Electron Spin Resonance Studies of Photo-oxidation by Metal Ions in Rigid Media at Low Temperatures Part 5.-Photo-oxidation by the Iron(m) Ion BY ALAN COX AND TERENCE J. KEMP* Department of Molecular Sciences, University of Warwick, Coventry CV4 7AL Received 21.d May, 1975 Iron(ur), both as its perchlorate and chloride salts, is an effective photo-oxidant for a wide range of organic compounds to give radical species readily trapped and detected by e.s.r. spectroscopy at 77 K. Its general behaviour parallels that of cerium(rv) photo-oxidation in that C-C fission processes are prominent with tertiary alcohols and carboxylic acids, although with simple alcohols, amides and certain other types of molecule, abstraction of a hydrogen atom from an activated C-H bond is found. The closely similar behaviour of the two salts towards the compounds we have examined suggests that Cl atoms are not essential intermediates in FeJII photo-oxidations, as has been previously suggested, although there are photo-oxidations by FeCl, in which chlorine is incorporated into the isolabte product.In Parts 1-3 1-3 we showed that photolysis of solutions of cerium(1v) salts in various organic media at 77 K using light absorbed only by CeIV species (usually a charge-transfer complex with the solvent) leads to the efficient production of solvent- derived radicals readily detectable by e.s.r. spectroscopy. Experiments with organic substrates dissolved in dilute perchloric acid and solutions of cerium(1v) salts gave essentially similar results. In Part 4 the behaviour of uranyl salts was examined under similar experimental conditions ; again, organic radicals were produced, but often of a type different from those encountered in cerium(1v) photosensitisation.In view of the considerable literature existing on the photo-induced reactions of iron@) salts with organic substrates, we report here the results of a brief survey of the e.s.r. spectra generated by photo-oxidation of a representative variety of organic substrates, mostly by iron(rn) perchlorate, making comparison with the behaviour of CeIV and Uvl, and with previous data on FelIr where this is available. EXPERIMENTAL The procedures described in Part 1 were used. The radiation from a 100 W point- source Xe arc was filtered through Pyrex. Solutions were made up in neat solvent where the organic molecule of interest was a liquid, otherwise saturated aqueous solutions were used.Where iron(m) perchlorate was comparatively insoluble, iron(m) chloride was used instead. [Fe1Ir] was typically 0.05 mol dm-3. RESULTS ALCOHOLS METHANOL A 1 : 2 : 1 triplet was produced with QH = 1.98 mT and g = 2.002 4 which, by comparison with our previous data and those in the literature. is assigned to CH20H radical. This result agrees with that of Kryukov et aLY6 who used both perchlorate and chloride salts. 2490A . COX AND T . J . KEMP 249 1 ETHANOL A basic five-line spectrum of binomial intensity distribution was produced in agreement with the result of Kryukov et u Z . , ~ which is identical to that found in both CeIV and Uvl photo-oxidations,'* and which is assigned to CH,tHOH.As with Cexy, however, additional small peaks appeared near the maxima of the second and third absorption lines (numbering from low field) indicative of the presence of a second, minor radical. PROPAN-~-OL Photoreaction was much slower in this instance and the spectrum was far less intense than that found with either of the lower alcohols or using CeIV as oxidant1 It consisted of a central singlet flanked by a number of weaker, but sharp and repro- ducible peaks. This substrate gave complex behaviour with CeIV and (in the liquid state) yielded only a very weak spectrum with Uv* as photo-oxidant. t-BUTYL ALCOHOL A relatively sharp 1 : 3 : 3 : 1 quartet was obtained with aH - 2.2 mT. runs traces were apparent of an additional 1 : 2 I 1 triplet with aH 1.89 mT.species are assigned to CH3 and -CH,C(CH,),OH respectively. In some These 2 - MET HY L B U T A N - 2 - 0 L( t - A M Y L A 1, C OH 0 L) Photoreaction was again rather slower than with CeIV. At 77 K a very clear spectrum of ethyl radical gradually emerged ; on warming the samples to - 150 K the spectrum changed to a 1 : 2 : 1 triplet suggestive of attack of C2H5 upon substrate to yield a CH2X species. BUTAN-2-OL A radical mixture was obtained comprising a central singlet and other peaks typical of C2H5 radical (major coupling 2.6 mT). 