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Nano-sheets of 2D magnetic and conducting Fe(II)/Fe(III) mixed-valence MOFs Samia Benmansour, Alexandre Abhervé, Patricia GómezClaramunt, Cristina Vallés-García, and Carlos J. Gomez-Garcia ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b08322 • Publication Date (Web): 17 Jul 2017 Downloaded from http://pubs.acs.org on July 19, 2017

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Nano-sheets of 2D magnetic and conducting Fe(II)/Fe(III) mixedvalence MOFs Samia Benmansour,* Alexandre Abhervé, Patricia Gómez-Claramunt, Cristina Vallés-García and Carlos J. Gómez-García*. Departamento de Química Inorgánica. Instituto de Ciencia Molecular (ICMol). Parque Científico. Universidad de Valencia. Catedrático José Beltrán, 2. 46980 Paterna Valencia, Spain. E-mail: [email protected] (CJGG); [email protected] (SB). KEYWORDS: Anilato-type ligands • Magnetic MOFs • Metal-organic frameworks • Multifunctional MOFs • Nanostructures. ABSTRACT: We report the synthesis, magnetic properties, electrical conductivity and delamination into thin nano-sheets of two anilato-based Fe(II)/Fe(III) mixed valence 2D-MOFs. Compounds [(H3O)(H2O)(phenazine)3][FeIIFeIII(C6O4X2)3]·12H2O; X = Cl (1) and Br (2) present a honeycomb layered structure with an eclipsed packing that generates hexagonal channels containing the water molecules. Both compounds show ferrimagnetic ordering at ca. 2 K coexisting with electrical conductivity (with room temperature conductivities of 0.03 and 0.004 S/cm). Changing the X group from Cl (1) to Br (2) leads to a decrease in the ordering temperature and in the room temperature conductivity that can be correlated with the decrease of the electronegativity of X. Despite the ionic charge of the anilato-based layers, these MOFs can be easily delaminated in thin nano-sheets with a few monolayers thickness.

INTRODUCTION Metal organic frameworks (MOFs) or porous coordination polymers (PCPs) are crystalline materials with a porous ordered network1-3 with applications in gas storage and separation,4,5 water adsorption,6 sensors,7 catalysis,8,9 photocatalysis for water splitting, CO2 reduction and organic reactions,10 luminescence,11 energy storage and transfer,12 biomedical applications,...13 Although porosity represents a clear advantage for these and other applications, it may become a drawback for the presence of cooperative properties as electrical conductivity and long range magnetic ordering. Thus, the synthesis of magnetic and/or conducting MOFs represents a fascinating challenge in materials chemistry which has started to be faced in the last years. Porous magnets are expected to find applications in magnetic sensors, low-density magnets, absorption/separation of magnetic molecules, etc. Despite their synthetic difficulties, some interesting examples of magnetic MOFs have been reported in the last decade,14-16 including some with ordering temperatures above 100 K.17-19 On the other hand, highly conducting MOFs may find applications in porous electrodes, sensors, electrocatalysts, fuel cells, capacitors, etc.20 Recent results have shown that it is 21-25 possible to prepare MOFs with high conductivity values, above those reported for covalent organic frameworks (COFs),26 1D nanoribons27 and coordination polymers.28 A more difficult challenge is the synthesis of MOFs presenting both cooperative properties (electrical conductivity and magnetic order) in the same lattice.29 Thus, although there are several examples of conducting ferro- and

