Synthesis, reactivity, and x-ray crystal structure of some mixed-ligand oxovanadium(V) complexes: First report of binuclear oxovanadium(V) complexes containing 4,4′-bipyridine type bridge

Article abstract of DOI:10.1021/ic051120g

Reaction of the tridentate ONO Schiff-base ligand 2-hydroxybenzoylhydrazone of 2-hydroxybenzoylhydrazine (H₂L) with VO(acac)₂ in ethanol medium produces the oxoethoxovanadium(V) complex [VO(OEt)L] (A), which reacts with pyridine to form [VO(OEt)L·(py)] (1). Complex 1 is structurally characterized. It has a distorted octahedral O₄N ₂ coordination environment around the V(V) acceptor center. Both complexes A and 1 in ethanol medium react with neutral monodentate Lewis bases 2-picoline, 3-picoline, 4- picoline, 4-amino pyridine, imidazole, and 4-methyl imidazole, all of which are stronger bases than pyridine, to produce dioxovanadium(V) complexes of general formula BH[VO₂L]. Most of these dioxo complexes are structurally characterized, and the complex anion [VO ₂L]^- is found to possess a distorted square pyramidal structure. When a solution/suspension of a BH[VO₂L] complex in an alcohol (ROH) is treated with HCl in the same alcohol, it is converted into the corresponding monooxoalkoxo complex [VO(OR)L], where R comes from the alcohol used as the reaction medium. Both complexes A and 1 produce the 4,4′-bipyridine-bridged binuclear complex [VO(OEt)L]₂(μ-4, 4′-bipy) (2), which, to the best of our knowledge, represents the first report of a structurally characterized 4,4′-bipyridine-bridged oxovanadium(V) binuclear complex. Two similar binuclear oxovanadium(V) complexes 3 and 4 are also synthesized and characterized. All these binuclear complexes (2-4), on treatment with base B, produce the corresponding mononuclear dioxovanadium(V) complexes (5-10).

Full text of DOI:10.1021/ic051120g

Inorg. Chem. 2006, 45, 5150−5161
Synthesis, Reactivity, and X-ray Crystal Structure of Some Mixed-Ligand
Oxovanadium(V) Complexes: First Report of Binuclear Oxovanadium(V)
Complexes Containing 4,4 -Bipyridine Type Bridge
′
Manas Sutradhar,† Gurunath Mukherjee,*,† Michael G. B. Drew,‡ and Saktiprosad Ghosh*,†
Department of Chemistry, UniVersity of Calcutta, UniVersity College of Science, 92 Acharya
Prafulla Chandra Road, Kolkata 700 009, India, and School of Chemistry, The UniVersity of
Reading, PO Box 224 Whiteknights, Reading RG6 6AD, United Kingdom
Received July 6, 2005
Reaction of the tridentate ONO Schiff-base ligand 2-hydroxybenzoylhydrazone of 2-hydroxybenzoylhydrazine (H2L)
with VO(acac)2 in ethanol medium produces the oxoethoxovanadium(V) complex [VO(OEt)L] (A), which reacts with
pyridine to form [VO(OEt)L‚(py)] (1). Complex 1 is structurally characterized. It has a distorted octahedral O4N2
coordination environment around the V(V) acceptor center. Both complexes A and 1 in ethanol medium react with
neutral monodentate Lewis bases 2-picoline, 3-picoline, 4- picoline, 4-amino pyridine, imidazole, and 4-methyl
imidazole, all of which are stronger bases than pyridine, to produce dioxovanadium(V) complexes of general formula
BH[VO2L]. Most of these dioxo complexes are structurally characterized, and the complex anion [VO2L]- is found
to possess a distorted square pyramidal structure. When a solution/suspension of a BH[VO2L] complex in an
alcohol (ROH) is treated with HCl in the same alcohol, it is converted into the corresponding monooxoalkoxo
complex [VO(OR)L], where R comes from the alcohol used as the reaction medium. Both complexes A and 1
produce the 4,4
represents the first report of a structurally characterized 4,4
Two similar binuclear oxovanadium(V) complexes 3 and 4 are also synthesized and characterized. All these binuclear
′
-bipyridine-bridged binuclear complex [VO(OEt)L]2(µ-4,4′-bipy) (2), which, to the best of our knowledge,
′
-bipyridine-bridged oxovanadium(V) binuclear complex.
complexes (2−4), on treatment with base B, produce the corresponding mononuclear dioxovanadium(V) complexes
(5−10).
Introduction
inhibition of phosphate-metabolizing enzymes7 and stim-
ulation of phosphomutases;8 (c) anticancer activity;9 (d)
insulin-mimetic activity10 of some vanadium(IV) and (V)
complexes; and (e) the catalytic activity of vanadium(V)
complexes in some N-O donor ligands in R-olifine poly-
merization.11 Works on V(V) complexes of ONO Schiff
bases in general are well-reported in the recent litera-
Coordination chemistry of vanadium received a shot in
the arm in recent times because of (a) the discovery of the
presence of vanadium in some sea squirts,1,2 mushrooms,3,4
and the vanadium-dependent enzymes nitrogenases5 and
haloperoxidases;6 (b) the involvement of vanadium in the
* To whom correspondence should be addressed. E-mail: gmchem@
rediffmail.com (G.M); ghosh_sakti@vsnl.net (S.G).
† University of Calcutta.
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‡ The University of Reading.
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S. N.; Helliwell, M.; Garner, C. D. Angew. Chem., Int. Ed. 1999, 38,
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5150 Inorganic Chemistry, Vol. 45, No. 13, 2006
10.1021/ic051120g CCC: $33.50
© 2006 American Chemical Society
Published on Web 05/18/2006

Binuclear OxoWanadium(V) with 4,4′-Bipyridine Bridge
ture.12-24,26-28 Stable mononuclear V(V) complexes of such
ONO donor ligands containing a coordinated alkoxide moiety
are not so numerous.17,20,21,23,25,54
In a continuation of our work on vanadium(IV) and (V)
complexes of multidentate N-O donor ligands,25,29-32 we
report the following findings in this work: (i) preparation,
structural characterization, and exploration of the reactivity
of the monooxoalkoxovanadium(V) complexes A and 1, in
which L2- is the dianion of the tridentate ONO donor ligand;
(ii) preparation and structural characterization of several BH-
[VO2L] type water soluble complexes, where B ) 4-meth-
ylimidazole, imidazole, 2-picoline, 3-picoline, 4-picoline, and
4-amino pyridine; (iii) preparation and study of three
binuclear vanadium(V) complexes in which N,N′-bidentate
linear spacers such as 4,4′-bipyridine act as bridging ligands.
Two of these binuclear complexes are structurally character-
ized by X-ray analysis. To the best of our knowledge, this
is the first report of a structurally characterized 4,4′-
bipyridine-bridged binuclear oxovanadium(V) complex. (iv)
Reversible transformations involving monooxoalkoxo and
dioxovanadium(V) complexes and the generation of different
oxoalkoxovanadium(V) species from a single dioxo precursor
under ambient conditions are also reported. Recent reports
on insulin-mimetic activity33,34 by vanadium(V) complexes
in some dianionic tridentate ONO ligands makes us opti-
mistic about the possible insulin-mimetic behavior of our
water-soluble complexes.
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Experimental Section
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Inorganic Chemistry, Vol. 45, No. 13, 2006 5151

