Scandium ion-promoted photoinduced electron-transfer oxidation of fullerenes and derivatives by p-chloranil and p-benzoquinone.

In the presence of scandium triflate, an efficient photoinduced electron transfer from the triplet excited state of C(60) to p-chloranil occurs to produce C(60) radical cation which has a diagnostic NIR (near-infrared) absorption band at 980 nm, whereas no photoinduced electron transfer occurs from the triplet excited state of C(60) (3C(60)) to p-chloranil in the absence of scandium ion in benzonitrile. The electron-transfer rate obeys pseudo-first-order kinetics and the pseudo-first-order rate constant increases linearly with increasing p-chloranil concentration. The observed second-order rate constant of electron transfer (k(et)) increases linearly with increasing scandium ion concentration. In contrast to the case of the C(60)/p-chloranil/Sc(3+) system, the k(et) value for electron transfer from 3C(60) to p-benzoquinone increases with an increase in Sc(3+) concentration ([Sc(3+)]) to exhibit a first-order dependence on [Sc(3+)], changing to a second-order dependence at the high concentrations. Such a mixture of first-order and second-order dependence on [Sc(3+)] is also observed for a Sc(3+)-promoted electron transfer from CoTPP (TPP(2-) = tetraphenylporphyrin dianion) to p-benzoquinone. This is ascribed to formation of 1:1 and 1:2 complexes between the generated semiquinone radical anion and Sc(3+) at the low and high concentrations of Sc(3+), respectively. The transient absorption spectra of the radical cations of various fullerene derivatives were detected by laser flash photolysis of the fullerene/p-chloranil/Sc(3+) systems. The ESR spectra of the fullerene radical cations were also detected in frozen PhCN at 193 K under photoirradiation of the fullerene/p-chloranil/Sc(3+) systems. The Sc(3+)-promoted electron-transfer rate constants were determined for photoinduced electron transfer from the triplet excited states of C(60), C(70), and their derivatives to p-chloranil and the values are compared with the HOMO (highest occupied molecular orbital) levels of the fullerenes and their derivatives.     

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.     

Thermochromism of the disproportionation equilibrium of pi-dimer radical anion complexes bridged by scandium ions

Semiquinone radical anion (Q(*-)) forms a stable pi-dimer with neutral p-benzoquinone (Q), bridged by two or three scandium ions (Sc(3+)) to afford Q(*-)-nSc(3+)-Q (n= 2,3), which is in disproportionation equilibrium with Q and hydroquinone (QH(2)). The number of binding scandium ions changes depending on temperature, causing a remarkable color change associated with the change in the ESR spectra.     

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).     

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.     

Binding modes in metal ion complexes of quinones and semiquinone radical anions: electron-transfer reactivity.

9,10-Phenanthrenequinone (PQ) and 1,10-phenanthroline-5,6-dione (PTQ) form 1:1 and 2:1 complexes with metal ions (M (n+)=Sc (3+), Y (3+), Mg (2+), and Ca (2+)) in acetonitrile (MeCN), respectively. The binding constants of PQ--M (n+) complexes vary depending on either the Lewis acidity or ion radius of metal ions. The one-electron reduced species (PTQ(-)) forms 1:1 complexes with M (n+), and PQ(-) also forms 1:1 complexes with Sc(3+), Mg(2+), and Ca(2+), whereas PQ(-) forms 1:2 complexes with Y(3+) and La(3+), as indicated by electron spin resonance (ESR) measurements. On the other hand, semiquinone radical anions (Q(-) and NQ(-)) derived from p-benzoquinone (Q) and 1,4-naphthoquinone (NQ) form Sc(3+)-bridged pi-dimer radical anion complexes, Q(-)--(Sc(3+))(n)--Q and NQ(-)--(Sc(3+))(n)-NQ (n=2 and 3), respectively. The one-electron reduction potentials of quinones (PQ, PTQ, and Q) are largely positively shifted in the presence of M (n+). The rate constant of electron transfer from CoTPP (TPP(2-)=dianion of tetraphenylporphyrin) to PQ increases with increasing the concentration of Sc(3+) to reach a constant value, when all PQ molecules form the 1:1 complex with Sc(3+). Rates of electron transfer from 10,10'-dimethyl-9,9'-biacridine [(AcrH)(2)] to PTQ are also accelerated significantly by the presence of Sc(3+), Y(3+), and Mg(2+), exhibiting a first-order dependence with respect to concentrations of metal ions. In contrast to the case of o-quinones, unusually high kinetic orders are observed for rates of Sc(3+)-promoted electron transfer from tris(2-phenylpyridine)iridium(III) [Ir(ppy)(3)] to p-quinones (Q): second-order dependence on concentration of Q, and second- and third-order dependence on concentration of Sc(3+) due to formation of highly ordered radical anion complexes, Q()--(Sc(3+))(n)--Q (n=2 and 3).     

