Synthesis, spectroscopic, crystal structures and photoluminescence studies of cadmium(II) complexes derived from di-2-pyridyl ketone benzoylhydrazone: Crystal structure of a rare eight coordinate cadmium(II) complex

Polyhedron 127 (2017) 84–96

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Synthesis, spectroscopic, crystal structures and photoluminescence studies of cadmium(II) complexes derived from di-2-pyridyl ketone benzoylhydrazone: Crystal structure of a rare eight coordinate cadmium (II) complex Daly Kuriakose, A. Ambili Aravindakshan, M.R. Prathapachandra Kurup ⇑ Department of Applied Chemistry, Cochin University of Science and Technology, Kochi 682 022, Kerala, India

a r t i c l e

i n f o

Article history: Received 23 November 2016 Accepted 25 January 2017 Available online 4 February 2017 Keywords: Aroylhydrazone Chelation Cadmium(II) complexes Bifurcated Crystal structures

a b s t r a c t Four cadmium(II) complexes, [CdL2] (1), [Cd(HL)Cl2] (2), [Cd(HL)Br2] (3) and [Cd(HL)(NO3)2(H2O)] (4) of di-2-pyridyl ketone benzoylhydrazone (HL) have been synthesized and characterized by different physico-chemical methods. The tetradentate aroylhydrazone acts as a tridentate ligand in all the complexes, bonding through iminol or amido oxygen, azomethine and pyridyl nitrogens. The molecular structures of all monomeric complexes were determined by single crystal X-ray diffraction studies. The complexes, [CdL2] (1) and [Cd(HL)Cl2] (2) got crystallized in a monoclinic space group whereas the other two complexes [Cd(HL)Br2] (3) and [Cd(HL)(NO3)2(H2O)] (4) in a triclinic crystal system. The complex [CdL2] (1) has a distorted octahedral geometry with two deprotonated monoanionic hydrazone moieties satisfying the coordination whereas in [Cd(HL)Cl2] (2) and [Cd(HL)Br2] (3), the pentacoordination around Cd(II) is satisfied by neutral N, N, O – chelated hydrazone and two anions chloride and bromide respectively. In [Cd(HL)(NO3)2(H2O)] (4), cadmium shows octacoordination and nitrate ion binds both in anisobidentate and bidentate fashion. The thermal stability and the nature of water molecule in complex [Cd(HL) (NO3)2(H2O)] (4) was studied. The nature of emission was found to be quenched in all complexes. Ó 2017 Elsevier Ltd. All rights reserved.

1. Introduction Hydrazones and their metal complexes have been widely investigated by researchers in the recent years due to their applications in diverse fields [1–6]. The interest remains unwavering and was further fueled by the inherent ability of hydrazones to exhibit tautomerism which in turn displays versatile coordination modes during complexation. The reaction parameters such as nature and denticity of the hydrazone, metal ion and its concentration and the pH of the medium [7,8] contributes to the coordinating ability of hydrazone and resulting in diverse geometries and nuclearities [9–11]. Among d10 metals, the heavy metal cadmium forms a plethora of complexes with wide variety of coordination numbers ranging from four to eight [12]. Recently cadmium(II) ion is also found to serve as the catalytic center in carbonic anhydrase [13]. Moreover the d10 metal ions also find potential applications as photoluminescence materials and the nature of fluorescence can be of Chelation Enhanced Fluorescence effect (CHEF) or Chelation ⇑ Corresponding author. Fax: +91 484 2575804. E-mail addresses: [email protected], [email protected] (M.R.P. Kurup). http://dx.doi.org/10.1016/j.poly.2017.01.041 0277-5387/Ó 2017 Elsevier Ltd. All rights reserved.

Enhanced Quenching effect (CHEQ) or both effects coupled with red or blue shift in emission band. Extensive studies have been done on the transition metal complexes of di-2-pyridyl ketone, a versatile polydentate ligand with three potential donor sites and the study began since 1967, when the first Cu(II) complex of di-2-pyridyl ketone was reported by Osborne and McWhinnie [14]. Transition metal complexes of di2-pyridyl ketone derived ligand systems have also gained considerable interest [9,15]. In this paper, we report the syntheses as well as structural characterization of four cadmium(II) complexes of di-2-pyridyl ketone benzoylhydrazone. The quenching nature in the emission spectra of aroylhydrazone as well as complexes were also studied. 2. Experimental 2.1. Materials All chemicals used were of analar quality and purchased from commercial source. Di-2-pyridyl ketone (Aldrich), benzhydrazide (Aldrich), cadmium(II) acetate dihydrate (E-Merck), cadmium(II)

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chloride (Aldrich), cadmium(II) bromide tetrahydrate (Aldrich) and cadmium(II) nitrate tetrahydrate (Qualigens) were used as supplied. Solvents, methanol and DMF were purchased from Spectrochem and used without further purification. The aroylhydrazone (HL), di-2-pyridyl ketone benzoylhydrazone was synthesized according to the reported procedure [16]. 2.2. Synthesis of aroylhydrazone (HL) and its cadmium(II) complexes 2.2.1. Synthesis of di-2-pyridyl ketone benzoylhydrazone (HL) The aroylhydrazone, HL was prepared by refluxing equimolar methanolic solutions of benzhydrazide (0.136 g, 1 mmol) and di2-pyridyl ketone (0.184 g, 1 mmol) for 4 h along with two drops of conc. HCl. The resulting solution was kept aside for slow evaporation at room temperature. Colorless crystals were obtained within a period of 2–3 days. The product obtained was filtered and dried. Yield: 0.222 g (55%). 2.2.2. Syntheses of cadmium(II) complexes 2.2.2.1. Synthesis of [CdL2] (1). Methanolic solutions of aroylhydrazone, HL (0.302 g, 1 mmol) and cadmium(II) acetate dihydrate (0.267 g, 1 mmol) were mixed and refluxed for three hours. The resulting yellow colored solution is allowed to stand for slow evaporation under room temperature. Yellow crystals are obtained within a period of a week. The crystals were then filtered off, washed with methanol and dried over P4O10 in vacuo (Scheme 1). For [CdL2] (1): Yield: 57% (0.407 g). Color: Yellow. Anal. Calc. for C36H26CdN8O2 (M.W.: 715.05 g mol
1) C, 60.47; H, 3.66; N, 15.67. Found: C, 60.63; H, 3.6; N, 15.49%. kM (DMF): 3.5 mho cm2 mol
1. 2.2.2.2. Syntheses of [Cd(HL)Cl2] (2) and [Cd(HL)Br2] (3). To the methanolic solution of aroylhydrazone, HL (0.302 g, 1 mmol), methanolic solution of appropriate cadmium(II) salt (1 mmol) was mixed and the resulting yellow colored solutions were refluxed for three hours. On cooling, yellow microcrystalline product of the respective compounds formed were filtered off and washed with methanol and dried. Single crystals suitable for X-ray analysis were obtained on recrystallization from DMF solution of the crude yellow microcrystals of 2 and 3. The crystals were filtered off, washed with methanol dried in vacuo over P4O10 (Scheme 1). For [Cd(HL)Cl2] (2): Yield: 71% (0.344 g). Color: Yellow. Anal. Calc. for C18H14N4OCdCl2 (M.W.: 485.64 g mol
1) C, 44.52; H, 2.91; N, 11.54. Found: C, 44.58; H, 2.83; N, 11.31%. kM (DMF): 22 mho cm2 mol
1. For [Cd(HL)Br2] (3): Yield: 77% (0.442 g). Color: Yellow. Anal. Calc. for C18H14N4OCdBr2 (M.W.: 574.54 g mol
1) C, 37.63; H, 2.46; N, 9.75. Found: C, 38.03; H, 2.73; N, 10.09%. kM (DMF): 26 mho cm2 mol
1. 2.2.2.3. Synthesis of [Cd(HL)(NO3)2(H2O)] (4). To the ethanolic solution of aroylhydrazone, HL (0.302 g, 1 mmol), ethanolic solution of cadmium(II) nitrate tetrahydrate (1 mmol, 0.308 g) was added, mixed and refluxed for three hours. On cooling, yellow block shaped single crystals suitable for X-ray analysis were formed within a period of five days. The crystals were separated and dried in vacuo over P4O10 (Scheme 1). For [Cd(HL)(NO3)2(H2O)] (4): Yield: 63% (0.351 g). Color: Yellow. Anal. Calc. for C18H16CdN6O8 (M.W.: 556.78 g mol
1) C, 38.83; H, 2.90; N, 15.09. Found: C, 38.92; H, 2.93; N, 14.01%. kM (DMF): 136 mho cm2 mol
1. 2.3. Physical measurements The elemental analyses of the aroylhydrazone and its cadmium (II) complexes were carried out using a Vario EL III CHNS analyzer. FT-IR spectra of the compounds were recorded on a JASCO – 4100

