1-alkyl-2-{(O-thioalkyl)phenylazo}imidazole complexes of PbII and their photochromic property

Article abstract of DOI:10.1002/zaac.201300173

Pb^II complexes of 1-alkyl-2-{(o-thioalkyl)phenylazo}imidazole (SRaaiNR'), [Pb(SRaaiNR')₂X₂] were characterized by spectroscopic studies. The single-crystal X-ray structure of [Pb(SEtaaiNEt) ₂Cl₂] (SEt

Full text of DOI:10.1002/zaac.201300173

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1-Alkyl-2-{(O-Thioalkyl)Phenylazo}Imidazole
Complexes of PbII and Their Photochromic
Property
ARTICLE in ZEITSCHRIFT FÜR ANORGANISCHE CHEMIE · AUGUST 2013
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ARTICLE
DOI: 10.1002/zaac.201300173
1-Alkyl-2-{(O-Thioalkyl)Phenylazo}Imidazole Complexes of PbII and Their
Photochromic Property
Shefali Saha Halder,[a] Suman Roy,[a] Tapan Kumar Mondal,[a] Rajat Saha,[b] and
Chittaranjan Sinha*[a][‡]
Keywords: PbII-thioalkylphenyl-azoimidazoles; X-ray diffraction; Photochromism; Activation energy; Effect of halide; Halogens
Abstract.
PbII
complexes
of
1-alkyl-2-{(o-thioalkyl)- erization of the coordinated azoimidazole. The rate of isomerization
phenylazo}imidazole (SRaaiNR’), [Pb(SRaaiNR’)2X2] were charac-
terized by spectroscopic studies. The single-crystal X-ray structure of
follows the sequence: [Pb(SRaaiNR’)2Cl2] Ͻ [Pb(SRaaiNR’)2Br2] Ͻ
[Pb(SRaaiNR’)2I2]. Quantum yields (φEǞZ) and the activation energy
[Pb(SEtaaiNEt)2Cl2]
(SEtaaiNEt
=
1-ethyl-2-{(o-thioalkyl)- (Ea) of the isomerization of the complexes are lower than observed for
phenylazo}imidazole) proved imidazolyl-N and –SEt coordination
forming unusual puckered eight member chelate rings. UV light irradi-
the free ligand. This can be explained by considering the molecular
assembly and the thus observed increase in mass and rotor volume of
ation of the complexes in DMF solution shows E-to-Z (E and Z refer the complexes. DFT and TDDFT calculations of optimized geometry
to trans and cis-configuration about –N=N–, respectively) photoisom- could explained the spectral properties and photochromic activity.
structure of 1-alkyl-2-(arylazo)imidazole about –N=N– upon
Introduction
light irradiation. This is known as photochromic activity. Pho-
Lead, a useful metal in power, nuclear, construction indus-
tochromic compounds have found application in different areas
tries, is a dangerous pollutant[1] to human health. Because of
such as liquid crystal alignment,[26] optical data storage,[27]
its borderline acidic property it is able to bind strongly with
non-linear optics,[28] photoswitching,[29] molecular-photonic
–SH function in enzymes, proteins and also with imidazolyl
[30]
devices
etc. Use of light as stimulus has enormous advan-
groups of biomolecules. Interaction of lead with biomolecules
may cause toxicity and sometime fatal to human health.[2,3]
However, lead may be utilized for the benefit of mankind fol-
lowing the design of proper materials.
tages over other agents like thermal, magnetic, mechanical,
electrical, redox etc. Light irradiation to some molecules may
reversibly transform one molecular structure to other structure
whose absorption spectra differ prominently.[31]
Aromatic diazo compounds are useful dyes and pigments
and have application in biology, medicine, and in the field of
non-linear optics, photoswitching and in the fabrication of op-
tical data storage devices.[4–7] The synthesis and characteriza-
tion of heteroarylazo dyes have drawn special attention.[8–11]
Our interest is to synthesize lead(II) arylazoimidazole com-
pounds. The molecule is π-acidic and its active function is an
azoimine group (–N=N-C=N–). We have studied the coordina-
tion chemistry[12–16] and analytical application[17,18] of aryl-
azoimidazoles. Our group has recently started to investigate
the coordination chemistry of PbII with 1-alkyl-2-(arylazo)-
imidazole.[19,20] One of the interesting properties of arylazo-
imidazoles and their complexes is photoisomerization,[21–25]
which is a reversible transformation between trans and cis
In continuation of our efforts, this work is concerned with
the photoisomerization of PbII complexes of 1-alkyl-2-{(o-
thio-alkyl)phenyl-azo}imidazole (SRaaiNR’) (Scheme 1). The
ligand is unusually chelated via imidazolyl-N and -SR to PbII.
Ligand, SRaaiNR’, has three potential donor centers, N(imid-
[32]
azole), N(azo) and –SR, and can serve as tridentate-N,N’,S
[25]
and bidentate-N,N’
donor agent. Single-crystal X-ray
structure determination of representative complex confirmed
the structure of the complex. Light irradiation of solutions of
* Prof. Dr. C. Sinha
E-Mail: c_r_sinha@yahoo.com
[a] Department of Chemistry
Jadavpur University
Kolkata – 700 032, India
[b] Department of Physics
Jadavpur University
Kolkata – 700 032, India
[‡] Present Address: Kalyani Govt. Engg. College
Kalyani, Nadia
Scheme 1. Synthesis of ligand and atom numbering of the ligand and
the complexes.
West Bengal – 741235, India
Z. Anorg. Allg. Chem. 2013, 639, (10), 1861–1870
© 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
1861

S. Saha Halder, S. Roy, T. K. Mondal, R. Saha, C. Sinha
ARTICLE
the complexes showed reversible photoisomerization.[25,32] of their van der Waals radii (Pb 2.02; Nazo 1.55 Å; sum of van
The effect of Pb–X (X = Cl, Br, I) and SRaaiNR’ on photo- der Waals radii = 3.57 Å) and the low chelate angle (vide in-
isomerization rate and quantum yields have been reported in fra). The Pb–N(1)–C(7)–N(3)–N(4)–C(1)–C(2)-S(1) puckered
this work. Theoretical calculation has been attempted on opti- chelate ring is essentially nonplanar (deviation Ͼ 0.14 Å). The
mized geometry of the complexes to rationalize the bonding bond parameters are listed in Table 1. The Pb–Nimidazole
and spectral properties of the complexes.
