Designing Multifunctional 5-Cyanoisophthalate-Based Coordination

Article pubs.acs.org/IC

Designing Multifunctional 5‑Cyanoisophthalate-Based Coordination Polymers as Single-Molecule Magnets, Adsorbents, and Luminescent Materials Jose M. Seco,† Itziar Oyarzabal,† Sonia Pérez-Yáñez,‡ Javier Cepeda,*,†,§ and Antonio Rodríguez-Diéguez*,∥ †

Departamento de Química Aplicada, Facultad de Química and §Departamento de Ciencia y Tecnología de Polímeros, Facultad de Química, Universidad del País Vasco/Euskal Herriko Unibertsitatea, UPV/EHU, 20018 San Sebastián, Spain ‡ Departamento de Química Inorgánica, Facultad de Ciencia y Tecnología, Universidad del País Vasco/Euskal Herriko Unibertsitatea, UPV/EHU, Apartado 644, E−48080 Bilbao, Spain ∥ Departamento de Química Inorgánica, Universidad de Granada, 18071 Granada, Spain S Supporting Information *

ABSTRACT: Detailed structural, magnetic, and photoluminescence characterization of a family of new compounds based on 5-cyanoisophthalate (CNip) ligand and several transition metal or lanthanide ions, namely, [Cu 3 (μ3-CNip) 2 (μH2O)2(μ3-OH)2]n (1), {[Co3(μ4-CNip)3(DMF)4]·∼2DMF}n (2), [Cd(μ4 -CNip) (DMF)]n (3), {[Ln2 (μ4-CNip)(μ3CNip)2(DMF)4]·∼DMF·H2O}n (4-Ln) (with LnIII = Tb, Dy, and Er), {[Gd6(μ3-CNip)5(μ4-CNip)3(μ-form)2(H2O) (DMF) 10 ]·∼3DMF·3H 2 O} n (5), {[Zn 32 (μ 4 -CNip) 12 (μCNip)12(μ4-O)8(H2O)24]·∼12DMF}n (6) (where DMF = dimethylformamide, form = formate), is reported. The large structural diversity found in the system may be explained mainly in terms of the coordination characteristics that are inherent to the employed metal ions, the coordination versatility of the dicarboxylic ligand and the synthetic conditions. Interestingly, some crystal structures (three-dimensional (3D) frameworks of 4-Ln and 5 and 3D network of 6) exhibit open architectures containing large solvent-occupied void systems, among which 5 reveals permanent porosity as confirmed by N2 adsorption measurements at 77 K. Magnetic direct current (dc) susceptibility data on compounds 1, 2, and 5 were measured. Moreover, compounds 2, 4-Dy, 4-Er, and 5 show slow magnetic relaxation, from which it is worth highlighting the effective energy barrier of 44 K at zero dc field for the dysprosium counterpart. Compound 5 also deserves to be mentioned given the few 3D Gd-organic frameworks reported examples. Photophysical properties were also accomplished at different temperatures, confirming both the fluorescent emission of 5-cyanoisophthalate ligands when coordinated to cadmium ions in 3 and their capacity to sensitize the long-lived fluorescence of the selected lanthanide ions in 4-Ln. Broken symmetry and time-dependent density functional theory computational calculations support the experimental luminescence and magnetic properties.

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INTRODUCTION Over the past two decades, there has been a substantial research effort on the construction of metal−organic frameworks (MOFs) not only for the diverse and fascinating structures and topologies they render but also for their interesting properties relevant to applications such as catalysis, gas adsorption, luminescence, and magnetism, among others.1 Although some indications for obtaining MOFs with particular topologies and/or properties have been elucidated by deconstructing their crystal structures into nodes and spacers through the classical concepts of reticular chemistry, a rational synthetic strategy to control the resulting structures and properties is still challenging.2 This is because there are different factors governing the self-assembling process of MOFs in addition to the organic ligand and metal salt themselves, such © XXXX American Chemical Society

as solvents, metal-to-ligand ratio, temperature, synthetic procedure, and so on.3 In fact, a remarkable number of previous works focusing on these aspects have already confirmed the importance of an exhaustive synthetic study on a given system to unravel the complete structural diversity.4 Nonetheless, it should not be ignored that the selection of metal ions and organic ligands yet plays an important role in tuning the structures and functionalities. In this line, ligands with carboxylate groups are widely employed for two main reasons: (i) their negative charge affords them with the capacity to bind strongly to metal centers while (ii) they introduce certain coordination flexibility that permits them adopting Received: July 29, 2016

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DOI: 10.1021/acs.inorgchem.6b01845 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry different coordination modes that fit into most of metal environments and crystal packing requirements.5 The chemical nature of the spacer is not a less important concern, since, for example, aromatic spacers afford greater rigidity than aliphatic ones owing to their limited geometric characteristics that may restrict the structural variability, a fact of major importance when only one ligand is employed. Furthermore, these ligands give the system the opportunity to finely tune the topology as well as the functionality of the framework (and consequently its properties) by simply introducing different substituents in the aromatic rings. Particularly for isophthalate (1,3-benzenedicarboxylate) ligand, which is a widely explored linker, it has been proved that changing the substituent at the aromatic 5-position is a successful tactic for enhancing the topological diversity.6 For instance, Zhou et al. demonstrated that increasing the steric hindrance of the substituent in a zinc isophthalate series leads to a progressive decrease of the dimensionality.7 Taking these considerations into account, we chose 5cyanoisopthtalate (CNip) as linker, for which an MOF (PCN236) has been recently reported,8 with the aim of studying the effect of the cyanide function on both the capacity to build open frameworks and to modulate their properties. In fact, this ligand had been previously employed to give rise to 5-(1Htetrazol-5-yl)isophthalate following the in situ Demko− Sharpless [2 + 3] cycloaddition reaction between organonitriles and sodium azide, in such a way that the resulting MOFs exhibited interesting photoluminescent and magnetic properties.9 Accordingly, transition metals and lanthanide ions were employed to exploit their different coordination characteristics as well as the properties derived from their diverse architectures. The former are acknowledged for their welldefined coordination geometries and strong coordination capacity, all which is usually translated into high structural stability, whereas the latter are known to yield more flexible and larger coordination geometries owing to their shielded 4f electrons that are hardly affected by crystal field. In any case, the presence of these paramagnetic centers may afford interesting magnetic properties to the resulting compounds that can be correlated with the crystal structure.10 In this sense, there has been a recent increase of publications reporting the use of some lanthanide ions to give single-molecule magnets (SMMs) due to their large magnetic moment and significant single-ion anisotropy derived from spin−orbit coupling.11 SMMs are discrete magnetic molecules composed of monoor polynuclear entities that exhibit a remarkable number of phenomena, such as low-temperature hysteresis, quantum tunneling, and so on.12 That is why the first examples correspond mainly to dysprosium-based isolated molecules, although this field is rapidly expanding to higher dimensional frameworks and other lanthanides such as gadolinium, terbium, or erbium, since the characterization of MOFs exhibiting slow relaxation of magnetization ensures obtaining new types of molecular magnetic materials.13 Nonetheless, the number of these kind of MOFs is still very scarce. Regarding the optical properties, the coordination of lanthanides to extended aromatic systems also allows these acting as sensitizers, such that the resulting MOFs may present enhanced luminescence.14 It should not be neglected that equally fascinating photoluminescent properties can be achieved by frameworks based on d10 metal centers.15 In addition, the coupling of accessible pores, in which different gas molecules can be adsorbed, with the magnetism provided by SMMs and/or with photoluminescent emissions, results in a dynamic magnetic/

luminescent coordination network that could bring in further functionalities to the hybrid material. All in all, herein we report the preparation and structural characterization of eight new coordination polymers consisting of the CNip ligand and transition/lanthanide metal ions, together with the adsorption, magnetic, and photoluminescent properties. In particular, alternating-current (ac) magnetic susceptibility measurements reveal the occurrence of slow relaxation of the magnetization in some 3d- or 4f-based compounds, which in turn also show intense and long-lived emissions. In addition to an exhaustive experimental analysis of these properties, the results were well-correlated with computational calculations that have helped in elucidating the mechanisms by which the overall magnetic and luminescent behaviors take place. Structural diversity was achieved in the system for the compounds that exhibit fascinating architectures of variable dimensionality, some of which contain solventoccupied accessible pores. A considerable effort was devoted to rationalize the structures with the synthetic conditions and the metal ions, which facilitates the success in upcoming works. Finally, and most importantly, some of these compounds present more than one property, a fact that opens a way for these multifunctional materials toward still undeveloped applications.

