Scandium ion-promoted photoinduced electron transfer from electron donors to acridine and pyrene. Essential role of scandium ion in photocatalytic oxygenation of hexamethylbenzene

Photoinduced electron transfer from a variety of electron donors including alkylbenzenes to the singlet excited state of acridine and pyrene is accelerated significantly by the presence of scandium triflate [Sc(OTf)(3)] in acetonitrile, whereas no photoinduced electron transfer from alkylbenzenes to the singlet excited state of acridine or pyrene takes place in the absence of Sc(OTf)(3). The rate constants of the Sc(OTf)(3)-promoted photoinduced electron-transfer reactions (k(et)) of acridine to afford the complex between acridine radical anion and Sc(OTf)(3) remain constant under the conditions such that all the acridine molecules form the complex with Sc(OTf)(3). In contrast to the case of acridine, the k(et) value of the Sc(OTf)(3)-promoted photoinduced electron transfer of pyrene increases with an increase in concentration of Sc(OTf)(3) to exhibit first-order dependence on [Sc(OTf)(3)] at low concentrations, changing to second-order dependence at high concentrations. The first-order and second-order dependence of k(et) on [Sc(OTf)(3)] is ascribed to the 1:1 and 1:2 complexes formation between pyrene radical anion and Sc(OTf)(3). The positive shifts of the one-electron redox potentials for the couple between the singlet excited state and the ground-state radical anion of acridine and pyrene in the presence of Sc(OTf)(3) as compared to those in the absence of Sc(OTf)(3) have been determined by adapting the free energy relationship for the photoinduced electron-transfer reactions. The Sc(OTf)(3)-promoted photoinduced electron transfer from hexamethylbenzene to the singlet excited state of acridine or pyrene leads to efficient oxygenation of hexamethylbenzene to produce pentamethylbenzyl alcohol which is further oxygenated under prolonged photoirradiation of an O(2)-saturated acetonitrile solution of hexamethylbenzene in the presence of acridine or pyrene which acts as a photocatalyst together with Sc(OTf)(3). The photocatalytic oxygenation mechanism has been proposed based on the studies on the quantum yields, the fluorescence quenching, and direct detection of the reaction intermediates by ESR and laser flash photolysis.     

Does bimolecular charge recombination in highly exergonic electron transfer afford the triplet excited state or the ground state of a photosensitizer?

The investigations were made on photoinduced electron transfer (ET) from the singlet excited state of rubrene (1RU*) to p-benzoquinone derivatives (duroquinone, 2,5-dimethyl-p-benzoquinone, p-benzoquinone, 2,5-dichloro-p-benzoquinone, and p-chloranil) in benzonitrile (PhCN) by using the steady state and time-resolved spectroscopies. The photoinduced ET produces solvent-separated type charge-separated (CS) species and the charge-recombination (CR) process between RU radical cation and semiquinone radical anions obeys second-order kinetics. Not only the CS species but also the triplet excited state of RU (3RU*) is seen in the transient absorption spectra upon laser excitation of a PhCN solution of RU and p-benzoquinone derivatives. The comparison of their time profiles clearly suggests that the CR process between RU radical cation and semiquinone radical anions to the ground state is independent from the deactivation of 3RU*. This indicates that the CR in a highly exergonic ET occurs at a longer distance with a large solvent reorganization energy, which results in faster ET to the ground state than to the triplet excited state that is lower in energy than the CS state. Photoinduced ET from 3RU* in addition from 1RU* also occurs when p-benzoquinone derivatives with electron-withdrawing substituents were employed as electron acceptors.     

