(N-Benzyl-bis-N′,N″-salicylidene)-cis-1,3,5-triaminocyclohex ane copper(ii): A novel catalyst for the aerobic oxidation of benzyl alcohol

Article abstract of DOI:10.1039/b512296c

Reaction of Cu(BF₄)₂·6H₂O with the N₃O₂ donor ligand H₂L (where H₂L = N-benzyl-N′,N″-di-tert-butyl-disalicyl-triaminocyclohexane) results in the formation of a novel Cu^II

Full text of DOI:10.1039/b512296c

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PAPER
www.rsc.org/dalton | Dalton Transactions
ꢀ
ꢀꢀ
(
N-Benzyl-bis-N ,N -salicylidene)-cis-1,3,5-triaminocyclohexane copper(II):
a novel catalyst for the aerobic oxidation of benzyl alcohol
a
a
a
a
b
Alison K. Nairn, Stephen J. Archibald, Rajiv Bhalla, Bruce C. Gilbert, Elizabeth J. MacLean,
b
a
Simon J. Teat and Paul H. Walton*
Received 30th August 2005, Accepted 14th October 2005
First published as an Advance Article on the web 28th October 2005
DOI: 10.1039/b512296c
ꢀ
ꢀꢀ
Reaction of Cu(BF
4
)
2
·6H
2
O with the N
3
O
2
donor ligand H
2
L (where H
2
L = N-benzyl-N ,N -di-tert-
II
butyl-disalicyl-triaminocyclohexane) results in the formation of a novel Cu L complex, 1. X-Ray
crystallography of 1 shows the Cu centre coordinated by two phenolate oxygens and two imine
II
˚
nitrogens in a distorted square plane with an elongated bond to the amine nitrogen (2.512 A) in the
axial position. EPR spectroscopy of 1 gives g values of g = 2.100, g = 2.025, and A
= 2.277, g
5.6 mT which are consistent with the distorted square pyramidal coordination environment
determined from the X-ray structure. UV/visible and electrochemical analysis of 1 shows that it
undergoes two reversible processes assigned to the successive oxidation of the phenolate oxygens to
1
2
3
1
=
1
phenoxyl radicals, the first at E 2
1
= 0.89 V (DE = 81 mV, vs. Ag/AgCl) and the second at E
1
2
= 1.13V
(
DE = 84 mV, vs. Ag/AgCl). Chemical oxidation of 1 results in the formation of a species, assigned as
1] which is EPR silent due to antiferromagnetic coupling between the Cu centre and the bound
phenoxyl radical. The oxidised species catalyses the oxidation of benzyl alcohol to benzaldehyde.
+
•
II
[
Introduction
alcohol to benzaldehyde. Such catalytic behaviour suggests that
the complex offers the potential to act as an effective oxidation
catalyst in other reactions.
1
,2
The fungal enzyme galactose oxidase (GOase) catalyses the
selective oxidation of primary alcohols to aldehydes with the
concomitant reduction of O
process requires two electrons and the enzyme does this with the
use of both a Cu /Cu and a tyrosyl/tyrosinate redox couple. In
recent years there has been considerable focus on the preparation
of small molecule models of the active site of GOase. These
complexes are of inherent interest as potential catalysts for
selective oxidation chemistry.
3
2
to H
2
O
2
. The overall oxidation
Results and discussion
I
II
4,5
Preparation of the novel N
3
2
O donor ligand is carried out by a
simple Schiff base condensation reaction of two equivalents of
a 3,5-derivatised salicylaldehyde (to give the phenolate moiety)
6,7
with the novel primary amine, I, to give the ligand H L (N-
2
ꢀ
ꢀꢀ
benzyl-N ,N -di-tert-butyl-disalicyl-triaminocyclohexane, Fig. 1).
