Dalton
Transactions
PAPER
Cite this: DOI: 10.1039/c6dt03425j
Received 1st September 2016,
Accepted 15th September 2016
DOI: 10.1039/c6dt03425j
www.rsc.org/dalton
Vanadium(IV and V) complexes of pyrazolone based
ligands: Synthesis, structural characterization and
catalytic applications†
Mannar R. Maurya,*
a
Bithika Sarkar,
a
Fernando Avecilla
b
and Isabel Correia*
c
The ONO donor ligands obtained from the condensation of 4-benzoyl-3-methyl-1-phenyl-
2-pyrazoline-5-one (Hbp) with benzoylhydrazide (H
2
bp-bhz I), furoylhydrazide (H
2
bp-fah II), nicotinoyl-
hydrazide (H
2
bp-nah III) and isonicotinoylhydrazide (H
2
bp-inh IV), upon treatment with [V
IV
O(acac)
2
], lead
to the formation of [V
IV
O(bp-bhz)(H
2
O)] 1, [V
IV
O(bp-fah)(H
2
O)] 2, [V
IV
O(bp-nah)(H
2
O)] 3 and [V
IV
O(bp-inh)
(H
2
O)] 4, respectively. At neutral pH the in situ generated aqueous K[H
2
V
V
O
4
] reacts with ligands I and II,
forming potassium salts, K(H
2
O)
2
[V
V
O
2
(bp-bhz)] 5 and K(H
2
O)
2
[V
V
O
2
(bp-fah)] 6, while ligands III and IV
give neutral complexes, [V
V
O
2
(bp-nah)] 9 and [V
V
O
2
(bp-inh)] 10, respectively. Acidification of aqueous
solutions of 5 and 6 with HCl also gives neutral complexes [V
V
O
2
(Hbp-bhz)] 7 and [V
V
O
2
(Hbp-fah)] 8,
respectively. Complexes 1–4, upon slow aerial oxidation in methanol, convert into monooxidovanadium(V)
complexes, [V
V
O(bp-bhz)(OMe)] 11, [V
V
O(bp-fah)(OMe)] 12, [V
V
O(bp-nah)(OMe)] 13 and [V
V
O(bp-inh)
(OMe)] 14, respectively. All complexeswere characterized by various spectroscopic techniques like FT-IR,
UV-visible, EPR (for complexes 1–4) and NMR (
1
H,
13
C and
51
V), elemental analysis, thermogravimetry and
single crystal X-ray diffraction (for complexes 5–10 and 12). In the solid state, all complexes characterized
by X-ray diffraction show the metal ion 5-coordinated in a distorted square pyramidal geometry.
Complexes 11–14 were tested as catalysts for the one-pot three-component (ethylacetoacetate, benz-
aldehyde and ammonium acetate) dynamic covalent assembly, via Hantzsch reaction, using hydrogen
peroxide as oxidant in solutionand under solvent-free conditions. The complexes are also active catalysts
for the oxidation of tetralin to tetralone with H
2
O
2
as oxidant. The influence of the amounts of catalyst
and oxidant, and solvent, temperature and time on the catalyzed reactions was investigated.
Introduction
Hantzsch 1,4-dihydropyridines (1,4-DHPs) are one of the most
significant groups of heterocyclic compounds because of their
pharmacological activity as calcium antagonists or agonists.
1
Substituted 1,4-DHPs are extensively used in other pharmaco-
logical applications such as antitumor,
2
bronchodilating
3
anti-
diabetic,
4
neurotropic
5
and antianginal.
6
First reported in
1882, the Hantzsch reaction is a convenient way to prepare
1,4-DHPs, involving the condensation of an aldehyde with 2
equivalents of a β-keto ester and a nitrogen donor.
7
Aromatization of DHPs has received considerable attention
because the conversion of 1,4-DHPs into the corresponding
pyridine derivatives is one of the main metabolic pathways of
the above drugs. 1,4-DHP based calcium-channel blockers are
oxidatively converted into pyridine derivatives by cytochrome
P-450 in the liver
8
and these show anti-hypoxic and anti-
ischemic activities.
9
In recent years, Bocker and co-workers
10
have studied the metabolism of Hantzsch 1,4-DHPs and
showed that the first metabolic step includes aromatization to
the corresponding pyridine derivative. Moreover, the products
of aromatization (substituted pyridines) have found use in the
treatment of atherosclerosis and various coronary diseases.
11
Therefore, the oxidative aromatization of DHPs has attracted
substantial attention from organic and medicinal chemists.
Numerous reagents and procedures have been suggested for
the oxidation of DHPs. Strong oxidizing reagents such as pyri-
dinium chlorochromate,
12
metallic salts e.g. Zr(NO
3
)
4
,
13
(NH
4
)
4
Ce(NO
3
)
6
,
14
urea nitrate, photochemical oxidation
15
silver salts, iodine monochloride, inorganic acidic salts,
16
† Electronic supplementary information (ESI) available: Details of EPR,
51
V NMR
and other spectroscopic studies. CCDC 1502023–1502029. For ESI and crystallo-
graphic data in CIF or other electronic format see DOI: 10.1039/c6dt03425j
a
Department of Chemistry, Indian Institute of Technology Roorkee, Roorkee-247667,
India. E-mail: rkmanfcy@iitr.ernet.in; Fax: +91 1332 273560; Tel: +91 1332 285327
b
Departamento de Química Fundamental, Universidade da Coruña, Campus de A
Zapateira, 15071 A Coruña, Spain
c
Centro de Química Estrutural, Instituto Superior Técnico, Universidade de Lisboa,
Av. Rovisco Pais 1, 1049-001 Lisboa, Portugal. E-mail: icorreia@ist.utl.pt;
Fax: +35 121 8464455; Tel: +351 21 8419177
This journal is © The Royal Society of Chemistry 2016 Dalton Trans.