Formation of a Cu(II)-phenoxyl radical complex from a Cu(II)-phenolate complex: A new model for galactose oxidase

Article abstract of DOI:10.1016/j.poly.2012.12.036

A new N₃O type tetradentate ligand (HL) [2,4-di-tert-butyl-6- ((4-(2-(6,8-di-tert-butyl-2H-benzo[e][1,3]oxazin-3(4H)-yl)ethyl)piperazin-1-yl) methyl)phenol] and its complex [CuL(CH₃OH)]ClO₄(CH ₃OH) have been synthesised. The complex has been crystallographically and spectroscopically characterised. It has been noticed that in the presence of acetonitrile the Cu(II) centers in the complex undergo reduction with a concomitant oxidation of the ligand. EPR and UV-Vis spectroscopic studies of the complex in acetonitrile indicate that this disproportionation proceeds via the formation of a Cu(II)-phenoxyl intermediate. The Cu(II)-phenoxyl complex is found to be stable in methanol in the presence of base. The oxidised products of the ligand are isolated and characterized. The paramagnetic centers in the Cu(II)-phenoxyl complexes were found to be weakly ferromagnetically coupled.

Full text of DOI:10.1016/j.poly.2012.12.036

Polyhedron 51 (2013) 222–227
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Polyhedron
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Formation of a Cu(II)–phenoxyl radical complex from a Cu(II)–phenolate complex:
A new model for galactose oxidase
a,
Rajib Kumar Debnath a, Apurba Kalita b, Pankaj Kumar b, Biplab Mondal b, , Jatindra Nath Ganguli
⇑
⇑
a Department of Chemistry, Gauhati University, Guwahati, Assam 781014, India
b Department of Chemistry, Indian Institute of Technology Guwahati, Assam 781039, India
a r t i c l e i n f o
a b s t r a c t
Article history:
A new N3O type tetradentate ligand (HL) [2,4-di-tert-butyl-6-((4-(2-(6,8-di-tert-butyl-2H-benzo[e]
[1,3]oxazin-3(4H)-yl)ethyl)piperazin-1-yl)methyl)phenol] and its complex [CuL(CH3OH)]ClO4
Received 4 October 2012
Accepted 21 December 2012
Available online 11 January 2013
(CH3OH) have been synthesised. The complex has been crystallographically and spectroscopically
characterised. It has been noticed that in the presence of acetonitrile the Cu(II) centers in the complex
undergo reduction with a concomitant oxidation of the ligand. EPR and UV–Vis spectroscopic studies
of the complex in acetonitrile indicate that this disproportionation proceeds via the formation of a
Cu(II)–phenoxyl intermediate. The Cu(II)–phenoxyl complex is found to be stable in methanol in the
presence of base. The oxidised products of the ligand are isolated and characterized. The paramagnetic
centers in the Cu(II)–phenoxyl complexes were found to be weakly ferromagnetically coupled.
Ó 2013 Elsevier Ltd. All rights reserved.
Keywords:
2,4-Ditert butyl phenol
Piperazine ethylamine
Manich condensation
Galactose oxidase
1. Introduction
and EPR studies. In acetonitrile the complex also shows catalytic
activity for the conversion of some aromatic alcohols to their cor-
The active sites of the two copper containing enzymes galactose
oxidase and glyoxal oxidase are known to contain a coordinated
tyrosyl radical [1–6]. Galactose oxidase (GO), the extracellular cop-
per-containing enzyme, is known to catalyse the two-electron oxi-
dation of primary alcohols to the corresponding aldehydes with a
simultaneous reduction of dioxygen to hydrogen peroxide [7–
13]. An X-ray crystallographic study [14] of the active site of the re-
duced inactive form of galactose oxidase shows the presence of a
mono-nuclear copper(II) center in a square-pyramidal coordina-
tion sphere comprised of two tyrosine, two histidine residues
and an acetate ion. Previous work shows that this Cu(II) center
along with a phenoxyl radical catalyze two electron redox chemis-
try at the mononuclear active site [9,15]. However, the mechanism
of the electron transfer processes and structural parameters which
play a key role in controlling the stability and reactivity of the
coordinated radical in the active form are still not very clear. Here
we report the spectroscopic studies of a Cu(II)–phenoxyl radical
complex which can mimic the enzymetic active sites. From the sin-
gle crystal structure of the complex it is seen that the complex is in
the phenoxide and not the phenoxyl form. Even in methanol the
complex shows the inactive form of the GO model, but in acetoni-
trile the complex gets activated and conversion takes place from
the phenoxide to the phenoxyl form, which is shown by UV–Vis
responding aldehydes [16].