We have previously prepared and studied a range of coordina-
tively saturated Fe complexes prepared from N
Derivatisation in these positions of the phenolate group has been
III
8,10
3
O
3
donor ligands
shown to enhance the stability of any subsequent radical species.
based on salicyl derivatives of 1,3,5-cis,cis-triiminocyclohexane
II
Complexation of the H
2
L ligand with Cu (BF
4
)
2
·6H
2
O occurs
8
III
ligands. The Fe complexes exhibit reversible ligand-based
oxidation processes to give bound phenoxyl radical species at
remarkably low oxidation potentials. The complexes are, however,
coordinatively saturated at the metal centre, limiting their ability
to act as oxidation catalysts. We have now developed this ligand
II
readily to give the corresponding Cu L complex (1), in 70%
yield (Fig. 1). 1 is purified by column chromatography on neutral
alumina to give the product as a green complex which can be
crystallised by slow evaporation of an alcohol solution of the
complex.
system further with the preparation of a novel unsymmetrical 1-
amino-3,5-diiminocyclohexane-based N
The X-ray structure of 1 shows a five-coordinate copper(II)
ion bound by two imine nitrogen atoms and the amine from the
tach and two phenolate oxygen atoms giving a distorted square
pyramidal coordination geometry (Fig. 2). The copper to imine
9
3
O
2
ligand. Our objective
is to couple the stable ligand-based redox properties with a redox-
active coordinatively unsaturated metal site (as in GOase), thus
providing a novel chemical system capable of oxidation catalysis.
Herein we describe the preparation and characterisation of a
copper complex using this ligand system, including the EPR, X-
ray structure, UV/visible studies, chemical oxidation and catalytic
studies. We show that the complex catalyses the oxidation of benzyl
˚
nitrogen atom bond lengths [av. 1.991(4) A] and the copper to
˚
phenolate oxygen atom bond lengths [1.936(3) A and 1.994(4)
˚
A] are within the expected ranges and provide the square base
˚
around the copper, the bond to the amine group [2.512(4) A]
is elongated and in the axial position. The sixth coordination
site is unoccupied. Structurally characterised unsymmetrical tach
11
complexes are limited to only a few examples. 1 is the first
a
Department of Chemistry, University of York, Heslington, York, UK YO10
example of a pentacoordinating tach-based ligand.
5
DD. E-mail: phw2@york.ac.uk; Tel: +44 1904 432500
b
The EPR spectrum (77 K) of 1 was recorded in frozen CH Cl
CCLRC Daresbury Laboratory, Daresbury, Warrington, Cheshire, UK
WA4 4AD. E-mail: e.j.maclean@dl.ac.uk; Tel: +44 1925 603193
2
2
solution (Fig. 3). Computer simulation of the spectrum gives
1
7 2 | Dalton Trans., 2006, 172–176
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II
t
Fig. 1 Preparation of the N
3 2
O donor ligand and the corresponding Cu L complex (1). Conditions: (i) 2 equivalents of 3,5-di- Bu-salicylaldehyde,
MeOH; (ii) Cu(BF O, MeOH.
4
)
2
·6H
2
1
13–18
pyramidal environment {a (dx2−y2 ) electronic configuration}.
The results suggest that the solution structure of the complex
is very similar to that found in the solid state structure. When
recording the EPR spectrum of the complex, it was noted that the
appearance of the spectrum changed when a coordinating solvent,
such as methanol, was employed. This suggests that the Cu centre
can accommodate a sixth ligand.
The UV/visible spectrum of 1 in CH
different bands, three of which (230, 258, 326 nm, e = 6000 to
70000 dm mol cm ) are assigned to ligand-based p to p*
2
Cl
2
(Fig. 4) shows five
3
−1
−1
1
charge transfer transitions (in reference to the free ligand). A
fourth band at 378 nm is assigned to a LMCT transition from the
II
3
−1
−1 14,15,17
phenolate to the Cu metal centre (e = 628 dm mol cm ).
3
−1
−1
A fifth band at 594 nm (e = 130 dm mol cm ) is assigned to a
metal-based transition.
12
Fig. 2 ORTEP view (50% probability ellipsoids) of the X-ray crystal
◦
structure of 1. Selected bond lengths ( A˚ ) and angles ( ): Cu(1A)–O(1A)
1
.936(3), Cu(1A)–N(1A) 1.991(4), Cu(1A)–N(3A) 2.512(4), Cu(1A)–
O(2A) 1.994(4), Cu(1A)–N(2A) 1.991(4); O(1A)–Cu(1A)–O(2A)
8
9
8
8
9
8
4.41(15), O(2A)–Cu(1A)–N(1A) 159.06(17), O(2A)–Cu(1A)–N(2A)
0.37(16), O(1A)–Cu(1A)–N(3A) 97.39(14), N(1A)–Cu(1A)–N(3A)
0.21(16) O(1A)–Cu(1A)–O(2A) 84.41(15), O(1A)–Cu(1A)–N(1A)
9.82(16), O(1A)–Cu(1A)–N(2A) 171.99(16), N(1A)–Cu(1A)–N(2A)
7.16(17), O(2A)–Cu(1A)–N(3A) 120.45(15), N(2A)–Cu(1A)–N(3A)
0.01(15).
