Inorganic Chemistry, Vol. 18, No. 10, 1979 2935
Notes
provide insight into the mechanism of induced electron transfer Infrared spectroscopy was used for further confirmation of azido coordination. The asymmetric N-N stretching frequency, expected in which an oxidizing agent causes the reduction of the metal between 2000 and 2100 cm-I, cannot be used, since selenocyanate center .5 would exhibit a C-N stretch in the same region. However, the Experimental Section symmetric N-N stretching frequency is expected between 1240 and [ C O ( N H ~ ) ~ O ~(C1OJ2 C H ] was prepared according to the method 1280 with no counterpart in the selenocyanate or bipyridine of Sebera and Taube;6 the spectral characteristics of the compound spectrum. A band was observed at 1246 cm-I, which would appear were identical with those reported in the literature. to be the v5(N-N-N) mode. Pulse radiolyses with optical absorption detection were conducted The selenocyanate reaction in particular, and [ R ~ ( b p y ) , ] ~ ~ by using the apparatus at the US. Army Natick Research and photosubstitution reactions in general, appears to merit additional Development Command;' pulse radiolyses with kinetic conductivity investigation aimed at unraveling both mechanistic and overall detection were performed at the Radiation Laboratory of the University stoichiometric details of the process. of Notre Dame.* Continuous radiolyses were conducted at Boston Registry NO. [Ru(bpy)3]C12,14323-66-9;[Ru(bpy)3]12,23792-07-6; University in 6oCoy-ray sources (dose rates -3.5 X 10l6and -7.0 [R~(bpy)~l(NO,),,71031-49-7; [Ru(bpy)J(mal), 71000-88-9; X 1017eV mL-'m i d ) ; the Fricke dosimeter9 was used to evaluate [Ru(bpy)3] (N3)2, 7 1000-89-0; [Ru(bpy)S] (CN),, 7 1050-17-4; the exact radiation dose received by the solutions. [Ru(bpy)J (SeCN)2, 7 1031-52-2; [Ru(bpy)J (I03)z, 7 1000-90-3; R u ( ~ P Y ) ~ ( N ~ ) ~ , The solutions were purged of air by saturation with N 2 0 (2.5 X R~(bpy)212,71048-70-9;R~(bpy)2(N02)2,29241-01-8; M); alternatively, the solutions were saturated with O2 (1.3 X 5 5 124-48-6; Ru(bpy),(mal), 7 1000-91-4; R u ( b p ~ ) ~ ( c N20506-36-9. )~, M). Reagent grade chemicals and Millipore water were used References and Notes to prepare the solutions. Kitson's methodlowas used for the analysis of Co2+(aq). UV-vis spectra were determined with a Cary 118 (1) F. P. Dwyer, H A. Goodwin, and E. C . Gyarfas, Aust. J . Chem., 16, 544 (1962). spectrophotometer. (2) N . R. Davies and T. L. Mullins, Aust. J. Chem., 20, 657 (1967). Results and Discussion J . Chem., 21, 915 (1968). (3) N . R. Davies and T. L. Mullins, Aust (4) G. M. Bryant, J. E. Fergusson, and H. K. J. Powell, Aust. J . Chem., The radiolysis of water and aqueous solutions proceeds 24, 257 (1971). according to the overall reaction H20 eaq- (2.8), OH ( 5 ) J. B. Godwin and T. J. Meyer, Inorg. Chem., 10,471 (1971). (6) F. R. Keene, D. J. Salmon, and T. J. Meyer, J . Am. Chem. Soc., 98, (2.8), H (0.55) where the numbers in parentheses represent 1884 (1976). the G values (number of radicals formed/ 100 eV of energy (7) F. P. Dwyer, H. A. Goodwin, and E. C. Gyarfas, Aust. J . Chem., 16, absorbed) of the species. NzO, H+, and O2 are effective 42 (1963). scavengers for e,q-;ll H and C02-also react with 02.4 (8) B. Bosnich and F. P. Dwyer, Aust. J . Chem., 19, 2229 (1966).
