Syntheses, structures and properties of μ3-carbonato bridged trinuclear zinc(II) complexes containing a tailored tetradentate amine

Polyhedron 32 (2012) 54–59

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Syntheses, structures and properties of l3-carbonato bridged trinuclear zinc(II) complexes containing a tailored tetradentate amine Sumitava Khan a, Subhasis Roy a, Kishalay Bhar a, Ramachandran Krishna Kumar b, Tapas Kumar Maji b,⇑, Barindra Kumar Ghosh a,⇑ a b

Department of Chemistry, The University of Burdwan, Burdwan 713104, India Chemistry and Physics of Materials Unit, Jawaharlal Nehru Center for Advanced Scientific Research, Jakkur, Bangalore 560064, India

a r t i c l e

i n f o

Article history: Available online 13 July 2011 Keywords: Trinuclear zinc(II) N,N0 -bis(3-aminopropyl)-1,2ethanediamine l3-Carbonate Synthesis X-ray structure

a b s t r a c t Two trinuclear zinc(II) compounds of the type [Zn3(l3-CO3)(L)3](X)4 [L = N,N0 -bis(3-aminopropyl)-1,2ethanediamine; X = ClO4 (1), PF6 (2)] have been prepared by reactions of 1:1 molar ratio of zinc(II) salt and L in open air at room temperature using appropriate counter anions. The compounds are characterized by microanalytical, spectroscopic, thermal and other physicochemical properties. Structures of 1 and 2 are solved by single crystal X-ray diffraction measurements. Structural study reveals that a l3-carbonate binds three zinc(II) centers; each metal center in 1/2 adopts a distorted trigonal bipyramidal geometry with a ZnN4O chromophore bound by four N atoms of the tetradentate amine (L) and one O atom of carbonate. The trinuclear units in 1/2 are engaged in N–H  O/N–H  F and C–H  F hydrogen bondings leading to different 3D network structures. Ó 2011 Elsevier Ltd. All rights reserved.

1. Introduction The activation of CO2 and its chemical fixation by metal complexes are of continued attention [1–7] to control concentration of this greenhouse gas reducing several serious environmental problems [8,9]. Development of effective chemical methods to absorb CO2 and to convert it [10] into useful materials by metal complexes is a challenge to the synthetic coordination chemists. Metalloenzymes like carbonic anhydrase, D-ribulose 1,5-bisphosospate carboxylase-oxygenase, non-heme iron in the photosynthetic system II, copper complexes of the cyclic peptide ascidiacyclamide, etc. [11–15] are well-known to have pivot role in activation and fixation of CO2. Many metal carbonate complexes are synthesized with different binding modes of the carbonate dianion [16–25]. In most cases, the carbonate complexes were obtained by addition of Na2CO3, NaHCO3 or bubbling CO2 to the reaction solution [16–19]. Only a few examples of fixation of CO2 from air [20–25] are known at present. A macrocyclic amidourea on dissolving in DMSO along with tetrabutyl ammonium fluoride, absorbs CO2 from the atmosphere to form a complex in which CO32 is held by a number of O  H–N bonds within the bowlshaped cavity of the macrocycle [23]. The reaction of copper(II) perchlorate and 1,2-bis(4-pyridyl)ethane (bpe) in basic aqueous ⇑ Corresponding authors. Tel.: +91 80 22082826; fax: +91 80 22082766 (T.K. Maji), tel.: +91 342 2533913; fax: +91 342 2530452 (B.K. Ghosh). E-mail addresses: [email protected] (T.K. Maji), [email protected] (B.K. Ghosh). 0277-5387/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.poly.2011.07.001

