Site-Dependent Substitutions in Mixed-Metal Metal–Organic

Article pubs.acs.org/cm

Site-Dependent Substitutions in Mixed-Metal Metal−Organic Frameworks: A Case Study and Guidelines for Analogous Systems Romain Sibille,*,† Thomas Mazet, Bernard Malaman, Qirong Wang, Emilie Didelot, and Michel François Institut Jean Lamour, CNRS and Université de Lorraine, BP 70239, 54506 Vandœuvre-lès-Nancy, France S Supporting Information *

ABSTRACT: Complex architectures are often found among metal−organic framework (MOF) compounds. The mixed-metal approach to this type of material offers an additional degree of structural complexity, and potential tunability of their properties, which remains largely unexplored. We present an in-depth investigation of the crystal chemistry of mixed-metal MOFs based on succinate linkers (C 4 H 4 O 4 ) and having the general formula (M′1−xM″x)5(OH)2(C4H4O4)4 with M′/M″ = Mn/Co, Fe/Co, and Mn/Fe. The distribution of the metallic elements over three crystallographic sites throughout the different substitutions is finely characterized by resonant contrast diffraction (RCD) experiments corroborated by neutron diffraction (ND) measurements. We observe a size-effect in the filling of the oxygen octahedra, leading to the existence of compounds in which a partial order of the cations over the different metallic sites exists for some compositions of the Co/ Mn solid solution. This points out the existence of complex structural phenomena potentially able to influence the physical behavior of such phases and that might, so far, have been overlooked in MOFs. In order to facilitate future studies on mixedmetal MOFs, we consider the possibility of using conventional single-crystal X-ray diffraction (SCXRD) to locate cations of close electronic densities in such cases. The comparison with the results from dedicated measurements based on synchrotron (RCD) and neutron (ND) radiations indicates guidelines for the use of laboratory SCXRD to address mixed-metal MOFs where metal distribution is fundamental to tuning physical properties.

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INTRODUCTION

Cationic substitutions have been extensively used in the chemistry of oxide and intermetallic compounds, allowing efficient tunings of their physical properties and emergence of unprecedented phenomena. This fruitful approach remains very little explored in the field of MOFs. This is probably related, to some extent, to the fact that metallic centers are generally less correlated in hybrid materials than in purely inorganic phases, which, as a consequence, limits the impact of substitutions on the overall physical behavior. Nevertheless, works dealing with cationic substitutions in MOFs exist. Depending on their crystal structure, i.e., on the number of available metallic sites and on the nature of the mixed cations, the substitution can either lead to heterometallic compounds3 in which metals are ordered on specific sites, or to the formation of solid solutions4 in which metals occupy, more or less statistically, one or several positions. The location of metallic elements in the most favorable cases has been determined by single crystal X-ray diffraction on heterometallic systems where the mixed metallic elements have very different crystallochemical properties.3b,d−h For solid solutions, the problem of metallic elements being distributed over several sites is much more complex and is often

Metal−organic frameworks (MOFs) are organic−inorganic hybrid solids in which a, usually low-dimensional, inorganic subnetwork is connected with appropriately functionalized organic molecules to form a 3D framework. The field of MOFs expands at a remarkable pace,1 in part because of the huge combinatorial possibilities offered by the amalgamation of organic and inorganic chemistry, but also because of their ability to produce interesting properties for both industrial applications and fundamental science. The exploration of their potential applicability has mostly focused on gas capture, luminescence, catalysis, sensing, cryogenic cooling, or drug delivery capabilities.1 One of their most striking structural features resides in the so-called reticular design approach,2a which, in short, consists in replacing a ligand while keeping intact the global architecture of the framework. Besides its beauty, this way of tuning framework materials is also capable of modifying their properties, e.g., their gas storage capabilities by varying the size of the pores. Another elegant and fascinating way of acting on MOFs’ structures and properties is the use of a guest, which can, for example, act on a flexible framework keeping its topological integrity upon deformation.2b−d Finally, the modification of the inorganic subnetwork of a MOF by using a different cation or by mixing cations is, of course, another way of influencing their structures and properties.3,4 © 2014 American Chemical Society

Received: September 28, 2014 Revised: November 22, 2014 Published: December 10, 2014 133

