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Field-Induced Slow Magnetic Relaxation and Anion/Solvent Dependent Proton Conduction in Cobalt(II) Coordination Pol-ymers PARTHA MONDAL, Bijoy Dey, Subhadip Roy, Siba Prasad Bera, Rajendar Nasani, Atanu Santra, and Sanjit Konar Cryst. Growth Des., Just Accepted Manuscript • DOI: 10.1021/acs.cgd.8b01080 • Publication Date (Web): 10 Sep 2018 Downloaded from http://pubs.acs.org on September 11, 2018

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Crystal Growth & Design

Field-Induced Slow Magnetic Relaxation and Anion/Solvent Dependent Proton Conduction in Cobalt(II) Coordination Polymers Partha Mondal,† Bijoy Dey, Subhadip Roy, Siba Prasad Bera, Rajendar Nasani, Atanu Santra, and Sanjit Konar* Department of Chemistry, Indian Institute of Science Education and Research Bhopal, Bhopal Bypass Road, Bhauri, Bhopal 462 066, India. E-mail: [email protected]; Fax: +91-755-6692392; Tel: +91-755-6692339. †

Undergraduate Researcher

Supporting Information Placeholder ABSTRACT: Three new coordination polymers (CPs), namely 2D {[Co(L)2(H2O)2](ClO4)2·3DMA.0.4H2O}n (1), 3D {[Co(L)2 (H2O)2]·(Cl)2}n (2) and 2D {[Co(L)2(NCS)2]}n (3) were obtained by the self-assembly of corresponding cobalt(II) salts and a linear semi-rigid linker bis(4-imidazol-1-yl-phenyl)diazene (L) designed with two terminal imidazole groups and an azo moiety in the middle. CPs 1 and 2 are composed of cationic framework which leads to incorporation of anion in the framework along with solvent molecules (only in 1), whereas CP 3 possesses a neutral framework. The cationic core adopts a cis and trans configuration in 1 and 2, respectively. The structures consist of 2D net with sql topology in 1, 8-fold interpenetrating 3-periodic network with dia topology in 2 and 3-fold interpenetrating 2-periodic network with sql (2,4L2) topology in 3. The structural diversity of CPs 1–3 is governed by several factors, including the different coordination ability of the anions, reaction conditions and intermolecular interactions. Using the method of molecular Voronoi polyhedral, all intermolecular interactions in CPs 1-3 have been analysed. Magnetic susceptibility measurements in the range 2-300 K reveal that in these CPs Co(II) ions behave as magnetically isolated centres with a significant orbital contribution to the magnetic moment. Alternating current (ac) measurements show signature of slow magnetic relaxation in these CPs. Ab initio investigations on simplified model structures indicate that CoII nodes in the CPs exhibit easy-plane magnetic anisotropy. Furthermore, CP 1 displays significant value of proton conductivity which reaches up to 3.96x10-4 S cm-1 at 800C and 95% relative humidity (RH).

INTRODUCTION The design and synthesis of coordination polymers are of immense interest not only because of their intriguing architectures and topologies, but also for their wide applicability in various fields such as proton conduction, gas storage and separation, molecular magnetism etc.1-13 In molecular magnetism, coordination polymers provide excellent examples to better understand some fundamental magnetic phenomena such as ferromagnetism, antiferromagnetism, ferrimagnetism, spin-canting, metamagnetic transition, etc.14-27 and their correlation with structure. Magnetism of metal-organic framework (MOF) materials containing paramagnetic metal centers, which are separated by adequately functionalized organic moieties, has emerged as a rapidly growing field of research as these materials can easily give rise to magnetic subnetworks of varied dimensions.28 In particular, SCM (single chain magnet) and SMM (single molecule magnet) based MOFs with slow magnetic relaxation have found significance in the fields of high-density magnetic memories and quantum computing devices.29-32 Following the finding of slow magnetic relaxation in a high-spin iron(II) complex,33 several 3d metal complexes have been investigated for SIM (Single-ion magnet) behaviour.34 Especially, Co(II) ion which may exhibit large magnetic anisotropy with a flexible zero-field splitting parameter depending on its coordination geometry and the distortion degree of its surroundings,35 has emerged as an promising candidate for building SIMs. Our research group has actively pursued the single-ion magnetism of Co(II) ions and reported mononuclear complexes with different (tetrahedral,36 trigonal bipyramidal,37 and square-pyramidal 38,39)

