substituents/functional groups make them excellent molecular building blocks for the construction of multifunctional tunable metal-organic frameworks (MOFs). An overview of the recent advances on benzoquinone-based MOFs, with a particular focus on key examples where magnetic and/or conducting properties are tuned/switched, even simultaneously, by playing with redox activity, is herein envisioned.


Introduction
Over several decades, metal-organic frameworks (MOFs) have been extensively studied [1] due to their unique supramolecular architectures, which lead to high porosity and interesting properties in magnetism [2], conductivity [3], photochromism [4,5], luminescence [6][7][8][9], etc. MOFs are coordination compounds formed by metal ions linked to organic ligands, forming an infinite array in one, two, or three dimensions (1D, 2D, and 3D) and offering a plethora of applications in different fields including gas storage, separation, catalysis, energy storage, sensing, biomedical applications, etc. [10][11][12][13][14]. Depending on a careful choice of metal ions/linkers [15], MOFs can also show a combination and/or even interplay of physical properties. A relatively new strategy [10,16,17] to enhance their physical properties, in particular, magnetism and conductivity, is the incorporation of redox activity. Redox activity can be promoted via various methods such as (i) a rational design of redox-active metals centers or linkers [18], (ii) post-synthetic modifications of metal ions or through ligand exchange, and (iii) encapsulation of redox-active guest ions in the pores of MOFs [19], leading to the formation of radical species, affecting the electronic properties of the organic linkers and, therefore, the physical properties of the related networks [20,21].
The benzoquinone/hydroquinone linkers represent a redox-active couple remarkably studied in technologically important materials such as electrodes [6,29], flow batteries [30], pseudo-capacitors [31], and materials used in artificial photosynthesis [32]. Benzoquinones are a class of naturally occurring organic compounds that possess two carbonyl groups C=O in the 1 and 4 position in an unsaturated six-membered ring [33]. Benzoquinones are usually electron deficient, and their benzoquinoid-like configuration undergoes a monoelectron reversible reduction to produce the para semiquinone radical species, which, in turn, could be further reduced to form the aromatic hydroquinone dianion (vide infra), as described in Scheme 1. The stability of the semiquinoid form could be influenced by different factors, such as the nature of the ring substituents, intra/intermolecular hydrogen bonding, solvent polarity, the presence of acidic or basic additives, and protonation [34][35][36][37][38][39]. Particularly, chemical tailoring of benzoquinone linkers, through the functionalization of the six-membered ring by various substituents (halo, nitro, amino, methoxy, etc.), can stabilize the radical anion [40], depending on the steric and electronic nature of the substituents, thus modulating the physical properties of the benzoquinone-based networks at a molecular level [41]. In fact, the presence of different substituents on the benzoquinoid ring can tune the one-electron reduction potential, as shown in Table 1. When the benzoquinones are functionalized with electron-donating substituents, i.e., methyl groups, the reduction potential is more negative depending on how many methyl groups are present, making these species more difficult to reduce. On the contrary, it is possible to observe an opposite trend when electron-withdrawing substituents, such as chlorine, are present. Therefore, benzoquinones containing chlorine substituents in the ring show more positive reduction potentials and consequently are easier to reduce [33].

Benzoquinone Derivatives (E NHE )
Chemical structure of anilates and the most common coordination modes.
On this basis, MOFs formed by metal nodes and benzoquinoid-based ligands, especially anilates, feature an ideal platform for the construction of porous redox materials with switchable conducting/magnetic properties [62], due to the changeover to the semiquinoid form. A pioneering study on a dhbq based-framework, reported by Abrahams et al. [63], revealed (i) the potential of benzoquinone to act as a suitable building block for constructing extended frameworks [64] and (ii) its capability to receive/lose electrons while keeping unchanged the supramolecular architecture. This study opened up unprecedented routes to tune the physical properties of extended frameworks through redox activity.
The present work focuses on key examples of the latest developments (from 2015 to date) on redox-active benzoquinone-based MOFs showing conducting and/or magnetic properties. The aim is to evidence the enhancement/switching of magnetism and/or conductivity due to a fine modulation of benzoquinone redox properties, highlighting the extreme versatility of this class of redox-active linkers in tailoring the physical properties of extended frameworks.

