3116
Intermediates in the Photochemistry of Carbonato-Amine Complexes of Cobalt(II1). CO; Radicals and the Aquocarbonato Complex’ Virgil W. Cope,2 Schoen-nan Chen, and Morton Z. Hoffman* Contributionfrom the Department of Chemistry, Boston University, Boston, Massachusetts 02215. Received August 5, 1972 Abstract: The 254-nm irradiation of neutral, buffered, solutions of CO(NH~)~CO~+ and Co(en)zCOaf produces Coaq2+with a relatively low quantum yield (0.06 and 0.02, respectively); Co(NH&C03+also is photoreduced. also ~ +generates CO(NH~)~(OH&~+ with a quantum yield of 0.11 =k 0.03. The The irradiation of C O ( N H ~ ) ~ C O flash photolysis of the three complexes yields a transient intermediate which has A., 600 nm and which is identified as the C0,- radical. The flash photolysis of the bidentate carbonato complexes also reveals a long-lived tail absorption in the 300-nm region which decays via pH-dependent fist-order kinetics. The behavior of this intermediate is identical with that of the aquocarbonato species proposed as an intermediate in the acid-catalyzed aquation of the bidentate carbonato complexes. It is proposed that photochemical excitation of these complexes in the intense ligand-to-metal charge-transfer band produces the charge-transfer singlet state (‘CT) which generates, via radiationless transition, the charge-transfer triplet state (CT)and a ligand excited state The 8cT state is the precursor to the oxidation-reduction products; protonation of the carbonato ligand in the L* state is viewed as causing ring opening and aquation.
a*).
I
t has been amply d e m ~ n s t r a t e d ~that - ~ the predompounds offer a unique opportunity to compare the inant process resulting from the irradiation of photochemical and thermal behavior of coordination amine complexes of Co(II1) in their intense ligandcomplexes. to-metal charge-transfer band is the generation of Co(NH3)d203+ decarboxylates quite rapidlyl5 in a Coas2+ and a free radical derived from the one-electron first-order manner for which kobsd = kl[H+]/([H+] oxidation of a ligand. In addition, other processes K2),where kl is the rate constant for the unimolecular arising from this excitation have been observed such decarboxylation of the protonated form of the complex as NHBaquation [ C O ( N H ~ ) ~loCand ~ ~ +C O ( N H ~ ) , N ~ ~ + ~ ] , Co(NHs)&03H*+ --t Co(NHa)bOH*++ Con (1) linkage isomerization [Co(NH&NOZz+l 1 and Coand Kz is the equilibrium constant for (NH&OzCH2+ 12], and ligand decomposition [Co(NH3)5Cz04+and C O ( N H ~ ) ~ C x]. ~ O ~In+ the case of CO(NH~)~CO~HZ+ Co(NHa)aCOa+ + H+ ( 2) the bidentate oxalato complex, no evidence of photoAt the low pH limit kl = 1.25 sec-l and pKz = 6.4. chemically induced ring opening, aquation, and subThe value of kobsd at p H >8 is as much as two orders sequent loss of oxalate was obtained.x As one part of magnitude slower than in acidic soIution.’6 The of our continuing study of the effect of multidentate activation parameters for reaction 2 are AH* = 17.0 chelation on the photochemistry of transition metal f 0.5 kcalmol-landAS* = -0.5 f l.Oeu.lB complexes, we chose to examine the carbonato-amine The aquation of C O ( N H ~ ) ~ C Ooccurs ~ + through recomplexes of Co(II1). These bidentate carbonato action with HzOand H 3 0 + and has been postulated to complexes are known to undergo thermal ring opening proceed via the formation of a ring-opened aquocarand aquation and most of the mechanistic details have bonato intermediate.” been well ~ 1 a r i f i e d . l ~ The ~ ~properties ~ of these comCo(NH3)aCOa+ + HZ0 J _ C ~ ~ - C ~ N H ~ ) ~ ( O H ) ( C (3) O~H)+
+
(1) This work was presented, in part, at the 163rd National Meeting of the American Chemical Society, Boston, Mass., April 1972. (2) Participant, NSF College Teachers Research Program, Summer 1971, from theDepartment of Chemistry, University of Michigan, Flint College, Flint, Mich. 48503. (3) V. Balzani and V. Carassiti, “Photochemistry of Coordination Compounds,” Academic Press, New York, N. Y . , 1970. (4) G. Caspari, R. G. Hughes, J. F. Endicott, and M. Z . Hoffman,
