Ligand-regulated unusual nickel clusters: A centrosymmetric dicubane Ni8 and a tetrahedral Ni10 cluster
Ying Zou , Qiang Gao , Na Sun , Songde Han , Xiaoyu Li , Guoming Wang
Owing to the atom-level precise molecular structures and special inorganic polymetallic cores, polynuclear metallic clusters possess the unique physicochemical properties, endowing wide application prospects in the luminescence, photoelectric catalysis, biological imaging, nano-electron and magnetism [1-4]. Now, the fruitful achievement has been gained for noble metal clusters (Cu, Ag, Au) [5-10], polyoxometalates (POMs) [11, 12], transition metal clusters [13, 14], rare earth metal clusters [15-17], titanium oxide clusters [18] and mixed metal clusters [19-21]. For transition metal clusters chemistry, this new era was triggered by the discovery of the first discrete Mn12 molecule with unique single-molecule magnet (SMM) property at the molecular scale [22]. After that, high-nuclearity transition metal clusters with large spin ground electronic states and magnetic anisotropies were pursued to obtain nanoscale excellent SMMs. Mn and Fe clusters have been proven to be two efficient sources of SMMs owing to the molecular anisotropies generated by the Jahn–Teller distortion of Mn(Ⅲ) and Fe(Ⅲ) in octahedral coordination geometry [19], exemplified by Mn84 [23], Mn49 [24], Mn30 [25], Mn26 [26], Mn24 [26], Mn9 [27], Mn6 [28], Fe19 [29], Fe11 [30] and Fe4 [31]. Besides, V, Cr, Co, Ni and heterometallic 3d-containing transition metal clusters also exhibit the potential capability as the SMMs [20, 32-34].
Nickel clusters can be seen as good candidate for SMMs due to their large single-ion zero-field splitting [35, 36]. Just like other metal clusters, polynuclear Ni clusters were generally isolated from complex reactive systems influenced by ligand, solvent, pH, temperature, counter-ions and others. An effective synthetic method is to choose polydentate chelate ligands to stabilize supramolecular aggregates of polymetallic ions. Calixarene derivate, polyalcohol acid, pyridine/pyrazolate and oxime have been proven to be prominent species. Liao group adopts the calixarenes derivate to construct a series of various Ni clusters capped different numbers of bowl-like ligands, including Ni8, Ni18, Ni20, Ni24, Ni32 and the largest Ni72 cluster [37-39]. By using bi(pyrazole-alcohol) and halogen-substituted pyrazolate, Sun group successfully obtained a Ni6 with ferromagnetic exchange and three cubic Ni8 clusters with different magnetic behaviors [40, 41]. Furthermore, the pyridine-2-amidoxime and R substituent modification ligands have suitable coordination configurations to construct Ni clusters [42-44]. Based on above ligands, two high-nuclearity Ni12 and Ni16 with multiple-decker structures were successfully constructed and they display ferromagnetic exchange interactions generating spin ground states of S = 6 for Ni12 and S = 8 for Ni14 [45]. In addition, by using an achiral citric acid, Güdel group synthesizes a catenulate Ni21 with a slow relaxation [33] and two Ni8 realizing the regulation of magnetic properties by changing the crystallization temperature [46].
Our group recently developed a N-tris(hydroxymethyl)methylglycine (H5thmmg) ligand to synthesize a cyclic Fe6 cluster and a bricky Fe18 cluster seen as a "Chinese knot", providing a potential ligand to construct high-nuclear transition metal clusters [47]. Here, the H5thmmg ligand was extended to Ni chemistry because of similar coordination configuration of Fe3+ and Ni2+ ions. Fortunately, an octanuclear Ni cluster has been successfully isolated under solvothermal condition. Its metallic inner is comprised of two centrosymmetric cubanes, Ni4(µ3-O)3(µ6-O) by sharing an O2− ion and the periphery is protected by six H3thmmg2− ligands. In order to regulate the architectures of the Ni clusters, adopting a mixed-ligand strategy, a new 2-mercapto-5-amino-1,3,4-thiadiazole (Hmat) auxiliary ligand with both N and S donor atoms was introduced. As expected, a disparate decanuclear nickel cluster was isolated. Its nickel metal core features a pudgy tetrahedron, surrounded by four apical H3thmmg2− ligands and 8 mat− ligands on the six sides of the tetrahedron. Furthermore, the magnetic properties of Ni8 and Ni10 clusters were investigated.
