Physico – Chemical Study of Transition Metal Complexes with Schiff’s Base derived from Naphthaldehyde and Substituted Aromatic Amines

Introduction

These  days considerable attention is being paid to the chemistry of complex compounds of Schiff’s base containing nitrogen and other donor atoms.1- 4 Schiff’s base offer a versatile and flexible series of ligands capable to bind with varity of metal ions to give complexes with varying properties. These complexes are biologically active⁵and have wide potential applications in many fields such as catalysis⁶, electrochemistry⁷ and medicines and have shown that metal complexes act as antitumour, antiviral, anti cancer10 and other many anti bacterial agents.

A large number of polydentate Schiff’s base compounds have been  synthesized and their complexes have been structurally characterized and extensively investigated. But little is known for their  stability in aqueous solution in which it is used. Hence, the title project have been under taken. Here in the stability constant of complexes of divalent transition metals i.e. Co(II), Ni(II), Cu(II) and Zn(II) with Schiff’s base ligands have been determined.

Experimental

Nitrate salts of divalent Co, Ni, Cu and Zn all were E. Merck. All other chemicals used were AnalR grade and used without further purification. Elemental analysis of metal salts were done by volumetric and gravimetric methods. Double distilled and deionised water was used throughout the experiment. All titrations were done in aqueous-dioxane medium in the ratio 3:2 (v/v). Dioxane was purified by standard method.

Schiff’s base ligands were synthesized by the condensation of  Naphthaldehyde with 7-Hydroxy naphthalene– 2 – amine. 3.5g of aldehyde in solution was mixed with nearly 3.0 g amine. The mixture was boiled under reflux in the presence of glacial acetic acid for about 2 hours. The solution was concentrated and cooled to 0⁰C. The product obtained was filtered, washed several times and re-crystallized from ethanol. The yield of product was nearly 2.6 g.

Scheme 1 Click here to View Scheme

 

Calvin – Bjerrum pH metric titration of acid, acid + ligand and acid + ligand + metal ions solutions were done at constant ionic strength of 0.1 M KNO₃ at  298 K temperature in an inert atmosphere of nitrogen.The same process of titration were repeated for all the four Co, Ni, Cu and Zn metal ions. The change in colour and appearance of turbidity at particular pH value were recorded simultaneously. The change in pH of the solutions with each addition of alkali was recorded in Table no. 2.

Results

A graph was plotted between pH meter reading [B] and volume of alkali added in each case, (Figure  – 1. )  Three titration curves obtained for each metal ions are acid titration curve (a), ligand titration curve (b) and complex titration curve (c) respectively.

Table 1: Concentrations Used In The Experiment

Metal / Ions	Vo (mL)	Y	NO	EO	T	T
Co (II)	100	1	1.0 (M)	1.0 x 10-2 (M)	2.4 x 10-3 (M)	5.0 x 10-4 (M)
Ni(II)	100	1	1.0 (M)	1.0 x 10-2 (M)	2.4 x 10-3 (M)	5.0 x 10-4 (M)
Cu(II)	100	1	1.0 (M)	1.0 x 10-2 (M)	2.4 x 10-3 (M)	5.0 x 10-4 (M)
Zn(II)	100	1	1.0 (M)	1.0 x 10-2 (M)	2.4 x 10-3 (M)	5.0 x 10-4 (M)

 

The values of volumes (V₁, V₂, & V₃) corresponding to the same pH values were read from acid, ligand and complex titration curves (a), (b) and (c) respectively obtained from the experiment at temperature 298 K   given in Figure – 1.

Table 2: Volume of alkali consumed in different titrations

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Ligand – HNNCI (L₁)                                 Temp. 298±1K

μ⁰= 0.10 (M) KNO₃                                   Water : dioxane medium (v/v) = 3:2

			pH – meter reading (B)
Vol. of alkali added in mL	H+	H++ L	H+ +L + Co(II)	H+ +L + Ni(II)	H+ + L + Cu(II)	H+  + L + Zn(II)
0.0	5.05	5.35	5.32	5.3	5.3	5.3
0.1	5.15	5.43	5.4	5.42	5.42	5.44
0.2	5.33	5.57	5.52	5.55	5.5	5.52
0.3	5.53	5.87	5.8	5.86	5.82	5.82
0.4	5.95	5.91	6.32	6.32	6.32	6.42
0.5	6.13	6.73	6.5	6.52	6.52	6.5
0.6	6.45	6.9	6.72	6.72	6.7	6.74
0.7	7.9	7.15	7.1	7.12	7.1	7.12
0.8	8.9	7.75	7.72	7.79	7.82	7.74
0.9	10.15	10.27	8.55	8.98	8.5	8.62
1.0	11.9	11.1	10.5	10.75	9.22	9.12
1.1	12.85	12.43	11.25	11.75	9.62	10.34
1.2	13.15	12.57	11.70	12.10	10.20	10.56

 

Figure 1: Experimental curve with ligand  HNNCI Click here to View Figure

 

Proton Ligand Stability Constant

The ligand titration curve is above the acid titration curve showing the basic nature of ligand and it is well separated from the acid titration curve at pH=6.3 at temp 298 K. The ligand curves run parallel to the acid titration curve indicating the smooth dissociation of the ligand.

