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Evidence for the photochemical formation of a high-valent 2A2u antimony porphyrin π-radical cation
Inorganic Chemistry Communications 3 Ž2000. 505–507 www.elsevier.nlrlocaterinoche
Evidence for the photochemical formation of a high-valent 2A 2u antimony porphyrin p-radical cation Gunther Knor ¨ ¨ ) UniÕersitat ¨ Regensburg, Institut fur ¨ Anorganische Chemie, 93040 Regensburg, Germany Received 24 June 2000
Abstract Irradiation of wSbVŽtpp.ŽOH. 2 xq Žtpp s dianion of 5,10,15,20-tetraphenylporphyrin. at 298 K in argon saturated dichloromethane Ø under steady state conditions results in the formation of the highly oxidising p-radical cation wSbVŽtpp .ŽOH. 2 x 2q with spectral 2 characteristics corresponding to a A 2u electronic ground state. q 2000 Elsevier Science S.A. All rights reserved. Keywords: Porphyrins; Radical cations; Oxidations; Photochemistry; Enzyme models
Numerous studies on the catalytic mechanisms of hemoproteins and model complexes that employ oxygen cosubstrates have established that a high-valent iron–oxo species coupled with a porphyrin p-radical cation Žindicated as compound I. is formed as a common intermediate Žfor general reviews, see Refs. w1–5x.. While in the redox cycles of peroxidases a one-electron transfer step reduces first the porphyrin radical of compound I, cytochrome P450 enzymes are thought to transfer the postulated ferryl oxygen atom to their substrates rather than simply serving as electron acceptors w6,7x. An issue of importance for understanding the diverse reactivity of oxidative heme enzyme intermediates and legitimate model compounds is the extent to which the porphyrin p-radical type Ž 2A 1u or 2 A 2u electronic configuration. can effect the inherent properties of the metal–oxo fragment of these species w8,9x. Besides some synthetic iron porphyrin derivatives w10x, photoexcited antimony porphyrin complexes can act as efficient catalysts mimicking the substrate transformations of several oxidoreductase enzymes w11x such as cytochrome P450-dependent monooxygenases. Although high-valent porphyrin p-radical cation intermediates have been proposed to play an important role in the photocatalytic cycles of antimony porphyrin mediated alkene oxygenations w12,13x, no previous attempts were made to further explore this possibility. The present communication
for the first time reports on the photochemical accumulation of a one-electron oxidised antimonyŽV. porphyrin. It also provides spectroscopic information about the ground state electronic structure of the powerful oxidant formed in this reaction.
The dihydroxy-coordinated complex wSbV Žtpp.ŽOH. 2 xCl, where tpp is the dianion of 5,10,15,20-tetraphenylporphyrin, was prepared as previously described w12,13x. This compound was chosen for the present investigation, because it readily forms an Sb s O oxometallate species wSbV Žtpp.ŽO.ŽOH.x upon deprotonation both in the electronic ground and excited state. Owing to the rather stable closed-shell d10 s 0 electronic configuration of the antimonyŽV. central-metal, a one-electron oxidation of wSbV Žtpp.ŽO.ŽOH.x should unambiguously lead to to the porphyrin Ø p-radical cation wSbV Žtpp .ŽO.ŽOH.xq or its correspon1
1
)
Tel.: q49-941-943-4484; fax: q49-941-943-4488.
Selected spectroscopic data: FD–MS m r z 767.7 ŽCalc. for C 44 H 30 N4 O 2 Sbq 767.1.. IR ŽCH 2 Cl 2 . 3570, 3580 cmy1 Ž n OyH .. UV–VIS wEtOH, lma x rnm Ž ´ rl moly1 cmy1 .x 314 Ž12900., 419 Ž316200., 552 Ž11800., 591 Ž6400..
