CdII Complexes with 2,6-Diaminopurine by

DOI: 10.1021/cg9007119

Unusual Polymeric ZnII/CdII Complexes with 2,6-Diaminopurine by Synergistic Coordination of Nucleobases and Polycarboxylate Anions: Binding Behavior, Self-Assembled Pattern of the Nucleobase, and Luminscent Properties

2009, Vol. 9 4933–4944

En-Cui Yang,* Ya-Nan Chan, Hui Liu, Zhi-Chao Wang, and Xiao-Jun Zhao* College of Chemistry and Life Science, Tianjin Key Laboratory of Structure and Performance for Functional Molecule, Tianjin Normal University, Tianjin 300387, People’s Republic of China Received June 24, 2009; Revised Manuscript Received August 4, 2009

ABSTRACT: To investigate the coordination behavior of 2,6-diaminopurine (Hdap) and to construct unique highdimensional nucleobase-based complexes, six Hdap-based ZnII/CdII polymers were obtained by incorporating aliphatic/ aromatic polycarboxylate as coligands under hydrothermal conditions and were fully structurally characterized. Significantly resulting from the synergistic coordination of nucleobase and polycarboxylate groups, they are a tetranuclear ZnII-core-based two-dimensional (2D) covalent layer for {[Zn4(μ2-Hdap)2(tp)3(μ3-OH)2] 3 2H2O}n (1), an eight-connected three-dimensional (3D) self-penetrating metal-organic framework (MOF) for [Zn2(μ2-Hdap)(tp)2]n (2), a 3D pillared-layer structure for {[Zn2(μ2Hdap)(tm)(μ2-OH)] 3 H2O}n (3), a one-dimensional (1D) linear double-chain motif for {[Zn(H2dap)(H2O)(btc)] 3 3H2O}n (4), a trinuclear CdII-cluster-based 2D aggregate for {[Cd3(H2O)2(μ3-dap)2(ap)2] 3 H2O}n (5), and a 1D Z-shaped chain for {[Cd(H2dap)(H2O)2(tp)] 3 0.5tp 3 H2O}n (6), respectively (H2tp = terephthalic acid, H3tm = trimesic acid, H3btc = 1,2,3benzenetricarboxylic acid, and H2ap=adipic acid). In the polymers, the neutral Hdap nucleobase presents bidentate μ2-N3, N9 and μ2-N7,N9 bridging modes to contribute to the tetranuclear ZnII cluster in 1 and the [Zn2(Hdap)]4þ subunit in 2 and 3, respectively. In contrast, acting as a terminal ligand by monodentate N9 and N7 binding patterns, the cationic H2dapþ ligand just completes the metal coordination sphere in 4 and 6. More interestingly, the anionic dap- molecule in 5 exhibits a tridentate μ3-N3,N7,N9 bridging manner to devote to both the aggregate of the three CdII cores within the trinuclear cluster and the extension of an infinite 2D aggregate. Thus, the binding patterns, the protonation degree, and its corresponding tautomeric forms of the Hdap nucleobase essentially dominate the polymeric nature of 1-6. Additionally, the repeatedly observed hydrogen-bonding interactions produced by the exocyclic amino/endocyclic imino groups of the nucleobase and the carboxylate groups of the coligand favorably stabilize the high-dimensional order supramolecular architectures. At room temperature, complexes 1-6 exhibit intense luminescent emissions originated from a Hdap-based intraligand and/or photoinduced charge transfer upon cation binding, which offers the possibility for their applications as relevant antivirasic prodrugs.

*Author to whom correspondence should be addressed. E-mail: [email protected] (X.-J.Z.); [email protected] (E.-C.Y.).

expected binding patterns similar to the adenine, because it is structurally related to a pyrimidine ring fused to an imidazole ring. Moreover, an extra exocyclic amino group of Hdap than adenine can further serve as functional sites for both the coordination3,4 and/or intra/intermolecular hydrogen-bonding sites,1 which, combined with the polyfunctional purine moiety, can reasonably produce novel complexes with intriguing framework-connectivity and significant Hdap-based applications. Thus, as a continuation to our crystallographically structural investigations on the transition metal-Hdap nucleobase complexes, and especially to construct unique polymeric complexes with the nucleobase,23,24 in the present contribution, Hdap was selected as a core building block to react with various aliphatic/ aromatic poly carboxylic acids and ZnII/CdII ions. Notably, the particular purpose for introducing the poly carboxylic acid as coligand is that negatively charged polycarboxylate anion is well-known to be an efficient ligand by its surprisingly binding patterns and hydrogen-bonding acceptor ability,35 which can favorably extend and/or consolidate the desired high-dimensional frameworks. As expected, a series of six polymeric structures with mixed Hdap and terephthalic acid (H2tp), trimesic acid (H3tm), 1,2,3-benzenetricarboxylic acid (H3btc), as well as adipic acid (H2ap) ligands, were respectively isolated under hydrothermal conditions and fully characterized by X-ray crystallography, FT-IR spectra, elemental analysis,

r 2009 American Chemical Society

Published on Web 09/16/2009

Introduction Recently, transition metal-heterocyclic nucleobase complexes are of great interest owing not only to the biological importance of the metal-nucleobase bonds, the functions of nucleic acids, and genetic information transfer but also to their structural diversity, molecular recognition model for nucleic acids and peptides, and potential applications as advanced functional materials.1-6 However, it is particularly difficult to gain structural information on such systems due to their low solubility and the difficulty of obtaining crystalline complexes. Therefore, to date, most of the reported complexes have been focused on the native adenine nucleobase7-24 and its diverse N-alkylated derivatives as the ligands.25-34 And the reported adenine-based complexes are common monomers,7,12-17,29-33 discrete oligomeric species,8,9,11,14,19-21,28,29 and scarcely high-dimensional polymeric structures.10,26,30 In contrast, the available structural information on the metal complex with 2,6-diaminopurine (Hdap) nucleobase in the Cambridge Structural Database is surprisingly limited, although N-rich and N-H-rich Hdap is one of the most closely related analogues of adenine. In principle, Hdap nucleobase can potentially represent the

