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Kinetics of aquation of Cis- and trans-tetrachlorodipyridineiridate(III) anions and chloride anation of cis- and trans-trichloroaquodipyridineiridium(III)
J. inorg, nucl. Chem., 1969, Vol. 31, pp. 2871 to 2882.
Pergamon Press.
Printed in Great Britain
KINETICS OF AQUATION O F CIS- A N D T R A N S TETRACHLORODIPYRIDINEIRIDATE(II I) A N I O N S AND CHLORIDE ANATION O F CIS- A N D TRANS-TRICHLOROAQUODIPYRIDINEIRIDIUM(III)* J. B. TIRSELL and C. S. G A R N E R Department of Chemistry, University of California, Los Angeles, Calif. 90024
(First received 9 December 1968; in revised form 27 January 1969) A b s t r a c t - T h e kinetics of the reactions trans-Ir(py)2C14-+H~O ,
kls k31
• trans-Ir(py)2(OHz)Cl3+ CI-
were studied in 0.01-3.7 F HCI or HCIO4 (/z := 0.01-3.7, NaCI and/or NaCIO4) at 110"00°C. In 0"5 F HCI (/z =: 3.7), k13 = (8 - 1) x 10-s sec -1 and k3~ = (4.4_+ 0.8) x 10-4 M -t sec -~, nearly independent of C0 (0-02-0"5 mF), [H +] (0-1-0-5 F, at 1'7 F C1-), and [CI-] (0-5-3'7 F, at0.5 F H+); increase of/x(0.5-3.7) increases k3t, but has only a small effect on k13. The kinetics of aquation of cis-lr(py)2C14- was examined in 1-2.5 F HCIO4 (~ = 1-3-7, NaCIO4) at 90-110°C. The products are 5% trans-, 28% cis(sun)-, and 67% cis(autoclave)- Ir(py)2(OH2)C13; the former is formed directly, but relative amounts of the latter two isomers are apparently determined by their faster isomerization. The total aquation rate constant is (1.3 _+0.1) × 10-4 sec -t at I10.00°C in 1 F HCIO4 (/z = 3.7), nearly independent of C0(0"02-1"6mF), [H+](1-2-5 F), and /z(1-3.7); Ea=34+--4kcal mole -t, logPZ (sec -1) = 15.5+2.3. Chloride anation of cis(autoclave)-lr(py)2(OH2)Cl3 is complicated by competing isomerization. At II0.00°C the anation rate constant appears to be ca. 2× 10-4M -~ sec -~ in 1 F H ÷ (/~ = 3-7) from 0.5-3.7 F CI-. Accurate u.v. and visible absorption spectra of the complexes are presented.
THE SYNTHESIS and general chemical behaviour of cis- and trans-tetrachlorodipyridineiridate(III) anions, cis- and trans-Ir(py)2Ch-, and the three geometric isomers of trichloroaquodipyridineiridium(III), Ir(py)2(OH2)Cl3, have been investigated by Delrpine, [ 1-3] but apparently no kinetic studies of their reactions have been reported. In connexion with a programme of studying the kinetics and stereochemistry of hydrolysis and anation of iridium(Ill) complexes, we undertook a kinetic investigation of the aquation of cis- and trans-Ir(py)2C4- and the chloride anation of the Ir(py)..,(OH2)CI3 isomers, which we report here. EXPERIMENTAL
Preparation of complexes. Commercial (NH4)~[IrCI6] (Fairmount Chemical Co., Inc., Newark, N. J.), found spectrographically to contain <0"05 per cent of any of the other Pt metals, was the source of all Ir compounds. The method of Kauffman[4] was used to prepare orange cis- and red *Contribution No. 2288. 1. M. DeMpine, (a) C.r. hebd. Sernc. Acad. Sci., Paris 175, 1075 (1922); (b) 200, 1373 (1935); (c) 233, 1156, 1533 (1951); (d) 234, 1721 (1952); (e) 236, 1713 (1953); (f) 238, 27 (1954); (g) 240, 2468 (1955); (h) 242, 27 (1956). 2. M. Delrpine, (a)Annls Chim. Phys. [9], 19, 5,145 (1923); (b) [11],4,271 (1935). 3. M. Delrpine, Recl Tray. Chim. Pays-Bas Belg. 59,486 (1940). 4. G. B. Kauffman, lnorg. Synth. 7,228 (1963). 2871
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J . B . T I R S E L L and C. S. G A R N E R
trans-pyH[lr(py)2Cl4]. Anal. Calcd. for C5H5NH[Ir(C5HsN)2CI4]: C, 31.48; H, 2.82: Ir, 33.58. Found: (trans): C, 31.87; H, 3.17; Ir, 33.67; (cis): lr, 33.76. The pyridinium salts were converted to the NH4 + (cis) or Na ÷ (trans) salts by treatment with a stoichiometric amount of NH4OH (cis) or NaOH (trans) on a steam bath for 5 min, followed by evaporation to crystals at ca. 20°C, recrystallization from hot HzO, then washing with EtOH and Et20. The method of Del~pine [2b] was employed to convert cis-NH4[Ir(py)2 C14] and trans-Na[Ir(py)2Cl4] to orange-yellow cis(autoclave)*- and orange-red trans-Ir(py)2(OH2)Cl~, respectively, except that aquation was carried out at 110°C (instead of 130°C) and for only 3 hr (cis) or 7 hr (trans) (instead of 1 or 0.5 hr, respectively). The crystals were air-dried. Anal. Calcd. for cis-[Ir(CsHsN)z(OH2)CI3] • 1.5 H20: Ir, 38.30. Found: Ir, 38.22-38.43. Calcd. for trans-[Ir(CsH~N)~(OH2)C13]HzO: C, 24.37; H,2.86; N,5.68; Ir, 39.00. Found: C,24.12; H,3.06; N, 5.67; Ir, 38.37-38.64. Orange-yellow cis(sun)-[Ir(py)~(OH2)Cl3].2H~O was made by modifying the Del~pine[lh] method as follows, A solution of cis-NH4[Ir(py)2Cl4] in H20 (1 g in 10 ml) was photolyzed at 33°C with a 250-W incandescent lamp at ca. 5 cm for 2 days. Fine crystals of a foreign substance (polymer?) formed and floated on the surface of the solution; these were decanted off. Orange-yellow crystals, which had separated and sunk to the bottom, were filtered off, washed with cold H20, then with EtOH and EhO, and air dried. Anal. Calcd. for [Ir(CsHsN)z(OH2)CIa]'2H20: Ir, 37"63. Found: Ir, 38"71-38.83. Other chemicals. Reagent grade Na2COz was neutralized with reagent grade HCIO4, and the solution evaporated to crystallization to prepare NaCIO4, which was recrystallized twice from H20. All other chemicals were reagent grade. The water was double distilled, then passed through a mixedbed cation-anion resin column and monitored for purity by electrical conductivity. Chemical analyses. Solid I r compounds were analyzed for I r as described previously, [5] and for C, H, and N with standard microanalytical procedures. Kinetic and stereochemical experiments. Attempts to follow reaction rates by determination of released CI- failed. Titration of CI- by AgNOa, either potentiometrically at 0°C in aqueous acetonedetergent solution [6] or by the Volhard method, gave erratic results in the presence of the Ir complexes. We were unable to separate CI- chromatographically for titration in the absence of Ir complexes; although CI- could be separated quantitatively from the Ir(py)zC4- isomers adsorbed on a NO3- Dowex A G I - X 8 (100-200 mesh) column from H20 solution, contamination with lr(py)z(OH2)CIa isomers (and in some cases, lr(pyh(OH2)Cl2 +) was always present. Even from0.3 F HCIO4, only ca. 94 per cent ofcis-Ir(py)2Cl4- could be adsorbed, the rest coming through with the C1-. Accordingly, the rates were followed spectrophotometrically. Even here band intensities in the visible absorption spectra (Fig. 1) and solubilities of some complexes were so low that this approach would have given large errors or, for the more soluble complexes, consumed large amounts of the complexes. Because cis- and trans-lr(py)2Cl4- form insoluble compounds when oxidized to lr(IV), we could not use the method of oxidizing reaction aliquots with CI2 (to give Ir(IV) complexes with band intensities normally 20-40 times those of the Ir(lll) analogues) found so useful by us in our earlier Ir(IIl) studies[5, 7, 8]. Hence, changes in the u.v. spectra with time were used. As Fig. 2 shows, the regions around 2 5 0 - 2 6 0 n ~ and 320-330m/z offer reasonable absorbancy changes; the former region, however, is subject to errors arising from absorption by pyridinium ion or other contaminants and was found less suitable than analysis at 320 m/z. Determination of the isomeric distribution of reaction products, however, was essentially impossible in the u.v. since the isosbestic points predicted from Fig. 2 were nearly the same for the various pairs of potential reaction products. Hence, in several runs substrate concentrations were • Deirpine[l-3] has used the terms "sun" and "autoclave" isomers to describe, respectively, the two isomers of Ir(py)s(OH2)CI3 made from aqueous cis-lr(py)2Ch- by a photochemical reaction with sunlight, and by heating in an autoclave to 130°C. These two isomers have the pyridine ligands cis to each other, with the water ligand trans to either a pyridine or chloro ligand; which of these two configurations is the "sun" and which the "autoclave" isomer has not been established. In trans-lr(py)z(OH2)C13 the pyridine ligands are trans to each other. 5. 6. 7. 8.
J. C. Chang and C. S. Garner, lnorg. Chem. 4,209 (1965). D.J. MacDonald and C. S. Garner, J. inorg, nucl. Chem. 18, 219 ( 1961). I.A. Poulsen and C. S. Garner, J.Am. chem. Soc. 84, 2032 (1962). A. A. EI-Awady, E. J. Bounsall, and C. S. Garner, lnorg. Chem. 6, 79 (1967).
Cis- and trans-tetrachlorodipyridineiridate(l 1i) anions WAVENUMBER.