The synthetic routes of the Ni8 and Ni10 clusters are displayed in Scheme S1 (Supporting information). Reaction of Ni(NO3)2·6H2O, H5thmmg ligand, N(CH2CH2O)3 and Et3N in EtOH solution under solvothermal condition produces green cubic crystals (Ni8). The Ni10 cluster has exactly the same synthetic condition except that a new 2-mercapto-5-amino-1,3,4-thiadiazole (Hmat) ligand was added. The thiadiazole ligands have testified to be a good class of ligands for the construction of transition metal clusters, such as Co20 [48], Ni20 [49], Ni9 [50], Ag11 [51], so Hmat is dominant auxiliary ligand to construct various Ni clusters. As expected, the Hmat participates in coordination in Ni10 cluster. In addition, the hydroxide is an ideal building-component for high-nuclearity metal clusters. Here, the triethylamine was added to adjust the alkaline condition generating more OH− to construct high-nuclearity Ni clusters. Triethanolamine as an auxiliary ligand was added in the syntheses of Ni8 and Ni10, whereas it was not embedded into the final structures. But it is essential for the formation of crystalline products of Ni8 and Ni10, because no crystals dissolved out when triethanolamine was removed.
The X-ray crystallographic analysis indicates Ni8 belongs to monoclinic space group C2/c and the asymmetric unit contains half of a Ni8 molecular cluster and one NO3−. As shown in Fig. 1a, the total Ni8 cluster is a cationic cluster with molecular formula [Ni8O(H3thmmg)6]2+ and two free NO3− counter anions. The Ni8 skeleton is comprised of 8 Ni2+, 1 O2− and 6 H3thmmg2− ligands and the Ni/O core is two centrosymmetric cubanes linked by sharing an O2− ion (Fig. 1c). All Ni2+ ions are six-coordinated distorted octahedral geometry with N atoms and different O atoms from H3thmmg2− ligands and an O2− ion (Fig. 1b). Ni1, Ni3 and Ni4 coordinate with 2 COO−, 2 OH−, 1 NH− and 1 O2−, and Ni2 is linked by 6 OH− from three H3thmmg2− ligands. The ranges of Ni-O and Ni-N bonds are 2.008-2.1735 Å and 2.120-2.126 Å. The O-Ni-O and O-Ni-N angles fall into the range of 83.40°-172.93° and 80.52°-163.06°. The H3thmmg2− ligand adopts only one μ7-η1: η1: η2: η3 chelating-bridging coordination mode (Fig. S1a in Supporting information) in the periphery to stabilize the core of Ni8 cluster. One O2− ion in the center of Ni8 generated in situ plays an important effect on the assembly of Ni8 cluster, which bridges six Ni2+ ions forming a typical octahedron (Fig. 1d) with Ni…Ni distances in the range of 3.037-3.074 Å.
As shown in Figs. 1e and f, the discrete Ni8 molecular clusters stack to a 3D network structure by hydrogen-bond interactions formed by H3thmmg2− ligand and free NO3− (Table S1 in Supporting information). NH− is hydrogen-bonded to free OH− of H3thmmg2− ligands with N…O distances for 2.932 and 2.836 Å and N…H-O angles for 156.06° and 159.88°. The NO3− counter anion makes a great contribution to the formation of supramolecular interactions, in which all three O atoms are as the acceptors of three coordinated OH− of H3thmmg2− ligands with O…O contacts at 2.608, 2.652 and 2.709 Å and O…H-O angles at 154.5°, 174.9° and 160.01°. Besides, the COO− can be as the acceptor of OH− of H3thmmg2− and forms the hydrogen bonds (O…O distances: 2.676, 2.706 and 2.780 Å; O…H-O angles: 173.04°, 165.69° and 159.29°).
