1,2,4-Triazole

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Tuning the Crystal Structure Dimensionality of Cobalt(II)/1,2,4-Triazole Complexes Romain Gautier, and Rodolphe Clérac Cryst. Growth Des., Just Accepted Manuscript • DOI: 10.1021/acs.cgd.6b01723 • Publication Date (Web): 15 Dec 2016 Downloaded from http://pubs.acs.org on December 16, 2016

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Crystal Growth & Design

Tuning the Crystal Structure Dimensionality of Cobalt(II)/1,2,4-Triazole Complexes Romain Gautiera,* and Rodolphe Cléracb,c a

Centre National de la Recherche Scientifique, CNRS-IMN, 44300 Nantes, France

b

CNRS, CRPP, UPR 8641, F-33600 Pessac, France.

c

Univ. Bordeaux, CRPP, UPR 8641, F-33600 Pessac, France.

KEYWORDS. Low-dimensional structure, Antiferromagnetism, 1,2,4-Triazole based cobalt complex, Hydrothermal synthesis

ACS Paragon Plus Environment

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ABSTRACT. To control the structure dimensionality of hybrid materials, a common approach consists in selecting ligands with specific bridging modes. Herein, we show that the bridging mode for some azole ligands with Co(II) metal ions can be tuned in-situ using different synthesis conditions. For example, the dimensionality of 1,2,4-triazole based materials could be controlled according to its (de)protonation. Under hydrothermal conditions, the deprotonation commonly occurs and leads to the tripodal moiety that favors high dimensional crystal structure. To target low-dimensional 1,2,4-triazole based cobalt materials, the deprotonation was prevented by carrying out hydrothermal syntheses under strong acidic conditions. With this strategy, two new compounds with low dimensional crystal structures were obtained: Co(Htrz)Cl2 (1) exhibiting one-dimensional coordination polymers and [Co3(Htrz)6(H2O)6][ZnBr4]3•9H2O (2) containing Co trinuclear complexes (Htrz = 1,2,4-triazole). Magnetic measurements showed weak antiferromagnetic interactions between Co(II) centers in both systems.

INTRODUCTION New magnetic materials based on transition metal ions have focused chemists' and physicists' attention in the past decades owing to their potential applications in data storage technologies. In this search, the development of different approaches to target crystal structures with specific arrangements of spins is of particular interest. For example, few strategies to design triangular arrangements of antiferromagnetically coupled spins for frustrated systems have been reported such as the use of secondary building units,1,2 or metal-centered oxyfluoride anions.3 The control of the dimensionality of the framework is also important in the design of single-molecule magnets and single-chain magnets.4–7 Different approaches have been proposed such as the use


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of monodentate vs. multidentate ligands,8 or templating cations such as ammonium cations for zeolites.9 By using ligands which are either bridging or terminal, one can control the metal's available coordination sites in order to modify the dimensionality of the structure. Thus, Hagrman et al. showed that the appropriate selection of the ligands allowed either to passivate the metal coordination spheres to target low-dimensional materials or to propagate the geometry of the metal ion through the bridging ligands. In this article, we showed that, for azole ligands, the dimensionality can also be controlled by rationalizing the synthetic conditions. Our attention was focused on the 3d metal ion / triazole / halide system that exhibits a rich and interesting chemistry owing to the large variety of possible coordination mode of the metal ions to the ligands and the potential applications.10,11 For example, the zero dimensional [Fe3(4-R-1,2,4-triazole)6(H2O)6](A)6•nH2O (with different anions A and substituents R) and one-dimensional [Fe(Htrz)3](BF4)2 compounds (Htrz = 1,2,4-Triazole) exhibit spin-crossover properties, which are potentially interesting for memory applications.12–17 In this system, our analysis of the previously reported crystal structures showed that, according to the (de)protonation of the ligand, different dimensionalities of the frameworks could be targeted. Hence, two new low dimensional materials could be synthesized by carrying out hydrothermal

method

under

strong

acidic

conditions:

Co(Htrz)Cl2

(1)

and

[Co3(Htrz)6(H2O)6][ZnBr4]3•(H2O)9 (2), and their magnetic properties were studied. As expected, compound 1 is formed of Co(Htrz)Cl2 coordination chains while compound 2 is formed of [Co3(Htrz)6(H2O)6]6+ trinuclear cations. Both materials show antiferromagnetic couplings mediated by the triazole ligand between Co(II) magnetic sites.