−
5
3
Fig. 4 UV/visible spectrum of a CH
recorded at room temperature (4 × 10 mol dm inset).
2
Cl
2
solution of 1 (4 × 10 mol dm )
−
4
3
The cyclic voltammogram (CV) of 1 in CH
no electrochemical processes in the negative region (between 0 and
1.8 V vs. Ag/AgCl, not shown). Two reversible processes on the
2
Cl
2
(Fig. 5); shows
−
CV timescale are apparent at positive potentials; the first at E
1
2
=
+
0
at E
.89 V (DE = 81 mV, 0.34 V vs. FeCp
2
/FeCp
2
) and the second
+
Fig. 3 Frozen solution EPR spectrum of 1 recorded at 77 K in CH
—) and simulated spectrum ( · · · · · · ).
2
Cl
2
1
2
= 1.13V (DE = 84 mV, 0.58 V vs. FeCp
2
/FeCp ). These
2
(
processes are observed at slightly lower potentials in MeCN, the
first at E
the second at E
Differential pulse experiments confirm that there is only one
electron involved in each of the two processes.
+
1
2
= 0.80 V (DE = 70 mV, 0.33 V vs. FeCp
2
/FeCp
2
) and
+
parameters of g
1
= 2.277, g
2
= 2.100, g
3
= 2.025, and A
1
=
1
2
= 1.03 V (DE = 80 mV, 0.56 V vs. FeCp
2
/FeCp ).
2
II
1
5.6 mT, values that are typical of a Cu centre coordinated by
three nitrogens and two phenolate oxygens in a distorted square-
This journal is © The Royal Society of Chemistry 2006
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3
−1
−1
(
e = 9860 dm mol cm ) which is assigned to a phenoxyl-
II 8
radical to Cu LMCT in analogy to the electronic spectrum of the
oxidised active form of GOase, which shows a distinctive feature
3
−1
−1
at 445 nm (e ≈ 50 000 dm mol cm ) likely to be due to a tyrosyl-
radical to Cu LMCT band. Parallel EPR experiments on the
oxidised [1] complex show that the addition of the oxidant to
the complex results in the complete loss of the EPR signal. This
loss of EPR activity strongly suggests that the bound phenoxyl-
radical species, which is produced when the complex is oxidised,
II
20
+
•
II
is anti-ferromagnetically coupled to the Cu centre.
We have completed a preliminary study into the aerobic
oxidation of benzyl alcohol to benzaldehyde using 1 as the
catalyst. We have followed the method used by Stack and co-
workers so that the two catalytic systems can be compared
14
directly. Approximately 4% of the substrate was added to the
reaction mixture in the deprotonated form (as the sodium salt,
−
4
−3
Fig. 5 Cyclic voltammogram (298 K) of 1 (1 × 10 mol dm ) in CH
2
Cl
2
PhCH ONa). The oxidation reactions were initiated by the final
2
−
3
−1
II
solution (0.5 mol dm [NBu ]BF ) recorded at 200 mV s
4
4
.
addition of the chemical oxidant, Cu (CF SO ) (1 : 1 ratio to 1), to
3
3 2
all the other components. The reactions were then monitored from
this point onwards by GC, the reactions were run in air at room
8
In reference to previous phenolate–tach complexes, both the
first and second reversible oxidation processes observed in the
CV of 1 are assigned to the production of first one and then a
second phenoxyl-radical. It appears, therefore, that 1 undergoes
two reversible ligand-based, phenolate to phenoxyl-radical redox
processes, and no metal-based processes are observed within the
temperature. Control reactions without the presence of the catalyst
II
show that the chemical oxidant, Cu (CF
3
SO
3
2
) can perform one
pseudo-catalytic turnover. The amount of benzaldehyde produced
II
by the Cu (CF
3
SO
3
2
) complex alone was taken into account when
the turnovers numbers for the oxidation reactions were calculated.