The values of n̄_A at various pH reading [B] was calculated from the acid and ligand titration curves and recorded in table 3.

The formation curve obtained from the plot of n̄_A vs [B] extends from 0.43 to 0.88 (Figure 2) at temp 298 K. The dissociation of ligand may be represented as

HL ⇌ H⁺ + L^–

The value of proton ligand stability constant was calculated by half integral method and it was further corroborated by linear plot method. (log n̄_A / (1 – n̄_A) vs [B] Figure. – 3.

Table 3: The values of n̄_A at various pH reading [B]

Ligand – HNNCI                                                                  Temp.:  298

μ⁰= 0.10(M) KNO₃                                                               Water : dioxane = 3:2(v/v)

[B]	V2 – V1	n̄A	log n̄A /1-n̄A)
5.2	0.005	0.8878
5.4	0.007	0.8840
5.6	0.007	0.8832
5.8	0.008	0.8812
6.0	0.008	0.8814
6.2	0.009	0.8742
6.4	0.009	0.8724
6.6	0.010	0.8682
6.8	0.014	0.8610
7.0	0.014	0.8442
7.2	0.014	0.8362
7.4	0.015	0.8360	1.2642
7.6	0.017	0.8282	1.2090
7.8	0.017	0.8196	1.0402
8.0	0.018	0.8121	1.0252
8.2	0.022	0.7986	0.8530
8.4	0.024	0.7964	0.7942
8.6	0.032	0.7882	0.7020
8.8	0.034	0.7834	0.6320
9.0	0.040	0.7602	0.4582
9.2	0.052	0.7120	0.3904
9.4	0.054	0.6794	0.3490
9.6	0.060	0.6555	0.2904
9.8	0.062	0.6274	0.2272
10.0	0.070	0.5996	0.4672
10.2	0.084	0.5674	0.4032
10.4	0.092	0.5322	0.3344
10.6	0.102	0.4883	0.2530
10.8	0.110	0.4782	0.0492
11.0	0.122	0.4672	0.0310
11.2	0.142	0.4394	-0.2050

 

Figure 2: Formation curve of ligand – HNNCI Plot of n̄A Vs [B] Click here to View Figure

 

Figure 3: Linear plot of log(log n̄A  / (1 – n̄A ) Vs [B] Click here to View Figure

 

The complex titration curve of the system crossed the ligand mixture curve at pH 5.35 for Co(II), pH 4.88 for Ni(II), at pH = 5.90 for Cu(II) and pH 5.68 for Zn(II) – ligand system indicating the start of complexation.

Metal titration curve run parallel to the ligand titration curve indicating the liberation of extra proton due to hydrolysis of metal ions.

In, Co (II) System

Precipitation was observed at pH 8.9. Hence in the calculation of  only the lower pH region of titration curve were used.

In Ni(II) System

The curve increased regularly up to pH 7.56  indicating constant rate of release of proton .No turbidity appears, which indicates that hydrolysis does not take place.

In Cu(II) System

Similarly in Cu(II)  system the curve increased regularly up to pH 9.356  indicating constant rate of release of proton .No turbidity appears,  indicating that hydrolysis does not take place

In ,Zn(ii) system

In case of Zn(II) system complex titration curve diverges at higher pH which indicates incomplete dissociation of ligand. Therefore for the calculation of n̄ only symmetrical region of the curve was used.

The value of n̄ calculated for these metals are

Co(II)              –           0.22 to 1.86

Ni(II)               –           0.40 to 1.96

Cu(II)              –           0.081 to 1.82

Zn(II)              –           0.12 to 1.72

As n̄ value did not go beyond 2 for any of the metal indicating the formation of ML and ML₂ type of complexes.

From the formation curve of n̄  vs P^L (Figure no.- 4a,4b,4c and 4d) the values of log K₁ and log K₂ were calculated in each case. It was further corroborated by mid point calculation method and linear plot of log n̄/ 1- n̄ vs P^L (Figure no.- 5a, 5b, 5c and 5d) and also by plot of log 2- n̄/n̄-1 vs  P^L (Graph no.-6a, 6b, 6c and 6d) at temperature 298 K.

