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pH dependent synthesis of structurally diverse praseodymium(III) coordination polymers based on isomeric ligands
pH dependent synthesis of structurally diverse praseodymium(III) coordination polymers based on isomeric ligands Qi Wu, Meng Jie Cao, Bo Wei, Yu Bai, He Tian, Jing Wang, Qiao Liu, Qiao Yun Li, Gao Wen Yang PII: DOI: Reference:
S1387-7003(15)30127-1 doi: 10.1016/j.inoche.2015.10.035 INOCHE 6156
To appear in:
Inorganic Chemistry Communications
Received date: Revised date: Accepted date:
22 September 2015 23 October 2015 27 October 2015
Please cite this article as: Qi Wu, Meng Jie Cao, Bo Wei, Yu Bai, He Tian, Jing Wang, Qiao Liu, Qiao Yun Li, Gao Wen Yang, pH dependent synthesis of structurally diverse praseodymium(III) coordination polymers based on isomeric ligands, Inorganic Chemistry Communications (2015), doi: 10.1016/j.inoche.2015.10.035
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
ACCEPTED MANUSCRIPT pH dependent synthesis of structurally diverse praseodymium(III) coordination polymers based on isomeric ligands
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Qi Wu, Meng Jie Cao, Bo Wei, Yu Bai, He Tian, Jing Wang, Qiao Liu, Qiao Yun Li*, Gao Wen Yang*
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*Qiao-Yun Li: Tel: +86-512-52251842; Fax: +86-512-52251842 Email: [email protected] *Gao Wen Yang:Tel: +86-512-52251842; Fax: +86-512-52251842 Email: [email protected] Jiangsu Key Laboratory of Advanced Functional Material, Department of Chemistry and Material Engineering, Changshu Institute of Technology, Changshu, 215500, P.R.China Abstract: Reactions of two isomeric ligands, 5-(n-pyridyl)tetrazole-2-acetato potassium salt (Kn-pytza, n=2,4) and PrCl3·6H2O under solvothermal conditions, afforded three new complexes, [Pr(2-pytza)(SO4)(H2O)3]·H2O(1), [Pr(4-pytza)3(H2O)2]·2H2O (2) and [Pr4(4-pytza)5(OH)4(H2O)7Cl]Cl2·4H2O (3), whose structures are controlled by not only the different positions of the nitrogen atom of the pyridine ring but also the pH value of the solvent system. These compounds have been characterized by elemental analysis, IR and single crystal X-ray diffraction. The X-ray analysis reveals that compound 1 is a two dimensional layer structure made up of a Pr2O6 binuclear unit and the tridentate bridging SO42- ligand; compound 2 is an unusual one dimensional ladder like chain structure consisting of a Pr2O10 binuclear unit and 4-pytza which acts as a tetradentate ligand via the pyridine-N and the carboxylate group in a μ1,1,3-COO bridging mode while compound 3 is an unprecedented two dimensional network composed of a tetranuclear cubane-shaped Pr4(OH)48+ cluster (SBU) and tridentate 4-pytza via the pyridine-N and the carboxylate group in a μ1,3-COO bridging mode. Various hydrogen bonds exist to assemble the 1D chains or 2D layers into 3D supramolecular network structures. Furthermore, the luminescence properties investigated at room temperature in the solid state show only intraligand emission for 1, both intraligand and characteristic peaks of Pr3+ for either compound 2 or 3. Keywords: pH; isomer; Pr(III); crystal structure; luminescence Considerable attention attached to lanthanide coordination compounds stems from not only their diversity in structure[1-4], but also their tunable application as advanced functional materials, in the field of luminescence, catalysis, gas storage, etc [5-8]. To the best our knowledge, on one hand, lanthanide elements tend to have much higher coordination numbers and more flexible coordination modes, compared to other metal ions. On the other hand, many factors, such as pH value, solvent system, temperature are likely to influence the eventual structures of the targeted molecules and add to the novelty of the structures. One of the key factors to obtaining the targeted molecules is via appropriate selection of the ligand whereas the researches concerning coordination compounds based on isomeric ligands are by no means satisfactory[9]. It is universally acknowledged that tetrazole-carboxylate ligands are outstanding candidates for construction of novel compounds in that they tend to display a variety of coordination modes and the -CH2- spacer between the tetrazole ring and the carboxylate group is able to offer flexible orientations of the carboxylate arm, allowing the formation of various framework