3 -METHY LHEXAN- 3 -0L Quite rapid photolysis occurred to yield a pure spectrum of ethyl radical. BENZYL ALCOHOL Quite rapid photolysis gave a broad singlet showing hyperfine coupling suggestive of a benzyl-type radical (possibly C6H&HOH).CARBOXYLIC ACIDS ACETIC ACID At 77 K a radical mixture (of approximately equal concentrations) was obtained assigned to .CH3 and CH2C02H from the couplings and g-factors. On warming, the methyl radical disappeared leaving -CH,C02H which displayed the anisotropy apparent in the spectrum of Ayscough et a1.' PROPIONIC ACID The principal feature was a five-line spectrum of binomial intensity distribution together with two quite sharp additional peaks with al, 2.6 mT, suggestive of ethyl2492 E . S . R . OF PHOTO-OXIDATION PRODUCTS radical. The quintet coupling of 2.35 mT suggests assignment to MeCHC0,H (the remaining peaks of the ethyl radical are submerged under those of the quintet). ISOBUTYRIC ACID An intense, very well-defined seven-line spectrum was produced with a binomial intensity distribution and a, 2.06 mT, due clearly to MezeCO2H (and not the isoprop yI radical).ISOVALERIC ACID An intense five-line spectrum resulted (a, - 1.89 mT) with the appearance of that for y-irradiated isobutyl halides suggestive described by Ayscough and Thomson of the formation of isobutyl radicals. VINYLACETIC ACID A very intense five-line spectrum of binomial intensity distribution and with aH = 1.51 mT was given, suggesting the formation of ally1 radical. MISCELLANEOUS SUBSTRATES FORMAMIDE An intense five-line spectrum was obtained, identical in appearance to that obtained by Bosco et aL9 for matrix-isolated CONH2 radical. (One proton coupling is too small to be resolved and one line of the six expected is submerged in the broad central peak.) Our couplings are slightly larger, aN = 2.3 mT, aH = 3.5 mT. DIMETHYLFORMAMIDE A very intense 1 : 2 : 1 triplet was produced with a, = 1.89 mT due to the radical HCON(Me)CH2*, identical to that found on y-radiolysis of this material at 77 K," PENTAN-3-ONE A quintet of binomial intensity distribution was given with aH = 2.20 mT assigned to the radical MecHCOEt.ACETALDEHYDE The spectrum consisted of a basic singlet centred on g,, = 2.001 7 showing considerable hyperfine structure to both high and low field with coupling of - 1.0 mT. The nature of this radical is obscure ; MeCO has gav = 2.0005 while CH2CH0 has = 2.07 mT. TETRAHYDROFURAN AND 2-METHYLTETRAHYDROFURAN Weak spectra exhibiting fine structure were given.They did not correspond to those expected of the a-radical, produced during y-radiolysis,1° although we did find cerium(1v) ammonium nitrate photo-oxidation of tetrahydrofuran to produce the a-radical, with an intensity pattern 1 : 2 : 2 : 2 : 1 (aav 1.89 mT). The a-radical, MeCHOEt has been observed during FeC1, photo-oxidation of diethyl ether,l and we also found it during photo-oxidation by cerium(iv) ammonium nitrate at 77 K with a, = 1.98 mT.A. COX AND T . J . KEMP 2493 LACTIC ACID An intense absorption was obtained at 77 K which we regard as due to a radical mixture. Its basic pattern is a 1 : 4 : 6 : 4 : 1 quintet, but the coupling constant varied somewhat from peak to peak, averaging to 1.67 mT which seems too low a figure to be attributed to CH,eHOH.Curiously, Poznyak et aZ.12 found aqueous ethylene glycol solutions of FeIrl complexes of lactate and