S. Saha (Halder) et al. / Polyhedron 97 (2015) 240–247
241
R'
N
structure shows pentacoordination about Zn(II), ZnN4S sphere.
Zn(II) is bridged by –SCN– groups with two adjacent Zn-centers
thus 1-D chain is formed (Fig. 2). The ligand, SMeaaiNEt, acts as
bidentate –N(imidazolyl), N(azo) chelator. In the bridging unit
N
N
R'
N
N
hv
N
N
S
N
R''
hv ' or
‘‘[Zn(SMeaaiNEt)(l-SCN)Zn(SMeaaiNEt)]’’ two SMeaaiNEt show
S
R''
slightly different chelate angles, ‘\Zn–N(imidazolyl)-N(azo)’,
\N(1)–Zn(1)–N(4), 70.62(1)° and \N(7)–Zn(2)–N(10), 68.26(4)°.
These angles have been supported by reported data [24–26]. In
the polymeric motif the Zn–N(imidazolyl) distances are closely
spaced, Zn(1)–N(1), 1.988(3) and Zn(2)–N(10), 1.982(3) Å. The
Zn–N(azo) lengths differ slightly in two Zn(SMeaaiNEt) motifs of
trans- (E)
cis-(Z)
Scheme 1. Isomerisation of 1-alkyl-2-{(o-thioalkyl)phenylazo}imidazole.
bridging dimmer, –Zn(l-SCN)2Zn–: Zn(1)–N(4)(azo), 2.572(4) Å
and Zn(2)–N(10)(azo), 2.695(2) Å. The Zn–N(azo) is longer than
Zn–N(imidazolyl) which may account the inherent distortion in
the geometry and weakening effect of Zn–N(azo) bond. The
ZnN4S is neither the trigonal bipyramid nor square pyramid;
N(1)–N(4)–S(5) forms a distorted trigonal plane (deviation 5.7°).
The bridge, –N(6)–C(14)–S(3)– is tilted with trigonal plane,
ZnN2S, by 98.8° while –N(5)–C(13)–S(2)– bridge makes 111.4°.
Bridging motif –N(11)–C(27)–S(5)– makes torsion angle 154.32°
with Zn(1) and 137.58° with Zn(2) which is in support of signifi-
cant distortion from linearity. The length of C–N bonds also differ
in the bridging and terminal –SCN–: N(11)–C(27), 1.150(5) Å;
N(5)–C(13), 1.141(5) Å; N(6)–C(14), 1.126(5) Å. The crystal packing
shows C–H---S(SCN) recognizable weak interaction and has gener-
ated 3D super-build (Fig. 3) network. Thiocyanato-S interacts with
imidazolyl-Hs, C(9)–H(9) (C(9)–H(9)---S(5): H(9)---S(5), 2.82,
C(9)---S(5), 3.648(4); \C(9)–H(9)---S(5), 148°) and C(23)–H(23)
(C(23)–H(23)---S(6): H(23)---S(6), 2.82, C(23)---S(6), 3.686(5);
\C(9)–H(9)---S(5), 156°; symmetry, 1 ꢁ x, 1/2 + y, 1/2 ꢁ z) to
develop 3D structure. The DFT optimized structure shows metric
parameters where bond lengths are elongated by 0.05–0.2 Å and
bond angles are extended by 1–3° (Supplementary Materials,
Table S2).
2-{(o-thioalkyl)phenylazo}imidazole and has been alkylated by
adding alkyl iodide (MeI, EtI) in dry THF solution in presence of
NaH to synthesize 1-alkyl-2-{(o-thioalkyl)phenylazo}imidazole
[SRaaiNR/(R = R/ = Me (2a); R = Me, R/ = Et (2b); R = Et, R/ = Me
(2c); R = R/ = Et (2d)]. The reaction of Zn(OAc)2 with SRaaiNR/ in
MeOH (1:2 mole proportion) followed by the addition of NH4SCN
(2 equiv.) isolates dark red colored crystalline compounds
[Zn(SRaaiNR/)(SCN)(
l-SCN)2]n (3a–3d). The composition of the
complexes has been confirmed by microanalytical data and by
other spectroscopic information. The solid state (KBr) IR
stretchings at 1622–1625 and 1428–1431 cmꢁ1 and are assigned
to
2330–2380 cm–1 may be assigned to
while bridging thiocyanato ( -SCN) group shows
2150 cmꢁ1. The (CS) is observed at 745–755 cmꢁ1 and d(NCS)
m
(C@N) and
m
(N@N), respectively [25]. A strong band at
(CN) of monodentate –SCN
(CN) at 2050–
m
l
m
m
may be assigned to 460–470 cmꢁ1. The 1H NMR spectra of
complexes in CDCl3 show signals at 7.4 and 7.2 ppm corresponding
to imidazolyl 4- and 5-H which have been supported by literature
reports [17,24,25]. Phenyl protons (8- to 11-H) of –S-(R)-C6H4
remain almost unperturbed. Although single crystal X-ray
structure of 3b (Fig. 1) shows chelated [Zn(N(imidazolyl), N(azo)]
motif of SMeaaiNEt but Zn–N(azo) bond length is very long (but
less than that of sum of the van der Waals radii of Zn(II), 1.39 Å
and N(azo), 1.55 Å) which might be the reason for not showing
significant proton signal movement compared to monodentate
Zn–N(imidazolyl) complexes [24] (see Scheme 2).
2.3. UV–Vis spectra and photochromism of [Zn(SRaaiNR/)(SCN)
(l-SCN)2]n (3a–3d)
The ligands, SRaaiNR/ absorb at 350–360 nm which is assigned
2.2. Molecular structure of [Zn(SMeaaiNEt)(SCN)(l-SCN)2]n (3b)
to p ?
p⁄ transition and weak bands at 420 and 450 nm [17,24,25].
The metal complexes show an additional weak absorption at
longer wavelength, 460–465 nm, which is referred to MLCT transi-
tion. The TD-DFT and DFT computation of one of the complexes
The molecular structure of [Zn(SMeaaiNEt)(SCN)(
is shown in Fig. 1. The bond parameters are given in Table 1. The
l-SCN)2]n (3b)
Fig. 1. Crystal structure of [Zn(SMeaaiNEt)(l-SCN)2]n (3b).