Sutradhar et al.
7.91-6.91 (m, 8H, C6H4), 8.68 (s, 1H, -CHdN), 11.18 (s, 1H,
NH), 11.76 (s, 1H, OH), 12.02 (s, 1H, OH). IR (KBr, cm-1): 3195
(νOH), 3075 (νNH), 1660 (νCdO), 1618 (νCdN).
O10V2: C, 57.64; H, 4.58; N, 9.17; V, 11.13. Found: C, 56.61; H,
4.56; N, 9.11; V, 11.10. λmax (CH3CN, nm (ꢀ, L M-1 cm-1)): 399
(16 959), 335 (21 035), 321 (20 728), 306 (19 389), 254 (33 318).
IR (KBr, cm-1): 3437 (νOH), 1253 (νC-O/enolic), 1604 (νCdN), 1048
(νN-N), 960 (νVdO).1H NMR (CDCl3, δ): 10.21 (s, 2H, OH), 8.48
(s, 2H, -CHdN), 8.41 (m, 4H, CH, C12H12N2), 7.93-6.82 (m,
22H, C6H4 and C12H12N2), 5.39 (q, 4H, CH2, OEt), 2.85 (s, 4H,
C2H4), 1.76 (t, 6H, CH3, OEt).
Synthesis of Mononuclear Oxovanadium(V) Complexes. [VO-
(OEt)L] (A). This complex was prepared by a reported method.36
Ligand H2L (0.26 g, 1.0 mmol) was dissolved in 30 mL of ethanol,
and 0.27 g (1.0 mmol) of VO(acac)2 was added to this solution.
On being stirred, the solution changed from a greenish color to
dark brown. After 6 h of being stirred in open air, the solution was
filtered; the filtrate was kept in air for 3 days, at which point dark
brown crystals were deposited. Yield: 70%. Anal. Calcd for
C16H15N2O5V: C, 52.42; H, 4.12; N, 7.64; V, 13.93. Found: C,
52.50; H, 4.21; N, 7.63; V, 13.87. λmax (CH3CN, nm (ꢀ, L M-1
cm-1)): 399 (13 452), 335 (15 913), 321 (15 468), 306 (14 364).
IR (KBr, cm-1): 3195 (νOH), 1245 (νC-O/enolic), 1602 (νCdN), 1038
(νN-N), 992 (νVdO).1H NMR (CDCl3, δ): 10.18 (s, 1H,OH), 8.59
(s, 1H, -CHdN), 7.98-6.78 (m, 8H, C6H4), 5.5 (q, 2H, CH2, OEt),
1.74 (t, 3H, CH3, OEt).
[VO(OEt)L(py)] (1). To 25 mL of an ethanol solution of [VO-
(OEt)L] (A) (0.37 g, 1.0 mmol) was added excess pyridine (6-8
mL), and the mixture was refluxed for 4 h on a steam bath. The
color of the solution slowly changed from dark brown to reddish
brown. The solution was filtered and allowed to evaporate slowly
in a refrigerator. After 8-10 days, yellowish brown single crystals
separated out that were suitable for X-ray diffraction analysis.
Yield: 70%. Anal. Calcd for C21H20N3O5V: C, 56.63; H, 4.49; N,
9.44; V, 11.46. Found: C, 56.60; H, 4.38; N, 9.50; V, 11.34. λmax
(CH3CN, nm (ꢀ, L M-1 cm-1)): 398 (9730), 335 (12 370), 321
(12 171), 306 (11 510), 254 (19 135). IR (KBr, cm-1): 3448 (νOH),
1264 (νC-O/enolic), 1604 (νCdN), 1036 (νN-N), 969 (νVdO).1H NMR
(CDCl3, δ): 10.19 (s, 1H, OH), 8.57 (s, 1H, -CHdN), 8.51 (br,
2H, CH), 7.91-6.85 (m, 11H, C6H4 and C6H5N), 5.42 (q, 2H, CH2,
OEt), 1.71 (t, 3H, CH3, OEt).
Synthesis of Dinuclear Oxovanadium(V) Complexes. [VO-
(OEt)L]2(µ-4,4′-bipy) (2). This complex was obtained as yellowish
brown crystals using the procedure adopted for complex 1 using a
30 mL ethanol solution of complex A (0.37 g, 1.0 mmol) and 0.80
g (0.50 mmol) of 4,4′-bipyridine. After 8-10 days, yellowish brown
single crystals separated out that were suitable for X-ray diffraction
analysis. Yield: 70%. Anal. Calcd for C42H38N6O10V2: C, 56.75;
H, 4.28; N, 9.46; V, 11.48. Found: C, 56.72; H, 4.26; N, 9.44; V,
11.42. λmax (CH3CN, nm (ꢀ, L M-1 cm-1)): 398 (22 375), 335
(29 013), 321 (28 693), 306 (27 455). IR (KBr, cm-1): 3435 (νOH),
1254 (νC-O/enolic), 1605 (νCdN), 1031 (νN-N), 954 (νVdO). 1H NMR
(CDCl3, δ): 10.26 (s, 2H, OH), 8.94 (s, 2H, -CHdN), 8.58 (m,
4H, CH, C10H8N2), 7.98-6.92 (m, 20H, C6H4 and C10H8N2), 5.36
(q, 4H, CH2, OEt), 1.68 (t, 6H, CH3, OEt).
[VO(OEt)L]2(µ-1,2-bis(4-pyridyl)ethylene) (3). This complex
was isolated as a dark brown crystalline solid by following the
method adopted in the preparation of 2 using 1,2-bis(4-pyridyl)-
ethylene in place of 4,4′-bipyridine. Yield: 80%. Anal. Calcd for
C44H40N6O10V2: C, 57.77; H, 4.38; N, 9.19; V, 11.16. Found: C,
56.74; H, 4.36; N, 9.15; V, 11.12. λmax (CH3CN, nm (ꢀ, L M-1
cm-1)): 399 (18 383), 334 (22 186), 294 (48 655), 285 (49 599).
IR (KBr, cm-1): 3433 (νOH), 1254 (νC-O/enolic), 1605 (νCdN), 1050
(νN-N), 963 (νVdO). 1H NMR (CDCl3, δ): 10.23 (s, 2H, OH), 8.87
(s, 2H, -CHdN), 8.57 (m, 4H,CH, C12H10N2), 7.91-6.85 (m, 22H,
C6H4, C2H2 and C12H10N2), 5.39 (q, 4H, CH2, OEt), 1.76 (t, 6H,
CH3, OEt).
Complexes A and 1-4 are soluble in alcohol, acetonitrile, and
dichloromethane but insoluble in water.
Synthesis of Dioxovanadium(V) Complexes. [4-Me imzH]-
[VO2L] (5). To 30 mL of an ethanol solution of A (0.37 g, 1.0
mmol) was added 4(5)-methyl imidazole (0.10 g, 1.2 mmol), and
the mixture was refluxed for 5 h on a steam bath. The color of the
solution slowly changed from dark brown to reddish yellow. The
solution was filtered and allowed to evaporate slowly in open air
at room temperature. After 4-5 days, a yellow crystalline product
separated out. On recrystallization from methanol, yellow crystals
suitable for X-ray diffraction studies were collected. This complex
is water-soluble. Yield: 80%. Anal. Calcd for C18H17N4O5V: C,
51.43; H, 4.05; N, 13.33; V, 12.14. Found: C, 51.38; H, 4.06; N,
13.30; V, 12.12. λmax (CH3CN, nm (ꢀ, L M-1 cm-1)): 399.5
(13 621), 335 (15 704), 321 (15 372), 305 (14 380). IR (KBr, cm-1):
3140 (νOH), 1259 ν(C-O/enolic), 1612 (νCdN), 1071 (νN-N), 943, 912
1
(νVdO). H NMR (D2O, δ): 8.68 (s, 1H, -CHdN), 8.31 (s, 1H,
CH, C4H6N2), 7.71-6.77 (m, 9H, C6H4 and C4H6N2), 2.17 (s, 3H,
CH3).
[4-Me imzH][VO2L]‚2H2O (5A). Orange-yellow single crystals
were obtained by recrystallizing the compound [Me imdzH][VO2L]-
(5) from water.Yield: 70%. Anal. Calcd for C18H21N4O7V: C,
47.37; H, 4.60; N, 12.28; V, 11.18. Found: C, 47.31; H, 4.63; N,
11.12; V, 11.16. λmax (CH3CN, nm (ꢀ, L M-1 cm-1)): 399 (12 821),
335 (15 314), 321 (15 042), 306 (14 780). IR (KBr, cm-1): 3151
(νOH), 1259 (νC-O/enolic), 1610 (νCdN), 1070 ν(N-N), 938, 908 (νVd
O).
[imzH][VO2L] (6). This compound was prepared using a
procedure similar to that adopted for 5 using imidazole in place of
4-methyl imidazole. Yield: 74%. Anal. Calcd for C17H15N4O5V:
C, 50.25; H, 3.69; N, 13.79; V, 12.56. Found: C, 50.21; H, 3.72;
N, 13.82; V, 12.55. λmax (CH3CN, nm (ꢀ, L M-1 cm-1)): 399
(18 732), 335 (21 540), 321 (20 880), 305 (19 149). IR (KBr, cm-1):
3120 (νOH), 1260 (νC-O/enolic), 1614 (νCdN), 1076 (νN-N), 947, 911
(νVdO).1H NMR (D2O, δ): 8.64 (s, 1H, -CHdN), 8.28 (s, 1H, CH,
C3H4N2), 7.68-6.64 (m, 10H, C6H4 and C3H4N2).
[2-picH][VO2L] (7). To 30 mL of an ethanol solution of A (0.37
g, 1.0 mmol) was added excess 2-picoline (6-8 mL), and the
mixture was refluxed for 5 h on a steam bath. The color of the
solution slowly changed from dark brown to yellow. The solution
was filtered and allowed to evaporate slowly at room temperature.
After 5-6 days, a yellow crystalline product separated out. Yield:
80%. Anal. Calcd for C20H18N3O5V: C, 55.68; H, 4.18; N, 9.74;
V, 11.83. Found: C, 55.62; H, 4.2; N, 9.72; V, 11.8. λmax (CH3-
CN, nm (ꢀ, L M-1 cm-1)): 399 (13 156), 335 (15 097), 321 (14 786),
306 (13 877), 256 (21 165). IR (KBr, cm-1): 3438 (νOH), 1220 ν
(
C-O/enolic), 1605 (νCdN), 1060 (νN-N), 938, 907 (νVdO).
1H NMR (CDCl3, δ): 10.39 (s, 1H, OH), 8.73 (m, 1H, CH,
C5H4N), 8.69 (s, 1H, -CHdN), 7.71-6.77 (m, 11H, C6H4 and
C5H4N), 2.90 (s, 3H, CH3).
[3-picH][VO2L] (8). This compound was prepared using the
same procedure described above for 7 but using 3-picoline in place
of 2-picoline. Yield: 75%. Anal. Calcd for C20H18N3O5V: C, 55.68;
H, 4.18; N, 9.74; V, 11.83. Found: C, 55.62; H, 4.2; N, 9.72; V,
11.8. λmax (CH3CN, nm (ꢀ, L M-1 cm-1)): 399 (10 746), 335
[VO(OEt)L]2(µ-1,2-bis(4-pyridyl)ethane) (4). It was isolated
as a dark brown crystalline solid following a procedure similar to
that adopted in the preparation of 2 using 1,2-bis(4-pyridyl)ethane
in place of 4,4′-bipyridine. Yield: 75%. Anal. Calcd for C44H42N6
5152 Inorganic Chemistry, Vol. 45, No. 13, 2006

