A Novel Acentric Three-Dimensional Network with Complicated Topology Hao-Ling Sun,† Bao-Qing Ma,† Song Gao,*,† and Stuart R. Batten*,‡ Beijing National Laboratory for Molecular Sciences, State Key Laboratory of Rare Earth Materials Chemistry and Applications, College of Chemistry and Molecular Engineering, Peking UniVersity, Beijing 100871, China, and School of Chemistry, P.O. Box 23, Monash UniVersity, Clayton, Victoria 3800, Australia
CRYSTAL GROWTH & DESIGN 2006 VOL. 6, NO. 6 1261-1263
ReceiVed January 19, 2006; ReVised Manuscript ReceiVed February 14, 2006
ABSTRACT: A novel noncentric three-dimensional network Cd3(NCS)6(pzmo)2 (1) (pzmo ) pyrazine-monoxide) with complicated topology has been designed and synthesized based on an asymmetric ligand pzmo in aqueous solution. The second-order nonlinear optical property measurement gives a second harmonic generation efficiency, which is approximately 4.0 times larger than that of potassium dihydrogen phosphate (KDP). Much effort has been focused on the design and controlled synthesis of organic and organo-inorganic hybrid materials through covalent,1 hydrogen-bonding,2 or other intermolecular interactions.3 The design of the ligands that may have versatile coordination modes, variable hydrogen-bonding configurations, and different kinds of intermolecular interactions plays important roles in the design of these materials. Furthermore, the connection mode of the ligands is one of the key factors governing the topology of the materials.4 Second-order nonlinear optical materials require crystallization in noncentrosymmetric space groups (those lacking inversion symmetry).5 For synthetic chemists, the rational preparation of noncentric networks is a challenge because most complexes crystallize in a centric space group.6 Noncentric coordination compounds with promising nonlinear optical (NLO) properties may be obtained either by employing an asymmetric bridging ligand to create acentric metal-ligand coordination polymers7 or by using an inherently asymmetric connection center,8 or by modifying the symmetry of molecular pillars to create polar hydrogen-bonded host frameworks;9 or by tailoring head-to-tail alignment of dipolar guests confined in organic host channels,10 which are predisposed to pack in an acentric space group. Our strategy of rational design and synthesis of noncentric coordination compounds is to employ the asymmetric ligand pyrazine-monoxide (pzmo) as the bridging ligand to create compounds with promising NLO properties. Pyrazinemonoxide (pzmo), unlike the centrosymmetric bridging of pyrazine11 and pyrazine-dioxide,12 possesses nitrogen and oxygen atoms in either end, which may act as coordination atoms resulting in two possible asymmetric bridging modes (Scheme 1), and thus has Scheme 1.
Figure 1. Local coordination geometry in the structure of 1. Note also the 1D chain subnet formed by the bridging of the cadmium atoms and the pzmo ligands. Cd atoms are shown in purple (different shades for Cd1 and Cd2), sulfur is in yellow, carbon is in green, nitrogen is in blue, and oxygen is in red. Hydrogen atoms are omitted for clarity.
Two Possible Asymmetric Bridging Modes of pzmo
Figure 2. The Cd/SCN subnet in the structure of 1.
application in constructing acentric networks. Herein, we present a noncentric three-dimensional network Cd3(NCS)6(pzmo)2 (1) based on asymmetric pzmo and Cd(NCS)2, which has a sixconnecting Cd ion and ten-connecting Cd dimers with very complicated topology. Compound 1 was prepared by mixing Cd(NCS)2 and pzmo in aqueous solution under stirring and heating.13 X-ray diffraction analysis reveals that compound 1 crystallizes in a noncentric space group Fdd2 and forms a three-dimensional (3D) network with complicated topology.14 1 contains two crystallographically unique * To whom correspondence should be addressed. (S.G.) E-mail: gaosong@ pku.edu.cn. (S.R.B.) E-mail:
[email protected]. † Peking University. ‡ Monash University.
