Crystallization condition-controlled assembly of oxygen-bridged tetranuclear and hexanuclear Ni(II) clusters: syntheses, structures and properties

Abstract

Crystallization of a Schiff base with nickel acetate in different solvents yields a tetranuclear (dicubane-like [Ni₄O₄] core with two missing vertices) and a hexanuclear cluster ([Ni₆O₆] core with two nickel(II) ions attached to a [Ni₄O₄] cubane through oxygen bridges). Metal centers within these clusters exhibit ferromagnetic and antiferromagnetic coupling interactions, respectively.

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During the past few decades, polynuclear compounds of paramagnetic metal ions have received considerable interests because of their potential application in the fields of molecular magnetic materials, especially those that can function as single-molecule magnets (SMMs).^1–4 Properties of these compounds is generally governed by the spin values of metal ions, the linkages of polydentate ligands and especially the arrangements of the metal ions through these bridges. A series of ligands, especially those that could provide alkoxo or phenoxo donor groups, such as 2-(hydroxymethyl)pyridine (hmp),^4a 2-(2-hydroxyethyl)pyridine (hep),^4b Schiff base^4c,d and salicylaldoxime as well as its various derivatives (Rsao),^4e have been proved effective to generate compounds with good magnetic properties. The assembly process is largely dependent upon the crystallization conditions, such as temperature, solvent, template, pH, guest, concentration and so on.

Recently, we have investigated a linear Mn₄ cluster and a novel Mn₉ cluster based on hmp and Mesao ligands, respectively. Both exhibit single molecular magnetic properties.^3e,f Along with our work in this field, we selected a Schiff base ligand, N,N′-bis(2-hydroxyphenyl-1-ethylidene)-1,3-diamino-2-hydroxypropane (H₃L₁), which possesses both alkoxo and phenoxo donor groups. To the best of our knowledge, only several Cu^II, Co^II/Co^III and Ni^II compounds have been reported assembled from this ligand very recently.⁵ In addition, nickel(II), which is known to have a large single-ion zero-field splitting (ZFS) and easily give rise to high spin molecules with high anisotropy depending on the coordination geometry and symmetry, often leads to ferromagnetic coupling.⁶ Herein, we obtained a tetranuclear, [Ni₄(L₁)₂(MeO)₂(MeOH)₂] (1), and a hexanuclear cluster, [Ni₆(L₁)₂(L₂)₂(OAc)₂]·3DMF (2) (H₂L₂ = N-2-hydroxyphenyl-1-ethylidene-1,3-diamino-2-hydroxypropane) through the reaction of this Schiff base ligand and nickel acetate in different crystallization conditions.‡

Reaction of Ni(OAc)₂·6H₂O with H₃L₁ in a 3 : 2 molar ratio in DMF gave a green solution from which green crystals of 1 and red crystals of 2 were obtained through the diffusion of CH₃OH and slow evaporation, respectively.

The single-crystal X-ray diffraction analysis indicates that 1 crystallizes in the monoclinic P2₁/c space group. As shown in Fig. 1, the core of 1 is composed of four Ni^II ions, two triply-deprotonated Schiff base ligands L₁³⁻, two μ³–MeO⁻ anions and two MeOH ligands. Ni1 and Ni1A adopt six-coordinated octahedron geometry with two μ³–MeO⁻ oxygens, one phenoxo oxygen, one imine nitrogen and one MeOH oxygen, respectively. Ni2 and Ni2A, however, are surrounded by one alkoxo oxygen, two phenoxo oxygens, one μ³–MeO⁻ oxygen and one imine nitrogen, respectively. The geometry could be described as much distorted square pyramidal with the Addison parameter τ of 0.523.⁷ Two η²:η¹:η²:η¹:η¹:μ³ Schiff base ligands and two μ³–MeO⁻ anions bridge four Ni^II ions into a defected face-shared dicubane-like [Ni₄O₄] core with two missing vertices. The Ni^II ions are located at the four corners. The intracluster Ni⋯Ni distances of Ni1⋯Ni2, Ni1⋯NiA and Ni2⋯Ni2A are 3.016(1), 3.100(1) and 5.155(5) Å, respectively. All the doubly bridged Ni–O–Ni angles are in the range of 95.02(2)–97.46(3)°. It should be noted that, although this kind of tetranuclear arrangement is quite common, this tetranuclear structure may be the second example in which five and six-coordinated Ni^II ions are involved since the first and similar example has been reported very recently.^5b