mandelate ions to photodecompose at 77 K (indicated by loss of the e.s.r. line of FelI1) but saw no production of RcHOH radicals until the matrix was warmed to 140 K ; this " delayed action '' in radical production was rationalised by the authors in terms of an [Fell-RcHOH] adduct which they believe to show no e.s.r. absorption; this thermally dissociates at 140 K into Fe" + RCHOH. DISCUSSION Thermodynamically Fe"' is intermediate in activity between Ce" and Uv' on which we have previously concentrated. l-' Whilst CeIV photo-oxidations of organic substrates involve light absorption in the charge-transfer band of the CeIv-substrate complex (A,,, - 300 nm or 361 kJ mol-l) together with net reduction of CeIv to Ce"', for which E" = 1.70 V in HC104 solution (1 mol dm-,), equivalent to a free energy change of 164 kJ mol-', the corresponding figures for UO$+ are Amax = 400 nm (or 271 kJmol-l) and E" = 0.05 V (or AGO = 4.8 kJ mol-') and for Fe3+ in HC104 solutions with added substrate, Amax x 350 nm (or 309 kJ mol-l) and E" = 0.772 V (or AGO = 74.5 kJ mol-l).Both as regards the photochemical and electrochemical terms, then, Fe"' might be expected to show behaviour intermediate between Ce'" and Uvl in its photochemical interaction with organic substrates. In its interaction with the two simplest alcohols, excited iron(1rr) ion, denoted FeI1I4:, behaves both as CeIV* in abstracting a hydrogen atom from the hydroxylic carbon atom to give RCHOH (R = H or Me).This takes place with both FeCI, and Fe(ClO,),, indicating the process to be due to a genuine attack by FelI1* rather than by intermediate formation of a chlorine atom,6 followed by a secondary attack of the latter upon the matrix. While both of these mechanisms are conceivable for FeCl,, the CI. atom mechanism cannot operate in the case of Fe(C104),, nor can it explain the formation of readily detectable alkyl radicals as a principal path- way in the Fe"'* oxidation of tertiary alcohols which is found with both Fe(C104), and FeCl,. These originate from a C-C cleavage photoprocess previously found with CeIV*, but not with Uvl*, viz., and Uv'* R1R2R3COH + FelI1* + R1R2R3&H + Fel1 (1) (2) + - R1R2R3COH 4 H+ + R1* + R2R3C=0 Towards carboxylic acids, FelI1* shows ambivalent behaviour.[steps (1) and (2) may be concerted). The radical derived from RC02H, where R = Me, Et, (Me),CHCH, and CH2=CHCH2, is predomi- nantly or exclusively R e , which comes from a process of oxidative decarboxylation found with CeIV* and PbtV* ; l4 with isobutyric acid, however, the seven-line spectrum must be due to (Me),CC02H, i.e., the route is one of hydrogen-atom abstraction [the possibility that the primary isopropyl radical is highly reactive towards the labile tertiary hydrogen atom at 77K is ruled out by the observation of Me,CH under these conditions during CeIV* oxidation], Clearly FelI1* behaves very much like CeIV* [and unlike Uvr';] in these particular oxidations, with the exception of isobutyric acid towards which it behaves like Uvl*.2484 E.S.R.OF PHOTO-OXIDATION PRODUCTS FelI1* attacks formamide and dimethylformamide by an H-abstraction process to give CONH2 and HCON(Me)cH2, resembling both CerV* and Uv1*.4 It is un- necessary, therefore, to invoke the intermediacy of C1 atoms in the FeC1, photo- reaction with dimethylformamide, although such intermediacy cannot be excluded. The significance of the anion has been demonstrated l6 in the FeCT3 photo-oxidation of succinic, glutaric and adipic acids, when the o-chloro-derivative of the correspund- ing mono-acid is produced, suggesting attack of H02C[CH2],CH2* upon unreacted FeGl3. Incorporation of the chlorine into certain of the products also occurs during the FeC13 photo-oxidation of diethyl ether.I1 With diethyl ketone, the MeCIHCO- CH,Me radical was produced, as with Uvl* and Ce'v*,3 although the latter gave additional peaks of C,IES.The acetyl radical given by