A close-packed Ti₆ and Ti₁₂ nanocluster superstructure

Abstract

A close-packed superstructure consisting of Ti₁₂ and Ti₆ in one single crystal is prepared via a solvothermal synthesis. The closest twelve neighboring Ti₆ clusters form a distorted cuboctahedron centered with a Ti₁₂ cluster. Such a structure imitates the stacking sequence of a face-centered cubic (FCC) metal. By employing the cuboctahedron as a basic unit, the whole single crystal is re-rendered.

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Introduction

How the atom and molecule self-assemble into a highly complex and functional entity through weak intermolecular interactions keeps fascinating the researchers in the fields of biochemistry, supermolecules, hybrid materials, etc.^1–4 Recently, the analysis of self-assembly of atomically precise nanoclusters with X-ray crystallography has drawn fast-increasing attention.^5,6 For example, Zeng et al. have revealed the directional packing of Au₂₄₆ nanoclusters with an orientational, rotational and translational order through hierarchical interactions.⁷ This research protocol sheds light on the complex assembly structure of nanoclusters at the atomic scale. Moreover, the surface organic ligand, which is usually “invisible” even with high-resolution transmission electron microscopy (TEM), can be clearly displayed using this methodology.⁸ With this information, the potential inter-cluster interactions and the specific self-assembly mechanism can be deduced with much more accuracy.⁹

Crystalline titanium-oxide clusters (TOCs) have attracted growing attention in recent years because of their rich structural diversity and broad applications.^10–13 They are desired molecular models for understanding the properties of titanium dioxide semiconductors.^14–17 To date, huge progress has been made in expanding the structural diversity, regulating the bandgap for photocatalysis and unravelling the host–guest exchange in TOCs.^18–21 However, the investigations of their self-assembly and inter-cluster interaction are relatively rare. In this aspect, TOCs are expected to attract more interest due to their structural diversity and compatibility with organic components, metals (e.g., lanthanide) and different clusters.^22–25 For example, Zhu and co-workers reported a co-crystallization of a Ti₁₂-oxo cluster and [Ti(DMF)₆] that are organized into a NaCl-like unit cell.²⁶ Zhang et al. discovered that the same Ti₁₂-oxo cluster and anionic copper halide could be organized into superstructures like those of MgCl₂ and AlP.²⁷ Unfortunately, the number of such examples is too limited to fully exemplify the aesthetic beauty and amazing complexity of TOC superstructures, leaving many questions to be answered. A prominent one is whether a close-packed structure can be achieved using TOCs.

The closest packing occurs prevalently in many metallic structures. For example, a face-centered cubic (FCC) lattice is adopted by Cu, Pd, Au and many other metals.²⁸ The closest packing of metal atoms maximizes the interactions between atoms and minimizes the energy of the system. In this work, we report the first example of a close-packed superstructure composed of Ti₁₂(μ₃-O)₁₄(OⁱPr)₁₈ (Ti₁₂) and Ti₆(μ₂-O)₆(2-pic)₆(OⁱPr)₆ (2-Hpic = 2-picolinic acid) (Ti₆) clusters in one single crystal. The structure has several obvious Q peaks at the symmetric position. The clusters may be balanced by these highly disordered components that cannot be resolved precisely. [Ti₁₂(μ₃-O)₁₄(OⁱPr)₁₈]·3[Ti₆(μ₂-O)₆(2-pic)₆(OⁱPr)₆] (denoted as [Ti₁₂ + Ti₆]) that imitates the stacking sequence of the closest-packed structure similar to that of a face-centered cubic (FCC) metal. We attempt to understand the origin of such a structure from the viewpoint of inter-cluster interactions. And by viewing the polyhedron composed of Ti₁₂ and Ti₆ as a basic unit, the single crystal of [Ti₁₂ + Ti₆] is represented by the stacking of such a polyhedron.

Results and discussion

The synthesis of [Ti₁₂ + Ti₆] involves a one-pot solvothermal reaction of titanium(IV) isopropoxide, i.e., Ti(OⁱPr)₄ and 2-picolinic acid in acetonitrile at 100 °C for 24 hours. The isolated colorless block-shaped single crystals were characterized by X-ray crystallographic analysis. The crystallographic data are listed in Table 1. Its structure is composed of two orderly arranged clusters, Ti₁₂ and Ti₆. Their detailed structures are respectively illustrated in Fig. 1. Ti₆ can be viewed as a 2-pyridinecarboxylate (2-pic) and isopropoxy co-protected hexagonal prism-shaped Ti-oxo cluster. The core of Ti₆ consists of two Ti₃(μ₃-O)₃ subunits connected by six 2-pic ligands. Each of the six Ti atoms adopts an octahedral coordination mode with six neighbouring O atoms (Fig. 1c). This type of core structure is widely observed for hexanuclear Ti-oxo clusters.^29–31Ti₁₂, on the other hand, is protected solely by isopropoxy. It is a Ti-oxo cage composed of 12 Ti atoms, 14 μ₃-oxygen atoms and 18 isopropoxy groups. Half of the 12 Ti atoms in the middle layer are five-fold coordinated with oxygen, forming six square pyramids (Fig. 1b and d). There are three TiO₆ octahedra on the top and bottom layers, respectively. They are connected to each other and to the mentioned square pyramids via edge-sharing. It is notable that if only the twelve Ti atoms are considered, they are in the shape of icosahedron (Fig. 1g). We believe this that arrangement is closely related to the mentioned close-packed superstructure of [Ti₁₂ + Ti₆] (vide infra). The FT-IR spectrum shows vibration bands at 1562 cm⁻¹ and 1404 cm⁻¹ that are assigned to ν_as(COO⁻) and ν_s(COO⁻) of the bridging 2-pic ligand (Fig. S1 and S3†).³² The UV-vis diffuse reflectance spectrum of the [Ti₁₂ + Ti₆] sample is shown in Fig. S2,† in which the band gap of [Ti₁₂ + Ti₆] is evaluated to be 2.6 eV.³³

