Synthesis, Mechanism and Kinetic Studies of Cobalt(II) Schiff base Complexes with Organotin(IV)chlorides

Introduction

Schiff bases form stable complexes with metals that perform an important role in biological systems [1]. Transition metal Schiff base complexes have many applications in the area of antitumoral, antiviral and antibacterial activity [2] and are also used as mimetic systems for enzyme models [3].

The tetraorganotin compounds show no evidence of Lewis acidity, the tin(IV) halides are strong Lewis acids which can produce 1:1 and 1:2 adducts in the presence of Lewis bases. The organotin halides R_nSnX_4-n are intermediate in Lewis acidity and found between R₄Sn and SnX₄. Generally the acidity is increased when the proportion of the halide is increased.

It is worth to note that till now no studies have been done on the kinetics and mechanism of the adduct formation between Co(II) tetraaza Schiff base complexes and the diorganotin(IV)dichlorides, although several studies have been done in our group on the thermodynamics of their adduct formation [4,5].

In this paper we have investigated the kinetics and mechanism of the interaction between the cobalt(II) tetraaza Schiff base complexes as donor and the organotin(IV)chlorides as acceptor. Cobalt(II) tetraaza Schiff base complexes such as [Co(ampen)], [Co(campen)], [Co(amaen)], [Co(cappn)] and [Co(appn)] were synthesized and characterized. The kinetics and mechanism of their adduct formation with diorganotin(IV)dichlorides, such as Me₂SnCl₂, Bu₂SnCl₂, Ph₂SnCl₂ and Ph₃SnCl as acceptors in DMF solvent were studied spectrophotometrically, and explained by an associative (A) mechanism.

Experimental

Reagents

1,2-ethylenediamine, 2-amino-5-chlorobenzophenone, 2-amino-benzophenone, 2-amino- acetophenone, 1,2-propylenediamine, methanol, chloroform, cobalt(ΙΙ)acetatetetrahydrate, dimethyltin(IV)dichloride, dibutyltin(IV)dichloride, diphenyltin(IV)dichloride triphenyltin(IV)chloride and acetonitrile were commercially obtained from Merck , Fluka or Acros and used without further purification.

Instrument

Light-absorption measurements in the region (300-700 nm) were made with a Perkin-Elmer-Uv-Vis spectrophotometer-Lambda 2, equipped with a Lauda-ecoline-RE thermostat. FT-IR spectra were run on a Shimadzu FTIR-8300 spectrophotometer. ¹¹⁹SnNMR spectra were recorded on a Bruker Avance DPX-400 spectrometer in CDCl₃ solvent. Elemental microanalyses (C.H.N.) were obtained using a Thermo Finnigan-CHNSO Analyzer.

Synthesis of Cobalt(II) Tetraaza Schiff Base Complexes

The four-coordinate complexes of N₄ type, [Co(amaen)], [Co(ampen)], [Co(campen)], [Co(appn)], [Co(cappn)], [Co(aptn)] and [Co(captn)] were prepared according to the literature [6] (Fig. 1). The IR and the Uv-Vis absorption spectra were in good agreement with the published data [6].

Fig1: The structure of [Co(amaen)] (1), [Co(ampen)] (2), [Co(campen)] (3), [Co(appn)] (4), [Co(cappn)] (5), [Co(aptn)] (6) and [Co(captn)] (7).  Click here to View figure

 

Synthesis of Adducts of Diorganotin(IV)Dichlorides with Co(ΙΙ) Tetraaza Schiff Base Complexes

0.1 mmol R₂SnCl₂  was dissolved in 5 ml acetonitrile and dropwisely added to 0.1 mmol of the Co(ΙΙ) complexes in 10 ml acetonitrile. The mixture was stirred for 2h at room temperature. During that time the red color solution was changed to green precipitate. Changing in color is due to the interaction of R₂SnCl₂ with Co(ΙΙ) complexes. The precipitate was filtered off and dried under vacuum (Fig. 2).

