Pyridyl and triazole ligands directing the assembling of zinc(II) into coordination polymers with different dimensionality through azides

Accepted Manuscript Pyridyl and triazole ligands directing the assembling of zinc(II) into coordination polymers with different dimensionality through azides Franz A Mautner, Christian Berger, Christian Gspan, Beate Sudy, Roland C Fischer, Salah S Massoud PII: DOI: Reference:

S0277-5387(17)30281-4 http://dx.doi.org/10.1016/j.poly.2017.04.012 POLY 12590

To appear in:

Polyhedron

Received Date: Accepted Date:

12 March 2017 11 April 2017

Please cite this article as: F.A. Mautner, C. Berger, C. Gspan, B. Sudy, R.C. Fischer, S.S. Massoud, Pyridyl and triazole ligands directing the assembling of zinc(II) into coordination polymers with different dimensionality through azides, Polyhedron (2017), doi: http://dx.doi.org/10.1016/j.poly.2017.04.012

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Pyridyl and triazole ligands directing the assembling a ssembling of zinc(II) into coordination polymers polymers with different dimensionality through azides azides Franz A. Mautner,a* Christian Berger,a Christian Gspan,a Beate Sudy,a Roland C. Fischer,b Salah S. Massoud c a)

Institut für Physikalische and Theoretische Chemie, Technische Universität Graz, A-8010 Graz, Austria

b)

Institut für Anorganische Chemie, Technische Universität Graz, Stremayrgasse 9, A-8010 Graz, Austria

c)

Department of Chemistry, University of Louisiana at Lafayette, P.O. Box 43700 Lafayette, LA 70504, USA (Received:

, accepted:

Keywords: Zinc; Coordination polymers, Azido complexes; Crystal structure; Fluorescence __________________________

* Corresponding author: [email protected], Tel. ++43 316-873-32270

)

ABSTRACT: Five coordination polymers catena-[(3-Hampy)2[Zn7(N3)16(3-ampy)4]·3H2O] (1), catena-[Zn2(µ2-4-HO-py)(µ1,1-N3)4]·H2O (2), catena-[Zn2(µ2-4-amtz)(µ1,1-N3)4] (3), catena-[Zn(5Et-2-Me-py)(µ1,1-N3)2] (4) and catena-[Zn(µ2-2-O-py-N-oxide)(µ1,1-N3)] (5), where 3-ampy = 3aminopyridine , 4-HOpy = 4-hydroxypyridine, 4-amtz = 4-amino-4H-1,2,4-triazole, 5-Et-2-Mepy = 5-ethyl-2-methylpyridine and 2-HO-py-N-oxide = 2-hydroxypyridine-N-oxide were synthesized and structurally characterized by single crystal X-ray crystallography. The complexes display different dimensionality depending on the nature of the coligand. The anionic polymer in complex 1 consists of 1D system of defective dicubanes with two different azido-bridging µ1,1-N3 and µ1,1,1N3 modes. Complexes 2 and 3 are isostructural and both exhibit 2D networks with µ(O,O)pyridinone bridging molecule in 2 or µ(N,N’)-bridging 4-amtz in 3, and µ1,1,-azide bridging. Complex 4 reveals a zigzag chains of 1D polyhedral through a doubly bridged µ1,1-N3 bonding mode, whereas in complex 5 a 1D polymeric chain is generated via singly bridged µ1,1-N3 bonding and hydrogen bonding results in the formation of supramolecular 2D system. The emission spectral properties of the parent free ligands and their corresponding Zn(II) complexes were investigated where enhancement luminescence emissions with strong red shifts were observed in complexes 2, 4 and 5.

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1. Introduction Great efforts have been directed over the last two decades for the construction new coordination polymers (CP) and polynuclear metal complexes. The design of these compounds necessitates the incorporation of bridging ligands into the reaction mixtures. The most common bridging ligands which have been used for this purpose include aromatic and heterocyclic polycarboxylates, [1,2] benzenoid aromatic oxocarbon dianions, CnOn2− (n = 4, squarate; n = 5, croconate) [3] and small pseudohalides (azide, N3 -; thiocyanate, NCS-; cyanate, OCN-) [4-9] as well as the larger molecular dicynamide ion, NCNCN- (dca) [10]. These ligands have the capability to simultaneously bind several metal ions and hence act as bridging linkers to assemble metal ions in different nuclearity and in 1-, 2-, or 3D extended polymeric networks resulted in the isolation of many coordination compounds with interesting molecular and crystalline architectures.[1-10] The inherent features of the structural topologies in these compounds led to interesting properties which made them useful in materials science for some potential applications such as molecular recognition in biological systems and medicine [11], molecular recognition [12], catalytic reactions [13], electrical conductivity [14], gas adsorption [15], gas storage [16], guest exchange [17], nonlinear optical (NLO) activity [18] and magnetism [19]. Azide ion, N3 - is probably one of the ions which have been extensively investigated because of its flexidentate nature to simultaneously bind several metal ions in different coordination modes. In addition to this property, the anion serves as an excellent effective ligand to propagate the magnetic interaction between the paramagnetic centers where magnetic coupling is known to occur through various modes [4-6]. The azido ligand has been used for the construction of many bridging coordination compounds with a wide range of bonding modes that range from bridging two to six metal ions simultaneously depending on the electronic nature of the central metal ion and the structural skeleton of the coordinated coligands, which in most cases determine the dimensionality and degree of nuclearity [4,5,20]. These various azide-bonding modes include µ1,3-N3 (end-to-end, EE) [4-6,21-23], µ1,1-N3 (end-on, EO) [4-6,24] and (µ1,3-N3)/(µ1,1-N3) (EE/EO) [25], µ1,1,3-N3 and µ1,1,1-N3 [26,27], µ1,1,1,1-N3 and µ1,1,2,2-N3, [6-28] as well as µ1,1,1,3,3,3-N3 [26-29]. Complexes with alternate µ1,3-N3 and µ1,1-N3 bridges were also reported in some systems [27,30-32]. The different bonding modes of bridged azide were recently summarized. [4(c),5,7]