Stepwise vs. concerted pathways in scandium ion-coupled electron transfer from superoxide ion to p-benzoquinone derivatives

Superoxide ion (O2˙-) forms a stable 1 : 1 complex with scandium hexamethylphosphoric triamide complex [Sc(HMPA)(3)(3+)], which can be detected in solution by ESR spectroscopy. Electron transfer from O2˙- -Sc(HMPA)(3)(3+) complex to a series of p-benzoquinone derivatives occurs, accompanied by binding of Sc(HMPA)(3)(3+) to the corresponding semiquinone radical anion complex to produce the semiquinone radical anion-Sc(HMPA)(3)(3+) complexes. The 1 : 1 and 1 : 2 complexes between semiquinone radical anions and Sc(HMPA)(3)(3+) depending on the type of semiquinone radical anions were detected by ESR measurements. This is defined as Sc(HMPA)(3)(3+)-coupled electron transfer. There are two reaction pathways in the Sc(HMPA)(3)(3+)-coupled electron transfer. One is a stepwise pathway in which the binding of Sc(HMPA)(3)(3+) to semiquinone radical anions occurs after the electron transfer, when the rate of electron transfer remains constant with the change in concentration of Sc(HMPA)(3)(3+). The other is a concerted pathway in which electron transfer and the binding of Sc(HMPA)(3)(3+) occurs in a concerted manner, when the rates of electron transfer exhibit first-order and second-order dependence on the concentration of Sc(HMPA)(3)(3+) depending the number of Sc(HMPA)(3)(3+) (one and two) bound to semiquinone radical anions. The contribution of two pathways changes depending on the substituents on p-benzoquinone derivatives. The present study provides the first example to clarify the kinetics and mechanism of metal ion-coupled electron-transfer reactions of the superoxide ion.     

Remarkable effects of counter ions on scandium ion-promoted electron transfer reactions

Scandium ion-promoted electron transfer reactions of p-benzoquinone are remarkably accelerated when tetrakis(pentafluorophenyl)borate anion is used instead of trifluoromethanesulfonate anion as the counter anion of scandium ion. Only a catalytic amount of scandium borate salt (Sc[B(C6F5)4]3) accelerates significantly the Diels-Alder reaction of 9,10-dimethylanthracene with p-benzoquinone, which proceeds via Sc(3+)-promoted electron transfer from the anthracene to p-benzoquinone.     

富勒烯(C60／C70)与N，N，N’，N’-四-(对甲苯基)-4，4’-二胺-1，1’-二苯硒醚(TPDASe)间光诱导电子转移过程的激光光解研究

富勒烯(C60／C70)与N，N，N’，N’-四-(对甲苯基)-4，4’-二胺-1，1’-二 苯硒醚(TPDASe)间在激光光诱导条件下，发生了分子间的电子转移过程．在可见- 近红外区(600-1200nm)，观测到了TPDASe阳离子自由基、富勒烯(C60／C70)激发三 线态和阴离子自由基，在苯腈溶液中，观测瞬态谱测定了电子从TPDASe转移到富勒 烯(C60／C70)激发三线态的量子转化产率(Φet^T)和电子转移常数(Ket)．     

Metal ion-catalyzed cycloaddition vs hydride transfer reactions of NADH analogues with p-benzoquinones

1-Benzyl-4-tert-butyl-1,4-dihydronicotinamide (t-BuBNAH) reacts efficiently with p-benzoquinone (Q) to yield a [2+3] cycloadduct (1) in the presence of Sc(OTf)(3) (OTf = OSO(2)CF(3)) in deaerated acetonitrile (MeCN) at room temperature, while no reaction occurs in the absence of Sc(3+). The crystal structure of 1 has been determined by the X-ray crystal analysis. When t-BuBNAH is replaced by 1-benzyl-1,4-dihydronicotinamide (BNAH), the Sc(3+)-catalyzed cycloaddition reaction of BNAH with Q also occurs to yield the [2+3] cycloadduct. Sc(3+) forms 1:4 complexes with t-BuBNAH and BNAH in MeCN, whereas there is no interaction between Sc(3+) and Q. The observed second-order rate constant (k(obs)) shows a first-order dependence on [Sc(3+)] at low concentrations and a second-order dependence at higher concentrations. The first-order and the second-order dependence of the rate constant (k(et)) on [Sc(3+)] was also observed for the Sc(3+)-promoted electron transfer from CoTPP (TPP = tetraphenylporphyrin dianion) to Q. Such dependence of k(et) on [Sc(3+)] is ascribed to formation of 1:1 and 1:2 complexes between Q(*)(-) and Sc(3+) at the low and high concentrations of Sc(3+), respectively, which results in acceleration of the rate of electron transfer. The formation constants for the 1:2 complex (K(2)) between the radical anions of a series of p-benzoquinone derivatives (X-Q(*)(-)) and Sc(3+) are determined from the dependence of k(et) on [Sc(3+)]. The K(2) values agree well with those determined from the dependence of k(obs) on [Sc(3+)] for the Sc(3+)-catalyzed addition reaction of t-BuBNAH and BNAH with X-Q. Such an agreement together with the absence of the deuterium kinetic isotope effects indicates that the addition proceeds via the Sc(3+)-promoted electron transfer from t-BuBNAH and BNAH to Q. When Sc(OTf)(3) is replaced by weaker Lewis acids such as Lu(OTf)(3), Y(OTf)(3), and Mg(ClO(4))(2), the hydride transfer reaction from BNAH to Q also occurs besides the cycloaddition reaction and the k(obs) value decreases with decreasing the Lewis acidity of the metal ion. Such a change in the type of reaction from a cycloaddition to a hydride transfer depending on the Lewis acidity of metal ions employed as a catalyst is well accommodated by the common reaction mechanism featuring the metal-ion promoted electron transfer from BNAH to Q.     

