Synthesis and characterization of binuclear [ONXO]-type amine-bis(phenolate) copper(II) complexes

Article abstract of DOI:10.1016/j.ica.2011.04.048

Dimeric copper(II) complexes [Cu^II₂L₂] of a series of amine-bis(phenolate) ligands (H₂L) bearing different non- or weakly coordinating side-arms such as ethyl (H₂L^Et), n-butyl (H₂L^Bu), thiomethyl (H₂L^SMe) and hydroxyl (H₂L^OH) were synthesized. They were characterized by X-ray crystallography, UV-Vis, IR and magnetic susceptibility measurements. X-ray analysis revealed complexes in which Cu(II) centers are surrounded by three phenolate oxygen atoms, an amine nitrogen atom and thiomethyl and hydroxyl coordinating groups in Cu₂L ^SMe₂ and Cu₂L^OH₂, respectively. Two phenolate bridges hold both copper atoms together to form binuclear [Cu^II₂L₂] complexes. To the best of our knowledge, Cu···Cu distances and Cu-O-Cu bridge angles of these complexes are the smallest values, which have been reported for phenoxo-bridged copper complexes to date. Phenolate moieties of the copper complexes can be electrochemically oxidized to phenoxyl radicals. Magnetic studies show that Cu₂L^Et₂ and Cu ₂L^Bu₂ are rare examples of phenolato-bridged Cu(II) dimer exhibiting ferromagnetic interaction (J = +26.62 and +38.65 cm ^-1 respectively). In addition, an antiferromagnetic exchange with J values of -65.88 and -2.77 cm^-1 was found for Cu₂L ^SMe₂ and Cu₂L^OH₂, respectively.

Full text of DOI:10.1016/j.ica.2011.04.048

Inorganica Chimica Acta 375 (2011) 158–165
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Inorganica Chimica Acta
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Synthesis and characterization of binuclear [ONXO]-type amine-bis(phenolate)
copper(II) complexes
a
b
b
Elham Safaei a, , Maryam Rasouli , Thomas Weyhermüller , Eckhard Bill
⇑
a Institute for Advanced Studies in Basic Sciences (IASBS), 45195 Zanjan, Iran
b Max-Planck Institute for Bioinorganic Chemistry, Stiftstraße 34-36, D-45470, Mülheim an der Ruhr, Germany
a r t i c l e i n f o
a b s t r a c t
Dimeric copper(II) complexes [CuII2L2] of a series of amine-bis(phenolate) ligands (H2L) bearing different
non- or weakly coordinating side-arms such as ethyl (H2LEt), n-butyl (H2LBu), thiomethyl (H2LSMe) and
hydroxyl (H2LOH) were synthesized. They were characterized by X-ray crystallography, UV–Vis, IR and
magnetic susceptibility measurements. X-ray analysis revealed complexes in which Cu(II) centers are
surrounded by three phenolate oxygen atoms, an amine nitrogen atom and thiomethyl and hydroxyl
Article history:
Received 16 November 2010
Received in revised form 16 February 2011
Accepted 30 April 2011
Available online 12 May 2011
coordinating groups in Cu2LSMe and Cu2LOH2, respectively. Two phenolate bridges hold both copper
2
Keywords:
Magnetism
Amine bis(phenolate)
Model complexes
Phenoxyl
atoms together to form binuclear [CuII2L2] complexes. To the best of our knowledge, CuꢀꢀꢀCu distances
and Cu–O–Cu bridge angles of these complexes are the smallest values, which have been reported for
phenoxo-bridged copper complexes to date. Phenolate moieties of the copper complexes can be electro-
chemically oxidized to phenoxyl radicals. Magnetic studies show that Cu2LEt2 and Cu2LBu2 are rare exam-
ples of phenolato-bridged Cu(II) dimer exhibiting ferromagnetic interaction (J = +26.62 and +38.65 cmꢁ1
respectively). In addition, an antiferromagnetic exchange with J values of ꢁ65.88 and ꢁ2.77 cmꢁ1 was
found for Cu2LSMe and Cu2LOH2, respectively.
2
Ó 2011 Elsevier B.V. All rights reserved.
1. Introduction
there is not any copper(II)–copper(II) spin interaction. In the
strongly interacting type, relatively strong copper(II)–copper(II)
Metalloenzymes control a wide range of functions in biological
systems. These metallo-biomolecules are coordination complexes
whose active sites, consist of one or more metal atoms and their
coordinated ligands. The metal ions in these coordination environ-
ments cooperate with each other to complete a catalytic function
[1–3]. Copper is one of these metal ions frequently found at some
of the enzymes’ active sites composed of donors from the side-
chains of amino acids such as histidine and tyrosine holding it [4–9].
The synthetic analog approach is based on the chemistry of the
enzyme’s active site. Recent studies provided Cu(II) complexes
with tripodal aminophenol ligands involving N3O, N2O2 and NO2
coordination spheres (N and O represent nitrogen and oxygen do-
nor molecules such as pyridine and phenol) as synthetic analogs of
tyrosine and histidine moieties of amino acids [10–20]. Significant
efforts have been made to provide coordination complexes as mod-
els for active sites of multi-copper enzymes [21–24]. On the other
hand, magneto chemistry of these multinuclear complexes is a
subject of current interest. These complexes are classified accord-
ing to their metal–metal spin interaction. These interactions are
termed superexchange because of the large distances involved
(3–5 Å) between the metal ions [25]. In the non interacting type,
spin interaction occurs. In weakly interacting systems, coupling be-
tween the electron spins of the two copper ions leads to antiferro-
magnetic or ferromagnetic behavior, depending on which
exchange pathway dominates spin interactions.
This study aimed to investigate the effect of an additional pen-
dent arm on the magnetic and redox properties of the resulting
complexes of amine-bis(phenol) ligands (Scheme 1). We synthe-
sized copper complexes of mentioned ligands with different side
arms such as alkyl and coordinating thiomethyl and hydroxyl moi-
eties. The coordination, magnetic and redox properties of related
binuclear Cu(II) complexes of ligands H2L are described.
