Redox chemistry of copper complexes with various salen type ligands

Article abstract of DOI:10.1016/j.ica.2012.08.016

The two essential redox processes in Cu(II) salen (and salan) complexes Cu(II)/Cu(I) and [PhO]/[PhO[-]] were studied by electrochemical and spectroelectrochemical (UV-Vis-absorption or EPR) techniques on a series of complexes in which the salen (salan) type ligands bear various substituents R = H, F, Ph, tBu or CH₃ on the linker C- or N-atoms or on the phenol core. The substitution pattern can be related to the stability of the phenoxy radicals. The geometry of the complexes (especially around the Cu atom) could be established by XRD for the new complex [(Me₂salhexF₄)Cu] {H₂(Me₂salhexF₄) = (1R,2R)-N,N′-bis(2- hydroxy-3,5-di-fluoro-acetophenonylidene)-cyclohexane-1,2-diamine} in the solid and for all other derivatives by EPR in fluid solution. Also, the application of the complexes in oxidation catalysis was tested using the oxidation of benzyl alcohol.

Full text of DOI:10.1016/j.ica.2012.08.016

Inorganica Chimica Acta 394 (2013) 237–246
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Redox chemistry of copper complexes with various salen type ligands
a
b
Katharina Butsch a,1, Thomas Günther b, Axel Klein a, , Kathrin Stirnat , Albrecht Berkessel ,
⇑
Jörg Neudörfl b
a Universität zu Köln, Department für Chemie, Institut für Anorganische Chemie, Greinstrasse 6, D-50939 Köln, Germany
b Universität zu Köln, Department für Chemie, Institut für Organische Chemie, Greinstrasse 4, D-50939 Köln, Germany
a r t i c l e i n f o
a b s t r a c t
The two essential redox processes in Cu(II) salen (and salan) complexes Cu(II)/Cu(I) and [PhO]Å/[PhO[ꢀ]]
were studied by electrochemical and spectroelectrochemical (UV–Vis-absorption or EPR) techniques on a
series of complexes in which the salen (salan) type ligands bear various substituents R = H, F, Ph, tBu or
CH3 on the linker C- or N-atoms or on the phenol core. The substitution pattern can be related to the sta-
bility of the phenoxy radicals. The geometry of the complexes (especially around the Cu atom) could be
established by XRD for the new complex [(Me2salhexF4)Cu] {H2(Me2salhexF4) = (1R,2R)-N,N0-bis(2-
hydroxy-3,5-di-fluoro-acetophenonylidene)-cyclohexane-1,2-diamine} in the solid and for all other
derivatives by EPR in fluid solution. Also, the application of the complexes in oxidation catalysis was
tested using the oxidation of benzyl alcohol.
Article history:
Received 17 April 2012
Received in revised form 11 July 2012
Accepted 20 August 2012
Available online 29 August 2012
Keywords:
Salen ligands
Copper(II)
Electrochemistry
EPR
Ó 2012 Elsevier B.V. All rights reserved.
Spectroelectrochemistry
Alcohol oxidation
1. Introduction
square pyramidal Cu(II) coordination geometry (at pH 7) [22] and
the coordinated tyrosyl group covalently bound to a thioether group.
Salen (2,20-Ethylenebis(nitrilomethylidene)diphenol) (Scheme 1)
and salen type ligands are known to form stable complexes with
various transition metals [1–7]. Many of them have been studied
with regard to their catalytic properties, especially in oxidation
catalysis [5,8–11]. An example of such a reaction is the two electron
oxidation of primary alcohols to the corresponding aldehydes
[1,8,12–16], which is e.g. known in nature from the metalloenzyme
Galactose Oxidase (GO) which occurs in the fungus Polyporus
circinatus [17]. The dehydrogenation of alcohols comprises the trans-
feroftwoelectrons, inGOtheyare providedbytwo cooperatingredox
pairs Cu(II)/Cu(I) and tyrosyl/tyrosinate ([Tyr]Å/[Tyrꢀ]) [18] with the
tyrosyl radical [Tyr]Å directly coordinated to the copper ion. The
alcohol oxidation is coupled to the formation of H2O2 using aerial
oxygen for reoxidation of the enzyme [19]. The electrochemical
potentials of both redox couples in GO are very low (0.16 V vs. NHE
The latter is thought to have a stabilising effect on the radical.
However, the exact way of stabilisation remains still under debate
[23]. In contrast to this, there is far less doubt that the coordination
geometry and (low) coordination number around the Cu(II) ion
facilitates the reduction of Cu(II) to Cu(I) [18,24].
Salen type ligands play an important role in developing func-
tional models of GO [10,12,15,25]. They provide an N2O2 donor
set, similar to that found in GO, with two phenolate O donor func-
tions which are capable of forming (meta)stable ligand radicals.
Additionally, the rigidity of the linker determines the coordination
behaviour towards transition metals, e.g. a rigid linker forces a
square planar binding of the N2O2 donor set.
A large number of Cu salen complexes have been successfully
applied in alcohol oxidation [1,8,12,26–28]. However, not all of
them catalyse alcohol oxidation in a GO like way. Homoleptic com-
plexes of ortho-aminophenols form biradical species, with both
radicals taking part in the reaction and leaving the coordinated
metal ion redox inactive,[1] while Cu-salen complexes with
+
[20] or –0.24 V vs. FeCp2/FeCp2 [21] for the Cu(II)/Cu(I) redox
+
couple and 0.41 V vs. NHE [20] or 0.01 V vs. FeCp2/FeCp2 [21] for
the [Tyr]Å/[Tyrꢀ] redox couple) and the thermodynamic gap between
both processes is small with only 0.25 V. GO owes these low redox
potentials to its peculiar molecular structure, showing a distorted
strongly
r-donating ligands containing amide [29] or imine func-
tions in the linker exhibit a comparably stable Cu(III) state instead
of a ligand centred radical (at least at low temperatures) [30]. In
contrast to this, ‘‘reduced’’ Cu salen complexes (containing ami-
no-functions in the linker; also called ‘‘salan’’) reliably form a
[Cu(II)-salÅ]+ species with a redox active metal ion and a ligand-
centred radical.
⇑
Corresponding author. Fax: +49 2214703933.
E-mail address: axel.klein@uni-koeln.de (A. Klein).
Present address: Stanford University, Department of Chemistry, Stack Lab 333 N-S
1
Axis Stauffer 2 107, Stanford, CA 94305-5080, United States.
0020-1693/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.ica.2012.08.016

238
K. Butsch et al. / Inorganica Chimica Acta 394 (2013) 237–246
different linker groups, containing imine functions as well as sec
linker
and tert amino functions. Some linker groups are further substi-
tuted using small (–CH3) or bulky (–Ph) groups. Each linker con-
tains a cyclohexane ring, thus all salen ligands are relatively
rigid, with all above mentioned consequences.
N
N
OH
HO
The phenol groups of the ligands carry phenyl, fluorine or tBu
substituents. Fluorine and alkyl substituents are expected to sup-
port phenoxyl radical formation by their electron donation ability
(F possesses a +M and a ꢀI effect), while the stabilising ability of
Ph groups is unequivocal, since a +I and a –M effect is attributed
to this group. Furthermore, the substituents exert different sterical
demand (tBu > Ph > F > H). These ligands were used to prepare
Cu(II) complexes. Their synthesis, analysis, spectroscopic and elec-
trochemical characterisation will be reported in detail and the re-
sults of catalytic test reactions (oxidation of benzyl alcohol) will be
discussed in view of these properties.
