Coordination Compounds of Indium. Part XXVI. Ligand Exchange Studies with Trifluoromethyl-~-diketonato(dimethyl)indium(III) compounds

H. L. CHUNG AND D. G. TUCK Department of Chernistr?, University of Windsor, Windsor, Ontario N9B 3P4

Received June 14. 1974

H. L. CHUXG and D. G. TUCK. Can. J. Chem. 52,3944 (1974). The ligand exchange between MezInL and H L (L = CF,. CO. CH. CO. R - anion; R =

methyl or t-butyl) has been studied by l9F n.m.r. spectroscopy, using benzene and di-isobutyl- ketone as solvents. The exchange is first order in both Me,InL and HL. Activation free energies were derived from mcasurenlents of the coalescence temperature. The results are compared with those of earlier studies. The rate-controlling process in the exchange is identified as the rotation of one monodentate diketonate ligand about a partial double bond prior to intra- molecular proton transfer to a second monodentate ligand.

H. L. CHUNG et D. G. TUCK. Can. J. Chem. 52.3944(1974). On etudie par spectroscopie r.m.n. du lgF l'echange de coordinats entre Me21nL et H L (L =

I'anion CF, . CO. CH . CO . R - ; R = methyle ou t-butyle) en se servant du benzene et de la di- isobutylcetone comme solvants. L'echange est du premier ordre pour Me21nL et pour HL. Des mesures de temperature de coalescence on deduit les energies libres d'activation. On compare ces res~~l ta ts avec ceux d'etudes anterieures. On identifie le rnecanisme qui contr6le la vitesse de l'echange comme etant la rotation du groupe dicetonate monodentate autour d'une liaison double partielle precedant le transfert intramolCculaire de protons a un second coordinat monodentate. [Traduit par le journal]

Introduction The study of the kinetics of ligand exchange

plays an important role in establishing an understanding of the behavior of the complexes of a metallic element. For indium, quantitative data in this field are sparse, despite the fact that rearrangement reactions occur easily with the- complexes of this element and have importait consequences in preparative work (2). In an earlier investigation (3), the exchange between InL, and trifluoromethyl-p-diketones HL (L = CF,. CO. C H . CO. R p ; R = methyl, i-butyl, phenyl, 2-naphthyl, and 2-thienyl) was studied in a variety of solvents. The exchange is fast and 19F n.m.r. methods proved to be a convenient way of obtaining kinetic data. The exchange was found to be first order in InL, concentration, but zero order in free ligand concentration, and the rate controlling process was identified as the

acetylacetonato(dimethyl)indiu~ii(III) was first reported by Coates and Whitcombe (4). The experimental methods used in the present work again involved I9F n.m.r. and the results are different from, but in general agreement with, those reported earlier (3). We may note that Glass and Tobias (5) have reported similar studies of, it~ter alia, Me,Ga(acac) and Me2T1- (acac) with free Hacac (acac = acetylacetonate anion) using proton n.m.r. methods, and identi- fied metal-oxygen bond cleavage as the rate- controlling process.

Some preparative work has also been carried out in the course of this study, and adducts of Me21n(tfac) (tfac = CF, . CO. CH. CO. CH,- anion) with some bidentate donors are de- scribed. The existence of such adducts bears upon the n~echanisn~ proposed for the ligand exchange process.

rotation of a monodkntate ligand about a partial Experimental double bond prior to intramolecular proton

Preparative transfer. Reagents and solvents were generally reagent grade We have now extended these studies to the materials; solvents were distilled over drying agents.

organo-indium species Me,InL, where L is Indium trichloride, prepared by passing chlorine gas again a trifluoromethyl-P-diketonate; the parent over indium metal heated to about 150 "C, was obtained

as a white powder. Trimethylindium was prepared (a) by the reaction of rnethvl lithium in ether 14.16 ml. 2.09 M)

'For Part XXV, see ref. 1. with indium trichloride (0.64 g, 2.90 mtnol) in'15 ml of

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CHUNG AND TUCK: COORDINATION COMPOUNDS OF INDIUM. PART XXVI 3945

TABLE 1. Analytical results and melting points for Me21n(CF3. CO . CH . CO . R) and adducts

Observed (%) Calculated (%) Melting point - --

Compound ( jc) C H In C H In

*N: observed, 6.09,; calcd., 6.29,. TP: observed, 9.1%; calcd., 8.99,. $See text.