2. Experimental
2.1. Materials and physical measurements
Reagents and analytical grade materials were obtained from
commercial suppliers and used without further purification, except
those for electrochemical measurements. Elemental analyses (C, H,
N) were performed by the Research Institute of Petroleum Industry
(RIPI). Fourier transform infrared spectroscopy on KBr pellets was
performed on
a FT IR Bruker Vector 22 instrument. NMR
⇑
measurements were done on a Bruker 250 instrument. UV–Vis
absorbance digitized spectra were collected using a CARY 100 Bio
Corresponding author. Tel.: +98 241 4153200; fax: +98 241 4153232.
E-mail address: safaei@iasbs.ac.ir (E. Safaei).
0020-1693/$ - see front matter Ó 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.ica.2011.04.048

E. Safaei et al. / Inorganica Chimica Acta 375 (2011) 158–165
159
method and refined by full-matrix least-squares techniques based
on F2
SHELXTL software package) [33].
tBu
X
N
tBu
(
2.2. Preparations
OH
HO
Ligands H2LEt, H2LBu and H2LSMe were prepared by procedures re-
ported in the literature [34,35]. The new ligands H2LOH and H2LOAC
were synthesized by procedures reported here. As aqueous formal-
dehyde was one of the reagents, in some cases no added solvent was
required beyond what was already present in the reagents solution.
tBu
tBu
2.2.1. Synthesis of H2LEt
A solution of 2,4-di-tert-butylphenol (5.0 g, 24.2 mmol), ethyla-
mine (0.67 ml, 12.1 mmol), and 36% aqueous formaldehyde
(1.62 ml, 19.4 mmol) in methanol (10 ml) was stirred and refluxed
for 24 h. The mixture was cooled, filtered and the residue was
washed with cold methanol, to give the product as a white powder.
The product was further purified by recrystallization from dichlo-
romethane/methanol solvent mixture (2:1). The solution remain-
ing after the removal of the solid was left to give more product
as a white powder (2.27 g, 39%). MP. 113.6 °C. 1H NMR: (CDCl3/
250 MHz): d 7.26 (s, 2H, C6H2), 6.96 (s, 2H, C6H2), 3.73 (s, 4H,
CH2), 2.68 (q, 2H, CH2), 1.38 (s, 18H, C(CH3)3), 1.22 (s, 18H,
Scheme 1. The structure of H2L.
spectrophotometer. Magnetic susceptibility data were measured
from powder samples of solid material in the temperature range
of 2–290 K by using a SQUID susceptometer with a field of 1.0 T
(MPMS-7, Quantum Design, calibrated with standard palladium
reference sample, error < 2%). Multiple-field variable-temperature
magnetization measurements were done at 1, 4, and 7 T also in
the range of 2–290 K with the magnetization equidistantly sam-
pled on a 1/T temperature scale. The experimental data were cor-
rected for underlying diamagnetism by use of tabulated Pascal’s
constants [26,27] as well as for temperature-independent para-
magnetism. The susceptibility and magnetization data were simu-
lated with our own package julX for exchange coupled systems
[28]. The simulations are based on the usual spin-Hamilton opera-
tor for binuclear complexes with two spins S1 = S2 = 1/2.
C(CH3)3), 1.06 (t, J = 7 Hz, 3H, CH3). IR (KBr):
m = 3240, 2953,
2872, 2860, 1460, 1330, 1300, 1220, 860 cmꢁ1
.
A synthesis of the ligand in aqueous media or under methanol-
free condition was tested (2.33 g, 40%).
2.2.2. Synthesis of H2LBu
A solution of 2,4-di-tert-butylphenol (5.0 g, 24.2 mmol), butyl-
amine (1.2 ml, 12.1 mmol) and 36% aqueous formaldehyde
(4.0 ml, 48.0 mmol) in 10 ml of methanol was stirred and refluxed
for 24 h. After cooling the mixture in the freezer overnight, the sol-
vent was removed. The residue was washed repeatedly with little
amounts of cold methanol to give the product as a white powder
(3.89 g, 63.2%). The product was further purified by recrystalliza-
tion from dichloromethane/methanol solvent mixture (2:1). MP
102.6 °C. 1H NMR: (CDCl3/250 MHz): d 7.24 (s, 2H, C6H2), 6.94 (s,
2H, C6H2), 3.70 (s, 4H, CH2), 2.55 (t, J = 7 Hz, 2H, CH2), 1.60 (m,
4H, CH2), 1.36 (s, 18H, C(CH3)3), 1.11 (s, 18H, C(CH3)3), 0.852 (t,
^
^
^
^
~
^
~
~
~
ꢀ
H ¼ ꢁ2J½S1 ꢀ S2ꢂ þ gbðS1 þ S2Þ ꢀ B
ð1Þ
where J is the spin coupling constant and g is the average of the
electronic g matrix components (kept equal for both Cu sites). Diag-
onalization of the Hamiltonian was performed with the routine
ZHEEV from the LAPACK Library [29] and the magnetic moments
were obtained from first order numerical derivative dE/dB of the
eigenvalues. The powder summations were done by using a 16-
point Lebedev grid [30,31]. Intermolecular interactions were con-
J = 7.75 Hz, 3H, CH3). IR (KBr):
m = 3240, 2960, 2900, 2850, 1493,
1350, 1230, 850 cmꢁ1
.
A synthesis of the ligand in aqueous media or under methanol-
free condition tested (2.64 g, 43%).
2.2.3. Synthesis of H2LSMe
This ligand was synthesized according to procedure reported in
the literature (2.36 g, 44.1%). MP. 116.8 °C. 1H NMR: (CDCl3/
250 MHz): d 7.89 (s, 2H, OH), 7.24 (s, 2H, C6H2), 6.93 (s, 2H,
C6H2), 3.67 (s, 4H, CH2), 2.76 (s, 4H, CH2), 2.01(s, 3H, CH3), 1.43
sidered by using a Weiss temperature,
H
W, as perturbation of the
temperature scale, kT0 = k(T ꢁ
HW) for the calculation. Powder
summations were done by using a 16-point Lebedev grid.
