Oxovanadium(IV) and -(V) complexes of dithiocarbazate-based tridentate schiff base ligands: Syntheses, structure, and photochemical reactivity of compounds involving imidazole derivatives as coligands

Article abstract of DOI:10.1021/ic020438v

The tridentate dithiocarbazate-based Schiff base ligands H₂L (S-methyl-3-((5-R-2-hydroxyphenyl)methyl)dithiocarbazate, R = NO₂, L = L²; R = Br, L = L³) react with [VO(acac)₂] in the presence of imidazole derivatives as coligands to form oxovanadium(IV) and cis-dioxovanadium(V) complexes. With benzimidazole and N-methylimidazole, the products are oxovanadium(IV) complexes, viz. [VOL³(Bzlm)]·0.5CH₃CN (1a) and [VOL(N-Melm)₂] (L = L³, 1b; L = L², 1c), respectively. In both 1a,b, the O and S donor atoms of the tridentate ligand are cis to the terminal oxo group (in the equatorial plane) and mutually trans, but the N donor atom is respectively cis and trans to the oxo atom, as revealed from X-ray crystallography. When imidazole or 4-methylimidazole is used as the ancillary ligand, the products obtained are water-soluble cis-dioxovanadium(V) complexes [VO₂L(R′-ImH)] (L = L³ and L², R′ = H and Me, 2a-d). These compounds have zigzag chain structures in the solid state as confirmed by X-ray crystallographic investigations of 2a,d, involving an alternating array of LVO₂^- species and the imidazolium counterions held together by Coulombic interactions and strong hydrogen bonding. Complexes 2a-d are stable in water or methanol. In aprotic solvents, viz. CH₃CN, DMF, or DMSO, however, they undergo photochemical transformation when exposed to visible light. The putative product is a mixed-oxidation divanadium(IV/V) species obtained by photoinduced reduction as established by EPR, electronic spectroscopy, and dynamic ¹H NMR experiments.

Full text of DOI:10.1021/ic020438v

Inorg. Chem. 2003, 42, 1508−1517
Oxovanadium(IV) and -(V) Complexes of Dithiocarbazate-Based
Tridentate Schiff Base Ligands: Syntheses, Structure, and
Photochemical Reactivity of Compounds Involving Imidazole Derivatives
as Coligands
Satyabrata Samanta,† Dipesh Ghosh,† Suman Mukhopadhyay,† Akira Endo,‡
,†
Timothy J. R. Weakley,§ and Muktimoy Chaudhury*
Department of Inorganic Chemistry, Indian Association for the CultiVation of Science, Kolkata
700 032, India, Department of Chemistry, Faculty of Science and Technology, Sophia UniVersity,
7-1 Kioi-cho, Chiyoda-ku, Tokyo 102-8554, Japan, and Department of Chemistry, UniVersity of
Oregon, Eugene, Oregon 97403
Received July 8, 2002
The tridentate dithiocarbazate-based Schiff base ligands H L(S-methyl-3-((5-R-2-hydroxyphenyl)methyl)dithiocarbazate,
2
R ) NO , L ) L2; R ) Br, L ) L3) react with [VO(acac)2] in the presence of imidazole derivatives as coligands
2
to form oxovanadium(IV) and cis-dioxovanadium(V) complexes. With benzimidazole and N-methylimidazole, the
products are oxovanadium(IV) complexes, viz. [VOL3(BzIm)]‚0.5CH CN (1a) and [VOL(N-MeIm)2] (L ) L3, 1b; L )
3
L2, 1c), respectively. In both 1a,b, the O and S donor atoms of the tridentate ligand are cis to the terminal oxo
group (in the “equatorial” plane) and mutually trans, but the N donor atom is respectively cis and trans to the oxo
atom, as revealed from X-ray crystallography. When imidazole or 4-methylimidazole is used as the ancillary ligand,
the products obtained are water-soluble cis-dioxovanadium(V) complexes [VO L(R′-ImH)] (L ) L3 and L2, R′ ) H
2
and Me, 2a−d). These compounds have zigzag chain structures in the solid state as confirmed by X-ray
crystallographic investigations of 2a,d, involving an alternating array of LVO - species and the imidazolium counterions
2
held together by Coulombic interactions and strong hydrogen bonding. Complexes 2a−d are stable in water or
methanol. In aprotic solvents, viz. CH CN, DMF, or DMSO, however, they undergo photochemical transformation
3
when exposed to visible light. The putative product is a mixed-oxidation divanadium(IV/V) species obtained by
photoinduced reduction as established by EPR, electronic spectroscopy, and dynamic 1H NMR experiments.
Introduction
indicated the presence of imidazole moiety (histidine) in the
vanadium active sites of these enzymes. In aqueous solutions,
the equilibrium and kinetics studies of many vanadate
reactions are found to be significantly altered by the presence
of imidazole in the reaction media.11,12 Also many phospho-
rylase enzymes are known to contain histidine residues that
The identities of vanadium biochromophores in haloper-
oxidase enzymes have been under close scrutiny by various
spectroscopic investigations.1-10 Many of these studies have
* To whom correspondence should be addressed. E-mail: icmc@
mahendra.iacs.res.in.
† Indian Association for the Cultivation of Science.
‡ Sophia University.
(6) Chasteen, N. D. In Biological Magnetic Resonance; Berliner, L. J.,
Reuben, J., Eds.; Plenum Press: New York, 1981; Vol. 3, pp 53-
119.
§ University of Oregon.
(1) (a) Vanadium in Biological Systems; Chasteen, N. D., Ed.; Kluwer
Academic Publishers: Dordrecht, The Netherlands, 1990. (b) Butler,
A.; Carrano, C. J. Coord. Chem. ReV. 1991, 109, 61. (c) Rehder, D.
Angew. Chem., Int. Ed. Engl. 1991, 30, 148.
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1991, 41, 171.
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Collison, D.; Garner, C. D.; Wever, R. FEBS Lett. 1988, 235, 93.
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Biochemistry 1989, 28, 7968.
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1508 Inorganic Chemistry, Vol. 42, No. 5, 2003
10.1021/ic020438v CCC: $25.00 © 2003 American Chemical Society
Published on Web 02/12/2003

OxoWanadium(IV) and -(V) Complexes
propriate reagents29 and distilled under nitrogen prior to their use.
All other chemicals were commercially available and used as
received.
coordinate to vanadium center.13 All these have generated
sufficient interest in recent times to understand the coordina-
tion chemistry of vanadium in biologically relevant ligand
environments involving imidazole as donor.14-18
Preparation of Complexes. [VOL3(BzIm)]‚0.5CH3CN (1a). To
an acetonitrile solution (15 mL) of [VO(acac)2] (0.27 g, 1 mmol)
was added 0.31 g of the ligand H2L3 (1 mmol) in 10 mL of the
same solvent. It was refluxed for 15 min to get a brown solution.