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EXPERIMENTAL SECTION Synthesis. The compounds were synthesized by hydrothermal method from mixtures of Co3O4 (Alfa Aeasar, 99.7%), Zn (Merck, 99.9%), 1,2,4-triazole (Sigma Aldrich, 98%), HCl (Alfa Aesar, 37%), and HBr (Alfa Aesar, 48%). Co(Htrz)Cl2 (1). A mixture of Co3O4 (0.66 mmol), Zn (7.64 mmol), 1,2,4-triazole (7.24 mmol), 3 ml 37% aqueous HCl (35.95 mmol) was placed in 23 mL Teflon-lined stainless steel autoclave. The autoclave was heated to 180°C for 24 hours and cooled to room temperature at 10°C/h. Blue single crystals were recovered by vacuum filtration. [Co3(Htrz)6(H2O)6][ZnBr4]3•(H2O)9 (2). A mixture of Co3O4 (0.66 mmol), Zn (7.64 mmol), 1,2,4-triazole (7.24 mmol), 3 ml 48% aqueous HBr (26.5 mmol) was placed in 23 mL Teflonlined stainless steel autoclave. The autoclave was heated to 180°C for 24 hours and cooled to room temperature at 10°C/h. Orange single crystals were recovered by vacuum filtration. Structure determination. Single-crystal X-ray diffraction was carried out with a Bruker-Nonius Kappa CCD diffractometer with monochromated MoKα radiation (λ = 0.71073 Å). The crystal to detector distances were 25 mm and 50 mm for compounds 1 and 2, respectively. Absorption corrections were processed with SADABS.18 The determinations of both structures were realized using direct methods and were completed by Fourier difference syntheses using SIR2004.19 The crystal structures (including the anisotropic displacement parameters) were refined using SHELXL-2013,20 and the Crystallographic Information Files were compiled with Olex2.12.21 The program PLATON enabled to check for additional symmetry elements.22 Table 1 summarizes the crystallographic data for compounds 1 and 2.


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Table 1. Crystallographic data for compounds 1 and 2.

Space group a /Å b /Å c /Å α /° β /° γ /° Volume /Å3 Z ρcalc g/cm3 Radiation 2θ range for data collection/° Reflections collected Data/restraints/parameters Goodness-of-fit on F2 Final R indexes [I ≥ 2σ(I)] Largest diff. peak/hole / e Å-3

1 C2/c 9.4530(19) 11.690(2) 6.6830(13) 90 131.22(3) 90 555.5(3) 1 2.3420 Mo Kα (λ = 0.71073) 13.14 to 69.94 15968 1218/0/38 1.111 R1 = 0.0406 wR2 = 0.1170 1.01/-1.21

2 R3c 18.444(1) 18.444(1) 28.950(2) 90 90 120 8528.8(7) 18 2.2989 Mo Kα (λ = 0.71073) 13.08 to 70 72280 8143/0/190 1.003 R1 = 0.0686 wR2 = 0.1006 2.14/-2.32

Magnetic measurements. The magnetic susceptibility measurements were obtained with a MPMS-XL Quantum Design SQUID magnetometer that works between 1.8 and 400 K with applied dc fields ranging from –7 to 7 T. Reproducible measurements were performed on polycrystalline samples (17.46 mg for compound 1 and 12.64 mg for compound 2), introduced in a sealed polyethylene bag (3 × 0.5 × 0.02 cm of 26.70 and 20.05 mg, respectively) and covered with mineral oil (to avoid torquing effect; 5.30 and 6.39 mg, respectively). Prior to the experiments, the field-dependent magnetization was measured at 100 K in order to confirm the absence of any bulk ferromagnetic impurities. Ac susceptibility measurements were made with an oscillating field of 3 Oe with a frequency of 1500 Hz. Above 1.8 K, the out-of-phase