The synthesis and properties of all three salan ligands (NH)2sal-
hex [37], (NH)2salhexPh2, [9c,38] and (NMe)2salhextBu4 [39] have
been reported before. In contrast to this, from the three salen li-
gands only salhexPh2 [5,37b,38,40] and has been synthesised
and characterised in depths, while Ph2salhex was only mentioned
in a brief report without giving details on the synthesis or analysis
[41], and Me2salhexF4 has not been reported so far.
Cu(II) complexes of the salan ligand (NMe)2salhextBu4 have not
been reported so far, but its derivatives (NH)2salhextBu4
[12b,28,30,42,43], (NH)2salentBu4 and (NMe)2salentBu4 (ethyl
instead of cyclohexyl as spacer) [25,44,45] have been synthesised
and investigated in detail in view of their electrochemical or cata-
lytic properties (oxygenation). For the salan ligand (NH)2salhex Cu
complexes have been only proposed so far, based on catalytic
behaviour of solutions of the ligand and Cu(OAc)2 [37a]. For the
ligand (NH)2salhexPh2 Ti complexes have been reported with
application in epoxidation catalysis, but no Cu derivatives
[9c,38]. For the salen ligand salhexPh2 Ti complexes are considered
as active catalysts in epoxidation reactions, however, no complexes
have been isolated so far [5,38,40]. Thus, in this contribution we
report for the first time the Cu(II) complexes of the salen and salan
ligands depicted in Scheme 2. All complexes have been character-
ised thoroughly by EPR spectroscopy, electrochemical and spectro-
electrochemical methods. Also preliminary catalytic test reactions
phenol
Scheme 1. Salen (2,20-Ethylenebis(nitrilomethylidene)diphenol).
Depending on the flexibility of the linker group, the coordina-
tion geometry of complexes with salen type ligands varies from
square planar to tetrahedrally distorted [12]. Generally, a tetrahe-
dral distortion of the complexes makes the complexes more
GO-like in that the distortion has a strong impact on the redox po-
tential of the copper ion (i.e. stabilising the Cu(I) state) and
supports overlapping of the molecular orbitals of the phenol ring
with the singly occupied Cu dx2ꢀy2 orbital. Such overlap results in
an anti-ferromagnetic coupling of the odd electrons [31], such as
in GO [27,32]. In contrast, Cu salen complexes with rigid linker
groups exhibit ferromagnetic coupling (S = 1), in line with the
Goodenough-Kanamori-Anderson rules [33]. In previous investiga-
tions it was found that anti-ferromagnetic spin coupling and
catalytic activity of Cu salen complexes go hand in hand [12].
It is well established, that the stability of the salen type phe-
noxyl radicals depends on the presence of stabilising substituents
in ortho- and para-position to the phenol OH group [25,34]. Very
often these substituents were bulky groups such as tBu, SiPr or
SPh [12,27]. Non-stabilised radical species have been reported to
lack any activity in alcohol oxidation [12]. However, under basic
condition catalytic alcohol oxidation has been observed for Cu
complexes with un-substituted salen ligands [16] and is has been
recognised that also the stabilisation of the Cu(I) species might
be crucial for the catalytic activity [28]. Substitution of the salen-
linker may have an impact on the stability of the phenoxyl radicals
and on the catalytic activity, since the rigidity and bulkiness of the
linker groups determine the geometry (and flexibility) around the
Cu atom [1,12,25,27,34a,35,36].
In this work, copper complexes of various chiral (R,R)-cyclo-
hexyl salen type ligands were examined (Scheme 2). They are char-
acterised by different ortho and para positioned substituents and
OH HO
HO
OH
N
N
NH HN
Ph
Ph
H2((NH)2salhex)
H2(Ph2salhex)
Ph
Bu
Ph
Ph
Ph
HO
HN
OH HO
OH
NH
N
N
H2((NH)2salhexPh2)
H2(salhexPh2)
F
t
Bu
F
F
t
t
Bu
HO
HO
N
OH
N
OH
N
t
Bu
F
N
H3C
CH3
H3C
CH3
H2(Me2salhexF4)
H2((NMe)2salhextBu4)
Scheme 2. Salen type ligands used in this study and their abbreviations.

K. Butsch et al. / Inorganica Chimica Acta 394 (2013) 237–246
239
are reported. The spectroscopic and electrochemical properties are
compared to those of Cu(II) complexes of related salen or salan
ligands such as [(salen)Cu] [46–50], [(salhex)Cu] [16,51],
[(salhextBu4)Cu], [12b,28,30,42,45,51,52] [(salentBu4)Cu], [45,48]
[((NMe)2salentBu4)Cu], [44] [((NH)2salhextBu4)Cu] [28,30,42,43],
[((NH)2salentBu4)Cu] [45] or [((NH)2salhexOEt)Cu] [9b] which
have been previously investigated in view of their structures, elec-
trochemistry, spectroscopy and catalytic properties.
product yields were determined by integration of the aldehyde
proton.
2.4. Materials and procedures
2-Hydroxybiphenyl-3-carbaldehyde [57] and the ligands H2-
(salhexPh2) [37b], H2((NMe)2salhextBu4) [39a,b] and H2((NH)2sal-
hex) [37b] were synthesised following published methods. All
reactions were carried out in oven-dried or flame-dried round bot-
tom flasks and the reactions were conducted under a positive pres-
sure of argon, unless otherwise stated. Stainless steel syringes or
cannulae were used to transfer air- and/or moisture-sensitive
liquids.
2. Experimental
2.1. Instrumentation
NMR spectra were recorded on a Bruker Avance II 300 MHz or a
Bruker DPX300 spectrometer, using a triple resonance 1H, nBB in-
verse probe. Unambiguous assignments of 1H and 13C resonances
was obtained from 1H NOESY, 1H COSY, gradient selected 1H, 13C
HSQC and HMBC experiments. All 2D NMR experiments were per-
formed using standard pulse sequences from the Bruker pulse pro-
gram library. Chemical shifts are stated relative to TMS. Infrared
(IR) spectra were recorded on a Perkin-Elmer Paragon 1000 FT-IR
spectrometer with ATR technique. Data are reported as: wavenum-
ber of absorption (cmꢀ1), intensity of absorption (s = strong,
2.4.1. (1R,2R)-N,N0-bis(2-hydroxybenzophenonylidene)cyclohexane-
1,2-diamine H2(Ph2salhex)
To
a
solution of (1R,2R)-(ꢀ)-trans-cyclohexane-1,2-diamine
(257 mg, 2.25 mmol, 1 eq) in anhydrous ethanol (8 mL) was added
2-hydroxybenzophenone (892 mg, 4.50 mmol, 2 eq). The resulting
yellow solution was refluxed for 48 h at 80 °C. After cooling to
ambient temperature, water (16 mL) was added whereupon a yel-
low precipitate was formed. The reaction mixture was stirred for
1 h and the precipitate was collected by suction filtration, washed
with small portions of cold ethanol and dried in vacuo to yield a
yellow powder (928 mg, 1.95 mmol, yield 87%). Mp = 86 °C;
m = medium, w = weak). HRMS data (Du = 0.002 u) and ESI-MS
analyses were recorded on a Finnigan MAT 900S instrument. UV–
Vis/NIR absorption spectra were measured on Varian Cary50 Scan
or Shimadzu UV-3600 photo spectrometers.