Semiquinone-Based MOFs
The incorporation of a semiquinone in a solid framework could be achieved through in situ or post-synthetic reduction of the benzoquinone derivatives. In 2016, Stock et al. [65] reported on the first example of permanently porous Al III -MOFs, of the formulas (CH 3 (2), containing ligands in both their dianionic form (dhbq 2− ) as well as in the semiquinonic form (dhbq 3− ). Interestingly, MOFs 1 and 2 were obtained by in situ reduction, using high-throughput methods, which consist of an automated solvothermal equipment with different ligand/Al stoichiometric ratios, in DMF solvent, for optimizing the synthesis conditions. MOFs 1 and 2 show specific surface areas of 1440 and 1430 m 2 g −1 , respectively [65]. A similar MOF was also reported by Harris et al. [62,66], by combining Fe II ions with H 2 Cl 2 An through a solvothermal reaction in DMF, leading to a novel porous semiquinoid antiferromagnet formulated as ((CH 3 ) 2 NH 2 ) 2 [Fe 2 (Cl 2 An) 3 ]·2H 2 O·6DMF (3). The chloranilate bridging ligand is simultaneously present in both benzoquinoid and semiquinoid forms, resulting from a spontaneous electron transfer from Fe II to Cl 2 An 2− , giving rise to a mixed-valence layered MOF [62]. The ligand coordinates in its bis-bidentate mode, which generates anionic layers where six metal ions are coordinate by the ligand forming a hexagonal motif, the typical honeycomb packing. The (CH 3 ) 2 NH 2 + cations both balance the charge and orient the anionic layers to an eclipsed structure, forming 1D hexagonal channels, which show a Brunauer-Emmett-Teller (BET) surface area of 1175 m 2 g −1 . MOF 3 shows antiferromagnetic interactions with a spontaneous magnetization below 80 K in its solvated form, while the magnetic ordering temperature decreases to 26 K in the desolvated form (3_desolv), which shows a BET surface area of 885 m 2 g −1 ( Figure 2) and a fully reversible structural contraction consistent with a "breathing" behavior. The high value of magnetic ordering temperature compared to other extended systems containing the same bridging ligand in its dianionic form [45] highlights the ability of semiquinone ligands to form porous magnets with enhanced magnetic coupling between metal ions (vide supra) [62]. Moreover, MOFs 3 and 3_desolv show conductivity values of σ = 1.4(7) × 10 −2 S/cm (Ea = 0.26(1) eV) and 1.0(3) × 10 −3 S/cm (Ea = 0.19(1) eV), respectively, proving the ability of benzoquinone derivatives to construct multifunctional MOFs, in which porosity, magnetism, and conductivity coexist [62]. Furthermore, H 2 Cl 2 An in MOF 3 could be fully reduced to its semiquinoid form via a post-synthetic approach by using cobaltocene (Cp 2 Co), which allows a single-crystalto-single-crystal chemical (one-electron) reduction, due to its porous crystalline structure, affording a 2D MOF, formulated as (Cp 2 Co) 1.43 ((CH 3 ) 2 NH 2 )1.57[Fe 2 Cl 2 An 3 ]·4.9DMF (4). Remarkably, the Tc can increase up to 105 K, a rare value for MOFs, attributable to the strong magnetic exchange interactions between metal ions mediated by the semiquinone radical form. Variable-field measurements show a magnetic hysteresis up to 100 K, which is consistent with the high Tc value (see Figure 3). The r.t. conductivity of MOF 4, on a pressed-pellet, has a value of σ = 5.1(3) × 10 −4 Scm −1 , probably due to the complete ligand reduction that remove the mixed-valence character of the MOF [66]. Variable-temperature conductivity data in the 300-160 K range fit well the Arrhenius law with E a = 0.34(1) eV, in agreement with the observed conductivity value of MOF 4 and lower than MOFs 3 and 3_desolv, supporting the complete chemical reduction.
Noteworthily, the coexistence of high magnetic ordering and electrical conductivity in the same material is rather unusual, as well as the capability of the quinoid MOF to retain its crystalline structure upon post-synthetic chemical reduction, demonstrating the potential of quinoid-based MOFs to provide a new generation of redox-active conducting magnets for future spintronics applications (vide infra).
These results show that the incorporation of semiquinoid ligands in an extended scaffold is a valuable strategy for developing multifunctional MOFs with improved electrical conductivity and temperature magnetic ordering, making quinone derivatives excellent candidates for constructing next-generation data processing and storage systems.

