Communication pubs.acs.org/crystal
Crystal Engineering with Modified 2‑Aminopurine and Group 12 Metal Ions Balaram Mohapatra† and Sandeep Verma*,†,‡ †
Department of Chemistry and ‡DST Thematic Unit of Excellence on Soft Nanofabrication, Indian Institute of Technology Kanpur, Kanpur 208016 (UP), India S Supporting Information *
ABSTRACT: This communication describes the synthesis and structure of zinc, cadmium, and mercury complexes of modified N9-substituted 2-aminopurine (2AP) analogues. 2AP is an inherently fluorescent heterocyclic nucleobase with wide applications in biochemical reactions. Herein, we report a one-dimensional (1D) polymeric helical chain and discrete arrangements for zinc complexes [C8H11N5Cl2Zn] (1) and [C12H15N5O5Zn] (2), a 1D polymeric helical chain for a Cd complex [C8H11CdI2N5] (3), and a unique example of s u p p o r t e d 2 A P - H g ( I I ) t w o - d i m e n s io n a l c l u s t e r s [C16H21Cl6Hg3N10] (4). The photoluminescence properties of free ligand L and complexes 1−4 are also presented.
Hg ions are d10 systems, which exhibit coordination numbers ranging from 2 to 8, with linear to distorted dodecahedral geometries. Complexes of these metal ions offer a route for the construction of luminescent metal complexes and electroluminescent materials for potential organic light-emitting diode (OLED) applications.6 Given their closed shell configuration, excited energy states of d10 complexes are based on ligandcentered and/or ligand-to-ligand charge transfer properties. However, there are some reported instances of ligand-to-metal charge transfer states as well.6e We have previously investigated nucleobase−metal ion interaction with Ag(I) and Cu(II) d10 systems, giving us further impetus to study metal-containing supramolecular frameworks with 2AP.7 We recently reported two interesting examples of Ag(I) and Hg(II) mediated cyclization of propargylated adenine and guanine nucleobases.8 The ligand used in this study, N9-propyl-2-aminopurine, was synthesized by using a modified literature procedure.9 Its colorless crystals were grown in aqueous methanol by slow evaporation, and the crystal refinement data belonged to the monoclinic system in (C2/c) space group. The asymmetric unit was composed of two ligand molecules, stabilized by intermolecular hydrogen bonding, along with a water molecule (Figure S1, Supporting Information). The modified 2AP ligand was further reacted with Zn(II), Cd(II), and Hg(II), and the coordination assemblies were studied by crystallographic analysis. Zn complex 1 was synthesized by reacting ligand with ZnCl2, and the block-shaped colorless crystals were grown in aqueous
2-Aminopurine (2AP), a structural analogue of the natural nucleobase adenine, is widely used as a fluorescent base in nucleic acids to follow biochemical transformations and for mutagenesis experiments, due to its ability to base pair with thymidine and cytosine.1 These mismatches understandably cause transition mutations when 2AP is added exogenously to Escherichia coli growth medium2 and also engender frequent occurrence of frameshift mutations.3 From photophysics point of view, the absorption maxima of 2AP is red-shifted compared to other natural nucleobases, making it simpler to achieve selective excitation of nucleic acids containing this modified base. Although 2AP nucleoside exhibits a finite fluorescence lifetime, detailed photophysical studies with nucleic acids are not possible due to lack of sufficient information concerning its solvation and interbase interactions.4 The movement of an exocyclic amino group from C6 to C2 position is responsible for the altered hydrogen bonding pattern, thus leading to noncognate base pairing. This substituent shift considerably improves the metal ion binding ability of N7 imino nitrogen, while completely suppressing metal ion interactions at the N3 position due to steric hindrance.5 Consequently, it has been noted that metal ion coordination, often leading to macrochelate formation, is highly facilitated at the N7 position in 2AP derivatives such as 9-[2phosphonomethoxy)ethyl]-2-aminopurine, while obliterating any interaction at N3, when compared to corresponding metal ion interactions with 9-[2-phosphonomethoxy)ethyl]adenine.5a We decided to investigate 2AP coordination with Zn(II), Cd(II), and Hg(II), as we envisaged interesting structural properties as well as careful modulation of photophysical behavior of this fluorescent nucleobase. Divalent Zn, Cd, and © XXXX American Chemical Society
Received: April 23, 2013 Revised: June 4, 2013
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Figure 1. Molecular and crystal structures of modified 2AP ligand and ORTEP diagram of the asymmetric unit of the complexes 1 and 2 drawn at 50% probability level (hydrogen atoms omitted for clarity).