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TG-DTA, and fluorescence emission. Structural determinations reveal that complexes 1 and 5 are tetranuclear ZnII and trinuclear CdII cluster-based two-dimensional (2D) covalent layers, respectively; complexes 2 and 3 are high-dimensional frameworks with an eight-connected 3D self-penetrating network for 2 and a 3D pillared-layer structure for 3. In contrast, a 1D linear double-chain and Z-shaped single-chain are observed for 4 and 6, respectively. In particular, the Hdap nucleobase in the six polymers presents three protonation degree (neutral Hdap molecule, cationic H2dapþ and anionic dap- ions), diverse tautomeric forms (H(N7)Hdap, H(N3)Hdap, H2(N3, N7)H2dapþ, H2(N1,N9)H2dapþ and dap-), and five interesting binding patterns (N7-, N9-, μ2-N7,N9-, μ2-N3,N9-, and μ3-N3,N7,N9-), which, along with the multidentate polycarboxylate anion, cooperatively contributes to the polymeric nature of the resulting complexes. Additionally, the six polymers with considerable thermal stability exhibit intense luminescent emissions and can be potentially applied as photoactive materials. Experimental Section Materials and Physical Measurements. All of the starting materials employed were commercially purchased (Hdap, H2tp, H3tm, H2ap, and H3btc were from Acros and other analytical-grade reagents were from Tianjin Chemical Reagent Factory) and used as received without further purification. Doubly deionized water was used for the conventional synthesis. Elemental analyses of carbon, hydrogen, and nitrogen were carried out with a CE-440 (Leeman-Laboratories) analyzer. Fourier transform (FT) IR spectra (KBr pellets) were taken on an Avatar-370 (Nicolet) spectrometer in the range 4000-400 cm-1. Thermogravimetric analysis (TGA) experiments were carried out on Shimadzu simultaneous DTG-60A compositional analysis instrument from room temperature to 800 C under N2 atmosphere at a heating rate of 5 C/min. Fluorescence spectra of the polymers in an acetonitrile solution were performed on a Cary Eclipse fluorescence spectrophotometer (Varian) equipped with a xenon lamp at room temperature. Hydrothermal Synthesis. All the complexes were successfully synthesized by a hydrothermal method. {[Zn4(μ2-Hdap)2(tp)3(μ3-OH)2] 3 2H2O}n (1). Zn(NO3)2 3 6H2O (6.0 mg, 0.02 mmol), Hdap (1.5 mg, 0.01 mmol), and H2tp (1.7 mg, 0.01 mmol) were dissolved in water (10 mL), and the initial pH value of the mixture was adjusted to ca. 6 by triethylamine. The mixture was then transferred into a parr Teflon-lined stainless steel vessel (23 mL) and heated to 160 C for 48 h under autogenous pressure. After the mixture was cooled to room temperature at a rate of 2 C 3 h-1, pale-yellow strip-shaped crystals suitable for X-ray analysis were obtained directly, washed with water and ethanol, and dried in air. Yield: 65% on the basis of ZnII salt. Anal. Calcd for C17H15N6O8Zn2: C 36.33, H 2.69, N 14.95%; Found: C 36.43, H 2.62, N 15.08%. FT-IR (KBr, cm-1): 3427 (s), 3388 (s), 3196 (w), 1681 (s), 1607 (w), 1568 (m), 1389 (s), 1288 (s), 739 (s), 523 (m). [Zn2(μ2-Hdap)(tp)2]n (2). The pale-yellow block-shaped crystals, 2, suitable for X-ray analysis were obtained directly by adopting the same procedures to 1, except that the cooling rate of the reaction mixture is increased to 6 C 3 h-1. Yield: 70% on the basis of ZnII salt. Anal. Calcd for C21H14N6O8Zn2: C, 41.41; H, 2.32; N, 13.80%. Found: C, 41.52; H, 2.35; N, 13.96%. FT-IR (KBr pellet, cm-1): 3425 (s), 3338 (s), 3230 (s), 3145 (w), 2910 (w), 1663 (m), 1601 (m), 1575 (s), 1506 (w), 1467 (m), 1386 (s), 1227 (w), 820 (m), 741 (m), 571 (w), 547 (w). {[Zn2(μ2-Hdap)(tm)(μ2-OH)] 3 H2O}n (3). A mixture containing Zn(NO3)2 3 6H2O (6.0 mg, 0.02 mmol), Hdap (1.5 mg, 0.01 mmol), and H3tm (1.1 mg, 0.005 mmol) were dissolved in water (10 mL), and the initial pH value was adjusted to ca. 6 by triethylamine. Then the mixture was placed in a parr Teflon-lined stainless steel vessel (23 mL) under autogenous pressure, which was heated to 160 C for 72 h, and then cooled to room temperature at a rate of 5 C 3 h-1. The pale-yellow strip-shaped single crystals suitable for X-ray analysis were obtained directly. Yield: 42% on the basis of ZnII salt. Anal. Calcd for C14H12N6O8Zn2: C 32.15, H 2.31, N 16.07%; Found: C

Yang et al. 32.25, H 2.41, N 16.04%. FT-IR (KBr pellet, cm-1): 3450 (s), 3341 (m), 3218 (m), 3133 (w), 1622 (s), 1563 (m), 1480 (m), 1437 (m), 1342 (s), 1213 (w), 979 (w), 843 (w), 754 (w), 723 (m), 551 (w). {[Zn(H2dap)(H2O)(btc)] 3 3H2O}n (4). A mixture containing Zn(NO3)2 3 6H2O (6.0 mg, 0.02 mmol), Hdap (1.5 mg, 0.01 mmol), and H3btc (2.1 mg, 0.01 mmol) were dissolved in 10 mL of H2O, and the initial pH value was adjusted to ca. 6 by triethylamine. Then the mixture was transferred into a parr Teflon-lined stainless steel vessel (23 mL) under autogenous pressure, which was heated to 140 C for 72 h, and then cooled to room temperature at a rate of 5 C 3 h-1. The mixture was filtered off and left to stand for several days. Yellow block-shaped crystals suitable for X-ray analysis were obtained within one week. Yield: 55% on the basis of ZnII salt. Anal. Calcd for C14H18N6O10Zn: C 33.92, H 3.66, N, 16.95%. Found: C, 33.88; H, 3.46; N, 17.05%. FT-IR (KBr pellet, cm-1): 3640 (w), 3468 (m), 1614 (s), 1559 (s), 1460 (s), 1395 (s), 1160 (w), 903 (w), 861 (w), 821 (w), 770 (w), 704 (m), 612 (w), 567 (w), 474 (w). {[Cd3(H2O)2(μ3-dap)2(ap)2] 3 H2O}n (5). The colorless blockshaped single crystals, 5, suitable for X-ray analysis were obtained directly by adopting the same procedures as that for 4, except that Zn(NO3)2 3 6H2O and H3btc were replaced by Cd(OAc)2 (6.17 mg, 0.02 mmol) and H2ap (1.46 mg, 0.01 mmol), respectively. Yield: 77% on the basis of CdII salt. Anal. Calcd for C11H16Cd1.5N6O5.5: C 27.03, H 3.30, N 17.19%. Found: C 27.30; H 3.38; N 17.28%. FT-IR (KBr, cm-1): 3474 (m), 3366 (s), 3074 (m), 2945 (w), 2843 (w), 1691 (s), 1617 (s), 1552 (s), 1471 (w), 1437 (m), 1382 (s), 1321 (w), 1287 (m), 1227 (m), 1160 (s), 792 (m), 653 (w), 614 (m). {[Cd(H2dap)(H2O)2(tp)] 3 0.5tp 3 H2O}n (6). A mixture containing Cd(NO3)2 3 6H2O (3.0 mg, 0.01 mmol), Hdap (1.5 mg, 0.01 mmol), and H2tp (2.55 mg, 0.015 mmol) was dissolved in water (10 mL), and the initial pH value was adjusted to ca. 6 by triethylamine. Then the mixture was transferred into a parr Teflon-lined stainless steel vessel (23 mL) under autogenous pressure, which was heated to 100 C for 48 h, and cooled to room temperature at a rate of 2 C 3 h-1. The colorless block-shaped single crystals suitable for X-ray analysis were obtained directly. Yield: 36% on the basis of CdII salt. Anal. calcd for C17H19N6O9Cd: C 36.22, H 3.40, N 14.91%; Found: C 36.25, H 3.46, N 14.88%. FT-IR (KBr pellet, cm-1): 3428 (m), 3340 (m), 3180 (m), 2965 (w), 1668 (s), 1607 (s), 1557 (s), 1389 (s), 1302 (s), 849 (m), 735 (m), 619 (m), 528 (m). X-ray Crystallography. Diffraction intensities for complexes 1-6 were collected on Bruker APEX-II CCD diffractometer equipped with graphite-monochromated Mo-KR radiation with a radiation wavelength of 0.71073 A˚ by using the j-ω scan technique at 296(2) K. There was no evidence of crystal decay during data collection. Semiempirical absorption corrections were applied (SADABS), and the program SAINT was used for integration of the diffraction profiles.36 The structures were solved by direct methods and refined with the full-matrix least-squares technique using the SHELXS-97 and SHELXL-97 programs.37 The oxygen atom (O9) of the lattice water molecule in 6 is disordered and varies between two positions with occupancies that were refined to 0.746 for O(9) and 0.254 for O(90 ). Anisotropic thermal parameters were assigned to all non-hydrogen atoms. All the hydrogen atoms of the ligands were found in the difference Fourier Map and refined with isotropic temperature factors. No attempts have been made to locate hydrogen atoms of the splitting water molecules. The crystallographic data and the hydrogen bond parameters are given in Tables 1 and 2. CCDC-729886 (1), 729887 (2), 729888 (3), 729889 (4), 729890 (5), and 729891 (6) 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.