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EXPERIMENTAL PROCEDURES

Chemicals. All the chemicals were of reagent grade and were used as commercially obtained. Synthesis of [Cu3(μ3-CNip)2(μ-H2O)2(μ3-OH)2]n (1). 0.1 mmol (24.9 mg) of Cu(NO3)2·3H2O and 0.1 mmol of H2CNip (19.1 mg) were dissolved in 10 mL of distilled water. The resulting solution was placed in a 25 mL Teflon-lined stainless steel autoclave under autogenous pressure at 150 °C for 24 h and then slowly cooled to room temperature. Blue well-defined single crystals were collected at open atmosphere and washed with water and methanol. Yield: 45% based on metal. Anal. Calcd for C18H12Cu3N2O12 (%): C, 33.84; H, 1.89; Cu, 29.84; N, 4.38. Found: C, 33.56; H, 1.95; Cu, 29.48; N, 4.13. Synthesis of {[Co3(μ4-CNip)3(DMF)4]·∼2DMF}n (2). Two milliliters of a dimethylformamide (DMF) solution containing 0.1 mmol of H2CNip (19.1 mg) was added over another solution of Co(NO3)2· 6H2O (2 mL, 29.1 mg). The resulting solution was placed in closed vial (8 mL of capacity), which was sonicated to help in the dissolution of the reagents, and heated to 95 °C for 24 h. Well-shaped purple single crystals of 2 were obtained. Yield: 50% based on metal. Anal. Calcd for C45H51Co3N9O18 (%): C, 45.70; H, 4.35; Co, 14.95; N, 10.66. Found: C, 45.92; H, 4.65; Co, 14.68; N, 10.23. Synthesis of [Cd(μ4-CNip)(DMF)]n (3). Two milliliters of a DMF solution of CdCl2 (0.1 mmol, 18.2 mg) were mixed with a solution containing 0.1 mmol of H2CNip (2 mL, 19.1 mg) in a vial. Then, 500 μL of a concentrated NH3 solution (%30 w/w) was added over the previous mixture such that the pH of the solution rose to 5.0, and a white precipitate was formed. Block-shaped colorless single crystals of 3 were obtained after heating the final suspension within the closed vial at 95 °C for 48 h. Yield: 65% based on metal. Anal. Calcd for C12H10CdN2O5 (%): C, 38.47; H, 2.69; Cd, 30.00; N, 7.48. Found: C, 38.80; H, 2.55; Cd, 29.78; N, 7.52. Synthesis of {[Ln2(μ4-CNip)(μ3-CNip)2(DMF)4]·∼DMF·H2O}n (4-Ln, LnIII = Tb, Dy, and Er). Single crystals of 4-Dy and polycrystalline samples of 4-Tb and 4-Er were obtained by mixing a DMF solution of the corresponding Ln(NO3)3·xH2O (0.1 mmol) and a DMF solution containing 0.1 mmol of H2CNip (19.1 mg). The resulting mixtures were placed in closed vial (8 mL capacity), sonicated and heated to 95 °C for 24 h. Yield: 75−85% based on metal. Anal. Calcd for C42H46N8O18Tb2 (%): C, 39.76; H, 3.65; Tb, 25.05; N, 8.83. Found: C, 40.09; H, 3.32; Tb, 24.83; N, 8.68. Anal. Calcd for C42H46Dy2N8O18 (%): C, 39.54; H, 3.63; Dy, 25.47; N, 8.78. Found: C, 40.05; H, 3.61; B


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Inorganic Chemistry Table 1. Crystallographic Data and Structure Refinement Details of All Compounds compound chem formula formula weight cryst system space group a (Å) b (Å) c (Å) α (deg) β (deg) γ (deg) V (Å3) Z GOFa Rint R1b/wR2c [I > 2σ(I)] R1b/wR2c (all data)

1 C18H12Cu3N2O12 638.92 triclinic P1̅ 5.7384(5) 9.5708(9) 10.0605(9) 65.924(3) 81.885(3) 86.470(3) 499.41(8) 1 1.052 0.0545 0.0373/0.0754 0.0585/0.0819

2 C45H51Co3N9O18 1182.75 monoclinic C2/c 9.466(7) 24.358(18) 22.393(16) 90 91.740(8) 90 5161(7) 4 0.939 0.0724 0.0543/0.1195 0.0885/0.1319

3 C12H10CdN2O5 374.62 orthorhombic Pna21 7.1291(2) 16.0748(6) 11.0375(4) 90 90 90 1264.88(7) 4 1.080 0.0318 0.0194/0.0422 0.0217/0.0428

4-Dy C42H46Dy2N8O18 1275.86 tetragonal I42̅ d 12.8070(8) 12.8070(8) 59.467(6) 90 90 90 9754(2) 8 1.053 0.0493 0.0418/0.0896 0.0478/0.0919

5 C113H125Gd6N21O53 3568.81 orthorhombic Pbca 34.967(2) 21.6125(12) 41.296(2) 90 90 90 31208(3) 8 1.019 0.1807 0.0973/0.2034 0.1952/0.2389

6 C252H204N36O140Zn32 8068.95 cubic Fm3̅ 32.767(2) 32.767(2) 32.767(2) 90 90 90 35181(7) 4 0.947 0.1947 0.0617/0.1418 0.0999/0.1528

S = [∑w(F02 − Fc2)2/(Nobs − Nparam)]1/2. bR1 = ∑∥F0| − |Fc∥/∑|F0|. cwR2 = [∑w(F02 − Fc2)2/∑wF02]1/2; w = 1/[σ2(F02) + (aP)2 + bP], where P = (max(F02,0) + 2Fc2)/3 with a = 0.0334 (1), 0.0635 (2), 0.0161 (3), 0.0366 (4-Dy), 0.1111 (5), 0.0811 (6), and b = 0.8077 (1), 1.1959 (3), 78.5214 (4-Dy), 207.3101 (5). a

Dy, 25.28; N, 8.49. Anal. Calcd for C42H46Er2N8O18 (%): C, 39.25; H, 3.61; Er, 26.03; N, 8.72. Found: C, 39.36; H, 3.54; Er, 25.85; N, 8.44. Synthesis of {[Gd 6 (μ 3 -CNip) 5 (μ 4 -CNip) 3 (μ-formate) 2 (H 2 O) (DMF)10]·∼3DMF·3H2O}n (5). Applying an identical solvothermal procedure detailed in the synthesis of 4-Ln but using gadolinium(III) nitrate hexahydrate as metal source led to growth of single crystals of 5. Yield: 75% based on metal. Anal. Calcd for C110H116Gd6N20O51 (%): C, 37.99; H, 3.36; Gd, 28.65; N, 8.06. Found: C, 38.08; H, 3.16; Gd, 28.58; N, 8.38. Synthesis of {[Zn32(μ4-CNip)12(μ-CNip)12(μ4O)8(H2O)24]·∼12DMF}n (6). Single crystals of 6 were obtained following the same synthetic procedure of 3 but replacing CdCl2 by Zn(NO3)2·4H2O. Yield: 75−85% based on metal. A rapid loss of crystallinity was observed for these single crystals after removal from their mother liquors, which rendered an amorphous product associated with the structural collapse as inferred from the powder X-ray diffraction (PXRD) analysis. Physical Measurements. Elemental analyses (C, H, N) were performed on an Euro EA Elemental Analyzer, whereas the metal content, determined by inductively coupled plasma (ICP-AES), was performed on a Horiba Yobin Yvon Activa spectrometer. IR spectra (KBr pellets) were recorded on a ThermoNicolet IR 200 spectrometer in the 4000−400 cm−1 spectral region. Magnetic susceptibility measurements were performed on polycrystalline samples of the complexes with a Quantum Design SQUID MPMS-7T or MPMS-XL5 (only for compound 2) susceptometers at an applied magnetic field of 1000 G. The susceptibility data were corrected for the diamagnetism estimated from Pascal’s Tables,16 the temperature-independent paramagnetism, and the magnetization of the sample holder. SMMs measurements were performed on a Physical Property Measurement System-Quantum Design model 6000 magnetometer under a 3.5 G ac field and frequencies ranging from 60 to 10 000 Hz. Thermal analyses (thermogravimetric (TG)/differential thermal analysis (DTA)) were performed on a TA Instruments SDT 2960 thermal analyzer in a synthetic air atmosphere (79% N2/21% O2) with a heating rate of 5 °C·min−1. Nitrogen physisorption data for 5 were recorded with an Autosorb iQ Quantachrome Instruments analyzer at 77 K, after activating the samples in vacuo for 6 h at 150 °C. The specific surface area was calculated from the adsorption branch in the relative pressure interval using the Brunauer−Emmett−Teller (BET) method17 and the consistency criteria proposed by Walton and Snurr that is commonly applied for MOFs,18 while the micropore volume was estimated by fitting the measured N2 isotherms with the t-plot method.19 A Varian Cary-Eclipse Fluorescence Spectrofluorimeter equipped with a xenon discharge lamp (peak power equivalent to 75 kW), Czerny−Turner