To this was then added an aqueous solution (15 mL) of benzimi-
dazole (0.14 g, 1.2 mmol), and the resulting mixture was further
refluxed for an additional 2 h when a deep green solution was
obtained. It was filtered and cooled at 0 °C for an overnight period
when a green microcrystalline product was deposited. The com-
pound was collected by filtration, washed with diethyl ether (4 ×
10 mL), and finally dried in vacuo. The product was recrystallized
from acetonitrile solvent. Yield: 0.22 g (44%). Anal. Calcd for
C17H14.5BrN4.5O2S2V: C, 40.11; H, 2.85; N, 12.38. Found: C, 39.9;
H, 2.8; N, 12.2. IR (KBr disk, cm-1): ν(C-N), 1598 s; ν(C-O/
phenolate), 1528 s; ν(VdOt), 960 s.
Over the past few years we have been working on the
coordination chemistry of oxovanadium species, using sulfur
containing tri- and tetradentate ligands.19-23 The use of
tridentate ligands in oxometalate chemistry has an intrinsic
advantage because of their ability to form the MOL primary
core, leaving open at least one or more coordination site(s)
for the acceptance of ancillary ligands to complete the
coordination geometry.24,25 We have exploited these coor-
dination possibilities for the syntheses of structurally and
kinetically interesting mono-, bi-, and polynuclear oxo-
vanadium complexes using dithiocarbazate-based Schiff base
molecules (H2L, L ) L1-L3) as tridentate ligands.19,20,22,23
[VOL3(N-MeIm)2] (1b). A solution of [VO(acac)2] (0.27 g, 1
mmol) in acetonitrile (20 mL) was added to an equimolar amount
of the ligand H2L3 (0.31 g, 1 mmol) taken in the same solvent (15
mL) and refluxed for 10 min. To the resulting brown solution was
added an aqueous solution (10 mL) of N-methylimidazole (0.18 g,
2.2 mmol), and the resulting solution was heated again at reflux
temperature for ca. 4 h to get a clear brown solution. It was filtered,
and the filtrate volume was reduced to ca. 20 mL by rotary
evaporation and finally cooled in a freezer at 0 °C for an overnight
period. A brown crystalline compound that deposited at this stage
was collected by filtration, washed with diethyl ether (4 × 10 mL),
and dried in vacuo. The product was recrystallized from acetonitrile
solvent. Yield: 0.26 g (48%). Anal. Calcd for C17H19BrN6O2S2V:
C, 38.20; H, 3.56; N, 15.73. Found: C, 37.6; H, 3.6; N, 15.2. IR
(KBr disk, cm-1): ν(C-N), 1592 s; ν(C-O/phenolate), 1536 s;
ν(VdOt), 954 s.
[VOL2(N-MeIm)2] (1c). This compound was prepared by
following essentially the same procedure as described above for
1b using H2L2 as the tridentate ligand. The brown crystalline product
was obtained in 54% yield. Anal. Calcd for C17H19N7O4S2V: C,
40.80; H, 3.80; N, 19.60. Found: C, 41.1; H, 3.7; N, 19.3. IR (KBr
disk, cm-1): ν(C-N), 1595 s; ν(C-O/phenolate), 1537 s;
ν(VdOt), 951 s.
[VO2L3(ImH)]∞ (2a). To a solution of [VO(acac)2] (0.27 g, 1
mmol) in acetonitrile (15 mL) was added stoichiometric amount
(1:1 mole ratio) of the ligand H2L3 (0.31 g), also taken in the same
solvent (15 mL). The solution was refluxed for 10 min and then
cooled to room temperature to get a brown solution. To this was
then added an aqueous solution (7 mL) of imidazole (0.08 g, 1.18
mmol), and the resulting mixture was further refluxed for 1 h. The
resulting green solution obtained at this stage was filtered and kept
in the air for several days becoming gradually yellow in color. It
was filtered, the filtrate volume was reduced to ca. 15 mL by rotary
Herein, we report the syntheses of oxovanadium(IV) and
-(V) complexes of the aforesaid tridentate ligands in the
presence of various imidazole derivatives as coligands.
Electronic and molecular structures of these compounds have
been examined in details. Photochemical reduction of the
cis-dioxovanadium(V) complexes into mixed-oxidation di-
vanadium(IV/V) products is also established.
Experimental Section
Materials. The tridentate ligands (H2L2 and H2L3)26,27 and the
precursor complex [VO(acac)2]28 were prepared by following the
reported methods. Reagent grade solvents were dried from ap-
(13) Lindqvist, Y.; Schneider, G.; Vihko, P. Eur. J. Biochem. 1994, 221,
139.
(14) Cornmann, C. R.; Kampf, J.; Pecoraro, V. L. Inorg. Chem. 1992, 31,
1981.
(15) Cornmann, C. R.; Kampf, J.; Lah, M. S.; Pecoraro, V. L. Inorg. Chem.
1992, 31, 2035.
(16) Calviou, L. J.; Arber, J. M.; Collison, D.; Garner, C. D.; Clegg, W. J.
Chem. Soc., Chem. Commun. 1992, 654.
(17) Keramidas, A. D.; Miller, S. M.; Anderson, O. P.; Crans, D. C. J.
Am. Chem. Soc. 1997, 119, 8901.
(18) Crans, D. C.; Keramidas, A. D.; Hoover-Litty, H.; Anderson, O. P.;
Miller, M. M.; Lemoine, L. M.; Pleasic-Williams, S.; Vandenberg,
M.; Rossomando, A. J.; Sweet, L. J. J. Am. Chem. Soc. 1997, 119,
5447.
(19) Dutta, S. K.; Kumar, S. B.; Bhattacharyya, S.; Tiekink, E. R. T.;
Chaudhury, M. Inorg. Chem. 1997, 36, 4954.
(20) Dutta, S. K.; Samanta, S.; Kumar, S. B.; Han, O. H.; Burckel, P.;
Pinkerton, A. A.; Chaudhury, M. Inorg. Chem. 1999, 38, 1982.
(21) Bhattacharyya, S.; Mukhopadhyay, S.; Samanta, S.; Weakley, T. J.
R.; Chaudhury, M. Inorg. Chem. 2002, 41, 2433.
(22) Dutta, S. K.; Samanta, S.; Mukhopadhyay, S.; Burckel, P.; Pinkerton,
A. A.; Chaudhury, M. Inorg. Chem. 2002, 41, 2946.
(23) Dutta, S. K.; Samanta, S.; Ghosh, D.; Butcher, R. J.; Chaudhury, M.
Inorg. Chem. 2002, 41, 5555.
(24) Craig, J. A.; Harlan, E. W.; Snyder, B. S.; Whitener, M. A.; Holm, R.
H. Inorg. Chem. 1989, 28, 2082.
(25) Topich, J.; Lyon, J. T., III. Inorg. Chem. 1984, 23, 3202.
(26) Dutta, S. K.; Tiekink, E. R. T.; Chaudhury, M. Polyhedron 1997, 16,
1863.
(27) Abbreviations used: H2L1, S-methyl-3-((2-hydroxyphenyl)methyl)-
dithiocarbazate; H2L2, S-methyl-3-((5-nitro-2-hydroxyphenyl)methyl)-
dithiocarbazate; H2L3, S-methyl-3-((5-bromo-2-hydroxyphenyl)methyl)-
dithiocarbazate; Hacac, acetylacetone; Im, imidazole; BzIm, benzimida-
zole; N-MeIm, N-methylimidazole; TEAP, tetraethylammonium per-
chlorate.