1387-7003r00r$ - see front matter q 2000 Elsevier Science S.A. All rights reserved. PII: S 1 3 8 7 - 7 0 0 3 Ž 0 0 . 0 0 1 3 3 - 7
506
G. Knor ¨ r Inorganic Chemistry Communications 3 (2000) 505–507
oxidation of metalloporphyrin complexes w18,19x. By analogy with other photooxidations in halocarbon solvents, these results are consistent with a photoinduced one-electron transfer process according to the following overall reaction, Eq. Ž1.: SbV Ž tpp . Ž OH . 2
q
q CH 2 Cl 2
™ Sb Žtpp . Ž OH. hn
Fig. 1. Electronic absorption spectra of 1.2=10y5 M wSbVŽtpp.ŽOH. 2 xCl in argon saturated CH 2 Cl 2 before Ž---. and after t s120 min of monochromatic irradiation at l s 313 nm Ž — ..
Ø ding protonated form wSbV Žtpp .ŽOH. 2 x 2q. High redox potentials in the range of E1r2 s 1.5–1.8 V vs. SCE are reported for the first reversible electrooxidations of antimonyŽV. tetraphenylporphyrin complexes with various axial ligands w12–14x. In order to study these reactive p-radical cation species at ambient temperature in solution, the presence of potential substrate molecules and nucleophiles must be avoided. Neat chlorocarbon solvents such as CHCl 3 or CH 2 Cl 2 fulfill these requirements, and simultaneously can act as one-electron acceptors for the photooxidation of metal complexes w15,16x. The formation of metalloporphyrin p-radical cations in chlorocarbon solution has been successfully demonstrated by g-radiolytic methods w17x and photochemical routes w18,19x. A similar approach was followed for the desired one-electron oxidation of wSbV Žtpp.ŽOH. 2 xCl. When solutions of this compound in degassed dichloromethane are irradiated in the porphyrin B ŽSoret. or Q band spectral regions at 405 nm or 546 nm,2 no significant photoreactivity is observed. Photolysis at 313 nm, however, leads to absorption spectral changes that are characterised by a continuous loss in intensity of the intraligand B and Q bands. A quantum yield of f ; 10y4 Ž lirr s 313 nm. can be estimated from the disappearance of the antimonyŽV. porphyrin Q band maximum at 552 nm. At the same time, a growth in intensity of broad bands near 450 nm and 700 nm occurs. The final spectrum recorded at the completion of the reaction is shown in Fig. 1. A general broadening of each of the bands in the absorption spectrum, together with a decrease in intensity, is a typical behaviour observed during the ligand centered
2
Photolysis of the samples was carried out in argon saturated solutions at 298K with a Hanovia XerHg 977 B-1 Ž1 kW. lamp in Teflon stoppered 1 cm quarz spectrophotometer cells. Monochromatic light was obtained by a Schoeffel GM 250-1 monochromator. Quantum yields were determined with a Polytec pyroelectric radiometer, which was calibrated and equipped with a RkP-345 detector.
V
2q
Ø
2
Ø
q CH 2 Cl q Cly
Ž 1.
The presence of several isosbestic points in the visible spectral region indicates that only one porphyrin photoproduct is formed. At lower wavelengths in the UV part of the spectrum the isosbestic points are rapidly lost in the course of the reaction. This is most probably due to the appearence of light-absorbing secondary products resulting Ø from radical termination reactions of the CH 2 Cl species formed. While in some related photoreactions of metal complexes a direct participation of halogenated solvents as photoactive species has been observed w16x, such a solvent-initiated process can be ruled out in the present case, since CH 2 Cl 2 does not absorb light under 313 nm irradiation. Furthermore, oxidation processes by chloromethyl radicals or Cl atoms can be considered as unimportant w17x since these species are very short-lived and the metalloporphyrin concentration is relatively low. It is suggested that the reactive excited state involved in the photooxidation of wSbV Žtpp.