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Now, it is known that d10 metal ions and in particular Zn(II) and Cd(II) ions in certain complexes or metal-organic frameworks (MOFs) can enhance or quench the fluorescence emission of their parent organic ligand from which complexes were derived.[7,33-35] The photoinduced electron transfer (PET) process in these complexes results from the excitation of the lone-pairs of electrons which are located in the donor atoms of the ligand. Upon complexation and/or chelation of the organic ligand with metal ion, the PET process may be suppressed or prevented and as a result the fluorescence intensity may be enhanced.[7,34-36]

However, enhancement of emission

fluorescence through metal chelation is not a very common phenomenon but instead quenching of fluorescence intensities is the one which observed in most cases.[34(b),37-39]

Quenching of

fluorescence intensities may result from the poor overlap of the orbitals of the coordinated N-donor ligand in the chelate ring, where such poor overlap makes the electron pairs in the higher energy orbitals more readily available for quenching the fluorescence.[38,39] In general, the fluorescence’s enhancement phenomenon is more interesting due to the possible use of the complexes as photochemical devises.[40] In a continuous effort to explore the chemistry of azido compounds and the luminescence properties of d10 metal ions,[7,35] a new series of Zn(II)-azido complexes were synthesized based on pyridyl {3-aminopyridine (3-ampy), 5-ethyl-2-methylpyridine (5-Et-2-Mepy), 4-hydroxypyridine (4HOpy), 2-hydroxypyridine-N-oxide (2-HO-py-N-oxide)} and 4-amino-4H-1,2,4-triazole (4-amtz) ligands. The complexes combine the structural features generated through the bridged azido groups and the fluorescence emission induced by different Zn(II)/coligands.

2. Experimental

2.1. Materials and physical measurements The pyridine derivative ligands used in this study were purchased from TCI and all other chemicals were of analytical grade quality. Hydrazoic acid was obtained by reaction of NaN3 and H2SO4 with a modified Kipp apparatus.[25] Emission Spectra were recorded with a Perkin-Elmer LS55 spectrofluorometer. IR spectra of complexes 1-3 were recorded with a Perkin-Elmer 325 (as KBr pellets), whereas complexes 4 and 5 were recorded with a Bruker Alpha P (platinum-cap). Elemental microanalyses were carried out with an Elementar Vario EN3 analyser. Caution:

4

Hydrozoic acid, salts of azide and their metal complexes are potentially explosive and should be handled with great care and in small quantities

2.2. Preparation

2.2.1. catena-[(3-Hampy)2[Zn7(N3)16(3-ampy)4]·3H2O] (1) The complex was prepared by dissolving 3-aminopyridine (0.43 g, 4.6 mmol) in an aqueous solution of Zn(N3)2 saturated with hydrazoic acid (20 mL, 17.8 mmol) at room temperature. After three days, the resulting colorless crystals which separated were collected by filtration and dried in air (yield: 0.94 g, 70%). Anal. Calcd. for C30H44N60O3Zn7 (1750.84 g/mol): C, 20.6; H, 2.5; N, 48.0%. Found: C, 20.5; H, 2.5; N, 48.3%. Selected IR bands (KBr, cm-1): 3537 (s,br), 3459 (s,br), 3358 (s), 3219 (m), 2168 (s,sh), 2099 (vs), 2076 (vs), 2037 (s,sh), 1628 (s), 1594 (s), 1505 (s), 1458 (s), 1350 (m), 1311 (m), 1279 (s) 1204 (w), 1143 (w), 1075 (w), 1029 (w), 807 (m), 698 (m), 660 (m), 605 (w), 519 (w). 2.2.2. catena-[Zn2(µ2-4-HOpy)(µ1,1-N3)4]·H2O (2) The complex was prepared by dissolving 4-hydroxypyridine (0.85 g, 8.8 mmol) in an aqueous solution of Zn(N3)2 saturated with hydrazoic acid (20 mL, 8.9 mmol Zn) at 80° C, then the resulting solution was allowed to stand at room temperature. After three days, the colorless crystals which separated were collected by filtration and dried in air (yield: 0.76 g, 42%). Anal. Calcd. for C5H7N13O2Zn2 (412.02 g/mol): C, 14.6; H, 1.7; N, 44.2%. Found: C, 14.5; H, 1.4; N, 44.5%. Selected IR bands (KBr, cm-1): 3305 (s,br), 2133 (vs), 2107 (vs), 1631 (s), 1575 (m), 1509 (vs), 1465 (s), 1305 (m), 1277 (w), 1253 (s); 1233 (w), 1193 (s), 1171 (m), 1084 (w), 998 (m), 859 (m), 835 (w), 791 (m), 709 (w), 693 (m), 591 (w), 557 (s), 525 (m), 509 (m), 423 (w). 2.2.3. catena-[Zn2(µ2-4-amtz)(µ1,1-N3)4] (3) The complex was prepared following the same procedure described for complex 2 except 4-amino4H-1,2,4-triazole was used instead of 4-hydroxypyridine. The colorless crystals which separated in the following day were collected by filtration and dried in air (yield: 1.23 g, 72%). Anal. Calcd. for C2H4N16Zn2 (382.99 g/mol): C, 6.3; H, 1.1; N, 58.5%. Found: C, 6.3; H, 1.0; N, 58.3%. Selected IR bands (KBr, cm-1): 3320 (s), 3213 (m), 3141 (m), 3122 (m), 2075 (vs), 1634 (m), 1534 (m), 1474 (w), 1389 (w), 1299 (m), 1205 (m), 1076 (m); 1023 (m), 989 (m), 917 (w), 883 (s), 801 (w), 738 (w), 680 (w), 611 (s), 429 (w).

