Defect-free chemical functionalization of magnetic monolayers based on coordination polymers
Chemical functionalization has demonstrated to be a powerful approach to tailor the physical and chemical properties of two-dimensional (2D) materials, to increase their processability and stability, to add new functionalities and, even, to create new 2D materials. However, this postsynthetic method -which involves the anchoring of molecules on the surface of an exfoliated 2D crystal -inevitably leads to defective materials, which lack long-range structural order. If defect-free functionalized monolayers are required, a radically new approach needs to be developed. Here we present a pre-synthetic method based on coordination chemistry that affords the isolation of crystalline functionalized monolayers. This involves functionalization of the ligand prior to the formation of the 2D material. The concept is illustrated using layered coordination polymers formed by reacting various benzimidazole derivatives with ferrocene. This surface tuneability, together with the robust magnetic and mechanical properties of these 2D materials, make them exceptional candidates for studying the magnetism in the 2D limit, as well as for developing membranes for selective molecular sensing.
<!>Synthesis and characterization of the layered coordination polymers<!>From bulk crystals to the 2D limit<!>Surface modification<!>Discussion<!>Methods<!>X-ray Structural<!>X-ray Powder Diffraction Measurements.<!>Magnetic<!>Materials and synthesis<!>Synthesis of [Fe(bimCl) 2 ] (MUV-1-Cl)<!>Synthesis [Fe(bimH) 2 ] (MUV-1-H)<!>Synthesis [Fe(bimCH 3 ) 2 ] (MUV-1-CH 3 )<!>Synthesis [Fe(bimBr) 2 ] (MUV-1-Br)<!>Synthesis [Fe(bimNH 2 ) 2 ] (MUV-1-NH 2 )<!>Crystal structure determinations and refinements<!>S6<!>X-Ray powder diffraction<!>Infrared spectroscopy<!>Thermal stability<!>Scanning electronic microscope images<!>Figure S11. Scanning electron micrograph of bulk-type MUV-1-Br, MUV-1-H, MUV-1-CH 3<!>Methods and instruments<!>Mössbauer Spectra<!>1-H and MUV-1-NH 2 an additional doublet with IS and QS consistent with high-spin<!>Magnetic structure determination<!>MUV-1-H MUV-1-Cl<!>S26<!>Methods and instruments<!>DC and AC measurements<!>S38<!>Two-dimensional approach and characterization<!>Mechanical delamination and optical images<!>Optical and Atomic Force Microscopy of the monolayer of MUV-1-Cl<!>Transmission Electronic Microscopy and AFM in MUV-1-X thin layers
<p>Since the isolation of graphene, there has been an explosion in the search for other atomically thin layers (aka. 2D materials). 1 Most of these graphenoid compounds are layered inorganic materials that in bulk show stacked structures with weak van der Waals interactions between adjacent sheets, but with strong covalent bonding within each sheet. These materials cover a wide range of physical properties including insulators (h-BN), 2 semiconductors (1T-ZrS 2 , 2H-MoS 2 or black phosphorous), [3][4][5] metals (1T-MoS 2 ), 6 and superconductors (Mo 2 C, 2H-NbSe 2 or 2H-TaS 2 ). [7][8][9] However, 2D magnetic materials (V 5 S 8 , FePS 3 , Cr 2 Ge 2 Te 3 or CrI 3 ) [10][11][12][13] have remained elusive until recently, due to their unstability at ambient conditions and the difficulty to experimentally detect magnetism in the 2D limit. In fact, this challenging issue has only been achieved so far in the most favorable case (Ising ferromagnet). 13 Recently, a new and less explored field has emerged with 2D organic polymers (including covalent organic frameworks, COFs, and conjugated aromatic polymers, CAPs), 14,15 and 2D coordination polymers (enclosing coordination polymers, CPs, and metal-organic frameworks, MOFs), 16 which have been employed for membranes 17 , gas separation 18 , electronics 19 and photoluminescence. 20 However, due to the fragility of the starting crystals, there are only a few examples that can be exfoliated micromechanically, although with limited lateral dimensions, 15,21 being liquid exfoliation the most common methodology. 17,22 This prevents application of these exfoliated materials in areas where high-quality layers are required (nanoelectronics, for example), the most recent uses being in the fields of catalyisis 23 and membranes. 24 A crescent interest in the field of 2D materials resides in their covalent functionalization, since surface engineering can lead to the rational modification of surface properties (hydrophobicity, adhesivity or wettability, among others), or electronic properties of the layers. 25,26 As the surface modifications are generally performed following a post-synthetic method, high rates of surface functionalization are often challenging due to the poor reactivity and solubility of the inorganic surfaces. 27 Therefore, the molecules are anchored to the surface in a random way leading to a defective and incomplete functionalization of the layered material. An alternative strategy for the chemical surface modification is the pre-synthetic method where the bulk layered material is chemically modified prior to the exfoliation. This approach, unfeasible in 2D inorganic materials, can be easily achieved in 2D coordination polymers due to the molecular composition of these compounds. Thus, through the rational choice of the functional substituent of the organic ligand, it is possible to achieve a periodically ordered decoration of the 2D coordination network, i.e. a defectfree functionalized 2D material (Figure 1). Here we present a new family of layered coordination polymers (LCPs), MUV-1-X (MUV = Material of the University of Valencia), of general formula [Fe(bimX) 2 ] (HbimX = derivative of benzimidazole, with X = H, Cl, Br, CH 3 , or NH 2 ) which exhibit cooperative magnetism and which are robust enough to be micromechanically exfoliated down to the monolayer, while preserving their crystallinity. The pre-synthetic modification of the materials through the convenient choice of the organic substituent gives rise to a surface tunability ranging from hydrophilic to hydrophobic, albeit maintaining the inner physical property (i.e. magnetic order) in all the derivatives of the family.