ŽOH. 2 xCl is of the chargetransfer-to-solvent ŽCTTS. type w15,16x. The rather low quantum yield and the observed wavelength dependence of the reaction given in Eq. Ž1. are consistent with this assignment, and are reflecting the high redox potentials connected with the one-electron oxidation of antimonyŽV. tetraphenylporphyrin complexes w14x. Absorption spectral features have been applied to characterise the electronic structure and predominant unpaired spin distribution of metalloporphyrin p-radical cations w20x. According to the four-orbital model of porphyrin spectra w21x, the a 1u and a 2u p-orbital ordering determines the ground state of the ring-oxidised species, with the Ža21u a12u . configuration resulting in a 2A 2u state and Ža22u a11u . in a 2 A 1u state, respectively. Analysis of the optical data obØ tained for the one-electron oxidised species wSbV Žtpp .ŽOH. 2 x 2q with a rather sharp Soret region and poorly resolved lowest energy absorption bands ŽFig. 1. clearly indicates a 2A 2u ground state w20x for the antimonyŽV. tetraphenylporphyrin p-radical cation. The same radical type can be expected for the corresponding deprotonated Ø metal–oxo species wSbV Žtpp .ŽO.ŽOH.xq which resembles the reactive compound I intermediates of hemoproteins. In this context it is interesting to note that owing to the interaction of the porphyrin radical and oxo orbitals via metal orbitals, A 2u type model compounds are considered
G. Knor ¨ r Inorganic Chemistry Communications 3 (2000) 505–507
to be more effective in cytochrome P450 related substrate oxidation processes than A 1u radicals w22x. Acknowledgements Financial support from the Fonds der Chemischen Industrie is gratefully acknowledged. References w1x D. Mansuy, Battioni, in: J. Reedijk ŽEd.., Bioinorganic Catalysis, Dekker, New York, 1993. w2x R.A. Sheldon ŽEd.., Metalloporphyrins in Catalytic Oxidations, Dekker, New York, 1994. w3x W.-D. Woggon, Top. Curr. Chem. 184 Ž1996. 39–96. w4x G. Loew, in: E.I. Solomon, A.B.P. Lever ŽEds.., Inorganic Electronic Structure and Spectroscopy, vol.II, Wiley, New York, 1999. w5x K.M. Kadish, K.M. Smith, R. Guilard ŽEds.., The Porphyrin Handbook, vol. IV, Academic Press, New York, 1999. w6x P.R. Ortiz de Montellano, Acc. Chem. Res. 31 Ž1998. 543–549. w7x Y. Goto, T. Matsui, S. Ozaki, Y. Watanabe, S. Fukuzumi, J. Am. Chem. Soc. 121 Ž1999. 9497–9502.
507
w8x K. Czarnecki, J.R. Kincaid, H. Fujii, J. Am. Chem. Soc. 121 Ž1999. 7953–7954. w9x K. Czarnecki, L.M. Proniewicz, H. Fujii, D. Ji, R.S. Czernuszewicz, J.R. Kincaid, Inorg. Chem. 38 Ž1999. 1543–1547. w10x A. Maldotti, L. Andreotti, A. Molinari, V. Carassiti, J. Biol. Inorg. Chem. 4 Ž1999. 154–161. w11x G. Knor, ¨ Coord. Chem. Rev. 171 Ž1998. 61–70. w12x G. Knor, ¨ A. Vogler, Inorg. Chem. 33 Ž1994. 314–318. w13x S. Takagi, M. Suzuki, T. Shiragami, H. Inoue, J. Am. Chem. Soc. 119 Ž1997. 8712–8713. w14x K.M. Kadish, M. Autret, Z. Ou, K.-y. Akiba, S. Masumoto, R. Wada, Y. Yamamoto, Inorg. Chem. 35 Ž1996. 5564–5569. w15x A. Vogler, H. Kunkely, Inorg. Chem. 21 Ž1982. 1172–1175. w16x P.E. Hoggard, Coord. Chem. Rev. 159 Ž1997. 235–243. w17x D.M. Guldi, P. Neta, P. Hambright, J. Chem. Soc., Faraday Trans. 88 Ž1992. 2013–2019. ˇ w18x J. Sima, in: J.W. Buchler ŽEd.., Metal Complexes with Tetrapyrrole Ligands, vol. III, Struct. Bonding 84 Ž1995. 135–193. w19x D. Chatterjee, E. Balasubramanian, J. Coord. Chem. 46 Ž1999. 467–470. w20x Z. Gasyna, M.J. Stillman, Inorg. Chem. 29 Ž1990. 5101–5109. w21x M. Gouterman, in: D. Dolphin ŽEd.., The Porphyrins, vol. III, Academic Press, New York, 1978. w22x Y. Tokita, K. Yamaguchi, Y. Watanabe, I. Morishima, Inorg. Chem. 32 Ž1993. 329–333.