J . Amer. Chem. Soc., 92, 6801 (1970). ( 5 ) J. F. Endicott, M. 2. Hoffman, and L. S , Beres, J . Phys. Chem., 74, 1021 (1970). (6) E. R. Kantrowitz, M. Z. Hoffman, and J. F. Endicott, ibid., 75, -1914 - - 11971) . -,. (7) E. R. Kantrowitz, M. Z . Hoffman, and K. M. Schilling, ibid., 76, 2492 (1972). (8) A. F. Vaudo, E. R. Kantrowitz. M. Z. Hoffman. E. Papaconstantinou, and J. F. Endicott, J . Amer. Chem. SOC.,94, 6655 (1972). (9) D. D. Campano, E. R. Kantrowitz, and M. Z. Hoffman, 164th
where k, = 1.3 X
sec-l and k-3 -10-l
sec-l
+
Co(NHa)4CO3+ H30++C ~ ~ - C ~ ( N H & ( O H ~ ) ( C O I(4) H)~+
with k4 = 1.5 M-l sec-1. The various acid-base forms of the aquocarbonato intermediate are related by the following equilibria cis-Co(NHJ a(OH&08H)z+ C~S-CO(NH~)~(OH)(CO~H)’ H+ ( 5 )
+
% - -
National Meeting of the American Chemical Society, New York, N. Y . , Sept 1972, Abstract INOR 23. (10) L. Moggi, N. Sabbatini, and V. Balzani, Garz. Chim. Ital., 97, 980(1967). (11) V. Balzani, R. Ballardini, N. Sabbatini, and L. Moggi, Inorg. Chem.. 7. 1398 (1968). (12).A’, F. Vaudo,’ E. R. Kantrowitz, and M. Z. Hoffman, J . Amer. Chem. SOC.,93,6698 (1971).
Journal of the American Chemical Society
c~s-CO(NH~)~(OH)(CO~H)+ cis-Co(NHs)r(OH)(COa)’
+ H+
(6)
(13) C. R. P. Mac-Coll, Coord. Chem. Reu., 4,147 (1969). (14) K. V. Krishnamurty, G . M. Harris, and V. S . Sastri, Chem. Reo., 70, 171 (1970). (15) T. P. Dasgupta and G. M. Harris, J . Amer. Chem. Soc., 90, 6360 (1968). (16) D. J. Francis and R. B. Jordan, ibid., 89, 5591 (1967); 91, 6626 (1969). (17) T. P. Dasgupta and G. M. Harris, ibid., 91,3207 (1969).
1 95:lO May 16, 1973
where pK6 is estimatedl8 to be -6 The decarboxylation reactions
-
3117
8.6.19
A phosphate buffer stock solution was prepared to pH 6.5 by the addition of concentrated NaOH to 0.4 M NaH2P04solution. The solutions for continuous photolysis were prepared from this stock Co(NHs)r(OHz)(COsH)'+ +Co(NHs)c(OHz)(OH)'+ COS (7) solution so that [total phosphate] = 0.02 M ; pH was adjusted using concentrated HC104 or NaOH and measured with a Beckman Co(NHs)r(OH)(COaH)++Co(NHa)r(OH)a+ COz (8) Expandomatic pH meter. have rate constants estimated to be k7 2 sec-l and All solutions were prepared using distilled water. In some of the flash photolysis experiments, the solutions were prepared from ks N 1 X 10-2 sec-l. Isomerization of the cis-aquotriply distilled water which had been radiolyzed and photolyzed. carbonato intermediate is too slow to be considered Photochemical Apparatus. The 254-nm photolysis units have in this competitive schemelgand the neutral deprotonated already been described! Quantum yields were determined relative product C O ( N H ~ ) ~ ( O H ) ( C Ois~ ) ~fairly resistant to to the photolysis of Co(NH3)s0zCCHa2+in 0.1 M HClO, taking $cow = 0.19.8 Flash photolysis was performed using a 22-cm hydrolysis or isomerization. l9tZ0 Because of the comoptical cell and procedures which have already been reported. plex ring-opening and decarboxylation mechanism, For monitoring the 300-nm region, a RCA 1P28 photomultiplier measured activation parameters reflect the combined was used; for the 600-nm region, a Hamamatsu R136 PM tube was contributions from the individual steps. However, operated at about 600 V. The linearity of the detection system the point must be made that the decarboxylation of the was checked and found to be satisfactory. A glass filter (for shortwavelength monitoring, Vycor filter; for long-wavelength, Corning aquocarbonato intermediate (reaction 7) is kinetically very similar to the direct decarboxylation of C O ( N H ~ ) ~ - 3-72) was always in place between the analyzing lamp (Osram 150w Xe lamp) and the cell and an electric shutter restricted the exC03+(reaction 1). posure of the solution to the light. The solutions used were very For C0(en)~C0~+, the same mechanism has been dilute ( 4 X lo+ M ) to