Adamson, A. W.

Vogler, Arnd

English

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Photosensitized decomposition of some cobalt ammines

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5943 Using the stopped-flow method, we have also inves- tigated the kinetics of the oxidation of Pt(en)z2+ (en = ethylenediamine) by IrC162- (eq 8) and, in this case also, Pt(en)2z+ + 21rC162- + 2C1- + trans-Pt(en)2CIz2+  2IrCI63- (8) found evidence for a stepwise mechanism involving an intermediate platinum(II1) species, Pt(en)23+ (or, alter- natively, Pt(en)zC1*+ or Pt(en)2Clz+). Over the initial concentration ranges, 6.2 X 10-6 to  2.5 X M Pt(en)22+, 1.3 X 10-j to 2.5 X M IrC162-, 0 to  1.0 X M IrCh3-, 10-6 to 0.5 M H+, and 0.1 to  1.0 M C1-, the kinetics of this reaction accurately obeyed the rate law described by eq 9, which is derived, - d[IrC162-] - - 2d[Pt(en)2z+] - - dt dt inhibition of such reactions by hexachloroiridate(1V) has been attributed to  rapid oxidation of platinum(III), but the evidence favoring the interpretations in question, while reasonable, has been for the most part indirect and qualitative. We now wish to  report quantitative evidence for the intermediate formation of platinum(II1) in the chemical oxidation of certain platinum(I1) com- plexes and to  describe some features of the chemistry of the intermediate platinum(II1) species as revealed by observation of their competitive reactions including oxidation by hexachloroiridate(1V). Our principal observations relate to  the oxidation of tetrachloroplatinate(I1) by hexachloroiridate(IV), ac- cording to eq 1, in aqueous sodium chloride-hydrochloric Ptcir2- + 21rci62- + 2c1- + Ptcis2- + 21rci63- (1)  acid solution. The stoichiometry of the reaction was established spectrophotometrically and the kinetics were determined, also spectrophotometrically, over the following initial concentration ranges of reactants and products: 4.0 X lo-5 to 2.5 X M PtClr2-, 1.3 x 10-4 to 8.0 x 10-4 M 1rci62-, o to 8.0 x 10-2 M 1rci63-, 0.05 to 1.0 M ci-, 10-5 to 1.0 M H+. Throughout this combination of concentration ranges the kinetics of the reaction were accurately represented by the rate law, expressed by eq 2, which is derived, -d[IrC162-] - -2d[PtCl,*-] - - _  - dt dt employing the steady-state approximation for PtC14-, on the basis of the mechanism depicted by eq 3-5. PtCI42- + IrC162- J_ PtCla- + 1rci63- (3) PtciI-  + 1rci62- + CI- -+ Ptcij- + I ~ W -  (4) PtClj- + c1- PtCl@2- ( 5 )  Our kinetic measurements at 25 O, and an ionic strength of 1.0 M ,  maintained with NaC104, yield the values kl = 0.62 i 0.03 M-l sec-I and k-l/k2 = 9.0 X M .  The rate was unaffected by variation of the con- centration of H+ between 10-s and 1 M and by the addition of up to M PtC16?-. An obvious variant of this mechanism, which is kinetically indistinguishable from that depicted above, is one in which the termolecular step 4 is replaced by a sequence of bimolecular steps (eq 6 and 7). This P t C k  + CI- PtClj2- (rapid equilibrium) ( 6 )  PtClj2- + IrCh2- + P t C k  + IrCI63- (7) variant leads to a rate law identical in form with eq 2,  in  which ky is replaced by kz’K. The observed value of 9.0 X M then refers to k-l/k2‘K, instead of to k-l/k2. Favoring this formulation is the recent direct observation4 of the formation of PtCls2- in aqueous solution by dissociation of PtC1G3- following electron capture by PtC162-.4 k1 k -1 k2 K k21 (1) R .  L. Rich and H.  Taube, J .  A m .  Chem. SOC., 76, 2608 (1954). (2) A. W. Adamson and A. H.  Sporer, ibid., 80, 3865 (1958). (3) A. J. Poe and M .  S .  Vaidya, J .  