solution yields a 3D compound, {[Cu3(CO3)2(bpe)3]2ClO4}n through the atmospheric fixation of CO2 composing Cu(CO3) kagomé layers pillared by the bpe linker that exhibits weak antiferromagnetic interaction in the kagomé layer and interlayer ferromagnetic coupling at low temperature [24]. Reactions of Ln(III) acetate (Ln = Pr and Nd) and a polydentate Schiff base in a mixture of methanol and acetonitrile resulted in a novel Ln10 aggregate with two Ln5 pentagons templated by l5-CO32 via spontaneous fixation of atmospheric carbon dioxide [25]. This work stems from our interest to develop a new system for effective fixation of atmospheric CO2. In the present endeavor, we examine this behavior to zinc(II) in combination with a tailored tetradentate amine, N,N0 bis(3-aminopropyl)-1,2-ethanediamine (L; Scheme 1). We have successfully isolated two trinuclear zinc(II) compounds of the type [Zn3(l3-CO3)(L)3](X)4 [X = ClO4 (1) and PF6 (2)] by reactions of a 1:1 molar ratio of zinc(II) salt and L in open air at room temperature using appropriate counter anions. The syntheses, characterizations, structures and properties of these new compounds are described below. 2. Experimental 2.1. Materials High purity N,N0 -bis(3-aminopropyl)-1,2-ethanediamine (Lancaster, UK), zinc(II) chloride (E. Merck, India) and potassium hexaflurophosphate (Fluka, Germany) were purchased from respective concerns and used as received. Zinc(II) perchlorate

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S. Khan et al. / Polyhedron 32 (2012) 54–59

N H

NH 2

N H

H 2N

Scheme 1. N,N0 -bis(3-aminopropyl)-1,2-ethanediamine (L).

hexahydrate was prepared [26] by treatment of the zinc(II) carbonate (E. Merck, India) with perchloric acid (E. Merck, India) followed by slow evaporation on a steam-bath, filtration through a fine glass-frit and preserved in a desiccator containing concentrated sulfuric acid (E. Merck, India) for subsequent use. All other chemicals and solvents used were AR grade. The synthetic reactions and work-up were done in open air. Caution! Perchlorate compounds of metal ions are potentially explosive [27] especially in the presence of organic ligands. Only a small amount of these materials should be prepared and handled with care. 2.2. Physical measurements Elemental analyses (carbon, hydrogen and nitrogen) were performed on a Perkin-Elmer 2400 CHNS/O elemental analyzer. IR spectra (KBr discs, 4000–400 cm1) were recorded using a Perkin-Elmer FTIR model RX1 spectrometer. Molar conductances were measured using a Systronics conductivity meter where the cell constant was calibrated with 0.01 M KCl solution and dry MeCN was used as solvent. Ground state absorption measurements (in MeCN) were made with a Shimadzu model UV-2450 spectrophotometer. Thermal behaviors were investigated with a Perkin-Elmer Diamond TG/DT analyzer heated from 40 to 740 °C under nitrogen.

The microanalytical and spectroscopic results of 2 obtained from both the methods (A and B) are similar. Anal. Calc. for C25H66N12O3F24P4Zn3 (2): C, 22.1; H, 4.9; N, 12.4. Found: C, 22.2; H, 4.9; N, 12.5%. IR (KBr, cm1): m(N–H) 3242; m(C–H) 2952, 2877; m(CO3) 1479, 1440; 739; m(PF6) 837, 558. KM (MeCN, ohm1 cm2 mol1): 420. UV–Vis (k, nm): 266, 228. 2.4. X-ray crystallographic study Suitable single crystals of 1 and 2 were mounted on a thin glass fiber and X-ray single crystal structural data were collected on a Bruker Smart-CCD diffractometer using graphite monochromated Mo Ka radiation (k = 0.71073 Å) at 100 K with the Oxford Cyrosystem Cobra low-temperature attachment. The program SAINT [28] was used for integration of diffraction profiles and empirical absorption corrections were made with SADABS [29] program. The structures were solved by SIR 92 [30] and refined by full matrix least squares method using SHELXL 97 [31]. The non-hydrogen atoms were refined anisotropically. All hydrogen atoms were located by Fourier analysis. In case of 1, there are three different ClO4 anions in the asymmetric unit as counter anion and among them two are in positional disordered states; the atoms Cl1, O2, O3 and Cl2, O4, O5 are in the disordered states and Cl1 and Cl2 sit at special positions with oxygen atoms at general positions. O2 atoms are generated by the inversion center situated at Cl1 and O4 atoms are generated by the inversion center at Cl2. Two three-fold axes pass through the atoms Cl1, O2 and Cl2, O4 of the perchlorate anions that generate other oxygen atoms. It is a three symmetry combination operations involving the inversion center and the two threefold axes that give rise to disorder in the perchlorate anions. In the same way, 2 contains three PF6 anions in the asymmetric unit out of which two are in positional disordered states; the atoms P3, F8, F9 are in disordered states and P3 is at special position with the fluorine atoms at general positions. F8 atoms are generated by the