monochromators, and R-928 photomultiplier tube were used to obtain the fluorescence spectra at room temperature. The photomultiplier detector voltage was 700 V, and the instrument excitation and emission slits were opened 5 nm. Additionally, a closed cycle helium cryostat enclosed in an Edinburgh Instruments FLS920 spectrometer was employed for steady-state photoluminescence (PL) measurements in the 10−250 K range. For steady-state measurements an IK3552R-G HeCd continuous laser (325 nm) and a Müller-Elektronik-Optik SVX1450 Xe lamp were used as excitation source. Photographs of irradiated single-crystal and polycrystalline samples were taken at room temperature in a micro-PL system included in an Olympus optical microscope illuminated with a HeCd laser or a Hg lamp. Single-Crystal X-ray Diffraction. X-ray data collection of suitable single crystals was done at 100(2) K on a Bruker VENTURE area detector equipped with graphite monochromated Mo Kα radiation (λ = 0.71073 Å) by applying the ω-scan method. The data reduction was performed with the APEX220 software and corrected for absorption using SADABS.21 Crystal structures were solved by direct methods using the SIR97 program22 and refined by full-matrix least-squares on F2 including all reflections using anisotropic displacement parameters by means of the WINGX crystallographic package.23 All hydrogen atoms were located in difference Fourier maps and included as fixed contributions riding on attached atoms with isotropic thermal displacement parameters 1.2 times or 1.5 times those of their parent atoms for the organic ligands and the water molecules, respectively. During the refinement of compounds 4-Dy, 5, and 6 the electron density at the voids was subtracted from the reflection data by the SQUEEZE procedure as implemented in PLATON program24 due to the presence of disordered solvent molecules. Details of the structure determination and refinement of compounds 1−6 are summarized in Table 1. Additional crystallographic information is available following the Supporting Information. The PXRD patterns were collected on a Phillips X’PERT powder diffractometer with Cu Kα radiation (λ = 1.5418 Å) over the range 5 < 2θ < 50° with a step size of 0.026° and an acquisition time of 2.5 s per step at 25 °C. Indexation of the diffraction profiles was made by means of the FULLPROF program (pattern-matching analysis)25 on the basis of the space group and the cell parameters found for single-crystal X-ray diffraction. Computational Details. Time-dependent density functional theory (TD-DFT) theoretical calculations were performed at 10 K using the Gaussian 09 package,26 using the Becke three-parameter hybrid functional with the nonlocal correlation functional of Lee− Yang−Parr (B3LYP)27 for all atoms but for the central cadmium cation, where the LANL2DZ28 basis set along with the corresponding effective core potential (ECP) was used, whereas 6-311G(d)29 was C


Article

Inorganic Chemistry Table 2. Selected Bond Lengths for Compound 1a

adopted for the remaining atoms. The latter has been proven to be an adequate method to describe luminescence of cadmium-based coordination compounds.30 Further description of a suitable model can be found in the Supporting Information. The 40 lowest excitation and emission energies were calculated on model 3 by the TD-DFT method. Gaussian results were analyzed using the GaussSum program package,31 and molecular orbitals were plotted using GaussView 5.32 A detailed description of the computational strategy adopted in this work to compute the magnetic coupling constant (Jcalc) values has been described elsewhere.33 One calculation was performed to determine the high-spin state and another to determine the low-spin broken symmetry state. The correctness of the latter state was ensured by means of its spin density distribution. Density functional theory was used to perform two separate calculations to evaluate the coupling constant of each compound, employing the aforementioned hybrid B3LYP functional and Gaussian-implemented 6-311G(d) basis set for all nonmetallic atoms and the corresponding LANL2DZ pseudopotentials for the metal atoms.

Cu1−O1(i) Cu1−O1(ii) Cu1−O1w Cu1−O11A Cu1−O31A a

1.998(2) 1.978(2) 2.302(3) 1.939(2) 1.918(2)

Cu2−O1 Cu2−O1(i) Cu2−O12A Cu2−O12A(i) Cu2−O1w Cu2−O1w(i)

1.962(2) 1.962(2) 1.958(2) 1.958(2) 2.503(3) 2.503(3)

Symmetries: (i) −x + 2, −y + 2, −z; (ii) x − 1, y, z; (iii) x + 1, y, z.

Å], all of which gives rise to chains running along the a axis. These chains are connected to each other through the dicarboxylic ligands (coordination mode a in Scheme 1), which establish a third bridge (of the Cu1−O−C−O−Cu2 type) within the chain in addition to the coordination of O31A atom to a Cu1 atom belonging to an adjacent layer. The noncoordinated carboxylate oxygen atom establishes an intramolecular hydrogen bonding interaction with the water and hydroxide ligands, thus forcing CNip ligands to remain essentially planar while they arrange establishing an angle of 37.7° with the mean plane of the layers (Figure 2). It is worthy to note that two symmetry-related CNip ligands are sequentially placed with a parallel face-to-face disposition along the chains, rendering a sort of laddered structure. The occurrence of several nodes (copper and tridentated ligands) prevents an easy topological description of layers of 1. In fact, an exhaustive analysis with the TOPOS program35 reveals a rodlike topological four-nodal network that can be described with the (3·42)2(3·43·52·63·7)2(32·42·52·62·72·82·93)(42·6)2 point symbol. Structural Description of {[Co3(μ4CNip)3(DMF)4]·∼2DMF}n (2). Compound 2 is also built by the pilling of 2D layers that exhibit a completely different connectivity compared to those of 1 owing to the coordination of DMF solvent molecules and the absence of water and hydroxide anions. In particular, layers of 2 grow from the junction of centrosymmetric trinuclear entities, which are considered as secondary building units (SBUs), by means of CNip ligands (Figure 3). Two Co2 atoms occupy the edges of the SBU and render a severely distorted octahedron (SOC = 2.57) formed by four carboxylate oxygen atoms of four CNip linkers and two terminal DMF molecules that prevent the SBU from further polymerization (Table 3). Co1 atom lies on the center of the trimer and exhibits an almost ideal octahedral environment (SOC = 0.32) established by an O6 donor set from six carboxylate oxygen atoms. Hence, the SBU is formed by six carboxylate bridges of six CNip ligands, two of which are crystallographically independent (showing coordination modes b and c in Scheme 1); that is, three CNip ligands bridge the metal centers with a Co1···Co2 distance of 3.51 Å. While two of these moieties describe simple Co−O−C−O−Co pathways, the third one establishes an additional chelating ring with the external Co2 atom, which explains the longer Co2−O12A bond. As a consequence, six dicarboxylic linkers arise from the SBU to join it with six adjacent units giving rise to 2D layers that spread along the crystallographic ac plane. In this sense, it is worth noting the twist of the carboxylate groups with respect to the rings of the CNip ligands (18.9 and 21.5°) to optimize π−π stacking interactions (Table S2). The topological analysis reveals that the network belongs to the Shubnikov sql type, which is also described by the (44·62) point symbol (Figure 4). On the one hand, within the sheets, vectors containing the

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RESULTS AND DISCUSSION Structural Description of [Cu3(μ3-CNip)2(μ-H2O)2(μ3OH)2]n (1). Crystal structure of compound 1 consists of a compact framework built from two-dimensional (2D) sheets that are held together by means of hydrogen-bonding interactions. The asymmetric unit contains two unequivalent copper(II) atoms, a bridging water molecule, and a μ3hydroxide anion in addition to a μ3-CNip ligand (Figure 1).

Figure 1. Excerpt of compound 1 showing the coordination environment of crystallographically independent copper(II) atoms. Color coding: C = gray, H = white, N = blue, M = green. Red, green, and blue lines stand for (I), (II), and (III) Cu···Cu pathways.

