Novel manganese(II) and cobalt(II) 2D polymers containing alternating chains with mixed azide and carboxylate bridges: crystal structure and magnetic properties

Abstract

Two coordination polymers formulated as [Mn₃(L¹) (N₃)₆(H₂O)₄] (1) and [Co₃(L²) (N₃)₆(H₂O)₅] (2) (L¹ = 1,1′-bis(4-carboxylatobenzyl)-4,4′-bipyridinium, L² = 1,2,4,5-tetrakis(4-carboxylatopyridinium-1-methylene)benzene) were synthesized and structurally and magnetically characterized. Both compounds 1 and 2 contain alternating chains constructed by azide and carboxylate bridges. The independent sets of bridges alternate in an A1A1BA1A1 sequence between adjacent Mn(II) ions in 1: [(EO-N₃)(OCO)] double bridges (EO = end-on) (denoted as A1) and (EO-N₃)₂ double bridges (denoted as B) and the alternation mode is in an A2A2BA2A2 sequence between adjacent Co(II) ions in 2: [(EO-N₃)(OCO)₂] triple bridges (denoted as A2) and (EO-N₃)₂ double bridges (denoted as B). The alternating chains are interlinked into 2D coordination layers respectively by the L¹ and L² ligands. Magnetic studies demonstrate that the [(EO-N₃)(OCO)] and (EO-N₃)₂ double bridges mediate antiferromagnetic coupling between Mn(II) ions in 1 while the [(EO-N₃)(OCO)₂] triple and (EO-N₃)₂ double bridges mediate ferromagnetic coupling between Co(II) ions in 2.

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1. Introduction

Molecular magnetic systems in which paramagnetic metal ions are bridged by short bridges have been intensively studied for decades to uncover the fundamental relationship between structures and magnetic behaviours and to obtain new magnetic materials with potential applications.¹ In this context, the azide ion (N₃⁻) has been used as the short bridges due to its diversity of coordination modes and magnetic mediating, and a great number of azide-bridged discrete polynuclear and infinite polymeric systems with different structural motifs and magnetic properties have been demonstrated,^2–4 including some long-range magnetic ordered materials and a few single molecule/-chain magnets (SMMs and SCMs).^2,4,5

An effective synthetic approach to obtaining new compounds with interesting magnetic properties is to construct such systems in which the paramagnetic metal ions are bridged by simultaneous azide and carboxylate bridges.^6–11 In this context, we have recently demonstrated that zwitterionic carboxylates as the selected ligands are a good synthetic approach to get the mixed azide- and carboxylate-bridged systems.^9–12 Using this approach, various magnetic systems based on mixed bridge polynuclear clusters,^9,10b Infinite chains and 2D layers,^10a,11a,b and even 3D frameworks have been obtained,^11c,d showing a variety of interesting magnetic behaviours, such as ferromagnetic (FM) coupling,^10a,11a,d topological ferrimagnetism,^11b single-chain magnetism,^11a,12a,d solvent-modulated magnetism^11f,12a and mixed metal single-chain magnetism.^12b,c As part of our systematic studies on mixed azide and carboxylate systems, here we report the syntheses, structure and magnetic properties of two 2D coordination polymers constructed by manganese(II)/cobalt(II), azide and two zwitterionic dicarboxylate/tetracarboxylate ligands, 1,1′-bis(4-carboxylatobenzyl)-4,4′-bipyridinium (L¹) and 1,2,4,5-tetrakis(4-carboxylatopyridinium-1-methylene)benzene (L²) (Scheme 1). Compounds 1 and 2 contain 2D layers respectively based on Mn(II) chains in which two different bridges, (EO-N₃)(OCO) (denoted as A1) and (EO-N₃)₂ (denoted as B), alternate in the A1A1BA1A1 sequence in 1, Co(II) chains in which two different bridges, (EO-N₃)(OCO)₂ (denoted as A2) and (EO-N₃)₂ (denoted as B), alternate in the A2A2BA2A2 sequence in 2. The magnetic studies of 1 show that the double (EO-N₃)(OCO) and double (EO-N₃)₂ bridges (Mn–N7–Mn = 96.1(4)° < 98°) mediate antiferromagnetic (AF) coupling while the triple (EO-N₃)(OCO)₂ and double (EO-N₃)₂ (Co–N4–Co = 99.7(6)° > 98°) bridges mediate FM coupling.