geometries, tetrahedral binuclear double-stranded helicates,40 and even coordination polymer based on octahedral Co(II) building blocks.41 The 2D coordination polymer, [Co(L)2(SCN)2·2CH3CN·2DMF]n (where L = 4′-(4methoxyphenyl)-4,2′:6′,4′′-terpyridine]) represents a particularly interesting example where the extended ligand-spacer prevents any magnetic interaction among the metal nodes thereby leading to isolated single-ion magnetic centres.41 Similar field-induced SIM behaviour has been also explored by other research groups in 1D,42-45 2D,46-53 and 3D 54-56 Co(II) based coordination polymers where the metal centres remain magnetically isolated. On the other hand, proton conducting coordination polymers have drawn great attention due to their potential use in fuel-cells.57,58 Three approaches have been mainly adopted to improve the proton conductivity of coordination polymers: 1) utilization of functionalized organic ligands (e.g., −COOH, −OH, –SO3H) to tune the hydrophilicity and acidity of the pore surface,59-61 2) encapsulation of proton carriers into the pores (e.g., NH4+, H3O+, HSO4−, H3PO4, carboxylate, triazole, imidazole molecules)62-67 and 3) the construction of ionic coordination polymers incorporating wellorganized counter ions in the porous channels.68,69 Surprisingly, it has been observed that much less attention has been paid in examining the proton conduction in cationic frameworks,70 which hold immense potential for enhanced proton conductivity due to the presence of counter anions and trapped guest molecules in the channels of the framework. Bearing the above points in mind and following our general interest in Co(II) based SIMs, we aimed to generate multi-dimensional frameworks ensuring the magnetic isolation of the paramagnetic

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Co(II) centres. Moreover, we intended to investigate the selfassembly regularity through variations in the anion to modulate the coordination microenvironment around Co(II) centres. We selected a linear semi-rigid ligand, bis(4-imidazol-1-ylphenyl)diazene (L) (Scheme 1) decorated with two terminal imidazole groups and an azo moiety in the middle which can adopt bis(monodentate)bridging coordination mode71,72 to react with different Co(II) salts. Herein, we report the syntheses, structural features and topological analysis of the following coordination polymers (CPs): 2D {[Co(L)2(H2O)2](ClO4)2·3DMA.0.4H2O}n (1), 3D {[Co(L)2(H2O)2]·(Cl)2}n (2) and 2D {[Co(L)2(NCS)2]}n (3). Magnetic measurements reveal that all the CPs display fieldsupported slow magnetic relaxation behaviour. In addition, proton conduction behaviour of CP 1 has been explored in detail.

Scheme 1. Representation of the ligand bis(4-imidazol-1ylphenyl)- diazene (L)

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in air (yield ~ 40% based on metal). Elemental analysis: Anal. Calcd for C38H28CoN14S2 (%): C, 56.78; H, 3.51; N, 24.40; Found (%): C, 56.85; H, 3.58; N, 24.46. FT-IR (KBr pellet, cm−1): 3120(m), 2077(s), 1599(s), 1515(s), 1304(s), 1057(m), 960(m), 846(m), 656(m).

