Chemical Science

Structural memory effect in solid-state transformationof kinetic coordination networks
The chemistry of porous coordination networks has
shown a rapid advance in the last 15 years owing to
the hybrid nature of their building blocks (i.e., metal
ions and organic ligands) [1]. The selective synthesisof kinetic over thermodynamic products is an important
aspect in porous coordination networks. In general,kinetic or thermodynamic structures can be preparedby fast or slow crystallization methods, respectively.
However, synthetic routes enabling such selectivity
as homogeneous products are not trivial. Fast
precipitation can generate metastable products but as
microcrystals that are not suitable for single-crystal
X-ray analysis. We have recently demonstrated that
“instant synthesis” can form kinetic microcrystalline
porous coordination networks suitable for ab initio
XRPD structure determination [2] and how new
porous structures could be obtained by the annealingof kinetic products [3].
Herein, we studied a kinetically controlled familyof isostructural porous coordination networks[(ZnX2)3(TPT)2]n•5.5(solvent) (where X = I, Br, and Cl),
where TPT is tris(4-pyridyl)triazine (1-3) (Fig. 1)
obtained by “instant synthesis” and studied its solidstate reactivity at high temperatures (above 573 K).
Insights on the solid-state reactions were obtainedusing state-of-the-art methods for ab initio XRPD
structure determination by synchrotron X-ray
diffraction at BL19B2 and BL02B2 beamlines [4].
Upon heating from 300 to 673 K, network 1
undergoes a crystalline-to-amorphous-to-crystalline(CAC) phase transition. In situ XRPD shows that 1changes to an amorphous phase at 473 K, and uponfurther heating, a new crystalline porous phase (1a)appears uniformly at 573 K and is stable up to 673 K(Fig. 1). We demonstrated the guest inclusion abilityof 1a by immersing it in nitrobenzene and exposing
1a to I2 vapor. Ab initio XRPD structure determination(BL19B2 beamline) indicated that 1a can reversibly
encapsulate nitrobenzene [3] and I2 [4].
The CAC reaction in 1 involves the guest removal
and concurrent shrinking of the networks, followed by
the unlocking of interpenetrated networks (cleavageof coordination bonds), and crystallization by bondformation to yield 1a (Fig. 2).
To test if the molecular prearrangement in 1 isnecessary for the formation of 1a, we performed
control experiments by mixing, grinding, and heating
using the same stoichiometric amounts of starting
materials of 1 as in instant synthesis. X-ray analysis
proved that 1a was not formed (Fig. 2).
Intrigued by the results, we investigated theisostructural networks 2 and 3. Microcrystalline 2
was heated to 573 K and monitored by in situ XRPD
analysis. The diffraction data shows also two phasetransitions through a CAC process. Elemental analysis
results suggest the formation of the compound[(ZnBr 2 ) 3 (TPT) 2 ] n •(H 2 O) (2a). The high quality
synchrotron XRPD data of 2a was recorded at
BL19B2 and structure determination was carried out
by ab initio XRPD followed by Rietveld refinement(Fig. 3) [4]. Compound 2a is an interpenetrated
network obtained after guest removal and molecularNNIntensity (arb. units)

NIsostructural 3D NetworksNNTPT

Nitrobenzene /Methanol

ZnX2t = 30 sec[(ZnX2)3(TPT)2]n • (solvent)NAmorphous Phase
473K673K(1) X = I
(2) Br(3) Cl

573 K
473 K
423 K300 K

51015202θθ (degree)2530

ECrystalline Phase
300K

(1)

Polycrystalline Phase
673K (1a)Fig. 1. Top: Fast crystallization of 1-3 by the “instant synthesis” method. Bottom: XRPDof 1 measured at different temperatures (left). Crystal structures of 1 and 1a and energy
plot depicting the transformation from kinetic to thermodynamic products upon heatingvia an amorphous phase. Interpenetrating networks are shown in blue and orange (right).