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FT-IR Spectrophotometer in the range 4000–400 cm
1 using KBr pellets. Electronic spectra in DMF solutions were recorded on a Thermo scientific evolution 201 UV–Vis double beam spectrometer in the 200–800 nm range. The emission properties were studied using Shimadzu RF-5301PC spectrometer (solution). The TG-DTG analysis of the complex 4 was carried out in a Perkin Elmer Pyris Diamond TG/DTA analyzer under nitrogen at a heating rate of 10 °C min
1 in the 50–700 °C range. The molar conductivities of the complexes in DMF (10
3 M) solutions at room temperature were measured using a Systronic model 303 direct reading conductivity bridge. 2.4. Crystallographic data collection and refinement Single crystals of the compounds [CdL2] (1), [Cd(HL1)Cl2] (2), [Cd(HL1)Br2] (3) and [Cd(HL)(NO3)2(H2O)] (4) with suitable dimensions were selected for X-ray diffraction measurements and mounted on a Bruker SMART APEXII CCD diffractometer, equipped with a graphite crystal, incident-beam monochromator and a fine focus sealed tube with Mo Ka (k = 0.71073 Å) radiation as the Xray source. The unit cell dimensions were measured and the data collection was performed. The programs SAINT and XPREP were used for data reduction and APEX2 and SAINT were used for cell refinement [17]. Absorption corrections were carried out using SADABS based on Laue symmetry using equivalent reflections [18]. The structure was solved by using SHELXS-97 direct methods and refined by fullmatrix least-squares refinement on F2 using SHELXL-2014/7 [19] as well as WinGX software package [20]. The molecular and crystal structures were plotted using ORTEP-3 [20] and DIAMOND version 3.2 g [21]. In complex 1, one of the four pyridyl rings was disordered over five closely positioned sets of sites (N2/C8/C9/C10/C11) with a siteoccupation factor of 0.684(19) for the major-occupied and 0.316 (19) for the minor-occupied sites. The hydrogens in pyridine ring and its disordered counterpart were fixed using HFIX instruction. The distances were restrained to be equal using SADI instruction. The connectivity array and planarity were restrained using DELU and FLAT instructions. In all cadmium(II) complexes, anisotropic refinement were performed for all non-hydrogen atoms and all H atoms on C were placed in calculated positions, guided by difference maps, with C–H bond distances 0.93–0.96 Å. H atoms were assigned as Uiso = 1.2 Ueq (1.5 for Me). In [Cd(HL1)Cl2] (2), [Cd (HL1)Br2] (3) and [Cd(HL)(NO3)2(H2O)] (4) the hydrogen atom H40 attached to N4 was located from Fourier map and restrained to a distance of 0.88 ± 0.01. The hydrogen atoms, H8A and H8B attached to the oxygen atom O8 of the coordinated water molecule in 4 were located from difference map and their distances and angles were restrained using DFIX and DANG instructions with distance restraint of O–H = 0.86 ± 0.01 Å and H  H = 1.36 ± 0.02 Å followed by refinement of their displacement parameters. Some reflections were omitted for all complexes owing to bad disagreement. The extinction coefficient have value of 0.02389 in complexes 2. The crystallographic data and structure refinement parameters for compounds 1–4 are given in Table 1. 3. Results and discussion Four cadmium(II) complexes of tetradentate di-2-pyridyl ketone benzoylhydrazone (HL) were synthesized and characterized by elemental analysis and molar conductivity measurements, IR, electronic and 1H NMR spectral studies. The reaction of cadmium (II) salts with aroylhydrazone in appropriate solvent yielded monomeric complexes 1–4. In all the complexes, the HL acts as a tridentate NNO donor ligand bonding through iminol and amido oxygen, azomethine nitrogen and one of the pyridyl nitrogen

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Scheme 1. Synthesis of cadmium(II) complexes 1, 2, 3 and 4.

thereby leaving a free potential donor site available for the coordination. The molecular structures of complexes 1–4 were determined by single crystal X-ray diffraction studies and the coordination geometries of the cadmium(II) center in complexes were found to possess varying topologies like square pyramidal (2 and 3), octahedral (1) and triangulated dodecahedral (4) accomplished by the reaction of aroylhydrazone with different metal salts. The thermal stability as well as presence of coordinated water molecule in complex 4 were confirmed by TG analysis. The photoluminescent behavior of all complexes was also investigated. All the cadmium(II) complexes prepared are stable towards air and moisture at room temperature. They are insoluble in water,

methanol and ethanol but soluble in dipolar aprotic solvents like DMF and DMSO. The partial elemental analyses for the complexes are in good agreement with the given molecular formulae, which was further authenticated by single crystal X-ray diffraction studies. The molar conductivity measurements of ligand as well as complexes were measured in DMF (10
3 M) at room temperature. The values suggest that the ligand as well as complexes except 4 are non-electrolytic nature and remain un-dissociated in DMF [22]. The high molar conductivity value of 136 mho cm2 mol
1 corresponds to the 1:2 electrolytic nature of complex 4 and be attributed to a partial dissociation and/or partial replacement of the anion in the solvent molecules [22,23].