Fig2: The proposed structure of the adduct fromed in the kinetic reaction of R2SnCl2 with Co(ΙΙ) tetraaza Schiff base complexes (R=Me, Bu, Ph, R1=Me, Ph, R2=H, Cl,R3=Me).  Click here to View figure

 

Kinetic Measurement

A solution from each Co(ΙΙ) complex with certain concentration 6.4×10^-5 M was prepared. In a typical measurement, 2.5 ml of this solution was transferred into the thermostated cell compartment of the Uv-Vis instrument, which was kept at constant temperature (runs from 10 – 40( ± 0.1ºC)) by circulating water. Excess concentration in the range (1×10^-4– 4×10^-2 M) of a given R₂SnCl₂ and (1.3×10^-3– 1.1×10^-1 M) of Ph₃SnCl acceptor was added to this solution by Hamilton µL syringe.

The absorption measurements were carried out at various wavelengths where the different in absorptions were the maximum. The formed adduct shows absorption different from the donor, while the acceptors show no absorption at those wavelengths. The pseudo- first order rate constants k_obs (s^-1) were calculated by fitting the data to ln [(A_t – A_∞)/(A₀ – A_∞)] = k_obst (where A_t = absorbance at time t; A₀ = absorbance at t = 0; A_∞ = absorbance at t = ∞) by means of a linear least-squares computer program. The second-order rate constants k₂ (M^-1s^-1) were obtained from the slope of the linear plots of k_obs vs. [A] (acceptor concentration). The standard deviation values of the rate constants were obtained duplicate readings.

The activation parameters ΔH^#, ΔS^# were obtained from the linear Eyring plots of ln(k₂/T) vs. 1/T at four different temperatures in the range (10-40ºC) using linear least squares computer program. Also the ΔG^# values were computed by the same program using equation ΔG^# =ΔH^# -TΔS^# at T=313K.

Results And Discussion

Spectral Characterization

Electronic Spectra

Absorption spectra of the Co(ΙΙ) complexes and their adducts with diorganotin(IV)dichlorides were examined over the range 300-700 nm, in DMF solvent and the results are summarized in (Table 1). In Co(ΙΙ) tetraaza Schiff base complex spectrum several intense absorption bands are observed in the Uv-Vis regions. By addition of R₂SnCl₂ to a solution of Co(ΙΙ) tetraaza Schiff base complex in DMF the original peaks of Co(ΙΙ) tetraaza Schiff base complex are changed. The electronic spectra of adducts formed at the end of the kinetic runs were the same as the electronic spectra of the respective separately synthesized adducts (Fig.3).

Table 1. The Uv-Vis bands λ_max (nm) of the cobalt(ΙΙ) tetraaza complexes and their adducts with diorganotin(IV)dichlorides in DMF.

Complex	λ (nm)
[Co(amaen)]	371, 428, 540
[Co(amaen)].(Ph2SnCl2).H2O	366 (sh), 426
[Co(amaen)].(Me2SnCl2).H2O	427 (sh)
[Co(amaen)].(Bu2SnCl2).H2O	361 (sh), 423 (sh)
[Co(campen)]	387, 442, 552
[Co(campen)].(Ph2SnCl2).H2O	384, 440, 554
[Co(campen)].(Me2SnCl2).H2O	381, 439, 551
[Co(campen)].(Bu2SnCl2).H2O	380, 439, 551
[Co(ampen)]	377, 436, 542
[Co(ampen)].(Ph2SnCl2).H2O	369 (sh), 431
[Co(ampen)].(Me2SnCl2).H2O	431 (sh)
[Co(ampen)].(Bu2SnCl2).H2O	370 (sh), 431
[Co(appn)]	479, 438, 541
[Co(appn)].(Ph2SnCl2).H2O	470 (sh), 432, 539 (sh)
[Co(appn)].(Me2SnCl2).H2O	373 (sh), 432, 538 (sh)
[Co(appn)].(Bu2SnCl2).H2O	369 (sh), 431, 536 (sh)
[Co(cappn)]	387, 442, 554
[Co(cappn)].(Ph2SnCl2).H2O	384, 440, 553
[Co(cappn)].(Me2SnCl2).H2O	370 (sh), 440, 552 (sh)
[Co(cappn)].(Bu2SnCl2).H2O	384, 439, 551

