Coordination Programming of Two-Dimensional Metal Complex

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Invited Feature Article

Coordination Programming of Two-dimensional Metal Complex Frameworks Hiroaki Maeda, Ryota Sakamoto, and Hiroshi Nishihara Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.6b00156 • Publication Date (Web): 26 Feb 2016 Downloaded from http://pubs.acs.org on February 28, 2016

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Coordination Programming of Two-dimensional Metal Complex Frameworks Hiroaki Maeda,† Ryota Sakamoto,†,‡ Hiroshi Nishihara†,* †

Department of Chemistry, School of Science, The University of Tokyo, 7-3-1, Hongo,

Bunkyo-ku, Tokyo 113-0033, Japan ‡

JST-PRESTO, 4-1-8, Honcho, Kawaguchi, Saitama 332-0012, Japan

ABSTRACT: Since the discovery of graphene, two-dimensional materials with atomic thickness have attracted much recent attention because of their characteristic physical and chemical properties. Recently, coordination nanosheets (CONASHs) came into the world as new series of two-dimensional frameworks, which can show various functions based on metal complexes formed by numerous combinations of metal ions and ligands. This Feature Article provides an overview of recent progress in synthesizing CONASHs and in elucidating their intriguing electrical, sensing and catalytic properties. We also review recent theoretical studies on the prediction of the unique electronic structures, magnetism and catalytic ability of materials based on CONASHs. Future prospects for applying CONASHs to novel applications are also discussed.

Introduction

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Two-dimensional (2D) materials have recently attracted much attention, with graphene one of the most exciting areas of research since its discovery in 20041 due to its unique properties at room temperature, such as extremely high atomic mobility2 and quantum Hall effect.3 These properties arise from graphene’s unique Dirac cone-type electronic structure that lead to metal-like behavior. The electronic structure of graphene is subtly perturbed by small modifications such as the formation of a double layer due to interlayer interactions4 and doping with non-carbon elements.5 Significant current research on inorganic 2D materials has examined such as metal oxides,6,7 chalcogenides,8–10 and hydroxides,11–14 with molybdenum disulfide being of particular interest. Applications of inorganic 2D materials containing graphene have been extensively investigated, and devices such as transistors,15–17 sensors,18,19 luminescent devices,20–22 thermal interface materials,23,24 and piezo elements25,26 have been proposed and demonstrated. One recent exciting topic in 2D materials is the bottom-up synthesis of soft nanosheets based on an organic framework such as covalent organic framework (COF) using covalent bonds27–30 and on a metal complex framework using coordination bonds. The latter allows the assembly of coordination nanosheets (CONASHs) that display two significant advantages: first, numerous combinations of metals and ligands are possible, providing various chemical structures with unique electronic properties, and second, most coordination reactions proceed under ambient conditions and at room temperature, making a bottom-up process easy and inexpensive (Figure 1).


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Figure 1. Concept behind coordination nanosheets (CONASHs). Various combinations of metals and ligands produce a range of functionalized CONASHs.

Reports prior to 2012 describing CONASHs focused on their characterization and structural analysis31–44 rather than on their unique physical and chemical functionalities. The first published attempts to realize functional CONASHs were reported by Nishihara and his coworkers.45–53 Kambe et al. in 2013 synthesized multi-layer and single-layer CONASHs comprising a Kagome lattice by coordination reactions of nickel ions with benzenehexathiol: multi-layer constructs were generated at a liquid–liquid interface, and single-layer constructs were generated at a gas–liquid interface.45 Immediately following this publication, a theoretical prediction of the topological insulating behavior of this CONASH was reported.54 The synthesis, properties, and functions of CONASHs have subsequently been extensively


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studied both experimentally and theoretically. In the present Feature Article, we focus on the developing field of CONASHs.

1. Metalladithiolene CONASHs Metalladithiolene has a five-membered ring structure comprising a metal ion and a dithiolate ligand. The electronic communication between the π-orbital of dithiolate ligand and the d-orbital of metal ion produces quasi-aromaticity, the delocalization of π-electrons over the metalladithiolene ring. The unsaturated metal center results in redox activity,55–57 photofunction,58 and chemical reactivity,59–61 providing an interesting motif for CONASHs.

π-Conjugated multinuclear metalladithiolene systems allow very strong electronic communication among the metal centers, and their electronic structures can be easily controlled by changing the metal centers. A triangular trinuclear complex of ruthenium shows three-step one-electron oxidation due to the dithiolene moiety and three-step one-electron reduction due to the metal site, indicating very strong electronic communication among the metalladithiolene moieties.62 The degree of internuclear electronic communication in similar triangular trinuclear complexes of group 9 metals (e.g., cobalt, rhodium, and iridium) strong depends on the choice of the central metal.63 This section focuses on the first example of a bis(dithiolato)nickel (NiDT) CONASH and describes another CONASH, PdDT.

