A novel nonlinear optically active tubular coordination network based on two distinct homo-chiral helices

Article abstract of DOI:10.1039/b308007b

A novel tubular coordination network [Zn(spcp)(OH)] (spcp = 4-sulfanylmethyl-4′-phenylcarboxylate pyridine) with a modest SHG activity based on two types of homo-chiral helices was synthesized and characterized.

Full text of DOI:10.1039/b308007b

A novel nonlinear optically active tubular coordination network based on
two distinct homo-chiral helices
Lei Han, Maochun Hong,* Ruihu Wang, Junhua Luo, Zhengzhong Lin and Daqiang Yuan
State Key Laboratory of Structural Chemisry, Fujian Institute of Research on the Structure of Matter,
Chinese Academy of Sciences, Fujian, Fuzhou, 350002, China. E-mail: hmc@ms.fjirsm.ac.cn
Received (in Cambridge, UK) 13th July 2003, Accepted 28th August 2003
First published as an Advance Article on the web 11th September 2003
A novel tubular coordination network [Zn(spcp)(OH)] (spcp
= 4-sulfanylmethyl-4A-phenylcarboxylate pyridine) with a
modest SHG activity based on two types of homo-chiral
helices was synthesized and characterized.
oxygen (O(2)) is significantly larger than other bond angles
(104.7(3)°–109.67(16)°). The bridging Zn–O–Zn angle is
137.6(4)°, similar to those found in other zinc complexes
containing bridging hydroxo oxygen atoms.7 The Zn–N dis-
tance is 2.081(8) Å and the Zn–O distances range from 1.910(7)
Å to 1.960(6) Å.
Crystal engineering based on noncentrosymmetric metal–
organic frameworks (MOFs) by using asymmetric bridging
ligands as building blocks has attracted much attention owing to
its potential applications as second-order nonlinear optical
(NLO) materials.1–4 The utilization of asymmetric bridging
ligands can introduce electronic asymmetry (push–pull effect),
which is essential for a second harmonic generation (SHG)
response. At present, there are numerous examples with three-
dimensional interpenetrated diamondoid2 or two-dimensional
grid3 structural motifs assembled by combining unsymmetrical,
rigid, and linear pyridinecarboxylate ligands and d10 metal ions
(in particular Zn2+ and Cd2+) for their favorable optical
transparency. However, the helical and other recurring coor-
dination networks used in NLO materials still remain few
explored.4 Widely found in nature, helical structural motifs
exhibit axial chirality, and can crystallize in noncentrosym-
metric space groups.5 We believe that the introduction of
chemical interaction sites in asymmetric bridging ligands is a
challenging subject in developing new polar and chiral systems,
and aesthetic structural motif with fascinating functions can be
obtained.
Toward the goal of designing NLO materials with new
noncentrosymmetric MOFs containing chemical interaction
sites, we are currently taking an approach of self-assembly
reaction between metal ions and sulfur-containing asymmetri-
cal linking ligands,6 where sulfide moieties are well known as
redox active functional groups that could enhance electronic
asymmetry. Here we report a novel two-dimensional tubular
coordination polymer, [Zn(spcp)(OH)] (spcp = 4-sulfanylme-
thyl-4A-phenylcarboxylate pyridine), together with its SHG
response, fluorescent property and thermal stability data. The
structure of the polymer is an alternating assembly of two
distinct homo-chiral helices with sulfide sites.
A striking structural feature of this polymer is the alternating
assembly of the two distinct homo-chiral helices along the
crystallographic b axis (Fig. 2). One helix is formed by hydroxo
bridging Zn(II) atoms. The Zn…Zn distance bridged by a
hydroxo anion is about 3.573 Å, and the pitch of the helix is
about 5.824 Å. The other type of helix is constructed by spcp
bridged between the Zn(II) centers, displaying an opposite
helical orientation to the former helix. The Zn…Zn distance
bridged by spcp is about 11.527 Å, and the pitch of the helix is
identical with the former. The dimension of the [Zn-spcp]
helical tube is about 1.0 3 0.7 nm, which is close to those
observed in others8 due to the large size of the spcp ligand.
Furthermore, the formation of [Zn-spcp] helical tube is due to
the sp3 configurations of C and S of the –CH2S– spacer, forcing
the spcp ligand to be nonlinear and generating the twisty helical
conformation. The C–C–S (117.1(8)°) and C–S–C (104.5(5)°)
angles of spcp and the phenyl/pyridyl rings dihedral angle
(79.7°) in the ligand clearly depict the nonlinear configuration
of spcp in the complex. These two distinct homo-chiral helices
are in an orderly arrangement with the zinc atoms functioning as
hinges, and a novel two-dimensional layer in the bc plane stacks
along crystallographic a axis in an interlocking fashion. For
each tubular helix, only one helical orientation is involved in the
crystal structure, which crystallizes in the asymmetric space
group P21. One of two lone pair electrons of the sulfide group
in spcp may locate parallel to the layer, while the other
protrudes from the layer. As a result, the two lone pairs of
electrons of the sulfide are stranded in an asymmetric
environment, that is to say, the sulfide groups are located in the
homo-chiral helices in each crystal. Although the bulk product
is racemic because the complex was derived from spontaneous
resolution from achiral components without any chiral sources,
each crystal, however, has a chirality that could be applicable as
functional materials.7
We have performed quasi Kurtz powder second harmonic
generation measurements9 (l = 1064 nm) on the complex to
The Hspcp ligand in the Zn(II) complex was prepared by
mixing 4-mercaptopyridine and a-bromo-p-toluic acid in water
at refluxing temperature. After cooling down to room tem-
perature, the resulting crystals were filtered out and were
confirmed to be Hspcp.† Colorless crystals of the
[Zn(spcp)(OH)] complex were obtained by diffusion of an
aqueous solution of Hspcp into a methanol solution of
Zn(NO3)2·6H2O in the presence of triethylamine (yield 65%).
The structure of the complex was identified by satisfactory
elemental analysis, IR and X-ray diffraction.‡ As shown in Fig.
1, each zinc atom locates at a distorted tetrahedral environment
with two hydroxo oxygen atoms, a pyridine nitrogen atom, and
a carboxylate oxygen atom. The bond angle (120.2(4)°)
between the hydroxo oxygen (O(3A)) and the carboxylate
Fig. 1 A view of the coordination arrangement of the zinc centers of the
complex. Thermal ellipsoids for the nonhydrogen atoms were drawn at the
30% probability level. Hydrogen atoms were omitted for clarity. Selected
bond lengths (Å) and angles (°): Zn(1)–O(2) 1.961(6), Zn(1)–O(3) 1.922(8),
Zn(1)–N(1A) 2.081(8), O(3A)–Zn(1)–O(3) 109.67(16), O(3A)–Zn(1)–
O(2) 120.2(4), O(3)–Zn(1)–O(2) 105.5(3), O(3A)–Zn(1)–N(1A) 108.2(3),
O(3)–Zn(1)–N(1A) 107.9(4), O(2)–Zn(1)–N(1A) 104.7(3), Zn(1A)–O(3)–
Zn(1) 137.6(4).
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This journal is © The Royal Society of Chemistry 2003

