Received 11 May 2011; revised and accepted 1 July 2011

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1 Indian Journal of Chemistry Vol. 50A, Sept-Oct 2011, pp Zinc and cadmium complexes of a Schiff base ligand derived from diaminomaleonitrile and salicylaldehyde: Syntheses, characterization, photoluminescence properties and DFT study Averi Guha a, Jaydeep Adhikary a, Tapan Kumar Mondal b & Debasis Das a, * a Department of Chemistry, University of Calcutta, 92, APC Road, Kolkata , India b Department of Chemistry, Jadavpur University, Kolkata , India dasdebasis2001@yahoo.com Received 11 May 2011; revised and accepted 1 July 2011 Zn(II) and Cd(II) complexes of a 1+1 condensed Schiff base derived from salicylaldehyde and diaminomaleonitrile has been synthesized and characterized. Electronic and photoluminescence spectral properties of the complexes as well as the free ligand have been investigated and a thorough DFT study has been performed to rationalize the spectral behavior. Keywords: Coordination chemistry, Zinc, Cadmium, Schiff bases, Diaminomaleonitrile, Fluorescence properties Diaminomaleonitrile (DAMN, 2,3-diamino-cis-2- butenedinitrile) with the formula (H 2 N)(NC)C=C(CN)(NH 2 ) appears to be symmetric and has been known as an unsaturated electron rich ligand as well as a tetramer of hydrogen cyanide, resulting from the polymerization of HCN under basic conditions 1. The reaction of DAMN with aromatic aldehydes is very interesting as in every case 1+1 condensed Schiff base is obtained even in presence of excess of aldehyde. 2 However, these compounds are important as synthetic intermediates and recently being used in various fields such as in pharmacology 3, synthesis of conjugated linear polymers 4 and in the thermostable optical material industry 5. Interestingly, coordination chemistry of Schiff bases derived from DAMN is not well explored. Only a few well characterized complexes of DAMN-based Schiff bases are known to us. Thompson et al. 6 for the first time reported the crystal structures of 1+1 condensed Schiff base (HL) derived from DAMN and salicylaldehyde and some metal complexes of DAMN related Schiff bases. However, in spite of the highly conjugated nature of HL, photoluminescence property of the ligand (HL) as well as its d 10 metal complexes is not yet evaluated. In this report we present the synthesis and solution phase characterization of 1+1 condensed Schiff base derived from DAMN and salicylaldehyde and its luminescence property. Syntheses of Zn(II) and Cd(II) complexes of this Schiff base, their characterization,

2 1464 INDIAN J CHEM, SEC A, SEPT-OCT 2011 fluorescence property and DFT studies to interpret the spectral behaviors of the ligand as well as these two complexes is also reported. Materials and Methods All chemicals were obtained from commercial sources and used as received. Solvents were dried according to standard procedures and distilled prior to use. High purity 2,3-diamino-cis-2-butenedinitrile (DAMN) and salicylaldehyde were purchased from Fluka and Lancaster Chemical Company Inc., respectively. Cadmium(II) acetate and zinc(ii) acetate were purchased from Aldrich Chemical Company Inc. and used as received. All other chemicals were of AR grade. Elemental analyses (carbon, hydrogen, and nitrogen) were performed using a Perkin-Elmer 240C elemental analyzer. Infrared spectra ( cm 1 ) were recorded in KBr at 300 K using a Shimadzu FTIR-8400S. Electronic spectra ( nm) were obtained at 25 C using a Hitachi U-3501 spectrophotometer in methanol. Fluorescence spectra of the ligand and complexes were recorded in solution using methanol as a solvent with a Perkin-Elmer (model LS 55) luminescence spectrometer. The electrospray mass spectra were recorded on a Micromass Q-TOF mass spectrometer. 1 H NMR spectra (300 MHz) were recorded in (CD 3 ) 2 SO solvent on a Bruker AV300 Supercon NMR spectrometer using the solvent signal as the internal standard in a 5 mm BBO probe. Synthesis of the ligand (HL) To an ethanolic solution (10 ml) of salicylaldehyde (0.244 g, 2 mmol), an ethanolic solution (5 ml) of DAMN (0.126 g, 2 mmol) was added dropwise with constant stirring. The stirring was continued further overnight. The yellow monoamine which precipitated out, was filtered, washed with ethanol and dried under vacuum (Yield: 45 %). Preparation of Zn(II) complex (1) An ethanolic solution (20 ml) of Zn(OAC) 2 ( g, 2.10 mmol) was added to an ethanolic solution of the ligand, HL ( g, 2 mmol) with constant stirring. The resulting mixture was then refluxed for 4 hrs. The intense red-purple solution deposited a deep purple solid, which was filtered off, washed with ethanol, and dried under vacuum. Yield: 86 %. Anal. (%): Calcd for [C 13 H 10 N 4 O 3 Zn] (1): C, 46.52; H, 3.00; N, Found: C, 46.45; H, 3.08; N, Preparation of Cd(II) complex (2) This was prepared adopting similar procedure where Cd(OAC) 2 ( g, 2.10 mmol) was used instead of Zn(OAC) 2. Yield 81 %. Anal. (%): Calcd for [C 15 H 18 N 4 O 5 Cd] (2): C, 40.33; H, 4.06; N, Found: C, 40.23; H, 5.02; N, Computational details All computations were performed using the Gaussian03 (G03) software package 7. Full geometry optimizations were carried out using the DFT computational technique at the B3LYP level 8,9 for HL, [Zn(L)(OAc)] (1) and [Cd(L)(OAc)(MeOH) 2 ] (2) in singlet ground state (S = 0). The lanl2dz basis set with effective core potential was employed for the Zn and Cd atoms 10 and the standard 6-31G(d) basis for N, O, C and H atoms. The vibrational frequency calculations were performed to ensure that the optimized geometries represent the local minima of potential energy surface and there were only positive eigen values. The lowest 30 singlet-singlet vertical electronic excitations based on B3LYP optimized geometries were computed using the time-dependent density functional theory (TD-DFT) formalism in methanol using conductor-like polarizable continuum model (CPCM) with the same B3LYP level and basis sets. GaussSum 17 was used to calculate the fractional contributions of various groups to each molecular orbital. Results and Discussion Synthesis and characterization The Schiff base ligand HL has been synthesized by 1+1 condensation of DAMN and salicylaldehyde in ethanol. It has been characterized by common physicochemical techniques. IR spectrum of the half-condensed ligand shows major NH stretches at 3416, 3307, and 3193 cm -1 and a strong CN stretching band at 2203 cm -1, with a sharp, weak shoulder at very high energy 2363 cm -1. The bands at 1627 cm -1 and 1566 cm 1 are due to the imine (C=N) and skeletal vibrations, respectively. 1 H NMR spectrum of the ligand exhibits the peaks at δ (s, 1H; for phenolic -OH), δ (s, 1H, for imine H), δ (m, 1H, aromatic H), δ (s, 2H, for NH 2 ), δ (m, 1H, aromatic H) and δ (m, 1H, aromatic H), which corroborate well with the proposed structure of HL. The Zn(II) and Cd(II) complexes of ligand HL have been synthesized by adding the corresponding acetate

3 GUHA et al.: Zn(II)/Cd(II) COMPLEXES OF SCHIFF BASE FROM SALICYALDEHYDE + DAMN 1465 salts to the ethanolic solution of the ligand. Elemental analyses suggest that the molecular compositions of the two complexes are ZnL(OAc) and CdL(OAc)(MeOH) 2, respectively. The Zn(II) complex (1) shows no ν NH absorption but a major ν CN stretch at 2214 cm -1, with a weak shoulder at 2374 cm -1. Similarly, the Cd(II) complex (2) exhibits a major ν CN stretch at 2214 cm -1, with a weak peak at 2381 cm -1. Due to complexation the phenolic OH gets deprotonated and the peak at δ is absent in the 1 H NMR spectra in both the zinc and cadmium complexes. The peak due to two NH 2 protons gets Fig. 1 Absorption spectrum of ligand (HL) in methanol medium. shifted to a much downfield region as compared to its original δ7.803 position due to metallation. In order to get the molecular compositions of the complexes in solution we have performed ESI-MS experiments. Complex (1) exhibits base peak at amu, which suggests that the most probable composition of the species is [ZnL+Na] + (calcd m/z = amu). For complex (2), the base peak is observed at amu, and therefore, the most probable composition of the species is [CdL(MeOH) 3 ] + (calcd m/z = amu). Here it is noteworthy that we tried to synthesize the corresponding Hg(II) complex but failed. Electronic and fluorescence properties of the ligands and complexes The ligand in methanol medium shows three absorption band maxima at 207, 259 and 379 nm with a shoulder at 396 nm arising due to π π* transitions (Fig. 1). On excitation at 207 nm and 259 nm same emission spectrum with λ ex at 294 nm is obtained (Fig. 2). Interestingly for 379 nm excitation, emission occurs at 441 nm (Fig. 2). The UV-vis spectrum of zinc complex in methanol shows four bands at 306, 362, 468 and 553 nm (Fig. 3). The bands at 306 nm and 362 nm are ligand centered arising due to intra-ligand charge transfer, whereas, 468 and 553 nm bands are due to LMCT. On excitation at 306 nm, no fluorescence activity is noticed, but when the excitation is done at 362 nm, blue fluorescence with band maxima at 423 nm is observed (Fig. 4). Interestingly, on excitation separately at 468 nm and 553 nm the same Fig. 2 Emission spectra of the ligand, (HL) on excitation at 259 nm (curve 1) and 379 nm (curve 2) in methanol medium. Fig. 3 Absorption spectra of the Zn(II) (curve 1) and Cd(II) (curve 2) complexes in methanol medium.