ensure homogeneous absorption of the photolyzing flash and the distribution of transient species in the cell. found to hold1*with k7 2 sec-l and pK5 5.3.l9 Transient absorption spectra were obtained from the optical density Because the slow ring opening of the bidentate carof the absorption measured 50 gsec after the start of the flash as a bonate is the rate determining step, the presence and function of monitoring wavelength. Rate constants were calcudisappearance of the aquocarbonato intermediate has lated from a least-square computer fit of the data to the appropriate not been directly observable in the thermal reaction rate law. Procedures. Because of the acid-catalyzed thermal aquation of in neutral or mildly acidic solution. In 2 M HCI04, these carbonato complexes, the solid salt was added to the buffer where the H30+-catalyzed ring-opening step would be solution at the last moment and all subsequent procedures were somewhat faster than the subsequent decarboxylacarried out as rapidly as possible. In the continuous photolysis tion of the intermediate, experiments using stoppedexperiments, we found the optimum conditions for the bidentate flow techniques*' have shown the formation and complexes to be pH 6.4 and 2 X 10-3 M complex in the phosphate buffer. Under these conditions the thermal aquation is relatively decay of the intermediate from Co(tren)C03+ with the slow and the solution does not change by more than 0.2 unit during complex consecutive reaction kinetics analyzed in photolysis. Photolyses were run to 20-25 completion (aliquots terms of the estimated rate constants. From these were taken periodically for CO,,~+analysis) and no Co(I1) hydroxide experiments it is known that the spectra of the proor phosphate precipitated. Plots of Coaq2+yield DS. time, from which $cow values were obtained, were linear. Coa92+analyses posed aquocarbonato species in the ultraviolet region were performed using the thiocyanate complexation method deis more intense than that of the chelated carbonato scribed previous1y;e the presence of the phosphate buffer had no complex. effect on the analytical procedure. Analysis for CoZ+ was carried In this paper we report the results of the continuous out immediately. The photolyte was first acidified with HClO4 to and flash photolysis of carbonato-amine complexes pH 1 in order to destroy the carbonato substrate and to dissolve any microcrystals of Co(I1) salts. In the flash photolysis experiments, of Co(II1) [Co(NH3)&03+, C O ( N H ~ ) ~ C O Co(en)z~+, solutions were prepared without the complex which was added as COa+]and the observation of photoinduced ring openthe solid immediately prior to the experiment. This was particuing of the chelated ligand. larly critical at pH 4 or at elevated temperatures where the useful life of the solution was about 5 min. Alternatively, in neutral or Experimental Section mildly alkaline solution, a fresh stock solution or 0.01 M COChemicals. Co(NHa)&03+ (as the nitrate salt) was prepared (NH3)4C03+was used to prepare the solutions for flashing. The according to literature procedures22 as was CO(NH~)~CO~+ (as the solutions for flashing were prepared in less than 1 min and the stock chloride salt).23 Conversion of the bidentate complex to the persolution was discarded within 2 hr of preparation. Under these chlorate salt was accomplished by dissolving the chloride salt in a experimental conditions thermal aquation is never an important minimum volume of water at 60",filtering quickly, and adding the feature. For C O ( N H ~ ) ~ C Otl/, ~ + , 80 min for thermal aquation filtrate to an equal volume of 50% NaClO, solution (w/v). A at pH > 5 ; even at pH 4, ti/, > 30 min. Solutions were rarely used bri&t purple solid was obtained after 1 hr at 0" which was filtered, for as much as 15 min and considerably less at lower pH values and washed with alcohokther, and air dried. Recrystallization did elevated temperatures. Thus, we are confident that our bidentate not