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D. Kuriakose et al. / Polyhedron 127 (2017) 84–96 Table 1 Crystallographic data and structure refinement for 1, 2, 3 and 4. Parameters

[Cd(L)2] (1)

[Cd(HL)Cl2] (2)

[Cd(HL)Br2] (3)

[Cd(HL)(NO3)2(H2O)] (4)

Empirical formula Formula weight T (K) Crystal system Space group

C36H26CdN8O2 715.06 293(2) K monoclinic P21/c

C18H14CdCl2N4O 485.64 293(2) K monoclinic P21/n

C18H14CdBr2N4O 574.54 293(2) K triclinic P1

C18H16CdN6O8 556.78 296(2) K triclinic  P1

11.0769(4) 11.1238(3) 26.7736(9) 90 98.6850(10) 90 3261.14(18) 4 1.456 0.715 1448 0.35  0.30  0.25 1.986–28.27°
14  h  14
14  k  13
35  l  35 24 516/8083 [Rint = 0.0258]

9.3428(4) 13.4557(8) 14.7105(7) 90 93.8480(10) 90 1845.15(16) 4 1.748 1.488 960 0.3  0.25  0.20 2.93–28.18°
11  h  12
17  k  16
19  l  19 13 632/4545 [Rint = 0.0258]

8.7232(4) 9.9885(6) 12.1936(11) 104.589(3) 98.629(2) 108.261(2) 945.62(11) 2 2.018 5.395 552 0.30  0.25  0.20 2.265–28.243°
6  h  11
13  k  13
16  l  15 8565/4688 [Rint = 0.0224]

8.2232(5) 10.1525(7) 13.7080(11) 70.886(3) 77.467(3) 83.342(3) 1054.32(13) 2 1.754 1.095 556 0.5  0.45  0.40 2.54 to 24.99°
9  h  8
12  k  11
16  l  16 6599/3636 [Rint = 0.0249]

25.242 (99.9%) Semi-empirical from equivalents 0.836 and 0.779 8019/114/440 0.921 R1 = 0.0398, wR2 = 0.1047 R1 = 0.0671, wR2 = 0.1226 1.403 and
0.469

28.18 (99.3%) Semi-empirical from equivalents 0.743 and 0.646 4518/1/240 1.030 R1 = 0.0263, wR2 = 0.0716 R1 = 0.0338, wR2 = 0.0789 0.429 and
0.569

25.242 (99.2%) Semi-empirical from equivalents 0.349 and 0.226 4688/1/239 0.961 R1 = 0.0321, wR2 = 0.0680 R1 = 0.0566, wR2 = 0.0774 0.867 and
0.574

24.99 (98.0%) Semi-empirical from equivalents 0.645 and 0.584 3707/4/310 0.968 R1 = 0.0253, wR2 = 0.0705 R1 = 0.0294, wR2 = 0.0759 0.412 and
0.355

Cell parameters a (Å) b (Å) c (Å) a (°) b (°) c (°) V (Å3) Z Dcalc (q) (mg m
3) Absorption coefficient, l (mm
1) F(0 0 0) Crystal size (mm3) h range for data collection Limiting indices

Reflections collected/Unique Reflections (Rint) Completeness to h Absorption correction Maximum and minimum transmission Data/restraints/parameters Goodness-of-fit on F2 Final R indices [I > 2r (I)] R indices (all data) Largest difference peak and hole (e Å
3)

R1 = R||Fo|
|Fc||/R|Fo| wR2 = [Rw(F2o
F2c )2/Rw(F2o)2]1/2.

Fig. 1. ORTEP plot of [Cd(L)2] (1) along with atom numbering scheme of the non-hydrogen atom. Displacement ellipsoids are drawn at 30% probability.

3.1. Description of crystal structures 3.1.1. Molecular structure of [CdL2] (1) The molecular structure of [CdL2] (1) is depicted in Fig. 1 along with the atom numbering scheme and selected bond lengths (Å) and bond angles (°) are shown in Table 2. The monomeric bisligated complex crystallized in P21/c space group with four crystallographic identical molecules occupying the monoclinic unit cell.

The hexacoordination around Cd(II) ion can be best described with a CdN4O2 chromophore comprising of two equivalent mono-anionic ligands (Fig. 1). The Cd atom is coordinated by azomethine nitrogens (N3 and N7), iminolate oxygens (O1 and O2) and one of the two pyridyl nitrogens (N1 and N5) from each aroyl hydrazone molecule. The oxygen atom of the aroylhydrazone ligand coordinates the metal atom in iminolate form through iminolization followed by

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Table 2 Selected bond lengths (Å) and bond angles (°) of compounds 1–4. [Cd(L)2] (1)

[Cd(HL)Cl2] (2)

[Cd(HL)Br2] (3)

[Cd(HL)(NO3)2(H2O)] (4)

Bond lengths (Å) N(1)–Cd(1) N(3)–Cd(1) N(5)–Cd(1) N(7)–Cd(1) O(1)–Cd(1) O(2)–Cd(1) C(6)–N(3) C(24)–N(7) N(3)–N(4) N(7)–N(8) C(12)–N(4) C(30)–N(8) C(12)–O(1) C(30)–O(2)

2.364(3) 2.282(2) 2.409(3) 2.295(2) 2.240(2) 2.255(3) 1.281(4) 1.286(4) 1.379(3) 1.364(3) 1.328(4) 1.342(4) 1.268(4) 1.265(3)

Cl(1)–Cd(1) Cl(2)–Cd(1) N(1)–Cd(1) N(3)–Cd(1) O(1)–Cd(1) C(6)–N(3) N(3)–N(4) C(12)–N(4) C(12)–O(1)

2.4040(7) 2.4245(7) 2.363(2) 2.3548(18) 2.3896(17) 1.288(3) 1.364(3) 1.356(3) 1.231(3)

Br(1)–Cd(1) Br(2)–Cd(1) N(1)–Cd(1) N(3)–Cd(1) O(1)–Cd(1) C(6)–N(3) N(3)–N(4) C(12)–N(4) C(12)–O(1)

2.5503(5) 2.5137(5) 2.340(3) 2.395(3) 2.444(3) 1.295(4) 1.359(4) 1.362(4) 1.228(5)

O(5)–Cd(1) O(6)–Cd(1) O(8)–Cd(1) N(1)–Cd(1) O(5)–Cd(1) N(3)–Cd(1) O(1)–Cd(1) O(2)–Cd(1) O(4)–Cd(1) C(6)–N(3) N(3)–N(4) C(12)–N(4) C(12)–O(1) O(2)–N(5) O(3)–N(5) O(4)–N(5) O(5)–N(6) O(6)–N(6) O(7)–N(6)

