Novel Alternating Ferro-Ferromagnetic Two-Dimensional (4,4) and

Mar 6, 2007 - Two novel two-dimensional (2D) and one three-dimensional (3D) coordination polymers [Ni(HTTG)(H2O)2]n (1), [Co(HTTG)(H2O)2]n (2), and ...
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Novel Alternating Ferro-Ferromagnetic Two-Dimensional (4,4) and Photoluminescent Three-Dimensional Interpenetrating PtS-Type Coordination Networks Constructed from a New Flexible Tripodal Ligand as a Four-Connected Node

CRYSTAL GROWTH & DESIGN 2007 VOL. 7, NO. 4 747-754

Suna Wang,† Junfeng Bai,*,† Hang Xing,† Yizhi Li,† You Song,† Yi Pan,† Manfred Scheer,‡ and Xiaozeng You† The State Key Laboratory of Coordination Chemistry & School of Chemistry and Chemical Engineering, Nanjing UniVersity, Nanjing 210093, P. R. China, and Institut fu¨r Anorganische Chemie der UniVersita¨t Regensburg, 93040 Regensburg, Germany ReceiVed October 31, 2006; ReVised Manuscript ReceiVed January 17, 2007

ABSTRACT: Two novel two-dimensional (2D) and one three-dimensional (3D) coordination polymerss[Ni(HTTG)(H2O)2]n (1), [Co(HTTG)(H2O)2]n (2), and [Cd(HTTG)]n (3)shave been hydrothermally synthesized by self-assembly of an unexplored tripodal ligand H3TTG (N,N′,N′′-1,3,5-triazine-2,4,6-triyltris-glycine) and corresponding metal salts. X-ray diffraction analysis reveals that the three polymers exhibit novel frameworks due to diverse coordination conformations and different acid forms of the flexible tricaboxylate ligands. The isostructural complexes 1 and 2 possess 2D frameworks with M(H2O)2(CO2)2 chains bridged by the ligands, and the resulting 3D networks were extended through R22(8) hydrogen bonds between the undeprotonated carboxylate groups and the triazine rings as well as π‚‚‚π interactions between the triazine rings. Complex 3 shows a 3D interpenetrating compact network with inorganic Cd2O2 chains interconnected by the flexible organic ligands. Topological analysis indicates that in all three complexes the tripodal ligand acts as a four-connected square-planar node, linking four-connected square-planar or tetrahedralconnected metal centers to form interesting two-dimensional (2D) (4,4) (in 1 and 2) and three-dimensional (3D) 2-fold interpenetrating PtS topologies (in 3), respectively. Interestingly, both 1 and 2 exhibit ferromagnetic interactions within the chains bridged by the syn-anti carboxylate groups and the water molecules alternately, and complex 3 displays strong photoluminescent properties at 413 nm due to the coordination of the ligands with the metal centers. Introduction Self-assembly of metal-organic frameworks (MOFs) has attracted more attention in the fields of supramolecular chemistry and crystal engineering in recent years owing to potential applications, as well as their intriguing variety of architectures.1-5 In particular, exploring highly symmetrical multitopic ligands and suitable metal salts to construct supramolecular architectures is of higher interest.6,7 Rigid multitopic ligands are often employed in a design strategy to construct coordination complexes with special topologies due to the predictability of the resulting networks. Many frameworks that exist in nature with topologies such as boracite, CaF2, CdSO4, NbO, quartz, rutile, and so on have been successfully obtained in metal-organic frameworks.8-11 Flexible ligands, however, can adopt different conformations and coordination modes according to the geometric requirements of different metal ions. Topological analysis of the structures of them is not easy to be anticipated and identified, and thus only a few examples with simple bisdentate and tripodal ligands have been reported on them.12 Recently, we have reported four novel two-dimensional (2D) and three-dimensional (3D) cadmium(II) compounds with unusual three-, eight-, and ten-connected topologies based on a flexible tripodal ligand, 1,3,5-tris(carboxymethoxy)benzene (TCMB), and different pyridyl-containing coligands.13 In order to extend our work in this field, another unexplored tripodal ligand, N,N′,N′′-1,3,5-triazine-2,4,6-triyltris-glycine (H3TTG), attracts our great attention with additional interesting characteristics: (1) it also holds flexibility because of the presence of * To whom correspondence should be addressed. E-mail address: [email protected]. † Nanjing University. ‡ Universita ¨ t Regensburg.