Binuclear OxoWanadium(V) with 4,4′-Bipyridine Bridge
Table 1. Crystal Data and Structure Determination Summary of
Complexes 1-3
(12 348), 321 (11 902), 307 (10 767), 256.5 (16 306). IR (KBr,
cm-1): 3452 (νOH), 1230 (νC-O/enolic), 1606 (νCdN), 1061 (νN-N),
958, 905 (νVdO).1H NMR (CDCl3, δ): 10.50 (s, 1H, OH), 8.72
(br, 1H, CH, C5H4N), 8.61 (s, 1H, -CHdN), 7.71-6.77 (m, 11H,
C6H4 and C5H4N), 2.43 (s, 3H, CH3).
1
2
3a
empirical formula
fw
C21H20N3O5V C42H38N6O10V2 C22H20N3O5V
445.34
888.66
457.35
cryst syst
space group
a (Å)
b (Å)
c (Å)
R (deg)
â (deg)
γ (deg)
V (Å3)
triclinic
P1h (No. 2)
10.540(12)
13.682(15)
15.952(17)
101.73(1)
105.23(1)
101.82(1)
2090(4)
4
monoclinic
P21/c (No.14) P1h
triclinic
[4-picH][VO2L] (9). This compound was prepared using the
same procedure mentioned above for 7 but using 4-picoline in place
of 2-picoline. Yield: 80%. Anal. Calcd for C20H18N3O5V: C, 55.68;
H, 4.18; N, 9.74; V, 11.83. Found: C, 55.62; H, 4.2; N, 9.72; V,
11.8. λmax (CH3CN, nm (ꢀ, L M-1 cm-1)): 399 (12 027), 334
(14 211), 321 (13 786), 225 (18 387). IR (KBr, cm-1): 3089 (νOH),
1251 (νC-O/enolic), 1602 (νCdN), 1062 (νN-N), 955, 937 (νVdO).1H
NMR (CDCl3, δ): 10.66 (s, 1H, OH), 8.72 (s, 1H, -CHdN), 8.59
(m, 2H, CH, C5H4N), 7.60-6.93 (m, 10H, C6H4 and C5H4N), 2.44
(s, 3H, CH3).
[4-ampyH][VO2L] (10). This compound was prepared using a
procedure similar to that adopted for 5 using 4-amino pyridine in
place of 4-methyl imidazole. Yield: 75%. Anal. Calcd for
C19H17N4O5V: C, 52.77; H, 3.94; N, 12.96; V, 11.80. Found: C,
52.72; H, 3.98; N, 12.98; V, 11.78. λmax (CH3CN, nm (ꢀ, L M-1
cm-1)): 399 (6611), 335 (7651), 321 (7461), 306 (6922), 260
(16 311). IR (KBr, cm-1): 3160 (νOH), 1265 (νC-O/enolic), 1610 (νCd
N), 1065 (νN-N), 954, 912 (νVdO).1H NMR (CDCl3, δ): 10.15 (s,
1H, OH), 8.59 (s, 1H, -CHdN), 7.87-6.22 (m, 12H, C6H4 and
C5H4N), 3.62 (s, 2H, NH2).
12.740(14)
10.200(13)
17.159(19)
(90)
109.86(1)
(90)
2097(4)
2
1.407
10.681(14)
12.596(14)
15.086(14)
105.45(10)
90.92(1)
104.35(1)
1888
Z
D
3
calcd (g cm-3
)
1.415
7350/0/546
5920
1.207
data/restraints/params
no. of obsd reflns
[I > 2σ(I)]
3304/0/273
2447
R(F), wR(F) (obsd data)b 0.0540,0.1254 0.1106, 0.2998
R(F2), wR(F2) (all data)b 0.0707,0.1342 0.1344, 0.3148
Rint
peaks in final difference 0.55, -0.34
map (e A-3
GOF on F2
0.029
0.062
1.61,-0.73
)
1.02
1.089
a Complex 3 was found to be disordered. b R ) ∑||Fo| - |Fc||/∑|Fo|;
awR(F2) ) [∑w(|Fo| - |Fc| )2/∑w|Fo| ]1/2
.
2
2
4
1
spectrophotometer. H NMR spectra were recorded on a Bruker
model DPX300 advance spectrometer. Electrochemical measure-
ments were performed with a PAR model 362 scanning potentiostat,
and cyclic voltammograms were recorded at 25 °C in the designated
solvent under dry dinitrogen with the electroactive component at
ca. 1 × 10-3 M. Tetraethylammonium perchlorate (NEt4ClO4, 0.1
M) was used as the supporting electrolyte. A three-electrode
configuration was employed with a platinum working electrode,
calomel reference electrode, and platinum auxiliary electrode. The
ferrocene/ferrocenium (Fc/Fc+) couple was used as the internal
standard.38 Molar conductance values of the complexes in aqueous
solution were measured with a Systronics 304 digital conductivity
meter. Room-temperature magnetic susceptibilities of complexes
were measured in the polycrystalline state on a PAR 155 sample
vibrating magnetometer using Hg[Co(SCN)4] as the calibrant.
Crystal Structure Determinations. Diffraction data for suitable
single crystals of complexes 1, 2, 5, 5A, 8, and 10 were measured
with Mo KR radiation using the MARresearch image plate system.
The crystals were positioned 70 mm from the image plate. A total
of 95 frames was measured at 2° intervals with a counting time of
2 min. Data analyses were carried out with the XDS program.39
The structures were solved using direct methods with the SHELX-
86 program.40 The non-hydrogen atoms were refined with aniso-
tropic thermal parameters. The hydrogen atoms bonded to carbon
and nitrogen were included in geometric positions and given thermal
parameters equivalent to 1.2 times those of the atom to which they
were attached. Absorption corrections were carried out using the
DIFABS program.41 The structures were refined on F2 using
SHELXL.42 The structures were generated using ORTEP-3.43
Crystallographic data are summarized in Tables 1 and 3.
All complexes 2-10 were also obtained by the same procedure,
using complex 1 instead of complex A as the starting material. All
these dioxovanadium(V) complexes are fairly soluble in water but
not in alcohol or acetonitrile.
Synthesis of Monooxoalkoxovanadium(V) Complexes from
Dioxovanadium(V) Complexes. Preparations of two representative
oxoalkoxo (methoxo and butoxo) complexes are describe here.
[VO(OMe)L] (11). To 20 mL of a methanolic suspension of
[4-ampyH][VO2L] (10) (0.43 g, 1.0 mmol) was added methanolic
solution of HCl to lower the pH to ∼4, and the mixture was stirred
at room temperature for 2 h. The reaction mixture was filtered and
allowed to evaporate slowly at room temperature. After 2-3 days,
reddish brown crystalline product separated out. Yield: 68%. Anal.
Calcd for C15H13N2O5V: C, 51.14; H, 3.69; N, 7.95; V, 14.49.
Found: C, 51.10; H, 3.72; N, 7.92; V, 14.46. λmax (CH3CN, nm (ꢀ,
L M-1 cm-1)): 399 (9598), 334 (11 616), 321 (11 345), 306
(10 534). IR (KBr, cm-1): 3180 (νOH), 1238 (νC-O/enolic), 1604
(νC)N), 1034 (νN-N), 988 (νVdO).1H NMR (CDCl3, δ): 10.17 (s,
1H,OH), 8.59 (s, 1H, -CHdN), 7.99-6.80 (m, 8H, C6H4), 5.34
(s, 3H, CH3, OMe).
[VO(OnBu)L] (12). This compound was prepared using the same
procedure mentioned above for 11 but using n-butanol in place of
methanol. Yield: 65%. Anal. Calcd for C18H19N2O5V: C, 54.82;
H, 4.82; N, 7.11; V, 12.94. Found: C, 54.80; H, 4.78; N, 7.13; V,
12.91. λmax (CH3CN, nm (ꢀ, L M-1 cm-1)): 399 (9917), 334
(11 844), 321 (11 598), 306 (10 799). IR (KBr, cm-1): 3187 (νOH),
1251 (νC-O/enolic), 1605 (νCdN), 1048 (νN-N), 995 (νVdO).1H NMR
(CDCl3, δ): 10.18 (s, 1H, OH), 8.58 (s, 1H, -CHdN), 7.96-6.93
(m, 8H, C6H4), 5.59 (t, 2H, CH2, OBu), 2.05 (m, 2H, OBu), 1.5
(m, 2H, CH2, OBu), 1.06 (t, 3H, CH3, OBu).
Complexes 11 and 12 are soluble in alcohol and acetonitrile but
insoluble in water.
Results and Discussion
Our attempt to prepare adducts of the type [VO(OEt)L‚
B] (where B is a monodentate Lewis base) led to an
interesting observation and guided us to a general method
of generating dioxovanadium complexes from the mon-
oxoalkoxoV(V) complexes. It was found that adduct forma-
tion took place with pyridine (pKa ) 5.48), and the complex
Physical Measurements. Elemental analyses (C, H, and N) were
done with a Perkin-Elmer 2400 CHNS/O analyzer, and vanadium
contents (%) of the complexes were determined gravimetrically as
V2O5.37 Electronic spectra of the complexes in acetonitrile were
recorded on a Hitachi U-3501 spectrophotometer, and IR spectra
(as KBr pellets) were recorded using a Perkin-Elmer RXI FT-IR
Inorganic Chemistry, Vol. 45, No. 13, 2006 5153

