COMMUNICATION
www.rsc.org/chemcomm | ChemComm
Hydroxylation of benzophenone with ammonium phosphomolybdate
in the solid state via UV photoactivationw
a
a
a
a
a
Mrinmoyee Basu, Suresh Sarkar, Surojit Pande, Subhra Jana, Arun Kumar Sinha,
a
a
b
a
Sougata Sarkar, Mukul Pradhan, Anjali Pal and Tarasankar Pal*
Received (in Cambridge, UK) 23rd March 2009, Accepted 1st October 2009
First published as an Advance Article on the web 15th October 2009
DOI: 10.1039/b905718h
UV photoactivation of a mixture of benzophenone and ammonium
phosphomolybdate (APM) in the solid state splits adsorbed
moisture, resulting in selectively hydroxylated benzophenone
and leaving an electron trapped in green (reduced) solid APM.
In a typical experiment, 0.25 g of benzophenone, 1 ml of
ethanol and 0.5 g of as-prepared APM were mixed together.
The slurry in a Petri dish was dried in air and the dry mass was
kept under UV light of wavelength 365 nm, 15 W (at a distance
of 3 cm from the light source) for 12 h for photoactivation
(B30 1C) (high-power UV light removed the adsorbed
moisture and hence water splitting was not observed and no
hydroxylation of benzophenone took place). Visually we
observed that the yellow mass changed color to green. For
maximum yield, the exposed mixture was again treated with an
alcoholic suspension of 0.5 g of APM and was air dried as
before. The mixture was again irradiated with the same UV
light source for 8 h. The photoproduct was separated from the
green solid with 5 ml ethanol by centrifugation. The residue
was washed and dried and then treated with dilute nitric acid
for reuse. Similar photoactivation experiments were carried
out in different alcohols, dichloromethane and also in diethyl
ether. Every time, 2- and 3-hydroxybenzophenone were
identified by thin layer chromatography as the oxidation
products, and subsequently these two isomers were separated
by column chromatography using a solvent mixture of
petroleum ether–ethyl acetate (20 : 1). Finally, the separated
products were characterized by UV-visible spectroscopy,
NMR, FTIR and GC-MS studies. Quantification of the
product yield was obtained from HPLC.
1
The term polyoxometalates (POMs) is applied to an extremely
large group of generally anionic clusters with frameworks built
from transition metal oxo anions linked by shared oxide ions.
The POMs have recently gained significant interest owing to
their versatile architecture, facile synthesis, material properties,
2
–4
and especially their catalytic activity and redox properties,
and most of the elements in the periodic table can be incorporated
1
into the structural framework of these compounds. Among
the first POMs to be structurally characterized were the
Keggin ions, for example the insoluble phosphomolybdate
ion employed here. The Keggin core has a special ability to
accept one or several electrons, which remain delocalized
within the Keggin structure without causing any structural
5
change. As a result, after electron trapping the Keggin ion can
act as a reducing agent, or before reduction it can serve as an
oxidizing agent. The reduced Keggin species frequently posses
a deep blue color, which justifies their name ‘‘heteropoly blues’’.
Keeping this background in mind, we have successfully
photoactivated an intimate mixture of benzophenone and
ammonium phosphomolybdate (APM) in the solid state,
whereby benzophenone is oxidized by the phosphomolybdate
ion with the introduction of a hydroxyl group into the 2- or
Benzophenone is a prototypical aromatic carbonyl compound
that has been extensively studied to better understand its
7
photophysics and photochemistry. During photoactivation,
3
-position of the aromatic moiety. As a consequence APM
becomes green and contains trapped electrons. Hydroxylation
of benzophenone has been reported previously in the presence
an excited state of the ketone extracts a hydrogen atom from a
hydrogen donor and the ultimate reaction products are
formed through coupling and disproportionation reactions
from the as-obtained radicals. In the case of benzophenone
6
of reagents or supports containing hydroxyl groups, but in
this study, the source of the hydroxyl groups is simply
available moisture. The present report is new and full of
promise. Further, to the best of our knowledge, this is the
first example of selective photoconversion of benzophenone
to 2- and 3-hydroxybenzophenone as a consequence of
decomposition of available water molecules.
the lowest singlet (S
1
np*) and triplet (T pp*) states are
2
separated by a small energy gap and are strongly coupled via
8
a spin–orbit interaction. Due to this small energy gap and the
strong spin-orbit coupling, upon irradiation, state T
produced with a large yield close to unity. The T state is
is
1
1
typically responsible for all the major photochemical reactions
of benzophenone. Benzopinacol is the product of the photo-
lysis of benzophenone alone under oxygen-free condition,
while in aerated conditions, hydroxylated benzophenones are
a
Department of Chemistry, Indian Institute of Technology,
Kharagpur-721302, India. E-mail: tpal@chem.iitkgp.ernet.in
Department of Civil Engineering, Indian Institute of Technology,
Kharagpur-721302, India
b
6
produced with a very low quantum yield.
w Electronic supplementary information (ESI) available: 1: Schematic
representation of the photochemical reaction pathway, 2: FTIR
spectra of benzophenone and 2- and 3-hydroxybenzophenone, 3:
GC-MS spectra of 2- and 3-hydroxybenzophenone and 4: EDS
analysis of (a) yellow and (b) green APM i.e., before and after UV
light irradiation. See DOI: 10.1039/b905718h
It was not previously known that APM, a very simple and
low cost compound, a waste product of the undergraduate
laboratory, can show such a clean and effective process of
hydroxylation of benzophenone in the presence of UV-light
(365 nm) and moisture (Fig. 1). Also of interest here is that the
This journal is ꢀc The Royal Society of Chemistry 2009
Chem. Commun., 2009, 7191–7193 | 7191
Basu, Mrinmoyee