Results and Discussion Syntheses and IR Spectra. It is well-known that purine can undergo a variety of acid-base reactions and exists as different tautomers, which offers the possibility of the Hdap nucleobase as a neutral, anionic, and/or cationic ligand. In fact, the Hdap nucleobase in polymers 1-6 prefers a slightly weak acidic environment, because the initial pH of the

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Table 1. Crystal Data and Structure Refinement for 1-6a 1

2

3

4

5

empirical formula

C17H15N6O8Zn2

C21H14N6O8Zn2

C14H12N6O8Zn2

C14H18N6O10Zn

Fw crystal syst space group cryst size/mm a [A˚] b [A˚] c [A˚] R [] β [] γ [] V [A˚3] Z Dc [g/cm3] μ [mm-1] F(000) limiting indices

562.09 monoclinic C2/c 0.25 0.24 0.22

609.12 monoclinic P21/n 0.25 0.15 0.14

523.04 monoclinic P21/c 0.27 0.18 0.17

495.71 triclinic P1 0.25 0.22 0.20

C11H16Cd1.5N6O5.5 488.90 monoclinic P21/c 0.25 0.24 0.20

563.78 monoclinic P21/c 0.16 0.13 0.12

9.0571(9) 16.8080(16) 10.9333(10) 90.00 98.9910(10) 90.00 1643.9(3) 4 2.113 2.985 1048 -8 e h e 10, -19 e k e 15, -13 e l e 12 2894/0/271 1.041 0.0134 0.0226, 0.0603 0.0256, 0.0620 0.279, -0.446

9.9718(6) 10.0083(6) 10.4759(6) 81.9930(10) 69.0970(10) 70.4920(10) 920.39(9) 2 1.789 1.407 508 -10 e h e 11, -9 e k e 11, -12 e l e 12 3228/0/280 1.037 0.0113 0.0274, 0.0732 0.0299, 0.0746 0.369, -0.347

9.4679(6) 11.8326(7) 14.2259(9) 90.00 90.7110(10) 90.00 1593.60(17) 4 2.038 2.059 960 -10 e h e 11, -12 e k e 14, -16 e l e 16 2801/0/223 1.012 0.0123 0.0169, 0.0446 0.0176, 0.0451 0.644, -0.433

11.0461(3) 24.5548(6) 7.8992(2) 90.00 106.97 90.00 2049.29(9) 4 1.827 1.132 1132 -13 e h e 11, -29 e k e 26, -6 e l e 9 3616/6/308 1.054 0.0159 0.0217, 0.0553 0.0240, 0.0567 0.431, -0.397

)

)

19.8378(15) 7.3203(4) 11.0364(8) 19.1072(10) 18.3362(14) 15.7652(8) 90.00 90.00 91.2290(10) 101.7870(10) 90.00 90.00 4013.6(5) 2158.6(2) 8 4 1.860 1.874 2.452 2.288 2264 1224 -23 e h e 23, -8 e h e 8, -22 -13 e k e 13, e k e 22, -13 e l -21 e l e 9 e 18 data/restraints/params 3538/0/298 3809/0/334 GOF 1.045 1.040 0.0164 0.0221 Rint 0.0313, 0.0777 0.0234, 0.0618 R1a, wR2b [I > 2σ(I)] 0.0358, 0.0805 0.0254, 0.0630 R1, wR2 [all data] 0.778, -0.658 0.333, -0.710 residuals [e A˚-3] P P P a R1 = Fo|-|Fc /|Fo|. b wR2 = [ w(Fo2-Fc2)2/ w(Fo2)2]1/2.

6 C17H19CdN6O9

medium is consistently adjusted to ca. 6 by triethylamine. Such a pH value is much different from the previous reactions performed in NaOH10,18,21,24 or hydrochloric acid solutions7,12,22,23 and is favorable for the spontaneous adjustment of the Hdap nucleobase between the protonation degree and the corresponding proton tautomers. As a result, the free Hdap nucleobase exists as a neutral molecule (Hdap) in complexes 1-3 with a simultaneous proton shift from N9 to N7 in 1 and from N9 to N3 in both 2 and 3. In contrast, a cationic H2dapþ ligand is observed in polymeric 4 and 6 with dissociable proton on N3 and N7 in 4 and on N1 and N9 in 6. Alternately, an anionic dap- species is formed in 5 with the deprotonation at the most basic N9 site. On the other hand, in contrast to the previous preparation methods adopted in the adenine-based complexes, such as the typically conventional evaporation,7,9-18,20,21,24,26-32 the in situ core-controlled expansion,8 temperature-controlled sublimation,38 diffusion/layering method,26 and the multiple-step reaction of metal complex with nucleobase,27 the hydrothermal technique herein has been proven to be very efficient for the construction of the polymeric nucleobase-based mixedligand complexes.23 Indeed, polymers 1-6 are directly prepared from the hydrothermal conditions by careful control of the reaction temperature and the cooling rate of the reaction mixture (see Scheme 1). Additionally, the different stoichiometries of metal ion and mixed-ligand play a key role on the growth and crystallization of the target polymers. In the FT-IR spectra, strong bands appearing above 3000 cm-1 for 1-6 should be ascribed to the stretching vibrations of O-H and/or N-H, suggesting the presence of an amino group of the nucleobase, coordinated/lattice water molecule, and/or hydroxy group.39 The weak aromatic C-H vibrations are located at ca. 2900 cm-1. The absence of the characteristic band at ca. 1700 cm-1 for 1-6 consistently confirms the full deprotonation of the four carboxylic acids by triethylamine during the reaction process.39 Additionally, the multiple bands appearing at 1600 - 1400 cm-1 are closely

