MnII and CoII Coordination Polymers Showing Field-Dependent

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MnII and CoII Coordination Polymers Showing FieldDependent Magnetism and Slow Magnetic Relaxation Behavior SOUMYABRATA GOSWAMI, Gregory Leitus, Bharat Kumar Tripuramallu, and Israel Goldberg Cryst. Growth Des., Just Accepted Manuscript • DOI: 10.1021/acs.cgd.7b00696 • Publication Date (Web): 17 Jul 2017 Downloaded from http://pubs.acs.org on July 19, 2017

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Crystal Growth & Design

MnII and CoII Coordination Polymers Showing Field-Dependent Magnetism and Slow Magnetic Relaxation Behavior Soumyabrata Goswami,† Gregory Leitus,‡ Bharat Kumar Tripuramallu,† and Israel Goldberg*† †

School of Chemistry, Sackler Faculty of Exact Sciences, Tel-Aviv University, Ramat-Aviv, 69978 TelAviv, Israel ‡ Department of Chemical Research Support, Weizmann Institute of Science, Rehovot 76100, Israel

E-mail addresses: [email protected]; [email protected].

Abstract: Four new MnII and CoII-containing magnetic coordination polymers: [{Mn(Br-isa)(bpe)·½H2O}n (1), {Co(Br-isa)(bpe)1.5·½H2O}n (2), [{Mn(Br-isa)(4-bpmh)}4·6H2O]n (3) and [{Co(Br-isa)(4bpmh)}2·2½H2O]n (4)] [isa = isophthalic acid, bpe = 1,2-bis-(4-pyridyl)ethylene and 4-bpmh = N,N’-bispyridine-4-yl-methylene-hydrazine] have been synthesized at room temperature, using 5-bromo isophthalic acid (Br-H2isa) and two different N-donating ancillary ligands. The complexes

have

been

characterized

by

single-crystal

X-ray

diffraction

and

other

physicochemical techniques. Structure determination reveals two dimensional (2D) coordination network architectures for all the complexes. In 1, 3 and 4, MnII and CoII dinuclear units are connected via Br-H2isa ligands to form infinite 1D chains. The ancillary N, N'-donor spacer ligands interconnect the 1D chains into 2D coordination layers. Complex 2, on the other hand, can be viewed as being composed of cationic [{Co(bpe)}4]8+ square units that are joined by anionic Br-isa2- bridges into a 2D grid-like framework. Topology analysis shows sql/Shubnikov tetragonal plane net topology for complexes 1, 3 and 4, and SP 2-periodic net (4, 4) Ia topology for complex 2. Complexes 1 and 3 show field-dependent change in magnetic behavior which is confirmed from the susceptibility measurements at varying fields, field-dependent magnetization measurements, as well as from hysteresis data. Complex 2 exhibits slow magnetization relaxation phenomenon manifested by the AC susceptibility measurements at different temperatures and frequencies. Finally, complex 4 exhibits a magnetic feature that can be interpreted as antiferromagnetic exchange interactions between two syn-syn carboxylate-bridged CoII atoms. 1 ACS Paragon Plus Environment


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Introduction: Metal-organic coordination polymers (CPs) have gained continuing attention in the field of crystal engineering and supramolecular chemistry. Such compounds show diverse and readily tailored structures, and could be suitable for a wide range of potential applications. Their use in heterogeneous catalysis, gas storage, gas separation and sensing has been well documented.1-8 Another area where the CPs have drawn considerable interest is in the field of molecular magnetism.9-17 The magnetic properties of the CPs can be fine-tuned by suitable modification of their chemical composition (varying the paramagnetic metal or metal cluster nodes and of the organic linkers). This in turn affects the coordination environments around the metal centers, and of the magnetic exchange pathways.14-20

Aromatic multicarboxylate ligands are perfect

coordinating reagents to metal center(s) (while accounting simultaneously for charge balance) and are widely employed in the synthesis of multi-dimensional CPs. They are capable of transmitting magnetic interactions between the paramagnetic ions.9,21-23 The inclusion of ancillary N-donor ligands along with the carboxylate ligands is an interesting modification of the synthetic strategy. The coordination complementarity or competition between carboxylate and N-donating ligands can significantly diversify the structural and magnetic features of the resulting CPs.24,25 The nature of the paramagnetic metal ions play also a vital role in regulating the magnetic behaviors in CPs. In this respect, the isotropic MnII ions and the highly anisotropic CoII ions (with large residual orbital contribution) impart interesting magnetic properties to the CPs.26-32 We report in this work on four new MnII and CoII-based CPs that have been synthesized using 5-bromoisophthalic acid (Br-H2isa) as bicarboxylic acid along with N, N’-bipyridyl derivatives as the ancillary ligands (Scheme 1). The Br-H2isa ligand, with 120° angle between two carboxylic groups, has been used earlier as a useful building block in generating attractive polymeric architectures.33-38 Moreover, the electron-withdrawing Br-substituent may take part in secondary hydrogen or halogen bonding to reinforce the supramolecular structure that forms. The bipyridyl linkers introduce an additional binding dimension to the complexes. The CPs reported

in

this

isa)(bpe)1.5·½H2O]

paper n

(2),

are

formulated

as

[Mn(Br-isa)(bpe)·½H2O]n

[{Mn(Br-isa)(4-bpmh)}4·6H2O]n

(3)

and

(1),

[Co(Br-

[{Co(Br-isa)(4-

bpmh)}2)·2½H2O]n (4) and they all display two dimensional (2D) coordination features. The structures have been determined by X-ray diffraction techniques (SCXRD and PXRD), and 2 ACS Paragon Plus Environment

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further characterized by elemental analysis, FT-IR spectroscopy and thermogravimetric analysis (TGA). Magnetic measurements reveal field-dependent magnetic behaviour for complexes 1 and 3, slow magnetic relaxation phenomenon in complex 2 and antiferromagnetic behaviour for complex 4.

Scheme 1. Schematic representation for the synthesis of complexes 1-4.

Experimental Section (a) Materials and reagents: All the reagents, MnCl2·4H2O, CoCl2·6H2O, 5-bromoisophthalic acid (H2Br-isa), 1,2-bis(4-pyridyl) ethylene (bpe) and solvents were commercially available and were used as obtained. Ligand 4-bpmh (N,N’-bispyridine-4-yl-methylene-hydrazine) was synthesized according to the reported procedure39 using 4-pyridine carboxaldehyde and hydrazine monohydrate.