Cu1 displays a square pyramid (SPY) environment established by two carboxylate oxygen atoms and two hydroxide anions occupying the basal plane, whereas an apically elongated water molecule completes the slightly distorted polyhedron (SSPY = 1.36).34 Cu2 lies on the inversion center and describes a distorted octahedron (SOC = 2.55) formed by symmetrically related carboxylate oxygen, coordination water, and hydroxide ligands (Table 2). As a consequence, copper atoms are sequentially joined by three different types of pathways: (I) a triple one consisting of two μ-oxo (involving a water and a hydroxide anion) and syn− syn carboxylate bridges [Cu1···Cu2 of ca. 3.08 Å], (II) a double one formed by two μ-OH bridges [Cu1···Cu1(iii) of ca. 3.06 Å], and (III) a single μ-OH bridge [Cu2···Cu1(iii) of ca. 3.42 D


Article

Inorganic Chemistry Scheme 1. Coordination Modes of the CNip Ligand in Compounds 1−6a

a (a) μ3-CNip-κO:κO′:κO″, (b) μ4-CNip-κO:κO′:κO″:κO‴, (c) μ4-CNip-κ2O,O′:κO′:κO″:κO‴, (d) μ3-CNip-κ2O,O′:κO″:κO‴, (e) μ-CNipκO:κO″. Bar chart indicates the percentage of hits showing the coordination mode for 5-cyanoisophthalate ligand.

Figure 2. View of the packing of compound 1 showing the hydrogen-bonding interactions (dashed plum lines).

dimensional channels that stand for the 22% of the total volume of the cell (Figure S6). Structural Description of [Cd(μ4-CNip)(DMF)]n (3). Compound 3 presents a 3D structure in which the crystallographically independent Cd1 atom is coordinated to a DMF molecule in addition to CNip ligands. The metal atom is placed in the center of a severely distorted trigonal prism (STPR = 6.64) established by five carboxylate oxygen atoms and an oxygen

metal atoms in the SBUs are oriented with an angle of 45° with regard to the mean plane. On the other hand, DMF molecules get exposed toward the inner space of the rings established in the network. The layers are then piled on one another in a regular AB fashion, which gives rise to an overall threedimensional (3D) packing that encloses discrete voids filled with dimethylformamide molecules. A detailed analysis with PLATON software estimates that these voids group in oneE


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Inorganic Chemistry

Given the large hapticity, CNip ligands are no longer planar, because one of its carboxylate groups shows a noticeable twist of 13.8°, whereas the other group remains almost coplanar owing to the occurrence of the μ-O bridge. The junction of the cadmium atoms by means of the carboxylate moieties leads to rodlike chains running along the crystallographic a axis, from which emerge two coordinated DMF molecules. Then, the chains are linked to one another by the ligands in such a way that the resulting compact 3D backbone that contains no solvent-accessible voids displays a square grid in the bc plane. A deeper simplification of the crystal structure reveals a two-nodal (metal and ligand) network with the pts topology and the (32· 62·72)(34·42·64·75) point symbol. Structural Description of {[Ln 2 (μ 4 -CNip)(μ 3 CNip)2(DMF)4]·∼DMF·H2O}n (4-Ln, LnIII = Dy, Tb, and Er). The three title compounds are isostructural and crystallize in the tetragonal I4̅2d space group, and only crystal structure of 4-Dy will be described in detail. 4-Dy consists of an open 3D framework built from [Dy2(CNip)4(DMF)4] dinuclear building units. Within the entities, two symmetrically equivalent dysprosium atoms are quadruply bridged by carboxylate groups of CNip ligands that place them at a Dy···Dy distance of 4.43 Å in the paddle-wheel-like entity (Figure 6). Dy1 atom exhibits an O8 donor set that resembles an almost ideal square antiprism (SSAPR = 1.57), which is established by the oxygen atoms of chelating carboxylate groups and two DMF oxygen atoms in addition to the previously mentioned intra-dinuclear bridging atoms (Table 5). The dinuclear units are joined to each other by two sorts of CNip linkers that display bis-bridging and bridging−chelating coordination modes (b and d, respectively, see Scheme 1). On the one hand, two adjacent and slightly disordered bis-bridging ligands emerge almost perpendicular from the building unit to link to two surrounding units. The remaining two bridging ligands on the opposite side of dimer establish a chelating ring with Dy atoms belonging to different units. On the other hand, two additional units are in turn joined to the reference by means of equivalent chelating rings. As a result, each unit is connected to six neighboring ones leading to a 3D network that

Figure 3. Trinuclear SBU of compound 2 showing the coordination environments of the two crystallographically independent Co atoms (hydrogen atoms were omitted for clarity). Double dashed lines stand for π−π interactions.

Table 3. Selected Bond Lengths for Compound 2a Co1−O11A Co1−O11A(i) Co1−O12B Co1−O12B(i) Co1−O32A(ii) Co1−O32A(iii)

2.144(4) 2.144(4) 2.089(4) 2.089(4) 2.012(4) 2.012(4)

Co2−O11A Co2−O12A Co2−O11B Co2−O31A(iii) Co2−O1D Co2−O1E

2.102(3) 2.353(4) 2.011(4) 2.081(4) 2.080(4) 2.054(5)

a Symmetries: (i) −x, −y, −z + 1; (ii) −x − 1, −y, −z + 1; (iii) x + 1, y, z.

atom belonging to the solvent molecule (Figure 5 and Table 4). Thus, the tetradentate CNip ligand exhibits the μ 4 κ2O,O′:κO′:κO″:κO‴ coordination mode by which it bridges two cadmium centers through each carboxylate moiety (showing bridging or chelating-bridging local modes) imposing Cd···Cd distances of 3.66 Å.

Figure 4. View of the packing of layers in 2 along b axis and a caption of tetragonal plane network. F


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Inorganic Chemistry

Figure 5. View of the packing of 3 along a axis showing the Cd-carboxylate rodlike chain and an excerpt of the coordination environment.

Table 4. Selected Bond Lengths for Compound 3a Cd1−O11A Cd1−O12A Cd1−O12A(i)

2.379(3) 2.517(3) 2.262(4)

Cd1−O31A(ii) Cd1−O32A(iii) Cd1−O1B

can be described with the (49·66) point symbol and the snw topology, which is a very less common topology for which only few records have been reported36 (see Figure S7). The overall architecture contains segregated voids occupied by DMF molecules that account for the 13.3% of the unit cell volume as estimated by PLATON.24 Despite the relatively small solvent-available volume entrapped in the framework, the cyanide substituents of CNip ligands are still somewhat free to swing with regard to the strongly anchored bis-bridging carboxylate moieties. Interestingly, this framework has a strong resemblance to a previously published Dy-based MOF37 that contains a larger void volume (24.7%). Both frameworks share the topological class given the identical connectivity imposed by CNip ligands. Therefore, the smaller porosity observed for 4 is attributed to the slight geometrical differences present among them, particularly for the arrangement of the coordinated DMF molecules. Structural Description of {[Gd6(μ3-CNip)6(μ4-CNip)2(μformate)2(H2O) (DMF)10]·∼3DMF·3H2O}n (5). Crystal architecture of compound 5 may be considered as a structural derivative of 4, since its 3D framework consists not only of equivalent [Gd2(CNip)4(DMF)4]2− dinuclear units but also of tetranuclear [Gd4(CNip)8(form)2(DMF)4(H2O)]6− (form = formate) units that, in turn, can be viewed as the result from the fusion of two dinuclear units. In contrast to the building unit of 4, the dinuclear unit lacks an inversion center and renders different GdO8 coordination polyhedra for Gd1 and Gd2 atoms (SSAPR = 1.64 and STDD = 1.35 for Gd1 and Gd2, respectively) in spite of their indistinguishable environment from a chemical point of view (Figure 7 and Table 6). The tetranuclear unit is much more irregular than the latter given the large dissimilarity between the two Gd2 subunits that form it. In particular, the first subunit (subunit-1) keeps the paddle-wheel shape due to the four carboxylate moieties bridging the metal centers (Gd3··· Gd6 of 4.36 Å), whereas the one established between Gd4 and Gd5 atoms (subunit-2 in advance) only contains three

2.228(2) 2.251(2) 2.289(3)

a Symmetries: (i) −x, −y, −z + 1; (ii) −x − 1, −y, −z + 1; (iii) x + 1, y, z.

Figure 6. Fragment of framework of 4-Dy showing the dinuclear entity established by four bridging ligands. Disordered parts of some of the CNip molecules are represented by transparent bonds.

Table 5. Selected Bond Lengths for Compound 4 Dy1−O11A Dy1−O12A Dy1−O11B Dy1−O12B

2.382(5) 2.202(5) 2.220(5) 2.374(5)

Dy1−O31A Dy1−O32A Dy1−O1D Dy1−O1E

2.526(5) 2.391(6) 2.406(5) 2.368(6) G


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Inorganic Chemistry

Figure 7. (a) Dinuclear and (b) tetranuclear building units of 5. (c) View of the packing along b axis showing the 2D void system.