(N(1/5)) bonds are sufficiently strong and the distances are
comparable with lengths reported in literature.[20] The Pb–S
bond lengths are Pb(1)–S(1/2), 3.351/3.338 Å.[33,34] The Nazo
atoms (N(4/8)) could be forced to bind to PbII and the Pb(1)–
N(4/8) (3.151/3.090 Å) bond lengths are much longer than pre-
viously reported distances (2.752(12) Å,[20]) which is also un-
favorable due to the small chelate angles (ЄN(1)–Pb(1)–N(4),
55.81°; ЄN(5)–Pb(1)–N(8), 53.81°). Thus, an eight-membered
chelated structure may be considered. The Pb–S(–R) bonding
Results and Discussion
Syntheses of the Complexes
1-Alkyl-2-{(o-thioalkyl)phenylazo}imidazoles [SRaaiNR’]
(R = R’ = Me (2a); R = Me, R’ = Et (2b); R = Et, R’ =
Me (2c); R = R’ = Et (2d)] were synthesized by literature
procedure.[32] The reaction of hot water solution or EGME
solution of PbX2 and the ligand SRaaiNR’ in MeOH in 1:2
molar ratio yielded orange-red crystals of composition
[Pb(SRaaiNR’)2X2]. Microanalytical data confirmed the com-
position of the complexes. The structures were established by
single-crystal X-ray diffraction studies.
Structure of [Pb(SEtaaiNEt)2Cl2] (3d)
The molecular structure of 3d is shown in Figure 1. The
structure shows interesting unusual Nimidazole (N), S–Et eight-
membered metal coordination whereas Nazo (N’) is weakly
bonded and appears as an in-between N,S-chelator, 1-ethyl-
2-{(o-thioethyl)phenylazo}, SEtaaiNEt. The charge balance is
carried out by two Pb–Cl bonds. Thus, the PbII atom shows
PbN2S2Cl2 coordination whereas the two Nazo atoms are
Figure 1. ORTEP plot (40% probability ellipsoid) of
weakly bonded (Pb–Nazo, 3.151/3.090 Å) as evident from sum [Pb(SEtaaiNEt)2Cl2] (3d).
Table 1. Selected bond lengths /Å and angles /° for [Pb(SEtaaiNEt)2Cl2] (3d).
Bond length /Å
#
†
[Pb(SEtaaiNEt)2Cl2]
Expt.
[Pb(SEtaaiNEt)2Cl2]
Theo.
[Pb(SEtaaiNEt)2Br2]†
Theo.
[Pb(SEtaaiNEt)2I2]†
Theo.
Theo.
Pb(1)–N(1)
Pb(1)–N(5)
Pb(1)–X(1)
Pb(1)–X(2)
Pb(1)–S(1)
Pb(1)–S(2)
N(3)–N(4)
N(7)–N(8)
S(1)–C(2)
S(1)–C(12)
2.631(7)
2.654(7)
2.669(3)
2.638(3)
3.351(3)
3.338(2)
1.245(9)
1.272(9)
1.793(12)
1.441(8)
2.656
2.707
2.609
2.611
3.814
3.938
1.267
1.268
1.780
1.839
2.666
2.713
2.611
2.609
3.918
3.766
1.267
1.268
1.837
1.781
2.682
2.698
2.824
2.822
3.801
3.877
1.267
1.268
1.781
1.839
2.688
2.713
3.075
3.081
3.720
3.775
1.268
1.268
1.780
1.838
Bond angle /°
Expt.
Theo
Theo
Theo
Theo
N(1)–Pb(1)–X(2)
N(1)–Pb(1)–N(5)
X(2)–Pb(1)–N(5)
N(1)-Pb(1)- X(1)
X(2)–Pb(1)–X(1)
N(5)–Pb(1)–X(1)
C(20)–N(5)–Pb(1)
C(21)–N(5)–Pb(1)
C(7)–N(1)–Pb(1)
C(8)–N(1)–Pb(1)
83.99(17)
164.0(2)
86.11(19)
85.42(17)
96.26(12)
83.16(17)
122.0(6)
128.9(7)
124.1(5)
129.4(7)
83.65
161.6
85.22
85.19
102.91
83.06
123.07
127.3
124.63
127.33
83.52
160.56
84.68
84.59
102.6
85.68
165.0
84.86
85.38
104.37
85.73
125.55
125.99
125.82
125.95
87.22
169.94
85.85
87.22
105.53
88.78
82.98
123.08
127.08
124.88
127.11
125.7
126.66
125.57
126.38
#
†SEtaaiNEt acts as monodentate Nimidazole donor. SEtaaiNEt acts as bidentate Nimidazole,Sthioether chelator
1862
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© 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
Z. Anorg. Allg. Chem. 2013, 1861–1870

1-Alkyl-2-{(O-Thioalkyl)Phenylazo}Imidazole Complexes of PbII
1
is confirmed by H NMR spectroscopy; the signals of –S–Me/
Et are shifted downfield compared with free ligand data.[32]
The [Pb(N,S)2Cl2] units are connected by hydrogen bonding
with Pb–Cl bonds of imidazolyl-H/aryl-H of neighboring mo-
lecules to constitute supramolecular networks (Figure 2 and
Figure 3). Out of two Pb–Cl bonds the Cl(1) atom is triply
hydrogen bonded, Cl(1)···H(5), H(9), H(22) and Cl(2) is dou-
bly hydrogen bonded Cl(2)···H(4), H(19). The bonding param-
eters are Cl(1)···H(5)–C(5):C(5)–H(5), 0.93(4) Å, Cl(1)···H(5),
2.73(2) Å, C(5)···Cl(1), 3.633(15) Å; ЄCl(1)···H(5)–C(5),
164.31(1)° (symmetry, –x, –y, 1–z); Cl(1)···H(9)–C(9):C(9)–
H(9), 0.93(2) Å, Cl(1)···H(9), 2.80(6) Å, C(9)···Cl(9),
3.632(11) Å and ЄCl(1)···H(9)–C(9), 149.5(1)° (symmetry, –
x, –1–y, 1–z); Cl(1)···H(22)–C(22):C(22)–H(22), 0.93(1) Å,
Cl(1)···H(22), 2.73(7) Å, C(22)···Cl(1), 3.570(12) Å and
ЄCl(1)···H(22)–C(22), 149.5(6)° (symmetry, –x, –1–y, 2–z);
and Cl(2)···H(4)–C(4):C(4)–H(4), 0.93(3) Å, Cl(2)···H(4),
2.74(3) Å, C(4)···Cl(2), 3.557(15) Å and ЄCl(2)···H(4)–C(4),
146.62(1)° (symmetry, x, 1+y, z); Cl(2)···H(19)–C(19):C(19)–
Figure 3. 3D supramolecular structure formed by hydrogen bonding
interactions. Each monomeric units are connected by supramolecular
C–H···π interactions leading to the formation of 2D supramolecular
sheet structure, Hydrogen bonding interactions also connect the mono-
meric units to form 3D supramolecular structure.