related to the aromatic CdC skelton vibrations, the asymmetric and symmetric stretching vibrations of the carboxylated groups, and the stretching vibrations of the heterocyclic ring, which are overlapped together and hard to distinguish clearly. Crystal Structure of {[Zn4(μ2-Hdap)2(tp)3(μ3-OH)2] 3 2H2O}n (1). As shown in Figure 1a, polymer 1 is a tetranuclear ZnII core-based 2D covalent layer solely extended by tp anions, in which the neutral Hdap nucleobase alternately appends on both sides of the layer. The cationic [Zn4(μ2Hdap)2(μ3-OH)2]6þ core is a centrosymmetric dimer, containing four ZnII atoms, a pair of neutral Hdap nucleobases, and two symmetry-related μ3-OH- groups. Both crystallographically independent ZnII atoms are pentacoordinated, exhibiting a distorted trigonal-bipyramidal geometry for Zn1 and flattened square-pyramidal geometry for Zn2. For Zn1, two carboxylate O atoms (O4 and O5) from two different tp anions and one μ3-OH- are located at the basal plane. An imidazole N9 donor from Hdap nucleobase and one carboxylate O6 atom from the tp ligand occupy the apical positions, respectively. The distance of Zn1 3 3 3 O3 is much longer than the other Zn1-O/N bond lengths (2.487(3) A˚ vs. 1.957(2)-2.020(3) A˚, Table S1, Supporting Information). In contrast, Zn2 is surrounded by two μ3-OH(O7 and O7A) and two carboxylate O atoms (O1 and O6A) located at the basal plane and one pyrimidinic N3 atom occupying the apical position (Figure 1 and Figure S1, Supporting Information). A pair of μ3-OH- groups doubly bridge three ZnII atoms from the adjacent asymmetric units to generate a centrosymmetric tetranuclear [Zn4(μ2-Hdap)2(μ3-OH)2]6þ core, in which a pair of neutral Hdap nucleobases connects the Zn1, Zn2A and Zn1A, Zn2 in a bidentate μ2-N3,N9 bridging mode (Figure 1a and Figure S2, Supporting Information). Notably, because of the formation of Zn-N9 and Zn-N3 coordination bonds, the proton on the most basic N9 of the neutral Hdap nucleobase was

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Table 2. Selected Hydrogen Bond Lengths (A˚) and Angles () for 1-6a d (D-H)

d (H 3 3 3 A)

d (D 3 3 3 A)

— DHA

N6-H6A 3 3 3 O2#1 N2-H2B 3 3 3 O1#2 N6-H6B 3 3 3 O8#4 N7-H70 3 3 3 O8#4 O8-H8A 3 3 3 O3 O7-H700 3 3 3 O2#3 O8-H8B 3 3 3 O2#3

0.86 0.86 0.86 0.86 0.85 0.85 0.85

1 2.11 1.94 2.31 1.93 2.44 2.40 1.98

2.860(4) 2.781(4) 3.137(6) 2.738(4) 2.724(7) 3.037(5) 2.793(1)

145 164 162 157 101 132 160

N3-H4 3 3 3 O7#1 N6-H6A 3 3 3 O2#2 N6-H6B 3 3 3 O5 N2-H2A 3 3 3 O1#3 N2-H2B 3 3 3 O3#4

0.86 0.86 0.86 0.86 0.86

2 1.88 2.17 2.11 2.10 2.46

2.739(2) 3.003(2) 2.912(2) 2.933(2) 3.231(2)

173 162 155 163 149

N3-H3 3 3 3 O2#3 N6-H6B 3 3 3 O4#4 N2-H2A 3 3 3 O3#2 O7-H7 3 3 3 N1#1 N2-H2B 3 3 3 O8#3 O8-H8B 3 3 3 O6

0.86 0.86 0.86 0.85 0.86 0.85

3 1.70 2.08 2.24 1.95 2.36 2.15

2.559(2) 2.935(3) 2.981(3) 2.799(3) 3.063(3) 2.993(3)

176 172 144 177 139 172

N2-H2B 3 3 3 O3#7 N3-H3 3 3 3 O5#7 N6-H6B 3 3 3 O10#2 N6-H6A 3 3 3 O7#3 N2-H2A 3 3 3 O9#3 N7-H7 3 3 3 O10#2 O8-H8A 3 3 3 O4#4 O8-H8B 3 3 3 N1#5 O7-H7A 3 3 3 O2#1 O7-H7B 3 3 3 O8#2 O9-H9A 3 3 3 O2#2 O10-H10B 3 3 3 O4#6 O9-H9B 3 3 3 O8#1 O10-H10A 3 3 3 O9

0.90 0.86 0.86 0.86 0.90 0.86 0.85 0.85 0.85 0.85 0.85 0.85 0.85 0.85

4 2.19 1.78 2.14 2.22 2.05 1.89 1.86 2.11 2.07 1.92 1.94 1.91 1.97 1.86

3.077(9) 2.587(4) 2.978(3) 2.951(3) 2.912(3) 2.695(3) 2.687(2) 2.949(3) 2.834(2) 2.741(3) 2.782(3) 2.728(2) 2.777(3) 2.704(3)

170 157 163 144 160 154 163 167 150 162 169 162 157 176

N2-H2B 3 3 3 N7#2 N6-H6B 3 3 3 O2#4 N6-H6A 3 3 3 N1#3 O5-H500 3 3 3 O6 O5-H50 3 3 3 O4#1 O6-H600 3 3 3 O5#5

0.86 0.86 0.86 0.85 0.85 0.85

5 2.32 1.97 2.01 2.01 1.99 2.09

3.131(3) 2.799(3) 2.869(3) 2.733(7) 2.816(3) 2.733(7)

158 162 172 143 163 131

N2-H2B 3 3 3 O4#4 N6-H6B 3 3 3 O4 N2-H2A 3 3 3 O5#4 N6-H6A 3 3 3 O6#4 N1-H1 3 3 3 O6#4 N9-H9 3 3 3 O9#5 O7-H7A 3 3 3 O1#1 O7-H7B 3 3 3 O3#1 O8-H8A 3 3 3 O3#2 O8-H8B 3 3 3 O5#3

