Research Journal of Chemical Sciences ______________________________________________ ISSN 2231-606X Vol. 5(10), 43-50, October (2015) Res. J. Chem. Sci. International Science Congress Association 43 New Ferromagnetically Coupled Dicopper (II) Complexes with – bonding bridgeRan Bahadur Yadav Applied Chemistry Department, Faculty of Technology and Engineering, Kalabhavan, The Maharaja Sayajirao University of Baroda, Vadodara- 390001, INDIA Available online at: www.isca.in, www.isca.me Received 1st October 2015, revised 13th October 2015, accepted 17th October 2015 AbstractFour ternary binuclear complexes, [Cu(A),pichx](ClO (1 and 2) and [Cu(A)pichx](3 and 4) have been synthesized. In the complexes two metal centers are held together by - bonding bridge i.e. pichx= N,N’-bis(2-pyridylcarbonyl)-1,6-diaminohexane and A2 is 2,2’-bipyridyl or 1,10-phenanthroline or 2-hydroxybenzoic acid or 5-bromo-2-hydroxybenzoic acid coordinated to the metal ions as non-bridging ligands. The complexes were characterized by elemental analyses, conductance, magnetic susceptibility measurement, IR and electronic spectroscopy. The ESI-mass of the complex is consistent with the binuclear formulation. The complexes are observed to undergo a weak to moderate ferromagnetic coupling between two copper (II) ions. Keywords: Binucleating ligand, binuclear Cu (II) complexes, – bonding bridges and magnetic properties. Introduction The field of molecular magnetism has seen giant leaps since the 1980s, from the search for molecule-based magnets to the achievements of single-molecule magnets and multifunctional materials1–6. It is well established that the extent of exchange coupling between two copper (II) centers depends on various structural features such as the coordination geometry of the copper ions, energy of the interacting orbitals and on the variation in geometrical parameters such as metal – ligand bond length, M-L-M bridging angle, dihedral angle between the metal coordination plane and the degree of planarity of the bridging unit5-12. In most cases this type of exchange through multi atomic bridges has been shown to take place through the bridging ligand, possessing - orbitals. However, it has been suggested that the – orbitals13, 14 can participate in the super exchange over a long distance in multiatomic bridges and lead to a spin exchange15-17, yet such interactions are very weak. In order to study magnetic exchange between paramagnetic metal centers through saturated moiety, new ternary binuclear complexes, [Cu(A),pichx](ClO4 and [Cu(A),pichx] have been synthesized. The ligand, pichx = N,N’-bis(2-pyridylcarbonyl)-1,6-diaminohexane (pichx) and A = 2,2’-bipyridyl or 1,10-phenanthroline or 2-hydroxybenzoic acid or 5-bromo-2-hydroxybenzoic acid. The complexes have been characterized based on elemental analysis and spectral properties. Material and Methods Ethyl-2-pyridinecarboxylate, 1,6-diaminohexane, 2,2’-bipyridine, 1,10-phenanthroline, 2-hydroxybenzoicacid, 5-bromo-2-hydroxybenzoic acid, cupric acetate monohydrate and sodium perchlorate were obtained from Merck. All of these were of A. R. grade and were used as received. All solvents were distilled twice before use. Synthesis of binucleating ligand, N, N’-bis (2-pyridylcarbonyl)-1,6-diaminohexane (pichx): A solution of ethyl-2-pyridinecarboxylate (25 mmol, 3.37 ml) was placed in a flask equipped with water condenser and a magnetic stirrer. To this was added a solution of 1,6-diaminohexane (12.5 mmol, 1.45 gms). The mixture was allowed to reflux for eight hours. A pale brown solid compound separated out on cooling. The solid product obtained was crystallized from 50:50 CHCl and petroleum ether. Finally, the compound was washed with 20 ml (in five portions) distilled water and 10 ml diethylether and dried in air. The yield was 3.1978 gms (40%) and mpt 98-100 C. It was characterized by IR (KBr, cm-1) 3371, 3068, 2924, 2853, 1657, 1593. Elemental analysis, calculated for formula 1822 : Found (Calc.) C 66.26 (66.26), H 6.74 (6.47), N 17.18 (17.09) and Chemical shift values in H NMR recorded in CDCl solution: ()1.473 (t, 2H (CH)); 1.640 (t, 2H (CH)); 3.461 (m, 2H (CH)); 7.446 (m, H (PyH)); 7.856 (m, H (PyH); 8.089 (s, H (NH)); 8.196 (d, H (PyH); 8.545 (m, H (PyH) Chemical shift values in 13C NMR: () 26.68 (C (CH); 29.56 (C (CH); 39.31 (C (CH); 122.16 (C (Py)); 126.05 (C (Py)); 137.33 (C (Py)); 148.01 (C (Py)); 149.99 (C (Py)); 164.26 (C (C=O)). Synthesis of binuclear [Cu(A),pichx](ClO type complexes, (1 and 2): Copper (II) acetate monohydrate (1.66 mmol, 0.333 gms) in 20 ml CHOH was taken in three neck flat bottom flask. To this was added 2,2’-bipyridine (1.66 mmol, 0.2605 gms) in 15 ml CHOH and pichx (0.83 mmol, 0.2716 gms) in 15 ml CHOH simultaneously drop wise over 1 hour. Research Journal of Chemical Sciences ___________________________________________________________ ISSN 2231-606XVol. 5(10), 43-50, October (2015) Res. J. Chem. Sci. International Science Congress Association 44 The reaction mixture was allowed to reflux for two hours. The excess of CHOH was distilled out from reaction mixture, 20 ml distilled water followed by sodium perchlorate monohydrate (3.33 mmol, 0.468 gms) in 10 ml distilled water were added. The solution was allowed to digest for 30 minutes on water bath. Light blue coloured solid separated on cooling was filtered, washed thoroughly with 50 ml distilled water followed by 25 ml ethanol and dried in air. Complex, 2was prepared by following similar procedure as above and using pichx (0.83 mmol, 0.2716 gms) and 1,10-phenathroline (0.3306 gm, 1.67 mmol) respectively, (Scheme 1). Synthesis of binuclear [Cu(A),pichx] type complexes, (3 and 4): A solution of copper (II) acetate monohydrate (0.333 gms, 1.66 mmol) in 20 ml methanol was taken in a three neck flask. Solution of 2-hydroxybenzoic acid (1.67 mmol, 0.230 gms) and pichx (0.83 mmol, 0.2716 gms), each in 15 ml methanol, were added dropwise with constant stirring over 1 hour to the hot refluxing solution of copper (II) acetate. The solution was allowed to reflux for further 2 hours where upon a micro crystalline solid separated out. It was filtered, washed thoroughly with 30 ml methanol and dried in air (). Complexes, 4 was prepared by following similar procedure as above and using pichx (0.83 mmol, 0.2716 gms) and 5-bromo-2-hydroxybenzoic acid (1.67 mmol, 0.3616 gms). (Scheme-1). Physical measurements: The electronic spectra of the complexes in UV-Vis region were recorded in methanolic solutions using Perkin Elmer Lambda 35, UV – Visible spectrometer. IR spectra (as KBr pellets) were recorded on Perkin Elmer FT-IR, spectrum RX1. NMR of the ligand was recorded on Brucker 400 MHZ. The ESI-MS spectrum of Complex, [Cu(phen),pichx](ClO was recorded on a THERMO Finnign LCQ Advantage max ion trap mass spectrometer in acetonitrile solution. Specific conductivity of the complexes 1 and 2 in DMF solution having 1.0 mmolar concentration was measured using a Toshniwal conductivity bridge. Magnetic susceptibility of the complexes at room temperature was measured in the powder form using the Faraday method with magnetic field of 0.8 Tesla. Magnetic moments per atom were calculated using following formula, eff = 2.808( T Χ 10-6/2)1/2 Where, m = Molar magnetic susceptibility per metal atom and other terms have their usual meanings. Pascal corrections have been incorporate. Results and Discussion Elemental analysis of the purified ligand is agreeable with suggested empirical formula. The H NMR and 13C NMR of the ligand have all features expected for the proposed structure(figure-1a, 1b and figure-2). The binucleating ligand, bis(picolinamide) is ambidentate and can coordinate with the metal ion either (1) through the amide nitrogen or (2) the amide oxygen. In both cases, it will form five membered chelate rings with the metal ion (figure-3a and 3b). In order to verify the preference for coordination, the ligand geometry was optimized by semi-empirical Quantum Mechanical (PM3) calculations and the energy, electrostatic potentials and electron densities were calculated. The map of electrostatic potential over electron density shows maximum electron density (figure-4)over pyridine nitrogen and amide oxygen directed towards each other in a way to facilitate the coordination of a metal ion at this site and confirms the coordination through amide nitrogen. (Ligand, pichx)NH(CHNH AAAA NHNHCuCu (CH (ClO+ 2 Cu(CHCOO).H1. 2A, 2. CHOH, Reflux, 2 - 3 hours3. NaClO.HO (aq)digest AAAA NHNHCuCu (CH 1. 2A2. CHOH, Reflux, 2 - 3 hours1 : A = 2,2'-bipyridine. 2 : A = 1,10-phenanthroline.(Complexes: 1 and 2)3 : A = 2-hydroxybenzoic acid.4 : A = 5-bromo-2-hydroxybenzoic acid.