Synthesis, crystal structure and photo-induced isomerization of [N,N′-bis(4-fluorobenzylidene)ethylenediamine]bromo(triphenyl-phosphine) copper(I)

Article abstract of DOI:10.1016/j.poly.2013.06.016

The title Schiff base complex is a model compound for photoisomerization reactions which occur in photobiological processes such as vision. The Schiff base ligand and the complex have been prepared in a one-pot synthesis. In the crystal structure of the complex, copper has a distorted tetrahedral coordination. Photochemical investigations in solution reveal time-resolved spectroscopic changes that are interpreted in terms of a transformation from E to Z configuration of the C=N bonds of the coordinated Schiff base ligand. Application of multivariate curve resolution and non-linear least squares curve fitting to the spectroscopic profiles provides a rate constant of 0.11 min ¹ for the photoisomerization and a quantum yield of 0.246.

Full text of DOI:10.1016/j.poly.2013.06.016

Polyhedron 62 (2013) 61–65
Contents lists available at SciVerse ScienceDirect
Polyhedron
journal homepage: www.elsevier.com/locate/poly
Synthesis, crystal structure and photo-induced isomerization
of [N,N0-bis(4-fluorobenzylidene)ethylenediamine]bromo(triphenyl-
phosphine)copper(I)
b
c
d
Kazem Barati a, , Mohammad Hossein Habibi , Morteza Montazerozohori , Hooshang Shafieyan ,
⇑
Ross W. Harrington e, William Clegg e,
⇑
a Young Researchers Club, Yasooj Branch, Islamic Azad University, Yasooj, Iran
b Department of Chemistry, University of Isfahan, Isfahan 81746-73441, Iran
c Chemistry Department, Yasouj University, Yasouj 75914–353, Iran
d Faculty of Sciences, K.N. Toosi University of Technology, Tehran, Iran
e School of Chemistry, Newcastle University, Newcastle upon Tyne NE1 7RU, UK
a r t i c l e i n f o
a b s t r a c t
Article history:
The title Schiff base complex is a model compound for photoisomerization reactions which occur in pho-
tobiological processes such as vision. The Schiff base ligand and the complex have been prepared in a one-
pot synthesis. In the crystal structure of the complex, copper has a distorted tetrahedral coordination.
Photochemical investigations in solution reveal time-resolved spectroscopic changes that are interpreted
in terms of a transformation from E to Z configuration of the C@N bonds of the coordinated Schiff base
ligand. Application of multivariate curve resolution and non-linear least squares curve fitting to the spec-
troscopic profiles provides a rate constant of 0.11 minꢀ1 for the photoisomerization and a quantum yield
of 0.246.
Received 31 January 2013
Accepted 13 June 2013
Available online 22 June 2013
Keywords:
Schiff base
Copper(I) complex
Tetrahedral coordination
Photo-induced isomerization
Rate constant
Ó 2013 Published by Elsevier Ltd.
1. Introduction
work is related to copper(I) complexes with mixed ligands,
Cu(NN)XY [5,6]. Photochemical Z/E isomerizations of C@C [7] and
Schiff-base metal complexes are of interest in inorganic chem-
istry and have been studied extensively; the Cambridge Structural
Database (version 5.34, November 2012) [1] contains over 70 en-
tries having the Schiff base skeleton shown in Scheme 1 with any
substituents allowed on the aromatic rings but none on the ali-
phatic chain, and has over 1300 metal complexes of such ligands.
Complexes of transition metal ions with bidentate Schiff bases con-
taining nitrogen and oxygen donor atoms play an important role in
biological systems and represent interesting models for metalloen-
zymes that catalyze the reduction of nitrogen and oxygen [2].
Schiff-base metal complexes have industrial, antifungal and bio-
logical applications [3]. The steric, electronic, and conformational
effects imparted by the coordinated ligands play an important role
in modifying the properties of their metal complexes. A thorough
understanding of these effects will serve as the basis for a rational
design of complexes with specific and predictable properties.
Although reports on [CuI(N4)] complexes are numerous [4], little
N@N [8] double bonds have been investigated thoroughly, but only
limited information is available on photoisomerization of C@N
double bonds [9]. Multivariate curve resolution-alternating
least squares (MCR-ALS) has already been applied to very diverse
second-order calibration problems, as for example, series of titra-
tions, chromatographic runs, or kinetic data [10–13]. We report
here the synthesis, structural characterization, and photo-induced
E/Z isomerization of (fb2en)bromo(triphenylphosphine)copper(I)
(1) (Scheme 1), where fb2en is the ligand N,N0-bis(4-fluoro-
benzylidene)ethylenediamine.
2. Experimental
2.1. Materials and physical measurements
All chemicals used were purchased from commercial sources
and used as received without further purification. The infrared
spectrum was obtained with a Shimadzu IR-435 spectrophotome-
ter using a KBr pellet. The room-temperature visible absorption
spectrum was recorded with a Shimadzu 160 spectrophotome-
ter. NMR spectra were recorded on a Bruker AW spectrometer
⇑
Corresponding authors. Tel.: +98 311 7932707; fax: +98 311 6689732
(K. Barati), tel.: +44 191 222 6649; fax: +44 191 222 6929 (W. Clegg).
E-mail addresses: barati111@yahoo.com (K. Barati), bill.clegg@ncl.ac.uk
(W. Clegg).
0277-5387/$ - see front matter Ó 2013 Published by Elsevier Ltd.
http://dx.doi.org/10.1016/j.poly.2013.06.016