Fig. 1 View of the molecular structure of 1. Hydrogen atoms have been omitted for clarity. Symmetry codes: A, −x, 1−y, −z.

Compound 2 crystallizes in the orthorhombic Pbcn space group and is a novel hexanuclear cluster which features a novel [Ni₄O₄] core with two Ni^II ions attached to a [Ni₄O₄] cubane through oxygen bridges (Fig. 2). Interestingly, the Schiff base ligand in this case undergoes partial hydrolysis of the imine bond during the crystallization and results in another Schiff base ligand, N-2-hydroxyphenyl-1-ethylidene-1,3-diamino-2-hydroxypropane (H₂L₂). The hydrolysis of Schiff base in the presence of a metal ion has been reported, while the probable reasons for the above reaction are still unknown.⁸ Considering the same reaction, this hydrolysis may take place in a different crystallization process. The addition of MeOH and the slow evaporation of DMF at room temperature may prevent/promote the probability of this hydrolysis process.

Fig. 2 View of the molecular structure of 2. Hydrogen atoms have been omitted for clarity. Symmetry codes: A, 1−x, y, 0.5−z.

The asymmetric unit of 2 consists of three crystallographically independent Ni^II ions, one triply-deprotonated Schiff base ligands (L₁³⁻), one doubly-deprotonated Schiff base ligand (L₂²⁻) and one syn–syn bisdentate acetate ion as well as one and a half DMF lattice molecules. Three metal centers exhibit different coordination environment. Ni1 and Ni2 adopt six-coordinated distorted octahedron geometry. The former is surrounded by one imine nitrogen, one μ²-alkoxo oxygen and two μ³-phenoxo oxygen of two L₁³⁻, one μ³-alkoxo oxygen of L₂²⁻ as well as one acetate oxygen. The latter, however, is coordinated with one μ³-phenoxo oxygen of L₁³⁻, one imine nitrogen, one η¹-phenoxo oxygen and two μ³-alkoxo oxygens of two L₂²⁻ as well as one acetate oxygen. Ni3 is four-connected and the square plane geometry is furnished by one amine nitrogen of L₂²⁻, one μ²-phenoxo oxygen, one imine nitrogen, one η¹-phenoxo oxygen of L₁³⁻. As a result, two Schiff base ligands L₁³⁻ and L₂²⁻ act as η³:η¹:η²:η¹:η¹: μ⁴ and η¹:η¹:η³:η¹: μ⁴ bridges to connect six Ni(II) centers, forming a novel hexanuclear [Ni₆O₆] cluster in which two Ni^II ions is bridged with the [Ni₄O₄] cube through η²-alkoxo oxygen atoms. Within the cube subunit the Ni⋯Ni distances and the oxygen doubly-bridged Ni–O–Ni angles are in the range of 2.973(2)–3.335(2) Å and 88.67(18)–103.1(2)°, respectively. The Ni2⋯Ni3 separation across the η²-alkoxo oxygen of L₂²⁻ ligand is 3.312(2) Å with the Ni–O–Ni angle of 114.3(3)°. The family of Ni₆ compounds is growing rapidly with various topologies, such as lines, hexagons, open books, cycles with chair conformation, Chinese-lantern-like objects and face-shared distorted dicubanes with the corners of the cubane missing and so on.⁹ It should be noted that only one compound has very recently been reported to have a similar metal arrangement^9g and this compound is also the first hexanuclear cluster involving both six- and four-coordinated Ni^II ions.

