Notes. Synthesis, characterization and DFT calculations of N,O Schiff base complex of copper(ii)

Indian Journal of Chemistry Vol. 54A, December 2015, pp. 1451-1458 Notes Synthesis, characterization and DFT calculations of N,O Schiff base complex of copper(ii) Sudipto Dey a, Koushik Ghosh a, Shibashis Halder a, Corrado Rizzoli b & Partha Roy a, * a Department of Chemistry, Jadavpur University, Jadavpur, Kolkata 700 032, India Email: proy@chemistry.jdvu.ac.in b Universita' degli Studi di Parma, Dipartimento di Chimica, Parco Area delle Scienze 17/A, I-43124 Parma, Italy Received 20 June 2015; revised and accepted 23 November 2015 Mononuclear copper(ii) complex of the Schiff base, 2-methoxy-6-(3-morpholinopropyl-iminomethyl) phenol, [Cu(HL) 2 ](ClO 4 ) 2 ].H 2 O (1) has been synthesized and characterized by elemental analysis and different spectroscopic techniques. The structure of (1) is confirmed by single crystal X-ray diffraction analysis. The complex (1) crystallizes in the monoclinic space group P21/n. The crystal structure of the mononuclear complex exhibits two Schiff base ligands bound to the copper atom and two perchlorate ions as non-coordinating species. Both the ligands in (1) are zwitterions where nitrogen atom of the morpholine ring is protonated. Fluorescence spectral study shows that HL displays an emission band at 535 nm on excitation at 418 nm. The presence of Cu 2+ ion in complex (1) quenches its emission intensity. DFT and TDDFT calculations have been carried out to investigate the spectral transitions. Keywords: Coordination chemistry, Schiff bases, Density functional calculations, NO ligands, Fluorescence, Crystal structure, Copper Over the last few decades, a significant amount of research work has been done on Schiff bases and their complexes in the fields of coordination chemistry, materials chemistry and biological chemistry. 1-4 Schiff base ligands are generally synthesized by the condensation of primary amines and aldehydes. The resultant imine nitrogen along with other donor atoms participates in binding with metal ions via donation of lone pair(s) of electrons. Ketones can also form Schiff base ligands when they are reacted with primary amines. Schiff base ligands with various number of donor atoms are designed according to the electronic requirements of metal ions and the expected properties of the complex. Transition metal complexes with Schiff base not only display their intriguing chemical structures but also many important properties and applications. Besides being employed as ligands for complex formation, Schiff bases have been explored, for instance, as sensors, conductors, etc. 5 A huge number of transition metal complexes with various Schiff base ligands have been reported. 3,6-12 Schiff bases can stabilize different oxidation states of various metal ions, thereby offering the possibility to control the properties of transition metal complex in magnetism, catalysis, electronic spectra, etc. The use of appropriate bridging ligands, reaction conditions and choice of suitable metal center to make coordination compounds may lead to the formation of metal complexes with different nuclearity and dimensionality. However, mononuclear transition metal complexes can be useful to study different enzyme mimics in vitro. 13 Salicyldahyde along with its derivatives has been used widely to synthesize a number of Schiff base ligands and their complexes. A Schiff base ligand was prepared by the condensation of 3-methoxysalicylaldehyde and 4-(2-aminoethyl) morpholine in 1:1 ratio. 