Outer Sphere Oxidations of Alcohols and Formic Acid ...

Outer Sphere Oxidations of Alcohols and Formic Acid by Charge Transfer Excited States of Iron(II1) Species1

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JOHNH . CAREYAND COOPERH. LANGFORD~ Department of Chemistry, Carleton University, Ottawa, Canada K I S 5B6 Received February 21, 1975 JOHNH. CAREY and COOPER H. LANGFORD. Can. J. Chem..53,2436 (1975). When methanol, 2-propanol, and formic acid are used as scavengers in the ligand to metal charge transfer (1.m.c.t.) photolysis of Fe(OH2),3+, Fe(OH2),C12+, Fe2(OH2)8(OH)24+,or FeEDTA, there is a linear relationship between quantum yield for Fe(I1) production and scavenger concentration, [S], at higher [S] values. Extrapolation of the linear portions to [S] = 0 gives an intercept corresponding to the limiting yields observed for scavenging with tert-butyl alcohol. Butanol scavenging at the limit has been shown to give the primary free radical yields from photolysis of aquo iron(II1) species. Nuclear magnetic resonance relaxation time studies show that alcohols do not coordinate to Fe(II1) and calculations from known stability constants indicate that formic acid does not coordinate under the experimental conditions. The increase of Fe(I1) yields with [S] is attributed to an outer sphere oxidation of noncoordinated organic species by the charge transfer excited states of Fe(II1) species. There is no discrimination among the organic reductants. The results may be understood without postulating a long lifetime for the Fe(II1) 1.m.c.t. states if the reaction is assumed to occur only with organic molecules in encounter with the Fe(II1) complex at the time of excitation. Organic products were formaldehyde from methanol oxidation and acetone from 2-propanol oxidation. The Fe(I1):formaldehyde stoichiometry was 2: 1. JOHNH. CAREY et COOPER H. LANGFORD. Can. J. Chem. 53,2436 (1975). La photolyse du transfert de charge du ligand au metal (t.c.1.m.) de Fe(OH2)63+,Fe(OH2)5C12+,Fe(OH2)8(OH)24+ou de FeEDTA est Ctudik en se servant du methanol, du propanol-2 et de I'acide formique comme agents piegeurs. On observe une relation linkire entre le rendement quantique de la formation de Fe(I1) et la concentration [S] en agent piegeur, pour les fortes concentrations en [S]. L'extrapolation [S] = 0 des portions lineaires permet de retrouver les rendements limites observes dans le cas du piegeage par I'alcool tert-butylique. I1 a ete montrC qu'a la limite, le piegeage par l'alcool tert-butylique conduit aux rendements en radical libre primaire, a partir de la photolyse d'esphs aquo fer(II1). Les etudes du temps de relaxation par r.m.n. montrent que les alcools ne se coordonnent pas au Fe(II1). Les calculs bases sur les constantes de stabilite connues indiquent que I'acide formique ne se coordonne pas non plus dans ces conditions experimentales. L'augmentation du rendement en Fe(I1) avec [S] est attribue a une oxydation externe, par transfert de charge des Ctats excites des especes Fe(III), de composes organiques non coordonnes. I1 n'y a pas de distinction entre les reducteurs organiques. Ces resultats peuvent &treinterpret& sans avoir a postuler une longue d u r k de vie pour les Ctats t.c.1.m. de Fe(II1) si I'on admet que la reaction se fait seulement entre des mol&ules organiques rencontrant un complexe de Fe(II1) au moment de l'excitation. Les produits organiques Ctaient la formaldehyde provenant de I'oxydation du methanol et I'acetone venant de I'oxydation du propanol-2. La stoechiometrie de Fe(I1):formaldehyde est 2:l. [Traduit par le journal]

Introduction In many photochemical reactions, those of charge transfer excited states of metal complexes included, identification of radicals produced in primary photo processes and determination of primary quantum yields requires introduction of scavengers. Frequently, rather large scavenger concentrations (large [ S ] ) are required to achieve complete scavenging of the desired 'We thank the National Research Council of Canada for financial support. 2To whom correspondence should be addressed.

radical. But, there is a special feature of large scavenger concentrations. The scavengers may frequently be the encounter partner in the solvent cage of the molecule absorbing light, a phenomenon described in an organic photochemical example by Waits and Hammond (1). Encounter with scavengers can be important at surprisingly low concentration. Modeling encounter equilibrium between a complex ion and a molecule using the Eigen-Fuoss equation which is successful in the description of complex formation reactions (2), assumption of a center to center

