Photon Factory Activity Report 2014 #32 (2015) B
AR-NW2A/ 2014G008

Single-crystal growth of coordination networks via the gas phase
Tatsuhiro Kojima, Wanuk Choi, and Masaki Kawano*
Division of Advanced Materials Science, Pohang University of Science and Technology (POSTECH),
Pohang 790-784, Korea
1 Introduction
To achieve high performance of various applications inporous coordination polymers (PCPs) or metal-organic
frameworks (MOFs), not only functionality but also
thermal stability of networks is important. To create
thermally stable networks, it is essential to develop
networking methods. Most of the MOFs have been
prepared from solution phase reaction such as
solvothermal synthesis because solvent readily assists informing a coordination bond and making a large pore.
However, sometimes the network containing a number ofsolvents in a large pore is so fragile that it cannot retain aframework.
Therefore, we paid attention to thenetworking via gas phase reaction as a solvent-freemethod. Herein we report the single crystal growth ofthermally stable coordination network via gas phase.1
2 Experiment
We observed rapid vaporization of starting materials
and deposition of white crystalline powder on the insidesurface of the glass ampule within 10 seconds on rapidly
heating of ZnI2 and TPT (TPT = 2,4,6-tris(4pyridyl)triazine) together (at the mole ratio of 2:1) at 850
K under reduced pressure (~0.1 Torr) in a glass ample.
Afterward, the white crystalline powder turned stepwise
light yellow to dense yellow for 2 min. Three kinds ofcrystalline phases were found by powder X-ray
diffraction. By optimizing experimental conditions oftemperature, reaction time, and mole ratio of ZnI2 andTPT, we successfully isolated each single crystal,[(ZnI2)(TPT)]n (1), [(ZnI2)3(TPT)2]n (2), and [(ZnI2)(μI)(ZnI)(TPT)]n (3) in Table 1.
The single crystal X-ray structure analysis revealed eachstructural detail. 1 has a stacking structure of 1D
undulated chains composed of ZnI2 and TPT,[(ZnI2)(TPT)]n. 2 has a stacking structure of 1D saddletype unit. Two TPTs connected by ZnI2 form a saddletype unit and each saddle-type unit forms a 1D chain
through ZnI2 connectors, [(ZnI2)3(TPT)2]n. Thanks to thesaddle-type unit, 1D channels form in the structure withpore volume of 293.4 Å3 per unit cell. To the best of ourknowledge, this is the first example of single crystal
growth of porous coordination network via gas phase.
The single crystal X-ray structure analysis of 3 revealed a
new type of iodide single-bridging coordination mode,(ZnI2)(μ-I)(ZnI).
Crystal structure analysis provides an insight of relative
thermal stability of each network regarding bonddissociation energy. TG curves support each thermal
stability by the decomposition temperatures, 1 < 2 < 3.

This result indicates that, to control each phase, reaction
temperature and mole ratio of ZnI2 and TPT are crucial inselectivity of single crystal growth as shown in Table 1.
Therefore, the energy diagram for three-step reactions inscheme 1 was proposed.

Scheme 1. Proposed energy diagram via gas phase.
Table 1. Dependence of selectivity of single crystal growth onreaction temperature and mole ratioa
Mole ratioofZnI2:TPT

673

708

723

743

788

843

1:1
3:2
2:1

1×b×b11, 221, 21, 22222×c×c2, 3d×c×c
3d

Reaction temperature / K[a] All reaction times (except 2:1, 788K and 843K) are 2hours leading to thermal equilibrium state. [b] No reaction. [c]Decomposition. [d] 5 min

In summary, we have successfully prepared three kindsof single crystals via gas phase. Especially, 2 is the firstexample of single crystal growth of a porous coordination
network via gas phase, which can trap small molecules ina pore. Furthermore, the equilibrium among threenetworks can be controlled by temperature andstoichiometry. This synthetic method via gas phase can beapplied to the materials which cannot be available insolution phase, for example, because of low solubility.
References[1] T. Kojima et al., Chem. Commun., 2014, 50, 1379313796.
*mkawano@postech.ac.kr