(b) Syntheses: (i) Synthesis of [Mn(Br-isa)(bpe)·½H2O]n (1): 5 mL aqueous solution of Na2(Br-isa) (0.1 mmol, 0.028 g) was gradually mixed with 5 mL acetonitrile solution of bpe (0.1 mmol, 0.0182 g) ligand under continuous stirring and the resulting solution was further stirred for 15-20 minutes to mix uniformly. MnCl2·4H2O (0.1 mmol, 0.0217 g) was separately dissolved in 10 mL of acetonitrile. Then 2 mL of this metal solution was slowly and carefully layered over 2 mL of the mixed ligand solution keeping 2 mL of a 1:1 mixture of water and acetonitrile as a third middle layer to slow down the diffusion, in narrow glass tubes. The tubes were left undisturbed. After few days, yellow colored, rod-shaped crystals suitable for X-ray diffraction were obtained on the walls of the tubes. The crystals were collected and washed with cold CH3CN and air-dried (Yield 3 ACS Paragon Plus Environment


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75% based on Mn). Elemental analysis (for the dehydrated sample): Calcd. (%): C 50.03, H 2.73, N 5.83; Found (%): C 50.12, H 2.64, N 5.91; FT-IR (cm-1): 3434 (m), 3059 (w), 3023 (s), 2928 (w), 1605 (s), 1543 (s), 1527 (m), 1457 (s), 1408 (s), 1382 (m), 1277 (m), 1201 (m), 1185 (m), 1128 (w), 1053 (m), 997 (w), 974 (w), 903 (s), 845 (w), 736 (m), 682 (w), 550 (m), 519 (w) (Figure S1).

(ii) Synthesis of [Co(Br-isa)(bpe)1.5·½H2O]n (2): Complex 2 was obtained by the same procedure as that for the preparation of compound 1 except for using CoCl2·6H2O (0.1 mmol, 0.0237 g) instead of MnCl2·4H2O. Pink rhombus-shaped crystals suitable for X-ray diffraction were formed on the walls of the test tube after a few days. These were collected and washed with cold CH3CN and air-dried (Yield 85% based on Co). Elemental analysis (for the dehydrated sample): Calcd. (%): C 54.28, H 3.15, N 7.30; Found (%): C 54.39, H 3.03, N 7.21; FT-IR (cm1

): 3430 (m), 3055 (w), 3027 (s), 2919 (w), 1610 (s), 1548 (s), 1530 (m), 1461 (s), 1410 (s), 1382

(m), 1275 (m), 1199 (m), 1150 (m), 1050 (m), 1000 (w), 975 (w), 900 (s), 850 (w), 732 (m), 680 (w), 556 (m), 521 (w) (Figure S1).

(iii) Synthesis of [{Mn(Br-isa)(4-bpmh)}4·6H2O]n (3): Complex 3 was obtained by the same procedure as that for the preparation of compound 1 except for using the ligand 4-bpmh (0.1 mmol, 0.0210 g) instead of bpe. Yellow rod-like crystals were obtained after few days, which were collected and washed with cold CH3CN and air-dried (Yield 72% based on Mn). Elemental analysis (for the dehydrated sample): Calcd. (%): C 46.85, H 2.65, N 10.93; Found (%): C 46.97, H 2.71, N 11.01; FT-IR (cm-1): 3439 (m), 3061 (w), 2932 (w), 1625 (m), 1610 (s), 1549 (s), 1452 (s), 1412 (s), 1381 (m), 1320 (m), 1280 (m), 1245 (m), 1187 (m), 1130 (w), 1051 (m), 1000 (w), 973 (w), 950 (m), 839 (w), 815 (m), 740 (m), 678 (w), 650 (m), 525 (w) (Figure S1).

(iv) Synthesis of [{Co(Br-isa)(4-bpmh)}2·2½H2O]n (4): Complex 4 was obtained by the same procedure as that for the preparation of compound 3 except for using CoCl2·6H2O (0.1 mmol, 0.0237 g) instead of MnCl2·4H2O. Pink rhombus-shaped crystals, formed on the walls of the test tube, were collected and washed with cold CH3CN and air-dried (Yield 80% based on Co). Elemental analysis (for the dehydrated sample): Calcd. (%): C 46.90, H 2.56, N 10.94; Found (%): C 46.93, H 2.65, N 10.99; FT-IR (cm-1): 3440 (m), 3063 (w), 2930 (w), 1622 (m), 1615 (s), 4 ACS Paragon Plus Environment

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1550 (s), 1460 (s), 1412 (s), 1378 (m), 1318 (m), 1283 (m), 1244 (m), 1190 (m), 1130 (w), 1057 (m), , 985 (m), 948 (m), 828 (m), 738 (m), 682 (w), 644 (m), 515 (w) (Figure S1).

(c) Physical measurements: Elemental analysis (% C, H, N) was performed with a Thermo Flash EA-1112 Elemental analyzer. Fourier-transform (FT)-IR spectra were recorded on a Bruker Tensor 27 system spectrophotometer in ATR mode. Powder X-ray diffraction data were recorded on Bruker D8 Advance diffractometer using CuKα radiation (λ = 1.54056 Å) over a 2θ range of 5-50° at a scan rate of 1° min-1.

Single crystal X-ray diffraction measurement: The X-ray measurements [Bruker-ApexDuo diffractometer, IµS microfocus MoKα radiation] were carried out at ca. 110 (2) K on crystals coated with a thin layer of amorphous oil. These structures were solved by direct and Fourier methods and refined by full-matrix least-squares (using standard crystallographic software: SHELXT-2014, SHELXL-2014).40-42 The TOPOS software package43, 44 was used to analyse the topological features of the coordination polymers. The integrity of the crystalline products 1-3 was confirmed in each case by repeated measurements of the unit-cell dimensions from different single crystallites, as well as by matching between the simulated and experimental powder X-ray diffraction patterns (Figure S2). Structures 1-4 were found to contain severely disordered crystallization solvent (water) within the intra-lattice voids, which could not be modeled by discrete atoms. Correspondingly, the contribution of the disordered solvent moieties was subtracted from the diffraction pattern by the Squeeze procedure and PLATON software.42 The crystal data of the analysed structures (the solvent content was assessed from the number of the Squeeze-excluded electrons, in combination with weight loss data from the TGA experiments) are presented below: (1) C20H13BrMnN2O4·½H2O, Mr = 489.18, triclinic, space group P-1 (No. 2), a = 8.9588(19), b = 10.284(2), c = 11.6806(19) Å, α = 79.607(11)°, β = 83.347(5)°, γ = 75.262(11)°, V = 1020.9(3) Å3, T = 110 K, Z = 2, µ (Mo Kα) = 2.63 mm−1, ρ (calcd.) = 1.591 g·cm−3, 8858 reflections measured to θ = 24.92°, of which 3489 were unique (Rint = 0.049) and 2558 with I > 2σ (I). Final R1 = 0.091 (wR2 = 0.230) for the 2558 data above the intensity threshold, and R1 = 0.117 (wR2 = 0.242) for all unique data. CCDC 1550457. 5 ACS Paragon Plus Environment