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component of the ac susceptibility was found to be zero. The magnetic data were corrected for the sample holder, the mineral oil and the intrinsic diamagnetic contribution. Thermal measurements. Differential Scanning Calorimetry and thermogravimetric measurements were carried out with a Jupiter STA 449 F3 Instrument under air from 25°C to 800°C with an heating rate of 5°C/min. Spectroscopy. UV/Visible Spectroscopy: Diffuse reflectance measurements were conducted for both compounds on a Varian Cary 5G spectrophotometer equipped with a 60 mm integrating sphere from 250 to 2500 nm. Absorption spectra were obtained from reflectance data using the Kubelka-Munk function (a/S = (1-R)2/2R) where a stands for the absorption coefficient, S the scattering coefficient and R the reflectance. FT-IR Spectroscopy: After mixing with KBr and pressed into pellets, the FT-IR spectra (with substraction of background spectrum) were recorded using a Bruker Vertex 70 Instrument from 400 to 4000 cm-1. A resolution of 4 cm-1 was used and 100 scans were collected.

RESULTS Synthesis and Structure description The two compounds were synthesized by hydrothermal technique in a Teflon-lined stainless steel autoclave at 180°C from mixtures of Co3O4, Zn metal, 1,2,4-triazole ligand and aqueous HCl or HBr for 1 and 2 respectively. After one day, the autoclave was cooled down to room temperature (at 10°C/h) and crystals of the compounds were simply isolated by filtration.


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The complex 1 crystallizes in the C2/c space group (Figure 1) with a crystal structure containing one-dimensional coordination polymers running along c axis. One triazole ligand assisted by two chlorine anions are bridging the Co metal ions along the chain leading to an octahedral coordination sphere of the unique centrosymmetric Co crystallographic site. The bond distances around the Co metal ions are typical of a high spin Co(II) center, Co1-Cl1 = 2.4899(8) and 2.4918(13) Å, Co1-N1 = 2.101(2) Å. The bond valence sum (BVS; Co: 1.75) confirms this observation in agreement with the magnetic properties discussed below. The π-stacking between triazole ligands is responsible for the packing of these chains in this compound. The complex 2 crystallizes in the noncentrosymmetric space-group R3c (Figure 2). The noncentrosymmetry could not be confirmed by SHG measurements but no additional symmetry elements were found using the program PLATON. The crystal structure is composed of trinuclear [Co3(Htrz)6(H2O)6]6+ cations, [ZnBr4]2- anions and water molecules, which are isolated in a hydrogen bond network. In the tetrahedral zinc bromide anions, the Zn1-Br bond distances range from 2.3851(16) to 2.4256(21) Å (the BVS confirms the +2 oxidation state, 2.01, of the Zn metal ions). Trinuclear [Co3(Htrz)6(H2O)6]6+ complexes are built of three crystallographic independent octahedral Co metal ion sites. The Co-N and Co-Ow bond lengths are in the range 2.1048(74) Å < dCo-N < 2.1560(73) Å and 2.1151(75) Å < dCo-Ow < 2.1339(78) Å, respectively. The BVS for Co1, Co2 and Co3 are, respectively, 1.558, 1.81 and 1.77 in agreement with a +2 oxidation state and with the magnetic properties described below.


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Figure 1. View of compound 1 (a) along the c axis and (b) perpendicular to the chains. Blue octahedra represent cobalt centered units. Color code: Cu, light blue; Cl, green; N, blue; C, grey.

