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Using tridentate ligands, Ni (II) complexes of the composition NiL1X2 and NiL2X2 (where L1 = p-hydroxybenzylidene-2-aminothiazole, L2 = p-hydroxybenzylidene-2-amino-6-methylpyridine and X = SO4-, CH3COO- and NO3-) have been prepared. Ligands were characterized with the help of elemental analyses, IR, and mass spectral studies while complexes were characterized on the basis of elemental analyses, magnetic susceptibility measurements, IR and NSH treatment. The studies showed that the complexes are of high spin type having magnetic moment corresponding to three unpaired electrons and indicating distorted octahedral environment around the central metal ion.

Synthetic and Structural Investigations of Ni(II) Complexes of Tridentate Ligands

Nickel is the twenty-second most abundant element by weight in the earth’s crust. It is silvery-white, hard, malleable and ductile metal that takes on a high polish. It occurs usually in combination with sulphur and iron in pentlandite, with sulphur in millerite, with arsenic in the mineral nickeline and with arsenic and sulphur in nickel glance[7]. Alex Constedt of Sweden first discovered nickel in 1751. It has melting point of 1453oC, boiling point of 2913oC and density of 8.902 gcm-3. Nickel (II) ion has a 3d8 valence electron configuration. This give rise to the Russel-Saunders terms (in order of increasing energy) 3F, 1D, 3P, 1G, 1S. The triplet term 3F4 represents the electronic ground state of the gaseous ion and triplet terms derived from this usually represent the ground state of nickel (II) compounds. The bonding in transition metal complexes2 is considered to be electrostatic (ion-ion or ion-dipole), and if a point charge (or a point charge-point dipole) model is used, relative energies of the d-orbitals of the central metal ion can be calculated. This lead to the splitting of d-orbitals for an octahedral complex with the dx2-y2 and dz2 (eg) more unstable than the dxy, dyz and dzx (t2g) orbitals. Sabastigam and coworkers[9] have reported the electronic spectra of nickel (II) complexes of Mannich bases derived from piperidinomethyl urea, exhibiting three bands at ~ 10,000 cm-1 due to 3A2g → 3T2g (ν1), ~ 16,000 cm-1 due to 3A2g → 3T1g (F) (ν2), and ~ 27,000 cm-1 due to 3A2g → 3T2g (P) (ν3) indicating the octahedral nature of nickel (II) complexes. Some nickel (II) complexes of Mannich base N,N-bis (morpholinobenzyl)urea (L) were synthesized and characterized by Prabhu and coworkers[10]. Halli et. al.[11] prepared some octahedral complexes of nickel (II) ion. The symmetry of the complexes was confirmed on the basis of magnetic measurements, IR and electronic spectral studies. Nickel (II) complexes of bifunctional bis(piclolyl)amino ligand derived from glycine were reported by Niklas and coworkers[12]. Sharma[13] have synthesized nickel (II) and cobalt (II) complexes with some new fused heterocycles. On the basis of magnetic moment and electronic spectral data, they concluded that these complexes possess high spin octahedral geometry. Mane and Shirodhkar[14] synthesized nickel (II) complexes with bidentate ligands. They calculated various ligand field parameters and on the basis of these parameters they reported the octahedral geometry for nickel (II) complexes. Mohanan and Murukan[15] synthesized the complexes of manganese (II), iron (II), cobalt (II), nickel (II), copper (II) and zinc (II) with bishydrazone and characterized them with the help of elemental analysis, molar conductance, magnetic susceptibility, UV-visible, IR and NMR spectral studies. They have reported tetrahedral geometry for nickel (II) complexes. Recently, a number of research papers[16-17] have reported on nickel (II) complexes with ON/ONS/ONNO/NS type donor system. The electronic spectral data were suggestive of distorted octahedral geometry of these complexes. In this chapter, the structure and stereochemistry of nickel (II) complexes with Mannich base derived from p-hydroxybenzaldehyde and 2-aminothiazole/2-amino-6-methylpyridine have been discussed. Various ligand and crystal field parameters have been calculated with the help of electronic spectra and Normalized Spherical Harmonic (NSH) Hamiltonian theory[17] have been used to assign the structure of the synthesized complexes.