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(2) C26H18BrCoN3O4·½H2O, Mr = 584.27, triclinic, space group P-1 (No. 2), a = 9.8951(3), b = 10.8297(3), c = 13.6404(4) Å, α = 108.815(2)°, β = 92.487(1)°, γ = 112.137(1)°, V = 1258.74(7) Å3, T = 110 K, Z = 2, µ (Mo Kα) = 2.31 mm−1, ρ (calcd.) = 1.542 g·cm−3, 23588 reflections measured to θ = 28.61°, of which 6366 were unique (Rint = 0.031) and 5272 with I > 2σ (I). Final R1 = 0.045 (wR2 = 0.118) for the 5272 data above the intensity threshold, and R1 = 0.056 (wR2 = 0.124) for all unique data. CCDC 1550458. (3) 2(C40H26Br2Mn2N8O8·6H2O, Mr = 2158.88, triclinic, space group P-1 (No. 2), a = 10.1644(5), b = 15.8403(10), c = 33.7485(16) Å, α = 101.567(3)°, β = 94.610(3)°, γ = 95.153(3)°, V = 5274.9(5) Å3, T = 110 K, Z = 2, µ (Mo Kα) = 2.05 mm−1, ρ (calcd.) = 1.359 g·cm−3, 46163 reflections measured to θ = 25.21°, of which 18739 were unique (Rint = 0.051) and 12993 with I > 2σ (I). Final R1 = 0.082 (wR2 = 0.192) for the 12993 data above the intensity threshold, and R1 = 0.117 (wR2 = 0.209) for all unique data. CCDC 1550459. (4) C40H26Br2Co2N8O8·2½H2O, Mr = 1069.41, triclinic, space group P-1 (No. 2), a = 10.0929(8), b = 15.6022(11), c = 17.0492(13) Å, α = 68.281(3)°, β = 75.743(3)°, γ = 84.468(3)°, V = 2417.3(3) Å3, T = 110 K, Z = 2, µ (Mo Kα) = 2.40 mm−1, ρ (calcd.) = 1.468 g·cm−3, 25194 reflections measured to θ = 25.12°, of which 8538 were unique (Rint = 0.037) and 6725 with I > 2σ (I). Final R1 = 0.059 (wR2 = 0.142) for the 6725 data above the intensity threshold, and R1 = 0.077 (wR2 = 0.148) for all unique data. CCDC 1550460.

Magnetic measurements: Temperature and field dependent magnetic moments of the powdered samples were measured using SQUID magnetometer MPMS3 (LOT-Quantum Design inc.). The samples were measured using the VSM (vibrating sample magnetometry) mode. The magnetic moment (M) was normalized to mole using the respective chemical formulae of the complexes. For complexes 1 and 3, the dependence of the magnetic moment on temperature in the range of 2K ≤ T ≤ 300K was analyzed by applying different magnetic fields of H = 0.02, 0.5 and 6 T. Both zero field cooled (ZFC) and field cooled (FC) modes were applied. Magnetic-hysteresis data (M vs. H) were measured at temperatures of T = 2, 3, 5 and 300 K. During the measurements H was varied at the interval -7T ≤ H ≤ +7T. In addition to DC, AC measurements at different temperatures and frequencies were performed for complex 2. The derived

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susceptibilities were corrected for the diamagnetism of the samples, estimated from Pascal’s tables.45

Results and Discussion: Structural description of [Mn(Br-isa)(bpe)·½H2O]n (1): Complex 1 crystallizes in triclinic P-1 space group with one MnII ion, one Br-isa ligand, one bpe linker (in anti-conformation) and half molecule of the disordered water solvent, constituting the asymmetric unit (Figure 1a). MnII center is ligated by four O atoms from three different Br-isa2ligands in the equatorial position and two N atoms from two trans-positioned bpe ligands at axial positions (Figure 1b) with N1-Mn1-N2 angle of 174.4 (4)°. Analysis of the coordination geometries around the MnII center, using the SHAPE 2.1 program,43 showed that the ion resides in a distorted octahedral environment (Oh symmetry) (Figure S3a) with a CShM (continuous shape measure) value of 3.950 (detail geometric analysis is described in Table S1). The Mn−N bond lengths are in the range of 2.239(9) - 2.268(9) Å while the Mn−O bond lengths are in the range of 2.120(7) - 2.356(7) Å. Two adjacent MnII centers are bridged by two carboxyl groups from two Br-isa ligands forming a dinuclear [Mn2(CO2)4N4] building unit, which resembles a two-blade paddlewheel (Figure 1b) with Mn···Mn distance of 4.22(3) Å. These dinuclear paddlewheel building units are then connected to each other in one dimension (along crystallographic b-axis) via other Br-isa ligands in (κ2)-(κ1-κ1)-µ3 coordination mode (Figure S3b), to generate an infinitely extended 1D chain (Figure 1c). The 1D Mn2(Br-isa) chains are then cross linked via auxiliary N-donating bpe linkers to ultimately yield a neutral 2D Mn-organic network (Figure 1d). Structural analysis shows that the separations between two adjacent [Mn2(CO2)4N4] units (internode separation) is 10.28 Å along the Br-isa ligands and 13.87 Å along the bpe linkers. The bpe ligand exhibits a slightly twisted conformation with a dihedral angle of 20.93° between the planes of the two pyridyl rings (Figure S4), which results in the formation of an undulated 2D layer (Figure S5). The layers finally form 3D supramolecular architecture through interlayer C-H···Br hydrogen bonding interactions (Figure S6) with C-H···Br distances of 3.79 Å (Table S2). From the viewpoint of network topology each Mn dinuclear unit can be considered as a 4connected node, whereas the Br-isa ligand and the dinuclear unit-bound N-donating pillar 7 ACS Paragon Plus Environment


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ligands can both be viewed as linear linkers (Figure 2a). Consequently, the entire structure of 1 consists of parallel flat layers that can be simplified as 4-connected uninodal nets with sql/Shubnikov tetragonal plane net topology having point symbol 44.62 (Figure 2b).