Scheme 1 1,1′-Bis(4-carboxylatobenzyl)-4,4′-bipyridinium (top, L¹); 1,2,4,5-tetrakis(4-carboxylatopyridinium-1-methylene)benzene (bottom, L²).

2. Experimental section

2.1 General procedure

The reagents and the organic ligand L¹ (1,1′-bis(4-carboxylatobenzyl)-4,4′-bipyridinium) were obtained from commercial sources and used without further purification. The method used for preparing 1,2,4,5-tetrakis(4-carboxylatopyridinium-1-methylene) benzene tetrahydrobromide ([H₄L]Br₄) is similar to that for (1,4-bis(4-carboxylatepyridinium-1-methylene)-benzene).¹³

CAUTION! Although not encountered in our experiments, azido compounds of metal ions are potentially explosive. Only a small amount of the materials should be prepared, and it should be handled with care.

2.2 Synthesis of [Mn₃(L¹)(N₃)₆(H₂O)₄] (1)

[H₂L¹]Cl₂ (0.0248 g, 0.05 mmol) and sodium azide (0.13 g, 2 mmol) were dissolved into a mixture of methanol (4 mL) and water (4 mL), and then the solution was added into the methanol solution (3 mL) of Mn(OAc)₂·4H₂O (0.1 mmol, 0.0245 g). The mixture was stirred for a few minutes and filtered off. Slow evaporation of the filtrate at room temperature afforded yellow crystals of 1 after two days. The crystals were collected by filtration, washed by water and methanol, and dried in air. Yield: 65% based on L¹. Elem anal. calcd (%) for C₂₆H₂₈Mn₃N₂₀O₈: C, 34.19; H, 3.09; N, 30.67%. Found: C, 34.53; H, 2.74; N, 30.24%. Main IR bands (KBr, cm⁻¹): 2078s [ν(N₃)], 2052s [ν(N₃)], 1639s [ν_as(COO)], 1596s, 1546s, 1446m, 1398s [ν_s(COO)], 1335m, 1301w, 1161m, 772s, 643m.

2.3 Synthesis of [Co₃(L²)(N₃)₆(H₂O)₅] (2)

Co(NO₃)₂·6H₂O (0.1164 g, 0.4 mmol), [H₄L²]Br₄ (0.051 g, 0.1 mmol) and sodium azide (0.065 g, 1.0 mmol) were dissolved in the mixture solvent of methanol and water (4 mL/2 mL) and transferred to a 23 mL Teflon-lined autoclave. After being stirred in air for 20 min, the mixture was heated to 80 °C at the speed of 20 °C h⁻¹ and kept for 3 days. After cooling to room temperature at the speed of 5 °C h⁻¹, red crystals of 2 were collected in a 60% yield based on L². Elem anal. calcd (%) for C₃₄H₃₆Co₃N₂₂O₁₃: C, 35.90; H, 3.19; N, 27.09%. Found: C, 35.58; H, 3.52; N, 26.75%. IR bands (KBr, cm⁻¹): 2075s [ν(N₃)], 2043s [ν(N₃)], 1626s [ν_as(COO)], 1565m, 1452m, 1384s [ν_s(COO)], 1022w, 789w.

2.4 Physical measurements

Elemental analyses were determined on an Elementar Vario ELIII analyzer. The Fourier transform infrared (FTIR) spectra (KBr disk) were recorded in the range 500–4000 cm⁻¹ using KBr pellets on a TENSOR 27 FI-IR spectrophotometer. Powder X-ray diffraction pattern (PXRD) was carried out on a EMPYREAN PANALYTICAL apparatus. Magnetic measurements were performed on a Quantum Design MPMS XL7 SQUID magnetometer.