Proposed structures of MB₁ and MB₂

Experimental

All the chemicals used were of A.R. grade. The purity of these compounds was checked by thin layer chromatography (TLC). The spots were developed exposing the slides in iodine vapor chamber.

Synthesis of Ligands

(a)   Preparation of Schiff Bases

(i)             Preparation of p-hydroxybenzylidene-2-iminothiazole (SB₁)

1.22gm 4-Hydroxybenzaldehyde and 1.0gm 2-aminothiazole were dissolved in 20ml ethanol and refluxed for 14 hours over water bath using water condenser. The obtained solution was allowed to cool at room temperature. The concentrated solution was cooled in refrigerator and obtained product was filtered, washed with ether and dried under reduced pressure over anhydrous calcium chloride.

(ii)            Preparation of p-hydroxybenzylidene-2-imino-6-methylpyridine (SB₂):

1.22gm 4-Hydroxybenzaldehyde in ethanol was mixed with an ethanolic solution of 1ml of 2-amino-6-methylpyridine (dissolved in 20ml ethanol). Mixture was refluxed for 12 hours over a water bath using water condenser. The obtained solution was allowed to cool at room temperature and the concentrated solution was cooled in refrigerator for 24 hours. The obtained product was filtered, washed with acetone several times and followed by ether. It was recrystallized with absolute alcohol and dried under reduced pressure over anhydrous calcium chloride.

(b)   Preparation of Mannich Bases

(i)   Preparation of p-hydroxybenzylidene-2-aminothiazole (MB₁) derived from p-hydroxybenzylidene-2-iminothiazole (SB₁):

The titled Mannich Base was prepared by stirring p-hydroxybenzylidine-2-iminothiazole (2.04 gm) with 20ml of methanol. The product is then cooled to 0^oC and sodiumborohydride (0.2gm) was added over a period of 1 hour in three or four instalments. Slowly the temperature was raised to room temperature. A dark brown solution resulted and then solvent was slowly evaporated. A solid colored powder was obtained. It was then washed with ethanol and dried in air, a deep brown colored crystals were obtained.

The mass spectra of the ligand exhibits m/z values: 205, 189, 107, 122, 113 and 99 assignable to C₁₀H₁₀N₂OS, C₁₀H₁₀N₂S, C₇H₇O, C₇H₈NO, C₄H₅N₂S and C₃H₃N₂S molecular ion.

(ii)   Preparation of p-hydroxybenzylidene-2-amino-6-methylpyridine (MB₂) derived from p-hydroxybenzylidene-2-imino-6-methylpyridine (SB₂):

The titled Mannich Base was prepared by stirring p-hydroxybenzylidine-2-amino-6-methylpyridine (2.12gm) with 20ml of methanol. The product was then cooled to 0^oC and then sodiumborohydride (0.2gm) was added with it in three or four installments with continuous stirring over a period of 1 hour. Slowly the temperature was raised to room temperature. A light brown colored solution resulted and then solvent was slowly evaporated. A solid chocolate colored powder is obtained. It was then washed with alcohol and dried in air.

The mass spectra of the ligand exhibits m/z values: 214, 198, 176, 121, 107 and 93 assignable to C₁₃H₁₄N₂O, C₁₃H₁₄N₂, C₁₂H₁₂N₂O, C₇H₉N_2, C₆H₇N_2,  and C₆H₇N molecular ion.

(c)  Preparatiom of Metal Complexes

                 A general method was used for the preparation of trivalent nickel (II) complexes. The nickel acetate/ sulphate/ nitrate were used in 1:2::metal: ligand ratio with MB₁ and MB₂. Ethanol and water were used as solvent. The resulting mixture was stirred for 15-20 minutes and refluxed for 3-4 hours on a water bath. The precipitated complex was filtered and washed with ethanol, ether and dried in air. (Table: 1). 

