Synthesis, structure and solution chemistry of dioxidovanadium(V) complexes with a family of hydrazone ligands. Evidence of formation of centrosymmetric dimers via H-bonds in the solid state

Article abstract of DOI:10.1016/j.ica.2010.03.053

[V^IVO(acac)₂] reacts with the methanol solution of tridentate ONO donor hydrazone ligands (H₂L^1-4, general abbreviation H₂L; are derived from the condensation of benzoyl hydrazine with 2-hydroxyacetophenone and its 5-substituted derivatives) in presence of neutral monodentate alkyl amine bases having stronger basicity than pyridine e.g., ethylamine, diethylamine, triethylamine and piperidine (general abbreviation B) to produce BH⁺[VO₂L]^- (1-16) complexes. Five of these sixteen complexes are structurally characterized revealing that the vanadium is present in the anionic part of the molecule, [VO₂L]^- in a distorted square pyramidal environment. The complexes 5, 6, 15 and 16 containing two H-atoms associated with the amine-N atom in their cationic part (e.g., diethylammonium and piperidinium ion) are involved in H-bonding with a neighboring molecule resulting in the formation of centrosymmetric dimers while the complex 12 (containing only one hydrogen atom in the cationic part) exhibits normal H-bonding. The nature of the H-bonds in each of the four centrosymmetric dimeric complexes is different. These complexes have potential catalytic activity in the aerial oxidation of l-ascorbic acid and are converted into the [VO(L)(hq)] complexes containing VO³⁺ motif on reaction with equimolar amount of 8-hydroxyquinoline (Hhq) in methanol.

Full text of DOI:10.1016/j.ica.2010.03.053

Inorganica Chimica Acta 363 (2010) 2296–2306
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Inorganica Chimica Acta
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Synthesis, structure and solution chemistry of dioxidovanadium(V) complexes
with a family of hydrazone ligands. Evidence of formation of centrosymmetric
dimers via H-bonds in the solid state
Bipul Mondal a, Michael G.B. Drew b, Tapas Ghosh a,
*
a Department of Chemistry, Ramakrishna Mission Vivekananda Centenary College, Rahara, Kolkata 700118, India
b Department of Chemistry, The University of Reading, PO Box 224 Whiteknights, Reading RG6 6AD, UK
a r t i c l e i n f o
a b s t r a c t
[VIVO(acac)2] reacts with the methanol solution of tridentate ONO donor hydrazone ligands (H2L1–4, gen-
eral abbreviation H2L; are derived from the condensation of benzoyl hydrazine with 2-hydroxyacetophe-
none and its 5-substituted derivatives) in presence of neutral monodentate alkyl amine bases having
stronger basicity than pyridine e.g., ethylamine, diethylamine, triethylamine and piperidine (general
abbreviation B) to produce BH+[VO2L]ꢀ (1–16) complexes. Five of these sixteen complexes are structur-
ally characterized revealing that the vanadium is present in the anionic part of the molecule, [VO2L]ꢀ in a
distorted square pyramidal environment. The complexes 5, 6, 15 and 16 containing two H-atoms associ-
ated with the amine-N atom in their cationic part (e.g., diethylammonium and piperidinium ion) are
involved in H-bonding with a neighboring molecule resulting in the formation of centrosymmetric
dimers while the complex 12 (containing only one hydrogen atom in the cationic part) exhibits normal
H-bonding. The nature of the H-bonds in each of the four centrosymmetric dimeric complexes is differ-
Article history:
Received 13 January 2010
Received in revised form 16 March 2010
Accepted 19 March 2010
Available online 27 March 2010
Keywords:
Mixed-ligand oxidovanadium(V) complexes
Hydrazone complexes
Crystal structures
Hydrogen bonded centrosymmetric dimers
Reactivity
ent. These complexes have potential catalytic activity in the aerial oxidation of L-ascorbic acid and are
converted into the [VO(L)(hq)] complexes containing VO3+ motif on reaction with equimolar amount
of 8-hydroxyquinoline (Hhq) in methanol.
Ó 2010 Elsevier B.V. All rights reserved.
1. Introduction
vanadate and phosphate have been proposed to be responsible in
explaining the mode of action in biological systems [3]. Such prop-
Among the three (viz., +III, +IV and +V) physiologically impor-
tant oxidation states of vanadium, the +V state has received con-
siderable attention. The research interest on this oxidation state
is largely fueled not only due to its interesting chemistry associ-
erties of vanadium along with its coordinative flexibility make it
biologically important.
Hydrogen bonding plays significant roles in several biological
processes e.g., in the regulation of metal ion reactivity as well as
in the recognition and transport of various small molecules and
ions [4–6], the catalytic cycle of haloperoxidase activity has been
suggested to proceed through hydrogen bonding network [7–10],
etc. In presence of appropriate hydrogen bonding donors, the
hydrogen bonding is a general feature of vanadium(IV) and vana-
dium(V) complexes [11–17]. In general, these examples lead to for-
mation of hydrogen bonded molecular assemblies ranging from
simple dimers to three-dimensional networks.
Hydrazone ligands derived from the condensation of aliphatic/
aromatic acid-hydrazide with aromatic 2-hydroxycarbonyl com-
pounds are important tridentate ONO donor chelating ligands con-
taining two intermediate basic phenolic and amide groups and one
neutral imine moiety have the ability to stabilize the +V state of
vanadium. As a part of our programme on the study of oxidovana-
dium(IV) and oxidovanadium(V) complexes with a family of ben-
zoyl hydrazone ligands (H2L1–4, general abbreviation H2L) it has
been found that these ligands exclusively formed [VV2O3L2]
ated with its ability to exist in three different motifs viz., VO3+
,
4+
+
V2O3 and VO2 but also its involvement in many catalytic pro-
cesses. Vanadium easily switches between the oxidation states
+IV and +V and their relative stability depend upon the basicity
of the coordinated ligands. The redox potential at pH 7 for the cou-
ple H2VOꢀ þ 4Hþ þ e ꢀ VO2þ þ 3H2O amounts to ꢀ0.341 V and in
4
this range the vanadyl (VO2+) is oxidized to vanadate under aerobic
conditions and vanadate reduced to vanadyl by cellular compo-
nents such as cysteine containing peptides (glutathione) and pro-
teins, ascorbate, NADH and phenolic compounds [1]. The main
species present under physiological aerobic conditions is the acid–
base pair H2VOꢀ ꢀ HVO42ꢀ þ Hþ (pKa = 8.1) [2] and sufficiently
4
strong chelating ligands are required to stabilize the cationic VV
species in solution around pH 7. The chemical similarities between
* Corresponding author. Tel.: +91 3325685674.
E-mail address: ictg_64@yahoo.co.in (T. Ghosh).
0020-1693/$ - see front matter Ó 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.ica.2010.03.053

B. Mondal et al. / Inorganica Chimica Acta 363 (2010) 2296–2306
2297
complexes [18] (L2ꢀ is the dianion of the tridentate hydrazone li-
gand) on reaction with equimolar amount of [VIVO(acac)2] (acacꢀ
representing the acetylacetonate ion) in a non-hydroxylic solvent
like acetone, CH2Cl2, CHCl3, etc., while the similar reaction mixture
in methanol afforded either [VVO(L)(OCH3)] or [VVO(L)(OCH3)]2
complexes (depending upon the substituent in the aryloxy ring
of the hydrazone ligands) [19]. However, in presence of excess pyr-
idine (py), the methanolic solution of [VIVO(acac)2] and H2L yielded
the quaternary [VVO(L)(OCH3)(py)] complexes [20]. This work has
been done with view to studying the type of complexes formed
by the reaction of equimolar mixture of [VIVO(acac)2] and H2L in
presence of excess monodentate neutral amine bases having high-
er basicity than pyridine. In this paper we report the following
findings of this work: (i) synthesis and characterization of sixteen
dioxidovanadium(V)-hydrazone complexes; (ii) the solid state
characterization of centrosymmetric dimers that are formed via
H-bonding between the cationic and anionic part of the molecules
of four complexes which to the best of our knowledge, is the first
report of structurally characterized H-bridged centrosymmetric di-
mers of dioxidovanadium(V)-hydrazone complexes; (iii) catalytic
5.3; N, 11.0. Found: C, 53.2; H, 5.1; N, 10.9%. IR (KBr,
m
max/cmꢀ1):
1589 (C@Nazomethine), 1365 (C–Ophenolic), 1238 (C–Oenolate), 1032
(N–N), 897, 862 (V@O); 2779-3092 (N–H). KM (DMF) = 58 Xꢀ1
cm2 molꢀ1. kmax (DMSO)/nm: 386 ( /dm3 molꢀ1 cmꢀ1 11,235),
e
316 (12,400). 1H NMR (DMSO-d6, d/ppm): 6.79 (brd, 1H, H-3,
J = 8.0, Hz), 7.56–7.59 (m, 2H, H-4, H-13), 7.30 (brt, 1H, H-5,
J = 7.8 Hz), 7.76 (brd, 1H, H-6, J = 7.8 Hz), 2.86 (s, 3H, 3 ꢂ H-8),
8.05 (dd, 2H, H-11, H-15, J = 7.6, 2.30 Hz), 7.45–7.47 (m, 2H, H-
12, H-14), 2.80–2.85 (m, 2H, 2 ꢂ H-16), 1.14 (t, 3H, 3 ꢂ H-17,
J = 7.3 Hz).
