or dimensionality

Polyhedron 93 (2015) 46–54

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Polyhedron journal homepage: www.elsevier.com/locate/poly

Mercury (II) coordination complexes bearing Schiff base ligands: What affects their nuclearity and/or dimensionality Ghodrat Mahmoudi a,⇑, Masoumeh Servati Gargari a, Farhad Akbari Afkhami a, Christos Lampropoulos b,⇑, Marjan Abedi c, Sergio A. Corrales b, Ali Akbar Khandar d, Joel Mague e, Don Van Derveer f, Barindra Kumar Ghosh g,⇑, Asad Masummi a a

Young Researchers And Elite Club, Tabriz Branch, Islamic Azad University, Tabriz, Iran Department of Chemistry, University of North Florida, Jacksonville, FL 32224, USA Department of Chemistry, Faculty of Science, University of Mohaghegh Ardabili, P.O. Box 56199-11367, Ardabil, Iran d Department of Inorganic Chemistry, Faculty of Chemistry, University of Tabriz, P.O. Box 5166616471, Tabriz, Iran e Department of Chemistry, Tulane University, New Orleans, LA 70118, USA f Department of Chemistry, Clemson University, Clemson, SC 29634-0973, USA g Department of Chemistry, The University of Burdwan, Burdwan 713 104, India b c

a r t i c l e

i n f o

Article history: Received 18 February 2015 Accepted 30 March 2015 Available online 3 April 2015 Keywords: Coordination polymer Mercury(II) halide/pseudohalide Tetradentate Schiff base Synthesis X-ray structure

a b s t r a c t A series of seven new mercury (II) Schiff base compounds were synthesized and characterized using single-crystal X-ray crystallography and other spectroscopic and physical methods. The complexes feature the ligands L1 and L2 [L1 = (N,N-bis(pyridine-2-yl)benzylidene)-2,20 -dimethylpropane-1,3-diamine; L2 = (N,N-bis(pyridine-2-yl)benzylidene)butane-1,4-diamine], and are either dinuclear, namely complex [Hg(L1)(l-Cl)HgCl3] (1), mononuclear, namely [Hg(L1)Br2] (2), [Hg(L1)I2] (3), [Hg(L1)(SCN)2] (4), polynuclear polymeric, namely [Hg8(l-L2)2(l-Cl)6(l3-Cl)4Cl4]n (5), [Hg2(L2)Br4]n (6) or molecular dimeric, namely [Hg2(L2)I4] (7). The syntheses of these complexes were achieved using mild solvothermal conditions, in a locally-developed apparatus. The microanalyses and spectroscopic results corroborate the structural data from single crystal X-ray diffraction; the latter proved the versatility of Hg(II) coordination chemistry, since simple variation of the Schiff base chelate or the halide/pseudohalide counterion resulted in distinctively different complexes. In the dinuclear compound 1, the asymmetric unit contains two different mercury(II) centers with tetrahedral and square pyramidal geometries, whereas in 2–4 the metal ion adopts a distorted octahedral geometry. The coordination polymers 5 and 6 have 3D and 1D structures, respectively, in which each mercury(II) center is five-coordinate with a square pyramidal geometry, whereas both Hg(II) centers in 7 are tetrahedral. Ó 2015 Elsevier Ltd. All rights reserved.

1. Introduction The chemistry of mercury and its compounds has attracted the interest of both the chemistry and chemical engineering communities over the years, due to its potential applications in the paper industry, as a preservative, in fluorescent lamps, in cosmetics, paints, in sensors, as well as for mercury batteries [1–5]. The versatility of this chemistry is evident from the ample variations in the coordination environment of Hg(II), and the formation of a plethora of coordination frameworks [6–16]. A number of these complexes have been synthesized using a variety of organic

⇑ Corresponding authors. E-mail addresses: [email protected] (G. Mahmoudi), [email protected] (C. Lampropoulos), [email protected] (B.K. Ghosh). http://dx.doi.org/10.1016/j.poly.2015.03.035 0277-5387/Ó 2015 Elsevier Ltd. All rights reserved.

ligands, in the presence of ancillary units, either inorganic or organic. The resulting compounds often exhibit varied nuclearities; thus, ligands with different denticities and/or linking properties have been instrumental in the isolation of a number of mono-, di-, oligo- and poly-nuclear complexes and coordination networks, including supramolecular aggregates based on weak attractive non-covalent forces, i.e., hydrogen bonds, and p–p stacking interactions. It is the subtle steric and/or electronic control of the frameworks which motivated the present study of azines [17–19] as organic ligands in Hg(II) chemistry. Schiff-type ligands have proven to be particularly versatile, since they form strong coordination bonds and allow for a variety of weak, multiple and lateral non-covalent interactions [20–22]. Different halides and pseudohalides are also added as terminal and/or bridging units, in reactions of mercury salts and organic

G. Mahmoudi et al. / Polyhedron 93 (2015) 46–54

azines; they are meant to either complete the coordination sphere of the metal ion, or to induce bridging between complexes into polymeric networks. Subtle variations in the reaction mixtures, such as the identity or stoichiometric amounts of the halide/pseudohalide ancillary ligand and or the steric/electronic characteristic of the azine, have lead to very different products in the past, including complexes of not only different nuclearities, but also dimensional polymers (supramolecular or covalently-bound 1D chains, and 2D, or 3D networks) [23–33]. Recently, we reported syntheses and X-ray structures of a number of mercury(II) compounds of various nuclearities and architectures [21,34,35], from the use of N,N0 -(bis-(pyridin-2-yl)benzylidene)-1,2-ethanediamine. To extend this work and to investigate the effects of the organic ligand on the resulting coordination complexes, two structurally related ligands (Scheme 1) have been synthesized and used in Hg(II) chemistry. As per our previous interest in investigating the influence of anions on the structures of Hg(II) coordination complexes and polymers, the syntheses, structures, spectroscopic and photophysical properties of seven new such compounds are reported herein.

