Catalytic cycle of a divanadium complex with salen ligands in O2 reduction: Two-electron redox process of the dinuclear center (salen = N,N′-ethylenebis(salicylideneamine))

Article abstract of DOI:10.1021/ja9617799

In an attempt to provide confirmation for the postulated mechanism of O₂ reduction in vanadium-mediated oxidative polymerization of diphenyl disulfide, a series of divanadium complexes containing salen ligand (salen = N,N′-ethylenebis(salicylideneamine)) were prepared, characterized, and subjected to reactivity studies toward dioxygen. A divanadium(III, IV) complex, [(salen)VOV(salen)][I₃] (II), was yielded both by treatments of solutions of [(salen)VOV(salen)][BF₄]₂ (I) in acetonitrile with excess tetrabutylammonium iodide and by electroreduction of I followed by anion exchange with tetrabutylammonium triiodide. The complex II was characterized by a near-infrared absorption at 7.2 × 10³ cm^-1 (∈ = 60.1 M^-1 cm^-1 in acetonitrile) assigned to an intervalence transfer band. A crystallographically determined V(III)-V(IV) distance of 3.569(4) A? is consonant with the classification of II as a weakly coupled Type II mixed-valence vanadium (α = 3.0 × 10^-2). Oxidation of the cation [(salen)VOV(salen)]⁺ with O₂ in dichloromethane yielded spontaneously the deep blue, mixed valent, divanadium(IV, V) species [(salen)VOVO(salen)]⁺ which was structurally characterized both as its triiodide (III) and perchlorate (IV) salts. Crystal data for III: triclinic space group P1 (no. 2), a = 14.973(2) A?, b = 19.481(2) A?, c = 14.168(2) A?, α = 107.00 (1)°, β= 115.56(1)°, γ = 80.35(1)°, V = 3561.3(9) A?³, Z = 4, D_calc = 1.953 g/cm³, μ (MoKα) = 31.74 cm^-1, final R = 0.057 and R_w = 0.065. Crystal data for IV: triclinic space group P1 (no. 2), a = 11.923(3) A?, b = 14.25(1) A?, c = 11.368(7) A?, α = 112.92(5)°, β = 92.76(4)°, γ = 99.13(4)°, V = 1743(1) A?³, Z = 2, D_calc = 1.537 g/cm³, μ (CuKα) = 57.69 cm^-1, final R = 0.042 and R_w = 0.061. The complexes III and IV were deoxygenated in strongly acidic nonaqueous media to produce [(salen)VOV(salen)]³⁺ as a high-valent complex whose reversible two-electron redox couple (VOV³⁺/VOV⁺) at 0.44V vs Ag/AgCl has been confirmed. Its ability to serve as a two-electron oxidant provided a unique model of a multielectron redox cycle in oxidative polymerization.

Full text of DOI:10.1021/ja9617799

J. Am. Chem. Soc. 1996, 118, 12665-12672
12665
Catalytic Cycle of a Divanadium Complex with Salen Ligands
in O2 Reduction: Two-Electron Redox Process of the Dinuclear
Center (salen ) N,N′-Ethylenebis(salicylideneamine))
Kimihisa Yamamoto, Kenichi Oyaizu, and Eishun Tsuchida*,†
Contribution from the AdVanced Research Institute for Science & Engineering, Department of
Polymer Chemistry, Waseda UniVersity, Tokyo 169-50, Japan
ReceiVed May 28, 1996X
Abstract: In an attempt to provide confirmation for the postulated mechanism of O2 reduction in vanadium-mediated
oxidative polymerization of diphenyl disulfide, a series of divanadium complexes containing salen ligand (salen )
N,N′-ethylenebis(salicylideneamine)) were prepared, characterized, and subjected to reactivity studies toward dioxygen.
A divanadium(III, IV) complex, [(salen)VOV(salen)][I3] (II), was yielded both by treatments of solutions of [(salen)-
VOV(salen)][BF4]2 (I) in acetonitrile with excess tetrabutylammonium iodide and by electroreduction of I followed
by anion exchange with tetrabutylammonium triiodide. The complex II was characterized by a near-infrared absorption
at 7.2 × 103 cm-1 (ꢀ ) 60.1 M-1 cm-1 in acetonitrile) assigned to an intervalence transfer band. A crystallographically
determined V(III)-V(IV) distance of 3.569(4) Å is consonant with the classification of II as a weakly coupled Type
II mixed-valence vanadium (R ) 3.0 × 10-2). Oxidation of the cation [(salen)VOV(salen)]+ with O2 in
dichloromethane yielded spontaneously the deep blue, mixed valent, divanadium(IV, V) species [(salen)VOVO-
(salen)]+ which was structurally characterized both as its triiodide (III) and perchlorate (IV) salts. Crystal data for
III: triclinic space group P1h (no. 2), a ) 14.973(2) Å, b ) 19.481(2) Å, c ) 14.168(2) Å, R ) 107.00 (1)°, â )
115.56(1)°, γ ) 80.35(1)°, V ) 3561.3(9) Å3, Z ) 4, Dcalc ) 1.953 g/cm3, µ (MoKR) ) 31.74 cm-1, final R ) 0.057
and Rw ) 0.065. Crystal data for IV: triclinic space group P1h (no. 2), a ) 11.923(3) Å, b ) 14.25(1) Å, c )
11.368(7) Å, R ) 112.92(5)°, â ) 92.76(4)°, γ ) 99.13(4)°, V ) 1743(1) Å3, Z ) 2, Dcalc ) 1.537 g/cm3, µ (CuKR)
) 57.69 cm-1, final R ) 0.042 and Rw ) 0.061. The complexes III and IV were deoxygenated in strongly acidic
nonaqueous media to produce [(salen)VOV(salen)]3+ as a high-valent complex whose reversible two-electron redox
couple (VOV3+/VOV+) at 0.44V vs Ag/AgCl has been confirmed. Its ability to serve as a two-electron oxidant
provided a unique model of a multielectron redox cycle in oxidative polymerization.
Introduction
involve the use of multinuclear complexes having O2 coordina-
tion sites. Anson and co-workers found that four-electron
reduction of O2 is accomplished at 0.30 V vs SCE by using a
tetraruthenated cobalt tetrapyridylporphyrin, with continued
diligence toward unraveling the mechanism.4 Our interest is,
on the other hand, directed toward the application of well-
defined four-electron reduction systems of O2 to molecular
syntheses.5 Indeed, a number of important biological and
industrial processes involve metal-catalyzed oxidation of organic
substrates, and, for this purpose, the use of dioxygen as the
ultimate oxidant has obvious advantages in terms of cost and
handling use.6
The capacity of metalloenzymes to transduce energy Via
synergetic electron transfer (ET) for the assimilative reductions
of small molecules such as carbon dioxide and dinitrogen and
O2-oxidations in dissimilatory processes1 poses a considerable
challenge to modeling the biological processes. As recent
topics, the X-ray structures of cytochrome c oxidases which
provoke four-electron reduction of O2 to H2O attract much
attention2 though the structures of the intermediate O2 adducts
or their catalytic mechanisms have not been realized yet. As
to the four-electron reduction system of O2, much effort has
been spent to establish an effective electrocatalyst which
operates near the thermodynamic potential.3 Recent trends
We have established the novel synthetic route of poly(p-
phenylene sulfide), an important engineering plastic, by the
oxovanadium-catalyzed O2-oxidative polymerization of diphenyl
disulfide.7 Bis(acetylacetonato)oxovanadium(IV), VO(acac)2,
acts as an excellent catalyst in nonaqueous acidic media, but
† CREST investigator (1996-2000), JSTC.