62
K. Barati et al. / Polyhedron 62 (2013) 61–65
Scheme 1. Synthesis of [CuBr(fb2en)(PPh3)] (1).
operating at 400 MHz. A high-pressure mercury lamp with grating
monochromator was used in the photoisomerization study. Singu-
lar value decomposition (SVD), multivariate curve resolution and
non-linear least squares curve fitting were performed with
MATLAB version 7.0.
(60 mg, 0.23 mmol) in acetonitrile (5 ml) was added to the above
solution, which was stirred for an additional 20 min. Slow evapora-
tion of the solvent under vacuum gave the title compound (1) as yel-
low crystals: yield 136 mg, 85%. IR (KBr pellet, 400–4000 cmꢀ1) 1620
(C@N). UV [chloroform; kmax in nm (log
e
in L molꢀ1 cmꢀ1)]: 274
(6.44), 295 (6.47), 379 (6.59). 1H NMR (CDCl3) 3.86 (s, 4H, NCH2
CH2N); 7.02–7.75 (m, 23H, ArH); 8.61 (s, 2H, 2CH@N).
2.2. Synthesis of [CuBr(fb2en)(PPh3)] (1)
At ambient temperature, a chloroform solution (5 ml) of ethylene-
diamine (14 mg, 0.23 mmol) was added to 4-fluorobenzaldehyde
(57 mg, 0.46 mmol) in chloroform (10 ml) with stirring. After
half an hour, a mixture of CuBr (30 mg, 0.23 mmol) and PPh3
2.3. X-ray diffraction analysis
Single-crystal diffraction for (1) were measured at 150 K on a
Bruker SMART 1 K CCD diffractometer with Mo
Ka radiation
Fig. 1. Molecular structure of (1) with 40% probability displacement ellipsoids and atom labels.