KK
2873
18
4O i
E
o T
x" Z >(J Z
<
~n n~
o~o O0 m re" ..J 0
~E 10
\
\ -400
450 500 WAVELENGTH,
sso
600
mp
Fig. 1. Visible absorption spectra of chloro dipyridine iridium(Ill) complexes in 1 F HC104 at 20-25°C: TTC, trans-Ir(py)2Cl4-; CTC, cis-Ir(py)2CI4-; TTCA, transIr(py)2(OH2)CI3; CATCA, cis(autoclave)-Ir(py)2(OH2)Cl:6 CSTCA, cis(sun)-Ir(py)2(OHm)C13. adjusted as high as possible to permit a search for isosbestic points in the visible region, where the differences predicted from Fig. I are much greater, despite the relatively low intensities involved. Reaction solutions were prepared by dissolution of weighed Ir complex in solutions of appropriate acid concentrations and ionic strengths, and sealed inside Pyrex ampoules. These were wrapped with A1 foil to exclude light, heated in 2-5 min to within a few degrees of bath temperature, then placed in thermostat baths at 90.00 ± 0'03,100.00 ± 0.08, or 110.00 ± 0.05°C. Samples were quenched in ice at known times. With a few exceptions, kinetic run samples were diluted with H20 or acid to give solutions 1 F in HC104 for the u.v. spectral scans in a Cary Model 15 spectrophotometer. Matched 1-, 5-, or 10-cm quartz cells were used, with the reference cell normally filled with the same solution except for absence of Ir species. Molar absorbaney index plots. These were calculated from spectra of fresh solutions of tLe weighed lr complexes, using a Cary 15 spectrophotometer and matched quartz cells. RESULTS AND DISCUSSION
Visible and ultraviolet absorption spectra. Absorption maxima, and in some cases spectral plots, have been reported for the complexes,[9, 10] but these were too incomplete or inaccurate for use in spectral study of the kinetics and stereochemistry. In Figs. 1 and 2 we give the visible and u.v. absorption spectra, respectively, of the complexes measured in our laboratory. As expected, substitution of ligand H20 for ligand C1 shifts the d-d bands to higher energies (Fig. 1). Wavelengths of the absorption bands agree to within 2 mt~ and their molar 9. 1. Yasuo and Y. Kondo, J. chem. Soc. Japan (Pure Chem. Sec.) 72,787 ( 1951 ). 10. C. K. Jergensen,Acta chem. scand. 11, 151 (1957).
2874
J . B . T I R S E L L and C. S. G A R N E R
46
WAVENUMBER, KK
42
38
34
30
~TTCA
TE9 x"
UJ7 >-6 0 Z
o
o ~2 X ~t
250
300
350
400
WAVELENGTH, mlJ Fig. 2. Ultraviolet absorption spectra of complexes of Fig. 1 in 1 F HCIO4 at 20-25°C.
absorbancy indices to within 5-15 per cent with values reported by JCrgensen, [10] except for cis(autoclave)- and trans-Ir(py)2(OH2)Cla, for which we find bands at 452 m/z (aM, 24"4) and 495 m/z (aM, 9.7), respectively, vs. ca. 445 m/~ (aM, 28) and ca. 475 m/z (aM, 17) given by JCrgensen.'* Isomerization, polymerization, and oxidation o f trans-lr(py)2Cl4-. Delrpine [2a] has reported that trans-Ir(py)2C14- is oxidized to insoluble deep violet Ir(py)2Cl4 (presumably the trans isomer) almost immediately by C12, Br2, 15 F HNOs, and aqua regia at room temperature. We find that oxidation occurs slowly in 1 F HCIOa at 90-110°C, but apparently some catalytic impurity is involved since in two cases we were able to carry out aquation kinetic runs in 1 F HCIO4, one at 90°C and one at 110°C, without any evidence of oxidation. At 20-25°C, HC104 of concentration 2-5 F or greater oxidizes trans-Ir(py)2Cl4- overnight. Evidence that oxidation is occurring, rather than polymerization, is found in the fact that the deep violet precipitates obtained this way could be reduced in acidic solution to a soluble complex by the action of HzO~, I-, or Fe z+. Delrpine[2b] has stated that trans-Ir(py)2Cl4- in H20 heated at 130°C for 1 hr generates an insoluble rose polymer, [Ir(py)zCla],, in addition to transIr(py)2(OH2)CIa. Although we have seen a rose precipitate forming in acidic solutions at 110°C after prolonged heating, neither oxidation nor polymerization *In Ref. 10 JOrgensen states that "trans- and cis-(autoclave)-Ir py2Cla(H20) are so weakly soluble in water that it is difficult to observe the ligand field triplet transition." For the trans complex the 495-m/~ band had intensities of only 0.1-0.3 absorbancy unit in the solutions we measured; absorbancies o f ca. 1 unit were possible for the cis complex.
Cis- and trans-tetrachlorodipyridineiridate(I I I) anions
2875
was observed during the reaction times on which we have based our kinetic calculations. We made a search for possible isomerization of trans-Ir(py)~Cl4- in 1 F H C I 4 F LiCI at 100°C and in 0.5 F H C 1 - 3 , 2 F NaCI at 110°C to see if isomerization would interfere with aquation and anation studies. The high CI- concentration represses aquation, our aquation-anation stereochemical and rate data (vide infra) implying an equilibrium under these conditions with ca. 93-97% trans-Ir(py)2C14and 7-3% trans-Ir(py)2(OH2)Cl3. In 137 min at 100°C intensities in the 320-m/x region decreased ca. 0.5 per cent; an absorbancy decrease there is expected upon reaction to produce any other complex of Fig. 2 (or upon polymerization or oxidation). Intensity increases of ca. 1 and 5 per cent in 17 and 475 min at 110°C at 465 m/z were observed, compatible with the above predicted extent of conversion to trans-Ir(py)2(OH2)Cls. However, conversion to any other component of Fig. 2 would also be compatible (but not polymerization or oxidation). Although at least one isosbestic point in the visible would occur if a single product were formed (see Fig. 1), no isosbestic points were observed. Moreover, little change in absorbancy occurred above 515 m/x, although decreasing absorbancy there would be expected. These anomalies may be partly due to the necessarily low intensities (A <~ 0.3; see Experimental), or possibly the trans-lr(py)2Cl4- contained a small percentage of some impurity. If we take 1 per cent as a maximum extent of isomerization of trans-Ir(py)2C14- (corresponding to the total inclease in absorbancy at 465 m/z in 475 min assuming the only change is isomerization), we obtain k12* = 4 × 10-7 sec -1 for an upper limit of the first-order isomerization rate constant at 110°C. This is insufficient to interfere with the aquation and anation kinetic studies. Isomerization, polymerization, and oxidation of cis-lr(py)2Cl4-. Delrpine[2a] found that C12, aqua regia, or fuming HNO3 in.large excess (but not Br2) oxidizes cis-Ir(py)2Cl4- to insoluble black Ir(py)2Cl4 (presumably the cis isomer). We found no evidence in our kinetic or stereochemical runs for any oxidation of cislr(py)2Ch-.
Formation of a deep orange polymer, [Ir(py)zCl3]n, on heating solid cispyH[Ir(py)2CI4] at 140°C was noted by Delrpine,[2b] but no polymer formation from aqueous solution was reported. No opalescence was seen by us in our cisIr(py)2Cl4- aquation kinetic studies, although a trace of precipitate was noted on heating the complex in H20 at 110°C for 27 hr. An isomerization search was made in 1 F H C I - 2 . 7 F NaCI at I I0°C, in which medium our kinetic and stereochemical aquation results (vide infra) suggest aquation of cis-Ir(py)2Cl4- with a half-life of ca. 17 min to an equilibrium state of ca. 80% cis-Ir(py)2Cl4- and ca. 20% cis(autoclave)-Ir(py)z(OH2)Cls (perhaps with a smaller percentage of cis(sun)-Ir(py)z(OHz)Cl3 supplanting some of the cis(autoclave) isomer). The u.v. and visible spectra taken at 0, 25, and 1145 min showed changes qualitatively compatible with this extent of aquation. As Fig. 1 shows, concurrent isomerization of cis-Ir(py)zCI4- to any significant extent would disrupt the changes seen and, in particular, would cause the absorbancies in the *Rate constant k12 is for direct production of complex 2 from complex 1 of Fig. 4; this notation is used for all other k values given.
2876
J.B. TIRSELL and C. S. GARNER
460-m/z region to decrease appreciably. Such was not observed, even on extending the reaction time to 1960 min. Conservatively taking 5% isomerization as the maximum possible in 1960 min, we estimate 5 × l0 -7 sec -1 at 110°C for an upper limit of the isomerization rate constant. This is too small to allow significant interference with the aquation and anation kinetic studies. Isomerizatlc,n and polymerization o f Ir(py)~(OH2)Cla isomers. Polymerization of cis(autoclave)- and cis(sun)-Ir(py)2(OH2)Cl3 hydrated solids to insoluble deep orange [h(py)2Cl3]n and of solid trans-[Ir(py)2(OH2)Cl3]'H20 to insoluble dark [lr(py)2CI3]2 at 140-150°C was reported by Deldpine[2b]. We have noted no insoluble substances forming during our kinetic runs with cis-lr(py)2(OH2)Cla isomers in aqueous solution; with trans-Ir(py)2(OH2)Cl3 in aqueous solutions we observed opalescence in some runs, but only after 25-100 half-times of the anation reactior. A search was made for possible isomerization of cis(sun)-Ir(py)2(OH2)Cl3 in 2.5 F H C I O 4 - - 1 . 2 F NaCIO4 at 110°C by scanning the visible spectra at 0, 30, 76, and 120 min. The first three scans gave an isosbestic point at 451 m/z (aM, 24"2), in excellent agreement with 451 m/z (aM, 24"4) expected from Fig. 1 for isomerization to the cis(autoclave) isomer (a second isosbestic point expected at 524 m/z was not observed, but aM is only 4 and the actual crossing of the cis(sun) and cis(autoclave) curves in that region is not certain); isomerization to the trans isomer would have given an isosbestic point at 498 m/x (aM, 9"6). The shape of the 76-min scan and its molar absorbancy indices at 430-470 m/x were found by trial-and-error calculations to agree with those predicted for isomerization lo an equilibrium mixture of 70% cis(autoclave) and 30% cis(sun) isomers. The 120-min scan had the same shape, but was displaced to slightly lower absorbancies, suggesting onset of polymerization or oxidation, since aquation should result in a shift in wavelength of the absorption band and is probably too slow to compete in this reaction time. We cannot be sure the equilibrium ratio has been correctly determined, but if we assume equilibrium at 30% cis(sun) isomer, we calculate for the isomerization:
cis(autoclave)-Ir(py)2(OH2)Cla .
k54 k45
• cis(sun)-Ir(py)2(OH2)C13
(1)
an equilibrium quotient Ki = k54/k45 -- [cis(sun)] / [cis(autoclave)] -- 0.30/0.70 -- 0.43 at ll0°C. From the cha0ge in spectra with time, we estimate the half-time of approach to equilibrium as ca. 20 min, which together with Ki gives k54 ~ 2 X 10 - 4 sec -1 and k45 ~ 4 × 10-4 sec -~ at 110°C (accurate determination of h/2 was impossible since equilibrium was never attained).
