Photoredox Decomposition of [Cobalamin-^-NC-Fen ...

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aquocobalamin chloride (hydroxocobalamin hy drochloride from Sigma) with equimolar amounts of K4[Fe(CN)6] in water [5]. Solutions of K3[coba- lamin-[i ...
Photoredox Decomposition of [Cobalamin-^-NC-Fen(CN)5]3* Induced by Metal-to-Metal Charge Transfer Excitation H. Kunkely, A. Vogler* Institut für Anorganische Chemie, Universität Regensburg, D-93040 Regensburg, Germany Z. Naturforsch. 51b, 2 4 5 -2 4 8 (1996); received July 7, 1995 Vitamin B I2, Photolysis, Electron Transfer, Cyano Complexes, Iron In the presence of oxygen aqueous solutions of the binuclear complex [cobalamin-^-NCFe(C N )5]3' undergo a redox photolysis (0 ~ 2 x 10'3 at Airr = 405 nm) which yields aquocobalamin and [Fe(C N)6]3\ It is suggested that this photoreaction is induced by direct metal-tometal charge transfer (M M CT) excitation. On the contrary, [cobalamin-fx-NC-Ru(CN)5]3' is photo-inert because a reactive MMCT state is not accessible.

Introduction The interaction of aquocobalamin (vitamin B 12a) with cyanoferrate complexes has been studied by several groups [1-5]. Initially these in­ vestigations were stimulated by the intention to use vitamin B 12 as an antidote for cyanide poison­ ing induced by the hypotensive com pound nitroprusside [6]. A nother interesting feature of such binuclear complexes should be the charge transfer (CT) interaction of C o(III) and Fe(II). Optical m etal-to-metal charge transfer (MMCT) of simple binuclear complexes which contain a reducing and an oxidizing metal center has been investigated extensively [7-9]. Photoredox reactions induced by MMCT excitation were also observed for binu­ clear complexes which contain C o(III) as acceptor and Fe(II) as [Fe(CN)6]4- or R u(II) as [R u(C N )6]4' as donor [7-11]. Although photoredox processes of porphyrin [12-14] and corrin [15] complexes are of considerable importance, they have not yet been reported to occur upon direct MMCT excita­ tion. This is, however, not surprising since the de­ tection of optical MMCT is ham pered by the pres­ ence of intense porphyrin or corrin intraligand (IL) absorptions which extend from the U V to the red spectral region. Any other bands of different origin are then difficult to identify. As a suitable candidate for the detection of a photoactive MMCT transition in binuclear complexes contain­ ing porphyrin or corrin ligands we selected the an­

ion [cobalamin-(i-NC-Fe(CN)5]3* [5] (abbrevia­ tion: [B12-FeH(CN)6]3"). For comparison the corresponding ruthenium complex [B12-R un (CN)6]3' was investigated, too. Experimental Section M aterials

An aqueous solution of K3[cobalamin-^i-NCFe(CN)5] was prepared in situ by the reaction of aquocobalamin chloride (hydroxocobalamin hy­ drochloride from Sigma) with equim olar amounts of K4[Fe(CN)6] in water [5]. Solutions of K3[cobalamin-[i,-NC-Ru(CN)5] were obtained by the same procedure using K4[Ru(CN)6] instead of K4[Fe(CN)6], P h otolyses

The light source was an Osram H B O 100 W/2 or a Hanovia Xe/Hg 977 B -l (1 kW) lamp. M ono­ chromatic light was obtained by means of the Schott PIL/IL interference filters 366, 405, 436 and 546 nm or by a Schoeffel GM 250/1 high-intensity monochromator. The photolyses were carried out in aqueous solutions in 1 cm spectrophotom eter cells at room tem perature. Progress of the pho­ tolyses were m onitored by UV-visible spectropho­ tometry. For quantum yield determ inations the concentrations of the complexes were such as to have essentially complete light absorption. The to­ tal am ount of photolysis was limited to less than 5% to avoid light absorption by the photoproduct. A bsorbed light intensities were determ ined by a Polytec pyroelectric radiom eter which was cal­ ibrated and equipped with a RkP-345 detector.