2.2.2. (eaH)+[VVO2(L2)]ꢀ (2)
Yield: 0.35 g (89%). Anal. Calc. for C18H22N3O4V: C, 54.7; H, 5.6;
N, 10.6. Found: C, 54.4; H, 5.5; N, 10.5%. IR (KBr,
(C@Nazomethine), 1360 (C–Ophenolic), 1242 (C–Oenolate), 1032 (N–N),
933, 860 (V@O); 2712-2980 (N–H). KM (DMF) = 56
ꢀ1 cm2 molꢀ1
kmax (DMSO)/nm: 395 (
/dm3 molꢀ1 cmꢀ1 9,617), 320 (12,556). 1H
m
max/cmꢀ1): 1581
X
.
e
NMR (DMSO-d6, d/ppm): 6.69 (d, 1H, H-3, J = 8.1 Hz), 7.11 (brd, 1H,
H-4, J = 8.1 Hz), 2.28 (s, 3H, 5-CH3), 7.44–7.46 (m, 2H, H-6, H-13),
2.86 (s, 3H, 3 ꢂ H-8), 8.04 (dd, 2H, H-11, H-15, J = 7.8, 2.7 Hz),
7.54–7.58 (m, 2H, H-12, H-14), 2.79-2.83 (m, 2H, 2 ꢂ H-16), 1.13
(t, 3H, 3 ꢂ H-17, J = 7.2 Hz).
activity of these complexes in the aerial oxidation of
L-ascorbic acid
and (iv) the transformation of these VO2+-complexes to the VO3+
-
complexes at ambient condition. We have recently reported [18]
the dioxidovanadium(V) complexes of the type K(H2O)+[VO2(L)]ꢀ
containing this family of hydrazone ligands without any structural
characterization which are synthesized by the reaction of
[V2O3(L)2] complexes with KOH in methanol. A significant number
of such type of VO2+-complexes containing ONO donor hydrazone
ligands are also available in the literature [11–15,7,18,21–25]. Such
studies are important in connection with several catalytic [7–
10,26–35] and medicinal [36–49] activities of vanadium
complexes.
2.2.3. (eaH)+[VVO2(L3)]ꢀ (3)
Yield: 0.35 g (86%). Anal. Calc. for C18H22N3O5V: C, 52.6; H, 5.4;
N, 10.2. Found: C, 52.4; H, 5.3; N, 10.1%. IR (KBr,
(C@Nazomethine), 1356 (C–Ophenolic), 1290 (C–Oenolate), 1030 (N–N),
964, 932 (V@O); 3126-2804 (N–H). KM (DMF) = 55
ꢀ1 cm2 molꢀ1
kmax (DMSO)/nm: 410 (
/dm3 molꢀ1 cmꢀ1 12,076), 316 (17,812).
m
max/cmꢀ1): 1593
X
.
e
1H NMR (DMSO-d6, d/ppm): 6.75 (brd, 1H, H-3, J = 8.7 Hz), 6.97
(dd, 1H, H-4, J = 8.7, 2.7 Hz), 3.77 (s, 3H, 5-OCH3), 7.21 (d, 1H, H-
6, J = 2.7 Hz), 2.85 (s, 3H, 3 ꢂ H-8), 8.06 (brd, 2H, H-11, H-15,
J = 6.6 Hz), 7.46–7.48 (m, 2H, H-12, H-14), 7.56–7.59 (m, 1H, H-
13), 2.80–2.82 (m, 2H, 2 ꢂ H-16), 1.14 (t, 3H, 3 ꢂ H-17, J = 7.2 Hz).
2. Experimental
2.2.4. (eaH)+[VVO2(L4)]ꢀ (4)
2.1. Materials
Yield: 0.39 g (94%). Anal. Calc. for C17H19N3O4ClV: C, 49.1; H,
4.6; N, 10.1. Found: C, 48.9; H, 4.5; N, 10.0%. IR (KBr, m
max/cmꢀ1):
[VIVO(acac)2] [50] and the hydrazone ligands (H2L1–4) [51] were
synthesized by the reported methods. Reagent grade solvents were
dried and distilled prior to use. All other chemicals were reagent
grade, obtained from commercial sources (Loba company, India)
and used without further purification. Spectroscopic grade solvents
were used for spectral measurements.
1595 (C@Nazomethine), 1358 (C–Ophenolic), 1296 (C–Oenolate), 1030
(N–N), 935, 874 (V@O); 2783-2991 (N–H). KM (DMF) = 51 Xꢀ1
cm2 molꢀ1. kmax (DMSO)/nm: 392 ( /dm3 molꢀ1 cmꢀ1 9,698), 318
e
(11,377). 1H NMR (DMSO-d6, d/ppm): 6.81 (d, 1H, H-3, J = 8.4 Hz),
7.31 (brd, 1H, H-4, J = 8.4 Hz), 7.74 (s, 1H, H-6), 2.85 (s, 3H,
3 ꢂ H-8), 8.04–8.06 (m, 2H, H-11, H-15), 7.47–7.49 (m, 2H, H-12,
H-14), 7.57–7.59 (m, 2H, H-12, H-14), 2.79-2.81 (m, 2H, 2 ꢂ H-
16), 1.13 (t, 3H, 3 ꢂ H-17, J = 7.2 Hz).
2.2. Synthesis of dioxidovanadium(V) complexes, (1ꢀ16)
All the sixteen (1–16) cis-VO2+ complexes containing the hydra-
zone ligands H2L1ꢀ4 reported in this work have been synthesized
by a simple general method using [VIVO(acac)2] as the starting
material in presence of excess monodentate alkyl amine bases like
ethylamine (ea), diethylamine (dea), triethylamine (tea) and piper-
idine (pip).
2.2.5. (deaH)+[VO2(L1)]ꢀ (5)
Yield: 0.39 g (96%). Anal. Calc. for C19H24N3O4V: C, 55.7; H, 5.9;
N, 10.3. Found: C, 55.6; H, 5.7; N, 10.0%. IR (KBr, m
max/cmꢀ1): 1601
(C@Nazomethine), 1367 (C–Ophenolic), 1250 (C–Oenolate), 1059 (N–N),
935, 897 (V@O); 2827–2988 (N–H). KM (DMF) = 54 Xꢀ1
cm2 molꢀ1. kmax (DMSO)/nm: 387 ( /dm3 molꢀ1 cmꢀ1 10,255),
e
316 (11,001). 1H NMR (DMSO-d6, d/ppm): 6.77–6.83 (m, 2H, H-3,
H-5), 7.29 (t, 1H, H-4, J = 7.7 Hz), 7.74 (d, 1H, H-6, J = 7.8 Hz),
2.83 (s, 3H, 3 ꢂ H-8), 8.02–8.04 (m, 2H, H-11, H-15), 7.43–7.45
(m, 3H, H-12, H-13, H-14), 2.89 (q, 4H, 2 ꢂ H-16, 2 ꢂ H-18,
J = 7.2 Hz), 1.13 (t, 6H, 3 ꢂ H-17, 3 ꢂ H-19, J = 7.2 Hz).
