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

The kinetics and mechanism of the adduct formation of diorganotin(IV)dichlorides (R2SnCl2) where R=Ph, Me, Bu and triphenyltin(IV)chloride with Co(II) tetraaza Schiff base complexes such as: [Co(ampen)] {[N,N’-ethylenebis-(o-amino--phenylbenzylideneiminato)cobalt(II)]}, [Co(campen)] {[N,N’-ethylenebis-(5-chloro-o-amino--phenylbenzylideneiminato)cobalt(II)]} and [Co(amaen)] {[N,N’-ethylenebis-(o-amino--methylbenzylideneiminato)cobalt(II)]}, [Co(appn)] {[N,N’-1,2-propylenebis-(o-amino--phenylbenzylideneiminato)cobalt(II)]}, [Co(cappn)]{[N,N’-1,2proylenebis-(5-chloro-o-amino--phenylbenzylideneiminato-)cobalt(II)]} were studied spectrophotometrically. The kinetic parameters and the rate constant values show the acceptor tendency trend for the organotin(IV)chlorides as follow: Ph2SnCl2 > Me2SnCl2 > Bu2SnCl2> Ph3SnCl Adducts have been separately synthesized and fully characterized by 119SnNMR, IR, Uv-Vis spectra and elemental microanalysis (C.H.N) methods. The trend of the rate constants for the adduct formation of the cobalt complexes with a given tin acceptor decreases as follow: [Co(amaen)] > [Co(appn)] > [Co(ampen)] > [Co(cappn)] > [Co(campen)]. The linear plots of kobs vs. the molar concentration of the organotin(IV) chlorides, the high span of the second order rate costant k2 values and the large negative values of S and low H# suggest an associative (A) mechanism for the acceptor-donor adduct formation. Also [Co(aptn)]{[N,N’-1,3-propylenebis-(o-amino-phenylbenzylideneiminato)cobalt(II)] and [Co(captn)]{[N,N’-1,3-proylenebis-(5-chloro-o-aminophenylbenzylideneiminato)cobalt (II)]} were synthesized and characterized but their kinetics with R2SnCl2 were very fast that we were unable to follow them using the conventional methods.


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, adducts in the presence of Lewis bases.The organotin halides R n SnX 4-n are intermediate in Lewis acidity and found between R 4 Sn and SnX 4 .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 Me 2 SnCl 2 , Bu 2 SnCl 2 , Ph 2 SnCl 2 and Ph 3 SnCl as acceptors in DMF solvent were studied spectrophotometrically, and explained by an associative (A) mechanism.

Synthesis of Adducts of Diorganotin(IV) dichlorides with Co(II) Tetraaza Schiff Base Complexes
0.1 mmol R 2 SnCl 2 was dissolved in 5 ml acetonitrile and dropwisely added to 0.1 mmol of the Co(™™) 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 R 2 SnCl 2 with Co(™™) complexes.The precipitate was filtered off and dried under vacuum (Fig. 2).

Kinetic Measurement
A solution from each Co(II) 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 R 2 SnCl 2 and (1.3×10 -3 -1.1×10 -1 M) of Ph 3 SnCl 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 " )/(A 0 -A " )] = k obs t (where A t = absorbance at time t; A 0 = absorbance at t = 0; A " = absorbance at t = ) by means of a linear least-squares computer program.The second-order rate constants k 2 (M -1 s - 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 H # , S # were obtained from the linear Eyring plots of ln(k 2 /T) vs. 1/T at four different temperatures in the range (10-40ºC) using linear least squares computer program.Also the G # values were computed by the same program using equation G # = H # -T S # at T=313K.

Spectral Characterization Electronic Spectra
Absorption spectra of the Co(II) 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(II) tetraaza Schiff base complex spectrum several intense absorption bands are observed in the Uv-Vis regions.By addition of R 2 SnCl 2 to a solution of Co(II) tetraaza Schiff base complex in DMF the original peaks of Co(II) 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).

SnNMR Spectra
Hole ek and coworkers studies show that the di-and tri-organotin 119 SnNMR 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][8][9][10] .The adducts formed in the present investigation exhibit the 119 Sn spectra in the range -110 to -182 ppm , suggesting that the tin atom is five-coordinated (Table 2).

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 H 2 O present 11 in adduct formed (Table 3).

Elemental Microanalysis
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(II) tetraaza: R 2 SnCl 2 with one coordinated H 2 O molecule (Table 4).

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].
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  and with [Co(amaen)] were at (574, 343 nm).As an example, the variation of the electronic spectra for [Co(amaen)], reacted with excess Ph 2 SnCl 2 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.
Adduct entities were obtained from the reaction of the acceptors with the donors, according to Eq. ( 1): 3.0 (2.0) 3.5 (0.0) 59.0 (1.4) 40°C 0.9 (0. 3.9 (0.9    The rate law of Eq. ( 2) is compatible with the adduct formation according to Eq. ( 1): The activation parameters, H # and S # (Table 25) were calculated as a function of temperature by Eyring Eq. (3): A typical linear Eyring plots of ln(k 2 /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 Ph 3 SnCl (Lewis acid) in DMF is shown in Fig. 7.The low "H # values and the large negative DS # values is compatible with (A) mechanism.Also the linear plots of k obs vs. R 2 SnCl 2 and or Ph 3 SnCl the high span of k 2 values differing with the nature of the acceptors suggest an associative (A) mechanism.It is clear that the electron withdrawing groups (Ph-) on the tin center makes Ph 2 SnCl 2 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 k 2 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 k 2 values for the ligands entry show high span, suggesting the dependence of rate on the nature of the ligand.The low H# values and the large negative S # ?values are compatible with

CONCLUSIONS
The results of this work can be summarized as follow: 1. The H 2 O or [Co (L)].(Ph 3 SnCl).H 2 O ...(1) numbers in parentheses are the standard deviations of kobs.

Fig. 6 :Fig. 7 :
Fig. 6: The Eyring plots for the reaction of different acceptors (Lewis acid) with Co (ampen) in DMF (A) mechanism.Also the linear plots of k obs vs. [R 2 SnCl 2 ], [Ph 3 SnCl] and the high span of k 2 values differing with the nature of the acceptors suggest an associative (A) mechanism.The obtained trend shows that, k 2 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 k 2 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 k 2 decrease according to the sequence: [Co(amaen)] > [Co(appn)] > [Co(ampen)] > [Co(cappn)] > [Co(campen)].

Table 25 : Activation Parameters a G # , H # and S # for the Reaction of Cobalt(™™) Complexes with Diorganotindichlorides in DMF
relative Lewis acidities of different organotin(IV)chlorides toward a given Co(™™) tetraaza Schiff base complex according to the k 2 values follow the trend: Ph 2 SnCl 2 > Me 2 SnCl 2 > Bu 2 SnCl 2 > Ph 3 SnCl.2. The linear plots of k obs vs. [R 2 SnCl 2 ], the high span of k 2 values and the large negative DS # ?