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2.2.4. catena-[Zn(5-Et,2-Me-py)(µ1,1-N3)2] (4) ZnSO4.7H2O (0.29 g, 1 mmol), NaN3 (0.26 g, 4 mmol) and 5-ethyl-2-methylpyridine (0.12 g, 1 mmol) were dissolved in 22 mL H2O at 60° C. After slow cooling of the clear solution to room temperature small amount of white needles were separated (yield: 0.19 g, 70%). These were collected by filtration and dried in air. Anal. Calcd. for C8H11N7Zn (270.63 g/mol): C, 35.5; H, 4.1; N, 36.2%. Found: C, 35.3; H, 4.1; N, 36.1%. Selected IR bands (ATR-IR, cm-1): 2090 (vs), 2063 (vs), 1503 (s), 1444 (m), 1345 (m), 1286 (m), 1192 (m), 1138 (s), 1065 (w), 925 (m), 849 (m), 811 (m), 757 (m), 663 (m), 601 (m), 430 (w). 2.2.5. catena-[Zn(2-O-py-N-oxide)(µ1,1-N3)(H2O)] (5) A mixture of ZnSO4.7H2O (2.59 g, 9 mmol), 2-hydroxypyridine-N-oxide (0.33 g, 3 mmol) and NaN3 (1.17 g, 18 mmol) were dissolved in H2O (14 mL) and the resulting solution was allowed to crystallize at room temperature. In the following day, the colorless crystals which separated were collected by filtration and dried in air (yield: 0.45 g, 64%). Anal. Calcd. for C5H6N4O3Zn (235.53 g/mol): C, 25.5; H, 2.6; N, 23.8. Found: C, 25.3; H, 2.5; N, 23.7%. Selected IR bands (ATR-IR, cm1

): 3213 (s,br), 2563 (w), 2112 (vs), 2064 (vs), 1621 (s), 1549 (w), 1512 (vs), 1446 (m), 1363 (m),

1285(m), 1230 (w), 1180 (s), 1148 (w), 1109 (m), 924 (w), 884 (m), 846 (w), 783 (w), 741 (m), 690 (w), 609 (m), 545 (m), 448 (w).

2.3. X-ray crystallography The X-ray single-crystal data of compounds 1-5 were collected on a Bruker-AXS APEX CCD diffractometer at 100(2) K. The crystallographic data, conditions retained for the intensity data collection and some features of the structure refinements are listed in Table 1. The intensities were collected with Mo-Kα radiation (λ= 0.71073 Å). Data processing, Lorentz-polarization and absorption corrections were performed using APEX, and the SADABS computer programs.[41] The structures were solved by direct methods and refined by full-matrix least-squares methods on F2, using the SHELXTL [42] program package. All non-hydrogen atoms were refined anisotropically. The hydrogen atoms were located from difference Fourier maps, assigned with isotropic displacement factors and included in the final refinement cycles by use of geometrical constraints. Hydrogen atoms of disordered lattice water molecules in 1 and 2 were omitted. Molecular plots were performed with the Mercury program.[43]

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Please insert Table 1 close to here