Aquation of trans-lr(py)2Cl4- and Cl- anation o f trans-lr(py)2(OH2)Cl3. Spectral scans taken during aquation of trans-Ir(py)2Cl4- at 110°C up to ca. 80 per cent aquation exhibited isosbestic points in the u.v. and visible in good agreement with values predicted from Fig. 1 and 2 for aquation to trans-Ir(py)2(OH2)CIs as sole Dr product; in the visible spectra isosbestic points were observed at 451-453 ~ (aM, 6"6--6"8) and 504-507 m/~ (aM, 9"2--9"9) VS. predicted values of 453 m/~ (aM, 6"9) and 506 rn~ (aM, 9"3). Significant aquation to either or both cis-Ir(py)2(OH2)C13 isomers would seriously alter the first of the above two isosbestic points.
Cis- and trans-tetrachlorodipyridineiridate( I I 1) anions
2877
In CI- anation of trans-Ir(py)2(OH2)Cl3 at I10°C the expected isosbestic points corresponding to anation solely to trans-lr(pyhCl4- were found in the u.v. scans. In the visible region scans taken at 9 and 13 min gave "isosbestic points" at 450 mix (aM, 6"6) and 506 rn~ (aM, 8"8), scans at 13 and 26min gave values of 453 and 504 m/x, and scans at 26 and 40 min gave 454 and 521 m/~. Under the conditions involved, the reaction half-time was 13 min, so beyond two half-times some reaction is occurring in addition to anation to trans-Ir(pyhC14-. Opalescence due to polymer formation (and/or to possible oxidation if C104- was present) was sometimes noted, but only after 25 to 100 times the anation half-time. Del6pine[3] reported that prolonged treatment of translr{py)dOH2)Cl3 with 1.2 F HCi on a steam bath gave trans-H[lr(py)2C14] and some polymer. An alternative but less probable explanation for the shifting "isosbestic points" observed after 26 min at 110°C lies in the possibility of a small contribution from aquation of trans-lr(pyh(OH2)CI3, even in the sometimes high concentrations of CI- involved. We allowed trans-Ir(py)2Cl4- in 1 F HCIO4 to aquate at 110°C for 25 half-times (95 hr) then diluted an aliquot four-fold with 0.001 F HCIO4, charged the solution onto a 10-cm × l-cm diameter column of H ~ Dowex AG50W-X8 (100-200 mesh), eluted off anionic and neutral species with 50ml of 0.01 F HCIO4, then eluted what we believe to be Ir(py)dOH2hCl2 + (unknown configuration) with 50 ml of 0.2 F HCIO4. The elution behaviour is characteristic of a trans-dichloro species of 1+ charge. The effluent was too dilute to measure the visible absorption spectrum, but an absorption band was noted at 295 m/z. Recently Lar6ze[11] has reported the isolation of two solids. [lr(py)z(OH.,)zC12]CI and [Ir(py)2(OH,,)2CI2]CI.H,,O, obtained from translr(py)2{OHOCla via base hydrolysis thermally and photochemically, respectively. No u.v. spectra were given but a shoulder was reported at 465 m/x (aM, 8"5) and 470 m/z CaM.8"3), respectively. Since both cis-Ir(py).,(OH2)Cl:] isomers have their visible absorption bands at lower wavelengths, whereas trans-lr(py),,(OH,,)C13 has its band at 495 m/z, these two preparations of Lar~ze are probably mainly transdipyridine complexes (not necessarily pure, although two such geometric isomers are possible, as well as three in which the pyridine ligands are cis to each other). The kinetic aquation and anation data at 320 m/.t were plotted in terms of the relation: 2"30 log [(Ao -- Ae)/(A - Ae)] = (k,:~+ ka,[Cl-l)t
{2}
where Ao and A are the optical absorbancies at time zero and time t, respectively, Ae is the equilibrium absorbancy, [C1-] was taken equal to the total CI- formality (this remained essentially constant in each anation run), and k13 and k3, are the first-order aquation and second-order anation rate constants for the reactions kl.l
trans-Ir(py)2C14- + H 20 ~---- trans-lr(py)2(OH2)Cl:~ + CF. k31
(3)
For the first run of Table 1 (no CI- initially), Ae was calculated from the spectrum of trans-lr(py)2(OHjC13; for the other runs Ae was evaluated experimentally 11. F. Lar~ze, C.r. hebd. Se6nc. Acad. Sci., Paris 265, 307 (1967).
2878
J . B . TIRSELL and C. S. G A R N E R
from the last scan showing little further change in A. Reaction half-times were read off the plots and the total pseudo first-order rate constant, kla + k31[Cl-], taken as 0"693/h/2. Resolution into values of k13 and kal was effected using the relations: K t = kls/k31 = [CI-](Ao - - A e ) / ( A e - - A = ) (4) Kt = kla//k3~ = [C1-](Ae- A = ) / ( A o - Ae)
(5)
where Equation (4) was used when starting with trans-Ir(py)2Cl4-, calculating A= from the spectrum of trans-lr(py)2(OH2)Cl3, and Equation 5 was used when starting with trans-Ir(py)z(OH2)Cl3, calculating A® from the spectrum of transIr(py)~C4-. Rate plots varied in quality, some being linear to ca. 80 per cent reaction to equilibrium and some showing considerable scatter of points. Values of K t, k13 and kal are given in Table 1. Focusing first on runs 7-12 at constant [H ÷] and constant ionic strength, we see that Kt doubles with a 7-fold increase of [CI-] from 0-5 to 3.7 F. The experimental errors are large, a reflection in part of the fact that the isosbestic points in the u.v. spectra were generally not fully sharp. Moreover, substitution of CI- for CIO4- may change the ionic environment at the high (constant) ionic strength, although ion pairing effects on k31 should be small since the reactants are both anions; the ion atmosphere effect may not be large, since comparison of run 12 (/z controlled with NaCl) with runs 13 and 14 (/., controlled with LiC1) suggests that substitution of LiCI for NaCl Table 1. Rate constants for aquation of trans-Ir(py)2C14- and CI- anation of trans-Ir(py)2(OHz)Cls in the dark at 110.00°C
[H+] * (F)
[Cl-]t (F)
Co*
/z§
Run
(mF)
(F)
1 2 3 4 5 6 7 8 9 l0 11 12 13 14 15 16
1.00 0.010 0.095 0-498 0-504 0" 100 0.498 0-498 0.498 0.510 0"498 0.498 0.498 0-498 1.70 3"70
0.010 0-095 0"498 0.504 1"70 0.498 0.498 1.10 1-71 2"49 3.70 3-70 3"70 1.70 3"70
1.4 0.019 0-024 0-11 0.021 0"39 0-45 0.19 0.11 0.40 0"40 0.34 0-11 0" l 1 0.39 0.44
1.0 0.010 0.095 0.498 0.504 3"70 3.68 3-70 3.70 3.71 3.69 3.70 3.70 3'70 3.70 3.70
105k13 Kt
0" 15 0.21 0" 15 0-18 0-13 0" 14 0.18 0-18 0.26 0.26 0.22 0.22 0.21 0.18
104k31
(sec-0(M -1 sec-0 7"1 3"5 6.9 7.1 4.8 7-0 5"8 7"4 9-5 8.7 8.4 8"6 9.0 10 15 12
2.8 3-4 3-3 4.0 4"6 5-3 5.3 4.8 3.2 3.3 4.2 4.7 7.2 6.8
*HCIO4 in run 1; HC1 in all others. tHCI in runs 1-5, 7, 8, 15, 16; H C I + N a C I in runs 6, 9-12; H C I + LiCl in runs 13, 14. :Hnitiai complex is trans-[Ir(py)z(OHz)Cla].H~O, except in run 1 (trans-pyH[Ir(py)2Cl4]) and runs 2, 3 (trans-Na[Ir(py)zC4]). §Ionic strength; NaCIO4 added in runs 6-11, 15.
Cis-and trans-tetrachlorodipyridineiridate(11!) anions
2879
affects Kt less than does doubling [CI-]. Variations in k13 and k31 in runs 7-12 are much smaller. Analysis of the rate data of several experiments at 250 m/~ gave essentially the same values of kla and k31 as at 320 m~. Within experimental error the average value of k,3 from runs 7 - 1 2 is compatible with k13 determined directly in run I. Accuracy in k~3 could have been improved if oxidation of trans-Ir(py)2CI4- by CIO4- had not generally interfered with direct aquation runs (vide ante; ClO4- in runs 6-11 and run 15 did not interfere since anation reaction times were short). We ignore the trend in Kt, and take as average values k~3 = (8 +- 1) × l0 -5 s e c - ' and k3~ = ( 4 . 4 + 0 . 8 ) x 1 0 - 4 M - l s e c -1 in 0 . 5 F HCI ( / z = 3.7, NaCI) at II0°C over the range 0.5-3.7 F C1-. Comparison of run 6 with runs 10 and 15 (1-70 F CI-, ~ = 3.7) shows no effect (within experimental error) on k~a or ks~ of increasing [H +] from 0.1 to 0-5 F, and at least within this range the reactions may be considered as aquation and anation, uncomplicated by base hydrolysis or formation of hydroxo complexes. T h e source of the apparently greater rates in 1.7 and 3.7 F H + is not known; it may be a salt effect in controlling/z. Increase of ionic strength from 0.5 to 3.7 F in 0.5 F HC1 (runs 4, 7, 8) appears not to affect k~3 within experimental error; although the errors are relatively large, k3~ (see also run 3) appears to be increased somewhat, as expected from the BrCnsted-Bjerrum theory, which is only a poor approximation at the high ionic strengths involved. Aquation of cis-lr(py)2CI4- and CI- anation of cis-lr(py)2(OH2)Cla. Spectral scans made during aquation of cis-lr(py)2Cl4- at 90 and 110°C failed to give isosbestic points in the visible region, whereas Fig. 1 predicts at least one isosbestic point for conversion of cis-lr(py)2C14- to any one of the three geometric isomers of Ir(py)2(OHz)CI3. Trial-and-error calculations show that the actual visible spectral scan behaviour, shown in Fig. 3, can be reproduced if it be assumed that the aquation products form in the fixed ratio of 67% cis(autoclave)-, 28% cis(sun)-, 5% trans-Ir(py)2(OH2)Clz (this product ratio also gives isosbestic points in the u.v. spectrum in accord with those observed experimentally), 0"5
o-o
I
I
I
I
460
500
WAVELENGTH, mp Fig. 3. Change in spectrum (420-500 m/z) during aquation of 1.2 mE cis-lr(py)~C14in 1 F HC104 at ll0°C; reading down at 440 n~, reaction times are 0, 30, 60, 90 and 300 rain.