Physical Measurements. The elemental analyses were carried out on Elementar Micro vario Cube elemental analyzer. FT–IR spectra (4000–400 cm–1) were recorded on KBr pellets with a Perkin Elmer Spectrum BX spectrometer. Thermogravimetric analysis (TGA) was carried out (Perkin Elmer) in the temperature range of 30–900 °C (heating rate 5 °C min –1). Powder X–ray diffraction (PXRD) data were collected on a PANalytical EMPYREAN instrument using Cu–Kα radiation. Alternating-current (AC) impedance analysis measurements were carried out with a Solartron SI 1260 impedance analyzer using the conventional quasi-four-probe method on a pellet sample (diameter of 13 mm and thickness of ≈0.85−0.9 mm). The temperature and humidity were controlled by a programmable humidification chamber (JEIOTECH, TH-PE series). Water vapor adsorption studies were performed using a BELSORP MAX (BEL JAPAN) volumetric adsorption analyzer.

Magnetic Measurements.

EXPERIMENTAL SECTION Materials. All the reagents and solvents employed were commercially available and used without further purification unless otherwise noted. bis(4-imidazol-1-yl-phenyl)diazene was synthesized according to literature method.71

Caution! Perchlorate salts of metal complexes with organic ligands are potentially explosive. Only a small amount of material should be prepared, and it should be handled with care.

Synthesis. {[Co(L)2(H2O)2](ClO4)2·3DMA.0.4H2O}n (1). A hot DMA solution (3 mL) of ligand L (50 mg, 0.16 mmol) was added to MeOH/H2O mixed solution (1:1, 2 mL) of Co(ClO4)2·6H2O (58 mg, 0.16 mmol). On slow evaporation of the filtrate at room temperature, red colored crystals were obtained. The crystals were separated and washed with methanol and air–dried (yield ~ 49% based on metal). Elemental analysis: Anal. Calcd for C48H57.8Cl2CoN15O13.4 (%): C, 48.48; H, 4.90; N, 17.67; Found (%): C, 48.56; H, 4.94; N, 17.63. FT-IR (KBr pellet, cm−1): 3404(br), 3138(m), 1603 (m), 1517(s), 1304(s), 1089(s), 1117(s), 963(m), 846(s), 623(s). {[Co(L)2(H2O)2]·(Cl)2}n (2). A solution of CoCl2·6H2O (38 mg, 0.16 mmol) in acetonitrile (3 mL) was carefully layered over an DMA solution (3 mL) of the ligand L (50 mg, 0.16 mmol) in a glass tube using a mixture of DMA and acetonitrile as a buffer layer. Orange colored single crystals were obtained from the junction of the layers after one week. The crystals were separated and washed with acetonitrile and air–dried (yield ~ 45% based on metal). Elemental analysis: Anal. Calcd for C36H32Cl2CoN12O2 (%): C, 54.42; H, 4.06; N, 21.15; Found (%): C, 54.37; H, 4.12; N, 21.11. FT-IR (KBr pellet, cm−1): 3407(br), 3110(m), 1712(s), 1602(m), 1516(s), 1303(s), 1117(m), 1063(m), 849(m), 655(m). {[Co(L)2(NCS)2]}n (3). A mixture of Co(SCN)2 (28 mg, 0.16 mmol), ligand L (50 mg, 0.16 mmol) and solvent mixture of MeOH/H2O/DMA (1:1:3, 5 mL) were stirred at room temperature for 10 min. Thereafter, the whole mixture was transferred into a 10 mL Teflon-lined stainless steel vessel and heated at 160 °C for 3 days, followed by cooling to room temperature. Orange-red colored single crystals suitable for X-ray diffraction were collected by filtration, washed with acetonitrile several times, and dried

Variable temperature (2-300 K) direct current (dc) magnetic susceptibility measurements under an applied field of 0.1 T and variable field (0-7 Tesla) magnetization measurements at low temperatures in the range of 2-10 K were performed by using a SQUID VSM magnetometer (Quantum Design).The measured values were corrected for the experimentally measured contribution of the sample holder and the derived susceptibilities were corrected for the diamagnetism of the samples, estimated from Pascal's tables.73 Variable temperature (2-10 K) alternating current (ac) magnetic susceptibility measurements under 0 Oe and 2000 Oe were carried out with SQUID VSM magnetometer. Ac and dc measurements were performed by crushing the crystals and restraining the sample in order to prevent any displacement due to its magnetic anisotropy.