3. Results and discussion

3.1. Synthesis and IR spectra The reactions of an aqueous saturated solution of hydrazoic acid, HN3 containing Zn2+ with 3aminopyridine (3-ampy), 4-hydroxypyridine (4-HO-py) or 4-amino-4H-1,2,4-triazole (4-amtz) afforded the polymeric crystalline compounds catena-[(3-Hampy)2[Zn7(N3)16(3-ampy)4]·3H2O] (1), catena-[Zn2(µ2-4-HO-py)(µ1,1-N3)4]·H2O (2) or catena-[Zn2(µ2-4-amtz)(µ1,1-N3)4] (3), respectively. The two complexes catena-[Zn(5-Et-2-Me-py)(µ1,1-N3)2] (4) and catena-[Zn(2-O-py-N-oxide)(µ1,1N3)(H2O)] (5) were obtained by the reactions of aqueous solutions containing ZnSO4, NaN3 and the corresponding ligand 5-ethyl-2-methylpyridine (5-Et-2-Mepy) and 2-hydroxypyride-N-oxide (2HO-py-N-oxide), respectively. The complexes, which were isolated in moderate yields (62-72%) except complex 2 was obtained in 42%, were characterized by the elemental microanalyses, IR and by X-ray crystallography. Interestingly, the use of hydrazoic acid in complex 1 allows the protonation of some of the 3-aminopyridine ligand and resulted in the formation of complex 1 with unique cluster composition (see X-ray section). However, this was not the case in complexes 2 and 3. Under comparable conditions, a Mn(II) complex with structure composition similar to 1 was obtained when 5-ethyl-2-methylpyridine was used as a coligand.[44] In contrast, the same ligand (5Et-2-Me-py) was not protonated when it was used with Zn(N3)2 saturated with HN3 solution. The IR spectra of the complexes under investigation display general characteristic features as expected for the azido compounds: two strong absorption bands in the 2130-2060 cm-1 region for complexes 1, 2, 4 and 5, and a single strong band at 1075 cm-1 for complex 3 due to the asymmetric stretching frequencies, νas(N3) of the coordinated azido ligands. Probably, we should emphasize that the number of stretching frequency bands νas(N3) cannot be considered as a criterion to differentiate between the azido-bridging bonding modes.[7] The complexes exhibit also medium-strong intensity bands in the 1300-1140 cm-1 region which can be assigned to the symmetrical stretching vibration, νs(N3). Complexes 1, 2 and 5 exhibit a strong broad band over the frequency range 3460-3210 due to ν(O-H) of the coordinated aqua ligand and/or the hydroxyl group in 2. The strong band observed in complex 5 at 1180 cm-1 is assigned to ν(ON) of the chelated 2-hydroxypyridyl-N-oxide.

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3.2. Description of the crystal structures 3.2.1. catena-(3-Hampy)2[Zn7(N3)16(3-ampy)4]·3H2O (1) A perspective view of 1, together with the atom numbering scheme is given in Fig. 1, and selected bond parameters are given in Table S1. The structure of compound 1 consists of polymeric anionic [Zn7(N3)16(3-ampy)4]2n- cluster chains, protonated 3-Hampy+ counter cations, and partially disordered lattice water molecules. The centro-symmetric building blocks of the 1D system are octanuclear cluster units, which have common Zn(4) centers, located on 2-fold axis. The inner core of the building blocks may be described as defective dicubanes with one unoccupied corner in each cube (Fig. 1a,b). The common face of the dicubane is formed by Zn(1), N(11), and their centrosymmetrically related Zn(1’) and N(11’) atoms. The other corners of the defective dicubane are occupied by Zn(2), Zn(2’) and N(21), N(31), N(21’) and N(31’) atoms. The octahedral Zn(1) and Zn(2) centers are pairwise bridged by two µ1,1-azide groups to four ZnN4 tetrahedra of the outer sphere of the octa-nuclear cluster units (Fig. 1a,b). Only Zn(2) and Zn(3) centers are each ligated by one neutral terminal 3-aminopyridine molecule via its pyridine ring N atom [Zn(2)-N(1) = 2.0858(16), Zn(3)-N(3) = 2.0364(17) Å], all other sites of the Zn(ii) polyhedra are occupied by azide groups. The N(11) atom of central azide group (N11)-N(12)-N(13) is ligated to Zn(1), Zn(2) and Zn(1’) and thus acting as µ1,1,1-azide bridge. Zn(3) center is ligated by N(81) atom of a terminal azide group, all other azide groups act in the µ1,1-bridging mode. The Zn-N(N3) bond distances within the ZnN6 octahedra range from 2.1265(16) to 2.2290(16) Å, the Zn-N(N3) bond distances within the ZnN4 tetrahedra are shorter (1.9937(16) to 2.0101(16) Å). The µ1,1,1-azide bridge forms Zn-N(11)-Zn and Zn-N(11)-N(12) bond angles of 97.80(6), 97.30(6), 98.90(6)°, and 112.82(12), 121.39(12) and 123.64(13)°, respectively. The µ1,1,-azide bridges form Zn-N-Zn and Zn-N-N bond angles that vary from 101.84(7) to 126.71(8)°, and from 113.69(13) to 121.63(13)°, respectively. The azide groups are asymmetric with mean ∆(N-N) of 0.073 Å, and their N-N-N bond angles vary from 176.7(3) to 179.01(19)°. The Zn···Zn distances within the octa-nuclear cluster units are: Zn(1)···Zn(1’) = 3.3312(5), Zn(1)···Zn(2) = 3.3380(5), Zn(1)···Zn(2’) = 3.3817(5), Zn(1)-Zn(4’) = 3.6067(4), Zn(1)···Zn(3) = 3.6667(5), Zn(2)···Zn(3) = 3.3697(5), Zn(2)-Zn(4) = 3.7571(4), Zn(2) ···Zn(2’) = 5.8361(6) Å.