m
max(ATR)/cmꢀ1 2929, 2856, 2160, 2029, 1977, 1604, 1571, 1492,
1442, 1300, 1253, 1147, 964, 827, 752, 719, 693, 646; dH(300 MHz;
CDCl3) 1.12–1.18 (2 H, m), 1.43–1.46 (2 H, m), 1.63–1.71 (4 H, m),
3.49–3.52 (2 H, m), 6.59–6.64 (2 H, m), 6.70–6.73 (2 H, m), 6.98–
7.01 (2 H, m), 7.13–7.15 (2 H, m), 7.23–7.28 (2 H, m), 7.32–7.35
(2 H, m), 7.48–7.52 (4 H, m), 7.61–7.65 (2 H, m), 15.29 (2 H,
s(br)); dC(75 MHz; CDCl3) 23.7 (t, CH2), 32.4 (t, CH2), 64.7 (d, CH),
117.3 (d, CH), 117.7 (d, CH), 119.9 (s, Cq), 126.7 (d, CH), 128.1 (d,
CH), 128.3 (d, CH), 128.8 (d, CH), 129.1 (d, CH), 131.8 (d, CH),
132.2 (d, CH), 133.8 (s, Cq), 162.8 (s, Cq), 173.6 (s, Cq); m/z (ESI-
MS) 475 ([M+H]+. C32H30N2O2 requires 475), 497 ([M+Na]+) and
475 ([M+K]+); (HRꢀMS, ESI) 475.235 ([M+H]+ C32H30N2O2 requires
475.235).
Elemental analyses were carried out using a HEKAtech CHNS
EuroEA 3000 Analyzer. EPR spectra were recorded in the X-band
on a Bruker System ELEXSYS 500E equipped with a Bruker Variable
Temperature Unit ER 4131VT (500–100 K or an Oxford Instruments
helium-cryostat (300–4 K); the g values were calibrated using a
dpph sample. Simulation of the EPR spectra were performed using
the Bruker SimFonia. Electrochemical experiments were carried
out in 0.1 M nBu4NPF6 solutions using a three-electrode configura-
tion (glassy carbon working electrode, Pt counter electrode, Ag/
AgCl pseudo reference) and an Autolab PGSTAT30 potentiostat
and function generator. The ferrocene/ferrocenium couple (FeCp2/
FeCp2+) served as internal reference. UV–Vis spectroelectrochemi-
cal measurements were performed with an optically transparent
thin-layer electrochemical (OTTLE) cell [53].
2.4.2. (1R,2R)-N,N0-Bis(2-hydroxy-3,5-di-fluoro-acetophenonylidene)-
cyclohexane-1,2-diamine H2(Me2salhexF4)
To
a
solution of (1R,2R)-(ꢀ)-trans-cyclohexane-1,2-diamine
(299 mg, 2.62 mmol, 1 eq) in anhydrous ethanol (15 mL) was
added 1-(3,5-difluoro-2-hydroxy-phenyl)-ethan-1-one (900 mg,
5.23 mmol, 2 eq). The resulting yellow solution was refluxed for
18 h. After cooling to ambient temperature, water (8 mL) was
added whereupon a yellow precipitate was formed. The reaction
mixture was stirred for 1 h at 298 K and the precipitate was col-
lected by suction filtration, washed with water and small portions
of cold ethanol and dried in vacuo to yield a yellow powder
2.2. Crystal structure determination
The measurements were performed at 293(2) K using graphite-
monochromatised Mo K
a radiation (k = 0.71073 Å) on IPDS II
(STOE and Cie.). The structures were solved by direct methods
using SHELX-97 and WinGX (SHELXS-97) [54] and refined by full-ma-
trix least-squares techniques against F2
(SHELXL-97) [55]. The
(874 mg, 2.07 mmol, yield 79%). Mp = 154 °C;
m
max(ATR)/cmꢀ1
numerical absorption corrections (X-RED V1.22; Stoe & Cie, 2001)
were performed after optimising the crystal shapes using X-SHAPE
V1.06 (Stoe & Cie, 1999) [56]. The non-hydrogen atoms were re-
fined with anisotropic displacement parameters. H atoms were in-
cluded by using appropriate riding models.
3043, 2939, 2160, 1595, 1479, 1448, 1350, 1269, 1116, 844, 800,
584; dH(300 MHz; CDCl3) 1.46–1.53 (2 H, m), 1.67–1.53 (2 H, m),
1.91–1.99 (4 H, m), 2.29 (6 H, s), 3.88–3.91 (2 H, m), 6.86–6.92
(4 H, m); dC(75 MHz; CDCl3) 14.8 (q, CH3), 24.0 (t, CH2), 32.1 (t,
CH2), 62.9 (d, CH), 107.5 (d, CH), 108.6 (d, CH), 118.7 (s, Cq),
150.4 (s Cq), 153.6 (s, Cq), 153.7 (s, Cq); 170.9 (s, Cq); m/z (ESI-
MS) 423 ([M+H]+ C22H22F4N2O2 requires 423), 445 ([M+Na]+) and
461 ([M+K]+); (HR-MS, ESI) 423.169 ([M+H]+ C22H22F4N2O2
requires 423.169).
2.3. Catalytic test reactions
A solution of 116 mg (0.23 mmol, 0.5 eq) [(Cu(OTf))2(l-tolu-
ene)] with 0.9 mmol (2.0 eq) ligand in 0.5 mL pure MeCN was pre-
pared. 0.5 g benzyl alcohol was mixed with 180 mg powdered
NaOH. Both mixtures were combined and vigorously stirred at
298 K. Samples of the reaction mixtures were taken, mixed with
CD2Cl2 upon which a green-brown precipitate was formed. The
remaining solution was isolated and submitted to NMR spectro-
scopic analysis. 1H NMR spectra (300 MHz) were recorded and
2.4.3. (1R,2R)-N,N0-Bis(3-phenylsalicyl)cyclohexane-1,2-diamine
H2((NH)2salhexPh2)
To a solution of (1R,2R)-N,N0-bis(3-phenylsalicylidene)cyclo-
hexane-1,2-diamine (2.0 g, 4.22 mmol, 1 eq) in methanol was
added sodium borohydride (479 mg, 12.7 mmol, 3 eq) in small

240
K. Butsch et al. / Inorganica Chimica Acta 394 (2013) 237–246
portions. The colourless solution was stirred for 3 h at 298 K. Water
(20 mL) was added and the mixture was extracted three times
using ethyl acetate. The combined organic layers were washed
with brine and dried over anhydrous magnesium sulphate. The sol-
vent was evaporated under reduced pressure to yield a colourless
70%). Anal. Calc. for C32H32ZnO2N2: C, 70.23; H, 5.99; N,
5.24%Found: C, 70.43; H, 5.95; N, 5.27%. dH(300 MHz; C3D6O)
1.00 (4 H, m), 1.74 (4 H, m), 2.51 (2 H, m), 3.37 (2 H, s(br)), 3.98
(2 H, s(br)), 6.46 (2 H, s(br)), 6.81 (2 H, s(br)), 7.07 (4 H, s(br)),
7.41 (4 H, m), 7.57 (2 H, s(br)) 7.66 (2 H, s(br)).
powder (1.53 g, 3.11 mmol, yield 74%). Mp = 78 °C;
mmax(ATR)/
cmꢀ1 3298, 2927, 2854, 2360, 1591, 1498, 1460, 1427, 1230,
1087, 1028, 918, 827, 754, 696; dH(300 MHz; CDCl3) 1.22–1.26 (4
H, m), 1.69–1.71 (2 H, m), 2.14–2.18 (2 H, m), 2.45–2.47 (2 H,
m), 3.95 (2 H, d, J 14.0), 4.07 (2 H, d, J 14.0), 6.86 (2 H, t, J 7.6),
6.95 (2 H, d, J 7.6), 7.26–7.33 (4 H, m), 7.40 (4 H, t, J 7.6), 7.58 (4
H, d, J 7.6); dC(75 MHz; CDCl3) 24.1 (t, CH2), 30.3 (t, CH2), 49.6 (t,
CH2-N), 59.5 (d, CH), 119.2 (d, CH), 123.3 (s, Cq), 126.7 (d, CH),
127.6 (d, CH), 128.0 (d, CH), 129.0 (s, Cq), 129.3 (d, CH), 129.8 (d,
CH), 138.3 (s, Cq), 154.9 (s, Cq); m/z (ESI-MS) 479 ([M+H]+
C32H34N2O2 requires 478.62) and 501 ([M+Na]+); (HR-MS, ESI)
479.269 ([M+H]+ C32H34N2O2 requires 479.269).