2.5 Crystal data collection and refinement

Diffraction intensity data were collected at 293 K on a Bruker APEX II diffractometer equipped with a CCD area detector and graphite-monochromated Cu Kα radiation (λ = 1.54184 Å). Empirical absorption corrections were applied using the SADABS program.¹⁴ The structures were solved by the direct method and refined by the full-matrix least-squares method on F², with all non-hydrogen atoms refined with anisotropic thermal parameters.¹⁵ All the hydrogen atoms attached to carbon atoms were placed in calculated positions and refined using the riding model. The hydrogens attached to water molecules were located from the difference Fourier maps and refined isotropically. Crystallographic data for 1 and 2 have been deposited at the Cambridge Crystallographic Data Center with the deposition number of CCDC 1470719 and 1470720. A summary of the crystallographic data, data collection, and refinement parameters for complexes 1 and 2 is provided in Table 1. The selected bond lengths and angles are given in Tables 2 and 3.

Table 1 Crystal data and structure refinements for compounds 1 and 2

Compound	1	2
Empirical formula	C26H28Mn3N20O8	C34H36Co3N22O13
Formula weight	913.5	1137.64
Crystal system	Monoclinic	Triclinic
Space group	P21/c	P1̄
a, Å	11.438(2)	10.0434(13)
b, Å	16.436(3)	11.0274(13)
c, Å	10.711(2)	11.2225(13)
α, °	90	71.812(10)
β, °	115.22(3)	76.599(11)
γ, °	90	72.559(11)
V, Å^3	1821.6(7)	1113.4(2)
Z	2	1
ρcalcd, g cm^−3	1.665	1.697
μ, mm^−1	9.008	9.434
Unique reflections	3243	3921
Rint	0.1018	0.1051
S on F^2	0.999	0.936
R1, wR2 [I > 2σ(I)]	0.1134, 0.2664	0.1247, 0.2780
R1, wR2 (all data)	0.1936, 0.3664	0.2374, 0.3991

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Table 2 Selected bond lengths (Å) and angles (°) for compound 1^a

1
a Symmetry codes: A: 2 − x, −y, 1 − z; B: 2 − x, −y, 2 − z; C: 1 − x, 1 − y, 1 − z.
Mn1–O3A	2.167(9)	Mn1–N4	2.183(9)
Mn1–O1	2.195(8)	Mn1–N7B	2.237(11)
Mn1–N7	2.277(9)	Mn1–O2	2.304(9)
Mn2–O4	2.134(7)	Mn2–N1	2.255(12)
Mn2–N4	2.265(10)	N7–Mn1B	2.237(11)
O3–Mn1A	2.167(9)
O3A–Mn1–N4	96.9(3)	O3A–Mn1–O1	100.5(3)
N4–Mn1–O1	89.7(4)	O3A–Mn1–N7B	88.1(4)
N4–Mn1–N7B	98.9(4)	O1–Mn1–N7B	167.0(3)
O3A–Mn1–N7	87.5(3)	N4–Mn1–N7	174.9(4)
O1–Mn1–N7	86.8(3)	N7B–Mn1–N7	83.9(4)
O3A–Mn1–O2	167.7(3)	N4–Mn1–O2	90.9(3)
O1–Mn1–O2	89.0(3)	N7B–Mn1–O2	81.3(3)
N7–Mn1–O2	85.2(3)	O4–Mn2–O4A	180.0(4)
O4–Mn2–N1A	87.5(4)	O4–Mn2–N4	92.0(3)
O4–Mn2–N1	92.5(4)	N1A–Mn2–N4	89.8(4)
N1A–Mn2–N1	180.0(2)	O4–Mn2–N4A	88.2(3)
O4A–Mn2–N4	88.2(3)	N1A–Mn2–N4A	90.6(4)
N1–Mn2–N4	90.2(4)	N4–Mn2–N4A	180.0(1)
N1–Mn2–N4A	89.4(4)	Mn1B–N7–Mn1	96.1(4)
Mn1–N4–Mn2	116.7(5)