(28) Rowe, R. A.; Jones, M. M. Inorg. Synth. 1957, 5, 113.
(29) Perrin, D. D.; Armarego, W. L. F.; Perrin, D. R. Purification of
Laboratory Chemicals, 2nd ed.; Pergamon: Oxford, England, 1980.
Inorganic Chemistry, Vol. 42, No. 5, 2003 1509

Samanta et al.
Table 1. Relevant Crystal Data for [VOL3(BzIm)]‚0.5CH3CN (1a), [VOL3(N-MeIm)2] (1b), [VO2L3(ImH)]∞ (2a), and [VO2L2(4-MeImH)]∞ (2d)
1a
1b
2a
2d
composn
fw
C17H14.5BrN4.5O2S2V
508.8
C17H19BrN6O2S2V
534.3
C12H12BrN4O3S2V
455.22
C13H14N5O5S2V
435.4
space group
orthorhombic
P21212 (No. 18)
15.4564(11)
31.732(2)
monoclinic
P21/c (No. 14)
9.1958(18)
20.970(7)
12.1575(27)
111.51(2)
2181(1)
monoclinic
P21/a (No. 14)
6.6680(3)
13.968(2)
18.441(3)
93.30(3)
1714(1)
monoclinic
C2/c (No. 15)
22.401(4)
12.650(3)
13.836(9)
115.48(4)
3539(3)
8
1.634
23
0.710 73
8.32
a, Å
b, Å
c, Å
â, deg
V, Å3
Z
8.3310(2)
4085.9(7)
8
1.654
23
0.710 73
26.7
4
4
F
calcd, g cm-3
1.627
1.764
T, °C
22
22
λ, Å
0.710 73
25.1
0.710 73
31.2
µ, cm-1
rel transm coeff(ψ scans)
no. obsd reflcns [I g σ(I)]
tot. indepdt reflcns
R(F), wR(F) (obsd data)a
R(F2), wR(F2) (all data)a
0.795-1.000
0.670-1.000
0.675-1.000
0.89-1.00
2438
2493
2090
2465
4050
3725
2996
3115
0.059, 0.052
0.102, 0.105
0.058, 0.046
0.084, 0.094
0.055, 0.057
0.091, 0.110
0.041, 0.042
0.063, 0.085
2
2
4
a R ) ∑||F0| - |Fc||/∑|F0|; wR(F2) ) [∑w(|F0| - |Fc| )2/∑w|F0| ]1/2
.
evaporation, and finally cooled in a refrigerator at 4 °C for an
overnight period to get a yellow crystalline product. It was collected
by filtration, washed with diethyl ether, and finally recrystallized
from methanol/water (3:1 v/v) mixture. Yield: 0.27 g (60%). Anal.
Calcd for C12H12BrN4O3S2V: C, 31.65; H, 2.64; N, 12.31. Found:
C, 32.2; H, 2.6; N, 12.2. IR (KBr disk, cm-1): ν(C-N), 1592 s;
ν(C-O/phenolate), 1537 s; ν(VdOt) 932 s, 835 s.
electrodes were used for these measurements. The ferrocenium/
ferrocene (Fc+/Fc) couple was used as the internal standard.
X-ray Crystallography. Diffraction-quality crystals of [VOL3-
(BzIm)]‚0.5CH3CN (1a) (green prism, 0.05 × 0.14 × 0.47 mm
obtained from CH3CN solution), [VOL3(N-MeIm)2] (1b) (dark red-
brown tablet, 0.21 × 0.41 × 0.43 mm obtained from CH3CN/CH3-
OH (1:1 v/v) mixture), [VO2L3(ImH)]∞ (2a) (yellow plate, 0.05 ×
0.36 × 0.58 mm obtained from CH3OH/H2O mixture), and [VO2L2-
(4-MeImH)]∞ (2d) (amber prism, 0.20 × 0.23 × 0.43 mm obtained
from CH3CN/CH3OH (1:1 v/v) solution)) were obtained from the
designated solvents by slow evaporation. The orientation parameters
and cell dimensions were obtained from the setting angles of an
Enraf-Nonius CAD-4 diffractometer fitted with graphite-mono-
chromatized Mo KR radiation for 25 centered reflections in the
range 13.2° e θ e 14.7°, 13.3° e θ e 14.9°, 12.9° e θ e 14.8°,
and 12.2° e θ e 14.5° for 1a, 1b, 2a, and 2d, respectively. The
crystal data and the final residuals are summarized in Table 1.
Absorption corrections based on azimuthal scans (“ψ-scans”) were
applied.
[VO2L2(ImH)]∞ (2b). This complex was prepared from [VO-
(acac)2] (0.27 g, 1 mmol), ligand H2L2 (0.27 g, 1 mmol), and
imidazole (0.08 g, 1.18 mmol) using the methodology analogous
to that described above for 2a. Yield: 54%. Anal. Calcd for
C12H12N5O5S2V: C, 34.20; H, 2.85; N, 16.63. Found: C, 34.3; H,
2.9; N, 17.0. IR (KBr disk, cm-1): ν(C-N), 1605 s; ν(C-O/
phenolate), 1533 s; ν(VdOt), 958 s, 842 s.
[VO2L3(4-MeImH)]∞ (2c). This compound was synthesized
using a similar method as employed for 2a by using 4-methylimi-
dazole instead of imidazole. Yield: 49%. Anal. Calcd for C13H14-
BrN4O3S2V: C, 33.26; H, 2.98; N, 11.94. Found: C, 33.5; H, 3.0;
N, 12.0. IR (KBr disk, cm-1): ν(C-N), 1598 s; ν(C-O/phenolate),
1536 s; ν(VdOt), 915 s, 820 s.
For 1a, the only systematic absences were: hk0, h odd, and 0k0,
k odd. The implied symmetry elements in turn required a glide
plane normal to a, although there were no absences in the 0kl
reflections. It was assumed that the h00 absences, but not the
apparent hk0 absences, were genuine, i.e., that the space group was
P21212. A SIR 92 E-map31 then showed all non-hydrogen atoms of
two independent but very similar molecules with coordinates of
the corresponding atoms related approximately as x, y, z and 0.5 +
x, y, 0.40 - z (i.e. by a pseudo a glide normal to c). The C and N
atoms of two independent CH3CN molecules on crystal diad axes
were refined isotropically. Their parameters had to be fixed in the
last cycles to ensure convergence.
[VO2L2(4-MeImH)]∞ (2d). This compound was obtained in 51%
yield by following the same procedure as for 2a but using H2L2
and 4-methylimidazole as the ligands. Anal. Calcd for C13H14N5-
O5S2V: C, 35.86; H, 3.22; N, 16.09. Found: C, 36.1; H, 3.3; N,
16.2. IR (KBr disk, cm-1): ν(C-N), 1600 s; ν(C-O/phenolate),
1553 s; ν(VdOt), 953 s, 833 s.
