Synthesis and Properties of Dinuclear Imine Complexes of Zn(II) and Cd(II) by Novel Route

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1 DOI: /cst Chemical Science Transactions ISSN: , 7(3), RESEARCH ARTICLE Synthesis and Properties of Dinuclear Imine Complexes of Zn(II) and Cd(II) by Novel Route VASISHTA D. BHATT 1* and SAMAT R. RAM 2 1 Department of Chemistry, School of Sciences,Gujarat University, Ahmedabad , Gujarat, India 2 P.B. Science College, Kapadwanj- Gujarat, India vdbhatt@gujaratuniversity.ac.in Received 29 December 2017 / Accepted 30 January 2018 Abstract: Mononuclear complexes of o-vanillin and 2-aminophenol with Zn(II) and Cd(II) were used as precursor to obtain new homo and heterodinuclear imine complexes. The complexes obtained by this new route using inter-complex reaction were then characterized by methods like elemental analysis, IR spectroscopy and thermogravimetric analysis. The powder x-ray diffractogram indicated crystalline nature with tetragonal symmetry of the complexes. The molecular ion peaks in the mass spectra supported the structures of dinuclear complexes. Finally, the structures of the complexes were confirmed using 1 H and 13 C NMR spectroscopy. A mechanism of the complex formation was proposed. Keywords: Dinuclearcomplex, Inter-complex reaction, Schiff base Introduction The diamagnetic nature of metal complexes of Zn(II) and Cd(II) offer an ease for the determination of structure with the help of magnetic resonance studies. Thus, these complexes have carved a niche in studying the structural aspects of coordination chemistry. Also, owing to the completely filled configuration of d-orbitals, these complexes exhibit distinguished coordinative properties 1. Numerous mononuclear metal complexes obtained using Schiff bases and other polydentate ligands have been well documented 2. The homodinuclear complexes of these metals as expected exhibit enhanced properties compared to their mononuclear analogues. Whereas, the heterodinuclear complexes show novel properties originating from the interactions between the metal ions 3,4. The preparation and characterization of such dinuclear complexes requires insightful conditions of reaction and selection of moieties. Mononuclear complexes having ligands with unutilised functionality have been used successfully to prepare homo and heterodinuclear complexes. Here, a complex containing unutilized functionality in ligand is considered as a ligand and named as metal organic ligand (MOL) 5,6. Under suitable conditions, this MOL can further coordinate with a different metal ion resulting in formation of mixed metal complexes.