a -NHCH2- spacer between the aromatic ring and carboxylate moiety that may bend to require its conformation to coordinate to metal centers; (2) the multifunctional coordination sites provide a high likelihood for generation of structures with high dimensions; (3) the multicarboxylate groups can be deprotonated in different degrees to generate diverse acid modes, and nitrogen atoms on the triazine can also be protonated, which allow various, acid-dependent coordination modes; (4) the presence of O and N atoms and aromatic rings may form hydrogen bonds and stacking interactions, to extend and stabilize the whole framework. As a result, rich topologies, including coordination modes, packing fashions, and dimensionalities of supramolecular coordination solids may result from this interesting ligand. Herein, we report three novel 2D and 3D coordination networks based on it: [Ni(HTTG)(H2O)2]n (1), [Co(HTTG)(H2O)2]n (2), and [Cd(HTTG)]n (3). Among them, complexes 1 and 2 are isostructural, and both of them exhibit interesting 2D frameworks with M(H2O)2(CO2)2 chains bridged by the incompletely deprotonated ligands. The unusual R22(8) hydrogen bonds between the undeprotonated carboxylate groups and the triazine rings and π‚‚‚π stacking interactions between the aromatic rings extend the whole structure into a 3D network. However, complex 3 shows a 2-fold interpenetrating 3D compact network containing one-dimensional M2O2 chains, in which the ligand adopts a different acid form with the nitrogen atom protonated. Further investigation of the topologies indicates that the tripodal ligand can be identified as a four-connected square-planar node, linking square-planar (1 and 2) or tetrahedral (3) metal nodes to form four-connected 2D (4,4) and 3D interpenetrating PtS metal-organic frameworks, respectively. Both 1 and 2 exhibit ferromagnetic interactions in which the rare S ) 1 alternating ferro-ferromagnetic chains of the Ni(II)

10.1021/cg0607708 CCC: $37.00 © 2007 American Chemical Society Published on Web 03/06/2007

748 Crystal Growth & Design, Vol. 7, No. 4, 2007

Wang et al. Scheme 1. Coordination Modes of the Ligand (a) in Complexes 1 and 2 and (b) in Complex 3.

Figure 1. Coordination environment of the nickel(II) ion in 1. Hydrogen atoms have been omitted for clarity. Selected bond information is listed in Table 2. Symmetry codes: A, 2 - x, 1 - y, 1 - z; B, x, y, 1 + z; C, 1 - x, 1 - y, z.

centers in 1 were well fitted. Moreover, complex 3 displays strong photoluminescent properties at 413 nm due to chelating of the ligand with the metal centers. Results and Discussion Syntheses of the Complexes. Due to the low solubility of the ligand, a hydrothermal method was employed in our syntheses, and different variations of starting materials, solvents, reaction temperature, pH, and molar ratio have been investigated in detail. In order to synthesize complex 1, salts with different anions, such as NO3-, Cl-, and SO42-, have been used, and only newly precipitated Ni(OH)2 was suitable for the isolation of crystals for X-ray diffraction. For complex 2, use of CoCl2‚ 6H2O led to crystals, and the ratios of the base as well as temperature from 80 to 140 °C had no effect on the resulting complex, suggesting the complex that we obtained may be very stable in these hydrothermal conditions. For complex 3, other cadmium salts, such as Cd(NO3)2‚6H2O, instead of Cd(OAc)2‚ 2H2O only afforded precipitates, indicating that the OAc- anion may play an important role in the formation of the crystals. Seemingly, different metal ions may lead to different coordination of the ligand and thus the resulting framework. Structure Description. The X-ray crystal structure analyses of 1 and 2 reveal that they are isostructual with the general formula [M(C9H10N6O6)(H2O)2] and show the expected lattice expandability on going from divalent nickel with an ionic radius of 0.83 Å to cobalt with 0.89 Å. [Ni(HTTG)(H2O)2]n (1). As illustrated in Figure 1, the fundamental building unit of complex 1 consists of one crystallographically independent Ni(II) center. The six-coordinated center exhibits a slightly distorted octahedral geometry {NiO6}, with four oxygen atoms (O1, O1A, O5B, O6C) from two carboxylate groups of four separated TTG ligands composing the equatorial plane and two aqua molecules completing the six-coordinated sphere at the apical positions. The Ni-O distances in the plane are in the range from 2.039(3) to 2.162(3) Å, and the aqua molecules are coordinated to metal ions with distances Ni1-O1w and Ni1-O2w of 2.011(3) and 2.054(4) Å, respectively, indicating the compressed octahedral environment of the metal ion. The ligand is incompletely deprotonated as depicted in Scheme 1a. Due to flexibility, three of the arms show significant deviation from the central aromatic ring. The undeprotonated carboxylate -COOH is coplanar with the central triazine ring, while the other two arms are arranged above and below the