Physical Measurements. Details of elemental (C, H, and N)
analyses and IR and UV-vis spectral measurements were described
1
elsewhere.20,30 The H NMR spectra were recorded on a Bruker
model Avance DPX 300 spectrometer. Magnetic moments of the
powdered polycrystalline samples at room temperature were
calculated from the data obtained on a PAR 155 vibrating-sample
magnetometer. X-band EPR spectra in solution at room temperature
and in the frozen state (77 K) were recorded on a Bruker model
ESP 300 E spectrometer equipped with standard Bruker attachments
and also on a JES-3X spectrometer equipped with a JEOL
ES-PRIT 330 data processing system. Cyclic voltammetric mea-
surements were performed on a PAR electrochemical analyzer
(model 250/5/0) in dry acetonitrile under purified dinitrogen with
TEAP as the supporting electrolyte. A glassy carbon working, a
platinum wire counter and saturated calomel reference (SCE)
For 1b and 2a,d, the systematic absences together with the centric
distribution of intensities indicated the space groups P21/c, P21/a,
and C2/c, respectively. Absorption corrections based on azimuthal
scans (“ψ-scans”) were applied. A SIR 92 E-map31 showed all the
non- hydrogen atoms.
Hydrogen atoms were included at positions recalculated after
each cycle of refinement [B(H) ) 1.2Beq(C); d(C-H) ) 0.95 Å].
The final difference syntheses were featureless in all cases. The
TEXSAN program suite32 was used in all calculations.
(30) Bhattacharyya, S.; Weakley, T. J. R.; Chaudhury, M. Inorg. Chem.
1999, 38, 633.
(31) Altomare, A.; Cascarano, G.; Giacovazzo, C.; Burla, M. C.; Polidori,
G.; Camalli, N. J. Appl. Crystallogr. 1994, 24, 435.
1510 Inorganic Chemistry, Vol. 42, No. 5, 2003

OxoWanadium(IV) and -(V) Complexes
Scheme 1a
Figure 1. ORTEP diagram of [VOL3(BzIm)]‚0.5CH3CN (1a) with 30%
thermal probability ellipsoids showing the atomic numbering scheme.
CH3CN is omitted for clarity.
the protonated imidazolium counterions, held together by
strong Coulombic and hydrogen bonding interactions as
revealed from X-ray crystallography (see later). A similar
strategy of bonding interactions has been systematically
utilized in crystal engineering to build up organic supra-
molecular architectures.33
Compound 1a, when allowed to undergo aerial oxidation
in aqueous acetonitrile medium, generated after ca. 1 week
time a greenish yellow solution, from which we were unable
to isolate the oxidized cis-dioxo product in analytically pure
form. Both 1b,c, on the other hand, are resistant to similar
aerial oxidation under comparable experimental conditions.
Thus, the basicity in the reaction medium, as influenced by
the ancillary imidazole ligand, is a relevant factor in the
a Key: (i) 1:1:1 molar proportions of [VO(acac)2], H2L3, and benzimi-
dazole; (ii) 1:1:2 molar proportions of [VO(acac)2], H2L2 (or H2L3), and
N-methylimidazole; (iii) 1:1:1 molar proportions of [VO(acac)2], H2L2 (or
H2L3), and imidazole (or 4-methylimidazole). All reactions were carried
out in refluxing CH3CN/H2O mixture. In reaction iii, prolonged exposure
to atmospheric oxygen is required to get the cis-dioxovanadium(V) products.
+
oxidation to VO2 species.34
Photochemical Study. Clear yellow solutions of the complexes
(2a-d) in CH3CN were placed in quartz cell and purged with argon
for 20 min. A tungsten filament lamp (60 W) was used as a visible
light source to irradiate these solutions which gradually turned green
with the passage of time during the photolysis. The green solutions
obtained after different intervals of exposure time were used for
subsequent characterization.
IR spectra of the complexes are summarized in the
Experimental Section. In each case, the spectrum displays
the characteristic bands of the coordinated tridentate ligands.19
In addition, a strong two-band pattern observed for 2a-d in
the 960-840 cm-1 region is the signature of the cis-
dioxovanadium(V) moiety.35 A single sharp band due to
VdOt stretch is, however, observed at ca. 955 cm-1 for the
vanadyl complexes (1a-c).
Results and Discussion
Syntheses. The synthetic strategy is outlined in Scheme
1. The oxovanadium(IV) complexes (1a-c) have been
isolated as shining crystals, green to deep brown in color, in
moderate yields (44-54%) by metathetical reactions involv-
ing stoichiometric amounts of [VO(acac)2], the tridentate
ligand (H2L2 or H2L3), and the respective imidazole base
(benzimidazole for 1a and N-methylimidazole for 1b,c) in
acetonitrile/water medium at reflux temperature. When
imidazole or 4-methylimidazole are used as the ancillary
base, the green oxovanadium(IV) compounds that formed
initially do not crystallize out because of their higher
solubilities and undergo slow oxidation when allowed to
stand in the air for several days. The products (2a-d)
obtained are yellow crystalline solids with cis-dioxovana-
dium(V) center [LVO2(R′-ImH)]∞. The mandatory steps in
the preparative procedure of 2a-d are the presence of water
in the reaction media and their subsequent exposure to
atmospheric oxygen as confirmed by several control experi-
ments. The products obtained are water-soluble compounds
with zigzag chain structure and involve an alternating array
Description of Crystal Structures. Figures 1 and 2
display the perspective views of the vanadyl complexes 1a,b,
respectively. Their selected metrical parameters are given
in Table 2. The asymmetric unit of 1a contains two
independent molecules; however, there are only minor
conformational differences between them. The unit cell
comprises eight molecules. The vanadium atom in each
molecule is five-coordinate, existing in a distorted square
pyramidal geometry in which the basal plane is defined by
S(1), N(2), and O(2) atoms, derived from the tridentate
ligand, and the N(3) atom from benzimidazole, the mono-
dentate ancillary ligand. The apical position is occupied by
the oxo atom O(1). The deviations from the least-squares
plane through the S(1), N(2), O(2), and N(3) atoms compris-
ing the square plane are -0.016(4), 0.093(10), -0.067(8),
and 0.094(10) Å, respectively and the vanadium atom lies
0.639 Å out of this plane toward the apical oxo atom O(1).
The trans O(2)-V-S(1) and N(2)-V-N(3) angles are
142.0(3)° [141.7(3)° for the second molecule] and 148.9-
-
of the anionic cis-dioxovanadium(V) [LVO2 ] species and
(33) Fe´lix, O.; Hosseini, M. W.; De Cian, A.; Fischer, J. Angew. Chem.,
Int. Ed. Engl. 1997, 36, 102.
(32) TeXsan Software for Single-Crystal Structure Analysis, version 1.7;
Molecular Structure Corp.: 3200A Research Forest Drive, The
Woodlands, TX 77381, 1997.
(34) Ghosh, S.; Nanda, K. K.; Addison, A. W.; Butcher, R. J. Inorg. Chem.
2002, 41, 2243.
(35) Li, X.; Lah, M. S.; Pecoraro, V. L. Inorg. Chem. 1988, 27, 4657.
Inorganic Chemistry, Vol. 42, No. 5, 2003 1511

Samanta et al.
Figure 2. ORTEP diagram of [VOL3(N-MeIm)2] (1b) with 30% thermal
probability ellipsoids showing the atomic numbering scheme.