</p><!><p>The solvent-free reaction of 5-chlorobenzimidazole (HbimCl) and ferrocene in a sealed tube yields large laminar colorless crystals of formula [Fe(bimCl) 2 ] (abbreviated as MUV-1-Cl), consisting of distorted tetrahedral Fe II centers connected by 5-chlorobenzimidazolate bridges (Figures 2a and 2b) forming a neutral layered coordination polymer extended in the ab plane. These layers weakly interact with each other through van der Waals interactions (Cl•••Cl interactions). The compound is related to the diamagnetic Zn analogue [Zn(bimH) 2 ], which was obtained via decomposition of a 3D MOF. 17 In these atomically thin layers, the Fe II atoms are located in the inner part of the layers whereas the Cl substituents are positioned at the surface (Figure 2a), thus playing an active role in the chemical behavior of the molecular interface. A close-up view with scanning electron microscopy (SEM) evidences the layered structure of MUV-1-Cl as the stacking of multiple sheets (Figure 2c), with a typical size of 400 µm approximately, whose purity is confirmed by powder X-ray diffraction (Figure S8). The room temperature infrared and Raman spectra present coincident peaks, indicating a non-centrosymmetric structure (Figure S40). Furthermore, room temperature Mossbauer spectra consist of two doublets with the same relative area for Fe II , indicating the presence of two different Fe II sites with the same multiplicity (Figure S12). These features are consistent with the space group C2, although the presence of a racemic mixture causes a better refinement of the single crystal X-ray data in the C2/c space group. This is related to the weak interlayer interactions, which prevent long range order in the c direction, but which is advantageous for exfoliation. The short pathway between the iron centers, provided by the 5-chlorobenzimidazolate bridges, facilitates the presence of magnetic exchange in MUV-1-Cl, as evidenced in the plot of the molar magnetic susceptibility (χ m ) as a function of the temperature (Fig. 2d and Supp. section S5.2).</p><p>The χ m vs. T curve exhibits a broad maximum at 30.0 K and a sharp peak at ca. 20 K. In the plot of χ m T vs. T, a χ m T value of 2.94 emu•mol -1 •K is observed at room temperature, in agreement with that expected for a high-spin Fe II , S = 2 (3.0 emu•mol -1 •K where g = 2.0), which decreases upon cooling down to 20 K (Figure S17). The behavior in this region indicates the presence of dominant antiferromagnetic exchange coupling between the Fe II centres, confirmed by a negative Curie-Weiss temperature, θ = -80.6 K (Figure S16). These data can be quantitatively reproduced using a model for a quadratic-layer Heisenberg antiferromagnet 28  ) (see Figure 2d and Figure S15). The resulting exchange parameter is J = -22.9 cm -1 . The abrupt increase in both χ m and χ m T suggests the occurrence of a magnetic transition towards a canted spin structure below T N = 20 K. The presence of an out-of-phase susceptibility signal below 20 K in the ac measurements confirms this kind of magnetic order (see Figure 2e and Figure S18). Powder neutron diffraction patterns collected at 1.8 K and 30 K, i.e. below and above the ordering temperature (Supp. section 5.1), provide a further support on the antiferromagnetic ordering in this material. Still, in the present case these measurements are unable to detect the presence of spin canting, which is below the limit of detection of these measurements (see Supp. section 5.1), mainly due to the small value of the ferromagnetic component resulting from canting (ca. 10 -5 µB, as estimated from susceptibility measurements).</p><!><p>The layered nature of MUV-1-Cl and the absence of strong intermolecular interactions between adjacent layers prompted us to explore the possibility of mechanically exfoliating this material by the so-called Scotch-tape method, a conventional approach usually employed for graphene and other two-dimensional materials, but rather scarce in coordination polymers due to the fragile nature of the crystals. 21 The advantage of this dry method over liquid exfoliation is the potential achieving of atomically thin-layers with high crystallinity and larger lateral sizes.</p><p>Bulk crystals of MUV-1-Cl were exfoliated using a plastic tape (Ultron Systems) and deposited onto a silicon substrate with 285 nm of thermally grown SiO 2 . As a result, a plethora of flakes with a remarkable well-defined rectangular shapes (lateral dimensions > 1 µm) and different thicknesses (ranging from a single layer up to hundreds of nanometers) were obtained, as can be clearly seen by optical and atomic force microscopies (Figures S22-28), thus being air-stable. This contrasts with the lower quality nanosheets obtained in a Zn analogue compound by liquid exfoliation. 17 Remarkably, a monolayer of dimensions 4 x 1.5 µm 2 has been isolated (see Figure 3a and Supplementary Section 6.2). The crystallinity, chemical composition and integrity of the exfoliated flakes was confirmed through TEM and Raman studies on specimens of different thicknesses (see Figures 3b and 3d and Supp. section 6). To the best of our knowledge, this is the first time that selected-area electron diffraction (SAED) patterns have been obtained in a top-down micromechanically exfoliated LCP (Figure 3d), and previously this has only been achieved with inorganic materials, mainly members of transition metal dichalcogenides. 