plane. In addition, two different coordination modes are observed: one is µ2-η2-bridging (one oxygen atom coordinates to two metal atoms), and the other is µ2-η1:η1-bridging (each oxygen atom coordinates to one metal atom, and the carboxylate group coordinates to two metal atoms). Thus, the whole ligand acts as a µ4-bridge linking four nickel(II) centers. Interestingly, the four metal centers are almost in the same plane with a slight deviation of 0.2106 Å. It is interesting to note that the metal ions are connected in such a way that one-dimensional metal chains are formed along the a-axis, in which two different rings are arranged alternately (Figure 2). The µ2-η2 carboxylate groups (O5 and O6 carboxylate group) connect the metal centers in syn-anti mode, resulting in eight-membered rings {Ni2(OCO)2} with the two metal atoms and four oxygen atoms arranged in a chairlike conformation. The neighboring metal centers Ni1 and Ni1A (symmetry code: A, 1 - x, 1 - y, 1 - z.) are separated with a distance of 4.664(2) Å. These rings are bridged further by the µ2-η1:η1 groups

Figure 2. 2-D layer structure in 1, showing one-dimensional M-O2M-(OCO)2 alternating chains bridged by the TGG ligand along the a-axis. The dotted line shows the π‚‚‚π stacking interactions.

Novel 2D and 3D Interpenetrating PtS-Type Networks

Crystal Growth & Design, Vol. 7, No. 4, 2007 749

Figure 4. Simple representation of the 2D (4,4) layer in 1, showing the connectivity of the four-connected ligands (black node) and the metal centers (green node).

Figure 3. R22(8)-type hydrogen bonds (pink lines) between the adjacent layers of 1. Only the hydrogen atoms that generate the hydrogen bonds are shown for clarity. Blue and green polyhedrons represent the edgesharing NiO6 octahedrons in two different layers.

(O1 and O2 carboxylate group), and four-membered rings {Ni2(µ2-O1)2} are formed with the Ni1‚‚‚Ni1B distance and Ni1O1-Ni1B angle (symmetry code: B, 2 - x, 1 - y, 1 - z) of 3.207(1) Å and 100.04°, respectively. Although many transition metal carboxylate-bridged complexes have been reported up to now, such alternating syn-anti carboxylate and monoatombridged infinite chains are very limited.14 Additionally, the alternating chain structure can be envisaged as edge-sharing NiO6 octahedral dimers bridged by syn-anti carboxylate groups as illustrated in Figure 3. Furthermore, the neighboring metal chains are connected through the ligands to form a 2D undulating sheet along the ac plane, in which adjacent ligands are arranged antiparallel with a centroid-to-centroid distance between triazine rings of 3.30 Å, suggesting strong π‚‚‚π stacking interactions. And the nearest metal-metal distance in different chains bridged by one organic ligand is ca. 10.698(7) Å. Notably, significant hydrogen bonding interactions exist between the adjacent sheets (Figure 3). The undeprotonated carboxylate groups -COOH can interact with the nitrogen atoms in the triazine rings and the conjoined -NH- group, forming interesting ring structures with two kinds of hydrogen bonds, represented by R22(8).15a The N‚‚‚O distance (2.651(4)-2.839(5) Å) and O-H‚‚‚N (or N-H‚‚‚O) angles (160.5°-166.8°) are both within ranges of those reported hydrogen bonds. As far as we know, similar hydrogen bonds are common in organic systems, but few examples have been documented in metalorganic frameworks up to now.15 Additionally, the packing of the whole structure is further reinforced by the hydrogen bonds between the carboxylate groups and the coordinated aqua molecules (see Supporting Information). Due to the condensed packing, no solvent molecules are observed in the network. In order to identify the connectivity between the ligands and the metal centers, a representation of one layer is illustrated in

Figure 5. Coordination environment of the cadmium(II) ion in 3. Hydrogen atoms have been omitted for clarity. Selected bond information is listed in Table 2. Symmetry codes: A, 1 - x, 1 - y, z; B, -x, -y, -z; C, 1 + x, -1 + y, z.

Figure 4. Only two arms of the ligand coordinate with the metal centers, and the undeprotonated carboxylate group can be ignored from the view of topology. The tripodal ligand plays a similar role as a tetradentate ligand and can be viewed as a planar four-connected node actually. The metal center, in the meantime, connects with four such nodes in the plane and also acts a planar four-connected node. Accordingly, the whole topology is represented as (4,4). Though this kind of network is common in metal-organic frameworks,16,12b most of them are based on bisdentate spacers or tetradentate ligands; tripodal ligands as four-connected nodes have never been reported up to now. [Co(HTTG)(H2O)2]n (2). Complex 2 is isostructural to 1. The Co(II) center adopts octahedral coordination geometry with the Co-O distances ranging from 2.077(5) to 2.191(2) Å, indicating comparably regular octahedral geometry. Similar syn-anti carboxylate bridged chains are formed along the c-axis. [Cd(HTTG)]n (3). Single-crystal X-ray diffraction reveals that the fundamental unit of complex 3 consists of one crystallographically independent cadmium(II) ion. As illustrated in Figure 5, two chelating carboxylate groups (O1, O2, O3A, O4A) from different TTG ligands, as well as one monodentate carboxylate oxygen atom (O3B), occupy the equatorial positions around the Cd1 center, and two carboxylate oxygen (O6, O6C) atoms of separate ligands hold the apical positions. Thus, the metal center is surrounded by four different ligands and the coordination geometry can be best described as a heptacoordi-