2 Chem Sci Trans., 2018, 7(3), We report here, a novel approach of synthesizing dinuclear complexes. It has been hypothesized that the coordinated ligands of two metal chelates can be reacted to obtain a new metal chelate. In the present work, we have allowed to react, two such complexes under the conditions that permit coordinated NH 2 to react with the coordinated CHO group. Here, as ionic bonds of the precursor do not dissociate, metal-ligand bonding in both the complexes remained intact. The coordinated amino and aldehyde groups reacted to form Schiff base dinuclear complex. Thepresent work is extension of our previous work on synthesis of dinuclear complexes of Co(II), Ni(II) and Cu(II) Due to the paramagnetic nature of the complexes in our earlier investigation, the NMR spectra of the complexes could not be reported. Thus, diamagnetic homo and heterodinuclear complexes of Zn(II) and Cd(II) were synthesized and characterized. Experimental o-aminophenol and o-vanillin ( 99.0%) were purchased from Sigma-Aldrich. Zinc acetate, cadmium acetate, sodium hydroxide and solvents ( 99.0%) were purchased from E-Merck Ltd, Mumbai (India). The purification was done according to the needs through known procedures 13. Measurements Elemental analysis (C, H and N) was carried out on PERKIN ELMER, Series II, 2400 CHNS/O analyzer. The metal composition of the complexes was determined by EDTA titration subsequent to the decomposition of metal complexes with an acid mixture of HClO 4, H 2 SO 4 and HNO 3 (1:1.5:2.5). The amount of zinc content in Schiff base complex Zn 2 (SB) 2 (H 2 O) 2 was determined by direct EDTA titration method 14. The amount of cadmium content in Cd 2 (SB) 2 (H 2 O) 2 was determined by reaction with excess EDTA and back titrated with standard zinc solution 15. The amount of zinc and cadmium in heterometallic complexes was obtained by known volumetric method 16,17. Infrared spectra were recorded on a Fourier transform infrared (FTIR), GX FT-IR PERKIN ELMER, in the range cm -1 by making a KBr pellet of the compound. Thermogravimetric analysis was carried out on METTLER TOLEDO. The mass spectra are obtained using API QSTAR Pulsar LC-MS quadruple and TOF based single mass spectrometer. In case of the Zn- Cd complexes, mass spectra peaks based on isotope ratio, Zn %, Zn %, Zn % and the mass of the most intense isotope peak is given. 1 H NMR and 13 C NMR were carried out on Bruker 400 MHz FT-NMR spectrometer. Powder XRD was obtained on XRD Diffractometer (powder) Philips Xpert MPD. Synthesis The synthesis consisted of three steps as shown below: In the first step, a solution of o-vanillin (0.612 g, 0.2 M) in absolute alcohol (~20 ml) was prepared and a solution of zinc acetate/cadmium acetate (0.438/0.533 g, 0.1 M) in rectified spirit (~20 ml), were mixed and stirred for 2 hours to obtain a four coordinated complex viz. M(O-VAN) 2 in solution. The reaction is shown in Eq. 1. M + 2(O-VAN) M(O-VAN) 2 (1) In the second step, a solution of o-aminophenol (0.436 g, 0.2 M) in absolute alcohol (~20 ml) was prepared and a solution of zinc acetate/cadmium acetate (0.438/0.533 g, 0.1 M) in rectified spirit (~20 ml), were mixed and stirred for 2 hours to obtain a four coordinated complex viz. M (2-AP) 2 in solution. The reaction is shown in Eq. 2. M + 2(2-AP) M (2-AP) 2 (2)