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structures[10-12]. In addition, the abundant nitrogen and oxygen atoms may participate in the formation of hydrogen bonds, stabilizing the supramolecular assemblies. Inspired by the latest work which have reported coordination compounds based on pH values or isomeric ligands[9, 13-15], we have selected two isomeric bifunctional tetrazole-carboxylate ligands, 5-(n-pyridyl)tetrazole-2-acetato potassium salt (Kn-pytza, n=2,4), to react with PrCl3·6H2O under different pH values, and three novel coordination compounds have been obtained (Scheme1). The different positions of the nitrogen atom of the pyridyl ring can show quite distinct coordination modes. We assume that 2-pytza often adopts N(pyridyl), N(tetrazolyl) chelating mode [16-17] while 4-pytza can act as a bridging ligand via pyridyl-N and carboxylate-O atoms. Herein, we will report their synthesis, crystal structures and luminescence properties. In this work, two isomeric ligands 5-(n-pyridyl)tetrazole-2-acetato potassium salt (n=2,4) have been selected to be reacted with PrCl3·6H2O under different pH values in order to investigate whether or not the different positions of the nitrogen atom of the pyridine ring and the pH values will influence the final structures of our targeted molecule. Three novel compounds were prepared successfully. The structure of compound 1 remains unchanged when the pH value increases or decreases. However, compound 2 can only be obtained when the pH value of the system retains a relatively low level (pH=4). It is interesting that hydroxyl anions are included in the molecule, which is correspondent to the synthetic strategy, to some extent. Compounds 1-3 are stable towards oxygen and moisture. The elemental analysis of 1-3 are consistent with their chemical formula. In the IR spectra of 1-3, the characteristic bands of carboxylate groups appeared in the usual region at 1601–1658 cm-1 for the asymmetric stretching vibrations and at 1302-1392 cm-1 for the symmetric stretching vibrations[20]. Peaks at (3418-3435cm-1) are ascribed to the O-H vibration of both coordinated and uncoordinated water. The X-ray diffraction reveals that compound 1 crystallizes in monoclinic lattice space group P21/c. As shown in FigS1, each Pr(III) center is nine-coordinated by nine oxygen atoms, of which three are from three water molecule, three from two carboxylate groups of two 2-pytza ligands, and the remaining three from three independent sulfate anions, forming a distorted PrO9 monocapped square antiprism coordination arrangement. Two neighboring Pr(III) centers are doubly bridged by two carboxylate groups in a μ1,1,3-COO mode to form a binuclear unit. Each sulfate anion acts as a tridentate bridging ligand to connect adjacent binuclear units, displaying a two dimensional network extending along the bc plane (Fig1). The Pr-O distances range from 2.433 to 2.645Å, which are in good agreement with those of the previously reported praseodymium compounds[8]. Compared with [Pr(3-pytza)2Cl(H2O)2], the coordination mode of 2-pytza is analogous to that of 3-pytza, but the structure is quite different since the sulfate anions not only balances the charge of Pr(III) but also increases the dimension of the structure. Contiguous layers are further held together by various hydrogen bonds to generate a three dimensional supramolecular network (TableS3). Fig 1 here Compound 2 crystallizes in triclinic lattice space group Pī. Each Pr(III) in a distorted PrO8N monocapped square antiprism coordination arrangement is surrounded by two oxygen atoms from two water molecules (O7, O8), six oxygen atoms from five carboxylate groups of five 4-pytza ligands (O2,O3,O3A,O4A,O5,O6A) and a nitrogen atom from the pyridyl ring (N10B) (FigS2). 4-pytza shows three different coordination modes in the molecule. First, two 4-pytza anions adopt a monodentate mode to coordinate to two Pr(III) centers by one of the carboxylate oxygen atom