Figure 2. (a) Intermolecular N2−H2A···N3 hydrogen bonding interactions of polymeric complex 1 (propyl groups, C8−H and C6−H are omitted for clarity). (b) Purine−purine interaction and H-bonding interaction between water molecules and acetate groups of complex 2 (propyl groups and methyl groups are omitted for clarity). (c, d) Coordination environments are shown around the “Zn” atom in complexes 1 and 2.
triclinic system with (P1̅) space group. In the asymmetric unit, the Zn(II) ion was neutralized by two acetate groups, and it was found hexacoordinated to four oxygens from the acetate group, a water molecule, and N7 nitrogen of the ligand, exhibiting a distorted-octahedral geometry (Figure 2d). Careful investigation of the crystal lattice of 2 revealed a hydrogen bonding interaction between purine bases through the involvement of corresponding Watson−Crick faces (N2−H2B···N1) (Figure 2b). In the case of complex 1, N1 nitrogen was involved in metal coordination, but in complex 2 only N7 nitrogen is involved in coordination. N1 nitrogen prefers to hydrogen bond with the exocyclic amino group with a hydrogen bonding (N2−H2B···N1) distance and bond angle of 2.14 Å and 171°, respectively. The purine amino nitrogen (N2) also participates in hydrogen bonding with O4 of the acetate group (N2− H2A···O4). Interestingly, acetates and water molecule also engender secondary hydrogen bonding interactions forming a two-dimensional (2D) network in the crystal lattice (Figure 2b).
methanolic solution in about a week. Single crystal data of [C8H11N5Cl2Zn] belonged to the Monoclinic system in (P21/ n) space group. The asymmetric unit of 1 was composed of a Zn atom, two chlorides, and one ligand (Figure 2c). Zn coordination in complex 1 exhibited a distorted tetrahedral geometry, where the metal ion was coordinated to two nitrogen atoms (N1 and N7) of ligand and two chlorides. A onedimensional (1D) zigzag polymeric structure was noted in the lattice (Figure 2a). The bond angle between N7−Zn−N1 was 101.5°, whereas the bond distances varied from 2.07 to 2.08 Å. Interestingly, purine−purine self-association was supported via intermolecular hydrogen bonding (N2−H2A···N3). The observed hydrogen bonding (N2−H2A···N3) distance and bond angle is 2.38 Å and 178°, respectively. In another experiment, we decided to react and crystallize modified 2AP with zinc acetate. This effort led to the formation of a discrete complex 2, where we obtained block-shaped colorless crystals by slow evaporation from their methanolic solution. Refinement data of [C12H15N5O5Zn] belonged to the B
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Figure 3. (a) Hexameric crystal packing of complex 3 with purine−purine hydrogen bonding interaction and (b)tetrahedral coordination of “Cd” are shown in the figure (propyl groups are omitted for clarity).
metries within the crystal lattice (Figure 4d). One μ-Cl− bridges three Hg(II) ions, whereas the other bridges two Hg(II) atoms. The bond distance between Hg(II) and Cl− varied from 2.31 to 2.93 Å, which is less than the sum of their van der Waals bond distance (3.2 Å).10 This bridging manifests organization of two fused cyclic mercuric clusters: one being an eight-membered (Hg4Cl4) and the other a six-membered (Hg3Cl3) (Figure 4c). The cluster formation was strongly supported by N7−HgII−N7 dimer, with the observed Hg−N7 bond distance varying from 2.09 to 2.10 Å with nearly linear bond angles. Such mercuric clusters are unique and have not been previously reported for purine derivatives to the best of our knowledge. Hg(II) ion has the potential to interact with DNA bases through various coordination sites.11 Lippert and co-workers have studied interaction of Hg(II) with different purine analogues and reported novel molecular motifs with varied coordination behavior.12 Halogenmercurate(II) complexes generally form three types of halide bridging motifs (μ2single, μ2double, μ3triple). The well-studied singly halogen−bridged chain [Hg−X−Hg−X]n, where each Hg atom is coordinated by an organic ligand and two terminal chlorides and one bridging halogen atom leads to form an 1D infinite chain,13 whereas the double-bridge (μ2) motif [Hg−(X)2−Hg]n, where two halogens are involved in bridging with two Hg atoms, afford 1D or 2D polymeric structure.14 However, there are a few reports about the triple-bridge (μ3) motif (Hg3Cl3) cluster15 and also cubanelike 2D supramolecular Hg−Cl−Hg cluster.16 But to the best of our knowledge, fused Hg3Cl3 