1.1 Benzenehexathiol-based bis(dithiolato)nickel (NiDT) CONASH d8-Metal ions such as Ni2+ form a tetra-coordinated square planar structure, allowing the formation of bis(dithiolato)nickel units by a simple reaction of benzenehexathiol with Ni(II)


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ions, which thus affords two-dimensional metal complex polymers. Bis(dithiolato)nickel forms three oxidation states: −2 (formally Ni(II)), −1 (Ni(III)), and 0 (Ni(IV)). These oxidation states interchange reversibly, suggesting that metalladithiolene π-conjugated nanosheets might have unique physical properties. A liquid–liquid interfacial of aqueous nickel acetate (upper layer) and organic benzenehexathiol (lower layer) gave a black film ca. 1 µm thick at the interface after standing overnight (Figure 2).45 The isolated multilayer film (called micro-NiDT) exhibited a metallic luster, and had a 3:1 Ni(III)-to-Ni(IV) ratio, indicating the formation of a mixed valence state.

Figure 2. Schematic illustration of the NiDT CONASH structure (left) and a photograph of micro-NiDT synthesized at a liquid–liquid interface. Adapted with permission from J. Am. Chem. Soc. 2013, 135, 2462-2465. Copyright 2013 American Chemical Society.


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Transmission electron microscopy (TEM) imaging of the edge of this multi-layer sheet showed a layered structure, and selected area electron diffraction (SAED) analysis showed a hexagonal diffraction pattern. This pattern indicated a hexagonal lattice with 1.4 nm cell length, consistent with the model structure (Figure 3). Powder X-ray diffraction (XRD) with synchrotron radiation revealed that the diffraction pattern corresponds to the crystalline state of the layered structure. The diffractions agreed with the simulated model based on a staggered hexagonal structure with the P63/mmc space group with in-plane lattice lengths of 1.41 nm and an interlayer distance of 0.38 nm.

Figure 3. (a) SAED pattern (left) and TEM image of micro-NiDT. (b) Experimental (red solid line) and simulated powder XRD patterns (black bars) and estimated stacking structure of NiDT (inset). Adapted with permission from J. Am. Chem. Soc. 2013, 135, 2462-2465. Copyright 2013 American Chemical Society.


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Very thin (atomic thickness) films can also be produced by interfacial coordination reactions in which the amounts and concentrations of the starting materials at the gas–liquid interface were controlled to form a submonolayer film. The film can be transferred onto a plate such as HOPG; subsequent AFM images showed CONASH films 0.6 nm thick, which exactly corresponds to the thickness of a single layer. We named this single-layer product nano-

NiDT. STM showed a hexagonal structure with a lattice distance of about 4.5 nm, and a distance of 1.4 nm between the metal atoms comprising the hexagonal structure. This hexagonal structure can be explained by a moiré structure overlapping the image of the metalladithiolene and graphite planes, similar to the case of graphene (Figure 4).

Figure 4. (a) STM image of nano-NiDT on HOPG and cross-sectional analysis at a red line (inset). (b) Enlarged scan image of the area outlined by the white square in (a), the result of fast Fourier transform (FFT) analysis (upper-right inset), and a FFT-filtered image (lowerleft). Adapted with permission from J. Am. Chem. Soc. 2013, 135, 2462-2465. Copyright 2013 American Chemical Society.


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Multiple nano-NiDT layers can be stacked in a step-wise manner using the Langmuir– Schäfer method. The vis–NIR spectra of these multiple layers exhibit broad absorption over the entire visible and near infrared region, suggesting a unique electronic structure.46 Both

micro- and nano-NiDT undergo a reversible redox reaction due to the Ni(IV)/Ni(III) couple. NiDT in the reduced state can be synthesized using Ni(III) and in the oxidized state can be synthesized using Ni(IV). The reduced and oxidized states of NiDT can be synthesized by chemical reduction of the as-prepared sample in the Ni(IV)/Ni(III) mixed-valence state using NaTCNQ, and by oxidation with tris(4-bromophenyl)aminium hexachloroantimonate, respectively.47 The electrical conductivities of the as-grown Ni(III)Ni(IV) mixed-valence film and oxidized Ni(IV) film at 300 K were 2.8 S cm−1 and 160 S cm−1, consistent with the metallic nature indicated by the band structure of the multilayered film. These results indicate that the nickelladithiolene π-nanosheet is redox active and electronically conducting

1.2 Other bis(dithiolato)metal CONASHs Pal et al. synthesized a bis(dithiolato)palladium nanosheet (called PdDT) incorporating heavier Pd (below Ni in group 10), which thus should produce strong spin orbit coupling.48 A black multilayer film was formed at liquid–liquid interface of the aqueous Pd(II) upper layer and benzenhexathiol ligand in chloroform as the bottom layer, and a single-layer PdDT was also synthesized using gas-liquid interfacial reaction technique. Microscopy (TEM, AFM, and STM) showed a sheet-like morphology, and the IR and X-ray photoelectron (XP) spectra indicated mixed-valence metal complex formation with an 81:19 Pd(IV)-to-Pd(III) ratio.