Figure 1. (a) Asymmetric unit of complex 1; (b) Representation of dinuclear paddlewheel building unit; (c) 1D chain formed of Mn-dimers connected by Br-isa ligands; (d) 2D network formed of 1D Mn2(Br-isa) chains and bpe linkers. Colour Codes: grey C, red O, blue N, dark grey H, cyan Mn and olive-green Br. (The disordered water solvent has been omitted).


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Figure 2. (a) View of the Mn dinuclear unit as a 4-connected node; (b) Representation of sql/Shubnikov tetragonal plane net topology in complex 1.

Structural description of [Co(Br-isa)(bpe)1.5·½H2O]n (2): Compound 2 crystallizes in the triclinic P-1 space group with the asymmetric unit made up of one CoII ion, one Br-isa ligand, one full bpe linker (in anti-conformation) half bpe linker and half disordered water species (Figure 3a). The Co atom binds to three O-atoms from two Br-isa2ligand and three N atoms from three different bpe linkers (Figure 3b). Analysis of the coordination geometry around the CoII center by using the SHAPE 2.1 program reveals that it can best be described as distorted octahedron (Oh symmetry) (Figure S7a) with a CShM value of 4.923 (detail geometric analyses are described in Table S1). The Co−N bond lengths are in the range of 2.151(2) - 2.166(2) Å while the Co−O bond lengths are in the range of 2.069(2) 2.257(2) Å. The Co···Co distance bridged by Br-isa and bpe ligands are 9.89 Å and 13.73 Å, respectively.



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Figure 3. (a) Asymmetric unit of complex 2; (b) View of coordination environment around the Co atom; (c) Co(Br-isa)(bpe) 1D chain viewed as an infinite array of metallocycles extending along the a-direction in 2. (The disordered water solvent has been omitted). The diprotic Br-isa2- ligand connects two Co atoms in κ2-κ1-µ2 coordination fashion (Figure S7b) and gives rise to a Co(Br-isa) chain; two such chains are subsequently connected by the bpe linker to form a wider Co(Br-isa)(bpe) 1D chain, which can be viewed as an infinite array of metallocycles extending along the a-direction (Figure 3c). Each metallocycle is formed of four Co atoms, two Br-isa ligands and two bpe linkers (non-twisted), with dimensions of 9.89 x 13.73 Å. Interestingly, the Co(Br-isa)(bpe) metallocycle chains are parallel-stacked on top of each other and are further cross-linked via other bpe linkers of length 13.64 Å along the cdirection of the crystal (Figure 4a). These bpe linkers remain in a twisted geometry with a dihedral angle of 66.06° between two pyridyl rings. Therefore, unlike 1, the bpe linkers can be


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found here in two forms, those that form the metallocycle are non-twisted and the others that inter-link between the metallocycles are twisted. Moreover, the dihedral angle has also appreciably increased (from 20.93° in 1 to 66.06° in 2) on replacing the Mn metal center by Co. Structure 2 can be viewed also as composed of cationic [{Co(bpe)}4]8+ square units that are joined by anionic Br-isa2- bridges (Figure 4b), to yield 1D 'nanotubes' that extend along the crystallographic a-axis. The overall architecture is then a 2D grid-like network in the ac-plane made up of these 1D nanotubes. The networks contain voids of cubic shape of approximate (atom-to-atom) dimensions of 9.89 x 13.64 x 13.73 Å (Figure S8). This type of nanotubular structure constructed by covalent linkage of molecular squares is very rare and imparts uniqueness to this complex. Another fascinating structural feature of 2 is that each nanotube, which is aligned parallel to the a-axis, is interlocked in a parallel fashion (along the b-axis) with two nearest neighbours giving rise to a 2D polycatenated layer (Figure 4c). This kind of entanglement is also quite uncommon as most of the known examples are based exclusively on infinite 1D molecular ladders.47-49



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Figure 4. (a) Representation of Co(Br-isa)(bpe) metallocycle chains stacked on top of each other and cross linked via other bpe linkers; (b) Cationic [{Co(bpe)}4]8+ square units joined by anionic Br-isa2- bridges to give 1D nanotubes; (c) View of parallel polycatenation of three adjacent networks; (d) Representation of 5-connected SP 2-periodic net (4, 4) Ia topology for complex 2.

Topological analysis reveals that each Co center represents a 5-connected node and the Br-isa and bpe ligands act as linear linkers. Therefore, 2 exhibits a 2D-layered network with a uninodal 5-connected SP 2-periodic net (4, 4) Ia topology (Figure 4d) with point symbol 48.62. In an attempt to understand the stabilization factors for the interpenetrated networks, supra-molecular interactions were analyzed in detail and presented in Figure S9. It is observed that the nets are involved in C–H···O interactions comprising the uncoordinated carboxylate oxygen of the Brisa2- ligand with the pyridyl hydrogen atoms of the bpe ligand (Table S3). The Br atom from the Br-isa2- moiety also forms intermolecular C–H···Br interactions with the pyridyl hydrogen atoms with C-H···Br distances of 3.793 Å. In addition to these intermolecular H-bonding interactions, 12 ACS Paragon Plus Environment

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π···π interactions between the phenyl rings of the Br-isa and ethylene group of the bpe linker with c.g···c.g perpendicular distance of 3.373 Å are observed.

