摘要:
J. CHEM. SOC. PERKIN TRANS. II 1986 Photoelectron-transf er Reactions of Flavin Analogues with Tetra-al kylti n Compounds Shunichi Fukuzumi, Sadaki Kuroda, and Toshio Tanaka Department of Applied Chemistry, Faculty of Engineering, Osaka University, Suita, Osaka 565,Japan The fluorescence of flavin analogues (3-methyl-1 0-phenylisoalloxazine and 3-methyl-I O-phenyl-5- deazaisoalloxazines) in the absence and presence of Mg2+ion in acetonitrile was quenched by the electron-transfer reactions with tetra-alkyltin compounds. The quenching rate constants as well as the rate constants for electron-transfer reactions of the tetra-alkyltin compounds with iron(111) complexes [Fe(N-N),I3+ (N-N = 2,2'-bipyridine and various substituted 1,I0-phenanthrolines) agree with those calculated by using the Marcus theory for outer-sphere electron-transfer reactions over a wide spread of values of the Gibbs energy change from the highly exothermic to the endothermic region.The intrinsic barrier h for the electron-transfer reactions of tetra-alkyltin compounds is found to be significantly large, i.e., h = 170 kJ mol-', compared with those of organic compounds (typically, h = 40 kJ mol-'). A flavin analogue (3-methyl-I 0-phenylisoalloxazine) catalyses the photo-oxidation of tetra-alkyltin compounds by oxygen in the presence of Mgz+ion in acetonitrile, which proceeds via the photoelectron-transfer from tetra-alkyltin compounds to the excited state of the flavin. Photoelectron-transfer reactions are receiving increased atten- tion in the field of inorganic' as well as organic chemi~try.~.~ QThe basic concept and its application have been extended not only to the field of biomimetic chemistry but also to that of biochemi~try.~In fact, photoelectron-transfer reactions of flavin, which is a principal coenzyme of biochemical redox reactions, with various organic reductants have been extensively In contrast, relatively little is known of photo- electron-transfer reactions of organometallic compounds, especially alkylmetals, despite the fact that many alkylmetals are known to be good electron donors.' In this paper, we report photoelectron-transfer reactions of flavin analogues [3-methyl- 10-phenylisoalloxazine (l), 3-methyl-10-phenyl-5-deazaisoalloxazine(2a), 3-methyl-8-chloro-10-phen yl-5-deazaisoalloxazine (2b), 3-methyl-7-nitro- l0-phenyl-5-deazaisoalloxazine(2c)l with tetra-alkyltin com- pounds R,Sn (R = Me, Et, Pr', Bu") in the absence and presence of Mg2+ ion in acetonitrile (MeCN). The redox potentials of R,Sn can be tuned by changing the alkyl group,' and those of the singlet-excited states of flavin analogues are changed by the complex formation with Mg2+ ion in MeCN.7 Since an outer-sphere electron-transfer mechanism has well been established for electron-transfer reactions from R4Sn to +iron(rI1) complexes [Fe(N-N),I3 (N-N = 2,2'-bipyridine and various substituted l,lO-phenanthr~lines),~ the present study provides an opportunity to establish the functional dependence of the rate constants for electron-transfer reactions of R,Sn over a wide range of the Gibbs energy change of electron transfer ranging from the highly exothermic photoelectron- transfer reactions to the endothermic thermal electron-transfer reactions. Then, the applicability of the Marcus theory, which is known to be particularly useful for predicting electron-transfer rates when the Gibbs energy change of electron-transfer is nearly zero," will be tested for the electron-transfer reactions of R,Sn over a wide spread of values of the Gibbs energy change of electron transfer.We report also the photo-oxidation of R,Sn by oxygen, catalysed by a flavin analogue (1)-Mg2+ complex in MeCN, which is initiated by photoelectron transfer from R,Sn to the excited state of (1). Experimental Materials.