2.3. Preparation of the complexes 2.3.1. [Zn3(l3-CO3)(L)3](ClO4)4 (1) L (0.174 g, 1 mmol) in methanol (10 cm3) was added slowly to a Zn(ClO4)26H2O (0.372 g, 1 mmol) solution (10 cm3) in the same solvent. After filtration through a fine glass-frit, the supernatant colorless solution was kept in open air for slow evaporation. Rectangular crystals of 1 that deposited within a week were separated by filtration and dried in vacuo over silica gel indicator. Yield: 0.52 g (45%). Anal. Calc. for C25H66N12O19Cl4Zn3 (1): C, 25.5; H, 5.6; N, 14.3. Found: C, 25.7; H, 5.6; N, 14.5%. IR (KBr, cm1): m(N–H) 3220; m(C–H) 2924, 2874; m(CO3) 1482, 1439, 837, 736; m(ClO4) 1150, 1119, 1082, 625. KM (MeCN, ohm1 cm2 mol1): 425. UV–Vis (k, nm): 265, 229. 2.3.2. [Zn3(l3-CO3)(L)3](PF6)4 (2) 2.3.2.1. Method A. A methanolic solution (10 cm3) of L (0.174 g, 1 mmol) was added slowly to a solution (10 cm3) of ZnCl2 (0.136 g, 1 mmol) in the same solvent; to this mixture a methanolic solution (30 cm3) of KPF6 (0.736 g, 4 mmol) was added. The resulting colorless solution was filtered and left undisturbed in air for slow evaporation. After a week colorless rectangular shaped crystals that separated were collected in pure form as described in 1. Yield: 0.82 g (70%). 2.3.2.2. Method B. The compound 2 was also isolated by metathesis of 1 with KPF6 in 1:4 molar ratios in a methanolic solution (40 cm3) with constant stirring for 45 min at room temperature. The resulting colorless solution was filtered and left undisturbed in air for slow evaporation. Colorless shining microcrystals of 2 were obtained in almost quantitative yield.

Table 1 Crystallographic data for 1 and 2. Crystal parameters

1

2

Formula Formula weight Crystal system Space group a (Å) c (Å) V (Å3) k (Å) qcalcd (g cm3) Z T (K) l (mm1) F(0 0 0) Crystal size (mm3) h Ranges (°) h/k/l Reflections collected Independent reflections Tmaximum and Tminimum Data/restraints/ parameters Goodness-of-fit (GOF) on F2 Final R indices [I > 2r(I)]

C25H66N12O19Cl4Zn3 1176.81 trigonal  R3

C25H66N12O3F24P4Zn3 1359.14 trigonal  R3

21.2386(7) 18.0909(7) 7067.1(4) 0.71073 1.659 6 100(2) 1.823 3660 0.10  0.08  0.06 1.58–30.55 30, 29/30, 30/25, 25 26 765 4794 0.9916 and 0.7690 4794/0/203

21.8281(4) 18.6708(5) 7704.2(3) 0.71073 1.757 6 100(2) 1.645 4140 0.10  0.08  0.08 1.87–32.95 32, 32/31, 23/28, 27 14 793 5824 0.9816 and 0.7892 5824/4/209