and Hg4Cl4 motifs, with purine ligand involvement, are not reported in the literature. Further inspection of the crystal lattice revealed strong intermolecular hydrogen bonding through Watson−Crick face (N2A−H2A2···N1A) resulting a 3D supramolecular frames (Figure 4a). N2A−H2A2···N1A hydrogen bonding distance is 2.21 Å, and the bond angle was observed to be 172°. Weak Hg···Hg interactions further stabilized the polymeric complex. The separation is larger than the Hg···Hg distance in metallic mercury (3 Å) (Figure S5), but smaller than between the two mercury atoms that varied from 4.10 to 4.27 Å which is smaller than the van der Waals radii of the two atoms (5 Å).17 The separation between two purine rings is 7.29 Å. The important
Interaction of cadmium iodide with ligand afforded blockshaped colorless single crystals through slow evaporation from aqueous methanolic solution. Crystal refinement showed that complex 3 crystallized in a monoclinic space group (P21/c). A closer inspection revealed a bidentate coordination mode with N1 and N7 ring nitrogens being coordinated to two cadmium ions in a head-to-tail fashion, resulting in a 1D polymeric chain. Adjacent helical chains interacted with each other with the help of hydrogen bonding interaction with the N3 nitrogen and exocyclic C2 amino group (N2−H2A···N3) resulting in a polymeric structure (Figure 3). The bond distance and bond angles between N2−H2A···N3 are 2.22 Å and 174°, respectively. Table 1. Selected Hydrogen Bonding Present in Complexes 1−4a D−H···A# N2−H2A···N3i N2−H2B···N1 N2−H2A···O4 O1W−H1W···Oii O1W−H2W···O3iii N2−H2A···N3iv N2A−H2A2···N1Av
D···A Complex 3.241(4) Complex 2.997(6) 3.001(6) 2.730(6) 2.687(6) Complex 3.075(4) Complex 3.076(2)
H···A
D−H···A
1 2.38
178
2.141 2.195 1.84(9) 1.79(8)
171 157 172(8) 171(8)
2.221
174
2.21
172
2
3 4
Symmetry code: (i) 1 − x, 1 − y, 1 − z, (ii) 1 − x, 1 − y, 1 − z, (iii) 2 − x, −y, 1 − z, (iv) 1 − x, 1 − y, −z. (v) −x, 2 − y, 1 − z.
a
Complex 4 was synthesized by reacting ligand with HgCl2 in aqueous methanolic solution at room temperature, followed by slow evaporation to afford crystal formation. Crystal refinement data [C16H21Cl6Hg3N10] belonged to the triclinic system in (P1̅) space group. The asymmetric unit of complex 4 consisted of four Hg(II), six Cl−, and two ligand molecules, out of which two have occupancy of one and the rest have an occupancy of half. The overall charge was neutralized by six Cl− anions present in the unit cell. These chloride anions act as bridges and support the formation of mercury clusters where Hg(II) exhibits both tetrahedral and octahedral coordination geoC
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Figure 4. (a) 3D crystal lattice packing present in the complex 4. (b) Part of the crystal lattice forming H-bonding through Watson−Crick faces. (c) 2D lattice containing six- and eight-membered ring with purine ligand viewed along the a axis. (d) Part of the crystal lattice of complex 4 showing octahedral and tetrahedral arrangements.
Figure 5. PXRD spectrum of complexes 1−4. (a) Synthesized and (b) simulated at 100 K.
bond angles and bond distances are given in the Supporting Information. The PXRD observations for complexes 1−4 suggests that the diffraction lines of complexes 1−4 are consistent with their respective simulated PXRD patterns formed at 100 K (Figure 5). According to this information, complexes 1−4 belong to
Fluorescent properties of 2AP led us to investigate the photophysical properties of ligand and complexes 1−4. Figure 6 illustrates photoluminescence properties of ligand L along with complexes 1−4, studied at room temperature. Ligand L exhibited a strong near UV emission at 372 nm and a weak visible emission at 478 nm, upon excitation at 330 nm. Ligand L emission could arise from the lowest excited π−π* state and a second excited n−π* state transitions.18 Complex 1 was slightly
space group P21/n, P1̅, P21/c, and P1̅, respectively. D
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS We thank the single-crystal CCD X-ray facility at CESE department IIT Kanpur. B.M. acknowledges CSIR, India, for a fellowship. Mr Asit Prakash and Prof. Monica Katiyar, Department of Materials Science & Engineering, and Samtel Centre for Display Technologies, IIT Kanpur, India, are thanked for their help in photoluminescence studies. Financial support of this work by DAE-SRC Outstanding Investigator Award and J. C. Bose National Fellowship (DST) to S.V., is gratefully acknowledged.