Crystal Data Collection and Structure Determination. Intensity data were collected on a Brüker APEX-II CCD diffractometer using a graphite monochromated Mo-Kα radiation (λ = 0.71073 Å) at 120 K. Data collections were performed using φ and ω scan. Olex274 was used as the graphical interface and the structures were solved with the ShelXT75 structure solution program using intrinsic phasing. The models were refined with ShelXL75 with full matrix least squares minimisation on F2. All non-hydrogen atoms were refined anisotropically. In CP 1, there is minor disorder in perchlorate anions and one of the three DMA solvates, which has been treated using standard techniques. Complete lists of the applied restraints, the original diffraction file as well as the model itself are embedded in the deposited CIF files. Crystallographic data for CPs 1-3 have been summerized in Table S1. Bond lengths and angles are listed in Tables S2−S7.

Topological and Voronoi-Dirichlet Polyhedral Analysis. Topological analysis was performed with the ToposPro program package and the TTD collection of periodic network topologies.76 The RCSR three-letter codes 77 were used to designate the network topologies. Those nets that are absent in the RCSR are designated with the TOPOS NDn nomenclature,78 where N is a sequence of coordination numbers of all non-equivalent nodes of the net, D is periodicity of the net (D=M, C, L, T for 0-,1-,2-,3periodic nets), and n is the ordinal number of the net in the set of all non-isomorphic nets with the given ND sequence. The Voronoi-Dirichlet polyhedral analysis79-84 was also carried out for CPs 1-3. The analysis was performed using the Dirichlet program software package within ToposPro. Voronoi polyhedra

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were calculated for each atom of the crystal structure. The belonging of the atom to the valence-bound fragment was then determined. Bonded fragments were the metal complex structure (ligands and metal ions coordinating them), solvate molecules and counterions. Only contacts of fragments of the metal complex structure were considered. Contacts were divided into valence and nonvalence interactions.

stitched, the whole structure can also be described as 2D+2D parallel polycatenated array. Layers are oriented in [112] direction. The pattern of catenation (Figure 3c and 3d) has been characterized by topological type of extended ring net87 and feature 12,16T2 topology. In 1, the shortest intra-net Co…Co distance is 10.338 Å, while the shortest inter-net Co…Co distance is of 7.309 Å.

RESULTS AND DISCUSSION Structural and Topological Description. The compound {[Co(L)2(H2O)2](ClO4)2·3DMA.0.4H2O}n (1) is a 2D coordination polymer, the structure of which is composed of the repeating cationic [Co(L)2(H2O)2] units and perchlorate anions along with crystallization DMA and water molecules. The sixcoordinate Co(II) atom in the cationic units in 1 shows a distorted octahedral {CoN4O2} environment filled by four N- atoms of the ligand L and two water molecules (Figure 1). Out of four coordinated N-atoms, two N-atoms occupy the axial positions and two N-atoms occupy equatorial positions, while the two O atoms of water molecules occupy the equatorial positions. It is a cis-isomer. Crystal chemical formula of the complex is AB22M12, where B corresponds to L ligands and M corresponds to aqua ligands.85 The Co−N distances are in the 2.1129(19)−2.1523(19) Å range; Co−O distances are 2.1240(17) and 2.1420(17) Å. Using the Shape 2.1 program,86 the continuous shape measure value (CShM) related to the ideal octahedron (Oh symmetry) for the Co(II) centre in 1 was calculated to be 0.151.

Figure 2. (a) A fragment of the crystal structure merged with the corresponding 4-c underlying net sql. H-atoms are omitted for clarity. (b) System of H-bonding in 1. Minor disordered components and H-atoms (which do not participate into forming Hbonds) are not shown for clarity.