Figure 3. Molecular structure and atom-numbering scheme for the
monomeric unit of [VO2L3(ImH)]∞ (2a) with thermal ellipsoids drawn at
the 30% probability level.
Table 2. Selected Bond Lengths (Å) and Angles (deg) for Complexes
1a,b
1a
1b
Bond Lengths
V-O(1)
V-S(1)
V-O(2)
V-N(2)
V-N(3)
V-N(5)
1.599(8), 1.609(8)
2.359(4), 2.348(4)
1.896(7), 1.911(7)
2.056(10), 2.073(9)
2.071(10), 2.063(9)
1.603(3)
2.423(2)
1.938(3)
2.238(4)
2.114(4)
2.119(4)
Bond Angles
O(2)-V-S(1)
N(2)-V-N(3)
O(1)-V-O(2)
O(1)-V-N(2)
O(1)-V-S(1)
O(1)-V-N(3)
O(2)-V-N(2)
N(2)-V-S(1)
S(1)-V-N(3)
N(3)-V-O(2)
N(5)-V-N(3)
N(5)-V-O(1)
N(5)-V-N(2)
N(5)-V-S(1)
N(5)-V-O(2)
142.0(3), 141.7(3)
148.9(4), 149.9(4)
108.5(4), 108.5(4)
104.6(4), 104.7(4)
109.3(4), 109.7(4)
106.2(4), 105.2(4)
87.3(4), 87.2(3)
79.3(3), 80.0(3)
86.7(3), 87.7(3)
86.9(4), 85.6(3)
159.79(9)
88.2(1)
106.2(1)
170.6(2)
94.0(1)
92.9(2)
83.2(1)
76.6(1)
89.8(1)
89.7(1)
173.8(2)
93.0(2)
86.2(1)
91.6(1)
86.9(1)
Figure 4. Molecular structure and atom-numbering scheme for the
monomeric unit of [VO2L2(4-MeImH)]∞ (2d) with thermal ellipsoids drawn
at the 30% probability level.
Å. Of the three V-N distances in this molecule, the V-N(2)
bond length (2.238(4) Å) has undergone a modest elongation
due to trans labilizing influence of the terminal oxo group
as observed in other oxovanadium complexes.37 While N(5)-
V-N(3) trans angle 173.8(2)° is close to the one required
for an ideal square plane, some distortion is manifested in
the O(2)-V-S(1) angle of 159.79(9)°, due to the restricted
bite angle of the tridentate ligand.
The molecular structures of 2a,d are shown in Figures 3
and 4, respectively. Important interatomic parameters are
listed in Table 3. Strong hydrogen bonds link the imidazo-
lium cations and LVO2- anions in both structures into zigzag
chains, along the crystal screw axis in 2a (Figure 5) and
along the c glide direction in 2d (Figure 6).
(4)° [149.9(4)°], respectively. The VdOt distance 1.599(8)
Å [1.609(8) Å] is not exceptional.36
Compound 1b crystallizes in a monoclinic space group
P21/c with four molecules accommodated in the unit cell. It
has an octahedral geometry (Figure 2). Two terminal donor
atoms O(2) and S(1) from the tridentate ligand along with
N(3) and N(5) atoms from the monodentate N-methylimi-
dazole molecules define the equatorial plane and lie -0.153-
(3), -0.116(2), 0.134(4), and 0.135(4) Å, respectively, out
of the least-squares plane through them. The terminal oxo
atom O(1) along with N(2) from the tridentate ligand occupy
the axial positions and form an O(1)-V-N(2) angle of
170.6(2)°. The V atom is displaced from the mean equatorial
plane toward the vanadyl oxygen atom (O(1)) by 0.241(1)
In each case, the vanadium(V) centers exist in a distorted
square pyramidal geometry (Figures 3 and 4), with four basal
(36) (a) Vlahos, A. T.; Tolis, E. I.; Raptopoulou, C. P.; Tsohos, A.; Sigalas,
M. P.; Terzis, A.; Kabanos, T. A. Inorg. Chem. 2000, 39, 2977. (b)
Tsaramyrsi, M.; Kaliva, M.; Salifoglou, A.; Raptopoulou, C. P.; Terzis,
A.; Tangoulis, V.; Giapintzakis, J. Inorg. Chem. 2001, 40, 5772.
(37) (a) Vergopoulos, V.; Priebsch, W.; Fritzsche, M.; Rehder, D. Inorg.
Chem. 1993, 32, 1844. (b) Nakajima, K.; Kojima, M.; Toriumi, K.;
Saito, K.; Fujita. J. Bull. Chem. Soc. Jpn. 1989, 62, 760.
1512 Inorganic Chemistry, Vol. 42, No. 5, 2003

OxoWanadium(IV) and -(V) Complexes
Figure 5. Extended chain structure of [VO2L3(ImH)]∞ (2a) due to
hydrogen bonding along the crystal screw axis.
Figure 6. Extended chain structure of [VO2L2(4-MeImH)]∞ (2d) due to
hydrogen bonding along the c-glide plane.
Table 3. Selected Bond Lengths (Å) and Angles (deg) for the
Complexes 2a,d
2a
2d
Bond Lengths
2.364(2)
V-S(1)
V-O(1)
V-O(2)
V-O(3)
V-N(2)
2.342(1)
1.666(2)
1.604(2)
1.902(2)
2.176(2)
1.643(3)
1.619(3)
1.888(4)
2.174(4)
Bond Angles
87.2(1)
S(1)-V-O(1)
S(1)-V-O(2)
S(1)-V-O(3)
S(1)-V-N(2)
O(1)-V-O(2)
O(1)-V-O(3)
O(1)-V-N(2)
O(2)-V-O(3)
O(2)-V-N(2)
O(3)-V-N(2)
87.98(8)
107.97(8)
139.72(7)
77.45(6)
107.3(1)
97.94(9)
156.9(1)
108.1(1)
94.38(9)
82.50(8)
106.4(1)
143.4(1)
76.5(1)
107.2(2)
96.9(2)
151.4(2)
107.0(2)
99.9(2)
Figure 7. Hydrogen-bonded chain of [VO2L3(ImH)]∞ (2a) projected down
the crystal screw axis, showing the nearly eclipsed imidazole rings.
are symmetrically attached to NH protons of the two
neighboring imidazolium moieties (Figure 5), the corre-
sponding mode of attachment of the L2VO2- units in 2d is,
however, asymmetric and takes place through the participa-
tion of a single terminal oxo group (the equatorial one),
keeping the other free (Figure 6). The presence of a methyl
group in the imidazole moiety seems to influence this
asymmetric binding in 2d. In 2a, the two terminal oxo atoms
attached to the NH protons of an imidazolium moiety are
not exactly the same. While one of these is an apical oxo
atom, the other happens to be an equatorial one coming from
the immediately neighboring vanadium center. Since in an
LVO2- unit these two oxo atoms are nearly orthogonal (O-
V-O angle being 107° (Table 3)), that allows the neighbor-
ing imidazole moieties to nearly eclipse one another when
viewed down the crystal screw axis (Figure 7).
