Mercury coordination polymers with flexibleethane-1,2-diyl-bis-(pyridyl-3-carboxylate):Synthesis, structures, thermal and luminescent properties

Javier Vallejos a, Iván Brito b,n, Alejandro Cárdenas c, Jaime Llanos a, Michael Bolte d,Matías López-Rodríguez e

a Departamento de Química, Universidad Católica del Norte, Av. Angamos 0610, Antofagasta, Chileb Departamento de Quimica, Universidad de Antofagasta, Av. Angamos 601, Antofagasta, Chilec Departamento de Física, Universidad de Antofagasta, Av. Angamos 601, Antofagasta, Chiled Institut für Anorganische Chemie der Goethe—Universität Frankfurt, Max-von-Laue-Strasse 7, D-60438 Frankfurt am Main, Germanye Instituto de Bio-Orgánica “Antonio González”, Universidad de La Laguna, Astrofísico Francisco Sánchez N1 2, La Laguna, Tenerife, Spain

a r t i c l e i n f o

Article history:Received 20 November 2013Received in revised form1 March 2014Accepted 9 March 2014

Keywords:Hg(II) coordination polymerSupra-molecular structureLuminescent and thermal properties

a b s t r a c t

The reaction of the flexible ligand, ethane-1,2-diyl-bis-(pyridyl-3-carboxylate), (L) with HgI2 and HgBr2salts under the same experimental conditions leads to the formation of two coordination polymers withdifferent motifs: {[Hg(L)(Br2)]}n (1) and {[Hg(L)(I2)]}n (2).

In both compounds, the ligand, (L) acts in a μ2-N:N0-bidentate fashion to link HgBr2 and HgI2 units toform a linear and helical chain motif, along [1 0 0] for (1) and [0 0 1] for (2). The ethylene moiety of (L)has gauche and trans conformation in compounds (1) and (2), respectively. The flexible conformation ofL produces differences in the optical and crystal properties of the two compounds.

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1. Introduction

Coordination polymers containing metal nodes and organic ligandshave attracted tremendous attention over the past 30 years becauseof their wide application as functional materials [1,2]. The synthesisof coordination polymers by the judicious choice of organic spacersand metal centers can be an efficient method to obtain new types ofluminescent materials, especially for d10 or d10–d10 systems of metalcenters [3,4]. The most important motifs among one-dimensionalcoordination polymers are linear chains, zig-zag chains, doublechains, ladder chains, fish-bone chains and helix and double helixchains. The group 12 elements are rather a special case whenconsidering the chemistry of the main group elements. Supramole-cular structures that contain mercury(II) seem to have much more incommon with main group elements, in part because they tend toform structures with low-coordinate linear or other distorted coor-dination geometries [5].

One special class of such compounds is coordination polymersbased on Hg(II) halides and aromatic N-donor ligands. These show

great structural diversity, in part arising from the adjustablecoordination numbers and geometries of d10 Hg(II) centers, whichare particularly suited to the construction of coordination poly-mers and networks [6–9]. On the other hand, owing to the abilityof organic materials to affect wavelength emissions, syntheses ofcoordination polymers by judicious selection of organic spacersand metal ion centers (such as Zn(II), Cd(II), Hg(II), Ag(I), Cu(I)) canbe an efficient method to obtain new types of luminescentmaterials [10–14].

We have been investigating L ligand (ethane-1,2-diyl bis(pyridine-3-carboxylate) for its versatile and luminescent properties [15]. Froma structural point of view, it should be pointed out that L ligand,unlike rigid bi-pyridines, possesses flexibility owing to the presenceof a –CH2-spacer between the pyridine-carboxylate rings.

This paper forms part of our continuing study on the synthesis,structural characterization and photo-physical properties of hybridmaterials based on d10 ions and flexible ligands. In this report, wehave focused our investigation on two Hg(II)-containing coordinationpolymers and their luminescent properties, due to the attractiveproperties of mercury(II) compounds in terms of their potentialapplications in the paper industry, paints, cosmetics, preservatives,thermometers, manometers, energy efficient fluorescent light bulbsand mercury batteries (although somewhat limited due to mercury'stoxicity), and considering studies on the formation of polymers with

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n Corresponding author. Tel.: þ56 55 2637814; fax: þ56 55 637457.E-mail address: ivanbritob@yahoo.com (I. Brito).