Figure 2. View of (a) the packing of [Co3(Htrz)6(H2O)6]6+ cations and [ZnBr4]2- anions in compound 2 and (b) the trinuclear [Co3(Htrz)6(H2O)6]6+ cation along the b axis. Blue and green polyhedra represent cobalt centered cations and zinc centered anions, respectively. Color code: Cu, light blue; O, red; N, blue; C, grey.


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Magnetic Measurements At room temperature, the χT products are 3.0 cm3 K mol-1 and 8.5 cm3 K mol-1 for compounds 1 and 2, respectively (Figure 3). These values are in good agreement with the expected values for the presence of one and three high spin Co(II) (S = 3/2) ions, respectively, with g factor around 2.5-2.6. In 1, when the temperature is lowered, the χT product at 1000 Oe slightly decreases to 2.6 cm3 K mol-1 till 100 K and a more pronounced decreases to 0.7 cm3 K mol-1 is observed down to 1.8 K. In 2, the χT product at 1000 Oe decreases significantly with lowering the temperature to 1.7 cm3 K mol-1 down to 1.8 K. These thermal behavior of the χT product for these compounds is clearly a combination of the weak antiferromagnetic interactions between Co(II) centers, much stronger in 2 than in 1, and also the presence of spin-orbit coupling well known in Co(II) systems. This latter effect results in the splitting of the energy levels arising from the 4T1g ground term that finally stabilizes a doublet ground state.23 The temperature dependence of the χT product for 1 and 2 was fitted to a Curie-Weiss law (Figure S2) leading to Curie constants of 3.2 and and 10.2 cm3K/mol and Weiss constants of -25.2 and -52.9 K considering data above 50 and 30 K, respectively. The Curie constants are well in agreement with the room temperature χT product and confirm the presence of one and three high spin Co(II) (S = 3/2) ions, respectively, with g factor of 2.61(5) for 1 and 2.69(5) for 2. The Weiss constants confirm the presence of significant antiferromagnetic interaction between spin carriers and that the coupling in 2 is approximately the double of the one in 1. The field dependence of magnetization at low temperatures reveals a relative rapid increase of the magnetization at low fields and then a linear variation without clear saturation (Figures S3 and S4). The magnetizations at 7 T are 2.0 µB for 1 and 2.4 µB for 2. These values are in agreement with the residual presence of the equivalence of only one Co(II) center as expected in 2, if the Co spins


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are antiferromagnetically coupled in the trinuclear complex. In 1, the presence of one Co(II) magnetic center at high field is probably the result of the magnetic field that overcomes the weak antiferromagnetic interaction. This effect is indeed seen around 5400 Oe with an inflection point in the M vs H data that is better detected on the field derivative curve shown in Figure S3. The high field linear variations of the magnetization suggest the expected presence of a significant anisotropy or low lying excited states in both compounds, which is further confirmed by the nonsuperposition of the M vs H/T plots at different fields (Figures S3 and S4). Slow dynamics of the magnetization was not observed on the field dependence of the magnetization above 1.8 K for both compounds as also confirmed by the ac susceptibility mesurements in zero dc field.

Figure 3. Temperature dependence of the χT product at 0.1 T (χ is defined as magnetic susceptibility equal to M/H per mole of complex) for 1 (black dots) and 2 (red dots).


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Spectroscopy and thermal measurements Below 400 nm, the diffuse reflectance spectra show a strong absorption attributed to π−π∗ intraligand transitions (Figure S5). As expected from the crystal structures, FTIR spectra show the presence of 1,2,4-triazole ligand in 1 and 2, with characteristic IR bands between 500 and 1700 cm-1), and water molecules (at 3400 cm-1) for 2 (Figure S6). The thermogravimetry analysis show different decomposition processes for 1 and 2 (Figure 4). In compound 1, the weight variation from 400 to 540°C was identified by mass spectroscopy as corresponding to the loss of H2O, NH3 and CO2 while between 540 to 640°C, HCl is lost. The total mass variation (62.4%) is in agreement with the presence of cobalt oxide (CoO, calculated loss: 61.7%) at the end of the thermal treatment. For 2, the total weight variation of 85.7% corresponds to the loss of NH3, H2O, CO2 and HBr in agreement with the presence of Co and Zn metals as products of the thermal treatment (calculated loss: 81.2%).