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Chemistry of Materials

CoCl2·6H2O (0.98 g, 4.10 mmol), MnCl2·4H2O (0.81 g, 4.10 mmol); 6, x = 0.66 (0.62), CoCl2·6H2O (0.66 g, 2.79 mmol), MnCl2·4H2O (1.10 g, 5.40 mmol); 7, x = 0.80 (0.74), CoCl2·6H2O (0.39 g, 1.64 mmol), MnCl2·4H2O (1.30 g, 6.56 mmol). SS-FexCo1−x. Compound 8, x = 0.20 (0.16), CoCl2·6H2O (1.56 g, 6.56 mmol), FeCl2·4H2O (0.33 g, 1.64 mmol); 9, x = 0.33 (0.42), CoCl2·6H2O (1.31 g, 5.5 mmol), FeCl2·4H2O (0.54 g, 2.70 mmol); 10, x = 0.5 (0.55), CoCl2·6H2O (0.97 g, 4.1 mmol), FeCl2·4H2O (0.81 g, 4.1 mmol); 11, x = 0.66 (0.70), CoCl2·6H2O (1.29 g, 5.4 mmol), FeCl2·4H2O (0.55 g, 2.79 mmol); 12, x = 0.80 (0.82), CoCl2·6H2O (0.39 g, 1.64 mmol), FeCl2·4H2O (1.30 g, 6.56 mmol). SS-MnxFe1−x. Compound 13, x = 0.20 (0.31), FeCl2·4H2O (1.30 g, 6.56 mmol), MnCl2·4H2O (0.32 g, 1.64 mmol); 14, x = 0.33 (0.48), FeCl2·4H2O (1.12 g, 5.65 mmol), MnCl2·4H2O (0.54 g, 2.71 mmol); 15, x = 0.66 (0.82), FeCl2·4H2O (0.55 g, 2.79 mmol), MnCl2·4H2O C4H6O4 (1.07 g, 5.41 mmol). Each compound was synthesized according to the following procedure: the succinic dicarboxylic acid (1.6 g, 13.5 mmol) was reacted with KOH (1.8 g, 3.2 mmol) in 10 mL of distillated water. A limpid solution was consequently obtained, and the pH reached a value of about 13. The metallic cations were then directly added to the solution under magnetic stirring. The pH of the resulting mixture was finely adjusted by KOH (1 M) droplets to 8.3. The magnetic stirring was maintained during 15 min, and the pH was maintained during this time. The mixture was finally transferred into a Teflon-walled stainless steel autoclave. Following this first step, the various compounds (1− 15) were obtained by a hydrothermal treatment at 150 °C. After 5 days, the autoclave was naturally cooled in air and opened under ambient atmosphere. The reaction product was collected by centrifugation, washed twice with a mixture of distilled water and ethanol (1/1), and then dried at room temperature. Resonant Contrast Diffraction (RCD). For a good introduction to the RCD method the reader can refer to ref 7. Powder X-ray diffraction experiments were carried out at Swiss Light Source using the MSX04SA beamline (powder station).8a Diffraction patterns of the samples were measured in glass capillaries (ϕ = 0.3 mm) with the Debye−Scherrer geometry and a multistrip detector.8b Three powder diffraction patterns were recorded for each mixed-metal sample (4− 15): two patterns using X-ray energies near the K edge of each metallic element (I, λMn = 1.8982; II, λFe = 1.7444; III, λCo = 1.6095 Å), i.e., at incident energies for which the scattering factors vary by resonant effects, and a third one at a higher energy (IV, λHighEn = 0.8268 Å), i.e., far from any absorption edge. Anomalous X-ray scattering factors as a function of energy in the vicinity of the absorption thresholds of Mn, Fe, and Co are shown on Figure S1, Supporting Information. The end members of the solid solutions (compounds 1−3) were recorded with λ = 0.8185 Å. The one-dimensional detector allows measuring entire diffraction patterns over 120° 2θ in a few seconds. For each sample, four diffraction patterns corresponding to four, slightly shifted, positions of the detector were measured in order to improve signalto-noise ratio. These diffraction patterns were recorded via the local software piloting the diffractometer and averaging the data acquired around 10 s per position. The energies were calibrated using a standard silicon sample from NIST. The atomic scattering factors used in the Fullprof software are reported in Table S1, Supporting Information. They were determined using the DISPANO V.2 program9 and the Brenann and Cowan data.10 The effects of anomalous X-ray scattering on the relative intensities of the diffracted beams is illustrated in Figure 1, for (Co0.5Mn0.5)5(OH)2(C4H4O4)4 5, with the patterns recorded at λMn and λCo. Le Bail Decomposition. The line profiles were modeled using the Thompson−Cox−Hastings function as implemented in the Fullprof_Suite software.11 The instrumental function was measured by using a small line-width standard sample (NAC; Na2Ca3Al2F4, NIST) as a reference. The line broadening was interpreted by microstructural effects (size and microstrain). For each pattern (I, II, III, and IV), according to the monoclinic P21/c symmetry, four lattice parameters and the zero position are refined. The number of parameters for crystallite size, strain, and background depends on the pattern (see