TABLE 2. Nuclear magnetic resonance spectra of CF, . CO . CH, . CO . R, Me,In(CF3. CO. C H . CO. R), and adducts of Me,In(tfac) in CDCI3 at 35"

'H Chemical shifts

- (Hz)"

19F Chemical R y-proton CH3-In shifts (Hz)"

Adducts: bipyd 140 355 12,24 phene 146 357 12,29 diphosf 133 348 9,14

ORelative to external TMS; spcctrometer frequency 60 MHz. bRelative to external CF,.COOD; spcctrometer frequency 56.4 MHz. 'Center peak of multiplet. dMultiplet centered at 486 for free hipy appears at 501 Hz in complex. =Multiplet centered at 480 for free phen appears at 497 IIz in complex. fDiphos CH, triplet unchanged in complex at 137 Ilz.

ether, followed by distillation in cacuo (6) or (b) by heating a mixture of dimethylmercury (12 g) and indium metal (7.6 g) a t 135" for 1 week (7).

The three trifluoromethyl-13-diketonato(dimethy1)in- dium(II1) compounds (Me,In(CF3. CO . CH . CO . R) ; R = methyl, t-butyl, or phenyl) were prepared by mixing Me31n and the appropriate parent diketone in a I : 1 molar ratio in ether. The white solids obtained after evaporation of the solvent were dried in cacuo. The R = phenyl com- pound was not used in the kinetic studies, since the sub- stance is extremely hygroscopic and some difficulty was experienced in obtaining good crystalline material. Al- though the indium analysis of the freshly prepared material was satisfactory, it was not possible to obtain reliable carbon and hydrogen values because of the deliquescence.

Adducts of Me21n(CF3. CO. CH . CO . CH3) with the bidentate ligands 2,2'-bipyridyl (bipy), I ,lo-phenanthro- line (phen), and 1,2-bis(dipheny1phosphino)ethane (di- phos) were prepared by mixing chloroform solutions of the compounds concerned. The 1 : 1 adducts were re- crystallized from chloroform - petroleum ether and dried in vacuo. Analytical results for all the compounds con- cerned are presented in Table 1. Melting points are un-

corrected. Indium analysis was not possible for Me21nL- (diphos), since treatment with concentrated nitric acid gave turbid solutions which were not suitable for atomic absorption work, but satisfactory analyses were reported for C, H, and P.

Nuclear Magnetic Resonance Spectroscopy 'H and 19F n.m.r. spectra were obtained with a Jeolco

JNM-C-60 HL high resolution nuclear magnetic reson- ance spectrometer operating at oscillator frequencies of 60 or 56.4 MHz. Resonance frequencies were measured with reference to external standards of TMS and CF3- COOD, respectively. A large scale expansion and slow sweep time were employed in order to ensure accurate values in measurement of line-widths and coalescence temperatures. The temperature variation experiments employed a variable temperature controller, previously calibrated against a thermometer inserted in the n.m.r. sample chamber.

Kinetic Studies As in the earlier work (3), the I9r ' chemical shifts are

much larger than those for the proton resonance (see Table 2), and the kinetic studies are therefore based entirely on I9F measurements. Two series of experiments

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3946 C A N . J . CHEM. VOL. 5 2 , 1974

were carried out. 111 the first o f these, the order o f the reaction was determined by measuring the line-width, and hence the 19F n.m.r. lifetime, as a function o f the concen- tration o f complex and free ligand respectively. In the second series, the free energies o f activation for the ex- change in differing solvents were obtained from measure- ments o f the coalescence temperature o f solutions con- taining equimolar amounts o f complex and ligand. The d~fferences in the I9F chemical shifts o f the t n o reactants (Av in Table 4) were taken from measurements o f the spectra o f separate solutions o f the individual species in the solvent in question at 35" ( c .5 ref. 3).

Moleclrlar Weight Studies The molecular weights o f Me21n(CF,. CO . CH . CO . -

CH,) and its diphos adduct were measured in benzene in order to establish the degree o f association, using a Hewlett-Packard 302B vapor pressure osniometer. Re- sults were obtained at four different concentrations; benzil was used as a standard for calibration.