Mechanism of scandium ion catalyzed Diels-Alder reaction of anthracenes with methyl vinyl ketone

Rates of Diels-Alder cycloadditions of anthracenes with methyl vinyl ketone (MVK) are accelerated significantly by the presence of scandium triflate [Sc(OTf)3]. Sc(OTf)3 also promotes photoinduced electron-transfer reactions from various electron donors to MVK significantly. Comparison of the promoting effect of Sc(OTf)3 in photoinduced electron-transfer reactions of MVK with the catalytic effect of Sc(OTf)3 in the Diels-Alder reaction of 9,10-dimethylanthracene with MVK has revealed that the MVK-Sc(OTf)3 complex is a reactive intermediate in both the Diels-Alder and photoinduced electron-transfer reactions. The observed second-order rate constants of the Sc(OTf)3-catalyzed Diels-Alder reactions of anthracenes with MVK are by far larger than those expected from the observed linear Gibbs energy relation for the Diels-Alder reactions of anthracenes with stronger electron acceptors than MVK, which are known to proceed via electron transfer. This indicates that the Sc(OTf)3-catalyzed Diels-Alder reactions of anthracenes with MVK does not proceed via an electron-transfer process from anthracences to the MVK-Sc(OTf)3 complex.     

Study of photoinduced electron transfer between [60]fullerene and proton-sponge by laser flash photolysis: addition effects of organic acid

Photoinduced electron-transfer processes between fullerene (C60) and 1,8-bis(dimethylamino)naphthalene, which is called a proton-sponge (PS), have been investigated by means of laser flash photolysis in the presence and absence of CF3CO2H. For a mixture of C60 and PS, the transient absorption spectra showed the rise of the C60 radical anion with concomitant decay of the C60 triplet (3C60), suggesting that photoinduced intermolecular electron transfer occurs via 3C60 in high efficiency in polar solvent. For a covalently bonded C60-PS dyad, photoinduced intramolecular charge-separation process takes place via the excited singlet state of the C60 moiety, although charge recombination occurs within 10 ns. For both systems, electron-transfer rates were largely decelerated by addition of a small amount of CF3CO2H, leaving the long-lived 3C60. These observations indicate that the energy levels for charge-separated states of the protonated PS and C60 become higher than the energy level of the 3C60 moiety, showing low donor ability of the protonated PS. Thus, intermolecular electron-transfer process via 3C60 for C60-PS mixture and intramolecular charge-separation process via 1C60-PS for C60-PS dyad were successfully controlled by the combination of the light irradiation with a small amount of acid.     