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(O2); then the two independent Pr(III) centers are doubly bridged by two bidentate 4-pytza ligands in a μ1,3-COO mode to form a binuclear unit; eventually adjacent binuclear units are bridged by four tetradentate 4-pytza anions via its nitrogen atom of the pyridyl ring and the carboxylate group in a μ1,1,3-COO mode, thereby giving an unusual one dimensional ladder like chain extending along the c axis (Fig2). The Pr-O distances from 2.410 to 2.671Å are approximately correspondent to those of compound 1 or [Pr(atza)2(CH3OH)(H2O)Cl] (atza=5-aminotetrazole-1-acetato)[4]. In contrast with [Pr(3-pytza)2Cl(H2O)2] in which 3-pytza only acts as a tridentate ligand via its carboxylate group in a μ1,1,3-COO bridging mode, 4-pytza displays more coordination modes and therefore shows a complicated one dimensional chain. Compared with 1, however, the sulfate anions are not included in the molecule, this is probably due to the nature of the ligand. Furthermore, neighboring one dimensional chains are self-assembled by various O···H or N···H hydrogen bonds to form a three dimensional supramolecular network (TableS3). Fig 2 here Compound 3 crystallizes in monoclinic lattice space group P21/n. As is illustrated in FigS3, Pr(1)-Pr(4) are all eight-coordinated and display distorted square antiprism coordination arrangements. Pr(1) is eight-coordinated by three oxygen atoms from three hydroxyl anions (O11,O12,O14), two atoms from two water molecules (O17,O18) and three oxygen atoms (O2,O8,O10) from three carboxylate groups of three independent 4-pytza ligands (FigS3a); Pr(2) by three hydroxyl oxygens (O11,O13,O14), two water oxygens (O15,O16), two carboxylate oxygens (O1,O4) and one nitrogen atom (N65A) (FigS3b); Pr(3) by three hydroxyl oxygens (O12,O13,O14), one water oxygen (O21), three carboxylate oxygens (O3,O5,O9) and one chloride anion (Cl3) (FigS3c); The coordination environment of Pr(4) is similar to that of Pr(2) (FigS3d). Each hydroxyl anion acts as a tridentate ligand to connect to three Pr(III) centers to form a distorted cubane-shaped Pr4(OH)48+ unit in which four praseodymium atoms are at the corners of a slightly irregular tetrahedron(Fig3a), with four hydroxide oxygen atoms occupying positions outside the faces of the praseodymium tetrahedron. Then Pr(1) and Pr(2), Pr(1) and Pr(4), Pr(2) and Pr(3) are bridged by three bidentate 4-pytza ligand in a μ1,3-COO mode, respectively . These tetranuclear Pr4(OH)48+ clusters, which behave as SBUs, are bridged by tridentate 4-pytza via one pyridyl nitrogen and carboxylate group in a μ1,3-COO bridging mode to form a two dimensional layer extending along the bc plane (Fig3). The Pr-O distances are from 2.355 to 2.483Å, which are a little shorter than those of [Pr(atza)2(CH3OH)(H2O)Cl](atza=5-aminotetrazole-1-acetato)[4] The structure is unprecedented among praseodymium compounds, to the best of our knowledge. It is worthwhile to point out compound 3 is obtained under relatively higher pH value, which is correspondent to the existence of the hydroxyl anions in the molecule. Compared to 2, one more coordination mode of 4-pytza is found in compound 3, and the structure changes from one to two dimensional as the pH value increases. However, although compounds 1 and 3 are both two dimensional, the structures are quite different. There exist one coordinated chloride anion and two uncoordinated ones in compound 3 while 1 contains sulfate anions. It is interesting that sulfate anions do not exist in the molecule, which is probably assigned to the stereo effect. Adjacent layers are further held together by various hydrogen bonds to form a three dimensional network (TableS3). Fig 3 here Lanthanide coordination compounds usually show interesting luminescence properties [19-20]. In view of their potential applications in the photochemistry, we are dedicated to
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investigating the luminescence properties of compounds 1-3 at room temperature in the solid state(FigS4). Upon excitation at 328, 370 nm, H2-pytza (H2-pytza=5-(2-pyridyl)tetrazole-2-acetic acid) and K4-pytza exhibit photoluminescence with maximum intensity at 375, 429 nm, respectively. Compound 1 only shows maximum intensity at 342 nm upon excitation at 275 nm, which can be tentatively assigned to the intraligand emission[9]. Compounds 2 and 3 show maximum intensity at 437, 457, 476, 491 nm and 429, 457, 479, 496 nm upon excitation at 376 and 370 nm, respectively. The peaks at 437 and 429 nm are attributed to the ligand centered emission since similar peaks at 429 nm was observed for the free ligand K4-pytza. The other peaks can be ascribed to the transitions of 1I6-3H4 (457 nm for both 2 and 3), 3P1-3H4 (476 nm for 2, 479 nm for 3), 3P0-3H4 (491 nm for 2, 496 nm for 3). The luminescence of compounds 2 resembles that of 3. Compared to the luminescence of compound 1 or [Pr(3-pytza)2Cl(H2O)2] [9], compounds 2 and 3 exhibit not only the ligand centered luminescence, but also the characteristic peaks of Pr3+, showing that the isomers really have an impact on the luminescence.