The newly synthesized nickel (II) complexes of MB₁ and MB₂ were of different shades of brown and green color and soluble in DMF/DMSO but sparingly soluble in ethanol, acetone and water. The analytical estimations suggest that these complexes containing two molecules of Mannich base are coordinated to each nickel atom in 1:2 metal: ligand stoichiometry.( Table: 1).

                                                                       Table-1

Physical Properties, Analytical Data and Magnetic Moment Values of Metal Complexes of MB₁ and MB₂     

S. No.	Ligand/Complexes	M.P. ( oC )	Formula Weight	Color	Percentage of Elements (Found/Calculated)					μeff. (B.M.)
					C	H	N	S	M
1.	C10H10N2OS	150	206	Dark brown	57.92/ 58.25	4.80/ 4.85	13.20/ 13.59	15.50/ 15.53	-	-
1.1	Ni(C10H10N2OS)2 .(OAc)2	275	588.6	Light Brown	48.20/ 48.92	4.30/ 4.41	9.10/ 9.51	10.80/ 10.87	9.35/ 9.95	2.80
1.2	Ni(C10H10N2OS)2 .(NO3)2	270	594.6	Brown	40.30/ 40.36	3.06/ 3.36	14.10/ 14.12	10.56/ 10.76	9.80/ 9.85	2.96
1.3	Ni(C10H10N2OS)2 .SO4	260	566.6	Light Brown	42.30/ 42.35	3.40/ 3.52	9.80/ 9.88	16.80/ 16.94	10.30/ 10.34	3.12
2.	C13H14N2O	190	214	Light brown	72.80/ 72.98	6.50/6.54	13.56/ 13.08	-	-	-
2.1	Ni(C13H14N2O)2 .(OAc)2	270	604.6	Light Brown	58.86/ 59.54	5.60/5.62	9.15/ 9.26	-	8.95/ 9.69	3.21
2.2	Ni(C13H14N2O)2 .(NO3)2	260	610.6	Brown	51.25/ 51.09	4.10/4.58	13.50/ 13.76	-	9.10/ 9.59	3.45
2.3	Ni(C13H14N2O)2 .SO4	265	582.6	Light Brown	53.20/ 53.55	4.75/4.81	9.50/ 9.61	-	9.97/ 10.05	3.57

Magnetic Measurements

The magnetic moment values for all the present complexes were measured at room temperature and lie between 2.80-3.20 B.M. The values of magnetic moments of these complexes were shown in Table:- 1. The observed magnetic moment values have good relation with the complexes having ²A_2g or  ³B_1g ground term and these values also depend on the magnitude of the orbital contribution expected for similar hexa-coordinated nickel (II) ions. The observed values were slightly higher than the spin only value (2.83 B.M.), probably due to slight distortion from pure octahedral to tetragonal symmetry.

Infra-Red Spectral Studies

In the infra-red spectra of SB1 and SB2 show the bands at 1605-1610 cm-1 due to azomethine which were disappeared in the spectrum of MB1 and MB2 are new bands were observed in the region 3358-3365 cm-1 due to the secondary amino group . In Nickel (II) complexes , these bands were shifted to higher frequency region by 9-30 cm-1 indicating the involvement of nitrogen of secondary amino group in complexation with metal ions. This is confirmed by the appearance of band at 472-484 cm-1 due to v (M-N) vibrations. Table - 2(a) and 2(b).