Sutradhar et al.
Table 2. Selected Bond Lengths (Å) and Angles (deg) in Complexes 1
and 2
both CH3CN and CH2Cl2 solutions. This is probably due to
the symmetrical character of the bridging ligands used and
the fact that the structures of the dinuclear complexes are
centrosymmetric. Then we tried unsymmetrical ligand 4-ami-
nopyridine, which has the potential to act as a neutral
(exobidentate) bridging ligand like 4,4′-bipyridine. But the
resulting complex, instead of being the expected dinuclear
one, was found to be a mononuclear dioxo species BH-
[VO2L] (BH+ ) monoprotonated 4-amino pyridine proto-
nated at the pyridine nitrogen). When we looked into the
structure of 2, we found that the 4,4′-bipyridine moiety is
firmly attached to the respective V(V) centers, which is
reflected in the much shorter V(V)-N(bipy) bond distance
compared to the V(V)-N(pyridine) bond distance present
in 1. This dinuclear species 2 is very stable and is formed
even if 4,4′-bipyridine in kept in large excess (1:10) in
alcohol and either of the complexes A or 1 is added very
slowly to it as a very dilute alcohol solution under ice cold
conditions. Our attempts to prepare a mono bipy adduct did
not succeed.
1a
1b
2
V(1)-O(21)
V(1)-O(2)
V(1)-O(13)
V(1)-O(22)
V(1)-N(10)
V(1)-N(25)
1.586(3)
1.855(3)
1.976(3)
1.785(3)
2.131(3)
2.476(4)
1.580(4)
1.850(4)
1.968(4)
1.791(4)
2.152(4)
2.426(4)
1.581(7)
1.850(7)
1.982(7)
1.799(7)
2.139(7)
2.385(8)
O2-V1-O13
O2-V1-O21
O2-V1-O22
O2 -V1-N10
O2-V1-N25
O13-V1-O21
O13-V1-O22
O13-V1-N10
O13-V1-N25
O21-V -O22
O21-V -N10
O21-V -N25
O22-V1-N10
O22-V1-N25
N10-V1-N25
151.85(10)
100.32(14)
104.38(11)
83.09(11)
82.31(11)
98.68(13)
91.58(10)
74.89(10)
77.38(11)
102.24(13)
94.44(12)
175.02(11)
160.01(11)
81.06(13)
81.64(11)
150.87(13)
100.75(15)
104.22(14)
82.55(12)
81.26(12)
98.54(14)
93.25(13)
74.95(12)
77.65(11)
100.89(15)
92.57(14)
174.27(12)
163.36(13)
83.70(13)
82.37(12)
151.7(3)
101.6(3)
105.0(3)
83.9(3)
79.8(3)
97.7(3)
90.8(3)
74.7(3)
79.8(3)
102.2(3)
92.6(3)
176.4(3)
160.6(3)
80.5(3)
84.3(3)
When 2 is refluxed in alcohol with a base B in a 1:2
molar ratio, the product obtained is the monomeric dioxo
[BH]+[VO2L]- species and not the expected {[VO2L]2(µ-
4,4′bipy)}2- type dinuclear complexes. Our attempt to
prepare the 4,4′-bipyridine-bridged dioxovanadium(V) di-
nuclear species did not succeed. When we took complex 2
in ethanol and first refluxed with imidazole (in a 1:2 molar
ratio), yellow [imzH][VO2L] (6) was precipitated. Thereafter,
careful addition of ethanolic HCl to this mixture until the
pH of the reaction mixture became ∼4 resulted in the
reddish-brown solution depositing golden yellow crystals
when kept at room temperature for 2-3 days. These crystals
were identified as the starting dinuclear species 2. Similar
observations were also recorded in the cases of complexes
3 and 4. The above results point to the reluctance of the
dioxo species to form the dinuclear-bridged complexes and
also indicate the urge of the oxoalkoxo complexes to form
dinuclear-bridged species. Thus complexes 5 and 6-10 can
also be prepared from any one of dinuclear complexes 2-4.
Description of Crystal Structures. (a) [VO(OEt)L(py)]
(1). Figure 1 displays a perspective view of the vanadium-
(V) mixed-ligand complex 1, and bond parameters are
included in Table 2. There are two molecules in the
asymmetric unit, named 1a and 1b, with very similar
geometries. The vanadium atom in the molecule is hexaco-
ordinated, existing in a distorted octahedral environment in
which the basal plane is made of phenolic oxygen O(2),
enolic oxygen O(13), imine nitrogen N(10) from the triden-
tate ligand, and O(22) from the ethoxide oxygen. Unequal
lengths of the O(2)-V(1), O(21)-V(1), O(22)-V(1), O(13)-
V(1), N(10)-V(1), and N(25)-V(1) bonds as well as
unequal angles generated by these bonds at the V(1) acceptor
center point to a significant distortion of the coordination
octahedron.
[VO(OEt)L‚(py)] was structurally characterized by the single-
crystal X-ray diffraction technique. However, when any one
of the other bases, such as 2-picoline (pKa ) 6.13), 3-picoline
(pKa ) 6.02), 4-picoline (pKa ) 6.17), imidazole (pKa )
7.33), 4-methyl imidazole (pKa ) 7.61), and 4-amino
pyridine (pKa )9.53), all of which are stronger bases than
pyridine (pKa ) 5.48),44 is used, the product isolated is the
dioxovanadium complex BH[VO2L]. Similar observations
were reported previously.45 All complexes 1-12 are found
to be diamagnetic, as expected for a d0 system. Molar
conductance values of these dioxovanadium(V) complexes
in water (Table 6) support their 1:1 electrolytic nature.46,47
Structure determinations of these dioxovanadium(V) com-
plexes have firmly established these findings.
These dioxovanadium(V) complexes were found to un-
dergo an interesting transformation. When a suspension of
any one of them in an alcohol (ROH) was treated with a
solution of HCl in the same alcohol, it was found to be
converted into the corresponding monooxoalkoxo species
[VO(OR)L], in which the R-group comes from the alcohol
used as the reaction medium under ambient conditions. So
this process could be utilized as a general method for
preparing [VO(OR)L] type oxoalkoxo complexes from any
BH[VO2L] species using different alcohols as the reaction
medium under ambient conditions. Reaction of complexes
A and 1 with the 4,4′-bipy type bridging ligands in a 2:1
molar ratio in refluxing ethanol led to isolation of the
binuclear 4,4′-bridged complexes 2-4, of which 2 and 3 were
structurally characterized. All of the above reactions are
presented in Scheme 1.
One of our objectives for preparing dinuclear vanadium-
(V) complexes, in which two V(V) acceptor centers are
bridged by 4,4′-bipyridine and related ligands, is to get access
to the domain of mixed-valence divanadium(IV,V) com-
plexes48 through selective electrochemical reduction of one
of the two V(V) centers to the V(IV) state. Unfortunately,
all three such dinuclear complexes 2-4 are found to undergo
simultaneous one-electron reduction at both V(V) centers in
The Schiff-base ligand salicylaldehydehydrazone of 2-hy-
droxybenzoylhydrazine (H2L) forms one six-membered and
another five-membered chelate ring at the V(V) acceptor
center, with the corresponding bite angles being 83.1(1) and
5154 Inorganic Chemistry, Vol. 45, No. 13, 2006