29,30 There are some features that could be associated to diffuse scattering, resulting from the presence of a modulated commensurate structure in the bulk material (see Supporting Information sections 2 and 6.4). The indexation of these patterns indicates that they correspond to the zone axis [0 0 1], confirming that the exfoliated flakes consist of one or several layers whose stacking along the c axis forms the original bulk material. The close resemblance between the experimental and the simulated patterns (Figure 3d and Supp. section 6.4) provides further evidence of that. Moreover, the well-defined Bragg peaks observed in the SAED patterns point out that the exfoliation process does not damage the structure of the LCP, and the obtained flakes retain the crystallinity of the bulk material. Note that some of the SAED patterns were obtained from layers as thin as 20 nm, that is a remarkable result due to the molecular nature of the material. It was not observed any thickness-dependence of the unit cell or Raman peaks shift, compared to their bulk counterparts.</p><p>The magnetic order of the atomically thin-layers deposited in a silicon substrate was inspected by Magnetic Force Microscopy at Low-Temperatures (LT-MFM) in a dual-pass mode configured with a phase locked loop (PLL) feedback on the cantilever oscillation (see Supp. section 6.5). In this configuration, the magnetic signal is related to the frequency shift (Δf) in such a way that an attractive tip-sample interaction is characterized by Δf < 0 (red contrast) and a repulsive tip-sample interaction is characterized by Δf > 0 (blue contrast). 31 For simplicity in the comparison of the magnetic images, we have defined in this work the frequency shift to be 0 (green contrast) in the silicon substrate. 32 Below the order temperature, the flakes are imaged with a smoothly red contrast, indicative of an attractive tip-sample interaction, that disappears at temperatures above T N (Figure 3c), in agreement with a transition from a canted AF structure to a paramagnetic phase. In addition, as expected from the magnetic structure of the material (weak ferromagnet due to an antiferromagnetic canted structure), this contrast is very weak and furthermore it remains unaffected by the reversal of an applied in-plane external field (from +2 T to -2 T). Hence, from these measurements we cannot conclude that the signal has a pure magnetic component as it may be also influenced by other forces as the electrostatic ones. Low-temperature nano-MOKE measurements and Kelvin-probe microscopy on individual sheets will be performed in the future to get further information on this issue.</p><!><p>Surface engineering is a demanding challenge in 2D materials since it permits to tune the surface chemistry at will (modifying, for example, the hydrophobicity/hydrophilicity) and, subsequently, opens the door for the use of these systems in an industrial and technological relevant scale. However, many of the reported processes to modify the surface of 2D materials lead to defective materials that change their electronic or magnetic properties 33 because they generally involve post-synthetic methods. In order to circumvent these problems, we present a pre-synthetic strategy for modifying the surface properties of MUV-1-Cl with retention of the physical properties  The atoms at the surface can be changed by proper chemical design in a pre-synthetic method in order to obtain different surface behaviours (changing the water contact angle) without the modification of the magnetic properties.</p><p>The structural and magnetic properties of all these compounds have been characterized by Xray diffraction, SQUID measurements, Mössbauer and Raman spectroscopies, SEM and TEM (Supplementary info). Remarkably, the magnetic properties of the whole MUV-1-X family remains unaltered under the different surface modifications, as can be seen in Supp. Table 4 and Supp. Section 5.2. In addition, all the members from the MUV-1-X family have been exfoliated successfully by mechanically methods and characterized in detail as MUV-1-Cl (see Supplementary Section 6).</p><p>The ability to tailor the functionality of MUV-1-X, combined with its mechanical resonant readout makes it attractive for sensing applications, because the resonance frequency and Q-factor of these suspended membranes are expected to depend strongly on the mass and adhesion of captured molecules. To demonstrate this concept, we exfoliate and transfer thin flakes of MUV-1-Cl on top of a Si/SiO 2 substrate with prepatterned circular cavities. In Figure 5a  comparable to those reported in nanomechanical resonators made of graphene and other twodimensional materials. 34,35 The lower Q factors may be due to viscoelastic damping in this polymer material. Because of the large diameter-to-thickness ratio, the resonance frequencies of these nanodrums are mainly determined by the bending rigidity of the material. For the given resonance frequencies and geometry of the drums, we thus estimate the Young's modulus of MUV-1-Cl to be 3 -7 GPa, which is almost three orders of magnitude lower than that of graphene and similar to that previously reported for a layered coordination polymer measured by AFM nanoindentation. 36 The results show that these polymer resonators, despite their low Young's modulus and despite potential viscous damping mechanisms, have surprisingly high Q factors and resonance frequencies, which are comparable to graphene membranes. In combination with their low weight and tunable functionalization these materials can be used to create future platforms for resonance based sensing.