6655
Intermediates in the Photochemistry of Amine-Oxalate Complexes of Cobalt (111) in Aqueous Solution' Anthony F. VaudoYzaEvan R. Kantrowitz,2aMorton Z. Hoffman,*2n Elias Papaconstantinou,2aand John F. Endicott2b Contributionf r o m the Departments of Chemistry, Boston University, Boston, Massachusetts 02215, and Wayne State University, Detroit, Michigan 48202. Received March 23, 1972 Abstract: The 254-nm photolysis of (NH3)4Co(C204)i and (NH3)sCo(C204)+ generates Co2+and COn in a 1 : 1 ratio. The values of +c02+ in deoxygenated acidic solution (pH 1) are 0.90 and 0.65, respectively; for the mono-
is increased somewhat (0.84) when the free end of the ligand is deprotonated (pK, = reduces these & z + values by a factor of 2. In the case of (en)+Zo(C204)+, the yield of Co2+ is independent of the presence of O2at pH 1, is somewhat dependent on O2presence at pH 2.7-2.9, and is increased during a post-irradiation period. At pH 1, the post-irradiation formation of Co2f is via fist-order kinetics with k = 8.7 x 10-5 sec-l at 25". The C02/Co*+ratio also shows this post-irradiation behavior, decreasing from 2.2 to 1.5 over a 24-hr period following irradiation. In addition, a new product is obtained by ion-exchange chromatography which is identified as the C-bonded formato linkage isomer, (en)z(H20)CoC02H2+ (pK, = 2.6), and which spontaneously decays to Coz+ with an activation energy of about 21 kcal mol-'. Flash photolysis of these complexes reveals two transient species, the longer lived of which is assigned as the C-bonded formato linkage isomer of that complex arising from heterolytic C-C bond scission of the oxalate ligand and rotation of the resulting "carbene" within the coordination sphere of the complex. The excited state initially populated by the absorption of radiation is seen to undergo radiationless transition to at least two other electronic states: a charge-transfer excited state which produces the direct primary yield of Co2+and the accompanying oxalate radical, and a ligand excited state which gives rise to the decomposition of the oxalate within the coordination sphere. In (en)Ko(GO4)+,the ratio of the formation of the C-bonded formato species to the direct, primary generation of Co2+is -4. The observed photochemistry can be explained in terms of the radicals accompanying the direct primary generation of Co2+and the intramolecular ligand-to-metal electron transfer in the thermal decay of the C-bonded formato species. The ammine-oxalate complexes are reduced by the protonated and deprotonated forms of and C 0 2 - radicals producing a secondary source of Co2+; O2scavenges these radicals. The en complex is apparently inert to the protonated forms of these radicals but can be reduced by the basic forms. These reactivity differences are in accord with the redox potentials of the species involved. dentate oxalate complex,
2.2). The presence of
0 2
T
he photochemistry of oxalate complexes of transition metals has had a long and honorable history. However, despite the importance of these compounds as standard chemical actinometers, many of the mechanistic details of the oxidation of the coordinated oxalate and the reduction of the metal center have not been established. In particular, the reaction intermediates remain virtually uncharacterized. Interestingly, the mechanisms proposed to account for the redox behavior of trivalent-metal oxalates are identical if allowance is made for secondary reactions of the reduced metal.3 Using C O I I I ( C ~ O ~ )as ~ ~ an - e ~ a m p l e ,the ~ generally accepted mechanism proceeds via the generation of a radical in the primary photochemical act through one-equivalent oxidation of C2042-followed by the efficient scavenging of the radical by the substrate. hu
(1)
c 0 x r 1 ( c ~ o ~--t ) ~ 3 -c011(czo~)z2-f CnOaCO"(CzOa)z2-
CzOa-
-3 C O 2 +
+ C0111(C20r)33-
----f CO"
(2)
f 2c~o4~-f 3C20a2- f 2coz
(3)
The fact that the quantum yield of Co2+ production, q5c02+, is less than unity has been attributed to the in(1) Presented in part at the 161st National Meeting of the American Chemical Society, Los Angeles, Calif., March 1971. (2) (a) Boston University; (b) Wayne State University. (3) V. Balzani and V. Carassiti, "Photochemistry of Coordination Compounds," Academic Press, New York, N. Y . , 1970. (4) T. B. Copestake and N. Uri, Proc. Roy. Soc., Ser. A , 228, 252 ( 1955).