Voltammetric measurements were made with a computer con-
trolled Auto Lab electrochemical system (ECO Chemie, Ultrecht,
The Netherlands) equipped with a PGSTA 30 model and driven
by GPES (ECO Chemie). A glassy carbon electrode with a surface
area of 0.756 cm2 was used as a working electrode and a platinum
wire served as the counter electrode. The reference electrode was
an Ag wire as the quasi reference electrode. Ferrocene was added
as an internal standard and potentials were referenced versus the
ferrocenium/ferrocene couple (Fc+/Fc).
(s, 18H, C(CH3)3), 1.29 (s, 18H, C(CH3)3). IR (KBr):
m = 3250, 2975,
2840, 2800, 1495, 1360, 1250, 875 cmꢁ1
.
2.2.4. Synthesis of H2LOH
A solution of 2,4-di-tert-butylphenol (5.0 g, 24.2 mmol), etha-
nolamine (0.73 ml, 12.1 mmol) and 36% aqueous formaldehyde
(4.0 ml, 48.0 mmol) in 10 ml of methanol was stirred and refluxed
for 24 h. After cooling the mixture in the freezer overnight, the sol-
vent was removed. The residue was washed repeatedly with little
amounts of cold methanol to give the product as a white powder
(1.76 g, 29.3%). The product was further purified by recrystalliza-
tion from dichloromethane/ methanol solvent mixture (2:1). MP.
103.5 °C. 1H NMR: (CDCl3/250 MHz): d 7.24 (s, 2H, C6H2), 6.91 (s,
2H, C6H2), 3.89 (t, J = 5 Hz, 2H, CH2), 3.79 (s, 4H, CH2), 2.76 (t, 2H,
CH2), 1.47 (s, 18H, C(CH3)3), 1.19 (s, 18H, C(CH3)3). IR (KBr):
The X-ray data for the reported complexes were collected on a
Bruker-Nonius Kappa CCD diffractometer at 100(2) K using graph-
ite-monochromated Mo K
a radiation (k = 0.71073 Å). Final cell
constants were obtained from a least-squares fit of all integrated
reflections. Intensity data were corrected for Lorentz and polariza-
tion effects. The data sets were corrected for absorption (SADABS
,
Bruker-Nonius 2004) [32]. The structures were solved by direct
m
= 3400, 2960, 2890, 2810, 1470, 1320, 830 cmꢁ1
.

160
E. Safaei et al. / Inorganica Chimica Acta 375 (2011) 158–165
A synthesis of the ligand in aqueous media or under methanol-
free condition tested (2.05 g, 34.2%).
nm (e m = 2980, 2850,
, Mꢁ1 cmꢁ1): 474 (3050), 703 (580). IR (KBr):
2830, 1520, 1370, 1260, 880 cmꢁ1
.
2.2.5. Synthesis of H2LOAC
2.2.9. Synthesis of Cu2LOH
2
Acylation of H2LOH was done according to the procedure re-
ported in the literature [36]. A mixture of acetylchloride (0.18 ml,
2 mmol), H2LOH (0.497 g, 1 mmol) and 0.0017 g, (0.0016 mmol) of
N,N0,N00,N000-tetramethyl-tetra-2,3-pyranoporphyrazinatocopper(II)
[Cu(2,3-tmtppa)]4+, in 5 ml of dichloromethane was stirred for 4 h
at room temperature. Water and n-hexane were added to the mix-
ture so that the catalyst was extracted to the water phase and
product was left in the organic phase. After removing solvent,
the viscose yellowish material cooled in the freezer overnight.
The residue was triturated with cold methanol, filtered and
washed thoroughly with cold methanol to give the product as a
white powder (1.76 g, 29.3%). The product was further purified
by recrystallization from a dichloromethane/methanol (2:1) sol-
vent mixture. MP. 136.9 °C. 1H NMR: (CDCl3/TMS-250 MHz): d
7.72 (s, 2H, OH), 7.23 (s, 2H, C6H2), 6.91 (s, 2H, C6H2), 4.27 (s, 2H,
CH2), 3.73 (s, 4H, CH2), 2.83 (t, 2H, CH2), 2.17 (s, 3H, CH3), 1.4 (s,
18H, C(CH3)3), 1.28 (s, 18H, C(CH3)3).
H2LEt (0.539 g, 1 mmol) was added to a solution of sodium
methoxide (0.15 ml, 3 mmol) in methanol (50 ml). The solution
was stirred for 10 min at room temperature. Then Cu(OAC)2.2H2O
(0.199 g, 1 mmol) was added to the solution. The mixture was re-
fluxed for 120 min. Meanwhile, the color changed to red brown
and some light brown precipitate appeared. The solution was fil-
tered and washed with methanol. Brown crystals were isolated
from dichloromethane/ methanol (2:1). Yield = 0.2 g (35.4%) Anal.
Calc. for C68H102Cu2N2O8 (1201.08 g/mol): C, 66.7; H, 8.8; N, 2.3.
Found: C, 67.9; H, 8.5; N, 2.3%. UV–Vis in DMF: kmax, nm (
Mꢁ1 cmꢁ1): 465 (2920), 557 (708). IR (KBr):
= 2975, 2820, 2800,
1480, 1300, 820 cmꢁ1
e,
m
.
3. Results and discussion
The synthesis of the ligands H2LEt, H2LBu, H2LSMe and H2LOH is
based on a single-step Mannich condensation of the amine with
formaldehyde, and 2,4-di-tert-butyl phenol. Dinuclear copper com-
plexes were easily formed in good yields by refluxing methanolic
solutions of the ligands with copper acetate and a base (sodium
methoxide or triethylamine). In the case of H2LOH, an insoluble
polymeric material was formed. To prevent polymerization of this
complex through hydroxyl side arm of the mentioned ligand, the
hydroxyl group was selectively protected in the presence of phenol
groups by acylation before reaction with copper salt. The ester
group in the pendant arm(H2LOH) seems to be hydrolized to hydro-
xyl via nucleophilic attack of water during complexation.