3. Results and discussion
3.1. Synthesis and structure of the salen ligands and their copper
complexes
While the ligands H2((NH)2salhex) [37], H2((NH)2salhexPh2)
[9c,38], H2((NMe)2salhextBu4), [39] and H2(salhexPh2) [5,37b,38,
40], have been reported before, for H2(Ph2salhex) [41] only a tita-
nium complex is described in the literature (thus the synthesis of
H2(Ph2salhex) is reported herein). H2(Me2salhexF4) is a new salen
ligand and its synthesis is outlined in the Experimental part as well
as the synthesis of H2((NH)2salhexPh2) which was obtained
through a modified synthesis. All ligands were characterised by ele-
mental analysis, NMR, UV/Vis absorption spectroscopy, and cyclic
voltammetry (CV). The ligands H2((NH)2salhex) and H2(Me2sal-
hexF4) were obtained as single crystals by recrystallisation from
methanol and were analysed by XRD. Both ligands crystallise in
the orthorhombic space group P212121 and both molecules (Fig. 1
and Supplementary material) do not provide the O,N,N,O binding
pocket; one phenol ring lies above and one beneath the cyclohex-
ane ring plane. Details on the crystal structure solution and refine-
ment as well as graphics of the crystal packing can be found in the
Supplementary material.
The Cu complexes were prepared by reacting the ligands and
Cu(OAc)2 in 1:1 stoichiometry in methanol solution. The com-
plexes [((NMe)2salhextBu4)Cu] (green), [((NH)2salhex)Cu] (dark
green), [((NH)2salhexPh2)Cu] (green), [(Me2salhexF4)Cu] (green),
[(Ph2salhex)Cu] (brown) and [(salhexPh2)Cu] (brown) were ob-
tained in moderate to high yields (79–33%) and in high purity as
inferred from elemental analyses. For comparison, also the zinc
complex [((NH)2salhexPh2)Zn] was prepared and included into
our studies.
2.4.4. General Procedure for the synthesis of salen type complexes
1 eq ligand and 1 eq Cu(OAc)2ꢁH2O (or Zn(OAc)2ꢁ2H2O) were
dissolved in methanol and both solutions were mixed at 298 K.
The mixture was stirred at 298 K for 16 h. While [((NH2)sal-
hexPh2)Cu] and [(salhexPh2)Cu] precipitated and were filtered off
and washed with small portions of methanol, all other complexes
were isolated by removing the solvent under vacuum and recrys-
tallisation from acetone solution.
2.4.4.1. [((NMe2)salhextBu4)Cu]. Fifty eight milligram (0.1 mmol)
H2((NMe2)salhextBu4) and 20 mg (0.1 mmol) Cu(OAc)2ꢁH2O were
reacted to yield dark green needle shaped crystals (24 mg,
0.04 mmol, yield 40%). Anal. Calc. for C38H60CuO2N2: C, 71.26; H,
9.44; N, 4.37%. Found: C, 71.25; H, 9.45; N, 4.34%.
2.4.4.2. [((NH)2salhex)Cu]. Thirty ove milligran (0.1 mmol) H2-
((NH)2salhex) and 20 mg (0.1 mmol) Cu(OAc)2ꢁH2O were reacted
to yield reddish-brown crystalline needles (26 mg, 0.07 mmol,
yield 70%). Anal. Calc. for C20H24CuO2N2: C, 61.92; H, 6.24; N,
7.22%. Found: C, 62.01; H, 6.28; N, 7.22%.
The complex [(Me2salhexF4)Cu] could be crystallised from
methanol and single crystals suitable for XRD were obtained. The
crystal structure was solved and refined in the monoclinic space
group C2 (Fig. 1, and Supplementary material). In the crystal, the
copper complex molecules are stacked along the c axis with large
Cuꢁ ꢁ ꢁCu distances of 3.820(1) Å and 3.865(1) Å. No intermolecular
interaction such as hydrogen bridges were found in the crystal
structure. The stacked molecules leave tunnels in the structure
along the c axis. The tunnels have ellipsoid cross-sections and were
‘‘coated’’ with fluorine atoms. The observed residual electron den-
sity found in these tunnels was assigned to small amounts of sol-
vent molecules and the solvent correction tool of Platon (SWAT)
was used during refinement. Assignment and refinement of the
solvent molecules was impossible due to statistic distribution of
these molecules. In the two independent molecules, the geometry
2.4.4.3. [((NH2)salhexPh2)Cu]. Hundred milligram (0.2 mmol) H2-
((NH2)salhexPh2) and 40 mg (0.2 mmol) Cu(OAc)2ꢁH2O were re-
acted to yield a blue-green powder (45 mg, 0.08 mmol, yield
40%). Anal.Calc. for C32H32CuO2N2: C, 71.15; H, 5.97; N, 5.19%.
Found: C, 71.22; H, 5.99; N, 5.22%.
2.4.4.4. [(Me2salhexF4)Cu]. Fourty milligram (0.1 mmol) H2(Me2sal-
hexF4) and 20 mg (0.1 mmol) Cu(OAc)2ꢁH2O were reacted to yield
dark green crystals (36 mg, 0.075 mmol, yield 75%). Anal. Calc. for
C22H20CuO2N2F4: C, 54.60; H, 4.17; N, 5.79%. Found: C, 54.62; H,
4.14; N, 5.77%.
2.4.4.5. [(Ph2salhex)Cu]. Fifty five milligram (0.12 mmol, 1 eq) H2-
(Ph2salhex) and 24 mg (0.12 mmol, 1 eq) Cu(OAc)2ꢁH2O were re-
acted to yield a brown powder (20 mg, 0.04 mmol, yield 33%).
Anal. Calc. for C32H28CuO2N2: C, 71.69; H, 5.26; N, 5.23%. Found:
C, 71.75; H, 5.24; N, 5.24%.
around the Cu atoms is square planar (
R of angles around
Cu = 363.9(3) or 360.1(3)°, respectively) and also with negligible
distortion in the linker group. The Cu–O distances are 1.859(6)
(molecule 1) Å 1.834(7) and 1.855(5) Å (molecule 2) respectively
which is common for copper coordinated to phenolate groups
[9b,45,47,51,58,59].
2.4.4.6. [(salhexPh2)Cu]. Hundred milligram (0.21 mmol) H2(sal-
hexPh2) and 42 mg (0.21 mmol) Cu(OAc)2ꢁH2O were reacted to
yield a light brown powder (86 mg, 0.16 mmol, yield 76%). Anal.