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the Hg2þ ion are disproportionately sparse when compared withthose on Zn2þ and Cd2þ metals.

2. Experimental

2.1. Materials and physical measurements

All reagents were purchased commercially and used withoutpurification except the ligand, which was synthesized by us [15].For compounds (1) and (2), FTIR spectra were obtained with aNicolet Avatar 330 spectrometer, using KBr pellets, in the range of4000–400 cm�1. The UV–vis spectra were recorded on a PerkinElmer Lambda 20 Spectrophotometer with a diffuse reflectancesphere in the range of 200–400 nm in the solid state at roomtemperature. Thermal stability (TG-DTA) studies were performedwith a DTG-60 H (Shimadzu) thermal analyzer from room tem-perature to 800 1C under N2 with a heating rate of 10 1C/min.The photoluminescence spectra were measured using the JASCOFP-6500 spectrofluorometer. All spectra were registered at room

temperature. In order to compare the photoluminescence inten-sity, the amount of samples was the same in all experiments.

2.2. Synthesis of {HgBr2(L)}n (L¼ethane-1,2-diyl bis(pyridine-3-carboxylate) (1)

Single crystals of the title compound suitable for X-ray diffrac-tion studies were obtained from a HgBr2 solution (56 mg,0.1 mmol) in H2O (3 ml), which was added to a CH3CN (3 ml)solution of L (27 mg, 0.1 mmol). The clear solution was kept formany days at room temperature until crystals formed. Colorlesscrystals of [Hg(L)(Br2)]1 were collected with approximately 75%yield. IR (KBr, cm�1): 3454 (w), 3101 (w), 3070 (w), 2964 (w), 2375(w), 1731 (s), 1598 (m), 1479 (m), 1112 (m).

2.3. Synthesis of {HgI2(L)}n (L¼ethane-1,2-diyl bis(pyridine-3-carboxylate) (2)

Single crystals of the title compound suitable for X-ray diffrac-tion studies were obtained from a HgI2 solution (45 mg, 0.1 mmol)in H2O (3 ml), which was added to a CH3CN (3 ml) solution of L(27 mg, 0.1 mmol). The clear solution was kept for many daysat room temperature until crystals formed. Colorless crystalsof [Hg(L)(I2)]1 were collected with approximately 75% yield.IR (KBr, cm�1): 3426 (w), 3057 (w), 3029 (w), 2959 (w), 1716 (s),1596 (m), 1436 (m), 1112 (m).

All above mentioned compounds were obtained as singlecrystals and they were washed with methanol and dried at roomtemperature for 3 h and used for X-ray experiments.

2.4. X-ray crystallography

Single crystal X-ray diffraction data were recorded on a STOEIPDS II two-circle-diffractometer with Genix 3D multilayer opticsand Mo-Kα radiation (λ¼0.71073 Å) at 173 K. for both compounds.Absorption corrections were made with multi-scan [16]. Bothcrystal structures were solved by direct methods and refinedemploying full-matrix least squares on F2 using SHELXL-97 [17].All non-hydrogen atoms were refined anisotropically; hydrogenatoms were located geometrically. The CCDC number for (1) and(2) are 944923 and 944924, respectively. The crystal data andstructure refinements of (1) and (2) are summarized in Table 1 andselected bond lengths and angles are listed in Table 2.

Table 1Details of crystal data and structure refinement parameters for (1) and (2).