13C NMR: (CDCl3/TMS-250 MHz):
d 152.28 (C@O), 141.46,
123.75, 121.07, 57.47 (ArCH2), 51.73 (CH2), 34.91(C(CH3)3), 34.16
(C(CH3)3), 31.61 (C(CH3)3), 29.61 (C(CH3)3), 20.86 (CH2). IR (KBr):
m
= 3420, 2970, 2900, 1720, 1473, 1325, 840 cmꢁ1
.
2.2.6. Synthesis of Cu2LEt
2
H2LEt (0.481 g, 1 mmol) was added to a solution of triethyl-
amine (0.54 g, 10 mmol) in methanol (50 ml). The solution was
stirred for 10 min at room temperature. Then Cu(OAC)2.2H2O
(0.199 g, 1 mmol) was added to the solution. The mixture was re-
fluxed for 60 min whereupon a color changed to red brown oc-
curred and some light brown precipitate was formed. The
solution was filtered and washed with methanol. Dark-brown crys-
tals were isolated from a dichloromethane/methanol mixture
In IR spectra of most complexes, the strong and sharp band at fre-
quencies higher than 3000 cmꢁ1 for the
m
OH stretch of ligands were
disappeared, provingthe coordination ofphenol groupsto themetal.
(2:1). Yield = 0.37 g (68.2%). Anal. Calc. for
(1086.52 g/mol): C, 69.6; H, 8.8; N, 3.7. Found: C, 70.8; H, 9.0; N,
2.6%. UV–Vis in DMF: kmax, nm (
, Mꢁ1 cmꢁ1): 431 (1700), 669
(633). IR (KBr):
= 2920, 2900, 2850, 1495, 1290, 855 cmꢁ1
C64H98Cu2N2O4
3.1. X-ray data collection and structure determination of Cu2LEt
Cu2LBu2, Cu2LSMe and Cu2LOH
,
2
2
2
e
m
.
Dark brown crystals of Cu2LEt2, Cu2LBu2, Cu2LSMe and Cu2LOH
2
2
for X-ray analysis were obtained from methanol/dichloromethane
(1:2) solution. Details of data collection and refinement of the
structures are summarized in Table 1. Selected bond lengths and
angels for Cu2LEt2, Cu2LBu2, Cu2LSMe and Cu2LOH are presented in
Tables 2–5. A complete listing of bond lengths and angles for these
complexes can be found in the Supplementary material.
2.2.7. Synthesis of Cu2LBu
2
H2LEt (0.509 g, 1 mmol) was added to a solution of triethyl-
amine (0.27 ml, 3 mmol) in methanol (50 ml). The solution was
stirred for 10 min at room temperature. Then Cu(OAC)2.2H2O
(0.199 g, 1 mmol) was added to the solution. The mixture was re-
fluxed for 60 min. Meanwhile, the color changed to red brown
and some light brown precipitate appeared. The solution was fil-
tered and washed with methanol. Dark-brown crystals were iso-
lated from dichloromethane/ methanol (2:1). Yield = 0.4 g
(70.4%). Anal. Calc. for C68H106Cu2N2O4 (1142.63 g/mol): C, 71.1;
2
2
All compounds reported in this study are of the general formula
[L2Cu2]0. Complexes with ligands having alkyl groups attached to
the central nitrogen consist of two phenolato bridged square pla-
nar copper centers in a N3O coordination environment showing a
considerable tetrahedral distortion. Compounds carrying side
chains with an additional coordinating function (Cu2LSMe2 and Cu2-
LOH2) form pentacoordinated species with distorted square pyrami-
dal geometry binding the additional donor in an apical position.
Structural analysis of the reported dimeric complexes revealed
short CuꢀꢀꢀCu distances being 2.6710(4), 2.6800(4), 2.8157(2) and
2.7206(10) Å for Cu2LEt Cu2LBu Cu2LSMe2, Cu2LOH2, respectively.
H, 9.1; N, 3.0. Found: C, 71.5; H, 9.3; N, 2.4%. UV–Vis in DMF: kmax
,
nm (e m = 2960, 2920,
, Mꢁ1 cmꢁ1): 436 (1790), 668 (649). IR (KBr):
2860, 1500, 1360, 1220, 840 cmꢁ1
.
2.2.8. Synthesis of Cu2LSMe
2
H2LEt (0.527 g, 1 mmol) was added to a solution of triethyl-
amine (0.27 ml, 3 mmol) in methanol (50 ml). The solution was
stirred for 10 min at room temperature. Then Cu(OAC)2ꢀ2H2O
(0.199 g, 1 mmol) was added to the solution. The mixture was re-
fluxed for 60 min. Meanwhile, the color changed to red brown
and some light brown precipitate appeared. The solution was fil-
tered and washed with methanol. Red-brown crystals were iso-
lated from dichloromethane/methanol (2:1). Yield = 0.41 g
(69.2%) Anal. Calc. for C66H102Cu2N2O4S2 (1178.70 g/mol): C, 66.7;
2,
2,
The Cuꢀ ꢀ ꢀCu distances found in Cu2LEt Cu2LBu are, to the best of
2,
2
our knowledge, the shortest which have ever been detected in
bis(l-phenoxo)dicopper(II) complexes to date. These short dis-
tances arise from a distinct folding of the two square planar moie-
ties along the O–O vector in the central Cu2O2 ring. The calculated
angle between the two mean planes containing the Cu ion and the
two phenoxo oxygen atoms is about 63° in both cases (63.7° and
62.2° for Cu2LEt and Cu2LBu2, respectively) which allows the Cu
2
H, 8.7; N, 2.4. Found: C, 67.3; H, 8.6; N, 2.4%. UV–Vis in DMF: kmax
,
ions to come very close.