The cluster units are connected via their common Zn(4) centers to

polymeric chains oriented along the [1 0 1] direction of the monoclinic unit cell (Fig. 1c). The

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polymeric chains are separated by protonated 3-Hampy+ ions, which form hydrogen bond of type NH···O to O(1) donor atom of water molecule [N(5)···O(1) = 2.730(3) Å]. The lattice water molecule forms hydrogen bonds of type O-H···N to adjacent N(81) and N(23) atoms [2.966(3) and 3.033(3) Å]. The amino group of the neutral and the protonated 3-Hampy+ molecules form hydrogen bonds of type N-H···N to non-coordinated N atoms of azido ligands (Table S6) to generate a supramolecular 3D network structure. Please insert Fig. 1(a,b,c) close to here 3.2.2. catena-[Zn2(µ2-4-HOpy)(µ1,1-N3)4]·H2O (2). A perspective view of 2, together with the atom numbering scheme is given in Fig. 2, and selected bond parameters are given in Table S2. The asymmetric unit of compound 2 consists of two Zn(ii) centers, four azide groups, one 4-hydroxypyridine molecule (4-HOpy) and one disordered lattice water molecule. Each penta-coordinated Zn(ii) center is ligated by O(1) atom of the 4-HOpy molecule, which acts in its tautomeric keto-form as µ(O,O)-pyridinone bridging molecule. The remaining sites are located by azide groups, which act in µ1,1,-azide bridging mode. Both Zn(ii) centers are further linked via N(11) atom of EO-azide bridge N(11)-N(12)-N(13) to form fourmembered Zn2ON rings [Zn(1)-O(1)-Zn(2) and O(1)-Zn-N(11) bond angles are 96.56(6), 76.65(7) and 78.33(6)°]. The ZnN4O chromophores have distorted trigonal bipyramidal geometry (TBP) with τ values of 0.59 and 0.78, for Zn(1) and Zn(2), respectively [τ–values of 0 and 1 refer to ideal square pyramidal (SP) and TBP geometry).[45] The axial sites are occupied by O(1) and N(31) and N(21b), respectively. The Zn-N/O bond lengths vary from 1.9938(17) to 2.2260(18) Å. Centrosymmetric tetra-nuclear Zn(ii) rings are formed via N(41) and N(41a) atoms of two single EO-azide bridges. The tetra-nuclear Zn(ii) rings are further connected via di-EO azide bridges to generate a 2D system of tetra-nuclear Zn(ii) and octa-nuclear Zn(ii) rings (Figs. 2b,c) In terms of network analysis47 the topology of 2 may be described as a distorted “4.82” 2D net (Fig. 2d) which is oriented along the b- and c-axis of the monoclinic unit cell. The Zn(1)..Zn(2c), Zn(1)..Zn(2), Zn(1)..Zn(2c) distances via EO-azide bridges are 3.2054(3), 3.2415(3), 3.5816(4) Å, respectively. The Zn(1)..Zn(1a) and Zn(2)..Zn(2a) separations in the tetra-nuclear Zn(ii) rings are 4.7405(4) and 4.9191(4), Å, respectively. The Zn..Zn’ separations between pairs of opposite Zn(ii) centers within the octa-nuclear Zn(ii) rings are: 7.6188(5) and 9.5518(5) Å, for Zn(1) centers; 7.9115(5) and 9.2336(5) Å, for Zn(2) centers, respectively. The Zn-N-Zn and Zn-N-N bond angles of the EO azide 9

bridges vary from 97.81(7) to 125.58(9) and from 113.70(13) to 135.68(14)°, respectively. The azide ligands are asymmetric with a mean ∆(N-N) of 0.072 Å, and their N-N-N bond angles vary from 178.5(2) to 179.2(2)°. Hydrogen bonds of type N-H···N are formed from N(1) donor atom of the 4pyridinone to adjacent N(23) and N(33) acceptors [2.941(3) and 3.110(3) Å] to generate a supramolecular 3D system (Table S6). Please insert Fig. 2 (a,b,c,d) close to here

3.2.3. catena-[Zn2(µ2-4-amtz)(µ1,1-N3)4] (3) A perspective view of 3, together with the atom numbering scheme is given in Fig. 3, and selected bond parameters are given in Table S3. The complex 3 forms an iso-structural 2D system with fused tetra- and octa-nuclear Zn(ii) rings as complex 2, where the µ(O,O)-pyridinone bridge is replaced by the

µ(N,N’)-bridging

4-amino-4H-1,2,4-triazole

molecule

(4-amtz).

The

resulting

ZnN5

chromophores have distorted TBP geometry with τ values of 0.76 and 0.69, for Zn(1) and Zn(2), respectively.[45] The axial sites are occupied by N(1), N(21), and by N3, N(31b) [N(1)-Zn(1)-N(21) = 170.12(4), N(3)-Zn(2)-N(31b) = 172.89(4)°]. The Zn-N bond distances vary from 2.0101(10) to 2.2310(10) Å. The five-membered Zn2N3 rings have following bond parameters: Zn(1)-N(1)-N(3) = 119.68(7), Zn(2)-N(3)-N(1) = 122.50(7), N(11)-Zn(1)-N(1) = 86.98(4), N(11)-Zn(2)-N(3) 87.15(4)°. The Zn(1)..Zn(2c), Zn(1)..Zn(2), Zn(1)..Zn(2c) distances via EO-azide bridges are 3.2368(2), 3.5946(2), 3.6020(2) Å, respectively. The Zn(1)..Zn(1a) and Zn(2)..Zn(2a) separations in the tetranuclear Zn(ii) rings are 4.8453(2) and 5.3212(2), Å, respectively. The Zn..Zn’ separations between pairs of opposite Zn(ii) centers within the octa-nuclear Zn(ii) rings are: 7.7929(3) and 9.9406(4) Å, for Zn(1) centers; 7.8549(3) and 9.4214(4) Å, for Zn(2) centers, respectively. The Zn-N-Zn and ZnN-N bond angles of the EO azide bridges vary from 98.69(4) to 124.04(5) and from 115.89(8) to 128.40(9)°, respectively. The azide groups are asymmetric with a mean value ∆(N-N) of 0.076 Å, and their N-N-N bond angles vary from 177.72(13) to 179.16(12)°. Hydrogen bonds of type NH···N are formed from N(4) donor atom of the 4-amino-4H-1,2,4-triazole molecule to adjacent N(13) and N(23) acceptors [3.2151(16) and 3.0594(19) Å] to generate a supramolecular 3D system (Table S6). Please insert Fig. 3 (a,b) close to here

