Helical and network coordination polymers based on a novel C2-symmetric ligand: SHG enhancement through specific metal coordination

Article abstract of DOI:10.1039/b316931f

Cu(I) and Ag(I) coordination polymers of an axially chiral push-pull ligand possess respectively 2-D network and helical structures and the coordination mode strongly influences the solid state SHG reduction/enhancement with respect to the free ligand.

Full text of DOI:10.1039/b316931f

Helical and network coordination polymers based on a novel
C2-symmetric ligand : SHG enhancement through specific metal
coordination†
S. Philip Anthony and T. P. Radhakrishnan*
School of Chemistry, University of Hyderabad, Hyderabad 50046, India. E-mail: tprsc@uohyd.ernet.in;
Fax: +91-40-2301-2460; Tel: +91-40-2301-1068
Received (in Cambridge, UK) 6th January 2004, Accepted 20th February 2004
First published as an Advance Article on the web 25th March 2004
Cu(
I
) and Ag(
I
) coordination polymers of an axially chiral
161.8°) hydrogen bonds lead to an extended supramolecular
network (Fig. 1). In contrast to the nitro analog BNDC,12 BCDC
does not form any helical superstructure. Layering a solution of
BCDC in THF on top of a solution of [Cu(CH3CN)4]PF6 in
dichloromethane yielded crystals of [Cu(BCDC)]PF6 at the inter-
“push–pull” ligand possess respectively 2-D network and
helical structures and the coordination mode strongly influ-
ences the solid state SHG reduction/enhancement with respect
to the free ligand.
13
face. The crystals belong to the P22121 space group with half the
The flexibility and versatility inherent in coordination polymer
design facilitates the realization of a variety of supramolecular
organizations and a wide range of materials attributes such as
electrical conductivity, magnetism and optical characteristics.1
Nonlinear optical (NLO) properties2 of organized molecular
assemblies is one of the key targets of current investigations in
materials chemistry. Appreciable thermal stability and optical
transparency that can be achieved through a judicious choice of
metal ions and ligands makes coordination polymers particularly
attractive candidates for NLO applications.3 Coordination of metal
atoms has been shown to enhance the nonlinearity at the molecular4
and bulk5,6 levels. The possibility of fine-tuning the hyper-
polarizability, b, at the molecular level is an important advantage
for realizing quadratic effects such as second harmonic generation
(SHG). A variety of metals such as Zn and Cd as well as transition
elements like W, Ni, Cu and Ru and ligands based on Schiff bases,
pyridines and stilbenes have been employed.3,7–9 The quest for
unravelling fundamental structure–property correlations in ex-
tended structures warrants the exploration of novel ligand–metal
combinations that can lead to varied polymer architectures and
facilitate the development of simple approaches to the inter-
pretation of NLO characteristics.
molecule and a molecule of THF in the asymmetric unit.11 Cu(
) lies
I
on a 2-fold axis and is tetrahedrally coordinated to the two amino
nitrogens of BCDC and the cyano nitrogens of two other BCDC’s
leading to an extended 2-D network (Fig. 2).† The individual
strands in the network have a zig-zag structure; no helical
superstructure is observed. The coordination of the amino groups to
Cu( ) is facilitated by the exo orientation of the cyanophenyl groups
I
2
in BCDC. PF6 ions and THF appear in the channels of the
network. Mixing acetone solutions of BCDC and AgClO4 produced
[Ag(BCDC)]ClO4. Crystals grown by slow evaporation of acetoni-
trile solution are found to belong to the P212121 space group with
two molecules in the asymmetric unit.11 Unlike the majority of
known Ag(
coordination of two cyano nitrogens around Ag(
the cyanophenyl groups in BCDC are now oriented endo, leading to
a helical polymer chain formed through the Ag( ) links (Fig. 3).†
The ClO4 ions reside in the helical channel. Both the Cu( ) and
Ag( ) complexes are transparent (lmax [lcut-off] = 362.2 [392.4] nm
and 348.6 [367.8] nm respectively).