14 Cadmium(II) 15 and cobalt(iii) 16 metal ions are reported to form mononuclear complexes with the ligand. Recently Sasmal et al. 17 reported a dinuclear complex of Ni(II) with this ligand and extensive magnetic studies were carried out revealing the moderate ferromagnetic interaction of the metal centers in the compound. We report herein the synthesis and characterization of a mononuclear copper(ii) complex with HL ([Cu(HL) 2 ](ClO 4 ) 2.H 2 O (1) (HL = 2-methoxy-6- (3-morpholinopropyliminomethyl)phenol) with N,O donor atoms. The use of HL as ligand is reported here for the first time. All compounds were characterized by elemental analysis, FT-IR, UV-vis and mass spectroscopy. The structure of one of the copper(ii) complex was confirmed by single crystal X-ray diffraction studies. Fluorescence properties of the ligands and its complex were also studied. Theoretical calculations have been performed to investigate the spectral transitions. Experimental 4-(3-Aminopropyl)morpholine and vanillin were purchased from Sigma Aldrich and used without further purification. All other reagents were obtained from commercial sources and used as received. Solvents used for spectroscopic studies were purified

1452 INDIAN J CHEM, SEC A, DECEMBER 2015 and dried by standard procedures before use. 18 NMR spectra of the compounds were recorded on Bruker 300 MHz spectrometer using DMSO-d 6 as solvent. FT-IR spectra were obtained on a RX-1 Perkin Elmer spectrophotometer with samples prepared as KBr pellets. Elemental analysis was carried out with a 2400 Series-II CHN analyzer, Perkin Elmer, USA. The ESI-MS spectra were recorded on Qtof Micro YA263 mass spectrometer. Absorption spectra were recorded on a Lambda 25 Perkin Elmer spectrophotometer. Caution: Perchlorate salts are potential explosive. Although no accident was encountered, but small amount of perchlorate salt should be handled with care. The ligand, 2-methoxy-6-(3-morpholinopropy liminomethyl)phenol (HL) was synthesized following a published procedure with a slight modification. 15 To a 10 ml methanolic solution of 4-(3-aminopropyl) morpholine (1.0 mmol, 0.144 g) 0.278 g), was added vanillin (1.0 mmol, 0.152 g) dissolved in 10 ml of methanol. The mixture was stirred for 30 min, was refluxed for 4 h and cooled to room temperature. A yellow solid was collected from the solution by filtration. Yield: 92%. Anal. Calc. (%) for C 15 H 122 N 2 O 3 : C, 64.73; H, 7.97; N, 10.06; Found: C, 64.78; H, 8.03; N, 9.97. 1 H NMR (300 MHz, DMSO-d 6 ; δ): 1.72-1.81 (m, 2H, -CH 2 ), 2.32-2.34 (overlapped, 6H, -CH 2 ), 3.53-3.6281 (m, 6H, -CH 2 ), 3.75 (s, 3H, OCH 3 ), 6.74 (t, J = 7.9 and 7.8 Hz, 1H, Ar) 9.98 (dd, J = 1.9 and 7.8 Hz, 2H, Ar), 8.51 (s, 1H, HC=N), 13.83 (s, 1H, OH). HRMS: m/z (ESI) 279.16 (C 15 H 23 N 2 O 3 + requires 279.16). For the synthesis of [Cu(HL 2 ) 2 ](ClO 4 ) 2.H 2 O (1), copper(ii) perchlorate hexahydrate (0.370 g, 1.0 mmol) in 10 ml of methanol was added slowly to 20 ml methanolic solution of the Schiff base ligand (HL) (0.528 g, 2.0 mmol). The mixture was stirred for 45 min, then refluxed for 2 h, cooled to room temperature and filtered to remove any precipitate or suspended materials. The filtrate was kept at room temperature for slow evaporation of the solvent. Dark green color crystals of complex (1), suitable for single crystal X-ray diffraction analysis, were grown within a few days. Yield: 0.522 g, 72%. Anal. Calc. (%) for C 30 H 46 N 4 O 15 Cl 2 Cu: C, 43.04; H, 5.54; N, 6.69; Found: C, 42.92; H, 5.52; N, 6.62. Details of the data collection and refinement parameters for complex (1) are summarized in Table 1. The diffraction experiments were carried out on a Bruker SMART 100 CCD diffractometer using graphite monochromated Mo Kα radiation at 294(2) K. Data were processed using the Bruker SAINT package. 