Table- 2(A)

Important Infra-Red FrequencieS (Cm^-1) of MB1₁ and  Its Metal Complexes

S. No.	Ligand/Complexes	Ligand Modes						Coordination Modes
		υ(OH)	υ(CH2-NH)	υ(C-O) Phenolic	υ(C=N) cyclic	υ(C-N ) cyclic	υ(C-S-C) thiazole	υ(M-O)	υ(M-N)	υ(M-S)
1 1.1 1.2 1.3	C10H10N2OS Ni(C10H10N2OS)2 (OAc)2 Ni(C10H10N2OS)2 (NO3)2 Ni(C10H10N2OS)2 SO4	3452 - - -	3358 3388 3385 3386	1230 1244 1248 1242	1513 1516 1518 1516	1350 1350 1351 1354	830 848 836 848	- 516 547 545	- 472 478 481	- 342 365 345

 Table – 2 (b)

Important Infra-Red Frequencies (cm^-1) of MB₂  and Its Metal Complexes

S.No.	Ligand/Complexes	Ligand Modes				Coordination Modes
		υ(OH)	υ(CH2=NH)	υ(C-O) Phenolic	υ(C-N-C) pyridine	υ(M-O)	υ(M-N)	υ(M-N) pyridine
1 1.1 1.2 1.3	C13H14N2O Ni(C13H14N2O)2 (OAc)2 Ni(C13H14N2O)2 (NO3)2 Ni(C13H14N2O)2 SO4	3520 - - -	3365 3384 3389 3380	1480 1465 1467 1468	1498 1512 1512 1519	- 542 548 545	- 484 480 482	- 518 510 509

Electronic Spectral Studies 

                In the present work, the observed electronic spectra of newly synthesized divalent nickel complexes were recorded in DMF at room temperatures. The observed electronic transitions  and the ligand field spectral data of nickel (II) complexes is presented in Table:- 3. The electronic spectra of all the complexes show similar features.

                Here, we are reporting the calculated values of the tetragonal parameters for all nickel (II) complexes. The electronic spectra indicate that the three transitions can be assigned i.e. ³A_2g   (³T_, P), ³E_g  (³T, P) and ³A_2g  (³T, F). This indicates that the high energy bands arise from the P-state and assignment of ³A_2g (P), ³E_g(P) is predictable in the view of both Mannich bases (ligands).

                Taking Ni (C₁₀H₁₀N₂OS)₂ (CH₃COO)₂ as an example, the pronounced tetragonal splitting for the excited level ³T_2g (F) and ³T_1g (F) has been observed. For ³T_2g splitting, these bands were observed in the region 9107 and 10917 cm^-1 and for ³T_1g (F) splitting, these bands were observed in the region 14513 and 16286 cm^-1 respectively.

                A number of theoretical methods have been developed for calculating the transition energy and various crystal or ligand field parameters but in the present work, all newly synthesized nickel (II) complexes of MB₁ and MB₂ were treated with Normalised Spherical Harmonic (NSH) Hamiltonian theory.

Table - 3

Ligand Field Spectral Data (9000 – 30,000 Cm^-1) of Nickel (Ii) Complexes of Mb₁ And Mb₂

S. No	Complex	3B1g   3B2g(νB)	3B1g   3Eg(νE)	3B1g   Eg (Sh)	3A2g (F)   3T1g(F) ν2	3A2g (F)   3T2g(P) ν3
1	Ni(C10H10N2OS)2.(OAc)2	9107	10917	14513	16286	26178
2	Ni(C10H10N2OS)2.(NO3)2	9166	10989	14599	16286	25974
3	Ni(C10H10N2OS)2 . SO4	9268	11050	14306	16026	26883
4	Ni(C13H14N2O)2.(OAc)2	9191	10965	14535	16340	25706
5	Ni(C13H14N2O)2 .(NO3)2	9140	10893	14641	16447	26455
6	Ni(C13H14N2O)2.. SO4	9132	10929	14556	16393	26246

Normalised Spherical Harmonic (NSH) Hamiltonian Theory Applied on Nickel (II) Complexes of MB₁ and MB₂  

           NSH Hamiltonian is basically used as a means of analyzing the electronic spectra of both cubic and non-cubic metal complexes. It explains the magnitudes of each term in the perturbation. Hamiltonian is entirely independent of the magnitude of any other term. Each term is projected octahedral vector component and therefore, is normalized for finite 3-dimensional geometries: to distinguish upper case nomenclature (eg. DQ, DS, DT etc.) will be employed instead of the lower case parameters (eg. Dq, Ds, Dt etc.). Using the NSH Hamiltonian and the projected wave function, matrix elements for any dⁿ or fⁿ configuration in any geometry, can be simply constructed for a D_4h molecule.

