A facile synthesis of mixed ligand Cu(II) complexes with salicylaldehyde and salicylaldimine ligands and their X-ray structural characterization

Article abstract of DOI:10.1016/j.ica.2011.12.009

High yield synthetic routes to the preparation of new mixed ligand Cu(II) complexes (1-7) derived from a sterically hindered salicylaldimine and the corresponding salicylaldehyde have been developed. The complexes could be obtained either forming a salicylaldimine preligand or two optional Cu(II) complex precursors. One-pot synthesis could also be used. The complexes have been fully characterized by means of elemental analysis, UV-Vis and IR spectroscopy. Crystal structures obtained for four [(3,5-di-tert- butylsalicylaldehydato)(N-R-3,5-di-tert-butylsalicylaldiminato)]Cu(II) (where R = phenyl (1), isopropyl (3), benzyl (5) and 2-phenylethyl (6)) complexes show that three of them are cis-isomers and one is a trans-isomer with respect to the phenolic O-atoms. Complexes 1, 5 and 6 form preferably loose dimers while 3 favors to exist in a loose polymeric structure in a crystal. In all these polynuclear forms the geometry around the Cu(II) ions is a distorted octahedron.

Full text of DOI:10.1016/j.ica.2011.12.009

Inorganica Chimica Acta 384 (2012) 275–280
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Inorganica Chimica Acta
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A facile synthesis of mixed ligand Cu(II) complexes with salicylaldehyde
and salicylaldimine ligands and their X-ray structural characterization
⇑
Jahir Uddin Ahmad, Minna T. Räisänen, Martin Nieger, Markku Leskelä, Timo Repo
Department of Chemistry, Laboratory of Inorganic Chemistry, P.O. Box 55, University of Helsinki, FI-00014 Helsinki, Finland
a r t i c l e i n f o
a b s t r a c t
Article history:
High yield synthetic routes to the preparation of new mixed ligand Cu(II) complexes (1–7) derived from a
sterically hindered salicylaldimine and the corresponding salicylaldehyde have been developed. The
complexes could be obtained either forming a salicylaldimine preligand or two optional Cu(II) complex
precursors. One-pot synthesis could also be used. The complexes have been fully characterized by means
of elemental analysis, UV–Vis and IR spectroscopy. Crystal structures obtained for four [(3,5-di-tert-buty-
lsalicylaldehydato)(N-R-3,5-di-tert-butylsalicylaldiminato)]Cu(II) (where R = phenyl (1), isopropyl (3),
benzyl (5) and 2-phenylethyl (6)) complexes show that three of them are cis-isomers and one is a
trans-isomer with respect to the phenolic O-atoms. Complexes 1, 5 and 6 form preferably loose dimers
while 3 favors to exist in a loose polymeric structure in a crystal. In all these polynuclear forms the geom-
etry around the Cu(II) ions is a distorted octahedron.
Received 3 November 2011
Received in revised form 8 December 2011
Accepted 8 December 2011
Available online 16 December 2011
Keywords:
Schiff base
Cu(II) complex
Salicylaldimine
Crystal structure
Mixed ligand complex
Ó 2011 Elsevier B.V. All rights reserved.
1. Introduction
Structures of metal complex catalysts are constantly modified
in different ways to achieve a better understanding over the func-
Mixed ligand metal complexes are well-known to play a signif-
icant role in various biological systems such as galactose oxidase
(GO), vitamin B12, chlorophyll, laccases and hemoglobin [1]. These
naturally occurring mixed ligand Cu, Co, Fe and Mg complexes
have at least two different ligand moieties or in a case the ligand
is a single macromolecule, it consists of two or more different
kinds of donor sets of atoms. Synthetic mixed ligand Cu, Ni, Co,
Mn and Cd complexes have been widely studied as well. Such
complexes have traditionally bioactive compounds, e.g. purine,
aminoacid, bipyridine, phenanthroline or imidazole as ligands
and some of them have very promising antimicrobial activities
[2]. Mixed ligand complexes are biologically more active than
their constituting ligands or the corresponding homoligated bis-
complexes [2g–h] which is another important motivation for the
studies.