CeIV* attack on acetalde- hyde was not found with FelI1*, which gave a spectrum yet to be assigned. To summarise, FelI1 is an effective photo-oxidant for a wide range of organic substrates, behaving more like CeIV than Uvl although differing from the former in points of detail. Its effectiveness does not depend on the presence of chloride ligands to give reactive Cb intermediates, as iron(zI1) perchlorate yields the same radicals as does FeCi, and moreover in certain oxidations the product radicals are inconsistent with attack by CI-. We thank the S,R.C. for a grant to purchase the spectrometer, and Mr.S. H. Jenkins for both recording the spectra and maintenance of the spectrometer. I D. Greatorex and T. J. Kemp, Trms. Faraday SOC., 1971,67, 56. D. Greatorex and T. J. Kemp, Trans, Favaday SOC., 1971, 67, 1576. D. Greatorex and T. J. Kemp, J.C.S. Fmadzy I, 1972, 68, 121. D. Greatorex, R. J. HiII, T. J. Kemp and T. J. Stone, J.C.S. Faraday f, 1972, 68,2059. V. Balzani and V. Carassiti, Photochemistry of Cmrdination Compounds (Academic Press, New York and London, 1974), chap. 10. A. I. Kryukov, L. V. Nazarova and B. Y. Dain, Ukrain. khim. Zhur., 1963,29, 812. P. B. Ayscough, K. Mach, J. P. Oversby and A. K. Roy, Truns. Fmaday Soc., 1971,67,360. P. B. Ayscough and C. Thomson, Trans. Faraday Soc., 1962,58, 1,477. S. R. Bosco, A Circillo and R. B.Timmons, J. Amer. Chem. SOC., 1969, 91, 3140. S. A. Ivanitskaya, A. N. Korol and A. I. Kryukov, Ukrdn. khim. Zhur.,, 1973,39,1248. 1260. lo V. 1. Trofimov and I. I. Chkeidze, High Energy Chem., 1967, 1, 282. l2 A. L. Poanyak, G. A. Shaghitanova and S. I. Arzhankov, Russ. J. lnorg. Chem., 1970, 15, l 3 W. M. Latimer, Oxidation Potentials (Prentice-Hall, New York, 2nd edn., 1952). l4 K. Heusler and H. Loeliger, Heto. Chim. Aci'a, 1969,52,1495. l 5 F. S. Dainton and R. G. Jones, Trans. Faraduy SOC., 1967,63, 1512. l6 J. A. Kuhnle, R. E. Lundin and A. C. Waiss, Jr., Chem. Comm., 1972, 287. Electron Spin Resonance Studies of Photo-oxidation by Metal Ions in Rigid Media at Low Temperatures Part 5.-Photo-oxidation by the Iron(m) Ion BY ALAN COX AND TERENCE J. KEMP* Department of Molecular Sciences, University of Warwick, Coventry CV4 7AL Received 21.d May, 1975 Iron(ur), both as its perchlorate and chloride salts, is an effective photo-oxidant for a wide range of organic compounds to give radical species readily trapped and detected by e.s.r.spectroscopy at 77 K. Its general behaviour parallels that of cerium(rv) photo-oxidation in that C-C fission processes are prominent with tertiary alcohols and carboxylic acids, although with simple alcohols, amides and certain other types of molecule, abstraction of a hydrogen atom from an activated C-H bond is found. The closely similar behaviour of the two salts towards the compounds we have examined suggests that Cl atoms are not essential intermediates in FeJII photo-oxidations, as has been previously suggested, although there are photo-oxidations by FeCl, in which chlorine is incorporated into the isolabte product.In Parts 1-3 1-3 we showed that photolysis of solutions of cerium(1v) salts in various organic media at 77 K using light absorbed only by CeIV species (usually a charge-transfer complex with the solvent) leads to the efficient production of solvent- derived radicals readily detectable by e.s.r. spectroscopy. Experiments with organic substrates dissolved in dilute perchloric acid and solutions of cerium(1v) salts gave essentially similar results. In Part 4 the behaviour of uranyl salts was examined under similar experimental conditions ; again, organic radicals were produced, but often of a type different from those encountered in cerium(1v) photosensitisation.In view of the considerable literature existing on the photo-induced reactions of iron@) salts with organic substrates, we report here the results of a brief survey of the e.s.r. spectra generated by photo-oxidation of a representative variety of organic substrates, mostly by iron(rn) perchlorate, making comparison with the behaviour of CeIV and Uvl, and with previous data on FelIr where this is available. EXPERIMENTAL The procedures described in Part 1 were used. The radiation from a 100 W point- source Xe arc was filtered through Pyrex. Solutions were made up in neat solvent where the organic molecule of interest was a liquid, otherwise saturated aqueous solutions were used. Where iron(m) perchlorate was comparatively insoluble, iron(m) chloride was used instead.