18812
J. Phys. Chem. C 2007, 111, 18812-18820
Enhancements in the Electron-Transfer Kinetics of Uranium-Based Redox Couples Induced by Tetraketone Ligands with Potential Chelate Effect Tomoo Yamamura,*,† Kenji Shirasaki,† Hironori Sato,† Yoshiyuki Nakamura,†,‡ Hiroshi Tomiyasu,§ Isamu Satoh,† and Yoshinobu Shiokawa† Institute for Materials Research, Tohoku UniVersity, Sendai, Miyagi 980-8577, Japan, Chemical Resources Laboratory, Tokyo Institute of Technology, Kanagawa 226-8503, Japan, and Nippon TMI Company, Ltd., Matsumoto, Nagano 390-1242, Japan ReceiVed: September 10, 2007
Acetylacetone shows dissociative electron transfer when it is complexed with metal ions. In an effort to improve the kinetics of electron transfer of uranium-based complexes for application to a redox-flow battery, improvement by dimerization of acetylacetone into tetraketones with potential chelate effect is examined and discussed quantitatively. For the monomer acetylacetone, electron transfer of uranium complexes reveals an ECE mechanism and an inner-sphere reaction on the electrode surface. By using tetraketones, 8-oxo-2,4,12,14-tetraoxapentadecane and m-bis(2,4-dioxo-1-pentyl)benzene, the electron transfer of tetraketones with U(VI)/U(V) and U(IV)/U(III) shows rapid kinetics based on the E mechanism. In clear contrast to U(acac)4, the electrochemically reduced species of the U(IV) complex with tetraketone is stable during potential cycling. These results are also supported by NMR of tetraketones with U(VI) and U(IV); each acetylacetone site to uranium atom is stable at -40 ∼ +40 °C. The results demonstrate a remarkable enhancement of the stability of the reduced form of the metal center and thereby an improvement of the redox kinetics by the chelate effect. The tetraketones are eminently suitable for use in the active materials of a high-efficiency redox-flow battery.
1. Introduction The metal β-diketonates with chemical functionalities and toughness for light and heat have been applied to electroluminescence (EL) materials1-3 and precursors of metallorganic chemical vapor deposition (MOCVD) for oxide materials.4-6 Our recent study has focused on the application of the β-diketonates for the active materials of a redox-flow battery in use of leveling of electric power generated by natural energy such as wind power. However, the Vanadium redox-flow battery presently in operation7 consisting of aqueous electrolytes (VO2+/ VO2+ for positive and V3+/V2+ for negative electrolytes) suffers from low energy-efficiency because of the large over-voltage due to slow kinetics of the electron transfer (ET) especially of the VO2+/VO2+ couple requiring oxygen transfer.8 With this respect, we have focused our attention on the inherent nature of light actinides (An) that there are two couples of isostructural ions, namely Ann+ (n ) 3 and 4; n is oxidation number) and AnO2(n-4)+ (n ) 5 and 6). Within the couples of the isostructural ions, the rapid kinetics of ET is expected to lower the electrochemical overvoltage according to the Butler-Volmer law.9 Among the light actinides, depleted uranium, derived as a major byproduct of the enrichment process of nuclear fuel, has been deposited and its amount will exceed two million tons in the world by 2015. Because the conventional use of the depleted uranium in counterweights, shielding materials, and depleted uranium shell were insufficient and/or unsuccessful,10 substantial efforts have been paid for potential applications to * Corresponding author. Fax: +81-22-215-2121; e-mail: yamamura@ imr.tohoku.ac.jp. † Tohoku University. ‡ Tokyo Institute of Technology. § Nippon TMI Company, Ltd.