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Table 3 Selected bond lengths (Å) and angles (°) for compound 2^a

2
a Symmetry codes: A: −x, 1 − y, 3 − z; B: 1 − x, −y, 2 − z; C: −1 + x, 1 + y, 1 + z; D: −1 − x, 1 − y, 3 − z.
Co1–N1	2.093(11)	Co1–N4	2.078(12)
Co1–O4C	2.095(8)	Co1–N7	2.094(13)
Co1–O2	2.184(10)	Co1–N4D	2.223(12)
Co2–O3B	2.082(9)	Co2–O1	2.016(9)
Co2–N7	2.135(12)	Co1–N4–Co1D	99.7(6)
N1–Co1–N4	94.9(5)	N1–Co1–O4C	179.4(4)
N4–Co1–O4C	85.0(4)	N1–Co1–N7	92.6(5)
N4–Co1–N7	168.8(5)	O4C–Co1–N7	87.4(4)
N1–Co1–O2	83.2(4)	N4–Co1–O2	94.0(4)
O4C–Co1–O2	97.4(3)	N7–Co1–O2	95.1(4)
N1–Co1–N4D	94.7(4)	N4–Co1–N4D	80.3(5)
O4C–Co1–N4D	84.6(4)	N7–Co1–N4D	90.9(4)
O2–Co1–N4D	173.8(4)	O1A–Co2–O1	180.0(2)
O1A–Co2–O3B	92.4(4)	O1–Co2–O3B	87.8(4)
O1A–Co2–O3	87.6(4)	O1–Co2–O3C	92.4(4)
O3B–Co2–O3C	180.0(2)	O1A–Co2–N7A	92.1(4)
O1–Co2–N7A	87.9(4)	O3B–Co2–N7A	92.7(4)
O3C–Co2–N7A	87.3(4)	O1A–Co2–N7	87.9(4)
O1–Co2–N7	92.1(4)	O3B–Co2–N7	87.3(4)
O3C–Co2–N7	92.7(4)	N7A–Co2–N7	180.0(5)

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3. Results and discussion

3.1 Crystal structure

Compound 1. The structure of 1 was determined by single crystal X-ray analyses. It is in the monoclinic P2₁/c space group and exhibits 2D coordination network based on the chain motifs with alternating bridges (Fig. 1). The selected bond distances and angles are listed in Table 2. The coordination environments of the metal ions are shown in Fig. 1a. There are two crystallographically independent Mn(II) ions (Mn1 and Mn2) in 1. Mn1 assumes a distorted mer-octahedral [N₃O₃] geometry defined by three azide atoms (N4A, N7A and N7B), two water molecules (O1A and O2A) and one carboxylate oxygen atom (O3). Mn2 adopts the centrosymmetric trans-octahedral [N₄O₂] geometry with four azide nitrogen atoms (N1 and N1A, N4 and N4A) at the equatorial plane and two equivalent oxygen atoms (O4 and O4A) at axial positions. The Mn–N/O bond distances for both Mn1 and Mn2 fall in the range of 2.134(7)–2.304(9) Å. There are two kinds of bridging moieties between neighbouring Mn(II) ions. One is the (OCO)(EO-N₃) double bridges (A1) between Mn1 and Mn2 with Mn–N4–Mn = 116.7(5)° and Mn⋯Mn = 3.787(2) Å, and the other is the centrosymmetric (EO-N₃)₂ double bridges (B) between two Mn1 ions, with Mn–N7–Mn = 96.1(4)° and Mn⋯Mn = 3.358(3) Å. The Mn–N, Mn–N–Mn and Mn⋯Mn parameters are comparable to those which have been observed in a few Mn(II) compounds.^3e,6c,11b,d The metal ions are linked by the bridges alternating in an A1–A1–B–A1–A1 sequence to give a 1D chain along the a direction (Fig. 1b).

