234 J. Burgess and E.-E. A. Abu-Gharib Transition Met. Chem. 9, 234-236 (1984)

Solubilities of Salts of Cobalt(III), Chromium(III), and Iron(II) Complexes in Aqueous Methanol; Transfer Chemical Potentials of Anions

John Burgess* and Ezz-Eldin A. Abu-Gharib**

Chemistry Department, University of Leicester, Leicester LE1 7RH, U.K.

Summary

We report solubilities of a variety of salts of cobalt(Ill), chromium(III), and iron(II) complexes in methanol-water mixtures at 298.2K. From these solubilities and published transfer chemical potentials for the complex cations we are able t o derive transfer chemical potentials for such anions as nitrate, thiosulphate, peroxodisulphate, dithionate, thiocyan- ate, and antimonyl tartrate. Transfer chemical potentials for several hexahalogenometallate anions, and tetrachloro- platinate(II), are derived from published solubilities. A com- parative picture of transfer chemical potentials for anions is thus available, with the transition metal complex anions in the overall context of anions and their solvation characteristics in methanol-water mixtures.

Introduction

The establishment of satisfactory sets of transfer chemical potentials based on the TATB assumption [~mp, e(AsPh +) = 5m~te(BPh;-)] for simple ions in methanol-water mixtures enabled us to derive transfer chemical potentials for a variety of cobalt(III) and chromium(III) complexes (z). Now we show how these results, plus known 5m~te[Fe(phen)32+] values, enable us to obtain transfer chemical potentials for a variety of

anions from solubility measurements on appropriate complex salts. These results, taken with transfer chemical potentials for a variety of hexahalogenometallate anions derived from pub- lished solubilities, enable us to build up a picture of the ther- modynamics of preferential solvation of anions. This picture can be compared with similar pictures for cobalt(III) and chromium(III) complexes (1), and for a range of low-spin iron(II)-diimine complexes (2).

Results and Discussion

Solubilities determined in the course of this investigation are reported in Table 1. Transfer chemical potentials for these salts were calculated on the assumption that the ratio of mean activity coefficients in water and aqueous methanol was in all cases unity. As discussed earlier (~/, in the present context this is probably satisfactory for Iz+z_l~< 3. Using the transfer chem- ical potentials for the various cationic complexes recently derived (1) on the TATB assumption (3), we have estimated transfer chemical potentials for a variety of anions (Table DI*). These results are plotted, with those for a selection of other anions (converted, where necessary, to the TATB assumption) in Figure 1. Transfer chemical potentials for the sulphate ion were derived from published solubilities of potas-

Table 1. Solubilities of salts containing cobalt(III), chromium(III), and iron(II) complex cations in MeOH-H20 solvent mixtures at 298.2K.

Compound ?~(e) 104 soly/mol dm -3 in vol % MeOH 0 10 20 40 60

[Co(en)2(ox)](NCS) [Co(en)2(ox)lz(S203) �9 3 H20 [Co(en)2(ox)]2(S2Os) �9 5 H20

trans-[Co(py)4C12]2(S206 ) �9 10H20

[Co(NH3)6] (NO3)3

trans-[Co(en)2C12](N03)

[Cr(urea)~]2(SO4)3 �9 10H20 [Cr(urea)6]2(S~O3)3 �9 3 HzO [Cr(urea)6]z(S~O6)3" 2I-I20 [Cr(urea)612(S2Os)3

[Fe(phen)3](NCS)2 [Fe(phen)3][BPb3(CN)]2 [Fe(phen)3][Sb2(tartrate)2] �9 8HzO

498(103) 82 51 35 20 498(206) 108 32 9.0 3.9 498(206) 24 11.5 5.2 1.6

638(90) 50 81 129 207

475(58) 650 b) 353 183 81

618(37) ") 238 174 130 89

625(97) 214 162 118 57 47 c~ 24 12 4.7 2.6 10.5 9.0 7.5 5.5 22 19 18.5 17.5 14.5

510(11100) 27 60 228 605 0.43 2.7

1.5 2.2 3.2

a) Our value of e = 37.2 agrees with C: F. Wells, J. Chem. Soc., Faraday Trans. I, 78, 619 (1982), though not with some earlier punished values at this wave-length; b) this result compares well with values of 0.0202 and 0.052 moldm -3 at 273.2 and 293.2K [J. N. BrSnsted and A. Petersen, J. Am. Chem. Soc., 43, 2265 (1921)], slightly less well with 0.013 moldm -3 at 303.2K [A. Benratb, Z. Anorg. Chem., 151, 343 (1926)]; c) cf. 0.006 moldm -3 at 293.2K [E. Wilke-D6rfurt and K. Niederer, Z. Anorg. Chem., 184, 145 (1929)].