catalysis, hydrogen storage alloys, and magnetic materials although none of them has been put to practical use. Taking account of the electrochemical nature of uranium, we have proposed a uranium redox-flow battery with a potentially high energy-efficiency on the basis of three essential conditions: (i) the two couples of rapid ET, U3+/U4+ and UO2+/UO22+, are used,11,12 (ii) aprotic solvents are required to prevent uranium(V) from disproportionation,13-19 and (iii) organic ligands are necessary to improve the solubilities of their uranium complexes especially of U(IV) complexes.12 The β-diketonates of uranium are the first candidates for the active materials studied. Similar to β-diketonates of d-transition metals,20 cyclic voltammetry of β-diketonates of U(VI)/U(V)12,21 and U(IV)/ U(III)12,22,23 displays multiple couples of peaks and suggests dissociative ET,24 that is, ET followed by chemical reaction (EC), due to smaller effective charge carried by the reduced metal center. Assuming enhanced stability of the reduced complex, the electromotive force (EMF) of the single-unit cell of the battery is estimated more than 1.0 V from the difference between E1/2 for two-coordinated complexes of U(VI)/U(V) and that for four-coordinated ones of U(IV)/U(III).12 This value is comparable to 1.2 V of the vanadium battery. One reasonable strategy is the use of the chelate effect by cyclic or multidentate ligands such as crown ethers and cryptands, which form stable complexes even with alkaline metal ions with the least complexing abilities. Similarly, in order to stabilize the complexes with U(V) and U(III), we have initiated our study on the kinetic effect of dimerization of acetylacetone to 8-oxo-2,4,12,14tetraoxapentadecane (1) and m-bis(2,4-dioxo-1-pentyl)benzene (2) (Scheme 1).25-27 We have performed a mechanistic study of the monomer acetylacetone-based complex by dependence on free ligand
10.1021/jp077243z CCC: $37.00 © 2007 American Chemical Society Published on Web 11/29/2007
Uranium-Based Redox Couples SCHEME 1: Dimerization of Acetylacetone into Tetraketones
concentration ([acac-]) and on electrode materials. On the basis of this study, the kinetics of ET of its dimerized tetraketonebased complexes is quantitatively compared with the reference of acetylacetone by using an example of complexes with U(VI) and U(IV). The results are reported in this paper. 2. Experimental Section 2.1. General Details. N,N-dimethylformamide (DMF), dimethyl sulfoxide (Me2SO), and acetonitrile (CH3CN) were used after being purified according to the standard methods.28 Tetrabutylammonium perchlorate (TBAP) was used as received from Tokyo Kasei Kogyo Co., Ltd., Japan. Acetylacetone was used after distillation, but other chemicals purchased from Wako Pure Chemical Industries, Ltd., Japan, were used without further purification. Two tetraketone ligands, 1 and 2, were prepared according to the methods published previously,26 whose method is the slightly modified method reported by Albert and Cram.25,29 NMR measurements, 1H and 13C NMR, 1H-1H, and 1H-13C COSY measurements were carried out by using JEOL JNMLA-500 in CDCl3. Elemental analysis of prepared complexes was carried out by using a CHNSO elemental analyzer (Thermo Quest Italia Flash EA1112) and an ICP-AES spectrophotometer (Shimadzu Corp., ICPS-7500). Acetylacetonates of U(VI),12 U(IV),23 and Th(IV)23 were prepared and purified by the method reported previously. Elemental analysis and NMR measurements were carried out, and the latter results were included in Table 1. 2.2. Preparation of Complexes. UO21(C2H5OH)1.5 (Referred to as UO21). To aqueous solution of uranyl nitrate (2 mmol), ethanolic solution of ligand 1 (0.81 g, 2.98 mmol) was added. Aqueous sodium hydroxide (3 mol dm-3; mol dm-3 is abbreviated as M) was added to this solution dropwise while stirring the solution rigorously until its acidity reached pH 2. The coarse product of red-orange solid was recrystallized from ethanol to give a red-orange solid. Elemental analysis: Calcd.: U, 39.19%; C, 33.61%; H, 4.81%. Found: U, 39.59%; C, 33.60%; H, 4.47%. M.W.: 607.441. Yield: 56.5% (ligand-based). NMR: See Table 1. Soluble in CHCl3, CH2ClCH2Cl, ethanol, 1,4-dioxane, DMF, and Me2SO. Fairly soluble in benzene, toluene, and acetone. UO22(C2H5OH)2(H2O)0.5 (Referred to as UO22). To aqueous solution of uranyl nitrate (4 mmol), ethanolic solution of ligand