I
)-cyano complexes,14 the present one shows linear
). Significantly,
I
I
2
I
I
We have carried out Kurtz–Perry SHG measurements15 on
BCDC and the metal complexes; SHG variation with particle size
indicates phase-matchable behavior in all cases. BCDC shows a
Donor–acceptor substituted conjugated systems generally pos-
sess large b which can be enhanced further by the coordination of
electropositive metals at the acceptor site. Considering this, and the
fact that homochirality ensures the formation of noncentrosym-
metric lattice structures, we have investigated the deployment of
N,NA-bis(4-cyanophenyl)-(1R,2R)-diaminocyclohexane (BCDC)10
as a novel C2-symmetric ligand with a “push–pull” framework. The
axial symmetry coupled with the multiple ligation sites of BCDC
should allow the fabrication of different coordination polymer
topologies. The optical transparency and the high probability of
generating coordination polymers prompted the choice of Cu(
I) and
Ag( ) for complexation. We report the crystal structures and SHG
I
capability of the free ligand as well as the two coordination
polymers and analyze the interesting trends that they exhibit. The
modes of coordination are strikingly different with the two metals
leading to 2-D network and helical structures respectively. These
novel materials demonstrate the critical role of the coordination
mode and the polymer structure on the SHG.
Fig. 1 Molecular structure of BCDC and packing in the BCDC·H2O crystal;
C (grey), N (blue), O (red), H (white); H-bonds (broken line).
BCDC was synthesized by the reaction of (1R,2R)-diaminocy-
clohexane with 4-fluorobenzonitrile.10 Crystals grown by diffusion
of hexane into ethyl acetate solution are found to belong to the
P212121 space group with one molecule in the asymmetric unit.11‡
A water molecule included in the unit leads to N–H…O (rN7…O25
= 3.035 Å, qN7–H7…O25 = 154.9°) and O–H…N (rO25…N24
2.999 Å, qO25–H25A…N24 = 122.7°) hydrogen bond bridges which
together with direct N–H…N (rN8…N24 = 3.136 Å, qN8–H8…N24
=
=
† Electronic supplementary information (ESI) available: experimental,
crystallographic and computational details. See http://www.rsc.org/supp-
data/cc/b3/b316931f/
Fig. 2 2-D network structure in [Cu(BCDC)]PF6·THF (bc plane); PF62 and
THF are omitted for clarity; C (grey), N (blue) Cu (orange).
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C h e m . C o m m u n . , 2 0 0 4 , 1 0 5 8 – 1 0 5 9
T h i s j o u r n a l i s © T h e R o y a l S o c i e t y o f C h e m i s t r y 2 0 0 4

moderate value saturating at 0.3 U (1 U = SHG of urea).
Complexation of Cu( at the amino and cyano sites in
[Cu(BCDC)]PF6 leads to a small reduction in the SHG to 0.2 U.
Interestingly, the complexation of Ag( ) at the cyano sites alone in
provides meaningful insight into the trends of molecular non-
linearity in these polymeric systems and suggests a convenient
approach to the design of coordination polymers for SHG
applications.
I)
I
[Ag(BCDC)]ClO4 causes a nearly ten-fold increase in the SHG to
2.9 U; the SHG of some of the helical coordination polymers
reported earlier6,16 is ~ 1 U. The bulk SHG from materials is
controlled by the molecular hyperpolarizability as well as the
molecular organization. The impact of the molecular organization
in the extended polymeric structure is difficult to quantify; the
helical structure possibly enhances the bulk SHG.12 The anion can
influence the formation of the extended structures. However,
investigation of the hexafluorophosphate and perchlorate salts of
Financial support from DST, New Delhi (Swarnajayanti Fellow-
ship) and the use of the National Single Crystal Diffractometer
Facility (DST) at the School of Chemistry, University of Hyder-
abad are acknowledged with gratitude. SPA thanks the CSIR, New
Delhi for a senior research fellowship.
Notes and references
‡ CCDC 228200–228202. See http://www.rsc.org/suppdata/cc/b3/
the Cu(
I) complex and nitrate, acetate and perchlorate salts of the
b316931f/ for crystallographic data in .cif format.