K. Barati et al. / Polyhedron 62 (2013) 61–65
63
Table 1
(0.71073 Å); absorption corrections were based on repeated and
symmetry-equivalent data. The structure was solved by direct
methods and refined on all unique F2 values, with anisotropic dis-
placement parameters and with constrained riding isotropic H
atoms.
Yellow block crystal, 0.34 ꢁ 0.34 ꢁ 0.30 mm; C34H29BrCuF2N2P,
M = 678.0; monoclinic, space group P21/c, a = 19.1663(18), b =
13.2456(13), c = 11.9120(11) Å, b = 90.004(2)°, V = 3024.1(5) Å3,
Z = 4. 26681 reflections measured, 7368 unique, Rint = 0.0274,
Selected bond lengths (Å) and angles (°) for complex 1.
Cu–Br
2.4789(3)
2.1322(14)
117.682(15)
99.22(4)
Cu–P
Cu–N(2)
Br–Cu–N(1)
P–Cu–N(1)
N(1)–Cu–N(2)
2.2048(5)
2.0912(15)
107.24(4)
117.69(4)
84.07(6)
Cu–N(1)
Br–Cu–P
Br–Cu–N(2)
P–Cu–N(2)
125.10(4)
6178 reflections with F2 > 2
refined parameters, R [F, F2 > 2
r
, data 99.9% complete to h = 26°. 370
r
] = 0.0252, Rw [F2, all data] =
0.0629, goodness of fit = 1.01 on F2, final difference map extremes
+0.39 and ꢀ0.32 e Åꢀ3. Programs were Bruker SMART
, SAINT and SADABS
for data collection and processing, SHELXTL for structure solution,
refinement and graphics [14–16].
3. Results and discussion
3.1. Crystal structure of [CuBr(fb2en)(PPh3)] (1)
Compound (1) is a copper(I) complex with a distorted tetrahe-
dral coordination geometry for the N2PBr donor set (Fig. 1).
Selected bond lengths and angles are given in Table 1. The distor-
tion from ideal tetrahedral geometry is due mainly to the restricted
bite angle of the chelating Schiff base ligand: the N(1)–Cu–N(2) an-
gle is only 84.07(6)° while other angles range from 99.22(4)° to
125.10(4)°. There is also a twisting distortion, with a dihedral angle
of 83.7° (ideal 90°) between the CuN2 and CuBrP planes. The Cu–Br
bond length of 2.4789(3) Å in (1) is somewhat greater than those in
closely related copper(I) complexes with N-bidentate, Br and PPh3
ligands, for example 2.4400(5) Å in [CuBr(ca2en)(PPh3)] [17] and
2.3812(3) Å in [CuBr(bipy)(PPh3)] [18]. The Cu–P distance of
2.2048(5) Å agrees well with those in related complexes [5]. The
internal geometry of the ligands is unremarkable. The CuNCCN
chelate ring has approximately a twist conformation with atoms
C(8) and C(9) on opposite sides of the CuN2 plane, and this ligand
as a whole is bowed, with a dihedral angle of 44.7° between the
Fig. 2. UV–Vis absorption spectroscopic changes for 0.10 mM (1) in chloroform
with UV irradiation. From a to q, each irradiation time is 2 min, total 40 min.
two aromatic rings. There is intramolecular
Schiff base benzene rings (containing C(1)) and one of the phosphine
phenyl rings (containing C(29)), with
centroidꢂ ꢂ ꢂcentroid
p-stacking of one of the
Scheme 2. Photoisomerization mechanism of (1) in chloroform.
a
Scheme 3. Photoisomerization mechanism of the ligand in chloroform.