Figure 6. Normalized solid state luminescence spectra of L and complexes 1−4 at room temperature.
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(1) (a) O’Neill, M. A.; Barton, J. K. J. Am. Chem. Soc. 2002, 124, 13053−13066. (b) Barbieri, C. M.; Kaul, M.; Pilch, D. S. Tetrahedron 2007, 63, 3567−3574. (c) Hall, K. B. Methods Enzymol. 2009, 469, 269−285. (d) Jean, J. M.; Hall, K. B. Biochemistry 2002, 41, 13152− 13161. (e) Jean, J. M.; Hall, K. B. Proc. Natl. Acad. Sci. U. S. A. 2001, 98, 37−41. (f) Srivatsan, S. G.; Sawant, A. A. Pure Appl. Chem. 2011, 83, 213−232. (2) (a) Pershing, D. H.; McGinty, L.; Adams, C. W.; Fowler, R. G. Mut. Res. 1981, 83, 25−27. (b) Pitsikas, P.; Patapas, J. M.; Cupples, C. G. Mut. Res. 2004, 550, 25−32. (3) (a) Cupples, C. G.; Cabrera, M.; Cruz, C.; Miller, J. H. Genetics 1990, 125, 275−280. (b) Baase, W. A.; Jose, D.; Ponedel, B. C; Von Hippel, P. H.; Johnson, N. P. Nucleic Acids Res. 2009, 37, 1682−1689. (c) Marcheschi, R. J.; Mouzakis, K. D.; Butcher, S. E. ACS Chem. Biol. 2009, 4, 844−854. (4) (a) Neely, R. K.; Magennis, S. W.; Dryden, D. T. F.; Jones, A. C. J. Phys. Chem. B 2004, 108 (17), 606−610. (b) Neely, R. K.; Magennis, S. W.; Parsons, S.; Jones, A. C. ChemPhysChem 2007, 8, 1095−1102. (c) Bonnist, E. Y. M.; Jones, A. C. ChemPhysChem 2008, 9, 1121− 1129. (5) (a) Fernandez-Botello, A.; Operschall, B. P.; Holy, A.; Moreno, V.; Sigel, H. Dalton Trans. 2010, 39, 6344−6354. (b) DominguezMartin, A.; Choquesillo-Lazarte, D.; Gonzalez-Perez, J. M; Castineiras, A.; Niclos-Gutierrez, J. J. Inorg. Biochem. 2011, 105, 1073−1080 and references cited therein. (c) Dominguez-Martin, A.; Brandi-Blanco, M. D. P.; Matilla-Hernandez, A.; Bakkali, H. E.; Nurchi, V. M.; GonzalezPerez, J. M.; Castineiras, A.; Niclos-Gutiérrez, J.; Coord. Chem. Rev. 2013, DOI: 10.1016/j.ccr.2013.03.029. (6) (a) Evans, R. C.; Douglas, P.; Winscom, C. J. Coord. Chem. Rev. 2006, 250, 2093−2126. (b) Wang, S. Coord. Chem. Rev. 2001, 215, 79−98. (c) Yam, V. W. W. Acc. Chem. Res. 2002, 35, 555−563. (d) Carol, P.; Sreejith, S.; Ghosh, A. Chem. Asian J. 2007, 2, 338−348. (e) Barbieri, A.; Accorsi, G.; Armaroli, N. Chem Commun. 2008, 2185−2193. (7) (a) Verma, S.; Mishra, A. K.; Kumar, J. Acc. Chem. Res. 2010, 43, 79−91. (b) Mishra, A. K.; Prajapati, R. K.; Verma, S. Dalton Trans. 2010, 39, 10034−10037. (c) Purohit, C. S.; Mishra, A. K.; Verma, S. Inorg. Chem. 2007, 46, 8493−8495. (d) Mishra, A. K.; Verma, S. Inorg. Chem. 2010, 49, 8012−8016. (e) Pandey, M. D.; Mishra, A. K.; Chandrasekhar, V.; Verma, S. Inorg. Chem. 2010, 49, 2020−2022. (f) Purohit, C. S.; Verma, S. J. Am. Chem. Soc. 2007, 129, 3488−3489. (g) Mishra, A. K.; Purohit, C. S.; Kumar, J.; Verma, S. Inorg. Chim. Acta 2009, 362, 855−860. (h) Mishra, A. K.; Purohit, C. S.; Verma, S. CrystEngComm 2008, 10, 1296−1298. (i) Srivatsan, S. G.; Verma, S. Chem. Commun. 2000, 515−516. (j) Mishra, A. K.; Verma, S. Inorg. Chem. 2010, 49, 3691−3693. (k) Madhavaiah, C.; Verma, S. Chem. Commun. 2003, 800−801.