1.102

1.242

R = 0.0596 and wR = 0.1846 R = 0.0789 and wR = 0.1991 1.849 and 1.490

R = 0.0846 and wR = 0.1703 R = 0.1006 and wR = 0.1746 1.117 and 1.178

R indices (all data) Largest peak and hole (e Å3)

P P P P Weighting scheme: R = ||Fo|  |Fc||/ |Fo|, wR2wR = [ w(F 2o  F 2c )2/ w(F 2o )2]1/2, 2 2 2 calcd w = 1/[r (F o ) + (0.1197P) + 12.5748P] for 1; calcd w = 1/[r2(F 2o ) + (0.0046P)2 + 145.1216P] for 2; where P = (F 2o þ 2F 2c )/3.

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inversion center at P3. There are two three-fold axes that pass through P3, F8 which generate the other fluorine atoms. These three operations result in the disorder in 2. All calculations were carried out using SHELXL 97, SHELXS 97 [32], PLATON 99 [33] and MERCURY 1.4.1 [34]. Crystal data and structure refinement parameters for 1 and 2 are summarized in Table 1. 3. Results and discussion 3.1. Synthesis and formulation The trinuclear compound [Zn3(l3-CO3)(L)3](ClO4)4 (1) was obtained in good yield through reaction of a 1:1 molar ratio of zinc(II) perchlorate hexahydrate and L in open air at room-temperature. For preparation of 2, four equivalents of potassium hexafluorophosphate was added to the reaction mixture containing 1:1 molar ratio of zinc(II) chloride and L. The formation of carbonato complexes 1 and 2 may presumably be due to intake of CO2 from atmosphere by the reactive zinc(II) amine species that leads to transformation of CO2 into CO32 in methanolic solution followed by coordinating three metal centers through l3-bridging mode. The compound 2 was also isolated by metathesis of 1 with potassium hexafluorophosphate in methanolic solution. All the reactions were reproducible as was evident from repetitive microanalytical results, solution electrical conductivity data, spectral behaviors and other physicochemical properties. The reactions are summarized in the following Eqs. (1)–(3): MeOH

ZnðClO4 Þ2  6H2 O þ L ! ½Zn3 ðl3 -CO3 ÞðLÞ3 ðClO4 Þ4 air

ð1Þ

ð1Þ

MeOH

ZnCl2 þ L þ KPF6 ! ½Zn3 ðl3 -CO3 ÞðLÞ3 ðPF6 Þ4 air

ð2Þ

ð2Þ

MeOH

½Zn3 ðl3 -CO3 ÞðLÞ3 ðClO4 Þ4 ! ½Zn3 ðl3 -CO3 ÞðLÞ3 ðPF6 Þ4 ð1Þ

Fig. 1. Molecular structure of [Zn3(l3-CO3)(L)3]4+ in 1 and 2 with atom labeling schemes. Symmetry operations: a = y, x  y, z (for 1) and 1  y, 1 + x  y, z (for 2); b = x + y, x, z (for 1) and 2  x + y, 1  x, z (for 2).

