Luminescent coordination polymers of 2,2′-bipyrimidine and mercury(II) salts: A structural and computational study

Polyhedron 104 (2016) 25–36

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Luminescent coordination polymers of 2,20 -bipyrimidine and mercury(II) salts: A structural and computational study Emanuele Priola a,b, Elisabetta Bonometti a,b, Valentina Brunella a, Lorenza Operti a,b, Eliano Diana a,b,⇑ a b

Dipartimento di Chimica, Università di Torino, Via Pietro Giuria 7, Turin, Italy CrisDi-Interdepartmental Center for Crystallography, University of Torino, Turin, Italy

a r t i c l e

i n f o

Article history: Received 16 September 2015 Accepted 30 October 2015 Available online 26 November 2015 Keywords: Photoluminescence Supramolecular chemistry Coordination polymers 2,20 -Bipyrimidine Mercury halogenides

a b s t r a c t We report here the synthesis and structural, thermal and vibrational characterization of five complexes of the 2,20 -bipyrimidine ligand (bpym) with HgX2, X = Cl, Br, I, CN, SCN, with the aim of investigating the factors that could influence the supramolecular chemistry of mercury(II). The combination of the 2,20 bpym ligand, a bidentate molecule and bridging anions together with a d10 metal like mercury(II) has given different polymeric structures with 1D and 3D dimensionalities. A computational modelling, compared to statistical analysis of the reported structures, has allowed the rationalisation of the differing contributions of the bpym, halide and pseudohalide ligands to the supramolecular architecture. The solid state photoluminescent properties of the complexes have been investigated and an interpretation based on computational models has been proposed. Ó 2015 Elsevier Ltd. All rights reserved.

1. Introduction Self-assembly of small building blocks to give supramolecular architectures is a vibrant area of research [1]. An interesting method of constructing complex molecules employs ‘‘dynamic” coordination chemistry, with labile metal centers and bridging ligands used to form the primary structure, and weak, non-covalent interactions like hydrogen bonding, p–p stacking, metallophilic interactions (especially for d10 metal compounds) [2] or secondary interactions in the metal sphere, exploited to organize these primary structures into supramolecular materials [3]. The supramolecular chemistry of the Hg(II) cation is a perfect candidate in this dynamical crystal engineering approach; the peculiar behavior of this heavy d10 metal center is that its closed shell electronic configuration cannot be stabilized by crystal fields effects to a specific geometry and this allows strong distortions in the platonic polyhedron typical of transition metals and the presence of further weaker interactions in the coordination environment. These distortions are affected by the packing forces and by the donor strength of the coordinated ligands. Furthermore, the strong relativistic behavior of this metal allows a stable linear coordination, strong covalent bonds with many anionic or neutral ligands and the appearance of metallophilic interactions. The patterns

⇑ Corresponding author at: Dipartimento di Chimica, Università di Torino, Via P. Giuria 7, 10125 Torino, Italy. Tel.: +39 0116707572; fax: +39 0116707855. E-mail address: [email protected] (E. Diana). http://dx.doi.org/10.1016/j.poly.2015.10.059 0277-5387/Ó 2015 Elsevier Ltd. All rights reserved.

obtained in the supramolecular structures of Hg(II) therefore are the result of a subtle balance of weak forces, and their control is quite difficult without a selective choice of different building blocks [4]. These effects have been investigated in a series of complexes prepared by the addition of an ancillary ligand to an inorganic cation–anion framework. We have studied the coordination of the 2,20 -bipyrimidine ligand (bpym) to some HgX2 salts, with X = Cl, Br, I, CN and SCN. Some of these complexes have already been reported, but not fully characterized [5]. These anions usually can bridge metal centers, but their arrangements are strongly influenced by the other constituents because of the peculiarities of Hg(II) cited above, and the bridging bonds are weaker than the typical interactions of coordination polymer [6]. We have analyzed these effects through a structural, vibrational, thermal and computational study. Furthermore, this aromatic ligand was studied deeply for its interesting photochemical and photophysical properties: the presence of three low-energy p⁄ antibonding levels make this molecule a good p acceptor [7], able to give high-intensity room temperature luminescence emissions. However, few studies have been reported about the luminescence of d10 metal complexes of bpym, and they have all been focused on Zn(II) and Cu(I). In the zinc complexes, metal–ligand interactions are interpreted not unambiguously: their effect seems to have a modest effect on the electronic transitions of the ligand itself [8], or a MLCT is invoked [9]. For Cu(I) complexes of POP (bis{2-(diphenylphosphanyl)phenyl}ether) and phosphines, a huge increase in the intensity of the luminescence and the apparition of new MLCT

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bands are reported [10]. However, all the studies mainly focused on molecular complexes and to our knowledge no detailed investigations have been reported on the luminescence of coordination polymers of bpym with d10 metals. With this background, we measured and analysed the luminescence of the title compounds.

found in Table 1. The main distances and angles are reported in Table 2. As expected, the highest peaks and deepest holes for these mercury complexes are slightly high, but the residual densities are close to heavy mercury atoms.

2. Experimental

2.4. Spectral analysis

2.1. Chemicals and reagents

FTIR and FT-Raman spectra were obtained with a Bruker Vertex 70 spectrophotometer, equipped with the RAMII accessory. The spectra were recorded with a resolution of 2 cm1. Infrared spectra were obtained from KBr pellets. Raman spectra were obtained from crystalline sample, by exciting with a 1064 nm radiation. The UV–Vis spectra were recorded on crystalline samples employing a Perkin Elmer Lambda 900 spectrophotometer in the reflectance mode. The photoluminescence measurements were recorded using a Fluorolog F2 Horiba/Jobin–Yvon spectrofluorometer.