2.2.1. (eaH)+[VVO2(L1)]ꢀ (1)
To a warm methanolic solution (20 mL) of H2L1 (0.254 g,
1 mmol) was added a methanolic solution (10 cm3) of [VIVO(acac)2]
(0.265 g, 1 mmol) with stirring whereby a brown solution was ob-
tained. To this brown solution a methanolic solution (10 cm3) of
ethylamine (ꢁ1 cm3) was added with continuous stirring at room
temperature. Immediately the color of the solution changed to in-
tense yellow. This mixture was then stirred for 1 h at ꢁ60 °C and
then kept for slow evaporation at room temperature. After several
days a reddish yellow crystalline product was obtained which was
filtered followed by washing with methanol and dried over silica
gel. Yield: 0.35 g (93%). Anal. Calc. for C17H20N3O4V: C, 53.5; H,
2.2.6. (deaH)+[VO2(L2)]ꢀ (6)
Yield: 0.40 g (94%). Anal. Calc. for C20H26N3O4V: C, 56.7; H, 6.2;
N, 9.9. Found: C, 56.7; H, 6.1; N, 9.8%. IR (KBr,
(C@Nazomethine), 1358 (C–Ophenolic), 1244 (C–Oenolate), 1065 (N–N),
941, 899 (V@O); 2706–2984 (N–H). KM (DMF) = 53
ꢀ1 cm2
molꢀ1. kmax (DMSO)/nm: 396 ( /dm3 molꢀ1 cmꢀ1 10,484), 320 (
dm3 molꢀ1 cmꢀ1 13,094). 1H NMR (DMSO-d6, d/ppm): 6.68 (d,
m
max/cmꢀ1): 1582
X
e
e/

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B. Mondal et al. / Inorganica Chimica Acta 363 (2010) 2296–2306
1H, H-3, J = 8.3 Hz), 7.11 (d, 1H, H-4, J = 8.3 Hz), 2.26 (s, 3H, 5-CH3),
7.53 (s, 1H, H-6), 2.82 (s, 3H, 3 ꢂ H-8), 8.01–8.03 (m, 2H, H-11, H-
15), 7.43-7.45 (m, 3H, H-12, H-13, H-14), 2.79–2.83 (q, 4H, 2 ꢂ H-
16, 2 ꢂ H-18, J = 7.2 Hz), 1.14 (t, 6H, 3 ꢂ H-17, 3 ꢂ H-19, J = 7.2 Hz).
(q, 6H, 2 ꢂ H-16, 2 ꢂ H-18, 2 ꢂ H-20, J = 7.3 Hz), 1.14 (t, 9H,
3 ꢂ H-17, 3 ꢂ H-19, 3 ꢂ H-21, J = 7.3 Hz), 10.02 (brs, 1H, N-3).
2.2.12. (teaH)+[VO2(L4)]ꢀ (12)
Yield: 0.37 g (79%). Anal. Calc. for C21H27N3O4ClV: C, 53.5; H,
2.2.7. (deaH)+[VO2(L3)]ꢀ (7)
5.8; N, 8.9. Found: C, 53.3; H, 5.6; N, 8.8%. IR (KBr, m
max/cmꢀ1):
Yield: 0.38 g (87%). Anal. Calc. for C20H26N3O5V: C, 54. 7; H, 6.0;
1597 (C@Nazomethine), 1364 (C–Ophenolic), 1240 (C–Oenolate), 1072
N, 9.6. Found: C, 54.4; H, 5.9; N, 9.5%. IR (KBr, m
max/cmꢀ1): 1587
(N–N), 953, 876 (V@O); 2592-2976 (N–H). KM (DMF) = 43 Xꢀ1
cm2 molꢀ1. kmax (DMSO)/nm: 384 ( /dm3 molꢀ1 cmꢀ1 7,464), 310
e
(C@Nazomethine), 1362 (C–Ophenolic), 1275 (C–Oenolate), 1061 (N–N),
943, 895 (V@O); 2710–2984 (N–H). KM (DMF) = 50 Xꢀ1
(13,629). 1H NMR (CDCl3, d/ppm): 6.97 (d, 1H, H-3, J = 7.3 Hz),
7.38–7.42 (m, 4H, H-4, H-12, H-13, H-14), 7.70 (d, 1H, H-6,
J = 2.4 Hz), 2.92 (s, 3H, 3 ꢂ H-8), 8.12–8.16 (m, 2H, H-11, H-15),
3.35 (q, 6H, 2 ꢂ H-16, 2 ꢂ H-18, 2 ꢂ H-20, J = 6.9 Hz), 1.50 (t, 9H,
3 ꢂ H-17, 3 ꢂ H-19, 3 ꢂ H-21, J = 6.9 Hz), 12.17 (brs, 1H, N-3).
cm2 molꢀ1. kmax (DMSO)/nm: 410 ( /dm3 molꢀ1 cmꢀ1 9,482), 315
e
(14,051). 1H NMR (DMSO-d6, d/ppm): 6.75 (d, 1H, H-3, J = 8.9 Hz),
6.98 (dd, 1H, H-4, J = 8.9, 2.9 Hz), 3.75 (s, 3H, 5-OCH3), 7.21 (d,
1H, H-6, J = 2.9 Hz), 2.84 (s, 3H, 3 ꢂ H-8), 8.03-8.05 (m, 2H, H-11,
H-15), 7.45–7.47 (m, 3H, H-12, H-13, H-14), 2.91 (q, 4H, 2 ꢂ H-
16, 2 ꢂ H-18, J = 7.3 Hz), 1.15 (t, 6H, 3 ꢂ H-17, 3 ꢂ H-19, J = 7.3 Hz).
2.2.13. (pipH)+[VO2(L1)]ꢀ (13)
Yield: 0.34 g (81%). Anal. Calc. for C20H24N3O4V: C, 57.0; H, 5.7;
2.2.8. (deaH)+[VO2(L4)]ꢀ (8)
N, 10.0. Found: C, 56.9; H, 5.6; N, 9.9%. IR (KBr,
(C@Nazomethine), 1365 (C–Ophenolic), 1251 (C–Oenolate), 1028 (N–N),
965, 891 (V@O); 2738-2955 (N–H). KM (DMF) = 57
ꢀ1 cm2 molꢀ1
kmax (DMSO)/nm: 380 (
/dm3 molꢀ1 cmꢀ1 9,518), 308 (16,020). 1H
m
max/cmꢀ1): 1592
Yield: 0.40 g (91%). Anal. Calc. for C19H23N3O4ClV: C, 51.4; H,
5.2; N, 9.5. Found: C, 51.3; H, 5.1; N, 9.4%. IR (KBr, m
max/cmꢀ1):
X
.
1595 (C@Nazomethine), 1362 (C–Ophenolic), 1240 (C–Oenolate), 1067
e
(N–N), 947, 920 (V@O); 2714–2986 (N–H). KM (DMF) = 47 Xꢀ1
NMR (CDCl3, d/ppm): 7.02 (dd, 1H, H-3, J = 8.1, 0.6 Hz), 7.32–7.40
(m, 4H, H-4, H-12, H-13, H-14), 6.93 (dt, 1H, H-5, J = 7.5, 0.6 Hz),
7.78 (dd, 1H, H-6, J = 7.5, 0.8 Hz), 2.99 (s, 3H, 3 ꢂ H-8), 8.17 (dd,
2H, H-11, H-15, J = 6.5, 0.8 Hz), 3.36 (s, 4H, 2 ꢂ H-16, 2 ꢂ H-20),
1.78 (s, 4H, 2 ꢂ H-17, 2 ꢂ H-19), 1.51 (s, 2H, 2 ꢂ H-18), 9.20 (brs,
2H, 2 ꢂ N-3).
cm2 molꢀ1. kmax (DMSO)/nm: 393 ( /dm3 molꢀ1 cmꢀ1 14,883),
e
317 (16,281). 1H NMR (DMSO-d6, d/ppm): 6.83 (d, 1H, H-3,
J = 8.8 Hz), 7.32 (dd, 1H, H-4, J = 8.8, 2.3 Hz), 7.74 (d, 1H, H-6,
J = 2.3 Hz), 2.84 (s, 3H, 3 ꢂ H-8), 8.03–8.06 (m, 2H, H-11, H-15),
7.46–7.48 (m, 3H, H-12, H-13, H-14), 2.91 (q, 4H, 2 ꢂ H-16,
2 ꢂ H-18, J = 7.3 Hz), 1.15 (t, 6H, 3 ꢂ H-17, 3 ꢂ H-19, J = 7.3 Hz).