ferrimagnets,17,30-36 only a few of them present a porous These few examples include structure.37,38 [CoII3(lac)2(pybz)2]·2.7I2 (pybz = 4-(pyridin-4-yl)benzoate, lac = lactate), that shows a porous structure where the uptake of I2 results in oxidation of the Co(II) centers giving rise to a room temperature conductivity of 7x10-6 S/cm and an antiferromagnetic order at TN = 8 K.37 Other remarkable examples are the two related compounds (NMe2H2)2[FeIII2(C6O4Cl2)3]·2H2O·6DMF (A) and (Cp2Co)1.43(NMe2H2)1.57[FeIII2(C6O4Cl2)3]·4.9DMF (B) (C6O4Cl22- = chloranilato dianion). These MOFs present 2D lattices with hexagonal channels containing solvent molecules and with 2/3 (in A) or all (in B) the chloranilato ligands reduced in their semiquinone radical form (Scheme 1). The unpaired delocalized electrons in the anilato rings give rise to semiconducting behaviors with room temperature conductivities of 1.4x10-2 and 5.1x10-4 S/cm and to magnetic order with Tc = 80 and 105 K, in A and B, respectively. Furthermore, when the solvent molecules are removed in A, Tc and the conductivity decrease to 26 K and 1.0x10-3 S/cm.38,39 The structure of compounds A and B is similar to those previously reported in the series: (i) Na2(H2O)24[MII2(C6O4Cl2)3] (MII = Mn and Cd),40 (ii) [(H3O)2(phz)3][MII2(C6O4Cl2)3]·G (phz = phenazine, Scheme 1); MII = Cu, Cd, Zn, Co and Mn; G = H2O and acetone),41,42 (iii) [(H3O)(phz)3][MnIIMIII(C6O4X2)3].G, (MIII = Cr and Fe; X = Cl and Br; G = H2O and acetone),43 (iv) [NBu4][MnIICrIII(C6O4Cl2)3]43 and (v) A[MnIICrIII(C6O4Cl2)3] (A = [FeIII(sal2-trien)]+, [FeIII(4-OH-sal2-trien)]+, [FeIII(sal2epe)]+ and [FeIII(5-Cl-sal2-trien)]+.44 Albeit, in these MII2 and MnIIMIII families the anilato ligands are oxidized and, although some members of the the MnIIMIII series presents

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long range ferrimagnetic ordering in the range 5.5-11.0 K, none of them shows electrical conductivity. Another interesting compound presenting conductivity and magnetic ordering is (NBu4)2[Fe2III(C6O4H2)3] (C). In this compound also 2/3 of the bridging ligands (dhbq, X = H, Scheme 1) are reduced.36 This compound presents an interpenetrated (10,3)-a 3D lattice also previously reported for [NBu4]2[MII2(C6O4H2)3] (MII = Mn, Fe, Ni, Co, Zn and Cd), [NBu4]2[Mn2(C6O4Cl2)3]45 and [NBu3Me]2[NaCr(C6O4Br2)3].46 Compound C is a good conductor (σ300K = 0.16 S/cm) and shows a long range magnetic ordering below 8 K but it is not porous.36 X

X

N

O

O-

N

-O

O

phenazine (phz)

X X = H, Cl, Br, I,

+ e-e

-O

O-

-O

O X

Scheme 1. (left) Phenazine. (right) Quinone and semiquinone forms of the derivatives of the 2,5-dihydroxy1,4-benzoquinone dianion Once the desired cooperative magnetic and electrical properties are incorporated in a MOF, it is necessary to process these materials into nano-sheets in order to prepare flexible and transparent electronic and optoelectronic devices47,48 or for their use in other applications as membranes and molecular sieves.49 This is a disadvantage of many MOFs since their formation is irreversible and cannot be solubilized or dispersed as nanoparticles or nano-flakes in order to deposit them as thin films. In order to overcome this problem, different bottom-up techniques as liquid phase epitaxy,50 chemical vapour deposition,51 layer-by-layer,52 LangmuirBlodgett,53 spin-coating, drop casting, inkjet printing,...54 have been used. However, these methods need a selective control of the growth direction to prevent the growth in the vertical direction while keeping a good control of the structure, thickness and crystallinity of the deposited material.47 Although still are very limited, there are some successful examples of the top-down approach to prepare thin films of MOFs with good yields and homogeneous coverage,47,55-58 including one example with anilato-based magnets using the Scotch-tape method.59 Here we present two MOFs with magnetic ordering and a high room temperature conductivity of 0.03/0.004 S/cm that can be easily delaminated to prepare nano-sheets of a few monolayers thickness. These conducting and magnetic MOFs are based on mixed valence Fe(II)/Fe(III) coordination polymers and constitute, as far as we know, the first examples of conducting and magnetic MOFs that can be delaminated into a few monolayers nano-sheets.