neglected.4d,e In most cases the mixed elements are close in the periodic table, and thus have too low X-ray diffusion contrast to be distinguished by standard X-ray diffraction. We have already considered this problem in the (Co1−xFex)2(OH)2(C8H4O4) solid solution where Co and Fe share two octahedral sites.4f,g We had successfully used the resonant contrast diffraction (RCD) method, also termed as multiwavelength anomalous dispersion (MAD), and neutron diffraction (ND) on powder samples. More recently, the former method was used on single crystals of Mn3[(Mn4Cl)3(BTT)8]2 (BTT = 1,3,5 benzentristetrazolate)5 to locate Fe, Cu, and Zn over two metallic sites. In the present work we employ the same approach as before,4f,g selecting the already well-known M5(OH)2(C4H4O4)4 system6,4e for several reasons: (i) the structure exists for M = Fe, Co, and Mn and allows the synthesis of bimetallic solid solutions;4e (ii) it contains three crystallographically independent sites for the metallic elements; and (iii) their hydrothermal synthesis provides crystallites suitable for single-crystal X-ray diffraction (SCXRD). (M′1−xM″x)5(OH)2(C4H4O4)4 samples with nominal compositions xnominal = 0.2, 0.33, 0.5, 0.66, and 0.8 have been synthesized for the three possible M′/M″ couples (Fe/Co, Co/ Mn, and Fe/Mn). The three series of samples have been thoroughly characterized by SCXRD, ND, and RCD using synchrotron powder diffraction at incident energies in the vicinity of the K absorption edges of the metals. This article first deals with the crystal chemistry of these mixed-metal MOFs. In particular, we show that size-effects dominate the filling of the three metallic sites in these bimetallic compounds, leading to strong differences in the ratios of the two metals among the crystallographic positions. Second, we address more general concerns about the appropriate way of dealing with the crystal chemistry of MOFs mixing metals of very close electronic densities. For this purpose we compare the results from conventional SCXRD with those from the large-scale facilities techniques, with the aim of providing a case study and general guidelines for the broad community of researchers dealing with MOFs.

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EXPERIMENTAL SECTION

Synthesis and Chemical Analysis. All chemicals were commercially available from Aldrich and used as received. The monometallic phases M5(OH)2(C4H4O4)4 with M = Mn, Fe, and Co and the bimetallic solid solutions (Co1−xMnx)5(OH)2(C4H4O4)4, SSCo 1‑x Mn x ; (Co 1−x Fe x ) 5 (OH) 2 (C 4 H 4 O 4 ) 4 , SS-Co 1‑x Fe x ; and (Fe1−xMnx)5(OH)2(C4H4O4)4, SS-Fe1‑xMnx, were synthesized from succinic dicarboxylic acid C4H6O4 (1.6 g, 13.5 mmol) and metallic salts, the quantities of which are given below. Powder samples 4−15 were observed by a JEOL J7600F field-effect SEM and the ratios M′/ M″ were determined by EDS analysis. EDS analyses were performed at 15 kV (1.2 nA) using an INCA system from Oxford Instrument (20 mm2 SDD detector). It is worth noting the existence of interference between the signals from Fe Kα (6.404 keV) and Mn Kβ (6.49 keV). However, Fe Kβ and Mn Kα lines being exempt of such interferences, it is possible to provide a satisfactorily deconvolution of the two signals by optimizing the line profiles on the end members of the series, which were used as standards. A similar approach was employed for the treatment of the signals from Fe Kβ (7.058 keV) and Co Kα (6.930 keV). In the following both xnominal and xEDS (in brackets) are indicated. Monometallic Phases. Mn5(OH)2(C4H4O4)4 1 (MnCl2·4H2O, 1.62 g, 8.2 mmol), Fe5(OH)2(C4H4O4)4 2 (FeCl2·4H2O, 1.63 g, 8.2 mmol), and Co5(OH)2(C4H4O4)4 3 (CoCl2·6H2O, 1.95 g, 8.2 mmol). SS-MnxCo1−x. Compound 4, x = 0.20 (0.19), CoCl2·6H2O (1.56 g, 1.64 mmol), MnCl2·4H2O (1.56 g, 6.56 mmol); 5, x = 0.50 (0.58), 134

DOI: 10.1021/cm503570f Chem. Mater. 2015, 27, 133−140

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Figure 2. Rietveld plot for (Mn0.5Co0.5)5(OH)2(C4H4O4)4 5 (λMn = 1.8982 Å, SLS data).