Analysis of the products by gas chromatography confirm the
oxidation of benzyl alcohol to benzaldehyde is catalysed by 1
1
4,15,17
redox range of the experiment.
of redox behaviour was observed with a N
based system prepared by Tolman and co-workers. In this case,
Another example of this type
3
O
2
triazacyclononane-
16
[
average of 44 turnovers in 24 h (∼2% of substrate), 160 turnovers
II
II
in 144 h (∼8% of substrate) at 295 K]. Oxidation of benzyl alcohol
is likely to occur on the coordinated alcoholate anion rather than
directly on the alcohol itself. We note, however, that up to 8%
both the Zn and Cu complexes undergo analogous oxidation
chemistry; the system also incorporates the same di-tert-butyl
substitution pattern around the phenolate ring. Stack and co-
workers also observed similar behaviour from a binapthyl salen-
of the substrate is oxidised, greater than the 4% of PhCH
which is added to the reaction. This shows that whilst PhCH
2
ONa
ONa
II
14,17
type N
2
O
2
-donor Cu complex.
2
is necessary to allow coordination of the substrate to the complex,
benzyl alcohol is the species which is the principal substrate for
the oxidation reaction. Tests carried out in the absence of air show
much reduced oxidation activity indicating that the oxidant of 1
to give the active [1] species is molecular oxygen, although we
cannot rule out the possibility that hydrogen peroxide—which may
be produced as a product of the reaction—also plays an oxidative
role.
The first oxidation process observed for 1 can be carried
II
out chemically by addition of Cu (CF
Ag/AgCl in MeCN, 0.40 V vs. FeCp
3
SO
3
)
2
(E
1
2
= 0.95 V vs.
+
19
2
/FeCp
2
). When the
oxidant is added to a dilute pale green MeCN solution of 1 an
instantaneous colour change to intense dark emerald green is
observed. The UV/visible spectrum of the dark emerald green
+
•
+
•
solution of [1] (Fig. 6) shows a strong absorption band at 390 nm
There are several examples of oxidation catalysts based on
GOase which aerobically oxidise primary alcohols to aldehydes.
t
Copper complexes of thio-bis(2,4-di- Bu-phenol), seleno-bis(2,4-
t
t
di- Bu-phenol), bis(2-hydroxy-3,5-di- Bu-phenyl)amine and bis(2-
t
hydroxy-3,5-di- Bu-phenyl)phenylenediamine by Wieghardt and
co-workers incorporate features seen in the active site of GOase,
such as coordinated phenoxyl radicals, but all but the last are not
6g,21
direct structural models of the active site.
These complexes act
as active catalysts for the aerobic oxidation of a range of different
primary alcohols, the copper complexes which incorporate sulfur,
however, also oxidise secondary alcohols and carry out oxidative
C–C coupling reactions. The complexes incorporating selenium-
based ligands show better selectivity, oxidising benzyl alcohol to
6g
benzaldehyde with 95 turnovers in 24 h.
The activity of 1 is similar to the analogous bis(di-tert-butyl-
II
salicyl-imine)-binaphthyl Cu complex described by Stack and
Fig. 6 UV/visible spectrum of a MeCN solution of the chemically
co-workers which gives 40 turnovers in 20 h under the same
conditions. A bis(2-hydroxy-3,5-di- Bu-phenyl)ethylene-diamine
generated [1] species (1 × 10−4 mol dm ) recorded at room temperature.
+
•
−3
14
t
1
7 4 | Dalton Trans., 2006, 172–176
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copper complex, prepared by Pierre and co-workers, has been
evaluated for the catalytic oxidation of primary alcohols ROH
Synthesis
Preparation of 1.
22
H
2
L (70.1 mg, 0.10 mmol, 1.00 equiv)
to RCHO (where R is Me, Et, Pr and Bu). The complex is
selective for the aerobic oxidation of primary alcohols and is
reported to give a maximum number of turnovers of 30. Later
work by the same group showed that the copper complex of an
was dissolved in MeOH (10 mL) to give an orange solution.
