38414-01-6Relevant articles and documents
OFF-OFF-ON switching of fluorescence and electron transfer depending on stepwise complex formation of a host ligand with guest metal ions
Yuasa, Junpei,Fukuzumi, Shunichi
, p. 566 - 575 (2008)
Stepwise complex formation is observed between 2,3,5,6-tetrakis(2-pyridyl) pyrazine (TPPZ) and a series of metal ions (Mn+ = Sc3+, Y3+, Ho3+, Eu3+, Lu3+, Nd 3+, Zn2+, Mg2+, Ca2+, Ba 2+, Sr2+, Li+), where TPPZ forms a 2:1 complex [(TPPZ)2-Mn+] and a 1:1 complex [TPPZ-Mn+] with Mn+ at low and high concentrations of metal ions, respectively. The fluorescence intensity of TPPZ begins to increase at high concentrations of metal ions, when the 2:1 (TPPZ)2-Mn+ complex is converted to the fluorescent 1:1 TPPZ-Mn+ complex. This is regarded as an OFF-OFF-ON fluorescence sensor for metal ions depending on the stepwise complex formation between TPPZ and metal ions. The fluorescence quantum yields of the TPPZ-Mn+ complex vary depending on the metal valence state, in which the fluorescence quantum yields of the divalent metal complexes (TPPZ-M2+) are much larger than those of the trivalent metal complexes (TPPZ-M3+). On the other hand, the binding constants of (TPPZ)2-Mn+ (K1) and TPPZ-Mn+ (K2) vary depending on the Lewis acidity of metal ions (i.e., both K1 and K2 values increase with increasing Lewis acidity of metal ions). Sc3+, which acts as the strongest Lewis acid, forms the (TPPZ)2-Sc3+ and TPPZ-Sc3+ complexes stoichiometrically with TPPZ. In such a case, OFF-OFF-ON switching of electron transfer from cobalt(II) tetraphenylporphyrin (CoTPP) to O 2 is observed in the presence of Sc3+ and TPPZ depending on the ratio of Sc3+ to TPPZ. Electron transfer from CoTPP to O 2 occurs at Sc3+ concentrations above the 1:2 ratio ([Sc3+]/[TPPZ]0 > 0.5), when the (TPPZ) 2-Sc3+ complex is converted to the TPPZ-Sc3+ complex and TPPZ-(Sc3+)2, which act as promoters of electron transfer (ON) by the strong binding of O2?- with Sc3+. In sharp contrast, no electron transfer occurs without metal ion (OFF) or in the presence at Sc3+ concentrations below the 1:2 ratio (OFF), when the (TPPZ)2-Sc3+ complex has no binding site available for O2?-.
Metal ion-catalyzed cycloaddition vs hydride transfer reactions of NADH analogues with p-benzoquinones
Fukuzumi,Fujii,Suenobu
, p. 10191 - 10199 (2007/10/03)
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 = OSO2CF3) in deaerated acetonitrile (MeCN) at room temperature, while no reaction occurs in the absence of Sc3+. 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 Sc3+-catalyzed cycloaddition reaction of BNAH with Q also occurs to yield the [2+3] cycloadduct. Sc3+ forms 1:4 complexes with t-BuBNAH and BNAH in MeCN, whereas there is no interaction between Sc3+ and Q. The observed second-order rate constant (kobs) shows a first-order dependence on [Sc3+] at low concentrations and a second-order dependence at higher concentrations. The first-order and the second-order dependence of the rate constant (ket) on [Sc3+] was also observed for the Sc3+-promoted electron transfer from CoTPP (TPP = tetraphenylporphyrin dianion) to Q. Such dependence of ket on [Sc3+] is ascribed to formation of 1:1 and 1:2 complexes between Q?- and Sc3+ at the low and high concentrations of Sc3+, respectively, which results in acceleration of the rate of electron transfer. The formation constants for the 1:2 complex (K2) between the radical anions of a series of p-benzoquinone derivatives (X - Q?-) and Sc3+ are determined from the dependence of ket on [Sc3+]. The K2 values agree well with those determined from the dependence of kobs on [Sc3+] for the Sc3+-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 Sc3+-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(ClO4)2, the hydride transfer reaction from BNAH to Q also occurs besides the cycloaddition reaction and the kobs 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.
Efficient reduction of dioxygen with ferrocene derivatives, catalyzed by metalloporphyrins in the presence of perchloric acid
Fukuzumi, Shunichi,Mochizuki, Seiji,Tanaka, Toshio
, p. 2459 - 2465 (2008/10/08)
Reduction of dioxygen with ferrocene derivatives (Fc) is catalyzed by metalloporphyrins (MTPP+: M = Co, Fe, Mn; TPP = tetraphenylporphyrin) or Co(TIM)3+ (TIM: a tetraaza macrocyclic ligand) in the presence of HClO4 in acetonitrile (MeCN). Electron transfer from Fc to MTPP+ is the rate-determining step for the MTPP+-catalyzed oxidation of Fc by dioxygen, when the rate is independent of the concentration of dioxygen or HClO4. On the other hand, the rate of electron transfer from Fc to Co(TIM)3+ is accelerated by the presence of HClO4 and dioxygen. The rates of these electron-transfer reactions are discussed in light of the Marcus theory of electron transfer to distinguish between outer-sphere and inner-sphere electron-transfer processes. The strong inner-sphere nature of metalloporphyrins in the electron-transfer reactions with dioxygen in the presence of HClO4 plays an essential role in the catalytic reduction of dioxygen.