2.301(3) 2.812(3) 2.294(2) 2.344(2) 2.301(3) 2.384(2) 2.385(2) 2.443(2) 2.424(2) 1.288(3) 1.356(3) 1.361(3) 1.229(3) 1.261(4) 1.210(4) 1.254(3) 1.230(4) 1.201(4) 1.231(4)

Bond angles (°) [Cd(L)2] (1) N(3)–Cd(1)–N(7) O(1)–Cd(1)–N(1) O(2)–Cd(1)–N(5) O(2)–Cd(1)–N(3) O(1)–Cd(1)–N(7) O(1)–Cd(1)–O(2) O(2)–Cd(1)–N(1) O(1)–Cd(1)–N(5) N(3)–Cd(1)–N(5) N(7)–Cd(1)–N(1) N(1)–Cd(1)–N(5) O(1)–Cd(1)–N(3) N(3)–Cd(1)–N(1) O(2)–Cd(1)–N(7) N(7)–Cd(1)–N(5)

[Cd(HL)Cl2] (2) 155.76(8) 139.61(8) 137.32(9) 129.20(8) 125.66(8) 102.85(9) 99.79(10) 94.46(8) 93.32(8) 93.49(9) 90.99(10) 69.88(8) 69.86(8) 69.47(8) 68.73(8)

[Cd(HL)Br2] (3) N(1)–Cd(1)–O(1) N(3)–Cd(1)–Cl(1) Cl(1)–Cd(1)–Cl(2) N(3)–Cd(1)–Cl(2) N(1)–Cd(1)–Cl(1) O(1)–Cd(1)–Cl(2) N(1)–Cd(1)–Cl(2) O(1)–Cd(1)–Cl(1) N(3)–Cd(1)–N(1) N(3)–Cd(1)–O(1)

[Cd(HL)(NO3)2(H2O)] (4) 133.90(6) 129.49(6) 115.41(2) 114.91(5) 105.95(6) 102.18(5) 101.58(6) 98.55(5) 67.57(7) 66.68(6)

Br(1)–Cd(1)–Br(2) N(1)–Cd(1)–O(1) N(3)–Cd(1)–Br(2) N(1)–Cd(1)–Br(1) N(1)–Cd(1)–Br(2) N(3)–Cd(1)–Br(1) O(1)–Cd(1)–Br(1) O(1)–Cd(1)–Br(2) N(3)–Cd(1)–N(1) N(3)–Cd(1)–O(1)

135.08(18) 132.04(10) 122.99(7) 105.58(7) 103.85(7) 99.52(7) 92.58(8) 92.17(7) 67.38(9) 66.01(9)

N(3)–Cd(1)–O(2) O(6)–Cd(1)–O(8) O(5)–Cd(1)–O(8) N(3)–Cd(1)–O(4) O(1)–Cd(1)–O(4) N(1)–Cd(1)–O(1) N(1)–Cd(1)–O(2) O(6)–Cd(1)–O(1) O(5)–Cd(1)–O(4) O(6)–Cd(1)–N(3) O(5)–Cd(1)–N(1) O(8)–Cd(1)–N(1) O(1)–Cd(1)–O(2) O(5)–Cd(1)–O(2) O(5)–Cd(1)–N(3) O(6)–Cd(1)–O(2) O(8)–Cd(1)–O(2) O(8)–Cd(1)–N(3) N(1)–Cd(1)–O(4) O(6)–Cd(1)–O(4) O(8)–Cd(1)–O(1) O(8)–Cd(1)–O(4) O(5)–Cd(1)–O(1) O(6)–Cd(1)–N(1) N(1)–Cd(1)–N(3) N(3)–Cd(1)–O(1) O(4)–Cd(1)–O(2) O(6)–Cd(1)–O(5)

159.43(9) 157.45(10) 154.43(9) 141.20(7) 140.67(7) 134.48(7) 131.13(8) 121.99(8) 119.67(10) 110.87(9) 103.01(10) 97.35(10) 94.08(8) 92.90(13) 89.59(12) 85.20(10) 85.09(11) 83.91(10) 80.40(7) 79.26(8) 79.02(9) 78.71(10) 75.68(8) 74.19(9) 67.71(7) 66.79(7) 52.07(7) 46.56(9)

deprotonation confirmed by the double bond nature of Nimine@C (1.328(4) in C12–N4 and 1.342(4) Å in C30–N8 bonds) and single bond nature of C–Oiminolate (1.269(3) in C12–O1 and 1.265(3) Å in C30–O2 bonds) bond lengths (Table 2) [24,25]. The torsion angles [C5–C6–N3–N4,
180.0(2)°; C23–C24–N7–N8,
179.5(2)°; N3–N4–C12–O1, 3.3(4)°; and N7–N8–C30–O2,
0.5(4)°] observed support the formation of E, Z isomer (trans, cis isomer) of the iminolic form of the ligand along C6 = N3 and C24 = N7 respectively facilitating the coordination of deprotonated aroylhydrazone to the cadmium(II) center. The two monodeprotonated aroyl hydrazone coordinates to the cadmium center forming four fused five membered chelate rings with intraligand bite angles more acute in the range 68–70° (N1–Cd1–N3, 69.86(9)°, N3–Cd1–O1, 69.88(8)°, N7–Cd1–N5, 68.73(8)° and N7–Cd1–O2, 69.46(8)°) and the bicyclic chelate system (Cd1, N1, C6, N3, N4, C12, O1) possess a dihedral angle of 85.473(43)° with its counterpart (Cd1, N5, C23, C24, N7, N8, C30, O2) suggesting non-coplanarity. Each pair of five membered chelate rings formed by the coordination of aroylhydrazone with the

metal center possess a dihedral angles of 2.01(11) and 5.92(11)° between each other which shows the rigidity of the bonds as well as non-coplanar nature of metallocycles during complexation. The distortion from the ideal octahedral geometry can be viewed from the large deviation of trans and cis angles (155.76(9)° and 69.46(8)° respectively) formed around metal center. The bond distances to cadmium are in the order Cd–N(azo) < Cd–O(iminolate) < Cd–N(py). The shortest coordinate bonds are to central imine N donors [N (3)–Cd(1), 2.282(2); N(7)–Cd(1), 2.295(2)] and iminolate oxygens O(1)–Cd(1), 2.239(2); O(2)–Cd(1), 2.254(2)] and while those to pyridyl nitrogens [N(1)–Cd(1), 2.364(3); N(5)–Cd(1), 2.410(3)] are significantly weaker. Ring puckering analyses and least square plane calculations show that the two pairs of fused five-membered chelate rings surrounding the cadmium(II) center are puckered. The five membered metallocycle Cg(1) (comprises of Cd1, O1, C12, N4 and N3), Cg(2) (comprises of Cd1, O2, C30, N8 and N7), Cg(3) (comprises of Cd1, N1, C5, C6 and N3) and Cg(4) (comprises of Cd1, N5, C23, C24 and N7) are puckered [26] with puckering parameters,