2880
J . B . T I R S E L L and C. S. G A R N E R
the ratio of cis(sun) to cis(autoclave) isomers being so calculated as to preserve the K~= 0.43 isomerization quotient at l l0°C discussed above. This ratio of products not only accounts for the shape changes of the spectra but also predicts that the narrowest gap between the spectral scans in the 420-500 ro4z region occurs at 455-458 m/~ (aM, 23"6--23"3), where it is found experimentally. The 5% of trans product must be formed directly. If it were formed by faster isomerization from either the cis(autoclave) or cis(sun) isomer (the postulated isomerization would have to be faster than the aquation in order to avoid getting isosbestic points in the early stages of reaction), product distribution would be nearly identical in cis- and in trans-Ir(py)2C14- aquation, whereas as shown earlier the latter gives ca. 100% trans product. Whether cis-Ir(py)2Ch- aquates directly to cis(autoclave) or cis(sun) isomer or both is not known, since the isomerization between them is faster on the aquation time scale. Accordingly, the rate constants for aquation of cis-Ir(py)2C14- were obtained by plotting the data at 320 m/z in terms of the relation: 2.30 log [(Ao - A=)/(A - A=)] = k2345t
(6)
where A~ was calculated for a 67% cis(autoclave)-, 28% cis(sun)-, 5% transIr(py)2(OHz)Cla mixture from the spectra of the pure components, and kz345 is the total first-order rate constant for aquation of cis-Ir(py)2Cl4- to this mixture.* Rate plots were linear to 50-75 per cent reaction. Values of k2345 are given in Table 2. Comparison of runs 4 and 5 with runs 9-12 indicates that k2345increases only ca. 15 per cent on increasing [H +] from 1-0 to 2.5 F HC104 at ll0°C (/~ = 3.7, NaCIO4). In view of the high ionic strength, we may safely assume the reaction is aquation with essentially no contribution from base hydrolysis or acid catalysis. Comparison of runs 1, 2, 4, and 5 at 110°C, and runs 17-20 at 90°C, shows that increase of ionic strength from 1.0 to 3.7 F alters k2345 negligibly, as is often found for aquation reactions, even at such high ionic strengths. Taking weighted averages of 105kz345 of 13.0, 3.45, and 1.13 at 110.00, 100.00, and 90.00°C, respectively, in 1 F HCIO4 (/x = 3.7, NaCIO4), we obtain a fair Arrhenius plot, from which E~ = 34___4 kcal mole -1 and log PZ (sec -1) = 15.5 ___2.3. Since cis-Ir(py)zCl4- does not aquate directly to a single product, this E~ is only a composite of the true E~ values for the individual reactions participating. Since 5 per cent of the total reaction produces trans-Ir(py)~(OH2)Cls, we take k2a = 0"050k2345 in 1 F HCIO4 (/x = 3.7, NaCIO4), giving 10~k23 = 0.65, 0.17, and 0.056 sec -1 at 1 10, 100 and 90°C, respectively. In CI- anation of cis-lr(py)2(OH2)Cl3, preliminary calculations indicated that reversible anation occurs under the conditions of [CI-] involved, at a rate comparable with reversible isomerization to the cis(sun) isomer, which itself is probably capable of reversibly anating to cis-Ir(py)zCl4- at a comparable rate. To treat the kinetic data properly, a complex computer program would be *Should our value o f the isomerization quotient K~ be somewhat too large, values o f k2345 are not greatly affected since molar absorbancy indices of cis(autoclave)- and cis(sun)-lr(py)~(OH2)Cl3 are almost the same at 320 rn~; ca. 5% trans-Ir(py)2(OH2)Cl3 is still required in order to fit the spectral scans.
Cis- and trans-tetrachlorodipyridineiridate( I i I) anions
2881
needed, together with a knowledge of the concentration of either the cis(autoclave) or cis(sun) isomer as a function of time (which cannot be known from absorbancy changes alone). The problem is further complicated by the fact that aquation of cis-Ir(py)2Cl4-, produced in the anation, also generates 5% translr(py)dOH2)CI3 at longer reaction times. Indeed, the visible spectral scans taken at 0-475 rain in 2.4 F CI- at 110°C show shifting "isosbestic points" and it is clear that a true equilibrium is not attained in useful reaction times. In an attempt to estimate the anation rate constant at 110°C, we analyzed three anation runs using a relation analogous to Equation 2, with Ae taken as A at a time when only Table 2. Rate constants for aquation of cis-lr(py)2Cl4- and CI anation of cis-Ir(py)dOHz)C13 in the dark Temp
[H+] *
[C}-]?
Co-~
/z§
10~k2aa5 104k52
Run
(°C)
(F)
(F)
(mF)
(F)
(sec-~)(M -~ sec-')
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20
110.00 110.00 110.00 110.00 110.00 110-00 110.0~ 110-00 110.00 110-00 110.00 110"00 100.00 100.00 100.00 90.00 90-00 90.00 90.00 90.00
0.984 1.00 2-05 1.00 1-00 1-00 1'00 1.00 2"50 2'50 2"50 2-50 1-00 1.00 1-00 0-982 1.00 1.00 1.00 1.00
0-024 1.4 1.6 0.50 0-49 0-I1 0.11 0.50 0.50 0.50 1.4 7.0 0.020 0-024 0-018 0.024 0"020 1.4 0.50 3.5
0-984 1.00 3.25 3-70 3'70 3.70 3.70 3.70 3.70 3.70 3-70 3-70 1.00 1-00 1.00 0.982 1.00 1.00 3.70 3- 70
0-50 2.35 3-70
13.4 13-3 13-2 11.4 12.9 (13.0) It ( 13.0)11 (13.0) II 15.1 14.9 15.7 19-6 3-73 3.44 3.19 1.44 1.09 1.06 1.09 I. 20
2.9 1-7 1-4
*HCIO4 in all runs, except HCI in runs 7, 8. tNaC1 in run 6; HCI + NaCI in runs 7, 8. :~lnitial complex is cis-NH4llr(py)2C14], except in runs 2, 11, 12, 18, 20 (cis-pyH[Ir(py)2Cl4]) and runs 6-8 (cis(autoclave)-llr(py)z(OH2)C13]" 1"5-H20). §Ionic strength; NaCIO4 added in runs 3-7, 9-12, 19-20, NaC1 added in runs 6-8. IINormalized value.
relatively small absorbancy changes were occurring (much larger changes in A would occur ultimately after appreciable trans-lr(py)z(OH2)C13 is formed). The half-time of approach to this pseudo equilibrium was used to estimate k52 for CIanation of cis-lr(py)2(OH2)Cl3, taking 1.30 × 10-4 sec-' (a weighted average of the values from aquation in 1 F HC104-2.7 F NaCIO4) as a normalized value of the cis-Ir(py)zC4- aquation rate constant. These values of k~2 (see Table 2) show a 2-fold variation over the 7-fold variation of [CI-] from 0.5 to 3.7 F; the average,
2882
J . B . T I R S E L L and C. S. G A R N E R
2 × 10-4 M -1 sec -1, may be considered as a probable upper limit for both k52 and lq2 at 110°C. A single kinetic run of aquation of cis(autoclave)-Ir(py)2(OH2)Cla in 1 F HCIO4 at 110°C suggested that k for this aquation is probably <7 × 10-6 see -1. Reaction scheme and mechanisms. In Fig. 4 we present a reaction scheme relating the rate constants obtained at l l0°C. The scheme ignores polymer formation, which occurs at long reaction times. We see that cis-Ir(py)2CIc aquates at 110°C with a rate approximately twice that of the trans isomer. This behaviour is similar to that of cis- and transIr(OH2)2CIc, for which the aquation rates are approximately equal at 80°C,[8] but dissimilar to the factors of approximately 10 or more commonly found for aquation rates of many octahedral cobalt(Ill) and chromium(Ill) complexes. Extrapolation of the 50-80°C rate constants of EI-Awady, Bounsall, and Garner [8] for trans-Ir(OH2)2Cl4- aquation to 110°C gives 8 × 10-4sec -1, which is ten times the aquation rate of trans-lr(py)2C4-. Since essentially no studies have been made of pyridine as an "orienting ligand"[12] in octahedral substrates, we are not prepared to explain the lower rate of the dipyridine complex without further evidence. It is interesting that both trans-Ir(OH2)2C1c [8] and trans-Ir(py)2C1c aquate with ca. 100 per cent retention of configuration. Aquation via a square pyramidal transition state would be compatible with this. On the other hand, the cis isomers aquate with stereochemical change (at 80°C, cis-Ir(OH2)2Clc aquates 60 per cent to 1,2,3- and 40 per cent to 1,2,6-Ir(OH2)2C13 [8]). Aquation ofcis-Ir(py)2CIc via a trigonal bipyramidal transition state predicts a statistical ratio of 17% trans-, 33% cis(sun)-, and 50% cis(autoclave)-Ir(py)2(OH2)Cl3 (assuming here that the latter has a cis-dipyridine-trans-dichloro configuration), whereas a square pyramidal transition state statistically gives 50% cis(sun) and 50% cis (autoclave) product, ignoring isomerization. Electrostatic and other factors render such statistical predictions uncertain, but the 5% trans-Ir(py)2(OH2)Cl3 found in cis-Ir(py)2CIc aquation is suggestive of reaction via a trigonal bipyramidal structure.
(1) trans-Ir(py)2CIc . I~
0'8 4.4
(3) trans-Ir(py)2(OH2)C13 --~
~
(4) cis(sun)-Ir(py)2(OH2)Cl3 --~,
<0.004 <0-005 0 . 0 6~5 A _1 . 2 /
,
%
/////"'~ / ~ ~1.2 (2) cis-Ir(py)2Cl, . . . . .
2?
-4
~r--2
(5) cis(autoclave)-Ir(py)~(OH)2Cla --~
Fig. 4. Rate constants (10' k, sec -1 or M -1 sec -1) for aquation, isomerization, and CIanation reactions in 0.5-2.5 F H + (/z = 3.7, NaC104 and/or NaC1) at 110°C.
Acknowledgement-This work was partly supported by the U.S. Atomic Energy Commission under Contract AT(I 1-1)-34, Project 12, with the University. This paper constitutes Rep. No. U C L A 34P12-72 to the A.E.C. 12. C. lngold, R. S. Nyholm and M. L. Tobe, Nature, Lond. 187,477 (1960).