Fig. 3 Fluorescent emission spectrum of [Zn(spcp)(OH)] complex in solid
state at room temperature.
This work was supported by the Nation Natural Science
Foundation of China and the Natural Science Foundation of
Fujian Province.
Notes and references
†
Anal: Calc. for Hspcp: C, 63.65; H, 4.52; N, 5.71. Found: C, 63.58; H,
4.61; N, 5.57; 1H NMR (500 MHz, DMSO-d6): d 4.454 (s, 2H, –CH2–), 7.31
(d, J = 4.5 Hz, 2H, –C5H4N), 8.34 (d, J = 4 Hz, 2H, –C5H4–), 7.56 (d, J
= 8 Hz, 2H, –C6H4–), 7.88 (d, J = 8.5 Hz, 2H, –C6H4–); IR (KBr, cm21):
1630(s), 1590(vs), 1546(s), 1385(vs), 1110(m), 1091(m), 1018(m), 799(s),
746(s), 635(m), 536(w), 492(s).
Anal: Calc. for [Zn(spcp)(OH)]: C, 47.80; H, 3.39; N, 4.29. Found: C,
47.68; H, 3.15; N, 4.20; IR (KBr, cm21): 3371(vs), 2925(w), 2898(w),
1600(vs), 1560(vs), 1486(m), 1382(vs), 1226(m), 1066(m), 1025(m),
810(s), 761(s), 738(m), 598(m), 497(m).
‡ Crystal data for [Zn(spcp)(OH)]: C13H11NO3SZn, M = 326.66, mono-
clinic, space group P21, a = 10.9901(12), b = 5.8238(7), c = 12.5648(13)
Å, b = 112.091(3)°, V = 745.16(14) Å3, Z = 2, m (Mo–Ka) = 1.789
mm21, Dc = 1.456 g cm23. The structure, refined on F2, converged for
1888 unique observed reflections with I = 2s(I) to give R1 = 0.0551 and
wR2 = 0.1668 and S = 1.207. CCDC 214766. See http://www.rsc.org/
suppdata/cc/b3/b308007b/ for crystallographic data in .cif or other elec-
tronic format.
Fig. 2 A view of the two types of homo-chiral helices (top) and the two-
dimensional tubular network formed by the alternating assembly (middle),
and a schematic showing the regular structure (bottom).
confirm its acentricity as well as to evaluate its potential as
second-order NLO material. Preliminary experimental results
show that the complex displays a modest powder SHG
efficiency approximately 5 times higher than that of techno-
logically useful potassium dihydrogen phosphate (KDP), and
represents the first NLO-active bulk solid based on two-
dimensional tubular coordination polymer alternating assem-
bled by two types of homo-chiral helices with sulfide sites. The
complex also exhibits remarkable thermal stability. TGA
analyses show that it has an onset temperature for decomposi-
tion above 300 °C. The stability of the complex makes it
potential candidates for practical applications.
The other interesting characteristic of this complex is its
blue–green fluorescent property. Excitation of solid samples at
l = 350 nm produces an intense luminescence with peak
maximum at 490 nm (Fig. 3), which probably originates from a
ligand-centred n–p* or p–p* process including significant
charge transfer character induced by the polar cation, mixed
with hydroxo-zinc helix promoting the couplings of metal
atoms through delocalized electron-rich systems. This lumines-
cence is quite different from blue luminescence of the
previously reported zinc polymers.2
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In summary, we have developed a synthetic approach toward
chiral solids based on 2D tubular coordination network using
sulfur-containing asymmetrical linking ligands. This work will
open the door for further exploration of other aesthetic
structural motifs and NLO-active polymeric networks.
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