Selective formation of benzo[c]cinnoline by photocatalytic reduction of 2,2′-dinitrobiphenyl using TiO2 and under UV light irradiation

Article abstract of DOI:10.1039/c5cc02713f

Photocatalytic reduction of 25 μmol 2,2′-dinitrobiphenyl in 50% aqueous iso-propanol and 50 mg P25-TiO₂ under an argon atmosphere and 20 h UV light irradiation selectively produced 23.8 μmol of benzo[c]cinnoline (95%), and 2,2′-biphenyldiamine (5%) whose amount gradually increased with the irradiation time beyond 20-24 h due to further reduction of benzo[c]cinnoline. This journal is

Full text of DOI:10.1039/c5cc02713f

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Selective formation of benzo[c]cinnoline by
photocatalytic reduction of 2,20-dinitrobiphenyl
using TiO2 and under UV light irradiation†
Cite this: Chem. Commun., 2015,
51, 8500
Received 1st April 2015,
Accepted 10th April 2015
Jaspreet Kaur and Bonamali Pal*
DOI: 10.1039/c5cc02713f
www.rsc.org/chemcomm
Photocatalytic reduction of 25 lmol 2,20-dinitrobiphenyl in 50% withdrawing effect and the position of the –NO2 group in the
aqueous iso-propanol and 50 mg P25-TiO2 under an argon atmo- benzene moiety. However, little information has been known to
sphere and 20 h UV light irradiation selectively produced 23.8 lmol date about the reduction behaviour of –NO2 groups present in two
of benzo[c]cinnoline (95%), and 2,20-biphenyldiamine (5%) whose separate benzene rings of biphenyl compounds.
amount gradually increased with the irradiation time beyond
In this context, this research demonstrated for the first time
that photocatalytic reduction of DNBP led to highly selective
yield of BC (Scheme 1) along with a small amount of other
20–24 h due to further reduction of benzo[c]cinnoline.
Benzo[c]cinnoline (BC) is a very important organic compound amino derivatives by P25-TiO2 (Degussa, a mixture of 70%
used in manufacturing of dyes, electrochromic polymers, anatase and 30% rutile) under controlled UV light irradiation.
coloured polyamide fibers and has microbial and herbicidal The reduction was carried out in a rubber capped-sealed test
activities.1–3 It is typically synthesized by reduction of 2,20- tube containing 5 mL aqueous iso-propanol (IPA, 50 vol%),
dinitrobiphenyl (DNBP)4,5 with RANEYs Ni6 and Pd-C,7,8 Zn,9 50 mg P25-TiO2 (TiO2) and 25 mmol DNBP suspension under an
Fe+2 oxalate,10 Fe,11 LiAlH4,12,13 triethyl phosphite,14 Fe-carbonyl,15 argon atmosphere with constant magnetic stirring and UV light
and by electrochemical16 methods. These conventional methods (125 W Hg arc, 10.4 mW cmꢀ2) irradiation for different time
require harsh conditions such as strong reducing agents, high periods. The reaction products are quantitatively analysed by
pressure and temperature, expensive solvents, and leave toxic HPLC, GC-MS and NMR techniques (ESI†).
byproducts. Hence, a greener, environmentally benign and mild
Fig. 1a and b show the GC chromatographs of the authentic
photocatalytic reaction process using a nontoxic TiO2 catalyst sample showing a single peak of DNBP (25 mmol) at tR = 14.5 min
under light irradiation could have beneficial advantages for the and BC (7.5 mmol) at tR = 13 min. Fig. 1c reveals that after DNBP
highly selective production of BC from DNBP reduction under reduction by TiO2 during 8 h irradiation, a number of GC peaks
ambient conditions.
