New Polydentate Ligands Containing a 1,8-Naphthyridine

2-(o-Hydroxyphenyl)-1,8-naphthyridine (HN),


Results and Discussion
The complexes were prepared by heating together methanol solutions of the appropriate ligand and metal acetate.All the complexes prepared are non-hygroscopic and stable at room temperature.The complexes of HN and HMPN are insoluble in methanol, ethanol, benzene and THF, but soluble in DMSO and DMF.The characterization data are presented in Table 1.The molar conductance values of the complexes of HN and HMPN (10 -3 M solution in DMF) are found to be in the range 8-15 ohm - 1 cm 2 mol -1 .These low values indicate that the complexes are non-ionic [15].The complexes of BN are insoluble in common solvents and hence molar conductance could not be measured, but this very insolubility indicates that the complexes are neutral [15].

Mass Spectra
The ligands HN, HMPN and BN exhibit their highest mass peaks at m/z 223, 254 and 246, respectively, corresponding to the [M+1] + ions.The conspicuous fragments in the mass spectra of all the HN, HMPN, and BN complexes correspond to the ions [M(L) 2 (H 2 O) 2 ] ‫ך‬ + (L = HN and HMPN) and [M(BN) 2 (OAc) 2 ] ‫ך‬ + , respectively.Further, the interaction of the ligands with the metal ions is confirmed by the appearance of the fragments corresponding to [M(L) 2 ] ‫ך‬ + and [M(BN) 2 ] ‫ך‬ + .The presence of lower mass fragments that also appeared in the spectra of the ligands substantiates the presence and coordination of the ligands to the metal ions.The analytical data presented in Table 1 confirm the formulae of the complexes of the three ligands as and [M(BN) 2 (OAc) 2 ], where M = Mn(II), Co(II), Ni(II), Cu(II) or Zn(II) [16,17].

Thermal analysis
The thermograms of the complexes of HN and HMPN exhibit weight losses in two different temperature regions, one in the region of 150 -250 °C and the other between 260 and 800 °C.Endothermic peaks were observed in the low temperature region of 150 -250 °C in the DTA thermograms of all complexes, indicating that there are water molecules in the complexes.The loss of matter in this temperature region corresponds to two water molecules and the loss in this temperature region confirms that the water molecules are coordinated to the metal ion [18].In the high temperature region, the DTA curves show exothermic peaks which indicate the loss of organic matter [19], and in all the cases the residue left corresponds to the respective metal oxides [11].These results confirm that the formula of the complexes is [M(L) 2 (H 2 O) 2 ] (L = HN and HMPN).Thus HN and HMPN are expected to act as bidentate uninegative ions and the coordination number of the metal ions is six.Perusal of the thermal analyses data of the metal complexes of BN indicates that they decompose in a single stage.This decomposition occurs between 208 -800 o C with the loss of two acetate ions and two ligand molecules [19].The experimental data of the thermal analyses are in good agreement with the theoretical values and also with the results of the elemental analyses.These results confirm that the formula of the complexes is [M(BN) 2 (OAc) 2 ].Thus, BN is expected to act as a bidentate neutral ligand and the coordination number of the metal ions is six.The thermal stability of the complexes of HN, HMPN and BN was found to follow the order Mn(II) < Co(II) ≈ Ni(II) < Cu(II) > Zn(II).