Binuclear OxoWanadium(V) with 4,4′-Bipyridine Bridge
Table 3. Crystal Data and Structure Determination Summary of Complexes 5, 5A, 8, and 10
5
5A
8
10
empirical formula
fw
C18H17N4O5V
420.30
C18H21N4O7V
456.30
C20H18N3O5V
431.31
C19H17N4O5V
432.31
cryst syst
space group
a (Å)
b (Å)
c (Å)
R (deg)
â (deg)
γ (deg)
V (Å3)
triclinic
monoclinic
P21/a (No.14)
15.151(17)
7.078(10)
18.68(2)
(90)
99.16(1)
(90)
2090(4)
monoclinic
P21/n (No.14)
11.161(14)
14.194(17)
24.40(3)
(90)
100.47(1)
(90)
3801(8)
triclinic
P1h (No.2)
8.622(9)
10.431(12)
12.233(14)
111.98(1)
93.82(1)
109.00(1)
942.1(18)
2
P1h (No.2)
9.662(12)
10.443(12)
10.640(12)
89.40(1)
63.54(1)
88.40(1)
961(2)
Z
D
4
8
2
calc d(g cm-3
)
1.482
1.529
1.507
1.494
data/restraints/params
no. of obsd reflns [I > 2σ(I)]
R(F), wR(F2) (obsd data)a
R(F2), wR(F2) (all data)a
Rint
2845/0/255
1434
0.1177, 0.2245
0.2044, 0.2722
0.099
3252/0/273
2723
0.1310, 0.2581
0.1497, 0.2666
0.085
5309/0/528
4554
0.1102, 0.2436
0.1287, 0.2533
0.078
3364/0/264
2850
0.0476, 0.1089
0.0615, 0.1152
0.030
peaks in final difference map (e A-3
GOF on F2
)
0.38, -0.33
1.06
1.44,-0.45
1.251
0.45-0.39
1.217
0.40-0.32
1.023
a
2
2
4
a R ) ∑||Fo| - |Fc||/∑|Fo|; wR(F2) ) [∑w(|Fo| - |Fc| )2/∑w|Fo| ]1/2
.
Table 4. Selected Bond Distances (Å) and Angles (deg) in 5, 5A, 8,
and 10
Table 5. Hydrogen Bond Distances (Å) and Bond Angles (deg) of
Complexes 1, 2, 5, 5A, 8, and 10
5
5A
8a
8b
10
D-H‚‚‚A
d(H‚‚‚A)
DHA d(D‚‚‚A)
Complex 1a
145
symmetry
V(1)-O(21)
V(1)-O(22)
V(1)-O(2)
V(1)-O(13)
V(1)-N(10)
1.633(9)
1.637(9)
1.911(8)
1.981(8)
2.157(9)
1.627(9)
1.603(9)
1.901(7)
1.989(7)
2.106(8)
1.612(7)
1.638(6)
1.898(7)
1.980(6)
2.129(6)
1.602(7)
1.649(8)
1.902(6)
1.994(6)
2.156(6)
1.630(3)
1.630(4)
1.892(3)
1.982(3)
2.151(4)
O20-H20‚‚‚ N11
O20-H20‚‚‚ N11
O20-H20‚‚‚N11
1.87
2.59
Complex 1b
1.89
146
2.607
2.628
Complex 2
144
O2-V1-O13 150.6(4)
O2-V1-O21 103.1(4)
O2-V1-O22 94.1(4)
O2-V1-N10 80.9(3)
O13-V-O21 101.0(4)
O13-V-O22 93.6(4)
O13-V-N10 75.9(3)
O21-V-O22 109.4(5)
O21-V-N10 108.0(4)
O22-V-N10 142.5(4)
156.0(3)
156.0(2)
147.9(2)
154.60(9)
101.49(10)
96.77(12)
82.58(9)
97.30(10)
92.75(11)
73.67(8)
109.78(14)
117.02(12)
132.38(12)
1.92
100.3(4)
96.1(4)
82.2(3)
96.4(4)
94.9(4)
74.4(3)
108.1(4)
121.2(3)
130.3(4)
100.5(3)
97.7(3)
82.5(2)
94.4(3)
95.1(3)
73.8(2)
108.7(3)
119.4(3)
131.1(3)
104.8(3)
97.9(3)
82.2(2)
101.3(3)
91.2(3)
73.5(2)
108.6(4)
104.1(3)
146.1(3)
Complex 5
145
121
165
151
O20-H20‚‚‚N11
N25-H25‚‚‚O20
N23-H23‚‚‚O2
N25-H25‚‚‚O13
1.85
2.62
2.19
2.34
2.567
3.154
3.034
3.122
x - 1, y - 1, z - 1
2 - x, 2 - y, 2 - z
Complex 5A
144
O20-H20‚‚‚N11
O29-H29a‚‚‚O30
O29-H29b ‚‚‚O22
O30-H30a‚‚‚O2
O30-H30a‚‚‚O22
O30-H30b‚‚‚O13
O30-H30b‚‚‚O21
N23-H23‚‚‚O21
N25-H25‚‚‚O29
1.84
1.88
2.11
2.18
2.47
2.36
2.48
1.86
1.91
2.557
2.698
2.862
2.910
3.224
3.058
3.235
2.718
2.761
162
147
144
148
140
149
174
168
-x + 1/2, y - 1/2, -z
-x + 1/2, y - 1/2, -z
-x + 1/2, 1/2 + y, -z
-x + 1/2, 1/2 + y, -z
1 - x, -y, -z
74.9(1)° in 1a and 81.3(1) and 74.9(1)° in 1b. The two axial
positions are occupied by the O(21) oxo atom and N(25)
atom of the secondary ligand pyridine. The short V(1)-O(21)
distances of 1.586(3) and 1.580(4) Å in 1a and 1b,
respectively, indicate the presence of a vanadium-oxygen
double bond (VdO), which is commonly found in five- and
six-coordinate octahedral complexes of vanadium(IV) and
(V). In both molecules a and b of complex 1, the four V-O
bond lengths are unequal with the VdO bond being the
shortest and the V-O (enolate oxygen O(13)) bond the
longest. The vanadium-oxygen bond lengths follow the
order V-O(oxo) < V-O(alkoxide) < V-O(phenoxide) <
O(enolate). These data indicate stronger binding of the alkoxo
group compared to those of phenoxo and enolate oxygen
atoms, which is not uncommon for V(V) complexes of
related ligands.49-51 The V(1)-N(25) bond distances at
2.476(4) and 2.426(4) Å are significantly longer than the
other V(1)-N(10) bonds at 2.131(3) and 2.152(4) Å,
indicating that the secondary ligand (pyridine) is weakly
bonded to the central vanadium atom.
-x + 1/2, 1/2 + y, -z
Complex 8a
147
O20-H20‚‚‚N11
N23-H23‚‚‚O22
1.86
1.78
2.585
2.625
166
Complex 8b
146
O20-H20‚‚‚N11
N23-H23‚‚‚O22
1.87
1.83
2.593
2.676
167
Complex 10
147
O20-H20‚‚‚N11
N23-H23‚‚‚O22
N23-H23‚‚‚O20
N29-H29a‚‚‚O21
N29-H29b‚‚‚O21
1.883
2.120
2.642
2.078
2.209
2.607
2.829
3.119
2.896
2.969
139
116
159
147
x, 1 + y, z
-x + 1, -y + 1, -z
-x, -y + 1, -z + 1
symmetry. This symmetry is reflected in the electrochemical
behavior of 2, with the two V(V) centers undergoing
reduction at the same potential. The molecular structure and
atom-labeling scheme of 2 are illustrated in Figure 2, and
dimensions are provided in Table 2.
(b) [VO(OEt)L]2(µ-4,4′-bipy) (2). The golden yellow
complex [VO(OEt)L]2(µ-4,4′-bipy) (2) crystallizes such that
the dinuclear molecule contains a crystallographic center of
Each half of the binuclear complex closely resembles the
structure of 1, which essentially consists of the same donor
Inorganic Chemistry, Vol. 45, No. 13, 2006 5155