All chemicals were used as purchased without further purification: HgCl2 (99.9%, Schiapparelli), HgI2 (99.0%, Sigma–Aldrich), HgBr2 (99.0%, Sigma–Aldrich), Hg(CN)2 (99.0%, Aldrich), Hg(SCN)2 (99.0%, Fluka), 2,20 -bipyrimidine (bpym) (99.0%, Sigma–Aldrich). 2.2. Synthesis of [Hg2(BPYM)X4]x (X = Cl (1), Br (2), I (3), CN (4), SCN (5)) Lanza (for the chloride binuclear complex) [5a] and Jarabat (for the halide binuclear complexes) [5b] proposed a synthetic procedure consisting of mixing solutions of the inorganic salt and the organic ligand in water, ethanol or acetone (depending on the solubility of the salt) to obtain powder pure compounds. We decided to modify this methodology in order to obtain crystalline samples: (a) A solution of bpym (50 mg, 0.32 mmol) in 2 mL of acetonitrile and a solution of HgX2 (X = Cl, Br, I, CN, SCN) (0.64 mmol) in 20 mL of acetonitrile were mixed, with the immediate formation of a precipitate (not relevant for the cyanide and chloride salts), and the yellow mixture obtained was refluxed for 2 days, until the solution became orange and transparent. The solution was then concentrated at 25 °C and orange prismatic crystals were obtained and separated, which were suitable for X-ray analysis. For better crystals of 5, the initial small acicular crystalline product was redissolved in acetone at room temperature; after a week, prismatic orange crystals were obtained. (b) We also performed the synthesis of the various complexes under solvothermal conditions, with the previous quantity of reagents mixed in 100 mL of acetonitrile. The reaction was performed at 150 °C for 12 h, and after cooling gave the same crystalline phases as obtained with the first procedure. All the products have been analysed by powder X-ray diffraction and Raman spectroscopy to check their purity, and there are no relevant peaks of other phases or impurities. 2.3. X-ray crystallography Data of single crystals of compounds 1, 2, 3, 4 and 5 have been collected on a Gemini R Ultra diffractometer [11]. All the data were collected using graphite-monochromated Mo Ka radiation (k = 0.71073 Å) with the x-scan method. Cell parameters were retrieved using CrysAlisPro [12] software, and the same program has been used for performing data reduction, with corrections for Lorenz and polarizing effects. Scaling and absorption corrections were applied by the CrysAlisPro [12] multi-scan technique. All the structures were solved by direct methods using SHELXS-97 [13] and refined with full-matrix least-squares on F2 using SHELXL-97 [13]. All non-hydrogen atoms were anisotropically refined. Hydrogen atoms were located in the final Fourier-difference maps and refined with coordinates and Uiso free (compounds 1, 4 and 5) or calculated and riding on the corresponding C atom (compounds 2 and 3). Structural illustrations have been drawn with CrystalMakerÒ: a crystal and molecular structures program for Mac and Windows [14]. The powder diffraction measurements have been recorded with the same instrument, using Cu(Ka) radiation on the milled samples. Crystal data and refinement results can be

2.5. Thermal analysis A differential scanning calorimeter (DSC Q200, TA Inc.), provided with a cooling system RCS90, was used to collect the DSC thermograms. The DSC measurements were performed with a closed aluminum pan under a nitrogen atmosphere (50 cm3/min) and with a 10 °C/min heating rate, from 20 to 180 °C. Thermal stability was evaluated with a Hi-Res thermo gravimetric analyzer (TGA Q500 balance, TA Inc.) on 10–15 mg samples contained in alumina pans, with a 10 °C/min heating ramp from 50 to 700 °C under a 100 cm3/min nitrogen flow.

2.6. Computational analysis DFT calculations of molecular models were carried out with the Gaussian09 package [15] by employing the B3PW91 hybrid functional. We used a 6-31++g(2d,2p) basis set for H, C, N, O, S and Cl atoms, and the ECP60MWB quasi-relativistic pseudopotential and a (8s7p6d2f1g)/[6s5p3d2f1g] basis set for Hg [16] and the ECP46MWB pseudopotential [17] and a (14s10p3d1f)/[3s3p2d1f] basis set for iodine [18]. All the structures were optimized to an energy minimum and the harmonic vibrational frequencies were computed. The electronic spectra were computed by means of the Hartree–Fock configuration interaction method with single and double excitations, on optimized geometries obtained with a 6-31g(d) basis set for H, C, N, O, S, Cl and Br atoms and the LANL2DZ basis set and pseudopotential for Hg and I atoms. The band structures of the crystalline solids have been computed with the Crystal09 [19] package, at the Hartree–Fock level, with a 6-31g(d) basis set for light elements and the previously cited ECP pseudopotential and basis set for Hg and I atoms.

3. Results 3.1. X-ray structures 2,20 -bipyrimidine has four nitrogen atoms that act as bases towards the Hg(II) ions, giving rise to structures containing two HgX2 (X = Cl, CN, I, SCN, Br) moieties for each ligand (Fig. 1). Complexes 1, 2, 3 and 5 lay on a crystallographic inversion center, while complex 4 lays on a crystallographic mirror plane. In all the complexes the Hg(II) ion has a distorted tetrahedral coordination.

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E. Priola et al. / Polyhedron 104 (2016) 25–36 Table 1 Summary of the crystallographic data for all the complexes.

Empirical formula Formula weight T (K) k (Å) Crystal system Space group Unit cell dimensions a (Å) b (Å) c (Å) b (°) V (Å3) Z Calculated density (g/cm3) Abs. coeff. l (mm1) F(0 0 0) Crystal size (mm) h range for data collection (°) Limiting indices

Refl. collected/unique Completeness to theta Refinement method Data/restrains/parameters Goodness-of-fit (GOF) on F2 Final R indices[I > 2r(I)] R indices (all data) Extinction coefficient Largest difference peaks and hole (e Å3) Empirical formula Formula weight T (K) k (Å) Crystal system Space group Unit cell dimensions a (Å) b (Å) c (Å) b (°) V (Å3) Z Calculated density (g/cm3) Abs. coeff. l (mm1) F(0 0 0) Crystal size (mm) h range for data collection (°) Limiting indices

Refl. collected/unique Completeness to theta Refinement method Data/restrains/parameters Goodness-of-fit (GOF) on F2 Final R indices[I > 2r(I)] R indices (all data) Extinction coefficient Largest difference peaks and hole (e Å3)