2.2.14. (pipH)+[VO2(L2)]ꢀ (14)
2.2.9. (teaH)+[VO2(L1)]ꢀ (9)
Yield: 0.34 g (79%). Anal. Calc. for C21H26N3O4V: C, 57.9; H, 6.0;
Yield: 0.38 g (87%). Anal. Calc. for C21H28N3O4V: C, 57.7; H, 6. 5;
N, 9.7. Found: C, 57.8; H, 5.9; N, 9.6%. IR (KBr, m
max/cmꢀ1): 1594
N, 9.6. Found: C, 57.4; H, 6.4; N, 9.4%. IR (KBr, m
max/cmꢀ1): 1599
(C@Nazomethine), 1361 (C–Ophenolic), 1243 (C–Oenolate), 1029 (N–N),
(C@Nazomethine), 1367 (C–Ophenolic), 1250 (C–Oenolate), 1065 (N–N),
907, 860 (V@O); 2725–2942 (N–H). KM (DMF) = 55 Xꢀ1
935, 897 (V@O); 2737–3057 (N–H). KM (DMF) = 50 Xꢀ1
cm2 molꢀ1. kmax (DMSO)/nm: 386 ( /dm3 molꢀ1 cmꢀ1 9,202), 303
e
cm2 molꢀ1. kmax (DMSO)/nm: 372 ( /dm3 molꢀ1 cmꢀ1 7,498), 308
e
(18,173). 1H NMR (CDCl3, d/ppm): 6.91 (d, 1H, H-3, J = 8.3 Hz),
7.15 (dd, 1H, H-4, J = 8.3, 0.8 Hz), 7.56 (d, 1H, H-6, J = 0.8 Hz),
2.35 (s, 3H, 5-OCH3), 2.97 (s, 3H, 3 ꢂ H-8), 8.16 (dd, 2H, H-11, H-
15, J = 6.8, 0.7 Hz), 7.37–7.42 (m, 3H, H-12, H-13, H-14), 3.36 (s,
4H, 2 ꢂ H-16, 2 ꢂ H-20), 1.79 (s, 4H, 2 ꢂ H-17, 2 ꢂ H-19), 1.53 (s,
2H, 2 ꢂ H-18).
(12,427). 1H NMR (CDCl3, d/ppm): 7.02–7.07 (m, 2H, H-3, H-5),
7.36 (brd, 1H, H-4, J = 8.7 Hz), 7.72 (brd, 1H, H-6, J = 9.0 Hz), 3.00
(s, 3H, 3 ꢂ H-8), 7.73-7.76 (m, 2H, H-11, H-15), 7.65 (brd, 2H, H-
12, H-14, J = 7.8 Hz), 7.19 (brd, 1H, H-13, J = 7.8 Hz), 3.46 (q, 6H,
2 ꢂ H-16, 2 ꢂ H-18, 2 ꢂ H-20, J = 7.2 Hz), 1.15 (t, 9H, 3 ꢂ H-17,
3 ꢂ H-19, 3 ꢂ H-21, J = 7.2 Hz), 9.54 (brs, 1H, N-3).
2.2.15. (pipH)+[VO2(L3)]ꢀ (15)
2.2.10. (teaH)+[VO2(L2)]ꢀ (10)
Yield: 0.37 g (82%). Anal. Calc. for C21H26N3O5V: C, 55.9; H, 5.8;
Yield: 0.33 g (74%). Anal. Calc. for C22H30N3O4V: C, 58.5; H, 6.7;
N, 9.3. Found: C, 55.8; H, 5.7; N, 9.2%. IR (KBr,
(C@Nazomethine), 1360 ( C–Ophenolic), 1217 (C–Oenolate), 1032 (N–N),
952, 915 (V@O); 2714–2949 (N–H). KM (DMF) = 53 Xꢀ1
cm2 molꢀ1. kmax (DMSO)/nm: 405 ( /dm3 molꢀ1 cmꢀ1 8,084), 307
m
max/cmꢀ1): 1590
N, 9.3. Found: C, 58.4; H, 6.6; N, 9.2%. IR (KBr, m
max/cmꢀ1): 1582
m
(C@Nazomethine), 1367 (C–Ophenolic), 1244 (C–Oenolate), 1067 (N–N),
943, 897 (V@O); 2706–2984 (N–H). KM (DMF) = 48 Xꢀ1
cm2 molꢀ1. kmax (DMSO)/nm: 384 ( /dm3 molꢀ1 cmꢀ1 7,569), 309
e
e
(10,360). 1H NMR (CDCl3, d/ppm): 6.99 (s, 2H, H-3, H-4), 3.83 (s,
3H, 5-OCH3), 7.24(s, 1H, H-6), 2.96 (s, 3H, 3 ꢂ H-8), 8.16 (d, 2H,
H-11, H-15, J = 0.6 Hz), 7.40 (s, 3H, H-12, H-13, H-14), 3.34 (s, 4H,
2 ꢂ H-16, 2 ꢂ H-20), 1.81 (s, 4H, 2 ꢂ H-17, 2 ꢂ H-19), 1.55 (s, 2H,
2 ꢂ H-18), 9.25 (brs, 2H, 2 ꢂ N-3).
(15,680). 1H NMR (DMSO-d6, d/ppm): 6.70 (d, 1H, H-3, J = 8.4 Hz),
7.12 (brd, 1H, H-4, J = 8.4 Hz), 2.29 (s, 3H, 5-CH3), 7.54 (s, 1H, H-
6), 2.84 (s, 3H, 3 ꢂ H-8), 8.04-8.06 (m, 2H, H-11, H-15), 7.44–7.47
(m, 3H, H-12, H-13, H-14), 2.93 (q, 6H, 2 ꢂ H-16, 2 ꢂ H-18,
2 ꢂ H-20, J = 6.9 Hz), 1.16 (t, 9H, 3 ꢂ H-17, 3 ꢂ H-19, 3 ꢂ H-21,
J = 6.9 Hz).
2.2.16. (pipH)+[VO2(L4)]ꢀ (16)
2.2.11. (teaH)+[VO2(L3)]ꢀ (11)
Yield: 0.33 g (71%). Anal. Calc. for C22H30N3O5V: C, 56.5; H, 6.5;
N, 9.0. Found: C, 56.5; H, 6.3; N, 8.9%. IR (KBr,
Yield: 0.38 g (83%). Anal. Calc. for C20H23N3O4ClV: C, 52.7; H,
5.1; N, 9.2. Found: C, 52.5; H, 5.0; N, 9.1%. IR (KBr, m
max/cmꢀ1):
m
max/cmꢀ1): 1585
1582 (C@Nazomethine), 1369 (C–Ophenolic), 1243 (C–Oenolate), 1029
(C@Nazomethine), 1363 (C–Ophenolic), 1246 (C–Oenolate), 1069 (N–N),
(N–N), 963, 920 (V@O); 2724–2950 (N–H). KM (DMF) = 49 Xꢀ1
942, 901 (V@O); 2725–2992 (N–H). KM (DMF) = 47 Xꢀ1
cm2 molꢀ1. kmax (DMSO)/nm: 388 ( /dm3 molꢀ1 cmꢀ1 7,349), 304
e
cm2 molꢀ1. kmax (DMSO)/nm: 387 (
e
/dm3 molꢀ1 cmꢀ1 7,748), 312
(12,867). 1H NMR (CDCl3, d/ppm): 6.97 (d, 1H, H-3, J = 0.6 Hz),
7.27–7.42 (m, 4H, H-4, H-12, H-13, H-14), 7.73 (s, 1H, H-6), 2.95
(s, 3H, 3 ꢂ H-8), 8.14 (d, 2H, H-11, H-15, J = 0.6 Hz), 3.34 (s, 4H,
2 ꢂ H-16, 2 ꢂ H-20), 1.79 (s, 4H, 2 ꢂ H-17, 2 ꢂ H-19), 1.55 (s, 2H,
2 ꢂ H-18), 9.23 (brs, 2H, 2 ꢂ N-3).