Evidence for the photochemical formation of a high-valent 2A 2u antimony porphyrin p-radical cation Gunther Knor ¨ ¨ ) UniÕersitat ¨ Regensburg, Institut fur ¨ Anorganische Chemie, 93040 Regensburg, Germany Received 24 June 2000
Abstract Irradiation of wSbVŽtpp.ŽOH. 2 xq Žtpp s dianion of 5,10,15,20-tetraphenylporphyrin. at 298 K in argon saturated dichloromethane Ø under steady state conditions results in the formation of the highly oxidising p-radical cation wSbVŽtpp .ŽOH. 2 x 2q with spectral 2 characteristics corresponding to a A 2u electronic ground state. q 2000 Elsevier Science S.A. All rights reserved. Keywords: Porphyrins; Radical cations; Oxidations; Photochemistry; Enzyme models
Numerous studies on the catalytic mechanisms of hemoproteins and model complexes that employ oxygen cosubstrates have established that a high-valent iron–oxo species coupled with a porphyrin p-radical cation Žindicated as compound I. is formed as a common intermediate Žfor general reviews, see Refs. w1–5x.. While in the redox cycles of peroxidases a one-electron transfer step reduces first the porphyrin radical of compound I, cytochrome P450 enzymes are thought to transfer the postulated ferryl oxygen atom to their substrates rather than simply serving as electron acceptors w6,7x. An issue of importance for understanding the diverse reactivity of oxidative heme enzyme intermediates and legitimate model compounds is the extent to which the porphyrin p-radical type Ž 2A 1u or 2 A 2u electronic configuration. can effect the inherent properties of the metal–oxo fragment of these species w8,9x. Besides some synthetic iron porphyrin derivatives w10x, photoexcited antimony porphyrin complexes can act as efficient catalysts mimicking the substrate transformations of several oxidoreductase enzymes w11x such as cytochrome P450-dependent monooxygenases. Although high-valent porphyrin p-radical cation intermediates have been proposed to play an important role in the photocatalytic cycles of antimony porphyrin mediated alkene oxygenations w12,13x, no previous attempts were made to further explore this possibility. The present communication
for the first time reports on the photochemical accumulation of a one-electron oxidised antimonyŽV. porphyrin. It also provides spectroscopic information about the ground state electronic structure of the powerful oxidant formed in this reaction.
The dihydroxy-coordinated complex wSbV Žtpp.ŽOH. 2 xCl, where tpp is the dianion of 5,10,15,20-tetraphenylporphyrin, was prepared as previously described w12,13x. This compound was chosen for the present investigation, because it readily forms an Sb s O oxometallate species wSbV Žtpp.ŽO.ŽOH.x upon deprotonation both in the electronic ground and excited state. Owing to the rather stable closed-shell d10 s 0 electronic configuration of the antimonyŽV. central-metal, a one-electron oxidation of wSbV Žtpp.ŽO.ŽOH.x should unambiguously lead to to the porphyrin Ø p-radical cation wSbV Žtpp .ŽO.ŽOH.xq or its correspon1
1
)
Tel.: q49-941-943-4484; fax: q49-941-943-4488.
Selected spectroscopic data: FD–MS m r z 767.7 ŽCalc. for C 44 H 30 N4 O 2 Sbq 767.1.. IR ŽCH 2 Cl 2 . 3570, 3580 cmy1 Ž n OyH .. UV–VIS wEtOH, lma x rnm Ž ´ rl moly1 cmy1 .x 314 Ž12900., 419 Ž316200., 552 Ž11800., 591 Ž6400..
1387-7003r00r$ - see front matter q 2000 Elsevier Science S.A. All rights reserved. PII: S 1 3 8 7 - 7 0 0 3 Ž 0 0 . 0 0 1 3 3 - 7
506
G. Knor ¨ r Inorganic Chemistry Communications 3 (2000) 505–507
oxidation of metalloporphyrin complexes w18,19x. By analogy with other photooxidations in halocarbon solvents, these results are consistent with a photoinduced one-electron transfer process according to the following overall reaction, Eq. Ž1.: SbV Ž tpp . Ž OH . 2
q
q CH 2 Cl 2
™ Sb Žtpp . Ž OH. hn
Fig. 1. Electronic absorption spectra of 1.2=10y5 M wSbVŽtpp.ŽOH. 2 xCl in argon saturated CH 2 Cl 2 before Ž---. and after t s120 min of monochromatic irradiation at l s 313 nm Ž — ..