 

Fig3: The electronic spectra of the four-coordinate Co(amaen) (1), the separately synthesized adduct of Bu2SnCl2 with Co(amaen) (2) and the end of the kinetics of Co(amaen) with Bu2SnCl2 (3).  Click here to View figure

 

¹¹⁹SnNMR Spectra

Holeĉek and coworkers studies show that the di- and tri-organotin ¹¹⁹SnNMR spectra can be used as an indicator of the coordination number of the tin atom. In the ranges of +200 to -60, -90 to -190, -210 to -400, -440 to -540 ppm, the coordination number of the tin atom is four, five, six and seven, respectively [7-10]. The adducts formed in the present investigation exhibit the ¹¹⁹Sn spectra in the range -110 to -182 ppm , suggesting that the tin atom is five-coordinated (Table 2).

Table 2. ¹¹⁹SnNMR characterization of some cobalt(ΙΙ) adducts with diorganotindichlorides

119Sn(δ,ppm)	Complex
-182.42	[Co(amaen)].(Ph2SnCl2).H2O
-176.35	[Co(amaen)].(Me2SnCl2).H2O
-168.33	[Co(amaen)].(Bu2SnCl2).H2O
-110.12	[Co(campen)].(Ph2SnCl2).H2O
-120.39	[Co(campen)].(Me2SnCl2).H2O
-145.65	[Co(campen)].(Bu2SnCl2).H2O
-165.54	[Co(ampen)].(Ph2SnCl2).H2O
-143.25	[Co(ampen)].(Me2SnCl2).H2O
-151.18	[Co(ampen)].(Bu2SnCl2).H2O
-1889.0	[Co(appn)].(Ph2SnCl2).H2O
-166.0	[Co(appn)].(Me2SnCl2).H2O
-179.0	[Co(appn)].(Bu2SnCl2).H2O
-142.0	[Co(cappn)].(Ph2SnCl2).H2O
-120.0	[Co(cappn)].(Me2SnCl2).H2O
-166.0	[Co(cappn)].(Bu2SnCl2).H2O

 

IR Spectra

The stretching vibration of the azomethine group (C=N) is observed in the range (1610-1680 cm^-1). The stronger bands in the range (1440-1600 cm^-1) are due to the skeleton stretching vibration of C=C of the benzene ring. The band at about 400 cm^-1 can be assigned to the (Co-N) vibration. This band is the characteristic band of the complexes and was not found in the free Schiff base ligand spectra. The results show that, by formation of adducts, the C=N vibrations are shifted toward higher frequencies about (13-25 cm^-1). The ring skeletal vibrations (C=C) are weakly affected by complexation. By formation of adducts, the (N-H) vibrations have been shifted toward higher frequencies. In the IR spectra of adducts, vibration bands in the range (3400-3450 cm^-1) as well as at (704-725 cm^-1) is assigned to (O-H) stretching of the coordinated H₂O present [11] in adduct formed (Table 3).

Table3: The IR bands characteristic of Co(ΙΙ) tetraaza complexes and their adducts with diorganotin(IV)dichlorides.  Click here to View table

Elemental Microanalysis&nbsp

Elemental analysis of the synthesized products have good agreement with the proposed adducts of the kinetic runs with the stochiometric composition 1:1 for Co(ΙΙ) tetraaza: R₂SnCl₂  with one coordinated H₂O molecule (Table 4).