2. Results and discussion
The ligand HL has been synthesized using the previously re-
ported methods of a one-pot Mannich synthesis [17–19]. The for-
mation of the ligand has been confirmed by various
spectroscopic techniques like FT-IR, 1H and 13C NMR, elemental
analysis and single crystal structure. An X-ray quality single crystal
of the ligand was grown by slow diffusion of a dichloromethane
solution of the ligand into hexane, followed by slow evaporation.
The Ortep diagram for the neutral HL molecule is shown in Fig. 1.
2.1. Structure and spectroscopic properties of the complex
The green complex 1 has been synthesized by the reaction of
hexa aqua Cu(II) perchlorate with LH in methanol. The complex
was obtained as green microcrystalline precipitates on keeping
the reaction mixture at 0 °C for overnight. The formation of the
complex has been confirmed by various spectroscopic studies,
namely FT-IR, EPR, ESI-Mass, magnetic moment studies, micro-
analysis and finally by single crystal structure determination. The
mono-cationic complex shows satisfactory microanalytical data
and behaves as a 1:1 electrolyte in methanol solution.
⇑
Fig. 2 represents the ORTEP diagram of complex 1. The X-ray
quality crystals were grown by dissolving the complex in HPLC
Corresponding authors.
E-mail address: jatin_ganguli_gu@yahoo.co.in (J.N. Ganguli).
0277-5387/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.poly.2012.12.036

R.K. Debnath et al. / Polyhedron 51 (2013) 222–227
223
C20
C21
C9
C22
C18
N1
N3
N2
O2
C31
C10
C11
C7
C2
C17
C8
O1
C19
C32
C29
C30
C1
C23
C24
C16
C28
C3
C4
C33
C6
C15
C14
C13
C27
C5
C25
C26
C12
C37
C34
C35
C36
Fig. 1. Ortep plot of LH (50% ellipsoid plot). Hydrogens have been removed for clarity.
Fig. 2. ORTEP diagram for complex 1 (methanol and perchlorate are removed for clarity).
graded methanol and keeping the solution at 25 °C for three days.
The crystal structure reveals that the complex is in a square-pyra-
midal coordination sphere in which the Cu(II) center is surrounded
by three nitrogen and one oxygen atom from the ligand moiety in a
square plane, and the fifth coordination site is occupied by a meth-
complexes were 1497 and 1494 cmꢀ1 in complexes [CuII(3Lmet)H]+ꢀ
and [CuII(3Lmet)H2]2+ꢀ respectively. [(3Lmet)H2 = 1,4,7-tris(3-tertbu-
tyl-5-methoxy-2-hydroxybenzyl)1,4,7-triazacyclononane] [25]. In
the FT-IR spectra, the presence of the characteristic perchlorate
vibrations at ꢁ1100 and 625 cmꢀ1 indicates the monocationic
anol solvent. Here the basal angles are b = 162.93° and
thus the trigonality index ( ) value is 0.05 [ = (b–
a
= 159.79°,
complex. The molar conductivity: [KM = 137 X
ꢀ1 cm2 molꢀ1] indi-
s
s
a
)/60°] [20],
cates that the complex is a 1:1 electrolyte in methanol.
which indicates the complex is present almost in a square
pyramidal geometry. The Cu–Ophen distance is found to be
1.859(6) Å, which is about 0.1 Å shorter than that of the Cr(III)–
Ophen distance (1.943 Å) in a Cr(III)–phenoxyl radical complex
with the 1,4,7-tris(3-tertbutyl-5-methoxy-2-hydroxybenzyl)1,4,7-
triazacyclononane ligand [21]. This because in complex 1 the oxy-
gen of the phenol ring binds Cu in the phenolate form instead of
the phenoxyl form. The O1–C7 distance of 1.347 Å is little longer
compared to the reported distance in the above said phenoxyl rad-
ical complex. It is also on the higher side compared to the calculated
distance (1.257 Å) for the free phenoxyl radical, p-CH3–PhOꢀ , re-
ported by Hildebrandt et al. [22,23]. The crystallographic data for
both complex 1 and the ligand LH are supplied in Table 1. The bond
length and bond angle data table for complex 1 are supplied in Ta-
ble 2 (for the ligand see Supporting information Tables S1 and S2).