</p><!><p>In this work, we have presented a general strategy for the synthesis of crystalline layered coordination polymers (LCP) exhibiting a magnetic order and providing an unconventional route to tune at will the surface chemistry of the individual layers. In particular, by the election of different ligands, we have obtained a new family of LCP denoted as MUV-1-X (where X = Cl, Br, H, CH 3 , or NH 2 ), with surface properties ranging from hydrophobic to hydrophilic, that retain their magnetic properties under the different surface modifications. Interestingly and in contrast with most of the reported MOFs, large crystals of these coordination polymers have been grown using a solvent-free synthesis. Such a possibility together with the layered nature of these materials and the very weak van der Waals forces between the layers have permitted isolation of micrometer size atomically-thin flakes down to the monolayer by mechanical and dry exfoliation methods. The high crystallinity, integrity and chemical composition of these mechanically exfoliated layers has been confirmed by TEM and Raman studies on LCP of different thicknesses. On the other hand, and in contrast to what happens in the few examples of reported 2D ferromagnets based on solid state chemistry, 13 which have shown to be highly unstable in open air, this coordination chemistry approach has afforded the isolation of robust 2D magnetic materials. Such a robustness has also enabled us to study the mechanical properties of these atomically-thin layers, a possibility that is unprecedented for coordination polymers. These results open the door to the integration and application of 2D coordination polymers in different technological areas like microfluidics, nanoelectronics, coatings or molecular sensing and, last but not least, it brings new candidates for the study of magnetism and phase transitions in the two-dimensional limit.</p><!><p>Synthesis of MUV-1-X. Ferrocene (30 mg, 0.16 mmol) and benzimidazole (or derivates) (0.34 mmol) were combined and sealed under vacuum in a layering tube (4 mm diameter). The mixture was heated at 250 °C for 3 days to obtain crystals suitable for X-ray single-crystal diffraction. The product was allowed to cool to room temperature, and the layering tube was then opened. The unreacted precursors were extracted with acetonitrile and benzene, and the product was isolated as colorless crystals (yield 80 %). Phase purity was established by X-ray powder diffraction. EDAX analysis of MUV-1-Cl from SEM and TEM show in both cases a 70:30 ratio for Cl:Fe.</p><!><p>Studies. X-ray data for compounds MUV-1-H and MUV-1-Cl were collected at a temperature of 100 K using a synchrotron radiation at single crystal X-ray diffraction beamline I19 in Diamond Light Source, equipped with a Pilatus 2M detector and an Oxford Cryosystems nitrogen flow gas system. Data was measured using GDA suite of programs. X-ray data for compounds MUV-1-Br, MUV-1-CH 3 and MUV-1-NH 2 were collected at a temperature of 100/150 K using a Rigaku FR-X rotating anode diffractometer, equipped with Hybrid Photon detector HyPix-6000HE and an Oxford Cryosystems nitrogen flow gas system.</p><p>A summary of the data collection and structure refinements is provided in section 2 of the supplementary information.</p><!><p>Polycrystalline samples of MUV-1-X were lightly ground in an agate mortar and introduced into 0.5 mm borosilicate capillaries that were mounted and aligned in a Empyrean PANalytical powder diffractometer, using Cu Kα radiation (λ = 1.54056 Å).</p><p>Two repeated measurements were collected at room temperature (2θ = 5 − 30 °) for each compound and merged in a single diffractogram.</p><p>AFM. Optical images were obtained with a NIKON Eclipse LV-100 Optical microscope and AFM images were performed with a Nanoscope IVa Multimode Scanning Probe Microscope (Bruker, Karlsruhe, Germany) in tapping mode.</p><p>Raman. Raman spectra were acquired with a micro-Raman (model XploRA ONE from Horiba, Kyoto, Japan) with a grating of 2400 gr/mm, slit of 50 µm, and hole of 500 µm. The employed wavelengths were 532 nm, 638 nm, and 785 nm. The power density of the laser used for spectra measured at 532 nm was 5.25 mW/µm 2 (bulk crystals) and 170 µW/µm 2 (thin-layers), for spectra measured at 638 nm it was 7.58 mW/µm 2 (bulk crystals) and 410 µW/µm 2 (thin-layers), and for those spectra measured at 785 nm it was 7.2 mW/µm 2 (bulk crystals).</p><p>TEM. Several mechanical exfoliated flakes were transferred onto a grid with a membrane of amorphous SiN (50 nm thick) using a dry and deterministic method (that involves the use of a micromanipulator and PDMS/PPC polymers, as reported in ref. 37 ). TEM images and diffraction patterns were acquired with a JEOL JEM-2100F with a field emission gun operating at 200 kV. The simulated SAED patterns were generated with SingleCrystal software.</p><p>Contact Angle (CA) Measurements. Static water contact angle measurements of the samples were performed in air using a Rame-hart 200 standard goniometer equipped with an automated dispensing system. The initial drop volume was 0.17 µL, increased by additions of 0.08 µL.