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CAREY AND LANGFORD: OUTER SPHERE OXIDATIONS


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encounter distance of only 5 A gives an equilibrium constant, KE, of 0.3. To see the implication of this value, consider a dilute aqueous solution of Fe(OH2)63+,containing 1 M CH30H. In this circumstance, 25% of the complex ions Fe(OH2)63+ have an alcohol molecule in encounter (in the second coordination sphere or solvation sphere) at any time. This means that $ of all Fe(OH2)63+ complexes absorbing photons from the 254 nm line of a mercury lamp may be expected to arrive in the ligand to metal charge transfer (1.m.c.t.) excited state with an alcohol molecule as a nearest neighbor. Under these circumstances there is no need to assume that the 1.m.c.t. excited state has a substantial lifetime in order to account for a direct reaction between that state and the alcohol. In this report we describe the reaction at high [S] between the 1.m.c.t. excited states of several Fe(II1) species and oxidizable organic scavengers which do not penetrate the primary coordination sphere. We will analyze the results for these outer sphere redox processes in terms of the encounter model described above because they do not become prominent except at high [S] and because this model does not require a long life for the 1.m.c.t. excited states. The treatment is analogous to "static" quenching of emission (3). We note here that the alternative of "dynamic" quenching and a long-lived 1.m.c.t. excited state is not excluded. It simply seems less likely. Results and Discussion In the preceding paper (4), we noted that the quantum yield for the reaction shown in reaction 1 could be determined simply by using tertbutyl alcohol as a scavenger. When methanol,

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[scavenger]

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1.5

2.0

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M

FIG. 1. Quantum yields for production of Fe(1I) as a function of scavenger concentration using (a)tertbutyl alcohol, (m) formic acid, (A) isopropyl alcohol, and ( 0 )methanol as scavengers. Deoxygenated runs with isopropyl alcohol are shown by (A). The upper two curves refer to Fe(OH2),C12+.The lower two curves refer to F e ( o H ~ ) +6.~

error of the yield of Fe(I1) determined with tert-butyl alcohol scavenging in the limit of high alcohol concentration (0.130) where all hydroxyl radicals escaping the solvent cage are scavenged and none react with Fez+ to regenerate the initial complex. The increase in quantum yield for Fe(I1) production above this intercept involves a process not included in the kinetic schemes of the preceding paper. An immediate possibility is that methanol, 2-propanol, and formic acid form complexes with Fe(OH2)63 and that we observe photochemistry of these new species. (It is more difficult to entertain this suggestion for FeEDTA.) The idea may be tested easily in the case of formic acid and Fe(OH2),3+. The stability constant of the formatoiron(II1) complex is known (5) and is at 25". Less than 0.1% of the formate 8.5 x +

2-propanol, and formic acid are employed as scavengers, the quantum yield for Fe2+ production does not approach a limit at high [S]. Instead, it becomes linearly dependent upon [S]. This behavior is illustrated in Fig. 1 for acid solution of Fe(OH2)63+ irradiated at 254 nm and for Fe(OH2),C12+ irradiated at 350 nm. 1 Corresponding results for Fe2(OH2),(OH)24+ [scavenger] M irradiated at 350 nm and Fe(EDTA)OH2FIG. 2. Quantum yields for production of Fe(I1) as irradiated at 350 nm are shown in Figs. 2 and 3. a function of scavenger concentration for irradiation of The [S] = 0 intercept of the line in Fig. 1 is the dihydroxo-bridged dimer. Symbols for scavengers as seen to be 0.125. This is within experimental in Fig. 1. I

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5


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CAN. J. CHEM.

0

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2

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[Scavenger]

-

VOL. 53,

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3

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FIG.3. Quantum yields for Fe(I1) as a function o f scavenger concentration (symbols as in Fig. 1) for irradiation of FeEDTA.

would be coordinated at unit hydrogen ion activity. The medium in the photolysis experiments is 2.25 M HClO,. Formic acid is reacting in an outer sphere reaction in the sense that the effective photons cannot be exciting formato complexes. The similarity of behavior between formic acid and the alcohols suggests a similar assignment, but it is possible to be somewhat more specific. The Fe(OH2)63+ion is paramagnetic and its hydroxyl protons are exchanging rapidly with hydroxylic protons in the bulk solvent in this acidic medium. There is a single n.m.r. hydroxyl proton signal which is moderately broadened by the paramagnetic center. Now, as the alcohol is introduced, if alcohol molecules are excluded from the first coordination of Fe(III), the number of bulk hydroxylic sites will decrease without a corresponding decrease in the number of hydroxylic sites in the paramagnetic neighborhood of Fe(II1). The result will be enhanced paramagnetic broadening of the hydroxylic protons. In other words, if Fe(II1) is "preferentially solvated" in the coordination sphere by H 2 0 , the hydroxylic proton n.m.r. signal will become broader as alcohol concentration increases. (This discussion has neglected broadening arising from protons in the secondary solvation shell which may be as large as 5-10% of that from the primary coordination sphere.) Quantitative treatment of this effect has been given several times (6-8) and is moderately lengthy. Using the procedures indicated in the references, hydroxy proton line widths were calculated on the assumption that Fe(OH,):+ was the only iron complex present in solutions containing alcohols. In Fig. 4, experimental

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RG. 4. Observed n.m.r. linewidths us. predicted linewidths for (A) methanol, ( 0 )tert-butyl alcohol, and isopropyl alcohol.