-Preparations of flavin analogues (1) and (2a-c) were described el~ewhere.~.' A series of tetra-alkyltin (2) a; R' = R*= H b; R' = CL, R2= H c; R' = H, R2= NO2 compounds was prepared according to the standard pro- cedures.'.' Anhydrous metal perchlorates Mg(C104)2 and NaClO, were obtained commercially and dried before use.Potassium ferrioxalate used as an actinometer was prepared according to the literature,' and purified by recrystallization from hot water. Acetonitrile used as a solvent was purified and dried by the standard pr~cedure,'~ and stored on calcium hydride under nitrogen. Fluorescence-quenching Experiments.-Fluorescence meas-urements were carried out on a Hitachi 650-10s fluorescence spectrophotometer. Quenching experiments of the flavin ana- logue fluorescence in the absence and presence of Mg(ClO,), in MeCN were carried out by using tetra-alkyltin compounds as quenchers.Relative fluorescence intensities of a flavin analogue at the emission maximum [h,,,, for (1) 506, for (2a-c) 451 nm in the absence of Mg(ClO,),; h,,,. for (1) 493, for (2a) 439, for (2b) 434,for (2c) 443 nm in the presence of 0.10 mol dm-, Mg(ClO,),] were measured for MeCN solutions containing a flavin analogue and a quencher at a variety of concentrations. There was no change in the shape but the intensity of the fluorescence spectrum changed on addition of tetra-alkyltin compounds. The Stern-Volmer relationship was obtained between the ratio of the fluorescence intensity in the absence and presence of a quencher c/If and the quencher concentration [R,Sn] as expressed by equation (l), where k, is the quenching 26 J.CHEM. SOC. PERKIN TRANS. 11 1986 c/If = 1 + k,z[R,Sn] (1) rate constant and z is the fluorescence lifetime of flavin analogues [t for (1) 2.4, for (2a) 1.7, for (2b) 1.8, for (2c) 0.38 ns in the absence of Mg(ClO,),; for (1) 1.7, for (2a) 1.6,for (2b) 1.0, for (2c) 0.19 ns in the presence of 0.10 mol dmP3 Mg(C10,)2J.7 Determination of Redox Potentials of the Singlet-excited States of Flavin Analogues.-The redox potential of (1) was determined by the cyclic voltammetry measurements using a Hokuto Denko model HA-301 potentiostat-galvanostat at 298 K.The electrochemical reduction of (1) in MeCN with a platinum microelectrode under deaerated conditions was rever- sible and the redox potential of the couple (l)/(l-*) was determined as 0.76 V versus a standard NaCl calomel reference electrode (s.c.e.). The redox potential of the singlet-excited state of (1) Eo(l*)/(l-*) was determined as the sum of that of the ground state Eo(l)/(l -*) and the excitation energy AEo,o [equation (2)], where the AEo,ovalue was obtained as 2.64 eV Eo(l*)/(l-.) = Eo(l)/(l-*) + AEo,o (2) (1 eV = 1.60 x J) from the frequencies of the absorption and the fluorescence maxima of (1). Although the electro- chemical reduction of flavin analogues other tban (1) was irreversible, their Eo(Fl*/Fl -') values were determined based on the value of (1) since the relative values of Eo(Fl*/Fl-') referenced to (1) in the absence and presence of 0.10 mol dm-, Mg(ClO,), have been previously rep~rted.~ Photo-oxidation of Tetra-alkyltin Compounds.-The photo-oxidation of Me,Sn by oxygen, catalysed by a (ltMg2+ complex, was monitored by using a Japan Electron Optics JNM-PS-100 'H n.m.r.spectrometer. Typically, Me,Sn (6.2 x cm3) was added to an n.m.r. tube which contained a CD3CN solution (0.50 cm3) of a flavin analogue (1) (2.0 x lo--, mol dmT3) and Mg(ClO,), (0.10 mol dmP3). After the reactant solution was saturated with oxygen, it was irradiated with the visible light from an Ushio model U1-501 Xenon lamp through a Toshiba filter C-39A transmitting light of 350 nm < h < 470 nm.The relevant 'H n.m.r. data are 6 0.06 (12 H, s) for Me,Sn and 0.53 (9 H, s) and 3.77 (3 H, s) for the oxidation product MeO,SnMe,. The oxidation