2. Experimental 2.1. Materials The solvents and reagents used in this work were obtained from commercial sources and were used as received. Caution! Mercury and its compounds are toxic [36]. Only a small amount of these materials should be prepared and handled with care. 2.2. Physical measurements FT-IR spectra were recorded on a Bruker Tensor 27 FT-IR spectrometer. Microanalyses were performed using a Heraeus CHNO-Rapid analyzer. Melting points were measured on an Electrothermal 9100 apparatus and are uncorrected. 2.3. X-ray crystallography Relevant data on the collection and structure solutions are summarized in Table 1. Single crystals of 1–3 and 5–7 suitable for X-ray analyses were selected and crystallographic data were collected on a Bruker AXS SMART APEX CCD diffractometer using Mo Ka radia0

tion (k = 0.71073 Å A) in the x-scan mode. The detector frames were integrated by use of the program SAINT [37] and numerical absorption corrections were performed using the SADABS program [38]. Intensity data for 4 were collected using a Rigaku Mercury CCD detector and an AFC8S diffractometer. Data reduction including the application of Lp and absorption corrections used the CrystalClear program [39]. All the structures were solved by direct methods and refined by full matrix least-squares procedures using SHELXL [40]. All non-hydrogen atoms were refined with anisotropic displacement parameters and hydrogen atoms were placed in calculated positions when possible and included as riding

N

N

L

N

N

N

N

CH3 CH3

L1

N

N

N

N

N

L2

Scheme 1. The chemical structure of L, L1 and L2.

N

47

contributions with isotropic U values 1.2 times that of the atom to which they are bonded. 2.4. Syntheses The locally designed apparatus, in which complexes 1–7 were synthesized, has been extensively described in our previous work [41]. All reported yields are based on the Hg2+ starting material. 2.4.1. Ligands (N,N-Bis(pyridine-2-yl)benzylidene)-2,20 -dimethylpropane1,3-diamine (L1) is a new ligand; it was prepared from the condensation of 1:2 molar ratio of 2,2-dimethylpropylenediamine and 2-benzoylpyridine. L1: Anal. Calc. for C29H28N4: C, 80.51; H, 6.53; N, 12.95. Found: C, 80.54; H, 6.48; N, 12.98%. IR (KBr cm1) 698, 992, 1151, 1239, 1427, 1489, 1580, 1625, 3000, 3053. The synthetic details are the same as for the previously used (N,N-bis(pyridine-2-yl)benzylidene)butane-1,4-diamine (L2) ligand. The latter was prepared again by condensation of 1:2 molar ratio of 1,4diaminobutane and 2-benzoylpyridine [35]. 2.4.2. Synthesis of [Hg(L1)(l-Cl)HgCl3] (1) HgCl2 (0.5 mmol) and (N,N-bis(pyridine-2-yl)benzylidene)-2,20 dimethylpropane-1,3-diamine (L1) (0.5 mmol) were placed in the main arm of a branched tube. Methanol was carefully added to fill the arms. The tube was sealed and immersed in an oil bath at 60 °C while the branched arm was kept at ambient temperature. After 2 days, crystals of 1 formed in the cooler arm, and were filtered off, washed with acetone and ether, and dried in air. Yield: 87%. m.p. 225 °C. Anal. Calc. for C29H28Cl4Hg2N4: C, 35.70; H, 2.89; N, 5.74. Found: C, 35.65; H, 2.77; N, 5.65%. FTIR (cm1) selected bands: 708(s); 771(m); 1011(m); 1145(s); 1313(m); 1434(m); 1579(s); 1568(m); 1622(m); 2929(w); 2960(w); 3051(w). 2.4.3. Synthesis of [Hg(L1)Br2] (2) HgBr2 (0.5 mmol) and (N,N-bis(pyridine-2-yl)benzylidene)2,20 -dimethylpropane-1,3-diamine (L1) (0.5 mmol) were placed in the main arm of a branched tube. Methanol was carefully added to fill the arms. The tube was sealed and immersed in an oil bath at 60 °C while the branched arm was kept at ambient temperature. After 2 days, crystals of 2 that formed in the cooler arm, were filtered off, washed with acetone and ether, and dried in air. Yield: 83%. m.p. 208 °C. Anal. Calc. for C29H28Br2HgN4: C, 43.93; H, 3.56; N, 7.07. Found: C, 43.85; H, 3.37; N, 7.25%. FTIR (cm1) selected bands: 702(s); 796(s); 996(m); 1069(m); 1249(m); 1308(s); 1429(s); 1568(s); 1634(s); 2922(w); 2964(w); 3052(w). 2.4.4. Synthesis of [Hg(L1)I2] (3) HgI2 (0.5 mmol) and (N,N-bis(pyridine-2-yl)benzylidene)-2,20 dimethylpropane-1,3-diamine (L1) (0.5 mmol) were placed in the main arm of a branched tube. Methanol was carefully added to fill the arms. The tube was sealed and immersed in an oil bath at 60 °C while the branched arm was kept at ambient temperature. After 2 days, crystals of 3 that formed in the cooler arm, were filtered off, washed with acetone and ether, and dried in air. Yield: 91%. m.p. 182 °C. Anal. Calc. for C29H28HgI2N4: C, 39.27; H, 3.18; N, 6.32. Found: C, 39.25; H, 3.37; N, 6.65%. FTIR (cm1) selected bands: 701(s); 794(m); 994(m); 1248(m); 1307(m); 1428(m); 1567(s); 1627(s); 2921(w); 2967(w); 3050(w). 2.4.5. Synthesis of [Hg(L1)(SCN)2] (4) Hg(SCN)2 (0.5 mmol) and (N,N-bis(pyridine-2-yl)benzylidene)2,20 -dimethylpropane-1,3-diamine (L1) (0.5 mmol) were placed in the main arm of a branched tube. Methanol was carefully added to fill the arms. The tube was sealed and immersed in an oil bath at 60 °C while the branched arm was kept at ambient temperature.