J. Phys. Chem. C, Vol. 111, No. 50, 2007 18813 2 (1.11 g, 3.69 mmol) was added. When the mixture changed acidity to pH 2 by NaOH(aq) (3 M), the coarse product of orange solid was obtained. It was recrystallized from ethanol to give an orange solid. Elemental analysis: Calcd.: U, 35.45%; C, 39.35%; H, 4.95%. Found: U, 35.20%; C, 39.45%; H, 4.70%. M.W.: 671.528. Yield: 68.7% (ligand-based). NMR: See Table 1. Soluble in benzene, toluene, DMF, and Me2SO. Fairly soluble in acetone. U12(H2O) (Referred to as U12). To aqueous solution of uranium(IV) chloride (2 mmol), ethanolic solution of the ligand 1 (1.39 g, 5.16 mmol) was added. Aqueous sodium hydroxide (3 M) was added to this solution dropwise while stirring the solution rigorously until its acidity reached pH 4-5. After the oily coarse product recrystallized from CHCl3 at 10 °C, a darkgreen solid was obtained. Elemental analysis: Calcd.: U, 30.03%; C, 42.43%; H, 5.34%. Found: U, 30.34%; C, 42.26%; H, 5.34%. M.W.: 792.664. Yield: 21.9% (ligand-based). NMR: See Table 1. Soluble in benzene, toluene, CH3CN, and DMF. Fairly soluble in acetone and Me2SO. U22(H2O)1.5 (Referred to as U22). To aqueous solution of uranium(IV) chloride (4 mmol), ethanolic solution of the ligand 2 (2.42 g, 8.01 mmol) was added. When the acidity of the mixture changed to pH 4-5 by NaOH(aq) (3 M), the oily coarse product was obtained. After it was recrystallized from CHCl3 at 10 °C, an orange solid was obtained. Elemental analysis: Calcd.: U, 27.49%; C, 49.94%; H, 5.01%. Found: U, 27.40%; C, 49.98%; H, 4.72%. M.W.: 865.748. Yield: 24.0% (ligandbased). NMR: See Table 1. Solubility: >0.05 M for CH3CN, DMF, and THF; |E01| holds in this case, the overall reaction near E01 is UVIO2(acac)2 + e- f UVO2(acac) + acac-. The mechanism is supported by the similar dependence observed in the cyclic voltammograms of 1.0 mM of UO2(dmso)5(ClO4) in Me2SO/ 0.1 M TBAP on GC (Figure 2b). The addition of acetylacetone (Hacac) reduces the peak current of the oxidation wave at E ) -0.91 V versus Fc/Fc+ (a0) and an additional couple of redox waves appears at the potential identical to c1/ a1. This is consistently explained by a shift of the equilibrium, which reduces the concentration of UO2(dmso)5(ClO4)2 and in turn increases monoacetylacetonate complexes with the redox
SCHEME 3: Electrochemical Reactions Observed for Me2SO Solution of UO2(acac)2 and Related Reactions Observed in Ligand Addition Experiments (Shadowed Region)a
a Upper and downward arrows indicate adition and dissociation of acetylacetonate, respectively. Reactions associated with asterisks were observed on GC electrode and reaction of dashed arrow was observed on Pt electrode in the Me2SO /0.1 M TBAP solution containing UO2(acac)2 in Figure 3d and a, respectively.
couple of UVIO2(acac)-/UVO2(acac) at E1/2 ) -1.21 V versus Fc/Fc+ (c1/a1). For Li(acac) (Figure 2c), in addition to the behavior observed in the former case, further changes are observed: (i) a peak intensity of c2 decreases while the corresponding anodic one of a2 increases and (ii) a new reduction wave appears at Ep ) -1.49 V versus Fc/Fc+ (c3) without a corresponding anodic peak. These poorly synchronized changes between cathodic and anodic responses are explained by taking account of the kinetic and equilibrium conditions: (a) fast kinetics of dissociative equilibrium (C step) accompanied with ET (E process) at the cathodic scan and (b) the stability of the complex depends on the metal center, that is, larger formation stability with U(VI) than that with U(V). The change of the voltammograms caused is more significant for Na(acac) (not shown here) and prominent for K(acac) (Figure 2d). The change caused by 5 × 10-5 M of Li(acac) is comparable with the change caused by only 1 ×
18816 J. Phys. Chem. C, Vol. 111, No. 50, 2007
Yamamura et al. SCHEME 4: Inner-Sphere Mechanism Proposed for the Electron Transfer of UO2(acac)2 on GC
SCHEME 5: Electrochemical Reactions Observed for Me2SO Solution of UO2T, Where T ) 1 or 2
Figure 3. Cyclic voltammograms of Me2SO/0.1 M TBAP solution containing UO2(acac)2 (a, d), UO21 (b, e) and UO22 (c, f) complexes at Pt (a-c) and GC (d-f) electrodes. Concentration of uranium are 1.37 mM (a) and 1.44 mM (d) for UO2(acac)2, 0.92 mM (b) and 1.21 mM (e) for UO21 and 1.10 mM (c) and 1.39 mM (f) for UO22. Initial sweep direction is cathodic. Scan rate ranges from 0.025 to 0.2 V s-1.