19 Absorption corrections based on multi-scans using the SADABS software 19 were applied to the intensity data. The structures were solved by direct methods using SIR97 20 and refined with full-matrix least-squares on F 2 on all unique reflections using SHELX97-L. 21 All the non-hydrogen atoms of the complexes were refined anisotropically. In complex (1), the oxygen atoms of the perchlorate anions are disordered over two sets of orientations with site occupancies of 0.5. During the refinement, the Cl O and O...O distances within the anions were restrained to assume the same value (SADI instruction in SHELX97-L) and the displacement ellipsoids of the pairs of disordered oxygen atoms were constrained to be equal (EADP instruction in SHELX97-L). For both the complexes the water H atoms were placed at chemically meaningful positions (O H = 0.86 Å; O...O = 1.36 Å) and refined as riding, with U iso (H) = 1.5 U eq (O), and the H atom associated to the morpholine N atoms were located in a difference Fourier map and refined isotropically. The C-bound H atoms were placed in geometrically idealized positions and refined using a riding model approximation, with C H = 0.93-0.97 Å and with U iso (H) = 1.2 U eq (C) or 1.5 U eq (C) for methyl H atoms. DFT calculations on HL and complex (1) were fully optimized using Gaussian 03 program. 23 Here Table 1 Crystal data for complex (1) Complex (1) Formula C 30 H 44 CuN 4 O 6 2(ClO 4 ) H 2 O Formula wt 837.16 Temp. (K) 294(2) Color Black Crystal system monoclinic Space group P21/n a (Å) 14.832(5) b (Å) 15.849(6) c (Å) 15.958(6) β ( ) 101.347(5) V (Å 3 ) 3678(2) Z 4 Crystal dim. (mm) 0.36 0.25 0.21 Min. and max. transmission factors 0.770-0.848 F(000) 1748 D c (g cm -3 ) 1.512 Λ (Mo Kα) (Å) 0.71073 θ range (º) 1.71-25.25 Reflect. collect./unique/obs. 38393, 6647, 4286 Abs. corr. multi-scan R int 0.0604 Final R 1 index [I>2σ(I)] 0.0625 Final wr 2 index [all reflections] 0.2069 Goodness-of-fit 1.027

NOTES 1453 the B3LYP functional has been adopted along with the 6-31G basis set for H, C, N, O atoms and LANL2DZ effective core potentials and basis set for the Cu atom. Time dependent density functional theory (TDDFT) with B3LYP density functional was applied to study the low-lying excited states of the complex in methanol [34-37]. 24-27 The UV-vis spectra were computed from TDDFT calculations in methanol. Results and discussion Synthetic procedure for complex (1) has been given in Scheme 1. The Schiff base ligand was obtained by the reaction between one equivalent of o-vanillin and one equivalent of the amine by refluxing the mixture in methanol. The complex was obtained by the reaction of one equivalent of the ligand with one equivalent of metal salt. No external base was added to deprotonate the phenolic proton. Complex (1) contains one molecule of water in its crystal as solvent of crystallization. This water may be from the solvent in which the reaction was carried out. Alternative source of water could be the solvent (six water molecules) of crystallization of copper(ii) perchlorate hexahydrate. All these reactions were performed in aerobic conditions, so the environment may also be a potential source of the water molecule in crystal (1). FT-IR spectra of the complexes as well as the ligands were recorded with the samples directly by attenuated total reflectance (ATR) technique or with the samples by preparing KBr pellets. The FT-IR spectrum of HL exhibits a number of strong ν C H bands 6 at the range of 2800 3000 cm 1. The strong band at 1632 cm 1 may be attributed to the presence of the imine (C=N) bond. Complex (1) shows a strong band at 1618 cm 1 confirming the retention of the imine bond in the metal complex. Very strong peaks at 1081 cm 1 (stretching) and 622 cm 1 (bending) indicate the presence of the perchlorate ion. Fig. 1 The asymmetric unit of (1) with displacement ellipsoids drawn at the 30% probability level. Only one component of the disordered perchlorate anions is shown. Scheme 1 Table 2 Selected bond lengths and bond angles of complex (1) Bond lengths (Å) Cu1 O1 1.913(4) Cu1 O4 1.907(3) Cu1 N1 2.009(4) Cu1 N3 2.002(4) N2 C10 1.508(8) N2 C11 1.497(8) N2 C14 1.493(8) N4 C25 1.496(8) N4 C26 1.487(8) N4 C29 1.504(9) Bond angles ( ) O4 Cu1 O1 167.27(16) N1 Cu1 O1 90.99(16) N1 Cu1 O4 89.92(16) N3 Cu1 O1 89.15(16) N3 Cu1 O4 91.29(16) N3 Cu1 N1 173.89(18)