Photoinduced intermolecular electron transfer process of fullerene (C60) and amine-substituted fluorenes studied by laser flash photolysis

Photoinduced intermolecular electron transfer process of fullerene (C60) with 9,9-bis(4-triphenylamino)fluorene (BTAF) and 9,9-dimethoxyethyl-2-diphenylaminofluorene (DAF) in toluene and benzonitrile has been investigated by nanosecond laser photolysis technique in the visible/near-IR regions. By the selective excitation of C60 using 532 laser light, it has been proved that the electron transfer takes place from the ground states BTAF and DAF to the triplet excited state of C60 ((3)C60*) by observing the radical anion of C60 and radical cation of BTAF and DAF. It was observed that the electron transfer of BTAF/(3)C60* is more efficient than DAF/(3)C60* reflecting the effect of amine-substitutents of the fluorene moiety on the efficiency of the electron transfer process. On addition of a viologen dication (OV(2+)), the electron of the anion radical of C60 mediates to OV(2+) yielding the OV(+). These results proved that the photosensitized electron-transfer/electron-mediating processes have been confirmed by the transient absorption spectral method.     

Highly self-organized electron transfer from an iridium complex to p-benzoquinone due to formation of a pi-dimer radical anion complex triply bridged by scandium ions

Unusually high kinetic order was observed in self-organized Sc3+-promoted electron transfer from tris(2-phenylpyridine)iridium(III) [Ir(ppy)3] to p-benzoquinone (Q) in propionitrile, third-order with respect to the concentration of Sc3+ and second-order with respect to the concentration of Q, to produce a pi-dimer semiquinone radical anion complex that is triply bridged by three Sc3+ ions (Q*--3Sc3+-Q).     

Unusual role of oxygen in electron-transfer processes

Molecular oxygen's unique involvement in electron-transfer processes is demonstrated on a series of dyads between porphyrin derivatives and fullerene C60. It has been shown for the first time that oxygen can serve as an inhibitor of back electron transfer by enhancing intersystem crossing of a singlet radical ion pair into its triplet state. The effect is observed only when energy of the charge-separated state is lower than that of the locally excited triplet states. Due to the spin statistics, the reverse intersystem crossing is less efficient, allowing use of oxygen and other paramagnetic species for impeding charge recombination in various electron-transfer systems.     

Efficient charge separation in C60-based dyads: triazolino

Triazoline[4,5][60]fullerenes are strong electron acceptors that form with tetrathiafulvalene (TTF), a novel type of donor-acceptor dyad exhibiting efficient improved electron-transfer dynamics. In particular, a rapid photoinduced intramolecular electron transfer, forming a charge-separated state, is followed by a slow charge recombination to generate the fullerene triplet excited state in moderate quantum yields.     

Photoinduced electron transfer competitive with energy transfer of the excited triplet state of [60]fullerene to ferrocene derivatives revealed by combination of transient absorption and thermal lens measurements

The quenching processes of the exited triplet state of fullerene (3C60) by ferrocene (Fc) derivatives have been observed by the transient absorption spectroscopy and thermal lens methods. Although 3C60 was efficiently quenched by Fc in the rate close to the diffusion controlled limit, the quantum yields (phi(et)) for the generation of the radical anion of C60 (C60*-) via 3C60 were quite low even in polar solvents; nevertheless, the free-energy changes (deltaG(et)) of electron transfer from Fc to 3C60 are sufficiently negative. In benzonitrile (BN), the phi(et) value for unsubstitued Fc was less than 0.1. The thermal lens method indicates that energy transfer from 3C60 to Fc takes place efficiently, suggesting that the excited triplet energy level of Fc was lower than that of 3C60. Therefore, energy transfer from 3C60 to ferrocene decreases the electron-transfer process from ferrocene to 3C60. To increase the participation of electron transfer, introduction of electron-donor substituents to Fc (phi(et) = 0.46 for decamethylferrocene in BN) and an increase in solvent polarity (phi(et) = 0.58 in BN:DMF (1:2) for decamethylferrocene) were effective.     