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Q(2) = 0.085(2) Å, u(2) = 342.8(19)° for Cg(1), Q(2) = 0.085(2) Å, u (2) = 359(2)° for Cg(2), Q(2) = 0.088(3) Å, u(2) = 139(19)° for Cg(3) and Q(2) = 0.093(2) Å, u(2) = 198.8(19)° for Cg(4). The six membered pyridine ring Cg(6) (comprises of N2, C7, C11, C10, C9 and C8) and the disordered counterpart Cg(9) (comprises of N2A, C7, C11A, C10A, C9A and C8A) are also puckered with puckering parameters, Q(2) = 0.070(12) Å, u(2) = 304(14)° for Cg(6), Q(2) = 0.20(3) Å, u(2) = 305(8)° for Cg(9). The puckering of five membered metallocycles were also calculated in terms of pseudorotation parameter P and sm [27] and it was found that the metallocycles Cg(1) and Cg (4) adopt twisted conformation on N3–Cd1 and N5–Cd1 bonds respectively with pseudorotation parameter P = 136.5(16)° and s = 7.2(2)° for Cd1–O1 bond in Cg(1) and P = 9.2(16)° and s = 8.0 (2)° for Cd1–N5 bond in Cg(4). The metallocycles Cg(2) and Cg(3) adopt as an envelope on Cd1 for Cg(2) with P = 165.0(17)° and s = 7.0(2)° for reference bond Cd1–O2 and N3 for Cg(3) with P = 287.8(11)° and s = 9.9(3)° for reference bond Cd1–N1. Conformational analyses [28] shows the five membered metallocycles Cg (1) and Cg(4) adopts T-form (half chair) conformation whereas Cg (2) adopts E-form (envelope) conformation. 3.1.2. Molecular structures of [Cd(HL)Cl2] (2) and [Cd(HL)Br2] (3) The molecular structures of the complexes [Cd(HL)Cl2] (2) and [Cd(HL)Br2] (3) along with the atom numbering scheme are shown

89

in Figs. 2 and 3 and significant bond distances and bond angles are shown in Table 2. X-ray diffraction studies reveal that both compounds are isostructural species whose structural geometry is defined as distorted square pyramidal around Cd(II) centre coordinated by pyridyl nitrogen N1, azomethine nitrogen N3, amido oxygen atom O1 of the aroylhydrazone moiety and two anions (Cl(1) and Cl(2) for 2 and Br(1) and Br(2) for 3). The complex [Cd(HL)Cl2] (2) crystallizes with four crystallographic identical molecules occupying the monoclinic unit cell with P21/n space group whereas [Cd(HL)Br2] (3) crystallizes in tri with two molecules in the unit cell. The clinic space group P1 structural studies of the halide complexes reveal that the distances of Cd–N (average 2.3675(3) Å) and Cd–O (2.444(3) Å) in complex 3 are slightly longer than that of complex 2 (2.3589(10) Å and 2.3896 (17) Å. It may be attributed to the ligand field strength of halide ligand (Br < Cl) [29]. The C(12)–O(1) bond length is found to be 1.231(3) Å in 2 and 1.228(4) Å in 3 which is very close to formal C@O length of 1.23 Å confirms the coordination of aroyl hydrazone, HL to the central metal via amido form (Table 2) [30]. The slight variation in C6 = N3 and N3–N4 bond lengths in complexes as compared to the aroylhydrazone attributed to the extensive delocalization over aroylhydrazone moiety during complexation. The bond distances to cadmium are in the order Cd–N(azo) < Cd–N(py) < Cd–O(ketoxy) for both complexes 2 and 3. The difference

Fig. 2. ORTEP plot of [Cd(HL)Cl2] (2) along with atom numbering scheme of the non-hydrogen atom. Displacement ellipsoids are drawn at 30% probability.

Fig. 3. ORTEP plot of [Cd(HL)Br2] (3) along with atom numbering scheme of the non-hydrogen atom. Displacement ellipsoids are drawn at 30% probability.

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between the Cd–Xanion bond distances are 0.0205 and 0.0366 Å for complexes 2 and 3 respectively. The torsion angle values of -179.0 (2)° in 2 and 175.9(3)° in 3 observed for C5–C6–N2–N3 moiety supports the E isomer of the amido form of the principle ligand on coordination. The distortion in the five coordinated systems is expressed with trigonality index (Addison’s parameter) s = (ba)/60 [31] (for perfect square pyramidal and trigonal bipyramidal geometries the values of s are zero and unity respectively). The s values of 0.0735 and 0.0507 in [Cd(HL)Cl2] (2) and [Cd(HL)Br2] (3) respectively indicate that the irregular coordination geometry of the complexes 2 and 3 in the solid state is 7.35% and 5.07% along the pathway of distortion from square pyramidal to trigonal bipyramidal. In both complexes 2 and 3, the neutral aroylhydrazone coordinates to the cadmium center forming two sets of five membered metallocycles with more acute intraligand bite angle values of 67.57(7) (N1–Cd1–N3) and 66.68(6)° (N3–Cd1–O1) for 2 and 67.38(9) (N1–Cd1–N3) and 66.01(9)° (N3–Cd1–O1) for 3 displaying distortion in the coordination geometry. The dihedral angle values of 3.36(9) and 8.66(16)° in 2 and 3 respectively formed between two five-membered chelate rings fused along the Cd1– N3 junction during complexation shows the non-coplanar nature of metallocycles. Cremer and Pople ring puckering analysis [26] shows that in complex 2, the five membered metallocycle Cg(2) (comprises of Cd1, N1, C5, C6 and N3) is puckered with Q(2) = 0.1221(19) Å and u(2) = 207.1(10)° for Cg(2) whereas in complex 3, both the metallocycles Cg(1) and Cg(2) (comprises of Cd1, N1, C5, C6 and N3) are puckered with Q(2) = 0.237(6) Å and u(2) = 172.325(7)° for Cg(1) and Q(2) = 0.244(6) Å and u(2) = 312.6(10)° for Cg(2). The pseudorotation parameter P and sm, parameters [27] were also calculated for both complexes and it was found that the metallocycle Cg(2) adopts an envelope on N1 with P = 20.9(9)° and s = 11.6(1)° for reference bond Cd1–N1 in 2 whereas in 3, metallocycle Cg(1) adopts an envelope conformation on Cd1 with P = 331.8° and s = 19.1° for reference bond Cd1–O1 and Cg(2) adopts a twisted form on N3–C6 bond with P = 102.1° and s = 27.7° for reference bond Cd1–N1. 3.1.3. Molecular structure of [Cd(HL)(NO3)2(H2O)] (4) The molecular structure of [Cd(HL)(NO3)2(H2O)] (4) is depicted in Fig. 4 along with the atom numbering scheme and selected bond lengths (Å) and bond angles (°) are shown in Table 2. The mono space group meric eight coordinated complex crystallizes in P 1