0.90 0.86 0.90 0.86 0.86 0.86 0.85 0.85 0.85 0.85

6 2.28 2.17 1.99 2.47 1.77 2.03 2.27 1.95 2.02 1.92

3.045(3) 2.923(3) 2.878(3) 3.137(3) 2.615(2) 2.877(3) 2.780(3) 2.778(3) 2.863(3) 2.750(3)

143 147 169 135 166 170 118 164 171 166

D-H 3 3 3 A

a Symmetry codes for 1: #1 -1/2 þ x, 3/2 - y, -1/2 þ z; #2 - 1/2 þ x, 1/2 - y, - 1/2 þ z; #3 1/2 - x, 1/2 - y, 1 - z, #4 x, 1 þ y, z; for 2 #1 x - 3/2, - y þ 1/2, z þ 1/2, #2 3/2 - x, - 1/2 þ y, 1/2 - z, #3 - 1/2 þ x, 1/2 - y, - 1/2 þ z, #4 - x þ1/2, y - 1/2, - z þ 2; for 3 #1 - x þ 2, y - 1/2, - z þ 3/2, #2 x þ 1, - y þ 3/2, z - 1/2, #3 - x þ 2, - y þ 1, - z þ 1, #4 - x þ 1, - y þ 1, - z þ 2; for 4 #1 x, y - 1, z, #2 - x þ 1, - y þ 1, - z þ 1, #3 - x, - y þ 1, - z þ 1, #4 x, y, z þ 1, #5 x þ 1, y, z; #6 - x þ 1, - y þ 1, - z; #7 - 1þ x, y, z; for 5 #1 x, - y þ 3/2, z þ 1/2, #2 x, - y þ 1/2, z þ 1/2, #3 - x, - y, - z þ 1, #4 x, - y þ 1/2, z - 1/2; #5 1 - x, 1/2 þ y, - z þ 1/2; for 6 #1 x, - y þ 3/2, z þ 1/2, #2 x, - y þ 3/2, z - 1/2, #3 x, y, z - 1, #4 - x þ 2, - y þ 1, - z þ 1, #5 - x þ 1, - y þ 1, - z þ 1.

shifted to N7. Consequently, the first tautomer of the neutral Hdap nucleobase, H(N7)Hdap, is generated. However, only the Zn-N3 coordination bond is reinforced by intramolecular interligand N2-H2B 3 3 3 O1 hydrogen-bonding interactions, and not any interligand N7-H70 3 3 3 A interactions to strengthen the Zn-N9 coordination bond (see Table 2, A = H-acceptor atom of a counter ligand). Notably, within the [Zn4(μ2-Hdap)2(μ3-OH)2]6þ core, the

Zn1 3 3 3 Zn2A separation by the Hdap is 3.155(5) A˚ and 0.704 A˚ shorter than Zn1 3 3 3 Zn2 bridged by μ3-OH in the asymmetric unit. The tetranuclear ZnII cores are then covalently extended by six tp anions to form a 2D wavelike layer with the Hdap nucleobases locating alternately on both sides of the layer (Figure 1a). Obviously, the two crystallographically independent tp linkers (the gray and purple one) exhibit

Article Scheme 1. Summary of the Hydrothermal Conditions in the Preparation of 1-6


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two common binding modes: bidentate chelating-monodentate for the gray one, and a bis-bidentate bridging manner for the purple one. Their dihedral angle is 75.39(5)o. Additionally, O7-H700 3 3 3 O2 interactions (see Table 2) between the μ3-OH- and the carboxylate group of tp anion further stabilize the four ZnII cores (Figure 1a) and even the infinite layer structure. Furthermore, the parallel 2D layers

Figure 1. (a) Infinite 2D layer of 1 and the local coordination environment of ZnII atoms (H atoms were omitted for clarity, symmetry codes: A = 1 - x, y, 0.5 - z). (b) 3D hydrogen-bonded supramolecular structure of 1 with lattice water molecules entrapped in them.

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Figure 2. (a) Local coordination environments of ZnII atoms in 2 (H atoms were omitted for clarity, symmetric codes: A - 1.5 þ x, 0.5 - y, 0.5 þ z; B: 1 - x, - y, 2 - z; C 2 - x, - y, 1 - z). (b) The rhombus layer of 2 with intralayer N-H 3 3 3 O hydrogen-bonding interactions. (c) The connection mode of Hdap and the second crystallographically independent tp anion. (d) 3D architecture of 2 and its interpenetrating R-Po-type topological structure.

are packed together by a pair of interlayer hydrogen-bonding interaction (N6-H6A 3 3 3 O2) between the exocyclic amino group of Hdap and the carboxylate group of tp, which leads to a 3D supramolecular network (Figure 1b and Table 2). One lattice water molecule was entrapped in the packing structure by 4-fold hydrogen-bonding interactions (N6-H6B 3 3 3 O8, N7-H70 3 3 3 O8, O8-H8A 3 3 3 O3, and O8-H8B 3 3 3 O2, see Table 2). Thus, the H(N7)Hdap tautomer in 1 binds a metal on N3 and N9 and uses four -N-H bonds as a H-donor to construct a high-dimensional order structure (Figure S3, Supporting Information). Crystal Structure of [Zn2(μ2-Hdap)(tp)2]n (2). Although prepared from the same reactants as 1, 2 is an eight-connected 3D self-penetrating MOF jointly extended by Hdap and tp ligands. The asymmetric unit of 2 contains two crystallographically independent ZnII atoms, two tp anions (shown in pink and blue in Figure 2, respectively), and one neutral Hdap molecule with the concomitant proton migration from the most basic N9 to the less basic heterocyclic N3. Thus, the second tautomer named H(N3)Hdap was obtained. As shown in Figure 2a, both ZnII atoms are in distorted tetrahedrons, being constructed by three carboxylate O atoms from three different tp anions and one imidazole N atom from Hdap nucleobase. All the Zn-O and Zn-N bond distances (Table S2, Supporting Information) are

comparable to those documented values.40 Zn1 and Zn2 atoms in the asymmetric unit are joined together by Hdap nucleobase in a bidentate μ2-N7,N9 bridging mode to generate a binuclear [Zn2(μ2-Hdap)]4þ subunit. As expected, the Zn-N9 and Zn-N7 coordination bonds are collectively reinforced by 2-fold N3-H4 3 3 3 O7 and N6-H6B 3 3 3 O5 hydrogen-bonding interactions resulting from the protonation of the N3 site of Hdap nucleobase (Figure 2a). The subunits are infinitely extended by the carboxylate groups of three crystallographically equivalent tp anions (shown in pink in Figure 2) in the ab plane to generate a 2D (4 4) rhombus layer structure (Figure 2b). The adjacent 2D layers are further interlinked by the second crystallographically independent tp anions (shown in blue in Figure 2) to lead to a 3D coordination framework of 2 with three N-H 3 3 3 O hydrogen-bonding interactions between the exocyclic amino group of Hdap and the carboxylate of tp (see Table 2 and Figure 2d). Thus, the H(N3)Hdap tautomer in 2 presents an extra hydrogen-bonding interaction (N2-H2A 3 3 3 O1) compared with the H(N7)Hdap tautomer in 1 (Figure S4, Supporting Information). On the other hand, although the two crystallographically independent tp anions show only one binding mode (bidentate bridging and monodentate mode), they play different roles upon the construction of a high-dimensional coordination framework of 2. As compared to the first tp anion, the second one just links the