(a)

linewidths are plotted against calculated linewidths. Any disagreement is in the direction of lines too broad which is inconsistent with alcohol coordination and probably explained by the neglect of outer sphere broadening. Like the formic acid reactions, reactions with methanol and %-propano1 do not arise from excitation of the inner sphere complexes of the substrate undergoing oxidation. The next result to which attention should be drawn is the similarity of the dependence of quantum yields for Fe(I1) production on [S] in various cases. When Fe(OH2)63+is excited at 254 nm, slopes for methanol, 2-propanol, and formic acid are indistinguishable! In turn, this value is indistinguishable from the slope describing experiments in which Fe(OH2),C12+ is irradiated at 350 nm. The simplest interpretation of an equality of reactivity over several systems is the assumption that all have reached an "upper limit" of some sort. That is, we wish to test the assumption that all of the three organic molecules are oxidized with a quantum yield approaching unity (the theoretical upper limit) for the reactive excited state, not necessarily the primary one. Looking back at the argument in the introduction, it is apparent that the entity reacting with a quantum yield of unity cannot be the one-to-one encounter species (or outer sphere


CAREY AND LANGFORD: OUTER SPHERE OXlDATIONS

complex) involving the primary excited state if the Fuoss-Eigen equation is approximately correct. According to that equation, the species with one alcohol (or formic acid) molecule in the solvation shell of Fe(OH2)63+would account for 25% of all Fe(OH2)63+ species at 1 M alcohol. The implication would be a primary radical quantum yield of the order of 0.25 rising to nearly 0.5 at 2 M alcohol3 and approaching a limit shortly thereafter contrary to the results in Fig. 1. To see the weakness of this simple conception, it is useful to consider Fe(OH2),C12+ first. In the case of Fe(OH2),C12+ the c.t. transition occurs primarily between C1- and Fe3+ leading to a "chlorine atom" center as oxidant at least in so far as the release of C1 atoms to the solution (4) implies the nature of the primary excited state. If a molecule of an oxidizable substance, e.g. methanol, is in encounter with (or solvates) F e ( 0 ~ , ) , C l ~ +on the side away from the C1, it is unlikely to be oxidized by a short-lived excited state. The quantum yield for oxidation will not be expected to approach unity until all outer sphere sites are occupied by the oxidizable species (e.g. methanol). Thus, before we can estimate the limiting reactivity of an excited state of Fe(OH2),C12+ we need to estimate a statistical factor for the chance that an alcohol molecule in encounter occupies the reactive site. On the assumption that the reactive site is unique and that distribution of alcohol molecules over outer sphere sites is random, the correct statistical factor is simply the outer sphere solvation number which has been variously estimated as lying between 4 and 12. A similar statistical factor was introduced by Langford and Muir and considered by Pearson and Ellgen in discussion of the interchange mechanism of thermal ligand substitution at Co(II1) and Ni(I1) centers (9). The factor which seems to apply in thermal reactions is between 5 and 10. On this basis, the 1.0 M value of the primary radical yield, or the slope of the primary radical quantum yield - alcohol concentration function should lie between 0.025 and 0.050. If these are correct, the experimental slopes for Fe(I1) production (2 x primary radical) should lie between 0.050 and 0.100. In other words, we estimate a slope for a reaction 3Recall that a primary radical yield of 0.25 implies an experimental Fe(I1) yield of 0.50.

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of unit efficiency nearly twice the experimental value in Fig. 1. It is important to recall that there was a strong reason to look for a way of regarding the slopes in Fig. 1 as representing unit efficiency. There is a very significant lack of discrimination among a fairly wide variety of oxidizable organic species. If the calculations of the previous paragraph are correct, they represent a limiting efficiency of about 50%. This could be rationalized if the state reacting with unit efficiency is not the primary excited state but some successor reached with comparable efficiency in all cases. Such a proposal was made by Endicott (10) to explain the behavior of acetatopentaamminecobalt(III) on c.t. excitation. Co(I1) yields (but not acetate decomposition yields) increased in this case on addition of isopropyl alcohol approaching a limiting value of 0.5. However, the calculations in the previous paragraph depend upon the Fuoss-Eigen equation. That equation is based on a simple hard sphere encounter theory and a factor of two error is not unexpected. If the Fe(II1) complexes are preferentially solvated by water to a slight extent, the observed slope could well correspond to a limiting yield of unity. We note that the alternative of a dynamic mechanism (as opposed to the static encounter mechanism outlined above) requires that the rate constant for reaction of a longer lived excited state with a scavenger must be the same for excited states of at least two Fe(II1) complexes of different structure and for the several different scavengers. The relative implausibility of this is one reason for focusing attention on encounter. Returning to Fe(OH2)63+,we ask what is the geometrically suitable site corresponding to that of Fe(OH2),CI2+ ? This is not a major difficulty. The six water molecules are equivalent only in the geometry of the ground state. It was to be expected (although there has not been experimental evidence before) that the vibrationally equilibrated excited state would be distorted in such a way that a particular water molecule has become the electron deficient one which might be visualized as "on its way toward oxidation". Having speculated on the excited state responsible for the outer sphere oxidation, its scope is worth comment. The reaction has appeared following excitation of the c.t. transitions of all iron(II1) species we have irradiated at either


