Spectroscopic Characterisation of Metal Complexes with Tetradentate Ligand

A Schiff’s base containing tetradenticity N,N-bis-(3-hydroxyquinoxaline)2-carboxidine-1,8-diaminenaphthalene, henceforth abbreviated as NBHCN, has been prepared by condensation of two moles of 3-hydroxyquinoxaline-2-carboxaldehyde with one mole of 1,8-diaminonaphthalene. This Schiff’s base has been made to undergo complexation with Co(II) and Ni(II) metal ions. On the basis of their elemental analysis and molar conductivity values, the complexes have been formulated as [M(NBHCN)X2 ] where X stands for water, pyridine and α-picoline which act as secondary ligands. The comparison of infrared spectra of metal complexes with that of NBHCN (Schiff’s base ligand) indicates its coordination through two azomethine nitrogen and two deprotonated hydroxyl oxygen atoms. Thus, the ligand acts as a bivalent tetradentate anionic one joining through four potential sites to the metal ions. The magnetic moments of Co(II) complexes have been found in the range of 4.90–4.91 Bohr Magnetone (BM) which is greater than three unpaired electrons in spin free octahedral complexes of Co(II) metal ion. The slightly excess value of magnetic moment of Co(II) complexes from μs = 3.87 BM, where μs is magnetic moment due to spin-only motion, may be attributed to T1g ground state of 4F term of d-system in octahedral symmetry, being orbitally triply degenerate, makes sufficient contribution to the magnetic moment of the complexes. The octahedral symmetry of Co(II) complexes has further been confirmed by their electronic spectra which display three bands due to T1g(F)→ T2g(F), T1g(F)→ A2g(F) and T1g(F)→T1g(P) spin-allowed transitions. The magnetic moment values of Ni(II) complexes have been found in the range of 3.0–3.2 BM which is also greater than its ms value of 2.818 BM, corresponding to two unpaired electrons under octahedral symmetry. The appearance of three bands in their electronic spectra due to A2g → T2g, A2g → T1g(F) and A2g → T1g(P) spin-allowed transitions is indicative of octahedral symmetry around Ni(II). Their ν2 /ν1 values also support octahedral geometry Characterisation of Metal Complexes 2 around metal ions and decrease in Racah parameters B from free metal ions to complexes is indicative of appreciable covalent character in M-L bonds.


inTroduCTion
Transition metal complexes of Schiff's base macrocycle have been of considerable interest in terms of structural complexity and biological functions. 1,2 Tetradentate Schiff's base ligands are able to coordinate with many different metal ions forming stable compounds and some of these compounds have been recognised as oxygen carrier. 3,4 The tetradentate Schiff's base complexes of transition metal ions have shown an exponential increase in inorganic catalysis for various organic preparations. [5][6][7][8] The importance of coordination compounds of multidentates Schiff's base is widely known in pharmaceutical (antitumor, antiviral, antituberculosis and antimicrobial) and industrial (analytical reagent, polymer, coatings and fluorescent material) fields. 9-13 N-hetrocycles like quinoxaline, quinozoline and their analogues are important hetrocycles that are present in many naturally occurring alkaloids and possess interesting pharmacological activities like antihypertensive, analgesic, anti-inflammatory, anticancer and anti-HIV activities. [14][15][16][17][18] The Schiff's bases containing quinoxaline units are also known to exhibit wonderful pharmacological properties. The literature, however, reveals that no work has been reported on the metal complexes of Schiff's base derived from quinoxalinealdehyde and naphthalene-1,8-diamine. Therefore, we report herein the synthesis and characterisation of the Schiff's base derived from 3-hydroxyquinoaxaline-2-carboxaldehyde and 1,8-diaminonaphthalene and its metals complexes with Co(II) and Ni(II) metal ions in which the tetradenticity of the ligand may lead to the formation of macromolecule type coordination compounds with interesting spectral data.