X Abstract published in AdVance ACS Abstracts, December 1, 1996.
(1) (a) Brudvig, G. W.; Thorp, H. H.; Grabtree, R. H. Acc. Chem. Res.
1991, 24, 311. (b) Biological Nitrogen Fixation; Stacey, G., Burris, R. H.,
Evans, H. J., Eds.; Chapman & Hill: New York, 1992. (c) Biology and
Biochemistry of Nitrogen Fixation; Dilworth, M. J., Glenn, A. R., Eds.;
Elsevier: Amsterdam, 1991. (d) Manganese Redox Enzymes; Pecoraro, V.
L., Ed.; VCH Publishers Inc.: New York, 1992.
(4) (a) Shi, C.; Anson, F. C. J. Am. Chem. Soc. 1991, 113, 9564. (b)
Shi, C.; Anson, F. C. Inorg. Chem. 1995, 34, 4554. (c) Steiger, B.; Anson,
F. C. Inorg. Chem. 1995, 34, 3355. (d) Steiger, B.; Anson, F. C. Inorg.
Chem. 1994, 33, 5767. (e) Shi, C.; Anson, F. C. Inorg. Chem. 1992, 31,
5078. (f) Steiger, B.; Shi, C.; Anson, F. C. Inorg. Chem. 1993, 32, 2107.
(5) (a) Yamamoto, K.; Shouji, E.; Nishide, H.; Tsuchida, E. J. Am. Chem.
Soc. 1993, 115, 5819. (b) Yamamoto, K.; Shouji, E.; Suzuki, F.; Kobayashi,
S.; Tsuchida, E. J. Org. Chem. 1995, 60, 452. (c) Yamamoto, K.; Kobayashi,
S.; Shouji, E.; Tsuchida, E. J. Org. Chem. 1996, 61, 1912.
(6) (a) Thiyagarajan, B.; Kerr, M. E.; Bruno, J. W. Inorg. Chem. 1995,
34, 3444. (b) Dioxygen ActiVation and Homogeneous Catalytic Oxidation;
Simandi, L. I., Ed.; Elsevier: Amsterdam, 1991; Vol. 66. (c) Sheldon, R.
A.; Kochi, J. M. Metal-Catalyzed Oxidation of Organic Compounds;
Academic Press: New York, 1981.
(2) (a) Iwata, S.; Ostermeier, C.; Ludwig, B.; Michel, H. Nature 1995,
376, 660. (b) Tsukihara, T.; Aoyama, H.; Yamashita, E.; Tomizaki, T.;
Yamaguchi, H.; Shinzawa-Itoh, K.; Nakashima, R.; Yaono, R.; Yoshikawa,
S. Science 1995, 269, 1069.
(3) (a) Chang, C. K.; Liu, H. Y.; Abdalmuhdi, I. J. Am. Chem. Soc. 1984,
106, 2725. (b) Ni, C.-L.; Abdalmuhdi, I.; Chang, C. K.; Anson, F. C. J.
Phys. Chem. 1987, 91, 1158. (c) Shi, C.; Anson, F. C. J. Electroanal. Chem.
1990, 293, 165. (d) Zhang, J.; Anson, F. C. J. Electroanal. Chem. 1992,
341, 323. (e) Shi, C.; Anson, F. C. Inorg. Chem. 1990, 29, 4298. (f) Collman,
J. P.; Denisevich, P.; Konai, Y.; Marrocco, M.; Koval, C.; Anson, F. C. J.
Am. Chem. Soc. 1980, 102, 6027. (g) Collman, J. P.; Kim, K. J. Am. Chem.
Soc. 1986, 108, 7847.
S0002-7863(96)01779-9 CCC: $12.00 © 1996 American Chemical Society

12666 J. Am. Chem. Soc., Vol. 118, No. 50, 1996
Yamamoto et al.
Method 3. A procedure essentially identical to that described by
Leigh et al.10 was followed with slight modifications. A solution of I2
(1.9 g, 7.5 mmol) in tetralin (70 mL) was added dropwise to boiling
tetralin (40 mL) under an atmosphere of pure argon. The generated
gaseous HI was introduced with argon to a green suspension of VO-
(salen) (2 g, 6 mmol) in anhydrous CH3CN (50 mL). A small amount
of I2 contaminated in HI/argon proved to produce unidentified side
products as brown solid; the amount of iodine in the gas was minimized
by using the washing bottle cooled with dry ice which was placed before
the reaction vessel. The suspension gradually turned dark brown. After
all I2 was added, the reaction mixture was refluxed for 30 min and
then filtered to remove a little amount of brown solid. The filtrate
was stored in a drybox under an atmosphere of pure argon at 20 °C.
After 1 day, the produced dark red needles were collected, washed
with anhydrous Et2O, and dried in Vacuo. The product was identified
as in Method 1.
mechanistic analysis of the redox process involved in the
catalytic cycle is made difficult because of the lability of the
â-diketone ligand.8 We previously described the electron
transfer chemistry of a more stable model of the acetylacetonato
catalyst, VO(salen) (salen ) N,N′-ethylenebis(salicylideneam-
ine)), which revealed that a µ-oxo divanadium(IV) complex
([(salen)VOV(salen)]2+, VOV2+) is formed in strongly acidic
nonaqueous media.9 The two-electron redox couple of the
complex (VOV3+ + 2e- a A˜ VOV+) has been defined in CH3-
CN by electrochemical measurements.9
Since the oxidative polymerization of diphenyl disulfide
catalyzed by VO(acac)2 results in a selective formation of
thioether bonds without any oxygenated compounds such as
sulfoxides and/or sulfones, we should note that H2O should be
produced predominantly by the reduction of O2 catalyzed by
the vanadium complex without the formation of partially-
reduced side products such as H2O2. The present paper reports
a novel catalysis of the divanadium complex with salen ligands
in the reduction of O2 to H2O with determination of the catalytic
mechanism. The syntheses and the X-ray structures of the
divanadium complexes are described, and the V(III)/O2 chem-
istry is detailed. These results also provide an additional insight
to the unique vanadium(III)/oxovanadium(V) redox chemistry
with possible relevance to metal monooxygenases.
B. [(salen)VOVO(salen)][I3] (III). O2 gas was swept through a
solution of II (0.011 g, 0.01 mmol) in CH2Cl2 (100 mL) for 1 day and
then the solvent was replaced by CH3CN (5 mL). Crystallization by
vapor diffusion of Et2O into the solution yielded dark brown plates of
III in nearly quantitative yield as identified by X-ray crystallography
and elemental analysis. FTIR (KBr, cm-1): 1618 (νCdN), 981 (νVdO).