(14,540). 1H NMR (CDCl3, d/ppm): 7.00–7.04 (m, 2H, H-3, H-5),
7.32 (d, 1H, H-4, J = 8.3 Hz), 3.84 (s, 3H, 5-OCH3), 7.25 (s, 1H, H-
6), 2.98 (s, 3H, 3 ꢂ H-8), 7.72-7.75 (m, 2H, H-11, H-15), 7.64 (brd,
2H, H-12, H-14, J = 7.6 Hz), 7.20 (brd, 1H, H-13, J = 7.6 Hz), 3.45

B. Mondal et al. / Inorganica Chimica Acta 363 (2010) 2296–2306
2299
2.3.1. Reaction with
L
-ascorbic acid
2.5. Crystallographic data collection and refinement
The reduction of (eaH)+[VO2(L1)]ꢀ (1) by
L-ascorbic acid was
studied by electronic spectroscopy in either methanol or DMSO
solution. Ascorbic acid (0.04 mmol, 14.2 mg) was added to a
(eaH)+[VO2(L1)]ꢀ solution (7.973 ꢂ 10ꢀ4 M, 10 mL) ((eaH)+
[VO2(L1)]ꢀ/ascorbic ratio, molar ratio 1:10). The spectrum of the
resulting solution was measured in the range 400–800 nm both
before and after the addition and monitored with time until
the initial spectra (i.e., before addition of ascorbic acid) was
obtained. Other complexes 2–16 also displayed similar
observation.
Good quality single crystals of 5, 6, 12, 15 and 16 were obtained
from the respective reaction mixture upon slow evaporation of the
solvent (methanol) at room temperature and they were used for
structural analysis. X-ray data were measured with Mo Ka radiation
at 150 K using the Oxford Diffraction X-Calibur CCD System. Data
analysis was carried out with the Crysalis [52] program and the
structures were solved with the SHELXS-97 program [53]. For all five
structures, the non-hydrogen atoms were refined with anisotropic
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 with the AB-
SPACK program [54]. The structures were refined on F2 using the SHEL-
XL-97 program [53]. Crystallographic data are collected in Table 1.
2.3.2. Reaction with 8-hydroxyquinoline(Hhq)
To a solution of 1 (0.10 g, 0.26 mmol) in 20 mL methanol was
added Hhq (0.04 g, 0.28 mmol) dissolved 5 mL methanol. The reac-
tion mixture was stirred at room temperature for 4 h. A deep violet
solution was obtained which was kept for slow evaporation at
room temperature. A shiny black microcrystalline compound was
obtained after several days. The product was filtered, washed with
methanol and dried over silica gel. Yield: 0.10 g (83%). This com-
pound was found to be identical in all respect with the complex
[VVO(L1)(hq)] which was reported recently [51] from this labora-
tory synthesized by different method. Other complexes 2–16 be-
haved similarly.
3. Results and discussion
3.1. Synthesis
In an attempt to synthesize quaternary complexes of the type
[VVO(L)(OCH3)(B)] like [VVO(L)(OCH3)(py)] [20] complexes with
monodentate N-donor organic amine bases (B) having higher basi-
city than pyridine (py) (pKa = 5.48) provides an interesting observa-
tion which encouraged as to rationalize a general synthetic method
of generating dioxidovanadium(V) complexes incorporating hydra-
zone ligands (containing one phenolic, one enolic and one neutral
imine moiety). Four monodentate neutral alkyl amine bases viz.,
ethylamine (ea, pKa = 10.64), diethylamine (dea, pKa = 10.98), tri-
ethylamine (tea, pKa = 10.75) and piperidine (pip, pKa = 11.12)
[55], which are of higher basic than pyridine and four benzoyl
hydrazone ligands (H2L1–4, Scheme 1) have been used in this study.
The dioxidovanadium(V) complexes 1–16 have been synthe-
sized by the equimolar reaction of [VIVO(acac)2] and H2L in pres-
ence of B. Considering aerial oxygen being the oxidizing agent
the formation of these dioxido complexes may be represented as:
2.4. Physical measurements
Elemental analyses were performed on a Perkin-Elmer 2400
CHNS/O analyzer. Electronic spectra (in DMSO of 1–12 and in
CH2Cl2 of 13–16 complexes) were recorded on a Hitachi U-3501
spectrophotometer and IR spectra (as KBr pellets) were recorded
on a Perkin-Elmer 782 spectrophotometer. The 1H NMR spectra
were recorded either in CDCl3 (of 9, 11, 12 and 13–16 complexes)
or in DMSO-d6 (of 1–8 and 10 complexes) on a Bruker AM 300L
(300 MHz) superconducting FT NMR spectrophotometer. Conduc-
tivity measurements of all the complexes were performed at
298 K in DMF solution with the Systronics 304 digital conductivity
meter. The EPR spectra (X-band) of the reduced species were re-
corded on a E-112 Century series Varian E-102 Microwave bridge
spectrometer. Tetracyanoethene (tcne) (g = 2.0027) was used to
calibrate the EPR spectra.
h
i
1
2
2 VIVOðacacÞ2 þ 2H2L þ 2B þ O2 þ H2O
h
i
! 2BHþ VVO2ðLÞ ꢀ þ 4Hacac;
ð1Þ
where, Hacac is the acetylacetone.
Table 1
Crystal data and structure determination summary of the complexes 5, 6, 12, 15 and 16.
5
6
12
15
16
Empirical formula
Formula weight
Crystal color
Crystal system
Space group
a (Å)
C19H24N3O4V
409.35
golden yellow
monoclinic
P21/c
C20H26N3O4V
423.38
golden yellow
monoclinic
P21/c
C21H27ClN3O4V
471.85
golden yellow
monoclinic
P21/n
C21H26N3O5V
451.39
golden yellow
C20H23N3O4ClV
455.80
golden yellow
monoclinic
P21/n
triclinic
ꢀ
P1
8.3618(7)
19.2985(9)
12.0881(10)
90
95.285(8)
90
1942.4(2)
4
1.400
8.8945(6)
18.6042(13)
12.2166(8)
90
101.235(6)
90
1982.8(2)
4
1.418
13.818(2)
8.4821(16)
19.171(2)
90
99.532(11)
90
2215.9(6)
4
1.414
8.5295(9)
11.6703(11)
11.9623(13)
67.671(9)
85.721(9)
69.120(9)
1026.58(18)
2
9.4002(5)
21.3238(17)
10.0002(11)
90
94.070(7)
90
1999.5(3)
4
1.514
b (Å)
c (Å)
a
(°)
b (°)
c
(°)
V (Å3)
Z
qcalc. (g cmꢀ3
)
1.460
0.522
l
(mmꢀ1
)
0.540
0.532
0.600
0.662
F(0 0 0)
856
888
984
472
944
Data/restraints/parameters
5590/0/247
0.813
5699/0/257
1.066
6342/0/275
0.909
4281/0/273
0.965
5719/0/263
0.738
Goodness-of-fit on F2
R indices [I > 2
R indices (all data) R1, wR2
r
(I0)] R1, wR2
0.0466, 0.1196
0.0680, 0.1319
0.0848, 0.2207
0.1205, 0.2346
0.0555, 0.1351
0.0865, 0.1460
0.0380, 0.0955
0.0558, 0.0995
0.0548, 0.1672
0.0771, 0.1985

2300
B. Mondal et al. / Inorganica Chimica Acta 363 (2010) 2296–2306
4
3
1
2
5
R
R
OH
OH
N
6
7
C
C
N
H
OH
O
9
C
H38C
H3C
N
N
11
C
10
12
13
15
14
(Keto form)
(Enol form/active)
H2L
R
H2L1
H2L2
H2L3
H2L4
H
CH3
OCH3
Cl
Scheme 1. Ligands employed in this study.
Similar observation was reported previously [17]. Complexes 1-
16 are found to be diamagnetic indicating the +V state (d0 system)
of the vanadium. Molar conductance values of these dioxidovana-
dium(V) complexes (vide experimental section) in dimethylform-
amide (DMF) support their 1:1 electrolytic nature [56,57].