TABLE 3. "F n.m.r, line-widths at half-height as a function o f concentration in the system

Me,In(tfac)/Htfac/di-isobutylketone at - 20 "C

( a ) Varying Me,In(tfac), constant Htfac (0.094 n ~ m o l )

Line-width (Hz)* Me,In(tfac) added

(mmol) Htfac Me21n(tfac)

Results and Discussion (b) Varying Htfac, constant Me,In(tfac) (0.158 mmol)

The background theory to this work has been Line-width (H7)*

discussed in the previous paper (3) and need not Htfac be repeated here. added (mmol) Htfac Me,In(tfac)

19F Nuclear Magnefic Resonance Lifetime Measurements

The order of the exchange reaction can be established by following the lifetimes of the in- dividual species as a function of reactant con- centration, measuring the n.m.r. linewidths below the coalescence temperature. The system Me,In(CF, .CO . C H . CO. CH,)/CF, .CO .CH,. CO.CH, (= Me,In(tfac)/Htfac) in di-isobutyl- ketone was chosen for this purpose, since the 19F resonances are well-resolved (Av = 49 Hz at -20 "C) below the coalescence tempera- ture (1 6 "C). An additional convenience is that the indium complex is air stable. The results are summarized in Table 3. The changes in the volume of the solution due to addition of com- plex (or ligand) were sufficiently small that they could be neglected, so that concentrations were taken to be proportional to the quantities added.

Table 3a establishes that the half-width of the 19F resonance due to Me,In(tfac) is constant (average value 2.5 * 0.2 Hz) under the experi- mental conditions, whereas the corresponding quantity for free Htfac is a linear function of the concentration of that substance. Similarly, the results in Table 36 show a constant half- width for Htfac (average 3.9 * 0.1 Hz), and values for Me,In(tfac) which are linearly de- pendent on the complex concentration. The rate- law for the exchange between Me,In(tfac) and

0 - 1.2 0.085 4 .2 2.7 0.122 3.8 3.4 0.170 3.9 4 .6 0.200 3 . 8 4 .8 0.220 3.9 5 .9 0.249 3.9 6.1 0.294 3.9 6.9

*Estimated accuracy on lnd~vldual values i 0.2 Hz.

Htfac in di-isobutylketone at -20 "C is there- fore of the form

This expression is in marked contrast to that reported for the exchange between the tris- chelate complex In(tfac), and the free ligand, for which situation the rate-law was found (3) to be

We return to this difference below.

Experitnental Free Energies of Actiration The studies by Glass and Tobias (5) on ligand

exchange with the gallium and thallium ana- logs of Me,In(acac) gave values for EA, whereas the studies of In(tfac), and similar compounds lead to AG*. For reasons discussed earlier (31, we have restricted the present series of measure- ments to those systems in which coalescence of the two resonances could be observed, so that AG* values are again obtained. At coalescence one has

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CHUNG AND TUCK: COORDINATION COMPOUNDS OF INDIUM. PART XXVI 3947

TABLE 4. Free energies of activation for ligand exchange in Me,In(CF, . CO . CH . CO . R)/CF, . CO . CH, . CO . R systems

Benzene Di-isobutylketone

Av(IgF) T, Concentration AG* Av(19F) T, Concentration AG * R (HZ) ("c) (M) (kcal mol-') (Hz) ('C) (M) (kcal mol-')

eH3 53 32 0.34 13.4 58 16 0.46 12.8 t-C4H9 3 8 40 0.33 14.0 45 32 0.23 13.3

[3 I 117, = JZAv This process was shown to be in keeping with the known coordination chemistry of indium(II1)

where z, is the average lifetime of one reactant (31, and with the mechanism proposed for other molecule A, and AV is the chemical shift differ- similar exchange systems (10). The steady-state ence between the species A and B (8, 9). For a concentration of these five-coordinate indium- second-order process, as established in eq. (111) complexes is undetectably small (3, 11). above, Although such a process is readily invoked

[4I TA = llkc[Al for tris-chelate complexes, it seems an unlikely first step in the case of the four-coordinate

and AG* can then be obtained by substitution M ~ , I ~ L complexes. ~ h ~ ~ ~ - ~ ~ ~ ~ d i ~ ~ t i ~ ~ is rela- into the Eyring rate equation tively rare in indium(II1) chemistry, and those

hk, species which are formally InX, are strong [jl -AGi = RT, In --

K ~ T , electron pair acceptors, readily increasing the coordination number of the metal atom to four,

in which kc is the rate constant at the coales- five, or six by the coordination of neutral or cence temperature T,, k is the Boltzmann con- anionic ligands (12). It therefore seems most stant, and K is the transmission coefficient, to unlikely that the process which a value of 0.5 is assigned. The values of Me AG* obtained by this procedure are given in [71