Figure 1. Effects of anomalous X-ray diffusion on the relative intensities of (Co0.5Mn0.5)5(OH)2(C4H4O4)4 5 powder diffraction pattern. Data recorded at λMn = 1.8982 Å (bottom) and λCo = 1.6095 Å (top).

were due to strong fluorescence effects affecting the quality of the diffraction patterns. Neutron Diffraction (ND). Powder neutron diffraction experiments were carried out at the Institut Laue Langevin (ILL) in Grenoble, France, using the D1B two-axis diffractometer (λ = 2.52 Å, steps of 0.1°). Diffraction patterns of the 12 bimetallic MOFs 4−15 were recorded at temperatures in the range 50−100 K using a standard “orange” helium cryostat. The analysis of the data was performed by Rietveld refinements using the Fullprof software.11 Crystallographic data and refinement parameters are reported in Tables S5−S7, Supporting Information. A typical neutron Rietveld diagram is presented in Figure 3. Taking advantage of the well contrasted

Tables S2−S4, Supporting Information). Rp and Rwp factors converge satisfactory to values lower than 4%. Three-Pattern Rietveld Refinement. Refinement parameters are reported in Tables S2−S4, Supporting Information, for the three series SS-MnxCo1‑x, FexSS-Co1‑x, and SS-MnxFe1‑x, respectively. The starting structural model was taken from our single crystal data (vide inf ra). Eighty-six to 106 parameters were refined, depending on the compound (4−15). For all samples, 18 intensity-dependent parameters were refined, including nine atomic coordinates, one isotropic thermal factor for the metals and the oxygen atoms of the hydroxide groups, six Euler angles for the two independent succinate molecules, and two occupancy factors for the metallic sites. Full occupancy of M1(2a), M2(4e), and M3(4e) was assumed during the M1 M2 M2 M3 M3 refinements, i.e., xM1 M′ + xM″ = 1, xM′ + xM″ = 1, and, xM′ + xM″ = 1, being the proportion of metal M on the site. Moreover, global xsite M compositions “x” were fixed to xEDS during the refinements by using M2 M3 EDS , which takes into the constraint (xM1 M /2 + xM + xM )/2.5 = x account the multiplicities of the metal positions. This led us to refine only two occupancy factors for each sample. To lower the number of refined parameters, thermal displacements of the two nonequivalent succinate molecules treated as rigid bodies were refined with the TLS subroutine for Rietveld refinements,12 instead of using one thermal factor for each atom of the molecules. In our case, only the translational part T of the TLS routine was used. Thus, four independent thermal parameters for each succinate molecule (total of eight parameters for the TLS procedure) were refined. Absorption effects due to the finite thickness of the samples had the same influence on refined scale and occupancy factors. To avoid systematic errors on the distribution of the elements, a sampledependent absorption correction coefficient was applied to correct the intensities. The values of μ × r used for the refinements consider that the powder density in the capillaries is one-third of the theoretical one. μ × r coefficients varied in the range [0.30−1.6] and, in most cases, were lower than unity (see Tables S2−S4, Supporting Information). A higher μ × r value was systematically obtained for the patterns recorded at the intermediate energy. A representative Rietveld plot is shown in Figure 2. The Rietveld refinements converged satisfactorily with RBragg values in the 5−12% range depending on the sample (1− 15) and on the pattern (I−IV). Higher RBragg values (Table S2, Supporting Information) for samples 5, 6, 7, and 12 recorded with λCo (pattern III), in SS-MnxCo1‑x and SS-FexCo1‑x series, respectively,

Figure 3. Rietveld plot of (Mn0.5Co0.5)5(OH)2(C4H4O4)4 5 from neutron data (λ = 2.52 Å, ILL data) given as an example. neutron scattering lengths of Fe, Co, and Mn (bFe = 0.96 × 10−12 cm, bCo = 0.25 × 10−12 cm, and bMn = −0.36 × 10−12 cm), the occupancy factors of the three metallic sites M1, M2, and M3 (namely, the ratio Co/Fe in 4−7, Co/Fe in 8−12, and Fe/Mn in 13−15) were refined. The global metallic composition x was constrained to the experimental xEDS value. For each composition, the structural model used was the one obtained from the RCD experiment. This was done in order to avoid 135