Supramolecular carbon nanotube-fullerene donor-acceptor hybrids for photoinduced electron transfer

Photoinduced electron transfer in a self-assembled single-wall carbon nanotube (SWNT)-fullerene(C60) hybrid with SWNT acting as an electron donor and fullerene as an electron acceptor has been successfully demonstrated. Toward this, first, SWNTs were noncovalently functionalized using alkyl ammonium functionalized pyrene (Pyr-NH3+) to form SWNT/Pyr-NH3+ hybrids. The alkyl ammonium entity of SWNT/Pyr-NH3+ hybrids was further utilized to complex with benzo-18-crown-6 functionalized fullerene, crown-C60, via ammonium-crown ether interactions to yield SWNT/Pyr-NH3+/crown-C60 nanohybrids. The nanohybrids were isolated and characterized by TEM, UV-visible-near IR, and electrochemical methods. Free-energy calculations suggested possibility of electron transfer from the carbon nanotube to the singlet excited fullerene in the SWNT/Pyr-NH3+/crown-C60 nanohybrids. Accordingly, steady-state and time-resolved fluorescence studies revealed efficient quenching of the singlet excited-state of C60 in the nanohybrids. Further studies involving nanosecond transient absorption studies confirmed electron transfer to be the quenching mechanism, in which the electron-transfer product, fullerene anion radical, was possible to spectrally characterize. The rates of charge separation, kCS, and charge recombination, kCR, were found to be 3.46 x 10(9) and 1.04 x 10(7) s-1, respectively. The calculated lifetime of the radical ion-pair was found to be over 100 ns, suggesting charge stabilization in the novel supramolecular nanohybrids. The present nanohybrids were further utilized to reduce hexyl-viologen dication (HV2+) and a sacrificial electron donor, 1-benzyl-1,4-dihydronicotinamide, in an electron-pooling experiment, offering additional proof for the occurrence of photoinduced charge-separation and potential utilization of these materials in light-energy harvesting applications.     

Scandium ion-enhanced oxidative dimerization and N-demethylation of N,N-dimethylanilines by a non-heme iron(IV)-oxo complex

Oxidative dimerization of N,N-dimethylaniline (DMA) occurs with a nonheme iron(IV)-oxo complex, [Fe(IV)(O)(N4Py)](2+) (N4Py = N,N-bis(2-pyridylmethyl)-N-bis(2-pyridyl)methylamine), to yield the corresponding dimer, tetramethylbenzidine (TMB), in acetonitrile. The rate of the oxidative dimerization of DMA by [Fe(IV)(O)(N4Py)](2+) is markedly enhanced by the presence of scandium triflate, Sc(OTf)(3) (OTf = CF(3)SO(3)(-)), when TMB is further oxidized to the radical cation (TMB(?+)). In contrast, we have observed the oxidative N-demethylation with para-substituted DMA substrates, since the position of the C-C bond formation to yield the dimer is blocked. The rate of the oxidative N-demethylation of para-substituted DMA by [Fe(IV)(O)(N4Py)](2+) is also markedly enhanced by the presence of Sc(OTf)(3). In the case of para-substituted DMA derivatives with electron-donating substituents, radical cations of DMA derivatives are initially formed by Sc(3+) ion-coupled electron transfer from DMA derivatives to [Fe(IV)(O)(N4Py)](2+), giving demethylated products. Binding of Sc(3+) to [Fe(IV)(O)(N4Py)](2+) enhances the Sc(3+) ion-coupled electron transfer from DMA derivatives to [Fe(IV)(O)(N4Py)](2+), whereas binding of Sc(3+) to DMA derivatives retards the electron-transfer reaction. The complicated kinetics of the Sc(3+) ion-coupled electron transfer from DMA derivatives to [Fe(IV)(O)(N4Py)](2+) are analyzed by competition between binding of Sc(3+) to DMA derivatives and to [Fe(IV)(O)(N4Py)](2+). The binding constants of Sc(3+) to DMA derivatives increase with the increase of the electron-donating ability of the para-substituent. The rate constants of Sc(3+) ion-coupled electron transfer from DMA derivatives to [Fe(IV)(O)(N4Py)](2+), which are estimated from the binding constants of Sc(3+) to DMA derivatives, agree well with those predicted from the driving force dependence of the rate constants of Sc(3+) ion-coupled electron transfer from one-electron reductants to [Fe(IV)(O)(N4Py)](2+). Thus, oxidative dimerization of DMA and N-demethylation of para-substituted DMA derivatives proceed via Sc(3+) ion-coupled electron transfer from DMA derivatives to [Fe(IV)(O)(N4Py)](2+).     