EXPERIMENTAL SECTION General Considerations. All the chemicals were commercially available and used as received. The reactions were performed in open air. [(H3O)(H2O)(phz)3][FeIIFeIII(C6O4Cl2)3]·12H2O (1). Single crystals of 1 were obtained by carefully layering, at room temperature, a degased solution of [PEtPh3]3[Fe(C6O4Cl2)3]60 (15.5 mg, 0.01 mmol) in acetonitrile (4.5 mL) on top of a degased solution of phenazine (54 mg, 0.3 mmol) in THF (2 mL) and a degased solution of Fe(BF4)2.6H2O (55 mg, 0.16

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mmol) in water (2.5 mL). The solution was sealed and allowed to stand for about two months to obtain black hexagonal prismatic crystals (Figure S1, supporting information). Crystals suitable for X-ray diffraction were filtered and air dried. This compound could also be obtained in larger quantities as a polycrystalline solid by carefully layering, at room temperature, three different degased solutions. The top solution contains 0.40 g (0.3 mmol) of (NBu4)3[Fe(C6O4Cl2)3]43 in 50 mL of acetonitrile. The middle solution was prepared by dissolving 1.30 g (9 mmol) of phenazine in 50 mL of THF. The bottom solution was prepared by dissolving 1.42 g (4.2 mmol) of Fe(BF4)2·6H2O in 50 mL of water. The solution was allowed to stand sealed for about three and a half months to obtain 0.382 g of a black shiny microcrystalline solid (Yield = 98.5 %). Elemental analyses (%) calculated for 1: C, 43.49; H, 3.50; N, 5.51. Found: C, 43.69; H, 3.37; N, 5.54. FT-IR (νmax/cm-1, KBr pellet): 3422(s), 1626(m), 1482(s), 1355(s), 1150(w), 1116(m), 1066(w), 1003(m), 910(w), 853(m), 825(w), 757(m), 608(w), 593(w), 573(w), 504(w), 471(w), 411(w). The phase purity of this compound was confirmed by powder X-ray diffraction analysis (Figure S3). [(H3O)(H2O)(phz)3][FeIIFeIII(C6O4Br2)3]·12H2O (2). Single crystals of 2 (Figure S2) were obtained with a similar method to 1 but using [PEtPh3]3[Fe(C6O4Br2)3]60 (18.2 mg, 0.01 mmol) instead of [PEtPh3]3[Fe(C6O4Cl2)3]. This compound could also be obtained in larger quantities as a polycrystalline solid by carefully layering, at room temperature, three different degased solutions. The top solution contains 1.00 g (0.6 mmol) of (NBu4)3[Fe(C6O4Br2)3]43 in 100 mL of acetonitrile. The middle solution was prepared by dissolving 2.60 g (18 mmol) of phenazine in 100 mL of THF. The bottom solution was prepared by dissolving 2.84 g (8.4 mmol) of Fe(BF4)2·6H2O in 100 mL of water. The solution was allowed to stand for about three and a half months to obtain 0.748 g of a black shiny microcrystalline solid (Yield = 80.0 %). Elemental analyses (%) calculated for 2: C, 36.17; H, 2.98; N, 4.69. Found: C, 36.55.; H, 3.04; N, 4.82. FT-IR (νmax/cm-1, KBr pellet): 3441(w), 1624(w), 1482(s), 1353(s), 1148(w), 1126(m), 1065(w), 988(w), 911(w), 813(m), 758(m), 593(w), 554(w), 499(w), 465(w), 411(w). The phase purity of this compound was confirmed by powder X-ray diffraction analysis (Figure S3). X-ray Structure Determination. Suitable single crystals of compounds 1 and 2 were mounted on a glass fiber using a viscous hydrocarbon oil to coat the crystals and then transferred directly to the cold nitrogen stream for data collection. X-ray data were collected at 120 K on a Supernova Agilent Technologies diffractometer equipped with a graphitemonochromated Enhance (Mo) X-ray Source (λ = 0.71073 Å). The program CrysAlisPro, Agilent Technologies Ltd., was used for unit cell determinations and data reduction. Empirical absorption correction was performed using spherical harmonics, implemented in the SCALE3 ABSPACK scaling algorithm. Crystal structures were solved by direct methods with the SIR92 program,61 and refined against all F2 values with the SHELXL-2014 program,62 using the WinGX2014.1 graphical user interface.63 All non-hydrogen atoms were refined anisotropically and hydrogen atoms were placed in calculated positions and refined isotropically with a riding model. Data collection and refinement parameters are given in Table 1.