1454 INDIAN J CHEM, SEC A, DECEMBER 2015 An ORTEP diagram of complex (1) is shown in Fig. 1. Selected bond lengths and bond angles of complex (1) are given in Table 2. The asymmetric unit contains two Schiff base ligands (HL), one copper atom, two disordered perchlorate anions and a water molecule of crystallization. The ligands coordinating the copper metal assume a zwitterionic form where the N atom of morpholine ring is protonated. The copper atom is coordinated to the phenolate oxygen atom and the imine nitrogen atom of both ligands in a remarkably tetrahedrally distorted square-planar geometry, with the metal displaced by 0.1136(7) Å from the mean plane of the donor atoms (r.m.s. deviation = 0.1706 Å). The Fig. 2 Packing diagram of (1) showing the hydrogen bonding network (dashed lines). Hydrogen atoms not involved in hydrogen bonds are omitted. Only one component of the disordered perchlorate anion is shown. cis donor-metal-donor bond angles vary from 89.15(16) to 91.29(16) whereas the trans angles are 167.27(16) and 173.89(18). The donor-metal bond distances are in good agreement with the published values. 10-12. The sixmembered chelating rings have a half-boat conformation, with copper displaced by 0.4853(7) and 0.4651(7) Å from the O1/C1/C6/C7/N1 (r.m.s. deviation = 0.0256 Å) and O4/C16/C21/C22/N3 (r.m.s. deviation = 0.0592 Å) mean planes. A packing diagram of complex (1) is given in Fig. 2. The morpholinium H atoms in (1) are engaged as donors in the intracomplex bifurcated hydrogen bonds involving the methoxyphenolato oxygen atoms of the other ligand (Fig. 2) as acceptors. In addition, crystal stabilization is enhanced by intemolecular C H...O and O H...O hydrogen bonds between methylene, aromatic and water H atoms as donors and oxygen atoms of the perchlotrate anions and morpholinium groups as acceptors. UV-vis spectra of HL and its copper(ii) complex were recorded in methanol at room temperature (Fig. 3). UV-vis spectrum of HL in methanol shows bands at 261, 293, 331 and 418 nm. Complex 1 shows peak at 381 nm, which could be attributed to the LMCT. A broad band in the region of 750-850 nm may be due to the d-d transition in the metal complex. The fluorescence spectra of HL and complex (1) were recorded in methanol at room temperature (Fig. 4). HL displays an emission band at 535 nm when excited at 418 nm. This may be tentatively assigned to the (π π*) intraligand fluorescence. However, no observable fluorescence intensity was noticed for complex (1) when it was excited at 418 nm. This may be due an electron or energy transfer between the Cu 2+ ion and HL which is known as the fluorescence quenching mechanism. 22 Fig. 3 UV-visible spectra of HL and complex (1) in methanol. [Curve 1, HL; Curve 2, Complex (1)]. Fig. 4 Fluorescence spectra of HL and complex (1) in methanol. [Curve 1, HL; Curve 2, complex (1)].

NOTES 1455 The geometries of HL and complex (1) were fully optimized by DFT/B3LYP method using the Gaussian 03 program. The experimental data and calculated structural parameters are reasonably similar. The contour plots of preferred frontier molecular orbitals of HL and complex (1) are shown in Figs 5, 6 and 7. Fig. 5 Contour plots of some selected Frontier molecular orbitals of HL. Fig. 6 Contour plots of some selected α-spin frontier molecular orbitals of complex (1). Fig. 7 Contour plots of some selected β -spin frontier molecular orbitals of complex (1).