* Reprint requests to Prof. Dr. A . Vogler. 0 932-0776/96/0200-0245 $ 06.00

© 1996 Verlag der Zeitschrift für Naturforschung. A ll rights reserved.

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H. K u n k ely-A . Vogler • Photoredox D ecom position of (Cobalamin-ia -N C -F e II(C N )5]3_

Instrum entation

Absorption spectra were measured with a Hew­ lett Packard 8452A diode array spectrometer.

Results and Discussion The electronic absorption spectra of [B12Fe(CN)6]3" (Amax = 537 nm, e = 10 400; Amax = 508 nm, £ = 10 100; Amax = 410 nm, e = 4 700; Amax = 356 nm, e = 31 350 M 1 c m 1) and [B12-Ru(CN)6]3(Fig.l) are nearly identical. The ruthenium com­ plex displays band maxima at X - 536 (10 200), 506 (10 100), 410 (4 700), and 355 nm (30 700). In anal­ ogy to many other cyanide-bridged complexes [7 9] the anions [B12-M (CN)6]3" are expected to show absorptions which separately can be attributed to the m ononuclear components B 12-NC and [M (CN)6]4'. Both species constitute the binuclear complexes. In addition, MMCT bands should ap­ pear. Com pared to [B12-Fe(CN)6]3" the MMCT transition of [B12-Ru(CN)6]3‘ requires a higher en­ ergy since [Ru(CN)6]4~ is less reducing than [Fe(CN)6]4' (ZlE = 0.5 V or 4033 cm '1) [7], Since both [M(CN)6]4" complexes do not absorb above 340 nm [11], the bands at longer wavelength must be assigned to transitions within the B 12 moiety and possibly to MMCT transitions from M(II) to Co(III). However, the close similarity of the ab­ sorption spectra of [B12-Fe(CN)6]3' and [B12R u(C N )6]3' is only consistent with the assumption that all long-wavelength bands of both complexes can be attributed to the B 12 moiety. From the com­ parison with other B 12 complexes [15] such as cyano and aquocobalamin it follows that these ab­ sorptions of [B12-M (CN)6]3‘ are exclusively of the IL (corrin) type. Most prom inent are the a, ß , and y (Soret) bands (Fig. 1). MMCT absorptions of [B12-M (CN)6]3" are expected to occur in the same spectral region. Such bands were identified for [(NC)5Coiii-h-CN-Mii(CN)5]6- at Amax = 385 nm (£ - 630) with M = Fe and Amax = 312 nm {e - 460) with M = Ru [11], and for [(NH3)5Con V C N - R u 11(CN)5]" at Amax = 375 nm (e - 690) [10]. However, in the case of [B12-M (CN)6]3' these MMCT bands which should show up at different energies are ap­ parently hidden under the intense corrin IL ab­ sorptions. Generally, m ononuclear B 12 complexes are light sensitive [15,16-20]. The photoreactivity depends

Fig. 1. Electronic absorption spectrum of 5.27 x 10'5 M [cobalamin-(^-NC-Ru(CN)5]3" in water at room temper­ ature; 1-cm cell.