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Table 1. Crystallographic data for 1 and 2 Compound

1

2

Empirical formula Formula weight g/mol

C54H53N6O26Fe2Cl6 1526.43

C54H53N6O26Fe2Br6 1793.14

Crystal system Space group Temp, K

Trigonal P-31m 120

Trigonal P-31m 120

Wavelength, Å a, Å

0.71073 13.6909(3)

0.71073 13.7841(3)

b, Å c, Å

13.6909(3) 9.1968(3)

13.7841(3) 9.2424(3)

α, deg β, deg γ, deg

90

90

90

90

120

120

1492.90(8)

1520.80(8)

R1 (I > 2σ(I))a

1.665 0.0381

1.926 0.0481

wR2 (all)b GoF

0.1107 1.093

0.1433 1.071

V, Å3 3

dcalc, g/cm

a

R1 = Σ||Fo|–|Fc||/Σ|Fo|; bwR2 = [Σw(Fo2–Fc2)2/Σw(Fo2)2]1/2; [c]S(F2) = [w(Fo2–Fc2)2/(n + r – p)]½

Powder X-ray Diffraction Analysis. The X-ray powder diffractograms (Figure S3) were collected for polycrystalline samples of 1 and 2 slightly ground in an agate mortar using 0.5 mm glass capillaries that were mounted and aligned on a Empyrean PANalytical powder diffractometer, using CuKα radiation (λ = 1.54056 Å). Two scans were collected at room temperature in the 2θ range 5-40º and merged in a single diffractogram. Magnetic Measurements. Magnetic measurements were performed with a Quantum Design MPMS-XL-5 SQUID magnetometer in the 2 to 300 K temperature range with an applied magnetic field of 0.1 T on polycrystalline samples of 1 and 2 (with masses of 13.67 and 14.50 mg, respectively). Hysteresis cycles of the isothermal magnetization were performed at 1.8 K in the field range from -5 to 5 T. AC susceptibility measurements were performed in the temperature range 1.8-5 K with an alternating field of 0.395 mT oscillating at frequencies in the range 1-1000 Hz. Susceptibility data were corrected for the sample holder and for the diamagnetic contribution of the salts using Pascal’s constants.64 Electrical Conductivity Measurements. The thermal dependence of the DC electrical conductivity was measured with the two contacts method on four single crystals of compounds 1 and 2 in the temperature range 2-300 K. The contacts were made with Pt wires (25 m diameter) using graphite paste. The samples were measured in a Quantum Design PPMS-9 equipment connected to an external voltage source (Keithley model 2400 source-meter) and amperometer (Keithley model 6514 electrometer). All the conductivity quoted values have been measured in the voltage range where the crystals are Ohmic conductors. The cooling and warming rates were 0.5 or 1 K/min. The samples were measured with the current along the hexagonal faces of the single crystals

(parallel to the ab plane, σpar) or perpendicular to them (along the c direction, σper) (Figure S9). Other Physical Measurements. FT-IR spectra were performed on KBr pellets and collected with a Nexus-Nicolet 5700 spectrophotometer (see supporting information). Elemental C, H, N analyses were performed with a CE instrument EA 1110 CHNS analyser.