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G. Mahmoudi et al. / Polyhedron 93 (2015) 46–54

Table 1 Crystallographic data and refinement parameters for 1–7.

Formula F.W. T (K) Crystal System Space group a (Å) b (Å) c (Å) a (°) b (°) c (°) Volume (Å3) Z Dcalc (Mg/m3) Radiation (k, Å) Independent reflections R1 wR2

1

2

3

4

5

6

7

C29H28Cl4Hg2N4 975.53 100 (2) monoclinic P21/c 15.615(2) 9.884(1) 20.136(3) 90 100.623(2) 90 3054.3(7) 4 2.121 0.71073 7991 0.0246 0.0545

C29H28Br2HgN4 792.96 100 (2) monoclinic C2/c 21.447(2) 12.1635(13) 10.7639(11) 90.00 97.8250(10) 90.00 2781.8(5) 4 1.893 0.71073 4208 0.0405 0.0846

C29H28HgI2N4 886.94 100 (2) monoclinic C2/c 21.583(2) 12.5873(13) 10.7347(12) 90.00 98.681(2) 90.00 2883.0(5) 4 2.043 0.71073 4240 0.0337 0.0594

C31H28HgN6S2 749.30 173 (2) triclinic  P1

C28H26Cl8Hg4N4 1504.49 100 (2) monoclinic P21/n 16.533(2) 10.0741(14) 21.535(3) 90.00 96.023(2) 90.00 3567.1(9) 4 2.801 0.71073 10 553 0.0517 0.0849

C28H26Br4Hg2N4 1139.31 100 (2) monoclinic P21/c 16.3375(16) 9.1644(9) 20.670(2) 90.00 95.8940(10) 90.00 3078.5(5) 4 2.458 0.71073 9259 0.0513 0.0865

C28H26Hg2I4N4.2.86O 1373.07 100 (2) monoclinic P21/n 12.2888(13) 23.061(3) 14.3057(15) 90.00 111.5730(14) 90.00 3770.2(7) 4 2.419 0.71073 12 460 0.0643 0.1415

11.0078(12) 12.2732(3) 12.5120(7) 67.857(12) 80.84(2) 69.816(14) 1468.73(18) 2 1.705 0.71073 5404 0.0300 0.0584

P P R1 = [ ||Fo|  |Fc||]/ |Fo| (based on F). P P wR2 = [[ w(|F2o  F2c |)2]/[ w(F2o)2]]1/2 (based on F2).

After 2 days, crystals of 4 that formed in the cooler arm, were filtered off, washed with acetone and ether, and dried in air. Yield: 75%. m. p. 194 °C. Anal. Calc. for C31H28HgN6S2: C, 49.69; H, 3.77; N, 8.56. Found: C, 49.55; H, 3.47; N, 8.65%. FTIR (cm1) selected bands: 708(s); 774(m); 899(m); 1249(m); 1464(m); 1570(m); 1624(m); 2112(s); 2925(w); 2955(w); 3060(w).

bands: 699(s); 768(m); 1001(m); 1251(m); 1310(s); 1433(s); 1618(m); 2844(m); 2914(m); 3053(m).

2.4.6. Synthesis of [Hg8(l-L2)2(l-Cl)6(l3-Cl)4Cl4]n (5) HgCl2 (0.5 mmol) and (N,N-bis(pyridine-2-yl)benzylidene)butane-1,4-diamine (L2) (0.5 mmol) were placed in the main arm of a branched tube. Methanol was carefully added to fill the arms. The tube was sealed and immersed in an oil bath at 60 °C while the branched arm was kept at ambient temperature. After 2 days, crystals of 5 that formed in the cooler arm, were filtered off, washed with acetone and ether, and dried in air. Yield: 94%. m.p. 260 °C. Anal. Calc. for C28H26Cl8Hg4N4: C, 22.35; H, 1.74; N, 3.72. Found: C, 22.25; H, 1.67; N, 3.57%. FTIR (cm1) selected bands: 703(s); 773(m); 1014(m); 1254(m); 1436(s); 1587(m); 1632(s); 2852(w); 29 290(w); 3070(w).

Ligands L1 and L2 were synthesized by refluxing 2,20 -dimethylpropane-1,3-diamine and 1,4-diaminobutane, respectively with two equivalents of 2-benzoylpyridine in absolute ethanol. The yields were almost quantitative in all cases. One-pot syntheses using a 1:1 molar ratio of mercury(II) salts and ligands L1 or L2 in MeOH afforded the mono-, di- and polynuclear mercury(II) compounds 1–7 in good yields. These syntheses were performed under mild solvothermal conditions in a locally-developed glassware apparatus, involving a branched tube, where the main glass chamber is immersed in a 60 °C oil bath, while the branched part of the tube is outside the bath at ambient temperature; more details about this synthetic/crystallization apparatus are presented elsewhere [41]. The new complexes were characterized using microanalytical (C, H and N), spectroscopic and other physicochemical studies. The microanalytical data support the formulas given for 1–7. The compounds melt in the 164–260 °C temperature range, as evident from melting point measurements and TGA data; no other features were present in the TGA/DTA and as such they are not reported. In the IR spectra, the organic ligands bound to the metal exhibit m(C@N) + m(C@C) stretching vibrations [42] in the range 1650–1560 cm1. The mas(N@C@S) and m(CAS) stretches of S-coordinated thiocyanate [27] appear as strong bands respectively at 2112 and 774 cm1 in 4. Additionally, a band pertaining to the deformation frequency d(NCS) is found at 456 cm1. All other organic ligand vibrations are seen in the range 1600– 600 cm1.

