EFFECT OF DIFFERENT SOLVENT ... - Science Direct

J. inorg, nucl. Chem. VoL 43, No. 10, pp. 2471-2480, 1981

0022-1902181/102471-10502.00[0 Pergamon Press Ltd.

Printed in Great Britain.

EFFECT OF DIFFERENT SOLVENT CHARACTERISTICS ON THE PROTON-LIGAND AND CuJ+-LIGAND EQUILIBRIUM AND FORMATION CONSTANTS OF ACETYL ACETONE IN VARIOUS MIXED AQUEOUS SOLVENTS N. KOLE and A. K. CHAUDHURY Chemistry Department, Burdwan University, Burdwan 713104, W,B., India (Received 1 October 1980; accepted for publication 16 January 1981)

Abstract--Complex equilibria of acetyl acetone (Ac-Ac) with proton and Cu 2+ ion has been measured in various mixed aqueous solvents, viz. dioxane-water, isopropanol-water, acetone-water, ethanol-water and methanol[CuL] were water. ~'K~" of Ac-Ac, the equilibrium constant ~ = [CuL][H] [Cu][HL] and the formation constant / 3 ' = ~ measured in each of the solvent mixtures. The thermodynamic formation constants have also been determined. The . the reverse . . dielectric . . variation of th e v alues of log a K tH , log/3 and log/3, with of constant or mole frachon of the solvent was studied. The plots of log °K~" vs 1/~ or log/3' vs 1/e are explained in the light of solvent effects such as: (I) Change of dielectric constant of the mixed solvent. (2) Structuredness of water and change in hydrogen bonding in water by organic solvent. (3) Proton solvation of the organic solvent. Application of Fuoss expression and consideration of both electrostatic and non-electrostatic effects are made to explain the values of the constants. solvent-water mixtures with 0.1 M carbonate-free NaOH solution. The organic solvents were dioxane, isopropanol, acetone, ethanol and methanol. The titration cell was flushed with pure nitrogen gas previously saturated with the particular solvent used in the reaction cell. Calculations. The equilibrium constants for the reactions

INTRODUCTION

Solvent effects on proton-ligand and metal-ligand formation constants have been studied by many workers[I9]. Different physicochemical aspects should be taken into consideration to explain the values of proton ligand and metal ligand formation constants of complexes in different mixed aqueous solvents. In the studies of complex equilibria in different mixed aqueous solvents, the organic solvent certainly plays an important role. On the addition of organic solvent the structuredness of water changes[10] and the lattice structure of water is gradually broken down[l l]. When the concentration of the organic solvent becomes very high it solvates protons[12]. Moreover the change in proportion of water-organic solvent changes the dielectric constant of the medium and this has a pronounced effect on the values of the constants. According to Bates et al.[13] and Rorabacher et a/.[3] both the electrostatic and non-electrostatic effects must be considered to explain the change of the constants in these solvent mixtures. Gergely and Kiss[8] have studied the formation constants of proton, copper and nickel complexes of alanine in dioxane-water and methanol-water. They found that the plot of log K vs l/e was not linear. We now report a study on the interaction of Cu(II) with acetyl acetone in different mixed aqueous solvents of various compositions. The variation of logaK~ H, equilibrium constants (log jS) and formation constants (log 18') with the solvent composition is explained with the help of solvent effects. EXPERIMENTAL

All reagents were A.R. or G.R. grade. The pH measurements were made at 30±0.1°C with a bench model Cambridge pH meter using glass electrodes, pH titrations were done with the following solutions. (1) Mineral acid--this contained 0.0025 M HCIO4 and 0.1 M NaCIO4 in 20 ml of the various solvents. (2) Ligand solution--this was the same as the mineral acid solution but contained 0.005 M ligand in addition. (3) Metal solution--this was the same as ligand solution containing in addition 0.001 M cupric ion. The metal:ligand ratio was maintained at 1:5. The titrations were carried out in 0, 30, 40, 50, 60, 75 and 85% v/v organic

Cu2++ HL ~ CuL ÷ + H + and Cu 2~ + 2HL ~ CuL~ + 2H*

d)

were calculated by the method for monoprotic ligands[14]. Free ligand concentration was measured by the expression T ° [HL]=(HL--/iT~)

W°

VO + V""

(2)

The equilibrium constants BJ _ [CuLl[HI [Cu][HL] and /3: = [CuLJI[H]2 [ C ~ were calculated by the expression "="

E/~-

,.

n,~o

[HL]"

=0

{Sl

~[/'/]

(n - 1)[HL] (= Y) was plotted vs (2- fi)[HL] (= X) and points (~

-

I)[H]

falling on an approximate straight line were used for calculation. The constants were computed using the BURROUGHS 6700 system. Here essentially the standard least square technique was adopted; but it was so programmed that several least square values of constants were obtained by considering several sets of experimental points. Finally the mean of the several values was determined and the standard deviation calculated. The values of eKe" were measured with the help of the expression log PKI~ = B + log 1_t~A fi,a

(4)

using usual procedure. Practical constants were converted into stoichiometric constants by the expression[15], log CKj" = logfU~ + log r'Kj"

(5)

and logfU~ of different solvent compositions obtained as the intercept of the linear plot of pH vs B.