Infrared spectra
The IR spectra of ligands HN and HMPN exhibit characteristic absorptions at 1615 cm -1 ν (C=N) , 3425 cm -1 ν (O-H) and 1217 cm -1 ν (C-O) .The complexes of these ligands exhibit a broad absorption between 3600 and 3100 cm -1 , attributable to the ν (O-H) of the coordinated water molecules, Further, the absorption at 827 cm -1 indicates that the water molecules are coordinated [20].The ν (C=N) and ν (C-O) of the ligands are shifted to the lower frequency region by 10 to 30 cm -1 in the spectra of the complexes, confirming the coordination through ring nitrogen atom and the phenolic oxygen atom [21,22].
The IR spectrum of the BN ligand exhibits a characteristic absorption at 1581 cm -1 that may be assigned to the 1,8-naphthyridine ring ν (C=N) .This absorption was shifted to the low frequency side by 10 to 15 cm -1 in the spectra of the complexes, confirming the coordination through the nitrogen atoms of the naphthyridine and benzimidazole groups.The ν asym(COO-) and ν sym(COO-) stretching frequencies of free acetate ions are observed at 1560 and 1416 cm -1 , respectively.In the IR spectra of the complexes of BN, the acetate ion-stretching frequencies are found to have shifted to 1535-1555 and 1385-1380 cm -1 , respectively.The energy separation between ν asym(COO-) and ν sym(COO-) is found to be >144 cm -1 (150-185 cm -1 ) , and this indicates the monodentate nature of the acetate ion [22], since in the event of bidentate coordination, the energy separation is reported to be <144 cm -1 .Further, the IR spectra of the complexes exhibit two new bands in the far-IR region at 410-475 cm -1 and 500-525 cm -1 .These absorptions may be assigned to ν(M-N) and ν(M-O) respectively [6,15,23].

H-NMR spectra
The 1 H-NMR spectra of the ligands HN and HMPN and the diamagnetic Zn(II) complexes were recorded in d 6 -DMSO.The characteristic proton resonance signals of HN and HMPN are the phenolic H's at δ 15.08 s and 14.36 s ppm, the phenyl and naphthyridine ring H's at 9.11 -6.99 m and the methyl H's, in the case of HMPN, at 2.3 s ppm In the spectra of their Zn(II) complexes, the signals at δ 15.08 s in 5 and 14.36 s ppm in 10 disappeared, indicating the deprotonation of the phenolic proton and complex formation [24].A new signal at 5.55 ppm is observed in the spectra of the Zn(II) complexes of HN and HMPN.This may be assigned to the proton resonance signal of coordinated water molecules.The complexes of BN are insoluble in common organic solvents and hence 1 H-NMR spectra could not be recorded.

Magnetic moment and UV-Vis spectra
The evaluated magnetic moment values of the complexes are presented in the Table 1.The electronic spectra of the complexes of HN and HMPN in DMF solution and the reflectance spectra of the complexes of BN in solid state (the solubility of these complexes is very poor in common solvents) were recorded and are presented in Table 2.The high spin d 5 Mn(II) octahedral complexes (µ eff 5.9 B.M.) exhibit complex spin forbidden transitions from the 6 A 1g ground state.The Mn(II) complexes of all the ligands shows five absorptions characteristic of electronic transitions to the excited states 4 T 1g , 4 T 2g , 4 A 2g (D), 4 E 1g (D) and 4 T 1g (P).The high spin octahedral Co(II) and Ni(II) complexes exhibit magnetic moment values around 4.9 and 2.9 B. M. respectively and three transitions each in the electronic spectra.The Co(II) complex exhibits three electronic transitions from ground state 4 T 1g (F) to the excited states 4 T 2g (F), 4 A 2g (F) and 4 T 2g (P) respectively.In the case of Ni(II) complexes, the three observed absorptions can be attributed to the transitions [25], 3 A 2g 3 T 2g (P), 3 A 2g 3 T 1g (F) and 3 A 2g 3 T 1g (P).The parameters like β, B and Dq have been evaluated from the electronic spectral absorptions of Co(II) and Ni(II) complexes.The β values are found to be less than 1.0, indicating that the M-L bond is covalent [26].The magnetic moment values evaluated from the spectra [27] are in good agreement with the experimentally determined values (Table 1).The Cu(II) complexes of the ligands HN, HMPN, and BN, with µ eff = 1.94, exhibit a broad absorption centered at 15149 cm -1 , 14405 cm -1 and 15375 cm -1 respectively suggesting a distorted octahedral geometry [28].The diamagnetic Zn(II) (d 10 ) complexes do not exhibit any characteristic d-d transitions.However, the ligand absorptions in the UV region have shifted in the spectra of the complexes confirming the coordination of the ligands, HN, HMPN and BN to Zn(II) ions.

