10.1002/chem.201700048
Chemistry - A European Journal
of 1. In contrast, 2 was prepared as a pure phase (See
Experimental Section) with a yield of 85% based on Zn, allowing
us to further investigate its interesting properties. Single-crystal
structure analysis was performed to determine the structures and
chemical formulae (Table S2 and Figure S1),[9] which were
corroborated by elemental analysis (EA) and thermogravimetric
analysis (TGA) (Figure S3). The EA data confirmed the
stoichiometry of organic amines for 2. The found/calculated
percentages were C, 15.64/15.43; H, 4.61/4.20; and N, 3.15/2.99.
The structures of 1 and 2 share similar porous topology and
building module. The 2D structure of 1 crystallizes in the space
group C2/c and features a channel-like topology on the ab plane,
with nearest and farthest interlayer-free distances of 2.12 Å and
20.14 Å , respectively (Figure 1a). On the other hand, the 3D
framework of 2 crystallizes in the space group Ibca and features a
diamond-shaped 28R-channel structure with free pore size of
~10.6 Å (Figure 1b). The void space in 2 reached 50.8%, which is
higher than that in the 36R-channel structure of Cacoxenite
(44.5%).[10] Five particular modules―∞[GaF(HPO3)2]2– (block A),
(004) and (040) planes merged into one peak upon heating to
100°C (2-T1) (Figure 3b), indicating a higher symmetry. Peaks
from the pattern of 2-T1 were indexed,[11] revealing that the cell
∞[Ga2FZn(OH)5(HPO3)4]6- (block α), [Zn(HPO3)] (blocks B and
∞
β), and [Zn(HPO3)2(H2O)4]2– (block C)―comprise the structures
of 1 or 2 (Figure 1c). Blocks α and A are negatively-charged
zincogallium/gallium phosphite infinite chains; block C is a
negatively-charged zinc phosphite cluster; blocks β and B are
neutral polymorphs. Block B consists of a zinc phosphite 4R
infinite chain, while block β can be viewed as a rearrangement of
4R units into a new 8R channel (Figures S8 and S10). Since 1
needed a longer reaction time to form than 2, we speculated that
block α was likely formed of blocks A and C. A comparison of
thermodynamic or kinetic stability among these blocks has been
made. The thermodynamic stability sequence can be sorted as B >
α > C ≈ A > β; and the kinetic stability sequence can be sorted as
A ≈ B > C > β > α. The unique layer structure of 1 can be
described as an infinite linkage of αBα, while the wall structure of
2 is built from a sequence of A(BC)β(CB)A. Interestingly, the
connecting sequence in 2 was the same as that in 28R-NTHU-13,
but 2 possessed a lower symmetry (orthorhombic) than that of
28R-NTHU-13 (tetragonal). The successful preparation of 2
represents that using DADD was efficient in reproducing these
particular building modules. It is worth to note that in the
previously monoamine-templated NTHU-13 system, blocks A, B,
and C repeatedly appeared in the pore augmentation process,
while block β only appeared in 28R-NTHU-13, which was
templated by n-hexylamine. The elongation of carbon chain of
monoamines had no effect on the reproduction of block β.
However, by using DADD instead of monoamines, we
successfully produced block β again.
Figure 1. Crystal structures for 1 and 2. a) Quasi-channel structure of 1 along the c-
axis, showing the channel-like topology with nearest and farthest interlayer-free
distances. b) 28R channels along the a-axis, showing a diamond-shaped window
with a free diameter of 10.6 Å . c) Five inorganic building modules comprise
structures of 1 and 2. Tetrahedra of Zn in cyan, P in yellow, trigonal-bipyramid of Zn
in green, octahedra of Zn in blue, and Ga in pink.
The DADD templates were clearly resolved in both 1 and 2
(Figure 2). In 1, two types of protonated DADD were observed:
one bent and the other straight (Figure 2), and they had intra-
molecule N-to-N distances of 11.49 Å and 16.07 Å , respectively
(Figure S6). Similar to 1, two types of protonated DADD were
observed in 2: one with a longer N-to-N distance of 15.24 Å and
the other with a shorter N-to-N distance of 14.58 Å (Figure S6).
In both structures, the positively charged N heads interacted with
the inorganic framework through hydrogen bonds (donor to
acceptor distances are between 2.76 ~ 3.06 Å) (Figure S7).
TG analyses showed that removal of lattice water molecules
began at ca. 50°C for both 1 and 2, and could be completely
removed before 150°C (Figure S3). Variable temperature powder
X-ray diffraction (VT-PXRD) measurements showed that the
structure of 2 could sustain up to 200°C (Figure S11). From VT-
PXRD patterns, we observed that two peaks representing the
Figure 2. Schematic drawing of structure connectivity and the arrangement of
DADD in 1 (a) and 2 (b). Various stick colors represent DADD residing at various
depths along the channel.
transformed into a tetragonal system with a (or b) = 52.103 Å .
Toward 200°C, low-angle peaks shifted slightly to higher angles
(2-T2), suggesting shrinkage of the crystal cell. Interestingly, 2-T1
and 2-T2 were discovered to transform back into the orthorhombic
phase with exposure to humidity. The transformation between
tetragonal and orthorhombic phases was reversible by heating and
rehydration. To understand which part in the structure supported
the symmetry transformation, we replaced the Ga centers by Al
2
This article is protected by copyright. All rights reserved.