H(19), 0.93(1) Å, Cl(1)···H(19), 2.77(8) Å, C(19)···Cl(2), 0.6 Å than that of X-ray determined structures. The molecular
3.554(12) Å and ЄCl(2)···H(19)–C(19), 141.48(1)° (symmetry, functions are calculated and the electronic structure is assigned
to the molecule that was used to consign electronic transitions
observed in solution spectra. SEtaaiNEt has three donor cen-
ters – Nazo, Nimidazole, Sthioether and could act as monodentate
–1–x, –1–y, 2–z). These interactions form 2D supramolecular
sheet within the ab-plane, Figure 2. (symmetry: –x, –y, 1–z, x,
1+y, z). These 2D sheets are further connected by π···π interac-
tion with thioaryl and imidazole rings (C(13)–H(13C)···Cg4;
H···Cg4, 2.80 and ЄC(13)–H(13C)···Cg4, 137°. C(23)–
H(23B)···Cg4; H···Cg4, 2.82 and ЄC(23)–H(23B)···Cg4, 144°
(Cg4 refers to thioaryl group, C(14) to C(19)) to form 3D sup-
ramolecular structure, Figure 3.
Nimidazole(Pb(Nimidazole)2Cl2)orbidentateNimidazole,Sthioetherche-
lator (Pb(Nimidazole, Sthioether)2Cl2) or tridentate Nazo, Nimidazole
,
Sthioether chelators (Pb(Nimidazole, Nazo, Sthioether)2Cl2) (Fig-
ure 4). DFT calculations were performed for three isomeric
structures of [Pb(SEtaaiNEt)2Cl2] (3d). The calculation shows
that the energy of the bidentate Nimidazole, Sthioether chelated
complex (energy of X-ray structure, –86215.9917 eV) is closer
to a proposed monodentate Nimidazole coordinated structure
(–86215.9928 eV) while the energy of the tridentate Nazo
,
N
imidazole, Sthioether coordinated compound has higher energy
(–86187.554 eV). This results partly suggest remote feasi-
bility of tridentate behavior of SEtaaiNEt to PbII. However,
both monodentate Nimidazole coordinated (Pb(Nimidazole)2Cl2)
and bidentate Nimidazole, Sthioether, chelated (Pb(Nimidazole
,
Sthioether)2Cl2) complexes could exist and only the latter could
be structurally characterized. The procedure has been extended
to [Pb(SEtaaiNEt)2Br2] and [Pb(SEtaaiNEt)2I2] (Supporting
Information, Figure S2 and Figure S3) and the molecular func-
tions are calculated to explain the spectral and photochromic
properties.
Figure 2. 2D supramolecular sheet of 3d formed by C–H···π interac-
tions.
DFT computations were carried out with optimized geome-
tries of the complexes. The bond parameters are calculated and
comparison of calculated and X-ray crystallographically deter-
mined bond lengths and angles show good agreement
Figure 4. Optimized structures of (a) monodentate Nimidazole coordi-
nated, (b) bidentate Nimidazoles, Sthioether chelated and (c) Nimidazoles
,
(Table 1). The theoretical bond lengths are elongated by 0.02– Nazo, Sthioether tridentate type [Pb(SEtaaiNEt)2Cl2].
Z. Anorg. Allg. Chem. 2013, 1861–1870
© 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
www.zaac.wiley-vch.de
1863

S. Saha Halder, S. Roy, T. K. Mondal, R. Saha, C. Sinha
ARTICLE
UV/Vis Spectra and Photochromism
NEt (49%, 3d; 33%, 4d; 7%, 5d) and halogen (36% of Cl in
3d; 52% of Br in 4d and 84% of I in 5d) (Supporting Infor-
mation, Table S1). Other occupied MOs (HOMO-1, HOMO-2
etc) are composed of SEtaaiNEt and X (halogen). The LUMOs
(LUMO, LUMO+1, LUMO+2 etc.) are π* orbital delocalized
over the entire SEtaaiNET (98–100%). The energy of the MOs
is correlated in Figure 6. The calculated transitions are given
in Table 2 (Details are given in the Supporting Information,
Tables S2, S4, S6). The intensity of these transitions has been
assessed from oscillator strength (f). The calculated bands are
mainly contributed of ILCT, XLCT or admixture of them and
a small contribution of MLCT (MLCT, metal-to-ligand charge
transfer; XLCT, halide-to-ligand charge transfer, ILCT, intra-
ligand charge transfer where ligand refers to SEtaaiNEt). The
transitions in UV region (Ͻ 400 nm) are the mixture of ILCT
and XLCT. The observed transitions appear closer to the calcu-
lated wavelength in the complexes (Figure 7).