Cu(II) salicylaldimine complexes are a common class of com-
pounds actively used in various fields of catalysis. They are effi-
cient catalysts for instance in oxidation reactions [3]. As an
example, Cu(II) complexes based on 3,5-di-tert-butylsalicylaldi-
mines are highly efficient catalytic systems for the aerobic oxida-
tion of primary benzylic, allylic and heterocyclic alcohols under
mild reaction conditions [3e].
tioning of the catalyst systems and for development of more active
catalyst species. Geometry of the complexes can be tuned by the
choice of ligands and specifically by adjusting steric bulkiness of
the ligand backbone. In nonbridged Cu(II) Schiff base complexes
the metal ion is typically found in a distorted square-planar
geometry and the salicylaldimine ligands have adopted a trans-ori-
entation with respect to each other. Such complexes with cis-coor-
dinated ligands are rare [3e,4]. In contrast to extensive research
done with bis(salicylaldiminato)Cu complexes, mixed ligand Cu
complexes with salicylaldimine and salicylaldehyde ligands are
scarce [5,6] and only in one case the structure has been determined
by X-ray diffraction method [6].
Herein, we report the synthesis, characterization and X-ray
structures of a series of mixed ligand Cu(II) complexes derived
from 3,5-di-tert-butylsalicylaldehyde and various 3,5-di-tert-
butylsalicylaldimines.
2. Experimental
2.1. General
All chemicals were obtained from commercial suppliers and
were used without a further purification. Melting points were
determined in an electrothermal melting point apparatus. EI-mass
spectra were run with a JEOL JMS-SX 102 mass spectrometer (ion-
ization voltage 70 eV) from solid samples. IR and UV–Vis spectra
⇑
Corresponding author. Tel.: +358 9191 50194; fax: +358 9191 50198.
E-mail address: timo.repo@helsinki.fi (T. Repo).
0020-1693/$ - see front matter Ó 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.ica.2011.12.009

276
J.U. Ahmad et al. / Inorganica Chimica Acta 384 (2012) 275–280
were collected with a Perkin Elmer Spectrum GX spectrometer and
a Hewlett Packard 8453 spectrophotometer, respectively. Elemen-
tal analyses were made by an EA 1110 CHNS-OCE instrument.
into a green suspension but in any case the reaction mixture was
refluxed for 1 h. Complexes 1–7 were obtained in 60–90% yields.
2.4.4. Method proceeding through bis(salicylaldiminato)Cu(II)
complex (IV)
2.2. Synthesis of bis(3,5-di-tert-butylsalicylaldiminato)Cu(II) (A) and
Equimolar amounts of complex precursors A (53 mg, 0.1 mmol)
and B were added in 3 mL of toluene [5]. The resulting solution was
refluxed for 2 h followed by solvent removal with rotary evapora-
tor. The green solid obtained was dissolved in CH2Cl2 and DMSO
was introduced on the top of the solution. After several days, green
or yellowish green crystals separated providing the pure product.
Complexes 1–7 were obtained in 71–90% yields.
ligands
A detailed description of the synthesis of bis(3,5-di-tert-buty-
lsalicylaldiminato)Cu(II) complexes (A) and the corresponding
3,5-di-tert-butylsalicylaldimine (D) ligands can be found else-
where [3e,7,11b].
2.3. Synthesis of bis(3,5-di-tert-butylsalicylaldehydato)Cu(II) complex
precursor (B)
2.4.4.1.
[(3,5-Di-tert-butylsalicylaldehydato)(N-phenyl-3,5-di-tert
butylsalicylaldiminato)]Cu(II) (1). Respective yields with methods
I-IV were 61%, 67%, 63% and 72%. Brown crystals of 1 suitable for
X-ray structure determination were grown in refrigerator by slow
evaporation of n-hexane. mp 230–232 °C. Anal. Calc. for C36H47Cu-
NO3 (605.3 g/mol): C, 71.43; H, 7.83; N, 2.31. Found: C, 70.90; H,
Slight modifications were made to the literature procedure [8]
to synthesize bis(3,5-di-tert-butylsalicylaldehydato)Cu(II) (B). To
a stirred mixture of 3,5-di-tert-butylsalicylaldehyde (C) (468 mg,
2 mmol) and Et3N (2 equivalents) in MeOH (5 mL) was added drop-
wise a MeOH (30 mL) solution of Cu(OAc)2 (181.1 mg, 1 mmol).