Intermolecular and intracomplex photoinduced electron transfer from planar and nonplanar metalloporphyrins to p-quinones

The rate constants of intermolecular photoinduced electron transfer from triplet excited states of metalloporphyrins to a series of p-benzoquinone derivatives in benzonitrile were determined to examine the effects of the driving force, the metal, and the conformational distortion of the porphyrin ring on the reorganization energies (λ) of electron transfer by laser flash photolysis. The λ values were evaluated from the determined rate constants on the basis of the Marcus theory of electron transfer. The λ values of planar metalloporphyrins, [Al(TPP)(PhCOO)] and [Zn(TPP)] (TPP(2-)=tetraphenylporphyrin dianion), are approximately the same, but they are 0.27 eV smaller than those of the corresponding nonplanar (saddle-distorted) metalloporphyrins [Al(DPP)(PhCOO)] and [Zn(DPP)] (DPP(2-)=dodecaphenylporphyrin dianion) when they are compared for the same driving force of photoinduced electron transfer. The axial ligand PhCOO(-) of [Al(TPP)](+) and [Al(DPP)](+) was replaced by anthraquinone-2-carboxylate (AqCOO(-)) to afford the electron donor-acceptor complexes [Al(TPP)(AqCOO)] and [Al(DPP)(AqCOO)], respectively. The X-ray crystal structure of [Al(TPP)(AqCOO)] revealed strong coordination of AqCOO(-) to the Al(3+) ion of [Al(TPP)](+) and the existence of π-π interactions between AqCOO(-) and the porphyrin ring. In the case of the saddle-distorted [Al(DPP)(AqCOO)], however, the AqCOO(-) moiety is nearly perpendicular to the porphyrin ring. The photodynamics of intracomplex photoinduced electron transfer from the singlet excited state of [Al(TPP)](+) and [Al(DPP)](+) to the AqCOO(-) moiety were also examined in comparison with the intermolecular photoinduced electron-transfer reactions, and the determined rate constants were evaluated in light of the Marcus theory of electron transfer to reveal that the electron transfer is adiabatic in each case.     

Change in spin state and enhancement of redox reactivity of photoexcited states of aromatic carbonyl compounds by complexation with metal ion salts acting as Lewis acids. Lewis acid-catalyzed photoaddition of benzyltrimethylsilane and tetramethyltin via photoinduced electron transfer

The lowest excited state of aromatic carbonyl compounds (naphthaldehydes, acetonaphthones, and 10-methylacridone) is changed from the n,pi triplet to the pi,pi singlet which becomes lower in energy than the n,pi triplet by the complexation with metal ions such as Mg(ClO(4))(2) and Sc(OTf)(3) (OTf = triflate), which act as Lewis acids. Remarkable positive shifts of the one-electron reduction potentials of the singlet excited states of the Lewis acid-carbonyl complexes (e.g., 1.3 V for the 1-naphthaldehyde-Sc(OTf)(3) complex) as compared to those of the triplet excited states of uncomplexed carbonyl compounds result in a significant increase in the redox reactivity of the Lewis acid complexes vs uncomplexed carbonyl compounds in the photoinduced electron-transfer reactions. Such enhancement of the redox reactivity of the Lewis acid complexes leads to the efficient C-C bond formation between benzyltrimethylsilane and aromatic carbonyl compounds via the Lewis-acid-promoted photoinduced electron transfer. The quantum yield determinations, the fluorescence quenching, and direct detection of the reaction intermediates by means of laser flash photolysis experiments indicate that the Lewis acid-catalyzed photoaddition reactions proceed via photoinduced electron transfer from benzyltrimethylsilane to the singlet excited states of Lewis acid-carbonyl complexes.     

Photoinduced electron transfer between chlorophylls (a/b) and fullerenes (C60/C70) studied by laser flash photolysis.