The solution electronic spectra of the compounds were re-
corded in DMF in 200–600 nm. There are two bands in the
UV/Vis region at 360–370 and 410–420 nm. By comparison
with the spectra of the free ligands[32] and those of the PbII
complexes,[19,20] it may be concluded that these stem from in-
tramolecular charge-transfer transitions (nǞπ*, πǞπ*). These
observations are also supported by frontier molecular orbital
calculations using X-ray diffraction parameters.
The electronic configurations of three representative com-
plexes [Pb(SEtaaiNEt)2X2] (X = Cl (3d), Br (4d), I (5d)) were
calculated by DFT and TD-DFT computation. The molecular
functions have been used to explain the origin of transitions.
The selected MOs of the complexes are shown in Figure 5 and
composition of MOs is given in the Supporting Information,
Tables S1, S3, S5. The HOMOs of these three complexes are
composed of lead (15% 3d and 4d while 9% for 5d), SEtaai-
Figure 5. Selected MOs (HOMO-2, HOMO-1, HOMO, LUMO, Figure 6. Correlation amongst calculated energy levels of
LUMO+1, LUMO+2) of [Pb(SEtaaiNEt)2Cl2] (3d). [Pb(SEtaaiNEt)2X2] (X = Cl (3d), Br (4d), I (5d)).
Table 2. Selected list of excitation energies of the complexes in gas phase and the assignment of transitions.
Excitation energy /eV
Wavelength /nm
f
Key Transitions
Character
[Pb(SEtaaiNEt)2Cl2] (3d)
2.3626
2.3931
2.7401
2.8594
4.6498
4.7774
4.8737
524.77
518.09
452.47
433.61
266.64
259.52
254.40
0.0238
0.0255
0.0567
0.0552
0.0541
0.1002
0.0771
(83%)HOMOǞLUMO
ILCT, XLCT
ILCT, XLCT
ILCT
ILCT, XLCT
ILCT, XLCT
ILCT, XLCT
ILCT
(79%) HOMOǞLUMO+1
(69%) HOMO-1ǞLUMO
(74%) HOMO-2ǞLUMO
(88%) HOMOǞLUMO+2
(78%) HOMOǞLUMO+3
(50%) HOMO-1ǞLUMO+2
[Pb(SEtaaiNEt)2Br2] (4d)
2.2831
2.3199
4.5371
4.7862
4.8673
4.9416
543.06
534.44
273.27
259.05
254.73
250.90
0.0137
0.0162
0.0226
0.0516
0.0309
0.1082
(92%)HOMOǞLUMO
ILCT, XLCT
ILCT, XLCT
ILCT, XLCT
ILCT, XLCT
ILCT, XLCT
ILCT, XLCT
(91%) HOMOǞLUMO+1
(83%) HOMOǞLUMO+2
(68%) HOMO-1ǞLUMO+2
(81%) HOMO-2ǞLUMO+2
(72%) HOMOǞLUMO+4
[Pb(SEtaaiNEt)2I2] (5d)
4.0986
4.2712
4.6360
302.50
290.28
267.44
262.41
0.0309
0.0016
0.0462
0.0172
(40%) HOMOǞLUMO+2
(35%) HOMO-3ǞLUMO+2
(26%) HOMO-4ǞLUMO+2
(30%) HOMO-4ǞLUMO+3
XLCT
XLCT
XLCT
XLCT
4.7248
1864
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© 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
Z. Anorg. Allg. Chem. 2013, 1861–1870

1-Alkyl-2-{(O-Thioalkyl)Phenylazo}Imidazole Complexes of PbII
Figure 7. UV/Vis Spectrum (theoretical,___; experimental (DMF solu-
tion),---) of [Pb(SEtaaiNEt)2Cl2].
Figure 8. Spectral change of [Pb(SEtaaiNEt)2Cl2] (3d) in DMF upon
repeated irradiation at 360nm at 4 min interval at room temperature.
The E-to-Z (trans-to-cis) isomerization of free ligand,
SRaaiNR’ in acetonitrile (Figure 8) and [Pb(SRaaiNR’)2X2]
(Scheme 2) was investigated by irradiation of UV light in
DMF solution (Figure 9). The spectral pattern of free ligands
(SRaaiNR’) and their PbII complexes are very similar. Being
no transition metal ion, PbII does not contribute to the elec-
tronic configuration of coordinated SRaaiNR’, except the ener-
gies of the FMOs (frontier molecular orbitals) are reduced and
hence electronic spectral transitions are shifted to longer wave-
lengths by 5–10 nm. Photoisomerization of SRaaiNR’ was car-
ried out in acetonitrile. Because of very close polarity of DMF
(6.4 D) and acetonitrile (5.8 D) the photoisomerization of
SRaaiNR’ has been taken in acetonitrile and has compared
with results of DMF solution of the complexes. Absorption
spectra of the E-configuration of coordinated SRaaiNR’ in the
complexes, 3–5, in DMF are changing with isosbestic point
upon excitation to the cis configuration of the ligand. The
quantum yields were measured for the E-to-Z (φEǞZ) photo-
isomerization of these compounds in DMF on irradiation of
UV wavelength (Table 3). The φEǞZ values are significantly
dependent on the nature of the substituents, halide type and
molecular weight.[19,20,22–25] In presence of -Me and -Et
groups, the quantum yields are reduced compared to those of
the free ligands.
Scheme 2. Photochromism of free and coordinated SRaaiNR/.
Upon UV light irradiation, the trans (E) structure of
SRaaiNR’ changed to cis structure about the azo (–N=N–)
function and the cis molar ratio reached to Ͼ80% (Scheme 2).
UV light irradiation to the solution of the photochrome may
insist the rotation of the azo-aryl group (–N=N–Ar) which
isomerizes from trans (E) to cis (Z) form. The rates of photo-
Figure 9. Z Ǟ E isomerization of [Pb(SMeaaiNEt)2Cl2] (3b).