3 479 Chem Sci Trans., 2018, 7(3), In the last step, a solution of M (O-VAN) 2 was added to the refluxing solution of M' (2-AP) 2. The reaction mixture was refluxed for 6 hours in a water bath to obtain the product under slightly alkaline condition created by sodium hydroxide.the precipitates were then filtered, washed with ethanol and dried over fused CaCl 2. The reaction of the third step is depicted in Eq. 3. M(O-VAN) 2 + M'(2-AP) 2 MM'(SB) 2 (H 2 O) 2 (3) All the complexes were prepared by the method discussed above. The elemental analysis, NMR and mass spectral data of all complexes are reported here. Zn 2 (SB) 2 (H 2 O) 2 (1) Homo dinuclear complex Zn 2 (SB) 2 (H 2 O) 2 was obtained using above discussed method where both M and M' = Zn (II). Yield: 72.4%, m.p.: > 350 o C. Anal. Calc. for C 28 H 26 N 2 O 8 Zn 2 (649.27): C, 51.80; H, 4.04; N, 4.31; Zn, %. Found: C, 51.71; H, 3.96; N, 4.19; Zn, 20.06%, mass: m/z=648[c 28 H 26 N 2 O 8 Zn 2 64 ] +, 650 [C 28 H 26 N 2 O 8 Zn 2 66 ] +, 617 [M-H 4 O 2 +5H] +, 521 [M-H 4 O 2 -C 6 H 3 O] +, 418 [M-H 4 O 2 -C 7 H 5 O 2 NZn+5H] +, 385 [M-H 4 O 2 C 8 H 8 O 3 NZn+2H] +, 372[(SB)+Zn 2 +H] +, 244[(SB)+H] +. 1 H NMR (DMSO, 400 MHz) δ/ppm 3.78 (s, 3H, -OCH 3 ), 8.90 (s, 1H, C=N), (m, 7H, Ar.). 13 C NMR (DMSO, 400MHz) δ/ppm (2C, -OCH 3 ), (12C, Ar). Cd 2 (SB) 2 (H 2 O) 2 (2) Homo dinuclear complex Cd 2 (SB) 2 (H 2 O) 2 was obtained using same process where both M and M' = Cd (II). Yield: 70.1%, m.p.: > 350 o C. Anal. Calc. for C 28 H 26 N 2 O 8 Cd 2 (743.34) : C, 45.24; H, 3.53; N, 3.77;Cd, %. Found : C, 45.11; H, 3.63; N, 3.70 Cd, 30.00%. mass: m/z=744 [C 28 H 26 N 2 O 8 Cd ] +, 746 [C 28 H 26 N 2 O 8 Cd ] +, 714 [M-H 4 O 2 +5H] +, 617 [M-H 4 O 2 -C 6 H 3 O] +,468 [M-H 4 O 2 -C 7 H 5 O 2 NCd+5H] +, 435 [M-H 4 O 2 C 8 H 8 O 3 NCd+2H] +, 467[(SB)+Cd 2 +H] +, 244[(SB)+H] +. 1 H NMR (DMSO, 400 MHz) δ/ppm 3.76 (s, 3H, - OCH 3 ), 8.92 (s, 1H, C=N), (m, 7H, Ar.). 13 C NMR (DMSO, 400MHz) δ/ppm (2C, -OCH 3 ), (12C, Ar). ZnCd(SB) 2 (H 2 O) 2 (3) The heterodinuclear complex ZnCd(SB) 2 (H 2 O) 2 obtained in case of M = Zn(II), M' = Cd(II). Yield: 69.3%, m.p.: > 350 o C. Anal. Calc. for C 28 H 26 N 2 O 8 ZnCd(696.34) : C, 48.30; H, 3.76; Cd, 16.14; N, 4.02; Zn, 9.39%. Found: C, 48.18; H, 3.69; Cd, 16.05; N, 3.96; Zn, 9.30%, mass: m/z = 696 [C 28 H 26 N 2 O 8 Zn 64 Cd 114 ] +, 698 [C 28 H 26 N 2 O 8 Zn 66 Cd 114 /C 28 H 26 N 2 O 8 Zn 64 Cd 116 ] +, 698 [C 28 H 26 N 2 O 8 Zn 64 Cd 116 ] +, 700 [C 28 H 26 N 2 O 8 Zn 66 Cd 116 ] +, 665 [M-H 4 O 2 +5H] +, 569 [M-H 4 O 2 -C 6 H 3 O] +, 418 [M-H 4 O 2 -C 7 H 5 O 2 NCd+5H] +,385 [M-H 4 O 2 C 8 H 8 O 3 NCd+2H] +, 419[(SB) +ZnCd+H] +, 244[(SB)+H] +. 1 H NMR (DMSO, 400 MHz) δ/ppm 3.74 (s, 3H, -OCH 3 ), 8.88 (s, 1H, C=N), (m, 7H, Ar.). 13 C NMR (DMSO, 400MHz) δ/ppm (2C, -OCH 3 ), (12C, Ar). CdZn(SB) 2 (H 2 O) 2 (4) The heterodinuclear complex CdZn(SB) 2 (H 2 O) 2 obtained in case of M = Cd(II), M' = Zn(II). Yield: 69.0%, m.p.: > 350 o C. Anal. Calc. for C 28 H 26 N 2 O 8 CdZn(696.34) : C, 48.30; H, 3.76; Cd, 16.14; N, 4.02; Zn, 9.39%. Found: C, 48.21; H, 3.71; Cd, 16.02; N, 3.94; Zn, 9.27 %, mass: m/z = 696 [C 28 H 26 N 2 O 8 Cd 114 Zn 64 ] +, 698 [C 28 H 26 N 2 O 8 Cd 114 Zn 66 /C 28 H 26 N 2 O 8 Cd 116 Zn 64 ] +, 698 [C 28 H 26 N 2 O 8 Cd 116 Zn 64 ] +, 700 [C 28 H 26 N 2 O 8 Zn 66 Cd 116 ] +, 665 [M-H 4 O 2 +5H] +, 569 [M-H 4 O 2 -C 6 H 3 O] +, 468 [M-H 4 O 2 -C 7 H 5 O 2 NZn+5H] +, 434 [M-H 4 O 2 C 8 H 8 O 3 NZn+2H] +, 419[(SB)+CdZn+H] +, 244[(SB)+H] +. 1 H NMR (DMSO, 400 MHz) δ/ppm 3.77 (s, 3H, - OCH 3 ), 8.90 (s, 1H, C=N), (m, 7H, Ar.). 13 C NMR (DMSO, 400MHz) δ/ppm (2C, -OCH 3 ), (12C, Ar).