red-shifted in near UV region (382 nm) with a shoulder at 410 nm and a weak emission at 476 nm (λex = 330 nm), while complex 2 exhibited strong emission bands centered at 372 nm and weak emissions at 410 and 476 nm (λex = 330 nm). The appearance of 410 nm emission in complexes 1 and 2, which was absent in ligand L, could be attributed to the involvement of new excited state through Zn coordination.19 Besides, complexes 1 and 2 show slight enhancement in the emission maximum at 476 nm as compared to ligand alone. This observation could be ascribed to d10 metal ion enhanced fluorescence.20 However, complexes 3 and 4 show emission at 366 and 476 nm; 376 and 496 nm, respectively, by exciting at 330 nm. The emission maxima at 496 nm of the complex 4 were observed to be red-shifted by 20 nm and appeared to be welldefined more than other complexes. Preliminary observation suggests that the shifting of emission maxima might occur due to decrease in fluorophore separation upon binding to the Hg(II) ion.21 The complex 3 shows significantly weaker emission maxima at 476 nm, compared to others, perhaps due to the heavy atom effect of the iodide ligand.22 In conclusion, we have reported the coordination behavior of a series of heavy metals such as Zn(II), Cd(II), and Hg(II) with a modified purine analogue having a propyl group at the N-9 position. A 2D purine-based cyclic mercurous cluster is reported. In addition, we have also reported bidentate coordination modes of 2-aminopurine with Zn and Cd resulting in a 1D, zigzag, and helical coordination polymer, respectively. 3D and 2D supramolecular frameworks through hydrogen-bonding interactions with Hg(II) and Zn(II) complexes are also discussed. Superior photoluminescence properties of free ligand L and complexes with d10 metals were discussed, and it is possible to explore these effects for material applications.
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REFERENCES
ASSOCIATED CONTENT
S Supporting Information *
Synthetic procedures, characterizations, experiments, and crystal structure refinement parameters are given. X-ray crystallography data in cif format are also given. This material is available free of charge via the Internet at http://pubs.acs. org/. E
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(8) (a) Nagapradeep, N.; Verma, S. Chem. Commun. 2011, 47, 1755− 1757. (b) Prajapati, R. K.; Kumar, Verma, S. Chem. Commun. 2010, 46, 3312−3314. (9) Bailey, S.; Harnden, M. R.; Jarvesst, R. L.; Parkin, A.; Boyd, M. R. J. Med. Chem. 1991, 34, 57−65 Crystallographic data for complex 1: C8H11N5Cl2Zn, Mr = 313.49, T = 100(2) K, monoclinic, space group P 21/c, Z = 4, a = 13.142(5) Å, b = 6.888(5) Å, c = 13.799(3) Å, α = 90°(5), β = 109.193(5)°, γ = 90°(5), V = 1179.7(11) Å3, Dx = 1.765 Mg·m−3, F(000) = 632, μ = 1.765 mm−1, 8658 reflections collected, 2316 unique (Rint = 0.0375) which were used in all calculations, R and Rw [I > 2σ(I)] = 0.0295 and 0.0583, R and Rw (all data) = 0.0407 and 0.0613, GOF = 1.068. Crystallographic data for complex 2: C12H15N5O5Zn, Mr = 374.66, T = 100(2) K, triclinic, space group P1̅, Z = 2, a = 7.626(5) Å, b = 9.537(5) Å, c = 