The thermal Z–to–E (cis-to-trans) isomerization of the com-
isomerization (trans-to-cis (E-Z), φEǞZ
) increases with
decreasing electronegativity of X in the complexes (Table 3); plexes was followed by UV/Vis spectroscopy in MeCN at var-
rate follows [Pb(SRaaiNR’)2Cl2] (3) Ͻ [Pb(SRaaiNR’)2Br2] ied temperatures, 298–313 K and the activation energies were
(4) Ͻ [Pb(SRaaiNR’)2I2] (5) although the molar mass follows obtained (Table 4) from Eyring plots (Figure 10). In the com-
the order 3Ͻ4Ͻ5. Higher electronegativity of chlorine in 3 plexes the Eas are quite closer to respective free ligands, which
may help to enhance association strength by electrostatic inter- mean slow rate of Z-to-E thermal isomerization of the com-
action with neighboring molecules and may effectively in- plexes. The entropies of activation (ΔS*) are more negative in
crease rotor mass than that of 4 or 5.
the complexes than that of the free ligands. This is also de-
Z. Anorg. Allg. Chem. 2013, 1861–1870
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S. Saha Halder, S. Roy, T. K. Mondal, R. Saha, C. Sinha
Table 3. Excitation wavelength (λππ*), rate of E(trans)Ǟ Z(cis) conversion and quantum yield (ftǞc).
ARTICLE
Compd
λπ,π* /nm Isobastic points/nm
Rate of EǞ Z conversion ϫ 108 /s–1
(fEǞZ).
SMeaaiNMe♣
357
358
357
356
362
361
362
360
361
364
363
363
360
361
360
362
337
337
336
335
341
342
340
338
343
339
341
340
342
341
339
340
4.908 (2)
3.108 (4)
4.67 (1)
2.948 (2)
2.83 (2)
1.67 (3)
2.12 (2)
1.13 (9)
3.34 (5)
1.82 (7)
2.45 (4)
1.48 (5)
3.62 (3)
2.07 (4)
2.81 (1)
1.72 (4)
0.317 (1)
0.232 (3)
0.290 (4)
0.197 (4)
0.151 (3)
0.091 (6)
0.093 (7)
0.086 (8)
0.176 (2)
0.098 (7)
0.103 (1)
0.092 (8)
0.203 (6)
0.105 (1)
0.111 (5)
0.099 (5)
SMeaaiNEt♣
SEtaaiNMe♣
SEtaaiNEt♣
[Pb(SMeaaiNMe)2Cl2] (3a)
[Pb(SMeaaiNEt)2Cl2] (3b)
[Pb(SEtaaiNMe)2Cl2] (3c)
[Pb(SEtaaiNEt)2Cl2] (3d)
[Pb(SMeaaiNMe)2Br2] (4a)
[Pb(SMeaaiNEt)2Br2] (4b)
[Pb(SEtaaiNMe)2Br2] (4c)
[Pb(SEtaaiNEt)2Br2] (4d)
[Pb(SMeaaiNMe)2I2] (5a)
[Pb(SMeaaiNEt)2I2] (5b)
[Pb(SEtaaiNMe)2I2] (5c)
[Pb(SEtaaiNEt)2I2] (5d)
Data are collected from Ref. [24].
the following instruments: UV/Vis spectra with a Perkin–Elmer
Lambda 25 spectrophotometer. IR spectra (KBr disk, 4000–200 cm–1)
with a Perkin–Elmer RX-1 FTIR spectrophotometer; photoexcitation
experiments were carried out using a Perkin–Elmer LS-55 spectrofluo-
rimeter.
Syntheses
[Pb(SEtaaiNEt)2Cl2] (3d): A methanol solution of 1-ethyl-2-{(o-thi-
oethyl)phenylazo}imidazole (56 mg, 0.20 mmol) was added in drops
to hot water solution of PbCl2 (30 mg, 0.11 mmol) and stirred for 2 h
and heated to reflux for 2 h. The resultant reddish solution was col-
lected by filtration. Slow evaporation of the solution gave orange-red
crystals in a yield of 53.42 mg (62%).
Figure 10. Eyring plots of cis-to-trans thermal isomerization of (a)
SEtaaiNEt (2d) and (b) [Pb(SEtaaiNEt)2Cl2] (3d).
[Pb(SRaaiNR’)2X2] (X = Br and I) were synthesized by mixing solu-
tions of PbBr2 and PbI2 in EGME (ethylene glycol monomethyl ether)
and SRaaiNR’ in MeOH. The products were isolated in yields in the
range 60–70%.
fending the increase in rotor volume in the complexes. It is
our apprehension that light irradiation in UV wavelength (360–
364 nm) may cleave Pb–SR bonds while PbII–Nimidazole may
remain in hooking stage (as it is a stronger bond than Pb–SR)
and pendant thioalkylarylazo unit may suffer bond rotation to
assist photochromism. DFT and TD-DFT computation have
calculated that the transitions in UV region (Ͻ 400 nm) are
the mixture of ILCT and XLCT which assist for photochromic
reaction. Participation of X in the π···π* transition has indi-
rectly influencing the E-to-Z photoisomerization.
[Pb(SRaaiNR’)Br2] and [Pb(SRaaiNR’)I2] were synthesized following
a similar procedure using PbBr2 and PbI2 and SRaaiNR’ in methanol-
EGME solution (3:1, v/v) under reflux.
Microanalytical data of the complexes were as follow
[Pb(SMeaaiNMe)2Cl2] (3a) Anal. Found: C, 35.59; H, 3.22; N,
15.08%. Calc. for [PbC22H24N8S2Cl2]: C, 35.58; H, 3.23; N, 15.09 %.
FT-IR (KBr disc), ν(N=N), 1420; ν(C=N), 1571 cm–1. UV/Vis (DMF)
(λmax(nm)(10–3 ε (dm3 mol–1 cm–1): 362 (26.21), 410 (17.65). 1H
NMR (300 MHz, CDCl3), δ(ppm), (J(Hz)): 7.41 (br. s, 4-H), 7.24 (br.
s, 5-H), 7.35 (d, 7.0 Hz, 8-H), 7.44 (m, 9- and 10-H), 8.10 (d, 8.0 Hz,
11-H), 4.32 (s, 1-CH3), 2.60 (s, S-CH3).
Experimental Section
Materials
[Pb(SMeaaiNEt)2Cl2] (3b): Anal. Found: C, 37.38; H, 3.64; N,
14.55%. Calc. for [PbC24H28N8S2Cl2]: C, 37.40; H, 3.63; N, 14.54 %.
