Coordination Compounds of Indium. Part XV.' The Stability Constants of some Trifluoromethyl-/I-diketonate Complexes of Indium (111)

KEITH BOWDEN,' (MRS.) G. M. TANNER, AND D. G. TUCK Department of Chemistry, Simon Fraser University, Burnaby 2, British Columbia

Received March 7, 1972

The stability constants K, and K2 have been measured for the interaction of indium(II1) with the ligands R.CO.CH,.CO.CF, (R =2-fury], 2-thienyl, phenyl, 2-naphthyl, i-butyl, and I-butyl). The pK, values for these compounds, and for R = 3-pyridyl and methyl, are also reported. A conventional potentiometric technique was used ; the results refer to 46% aqueous acetone, at an ionic strength of approximately 0.1 M. The results are compared with published values for complexes of these ligands with divalent cations in other solvent systems.

Les constantes de stabilitt K, et K2 ont Cte mesurtes pour I'interaction de l'indium(II1) avec les ligands R.C0.CH2.C0.CF3 (R = furyl-2, thienyl-2, phenyl, naphthyl-2, isobutyl, et t-butyl). Les valeurs des pK, de ces composts et pour R = pyridyl-3 et mtthyle, ont t t t rapporttes. La technique potentiomttrique courante a t t t utilisee, les rtsultats ayant pour rtftrence l'acttone aqueuse a 46%, a une force ionique de 0.1 M approximativement. Les rtsultats sont cornparts aux valeurs publiees pour les complexes de ces ligands avec des cations divalents dans d'autres systtmes de solvants.

Canadian Journal of Chemistry, 50,2622 (1972)

The mechanism of the ligand exchange reaction

in a series of organic solvents has recently been investigated in this laboratory by n.m.r. tech- niques (l), using a number of l,l,l-trifluoro- methyl-p-diketonate ligands, 1, (CF3.C0.CH. C0.R)-. The free energy of activation for the exchange, and hence the rate of exchange, depends both on the particular solvent chosen and on the nature of the terminal group R (= methyl, i-butyl, phenyl, 2-naphthyl, 2-furyl, or 2-thienyl).

One possible mechanism for the exchange process [l] would involve dissociation of the complex into free ligand (or ligand anion) and a residue of lower coordination number. In order to investigate the significance of such processes, we have now measured the stability constants of a series of these chelate complexes. Results have been obtained for the pK, of the diketones themselves, and for the first and second stability constants using a simple potentiometric titration method; it proved impossible to obtain repro- ducible values for K3 due to the chemical instability of the parent diketones in strongly alkaline solution (see below). Measurements have been carried out for the ligands listed

'For Part XIV, see ref. 11. 'Permanent address: Department of Chemistry, Univer-

sity of Essex, Colchester, England.

above, and for R = 3-pyridyl and t-butyl. A number of solvents and solvent mixtures were examined in preliminary experiments, with approximately 50% aqueous acetone proving the most satisfactory in terms of solubility for both ligands and indium complexes.

There are relatively few references in the literature to stability constant measurements with these ligands, and none referring to com- plexes of Group IIIA cations. The ligand most widely studied has been 1 ,l,l-trifluoro-4(2- thieny1)-2,4-butanedione (TTA; 1, R = 2-thie- nyl), presumably because of the well established use of this compound in solvent extraction. Of special interest, however, are the studies of Van Uitert et al. (2) on the stability constants for the reaction of ~ a ' + , CuZ+, and Ni2+ with four of the ligands studied by us (1; R = methyl, naphthyl, thienyl, and furyl). Their results show many of the same general features found in the present work.

Experimental Materials

All the trifluoromethyl-8-diketonates were obtained from Eastman Organic Chemicals and were used as supplied. Acetone was Analytical Grade material.

Tetraethylammonium perchlorate was prepared by mix- ing hot aqueous solutions of tetraethylammonium bromide and sodium perchlorate, and recrystallized from water.

A weighed amount of indium metal (-1 mmol) was dissolved in concentrated aqueous hydrochloric acid, and the resultant solution evaporated to yield InCI,. - 3H20. This material was dissolved in 10 ml of 0.1 M perchloric acid in the preparation of the titration solution.

Can

. J. C

hem

. Dow

nloa

ded

from

ww

w.n

rcre

sear

chpr

ess.

com

by

UN

IVE

RSI

TY

OF

MIC

HIG

AN

on

11/1

0/14

For

pers

onal

use

onl

y.