Table 4: Values of  and P^L at various [B] values

Co (II) + HNNCI                                                                   Temp: 298  1 K

μ⁰ = 0.10 (M) KNO₃                                                           Water: Dioxane  = 3:2(v/v)

[B]	V3 – V2	n̄	PL
5.0	0.004	0.2222	8.1650
5.2	0.006	0.3244	7.9762
5.4	0.010	0.4265	7.7862
5.6	0.018	0.5914	7.6014
5.8	0.032	0.5764	7.4124
6.0	0.042	0.7870	7.2412
6.2	0.052	1.8662	7.0634
6.4	0.060	1.2842	6.8858
6.6	0.072	1.58404	6.7092
6.8	0.074	1.7964	6.5364
7.0	0.090	1.8682	6.3742

 

Table 5: Values of P^L at various values of  log n̄/(1-n̄)  and log (2 -n̄/(n̄-1)  

Co (II) + HNNCI                                                                   Temp : 298  1 K

μ⁰ = 0.10 (M) KNO₃                                             Water: Dioxane  = 3:2(v/v)            

log n̄/(1-n̄)	PL	log (2 -n̄/(n̄-1)	PL
-0.9560	8.9754	0.0262	7.7092
-0.4134	8.7850	-0.3622	7.5364
0.4212	8.4132
0.7904	8.2402

 

Table 6: Values of n̄ and P^L at various [B] values

Ni (II) + HNNCI                                                                    Temp : 298  1K

μ⁰= 0.10 (M) KNO₃                                                     Water: Dioxane  = 3:2(v/v)

[B]	V3 – V2	n̄	PL
5.0	0.004	0.0406	8.1590
5.2	0.006	0.1012	7.9644
5.4	0.008	0.2454	7.7764
5.6	0.014	0.4082	7.5930
5.8	0.022	0.6150	7.4124
6.0	0.034	0.8036	7.2322
6.2	0.042	0.9932	7.0520
6.4	0.052	1.2306	6.8786
6.6	0.062	1.4634	6.7068
6.8	0.086	1.6972	6.5368
7.0	0.094	1.9684	6.3734

 

Table 7: Values of P^L at various values of  logn̄/(1-n̄)  and log (2 -n̄/(n̄-1)  

Ni (II) + HNNCI                                                            Temp : 298  1K

μ⁰= 0.10 (M) KNO₃                                                     Water: Dioxane  = 3:2(v/v)

logn̄ /(1-n̄ )	PL	log (2-n̄) / (n̄ – 1)	PL
-0.9454	8.9640	0.6226	7.8782
-0.4882	8.7770	0.5626	7.7064
-0.1606	8.5924	0.4622	7.5362
0.2040	8.4124	0.3216	7.4832
0.6114	8.2322	0.2182	7.3218

 

Table 8: Values of n̄ and P^L at various [B] values

Cu (II) + HNNCI                                                       Temperature 298 K

μ⁰= 0.10 (M) KNO₃                                                  Water: Dioxane  = 3:2(v/v)

[B]	V3 – V2	n̄	PL
6.2	0.006	0.0814	8.5624
6.4	0.010	0.1640	8.3702
6.6	0.012	0.2672	8.1794
6.8	0.014	0.3722	7.9890
7.0	0.020	0.4796	7.7994
7.2	0.024	0.6150	7.6124
7.4	0.030	0.7632	7.4270
7.6	0.042	0.9412	7.2462
7.8	0.052	1.1434	7.0684
8.0	0.060	1.3270	6.8912
8.2	0.064	1.5364	6.7152
8.4	0.080	1.8240	6.5526

 

Table 9: Values of P^L at various values of  log n̄/(1-n̄)  and log (2 -n̄/(n̄-1)  

Cu (II) + HNNCI                                                                Temperature 298 K

μ⁰= 0.10 (M) KNO₃                                                          Water: Dioxane  = 3:2(v/v)

logn̄ /(1-n̄ )	PL	log (2-n̄) / ( n̄- 1)	PL
-0.7056	8.3704	0.7752	7.0684
-0.4360	8.1796	0.3114	6.8902
-0.2262	7.9892	-0.0642	6.7152
-0.0353	7.7994	-0.6710	6.5364
0.2044	7.6130	-0.7128	6.3281
0.5090	7.4284	-0.8214	5.9216

 

Table 10: Values of n̄ and P^L at various [B] values

Zn (II) + HNNCI                                                                 Temp:  298  1K

μ⁰= 0.10 (M) KNO₃                                                          Water: Dioxane  = 3:2(v/v)