2471

2472

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2473

2474

N. KOLE and A. K. CHAUDHURY

Thermodynamic proton-!igand formation constants were determined using the relation log "K !u = log rK iH - 2 log 2/_+

(6)

and ~,~_the mean activity coefficients for different solvent compositions of methanol-water, ethanol-water and dioxane-water (0-75%) v/v were obtained from the interpolation of the values of 3', for HCI[16, 17]. For isopropanol-water and acetone water mixtures the mean activity coefficients were calculated from the extended Debye Huckel equation[18] log~/~ = -

[Z,Z211.29x 106V~-~/[,. 35.56a°V~-~ '~ (eT)3n -/t \ , ~ ~ ] (7)

where tx = ionic strength, T = absolute temperature, Z~ and Z~ are charges of positive and negative ions and a ° the mean ionic diameter. The above expression gave the values of 3,_+for different solvent compositions of methanol-water, ethanol-water and dioxane-water which agreed with the values obtained experimentally[16,17] when the value of 6°,~ was used for a °. Therefore 7± for acetone-water and isopropanol-water mixtures was calculated using this value. The formation constants /3i and /3~ were calculated by the relation

log/31= log/3i-log PK~H

and log/32= log/3"-2 log PKIn.

(8)

The values of log/3[ and log/3~ of the complexes in water and in 50% dioxane were also calculated by the method of Irving and Rossotti[ 15] using the expression e ~

fi

dioxane-water > isopropanol-water > acetone-water > ethanol-water > methanol-water. However, for the same v/v composition or same mole fraction log OK~H in different solvent-water mixtures follows the sequence: dioxane-water > acetone-water > isopropanol-water > ethanol-water > methanol-water.

[CuL] [HI[L] /3' [Cu][L] [HLI ..

The values of log/31 and log/3~ in 75% v/v dioxane-water are very close to the results obtained by Al-Niaimi[20]. The values of thermodynamic stability constants are given in Table 2. From the data it is observed that log'~K~u increases with increasing percentage of organic solvent in the media. Figure 1 shows that plots of log "K~n vs 1/E give straight lines in the region of 0-60% v]v for each solvent-water mixture, whilst straight lines of different slopes are obtained in the region of 60-85% v/v. Figure 2 illustrates the linear dependence of log aK~n on mole fraction in all organic solvent-water mixtures in the entire region studied. For a particular composition of solvent water mixture, i.e. same v/v or same mole fraction, I/E changes in the following order:

(2- fi)[L] o = (a - 1) m-/3,.

The results obtained by the new method[14] were compared with those obtained by former method[15] and with the published data. Thermodynamic formation constants were calculated by the relations[19,20]

The proton ligand formation constant in a mixed aqueous solvent may be influenced by different solvent characteristics, namely, (1) Dielectric constant of the mixed solvent. (2) Structuredness of water and change in hydrogen bonding in water by organic solvent. (3) Protonation of the organic solvent. Bates et al.[13] and Rorabacher et a/.[3] explained the change of log K n with solvent composition considering both the electrostatic and non-electrostatic effects. They concluded that non-electrostatic phenomena become increasingly important in solvents containing greater than 50% methanol. Protonation is viewed [3] as a two-step process, K,,~ H~ . . . L _ b : K,-L " HL+I_ b. (10) H + + L -b ~..-._._z"

log "K I = log K i-4 log 7_+ log "/3~= log/3.~-6log 3,_+

Solvent separated ion pair (9)

and 3'* de[ermined using the expression (7) and taking the value of a = 10A[19,21]. However the thermodynamic formation constants contain inaccuracies[22] and it is incorrect to calculate the thermodynamic formation constants in mixed solvents containing 70% dioxane or more and at ionic strength 0.1 M[23,24]. So these values must be considered with reservation.