Table4: Elemental analytical data of tetraaza Schiff base complexes and their diorganoltin(IV)dichloride adducts.  Click here to View table

 

Kinetic Studies

Tables (5-24) show the rate constants and the activation parameters for the kinetic interaction between diorganotin(IV)dichlorides and triphenyltin(IV)chloride with the cobalt(II) tetraaza Schiff base complexes in DMF as solvent at various temperatures. The kinetics were followed under pseudo-first-order conditions for the acceptor concentration; the donor concentration was kept constant at 6.4×10^-5 M, and the excess concentration of each acceptor was varied in the range 1×10^-4– 4×10^-2 M. The results in Tables (5-24) show that there is a linear rate dependence on the concentration of the acceptor [A].

Table5: Pseudo-First-Order Rate Constants 103kobsa (s-1) for the Reaction of Co(campen) with Ph2SnCl2 in DMF at Different Temperatures. [Complex]= 6.4×10-5M  Click here to View table

 

Table6: Pseudo-First-Order Rate Constants 103kobsa (s-1) for the Reaction of Co(campen) with Me2SnCl2 in DMF at Different Temperatures. [Complex]= 6.4×10-5M  Click here to View table

 

Table7: Pseudo-First-Order Rate Constants 103kobsa (s-1) for the Reaction of Co(campen) with Bu2SnCl2 in DMF at Different Temperatures. [Complex]= 6.4×10-5M  Click here to View table

 

Table8: Pseudo-First-Order Rate Constants 103kobsa (s-1) for the Reaction of Co(ampen) with Ph2SnCl2 in DMF at Different Temperatures. [Complex]= 6.4×10-5M  Click here to View table

 

Table9: Pseudo -First-Order Rate Constants 103kobsa (s-1) for the Reaction of Co(ampen) with Me2SnCl2 in DMF at Different TTemperatTemperature. [Complex]= 6.4×10-5M  Click here to View table

 

Table10: Pseudo-First-Order Rate Constants 103kobsa (s-1) for the Reaction of Co(ampen) with Bu2SnCl2 in DMF at Different Temperatures. [Complex]= 6.4×10-5M  Click here to View table

 

Table11: Pseudo-First-Order Rate Constants 103kobsa(s-1) for the Reaction of Co(amaen) with Ph2SnCl2 in DMF at Different Temperatures. [Complex]= 6.4×10-5M  Click here to View table

 

Table12: Pseudo-First-Order Rate Constants 103kobsa(s-1) for the Reaction of Co(amaen) with Me2SnCl2 in DMF at Different Temperatures. [Complex]= 6.4×10-5M  Click here to View table

 

Table13: Pseudo-First-Order Rate Constants 103kobsa (s-1) for the Reaction of Co(amaen) with Bu2SnCl2 in DMF at Different Temperatures. [Complex]= 6.4×10-5M  Click here to View table

 

Table14: Pseudo-First-Order Rate Constants 103kobsa (s-1) for the Reaction of Co(cappn) with Ph2SnCl2 in DMF at Different Temperatures. [Complex]= 6.4×10-5M  Click here to View table

 

Table15: Pseudo-First-Order Rate Constants 103kobsa (s-1) for the Reaction of Co(cappn) with Me2SnCl2 in DMF at Different Temperatures. [Complex]= 6.4×10-5M  Click here to View table

 

Table16: Pseudo-First-Order Rate Constants 103kobsa (s-1) for the Reaction of Co(cappn) with Bu2SnCl2 in DMF at Different Temperatures. [Complex]= 6.4×10-5M Click here to View table

 

Table17: Pseudo-First-Order Rate Constants 103kobsa (s-1) for the Reaction of Co(appn) with Ph2SnCl2 in DMF at Different Temperatures. [Complex]= 6.4×10-5M  Click here to View table

 

Table18: Pseudo -First-Order Rate Constants 103kobsa (s-1) for the Reaction of Co(appn) with Me2SnCl2 in DMF at Different TTemperat Temperature. [Complex]= 6.4×10-5M  Click here to View table

 

Since most of the kinetic studies [12] that have been published till now were carried out in interfering (i.e. coordinating) solvents, it is necessary to investigate the nature of this interference before the mechanistic assignment is secure [13]. Plots of k_obs vs. R₂SnCl₂ and plots of k_obs vs. Ph₃SnCl exhibit an almost zero intercept. The present studies indicate that solvolysis of Co(II) tetraaza in DMF is negligible. Typical plots of k_obs vs. Ph₂SnCl₂ molar concentrations [A] for [Co(ampen)] at different temperatures are shown in Fig.4.