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Figure 3. (a) Local coordination environment of ZnII atoms in 3 (H atoms were omitted for clarity, symmetry codes: A = 1 - x, 1 - y, 2 - z; B = 2 - x, 1 - y, 1 - z; C = 1 þ x, y, z; D: 1 - x, 0.5 þ y, 1.5 - z). (b) 2D layer of 3 with intralayer N-H 3 3 3 O hydrogen-bonding interactions. (c) 3D pillared-layer framework of 3.

adjacent binuclear [Zn2(μ2-Hdap)]4þ subunits into a 1D ribbon, rather than a 2D layer (Figure 2b,c). Topologically, when the [Zn4(COO)4] core is considered as a single node, tp and Hdap as two different connectors, the coordination framework of 2 can be simplified into a 3D network sustained by irregular eight-connected nodes. Further analysis indicates that tp connectors link the same neighboring [Zn4(COO)4] core into two identical interpreting six-connected R-Po-type structures. Then the Hdap connectors link the two R-Po-type structures to form a scarcely eight-connected 3D self-penetrating MOF. To the best of our knowledge, this is the first 3D self-penetrating example involved nucleobase-based transitional metal complexes. Comparisons between the crystal structure and the preparation conditions of 1 and 2 suggest that the sole difference in the cooling rate of the mixture has significant influences on the coordination polyhedrons of the ZnII ion, the binding modes of the both ligands, and the proton tautomers of the neutral Hdap nucleobase, which undoubtedly result in different framework packing. Crystal Structure of {[Zn2(μ2-Hdap)(tm)(μ2-OH)] 3 H2O}n (3). Because of the replacement of binary H2tp by the ternary H3tm coligand, 3 has a dense 3D pillared-layer coordination framework triply supported by Hdap, tm, and μ2-OH-. However, the coordination polyhedrons of ZnII atoms (see Figure 3a and Table S3, Supporting Information), the binding mode, the protonation degree (Hdap), and the proton tautomer of the Hdap nucleobase (H(N3)Hdap) in 3 are similar to those in 2. Therefore, herein, we only focus on the structural difference of 3 from that of 2. As shown in Figure 3b, the binuclear [Zn2(μ2-Hdap)]4þ subunits observed in 3, were linked together by the coordination bonds of ZnII atom and the carboxylate group O atoms of tm (O1 and O4), leading to a 1D linear ribbon along the crystallographic c-direction. Interestingly, both tm and Hdap ligands locate just on the one side. Such an arrangement of

the mixed ligands probably results from the 2-fold interligand hydrogen-bonding interactions between the exocyclic amino/endocyclic imino groups of Hdap and carboxylate group of tm (N6-H6B 3 3 3 O4, N3-H3 3 3 3 O2, see Table 2 and Figure 3b), which also favors the metal-nucleobase bonds (Zn-N7 and Zn-N9). Two unparallel ribbons are aggregated together by the coordination bonds of ZnII and the third carboxylate O6 atom of tm to generate a 2D covalent plane with inter-ribbon N2-H2A 3 3 3 O3 recognition interactions (see Figure 3b). Undoubtedly, the formation of the covalent 2D layer is closely related to the orientation and the number of the functional carboxylate groups in the tm ligands. Then, a dense pillared-layer 3D framework with ca. 3.4442(2) A˚ separation was unexpectedly observed, which is doubly pillared by the bridging binding modes of μ2-OH- and the carboxylate group of tm in 3 (Figure 3c). Notably, in addition to acting as a pillar, the μ2-OH is also an H-donor and can produce an unusual interligand hydrogen-bonding interaction with the N1 of Hdap.4 Thus, the H(N3)Hdap tautomer in 3 can act either as H-donor or as H-acceptor, engaging in five hydrogen-bonds with the coligands and/or lattice water molecule (Figure S5, Supporting Information). Crystal Structure of {[Zn(H2dap)(H2O)(btc)] 3 3H2O}n (4). Different from the monoclinic space group of 1-3, 4 crystallizes in the triclinic P1 space group (Table 1), featuring a 1D double-chain motif bridged by btc anions. As shown in Figure 4a, the sole ZnII atom in the asymmetric unit is surrounded by one imidazole N9 from a cationic H2dapþ nucleobase, one coordinated water molecule, and three carboxylate O atoms from three separate btc anions, exhibiting a distorted trigonal-bipyramidal polyhedron (Table S4, Supporting Information). Rather than the bidentate bridging modes of neutral Hdap in 1-3, the cationic H2dapþ tautomer (H2(N3,N7)H2dapþ) with the ionizable protons on both N3 and N7 atoms, only presents its imidazole N9 donor

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Figure 4. (a) Local coordination environment of ZnII atom in 4 (H atoms were omitted for clarity, symmetry codes: A = 1 - x, 1 - y, - z; B = -x, 1 - y, -z). (b) 1D double-chain of 4 bridged by btc ligand. (c) 3D supramolecular architecture constructed from O - H 3 3 3 O hydrogenbonding interactions with a boat-shaped water ring entrapped in it.

to coordinate with the ZnII atom in a monodentate fashion. Because of the protonation on N3 of the nucleobase, the Zn-N9 coordination bond is doubly enhanced by the intramolecular interligand N3-H3 3 3 3 O5 and N2H2B 3 3 3 O3 hydrogen-bonding interactions (Table 2). Two carboxylate groups at 1 and 3 positions of the btc anion connect the adjacent ZnII atoms along the a-direction in a monodentate fashion to generate a 1D linear chain. Two adjacent chains are further arranged in an unparallel manner by the coordination bonds of ZnII atom and the carboxylate O3 in the 2 position of btc anion, forming a 1D double chain (Figure 4b). Thus, the three adjacent carboxylate groups consistently display a monodentate mode, which, along with the benzene ring, collectively contribute to the formation of the 1D double-chain structure. And the H2dapþ nucleobase within the double-chain is just a terminal ligand to complete the metal coordination sphere. As shown Figure 4c, the double-chain and three lattice water molecules (O8, O9, and O10) are further assembled into a 3D H-bonded supramolecular network by abundant hydrogen-bonding interactions (O-H 3 3 3 O, O -H 3 3 3 N, N-H 3 3 3 O). As a result, a closed chair-shaped cluster composed of six lattice water molecules and two carboxylate groups of btc anions was surprisingly observed to be entrapped in the supramolecular structure by the N2H2A 3 3 3 O9, N7-H7 3 3 3 O10, O8-H8B 3 3 3 N1, and other O-H 3 3 3 O hydrogen-bonding interactions (Figure 4c). That is to say the N1 atom of the cationic H2dapþ nucleobase can also act as an H-acceptor to produce suitable hydrogen bonding interactions with counter-ligands (Figure S6, Supporting Information). Crystal Structure of {[Cd3(H2O)2(μ3-dap)2(ap)2] 3 H2O}n (5). Complex 5 is a trinuclear CdII cluster-based 2D aggregate solely extended by an anionic dap- nucleobase. As shown in Figure 5a, the trinuclear CdII cluster is a dimer of two crystallographically asymmetric units composed of one and half a CdII ions, one ap anion, one bridging dap nucleobase, and one coordinated water molecule. Cd1 is seven-coordinated by four oxygen atoms from two chelating