Figure 3. (a) A fragment of the initial crystal structure. Layers are shown by different colors. (b) A fragment of the corresponding 5c underlying net SP 2-periodic (4,4)Ia. (c,d) Different views on polycatenated array.

Figure 1. Structure of the fragment of the cationic core of CP 1. Anion and solvent molecules are not shown. Coordination polyhedral for Co(II) atom in 1 is shown in Inset. Examination of only valence bonds in the structure reveals that aqua ligands are presented by 1-c nodes and they can be removed from the underlying net. L ligands are bridging and presented by 2-c nodes, the replacement of 2-c nodes by straight edges leads to simplest underlying 2D net of sql topology in standard representation (Figure 2a). H-bonding pattern in 1 is complex in nature. One coordinated aqua ligand is H-bonded to water molecule, perchlorate anion and DMA molecule. Another one is H-bonded to two DMA molecules (Figure 2b). Considering H-bonds, it can be found that 2D layers are additionally stitched by the edge, which corresponds to the center of the mass of the outside molecules. The whole topology of this network is 5-c SP 2-periodic net (4,4)Ia (Figure 3a and 3b). Now, as layers are additionally

The 3D structure of {[Co(L)2(H2O)2]·(Cl)2}n (2) bears polymeric cationic [Co(L)2(H2O)2] units and chloride anions (Figure 4). The cationic unit in 2 is very similar to 1, except that it is a transisomer. Similar to 1, crystal chemical formula of the complex is AB22M12.85 The six-coordinate Co(II) atom in the cationic units in 2 reveals a less distorted octahedral {CoN4O2} environment (minimum CShM value 0.039) compared to 1, aqua ligands are located in axial positions and L ligands occupy equatorial plane. The Co−N and Co−O distances are ca. 2.125(2) Å and 2.130(2) Å, respectively. Likewise CP 1, in 2 aqua ligands are represented by 1-c nodes and bridging L ligands act as 2-c nodes. The resulting underlying 3D net is classified as dia in standard representation (Figure 5a). The structure consists of 8-fold interpenetrating 3periodic coordination networks (Figure 5b).



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underlying net. (c) A fragment of the underlying net with 6T4 topology in the standard representation and (d) its 6-c node.

Figure 4. Structure of the fragment of the cationic core of CP 2. Coordination polyhedral for Co(II) atom in 2 is shown in Inset.

The compound {[Co(L)2(NCS)2]}n (3) discloses a 2D metal−organic network, it is however neutral and assembled from the monocobalt(II) ions, thiocyanate ligands and L spacers (Figure 7). Likewise 1 and 2, crystal chemical formula of the complex is AB22M12, where B corresponds to inequivalent L ligands and M corresponds to thiocyanate ligands.85 3 possesses six-coordinated Co(II) centers with a distorted octahedral {CoN6} geometry (minimum CShM value 0.144). Thiocyanate ligands occupy the axial positions and L ligands are located in equatorial plane. The Co−N distances range 2.133(3)−2.173(3) Å.

Figure 5. (a) A fragment of the crystal structure and the corresponding 4-c underlying net dia. H-atoms are omitted for clarity. (b) An interpenetrated array consists of dia frameworks. Examining the pattern formed by H-bonds, it can be found that aqua ligands and chlorine atoms participate in H-bonding. It leads to the fact that frameworks are stitched by additional edges. The topology of the 6-c underlying net is 6T4 (Figure 6). In 2, the shortest intra-net and inter-net Co…Co distances are 19.477 and 8.042 Å, respectively.

Figure 7. Structure of the fragment of CP 3. Coordination polyhedral for Co(II) atom in 3 is shown in Inset.