The temperature dependence of magnetic susceptibilities of 1 and 2 was measured on powdered polycrystalline samples in the temperature range of 2.5–300 K in a magnetic field of 2kOe and shown as χ_MT vs. T plot in Fig. 3 and 4, respectively. Interestingly, the Ni^II ions within these two compounds exhibit ferromagnetic and antiferromagnetic coupling interactions, respectively.

Fig. 3 Top: Plot of χ_MT vs. T of 1 in an applied field of 2kOe. The solid lines represent the best-fit calculations. Bottom: The magnetic exchange interactions between the metal centers.

Fig. 4 Top: Plot of χ_MT vs. T of 2 in an applied field of 2kOe. The solid lines represent the best-fit calculations. Bottom: The magnetic exchange interactions between the metal centers. Ni3 and Ni3A are omitted due to the diamagnetic properties.

The χ_MT of 1 is equal to 5.89 cm³ mol⁻¹ K at 300 K, much larger than the spin-only value for four isolated S = 1 Ni^II center with g = 2.20 (4.8 cm³ mol⁻¹ K). Upon cooling, the value increases gradually and reaches the maximum of 12.78 cm³ mol⁻¹ K at 9.0 K, which is consistent with 12.1 cm³ mol⁻¹ K based on ground state spin S_T = 4 and g = 2.20, indicating ferromagnetic exchange between Ni^II ions is dominant in this compound. Below this temperature, the value of χ_MT sharply drops to 10.53 cm³ mol⁻¹ K at 1.8 K, which may be attributed to the weak intertetramer antiferromagnetic interactions and/or significant zero-field splitting as well as Zeeman effects. The interactions between adjacent tetranuclear clusters are very weak, and only the intratetramer coupling interactions are considered when trying to simulate the experimental data. Two different coupling constants were considered: (i) J₁, along the sides of the rhomb, exchange mediated by phenolate oxygen and alkoxo oxygen bridges; and (ii) J₂, along the short diagonal, exchange resulting from two alkoxo bridges. When the ZFS (D) is taken into account, the Heisenberg spin Hamiltonian was written as: H = -2J₁(S₁S₂ + S_1AS₂ + S₁S_2A + S_1AS_2A)-2J₂S₁S_1A-D[S_z²-S(S + 1)/3)]. The best fits with the parameters of g = 2.314(2), J₁ = + 5.87(8) cm⁻¹, J₂ = + 12.3(4) cm⁻¹ and |D| = 0.056(1) cm⁻¹ are consistent with the ferromagnetic interactions deduced from the plot (see ESI†).

The χ_MT of 2 at room temperature is 4.77 cm³ mol⁻¹ K, which is in good agreement with the expected value for the presence of four isolated nickel(II) metal ions with S = 1 and g = 2.20. Upon cooling, the value remains almost constant until about 100 K. From then on, χ_MT decreases sharply and reaches the minimum of 0.08 cm³ mol⁻¹ K at 2.5 K, indicating antiferromagnetic exchange in this compound. The rapid decrease in the low temperature may also be attributed to the zero-field splitting, Zeeman effects or both. Due to the diamagnetic properties of planar coordinated Ni3 and Ni3A ions, only the coupling interactions between the four Ni(II) centers within the cubane were considered: (i) J₁, exchange mediated by one syn–syn acetate and two alkoxo bridges, (ii) J₂, exchange resulting from two alkoxo bridges, and finally (iii) J₃ and J₄, exchange across two alkoxo bridges. As a consequence, the following Heisenberg spin Hamiltonian has been used to calculate the magnetic susceptibility: H = −2J₁(S₁S₂ + S_1AS_2A) − 2J₂(S₁S_2A + S_1AS₂) − 2J₃S₁S_1A − 2J₄S₂S_2A. The magnetic anisotropy and weak inter-compound interactions are completely neglected in this approximate model. The data were fitted by MAGPAG program.¹⁰ In order to avoid overparameterization of the simulation, we simplified the problem by considering J₂, J₃ and J₄ are equal and obtained the best fits with J₁ = 0.66, J₂ = J₃ = J₄ = −0.51 cm⁻¹ and g = 2.20(1).