at tR = 11.6, 12.9, 15.7 and 16.9 min including few smaller peaks
Nitroaromatics reduction by TiO2 under UV irradiation has evolved due to the formation of various reduction products. The
been recently reported17,18 to be feasible due to its higher peak height at tR = 14.2 min for DNBP is decreased due to its
conduction band energy (ꢀ0.85 V) than the reduction potential complete reduction, and with the passage of 15–24 h irradiation
ꢀ0.5 V vs. SCE of the –NO2 group.19,20 Nitrobenzene reduction (Fig. 1d–f), the intensity of all the peaks is reduced except for
to aniline and azoxybenzene by photoirradiated CdS, WO3, a smaller peak at tR = 11.6 min, and a major peak at tR = 12.9–
ZnO, TiO2, and SiO2@Rh–CdS composites,21–25 formation of 13.0 min is found to be significantly enhanced (Fig. 1e) after DNBP
4-ethoxy-1,2,3,4-tetrahydroquinoline26 and cyclic imino acids,27 reduction for 20 h irradiation. The peak at tR = 15.7 min is not
synthesis of 2-methylpiperazine by (TiO2 or CdS)–zeolites,28 etc. observed after 15 h, whereas the peak intensity at tR = 11.6 min is
are demonstrated. Photocatalytic reduction of m-dinitrobenzene
for selective formation of m-phenylenediamine (100%) by P25-TiO2
and m-nitroaniline (100%) by rutile TiO2, and p-dinitrobenzene
to a mixture of products is reported29 to be due to the electron
School of Chemistry and Biochemistry, Thapar University, Patiala, Punjab, India.
E-mail: bpal@thapar.edu; Fax: +91 175 2364498; Tel: +91 175 2393491
Scheme 1 Photocatalytic reduction of DNBP to BC by TiO2 under UV
† Electronic supplementary information (ESI) available: HPLC, GC, GC-MS and
irradiation.
NMR analysis techniques. See DOI: 10.1039/c5cc02713f
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Fig. 1 GC pattern of authentic (a) DNBP (25 mmol), (b) BC (7.5 mmol) and photoreduction products of DNBP (25 mmol) by TiO2 after (c) 8 h, (d) 15 h,
(e) 20 h, (f) 24 h UV irradiation.
seen to be almost the same up to 20 h and beyond that it is notably
increased with a simultaneous decrease in the peak height at
tR = 12.9 min after 20–24 h UV light irradiation.
The mass spectrum analysis of authentic DNBP (Fig. 2a) and
its reduction products confirmed the formation of BC (Fig. 2b)
at tR = 12.9 min, biphenyldiamine (BPD, Fig. 2c) at tR = 11.6 min,
and benzo[c]cinnoline dioxide (BCD, Fig. 2d) at tR = 15.7 min
during DNBP reduction by TiO2 for different time periods. Fig. 3a
shows the variation in the peak heights of three major reduction
products BCD, BC and BPD with decreased DNBP concentration
and increased UV irradiation time. It is found that DNBP almost
selectively reduced to BC along with the formation of a small
amount of BPD after 20 h. It further revealed that BCD is formed
up to 15 h irradiation, whereas BPD is increased with a decrease
in the amount of BC formed beyond 20–24 h reduction. By
comparing 7.5 mmol commercial authentic BC samples, the yield
and selectivity of BC obtained by DNBP (25 mmol) reduction using
Fig. 3 (a) Time course graph (peak height variation) of different products
of DNBP reduction and (b) BC yield and selectivity as a function of UV
irradiation time.
TiO2 under UV irradiation for 8–24 h gradually increased from
67% to a maximum of 23.8 mmol (95%) of BC during 20 h and
thereby, remarkably decreased beyond 20–24 h irradiation due to
its self-reduction to BPD. HPLC analysis (Fig. S1, ESI†) also
revealed the formation of B23.7 mmol of BC. In a blank test,
no such reduction of DNBP occurred without the TiO2 catalyst
Fig. 2 Mass spectral analysis of DNBP and its reduction products.
after 20 h of UV light irradiation (Fig. S2, ESI†), thus confirming
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the photocatalytic nature of DNBP reduction reaction. The repro- DNBP by irradiated TiO2 required 8eꢀ for the BC formation
ducibility was further judged by another set of experiments through various intermediate reduction steps. It is also observed
carried out under similar conditions, which produced B23.2 mmol that the reduction process is accompanied by the simultaneous
of BC (Fig. S3c, ESI†) relative to 23.8 mmol in the first run.