Pergamon Press.
Printed in Great Britain
KINETICS OF AQUATION O F CIS- A N D T R A N S TETRACHLORODIPYRIDINEIRIDATE(II I) A N I O N S AND CHLORIDE ANATION O F CIS- A N D TRANS-TRICHLOROAQUODIPYRIDINEIRIDIUM(III)* J. B. TIRSELL and C. S. G A R N E R Department of Chemistry, University of California, Los Angeles, Calif. 90024
(First received 9 December 1968; in revised form 27 January 1969) A b s t r a c t - T h e kinetics of the reactions trans-Ir(py)2C14-+H~O ,
kls k31
• trans-Ir(py)2(OHz)Cl3+ CI-
were studied in 0.01-3.7 F HCI or HCIO4 (/z := 0.01-3.7, NaCI and/or NaCIO4) at 110"00°C. In 0"5 F HCI (/z =: 3.7), k13 = (8 - 1) x 10-s sec -1 and k3~ = (4.4_+ 0.8) x 10-4 M -t sec -~, nearly independent of C0 (0-02-0"5 mF), [H +] (0-1-0-5 F, at 1'7 F C1-), and [CI-] (0-5-3'7 F, at0.5 F H+); increase of/x(0.5-3.7) increases k3t, but has only a small effect on k13. The kinetics of aquation of cis-lr(py)2C14- was examined in 1-2.5 F HCIO4 (~ = 1-3-7, NaCIO4) at 90-110°C. The products are 5% trans-, 28% cis(sun)-, and 67% cis(autoclave)- Ir(py)2(OH2)C13; the former is formed directly, but relative amounts of the latter two isomers are apparently determined by their faster isomerization. The total aquation rate constant is (1.3 _+0.1) × 10-4 sec -t at I10.00°C in 1 F HCIO4 (/z = 3.7), nearly independent of C0(0"02-1"6mF), [H+](1-2-5 F), and /z(1-3.7); Ea=34+--4kcal mole -t, logPZ (sec -1) = 15.5+2.3. Chloride anation of cis(autoclave)-lr(py)2(OH2)Cl3 is complicated by competing isomerization. At II0.00°C the anation rate constant appears to be ca. 2× 10-4M -~ sec -~ in 1 F H ÷ (/~ = 3-7) from 0.5-3.7 F CI-. Accurate u.v. and visible absorption spectra of the complexes are presented.
THE SYNTHESIS and general chemical behaviour of cis- and trans-tetrachlorodipyridineiridate(III) anions, cis- and trans-Ir(py)2Ch-, and the three geometric isomers of trichloroaquodipyridineiridium(III), Ir(py)2(OH2)Cl3, have been investigated by Delrpine, [ 1-3] but apparently no kinetic studies of their reactions have been reported. In connexion with a programme of studying the kinetics and stereochemistry of hydrolysis and anation of iridium(Ill) complexes, we undertook a kinetic investigation of the aquation of cis- and trans-Ir(py)2C4- and the chloride anation of the Ir(py)..,(OH2)CI3 isomers, which we report here. EXPERIMENTAL
Preparation of complexes. Commercial (NH4)~[IrCI6] (Fairmount Chemical Co., Inc., Newark, N. J.), found spectrographically to contain <0"05 per cent of any of the other Pt metals, was the source of all Ir compounds. The method of Kauffman[4] was used to prepare orange cis- and red *Contribution No. 2288. 1. M. DeMpine, (a) C.r. hebd. Sernc. Acad. Sci., Paris 175, 1075 (1922); (b) 200, 1373 (1935); (c) 233, 1156, 1533 (1951); (d) 234, 1721 (1952); (e) 236, 1713 (1953); (f) 238, 27 (1954); (g) 240, 2468 (1955); (h) 242, 27 (1956). 2. M. Delrpine, (a)Annls Chim. Phys. [9], 19, 5,145 (1923); (b) [11],4,271 (1935). 3. M. Delrpine, Recl Tray. Chim. Pays-Bas Belg. 59,486 (1940). 4. G. B. Kauffman, lnorg. Synth. 7,228 (1963). 2871
2872
J . B . T I R S E L L and C. S. G A R N E R
trans-pyH[lr(py)2Cl4]. Anal. Calcd. for C5H5NH[Ir(C5HsN)2CI4]: C, 31.48; H, 2.82: Ir, 33.58. Found: (trans): C, 31.87; H, 3.17; Ir, 33.67; (cis): lr, 33.76. The pyridinium salts were converted to the NH4 + (cis) or Na ÷ (trans) salts by treatment with a stoichiometric amount of NH4OH (cis) or NaOH (trans) on a steam bath for 5 min, followed by evaporation to crystals at ca. 20°C, recrystallization from hot HzO, then washing with EtOH and Et20. The method of Del~pine [2b] was employed to convert cis-NH4[Ir(py)2 C14] and trans-Na[Ir(py)2Cl4] to orange-yellow cis(autoclave)*- and orange-red trans-Ir(py)2(OH2)Cl~, respectively, except that aquation was carried out at 110°C (instead of 130°C) and for only 3 hr (cis) or 7 hr (trans) (instead of 1 or 0.5 hr, respectively). The crystals were air-dried. Anal. Calcd. for cis-[Ir(CsHsN)z(OH2)CI3] • 1.5 H20: Ir, 38.30. Found: Ir, 38.22-38.43. Calcd. for trans-[Ir(CsH~N)~(OH2)C13]HzO: C, 24.37; H,2.86; N,5.68; Ir, 39.00. Found: C,24.12; H,3.06; N, 5.67; Ir, 38.37-38.64. Orange-yellow cis(sun)-[Ir(py)~(OH2)Cl3].2H~O was made by modifying the Del~pine[lh] method as follows, A solution of cis-NH4[Ir(py)2Cl4] in H20 (1 g in 10 ml) was photolyzed at 33°C with a 250-W incandescent lamp at ca. 5 cm for 2 days. Fine crystals of a foreign substance (polymer?) formed and floated on the surface of the solution; these were decanted off. Orange-yellow crystals, which had separated and sunk to the bottom, were filtered off, washed with cold H20, then with EtOH and EhO, and air dried. Anal. Calcd. for [Ir(CsHsN)z(OH2)CIa]'2H20: Ir, 37"63. Found: Ir, 38"71-38.83. Other chemicals. Reagent grade Na2COz was neutralized with reagent grade HCIO4, and the solution evaporated to crystallization to prepare NaCIO4, which was recrystallized twice from H20. All other chemicals were reagent grade. The water was double distilled, then passed through a mixedbed cation-anion resin column and monitored for purity by electrical conductivity. Chemical analyses. Solid I r compounds were analyzed for I r as described previously, [5] and for C, H, and N with standard microanalytical procedures. Kinetic and stereochemical experiments. Attempts to follow reaction rates by determination of released CI- failed. Titration of CI- by AgNOa, either potentiometrically at 0°C in aqueous acetonedetergent solution [6] or by the Volhard method, gave erratic results in the presence of the Ir complexes. We were unable to separate CI- chromatographically for titration in the absence of Ir complexes; although CI- could be separated quantitatively from the Ir(py)zC4- isomers adsorbed on a NO3- Dowex A G I - X 8 (100-200 mesh) column from H20 solution, contamination with lr(py)z(OH2)CIa isomers (and in some cases, lr(pyh(OH2)Cl2 +) was always present. Even from0.3 F HCIO4, only ca. 94 per cent ofcis-Ir(py)2Cl4- could be adsorbed, the rest coming through with the C1-. Accordingly, the rates were followed spectrophotometrically. Even here band intensities in the visible absorption spectra (Fig. 1) and solubilities of some complexes were so low that this approach would have given large errors or, for the more soluble complexes, consumed large amounts of the complexes. Because cis- and trans-lr(py)2Cl4- form insoluble compounds when oxidized to lr(IV), we could not use the method of oxidizing reaction aliquots with CI2 (to give Ir(IV) complexes with band intensities normally 20-40 times those of the Ir(lll) analogues) found so useful by us in our earlier Ir(IIl) studies[5, 7, 8]. Hence, changes in the u.v. spectra with time were used. As Fig. 2 shows, the regions around 2 5 0 - 2 6 0 n ~ and 320-330m/z offer reasonable absorbancy changes; the former region, however, is subject to errors arising from absorption by pyridinium ion or other contaminants and was found less suitable than analysis at 320 m/z. Determination of the isomeric distribution of reaction products, however, was essentially impossible in the u.v. since the isosbestic points predicted from Fig. 2 were nearly the same for the various pairs of potential reaction products. Hence, in several runs substrate concentrations were • Deirpine[l-3] has used the terms "sun" and "autoclave" isomers to describe, respectively, the two isomers of Ir(py)s(OH2)CI3 made from aqueous cis-lr(py)2Ch- by a photochemical reaction with sunlight, and by heating in an autoclave to 130°C. These two isomers have the pyridine ligands cis to each other, with the water ligand trans to either a pyridine or chloro ligand; which of these two configurations is the "sun" and which the "autoclave" isomer has not been established. In trans-lr(py)z(OH2)C13 the pyridine ligands are trans to each other. 5. 6. 7. 8.
J. C. Chang and C. S. Garner, lnorg. Chem. 4,209 (1965). D.J. MacDonald and C. S. Garner, J. inorg, nucl. Chem. 18, 219 ( 1961). I.A. Poulsen and C. S. Garner, J.Am. chem. Soc. 84, 2032 (1962). A. A. EI-Awady, E. J. Bounsall, and C. S. Garner, lnorg. Chem. 6, 79 (1967).
Cis- and trans-tetrachlorodipyridineiridate(l 1i) anions WAVENUMBER.