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CAN. J . CHEM. VOL. 53, 1975

2537 or 3500 A. The list includes Fe(OH2)63+, Fe(OH2),C12+, Fe2(OH2),(OH)24+, and Fe(EDTA). (We shall present evidence elsewhere that the reaction extends to Co(EDTA).) It is not without precedent. In alcohol glasses, irradiation of cyano (11) and nitrosyl (12) complexes of Fe(II1) has been shown by e.s.r. to produce radicals derived by removal of a-hydrogens and Fe(I1). The present examples differ mainly in that net reactions are observable in fluids near room temperature. This probably indicates that recombination reactions of Fe(I1) species and radicals dominate the other systems. Oxidizable organic species so far appear to include methanol, 2-propanol, and formic acid, but not tert-butyl alcohol. It seems likely that an H on the carbon bearing the oxygen is a requirement., The reaction scheme may be written as follows.

?H

+

Fe(OH2)62+

jC=O

+ H+

R' 4A referee has suggested the following intermediate structure for a scavenger assisted reaction:

R' , H i.e. minimum strain via six member ring. Obviously tert-BuOH cannot form such a structure. We thank the referee.

In agreement with this scheme gas chromatography on CCl, or CHCI, extracts from irradiation of F~(OH,),,+ identified as organic products only formaldehyde from methanol and acetone from 2-propanol. 2,CDinitrophenylhydrazone derivatives of these were also isolated. Spectrophotometric determination of the formaldehyde yield via the examination of the 2,4-dinitrophenylhydrazone in basic media (12) indicates a stoichiometry of two moles Fe(I1) per mole of formaldehyde (f10%) for irradiations in presence of methanol, as required.

Experimental Materials and solutions were prepared as described in ref. 4. Irradiation procedures were also indicated there. The temperature was 35°C in the reaction vessels. Analyses were carried out as before (4) except that assay of formaldehyde produced was by the spectrophotometric procedures of Lappin and Clark (13). The n.m.r. procedures for preferential solvation studies have been extensively described (6-8). The treatment is lengthy and will not be reproduced here. Spectra used here were recorded on a Varian T-60 at 25 "C at an Fe(OH&concentration of 0.0200 M and an alcohol concentration range corresponding to that in irradiation experiments, 0.5 to 3.5 M.

I. H. P. WAITsand G. S. HAMMOND. J. Am. Chem. Soc. 86, 191l(1964). 2. M. EIGEN.Pure Appl. Chern. 6, 97 (1963); Z. Phys. Chem. Frankfurt am Main, 1, 176 (1954). I n Creation and detection of the ex3. P. J. WAGNER. cited state, Vol. 1, Part A. Edited by A. A. Lamola, Marcel Dekker, New York. 1971. and J. H. CAREY.Can. J. Chem. 4. C. H. LANGFORD This issue. and N. K. BRIDGE. J. Phys. Chem. 5. J. H. BAXENDALE 59,783 (1955). 6. L. S. FRANKEL, T. R. STENGLE, and C. H. LANGFORD.Can. J. Chem:46,3183 (1968). T. R. STENGLE, and C. H. LANG7. L. S. FRANKEL, FORD.J. Phys. Chem. 74, 1645(1970). S. BEHRENDT, and 8. V. S. SASTRI,R. W. HENWOOD, C. H. LANGFORD. J. Am. Chem. Soc. 94,753 (1972). ~ ~MUIR. W . J. Am. Chem. SOC. 9. C. H. L A N G F O R D ~R. 89,3141 (1967). Israel J. Chem. 8,209(1970). 10. J. F. ENDICOTT. 11. M . C . R . ~ Y M O N S , D . X . W E S T , ~ ~ ~ ~ . G . W ~ L J. Phys. Chem. 78, 1335(1974). 12. M. C. R. SYMONS, D. X. WEST,and J. G. WILKINSON. J. Chem. Soc. Chem. Commun. 917(1973). Anal. Chem. 23,541 13. G. R. LAPPINand L. C. CLARK. (1951).