After 1 h, the formed brown microcrystals were isolated by filtra-
tion and recrystallized from a CH2Cl2–DMSO mixture (1:1, v/v) to
obtain brown plate-like crystals of 1 (466 mg, 0.88 mmol, 88%
yield, mp 260–263 °C). Crystals suitable for X-ray diffraction stud-
ies were grown by slow layer diffusion of CH2Cl2 into a DMSO solu-
tion of B. The structure is a monoclinic polymorph1 of the published
orthorhombic structure [8]. Anal. Calc. for C30H42CuO4 (530.2 g/mol)
C, 67.96; H, 7.98. Found: C, 67.81; H, 7.96%. IR (cmꢀ1): 2950–2869
7.74; N, 2.36%. IR (cmꢀ1): 2951–2865 (
1614 ( C@O), 1595 ( C@N), 1429 (
415, 488, 627.
m
C–H from tert-butyl groups),
m
m
mC–O). UV–Vis kmax (nm): 297, 365,
2.4.4.2.
[(3,5-Di-tert-butylsalicylaldehydato)(N-(p-methylphenyl)-
3,5-di-tert-butylsalicylaldiminato)]Cu(II) (2). Respective yields with
methods I–IV were 85%, 88%, 83% and 78%. mp 223–225 °C. Anal.
Calc. for C38H49CuNO3 (619.3 g/mol): C, 71.75; H, 7.97; N, 2.26.
Found: C, 72.06; H, 7.82; N, 1.96%. IR (cmꢀ1): 2950–2868 (mC–H
from tert-butyl groups), 1605 (
mC@O), 1589 (mC@N), 1429 (mC–O).
(mC–H from tert-butyl groups), 1613 (mC@O), 1422 (mC–O). UV–Vis kmax
UV–Vis kmax (nm): 296, 365, 407, 488, 627.
(nm): 284, 347, 365 (sh), 627, 657. EI MS: m/z 530–532 with appro-
priate isotopic ratio for [C30H42CuO4]+.
2.4.4.3. [(3,5-Di-tert-butylsalicylaldehydato)(N-isopropyl-3,5-di-tert-
butylsalicylaldiminato)]Cu(II) (3). Respective yields with methods
I–IV were 81%, 93%, 90% and 79%. mp 207–210 °C. Anal. Calc. for
2.4. Synthesis of mixed ligand complexes
C
33H49CuNO3 (571.3 g/mol): C, 69.38; H, 8.65; N, 2.45. Found: C,
69.42; H, 8.49; N, 2.62%. IR (cmꢀ1): 2957–2867 (
C–H from tert-bu-
tyl groups), 1621 ( C@O), 1600 ( C@N), 1412 ( C–O). UV–Vis kmax
(nm): 284, 332, 395 (sh), 630.
2.4.1. Method proceeding through imine preligand formation (I)
Typically, a MeOH (20 mL) solution of Cu(CH3COO)2 (0.5 mmol)
was added slowly to a 1:1 mixture of preligand D (0.5 mmol) and C
(0.5 mmol) in MeOH (10 mL). The resulting mixture was then stir-
red for 1 h at room temperature and in case of complexes 1–4 it
was further refluxed for 2 h. A green solid formed in the reaction
mixture and was separated with a suction filtration. Pure product
was obtained after recrystallization from CH2Cl2–DMSO–MeOH.
Complexes 1–6 were obtained in 60–85% yields.
m
m
m
m
2.4.4.4.