Fig. 1 (a) Local coordination environments of the Mn²⁺ centers in 1 with the atom labelling scheme. The thermal ellipsoids were drawn at 30% probability. Symmetry codes: (A) 2 − x, −y, 1 − z; (B) 2 − x, −y, 2 − z; (C) 1 − x, 1 − y, 1 − z. (b) 1D alternate chain along the a direction in 1. (c) 2D network formed by the L¹ ligands connecting the chains.

The L¹ ligand in the structure is centrosymmetric and the two benzene rings from two benzylbenzoic acid locate on both sides of the plane formed by two pyridine rings of 4,4′-bipyridine, the equal dihedral angle being 110.559(1)°. Each ligand L¹ serves as a μ₄ bridge between two different chains, with two carboxylate groups in the bidentate bridging mode. Thus, a 2D layer is formed extending along the (002) plane (Fig. 1c). The nearest interchain Mn⋯Mn distances within the layer is 20.024(3) Å.

Compound 2. X-ray analyses revealed that compound 2 also exhibits 2D coordination networks based on the chain motifs with alternating bridges which is somewhat similar with that in 1 (Fig. 2). The selected bond distances and angles are listed in Table 3. There are two crystallographically independent Co(II) ions in 2. The Co2 ion is located at an inversion center and assumes a trans-octahedral [N₂O₄] coordination geometry completed by four equatorial carboxylate oxygen atoms (O1, O1A, O3B and O3C, Co–O = 2.016(9) and 2.082(9) Å) and two axial azide nitrogen atoms (N7 and N7A, Co–N = 2.135(12) Å). Co1 is ligated by four azide ions (N1, N4, N4D and N7) and two carboxylate oxygen atoms (O2 and O4C) in a [N₄O₂] cis-octahedral geometry with Co–N bond lengths ranging from 2.078(12) to 2.223(12) Å, Co–O bond lengths ranging from 2.095(8) to 2.184(10) Å. There are also two kinds of bridging moieties between neighbouring Co(II) ions. One is the (OCO)₂(EO-N₃) triple bridges (A2) between Co1 and Co2 with Co–N7–Co = 114.5(6)° and Co⋯Co = 3.556(2) Å, and the other is the centrosymmetric (EO-N₃)₂ double bridges (B) between two Co1 ions, with Co–N4–Co = 99.7(6)° and Co⋯Co = 3.290(3) Å. The metal ions are linked by the bridges alternating in an A2A2BA2A2 sequence to give a 1D chain along the a direction (Fig. 2b).

Fig. 2 (a) Local coordination environments of the Co²⁺ centers in 2 with the atom labelling scheme. The thermal ellipsoids were drawn at 30% probability. Symmetry codes: (A) −x, 1 − y, 3 − z; (B) 1 − x, −y, 2 − z; (C) −1 + x, 1 + y, 1 + z; (D) −1 − x, 1 − y, 3 − z. (b) 1D alternate chain along the a direction in 1. (c) 2D network formed by the L² ligands connecting the chains.

The L² ligand in the structure of 2 is noncentrosymmetric and the four pyridinium rings are nearly perpendicular to the benzene ring and the dihedral angles being 109.489(1)°–109.503(1)°. Each ligand L² serves as a μ₆ bridge between two neighbouring chains, with all four carboxylate groups in the bidentate bridging mode and thus the chains are interlinked by the L² ligands into 2D layer along the bc plane (Fig. 2c). The nearest interchain Co⋯Co distances within the layer is 15.812(4) Å.

3.2 Magnetic properties

Compound 1. The magnetic susceptibility (χ) of 1 was measured on a crystalline sample under 1 kOe in the temperature range of 2–300 K (Fig. 3). The χT value at 300 K is about 12.74 emu K mol⁻¹, slightly lower than the spin-only value (13.13 emu K mol⁻¹) for three uncoupled Mn(II) ions with g = 2.00. As the temperature is lowered, the χT value decreases continuously, while the χ value first increases to a narrow plateau (0.26 emu mol⁻¹) at about 12.2 K, and finally shows a slight rise to 0.33 emu mol⁻¹ at 2 K. The data above 28 K follow the Curie–Weiss law with C = 14.05 emu K mol⁻¹ and θ = −30.83 K, indicating AF coupling between Mn(II) centers. The slight rise of χ below 7.6 K can be attributed to the presence of a small amount of paramagnetic impurities.