CANADIAN JOURNAL OF CHEMISTRY. VOL. 50. 1972 2623

TABLE 1. pK, values for some I, I, I-trifluoromethyl-8-diketones in aqueous acetone --

pK, for RCOOH Literature pK, for pK, for RCOOH in 80% methyl

R P K ~ values CH3 .C0 .CH2.C0 .Ra in waterb cellosolvec

3-Pyridyl

Methyl

Phenyl

2-Naphthyl

"Reference 3: 75% dioxan, 30". bReference 4. 'Reference 5. dReference 6: 50% dioxan, 20". 'Reference 2: 75% dioxan, 30". 'Reference 7: 757' dioxan, 20°, ionic strength -0.1 ; ref. 76 is also quoted in ref. 8, p. 577. #Reference 7a: 75% dioxan (corr. to H,O), 20°, ionic strength -0.1. lReference 9: 25", ionic strength -0; also quoted in ref. 8, p. 590. 'Reference 10: 75% dioxan, 30".

Apparatus and Procedures Van Uitert et al., all measurements were carried out as For measurements of PK,, p H changes were recorded rapidly as possible but despite this, results at high p H were

with a Corning Model 10 pH meter and a glass electrode, irreproducible and no K, values could be obtained in highly calibrated for work in aqueous acetone. Solutions were alkaline solution. thermostated to 25 + 0.2". The results so obtained (Table 1) refer to -0.1 M ionic strength tetraethylammonium Of

perchlorate in 46% aqueous acetone (by volume) at a In the first series of experiments, P K ~ values were

diketone concentration of 0.02 M. obtained graphically; duplicate experiments agreed to The bulk phase in the stability constant measurements *0.02 pK units.

was 90 ml acetone, 100 ml aqueous (0.2 M ) tetraethyl- The potentiometric results in the stability constant

ammonium perchlorate solution, and 10 ml 0.1 M aqueous experiments were treated by a least-squares computer

perchloric acid containing 1 mmol In(II1) as the hydrated Program (IBM 360150) to give of E, the average chloride. This mixture was titrated with an acetone solution ligand number, and hence 8, and 8,. No significant changes of the ligand, previously neutralized with concentrated were found when of 81 and 8 2 were computed after

aqueous tetraethylammonium hydroxide solution and removing those experimental values, showing high devia-

made up to 10 ml with acetone, ~h~ mid-point of the tions in the least-squares analysis. The values in Table 2

titration curve therefore refers to a solution of approxi- are the mean at least and are

mately 0.1 M total ionic strength, in 46% aqueous acetone assigned a probable error f 0.05 units.

(by volume). Van Uitert et al. (2) have commented on the low chemical TABLE 2. Stability constants for

stability of 1,1,1-trifluoromethyl-8-diketones, and the indium(II1) complexes of some experimental problems resulting from this instability, which 1,1,1-trifluoromethyl-8-diketonates they ascribe to hydrolysis. While this fission reaction is likely to be of general importance for the diketones, we R observed that in the case of the methyl compound, the

log K, log K2

addition of excess tetraethylammonium hydroxide to an 2-Fury1 5.93 5.45 acetone solution of the diketone produced a red oil, and we 2-Thienyl 5.97 5.76 were therefore unable to carry out any stability constant Phenyl 5.85 5.95 measurements with this ligand. This red product probably 2-Naphthyl 6.93 6.65 arises from an intermolecular base-catalyzed condensation i-Butyl 6.78 6.40 between the acidic methyl group and the highly electrophilic t-Butyl 6.85 6.56 carbonyl group a to the trifluoromethyl group. Following

Can

. J. C

hem

. Dow

nloa

ded

from

ww

w.n

rcre

sear

chpr

ess.

com

by

UN

IVE

RSI

TY

OF

MIC

HIG

AN

on

11/1

0/14

For

pers

onal

use

onl

y.

2624 BOWDEN ET AL.: COORDINATION COMPOUNDS OF IN. PART XV

Results and Discussion

pK, Measurements The pKa values for the trifluoromethyl dike-

tones studied are given in the Table 1, together with earlier results in the literature, and relevant data for other acidic compounds containing the same substituent groups.

We note first that the pKa values of fl- diketones are strongly dependent on the solvent involved; Van Uitert et al. (10) have published graphs showing the changes in the measured pKa with composition in water:dioxane mix- tures. In view of this, a detailed comparison of our results with those of other workers is not productive, but we note that the order of pKa values is the same in both columns 2 and 3 of Table 1, with the sole exception of the methyl/ fury1 pair. The differences here are however small, and this apparent anomaly may well arise from errors in the measurements for CF,.CO.CH,.CO.CH, due to the instability of this compound in alkaline solution (see Experimental section). Similar comments apply to the comparison of the present pKa results with the values for the CH,.CO.CH, .CO.R dike- tones (column 4, Table 1).

Table 1 also shows that the various substi- tuent groups investigated have substantially parallel effects on the pKa's of CF,.CO.CH,. CO.R, CH3.C0.CH2.C0.R, and R.COOH over the relatively small 'range of values in- volved. Such linear correlations are to be expected, provided that there are no other compensating effects. The results in Table 1 refer to a range of aliphatic, aromatic, and heterocyclic substituents, so that a strictly linear relationship between the present results and the values in columns 5 and 6 of Table 1 would be unlikely. It is, however, of interest to note that the pKa values of the CF, .C0.CH2. C0.R compounds do in fact depend signif- icantly on the nature of R. This suggests that the structure of the anion involves considerable distribution of the negative charge onto all three possible sites (i.e., the central carbon atom and both oxygens), despite the presence of the very strongly electron-withdrawing tri- fluoromethyl group, which might have been expected to localize the anionic charge on sites proximate to this group.

