from the literature reports that the existence of solvent, change
of solvent or temperature can alter heterochiral/homochiral
7d,10c,12,13
crystallization.
Quasi-Kurtz powder second harmonic generation (SHG)
measurements14 show that 2 and 3 display modest powder SHG
efficiencies approximately 0.4 and 0.5 times than that potassium
dihydrogen phosphate (KDP), respectively. Unfortunately, we
were unable to clarify whether the bulk materials were homochi-
ral.
In summary, this work represents a successful example of
ligand-controlled homochiral crystallization of coordination
polymer chains via structural variation of achiral, asymmetric
bridging ligands. This implies that upon appropriate design of
analogous asymmetric bridging ligands, homochiral crystalliza-
tions could be achieved and may lead to potential applications.
Further study on the related coordination polymers is in
progress.
This work was supported by the National Natural Science
Foundation of China (No. 20131020) and the Scientific and
Technological Bureau of Guangdong Province (No. 04205405).
Notes and references
‡ Crystal data for 1 (C12H8CdI2N4O): monoclinic, space group C2/c,
˚
M = 590.42, a = 33.006(7), b = 7.412(3), c = 13.816(3) A, b =
◦
3
111.460(3) , V = 3146(1) A , Z = 8, l = 5.314 mm−1, R1 = 0.0360,
˚
wR2 = 0.0947. Crystal data for 2 (C12H8CdI2N4O): monoclinic, space
˚
group P21, M = 590.42, a = 7.057(7), b = 14.116(1), c = 8.743(9) A,
◦
b = 112.36(2) , V = 805.5(14) A , Z = 2, l = 5.188 mm−1, R1 = 0.0342,
3
˚
wR2 = 0.0890, Flack x = −0.03(6). Crystal data for 3 (C18H24CdI2N6O4):
orthorhombic, space group P212121, M = 754.63, a = 8.295(1), b =
3
−1
˚
˚
14.724(2), c = 21.236(2) A, V = 2593.8(5) A , Z = 4, l = 3.26 mm
,
R1 = 0.0453, wR2 = 0.0983, Flack x = 0.02(3). CCDC reference numbers
for crystallographic data in CIF or other electronic format.
Fig. 2 Perspective views of the chiral chains in 1 (a), 2 (b) and 3 (c).
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chains in a zipper-like fashion. It is notable that 1 could be
isolated in both MeOH and DMF solutions, which implies 1
has the optimized molecular packing fashion.
Each metal atom in 2 is coordinated in a square-pyramidal
˚
geometry [Cd–N 2.398(11)–2.469(11) A, Cd–I 2.717(3)–2.745(2)
◦
◦
˚
A, N–Cd–N 68.4(4)–143.5(4) , N–Cd–I 90.6(3)–140.8(2) , I–
Cd–I 118.63(6)◦] (Fig. 1b), very similar to that in 1. Interestingly,
2 crystallizes inthechiralspace groupwith each(P)right-handed
helical chain related by a 21-screw axis in the b direction, as
˚
shown in Fig. 2b. The pitch of the helix in 2 (14.116(1) A)
is almost twice than that of 1, attributable to the structural
difference between L1 and L2. Consequently, the packing
mode of 2 is different, neighbouring chains in 2 do not have
significant p–p stacking interaction, and they are homochiral
(Fig. S2†).
5 J. S. Seo, D. Whang, H. Lee, S. I. Jun, J. Oh, Y. J. Jeon and K. Kimoon,
Nature, 2000, 404, 982.
In 3, the metal atom exhibits a distorted octahedral geometry
˚
˚
6 (a) A. Akasaka, K. Biradha, S. Sakamoto, K. Yamaguchi and M.
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[Cd–N 2.360(5)–2.474(5) A, Cd–I 2.790(◦1)–2.878(1) A, Cd–
˚
O 2.463(5) A, N–Cd–N 68.9(2)–155.5(2) , N–Cd–I 86.4(1)–
160.9(1)◦, I–Cd–I 109.15(2)◦, N–Cd–O 75.0(2)–83.1(2)◦, O–Cd–
I 88.7(1)–161.2(1)◦], being additionally ligated by one aqua
ligand (Fig. 1c). Although the coordination geometry has been
drastically changed, similar to those in 2, homochiral, (P) right-
handed helices exist in 3 (Fig. 2c) and are related by the 21-screw
˚
axes in the b direction. The pitch of 3 is only longer by 0.608 A
than that of 2, which is attributed to the increase of coordination
number in 3. Different from those in 2, neighbouring chains
in 3 have interchain p–p stacking interactions (offset face-to-
˚
face, distances between 2-pyridyl and 4-pyridyl rings 3.64 A),
being different from that in 1 (Fig. S3†). Also interestingly, two
˚
unusual C–H · · · I hydrogen bonds (C–H · · · I 4.03 and 4.08 A,
C–H · · · I 168 and 165◦) (Fig. S4†) between chains are found in 3,
which dominate the molecular packing of 3. Therefore, we may
be able to conclude that the structure of the bridging ligand is
critically important, compared to the solvent and coordination
geometry. This observation represents a new strategy different
11 (a) Y. Cui, H. L. Ngo and W. Lin, Chem. Commun., 2003, 1388; (b) A.
Jouaiti, M. W. Hosseini and N. Kyritsakas, Chem. Commun., 2002,
D a l t o n T r a n s . , 2 0 0 5 , 4 2 4 – 4 2 6
4 2 5