</p><p>MFM. Magnetic images were recorded with a commercial LT-MFM (attoMFM I, Attocube Systems AG) with a commercial magnetic-coated cantilever (Nanosensors PPP-MFMR). The sample was cooled down with an applied external out-of-plane field of +1 T, to assure that the tip magnetization was out-of-plane. We performed a dual pass measurement configured with a phase locked loop (PLL) feedback on the cantilever oscillation. In this configuration, every line of the image is scanned twice:</p><p>in the first one, the topography is recorded in a tapping mode and, in the second one, the tip follows the first acquired profile but retracted a z lift distance. In the PLL mode, the magnetostatic tip-sample interaction is given by the frequency shift (Δf). Since the magnetic contrast is measured as the shift of the frequency f from a reference value, typically the free resonance frequency f0, we have defined in this work, for simplicity, the shift to be 0 in the silicon substrate. A negative frequency shift indicates an attractive tip-sample interaction (∆ f < 0, red contrast) while a positive frequency shift indicates a repulsive tip-sample interaction (∆ f > 0, blue contrast). Laser interferometry measurements. The motion of the MUV-1-X mechanical resonators was measured in vacuum, using a laser interferometry setup, similar to the one reported in ref. 34 . A modulated blue laser is used to sequentially heat up the membrane, bringing it into motion. A different, red laser is focused onto the suspended membrane and is partly reflected by the membrane and partly by the silicon chip underneath. As the membrane moves, the effective cavity depth is modulated. This modulates the intensity of the reflected laser beam by interference, which is captured by a photodiode. The measurements were done in a homodyne detection scheme, using a vector network analyser.</p><!><p>All data will be available from the corresponding authors upon request.</p><!><p>In this section, it is described the synthesis of the family of compounds [Fe(bimX) 2 ] (MUV-1-X), where X = Cl, H, CH 3 , Br and NH 2 . A general scheme of the synthetic procedure is presented in Figure S1.</p><!><p>Ferrocene (30 mg, 0.16 mmol) and 5-chlorobenzimidazole (49 mg, 0.34 mmol)</p><p>were combined and sealed under vacuum in a layering tube (4 mm diameter). The mixture was heated at 250 °C for 3 days to obtain colorless crystals suitable for X-ray singlecrystal diffraction. The product was cooled down naturally to room temperature, and the layering tube was then opened. The unreacted precursors were extracted with acetonitrile and benzene, and the main compound was isolated as colorless crystals (yield 80 %).</p><p>Phase purity was established by X-ray powder diffraction.</p><!><p>Ferrocene (30 mg, 0.16 mmol) and benzimidazole (40 mg, 0.34 mmol) were combined and sealed under vacuum in a layering tube (4 mm diameter). The mixture was heated at 250 °C for 3 days to obtain colorless crystals suitable for X-ray single-crystal diffraction. The product was cooled down to room temperature, and the layering tube was then opened. The unreacted precursors were extracted with acetonitrile and benzene, and</p><p>the main compound was isolated as colorless crystals (yield 80 %). Phase purity was established by X-ray powder diffraction.</p><!><p>Ferrocene (30 mg, 0.16 mmol) and 5-methylbenzimidazole (43 mg, 0.34 mmol)</p><p>were combined and sealed under vacuum in a layering tube (4 mm diameter). The mixture was heated at 250 °C for 3 days to obtain colorless crystals suitable for X-ray singlecrystal diffraction. The product was cooled down to room temperature, and the layering tube was then opened. The unreacted precursors were extracted with acetonitrile and benzene, and the main compound was isolated as colorless crystals (yield 80 %). Phase purity was established by X-ray powder diffraction.</p><!><p>Ferrocene (30 mg, 0.16 mmol) and 5-bromobenzimidazole (63 mg, 0.34 mmol)</p><p>were combined and sealed under vacuum in a layering tube (4 mm diameter). The mixture was heated at 250 °C for 3 days to obtain colorless crystals suitable for X-ray singlecrystal diffraction. The product was cooled down to room temperature, and the layering tube was then opened. The unreacted precursors were extracted with acetonitrile and benzene, and the main compound was isolated as colorless crystals (yield 80 %). Phase purity was established by X-ray powder diffraction.</p><!><p>Ferrocene (30 mg, 0.16 mmol) and 5-aminobenzimidazole (43 mg, 0.34 mmol)</p><p>were combined and sealed under vacuum in a layering tube (4 mm diameter). The mixture was heated at 250 °C for 3 days to obtain colorless crystals suitable for X-ray singlecrystal diffraction. The product was cooled down to room temperature, and the layering tube was then opened. The unreacted precursors were extracted with acetonitrile and benzene, and the main compound was isolated as colorless crystals (yield 40 %). Phase purity was established by X-ray powder diffraction. Cryosystems nitrogen flow gas system.</p><!><p>X-ray data was processed and reduced using CrysAlisPro suite of programmes.</p><p>Absorption correction was performed using empirical methods (SCALE3 ABSPACK) based upon symmetry-equivalent reflections combined with measurements at different azimuthal angles. [2][3][4] The crystal structures were solved and refined against all F 2 values using the SHELXL and Olex 2 suite of programmes. 5,6 Despite that the coordination polymers are intrinsically chiral, the centrosymmetric space group C2/c was found as result of the racemic distribution of the disordered layers of the MUV-1-H, MUV-1-Cl, MUV-1-CH 3 and MUV-1-NH 2 coordination polymers. As Figure S2 shows, the 2D coordination polymers present a modulated commensurate structure with a q vector of (½, ½, 0).</p><p>These results are consistent with the electron diffraction images displayed in Section 6.4. C-N, C-X (X = N, Cl and Br) and C-C distances were restrained using DFIX and SADI.</p><!