1456 INDIAN J CHEM, SEC A, DECEMBER 2015 Fig. 8 Frontier molecular orbitals involved in the UV-vis absorption of complex (1). Table 3 Vertical electronic excitations calculated for HL in methanol Excitation (ev) Electronic transition state Excitation (nm) Osc. strength (f) Key transitions CI Transition assignment 3.7801 S 0- S 1 327.99 0.0013 HOMO LUMO (99%) 0.70457 ILCT 4.2315 S 0- S 3 293.00 0.0076 HOMO-3 LUMO (87%) 0.65913 ILCT HOMO-1 LUMO (03%) 0.13101 ILCT 4.7836 S 0- S 4 259.19 0.4599 HOMO-2 LUMO (81%) 0.63858 ILCT Excitation (ev) Electronic transition state Table 4 Vertical electronic excitations calculated for complex (1) in methanol Excitation (nm) Osc. Strength (f) Key transitions 3.1589 S 0- S 14 392.49 0.0020 HOMO(α) LUMO(α) (86%) 0.65722 HOMO(β) LUMO(β) (90%) 0.67208 3.2171 S 0- S 15 385.39 0.0010 HOMO-5(α) LUMO(α) (05%) 0.1685 HOMO-3(α) LUMO+1(α) (03%) 0.1718 HOMO-9(β) LUMO(β) (04%) 0.1453 HOMO-2(α) LUMO(α) (03%) 0.7439 HOMO-5(β) LUMO+2(β) (05%) 0.1662 HOMO-1(β) LUMO+1(β) (90%) 0.1528 3.2508 S 0- S 16 381.39 0.0027 HOMO-5(α) LUMO(α) (02%) 0.1017 HOMO-3(α) LUMO+1(α) (02%) 0.1084 HOMO-1(α) LUMO(α) (100%) 0.8964 HOMO(α) LUMO(α) (02%) 0.1030 HOMO-2(α) LUMO(α) (03%) 0.1328 3.2882 S 0- S 17 377.06 0.0014 HOMO-2(α) LUMO(α) (04%) 0.1705 HOMO-1(β) LUMO+1(β) (100%) 0.9430 HOMO(β) LUMO+2(β) (04%) 0.1749 3.3034 S 0- S 18 375.33 0.0027 HOMO-4(α) LUMO(α) (03%) 0.1179 HOMO-2(α) LUMO(α) (100%) 0.9287 HOMO-2(α) LUMO(α) (02%) 0.1055 CI

NOTES 1457 To get a better understanding of the electronic transitions of HL and complex (1), the time dependent density functional theory, i.e., TDDFT calculations have been carried out at the B3LYP associated with the conductor-like polarizable continuum model (CPCM) method in methanol using the optimized geometry of the ground (S 0 ) state. The ligand, HL, shows moderately intense peak at 327.99 nm (331 nm experimental) (ILCT), at 293.00 nm (293 nm experimental) (ILCT) and at 259.19 nm (261 nm experimental) (ILCT) (Table 3). The complex (1) shows broad peak at 381.39 nm (expt. 381 nm) (Fig. 8 and Table 4). In the present study, we have isolated a mononuclear complex of Cu(II) (complex 1) with an N,O donor Schiff base ligand (HL, 2-methoxy-6-(3- morpholinopropyliminomethyl)phenol). HL and complex (1) were characterized by a number of techniques. The structure of the complex (1) was confirmed by single crystal X-ray analysis. It shows that two ligands were attached to one metal center and it is interesting to note that the ligand in complex exists as zwitterions. Emission spectral analysis revealed that HL exhibited emission peak at 535 nm on excitation at 418 nm at the same time emission of metal complex was quenched remarkably. Supplementary data CCDC 1046214 for complex (1) contains the supplementary crystallographic data for complex (1). These data can be obtained free of charge from the Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif. Acknowledgement PR acknowledges the Department of Science and Technology, New Delhi, India for financial support. SD and SH wish to thank University Grants Commission (UGC), New Delhi, India and Council of Scientific and Industrial Research (CSIR), New Delhi, India respectively for their fellowships. References 1 Gupta K C & Sutar A K, Coord Chem Rev, 252 (2008) 1420. http://www.sciencedirect.com/science/article/pii/s001085450 7002093. 