Stability Constant Measurements The stability constants K, and K, obtained

in the present experiments are shown in Table 2. It is clear that all the complexes are thermo- dynamically very stable; the log fl, values are in the range 11-13, from which one may reasonably deduce that log fl, > 15. The impli- cations of this finding for the kinetic studies are discussed elsewhere (1). As in other studies with these trifluoromethyl-fl-diketones (2), the K, values are only slightly smaller than those for K1. The experimental errors for K, are greater than for K,, since the former arise from experimental results in alkaline conditions, where ligand decomposition is an important consideration. For the phenyl compound, we find K, slightly larger than K,; this is almost certainly an artefact arising from the reasons just outlined.

It is instructive to compare the results in Table 2 with those reported for the interaction of some of these ligands with BaZ+, Ni2+, and Cu2+ (2) (see Table 2). A dependence of, for example, pK, on pKa is generally found in such studies; in particular, the stability constants of fl-diketonate complexes show a monotonic increase with pKa for a variety of alkyl and aromatic terminal groups, over a pK, range of 6-14 (and a corresponding pK, range, for Ni2+, of 4- 12). Within this wide range however some non-linear relationships appear, and it is interesting to note that this is the case for complexes of certain of the present ligands with the three M2+ ions studied by Van Uitert et al. (2). A graph of the pKa values against pK, (Tables 1 and 2) demonstrates that there are also significant deviations from a linear relationship for the indium(II1) complexes. The results may of course have been affected by the instability of the ligands in alkaline solution. Alternatively, this non-linearity may be evidence of some unusual thermodynamic factors in these systems; in particular, steric interactions between CF, and R groups in a constrained conformation around the metal could lead to destabilization relative to the free ligand anion.

We note finally that the failure in earlier work (1) to obtain the In(py.CO.CH.CO. CF,), complex was presumably due to the choice of inappropriate experimental condi- tions; the extrapolated values for pK,, etc.,

Can

. J. C

hem

. Dow

nloa

ded

from

ww

w.n

rcre

sear

chpr

ess.

com

by

UN

IVE

RSI

TY

OF

MIC

HIG

AN

on

11/1

0/14

For

pers

onal

use

onl

y.

CANADIAN JOURNAL OF CHEMISTRY. VOL. 50, 1972 2625

which can be obtained from a comparison of E. P. SERJEANT. Ionization constants of acids and bases. Methuen and Co. Ltd., London. 1962. and show lhat this

5. W. SIMON et o l Zusammenstellung von scheinbaren be thermodynamically stable. dissoziation-konstanten in losunamittelsvstem methvl-

This work was supported in part by an Operating Grant (to D.G.T.) from the National Research Council of Canada. We wish to thank Mr. R. C. Ferguson for advice on computer programs.

1. (MRS.) G. T. TANNER, D. G. TUCK, and E. J. WELLS. Unpublished results.

2. L. G. VAN UITERT, W. C. FERNELRTS, and B. E. DOUGLAS. J. Am. Chem. Soc. 75, 457 (1953).

3. L. G. VAN UITERT, W. C. FERNELIUS, and B. E. DOUGLAS. J. Am. Chem. Soc. 75, 2736 (1953); 75, 2739 (1953).

4. (a) G. KORTUM, W. VOGEL, and K. ANDRUSSOW. Pure Appl. Chem. 1, 163 (1960). (b ) A. ALBERT and

- cellosolve-wasser. Vols. I, 11, and 111. Juris-Verlag, Zurich. 1959, 1962, 1963.

6. M. CALVIN and K. W. WILSON. J. Am. Chem. Soc. 67, 2003 (1945).

7. (a) L. G. VAN UITERT, W. C. FERNELRTS, and B. E. DOUGLAS. AEC Report NYD-626. 1951. (b ) L. G . VAN UITERT. Ph.D. Thesis. Pennsylvania State University, 1951.

8. Stability constants. Chemical Society Special Publica- tion No. 17, London. 1964.

9. M. CALVIN. Unpublished result. 10. L. G. VAN UITERT, C. G. HAAS, W. C. FERNELIUS, and

B. E. DOUGLAS. J. Am. Chem. Soc. 75, 455 (1953). 11. J. S. POLAND and D. G. TUCK. J. Organometal.

Chem. In press.

Can

. J. C

hem

. Dow

nloa

ded

from

ww

w.n

rcre

sear

chpr

ess.

com

by

UN

IVE

RSI

TY

OF

MIC

HIG

AN

on

11/1

0/14

For

pers

onal

use

onl

y.