FT-IR (KBr disc), ν(N=N), 1425; ν(C=N), 1572 cm–1. UV/Vis (DMF)
(λmax(nm)(10–3 ε (dm3 mol–1 cm–1): 361 (28.45), 411(19.09). 1H NMR
(300 MHz, CDCl3), δ(ppm), (J(Hz)): 7.42 (br. s, 4-H), 7.23 (br. s, 5-
H), 7.32 (d, 7.0 Hz, 8-H), 7.43 (m, 9- and 10-H), 7.98 (d, 8.0 Hz,11-
H), 4.48 (q, 6.60 Hz, 1-CH2), 1.65 (t, 7.30 Hz, (1-CH2)-CH3)), 2.67 (s,
S-CH3).
PbCl2, PbBr2 and PbI2 were obtained from Loba Chemicals, Bombay,
India 1-alkyl-2-{(o-thio-alkyl)phenylazo}imidazole were synthesized
by reported procedure.[32] All other chemicals and solvents were rea-
gent grade as received.
Physical Measurements
Microanalytical data (C, H, N) were collected on Perkin–Elmer 2400 [Pb(SEtaaiNMe)2Cl2] (3c): Anal. Found: C, 37.41; H, 3.62; N,
CHNS/O elemental analyzer. Spectroscopic data were obtained using 14.52%. Calc. for [PbC24H28N8S2Cl2]: C, 37.40; H, 3.63; N, 14.54 %.
1866
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© 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
Z. Anorg. Allg. Chem. 2013, 1861–1870

1-Alkyl-2-{(O-Thioalkyl)Phenylazo}Imidazole Complexes of PbII
Table 4. Rate and activation parameters for Z Ǟ E thermal isomerization.
Compd T /K
Rate of thermal Z Ǟ E conversion ϫ 104 /s–1
Ea /kJmol–1 ΔH* /kJ·mol–1 ΔS* /J·mol–1K–1 ΔG* /kJ·mol–1
3a
3b
3c
3d
4a
4b
4c
4d
5a
5b
5c
5d
298
303
308
313
298
303
308
313
298
303
308
313
298
303
308
313
298
303
308
313
298
303
308
313
298
303
308
313
298
303
308
313
298
303
308
313
298
303
308
313
298
303
308
313
298
303
308
313
7.14
16.03
14.68
14.93
12.55
18.15
16.64
16.89
14.77
20.11
18.16
18.35
15.24
13.49
12.14
12.39
10.01
15.61
14.1
–259.93
–264.12
–263.27
–270.84
–252.66
–257.43
–256.52
–263.23
–246.04
–252.22
–251.57
–261.57
77.47
78.77
80.07
81.37
78.72
80.04
81.36
82.68
78.46
79.78
81.1
82.41
80.72
82.07
83.43
84.78
75.31
76.57
77.83
79.1
76.73
78.02
79.30
80.59
76.46
77.74
79.02
80.30
78.45
79.77
81.09
82.40
73.34
74.57
75.8
77.03
75.18
76.44
77.70
78.96
74.98
76.24
77.50
78.76
77.96
79.27
80.58
81.88
7.854
8.671
9.752
7.385
8.198
8.75
9.91
7.401
8.212
8.785
9.978
7.768
8.555
9.008
10.00
7.266
8.081
9.147
10.30
7.533
8.312
9.241
10.4
7.587
8.347
9.367
10.5
7.899
8.819
9.461
10.6
7.312
8.145
9.512
10.7
7.611
8.491
9.589
10.8
7.647
8.511
9.601
10.9
7.995
8.892
9.641
10.8
14.35
12.23
17.57
15.62
15.81
12.70
FT-IR (KBr disc), ν(N=N), 1427; ν(C=N), 1573 cm–1. UV/Vis (DMF) [Pb(SMeaaiNMe)2Br2] (4a): Anal. Found: C, 31.76; H, 2.90; N,
(λmax(nm)(10–3 ε (dm3 mol–1 cm–1): 362 (27.21), 412 (20.52). 1H 13.49%. Calc. for PbC22H24N8S2Br2: C, 31.77; H, 2.89; N, 13.48 %.
FT-IR (KBr disc), ν(N=N), 1421; ν(C=N), 1622 cm–1. UV/Vis (DMF)
(λmax(nm)(10–3 ε (dm3 mol–1 cm–1): 361 (18.55), 410 (18.11). 1H
NMR (300 MHz, CDCl3), δ(ppm), (J(Hz)): 7.40 (br. s, 4-H), 7.23 (br.
s, 5-H), 7.36 (d, 7.0 Hz, 11-H), 7.55 (m, 9- and 10-H), 7.84 (d, 8.0 Hz),
4. 72 (s, 1-CH3), 2.55 (s, S-CH3).
NMR (300 MHz, CDCl3), δ(ppm), (J(Hz)): 7.40 (br. s, 4-H), 7.23 (br.
s, 5-H), 7.34 (d, 7.0 Hz, 8-H), 7.46 (m, 9- and 10-H), 7.94 (d, 7.9 Hz,
11-H), 4.28 (s, 1-CH3), 3.01 (q, 7.2 Hz, S-CH2), 1.31 (t, 7.35 Hz, (S-
CH2)-CH3).
[Pb(SEtaaiNEt)2Cl2]: (3d): Anal. Found: C, 39.07; H, 4.02; N,
14.04%.Calc. for [PbC26H32N8S2Cl2]: C, 39.09; H, 4.01; N, 14.03 %.
FT-IR (KBr disc), ν(N=N), 1490; ν(C=N), 1572 cm–1. UV/Vis (DMF)
(λmax(nm)(10–3 ε (dm3 mol–1 cm–1): 360 (34.60), 409 (22.26). 1H
NMR (300 MHz, CDCl3), δ(ppm), (J(Hz)): 7.41 (br. s, 4-H), 7.22 (br.
s, 5-H), 7.35 (d, 7.0 Hz, 8-H), 7.45 (m, 9- and 10-H), 7.97 (d, 8.0 Hz,
11-H), 4.59 (q, 6.7 Hz, 1-CH2), 1.60 (t, 7.2 Hz, (1-CH2)-CH3), 3.04
(q, 7.2 Hz, S-CH2), 1.39 (t, 7.36 Hz, (S-CH2)-CH3).