with two molecules in the unit cell. The coordination geometry around the Cd(II) centre is distorted triangulated dodecahedra [32] defined by neutral tridentate HL ligand through pyridyl nitrogen, azomethine nitrogen and amido oxygen atoms, one water molecule and two nitrate ions in which one nitrate is anisobidentately coordinated through oxygen atoms and other is bidentately coordinated [33,34]. The hydrazone ligand in an N,N,O-fashion and O2 donor set of bidentately coordinated nitrate occupies the equatorial coordination plane whereas the oxygen atoms of solvent water as well as anisobidentate nitrate ion occupy the apical positions and completes the coordination geometry. All the distances around the cadmium atom are in the expected ranges for octacoordinated cadmium in distorted triangulated dodecahedral arrangement, although the inter-axial and equatorial angles around the metal differ from the ideal values of 180° and 72° respectively (Table 2). In the complex 4, three of the five equatorial angles are smaller than the ideal value and the other two angles are greater showing the degree of distortion from the ideal polyhedron. The sum of the equatorial angles is very close to the ideal value (360°), which ensures the planarity of the equatorial plane. The axial O6–Cd1– O8 and O5–Cd1–O8 bond angles of 157.45(10)° and 154.40(10)° indicates deviation from a linear configuration. Based on criteria for assigning nitrate binding modes proposed by Kleywegt et al. nitrate ion can bind to the central metal atom via monodentate, where only one oxygen atom bound to the metal, bidentate mode where nitrate coordinate through two oxygen atom with nearly same M–O distances or in aniso bidentate fashion, in which among two oxygen atoms bind to central metal atom one is significantly longer [34,35] (Fig. 5 and Table 3). The bidentate and anisobidentate coordination modes of nitrate ion in complex 4 were authenticated by differences in M–O, M–N bond distances and M–N–O bond angles. The Cd–Nimine and Cd– NPy bond lengths are found to be 2.384(2) and 2.344(2) Å respectively and are comparable to the same in the literature. The C(12)–O(1) bond length (1.229(3) Å) is very close to formal C@O length of 1.23 Å confirms the coordination of principal ligand, HL to the central metal via amido form (Table 2) [30]. The extensive delocalization over aroylhydrazone moiety during complexation as compared to the aroylhydrazone was supported by the slight variation in C6 = N3 and N3–N4 bond lengths in complex. The torsion angle value of 177.6(2)° in 4 observed for C5–C6– N3–N4 moiety supports the E isomer of the amido form of HL on

Fig. 4. ORTEP plot of [Cd(HL)(NO3)2(H2O)] (4) along with atom numbering scheme of the non-hydrogen atom. Displacement ellipsoids are drawn at 30% probability.

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other through C20–H20  N4 with H  A distance of 2.48 Å to form

Fig. 5. Parameters used to define the coordination preferences of the nitrato ligands, adapted from Kleywegt et al.

Table 3 Geometric parameters for the nitrate ligand coordination in [Cd(HL)(NO3)2(H2O)] (4).

l2-l1 A1-A2 l3-l2 A3

Mode NO3 (1)

Mode NO3 (2)

0.019 1.13 0.3977 172 Bidentate

0.511 25.3 0.1545 165.7 Monodentate

coordination. The more acute intraligand bite angle values of 67.71 (7) (N1–Cd1–N3) and 66.79(7)° (N3–Cd1–O1) was observed for 4. The neutral aroyl hydrazone coordinates to the cadmium center forming a pair of five membered metallocycles fused along the Cd1–N3 junction with dihedral angle of 1.88(5)°. 3.1.4. Supramolecular aspects in the complexes 1–4 Although there are no classical hydrogen bonds found in the complex [Cd(L)2] (1), the crystal packing is supported by non-classical intra and intermolecular hydrogen bonding interactions (Table S1, Fig 6). The intramolecular hydrogen bonds C22– H22  N6 and C4–H4  N4, adds to the rigidity of the molecular structure. The iminolate oxygen atoms, O1 and O2 acts a hydrogen bond acceptors in C15–H15  O2 and C27–H27  O1 hydrogen bonding interactions for associating adjacent complex motifs to form a layer type structure arranged in particular direction when viewed along crystallographic ‘b’ axis. The C15–H15  O2 interaction generates a R22 ð16Þ motif. This layered structures is linked each

supramolecular R22 ð14Þ network. The interaction parameters are listed in Table S2. In the unit cell, the molecules are packed in the lattice in a zig-zag manner along the ‘a’ axis (Fig. S1) and the packing is stabilized by noncovalent interactions viz, p  p and intermolecular C–H  p interactions (Table S2, Figs. 7 and 8). The five membered metallocycle, Cg(1) (comprises of Cd(1), O(1), C(12), N(4), N(3)) is engaged in p  p interaction with six membered aromatic ring Cg(9) (comprises of C(13), C(14), C(15), C(16), C(17), C(18)) with a distance of 3.868(2) Å. The five membered metal chelate ring Cg(3) (comprises of Cd(1), O(1), C(12), N(4), N(3)) and one of the pyridine ring Cg(5) (comprises of N(1), C(1), C(2), C(3), C(4), C(5)) also participates in p  p interactions with Cg(4) (comprises of C(13), C(14), C(15), C(16), C(17), C(18)) and Cg(5) of the neighboring units at distances of 3.3049(15) and 3.704(2) Å respectively. The Cg(1) is involved in C–H  p interaction with the hydrogen atom H19 attached to C19 of one of the pyridine ring of the aroylhydrazone moiety. The hydrogen atoms, H1 and H3 attached to the carbon atoms C1 and C3 of the pyridine part of the aroylhydrazone moiety interacts with metallocycle Cg(2) (comprises of Cd(1), O(2), C(30), N(8), N(7)) and pyridine ring Cg(7) (comprises of N(5), C(19), C(20), C(21), C(22), C(23)) adjacent molecules through C1–H1  Cg(2) and C3–H3  Cg(7) interactions. In complex [Cd(HL)Cl2] (2), the packing diagram shows an ordered arrangement of molecules connected via a network of bifurcated hydrogen bonds in which N4 acts as hydrogen bond donor for N2 (N4–H40   N2) and Cl1 (N4–H40   Cl1) acceptors (Table S1 Fig. 9). The N(4)–H(40 )  N(2) hydrogen bond results in S(6) motif. In addition to the aforementioned S(6) motif, an intermolecular N(4)–H(40 )  Cl(1) (Symmetry code:
x + 1,
y,
z + 2) interaction is observed which generates a hydrogen bonded inversion dimer with an R22 ð10Þ motif [36]. The crystal packing is also augmented by non-classical intermolecular hydrogen bonding interactions like C3–H3  O1 and C10–H10  Cl2 (Table S1). These classical and non-classical intra and inter molecular hydrogen bonding interactions contributes to the three dimensional supramolecular architecture of the complex.