RESULTS AND DISCUSSION Synthesis and structure. Both compounds have been prepared by slow diffusion of a degased Fe(II) solution and a degased solution of the Fe(III) precursor complex [Fe(C6O4X2)3]3- (X = Cl and Br) to assure the presence of both oxidation states of Fe in the final compounds. The IR spectra confirm the presence of bis-chelated anilato ligands and, most important, show that they are not reduced (see Figures S12S15), in agreement with the structural and magnetic data (see below). The crystal structure of compounds [(H3O)(H2O)(phz)3][FeIIFeIII(C6O4X2)3]·12H2O; X = Cl (1) and Br (2) show that they are isostructural to the series [(H3O)(H2O)(phz)3][MnIIMIII(C6O4X2)3]·G (MIII = Cr and Fe; G = H2O and acetone, X = Cl and Br)43 and closely related to the structure of the series [(H3O)2(phz)3][MII2(C6O4Cl2)3].G (MII = Mn, Cu, Cd, Zn, Co and Mn; G = H2O and acetone).41,42 The structure consists of cationic layers formulated as [(H3O)(H2O)(phz)3]+ alternating along the c direction with anionic layers with formula [FeIIFeIII(C6O4X2)3]- (X = Cl (1) or Br (2)) (Figure 1a). The distance between two anionic consecutive layers corresponds to the c parameter (9.1968(3) Å for 1 and 9.2424(3) Å for 2). The anionic layers show the well known hexagonal honeycomb lattice with Fe(II) and Fe(III) ions alternating in the vertices of the hexagons with C6O4X22- linkers as the sides (Figure 1b). Each Fe(II) or Fe(III) center is located on a C3 axis surrounded by three C6O4X22ligands with a local D3 symmetry and a propeller-like arrangement (Figure 1c). Consecutive Fe centers present opposite chiralities to form the planar honeycomb lattice. Since the structure is centro-symmetric (as observed in the MnIIFeIII and FeIIIFeIII derivatives)39,43 and the Fe centers are located on the C3 axis, there is only one crystallographically independent Fe center in 1 and 2. Note that this fact does not necessarily implies that the two Fe centers are strictly equivalent. The crystallographic equivalence may arise from a different disposition of the MII and MIII centers in different layers, as observed in the MnFe derivative (where the two metals are different but appear as equivalent).43 In fact, 1 and 2 are class II mixed valence compounds,65 as will be shown below, and at 120 K the electron is fully delocalized, rendering both Fe centers equivalent. The Fe-O bond distances in both compounds (2.0634(19) and 2.076(3) Å for 1 and 2, respectively) are in between those found for the FeIIIFeIII (2.020(4)-2.028(5) Å)38,39 and FeIIFeII derivatives (2.140(8) and 2.141(3) Å in two closely related 1D structures),66,67 in agreement with the expected average charge of +2.5 in 1 and 2 (Figure S4 and Table S1). In fact, the bond valence sum calculations68 give oxidation states of 2.465 for 1 and 2.381 for 2 when assuming Fe(II) centers and 2.637 for 1 and 2.547 for 2 when assuming Fe(III). Furthermore, although it could be argued that the two Fe centers could be Fe(III) ions (with the corresponding reduction of one of the three C6O4X22ligands to the radical C6O4X23-, as observed in the [FeIIIFeIII(C6O4Cl2)3]2- and [FeIIIFeIII(C6O4Cl2)3]3- lattices),38,39 the C-O and C-C bond distances in the ligands clearly exclude



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this possibility since the reduction of the ligand implies an elongation of the C-O bonds and a shortening of the C-C ones (Table S1). Thus, the C-O bond distances (1.268(3) and 1.262(6) Å in 1 and 2, respectively) are shorter than those found in the reduced chloranilato ligands (1.280(6) Å when the charge is -2.67 and 1.298(9) Å for a charge of -3, Figure S5). Additionally, the C-C bond distances between the two carbon atoms connected to the oxygen atoms in 1 and 2 (1.529(5) and 1.534(9) Å, respectively) are longer than those observed when the chloranilato ligands are reduced and present average charges of -2.67 or -3 (1.480(9) and 1.431(16) Å, respectively, Table S1 and Figure S6). All these data support the average oxidation state of +2.5 for the Fe atoms and a charge of -2 for the three anilato ligands in 1 and 2 and suggests that the reduction of the chloranilato ligand by the Fe(II) ions only takes place under hydrothermal conditions.

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H-bonds between the water molecules and the phenazine molecules are drawn as red thin lines Color code: C = gray, N = blue, O = red.

The eclipsed packing of the hexagonal layers gives rise to large hexagonal pores running along the c direction with 15.8073(2) Å diameter in 1 and 15.9173(4) Å in 2 (measured as the longest Fe-Fe distance in the hexagon). These channels represent ca. 21 % of the total space (Figure 3). These hexagonal channels are slightly larger in 1 and 2 than in the related [FeIIIFeIII(C6O4Cl2)3]2- lattice (where the Fe-Fe distance is 15.6614(6) Å), in agreement with the larger bond distances expected for Fe(II) compared with Fe(III) ions. The X-ray analysis and the thermogravimetric analysis (Figure S7) show the presence of 12 water molecules disordered in the hexagonal channels in both compounds. Half of these molecules are coplanar to the anionic layer and the other half with the cationic layer although the exact position of these water molecules cannot be located since they are disordered and appear on special positions (a mirror plane for O3W and a two-fold axis for O2W). These guest molecules can be partly exchanged with other molecules or partially removed without collapsing the structure (Figures S16 and S17), as already observed in other similar [M2(C6O4Cl2)3]n- 2D lattices.39,41