[Pb(SMeaaiNEt)2Br2] (4b): Anal. Found: C, 33.54; H, 3.25; N,
13.02%. Calc. for PbC24H28N8S2Br2: C, 33.53; H, 3.26; N, 13.04 %.
FT-IR (KBr disc), ν(N=N), 1422; ν(C=N), 1621 cm–1. UV/Vis (DMF)
(λmax(nm)(10–3 ε (dm3 mol–1 cm–1): 364 (31.59), 410 (21.32). 1H
NMR (300 MHz, CDCl3), δ(ppm), (J(Hz)): 7.41 (br. s, 4-H), 7.20 (br.
s, 5-H), 7.33 (d, 7.0 Hz, 8-H), 7.52 (m, 9- and 10-H), 7.88 (d,
Z. Anorg. Allg. Chem. 2013, 1861–1870
© 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
www.zaac.wiley-vch.de
1867

S. Saha Halder, S. Roy, T. K. Mondal, R. Saha, C. Sinha
8.0 Hz,11-H), 4.47 (q, 7.0 Hz, 1-CH2), 1.69 (t, 7.30 Hz, of ω scans. Unit cell parameters were determined from least-squares
ARTICLE
(1-CH2)-CH3)), 2.64 (s, S-CH3).
refinement of setting angles (θ) within the range 1.62 Յ θ Յ 25.73°.
Out of 6087collected data 4584 were used for structure solution. The
hkl ranges are –14 Յ h Յ 14, –13 Յ k Յ 14, –16 Յ l Յ 15. Data
were corrected for Lorentz polarization effects and for linear decay.
Semi-empirical absorption corrections based on ψ-scans were applied.
The structures were solved by direct method using SHELXS-97[35] and
successive difference Fourier syntheses. All non-hydrogen atoms were
refined anisotropically. The hydrogen atoms were fixed geometrically
and refined using the riding model. All calculation were carried out
using SHELXL-97,[36] PLATON-99,[37] and ORTEP-32[38] programs.
[Pb(SEtaaiNMe)2Cl2] (4c): Anal. found: C, 33.52; H, 3.27; N,
13.03%. Calc. for PbC24H28N8S2Cl2: C, 33.53; H, 3.26; N, 13.04 %.
FT-IR (KBr disc), ν(N=N), 1419; ν(C=N), 1573 cm–1. UV/Vis (DMF)
(λmax(nm)(10–3 ε (dm3 mol–1 cm–1): 363 (26.05), 411 (21.58). 1H
NMR (300 MHz, CDCl3), δ(ppm), (J(Hz)): 7.42 (br. s, 4-H), 7.20 (br.
s, 5-H), 7.35 (d, 7.0 Hz, 8-H), 7.56 (m, 9- and 10-H), 7.83 (d, 8.0 Hz,
11-H), 4.30 (s, 1-CH3), 3.00 (q, 7.4 Hz, S-CH2), 1.30 (6.2 Hz, (S-CH2)-
CH3).
[Pb(SEtaaiNEt)2Br2] (4d): Anal. found: C, 35.18; H, 3.60; N,
12.61%. Calc. for PbC26H32N8S2Br2: C, 35.17; H, 3.61; N, 12.62 %.
FT-IR (KBr disc), ν(N=N), 1420; ν(C=N), 1590 cm–1. UV/Vis
(CH3CN) (λmax(nm)(10–3 ε (dm3 mol–1 cm–1): 363 (31.42), 408
(22.68). H NMR (300 MHz, CDCl3), δ(ppm), (J(Hz)): 7.41 (br. s, 4-
H), 7.22 (br. s, 5-H), 7.32 (d, 7.0 Hz), 7.53 (m, 9- and 10-H), 7.86 (d,
Table 5. Summarized crystallographic data for [Pb(SEtaaiNEt)2Cl2]
(3d).
[Pb(SEtaaiNEt)2Cl2] (3d)
1
Empirical formula
Formula weight
C26H32Cl2N8S2Pb
798.81
8.0 Hz, 11-H), 4.52 (q, 7.25 Hz, 1-CH2), 1.62 (t, 7.2 Hz, (1-CH2)- Temperature /K
293(2)
Crystal system
Space group
Triclinic
P1
CH3), 3.04 (q, 7.3 Hz, S-CH2), 1.35 (t, 6.16 Hz, (S-CH2)-CH3).
¯
Crystal size /mm
Unit cell dimensions
a /Å
b /Å
c /Å
α /°
β /°
0.26 ϫ 0.22 ϫ 0.20
[Pb(SMeaaiNMe)2I2] (5a): Anal. found: C, 28.55; H, 2.60; N, 12.10
%. Calc. for PbC22H24N8S2I2: C, 28.54; H, 2.59; N, 12.11 %. FT-
IR (KBr disc), ν(N=N), 1412; ν(C=N), 1535 cm–1. UV/Vis (DMF)
(λmax(nm)(10–3 ε (dm3 mol–1 cm–1): 360 (16.11), 411 (15.45). 1H
NMR (300 MHz, CDCl3), δ(ppm), (J(Hz)): 7.41 (br. s, 4-H), 7.28 (br.
s, 5-H), 7.31 (d, 7.0 Hz, 8-H), 7.55 (m, 9- and 10-H), 7.80 (d, 8.0 Hz,
11-H), 4.14 (s, 1-CH3), 2.52 (s, S-CH3).