Fig. 6. Packing diagram showing the hydrogen bonding interactions in [CdL 2] (1).

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Fig. 7. p–p interactions between the molecules for [CdL2] (1).

Fig. 8. C–Hp interaction between the molecules for [CdL2] (1).

Similarly in [Cd(HL)Br2] (3), the amide proton N4 is in close proximity to the nitrogen atom N2 of the uncoordinated pyridine ring and participates in a classical intramolecular hydrogen bond of the type N–H  N resulting in an S(6) motif. The non-classical (C18–H18  O1) hydrogen bonding interactions generated S(5) motif also supports the packing of molecules (Table S1, Fig. 10). The non-covalent p  p as well as Cd(1)–Cl(1)  Cg(4) (symmetry coordinates 1
x, 1
y, 1
z) interactions in complex 2 reinforce the packing of complex whereas in 3, only p  p interactions contributes to the packing of molecules (Table S2). The five membered metallocycle, Cg(1) (comprises of Cd(1), O(1), C(12), N(4), N(3)) is engaged in p  p interaction with Cg(1) of another complex motif with a distance of 3.6814(11) Å (Fig. S2). The six membered Cg(3) (comprises of N(1), C(1), C(2), C(3), C(4), C(5)) pyridine ring participates in p  p interaction with Cg(5) (comprises of C(13), C(14), C(15), C(16), C(17), C(18)), (centroid

separation 3.8498(16)) aromatic part of aroylhydrazone moiety of complex motif (Fig. S2). One of the chloride ion, Cl1 attached to the cadmium center interacts with uncoordinated pyridine ring Cg(4) (comprises of N(2), C(7), C(8), C(9), C(10), C(11)), through Cd–Cl  p interaction of the neighboring unit thereby stabilize packing of the molecule (Fig. 11 and Table S2). In [Cd(HL)Br2] (3), the six membered ring Cg(3) (comprising of atoms N(1), C(1), C(2), C(3), C(4), C(5)) is involved in p  p interaction with Cg(3) as well as aromatic six membered ring Cg(5) (comprising of atoms C(13), C(14), C(15), C(16), C(17), C(18)) of the neighboring units. The packing of molecules in [Cd(HL)(NO3)2(H2O)] (4) is enriched by classical and non-classical intra and inter molecular hydrogen bonding interactions. The classical N4–H40   N2 and non-classical C1–H1  O4 interactions enhance the rigidity of the complex motif. The intramolecular hydrogen bond is observed between amide proton and nitrogen atom of the uncoordinated pyridyl ring. The hydrogen atoms, H8A and H8B of the coordinated water molecule are involved in the hydrogen bonding interactions. The hydrogen atom, H8A attached to the O8 of the coordinated water participates in the trifurcated hydrogen bonding interaction with three acceptors, O7, O6 and N6. The strong intermolecular hydrogen bonds lead to 1D supramolecular architecture propagating along ‘a’ axis (Table S1 and Fig 12). This 1D supramolecular architecture develops into 3D via non classical hydrogen bonds. In addition to this, hydrogen bonding interations and N(5)–O(3)  Cg(1) interactions are also found in the complex (Table S2 and Fig. S5). 3.2. Thermogravimetric studies Thermogravimetric analyses of the complexes gave information concerning the thermal stability and the nature of water molecules in the complex [37,38]. The thermal decomposition pattern of complex 4 is shown in Fig. S5. The thermal stability of complex 4 was investigated by TG/DTA measurements and the curves were obtained at a heating rate of 10 °C min
1 in air atmosphere over the temperature range of 0–800 °C. The complex 4 is stable up to 114 °C since there is no weight loss observed. The thermal decomposition pattern can be ascribed to three weight losses. The first weight loss of 3.26% (Calcd. 3.23%) observed between 114–172 °C indicating the

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Fig. 9. Packing diagram showing the hydrogen bonding interactions in [Cd(HL)Cl2] (2).

Fig. 10. Packing diagram showing the hydrogen bonding interactions in [Cd(HL)Br2] (3).

endothermic removal of one coordinated water molecule from the compound. The second stage decomposition starts from 173 °C and ends at 312 °C with DTA endothermic peak at 188 °C. It corresponds to the loss of organic moiety of the complex and two molecules of nitrogen dioxide (weight loss of 68.25 (Calcd. 73.19%)) from the complex. The third weight loss observed between 386 °C and 685 °C corresponding to removal of one molecule of oxygen (weight loss of 10.55 (Calcd. 11.08%)) leaving CdO as metallic residue. 3.3. IR spectra The tentative assignments of the significant IR spectral bands of aroylhydrazone, HL and its cadmium(II) complexes are displayed

in the Table S3. A comparison of the IR spectra of hydrazone and metal complexes shows that significant variations have occurred in the characteristic frequencies upon complexation. The spectrum of aroylhydrazone exhibits bands at 3448 (broad and medium) and 1691 cm
1 which can be assigned to m(N–H) and m(C@O) stretching vibrations respectively. The absence of m(N–H) stretching mode in the spectrum of complex 1 suggests that the ligand loses this proton on complexation, thus act as a monoanionic deprotonated ligand [30,39]. Interestingly in complexes 2–4, the presence of broad bands in the range 3450–3300 cm
1 along with sharp bands in the region 1600–1650 cm
1 corresponding to the persistence of m(N–H) and m(C@O) stretching vibrations indicates that the aroylhydrazone, HL is coordinated in the neutral amido form in these complexes [40,41]. In complex 1, the absence of m(C@O) stretching

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Fig. 11. Halidep interactions for [Cd(HL)Cl2] (2).

Fig. 12. Packing diagram showing the hydrogen bonding interactions in [Cd(HL)(NO3)2(H2O)] (4).

vibration confirms the coordination through iminolate oxygen [42]. The presence of a band at 1509 cm
1 for the complex 1 is assigned to newly formed m(C@N) after iminolization [39,43]. The shift in m(C@N) stretching frequency observed at 1580 cm
1 in the ligand to 1551, 1520, 1590 and 1509 cm
1 for complexes 1, 2, 3 and 4 respectively [44] supports coordination via azomethine nitrogen. It is also evident from shift in m(N–N) bands in the region 1152–1133 cm
1. Variations in ring breathing vibrations of

pyridine ring bands in the range 1470–1420 cm
1 confirms involvement of the pyridine nitrogen in coordination as indicated by the variations observed in the ring breathing vibrations of pyridine ring and the in-plane ring deformation band of pyridine in the range 807–690 cm
1. In the IR spectrum of the complex 4, the nitrato group stretching vibrations were observed at 1463, 1292 and 1032 cm
1 [45]. These three stretching modes suggest that nitrate ions coordinate