Fig4: The plots of kobs vs. Ph2SnCl2 molar concentrations [A] for [Co(ampen)] at different temperatures. Click here to View figure

 

Table19: Pseudo-First-Order Rate Constants 103kobsa (s-1) for the Reaction of Co(appn) with Bu2SnCl2 in DMF at Different Temperatures. [Complex]= 6.4×10-5M  Click here to View table

 

Table20: Pseudo-first-order rate constants 103kobsa (s-1) for the reaction of Co(amaen) with Ph3SnCl in DMF at different temperatures. [Complex]= 6.4×10-5M Click here to View table

 

Table21: Pseudo-first-order rate constants 103kobsa (s-1) for the reaction of Co(appn) with Ph3SnCl in DMF at different temperatures. [Complex]= 6.4×10-5M  Click here to View table

 

Table22: Pseudo-first-order rate constants 103kobsa (s-1) for the reaction of Co(ampen) with Ph3SnCl in DMF at different temperatures.[Complex]= 6.4×10-5M  Click here to View table

 

Table23: Pseudo-first-order rate constants 103kobsa (s-1) for the reaction of Co(cappn) with Ph3SnCl in DMF at different temperatures. [Complex]= 6.4×10-5M  Click here to View table

 

Table24: Pseudo-first-order rate constants 103kobsa (s-1) for the reaction of Co(campen) with Ph3SnCl in DMF at different temperatures.[Complex]= 6.4×10-5M  Click here to View table

 

Table25: Activation Parametersa ∆G#, ∆H# and ∆S# for the Reaction of Cobalt(ΙΙ) Complexes with Diorganotindichlorides in DMF  Click here to View table

 

The isosbestic points observed for the reaction of R₂SnCl₂ with [Co(campen)] were at (588, 354 nm), with [Co(ampen)] were at (576, 343 nm) and with [Co(amaen)] were at (574, 343 nm). As an example, the variation of the electronic spectra for [Co(amaen)], reacted with excess Ph₂SnCl₂ at 30ºC in DMF is shown in Fig. 5. The isosbestic points show that there is only one reaction in progress. The same procedure was followed for other systems and similar results were observed.

Fig5: The absorption spectra of the kinetics of [Co(amaen)], with twelve fold excess Ph2SnCl2 at 30ºC in DMF with the isosbestic points.  Click here to View figure

 

Adduct entities were obtained from the reaction of the acceptors with the donors, according to Eq. (1):

[Co(L)]^.H₂O + R₂SnCl₂or Ph₃SnCl→[Co (L)]^.(R₂SnCl₂)^.H₂O or  [Co (L)].(Ph₃SnCl).H₂O      (1)

where L = campen, ampen, amaen, appn, cappn and R = Me, Bu, Ph.

The rate law of Eq. (2) is compatible with the adduct formation according to Eq. (1):

k_obs= k₂ [A]             (2)

The activation parameters, DH^# and DS^# (Table 25)were calculated as a function of temperature by Eyring Eq. (3):

ln (k₂/T)= -ΔH^#/RT + ΔS^#/R + 23.8           (3)

A typical linear Eyring plots of ln(k₂/T) vs. 1/T at four different temperatures in the range (10-40ºC) for Co(ampen) donor with different acceptors is shown in Fig. 6. The Eyring plots for the reaction of different donors with the acceptor Ph₃SnCl (Lewis acid) in DMF is shown in Fig. 7.The low ΔH^# values and the large negative DS^# values is compatible with (A) mechanism. Also the linear plots of k_obs vs. R₂SnCl₂ and or Ph₃SnCl the high span of k₂ values differing with the nature of the acceptors suggest an associative (A) mechanism.