><p>The atomic displacement parameters (adp) of the ligands have been restrained using RIGU and SIMU, and constrained using EADP commands.</p><p>CCDC 1582347-1582351 contains the supplementary crystallographic data for this paper with the exception of MUV-1-Br that was not included due to the poor R  with a grating of 2400 gr/mm, slit of 50 µm, and hole of 500 µm. The employed wavelengths were 532 nm, 638 nm, and 785 nm. The power density of the laser used for spectra measured at 532 nm was 5.25 mW/µm 2 (bulk crystals) and 170 µW/µm 2 (thinlayers), for spectra measured at 638 nm it was 7.58 mW/µm 2 (bulk crystals), and for those spectra measured at 785 nm it was 7.2 mW/µm 2 (bulk crystals). S15</p><!><p>In Figure S8 the experimental X-ray powder diffraction patterns (in color) are summarized and compared with the theoretical diffractogram (in black). It is worth to mention the high correlation between the experimental and the theoretical diffractograms.</p><!><p>In Figure S9, the infrared spectra of bulk crystals of the family MUV-1-X are presented.</p><!><p>The thermal stability of all the compounds was determined by thermal gravimetric analysis at a heating rate of 5 o C min -1 . The results are shown in Figure S10.</p><!><p>The layered structure of MUV-1-X was clearly identified by SEM images. As it can be seen in Figure S11, well-defined rectangular edges are the main characteristic of these compounds.</p><!><p>and MUV-1-NH 2 .</p><!><p>Mössbauer spectra were collected in transmission mode using a conventional constant-acceleration spectrometer and a 25 mCi 57 Co source in a Rh matrix. The velocity scale was calibrated using α-Fe foil. Isomer shifts, IS, are given relative to this standard at room temperature. The absorber was obtained by packing the powdered samples into a perspex holder. The spectra were fitted to Lorentzian lines using a non-linear leastsquares method. 7</p><!><p>All the spectra (Figure S12) show two quadrupole doublets with isomer shifts relative to metallic α Fe at 295 K, IS (Table S2) and quadrupole splittings, QS, typical of Fe 2+ in the S=2 spin state and in tetrahedral coordination. 8 In two of the samples, MUV-</p><!><p>Fe 3+ is also present (3% in MUV-1-H and 7% in MUV-1-NH 2 ).</p><p>The estimated relative areas for both Fe 2+ doublets are not very different. This suggests that there are two different sites for Fe 2+ with the same multiplicity.  were mounted in a cylindrical vanadium can of 6 mm diameter and inserted in a standard ILL-Orange cryostat. Diffraction patterns were collected below and above T N (1.8 K and 30 K) with acquisition times of, typically, 3 -4 h and normalized to the incident flux. The evolution of the magnetic ordering was also followed by a thermodiffractogram in the 2 K -30 K temperature range at a continuous heating rate of 0.1 K/ min and with acquisition times of 5 min. The programs of the Fullprof Suite were used for the data analysis 9 (the programs of the FullProf Suite can be obtained at http://www.ill.eu/sites/fullprof).</p><!><p>The powder neutron diffraction patterns of compounds MUV-1-H and MUV-1-Cl collected at 1.8 K and 30 K (below and above the ordering temperature) are presented in Figure S13a and S13b. The high temperature patterns fairly correspond with the structure determined by X-ray diffraction, although only the cell parameters were refined and no further structural refinement could be undertaken due to the insufficient Qresolution (since the neutron diffraction experiment was designed for magnetic structure determination). Notwithstanding the high overall background due to the elevated hydrogen content of the compounds, and despite the low magnetic density of the material, the onset of long-range magnetic order is evidenced by the appearance of new weak reflections at the lowest temperature. The observed magnetic contribution can be indexed S23 for both compounds with a magnetic propagation vector k = (0, 1, 0) relative to the C centered crystallographic unit cell. Representation analysis as described by Bertaut has been used to obtain possible configurations of the magnetic structure compatible with this magnetic propagation vector and the C2/c space group of the racemic crystal structures (in order to avoid overparametrization, this initial symmetry analysis has been done based on the C2/c racemic structures instead of the noncentrosymmetric C2; see below).</p><p>Table S3. Parameters of the magnetic structure of MUV-1-H and MUV-1-Cl obtained from the analysis of the neutron diffraction data. f and q are the angles of the magnetic moment in spherical coordinates.</p><!><p>Magnetic Moment (µ B ) 3.30 (7)  3.73( 6)</p><p>-52 (4) -47 ( 2)</p><p>237 ( 4 The consistency of this solution has been checked by a simulated annealing procedure without any symmetry restrictions, using the integrated intensities of the magnetic reflections. The refinement of this model, with the magnetic moments described in spherical coordinates (Table S3), gives magnetic moments of 3.30 (7) µ B and 3.73(6) µ B for MUV-1-H and MUV-1-Cl, respectively, nicely matching the expected for iron(II) in high-spin configuration. The orientation of the moments in the ab-plane is similar for both compounds (component f in Table S3 and Figures S13c,d). It should be reminded, however, that since the a and b axes have very similar values, we have low sensitivity to the orientation in the ab-plane. Concerning the out-of-plane component, it is more relevant for the MUV-1-H case (component q in Table S3 and Figures S13c,d), with a more marked tendency of the magnetic moments to be aligned with one of the Fe-N bonds. Nevertheless, although the spin arrangement model is quite consistent, the results concerning the actual orientation of the magnetic moments should be taken with care, due to the weak magnetic signal that affects the quality of the refinements, in particular for the MUV-1-H case.