Crystal 1

Crystal 2

Crystal 3

C4H3Cl2HgN2 350.57 293(2) 0.71073 monoclinic P21/n

C4H3Br2HgN2 251.14 273(2) 0.71073 monoclinic P21/n

C4H3HgI2N2 533.47 293(2) 0.71073 orthorhombic Pnma

3.9406(2) 18.935(1) 9.5633(6) 101.89(1) 698.27(7) 4 3.335 22.709 620 0.113  0.128  0.295 3.89–32.27 5 6 h 6 5 27 6 k 6 28 13 6 l 6 14 14 437/2332 [R(int) = 0.0777] 94.1% Full matrix least square on F2 2332/54/83 0.951 R1 = 0.0356, wR2 = 0.0712 R1 = 0.0534, wR2 = 0.0791

4.1032(3) 19.427(2) 9.7526(6) 102.069(7) 760.23(10) 7 3.840 30.668 764 0.05  0.2  0.4 3.7956–29.2834 4 6 h 6 4 23 6 k 6 23 11 6 l 6 11 1339/1160 [R(int) = 0.0413] 99.7% Full matrix least square on F2 1160/0/82 1.070 R1 = 0.0372, wR2 = 0.0782 R1 = 0.0451, wR2 = 0.0818

7.014(1) 19.269(4) 12.399(3) 1675.8(6) 8 4.229 25.657 1816 0.093  0.134  0.332 3.29–29.25 9 6 h 6 9 25 6 k 6 26 16 6 l 6 16 26 105/2258 [R(int) = 0.0566] 96.3% Full matrix least square on F2 2258/0/92 1.077 R1 = 0.0391, wR2 = 0.1066 R1 = 0.0525, wR2 = 0.1158

1.821 and 2.507

1.440 and 1.199

2.204 and 2.235

Crystal 4 C6H4HgN4O0.5 340.72 293(2) 0.71073 monoclinic C2/c

Crystal 5 C12H6Hg2N8S4 791.67 293(2) 0.71073 monoclinic P21/c

12.07064(4) 7. 1711(2) 17.7626(5) 91.915(3) 1614.73(8) 8 2.803 19.008 1216 0.060  0.065  0.160 3.42–29.21 17 6 h 6 17 9 6 k 6 9 24 6 l 6 24 12 544/2045 [R(int) = 0.058] 93.8% Full matrix least square on F2 2045/87/110 1.037 R1 = 0.0232, wR2 = 0.0452 R1 = 0.0361, wR2 = 0.0503 0.00123 (6) 1.013and 0.929

10.5327(4) 11.7056(4) 16.0828(6) 104.262(4) 1921.76(12) 4 2.736 38.324 1432 0.116  0.496  0.044 3.36–25.03 12 6 h 6 10 13 6 k 6 11 15 6 l 6 19 3294/2857 [R(int) = 0.0256] 96.9% Full matrix least square on F2 2857/198/235 1.026 R1 = 0.0230, wR2 = 0.0470 R1 = 0.0299, wR2 = 0.0448

3.2. Crystal packing 3.2.1. Complexes 1 and 2 The coordination sphere of the Hg(II) ion could be analyzed by means of two coordination numbers: the characteristic coordination number, defined by the nearest neighbors atoms, and the effective coordination number, obtained by including all ligands within the Van Der Waals radii sum (Fig. 2) [20]. This distinction is due to the strong tendency of Hg to have linear or low coordination number [21]. In 1 and 2, the mercury center has a character-

0.739 and 0.576

istic coordination number of four, defined by the N–Hg bonds and strong Hg–Cl(1) and Hg–Cl(2) interactions (or Hg–Br(1) and Hg–Br(2) in 2), as highlighted by the Hg–Cl distances (Hg–Br in 2) (see Table 2). The effective coordination number is obtained by adding two weaker additional interactions between the Hg(II) ion and the atoms Cl(1ii) and Cl(2iii) (3.096(2) and 3.219(2) Å) (or Br(1ii) and Br(2iii) (3.228(1) and 3.445(1) Å) in 2) (symmetry codes: (ii) = x  1, y, z, (iii) = x + 2, y, z). The octahedron generated by the effective coordination number is highly distorted, and this is typical of the chemistry of d10 metals because of the absence of effects

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Table 2 Bond lengths (Å) and angles (°) of the complexes (symmetry codes for the structures: complex 1 (i) = x + 2, y, z + 1, complex 2 (i) = x + 1, y, z + 1, complex 3 (i) = x, y + 1, z, complex 4 (i) = x + 1/2, y + 1/2, z + 1, complex 5 (i) = x + 1, y + 1, z, (ii) = x, y, z. Complex 1 X = Cl

Complex 2 X = Br

Complex 3 X = I

Complex 4 x = CN

Complex 5 (molec. 1) X = SCN

Complex 5 (molec. 2) X = SCN

Hg(1)–Cl(2) 2.3483(14) Hg(1)–Cl(1) 2.3600(15) Hg(1)–N(2i) 2.575(5) Hg(1)–N(1)2.617(4)

Hg(1)–Br(2) 2.4619(12) Hg(1)–Br(1) 2.4831(11) Hg(1)–N(2i) 2.593(8) Hg(1)–N(1) 2.640(9)

Hg(1)–I(1) 2.6064(8) Hg(1)–I(2) 2.6077(9) Hg(1)–N(2) 2.681(7) Hg(1)–I(1i) 3.3299(9)

Hg(1)–C(6) 2.048(5) Hg(1)–C(5) 2.068(5) Hg(1)–N(2i) 2.641(3) Hg(1)–N(1) 2.650(3)

Hg(1)–S(1)2.4153(13)

Hg(2)–S(4) 2.3856(17) Hg(2)–S(3) 2.4169(14) Hg(2)–N(4ii) 2.523(4) Hg(2)–N(3) 2.535(4)

Cl(2)–Hg(1)–Cl(1) 147.53(6) N(2i)–Hg(1)–N(1) 63.09(14)

Br(2)–Hg(1)–Br(1) 146.34(4) N(2i)–Hg(1)–N(1) 62.3(2)

I(1)–Hg(1)–I(2) 160.23(3) Hg(1)–I(1)–Hg(1i) 91.01(3)

