A doubly end-on azido-bridged trinuclear Cu(II) complex: Synthesis, spectral and DFT functional studies

Article abstract of DOI:10.3184/174751911X12972790588178

A trinuclear, doubly azido-bridged Cu(II) complex, [Cu_1.50(L) (N₃)₃(CH₃OH)]₂, (LH = [(CH ₃)₂NCH₂CH₂N=CHC₆H ₃(OH) (OMe)]), has been syn

Full text of DOI:10.3184/174751911X12972790588178

140 RESEARCH PAPER
MARCH, 140–143
JOURNAL OF CHEMICAL RESEARCH 2011
A doubly end-on azido-bridged trinuclear Cu(II) complex: synthesis,
spectral and DFT functional studies
Amitabha Dattaa, Kuheli Dasb, Wen-Yen Huanga, Jui-Hsien Huanga*, Fu-Xing Liaoa and
Ching-Han Hua
aDepartment of Chemistry, National Changhua University of Education, Changhua 50058, Taiwan
bDepartment of Chemistry, National Chung-Hsing University, 250 Kuo-Kuang Road, Taichung, Taiwan
A trinuclear, doubly azido-bridged Cu(II) complex, [Cu1.50(L)(N3)3(CH3OH)]2, (LH = [(CH3)2NCH2CH2N=CHC6H3(OH)
(OMe)]), has been synthesised and fully characterised by elemental analyses, IR, UV-Vis, EPR and DFT studies. Its
single crystal X-ray structure reveals that adjacent CuII ions are linked by double end-on azido-bridges; thus the full
molecule is generated by the site symmetry of a crystallographic two-fold rotation axis.
Keywords: copper(II), Schiff base, azide bridge, X-ray structure
Schiff base complexes of transition metals have been widely
employed in the development of heterogeneous catalysis,1
molecular electronics, single-molecule-based magnetism,
and photochemistry.2–5 Their physical, optical, and electronic
properties have been explored in different coordination
environments with organic chelators, blockers, and suitable
bridging units.6–8 Such multidentate organic ligands9–11 can
accommodate one, two or more metal centres and thus may
provide the basis of models for active sites of biological sys-
tems.12–14 In recent years, an increasing effort has been focused
on the preparation of mono- or di-nuclear mixed- ligand transi-
tion metal complexes containing neutral or chelating nitrogen-
donor ligands. Early and late transition metal complexes of
this type and particularly those of copper have extensively
been used as catalysts for a wide variety of reactions, including
olefin polymerisation15–17 and dioxygen activation.18–20 Triden-
tate Schiff bases with N2O donor sets have been preferred
because such ligands effectively act as chelates and can block
the coordination sites of metal ions, leaving the metal ion coor-
dinatively unsaturated. In order to saturate the coordination
number of metal ions preferring square planar, square pyrami-
dal and octahedral geometry, different bridging ligands (N3−,
NCO−, NCS−, acetates, carboxylates) have been used, giving
rise to dinuclear and multinuclear complexes.21–26 As bridging
ligands, depending upon the steric and electronic demand
of coligands, the pseudohalide-bridged copper(II) complexes
can change their coordination to either an end-on (µ-1,1) or
an end-to-end (µ-1,3) mode.27,28 Moreover, various interesting
topologies have been prepared using different transition and
non-transition metal ions as templates with Schiff bases as
blocking units and pseudohalide-like azide as bridging ligand.
Azide ion is known for its versatility as bridging ligand since it
can bind two metal centres in end-on and/or end-to-end fash-
ions.29,30 Such azido-bridged copper(II) complexes are of great
interest to biologists and bioinorganic chemists investigating
the structure and role of active sites in copper proteins. Elec-
tron paramagnetic resonance (EPR) spectroscopy can notably
indicate large coupling through intensity variation and can
also extract small coupling with the use of multifrequency
experimentation.31–34
A representation of the molecular structure of complex 1
is depicted in Fig. 1 and selected bond lengths and angles are
shown in Table 2. Complex 1 features a doubly end-on azido-
bridged Cu(II) trinuclear unit as shown in Fig. 1. The solid
crystallises in the monoclinic unit cell of the C2/c space group
with the tricopper unit situated in a special position of 0, y,
1/4 such that only half of the tricopper complex exists in an
asymmetric unit. The full molecule is generated by the site
symmetry of a crystallographic two-fold rotation axis. The
coordination environment of the central copper atom can be
described as a distorted octahedron. It is coordinated by two
cis- methanol molecules and four bridging azide ligands.
The two unique quasi-linear bond angles at this copper atom
are 168.51(10) and 170.79(15)°. The copper atom at the flank
also has distorted octahedral coordination geometry. It is
ligated by one terminal and two cis-bridging azide ligands and
the tridentate ONN ligand via the phenolato-oxygen atom, the
amine and the imine nitrogen atoms. The extent of distortion
at this copper atom is less than that at the central copper atom
as evidenced by the three larger quasi-linear bond angles,
which are 173.63(12), 176.91(11), and 175.18(12)°. The bond-
ing distances and angles between the tridentate ONN ligand to
this copper atom are entirely normal with respect to similar
compounds in the literature.34,35 The Cu–N distance from the
terminal azide is short [1.942(3) Å] compared with those from
the bridging azide [1.966(3)–2.213(3) Å]. These Cu–N dis-
tances are within the normal range of similar compounds.36–38
We now describe the synthesis1 and X-ray structural charac-
terisation of a new doubly end-on azido-bridged trinuclear
copper complex [Cu1.50(L)(N3)3(CH3OH)]2, [L = (2-[{[2-(dime
thylamino)ethyl]imino}methyl]-6-methoxyphenol)] (1) incor-
porated with a tridentate Schiff-base.35 The complex has been
characterised by elemental analysis, IR and UV-Vis spectra,
EPR and DFT functional studies. The structure of the complex
was determined by X-ray diffraction using single crystals.
Fig. 1 Representation of the molecular structure of compound
1. Ellipsoids are drawn from 40% probability level. Hydrogen
atoms have been omitted for clarity.
* Correspondent. E-mail: juihuang@cc.ncue.edu.tw