change the absorption spectrum of the complex which comcarbonato solutions contained < l o % thermal aquation products pared very well with the literature.24 at the time they were flashed. For Co(NH3)jC03+, the flash solu[Co(en)lC03]C104was prepareda5 from [Co(en)zCl]Cl which in tion was prepared directly from the solid. All solutions were disturn had been prepared by the procedure of BailaraZb The speccarded after one flash. Since 0 2 had no effect on the kinetics of the trum of the complex was unchanged by recrystallization but showed transient species, air-saturated solutions were used directly as preslight deviations from the literature value94 (found: A,, 511, pared, The ionic strength of the solution was established by the 359 nm, emax 129, 118 M-l cm-l; literature: A,, 514, 362 nm, phosphate buffer, added NaCIO4, or the NaOH in alkaline solution. emax 135,123 M-1cm-1). Ion exchange separation of the photolysis products was carried out on a Dowex 50-4X, 200-400 mesh resin in a 5 X 0.6 cm column. The resin was first washed with 15 ml of 1 M NaC104to convert it to (18) V. S. Sastri and G. M. Harris, J. Amer. Chem. Soc., 92,2943 (1970). the Na+ form, then washed with 5 ml of water. This final eluate (19) H. Scheidegger and G. Schwarzenbach. Chimiu, 19, 166 (1965). (20) M. E. Farago, Coord. Chem. Rev., 1,66 (1966). was neutral to pH paper. A sample of the neutral photolyte solu(21) T. P. Dasgupta and G. M. Harris, J. Rmer. Chem. SOC.,93, 91 tion (or, alternatively, an unphotolyzed blank rolution) was placed (1971). on the column and washed with two 1-ml portions of water. The (22) F. Basolo and R. K. Murmann, Inorg. Syn., 4,171 (1953). column was then eluted with 0.2 M NaC104 which caused the ma(23) A. B. Lamb and E. B. Damon, J. Amer. Chem. SOC.,59, 383 terial on the solumn to separate into a moving purple band, CO(1937). (NH3)4C03+,and a stationary orange band which could only be (24) Y. Shimura and R. Tsuchida, Bull. Chem. SOC.Jup., 28, 572 eluted at higher NaC104 concentrations. The orange band, the (1 9 5 5). elution characteristics of which indicated that its charge was greater (25) M. Linhard and G. Stirn, 2.Anorgun. Chem., 268, 105 (1952). than 2+, was identified as CO(NH~)~(OH?)Z~+. The total CO (26) J. C. Bailar, Inorg. Syn., 2,222 (1946).
and pKe
-
-
+
+
-
-
Cope, Chen, Hoffman 1 Photochemistry of Cobalt(II1) Complexes
3118 007
I
I
1
matography enabled a determination of the quantum yield of photoaquation to be made. Since the extent of thermal aquation of an unphotolyzed 2 X 10-8 M C O ( N H ~ ) ~ C Osolution ~+ at pH 6.4 and room temperature (handled in exactly the same way as a photoand represented allyzed solution) was about 35 most ten times the extent of apparent photoaquation, a reliable estimate of the quantum yield of photoaquation could not be made under these conditions. Rather, the solutions were prepared and photolyzed at 5". Table I1 shows the results of these duplicate ex-
I
0.05 a06
0.04
-
G 0.03-
Table J.I. Determination of Photoaquation Quantum Yielda
0.02[ 0.01
'210
2hO
2$0
360
Initial concentration of Co(NHa)rCOa+ [Co(NHa)~COa+] remaining after 8-min exposure Thermal aquation (9 1%) CosqZ+yield Photoaquation yield
3;O
A , nm
Figure 1. The spectrum of the long-lived intermediate from the M Co(Nk&)4COs+, pH 6.4. Optical flash photolysis of 7.4 X density read 50 piec after the start of the flash.
#cow =
dJaq a
present before photolysis and in the eluted purple band was determined by converting the material to Co,,Z+ by boiling the solution in concentrated NaOH, acidifying, and analyzing for Coas2+in the usual way.@ Thus, by knowing the loss of carbonato complex by thermal aquation and photoreduction, an estimate of the yield of photoaquation could be made. Gas analysis for noncondensable gases was performed by gas chromatography on solutions photolyzed in vacuo.@
=
2.11 X leaM 1.30 x 0.19 X 0.23 X 0.39 X
10-3 M l(r3 M
l(r3M l(r3 M
0.07
0.11 f 0.03
Results of duplicate determinations; T = 5 " .