Structure determinations of five of these complexes by single crys-
tal X-ray diffractrometry have firmly established these findings.
part of the molecule along with the coordinated hydrazone ligand
while the cationic part consists of the monoprotonated form of the
respective alkyl amine base and (iii) complexes 5, 6, 15 and 16 con-
taining two hydrogen atoms associated with the amine nitrogen
are centrosymmetric dimers which are formed via the intermolec-
ular hydrogen bonding between the cationic and anionic part of
the neighboring molecules (vide infra) while complex 12 contain-
ing one hydrogen atom linked with the amine nitrogen is mono-
meric exhibiting normal H-bonding between the cationic and
anionic part of the molecule.
3.2. IR spectra of the complexes (1 –16)
The tridentate dinegative mode of binding of the deprotonated
hydrazone ligands with vanadium in their enol form in all these
complexes is evident from the disappearance of three ligand char-
acteristic bands [51] in the regions 1646–1651, 2924–2989, and
The numbering system for the five complexes is similar. So, a
representative molecular structure of 5 is displayed in Fig. 1 show-
ing the atom-numbering scheme along with the cationic and anio-
nic part and selected bond parameters of five complexes are given
in Table 2. Vanadium atom is present in the anionic part of the
molecules and its coordination sphere is of the VO4
the coordination geometry at the metal center can best be fitted
to a distorted square pyramid with one of the two oxido oxygens
at the apex. The square plane consists of a phenolic oxygen O(3),
an enolic oxygen O(4) and an imine nitrogen N(1) from the fully
deprotonated enolic form of the respective hydrazone ligand and
3215-3240 cmꢀ1 due to
and the appearance of a new
1296 cmꢀ1 region. The characteristic
are observed in the 1581–1601 cmꢀ1 and 1028–1072 cmꢀ1 region,
respectively. The shifting of (C@N) stretch towards the lower
m
(C@O),
(CꢀO) enolate band in the 1217–
(C@N) and
(NꢀN) stretches
m
(NꢀH) and
m
(OꢀH), respectively
N type and
m
m
m
m
wave number from the free ligand [51] also supports the coordina-
tion of azomethine nitrogen [17–25,51] to the metal. The appear-
the other oxido oxygen. The r.m.s. deviation of the four contribut-
ing atoms (r) is in the range 0.008–0.212 ÅA (Table 3). The displace-
ance of the characteristic
energy region is due to diminished repulsion between the lone
m
(NꢀN) band in the appreciably higher
0
ment (b) of the vanadium atom from this basal plane towards the
axial oxido oxygen atom is ꢁ0.50 Å (Table 3). The unequal bond
lengths of VꢀO(1), VꢀO(2), VꢀO(3), VꢀO(4) and VꢀN(1) bonds
as well as unequal bond angles generated by these bonds at the
V-acceptor center indicate a significant distortion from the perfect
square-pyramidal geometry. The amount of distortion can be
quantitatively estimated from the average O(axial)ꢀVꢀO/N(equa-
pairs of adjacent nitrogen atoms upon coordination. The strong
band in the 1353–1369 cmꢀ1 region is assigned to
late) [16] stretching.
m
(CꢀO) (pheno-
In addition to these bands, all these sixteen complexes also ex-
hibit two sharp bands in the 896–965 cmꢀ1 and 829–932 cmꢀ1 re-
gions which are attributed to antisymmetric and symmetric
+
stretching respectively of cis-VO2 motif [16–18,21,25,58,59].
torial) angle (
60°, where
largest ligand–metal–ligand angles). The average O(axial)ꢀVꢀO/
N(equatorial) angle and the parameter values are given in Table
c
) [62] and also from
s parameter [63] (equal to D/
Complexes 1ꢀ16 exhibit
a number of bands in the 2592–
3157 cmꢀ1 region which are assigned to the NꢀH stretching. The
presence of multiple bands and the band width suggest that
the NꢀH hydrogens are involved in hydrogen bonding [60,61].
Characteristic IR spectral data are collected in the experimental
section.
D
is the difference between the largest and next-to-
s
3. For idealized square-pyramid these values are respectively 102°
and zero. The deprotonated hydrazone ligands present in these five
molecules form a six-membered and a five-membered chelate ring
at the V(V) center with the corresponding bite angles being respec-
tively ꢁ81.0° and ꢁ74.0° (exact values are collected in Table 2). The
atoms in the five-membered ring are coplanar with an r.m.s. devi-
ation (d) of 0.022–0.071 Å (Table 3) and the six-membered ring is
folded.
3.3. Structure of the complexes (5), (6), (12), (15) and (16)
The structural analysis of five complexes reveals the following
three general features: (i) both cationic and anionic parts are pres-
ent in each of the molecules; (ii) vanadium is present in the anionic

B. Mondal et al. / Inorganica Chimica Acta 363 (2010) 2296–2306
2301
Fig. 1. ORTEP plot of complex 5 showing the atom-labeling scheme with thermal ellipsoids drawn at 50% probability level.
Table 2
Selected bond distances (Å) and angles (°) for complexes 5, 6, 12, 15 and 16.
5
6
12
15
16
V–O(1)
V–O(2)
V–O(3)
V–O(4)
1.627(1)
1.643(1)
1.879(1)
1.958(1)
2.172(2)
107.91(7)
105.82(7)
106.52(7)
96.17(6)
90.73(7)
142.93(6)
104.20(7)
147.10(7)
81.66(6)
73.28(6)
115.78(12)
1.617(3)
1.658(3)
1.865(3)
1.919(3)
1.607(2)
1.666(2)
1.883(2)
1.958(2)
2.181(2)
109.35(9)
105.96(8)
104.54(8)
95.34(8)
91.56(8)
144.36(8)
103.71(8)
146.35(8)
81.22(8)
74.00(7)
114.49(14)
1.624(2)
1.631(2)
1.877(2)
1.940(1)
2.168(2)
109.41(10)
101.40(8)
98.62(7)
94.38(8)
94.84(7)
153.72(6)
116.06(9)
134.30(8)
81.44(6)
74.53(6)
113.75(11)
1.610(2)
1.645(2)
1.932(2)
1.969(2)
2.152(2)
109.98(11)
103.32(9)
103.81(9)
94.33(9)
V–N(1)
2.176(3)
O(1)–V–O(2)
O(1)–V–O(3)
O(1)–V–O(4)
O(2)–V–O(3)
O(2)–V–O(4)
O(3)–V–O(4)
O(1)–V–N(1)
O(2)–V–N(1)
O(3)–V–N(1)
O(4)–V–N(1)
N(2)–N(1)–V
106.54(15)
107.88(14)
103.33(14)
95.20(13)
92.62(14)
144.03(12)
102.46(14)
150.26(14)
81.88(12)
74.25(12)
113.7(2)
93.25(8)
147.30(8)
108.37(10)
141.43(9)
80.68(8)
73.71(7)
115.11(15)
0
Table 3
Structure characterization parameters of complexes 5, 6, 12, 15 and 16.
O(1) of 3.004(2) ÅA with a NꢀHꢃꢃꢃO angle of 129°, while this is weak-
er than the former two but it is significantly stronger than the third
hydrogen bond in 15 and 16 (vide infra). However concomitant
with this is a weakening of the other hydrogen bond from H(3a),
Complex
r
(Å)
b (Å)
c
(°)
s
d (Å)
5
6
12
15
16
0.008
0.017
0.015
0.212
0.082
0.523
0.492
0.523
0.508
0.530
106.1
105.05
105.9
106.4
106.4
0.069
0.104
0.033
0.324
0.098
0.022
0.071
0.047
0.022
0.065
certainly as far as the subtended angle is concerned. Distances in
0
6 (Fig. 3) are N(3)ꢀH(3b)ꢃꢃꢃ0O(2)(ꢀx,1 ꢀ y, 1 ꢀ z) 2.710(4) ÅA and
N(3)ꢀH(3a)ꢃꢃꢃO(2) 2.892(4) 0ÅA. The additional weaker bond is
N(3)ꢀH(3a)ꢃꢃꢃO(1) 3.048(4) ÅA. Thus for 6, the hydrogen H(3a) is
bifurcated, bonded to both terminal oxygen atoms O(1) and O(2)
of one molecule. The dimers in 5 and 6 are different from those
in 15 and 16 in that the hydrogen bonds are formed solely to the
terminal oxygen atoms O(1) and O(2) and not to the phenolic oxy-
gen atom O(3) (vide infra). However they differ from each other as
is apparent from Figs. 2 and 3. Full dimensions for the hydrogen
bonds are given in Table 4.
r
= r.m.s. deviation of the four contributing atoms consisting of the square plane;
b = displacement of vanadium atom from basal plane towards the axial oxido
oxygen; = average of O(axial)–V–O/N(equatorial) angle; /60°, where is the
c
s
=
D
D
difference between the largest and next-to-largest ligand–metal–ligand angles;
d = r.m.s. deviation of five-membered chelate ring from co-planarity.