11.691(5)
12.083(5)
13.185(5)
78.368(5)
72.962(5)
67.843(5)
1640.5(12)
γ /°
V /Å3
[Pb(SMeaaiNEt)2I2] (5b): Anal. found: C, 30.21; H, 2.93; N, 11.76
%. Calc. for PbC24H28N8S2I2: C, 30.22; H, 2.94; N, 11.75 %. FT-
IR (KBr disc), ν(N=N), 1414; ν(C=N), 1532 cm–1. UV/Vis (DMF)
(λmax(nm)(10–3 ε (dm3 mol–1 cm–1): 361(27.32), 410(17.23). 1H NMR
(300 MHz, CDCl3), δ(ppm), (J(Hz)): 7.40 (br. s, 4-H), 7.23 (br. s, 5-
H), 7.34 (d, 7.0 Hz, 8-H), 7.56 (m, 9- and 10-H), 7.82 (d, 8.0 Hz,11-
H), 4.51 (q, 7.0 Hz, 1-CH2), 1.63 (t, 7.3 Hz, (1-CH2)-CH3)), 2.59 (s,
S-CH3).
Z
λ /Å
2
0.71073
5.461
1.617
μ (Mo-Kα) /mm–1
Dcalc /mg·m–3
hkl range
–14 Յ h Յ 14, –13 Յ k Յ 14, –16 Յ l
Յ 15
1.62–25.73
346
6087
θ range /°
Refine parameters
Total reflection
Unique data [I Ͼ 2σ (I)] 4584
[Pb(SEtaaiNMe)2I2] (5c): Anal. found: C, 30.23; H, 2.95; N, 11.76%.
Calc. for PbC24H28N8S2Cl2: C, 30.22; H, 2.94; N, 11.75 %. FT-IR
(KBr disc), ν(N=N), 1413; ν(C=N), 1534 cm–1. UV/Vis (CH3CN)
(λmax(nm)(10–3 ε (dm3 mol–1 cm–1): 360(25.21), 412(17.88). 1H NMR
(300 MHz, CDCl3), δ(ppm), (J(Hz)): 7.43 (br. s, 4-H), 7.27 (br. s, 5-
H), 7.32 (d, 7.0 Hz, 8-H), 7.55 (m, 9- and 10-H), 7.81 (d, 8.0 Hz, 11-
H), 4.17 (s, 1-CH3), 3.00 (q, 7.3 Hz, S-CH2), 1.29 (t, 7.0 Hz, (S-CH2)-
CH3).
a)
R1 [I Ͼ 2σ (I)]
wR2
Goodness of fit
0.0500
0.1409
1.017
b)
2
2
4 1/2
a) R = Σ|Fo–Fc|/ΣFo. b wR = [Σw(Fo –Fc )/ΣwFo ] are general but
2
2
w are different, w = 1/[σ2(F2) + (0.0869P)2] where P = (Fo + 2Fc )/
3.
[Pb(SEtaaiNEt)2I2] (5d): Anal. found: C, 31.79; H, 3.27; N, 11.40%.
Calc. for PbC26H32N8S2I2: C, 31.80; H, 3.26; N, 11.41 %. FT-IR (KBr
disc), ν(N=N), 1415; ν(C=N), 1533 cm–1. UV/Vis (CH3CN) (λmax(nm)
Photometric Measurements
Absorption spectra were taken with a PerkinElmer Lambda 25 UV/Vis
Spectrophotometer in a 1 ϫ 1 cm quartz optical cell maintained at
25 °C with a Peltier thermostat. The light source of a PerkinElmer LS
55 spectrofluorimeter was used as an excitation light, with a slit width
of 10 nm. An optical filter was used to cut off overtones when neces-
sary. The absorption spectra of the cis isomers were obtained by ex-
trapolation of the absorption spectra of a cis-rich mixture for which
the composition is known from H NMR integration. Quantum yields
(φ) were obtained by measuring initial trans-to-cis isomerization rates
(ν) in a well-stirred solution within the above instrument using the
(10–3
ε
(dm3 mol–1 cm–1): 362(28.11), 409(21.05). 1H NMR
(300 MHz, CDCl3), δ(ppm), (J(Hz)): 7.42 (br. s, 4-H), 7.22 (br. s, 5-
H), 7.33 (d, 7.0 Hz, 8-H), 7.56 (m, 9- and 10-H), 7.80 (d, 8.0 Hz, 11-
H), 4.44 (q, 7.30 Hz, 1-CH2), 1.61 (t, 7.3 Hz, (1-CH2)-CH3), 3.05 (q,
7.4 Hz, S-CH2), 1.30 (t, 5.30 Hz, (S-CH2)-CH3).
1
X-ray Diffraction
Single crystals suitable for data collection were grown from slow evap-
oration of the complexes in methanol. The crystal data and details of equation, ν = (φ I0/V)(1–10–Abs) where I0 is the photon flux at the front
the data collections are given in Table 5. A suitable single crystal of of the cell, V is the volume of the solution, and Abs is the initial
[Pb(SEtaaiNEt)2Cl2] (3d) (0.26 ϫ 0.22 ϫ 0.20 mm) was mounted on absorbance at the irradiation wavelength. The value of I0 was obtained
a Bruker SMART APEX CCD diffractometer (graphite monochro-
by using azobenzene (φ = 0.11 for π–π* excitation [39]) under the same
mated Mo-Kα radiation, λ = 0.71073 Å) and data were collected by use
irradiation conditions
1868
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© 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
Z. Anorg. Allg. Chem. 2013, 1861–1870

1-Alkyl-2-{(O-Thioalkyl)Phenylazo}Imidazole Complexes of PbII
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Conclusion
PbII complexes of 1-alkyl-2-{(o-thioalkyl)phenylazo}imida-
zole (SRaaiNR’) are described, [Pb(SRaaiNR’)2X2] (X = Cl,
Br, I). The structures of the complexes have been confirmed
by single-crystal X-ray structure study in representative cases.
Photochromism of the complexes are examined by UV light
irradiation in DMF solution and the results are compared with
free ligands data. The decrease in electronegativity of X in-
creases the rate of E-to-Z photoisomerization The Z-to-E isom-
erization is thermally driven process. The activation energy
(Eas) of Z-to-E isomerization has been calculated. The slow
rate of isomerization in complexes may be due to higher rotor
volume than that of free ligands. DFT and TD-DFT computa-
tion using optimized geometry has supported experimental
structure data and have explained spectral and photochromic
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Acknowledgments
Financial support from Department of Science & Technology, New
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