efficiency of the primary reaction 1. The presence of a radical has been inferred from experiments involving the polymerization of vinyl monomer and the reduction of Hg(l1). It is important to note that O2 and [substrate] were observed to have no effect on q5cO2+which was also independent of the intensity of absorbed radiation, I,. These features could result if reaction 3 were much more rapid than any scavenging of the radical by O2 or the bimolecular combination and/or disproportionation of the radicals. Thus, q5c02+has been assumed to be twice that of the primary process 1 involving the homolytic scission of the Co-0 b0nd.j It must be pointed out that in n o previous study has the assumed efficiency of reaction 3 actually been demonstratedexcept by analogy to the Fe(C204)33-case6 in which +Fez< > 1. The flash photolysis results of Parker and Hatchard' did not produce any insight into the mechanism of the they observed an initial inC O ( C ~ O3-~ )photolysis; ~ stantaneous rise in absorption at 313 nm followed by a rapid and then slow fall in absorption. They suggested that the rapid reaction is the bimolecular reaction 3, and the slow reaction represents (2) or dissociation of the product of (3) although they recognized that there are other possibilities. Recently Gross reported8 that (5) G. B. Porter, J. G. W. Doering, and S. Karanka, J . Amer. Chem. SOC.,84, 4027 (1962). (6) C. G. Hatchard and C. A. Parker, Proc. Roy. SOC.,Ser. A , 235, 518 (1956). (7) C. A. Parker and C. G. Hatchard, J . Phys. Chem., 63, 22 (1959). (8) R. C. Gross, Abstracts of the Sixth International Conference of Photochemistry, Bordeaux, France, Sept 1971.
Hoffman, et al. 1 Photochemistry of Amine-Oxalate Complexes of Co(III)
6656
k3 = 4 X IO4 M-1 sec-l. However, based on our own investigation of this c o m p l e ~ we , ~ estimate k3 to be at least four orders of magnitude higher than this value. The C204-radical generated in reaction 1 is believed’O to be relatively stable with its decomposition to give COS- and COS at a rate slow compared to reaction 3. The reaction of OH radicals generated pulse radiolytically with Cz042-in aqueous solution” produces a radical which has an absorption spectrum similar to that of COz- consisting of a maximum at 235 nm with a long tail down to 350 nm; the decay of C0,- is second order with 2k = 1.5 X IO9 M-I sec-l. Using the extinction coefficient of COS-, the rate constant of the oxalate decay was found to be identical with that of COS-. Using similar pulse radiolysis techniques, Getoff, et aZ.,12determined 2k4 = 9.6 X IO8 M-I sec-l. 2C204-
+2c02 + C2042-
(4)
In acidic solution, where the radical species would be expected to be protonated, the second-order rate constant is believedI2 to be several orders of magnitude lower although the experimental data are unclear on H+ is 3.9 i this point. The pK, for COzH $ COS0.313 with the decay of both the protonated and deprotonated radicals following identical kinetics” although the COyHradical disproportionates to CO, and H C 0 2 H while C02- combines to yield oxalate.14 Cot- reacts rapidly with OSuia electron transferI3 and presumably CO,H is also quenched by 0 2 . COS- is a stronger reducing agent than C02H,’j and presumably the same characteristics are shown by C204-/C204H although no direct determinations have been made. Way and FilipesculGdetermined 6co2-for the photolysis of ammine-oxalate complexes of Co(II1) at a number of wavelengths including 254 nm. They invoked the same general mechanism for these complexes, (1)-(4), although their data could not provide a test of the mechanism. Furthermore, there is no indication that their photolyses, performed in I-cm