83.4(2)
Hydrogen Bond Geometry
A
H
B
A‚‚‚B
A-H
H‚‚‚B
A-H‚‚‚B
2a N(3) H(8)
O(2) 2.743(5) 0.9 5
O(1) 2.709(5) 0.9 5
2d N(4) H(11) O(1) 2.744(3) 0.9 3(4) 1.83(4)
N(5) H(9) O(1) 2.931(4) 0.8 1(4) 2.13(4)
1.82
1.78
162.7
164.1
168(4)
166(4)
N(4) H(9)
positions being occupied by the donors S(1), N(2), and O(3)
from the coordinated tridentate ligands and one of the
terminal oxo groups O(1). The axial site is taken up by the
remaining oxo atom O(2) of the VO2 core. In 2a, O(2) forms
angles in the range 99.9(2)-107.2(2)° with the basal plane
(corresponding angles for 2d vary between 94.38(9) and
108.1(1)°). The trans angles O(1)-V-N(2) 151.4(2)° [156.9-
(1)°] and S(1)-V-O(3) 143.4(1)° [139.72(7)°] are reason-
ably compressed, and the central vanadium atom is shifted
by 0.541(1) Å [0.558(1) Å] from the basal plane toward the
apical oxygen atom.
Details of the hydrogen bond geometry in 2a,d are also
shown in Table 3.
Magnetism and EPR. Oxovanadium(IV) complexes 1a-c
are magnetically straightforward. At room temperature (298
K) the observed values of moments 1.69-1.71 µB (Table 4)
are as expected for a simple S ) 1/2 paramagnet with dxy-
based ground state.39
An interesting structural feature of these molecules is the
-
presence of a bridging LVO2 unit which behaves like an
inorganic analogue of a bridging carboxylate group.38 While
in 2a the individual terminal oxo atoms of the L3VO2- units
(38) Giacomelli, A.; Floriani, C.; De Souza Duarte, A. O.; Chiesi-Villa,
(39) Collison, D.; Gahan, B.; Garner, C. D.; Mabbs, F. E. J. Chem. Soc.,
Dalton Trans. 1980, 667.
A.; Guastini, C. Inorg. Chem. 1982, 21, 3310.
Inorganic Chemistry, Vol. 42, No. 5, 2003 1513

Samanta et al.
Table 4. Magnetic Moments and the Spin-Hamiltonian Parameters for
the Vanadium(IV) Complexes (1a-c) and the Photoreduced Products
(2A,D)
Table 5. Summary of Electronic Spectroscopic Data for the Complexes
in Solution
solvent
CH3CN
λ
max, nm (ꢀ, mol-1 cm2)
c
104 A ,b
104A|c
104A
1a
1b
721 (220), 523 (190), 400 (10 500), 343 (7400),
299 (15 500), 222 (37 300)
692 (100), 522 (145), 403 (12 800), 346 (10 400),
301 (18 200)
707 (30), 517 (90), 400 (10 700), 340 (7500),
300 (12 500), 218 (34 500)
694 (30), 520 (100), 403 (15 900), 345 (11 050),
298 (18 700)
706 (30), 517 (210), 382 (17 000), 336 (14 400),
306 (25 800), 215 (27 700)
400 (8100), 283 (20 800), 232 (27 500)
403 (8800)
386 (8100), 300 (27 300), 228 (34 900)
382 (14 800), 311 (24 100), 229 (23 500)
381 (11 200)
365 (sh), 314 (29 300), 227 (21 000)
401 (8500), 285 (21 200), 233 (28 200)
401 (9900)
a
b
c
µeff
g
g|c
g
cm-1
cm-1
cm-1
1a
1b
1c
2A
2D
1.69
1.71
1.70
1.978
1.988
1.989
1.945
1.961
1.959
1.972
1.974
1.988
1.995
1.998
88
51
52
45
46
158
103
105
54
32
32
CH2Cl2
CH3CN
CH2Cl2
CH3CN
a Measured at room temperature with powdered polycrystalline samples.
b From room-temperature spectra in CH2Cl2/toluene (1:1 v/v) solution (for
1a-c) and in CH3CN (for 2A,D). c From frozen solution (77 K) spectra.
1c
2a
CH3CN
acetone
methanol
CH3CN
acetone
methanol
CH3CN
acetone
CH3CN
acetone
methanol
2b
2c
2d
382 (17 200), 311 (28 100), 222 (31 000)
379 (13 200)
370 (sh), 308 (24 250), 223 (24 050)
EPR spectrum (in frozen solution, 77 K) of a representative
octahedral complex (1c) is displayed in Figure 8c. Such lower
A values are rarely observed for six-coordinated vanadium-
(IV) complexes and have been explained as due to delocal-
ization of the odd electron over the sulfur-containing ligand
environment.42
Electronic Spectra. The electronic absorption spectra of
the complexes are summarized in Table 5. All the vanadium-
(V) complexes (2a-d) display an intense to medium-
intensity electronic spectral band in the near-UV region. In
acetonitrile and acetone, this band appears at ca. 400 nm in
2a,c and blue-shifted to ca. 380 nm in 2b,d when the para
substituent in the tridentate ligand framework is changed
from bromo to nitro group. We believe this band is
originating from ligand-to-metal charge-transfer (LMCT),
arising from the phenolate oxygen to an empty d-orbital of
the vanadium(V) center. Of particular interest here is the
position of this LMCT band in methanol which suffers a
major blue-shift compared to what has been observed in a
weakly coordinating solvent, viz. acetonitrile or acetone
(Table 5). We believe this spectral change is originating from
the participation of methanol in coordination to vanadium
Figure 8. Solution (CH2Cl2/toluene, 1:1 v/v) EPR spectra (X-band) of
(a) [VOL3(BzIm)]‚0.5CH3CN (1a) at room temperature (298 K), (b) 1a in
frozen solution (77 K), and (c) [VOL2(N-MeIm)2] (1c) in frozen solution
(77 K).
EPR spectra of the oxovanadium(IV) complexes look
essentially identical. Spectra for 1a measured both in the
fluid (CH2Cl2/toluene, 1:1 v/v) as well as in the frozen
solution (77 K) are shown in Figure 8. At room temperature,
the spectrum (Figure 8a) shows typical eight-line pattern [ g
) 1.978 and A ) 88 × 10-4 cm-1 (Table 4)], characteristic
of an unpaired electron being coupled to the vanadium
nuclear spin (51V, 99.76 atom %, I ) 7/2). In the frozen
solution, however, the spectrum displays well-resolved axial
anisotropy with two sets of eight-line patterns. Corresponding
spin-Hamiltonian parameters (g|, 1.959; A|, 158 × 10-4 cm-1;
g , 1.988; A , 54 × 10-4 cm-1) are typical of square
pyramidal oxovanadium(IV) complexes under comparable
sulfur-containing donor environments.40,41 Of particular
interests are the subtle but interesting differences in the EPR
parameters of 1a-c, summarized in Table 4. For the
octahedral complexes (1b,c), the anisotropic hyperfine
parameters have significantly lower values (A|, 105 × 10-4
cm-1; A , 32 × 10-4 cm-1) compared to the corresponding
values obtained with the square pyramidal complex 1a. The
1
as corroborated also by H NMR experiment (see later).