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SAED determined that the in-plane cell length was 1.5 ± 0.1 nm and that the hexagonal structure was similar to that of NiDT. Bis(dithiolato)metal CONASHs have been synthesized in other research groups using benzenhexathiol (BHT) and 2,3,6,7,10,11-triphenylenehexathiol (THT) as ligands, and various metal ions such as Pt(II), Cu(II), Ni(II) and Co(II).64,

72, 78, 82

In addition,

2,3,6,7,10,11-hexaaminotriphenylene (HATP) and 2,3,6,7,10,11-triphenylenehexaol (THO) are known as available components to form bis(dithiolato)metal CONASH derivatives (Table 1).65, 70, 71 These CONASHs can be synthesized by the liquid-liquid or gas-liquid interfacial reaction methods and the one-phase solution reaction. The gas-liquid interfacial synthesis is the suitable technique to form thin films with sub- to several nanometer thicknesses while the reaction at the liquid-liquid interface usually gave thick films. In the case of the reaction in one-phase solution, powder or solid samples are obtained. Although the morphologies of samples depend on the preparation methods, all of synthetic methods give crystalline structures which can be detected by powder X-ray diffraction (PXRD), SAED or grazing incident XRD (GIXRD). In the next section, we focus on the syntheses and functions of metalladithiolene CONASHs and their derivatives.


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Table 1. Combinations of metals and ligands reported to date for synthesizing CONASHs and the properties of synthesized CONASHs. Metal source

NiDT

PdDT

Ligand

Synthesis

Ni(OAc)2

a, b

K2PdCl4

a, b

Lateral size

Thickness

Crystallinity

a: ~100 µm

a: 1-2 µm

○

b: −

b: 0.6 nm

(PXRD and SAED)

a: 2-3 µm

○

b: 4-10 nm

(PXRD and SAED)

−

Property

Ref.

Conductivity

45-47

−

48

O C3H7 C3H7

O

PtTHT

S S

S

Pt(MeCN)2Cl2

O C 3H 7

c S O

−

−

C 3H 7 C3H7 C3H7

S

○

Ion exchange

(PXRD)

Conductivity

64

O

S O

NiHATP

NiCl2·6H2O

c

−

~500 nm

○

Conductivity

65

Chemiresistive sensor

70, 71

(PXRD)

CuTHO, CuHATP, NiHATP

CuSO4·5H2O, NiCl2·6H2O

c

−

−

○ (PXRD)


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X = OH, NH2·HCl

2

CuBHT

Cu(NO3)2

a

> 1 cm

15-500 nm

NiTHT

Ni(NO3)2

b

> 1 mm2

0.7-0.9 nm

○

Conductivity

(GIXRD and PXRD)

High mobility

○

Hydrogen evolution

78

Hydrogen evolution

82

(PXRD)

CoBHT, CoTHT

[Co(MeCN)6][BF4]2, CoCl2·6H2O

or

72

a (organic phase was evaporated.)

−

○

−

(PXRD and SAED)

a: liquid-liquid interfacial reaction, b: gas-liquid interfacial reaction, c: reaction in a one phase solution.


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2. Recent progress in experimental research on CONASHs Following our synthesis of NiDT CONASH, we also reported functional CONASHs such as electrochromic bis(terpyridine)iron and cobalt CONASHs,50 and a bis(dipyrrinato)zinc CONASH51 that generates a photocurrent. These CONASHs were generated by using coordination chemistry approaches different from that used to synthesize the NiDT CONASH. Various groups also began to study CONASHs and to exploit the new research field of 2D materials. In this section, we focus on recent progress in experimental research on CONASHs based on metalladithiolene and metalladiimine complex skeletons (Table 1). As described above, the metal centers in metalladithiolene-based multinuclear clusters experience strong interactions that provide unique electronic properties. The structures of metalladiimine complexes are isoelectronic with those of metalladithiolene complexes, and thus should exhibit interesting functional characteristics. Cui and Xu synthesized a multilayer bis(dithiolato)platinum CONASH (PtTHT) by refluxing a mixture of Pt(CH3CN)2Cl2 and an in-situ-generated sodium triphenylene hexathiolate (Na6THT).64 The chemical composition of PtTHT was determined to be Na0.9Pt1.5THT by inductively-coupled plasma atomic emission spectroscopy and elemental analysis. The powder XRD pattern showed a 2D honeycomb structure with staggered stacking of the neighboring layers, and the formation of a bis(dithiolato)platinum complex was confirmed from the XP and IR spectra. In addition, the Brunauer–Emmett–Teller (BET) surface area of 329 m2 g−1, estimated from N2 sorption isotherms, revealed a porous structure. The authors also achieved an ion exchange replacement ratio of Na+ to Cs+ or Li+ of over 90% without significantly changing the crystalline structure. The electrical conductivity of