[(3,5-Di-tert-butylsalicylaldehydato)(N-cyclohexyl-3,5-di-
tert-butylsalicylaldiminato)]Cu(II) (4). Respective yields with meth-
ods I–IV were 60%, 53%, 60% and 71%. mp 215–218 °C. Anal. Calc.
for C36H53CuNO3 (611.4 g/mol): C, 70.73; H, 8.74; N, 2.29. Found:
C, 70.48; H, 8.49; N, 2.24%. IR (cmꢀ1): 2951–2869 (mC–H from tert-
butyl groups), 1614 (
(nm): 284, 337, 390 (sh), 486, 624.
mC@O), 1590 (mC@N), 1422 (mC–O). UV–Vis kmax
2.4.2. Method proceeding through bis(salicylaldehydato)Cu(II)
complex (II)
Complex precursor A (265 mg, 0.5 mmol) was suspended in
EtOH (10 mL) and the corresponding amine (0.5 mmol) was added.
The resulting mixture was stirred for 1 h at room temperature and
then refluxed for 2 h. Complexes 1–7 were obtained in 50–93%
yields.
2.4.4.5.
[(3,5-Di-tert-butylsalicylaldehydato)(N-benzyl-3,5-di-tert-
butylsalicylaldiminato)]Cu(II) (5). Respective yields with methods
I–IV were 84%, 88%, 79% and 82%. mp 199–200 °C. Anal. Calc. for
C
38H49CuNO3 (619.3 g/mol): C, 71.75; H, 7.97; N, 2.26. Found: C,
71.92; H, 8.01; N, 2.55%. IR (cmꢀ1): 2949–2866 (
C–H from tert-bu-
tyl groups), 1622 ( C@O), 1599 ( C@N), 1413 ( C–O). UV–Vis kmax
(nm): 285, 327, 390, 630.
m
m
m
m
2.4.3. One-pot synthesis (III)
A
methanol (30 mL) solution of Cu(CH3COO)2 (181.1 mg,
1 mmol) was added dropwise to a stirred MeOH (5 mL) solution
of C (468 mg, 2 mmol) and Et3N (2 equivalents). After stirring at
room temperature for 15 min a brown solid was formed and the
corresponding amine (1 mmol) was then added dropwise. In the
case of most complexes, at this stage the brown solid converted
2.4.4.6. [(3,5-Di-tert-butylsalicylaldehydato)(N-(2-phenylethyl)-3,5-
di-tert-butylsalicylaldiminato)]Cu(II) (6). Respective yields with
methods I–IV were 73%, 79%, 90% and 83%. mp 226 °C. Anal. Calc.
for C38H51CuNO3 (633.4 g/mol): C, 72.06; H, 8.12; N, 2.21. Found:
C, 71.73; H, 8.13; N, 2.09%. IR (cmꢀ1): 2955–2868 (mC–H from tert-
butyl groups), 1620 (
(nm): 285, 326, 390, 632.
mC@O), 1601 (mC@N), 1464 (mC–O). UV–Vis kmax
1
Orange crystals, C30H42CuO4, M = 530.18, crystal size 0.20 ꢁ 0.15 ꢁ 0.10 mm,
monoclinic, space group P21/n (No. 14): a = 11.603(1) Å, b = 8.066(1) Å,
c = 15.399(1) Å, b = 106.53(1)°, V = 1393.5(2) Å3, Z = 2,
q ,
(calcd) = 1.264 Mg mꢀ3
2.4.4.7. [(3,5-Di-tert-butylsalicylaldehydato)(N-(n-hexyl)-3,5-di-tert-
butylsalicylaldiminato)]Cu(II) (7). Respective yields with methods
II–IV were 84%, 75% and 80%. mp 170–173 °C. Anal. Calc. for
F(0 0 0) = 566,
l , 6574 reflections (2hmax = 50°), 2450 unique
= 0.815 mmꢀ1
[Rint = 0.038], 160 parameters, R1 (I > 2r(I)) = 0.053, wR2 (all data) = 0.133,
GOOF = 1.12, largest diff. peak and hole 1.352 and ꢀ0.364e Åꢀ3
.