Ø ding protonated form wSbV Žtpp .ŽOH. 2 x 2q. High redox potentials in the range of E1r2 s 1.5–1.8 V vs. SCE are reported for the first reversible electrooxidations of antimonyŽV. tetraphenylporphyrin complexes with various axial ligands w12–14x. In order to study these reactive p-radical cation species at ambient temperature in solution, the presence of potential substrate molecules and nucleophiles must be avoided. Neat chlorocarbon solvents such as CHCl 3 or CH 2 Cl 2 fulfill these requirements, and simultaneously can act as one-electron acceptors for the photooxidation of metal complexes w15,16x. The formation of metalloporphyrin p-radical cations in chlorocarbon solution has been successfully demonstrated by g-radiolytic methods w17x and photochemical routes w18,19x. A similar approach was followed for the desired one-electron oxidation of wSbV Žtpp.ŽOH. 2 xCl. When solutions of this compound in degassed dichloromethane are irradiated in the porphyrin B ŽSoret. or Q band spectral regions at 405 nm or 546 nm,2 no significant photoreactivity is observed. Photolysis at 313 nm, however, leads to absorption spectral changes that are characterised by a continuous loss in intensity of the intraligand B and Q bands. A quantum yield of f ; 10y4 Ž lirr s 313 nm. can be estimated from the disappearance of the antimonyŽV. porphyrin Q band maximum at 552 nm. At the same time, a growth in intensity of broad bands near 450 nm and 700 nm occurs. The final spectrum recorded at the completion of the reaction is shown in Fig. 1. A general broadening of each of the bands in the absorption spectrum, together with a decrease in intensity, is a typical behaviour observed during the ligand centered
2
Photolysis of the samples was carried out in argon saturated solutions at 298K with a Hanovia XerHg 977 B-1 Ž1 kW. lamp in Teflon stoppered 1 cm quarz spectrophotometer cells. Monochromatic light was obtained by a Schoeffel GM 250-1 monochromator. Quantum yields were determined with a Polytec pyroelectric radiometer, which was calibrated and equipped with a RkP-345 detector.
V
2q
Ø
2
Ø
q CH 2 Cl q Cly
Ž 1.
The presence of several isosbestic points in the visible spectral region indicates that only one porphyrin photoproduct is formed. At lower wavelengths in the UV part of the spectrum the isosbestic points are rapidly lost in the course of the reaction. This is most probably due to the appearence of light-absorbing secondary products resulting Ø from radical termination reactions of the CH 2 Cl species formed. While in some related photoreactions of metal complexes a direct participation of halogenated solvents as photoactive species has been observed w16x, such a solvent-initiated process can be ruled out in the present case, since CH 2 Cl 2 does not absorb light under 313 nm irradiation. Furthermore, oxidation processes by chloromethyl radicals or Cl atoms can be considered as unimportant w17x since these species are very short-lived and the metalloporphyrin concentration is relatively low. It is suggested that the reactive excited state involved in the photooxidation of wSbV Žtpp.