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the as-made, activated and I2-treated CONASHs determined using a two-probe configuration was on the order of 10−4 S cm−1. Dincă and his coworkers synthesized a conductive bis(diimine)nickel CONASH (NiHATP) from a heated aqueous solution of NiCl2·6H2O and HATP·6HCl with NH3 aqueous solution (Figure 5a, M = Ni, X = NH).65 The obtained black powder has a slipped parallel structure as revealed by its powder XRD pattern, extended X-ray absorption fine structure (EXAFS), and by DFT calculation. Two-probe conductivity measurement of the pelletized sample provided a conductivity of 2 S cm−1, while evaluation of the multilayer film sample by the van der Pauw method showed a much higher conductivity of 40 S cm−1 at room temperature (Figure 5b). These values are of the same order of magnitude or higher than those of organic conductors and MOFs.66–69 Dincă group also developed a chemiresistive sensor using a bis(diimine)copper CONASH (CuHATP) that provided a bulk conductivity of 0.2 S cm−1 in the pelletized sample.70 The device worked as an ammonia vapor detector capable of detecting 0.5 ppm ammonia by exhibiting a change in the current (Figure 5c). This group’s most recent work described the chemiresistive behaviors of multilayer bis(diimine)metal CONASH analogs, CuTHO, CuHATP and NiHATP against various volatile organic compounds.71


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Figure 5. (a) Chemical structure of CuTHO, CuHATP and NiHATP reported in references 65, 70 and 71. (b) Temperature-dependent conductivity measurements obtained using the van der Pauw method of a ~500 nm thick NiHATP (M = Ni, X = NH) on quartz. (c) Chemiresistive response of CuHATP (M = Cu, X = NH) to 0.5, 2, 5 and 10 ppm NH3 gas. Reprinted with permission from J. Am. Chem. Soc. 2014, 136, 8859-8862. Copyright 2014 American Chemical Society. Reproduced with permission from Angew. Chem. Int. Ed. 2015, 54, 4349-4352. Copyright 2015 Wiley-VCH Verlag GmbH & Co. KGaA.

Huang et al. synthesized a highly electrically conductive CONASH (CuBHT) by a liquid– liquid interfacial reaction using an aqueous solution of Cu(II) and a BHT dichloromethane solution.72 Structural analysis based on the grazing incident XRD (GIXRD) pattern, and component analysis based on electron probe microanalysis, elemental analysis, and XP spectroscopy showed that the two-dimensional hexagonal lattice had the formula


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[Cu3(C6S6)]n with lattice parameters a = b = 8.45 Å. Comparison of the total energy values of various structures estimated by DFT provided the coordination network structure shown in Figure 6, in which one BHT molecule is linked with six BHT molecules via six Cu atoms. From the electrical conductivity evaluation of the film with 15 – 500 nm thickness, conductivity values of up to 1580 S cm−1 were obtained using the four-probe method at room temperature, which is higher than that reported for conductive coordination polymers.73,74 This remarkable conductive ability was attributed to the strong π–d interaction between the metal ions and ligands and to the delocalized electrons in the two-dimensional system. This material also possesses both superior electron (116 cm2 V−1 S−1) and hole (99 cm2 V−1 S−1) mobilities.

Figure 6. Estimated structure of the 2D hexagonal lattice of [Cu3(C6S6)]n.

Metalladithiolenes are known for their hydrogen-generating catalytic ability,75–77 and metalladithiolene CONASHs exhibit significant conductivity, suggesting that CONASHs can be used as catalytic electrodes. The electrocatalytic evolution of hydrogen using


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bis(dithiolato)nickel and bis(dithiolato)cobalt CONASHs has been demonstrated by two groups. Dong et al. synthesized a single-layer bis(dithiolato)nickel CONASH (NiTHT) using a Langmuir–Blodgett trough, and transferred the monolayer onto an electrode.78 This CONASH exhibited electrocatalytic hydrogen evolution, with an overpotential of 110 mV, an operating potential of 333 mV at 10 mA cm−2 (Figure 7a), and an exchange current density during the hydrogen evolution reaction of 6 × 10−4 mA cm−2. These values reveal that the hydrogen evolution catalytic ability of bis(dithiolato)nickel CONASH is greater than those of nickel and cobalt complexes grafted onto carbon nanotubes79,80 or of nitrogen and phosphorus dual-doped graphene.81 Clough et al. synthesized two types of multilayer bis(dithiolato)cobalt CONASHs (CoBHT and CoTHT) using BHT and THT as bridging ligands using a liquid– liquid interfacial reaction.82 The CoBHT provided a current density of 10 mA cm−2 at an overpotential of 340 mV (Figure 7b), and achieved a two-orders higher maximum average surface catalyst concentration, 3.7 × 10−6 molCo cm−2, than the reported maximum catalyst loading.