2.4.7. Synthesis of [Hg2(l-L2)(l-Br)2Br2]n (6) HgBr2 (0.5 mmol) and (N,N-bis(pyridine-2-yl)benzylidene)butane-1,4-diamine (L2) (0.5 mmol) were placed in the main arm of a branched tube. Methanol was carefully added to fill the arms. The tube was sealed and immersed in an oil bath at 60 °C while the branched arm was kept at ambient temperature. After 2 days, crystals of 6 that formed in the cooler arm, were filtered off, washed with acetone and ether, and dried in air. Yield: 78%. m.p. 220 °C. Anal. Calc. for C28H26Br4Hg2N4: C, 29.52; H, 2.30; N, 4.92. Found: C, 29.46; H, 2.33; N, 4.62%. FTIR (cm1) selected bands: 702(s); 793(m); 1009(m); 1251(s); 1311(s); 1434(s); 1582(s); 1622(s); 2853(w); 2924(m); 3060(w). 2.4.8. Synthesis of [Hg2(L2)I4] (7) HgI2 (0.5 mmol) and (N,N-bis(pyridine-2-yl)benzylidene)butane-1,4-diamine (L2) (0.5 mmol) were placed in the main arm of a branched tube. Methanol was carefully added to fill the arms. The tube was sealed and immersed in an oil bath at 60 °C while the branched arm was kept at ambient temperature. After 2 days, crystals of 7 that formed in the cooler arm, were filtered off, washed with acetone and ether, and dried in air. Yield: 80%. m.p. 164 °C. Anal. Calc. for C28H26Hg2I4N4 2.86O: C, 24.74; H, 1.93; N, 4.12. Found: C, 24.69; H, 1.87; N, 4.35%. FTIR (cm1) selected

3. Results and discussion 3.1. Synthesis and spectroscopic results

3.2. Structural studies for 1–7 3.2.1. [Hg(L1)(l-Cl)HgCl3] (1) The molecular structure of 1 is given in Fig. 1. Relevant bond distances and angles may be found in Table 2. The structure can be described as units of l-Cl-connected square pyramidal (sp) and tetrahedral Hg centers, judged from their tau parameters as defined by Addison et al. [43] [s5 = 0.11 for Hg(1) and s4 = 0.80 for Hg(2)]. Three of the four Cl ions around Hg(2) act as terminal

G. Mahmoudi et al. / Polyhedron 93 (2015) 46–54

49

Fig. 1. Partially labeled X-ray crystal structure of complex 1. Color code: HgII purple, Cl green, N blue, C grey. (Colour online.)

ligands (Cl2, Cl3, and Cl4) with Hg–Cl distances: 2.381(1), 2.409(1) and 2.464(1) Å; the fourth one (Cl1) bridges [Hg–Cl distance: 2.851(1) Å], while the Hg1–Cl1–Hg2 bridging angle measures 93.65(3)°. The sp Hg/N coordination sites have bond lengths comparable to literature values [34]: the HgN (imine) [2.330(3), 2.333(3) Å] and Hg–N(pyridyl) [2.378(3), 2.436(3) Å]. Hg(1), two imine N atoms and the two pyridine N atoms lie out of the mean planes defined by N(1)N(2)Hg(1)N(3)N(4) by 1.003, 0.047 and 0.041 Å, respectively towards the bridging halide (Cl1). The Hg. . .Hg distance bridged by the l-Cl anion is 3.864 Å.

3.2.2. [Hg(L1)Br2] (2), [Hg(L1)I2] (3) and [Hg(L1)(SCN)2] (4) ORTEP representations of mononuclear species [Hg(L1)Br2] (2), and [Hg(L1)(SCN)2] (4) are shown in Fig. 2. Complex 3 is nearly isostructural to 2, and is not illustrated. Selected bond distances and angles relevant to the Hg(II) coordination spheres in 2, 3, and 4 can be found in Table 3. In each case, the metal ion is octahedrally coordinated, with the coordination sites occupied by (a) two halide atoms (for 2 and 3), or pseudohalide groups (in the case of 4), and (b) the nitrogen atoms of the chelating ligand. The octahedral coordination sphere of the Hg(II) ion is particularly strained for all three complexes, exhibiting angles ranging between 62.6(1) and 169.1(1) degrees for 2, 61.0(1)–171.7(1) degrees for 3, and 62.9(1)–167.0(1) degrees for 4. The ligand L1, upon coordination, orients the two pyridyl N atoms across from each other (trans), which forces the halides (in 2 and 3) or thiocyanates (in 4) to be in a strained cis disposition. The HgNimine distances found in 2 (2.543(3) Å, 3 2.566(3) Å), and 4 (2.540(3) Å/2.533(3) Å) are

Table 2 Selected bond lengths (Å) and bond angles (°) for [(L)Hg(l-Cl)HgCl3] (1). Bond lengths Hg1–N1 Hg1–N2 Hg1–N3 Hg1–N4 Hg1–Cl1

2.378(3) 2.333(3) 2.330(3) 2.436(3) 2.434(1)

Bond angles N1–Hg1–N2 N1–Hg1–N3 N1–Hg1–N4 N1–Hg1–Cl1 N2–Hg1–N3 N2–Hg1–N4 N2–Hg1–Cl1 N3–Hg1–N4 N3–Hg1–Cl1

70.77(10) 131.42(10) 95.41(11) 105.28(8) 82.08(10) 127.21(10) 138.33(7) 69.83(10) 121.33(7)