JOURNAL OF CHEMICAL RESEARCH 2011 141
The non-bonding Cu···Cu distance of 3.224(1) Å is long
compared with those in similar copper azide complexes.36–38
For example, the Cu···Cu distance in the relevant [Cu(μ1,1-
N3)2(N3)4]2− anion is 3.126(2) Å.38 The Cu–N(azide)–Cu angle
of 99.4(1)° is markedly wider than that of 89.1° in the dicopper
complex bearing a structurally very similar tridentate ONN
ligand and bridging azido ligands reported earlier,36 but is
slightly smaller than that of 103.2(2)° in another related dic-
opper azide complex also reported by us previously.37 One
of the bridging azide ligands is noticeably bent with a short
N–N–N bond angle of 168.2(6)°, compared with those of
174.7(4) and 179.1(4)° at the other two azide ligands. A strong
intramolecular hydrogen bond of the type O–H···O exists
between the proton on a coordinated methanol molecule and
the phenolato-oxygen atom of the tridentate ligand with the
H···O contact distance of 1.80(4) Å and O–H···O contact angle
of 160(4)°. A short non-classical intermolecular hydrogen
bond of the type C–H···N also exists between a methylene
hydrogen atom on the tridentate ligand and a nitrogen atom on
the bridging azide ligand, linking tricopper complexes into
infinite one-dimensional chains. The H···N contact distance
and angle are 2.33 Å and 152° respectively. A weaker, non-
classical intermolecular hydrogen bonding of similar C–H···N
type further links these chains into a densely-packed infinite
three-dimensional network. The contact distances and angles
of these interactions fall into ranges of 2.46–2.53 Å and 106–
154°, respectively. The crystallographic packing diagram is
shown in Fig. 2 along the b-axis.
charge-transfer transitions. The UV absorption bands observed
at 362 nm can be assigned to ligand-to-metal charge-transfer
transitions. The spectrum also shows a low-intensity absorp-
tion band at 558 nm, originating from d–d transitions.40–41
The experimental powder EPR spectrum (X-band, recorded
at room temperature) is composed of allowed (∆MS = 1)
transitions centred at g = 2.021 and of a weak intensity,
nominally forbidden, half-field (∆MS = 2) peak observable at
g = 4.682.
Compound 1 has been investigated by theoretical calcula-
tions. For this purpose, the three-parameter hybrid of exact
exchange and Becke’s exchange energy functional has been
used,42 plus Lee, Yang, and Parr’s gradient-corrected correla-
tion energy functional43 (B3LYP). The 6-31G(d) basis set was
applied in all computations. The Gaussian03 suite of programs
was used in our study.45 For the copper coordination compound
1, we found that the triplet state is significantly lower in energy
than its singlet and quintet counterparts. In addition, we have
located two isomers; among them the symmetric isomer lies
at 22.3 kcal mol−1 above the asymmetric isomer. In general,
the theoretically-predicted geometrical parameters are in good
agreement with those resolved by X-ray diffraction studies.
The geometries predicted by DFT (first entry) and those
resolved from the X-ray data (second entry) are presented and
compared in Figs 3a and b.
Experimental
Synthesis of the Schiff base ligand: [(CH3)2NCH2CH2N=CHC6H3
(OH)(OMe)] (LH) was obtained by refluxing a methanolic solution
(25 mL) of O-vanillin (1 mmol) and 2-dimethylaminoethylamine
(1 mmol) for 30 minutes. The resulting orange–yellow solution
containing the ligand was used without further purification.
1: To a methanolic solution (20 mL) of copper trichloroacetate
(0.388 g, 1 mmol), LH (1 mmol) was added, which produced immedi-
ately an intensely green solution. The solution was then heated to
boiling and then an aqueous solution (10 mL) of sodium azide
(0.195 g, 3 mmol) was added dropwise slowly over 15 mins to the hot
solution. After the completion of addition of sodium azide, the result-
ing solution was refluxed for another 10 min. On cooling and after
slow evaporation of the brown solution, dark brown, rectangular
shaped single crystals of the complex (1) separated out in 3 days.
The crystals were filtered off, washed with water and dried in air
In the IR spectra of 1, a peak at 1626 cm−1 can be assigned
as the stretching frequency of the azomethine group of the
Schiff base. A well-resolved strong band at 2044 cm−1 and a
bifurcated peak at 2066 cm−1 were also observed and assigned
to νN=N of the azide group coordinated to two different metal
centres and to terminal ligands respectively.39 The ligand
coordination to the copper centre is substantiated by two
bands appearing for 1, at 412 and 423 cm−1 attributable to νCu-N
and νCu-O, respectively. The bands in the range, 3544 and
1631 cm−1 are attributable to O–H stretching and bending of
water ligands.39
The electronic spectrum of
1 exhibits two strong
absorption bands at 232 and 281 nm, which are due to ligand
Fig. 2 Crystallographic packing diagram of 1.