Figure 7. (a) Electrocatalytic hydrogen gas evolution using NiTHT (black solid line) and a glassy carbon disk electrode (black dashed line) in 0.5 M H2SO4 aqueous solution. (b) Polarization curves obtained using a glassy carbon electrode (black dashed line), CoBHT


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(red solid line), and CoTHT (blue solid line) in pH 1.3 H2SO4 aqueous solution. Adapted with permission from Angew. Chem. Int. Ed. 2015, 54, 12058-12063. Copyright 2015 WileyVCH Verlag GmbH & Co. KGaA. Adapted with permission from J. Am. Chem. Soc. 2015, 137, 118-121. Copyright 2015 American Chemical Society.

CONASH research is still in its infancy, and so there are fewer experimental investigations than for inorganic 2D materials. However, CONASHs are attractive due to their enormous potential for providing materials with novel physical properties that can be tuned by combining various metals and ligands. Various kinds of functional CONASHs will thus be produced over the next decade. Concurrent with these advances, theoretical research has progressed rapidly to unveil the properties of CONASHs and discover new functions. The next section focuses on recent theoretical research on CONASHs.

3. Recent progress in theoretical research on CONASHs Theoretical calculations are a valuable tool for predicting the properties and elucidating the electronic band structures of CONASHs. Our report of the synthesis of NiDT led to various calculations to estimate its band structure and predict its functions. One of the most attractive properties of the NiDT CONASH predicted by DFT calculations is that it is a 2D topological insulator. This indicates the existence of a robust conducting edge on the boundary of a normal insulator, and holds potential for applications in spintronics and quantum computation devices. Prior to this, experimentally identified topological insulators were limited to inorganic materials such as HgTe83,84 and Bi(111) on Bi2Te3,85 Bi2Se3,86 and Bi2Te3.87 Computational chemistry can explore organometallic topological insulator candidates,88–90


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but the synthesis of the proposed structures remains very challenging. Liu and his coworkers at the University of Utah calculated the electronic structure of our single-layer NiDT CONASH using first-principles calculations, including spin-orbit coupling, and remarkably found that it is likely an organic topological insulator. This is the first candidate of organic topological insulator which can be really synthesized (Figure 8).54


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Figure 8. (a) Chemical structure of NiDT CONASH. The white solid lines show the unit cell (Ni3C12S12). (b) 2D band structure of NiDT. (c) Close-up of the band structure around the spin-orbit coupling gaps. Adapted with permission from Nano Lett. 2013, 13, 2842-2845. Copyright 2013 American Chemical Society.

Magnetism is a very attractive research topic in coordination chemistry, and one aspect of particular interest is what kind of magnetic state is produced when magnetic complexes are aligned in a regulated 2D system. The magnetic property of a bis(dithiolato)manganese CONASH was predicted by the Zhao group using first-principles calculations.91 A unit cell of a monolayer CONASH, Mn3C12S12, has a ferromagnetic spin lattice with S = 3/2. The spins in the unit cell interact and form long-range ferromagnetic ordering mediated by p–d hybridization resulting from the π-conjugated Kagome lattice. The Curie temperature estimated by Monte Carlo simulation is ca. 212 K (Figure 9). In addition, Liu and Sun predicted that replacement of the sulfur atom with a NH group would enhance the p–d exchange interactions by decreasing the lattice constant, resulting in a Curie temperature of ca. 450 K.92


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Figure 9. (a) Spin-polarized electron density distribution in a 2×2 superlattice of a Mn3C12S12 monolayer. (b) The temperature dependency of the magnetic moment in a unit cell of a Mn3C12S12 monolayer as estimated by Monte Carlo simulation. The inset shows the simulated curve for heat capacity (Cv) as a function of temperature. Adapted from Nanoscale 2013, 5, 10404-10408 with permission from The Royal Society of Chemistry.

First-principles calculations of a square planar CONASH monolayer composed of octaaminonaphthalene and metal (iron, chromium, or cobalt) ions were conducted by Mandal and Sarkar, and showed that the half-metallic nature of the 2D metal–organic crystals of Fe and Cr results in remarkable 100% spin-filtering efficiency, indicating that these CONASHs are good building blocks for spintronic devices.93 Discussion of the physical properties of 2D materials often focuses on their electronic band structures. Comparable to the relationship between graphite and graphene, a thin 2D material one-to-several layers thick will have a different characteristic electronic state from the same material in the bulk state, and these altered characteristics contribute to the unique functions of the 2D material. Nanosheet components also play an important role in tuning the physical properties of a thin material. Chen et al.94 evaluated the effects of film thickness and of the specific metal atom in a framework on the electronic band structure of the material. Specifically, they used DFT methods to calculate the bulk and sheet states of metal–organic frameworks comprising 2,3,6,7,19,11-hexaiminotriphenylene together with either nickel or copper (NiHATP and CuHATP). Both bulk materials have strong π–π interactions between neighboring layers and weak metal–metal interactions, and displayed metallic band structures, consistent with the experimentally measured conductivity of NiHATP