Me\In yo) -----L \ ~n-0-0 / \ - / Table 4. Me 0 Me

Reaction Mechanism can lead to any significant formation of three Rapid ligand exchange occurs in each of the coordinate species in solution. Glass and Tobias

systems studied. We may also report here that (5) suggest that this process occurs with the accurate measurements could not be made in gallium analog, but this step is not in fact dimethylsulfoxide solution because the coales- crucial to their argument for gallium, although cence temperature lies below the accessible tem- it is a likely process in such six-coordinate perature range (freezing point of dmso + 18.5 species as Me,Sn(acac),, which they also discuss "C), in each case, so that the values of AG * must in their paper. lie below 12.5 kcal mol-I (Av = 125 Hz). Simi- In establishing the mechanism, we first note larly, experiments with acetonitrile did not lead that Me,In(tfac) in benzene is a monomer to detectable splitting of the single peak down (mol. wt. found 289; calcd. 298), and it seems to - 38" in the case of R = t-butyl (Av = 28 Hz; reasonable to assume that the same holds for AG* < 10.4 kcal mol-I). the t-butyl compound. Since benzene is a poor

The primary step in the ligand exchange donor towards indium(III), the formation of mechanism for the six-coordinate InL, com- dimeric or higher species in donor solvents is plexes was assumed to be the opening of one unlikely, and the dimethyl complexes are there- chelate ring to give a five-coordinate species with fore taken to be monomeric under the condi- one monodentate ligand, tions of the present experiments. A second im-

T O portant point is that Me,In(tfac) is an acceptor O\ I which forms adducts with donor ligands (see

[61 In o/l"O-O below). In the light of these experimental ob- LO servations the proposed mechanism is

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3948 C A N . J . CHEM. VOL. 5 2 , 1974

The concentration of the proposed neutral five- coordinate intermediate [Me,In(tfac)(Htfac)] is clearly given by the concentration product K , [Me,In(tfac)][Htfac], and the experimentally determined kinetic order could be taken as identifying the formation of this species as the rate controlling step. There is, however, some inforniation on the analogous 4 tt 5-coordina- tion change in anionic indium(II1) and thallium- (111) chloride complexes. and it seems that the equilibrium

has a small equilibrium constant (K = 4.5 for Tl(II1)) (13). Similarly, it has been argued that preparative evidence points to small energy differences between InCIs2- and InC1,-, and between InX,L and InX3L2 adducts (12, 14, 15). While these statements apply to equilibrium studies, there is equally no evidence for a high activation energy barrier in such processes: in- deed such evidence as exists implies that change of coordination number requires little activa- tion energy in indium(II1) systems. A Illore obviously relevant system is the 6 -+ 5-coordina- tion change in the isomerization of In(tfac),, where the activation free energy is less than 11.0 kcal mol-I ( l l ) , which is lower than the activation free energy for liga~ld exchange in either In(tfac), or Me,In(tfac). We therefore assume that the formation of the intermediate [Me,ln(tfac)(Htfac)] is not the rate-controlling process in the present system, that the forward reaction (step I ) is fast, and the equilibrium con-

stant for the formation of the intermediate is small.

This leaves either the ring-opening process (step 2) or the intramolecular proton transfer (step 3) as possible rate-controlling processes. Without repeating the arguments presented in ref. 3, it seems that the more likely process is step 3, in which the partial double-bond charac- ter of the rnonodentate ligand system hinders the rotation which must precede proton trans- fer froin one monodentate diketonate ligand to the other. The rate is given by:

in agreement with the experimental result (es. 1).

The experiinentally determined activation free energies displayed in Table 4 are some 3-6 kcal mol-' lower than those reported previously for exchange in InL,/HL systems. This seems not unreasonable, since the ligand rotation process in the InL, case is occurring within the con- straints of a six-coordinate species, whereas in the present work the transition state is in the more open four-coordinate complex. The order of activation free energies with respect to R and solvent is similar to that found in the earlier work: in particular, basic solvents enhance the rate of exchange, presumably by acting as a bridge in the intramolecular proton transfer process. In general, the kinetic evidence on ex- change in InL,/HL and Me,lnL/HL systems supports a simple ~nechanism which applies to both systems.