2 Gupta K C, Sutar A K & Lin C -C, Coord Chem Rev, 253 (2009) 1926. http://www.sciencedirect.com/science/article /pii/s0010854509000885. 3 Vigato P A, Peruzzo V & Tamburini S, Coord Chem Rev, 256 (2012) 953. http://www.sciencedirect.com/science/ article/pii/s0010854512000069. 4 Raman N, Raja S J & Sakthivel A, J Coord Chem, 62 2009) 691. http://www.tandfonline.com/doi/full/10.1080/ 00958970802326179#.VYVLl_mqqko. 5 Jia Y & Li J, Chem Rev, 115 (2015) 1597. http://pubs. acs.org/doi/abs/10.1021/cr400559g. 6 Roy P, J Coord Chem, 62 (2009) 2003. http://www.tandfonline.com/doi/full/10.1080/009589709027 51888. 7 Halder S, Dey S, Rizzoli C & Roy P, Polyhedron, 78 (2014) 85. http://www.sciencedirect.com/science/article/pii/ S0277538714002393. 8 Kundu S, Roy S, Bhar K, Sutradhar D, Mitra P & Ghosh B K, Indian J Chem, 52A (2013) 1404. http://nopr.niscair. res.in/handle/123456789/23100. 9 Huang Y, Wang W X & Zhou X J, Indian J Chem, 53A (2014) 793. http://nopr.niscair.res.in/handle/123456789 /29063. 10 Roy P & Manassero M, Dalton Trans. 39 (2010) 1539. http://pubs.rsc.org/en/content/articlelanding/2010/dt/b914017 d#!divabstract. 11 Jana M S, Dey S, Priego J L, Jiménez-Aparicio R, Mondal T K & Roy P, Polyhedron, 59 (2013) 101. http://www.sciencedirect. com/science/article/pii/s027753871300329x. 12 Roy P, Dhara K, Chakraborty J, Nethaji M & Banerjee P, Indian J Chem, 46A (2007) 1947. http://www.niscair.res.in/ sciencecommunication/researchjournals/rejour/ijca/ijca2k7/ij ca_dec07.asp#p6. 13 Desbouis D, Troitsky I P, Belousoff M J, Spicciaa L & Graham B, Coord Chem Rev, 256 (2012) 897. http://www.sciencedirect.com/science/article/pii/s001085451 1002657. 14 Petek H, Albayrak C, Đskeleli N O, Ağar E & Şenel Đ, J Chem Crystall, 37 (2007) 285. http://link.springer.com/ article/10.1007/s10870-006-9175-4. 15 Dai C H & Mao F -L, Synth React Inorg, Met-Org, Nano- Met Chem, 42 (2012) 537. http://www.tandfonline.com/doi/ full/10.1080/15533174.2011.613435. 16 Wang P, Li Q L, Ruan Q & Su Y Q, Russian J Coord Chem, 37 (2011) 935. http://link.springer.com/article/10.1134/ S1070328411110121. 17 Sasmal S, Hazra S, Kundu P, Dutta S, Rajaraman G, Saňudo E C & Mohanta S, Inorg Chem, 50 (2011) 7257. http://pubs.acs.org/doi/abs/10.1021/ic200833y. 18 Perrin D D, Armarego W L F & Perrin D R, Purification of Laboratory Chemicals, (Pergamon Press, Oxford, U K) 1980. 19 SAINT, ver. 7.51A and SADABS Ver. 2007/4, (Bruker AXS Inc., Madison, Wisconsin, USA) 2008. 20 Altomare, Burla M C, Camalli M, Cascarano G, Giacovazzo C, Guagliardi A, Moliterni A G G, Polidori G & Spagna R, J Appl Cryst, 32 (1999) 115. http://journals.iucr.org/j/ contents/backissues.html. 21 Sheldrick G M, Acta Cryst, A64 (2008) 112. http://journals.iucr.org/a/issues/2008/01/00/sc5010/index.html. 22 Sarkar M, Banthia S & Samanta A, Tetrahedron Lett, 47 (2006) 7575. http://www.sciencedirect.com/science/article/ pii/s0040403906017096. 23 Gaussian 03, rev. C.02, (Gaussian, Inc., Wallingford, CT) 2004.

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