B	V3 – V2	n̄	PL
6.0	0.006	0.1226	7.3646
6.2	0.018	0.1852	7.1726
6.4	0.010	0.2914	6.9824
6.6	0.012	0.3750	6.7882
6.8	0.020	0.5092	6.6032
7.0	0.024	0.6366	6.4162
7.2	0.030	0.8344	6.2350
7.4	0.042	1.0352	6.0564
7.6	0.052	1.2612	5.8832
7.8	0.076	1.6668	5.7292
8.0	0.084	1.7215	5.5264

 

Table 11: Values of P^L at various values of  log n̄/(1-n̄)  and log (2 -n̄/(n̄-1) 

Zn (II) + HNNCI                                                              Temp:  298  1K

μ⁰= 0.10 (M) KNO₃                                                        Water: Dioxane  = 3:2(v/v)

log n̄ /(1-n̄ )	PL	log (2-n̄) / (n̄ – 1)	PL
-0.6424	8.1722	0.4526	6.8822
-0.3860	7.9816	-0.2626	6.7296
-0.2212	7.6896	-0.2012	6.5812
0.2430	7.3152	-0.1628	6.3158
0.7024	7.0125	-0.0831	6.2817

 

Figure 4a  Click here to View Figure

Figure 4b Click here to View Figure

Figure 4c Click here to View Figure

Figure 4d Click here to View Figure

Figure 5a Click here to View Figure

Figure 5b Click here to View Figure

 

Figure 5c   Click here to View Figure

Figure 5d                                                                                              Click here to View Figure

Figure 6aClick here to View Figure

Figure 6b                                                                                                              Click here to View Figure

Figure 6c                                                                                               Click here to View Figure

Figure 6d Click here to View Figure

 

The values of protonation constant and stepwise stability constant obtained by different computational methods   at temperatures 298 K are summarized in table no. 12

The different methods used are :-

a)     Half – integral method

b)     Mid – point calculation method

c)     Straight line plot method.

Table 12: Values of protonation constant of ligand and stepwise stability constant of complexes of Co(II), Ni(II), Cu(II) and Zn(II) with ligand  HNNCI

SystemMetal ions	Methods	Ligand HNNCI
		log K1                               log K2
HNNCI(L1)	Abc	10.96 – 10.96
Co (II)	Abc	7.56 7.58 7.62	6.64 6.66 6.68
Ni (II)	Abc	7.52 7.44 7.46	6.66 6.62 6.68
Cu (II)	Abc	6.76 6.64 6.84	5.76 5.04 5.78
Zn (II)	Abc	6.62 6.54 6.68	5.76 5.72 5.86

Table 13: Stepwise and over all stability constant of complex compounds of various metals with ligand HNNCI at temperature 298K

Water – Dioxane medium (v/v) = 3:2                   μ⁰ = 0.10(M) KNO₃

System	Ligand- MNNCI (L1)
	log K1	log K2	log
HNNCI (L1)	10.98	–	10.98
Co (II)	6.76	5.70	12.46
Ni (II)	7.48	6.68	14.10
Cu ( II)	7.58	6.66	14.20
Zn (II)	6.60	5.78	12.34

The order of stability constant of various metals for the given ligand

HNNCI are     –    Cu(II) > Ni(II) > Co (II) > Zn(II)

The values of stepwise stability constants and over all stability constants are given in table no. 13

For the given ligand the stability constants of metals show the sequence

Cu(II) > Ni(II) > Co(II) > Zn(II)

This is natural order given by Irving – William. A theoretical justification of the order of stability constants follows from the consideration of the reciprocal of the ionic radii and 2^nd ionization enthalpy of metal. Calvin – Bjerrum  titration technique modified by Irving and Rossotti was used to determine the practical proton ligand and metal ligand stability constants at constant ionic strength maintained by using dilute KNO₃ solution. Irving and Rossotti pointed out that the formation constant of metal chelates can be obtained without converting the pH – meter reading [B] to stoichiometric hydrogen ion concentration and without knowing the stiochiometric concentration of neutral salts added to maintain ionic strength. This method is valid for both aqueous and non-aqueous medium.

The nitrate (NO⁻₃) ion has very slight complexing tendency. Therefore competition between nitrate ion and the ligand under study is of no importance.

The stability of the chelates is greatly affected by the electron density around the imino nitrogen ( – C = N – ). Higher the electron density around the nitrogen atom, stronger is the metal ligand bond.

The difference between the successive stepwise stability constant is large, which suggest that the formation of ML and ML₂ chelates take place.  The results obtained are in conformity of our previous studies^12-15 and other workers^16-17.

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