KK
2873
18
4O i
E
o T
x" Z >(J Z
<
~n n~
o~o O0 m re" ..J 0
~E 10
\
\ -400
450 500 WAVELENGTH,
sso
600
mp
Fig. 1. Visible absorption spectra of chloro dipyridine iridium(Ill) complexes in 1 F HC104 at 20-25°C: TTC, trans-Ir(py)2Cl4-; CTC, cis-Ir(py)2CI4-; TTCA, transIr(py)2(OH2)CI3; CATCA, cis(autoclave)-Ir(py)2(OH2)Cl:6 CSTCA, cis(sun)-Ir(py)2(OHm)C13. adjusted as high as possible to permit a search for isosbestic points in the visible region, where the differences predicted from Fig. I are much greater, despite the relatively low intensities involved. Reaction solutions were prepared by dissolution of weighed Ir complex in solutions of appropriate acid concentrations and ionic strengths, and sealed inside Pyrex ampoules. These were wrapped with A1 foil to exclude light, heated in 2-5 min to within a few degrees of bath temperature, then placed in thermostat baths at 90.00 ± 0'03,100.00 ± 0.08, or 110.00 ± 0.05°C. Samples were quenched in ice at known times. With a few exceptions, kinetic run samples were diluted with H20 or acid to give solutions 1 F in HC104 for the u.v. spectral scans in a Cary Model 15 spectrophotometer. Matched 1-, 5-, or 10-cm quartz cells were used, with the reference cell normally filled with the same solution except for absence of Ir species. Molar absorbaney index plots. These were calculated from spectra of fresh solutions of tLe weighed lr complexes, using a Cary 15 spectrophotometer and matched quartz cells. RESULTS AND DISCUSSION
Visible and ultraviolet absorption spectra. Absorption maxima, and in some cases spectral plots, have been reported for the complexes,[9, 10] but these were too incomplete or inaccurate for use in spectral study of the kinetics and stereochemistry. In Figs. 1 and 2 we give the visible and u.v. absorption spectra, respectively, of the complexes measured in our laboratory. As expected, substitution of ligand H20 for ligand C1 shifts the d-d bands to higher energies (Fig. 1). Wavelengths of the absorption bands agree to within 2 mt~ and their molar 9. 1. Yasuo and Y. Kondo, J. chem. Soc. Japan (Pure Chem. Sec.) 72,787 ( 1951 ). 10. C. K. Jergensen,Acta chem. scand. 11, 151 (1957).
2874
J . B . T I R S E L L and C. S. G A R N E R
46
WAVENUMBER, KK
42
38
34
30
~TTCA
TE9 x"
UJ7 >-6 0 Z
o
o ~2 X ~t
250
300
350
400
WAVELENGTH, mlJ Fig. 2. Ultraviolet absorption spectra of complexes of Fig. 1 in 1 F HCIO4 at 20-25°C.
absorbancy indices to within 5-15 per cent with values reported by JCrgensen, [10] except for cis(autoclave)- and trans-Ir(py)2(OH2)Cla, for which we find bands at 452 m/z (aM, 24"4) and 495 m/z (aM, 9.7), respectively, vs. ca. 445 m/~ (aM, 28) and ca. 475 m/z (aM, 17) given by JCrgensen.'* Isomerization, polymerization, and oxidation o f trans-lr(py)2Cl4-. Delrpine [2a] has reported that trans-Ir(py)2C14- is oxidized to insoluble deep violet Ir(py)2Cl4 (presumably the trans isomer) almost immediately by C12, Br2, 15 F HNOs, and aqua regia at room temperature. We find that oxidation occurs slowly in 1 F HCIOa at 90-110°C, but apparently some catalytic impurity is involved since in two cases we were able to carry out aquation kinetic runs in 1 F HCIO4, one at 90°C and one at 110°C, without any evidence of oxidation. At 20-25°C, HC104 of concentration 2-5 F or greater oxidizes trans-Ir(py)2Cl4- overnight. Evidence that oxidation is occurring, rather than polymerization, is found in the fact that the deep violet precipitates obtained this way could be reduced in acidic solution to a soluble complex by the action of HzO~, I-, or Fe z+. Delrpine[2b] has stated that trans-Ir(py)2Cl4- in H20 heated at 130°C for 1 hr generates an insoluble rose polymer, [Ir(py)zCla],, in addition to transIr(py)2(OH2)CIa. Although we have seen a rose precipitate forming in acidic solutions at 110°C after prolonged heating, neither oxidation nor polymerization *In Ref. 10 JOrgensen states that "trans- and cis-(autoclave)-Ir py2Cla(H20) are so weakly soluble in water that it is difficult to observe the ligand field triplet transition." For the trans complex the 495-m/~ band had intensities of only 0.1-0.3 absorbancy unit in the solutions we measured; absorbancies o f ca. 1 unit were possible for the cis complex.
Cis- and trans-tetrachlorodipyridineiridate(I I I) anions
2875
was observed during the reaction times on which we have based our kinetic calculations. We made a search for possible isomerization of trans-Ir(py)~Cl4- in 1 F H C I 4 F LiCI at 100°C and in 0.5 F H C 1 - 3 , 2 F NaCI at 110°C to see if isomerization would interfere with aquation and anation studies. The high CI- concentration represses aquation, our aquation-anation stereochemical and rate data (vide infra) implying an equilibrium under these conditions with ca. 93-97% trans-Ir(py)2C14and 7-3% trans-Ir(py)2(OH2)Cl3. In 137 min at 100°C intensities in the 320-m/x region decreased ca. 0.5 per cent; an absorbancy decrease there is expected upon reaction to produce any other complex of Fig. 2 (or upon polymerization or oxidation). Intensity increases of ca. 1 and 5 per cent in 17 and 475 min at 110°C at 465 m/z were observed, compatible with the above predicted extent of conversion to trans-Ir(py)2(OH2)Cls. However, conversion to any other component of Fig. 2 would also be compatible (but not polymerization or oxidation). Although at least one isosbestic point in the visible would occur if a single product were formed (see Fig. 1), no isosbestic points were observed. Moreover, little change in absorbancy occurred above 515 m/x, although decreasing absorbancy there would be expected. These anomalies may be partly due to the necessarily low intensities (A <~ 0.3; see Experimental), or possibly the trans-lr(py)2Cl4- contained a small percentage of some impurity. If we take 1 per cent as a maximum extent of isomerization of trans-Ir(py)2C14- (corresponding to the total inclease in absorbancy at 465 m/z in 475 min assuming the only change is isomerization), we obtain k12* = 4 × 10-7 sec -1 for an upper limit of the first-order isomerization rate constant at 110°C. This is insufficient to interfere with the aquation and anation kinetic studies. Isomerization, polymerization, and oxidation of cis-lr(py)2Cl4-. Delrpine[2a] found that C12, aqua regia, or fuming HNO3 in.large excess (but not Br2) oxidizes cis-Ir(py)2Cl4- to insoluble black Ir(py)2Cl4 (presumably the cis isomer). We found no evidence in our kinetic or stereochemical runs for any oxidation of cislr(py)2Ch-.
Formation of a deep orange polymer, [Ir(py)zCl3]n, on heating solid cispyH[Ir(py)2CI4] at 140°C was noted by Delrpine,[2b] but no polymer formation from aqueous solution was reported. No opalescence was seen by us in our cisIr(py)2Cl4- aquation kinetic studies, although a trace of precipitate was noted on heating the complex in H20 at 110°C for 27 hr. An isomerization search was made in 1 F H C I - 2 . 7 F NaCI at I I0°C, in which medium our kinetic and stereochemical aquation results (vide infra) suggest aquation of cis-Ir(py)2Cl4- with a half-life of ca. 17 min to an equilibrium state of ca. 80% cis-Ir(py)2Cl4- and ca. 20% cis(autoclave)-Ir(py)z(OH2)Cls (perhaps with a smaller percentage of cis(sun)-Ir(py)z(OHz)Cl3 supplanting some of the cis(autoclave) isomer). The u.v. and visible spectra taken at 0, 25, and 1145 min showed changes qualitatively compatible with this extent of aquation. As Fig. 1 shows, concurrent isomerization of cis-Ir(py)zCI4- to any significant extent would disrupt the changes seen and, in particular, would cause the absorbancies in the *Rate constant k12 is for direct production of complex 2 from complex 1 of Fig. 4; this notation is used for all other k values given.
2876
J.B. TIRSELL and C. S. GARNER
460-m/z region to decrease appreciably. Such was not observed, even on extending the reaction time to 1960 min. Conservatively taking 5% isomerization as the maximum possible in 1960 min, we estimate 5 × l0 -7 sec -1 at 110°C for an upper limit of the isomerization rate constant. This is too small to allow significant interference with the aquation and anation kinetic studies. Isomerizatlc,n and polymerization o f Ir(py)~(OH2)Cla isomers. Polymerization of cis(autoclave)- and cis(sun)-Ir(py)2(OH2)Cl3 hydrated solids to insoluble deep orange [h(py)2Cl3]n and of solid trans-[Ir(py)2(OH2)Cl3]'H20 to insoluble dark [lr(py)2CI3]2 at 140-150°C was reported by Deldpine[2b]. We have noted no insoluble substances forming during our kinetic runs with cis-lr(py)2(OH2)Cla isomers in aqueous solution; with trans-Ir(py)2(OH2)Cl3 in aqueous solutions we observed opalescence in some runs, but only after 25-100 half-times of the anation reactior. A search was made for possible isomerization of cis(sun)-Ir(py)2(OH2)Cl3 in 2.5 F H C I O 4 - - 1 . 2 F NaCIO4 at 110°C by scanning the visible spectra at 0, 30, 76, and 120 min. The first three scans gave an isosbestic point at 451 m/z (aM, 24"2), in excellent agreement with 451 m/z (aM, 24"4) expected from Fig. 1 for isomerization to the cis(autoclave) isomer (a second isosbestic point expected at 524 m/z was not observed, but aM is only 4 and the actual crossing of the cis(sun) and cis(autoclave) curves in that region is not certain); isomerization to the trans isomer would have given an isosbestic point at 498 m/x (aM, 9"6). The shape of the 76-min scan and its molar absorbancy indices at 430-470 m/x were found by trial-and-error calculations to agree with those predicted for isomerization lo an equilibrium mixture of 70% cis(autoclave) and 30% cis(sun) isomers. The 120-min scan had the same shape, but was displaced to slightly lower absorbancies, suggesting onset of polymerization or oxidation, since aquation should result in a shift in wavelength of the absorption band and is probably too slow to compete in this reaction time. We cannot be sure the equilibrium ratio has been correctly determined, but if we assume equilibrium at 30% cis(sun) isomer, we calculate for the isomerization:
cis(autoclave)-Ir(py)2(OH2)Cla .
k54 k45
• cis(sun)-Ir(py)2(OH2)C13
(1)
an equilibrium quotient Ki = k54/k45 -- [cis(sun)] / [cis(autoclave)] -- 0.30/0.70 -- 0.43 at ll0°C. From the cha0ge in spectra with time, we estimate the half-time of approach to equilibrium as ca. 20 min, which together with Ki gives k54 ~ 2 X 10 - 4 sec -1 and k45 ~ 4 × 10-4 sec -~ at 110°C (accurate determination of h/2 was impossible since equilibrium was never attained).