Anal. Calcd for C32H28N4O6V2I3: C, 36.70; H, 2.69; N, 5.35. Found:
C, 36.54; H, 2.60; N, 5.20. FABMS (m/e): 1047. UV (λmax, nm, in
CH2Cl2): 365, 282, 240. ESR (eight-line signal, in CH2Cl2): g0 )
1.98, A0 ) 99.1 (G).
C. [(salen)VOVO(salen)][ClO4] (IV). The anhydrous 100 mL
CH2Cl2 solution of I (0.0082 g, 0.01 mmol) with tetrabutylammonium
perchlorate (3.42 g, 0.01 mol) was reduced by a carbon felt cathode at
0.3 V vs Ag/AgCl under an atmosphere of dry argon. After the
coulometric reduction, O2 gas was passed through the resulting solution
for 1 day. Then the solvent was replaced by CH3CN (5 mL). Slow
diffusion of Et2O into this solution caused the crystallization of the
product as small black prisms of IV as identified by X-ray crystal-
Experimental Section
Materials. All solvents were purified by distillation. Tetrabutyl-
ammonium tetrafluoroborate was obtained from Wako Chem. Co. and
recrystallized from the mixture of benzene and ethyl acetate. (N,N′-
Ethylenebis(salicylideneaminato))oxovanadium(IV) (VO(salen)), (N,N′-
ethylenebis(salicylideneaminato))oxovanadium(V) tetrafluoroborate (VO-
(salen)BF4), and (µ-oxo)bis[(N,N′-ethylenebis(salicylideneaminato))-
vanadium(IV)] tetrafluoroborate ([VOV(salen)2][BF4]2, VOV2+) (I) was
prepared as previously described.9
Preparation of Compounds. A. [(salen)VOV(salen)][I3] (II).
Method 1: Reduction of I by Iodide. To a suspension of I (0.082 g,
0.1 mmol) in anhydrous CH3CN (5 mL) was added tetrabutylammonium
iodide (0.369 g, 1 mmol) under an atmosphere of pure argon. As the
reducing agent dissolved, the color of the solution changed from dark
red to dark green. After 4 h of stirring, the solvent was removed under
vacuum, leaving a black residue. This residue was extracted with
anhydrous CH3CN (10 mL) under pure argon. Et2O was diffused into
the extract to yield the product II as dark red crystals. The product
was identified by X-ray crystallography and elemental analysis. FTIR
(KBr, cm-1): 1616 (νCdN), 978 (νVdO). Anal. Calcd for C32H28-
N4O5V2I3: C, 37.27; H, 2.74; N, 5.43. Found: C, 35.80; H, 2.52; N,
5.71. NIR (νmax, cm-1, in CH3CN): 7.1 × 103. UV (λmax, nm, in CH2-
Cl2): 367, 280, 240. ESR (eight-line signal, in CH2Cl2): g0 ) 1.98,
A0 ) 100 (G).
lography and elemental analysis. FTIR (KBr, cm-1): 1093 (νClO ), 981
4
(νVdO). Anal. Calcd for C34H31ClN5O10V2: C, 50.61; H, 3.87; N, 8.68.
Found: C, 50.20; H, 3.82; N, 8.32. UV (λmax, nm, in CH2Cl2): 575,
371, 260, 190. ESR (eight-line signal, in CH2Cl2): g0 ) 1.98, A0 )
97.5 (G).
Measurements. All measurements were performed in a drybox
under an atmosphere of dry argon. Electrochemical measurements were
carried out in a conventional two-compartment cell. A glassy carbon
disk-platinum ring was used as a working electrode and polished before
each experiment with 0.05-µm alumina paste. The auxiliary electrode,
a coiled platinum wire, was separated from the working solution by a
fine-porosity frit. The reference electrode was a commercial Ag/AgCl
electrode immersed in a salt bridge consisting of 0.1 mol/L tetrabutyl-
ammonium tetrafluoroborate, which was placed in a main cell compart-
ment. The formal potential of the ferrocene/ferrocenium couple in
dichloromethane was 0.34 V vs this reference electrode. All potentials
are quoted with respect to this Ag/AgCl reference electrode. A Nikko
Keisoku DPGS-1 dual potentiogalvanostat and a Nikko Keisoku NFG-3
universal programmer were employed with a Graphtec WX2400 X-Y
recorder to obtain the voltammograms. Coulometric exhaustive
electrolysis was performed using a Nikko Keisoku NDCM-1 digital
coulomb meter. UV-vis spectra were obtained using a Shimadzu UV-
2100 spectrophotometer. Infrared spectra were obtained using a JASCO
FT-IR 5300 as potassium bromide pellets. Near-infrared spectra were
obtained using a Shimadzu UV3101PC spectrophotometer. A quartz-
glass cell with 0.2-cm optical path length was employed. FABMS
spectrum was obtained using a VGZAB-HF Spectrometer with m-
nitrobenzyl alcohol as the matrix material.
Method 2: Electroreduction of I. Electroreduction of I (2.1 mg,
0.0025 mmol) in CH2Cl2 (25 mL) with tetrabutylammonium tetrafluo-
roborate (0.82 g, 2.5 mmol) was performed by coulometric electrolysis
at 0.3 V vs Ag/AgCl using a carbon felt as a working electrode.
Removal of solvent followed by addition of tetrabutylammonium
triiodide (0.016 g, 0.025 mmol) and crystallization from CH3CN/Et2O
yielded the product II as dark red crystals which was identified as in
Method 1.
(7) (a) Tsuchida, E.; Yamamoto, K.; Jikei, M.; Nishide, H. Macromol-
ecules 1989, 22, 4138. (b) Yamamoto, K.; Tsuchida, E.; Nishide, H.; Jikei,
M.; Oyaizu, K. Macromolecules 1993, 26, 3432. (c) Tsuchida, E.;
Yamamoto, K.; Oyaizu, K.; Suzuki, F.; Nishide, H.; Hay, A. S.; Wang, Z.
Y. Macromolecules 1995, 28, 409. (d) Oyaizu, K.; Iwasaki, N.; Yamamoto,
K.; Nishide, H.; Tsuchida, E. Bull. Chem. Soc. Jpn. 1994, 67, 1456. (e)
Yamamoto, K.; Jikei, M.; Oyaizu, K.; Suzuki, F.; Nishide, H.; Tsuchida,
E. Bull. Chem. Soc. Jpn. 1994, 67, 251.
X-ray Crystallography. Dark red needle crystals of II and black
prismatic crystals of III and IV were grown from acetonitrile solutions
of the desired complexes after layering with ether. Following
microscopic examination in air in each case, a suitable crystal was
mounted on a glass fiber at room temperature. All measurements were
made on a Rigaku AFC5R diffractometer with a 7.5 kW rotating anode
generator and graphite monochromated MoKR radiation (λ ) 0.71069
(8) (a) Patel, K. S.; Kolawole, G. A. J. Coord. Chem. 1986, 15, 137. (b)
Selbin, J. Chem. ReV. 1965, 65, 153. (c) Selbin, J. Coord. Chem. ReV. 1966,
1, 293.
(9) Tsuchida, E.; Yamamoto, K.; Oyaizu, K.; Iwasaki, N.; Anson, F. C.
Inorg. Chem. 1994, 33, 1056.