E. Safaei et al. / Inorganica Chimica Acta 375 (2011) 158–165
161
Table 1
Crystal data and structure refinement for Cu2LEt Cu2LBu2, Cu2LSMe and Cu2LOH
.
2,
2
2
Identification code
Cu2LEt
Cu2LBu
Cu2LSMe
Cu2LOH
2
2
2
2
Empirical formula
Formula weight
Temperature (K)
Wavelength (Å)
Crystal system
Space group
Unit cell dimensions
a (Å)
C64H98Cu2N2O4
1086.52
100(2)
0.71073
orthorhombic
Pcca
C68H106Cu2N2O4
1142.63
100(2)
0.71073
monoclinic
P21/c
C66H102Cu2N2O4S2
1178.70
100(2)
C64H98Cu2N2O6
1118.52
100(2)
0.71073
orthorhombic
Pcca
0.71073
triclinic
ꢀ
P1
28.995(2)
10.9773(6)
19.0806(9)
90.00
6073.1(6)
4
1.188
0.746
2344
16.558(2)
19.664(2)
21.441(3)
109.072(3)
6597.9(14)
4
1.150
0.690
13.7277(10)
14.3528(11)
18.7074(14)
82.911(2)
3272.7(4)
2
1.196
0.759
1268
28.891(6)
10.817(2)
19.128(4)
90.00
5978(2)
4
1.243
0.762
b (Å)
c (Å)
b (°)
Volume (Å3)
Z
Calculated density (Mg/m3)
Absorption coefficient (mmꢁ1
F(0 0 0)
)
2472
2408
Crystal size (mm)
Theta range for data collection (°)
Index ranges
0.60 ꢃ 0.30 ꢃ 0.18
3.00–32.50
ꢁ43 6 h 6 43
–16 6 k 6 16
–28 6 l 6 28
100 874/10 975
[Rint = 0.0438]
10 975/10/360
1.018
0.05 ꢃ 0.05 ꢃ 0.04
2.01–31.00
ꢁ23 6 h 6 23
–28 6 k 6 28
–31 6 l 6 31
165 828/21 005
[Rint = 0.0519]
21 005/19/737
1.109
0.06 ꢃ 0.06 ꢃ 0.04
0.08 ꢃ 0.07 ꢃ 0.04
2.13–23.51
ꢁ32 6 h 6 32
–12 6 k 6 12
–21 6 l 6 21
76 923/4382
[Rint = 0.1586]
4382/20/366
1.106
1.75–35.00
ꢁ22 6 h 6 22
–23 6 k 6 23
–30 6 l 6 30
199 490/28 692
[Rint = 0.0363]
28 692/7/721
1.067
Reflections collected/unique
Data/restraints/parameters
Goodness-of-fit on F2
Final R indices [I > 2
R indices (all data)
r
(I)]
R1 = 0.0454, wR2 = 0.1117
R1 = 0.0575, wR2 = 0.1248
0.588 and ꢁ1.402
R1 = 0.0392, wR2 = 0.1003
R1 = 0.0567, wR2 = 0.1172
1.380 and ꢁ0.596
R1 = 0.0277, wR2 = 0.0722
R1 = 0.0396, wR2 = 0.0809
0.658 and ꢁ0.481
R1 = 0.0504, wR2 = 0.1140
R1 = 0.0567, wR2 = 0.1172
0.579 and ꢁ0.484
Largest difference peak and hole (e Aꢁ3
)
Table 2
Table 3
Selected bond lengths (Å) and angles [°] for
Cu2LBu
Selected bond lengths (Å) and angles (°) for
Cu2LEt
.
2
.
2
Identification code
Cu2LEt
Identification code
Cu2LBu
2
2
Cu(1)–O(17)
Cu(1)–O(1)
Cu(1)–N(9)
Cu(1)–Cu(1)#1
1.8513(10)
1.9373(9)
2.0177(14)
2.6710(4)
2.0268(12)
94.33(5)
94.59(5)
84.69(4)
168.52(5)
75.10(5)
97.13(5)
155.14(5)
49.07(4)
126.35(5)
110.04(4)
46.24(3)
Cu(1)–O(57)
Cu(1)–O(41)
Cu(1)–N(49)
Cu(1)–Cu(2)
1.8457(11)
1.9228(10)
2.0290(13)
2.6800(4)
1.9240(10)
155.08(5)
168.35(5)
85.20(4)
96.94(5)
75.92(4)
95.69(5)
93.78(5)
Cu(1)#1–O(1)
Cu(2)–O(1)
O(1)–Cu(1)–N(9)
O(1)#1–Cu(1)–O(17)
Cu(1)–O(1)–Cu(1)#1
O(17)–Cu(1)–O(1)
O(1)#1–Cu(1)–O(1)
O(17)–Cu(1)–N(9)
O(1)#1–Cu(1)–N(9)
O(1)–Cu(1)–Cu(2)
O(17)–Cu(1)–Cu(2)
N(9)–Cu(1)–Cu(2)
O(1)#1–Cu(1)–Cu(2)
O(1)–Cu(1)–N(49)
O(57)–Cu(1)–O(41)
Cu(1)–O(1)–Cu(2)
O(17)–Cu(2)–O(41)
O(41)–Cu(1)–O(1)
O(17)–Cu(2)–N(9)
O(41)–Cu(1)–N(49)
O(1)–Cu(2)–Cu(1)
O(17)–Cu(2)–Cu(1)
N(9)–Cu(2)–Cu(1)
O(41)–Cu(2)–Cu(1)
Cu(1)–O(1)
49.12(3)
130.75(4)
108.88(4)
45.62(3)
2.0335(11)
Symmetry transformations used to generate
equivalent atoms:
#1 ꢁ x + 1/2, ꢁy, z for Cu2LEt
.
2
75.06(11). These angles are smaller than corresponding angles re-
ported in Refs. [38–40,22].