Compound (1) (2)

Empirical formula C14H12Br2HgN2O4 C14H12Hg I2N2O4

Formula weight (Daltons) 632.67 726.65Temperature (K) 173(2) 173(2)Wavelength ℓ (Å) 0.71073 0.71073Crystal system Monoclinic MonoclinicSpace group C2/c P 21/n

Unit cell dimensions (Å, 1)a (Å) 22.7861(14) 5.2933(3)b (Å) 4.9181(4) 28.0140(10)c (Å) 15.3032(10) 12.4125(6)α (1) 90 90β (1) 104.238(5) 95.283(4)γ (1) 90 90Volume (Å3) 1662.3(2) 1832.79(15)Z 4 4Calculated density (g/cm3) 2.528 2.633Absorption coefficient (mm�1) 14.086 11.785Absorption correction Multi-scan Multi-scanTmin, Tmax 0.1559; 0.2431 0.181; 0.357F(0 0 0) 1168 1312Rint 0.0882 0.1164Goodness-of-fit on F2 1.111 1.230R1a[I42s(I)] 0.0354 0.0335

wR2b 0.0915 0.0867

aR1∑jjF0j�jFc jj=∑jFoj; bwR2 ¼∑f½wðFo2�Fc2Þ2�=∑½wðFo2Þ�2g12

Table 2Bond lengths [Å] and angles [1] for (1) and (2).

Compound (1) Compound (2)

Hg(1)–N(13)#1 2.413(3) Hg(1)–N(21) 2.460(5)Hg(1)–N(1) 2.495(19) Hg(1)–N(11) 2.470(5)Hg(1)–I(2) 2.6514(17) Hg(1)–I(2) 2.6649(4)Hg(1)–I(1) 2.6604(17) Hg(1)–I(1) 2.6505(4)Hg(1)–N(13) 2.413(3) N(21)–Hg(1)–N(11) 98.24(15)Hg(1)–Br(1) 2.4916(4) N(21)–Hg(1)–I(2) 99.81(11)Hg(1)–Br(1)#1 2.4916(4) N(11)–Hg(1)–I(2) 98.73(11)N(13)#1–Hg(1)–N(13) 102.62(17) N(21)–Hg(1)–I(1) 98.61(11)N(13)#1–Hg(1)–Br(1) 97.79(8) N(11)–Hg(1)–I(1) 99.62(11)N(13)–Hg(1)–Br(1) 99.38(8) I(2)–Hg(1)–I(1) 151.753(14)N(13)#1–Hg(1)–Br(1)#1 99.38(8)N(13)–Hg(1)–Br(1)#1 97.79(8)Br(1)–Hg(1)–Br(1)#1 152.38(2)

Symmetry code: #1 �xþ1,y,�zþ3/2.

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3. Results and discussion

3.1. Crystal structure of (1)

Complex (1) crystallizes in the monoclinic form with spacegroup C2/c and has an infinite zig-zag chain structure. The Hg(II)atoms lie on a special position in crystallographic two-fold axis.The crystal structure of (1) with the coordination environment ofthe Hg(II) atom is depicted in Fig. 1. It can be clearly seen that eachHg(II) atom has four-coordinate and adopts a distorted tetrahedralgeometry with two symmetry-related N atoms and two Br atoms,with Hg–Br1 and Hg–N1 bond distances of 2.4916(4) and 2.413(3) Å,respectively. The bond angles of Br1–Hg–Br1#1(#1: �xþ1, y,�zþ3/2), Br–Hg–N, N1–Hg–N1#1 are in the range of 97.79(8)–152.38(2)1, in good agreement with those for other Hg(II)

coordination polymers [18]. The Hg–Hg#2 (#2:1/2 �x, 1/2 �y,1�z) separation across (L) is 14.2175(12) Å. The dihedral anglebetween pyridine rings is 89.4(2)1.

In the crystal structure the zig-zag chains run along [1 0 0] (seeFig. 2). Interactions between neighboring chains are not observed.