Micro Analytical data and Molar Conductivity
The micro analytical data and molar conductivity of ligand and complexes are presented in Table 1.

ir Spectra
The important and significant IR bands before and after complexation have been detected and assigned. The free ligand displays two broad bands at 3430 cm −1 and 3420 cm −1 which may be assigned to H-bonded v OH of enolic tautomeric form and H-bonded v N-H in amide tautomeric form of the three ligand. 21,22 It shows that the free ligand exists in tautomeric form which is further confirmed by the absence of medium band at 1690 cm −1 due to v c=o of ketoform. 23−26 In the IR spectra of the complexes, all these three bands disappear which indicates the dehydrogenation of phenolic OH group and coordination through deprotonated oxygen. 27−30 The azomethine group ν CH=N absorbs sharply at 1600 cm −1 in the IR spectra of free ligand. 31,32 The other sharp band at 1520 cm −1 is fairly assigned to endocyclic ν C=N vibration of quinoxaline ring. The former band shifts to higher frequency and appears at 1640-1650 cm −1 while the second band remains intact at 1590 cm −1 in the spectra of complexes. It clearly predicts that azomethine nitrogen is involved in the coordination. The endocyclic nitrogen does not participate in coordination to the metal ions. 33−35 The abnormal increase in ν C=N after coordination to the metal ions may be attributed to the extensive delocalisation of π-electrons in fully conjugated Schiff's base ligand. Thus, the IR spectra of complexes on comparison with that of free ligand reveals that the ligand coordinates through two azomethine nitrogen and two deprotonated phenolic OH groups. The coordination through nitrogen and oxygen is further confirmed by the appearance of two new bands, i.e., at 470−460 cm −1 and at 430−420 cm −1 which may be assigned to ν M-O and ν M-N , respectively. 36 The appearance of two new bands at 3390 cm −1 and 900 cm −1 in spectra of complexes no. 1 and 4 as in the table may be due to stretching and rocking mode of vibration of coordinated water. 37 The appearance of new bands at 760 cm −1 in IR spectra of complexes no. 2 and 5, and at 765 cm −1 in complexes no. 3 and 6 show the presence of pyridine and α-picoline within the coordination sphere of these complexes. 38

Magnetic and Electronic Spectra
The magnetic moments of complexes have been given in Table 1. The Co(II) complexes exhibit magnetic moment of 4.90-4.91 BM. These values are higher than the values for three unpaired electrons in high spin octahedral complexes, which may be due to orbital contribution from triply degenerate ground state 4 T 1g .
These complexes display three bands, in their electronic spectra, which may be assigned as below: The various crystal field parameters derived from electronic spectral bands of Co(II) complexes using Tanabe Sugano diagram are given in Table 2. The values of the various crystal field parameters are in good agreements with values reported for slightly distorted octahedral complexes of Co(II). 39−41 The magnetic moment of Ni(II) complexes are found in between 3.0-3.2 BM. The value is greater than μ s (2.83 BM) corresponding to two unpaired electrons in octahedral Ni(II) complexes. This may be due to second order Zeeman effect, under which µ effective is given by μ eff = μ so 10 4 1 Dq m d n as λ for Ni(II), a d 8 -system is negative (−ve) and hence μ eff becomes greater than μ s value. 42 The electronic spectra of Ni(II) complexes display four bands which show strong tetragonal distortion in complexes due to which 3 T 2g and 3 T 1g of cubic field terms of 3 F ground state term further undergo splitting and hence the assignment of bands may be given to the following spin allowed transitions.
On the basis of these bands, the values of various crystal field parameters like Dq (x,y) , Dq (z) , D s and D t , the extent of distortion from octahedral symmetry were derived and values have been given in Table 3 below:

ConCLuSion
The present study reveals that the Schiff's base NBHCN contains four potential sites for coordination to metal ions. Thus, it behaves as bianionic tetradentate coordinating through two azomethine nitrogen and two deprotonated phenolic oxygen. On the basis of magnetic moment values and electronic spectra of Co(II) and Ni(II), they are found to be octahedral with a little tetragonal distortion. The four coordinated sites of the ligand lie in XY plane while water, pyridine and α-picoline are tagged along Z-axis causing distortion in octahedral symmetry. On the basis of Dt. values in Ni(II) complexes, the distortion power of Z-axis ligands may be given in the following order: H 2 O > pyridine > α-picoline. The tentative structure of complexes may be given as the following:

ACKnoWLEdGEMEnTS
One of the authors, Mr. Sahdeo Kumar is thankful to University Grant Commission, New Delhi, India for awarding the Rajiv Gandhi National Fellowship (RGNF-2014-15-SC-BIH-68680).