[Fe1Ir] was typically 0.05 mol dm-3. RESULTS ALCOHOLS METHANOL A 1 : 2 : 1 triplet was produced with QH = 1.98 mT and g = 2.002 4 which, by comparison with our previous data and those in the literature. is assigned to CH20H radical. This result agrees with that of Kryukov et aLY6 who used both perchlorate and chloride salts. 2490A . COX AND T . J . KEMP 249 1 ETHANOL A basic five-line spectrum of binomial intensity distribution was produced in agreement with the result of Kryukov et u Z . , ~ which is identical to that found in both CeIV and Uvl photo-oxidations,'* and which is assigned to CH,tHOH. As with Cexy, however, additional small peaks appeared near the maxima of the second and third absorption lines (numbering from low field) indicative of the presence of a second, minor radical.PROPAN-~-OL Photoreaction was much slower in this instance and the spectrum was far less intense than that found with either of the lower alcohols or using CeIV as oxidant1 It consisted of a central singlet flanked by a number of weaker, but sharp and repro- ducible peaks. This substrate gave complex behaviour with CeIV and (in the liquid state) yielded only a very weak spectrum with Uv* as photo-oxidant. t-BUTYL ALCOHOL A relatively sharp 1 : 3 : 3 : 1 quartet was obtained with aH - 2.2 mT. runs traces were apparent of an additional 1 : 2 I 1 triplet with aH 1.89 mT. species are assigned to CH3 and -CH,C(CH,),OH respectively. In some These 2 - MET HY L B U T A N - 2 - 0 L( t - A M Y L A 1, C OH 0 L) Photoreaction was again rather slower than with CeIV.At 77 K a very clear spectrum of ethyl radical gradually emerged ; on warming the samples to - 150 K the spectrum changed to a 1 : 2 : 1 triplet suggestive of attack of C2H5 upon substrate to yield a CH2X species. BUTAN-2-OL A radical mixture was obtained comprising a central singlet and other peaks typical of C2H5 radical (major coupling 2.6 mT). 3 -METHY LHEXAN- 3 -0L Quite rapid photolysis occurred to yield a pure spectrum of ethyl radical. BENZYL ALCOHOL Quite rapid photolysis gave a broad singlet showing hyperfine coupling suggestive of a benzyl-type radical (possibly C6H&HOH). CARBOXYLIC ACIDS ACETIC ACID At 77 K a radical mixture (of approximately equal concentrations) was obtained assigned to .CH3 and CH2C02H from the couplings and g-factors.On warming, the methyl radical disappeared leaving -CH,C02H which displayed the anisotropy apparent in the spectrum of Ayscough et a1.' PROPIONIC ACID The principal feature was a five-line spectrum of binomial intensity distribution together with two quite sharp additional peaks with al, 2.6 mT, suggestive of ethyl2492 E . S . R . OF PHOTO-OXIDATION PRODUCTS radical. The quintet coupling of 2.35 mT suggests assignment to MeCHC0,H (the remaining peaks of the ethyl radical are submerged under those of the quintet). ISOBUTYRIC ACID An intense, very well-defined seven-line spectrum was produced with a binomial intensity distribution and a, 2.06 mT, due clearly to MezeCO2H (and not the isoprop yI radical).ISOVALERIC ACID An intense five-line spectrum resulted (a, - 1.89 mT) with the appearance of that for y-irradiated isobutyl halides suggestive described by Ayscough and Thomson of the formation of isobutyl radicals. VINYLACETIC ACID A very intense five-line spectrum of binomial intensity distribution and with