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Figure 4. Thermogravimetry analysis of (a) 1 (experimental total weight loss: 62.4%) and (b) 2 (experimental total weight loss: 85.7%).

DISCUSSION Triazole ligands based materials: dimensionality and magnetic properties. The chemistry of triazole based materials is very diverse owing to the large variety of possible coordination modes of the organic ligands to the transition metals.10,11 Among the complexes of 1,2,4-triazole, compounds which exhibit trinuclear structure such as [Ni3(Htrz)6(H2O)6](NO3)6,24 onedimensional (1D) coordination arrangements such as Cu(Htrz)Cl2,25 two-dimensional (2D)


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coordination structures such as the series of materials [M(Htrz)2(NCS)2] (M = Mn, Fe, Co, Ni Cu and Zn),26 or a three-dimensional (3D) network such as Co2(trz)3Cl have been reported.27 Some of these complexes exhibit interesting spin crossover properties like the well-known 1D compounds [Fe(Htrz)2(trz)]BF4 and [Fe(Htrz)3](BF4)2 which have been extensively studied.14,28– 32

In these compounds, the Fe(II) centers are bridged by three triazoles. The materials exhibit

spin crossover and thermochromism often associated with a phase transition and thermal hysteresis effects. Among the 1,2,4-triazole halide or thiocyanate based materials, very few cobalt compounds have been reported. To our knowledge, only the 2D Co(Htrz)2(NCS)2 system synthesized by Engelfriet et al., and the 3D Co2(trz)3Cl compound synthesized by Ouellette et al. have been previously characterized.27,33 In the Co2(trz)3Cl material, weak ferromagnetism resulting from antiferromagnetic interactions between canted Co(II) centers was observed. The triazole ligand, in which all three N-donor positions are coordinated, was described as the key to increase the dimensionality of this system and to lead to the observed ferromagnetic behavior. Cobalt complexes with 4-substituted triazoles in low dimensional crystal structure, in which antiferromagnetic couplings between cobalt centers are usually found, have also been investigated.34–36 However, as far as we know, no low dimensional unsubstituted 1,2,4 triazole ligands based cobalt materials have been synthesized up to now. Triazole as a bridging ligand and its influence on the structure dimensionality. The analysis of the 1,2,4-triazole / metal ion systems shows that the dimensionality of the frameworks is, in most cases, influenced by the bridging mode.37,38 Thus, the crystal structure in which the 1,2,4triazole ligand is in the N1,N2 bridging mode is more likely to be of lower dimension than when the N1,N2,N4 bridging mode is observed. For this reason, the 4-substituted triazole ligands, which can only coordinate through N1 and N2 sites, commonly lead to materials which exhibit


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polynuclear complexes or chains. When protonated, the 1,2,4 triazole will also more likely lead to low dimensional crystal structures owing to the impossible coordination of metals to the protonated N4 site. For example, many complexes with protonated 1,2,4-triazoles are reported to be isolated molecular entities such as [Ni3(Htrz)6(H2O)6](NO3)6•2H2O,24 1D systems such as CuCl2(Htrz),25 or 2D compounds such as Cu3(Htrz)2Br3,39 while complexes with deprotonated 1,2,4-triazole ligand are commonly reported to adopt 2D structures such as [Zn(trz)Cl],40 or 3D networks like for Cu(trz).41 Targeting low versus high dimensional 1,2,4-triazole ligand based magnetic materials. Under hydrothermal conditions, the deprotonation of 1,2,4 triazole commonly occurs even in the absence of bases.42–44 As mentioned previously, the deprotonated form of this ligand favors high dimensional frameworks owing to the N1, N2, N4 bridging mode and no low dimensional cobalt 1,2,4-triazole based materials have been reported so far. For this reason, our syntheses were carried out under strong acidic conditions. The pH-controlled change of the coordination mode has been previously investigated in polydentate carboxylate based materials.45–49 This approach enabled to tune the dimensionality of the related networks. However, this approach have not been explored up to now in systems with azole ligands. By preventing the deprotonation of the 1,2,4- triazole ligand in our syntheses, we could obtain low dimensional structures. Compound 1 is built of Co(Htrz)Cl2 chains similar to those observed in CuCl2(Htrz),25 while compound 2 exhibits the same trinuclear [Co3(Htrz)6(H2O)6]6+ cations as in [Ni3(Htrz)6(H2O)6](NO3)6.24 The crystal structure of 1 and 2 can also be compared to the 3D Co2(trz)3Cl compound reported by Ouellette et al.27 In this latter compound, chains of {Co(trz)3} are connected to each others owing to coordination of the 1,2,4-triazolate ligand through the N4 binding site. In compounds 1 and 2, this site is terminal owing to its protonation which prevents the propagation of the coordination