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Chemistry of Materials systematic errors when comparing the distribution of the metallic elements among the three crystallographic sites of the structure. The number of intensity-dependent parameters, which were refined for each pattern, was twelve: one scale factor, two occupancy factors, eight TLS parameters for the two succinate molecules, one isotropic thermal displacement factor for the three metallic sites, and the hydroxide groups. Using these neutron data the atomic coordinates of such large unit-cell structure could not be refined. Single Crystal X-ray Diffraction (SCXRD). A single-crystal suitable for X-ray diffraction was chosen for each sample 1−15 and mounted on a Kapton mount (MicroMounts, MiTeGen, New York, USA). Diffraction data were collected on a Bruker Apex II CCD diffractometer with MoKα radiation (λ = 0.7107 Å). Data reduction was performed by SAINT program.13 The structure was refined by fullmatrix least-squares on F2 (SHELXL97).14 Good resolution of the various collected data was assumed, ranging from 0.52 to 0.68 Å. Atomic positions were refined including anisotropic temperature factors for non-H atoms. H atoms were geometrically positioned using a riding model with the “HFIX” routine. For SS-MnxCo1‑x (samples 3−7), the occupancy factors of the metallic sites were refined, fixing x to xEDS value. For SS-FexCo1‑x (samples 8−12) and SS-FexMn1‑x (samples 13−15), occupancy factors of the metallic sites were fixed to the values given by the RCD experiments. Details of all single-crystal refinement parameters are reported in Table S8−S10, Supporting Information. Very good convergence was observed with R1 and wR2 values of 2−3% and 5−7%, respectively.

ligands around the metal positions M2 and M3 clearly form irregular octahedra: there are three relatively short distances (∼2 Å), two intermediate distances (∼2.15 Å), and a longer one (∼2.3 Å). Among these oxygen atoms, five belong to five distinct carboxylate molecules (Ocarb) and one to a hydroxide group (OH−). The octahedron formed by Ocarb and OH− groups around M1 site is more regular, with M1−O distances around 2.1 Å. The M−OH− distances are always shorter than the M−Ocarb distances. The hydroxide group OH− links the three metallic positions (μ3-OH−). Lattice Parameters Variation versus Composition. Variation of the lattice parameters using the very accurate synchrotron data recorded at high energy (IV, λ = 0.8268 Å) is shown, as a function of xEDS, in Figure 5. The relative values of

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RESULTS AND DISCUSSION Before going into detail of our results, lets first consider the crystal structure of M5(OH)2(C4H4O4)4 MOFs. It is made of a 2D inorganic subnetwork of edge-sharing M1−3(O6) octahedra (see Figure 4). The asymmetric unit is composed of three metallic atoms, one hydroxide group, and two succinate molecules. Among these two molecules, one pillars the 2D inorganic planes along [100] direction, while the other one links octahedra belonging to the same (b,c) plane. One of the metallic atoms sits on an inversion center, site M1(2a), the two others are on general positions, M2(4e) and M3(4e). The

Figure 5. Monoclinic lattice parameters a, b, c, β, and volume in solid solutions SS-MnxCo1‑x (squares), SS-FexCo1‑x (circles), and SSMnxFe1‑x (triangles), as a function of xEDS.

the lattice volumes for the three series are in agreement with the size of the divalent cation radii for octahedral Ocoordination, high spin state, and r(O2−) = 1.26 Å:15 r(Fe2+) = 0.78 Å, r(Co2+) = 0.745 Å, and r(Mn2+) = 0.83 Å. For SSFexCo1‑x and SS-MnxFe1‑x, the quasi linear variation of a, b, and c parameters and volume as a function of xEDS demonstrates a behavior following a Vegard’s law. For SS-MnxCo1‑x, the measured points deviate significantly from linearity. Site Occupancies from RCD and ND. Occupancy factors of sites M1, M2, and M3 in SS-MnxCo1‑x, SS-FexCo1‑x, and SSMnxFe1‑x are reported in Tables S11−S13, Supporting Information, respectively. Their evolution versus xEDS is graphically represented in Figure 6. Globally, the results obtained from the two powder methods are in good agreement (RCD, black squares; ND, red plain circles). The accuracy of the occupancy factors is generally better with RCD than with ND, as can be seen from standard deviations that are

Figure 4. Structure of M5(OH)2(C4H4O4)4 MOFs. (Top) General view of the inorganic planes pillared by succinate molecules. (Bottom) Details of the inorganic planes with the octahedral M1 (yellow), M2 (pink), and M3 (blue) sites. For clarity the intralayer succinate molecule is not represented. 136