750 Crystal Growth & Design, Vol. 7, No. 4, 2007

nated pentagonal bipyramidal. The average Cd-O and Cd-N distances are ca. 2.376(4) and 2.338(5) Å, respectively, which are slightly larger than those in other Cd-OFs.17 The mean derivation of the equatorial plane is 0.3561 Å, suggesting that the pentagonal bipyramidal is much distorted. The ligand is also incompletely deprotonated in this case; it shows a significantly different coordination fashion from those in complexes 1 and 2 and exhibits its flexibility further. As shown in Scheme 1b, three carboxylate groups all coordinate with the metal centers, showing significant deviation from the central triazine ring through different coordination modes: the group below the aromatic plane adopts µ2-η2:η1 (one oxygen atom coordinates to two metal atoms, one coordinates to one metal atom, and the whole group coordinates to two metal atoms), the other two arms bend in the opposite side through binding to two metal centers with µ2-η2 and µ2-η1:η1-bridging coordination modes, respectively. It is noteworthy that one nitrogen atom of the triazine ring is protonated, which can form hydrogen-bonding interactions with the carboxylate oxygen atom with N2‚‚‚O2i distance and N2-H2A‚‚‚O2i angle (symmetry code: x, -y + 1, z - 1/2) of 2.670(1) Å and 152.9°, respectively. As a result, the whole ligand also acts as a µ4-bridge, connecting four metal centers. Interestingly, these metal atoms are in strict planar arrangement. Different from complexes 1 and 2, the connectivity of the ligands and metal centers in this complex results in a 3D framework, which can be described as inorganic Cd2O2 chains interconnected by the flexible organic ligands. Carboxylate groups of the ligands bridge the adjacent Cd(II) centers running along the c-axis, forming the Cd2O2 inorganic chains (Figure 6a). Within the chain, Cd(II) ions are arranged with a little derivation from the line, and each pair of Cd(II) ions are bridged by two η2-O bridges (O3 and O6) of two carboxylate groups from discrete ligands to form a [Cd2O2] rhombic unit with a metal-metal separation of 3.724(4) Å. The Cd1-O3ii-Cd1iii and Cd1-O6iii-Cd1iii (symmetry codes: ii, 1 + x, 1 - y, 1/2 + z; iii, +x, -y, 1/2 + z) angles are 101.9° and 106.9°, respectively. These units form the chain by sharing their two apexes (Cd2+ ions) of acute angle. The adjacent units are not in the same plane, and they are linked to each other with the dihedral angle of 20.9°. Each [Cd2O2]n chain is connected with other four chains through the ligands. In this form, large scalelike pores M4L4 are formed with dimensions of 16.733(3) × 19.283(3) Å2 based on the distances of the opposite metal centers of Cd1‚‚‚Cd1iv and Cd1v‚‚‚Cd1vi (symmetry codes: iv, -2 + x, -1 - y, -1/2 + z; v, -1 + x, 1 - y, -1/2 + z; vi, -1 + x, -1 - y, -1/2 + z) (Figure 6b). As expected, due to the large cavities, two of the repeating networks pass through each other, resulting in the formation of the 2-fold interpenetrating framework. Accordingly, the pores are reduced to a large extent and no solvent molecules are found. Interestingly, within the highly condensed structures, hydrogen bonds between nitrogen atoms of the -NH- groups and oxygen atoms of the carboxylate groups of different networks are observed (See Supporting Information). Furthermore, the adjacent aromatic rings from different moieties of interpenetrating units are separated with a centroid-to-centroid distance of 3.70 Å, indicating π‚‚‚π stacking interactions between them. These weak interactions perhaps could be considered as driving forces for interpenetration of the whole network. Similarly, for the topology of this 3D network, suitable connectors should be confirmed. Though the ligand shows different coordination modes and conformations from those in 1 and 2, it also acts as a µ4-bridge, linking four planar metal

Wang et al.