Sutradhar et al.
imzH]+[VO2L]-‚2H2O for 5A, [3-picH]+[VO2L]- for 8, and
[4-ampyH]+[VO2L]- for 10. There are two molecules in the
asymmetric unit of 8, marked 8a and 8b. In all four
structures, the complex anion [VO2L]- contains the V(V)
acceptor center surrounded by a O4N- donor set. As shown
in Table 4, structures 5 and 8b are close to being square
pyramidal in geometry, whereas 5A, 8a, and 10 are distorted
more toward trigonal bipyramidal geometry. A good indicator
of the distortion is provided by the O(22)-V(1)-N(10)
angle, which is 142.5(4)° in 5 and 146.1(3)° in 8b, whereas
it is 130.3(4)° in 5A, 131.1(3)° in 8a, and 132.4(1)° in 10.
However, all structures can be described adequately in
terms of distorted square pyramidal geometry. The dianionic
tridentate ligand (L2-) occupies three positions of the basal
plane, forming one five- and one six-membered chelate ring
around the V(V) center; the bite angles range from 73.5(2)
to 75.9(3)° and 80.9(3) to 82.6(1)° in the five structures.
There are two terminal oxygen atoms, O(21) and O(22), with
VdO distances in the range of 1.602(9)-1.649(8) Å. These
values are slightly longer than the values found in 1 and 2,
which contain only one terminal oxygen atom. The angle
between the two terminal oxygen atoms ranges from 108.1-
(4)-109.8(1)°. In the square pyramid, O(21) occupies an
axial position and O(22) an equatorial position, but there is
no significant difference between these VdO bond lengths.
Along with O(22), the basal plane is formed by two
equatorially placed oxygen atoms, phenolate oxygen O(2)
and enolate oxygen O(13), the fourth corner of the square
plane being occupied by azomethine nitrogen N(10). The
apical vanadyl oxygen subtends angles at the metal with the
atoms in the basal plane in the range of 94.4(3)-121.2(3)°,
with the smallest angles to O(13). In the ligand, the V-O
distances are significantly shorter than the V-N distance.
Such observations are reported previously49-51 and suggest
some π donation leading to partial multiple-bond character,
which is often observed for oxygen coordination in V(V)
complexes. An additional reason may well be that the
nitrogen atom is approximately trans to terminal multiple-
bonded oxygen O(22). It is also noted that the V-O bond
involving phenolate oxygen O(2) is shorter than the similar
bond involving enolate oxygen O(13).
The V(V) acceptor centers are all displaced from the
average basal plane defined previously toward apical oxo-
oxygen O(21) by a distance in the range 0.48-0.52 Å. Such
a displacement of the V(V) center makes it somewhat
inaccessible to a donor, which may approach from the
direction trans to this apical oxo-oxygen.
In all the structures, there is an intramolecular hydrogen
bond between O(20) and N(11). N‚‚‚H‚‚‚O bond distances
and other related details are given in Table 5. This hydrogen
bond enables the ligand to remain closely planar in the crystal
structures. The presence of cations in these crystal structures
gives rise to intermolecular hydrogen bonds between the
cations and anions; details are given in Table 5. Complex 5
contains the anion [VO2L]- and the 4-methyl imidazolium
cation [4-Me imzH]+. The two N-H groups on the cation
are both hydrogen bonded to the chelating oxygen atoms
O(2) and O(13) of two different anions. 5A also contains
Table 6. Molar Conductance and Cyclic Voltammetric Data of
Vanadium(V) Complexes
a
molar conductance ΛM
E1/2 (V)
compound
[VO(OEt)L] (A)
(Ω-1 cm2 M-1
)
(∆Epb (mV))
non-electrolytec
non-electrolytec
non-electrolytec
0.42 (150)d
0.415 (70)d
0.42 (60)d,e
[VO(OEt)L(Py)] (1)
[VO(OEt)L]2(µ-4,4′-bipy) (2)
[VO(OEt)L]2(µ-1,2-bis
(4-pyridyl)ethylene) (3)
[VO(OEt)L]2(µ-1,2-bis
(4-pyridyl)ethane) (4)
[4-Me imzH][VO2L] (5)
[4-Me imzH][VO2L]‚2H2O (5A)
[imzH][VO2L] (6)
non-electrolytec
0.435 (110)d,e
non-electrolytec
66.6f
0.405 (152)d,e
0.39 (138)d
0.409 (126)d
0.439 (108)d
0.442 (142)d
0.453 (109)d
0.445 (130)d
0.433 (74)d
0.416 (142)
0.408 (136)d
70.6f
71.1f
[2-picH][VO2L] (7)
[3-picH][VO2L] (8)
[4-picH][VO2L] (9)
67.0f
67.8f
69.1f
[4-ampyH][VO2L] (10)
[VO(OMe)L] (11)
69.3f
non-electrolytec
non-electrolytec
[VO(OnBu)L] (12)
a ΛM observed in 0.001 molar solution. b ∆Ep ) Epa - Epc. c In dry
methanol at 25 °C. Complexes A, 1-4, 11, and 12 are insoluble in water
but soluble in methanol. d In acetonitrile. e In dichloromethane; scan rate
of 50 mVs-1; E1/2 is calculated as the average of anodic (Epa) and cathodic
(Epc) peak potentials. f In water at 25 °C.
atoms attached to its V(V) center in the same geometric
pattern with closely matching dimensions. This is expected
because complex 2 is obtained from complex 1 by simply
replacing the pyridine nitrogen atom by one nitrogen atom
of the 4,4′-bipyridine bridging nitrogen ligand. The only
significant difference observed in the case of 2 is that the
V(1)-N(25) bond involving the vanadium acceptor center
and the N(25) aromatic nitrogen atom of 4,4′-bipy is shorter
(2.385(8) Å) than the corresponding V(1)-N(25) bonds in
1, which are 2.476(4) and 2.426(4) Å. This shortening
indicates comparatively stronger binding of 4,4′-bipy nitrogen
N(25) to the V(V) center and may be responsible for the
facile transformation of [VO(OEt)L(py)] to [VO(OEt)L]2-
(µ-4,4′-bipy). The two pyridine rings of 4,4′-bipy lie in the
same plane and are perpendicular to the equatorial planes
of the two halves of the two [VO(OEt)L] units. The
vanadium acceptor center in 2 is displaced by 0.30 Å toward
the oxo oxygen atom. Similar distances are found in the two
molecules of 1, e.g., 0.29 and 0.31Å in a and b, respectively.
(c) [VO(OEt)L]2(µ-1,2-bis(4-pyridine)ethylene) (3). The
structure of complex 3 was disordered. There were two
discrete molecules in the structure. One, shown in Figure 3,
contains a crystallographic center of symmetry. The second
is disordered over a center of symmetry with 50% occupancy.
It was not possible to refine the structure below an R1 value
of 0.18 and so the dimensions are not reported here.
However, as illustrated by the ORTEP diagram of 3 shown
in Figure 3, the structure of the ordered molecule exhibits
close structural similarity to 2 and indeed also contains a
crystallographic center of symmetry. This structure is
consistent with the spectral and electrochemical properties
of this complex.
(d) Structure of the Anionic [VO2L]- Complexes of
Type [BH]+[VO2L]-. ORTEP diagrams of the complexes
of type BH+[VO2L]- are presented in Figures 4-7, and the
relevant dimensions are listed in Table 4. The formulas for
the four complexes are [4-Me imzH]+[VO2L]- for 5, [4-Me
5156 Inorganic Chemistry, Vol. 45, No. 13, 2006