As shown in Fig. 2, the structure of 5 consists of a dimeric cen-
trosymmetric unit containing two cations and anions held together
Fig. 4 displays the H-bonded structure of 12. In this molecule, a
significant difference in two V@O distances is observed: VꢀO(1)
and VꢀO(2) are respectively 1.606(2) Å and 1.666(2) Å. The
longer VꢀO(2) bond is probably due to its involvement in inter-
molecular H-bonding with the N(3)ꢀH atom of the cationic part
with hydrogen bonds. There are two strong hydrogen bonds
0
N(3)ꢀH(3a)ꢃꢃꢃO(1)(2 ꢀ x, 1 ꢀ y, 2 ꢀ z) 2.827(2) ÅA and N(3)ꢀH(3b)ꢃꢃꢃ
0
O(2) 2.810(2) ÅA. There is an additional interaction N(3)ꢀH(3a)ꢃꢃꢃ

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B. Mondal et al. / Inorganica Chimica Acta 363 (2010) 2296–2306
Fig. 2. The centrosymmetric dimeric structure of 5 with ellipsoids at 50% probability. Hydrogen bonds are shown as dotted lines.
Fig. 3. The centrosymmetric dimeric structure of 6 with ellipsoids at 50% probability. Hydrogen bonds are shown as dotted lines.
0
(triethylammonium ion) of the molecule. The NꢀHꢃꢃꢃO bond dis-
tance and related details are given in Table 4. The VꢀO bond dis-
tances follow the order VꢀO(1) < VꢀO(2) < VꢀO(3) < VꢀO(4).
As shown in Fig. 5, the structure of 15 like 5 and 6 also consists
of a dimeric centrosymmetric unit containing two cations and an-
ꢃꢃꢃO(1)(ꢀx, 1 ꢀ y, ꢀz) at 2.720(2) AÅ. In addition there is a weak inter-
0
action from N(3)ꢀH(3a)ꢃꢃꢃO(3) at 3.151(2) ÅA but with a NꢀHꢃꢃꢃO an-
gle of 120°, this can represent only a weak bond. Full dimensions
for the hydrogen bonds are given in Table 4. Fig. 6 displays the
H-bonded dimeric structure of 16. This structure, like 15, also
forms a centrosymmetric dimer of two cations and anions con-
nected via hydrogen bonds. However in this case these involve only
ions held together with hydrogen bonds. There are two strong
0
hydrogen bonds N(3)ꢀH(3b)ꢃꢃꢃO(2) at 2.778(2) AÅ and N(3)ꢀH(3a)

B. Mondal et al. / Inorganica Chimica Acta 363 (2010) 2296–2306
2303
Table 4
Hydrogen bond distances (Å) and bond angles (o) of complexes 5, 6, 12, 15 and 16.
Complex
D–HꢃꢃꢃA
d(HꢃꢃꢃA)
\ DHA
d(DꢃꢃꢃA)
Symmetry element
5
5
5
6
6
6
12
15
15
15
16
16
16
N(3)–H(3a)ꢃꢃꢃO(1)
N(3)–H(3a)ꢃꢃꢃO(1)
N(3)–H(3b)ꢃꢃꢃO(2)
N(3)–H(3a)ꢃꢃꢃO(2)
N(3)–H(3a)ꢃꢃꢃO(1)
N(3)–H(3b)ꢃꢃꢃO(2)
N(3)–H(3)ꢃꢃꢃO(2)
N(3)–H(3b)ꢃꢃꢃO(2)
N(3)–H(3a)ꢃꢃꢃO(1)
N(3)–H(3b)ꢃꢃꢃO(3)
N(3)–H(3a)ꢃꢃꢃO(2)
N(3)–H(3b)ꢃꢃꢃO(3)
N(3)–H(3a)ꢃꢃꢃO(2)
2.07
2.36
1.97
2.07
2.34
1.82
1.81
1.90
1.85
2.60
1.92
2.01
2.43
141
129
154
151
136
171
175
166
162
120
152
156
116
2.827(2)
3.004(2)
2.810(2)
2.892(4)
3.048(4)
2.710(4)
2.718(3)
2.778(2)
2.720(2)
3.151(2)
2.749(3)
2.861(3)
2.941(3)
2 ꢀ x, 1 ꢀ y, 2 ꢀ z
ꢀx, 1 ꢀ y, 1 ꢀ z
ꢀx, 1 ꢀ y, ꢀz
ꢀx, ꢀy, 2 ꢀ z
Fig. 4. The H-bonded structure of 12.
one terminal oxygen atom O(2) together with the phenolic oxygen
atom O(3) but not the second terminal oxygen atom O(1). So the di-
mer in 16 is completely different from that in 15 as is apparent
from Fig. 5. Hydrogen bond distances are N(3)ꢀH(3a)ꢃꢃꢃO(2)
3.4. Proton NMR spectra of the complexes
1H NMR spectral data of all the complexes are given in the
experimental section. The DMSO-d6 solution of the ligands exhibits
signals at d ꢁ 11.00 and d ꢁ 13.00 ppm which are attributed
respectively to NH and OH protons [51]. These signals are absent
in all these dioxidovanadium(V) complexes suggesting their
involvement in tautomerisation of the keto form into enol form
and subsequent coordination of the enolic and phenolic oxygens
through deprotonation to vanadium.
0
0
(ꢀx, ꢀy, 2ꢀz) 2.749(3) ÅA and N(3)ꢀH(3b)ꢃꢃꢃO(3) 2.861(3) ÅA. There
0
is an additional close contact N(3)ꢀH(3a)...O(2) of 2.941(3) ÅA but
the NꢀHꢃꢃꢃO angle of 116° shows this can only be a weak
interaction. Full dimensions for the hydrogen bonds are given in
Table 4.
Thus, while all four molecules (viz. 5, 6, 15 and 16) containing
two hydrogen atoms associated with the amine nitrogen in the cat-
ionic part of the respective molecule, form centrosymmetric di-
mers constructed from two cations and two anions but the
pattern of hydrogen bonds is different in each case. In spite of best
efforts, none of the four dioxidovanadium(V) complexes 1ꢀ4 con-
taining the ethylammonium ion (having three hydrogen atoms
available for H-bonding) in the cationic part afforded good quality
single crystal suitable for structure determination by X-ray crystal-
lography, thus giving no opportunity to study the pattern of H-
bonding in these complexes.
Based on the IR and 1H NMR spectral data, it is reasonable to as-
sume the gross structural similarity of 1–4 complexes with that of
5, 6, 12, 15, and 16.
3.5. Electronic spectra of the complexes
The electronic spectra of the complexes 1ꢀ12 were recorded in
DMSO while that of complexes 13ꢀ16 in CH2Cl2 solution in the
300–800 nm region and the spectral data are given in the experi-
mental section. Complexes being of d0 system, no d–d transition

2304
B. Mondal et al. / Inorganica Chimica Acta 363 (2010) 2296–2306
Fig. 5. The centrosymmetric dimeric structure of 15 with ellipsoids at 50% probability. Hydrogen bonds shown as dotted lines.
Fig. 6. The centrosymmetric dimeric structure of 16 with ellipsoids at 50% probability. Hydrogen bonds shown as dotted lines.
band is expected in their electronic spectra and in fact, all these
complexes exhibit very intense transitions; the lowest energy band
vanadium(IV) which is further oxidized by the aerial oxygen to
the original dioxidovanadium(V) species. The net reaction is thus
in the 372–410 nm region is assigned to ligand-to-metal charge
an oxidation of L-ascorbic acid by atmospheric oxygen catalyzed
by V(V). This reaction was monitored by 1H NMR, EPR and elec-
tronic spectroscopy. These dioxidovanadium(V) complexes are
yellow in color in either DMSO or CH3OH. However, on addition of
*
transfer (LMCT) transition. The ligand characteristic
p
?
p
transi-
tion band of the azomethine group are observed in 308–323 nm re-
gion [18–20,25,51].