(10) Hughes, D. L.; Kleinkes, U.; Leigh, G. J.; Maiwald, M.; Sanders,
J. R.; Sudbrake, C. J. Chem. Soc., Dalton Trans. 1994, 2457.

DiVanadium Complex with Salen Ligands in O2 Reduction
Table 1. Summary of X-ray Crystallographic Data
J. Am. Chem. Soc., Vol. 118, No. 50, 1996 12667
complex
emp form
fw
[(salen)VOV(salen)][I3] (II)
[(salen)VOVO(salen)][I3] (III)
[(salen)VOVO(salen)][ClO4] (IV)
C32H28N4O5V2I3
2062.39
C32H28N4O6V2I3
1047.19
C34H31ClN5O10V2
806.98
cryst syst
space group
a (Å)
b (Å)
c (Å)
triclinic
triclinic
triclinic
P1h (no. 2)
13.436(4)
13.969(4)
12.046(4)
104.47(3)
115.98(2)
97.77(3)
1888(1)
P1h (no. 2)
14.973(2)
19.481(2)
14.168(2)
107.00(1)
115.56(1)
80.35(1)
P1h (no. 2)
11.923(3)
14.25(1)
11.368(7)
112.92(5)
92.76(4)
99.13(4)
1743(1)
R (deg)
â (deg)
γ (deg)
V (Å3)
3561.3(9)
4
Z
2
2
density (calcd)
crystal size (mm)
radiation
absn coeff µ
2θ max (deg)
no. of reflns colld
no. of unq reflns
no. of obsd reflns
params
1.813 g/cm3
0.2 × 0.1 × 0.4
MoKR (λ ) 0.71069 Å)
28.82 cm-1
50.0
1.953 g/cm3
0.4 × 0.1 × 0.3
MoKR (λ ) 0.71069 Å)
31.74 cm-1
50.0
1.537 g/cm3
0.2 × 0.2 × 0.1
CuKR (λ ) 1.54178 Å)
57.59 cm-1
110.1
6957
6646
3760 (I > 3.00σ(I))
415
13070
12541
6584 (I > 3.00σ(I))
847
4595
4377
3089 (I > 3.00σ(I))
497
refln/param ratio
R
Rw
9.06
0.086
0.119
7.77
0.057
0.065
6.22
0.042
0.061
goodness-of-fit
max peak in diff map
min peak in diff map
1.67
1.35
1.58
5.34 e-/Å3
-3.91 e-/Å3
2.67 e-/Å3
-1.25 e-/Å3
0.40 e-/Å3
-0.37 e-/Å3
Table 2. Selected Bond Lengths for [(salen)VOVO(salen)][I3]
(III) Characterized by X-ray Crystallography
Å) for II and III and with a 12 kW rotating anode generator and
graphite monochromated CuKR radiation (λ ) 1.54178 Å) for IV. Unit
cell parameters and an orientation matrix for data collection were
determined by least-squares refinements using the setting angles of 25
carefully centered reflections in the range 20 < 2θ < 25° for II and
III, and in 40 < 2θ < 41° for IV. The data were collected at a
temperature using the ω-2θ scan technique to a maximum 2θ value
of 55° for II and III and of 110° for IV. Scans were made at a speed
of 8°/min (in omega) for II and IV and at 16°/min (in omega) for III.
The weak reflections (I < 3σ(I) for II and III and I < 15σ(I) for IV)
were rescanned (up to 2 scans) and the counts were accumulated to
ensure good counting statistics. Stationary background counts were
recorded on each side of the reflection. The ratio of peak counting
time to background counting time was 2:1. The diameter of the incident
beam collimator was 1.0 mm. The crystal to detector distance was
258 nm, and the computer controlled detector aperture was set to 9 ×
13 mm (horizontal × vertical). The intensities of three representative
reflection were measured after every 200 reflections. No decay
correction was applied. The data were corrected for Lorentz and
polarization effects.
atom-atom
bond length
atom-atom
bond length
V(1)-O(1)
V(1)-O(3)
V(1)-N(2)
V(2)-O(4)
V(2)-O(6)
V(2)-N(4)
V(3)-O(8)
V(3)-N(5)
V(4)-O(9)
V(4)-O(11)
V(4)-N(7)
1.90(1)
1.623(6)
2.04(1)
1.83(1)
1.573(7)
2.08(1)
1.90(1)
2.05(1)
2.246(7)
1.823(8)
2.08(1)
V(1)-O(2)
V(1)-N(1)
V(2)-O(3)
V(2)-O(5)
V(2)-N(3)
V(3)-O(7)
V(3)-O(9)
V(3)-N(6)
V(4)-O(10)
V(4)-O(12)
V(4)-N(8)
1.894(8)
2.02(1)
2.214(6)
1.811(7)
2.07(1)
1.928(8)
1.613(7)
2.065(9)
1.85(1)
1.578(7)
2.11(1)
a Bond lengths are in Å. Estimated standard deviations are given
in parentheses.
All calculations were performed using the teXsan crystallographic
software package of Molecular Structure Corporation.12
Results and Discussion
Structure Solution and Refinement. The structure was solved by
heavy-atom Patterson methods and expanded using Fourier techniques.
Positions for most non-hydrogen atoms were visible on the initial E-map
with positions of the remaining atoms found on subsequent electron
density difference maps. The nonhydrogen atoms were refined
anisotropically. Hydrogen atoms were included but not refined. The
final cycle of full-matrix least-squares refinement11 was based on
observed reflections (I > 3σ(I)) and converged with unweighted and
Synthesis, Structure and Redox State of a µ-Oxo Diva-
nadium Complex (II). Electrochemical and structural studies
of oxovanadium complexes with Schiff-base ligands attract
particular attention because of their reversible one-electron redox
behaviors allowing possible applications to electrocatalysts. VO-
(salen) and its oxidized product VVO(salen)ClO4 crystallize
easily and their X-ray structures have been solved.13 The (µ-
oxo)divanadium(IV) complex [(salen)VOV(salen)][BF4]2 (I) can
be prepared as a black powder by the acidification of VO(salen)
with trifluoromethanesulfonic acid or trityl tetrafluoroborate in
nonaqueous media (2VO(salen) + 2H+ f VOV2+ + H2O) as
previously described.9 Attempts to obtain single crystals of I
met with failure. Reasoning that the isolation of I might be
disfavored in CH3CN most amenable for crystallization because
of the dissociation equilibrium with mononuclear complexes,9
we turned to the reduced complex of I.
weighted agreement factors of R ) ∑||Fo| - |Fc||/∑|Fo| and Rw
)
2
(∑w(|Fo| - |Fc|)2/∑wFo )1/2 as listed in Table 1. Plots of ∑w(|Fo| -
|Fc|)2 versus |Fo|, reflection order in data collection, sin θ/λ and various
classes of indices showed no unusual trends. Data collection and
structure solution parameters and conditions are listed in Table 1.
Selected bond lengths and angles for III are listed in Tables 2 and 3,
respectively, and those for IV are listed in Tables 4 and 5, respectively.
(11) Least-squares: Function minimized: ∑w(|Fo| - |Fc|)2, where w )
(σ(Fo)2 + (0.020(Fo))2)-1
.
(12) teXsan: Crystal Structure Analysis Package; Molecular Structure
Corporation: 1985 & 1992.