Fig. 3 Temperature dependence of χ and χT for 1. The solid lines represent the fits to the model for alternating chains (see the text).

According to the structure data, the system can be magnetically treated as chains with alternating (OCO)(EO-N₃) double bridges and (EO-N₃)₂ double bridges, as illustrated in Scheme 2. The interactions through the long organic ligands are negligible. The magnetic susceptibility of such chains in the classical-spin approximation with alternating J₁–J₁–J₂ interactions has been proposed as eqn (1):¹⁶

χ = [Ng²β²S²/(3kT)][(3 + 4u₁ + 2u₂ + 4u₁u₂ + 2u₁² + 3u₁²u₂)/(1 − u₁²u₂)²](1 − ρ) + [3/2S(S + 1)/T]ρ (1)

where u_i = coth[J_iS(S + 1)/kT] − kT/[J_iS(S + 1)] (i = 1, 2; S = 5/2).

Scheme 2 The alternating bridges and the coupling scheme in 1.

The magnetic susceptibility data in the whole temperature range 2–300 K were fitted by the above expression and the best fits led to J₁ = −2.61(1) cm⁻¹, J₂ = −2.26(1) cm⁻¹ and ρ = 0.016 with g fixed at 2.00. The ρ parameter representing the molar fraction of the paramagnetic impurity (assumed to be three monoclear Mn(II) species, χ_impurity = 3 × 4.375/T) has been included to account for the observed rise in χ below 7.6 K. The parameters suggest that the interactions through the double (OCO)(EO-N₃) and (EO-N₃)₂ bridges are both AF. The (OCO)(EO-N₃) double bridges have been found in a few Mn(II) compounds with similar Mn–N–Mn angles (∼120°) and similar interaction parameters (−1.5 to −4.8 cm⁻¹) reported elsewhere by some of us^11b,d and others.^3e,6c For the EO azide bridge, it is believed that the nature of the magnetic coupling is dependent on the Mn–N–Mn bridging angle (θ). Density function theory (DFT) calculations on Mn(II) systems suggested that the coupling is ferromagnetic for θ > 98°,¹⁷ as observed in a number of Mn(II) species containing the double EO azide bridges with θ = 100–105°.^2a,3d,18 However, the Mn–N–Mn bridging angle (Mn1–N7–Mn1) is 96.1(4)° (<98°) in 1, which accounts for the AF interaction. We noticed that the example falling in the AF reqime (θ < 98°) is still very rare.^9b

Compound 2. The temperature-dependent magnetic susceptibility of 2 is shown in Fig. 4. The χT value at room temperature is 9.85 emu mol⁻¹ K per Co₃ unit, much higher than the spin-only value of 5.64 emu mol⁻¹ K for three S = 3/2 ions, indicating the significant orbital contribution typical of high-spin octahedral Co(II). The χ⁻¹ versus T plot above 50 K follows the Curie–Weiss law with C = 10.0 emu K mol⁻¹ and θ = −4.2 K. It must be noted that the small negative Weiss constant is not necessarily indicative of AF interactions because the thermal magnetic behaviour is strongly influenced by the spin–orbital coupling intrinsic to a single octahedral Co(II) ions.^1a,f,19 As the temperature is lowered, while χ value increases monotonically, χT value first decreases very slowly to a broad minimum around 50 K, then increases to a maximum of 38.72 emu K mol⁻¹ at 2 K. Such a complex temperature-dependent behaviour of χT is the consequence of the interplay and competition of several concurrent effects, including the single-ion effects (first-order spin–orbital coupling and ligand-field distortion of Co(II) sites) and intrachain magnetic interactions between Co(II) ions.²⁰ The increase of χT value in the low temperature range from 50 K to 2 K suggests that dominant FM coupling between neighbouring Co(II) ions are mediated through the (OCO)₂(EO-N₃) triple bridges and (EO-N₃)₂ double bridges, while the slight fall of the χT value from 300 to 50 K and the negative θ value above 50 K suggest that FM effect is overcompensated by the single-ion effects in the high temperature range.