The IR spectrum of complex 1 in a KBr disc (Supporting infor-
mation Figure S9) exhibits a sharp band at 1469 cmꢀ1 which can
be attributed to the C1–O stretching vibrations from the phenolate
group. The reported values for Cu(II)–coordinated phenoxyl radical
2.2. UV-Vis spectral studies
Fig.
acetonitrile. The green solution exhibits the
and / d–d transition transition in the visible region at 650 nm
3
shows the electronic spectrum of complex
1 in
phen ? Cu(II)
O
(
(
e
e
= 1210 Mꢀ1 cmꢀ1). The intense absorption at 400 nm
= 3525 Mꢀ1 cmꢀ1) is characteristic for the p⁄ transition of
p
?
phenoxyl radicals. These are in agreement with the earlier reported
phenoxyl complexes [18,25].
The same
p–
p⁄ phenoxyl transitions in complexes [Cu(L)]2ꢀ2+
and [Cu(LH)]ꢀ2+ (H2L = 1-ethyl-4,7-bis(3-tert-butyl-5-methoxy-2-
hydroxybenzyl)-1,4,7-triazacyclononane] are reported to appear
at 431, 485 and 431 nm, respectively [24]. It has been found that
these phenoxyl radical species are thermally unstable. In addition,
they are found to be stable in the presence of base (NaOEt) (Fig. 4)
[22].
We have found that the complex is stable in methanol, but in
acetonitrile solvent it readily turns colourless in air at room tem-

224
R.K. Debnath et al. / Polyhedron 51 (2013) 222–227
Table 1
0.25
Crystallographic data for the complex [CuL(CH3OH)]ClO4ꢂCH3OH and LH.
[CuL(CH3OH)]ClO4ꢂCH3OH
39H65ClCuN3O8
802.93
monoclinic
P21/c
LH
Formula
Molecular weight
Crystal system
Space group
T (K)
C
C37H59N3O2
577.87
monoclinic
P21/c
296(2)
296(2)
k (Å)
a (Å)
b (Å)
c (Å)
0.71073
11.598(2)
10.1758(18)
37.589(6)
90.00
0.71073
5.9429(6)
39.758(3)
15.7862(16)
90.00
a
(°)
b (°)
100.094(11)
90.00
4367.5(13)
4
1.221
0.610
101.015(6)
90.00
3661.2(6)
4
1.048
0.064
0
340
c
(°)
V (Å3)
440
540
640
740
840
940
Z
Wavelength (nm)
Dcalc (Mg mꢀ3
Absorption
)
Fig. 3. UV–Vis spectrum of complex 1 in acetonitrile.
coefficient (mmꢀ1
)
F(000)
Total no. of
1720
10867
1272.0
9029
reflections
Reflections, I > 2
1.8
r(I)
5054
8456
Max. 2h (°)
28.28
28.44
Ranges (h, k, l)
ꢀ10 6 h 6 10, ꢀ8 6 k 6 9,
ꢀ7 = h 6 7,
ꢀ53 k 6 53
ꢀ20 l 6 20
98.20
ꢀ34 6 l 6 34
Complete to 2h (%)
99.00
Refinement method
full-matrix least-squares on full-matrix least-
F2
squares on F2
Data/restraints/
parameters
8249/0/360
0.2101
wR2(all data)
Goodness-of-fit
(GOF) on (F2)
0.1776
1.112
0.956
0.0737
R indices [I > 2
R indices (all data)
r
(I)]
0.0798
0.0983
0.2123
0
350
450
550
650
750
850
950
1050
Wavelength (nm)
perature and yields the colorless crystals of [Cu(CH3CN)4]ClO4 and
the modified ligands L0 (6,8-di-tert-butyl-3,4-dihydro-3-(2-(pip-
erazin-1-yl)ethyl)-2H-benzo[e][1,3]oxazine) and 3,5-di-tert-bu-
tyl-2-hydroxy-benzaldehyde. This reaction has also been found
to take place under dinitrogen. Similar results have been reported
Fig. 4. The UV–Vis spectrum of the Cu(II)–phenoxyl radical complex generated
from complex 1 in methanol in the presence of NaOEt.
by Yamauchi and coworkers [18,26]. It should be noted that these
reductions are found to be associated with the concomitant
decomposition of the ligand HL to L0 along with 3,5-di-tert-butyl-
2-hydroxybenzaldehyde (Scheme 1) (Supporting information,
Figs. S16–20).