When a new 2-mercapto-5-amino-1,3,4-thiadiazole (Hmat) ligand was added into the synthesis condition of Ni8, a new ten-nuclearity Ni cluster was isolated. It crystallizes in triclinic P-1 space group and the asymmetric unit contains an intact Ni10 cluster molecule comprised of 10 Ni2+ ions, 1 O2−, 2 OH−, 4 H3thmmg2− and 8 mat− (Fig. 2a). The Ni10 metal skeleton looks like a pudgy tetrahedron (Fig. 2b). Its four vertexes are four Ni2+ ions and the remanent six Ni2+ ions are located in the tetrahedral cavity in which four Ni2+ ions form a Ni4 square connecting with the other branched 2 Ni2+ ions on opposite sides of the square by OH− and mat− ligands. A O2− ion is trapped into the center of the Ni4 square adopting a m4 coordination mode with the Ni-O distances in the range of 2.132(4)-2.197(4) Å. Obviously, the total Ni10 metal skeleton is protected by one O2− in the middle of Ni4 square, two OH−, 4 H3thmmg2− and 8 mat− ligands. Two OH− connect the middle Ni4 square and the other two nickel ions in the tetrahedral cavity. Four H3thmmg2− ligands are located at the four vertices of the tetrahedron, coordinating with the four nickel ions at the vertices (Fig. 2c). 8 mat− ligands are all on the six sides of the tetrahedron, in which four mat− ligands are attached on the four sides with approximately equal length and the remaining a longer and a shorter side fix two mat− ligands in the both sides (Fig. 2d).
Same as the Ni8 cluster, all Ni2+ ions adopt six-coordinated distorted octahedral geometry with N, S and O atoms (Fig. 2e). Four Ni2+ ions of the intermediate square are combined by three O atoms (1 O2−, 1 OH− and 1 OH−H3thmmg2−), two sulfydryl from two mat− ligands and one N atom of mat− ligand. Two branched Ni2+ ions on opposite sides of the square in the tetrahedral cavity have a same NiO2N3S coordination mode with 1 OH−, 1 H3thmmg2− and 4 mat− ligands. Four vertical Ni2+ ions of the tetrahedron have two kinds of coordination modes, NiO3N3 from one H3thmmg2− and 2 mat− ligands and NiO3N2S from 1 H3thmmg2− and 2 mat− ligands. Besides, four H3thmmg2− ligands have two kinds of connection modes, namely, μ5-η1: η1: η1: η2 and μ6-η1: η1: η1: η3 (Figs. S1b and c in Supporting information). The mat− ligands have μ3-η1: η1: η1 and μ4-η1: η1: η2 connection modes (Figs. S1e and f in Supporting information).
Similarly, the three-dimensional supramolecular structure of the Ni10 cluster is self-assembly driven by hydrogen-bond interactions deriving from the free -NH2 of mat− ligands, OH− and COO− of H3thmmg2− (Fig. 2f and Table S2 in Supporting information). Every -NH2 of eight mat− ligands is involved in the formation of the hydrogen-bond network, in which six COO− and two OH− of H3thmmg2− are as the acceptors to form N-H…O interactions (N…O lengths: 2.723-3.033 Å; N-H…O angles: 121.72°-166.24°). Moreover, two coordinated OH− of two H3thmmg2− are hydrogen-bonded to two COO− of two H3thmmg2− from another two Ni10 cluster molecules and O…O contacts are at 2.610 and 2.565 Å, while O…H-O angles are at 155.67° and 139.45°. The above hydrogen-bond interactions make each Ni10 molecule linked with the adjacent another four cluster molecules, stretching to form a supramolecular structure.
Fourier transform infrared (FT-IR) spectra of Ni8 and Ni10 were measured in the range of 4000-500 cm−1. Combined with Fig. S4a (Supporting information) and the analysis of the knowledge learned, it is concluded that the broadband absorptions at 3405 cm−1 and 3272 cm−1 are the stretching vibrations of N-H and O-H from the H5thmmg ligand. And the absorption peaks at 2930 cm−1 and 2865 cm−1 are corresponding to the intramolecular saturated C-H stretch. The bending vibration of the C=O and C-O from -COO−are at 1625 cm−1 and 1024 cm−1. The bending vibration of saturated C-H bond is at 1384 cm−1. FT-IR spectrum of Ni10 is similar to that of Ni8, except for the addition of C=N bending vibration from Hmat ligand at 1530 cm−1 and the adsorption of CO2 from air at 2360 cm−1 (Fig. S4b in Supporting information). The thermogravimetries of Ni8 and Ni10 were studied by dry solid samples under N2 atmosphere from 30 ℃ to 800 ℃. As shown in Fig. S6a (Supporting information), the Ni8 and Ni10 display the weight loss in 30-90 ℃ and 30-110 ℃ respectively, corresponding to the loss of ethanol molecules. The skeletons of Ni8 and Ni10 remain stable to 290 ℃ and 240 ℃. Subsequently, the obvious weight loss could be found owing to the collapse of the frameworks.