The reduction of I by tetrabutylammonium iodide (method
1) produced a mixed-valent, µ-oxo divanadium(III, IV) complex
[(salen)VIVOVIII(salen)][I3] (II). X-ray analysis revealed the
(13) (a) Riley, P. E.; Pecoraro, V. L.; Carrano, C. J.; Bonadies, J. A.;
Raymond, K. N. Inorg. Chem. 1986, 25, 154. (b) Bonadies, J. A.; Butler,
W. M.; Pecoraro, V. L.; Carrano, C. J. Inorg. Chem. 1987, 26, 1218.

12668 J. Am. Chem. Soc., Vol. 118, No. 50, 1996
Yamamoto et al.
Table 3. Selected Bond Angles for [(salen)VOVO(salen)][I3] (III)
Characterized by X-ray Crystallography
atom-atom-atom
angle
atom-atom-atom
angle
O(1)-V(1)-O(2)
O(1)-V(1)-N(1)
O(2)-V(1)-O(3)
O(2)-V(1)-N(2)
O(3)-V(1)-N(2)
O(3)-V(2)-O(4)
O(3)-V(2)-O(6)
O(3)-V(2)-N(4)
O(4)-V(2)-O(6)
O(4)-V(2)-N(4)
O(5)-V(2)-N(3)
O(6)-V(2)-N(3)
N(3)-V(2)-N(4)
O(7)-V(3)-O(9)
O(7)-V(3)-N(6)
O(8)-V(3)-N(5)
O(9)-V(3)-N(5)
N(5)-V(3)-N(6)
O(9)-V(4)-O(11)
O(9)-V(4)-N(7)
88.5(4) O(1)-V(1)-O(3)
87.2(4) O(1)-V(1)-N(2)
110.8(4) O(2)-V(1)-N(1)
87.7(4) O(3)-V(1)-N(1)
103.3(4) N(1)-V(1)-N(2)
83.4(3) O(3)-V(2)-O(5)
174.0(5) O(3)-V(2)-N(3)
79.0(3) O(4)-V(2)-O(5)
98.8(4) O(4)-V(2)-N(3)
156.5(3) O(5)-V(2)-O(6)
158.3(4) O(5)-V(2)-N(4)
92.9(4) O(6)-V(2)-N(4)
75.9(4) O(7)-V(3)-O(8)
105.3(4) O(7)-V(3)-N(5)
149.3(3) O(8)-V(3)-O(9)
146.7(4) O(8)-V(3)-N(6)
105.0(5) O(9)-V(3)-N(6)
77.9(4) O(9)-V(4)-O(10)
82.4(3) O(9)-V(4)-O(12)
83.8(3) O(9)-V(4)-N(8)
106.1(4)
149.8(3)
144.4(3)
104.4(4)
78.8(5)
82.8(3)
81.7(3)
106.8(4)
86.4(4)
101.8(4)
86.3(4)
97.3(5)
90.3(4)
87.1(4)
107.7(5)
88.0(4)
104.4(4)
82.7(3)
175.8(4)
80.0(4)
97.5(4)
156.0(3)
158.4(5)
92.0(4)
76.2(4)
162.8(6)
Figure 1. ORTEP view (50% probability ellipsoids) of a cation in
[(salen)VOV(salen)][I3] (II).
O(10)-V(4)-O(11) 109.0(5) O(10)-V(4)-O(12)
O(10)-V(4)-N(7) 85.5(4) O(10)-V(4)-N(8)
O(11)-V(4)-O(12) 101.4(4) O(11)-V(4)-N(7)
O(11)-V(4)-N(8)
O(12)-V(4)-N(8)
V(1)-O(3)-V(2)
85.1(5) O(12)-V(4)-N(7)
98.5(5) N(7)-V(4)-N(8)
166.6(5) V(3)-O(9)-V(4)
Figure 2. ORTEP diagram of the unit cell of II viewed down the
a-axis.
a Angles are in degrees. Estimated standard deviations are given in
parentheses.
of complex as expected for quantitative reduction to VOV+ (eq
1).9
Table 4. Selected Bond Lengths for [(salen)VOVO(salen)][ClO4]
(IV) Characterized by X-ray Crystallography
atom-atom
bond length
atom-atom
bond length
[(salen)VIVOVIV(salen)]2+ + e- f
V(1)-O(1)
V(1)-O(3)
V(1)-N(2)
V(2)-O(4)
V(2)-O(6)
V(2)-N(4)
1.910(3)
1.618(3)
2.056(4)
1.822(3)
1.597(4)
2.091(4)
V(1)-O(2)
V(1)-N(1)
V(2)-O(3)
V(2)-O(5)
V(2)-N(3)
1.899(3)
2.042(3)
2.250(3)
1.809(3)
2.081(4)
[(salen)VIVOVIII(salen)]+ (1)
The crystallization of the reduced species was performed in
CH3CN/Et2O in the presence of added tetrabutylammonium salts
such as perchlorate, hexafluorophosphate, bromide, iodide, and
triiodide. Good crystals were obtained only for the triiodide
salt.
a Bond lengths are in Å. Estimated standard deviations are given
in parentheses.
The crystal structure of II was essentially the same as that
reported by Leigh et al.10 The previous study10 involved the
addition of HI to solutions of VO(salen) in acetonitrile. We
could also obtain the complex II by the analogous method with
slight modifications (method 3). It can be reasoned that
acidification of VO(salen) with HI would first result in dimer-
ization of VO(salen)9 and then in subsequent reduction by iodide
to produce II. The phenolate oxygen in a salen ligand often
serves as a bridging ligand; the typical example is provided by
Cu(salen) which crystallizes in a dimeric arrangement with non-
planarity of the central coordinated group.15 Indeed, the
molecular structure of II can also be taken as tetravanadium
complex, as in the previous work,10 with the center of symmetry
being located at the center of the molecule (Figure 2). But since
O(6) in the adjacent cation is located as long as 2.101(9) Å
beyond V(2), and O(5) deviates from the N(3)N(4)O(4) plane
by only 0.472 Å, the cation structure can be regarded as a
dinuclear complex, rather than a tetranuclear complex, with O(5)
and O(6) axially located near vanadium in the octahedral
arrangement. It is known that vanadium(III) prefers an octa-
hedral arrangement, whereas vanadium(IV) is more stable in
square pyramidal geometry. The V(2) atom in the monocation
structure, surrounded by the four-coordinating atoms of a salen
ligand, extends only 0.059 Å above the best N2O2 least-square
plane with O(3)-V(2)-O(6) angle of 170.4(4)°. The arrange-
Table 5. Selected Bond Angles for [(salen)VOVO(salen)][ClO4]
(IV) Characterized by X-ray Crystallography
atom-atom-atom
angle
atom-atom-atom
angle
O(1)-V(1)-O(2)
O(1)-V(1)-N(1)
O(2)-V(1)-O(3)
O(2)-V(1)-N(2)
O(3)-V(1)-N(2)
O(3)-V(2)-O(4)
O(3)-V(2)-O(6)
O(3)-V(2)-N(4)
O(4)-V(2)-O(6)
O(4)-V(2)-N(4)
O(5)-V(2)-N(3)
O(6)-V(2)-N(3)
N(3)-V(2)-N(4)
89.5(1)
86.4(1)
110.2(1)
87.8(2)
104.0(1)
82.5(1)
173.0(1)
77.2(1)
99.4(2)
155.0(1)
159.9(2)
91.2(2)
76.9(1)
O(1)-V(1)-O(3)
O(1)-V(1)-N(2)
O(2)-V(1)-N(1)
O(3)-V(1)-N(1)
N(1)-V(1)-N(2)
O(3)-V(2)-O(5)
O(3)-V(2)-N(3)
O(4)-V(2)-O(5)
O(4)-V(2)-N(3)
O(5)-V(2)-O(6)
O(5)-V(2)-N(4)
O(6)-V(2)-N(4)
V(1)-O(3)-V(2)
105.9(2)
148.9(1)
144.9(1)
104.5(2)
78.3(1)
84.1(1)
82.2(1)
106.7(1)
86.0(1)
101.7(2)
85.8(1)
99.1(2)
169.1(2)
a Angles are in deg. Estimated standard deviations are given in
parentheses.