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.     

Formation of Metal Oxyfluorides from Specific Metal Reactions with Oxygen Difluoride: Infrared Spectroscopic and Theoretical Investigations of the OScF2 Radical and OScF with Terminal Single and Triple Sc?O Bonds

The scandium oxydifluoride free radical, OScF₂, is produced by the spontaneous, specific reaction of laser ablated Sc atoms with OF₂ in solid argon and characterized by using matrix infrared spectroscopy and theoretical calculations. The OScF₂ molecule is predicted to have C_2v symmetry and a ²B₂ ground state with an unpaired electron located primarily on the terminal oxygen atom, which makes it a scandium difluoride molecule coordinated by a neutral oxygen atom radical in forming the Sc? O single bond. The closed shell singlet OScF molecule with an obtuse bent geometry has a much shorter Sc? O bond of 1.682 Å than that of the OScF₂ radical (1.938 Å) on the basis of B3LYP calculations. The Sc? O bond in OScF consists of two covalent bonds and a dative bond in which the oxygen 2p_π lone pair donates electron density into an empty Sc 3d orbital thus forming a triple oxo bond. Density functional calculations suggest it is highly exothermic for fluorine transfer from OF₂ to scandium, which favors the formation of the OScF₂ radical species as well as the OScF molecule after fluorine loss.     

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+).     

Effects of metal ions on photoinduced electron transfer in zinc porphyrin-naphthalenediimide linked systems

Zinc porphyrin-naphthalenediimide (ZnP-NIm) dyads and zinc porphyrin-pyromellitdiimide-naphthalenediimide (ZnP-Im-NIm) triad have been employed to examine the effects of metal ions on photoinduced charge-separation (CS) and charge-recombination (CR) processes in the presence of metal ions (scandium triflate (Sc(OTf)(3)) or lutetium triflate (Lu(OTf)(3)), both of which can bind with the radical anion of NIm). Formation of the charge-separated states in the absence and in the presence of Sc(3+) was confirmed by the appearance of absorption bands due to ZnP(.) (+) and NIm(.) (-) in the absence of metal ions and of those due to ZnP(.) (+) and the NIm(.) (-)/Sc(3+) complex in the presence of Sc(3+) in the time-resolved transient absorption spectra of dyads and triad. The lifetimes of the charge-separated states in the presence of 1.0 x 10(-3) M Sc(3+) (14 micros for ZnP-NIm, 8.3 micros for ZnP-Im-NIm) are more than ten times longer than those in the absence of metal ions (1.3 micros for ZnP-NIm, 0.33 micros for ZnP-Im-NIm). In contrast, the rate constants of the CS step determined by the fluorescence lifetime measurements are the same, irrespective of the presence or absence of metal ions. This indicates that photoinduced electron transfer from (1)ZnP(*) to NIm in the presence of Sc(3+) occurs without involvement of the metal ion to produce ZnP(.) (+)-NIm(.) (-), followed by complexation with Sc(3+) to afford the ZnP(.) (+)-NIm(.) (-)/Sc(3+) complex. The one-electron reduction potential (E(red)) of the NIm moiety in the presence of a metal ion is shifted in a positive direction with increasing metal ion concentration, obeying the Nernst equation, whereas the one-electron oxidation potential of the ZnP moiety remains the same. The driving force dependence of the observed rate constants (k(ET)) of CS and CR processes in the absence and in the presence of metal ions is well evaluated in terms of the Marcus theory of electron transfer. In the presence of metal ions, the driving force of the CS process is the same as that in the absence of metal ions, whereas the driving force of the CR process decreases with increasing metal ion concentration. The reorganization energy of the CR process also decreases with increasing metal ion concentration, when the CR rate constant becomes independent of the metal ion concentration.     