Structural description of [{Mn(Br-isa)(4-bpmh)}4·6H2O]n (3): Complex 3 also crystallizes in triclinic P-1 space group with the asymmetric unit consisting of four MnII centers which are crystallographically independent, four Br-isa ligand, four 4-bpmh ligands and about five molecules of the water solvent (only one water could be located reliably; Figure 5a). The Mn centers are coordinated to four O atoms from three different Br-isa2- ligands in the equatorial position and to two N atoms from two 4-bpmh ligands in the axial positions. Coordination geometry around the MnII centers has been calculated using the SHAPE 2.1 program that shows distorted octahedron coordination environment (Oh symmetry) with CShM values of 4.206, 4.499, 3.373 and 4.948 (Table S1). There are two crystallographically independent Mn dimers in the structure, one containing Mn (1) and Mn (4), and the other containing Mn (2) and Mn (3). The two Mn centers in each dimer are bridged by two carboxylate groups from the Br-isa2- ligands in syn-syn mode with Mn···Mn distance of ~4.0 Å. The 4-bpmh ligands coordinated to Mn1 and Mn4 remains almost in an eclipsed position (Figure 5b). On the other hand, unlike the Mn1-Mn4 dimer, the 4-bpmh ligands bound to Mn2 and Mn3 remain in opposite conformations (Figure 5b). These 4-bpmh ligands also display a slightly twisted form with a dihedral angle of 20.59° between the planes of two pyridyl rings. The Mn dimers are interconnected via other Br-isa2- ligands in (κ2)-(κ1-κ1)-µ3 coordination mode to form an infinitely extended 1D chain along the crystallographic a-axis. The 1D Mn2(Br-isa) chains are further linked by 4-bpmh linkers to generate neutral 2D Mn-organic networks (Figure 5c). Due to the presence of two independent Mn dimers and different conformations of the 4-bpmh linker ligand, two kinds of 2D networks are observed that are arranged alternately in the packing diagram (Figure S10). Separations between two adjacent Mn dimers are 10.164 Å along the Brisa ligands and 15.840 Å along the 4-bpmh linkers.



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Figure 5. (a) Asymmetric unit of complex 3 (the four disordered water molecules are excluded); (b) Two crystallographically independent Mn dimers and the different conformations of the 4bpmh linkers attached to them; (c) 2D network formed of 1D Mn2(Br-isa) chains and 4-bpmh linkers; (d) Representation of sql/Shubnikov tetragonal plane net topology for complex 3. Topological analysis of 3 was simplified by assuming that the Mn-dimer represents a 4connected node, while the Br-isa moiety and the N-donating (metal-bound) pillar ligands behave as linear linkers. Consequently, the overall structure consists of parallel plane layers that can be considered as 4-connected uninodal nets, with sql/Shubnikov tetragonal plane net topology (Figure 5d) with point symbol 44.62. Further characteristics of the crystal structure involve intricate intra- and inter-net H-bonding interactions as well as π···π stacking interactions that impart stability to the 3D structure (Figure S11). Inter-net and intra-net C-H···O interactions are formed by carboxylate oxygen atoms of the Br-isa ligand with the pyridyl hydrogen atoms of the 14 ACS Paragon Plus Environment

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4-bpmh ligand, with C-H···O distances in the range of 2.85 – 3.47 Å (Table S4). The Br atom from the Br-isa moiety (Br1) also forms strong C–H···Br interactions with one of the pyridyl Hatoms (C-H···Br = 3.029 Å). In addition, π···π interaction operate between phenyl rings of the Br-isa ligands of two neighboring nets (c.g···c.g distances of ~4.0 Å).

Structural description of [{Co(Br-isa)(4-bpmh)}2·2½H2O]n (4): The asymmetric unit of complex 4 (space group P-1) consists of two CoII ions, two Br-isa ligands, two 4-bpmh linkers (in anti-conformation) and 2½ molecules of the water solvent (Figure 6a). Each of the CoII centers are coordinated by four carboxylate O atoms from three different Br-isa2- ligands and two N atoms from two different 4-bpmh linkers. Analysis of the coordination geometry around the CoII center (SHAPE 2.1 program) showed that the ion resides in a distorted octahedral environment (Oh symmetry) with a CShM value of 2.530 (Table S1). As in 1 and 3, one carboxylate group from each of two deprotonated Br-isa ligands bridges between two Co centers, forming a [Co2(CO2)2] dimer (Co···Co distance is 4.151 Å). The latter is connected to a nearby dimer via another carboxylate group of the Br-isa ligand in (κ2)-(κ1-κ1)-µ3 bridging mode. This generates an infinite Co2(Br-isa) 1D chain along the crystallographic a-axis (Figure 6b). The 1D chains are then inter-connected via 4-bpmh linkers through the Co centers forming a 2D Co-organic network in the ab-plane (Figure 6c). The dihedral angle between the planes of two pyridyl rings of the 4-bpmh ligand is markedly reduced to 11.06° with respect to the value observed in complex 3. The separation between two adjacent dimers is 10.093 Å along the Br-isa ligand, and 15.602 Å along the 4-bpmh linkers. When viewed down the a-axis, the 2D layers are arranged in a very compact manner revealing weak intra- and inter-layer supramolecular interactions (Figure S12). Short C-H···O contacts are formed between carboxylate O atoms of the Br-isa ligand with the pyridyl H atoms of the 4-bpmh linkers as well as the H-atoms of phenyl rings of neighboring Br-isa ligands (Table S5). Then, C-H···π interactions are observed between H-atoms of phenyl rings of a Br-isa ligand with the C=N of the 4-bpmh linkers of adjacent layers. From the topological point of view, in complex 4 the Co-dimers act as 4-connected nodes and the Br-isa and 4-bpmh ligands serve as 2-connected linkers. The overall structure consists of



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parallel plane layers that can be simplified as 4-connected uninodal nets with sql/Shubnikov tetragonal plane net topology (Figure 6d) having symbol 44.62.

Figure 6. (a) Asymmetric unit of complex 4 (the disordered water species are omitted); (b) 1D chain formed of Co-dimers connected by Br-isa ligands; (c) 2D network formed of 1D Co2(Brisa) chains and 4-bpmh linkers; (d) Representation of sql/Shubnikov tetragonal plane net topology for complex 4. Br-isophthalic acid along with different types of auxiliary ligands have been previously used by different groups to prepare a variety of coordination polymers.33-38 However, the present crystal structures reveal marked differences as compared to previous literature reports in various aspects such as in the binding modes of the carboxylate group, observed coordination environments around the metal centers, topological features, supramolecular interactions, 16 ACS Paragon Plus Environment