The oxovanadium(IV) complexes (1a-c) have closely
similar electronic spectra in acetonitrile and dichloromethane
solution. Like the vanadium(V) complexes (2a-d), these
spectra show an intense LMCT [PhO f V(dπ)] band in the
382-403 nm range. In addition, these complexes also display
two ligand field absorptions of moderate intensities (ꢀ, 30-
220 mol-1 cm2) in the 692-721 and 517-523 nm range,
2
2
,
corresponding to the transitions dxy f dxz, dyz and dxy f dx -y
respectively, as expected from an idealized square pyramidal
(C4V) oxovanadium(IV) species.43 For the octahedral com-
plexes 1b,c, the presence of a sixth donor ligand, trans to
(40) Dickson, F. E.; Kunesh, C. J.; McGinnis, E. L.; Petrakis, L. Anal.
Chem. 1972, 44, 978.
(41) Klich, P. R.; Daniher, A. T.; Challen, P. R.; McConville, D. D.;
Youngs, W. J. Inorg. Chem. 1996, 35, 347.
(42) Davidson, A.; Edelstein, N.; Holm, R. H.; Maki, A. H. J. Am. Chem.
Soc. 1964, 86, 2799.
(43) Hahn, C. W.; Rasmussen, P. G.; Bayo´n, J. C. Inorg. Chem. 1992, 31,
1963.
1514 Inorganic Chemistry, Vol. 42, No. 5, 2003

OxoWanadium(IV) and -(V) Complexes
Table 6. 1H NMR Spectral Data (δ, ppm)a for the Vanadium(V) Complexes in Different Solvents at Room Temperature
2a
2c
2d in
methanol-d4
8.84, 8.77 s
acetone-d6
acetonitrile-d3
methanol-d4
8.84, 8.77 s
acetonitrile-d3
acetonitrile-d3
integration
assgnt
8.95 s
8.88 s
8.90 s
9.03 s
1H
H4
8.26 s
8.86 s
8.51 s
8.34 s
8.43 s
8.54 s
1H
H6
7.63, 7.73 s
7.69 s
7.69 s
7.63, 7.72 s
7.71 s
8.24 s
1H
H3
7.40 (7.8), 7.53 (8.8) d
7.24 s
6.82 (8.8), 6.78 (8.8) d
2.50, 2.48 s
7.37 (8.8) d
7.63 s
6.72 (9.0) d
2.42 s
7.50 (6.0) d
7.41 s
6.86 (9.0) d
2.57 s
7.40 (8.7), 7.52 (8.8) d
7.02 s
6.81 (8.5), 6.78 (8.9) d
2.48, 2.50 s
7.49 s
7.13 s
6.87 (9.0) d
2.54 s
8.16 s
7.05 s
6.99 (12.0) d
2.53 s
1H
H2
H9, H8
H1
2H/1Hb
1H
3H
H5
2.22 s
2.33 s
1.27 s
3H
H11
a Chemical shift (δ) relative to internal TMS. Proton labels are as in Figure 9: s, singlet; d, doublet. Values in the parentheses represent coupling constants
(J in Hz). b Complexes 2c,d have a methyl substitution in the imidazole ring.
Figure 10. 300 MHz 1H NMR spectrum of [VO2L3(ImH)]∞ (2a) in
methanol-d4 solution at 25 °C showing features due to solvation equilibrium.
this is due to an equilibrium (eq 1) involving a weak
coordination of the solvent methanol as established also by
electronic spectral studies (vide supra). In fact, the
Figure 9. 300 MHz 1H NMR spectrum of [VO2L3(ImH)]∞ (2a) in acetone-
d6 solution at 25 °C.
corresponding isoelectronic molybdenum(VI) compound
[MoO2L1(CH3OH)] has been isolated and crystallographically
the terminal VdO bond, has a direct influence so that the
d-d transitions here are systematically blue-shifted compared
characterized.44
to the corresponding band positions obtained with the square-
pyramidal complex 1a (Table 5).26
[VO2(L3)(ImH)] + CH3OH h
[VO2(L3)(CH3OH)](ImH) (1)
All the remaining bands in the UV region are due to
intraligand transitions.
In dry acetonitrile, fresh yellow solutions of 2a-d
gradually turn green when exposed to visible light. Progress
1
1H NMR Spectroscopy. H NMR spectra of the vanadi-
um(V) complexes, taken in a number of solvents, are
of such photochemical reaction has been monitored by
summarized in Table 6. Free tridentate ligands have two
dynamic 1H NMR experiments. It appears that all the
broad resonances at ca. 10.9 and 10.5 ppm region due to
prominent spectral features except those due to imidazole
phenolic OH and NH protons, respectively. These signals
ring CH protons have undergone gradual line-broadening
with concomitant loss in spectral resolution as the exposure
time is increased. Results indicate the formation of para-
are absent in the spectra of the complexes indicating that
the tridentate ligands are dianionic. The spectrum of 2a in
acetone-d6 (Figure 9) involves two singlets at 8.86 and 7.63
magnetic species in solution as a probable product by the
ppm due to imidazolium ring CH protons. The NH protons
reduction of VO2L- species through a photochemical path-
of the imidazolium ring are not clearly visible in the
way as confirmed by control experiments.22 Similar photo-
spectrum. Two other singlets at 8.95 and 2.42 ppm are
induced reduction also occur in other aprotic donor solvents
characteristics of the azomethyne and SCH3 protons, respec-
such as DMF and DMSO.
Characterization of the Photoreduced Product. Despite
our repeated attempts, we remained unsuccessful in isolating
the green photoreduced products (2A-D) in the solid state.
In solution, however, the photoreduced products are all EPR
active unlike their cis-dioxovanadium(V) precursors (2a-
tively. Remaining peaks in the aromatic region appear with
expected multiplicities as outlined in Table 6
The spectrum of 2a in methanol-d4 looks a little more
complicated as shown in Figure 10. A careful scrutiny of
this figure reveals that the signals due to protons belonging
to the tridentate ligand (L3) appear in pair with expected
multiplicities while those due to imidazolium ring CH
protons (at 8.26 and 7.24 ppm) appear only once. We believe
(44) Dutta, S. K.; McConville, D. B.; Youngs, W. J.; Chaudhury, M. Inorg.
Chem. 1997, 36, 2517.
Inorganic Chemistry, Vol. 42, No. 5, 2003 1515

Samanta et al.
Figure 11. X-band EPR spectrum of a photoreduced solution of [VO2L3-
(ImH)]∞ (2a) in dry acetonitrile at room temperature after 12 h exposure to
visible light.
Figure 13. Traces of cyclic voltammograms recorded with a solution of
2d in dry acetonitrile during the progress of photoreduction: (a) fresh
solution; (b) after 3 h; (c) after 6 h; (d) after 12 h exposure to visible light
(potentials vs SCE, 0.1 M TEAP at a glassy carbon working electrode,
scan rate 50 mV s-1).
plex previously reported by us.20 The absorption in the visible
region appears due to ligand-field transition(s), localized on
the oxovanadium(IV) center of this mixed-oxidation diva-
nadium(IV/V) species.