molecular structure of the complex as a triiodide salt as shown
in Figure 1. As an alternative reduction method, controlled
potential reduction of the dimer I at 0.3 V was carried out
(method 2). The redox potential of the complex I is located at
0.44 V vs Ag/AgCl in CH2Cl2,14 and controlled potential
reduction at 0.3 V under N2 consumed one electron per mole
(14) The formal potential of the ferrocene/ferrocenium complex was 0.34
V vs this reference electrode. All potentials are quoted to this Ag/AgCl
reference electrode.
(15) Hall, D.; Waters, T. N. J. Chem. Soc. 1960, 2644.

DiVanadium Complex with Salen Ligands in O2 Reduction
J. Am. Chem. Soc., Vol. 118, No. 50, 1996 12669
Figure 4. ORTEP view (30% probability ellipsoids) of cations in
asymmetric unit of [(salen)VOVO(salen)][I3] (III).
transition band was observed at νmax ) 7.2 × 103 cm-1 in
acetonitrile with half band width ∆1/2 of 3.0 × 103 cm-1 and
16
molar absorption coefficient ꢀmax of 60.1 M-1 cm-1 at the νmax
.
The crystal structure provided the non-bonding vanadium(III)-
to-vanadium(IV) distance of 3.569(4) Å in the dinuclear
complex. The interaction factor R of the mixed valent state
was determined to be 3.0 × 10-2 17
.
The complex II was
classified into a Type II mixed-valent complex: The optical
and electronic properties of II are based not only on the
constituent components but also on their interaction.18
Syntheses and Structures of Divanadium Complexes (III,
IV). It is well-known that the reactivity of vanadium complexes
toward O2 is responsive to the redox state and the coordination
structure. A typical example is provided by acetylacetonate
complexes: a six-coordinate vanadium(III) complex V(acac)3
is oxidized by O2 whereas a five-coordinate vanadium(IV)
complex, VO(acac)2, is stable in air. The structure of II
containing the five-coordinate vanadium(III) prompted us to
examine the reaction with O2. The oxidation of II by O2 in
anhydrous CH2Cl2 proceeded almost stoichiometrically to
produce a divanadium(IV, V) complex (III).19 The crystalliza-
tion of III was performed in CH3CN/Et2O to produce dark
brown crystals. X-ray diffraction studies of the crystallized
product revealed the formation of [(salen)VOVO(salen)][I3]
complex (Figure 4). The structure consists of two crystallo-
graphically independent molecules per asymmetric unit. The
V(2) and V(4) atom, surrounded by the four coordinating atoms
of a salen ligand (N2O2), extends 0.270 Å and 0.262 Å,
respectively, above the best N2O2 least-squares plane. The
Figure 3. Near-infrared spectra of solutions prepared by dissolving
(a) 5.0 mmol/L divanadium(III, IV) complex (II); (b) 5.0 mmol/L
divanadium (IV, V) complex (III) (dotted curve), 5.0 mmol/L III +
10 mmol/L CF3SO3H (solid curve); (c) 5.0 mmol/L divanadium
complex (I) + 0.1 mol/L tetrabutylammonium tetrafluoroborate fol-
lowing controlled potential oxidation at 0.7 V which consumed one
electron per molecule of dimer. The solvent was anhydrous CH3CN.
Optical path length ) 0.2 cm. The baselines are corrected by subtracting
the spectrum of CH3CN.
(16) Our previous study (reference 9) revealed that the µ-oxo divanadium
cation is substantially dissociated in acetonitrile solutions. The dissociation
equilibrium of the divanadium(III, IV) complex has been established as K
) [VO(salen)][V(salen)+]/[(salen)VOV(salen)+] ) 13 mM. The molar
absorption coefficient of the intervalence transition band was determined
with regard to the equilibrium concentration of the undissociated dimer
ment around V(2) is pseudo octahedral. In contrast, V(1) atom
lying as far as 0.534 Å above the N2O2 plane and O(3) 1.646-
(10) Å beyond the V(1) atom represent a typical square-
pyramidal coordination arrangement. The bond length of the
vanadium atom and the equatorial phenolate oxygen of the
ligand are indicative of the oxidation state, since the V-O(phe-
nolate) bond is known to shrink as the vanadium is oxidized.
The V(1)-O(phenolate) bond length (V(1)-O(1), 1.885(9) Å;
V(1)-O(2), 1.927(9) Å) is shorter than that of V(2)-O(phe-
nolate) bond length (V(2)-O(4), 1.890(9) Å; V(2)-O(5), 1.941-
(10) Å). Thus V(1) and V(2) are ascribed to vanadium(IV) and
vanadium(III), respectively.
which is responsible for the electronic transition concerned.
-1
(17) (a) R2 ) (4.5 × 10-4)ꢀmax
∆ ν
1/2
r-2, where R is the interaction
parameter and r is the distance separating the interacting donor-acceptor
moieties. The extent of interaction is defined for the ground state of the
complex as ψG ) (1-R2)1/2φi + Rφj where φi and φj are wave functions
for the donor-acceptor components of the mixed-valence system. Cowan,
D. O.; Vanda, C. L.; Park, J.; Kaufman, F. Acc. Chem. Res. 1973, 6, 1. (b)
Allen, G. C.; Hush, N. S. Prog. Inorg. Chem. 1967, 8, 357. (c) Hush, N. S.
Prog. Inorg. Chem. 1967, 8, 391. (d) Hush, N. S. Coord. Chem. ReV. 1985,
64, 135.
(18) Robin, M. B.; Day, P.; AdV. Inorg. Chem. Radiochem. 1967, 10,
247.
(19) The oxidation of II by O2 obeyed a rate law pseudo second-order
in II, which suggested a mechanism of oxidation involving a bimolecular
reaction to produce an O2-adduct such as peroxo tetramer as the rate limiting
step. The homolytic scission of the intermediate is a likely reaction to
generate two molecules of the VOVO+ cation.