The magneto-structural corrections for double oxo bridged Ni^II compounds have been well established.¹¹ The coupling is largely dependent upon the Ni–O–Ni angle: the magnetic coupling is expected to be ferromagnetic when the angles are close to 90°, and a change from ferromagnetic to antiferromagnetic exchange coupling corresponds to a Ni–O–Ni angle of about 98–99°. An inspection of the structure of these compounds allows us to account for the observed ferromagnetic/antiferromagnetic coupling. All Ni–O–Ni angles within the tetramer of 1 are in the range of 95.02–97.46°. In 2, however, most Ni–O–Ni angles are above 99° (in the range of 100.6–103.1°) except the triply bridged Ni1 and Ni2 metals with the Ni–O–Ni angles in the range of 88.6–90.7°. Of course, the magnitude of the exchange coupling is also influenced by the environment of the Ni centers, such as the Ni–O distances, the dihedral angle in the Ni₂O₂ unit and so on.

In summary, crystallization of a Schiff base ligand with Ni(II) ions under different conditions led to the formation of a tetranuclear and a hexanuclear cluster, respectively. The former exhibits a defect dicubane-like core with two missing vertices, while the latter features a [Ni₆O₆] core with two Ni(II) ions attached to a [Ni₄O₄] cubane through oxygen atoms. Although the bridges are all the oxygen atoms, two compounds demonstrate overall different ferromagnetic and antiferromagnetic coupling interactions between the Ni(II) ions. Solvent plays an important role on the resulting structures and consequently the magnetic coupling interactions.

This work was supported by the National Natural Science Foundation of China (grant No. 20671048 and 21041002) and the Shandong Taishan Scholar Fund.