Table 2:
Electronic spectral data of transition metal complexes of ligands HN, HMPN and BN.

Electron Spin Resonance Spectra
The ESR spectra of the Cu(II) complexes were recorded at liquid nitrogen temperature and the bonding parameters have been evaluated using Kneubuhl's approximation.The g || , g ⊥ and g ave values are found to be: The λ values of the complexes are found to be less than that of the free ion value (829 cm -1 ), suggesting a considerable mixing of ground and excited state terms [29].The same is evident from slightly higher magnetic moment values for Cu(II) complexes of HN, HMPN

Conclusions
Based on the reported results, it may be concluded that HN and HMPN act as bidentate uninegative ligands, coordinating through one of the naphthyridine nitrogen atom and the deprotonated phenolic oxygen.BN acts as bidentate neutral ligand coordinating through the naphthyridine and benzimidazole nitrogen atoms.In a majority of unsubstituted 1,8-naphthyridine complexes, the ligand is found to act as a bridging group coordinating to the two metal ions through two nitrogen atoms of the two rings [30], but the presence of an electron pair donating atom on the phenyl rings at the second position, as reported in the present work, which facilitates formation of six member rings, shifts the nitrogen atom of the other ring away from the coordination site.In the present investigations, all the complexes are found to be mononuclear, based on the mass spectral data.The coordination number six is attained by coordination with the two bidentate ligand ions/molecules and to two water molecules in the case of HN and HMPN ions and to two acetate ions in the case of BN.Based on the physicochemical and the spectral studies the tentative structures proposed for the complexes are shown in Figures 1A-1C.

General
All the chemicals, the hydrated metal acetates and solvents used were purchased from E-Merck India Ltd.The chemicals and solvents were of AR grade and were purified before use wherever required by standard methods.The metal ion contents in the complexes were determined by complexometric titrations [31] using EDTA.Carbon, hydrogen and nitrogen were determined using Perkin-Elmer-2401 CHN analyzer.Magnetic susceptibility measurements were made on Gouy balance at room temperature.Molar conductance measurements were made in DMF using Digisun Digital DI-909 instrument.IR spectra were recorded on Perkin-Elmer BX series spectrometer using KBr pellets in the range of 4000-600 cm -1 and as Nujol mulls between CsI plates in the far-IR region.The electronic spectra were recorded on Shimadzu UV-2401PC Spectrophotometer. 1 H-NMR spectra were recorded on JEOL 200 MHz spectrometer.Mass spectra (FAB) were recorded on a Jeol JMS D-300 spectrometer.TGA, DTA and DSC thermograms of the complexes were obtained in the temperature range of 50 -1000ºC with a heating rate of 20 ºC min -1 in nitrogen atmosphere using Mettler TGA 851 and Mettler DSC 822E instruments.

Preparation of complexes
The ligands HN, HMPN and BN were prepared by the methods reported elsewhere [32].These compounds were recrystallized from ethanol and the spectral properties were compared with the reported data.The ligands HN, HMPN and BN (2 mmol, 444, 508 and 492 mg, respectively) were dissolved in methanol (20 mL) and to this the metal acetate (1.0 mmol) dissolved in methanol (20 mL) was added slowly with constant stirring over a period of 10 min.The reaction mixture was refluxed for 4-5 hours.The solid complexes obtained were collected on a fine frit filter and washed with cold methanol and then with acetone.The complexes were dried over fused calcium chloride in a vacuum desiccator.The yields of the complexes were in the range 60-80%.

Table 1 :
Characterization data of the complexes of