KPF6

ð3Þ

ð2Þ

The moisture-insensitive complexes are stable over long periods of time in powdery or crystalline state and are soluble in water and in a wide range of common organic solvents such as methanol, acetonitrile, dimethylformamide, dimethylsulfoxide. In MeCN solutions, 1 and 2 show conductivity values [KM, ohm1 cm2 mol1: 425 (1), 420 (2)] corresponding to 4:1 electrolytic behavior [35]. In IR spectra, m(N–H) stretching frequencies of the –NH2 groups of L in 1 and 2 are observed at 3230 cm1. Several weak bands in the range 2870–2960 cm1 assignable to aliphatic C–H stretching vibration are routinely observed [36] in both the complexes. The asymmetric m3 stretching vibrations of carbonate [37] are seen at 1482 and 1439 cm1 for 1 and 1479 and 1440 cm1 for 2. The in-plane m4 deformation of m(CO3) is found at 736 cm1 (in 1) and 739 cm1 (in 2). Another band at 837 cm1 in 1 is observed due to the out-of-plane deformation m2 of m(CO3); but in 2 such type of m2 deformation is obscured presumably due to the overlap with m(PF6) stretches. The results are confirmed by X-ray structure determinations of the compounds. The presence of perchlorate stretches at 1150, 1119, 1082 and 625 cm1 for 1 and hexafluorophosphate bands at 837 and 558 cm1 for 2 are indicative of non-coordination [38] to the metal centers. The colorless solutions of 1 and 2 show bands at 265 and 230 nm assignable [39] to ligand based transitions. 3.2. X-ray crystal structures of [Zn3(l3-CO3)(L)3](ClO4)4 (1) and [Zn3(l3-CO3)(L)3](PF6)4 (2) The molecular structure of the tetracationic trinuclear core in 1 and 2 with the atom labeling scheme is given in Fig. 1 and the

Fig. 2. (a) A short segment of 1D chain in 1 formed through trifurcated N–H  O hydrogen bonds running along c-axis. (b) Perspective view of 3D network structure in 1 showing cyclic R24 (8) in Etter’s graphs notation.

perspective views of different crystalline architectures in 1 and 2 are shown in Figs. 2 and 3. Selected bond distances and bond angles relevant to the metal coordination spheres in 1 and 2 are shown in Table 2. The significant hydrogen bonding parameters of the complexes are set in Table 3. Structural analyses show that both the compounds consist of one trinuclear [(ZnL)3(l-CO3)]4+ cation (Fig. 1) and four perchlorates (in 1) or four hexafluorophosphates (in 2) as counter anions. The coordination polyhedron around each zinc(II) center in 1 and 2 is best described as a distorted trigonal bipyramid [s = 0.68 (for 1) and s = 0.78 (for 2)] geometry with a ZnN4O chromophore [40]. To the best of our knowledge, till date three other trinuclear zinc(II) compounds with l3-carbonato bridge containing tetradentate ligands viz. 1,4,7,10-tetraazacyclododecane [4], tri(pyridilmethyl)amine [5] and 1,4,7,10-tetraazacyclotridecane [6] are reported. Each zinc(II) center adopts a square pyramidal geometry in the first case [4], whereas for the latter two [5,6] trigonal bipyramidal geometry of the metal ion is the result similar to 1 and 2. The

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S. Khan et al. / Polyhedron 32 (2012) 54–59 Table 3 Hydrogen bond distances (Å) and angles (°) for compounds 1 and 2. Compound

Fig. 3. (a) A short segment of 1D chain in 2 formed through multiple N–H  F and C–H  F hydrogen bonds along c-axis. (b) Perspective view of the 3D network structure in 2.

Table 2 Selected bond distances (Å) and bond angles (°) for 1 and 2. Bond distances for 1

Bond angles for 1

C10–O1 Zn1–O1 Zn1–N1 Zn1–N2 Zn1–N3 Zn1–N4

O1–Zn1–N1 O1–Zn1–N3 O1–Zn1–N2 O1–Zn1–N4 N4–Zn1–N1 N1–Zn1–N3 N4–Zn1–N3 N2–Zn1–N1 N2–Zn1–N3 N4–Zn1–N2 O1–C10–O1a

89.47(11) 94.44(11) 130.17(11) 112.43(14) 92.36(13) 171.21(14) 93.42(15) 89.72(13) 81.74(14) 117.39(15) 120.0

Bond angles for 2 O1–Zn1–N1 O1–Zn1–N3 O1–Zn1–N2 O1–Zn1–N4 N4–Zn1–N1 N1–Zn1–N3 N4–Zn1–N3 N2–Zn1–N1 N2–Zn1–N3 N4–Zn1–N2 O1–C10–O1a