References

1. (a) R. Sessoli, D. Gatteschi, A. Caneschi and M. A. Novak, Nature, 1993, 365, 141; (b) L. K. Thompson, O. Waldmann and Z. Q. Xu, Coord. Chem. Rev., 2005, 249, 2677; (c) H. Miyasaka, A. Saitoh and S. Abe, Coord. Chem. Rev., 2007, 251, 2622; (d) C. E. Kostakis and A. K. Powell, Coord. Chem. Rev., 2009, 253, 2686; (e) T. Taguchi, W. Wernsdorfer, K. A. Abboud and G. Christou, Inorg. Chem., 2010, 49, 199.
2. (a) J. K. Tang, I. Hewitt, N. T. Madhu, G. Chastanet, W. Wernsdorfer, C. E. Anson, C. Benelli, R. Sessoli and A. K. Powell, Angew. Chem., Int. Ed., 2006, 45, 1729; (b) C. J. Milios, A. Vinslava, P. A. Wood, S. Parsons, W. Wernsdorfer, G. Christou, S. P. Perlepes and E. K. Brechin, J. Am. Chem. Soc., 2007, 129, 8; (c) P. H. Lin, T. J. Burchell, R. Clérac and M. Murugesu, Angew. Chem., Int. Ed., 2008, 47, 8848; (d) D. E. Freedman, D. M. Jenkins, A. T. Lavarone and J. R. Long, J. Am. Chem. Soc., 2009, 130, 2884; (e) T. Taguchi, W. Wernsdorfer, K. A. Abboud and G. Christou, Inorg. Chem., 2010, 49, 199.
3. (a) S. Wang, J. L. Zuo, H. C. Zhou, H. J. Choi, Y. X. Ke, J. R. Long and X. Z. You, Angew. Chem., Int. Ed., 2004, 43, 5940; (b) Y. Z. Zhang, W. Wernsdorfer, F. Pan, Z. M. Wang and S. Gao, Chem. Commun., 2006, 3302; (c) M. H. Zebg, M. X. Yao, H. Liang, W. X. Zhang and X. M. Chen, Angew. Chem., Int. Ed., 2007, 46, 1832; (d) M. Morimoto, H. Miyasaka, M. Yamashita and M. Irie, J. Am. Chem. Soc., 2009, 131, 9823; (e) D. C. Li, H. S. Wang, S. N. Wang, Y. P. Pan, C. J. Li, J. M. Dou and Y. Song, Inorg. Chem., 2010, 49, 3688; (f) S. N. Wang, L. Q. Kong, H. Yang, Z. T. He, Z. Jiang, D. C. Li, S. Y. Zeng, M. J. Niu, Y. Song and J. M. Dou, Inorg. Chem., 2011, 50, 2705.
4. (a) J. Zhang, P. Teo, R. Pattacini, A. Kermagoret, R. Welter, G. Rogez, T. S. A. Hor and P. Braunstein, Angew. Chem., Int. Ed., 2010, 49, 4443; (b) V. Mereacre, Y. H. Lan, R. Clérac, A. M. Ako, J. Hewitt lan, W. Wernsdorfer, G. Buth, C. E. Anson and A. K. Powell, Inorg. Chem., 2010, 49, 5293; (c) A. Ray, S. Banerjee, G. M. Rosair, V. Gramlich and S. Mitra, Struct. Chem., 2008, 19, 459; (d) Y. Lan, G. Novitchi, R. Clérac, J. K. Tang, N. T. Madhu, I. J. Hewitt, C. E. Anson, S. Brooker and A. K. Powell, Dalton Trans., 2009, 1721; (e) C. J. Milios, S. Piligkos and E. K. Brechin, Dalton Trans., 2008, 1809.
5. (a) S. Basak, S. Sen, G. Rosair, C. Desplanches, E. Garribba and S. Mitra, Aust. J. Chem., 2009, 62, 366; (b) S. Banerjee, M. Nandy, S. Sen, S. Mandal, G. M. Rosair, A. M. Z. Slawin, C. J. Gómez García, J. M. Clemente-Juan, E. Zangrando, N. Guidolin and S. Mitra, Dalton Trans., 2011, 40, 1652.
6. (a) R. Boca, Coord. Chem. Rev., 2004, 248, 757; (b) J. Krzystek, A. Ozarowski and J. Telser, Coord. Chem. Rev., 2006, 250, 2308; (c) C. Papatriantafyllopoulou, L. F. Jones, T. Nguyen, N. Matamoros-Salvador, L. Cunha-Silva, F. A. A. Paz, J. Rocha, M. Evangelisti, E. K. Brechin and S. P. Perlepes, Dalton Trans., 2008, 3153; (d) B. Biswas, U. Pieper, T. Weyhermüller and P. Chaudhuri, Inorg. Chem., 2009, 48, 6781.
7. A. W. Addison, T. N. Rao, J. Reedijk, J. van Rijn and G. C. Verschoor, J. Chem. Soc., Dalton Trans., 1984, 1349.
8. (a) G. Das, R. Shukla, S. Mandal, R. Singh and P. K. Bharadwaj, Inorg. Chem., 1997, 36, 323; (b) B. Sarkar, M. S. Ray, M. G. B. Drew, A. Figuerola, C. Diaz and A. Ghosh, Polyhedron, 2006, 25, 3084; (c) P. Mukherjee, M. G. B. Drew and A. Ghosh, Eur. J. Inorg. Chem., 2008, 3372; (d) S. Naiya, B. Sarkar, Y. Song, S. Ianelli, M. G. B. Drewe and A. Ghosh, Inorg. Chim. Acta, 2010, 363, 2488.
9. (a) C. H. Chien, J. C. Chang, C. Y. Yeh, G. H. Lee, J. M. Fang, Y. Song and S. M. Peng, Dalton. Trans., 2006, 3429; (b) C. Papatriantafyllopoulou, G. Aromi, A. J. Tasiopoulos, V. Nastopoulos, C. P. Raptopoulou, S. J. Teat, A. Escuer and S. P. Perlepes, Eur. J. Inorg. Chem., 2007, 2761; (c) M. Mikuriya, K. Tanaka, N. Inoue, D. Yoshioka and J. W. Lim, Chem. Lett., 2003, 32, 126; (d) V. Ovcharenko, E. Fursova, G. Romanenko, I. Eremenko, E. Tretyakov and V. Ikorskii, Inorg. Chem., 2006, 45, 5338; (e) A. N. Georgopoulou, C. P. Raptopoulou, V. Psycharis, R. Ballesteros, B. Abarca and A. K. Boudalis, Inorg. Chem., 2009, 48, 3167; (f) S. T. Zheng, D. Q. Yuan, H. P. Jia, J. Zhang and G. Y. Yang, Chem. Commun., 2007, 1858; (g) H. L. C. Feltham, R. Clérac and S. Brooker, Dalton Trans., 2009, 2965.
10. (a) J. J. Borras-Almenar, J. M. Clemente-Juan, E. Coronado and B. S. Tsukerblat, Inorg. Chem., 1999, 38, 6081; (b) J. J. Borrás-Almenar, J. M. Clemente-Juan, E. Coronado and B. Tsukerblat, J. Comput. Chem., 2001, 22, 985.
11. (a) A. P. Ginsberg, J. A. Bertrand, R. I. Kaplan, C. E. Kirkwood, R. L. Martin and R. C. Sherwood, Inorg. Chem., 1970, 10, 240; (b) M. A. Halcrow, J. S. Sun, J. C. Huffman and G. Christou, Inorg. Chem., 1995, 34, 4167; (c) J. M. Clemente-Juan, E. Coronado, J. R. Galan-Mascaros and C. J. Gomez-Garcia, Inorg. Chem., 1999, 38, 55; (d) U. Kortz, Y. P. Jeannin, A. Teze, G. Herve and S. Isber, Inorg. Chem., 1999, 38, 3670; (e) P. Mukherjee, M. G. B. Drew, C. J. Gómez-García and A. Ghosh, Inorg. Chem., 2009, 48, 5848; (f) B. Biswas, U. Pieper, T. Weyhermuller and P. Chaudhuri, Inorg. Chem., 2009, 48, 6781.