(2 S cm−1 for a

pelletized sample and 40 S cm−1 for a film) reported by Dincǎ. In the sheet state, NiHATP is


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an indirect semiconductor with a narrow band gap, while CuHATP retains metallic properties. The electronic band structures of CONASHs on another inorganic 2D materials is curious topic for the development of new functions by the hybridization of organic and inorganic nanosheets. Zhou calculated the electronic states of NiDT CONASHs on graphene and hexagonal boron nitride (h-BN).95 The bilayer structure of NiDT CONASH on h-BN exhibited stronger interactions and maintained the semiconducting electronic property of

NiDT CONASH, whereas a metallic band structure was observed in NiDT CONASH on graphene (Figure 10). This suggests that the choice of supporting substrate is important for obtaining a CONASH with the desired properties.


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Figure 10. Optimized unit cells and band structures of (a) NiDT CONASH/graphene, (b) NiDT CONASH/h-BN, and (c) NiDT CONASH. Adapted from RSC Adv. 2014, 4, 1336113366 with permission from The Royal Society of Chemistry.

The physical properties of 2D materials can also be modified by forming stacking double layers and distorted structures. The relationship between the band gap of a bilayer NiDT CONASH and its stacking nature was discussed by Shojaei and Kang;96 for example, a 0.15


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eV band gap was observed in the bilayer system stacked in an eclipsed fashion, but decreased to 0 eV when one of the layers was offset to form a staggered structure. A change in the electronic band structure of a NiHATP monolayer from a semiconducting state to a metallic state caused by biaxial strain was predicted by Tie and Chen.97 They defined the in-plane biaxial tensile strain as ε = (a – a0)/a0 × 100, where a and a0 are the lattice constants for the strained and flat structures, respectively. They found that the band gap of the NiHATP monolayer decreased as ε increased, and became zero at ε = 7%, because the applied strain weakened the Ni–N bond and thus weakened charge redistribution between the Ni and N atoms. An understanding of the reaction pathway and thermodynamics is helpful for identifying synthetic strategies for efficient, catalytically-active CONASHs. A reaction mechanism involving the addition of ethylene to NiDT CONASH monolayer was investigated computationally by Tang and Zhou,98 and showed that an ethylene molecule can adsorb onto S atoms in NiDT CONASH by cis-interligand S,S’-addition. This adsorption occurs because the lowest unoccupied state of the S atom p orbitals can accept electrons from the occupied p orbitals of ethylene (Figure 11). In addition, this ethylene–NiDT complex is destabilized by electron injection, and decomposes into an ethylene molecule and a reduced NiDT, demonstrating the possible utility of NiDT CONASH for reversible electrocatalytic ethylene absorption storage.


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Figure 11. Top-views and side-views of the lowest unoccupied state of (b) NiDT CONASH and (b) ethylene-adsorbed NiDT CONASH. Adapted with permission from J. Phys. Chem. C

2013, 117, 14125-14129. Copyright 2013 American Chemical Society.

Zhang et al. used DFT calculations to investigate the catalytic activity of bis(dithiolene)metal complex (metal = Fe, Co, Ni, Ru, Rh, Pd, Os, Ir, and Pt) nanosheeets for the oxygen reduction reaction (ORR).99 Bis(dithiolato)iridium CONASH was chosen as the best catalytic material, because it exhibits the smallest Gibbs free energy change of the rate determining step in the ORR and the optimal adsorption properties of ORR intermediates on the metal center. These theoretical predictions of CONASH properties will contribute to the strategic and smooth development of new CONASHs and aid the selection of CONASHs for future synthesis.

Summary


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The present Feature Article summarizes recent experimental and theoretical research on twodimensional coordination nanosheets (CONASHs) using methalladithiolene analogous. Several types of CONASHs have been made using either interfacial synthesis methods (liquid–liquid and gas–liquid) or one-phase coordination reactions. Diffraction studies have shown that most CONASHs exhibit a 2D regulated structure. These π-conjugated CONASHs exhibit strong conductivity, comparable in magnitude or higher than values reported for organic conductors and conducting metal–organic frameworks. They also exhibit hydrogen evolution catalytic activity and chemiresistive properties. Consequently, CONASHs can be used as catalysts and as sensing electrodes. Furthermore, theoretical calculations predict that methalladithiolene CONASH analogs should act as 2D organometallic topological insulators and as unique functional materials exhibiting ferromagnetism, catalytic activity, and a tunable electronic band structure owing to their sheet components, stacking, and biaxial strain. These recent discoveries will promote the expansion of research into 2D CONASHs and the development of practical devices based on them.