on the nature of the reactive excited state. For ex­ ample, in the case of B 12-CN (cyanocobalamin) corrin IL excitation is followed by the population of a ligand field LF excited state which is substitutionally labile and undergoes an aquation of the cyanide ligand [21], Organocobalamins such as methylcobalamin have available redox active CT states [22]. CT excitation leads to a homolytic splitting of the cobalt(III)-carbon bond yielding B i2r (Co11) and methyl radicals in the primary pho­ tochemical step [23], Although [B12-M (CN)6]3' might also be ex­ pected to undergo a photosubstitution with the formation of B 12a (B 12-H 20 + or aquocobalamin) and [M(CN)6]4~, such a photolysis was not ob­ served. This is, however, not surprising since B12a and [M(CN)6]4" would rapidly regenerate the bi­ nuclear complexes [5]. This reaction, which is also used to synthesize [B12-M (CN)6]3' (see above), is certainly facilitated by ion pairing of the aquoco­ balamin cation and the highly charged [M(CN)6]4~ anion. While [B12-R u(C N )6]3' was not light sensitive at all, aqueous [B12-Fe(CN)6]3' underwent a photore­ dox photolysis with (p ~ 2 x 10 3 if the irradiation was perform ed in the presence of oxygen with an excitation wavelength of A = 405 nm. Irradiations at other wavelengths were not effective. The slight spectral changes which accompanied the photoly­ sis were com patible with the formation of B 12H 20 + [21]. In addition [Fe(CN)6]3' was formed. Its presence in the irradiated solution was confirmed by its reaction with Fe2+ which yielded Prussian blue.

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H. K u n k ely-A . Vogler • Photoredox D ecom position o f (Cobalamin-iM -NC-FeII(C N ) 5 ]3~

It is concluded that the photolysis proceeds ac­ cording to the equations: [B 12-Fe(C N )6]3-

hv

B 12r + [FeHI(C N )6]3'

B 12r + [Fein(C N )6]3 [B 12-Fe” (C N )6]3oxygen B [B 12-H 20 ] + water

(1) (2) (3)

The primary photochemical step (1) is indicative of MMCT excitation which involves an electron transfer from Fe(II) to Co(III). As a consequence, the binuclear complex collapses to the radical pair B i2r /[FeHI(CN)6]3~, which can undergo a cage es­ cape. In the absence of oxygen [B12-Fen (CN)6]3' is effectively regenerated by an inner-sphere elec­ tron back transfer (2). In air-saturated solutions Co(II) (B 12r) is intercepted by oxygen yielding fi­ nally B 12a (3). The low overall efficiency of the photolysis seems to reflect the relatively slow reac­ tion of B 12r with oxygen [15]. According to this reaction scheme the photoly­ sis of [B12-Fe(CN)6]3~ proceeds in analogy to that of [(NC)5CoII1-CN-FeII(CN)5]6' which is also in­ duced by Fe(II) to C o(III) MMCT (Amax = 385 nm, £ = 630) [11] excitation. Although in distinction to [(NC)5Coni-CN-Fen(C N )5]6- a MMCT band was not detected for [B12-Fe(CN)6]3~, it is assumed that a MMCT band of the latter complex is present at the irradiation wavelength (~ 405 nm), but is hid­ den under the more intense corrin IL band (e = 4 600) at this wavelength. Nevertheless, a consider­ able fraction of the exciting light can still be ab­ sorbed by the MMCT band. Irradiation with light of other wavelengths apparently does not initiate the redox photolysis since in agreem ent with pre­ vious observations [7-11] only direct MM CT exci­

[1] A. R. Butler, C. G lidewell, Chem. Soc. Rev. 16, 361 (1987). [2] A. R. Butler, C. G lidewell, A. S. McIntosh, D. Reed, I. H. Sadler, Inorg. Chem. 25, 970 (1986). [3] A. F. Cuthbertson, C. Glidewell, A. R. Butler, A . S. McIntosh, Inorg. Chim. Acta 153, 93 (1987). [4] O. R. Leeuwenkamp, W. P. van Bennekom , E. J. van der Mark, A. Bult, Pharm. Weekbl. Sei. Ed. 6, 129 (1984) and references therein. [5] G. Stochel, R. van Eldik, H. Kunkely, A. Vogler, Inorg. Chem. 28, 4314 (1989). [6] M. A. Posner, R. E. Tobey, H. McElroy, A n esth esi­ ology 44, 157 (1976). [7] A. Vogler, A . H. Osman, H. Kunkely, Coord. Chem. Rev. 64, 159 (1985)