3.2. Crystal structure of (2)

Complex (2) crystallizes in the monoclinic system in the spacegroup P21/c. The crystal structure of (2) with the coordinationenvironment of the Hg(II) atom is shown in Fig. 3. It can be clearlyseen that each Hg(II) atom is in a distorted tetrahedral coordina-tion environment with two N atoms from two different ligandsand two iodides from HgI2, with Hg–I1, Hg–I2, Hg–N11 andHg–N21

i (i: �1/2þx,1/2�y,1/2þz) bond distances of 2.6505(4),2.6649(4), 2.470(5), 2.460(5) Å, respectively. The bond angles ofI2–Hg–I1, I–Hg–N, and N11–Hg–N21i are in the range of 98.24(15)–151.753(14)1 (Table 2), similar to those observed in reported Hg(II)complexes with N and I donors [18]. The Hg⋯Hgi separation across(L) is 9.947(3) Å. The ligand has a gauche conformation and linkstwo Hg(II) atoms using its two pyridine groups to give a 1D helicalstructure, as shown in Fig. 3. The dihedral angle between thepyridine rings is 86.3(3)1.

The crystal structure consists of one-dimensional HgI2–L helicalchains running parallel to the [0 0 1] axis that are stabilized bysingle intramolecular hydrogen bonds involving atom C12 as adonor, viz. C12–H12⋯O3i to form R(19) rings (labeled A in Fig. 4)These chains are linked to neighboring chains by C3–H3⋯O1ii(ii:�1�x,1�y,�z) weak hydrogen bonds to form R2

2(10) centrosym-metric rings (labeled B in Fig. 4), so these interactions betweenneighboring chains generate R4

4(36) centrosymmetric rings(labeled C in Fig. 4) [19]. These three types of rings alternate in a⋯ABCABC⋯ fashion to form a two dimensional supramolecularaggregate (Fig. 4).

Fig. 1. Coordination environment of Hg(II) in (1).

Fig. 2. Part of the crystal structure of (1) showing the zig-zag chain along [1 0 0]. Fig. 3. Coordination environment of Hg(II) in (2).

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The O–C–C–O dihedral angle of moiety L for compounds (1)and (2) is 180.0(4) and 66.7(6)1, respectively. The ligand ofcompound (2) retains the gauche conformation [20]; however,another compound has a trans conformation and this conforma-tional flexibility in the ligand permits it to obtain two coordinationpolymers, with two different motifs.

3.3. Powder X-ray diffraction analysis of compounds (1) and (2)

The purity and homogeneity of the bulk products of com-pounds (1) and (2) were determined by a comparison of simulatedand experimental X-ray powder diffraction patterns. The peakpositions of the experimental patterns for the complex nearlymatched with the simulated one generated from single-crystalX-ray diffraction data, as depicted in Fig. 5. The differences inintensity may be due to the preferred orientation of the powdersamples.

3.4. Photoluminescent properties

It is well known that polymeric coordination complexes withd10 metal ions and conjugated ligands may be regarded aspromising candidates for potential luminescent applications asligh-emmiting diodes (LEDs) [21,22]. Thus, UV/Vis and lumines-cence spectra of (L) and compounds (1)–(2) in the solid state werestudied. The UV/Vis spectra of (L) and (1)–(2) in the solid statedisplay intense absorption bands ranging from 220 to 330 nm(Fig. 6 left side), indicating that the electronic transitions aremostly π–πn, originating from the pyridyl group of (L). The emissionproperties of compounds (1) and (2) and the free ligand (L) were

investigated in the solid state at room temperature. As shown inFig. 6 (right side), L exhibits an intense photoluminescence emissionat 401 nm (λex¼360 nm). The emission of (1), collected on acrystalline bulks showed a band at 425 nm with a second lessintense band present at 468 nm (λex¼363 nm). A crystalline bulk of(2) emitted at 416 nm, with a second equally intensity band at436 nm (λex¼365 nm). The emissions of (1) and (2) are red-shiftedwith respect to L. Because in d10 metal ions with one or two positivecharge are what, the d-orbitals are contracted and therefore theelectrons in these orbitals are much less accessible for back bondingto π-acceptor ligands, and Hg(II) has a weak electro-acceptingnature with respect to the electron of L. So, presumably, theseemissions may be assigned to the intra-ligand charge transfer band(LLCT) [23]. Some differences could be observed in the emissionspectra of the complexes in comparison with the spectra of theligands, more redshift bands were detected in the former, andprobably these luminescence spectra could be the result of andadmixture of LMCT and LLCT in both coordination polymers [24].