C(6)–Hg(1)–C(5) 164.86(18) N(2i)–Hg(1)–N(1) 61.90(11)

that stabilize platonic polyhedra. The two aromatic rings of the 2,20 -bipyrimidine ligand are coplanar and are not distorted by the complexation, and these form a 1D ladder-type coordination polymer based on the weaker inter-unit Hg(II)  Cl(Br) interactions that grow along the a-axis (Fig. 3). The structures of 1 and 2 are isomorphous, with a slight distortion for the greater dimensions of the bromine atom with respect to the chlorine atom. The metal centers are connected through two bridging chloride ligands that form Hg2Cl2 asymmetric lozenge rings with angles of 84.75(7)° and 91.48(7)° centered at the Hg(1) atom and at the Cl(1) atom, and similarly for the bromine analogue 2. The distances between the metal centers (3.941(1) Å in 1 and 4.103(1) Å in 2) are not indicative of strong metallophilic interactions [22]. The ladder disposition of the inorganic chains is not so common in the chemistry of bpym, and only three examples are reported in the literature [23]. However, in mercury complexes, a lozenge disposition of bridging chloride ligands is quite common, and some 1D polymers with this ladder chain are reported, especially with pyrazine and its derivatives [24], while is less common for bromine compounds. 3.2.2. Complex 3 In complex 3, the Hg(1) atom maintains a characteristic coordination number of four, with very similar Hg–I and Hg–N bonds (see Table 2). Unlike 1 and 2, the effective coordination number is five, and the additional weaker bond is with the iodide of the nearest binuclear complex (d(Hg(1)–I(1i)) = 3.3299(9) Å). The coordination geometry is a trigonal bipyramid, lightly distorted in the central plane: the s-deformation parameter for the trigonality [25] is 0.99 (1 in a perfect trigonal bipyramid). The HgI interactions form a dimeric Hg2I4 fragment, which is quite common in the chemistry of iodomercurates [26,27]: in our complex, the rhomboidal planar Hg2I2 lozenge has non-symmetric bridging anions with angles of 91.01(3)° at the iodides and 89.93(3)° at the mercury atoms. The distance between the metal centres is 4.265(1) Å, indicating the lack of any mercurophilic interactions. With respect to complexes 1 and 2, the five atom Hg–N(1)–C(1)–C(2)–N(2) ring is not planar, having a half-chair conformation due to the position of the Hg(II) ion. This distortion is reflected in the organic ligand, which is bent along the C1–C2 inter-ring bond with a torsion of the rings of 2.79 (6)°. The association of the bridging bpym ligand with the iodide ligand creates a 1D ribbon-like coordination polymer that runs through weak interactions along the c-axis (Fig. 4). This is a typical structural pattern of the bpym ligand, which can form ribbons with or without the help of other anionic bridges [27]. 3.2.3. Complex 4 The Hg(1) atom has four short interactions with the two cyanide C atoms and with the N atoms of the bpym ligand (Table 2).

Hg(1)–S(2) 2.4441(13) Hg(1)–N(1i) 2.469(4) Hg(1)–N(2) 2.513(4) Hg(1)–N(31) 2.661(5) S(1)–Hg(1)–S(2) 138.84(5) N(1i)–Hg(1)–N(2) 66.16(14)

S(4)–Hg(2)–S(3) 153.18(5) N(4ii)–Hg(2)–N(3) 64.85(13)

The effective coordination number is five because of an interaction with the cyanide nitrogen atom of an adjacent molecule (d(Hg(1)– N(6ii)) = 2.91(5) Å) (symmetry code: (ii) = x + 1/2, y 1/2, z + 5/2). The polyhedron is highly distorted, but analysis of the s parameter, with a value of 0.89, suggests a coordination geometry more similar to a triangular bipyramid. The aromatic rings of the 2,20 -bipyrimidine ligand are coplanar and undistorted by the complexation. The water molecule is situated in the spaces between the bimetallic units of the structure and bridges the [Hg2(bpym) (CN)4] units along the a-axis through strong hydrogen bonds with the cyanide ligands (d(O(1)–N(6)) = 3.116(5) Å, (O(1)–H(1). . .N(6)) angle = 154.44(10)°) (Fig. 5). The importance of the hydrogen bond in the crystallochemistry of Hg(CN)2 derivatives is well established and it could be one of the main driving forces in the structure packing [28]. On the other hand, cyanide bridged mercury atoms are very rare: in 4 the N(6) atom has a weak interaction with Hg(1) (d(N(6)–Hg(1)) = 2.991(5) Å, (C(6)–N(6). . .Hg(1)) angle = 128.04 (10)°). This behavior is not common and has seldom been reported [29].

3.2.4. Complex 5 The structure of 5 shows two non-equivalent mercury atoms, Hg(1) and Hg(2), having both the characteristic coordination number of four and effective coordination number respectively of 5 and 6. The former has two strong interactions with the sulfur atoms of the coordinated thiocyanate ligands and with two nitrogen of a bpym ligand (Table 2), and a weaker interaction with the nitrogen atom of a neighbouring SCN ligand (d(Hg(1)–N(31)) = 2.661(5) Å). Hg(2) is similarly involved in four strong interactions with two sulfur atoms and the organic ligand (Table 2) and in two weaker interactions with the nitrogen atoms of two close SCN ligands (d(Hg(2)– N(11)) = 3.083(4) Å, d(Hg(2)-N(21)) = 2.771(4) Å). The coordination sphere of the Hg(1) atom forms a trigonal bipyramid (s = 1) and that of the Hg(2) atom is a distorted octahedron. The two non-equivalent organic ligands are planar and undistorted by the coordination. The four SCN ligands are two bridging and two terminal. This structure allows the propagation of an inorganic chain along the c-axis, which is connected to the nearest chains through two bpym ligands in opposite directions. These ligands spatially form a helix (Fig. 6), and constitute the organic matrix where the inorganic chains develop. This network defines a 3D coordination polymer, with a series of hollow fishbone-like channels along the b axis where the terminal SCN ligands are positioned. To our knowledge, this is the first 3D polymer built up by the Hg(SCN)2 moiety, although the bridging mode of this pseudohalide anion is well reported and many coordination polymers have been synthesized.

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Fig. 1. Molecular structures of complexes 1–5. Hydrogen atoms are omitted for clarity (ORTEP plot 50%).