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tation can induce this redox photolysis. In com par­ ison to [B12-Fe(CN)6]3' the MMCT absorption of [B12-R u(C N )6]3~must be shifted to higher energies by approximately 4033 cm'1 (see above). The MMCT band of the ruthenium complex would then coincide with the Soret absorption which has a very high intensity (e = 30 700). Accordingly, upon irradiation in this spectral region the fraction of light absorbed by the MMCT band of [B12Fe(CN)6]3' is negligible and a redox photolysis does not occur. In summary, [B12-Fe(CN)6]3~is an interesting ex­ ample of a binuclear complex of biological im por­ tance which undergoes a photoredox process upon direct MMCT excitation. There are indications that this type of CT interaction may also apply to other polynuclear metalloenzymes [24]. In this context it is of considerable interest that our re­ sults supplement a large num ber of observations on photoinduced electron transfer between por­ phyrins and various donors and acceptors (e.g. diads, triads) [25,26]. However, in these cases intra­ molecular electron transfer generally was not achieved by direct CT interaction, but by excited state electron transfer. For the majority of metalloporphyrins this process is not feasible since the excited states of the porphyrin ligands are rapidly deactivated by the intervention of lower-energy excited states of different origin such as LF states [12-14], A c k n o w led g m en ts

Support for this research by the Deutsche Forschungsgemeinschaft and the Fonds der Chem ­ ischen Industrie is gratefully acknowledged.

[8] A. Vogler, in M. A. Fox and M. Chanon (eds): Pho­ toinduced Electron Transfer, part D, p. 179, Elsevier, Amsterdam (1988). [9] A. Vogler, H. Kunkely, in K. Kalyanasundaram and M. Grätzel (eds): Photosensitization and Photoca­ talysis Using Inorganic and Organometallic Com ­ pounds, p. 71, Kluwer, Dordrecht (1993). [10] A. Vogler, H. Kunkely, Ber. Bunsenges. Phys. Chem. 79, 83 (1975). [11] A. Vogler, H. Kunkely, Ber. Bunsenges. Phys. Chem. 79, 301 (1975). [12] K. Kalyanasundaram, Photochemistry of Polypyri­ dine and Porphyrin Complexes, Academ ic Press, London (1992).

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H. K u n k ely -A . Vogler • Photoredox D ecom position o f (C obalam in-^ -N C -F en(C N ) 5 ]3~

[13] D. M. Roundhill, Photochemistry and Photophysics of Metal Complexes, Plenum Press, New York (1994). [14] H. Kenneth, S. Suslick. R. A. Watson, New. J. Chem. 16, 633 (1992). [15] J. M. Pratt, Inorganic Chemistry of Vitamin B 12. A c­ ademic Press, London (1972). [16] D. G. Brown, Prog. Inorg. Chem, 18, 177 (1973). [17] G. N. Schrauzer, Angew. Chem. Int. Ed. Eng. 15, 417 (1976). [18] H. P. C. Hogenkam p, in D. Dolphin (ed): B 12 p.295, Wiley, N ew York (1982). [19] C. Gianotti, in D. Dolphin (ed): B 12 p.393, Wiley, N ew York (1982).

[20] P. J. Toscano, L. G. Marzilli, Prog. Inorg. Chem. 31, 105 (1984). [21] A. Vogler, R. Hirschmann, H. Otto, H. Kunkely, Ber. Bunsenges. Phys. Chem. 80, 420 (1976). [22] H. Kunkely, A . Vogler, J. Organomet. Chem. 453, 269 (1993). [23] (a) J. F. Endicott, G. J. Ferraudi, J. Am. Chem. Soc. 99, 243 (1977); (b) J. F. Endicott, T. L. N etzei, J. Am. Chem. Soc. 101, 4000 (1979). [24] H. Kunkely, A. Vogler, Inorg. Chem. 34, 2468 (1995). [25] D. Gust, T. A. M oore, A . L. M oore, Acc. Chem. Res. 26, 198 (1993). [26] M. R. W asielewski, Chem. Rev. 92, 435 (1992).

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