Cd atoms, two unique pzmo ligands, and three unique SCN anions. The local geometry is shown in Figure 1. The Cd1 atoms lie on a 2-fold axis and coordinate to two SCN anions via the nitrogen atoms (Cd1-N6a ) Cd1-N6b ) 2.274(2) Å, a ) x - 1/2, y, z + 1/2; b ) -x + 1/2, -y + 1, z + 1/2), two further SCN anions via the sulfur atoms (Cd1-S3 ) Cd1-S3a ) 2.733(1) Å, a ) -x, -y + 1, z), and two different pzmo ligands via the nitrogen atoms (Cd1N2 ) 2.399(4), Cd1-N4 ) 2.414(4) Å). The Cd2 atoms occur in dimeric pairs (Cd2‚‚‚Cd2a ) 3.987 Å, a ) -x, -y + 1, z) in which the cadmium atoms are related by a 2-fold axis and connected together by µ-O atoms from two different pzmo ligands (Cd2-O1 ) 2.435(2), Cd2-O2a ) 2.390(2) Å, a ) x, y, z - 1). Each Cd2 atom also is ligated by four SCN anions, two via the nitrogen atoms (Cd2-N7a ) 2.226(3), Cd2-N5a ) 2.233(3) Å,
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Figure 3. A schematic view of the four-connected Cd/NCS network (purple) and its relationship to the (10,3)-b network (green). Each node of the (10,3)-b net lies at the center of a three-membered circuit of the fourconnected Cd/NCS net.
Communications membered rings are themselves treated as a single connecting unit, then the network becomes a three-connected net with the wellknown (10,3)-b (or ThSi2) topology.15 Thus, the four-connected net can be derived simply by replacing each three-connected node of (10,3)-b with a circuit of three four-connecting nodes, which join to other three-membered rings via shared nodes. The four-connected net is therefore uninodal and has the Schla¨fli symbol 32.104. The process of replacing single nodes with a collection of nodes to give a new topology has been referred to previously as decoration or augmentation;16 however, the process described here is subtly different from the previous examples given as the new collections of nodes are joined not by links but by sharing nodes. The oVerall topology of the structure, which is obtained from the intersection of the two previously menioned subnets, is shown in Figure 4. It is a complicated mix of three-connectors (pzmo ligands) and six-connectors (Cd1 and Cd2 cations), in a 2:3 ratio. The strength of the second harmonic generation (SHG) efficiency of compound 1 was measured using the microcrystalline powder of the complex based on the principles proposed by Kurtz and Perry.17,18 The observed SHG efficiency of this complex is 4.0 times that of potassium dihydrogen phosphate (KDP), which confirmed its acentricity. In conclusion, a novel noncentric three-dimensional network Cd3(NCS)6(pzmo)2 (1) (pzmo ) pyrazine-monoxide) with complicated topology has been revealed, and its second harmonic generation efficiency is 4.0 times that of KDP. Acknowledgment. Financially supported by the National Natural Science Foundation of China (Nos. 20125104, 20221101, 20490210), the National Basic Research Program of China (2006CB601102), and the Australian Research Council. Authors thank Prof. Mao-Chun Hong for the measurement of NLO properties. Supporting Information Available: CIF files for the compounds 1. This material is available free of charge via the Internet at http://pubs.acs.org.
References
Figure 4. Schematic representation of the overall network topology in 1; pzmo ligands are represented by the oxygen atoms (red); SCN ligands are represented as Cd‚‚‚Cd connections.
a ) x + 1/4, -y + 3/4, z - 1/4), and two via the sulfur atoms (Cd2-S1 ) 2.710(1), Cd2-S2 ) 2.672 (1) Å). Thus, all thiocyanate anions bridge two metals, while each pzmo ligand bridges three metals, one Cd1 via the nitrogen and two Cd2 atoms via the µ-O atom. The asymmetric bridging mode of pzmo is µ3-, which results in the overall acentric framework. The overall 3D network formed is very complicated and is perhaps best described in terms of two subnets. The first subnet is composed of one-dimensional (1D) chains formed by the bridging pzmo ligands (Figure 1). The chains lie on 2-fold axes and consist of alternating Cd1 atoms and (Cd2)2 dimeric pairs bridged by the pzmo ligands. The second subnet is formed by the bridging SCN anions, which bridge the cadmium atoms into an unusual four-connected network (Figure 2). This net is shown schematically in Figure 3. This unusual net is composed entirely of corner-sharing three-membered rings, each containing one Cd1 and two Cd2 atoms. If these three-
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(15) (16)
(17) (18)
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CG060037G
