A semi-flexible aminotriazine-based bis-methylpyridine ligand for the design of nickel(II) spin clusters

Abstract

The self-assembly of a semi-flexible aminotriazine-based bis-methylpyridine ligand, N²,N²-dibenzyl-N⁴,N⁶-di(pyridylmethyl)-1,3,5-triazine-2,4,6-triamine (H₂L), with NiCl₂ and NiBr₂ afforded two new nickel(II) clusters, (H₂NMe₂)₂[Ni₅(OH)₂(H₂L)₂Cl₁₀] (1) and [Ni₆(OH)₂(H₂L)₂Br₁₀(THF)₂] (2) showing a high spin ground state of S = 3.

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The development of new molecule-based magnets is an important research topic in the fields of chemistry and physics, due to their impressive structural diversity and intriguing physical properties as well as complicated magneto-structural correlations.¹ One of the major challenges in the area of molecular magnetism is the construction of a polynuclear metal cluster that exhibits interesting magnetic properties, such as the high-spin ground state and/or single-molecule magnet (SMM) behavior.^2–5 For the preparation of such metal clusters, using a polychelating ligand with an unused arm or a donor site has been recognized.^6,7 An alternative preparation method is to utilize the flexidentate behavior of a multidentate ligand and judicial choice of bridging ligands, such as carboxylate and azide.^8,9 Among the bridging ligands, halide ions have been known for their versatile bridging coordination modes that generate polymeric compounds.¹⁰

Poly-pyridyl ligands had a major impact in the field of supramolecular chemistry for decades, which have led to a variety of metal/ligand supramolecular ensembles to be obtained such as double and triple helices, grids, ladders, and so forth.¹¹ However, the flexible poly-pyridyl ligands are rarely exploited in the formation of polynuclear metal clusters; especially the resulting structures may potentially exhibit interesting magnetic properties.

We herein report the self-assembly of two Ni(II) clusters, (H₂NMe₂)₂[Ni₅(OH)₂(H₂L)₂Cl₁₀] (1) and [Ni₆(OH)₂(H₂L)₂Br₁₀(THF)₂] (2), using a semi-flexible aminotriazine-based bis-methylpyridine ligand, N²,N²-dibenzyl-N⁴,N⁶-di(pyridylmethyl)-1,3,5-triazine-2,4,6-triamine (H₂L). The designed ligand, H₂L (Scheme 1), contains an aminotriazine ring and two flexible methylpyridine arms, which could chelate metal ions into clusters 1 and 2, exhibiting an S = 3 spin ground state arising from the uncanceled spin arrangement of the antiferro- and ferromagnetic interactions in 1 and ferromagnetic interaction in 2, respectively.

Scheme 1 Schematic representation of H₂L ligand.

X-ray crystal structure analysis showed that 1 and 2‡ crystallize in the monoclinic space groups P2₁/n and in the triclinic space groups P1̄, respectively. In complex 1, the geometry of the centrosymmetric Ni^II₅ cluster can be described as two corner-sharing μ₃-OH-centred Ni^II₃ triangles with bowtie topology (Fig. 1). Two H₂L groups connect the central Ni^II atom (Ni1) and two peripheral metal ions in the two sides of a bow tie (Ni2 and Ni3) in a μ₃-H₂L-κ⁵-N,N′:N′′:N′′′,N′′′′ coordination mode, in which two methylpyridine groups exhibit in a trans-conformation. The base (Ni2⋯Ni3) of each triangle is bridged by two μ₂-Cl⁻ anions. The μ₃-OH⁻ group links the central Ni1 to the two peripheral metal ions on either side of the molecule and the O atom of OH⁻ lie out of the plane of the Ni₃ triangle about 0.402 Å. Peripheral ligations around each Ni centers are completed by terminal Cl⁻ anions.

Fig. 1 Crystal structure of the anion complex 1 (left) and its Ni^II₅ core structure (right). The Me₂NH₂ cations and H atoms were omitted for clarity.

The structure of complex 2 reveals a dimer of [Ni^II₃(μ₃-OH)(μ₃-Br)(μ₂-Br)₃]⁺ core which is connected by two bis-chelating H₂L ligands (Fig. 2). The structure [Ni^II₃(μ₃-OH)(μ₃-Br)(μ₂-Br)₃]⁺ adopts a near-equilateral Ni^II₃ triangle core, which is bonded by a μ₃-oxide (O1) and a μ₃-Br⁻ (Br1) on both sides of the central planar where the central OH⁻ and Br⁻ bridges are located 0.902 and 2.056 Å above the Ni₃ plane. Each base of the Ni^II₃ triangle is connected by μ₂-Br⁻ anions (Br2–Br4). Two H₂L ligands in complex 2 exhibit a μ₂-H₂L-κ⁴-N,N′:N′′,N′′′ coordination mode with trans-conformation of their two methylpyridine groups and connects the two Ni₃ triangles into a hexanuclear dimer of Ni₃ structure. Peripheral ligations around each Ni2 centers are ended by one terminal Br⁻ anion and one THF molecule.

