Potentiometric investigation mixed ligand complexes of Co(II), Ni(II) and Zn(II) with thiosalicylic acid and diethylenetriamine

A C T A U N I V E R S I T A T I S L O O Z I E N S I S FOLIA CHIMICA 9, 1991

Joanna Masłowska*, Jó z e f Szmich*

PQTENTIOMETRIC INVESTIGATION OF MIXED LIGAND COMPLEXES OF C o ( I I ) , N i( I I ) AND Z n ( I I ) WITH THIOSALICVLIC ACID AND DIETHVLENETRIAMINE**

The form ation of mixed ligand complexes of C o ( I I ) , N i( 11) and Z n ( I I ) with t h i o s a l i c y l i c acid H^SR and diethy- le n e tria m in e E in aqueous ethanol (50\ v/v) was in v e s t ig a t ­ ed by the p o ten tiom etric method at 29B and 313 K. The thermodynamic fu nctions A F °, AH0 and A S 0 have been evalu ated .

T h io s a lic y lic acid (H^SR) belongs to p h y s io lo g ic a lly important c la s s of compounds c a lle d mercaptans. The growing importance of mixed complexes of tr a n s itio n metals w ith polyamines and b io lo g i­ c a l l y a c tiv e compounds c o n tain in g sulphur was the reason of our stu d ies in th is f i e l d . In lit e r a t u r e there are only few papers on mixed complexes of ^ S R and ethylenediam ine [ l , 2 ], As there were no data about such systems with d ie th y le n e tria m in e , we considered worthwhile to extend our in v e s tig a tio n s to them.

The aim our present paper was the determ ination of stepwise s t a b i l i t y constants of forming complexes in order to fin d out the tendency and paths of mixed ch e la te s forming in systems c o n s is tin g of C o ( l l ) , H i( 11) or Z n ( I I ) , d ie th y le n e tria m in e ( E ) and t h io s a lic y ­ l i c acid (H2S R ), in clu d in g the e v a lu a tio n of th e ir thermodynamic fu n ctio n s AF^, AH0 and AS^.

* In s t it u t e of General Food Chem istry, Technical U n iv e r s ity of Ld d i, Poland.

EXPERIMENTAL

Reagents

T h io s a lic y lic acid H2SR (Merck A. G.) was p u r ifie d by r e c r y s t a l­ liz a t io n from methanol. O ieth ylenetriam in e E (Schuhardt A. G .) was p u r ifie d by vacuum d i s t i l l a t i o n and used as i t s HCIO^ s a l t . Metal p e rch lo ra te s were prepared by tre a tin g corresponding carbonates with p e rc h lo ric ac id . M(C1Q4) 2 so lu tio n s were standarized by com- plexom etric method [3 ], A ll the pH-metric t it r a t io n s were c a rrie d out with carbonate - free NaOH s o lu tio n .

Apparatus

The pH-metric t it r a t io n s were performed under n itrogen in a w ater-jacketed thermostated beaker. pH measurements were made with a 0P-208 R ad elkis pH-meter equipped with a glass ÜP-Q710 and QP-830 P calomel e le ctro d e s. The g lass e le ctro d e was c a lib ra te d w ith b u ffe rs of pH = 4.008, 6.865 and 9.210 at 298 K. The NaOH

.e lu tio n was added from m icroburette.

Measurement method and c a lc u la tio n procedure

, The procedure employed was to t i t r a t e a M (II)- H 2SR-E mixture of constant io n ic strength (p = 0 ,1 ) in the presence of HCIO^ with standard NaOH at 298 and 313 K. Aqueous ethanol (50% v/v) was used as s o lv e n t. The sample volume was 50 cm3.

For each system s e v e ra l s e rie s of t it r a t io n s were performed. In each one, a constant co ncentration of two components was kept, w hile the co ncen tratio n of th ird component was v a rie d . From p a r t i ­ c u la r s e r ie s of t i t r a t i o n curves, the co ncen tratio n of free uncomp- lexed components [H2SRj , [e] and [m] were found as described i n ' paper [4].. Having the co ncentrations we have c a lc u la te d s t a b i l i t y constants of complexes using MINIQUA0 program [5 ].

mation A F °, lig a t io n a l enthalpy AH0 and entropy changes A S0 were evaluated usirtg eqns ( l ) - ( 3 ) [6 ];

A F ° = -2,303 RT log K . C l)

„ 2,303 R T jT jd o g K2 - log K j)

A H --- --

(2)

i s 0 • -r -A f ° (3 )

We have presumed the AH^ values to be constant at the in v e s tig a ­ ted 298-313 K temperature range.