Table 6. Selected Bond Lengths for Compound 5a Gd1−O32B Gd1−O12C Gd1−O12E Gd1−O11F Gd1−O12F Gd1−O12H Gd1−O1M Gd1−O1P Gd4−O31D Gd4−O31E(iii) Gd4−O32E(iii) Gd4−O32F(ii) Gd4−O32G(iv) Gd4−O31H(v) Gd4−O32H(v) Gd4−O1R a

2.409(11) 2.298(14) 2.433(12) 2.462(12) 2.487(12) 2.294(10) 2.414(16) 2.369(16) 2.341(12) 2.407(10) 2.474(12) 2.296(12) 2.281(14) 2.398(11) 2.486(12) 2.368(15)

Gd2−O31B Gd2−O11C Gd2−O11D Gd2−O12D Gd2−O11E Gd2−O11H Gd2−O1L Gd2−O1U Gd5−O32D(ii) Gd5−O31F Gd5−O31G(i) Gd5−O1I Gd5−O1S Gd5−O1T Gd5−O3T Gd5−O1 V

2.253(13) 2.392(12) 2.500(11) 2.417(12) 2.269(11) 2.352(12) 2.357(14) 2.453(17) 2.315(15) 2.379(13) 2.346(12) 2.360(16) 2.406(14) 2.467(12) 2.577(12) 2.320(16)

Gd3−O11A Gd3−O12A Gd3−O31A(i) Gd3−O11B Gd3−O32C(ii) Gd3−O11G Gd3−O1K Gd3−O1w Gd6−O32A(i) Gd6−O12B Gd6−O31C(ii) Gd6−O12G Gd6−O1J Gd6−O1S Gd6−O3S Gd6−O1T

2.467(11) 2.457(13) 2.281(12) 2.292(13) 2.338(16) 2.379(12) 2.375(14) 2.423(13) 2.407(12) 2.321(13) 2.315(11) 2.255(13) 2.299(17) 2.448(13) 2.668(17) 2.432(12)

Symmetries: (i) −x + 1/2, y − 1/2, z; (ii) −x + 1, −y, −z + 2; (iii) x, −y + 1/2, z + 1/2; (iv) x + 1/2, −y + 1/2, −z + 2; (v) −x + 1, −y + 1, −z + 2.

= 2.58, SBTPR = 2.36 for Gd3, Gd4, Gd5, Gd6, respectively), among which it is worth mentioning the largely distorted inner Gd5 due to the higher diversity of coordinated ligands (see Supporting Information). Regarding the CNip ligands, seven linkers arise from the central core of the tetramer (three bisbridging and one bridging−chelating linkers from subunit-1, two bis-bridging and one bridging−chelating linkers from subunit-2) to join neighboring units. Moreover, the Gd4 unit serves as an anchorage point for three additional bridging− chelating linkers.

carboxylate bridges due to the presence of two DMF molecules coordinated to the inner Gd5. This fact seems to be responsible for the slightly larger Gd5···Gd4 distance of 4.65 Å. Both subunits are mutually linked by means of two formate anions acting as μ-κ2O,O′:κO′ bridging ligands between the central metal atoms, which impose a remarkably shorter Gd5···Gd6 of 4.11 Å. Gadolinium atoms occupying the edges of the Gd4 unit, that is, Gd3 and Gd4, complete their coordination shells by water, DMF, and chelating carboxylate groups of CNip ligands. As a consequence, all GdIII centers show different environments resembling different polyhedra (SSAPR = 0.95, STDD = 2.18, SSAPR H


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Inorganic Chemistry

ideal tetrahedron (ST = 0.12) surrounding Zn2 is established by three carboxylate oxygen atoms in addition to the oxide anion (Figure 8). The edges of the cage are shaped by 24 dicarboxylate ligands that are arranged in pairs owing to remarkable π−π interactions between their aromatic rings (Table S3). These edge-sharing linkers show two different coordination patterns (b and e modes in Scheme 1), among which the tetradentate molecule is somewhat shrunk to adapt to the arrangement of the metal centers by means of the bisbridging mode, which in turn imposes a shorter Zn···Zn distance within a cluster (3.09 vs 3.28 Å for Zn1···Zn2 and Zn1···Zn1, respectively). Moreover, it is worth mentioning that all CNip ligands leave their cyanide groups exposed outward the cage, in such a way that they could serve as promising interacting points with polar or polarizable small gas molecules.1h The cages are interconnected edge-to-edge via Ow−Hw··· Ocarb hydrogen-bonding interactions as well as π−π stacking interactions among antiparallel ligands belonging to neighboring cages (Figure 9). As a result, each cage is linked by 12 surrounding ones leading to an overall crystal building possessing the previously reported jcr3 topology found for the 3D packing of isolated [In6(μ-pzdc)12]6− entity (pzdc = 2,5pyrazinedicarboxylate),38 which features the (321·433·512) point symbol. The supramolecular assembling of the cubic cages of 6 leads to a large free volume (37.5% per unit cell) that is occupied by dimethylformamide molecules. However, a careful analysis of the void system of this compound reveals an apparent lack of connectivity between the big voids sited within

Hence, taking into account that the dimeric entity follows the same connectivity shown in compound 4, Gd2 and Gd4 building units join each other giving rise to a binodal network with 4,6T54 topology and the (45·6)(47·68) point symbol. The resulting 3D framework contains channels running along the crystallographic a and b axes that are intercrossed leading to a 2D void system, which contains the solvent molecules and accounts for 29.3% of the unit cell. Structural Description of {[Zn 32 (μ 4 -CNip) 12 (μCNip)12(μ4-O)8(H2O)24]·∼12DMF}n (6). Compound 6 consists of open discrete cubic-shaped metal−organic cages that are connected by means of hydrogen-bonding interactions. The centrosymmetric cages are built from the linkage of eight Zn4O clusters, sited at the vertices of the cube, through bridging CNip ligands. Zn4O clusters display the habitual tetrahedron and contain two crystallographically independent metal atoms (Table 7). Three equivalent Zn1 atoms occupy the external Table 7. Selected Bond Lengths for Compound 6a Zn1−O1 Zn1−O12A Zn1−O11B Zn1−O1w a

1.959(3) 2.021(6) 1.952(7) 2.008(8)

Zn2−O1 Zn2−O11A Zn2−O11A(i) Zn2−O11A(ii)

1.942(11) 1.971(7) 1.971(7) 1.971(7)

Symmetries: (i) −y + 1/2, z, −x + 1/2; (ii) −z + 1/2, −x + 1/2, y.

positions of the cluster and are tetrahedrally coordinated (ST = 0.53) to two carboxylate oxygen atoms, the central oxide anion, and a water molecule pointing outward the cage. The almost

Figure 8. Metal−organic cage of compound 6 showing the Zn4O cluster occupying the edges and coordination environments of the metal atoms (dashed red lines stand for π−π interactions). I


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Inorganic Chemistry

coordination mode c, which is a minor mode for the related and widely explored isophthalate ligands.39 On another level, isostructural 4-Dy, 4-Tb, 4-Er, and 5 compounds are characterized by 3D frameworks with a higher connectivity compared to the preceding structures derived from CNip ligands (showing modes b and d) in coordinating to lanthanide metals. On the one hand, structure of 4 features paddle-wheel-like [Ln2(CO2)4] cores that are joined not only through the linkers emerging from the SBU but also from additional linkers chelating both metal atoms. On the other hand, the larger and more spherical size of gadolinium(III) enables the fusion of the dinuclear units by means of formate (form) ions in compound 5, which are a well-known byproduct of the DMF hydrolysis,40 leading to tetranuclear units. The mutual interconnection of both SBUs in the open framework of 5 encloses an available void volume that almost doubles that of 4. Finally, applying longer reaction times under mild-medium solvothermal conditions of the solution containing zinc(II) nitrate and the dicarboxylic ligand allows the occurrence of oxide anions,41 which are rapidly assembled into Zn4O clusters. Hence, the key factor for obtaining the MOC-based structure of compound 6 instead of a pcu topological 3D framework, which is the most habitual net for Zn4O clusters,42 seems to be originated at the angular shape of the CNip linker in contrast to linear linkers such as terephtalate and its derivatives when following b coordination mode. In fact, the strategy for approaching MOCs with the analogous isophthalate ligand has already been reported.43 Hence, the polymerization is interrupted, and the resulting open cubic cage is packed by means of hydrogen-bonding interactions established by water molecules coordinated to the tetrahedral cluster. Accordingly, some of the linkers are forced to adopt the coordination mode e so that they can also act as hydrogen-bonding receptors. Porosity Analysis and Gas Adsorption Measurements. As previously stated, compounds 4−6 contain open crystal structures with remarkable void percentages spanning from discrete and isolated cavities to interconnected channels, in such a way that they could a priori support gas adsorption measurements. Nonetheless, compound 6 was rapidly discarded, since its hydrogen-bonded structure lacks enough robustness and collapses after removal of single crystals from the mother liquors. TG/DTA analyses confirm that only 5 remains stable upon release of the solvent molecules (showing a short plateau that agrees well with the observed bidimensional pore system), whereas the sample of compound 4-Dy seems to decompose after solvent release. In fact, N2 adsorption measurements were conducted at 77 K on both activated samples, and 4-Dy showed negligible uptakes although many attempts were performed (i.e., changing outgassing temperature, and increasing equilibration time). Interestingly, compound 5 exhibits a type I adsorption curve with a marked knee at very low relative pressures (P/P0 ≈ 0.01) followed by a plateau close to 70 cm3(STP) g−1 (BET surface of 260 m2 g−1), which is characteristic of microporous solids with a uniform pore-size distribution (Figure 10). The last subtle increase observed at high relative pressures may be related with capillary condensation owing to small particle size, which is in any case a minor contribution as confirmed by the porosity analysis (see Table 8). Static Magnetic Measurements. The temperaturedependent magnetic susceptibility data for compounds 1, 2, and 5 were measured on polycrystalline samples in the 5−300