Table 2
Selected bond lengths (Å) and bond angles (°) of complex 1.
Bond length (Å)
Complex 1
Bond angle (°)
Complex 1
Cu(1)–N(1)
Cu(1)–N(2)
Cu(1)–N(3)
Cu(1)–O(1)
Cu(1)–O(2)
N(1)–C(1)
N(1)–C(4)
N(1)–C(5)
N(2)–C(2)
N(2)–C(3)
N(2)–C(20)
N(3)–C(21)
N(3)–C(22)
N(3)–C(37)
O(1)–C(7)
O(2)–C(39)
O(3)–C(28)
O(3)–C(37)
Cl(1)–O(4)
Cl(1)–O(5)
Cl(1)–O(6)
Cl(1)–O(7)
O(8)–C(38)
1.971(7)
2.006(8)
2.052(7)
1.859(6)
N(1)–Cu(1)–N(2)
N(1)–Cu(1)–N(3)
N(1)–Cu(1)–O(1)
N(1)–Cu(1)–O(2)
N(2)–Cu(1)–N(3)
N(2)–Cu(1)–O(1)
N(2)–Cu(1)–O(2)
N(3)–Cu(1)–O(1)
N(3)–Cu(1)–O(2)
C(1)–N(1)–Cu(1)
C(1)–N(1)–C(4)
C(1)–N(1)–C(5)
C(4)–N(1)–Cu(1)
C(4)–N(1)–C(5)
C(5)–N(1)–Cu(1)
C(2)–N(2)–Cu(1)
C(2)–N(2)–C(3)
C(2)–N(2)–C(20)
C(3)–N(2)–Cu(1)
C(3)–N(2)–C(20)
C(20)–N(2)–Cu(1)
Cu(1)–N(3)–C(21)
Cu(1)–N(3)–C(22)
Cu(1)–N(3)–C(37)
C(21)–N(3)–C(22)
C(21)–N(3)–C(37)
C(22)–N(3)–C(37)
75.0(3)
159.8(3)
98.1(3)
96.6(3)
84.9(3)
162.9(3)
103.7(3)
100.6(3)
90.0(3)
104.8(6)
106.2(7)
113.0(7)
102.3(5)
114.4(7)
115.0(5)
104.0(6)
107.4(8)
113.3(8)
102.2(6)
116.7(8)
111.9(6)
103.3(5)
110.2(5)
114.4(5)
111.1(7)
110.4(7)
107.5(7)
2.3. EPR studies
The X-band EPR spectrum of complex 1 has been recorded in
methanol solution at room temperature and the complex displays
the expected characteristic axial spectrum (Supporting informa-
tion, Fig. S12). Fig. 5 exhibits the X-band EPR spectrum of complex
1 in acetonitrile solvent at room temperature. It is interesting to
note that in acetonitrile solvent the complex displays four distinct
lines, signals characteristic for a square-pyramidal Cu(II) center
2.470(7)
1.490(11)
1.485(11)
1.460(10)
1.460(12)
1.482(12)
1.502(11)
1.484(11)
1.474(11)
1.453(10)
1.347(10)
1.424(12)
1.393(10)
1.415(10)
1.391(12)
1.365(10)
1.379(9)
2
2
having a dx
ground state along with a sharp isotropic radical
ꢀy
signal in the X-band EPR spectrum at room temperature which is
characteristic of a spin triplet state with very weak ferromagnetic
coupling. The Cu(II) signals appear at gave = 2.11 and the sharp iso-
tropic signal is for the organic radical (g = 1.998) (phenoxyl) (Fig. 5)
[27–29]. The broad four line signal for Cu(II) and the radical signal
have been found to disappear with time and in equal proportions,
indicating an immediate reaction between the Cu(II) center and the
phenoxyl radical (Fig. 6). This further supports the formation of
only one phenoxyl radical, though there are two phenolate groups.