89.54(16) 94.12(15) 125.43(15) 111.14(16) 91.09(18) 172.15(17) 94.06(17) 90.12(17) 82.09(17) 123.42(18) 120.0(3)

1.285(2) 2.015(2) 2.135(3) 2.115(3) 2.196(3) 2.077(3)

Bond distances for 2 C10–O1 1.288(3) Zn1–O1 2.013(3) Zn1–N1 2.143(5) Zn1–N2 2.100(4) Zn1–N3 2.192(5) Zn1–N4 2.089(5)

Symmetry code: a = y, x  y, z (for 1) and 1  y, 1 + x  y, z (for 2).

coordination in 1/2 includes one tetradentate amine (L) ligated by its four N atoms (N1, N2, N3, N4) and one O atom (O1) of the carbonate anion. The equatorial positions are occupied by the two amine N atoms (N2 and N4) of L and one carbonate O atom (O1) with bond lengths in the range of 2.015(2)–2.115(3) Å in 1 and 2.013(3)–2.100(4) Å in 2; the remaining two amine N atoms (N1 and N3) occupy the axial positions with Zn–N distances Zn1–N1 2.135(3) Å (1) and 2.143(5) Å (2), and Zn1–N3 2.196(3) Å (1) and 2.192(5) Å (2). The Zn1–O1 distance is the shortest among all reflecting strong coordination of the anionic carbonate over neutral amine. Compared to the reported [4–6] compounds, the Zn–N bond

D–H  A c

D–H

H  A

D  A

D–H  A

1

N2–H2  O6 N3–H3  O2 N1–H9A  O4 N1–H9B...O6d N4–H10A...O7 N4–H10B...O7d

0.91 0.91 0.90 0.90 0.90 0.90

2.41 2.25 2.20 2.31 2.20 2.54

3.165(5) 3.157(6) 3.075(5) 3.196(5) 3.089(7) 3.193(6)

140 174 165 169 172 130

2

N1–H1  F7c N1–H2  F8a N1–H2  F9d N2–H3  F7e N3–H4  F1 N3–H4  F1a N4–H4C  F7c N4–H4D  F4c C2–H2A  F8d C3–H3A  F2 C3–H3A  F6 C3–H3B  F5e

0.90(3) 0.91(7) 0.91(7) 0.90(10) 0.89(7) 0.89(7) 0.90 0.90 0.97 0.97 0.97 0.97

2.39(4) 2.53(6) 2.24(7) 2.45(8) 2.35(7) 2.37(7) 2.32 2.34 2.41 2.45 2.45 2.45

3.223(5) 3.13(2) 3.139(5) 2.967(7) 3.118(6) 3.073(6) 3.058(6) 3.154(7) 3.16(2) 3.380(9) 3.271(9) 3.369(8)

154(6) 124(5) 171(5) 117(7) 145(6) 136(6) 139 151 134 161 143 157

Symmetry codes: c = 1/3 + x  y, 2/3 + x, 1/3  z; d = 2/3  x, 1/3  y, 1/3  z (for 1). a = 1  y, 1 + x  y, z; c = 5/3  x, 1/3  y, 4/3  z; d = 2  x,  y, 1  z; e = 2/3 + y, 1/3  x + y, 4/3  z (for 2)