AUTHOR INFORMATION

Corresponding Author *E-mail: [email protected] ACKNOWLEDGMENT The present Feature Article is chiefly supported by JST-CREST “Development of Atomic or Molecular Two-Dimensional Functional Films and Creation of Fundamental Technologies for Their Applications” (To H.N.) and JST-PRESTO “Hyper-nano-space design toward Innovative Functionality” (To R.S.). The present article is partly supported by Grants-in-Aid


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from MEXT of Japan (Nos. 26708005, 26107510, 26220801, 15H00862, 15K13654, areas 2406 [All Nippon Artificial Photosynthesis Project for Living Earth], 2506 [Science of Atomic Layers], and 2509 [Molecular Architectonics]). R.S. is grateful to Asahi Glass Foundation, Iketani Science and Technology Foundation, The MIKIYA Science and Technology Foundation, Yazaki Memorial Foundation for Science and Technology, Shorai Foundation for Science and Technology, The Hitachi Global Foundation, Kumagai Foundation for Science and Technology, Foundation for Interaction in Science & Technology (FIST), The Foundation Advanced Technology Institute, Izumi Science and Technology Foundation, The Foundation for The Promotion of Ion Engineering, LIXIL JS Foundation, and The Iwatani Naoji foundation for financial supports.

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(75) Zarkadoulas, A.; Field, M. J.; Papatriantafyllopoulou, C.; Fize, J.; Artero, V.; Mitsopoulou, C.A. Experimental and Theoretical Insight into Electrocatalytic Hydrogen Evolution with Nickel Bis(aryldithiolene) Complexes as Catalysts. Inorg. Chem. DOI: 10.1021/acs.inorgchem.5b02000. (76) Eckenhoff, W. T.; Brennessel, W. W.; Elsenberg, R. Light-Driven Hydrogen Production from Aqueous Protons using Molybdenum Catalysts. Inorg. Chem. 2014, 53, 9860-9869. (77) McNamara, W. R.; Han, Z.; Alperin, P. J.; Brennessel, W. W.; Holland, P. L. Eisenberg, R. A Cobalt–Dithiolene Complex for the Photocatalytic and Electrocatalytic Reduction of Protons. J. Am. Chem. Soc. 2011, 133, 15368-15371. (78) Dong, R.; Pfeffermann, M.; Liang, H.; Zheng, Z.; Zhu, X.; Zhang, J.; Feng, X. LargeArea, Free-Standing, Two-Dimensional Suparamolecular Polymer Single-Layer Sheets for Highly Efficient Electrocatalytic Hydrogen Evolution. Angew. Chem. Int. Ed. 2015, 54, 12058-12063. (79) Goff, A. L.; Artero, V.; Jousselme, B.; Tran, P. D.; Guillet, N.; Métayé, R.; Fihri, A.; Palacin, S.; Fontecave, M. From Hydrogenases to Noble Metal-Free Catalytic Nanomaterials for H2 Production and Uptake. Science 2009, 326, 1384-1387. (80) Andreiadis, E. S.; Jacques, P.-A.; Tran, P. D.; Leyris, A.; Chavarot-Kerlidou, M.; Jousselme, B.; Matheron, M.; Pécaut, J.; Palacin, S.; Fontecave, M.; Artero, V. Molecular engineering of a cobalt-based electrocatalytic nanomaterial for H2 evolution under fully aqueous conditions. Nat. Chem. 2013, 5, 48-53. (81) Zheng, Y.; Jiao, Y.; Li, L. H.; Xing, T.; Chen, Y.; Jaroniec, M.; Qiao, S. Z. Toward Design of Synergistically Active Carbon-Based Catalysts for Electrocatalytic Hydrogen Evolution. ACS Nano 2014, 8, 5290-5296.


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Biographies Hiroaki Maeda was born in Toyama prefecture, Japan in 1987. He received his B. Sc. in 2010, M. Sc. in 2012 and Ph. D degree in 2015 from The University of Tokyo. His resent research interest lies on the stepwise preparation of metal complex wires on electrodes and their electrochemical evaluation, the syntheses of coordination nanosheets using interfacial reactions.


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Ryota Sakamoto was born in Yamagata Village., Nagano, Japan in 1980. He graduated from The University of Tokyo (Japan) in 2002, and received his Ph.D. degree from the same university in 2007 under the supervision of Prof. Hiroshi Nishihara. Then he was appointed as an assistant professor at Tokyo University of Science (Japan), working with Prof. Takeshi Yamamura. In 2010 he moved to The University of Tokyo, joining Prof. Nishihara’s group again. His current research interest lies on the construction of molecule-based nanostructures, and photonic and electronic devices using thereof.

Hiroshi Nishihara received his B. Sc. degree in 1977, M. Sc. in 1979 and D. Sc. in 1982 from The University of Tokyo. He was appointed research associate of Department of Chemistry at Keio University in 1982, and he was promoted lecturer in 1990, and associate professor in 1992. Since 1996, he has been a professor of Department of Chemistry, School of Science at The University of Tokyo. He also worked as a visiting research associate of Department of Chemistry at The University of North Carolina at Chapel Hill (1987-1989), and as a researcher of PRESTO, JST (1992-1996). He received Docteur Honoris Causa from


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University of Bordeaux in 2011, Commendation for Science and Technology by MEXT in 2014, Japan Society of Coordination Chemistry Award in 2015, and Chemical Society of Japan Award in 2016.