4 Chem Sci Trans., 2018, 7(3), Results and Discussion IR spectral studies The IR spectra of the mononuclear reactant complexes and dinuclear complexes displayed some similarities and dissimilarities. Significant IR bands are shown in Table 1. The spectra of reactant complex M (O-VAN) 2 showed bands at 1635 cm -1 and 1033 cm -1 attributed to C = O stretching and OCH 3 group. A weak band at 435 cm -1 observed due to M-O stretching frequency 18. The spectra of reactant complex M'(2-AP) 2 showed band at 1598 cm -1 attributed to NH 2 bending and C-N stretching. The bands observed at 3324 cm -1 and 3260 cm -1 attributed to NH 2 asymmetric and symmetric stretching. A weak band at 565 cm -1 observed due to M-N stretching frequency. A band seenaround 3050 cm -1 was attributed to the aromatic ring vibrations in the spectra of both reactant complexes. An absence of abroad band in the region of 3400 cm -1 in the reactant complexes indicated the absence of any coordinated water molecule. Table 1. FT - IR spectral frequencies of complexes System ν C=N cm -1 ν O-H cm -1 ν M-O cm -1 ν M-N cm -1 δ H2O (coord.) cm -1 M (O-VAN) M (2-AP) Zn 2 (SB) 2 (H 2 O) Cd 2 (SB) 2 (H 2 O) Zn Cd (SB) 2 (H 2 O) Cd Zn (SB) 2 (H 2 O) In the spectra of MM'(SB) 2 (H 2 O) 2, peaks due to C=O stretching (1629 cm -1 ), NH 2 bending and NH 2 stretching (3324 cm -1 and 3260 cm -1 ) did not appear. New sharp and strong peaks at cm -1, cm -1 and cm -1 were the evidences of imine formation and new metal bond formation. The peak due to OCH 3 group in reactant complex retained its position without any shifting which indicated the refrainment of the -OCH 3 group from coordination. A new band appearing in the range 731 cm -1 was assigned to the wagging mode of the coordinated water. Thermal analysis Thermal degradation of the complexes was seen in the TGA thermogramsof the complexes. The TGA thermogram showed the thermal stability and degradation pattern of the complexes. The thermal degradation of all complexes were found to be similar types as shown in Figure 1. For Zn 2 (SB) 2 (H 2 O) 2 a negligible weight loss was observed below 150 o C because of small amounts of adsorbed water. Loss of two coordinated water molecules (H 4 O 2 ) with mass losses of (obs. = 5.44%, Calc. = 5.54%) was observed in the first step. In second step loss of the organic fraction, with mass loss (obs. = 74%, Calc. = 74.29%) in the region of 400 o C to 550 o C was observed. The residual weight was not considered. Mass spectra Mass spectra ESI-MS of the complexes were carried out in a polar aprotic solvent; acetonitrile. The spectra of Zn 2 (SB) 2 (H 2 O) 2 showed molecular ion peaks with its isotopes Zn 64, Zn 66 at m/z of 648 (M + ) and m/z of 650 (M+2) (Figure 2). This molecular ion peak was in line with the proposed structures of the complexes. Several peaks for fragments with m/z = 617, 521, 418, 385, 372 and 244 were also noticed, which corresponded to the stable fragments associated with a different number of H + ions shown in Figure 3.