To explore the magnetic exchange coupling of Ni8 and Ni10 clusters, direct current (dc) magnetic susceptibility was measured under a 1000 Oe dc magnetic field (Fig. 3). As shown in Figs. 3a and b, the experimental χmT values for Ni8 and Ni10 are 8.18 and 12.45 cm3 K/mol at 300 K, which is approximate to the calculated values for 8 and 10 magnetically isolated Ni(Ⅱ) centers (9.68 cm3 K/mol for Ni8; 12.1 cm3 K/mol for Ni10, with S = 1 and g = 2.2 per Ni(Ⅱ) ion) [52]. During cooling from 300–25 K, the χmT of Ni8 and Ni10 slightly decrease and the values correspondingly reach 2.06 and 5.13 cm3 K/mol at 25 K. Further decreasing the temperature to 2 K, for Ni8 and Ni10, the χmT values rapidly decrease to minimum 0.30 and 1.51 cm3/mol, which attributes to the stronger intramolecular antiferromagnetic interactions at low temperatures [53-55]. Furthermore, the χm−1 vs. T plots can be fitted with the Curie−Weiss law in the range of 60–300 K for Ni8 and 55–300 K for Ni10 (Figs. S2a and b in Supporting information), giving C = 13.58 mol/cm3; θ = −196.91 K (Ni8) and C = 15.54 mol/cm3; θ = −80.77 K (Ni10). The negative Weiss constants θ of Ni8 and Ni10 further confirm their antiferromagnetic interactions [56].
The field-dependent magnetization characterizations of Ni8 and Ni10 were recorded in the range of 0-5 T at 2 K to investigate the nature of the metamagnetism [57]. As shown in Figs. 3c and d, the plots of M vs. H for Ni8 and Ni10 exhibit the magnetization values gradually decrease at low field. The maximum magnetization values of Ni8 and Ni10 at 5 T are 0.80 and 4.14 Nβ much lower than the corresponding theoretical value 17.6 and 22.0 Nβ, also suggesting their antiferromagnetic interactions.
The temperature-dependent alternating current (ac) magnetic susceptibilities under Hdc = 0 Oe for Ni8 and Ni10 were measured at 500 and 1000 Hz. Demonstrably, no frequency dependent signals for in- and out-of-phase were observed (Figs. S3a and b in Supporting information), indicating no single-molecule magnet behaviors (SMMs).
In conclusion, by using the H5thmmg ligand under solvothermal condition, an 8-nuclear Ni cluster was isolated. And when the Hmat ligand was introduced into the synthesis of the Ni8, a 10-nuclear Ni cluster assembled by H3thmmg2− and mat− mixed ligands was obtained. The above results demonstrate the amino acid-derived H5thmmg ligand is an effective ligand for constructing polymetallic clusters. The Ni8 cluster has a cationic metallic skeleton built by two centrosymmetric cubanes Ni4(µ3-O)3(µ6-O) linked by sharing an O2− ion and the periphery is protected by six H3thmmg2− ligands. The metal core of Ni10 cluster is a pudgy tetrahedron, whose four vertexes are four Ni2+ ions and the remanent six Ni2+ ions are located in the tetrahedral cavity. Four H3thmmg2− ligands are located at the four vertices of the tetrahedron and 8 mat− ligands are all on the six sides of the tetrahedron. In addition, complexes Ni8 and Ni10 display the antiferromagnetic interactions.
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
We would like to gratefully acknowledge financial support from the National Natural Science Foundation of China (Nos. 22101148, 22071126) and the Natural Science Foundation of Shandong Province (No. ZR2021QB008).
Supplementary material associated with this article can be found, in the online version, at doi:
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Figure 1 (a) The total molecular structure of Ni8 cluster. (b) The coordination modes of all Ni2+ ions of the Ni8 cluster. (c) The bi-cubane Ni8O8 core. (d) The coordinated Ni6 octahedron with one O2− ions in the center. (e) and (f) The molecule packing and hydrogen bond interactions of Ni8 at different directions.
Figure 2 (a) The molecular structure of Ni10 cluster. (b) The tetrahedral Ni10O7S4 core. (c, d) The coordination positions of H3thmmg2− and mat− ligands in the Ni10 cluster. (e) The six-coordinated distorted octahedral geometry of all Ni2+ ions in the Ni10 cluster. (f) The hydrogen bond interactions of Ni10.
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