Hg2–Cl1 Hg2–Cl2 Hg2–Cl3 Hg2–Cl4

N4–Hg1–Cl1 Cl1–Hg2–Cl2 Cl1–Hg2–Cl3 Cl1–Hg2–Cl4 Cl2–Hg2–Cl3 Cl2–Hg2–Cl4 Cl3–Hg2–Cl4 Hg1–Cl1–Hg2

2.851(1) 2.464(1) 2.381(1) 2.409(1)

94.27(8) 88.53(3) 92.58(3) 112.99(3) 118.17(4) 106.99(4) 128.41(3) 93.65(3)

Fig. 2. (A) Partially labeled X-ray crystal structure of complex 2; (B) Partially labeled X-ray crystal structure of complex 4. Color code: HgII purple, Br aqua, S yellow, N blue, C grey. (Colour online.)

comparable to corresponding literature values from our previous work [34]; for a compound structurally related to 2 the Hg– Npyridine distances were 2.685(4) Å on average, for a complex resembling 3 they averaged 2.790(3) Å, while the respective values for a compound similar to 4 were 2.684(3) Å/2.670(3) Å. Furthermore, the Hg–X (X = Br, I, SCN) distances, namely 2.5336(4), 2.6844(3) and 2.4650(11)/2.4707(11) Å for 2, 3, and 4, respectively, are comparable to the mean value of the corresponding literature values [34]. The metric parameters of the Hg(L1)Br2] (2) and [Hg(L1)I2] (3) complexes show that a twofold axis bisects the complex, passing through the C13C130 bond and the Hg atom. 3.2.3. [Hg8(l-L2)2(l-Cl)6(l3-Cl)4Cl4]n (5) ORTEP representations of 5 are presented in Fig. 3, while selected bond distances and angles relevant to the metal coordination spheres can be found in Table 4. Complex 5 is a neutral threedimensional coordination polymer with a crystallographically-imposed inversion center. The asymmetric unit consists of two square pyramidal HgII ions (Hg1 and Hg2 with an HgN2Cl3 chromophore), which are chelated on each side of L2, via the ligand’s pyridyl (N1 and N4) and imine nitrogens (N2 and N3); there are also two more HgII ions (Hg3 and Hg4), which are bridged to Hg2 and Hg1, respectively, via l-Cl (Cl2 bridges Hg1 and Hg4, whereas Cl6 bridges Hg2 and Hg3). The metal ions Hg3 and Hg4 are also five-coordinate with a distorted square pyramidal geometry, with an HgCl5 chromophore (see Table 4). The structure of 5 can be better described as a dimer of [Hg4(l3Cl)2(l-L2)2(l-Cl)2Cl2] subunits, with dimerization being facilitated via two l3-Cl (Cl1 and Cl5), and two l-Cl (Cl5, and Cl8) bridges.

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G. Mahmoudi et al. / Polyhedron 93 (2015) 46–54

Table 3 Selected bond lengths (Å) and bond angles(°) for [Hg(L1)Br2] (2), [Hg(L1)I2] (3) and [Hg(L1)(SCN)2] (4). [Hg(L1)Br2] (2) Bond lengths Hg1–N2 Hg1–N20 Hg1–N1 Hg1–N10 Hg1–X Hg1–X0 Bond angles N1–Hg1–N10 N1–Hg1–N2 N1–Hg1–N20 N1–Hg1–X N1–Hg1–X0 N10 –Hg1–N2 N10 –Hg1–N20 N10 –Hg1–X0 N10 –Hg1–X N2–Hg1–N20 N2–Hg1–X0 N2–Hg1–X N20 –Hg1–X0 N20 –Hg1–X X–Hg1–X0

2.543(3) 2.543(3) 2.685(4) 2.685(4) 2.534(1) 2.534(1) 169.12(2) 62.58(1) 127.75(1) 87.16(7) 90.20(7) 127.76(1) 62.58(1) 90.20(7) 87.17(7) 73.72(2) 115.79(7) 87.27(7) 87.28(7) 115.79(7) 151.97(2)

[Hg(L1)I2] (3) 2.566(3) 2.566(3) 2.790(3) 2.790(3) 2.684(1) 2.684(1) 171.72(7) 60.96(7) 126.89(7) 86.80(6) 91.07(6) 126.89(7) 60.96(7) 91.07(6) 86.80(6) 74.55(8) 116.17(5) 88.20(5) 88.20(5) 116.17(5) 150.17(1)

the coordination sphere of the metal ions (Cl3 and Cl7 on Hg4 and Hg3, respectively), which in turn does not perturb the geometry as much as L2 does in the case of Hg1 and Hg2 coordination spheres.

[Hg(L1)(SCN)2] (4) Bond lengths Hg1–N1 Hg1–N3 Hg1–N2 Hg1–N4 Hg1–S1 Hg1–S2

2.540(3) 2.533(3) 2.684(3) 2.670(3) 2.465(1) 2.471(1)

Bond angles N1–Hg1–N3 N1–Hg1–N2 N1–Hg1–N4 N1–Hg1–S1 N1–Hg1–S2 N3–Hg1–N2 N3–Hg1–N4 N3–Hg1–S1 N3–Hg1–S2 N2–Hg1–N4 N2–Hg1–S1 N2–Hg1–S2 N4–Hg1–S1 N4–Hg1–S2 S1–Hg1–S2 Hg1–S1–C30 Hg1–S2–C31

74.59(9) 63.02(9) 128.74(10) 89.13(7) 112.20(7) 129.86(9) 62.93(9) 110.12(7) 93.12(7) 166.98(9) 95.32(7) 80.07(7) 80.37(7) 97.99(7) 152.27(4) 153(7) 169(8)

Symmetry code: 1  x, y, 3/2  z.