carboxylate groups (O1, O2, O3A, and O4A), two N atoms of two individual dap- anion (N3, N7B), and one coordinated aqua molecule, building distorted pentagonal bipyramidal CdN2O5 surroundings (Table S5, Supporting Information). In contrast, Cd2, locating at the inversion center, exhibits a distorted octahedron built from four carboxylate O atoms of two ap anions (O1, O3, O1A, and O3A) and two imidazole N atoms from a pair of dap nucleobases (N9 and N9A). Cd1, Cd2, and Cd1A are strictly linear with a bond angle of 180.0o. And the Cd1 3 3 3 Cd2 distance within the trinuclear cluster is 3.5819(2) A˚. Ap anion exhibits an unusual hexadentate η1:η2:η2:η1 binding fashion to hold three CdII cores together. The nucleobase in 5 adopts an anionic form (dap-) and represents the loss of the tautomerizable proton of the heterocyclic N atom from the free neutral Hdap. By tridentate μ3-N3,N7,N9 coordination mode, the dap- anion holds three CdII cores together and further extends the trinuclear CdII-cluster into an infinite 2D (4 4) layer in the crystallographic bc plane (Figure 5b). Notably, the neighboring trinuclear CdII clusters within a 2D plane are arranged in an chiral Λ and Δ way with the specific dihedral angles of 80.00(3)o (calculated by the Cd-dap plane), and cannot be overlapped through simply rotation operator. Side viewing of the 2D plane suggests that trinuclear CdII-clusters are parallel with each other just resulting from its unique arrangement, rather than being in a straight line. Additionally, the intralayer N2-H2B 3 3 3 N7, N6-H6B 3 3 3 O2, and O5-H50 3 3 3 O4 hydrogen-bonding patterns between the dap and ap/H2O molecule (Figure 5b and Table 2) positively stabilize the CdII-N bonds and even the 2D aggregate. The neighboring 2D planes are stacked together in a parallel way by a pair of “head to tail” N6-H6A 3 3 3 N1 hydrogen-bonding interactions between the nucleobases. Thus, all the N atoms of the dap anion in 5 have been utilized either as hydrogen-bond sites or as metal binding donors (see Figure S7, Supporting Information). Crystal Structure of {[Cd(H2dap)(H2O)2(tp)] 3 0.5tp 3 H2O}n (6). 6 is a 1D Z-shaped chain linked by tp anions. The asymmetric unit of 6 contains [Cd(H2dap)(H2O)2(tp)]þ

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Figure 6. 2D supramolecular layer of 6 assembled from interchain N-H 3 3 3 N hydrogen-bonding interactions.

Figure 5. (a) Trinuclear CdII-subunit and local coordination environment of CdII atoms in 5 (H atoms were omitted for clarity, symmetry codes: A = 1 - x, 1 - y, 2 - z; B = 2 - x, 1 - y, 1 - z; C = 1 þ x, y, z; D = 1 - x, 0.5 þ y, 1.5 - z). (b) 2D layer of 5 with intralayer N - H 3 3 3 O hydrogen-bonding interactions. (c) 3D supramolecular network by N-H 3 3 3 N hydrogen-bonding interactions.

cations, half a tp anion with an inversion center for charge compensation, and one distorted lattice water molecule. The CdII atom adopts a slightly distorted octahedral geometry (Table S6, Supporting Information), coordinated by three carboxylate O atoms from two different tp anions and one imidazole N7 atom of a cationic H2dapþ nucleobase in an equatorial plane, and two coordinated water molecules in the axial positions (Figure 6 and Figure S8, Supporting Information). Amazingly, the binding mode and the protonation site of the cationic H2dapþ nucleobase in 6 are different from those in 4 (monodentate N7 binding in 6 vs. monodentate N9 binding in 4; and H2(N1,N9)H2dapþ in 6 vs. H2(N3,N7)H2dapþ in 4), although both the H2dapþ nucleobases act as the terminal ligand. In addition, the CdII-N7 bond is reinforced by intramolecular interligand N6-H6B 3 3 3 O4 interactions. The tp anion acts as a bridge connecting the adjacent CdII atoms in monodentate and bidentate chelating modes to lead to a 1D Z-shaped chain

with the terminal H2dapþ ligands alternately locating on either side of the CdII-tp chain. Furthermore, a 2D supramolecular layer was formed by intermolecular N2-H2B 3 3 3 O4 hydrogen-bonding interactions between the exocyclic amino group of H2dapþ and carboxylate group of tp anion. And the remaining N-H groups of H2dapþ act as H-donors to recognize the lattice tp anions to form a 3D supramolecular framework along with the classic O-H 3 3 3 O hydrogen-bonding interactions between the coordinated water molecules and carboxylate groups (see Table 2 and Figures S9-S10, Supporting Information). Polymeric Nature by Synergistic Binding of the Nucleobase and Polycarboxylate Anion. Six novel Hdap-based complexes ranged from a low-dimensional chain to a scarcely high-dimensional self-penetrating framework have been isolated under hydrothermal conditions by the appropriate combination of polycarboxylate anions and ZnII/CdII ions. Structural analysis confirms that the polymeric nature is significantly tuned by the synergistic binding of nucleobase and polycarboxylate anion, the protonation, and hydrogen bonding character of the nucleobase. As shown in Figure 7, the neutral nucleobase in 1 adopts a bidentate bridging mode to the formation of the tetranuclear ZnII cluster in 1 (μ2-N3, N9) and the binuclear [Zn2(μ2-Hdap)]4þ subunit in 2 and 3 (μ2-N7,N9), which are infinitely extended by tp, tm, and/or μ2-OH (see Figure S11, Supporting Information). In contrast, the tridentate dap- anion in 5 (μ3-N3,N7,N9) play a dual role for both the aggregate of the three CdII cores and the prolongation of the 2D infinite layer. In 4 and 6, the btc and tp anions significantly contribute to the formation of the chain motif, which displays an impressive contrast with the monodentate nucleobase. It should be noted that the binding patterns of the nucleobase are related to its protonation degree and tautomeric forms. Additionally, the tp coligand in 1, 2, and 6 has exhibited three different binding modes to extend the covalent framework. On the other hand, in addition to the binding modes of the nucleobase, the extensively protonation degree (H(N7)Hdap, H(N3)Hdap, H2(N3,N7)H2dapþ, H2(N1,N9)H2dapþ and dap-) and tautomerism-dependent hydrogen-bonding patterns between the nucleobase and counter-ligands (see Figures S3-S7 and S10, Supporting Information) are responsible for the extra stabilization of the ZnII/CdII-N coordination bonds and even the resulting polymeric motif. Moreover, some selfassembled patterns are repeatedly observed (see Figure S12,

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Figure 7. The binding character of 2,6-diaminopurine in complexes 1-6.