Terminal NCS- ligands are presented by 1-c nodes and can be removed from the underlying net. L ligands are bridging and presented by 2-c nodes, the replacement of 2-c nodes by straight edges leads to simplest underlying net of sql topology. However, after removing 2-c nodes information about entanglements is lost; the straightedges of entangled nets are crossed. Therefore, for characterization of ring-catenations pattern, 2-c nodes should be kept that result in the underlying net classified by 2,4L2 topological type (Figures 8a and b). The structure consists of 3-fold interpenetrating 2-periodic coordination networks. Each ring of the net is catenated by four rings of the adjacent nets, and the pattern of catenation characterized by topological type of extended ring net87 has 8L1 topology (Figure 8c). This pattern of catenation (8L1) was previously determined for 11 3-fold interpenetrating 2D coordination polymers of sql (2,4L2) topology, and it is second among the most widespread patterns of catenation for such systems. The nearest intra-net and inter-net Co…Co distances are 18.82 and 9.066 Å, respectively.

Figure 6. (a) A fragment of the initial crystal structure. H-atoms, which do not participate into forming H-bonds are not shown for clarity. (b) A system of H-bonds forms additional edges of the


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Figure 9. The total areas of nonvalence contacts of the structures of CPs 1-3.

Figure 8. (a) A fragment of the crystal structure merged with underlying 2,4L2 nets. (b) A fragment of the interpenetrated underlying nets 2,4L2 in the standard representation. 4-c nodes correspond to Co(II) atoms, 2-c correspond to L ligands. (c) Mode of interpenetration of three 2,4L2 underlying nets in the crystal structure.

Crystal Chemical Analysis by means of VoronoiDirichlet Polyhedral. Weak intermolecular interactions are often instrumental in directing the self-assembly process in coordination polymers.88,89 Hence, intermolecular interactions in the crystal structures of CPs 1-3 have been examined in detail by the method of molecular Voronoi–Dirichlet polyhedral (VDP) which has been shown to be very effective in the analysis of intermolecular interactions in crystal structures of a variety of substances.82-84 In general, the distribution of valence contacts by their type is similar in these three CPs (Figure S1, Table S8). Due to the same set of ligands in structures of CPs 1 and 2, the total contact areas are almost identical. Contacts of H/O and O/Co disappear, and C/S contacts appear in CP 3 from the replacement of water with thiocyanate ion. Valence contacts C/C, H/C and C/N are the most probable. The difference in the areas of nonvalence contacts turned out to be more significant than for valence contacts (Figure 9, Table S9). In this case, the contacts H/H and H/C are the most probable. An increase in the H/H contact area leads to a decrease in the H/C contact area, as in CP 2. Conversely, a decrease in the H/H contact area results in an increase in the H/C contact area, as in CP 3. The presence of H…O hydrogen bonds in CP 1 and H…Cl in CP 2 is clearly visible in Figure 9. The distribution of N/H contacts in each structure is particularly unique (Figure 10). The smallest total area of N/H contacts corresponds to CP 2 and the maximum total contact area is realized in CP 3.

Figure 10. Intermolecular N/H contacts of a fragment from structures of CPs 1 (a), 2 (b) and 3 (c). The total areas of the intermolecular N-H contacts are 214.7, 200.9, 290.9 Å2, respectively. The faces of Voronoi-Dirichlet polyhedral are marked in blue for contacts N/H, and in gray for contacts H/N.

Static Magnetic (dc) measurements. Phase purity of CPs 1-3 was checked by powder XRD (Figure S2). Experimental powder XRD patterns matches well with the PXRD patterns simulated form the respective single crystal X-ray data, which indicates that the crystal structures are truly representative of the bulk material. The temperature dependence of the magnetic susceptibility for CPs 1-3 (Figure 11) was measured in the 2−300 K temperature range under an applied field of 0.1 T. The room temperature χMT products are 2.96, 2.8 and 3.04 cm3 K mol-1 for CPs 1–3 respectively. These values are much higher than expected for an isolated Co(II) high spin value for S = 3/2 of 1.875 cm3 K mol-1. This mismatch indicates significant orbital contribution and spin-orbit coupling (SOC) to the magnetic moment. Also, the values fall in the range of other reported values (2.1-3.8 cm3 K mol-1) for high spin octahedral Co(II) centers in distorted octahedral geometry.47-52,54,55 The χMT remains fairly constant up to 200 K below which it decreases to reach a mini-



mum value of 1.66, 1.48 and 1.87 cm3 mol-1 K for CPs 1–3 respectively. In all these cases, the decrease in the susceptibility below 200 K is more likely due to magnetic anisotropy or thermal depopulation of the excited states rather than antiferromagnetic interactions.54,90