4 1466 INDIAN J CHEM, SEC A, SEPT-OCT 2011 fluorescence spectrum is obtained with emitted band maxima at 632 nm (Fig. 4). Like zinc complex, cadmium complex also exhibits four UV-vis bands at 305 nm, 365 nm, 447 nm and 569 nm in methanol (Fig. 3). Excitation at 447 nm and 569 nm do not yield any luminescence. Excitation at 305 and 365 nm shows the same fluorescence behavior. The emission spectrum shows band maxima at 428 and 473 nm (Fig. 5). DFT and TD DFT calculations and correlation with electronic spectra To understand the molecular and electronic structure the proposed structure of the ligand (HL) and the complexes [Zn(L)(OAc)] (1) and [Cd(L)(OAc)(MeOH) 2 ] (2) were optimized using DFT/B3LYP level of calculations. The optimized structures of HL, (1) and (2) are given in Fig. 6. In ligand HL, HOMO to HOMO-2 and HOMO-4 are composed of π symmetry molecular orbitals and HOMO-3 is non-bonding type. The lower energy unoccupied molecular orbitals are π * in character with the HOMO-LUMO gap is 3.42 ev (Fig. 7). In methanol, HL shows three sharp peaks at 376, 260 and 206 nm along with broad peaks at 395 and 326 nm. The TD DFT calculation has been performed using CPCM/B3LYP method in MeOH to assign the Fig. 4 Emission spectra of Zn(II) complex on excitation at 362 nm (curve 1), 468 nm (curve 2) and 553 nm (curve 3) in methanol medium. Fig. 5 Emission spectra of the Cd(II) complex on excitation at 305 nm (curve 1) and 365 nm (curve 2) in methanol medium. Fig. 6 DFT/B3LYP optimized structure of (a) ligand HL, (b) [Zn(L)(OAc)] (1) and (c) [Cd(L)(OAc)(MeOH) 2 ] (2).

5 GUHA et al.: Zn(II)/Cd(II) COMPLEXES OF SCHIFF BASE FROM SALICYALDEHYDE + DAMN 1467 Fig. 7 Contour plots of some selected molecular orbitals of HL in MeOH. Table 1 Calculated vertical electronic transition of HL in MeOH Excited state Excitation energy (ev) Wavelength (nm) Oscilltor strength (f) Key transitions λ max (Expt.) (71 %) HOMO LUMO 396, (78 %) HOMO-1 LUMO (41 %) HOMO-2 LUMO 260 (32 %) HOMO-1 LUMO (49 %) HOMO-1 LUMO+1 (18 %) HOMO-2 LUMO (70 %) HOMO-4 LUMO 206 Table 2 Calculated vertical electronic transitions of (1) in MeOH E excitation (ev) λ excitation (nm) Osc. strength (f) Key transitions Character λ max (nm) (Expt.) (98%)HOMO LUMO π π *, ILCT (94%)HOMO-2 LUMO π π *, ILCT (46%)HOMO-5 LUMO π π *, ILCT 362 (37%)HOMO-3 LUMO (34%)HOMO-5 LUMO π π *, ILCT (32%)HOMO-3 LUMO (84%)HOMO-2 LUMO+1 π π *, ILCT 306 Table 3 Calculated vertical electronic transitions of (2) in MeOH E excitation λ excitation Osc. strength (f) Key transitions Character λ max (Expt.) (93 %) HOMO LUMO π π *, ILCT (78 %) HOMO-1 LUMO π π *, ILCT (83 %) HOMO LUMO+1 π π *, ILCT (67 %) HOMO-1 LUMO+1 π π *, ILCT 305

6 1468 INDIAN J CHEM, SEC A, SEPT-OCT 2011 electronic bands. The vertical electronic transitions for HL are found at 384, 338, 203 nm corresponding to π π* transitions. In addition, two close strong transitions at 260 and 255 nm (Table 1) are also seen. The ligand emits at 294 and 441 nm upon excitation at 260 and 376 nm. Since the bands at 260 and 376 nm are due to π π* transitions, the emissions are also π π* origin. The occupied molecular orbitals of complex (1) and (2) are composed of ligand (HL) group of orbitals along with