Calc. for C32H28CuO2N2: C, 71.69; H, 5.26; N, 5.23%. Found: C,
71.62; H, 5.28; N, 5.28%.
3.2. EPR spectroscopy
The (parent) CuII complexes were analysed by EPR spectroscopy
from solid samples (microcrystalline powder) or MeCN solutions at
298 K or from glassy frozen Me-THF solutions at 115 K. Fig. 2
shows representative spectra and Table 1 lists the obtained data.
2.4.4.7. [((NH2)salhexPh2)Zn]. Hundred milligram (0.2 mmol) H2-
((NH2)salhexPh2) and 44 mg (0.2 mmol) Zn(OAc)2ꢁ2H2O were re-
acted to yield an off-white powder (74 mg, 0.14 mmol, yield

K. Butsch et al. / Inorganica Chimica Acta 394 (2013) 237–246
241
Fig. 1. ORTEP representation (50% probability level) of the molecular structure of H2(Me2salhexF4) (left) and [(Me2salhexF4)Cu] (one of the two independent molecules;
right); except for OH all H atoms were omitted for clarity.
Again, the ACu|| is smallest for the fluorine substituted complex,
while the AN\ is comparably large. Also, we find slightly larger
ACu|| values for the salan vs. the salen derivatives and slightly smal-
exp
ler values in DMF compared to Me-THF solutions, both in line with
earlier investigation [45,49]. Generally however, the obtained g
and A parameters do not vary markedly within the investigated
sim
series and the values largely agree with those of related salen or
salan Cu(II) complexes [28,45,49,58]. Importantly, for the behav-
iour of the complexes in solution we can state that regardless of
the solvent, the complexes retain largely their square planar geom-
etry observed in the solid state, which can be inferred from the
g||/A|| ratio which all lie around 100 cm for the salan and around
95 cm for the salen complexes (largely distorted systems exhibit
values up to 250 cm) [45,60]. Also, the solvents do not seem to
coordinate (in the axial position).
2700 2800 2900 3000 3100 3200 3300 3400
Field / G
Fig. 2. X-band EPR spectra of [(Me2salhexF4)Cu] and recorded in glassy frozen DMF
solution at 110 K with simulation below.
3.3. Electrochemical properties
X-band EPR spectra recorded at 298 K on microcrystalline sam-
ples showed broad, largely featureless resonances, while spectra in
MeCN solution (at 298 K) showed the four line pattern correspond-
ing to the hyper fine (ACu) coupling of the unpaired electron with
the nuclear spin of the 63/65Cu isotopes (I = 3/2, 69 and 31% nat.
abundance). The values for ACu were simulated and range from
82 to 90 G, which is typical for Cu(II) complexes of salen type
ligands [28,45,49,58], the smallest value was observed for the fluo-
rine substituted complex [(Me2salhexF4)Cu] in line with previous
findings [49]. The axial spectra recorded in glassy frozen Me-THF
or DMF solutions show large ACu for the g|| component, while the
g\ component reveals far smaller ACu but frequently resolved AN.
Electrochemical investigations were carried out on the free
ligands and on the copper complexes. Fig. 3 shows representative
examples and Table 2 lists the collected data (complete data in
the Supplementary material).
The free ligands exhibit irreversible oxidations in the range of
+0.27 V–+0.84 V, except for (NMe)2salhextBu4 (partly reversible
with a peak current ratio of 0.37). On cathodic scans irreversible
reductions at potentials lower than ꢀ2.2 V were observed.
On first view, the Cu complexes exhibit similar oxidation behav-
iour as the corresponding ligands. However, the oxidation waves
were markedly shifted to lower potentials and exhibit a far higher
Table 1
X-band EPR data of the salen Cu complexes.a
Salen ligand
giso or
g|| or g3
g\ g1,2
or
D
g
ACu A||/A\
AN\
Conditions
av.
(NMe)2salhextBu4
(NH)salhexPh2
(NH)2salhex
Ph2salhex
Me2salhexF4
salhexph2
(NMe)2salhextBu4
(NH)2salhex
Ph2salhex
Me2salhexF4
(NMe)2salhextBu4
(NH)2salhexPh2
(NH)2salhex
Me2salhexF4
salhexph2
2.107
2.093
2.090
2.106
2.105
2.107
2.102
2.096
2.108
2.102
2.115
2.109
2.101
2.107
2.103
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
87
87
85
87
82
88
194/31
195/34
186/33
181/22
184/22
182/33
183/31
177/25
194/24
–
–
–
–
–
–
11
15
16
15
11
12
13
15
15
MeCN/298 K
MeCN/298 K
MeCN/298 K
MeCN/298 K
MeCN/298 K
MeCN/298 K
Me-THF/115 K
Me-THF/115 K
Me-THF/115 K
Me-THF/115 K
DMF/110 K
2.221
2.211
2.223
2.228
2.236
2.231
2.227
2.236
2.213
2.048
2.038
2.051
2.039
2.055
2.048
2.038
2.043
2.048
0.173
0.173
0.185
0.189
0.181
0.183
0.189
0.193
0.165
DMF/110 K
DMF/110 K
DMF/110 K
DMF/110 K
a
Measured either in MeCN solution at 298 K or in glassy frozen Me-THF or DMF solution at 115 or 110 K. gav = averaged g value = (g1 + g2 + g3)/3 or (g|| + 2g\)/3;
or g|| – g\
Dg = g3–g1
.

242
K. Butsch et al. / Inorganica Chimica Acta 394 (2013) 237–246
ERed2
ERed1
EOx1
EOx2
EOx1
EOx2
1 µA
2 µA
-1.5
1.0
0.5
0.0
-0.5
0/+
-1.0
0.5
0.0
-0.5
0/+
E / V vs. FeCp2
E / V vs. FeCp2
Fig. 3. Cyclic voltammogramms of [(Me2salhexF4)Cu] (left) and [((NMe)2salhextBu4)Cu] (right) in MeCN/nBu4NPF6 solution at 298 K at 100 mV/s scan rate.
Table 2
Electrochemical data of salen Cu complexes [(ligand)Cu].a
Ligand
Eox2 (ligand)
Eox1 (ligand)
D
E
Ered1 Cu(II)/Cu(I)
Solvent (ref.)
ox2ꢀox1
(NMe)2salhextBu4
(NH)2salhextBu4
salhextbu4
salentbu4
Salen
0.35 (rev)
0.21 (rev)
0.65 (rev)
–
0.15 (rev)
0.08 (rev)
0.45 (rev)
0.42 (rev)
0.57 (irr)
0.38 (irr)
0.26 (irr)
0.60 (rev)
0.44 (p.rev)b
0.46 (irr)
0.20
0.13
0.20
–
–
–
–
–
–
–
ꢀ1.76 (rev)
<–1.5
<–1.5
ꢀ1.63 (rev)
ꢀ1.63 (rev)
ꢀ1.71 (rev)
ꢀ1.67 (rev)
ꢀ1.26 (rev)
ꢀ1.58 (rev)
ꢀ1.40 (rev)
MeCN/this work
CH2Cl2/ref. [28]
CH2Cl2/ref. [28]
DMF/ref. [48]
–
DMF/ref. [48]
(NH)2salhex
(NH)2salhexPh2
Me2salhexF4
Ph2salhex
0.91 (irr)
0.53 (irr)
0.88 (p.rev)b
0.84 (irr)
0.81 (irr)
MeCN/this work
MeCN/this work
MeCN/this work
MeCN/this work
DMF/this work
c
salhexph2
a
From cyclic voltammetry measured in MeCN/nBu4NPF6 at 298 K at 100 mV/s scan rate; Half-wave potentials E1/2 for reversible waves (rev), cathodic peak potentials Epc
+
for irreversible reductions (irr) in V vs. FeCp2/FeCp2
.
b
Partly reversible, 0.37 < peak current ratio < 0.92.