Chem. SOC., 2981 (1961). (4) G. E. .4dams, R. B. Broszkiewicz, and B. D. Michael, Trans. Faradaj, SOC., 64, 1256 (1968). (9) 2k1 ’k? ”[Pt(en)z2+][IrC16 2-]2[ C1-] k-1’[ Ir C W ]  + kz”[IrCl6 2-][ C1-] employing the steady-state approximation for Pt(en)23+, for the mechanism depicted by eq 10 and 11. Our kinetic Pt(en)22+ + IrCls2- Pt(en)23+ + IrC163- (10) Pt(en)*a+ + IrClG2- + 2C1- + Pt(en)zC122+  IrC163- (11) measurements (also at 25” and 1.0 M ionic strength, maintained with NaC104) yield the values kl’ = (1.40 i 0.05) X IO5 M-I sec-l and k-l’/k2’’ = 2.0 X The -105-fold difference between the value of kl for PtClr2- and kl’ for Pt(en)zz+ is in the direction expected from the difference between the charges of the two reactants. Preliminary kinetic observations also point to similar mechanisms for the oxidations of PtC13(C2H4)- and PtC12(0H2)(C2H4) by TrCh2-. The reactivities of these complexes are lower than those of PtClJ2- and Pt(en)22+, presumably reflecting stabilization of the + 2 oxidation state of platinum by the coordinated ethylene. The oxidation of these complexes is accompanied by release of the ethylene which does not itself undergo ~ x i d a t i o n . ~  Acknowledgment. Support of this work through grants from the National Science Foundation and the Advanced Research Projects Agency is gratefully acknowledged. One of us (M. P.) also thanks the U. S. Government Fulbright-Hays Program for a travel grant and the University of Zagreb for a leave of ab- sence. kl ’  k2“ M 2 .  ( 5 )  P. B. Chock and J. Halpern, unpublished results. Jack Halpern, Marijan Pribanic‘ Department of Chemistry, Unicersity of Chicago Chicago, Ilhois 60637 Receiced August 14, 1968 Photosensitized Decomposition of Some Cobalt Ammines Sir : We wish to report the finding that the redox decompo- sition of C O ( N H ~ ) ~ ~ +  and C O ( N H ~ ) ~ ( H ~ O ) ~ +  in various aqueous and organic solvents is photosensitized by organic compounds known to have relatively stable triplet excited states. This effect, while forshadowed Communications to the Editor 5944 Table I. Systems Showing Sensitized Redox Decomposition of Co(II1) Ammines. Semi tizer Solvent Complex Wavelength, mp Benzil 80 % ethanol-water [ C O ( N H , ) ~ ( H Z ~ ) I [ C ~ O ~ ] ~  350-370 trans-Stilbene-4-carboxyIic acid 80% ethanol-water [CO(NH~)~(HZO)][C~O~], 350-370 3 M aqueous NaOH Co(NH&C13 350-370 Dimethylacetamide ~ C O ( N H ~ ) ~ ( H Z O ) I ~ ~ ~ O ~ I ~  350-370 Benzophenone 50% ethanol-water CO(NH3)&13 Biacetyl 50% ethanol-water CO(NH3)&13 405-41 5 Water Co(NH&C13 405-41 5 Cu. 0.01 M sensitizer and complex. In very low efficiency. by previous observations that coordination compounds can quench organic triplet state does not seem to have been reported before. A ,  mu Figure I .  Absorption spectra : (A) C O ( N H ~ ) ~ C ~ ~ ,  (B) correspond- ing maxima for [CO(NH, )~(H~O)] [CI~~]~ ,  (C) benzophenone, (D) biacetyl. The latter two spectra are for 50% ethanol-water solu- tions. > v)  z W z t - FLUORESCENCE 1 PHOSPHORESCENCE A ,  mp Figure 2. Emission spectra for 0.15 M biacetyl in 5097, ethanol- water solution: (I) biacetyl alone; (11) with added 0.005 M Co- (NH&C13. (1) A. J. Fry, R. S. H. Liux, and G. S. Hammond, J .  Am. Chem. Soc., ( 2 )  L. Lindquist, Acta Chem. Scand., 20, 2067 (1966). (3) See H. Linschitz and L. Pekkarinen, J .  