ŽOH. 2 xCl is of the chargetransfer-to-solvent ŽCTTS. type w15,16x. The rather low quantum yield and the observed wavelength dependence of the reaction given in Eq. Ž1. are consistent with this assignment, and are reflecting the high redox potentials connected with the one-electron oxidation of antimonyŽV. tetraphenylporphyrin complexes w14x. Absorption spectral features have been applied to characterise the electronic structure and predominant unpaired spin distribution of metalloporphyrin p-radical cations w20x. According to the four-orbital model of porphyrin spectra w21x, the a 1u and a 2u p-orbital ordering determines the ground state of the ring-oxidised species, with the Ža21u a12u . configuration resulting in a 2A 2u state and Ža22u a11u . in a 2 A 1u state, respectively. Analysis of the optical data obØ tained for the one-electron oxidised species wSbV Žtpp .ŽOH. 2 x 2q with a rather sharp Soret region and poorly resolved lowest energy absorption bands ŽFig. 1. clearly indicates a 2A 2u ground state w20x for the antimonyŽV. tetraphenylporphyrin p-radical cation. The same radical type can be expected for the corresponding deprotonated Ø metal–oxo species wSbV Žtpp .ŽO.ŽOH.xq which resembles the reactive compound I intermediates of hemoproteins. In this context it is interesting to note that owing to the interaction of the porphyrin radical and oxo orbitals via metal orbitals, A 2u type model compounds are considered
G. Knor ¨ r Inorganic Chemistry Communications 3 (2000) 505–507
to be more effective in cytochrome P450 related substrate oxidation processes than A 1u radicals w22x. Acknowledgements Financial support from the Fonds der Chemischen Industrie is gratefully acknowledged. References w1x D. Mansuy, Battioni, in: J. Reedijk ŽEd.., Bioinorganic Catalysis, Dekker, New York, 1993. w2x R.A. Sheldon ŽEd.., Metalloporphyrins in Catalytic Oxidations, Dekker, New York, 1994. w3x W.-D. Woggon, Top. Curr. Chem. 184 Ž1996. 39–96. w4x G. Loew, in: E.I. Solomon, A.B.P. Lever ŽEds.., Inorganic Electronic Structure and Spectroscopy, vol.II, Wiley, New York, 1999. w5x K.M. Kadish, K.M. Smith, R. Guilard ŽEds.., The Porphyrin Handbook, vol. IV, Academic Press, New York, 1999. w6x P.R. Ortiz de Montellano, Acc. Chem. Res. 31 Ž1998. 543–549. w7x Y. Goto, T. Matsui, S. Ozaki, Y. Watanabe, S. Fukuzumi, J. Am. Chem. Soc. 121 Ž1999. 9497–9502.
507
w8x K. Czarnecki, J.R. Kincaid, H. Fujii, J. Am. Chem. Soc. 121 Ž1999. 7953–7954. w9x K. Czarnecki, L.M. Proniewicz, H. Fujii, D. Ji, R.S. Czernuszewicz, J.R. Kincaid, Inorg. Chem. 38 Ž1999. 1543–1547. w10x A. Maldotti, L. Andreotti, A. Molinari, V. Carassiti, J. Biol. Inorg. Chem. 4 Ž1999. 154–161. w11x G. Knor, ¨ Coord. Chem. Rev. 171 Ž1998. 61–70. w12x G. Knor, ¨ A. Vogler, Inorg. Chem. 33 Ž1994. 314–318. w13x S. Takagi, M. Suzuki, T. Shiragami, H. Inoue, J. Am. Chem. Soc. 119 Ž1997. 8712–8713. w14x K.M. Kadish, M. Autret, Z. Ou, K.-y. Akiba, S. Masumoto, R. Wada, Y. Yamamoto, Inorg. Chem. 35 Ž1996. 5564–5569. w15x A. Vogler, H. Kunkely, Inorg. Chem. 21 Ž1982. 1172–1175. w16x P.E. Hoggard, Coord. Chem. Rev. 159 Ž1997. 235–243. w17x D.M. Guldi, P. Neta, P. Hambright, J. Chem. Soc., Faraday Trans. 88 Ž1992. 2013–2019. ˇ w18x J. Sima, in: J.W. Buchler ŽEd.., Metal Complexes with Tetrapyrrole Ligands, vol. III, Struct. Bonding 84 Ž1995. 135–193. w19x D. Chatterjee, E. Balasubramanian, J. Coord. Chem. 46 Ž1999. 467–470. w20x Z. Gasyna, M.J. Stillman, Inorg. Chem. 29 Ž1990. 5101–5109. w21x M. Gouterman, in: D. Dolphin ŽEd.., The Porphyrins, vol. III, Academic Press, New York, 1978. w22x Y. Tokita, K. Yamaguchi, Y. Watanabe, I. Morishima, Inorg. Chem. 32 Ž1993. 329–333.