Alternately, the structure can be viewed as a chair conformation of the above dimer, with the subunits in the [Hg4(l3-Cl)2(l-L2)2(lCl)2Cl2]2 being related by crystallographically imposed inversion symmetry. This [Hg4(l3-Cl)2(l-L2)2(l-Cl)2Cl2]2 structural building unit also exhibits off-set p-stacking to neighboring units via the outermost pyridyl ring of L2. Finally, it can be envisioned as a three-dimensional network, the formation of which is allowed because of the flexibility of L2 and the coordination ability of the halides, which act as a ‘‘hinges’’ leading to a step-wise 3D architecture. The coordination geometry of the HgII centers is of also of interest, since there are metal ions with heavily distorted coordination environments. In the pentacoordinated Hg1 or Hg2 centers, two of the coordination positions are occupied by two N atoms of one pyridyl and one imine moieties on one side of the ligand [Hg–N bond distances: 2.272(5), 2.354(5) Å and (2.250(5), 2.356(5) Å for Hg1 and Hg2, respectively]; the other coordination sites are filled by three Cl donors, of which two are l3-Cl [Hg–Cl distances: 2.511(1), 2.862(1), and 2.931(1), 2.980(1) Å for Hg1 and Hg2, respectively], and one being a l-Cl [2.483(1) and 2.482(1) Å for Hg1 and Hg2, respectively]. The square pyramidal coordination sphere is highly distorted; it is characteristic that the mean plane defined by the L2-chelating ring on Hg2 and the respective plane defined by chloride linkers form a nearly 50° angle between them, which is also reflected by the Hg20 –Hg2–C17 angle, which measures 139° and the 144° Hg4–Hg1–C6 angle. It is believed that the bulk of L2 dictates this strained geometry, and as such it would be interesting to investigate the effect of ligand bulk even further; such efforts are ongoing. The pentacoordinated Hg(3) and Hg(4) ions, also exhibit distorted square pyramidal geometry with three l–Cl [Hg–Cl distances: 2.346(2), 2.766(2), 3.012(2), and 2.333(1), 2.805(1), 3.020(2) Å for Hg(3) and Hg(4), respectively Å], one l3Cl [Hg–Cl distances:2.931(1) and 3.018(1) Å for Hg(3) and Hg(4) Å, respectively] and one terminal chloride ligand [Hg–Cl distance: 2.308(1) and 2.306(2) Å for Hg(3) and Hg(4), respectively]. The Hg. . .Hg distances bridged by the Cl anions are 3.874, 3.894, 3.998, 4.059, 4.109 Å. The distortion in Hg3 and Hg4 is not so pronounced, possibly due to the presence of a terminal chloride within

3.2.4. [Hg2(l-L2)(l-Br)2Br2]n (6) In our attempt to investigate the effect of the halide on the structure, the co-ligand from Cl, which has a small atomic radius, was changed to the comparatively large Br. This change led to the isolation of 6, which is a one-dimensional chain based on a dinuclear subunit, namely the [Hg2(l-L2)Br2]2+ cation linked via Br anions. ORTEP representations of 6 are presented in Fig. 4, while selected bond distances and angles relevant to the metal coordination spheres can be found in Table 5. In this complex each HgII is five-coordinated with a distorted square pyramidal geometry [s5 = 0.28 for Hg1 and 0.22 for Hg2] surrounded by one pyridyl nitrogen Npyridine [Hg–Npyridine = 2.359(5) and 2.377(5) Å for Hg1 and Hg2, respectively], one imine nitrogen atom, namely Nimine [Hg–Nimine = 2.393(5) and 2.341(5) Å for Hg1 and Hg2, respectively], and three Br. It is worth noting that the Hg-N bond distances are longer for 6 as compared to 5. Among the three Br anions around each HgII in 6, one acts as a terminal ligand [Hg– Br distances: 2.547(1) and 2.518(1) Å for Hg1 and Hg2, respectively] and two others act as bridging ligands [Hg–Br distances: 2.734(1), 2.742(1) and (2.725(1), 2.884(1) Å for Hg1 and Hg2, respectively]. Thus the Hg–(Br)2–Hg moiety in the dinuclear complex resembles a non-planar parallelogram (Fig. 4), with the Hg. . .Hg distance across the bridging halide anions being 4.06 Å. The Br–Hg–Br bond angles within the two Hg2Br2 parallelograms are 86.42(1)°, 83.86(1)°, while the Hg–Br–Hg angles are 95.35(2)°, and 91.65(2)°.

Fig. 3. (A) The labeled asymmetric unit in 5, showing the connections to the neighboring Hg4 units (dashed bonds); (B) Partially labeled X-ray crystal structure of the (Hg4)2 in 5 (aromatic units have been omitted for clarity). Color code: HgII purple, Cl green, N blue, C grey. (Colour online.)

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Table 4 Selected bond lengths (Å) and bond angles (°) for [Hg8(l-L2)2(l-Cl)6 (l3-Cl)4Cl4]n (5). Bond lengths Hg1–N1 Hg1–N2 Hg1–Cl1 Hg1–Cl10 Hg1–Cl2 Hg2–N3 Hg2–N4 Hg2–Cl5 Hg2–Cl500 Hg2–Cl6 Bond angles N1–Hg1–N2 N1–Hg1–Cl1 N1–Hg1–Cl10 N1–Hg1–Cl2 N2–Hg1–Cl1 N2–Hg1–Cl10 N2–Hg1–Cl2 Cl1–Hg1–Cl10 Cl2–Hg1–Cl1 Cl2–Hg1–Cl10 N3–Hg2–N4 N3–Hg2–Cl5 N3–Hg2–Cl500 N3–Hg2–Cl6 N4–Hg2–Cl5 N4–Hg2–Cl500 N4–Hg2–Cl6 Cl5–Hg2–Cl500 Cl5–Hg2–Cl600 Cl5–Hg2–Cl6