Figure 6. View of (a) Cd2O2 chains along the c-axis in 3 and (b) large scale-like pores along the ab plane with the ball-and-stick representation. Hydrogen atoms are omitted for clarity.

centers, and thus can be considered as a four-connected planar node as well. While the metal centers in this case are connected by four ligands in a tetrahedral geometry, a different topology with equal numbers of square-planar and tetrahedral geometries is formed, which can be regarded as an interpenetrating PtS topology with the Schla¨fli symbol of 4284 (Figure 7a,b).18 A design strategy has been developed that uses a tetradentate ligand to act as a four-connecting framework node that encourages the formation of four-membered rings when bound to tetrahedral metal cations. Some examples have been successful, while those based on a potentially three-connected tripodal ligand are never reported to our best knowledge.19 Significantly, the diversity in the orientation of the ligands and the metal centers changes the network topology. FT-IR Spectra and Thermogravimetric Analyses. The IR spectra show features attributable to the carboxylate stretching vibrations of the complexes. Main differences lie in the signals in the range of 1760-1670 cm-1. The presence of such signals in complexes 1 and 2 indicates the incomplete deprotonation of the H3TTG ligand, while the absence of them in complex 3 suggests the complete deprotonation of the ligand. The characteristic bands of carboxylate groups are shown in the range 1560-1620 cm-1 for asymmetric stretching and 1370-1490 cm-1 for symmetric stretching. Weak absorptions observed in the range of 2900-2950 cm-1 can be attributed to the υCH2 of the TTG ligand. The broad bands at ca. 3300 cm-1 are ascribed to the vibration of the water ligands in the complexes 1 and 2. Though isostructural, the TGA curves of complexes 1 and 2

Novel 2D and 3D Interpenetrating PtS-Type Networks

Crystal Growth & Design, Vol. 7, No. 4, 2007 751

Figure 8. Magnetic susceptibility of 1 in the form of χMT. The solid line corresponds to the best fit to the expression calculated for an alternating ferrro-ferromagnetic Heisenberg chain of S ) 1 spins.

Scheme 2.

Figure 7. (a) PtS topology of 3 and (b) two-fold interpenetrating PtS topology of 3.

are slightly different: a sharp weight loss at ca. 260 °C may suggest the loss of the coordinated water molecules and further the decomposition of complex 1, while for complex 2, this loss of the coordinated water molecules from 200 to 300 °C was significant (2H2O per unit, calcd 9.16, found 9.80), and the rapid weight loss occurred at ca. 340 °C, indicating the decomposition of the whole structure. Due to the condensed structures, complex 3 exhibits high stability without any weight loss until the temperature reaches 370 °C. Magnetic Properties. Temperature-dependent magnetic susceptibility measurements on polycrystalline samples of 1 and 2 at an applied field of 2 kOe indicate that they exhibit ferromagnetic coupling between the metal centers. As described in Figure 8, at room temperature, the value of χMT of complex 1 is equal to 1.31 cm3 mol-1 K, larger than the spin-only value for an isolated S ) 1 NiII center (1.0 cm3 mol-1 K), which might be caused by the spin-orbit coupling characteristic for nickel(II) complexes with a 3A2g ground state resulting in an increasing g factor.20 Upon cooling, the value remains constant until the temperature is about 50 K and then increases gradually, reaching the maximum of 4.14 cm3 mol-1 K at 3.0 K, indicating that ferromagnetic exchange is dominant in this complex. Below 3.0 K, the value of χMT sharply drops to 3.64 cm3 mol-1 K at 1.8 K, attributed to the significant zero-field splitting and Zeeman effect.21 An inspection of the structure of the complex allows us to account for the observed ferromagnetic coupling. Because of the long metal-metal distance of ca. 8.9 Å, the coupling interactions between the metal centers separated by the triazine rings of the ligands should be negligible. Then, two kinds of

Magnetic Exchange Pathways between the Metal Centers in Complex 1

coupling parameters should be responsible for the resulting magnetic properties of complex 1: magnetic exchange in each Ni2O2 dimer and syn-anti carboxylate bridged Ni2(COO)2 dimer. It is well-known that for such syn-anti carboxylate group, mediate ferromagnetic interactions may result,22 while for the η2-O atom bridged dimer, most of the reported similar compounds exhibit antiferro- or ferromagnetic interactions, depending on the Ni-O-Ni angle to a large extent. For NiO-Ni angles around 90°, ferromagnetic exchange is dominant, while for Ni-O-Ni angles around 120°, antiferromagnetic exchange is dominant.23 In this case, the Ni-O-Ni angle is 100.0°, and ferromagnetic exchange interactions were expected. Thus, both of the possible magnetic exchange pathways (see Scheme 2) lead to ferromagnetic interactions, further giving birth to the overall ferromagnetic characters of the complex. In order to evaluate the exchange interactions between neighboring nickel(II) atoms, the susceptibility measurements were fitted in the high-temperature region by using a mode for Heisenberg S ) 1 alternating ferromagnetic chains. In contrast to the well-studied S ) 1 alternating antiferromagnetic or alternating ferro-antiferromagnetic chains, magnetic susceptibility of S ) 1 chains with simultaneously different ferro- and ferromagnetic coupling is scarcely investigated.24 The Hamiltonian for the Heisenberg alternating ferro-ferromagnetic chain can be written as follows. In this approach, the zero-field splitting for nickel(II) is not taken into account, assuming that its effect is significant only at very low temperatures. N-1