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Figure 6. Occupancy factors of site M1, M2, and M3 of solid solutions SS-MnxCo1‑x, SS-FexCo1‑x, and SS-MnxFe1‑x versus xEDS, from various diffraction techniques (RCD, ND, and SCXRD).

deduced from the interatomic distances in the monometallic compounds. Thus, site occupancies can be globally explained by size effects. The biggest site, M2, incorporates the divalent cations Mn2+, Fe2+, and Co2+ independently from their radius. In other words, this site is filled proportionally to the concentrations of the elements. The filling of the two smaller sites, M1 and M3, is very different. M1, which is the smallest site, more easily incorporates the smallest cation. This is verified for the three investigated series: Co2+ in SS-MnxCo1‑x and Fe2+ in SS-FexCo1‑x and SS-MnxFe1‑x. The filling of M3, which is of intermediate size, shows the inverse behavior of M1, i.e., M3 preferably incorporates the biggest cation. As a consequence of these size effects, we remark a large affinity of Co and Mn for M1 and M3, respectively, meaning that the affinity of Fe for the various sites depends on the second metal to which it is associated in the solid solutions. If associated with Co in SS-FexCo1‑x, Fe preferentially occupies M3 rather than M1, while, if associated with Mn in SS-MnxFe1‑x, Fe preferentially fills M1 site. Effects of crystal-field stabilization (CFS) probably play a role in the distribution of Mn2+, Fe2+, and Co2+ over the sites M1−3, but our results clearly indicate dominant size effects. Finally, it is worth noticing that the striking changes of slope observed in the filling of the different metallic sites as a function of composition cannot be explained by the uncertainties on the abscissa of the data points (xEDS). Structural Formulation and Partially Ordered Compounds. For the three series of solid solutions (compounds 4−15), the structural formula obtained from the RCD experiments can be written as listed in Table 1. We cannot evidence any fully ordered compound in the compositions investigated in this work. However, compounds 4−6 (SSMnxCo1‑x) can be considered as partially ordered as M1 site is almost fully occupied by Co atoms. Thus, for compositions xMn = 0−0.5 in SS-MnxCo1‑x, the structural formula (Co)M1(Co/ Mn)2M2(Co/Mn)2M3(OH)2(C4H4O4)4 can be written, with

systematically lower by a factor of 2 to 4 for the former method (see Tables S11−S13, Supporting Information). The best agreement between these two techniques is obtained for the SS-MnxFe1‑x series, for which neutron scattering contrast between the two metallic elements, Δnucl = bM′ − bM″, is the largest: Δnucl (Fe/Mn) = 1.32 barns, compared to Δnucl (Co/ Mn) = 0.61 and Δnucl (Co/Fe) = 0.71 barns. RCD results, being more accurate for the location of metals in these mixedmetal MOFs, will be considered in the following. SS-MnxCo1−x (Table S11). The occupancy factor of M2 site by Mn atoms increases proportionally with composition xMn (Figure 6b). In contrast, those of sites M1 and M3 increase in a drastically less regular way: (i) the filling of M1 by Mn atoms only starts to significantly increase for Mn contents larger than xMn ≈ 0.6 (Figure 6a), and (ii) the occupancy factor of M3 by Mn atoms increases with composition for xMn increasing from 0 up to 0.6, the value at which M3 is almost fully occupied by Mn (Figure 6c). We can deduce from these results that cobalt atoms have a large affinity for M1, whereas Mn obviously prefers M3. Dashed lines in Figure 6 emphasize the abrupt changes observed in the site-fillings versus x. SS-FexCo1−x (Table S12). Fe atoms significantly substitute Co on M1 for Fe contents larger than xFe ≈ 0.4 (Figure 6d). The Fe filling of M2 increases monotonously with xFe over the entire composition range (Figure 6e). The filling of M3 site as a function of Fe composition shows a change of slope at xFe ≈ 0.4 (Figure 6f), corresponding to what is observed for M1 site. SS-MnxFe1−x (Table S13). Filling M3 by Mn atoms (Figure 6i) is easier than filling M1 (Figure 6g). It is in agreement with the large affinity of Mn for M3 observed in SS-MnxCo1‑x. The filling of M1 and M3 by Mn presents a discontinuity at xMn ≈ 0.7, while the filling of M2 site, as for the other two series, is proportional to the composition. All sites remain substantially disordered for the intermediate compositions. Comments. Independently on the considered 3d transition metal (Co, Fe, or Mn), the size of the three octahedral sites in M5(OH)2(C4H4O4)4 increases in the order M1 > M3 > M2, as 137