Formation and characterization of the oxygen-rich scandium oxide/dioxygen complexes ScOn (n = 4, 6, 8) in solid argon

The reactions of scandium atoms and O(2) have been reinvestigated using matrix isolation infrared spectroscopy and density functional theory calculations. A series of new oxygen-rich scandium oxide/dioxygen complexes were prepared and characterized. The ground state scandium atoms react with dioxygen to form OSc(eta(2)-O(3)), a side-on bonded scandium monoxide-ozonide complex. The OSc(eta(2)-O(3)) complex rearranges to a more stable Sc(eta(2)-O(2))(2) isomer under visible light irradiation, which is characterized to be a side-on bonded superoxo scandium peroxide complex. The homoleptic trisuperoxo scandium complex, Sc(eta(2)-O(2))(3), and the superoxo scandium bisozonide complex, (eta(2)-O(2))Sc(eta(2)-O(3))(2), are also formed upon sample annealing. The Sc(eta(2)-O(2))(3) complex is determined to have a D(3h) symmetry with three equivalent side-on bonded superoxo ligands around the scandium atom. The (eta(2)-O(2))Sc(eta(2)-O(3))(2) complex has a C(2) symmetry with two equivalent side-on bonded O3 ligands and one side-on bonded superoxo ligand.     

Experimental and theoretical studies of 45Sc NMR interactions in solids

Solid-state 45Sc NMR spectroscopy, ab initio calculations, and X-ray crystallography are applied to examine the relationships between 45Sc NMR interactions and molecular structure and symmetry. Solid-state 45Sc (I = 7/2) magic-angle spinning (MAS) and static NMR spectra of powdered samples of Sc(acac)3, Sc(TMHD)3, Sc(NO3)3.5H2O, Sc(OAc)3, ScCl3.6H2O, ScCl3.3THF, and ScCp3 have been acquired. These systems provide a variety of scandium coordination environments yielding an array of distinct 45Sc chemical shielding (CS) and electric field gradient (EFG) tensor parameters. Acquisition of spectra at two distinct magnetic fields allows for the first observations of scandium chemical shielding anisotropy (CSA). 45Sc quadrupolar coupling constants (CQ) range from 3.9 to 13.1 MHz and correlate directly with the symmetry of the scandium coordination environment. Single-crystal X-ray structures were determined for Sc(TMHD)3, ScCl3.6H2O, and Sc(NO3)3.5H2O to establish the hitherto unknown scandium coordination environments. A comprehensive series of ab initio calculations of EFG and CS tensor parameters are in excellent agreement with the observed parameters. Theoretically determined orientations of the NMR interaction tensors allow for correlations between NMR tensor characteristics and scandium environments. Solid-state 45Sc, 13C, and 19F NMR experiments are also applied to characterize the structures of the microcrystalline Lewis acid catalyst Sc(OTf)3 (for which the crystal structure is unknown) and a noncrystalline, microencapsulated, polystyrene-supported form of the compound.     

Thermochromism of metal ion complexes of semiquinone radical anions. Control of equilibria between diamagnetic and paramagnetic species by Lewis acids