H)-

(2J1S2iS2i+1 + 2J2S2iS2i-1) ∑ i)1

(1)

Using this model in a molecular-field approximation, we obtain best fits with the parameters of g ) 2.2, J1 ) 9.4 cm-1, J2 ) 3.7 cm-1, and zJ′ ) -0.20 cm-1, consistent with the ferromagnetic interactions deduced from the plot.

752 Crystal Growth & Design, Vol. 7, No. 4, 2007

Wang et al.

Figure 10. Photoluminescence spectra of 3 at room temperature. Figure 9. Magnetic susceptibility of 2 in the form of χMT verus T.

Though isostructual to complex 1, complex 2 shows different magnetic character due to the larger spin-orbital coupling of the cobalt(II) ion. As shown in Figure 9, the χMT value at 300 K was 3.75 cm3 mol-1 K, larger than that expected for the spinonly case (χMT ) 1.87 cm3 mol-1 K, S ) 3/2), which may be caused by an important orbital contribution. And the value decreases gradually, reaching the minimum of 2.98 cm3 mol-1 K at about 30 K, then increases sharply until the maximum of 4.02 cm3 mol-1 K at 3.0 K, and drops with the value of 3.98 cm3 mol-1 K at 2.0 K. The decrease in χMT down to 30 K basically corresponds to single-ion behavior. It accounts for the splitting of the 4T1g term into six Kramers doublets as a consequence of the combined effect of spin-orbit coupling and distortion from octahedral symmetry.21a However, the observed increase below this temperature may come from a ferromagnetic CoII-CoII exchange interaction because such spin-orbit coupling of the cobalt(II) ion can normally lead to a decrease at low temperatures. Two similar kinds of exchange pathways established the dominative magnetism: the η2-oxygen and the syn-anti carboxylate group. A more plausible explanation is that Co2+ cations along the chain are ferromagnetically coupled, as expected from the Goodenough-Kanamori rules25 for a CoO-Co exchange angle of 99.3°, which is in the range in which the Co‚‚‚Co ferromagnetic pathways are dominant as a consequence of an accidental orthogonality of the magnetic orbitals that are pointing toward the bridging oxygen atoms , for the syn-anti bridging with a Co‚‚‚Co distance of 4.705(3) Å, weak ferromagnetic coupling interactions may result.26 Considering the complicated facts of the two-dimensional structure and the addition of the spin-orbit coupling, as well as zero-field splitting effects in the Co(II) ions, further attempts to fit the χMT data were unsuccessful. Photoluminescent Properties. The emission spectra of complex 3 in the solid state were measured at room temperature (Figure 10). The complex shows strong fluorescent emissions at 413 nm upon excitation at λ ) 360 nm and significant enhancement in the fluorescence intensity is realized compared with the free ligand, which has almost no fluorescence properties. The enhancement of luminescence in d10 complexes may be attributed to the ligation of the ligand to the metal center. The coordination enhances the “rigidity” of the ligand and thus reduces the loss of energy through a radiationless pathway.27 Furthermore, π‚‚‚π interactions existing between the adjacent aromatic rings may also be favorable to reduction of the energy of π-π* transition to some extent and thus help the photoluminescence.28 Thus, this complex may be suitable as an excellent candidate of blue-fluorescent materials.

Table 1. Crystal Data and Structure Refinement Information for the Complexes complex emprical formula formula weight space group a [Å] b [Å] c [Å] R [deg] β [deg] γ [deg] V [Å3] Z Dcalcd [g cm-3] µ [mm-1] θ range index ranges R1, wR2a [I > 2σ(I)] R1, wR2a [all data] GOF a

1 C9H14N6O8Ni 392.97 P1h 7.728(2) 8.641(2) 11.481(3) 104.401(5) 104.241(5) 107.558(5) 663.8(3) 2 1.966 1.524 2.6-24.4 -9 e h e 9 -10 e k e 10 -14 e l e 14 0.0535, 0.1131

2 C9H14N6O8Co 393.19 P1h 7.809 (2) 8.672(2) 11.560(3) 107.434(5) 101.067(5) 107.813(6) 675.5(3) 2 1.933 1.332 2.6-27.4 -5 e h e 9 -10 e k e 10 -14 e l e 14 0.0375, 0.0904

3 C9H10N6O6Cd 410.63 Pc 8.367(2) 9.642(2) 7.396(2) 90.00 94.901(4) 90.00 594.5(2) 2 2.294 1.885 2.4-27.7 -10 e h e 10 -11 e k e 11 -9 e l e 9 0.0565, 0.1182

0.0621, 0.1169

0.0474, 0.0945

0.0646, 0.1205

1.017

1.009

1.031

R1 ) ∑||Fo| - |Fc||/|Fo|; wR2 ) [∑w(∑Fo2 - Fc2)2/∑w(Fo2)2]1/2.