lengths are similar, but Zn–O distances are somewhat larger for 1/ 2. Each metal(II) center deviates 0.015 Å (in 1) and 0.009 Å (in 2) from the mean basal plane. The degrees of distortion from ideal trigonal bipyramidal geometry are reflected in the equatorial [112.43(14)–130.17(11)° (for 1) and 111.14(16)–125.43(15)° (for 2)] and axial [171.21(14)° (for 1) and 172.15(17)° (for 2)] bond angles. In the trinuclear units Zn  Zn separations are 4.876 Å in 1 and 4.971 Å in 2. The metal bound carbonate is essentially planar as reflected in its bond angles and bond distances (Table 2) as is found in literature [4–6]. In the packing of 1/2, the trinuclear units are engaged in multiple cooperative N–H  O/N–H  F and C–H  F hydrogen bonds involving H atoms of –NH–, NH2– and –CH2– groups in L and O/F atoms in ClO4/PF6 counter anions that give rise to different 3D network structures (Figs. 2 and 3). A 1D chain structure (Figs. 2a and 3a) running along crystallographic c-axis is the result in 1/2 through hydrogen bonds of types trifurcated N–H  O [N3– H3  O2 2.25 Å, 174°; N1–H9A  O4 2.20 Å, 165°; see Table 3]/ 0 cooperative N–H  F and C–H  F [N1–H2  F8a 2.53(6) Å A, 124(5)°; N1–H2  F9d 2.24(7) Å, 171(5)°; C2–H2A  F8d 2.41 Å, 134°; see Table 3], respectively. The chains are again associated through other N–H  O in 1 and N–H  F and C–H  F in 2 hydrogen bonds (Table 3) in bc-plane leading to different 3D network structures (Figs. 2a and 3b). Interestingly, in 1 a cyclic motif R24 (8) in Etter’s graphs notation [41] between two trinuclear units of two adjacent 1D chains and two perchlorates is formed by N–H  O hydrogen bonds [N4–H10A...O7 2.20 Å 172° and N4–H10B...O7d 2.54 Å, 130°; see Table 3] involving two H atoms (H10A and H10B) of the –NH2 group on one trinuclear unit and one O atom (O7) of a perchlorate ion and vice versa.

3.3. Thermogravimetric study To examine thermal stabilities of 1 and 2, thermogravimetric and differential thermal analyses (TG–DTA) were made between 40 to 740 °C in a static atmosphere of nitrogen. The TG curve (Fig. 4a) indicate that the complex 1 is stable up to 278 °C and then releases the carbonate moiety (weight loss: observed 8.2%; calc. 5.1%) in the temperature range 278–306 °C. The second weight loss (observed 39.2%; calc. 44.4%) in the range of 306–467 °C is due to

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S. Khan et al. / Polyhedron 32 (2012) 54–59

Fig. 4. Thermal behaviors of (a) compounds 1 and (b) 2.

the simultaneous decomposition of the three polyamine (L) molecules with an exothermic effect at 353 °C. Compound 2 shows thermal stability up to 265 °C and TG curve (Fig. 4b) indicates that the carbonate moiety is departed (weight loss: observed 8.9%; calc. 4.4%) in the temperature range 265–315 °C. The second weight loss (observed 48.5%; calc. 38.5%) between 315 and 377 °C is due to the removal of three polyamine molecules with an endothermic effect at 334 °C. In summary, thermogravimetric analyses show that compounds 1 and 2 have reasonable thermal stabilities and decompose above the temperature range 260–280 °C.

dimensionality in crystalline state. We are now active to investigate such fixation of this green house gas to some paramagnetic metal ions such as copper(II), nickel(II), cobalt(II) and manganese(II) in combination with L and other polydentate amines. Acknowledgements BKG thanks the DST and CSIR, New Delhi, India for financial support. TKM acknowledges financial support from JNCASR, Bangalore, India. S.K., K.B. and S.R. are grateful to CSIR and UGC, New Delhi, India for fellowships.

4. Conclusion This work demonstrates that the organic polydentate amine in zinc(II) bound state triggers carbon dioxide absorption from the atmosphere and fixation to the metal ion through chemical conversion. X-ray structural study shows presence of the l3-carbonate bridge that leads to a new trinuclear entity. Different multiple intermolecular hydrogen bonds in 1 and 2 lead to promotion of

Appendix A. Supplementary data CCDC 803936 and 803937 contain the supplementary crystallographic data for 1 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,

S. Khan et al. / Polyhedron 32 (2012) 54–59

Cambridge CB2 1EZ, UK; fax: (+44) 1223-336-033; or e-mail: [email protected].

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