Figure 9. (a) Connectivity among the cages. (b) Caption of supramolecular interactions linking two cages. (c) Packing of 6 showing the voids.

the cages and the small spherical voids created among them, which prevents its use for adsorption of gas molecules such as N2, CO2, and so on. Synthetic and Structural Details on the M/CNip System. A series of hydrothermal and solvothermal syntheses using CNip and different metal salts as reagents gives rise to eight compounds. X-ray diffraction analyses reveal a wide diversity of crystal structures ranging from isolated metal− organic cages (MOCs) to 3D frameworks owing to the versatility of CNip ligand, which, as most dicarboxylic ligands, may adopt a large number of binding patterns given the great coordination capacity of its carboxylate groups. In particular, it adopts the five coordination modes shown in Scheme 1 to tailor the architectures of compounds 1−6. Notwithstanding the fact that this is a major cause for such a structural diversity, a careful inspection of the synthetic scheme concludes that the coordination mode is, indeed, a consequence of three key factors: employed metal, solvent, and reaction time. To start with, hydrothermal reaction of copper(II) nitrate and CNip allows the presence of hydroxide anions that are easily seized by Cu atoms, which are brought together due to the trend of hydroxide anion to form bridges. This fact, in addition to its small size, permits other ligands (solvent itself and CNip) to be coupled into rodlike metal−organic chains that are eventually joined into the 2D sheets of 1. Thus, coordination mode a of CNip seems to be a consequence of the occurrence of additional bridging ligands with good hydrogen-bonding donors. However, using a metal ion with a marked preference for octahedral environments, such as cobalt(II), does not generate the preceding structural motif but renders trimeric building units. These units contain a less distorted central metal environment established by carboxylate moieties, which forces CNip ligands to coordinate slightly tilted, in such a way that some of the carboxylate oxygen atoms act as bridges and b and c coordination modes are adopted within the layers of compound 2. Nevertheless, such a structural arrangement also delimits the nuclearity of the building unit, so DMF molecules occupy its edges to complete the coordination shell of the metals. Quite the opposite, the 3D backbone of compound 3 builds from infinite metal-carboxylate rods because of the presence of cadmium atoms featuring a larger ion size in addition to high coordination plasticity that allows them to adopt multiple environments. All of this contributes to a better accommodation of the local chelating−bridging pattern of J


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Inorganic Chemistry

Figure 10. N2 adsorption and desorption curves of compound 5 at 77 K.

Table 8. Analysis of the Porosity of 5 Sample (6 h, 1 × 10−4 atm, 150 °C) sample

SBETa

Smicrob

Sextb

Vtc

Vmicrob

Vextd

5

258

211

47

0.121

0.081

0.040

BET specific surface area (m2 g−1). bMicropore surface area (Smicro) and volume (Vmicro) and external surface area (Sext) are estimated from the t-plot calculation (calculation of microporosity contribution includes all pores 98° but ferromagnetic for smaller angles, some recent calculations using different density functional methods have shown that the nature of magnetic interaction depends on many structural parameters (Cu−O distance, out of plane displacement of the hydroxo hydrogen atom, nonplanarity of the Cu2O2 core, and distance of copper to the basal plane).47 However, the ferromagnetic interaction computed for the first superexchange pathway (J1 = +24 cm−1) may be explained by means of the orbital counter-complementarity48 between the μ-syn,syncarboxylate bridge and the hydroxo bridge with φ > 98° and/ or the coexistence of the μ-OH2 bridge with φ > 79.6°. In fact, the calculated weak value agrees with previous suggestions that conclude that the strength of the coupling diminishes with the trend: O2− > OH− > H2O.49 The calculated spin density distributions of the ground (S = 1) and excited (S = 0) states of both pathways (Figure 12) show that the unpaired electron occupies the dx2−y2 orbital, as expected for elongated octahedral or square pyramidal environments. Therefore, the net antiferromagnetic behavior seems to originate from the simple superexchange path (J3 = −11 cm−1) and the weaker interchain interactions through the CNip ligands. This overall magnetic behavior is similar to that previously reported for the K


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Inorganic Chemistry

orbit coupling/site distortion and the anti-ferromagnetic exchange interactions, respectively. Best fitting of the equation, performed over the 5−300 K range to avoid the contribution of ferromagnetic exchange interactions, can be seen in Figure 11b. As observed, A + B sum (11.63 cm3 K mol−1) is very close to the Curie parameter, E1/κ = +32(1) K is within the range covered by similar cobalt(II) compounds,54 and −E2/κ = −0.11(3) K confirms the presence of weak anti-ferromagnetic interactions. Coming back to the crystal structure of 2, which grows from carboxylate bridged trinuclear cores that are eventually joined together through carboxylate ligands, both weak anti-ferromagnetic or ferromagnetic interactions can be expected depending on the μ-syn,syn-CO2 bond angles and the CoII−O−CoII angle, taking into account that the abovementioned countercomplementarity phenomenon can take place as well.55 To gain a deeper insight into the magnetic properties of this compound, DFTUB3LYP calculations were performed on a suitable model in which the three carboxylate bridges of a dimer were simplified taking into account the centrosymmetric character of the Co3 core (see Supporting Information). The obtained value (J = −5 cm−1) is in good agreement with the overall experimental behavior, which seems to indicate that the observed weak ferromagnetism at low temperatures is due to weaker intermolecular interactions. The temperature dependence of χMT for gadolinium-based compound 5 shows a stable value of 7.95 cm3 mol−1 K (in good agreement with a magnetically isolated GdIII ion with local S = 7/2 and g = 2.0) from room temperature to 35 K, below which it sharply decreases to 7.645 cm3 mol−1 K (Figure 13). This

Figure 12. Calculated spin-density distributions for ground and excited states for (a) triply and (b) doubly bridged superexchange pathways in 1.

compound of [Cu3(TAcO)4(H2O)2(OH)2]n·4H2O formula (where TAcO = thimine-1-acetic acid), which bears a strong structural resemblance.50 The ground state of a high-spin octahedral cobalt(II) is 4T1g, such that magnetic moments of the resulting complexes tend to show a considerable temperature dependence that is difficult to interpret owing to the partial quenching of the orbital momentum.51 This fact brings a large spin−orbit coupling that provokes these systems to exhibit substantial zero-field splitting that, in turn, causes a high magnetic anisotropy. The χMT versus T plot for 2 (Figure 11b) shows that, at room temperature, the χMT product is equal to 9.51 cm3 mol−1 K, a value that is largely higher than that expected for three magnetically isolated spin triplets (g = 2.01) in octahedral coordination geometry (5.61 cm3 mol−1 K), which arises from the strong orbital contribution of the octahedral CoII ions.52 χMT exhibits a very light decrease cooling the sample to 140 K, where it starts a sharper and continuous drop to reach a minimum value of 5.43 cm3 mol−1 K at 5 K. Then, it shows an abrupt increase to a value of 5.86 cm3 mol−1 K at 2 K. Thus, the behavior of χMT with temperature may be ascribed to the occurrence of three main factors. The strong initial decrease is governed by net anti-ferromagnetic exchange interactions in addition to the spin−orbit coupling, whereas the low temperature rise indicates that a small spontaneous magnetization emerges in the overall anti-ferromagnetic system. This fact is reasonable for a trinuclear Co3 entity in which the predominantly anti-ferromagnetic coupled spins leads inevitably to an uncoupled cobalt center (as observed from the χMT value at 5 K), which may, in turn, interact ferromagnetically with neighboring uncoupled centers through the CNip ligands. Fitting of the χM−1 values to the Curie−Weiss law in the 100− 300 K temperature range gives values of C = 12.47 cm3 K mol−1 and θ = −12.2 K, which confirms the above-mentioned interactions. Considering the strong spin−orbit coupling in this compound, the following simple phenomenological equation by Rueff and co-workers53 (eq 1) was employed to obtain an estimate of the anti-ferromagnetic exchange interaction: χM T = A exp( −E1/κT ) + B exp(−E2 /κT )