spectrophotometer cells, were carried out in the absence of 02. Indeed, we had previously reported” that &olT is reduced by a factor of about 2 when OS rather than Nz is bubbled through (NH3)4Co(Ca04)+solutions during 254-nm irradiation. This present study was undertaken to examine the ultraviolet photochemical behavior of (NH3)4Co(C20$+, (en),Co(C,O,)+ (where en = ethylenediamine), and (NH3)&o(CSO4)+ in aqueous solution in order to clarify some of the aspects of the general mechanism. Unlike C O ( C ~ O ~ )which ~ ~ - undergoes H+-catalyzed thermal oxidation-reduction to give Co Sf, these amineoxalate complexes are stable even in hot, acidic solutions. Because of the lower reduction potentials of
+
(9) N. S. Rowan, R. M. Milburn, and M. 2.Hoffman, Inorg. Chem., 2272 (1972). (10) 2. Simon, Reo. Roum. Chim., 14, 705 (1969). (11) P. Neta, M. Simic, and E. Hayon, J . Phys. Chem., 73, 4207 (1969). (12) N. Getoff, F. Schworer, V. M. Markovic, K. Sehested, and S.0. Nielsen, ibid., 75, 749 (1971). (13) A. Fojtik, G. Czapski, and A. Henglein, ibid., 74,3204 (1970). (14) H. Fricke, E. J. Hart, and H. P. Smith, J . Chem. Phys., 6, 229 (1938); E. J. Hart, J. Amer. Chem. SOC.,83,567 (1961). (15) J. Lilie, G. Beck, and A. Henglein, Ber. Bunsenges Phys. Chem., 75, 458 (1971). (16) H. Way and N. Filipescu, Inorg. Chem., 8, 1609 (1969). (17) J. F. Endicott, M. 2.Hoffman, and B. L. Mollicone, Abstracts, 152nd National Meeting of the American Chemical Society, New York, N. Y., Sept 1966, No. V38. 11,
Journal of the American Chemical Society 1 94:19
these mixed ligand complexes, we expected that the radical-substrate reaction 3 would be slowed down. The generality of the effect of O2 on +co2- for these complexes1* indicated that the quantum yield of reaction 1 could be evaluated without resort to mechanistic hypothesis; the proof of the existence of a radical offered the possibility of studying its chemistry. Finally, the paucity of published flash photolysis data on oxalate systems and the potential this technique has when applied to Co(II1) complexes19 encouraged us to examine the amine-oxalate complexes in detail. We hoped that a critical test of the time-honored mechanism could be made. Experimental Section Materials. The chloride and perchlorate salts of (NH& Co(GOd)+, (en)Ko(C20a)+, and (NH3)sCo(Ct04)’ were prepared using procedures described in the literature. 20-zz The perchlorate salts were used initially until the insensitivity of the photolysis process to the presence of C1- was established. The spectral characteristics of the compounds agreed to within a few per cent of the literature values and elemental analysis indicated their general purity. All solutions were prepared using distilled and deionized water and reagent grade chemicals. Apparatus and Procedures. The 254-nm radiation was generated by low-pressure mercury resonance lamps; the various units coverto 1 X ing a range of incident intensities (lo)from 6.6 X einstein I.-’ min-’ have already been described. 23--25 Primary actinometry was carried out using uranyl oxalatezs or potassium ferrioxalate.6 Flash photolysis was performed using a Xenon Corp. Model 720 unit with xenon-filled flash lamps (500 J; lie time = 30 rsec) and an optical cell 22 cm in length, The 150-W xenon analyzing lamp was operated either off a bank of batteries or off a controlled voltage power supply. The output of the photomultiplier tube was displayed on a Tektronix Model 545 storage oscilloscope. For transient half-lives of greater than 5 sec, a chart recorder was used. Spectra were recorded on a Cary 14, 16 or Unicam 800 spectrophotometer. Solutions were prepared from the solid immediately before use. If