86Interpenetrated

Not Interpenetrated
473 K

573 K

Crystalline-toAmorphous

Amorphous-toCrystalline

3 Znl2 + 2 TPT

Grind; Δ[(Znl2)3(TPT)2]n • (solvent)

(1)

1a

[(Znl2)3(TPT)2]nΔ

25000[(Znl2)3(TPT)2]n(1a)Intensity (counts)

1

process is an intermediate state retaining structural
information from the kinetic structure that is passed toa more stable structure upon heating. Clearly, such a
detailed structural analysis of organic-inorganic
materials is trivial in single-crystal X-ray analysis butnot in ab initio XRPD analysis.

Orthorhombic Pbca
a=23.089(2) Å b =22.702(2) Åc =15.646(1) Å V=8201(1) Å3
T=100 K
λ=1.3 ÅR WP = 5.39% R F = 2.79%

20000
15000Fig. 2. Top: Cartoon showing the structuraltransformation from 1 to 1a. Middle: control
experiments showed that it is not possible to obtain
the porous network 1a if the building blocks (ZnI2and TPT) are mixed, grinded, and heated. Bottom:
the molecular prearrangement in 1 is necessary toform 1a, which is only formed upon heating.

10000
5000
05rearrangement (Fig. 3). To determine whether
the unlocking of networks occurs, we investigated a
non-interpenetrated (2b) network (not shown here,
see Ref. [4]) with the same chemical composition of 2.
Heating 2b to 553 K yields the same XRPD pattern
and therefore the crystal structure of 2a [4]. Clearly,
the cleavage and formation of coordination bonds
occur during the CAC transformation from noninterpenetrated (2b) to interpenetrated (2a).
Control experiments performed by mixing thestarting materials using the same stoichiometry andgrinding followed by annealing did not form 2a,indicating that the atomic-level preorganizationin the starting solids is crucial for the formation of
the structures obtained after annealing. To ourunderstanding, in the amorphous phase, structural
information is retained and is passed via a typeof memory effect from metastable (2) to a morestable (2a) (Fig. 3, bottom).
Further heating to 723 K and slow cooling of 2
produced single crystals with a new bromide-bridged
network. In this case, the high temperature induced acertain level of calcination and, most likely, partial meltof 2 allowed crystallization upon slow cooling. A
similar behavior was observed for the chloride version
3 (see Ref. [4]), which also through a CAC process
formed a new phase (nonporous desolvated structure)that was unstable; thus, it transformed to a new
chloride-bridged network at 723 K. The different
solid-state reactivity can be attributed to the different
halides in 1-3.
In summary, we demonstrated that the frequently
observed limitation in the single-crystal X-ray analysis
of crystalline solids with good crystallinity and sizehas been tackled using a state-of-the-art ab initio
synchrotron XRPD structure solution. In a type ofmemory effect, the amorphous phase in the CAC

10152025302θθ (degree)

354045(2a)Retention of Structural Information from 2 via
Amorphous Transition: Memory Effect2473 K

3813 18 23 28 33 382θθ (degree)

573 K

2aFig. 3. Top: Experimental (red), calculated (paleblue), and difference (dark-blue) XRPD profiles fromthe final Rietveld refinement of 2a. Middle: crystalstructure of 2a with the two interpenetrated circuits ingreen and red (side view) and viewed along the c-axis(right). Bottom: XRPD showing the amorphous phaseupon heating 2 and formation of 2a. The very broad“peaks” indicate that the long-range order allows thepassing of structural information from 2 to 2a.

Javier Martí-Rujas a and Masaki Kawano b,*a Italian Institute of Technology, Centre forNano Science and Technology, Italyb The Division of Advanced Materials Science,

Pohang University of Science and Technology, Korea*E-mail: mkawano@postech.ac.kr

References[1] S. Kitagawa et al.: Angew. Chem. Int. Ed. 43 (2004) 2334.
[2] M. Kawano et al.: Angew. Chem. Int. Ed. 47 (2008) 1269.
[3] K. Ohara et al.: J. Am. Chem. Soc. 131 (2009) 3860.
[4] J. Martí-Rujas, N. Islam, D. Hashizume, F. Izumi, M.
Fujita and M. Kawano: J. Am. Chem. Soc. 133 (2011) 5853.

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