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Footnotes

† Electronic supplementary information (ESI) available: Additional figures of the compounds. CCDC reference numbers 795063–795064. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c2ra01219g

‡ Syntheses of 1 and 2: H₃L₁ (0.1903 g, 0.58 mmol) and three-ethyl amine (3 ml) were dissolved in 50 ml DMF. The solution was refluxed for 30 min under stirring. Then, Ni(OAc)₂·4H₂O (0.2489 g, 1 mmol) was added. This mixture was refluxed for 4 h under stirring. The green crystals of 1 were grown by slow diffusion of methanol into DMF solution. The red crystals of 2 were obtained from the DMF solution by slow evaporation at room temperature after one month. Crystal structure for 1: C₄₂H₅₂N₄O₁₀Ni₄, Mr = 1007.72, monoclinic, space group P2₁/c, a = 13.0082(15), b = 12.0689(12), c = 13.7934(17) Å, β = 107.5830(10), V = 2064.3(4) ų, Z = 2, Dc = 1.621 g cm⁻³, F(000) = 1048, R₁ = 0.0431, wR₂ = 0.1028 for 3643 reflections with I > 2σ(I), GOF = 1.000. 2: C₇₃H₉₃N₁₁O₁₇Ni₆, Mr = 1748.84, orthorhombic, space group Pbcn, a = 27.556(3), b = 16.8879(16), c = 17.3304(19) Å, V = 8065.0(14) ų, Z = 4, Dc = 1.440 g cm⁻³, F(000) = 3648, R₁ = 0.0829, wR₂ = 0.2091 for 7131 reflections with I > 2σ(I), GOF = 1.000.

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