deoxygenation was required, the solution was purged with Cr2+scrubbed Nz before irradiation. During continuous photolysis, gas bubbling and magnetic stirring ensured the homogeneity of the solution. Temperature was controlled by air or water thermostating. Samples could be removed after a desired exposure time by means of a syringe and a Teflon needle passing through a rubber serum cap. Cot+ determinations were performed as soon after photolysis as possible, generally within 0.5 hr. Gaseous products were detected and determined by gas chromatography from photolyses performed iiz oacuo using a F & M Model 810 instrument with thermal conductivity detection and a 12-ft molecular sieve column. Coz+was determined using a modification of Kitson’s method:27 4 ml of the solution containing Co2+( < 5 X M ) and 8 ml of a 50z solution of NH,SCN were diluted to 25 ml with acetone, and the absorbance at 620 nm was recorded ( 6 6 2 0 1.86 X lo3M-’ cm-l 1. Cation-exchange resin (Dowex 50W-4X, 200-400 mesh, H+ form) was used for the separation of complex products. For the determination of +c02 + from continuous photolysis, the yield of Co2+ as a function of exposure time was determined for the first 20-25z of reaction; plots were generally linear with evidence of the fall-off in rate as the substrate was depleted and I , decreased. The initial rate of the reaction was determined from such zero-order (18) B. L. Mollicone, M.A. Thesis, Boston University, 1966. (19) G. Caspari, R. G. Hughes, J. F. Endicott, and M. Z. Hoffman, J . Amer. Chem. SOC.,92,6801 (1970). (20) W. G. Palmer, “Experimental Inorganic Chemistry,” Cambridge University Press, New York, N. Y., 1954, p 547. (21) A. Werner and A. Wilmos, 2.Anorg. Chem., 21, 145 (1899). (22) P. Saffir and H. Taube, J . Amer. Chem. Soc., 82, 13 (1960). (23) J. F. Endicott and M. 2.Hoffman, ibid., 87,3348 (1965). (24) J. F. Endicott, M. 2.Hoffman, and L. S. Beres, J . PhJJS. Chem., 74, 1021 (1970). (25) E. R. Kantrowitz, M. Z. Hoffman, and J. F. Endicott, ibid., 75, 1914 (1971). (26) W. A. Noyes, Jr., and P. A. Leighton, “The Photochemistry of Gases,” Reinhold, New York, N. Y., 1941. (27) R. E. Kitson, Anal. Chem., 22, 664 (1950).
September 20, 1972
6657
r
plots. Although primary actinometry had been performed on each photolysis unit and the dependence of I , on the optical density of the solution was known, secondary actinometry was performed at the time of each photolysis in order to account for any variation in the output of the lamps. Specifically, each photolysis run was bracketed by an irradiation of Co(NJ&)5CI2+or CO(NH~)~OZCCH~Z+ under identical conditions of solution absorbance. The values of & - o ~ + for these complexes, 0.17 and 0.19, respectively, are well established under a variety of experimental condition^.^^^ 2s The 0.8 values of $c0z- determined in this way are known at least to within s' &lo% 0 In flash photolysis, the solution was forced into the N2-flushed 0.6 optical cell from a storage flask by a positive pressure of Nz. All solutions were discarded after one flash. For experiments in which 238 nm4 the wavelength of the flash radiation was restricted using filter solutions, a cell with an outer jacket was employed. Experiments were normally conducted at ambient temperature; variation of tempera, ture was accomplished either by thermostating the storage flask 0.2 and/or the optical cell. Where the optical cell was not thermostated, the temperature of the flashed solution did not differ by more than 3" from that of the storage vessel. Thermostating of the opti0 cal cell ensured that the solution temperature was constant within 200 240 0.3 I
I
I
I
I
I
I
I
I
1
I
0,41 n
".