aH = 1.51 mT was given, suggesting the formation of ally1 radical. MISCELLANEOUS SUBSTRATES FORMAMIDE An intense five-line spectrum was obtained, identical in appearance to that obtained by Bosco et aL9 for matrix-isolated CONH2 radical. (One proton coupling is too small to be resolved and one line of the six expected is submerged in the broad central peak.) Our couplings are slightly larger, aN = 2.3 mT, aH = 3.5 mT. DIMETHYLFORMAMIDE A very intense 1 : 2 : 1 triplet was produced with a, = 1.89 mT due to the radical HCON(Me)CH2*, identical to that found on y-radiolysis of this material at 77 K," PENTAN-3-ONE A quintet of binomial intensity distribution was given with aH = 2.20 mT assigned to the radical MecHCOEt.ACETALDEHYDE The spectrum consisted of a basic singlet centred on g,, = 2.001 7 showing considerable hyperfine structure to both high and low field with coupling of - 1.0 mT. The nature of this radical is obscure ; MeCO has gav = 2.0005 while CH2CH0 has = 2.07 mT. TETRAHYDROFURAN AND 2-METHYLTETRAHYDROFURAN Weak spectra exhibiting fine structure were given. They did not correspond to those expected of the a-radical, produced during y-radiolysis,1° although we did find cerium(1v) ammonium nitrate photo-oxidation of tetrahydrofuran to produce the a-radical, with an intensity pattern 1 : 2 : 2 : 2 : 1 (aav 1.89 mT).The a-radical, MeCHOEt has been observed during FeC1, photo-oxidation of diethyl ether,l and we also found it during photo-oxidation by cerium(iv) ammonium nitrate at 77 K with a, = 1.98 mT.A. COX AND T . J . KEMP 2493 LACTIC ACID An intense absorption was obtained at 77 K which we regard as due to a radical mixture. Its basic pattern is a 1 : 4 : 6 : 4 : 1 quintet, but the coupling constant varied somewhat from peak to peak, averaging to 1.67 mT which seems too low a figure to be attributed to CH,eHOH. Curiously, Poznyak et aZ.12 found aqueous ethylene glycol solutions of FeIrl complexes of lactate and mandelate ions to photodecompose at 77 K (indicated by loss of the e.s.r. line of FelI1) but saw no production of RcHOH radicals until the matrix was warmed to 140 K ; this " delayed action '' in radical production was rationalised by the authors in terms of an [Fell-RcHOH] adduct which they believe to show no e.s.r.absorption; this thermally dissociates at 140 K into Fe" + RCHOH. DISCUSSION Thermodynamically Fe"' is intermediate in activity between Ce" and Uv' on which we have previously concentrated. l-' Whilst CeIV photo-oxidations of organic substrates involve light absorption in the charge-transfer band of the CeIv-substrate complex (A,,, - 300 nm or 361 kJ mol-l) together with net reduction of CeIv to Ce"', for which E" = 1.70 V in HC104 solution (1 mol dm-,), equivalent to a free energy change of 164 kJ mol-', the corresponding figures for UO$+ are Amax = 400 nm (or 271 kJmol-l) and E" = 0.05 V (or AGO = 4.8 kJ mol-') and for Fe3+ in HC104 solutions with added substrate, Amax x 350 nm (or 309 kJ mol-l) and E" = 0.772 V (or AGO = 74.5 kJ mol-l).Both as regards the photochemical and electrochemical terms, then, Fe"' might be expected to show behaviour intermediate between Ce'" and Uvl in its photochemical interaction with organic substrates. In its interaction with the two simplest alcohols, excited iron(1rr) ion, denoted FeI1I4:, behaves both as CeIV* in abstracting a hydrogen atom from the hydroxylic carbon atom to give RCHOH (R = H or Me). This takes place with both FeCI, and Fe(ClO,),, indicating the process to be due to a genuine attack by FelI1* rather than by intermediate formation of a chlorine atom,6 followed by a secondary attack of the latter upon the matrix.While both of these mechanisms are conceivable for FeCl,, the CI. atom mechanism cannot operate in the case of Fe(C104),, nor can it explain the formation of readily detectable alkyl radicals as a principal path- way in the Fe"'* oxidation of tertiary