periments from which is calculated c $ =~ 0.11 ~ f 0.03 = 0.07 which was determined relative based on to ferrioxalate actinometry. Flash Photolysis Studies. The flash photolysis of the three complexes generated a transient absorption at 600 nm that has been characterized as the COS- radical. 27-29 Results In addition, C O ( N H ~ ) ~ C Oand ~ + C 0 ( e n ) ~ C 0 ~gener+ Continuous Photolysis Studies. The quantum yields ated a long-lived transient species which absorbed in for the formation of Cow2+, +cog+, for the 254-nm the 300-nm region with no evident absorption maxirradiation of C O ( N H ~ ) ~ C Oand ~ + C0(en)~C0~+ are imum (Figure 1). Because of the absorption of the given in Table I together with some data for C O ( N H ~ ) ~ substrate, spectral measurements below 270 nm were not possible and depletion of the substrate was seen as a shift in the base line to lower absorbance after Table I. Quantum Yields of Co,,a+ Formation0 the flash. In the spectral region examined, the abPurge Additional sorption due to the transient exceeded the loss of subComplex gas solute dJCd+b strate in the flash. Co(NH3)&03+ did not yield a 0.06 Co(NHa)aCOa+ Nz corresponding long-lived absorption in the 300-nm 0 2 0.06 region. OZ 1 M 2-propanol 0.07 The full absorption was formed during the time of the 1 M 2-propanol N2 0.15 0.02 Co(en)KO3+ N2 flash and decayed very slowly via first-order kinetics 0.02 0 2 (Figure 2) which were independent of the presence OZ 1 M 2-propanol 0.01 of 02,2-propanol (0.1 M ) , and methanol (0.5 M). 1 M 2-propanol N2 0.03 The observed first-order rate constant, k o w , was 0.06c,d Co(NHs)sCOa+ Nz 0. 0 s dependent on the pH of the solution as shown in Figure Co(NHa)XOH&'+ N2 n.nic Co(enMOH2ha+ Na 3. For C O ( N H ~ ) ~ C Okobsd ~ + , exhibited a rather complex dependence on ionic strength. At pH 4.0 and [complex] = 2.0 X 1(r3 M ; 25.0'; pH 6.4; [total phosphate] = 0.02 M ; I , 6 X 1W4einstein 1.-l min-1; average of two 8.0, no dependence was found, but at pH 6.4, a plot determinations. Values known to 10.01. [complex] = 5.0 X of log k us. p ' / 2 (Figure 4) gave a linear plot with a l(r3 M ; single determinations. The other conditions remained slope of 1.1. Activation parameters, determined the same. pH 8.3. from a plot of In k/T us. I/T (Figure 5 ) for Co(NH3)dC03+ at pH 4.3, had values of AH* = 19.2 f 3.4 kcal mol-l and A S * = 5 I 10 eu. Because of the COS+, CO(NH&(OHZ)~~+, and Co(en)z(0Hz)z3+. The 254-nm photolysis of Co(NH3)&03+ at pH 6.4 in problems associated with the stability of the carbonato complex at this pH and elevated temperatures, the vacuo revealed the absence of N2, 02, and NO as gasdata show considerable scatter. Nevertheless, the eous products. The thermal yield of COz as well as the appreciable solubility of the gas at pH 6.4 rendered values quoted are considered reliable within the standard deviations indicated. its determination of no significance. Note that the presence of 0, had no effect on the values of +cop+ but that I M 2-propanol in the absence of O2 caused (27) V. W. Cope and M. Z . Hoffman, Chem. Commun., 227 (1972). (28) S.-N. Chen and M. Z. Hoffman, ibid., 991 (1972). the quantum yields to double. (29) S.-N. Chen, V. W. Cope, and M. Z. Hoffman, J . Phys. Chem., Separation of the products by ion exchange chro77, 1111 (1973). ~
-
Journal of the American Chemical Society 1 95:lO / May 16, 1973
31 19
-'"O
2
4
6
8
10
Time, sec.
Figure 2. First-order decay kinetics of the long-lived intermediate M C O ( N H ~ ) ~ C OpH ~ +6.4, , from the flash photolysis of 7.4 X 23 ', monitoring wavelength 300 nm.
Figure 3. First-order rate constants for the decay of the long-lived intermediate from CO(NH~)~CO~+ (0) and Co(en)pC08* (0). [complex] = 6.0-9.0 X M, ionic strength = 0.026-0.036 M, 23 '.