Am. Chem. Soc., 82, 241 1 88, 4781 (1966). (1 960). The above two complex ions are photosensitive in their own right. With 254-mk light, both undergo photoredox decomposition to Co(I1) and oxidized ammonia,* the first with a quantum yield of about 0.9 and the second with one of about 0.07. The absorption band irradiated is the first charge-transfer (CT) one in both cases, as indicated in Figure 1. Irradiation of either the first or the second ligand field band gives only a trace of decomposition, the reported quantum yields being about and 5 X 10-4,6 respectively. The photosensitivity of these two complexes is thus confined to the CT band. Figure 3. Data for the benzophenone-sensitized decomposition of CO(NH&~+ plotted according to eq 4: 0, nondeaerated solu- tions; 0, deaerated solution. Our general observations are summarized in Table I. In each case, irradiation produced Co(I1) with a quali- tatively high efficiency in the presence of the sensitizer, and not at all in its absence. The Co(I1) determinations were made by means of a colorimetric method involving complexation with thiocyanate ;’ Reineckate actinom- etry8 was used. Some quantitative measurements were made with biacetyl and with benzophenone as sensitizers. With 0.15 M biacetyl and 0.005 M C O ( N H ~ ) ~ ~ +  in 50% ethanol-water solution, the quantum yield for Co(1I) production was 0.056 at room temperature, based on the total absorbed intensity of 410-mp irradiating light. This wavelength corresponds to the first singlet-singlet transition of biacetyl, as shown in Figure 1. Biacetyl is unusual in showing both fluorescent and phosphorescent emission at room temperature, and the combined emis- (4) J. F. Endicott and M. Z. Hoffman, ibid., 87, 3348 (1965). ( 5 )  A. W. Adamson, Discussions Faraday Soc., 29, 163 (1960). (6) D. Klein and C. W. Moeller, Inorg. Chem., 4, 394 (1965). (7) R. E. Kitson, Anal. Chem., 22, 664 (1950). (8) E. Wegiier and A. W. Adamson,J. Am. Chem. Soc., 88,394 (1966). Journal of the American Chemical Society / 90.21 / October 9, 1968 5945 singlet-triplet CT transitions are not evident in the absorption spectra of Co(II1) complexes, presumably because of being hidden by more intense d-d ones. Our suggestion is then that the sensitized decompositions re- ported here are those of low-lying CT triplet states which happen not to  be easily populated by direct absorption of light. Acknowledgment. These investigations were sup- ported in part by Contract DA-3 1-124-ARO-D-343 with the U. S. Army Research Office, Durham. Arnd Vogler, Arthur W. Adamson Departmeiir of Chemistry, Uiiicersity of Soutlierii Califortiia Los Aiigeles, Califorilia 90007 Receiced July 23, I968 sion spectrum is shown in Figure 2 (as determined with 400-mp stimulating light, using an Aminco emission spectrometer). In the presence of the complex, how- ever, the phosphorescent or triplet-state emission is quenched, but not that of the first singlet state. The implication is that it is the triplet state that is active in the sensitization. In addition, a series of measurements was carried out using benzophenone as sensitizer and various concen- trations of complex. An oxygen effect is present at least in this particular system, since the yield for Co(I1) production rose from 0.064 to 0.36 on thorough de- gassing of the solution. Further, the quantum yield of the nondeaerated solutions decreases with decreasing complex concentration, presumably as a result of com- petition between deactivation of the sensitizer by the complex and by other processes. The following reac- tion scheme is thus suggested hv S-+S*- - - tT  (1) T+S (2) T + C + S + Co(I1) (3) where S denotes sensitizer and T its triplet state, pro- duced cia an excited singlet state s*, T being produced in over-all quantum yield 4. It is assumed that the complex, C, may reactively deactivate T. In the present case C diminishes with time; assuming d[T]/dt = 0, i.e., stationary-state kinetics, one obtains ki kx 1 /#)obsd = 1 /'$ + (kl/kZ@)(P/CO) (4) where q50bsd is the observed quantum yield for Co(I1) formation, Co is the initial complex concentration, and fl is given by P = [CO/(CO - C)] In (CO/C) ( 5 )  with