Figure 1. Structure of 1 (similar for 2): (a) Side view of the alternating cationic and anionic layers. (b) View of the honeycomb structure of the layers. (c) View of a hypothetical Fe(III) ion connected to three Fe(II) ions through three chloranilato bridges. Color code: C = gray, N = blue, O = red, Cl = light green, Fe(III) = dark green, Fe(II) = orange.

The cationic layers follow the same hexagonal symmetry displayed by the anionic ones with H3O+ and H2O molecules located in half of the vertices and the phenazine molecules connecting them through H-bonds (Figure 2a) forming a propeller-like structure for the cation [(H3O)(phz)3]+ (Figure 2b). Due to the low electron density of the H atoms and the presence of a C3 axis passing through the oxygen atoms, the H3O+ cations and the water molecules appear as crystallographically equivalent and the H atoms could not be located.

Figure 2. (a) View of the cationic layer in the ab plane. (b) View of a [(H3O)(phz)3]+ cation with the propeller-like geometry.

Figure 3. View of a hexagonal channel along the c direction showing the 12 water molecules inside. Color code: C = gray, N = blue, O = red, Cl = light green, Fe(III) = dark green, Fe(II) = orange.

Magnetic properties. Both compounds show a ferromagnetic Fe-Fe interaction, as can be deduced from the increase observed in the χmT product when the temperature is decreased (χm is the molar magnetic susceptibility per two Fe ions, Figure 4). The room temperature χmT value for both compounds (ca. 8.3 cm3 K mol-1) is close to the expected one for a high spin S = 5/2 Fe(III) ion plus a high spin S = 2 Fe(II) ion (the calculated value is ca. 8.0 cm3 K mol-1 assuming g = 2 for the Fe(III) ion and g = 2.2 for the Fe(II) ion). The ferromagnetic coupling observed in 1 and 2 may be explained by the presence of a charge delocalization between both Fe atoms that generates a double exchange.69 This double exchange produces the parallel alignment of the spins of both Fe centers thanks to the “aligning” effect of the delocalized electron. At low temperatures both compounds show maxima in χmT followed by an abrupt decrease of the χmT product, suggesting


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the presence of antiferromagnetic (ferrimagnetic in this case) intra-layer interactions. This ferrimagnetic coupling might be due to a progressive localization of the delocalized electron as the temperature is decreased: at low temperatures the electron becomes localized on one of the two Fe centers, giving rise to a ferrimagnetic coupling between a high spin Fe(III) center (S = 5/2) and a high spin Fe(II) center (S = 2). Note that this antiferromagnetic coupling between both localized Fe centers is the expected one for a chloranilato bridge connecting two different spin states.43 This result suggests that the delocalized electron becomes localized at low temperatures (class II, mixed-valence compounds)65 and this localization gives rise to a ferrimagnetic long range ordering of the Fe(II) (S = 2) and Fe(III) (S = 5/2) centers, as observed in the MnIICrIII derivatives.43,44

χ mT (cm3 K mol-1)

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16 14 12 10 8 6 4 2 0

16 14 12 10 8 6 4

0

very small coercive fields of ca. 1 mT, indicating that they are very soft magnets, as observed in the MnCr derivatives.43,44 The Q-band Electron Paramagnetic Resonance (EPR) spectra of both compounds confirm the electron delocalization at high temperatures and the progressive localization at low temperatures (Figure 6). Thus, both compounds are EPR silent at high temperatures and show the progressive appearing of a signal at low temperatures (ca. 20 and 40 K in 1 and 2, respectively, Figure 6). The signal can be attributed to the presence of S = 5/2 Fe(III) ions since S = 2 Fe(II) ions are expected to be ERP-silent given the high ZFS. The appearing of the EPR signal indicates that the delocalized electron becomes localized at low temperatures and confirms that both compounds are class II mixed valence compounds.65 The lower temperature required in the Cl derivative (1) to localize the electron, confirms the idea that the electron delocalization is better in the Cl derivative.