Figure 11. χMT versus T plot for CPs 1-3. The dc magnetization data in the form of the M vs H plots (M being the magnetization per CoII unit and H being the applied dc magnetic field) for CP 1 is given in Figure 12. In all cases, the magnetization curves reveal a rapid and steady increase of the magnetization at 2 K without clear indication of saturation at 7 T. The non-saturation, as well as the non-superimposition of the isothermal lines in M vs H/T (Figure S3) data at higher fields, clearly indicates the presence of significant magnetic anisotropy. M vs H and M vs H/T plots for CPs 2 and 3 are given in Figure S4.

Figure 12. M vs H (a) and M vs H/T (b) plots for CP 1.

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Dynamic Magnetic Measurement Studies. Aforementioned, CoII based MOFs with larger separation between the metal centers have gained interest recently due to their SIM like behavior.42-56 To study the slow relaxation of the magnetization dynamics, temperature dependent ac susceptibility measurements were done with 3.5 Oe ac field in 2-10 K temperature range. At zero applied dc field the CPs show no peaks in the out of phase ac data which can be due to fast resonant zero-field quantum tunneling of the magnetization (QTM) through the thermal relaxation barrier between the degenerate ground ±3/2 levels, which dominates other relaxation pathways in the absence of any applied field. To suppress the quantum tunneling external dc field of 2000 Oe was applied upon which frequency dependency of in phase and out of phase ac signals were observed (Figures 13a-c and S5). There is no maximum in the temperature dependence of out of phase ac data. Above 2 K, at frequencies reaching 750 Hz the nature of out-of-phase ac signals indicated slow relaxation of magnetization for all CPs. Temperature dependency of in phase (χ′) and out of phase (χ′′) ac data for CPs 2 and 3 are presented in Figures S6 and S7. The energy barrier is determined by assuming that there is only one characteristic relaxation process with one energy barrier and one time constant, which can be extracted from the following equation (eqn 1): ln(χ′′/χ′) = ln (ωτ0) + Ueff/KT (Figure 13d).90 The linear fitting of ln(χ′′/χ′) vs 1/T allows extraction of the energy barrier (Ueff) and relaxation time (τo) which are 4.29 K and 1.1 x 10-5 s for CP 1, 4.11 K and 6.8 x 10-6 s for CP 2 and 2.6 K and 6.3 x 10-5 s for CP 3. These values are in good agreement with reported Co(II) MOF based SIMs.54 Arrhenius plot for CPs 2 and 3 are given in Figure S8.

Proton Conductivity Studies. The presence of well-organized hydrogen–bonded coordinated water molecules along with lattice perchlorate ions and DMA molecules in the channel of the framework make the CP 1 appropriate to be screened for proton conduction. Although all the CPs 1-3 were investigated for proton conduction behaviour, only CP 1 exhibited significant proton conductivity, thus confirming the initial expectations. Hence, impedance data for 1 will be described in detail. The proton conductivity of the complex was evaluated by the AC impedance measurements of pelletized sample at anhydrous as well as controlled humidity conditions and different temperatures. The thermal stability of the complex was accessed from TGA (Figure S9). The proton conductivity of sample was negligible in low humidity and room temperature. But, after humidification at 95% relative humidity (RH) for 24 hours, CP 1 shows significant conductivity. With increase of temperature at 95% RH, proton conductivity increased with maximum value of 3.96 x 10-4 S cm-1 at 80 0C (Figures 14 and S10) which is comparable to that of the high conductive hydrated metal−organic framework (MOF) material.63-65

Figure 13. Temperature dependence of out of phase (χM′′) ac data for CPs 1 (a), 2 (b) and 3 (c) at 2000 Oe applied dc field.Plot of 6 ln(χM''/χM') versus T-1 for CP 1 (d). The solid lines represent fit to the Debye model according to equation 1.