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dimensionality etc. For example when Br-H2isa and bpp (bpp = 1,3-bi(4-pyridyl)propane) were used as the starting material by Han et al., {[Mn3(H2O)2(H-bpp)2(Br-isa)4]·H2O}n was obtained.50 The complex is made of linear Mn3 clusters in which Mn centers are found to be both five and six coordinated residing in MnO4N and MnO6 environments respectively. This is different from the current observations in 1-4. The Br-H2isa ligand shows two binding modes: one is µ4-bridge to link four Mn atoms and the other is µ2-bridge to link two Mn atoms. The bpp ligand assumes a unidentate coordination mode and the non-coordinated N atom is protonated and acts as the H-bond donor. The overall structure is a 1D coordination polymer that is linked by extensive H-bonds to afford a 2D layer. In contrast, complexes 1-4 reveal two dimensional (2D) coordination networks. Another example is {Zn(Br-isa)(bix)}n 51 [bix = 1,4-bis(imidazol-1ylmethyl)benzene], where Zn shows a distorted tetrahedral geometry and Br-isa2- and bix ligands link the Zn centers into a 2D corrugated sheet, unlike any of the complexes 1-4. The corrugated sheets polycatenate each other in a parallel manner yielding a 2D to 3D polycatenated net. Of further interest are the complexes [Ni(Br-isa)(bib)]n and [Co(Br-isa)(bip)]n

[bib = 1,4-

bis(imidazol)butane); bip = 1,3-bis(imidazol) propane],52 reported by Ma et al. The former complex shows a 2-fold interpenetrating 3D network based on the dinuclear Ni units, featuring a binodal (3,5)-connected net with a point symbol of (42·6)(42·65·83). The latter complex is a 3D chiral coordination framework characterized as a 4-connected net with (65·8) topology. Br-isa2here acts as a bis-monodentate bridging ligand to link Co centers to form a 1D right-handed helical chain. Both these complexes exhibit significantly different metal coordination, dimensionality and topology features with respect to those observed in 1-4. Evidently, the backbone of the organic N-donor ligands and the nature of the metal ions govern to large extent the resulting architecture of the metal–organic coordination polymers.

PXRD and thermal stability analysis: In order to check the phase purity of the bulk samples, powder X-ray diffraction (PXRD) analysis was carried out for complexes 1-4. As shown in Figure S2, all of the peaks displayed in the experimental patterns closely match those in the simulated patterns generated from singlecrystal diffraction data, which indicates that the analyzed compounds represent a single phase. To evaluate the thermal stabilities of compounds 1-4, thermogravimetric analysis (TGA) was carried out in the temperature range of 30-800 °C (Figure S13) under N2 flow with a heating rate 17 ACS Paragon Plus Environment


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of 10 °C min-1. Complex 1 shows a weight loss of ~1.81 wt % in the range of 55-110 °C that corresponds to the loss of one H2O molecule per unit-cell (calcd. 1.81 wt%). The desolvated framework begins to collapse from ~350 °C. For complex 2, the weight loss is ~1.50 wt % in the temperature range 50-105 °C. This also corresponds to the removal of guest H2O molecule per unit-cell (calcd. 1.52 wt%). The decomposition of the desolvated framework takes place in steps in a temperature range of 310-750 °C. In complex 3, the observed weight loss is ~4.84 % in the 50-100 °C range, corresponding to loss of six lattice H2O molecules per formula-unit (calcd. 5.00 wt%). The desolvated framework of 3 collapses in a stepwise manner within 300-750°C. The weight loss in complex 4 is ~3.83 % which corresponds to nearly 2½ guest H2O molecules/formula-unit (calcd. 4.21 wt%). After loss of the lattice solvents, the framework starts collapsing from about 310 °C in a stepwise fashion. For compounds 1, 2 and 4, the above observations match reasonably well the solvent content assessments from the residual electron count, with the disordered solvent excluded from the structural model (by the Squeeze procedure in PLATON software; see above).42 In 3, the residual diffraction data indicate considerably larger solvent content (the calculated solvent accessible voids in this structure account for more than one third of the crystal volume), possibly due to lattice-inclusion of the less volatile acetonitrile species as well.

Magnetic properties studies: Given the unpaired spins of the MnII and CoII ions, variable temperature (2–300 K) and variable field direct current (DC) magnetic measurements were carried out for all the complexes 1-4. For complex 2 the magnetization dynamics was investigated as well by alternating current (AC) susceptibility measurements as a function of temperature and frequency. Complexes 1 and 3 display similar magnetic behaviours as illustrated in Figure 7. The room-temperature χMT values for 1 and 3 are 4.31 and 4.16 cm3 mol-1 K respectively (Figure 7a) (χM = molar magnetic susceptibility), which are comparable to the spin-only value (4.38 cm3 mol-1 K) for a high spin MnII ion in an octahedral environment (S = 5/2, g = 2). With lowering of temperature, χMT initially decreases gradually up to around 30 K, below which it decreases sharply. This decline in the χMT value from 300 K is due to an antiferromagnetic interaction between two MnII ions in the dimer. The χMT vs. T plots are fitted according to Curie-Weiss law [χMT = C/(T - θ)] and the best 18 ACS Paragon Plus Environment

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fitting yield Curie constant C = 4.56 cm3 mol-1 K and Weiss temperature θ = -7.80 K for 1 and C = 4.26 cm3 mol-1 K and θ = -6.66 K for 3.

(a)

(b)

(c)



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Figure 7. (a) Plot of χMT vs. T for complexes 1 and 3 in the temperature range of 2–300 K; the red line indicates fitting according to Curie-Weiss law and the blue line using Equation (1). Inset: χM vs. T plots at different applied magnetic fields; (b) Isothermal magnetization curves at different low temperatures for 1 and 3; (c) Hysteresis curves for 1 and 3 at low temperature (2 K). Inset: First derivative plots (dM/dH) at the same temperature (2 K). The negative value of θ signifies antiferromagnetic interaction between the MnII ions bridged by carboxylate moiety. Magnitude of the magnetic exchange interaction is estimated quantitatively by considering a dinuclear MnII unit model since it is the building unit of the structures in the two complexes, 1 and 3. Thus, the χMT vs T plots are fitted according to the equation (1) evaluated from the dimer model of the Heisenberg-Dirac-van Vleck spin Hamiltonian H = –JS1S2.53 χM = 2Ng2β2/kT·(A/B)…………(1), where, A = e2x + 5e6x + 14e12x + 30e20x + 55e30x, B = 1 + 3e2x + 5e6x + 7e12x + 9e20x + 11e30x, g is Landé factor, x = J/kT and J is exchange integral. The fitting parameters are J = -0.715 cm-1 and g = 1.97 for 1 and J = -0.431 cm-1 and g = 1.95 for 3. The small negative J values indicate weak antiferromagnetic super-exchange interactions between the MnII ions mediated through carboxylate (O-C-O) bridges. However, the χM vs. T plots taken at different magnetic fields, reveal some interesting features (insets of Figure 7a) such as at lower fields of 0.02 T and 0.5 T distinct peaks are noticed in the plots at very low temperature (~ 5 K) that eventually disappear at higher applied magnetic fields. Moreover, the isothermal magnetization curves (M vs. H plots, Figure 7b) show distinct sigmoidal shapes at T = 2 K and 3 K, but at 5 K the sigmoidal nature of the curves disappeared. These features in the susceptibility and magnetization plots indicate that at higher applied magnetic fields (above 0.5 T), the weak antiferromagnetic interactions between two MnII centers in the dinuclear cluster is probably overcome by some other type of magnetic interaction between the Mn ions. The latter could be e.g. an inter-dimer Mn-Mn interaction that disrupts the intra-dimer Mn-Mn spin orientations at the higher magnetic fields, causing spin flips in the antiferromagnetic cascade of Mn-Mn dimers. The field dependent isothermal magnetization plots show that at 2 K and 6 T, the magnetization values obtained for 1 and 3 are 5.05 and 4.95 NµB respectively, that are consistent with the magnetization value for a single MnII ion (S = 5/2). The hysteresis measurement has also been performed (Figure 7c) and it shows distinct steps in the curves which are much prominently observed in the first derivative (dM/dH) 20 ACS Paragon Plus Environment