L-ascorbic acid (in excess), the yellow color immediately changed
3.6. Study of the reactivity of the complexes
to deep green and the characteristic narrow proton signals (Supple-
mentary Figure) of the coordinated ligand of each of the 1ꢀ16
complexes are converted to a broad signal which suggests that dur-
ing the reaction, the [VVO2(L)]ꢀ complexes are reduced to a corre-
sponding paramagnetic VIV species. The absence of proton signals
for the ligand strongly suggests that the ligand is still attached to
the V-atom even after reduction. This green solution is EPR active
3.6.1. Reaction with L-ascorbic acid
The catalytic activity of these cis-VO2+ complexes (1ꢀ16) in the
aerial oxidation of L-ascorbic acid has been studied in either meth-
anol or DMSO solution. In this catalytic process, at first the vana-
dium(V) species is reduced by the -ascorbic acid to
L

B. Mondal et al. / Inorganica Chimica Acta 363 (2010) 2296–2306
2305
Fig. 8. Electronic spectra (400–800 nm) of
a
7.973 ꢂ 10-4 mol dmꢀ3 methanol
solution of 5: (a) before the addition of ascorbic acid, (b) immediately after the
addition of excess (about 10 times than that of the complex) ascorbic acid, (c)
15 min after the addition of ascorbic acid, (d) 40 min after the addition of ascorbic
acid, (e) 80 min after the addition of ascorbic acid and (f) 720 min after the addition
of ascorbic acid.
Fig. 7. X-band EPR spectra of (deaH)+[VO2(L1)ꢀ, (5) after reduction with
L-ascorbic
acid in DMSO solution at 77 K; tcne = tetracyanoethene.
Table 5
EPR spectral dataa of selected complexes after reduction with ascorbic acid at 77 K in
DMSO solution.
tached completely from the metal center as revealed by 1H NMR
spectra, vide supra) of the original ligand by DMSO or methanol.
The reversible nature of this catalytic process has been moni-
tored by spectrophotometric study. The deep green solution ob-
Complex
gk(104 A /cmꢀ1
)
g? (104 A?/cmꢀ1
)
k
(eaH)+[VO2(L1)]ꢀ (1)
(deaH)+[VO2(L1)]ꢀ (5)
(teaH)+[VO2(L1)]ꢀ (9)
(pipH)+[VO2(L1)]ꢀ (13)
1.943(170.7)
1.949(170.3)
1.945(170.5)
1.941(170.1)
1.971(60.7)
1.974(60.7)
1.972(60.4)
1.971(60.6)
tained after the addition of L-ascorbic acid to the methanolic
+
yellow solution of these VO2 complexes exhibit a new band in
the visible region near 670 nm (Table 6) which is attributed to
a
Limits of error in g values: 0.002 and A values: 3 ꢂ 104 cmꢀ1
.
dxy ? dxz, dyz transition and the relatively large e value indicating
significant distortion in the structure. After keeping the solution
in the air for sometime, the new band disappeared gradually
(Fig. 8) and finally (after ꢁ12 h) the initial spectrum was obtained
indicating the re-oxidation of the reduced VIV-complex to the ori-
and display axial EPR spectra. The EPR parameters for the first
member of each of the four systems have been collected in Table
5 and a representative EPR spectra is displayed in Fig. 7. An analy-
sis of the EPR parameter data (Table 5) reveals the relationships:
gk < g? and A ꢄ A , which are characteristic of an axially com-
+
ginal VO2 complex thus fulfilling the criteria for a catalyst. The
Scheme 2 may represent this catalytic process.
Such type of reversible redox reaction of oxidovanadium(V)
complexes is very important in connection with their catalytic
activity.
k
?
1
pressed dxy configuration [64–67]. Using the additivity relation-
ship presented by Chasteen [68] and considering the reported
value of different moieties [69–71], the estimated A value for
k
these complexes containing one phenolic-O, one enolic-O, one
imine-N and one oxido-O in the equatorial positions as revealed
by the X-ray study is expected to be close to 167 ꢂ 10ꢀ4 cmꢀ1
which is slightly lower than the observed value (Table 5). Such a
higher observed value may be due to partial replacement of one
or two coordinating sites (because the original ligand is not de-
3.6.2. Reaction with 8-hydroxyquinoline(Hhq)
These dioxidovanadium(V) complexes are converted almost
quantitatively to the monooxidovanadium(V) complexes of the
+
A
+ 2
H
H2A
Table 6
Electronic spectral data (in the 400–800 nm region) of the complexes 1ꢀ16 in
methanol solution at 298 K after addition of L-ascorbic acid.
Complex
kmax (nm) (e )
/dm3 molꢀ1 cmꢀ1
[VVO (L)]-
[VIVO (L)]2-
2
2
2
2
(eaH)+[VO2(L1)]ꢀ (1)
(eaH)+[VO2(L2)]ꢀ (2)
(eaH)+[VO2(L3)]ꢀ (3)
(eaH)+[VO2(L4)]ꢀ (4)
674(375)
679(410)
680(391)
674(332)
(deaH)+[VO2(L1)]ꢀ (5)
(deaH)+[VO2(L2)]ꢀ (6)
(deaH)+[VO2(L3)]ꢀ (7)
(deaH)+[VO2(L4)]ꢀ (8)
677(363)
678(451)
674(361)
677(444)
(teaH)+[VO2(L1)]ꢀ (9)
(teaH)+[VO2(L2)]ꢀ (10)
(teaH)+[VO2(L3)]ꢀ (11)
(teaH)+[VO2(L4)]ꢀ (12)
677 (437)
674 (344)
674(362)
676(349)
H+
2
1/2
O2
+
H2O
(pipH)+[VO2(L1)]ꢀ (13)
(pipH)+[VO2(L2)]ꢀ (14)
(pipH)+[VO2(L3)]ꢀ (15)
(pipH)+[VO2(L4)]ꢀ (16)
676(300)
678(376)
678(350)
674(228)
H2A = L-Ascorbic acid; A = dehydro L-ascorbic acid
Scheme 2. Catalytic cycle for the aerial oxidation of L-ascorbic acid in presence of
[VVO2(L)]ꢀ species.

2306
B. Mondal et al. / Inorganica Chimica Acta 363 (2010) 2296–2306
type [VVO(L)(hq)] [51] (confirmed from their analytical and IR,
NMR and UV–Vis spectral data) when an equimolar amount of 8-
hydroxyquinoline is added to the methanolic solution of these
complexes. The pH of the resulting solution is ꢁ11 indicating the
formation of base in this reaction. So, the reaction may be repre-
sented as:
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537.
h
i
h
i
VVO2ðLÞ ꢀ þ Hhq ! VVOðLÞðhqÞ þ OHꢀ
ð2Þ
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4. Conclusion
A simple general method has been established for the synthesis
of cis-dioxidovanadium(V) complexes starting from its tetravalent
precursor. Sixteen cis-dioxidovanadium(V) complexes have been
synthesized by the equimolar reaction of [VIVO(acac)2] with H2L
in presence of excess amount of alkyl amine bases (pKa > 10.5).
The complexes containing two hydrogen atoms attached to the
amine-N atom in the cationic part of the molecules (e.g., 5, 6, 15
and 16) are centrosymmetric H-bonded dimers in the solid state
and the nature of H-bond is different in each of the four complexes.
However, the complex containing one H-atom associated with the
amine-N (e.g., 12) exhibit normal H-bonding between the cationic
and anionic part of the molecule. These complexes have the cata-
lytic activity in the aerial oxidation of L-ascorbic acid. This study
also indicates that these dioxidovanadium(V) complexes are easily
converted to the complexes with VO3+ motif on reaction with a
monobasic bidentate NOꢀ donor ligand (e.g., Hhq).
Acknowledgements
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Our sincere thanks are due to the University Grants Commis-
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to our College authority for providing research facilities.
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Appendix A. Supplementary material
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CCDC 739922, 739923, 739924, 739925 and 739926 contain the
supplementary crystallographic data for compounds 12, 15, 16, 5
and 6, respectively. These data can be obtained free of charge from
The Cambridge Crystallographic Data Centre via www.ccdc.cam.
ac.uk/data_request/cif. Supplementary data associated with this
article can be found, in the online version, at doi:10.1016/
j.ica.2010.03.053.
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