--
(9) R. Charonnat, Ann. Chim. (Paris), 16, 5 (1931). (10) C. F. Liu, N . C . Liu, and J. C. Bailar, Inorg. Chem., 3, 1197 (1964). (1 1) E. C. Johnson, B. P. Sullivan, D. J. Salmon, S. A. Adeyemi, and T. J. Meyer, Inorg. Chem., 17, 2211 (1978). (1 2) J. N. Demas, T. F. Turner, and G. A. Crosby, Inorg. Chem.,8,674 (1969). (13) A. A. Schilt, J . Am. Chem. SOC.,85, 904 (1963). (14) P. E. Hoggard and G. B. Porter, J . Am. Chem. Soc., 100,1457 (1978). (15) W. M. Wallace and P. E. Hoggard, submitted for publication. (16) C. R. Bock, T. J. Meyer, and D. G. Whitten, J . Am. Chem. SOC.,97, 2909 (1975). (17) W. Beck, W. P. Fehlhammer, P. Pollman, E. Schuierer, and K. Feldl, Chem. Ber., 100, 2335 (1967). (18) K. Nakamoto, "Infrared Spectra of Inorganic and Coordination Compounds", 2nd ed., Wiley-Interscience, New York, 1970.
e,,
+ N20
-
OH + N2 + OH(k = 8.7 x 109 M-1 s-1 ) eaq- H+ H ( k = 2.2 X 1Olo M-' s-')
+
-
eaq- + O2 Oz( k = 2.0 X 1O'O M-' s-l)
H +0 2 HO2 ( k = 2.0 X 1O'O M-' s-') HOz e H+ + O2 (pK, = 4.8)12
Contribution from the Department of Chemistry, Boston University, Boston, Massachusetts 02215
.coy + 0 2
- coz +
( k = 1.5 X lo9 M-'
0,
1
s-l l 3
The [CO(NH,),(O,C-*)]~+Coordinated Ligand-Radical Complex'
The pulse radiolysis (optical absorption) of buffered N20-saturated solutions of 5.0 X M CO(NH~)~O~CH~+ (pH 1-7) reveals pseudo-first-order bleaching of the substrate Nina V. Brezniak and Morton Z . Hoffman* absorbance in the microsecond time frame corresponding to a second-order disappearance with k 2 X lo9 M-' s-l. n Received April 24, 1979 other transient or permanent absorbances are 0 b ~ e r v e d . l ~ One of the first coordinated ligand-radical species to be Continuous radiolysis under the same conditions produces proposed in electron-transfer reactions was [Co(NH3),Co2+(aq) in quantitative yield (G = 6.1 f 0.3) independent (02C-.)]z+ which was postulated by Candlin and Halpern2 as of pH, radiation dose, [substrate], and [phosphate buffer]; the a n intermediate in the M n O i oxidation of C O ( N H ~ ) ~ O ~ C H ~absorption + spectra of the radiolyzed solutions show loss of in aqueous solution. They interpreted their observations in substrate stoichiometric with the formation of Co2+(aq). The terms of intramolecular electron transfer in the coordinated pulse radiolysis (kinetic conductivity) of 5.0 X M CoCOZ-complex to form Co2+(as) and COz in competition with (NH3)50zCH2+a t pH 4.3 reveals that the full yield of Co(II1) reaction with Mn04- to form C O ( N H ~ ) ~ O Hand * ~ +C02. The is generated within 2 p s as evidenced by the uptake of Ht from ratio of the rate constants of these reactions was evaluated but the release of NH3. In fact, the results are very similar to those their absolute values were not determined. obtained for the reaction of ea( with C O ( N H ~ ) ~ C ~The ~+.'~ Because of the efficient reactions of OH radicals and H release of two ammonias, and presumably the nonammonia atoms with HCOzH ( k = 1.5 X lo8 and 1.1 X lo6 M-'s-l 1 ligand, occurs with t ! / 2C 2 p s ; the fourth, fifth, and sixth and HCOZ-( k = 2.0 X lo9 and 5.0 X lo7 M-' s - ' ) ~ ,to ~ form ligands are released with t l j 2= 8, 80, and 500 p s , respectively. .C02-, we reasoned that OH and H should react with CoThe results demonstrate that the reaction of OH radicals (NH3)50zCH2+via H abstraction to form [Co(NH,),and H atoms with C O ( N H ~ ) ~ O ~ C leads H ~ quantitatively + to (02C-.)]2+. In this way we could test whether or not [CoCo2+(aq) and that the primary intermediate from the radical (NH3)5(02C-.)]2+engages in intramolecular electron transfer, interaction has a lifetime toward the generation of Co(I1) of evaluate the rate constants for the various processes, and 1 2 p s . The sequence of events given in eq 1-7 can be written