product was quantitatively con- verted into Me,SnOH and MeOH by treatment with an aqueous solution of NaI [equation (3)]. The products of photo- oxidation of Pr',Sn were analysed also by g.1.c. MeO,SnMe, + 21-+ 2H++ Me3SnOH + MeOH + I, (3) A standard actinometer (potassium ferrioxalate) was used for the quantum-yield determinations on the photo-oxidation of R,Sn by oxygen, catalysed by a (1)-Mg2+ complex in MeCN. The actinometry experiments were carried out under con-ditions such that both an actinometer and a (l)-Mg2+ complex absorb essentially all the incident light (>95%) through the filter (350 nm < h < 470 nm). The quantum yields of the photo-oxidation of Me,Sn were determined from the rate of formation of MeO,SnMe, measured by the iodometry accord- ing to equation (3), and those of Pri,Sn were determined from the rate of formation of acetone and isopropyl alcohol measured by g.1.c.Results and Discussion Fluorescence Quenching of Flavin Analogues with Tetra-alkyltin Compound.-The fluorescence of a flavin analogue may be quenched by electron-transfer reactions with tetra- Table 1. Rate constants /cobs for the fluorescence quenching of flavin analogues (1) and (2a-c) by R,Sn in the absence and presence of 0.10 mol dm-3 Mg(ClO,), and the redox potentials of the singlet-excited states of the flavin analogues Eo(Fl*/Fl -*) (versus s.c.e.) in MeCN log(k0,,/dm3 mol-' s-') of R,Sn Eo(Fl*/ A \ Entry Flavin Fl-')/V R = Me R = Et R = Pr' R = Bun (1) 1.96 b 9.31 10.03 9.49 (l)-Mg2+" 2.28 8.54 9.73 10.12 9.93 Pa) 1.77 b 8.69 10.04 8.75 (2akMg2+" 2.10 8.19 9.50 10.10 9.50 (2b) 1.88 b 8.81 10.15 9.30 (2b)-Mg2 + 2.22 b 9.77 10.28 9.83 (b) 1.95 b 9.47 10.02 9.31 +(2ckMg' 2.27 8.85 10.00 10.56 10.10 " In the presence of 0.10 mol dm-3 Mg(ClO,),.Too small to be determined accurately. alkyltin compounds R,Sn (R = Me, Et, Bu", Pr') in the absence and presence of Mg(ClO,), in MeCN [equation (4)] since Fl* + R,SnL F1-' + R,Sn+' (4) energy transfer from F1* to R,Sn is unlikely to occur because of the much higher excited states of R,Sn than those of F1.The quenching rate constants kobs obtained from the slopes of the Stern-Volmer plots [equation (l)] and the fluorescence lifetimes are listed in Table 1, which also shows the redox potentials of the singlet-excited states of flavin analogues Eo(Fl*/Fl-') in the absence and presence of 0.10 mol dm-, Mg(ClO,), in MeCN (see Experimental section). In the presence of 0.10 mol dm-3 Mg(C10,),, most flavin molecules are known to form the 1:l complex with Mg2+ ion [equation (5)].7*1Thus, the redox potentials of the singlet-excited states F1 + Mg2+&[Fl Mg2+] (5) of flavin analogues are shifted in the positive direction (ca. +0.2 V) by the complex formation with Mg2+ ion (Table 1). In fact, the kobs values for the electron-transfer quenching of F1* with Me,Sn, Et,Sn, and Bu",Sn increased on increasing the Mg2 + concentration, and attained a constant value at high concentrations of Mg2+ ion (e.g., 0.10 mol dmP3) in each tetra- alkyltin, while the kobs values with the strongest electron donor among R,Sn used in this study, i.e.Pri,Sn, are close to the diffusion rate constant 2.0 x 10" dm3 mot' s-l,12 as shown in Table 1. Such an increase of the kobs value in the presence of Mg2+ ion cannot be attributed to a salt effect since the addition of 0.10 mol dmP3 NaClO, or NBu",ClO, to the Fl-R,Sn system has caused essentially no effect on the quenching rate constant. Application of the Marcus Theory to Electron-transfer Reactions of Tetra-alkyltin Compounds.