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In summary, we are the first to report praseodymium coordination compounds based on Kn-pytza (n=2,4). Three novel compounds have been constructed successfully whose structures are controlled by not only various coordination modes of n-pytza (n=2,4) and the different positions of the nitrogen atom of the pyridine ring, but also simultaneously the pH value. When the pH value is low, compound 2 with an unusual one dimensional chain was obtained, however, when high, compound 3 formed with an intriguing two dimensional network based on a tetranuclear Pr4(OH)48+ clusters (SBU). The luminescence properties show intraligand emission for 1, either ligand centered emission or characteristic peaks of Pr3+ for both compounds 2 and 3. Our research results indicate the structures of praseodymium compounds are affected by isomers and Pr(III)-4-pytza compounds are sensitive to pH values. Therefore, other investigations on isomers are underway in our group.
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Fig1 The 2D network of 1 formed by hydrogen bonding interactions. Fig2The 1D chain structure of 2 formed by hydrogen bonding interactions Fig3 The 2D layer network of compound 3 extending along the bc plane. Hydrogen atoms are omitted for clarity. Fig.S1The coordination environment of Pr(III) of compound 1. Hydrogen atoms are omitted for clarity FigS2 The coordination environment of Pr(III) of compound 2. Hydrogen atoms are omitted for clarity; FigS3 The coordination environment of PrIII(1)-PrIII(4);(a) PrIII(1); (b) Pr(2)III; (c)PrIII(3); (d) PrIII(4). Hydrogen atoms are omitted for clarity. Fig.S4(a)The emission spectra of H2-pytza, K4-pytza and compound 1 at room temperature in the solid state (For H2-pytza, λex = 328nm, For K4-pytza, λex = 370nm, For 1, λex = 275nm); (b)The emission spectrum of compound 2 at room temperature in the solid state (λex = 376nm); (c) The emission spectrum of compound 2 at room temperature in the solid state (λex = 370nm).
Acknowledgment
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The authors acknowledge financial support from the Natural Science Foundation of Jiangsu Provi nce (Grant No. BK2012210), the Natural Science Foundation of the Jiangsu Higher Education Inst itutions of China(Grant No.10KJB430001) and the Opening Fund of Jiangsu Key Laboratory of A dvanced Functional Materials (Grant No.12KFJJ010). References [1]S.Y. Lin, C. Wang, L. Zhao, J.K. Tang, Chem Asian J, 9 (2014) 3558-3564
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[16]Q.Y. Li, G.W. Yang, X.Y. Tang, Y.S. Ma, F. Zhou, W. Liu, J. Chen, H. Zhou, Inorg. Chem. Comm. 13 (2010) 254. [17]B.J. Wang, J.H. Zou, W.X. Li, Z. Wang, B. Xu, S. Li, Y.S. Zhai, D.L. Zhu, Q.Y. Li, G.W. Yang, J. Organomet. Chem. 749 (2014) 428-432. [18]J. Yang, Y.T. Min, Y. Chen, J. Wang, H. Tian, L. Shen, Y.S. Zhai, G.W. Yang , J.H. Zou, Q.Y. Li , H.J. Cui, Inorg. Chim. Acta 419 (2014) 73-81. [19]H. Deng, Y.C. Qiu, Y.H. Li, Z.H. Liu, O. Guillou,Inorg. Chim. Acta 362 (2009) 1797-1804. [20]P. Zhu, W. Gu, M.L. Liu, H.B. Song, X. Liu, Y.Q. Gao, H.Y. Duan, S.P. Yan and D.Z. Liao, CrystEngComm, 11 (2009) 351-358.
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ACCEPTED MANUSCRIPT Graphical abstract
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Three novel praseodymium coordination compounds based on isomeric ligands 5-(n-pyridyl)tetrazole-2-acetato potassium salt (n=2,4) have been successfully prepared, whose structures are not only controlled by the different positions of the nitrogen atom of the pyridine ring, but also the pH values. The luminescence shows ligand centered emission or both intraligand emission and characteristic peaks of Pr3+.