Shimazaki et al. have reported that N2O2 type tripodal ligands with
stoichiometric amounts of Cu(II) perchlorate results in a Cu(II) phe-
noxyl radical and Cu(I) in a disproportionation reaction [18,26]. On
1.383(10)
1.343(15)

R.K. Debnath et al. / Polyhedron 51 (2013) 222–227
225
Scheme 1.
Synthetic procedures
N
OH
N
N
NH2
O
(a)
N
+
HCHO
OH
+
(b)
(c)
HN
(a) Ethanol and water in 4:1 ratio
(b) NEt
3 as catalyst
(c) Refluxed for 48 hr at 50 °C
Scheme 2.
the other hand, Stack et al have reported Cu(III) complexes by dis-
proportionation of Cu(II) [30]. Our present observations clearly
indicate the disproportionation through Cu(II)–phenoxyl radical
complex formation. The sharp signal for the phenoxyl radical has
been found to disappear, leaving the characteristic Cu(II) signals,
on addition of an equivalent proportion of another organic radical,
TEMPO (2,2,6,6-tetramethylpiperidinyloxy, a stable nitroxyl radi-
cal) to an acetonitrile solution of complex 1. In methanolic solution
addition of an equivalent proportion of TEMPO shows the presence
of a radical signal, indicating the absence of the phenoxyl radical in
methanol. This further indicates the co-existence of Cu(II) and the
phenoxyl radical in the system. From the single structure of com-
plex 1, the Cu1–O1–C7 bond angle is 123.7(5)° and the angle be-
tween the XY plane and the phenoxyl ring plane is 31.28°; hence
according to the Goodenough-Kanamori rules [23] of exchange
coupling, the ground state of the phenoxyl copper(II) complex 1
is 1 P St P 0.
in acetonitrile solvent (Table S3). The same reaction does not take
place in methanol, even after stirring for 24 h in air at room tem-
perature. However, addition of a few drops of acetonitrile to the
2.4. Galactose oxidase reactivity
A number of substituted benzyl alcohols have been oxidized
into the corresponding aldehydes when stirred with complex 1
Fig. 5. X-band EPR spectrum of complex 1 in acetonitrile.

226
R.K. Debnath et al. / Polyhedron 51 (2013) 222–227
Fig. 6. X-band EPR spectrum of the Cu(II)–phenoxyl radical complex generated from complex 1 in acetonitrile at room temperature. The signals were found to decrease in
intensity rapidly with time. Expanded view of the sharp signal in the inset.
methanolic solution leads to the oxidation. This can be attributed
to the fact that in the presence of acetonitrile the Cu(II–phenoxyl
radical forms, which actively takes part in the oxidation of the pri-
mary alcohols.
complexes. The X-Band Electron Paramagnetic Resonance (EPR)
spectra of complex 1 and of the reaction mixture were recorded
on a JES-FA200 ESR spectrometer, at room temperature as well
as at variable temperatures. Elemental analyses were obtained
from a Perkin Elmer Series II Analyzer. The magnetic moment of
complex 1 was measured on a Cambridge Magnetic Balance.
3. Conclusion
4.2. Crystal structure analysis
The present study demonstrates a new set of Cu(II) complexes
with an N3O-type ligand as a functional model for the GO active
site. It has been found that the Cu(II) centers in the complexes, in
the presence of acetonitrile, undergo reduction with a concomitant
oxidation of the ligand. The oxidised products of the ligand were
isolated and characterized. Spectroscopic studies indicate that this
disproportionation proceeds via the formation of a Cu(II)–phenoxyl
intermediate. Pyridine also exhibits same reaction as acetonitrile,
which indicates the involvement of the exergonic N-donor ligand
for the formation of the Cu(II)–phenoxyl complex. The Cu(II)–phe-
noxyl complex is found to be stable in methanol in the presence of
base. The radical complex also catalyzes primary alcohols to the
corresponding aldehydes. To the best of our knowledge, the pres-
ent study demonstrates the first example of the formation of a
Cu(II)–phenoxyl complex where the paramagnetic centers are very
weakly coupled.