We have also reported a similar bisphenoxo compound, Lhf2Cu2
(H2Lhf = N,N-bis(3,5-di-tert-butyl-2-hydroxybenzyl)-2-(amino-
methyl) tetrahydrofuran), with a CuꢀꢀꢀCu separation of 2.766 Å and
two related complexes of amine-bis(phenolate) ligands bearing
methoxyethyl and methylthiophen arms Lm2Cu2 and Lt2Cu2 with
CuꢀꢀꢀCu distances of 2.7868(6) and 2.7187(8) Å [23,37].
For other phenoxo bridged binuclear copper complexes [38–
40,22], larger separations of the copper centers of about 3 Å have
been observed.
In Cu2LEt2 (Fig. 1), the Cu1–O1, Cu1–O17 and Cu1–N9 bonds are
1.9373(9), 1.8513(10) and 2.0177(14) Å, respectively. The O17–
Cu1–O1 fragment is almost linear, the corresponding valence angel
being 168.52(5)°. The sum of angels around the bridging phenoxo
oxygen atoms is about 339 deg (for O1) which proves it to be pyra-
midal rather than planar (Table 2).
The Cu1–O1, Cu1–O57 and Cu1–N49 bonds in Cu2LBu (Fig. 2)
2
are at of typical values of 2.0335(11), 1.8457(11) and
2.0290(13) Å, respectively. The sum of angels around bridge oxy-
gen with the value of 336.23° proves the pyramidal coordination
rather than planar for O1 (Table 3).
The Cu1–O1–Cu1 bridge angles within the central four-mem-
bered Cu2O2 ring are 84.69(4), 85.20(4), 89.94(2) and 86.93(9)
deg in Cu2LEt2, Cu2LBu2, Cu2LSMe and Cu2LOH2, respectively and
The Cu1–O41, Cu1–O57, Cu1–N49 and Cu1–S76 bonds of
2
the O1–Cu1–O1 angles are at 75.10(5), 75.92(4), 73.70(2) and
Cu2LSMe are 1.9632(6), 1.8796(7), 2.0542(7) and 2.6905(3) Å,
2

162
E. Safaei et al. / Inorganica Chimica Acta 375 (2011) 158–165
Table 4
Selected bond lengths (Å) and angles (°) for
Cu2LSMe
.
2
Identification code
Cu2LSMe
2
Cu(1)–O(41)
Cu(1)–N(49)
Cu(1)–Cu(2)
Cu(2)–O(1)
1.9632(6)
2.0542(7)
2.8157(2)
1.9608(6)
148.35(3)
165.62(3)
89.94(2)
91.50(3)
73.70(2)
96.17(3)
93.84(3)
45.925(18)
116.41(2)
109.21(2)
44.203(17)
2.6905(3)
2.6669(3)
102.16(2)
121.124(18)
83.46(2)
O(1)–Cu(1)–N(49)
O(57)–Cu(1)–O(41)
Cu(1)–O(1)–Cu(2)
O(17)–Cu(2)–O(41)
O(41)–Cu(2)–O(1)
O(17)–Cu(2)–N(9)
O(41)–Cu(1)–N(49)
O(1)–Cu(2)–Cu(1)
O(17)–Cu(2)–Cu(1)
N(9)–Cu(2)–Cu(1)
O(41)–Cu(2)–Cu(1)
Cu(1)–S(76)
Cu(2)–S(36)
O(17)–Cu(2)–S(36)
O(41)–Cu(2)–S(36)
S(36)–Cu(2)–N(9)
N(49)–Cu(1)–S(76)
Fig. 1. ORTEP diagram and atom labeling scheme for complex Cu2LEt
.
2
83.59(2)
Table 5
Selected bond lengths (Å) and angles (°) for
Cu2LOH
.
2
Identification code
Cu2LOH
2
Cu(1)–O(17)
1.848(2)
Cu(1)–O(1)
1.932(2)
Cu(1)–N(9)
2.041(3)
Cu(1)–Cu(1)#1
Cu(1)#1–O(1)
2.7206(10)
2.022(2)
O(1)–Cu(1)–N(9)
O(1)#1–Cu(1)–O(17)
Cu(1)–O(1)–Cu(1)#1
O(17)–Cu(1)–O(1)
O(1)#1–Cu(1)–O(1)
O(17)–Cu(1)–N(9)
O(1)#1–Cu(1)–N(9)
O(1)–Cu(1)–Cu(1)#1
O(17)–Cu(1)–Cu(1)#1
N(9)–Cu(1)–Cu(1)#1
O(28)–Cu(1)–O(1)#1
Cu(1)–O(28)
93.70(11)
93.37(10)
86.93(9)
168.40(10)
75.06(11)
97.01(11)
151.02(11)
47.91(7)
123.65(8)
107.85(8)
139.67(12)
2.425(5)
Symmetry transformations used to generate
equivalent atoms:
#1 ꢁ x + 3/2, ꢁy + 1, z for Cu2LOH
.
2
Fig. 2. ORTEP diagram and atom labeling scheme for complex Cu2LBu
.
2
respectively (Fig. 3). The geometrical parameters
is 0.29 for Cu(1) and 0.22 Cu(2), respectively (Table 2). Comparing
s
for this complex
ing valence angel being 168.40(10)° (Table 5). The Cu1–O1, Cu1–
O17, Cu1–N9 and Cu1–O28 bonds are 1.932(2), 1.848(2),
2.041(3) and 2.425(5) Å respectively. Axial coordination of O28
(2.425(5) Å) is weaker than equatorial ligations. Two copper cen-
ters are separated (Cu1ꢀ ꢀ ꢀCu1#1) by 2.7206(10) Å. The angels
within the central four-membered Cu2O2 ring in the dimer are
O1#–Cu1–O1, 75.06(11) and Cu1–O1–Cu1#1 86.93(9)°. The sum
of angels around the bridging oxygen atom is 340.13° in this case.
these
mid (
s
values with corresponding values for ideal trigonal bipyra-
s
= 1) and square pyramidal ( = 0) suggest that this complex
s
has a square-pyramidal geometry.