Benzoquinone-Based MOFs
Very recently, Miyasaka et al. reported on post-synthetic generation of radical species via solid-state bulk electrochemistry technique, in a MOF containing diamagnetic benzoquinone derivatives, at the cathode of a lithium-ion battery system (LIB), producing a radical spin in the benzoquinone moiety and Li + insertion for preserving neutrality [74]. In this case, porosity is a fundamental requirement for host Li + ions. The precursor, formulated as (H 3 O) 2 (phz) 3 [Fe 2 (Cl 2 An) 3 ] (9), was obtained by the desolvatation of (H 3 O) 2 (phz) 3 [Fe 2 (Cl 2 An) 3 ]·(CH 3 COCH 3 ) n ·(H 2 O) n (9_solv). MOF 9 shows the typical honeycomb packing shown by anilates, with alternating anionic/cationic layers, where the counter cations [(H 3 O) 2 (phz) 3 ] 2+ are placed between the layers, acting as a templating agent that leads to 1D hexagonal channels along the c axis, conferring porosity to the network. MOF 9 shows paramagnetic behavior and short-range ferromagnetic correlations among Fe II ions, through a chloranilate linker, in the layered framework, as can be seen by lack of hysteresis in the magnetization vs. field (M-H) measurements, even at 5 K. With the ligand reduction due to the insertion of Li + ions, antiferromagnetic superexchange interactions between the radical anion Cl 2 An• 3− and Fe II ions took place, and the reduced MOF, formulated as (Li) 3   Remarkably, Harris et al. [75] reported on the simultaneous switching of magnetic and conducting properties induced by post-synthetic chemical reduction in a MOF of formula (Me 4 N) 2 [Mn 2 (Cl 2 An) 3 ]x.DMF, containing the diamagnetic chloranilate linker (10). When MOF 10 is soaked in a THF equimolar reducing solution of sodium naphthalenide and 1,2-dihydroacenaphthylene for several days, a semiquinoid-based MOF, of formula Na 3 (Me 4 N) 2 [Mn 2 (Cl 2 An) 3 ]3.9THF (11), is obtained. The reduction mechanism occurs via single-crystal-to-single-crystal process (vide infra), provoking the formation of Cl 2 An• 3− semiquinone radical form starting from diamagnetic Cl 2 An 2− , while the oxidation state of Mn II remains unchanged. Upon the conversion in the radical form of Cl 2 An 2− , a simultaneous change in both conductivity and magnetic properties is observed. Indeed MOF 10 shows a paramagnetic behavior above 1.8 K and a r.t. conductivity value of σ = 1.14(3) × 10 −13 Scm −1 (E a = 0.74(3) eV), whereas MOF 11 shows antiferromagnetic interactions between Mn II ions below 41 K, mediated by the semiquinone, and a r.t. conductivity value of σ = 2.27(1)× 10 −8 Scm −1 (E a = 0.489 (8) eV), a value 200,000 times higher than the respective benzoquinoid framework. Furthermore, by soaking MOF 11 in ferrocene (Cp 2 Fe + ) solution, a compound, formulated as Na((CH 3 ) 4 N)[Mn 2 (Cl 2 An) 3 ] 5.5THF 0.8CH 3 CN (12), is afforded, showing similar values of T c and σ as MOF 10 (oxidized compound), highlighting the reversibility of the redox process [75], as reported in Figure 6. The capability of benzoquinone-based MOFs to undergo reversible redox processes, which provoke a simultaneous switching of magnetic and transport properties, are worth being highlighted, as they may be considered suitable materials for future spintronic technologies.

Conclusions and Perspectives
The recent developments on redox-active benzoquinone-based MOFs, herein discussed, contribute to the ongoing research on the use of these materials for technologically relevant applications. It turns out that benzoquinoid/semiquinoid redox activity is a powerful strategy to tune their physical properties, in particular magnetism and/or conductivity. Porosity is a remarkable additional property and porous channels in benzoquinoid MOFs, allowing for the facile insertion/extraction of the electrolytes, which makes them promising materials for electrodes and rechargeable energy storage systems [76,77]. Furthermore, it has been highlighted that post-synthetic chemical redox reactions are a promising strategy to control ligand redox states in the MOF and the related changes of its conducting and magnetic properties, while the scaffold with metal-semiquinoid transitions provides tunable and delocalized electronic structures. Therefore, the whole redox control over MOFs is a very challenging task, specifically for their applications in the electronic devices realm. In more detail, redox control requires open-shell ligands and metals, frontier orbitals with similar energies, and maximal overlap to favor charge delocalization, and benzoquinones successfully match these requirements. Finally, the fabrication of semiquinoid MOFs, showing coexistence of simultaneously switchable conducting and magnetic properties, represents a forefront challenge for their potential applications in next-generation spintronic technologies, as magnetic transistors, terahertz information, and multifunctional chips, where data storage and information processing can occur at the same location.

Conflicts of Interest:
The authors declare no conflict of interest.