alcohols which is found with both Fe(C104), and FeCl,. These originate from a C-C cleavage photoprocess previously found with CeIV*, but not with Uvl*, viz., and Uv'* R1R2R3COH + FelI1* + R1R2R3&H + Fel1 (1) (2) + - R1R2R3COH 4 H+ + R1* + R2R3C=0 Towards carboxylic acids, FelI1* shows ambivalent behaviour. [steps (1) and (2) may be concerted). The radical derived from RC02H, where R = Me, Et, (Me),CHCH, and CH2=CHCH2, is predomi- nantly or exclusively R e , which comes from a process of oxidative decarboxylation found with CeIV* and PbtV* ; l4 with isobutyric acid, however, the seven-line spectrum must be due to (Me),CC02H, i.e., the route is one of hydrogen-atom abstraction [the possibility that the primary isopropyl radical is highly reactive towards the labile tertiary hydrogen atom at 77K is ruled out by the observation of Me,CH under these conditions during CeIV* oxidation], Clearly FelI1* behaves very much like CeIV* [and unlike Uvr';] in these particular oxidations, with the exception of isobutyric acid towards which it behaves like Uvl*.2484 E.S.R. OF PHOTO-OXIDATION PRODUCTS FelI1* attacks formamide and dimethylformamide by an H-abstraction process to give CONH2 and HCON(Me)cH2, resembling both CerV* and Uv1*.4 It is un- necessary, therefore, to invoke the intermediacy of C1 atoms in the FeC1, photo- reaction with dimethylformamide, although such intermediacy cannot be excluded.The significance of the anion has been demonstrated l6 in the FeCT3 photo-oxidation of succinic, glutaric and adipic acids, when the o-chloro-derivative of the correspund- ing mono-acid is produced, suggesting attack of H02C[CH2],CH2* upon unreacted FeGl3. Incorporation of the chlorine into certain of the products also occurs during the FeC13 photo-oxidation of diethyl ether.I1 With diethyl ketone, the MeCIHCO- CH,Me radical was produced, as with Uvl* and Ce'v*,3 although the latter gave additional peaks of C,IES. The acetyl radical given by CeIV* attack on acetalde- hyde was not found with FelI1*, which gave a spectrum yet to be assigned.To summarise, FelI1 is an effective photo-oxidant for a wide range of organic substrates, behaving more like CeIV than Uvl although differing from the former in points of detail. Its effectiveness does not depend on the presence of chloride ligands to give reactive Cb intermediates, as iron(zI1) perchlorate yields the same radicals as does FeCi, and moreover in certain oxidations the product radicals are inconsistent with attack by CI-. We thank the S,R.C. for a grant to purchase the spectrometer, and Mr. S. H. Jenkins for both recording the spectra and maintenance of the spectrometer. I D. Greatorex and T. J. Kemp, Trms. Faraday SOC., 1971,67, 56. D. Greatorex and T. J. Kemp, Trans, Favaday SOC., 1971, 67, 1576. D. Greatorex and T. J. Kemp, J.C.S. Fmadzy I, 1972, 68, 121. D. Greatorex, R. J. HiII, T. J. Kemp and T. J. Stone, J.C.S. Faraday f, 1972, 68,2059. V. Balzani and V. Carassiti, Photochemistry of Cmrdination Compounds (Academic Press, New York and London, 1974), chap. 10. A. I. Kryukov, L. V. Nazarova and B. Y. Dain, Ukrain. khim. Zhur., 1963,29, 812. P. B. Ayscough, K. Mach, J. P. Oversby and A. K. Roy, Truns. Fmaday Soc., 1971,67,360. P. B. Ayscough and C. Thomson, Trans. Faraday Soc., 1962,58, 1,477. S. R. Bosco, A Circillo and R. B. Timmons, J. Amer. Chem. SOC., 1969, 91, 3140. S. A. Ivanitskaya, A. N. Korol and A. I. Kryukov, Ukrdn. khim. Zhur.,, 1973,39,1248. 1260. lo V. 1. Trofimov and I. I. Chkeidze, High Energy Chem., 1967, 1, 282. l2 A. L. Poanyak, G. A. Shaghitanova and S. I. Arzhankov, Russ. J. lnorg. Chem., 1970, 15, l 3 W. M. Latimer, Oxidation Potentials (Prentice-Hall, New York, 2nd edn., 1952). l4 K. Heusler and H. Loeliger, Heto. Chim. Aci'a, 1969,52,1495. l 5 F. S. Dainton and R. G. Jones, Trans. Faraduy SOC., 1967,63, 1512. l6 J. A. Kuhnle, R. E. Lundin and A. C. Waiss, Jr., Chem. Comm., 1972, 287.