Another interesting feature of the photoreduced products
is their electrochemical response as studied by cyclic
voltammetry. Although the precursor complexes (2a-d) are
all electrochemically inactive in the potential range -1.0 to
+1.0 V vs SCE, their photoreduced counterparts display the
development of an oxidation process as revealed from the
voltammogram of 2D (Figure 13) shown as a representative
example, the corresponding precursor complex being 2d. On
the basis of comparison with the ferrocenium/ferrocene
couple (∆Ep ) 80 mV, ipc/ipa ) 1.0 at 50 mV s-1), this
oxidation process (∆Ep ) 90 mV, ipa/ipc ) 1.1 at 50 mV
s-1) may be described as nearly reversible46 involving one-
electron transfer (eq 2) as reported earlier with a related
mixed-oxidation (µ-oxo)divanadium(IV/V) compound.19 Be-
cause of a strong electron-withdrawing nitro group substitu-
tion in the ligand structure, this oxidation is more difficult
in 2D (E1/2 ) 0.63 V) relative to that with the bromo
derivative 2A (E1/2 ) 0.50 V, potentials are vs SCE).
Figure 12. Vis-NIR electronic absorption spectrum of a photoreduced
solution of 2a (∼1.41 × 10-3 M) in acetonitrile after 12 h exposure to
visible light.
d). The spectrum of a representative product 2A (corre-
sponding precursor complex being 2a) is shown in Figure
11 which reveals a 15-line feature ( g ) 1.945) at room
temperature with asymmetric distortions. The product appears
to have a coupled divanadium center with an odd interacting
electron generating the 15-line structure with A ) 45 ×
10-4 cm-1, which is almost half as much as that of a localized
spectrum ( A ) 89 × 10-4 cm-1) reported earlier20 for a
8
µ-oxo divanadium(IV/V) product with related ligand. This
observation, none the less, speaks in favor of a mutually
interacting divanadium(IV/V) species to be the product of
this photoinduced reduction.45 The observed asymmetry in
the spectrum (Figure 11) probably has its origin in a solvent-
dependent equilibrium process involving two magnetically
inequivalent structural forms of the divanadium(IV/V)
compound as reported elsewhere20 for a related system.
-
[LOVV-O-VIVOL]- y-e z [LOVV-O-VVOL]0 (2)
Results of EPR, electrochemical, and electronic spectral
studies seem to support the generation of a (µ-oxo)-
divanadium(IV/V) species during the photochemical trans-
formation of 2a-d in dry CH3CN solution.
Freshly prepared yellow solutions of 2a-d in CH3CN are
all optically transparent in the visible/near-IR region (Table
5). The green solutions of the photoreduced products, on the
other hand, absorb strongly in the 1300-500 nm region. The
spectrum of 2A in the relevant region is shown in Figure
12, as a representative example. It involves an absorption
maximum of moderate intensity at 982 nm followed by a
low-intensity shoulder, centered around 726 nm, the absorp-
tion ratio, A982/A726 being close to 1.50 after a 12 h exposure
time. The shape and position of this near-IR band is quite
similar to that of an intervalence charge-transfer band of an
authentic mixed-oxidation (µ-oxo)divanadium(IV/V) com-
Concluding Remarks
Reactions of [VO(acac)2] with the tridentate ONS ligands
(H2L2 and H2L3) in aqueous acetonitrile medium generate
oxovanadium(IV) (1a-c) and -(V) (2a-d) complexes in the
presence of various imidazole derivatives as coligands. With
imidazole and 4-methylimidazole, the products obtained are
cis-dioxovanadium(V) complexes [VO2L(R′-ImH)] (2a-d).
In the solid state, these complexes have interesting zigzag
chain structures as revealed from the X-ray crystallographic
investigation of 2a,d, involving an alternating array of LVO2-
(46) Brown, E. R.; Large, R. F. Electrochemical Methods. In Physical
Methods in Chemistry; Weissberger, A., Rossiter, B., Eds.; Wiley-
Interscience: New York, 1971; Part IIA, Chapter VI.
(45) Slichter, C. P. Phys. ReV. 1955, 99, 478.
1516 Inorganic Chemistry, Vol. 42, No. 5, 2003

OxoWanadium(IV) and -(V) Complexes
species and the corresponding imidazolium counterions, held
together by Coulombic interactions and strong hydrogen
bonding. These LVO2 species are stable in water and
dium(IV) complexes (1a-c) with interesting variations in
structure. In solution, 1a has a tendency to undergo aerial
oxidation at a much slower rate, while 1b,c are resistant to
similar aerial oxidation under comparable condition. We
believe the strength of the imidazole bases plays a crucial
role here in controlling the rate of aerial oxidation of the
LVIVO precursors. Stronger bases such as 4-methylimidazole
(basic pKa, 7.61) and imidazole (basic pKa, 6.95) are efficient
enough to stabilize the oxidized anionic cis-VO2 species by
forming salts of their symmetrical mesomeric cations.51
Benzimidazole (basic pKa, 5.5) and N-methylimidazole, on
the other hand, have a little or no capability to generate the
protonated symmetrical cation needed for the stability of the
oxidized cis-dioxovanadium(V) product.
-
methanol as appeared from 1H NMR and electronic spectral
studies. But in aprotic solvents such as CH3CN, DMF, or
DMSO, they undergo proton-coupled photoinduced reduction
(eqs 3 and 4) through the intermediate formation of hydroxo
species LVO(OH).35,47,48 The reduced LVIVO species, thus
-
formed, reacts with excess LVO2 units to generate the
mixed-oxidation divanadium(IV/V) products 2A-D (eq 5).49
Structurally characterized mixed-oxidation poly(oxo)vana-
date(IV/V) compounds prepared by photoinduced reduction
have been reported recently in the literature.50
LVVO2- + H+ h LVVO(OH)
(3)
Acknowledgment. Financial support received from the
Council of Scientific and Industrial Research, New Delhi,
is gratefully acknowledged. We thank Professor K. Nag for
many useful discussions.
hν
LVVO(OH) + H+ + e- y
\
z LVIVO + H2O
(4)
LVIVO + LVVO2- h [LVIVO-(µ-O)-OVVL]- (5)
Supporting Information Available: X-ray crystallographic files
in CIF format for compounds 1a,b and 2a,d. This material is
available free of charge via the Internet at http://pubs.acs.org.
When imidazole is replaced by benzimidazole or N-
methylimidazole, the products are mononuclear oxovana-
(47) Root, C. A.; Hoeschele, J. D.; Cornman, C. R.; Kampf, J. W.; Pecoraro,
V. L. Inorg. Chem. 1993, 32, 3855.
(48) Asgedom, G.; Sreedhara, A.; Kivikoski, J.; Kolehmainen, E.; Rao, C.
P. J. Chem. Soc., Dalton Trans. 1996, 93.
IC020438V
(50) Yamase, T.; Suzuki, M.; Ohtaka, K. J. Chem. Soc., Dalton Trans.
1997, 2463.
(49) Nishizawa, M.; Hirotsu, K.; Ooi, S.; Saito, K. J. Chem. Soc., Chem.
Commun. 1979, 707.
(51) ComprehensiVe Organic Chemistry; Vol. 4, Heterocyclic Chemistry;
Sammes, P. G., Ed.; Pergamon Press: Oxford, England, 1979; p 364.
Inorganic Chemistry, Vol. 42, No. 5, 2003 1517