142 JOURNAL OF CHEMICAL RESEARCH 2011
1
Charge & mult.
0 2 (non-Sym.)
0 2 (Sym.)
Rel. energy (kcal mol-1)
0.0
22.3
Rel. energy is ∆H298
.
Fig. 3 DFT predicted geometries (first entry) and those resolved from X-ray (second entry). (a) Non-symmetrical; (b) symmetrical.
(yield ca. 83%). A suitable, dark brown block crystal was selected
for X-ray data collection. Anal. Calcd for C13H22Cu1.50N11O3: C, 32.82;
H, 4.66; N, 32.39. Found: C, 32.61; H, 4.73; N, 32.21%.
Table 1 Crystallographic data for 1
Empirical formula
C13H22Cu1.50N11O3
Formula weight
Temperature
475.73
Crystallographic data
100(2) K
Details concerning crystal data, data collection characteristics
and structure refinement are summarised in Table 1. The crystal was
mounted on capillaries and transferred to a goniostat and data were
collected at 100 (2) K under a nitrogen gas stream on a Bruker SMART
CCD diffractometer with graphite-monochromated Mo-Kα radiation
and the ω:2θ scan technique applied within a θ range of 3.04–33.66°.
No significant crystal decay was found. Data were corrected for
absorption empirically by means of ψ scans. A total of 16660 reflec-
tions were collected from which 5933 independent [R(int) = 0.0200]
reflections were measured. The stability of the crystals was checked
by measuring standard reflections at fixed intervals during the
data collection. However, no significant loss of intensity was noted.
Data were processed using the CRYSALIS-CCD and –RED45
programs. The structure was solved by direct methods using the
SHELXTL PLUS46 system and refined by a full-matrix least-squares
methods based on F2 using SHELXL9347 using all 19,594 data to final
wR2 (on F2, all data) = 0.1658 and R1 (on F, with [I>2σ(I)]) = 0.0585.
The functions minimised were Σw[|Fo|2 − [|Fc|2]2, where w = [σ2(I) +
(0.0849P)2 + 0.3606P]−1 for 1 with P = (|Fo|2 +2|Fc|2)/3. The hydrogen
atom positions were calculated and they were constrained to idealised
geometries and treated as riding where the H atom displacement
parameter was calculated from the equivalent isotropic displacement
parameter of the bound atom.
Wavelength
0.77490 Å
Crystal system
Space group
Unit cell dimensions
Monoclinic
C2/c
a = 29.894(3) Å α = 90°
b = 8.2233(7) Å
β = 110.3550(10)°
c = 16.2041(16) Å γ = 90°
3734.6(6) Å3
Volume
Z
8
Density (calculated)
Absorption coefficient
F(000)
1.692 Mg m−3
2.233 mm−1
1956
Crystal size
0.23 × 0.05 × 0.01 mm3
3.04–33.66°
Theta range for data collection
Reflections collected
Independent reflections
Data / restraints / parameters
Goodness-of-fit on F2
Final R indices [I>2σ(I)]
R indices (all data)
Largest diff. peak and hole
19594
5637 [R(int) = 0.0402]
5637/1/262
1.065
R1 = 0.0585, wR2 = 0.1581
R1 = 0.0711, wR2 = 0.1658
1.460 and –1.600 e Å−3
(fax: +44-1223-336-033; E-mail deposit@ccdc.cam.ac.uk or http://
www.ccdc.cam.ac.uk).
Crystallographic data for structural analysis have been deposited
at the Cambridge Crystallographic Data Centre, bearing 790433.
Copies of this information may be obtained free of charge from
the Director, CCDC, 12 Union Road, Cambridge, CB2 IEZ, UK
The authors gratefully acknowledge financial support from the
National Science Council of Taiwan.

