P. Datta et al. / Polyhedron 71 (2014) 47–61
49
2 ꢀ y,1/2 + z position and another imidazolyl ring, Cg2 interacts
with phenyl ring Cg3 (Cg1: N(1)–C(1)–N(2)–C(3)–C(2); Cg2:
N(5)–C(18)–C(19)–N(6)–C(20); Cg3: C(4)–C(5)–C(6)–C(7)–C(8)–
C(9); Cg5: C(21)–C(22)–C(23)–C(24)–C(25)–C(26)). Both these
two interactions connect each discrete unit to form 1D supramo-
lecular chain along crystallographic c-axis, Fig. 4(a). The separation
between centroids are 3.615(3) and 3.642(3) Å and dihedral angles
are 6.3(3)° and 2.9(3)°, respectively. The molecule also contains C–
H---p
interaction which gives enhanced stability to the structure.
The hydrogen atom H(8) faces C–H---
p
interaction with the phe-
nyl ring, Cg6 at 1 ꢀ x,1/2 + y,1/2 ꢀ z position (Cg6: C(29)–C(30)–
C(31)–C(32)–C(33)–C(34)) and thus 2D supramolecular structure
forms in the bc-plane, Fig. 4(b). These 1D chains are further con-
nected by p---p interaction to form 3D supramolecular structure,
Fig. 5. The phenyl ring Cg3 interacts with Cg3 at 1 ꢀ x,ꢀy,1 ꢀ z po-
sition and the other phenyl ring Cg5 interacts with the other phe-
nyl ring at ꢀx,ꢀy,ꢀz position. The distances between centroids are
not significant (4.199(3) and 4.271(3)(3) Å).
2.3. IR and 1H NMR spectra
The bands in the FT-IR spectra of the complexes were assigned
on comparing with free ligand data and reported complexes
[26,27]. Moderately intense stretching at 1590–1600 and 1370–
1380 cmꢀ1 are due to
m
(C@N) and
stretch is observed for the compounds 4–12 at 1650–1655 cmꢀ1
and is assigned to (C@O) of coordinated DMF (Supplementary
m(N@N), respectively. A strong
m
materials, Table S2). The 1H NMR spectra of the complexes are re-
corded in DMSO-d6 and the signals are assigned by spin–spin inter-
action, the effect of substitution therein and on comparing with
previously reported data [26,27]. The atom numbering pattern is
shown in the structure (Scheme 2). Data (Table 2) reveal that the
signals in the spectra, in general, are downfield shifted compared
to the spectra of free ligand. The signal due to 7,11-H are shifted
to higher d by 0.3–0.5 ppm. This observation supports the exis-
tence of strong interaction between imidazolyl group of the ligand
and Zn(II) in the complexes. Aryl signals are shifted to lower field
on Me-substitution to the ring. This is due to electron donating
effect of Me- group. N(1)-R/ shows usual signal pattern as earlier.
The coordination of DMF to the complexes 4–12 are supported
by a proton signal to most down field (8.3–8.4 ppm) which is cor-
responding to –CHO and two –Me signals at 2.80–2.82 and 2.88–
2.90 of –NMe2.
Fig. 3. 1D supramolecular chain is formed by C-Hꢁ ꢁ ꢁCl hydrogen bonding interac-
tions in [Zn(MeaaiCH2Ph)(DMF)Cl2] (6b) [C(11)–H(11b)—Cl(2): H---Cl, 2.81; C----
Cl, 3.740(4); \C–H–Cl, 160°].
two ligands comparing to one ligand in 6b. The Zn(II) and N(azo)
short contact appears at 2.697 Å which is less than the sum of
van der Waals radii of Zn (vdW Radius, 1.39 Å) and azo-N (vdW
radius, 1.55 Å). However, the proposed distance is 0.15 Å longer
than previously reported result, 2.546(3) Å of [Zn(MeaaiMe)(H2-
O)Cl2] [27]. The AN@NA bond length, N(3)–N(4), 1.253(3) Å in
6b is closer to that of free ligand value (1.2518 (19) Å) [18,29]
while it differs in 21b and are lower than free ligand data (N(3)–
N(4), 1.221(5) and N(7)–N(8), 1.126(6) Å). The Zn–X distances are
2.2313(8) (Zn–Cl(1)), 2.224(9) Å (Zn–Cl(2)) in 6b and 2.5965(6) Å
(Zn–I(1)), 2.6133(6) Å (Zn–I(2)) in 21b. 1-Benzyl-2-(p-toly-
lazo)imidazole is nearly planar excluding –CH2C6H5 group. Imidaz-
olyl and p-tolyl rings are joined by AN@NA group and the dihedral
angle is 2.0–2.5°. The acute chelate angle may develop a strain that
is relieved partially by structural distortion and bond length elon-
gation. The molecular packing of 6b shows 1-D polymer (Fig. 3) in
which the network is constituted via hydrogen bonding of one of
the two Cls (Zn–Cl) and benzyl-CH2, C(11)–H(11b)----Cl(2)
(H(11b)---Cl(2), 2.81; C(11)----Cl(2), 3.740(4) and \C(11)–
H(11b)---Cl(2), 160° (symmetry, 1/2 + x,1/2 ꢀ y,1/2 + z). In 21b
2.4. UV–Vis spectra and photochromism
The absorption spectra were recorded in DMF solution for the
complexes in 200–900 nm (Fig. 6). The absorption band around
365–385 nm with a molar absorption coefficient on the order of
104 Mꢀ1 cmꢀ1 is assigned to pp⁄ band and a tail at 450–460 nm
corresponds to
a n
p⁄ transition (Supplementary materials,
Table S2).
Light irradiation at kmax to a DMF solution of the complex, the
absorption spectrum changes (Figs. 7 and 8) which is the signature
of trans(t)-to-cis(c) isomerization. The intense peak at kmax
decreases, which is accompanied by a slight increase at the tail
portion of the spectrum around 490 nm until a stationary state
(PSS-I) is reached. Subsequent irradiation at the newly appeared
longer wavelength peak reverses the course of the reaction and
the original spectrum is recovered up to a point, which is another
photostationary state (PSS-II) under irradiation at the longer
wavelength peak. The quantum yields of the trans-to-cis photoiso-
merization were determined using those of azobenzene [30] as
a standard and the results are tabulated in Table 3. Thermal
cis-to-trans isomerization rates of these arylazoimidazoles at
298–313 K are collected in Table 4.
the components are connected by
p---p interactions to form
supramolecular structure (Supplementary materials, Table S1).
Imidazolyl ring, Cg1 interacts with phenyl ring Cg5 at x,1/
Datta, Paramita