J.U. Ahmad et al. / Inorganica Chimica Acta 384 (2012) 275–280
277
C36H55CuNO3 (613.4 g/mol): C, 70.49; H, 9.04; N, 2.28. Found: C,
70.43; H, 9.22; N, 2.50%. IR (cmꢀ1): 2998–2864 (
C–H from tert-bu-
tyl groups), 1621 ( C@O), 1602 ( C@N), 1464 ( C–O). UV–Vis kmax
temperature. Complex B is then converted to the mixed ligand
complex simply by reacting it with the appropriate amine. In the
one-pot synthesis (III) which is done under refluxing conditions,
C is allowed to react with Cu(II) acetate in the presence of Et3N fol-
lowed by the addition of appropriate amine. Interestingly, method
IV produces the mixed ligand complexes readily when a 1:1 mix-
ture of complex precursors A and B is refluxed in toluene [5].
The complexes 1–7 were carefully analyzed by IR and UV–Vis mea-
surements and purities of the complexes were verified by elemen-
tal analysis and melting point measurements. These types of
complexes can be successfully used as a complex precursor for
synthesis of novel heteroligated bis(phenoxyimino)Cu complexes
which are active catalysts toward oxidation of variety of alcohols
[10]. Significantly, the catalysts worked efficiently also for second-
ary alcohols unlike other reported Cu/TEMPO (2,2,6,6-tetramethyl-
piperidinyloxyl) systems.
m
m
m
m
(nm): 284, 330, 366, 374 (sh), 487, 627.
2.5. X-ray crystallography studies
Single-crystal X-ray diffraction studies of 1, 3, 5, and 6 were car-
ried out on a Bruker–Nonius Kappa-CCD diffractometer using Mo
Ka radiation (k = 0.71073 Å). Direct methods (SHELXS-97) were used
for structure solution and refinement (SHELXL-97), full-matrix least-
squares on F2) [9]. Semi-empirical absorption corrections were ap-
plied, and H atoms were localized by difference electron density
determination and refined using a riding model. Details of the data
collection and structure refinement are given in Table 1. The struc-
tures of 3, 5, and 6 are disordered allowing only the discussion
about the conformation of the complexes and the crystal packing.
3.2. Spectroscopic characterization of mixed ligand complexes 1–7
UV–Vis spectra of the mixed ligand complexes 1–7 in toluene
3. Results and discussion
3.1. Synthesis of ligands and complexes
have p ?
p⁄ transitions of the aromatic rings in the range of
284–365 nm. A band at 390–415 nm can be assigned to n ? p⁄
electron transition of C@N (imine) [11]. It appears as a shoulder
in case of N-alkyl compounds (3, 4, 7) whereas N-phenyl and N-
alkylphenyl compounds (1, 2, 5, 6, 8) show it as a strong absorption
band. Bands appearing with low intensities in the 486–488 and
624–632 nm regions are related to d-d transitions of the metal
center and are characteristic for distorted square-planar Cu(II)
complexes [11].
The IR spectra of the studied complexes exhibit the characteris-
tic C@N stretching vibration in the 1589–1601 cmꢀ1 region which
is shifted to lower frequencies by 25–33 cmꢀ1 when compared to
spectra of the corresponding free salicylaldimine ligands (1614–
1634 cmꢀ1) [7]. This is indicative of Cu–N coordination bond for-
mation [12]. The band appearing in the 1605–1623 cmꢀ1 region
Salicylaldimines (D) used as preligands for synthesis of mixed
ligand Cu(II) complexes (1–7) are prepared in a typical condensa-
tion reaction between appropriate amine and 3,5-di-tert-butylsal-
icylaldehyde (C) [7,11b]. The complexes 1–7 with
C and D
ligands can be obtained via four different routes (marked as I–IV
in Scheme 1). The most facile approach for their synthesis is one-
pot synthesis (III) but on the other hand method II, which proceeds
via a formation of bis(salicylaldehydato)Cu(II) complex (B), pro-
vides a convenient means for production of libraries of mixed li-
gand complexes. The complexes are prepared in good 41–89%
yields, depending on the complex and the used method (see Sec-
tion 2 for the details).