Fig6: The Eyring plots for the reaction of different acceptors (Lewis acid) with Co(ampen) in DMF.  Click here to View figure

 

Fig7: The Eyring plots for the reaction of different donors with the acceptor Ph3SnCl (Lewis acid) in DMF.  Click here to View figure

 

The Acceptor Properties of the Diorganotin(IV)dichlorides and Triphenyltin(IV)chloride

Comparing the k₂ values, the following trend of acidity for the acceptors used was assessed: Ph₂SnCl₂ > Me₂SnCl₂ > Bu₂SnCl₂ > Ph₃SnCl.

It is clear that the electron withdrawing groups (Ph-) on the tin center makes Ph₂SnCl₂ a stronger Lewis acid than the others. This trend also indicates that replacing the methyl- by a more bulky butyl- group on the organotin(IV) compound causes a weakening of the interaction. The butyl- group can affect the interaction in two ways: 1) A more bulky butyl- group makes adduct formation unfavorable because of its greater steric hindrance than a methyl- group [14]. 2) Butyl- group, though, have better electron releasing properties to reduce the acid strength of the di-organotin(IV) Lewis acid but its steric effect would predominate and compensates the higher electronic effect. Although the electronic effect signifies that the k₂ values must be higher with the triphenyltin(IV)chloride , but its steric effect is predominating (see Table 7) [15]. These are compitable with the electronegativity of Ph=2.717, and Cl=3.0 in Pauling and 3.475 in Sanderson’s scales [16].

Comparing the Donor Property of the Cobalt(II) Tetraaza Schiff Base Complexes

The k₂ values for the ligands entry show high span, suggesting the dependence of rate  on the nature of the ligand. The low ΔH# values and the large negative ΔS^# values are compatible with (A) mechanism. Also the linear plots of k_obs vs. [R₂SnCl₂], [Ph₃SnCl] and the high span of k₂ values differing with the nature of the acceptors suggest an associative (A) mechanism.

The obtained trend shows that, k₂ values for reaction of  [Co(campen)] is the least because of two electron withdrawing chlorides on the aromatic rings and two phenyl groups while [Co(amaen)] show the highest k₂ values because of two electron donating methyl groups. It is clear that the existence of methyl groups on the imine group make the Schiff base complexes better donors. In [Co(ampen)], however there is two electron withdrawing phenyl groups on the imine group that makes it a weaker donor compared with [Co(amaen)]. In [Co(appn)] there is one methyl group on the ethylene group which makes it a better donor compared with [Co(ampen)]. The presence of two chloride groups on the aromatic rings makes it a weaker donor compared with [Co(appn)]. Therefore the second order rate constants k₂ decrease according to the sequence:

[Co(amaen)] > [Co(appn)] > [Co(ampen)] > [Co(cappn)] > [Co(campen)].

Conclusions

The results of this work can be summarized as follow

1. The relative Lewis acidities of different organotin(IV)chlorides toward a given Co(ΙΙ) tetraaza Schiff base complex according to the k₂ values follow the trend: Ph₂SnCl₂  >  Me₂SnCl₂  >  Bu₂SnCl₂  >  Ph₃SnCl.
2. The linear plots of k_obs vs. [R₂SnCl₂], the high span of k₂ values and the large negative DS^# values suggest an associative (A) mechanism.
3. The kinetic parameters and the second order rate constants k₂ show the following trend of the donor properties of Co(ΙΙ) tetraaza Schiff base complexes toward a given acceptor:

[Co(amaen)] > [Co(appn)] > [Co(ampen)] > [Co(cappn)] > [Co(campen)].

Acknowledgement

We are grateful to Jahrom University Research Council for its financial support.

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