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ACCEPTED MANUSCRIPT Highlights
Two isomers were prepared.
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The luminescence properties were investigated.
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Three Pr(III) compounds were prepared.
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S1387-7003(15)30127-1 doi: 10.1016/j.inoche.2015.10.035 INOCHE 6156
To appear in:
Inorganic Chemistry Communications
Received date: Revised date: Accepted date:
22 September 2015 23 October 2015 27 October 2015
Please cite this article as: Qi Wu, Meng Jie Cao, Bo Wei, Yu Bai, He Tian, Jing Wang, Qiao Liu, Qiao Yun Li, Gao Wen Yang, pH dependent synthesis of structurally diverse praseodymium(III) coordination polymers based on isomeric ligands, Inorganic Chemistry Communications (2015), doi: 10.1016/j.inoche.2015.10.035
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
ACCEPTED MANUSCRIPT pH dependent synthesis of structurally diverse praseodymium(III) coordination polymers based on isomeric ligands
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Qi Wu, Meng Jie Cao, Bo Wei, Yu Bai, He Tian, Jing Wang, Qiao Liu, Qiao Yun Li*, Gao Wen Yang*
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*Qiao-Yun Li: Tel: +86-512-52251842; Fax: +86-512-52251842 Email: [email protected] *Gao Wen Yang:Tel: +86-512-52251842; Fax: +86-512-52251842 Email: [email protected] Jiangsu Key Laboratory of Advanced Functional Material, Department of Chemistry and Material Engineering, Changshu Institute of Technology, Changshu, 215500, P.R.China Abstract: Reactions of two isomeric ligands, 5-(n-pyridyl)tetrazole-2-acetato potassium salt (Kn-pytza, n=2,4) and PrCl3·6H2O under solvothermal conditions, afforded three new complexes, [Pr(2-pytza)(SO4)(H2O)3]·H2O(1), [Pr(4-pytza)3(H2O)2]·2H2O (2) and [Pr4(4-pytza)5(OH)4(H2O)7Cl]Cl2·4H2O (3), whose structures are controlled by not only the different positions of the nitrogen atom of the pyridine ring but also the pH value of the solvent system. These compounds have been characterized by elemental analysis, IR and single crystal X-ray diffraction. The X-ray analysis reveals that compound 1 is a two dimensional layer structure made up of a Pr2O6 binuclear unit and the tridentate bridging SO42- ligand; compound 2 is an unusual one dimensional ladder like chain structure consisting of a Pr2O10 binuclear unit and 4-pytza which acts as a tetradentate ligand via the pyridine-N and the carboxylate group in a μ1,1,3-COO bridging mode while compound 3 is an unprecedented two dimensional network composed of a tetranuclear cubane-shaped Pr4(OH)48+ cluster (SBU) and tridentate 4-pytza via the pyridine-N and the carboxylate group in a μ1,3-COO bridging mode. Various hydrogen bonds exist to assemble the 1D chains or 2D layers into 3D supramolecular network structures. Furthermore, the luminescence properties investigated at room temperature in the solid state show only intraligand emission for 1, both intraligand and characteristic peaks of Pr3+ for either compound 2 or 3. Keywords: pH; isomer; Pr(III); crystal structure; luminescence Considerable attention attached to lanthanide coordination compounds stems from not only their diversity in structure[1-4], but also their tunable application as advanced functional materials, in the field of luminescence, catalysis, gas storage, etc [5-8]. To the best our knowledge, on one hand, lanthanide elements tend to have much higher coordination numbers and more flexible coordination modes, compared to other metal ions. On the other hand, many factors, such as pH value, solvent system, temperature are likely to influence the eventual structures of the targeted molecules and add to the novelty of the structures. One of the key factors to obtaining the targeted molecules is via appropriate selection of the ligand whereas the researches concerning coordination compounds based on isomeric ligands are by no means satisfactory[9]. It is universally acknowledged that tetrazole-carboxylate ligands are outstanding candidates for construction of novel compounds in that they tend to display a variety of coordination modes and the -CH2- spacer between the tetrazole ring and the carboxylate group is able to offer flexible orientations of the carboxylate arm, allowing the formation of various framework