Metal ion complexes of semiquinone radical anions exhibit different types of thermochromism depending on metal ions and quinones. Metal ion complexes of 1,10-phenanthroline-5,6-dione radical anion (PTQ(.-)) produced by the electron-transfer reduction of PTQ by 1,1'-dimethylferrocene (Me(2)Fc) in the presence of metal ions (Mg(2+) and Sc(3+)) exhibit the color change depending on temperature, accompanied by the concomitant change in the ESR signal intensity. In the case of Mg(2+), electron transfer from Me(2)Fc to PTQ is in equilibrium, when the concentration of the PTQ(.-)-Mg(2+) complex (lambda(max) = 486 nm) increases with increasing temperature because of the positive enthalpy for the electron-transfer equilibrium. In contrast to the case of Mg(2+), electron transfer from Me(2)Fc to PTQ is complete in the presence of Sc(3+), which is a much stronger Lewis acid than Mg(2+), to produce the PTQ(.-)-Sc(3+) complex (lambda(max) = 631 nm). This complex is in disproportionation equilibrium and the concentration of the PTQ(.-)-Sc(3+) complex increases with decreasing temperature because of the negative enthalpy for the proportionation direction, resulting in the remarkable color change in the visible region. On the other hand, the p-benzosemiquinone radical anion (Q(.-)) forms a 2:2 pi-dimer radical anion complex [Q(.-)-(Sc(3+))(2)-Q] with Q and Sc(3+) ions at 298 K (yellow color), which is converted to a 2:3 pi-dimer radical anion complex [Q(.-)-(Sc(3+))(3)-Q] with a strong absorption band at lambda(max) = 604 nm (blue color) when the temperature is lowered to 203 K. The change in the number of binding Sc(3+) ions depending on temperature also results in the remarkable color change, associated with the change in the ESR spectra.     

The hydration of the scandium(III) ion in aqueous solution and crystalline hydrates studied by XAFS spectroscopy, large-angle X-ray scattering and crystallography

The structures of the hydrated scandium(III) ion and of the hydrated dimeric hydrolysis complex, [Sc2(mu-OH)2]4+, in acidic aqueous solutions have been characterized by X-ray absorption fine structure (XAFS) and large-angle X-ray scattering (LAXS) methods. Comparisons with crystalline reference compounds containing hydrated scandium(III) ions in well characterized six-, seven- and eight-coordinated polyhedra have been used to evaluate the coordination numbers and configurations in aqueous solution. In strongly acidic aqueous solution the structure of the hydrated scandium(III) ion is found to be similar to that of the eight-coordinated scandium(III) ion with distorted bicapped trigonal prismatic coordinating geometry in the crystalline [Sc(H2O)(8.0)](CF3SO3)3 compound. The EXAFS data reveal for the solution, as for the solid, a mean Sc-O bond distance of 2.17(1) Angstrom to six strongly bound prism water molecules, 2.32(4) Angstrom to one capping position, with possibly another capping position at about 2.5 Angstrom. The LAXS study supports this structural model and shows furthermore a second hydration sphere with approximately 12 water molecules at a mean Sc...O(II) distance of 4.27(3) Angstrom. In less acidic concentrated scandium(III) aqueous solutions, the dimeric hydrolysis product, [Sc2(mu-OH)2(H2O)10]4+, is the predominating species with seven-coordinated scandium(III) ions in a double hydroxo bridge and five terminal water molecules at a mean Sc-O bond distance of 2.145 Angstrom. Hexahydrated scandium(III) ions are found in the crystal structure of the double salt [Sc(H2O)6][Sc(CH3SO3)6], which crystallizes in the trigonal space group R3[combining macron] with Z = 6 and the unit cell dimensions a = 14.019(2) and c = 25.3805(5) Angstrom. The Sc-O distances in the two crystallographically unique, but nearly identical, [Sc(H2O)6]3+ entities (both with 3[combining macron] imposed crystallographic symmetry) are 2.085(6) and 2.086(5) Angstrom, while the mean Sc-O distance in the near octahedral [Sc(OSO2CH3)6]3- entities (with three-fold symmetry) is 2.078 Angstrom.     

Sc2@C3v(8)-C82 vs. Sc2C2@C3v(8)-C82: drastic effect of C2 capture on the redox properties of scandium metallofullerenes

We describe the first example of scandium dimetallofullerenes, Sc(2)@C(3v)(8)-C(82), which has the same cage as the previously assigned scandium carbide cluster fullerene Sc(2)C(2)@C(3v)(8)-C(82) but they exhibit distinctly different electronic configurations and electronic behaviours, confirming the drastic influence of the internal C(2) unit.     