Ag( ) complex indicated that the SHG is largely independent of the
I
anion. A simple computational approach provides insight into the
influence of the conformation of BCDC and the coordination of the
metal ion on the b. The conformation of the cyanophenyl groups in
BCDC may be characterized by the torsion angle, t(Cphenyl–N–C*–
C*); the single crystal analysis shows that the t are 157.9, 164.9 and
89.3° in BCDC, [Cu(BCDC)]PF6 and [Ag(BCDC)]ClO4 re-
spectively, representing exo, exo and endo conformations. Previous
studies4 have employed semiempirical as well as ab initio methods
to compute b. We have computed the static hyperpolarizability of
these ligand structures using the AM1/TDHF17 method; the
geometries from crystal structure were used and H atom positions
alone were optimized. The influence of the metal ions were
assessed by placing point positive charges at the relevant sites
1 C. Janiak, Dalton Trans., 2003, 2781.
2 (a) Nonlinear Optical Properties of Organic Molecules and Crystals,
ed. D. S. Chemla and J. Zyss, Academic Press, New York, 1989, vol. 1,
2; (b) W. Nie, Adv. Mater., 1993, 5, 520; (c) J. Zyss and J. F. Nicoud,
Curr. Opin. Solid State Mater. Sci., 1996, 1, 533.
3 (a) J. Qin, D. Liu, C. Dai, C. Chen, B. Wu, C. Yang and C. Zhan, Coord.
Chem. Rev., 1999, 188, 23; (b) O. R. Evans and W. Lin, Acc. Chem.
Res., 2002, 35, 511.
4 (a) D. R. Kanis, P. G. Lacroix, M. A. Ratner and T. J. Marks, J. Am.
Chem. Soc., 1994, 116, 10089; (b) P. G. Lacroix, S. D. Bella and I.
Ledoux, Chem. Mater., 1996, 8, 541; (c) D. Roberto, R. Ugo, S. Bruni,
E. Cariati, F. Cariati, P. Fantucci, I. Invernizzi, S. Quici, I. Ledoux and
J. Zyss, Organometallics, 2000, 19, 1775; (d) K. Senechal, O. Maury, H.
Le Bozec, I. Ledoux and J. Zyss, J. Am. Chem. Soc., 2002, 124,
4560.
5 (a) S. P. Anthony and T. P. Radhakrishnan, Chem. Commun., 2001, 931;
(b) C. Evans and D. Luneau, J. Chem. Soc., Dalton Trans., 2002, 83.
6 (a) Y. R. Xie, R. G. Xiong, X. Xue, X. T. Chen, Z. Xue and X. Z. You,
Inorg. Chem., 2002, 41, 3323; (b) L. Han, M. Hong, R. Wang, J. Luo,
Z. Lin and D. Yuan, Chem. Commun., 2003, 2580.
(parameters for Ag(
I
) and Cu( ) are not available in this program).
I
The computed b are collected in Table 1. The trends are in tune with
classical push–pull concepts; they are further confirmed using ab
initio computations18 at the B3LYP/3-21G* level carried out on the
same structures, but now with Cu(
observed that the endo orientation of the phenyl rings leads to larger
b. More significantly, the b is reduced by the coordination of Cu(
at the cyano and amino groups whereas it is enhanced by
coordination of Ag( ) at the cyano groups alone. Most notably, the
I) and Ag(
I) metals in place. It is
7 N. J. Long, Angew. Chem., Int. Ed. Engl., 1995, 34, 21.
8 S. Di Bella, Chem. Soc. Rev., 2001, 30, 355.
I)
9 H. Le Bozec and T. Renouard, Eur. J. Inorg. Chem., 2000, 229.
10 Synthesis. BCDC was synthesised by the reaction of 4-fluorobenzoni-
trile with (1R,2R)-diaminocyclohexane in DMSO-K2CO3 at 140 °C for
10 h (see Supplementary Information† for details of synthesis and
characterisation of BCDC and the complexes). Synthesis of BCDC
using a more elaborate procedure has been reported: M. Kwit and J.
Gawronski, Tetrahedron, 2003, 59, 9323.
I
computed b parallel the observed SHG values; the molecular
nonlinearity appears to exert a dominant influence on the bulk
property.