X = Cl (1)

X = Br (2)

0

5 10 15 20 25 30 35 40

50

100

150 200 T (K)

250

300

Figure 4. Thermal variation of the χmT product for 1 and 2. Inset shows the low temperature region.

The chloranilate derivative (1) shows a faster increase in χmT than the bromanilate one (2) when the temperature is lowered, suggesting that the coupling in 1 is slightly stronger than in 2. This fact indicates that the magnetic coupling increases as the electronegativity of X increases. This behavior contrasts with the observed trend in the series [(H3O)(H2O)(phz)3][MnCr(C6O4X2)3] (X = H, Cl, Br and I).43 that shows a linear dependence of the strength of the magnetic coupling with the electronegativity of X explained by simple inductive effects. This different behavior may, nevertheless, be explained if we assume a double exchange mechanism: as the electronegativity of X increases, the electron delocalization through the anilato bridge becomes easier, resulting in a stronger magnetic coupling. The long range ferrimagnetic ordering is confirmed by AC susceptibility measurements that show the presence at 1 Hz of a non-zero out of phase (χm”) signal below ca. 2.4 K and 2.1 K for the Cl and Br derivatives, respectively (Figure 5). The slight frequency shift observed in χm” might be attributed to the presence of some Fe(III)/Fe(II) disorder in the anionic layer and/or to the progressive electron localization, which is a temperature-dependent phenomenon. The in-phase signal (χm’) shows a rounded maximum for 1, also slightly frequency dependent, in the temperature range 2.0-2.5 K (Figure 5a). This maximum is absent in 2 above 1.8 K (Figure 5b), confirming the higher ordering temperature in 1, in agreement with the stronger magnetic coupling in 1. A further confirmation of the long range ordering is provided by the presence of a hysteresis cycle in the magnetization at 1.8 K (Figure S8). Both compounds show

Figure 5. Thermal variation of the in phase (χm’, filled circles, left scale) and out of phase (χm”, empty circles, right scale) AC susceptibility at different frequencies for 1 (a) and 2 (b).

Figure 6. Q-band EPR spectra of compounds 1 (left) and 2 (right) at different temperatures.



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Electrical conductivity. The electron delocalization found in these compounds is further confirmed by the electrical conductivity measurements performed on several single crystals of both compounds with the current along the hexagonal faces of the single crystals (parallel to the ab plane, σpar) or perpendicular to them (along the c direction, σper). The σpar measurements show that both compounds are semiconductors with relatively high room temperature conductivities of ca. 0.03 and 0.003 S/cm for 1 and 2, respectively (Table S2 and Figure 7). As expected, the room temperature conductivity perpendicular to the hexagonal layers (σper) is much lower (ca. 1x10-4 and 6x10-6 S/cm, Table S2), resulting in σpar/σper anisotropy values of ca. 300 and 500 in 1 and 2, respectively. The thermal variation of σpar and σper shows a classical Arrhenius behavior for both compounds with two different semiconducting regimes (insets in Figure 7). The activations energies are all very low, in the range 14-115 meV, and are lower in the high temperature regime (14-60 meV) than in the low temperature one (100-116 meV) (Table S2). The higher conductivity values found for compound 1 (one order of magnitude higher) confirm the idea that the electron delocalization is favored in compound 1 (in agreement with the susceptibility and EPR results). Thus, in 1 the smaller electron density in the anilato ring (due to the higher electron withdrawing ability of Cl) is expected to favor the electron jumping from one to the other Fe atom, resulting in a higher electron delocalization and, accordingly, in a higher electrical conductivity.28 Finally, it is worth to mention that the σpar values in 1 and 2 (0.03 and 0.003 S/cm) are much higher than those found in a pressed pellet of an amorphous compound with a similar composition although with NBu4+ as cation (2x10-5 S/cm)70 and of a related compound containing 1/3 of reduced chloranilate anions and [(H3O)2(phenazine)3]2+ dications (4x10-5 S/cm).42

Figure 7. Thermal variation of the electrical resistivity for 1 (a) and 2 (b) along the hexagonal layers (1|| and 2||) and perpendicular to them (1⊥ and 2⊥). Insets show the Arrhenius plots. Solid lines are the fit to the Arrhenius law in the two semiconducting regimes.