11.045(5) Å, α = 85.705°(5), β = 87.256(5)°, γ = 80.423°(5), V = 789.4(8) Å3, Dx = 1.576 Mg·m−3, F(000) = 384, μ = 1.588 mm−1, 4428 reflections collected, 3016 unique (Rint = 0.0270) which were used in all calculations, R and Rw [I > 2σ(I)] = 0.0580 and 0.1563, R and Rw (all data) = 0.0686 and 0.2091, GOF = 1.164. Crystallographic data for complex 3: C8H11N5I2Cd, Mr = 543.42, T = 100(2) K, monoclinic, space group P121/c 1, Z = 4, a = 7.219(5) Å, b = 8.659(5) Å, c = 22.367(5) Å, α = 90(5)°, β = 90(5)°, γ = 90(5)°, V = 1398.0(13) Å3, Dx = 2.582 Mg·m−3, F(000) = 992, μ = 0.169 mm−1, 9398 reflections collected, 2610 unique (Rint = 0.0346) which were used in all calculations, R and Rw [I > 2σ(I)] = 0.0225 and 0.0558, R and Rw (all data) = 0.0242 and 0.0569, GOF = 1.087. Crystallographic data for complex 4: C16H21N10Cl6Hg3, Mr = 1167.90, T = 100(2) K, Triclinic, space group P1̅, Z = 2, a = 7.288(5) Å, b = 13.934(5) Å, c = 14.172(5) Å, α = 70.316°(5), β = 89.974(5)°, γ = 89.992°(5), V = 1355.1(12) Å3, Dx = 1.765 Mg·m−3, F(000) = 1058, μ = 17.572 mm−1, 6986 reflections collected, 4677 unique (Rint = 0.0316) which were used in all calculations, R and Rw [I > 2σ(I)] = 0.0875 and 0.225, R and Rw (all data) = 0.0967 and 0.2466, GOF = 1.022. More details about the crystallographic studies are given in the Supporting Information. (10) (a) Batten, S. R.; Harris, A. R.; Murray, K. S.; Smith, J. P. Cryst. Growth Des. 2002, 2, 87. (b) Canty, A. J.; Deacon, G. B. Inorg. Chim. Acta 1980, 45, L225. (11) (a) Charland, J.-P.; Viet, M. T. P.; St-Jacques, M.; Beauchamp, A. L. J. Am. Chem. Soc. 1985, 107, 8202−8211. (b) Buncel, E.; Norris, A. R.; Racz, W. J.; Taylor, S. E. Inorg. Chem. 1981, 20, 98−103. (c) Krumm, M.; Zangrando, E.; Randaccio, L.; Menzer, S.; Danzmann, A.; Holthenrich, D.; Lippert, B. Inorg. Chem. 1993, 32, 2183−2189. (d) Sletten, E.; Nerdal, W. Met. Ions Biol. Syst. 1997, 34, 479−501. (e) Onyido, I.; Norris, A. R.; Buncel, E. Chem. Rev. 2004, 104, 5911− 5929. (12) (a) Zamora, F.; Sabat, M.; Lippert, B. Inorg. Chim. Acta 1998, 267, 87−91. (13) Yang, X.-J.; Liu, X.; Liu, Y.; Hao, Y.; Wu, B. Polyhedron. 2010, 29, 934−940. (14) (a) Wang, X.-F.; Lv, Y.; Okamura, T.-A.; Kawaguchi, H.; Wu, G.; Sun, W.-Y.; Ueyama, N. Cryst. Growth Des. 2007, 7, 1125−1133. (b) Yang, X.-J.; Liu, X.; Liu, Y.; Hao, Y.; Wu, B. Polyhedron 2009, 29, 934−940. (c) Park, S.; Lee, S. Y.; Jo, M.; Lee, J. Y.; Lee, S. S. CrystEngComm 2009, 11, 43−46. (15) Kong, L.; Zhang, D.; Su, F.; Lu, J.; Li, D.; Dou, J. Inorg. Chim. Acta 2011, 370, 1−6. (16) Khavas, H. R.; Sadegh, B. M. M. Inorg. Chem. 2010, 49, 5356− 5358. (17) (a) Liu, L.; Wong, W.-Y.; Lam, Y.-W.; Tam, W.-Y. Inorg. Chim. Acta 2007, 360, 109. (b) Biswas, S.; Mostafa, G.; Steele, L. M.; Sarkar, S.; Dey, K. Polyhedron 2009, 28, 1010−1016. (18) Holmen, A.; Norden, B.; Albinsson, B. J. Am. Chem. Soc. 1997, 119, 3114−3121. (19) Callis, P. R. Annu. Rev. Phys. Chem. 1983, 34, 329−357. (20) Prodi, L.; Bolletta, F.; Montalti, M.; Zaccheroni, N. Eur. J. Inorg. Chem. 1999, 455−460. (21) Guo, X.; Qian, X.; Jia, L. J. Am. Chem. Soc. 2004, 126, 2272− 2273.
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