Virtually not soluble in MeCN.
c
Table 3
completely dried solvents (DMF/MeCN vs. H2O).[36,45,46,48] The
waves at more negative potentials (e.g. Ered2 in Fig. 3) are assigned
to ligand-centred reduction processes which lead to de-coordina-
tion of the ligand.[46] Consequently, at very negative potentials
the reduction waves of the uncoordinated salen2– can be observed
(E < ꢀ2.5 V).
UV–Vis Absorption data of the salen copper complexesa.
Complexes
k
[((NMe)2salhextBu4)Cu] 255 (9690), 297
434
623
(510)
595
(190)
611
(450)
613
(140)
616
(120)
(6570)
246 (8940), 278
(7030)
232 (8320), 318
(9330)
(820)
385
(680)
394
[((NH)2salhex)Cu]
[((NH)2salhexPh2)Cu]
[(Me2salhexF4)Cu]
[(Ph2salhex)Cu]
Within the series of our complexes the 3,5-tBu substitution sta-
bilises the Cu(II) phenoxy radical complexes as expected, while 3-
Ph substitution as in [(salhexPh2)Cu] does not provide stability.
Also, when comparing our cyclohexyl-bridged systems with ethy-
lenediamine derivatives (‘‘salen’’ in Table 2) no marked differences
were observed, the same is true when imine (salen) and amine (sal-
an) type of systems were compared. More surprising is, that the
imine-Ph substituted complex [(Ph2salhex)Cu] exhibits a partially
reversible first oxidation at rather low potential (for CVs see Sup-
plementary material). Not surprising is the completely reversible
system [(Me2salhexF4)Cu] containing a 3,5-F4-substitution pattern,
since the stabilising role of the 3,5-tBu groups lies mainly in ham-
pering follow-reactions of phenoxyl radicals [34,52]. The F substit-
uents very probably contribute in the same way to the radical
stability, although it cannot be excluded, that the imine CH3 substi-
tuent might play an additional stabilising role. As a consequence for
the presence of the electron-demanding F substituents, the oxida-
tion potentials are quite high for this complex.
(910)
264 (50770)
369
(6270)
352
257 (37740)
262 (26910)
(1970)
[(salhexPh2)Cu]
a
Absorption maxima
k in nm; extinction coefficients e
in L molꢀ1 cmꢀ1 in
parentheses, measured in MeCN.
degree of reversibility. Only for the complexes [((NH)2salhex)Cu],
[((NH)2salhexPh2)Cu] and [(salhexPh2)Cu] and for the Zn derivative
[((NH)2salhexPh2)Zn] (0.30 and 0.55 V) the oxidations are com-
pletely irreversible. On the cathodic scan markedly broadened,
but essentially reversible reduction waves were observed for all
Cu complexes while the Zn complex [((NH)2salhexPh2)Zn] does
not exhibit any reduction wave up to potentials lower than ꢀ2.5 V.
Based on comparison with previous work (Table 2)
[12,25,34b,46,48,50] we can assign the oxidations to phenolate-li-
gand centred processes (PhO–/PhOÅ), while the reduction processes
correspond to the Cu(II)/Cu(I) couple. The partly irreversible char-
acter of the latter and the broadening of the waves are in line with
earlier investigations and might be due to ligand exchange in not
Finally, it should be noted, that comparison of our results with
reported data is delicate in case where different solvents have been
employed, since it is obvious that the potentials are markedly
depending on the solvent. Our measurements using either MeCN
or DMF showed marked differences even between these two seem-
ingly similar solvents (see Supplementary material).

K. Butsch et al. / Inorganica Chimica Acta 394 (2013) 237–246
243
3.4. UV–Vis absorption spectroscopy
absorption bands for the corresponding complex [((NH)2-
salentBu4)Cu]+ were reported with maxima at 607, 527, and
413 nm in CH2Cl2 solution [28], or at 626 and 384 nm in CH2Cl2:-
DMSO (9:1) solution respectively [45], in line with the correspond-
ing generation of phenoxyl radicals and the expected negative
solvatochromism of these bands [25]. The long-wavelength band
at 1600 nm reported in the first of the two studies, indicating the
mixed-valent state (inter-valence charge transfer) could not be ob-
served by us, due to spectrometer limitations.
Upon further oxidation (second wave) of [((NMe)2salhextBu4)-
Cu] the 520 nm band vanishes, the 414 nm band increases and
the 292 nm band shifts to 302 nm, slightly intensifying. Very sim-
ilar observations have been reported before for the analogous
[((NH)2salentBu4)Cu]2+ species [28,45]. The latter state very proba-
bly represents both phenolate-moieties in the oxidised (phenoxy)
state (spectra in the Supplementary material).
Absorption spectra were recorded for all Cu complexes dis-
solved in MeCN from 200 nm to 800 nm (Table 3). All complexes
show two intense absorption bands in the UV range (assigned to
p–
p⁄ transition) and d-d absorption bands around 600 nm. In the
range from 300 to 450 nm, there are further absorption bands of
medium intensity.[61] For the complexes with amine linkers, these
bands lie at about 400 nm (
uted to the corresponding
e
p
< 1000 L molꢀ1 cmꢀ1) and are attrib-
–
p⁄ triplet absorptions, while com-
plexes with imine linkers show bands at around 350 nm
(e
> 1000 Lmolꢀ1 cmꢀ1), which are assigned to metal to ligand
charge transfer (MLCT) transitions. The spectrum of the zinc com-
plex [((NH)2salhexPh2)Zn] showing the long-wavelength absorp-
tion band (attributable to
p–
p⁄ transitions) at 307 nm supports
the assignment for the Cu complexes.
Anodic electrolysis of [(Me2salhexF4)Cu] leads to similar new
bands at 542, 337 and 313 nm. Upon further oxidation (second
wave) both the 542 and 337 nm band vanish, while the 313 nm
band increases markedly. Also here, we assign both processes to
the oxidation of the phenolate moieties. Interestingly, the charac-
teristic band for the phenoxy state lies markedly higher in energy
for the fluorine-substituted derivative than for the other com-
plexes, which can be explained by the electron-withdrawing effect
of the F substituents.
Rather surprising was, that the Zn(II) complex [((NH)2sal-
hexPh2)Zn] also allowed to monitor two separate oxidation pro-
cesses (Fig. 4). Both processes create species which exhibit
intense structured bands ranging from around 400 to 600 nm,
assignable to transitions within the phenoxyl radical ligands.