JOURNAL OF CHEMICAL RESEARCH 2011 143
Table 2 Selected bond lengths (Å) and angles (°) for 1
Cu(1)−N(2)
1.897(3)
1.965(3)
2.086(3)
Cu(1)−O(2)
1.902(2)
2.009(3)
2.097(3)
Cu(1)−N(9)
1.942(3)
2.020(3)
2.213(3)
Cu(1)−N(3)
Cu(1)−N(6)
Cu(1)−N(1)
Cu(2)−N(3)
Cu(2)−O(3)
Cu(2)−N(6)
N(2)−Cu(1)−O(2)
N(2)−Cu(1)−N(3)
O(2)−Cu(1)−N(1)
N(5)−N(4)−N(3)
N(11)−N(10)−N(9)
93.82(12)
173.63(12)
176.91(11)
168.2(6)
N(2)−Cu(1)−N(9)
O(2)−Cu(1)−N(3)
N(3)#1−Cu(2)−O(3)
Cu(1)−N(6)−Cu(2)
90.34(13)
89.09(10)
168.51(10)
99.44(12)
O(2)−Cu(1)−N(9)
N(9)−Cu(1)−N(6)
N(6)−Cu(2)−N(6)#1
N(8)−N(7)−N(6)
89.92(12)
175.18(12)
170.79(15)
179.1(4)
174.7(4)
23 A. Ray, S. Banerjee, R.J. Butcher, C. Desplanches and S. Mitra, Polyhedron,
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Received 24 November 2010; accepted 13 January 2011
Paper 1000444 doi: 10.3184/174751911X12972790588178
Published online: 23 March 2011
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