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structures[10-12]. In addition, the abundant nitrogen and oxygen atoms may participate in the formation of hydrogen bonds, stabilizing the supramolecular assemblies. Inspired by the latest work which have reported coordination compounds based on pH values or isomeric ligands[9, 13-15], we have selected two isomeric bifunctional tetrazole-carboxylate ligands, 5-(n-pyridyl)tetrazole-2-acetato potassium salt (Kn-pytza, n=2,4), to react with PrCl3·6H2O under different pH values, and three novel coordination compounds have been obtained (Scheme1). The different positions of the nitrogen atom of the pyridyl ring can show quite distinct coordination modes. We assume that 2-pytza often adopts N(pyridyl), N(tetrazolyl) chelating mode [16-17] while 4-pytza can act as a bridging ligand via pyridyl-N and carboxylate-O atoms. Herein, we will report their synthesis, crystal structures and luminescence properties. In this work, two isomeric ligands 5-(n-pyridyl)tetrazole-2-acetato potassium salt (n=2,4) have been selected to be reacted with PrCl3·6H2O under different pH values in order to investigate whether or not the different positions of the nitrogen atom of the pyridine ring and the pH values will influence the final structures of our targeted molecule. Three novel compounds were prepared successfully. The structure of compound 1 remains unchanged when the pH value increases or decreases. However, compound 2 can only be obtained when the pH value of the system retains a relatively low level (pH=4). It is interesting that hydroxyl anions are included in the molecule, which is correspondent to the synthetic strategy, to some extent. Compounds 1-3 are stable towards oxygen and moisture. The elemental analysis of 1-3 are consistent with their chemical formula. In the IR spectra of 1-3, the characteristic bands of carboxylate groups appeared in the usual region at 1601–1658 cm-1 for the asymmetric stretching vibrations and at 1302-1392 cm-1 for the symmetric stretching vibrations[20]. Peaks at (3418-3435cm-1) are ascribed to the O-H vibration of both coordinated and uncoordinated water. The X-ray diffraction reveals that compound 1 crystallizes in monoclinic lattice space group P21/c. As shown in FigS1, each Pr(III) center is nine-coordinated by nine oxygen atoms, of which three are from three water molecule, three from two carboxylate groups of two 2-pytza ligands, and the remaining three from three independent sulfate anions, forming a distorted PrO9 monocapped square antiprism coordination arrangement. Two neighboring Pr(III) centers are doubly bridged by two carboxylate groups in a μ1,1,3-COO mode to form a binuclear unit. Each sulfate anion acts as a tridentate bridging ligand to connect adjacent binuclear units, displaying a two dimensional network extending along the bc plane (Fig1). The Pr-O distances range from 2.433 to 2.645Å, which are in good agreement with those of the previously reported praseodymium compounds[8]. Compared with [Pr(3-pytza)2Cl(H2O)2], the coordination mode of 2-pytza is analogous to that of 3-pytza, but the structure is quite different since the sulfate anions not only balances the charge of Pr(III) but also increases the dimension of the structure. Contiguous layers are further held together by various hydrogen bonds to generate a three dimensional supramolecular network (TableS3). Fig 1 here Compound 2 crystallizes in triclinic lattice space group Pī. Each Pr(III) in a distorted PrO8N monocapped square antiprism coordination arrangement is surrounded by two oxygen atoms from two water molecules (O7, O8), six oxygen atoms from five carboxylate groups of five 4-pytza ligands (O2,O3,O3A,O4A,O5,O6A) and a nitrogen atom from the pyridyl ring (N10B) (FigS2). 4-pytza shows three different coordination modes in the molecule. First, two 4-pytza anions adopt a monodentate mode to coordinate to two Pr(III) centers by one of the carboxylate oxygen atom