Discussion The identification of C O ( N H ~ ) ~ ( O3+H ~ as) ~a product from the 254-nm irradiation of Co(NH3)&03+ adds ring opening and aquation to the growing list of processes that can result from charge-transfer excitation of Co(II1) complexes. However, regardless of the mechanistic origins of the final products, the quantum yield values given in Table I follow some trends that have been seen before. The slightly lower 0.7 4 values for the en complexes (compared to the tetra0.1 0.2 0.3 0.4 amines) are in accord with the generally lower values P4 for such chelated complexes. Tetraammine complexes with bismonodentate ligands (of the form C O ( N H ~ ) ~ X ~Figure ) 4. The dependence of the first-order rate constant for the also tend to show diminished values of 4c0~T.3The decay of the long-lived intermediate on ionic strength (varied with = 6.0-9.0 X M, pH 6.4, 23". NaClOi). [CO(NH~)~CO~+] other quantum yield values for the carbonato complexes can be rationalized in terms of the presence of the COS- radical generated in the photolysis. The sion-controlled rates, and 4c0z+ returns to its value radical is not affected by O2 and does not exhibit any in the absence of scavengers. Although COS- reacts (pc0z+ in reactivity toward C O ( N H ~ ) ~ C O29~ +Thus, . with COaq2+and CO(NHS)~(OH&~+ with rate constants the presence of Nz and O2 is constant and must reflect of 4.4 X lo6 and 1.4 X IO7 M-l sec-l, respe~tively,2~ the generation of Co2+ in the primary photochemical these reactions would be of only minor concern due processes. In the presence of a relatively high conto the relatively low extent of reaction (10-15z) in centration of 2-propanol, COO- is scavenged, perhaps the continuous photolysis. It is not known if the via H atom transfer (or, alternatively, electron transfer reaction of C0,- with C O ( N H ~ ) ~ ( O H eventually ~)~~+ fpllowed by deprotonation), to generate the (CH3)Zyields Coaq2+ as does the reaction of other oxidizing COH radical radials with Co(II1) complexes.33s34 As far as the cos- + (CH&CHOH+ (CH&eOH + COsH(9) COSCoaq2+reaction is concerned, the formation of a C032--Co(III) complex cannot be ruled out; a where kg 6 4.0 X IO4 M-l sec-'. The (CH&COH radical is well established as a good reducing agent30 similar reaction of free radicals with Craqa+has been shown to generate Cr(II1) transient intermediates. 35 capable of converting Co(II1) complexes to Coaqz+ with rate constants in the order of 10-7-108 M-' However, instability of the C032--c~(III) complex relative to Cow2+and the oxidation of water would be set-1. 3 L 3 2 expected so that no net loss of Coaq2+would be expe(CHs)zCOH Co(II1) ---f CO.,~+ + (CH&CO (10) rienced. Aquocarbonato Intermediate. The behavior of the Thus, the oxidizing COS- radical is converted into a 300-nm absorption observed in the flash photolysis is reducing radical by reaction 9 and (pcOz+ is doubled. Ip the presence of 2-propanol and 02,the (CH3)2- completely consistent with the assignment of the intermediate as the aquocarbonato complex generated COH radical is scavenged by Ozt probably at diffudirectly in the flash from the opening of the carbonate
+
+
(30) J. Lilie, G. Beck, and A. Henglein, Ber. Bunsenges. Phys. Chem., 75,458 (1971). (31) M. Z. Hoffman and M. Simic, J. Amer. Chem. Soc., 94, 1757 (1972). (32) H. Cohen and D. Meyerstein, ibid., 94,6944 (1972).
(33) H. Cohen and D. Meyerstein, ibid., 93,4179 (1971). (34) J. D. White and H. Taube, J . Phys. Chem., 74,4142 (1970). (35) H. Cohen and D. Meyerstein, Chem. Commun., 320 (1971).
Cope, Chen, Hoffman
Photochemistry of Cobalt(II1) Complexes
3120
3.2
33
' 0
34
lo3/ T Figure 5. A Eyring-Polanyi plot of the decay of the long-lived intermediate. [Co(NH&COs+]= 6.0-9.0 X M , ionic strength = 0.026 M , pH 4.0.