C now denoting the final complex concentration. The results are plotted according to eq 4 in Figure 3, and the intercept and slope give about unity and 0.013 for 4 and kl /k2 ,  respectively. If reaction 3 is diffusion controlled, kl should be about 6 X lo9 M-I ~ e c - ' , ~  which would then make kl about 8 X lo7 sec-I. This last value is similar to  those reported for other systems involving benzophenone. lo Our investigations are continuing, but some prelimi- nary discussion is appropriate here. The wavelengths of the stimulating light used, as well as those corre- sponding to the triplet-state energies of the various sensitizers, are much longer than the threshold wave- lengths for direct photodecomposition of these com- plexes. This situation plus the lack of any appreciable photosensitivity of the d-d bands themselves make it seem unlikely that energy transfer has occurred into the d-d or ligand field system. On the other hand, the first CT bands of these complexes lie at much higher energies than available here. The remaining possibility seems to  be that the sensitizers are interacting with CT triplet states of the complexes. Such triplet states must exist in principle, and their theoretical descrip- tion has been discussed to some extent." The actual (9) J. G. Calcert and J. N. Pitts, Jr., "Photochemistry," John Wiley (10) H. L. Backstrom and K.  Sandros, Acta Chem. Scand., 14, 48 (11) C. I<. Jprgensen, "Absorption Spectra and Bonding in Com- and Sons, Inc., New York, N. Y., 1966. (1 960). plexes,'' Pergamon Press, London, 1962. Studies of Free Radicals. 11. Chemical Properties of Nitronylnitroxides. A Unique Radical Anion Sir: In an earlier communication the preparation of the highly stable aromatic nitronylnitroxide radicals I11 (R = Ar) was described.' We now report on some unusual chemical properties of the nitronylnitroxide grouping. The alkylnitronylnitroxides I11 (R = alkyl) are pre- pared like their aromatic analogs by treatment of the bishydroxylamine I with aldehydes to  give tetrahydro- imidazoles 11, which are subsequently oxidized with aqueous sodium periodate or lead dioxide. The result- ing radicals are deep red solids which display esr spec- tra in solution having basic five-line patterns caused by coupling with two equivalent nitrogen nuclei. Further splitting of each line by the a-alkyl hydrogens usually is also observed together with occasionally resolved very weak coupling with the twelve methyl hydrogens (Table I). Table I. Coupling Constants (in gauss)5 of Nitronylnitroxides 111 in Benzene R ax aa-H a4.5-CH8 CHI 7.42  3.3  0.21 CH2CH3 7.42 2.0 (CH2)izCHs 7.45  2 . 1  CHiOH 7.48  2 . 2  0 . 2  CH(CH3)z 7 . 5  -0.9 CH(OC2H& 7.25 <o. 5 0 .24  a =k0.03 gauss. Although many of these radicals can be stored at room temperature for several weeks without decomposi- tion, the radicals bearing a hydrogens are somewhat less stable than their aromatic analogs. By comparison, however, the hydroxymethylnitronylnitroxide I11 (R = CH20H)z  was exceptionally labile and was found to undergo a gradual change on standing that could be greatly accelerated in alcoholic sodium hydroxide. The almost exclusive product of this reaction was a new red water-soluble radical, identified as 111, R = H,2  from its esr pattern of five doublets [ax = 7.26, (1) J. H. Osiecki and E. F. Ullman, J .  Am. Chem. SOC., 90, 1078 (2) Mass spectral and analytical data were in accord with the assigned (1968). structure. Communications to the Editor 

Photosensitized Decomposition of Some Cobalt Ammines

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Photosensitized Decomposition of Some Cobalt Ammines

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