RESULTS AND DISCU55IQN

Po to n tio m etric t i t r a t i o n data suggest the form ation of tern ary complexes, which was confirmed by the r e s u lt s of c a lc u la t io n s . The obtained s t a b i l i t y constants are shown in Tab. 1.

7 a b 1 .

The values of parameters c h a r a c t e r is t ic fo r s t a b i l i t y of complexes forming in system M (II)- H 2SR-E Metal ion _{109P M(SR)E}

( c a l c ) 109 P M(SR)E ( s t a t )

lofl *M(SR)E A 109 KM(SR)E A M(SR)£

C o ( I I ) 13.43 12.35 2.75 -0.49 1.08

N i ( I I ) 16.08 15.40 1.96 -1.83 0.68

Z n ( I I ) 15.23 14.79 1.48 -1.88 0.44

Using evaluated s t a b i l i t y constants co n c e n tratio n of metal com­ plexes have been c a lc u la te d from pH 4 to 10. These r e s u lts p lo tte d in F ig s . 1-3 show th at the mixed ligand complexes of (m(S R )E J type are predominant species at higher pH valu e s.

F ig . 1. V a ria tio n with pH of the composition a so lu tio n ( 1 : 1 : 1 ) Co( I I ) - H 2SR-E

F ig . 2. V a ria tio n with pH of the composition of a s o lu tio n ( 1 : 1 : 1 ) N i ( I I ) - H 2SR-E

Facto r A log K^(SR)E fliven by expression ( 4 ) :

A log KM(SR)E * 1o9 Pm(SR )E " (lc f l KM(SR) + 109 KHE^

is the measure of tendency fo r forming the tern ary complexes by the stepwise ad d itio n according to eqns (5 )- (6 ) or (7 )- (8 ) (th e charges of Ions are ommitted):

M + SR = [M (SR)J (5 )

[M(SR>] + E = [M (SR)E] (6 )

M + E 1 [ME] (7 )

[ME] + SR - [M (SR)E] (8 )

2 _

Taking in to account, th at the s t a b i l i t y of complexes of [MCSR)^ and [ME2] 2+ are r e l a t i v e l y high, the d is s p ro p o rtio n a tio n of these Complexes according to the equation (9 ) seems to be very l i k e l y way of forming mixed complexes:

[M<SR)2r 2 ♦ [Mt2] 2+ - [M (S R )E ]° (9 )

The tendency fo r i t can be measured by fa c to r log xM(s r)e W given by the expression (1 0 ):

lo 9 XM(SR)E = 2 l 0 9 ( i M(5R)E ' ( l o 9 ^ M (S R )2 * (10>

Since the s t a t i s t i c e ff e c ts are e s s e n tia l fa c to r for mixed lig an d complexes s t a b iliz a t io n [ 8 - li] we have also c a lc u la te d the s t a t i s t i c a l s t a b i l i t y constants

l 0 9 ( î M ( S R )E < a ta t) * 109 2 *

1/2(l0flP

The d iffe re n c e û m(SR )E 9*van bV the expression (1 2 ):

••

A M(SR)E ' l o 9 P M (S R )E ( c a l c ) " l 0 ° P M (SR )E^8 t a t )

is the measure of the a d d itio n a l, d if fe r e n t than s t a t i s t i c a l i n f l u ­ ence of ligands on the s t a b i l i t y of formed mixed complexes. The c a lc u la te d values are gathered in Tab. 1.

Negative A log values show th at the tendency of forming of mixed ch e la te s by the ste p v ise ad d itio n of both ligand s is rath er moderate. On the co n tra ry to i t the log xM(gR)£ values are p o s itiv e , what in d ic a te s that the d is sp ro p o rtio n a tio n of p a irs [m( S R ) 2] " 2 and

[ME2] 2+ is the main mechanism for forming mixed complexes.