10

3.2.4. catena-[Zn(5-Et-2-Me-py)(µ1,1-N3)2] (4) A perspective view of 4 together with the atom numbering scheme is given in Fig. 4a and selected bond parameters are given in Table S4. The coordination of the Zn(1) metal center by 5-Et-2-Mepy ligand and the four azide groups may be described as an distorted trigonal bipyramid (τ = 0.68) [45] with N(11b) and N(21) at the axial positions. The end-on bridging azido groups [N(11)-N(12)N(13)] and [N(11b)-N(12b)-N(13b)] bridge a pair of zinc ions [Zn(1) and Zn(1b)] related by an inversion center forming a planar four-membered Zn2N2 ring, whereas µ-1,1-azide ligands [N(21)N(22)-N(23)] and [N(21a)-N(22a)-N(23a)] bridge Zn(1) and Zn(1a) ions forming another Zn2N2 ring, giving rise to zigzag chains of polyhedra along the a-axis of the monoclinic unit cell (Figs. 4a, b). The Zn-N-Zn bond angles are 102.29(9) and 102.25(9)°, and the Zn-N-N bond angles range from 122.26(17) to 132.68(18)°. The three equatorial Zn-N bond lengths are in the range from 2.009(2) to 2.043(2) Å, the axial Zn(1)-N(11b) and Zn(1)-N(21) bond distances are longer, i.e. 2.288(2) and 2.160(2) Å, respectively. The two end-on bridging azido ligands are asymmetric [N(11)-N(12) = 1.214(3), N(12)-N(13) = 1.148(3); N(21)-N(22) = 1.218(3), N(22)-N(23) = 1.140(3) Å], and practically linear [N-N-N angles of 178.6(3) and 179.3(3)°]. The intra-chain Zn···Zn distances are 3.2700(8) and 3.3499(8) Å, whereas the shortest inter-chain metal-metal separation is 6.1573(13) Å. The N-coordinated 5-Et-2-Mepy molecule acts as a terminal ligand. Please insert Fig. 4 (a,b) close to here

3.2.5. catena-[Zn(2-O-py-N-oxide)(µ1,1-N3)(H2O)] (5) A section of the crystal structure of 5 is shown in Fig. 5a and selected bond parameters are listed in Table S5. Zn(1) is penta-coordinated by N(11), N(11’) of end-on bridging azido groups, O(3) of a terminal aqua ligand, O(1) and O(2) of a chelating deprotonated 2-hydroxypyridine-N-oxide molecule. The ZnN2O3 polyhedron has an intermediate geometry between square pyramid (SP) and distorted trigonal bipyramid (TBP) with a τ = 0.49.[45] The Zn-N/O bond distances are in the range from 2.0332(12) to 2.0684(14) Å. The O(1)-Zn(1)-O(2) chelate angle is 79.24(5)°, Zn(1) and O(3) deviate by 0.044 and 0.245 Å from the mean plane of the 2-O-pyridine-N-oxide anion. The single end-on azido bridges form polymeric chains along the b-axis of the unit cell (Fig. 5b). The µ1,1-azido bridges have the following bond parameters: N(11)-N(12) = 1.215(2), N(12)-N(13) = 1.142(2) Å;

11

Zn(1)-N(11)-Zn(1’) = 122.57(7)°; N(11)-Zn(1)-N(11’) = 111.08(5)°, Zn(1)-N(11)-N(12) = 118.22(11), Zn(1’)-N(11)-N(12) = 117.45(11), N(11)-N(12)-N(13) = 179.41(17)°. The intra-chain Zn(1)···Zn(1’) and Zn(1)···Zn(1”) distances are 3.6219(7) Å, the shortest Zn···Zn inter-chain separation is 5.1346(9) Å. The chelating 2-O-pyridine-N-oxide molecule have a Zn(1)-O(2)-N(1) bond angle of 111.72(9)°. The aqua ligand forms hydrogen bonds of type O-H···O to O(1) and O(2) atoms of 2-O-pyridine-N-oxide molecules to generate a supramolecular 2D system oriented along the a- and b-axis of the unit cell (Fig. 5b) [O(3)···O(2)(-x,-1/2+y,1/2-z) = 2.723(2) Å, O(3)H(31)···O(2) = 167(2)°; O(3)…O(1)(-1/2+x,y,1/2-z) = 2.669(2) Å, O(3)-H(32)···O(1) = 167(3)°]. Please insert Fig. 5(a,b) close to here In addition to the above mentioned dicubane Mn(II) azide 2D system [46] with 5-Et,2Me-py ligand also a Cu(II) complex was reported, which is isostructural to the title compound 4.[47] Azide complexes with 3-ampy ligand were observed as monomeric [Mn(3-ampy)4(N3)2] [48], dimeric [Cu(3-ampy)2(N3)(NO3)]2.EtOH [49], and polymeric [Mn2(3-ampy)4(N3)4(H2O)]n [50]. µ2-4-amtz bridging ligands were found in the trinuclear [Cu3(4-amtz)2(N3)6] and cationic chain [Cu(4amtz)2(N3)]nn+, [51], whereas as N-terminal ligand only in the 1D system of [Mn(4amtz)2(N3)2]n.[52] As far as we know no further azide complexes of 3d metal ions with organic ligands of 4-hydroxypyridine and 2-hydroxypyrdine-N-oxide were reported.