5
10
15
20
25
30
[H*]' x M-' Figure 6 . The variation of the observed first-order rate constant, kobrd, as a function of [H+]: double reciprocal plot. [Co(NH&COS+]= 5.5-9.4 X M , 24", ionic strength = 0.026 M ,
the diffusion-controlled limit (lolo M-l sec-l), it is ring with subsequent rapid aquation in the cis position. easily seen that this analysis is numerically valid. Reactions 5-8 will be applicable in the conversion of For Co(en)zCOS+, the data give the following rethis intermediate to the diaquo final product. sults: k, = 0.88 i 0.08 sec-' and K s = 2.0 0.1 X The data shown in Figure 3 are almost identical 10-6 M(pK5 = 5.7). with those of the thermal aquation of C O ( N H ~ ) ~ C O ~ + The Primary Process. The observation that the presented by Francis and Jordan l6 and Dasgupta intermediates from the flash photolysis are generated and Harris's and can be treated in terms of reactions within the lifetime of the flash indicates that all pro5-8 where the acid-base equilibria (reactions 5 and 6) cesses involving the precursors to these species are over are fast and reaction 8 is slow compared to reaction in less than 30 psec. The initial absorption of the 7. Then, kob.d k7[H+] K5 so that l/kobsd = radiation within the intense ultraviolet band produces a l/k7 K5/k7[H+]. Figure 6 shows the data for Cospin-allowed ligand-to-metal charge-transfer transition (NH3)X03+plotted in reciprocal form from which the in which a ligand electron is propagated into a molecular intercept at high acidity kobsd = k7 = 1.0 f 0.1 sec-l; orbital more closely associated with the metal center. from the slope, a value of K s = 1.0 f 0.2 X This primary CTTM excited state of singlet multiM (pK6 = 6.0) is obtained. The excellent correlation plicity ('CT) may decay to the ground state, but without between our observed values and those of Dasgupta the generation of detectable luminescence, 3 6 or undergo and Harris17 leaves little doubt that the species we are radiationless relaxation to other excited states which observing directly is the aquocarbonato intermediate. lead to the observable products. Some recent senThe spectrum of the intermediate we observe is at sitization results3' have shown that the oxidationleast consistent with that of the charge-transfer region reduction products in Co(II1) photochemistry arise of a Co(II1bamine complex with an absorbance in from a reactive charge-transfer triplet state (3CT) which the observed spectral region greater than that of Comay also undergo radiationless return to the ground (NH&C03+. Despite the uncertainty in our values of state in order to account for the less-than-unity value the activation parameters, the coincidence between of the quantum yield. Thus, in the cases reported these results and those for C O ( N H ~ ) ~ C Oinvolving ~+ here, the generation of CO,,~+ and the COS- radical direct decarboxylation15is extremely good. is attributed to the reactive decay of the 3CT state. At higher pH values (up to 8.9), the deviations from The lifetime of the intermediate C0(II)-CO3- species the plot of Figure 6 become large as kobsd approaches is clearly shorter than the instrumental time resolusec-l. In this pH a limiting value of about 5 X tion of the flash. The rapid aquation of the Co(I1) range the intermediate is in its Co(NH&(OH)(CO$H)+ intermediate would release the COS- radical into the form with somewhat greater stability due to the inbulk solution. terna! hydrogen bonding bridge between the ligands. l 4 The fact that ring opening and aquation is the preAt pH 4.3, kobndis independent of ionic strength which dominant product from the charge-transfer excitation is consistent with the unimolecular nature of reaction of the bidentate carbonato complexes may not be sur7. At pH 8.0, where no ionic strength dependence prising within the context of their thermal chemistry is also seen, the rate-controlling steps must be the unimolecular decarboxylation of CO(NH~)~(OH)- but it is a novel observation in the 254-nm photochemistry of Co(II1)-amine complexes. In the photo(C03H)+ and C O ( N H & ( ~ H ) ( C O ~ )At ~ . p H 6.4 where chemistry of the analogous bidentate oxalato coma positive ionic strength effect is observed (Figure 4), plexes, no ring opening was observed to occur.8 The the value of the slope indicates the kinetic importance only other cases of loss of a ligand other than NH3 of a reaction between two 1 (or - 1) charged species. at 254 nm that appear to have been observed are At this pH, where - 7 5 x of the intermediate is in the various dichloro- and aquochloro-tetraammine and bishydroxybicarbonato form protonation of this 1 species (36) P. D. Fleischauer and P. Fleischauer, Chem. Rev., 70, 199 (reverse of reaction 5) must occur at a rate greater (1970). than that of reaction 7 for the ionic strength effect to (37) P. Natarajan and J. F. Endicott, J . Amer. Chem. Soc., 94, 3635 be seen. Taking protonation rates as being at or near (1 972).