Overall, there are large shifts in the emissions between the freeligand L and the title compounds. Unsurprisingly, the presence of

Fig. 4. A view of the two-dimensional supramolecular aggregate, showing theformation of R (19), R22(10) and R4

4 (36) rings (labeled A, B and C, respectively). Hatoms not involved in the C–H⋯O interactions have been omitted. [Symmetrycodes: (i) �1/2þx,1/2�y,1/2þz; (ii) �1�x,1�y,�z].

Fig. 5. XRPD pattern calculated from single crystals of complexes (1) right-sideand (2) left-side (black line), as-synthesized of the complexes (1), (2) (blue line).(For interpretation of the references to color in this figure legend, the reader isreferred to the web version of this article.)

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the Hg(II) ions, which exert a strong electron-withdrawing effectas a consequence of their 2þ charge and attached halides, has asignificant influence on the emission properties of the ligand [24].Although these complexes have emission bands centered atsimilar wavelengths when they are excited at �360 nm, complex(1) emits a blue light (CIE coordinates 0.17, 0.13) at 298 K. Mean-while, (2) emits pale blue light (CIE coordinates 0.22, 0.24), as canbe seen in Fig. 7. We assume that this variation in color coordi-nates is due to half-width differences between complexes (1) and(2), with full-width half-maxima (FWHM) of about 150 and 95 nm,respectively.

3.5. Thermal analyses

The thermogravimetric behaviors of both polymers wereexamined by TGA on polycrystalline samples from 25 to 800 1Cunder flowing Nitrogen at a heating rate of 10 1C/min (Figs. 8 and 9).The thermogravimetric analysis revealed that (1) and (2) arethermally stable up to 169 and 219 1C, respectively. The thermaldecomposition process for compound (1) occurs in two stages. Thefirst-step weight loss occurs from 169 to 273 1C and corresponds tothe loss of two Bromine atoms and complete removal of the ligandmolecule (calcd. 68.7%; found 68.1%). The second-step weight lossoccurs from 273 to 800 1C, reflecting volatilization of the Hg atom.

The thermal decomposition process for compound (2) alsooccurs in two stages. The first-step weight loss occurs from 219

Fig. 6. Diffuse reflectance of (1), (2) and L (left-side) and emission spectra of (1),(2) and L (right-side).

Fig. 7. CIE-1931 chromaticity diagram showing the color of complexes (1) and (2).The ideal blue light point is at (0.147, 0.054). (For interpretation of the references tocolor in this figure legend, the reader is referred to the web version of this article.)

Fig. 8. TGA curve for compound (1).

Fig. 9. TGA curve for compound (2).

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to 273 1C and corresponds to the loss of two Iodine atoms andcomplete removal of the ligand molecule (calcd. 68.7%; found72.2%).The second-step weight loss occurs from 273 to 800 1C,again reflecting volatilization of the Hg atom.

3.6. Conclusion

The flexible conformation properties of L produced differencesin the optical and crystal properties of the compounds studied andshould presumably, lead to new, tunable, luminescent materials.

Supplementary material

Crystallographic data for the structural analyses have beendeposited with the Cambridge Crystallographic Data Center, CCDCNos. 944923 and 944924 for (1) and (2). The data can be obtainedfree of charge from The Director, CCDC, 12 Union Road, CambridgeCB21EZ, UK (Fax: þ44 1223 336033; E-mail: deposit@ccdc.cam.ac.uk or http://www.ccdc.cam.ac.uk).

Acknowledgments

The authors gratefully acknowledge financial support to Uni-versidad de Antofagasta for the purchase of a License to theCambridge Structural Database system 2013. J.V. thanks Universidadde Antofagasta for a PhD fellowship.

Appendix A. Supplementary materials

Supplementary data associated with this article can be found inthe online version at http://dx.doi.org/10.1016/j.jssc.2014.03.022.

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