Figure 13. χMT vs T plot of compound 5 showing best theoretical fit (red line) and different superexchange couplings.

sharp drop may be related to the zero-field splitting associated with the complex, Zeeman depopulation effects, or intermolecular anti-ferromagnetic interaction between gadolinium(III) centers. Anyhow, both the observed temperature-independent behavior (300−35 K) and the obtained values from the fitting of data to Curie−Weiss law (C = 7.9567 and θ = −0.0615) suggest that the coupling between Gd3+ ions is very weak, as previously reported for many polynuclear complexes.56 The 8 S7/2 ground state of the Gd3+ ions allows quantitatively

(1)

In this equation, A + B equals to the Curie constant, while E1 and E2 stand for the “activation energies” related with the spin− L


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Inorganic Chemistry analyzing their magnetic interactions by applying a spin-only Hamiltonian.57 Paying attention to the crystal structure, the 3D framework can be considered as the coexistence of isolated gadolinium(III) dimers and tetramers from a magnetic point of view given that the shortest interdimer/tetramer separation is quite large (ca. 8.29 Å). Thus, the magnetic behavior is the result from three superexchange couplings taking place at: (i) subunit 1, consisting of four μ-syn,syn carboxylates (J1); (ii) subunit 2, with three μ-syn,syn carboxylates (J2); and (iii) the internal junction between both subunits in the tetramer, established by a double μ-formate bridge (J3); and (iv) that within the dimeric entity that is equivalent to the first one (J4 = J1). The analysis of the magnetic data was done with Phi program58 by considering three exchange interactions through the pathways bridging among four consecutive local isotropic spin octets. The calculated curve reproduces fairly well the experimental one over the whole temperature range, giving least-squares best-fit parameters found in Table 9. At first sight, Table 9. Best Least-Squares Fits of the Experimental Curves and Theoretically or Computationally Calculated Magnetic Data compound

gJ (exp/ theor)a

5

1.99(1) /2.00

a

J1 (exp/comp)b

J2 (exp/ comp)b

J3 (exp/ comp)b

−0.075/−0.06

0.033/0.03

0.038/0.07

Figure 14. Temperature dependence of the out-of-phase χM″ signals for complex 4-Dy in zero (top) and 1000 (bottom) Oe applied fields. (insets) Arrhenius plots for the relaxation times.

Theoretical values obtained from J = L + S;

gJ =

3 2

+

S(S + 1) − L(L + 1) b . Experimental 2J(J + 1) −1

and DFT-computed values

temperatures and the deviation of relaxation times from linearity. Accordingly, high α values (within the 0.27 (8 K)− 0.42 (4 K) range) extracted from the Cole−Cole plots support the latter statement, while they also suggest the presence of different relaxation modes (Figure S20). In addition, as the relaxation times cannot be fitted to the equation that takes into account the simultaneous presence of Orbach and QTM processes (eq 4), other relaxation modes such as Raman must also be taking place in the analyzed temperature range. Nevertheless, the analysis of the high-temperature data and its subsequent fitting to the Arrhenius equation led to an effective energy barrier of 44 K, with τ0 = 1.5 × 10−7 s.

of the coupling constant (cm ).

very weak coupling interactions of different nature coexist within the structure, which indicates that a correlation could be taking place between J values. However, DFT calculations on suitable models to consider these superexchange pathways support the experimental J values and the overall weak antiferromagnetic behavior dominating in the compound (Figures S25 and S26). This very weak anti-ferromagnetic interaction is confirmed by the magnetization versus H data at 2.0 K, which closely follows the Brillouin function for a magnetically isolated gadolinium(III) atoms by rapidly reaching a saturation value of 7 μB at 5 T. Although it may seem surprising that similar superexchange pathways consisting of the same bridges [μoxo(carboxylate) and syn,syn-carboxylate] provide interactions of opposite nature, previous magneto-structural correlations on similar Gd2 cores point out that the ferromagnetic coupling increases according to the Gd−O−Gd (η) angle.59 Coming back to the crystal structure, it is observed that η is 113.8/ 115.5° for J2 and 106.7° for the most tilted syn,syn-carboxylates for J3 (owing to the coordination of a DMF molecule), which fulfill the aforementioned trend when compared to less distorted paddle-wheel-shaped J1 and J4 pathways (with all η below 100.3°). Dynamic Magnetic Properties. Spin dynamics were evaluated for cobalt (2) and lanthanide (4-Tb, 4-Dy, 4-Er, and 5) based compounds by means of ac susceptibility measurements. 4-Dy is the unique compound showing frequency dependence of the out-of-phase signals in the lack of an external direct-current (dc) field owing to the large anisotropy of the Dy(III) ions (Figures 14 and S19). As usually found for Dy-based compounds, quantum tunneling of the magnetization (QTM) is operative at the lowest temperatures, which is inferred from the tails in χ″M signals at low

t −1 = tQTM −1 + t0 exp( −Ueff /kBT )

(4)

To suppress the QTM, an external field of 1000 Oe was applied. Nonetheless, low-temperature tails do not disappear completely, although they are significantly reduced, indicating that the QTM has not been fully suppressed (Figure 14). The α values in the 0.39 (7.6 K)−0.58 (5.2 K) range are still too high for a single relaxation process (Figure S20), but the energy barrier is slightly increased to 49 K (τ0 = 4.4 × 10−8 s) after the application of the field. Compounds 2, 4-Er, and 5 also show out-of-phase susceptibility signals after the application of an external field, but with maxima at lower temperatures compared to 4-Dy (Figure S22), whereas 4-Tb shows no frequency-dependent signals. Moreover, the signal corresponding to 4-Er is not strong enough as to provide a further analysis. Note that the presence of this behavior is still rare in Gd complexes, with compound 5 being the fifth Gd-based MOF showing slow relaxation of the magnetization by far to the best of our knowledge.60 The occurrence of this behavior in systems with lower nuclearity is also unusual.61 Regarding complex 2, the observance of this behavior is quite surprising due to the intramolecular anti-ferromagnetic interactions between CoII M


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Inorganic Chemistry ions. However, these interactions lead to a ST = 3/2 ground state in the trinuclear entities, which are ferromagnetically coupled to neighboring uncoupled centers through the CNip ligands. Photoluminescence Properties. To gain access into additional physical properties of the compounds belonging to the M/CNip system, solid-state photoluminescence was studied for closed-shell metal and 4f-metal complexes, that is, compounds 3, 4-Dy, 4-Tb, 4-Er, and 5. Metal−organic frameworks based on d10 metal centers and carboxylate ligands with aromatic rings, such as CNip, are promising candidates for developing new materials for many applications in the field of light-emitting diodes and chemical sensors.62 When excited under UV source (325 nm) at 10 K, polycrystalline sample of 3 shows a broad and intense band with the maximum centered at 400 nm in addition to a shoulder peaking at 438 nm, in good agreement with the bright blue emission observed at the naked eye (Figure 15). At higher wavelengths, the intensity of the

unoccupied molecular orbital), whereas the second band benefits from three main transitions: HOMO−8 → LUMO, HOMO−7 → LUMO, and HOMO−5 → LUMO. On its part, the less intense transitions above 470 nm arise from HOMO−1 → LUMO+3. The representation of the molecular orbitals shows that HOMO levels mainly grow from the carboxylate groups of the ligands (Figures S37 and S38), so taking into account that π nature of LUMO and LUMO+3 molecular orbitals, whose lobes are centered over the aromatic rings, it can be pointed out that, on the one hand, the emission of this compound follows a ligand-centered charge transition (LCCT) mechanism. On the other hand, measuring the decay curve at the most intense emission band confirmed that the luminescence of this compound consists of a very short lifetime (τ of 6 ns), as usually reported for similar cadmium(II) compounds.64 On another level, lanthanide-based PL has proven a very useful tool not only in applications related with solid-state lighting65 but also in the area of biomedicine, as they are frequently employed as fluoroimmunoassays.66 Compounds 4Tb and 4-Dy display bright and pale green luminescence under excitation at 325 nm (Figure 16). The low-temperature spectrum of the terbium(III) compound has seven multiplets corresponding to the 5D4 → 7FJ (J = 6, 5, 4, 3, 2, 1, and 0) transitions, among which 5D4 → 7F5 (sensitive to the nature of the surrounding atoms) is the strongest one according to its largest probability for both electric-dipole- and magnetic-