Results Spectra. The spectra of all three complexes show the general features common to Co(II1)-amine complexes with an oxygen-bonded carboxylate ligand : ligand-field (d-d) bands at 505 and 360 nm (E -100 M-1 cm-l) and the intense (E > l o 3 M-l cm-l) chargetransfer bands in the ultraviolet. In the case of the bidentate oxalate complexes, (NH3)4Co(C204)+and (en)zCo(Cz04)+,the spectra were independent of p H up to 2 M HC104. The spectrum of (NH3)5Co(C204)+ showed a variation with pH in the same manner as had been described by Andrade and Taube.28 Figure 1 shows the appearance of a new absorption band in the 290-nm region as the pH is raised, indicative of the acid dissociation reaction (NH3)sCo(C204H)Z+ H+ for which pKa = 2.2. The (NH3)5Co(C204)+ common isosbestic point at 238 nm and the similarity of the spectra at pH 1.4 and in 2 M HC104 indicate the absence of any effect due to ionic strength. Continuous Photolysis. Extensive photolysis converted the Co(II1) quantitatively t o Co2+. Unbuffered solutions photolyzed at pH >3.5 became basic producing a green-brown precipitate of the hydrous oxide of Co(I1). No analyses were conducted on such solutions since the uncertainty in the actinometry due to the increased opacity of the system would render quantum yield values meaningless. Oxygen bubbled for 20 min through the photolyte containing Co2+ produced no discernible reoxidation to Co(II1) in acidic solutions. When photolyzed, the solutions of (NH3)4Co(C204)+ and (NH3)jCo(C~04)+ lost their pink color and became pale. In the case of (en)2Co(C204)+,the observation of a post-irradiation production of Co2+ and time-dependent color changes indicated that the system was more complex than originally imagined. For example, a solution at pH 2.9 under N2 purge turned orangegold after 8 min of photolysis. However, within 2-3 hr after photolysis, the solution had turned colorless. The effect was the same whether or not the solution, in a Pyrex flask, was exposed to fluorescent lights, When heated to 50°, the orange color bleached in less than 1 hr. At pH 1.2, the solution was orange-gold after photolysis and bleached, with the production of
+
(28) C. Andrade and H.Taube, Inorg. Chem., 5,1087 (1966).
m
280
340
A, nm
Figure 1. Uv spectrum of (NH3)jCo(C2Oa)+;[complex] = 6.65 X loM4M : (A) the acid form in 2 M HC104; (B) the basic form at pH 6.
additional Co2+, in 24 hr. The details of this postirradiation effect will be discussed in a later section. Quantum yields for Co2+ production for the three complexes were determined as a function of pH in the presence of Nz or O2 and are given in Table I. For Table 1. Co2+ Quantum Yields from the 254-nm Photolysis of Amine-Oxalate Complexes of CO(III)~ Complex*
M-1
cm-1
(NH3)aCo(C204)+4 . 2 X l o 3
(NH3),Co(C2O4)+ Function of pH; see Figure 1
(en)2Co(C20r)+
6 . 7 X lo3
-
pH
Gas purge
@cOz+
1.2 1.2 2.4 2.4 1.1
Nz
0.90 0.45 0.81 0.40 0.65
1.3 2.6 2.6 1.2 1.2 1 .o 1.o 2.9 2.7
0 2
0 2
NP 0 2
N2 N? 0 2
N? 0 2
NP 0 2
N? 0 2
0.33 0.84 0.41 0.084 0.084 0 . 36d 0.34d 0.27 0.19
[Complex] = 1.4 X 10-3einstein 1.-' min-l. I, 5 X 3.4 X 10-3 M. c Single determinations with error estimated to be & l o % ; Co2+ analyses performed within 0.5 hr of photolysis. Co2+ analyses performed 24 hr after photolysis.
(NH3)4Co(C204)+, the presence of up to 5 M methanol or 2-propanol had no effect on 4cOz+in N,. Furthermore, q5col+ was independent of [(NH3)4Co(C204)+] from 2.5 X le3to 1.8 X l e 2 M , ionic strength, and the presence of C1- (up to 0.1 M HCl). The presence of acetate buffer (up to l e 2 M ) in the pH 4-6 region had the effect of lowering 4cOz+by about 50z. Plots of log (rate of Co2+ production) us. log (Ia)over three orders of magnitude of I. gave straight lines with slopes greater than 0.9. Photolyses of the complexes at 254 nm in uucuo generated Co2+ and COz in quantities shown in Table 11. As well, gas chromatographic analysis revealed that were also produced. traces of Hz and CO
(