Single crystals were grown by slow diffusion followed by the
slow evaporation technique. The intensity data were collected using
a Bruker SMART APEX-II CCD diffractometer, equipped with fine
focus 1.75 kW sealed tube Mo K
a radiation (k = 0.71073 Å) at
273(3) K, with increasing (width of 0.3° per frame) at a scan speed
x
of 3 s/frame. The SMART software was used for data acquisition. Data
integration and reduction were undertaken with SAINT and XPREP
software [31]. Multi-scan empirical absorption corrections were
applied to the data using the program SADABS [32]. The structures
were solved by direct methods using SHELXS-97 and refined with
full-matrix least squares on F2 using SHELXL-97 [33]. All non-hydrogen
atoms were refined anisotropically. The hydrogen atoms were
located from the difference Fourier maps and refined. Structural
illustrations have been drawn with ORTEP-3 for Windows.
4.3. Synthetic procedures
4. Experimental
4.3.1. Synthetic procedure for the ligand (LH)
4.1. General
To a solution of 2,4-ditert-butylphenol (4.80 g, 0.023 mol) in
EtOH (15 ml) and water (3 ml) was added aqueous formaldehyde
(37%, 3.3 g, 0.041 mol), 1-(2-aminoethyl)piperazine (1.29 g,
0.010 mol) and triethylamine (V = 0.500 ml, 0.004 mol) for a cata-
lyst. The resulting solution was kept in an oil bath (at 50 °C) for
48 h (Scheme 2). The white precipitate that formed was filtered,
washed with cold MeOH and dried under vacuo over P2O5. Yield:
3.845 g (66.52%). The formation of the ligand has been authenti-
cated by its single crystal X-ray structure. An X-ray quality single
crystal of the ligand was grown by slow diffusion of a dichloro-
methane solution of the ligand into hexane followed by slow evap-
oration. 1H NMR (400 MHz), dppm: 1.38 (18H, s), 1.42 (9H, s), 1.44
(9H, s), 2.66 (2H, t), 2.95 (2H, t), 3.73 (2H, s), 4.08 (2H, s), 4.92
(2H, s), 6.80 (1H, s), 6.86 (1H, s), 7.19 (1H, s), 7.23 (1H, s). 13C
All reagents and solvents were purchased from commercial
sources and were of reagent grade. Acetonitrile was distilled from
calcium hydride. Deoxygenation of the solvent and solutions were
affected by repeated vacuum/purge cycles or bubbling with nitro-
gen for 30 min. UV–Vis spectra were recorded on a Perkin Elmer
Lambda-25 spectrophotometer. FT-IR spectra were taken on a
Perkin Elmer spectrophotometer with the samples prepared as
KBr pellets. Solution electrical conductivity was checked using a
Systronic 305 conductivity bridge. 1H NMR spectra were obtained
with a 400 MHz Varian FT spectrometer. Chemical shifts (ppm)
were referenced either with an internal standard (Me4Si) for
organic compounds or to the residual solvent peaks for the copper

R.K. Debnath et al. / Polyhedron 51 (2013) 222–227
227
NMR (100 MHz), dppm: 197.51, 159.25, 149.96, 143.11, 141.77,
137.73, 136.42, 132.02, 128.00, 125.49, 124.93, 122.71, 120.18,
35.17, 34.40, 32.09, 31.82, 31.50, 29.87, 29.47, 29.10, 22.85, 14.30
and elemental Anal. Calc. for C37H59N3O2: C, 77.21; H, 10.06; N,
7.34. Found: C, 77.23; H, 10.07; N, 7.32%. The (m + 1) ion peak at
578.47 in the ESI mass spectrum further supports the formulation
(Supporting Information Figures S1, S2, S3 and S4).
of charge via http://www.ccdc.cam.ac.uk/conts/retrieving.html, or
from the Cambridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB2 1EZ, UK; fax: (+44) 1223 336 033; or e-mail:
deposit@ccdc.cam.ac.uk.
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4.3.2. Synthesis of complex 1 in MeOH
0.370 g (1 mmol) of [CuII(H2O)6](ClO4)2 was dissolved in 10 ml
distilled MeOH. To this solution, 0.577 g (1 mmol) of the ligand
L1 was added slowly with constant stirring. The color of the solu-
tion turned green from light blue. Stirring was continued for 1 h
at room temperature. The volume of the solution was then reduced
to ꢁ2 ml. To this, 10 ml of benzene was added to make a layer on it
and the resulting solution was kept overnight in a freezer. This
resulted in green coloured crystals of complex 1. Yield: 0.594 g
(80.45%). Elemental Anal. Calc. for C39H65ClCuN3O8: C, 58.34; H,
8.16; N, 5.23; O, 15.94. Found: C, 57.93; H, 8.06; N, 5.43; O,
16.07%. Molar conductivity: [KM = 137
X
ꢀ1 cm2 molꢀ1].