TABLE 2: Half-Wave Potentials, Diffusion Constants, and Standard Rate Constants of U(VI)/U(V) Electron Transfer of UO21, UO22, and UO2(acac)2 in Dimethyl Sulfoxide complex UO21
UO22
UO2(acac)2 2+
electrode Pt GC PG(a plane) PG(c plane) Pt GC GCb PG(a plane) PG(c plane) Pt GC
E1/2/V vs Fc/Fc+ a
∆E / mVa
107 D/ cm2 s-1
103 k0/ cm s-1
-1.45 -1.45 -1.45 -1.44 -1.45 -1.46 -1.46 -1.46 -1.46 -1.44 -1.44
104 70 78 78 148 70 68 74 79 273 71
18.7 12.9 4.3 5.0 10.9 10.5 5.0 10.0 8.5 12.9 17.7
3.4 11 4.5 4.7 1.2 15 10 3.8 4.6 0.4 12
k0/k0 (Pt) 3.2 1.3 1.4 13 8.3 3.2 3.8 30
Values determined from voltammograms at V) 0.2 V s . Values determined in ref 27. a
-1 b
10-6 M of K(acac). The change due to the ligand addition is likely to depend on the free ligand (acac-) concentration, which becomes larger with decreasing formation constants of acetylacetone salts in Me2SO as Hacac (log K ) 13.41) . Li(acac) (4.76) . Na(acac) (2.57) > K(acac) (1.399).40 Depending on [acac-], successive equilibrium UO2(acac)n(2-n) corresponding to cn/an waves is generated (Scheme 3), and this is responsible for (i) the successive appearance and disappearance of the redox waves of c0/a0, c1/a1, c2/a2, and c3 and (ii) the potentials in the order of E1/2(c0/a0) > E1/2(c1/a1) > E1/2(c2/a2) > Epc(c3). It is worthwhile to note that the third coordination of acetylacetonate to uranyl(VI) ion was first detected as the cathodic waves c3 in this study. The cyclic voltammetry of UO2(acac)2 reveals a remarkable dependence on electrode materials. On Pt, the cyclic voltammograms of UO2(acac)2 (Figure 3a) display the feature of irreversibility; complete disappearance of the anodic peak a2. A replacement of the working electrode by GC (Figure 3d) improves the reversibility markedly; the ∆Ep values decreases to 71 mV (Table 2) and the undiscovered anodic peak of a2 appears. The marked dependence of ET on the electrode materials is evidence of an inner-sphere mechanism. The irreversibility on Pt is considered as no affinity of both the ligand and the complex onto the Pt surface. The improvement on GC
is most likely due to the affinity of the ligand or the complex onto the GC surface41 or the affinity of the acetylacetone-like oxidized surface of the carbon electrode42 (Scheme 4). 3.3. Enhanced Stability and ET Kinetics of UO2L (L ) 1, 2). Tetraketone complexes with U(VI), namely UO21 and UO22, show single redox couples c2/a2 on Pt (Figure 3b and c). The half-wave potentials of c2/a2 of E1/2) -1.44 V (UO21) and -1.45 V versus Fc/Fc+ (UO22) agree well with the main redox wave (c2/a2) of UO2(acac)2 at E1/2 ) -1.43 versus Fc/Fc+. This is understood as a typical example of the linear correlation of half-wave potential of U(VI)/U(V) with two-coordinated β-diketones with the acid dissociation constants (pKa).12 The constants determined in water-dioxane (1:1 (v/v)) are pKa1 ) 9.69, pKa2 )10.68 for 1 and pKa1 ) 9.68, pKa2 )10.63 for 2 and the first dissociation constants are known to agree well with the pKa of acetylacetone.26 The voltammograms have only a limited dependence on electrode materials (Figure 3b-e and c-f). The values of ∆Ep for the couple of peaks c2/a2 are 104 mV (UO21) and 148 mV (UO22) (Table 2), which are improved from 248 mV for UO2(acac)2. When the working electrode was replaced by GC, much smaller ∆Ep values of 70 mV are obtained for the two complexes. It is worth mentioning that the voltammograms of UO22 on the GC electrode show an additional small peak at c1 (Figure 3f) and the absence of such a peak on Pt. It is considered that the ET of UO22 is mostly outer-sphere on Pt but the affinity of the tetraketone ligand 2 on the surface is not negligible. When electrodes of highly oriented pyrolytic graphite, PG(a plane) and PG (c plane), are used, the values are 74-79 mV, which are comparable to those for GC. Considering that the ∆Ep is as small as 70 mV at V ) 0.2 V s-1 and the ratio of cathodic peak current to anodic one ipc/ipa is close to unity, the redox couples of UO21 and UO22 have the feature of the reversible or quasireversible process. Consequently, the tetraketone complex undergoes a simple ET reaction without following equilibrium with the ligand (C step) (Scheme 5). The standard rate constants determined for UO21 and UO22 show a dramatic improvement of reversibility from acetylacetone (Figure 3a; k0 ) 4 × 10-4 cm s-1) to tetraketones (Figure 3b and c; k0 ) (1.2-3.4) × 10-3 cm s-1) on Pt. On GC, values of k0 are equally improved (k0 > 10-2 cm s-1) from those on Pt, whereas a sign of the C reaction is observed in ligand 2 (Figure 3d). In terms of the inner-/outer-sphere mechanism, the sensitivity on the electrode surface is formulated as k0/k0(Pt) and tabulated in Table 2. The value of k0(GC)/k0(Pt) ) 3.2 for UO21 is quite smaller than that for UO2(acac)2 (30). This indicates that the ET of UO21 is regarded as the inner-sphere reaction. The value for UO22 is k0(GC)/k0(Pt) ) 13 and the sensitivity is weaker than UO2(acac)2 but stronger than UO21. In comparison with k0 ) 8.5 × 10-4 cm s-1 for V(IV)/V(V),8 which is the positive active materials in the vanadium redox-flow battery, the ET kinetics of U(VI)/U(V) is 12-18 times faster. 3.4. Mechanism and Kinetics of ET of M(acac)4 (M ) U, Th) in CH3CN and DMF. Cyclic voltammograms of CH3CN
Uranium-Based Redox Couples
J. Phys. Chem. C, Vol. 111, No. 50, 2007 18817 SCHEME 6: Electrochemical Reactions Observed for DMF Solution of U(acac)4.
Figure 4. Plots of Ψ vs V-1/2 for cyclic voltammograms of UO2(acac)2, UO21 and UO22 complexes dissolved in Me2SO solution containing 0.1 M TBAP obtained at GC electrode. Concentrations of the complexes are 1.44 × 10-3, 1.20 × 10-3, and 1.39 × 10-3 M.
Figure 5. Cyclic voltammograms of U(acac)4 and Th(acac)4 (1 × 10-3 M) in CH3CN or DMF containing 0.1 M TBAP on Pt electrode. Initial sweep direction is cathodic. Scan rate is 0.2 V s-1.
Figure 6. Cyclic voltammograms of Th12 (a), U12 (b), Th22 (c), and U22 (d) in DMF solution containing 0.1 M TBAP obtained at Pt electrode. Concentration of complexes are 1.08, 1.00, 1.05, and 2.46 × 10-3 M for a-d, respectively. Initial sweep direction is cathodic. Scan rate is 0.2 V s-1.