5 481 Chem Sci Trans., 2018, 7(3), Temp., o C Figure 1. TGA thermogram of the dinuclear complexes % Weight, % m/z Figure 2. Mass spectra of Zn 2 (SB) 2 (H 2 O) 2

6 Chem Sci Trans., 2018, 7(3), Figure 3. Fragmentation pattern for Zn 2 (SB) 2 (H 2 O) 2 complex 1 H NMR and 13 C NMR spectra The 1 H NMR spectrum of Zn 2 (SB) 2 (H 2 O) 2 (dimethyl sulfoxide, 400 MHz) showed two characteristic absorptions (singlet at δ=3.78 ppm) attributed to protons of OCH 3 group and (singlet at δ=8.90 ppm) attributed to coordinated imine proton (Figure 4). Signals were observed between δ values 6 to 7.6 ppm attributed to six aromatic protons.the spectra showed half of the total number of hydrogen as the structure is symmetrical. The peak observed for imine proton revealed the inter-complex Schiff base reaction. The 13 C spectrum of complex Zn 2 (SB) 2 (H 2 O) 2 (dimethyl sulfoxide, 400 MHz) showed characteristic absorption at lower δ value ppm attributed to two carbon of two OCH 3 group. Spectra showed absorption between δ values ppm attributed to twelve aromatic carbons. The peak due to the coordinated imine carbon of azomethine C=N was observed at high δ value 163 ppm. All four complexes showed a similar type of spectra.

7 483 Chem Sci Trans., 2018, 7(3), Powder X-RD Figure 4. 1 H NMR and 13 C NMR spectra of Zn 2 (SB) 2 (H 2 O) 2 Powder x-ray diffraction pattern showed sharp intense peaks suggesting the crystalline nature of all the complexes (Figure 5). All the reflections for complex Zn 2 (SB) 2 (H 2 O) 2 were indexed for h, k and l values (Table ) using methods reported in the literature 19. The observed inter planner spacing values i.e. d-spacing with 2θ angles were measured from experimental diffractogram and compared with standard reference pattern. The complex was found to be a tetragonal crystal having unit cell parameters such as a =4.9910, b= and c = with maximum deviation of θ = and α = 90, β = 90, γ = 90 at the wavelength of Table 2. Powder x-ray diffraction data for Zn 2 (SB) 2 (H 2 O) 2 h k l 2θ 2θ d d Intensity 2θ d (Obs.) (Cal.) (Obs.) (Cal.) (Obs.)

8 Chem Sci Trans., 2018, 7(3), Counts/s Position, θ Theta, copper(cu) Figure 5. Powder x-ray diffraction pattern of Zn 2 (SB) 2 (H 2 O) 2 Structure and bonding IR spectra of the samples clearly indicated the presence of imine group, M-O and M-N bonds as well as coordinated water. Elemental composition and thermogravimetric analysis suggested the formation of dinuclear complexes and two coordinated water molecules. Formation of dinuclear complexes was also evident from the mass spectra. 1 H NMR and 13 C NMR spectra supported the proposed structure. The proposed structure was obtained by minimization of energy using the molecular mechanical method. Reaction mechanism According to the well known mechanism of Schiff base formation between an amine and an aldehyde, the lone pair of electrons on nitrogen makes a nucleophilic attack on the carbonyl carbon of aldehyde resulting into the formation of a C=N bond and transfer of a proton from amine to carbonyl oxygen. This intermediate, when catalyzed with a weak acid undergoes a loss of water molecule forming a Schiff base. In the proposed reaction (Figure 6), both nitrogen and carbonyl oxygen of the reacting complexes were previously coordinated to the respective metal ions where the coordinated nitrogen did not have the sufficient electron density required for the nucleophilic attack. It was thought that the electron density required by the nitrogen was made available through the oxygen atom of the amino phenol via ring electrons. Thus, due to this induction of electron density, the nitrogen became electron rich and made a nucleophilic attack on the carbonyl carbon. Initially, the weakly acidic medium permitted protons from the medium to attack on the carbonyl oxygen of the aldehyde complex. This facilitated the nucleophilic attack by coordinated nitrogen. A cleavage in the coordinate covalent bond between nitrogen and zinc was followed by the formation of a bond between nitrogen and carbon. At this time, one of the protons on the nitrogen was returned to the medium. A heterolytic cleavage of N-H bond