RESULTS AND DISCUSSION The values of IogPK, r~, log/h, logfl2, log/3~ and log/3~ of the complexes and I/E values of the solvents are shown in Table 1. The values of the constants in water and in 50% dioxane computed by the former method[15] can be compared with the values obtained by the new method[14] and with the published data. The values calculated by the two methods are in excellent agreement with each other and also with the published data[25-27], log"Kj H values in 75% dioxane and in water are close to those in literature[19,28,29]. The value of log °/3~ in 75% v/v dioxane-water (22.49) differs from that obtained by Fernelius (23.66) et al. [28] but it is identical with the value reported by Rao and Mathur[19].

where K , = Ko~" KnL.

(11)

The value of Kos, representing the diffusion controlled equilibrium constant for ion pair formation can be calculated from the Fuoss equation[30], Kos = 4~ ' a 3 N a ' 10-3 e - ( Z H -ZL.eO2le .akT)

(12)

where, a = centre to centre distance of closest approach between the solvated proton and base in the ion pair preceding the proton jump. NA = Avogadro's number Z , and ZL = formal charges on the proton and base respectively eo = electronic change k = Boltzmann constant T = absolute temperature. The term Kn-L in eqn (12) represents the proton jump equilibrium occurring within the ion pair, presumably a

Solvent effect on the formation of Cu(II)~AcAc complexes

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2476

N. KOLE and A. K. CHAUDHURY D I O X A N E - WATER ISOPROPANOL- WATER A CETONE - WATER E THA NOL - WATER METHANOL-WATER

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II "6

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non-electrostatic term dependent primarily on the relative basicity of the solvent and the base with a contribution from the nature and orientation of the solvent molecules separating the solvated proton and base species in the ion pair. It also appears that the non-electrostatic part of the interaction is rekited to the proton acceptance property of the medium and proton solvation of the organic solvent discussed below. Taking the logarithm of the expression (11) with in-

clusion of the Fuoss expression for Kos (12) we have, log KH = log 47rN,~ 10-3 + 3 log a + log KH-L Z H Z L eo 2

1

2.303kTa E"

(13)

Putting the value of ZL (=--1) it is seen that with decreasing dielectric constant (e) of the medium log K/~

Solvent effecl on the formation of Cu(ll)--AcAc complexes increases. The variation of KH e may have less effect on the protonation constant as it occurs as logarithmic function. It is well established[10, 31] that the first addition of an alcohol or other organic co-solvent to water leads to an increase in structuredness or the degree of order in the system. Furthermore, the degree of structuredness must increase until it passes through a maximum. Beyond this composition the highly ordered solvent structure begins to collapse. The compositions that correspond to maximum structuredness at 30°C are close to 0.2 mole fraction of methanol-water, 0.17 mole fraction of ethanolwater and 0.1 mole fraction of acetone-water. This phenomenon is also expected in the case of isopropanol, but dioxane behaves differently from alcohols. In particular, water-dioxane mixture do not show marked maxima in many cases. Though the breakdown of water structure starts after the region of maximum structuredness, hydrogen bonding in water still remains to an appreciable extent up to some higher percentage of organic solvent. After that the tetrahedral lattice structure of water is gradually broken down[11], and owing to the denser packing and smaller extent of hydrogen bonding between water molecules, the stability of the hydroxonium ion increases and the proton donating property of the medium falls. This may imply that the proton accepting property of the medium increases. Gergely et a/.[8] have indicated that the dioxane molecules progressively breakdown the hydrogen bonded structure of the water whereas methanol can form hydrogen bonded associations with water. Thus it is expected that the exlent of hydrogen bonding in alcoholwater is greater than that in dioxane-water. When the proportion of organic solvent becomes sufficiently large in a water-organic solvent mixture proton solvation of the organic solvent molecules takes place. Braude[12] has reported that the basicities of the pure solvent molecules decreases in the following order

2477

Thus the proton solvation of pure acetone is least. The effects (2) and (3) depend upon the concentration of mixed aqueous media. Now the more a solvent accepts a proton the more the ligand acid is dissociated and so the ~K~H value will tend to decrease. The foregoing solvent effects influence the "K, H of ligand conjugate acid in the following manner: (a) With increase of solvent dielectric constant "K," of the ligand decreases and vice versa. (b) On decreasing the extent of hydrogen bonding in water by the organic solvent, the proton accepting property of water increases. So PK, ~ of the ligand decreases. (c) Increase in proton solvation by the organic solvent decreases the PK~H of ligand and vice versa. The linear variation of log"K, H with l/e for 0-60% v/v composition of all solvent-water mixtures indicates that the structuredness of water remains to an appreciable extent up to this composition. The protons are accommodated in the interstices of the tetrahedrat lattice. The values of "K~ c" and "a" (Table 3) are also close in this region for all solvent water mixtures. The plot of log"KL ~ vs I/E gives another straight line in the region 60-85% v/v solvent composition for all solvent water mixtures. In this region the aqueous solvation shell is dispersed by the interposition of the organic solvent molecules and the participation of the organic solvent molecules in the solvation of H30 ~ cannot be ignored. Values of "a" are higher than those in former case. The values of log K~ ~. in each case are given in Table 3. The slope of the straight line in this region is smaller in case of dioxane-water mixture leading to highest value of "a" and log K, ~. This is due to the fact that both the effects (b) and (c) lead to lowering of "K, H. The sequence for log"K, ~ values in the different solvents is almost the same as that of dielectric constants of the solvents except that acetone and isopropanol have exchanged places. This is probably due to the low proton solvation capacity of acetone. Equilibrium constants/~, and/32 are tabulated in Table 1.