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Ab initio studies.

Figure 14. Nyquist plot of CP 1 at different temperatures and 95% RH. The enhanced conductivity with an increase in humidity (Figure 15) could be due to the improved intergrain conduction by added surface water molecules, as the samples are ground powders which have been pressed into pellets. It suggests that conductivity value increases with the increase of humidity as number of water molecules significantly rises inside the molecular structure which in turn can establish strong hydrogen bonding with the coordinated aqua ligands, perchlorate anion and solvate molecules. Absorption of water molecule in the framework was also supported by water vapour adsorption study. From adsorption isotherm, the amount of adsorbed water vapour was found to be 97.74 mL.g-1 (Figures S11). The increase in the conductivity with the temperature is certainly caused by thermal activation of the water molecules, which help with the transport of protons in the channel. Furthermore, movement of solvent molecules (DMA and H2O) also tends to increase at high temperature facilitating proton conductivity.91

To gain a deeper insight into the magnetic behavior of the CPs, ab initio calculations of zero-field splitting parameters (D and E) have been carried out by using MOLCAS 8.0 software package.9294 In all the cases, the structures have been reduced to simplified unit (the azo unit has been replaced with capped NH2 group, Figure S13). ANO-RCC basis sets have been utilized for all the calculations: Co atoms (6s5p4d2f), N (4s3p2d1f), O (4s3p2d1f), S (5s4p3s2f), C (3s2p), H (2s). The calculations employed the second order Douglas-Kroll-Hess Hamiltonian, where scalar relativistic contractions were taken into account in the basis set. Two electron cholesky decomposition approach has been adopted to reduce computation time. After carrying out CASSCF (Complete Active State Self Consistent Field) calculations, spin-orbit coupling has been introduced as implemented in the SO-RASSI (Restricted Active Space State Interaction) approach. In all the cases, the active space is composed of seven d electrons of Co(II) with 5d orbitals where all the quadruplets (10) and doublet (40) states have been considered. The computed zero-field splitting parameters have been shown in Table 1. Computed low lying spin orbit energy, g values and tensors and D contributors and tensors are presented in Tables S10-S12. Table 1. Ab initio (Molcas 8.0) computed D, |E| (in cm-1) and g values for the ground state of CPs 1-3. CP

Dcalc

|E|calc

gXX

gYY

gZZ

1

157.6

7.4

1.88

2.21

2.78

2

85.9

23.7

2.03

2.27

2.93

3

109.5

8.9

2.74

2.57

1.94

The computed large and positive D values are indicative of easyplane magnetic anisotropy of the octahedral Co(II) centres in 1-3. Moreover, it has been observed that 1D, 2D and 3D Co(II) coordination polymers reported in literature exhibiting SIM behaviour with local Oh symmetry only display an easy-plane type of magnetic anisotropy.41,47-49,51,52,54,55 In theory, large zero-splitting D parameters should contribute towards a large energy barrier for reversal of magnetization. But, we need to consider the fact that spin relaxation mechanisms also depend on lattice effects which can lead to reduction of D values and can not be captured in calculations done on single-molecule.54 Figure 15. Humidity dependent proton conduction in CP 1 at 30 ˚C. To probe and gain more insight into the proton conduction mechanism, we calculated the activation energy (Figures S12) for proton conduction of CP 1 from the Arrhenius equation,

σ σ exp (2)

[σ = conductivity and k = Boltzmann constant], which was found to be Ea = 0.23 eV. It is well-known from the literature that low activation energies (

Solvent

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