</p><!><p>The proposed magnetic structure model does not allow a ferromagnetic component, and therefore is, in principle, not compatible with the magnetometry results that show ferromagnetism. Although the combination of Γ 1 and Γ 4 allows a ferromagnetic component along the b-axis, the propagation vector k = (0, 1, 0) and the fact of having a single independent magnetic atom results in a total compensation of the magnetic moments. However, we can describe exactly the same magnetic structure based on the Cc space group of the noncentrosymmetric crystal structure (with axis a and b doubled, Figure S14). In this case, the magnetic propagation vector is k = (0, 0, 0) and there are 4 iron sites in the structure. The restriction that keeps the moments strictly compensated is therefore lifted and the same magnetic representation used before (combining Γ 1 and Γ 4 ) allows a ferromagnetic component along the b-axis. Independent orientations in the abplane of Fe1-Fe4 and Fe2-Fe3 magnetic moments are allowed. In order to avoid excessive number of free parameters, we keep Fe1-Fe4 in the same orientation as in the previous model, and we allow Fe2-Fe3 to rotate in the ab-plane, obtaining a refinement with very similar quality (Figure S14) but with a net magnetic moment along b. It should be reminded, however, that since the a and b axes have very similar values, we have very low sensitivity to the orientation in the ab-plane. This fact, together with the weak magnetic signal implies that the results concerning the actual in-plane orientation of the magnetic moments should be taken with care. Consequently, it seems more reasonable to keep the more symmetry-restricted description of the magnetic structure, keeping in mind that a net component of the magnetic moment in the b-axis is however allowed.</p><!><p>Variable-temperature (2−300 K) direct current (dc) magnetic susceptibility measurements were carried out in an applied field of 1.0 kOe and variable field magnetization measurements up to ±5 T at 2.0 K. The susceptibility data were corrected from the diamagnetic contributions as deduced by using Pascal's constant tables.</p><p>Variable-temperature (16−23 K) alternating current (ac) magnetic susceptibility measurements in a ±4.0 G oscillating field at frequencies in the range of 1−997 Hz were carried out in a zero dc field.</p><p>All the measurements were performed with a SQUID magnetometer (Quantum Design MPMS-XL-5 & MPMS-XL-7).</p><!><p>The temperature-dependent susceptibility of polycrystalline samples under an applied field of 1000 Oe was measured in the range of 2-300 K (DC measurements shown in Figure S15-S20). Upon cooling the sample, the susceptibility increases until it is reached a broad maximum (denoted as T M in Table S4) and, after that, the susceptibility faintly decreases. Then, while decreasing the temperature, it is observed a sharped and marked peak that indicates the existence of long range order, attributed to spin-canting, as supported by the small abruptness observed in the magnetization curves as a function of the external magnetic fields (Figure S19). At lower temperatures, it is observed a minimum of the susceptibility followed by a tenuous increase of the susceptibility due to the existence of some paramagnetic impurities. This increase is especially significant for the compound MUV1-Br and, for clarity, the paramagnetic contribution has been modelled following a Curie law and subtracted from the raw data (Figure S20). In the S29 product χ m (T)•T as a function of the temperature (Figure S17) it is observed a progressively decrease of the value of χ m (T)•T while decreasing the temperature until it reaches a sharp peak at T N and the value of χ m (T)•T decreases again.</p><p>The magnetic coupling constant, J, was determined by the formalism developed by de Jongh and Miedema (equations 1 and 2) 10 and by fitting the susceptibility versus temperature curves with the Lines expansion (equation 3; see Figure S15) 11 for a quadratic-layer antiferromagnet with S = 2 based on the following Hamiltonian:</p><p>.</p><p>where k is the Boltzmann constant, S is the spin (S = 2 in the present case), N is the Avogadro´s number, g is the Landé factor (we have employed g = 2 for determining J from equation 2), µ B is the Bohr magneton and C n are coefficients of the Lines expansion. 11 The obtained values for J and g are summarized in Table S4 and are in accordance between them. Interestingly, it can be noted that the different functionalization does not change significantly the magnetic properties of the compound.</p><p>Moreover, the data of the χ m (T) curves were fitted according to the conventional Heisenberg Hamiltonian in the molecular field approximation in order to obtain the Curie S30 and Weiss constants (C and Θ, respectively) as reported in Table S4 and Figure S16. In all cases, the values of Θ indicate a strong antiferromagnetic coupling.</p><p>The temperature dependences of the ac magnetic susceptibilities (in-phase, χ m ´ and out-of-phase, χ m ´´) at frequencies were measured from 16 to 23 K, as shown in Figure 18. It can be observed a sharp peak at T N , obtained at the value when χ m ´´ differs from zero (Table S4), that indicates the onset of a long-range ordered state.</p><!><p>Table S4 . Magnetic properties of the MUV-1-X family.</p><!