water > dioxane > ethanol > acetone.

Table 3. Values of "'a" and Iog K . ; calculated from the plots of log"K, H vs I/e in different organic solventwater mixtures Solvent

Oompos&tion ('~3 v/v

Intercept

Slope

'a'

log KH.L

Methanol

0 - 60

8.066

71.50

3.35

~

9.09

-water

60-8~

8.%~9

h9.2~

I+.% ~

9.03

Ethanol

0 - 60

9.075

69.21

o 3.~6 A

9.06

-water

60-85

%638

~3.72

5.~9 ~

9.02

Acetone

0 - 60

7.hO8

120.~3

-water

60-85

7.7~3

I05.1~

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0 - 60

8.2~8

75.68

-water

60-85

9.363

~3.70

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0 - 60

7.95"3

90.~q

-water

60-85

9.595

37.96

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2478

N. KOLE and A. K. CHAUDHURY

They generally increase with decrease of dielectric constant or when the proportion of the organic solvent increases. In some cases in high proportions of organic solvent there is some deviation. For lower volume per cent (30, 40 and 50%) of organic solvent the sequence of log/32 is isopropanol-water > dioxane-water > ethanol-water > methanol-water > acetone-water. For higher volume per cent (75 and 85%) the order of log/32 is isopropanol-water > ethanol-water > methanol-water > dioxane-water > acetone-water. It is evident that the sequences have no relation with the 1/e values of the media. It appears that the proton solvation

property becomes important in this case. Thus isopropanol possibly solvates the proton strongly and least proton solvation occurs in case of acetone (effect 3). The values of formation constants (/3') are obtained from equilibrium constants (/3) and PK~H values. Formation constants increase with the decrease of the dielectric constant of the solvent water composition of any organic solvent. Figure 3 shows the variation of log/3~ with 1/e. The variation is almost linear from 0 to 60% v/v methanol-water, ethanol-water and acetonewater and another straight line is obtained in the region of 60-85% v/v of the above solvent-water mixtures. The plot for isopropanol-water gives a straight line in the region of 0-50% v/v composition and another one in the region of 50-85% v/v. For dioxane-water the plot of log B~ vs l/e is linear from 0-50% v/v composition but for higher dioxane content the plot deviates from linearity. It can be seen from Fig. 4 that the variation of log/3~ with mole fraction

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Solvent effect on the formation of Cu(II)--AcAc complexes

2479

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is similar in nature to that of log/3~ with 11~ (Fig. 3). For 30-40% v/v organic solvent water composition log/3" generally follows the following sequence. dioxane-water > isopropanol-water > acetone-water > ethanol-water > methanol-water. Thus the order follows the sequence of lie values of the solvents. For higher percentage of organic solvent content isopropanol and acetone exchange places. At 85% v/v acetone moves to the first place. However comparison of the values of thermodynamic formation constants (log"/31 and log"/3~) at any volume percent shows that the order of dielectric constant is generally followed at higher percentages of organic solvent isopropanol and acetone exchange places. Two intersecting straight lines are obtained in the plot of log a/3~ vs I/E (Fig. 5) for all organic solvent water mixtures. The region of composition that corresponds to the first straight line is 0-60% v/v for methanol-water, ethanol-water and acetone-water, and is 0-50% v/v for dioxane-water and isopropanol-water. The second straight line is obtained in the region of 60-85% v/v for the first three solvent-water mixtures and 50--80% v/v for the remaining two solvent water mixtures. Acknowledgement--We thank the UGC, New Delhi, for the award of a teacher fellowship (N. K.). REFERENCES

1. H. M. Irving and H. S. Rossotti, Acta Chem. Scan. 72, 10 (1956). 2. G. Faraglia, F. J. C. Rossotti and H. Rossotti, Inorg. Chim. Acta 4, 488 (1970).

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