solution of Th(acac)4 and U(acac)4 (Figure 5a and b) displayed a couple of large peaks at E1/2 ) -2.49 and -2.42 V versus Fc/Fc+, respectively, as well as a couple of peaks at around -1 V versus Fc/Fc+. We attributed these couples (c4/a4 and c3/a3) to the ET of four- and three-coordinated species and remaining anodic peaks more negative than a3 to the oxidation
of the acac- anion after the previous work.22 The origin of the main redox c4/a4 is considered as ligand-centered ET between M(acac)40 and M(acac)4- 43 after the similarity with the electrochemistry of the organometallic thorium complex22,44 and of a few thorium(III) species,45-47 as it is generally accepted.48 Cyclic voltammograms of U(acac)4 and Th(acac)4, first obtained in DMF (Figure 5c and d), are similar to those in CH3CN with the values of E1/2 for c4/a4 being shifted slightly more (60-80 mV) to negative potential than those in CH3CN. In our previous study, we have applied the ligand addition experiment (refer to Section 3.2) to the U(acac)4 system in Me2SO whose donor number (29.8) is close to that of DMF (26.6).49 In this system, successive occurrences of ET of n-coordinated complexes (n ) 0-4), that is U(acac)n(4-n)/(3-n), are observed. The cathodic peak potentials of Th(acac)4 and U(acac)4 in DMF corresponds to the redox potential of n ) 4 (-2.66 V vs Fc/ Fc+) in Me2SO. The voltammograms obtained in DMF (Figure 5c and d) show a feature of irreversibility. The anodic peak current at a4 is smaller than the cathodic one at c4 as it was in Me2SO. Because the standard rate constants for U(acac)4 of k0 ) 2.5 × 10-3 (on Pt) and 1.7 × 10-2 cm s-1 (on GC) are in the order of pseudo-reversible, the irreversibility cannot be attributed to irreversible ET process (Eirrev) but to ECirrev where a pseudo-reversible ET followed by an irreversible chemical step. Including the appearance of c3/a3 couples, the whole electrochemical reactions observed can be explained by the ECE mechanism (Scheme 6). Depending on the kinetics of the C step within the time scale of the cyclic voltammetry, the reaction may not be complete on the observation time scale (0 < ia/ic < 1) for a moderately fast C step. This means that the exergonicity of the irreversible step C is quite larger than that of Eirrev process only accompanying structural transformation.37 3.5. Enhancement of Stability and ET Kinetics of M(L)2 (M ) U, Th; L ) 1, 2). Cyclic voltammograms of ThL2 and UL2, where L is tetraketone 1 or 2, are shown in Figure 6. For uranium complexes, the E1/2 values of redox couple a4/c4 are determined as -2.47 V (U12) and -2.48 V versus Fc/Fc+ (U22), which are almost identical to E1/2(c4/c4) ) -2.46 V versus Fc/ Fc+ of U(acac)4. From the dependence of voltammograms of U12, U22, and U(acac)4 on the scan rate (0.050-2.0 V s-1) (Figure 7), the standard rate constants k0 are determined (Table 3) by the linear relationships found in the plots of Ψ versus V-1/2 (Figure 8). The value of k0 of a c4/a4 reaction of UL2 (L ) 1, 2) is determined to be from 0.34 × 10-3 (U22 on Pt) to 0.9-1.5 × 10-2 cm s-1 (on GC). The order of the k0 values are found to be similar to that of U(acac)4. A most striking improvement in the tetraketone-based complex from the acetylacetone-based complex is found in the potential cycling for a long period. During potential cycling between potential of -2.8 and -1.6 V versus Fc/Fc+, the reduction peak current decreases gradually (Figure 9a). This implies the gradual decrease in the U(acac)4 concentration during the potential cycling, presumably because the reduced U(III) complex may dissociate its ligand to produce a three-coordinated complex, which may not return effectively to the fourcoordinated U(IV) complex (Scheme 6). In other words, the
18818 J. Phys. Chem. C, Vol. 111, No. 50, 2007
Yamamura et al.
Figure 9. Cyclic voltammograms of U(acac)4 and U22 in DMF solution containing 0.1 M TBAP obtained at GC electrode.
Figure 7. Cyclic voltammograms of U(acac)4 (a), U12 (b), and U22 (c) in DMF solution containing 0.1 M TBAP obtained at GC electrode. Concentration of complexes are 1.02, 0.96, and 1.47 × 10-3 M for a-c, respectively. Initial sweep direction is cathodic. Scan rate ranges from 0.050 to 2.0 V s-1.
Figure 8. Plots of Ψ vs V-1/2 for cyclic voltammograms of U(acac)4, U12 and U22 complexes dissolved in DMF solution containing 0.1 M TBAP obtained at GC electrode. Concentrations of the complexes are 1.44 × 10-3, 9.55 × 10-4, and 1.47 × 10-3 M.
TABLE 3: Half-Wave Potentials, Diffusion Constants, and Standard Rate Constants of U(IV)/U(III) Electron Transfer of U12, U22, and U(acac)4 Determined in N,N-Dimethylformamide 107 D/ 103 k0/ E1/2/V vs complex electrode Fc/Fc+ a ∆E/mVa cm2 s-1 cm s-1 k0/k0(Pt) U12 U22 U(acac)4
Pt GC Pt GC Pt GC
-2.49 -2.47 -2.53 -2.48 -2.51 -2.46
b 84 244 104 172 85
58 82 89 109 30 59
b 15 3.4 8.8 2.5 17
2.6 6.8
Values determined from voltammograms at V ) 0.2 V s-1. b The k0 value was not evaluated because of the small ∆E value (