(Complexes: 3 and 4) Scheme-1 Synthesis of binuclear complexes Research Journal of Chemical Sciences ___ ______________________________ Vol. 5(10), 43-50, October (2015) International Science Congress Association 1 H NMR of binucleating ligand, 1 H NMR of binucleating ligand, ______________________________ ___________________ International Science Congress Association Figure-1a H NMR of binucleating ligand, pichx in CDClFigure-1b H NMR of binucleating ligand, pichx in CDCl3 (expanded) ___________________ _______ ISSN 2231-606X Res. J. Chem. Sci. 45 Research Journal of Chemical Sciences ___ ______________________________ Vol. 5(10), 43-50, October (2015) International Science Congress Association 13 C NMR of binucleating ligand, N Cu Possible mode of coordination of ligand Plot of electrostatic potential over elect ______________________________ ___________________ International Science Congress Association Figure-2 C NMR of binucleating ligand, pichx in CDCl3 N ORN Cu H Cu Figure-3a Figure-3b Possible mode of coordination of ligand Figure-4 Plot of electrostatic potential over elect ron densities of ligand, pichx (Red regions indicate region richer in electron densities) ___________________ _______ ISSN 2231-606X Res. J. Chem. Sci. 46 (Red regions indicate region richer in electron Research Journal of Chemical Sciences ___________________________________________________________ ISSN 2231-606XVol. 5(10), 43-50, October (2015) Res. J. Chem. Sci. International Science Congress Association 47 As the values in table-1 reveal, the complexes 1 and 2 correspond to the general formulae [Cu(A), pichx] (ClOandcomplexes 3 and 4 correspond the general formulae [Cu(A),pichx]. Complexes 1 and 2are found to be highly soluble in DMF, hence conductance studies were carried out in DMF to confirm the number of anions inside or outside the coordination sphere. Complexes show the conductance values between 290 – 300 (-1cmmol-1) corresponds to 1: 4 electrolyte, indicating that four anions are outside of the coordination sphere. Complexes 3 and 4 were having non-conductance nature. Electronic Spectra: The electronic spectra of the binuclear complexes in methanolic solutions show several bands in the range of 200 – 850 nm. Intense bands observed below 350 nm are due to interligand transitions. The weak and broad band observed in each complex between 684 – 688 nm can be assigned to the Laporte forbidden ligand field transitions. In a square planar environment Cu(II), a d metal ion, has A1g1g, 2g1g and E1g transitions, which have similar energy and hence remain merged to form a broad band. The transitions are usually seen as a broad band near 600 nm in normal copper complexes (table-2). The lower energy of these transitions in complexes is indicative of significant distortion from planarity to weaken the ligand field. Infrared spectra: The presence or absence of certain bands in the IR spectra have been, generally, utilized to illustrate the structure of the complexes. Most important bands in the complexes are listed in, table-2. In the ligand and complexes, absorption due to stretching of the amide N-H is observed between 3233 - 3371 cm-1. The absorption due to the stretching of amide �C=O is observed between 1631-1647 cm-1 in the binuclear complexes, 1 to 4. These are at lower energy compared to the free ligand value of 1657 cm-1. The shift in the amide (�C=O) towards lower energy in the complexes, indicates that the amide oxygen is involved in coordination with the Cu (II) ion. Also, the almost unaffected amide �N-H stretching frequency indicates that the amide nitrogen is not coordinated with the metal ion18. In the IR spectra of the complexes, 1 and 2, as of perchlorate is observed between 1090 – 1092 cm-1. There is no splitting of the ~1090 cm-1 band indicating that perchlorate is tetrahedral and ionic i.e. not coordinated with the metal ions19. Other characteristic bands of ligand include asymmetric stretching between 2927 – 2931 cm-1 and symmetric stretching between 2857 – 2870 cm-1 due to the presence of –CH groups. –C=C- stretching in aromatic ring is observed between 3067 – 3081 cm-1 and stretching of the �C=N (ring) appears between 1592 – 1601 cm-1. Thus, the IR spectra support the suggested structures of the complexes. Table–1 Reflux time, yields, elemental analysis, molar conductivity and magnetic moment of the binuclear complexes Comp No. Complexes Reflux Time (hours) Yield (%) Elemental analysis Obsd. (Calc) in %C H N Molar conductivity -1cmmol-1) Magnetic moment (B.M.) 1 [Cu(bipy),pichx](ClO 38 H 38 N 8 O 18 Cl 4 Cu 2 4 31 38.70 (39.20) 3.89 (3.26) 10.06 (9.63) 300 1.84 2 [Cu(phen),pichx](ClO 42 H 38 N 8 O 18 Cl 4 Cu 2 4 66 41.16 (41.62) 3.42 (3.14) 9.82 (9.25) 290 2.03 3 [Cu(salac),pichx] 32 H 30 N 4 O 8 Cu 2 2 50 50.49 (50.96) 4.05 (4.13) 7.98 (7.72) - 1.86 4 [Cu(Brsalac),pichx] 32 H 28 N 4 O 8 Br 2 Cu 2 3 91 43.02 (43.48) 3.71 (3.17) 6.85 (6.34) - 