In the method I, the complexes 1–7 are synthesized by reacting
the preformed ligand D, C and Cu(II) acetate in 1:1:1 molar ratio at
room temperature. In the method II, complex precursor B is first
obtained by reacting C and Cu(II) acetate in 2:1 molar ratio at room
is assignable to the mC@O. It is shifted to lower frequencies by 27–
45 cmꢀ1 when compared to spectra of the free aldehyde ligand
(1650 cmꢀ1). In the spectrum of complex precursor B mC@O is
Table 1
Crystallographic data for mixed ligand Cu(II) complexes 1, 3, 5 and 6.
1
3
5
6
Empirical formula
Formula weight
T (K)
C
36H47CuNO3
C33H49CuNO3
571.27
123(2)
C37H49CuNO3
619.31
123(2)
C38H51CuNO3
633.34
123(2)
605.29
123(2)
Crystal system
Space group
a (Å)
b (Å)
c (Å)
monoclinic
P21/c (No. 14)
12.745(1)
15.119(1)
17.806(1)
102.60(1)
3348.4(4)
4
orthorhombic
Pbcn (No. 60)
26.510(2)
10.996(1)
10.816(1)
monoclinic
P21/n (No. 14)
12.722(1)
16.106(1)
16.403(1)
101.80(1)
3290.0(4)
4
monoclinic
C2/c (No. 15)
24.669(5)
15.758(3)
9.631(2)
107.98(2)
3561.1(12)
4
b (°)
V (Å3)
3152.9(5)
4
1.203
0.724
1228
Z
Dcalc (g cmꢀ3
)
1.201
0.686
1292
1.250
0.699
1324
1.181
0.648
1356
Absorption coefficient (mmꢀ1
F(000)
)
Crystal dimensions (mm)
2hmax (°)
0.40 ꢁ 0.20 ꢁ 0.04
0.50 ꢁ 0.20 ꢁ 0.10
0.32 ꢁ 0.16 ꢁ 0.08
0.40 ꢁ 0.12 ꢁ 0.06
55
50
55
50
Number of collected data
Number of unique data
Rint
33138
7668
0.053
27876
2777
0.036
69524
7531
0.043
16756
3142
0.053
Number of parameters/restraints
370/0
0.043
0.105
167/3
0.073
0.169
342/34
0.039
0.093
177/0
0.043
0.091
Ra [I > 2
r(I)]
b
wR2
GOF
1.05
0.493/ꢀ0.712
1.10
1.047/ꢀ1.502
1.05
0.741/ꢀ0.436
1.07
0.626/ꢀ0.443
Largest difference peak and hole (e Aꢀ3
)
a
R =
wR2 = [
R
||Fo| ꢀ |Fc||/
R|Fo|.
b
2
R
[w(Fo ꢀ Fc2)2]/
R .
[w(Fo2)2]]1/2

278
J.U. Ahmad et al. / Inorganica Chimica Acta 384 (2012) 275–280
R
O
N
N
O
R
Cu
O
O
R-NH2
II
B
O
Cu
IV
N
O
O
R
Cu
A
lux
tOH, ref
E
toluene, reflux
O
B
O
III
I
R
Complex
.
t
.
r
1
C
,
,
2
)
2
H
Cu(OAc)2
c
)
MeOH, r.t.
.
A
t
c
.
r
O
CH3
O
2
(
,
u
A
e
C
H
O
O
M
e
(
,
M
u
3
4
5
6
7
R
N
N
C
3
t
.
E
1
O
OH
OH
D
C6H13
C
Scheme 1. Different synthetic routes to preparation of mixed ligand Cu(II) complexes 1–7. Methods
I and II proceed through salicylaldimine preligand and
bis(salicylaldehydato)Cu(II) complex, respectively. Method III is a one-pot synthesis whereas in method IV the mixed ligand complex is obtained via successive formations
of imine preligand and bis(salicylaldiminato)Cu(II) complex and by final addition of bis(salicylaldehydato)Cu(II).
observed at 1613 cmꢀ1. The disappearance of mOH vibration in the
spectra of the complexes is indicative of Cu–O coordination bond
formation [11b].
of mixed ligand Cu(II) complexes 1, 3, 5 and 6 were determined by
X-ray diffraction method (Figs. 1–4). In case of 3, 5, and 6 only the
conformation of the complexes and the crystal packing are dis-
cussed due to disorder. In 3 and 6 the ligands are disordered about
a twofold axis in a 1:1 ratio, while in 5 the ligands are disordered
about a mirror plane approximately in a 3:1 (77:23) ratio.