3.3. Luminescence properties of the complexes The luminescence properties of the ligands used in this study, in the liquid state (carefully dried at 100 mbar, 60°C, over silica gel) and their corresponding zinc(II) complexes, in the solid state were examined at room temperature. The ligand 3-aminopyridine and its complex 1 as well as 4-amino-4H-1,2,4-triazole its corresponding complex 3 did not show any detectable luminescence emission spectra. While 5-ethyl,2-methylpyridine ligand behaved in a similar fashion as the previous mentioned ligands 4-hydroxypyridine and 2-hydroxypyridine-N-oxide molecules resulted in weak to very weak broad emission spectral bands around 481 and 576 nm, respectively. In contrast, their Zn(II) complexes 2, 4, and 5 revealed pronounced very intense luminescence emission with strong

12

red shits, ∆λ = 169-227 nm. This data are summarized in Table 2 and are illustrated in Fig. 6. These spectral data clearly show very dramatic enhancement in the luminescence of these complexes relative to their parent free ligands. The observed strong luminescence spectra in these complexes can be assigned to the intra-ligand n → π* transition.[53] Table 2. Luminescence data Compound catena-[Zn2(µ2-4-HOpy)(µ1,1-N3)4]·H2O (2) catena-[Zn(5-Et-2-Me-py)(µ1,1-N3)2] (4) catena-[Zn(2-O-py-N-oxide)(µ1,1-N3)(H2O)] (5) 4-Hydroxypyridine (4-HOpy) (L2) 2-Hydroxypyridine-N-oxide (2-HO-py-N-oxide) (L5)

λex (nm) 289 328 375 366 390

λem (nm) 516 542 544 481 576

∆λ (nm) 227 214 169 115 186

Pease insert Fig. 6 (luminescence emission) close to here The strong fluorescence enhancement observed in 2, 4 and 5 may be due to the improved overlap in Zn-O and Zn-N bonds in the former two complexes, respectively, and to the chelation enhanced fluorescence (CHEF) in the five-membered chelate ring containing the deprotonated 2-Opy-N-oxide moiety in complex 5. In general, the enhancement of the emission intensities of the complexes may be due to the metal ligand chelation [27,38-40] or to the increase in conformational rigidity of the ligands upon coordination.[38,39] The lack of luminescence in complexes 1 and 3 is most likely attributed to poor overlap of the orbitals of the N-donors in the amino-pyridine and amino-triazole rings, which may result in quenching the fluorescence.[38,39] Probably, the electron donating amino groups in these ligands may reduce the effectiveness of the n-π* transition and hence the non-radiative decay within the intraligand (π -π*) excited state was not improved.[7,36]

4. Conclusion Five novel coordination polymers with different dimensionality and topological features: catena-[(3Hampy)2[Zn7(N3)16(3-ampy)4]·3H2O] (1 1), catena-[Zn2(µ2-4-HO-py)(µ1,1-N3)4]·H2O (2 2), catena-[Zn2(µ24-amtz)(µ1,1-N3)4]

(3 3),

catena-[Zn(5-Et-2-Me-py)(µ1,1-N3)2]

(4 4)

and

catena-[Zn(µ2-2-O-py-N-

oxide)(µ1,1-N3)] (5 5) were synthesized depending on the nature of the coordinated coligand. The luminescence emission spectra of the complexes 2, 4 and 5 revealed strong fluorescence enhancement compared to their parent free ligands with strong red shift of the bands in the visible

13

region. This finding is very encouraging to continue the study not only to synthesize more azidoZn(II) and -Cd(II) coordination polymers based on substituted pyridine compounds, but also to explore the structural properties inherent in these compounds that may lead to strong fluorescence enhancement.

Acknowledgement F.A.M. thanks J. Baumgartner and K. Gatterer (TU-Graz) for assistance.

Appendix A. Supplementary data CCDC 1527058 - 1527062 contain the supplementary crystallographic data for compounds 1 – 5, of this paper, respectively, in CIF file format. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033. Tables S1-S5 give selected bond parameters for compounds 1-5, respectively. Table S6 shows the hydrogen bond systems in complexes 1,2,3 and 5. Supplementary data

associated

with

this

article

can

be

found

in

the

online

version,

at

http://dx.doi.org/10.1016/j.poly.......