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(O2); then the two independent Pr(III) centers are doubly bridged by two bidentate 4-pytza ligands in a μ1,3-COO mode to form a binuclear unit; eventually adjacent binuclear units are bridged by four tetradentate 4-pytza anions via its nitrogen atom of the pyridyl ring and the carboxylate group in a μ1,1,3-COO mode, thereby giving an unusual one dimensional ladder like chain extending along the c axis (Fig2). The Pr-O distances from 2.410 to 2.671Å are approximately correspondent to those of compound 1 or [Pr(atza)2(CH3OH)(H2O)Cl] (atza=5-aminotetrazole-1-acetato)[4]. In contrast with [Pr(3-pytza)2Cl(H2O)2] in which 3-pytza only acts as a tridentate ligand via its carboxylate group in a μ1,1,3-COO bridging mode, 4-pytza displays more coordination modes and therefore shows a complicated one dimensional chain. Compared with 1, however, the sulfate anions are not included in the molecule, this is probably due to the nature of the ligand. Furthermore, neighboring one dimensional chains are self-assembled by various O···H or N···H hydrogen bonds to form a three dimensional supramolecular network (TableS3). Fig 2 here Compound 3 crystallizes in monoclinic lattice space group P21/n. As is illustrated in FigS3, Pr(1)-Pr(4) are all eight-coordinated and display distorted square antiprism coordination arrangements. Pr(1) is eight-coordinated by three oxygen atoms from three hydroxyl anions (O11,O12,O14), two atoms from two water molecules (O17,O18) and three oxygen atoms (O2,O8,O10) from three carboxylate groups of three independent 4-pytza ligands (FigS3a); Pr(2) by three hydroxyl oxygens (O11,O13,O14), two water oxygens (O15,O16), two carboxylate oxygens (O1,O4) and one nitrogen atom (N65A) (FigS3b); Pr(3) by three hydroxyl oxygens (O12,O13,O14), one water oxygen (O21), three carboxylate oxygens (O3,O5,O9) and one chloride anion (Cl3) (FigS3c); The coordination environment of Pr(4) is similar to that of Pr(2) (FigS3d). Each hydroxyl anion acts as a tridentate ligand to connect to three Pr(III) centers to form a distorted cubane-shaped Pr4(OH)48+ unit in which four praseodymium atoms are at the corners of a slightly irregular tetrahedron(Fig3a), with four hydroxide oxygen atoms occupying positions outside the faces of the praseodymium tetrahedron. Then Pr(1) and Pr(2), Pr(1) and Pr(4), Pr(2) and Pr(3) are bridged by three bidentate 4-pytza ligand in a μ1,3-COO mode, respectively . These tetranuclear Pr4(OH)48+ clusters, which behave as SBUs, are bridged by tridentate 4-pytza via one pyridyl nitrogen and carboxylate group in a μ1,3-COO bridging mode to form a two dimensional layer extending along the bc plane (Fig3). The Pr-O distances are from 2.355 to 2.483Å, which are a little shorter than those of [Pr(atza)2(CH3OH)(H2O)Cl](atza=5-aminotetrazole-1-acetato)[4] The structure is unprecedented among praseodymium compounds, to the best of our knowledge. It is worthwhile to point out compound 3 is obtained under relatively higher pH value, which is correspondent to the existence of the hydroxyl anions in the molecule. Compared to 2, one more coordination mode of 4-pytza is found in compound 3, and the structure changes from one to two dimensional as the pH value increases. However, although compounds 1 and 3 are both two dimensional, the structures are quite different. There exist one coordinated chloride anion and two uncoordinated ones in compound 3 while 1 contains sulfate anions. It is interesting that sulfate anions do not exist in the molecule, which is probably assigned to the stereo effect. Adjacent layers are further held together by various hydrogen bonds to form a three dimensional network (TableS3). Fig 3 here Lanthanide coordination compounds usually show interesting luminescence properties [19-20]. In view of their potential applications in the photochemistry, we are dedicated to
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investigating the luminescence properties of compounds 1-3 at room temperature in the solid state(FigS4). Upon excitation at 328, 370 nm, H2-pytza (H2-pytza=5-(2-pyridyl)tetrazole-2-acetic acid) and K4-pytza exhibit photoluminescence with maximum intensity at 375, 429 nm, respectively. Compound 1 only shows maximum intensity at 342 nm upon excitation at 275 nm, which can be tentatively assigned to the intraligand emission[9]. Compounds 2 and 3 show maximum intensity at 437, 457, 476, 491 nm and 429, 457, 479, 496 nm upon excitation at 376 and 370 nm, respectively. The peaks at 437 and 429 nm are attributed to the ligand centered emission since similar peaks at 429 nm was observed for the free ligand K4-pytza. The other peaks can be ascribed to the transitions of 1I6-3H4 (457 nm for both 2 and 3), 3P1-3H4 (476 nm for 2, 479 nm for 3), 3P0-3H4 (491 nm for 2, 496 nm for 3). The luminescence of compounds 2 resembles that of 3. Compared to the luminescence of compound 1 or [Pr(3-pytza)2Cl(H2O)2] [9], compounds 2 and 3 exhibit not only the ligand centered luminescence, but also the characteristic peaks of Pr3+, showing that the isomers really have an impact on the luminescence.