9 485 Chem Sci Trans., 2018, 7(3), resulted into the formation of imine which forced the breakage of C-O bond leaving a coordinated -OH group on the metal ion. Subsequently, the proton released due to the cleavage NH bond breaking was captured by the hydroxyl oxygen to form a coordinated water molecule. The imine nitrogen then readily donated an electron pair to the Zn(II) ion for the formation of a coordinate covalent bond. A similar sequence repeated on the other amine and aldehyde groups of the complex resulted into the formation of a symmetrical dinuclear Schiff base complex. Figure 6. Mechanism for synthesis of dinuclear complexes

10 Chem Sci Trans., 2018, 7(3), Conclusion Preparation of dinuclear complexes by a new synthetic route is strongly supported by analytical data. The formation of the imine group isevident from infrared spectra of complexes. Both elemental analysis and thermogravimetric analysis favour the formation of a dinuclear complex with one of the metal atom in diaquo form. Molecular ion peaks in the mass spectra along with 1 H NMR and 13 C NMR spectra leaves no doubt regarding the success of the proposed reaction and bonding aspects of the newly synthesized complexes. Inability to grow a single crystal allowed only powder x-ray diffraction studies which could give limited information regarding the geometry of the complex. Finally, the molecular mechanical method used for energy minimization corroborates the proposed structure of the complexes. A slight alteration in the classical Schiff base mechanism could reasonably explain the formation of the complexes. This report on the synthesis of dinuclear complexes via novel route is capable of opening a new era in the preparation of complexes with a lot more variations. Acknowledgement The authors are thankful to the UGC, New Delhi (India), for providing financial support in the form of a major research project to the corresponding author. The authors thank the Directors of Sophisticated Instrumentation Centre for Applied Research and Testing (SICART), Vallabh Vidyanagar, Sophisticated Analytical Instrument Facility (SAIF) IITChennai for providing analytical support. References 1. Saghatforoush L A, Aminkhani A, Ershad S, Karimnezhad G, Ghammamy S and Kabiri R, Molecules, 2008, 13(4), ; DOI: /molecules Bhatt V D, ICAIJ, 2008, 3(1), Lemaire M T, Tosha M Barclay, Laurence K Thompson and Robin G Hicks, Inorg Chim Acta, 2006, 359(9), ; DOI: /j.ica Kahn O, Angewandte Chemie Int Ed Engl., 1985, 24(10), ; DOI: /anie Paschke R, Inorg Chem., 2003, 42(25), ; DOI: /ic Busch D H, Adv Chem.,, 1962, 37, 1-18; DOI: /ba ch Bhatt V D and Ram S R, Chem Sci J., 2012, 63, Bhatt V D and Ram S R, Chem Sci Trans., 2013, 2(4), ; DOI: /cst Bhatt V and Ram S, Chem Sci Rev Lett., 2015, 4(14), Bhatt Vasishta D and Ram S, Int J Res Chem Environ., 2013, 3(2), Vasishta D and Bhatt S R, Chem Sci Trans., 2013, 2(4), ; DOI: /cst Ram S R and Bhatt V D, Chem Sci Trans., 2013, 2(3), ; DOI: /cst Armarego W L F and Perrin D D, Purification of Laboratory Chemicals. 4 th Ed., 1997: Butterworth-Heinemann, Oxford. 14. Skoog D A, Donald M West and James Holler F, Analytical Chemistry: An Introduction. 7 th Ed., 2000, Salt City, UT (USA): Brooks cole. 15. Ramsay C G, J Chem Educ., 1977, 54(11), 714; DOI: /ed054p Flaschka H and Butcher J, Microchem J., 1963, 7(4), ; DOI: / X(63)

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