The mixed-valent state was characterized by the near-infrared
spectrum of II as shown in Figure 3(a). An intervalence

12670 J. Am. Chem. Soc., Vol. 118, No. 50, 1996
Yamamoto et al.
perchlorate anion indicates an absence of vanadium-perchlorate
interaction, in contrast to the monomeric [VVO(salen)][ClO4]13
in which perchlorate is located in the proximity of the square-
pyramidal vanadium(V) atom with V-O(perchlorate) distance
of 2.456(3) Å. A comparison of the two structures suggest that
the contact between the perchlorate anion and the vanadium
atom is more impeded in IV containing six-coordinated vana-
dium(V) and square-pyramidal vanadium(IV).
The stoichiometry of reaction 2 is reminiscent of the
electroreduction of O2 to H2O. It is of interest that oxygenation
of vanadium(III) gives oxovanadium(V), whereas multiple one-
electron redox centers have been considered to be essential for
the incorporation of O2. An explanation for this can be found
in the high reactivity of the five-coordinate vanadium(III) and
the stability of the oxovanadium(V) species.
Figure 5. ORTEP view (50% probability ellipsoids) of a cation in
[(salen)VOVO(salen)][ClO4] (IV).
Deoxygenation of III and IV. It has been confirmed that
VIVO(salen) undergoes deoxygenation to give V(salen)2+ and
H2O in strongly acidic conditions.9 The reactivity of II toward
O2 (eq 2), with significant potential in leading to the selective
four-electron reduction of O2 to H2O, prompted the study on
the reaction of the [(salen)VOVO(salen)]+ cation with a strong
acid to undergo deoxygenation of the VOVO+ cation to VOV3+
species. As expected, the VdO stretching bands in the infrared
spectrum of III and IV (981 cm-1) disappeared when 2-fold
mole of CF3SO3H was added to the CH3CN solution. The
vanadium species produced in the presence of protons may be
attributable to [(salen)VOV(salen)]3+. The deoxygenation reac-
tion believed to be responsible for the generation of [(salen)-
VOV(salen)]3+ from [(salen)VOVO(salen)]+ is
arrangement around V(2) and V(4) is fairly distorted octahedral.
In contrast, V(1) atom lying as far as 0.550 Å above the N2O2
plane and O(3) being 1.622(7) Å beyond the V(1) atom represent
a typical square-pyramidal coordination structure. The V(2)
atom deviating from the N2O2 plane by 0.536 Å also represents
a square-pyramidal arrangement. It is known that the oxova-
nadium average VdO bond length is approximately 1.6 Å and
the oxidation state of the vanadium atom is hardly reflected in
this. Nevertheless, we notice that the 1.573(8)-Å V(2)-O(6)
distance and the 1.578(8)-Å V(4)-O(12) distance are closer to
the VdO bond length of oxovanadium(V) complexes such as
those of VO(salen)ClO4 (1.576(3) Å) and VO(salen)BF4 (1.577-
(1) Å) than to that of VIVO(salen) (1.583(5) Å). The bond length
of the vanadium atom and the phenolate oxygen of the ligand
are again indicative of the oxidation state. The shorter
V-O(phenolate) bond length around V(2) and V(4), and the
longer length around V(1) and V(3) (Table 2) support the [VIV-
O-VVdO]+ oxidation state. It may be the preference of
vanadium(V) to be six coordinate rather than five coordinate
that drives the formation of the VOVO+ cation. The weak axial
interaction with O(3) and O(9), as judged by the long V(2)-
O(3) and V(4)-O(9) bond length and the substantial deviations
of the V(2) and V(4) atoms from the N2O2 plane, are due to
the low residual basicity of O(3) and O(9) which are already
bound to vanadium(IV). The units of the two molecules are
[(salen)VOVO(salen)]+ + 2H+ f
[(salen)VOV(salen)]3+ + H2O (3)
Although the crystallization of [(salen)VOV(salen)]3+ com-
plex was unsuccessful, probably due to the enhanced reactivity,
the cation was detected in the mass spectrum (m/z: 651).
Moreover, the resulting acidic solution exhibited a weak
intervalence transition band in the near-infrared spectrum as
shown in Figure 3(b) with νmax of 7.1 × 103 cm-1, ∆1/2 of 3.2
× 103 cm-1 and ꢀmax of 10 M-1 cm-1 at the νmax. Our previous
study9 revealed that [(salen)VIVOVV(salen)]3+ cation is produced
by coulometric one-electron oxidation of I. Added support for
deoxygenation was provided by a good agreement between the
near-infrared spectra of the [(salen)VOV(salen)]3+ solution
(Figure 3(c)) and that of the solution of the VOVO+ cations
after treatment with CF3SO3H. Agreement between the UV-
vis. spectra was also confirmed (λmax ) 245 nm, ꢀmax ) 6.8 ×
104 M-1 cm-1). While the lack of X-ray data prevents further
determination of the electronic states, the formation of a mixed-
valent [(salen)VIVOVV(salen)]3+ cation is evidenced by means
of those spectroscopic measurements.
Homogeneous Catalysis of the Electroreduction of O2. In
attempts to develop catalytic reduction of O2, the vanadium
complex I with the well-defined catalytic cycle was employed
as a mediator. The electroreduction of O2 was performed with
the complex I in CH2Cl2 containing trifluoroacetic acid. Cyclic
voltammograms for the electroreduction of O2 at graphite
electrodes are shown in Figure 6(a). As expected, a large
catalytic current was observed at Ep ) 0.17 V vs Ag/AgCl.
The reduction of O2 seems to begin near the redox potential of
I (0.44 V vs Ag/AgCl). To elucidate more quantitatively the
stoichiometry of the reduction of O2, a rotating ring-disk
electrode was employed.20 Collection experiments of H2O2 at
-
separated by the almost linear I3 anion which is arranged
between the unit.
It follows that the VOV+ complex, produced by the stoichio-
metric electroreduction of I (eq 1), is also expected to give
VOVO+ cation by O2-oxidation. Thus, a solution of I in
anhydrous CH2Cl2 containing 0.1 M tetrabutylammonium per-
chlorate was subjected to controlled potential reduction at 0.3
V under N2 to prepare the solution of VOV+, and the resulting
solution was exposed to O2 gas. The oxygenated product was
crystallized after solvent-exchange with CH3CN/Et2O and
allowing the solution to stand for several days. X-ray diffraction
studies of the crystallized product IV revealed the composition
of [(salen)VOVO(salen)]+ cation (Figure 5), ClO4- anion, and
CH3CN.
2[(salen)VIV-O-VIII(salen)]+ + O2 f
2[(salen)VIV-O-VV(salen)dO]+ (2)
In the cation structure of IV, the V(1) atom lay out of the
best N2O2 least-squares plane by 0.553 Å, representing a square-
pyramidal coordination structure. The arrangement around V(2),
deviating 0.274 Å from the N2O2 plane, is distorted octahedral.
The V-O(phenolate) bond length again suggests the valence
state as [VIV-O-VVdO]+. Aside from the cation structure of
IV being very similar to that of III, the arrangement of the
(20) Bard, A. J.; Faulkner, L. R. Electrochemical Methods; John Wiley
& Sons: New York, 1980; Chapter 8.