* Author to whom all correspondence should be directed. ** On leave from the Faculty of Science, Sohag, Egypt.

* Copies of this Table may be obtained from the Editor.

0340-4285/84/0606--0234502.50/0 �9 Verlag Chemie GmbH, D-6940 Weinheim, 1984

Transition Met. Chem. 9, 234-236 (1984) Co m, Cr m and Fe H salt solubilities in H20 : MeOH 235

I 20 ~m# 0

kJ tool "T

fret6 a- o/

0 / 6C1" /

, ,o-P'-- o ~ r

20 ~,0 % MeOH ---*

Figure 1. Transfer chemical potentials 6~F e, for anions from H20 into aqueous MeOH; molar scale, 298.2 K.

sium sulphate (4), for the hexahydroxoantimonate(V) anion from the published solubilities of its sodium salt (5), for the naphthalene-2-sulphonate anion from the published sol- ubilities of its potassium salt (6). Transfer chemical potentials for chloride, picrate, and tetraphenylboronate are from Tis-

sier (3). Transfer chemical potentials for triphenylcyanoboron- ate between 0 and 40% methanol are derived from solubilities of its caesium salt(7); the value for 60% methanol is derived from the Table 1 solubilities of its Fe(phen) 2+ salt. Transfer chemical potentials for the chromone carboxylate anion, 4- oxo-4H-l-benzopyran-2-carboxylate, are derived from sol- ubilities of its Co(NH3)~ + salt (8).

Figure 1 shows the marked destabilisation of inorganic anions with hydrophilic exteriors as the proportion of methanol in the mixed solvent increases. As the size of the anion decreases in the series: $20~- > $2 O2- > $20~- > SO42-, the destabilising effect of the methanol increases, as one would expect on simple electrostatic grounds. The smaller destabilis- ing effect on simple uni-negative anions such as chloride, nitrate, and thiocyanate of adding methanol is also consistent with simple electrostatics. Indeed the transfer chemical poten- tials for the large uni-negative perchlorate (1) and perrhenate (from caesium perrhenate solubilities (9)) anions are very close to zero throughout the solvent composition range 0 to 60% methanol. The very hydrophobic tetraphenylboronate ion is, of course, much stabilised by the addition of methanol. It is interesting to see (Figure 1) how little effect the replacement

20

t ~ m l.J. 8

kJ mot "1

10

i

-10 l

2- S0~

J

/ o

$2062-

/ o / c t - S b (OH) 6- _

~ v ~ ~ A / A ~ N03-

_

NCS

- - J I I

�9 A . . . .

o~ ~ a

~ e ~ BPh3l[N)- O.~.

--- BPh4-

F i g u r e 2. Transfer chemical potentials, 6m~ | for hexahalogenometallate anions from H20 into aqueous MeOH; molar scale, 298.2 K.

236 J. Burgess and E.-E. A. Abu-Oharib Transition Met. Chem. 9, 234-236 (1984)

of one phenyl group by cyanide has. However, organic anions whose peripheries contain hydrophilic areas, such as naph- thalene-2-sulphonate, chromone carboxylate, and picrate, are much less strongly stabilised on increasing the proportion of methanol. And, at the extreme, the di-negative dinuclear anti- mony tartrate anion (1~ which has a number of hydrophili c centres around its periphery, is actually destabilised as the proportion of methanol increases.

The $20 n- group of salts serve to illustrate the limitations of transfer chemical potential determinations from solubilities. The values obtained from salts of 1+ cations and used in Figure 1 are probably reasonably correct; a set of estimates from solubilities of [Cr(urea)6] 3+ salts (Table 1) give similar relations between the anions, but all values suggest much less destabilisation on addition of methanol. We feel that this dis- crepancy arises from a high level of ion-pairing between the 3+ and 2 - ions; an ion-pair of charge 1+ would obviously be affected very much less than separate 3+ and 2 - ions.

In Figure 2 we have gathered together transfer chemical potentials for several hexahalogeno-anions of transition ele- ments, and for PtClZ4 -. These values were derived from sol- ubilities of K3IrC16 (n), KzIrC16 (n), KzPtC16 (at 293K) 02), CszReC16(9, 13) CszReBr6(9, 13), and KzPtC14 (14). They have all been calculated on the TATB single ion reference assumption. All these halogeno-anions are destabilised on transfer from water into aqueous methanol, as would be expected from their hydrophilic exteriors and their charge. The importance of charge is shown by the plot for IrC13- in comparison with IrC12-, the relevance of size by the ReC126 - and ReBr~- pair of curves. A line for six chloride ions has been included in Figure 2 to relate the hexachlorometallate anion trends to that for their constituent ligands.