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herometallic systems). We notice that the successful location of Co and Mn atoms in SS-MnxCo1‑x compounds was realized using data collection resolution lower than 0.7 Å and X-ray absorption parameter μ × r lower than unity. Analysis of Interatomic M−O Distances from SCXRD. Although the site occupancies cannot be reliably determined in the case of a one-electron difference, the analysis of the M−O distances as a function of composition gives information on the site fillings. Indeed, as the M−O distances are related to the ionic radii of the divalent cations occupying a given site, they also give indications on the occupancy factors. The interatomic distances around M1−3, for the three series, are reported in Tables S14−S16, Supporting Information, and their average values are graphically represented as a function of compositions in Figure 7. Comparison of Figures 6 and 7 shows that the

Table 1. Structural Formula of the Metal Cations in MixedMetal MOF Solid Solutions of General Formula (M′1−xM″x)5(OH)2(C4H4O4)4 as Obtained from the RCD Experiments sample

structural formula

4 5 6 7 8 9 10 11 12 13 14 15

(Co0.96Mn0.04)M1(Co1.68Mn0.32)M2(Co1.40Mn0.60)M3 (Co0.91Mn0.09)M1(Co0.80Mn1.20)M2(Co0.40Mn1.60)M3 (Co0.96Mn0.04)M1(Co0.74Mn1.26)M2(Co0.14Mn1.86)M3 (Co0.79Mn0.21)M1(Co0.36Mn1.64)M2(Co0.14Mn1.86)M3 (Co0.89Fe0.11)M1(Co1.72Fe0.28)M2(Co1.59Fe0.41)M3 (Co0.88Fe0.12)M1(Co1.12Fe0.88)M2(Co0.90Fe1.10)M3 (Co0.74Fe0.26)M1(Co0.85Fe1.15)M2(Co0.70Fe1.30)M3 (Co0.53Fe0.47)M1(Co0.50Fe1.50)M2(Co0.50Fe1.50)M3 (Co0.35Fe0.65)M1(Co0.33Fe1.67)M2(Co0.27Fe1.73)M3 (Fe0.87Mn0.13)M1(Fe1.42Mn0.58)M2(Fe1.21Mn0.79)M3 (Fe0.74Mn0.26)M1(Fe1.09Mn0.91)M2(Fe0.77Mn1.23)M3 (Fe0.42Mn0.58)M1(Fe0.31Mn1.69)M2(Fe0.17Mn1.83)M3

Co/Mn ratio on M2 and M3 sites depending on the value of xMn. SCXRD and the Location of Cations of Close Electronic Densities. The location of cations in mixedmetal MOF solid solutions will presumably play an important role in the future of hybrid materials. Therefore, using more accessible techniques to determine their occupancy factors is of interest. As already pointed out in the introduction, solid solutions very often mix cations having very close electronic densities because many MOF architectures are, for a given ligand or set of ligands, shared by cations having similar steric, electronic, and/or chemical properties, i.e., cations being close neighbors in the same row of the periodic table. This nearly equal electronic density hinders their location using conventional X-ray diffraction techniques. However, several factors speak in favor of using SCXRD for the considered problem: (i) if available, the single-crystal samples of many MOFs are very often of high quality, (ii) MOF materials are most of the time ideal in terms of X-ray absorption corrections, and (iii) for 3d transition metal cations the absence of very large Z elements produces a relatively homogeneous scattering density in the reciprocal space, favoring the determination of reliable structure factors. These advantages, which favor the acquisition of high quality SCXRD data for MOFs based on 3d transition metals, together with the excellent performances of modern singlecrystal X-ray diffractometers, lead us to consider this obvious technique. SS-Co1−xMnx by SCXRD. As can be seen from Figure 6a−c, the location of Mn and Co by single crystal X-ray diffraction in SS-MnxCo1‑x is in fair agreement with what we obtained from ND and RCD (within the accuracy of the measurements, see Table S11, Supporting Information). These results show that it is possible to locate, thanks to the above-mentioned conditions, elements having a difference as small as two electrons for MOFs of the 3d transition metal series. For SS-FexCo1‑x and SS-MnxFe1‑x (one-electron difference), the location of the metallic elements is not directly accessible by refining the occupancy factors. In these cases the X-ray diffusion contrast is too low. Consequently, a two-electron difference can be considered as a lower limit for the (direct) location of 3d transition metal cations mixed on the same crystallographic site of a MOF for which good quality single-crystals are available. We can extend this statement to MOFs in which metals do not share crystallographic positions as it is an easier case (ordered