><p>All the members of the family MUV-1-X were mechanically exfoliated by mechanical methods (as commonly known as the Scotch-tape method, developed for obtaining graphene from graphite) 12 , using an adhesive plastic film of 80 µm thick tape from Ultron Systems. The obtained flakes were deposited onto silicon substrates with 285 nm of thermally grown SiO 2 and inspected by different microscopies such us optical microscopy, atomic force microscopy, magnetic force microscopy and transmission electron microscopy.</p><!><p>In Figure S22, it is shown a prototypical exfoliation of MUV-1. It can be observed a plethora of different flakes with lateral sizes ranging from few microns to hundreds of microns and thicknesses from monolayer (ca., 1 nm) to 100-200 nanometers. Interestingly, as occurs in graphene and other 2D materials, the optical contrast (i.e., the color) of MUV-1 exhibits a thickness-dependence, where the yellowish flakes are the thickest ones while the purplish ones are the thinnest.</p><p>Optical images were obtained with a NIKON Eclipse LV-100 Optical microscope.</p><!><p>In this section, it is presented a more detail characterization of the monolayer shown in the main manuscript. In the top panel of Figure S23, it is observed optical images with a general and a reduced area view of the monolayer. Interestingly, it can be observed the colour dependence with the thickness, where yellow colours are the thickest flakes and purple colours denote the thinnest ones.</p><p>Moreover, in the bottom panel of Figure S23, it is shown the histogram profile that led to the number of layers determination in the atomic force microscopy image, taking into account that the thickness of one layer is ca. one nanometer.</p><p>After the mechanical exfoliation, the flakes were covered with some residues from the tape, as can be observed in Figure S24. In order to remove them and obtain a better surface quality, the samples were annealed at 110 ºC under vacuum (6•10 -6 mbar) during 12 hours. As a result, a less amount of surface contamination was presented over the flakes, as it can be compared in Figure S23 and Figure S24.</p><!><p>The chemical and structural nature of the atomically-thin layers of MUV-1-X, obtained by mechanical exfoliation, was inspected by transmission electron microscopy (TEM).</p><p>The mechanical exfoliated flakes on Si/285 nm SiO 2 were transferred to silicon nitride TEM membranes. The transferred was performed by dry methods and employing a polypropylcarbonate stamp, as reported in ref. 13 TEM images and diffraction patterns were acquired with a JEOL JEM-2100F with a field emission gun operating at 200 kV. The simulated SAED patterns were generated with SingleCrystal software.</p><p>Interestingly, the thickness of the measured areas by TEM was assessed by measuring the same flake with atomic force microscopy. To the best of our knowledge, this is the first time that this strategy is employed for a coordination polymer since, generally, atomically-thin layers of these materials are obtained by liquid exfoliation and drop-casted into TEM grids; therefore, making its analysis by AFM not suitable. Thanks to the robustness of MUV-1-X, these systems have been  We performed a dual pass measurement configured with a phase locked loop (PLL) feedback on the cantilever oscillation. In this configuration, every line of the image is scanned twice: in the first one, the topography is recorded in a tapping mode and, in the second one, the tip follows the first acquired profile but retracted a z lift distance. In the PLL mode, the magnetostatic tip-sample interaction is given by the frequency shift (Δf). Since the magnetic contrast is measured as the shift of the frequency f from a reference value, typically the free resonance frequency f 0 , we have defined in this work, for simplicity, the shift to be 0 in the silicon substrate. 14 A negative frequency shift indicates an attractive tip-sample interaction (∆ f < 0, red contrast) while a positive frequency shift indicates a repulsive tip-sample interaction (∆ f > 0, blue contrast).</p><p>We have focussed on a thin-flake of MUV-1-Cl with a thickness of 5.7 nm on a SiO 2 /Si substrate and lateral dimensions of, approximately, 4 µm x 2 µm, as can be seen Figure S35. At 4.2 K and a Zlift of 50 nm (Figure S36a), the flake gives a red contrast as a result of an attractive interaction between the tip and the flake. This red contrast persists in higher Zlift, as seen in Figure S36b with a Zlift of 100 nm. At temperatures above the order temperature, the magnetic signal vanishes (Figure S36c), although it is possible to distinguish the borders of the flake. Moreover, it was checked the influence of an in-plane external applied field. However, inside our experimental available range of ±2 T, it was not observed any significant change in the signal (Figure S37). The absence of modulation of the magnetic signal under the applied external magnetic fields could be understood due to a non-perfect alignment of the external field with the b crystallographic axis or to a compensation of the spin-canting in a racemic mixture (see Section 2).</p><p>We inspected as well thicker flakes on a graphene/SiO 2 /Si substrate at 4.2K, Zlift of 50 nm and under different applied external magnetic fields (Figure S38). At zero field, the flakes show a blue contrast that turns into a non-homogenous one when external magnetic fields are applied. These inhomogeneities are still under study and the results will be published elsewhere.</p><p>Although the material is ordered at 4.2 K, the absence of a clear modulation of the measured frequency shift by an applied external field does not let us conclude that the measured signal has a pure magnetic component and, then, it can be influenced by some other long-range forces, as the electrostatic one.</p>