Table 1 Crystal data and structure refinement for [Ti₁₂ + Ti₆]

Identification code	[Ti 12 + Ti6]
Empirical formula	C216H342N18O104Ti30
Formula weight	6292.06
Temperature/K	298(1)
Crystal system	Trigonal
Space group	R3̄
a/Å	30.0732(10)
b/Å	30.0732(10)
c/Å	29.0267(11)
α/°	90
β/°	90
γ/°	120
Volume/Å^3	22 734.6(17)
Z	3
ρ calc g cm^−3	1.379
μ/mm^−1	7.012
F(000)	9768.0
Crystal size/mm^3	0.5 × 0.02 × 0.01
Radiation	CuKα (λ = 1.54184)
2Θ range for data collection/	6.972 to 153.054
Index ranges	−35 ≤ h ≤ 36, −37 ≤ k ≤ 32, −36 ≤ l ≤ 36
Reflections collected	21 819
Independent reflections	10 015 [Rint = 0.0622, Rsigma = 0.0763]
Data/restraints/parameters	10 015/184/584
Goodness-of-fit on F^2	1.132
Final R indexes [I ≥ 2σ(I)]	R 1 = 0.1178, wR2 = 0.3106
Final R indexes [all data]	R 1 = 0.1774, wR2 = 0.3633
Largest diff. peak/hole/e Å^−3	0.77/−1.01

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Fig. 1 The structures of Ti₁₂ (Ti₁₂(μ₃-O)₁₄(OⁱPr)₁₈) (b and d) and Ti₆ (Ti₆(μ₂-O)₆(2-pic)₆(OⁱPr)₆) (a and c). The core structures of Ti₆ (e) and Ti₁₂ (f). (g) The icosahedron composed of 12 Ti atoms of Ti₁₂.

Since Ti₁₂ and Ti₆ co-crystallized in one single crystal, we analyzed their packing sequence. They co-crystallize in the R3̄ space group. Fig. 2a shows their packing modes in one unit cell containing three Ti₁₂ and nine Ti₆ clusters. Eight Ti₁₂ clusters occupy the vertices of the unit cell. Two Ti₆ clusters are located at the centers of two faces. And another eight Ti₆ clusters are found at the middle of the edges. The rest of the Ti₁₂ and Ti₆ clusters fill the interstitial positions. What is interesting is that if the closest neighbours of one Ti₁₂ cluster are analyzed, we will find twelve Ti₆ clusters forming a distorted cuboctahedron (Fig. 2b). It can be regarded as a closest-packed structure that mimics the FCC metal by viewing Ti₁₂ and Ti₆ as atoms. A comparison of FCC copper and [Ti₁₂ + Ti₆] is shown in Fig. 2c and d. They are similar but with a significant difference. The polyhedron of [Ti₁₂ + Ti₆] is not a standard cuboctahedron, because it is not composed of regular polygon faces. It consists of six 15.0 × 13.0 Å rectangles and eight triangles. The triangles include two equilateral triangles with 15.0 Å edges and six isosceles triangles with 13.0 Å long legs and 15.0 Å long base. Therefore, it cannot be categorized as an Archimedean solid but is better described as a slightly compressed cuboctahedron on two opposite ends. Note that such a local structure applies to each of the Ti₁₂ clusters in [Ti₁₂ + Ti₆]. This type of close packing of two Ti-oxo clusters in one crystal has not previously been reported.

Fig. 2 (a) The stacking of [Ti₁₂ + Ti₆] in one unit cell. (b and c) The local structure of one Ti₁₂ and twelve neighbouring Ti₆ clusters. (d) Face-centered cubic copper.