*
+
+
+
+
Journal of the American Chemical Society / 95.10 1 May 16, 1973
3121
ethylenediamine complexes. lo Aquation processes inThis protonation would weaken one of the Co-0 volving the nonamine ligand, albeit with very low bonds causing heterolytic scission and the formation quantum yields, appear to be the general case, howof the resultant aquocarbonato intermediate. Such ever, in the irradiation of the ligand field (d-d) absorpa sequence could not, of course, be observed in free tion bands of these complexes. 3 , 3 8 From the sensitizacarbonate; furthermore, the absorption exhibited by C032- in the 200-nm region is charge-transfer-totion studies3’ it has also been concluded that the aquasolvent in nature and irradiation results in ionization. 4O tion products arise from the ligand field excited states The results of this study can be summarized in of triplet spin multiplicity. It is still a matter of conScheme I. The quantum yields for the formation of jecture whether CTTM excited states deactivate uiu the ligand field states. Regardless, it is hard to acScheme I count for the relatively high yield of ring opening and -,+ aquation of bidentate carbonate on the basis that this process originates from the ligand field states of the complex either directly or indirectly populated. There are no obvious properties of carbonate that would enhance intersystem crossing or the reactivity of the L* d-d excited states. It is hard to visualize how the aquocarbonato interH+ mediate could arise except uiu heterolytic Co-0 bond co2+ + co3scission and rapid aquation at the vacant sixth coordination site. The properties of the transient absorp4NH3/2en tion at 300 nm in no way resemble those of the long(en), I lived intermediates observed in the flash photolysis of formato- and oxalato-amine complexes of CO(III).~*~ In the latter cases, a C-bonded formato species was generated by heterolytic C-C bond scission followed by linkage isomerization of the resulting fragment. There is absolutely no evidence that the mode of formation of the intermediate absorbing at 300 nm Coaq2+and the products of intramolecular ligand exin the case of the carbonato complexes is in any way citation are rather low indicating that radiationless related to the intermediate derived from the oxalato return to the ground state from the various excited complexes. Neither is there any evidence to show that states is quite facile. Inasmuch as the molecular it is not related to the initiation of the thermal aquaparameters that govern the efficiency of these processes tion process. are not known, it is sufficient only to note that the q5 that It has been pointed out a number of values are lower than the corresponding values for the various processes exhibited by Co(II1) complexes the analogous oxalato complexes;8 in particular, the are independent and arise from distinct excited states, carbonato complexes are rather inert to photoreducall linked uiu radiationless transitions from the extion. There is no evidence to indicate that Co(NH3),cited state populated in the primary absorption proCOS+ shows any photoaquation of the carbonato cess. Let L* be a ligand excited state not normally populated upon irradiation of the free ligand (due to ligand. Finally, it is important to note that O2 has no absorptivity restrictions) which can be generated Diu apparent effect on the quantum yields in these systems. radiationless transition from ‘CT. If this L* state Except where O2 scavenges radicals which contribute involved population of a x* orbital of the ketonic to the generation of Coaq2+in secondary reactions, C-0 bond system with an increase in electron density O2 generally does not affect the yield of Coaq2+from on the oxygen atom, rapid protonation by H30+ the photolysis of Co(II1) complexes. This fact imor the solvent could generate the same species that plies that O2 cannot quench the various excited states Dasgupta and Harris’? postulated, for the transition generated because of either a low intrinsic reactivity state of reaction 4. or very short lifetimes of the excited states. To date, 0 no transient absorption spectra have been observed (NH&Co’ \C=OHZ+ in conventional microsecond flash photolysis that can be attributed to these excited states. r
1
(38) However, ammonia aquation was observed in the 366-nm uradiation of Co(NHs)sOzCCHa2+6 and, although with less certainty, Co(NHs)aOzCH*+.7 (39) V. Balzani, L. Moggi, F. Scandola, and V. Carassiti, Inorg. Chem. Acra Rev., 1, 7 (1967).
12+
1
+
Acknowledgment. This research was supported by the National Science Foundation through Grant No. G P 11213 and the College Teachers Research Program. (40) E.Hayon and J. J. McGarvey, J . Phys. Chem., 71,1472 (1967).
Cope, Chen, Hoffman / Photochemistry of CobaIt(II~Complexes