2.272(5) 2.354(5) 2.511(1) 2.862(1) 2.483(1) 2.356(5) 2.250(5) 2.980(1) 2.931(1) 2.482(1) 72.26(2) 137.17(1) 84.54(1) 120.13(1) 101.51(1) 151.41(1) 110.57(1) 84.07(4) 101.98(5) 95.32(5) 71.93(2) 104.88(1) 154.27(1) 110.74(1) 135.55(1) 85.08(1) 126.52(1) 83.31(5) 92.04(4) 96.69(5)

Hg3–Cl4 Hg3–Cl5 Hg3–Cl6 Hg3–Cl7 Hg3–Cl8 Hg4–Cl1 Hg4–Cl2 Hg4–Cl3 Hg4–Cl4 Hg4–Cl8 Cl4–Hg3–Cl5 Cl4–Hg3–Cl6 Cl4–Hg3–Cl7 Cl4–Hg3–Cl8 Cl5–Hg3–Cl6 Cl5–Hg3–Cl7 Cl5–Hg3–Cl8 Cl6–Hg3–Cl7 Cl6–Hg3–Cl8 Cl7–Hg3–Cl8 Cl1–Hg4–Cl2 Cl1–Hg4–Cl3 Cl1–Hg4–Cl4 Cl1–Hg4–Cl8 Cl2–Hg4–Cl3 Cl2–Hg4–Cl4 Cl2–Hg4–Cl8 Cl3–Hg4–Cl4 Cl3–Hg4–Cl8 Cl4–Hg4–Cl8 Hg1–Cl1–Hg1 Hg1–Cl2–Hg4 Hg2–Cl5–Hg2 Hg2–Cl5–Hg3 Hg2–Cl5–Hg300 Hg2–Cl6–Hg3

3.012(2) 2.931(1) 2.766(1) 2.308(1) 2.346(1) 3.018(1) 2.805(1) 2.306(1) 2.333(1) 3.020(1) 163.09(5) 87.66(5) 93.52(6) 82.52(5) 87.63(4) 102.95(5) 82.57(5) 93.09(5) 101.88(6) 164.30(6) 85.60(4) 105.36(5) 80.27(5) 160.32(4) 93.61(5) 101.73(6) 88.57(5) 164.09(6) 93.75(6) 82.55(5) 95.93(4) 94.65(5) 96.69(5) 81.91(4) 110.98(5) 95.02(5)

Symmetry codes: 0 : 1  x, 1  y, 1  z, 00 : 1  x, 1  y, 2 – z.

3.2.5. [Hg2(L2)I4] (7) An ORTEP representation of the structure of 7 is shown in Fig. 5. Selected bond distances and bond angles are listed in Table 6. Coordination around the mercury(II) ions is defined by the N2I2 donor set. In each HgII the two N atoms originate from the chelating ligand, and two halides are also coordinated, resulting in a distorted tetrahedral environment [s4 = 0.76 for Hg(1) and s4 = 0.78 for Hg(2)], with bond distances of 2.393(7)–2.442(8) Å for Hg–N and 2.645(1)–2.661(1) Å for Hg–I. The Hg(1). . .Hg(2) separation within the moieties is 7.869Å [14]. In contrast with the Cl and Table 5 Selected bond lengths (Å) and bond angles (°) for [Hg2(l-L2)(l-Br)2Br2]n (6). Bond lengths Hg1–N5 Hg1–N6 Hg1–Br1 Hg1–Br2 Hg1–Br4 Bond angles N5–Hg1–N6 N5–Hg1–Br1 N5–Hg1–Br2 N5–Hg1–Br4 N6–Hg1–Br1 N6–Hg1–Br2 N6–Hg1–Br4 Br1–Hg1–Br2 Br1–Hg1–Br4 Br2–Hg1–Br4 N3–Hg2–N4

2.393(5) 2.359(5) 2.547(1) 2.734(1) 2.742(1) 69.97(2) 103.50(1) 87.48(1) 150.00(1) 105.91(1) 132.88(1) 93.31(1) 119.64(2) 105.04(2) 86.42(2) 69.49(2)

Hg2–N3 Hg2–N4 Hg2–Br3 Hg2–Br2 Hg2–Br4 N3–Hg2–Br3 N3–Hg2–Br2 N3–Hg2–Br4 N4–Hg2–Br3 N4–Hg2–Br2 N4–Hg2–Br4 Br3–Hg2–Br2 Br3–Hg2–Br4 Br2–Hg2–Br4 Hg2–Br2–Hg1 Hg2–Br4–Hg1

2.341(5) 2.377(5) 2.518(1) 2.725(1) 2.884(1) 117.97(1) 140.51(1) 86.09(1) 123.45(2) 86.87(1) 127.28(2) 101.38(2) 109.26(2) 83.86(1) 95.35(2) 91.65(1)

Fig. 4. (A) Partially labeled X-ray crystal structure of the asymmetric unit in 6, showing the points of polymerization (dashed bonds); (B) Partially labeled X-ray crystal structure of the polymer of dimers in 6. Color code: HgII purple, Br aqua, N blue, C grey. (Colour online.)