Figure 15. 10 K experimental (solid line) and calculated (dotted line) emission spectra showing the molecular orbitals involved in the most relevant transitions.

band drops very slowly to 600 nm exhibiting some small bands emerging at ca. 490, 540, and 575 nm. In fact, micro-PL images collected at room temperature using different excitation lines confirm the multicolored emission of the compound, which is clearly weakened with increasing irradiation wavelengths (Supporting Information). It is also outlined that, compared with the emission of the free H2CNip ligand, compound 3 shows a clear change in the intensity of the bands in addition to a very small hypsochromic shift, all of which is attributed to its coordination to cadmium atoms (Figures S29 and S30).63 With the aim of disclosing the PL mechanism of this compound, TDDFT calculations were performed on a suitable model of 3 (Figure S36). The calculated spectrum reproduces fairly well the experimental one showing small shifts for the two main bands (centered at 386 and 447 nm, respectively). These calculations show that after the singlet excitations (see Supporting Information), the emission consists of different processes. The region close to the maximum (∼400 nm, blue emission) mainly results from the intense HOMO−4 → LUMO+3 and HOMO−7 → LUMO transitions (HOMO = highest occupied molecular orbital; LUMO = lowest

Figure 16. 10 K emission spectra of 4-Tb and 4-Dy compounds under excitation at 325 nm. N


Article

Inorganic Chemistry

Zn4O clusters that give rise to metal−organic cages in the zinc compound (6). Despite the fact that most of compounds can be regarded as potentially porous materials, only the 3D framework of 5 shows a noticeable N2 uptake characteristic of microporous adsorbents in good agreement with its bidimensional pore system. Measurements of the magnetic susceptibility in static conditions indicate that anti-ferromagnetic interactions dominate the overall magnetic behavior of the compounds, except for the cobalt compound (2), for which weak ferromagnetic interactions are observed at very low temperature. Notwithstanding the latter, a detailed analysis by DFT-BS tool for the individual superexchange pathways of the compounds reveals that the coexistence of weak ferromagnetic and anti-ferromagnetic interactions are eventually competing with each other. This fact is in good agreement with ac susceptibility measurements, which confirm that many of these compounds show a slow relaxation of the magnetization. It must be highlighted that the activation energy barrier for the dysprosium compound is of 44 K at zero dc field, which slightly increases to 49 K after the application of an external field. Equally important is the fact that this behavior is also observed for a Gd-based MOF, since it exemplifies one of the few bifunctional porous-magnet reported. Unfortunately, given that out-of-phase signals observed for Co, Gd, and Er compounds appear at very low temperatures, a further treatment of the data and subsequent calculation of the energy barrier is precluded. With regard to the luminescent properties, cadmium- and lanthanide-based compounds display quite intense emissions in the visible spectrum. Particularly for the cadmium compound, TD-DFT calculations have allowed to surmise that its multicolored emission involves an LCCT mechanism. Emission decay curves monitored at the emission maxima have allowed estimating the radiative lifetimes, which consist of processes ranging from few nanoseconds (in cadmium compound) to the scale of millisecond (in the case of terbium). All in all, most of the compounds reported exhibit a dual-functional character (adsorptive-magnetic or magnetic-luminescent performances), which may address the challenge of developing smart materials with enhanced sensor capacity.

dipole-induced transitions. On the one hand, at room temperature, the spectrum does not show any significant change, but it solely broadens (losing all the structure of the multiplets) because of the increase in the kinetic (thermal) energy of bond electrons (Figure S31). On the other hand, when CNip ligands sensitize Dy3+ ions in 4-Dy compound, it displays an emission spectrum governed by the ligand fluorescence, which is observed as a wide band covering the 360−600 nm range. Nonetheless, four groups of signals in the 350−800 nm range, centered at 485, 575, 665, and 755 nm, are also distinguished. The 4F9/2 → 6H13/2 transition is the strongest one, as observed for most of dysprosium(III)containing organic polymers,67 whereas the 4F9/2 → 6H15/2 transition, with less intensity, is somewhat immersed into the band assigned to CNip ligand. At higher wavelengths, the remaining two transitions possess very low intensities but still remarkable as to be observed. For comparative purposes, the absence of such ligand fluorescence in the spectrum of 4-Tb signifies a more efficient intramolecular energy transfer from the coordinated ligand to Tb(III) than to Dy(III). Additionally, decay curves at 5D4 → 7F5 and 4F9/2 → 6H15/2, respectively for Tb and Dy compounds, were monitored at different temperatures to check the emissive stability of both systems (Table 10). A single-exponential function [It = A0 + A1 exp(−t/τ)] was Table 10. Thermal Evolution of the Lifetime Values for 3, 4Tb, and 4-Dy Compounds 3

4-Tb

temp (K)

τ (ns)

χ

10 50 100 150 200 250 300

6.8(1)

1.225

2

4-Dy

τ (μs)

χ

τ (μs)

χ2

971(7) 983(6) 984(5) 996(5) 989(7) 989(7) 1013(5)

1.257 1.310 1.243 1.130 1.189 1.249 1.195

22.2(3) 23.6(7) 24.6.(8) 25.0(8) 25.4(4) 24.9(4) 26.3(4)

1.014 1.021 1.066 1.102 1.138 1.200 1.011

2

employed for the fittings. At first glance, at 10 K 4-Tb exhibits a much longer lifetime (of nearly 1 ms) compared to that of 4-Dy (of few μs), in agreement with the expected behavior given the different energy gaps and subsequent quenching through nonradiative processes existing between the excited and ground levels in both lanthanides.68 Moreover, another remarkable difference arises from the fact that the lifetime of 4-Tb remains almost invariable over the whole temperature range, whereas that of 4-Dy subtly decreases.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.6b01845. Additional figures of crystal structures and data, thermogravimetric analysis, PXRD analyses, dc and ac magnetic susceptibility data, spin density distributions, photoluminescence spectra and lifetimes, TD-DFT computational results. (PDF) X-ray crystallographic information (CIF) X-ray crystallographic information (CIF) X-ray crystallographic information (CIF) X-ray crystallographic information (CIF) X-ray crystallographic information (CIF) X-ray crystallographic information (CIF)

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CONCLUSIONS A wide family composed of eight new compounds grown from the coordination of 5-cyanoisophthalate ligand to transition metal(II) and lanthanide(III) centers has been obtained under hydro/solvothermal conditions. Six different X-ray crystal structures of variable dimensionality (2D and 3D) and uncommon topologies are achieved according to the coordination versatility shown by the CNip ligand and the different coordination geometries adopted by the metal ions. In addition to these two facts, other controllable parameters of the synthetic conditions seem also to contribute to the selfassembling process that promotes such a structural diversity. This is the case of the in situ generation of formate anions by thermal decomposition of dimethylformamide, which act as coligands in the gadolinium compound (5) or the formation of

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. (J.C.) *E-mail: [email protected]. (A.R.D.) O


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

This work was supported by the Junta de Andalucı ́a (FQM1484 and FQM-195) and Univ. of the Basque Country (GIU14/01). I.O. and J.C. thank the Univ. of the Basque Country (UPV/EHU) for their postdoctoral fellowships. The authors are thankful for technical and human support provided by SGIker of UPV/EHU and European funding (ERDF and ESF). Notes

The authors declare no competing financial interest. Crystallographic data (excluding structure factors) for the structures reported in this paper have been deposited with the Cambridge Crystallographic Data Center as supplementary publication Nos. CCDC 1496877−1496882. Copies of the data can be obtained free of charge on application to the Director, CCDC, 12 Union Road, Cambridge, CB2 1EZ, U.K. (Fax: +44−1223−335033; e-mail: [email protected] or http:// www.ccdc.cam.ac.uk).

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ACKNOWLEDGMENTS J.C. acknowledges J. M. Ugalde for his guidance and support along the postdoctoral period. REFERENCES

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