-
334
0020-1669/79/1318-2935$01 .OO/O
0 1979 American Chemical Society
7
0
2936 Inorganic Chemistry, Vol. 18, No. 10, 1979
Notes
-
CO"'(NH,)~O~CH+ ~ +OH [ C O " ' ( N H ~ ) ~ ( O ~ C - . )+] ~H20 + (1) Co"'(NH3)502CH2+
-
+H
[ C O " * ( N H ~ ) ~ ( O ~ C - *+) ]H2 ~ + (2)
[ C O " ' ( N H ~ ) ~ ( O ~ C - . ) ] ~ +[ C O " ( N H ~ ) ~ O ~ C (low ] ~ +spin) (3) [ C O " ( N H ~ ) ~ O ~ (low C ] ~ spin) + [CO"(NH~)~O~C (high ] ~ + spin) (4)
-
-
[ C O " ( N H , ) ~ O ~ C ](high ~ + spin) C O ' ' ( N H ~ ) ~+~C+ 0 2 + 2 N H 3 ( 5 )
f1/2
COII(NH~)~'+
= 8 irs
1,,* =
CO''(NH~)~~+
-- + 1112
= 500
CO"(NH~)~+
NH3
80 ps
+ H30+
NH4+
PS
Co2+(aq) (6)
H20
(7) with k l 2 X lo9 M-' s-' , k 2 10' M-' s-l,16 k4 > lo6 s-',17 k j 1 lo6 s-l, and k7 = 4.3 X 1O'O M-' ss1.18 W e estimate k3 1 lo6 s-', with the lifetime of [ C O ( N H ~ ) ~ ( O ~ C - toward -)]~+ intramolecular electron transfer s. This conclusion is very resonable in light of what we know about the powerful reducing ability of C02-toward Co(II1) c o m p l e x e ~and ' ~ the lifetimes of ligand radicals coordinated to Co(II1) centers.20 In continuously radiolyzed 02-saturated solution containing 5.0 X M CO(NH~)~O~C and H 0.5 ~ + M N a H C 0 2 at p H 6, G(Co2+(aq)) = 1.3 which corresponds entirely to the direct reaction of eaq-, which escape scavenging by 02,with the complex with k8 5 X 10'O M-l s-'; COz- reacts preferentially with O2 than with C O ( N H ~ ) ~ O ~ C ( kH ~ +1 X lo8 M-' s-l).I9 This result establishes that C O ( N H ~ ) ~ O ~ C+Hea;~ + Co2+(aq) + 5 N H 3 - 0 2 C H (8) 02-radicals do not reduce C O ( N H ~ ) ~ O ~ C HWith ~ + . this result, we can interpret the G(Co2+(aq)) values obtained for the continuous radiolysis of 02-saturated solutions containing 5.0 X lo4 M C O ( N H , ) ~ O ~ C HG(Co2'(aq)) ~+; = 2.8, 3.1, and 3.8 at p H 1.0, 3.1, and 6.8, respectively. The results are in complete accord with reactions 1, 2, and 8 leading to Co2+(aq) and with [CO(NH,),(O,C-.)]~+ not being quenched by O2 in competition with reaction 3. If reaction 9 occurred to any
-
-
-
-
+
[CO(NH,>~(O~C-*)]*+0
2
+
+
[Co(NH3)502CI3+ + 0 2 (9)
significant extent, a diminution in the yield of Co2+(aq) and the formation of C O ( N H ~ ) ~ O H would ~ , ' be expected; these effects were not observed. It can be concluded that k9 has a value of -lo9 M-l s-l a s an upper limit. According to the mechanism of Candlin and Halpern,2 reaction 3 would be in competition with reaction 10 with k1O/k3
H20
[CO(NH,)~(O,C-.)]~+ + Mn04CO(NH~)~O+ H ~C ~0 +2 + Mn042- (10) = 3 X lo2 M-'. From our estimate of k,, a value of k l o I 3 X lo8 M-' s-' is obtained. However, the possibility remains H ~ + via that reaction of Mn04- with C O ( N H ~ ) ~ O ~ Coccurs attachment.21 Acknowledgment. The authors thank Dr. M . G. Simic (Natick) and Dr. P. Neta (Notre Dame) for assistance in performing the pulse radiolysis experiments. Registry No. [CO"'(NH~)~(O~C-.)]*+, 71214-99-8. References and Notes (1) Research supported by the National Science Foundation through Grant NO CHE76-21050
0020-1669/79/1318-2936$01.00/0