oxidation of iso-propanol (hole scavenger) to acetone (tR = 1.2 min)
The 1H-NMR (400 MHz, CDCl3) spectral analysis (Fig. S4, whose amount (Fig. S5, ESI†) increased with (20 to 24 h) irradia-
ESI†) shows the peaks observed for: DNBP authentic at d 8.20 tion time, and no H2 production is detected by photoexcited holes
(d, 2H, J = 8.24 Hz, ArH), 7.70 (t, 2H, J = 7.32 Hz, ArH), 7.60 (h+) in the valence band under UV irradiation. Furthermore, over
(t, 2H, J = 8.24 Hz, ArH), 7.30 (dd, 2H, J = 7.76 Hz, ArH); BC oxidation of acetone to CO2 was not observed as confirmed by GC
authentic (Fig. S4b, ESI†) at d 8.76–8.74 (m, 2H, ArH), 8.59–8.57 analysis (Fig. S6, ESI†).
(m, 2H, ArH), 7.94–7.89 (m, 4H, ArH). BC formed by DNBP
Thus, it is evident that DNBP undergoes intramolecular
photoreduction shows NMR (Fig. S4c, ESI†) peaks at d 8.77–8.74 reductive cyclization reactions by TiO2 because of the close
(m, 2H, ArH), 8.61–8.59 (m, 2H, ArH), and 7.94–7.91 (m, 4H, spatial proximity5 of the interacting NO2 groups that lie in two
ArH) that almost closely match (except for some small peaks different benzene rings separately relative to their location in
of other unidentified products) with the characteristic NMR the same benzene moiety in various dinitrobenzenes. Such
spectra of the authentic BC sample. BPD produced after 24 h redox combined photocatalytic reactions like selective photo-
irradiation showed peaks at d 7.54–7.50 (m, 4H, ArH), 7.30 reduction of o-dinitrobenzene to benzimidazole (96%) by TiO2,31
(t, 2H, J = 7.8 Hz, ArH) and 7.23 (d, 2H, J = 8.24 Hz, ArH), and a and formation of pipecolinic acid32 from L-lysine irradiation are
typical broad peak for NH2 protons at 4.23 (bs, 4H, NH2) is also reported to be highly efficient for practical applications. Depend-
seen in Fig. S4d (ESI†) which are quite different from the NMR ing on the position (1 : 2, 1 : 3 and 1 : 4) of –NO2 functionality29,31
spectra of DNBP and BC.
and the extent of the electron withdrawing effect of –NO2 groups
On the basis of the obtained detected reduction products, imparted to the benzene ring, dinitrobenzene is reduced to a
it is revealed that the rate determining step for DNBP photo- variety of reduction products by both P25-TiO2 and rutile TiO2
catalytic reduction using TiO2 is the selective formation of BC under controlled UV irradiation. However, the pure rutile TiO2
via some transient intermediates; nitroso and hydroxylamine phase does not have any photoreactivity for DNBP reduction
derivatives BCD, azoxy compounds and few unidentified products under the same experimental conditions.
by following reaction pathways in Scheme 2. It is reported25,30 that
The non-conventional production of BC from DNBP reduction
this intermediate sequence RNO2–RNO–RNHOH–RNH2 is gener- by TiO2 and UV light where the yield and selectivity could be
ally formed during many nitroaromatics photoreduction. GC simply tuned by light irradiation under ambient conditions
chromatographs (Fig. 1) also further support that these short lived without using any costly toxic solvent and reducing agents
products (e.g., BCD) disappeared with increasing irradiation time. is experimented for the first time using an inexpensive and
BPD production took place to a lesser extent via direct reduction of non-toxic titania catalyst. This photoreduction process could be
the hydroxylamine derivative (path-I) during initial hours (8–20 h), extended to synthesis of other N-heterocyclic compounds, which
and beyond that more amount of BPD (path-II) is formed probably requires harsh experimental conditions in conventional techniques.
due to self-reduction of as-produced BC during 8–20 h irradiation.
We are grateful to Prof. Ashok Kumar Malik, Punjabi Uni-
The photoexposure of TiO2 (band gap energy = 380–390 nm) with versity, Patiala, India, for GC-MS analysis.
UV light generates electrons in the conduction band and holes in
the valence band, where two NO2 groups of DNBP are sequentially
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8502 | Chem. Commun., 2015, 51, 8500--8503
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