These transfer chemical potential trends are of interest in their own right, in establishing a quantitative measure of selec- tive solvation, in this case by methanol or water. They are also of value in providing initial state data for the analysis of reac- tivity trends for reactions of transition metal complexes into initial state and transition state contributions, an essential pre- requisite in understanding solvent effects on rate constants 05). It is perhaps worth pointing out the essential role of transition metal complexes in determining 6mp,| ) and ~)mp,| -) values - salts of these anions with simple cations are far too soluble for use in solubility determinations for transfer chemi- cal potentials. It should also be mentioned that the establish- ment of ~m].t| -) and 6m~e(ReC12-) values is already pro- ving useful in obtaining transfer chemical potentials for transi- tion metal complex cations. These and other anions, such as PtC12- and $202-, form many sparingly soluble salts that are suitable for determining transfer chemical potentials. The 6m~t~ data have been used in analysing kinetic data on aquation of the trans-[Co(py)4Clz] + cation in alcohol-water mixtures (16). The ~m~xe(NCS -) data are being used in the building up of transfer chemical potential patterns as a func- tion of ligand size, nature, and substituent for iron(II)-diimine complexes (z). Sometimes the thiocyanate salts of such cations have better solubility characteristics, or crystallise in a better

form, than the more often precipitated perchlorates. Similarly some iron cations of this type can more easily be obtained pure as triphenylcyanoborates than tetraphenylborates. The availa- bility of transfer chemical potentials for iodide, perchlorate, thiocyanate, tetraphenylboronate, and triphenylcyanoboro- hate will enable us to obtain transfer chemical potentials for a variety of low-spin iron(II)-diimine cations in our detailed examination of solvation effects on reactivity in this area.

Experimental

The cobalt(III) and chromium(III) complexes were pre- pared by the methods cited earlier (1). The salts whose sol- ubilities are reported in this paper were prepared by double decomposition reactions in solution, and were recrystallised as required. Saturated solutions were generated by agitating an excess of solid with the appropriate solvent in a darkened vessel in a thermostatted bath. Absorbances of saturated solu- tions were measured (after appropriate dilution when neces- sary) on a Unicam SP8-100 or SPS00 Spectrophotometer. The complexes used here showed no significant variation of Vr~ or r with solvent composition in methanol-water mixtures.

Acknowledgements

We are grateful to the Royal Society for the award of Grants-in-aid towards the purchase of the spectrophotometers used in this investigation.

References

(1) M. J. Blandamer, J. Burgess and E, A. Abu-Gharib, TMC 1081. - (2) M. J. Blandamer, J. Burgess, E. A. Abu-Gharib, P. Guardado, C. D. Hubbard and F. Sanchez, to be submitted to TMC. - (3) C. Tissier, Comptes Rendus, 286C, 35 (1978). - (4) G..~kerl6f and H. E. Turck, J. Am. Chem. Soc., 57, 1746 (1935). - (2) M. J. Blandamer, J. Burgess and R. D. Peacock, J. Chem. Soc., Dalton Trans., 1084 (1974). - (6) A. P. Krasnoperova, B. S. German and V. S. Chernyi, Izvest. Vyssh. Ucheb. Zaved., Khim. Khim. TekhnoL, 16, 699 (1973). - (7) M. J. Blandamer, J. Burgess and F. M. Mekhail, Inorg. Chim. Acta, 81, 103 (1984). - (8) j. Burgess and E. A. Abu-Gharib, J. Chem. Research, (S) 8 (M) 0275 (1984). - (9) j. Burgess, N. Morton and J. C. McGo- wan, J. Chem. Soc., Dalton Trans., 1775 (1977). - (~0~ D. H. Temple- ton, A. Zalkin and T. Ueki, Acta Crystallogr., 21, A154 (1966); Inorg. Chem., 12, 1641 (1973).

(11) M. J. Blandamer, J. Burgess, S. J. Hamshere, C. White, R. I. Haines and A. McAuley, Can. J. Chem., 61, 1361 (1983). - (12) E. H. Archibald, W. G. Wilcox and B. G. Buckley, J. Am. Chem. Soc., 30, 747 (1908). - ( 1 3 ) j. Burgess and S. J. Cartwright, J. Chem. Soc., Dalton Trans., 100 (1975). - 04) M. J. Blandamer, J. Burgess, P. P. Duce, A. J. Duffield and S. J. Hamshere, Transition Met. Chem., 6, 368 (1981). - (15) See, e.g., A. J. Parker, Chem. Rev., 69, 1 (1969); M. J. Blandamer and J. Burgess, Pure Appl. Chem., 51, 2087 (1979); 54, 2285 (1982); 55, 55 (1983). - (16) I. M. Sidahmed and C. F. Wells, J. Chem. Soc., Dalton Trans., 2034 (1981).

(Received January 20th, 1984) TMC 1104