Even more surprising was, that the complexes [(Ph2salhex)Cu],
[((NH)2salhex)Cu] and [((NH)2salhexPh2)Cu] also revealed the
growth of a band attributable to the generation of phenoxyl radical
3.5. UV–Vis and EPR spectroelectrochemistry
UV/Vis spectroelectrochemical experiments, using an optical
transparent thin-layer electrochemical (OTTLE) cell, were carried
out on selected samples to support the assignment of the oxidation
processes. Representative spectra were shown in Figs. 4 and 5, fur-
ther spectra and a table summarising the results can be found in
the Supplementary material. For the complexes [((NMe)2sal-
hextBu4)Cu], [(Me2salhexF4)Cu] and [((NH)2salhexPh2)Zn] two oxi-
dation processes could be defined from the appearance of
isosbestic points and reversibility, which is in line with the results
from cyclic voltammetry, while for the other complexes only one
oxidation process could be monitored. The spectra recorded upon
anodic electrolysis of the complex [((NMe)2salhextBu4)Cu] at low
potentials reveal the growth of bands at 520, 414 and 292 nm,
while the 436, 297 and 255 nm bands decrease in intensity. The
400
500
600
700
800
200
300
400
500
600
700
800
200
300
400
500
600
700
800
wavelength / nm
wavelength / nm
Fig. 4. Absorption spectra of [((NH)2salhexPh2)Zn] during anodic electrolysis in MeCN/nBu4NPF6 at 298 K, from 0 to 0.4 V (left) and from 0.4 to 0.8 V (right).
300
400
500
300
400
500
wavelength / nm
wavelength / nm
Fig. 5. Absorption spectra of [(Ph2salhex)Cu] (left) and [((NH)2salhex)Cu] (right) measured during electrochemical oxidation at 2.0 V in MeCN/nBu4NPF6 at 298 K.

244
K. Butsch et al. / Inorganica Chimica Acta 394 (2013) 237–246
complexes (Fig. 5). For [(Ph2salhex)Cu] the partially reversible
behaviour allows the observation of the corresponding species
for a few minutes, while for [((NH)2salhex)Cu] and [((NH)2sal-
hexPh2)Cu] the signals vanished rapidly. Although both complexes
are not able to stabilise phenoxyl radicals (as indicated by irrevers-
ible oxidation waves), intense absorption bands around 450 nm
(undoubtedly caused by phenoxyl radicals) were observed in both
cases.
In previous work, the energy of the absorption bands assigned
to a Cu(II)-bound phenoxyl-radical (400–500 nm) was taken as a
measure for the radical stabilisation. The fully stabilised systems
were expected to show absorption maxima shifted to lower ener-
gies, while the non- and partly stabilised complexes were expected
to show charge transfer bands shifted to higher energy [25]. For the
complexes investigated herein, the energy of the absorption in-
creases for the series Me2salhexF4 < (NMe)2salhextBu4 < (NH)2sal-
hexPh2 < Ph2salhex < (NH)2salhex, and correlates roughly with
the substitution pattern, the low oxidation potential and the
reversibility of the oxidation(s). In this sense, the complexes of
Me2salhexF4 and (NMe)2salhextBu4 are at the lower end, and the
(NH)2salhex derivative at higher energy.
the Cu(II) species could not be observed in the optically transpar-
ent thin layer electrochemical (OTTLE) cell. To support our conclu-
sions we thus performed reductive EPR spectroelectrochemistry in
the same potential range and beyond. E.g. reducing a MeCN solu-
tion of the complex [(Me2salhexF4)Cu] at potential higher than –
0.8 V lead to the loss of the EPR signal typical for Cu(II) (Fig. 6).
Upon prolonged electrolysis at potential lower than –1.6 V, a
narrow unresolved signal evolves at giso = 2.0045, which might be
assigned to a ligand-centred radical [(Me2salhexF4Å3–)Cu(I)]2–. The
same experiment was carried out using the zinc complex [((NH)2-
salhexPh2)Zn], and a similar spectrum was obtained (giso = 2.0046;
for spectra see Supplementary material). Both EPR signals do not
show any hfs (at a resolution of 3 G) to 14N (nor to 1H), thus, the
N atoms of the ligands do not contribute markedly to electron den-
sity of the radical ligands singly occupied molecular orbital (SOMO).
3.6. Catalytic test reactions
A standard method to generate the catalytically active species is
to use a Cu(I) precursor complex in the presence of aerial oxygen,
as described in Eq. (1). This method has previously been applied to
generate the active enzyme from isolated apo Galactose Oxidase
(GO) in vitro [62].
In further experiments we tried to record EPR spectra of the oxi-
dised species, generated from the parent Cu(II) complexes (dis-
solved in MeCN) by chemical oxidation, using NO[BF4] as oxidant
[27]. The samples were oxidised at ambient temperature and then
measured at 110 K (glassy frozen solutions). For none of the com-
plexes reasonable signals were detected, which is in line with re-
cent observations on the related complex [((NH)2salentBu4)Cu]+
[27,45]. The ground state of Cu(II) phenoxyl radicals can be either
S = 1 if the unpaired electrons on Cu(II) and the phenoxyl radical
couple ferromagnetically, or S = 0 if an antiferromagnetic coupling
occurs (EPR silent). This can be rationalised from the orbital over-
lapping becoming ideal if the Cu–O–C angle is approximately 130°,
and the angle between the Cu coordination plane and the phenol
ring is about 90°.[25,31] In [(Me2salhexF4)Cu] the Cu–O–C angle
is found to be around 127° (XRD), while the plane angle mentioned
is 7.6°. Thus, the geometrical requirements for efficient antiferro-
magnetic coupling are not given, and the oxidised species thus
might a triplet ground state (S = 1). However, from our experi-
ments we cannot discriminate between the two cases (S = 0 or
S = 1), since also for the S = 1 case our experimental conditions
would not allow the detection of the corresponding signal [45].
To support the assignment on the reduction processes the com-
plexes [((NMe)2salhextBu4)Cu], [(Me2salhexF4)Cu] and [(NH)2sal-
hex)Cu] were submitted to cathodic electrolysis in the OTTLE
cell. Reduction leads in all three cases to spectra characterised by
CuðIÞ ꢀ ½OPhꢀꢂ þ O2 þ 2Hþ¡CuðIIÞ ꢀ ½OPhꢂ þ H2O2
ð1Þ
For the test reactions, the ligands were mixed with [(Cu(OTf))2
l-toluene)] and this catalyst solution was used at 10%mol concen-
(
tration. Benzyl alcohol was added and the reaction was performed
at room temperature in the presence of solid NaOH (see Section 2).
The product benzaldehyde was detected by 1H NMR spectroscopy
using the aldehyde proton to monitor the product concentration.
Samples were taken after 1 and 17 h reaction time. Table 4
summarises the results of the catalytic test reactions. Importantly,
in control experiments (under the same conditions) neither the free
ligands nor Cu(OTf)2 showed any catalytical activity.
We observed product formation for four of the salen complexes
(Table 4), only the complexes containing the salhexPh2 and the
(NH)2salhex ligand failed. After one hour the highest activity was
observed for the 3,5-tBu-substituted complex [((NMe)2sal-
hextBu4)Cu] (note that the catalyst concentration was only 3% in
this case) in line with earlier reports, concluding that 3,5-substitu-
tion on the phenol core stabilises the phenoxyl radicals and thus
promotes the catalytic reaction [12,25,27,28]. Also in line with this
rule is the behaviour of the fluorine substituted complex [(Me2sal-
hexF4)Cu] and to some extent the (NH)2salhexPh2 derivative, hav-
ing bulky Ph substituents in the three positions. The complex
[(Ph2salhex)Cu] obviously does not follow this rule but neverthe-
less shows catalytic activity comparable to the other derivatives.
On the other hand this behaviour is in line with the partly revers-
ible CV wave for the first oxidation of this complex and the high
rate of phenoxyl radical generation in our spectroelectrochemical
experiments (Fig. 5). Furthermore, also the un-substituted salen
a red shift of the MLCT bands, while the
p–
p⁄ absorption bands
are less affected, thus confirming the assignment for the reduction
Cu(II) to Cu(I). Unfortunately, the weak d–d absorption bands of
Table 4
Results of the catalytic test reactions for salen Cu complexes.