3.3. Crystal structures
Complexes 3, 5 and 6 crystallized as cis-isomers and 1 as a
trans-isomer with respect to phenolic O-atoms. In the structures
The crystal structures of the salicylaldimine preligands used in
this work have been published in our previous paper [7]. Structures
Fig. 1. (a) Molecular structure of the monomeric unit of 1 with the atom numbering scheme (displacement parameters are drawn at 50% probability level). Hydrogen atoms
are omitted for clarity. (b) Dimeric structure of 1 showing the intermolecular Cuꢂ ꢂ ꢂO and Cuꢂ ꢂ ꢂH contacts with dashed lines. Selected bond lengths (Å) and angles (°): Cu1–O1
1.8868(15); Cu1–O10 1.8963(15); Cu1–O80 1.9556(16); Cu1–N8 1.9637(19); O1–C1 1.310(3); C70–O80 1.255(3); C7–N8 1.301(3); N8–C9 1.448(3); Cu1–H (symmetry operator:
ꢀx + 1, y ꢀ ½, ꢀz + ½) 3.12; Cu1–O80 (symmetry operator: ꢀx + 1, ꢀy + 1, ꢀz + 1) 2.8181(16); O1–Cu1–O10 159.29(7); O1–Cu1–O80 86.99(7); O10–Cu1–O80 92.70(6); O1–Cu1–
N8 92.79(7); O10–Cu1–N8 92.84(7); O80–Cu1–N8 164.75(7).
Fig. 2. (a) Molecular structure of the monomeric unit of 3 with the atom numbering scheme (displacement parameters are drawn at 50% probability level). Hydrogen atoms
and the disorder about the twofold axis of 3 are omitted for clarity. (b) Polymeric structure of 3 showing the Cuꢂ ꢂ ꢂO, Cuꢂ ꢂ ꢂN and Cuꢂ ꢂ ꢂOꢂ ꢂ ꢂCuꢂ ꢂ ꢂN contacts with dashed lines.

J.U. Ahmad et al. / Inorganica Chimica Acta 384 (2012) 275–280
279
Fig. 3. (a) Molecular structure of the monomeric unit of 5 with the atom numbering scheme (displacement parameters are drawn at 50% probability level). Hydrogen atoms
and the minor disordered part of 5 are omitted for clarity. (b) Dimeric structure of 5 with dashed lines indicating the intermolecular Cuꢂ ꢂ ꢂN and Cuꢂ ꢂ ꢂH contacts.
Fig. 4. (a) Molecular structure of the monomeric unit of 6 with the atom numbering scheme (displacement parameters are drawn at 50% probability level). Hydrogen atoms
and the disorder about the twofold axis of 6 are omitted for clarity. (b) Dimeric structure of 6 where the dashed lines indicate the intermolecular Cuꢂ ꢂ ꢂO and Cuꢂ ꢂ ꢂH contacts.
two monoanionic, bidentate ligands are coordinated to the Cu(II)
which has adopted an octahedral geometry in their dimeric or
polymeric form. 3,5-Di-tert-butylsalicylaldehydato coordinates to
the metal center through its O,O-donor atoms and salicylaldimina-
to through its N,O-donor set. In most crystal structures reported for
nonbridged Cu(II) Schiff bases the salicylaldimine ligands have
adopted trans-orientation and a geometry around the metal center
is a distorted square-plane [13]. Complexes having cis-oriented li-
gands are sparce [3e,4].