II
Cu (BF
4
)
2
·6H
2
O (34.7 mg, 0.10 mmol, 1.00 equiv) was dissolved
in MeOH (10 mL) to give a very pale blue solution. The two
solutions were added together and an instantaneous colour change
to dark green was observed, the solution was then left to stir for
18 h. The solution was filtered and then evaporated to dryness
to yield a dark green glassy solid which was further dried under
vacuum (53.2 mg, 70%, 0.07 mmol). The complex can be purified
t
unsymmetrical pyridyl-bis(2-hydroxy-3- Bu-phenyl)amine ligand
catalyses the oxidation of benzylalcohol with approximately 300
6d
turnovers in 24 h.
compares favourably with existing aerobic catalysts, and—
1
compared to others—1 is notable for its longevity under catalytic
conditions. Future work will concentrate on the development of
this ligand system and the application of the complexes as catalysts
in a wide range of oxidation processes.
by column chromatography on neutral alumina (product R
.60 in CH OH) to give 1 as green crystals. Elemental analysis:
Cu: calc C 69.87, H 8.21, N 5.43, found C 69.44, H
f
=
0
3
C
45
H
63
N
3
O
4
+
63
+
65
8
.03, N 5.06%. MS (ES): m/z = 773 (M [ Cu]), 775 (M [ Cu]).
−1
IR/cm (KBr pressed disk): 3438 (m, b), 3201 (w), 2958 (s), 2907
Conclusions
(m), 2870 (m), 1645 (m), 1613 (s, –CH=N–), 1533 (w), 1466 (m),
1
7
417 (m), 1362 (m), 1254 (w), 1207 (m), 1156 (s), 1069 (s), 839 (w),
70 (w), 739 (w).
We have demonstrated the design and synthesis of a new Cu(II)
complex of a N
3
O donor ligand based on a bis-salicyl derivative of
2
cis,cis-1,3,5-triaminocyclohexane, using a new synthetic method
to prepare the unsymmetrical ligands. We suggest that this com-
plex exhibits reversible redox chemistry involving the successive
oxidation of the two coordinated phenolate groups to give two
bound phenoxyl radicals. The complex has also been found to
be an active catalyst and is effective for the oxidation of benzyl
alcohol to benzaldehyde.
Crystallography
II
Crystals of Cu L were grown by slow evaporation of a propan-
2
-ol solution of the complex. The X-ray diffraction data were
measured at Station 9.8 of the CCLRC Daresbury Laboratory
Synchrotron Radiation Source using a Bruker AXS SMART
CCD diffractometer, x rotation with narrow frames (synchrotron
radiation, k = 0.6883 A). Crystal structure analysis for (Cu L):
II
˚
C
48
H
70CuN
3
O
5
, M = 832.61, green plates, crystal dimensions
Experimental
0
.07 × 0.06 × 0.02 mm, triclinic, a = 13.2972(12), b = 15.5508(14),
◦ ◦ ◦
˚
General procedures
c = 23.798(2) A, a = 99.359(2) , b = 102.082(2) , c = 104.520(2) ,
3
¯
˚
cell volume = 4536.4(7) A , T = 150(2) K, space group P1, Z =
Solvents for synthesis were supplied by Fisons Ltd., or Fischer
Scientific International Company. Deuterated solvents were sup-
plied by Aldrich Chemical company and Goss Scientific. All other
reagents were supplied by either Aldrich Chemical Company Ltd.,
Lancaster Chemicals Ltd., Avocado Research Chemicals Ltd. or
Fluka Ltd. All the reagents were used without further purification
unless stated otherwise. FT-NMR spectra were recorded using a
JEOL EX270 MHz spectrometer, referencing of the peaks was
carried out using the residual protons in the solvent. Infrared
spectra were recorded using a Mattson Sirius Research Series
FTIR Spectrometer as KBr discs (pressed under 7 tonnes pres-
4
, 34089 reflections collected, 12962 unique, Rint = 0.0806. The
structure was solved using direct-methods in SHELXS and refined
using SHELXL, R1 = 0.0621 (for 8884 reflections with I > 2r(I)),
wR2 = 0.1684 (all data).
CCDC reference number 282729.