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Ltd., London, 1994.
4.3.3. Isolation of L10
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Copper, Chapman and Hall Inc., New York, 1993, p. 447.
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3017.
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1666.
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4263.
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Commun. (1996) 1671.
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Wieghardt, Inorg. Chim. Acta. 297 (2000) 265.
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Wieghardt, Angew. Chem. 37 (1998) 616.
185 mg (0.5 mmol) of [Cu(H2O)6](ClO4)2 were dissolved in 15 ml
acetonitrile in a round bottom flask. 289 mg (0.5 mmol) of the li-
gand HL was added to the flask and stirred for an hour. The transient
greenish solution became orange over the course of the reaction.
The volume of the solution was reduced using a rotavapor and kept
in a freezer overnight. [Cu(CH3CN)4](ClO4) was crystallized out. The
remaining part was then dried and subjected to column chromatog-
raphy. The 3,5-di-tert-butyl-2-hydroxybenzaldehyde was sepa-
rated by hexane. Yield (75%). The amine product, L1 , was eluted
0
with methanol:chloroform (v/v, 1:10) mixture. Yield (70%). 3,5-
DI-tert-butyl-2-hydroxybenzaldehyde: 1H NMR (400 MHz, CDCl3),
dppm: 1.33 (9H, s), 1.43 (9H, s), 7.35 (1H, s), 7.60 (1H, s), 9.86
(1H, s), 11.66 (1H, s); 13C NMR (100 MHz, CDCl3), dppm: 29.46,
31.52, 34.50, 35.21, 120.18, 128.05, 132.09, 95 137.67, 141.81,
159.29, 197.55. 6,8–Di-tert-butyl-3,4-dihydro-3-(2-(piperazin-1-
0
yl)ethyl)-2H-ebenzo[e][1,3]oxazine, L1 , 1H NMR (400 MHz, CDCl3),
dppm: 1.18 (9H, s), 1.27 (9H, s), 2.20 (4H, s), 2.63 (4H, t), 3.58 (4H, s),
3.97 (2H, s), 4.64 (2H, s), 6.69 (1H,s), 7.22 (1H, s); 13CNMR
(100 MHz, CDCl3), dppm: 30.01, 31.80, 34.10, 35.06, 45.17, 49.17,
56.01, 64.03 82.89, 122.18, 123.87, 125.26, 136.18, 140.02, 154.12
(Supporting information Figs. S16–S20).
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Jr., W.B. Tolman, J. Am. Chem. Soc. 119 (1997) 8217.
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(2002) 168.
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vol. 50, John Wiley and Sons Inc., NewYork, 2001, p. 151.
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Acknowledgements
Sokolowski, S. Steenken, T. Weyhermüller, K. Wieghardt, Chem. Eur. J.
(1997) 308.
3
The authors would like to thank CSIR, India for providing a
scholarship to RKD, DST-FIST, India for the X-ray diffractometer
facility, CIF, IIT Guwahati for NMR, Esi-Mass and EPR facilities.
[29] Ademir dos Anjos, A.J. Bortoluzzi, M.S.B. Caro, R.A. Peralta, G.R. Friedermann,
A.S. Mangrich, A. Neves, J. Braz. Chem. Soc. 17 (2006) 540.
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K.O. Hodgson, A. Llobet, T.D.P. Stack, Angew. Chem. 114 (2002) 3117.
[31] SMART, SAINT and XPREP, Siemens Analytical X-ray Instruments Inc., Madison,
Wisconsin, USA, 1995.
[32] G.M. Sheldrick, SADABS
Appendix A. Supplementary material
: software for Empirical Absorption Correction,
University of Gottingen, Institut fur Anorganische Chemieder Universitat,
Tammanstrasse 4, D-3400 Gottingen, Germany, 1999–2003.
[33] G.M. Sheldrick, SHELXS-97, University of Gottingen, Germany, 1997.
CCDC 837285 and 95786 contain the supplementary crystallo-
graphic data for HL and complex 1 These data can be obtained free