2.04 The values given in parentheses are theoretical values calculated from the molecular formulae. Table–2 Electronic absorptions (nm) and IR absorptions (cm-1) of the ternary binuclear complexes Comp. No. Uv-visible absorption nn(-NH) stretching aromatic stretching nn(-C-H) nnas(-CH) and nn(-CH) nn(�C=O) ring stretching nn(-C=N) miscellaneous frequencies 1 260, 300, 310, 684 3361 3083 2933, 2863 1647 1601(ClO)1092 2 214, 300, 310, 684 3341 3071 2932, 2857 1643 1598(ClO)1090 3 220, 271, 300, 315, 684 3368 3067 2931, 2862 1638 1601 - 4 238, 268, 306, 6883233 3071 2927, 2870 1631 1592 (C-Br)1097 Research Journal of Chemical Sciences ___________________________________________________________ ISSN 2231-606XVol. 5(10), 43-50, October (2015) Res. J. Chem. Sci. International Science Congress Association 48 Mass Spectra: The ESI-MS spectrum of the binuclear complex, [Cu(phen),pichx](ClO was recorded. The spectrum of complexis shown in figure-5. Inthe mass spectrum, peak at m/z = 1210.9 correspond to parent ion peak of the complex, [Cu(A),pichx](ClO. Subsequently, perchlorate ions and a phenantroline molecule are lost from parent ion and peak for resultant fragment, [Cu2 (phen)pichx]+ is observed at m/z = 632.8. Peak corresponding to [Cu(phen), pichx]+ is obtained at m/z = 568.0 with 100% relative abundance. An important peak correspond to binucleating ligand is observed at m/z = 389.2. This is formed due to protonation of the ligand molecule, [pichx+H]. Important fragments are listed in table-3. Table–3 Fragmentation pattern in the positive ion ESI-MS of [Cu2 (phen),pichx](ClO. m/z (%% relative abundance) Possible fragments 1210.9 (13) [Cu2 (phen),pichx](ClO 632.8 (15) [Cu2 (phen)pichx] 568.0 (100) [Cu (phen),pichx] 452.5 (34) [Cu (pichx)] 423.1 (70) [Cu(phen) 389.2 (44%) [Cu(pichx)] 327.2 (64%) [pichx+H] 243 (14%) [Cu(phen)] Peaks observed at various m/z values for the complex 2 strongly elucidate the formation of binuclear complexes with suggested molecular formulae. Magnetic Properties: The magnetic susceptibility of complexes was measured at room temperature using Faraday set up. The values of magnetic moment per atom are 1.84 – 2.04 B.M. (table-1). These higher values of magnetic moments than free copper (II) ion, indicate ferromagnetic interaction between two metal centres. Hendrickson and coworkers13 first time suggested that the – orbitals can participate in the super exchange over a long distance in multiatomic bridges. The role of – orbitals in superexchange interaction has been further supported by the study of spin exchange interaction in the binuclear complexes, bearing the saturated bridging moieties20. It is appropriate to recall at this stage that in multi atomic bridging ligands, some molecular orbital with appropriate energy and symmetry is usually available to mediate the exchange5-12. In order to examine this, systematic variations have been made in the non bridging ligand in the present complexes. The bridging ligand with –CH– group is common in all complexes while the non bridging secondary ligand changes from bipy, phen, salc and Brsalc. Results show that bulkier and more -bonding ligands can distort the metal coordination planes to a greater extent and hence lead to stronger ferromagnetism. D. Zhang et al.21 have observed similar dependence of the extent of magnetic exchange on the non bridging ligands in the complexes with oxalodiamide bridging groups. Figure - 5 ESI-MS spectra of binuclear complex, [Cu(phen),pichx](ClO. Research Journal of Chemical Sciences ___________________________________________________________ ISSN 2231-606XVol. 5(10), 43-50, October (2015) Res. J. Chem. Sci. International Science Congress Association 49 Conclusion In the present endeavour, binuclear complexes, possessing aliphatic bridging moiety have been synthesised and characterised. Room temperature magnetic studied revealed that a ferromagnetic interaction has been exits between the metal centers which is propagated through the - bonding orbitals of the aliphatic bridging moiety. Room temperature magnetic data indicated that bulkier and more -bonding ligands can create greater mismatch of atomic orbitals with Molecular orbitals of bridging ligand. Hence, results lead towards ferromagnetism. Acknowledgements I am thankful to Head, Department of Chemistry, Faculty of Science, The Maharaja Sayajirao University of Baroda, Vadodara for NMR and magnetic measurements. SAIF, CDRI Lucknow also acknowledged for ESI-Mass. 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