For crystallographic data in CIF or other electronic format see
DOI: 10.1039/b512296c
Oxidation chemistry
The catalytic oxidation reactions were carried out in air using 2 mL
of MeCN as a solvent at room temperature. 1,2-Dichlorobenzene
was used as an internal standard. In each experiment to 100% of
st
sure). The data obtained were processed using Microsoft Win 1
software. Mass spectra were recorded on a Fisons Instruments
◦
−3
−3
Autospec. using a 0 to 650 C temperature range. Elemental
PhCH
2
OH (1.94 mol dm ) there was 0.05% of 1 (0.97 mmol dm )
II
analyses were performed at the micro analytical laboratory at the
University of Manchester. EPR spectra were recorded on a JEOL
JES-RE1X Spectrometer (equipped with an X-band Gunn diode).
[and 0.05% of the initiator Cu (CF
5% of 1,2-dichlorobenzene present. 20 lL aliquots of the reaction
were removed after 2, 24, 48, 72 and 144 h and then quenched
3
SO
3
)
2
], 4% of PhCH ONa and
2
Low temperature spectra were recorded with the use of a liquid N
2
with 20 lL of Et
2
O. These small samples were then analysed by
dewar (at 77 K). UV/visible spectra were recorded on a Hitachi
U-3000 spectrometer using 1 cm quartz cells.
GC. Control reactions without the presence of the catalyst show
3
II
that the chemical oxidant, Cu (CF
3
SO
3
2
) can perform one pseudo-
Cyclic voltammetry was performed using a standard three-
electrode configuration with platinum working (0.5 mm diameter
disk) and counter electrodes and a Ag/AgCl reference which gave
catalytic turnover. The volume of benzaldehyde produced by the
II
Cu (CF
3
SO
3
2
) complex alone was taken into account when the
turnover numbers for the oxidation reactions were calculated.
Gas chromatographic experiments were performed on an
AMS94 apparatus fitted with a Alltach Econo-cap Carbowax
column (30 m by 0.25 mm, internal diameter 0.25 lm). The data
+
the FeCp/FeCp couple at 0.55 V (DE = 80 mV) using an EG
&
G potentiostat. All measurements were made in a nitrogen-
−
3
purged solution of either CH
2
Cl
2
/0.5 mol dm [n-Bu
4
N][BF ] or
4
−
3
◦
MeCN/0.2 mol dm [n-Bu
from 50 to 500 mV).
4
N][BF
4
], over a range of scan rates
were obtained using a column and injector temperature of 200 C,
(
the run time for each experiment was 45 minutes. The data were
This journal is © The Royal Society of Chemistry 2006
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processed using the JCL6000 software for Microsoft Windows
version 2.0, revision 26).
9 S. J. Archibald, A. K. Nairn, P. Timmins and P. H. Walton, Tetrahedron
Lett., 2005, 46, 6441–6443.
(
1
0 (a) B. Adam, E. Bill, E. Bothe, B. Goerdt, G. Haselhorst, K. Hildebrand,
A. Sokolowski, S. Steenken, T. Wehyerm u¨ ller and K. Wieghardt,
Chem.-Eur. J., 1997, 3, 308–319; (b) A. Sokolowski, E. Bothe, E. Bill, T.
Wehyerm u¨ ller and K. Wieghardt, Chem. Commun., 1996, 1671–1672;
Acknowledgements
(
c) J. M u¨ ller, T. Wehyerm u¨ ller, E. Bill, P. Hildebrandt, L. Ould-Moussa,
T. Glaser and K. Wieghardt, Angew. Chem., Int. Ed., 1998, 37, 616–
19; (d) A. Sokolowski, J. M u¨ ller, T. Wehyerm u¨ ller, R. Schnepf, P.
This work is kindly supported by the EPRSC. We would like
to thank Dr Vic Young from the University of Minnesota for
preparing suitable files of the twinned crystallographic data to be
refined using his program ROTWIN.
6
Hildebrandt, K. Hildebrand, E. Bothe and K. Wieghardt, J. Am. Chem.
Soc., 1997, 119, 8889–8900.
1
1 (a) L. Cronin, B. Greener, S. P. Foxon, S. L. Heath and P. H. Walton,
Inorg. Chem., 1997, 36, 2594–2600; (b) H.-S. Chong, F. M. Torti, S. V.
Torti and M. W. Brechbiel, J. Org. Chem., 2002, 67, 8072–8078; (c) H.-S.
Chong, S. V. Torti, R. Ma, F. M. Torti and M. W. Brechbiel, J. Med.
Chem., 2004, 47, 5230–5234.
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