Fig. 4 χT and χ⁻¹ vs. T plots for 2. The line represents the fit to the Curie–Weiss law.

The FM interaction is also supported by the isothermal magnetization measured at 2 K (Fig. 5), which rises very rapidly in the low field region. The magnetization of 2 is 6.68 Nβ at 70 kOe, which is in the usual range expected for three high-spin Co(II) ions with orbital degeneracy. It is noted that the magnetization value of 2 tends to increase upon further increasing the field above 70 kOe. This behaviour is typical of Co(II) systems with large magnetic anisotropy. Further magnetic measurements revealed no indication of long-range ordering (no hysteresis and no out-of-phase ac susceptibility signals) (Fig. 6).

Fig. 5 Magnetization curves at 2 K for compound 2.

Fig. 6 Ac susceptibilities of 2 measured at frequencies 1, 10, 100 and 1000 Hz under dc field of 0 Oe with a driving ac field of 3.5 Oe.

Considering that the (OCO)₂(EO-N₃) triple and (EO-N₃)₂ double bridges alternate in the triple–triple–double sequence, a J₁–J₁–J₂ alternating chain model should be used to evaluate the exchange parameters as illustrated in Scheme 3. The magnetic analyses of Co(II) systems is always complicated by the large anisotropy arising from spin–orbital coupling. To fit the magnetic susceptibility data of 2, an Ising model (with effective S_eff = 1/2 spin at low temperature) was attempted for the chain in 2, but the fitting turned out in vain. Nevertheless, it has been demonstrated that (OCO)₂(EO-N₃) triple bridges and (EO-N₃)₂ double bridges mediate FM coupling between Co(II) ions in some previous examples reported by us^11a,12a,b,d, and others.^5b,d,21

Scheme 3 The alternating bridges and the coupling scheme in 2.

Compound 2 and our previously reported compound [Co₃(L)₂(N₃)₄(H₂O)₂]_n·2nH₂O, (HL = 1-carboxymethylpyridinium-3-carboxylate)^12a with very similar bridges and bridging mode, which the former showed no indication of long-range magnetic ordering while the latter behaved as a single-chain magnet (SCM). It may be related with relatively weak FM coupling although interchain magnetic coupling was weak (with longer interchain distance than the latter compound) and it might be able to behave as a SCM at lower temperature than we can reach.

4. Conclusions

With azide ions and two zwitterionic dicarboxylate and tetracarboxylate ligands, we have successfully synthesized two 2D novel coordination networks based on Mn(II) (1) and Co(II) (2) chains with the (OCO)(EO-N₃) (A1) and (EO-N₃)₂ (B) bridges alternating in the A1A1BA1A1 sequence in 1 while with (OCO)₂(EO-N₃) (A2) and (EO-N₃)₂ (B) bridges alternating in the A2A2BA2A2 sequence in 2. It has been demonstrated that the (OCO)(EO-N₃) and (EO-N₃)₂ double bridges mediate AF coupling between Mn(II) ions in 1 while the (OCO)₂(EO-N₃) triple and (EO-N₃)₂ double bridges mediate FM coupling between Co(II) ions in 2. This work further demonstrates that the potential of using zwitterionic carboxylate and azide ligands to construct novel magnetic systems with mixed carboxylate and azide bridges.

Acknowledgements

We are thankful for the financial support from NSFC (21301087, 21361016), the Inner Mongolia autonomous region natural science fund project (2013MS0206), and Programs of Higher-level talents of Inner Mongolia University (SPH-IMU-30105-125135).

Notes and references

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Footnote

† CCDC 1470719 and 1470720. For crystallographic data in CIF or other electronic format see DOI: 10.1039/c6ra09143a

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