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Delamination as nano-sheets. Compounds 1 and 2 show a remarkable capacity to easily form nano-sheets by using simple wet methods (see supporting information). The best results were obtained with sonication for 15 minutes of a suspension of crystals of both compounds in methanol. In this simple way we obtained suspensions showing the Tyndall effect for both compounds (Figure S10) that were used for deposition by drop-casting on a silicon oxide substrate (300 nm SiO2/Si). Atomic force microscopy (AFM) and high resolution transmission electron microscopy (HR-TEM) show that in both cases we obtain homogeneous deposition of nanosheets with a smooth surface and a few-layers thickness (Figures 8a and 8e). Optical contrast is very similar in all the nano-flakes, indicative of a good homogeneity of the thickness of the exfoliated MOFs. This homogeneity is further confirmed by the profiles of the AFM images (Figures 8d and 8h). The optical microscope (Figures 8b and 8f) and the tapping-mode AFM images of the nano-flakes (Figures 8c and 8g) show lateral dimensions of several micrometers and thicknesses of ca. 7 nm for compound 1 and 2, corresponding to ca. 7 bilayers, since each double layer (anionic plus cationic) presents a thickness of ca. 1.0 nm (Figure S11).

Figure 8. (a,e) HR-TEM image (b,f) Optical microscopy image, (c,g) AFM image and (d,h) height profiles of deposited flakes of 1 and 2, respectively.


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CONCLUSIONS

ACKNOWLEDGMENTS

In summary, we have prepared two novel mixed-valence 2D MOFs: [(H3O)(H2O)(phenazine)3][FeIIFeIII(C6O4X2)3]·12H2O; X = Cl (1) and Br (2) that show that: (i) It is possible to combine two cooperative properties as electrical conductivity and magnetic ordering in a MOFs thanks to the electron delocalization provided by the mixed valence character of the [FeIIFeIII(C6O4X2)3]- anionic lattice. (ii) It is possible to modify the ordering temperature and the electrical conductivity of these 2D MOFs by chemical design simply changing the X group in the anilato ligand. (iii) It is possible to easily delaminate these 2D magnetic and conducting MOFs into nano-sheets with a few layers thickness and a few micrometers side dimensions. This last result is very unexpected since the layers are not neutral. The structure of these compounds indicate that they are both mixed valence Fe(III)/Fe(II) 2D MOFs with an average oxidation state of +2.5. The magnetic studies show that both compounds present ferromagnetic couplings due to the electron hopping (double exchange) and a ferrimagnetic long range ordering at very low temperatures when the electron becomes localized. These studies suggests that the electron delocalization is favored in the Cl compound. EPR studies confirm the magnetic susceptibility results and the higher electron delocalization in the Cl derivative. This higher electron delocalization is due to the higher electron withdrawing effect of the Cl atom and is further confirmed by the electrical conductivity measurements that show a higher conductivity in the Cl derivative (0.03 S/cm) than in the Br compound (0.004 S/cm). These compounds represent two of the very few known examples of conducting and magnetic MOFs and are, as far as we know, the first examples that have been delaminated into nanosheets with a few layers thickness.47,71

This work was supported by the Generalitat Valencia (project PrometeoII2014/076). P.G.C. thanks the Generalitat Valenciana for a predoctoral Grant.

ASSOCIATED CONTENT Supporting Information Supporting information includes: pictures of the crystals of compounds 1 and 2, X-ray powder diffractograms of compounds 1 and 2 and the simulated one from the single crystal X-ray structure, a comparative structural analysis of the Fe-O, C-O and C-C bond distances in compounds 1 and 2 and in all the reported iron complexes with chloranilato bridges, thermogravimetric analysis, hysteresis plot at 2 K of both compounds, electrical conductivity values and activation energies for both compounds, details of the delamination experiments and characterization of the thin films and IR spectra and the band assignation for compounds 1 and 2. The Supporting Information is available free of charge on the ACS Publications website.

AUTHOR INFORMATION Corresponding Author *[email protected] and [email protected]

Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.

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SYNOPSIS TOC We report the synthesis and characterization of two conducting magnetic MOFs whose ordering temperature and electrical conductivity can be easily modified by chemical design. These conducting and magnetic MOFs can be easily delaminated into nano-sheets with a few layers thickness and a few micro-meters side dimensions.


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Fe(III

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