-By applying the general scheme for the fluorescence quenching by electron transfer in MeCN to the present system (Scheme l),l6>"the k30R,Sn + 'F1 .A[R,Sn 'F1*] 5[R,Sn+' Fl-.] * decayk, I k,, Scheme 1.observed quenching rate constant kobs [equation (4)] may be expressed by equation (6), where k,, comprises all possible J. CHEM. SOC. PERKIN TRANS. II 1986 modes by which the radical ion pair disappears, in particular via the back electron-transfer to the triplet and/or ground states of F1as well as the rapid fragmentation of R,Sn+' in equation (7),9 R,Sn+' %R' + R3Sn+ (7) being approximately equal to the frequency factor. Under such conditions, equation (6) can be rewritten as (8), where AG:, 2.0 x 1o'O kobs = 1 + 0.25[exp(AGi3/R7') + exp(AG,,/RT)] (8) and AGZ3 are the activation Gibbs energy and the Gibbs energy change of the actual electron-transfer process, respectively.A theoretical basis for the outer-sphere electron-transfer process is well provided by the Marcus theory which predicts the dependence of AGi3 on AG23 as expressed by equation (9),'O (9) where h is the reorganization energy of the electron transfer. Thus, the kobsvalue can be theoretically calculated as a function of AG2, by using equations (8) and (9). On the other hand, AG23 is obtained from the difference in the redox potentials between R,Sn and F1* by equation (lo), where wp is the work term required to bring the products together to the mean separation in the activated complex, and the corresponding work term for the neutral reactants w, is neglected.Then, the logarithms of the observed rate constants in Table 1 are plotted as a function of AG23 which is obtained from the values of Eo(R,Sn+'/R,Sn)9 and Eo(Fl*/Fl-') by using equation (lo), taking into account a work term wp for the AGZ3= F[EO(R,Sn+'/R,Sn) -Eo(F1*/F1-m)]+ wp (10) R,Sn+'-Fl-' system at -9.6 kJ rn~l-','~ as shown in Figure 1. The log kobs values for the thermal electron-transfer reactions of R4Sn (R = Me, Et, Bun, Pr') and Et4Pb with +[Fe(N-N)3]3 (N-N = 2,2'-bibyridine and various substituted 1,lO-phenanthrolines) reported previously [equation (1 l)] R,Sn + [Fe(N-N),J3+ kob + R,Sn+' + [Fe(N-N)J2' (11) are also plotted as a function of AG23 in Figure 1.Thus, the experimental dependence of log kobsfor the electron-transfer reactions of R,Sn over a wide range of the Gibbs energy change of the electron transfer has been established in Figure 1, which covers the range from highly exothermic photoelectron-transfer to endothermic thermal electron-transfer reactions. The theoretical rate constants for the electron-transfer reactions calculated by using equations (8) and (9) with the h value of 170 kJ mol-' are shown by the solid line in Figure 1, which agrees well with the experimental results. Such agree- ments demonstrate that the Marcus theory can well be applied for the highly exothermic as well as endothermic electron- transfer reactions with the large reorganization energy assum- ing that k,, is equal to the frequency factor, although the applicability of the Marcus theory is generally believed to decrease as the Gibbs energy change of electron transfer becomes largely negative or positive.' * The large h value (1 70 kJ mol-') in the present case shows a marked contrast with a much smaller h value (40 kJ mol-l) for the photoelectron-transfer reactions of Fl with benzene derivative^.^ Thus, the intrinsic barrier for the electron-transfer reactions of R,Sn with the oxidants (oxidants Fl* and [Fe(N-N)3]3'), which may be \ "\L -5 I1 I I I I -1-5 -1-0 -0.5 0 0.5 (AG,~/FIJV Figure 1.Plots of the logarithms of the observed rate constants log kobs for the photoelectron-transfer reactions of R,Sn with flavin analogues in the absence and presence of 0.10rnol dm-, Mg2+ ion (0)(Table 1) and for the electron-transfer reactions of R,Sn (R = Me, Et, Bun,Pr') and Et,Pb with [Fe(N-N)J3' (a)in MeCN at 298 K9 versus the Gibbs energy change of the electron transfer (AGz3/F); the solid line shows the calculated dependence of log kobs on AG,,/F based on the Marcus theory, see text Table 2.Yields of MeO,SnMe, based on (1) (2.0 x lC3rnol dm-3) in the photo-oxidation of Me,Sn (0.10rnol dm-,) by oxygen, catalysed by (1) in the presence of Mg(CIO,), (0.10 mol dmW3) in CD3CN tlh Yield (%) 2 210 3 340 4 450 15 lo00 average for the reorganization energies for the self-exchange of the Ox-Ox -* and R,Sn-R,Sn+' systems, comprises mainly the latter system, where the contribution of the bond change in the inner co-ordination shell (Sn-R bond) upon electron transfer may be important since the reorganization energy of the solvent shells of R,Sn has been estimated as being much smaller (50-84 kJ mol-') than the observed h value (1 70 kJ mol-').Similar large reorganization energies for the electrochemical oxidation of a series of organocobaloximes have recently been reported, showing that the large h value reflects both electronic effects and steric distortions on the axial Co-C bond, where bigger changes in the transition from the reacting particle to the activation state are localized.' Flavin-catalysed Photo-oxidation of Tetra-alkyltin Com-pounds.-Irradiation of an oxygen-saturated CD,CN solution containing a flavin analogue (1) (2.0 x lW3 mol dmP3), Me,Sn (0.10 mol dm-3), and Mg(ClO,), (0.10 mol dm-3) with visible light of 350 nm < h < 470 nm results in the formation of Me02SnMe3 (see Experimental section) [equation (1 2)].Yields Me,Sn + o, hv (350nm < b < 470 nrn) MeO,SnMe, (12)[FIMg2+] of Me02SnMe3 based on the initial amount of (1) reach 1 OOO% in 15 h (Table 2), demonstrating that (1) in the presence of Mg2+ ion acts as a photocatalyst in the photo-oxidation of Me,Sn. It has been confirmed that neither thermal oxidation of M,Sn by oxygen nor photo-oxidation of Me,Sn in the absence of (1) occurs. In the flavin-catalysed photo-oxidation of Me,Sn, Mg2+ ion plays an essential role, since in the absence of Mg2 +- 28 J.CHEM. SOC. PERKIN TRANS. 11 1986 0 5 10 1O2[MeLSn) /mol dm3 Figure 2. Plot of quantum yields CD for the photo-oxidation of Me,Sn by oxygen, catalysed by (l)-MgZ+ (2.2 x lt4 mol dm-3) in MeCN at 298 K wrms the concentration of Me,Sn I I 0 50 100 Irradiation time (minl Figure 3. Plots of molar ratios of products [Me,C=O (0)and Pr'OH (.)I to the initial amount of (l)-MgZ+ (2.2 x mol dm3) versus irradiation time for the photo-oxidation of Pr',Sn by oxygen, catalysed by (l)-Mg2+ in MeCN ion, photo-oxidation has hardly been observed. The role of Mg2+ ion may not only be to increase the oxidizing ability of the excited state of (1) as indicated by the enhanced fluorescence quenching of F1 by R,Sn in the presence of Mg2 + ion (Table l), but also to stabilize (1) against irradiation of the visible light to prevent the photodegradation of (1) by forming the complex with Mg2+ ion [equation (5)].In fact, the quantum yield of the photodegradation of (1)-Mg2+ complex (ad 6.2 x lop4) is much smaller than that of a free flavin (1) (@d 1.6 x The quantum yield for the photo-oxidation of Me,Sn by oxygen, catalysed by a (l)-Mg2+ complex in MeCN, is proportional to the Me,Sn concentration (Figure 2). In contrast with the case of Me,Sn, the photo-oxidation of Pr',Sn by oxygen, catalysed by a (l)-Mg2+ complex, gives essentially no peroxyl compound Pri02SnPri3 but instead 0 5 10 lo2[ Pr' Sn] / mol Figure 4. Plots of quantum yields CD for the photochemical reaction of (l)-Mgz+ (2.2 x mol dm3) with Pr',Sn in the absence (e)and presence of oxygen (0)in MeCN at 298 K uersus the concentration of Pri4Sn approximately equal amounts of isopropyl alcohol and acetone [equation (13)] as shown in Figure 3.In addition, the quantum hv (350nm c h < 470 nm)Pri,Sn + o2 [FIMg"] Pr'OH + Me,C=O (13) yield for photo-oxidation as well as the photochemical reaction of a (l)-Mg2+ complex with Pr',Sn under a degassed condition is independent of the Pr',Sn concentration (Figure 4). The products of the