Fig. 2 Crystal structure of the complex 2 (left) and its Ni^II₃ core structure (right). The H atoms were omitted for clarity.

The solid-state, variable-temperature magnetic susceptibility measurements were performed on microcrystalline samples of complexes 1 and 2 in the 2–300 K range in a 1 kOe magnetic field, which was suspended in eicosane to prevent torquing.

For complex 1, the χ_MT value of 6.01 cm³ K mol⁻¹ at 300 K decreases gradually with decreasing temperature in the range of 300 to 70 K, then abruptly increases, reaching a maximum of 7.05 cm³ K mol⁻¹ at 10 K, and decreases to 4.38 cm³ K mol⁻¹ at 2 K (Fig. 3). The change in χ_MT value indicates that antiferromagnetic dominated in the Ni₅ unit with a non-canceled spin ground state and the χ_MT value at 10 K is consistent with S = 3 (g = 2.2). Below 10 K, the χ_MT values slowly decrease, probably due to weak intermolecular antiferromagnetic interactions, zero field splitting and/or small anisotropy.

Fig. 3 Plots of χ_MT versus T for 1 in an applied field of 1 kOe from 2.0 to 300 K. The solid line represents a least-squares fit of the data (see text). The inset shows a 2 K magnetization isotherm collected between 0 and 50 kOe.

In order to understand the magnetic coupling of complex 1, the magnetic susceptibility data were fitted using a Ni^II₅ Heisenberg–van Vleck model. Based on the structure analysis, the number of magnetic interactions can be reduced significantly: J₁ for Ni^II⋯Ni^II through one μ₃-OH and one H₂L bridgings and J₂ for Ni^II⋯Ni^II through one μ₃-OH⁻ and two μ₂-Cl⁻ bridges (see Fig. S5 in the ESI†), hence the Hamiltonian can be written as:

H = −2J₁(S₁S₂ + S₁S₃ + S₁S₄ + S₁S₅) − 2J₂(S₂S₃ + S₄S₅)

The χ_MT data could be well fitted by this Heisenberg–van Vleck model with the addition of an intermolecular interaction by the mean-field approximation (zJ′). The results from fitting the experimental data are shown as solid lines in Fig. 3, with final parameters being g = 2.30, J₁ = −11.7 cm⁻¹, J₂ = 3.5 cm⁻¹ and zJ′ = −0.10 cm⁻¹. This set of parameters leads to the conclusion that the ground state is S_T = 3 and the first excited state is S = 2 at 24 cm⁻¹ above the ground state (Fig. S6†). The estimated values, for the intracluster magnetic exchange interactions, indicate that the antiferro- and ferromagnetic interactions are provided within the Ni^II₅ cluster in 1, and are associated with an S = 3 spin ground state. Both interactions (J₁ and J₂) are close to the reported exchange interactions of Ni^II⋯Ni^II through the similar pathways.¹² The magnetization curve recorded at 2 K of complex 1 shows a continuous increase up to the saturation value of 6.3Nβ (Fig. 3 inset), which corresponds well to a ground-state spin S = 3, in agreement with the χ_MT data. However, this magnetization curve cannot be nicely fitted by the Brillouin equation for S = 3, probably due to the presence of intermolecular interaction, zero field splitting and/or anisotropy.