Aquation of trans-lr(py)2Cl4- and Cl- anation o f trans-lr(py)2(OH2)Cl3. Spectral scans taken during aquation of trans-Ir(py)2Cl4- at 110°C up to ca. 80 per cent aquation exhibited isosbestic points in the u.v. and visible in good agreement with values predicted from Fig. 1 and 2 for aquation to trans-Ir(py)2(OH2)CIs as sole Dr product; in the visible spectra isosbestic points were observed at 451-453 ~ (aM, 6"6--6"8) and 504-507 m/~ (aM, 9"2--9"9) VS. predicted values of 453 m/~ (aM, 6"9) and 506 rn~ (aM, 9"3). Significant aquation to either or both cis-Ir(py)2(OH2)C13 isomers would seriously alter the first of the above two isosbestic points.
Cis- and trans-tetrachlorodipyridineiridate( I I 1) anions
2877
In CI- anation of trans-Ir(py)2(OH2)Cl3 at I10°C the expected isosbestic points corresponding to anation solely to trans-lr(pyhCl4- were found in the u.v. scans. In the visible region scans taken at 9 and 13 min gave "isosbestic points" at 450 mix (aM, 6"6) and 506 rn~ (aM, 8"8), scans at 13 and 26min gave values of 453 and 504 m/x, and scans at 26 and 40 min gave 454 and 521 m/~. Under the conditions involved, the reaction half-time was 13 min, so beyond two half-times some reaction is occurring in addition to anation to trans-Ir(pyhC14-. Opalescence due to polymer formation (and/or to possible oxidation if C104- was present) was sometimes noted, but only after 25 to 100 times the anation half-time. Del6pine[3] reported that prolonged treatment of translr{py)dOH2)Cl3 with 1.2 F HCi on a steam bath gave trans-H[lr(py)2C14] and some polymer. An alternative but less probable explanation for the shifting "isosbestic points" observed after 26 min at 110°C lies in the possibility of a small contribution from aquation of trans-lr(pyh(OH2)CI3, even in the sometimes high concentrations of CI- involved. We allowed trans-Ir(py)2Cl4- in 1 F HCIO4 to aquate at 110°C for 25 half-times (95 hr) then diluted an aliquot four-fold with 0.001 F HCIO4, charged the solution onto a 10-cm × l-cm diameter column of H ~ Dowex AG50W-X8 (100-200 mesh), eluted off anionic and neutral species with 50ml of 0.01 F HCIO4, then eluted what we believe to be Ir(py)dOH2hCl2 + (unknown configuration) with 50 ml of 0.2 F HCIO4. The elution behaviour is characteristic of a trans-dichloro species of 1+ charge. The effluent was too dilute to measure the visible absorption spectrum, but an absorption band was noted at 295 m/z. Recently Lar6ze[11] has reported the isolation of two solids. [lr(py)z(OH.,)zC12]CI and [Ir(py)2(OH,,)2CI2]CI.H,,O, obtained from translr(py)2{OHOCla via base hydrolysis thermally and photochemically, respectively. No u.v. spectra were given but a shoulder was reported at 465 m/x (aM, 8"5) and 470 m/z CaM.8"3), respectively. Since both cis-Ir(py).,(OH2)Cl:] isomers have their visible absorption bands at lower wavelengths, whereas trans-lr(py),,(OH,,)C13 has its band at 495 m/z, these two preparations of Lar~ze are probably mainly transdipyridine complexes (not necessarily pure, although two such geometric isomers are possible, as well as three in which the pyridine ligands are cis to each other). The kinetic aquation and anation data at 320 m/.t were plotted in terms of the relation: 2"30 log [(Ao -- Ae)/(A - Ae)] = (k,:~+ ka,[Cl-l)t
{2}
where Ao and A are the optical absorbancies at time zero and time t, respectively, Ae is the equilibrium absorbancy, [C1-] was taken equal to the total CI- formality (this remained essentially constant in each anation run), and k13 and k3, are the first-order aquation and second-order anation rate constants for the reactions kl.l
trans-Ir(py)2C14- + H 20 ~---- trans-lr(py)2(OH2)Cl:~ + CF. k31
(3)
For the first run of Table 1 (no CI- initially), Ae was calculated from the spectrum of trans-lr(py)2(OHjC13; for the other runs Ae was evaluated experimentally 11. F. Lar~ze, C.r. hebd. Se6nc. Acad. Sci., Paris 265, 307 (1967).
2878
J . B . TIRSELL and C. S. G A R N E R
from the last scan showing little further change in A. Reaction half-times were read off the plots and the total pseudo first-order rate constant, kla + k31[Cl-], taken as 0"693/h/2. Resolution into values of k13 and kal was effected using the relations: K t = kls/k31 = [CI-](Ao - - A e ) / ( A e - - A = ) (4) Kt = kla//k3~ = [C1-](Ae- A = ) / ( A o - Ae)
(5)
where Equation (4) was used when starting with trans-Ir(py)2Cl4-, calculating A= from the spectrum of trans-lr(py)2(OH2)Cl3, and Equation 5 was used when starting with trans-Ir(py)z(OH2)Cl3, calculating A® from the spectrum of transIr(py)~C4-. Rate plots varied in quality, some being linear to ca. 80 per cent reaction to equilibrium and some showing considerable scatter of points. Values of K t, k13 and kal are given in Table 1. Focusing first on runs 7-12 at constant [H ÷] and constant ionic strength, we see that Kt doubles with a 7-fold increase of [CI-] from 0-5 to 3.7 F. The experimental errors are large, a reflection in part of the fact that the isosbestic points in the u.v. spectra were generally not fully sharp. Moreover, substitution of CI- for CIO4- may change the ionic environment at the high (constant) ionic strength, although ion pairing effects on k31 should be small since the reactants are both anions; the ion atmosphere effect may not be large, since comparison of run 12 (/z controlled with NaCl) with runs 13 and 14 (/., controlled with LiC1) suggests that substitution of LiCI for NaCl Table 1. Rate constants for aquation of trans-Ir(py)2C14- and CI- anation of trans-Ir(py)2(OHz)Cls in the dark at 110.00°C
[H+] * (F)
[Cl-]t (F)
Co*
/z§
Run
(mF)
(F)
1 2 3 4 5 6 7 8 9 l0 11 12 13 14 15 16
1.00 0.010 0.095 0-498 0-504 0" 100 0.498 0-498 0.498 0.510 0"498 0.498 0.498 0-498 1.70 3"70
0.010 0-095 0"498 0.504 1"70 0.498 0.498 1.10 1-71 2"49 3.70 3-70 3"70 1.70 3"70
1.4 0.019 0-024 0-11 0.021 0"39 0-45 0.19 0.11 0.40 0"40 0.34 0-11 0" l 1 0.39 0.44
1.0 0.010 0.095 0.498 0.504 3"70 3.68 3-70 3.70 3.71 3.69 3.70 3.70 3'70 3.70 3.70
105k13 Kt
0" 15 0.21 0" 15 0-18 0-13 0" 14 0.18 0-18 0.26 0.26 0.22 0.22 0.21 0.18
104k31
(sec-0(M -1 sec-0 7"1 3"5 6.9 7.1 4.8 7-0 5"8 7"4 9-5 8.7 8.4 8"6 9.0 10 15 12
2.8 3-4 3-3 4.0 4"6 5-3 5.3 4.8 3.2 3.3 4.2 4.7 7.2 6.8
*HCIO4 in run 1; HC1 in all others. tHCI in runs 1-5, 7, 8, 15, 16; H C I + N a C I in runs 6, 9-12; H C I + LiCl in runs 13, 14. :Hnitiai complex is trans-[Ir(py)z(OHz)Cla].H~O, except in run 1 (trans-pyH[Ir(py)2Cl4]) and runs 2, 3 (trans-Na[Ir(py)zC4]). §Ionic strength; NaCIO4 added in runs 6-11, 15.
Cis-and trans-tetrachlorodipyridineiridate(11!) anions
2879
affects Kt less than does doubling [CI-]. Variations in k13 and k31 in runs 7-12 are much smaller. Analysis of the rate data of several experiments at 250 m/~ gave essentially the same values of kla and k31 as at 320 m~. Within experimental error the average value of k,3 from runs 7 - 1 2 is compatible with k13 determined directly in run I. Accuracy in k~3 could have been improved if oxidation of trans-Ir(py)2CI4- by CIO4- had not generally interfered with direct aquation runs (vide ante; ClO4- in runs 6-11 and run 15 did not interfere since anation reaction times were short). We ignore the trend in Kt, and take as average values k~3 = (8 +- 1) × l0 -5 s e c - ' and k3~ = ( 4 . 4 + 0 . 8 ) x 1 0 - 4 M - l s e c -1 in 0 . 5 F HCI ( / z = 3.7, NaCI) at II0°C over the range 0.5-3.7 F C1-. Comparison of run 6 with runs 10 and 15 (1-70 F CI-, ~ = 3.7) shows no effect (within experimental error) on k~a or ks~ of increasing [H +] from 0.1 to 0-5 F, and at least within this range the reactions may be considered as aquation and anation, uncomplicated by base hydrolysis or formation of hydroxo complexes. T h e source of the apparently greater rates in 1.7 and 3.7 F H + is not known; it may be a salt effect in controlling/z. Increase of ionic strength from 0.5 to 3.7 F in 0.5 F HC1 (runs 4, 7, 8) appears not to affect k~3 within experimental error; although the errors are relatively large, k3~ (see also run 3) appears to be increased somewhat, as expected from the BrCnsted-Bjerrum theory, which is only a poor approximation at the high ionic strengths involved. Aquation of cis-lr(py)2CI4- and CI- anation of cis-lr(py)2(OH2)Cla. Spectral scans made during aquation of cis-lr(py)2Cl4- at 90 and 110°C failed to give isosbestic points in the visible region, whereas Fig. 1 predicts at least one isosbestic point for conversion of cis-lr(py)2C14- to any one of the three geometric isomers of Ir(py)2(OHz)CI3. Trial-and-error calculations show that the actual visible spectral scan behaviour, shown in Fig. 3, can be reproduced if it be assumed that the aquation products form in the fixed ratio of 67% cis(autoclave)-, 28% cis(sun)-, 5% trans-Ir(py)2(OH2)Clz (this product ratio also gives isosbestic points in the u.v. spectrum in accord with those observed experimentally), 0"5
o-o
I
I
I
I
460
500
WAVELENGTH, mp Fig. 3. Change in spectrum (420-500 m/z) during aquation of 1.2 mE cis-lr(py)~C14in 1 F HC104 at ll0°C; reading down at 440 n~, reaction times are 0, 30, 60, 90 and 300 rain.