I f only s t a t i s t i c a l e ff e c ts take p la ce , the value of log X should be 0.6 [12, 13]. In f a c t , i t is consid erab le much h igher, what seems to be caused by the n e u tra liz a tio n of ion charges during d is s p ro p o rtio n a tio n . So in the pH range f u l l f i l l i n g the co nd itio ns of forming simple 1 : 1 m etal-ligand complexes, the p a r t ic ip a tio n of te rn ary complexes in s o lu tio n is sm all, but at the higher pH va lu t », when 1 ; 2 m etal-ligand complexes can be formed, mixed ligand complexes dominate in the s o lu tio n .

The s t a b i l i t y order of mixed complexes in term of metal ion is in a good egreement with I r v i n g-W i 1 1 i a m s e rie s [1 4 ]t

C o ( II ) < N i ( I I ) > Z n ( I I )

The c a lc u la te d thermodynamic fu n ctio n s for the form ation of mixed complexes by the stepwise a d d itio n according to eqns (5 )- (6 ) are given in Tab. 2 and 3. I t can be seen th at the a d d itio n of H2SR as w e ll as E is accompanied with high n egative F ° va lu e s, what prove very strong d riv in g fo rce fo r complex form ation. The addi­ tio n of H2SR is endothermie on co n trary to the egzothermic a d d it io n " E to [M (S R )]. The entropy p o s itiv e changes fovour the complex f o r ­ mation. A S 0 values are much higher fo r the ad d itio n of the f i r s t ligand (H2SR) than the second one ( E ) . The values of 4 u , „ . r MCSRyt

T a b l e 2 [m(S R )] complexes thermodynamic fun ctio ns

Temp.

DO

Complex [C o (SR )] [n i ( S R ) ] [Z n (S R )] 298 109 KM(SR) A F ° [KJ/mole] AH0 [KJ/mole] ÛS0 [j/m ole • K] 5.81 -33.15 29.77 2)0.91 7.33 -41.83 48.82 304.20 8.19 -46.73 29.19 244.70 313 109 KM(SR) A F ° [KJ/mole] t H*3 [KJ/m ole] A S0 p/m ole • K] 6.06 -36.32 29.77 211.15 7. 74 -46.39 48.82 304.18 8.41 -50.41 29.19 244.73 T a b l e 3 [m(S R )E ] complexes thermodynamic fu n ctio ns

T emjji. i

C alcu lated value

Complex

[Co(,SR)dien] [N i(5 R )d ie n ] [Z n (S R )d ien ]

298 109 KM(SR) A F ° [KJ/m ole] AH° [KJ/mole] AS0 [J/m ole • k] 7.62 -43.48 -26,20 57.99 8.75 -49.93 -33.34 55.67 7.04 -40.17 -22.62 58.89 313 ! I/M 109 KM(SR)E A F ° [KJ/mole] AH° [KJ/m ole] AS0 [J/m ole • K] 7.40 -44.35 -26.20 57.99 p .47 -50.77 -33.34 55.69 6.85 -41.06 -22.62 58.91

(Tab. 1) are p o s it iv e , what confirm that in v e s tig a te d metal ions are b e tte r s ta b iliz e d by two d if f e r e n t , than by two ligands of the same kin d . The high values of evaluated thermodynamic fu n ctio ns may be a ttr ib u te d to the high strength of the m etal-sulphur and metal- -nitrogen bonds.

REFERENCES

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Joanna Masłowska, Jó z ef S;wnich

P0TENCJOMETRYCZNE BADANIA MIESZANYCH KOMPLEKSÓW C o ( I I ) , N i ( I I ) I Z n ( I I ) Z KWASEM TIOSALICYLOWYM I DIETYLEN0TRI AMINĄ

Przy użyciu metod potencjometrycznych badano powstawanie mie­ szanych kompleksów C o ( I I ) , N i ( I I ) i Z n ( I I ) z kwasem tio salicylo w ym (H->SR) i d ie ty le n o tria m in ą ( E ) w roztworach etanolowo-wodnych (5D^ v/v } w temp. 298 i 313 K. Obliczono towarzyszące powstawaniu kompleksów zmiany fu n k c ji termodynamicznych; A F °, 4 H ° i 6 S ° .