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plots (inset of Figure 7c). The steps also suggest additional magnetic exchange interactions at higher fields that overcome the antiferromagnetic interactions at lower fields. The magnetic behaviour of complex 2 in the form of χMT vs. T plot (Figure 8a) shows a room temperature χMT value of 3.50 cm3 mol-1 K, which is higher than the spin-only value (1.875 cm3 mol-1 K) for a magnetically isolated high-spin CoII ion in an octahedral environment. This is due to orbital contribution typical for the 4T1g ground state of octahedral high-spin CoII ion.54-56 Upon cooling from 300 K, the χMT value declines slowly up to around 75 K, below which it decreases in a relatively rapid manner and reaches a value of 2.08 cm3 mol-1 K at 1.8 K. In the region 10-20 K, a small plateau-like feature is observed in the plot. Since in the complex, the Co···Co interactions are almost negligible owing to large Co···Co distances, therefore the decrease in the value of χMT with temperature is mainly attributed to the spin-orbit coupling (SOC) effect of CoII ions in an octahedral ligand field.45, 57-59 The small plateau-like feature can be due to change in the SOC effect possibly caused by slight distortions in the coordination environment around the Co-ions at the low-temperature region.60

(a)

(b)

Figure 8. (a) χMT vs. T plot measured at 0.5 T for complex 2. The red line is the best fit according to equation 2; Inset: Magnetization (M/NµB vs. H) plots at different low temperatures. The red lines are the best fit according to equation (2); (b) M/NµB vs. H/T plots at the indicated temperatures.



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The presence of isolated CoII centers in the complex, provoked us to analyse the SOC effects on the magnetic susceptibility, for which the following Hamiltonian in equation (2) was employed:32, 45, 61 H = -αλ(LCoSCo) + ∆[Lz,Co2 - 1/3L(L + 1)] + µBH(-αLCo + gSCo)……………(2), where λ corresponds to the SOC parameter, α is the orbital reduction factor defined as α = −Aκ [κ represents the reduction of the orbital momentum owing to the delocalization of unpaired electrons and A stands for the admixture of the upper 4T1g state (4P) into the ground 4T1g state (4F)] and ∆ is the energy gap between 4A2 and 4E levels arising from the axial orbital splitting of the 4T1g ground state due to an axial distortion of the ideal Oh symmetry of CoII ion. Using the PHI program61 the best-fitting results obtained over the whole temperature range are: λ = -239 cm-1, α = 7.74, ∆ = 293 cm-1 and g = 2.31. The large positive value of ∆ gives an indication that only the lowest two Kramers doublets of the 4A2 ground term are thermally populated such that the energy gap between them corresponds to the axial zero field splitting (ZFS) within the quartet state. Hence, the magnetic properties have been investigated further, using the following spin Hamiltonian in equation 3 to get an insight into the ZFS parameters. H = D[Sz2 – S(S + 1)/3] + E(Sx2 – Sy2) + gµBS×B…………(3), where, D and E represents the single-ion axial and rhombic ZFS parameters, S is the spin operator, and B is the magnetic field vector. From the field dependent isothermal magnetization data (Figure 8a inset), the highest value of 2.55 NµB is obtained at 2 K and 7 T, that is well below the theoretical saturation value for an S = 3/2 system (Msat = 3.3 NµB for g = 2.2). This observation, in addition to the feature of non-superimposition of the isothermal magnetization plots in the M/NµB versus H/T plots (Figure 8b) indicates the presence of significant magnetic anisotropy in the system. The PHI program was used to determine the anisotropy parameters of the CoII center by simultaneous fitting of the susceptibility and magnetization curves. The best fits of the magnetization data give D = 54.7 cm-1, E = 0.9×10-3 cm-1 and g = 2.30. The magnetization dynamics is also investigated for complex 2 by AC susceptibility measurements at various temperatures and frequencies at 3.5 Oe AC field and in absence and presence of DC fields. It is observed that at zero DC field both the in-phase (χM’) and out-ofphase (χM”) susceptibilities show frequency dependence within a temperature range of 1-14 K; 22 ACS Paragon Plus Environment

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however, the in-phase signals (Figure S14) are relatively weaker as compared to the out-of-phase signals (Figure 9a). This frequency dependency of the AC susceptibility plots indicates the occurrence of slow relaxation of magnetization, a characteristic feature of the single molecule magnets (SMMs). Plots of χM” vs χM’ known as the Cole - Cole plots (Figure 9b inset) are obtained from the frequency-dependent AC susceptibility data. These plots are then fitted using the generalized Debye model63,64 to determine α, a parameter that represents the width of distribution of magnetization relaxation time (τ). The obtained α value of 0.74-0.82 indicates a narrow distribution of the relaxation time for 2. Moreover, at zero Oe DC field distinct peaks are observed in the χM” versus T plots, which helps in the determination of the effective energy barrier (Ueff) and relaxation time (τ0) for the magnetic relaxation phenomenon, according to the Arrhenius equation (4), ln(1/τ) = ln(1/τ0) – Ueff/kBT ………(4) where kB represents Boltzmann constant and 1/τ0 is the pre-exponential factor. The best linear fit according to the Arrhenius equation yields Ueff = 61.6 K and τ0 = 1.06 × 10-6 s that is comparable to those of other CoII SMMs reported previously.65-70 The τ0 value observed is relatively higher, giving a hint of the probable existence of direct and Raman spin-lattice relaxation processes in the complex.