3.2.5. Vibrational analysis The vibrational spectra of 1–5 in the 4000–400 cm1 region are largely characterized by the internal modes of the bpym ligand. The spectra of compounds 4 and 5 also show the stretching modes of the CN and SCN ligands respectively. A complete vibrational assignment for the 2,2-bipyrimidine ligand has been proposed [30] and recently revised on the basis of DFT computations [31]. A normal coordinate analysis and a DFT based assignment for the coordinated bpym ligand in the complex [Ru(bpym)3]3+ has been developed [32]. Infrared spectra are usually diagnostic of the coordination mode of the bpym ligand, especially in the 1600– 1500 cm1 region where the ring stretching modes appear [33].

In this work we have tried to make an assignment of the vibrational infrared and Raman spectra of 1–5 compounds by comparison with the spectra of the bpym molecule. The results are reported in Table S5. 3.2.6. 3200–2900 cm1 region In this region the main vibrational modes are m(CH). Crystalline bpym has two molecules in the unit cell, but its vibrational spectrum shows very little coupling, and the reported assignment has been done by considering molecular modes and a planar D2h symmetry. Six modes are expected, three infrared and three Raman. The main effect of metal coordination to these modes is a mean

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Fig. 2. Graphical representation of the characteristic coordination number and effective coordination number (a) and data for our compounds.

Fig. 5. Intermolecular interactions in solid 4. Non-fundamental hydrogen atoms are omitted for clarity (ORTEP plot 50%). Fig. 3. Crystal packing of 1 along the a direction. Hydrogen atoms are omitted for clarity (ORTEP plot 50%).

Fig. 4. Polymer expansion along the c-axis in 3. Hydrogen atoms are omitted for clarity (ORTEP plot 50%).

increment of 35 cm1, which could be attributed to the charge variation of the C–H bond as a consequence of the ligand?metal electron donation. A variation in the charge distribution along the C–H bonds is also suggested by the strong decrease of the band intensities of m(CH) in the metal complexes with respect to the free bpym molecule. 3.2.7. 2300–2000 cm1 region In this region there are the C–N stretches of the cyanide groups of 4. The Raman symmetric mode is at 2185 cm1 and the infrared asymmetric mode is at 2183 cm1. The modest coupling between the two m(CN) bands could be attributable to the bending of the C–Hg–C angle (164.85(18)°). With respect to m(CN) of Hg(CN)2 in solution [34], there is a red shift of 13 cm1, which can be attributed to the effect of the coordinated bpym ligand. In the same spectral region there are the m(C„N) modes of the thiocyanide ligand of 4, which show a Raman feature at 2123 cm1.

Fig. 6. Packing and helical disposition of the bpym ligands in 5. Hydrogen atoms are omitted for clarity (ORTEP plot 50%).

3.2.8. 1600–500 cm1 region The ring stretching modes and C–H in-plane and out-of-plane deformation modes are found in this region. A comparison with the vibrational frequencies of free bpym shows that mercury coordination has little effect on the ring modes, apart from a small

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increment of the frequencies. This upward shift has been also found in 4,40 -bipyridyl metal complexes, and has been explained by considering coupling with low wave number vibrations of metal–ligand bonds [35], especially with metal-nitrogen bonds. This upward shift is more pronounced when bpym is coordinated to lighter metal, such as Cu, Ni and Fe [36a–c]. Also pyridine bound to metal atoms shows an upward shift of the vibrational frequencies, which has been interpreted both in term of vibrational coupling with the metal-nitrogen bond and the nature of this bond [37]. The shift is greater for a stronger metalnitrogen interaction, especially in the presence of r-donation and p-back donation interactions. The Hg(II) atom probably behaves like the Ag(I) cation, where these interactions are weak, and this fact explains the modest vibrational shifts found in 1–5. 3.2.9. Thermal analysis We performed a TGA and DSC analysis on compounds 1–5 to study the reticular energy of the supramolecular framework and to investigate the effect of heating on the supramolecular architecture, in order to find if it could be the driving force of crystalline reorganization with the formation of polymorphs (Fig. S8). The DSC pattern of 1 reveals a melting endothermic peak at 145 °C, with successive degradation at 200 °C, confirmed by the TGA analysis. The DTGA shows at least two different reactions that occur simultaneously. Compound 4 shows a more complex pattern: the TGA profile indicates clearly a water loss at 110 °C, the DSC pattern shows dehydratation at 120 °C. At higher temperatures, an endothermic peak at 170 °C is attributable to a melting process. The compound degradation starts at 250 °C and a single step is clear; the DTGA shows that in this step three reactions occur. Complex 5 decomposes before melting at 200 °C. The decomposition in this case is more complex, with two degradation steps clearly separated and for each one the DTGA shows at least two reaction patterns. The compound’s melting point suggests that the reticular energy increases on going from 1 to 4, whereas 2, 3 and 5 decompose before their melting point. The degradation temperatures indicate stronger reticular interactions in the complexes than that of the ligand itself and a greater stability for the polymers based on Cl and Br, than that based on I, probably for the structures based on inorganic chain frameworks, and this is indicative of the structural isomorphism of 1 and 2. The pseudohalides are more stable, especially the thiocyanate; this can be explained for the stronger bond in the last case than that for all other counter anions. 3.2.10. Computational analysis In order to evaluate the effect of bpym coordination to HgX2 systems we performed a DFT analysis of some model compounds. For complex 1, we calibrated the method on the experimental geometric and spectroscopic (vibrational frequencies) parameters of HgCl2, and found the best parameters with the B3BPW91 hybrid functional with a quasi-relativistic pseudopotential for mercury (see experimental section for details). We started by examining the (HgCl2)2 dimer. In Fig. S7 the most interesting results are reported. The geometrical parameters are well reproduced and NBO analysis gives some interesting information on the reciprocal effect of the HgCl2 units. The computed Hg–Cl distance in HgCl2 is 2.273 Å (versus an experimental gas phase value of 2.252(5) Å) [38]; in the dimer we find an elongation of the Hg–Cl bond involved in intermolecular contacts. The charge distribution (Fig. S8) points out a little bond polarization and does not highlight any charge transfer between the HgCl2 units. The bond polarization is reflected by the Hg–Cl bond order (analyzed by means of the Wiberg bond order [39]): in HgCl2 it is 0.7436, while in (HgCl2)2 it is 0.6787 for the longer and 0.7706 for the shorter Hg–Cl bond.