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Table 1. Crystallographic data and processing parameters Compound

1

2

3

Empirical formula

C30H42N60O3Zn7

C5H5N13O2Zn2

C2H4N16Zn2

Formula mass

1748.97

410.00

382.99

System

Monoclinic

Monoclinic

Monoclinic

Space group

C2/c

P21/c

P21/n

a (Å)

19.7510(9)

10.8442(5)

10.7398(4)

b (Å)

26.9814(11)

10.8168(5)

11.2743(4)

c (Å)

13.6877(12)

11.6589(5)

10.7911(4)

α (°)

90

90

90

β (°)

117.143(1)

99.866(2)

114.294(1)

γ (°)

90

90

90

V (Å3)

6491.0(7)

1347.36(11)

1190.92(8)

Z

4

4

4

T (K)

100(2)

100(2)

100(2)

µ (mm-1)

2.629

3.592

4.050

Dcalc (Mg/m3)

1.790

2.021

2.136

Crystal size (mm)

0.25 × 0.17 × 0.12

0.29 × 0.21 × 0.14

0.32 × 0.23 × 0.15

θ max (°)

29.89

27.00

29.09

Data collected

47224

15109

17109

Unique refl. / Rint

9353 / 0.0345

2937 / 0.0318

3184 / 0.0304

Parameters

492

208

193

Goodness-of-Fit on F2

1.034

1.104

1.051

0.0303 / 0.0716

0.0208 / 0.0545

0.0156 / 0.410

0.79 / -0.32

0.37 / -0.43

R1 / wR2 (all data) 3

Residual extrema (e/Å ) 0.89 / -0.63

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Table 1. cont. Crystallographic data and processing parameters Compound

4

5

Empirical formula

C8H11N7Zn

C5H6N4O3Zn

Formula mass

270.63

235.53

System

Triclinic

Orthorhombic

Space group

P-1

Pbca

a (Å)

6.1573(11)

10.7856(13)

b (Å)

9.4390(18)

6.6210(11)

c (Å)

9.761(2)

22.4025(19)

α (°)

86.01(2)

90

β (°)

75.68(2)

90

γ (°)

81.54(2)

90

V (Å3)

543.36(19)

1599.8(4)

Z

2

8

T (K)

100(2)

100(2)

µ (mm-1)

2.244

3.049

Dcalc (Mg/m3)

1.654

1.956

Crystal size (mm)

0.24 × 0.18 × 0.12

0.34 × 0.30 × 0.20

θ max (°)

26.30

26.29

Data collected

4352

11472

Unique refl. / Rint

2171 / 0.0199

1624 / 0.0524

Parameters

147

126

Goodness-of-Fit on F2

1.173

1.057

0.0333 / 0.0724

0.0267 / 0.0729

R1 / wR2 (all data) 3

Residual extrema (e/Å ) 0.48 / -0.31

0.65 / -0.86

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FIGURES Figure 1. catena-(3-Hampy)2[Zn7(µ1,1-N3)16(3-ampy)4]·3H2O (1): (a) Perspective view of the core of Zn8 cluster subunit of 1. (b) Perspective view of a section of the [Zn7(µ1,1-N3)16(3ampy)4] cluster chain·with partial atom numbering scheme. (c) Packing plot of the polymeric cluster chains. Symmetry codes: (‘) 3/2-x,3/2-y,-z; (“) 2-x,y,1/2-z; (*) -1/2+x,3/2-y,-1/2+z. Figure 2. catena-{[Zn2(µ2-4-HOpy)(µ1,1-N3)4]·H2O}n (2): (a) Perspective view of a tetra-nuclear subunit with atom numbering scheme. Symmetry codes: (a) 1-x,1-y,1-z; (b) 1-x,-1/2+y,3/2-z; (c) 1-x,1/2+y,3/2-z; (d) x,3/2-y,-1/2+z; (e) x,1/2-y,-1/2+z. (b) The 2 D system of 2 formed by Znazide sublattice view onto layer. (c) View into layer system of 2. (d) Simplified view of the 2D layer showing the alternate arrangement of the tetra- and octanuclear Zn(ii) rings. Figure 3. catena-[Zn2(µ2-4-amtz)(µ1,1-N3)4] (3): (a) Perspective view of a tetra-nuclear subunit with atom numbering scheme. Symmetry codes: (a) 1-x,1-y,1-z; (b) 1/2-x,-1/2+y,1/2-z; (c) 1/2x,-1/2+y,1/2-z; (d) 1/2+x,1/2-y,1/2+z; (e) 1/2+x,3/2-y,1/2+z. (b) The 2D system of 2 formed by Zn-azide sublattice. Figure 4. a) Perspective view of a section of the 1D system of 4. Symmetry codes: (a) 1-x,-y,1-z; (b) 2-x,-y,1-z. b) Packing plot of 4. Figure 5. a) Perspective view of a section of the 1D system of 5. Symmetry codes: (‘) 1/2x,1/2+y,z; (“) 1/2-x,-1/2+y,z; (*) x,1+y,z. b) Packing plot of 5.

Figure 6. The fluorescence emission [intensity (arbitrary units) vs. wavelength (in nm)] of complexes 2, 4 and 5, and ligands 4-hydroxypyridine (L2) and 2-hydroxypyridine-N-oxide (L5).

23

A series of five 1D and 2D polymeric Zn(II) azide complexes based on pyridine derivative compounds were synthesized, structurally characterized, and their emission spectral properties were investigated.

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