The Cu1...Cu2 distance at 2.8157(2) Å within the dimer Cu2LSMe
2
is longer than those reported for the Cu2LEt and Cu2LBu analogs.
2
2
The central Cu2O2 ring has also a different geometry due to the
O41–Cu2–O1 and Cu1–O1–Cu2 angels being 73.70(2) and
89.94(2)°, respectively.
The sum of angels around the bridging oxygen atom O41 with a
3.2. Magnetic susceptibility measurements
value of 342.47° proves a sightly more planar coordination mode
compared to Cu2LEt and Cu2LBu
.
2
Magnetic susceptibility data for polycrystalline samples of com-
plexes Cu2LEt2, Cu2LBu2, Cu2LSMe2 and Cu2LOH2 were collected in the
temperature range 2–290 K in an applied magnetic field of 1 T.
2
The architecture of Cu2LOH is similar to that of Cu2LSMe2. The
2
two copper centers form a distorted square pyramidal coordination
environment with s value of 0.29 (Fig. 4).The CH2CH2OH group was
The effective magnetic moment (l
eff) for Cu2LEt2 decreases from
found to be disordered and only about 55% of the side arms are
loosely bound to the copper centers and about 45% are not coordi-
nated. The O1–Cu1–O17 fragment is almost linear, the correspond-
2.90 lB at 15 K to 2.62 lB along with increasing temperature to
290 K (Fig. 5), indicative of a relatively weak ferromagnetic cou-
pling between two copper centers (SCu = 1/2) with a resulting trip-

E. Safaei et al. / Inorganica Chimica Acta 375 (2011) 158–165
163
let ground state (S = 1). Simulation of the magnetic moment for
this complex yields J = +26.62 cmꢁ1, g = 2.16.
In the case of Cu2LBu a similar behavior is observed. The effec-
2
tive magnetic moment (leff) for this complex decreases from
3.02 lB at 10 K to 2.72 lB at 290 K (Fig. 5). This complex appears
to have triplet ground state (S = 1) due to ferromagnetic coupling
between two copper centers. Simulation of the magnetic moment
yields J = +38.66 cmꢁ1, g = 2.05.
The magnetic behavior of Cu2LSMe with temperature is com-
2
pletely different of Cu2LEt2, Cu2LBu complexes. The effective mag-
2
netic moment per molecule (leff) in this complex decreases to
0.20 lB at 2–90 K from the maximum value of 2.2 at 290 K
(Fig. 5). It demonstrates that there is antiferromagnetic coupling
between the two copper centers leading to a singlet ground state
(S = 0) (Fig. 5). Simulation of the magnetic moment yields
J = ꢁ65.88 cmꢁ1, g = 2.04.
The magnetic behavior of Cu2LOH2 is similar to that of Cu2LSMe
.
2
The effective magnetic moment (leff) with the constant value of
2.60 lB at 30–290 K decreases to 1.36 lB along with the drop in
temperature to 2 K (Fig. 5). This behavior can be attributed to a
weak antiferromagnetic coupling between two copper centers
with a singlet ground state (S = 0). Simulation of the magnetic mo-
ment yields J = ꢁ2.77 cmꢁ1, g = 2.14.
Fig. 3. ORTEP diagram and atom labeling scheme for complex Cu2LSMe
.
2
It is well known that in dinuclear copper(II) complexes with di-l-
hydroxo and di-l-alkoxo bridges, the singlet–triplet (S–T) energy
gap J correlates with the bridging oxygen Cu–O–Cu angel (
a
). The
following formula has been established for these cases.[41].
Jðcmꢁ1Þ ¼ ꢁ74
a
þ 7270
ð2Þ
If
a
is about 97.5°, J is equal to zero and the systems are uncou-
is smaller than 97.5°, the complex will show a ferromag-
pled. If
a
netic character and a triplet ground state. A bridging angle larger
than 97.5° leads to an antiferromagnetically coupled system with
a diamagnetic ground state (S = 0). A similar relationship was
found for bis-phenoxide bridged macrocyclic dicopper(II) com-
plexes [42].
2Jðcmꢁ1Þ ¼ ꢁ31:95
a
þ 2462
ð3Þ
Considering the relationship (Eqs. (2)–(5)) between the singlet–
triplet energy gap and the Cu–O(Ph)–Cu angel ( ) allows to find the
a
effect of structural factors of our complexes on the exchange inter-
actions between copper centers [42–46].
Fig. 4. ORTEP diagram and atom labeling scheme for complex Cu2LOH
.
2
2J ¼ ET ꢁ ES
J ¼ 2 ꢁ ðe1
where 2
netic contributions, respectively. The variation of
tances affects the overlap of Cu(3dxy)/O(2p) orbitals and therefore
changes e1 e2 (the energy difference between Cu(3dxy)/ O(2px)
and Cu(3dxy)/O(2py)) orbitals (Scheme 2). In Cu2LEt2, Cu2LBu com-
ð4Þ
ð5Þ
2
j
ꢁ
e2Þ =U
j
and (e1
ꢁ
e
2)2 terms describe the ferro- and antiferromag-
a
and CuꢀꢀꢀCu dis-
3.5
3
ꢁ
2
plexes, the small
2.67 and 2.68 Å, caused e1
quently, the ferromagnetic term 2
a
of 84.69° and 85.20° and CuꢀꢀꢀCu distances of
e2 term to become negligible. Conse-
dominates and complexes show
2.5
2
ꢁ
j
a ferromagnetic coupling with a triplet ground state.
The longer distance of Cu1ꢀ ꢀ ꢀCu2 of 2.8157 Å, the larger value of
1.5
1
bridge angel (a = 89.76) and also more planarity of Cu2O2 core in
Cu2LSMe due to the coordination of thiomethyl groups to copper
2
centers increase the antiferromagnetic contribution in this
complex.