                In the present nickel (II) complexes, the absolute ligand field parameters DQ, DS, DT, DQ^L, DQ^Z etc. have been calculated with the help of NSH Hamiltonian theory.

                The ratio of DT/DQ which gives the amount of distortion have been calculated and indicated that the structure of the present complexes lie somewhere between square planar and octahedral structure.

 The calculated and experimental results were in good agreement with the previously reported results which provide a strong support for tetragonal structure of nickel (II) complexes which is a sort of distorted octahedron in which two groups along the z-axis are at a larger distance from the central nickel (II) ion than the four groups along x and y axis. The results are listed in Table :- 4(a) and 4(b).

Table – 4(a)

Calculated Electronic Spectral Parameters (cm^-1) of Nickel (II) Complexes of MB₁

 

S. No.	Parameters	Complexes
		Ni(C10H10N2OS)2.(OAc)2	Ni(C10H10N2OS)2.(NO3)2	Ni(C10H10N2OS)2.SO4
1	Dt	208.65	208.34	203.65
2	Ds	338.59	324.57	329.09
3	DqXY	910.7	916.6	926.8
4	DqZ	1272.7	1281.2	1283.2
5	DQ	25040.1	25202.3	25482.7
6	DS	- 2370.13	- 2271.99	- 2303.63
7	DT	- 2803.94	- 2824.14	- 2760.56
8	DqE	1031.36	1038.13	1045.59
9	DQL	21722.43	21860.73	22216.36
10	DQZ	31675.43	31885.43	32015.37
11	DQA	18404.76	18519.16	18950.02
12	DQE	28357.76	28543.86	28749.03
13	- DT/DQ	0.111	0.112	0.108
14	dσ	895.72	877.49	875.47
15	dл	9.23	33.99	15.48
16	∆1	- 18.47	- 67.98	- 30.97
17	∆2	10313.61	10381.30	10614.98
18	∆3	2388.60	2339.97	2334.60
19	Dt/Ds	0.610	0.641	0.618

Table – 4(b)

Calculated Electronic Spectral Parameters (cm^-1) of NICKEL (II) COMPLEXES OF MB₂ 

S. No.	Parameters	Complexes
		Ni(C13H14N2O)2.(OAc)2	Ni(C13H14N2O)2.(NO3)2	Ni(C13H14N2O)2.SO4
1	Dt	202.74	200.34	205.37
2	Ds	343.07	342.73	348.95
3	DqXY	919.1	914.0	913.2
4	DqZ	1273.9	1264.6	1272.6
5	DQ	25271.07	25130.84	25108.84
6	DS	- 2401.49	- 2399.11	- 2442.65
7	DT	- 2748.23	- 2715.69	- 2783.88
8	DqE	1037.36	1030.86	1032.99
9	DQL	22019.32	21917.59	21814.90
10	DQZ	31774.56	31557.33	31696.70
11	DQA	18767.57	18704.34	18520.97
12	DQE	28522.81	28344.08	28402.77
13	- DT/DQ	0.108	0.108	0.110
14	dσ	894.74	889.73	908.49
15	dл	7.75	13.24	10.00
16	∆1	- 15.51	- 26.49	- 20.00
17	∆2	10373.64	10308.64	10329.98
18	∆3	2385.97	2372.61	2422.64
19	Dt/Ds	0.590	0.584	0.588

 

Studies made in present work are focused mainly on structure elucidation of nickel (II) complexes of MB1 and MB2. Different spectral studies viz. infra-red, electronic spectral studies, and NSH treatment proposed the distorted octahedral geometry for the nickel (II) complexes of both the Mannich bases. As Mannich bases and their metal complexes have wide scope as antimicrobial field, hence these metal complexes may be of great academic and commercial interest.

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