Figure 7. Average interatomic distances around M1−M3 sites versus composition in (a) SS-MnxCo1‑x, (b) SS-FexCo1‑x, and (c) SSMnxFe1‑x from single crystal data.

evolution of the interatomic distances as a function of composition is clearly similar to that of occupancy factors. For the three series and throughout the entire composition ranges, M2−O distances increase linearly with xMn in SSMnxCo1‑x and SS-MnxFe1‑x, or with xFe in SS-FexCo1‑x. M1−O and M3−O distances present discontinuities at the same x values deduced from the occupancy factors (RCD results): at xMn ≈ 0.6 for SS-MnxCo1‑x, at xMn ≈ 0.7 for SS-MnxFe1‑x, and at x ≈ 0.4 in SS-FexCo1‑x. Overall, the plots of Figure 7 agree with site-size-dependent substitutions in (M′ 1−x M″ x )(OH)2(C4H4O4)4 solid solutions.

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CONCLUSIONS In this work we have thoroughly examined the crystal chemistry of the (M′1−xM″x)5(OH)2(C4H4O4)4 series (M′, M″ = Mn, Fe, Co) of mixed-metal MOFs by SCXRD, ND, and RCD. The study about the distribution of the metallic elements over their three crystallographic sites has unambiguously shown a particular behavior throughout the substitutions. In short, the analysis of the data reveals that Co and Mn have pronounced affinities for M1 and M3, respectively, while Fe adapts its occupancy to the second metal. Moreover, we could evidence the existence of compositions for which Co and Mn are partially ordered, a novel observation for solid solutions of MOFs. These experimental facts can be explained by sizeeffects of the metallic sites relative to the cation radii. To the 138

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(SCMEM, Université de Lorraine) for her careful help concerning EDS analysis.

best of our knowledge, such an in-depth investigation of the crystal chemistry of MOF solid solutions is unique. The phenomena revealed by our detailed survey give an excellent illustration of the, largely unexplored, potential of the mixed-metal approach to hybrid frameworks. The plethora of MOFs that has been discovered over the last two decades constitutes a huge reservoir of phases whose properties have, for a number of them, the capability to be finely tuned, or sometimes drastically modified, by mixing metals in solid solutions. A typical example is the magnetic behaviors that could arise from the crystal chemistry described in this work. Indeed, assuming the monometallic compounds enter different types of magnetically ordered phases, novel magnetic orders could emerge from the solid solutions. Other concrete perspectives for mixed-metal MOF solid solutions include the consequences, on the dielectric properties, of the distribution of different cations in the solid. It is worth recalling the tremendous contribution of solid solutions in purely inorganic materials, for both insulating and metallic phases, suggesting a bright future for the similar approach to MOFs. Besides revealing this site-size dependency of the substitutions in the considered series of materials, we also tackle the more general question of suitable experimental methods for fine structural characterizations of mixed-metal MOF solid solutions. First, we reaffirm that the RCD method gives highly reliable results. Second, we point out the excellent capabilities of SCXRD using a modern laboratory machine for dealing with site occupancies in these compounds: a two-electron difference appears sufficient to obtain a good, direct estimate of the site occupancies. Although techniques based on synchrotron, or neutron, radiation will remain necessary for some delicate cases, we believe that the indications provided by this challenging case-study (three sites with rather similar environments) give indications for future works, especially using SCXRD, on mixed-metal MOF solid solutions and other heterometallic MOFs.

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ABBREVIATIONS MOFs, metal−organic frameworks; SS, solid solution; RCD, resonant contrast diffraction; ND, neutron diffraction; SCXRD, single-crystal X-ray diffraction

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ASSOCIATED CONTENT

S Supporting Information *

Crystallographic Information Files (CIF) of compounds 1−15. This material is available free of charge via the Internet at http://pubs.acs.org.

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REFERENCES

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Present Address †

Laboratory for Developments and Methods, Paul Scherrer Institut, 5232 Villigen PSI, Switzerland. Author Contributions

The manuscript was written through contributions of all authors. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors are grateful to Nicola Casati and Antonio Cervellino from Swiss Light Source for their help in the diffraction experiments with the synchrotron radiation. We are indebted to the Institut Laue Langevin (Grenoble, France) for the provision of research facilities. Our local contact (Silvia Capelli) is warmly acknowledged. We thank Sandrine Mathieu 139

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