Conclusions. From an unexplored tripodal ligand, N,N′,N′′1,3,5-triazine-2,4,6-triyltris-glycine (H3TTG), three novel 2D and 3D coordination networks have been hydrothermally synthesized and structurally characterized. The isostructural complexes of [Ni(HTTG)(H2O)2]n (1) and [Co(HTTG)(H2O)2]n (2) exhibit novel 2D (4,4) networks, whereas [Cd(TTG)]n (3) shows a 3D 2-fold interpenetrating PtS topology in which this flexible tripodal ligand acts as an unusual four-connector to connect the metal atoms in square-planar and tetrahedral geometries, respectively. In addition, complexes 1 and 2 exhibit interesting ferromagnetic interactions between adjacent metal centers, and complex 3 displays strong photoluminescent properties at 413 nm. In summary, our research demonstrates for the first time that the new flexible tricarboxylate ligand H3TTG could be a potential building block to construct novel supramolecular architectures with unusual topologies and interesting physical properties. Further investigation is ongoing. Experimental Section General Methods. H3TTG was prepared according to the previous literature.29 All of the other reagents were commercially available and used as purchased without further purification. The elemental analysis was carried out with a Perkin-Elmer 240C elemental analyzer. The FTIR spectra were recorded from KBr pellets in the range of 4000-400 cm-1 on a VECTOR 22 spectrometer. Thermal analyses were performed on a TGA V5.1A Dupont 2100 instrument from room temperature to 700 °C with a heating rate of 10 °C/min under flowing nitrogen, and

Novel 2D and 3D Interpenetrating PtS-Type Networks Table 2. Selected Bond Lengths (Å) and Angles (deg) for the Complexes Ni(1)-O(1) Ni(1)-O(1A) Ni(1)-O(5B) Ni(1)-O(6C) Ni(1)-O(1W) Ni(1)-O(2W)

2.130(3) 2.055(3) 2.162(3) 2.039(3) 2.011(3) 2.054(4)

Co(1)-O(1) Co(1)-O(1A) Co(1)-O(5B) Co(1)-O(6C) Co(1)-O(1W) Co(1)-O(2W)

2.099(7) 2.164(2) 2.191(2) 2.077(5) 2.088(2) 2.034(2)

Cd(1)-O(1) Cd(1)-O(2) Cd(1)-O(6) Cd(1)-O(3A) Cd(1)-O(4A) Cd(1)-O(3B) Cd(1)-O(6C)

2.420(7) 2.338(8) 2.313(8) 2.470(8) 2.435(9) 2.357(8) 2.306(7)

Complex 1a O(1B)-Ni(1)-O(1) O(1B)-Ni(1)-O(5C) O(6A)-Ni(1)-O(5C) O(6A) Ni(1)-O(1) O(1W)-Ni(1)-O(2W) O(1W)-Ni(1)-O(1) O(1W)-Ni(1)-O(5C) O(2W)-Ni(1)-O(1) O(2W)-Ni(1)-O(5C)

79.98(12) 101.43(11) 87.81(11) 90.82(12) 177.32(14) 88.93(13) 89.57(13) 93.36(13) 88.17(12)

Complex 2b O(6C)-Co(1)-O(1A) O(1)-Co(1)-O(1A) O(6C)-Co(1)-O(5B) O(1)-Co(1)-O(5B) O(2W)-Co(1)-O(1W) O(1W)-Co(1)-O(1A) O(6C)-Co(1)-O(1W) O(2W)-Co(1)-O(6C) O(2W)-Co(1)-O(1A)

90.73(8) 80.65(8) 88.03(8) 100.59(8) 177.08(9) 93.85(8) 88.25(8) 92.35(8) 89.00(8)

Complex 3c O(2)-Cd(1)-O(1) O(2)-Cd(1)-O(3A) O(4A)-Cd(1)-O(3A) O(3B)-Cd(1)-O(4A) O(3B)-Cd(1)-O(1) O(6C)-Cd(1)-O(6) O(6)-Cd(1)-O(3B) O(6)-Cd(1)-O(1) O(6)-Cd(1)-O(2) O(6C)-Cd(1)-O(4A) O(6C)-Cd(1)-O(3B) O(6C)-Cd(1)-O(2)