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Fig. 13. Fluorescence spectra of HL and its complexes 1, 2, 3 and 4 in DMF (10
5 M).

to the metal as a bidentate ligand. This was further substantiated by X-ray structure analysis which shows that one nitrate ions bind to Cd(II) as bidentate chelating ligands and the other nitrate ion as an anisobidentate ligand. 3.4. NMR spectra The 1H NMR assignment for the hydrazone ligand HL and its Cd (II) complexes were recorded in DMSO. In aroyl hydrazone HL, the signal at 15.15 ppm is ascribed to the proton attached to the nitrogen (N4). In complexes 2–4, the upfield shift of this signal was observed at 15.13–14.98 ppm corresponding to proton attached to N4 which in turn supports the coordination of aroylhydrazone to the metal ion in amido form. The intensity of these signals is also found to be decreased considerably on D2O exchange confirmed the above assignment [8]. Aromatic protons are observed as multiplets in the range 8.96–7.25 ppm. Complex 1 do not show any signal corresponding to 4N–H revealing that the ligand adopts iminol form, followed by deprotonation prior to coordination with metal ion.

426 and 427 nm for complexes 1, 2, 3 and 4 respectively upon binding to the transition metal ion (kex 360 nm) (Fig. 13). Due to the difficulty in oxidizing or reducing d10 configuration, no emission arising from metal centered excited states of LMCT or MLCT are expected. High similarity in the locations and profiles of emission peaks of the metal free ligands and complexes allows to attributes the emission of these complexes to intraligand (n ? p⁄ and p ? p⁄) transitions within the hydrazone derivative. Cadmium(II) complexes in general can enhance or quench the fluorescence emission of organic ligand. The poor overlap of the metal orbitals with the orbitals of the ligand N-donors of the chelate ring causes the ligand electron pairs which are located in higher energy orbitals more available for quenching the fluorescence. The decrease in the emission intensities and enhancement of luminescence emission maxima may be due to energy transfer possible from the excited state of the ligand to metal ions thus increasing the non-radiative transitions of the ligand excited state and decreasing the fluorescence intensity [49,50].

4. Conclusion 3.5. Electronic spectra and photolumniscence properties The electronic spectral assignments of hydrazone and the its cadmium(II) complexes in DMF solution are summarized in Table S4. The electronic spectrum of the aroylhydrazone shows two absorption band maxima at 324 and 277 nm arising due to p ? p transitions. The spectra of complexes 1–4 show bands at 269, 271, 269 and 268 nm respectively [46,47] and these bands are ligand centered arising due to intraligand transitions and suffered considerable shift in intensity and wavelength on coordination. The band at 391, 393, 392 and 391 nm for complexes 1–4 corresponds to charge transfer transitions [48]. The photolumniscence properties of the aroylhydrazone as well as cadmium(II) complexes were studied in DMF (10
5 M). On excitation at 310 nm, the aroylhydrazone shows emission at 412 nm. Upon complexation, the photoluminescence intensities of metal complexes are changed with respect to that of ligand. The observed excitation maxima for ligand at 412 nm was shifted to 425, 430,

In the present paper we have discussed the synthesis, spectral and structural characterization of four new cadmium complexes of di-2-pyridyl ketone benzoylhydrazone. The reaction of aroylhydrazone comprising of flexible coordinating NNO framework (amido/iminol forms) with different metal salts generated mononuclear complexes with diverse distorted coordination geometries like square pyramidal (2 and 3), octahedral (1) and triangulated dodecahedral (4). The amido-iminol tautomerism enables the proligand coordinating via deprotonated iminol form in [CdL2] (1) and neutral amido form in [Cd(HL)Cl2] (2), [Cd(HL) Br2] (3) and [Cd(HL)(NO3)2(H2O)] (4). By changing the counter ion the coordination type of the aroyl hydrazone and geometry around the Cd(II) center can be varied. In [Cd(HL)(NO3)2(H2O)] (4), the nitrate ion appears in anisobidentate as well as bidentate fashion. The thermal behaviour studies of complex [Cd(HL)(NO3)2(H2O)] (4) shows that the complex was stable up to 110 °C. The photolumniscence properties of the aroylhydrazone as well as

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cadmium(II) complexes were studied. The influence of hydrogen bonding and weak non covalent forces augmenting the crystal packing were also investigated. The crystal packing was stabilized by intra and intermolecular hydrogen bonds. The photoluminsence studies of the aroylhydrazone HL and its cadmium complexes show quenching. Acknowledgments DK acknowledges UGC, New Delhi, India for the award of a Junior Research Fellowship. AAA acknowledges CSIR for the award of a Senior Research Fellowship. The authors are thankful to the Sophisticated Analytical Instrumentation Facility, Cochin University of Science and Technology, Kochi, India for elemental analyses, 1H NMR and TG-DTG analyses and single crystal X-ray diffraction measurements. We also thank the School of Environmental studies, Cochin University of Science and Technology, Kochi, India for the photoluminescence studies. Appendix A. Supplementary data CCDC 1509951, 1509994, 1509993 and 1509995 contain the supplementary crystallographic data for the complexes [CdL2] (1), [Cd(HL)Cl2] (2), [Cd(HL)Br2] (3) and [Cd(HL)(NO3)2(H2O)] (4). 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.2017.01.041. References [1] R.N. Patel, Y. Singh, Y.P. Singh, R.J. Butcher, A. Kamal, I.P. Tripathi, Polyhedron 117 (2016) 20–34. [2] S. Mondal, B. Pakhira, A.J. Blake, M.G.B. Drew, S.K. Chattopadhyay, Polyhedron 117 (2016) 327–337. [3] M.G. Retamosa, E. Matador, D. Monge, J.M. Lassaletta, R. Fernández, Eur. J. Chem. 22 (2016) 13430–13445. [4] R.S. Nair, M. Kuriakose, V. Somasundaram, V. Shenoi, M.R.P. Kurup, P. Srinivas, Life Sci. 116 (2014) 90–97. [5] H.A. El-Wahab, M.A. El-Fattah, A.H. Ahmed, A.A. Elhenawy, N.A. Alian, J. Organomet. Chem. 791 (2015) 99–106. [6] Y. Zhang, T. Yang, B.-Y. Zheng, M.-Y. Liu, N. Xing, Polyhedron 121 (2017) 123– 129. [7] N.A. Mangalam, E.B. Seena, M.R.P. Kurup, Polyhedron 29 (2010) 3318–3323. [8] V.P. Singh, S. Singh, D.P. Singh, K. Tiwari, M. Mishra, R.J. Butcher, Polyhedron 56 (2013) 71–81. [9] M. Bakir, R. Conry, J. Coord. Chem. 69 (2016) 1244–1257. [10] J. Chakraborty, S. Thakurta, G. Pilet, D. Luneau, S. Mitra, Polyhedron 28 (2009) 819–825.

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