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Figure 1. Concept behind coordination nanosheets (CONASHs). Various combinations of metals and ligands produce a range of functionalized CONASHs. 177x114mm (300 x 300 DPI)


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Figure 2. Schematic illustration of the NiDT CONASH structure (left) and a photograph of micro-NiDT synthesized at a liquid–liquid interface. Adapted with permission from J. Am. Chem. Soc. 2013, 135, 24622465. Copyright 2013 American Chemical Society. 177x90mm (300 x 300 DPI)


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Figure 3. (a) SAED pattern (left) and TEM image of micro-NiDT. (b) Experimental (red solid line) and simulated powder XRD patterns (black bars) and estimated stacking structure of NiDT (inset). Adapted with permission from J. Am. Chem. Soc. 2013, 135, 2462-2465. Copyright 2013 American Chemical Society. 177x80mm (300 x 300 DPI)


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Figure 4. (a) STM image of nano-NiDT on HOPG and cross-sectional analysis at a red line (inset). (b) Enlarged scan image of the area outlined by the white square in (a), the result of fast Fourier transform (FFT) analysis (upper-right inset), and a FFT-filtered image (lower-left). Adapted with permission from J. Am. Chem. Soc. 2013, 135, 2462-2465. Copyright 2013 American Chemical Society. 177x89mm (300 x 300 DPI)


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Figure 5. (a) Chemical structure of CONASHs synthesized using nickel or copper ions and 2,3,6,7,10,11position-substituted triphenylene ligand reported in references 65, 70 and 71. (b) Temperature-dependent conductivity measurements obtained using the van der Pauw method of a ~500 nm thick CONASH (M = Ni, X = NH) on quartz. (c) Chemiresistive response of CONASH (M = Cu, X = NH) to 0.5, 2, 5 and 10 ppm NH3 gas. Reprinted with permission from J. Am. Chem. Soc. 2014, 136, 8859-8862. Copyright 2014 American Chemical Society. Reproduced with permission from Angew. Chem. Int. Ed. 2015, 54, 4349-4352. Copyright 2015 Wiley-VCH Verlag GmbH & Co. KGaA. 177x114mm (300 x 300 DPI)


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Figure 6. Estimated structure of the 2D hexagonal lattice of [Cu3(C6S6)]n. 85x74mm (300 x 300 DPI)


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igure 7. (a) Electrocatalytic hydrogen gas evolution using bis(dithiolato)nickel CONASH (black solid line) and a glassy carbon disk electrode (black dashed line) in 0.5 M H2SO4 aqueous solution. (b) Polarization curves obtained using a glassy carbon electrode (black dashed line), CONASH comprising Co ion and BHT (red solid line), and CONASH comprising Co ion and THT (blue solid line) in pH 1.3 H2SO4 aqueous solution. Adapted with permission from Angew. Chem. Int. Ed. 2015, 54, 12058-12063. Copyright 2015 Wiley-VCH Verlag GmbH & Co. KGaA. Adapted with permission from J. Am. Chem. Soc. 2015, 137, 118-121. Copyright 2015 American Chemical Society. 177x62mm (300 x 300 DPI)


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Figure 8. (a) Chemical structure of NiDT CONASH. The white solid lines show the unit cell (Ni3C12S12). (b) 2D band structure of NiDT. (c) Close-up of the band structure around the spin-orbit coupling gaps. Adapted with permission from Nano Lett. 2013, 13, 2842-2845. Copyright 2013 American Chemical Society. 177x180mm (300 x 300 DPI)


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Figure 9. (a) Spin-polarized electron density distribution in a 2×2 superlattice of a Mn3C12S12 monolayer. (b) The temperature dependency of the magnetic moment in a unit cell of a Mn3C12S12 monolayer as estimated by Monte Carlo simulation. The inset shows the simulated curve for heat capacity (Cv) as a function of temperature. Adapted from Nanoscale 2013, 5, 10404-10408 with permission from The Royal Society of Chemistry. 177x52mm (300 x 300 DPI)


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Figure 10. Optimized unit cells and band structures of (a) NiDT CONASH/graphene, (b) NiDT CONASH/h-BN, and (c) NiDT CONASH. Adapted from RSC Adv. 2014, 4, 13361-13366 with permission from The Royal Society of Chemistry. 177x178mm (300 x 300 DPI)


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Figure 11. Top-views and side-views of the lowest unoccupied state of (b) NiDT CONASH and (b) ethyleneadsorbed NiDT CONASH. Adapted with permission from J. Phys. Chem. C 2013, 117, 14125-14129. Copyright 2013 American Chemical Society. 177x74mm (300 x 300 DPI)


Coordination Programming of Two-Dimensional Metal Complex

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