54.1(2) 81.8(3) 53.6(3) 97.0(3) 85.0(3) 162.7(3) 72.6(3) 86.2(2) 84.3(3) 87.5(3) 91.3(3) 112.1(3)

a Symmetry codes for complex 1: A, x, y, z + 1; B, -x + 1, -y + 1, -z; C, -x + 2, -y + 1, -z + 1. b Symmetry codes for complex 2: A, -x + 2, -y + 2, -z + 1; B, -x + 1, -y + 1, -z; C, x, y + 1, z + 1. c Symmetry codes for complex 3: A, x + 1, -y + 1, z + 1/ ; B, x + 1, y 2 - 1, z; C, x, -y, z + 1/2.

the data are consistent with the structures. The emission/excitation spectra were recorded on a Hitachi 850 fluorescence spectrophotometer. Magnetic measurements were carried out on a Quantum Design MPMS5XL SQUID system. [Ni(HTTG)(H2O)2]n (1). In a general synthesis, a mixture of H3TTG (0.030 g, 0.1 mmol) and newly participated Ni(OH)2 (0.026 g, 0.1 mmol) in H2O (10 mL) was placed in a Parr Teflon-lined stainless steel vessel and heated to 140 °C for 48 h. Then the reaction system was cooled to room temperature slowly, and green block crystals of 1 were obtained. After filtration, the crystals were washed with water and dried in air (0.024 g, yield 60% based on H3TTG). C9H14N6O8Ni (392.97): calcd C 27.51, H 3.59, N 21.39; found C 27.33, H 3.46, N 21.36. IR (KBr pellet): 3364(s), 2937(w), 1688(m), 1627(s), 1544(s), 1520(w), 1410(s), 1391(s), 1296(m), 1184(vw), 1042(vw), 779(w), 612(w) cm-1. [Co(HTTG)(H2O)2]n (2). Similar procedures were performed to obtain orange crystals of complex 2, except that CoCl2‚6H2O was used instead of Ni(OH)2 and several drops of triethylamide were dropped before the reaction (0.026 g, yield 66% based on H3TTG). C9H14N6O8Co (393.19): calcd C 27.49, H 3.59, N 21.37; found C 27.30, H 3.47, N 21.28. IR (KBr pellet): 3368(s), 2937(w), 1687(m), 1625(s), 1543(s), 1520 (m), 1409(s), 1390(s), 1297(m), 1182(vw), 1039(vw), 779(w), 615(w) cm-1. [Cd(HTTG)]n (3). Similar procedures were performed to obtain orange crystals of complex 1, except that Cd(OAc)2‚2H2O was used instead of Ni(OH)2. The reaction system was cooled to room temperature to give colorless block crystals of 3, which were filtered, washed with water, and dried in air (0.022 g, yield 53% based on H3TTG. C9H10N6O6Cd (410.63): calcd C 26.32, H 2.45, N 20.47; found C 26.15, H 2.33, N 20.15. IR (KBr pellet): 3378(m), 2926(w), 1664(s), 1635(m), 1624(m), 1571(s), 1541(s), 1437(m), 1411(m), 1381(m), 1331(m), 1282(m), 1169(w), 783(w), 619(w) cm-1.

Crystal Growth & Design, Vol. 7, No. 4, 2007 753 X-ray Crystallographic Study. Suitable single crystals were selected for indexing, and intensity data were measured on a Siemens Smart CCD diffractometer with graphite-monochromated Mo KR radiation (λ ) 0.710 73 Å) at 298 K. The raw data frames were intergrated into SHELX-format reflection files and corrected using SAINT program. Absorption corrections based on multiscan were obtained by the SADABS program. The structures were solved with direct methods and refined with a full-matrix least-squares technique using the SHELXS-97 and SHELXL-97 programs, respectively.30 The coordinates of the non-hydrogen atoms were refined anisotropically, and the positions of the H-atoms were generated geometrically, assigned isotropic thermal parameters, and allowed to ride on their parent carbon atoms before the final cycle of refinement. Basic information pertaining to crystal parameters and structure refinement is summarized in Table 1, and selected bond lengths and angles are listed in Table 2. CCDC 625155-625157 for complexes 1-3 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge from The Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif.

Acknowledgment. This work is supported by the Talent Foundation of Jiangsu Province (Grant BK2006513), the Major State Basic Research Development Program (Grant No. 2006CB806104), Twenty-one Century Talent Foundation of the Ministry of Education, Foundation for the Returnee of the Ministry of Education, Measurement Foundation of Nanjing University, and National Natural Science Foundation of China (Grant No. 20301010). Supporting Information Available: Additional figures, tables, TGA curves of complexes 1-3. This material is available free of charge via the Internet at http://pubs.acs.org.

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