J. P. Candlin and J. Halpern, J . Am. Chem. Soc., 85, 2518 (1963). Farhataziz and A. B. Ross, Natl. Stand. Ref: Data Ser., Natl, Bur. Stand., No. 59 (1975). M. Anbar, Farhataziz, and A. B. Ross, il'atl. Stand. Ref: Data Ser., Natl. Bur. Stand., No. 51 (1975). H . Taube, "Electron Transfer Reactions of Complex Ions in Solution", Academic Press, New York, 1970. D. K. Sebera and H. Taube, J . Am. Chem. Soc., 83, 1785 (1961). P. Neta, M. Simic, and E. Hayon, J . Phys. Chem., 73, 4207 (1969). J. Lilie and R. W. Fessenden, J . Phys. Chem., 77, 674 (1973). M. S. Matheson and L. M. Dorfman, "Pulse Radiolysis", The M.I.T. Press, Cambridge, Mass., 1969. R. E. Kitson, Anal. Chem., 22, 664 (1950). M. Anbar, M. Bambenek, and A. B. Ross, Natl. Stand. Ref. Data Ser., Null. Bur. Stand., No. 43 (1973). B. H . J. Bielski and J. M. Gebicki, Ado. Radiat. Chem., 2, 177 (1970). M. Simic in "Fast Processes in Radiation Chemistry and Biology", G. E. Adams, E. M. Fielden, and B. D. Michael, Ed., Wiley, London, 1975. At pH 2 7 in unbuffered solution, absorbance changes due to the formation and nucleation of hydrous oxides of Co(I1) are observed occurring in the second-to-minute time frame. J. Lilie, N. Shinohara, and M. G. Simic, J . Am. Chem. Soc., 98, 6516 (1976). E. R. Kantrowitz, M. Z. Hoffman, and K. M. Schilling, J . Phys. Chem., 76, 2402 (1972). J. F. Endicott in "Concepts in Inorganic Photochemistry", A. W. Adamson and P. D. Fleischauer, Eds., Wiley, New York, 1975, Chapter 3 . M.Eigen, Angew. Chem., 75, 489 (1963). M. Z. Hoffman and M . Simic, Inorg. Chem., 12, 2471 (1973). M. G. Simic, M. Z. Hoffman, and N. V . Brezniak, J . Am. Chem. Soc., 99, 2166, (1977). Reference 5, p 92.
Contribution from the Inorganic Chemistry Laboratory, Oxford University, Oxford, England OX1 3QR
Temperature-Dependent Optical Spectra of Manganese Acetate: Intratrimer Exchange T. E. Wood, G. Kokoszka, and P. Day* Received March 16, 1979
The magnetic structure of manganous acetate tetrahydrate [ M ~ ( O A C )has ~ ] been the subject of study for over 15 years. The quasi-two-dimensional lattice configuration, in which the intraplanar exchange field is about 8000 G and the interplane exchange field is some 1000 G, had been recognized early in the history of these studies.'-4 Furthermore, the strength of these exchange fields is nearly independent of temperature in the liquid helium to room-temperature range.5 However, a rather interesting structural feature was revealed in the neutron diffraction and X-ray crystallographic studies of Burlet et ala6,' Within the layered structure the Mn(I1) ions are grouped into linear trimers. The coupling within the trimer is antiferromagnetic as is the coupling between planes, while the intertrimer exchange coupling is ferromagnetic. As noted above, the latter two interactions have been measured, but the strength of the former exchange interaction can only be qualitatively determined from shifts in the low-temperature EPR resonance field which are observed at temperatures below 30 K3 and from a slight curvature in the 1/x vs. T plot in the powder susceptibility measurements.' Two earlier optical studies have been concerned with Mn(OAc),. Mehra and Venkateswarlu8 (MV) have studied the spectrum in the 3000-6000 A region a t 300 and 77 K. There is little doubt but that their classification scheme of the major spectral features is correct. The two lowest energy bands above the 6Al ground state labeled A and B by M V are assigned as the transition to the 4T1(G)and 4T2(G)states. The next band (C) is relatively sharp with some narrow features already present a t 77 K and is assigned as the transition to 4Al(G) and 4E(G). It is this spectral region which was studied in detail by Tsujikawa a t low temperatures (down to 1.3 K)
0 1979 American Chemical Society