Photoinduced electron-transfer processes in the systems of chlorophylls (Chl) (chlorophyll-a [Chl-a] and chlorophyll-b) and fullerenes (C60/C70) in both polar and non-polar solvents have been investigated with nanosecond laser photolysis technique, observing the transient spectra in the visible/near-IR regions. By the excitation of Chl in benzonitrile (BN) it has been proved that electron transfer takes place from the triplet excited states of Chl to the ground states of C60/C70. By the excitation of C70 in BN electron transfer takes place from the ground states of Chl to the triplet excited state of C70. In both Chl the rate constants and quantum yields for the electron-transfer processes are as high as those of zinc porphyrins and zinc phthalocyanines, indicating that the long alkyl chains of Chl play no role in retarding the electron transfer. The rate constant for the electron-mediating process from the radical anion of C70 to octylviologen dication yielding the octylviologen radical cation was evaluated. The back electron-transfer process from the viologen radical cation to the radical cation of Chl-a takes place in a longer time-scale, indicating that a photosensitized electron-transfer/electron-mediating cycle is achieved.     

Scandium ion-promoted reduction of heterocyclic N=N double bond. Hydride transfer vs electron transfer

Hydride transfer from 10-methyl-9,10-dihydroacridine (AcrH(2)) to 3,6-diphenyl-1,2,4,5-tetrazine (Ph(2)Tz), which contains a N=N double bond, occurs efficiently in the presence of Sc(OTf)(3) (OTf = OSO(2)CF(3)) in deaerated acetonitrile (MeCN) at 298 K, whereas no reaction occurs in the absence of Sc(3+). The observed second-order rate constant (k(obs)) increases with increasing Sc(3+) concentration to approach a limited value. When AcrH(2) is replaced by the dideuterated compound (AcrD(2)), the rate of Sc(3+)-promoted hydride transfer exhibits the same primary kinetic isotope effect (k(H)/k(D) = 5.2+/-0.2), irrespective of Sc(3+) concentration. Scandium ion also promotes an electron transfer from CoTPP (TPP(2)(-) = tetraphenylporphyrin dianion) and 10,10'-dimethyl-9,9'-biacridine [(AcrH)(2)] to Ph(2)Tz, whereas no electron transfer from CoTPP or (AcrH)(2) to Ph(2)Tz occurs in the absence of Sc(3+). In each case, the observed second-order rate constant of electron transfer (k(et)) shows a first-order dependence on [Sc(3+)] at low concentrations and a second-order dependence at higher concentrations. Such dependence of k(et) on [Sc(3+)] is ascribed to formation of 1:1 and 1:2 complexes between Ph(2)Tz(*)(-) and Sc(3+) at the low and high concentrations of Sc(3+), respectively, which results in acceleration of the rate of electron transfer. The formation of 1:2 complex has been confirmed by the ESR spectrum in which the hyperfine structure is different from that of free Ph(2)Tz(*)(-). The 1:2 complex formation results in the saturated kinetic dependence of k(obs) on [Sc(3+)] for the Sc(3+)-promoted hydride transfer, which proceeds via Sc(3+)-promoted electron transfer from AcrH(2) to Ph(2)Tz, followed by proton transfer from AcrH(2)(*)(+) to the 1:1 Ph(2)Tz(*)(-)-Sc(3+) complex and the subsequent facile electron transfer from AcrH(*) to Ph(2)TzH(*). The effects of counteranions on the Sc(3+)-promoted electron transfer and hydride transfer reactions are also reported.     

Electron transfer reduction of a highly electron-deficient fullerene,C60F18

Electron transfer reduction of a highly electron-deficient fullerene, C60F18, to the defluorinated anion, C60F17- occurs efficiently by a relatively weak one-electron reductant, p-chloranil radical anion; the one-electron reduction potential of C60F18 is evaluated as 0.04 V (vs. SCE) by comparison of the rate constant for electron-transfer from 10,10'-dimethyl-9,9',10,10'-tetrahydro-9,9'-biacridine to C60F18 with those of other one-electron reductants.