100
A. Satheshkumar et al. / Journal of Organometallic Chemistry 750 (2014) 98e106
1,2-diamino-anthroquinone (1 g, 0.0042 mol) and ferrocene car-
boxaldehyde (0.8988 g, 0.0042 mol) in 5 mL of DMSO was stirred
and heated at 90 ꢁC under nitrogen atmosphere for 12 h. After
completion of reaction the reaction mixture was cooled to RT and
precipitate formed was filtered through the filter paper and washed
with cold ethanol to give dark brown solid (1.2 g, Yield ¼ 66.2%).
absorbance of the peak at 450 nm decreases progressively whilst a
new band at 575 nm appears and develops. A clear isosbestic point
was observed at 487 nm which indicated the formation of a stable
complex, between 1 and Fꢀ ions, with a certain stoichiometric ratio
i.e. only 1 and 1. FL species coexist. In other words, addition of
fluoride ions bathochromically shifts the lICT
(DlICT ¼ 125 nm).
1H NMR (300 MHz, DMSO-d6)
d
(ppm) 13.28 (s, 1H), 8.22 (t, 2H,
With the effect the color of the solution instantaneously changed
from yellow to intense red. Such an observed electronic spectral
change is due to the fact that NeH/Fꢀ is relatively a better electron
donor than free NeH in the ICT transition that occur in 1 [29,30].
Addition of cyanide ions to the solution of 1(6.25 ꢂ 10ꢀ5 M) in
DMSO caused similar changes in the UVeVis spectra (Fig. 2B) with a
DlICT of 128 nm.
J ¼ 3 Hz), 8.00 (s, 2H), 7.32 (d, 1H, J ¼ 7.8 Hz), 6.58 (d, 1H, J ¼ 7.8),
5.20 (s, 2H), 4.55 (s, 2H), 4.19 (s, 5H). LC-MS m/z (M þ H)þ calcd.
432.3, found 433.2. Elemental analysis: Anal. Calcd. for
C
25H16FeN2O2: C, 69.47; H, 3.73; N, 7.40. Found: C, 68.93; H, 3.82, N,
7.45.
The UVeVis spectrum of receptor 2 (6.25 ꢂ 10ꢀ5 M) in DMSO
exhibits an absorption band at 409 nm corresponds to the ICT
transition (Fig. 3A and B). The lICT of receptor 2 is much less than
that of receptor 1 (lICT ¼ 450 nm) indicating that the ICT tran-
sition is energetically less favored in 2 than that in 1. This may be
due to the fact that in 1, the imidazole N-atom is directly
attached to the quinone moiety whereas in 2 it is far away,
leading to a relatively less favored ICT transition. Parallel to the
observations made with receptor 1, during the titrations with
fluoride and cyanide ions the UVeVis band seen at 409 nm for 2
disappeared with a concomitant appearance of new band at
509 nm. A clear isosbestic point is observed at 448 nm suggesting
the coexistence of 2 and 2 anion species only in solution. Here
again, the bathochromic shift observed in the lICT is due to the
fact that the NeH/Fꢀ moiety is relatively a better electron donor
than free NeH in the ICT transition. However, the relatively lower
DlICT (¼ 99 nm) observed with the receptor 2 when compared to
the receptor 1 indicated that the order of the strength of inter-
action is 1. FL > 2. FL. The other anions like OAcꢀ, H2PO4ꢀ, NOꢀ3 ,
Clꢀ, Brꢀ and Iꢀ did not induce any obvious change in the UVeVis
spectra of both the receptors 1 and 2. Representative UVeVis
spectra are given in Fig. S1. Likewise, the intensity of absorbance
of the 1-Fꢀ complex showed no appreciable change in the
presence of other anions (Fig. S2).
3. Results and discussion
Facile condensation of ferrocene carboxaldehyde with 2,3-
diaminonaphthoquinone (A) in DMSO yielded receptor 1 and
with 1,2-diamino-anthroquinone gave receptor 2 (Scheme 2).
These receptors were characterized using elemental analysis and
1H NMR (see Experimental section). The anion sensing properties of
1 and 2 were screened in DMSO using UVeVis, 1H NMR, CV and DPV
techniques.
3.1. Visual detection
Visual inspection of solutions of receptors
1
and
2
(6.25 ꢂ 10ꢀ5 M) in DMSO before and after addition of 1 equivalent
of tetrabutyl ammonium salts of various anions such as Fꢀ, Clꢀ, Brꢀ,
Iꢀ, AcOꢀ, H2PO4ꢀ, NOꢀ3 and CNꢀ was carried out. As depicted in Fig. 1
solutions of 1 and 2 (in DMSO) turned yellow to intense red color
after the addition of fluoride and cyanide ions. Nevertheless, the
color remained unchanged after the addition of the other chosen
inorganic anions. This observation indicated that 1 and 2 simulta-
neously sense fluoride and cyanide ions. The receptor 2 exhibited a
change of color from red to bluish green with cyanide ions in
acetonitrileewater medium.
The stoichiometry of the interaction between the receptors (1
and 2) and Fꢀ/CNꢀ ions was determined using the electronic
spectral data. The Job’s plots for the receptors with these two an-
ions are shown in Fig. 4. In these cases, curves with a maximum at
0.5 mol fraction indicated the formation of 1:1 (receptor:anion)
complexes [31]. The association constant (K) for these complexes
was determined using the Scott equation (Figs. S3eS4) [32] and the
values thus obtained are collected in Table 1.
The results indicated that the binding constant of receptor
1-anion complex (DlICT ¼ 123 or 125 nm) is relatively higher than
that of 2-anion complex (DlICT ¼ 99 nm). This is due to the fact that,
in the receptor 1, since the H-bond donor unit (receptor unit, NeH)
is directly attached to the signaling unit (quinone moiety), the
H-atom of the NeH group may relatively be more acidic due to
enhanced ICT transition and thus interacts strongly with the an-
ions. The significance of such an assembly is that any small
perturbation in the electron density on the receptor unit (as a result
of anion binding) would cause noticeable change in the optical
behavior of signaling unit. Similar trend has been observed by us
with these two quinone based chemosensors with urea as receptor
unit in fluoride sensing [27].
3.2. Binding studies using UVeVis spectrophotometry
The electronic spectra of 1 at a concentration of 6.25 ꢂ 10ꢀ5 M in
DMSO upon addition of fluoride and cyanide ions are shown in
Fig. 2A and B, respectively. The receptor 1 had an absorption band
(log
3
¼ 3.59) at 450 nm which corresponds to the intramolecular
charge transfer (ICT) transition from imidazole N-atom to the
quinone moiety (n / p*) [28]. As shown in Fig. 2A, upon addition
of incremental amounts of fluoride ions to the receptor 1, the
3.3. 1H NMR titration studies
To have a closer look in to the mechanism of anion binding
properties of these two receptors, 1H NMR titration experiments
were carried out in DMSO-d6. As envisaged in the electronic
spectral study, in the receptor 1, the signal for the imidazole NeH
Fig. 1. Color change of receptors 1 and 2 with anions.