partial contribution from AcO - group. The unoccupied orbitals are π* orbitals of ligands. In methanol, the solution spectrum complex (1) shows bands at 533, 468, 362 and 306 nm and emits at 423 and 632 nm upon excitation at 362 and 468 nm. TD DFT result of (1) shows transitions at 537 and 467 nm correspond to HOMO LUMO and HOMO-2 LUMO transitions respectively. In addition, two strong peaks at 372 and 341 nm due to HOMO-3/HOMO-5 LUMO transitions (Table 2). For complex (2) the observed bands at 569, 447, 365 and 305 are due to intra-ligand, π π* transitions (Table 3) and the emissions at 429 and 468 nm upon excitation at 365 nm is also of π π* origin. Conclusions A Schiff base ligand, HL, has been synthesized by 1+1 condensation of diaminomaleonitrile and salicylaldehyde. The solution phase structure of the ligand has been evaluated. The ligand was chosen since it was expected to be highly fluorescent due to the presence of extra ethylenic double bond. Our study proves that the ligand is highly fluorescent and the fluorescence is due to π π* transition. Zn(II) and Cd(II) complexes of the ligand were prepared and characterized by routine physicochemical studies. Solution phase structures of the two complexes were determined by ESI-MS study. DFT and TD/DFT calculations have been performed to determine the origin of the spectral behavior of the ligand as well as the complexes. Supplementary Data Supplementary data associated with this article, viz., IR spectra, 1 H NMR spectra of the ligand and its complexes, ESI-MS spectra of the two complexes, co-relation of vertical electronic transitions calculated by TD DFT/B3LYP/CPCM method and some selected molecular orbitals participated in electronic transitions, tables containing energies and composition of some selected molecular orbitals of complexes (1) and (2), contour plots of some selected molecular orbitals of complexes (1) and (2), may be obtained from the authors on request. Acknowledgement The authors wish to thank the University Grants Commission, New Delhi, India (No \2008 (SR)) for financial support. AG is thankful to Shailabala Biswas Research Foundation, University of Calcutta, Kolkata, India, for financial support. References 1 Gryszkiewics-Trochimowski E, Rocz Chem, (1928) Robertson P S & Vaughan J, J Am Chem Soc, 80 (1958) Holger H & Benedikt B, Eur Pat Appl, (1989). 4 Johnson D M, Reybuck S E, Lawton R G & Ramussen P G, Macromolecules, 38 (2005) Nesterov V V, Antipin M Y, Nesterov V N, Penn B G, Frazier D O & Timofeeva T V, Crystal Growth Des, 4 (2004) MacLachlan M J, Park M K & Thompson L K, Inorg Chem, 35 (1996) Gaussian 03, rev. D.01, (Gaussian, Inc., Wallingford CT) Becke A D, J Chem Phys, 98 (1993) Lee C, Yang W & Parr R G, Phys Rev B, 37 (1988) Hay P J & Wadt W R, J Chem Phys, 82 (1985) Bauernschmitt R & Ahlrichs R, Chem Phys Lett, 256 (1996) Stratmann R E, Scuseria G E & Frisch M J, J Chem Phys, 109 (1998) Casida M E, Jamorski C, Casida K C & Salahub D R, J Chem Phys, 108 (1998) Barone V & Cossi M, J Phys Chem A, 102 (1998) Cossi M & Barone V, J Chem Phys, 115 (2001) Cossi M, Rega N, Scalmani G & Barone V, J Comput Chem, 24 (2003) O Boyle N M, Tenderholt A L & Langner K M, J Comput Chem, 29 (2008) 839.