Preparatiz.e Ecidence A large number of adducts of neutral and

anionic indium(II1) halides, pseudohalides, di- thiolates, etc.. has been described in the litera- ture (12), and, as noted above, the existence of such adducts supports the formation of the species [Me,In(tfac)(Htfac)] in step I of the reaction mechanism. It seemed worthwhile to confirm the acceptor properties of Me,In(tfac) by the preparation of addition compounds with 2.2'-bipyridyl, I , 10-phenanthroline, and 1,2-bis- (dipheny1phosphino)ethane. The 1 : 1 adducts are air-stable crystalline materials, white (diphos) or pale yellow (bipy, phen) in color, and soluble in chloroform. The 1,lO-phenanthroline com-

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CHUNG AND TUCK: COORDINATION COMPOUNDS OF INDIUM. PART XXVI 3949

plex is a monomer in benzene solution (mol. wt. found 522, calculated 478). The compound Me,In(tfac)(bipy) is the methyl analog of Cl,In(acac)(bipy), whose preparation was re- ported elsewhere (I), and whose structure (16) has been shown by X-ray methods to be that of the cis-isomer of a six-coordinate InO2N2Cl2 species, so that it seems safe to conclude that the adducts of Me21n(tfac) are also six-coordinate indium(Il1) complexes.

The 'H and 19F n.m.r. spectra of these addi- tion compounds are summarized in Table 2. In freshly prepared solutions, all three compounds show two resonances due to the methyl groups attached to indium, confirming the cis-stereo- chemistry proposed above, since only one reso- nance would be observed for a trcms-octahedral species. There are a number of interesting stereo- chemical problems with these InO,N,Me, com- pounds, but these problems are more suitably discussed in terms of the five- and six-coordinate adducts of dimethyl(acetato)indium(TII), since the crystal structure of this compound is known (17); these matters will be discussed in a future publication (18). In general, the preparative work described here, and that on the derivatives of Me,Sn(OAc) confirms the reality of the equilibrium I i n the proposed reaction mechan- ism.

This work was s~~pported in part by Operating Grants from the National Research Council of Canada.

I . J. G. CONTRERAS and D. G. TUCK. J. Chem. Soc. Dalton. 1249 (1974).

2. D. G. TUCK. Proceedings of C.N.R.S. Conference No. 191, Editions ~ L I C.N.R.S. 1970. p. 159.

3. G. M. TANNER, D. G. TUCK, and E. J. WELLS. Can. J. Chem. 50, 3950 (1972).

4. G. E. COATES and R. A. WHITCOMBE. J. Chem. Soc. 3351 (1956).

5. G. E. GLASS and R. S. TOBIAS. J. Organometal. Chem. 15, 481 (1968).

6. H. C. CLARK and A. L. PICKARD. J. Organometal. Chem. 13, 61 (1968).

7. L. M. DENNIS, R. W. WORK, and E. G. ROCHOW. J. Am. Cheni. Soc. 56, 1047 (1934).

3. J. A. POPLE, W. G. SCHNEIDER, and H. J. BERXSTEIN. High resolution nuclear magnetic resonance. Mc- Graw-Hill, New York. 1959. p.223.

9. H. S. GUTOWSKY and C. H. HOLM. J. Chem. Phys. 25, 1228 (1956).

10. A. C. ADAMS and E. M. LARSEN. Inorg. Chem. 5, 814 (1966).

11. R. C. FAY and T. S. PIPER. Inorg. Chem. 3, 348 (1964).

12. A. J. CARTY and D. G. TUCK. Prog. Inorg. Chem. In press.

13. D. F. SHRIVER and I. WHARF. Inorg. Chem. 8, 2167 (1969).

14. A. J. CARTY and D . G. TUCK. J. Chem. Soc. A, 1081 (1966).

15. J. GISLASOK, M. H. LLOYD, and D. G. TUCK. Inorg. Chem. 10, 1907 (1971).

16. J. G. COZITRERAS, F. W. B. EINSTEIN, and D. G. TUCK. Unpublished results.

17. F. W. B. EINSTEIN, M. M. Gilbert, and D. G. TUCK. J. Chem. Soc. Dalton, 248 (1973).

18. J. J. HABEEB and D. G. TUCK. Can. J. Chen~. This issue.

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