BCDC is shown to be a convenient ligand to fabricate fascinating
coordination polymer topologies. The simple computational model
11 Crystal data. BCDC·H2O : C20H22N4O, M = 334.42, orthorhombic, a
= 7.886(3), b = 15.27(3), c = 15.765(9) Å, U = 1898(4) Å3, T = 293
K, space group P212121 (no. 19), Z = 4, m(Mo-Ka) = 0.07 mm21, 3431
reflections measured, 3405 unique (Rint = 0.000), 1520 (I 4 2sI) used
for refinement. Final R(F2) = 0.0535 (I 4 2sI). [Cu(BCDC)]PF6·THF
: C28H36CuF6N4O2P, M = 669.12, orthorhombic, a = 9.5577(9), b =
10.4800(12), c = 15.639(2) Å, U = 1566.5(3) Å3, T = 293 K, space
group P22121 (no. 18, nonstandard), Z = 2, m(Mo-Ka) = 0.82 mm21
,
3086 reflections measured, 3030 unique (Rint = 0.000), 1530 (I 4 2sI)
Fig. 3 Helical polymeric chain in [Ag(BCDC)]ClO4 along the b axis;
decrease of atom size schematically represents progression down the axis;
ClO42 are omitted for clarity; C (grey), N (blue), Ag (cyan).
used for refinement. Final R(F2) = 0.0653 (I 4 2sI). Cu–amine and Cu–
cyano distances : Cu1–N3
= 2.213(6), Cu1–N4 = 1.897(6) Å.
[Ag(BCDC)]ClO4 : C20H20AgClN4O4, M = 523.72, orthorhombic, a =
9.488(3), b = 14.094(3), c = 31.354(4) Å, U = 4193.0(16) Å3, T = 293
K, space group P212121 (no. 19), Z = 8, m(Mo-Ka) = 1.12 mm21, 4401
reflections measured, 4368 unique (Rint = 0.000), 3794 (I 4 2sI) used
for refinement. Final R(F2) = 0.0445 (I 4 2sI ). Ag-cyano distances :
Ag1–N1 = 2.105(7), Ag1–N8 = 2.102(6), Ag1–N4 = 2.106(6), Ag1–
N5 = 2.092(6) Å.
Table 1 Computed static hyperpolarizabilities of BCDC (in the free form
and as ligands in the metal complexes) and the BCDC/metal systems; in the
AM1 computations, the metal ions are replaced by point charges.
Geometries are taken from the appropriate crystal structures with H atoms
alone optimized in the AM1 method
12 P. Gangopadhyay and T. P. Radhakrishnan, Angew. Chem., Int. Ed.,
2001, 40, 2451.
b/esu
13 G. J. Kubas, B. Nozyk and A. L. Crumbliss, Inorg. Synth., 1979, 2,
90.
Structure
AM1
B3LYP/3-21G*
14 Cambridge Crystallographic Database (version 5.24) shows 192 hits for
silver–cyano complexes; the number of complexes showing near linear
coordination (N…Ag….N angle = 160–180°) is only 6.
15 S. K. Kurtz and T. T. Perry, J. Appl. Phys., 1968, 39, 3798.
16 O. R. Evans, Z. Wang and W. Lin, Chem. Commun., 1999, 1903.
17 (a) M. J. S. Dewar, E. G. Zoebisch, E. F. Healy and J. J. P. Stewart, J.
Am. Chem. Soc., 1985, 107, 3902; (b) MOPAC93 J Fujitsu Inc., Japan;
(c) M. Dupuis and S. Karna, J. Comput. Chem., 1991, 12, 487.
18 Gaussian 03, Revision B.04. Gaussian, Inc., Pittsburgh PA, 2003.
BCDC
[Cu(BCDC)]PF6
Ligand
Ligand
Ligand/3Cu(
Ligand
7.49
4.26
3.85
9.66
22.35
4.74
3.38
1.49
6.53
62.60
a
b
I
)
[Ag(BCDC)]ClO4
Ligand/2Ag(
I
)
a Metal (charge) placed at the amino and the two cyano coordination sites.
b Metal (charge) placed at the two cyano coordination sites.
C h e m . C o m m u n . , 2 0 0 4 , 1 0 5 8 – 1 0 5 9
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