Fig. 7. Computed geometry of [(bpym)HgCl2)2].

The effect of bpym on HgCl2 has been evaluated by means of two model compounds, (bpym)(HgCl2)2 and its dimer (Fig. 7). By comparing the computed geometrical parameters of HgCl2 and (bpym)(HgCl2)2, we can see that the coordination of bpym to HgCl2 induces a lengthening of the Hg–Cl bond and a bending of the Cl– Hg–Cl angle higher than those found in (HgCl2)2. The analysis of the NBO charge distribution indicates a charge transfer of 0.101 e from bpym to Hg; the Wiberg bond index is 0.7230 for Hg–Cl and 0.0926 for HgN, while in (HgCl2)2 it is 0.0666 for HgCl. The donor–acceptor NBO analysis indicates a modest transfer from the chlorine LP to the acceptor orbitals of Hg (7.80 kcal/mol), whilst in 2,2-bipyrimidine the donor–acceptor interaction from the nitrogen LP and empty orbitals of mercury involves an energy of 12.0 kcal/mol. The (HgCl2)2 unit computed in the system [bpym (HgCl2)2]2 has Hg–Cl distances similar to the experimental values found for 1, slightly greater than those computed for (HgCl2)2 system and similar to that computed for (bpym)(HgCl2)2. Also the Cl–Hg–Cl angle is similar to that of the monomer, but wider than that found in 1. By considering that HgCl intermolecular interactions have little effect on the Cl– Hg–Cl angle, as seen in the (HgCl2)2 system, the main effect is attributable to the nitrogen-mercury interaction: 1 has a shorter N–Hg distance than that computed for (bpym)(HgCl2)2 and the dimer, and this could influence the Cl–Hg–Cl angle. With a greater bpym–bpym distance, the [(bpym)(HgCl2)2]2 model shows that the dimeric (HgCl2)2 fragment is retained and that the bpym–HgCl2 interaction is not different to that of monomer, as shown by the analogous charge transfer of 0.100 e from bpym to the HgCl2 unit. We performed DFT modelling for the iodide complex 3. As in the study of 1, we calibrated the choice of basis set on HgI2, and the computed Hg–I distance and vibrational frequencies are in reasonable accord with the experimental ones [40] (2.593 Å; 226, 152, 51 cm1 versus 2.553(2) Å; 235, 156, 49 cm1). The DFT modelling of (HgI2)2 shows that this fragment is substantially maintained in 3, apart from some geometry variations, as similar to those found in complex 1, due to the nitrogen–mercury interaction of bpym (Fig. S9); with a little lengthening of the Hg–I bond and a bending of the I–Hg–I angle. A significant difference of the iodine model with respect the chlorine model is that the computed HgI intermolecular distance is greater than the experimental value, the opposite of that found in the (HgCl2)2 system. A guess as to the charge distribution shows that in the HgI2 molecule there is a reduction of the bond charge separation with

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respect to HgCl2, but the experimental HgHg distance is smaller in 3 than in 1 (4.265(1) versus 4.333(1) Å). We can suppose that the reduction of electrostatic interactions in the HgI2 dimers are compensated by the most significant dispersion forces, not well represented in our computational method. A computation of the (HgI2)2 dimer with the inclusion of Grimme’s B97-D dispersion term gives a HgHg distance of 4.291 Å, very similar to the experimental value in 3, nevertheless the IHg intermolecular distance is overvalued at 3.667 Å. 3.2.11. Electronic spectra Due to the extreme low solubility of complexes 1–5, we only recorded the UV–Vis spectra in the solid state (Fig. 8). All the spectra show a common intense band in the UV region at ca. 235 nm, and two broader features in the 250–400 nm interval. According to its electron structure [41a,41b], the strongest higher energy part of the bpym spectrum is attributable to p ? p⁄ transitions, while the broader feature at longer wavelength is usually assigned to n ? p⁄ transitions [41b]. The complexation of bpym to HgX2 affects the energy of the N atom lone pairs, and this can explain the changes in the 250–400 nm region. In order to evaluate these effects we computed the electronic transitions of the optimized geometry by means of a configuration interaction Hartree–Fock with double correction (CIS(D)) method both for bpym and complexes 1–5. The optimized geometry of bpym is D2h, while those of 1–5 complexes are C2h. Most significant results and spectral assignments are summarized in Table S6. Analysis of the MO composition shows that the high energy transition is mainly p ? p⁄ of the bpym ligand, and is scarcely affected by the Hg(II) coordination. In complexes 1–3 the transition at higher wavelengths show an increasing halogen ? Hg charge transfer on passing from Cl to I. In 4 and 5 the 250– 400 nm region contains both n ? p⁄ and CN (or SCN) ? Hg transitions. Nevertheless, the interpretation of electronic spectra based only on an insulated molecule model could be too approximate to analyze the electron transitions of polymeric

coordination complexes. Actually, solid bpym has a broad weak features that span from 380 to 600 nm, which are responsible for the reddish color of the solid phase, not found in the solution spectra. These transitions are probably attributable to intermolecular charge transfer in the crystalline state, favored by p–p stacking. Complexes 1 and 3, where this interaction is stronger, show similar bands around 500 nm, whereas in complexes 2 and 4 these features are less strong. The strong band at 370 nm could be attributed to a ligand-to-ligand transition, similar to that found in polymeric (HgI2)2(4,40 -bipy) [42]. In order to evaluate the electronic transitions in the solid state, we examined the total density of states (DOS), derived from the band structure of the crystalline solid (obtained from a single point energy computation of the experimental geometry). In Fig. 9 we report the DOS and band structure of complexes 1 and 3, computed in the first Brillouin zone path (described in Fig. S11). In 1, the electron transitions from the top valence band to the bottom conduction band involve mainly C and N atoms, and are therefore interpretable as p ? p⁄ and n ? p⁄. For the complex 3, the DOS analysis highlights that the top valence band has a main contribution from iodine atoms, and the bottom conduction band involves mainly the nitrogen atoms. The electron transitions from these bands in crystalline 3 are therefore attributable to inter ligand excitations and can explain the strong absorption at 370 nm. All of complexes 1–5 are luminescent, and by excitation at 315 nm show two emissions at ca. 460 and 560 nm (Fig. S12). The bpym ligand has an emission at 574 nm, with a relevant stock shift with respect to the emission found in solution [43], where a fluorescence emission has been found at 350 nm on exciting at 320 nm. The stock shift could be explained by the relevant p–p stacking interactions in the crystalline state. The emission in the complexes can be attributed to the n ? p⁄ transition, and the scarce variability in the wavelength is imputable to the modest perturbation induced in the bpym ligand as a consequence of the coordination. The photoluminescence properties of d10 Hg(II) complexes are usually attributed to ligand centered transitions [44], and Hg(II) acts mainly as a stabilizer of the ligand, reducing the non-radiative decay.