A mechanistic dichotomy in scandium ion-promoted hydride transfer of an NADH analogue: delicate balance between one-step hydride-transfer and electron-transfer pathways

The rate constant (kH) of hydride transfer from an NADH analogue, 9,10-dihydro-10-methylacridine (AcrH2), to 1-(p-tolylsulfinyl)-2,5-benzoquinone (TolSQ) increases with increasing Sc(3+) concentration ([Sc(3+)]) to reach a constant value, when all TolSQ molecules form the TolSQ-Sc(3+) complex. When AcrH2 is replaced by the dideuterated compound (AcrD2), however, the rate constant (kD) increases linearly with an increase in ([Sc(3+)]) without exhibiting a saturation behavior. In such a case, the primary kinetic deuterium isotope effect (kH/kD) decreases with increasing ([Sc(3+)]). On the other hand, the rate constant of Sc(3+)-promoted electron transfer from tris(2-phenylpyridine)iridium [Ir(ppy)3]to TolSQ also increases linearly with increasing ([Sc(3+)]) at high concentrations of Sc(3+) due to formation of a 1:2 complex between TolSQ*- and Sc(3+), [TolSQ*--(Sc(3+)2], which was detected by ESR. The significant difference with regard to dependence of the rate constant of hydride transfer on ([Sc(3+)]) between AcrH2 and AcrD2 in comparison with that of Sc3+-promoted electron transfer indicates that the reaction pathway is changed from one-step hydride transfer from AcrH2 to the TolSQ-Sc3+ complex to Sc3+-promoted electron transfer from AcrD2 to the TolSQ-Sc3+ complex, followed by proton and electron transfer. Such a change between two reaction pathways, which are employed simultaneously, is also observed by simple changes of temperature and concentration of Sc3+.     

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.     

Binding of the two-valence-electron metal ions Sc(+), Ti(2+), and V(3+) to the second-row hydrides BeH(2), BH(3), NH(3), OH(2), and FH: a computational study

We report results from a computational study of the binding in complexes formed from one of the transition-metal ions Sc(+), Ti(2+), or V(3+), each of which has two valence electrons outside an argon core, and one of the second-row hydrides FH, OH(2), NH(3), BH(3), or BeH(2). The complexes that involve the electron-rich ligands FH, OH(2), and NH(3) have strong ion-dipole components to their binding. There are large stabilization energies for sigma-interactions that transfer charge from occupied lone-pair natural bond orbitals on the F, O, or N atom of the (idealized) Lewis structure into empty non-Lewis orbitals on the metal ions; these interactions effectively increase electron density in the bonding region between the metal ion and liganded atom, and the metal ions in these complexes act in the capacity of Lewis acids. The complexes formed from the electron-poor hydrides BH(3) and BeH(2) consistently incorporate bridging hydrogen atoms to support binding, and there are large stabilization energies for interactions that transfer charge from the Be-H or B-H bonds into the region between the metal ion and liganded atom. The metal ions in Sc(+)-BeH(2), Ti(2+)-BeH(2), Ti(2+)-BH(3), and V(3+)-BH(3) act in the capacity of Lewis acids, whereas the scandium ion in Sc(+)-BH(3) acts as a Lewis base.     

A distorted tetrahedral metal oxide cluster inside an icosahedral carbon cage. Synthesis, isolation, and structural characterization of Sc4(mu3-O)2@Ih-C80

The remarkably large cluster Sc4(mu3-O)2 has been obtained trapped inside an Ih-C80 cage by conducting the vaporization of graphite rods doped with copper(II) nitrate and scandium(III) oxide in an electric arc under a low pressure helium atmosphere with an added flow of air. The product has been isolated by chromatography and identified by high-resolution mass spectrometry. The structure of Sc4(mu3-O)2@Ih-C80 has been determined by X-ray crystallography on a crystal of Sc4(mu3-O)2@Ih-C80.NiII(OEP).2(C6H6). The Sc4(mu3-O)2 unit consists of a distorted tetrahedron of scandium atoms with oxygen atoms bridging two of its faces. The Sc-Sc distances range from 2.946(7) to 3.379(7) A.