2880
J . B . T I R S E L L and C. S. G A R N E R
the ratio of cis(sun) to cis(autoclave) isomers being so calculated as to preserve the K~= 0.43 isomerization quotient at l l0°C discussed above. This ratio of products not only accounts for the shape changes of the spectra but also predicts that the narrowest gap between the spectral scans in the 420-500 ro4z region occurs at 455-458 m/~ (aM, 23"6--23"3), where it is found experimentally. The 5% of trans product must be formed directly. If it were formed by faster isomerization from either the cis(autoclave) or cis(sun) isomer (the postulated isomerization would have to be faster than the aquation in order to avoid getting isosbestic points in the early stages of reaction), product distribution would be nearly identical in cis- and in trans-Ir(py)2C14- aquation, whereas as shown earlier the latter gives ca. 100% trans product. Whether cis-Ir(py)2Ch- aquates directly to cis(autoclave) or cis(sun) isomer or both is not known, since the isomerization between them is faster on the aquation time scale. Accordingly, the rate constants for aquation of cis-Ir(py)2C14- were obtained by plotting the data at 320 m/z in terms of the relation: 2.30 log [(Ao - A=)/(A - A=)] = k2345t
(6)
where A~ was calculated for a 67% cis(autoclave)-, 28% cis(sun)-, 5% transIr(py)2(OHz)Cla mixture from the spectra of the pure components, and kz345 is the total first-order rate constant for aquation of cis-Ir(py)2Cl4- to this mixture.* Rate plots were linear to 50-75 per cent reaction. Values of k2345 are given in Table 2. Comparison of runs 4 and 5 with runs 9-12 indicates that k2345increases only ca. 15 per cent on increasing [H +] from 1-0 to 2.5 F HC104 at ll0°C (/~ = 3.7, NaCIO4). In view of the high ionic strength, we may safely assume the reaction is aquation with essentially no contribution from base hydrolysis or acid catalysis. Comparison of runs 1, 2, 4, and 5 at 110°C, and runs 17-20 at 90°C, shows that increase of ionic strength from 1.0 to 3.7 F alters k2345 negligibly, as is often found for aquation reactions, even at such high ionic strengths. Taking weighted averages of 105kz345 of 13.0, 3.45, and 1.13 at 110.00, 100.00, and 90.00°C, respectively, in 1 F HCIO4 (/x = 3.7, NaCIO4), we obtain a fair Arrhenius plot, from which E~ = 34___4 kcal mole -1 and log PZ (sec -1) = 15.5 ___2.3. Since cis-Ir(py)zCl4- does not aquate directly to a single product, this E~ is only a composite of the true E~ values for the individual reactions participating. Since 5 per cent of the total reaction produces trans-Ir(py)~(OH2)Cls, we take k2a = 0"050k2345 in 1 F HCIO4 (/x = 3.7, NaCIO4), giving 10~k23 = 0.65, 0.17, and 0.056 sec -1 at 1 10, 100 and 90°C, respectively. In CI- anation of cis-lr(py)2(OH2)Cl3, preliminary calculations indicated that reversible anation occurs under the conditions of [CI-] involved, at a rate comparable with reversible isomerization to the cis(sun) isomer, which itself is probably capable of reversibly anating to cis-Ir(py)zCl4- at a comparable rate. To treat the kinetic data properly, a complex computer program would be *Should our value o f the isomerization quotient K~ be somewhat too large, values o f k2345 are not greatly affected since molar absorbancy indices of cis(autoclave)- and cis(sun)-lr(py)~(OH2)Cl3 are almost the same at 320 rn~; ca. 5% trans-Ir(py)2(OH2)Cl3 is still required in order to fit the spectral scans.
Cis- and trans-tetrachlorodipyridineiridate( I i I) anions
2881
needed, together with a knowledge of the concentration of either the cis(autoclave) or cis(sun) isomer as a function of time (which cannot be known from absorbancy changes alone). The problem is further complicated by the fact that aquation of cis-Ir(py)2Cl4-, produced in the anation, also generates 5% translr(py)dOH2)CI3 at longer reaction times. Indeed, the visible spectral scans taken at 0-475 rain in 2.4 F CI- at 110°C show shifting "isosbestic points" and it is clear that a true equilibrium is not attained in useful reaction times. In an attempt to estimate the anation rate constant at 110°C, we analyzed three anation runs using a relation analogous to Equation 2, with Ae taken as A at a time when only Table 2. Rate constants for aquation of cis-lr(py)2Cl4- and CI anation of cis-Ir(py)dOHz)C13 in the dark Temp
[H+] *
[C}-]?
Co-~
/z§
10~k2aa5 104k52
Run
(°C)
(F)
(F)
(mF)
(F)
(sec-~)(M -~ sec-')
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20
110.00 110.00 110.00 110.00 110.00 110-00 110.0~ 110-00 110.00 110-00 110.00 110"00 100.00 100.00 100.00 90.00 90-00 90.00 90.00 90.00
0.984 1.00 2-05 1.00 1-00 1-00 1'00 1.00 2"50 2'50 2"50 2-50 1-00 1.00 1-00 0-982 1.00 1.00 1.00 1.00
0-024 1.4 1.6 0.50 0-49 0-I1 0.11 0.50 0.50 0.50 1.4 7.0 0.020 0-024 0-018 0.024 0"020 1.4 0.50 3.5
0-984 1.00 3.25 3-70 3'70 3.70 3.70 3.70 3.70 3.70 3-70 3-70 1.00 1-00 1.00 0.982 1.00 1.00 3.70 3- 70
0-50 2.35 3-70
13.4 13-3 13-2 11.4 12.9 (13.0) It ( 13.0)11 (13.0) II 15.1 14.9 15.7 19-6 3-73 3.44 3.19 1.44 1.09 1.06 1.09 I. 20
2.9 1-7 1-4
*HCIO4 in all runs, except HCI in runs 7, 8. tNaC1 in run 6; HCI + NaCI in runs 7, 8. :~lnitial complex is cis-NH4llr(py)2C14], except in runs 2, 11, 12, 18, 20 (cis-pyH[Ir(py)2Cl4]) and runs 6-8 (cis(autoclave)-llr(py)z(OH2)C13]" 1"5-H20). §Ionic strength; NaCIO4 added in runs 3-7, 9-12, 19-20, NaC1 added in runs 6-8. IINormalized value.
relatively small absorbancy changes were occurring (much larger changes in A would occur ultimately after appreciable trans-lr(py)z(OH2)C13 is formed). The half-time of approach to this pseudo equilibrium was used to estimate k52 for CIanation of cis-lr(py)2(OH2)Cl3, taking 1.30 × 10-4 sec-' (a weighted average of the values from aquation in 1 F HC104-2.7 F NaCIO4) as a normalized value of the cis-Ir(py)zC4- aquation rate constant. These values of k~2 (see Table 2) show a 2-fold variation over the 7-fold variation of [CI-] from 0.5 to 3.7 F; the average,
2882
J . B . T I R S E L L and C. S. G A R N E R
2 × 10-4 M -1 sec -1, may be considered as a probable upper limit for both k52 and lq2 at 110°C. A single kinetic run of aquation of cis(autoclave)-Ir(py)2(OH2)Cla in 1 F HCIO4 at 110°C suggested that k for this aquation is probably <7 × 10-6 see -1. Reaction scheme and mechanisms. In Fig. 4 we present a reaction scheme relating the rate constants obtained at l l0°C. The scheme ignores polymer formation, which occurs at long reaction times. We see that cis-Ir(py)2CIc aquates at 110°C with a rate approximately twice that of the trans isomer. This behaviour is similar to that of cis- and transIr(OH2)2CIc, for which the aquation rates are approximately equal at 80°C,[8] but dissimilar to the factors of approximately 10 or more commonly found for aquation rates of many octahedral cobalt(Ill) and chromium(Ill) complexes. Extrapolation of the 50-80°C rate constants of EI-Awady, Bounsall, and Garner [8] for trans-Ir(OH2)2Cl4- aquation to 110°C gives 8 × 10-4sec -1, which is ten times the aquation rate of trans-lr(py)2C4-. Since essentially no studies have been made of pyridine as an "orienting ligand"[12] in octahedral substrates, we are not prepared to explain the lower rate of the dipyridine complex without further evidence. It is interesting that both trans-Ir(OH2)2C1c [8] and trans-Ir(py)2C1c aquate with ca. 100 per cent retention of configuration. Aquation via a square pyramidal transition state would be compatible with this. On the other hand, the cis isomers aquate with stereochemical change (at 80°C, cis-Ir(OH2)2Clc aquates 60 per cent to 1,2,3- and 40 per cent to 1,2,6-Ir(OH2)2C13 [8]). Aquation ofcis-Ir(py)2CIc via a trigonal bipyramidal transition state predicts a statistical ratio of 17% trans-, 33% cis(sun)-, and 50% cis(autoclave)-Ir(py)2(OH2)Cl3 (assuming here that the latter has a cis-dipyridine-trans-dichloro configuration), whereas a square pyramidal transition state statistically gives 50% cis(sun) and 50% cis (autoclave) product, ignoring isomerization. Electrostatic and other factors render such statistical predictions uncertain, but the 5% trans-Ir(py)2(OH2)Cl3 found in cis-Ir(py)2CIc aquation is suggestive of reaction via a trigonal bipyramidal structure.
(1) trans-Ir(py)2CIc . I~
0'8 4.4
(3) trans-Ir(py)2(OH2)C13 --~
~
(4) cis(sun)-Ir(py)2(OH2)Cl3 --~,
<0.004 <0-005 0 . 0 6~5 A _1 . 2 /
,
%
/////"'~ / ~ ~1.2 (2) cis-Ir(py)2Cl, . . . . .
2?
-4
~r--2
(5) cis(autoclave)-Ir(py)~(OH)2Cla --~
Fig. 4. Rate constants (10' k, sec -1 or M -1 sec -1) for aquation, isomerization, and CIanation reactions in 0.5-2.5 F H + (/z = 3.7, NaC104 and/or NaC1) at 110°C.
Acknowledgement-This work was partly supported by the U.S. Atomic Energy Commission under Contract AT(I 1-1)-34, Project 12, with the University. This paper constitutes Rep. No. U C L A 34P12-72 to the A.E.C. 12. C. lngold, R. S. Nyholm and M. L. Tobe, Nature, Lond. 187,477 (1960).