Figure 8. Fluorescence emission spectra of 1-6 in an acetonitrile solution at room temperature.

Supporting Information) in different compounds and have become one of the strategies for the synthesis of the desired polymers. Thermogravimetric Analysis. To explore the compositional thermal stability of the mixed ligands in the polymeric structures and further to establish the relationship among the thermal stability and their dimensions, structural motif, and the weak inter/intramolecular interactions, TGA of the complexes 1-6 were measured from room temperature to 800 C under an inert atmosphere (Figure S13, Supporting Information). As a result, complex 1, a tetranuclear ZnII cluster-based 2D layer, displays considerably higher thermal stability than the 3D self-penetrating coordination framework of 2, although they are both constructed from the same mixed ligands. The removal of the μ3-OH and lattice water molecules in 1 leads to the first weight loss of 6.6% between 267 and 301 C (calcd. 6.2%). And the next obvious weight loss began at 390 C and ended at 607 C, ascribed to the synchronous decomposition of Hdap and tp anions. In contrast, 2 without any lattice molecules is only thermally stable up to 170 C, and is unstable by ca. 97 C compared to 1. Then the next weight loss corresponding to the decomposition of the mixed ligands slowly lasted until ca. 559 C.

As compared with 1, the weight loss of the μ2-OH and lattice water molecules in 3 is not obvious and occurs very slowly between 90 and 370 C (obs: 5.8%, calcd. 6.7%). The next weight loss began at 370 C and ended at 564 C, ascribed to the loss of the neutral Hdap and btc anions. Complex 4 with a 1D double-chain structure loses its lattice and coordinated water molecules from room temperature to 145 C (expt. 10.7%, calcd. 10.9%). The next weight loss began at 307 C and ended at 530 C, ascribed to the consecutive decomposition of H2dapþ and btc ligands. The remaining residue of the polymers 1-4 is ZnO (expt. 29.0%, calcd. 29.6% for 1, expt. 27.7%, calcd. 26.7% for 2; expt. 30.1%, calcd. 31.1% for 3, and expt. 17.4%, calcd. 16.4% for 4, respectively). For the two CdII-based polymers, the first stage between 140 and 290 C for 5 (expt. 5.8% calcd. 5.5%) and from room temperature to 130 C for 6 (obs. 11.5%, calcd. 9.6%) are due to the loss of free and coordinated water molecules, respectively. The second obvious weight-loss began at 380 C and ended at 540 C for 5 and began at 213 C and ended at 292 C for 6, ascribed to the continuous removal of dap/ Hdapþ and ap/tp anions. No remarkable changes appeared upon further heating of the samples to 800 C, leaving CdO fragments as the final products (expt. 39.4%, calcd. 39.3% for 5, and expt. 22.4%, calcd. 22.7% for 6). Thus, it can be concluded that the coordinated water or OH- group in the cluster-based substructures of 1 and 5 show slightly high thermal stability than those containing free lattice water in polymers 3, 4, and 6. However, due to the undistinguished stage for the removal of the mixed ligands, Hdap nucleobase and organic polycarboxylate anion contribute to the compositional stability of polymers 1-6 to a comparable extent, although they play different roles upon the construction of the polymeric structures. On the other hand, the thermal stability cannot be well related with the dimensions of the polymers probably resulting from the different structure motifs. Luminescence Spectra. Polymeric complexes of metal cations with the d10 configuration, such as ZnII, and CdII, and polyazaheterocyclic ligands have been shown to possess interesting luminescent properties,41,42 which can be used

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as a DNA photocleavage agent, chemical probe, and cationinduced photoswitch. Thus, we investigate the emission spectra of the resulting complexes in an acetonitrile solution. As shown in Figure 8, 1 presents two intense emissions at 358 and 394 nm and 2 gives only one emission at 328 nm upon excitation at 299 nm. In contrast, the emission band of 3 is red-shifted to 406 nm upon excitation at 333 nm. Correspondingly, the other three complexes, 4-6 exhibit only one emission located at 344 nm for 4, 339 nm for 5, and 342 nm for 6 upon the excitation at 291 ( 2 nm. To thoroughly understand the nature of the emission band, the luminescence spectra of the isolated Hdap nucleobase was also measured at room temperature for comparison. The results indicate that Hdap nucleobase exhibits a relatively strong emission with a maximum at 343 nm upon excitation at 301 nm. Thus, except complex 3, the emission band ca. 343 ( 15 nm of the polymers should be ascribed to the intraligand charge-transfer of Hdap. And the slight shift compared to isolated Hdap nucleobase should be ascribed to the different protonated and/or deprotonated forms of the Hdap and the chelating coordination of the Hdap with metal ions. Additionally, the origin of the emissions at 394 nm for 1 and 406 nm for 3 might be attributed to the photoinduced charge transfer (PCT) of the nucleobase upon cation binding.43 The enhanced emission behavior for all the complexes opens a nice window to the potential applications of the Hdap-based complexes as antivirasic prodrugs. Conclusions In summary, we have presented the solid structures of a series of six 2,6-diaminopurine-based mixed-ligand ZnII/CdII polymers, their subunit-dependent thermal stability, and intense luminescent properties. Significantly, the unusual polymeric nature is jointly tuned by the synergistic coordination of Hdap nucleobase and polycarboxylate anion under hydrothermal conditions. Meanwhile, the binding character of the nucleobase in the solid state is thoroughly discussed from the viewpoints of the binding pattern, protonation degree, and the corresponding proton tautomer as well as the hydrogen-bonding ability. Acknowledgment. This present work was financially supported by the National Natural Science Foundation of China (20703030, 20871092, and 20973125), the Key Project of Chinese Ministry of Education (Grant No. 209003), and the Program for New Century Excellent Talents in University (NCET-08-0914), which are gratefully acknowledged. Supporting Information Available: X-ray data in CIF format, TG curves for 1-6, and additional figures and tables. This material is available free of charg evia the Internet at http://pubs.acs.org.

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