4. Discussion

Fig. 8. UV–Vis reflectance spectra of bpym and complexes 1–5.

We prepared five bpym derivatives of HgX2 with a differing synthetic approach to that used by Lanza and Jarabat [5]. They obtained the products in the form of a precipitate by mixing solutions of the reagents, whereas we used the ability of acetonitrile to dissolve, by heating and mixing, the precipitate that was obtained. From the orange solution formed, it was easy to grow single crystals by evaporation. Compound 1 is a 1D polymer obtained by insertion of bpym ligands in two columns formed by the polymerization of (HgCl2)2 units. The coordination of the bpym ligand to HgCl2 induces some geometrical changes: an elongation of the Hg–Cl bond and a bending of the Cl–Hg–Cl angle, while the geometrical parameters of the bpym ligand are predominantly unchanged. This is in accord with the modest vibrational variation of the ring modes in the coordinated bpym ligand with our computational model, which shows a very low Hg–N bond order. The weakness of the bpym–Hg interactions is a consequence of the low Lewis basicity of bpym. The mean Hg–N distance is 2.596 Å; a comparison with RR0 bpymHgCl2 [45], with a mean Hg–N distance of 2.316(8) Å and with bipyHgCl2 [46] where d(Hg–N) = 2.393(1) Å, shows a stronger interaction in these latter complexes. However, there is evidence of charge transfer from the organic ligand to the mercury center, both from an evaluation of the infrared intensities of the C–H stretching modes and from the computational model. However, the aromatic system

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33

Fig. 9. Band structure and DOS of complex 1 (top) and 3 (bottom).

does not seem significantly perturbed, as suggested by the small variations observed in the electronic transitions involving p molecular orbitals. The polymeric chain of HgCl2 retains the geometry arrangement of the crystalline Hg(II) chloride, with rows of slipped HgCl2 units (see Fig. 2). This disposition has been explained in terms of electrostatic interactions [46]. Seminal work of Kaupp and Schnering [21] showed the importance of the relativistic effect in the halogen–mercury bond and its influence in the reduction of electrostatic interactions in Hg–X bonds, and showed that the C2h arrangement of (HgCl2)2 units is a minimum. They also computed the dimerization energy for HgCl2 and found a modest value of 24.3 kJ/mol (MP2, counterpoise corrected); this value underlines the weak intermolecular interactions in crystalline HgCl2, as also evidenced by the weak sublimation enthalpy of 77.4 kJ/mol [21]. The (HgCl2)2 fragment has a bent Cl–Hg–Cl angle, both in the crystalline state and in the computed model. The bending of the X–Hg– X angle depends on intermolecular interactions of the Hg atom [47] and the modest effect of intermolecular HgCl interactions (the NBO charge of Hg varies from 0.922 in HgCl2 to 0.925 in (HgCl2)2) could explicate the small variation of the Cl–Hg–Cl angle. In Fig. S13 the distribution of the experimental distances and angles obtained from the CSD database, on searching for the lozenge shaped (HgCl2)2 dimer, can be seen. The histograms show

a maximum at 3.250 Å for Hg  Cl, 88° for Cl–Hg–Cl and 95° for Hg–Cl–Hg. These data are in good agreement with the DFT results reported before. However, the tendency of this salt to form inorganic chains, in spite of the weakness of the involved interactions, could explain the ladder form of this 1D coordination polymer. This topology is rare in the chemistry of the bpym ligand [23], and the constraint induced by the HgCl2 polymerization could make the p– p stacking interactions favorable. Compound 2 has a crystalline architecture substantially analogous to 1, with minor differences due to the bigger dimensions of the bromine atom with respect to the chlorine atom. The reported structures are not enough to allow a statistical analysis and a minor tendency to build up coordination polymers could be noticed. A different behavior is found in complex 3: it is describable as a ribbon of alternating (HgI2)2 dimers in a quasi-C2h symmetry and bpym units. The (HgI2)2 fragment is not found in crystalline polymorphs of HgI2, where tetrahedral and octahedral iodine coordination are preferred, and is reported in some coordination polymers of pyrazine derivatives [24,48]. A search of the CSD database for the lozenge shaped (HgI2)2 fragment point out a lesser tendency of HgI2 to generate this topology and to form polymeric chains, and with respect to HgCl2 and HgBr2 there is the presence of stronger Hg  I interactions (Fig. S14).

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Table 3 Polymeric structures obtained from HgX2 units and weak halogen–mercury bridging interactions (R(Hg. . .X) < sum of VdW radii) reported in the CSD (2015) database. Most coordination polymers containing the HgI2 unit propagate through the organic ligand. Fragment

Total structures

Polymers

Weak bridging non-bonded interactions

HgCl2 HgBr2 HgI2

1052 489 586

325 (31%) 141 (29%) 160 (27%)

195 (19%) 44 (9%) 14 (3%)