For complex 2, the value of χ_MT increases steadily from 4.24 cm³ mol⁻¹ K at 300 K as the temperature decreases to reach a maximum of 6.03 cm³ mol⁻¹ K at 18 K, and then decreases to 1.00 cm³ mol⁻¹ K at 2.0 K (Fig. 4). The χ_MT value at 300 K is slightly larger than 4.00 cm³ mol⁻¹ K, the expected value for a Ni^II₃ complex with noninteracting metal centers with g = 2.3. This behavior clearly indicates the ferromagnetic coupling within complex 2 and the decrease in χ_MT at low temperature (<28 K) is likely due to the intermolecular (Ni₃⋯Ni₃) interaction, the Zeeman effect or zero-field splitting in the ground state. In order to describe the coupling within the cluster, the magnetic susceptibility data were fitted using a Ni^II₃ Heisenberg–van Vleck model: H = −2J(S₁S₂ + S₂S₃ + S₁S₃) with an interunit interaction by the mean-field approximation (zJ′) (Fig. S7†). The data below 20 K were omitted in the fitting, because zero-field splitting and Zeeman effect likely dominate in this temperature range. The fitting result of dc data in 1 kOe gave the best fit parameters of g = 2.26, J = 8.10 cm⁻¹ and zJ′ = −0.50 cm⁻¹. This set of parameters gives the ground state of S_T = 3 and the first excited state S = 2 at −48 cm⁻¹ above the ground state (Fig. S8†). Although the magnetic interaction between Ni^II ions through such bridges (one μ₃-OH, one μ₃-Br and one μ₂-Br) has not been reported in the literature, it is believed that the ferromagnetic interactions ensue from 6-coordinate geometry and the Ni–X–Ni bridging angles close to 90°.¹³ The magnetization curve recorded at 2 K of 2 is shown in Fig. 4 inset, in which the magnetization slowly increases with the increase of field and becomes saturated around 50 kOe with a value of 5.65Nβ. The less rapid saturation of magnetization at low field may result from the antiferromagnetic interaction of Ni₃⋯Ni₃ interunit and the saturation magnetization value corresponds well to a ground-state spin S = 3, in agreement with the χ_MT data. Again, this magnetization curve cannot be well fitted by the Brillouin equation for S = 3, due to the presence of intermolecular interaction and/or zero field splitting.

Fig. 4 Plots of χ_MT versus T for 2 in an applied field of 1 kOe from 2.0 to 300 K. The solid line represents a least-squares fit of the data (see text). The inset shows a 2 K magnetization isotherm collected between 0 and 50 kOe.

To investigate whether 1 and 2 might be a SMM, ac susceptibility measurements were performed with a zero applied dc field. Representative results for 1 and 2 are shown in Fig. S9 and S10.† At lower temperatures, the in-phase signal (χ_M′T) increases to ∼6.8 and 6.5 cm³ K mol⁻¹ for 1 and 2, respectively, confirming the spin ground state of S = 3 for both complexes. For complex 1, a weak χ_M′′ signal appears below 5 K, which is indicative of a slow magnetic relaxation within 1. However, the peak maxima clearly lie in the temperatures below 1.8 K, the operating limit of our instrument. These data thus suggest that compound 1 indeed exhibits a slow magnetic relaxation or long-range magnetic ordering at temperatures below 1.8 K. In contrast, the complex 2 shows no SMM behavior from the absence of χ_M′′ signal.

In conclusion, the use of semi-flexible aminotriazine-based bis-methylpyridine ligands (H₂L) has allowed the access of two novel Ni clusters with interesting magnetic properties. The H₂L ligand represents a ‘proof of feasibility’ for the belief that such ligands may provide a rich source of new transition-metal clusters. Further studies are in progress.

Notes and references

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Footnotes

† Electronic supplementary information (ESI) available: Detailed experimental procedures, additional crystallographic diagrams and magnetic diagram. CCDC 947250 and 947251. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c3dt52903g

‡ The complexes analyzed as (C, H, N) 1, calcd (found): C, 42.52 (42.17); H, 4.26 (4.79); N, 14.40 (14.35)% and 2, calcd (found): C, 34.37 (34.03); H, 3.23 (3.34); N, 9.72 (9.72)%. Crystal-structure data for 1, C₆₂H₇₀Cl₁₀N₁₈Ni₅O₂, M = 1747.31, monoclinic, P2₁/n, a = 15.7038(12) Å, b = 9.9564(7) Å, c = 23.8274(18) Å, β = 93.4680(10)°, V = 3718.7(5) ų, T = 150(2) K, Z = 2. (R_int = 0.0426), 8215 parameters, R(R_w) = 0.0382(0.0865) with [I > 2σ (I)] and for 2, C₆₆H₇₄Br₁₀N₁₆Ni₆O₄, M = 2306.56, triclinic, P1̄, a = 11.9623(7) Å, b = 13.3874(8) Å, c = 14.2964(9) Å, α = 65.3520(10)°, β = 72.9400(10)°, γ = 75.1400(10)°, V = 1965.5(2) ų, T = 150(2) K, Z = 1. (R_int = 0.0291), 9051 parameters, R(R_w) = 0.0256(0.0481) with [I > 2σ(I)].

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