Binuclear OxoWanadium(V) with 4,4′-Bipyridine Bridge
Scheme 1a
a B ) 4-methyl imidazole (5), imidazole (6), 2- picoline (7), 3-picoline (8), 4-picoline (9), 4-amino pyridine(10). N-N ) 4,4′-bipyridine (2), 1,2-bis(4-
n
pyridyl)ethylene (3), 1,2-bis(4-pyridyl)ethane (4). R ) CH3 (11) and C4H9 (12).
Figure 1. Molecular structure and atom-numbering scheme for complex 1, with thermal ellipsoids drawn at the 30% probability level. Two equivalent
molecules in the asymmetric unit, 1a and 1b, are shown. The intramolecular hydrogen bond is shown as a dotted line.
the complex anion [VO2L]- and the 4-methyl imidazolium
cation [4-Me imzH]+, as well as two water molecules. The
two N-H groups both form hydrogen bonds; in this case,
one nitrogen is hydrogen bonded to terminal oxygen atom
O(22) of the anion, whereas the other is hydrogen bonded
to a water molecule. There are also several hydrogen bonds
between the water molecules and both terminal oxygen atoms
as well as one between each other. By contrast, 8 contains
two 3-methyl-pyridinium cations together with two anions.
Both N-H groups are hydrogen bonded to a terminal oxygen
atom of an anion. In 10, the cation is 4-amino-pyridinium.
The pyridinium nitrogen atom is hydrogen bonded to O(20),
whereas the two amine hydrogen atoms are hydrogen bonded
to the terminal oxygen atoms of two different anions.
1
IR and H NMR spectroscopy. IR spectra of all the
monooxoalkoxo complexes contain all the characteristic
bands of the coordinated tridentate anionic ligand given in
the Experimental Section. In addition, the ν(VdO) bands
are observed in the 918-995 cm-1 range.21,25,38,55 In the case
of the dioxovanadium(V) complexes, the only difference
Inorganic Chemistry, Vol. 45, No. 13, 2006 5157