Br complexes 5 and 6, the halides in 7 are all terminal. The latter constitutes further proof that investigating the halide dependence of HgII products bearing ornate organic ligands/chelates is indeed worthwhile, since the coordination environment around the metal centers can largely affect the structure and/or the properties of the resulting materials (vide infra). In the lattice of 7, there are also a number of H2O molecules, which are all distorted. This is likely the result of inherent moisture in the environment in combination with the water content of the solvents used. As such, the same synthesis was attempted with freshly distilled solvent, and the product which crystallized was a nearly isostructural compound, the crystal structure of which did not include any lattice solvents. In both 7 and the solvent-free version of the structure, there are packing forces present, namely p–p stacking interactions between the pyridyl rings of neighboring compounds (the distance between the mean planes calculated for the pyridyl rings: 3.76 Å); this distance does not significantly change between the two isostructural versions of 7, and lies within the 3r criterion. The ability of 7 to accommodate, or not, a large amount of solvent molecules in the crystal lattice, suggests structural flexibility of the lattice; if this property is reversible (solvent exchange) it could lead to gas storage properties for the material, or allow co-crystallization of different molecular units in between the layers of 7. Such hybrid features are very desirable for molecule-based materials, since one can interleave magnetic or other units in between diamagnetic molecules, and allow for greater separation of i.e., molecular magnets [44]. Such possibilities are currently under investigation.

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Fig. 5. (A) Partially labeled X-ray crystal structure of the asymmetric unit in 7; (B) Packing diagram of 7, including the mass of disordered water molecules; (C) Different view of the packing diagram of 7, showing the layers of disordered water molecules. Color code: HgII purple, I yellow, N blue, C grey, O red. (Colour online.) Table 6 Selected bond lengths (Å) and bond angles (°) for [Hg2(L2)I4] (7). Bond lengths Hg1–N3 Hg1–N4 Hg1–I4 Hg1–I6 Bond angles N3–Hg1–N4 N3–Hg1–I4 N3–Hg1–I6 N4–Hg1–I4 N4–Hg1–I6 I4–Hg1–I6

2.393(7) 2.429(8) 2.661(1) 2.646(1) 68.70(3) 110.52(2) 109.56(2) 100.85(2) 110.76(2) 135.66(2)

Hg2–N1 Hg2–N2 Hg2–I3 Hg2–I5 N1–Hg2–N2 N1–Hg2–I3 N1–Hg2–I5 N2–Hg2–I3 N2–Hg2–I5 I3–Hg2–I5

2.442(8) 2.396(7) 2.655(1) 2.645(1) 68.90(3) 101.69(2) 109.46(2) 110.13(2) 107.76(2) 137.45(2)

3.3. Insights on the coordination behavior of the ligands The ligand L1 seems to prefer chelation and thus leads to mostly mononuclear species, whereas L2 is prone to bridging metal ions. This is likely due to the additional flexibility and rotational freedom of L2. The structures in the present work constitute proof, since in the L2-containing compounds a trans disposition of the chelate moieties allows for bridging; such a behavior is not

observed in the complexes of L1, in which the ligand acts only as a chelate. This behavior can be attributed to the steric hinderance originating from the two methyl groups on the L1 carbon chain backbone. However, the argument about the bridging preference of L2 is not conclusive; in previous coordination chemistry studies of L2 with a variety of transition metals it was shown that L2 can indeed act as an effective chelate. Several L2-containing complexes have been reported in the literature, some of which contain solely chelating L2 ligand(s) with different metal ions, such as Ni2+, Co3+, Zn2+, Cu2+, Cd2+, Mn2+ [45]. There is one known example of a Cd2+ dimeric complex where L2 acts as both a chelating agent and a bridging ligand [45h], and there are two previously reported complexes containing solely the bridging mode of L2, i.e., one dimeric Hg2+ compound and one Cd2+ polymer [35]. 3.4. Luminescence properties The photoluminescence behavior of the free Schiff base ligand (L1) and the compounds (1–7) were examined in the solid state at room temperature (298 K); the photoluminescence behavior of the Schiff base ligand L2 was previously reported [35]. The typical

Fig. 6. Solid-state fluorescence behavior at 298 K with kex = 320 nm of the ligand L1 and compounds 1–4.

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Fig. 7. Solid-state fluorescence behavior at 298 K with kex = 320 nm of the compounds 5–7; the L2 fluorescence plot can be found in reference [35].

spectral patterns are shown in Figs. 6 and 7. The nature of emission bands in 1–7 is very much similar to that of the free Ligand indicating intraligand p ? p⁄ transitions [46]. The increase in emission intensity of 3, 4 and 5 as compared to the free ligand is due to the increase in conformational rigidity of L upon coordination. In contrast, 1, 2, 6 and 7 show a less intense emission compared to that in free ligand, which is likely due to the quenching effect of the chloride and bromide co-ligands [34]. As a caveat it is also noted that different coordination environments around HgII, and the various non-covalent interactions (vide supra) present in the crystal structures of 1–7 (i.e., weak p–p interactions between the many aromatic groups) may also affect the emission intensity as well as the wavelength. 4. Conclusion In summary, in the present work the syntheses and X-ray crystal structures of seven new mercury(II) complexes are reported. These complexes have different nuclearities and dimensionalities, and they all contain Schiff base chelates with flexible arms and halide/pseudohalide ions. The halide/pseudohalide effect on the resulting structures is investigated, and the results provide proofof-principle for further investigation of HgII coordination chemistry. The choice of Hg(II) as the metal ion is based on its softer character, as compared to its Zn(II) and Cd(II) analogues; the latter property promotes the formation of less predictable metal ion coordination environments, and complex architectures [47]. This work beautifully portrays the versatility of the Schiff-type ligands, and the sensitivity of their chemistry to the presence and identity of the co-ligand. This is also partially due to the inherent flexibility of the chelates used, in which the two pyridyl rings rotate freely around the symmetrical ethylenic (–CH2–CH2–) arm. As such, it is anticipated that the general family of Schiff base and/or azine ligands, will continue to produce coordination complexes, clusters, and polymeric species bearing new structural motifs in transition metal chemistry. Acknowledgements We are grateful to the Young Researchers and Elite Club, Tabriz Branch, Islamic Azad University, for the financial support of this research. C. Lampropoulos acknowledges the University of North Florida and the Cottrell College Science Award from the Research

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