64
K. Barati et al. / Polyhedron 62 (2013) 61–65
distance of 3.596 Å, an interplanar distance of approximately
3.38 Å, and a dihedral angle of 1.9° between the rings. The phos-
phine has approximately the form of a three-bladed propeller,
the dihedral angles between pairs of rings being 69.3°, 71.0° and
75.3°. There are no unusually short intermolecular contacts, the
to a 2.86 Å intramolecular Brꢂ ꢂ ꢂH contact to the ring containing
C(1). This ring and a symmetry-equivalent also have mutual
Fꢂ ꢂ ꢂH contacts of 2.51 Å.
3.2. Photoisomerization of (1)
most significant probably being a CHꢂ ꢂ ꢂ
p interaction between
C(9)–H(9A) and the Schiff base ligand ring containing C(1), with
CHꢂ ꢂ ꢂcentroid = 2.57 Å and an angle of 150° at H(9A); this and a
symmetry-related interaction link pairs of molecules into centro-
symmetric dimers. The Br atom forms its shortest contacts of
3.83–4.00 Å with H atoms of neighbouring molecules, in addition
The spectrum of the free ligand shows two major peaks at 251
and 301 nm, identified as characteristic
p ?
p⁄ and n ? p⁄ absorp-
tion bands, and the spectrum of the Cu(I) complex shows two ma-
jor peaks, one at 295 nm that is related to a ligand-centred
transition of the coordinated E isomer of fb2en in solution, and an-
other at 379 nm which is identified as MLCT. On irradiating a chlo-
roform solution of (1) with a high-pressure mercury lamp, the
absorption maximum of complex (1) at 295 nm decreases in inten-
sity and that at 379 nm increases, with an isosbestic point at
354 nm (Fig. 2). The shift indicates the transformation from E to
Z configuration of the C@N bonds of the coordinated ligand in solu-
tion (Scheme 2).
The isosbestic point in Fig. 2 indicates that an equilibrium is
established in solution; no other molecules are involved the photo-
isomerization process. The mechanisms of photo-induced E/Z
isomerization of C@N bonds have been extensively investigated
over the last four decades [19–22]. The mechanism for the photo-
chemical interconversion of imine diastereomers has been consid-
ered in terms of either a planar inversion mechanism or a rotation
mechanism. The rotation or torsion mechanism involves twisting
about the C@N double bond. In order to bring about this change
in geometry, there must be a reduction in the double bond charac-
ter of the imine bond in the transition state relative to the ground
state. If four substituent groups on the double bonds in the ligand
are all different, there are four isomers, viz. (EE), (EZ), (ZE), and (ZZ),
Fig. 3. Expanded ranges of isomerization spectra of (1) in chloroform solution
starting with the pure EE isomer at 293 K.
Fig. 4. Expanded ranges of isomerization spectra of: (a, b) 1,4-di(4-alkoxyphenyl)butadiene in degassed toluene solution [23,24]; (c) N,N0-bis(4-dimethylaminobenzyli-
dene)1,2-diaminoethane; and (d) N,N0-bis(4-dimethylaminobenzylidene)1,3-diaminopropane in chloroform starting with pure EE isomers at 293 K.

K. Barati et al. / Polyhedron 62 (2013) 61–65
65
where I0,k is the incident light intensity at a specific wavelength
(E lꢀ1 sꢀ1). According to the data provided by the Southern New
England Ultraviolet Company, intensity at 350 nm is 3.11 ꢁ 10ꢀ6
(24 W) E lꢀ1 sꢀ1
, ek = molar absorptivity of compound at 350 nm,
and l = cell path length (1 cm). The quantum yield value obtained
for (1) is 0.246.
4. Conclusion
The Schiff base ligand fb2en forms the distorted tetrahedral cop-
per(I) complex (1) with a small bite angle for the bidentate ligand.
The crystal structure shows an EE configuration for the Schiff base
ligand. Photoisomerization leads to a ZZ configuration in solution,
as indicated by time-resolved analysis of spectra, which also pro-
vides a quantum yield and rate constant.
Acknowledgments
Fig. 5. The concentration profiles of EE (A) and ZZ (B) isomers of (1) in chloroform
(MCR-ALS (Ç), and fitted exponential functions (solid lines)).
The authors thank the EPSRC (UK) and Isfahan University Centre
of Excellence (Catalysts and Fuel Cells) for financial support.
but here we have only three, (EE), (EZ = ZE), and (ZZ) (Scheme 3).
The reaction EE M ZZ does not pass through the EZ isomer, as is
shown by the spectra in Fig. 3. In this photoisomerization an
iso- sbestic point is observed at 354 nm (i.e. the EE–ZZ spectra
cross-over). In contrast to this the cross-overs of the reaction spec-
tra induced by direct irradiation range from 305 nm (the cross-
over of the EE–EZ spectra) to 307 nm (Fig. 4(d)). This behaviour is
caused by the three-component reaction sequence EE M EZ M ZZ
[23–25].
Appendix A. Supplementary material
Crystallographic data have been deposited with the Cambridge
Crystallographic Data Centre, CCDC No. 658010 for (1). Copies of
this information may be obtained free of charge from The Director,
CCDC, 12 Union Road, Cambridge CB2 1EZ, UK (fax: +44 1223 336
033; e-mail: deposit@ccdc.cam.ac.uk or www: http://
www.ccdc.cam.ac.uk).
3.3. Kinetic constant and quantum yield
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k
/ ¼
ð1Þ
2:303I0;kekl