0.5
0
The coupling constant value in Cu2LSMe is considerably low
2
compared to those reported (|J| > 100 cmꢁ1) for similar phenolato
0
50
100
150
200
250
300
bridge copper complexes [38–40,22]. It can be attributed to the
lower bridge angel (a < 97.5°) of this complex in comparison to
others and more effective ferromagnetic contribution to the overall
magnetic moment of the complex.
T/ K
Fig. 5. Magnetic measurements for complexes Cu2LEt2 (N), Cu2LBu2 (d), Cu2LSMe2 (j)
and Cu2LOH (ꢄ).
2

164
E. Safaei et al. / Inorganica Chimica Acta 375 (2011) 158–165
The CV and DPV voltammograms observed with Cu2LEt
,
2
Cu2LSMe2 and Cu2LOH2 revealed two oxidations with a similar small
difference in their redox potentials, and three oxidations for
Cu2LBu (Table 6 and Fig. 6). The similarity of Eox values for all
2
complexes in the range of 0.5–1.0 V suggests the same oxidation
Table 6
Electrode potentials (in V) for oxidation and reduction of complexes Cu2LEt2, Cu2LBu
,
2
Cu2LSMe and Cu2LOH measured at ambient temperature in CH2Cl2 solutions and
2
2
referenced vs. the Fc+/Fc couple.
Complex
Ered/V
E1ox/V
E2ox/V
E3ox/V
Cu2LEt
–
–
–
–
0.42b
0.50b
0.39b
0.41b
0.96a
0.73a
0.79b
1.04b
–
2
Cu2LBu
1.01a
2
Cu2LSMe
–
–
2
Cu2LOH
2
a
Irreversible reaction, peak potential is given.
Electrochemical quasi-reversible reaction.
b
Scheme 2. Magnetic molecular orbitals for Cu–O(Ph)–Cu framework [42].
We already showed for a similar copper dimer that coordination
of an amine-bis(phenol) ligand which bears a tetrahydrofuran side
arm affects
a
, CuꢀꢀꢀCu separation and consequently exchange cou-
pling (CuꢀꢀꢀCu separation of 2.766 Å,
a
= 88°, J = ꢁ12.0 cmꢁ1) [23].
A comparison of these values with the corresponding ones for phe-
nolato bridged copper complex of bis(phenol) amine ligand with
an uncoordinated thiophene arm (CuꢀꢀꢀCu separation of 2.7187 Å,
a
= 89.23°, J = 17.94 cmꢁ1) confirms the influence of side arm coor-
dination on structural parameters and consequently magnetic
properties [23,37].
The smaller value of
a (a = 86.93) compared to
in Cu2LOH
2
Cu2LSMe2 caused higher ferromagnetic contribution in the final sin-
glet–triplet (S–T) energy gap J (J = ꢁ2.77 cmꢁ1). It demonstrates
that in this complex, ferromagnetic term 2j is nearly equal to anti-
ferromagnetic term
(e1
ꢁ
e
2)2/U. Indeed the small angel of
a
= 86.93 for this complex is almost the crossover point from anti-
ferromagnetic to ferromagnetic behavior.
Variation of dihedral angel between two adjacent CuO2 planes
(d) affects e1
ꢁ
e2. The small dihedral angels between two CuO2
planes in Cu2LEt2, Cu2LBu2, Cu2LOH2, Cu2LSMe (63.71, 62.2, 56.18
2
and 59.72, respectively) are indicative for a considerable distortion
from the coplanarity of the two square planes and hence the two
dx2
magnetic orbitals. It caused the decreasing of e1
ꢁ
e2 and
ꢁy2
the observed ferromagnetic contribution to the overall exchange
interactions. The low value of coupling constant J in Cu2LSMe can
2
be attributed to this criteria.
The sum of the angles at the bridging phenoxide oxygen atoms
are lower than 360° indicating a pyramidal oxygen binding mode
which consequently increases the contribution of the ferromag-
netic (2j) term.
3.3. Electrochemistry
Cyclic and differential pulse voltammograms (CV and DPV) of
complexes Cu2LEt2, Cu2LBu2, Cu2LSMe and Cu2LOH were recorded
2
2
in CH2Cl2 solutions containing 0.1 M [(nBu)4N]ClO4 as supporting
electrolyte. Prior to the measurement, the GC electrode was pol-
ished with 0.1 lm alumina powder and washed with distilled
water. The voltage scan rate was set at 50 mV sꢁ1. The solutions
were deoxygenated by bubbling nitrogen gas through them for
10 min. Ferrocene was added as an internal standard and poten-
tials were referenced versus the ferrocenium/ ferrocene couple
(Fc+/Fc).
Fig. 6. Cyclic and differential pulse voltamogramms of Cu2LEt2, Cu2LBu2, Cu2LSMe
2
and Cu2LOH in CH2Cl2 at room temperature.
2

E. Safaei et al. / Inorganica Chimica Acta 375 (2011) 158–165
165
mechanism for all complexes. As the further oxidation of copper
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The small CuꢀꢀꢀCu separations and Cu–O(Ph)–Cu bridging angles
of these complexes compared to other similar bis(l-phen-
oxo)dicopper(II) compounds reported in the literature make them
unique.
Cu2LSMe and Cu2LOH show a remarkably weak antiferromag-
2
2
netism while Cu2LEt and Cu2LBu exhibit ferromagnetic exchange
2
2
interactions.
Structural changes (most importantly CuꢀꢀꢀCu distance, Cu–
O(Ph)–Cu angle changes) were induced by variations of the side
arms of the amine-bis(phenol) ligand, which in turn affects wether
a diamagnetic ground state (S = 0) or a triplet state (S = 1) is
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Authors are grateful to the Institute for Advanced Studies in Ba-
sic Sciences (IASBS) and Max-Planck-Institute for Bioinorganic
Chemistry. E. Safaei gratefully acknowledges the support by the
Institute for Advanced Studies in Basic Sciences (IASBS) Research
Council under Grant No. G2011IASBS127. Thanks are due to Mr.
Andreas Göbels and Mrs. Heike Schucht (MPI-BAC) for their skillful
technical assistance.
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