In order to gain insight into why [Ti₁₂ + Ti₆] adopt such an unusual stacking structure, we focused on the inter-cluster interactions. In the study of self-assembly of other types of nanoclusters, such as Au₂₄₆,⁷ weak interactions were determined as critical factors. In [Ti₁₂ + Ti₆], as mentioned above, the twelve Ti atoms of Ti₁₂ are in the shape of an icosahedron (Fig. 1g). Each of the top three Ti atoms is bonded to an isopropoxy group. And between every two adjacent Ti atoms, another isopropoxy is bridged. In all, there are six isopropoxy groups for the three Ti atoms (Fig. 3c and d). Two neighbouring isopropoxy groups form multiple C–H⋯π interactions with two pyridine rings of one Ti₆ (distances ranging from 3.3 to 3.8 Å). Therefore, there are three appropriately oriented Ti₆ clusters getting involved in such interactions. The arrangement is similarly found for the three bottom Ti atoms. For the six Ti atoms in the middle layer, they each bond with one isopropoxy. The hydrogen of the isopropoxy forms a C–H⋯π interaction with the pyridine rings of one adjacent Ti₆ with a H⋯π distance of 3.8 Å (Fig. 3b). There are six Ti₆ clusters of this type. If viewed perpendicularly, the middle layer is roughly in a C₆ symmetry. Therefore, the presence of 12 Ti₆ clusters around Ti₁₂ forming a distorted cuboctahedron likely originates from the specific ligand interaction between the clusters. To maintain such interactions, the pyridine rings of Ti₆ have to be properly oriented. Because of that, there are actually three orientations for twelve Ti₆ clusters. There are four Ti₆ clusters in each orientation.

Fig. 3 (a) The structure of the isopropoxy group and Ti₁₂ core. (b) The C–H⋯π interactions between isopropoxy groups bonded to six Ti atoms in the middle of Ti₁₂ and neighbouring Ti₆. (c and d) The C–H⋯π interactions between the isopropoxy groups of the top of the Ti₁₂ and neighbouring Ti₆.

The single crystal of [Ti₁₂ + Ti₆] can be viewed as the orderly stacking of Ti₁₂ and Ti₆. The most straightforward way for characterization is analyzing the unit cell, which reflects both the content and symmetry elements of [Ti₁₂ + Ti₆]. We can also redraw [Ti₁₂ + Ti₆] by viewing the above distorted octahedron as a basic building block. The stacking of such polyhedra is illustrated in Fig. 4. For ease of viewing, the polyhedra are rendered with two colours. Ti₁₂ is represented by a green sphere located in the center of the polyhedron. And yellow sphere is Ti₆, which is the vertices of the polyhedron. The distorted cuboctahedron has six rectangle faces. By face sharing, each cuboctahedron is connected to six neighbouring cuboctahedra. And this principle applies to every cuboctahedron. Following such a packing style, the resulting superstructure shown in Fig. 4c is obtained. In this 3 × 3 × 3 unit composed of 27 cuboctahedra, there are 27 Ti₁₂ and 144 Ti₆ clusters. Fig. 4d also provides a vertical view of the green cuboctahedron layer. It is clear that the cuboctahedra of Ti₁₂ and Ti₆ are regularly and tightly packed together.

Fig. 4 (a and b) The stacking structure of one Ti₁₂ and twelve neighbouring Ti₆ clusters. (c) A 3 × 3 × 3 cells consisting of 27 cuboctahedra. (d) A vertical view of the green cuboctahedron layer.

On the other hand, if we view Ti₆ as a center, its neighbouring clusters also form a cuboctahedron. The 12 vertices include four Ti₁₂ and eight Ti₆ clusters (Fig. S4†). Such a result stems from the fact that the packing mode of [Ti₁₂ + Ti₆] matches the face-centered cubic lattice. An alternative intercluster topology is illustrated in Fig. 5. The eight corners and four face centers of the cube are arranged by Ti₆ clusters. The remaining two face centers are occupied by Ti₁₂ clusters. The above results indicate that we can regard the cuboctahedron as a supercell, which is more straightforward to understand the single crystal structure of [Ti₁₂ + Ti₆].

Fig. 5 The comparison between (a) the packing structure of [Ti₁₂ + Ti₆] and (b) the face-centered cubic copper.

Conclusions

In summary, the first example of a close-packed superstructure composed of Ti₁₂ and Ti₆ clusters is presented. It is synthesized via a one-step solvothermal synthesis. The detailed structure is revealed by X-ray crystallography. The twelve nearest neighbouring Ti₆ clusters form a distorted cuboctahedron, which is similar to the face-centered cubic metal. Our further analysis indicates that such a local structure can be induced by the weak interactions between the ligands of Ti₁₂ and Ti₆. Moreover, the whole single crystal can be redrawn by employing the cuboctahedron as a basic unit. The findings described in this paper are expected to stimulate the research on the superstructure of Ti-oxo clusters.

Author contributions

Huan Li and Xiaoqin Cui: synthesis, structural analysis, and writing of the paper. Xin Li, Dong Jing and Yuan Wang: data collection on an X-ray diffractometer and crystal structure refinement. Huan Li and Xian-Ming Zhang: conceptualization, funding, and revision of the paper. All authors have discussed the results and contributed to the final manuscript.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

We express our gratitude to the National Natural Science Foundation of China (21972080, 21503123, 91961201, and 21871167), Sanjin Scholar, Shanxi “1331 Project” for financial support.

Notes and references

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Footnotes

† Electronic supplementary information (ESI) available: CCDC 2204197. For ESI and crystallographic data in CIF or other electronic format see DOI: https://doi.org/10.1039/d2ce01198k

‡ These authors contribute equally to this work.

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