Sutradhar et al.
Figure 2. Molecular structure and atom-numbering scheme for complex 2, with thermal ellipsoids drawn at the 30% probability level. Intramolecular
hydrogen bonds are shown as dotted lines.
coordinated ligand, the spectrum of 11 exhibited one singlet
at 5.34 ppm that corresponds to three protons of the OCH3
group. Similarly, the spectrum of 12 displayed the charac-
teristic proton signals of the n-butoxy group at 1.06, 1.5,
2.05, and 5.59 ppm that correspond to a triplet of three
protons, multiplet of two protons, multiplet of two protons,
and triplet of two protons, respectively.
Electronic Spectra. The electronic spectral data of the
free ligand (H2L) and the vanadium(V) complexes are given
in the Experimental Section. The electronic spectra of all
the vanadium(V) complexes (1-12) in CH3CN solution
exhibit one intense absorption band in the 399-398 nm
region, which is assignable to L-V (dπ) LMCT transition,25
and three other absorption bands in the 335-285 nm region,
which correspond to intraligand transitions.
Figure 3. ORTEP view of the ordered molecule in complex 3. Intramo-
lecular hydrogen bonds are shown as dotted lines.
Electrochemistry. Cyclic voltammetric data for all the
complexes are presented in Table 6. Cyclic voltammograms
of monooxoalkoxo complexes A, 11, and 12 in CH3CN
exhibit irreversible one-step, one-electron responses around
+0.41 V, and complex 1 shows a reversible one-step one-
electron response at +0.415 V, corresponding to V(V)/V(IV)
reduction. The binuclear V(V) complexes are usually ex-
observed is the appearance of two ν(VdO) modes that
correspond to the symmetric and antisymmetric pair of cis
VdO groups.50,52 The positions of these two ν(VdO) bands
differ very little from complex to complex, because complex
anion [VO2L]- is the same in all the complexes; it differs
only in the cation outside the coordination zone of the V(V)
acceptor center.
pected to undergo a two-step, one-electron reduction, with
1H NMR data of all the complexes are given in the
the first step leading to the formation of the mixed-valence
Experimental Section. Complexes [VO(OMe)L] (11) and
[VO(OnBu)L] (12) were identified by their respective NMR
spectra in CDCl3. Apart from all the proton signals of the
V(V)-V(IV) species. But contrary to such an expectation,
binuclear complexes 2-4 were found to undergo only one-
step reduction in both CH3CN and CH2Cl2 in the +0.40-
0.435 V potential range, which is reversible in the case of 2
(55) Nakamoto, K. Infrared and Raman Spectra of Inorganic Coordination
Compounds, Wiley: New York, 1984; p 341.
but irreversible for 3 and 4. The centrosymmetric structure
5158 Inorganic Chemistry, Vol. 45, No. 13, 2006

Binuclear OxoWanadium(V) with 4,4′-Bipyridine Bridge
Figure 4. Molecular structure and atom-numbering scheme for complex 5, with thermal ellipsoids drawn at the 30% probability level. Hydrogen bonds are
shown as dotted lines.
Figure 5. Molecular structure and atom-numbering scheme for complex 5A, with thermal ellipsoids drawn at the 30% probability level. Hydrogen bonds
are shown as dotted lines.
of 2 confirms that the coordination environments of the two
V(V) centers are identical and explains their simultaneous
reduction at the same potential. The similar values of E1/2
for the monooxoalkoxo complexes can be related to similarity
in the donor environments of the V(V) centers in each
complex.
Inorganic Chemistry, Vol. 45, No. 13, 2006 5159

Sutradhar et al.
Figure 6. Molecular structure and atom-numbering scheme for complex 8, with thermal ellipsoids drawn at the 30% probability level. Two equivalent
molecules in the asymmetric unit, 8a and 8b, are shown. Hydrogen bonds are shown as dotted lines.
Figure 7. Molecular structure and atom-numbering scheme for complex 10, with thermal ellipsoids drawn at the 30% probability level. Hydrogen bonds
are shown as a dotted line.
In all the dioxo complexes, the V(V)/V(IV) reductions take
place in the +0.40-0.45 V region. The E1/2 values are close
to each other and also close to the E1/2 values of the
monooxoalkoxo complexes because of the similarity in the
coordination environments of the V(V) center in both the
classes. Three representative cyclic voltammograms of the
three different types of complexes (1, 2, and 10) are presented
in Figure 8.
5160 Inorganic Chemistry, Vol. 45, No. 13, 2006

Binuclear OxoWanadium(V) with 4,4′-Bipyridine Bridge
(OEt)L‚(py)] (1), which, along with complex [VO(OEt)L]
(A), acted as the precursor of a number of dioxovanadium-
(V) complexes with the general formula BH[VO2L], where
B is a neutral monodentate Lewis base. It is established that
some neutral monodentate donors, such as imidazole,
4-methyl imidazole, 2-picoline, 3-picoline, 4-picoline, and
4-amino pyridine, all of which are stronger bases than
pyridine, caused such a transformation. These monomeric
dioxovanadium complexes can also be prepared from di-
nuclear complexes 2-4 by refluxing with excess base B.
All the structurally characterized dioxovanadium(V) com-
plexes are found to possess a significantly distorted square
pyramidal donor environment. The presence of two asym-
metric units in the unit cell of each complex is to be noted.
It is also confirmed experimentally that such a transformation
is reversed on acidification in an alcohol medium. An
interesting aspect of the last reaction is that it serves as a
general method for preparing [VO(OR)L] type complexes
starting from any one of the BH[VO2L] type complexes
using different alcohols. The presence of different alkoxo
groups is confirmed from NMR data for the respective
complexes. Both A and 1 produced the same binuclear
complex [VO(OEt)L]2(µ-4,4′-bipy) (2), which we believe to
be the first report of a structurally characterized µ-4,4′-
bipy-bridged binuclear vanadium(V) complex. Two other
bridged binuclear vanadium(V) complexes, 3 and 4, are also
reported.
Acknowledgment. M.S. is grateful to the Council of
Scientific and Industrial Research (New Delhi) for the award
of a junior research fellowship. We thank Dr. P. K.
Bhattacharya, IICB, Kolkata for useful discussion.
Supporting Information Available: Crystallographic data for
the structural analysis of the complexes (1, 2, 5, 5A, 8, and 10)
have been deposited in the Cambridge Crystallographic Data Centre,
with CCDC No. 276166 (1), 276167(2), 276168(5), 276169(5A),
276170(8) and 276171 (10). Copies of this information may be
obtained free of charge from The Director, 12 Union Road,
Cambridge, CB2 IEZ, UK (fax: 44-1223-336033; E-mail:
deposit@ccdc.cam.ac.uk or www: http://www.ccdc.cam.ac.uk).
This material is also available free of charge via the Internet at
http://pubs.acs.org.
Figure 8. Cyclic voltammograms of (a) complex 1 (b) complex 2 and (c)
complex 10 in CH3CN (0.1 M TEAP) at a platinum electrode with a scan
rate of 50 mV s-1; the potential was recorded vs SCE.
Conclusion
This work reports the preparation and structural charac-
terization of the monooxoalkoxovanadium(V) complex [VO-
IC051120G
Inorganic Chemistry, Vol. 45, No. 13, 2006 5161