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In summary, we are the first to report praseodymium coordination compounds based on Kn-pytza (n=2,4). Three novel compounds have been constructed successfully whose structures are controlled by not only various coordination modes of n-pytza (n=2,4) and the different positions of the nitrogen atom of the pyridine ring, but also simultaneously the pH value. When the pH value is low, compound 2 with an unusual one dimensional chain was obtained, however, when high, compound 3 formed with an intriguing two dimensional network based on a tetranuclear Pr4(OH)48+ clusters (SBU). The luminescence properties show intraligand emission for 1, either ligand centered emission or characteristic peaks of Pr3+ for both compounds 2 and 3. Our research results indicate the structures of praseodymium compounds are affected by isomers and Pr(III)-4-pytza compounds are sensitive to pH values. Therefore, other investigations on isomers are underway in our group.
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Fig1 The 2D network of 1 formed by hydrogen bonding interactions. Fig2The 1D chain structure of 2 formed by hydrogen bonding interactions Fig3 The 2D layer network of compound 3 extending along the bc plane. Hydrogen atoms are omitted for clarity. Fig.S1The coordination environment of Pr(III) of compound 1. Hydrogen atoms are omitted for clarity FigS2 The coordination environment of Pr(III) of compound 2. Hydrogen atoms are omitted for clarity; FigS3 The coordination environment of PrIII(1)-PrIII(4);(a) PrIII(1); (b) Pr(2)III; (c)PrIII(3); (d) PrIII(4). Hydrogen atoms are omitted for clarity. Fig.S4(a)The emission spectra of H2-pytza, K4-pytza and compound 1 at room temperature in the solid state (For H2-pytza, λex = 328nm, For K4-pytza, λex = 370nm, For 1, λex = 275nm); (b)The emission spectrum of compound 2 at room temperature in the solid state (λex = 376nm); (c) The emission spectrum of compound 2 at room temperature in the solid state (λex = 370nm).
Acknowledgment
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The authors acknowledge financial support from the Natural Science Foundation of Jiangsu Provi nce (Grant No. BK2012210), the Natural Science Foundation of the Jiangsu Higher Education Inst itutions of China(Grant No.10KJB430001) and the Opening Fund of Jiangsu Key Laboratory of A dvanced Functional Materials (Grant No.12KFJJ010). References [1]S.Y. Lin, C. Wang, L. Zhao, J.K. Tang, Chem Asian J, 9 (2014) 3558-3564
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[16]Q.Y. Li, G.W. Yang, X.Y. Tang, Y.S. Ma, F. Zhou, W. Liu, J. Chen, H. Zhou, Inorg. Chem. Comm. 13 (2010) 254. [17]B.J. Wang, J.H. Zou, W.X. Li, Z. Wang, B. Xu, S. Li, Y.S. Zhai, D.L. Zhu, Q.Y. Li, G.W. Yang, J. Organomet. Chem. 749 (2014) 428-432. [18]J. Yang, Y.T. Min, Y. Chen, J. Wang, H. Tian, L. Shen, Y.S. Zhai, G.W. Yang , J.H. Zou, Q.Y. Li , H.J. Cui, Inorg. Chim. Acta 419 (2014) 73-81. [19]H. Deng, Y.C. Qiu, Y.H. Li, Z.H. Liu, O. Guillou,Inorg. Chim. Acta 362 (2009) 1797-1804. [20]P. Zhu, W. Gu, M.L. Liu, H.B. Song, X. Liu, Y.Q. Gao, H.Y. Duan, S.P. Yan and D.Z. Liao, CrystEngComm, 11 (2009) 351-358.
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Figure 1
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Figure 3
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ACCEPTED MANUSCRIPT Graphical abstract
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Three novel praseodymium coordination compounds based on isomeric ligands 5-(n-pyridyl)tetrazole-2-acetato potassium salt (n=2,4) have been successfully prepared, whose structures are not only controlled by the different positions of the nitrogen atom of the pyridine ring, but also the pH values. The luminescence shows ligand centered emission or both intraligand emission and characteristic peaks of Pr3+.
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ACCEPTED MANUSCRIPT Highlights
Two isomers were prepared.
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The luminescence properties were investigated.
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Three Pr(III) compounds were prepared.
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