DiVanadium Complex with Salen Ligands in O2 Reduction
J. Am. Chem. Soc., Vol. 118, No. 50, 1996 12671
Scheme 1
product in O2 reduction, not as an intermediate.22 Thus the
complex I proved to be an effective electrocatalyst for the direct
four-electron reduction of O2 to H2O.23
The need for the four-electron reduction of O2 as pointed
out earlier is the capacity of forming an intercalated peroxo
complex such as the Co-O-O-Co complex using the difacial
cobalt porphyrins and the dimeric iridium porphyrin.3 In recent
studies, Anson and co-workers have demonstrated that the
coordination of more than two Ru(NH3)5 groups to the meso
pyridine ligands of the cobalt porphyrin ring is effective in
accessing the four-electron reduction pathway.4 The use of
oxovanadium complex proved valuable in this study though it
remains to be determined how protons take part in the
oxygenation of vanadium to accelerate the overall catalytic
reaction. Nevertheless under conditions where oxygenation of
vanadium(III) to oxovanadium(V) followed by acid-promoted
deoxygenation proceeds, a reasonable yield of H2O was obtained
in the electroreduction of O2. Since the oxygenation-deoxy-
genation process of vanadium(III) yields two-electron-oxidized
vanadium(V), it can be stated that the one-step two-electron
redox couple of [(salen)VOV(salen)]3+/+ which can transfer
isoenergetically two electrons plays an essential role on the
catalytic activity.
Figure 6. (a) Cyclic voltammogram obtained for a solution prepared
by dissolving 0.5 mmol/L of I in anhydrous CH2Cl2 saturated with O2
or N2 in the presence of 0.1 mol/L trifluoroacetic acid at glassy carbon
electrode (φ ) 6 mm); supporting electrolyte ) 0.1 mol/L tetrabutyl-
ammonium tetrafluoroborate; scan rate ) 50 mV/s. (b) Rotating ring-
disk voltammogram in the same solution using a platinum ring-glassy
carbon disk electrode; the solid curves were recorded under O2 and
the dotted curves under N2; rotating speed ) 100 rpm; scan rate ) 5
mV/s; disk current shown above, ring current below, the x axis. Ring
potential: dotted curve (under N2), +0.5V; solid curve (under O2) iR1
,
+0.5V; iR2, +1.4 V. The potentials are referenced against Ag/AgCl.
The anodic wave for the oxidation of H2O2 in acidic CH2Cl2 was at Ep
) +1.3 V vs this Ag/AgCl.
Conclusions
We report herein the discovery of a catalytic cycle containing
[(salen)VOV(salen)]+ (II) and [(salen)VOVO(salen)]+ (III, IV),
well-defined electrocatalysts for the four-electron reduction of
O2. This result encourages us to postulate that a similar catalytic
cycle can also be applied to VO(acac)2 by which it can catalyze
the O2-oxidative polymerization of diphenyl disulfide. Thus
the overall redox cycle can be described as Scheme 1. The
cycle involves coordination of O2 to the [(salen)VIVOVIII-
(salen)]+ cation through which oxidation to the [(salen)VIV-
OVVO(salen)]+ cation takes place. As a result of metal-oxo
bond formation (LMx + 1/2 O2 f LMx+2dO), the resulting
complex contains two-electron oxidized vanadium(V) and a fully
reduced oxo ligand. It is likely that the oxygenation process is
promoted by acid. As a next step, the VOVO+ cation is
deoxygenated to produce the high-valent [(salen)VIVOVV-
(salen)]3+ complex which will act as a two-electron oxidant for
substrates regenerating the low-valent starting complex [(salen)-
VIVOVIII(salen)]+. The unprecedented reactivity of II toward
the ring electrode (iR2) revealed that the vanadium complex
resulted in a catalysis to allow four-electron reduction of O2 to
H2O predominantly. The ratio of the ring to disk currents (iR2/
iD) in Figure 6(b) corresponds to the reduction of more than
90% of the O2 molecules to H2O.21 The complex I showed
virtually no catalytic activity for the reduction of H2O2, which
indicate that a small amount of H2O2 is formed only as a parallel
(21) (a) Dotted curves in Figure 6(b) show the current-potential responses
obtained at the rotating ring-disk electrode in the absence of O2. The
collection efficiency of the ring maintained at 0.5 V at the electrode rotation
rate of 100 rpm (N ) 0.27) was slightly less than the intrinsic value of the
electrode (N0 ) 0.38) which was determined in independent experiments
with ferrocene, implying that VOV+ was unstable in acidic media.
Nevertheless in the presence of excess O2 (solid curves), the plateau disk
current iD increased with a significant decrease in the plateau ring current
i
R1 measured at 0.5 V where VOV+ is oxidized, which confirms that VOV+
is partially consumed by reaction with O2 during the time required for its
transfer from the disk to the ring electrode. Collection experiments of H2O2
at the ring electrode maintained at 1.4 V (iR2) were also performed, which
revealed that the vanadium complex resulted in a catalysis that causes most
of the O2 to be reduced by four-electrons. The ratio of the ring to disk
currents (iR2/iD) in Figure 6(b), normalized for the perturbed collection
efficiency of the ring under O2 (iR1/iD), corresponds to the reduction of
more than 90% of the O2 molecules to H2O. (b) Haberland, D.; Landsberg,
R. Z Electrochem. 1966, 70, 724.
(23) The produced H2O was detected by 1H-NMR (CDCl3, δ ) 1.5 ppm)
as follows. The electrolyte solution as in Figure 6 was subject to controlled
potential reduction at 0.1 V with a carbon felt electrode to allow catalytic
electroreduction of O2. As a pretreatment for NMR measurement, the
resulting solution was oxidized at 0.7 V to diminish the paramagnetic
vanadium(IV) species. Analysis for H2O in the resulting solution was
performed by NMR with CDCl3 as a solvent, which revealed that a
reasonable amount of H2O was formed.
(22) Cyclic voltammograms for a solution of I containing aqueous H2O2
in acidic CH2Cl2 under argon atmosphere showed no catalytic current for
the reduction of H2O2.

12672 J. Am. Chem. Soc., Vol. 118, No. 50, 1996
Yamamoto et al.
O2 and deoxygenation of III and IV by protons are suggested
as being a key to the performance of these complexes as catalyst
for reduction of O2 to H2O. The catalytic cycle involves one-
step two-electron transfer ([(salen)VIVOVV(salen)]3+ + 2e- a
[(salen)VIVOVIII(salen)]+) which would also have essential roles
on the catalysis. It may safely be stated that the transfer of
multiple isoenergetic electrons is advantageous in the redox
reaction: the two-electron oxidation of diphenyl disulfide
produces phenyl sulfenyl cation as an active species of the
oxidative polymerization. It is a unique model of a catalyst
operating in the four-electron reduction of O2 to H2O in acidic
organic media.
Corporation for his competent assistance in crystallography. This
work was partially supported by the Grant-in-Aid for Scientific
Research (Nos. 07215282, 07651006, and 08875188) and
International Scientific Research Program (Joint Research No.
08044174) from the Ministry of Education, Science, Sports and
Culture, Japan.
Supporting Information Available: Tables giving atomic
coordinates, equivalent isotropic thermal parameters, and aniso-
tropic displacement parameters for III and IV (12 pages). See
any current masthead page for ordering and Internet access
instructions.
Acknowledgment. We thank Prof. Fred Anson of Caltech
for helpful discussions. We also thank Dr. M. Shiro of Rigaku
JA9617799