(a)

(b)

Figure 9. (a) Temperature dependence of χM″ signals for complex 2 under zero Oe DC field; (b) Arrhenius fitting plot; inset: Cole-Cole plots. Solid lines represent the best fits.



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The magnetic behavior of complex 4 has been investigated as well, and the results are illustrated in Figure 10. The measured χMT value at 300 K equals 5.04 cm3 mol-1 K, which is larger than the spin only value of 3.75 cm3 mol-1 K for two uncoupled spins (g = 2.0 and S = 3/2). This is due to the orbital contribution of the high spin CoII ion in an octahedral coordination environment. Upon lowering the temperature, χMT value first decreases gradually up to ~75 K and then sharply to reach a minimum value of 1.87 cm3 mol-1 K at 1.8 K. This feature can be attributed jointly to the antiferromagnetic exchange interactions between two syn-syn carboxylate bridged CoII atoms as well as the spin-orbit coupling effect of the CoII ions. In the case of carboxylate bridged CoII ions forming an isolated spin dimer system, the magnetic susceptibility data can be fitted according to the following equations 5a and 5b, based on the spin Hamiltonian H = λL·S 71 χM' = (2Ng2β2 / kT) [3 + exp(-25J/9kT)]-1 …………(5a) χM = χM' / [1 − (2zJ' / Ng2β2)χM' ] …….....(5b), where, zJ' represents the inter-dimer interaction. Using the PHI program, the best fit was obtained with g = 2.37, J = -1.56 cm-1 and zJ' = -0.002 cm-1. The small negative J and zJ' values confirm a weak antiferromagnetic interaction between the Co centers in the dimer and also weak inter-dimer interaction respectively in the complex. In order to further investigate the magnetic behavior of 4, the field dependence of the magnetization has been performed at 2, 3 and 5 K in the field range of 0-7 T (Figure 10b). M/NµB values increased gradually until 7 T and the maximum value obtained at 2 K and 7 T is 4.0 NµB. This value can be rationalized by considering that at the low temperature studied, the ions behave as effective spin doublets, i.e. Seff = ½, which might then have geff = 4. Subsequently, an uncoupled pair of such ions would have a saturation magnetisation of ca. 4 NµB.


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(a)

(b)

Figure 10. (a) χMT vs. T plot measured at 0.5 T for complex 4. The red line is the best fit according to equation 5; (b) Magnetization (M/NµB vs. H) plots at different low temperatures.

Conclusion The self-assembly reactions of bent diprotic ligand 5-bromo isophthalic acid (Br-H2isa) with pyridyl N, N'-donor ligands and paramagnetic MnII, CoII cations have yielded four new coordination polymers with interesting magnetic properties. Structure determination reveals two dimensional (2D) network architectures for all the complexes. Topology analysis shows sql/Shubnikov tetragonal plane net topology for complexes 1, 3 and 4 and SP 2-periodic net (4, 4) Ia topology for complex 2. All the CPs show good thermal stability at elevated temperature. The magnetic behaviours of all the complexes have been examined in detail. For complexes 1 and 3 it is observed that the weak antiferromagnetic interactions between two MnII centers in the dinuclear unit occurring at lower fields is superseded by other type of magnetic interaction such as inter-dimer interaction at higher magnetic fields. Complex 2 shows a slow magnetization relaxation behaviour ascribed mainly to the strong magnetic anisotropy of the uncoupled CoII ions and this is evidenced from the AC susceptibility measurements at various temperatures and frequencies. The magnetic susceptibility data of 4 with Co2 units in the structure shows an antiferromagnetic coupling within the Co2 units. The results of this work, therefore, demonstrate the value of mixed ligand approach and the paramagnetic metal ions in the development of coordination polymers with attractive structural and magnetic features and should inspire further study in this research field. 25 ACS Paragon Plus Environment


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Associated Content Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/ xxxx Additional illustrations of molecular structures and coordination patterns, hydrogen bonding diagrams and tables, FT-IR spectra, PXRD patterns, TGA diagrams and magnetic plots. Accession Codes CCDC 1550457-1550460 contain the supplementary crystallographic data for this paper.

Author Information ORCID Soumyabrata Goswami: 0000-0002-9646-1439

Israel Goldberg: 0000-0002-3117-0534

Acknowledgments The authors gratefully acknowledge financial support from the Israel Science Foundation (Grant No. 108/12).

References: (1) Lee, J.; Farha, O. K.; Roberts, J.; Scheidt, K. A.; Nguyen, S. T.; Hupp, J. T. Chem. Soc. Rev. 2009, 38, 1450–1459. (2) Corma, A.; Garcia, H.; Llabrés, F. X.; Xamena, I. Chem. Rev. 2010, 110, 4606–4655. (3) Goswami, S.; Jena, H. S.; Konar, S. Inorg. Chem. 2014, 53, 7071–7073. (4) Murray, L. J.; Dinca, M.; Long, J. R. Chem. Soc. Rev. 2009, 38, 1294–1314; (5) Li, J. R.; Kuppler, R. J.; Zhou, H. -C. Chem. Soc. Rev. 2009, 38, 1477–1504; (6) Creno, L. E.; Leong, K.; Farha, O. K.; Allendrof, M.; Van Duyne, R. P.; Hupp, J. T. Chem. Rev. 2012, 112, 1105–1125.


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For Table of Contents Use Only MnII and CoII Coordination Polymers Showing Field-Dependent Magnetism and Slow Magnetic Relaxation Behavior Soumyabrata Goswami,† Gregory Leitus,‡ Bharat Kumar Tripuramallu,† and Israel Goldberg*†

Four new MnII and CoII based magnetic coordination polymers (CPs) have been synthesized based on diprotic carboxylate and pyridyl N, N′-donor spacer ligands. The compounds have been fully characterized by analytical and spectroscopic methods. In-detail magnetic property studies have been performed that show a field dependent magnetic behaviour for complexes 1 and 3; slow magnetization relaxation behaviour for complex 2 and antiferromagnetic behaviour for 4.


MnII and CoII Coordination Polymers Showing Field-Dependent

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