In 1 the Cu(II) centers of two closest molecules are bridged by
their aldehydato O atoms with the intermolecular Cuꢂ ꢂ ꢂO distances
of 2.8181(16) Å (Fig. 1b). The distorted octahedral coordination
sphere around Cu(II) is completed with tert-butyl H atom of alde-
hydato moiety (from the neighboring dimer) the Cuꢂ ꢂ ꢂH distances
being 3.12 Å. The Cu–Nimine and Cu–Ophenolic bond distances of 1
are in the ranges observed for similar structures [13]. The Cu–
Ocarbonyl bonds are significantly longer than the Cu–Ophenolic bonds
as observed also in analogous bis(salicylaldehydato)Cu(II) com-
plexes [14].1
Cu–Oꢂ ꢂ ꢂCu–O dimer and a mixed Cu–Oꢂ ꢂ ꢂCu–N dimer exist in 9:6:1
ratio. The same dimeric pattern can be observed in 6 in 1:2:1 ratio
(Fig. 4b). The Cuꢂ ꢂ ꢂO distances in 5 and 6 are 2.826(8) and
3.683(2) Å whereas the Cuꢂ ꢂ ꢂN distances are 3.295(3) and
3.852(9) Å, respectively.
4. Conclusions
A series of novel mixed ligand Cu(II) complexes with 3,5-di-tert-
butylsalicylaldimine and the corresponding aldehyde ligands have
been synthesized in good to excellent yields using the developed
synthetic methods. The simplest way to prepare the complexes is
a one-pot synthesis. Two other procedures are based on the forma-
tion of a complex precursor, either bis(salicylaldehydato)Cu(II) or
bis(salicylaldiminato)Cu(II). The fourth used method proceeds
through salicylaldimine preligand. The crystal structures of four
complexes, determined by X-ray diffraction studies, show that
three of the complexes are cis-isomers and one is a trans-isomer
with respect to the phenolic O-atoms. In all structures the Cu(II)
center has adopted a distorted octahedral geometry in their di-
meric or polymeric form. In three structures the Cu(II) complexes
from loose dimers in a crystal while with the fourth studied com-
plex loose polymers are observed. These types of mixed ligand
complexes can provide convenient synthetic routes to heteroligat-
ed bis(phenoxyimino)Cu complexes.
Complex 3 forms a polymeric structure where intermolecular
Cuꢂ ꢂ ꢂO (3.811(3) Å), Cuꢂ ꢂ ꢂN (4.615(7) Å) as well as Cu–Oꢂ ꢂ ꢂCu–N
contacts can be found (Fig. 2b). In 5 and 6 the apical positions of
the octahedral geometry are occupied by H atoms (Cuꢂ ꢂ ꢂH 2.95 Å
in 5 and 3.10 Å in 6) (Fig. 3b). The complex 5 forms dimers mainly
via Cu–Nꢂ ꢂ ꢂCu–N coordination bonds but due to the disorder also a

280
J.U. Ahmad et al. / Inorganica Chimica Acta 384 (2012) 275–280
(f) A.A. Osunlaja, N.P. Ndahi, J.A. Ameh, Afr. J. Biotechnol. 8 (2009) 4;
Acknowledgments
(g) P.P. Dholakiya, M.N. Patel, Synth. React. Inorg. Met.-Org. Chem. 34 (2004)
383;
We gratefully acknowledge financial support from the Graduate
School of Inorganic Materials Chemistry (J.U.A.). M.N. thanks the
Academy of Finland (Project No. 128800).
(h) P.P. Dholakiya, M.N. Patel, Synth. React. Inorg. Met.-Org. Chem. 32 (2002)
819.
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Appendix A. Supplementary material
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(e) J.U. Ahmad, P.J. Figiel, M.T. Räisänen, M. Leskelä, T. Repo, Appl. Catal., A 371
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Repo, Adv. Synth. Catal. 351 (2009) 2625.
CCDC 794404, 848926, 848927, 848928 and 799405 contain the
supplementary crystallographic data for complexes B, 1, 3, 5 and 6,
respectively. These data can be obtained free of charge from The
Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/
data_request/cif.
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