Salen ligand
% product after
1 h
% product after
17 h
Turn over
number
(NH)2salhexPh2
Me2salhexF4
Ph2salhex
Salhexph2
(NH)2salhex
(NMe)2salhextBu4
25.6
9.1
67.6
81.5
7.1ꢁ10ꢀ4
2.5ꢁ10ꢀ4
4.8
70.0
1.3ꢁ10ꢀ4
a,b
a,b
-
100 G
b
b
–
3.3c
38.8c
3.1ꢁ10ꢀ4
a
b
c
Extensive precipitation occurred during the reaction.
No product was observed.
The reaction was performed with 3%mol instead of 10%mol
Fig. 6. X-band EPR spectra of [(Me2salhexF4)Cu] measured during electrochemical
reduction from 0 to ꢀ1.3 V (first reduction) in MeCN/nBu4NPF6 at 298 K.
.

K. Butsch et al. / Inorganica Chimica Acta 394 (2013) 237–246
245
complex [(salPrOH)Cu] (PrOH = –CH2–CH(OH)–CH2– as linker) has
been reported to show considerable activity in the oxidation of pri-
mary alcohols und very basic conditions [16]. For this system it
was concluded, that also the stabilisation of the Cu(I) species might
be crucial for the catalytic activity [28]. In our experiments the un-
substituted complex [((NH)2salhex)Cu] gave no benzaldehyde
product, while for the complex [(salhexPh2)Cu] extensive precipi-
tation probably explains the lack of activity.
Not unexpectedly, the product yield was far higher for all four
active complexes after 17 h, which points to a slow formation of
the catalytically active species. Previous investigations gave evi-
dence for a substrate-binding equilibrium (second order kinetics),
which markedly influences the reaction rate [12b,28]. Very re-
cently, for the complex [(salhextBu2OMe2)Cu] very rapid decompo-
sition of the complex in the presence of benzyl alcohol, very low
substrate affinity and restoration of the complex upon addition
of NEt3 confirmed that the reaction proceeds via Eq. (2) [28,52].
in solution (from EPR in glassy frozen solution) the complexes ex-
hibit a relatively rigid square planar geometry for the Cu atom
which does not allow antiferromagnetic coupling of the unpaired
electron localised in the phenoxyl ligand and the Cu(II) ion. Largely
reversible reduction waves for all complexes were assigned to the
Cu(II)/Cu(I) couple which could be substantiated by EPR and UV–
Vis spectroelectrochemistry.
Very preliminary catalytic test reactions (oxidation of benzyl
alcohol by aerial O2), [((NMe)2salhextBu4)Cu], [(Me2salhexF4)Cu],
[(Ph2salhex)Cu] and [((NH)2salhexPh2)Cu] revealed reasonable
activities, although compared to established salen Cu catalysts,
the overall reaction rates of these complexes are low. Interestingly,
amongst the active catalysts in our series, we find imine as well as
amine systems, stabilised and non-stabilised phenol cores and the
correlation of the activity with the structures is not straightfor-
ward. Neither the rigidity of the linker group nor the stabilisation
of the phenoxyl radicals by substituents seems to be an exclusion
criterion. While the activities of the substituted systems [((NMe)2-
salhextBu4)Cu], [(Me2salhexF4)Cu] and [((NH)2salhexPh2)Cu] were
not unexpected in terms of the established rules (substituent-sta-
bilised systems are active catalyst), the good catalytic performance
of [(Ph2salhex)Cu] was unexpected. However, if catalytic activity
correlates with radical stability, the activity of [(Ph2salhex)Cu] is
in line with the partially reversible character of the oxidation ob-
served for this complex. Furthermore, the failure of [((NH)2sal-
hex)Cu] and [(salhexPh2)Cu] to perform catalysis goes along with
the irreversibility of the oxidation of these two complexes. There-
fore, electrochemical reversibility seems to be a reliable tool for the
estimation of catalytic activity of such complexes, whereas predic-
tions based on the substitution pattern might exclude reasonable
candidates for catalysis. Strangely, the behaviour of the system
[((NH)2salhexPh2)Cu] (irreversible oxidation, good catalysis) is
not in line with this correlation. However, in this case we have evi-
dence that the catalytic mechanism for this complex is different
from the other complexes and future studies will try to allow more
comprehensive and unequivocal conclusions.
2½ðsalhextBu2OMe2ÞCuꢂþ
þ PhCH2OH¡2½ðHsalhextBu2OMe2ÞCuꢂþ þ PhCHO
ð2Þ
Interestingly, the complex [((NH)2salhexPh2)Cu] showed rather
high product concentration after 1 h reaction time, but markedly
dropped in catalytic activity compared to the other derivatives
over a longer period of time, which might represent some evidence
for different substrate-binding abilities of the investigated Cu com-
plexes. Detailed studies on the reaction kinetics will thus be car-
ried out in the near future.
From the calculated turn over numbers (Table 4), we can con-
clude that all our complexes catalyse the oxidation reaction at
rather low reaction rates compared to systems as 1,10-binaphtyl
linked [(salbntBu4)Cu] [12a], but similar to other GO model sys-
tems [14]. Frequently, the paramagnetic ground state (S = 1) of
such complexes is discussed as one of the reasons for the reluctant
reaction [25].
4. Conclusions
Acknowledgements
A number of salen-Cu complexes bearing different combina-
tions of substituents and linkers were synthesised and investigated
in detail. All complexes show two electrochemical oxidation pro-
cesses at comparably low potentials, the lowest values lie around
0.3 V. The first oxidation of [((NMe)2salhextBu4)Cu], [(Me2sal-
hexF4)Cu] occurs fully reversibly, while for the complex [(Ph2sal-
hex)Cu] partly reversible behaviour is observed. For all other
complexes the oxidation is completely irreversible on the time-
scale of the CV experiment. For the first two complexes the revers-
ible behaviour can be related to the presence of substituents in the
3,5-position of the phenol-core, stabilising the phenoxyl radical
generated by the one-electron oxidation process. Consequently,
[((NMe)2salhextBu4)Cu] and [(Me2salhexF4)Cu] exhibit a second
oxidation process at slightly higher potentials, which is fully
reversible for [((NMe)2salhextBu4)Cu] and partly reversible for
[(Me2salhexF4)Cu], which might be due to the far higher oxidation
potential required for the latter. The second oxidation processes for
all other complexes were irreversible. Spectroelectrochemical
(UV–Vis) investigations clearly showed that these oxidation pro-
cesses are due to the formation of phenoxyl radical species. The en-
ergy of the observed phenoxyl-to-metal charge transfer band
correlates roughly with the stability of the radicals as inferred from
the reversibility of the redox waves and assumed from the substi-
tution pattern. Corresponding spectroelectrochemical EPR experi-
ments did not show signals for the oxidised complexes
[(PhOÅ)Cu(II)]Å+, although a triplet ground state can be assumed
from the geometry of the complexes. In both the solid state (from
XRD of [(Me2salhexF4)Cu] reported herein, others previously) and
We would like to thank Dr. Ingo Pantenburg and Ms. Ingrid
Müller, University of Cologne, for the collection of crystal data
and the ‘‘Studienstiftung des Deutschen Volkes’’ for financial sup-
port (KB).
Appendix A. Supplementary material
Supplementary data associated with this article can be found, in
the online version, at http://dx.doi.org/10.1016/j.ica.2012.08.016.
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