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in three dimensions (Figure 5). Similarly to 1 and 2, antiferromagnetic interactions occur between Co(II) centers in Co2(trz)3Cl. However, the arrangement of the metal centers in the 3D network leads to a competition between different antiferromagnetic interactions resulting in a canted antiferromagnetic ordered phase below 9 K.

Figure 5. Comparison of the crystal structures of (a) Co2(trz)3Cl reported by Ouellette et al.,27 (b) 1 and (c) 2. Co octahedral and tetrahedral sites are represented in blue and grey colors, respectively. Nitrogen binding and non-binding site are represented in blue and yellow color, respectively. In compounds 1 and 2, the protonation of N4 sites prevents the propagation of the coordination in three dimensions.


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CONCLUSION The triazole containing compounds, Co(Htrz)Cl2 and [Co3(Htrz)6(H2O)6][ZnBr4]3•9H2O, were synthesized by hydrothermal methods under strong acidic conditions. These conditions were shown to favor the stabilization of the protonated bidentate 1,2,4 triazole ligand and low dimensional,

molecular

(0D,

trinuclear

complex)

and

1D,

structures

in

[Co3(Htrz)6(H2O)6][ZnBr4]3•9H2O and Co(Htrz)Cl2, respectively. The magnetic properties of these compounds are dominated by weak antiferromagnetic interactions between Co(II) centers and the presence of magnetic anisotropy intrinsic to the octahedral cobalt(II) sites. This report and these materials illustrate how tuning the (de)protonation of a given ligands on specific binding sites allows the chemists to favor specific bridging modes which appears as a powerful method towards the design of new hybrid materials with unexplored dimensionality.

ASSOCIATED CONTENT Supporting

Information.

Crystallographic

Details

Information

on

Files

magnetic for

measurements,

compounds

Figures

Co(Htrz)Cl2

S1-S6,

and

(1)

and

[Co3(Htrz)6(H2O)6][ZnBr4]3•9H2O (2). This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author *[email protected].


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

ACKNOWLEDGMENT This work was supported by the University of Bordeaux, the ANR, the Région Aquitaine and the CNRS and the GdR MCM-2. We thank Stéphane Grolleau for thermal measurements on both materials.

REFERENCES (1) (2) (3) (4) (5) (6) (7) (8) (9) (10) (11) (12) (13) (14)

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For Table of Contents Use Only Title: Tuning the Crystal Structure Dimensionality of Cobalt(II)/1,2,4-Triazole Complexes Authors: Romain Gautier and Rodolphe Clérac

To target low dimensional cobalt triazole based materials, the deprotonation of the triazole ligand has been prevented by carrying out hydrothermal synthesis under strong acidic conditions. Two new hybrid materials with the formula Co(Htrz)Cl2 and [Co3(Htrz)6(H2O)6][ZnBr4]3•9H2O were obtained and their magnetic properties were studied.


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1,2,4-Triazole

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