photo-oxidation of R,Sn [equation (12) and (1 3)] as well as the dependences of @ on the R,Sn concentr- ation (Figures 2 and 4) can be explained by the radical chain mechanism (14)-( 17), similar to that previously established for autoxidations of alkylborons 2o and alkylzirconocenes.21 In reaction (14) Qi is the quantum yield for the photoinitiation step Initiation R,Sn + [FI*Mg2+]-% R' + R,Sn+ + [FI-'Mg2+] (14) Propagation R' + 02%R02* (15) R02' + R,Sn R0,SnR3 + R' (16) Termination 2R0,' decay (17) [equation (14)J Since the one-electron oxidation of R,Sn results in the facile fragmentation of R,Sn+' to yield R' [equation (7)],9 the autoxidation of R,Sn may proceed by the radical chain reactions (15) and (16).The terminatior, step of primary and secondary alkylperoxyl radicals is known to give equal amounts of alcohol and ketone in reaction (17).22 By applying the steady-state approximation to the radical species involved in equations (14)-(17), the quantum yield for the photo-oxidation of R,Sn @ is given by equation (18), where @ = mi + kp(@i/2k,In)1/2[R,Sn] (18) In is the light intensity absorbed by a (1)-Mg2+ complex.In the case of Me,Sn, the term for the chain-propagation step kp(@i/2k,In)'i2[Me,Sn] may be much larger than that for the photoinitiation step mi in equation (18), and thereby @ is proportional to [Me,Sn) whereas mi is independent of [Me,Sn], as shown in Figure 2. Conversely, in the case of Pri,Sn, the photoinitiation step becomes dominant with little contribution J. CHEM. SOC. PERKIN TRANS. 11 1986 from the chain-propagation step, to give mainly the termination products (isopropyl alcohol and acetone), and thus which is equal to Qi is independent of [Pr',Sn] as shown in Figure 4.Such a difference between the photo-oxidation of Me,Sn and that of Pr',Sn may be ascribed to the more efficient photoinitiation in the case of Pr',Sn than the case of Me,Sn as predicted by the much faster fluorescence quenching of a (It Mg2+ complex by Pr',Sn than by Me,Sn (Table 1) as well as the higher reactivity of the primary alkylperoxyl radical (MeO,') than the secondary alkylperoxyl radical (Pri02*) in the propagation step (16).22 The constant Qjvalue in the case of Pr',Sn (Figure 4)suggests that the quenching process of the excited state [F1*Mg2+] by Pr',Sn [equation (14)] is much faster than the other decay processes by the radiation and non-radiation pathways under the experimental conditions of Figure 4.Then, the minimum lifetime of the excited state involved in the photoinitiation step [equation (14)] can be estimated as T 9 10 ns by using the relation kdiffz[R,Sn] 9 1, where the diffusion rate constant kdiff = 2.0 x 10'~dm3 mol-' s-' and [Pr',Sn] 2 5.0 x mol dm-' (Figure 4). Since the fluorescence lifetime of [(1*)-Mg2+](z 1.7 ns) is much shorter than the minimum lifetime of the reactive excited state, the triplet state of the Aavin is the most plausible reactive species for the photoinitiation step (14) as in the case of most photochemical reactions of flavin analogues.23 Since the photo-oxidation of Pr',Sn which has no radical chain character proceeds in the presence of a catalytic amount of F1 (Figure 3), the photocatalyst may be regenerated by the fast oxidation of the reduced species [F1-'Mg2+] by oxygen [equation (19)]. In fact, the second-order rate constant for the [FI-'Mg*+] + O25[F1Mg2'] + 02-* reaction of FI-' with 0, has been reported to be ca.lo8 dm3 mol-l s-l 24 In conclusion, R,Sn can readily be oxidized by the photo- electron-transfer reactions with the excited states of flavin analogues, resulting in the facile fission of the Sn-R bond to give alkyl radical R', competing well with the back electron-transfer processes. 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ISSN:1472-779X
DOI:10.1039/P29860000025
出版商:RSC
年代:1986
数据来源: RSC