From a statistical analysis of the structures reported in the CSD database (Table 3), there seems to be an indication of general behavior: (1) on moving from chlorine to iodine there is a minor tendency to form coordination polymers; (2) at the same time it is possible to notice a decrease of the tendency to generate a lozenge shaped topology based on weak non-bonding interactions that expand the effective coordination number of the Hg(II) ion. This affects the topology of coordination polymers 1, 2 and 3: 1 and 2 have a ladder-like chain expansion, while 3 shows a ribbon type of polymerization with a lack of p–p interactions between the aromatic rings. A few compounds with Hg(CN)2 coordinated to other N-ligands have been reported, and only four are with heterocyclic compounds [29,49]. The interaction of Hg(CN)2 with bpym in 4 does not induce strong bond variations, apart from the bending of the C–Hg–C angle and a modest lengthening of the C–Hg and C–N bonds, (responsible for the red-shift of m(CN) from 2197 to 2177 cm1 [34]). The N–Hg mean distance of 2.64 Å is higher than that in the 4,40 -bipyridine adduct [42] (2.449(5) Å), and this confirms the weaker Lewis base behaviour of the bpym ligand. The geometric requirement of the CN ligand to form a bridging connection with a quasi-linear arrangement of M–CN–M and the steric hindrance of the (bpym)Hg(CN)2 unit probably make the formation of a polymeric structure with the cyanide ligand less favourite with respect to the halogens. The crystal structure of compound 5 confirms the well documented [50] tendency of the SCN ligand to form coordination polymers with Hg(II) ions, and this is made easier by the stronger ambidentate nature of SCN with respect to the other anions investigated in this study. This is evidenced by TGA data, which reports a higher decomposition temperature of 5 than found for 1–4. However, bidentate ancillary ligands with Hg(SCN)2 have been seldom reported, and usually coordination polymers of Hg(SCN)2 present 1D and 2D architectures. In the case of the complex with the 2,20 -bipyrimidine ligand, a 3D structure is formed, where the inorganic framework is maintained and the organic ligand acts as a ‘‘propagator” that allows connections with other chains. This crystalline architecture is representative of the importance of the selective choice of organic ligand to lead to the required dimensionality of the final product. 5. Conclusions In this work we have studied the crystal engineering versatility of a k-N bidentate organic ligand, 2,20 -bipirymidine, with some typical d10 building blocks, like halides and pseudohalides of Hg (II). We have performed a structural characterization of the complexes by means of single crystal XRD methods, computational, spectroscopic and thermal analysis, with special attention to the analysis of the subtle compensation of forces typical in the chemistry of mercury complexes. The overall results are quite interesting for a deeper comprehension of the behaviour of the bpym ligand and for the comprehension of the strength of the interactions in organometallic derivatives of mercury salts. It is possible

to deduce that in the reported complexes the interactions between the 2,20 -bipyrimidine ligand and Hg(II) ion are quite weak, as demonstrated by the minor structural effects on metal coordination and by the vibrational spectra. The thermal behaviour of the complexes shows stronger interactions in the thiocyanate complex, the only example of a clearly bidentate anion, and weaker forces in the other compounds. On the other hand, it is clear that the tendency of the halide anions to bridge and to form weakbonded dimers is a strong driving force in the formation of the coordination polymers and is responsible for the major structural variations in the coordination sphere of the mercury atoms. We were able to rationalize the behaviours of the different halides through a computational and statistical analysis of literature data. With a heavier halide we noticed a lesser tendency to form coordination polymers. The role of the bidentate ligand to maximise the dimensionality of the supramolecular structure is relevant, and permits the formation of 1D and 3D coordination polymers. The electronic and photoluminescence properties of the bpym complexes have been investigated and analysed using molecular and solid state periodic computational modelling. This approach shows the increasing contribution of the halide to the main electron excitation on passing from chlorine to iodine, and it can be noticed that the insulated molecule modelling is not always the optimal choice to interpret solid state photoluminescence properties. A knowledge of the band structure and exciton’s transmission through p– p stacking and are fundamentals for the correct interpretation of the electronic transition in organometallic compounds analysed in the solid state, especially for coordination compounds that do not have intrinsically a molecular 0D-nature. Acknowledgements The authors are grateful to prof. Giuliana Gervasio for her precious aid in the crystal structure determination and for her suggestions. Appendix A. Supplementary data CCDC 1028414–1028417, 1412102 contains the supplementary crystallographic data for the compounds 1, 3, 4, 5 and 2. These data can be obtained free of charge via http://www.ccdc.cam.ac. uk/conts/retrieving.html, or from the Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: (+44) 1223-336-033; or e-mail: [email protected]. Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.poly.2015.10.059. References [1] (a) S. Batten, R. Robson, Angew. Chem., Int. Ed. 37 (1998) 1460; (b) J.W. Steed, J.L. Atwood, Supramolecular Chemistry, VCH, New York, 2000; (c) G.F. Sweigers, T.J. Malefeste, Chem. Rev. 100 (2000) 3483; (d) S. Leinninger, B. Olenyuk, P.J. Stang, Chem. Rev. 100 (2000) 853; (e) M.D. Ward, J.A. McCleverty, J.C. Jeffery, Coord. Chem. Rev. 222 (2000) 251; (f) C. Kaes, A. Katz, M.W. Hosseini, Chem. Rev. 100 (2000) 3553; (g) B.J. Holliday, C.A. Mirkin, Angew. Chem., Int. Ed. 4 (2001) 2022; (h) A.N. Khlobystov, A.J. Blake, N.R. Champness, D.A. Lemenovskii, A.G. Majouga, N.V. Zyk, M. Schröder, Coord. Chem. Rev. 222 (2001) 155; (i) L. Carlucci, G. Ciani, D.M. Proserpio, Coord. Chem. Rev. 246 (2003) 247; (j) A.M. Beatty, Coord. Chem. Rev. 246 (2003) 131; (k) H.W. Roesky, M. Andruh, Coord. Chem. Rev. 236 (2003) 91; (l) O. Mamula, A. Von Zelewsky, Coord. Chem. Rev. 242 (2003) 87; (m) S.L. James, Chem. Soc. Rev. (2003) 276. [2] (a) H. Schmidbaur, Angew. Chem. Int., Ed. Engl. 15 (1976) 728; (b) H. Schmidbaur, W. Graf, G. Müller, Angew. Chem. Int., Ed. Engl. 27 (1988) 417; (c) A. Kolb, P. Bissiier, H. Schmidbaur, Inorg. Chem. 32 (1993) 5132; (d) R.L. White-Morris, M.M. Olmstead, F. Jiang, D.S. Tinti, A.L. Balch, J. Am. Chem. Soc. 124 (2000) 2327; (e) S.P. Yang, X.M. Chen, L.N. Ji, J. Chem. Soc., Dalton Trans. (2000) 2337; (f) W.J. Hunks, M.C. Jennings, R.J. Puddephatt, Inorg. Chem. 39 (2000) 2699;

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