Reaction behaviour of dinuclear copper(I) complexes with m-xylyl-based ligands towards dioxygen

Article abstract of DOI:10.1039/b406329p

Intramolecular ligand hydroxylation was observed during the reactions of dioxygen with the dicopper(I) complexes of the ligands L¹ (L ¹ = α,α-bis[(2-pyridylethyl)amino]-m-xylene) and L³ (L³ = α,α-bis[N-(2-pyridy

Full text of DOI:10.1039/b406329p

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F U L L P A P E R
Reaction behaviour of dinuclear copper(I) complexes with
m-xylyl-based ligands towards dioxygen
Simon P. Foxon,a Diana Utz,a Jörg Astner,a Siegfried Schindler,*a Florian Thaler,b
Frank W. Heinemann,b Günter Liehr,b Jhumpa Mukherjee,c V. Balamurugan,c
Debalina Ghoshc and Rabindranath Mukherjee*c
a
Institut für Anorganische und Analytische Chemie, Justus-Liebig-Universität Gießen,
Heinrich-Buff-Ring 58, 35392, Gießen, Germany.
E-mail: Siegfried.Schindler@anorg.chemie.uni-giessen.de; Fax: +49-641-9934149;
Tel: +49-641-9934140
b
Institut für Anorganische Chemie, Universität Erlangen-Nürnberg, Egerlandstrasse 1,
91058, Erlangen, Germany
Department of Chemistry, Indian Institute of Technology, Kanpur, 208016, India.
c
E-mail: rnm@iitk.ac.in; Fax: +91-512-2597437; Tel: +91-512-2597437
Received 27th April 2004, Accepted 3rd June 2004
First published as an Advance Article on the web 30th June 2004
Intramolecular ligand hydroxylation was observed during the reactions of dioxygen with the dicopper(I) complexes
of the ligands L1 (L1 = ,′-bis[(2-pyridylethyl)amino]-m-xylene) and L3 (L3 = ,′-bis[N-(2-pyridylethyl)-N-(2-
pyridylmethyl)amino]-m-xylene). The dinuclear copper(I) complex [Cu2L3](ClO4)2 (3) and the dicopper(II) complex
[Cu2(L1–O)(OH)(ClO4)]ClO4 (1) were characterized by single-crystal X-ray structure analysis. Furthermore, phenolate-
bridged complexes were synthesized with the ligand L2–OH (structurally characterized [Cu2(L2–O)Cl3] (7) with L2 = ,′-
bis[N-methyl-N-(2-pyridylethyl)amino]-m-xylene; synthesized from the reaction between [Cu2(L2–O)(OH)](ClO4)2
(2) and Cl−) and Me–L3–OH: [Cu2(Me–L3–O)(-X)](ClO4)2·nH2O (Me–L3–OH = 2,6-bis[N-(2-pyridylethyl)-N-(2-
pyridylmethyl)amino]-4-methylphenol and X = C3H3N2− (prz) (4), MeCO2− (5) and N3− (6)). The magnetochemical
characteristics of compounds 4–7 were determined by temperature-dependent magnetic studies, revealing their
antiferromagnetic behaviour [−2J (in cm−1) values: −92 for 4, −86 for 5 and −88 for 6; −374 for 7].
Introduction
Modelling of the copper enzyme tyrosinase (a monooxygenase
causing hydroxylation of monophenols and subsequent oxidation
of catechols to quinones)1–5 was first performed successfully by
Karlin and coworkers who found that an intramolecular ligand
hydroxylation of the complex [Cu2(R–XYL–H)]2+ during its
reaction with dioxygen occurred (Fig. 1).6,7
Fig. 2 Intramolecular ligand hydroxylation of [Cu2(H–BPB–H)S2]2+.
intramolecular hydroxylation reaction was less sensitive towards
ligand modifications.15,22–32
The reaction of dioxygen with complexes derived from
reduction of the imine bonds of such complexes (e.g. [Cu2(H–
BPB–H)S2]2+ in Fig. 2) has not been studied so far in detail,
although phenolate bridged binuclear copper(II) complexes with
such amine ligands are well known (a few examples are given in
the references).33–37
Therefore, in our efforts to gain a better understanding of the
intramolecular ligand hydroxylation reactions of xylyl-bridged
dicopper complexes, we investigated the reactivity of dioxygen
towards the dinuclear copper(I) complex of the ligand ,′-bis[(2-
pyridylethyl)amino]-m-xylene (L1, Scheme 1), the reduced form
of the imine H–BPB–H (Fig. 2). Additionally we analysed the
reaction of dioxygen with the copper(I) complex of the ligand ,′-
bis[N-(2-pyridylethyl)-N-(2-pyridylmethyl)amino]-m-xylene (L3)
(Scheme 1) which differs from R–XYL–H (Fig. 1) by two shorter
ligand “arms”, leading to the formation of two smaller chelate rings
in the metal complex.
Fig. 1 Intramolecular ligand hydroxylation of [Cu2(R–XYL–H)]2+.
A detailed kinetic analysis performed at low temperature as well
as a resonance Raman study revealed the formation of a -2:2-
peroxo complex as an intermediate.8–11 However, it has been shown
that bis--oxo copper units are also capable of performing ligand
hydroxylation reactions.12
At present it remains unclear why substitution of the pyridine
groups in [Cu2(R–XYL–H)]2+ with pyrazole or benzimidazole
donors completely suppresses the above reaction, while with
triazacyclononane units the intramolecular ligand hydroxylation
reaction occurs.13–21
Furthermore, similar and structurally related binuclear copper(I)
imine complexes also showed intramolecular ligand hydroxylation
when reacted with dioxygen (Fig. 2), however, in these cases the
T h i s j o u r n a l i s
©
T h e R o y a l S o c i e t y o f C h e m i s t r y 2 0 0 4
D a l t o n T r a n s . , 2 0 0 4 , 2 3 2 1 – 2 3 2 8
2 3 2 1

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Table 1 Crystallographic data for 1, 3 and 7
1
3
7
Molecular formula
Mr
C22H28Cl2Cu2N4O11
722.46
C37H42Cl2Cu2N6O9
912.75
C24H29Cl3Cu2N4O
622.94
T/K
183(2)
100(2)
293(2)
Radiation used (/Å)
Crystal system
Space group
Mo-K (0.71073)
Triclinic
P1
Mo-K (0.71073)
Triclinic
P1
Mo-K (0.71073)
Triclinic
P1
a/Å
b/Å
c/Å
/°
10.570(5)
12.130(5)
12.360(5)
68.590(5)
73.080(5)
71.198(5)
1369(1)
2
8.8457(5)
12.839(2)
18.430(2)
75.861(6)
78.533(5)
85.776(6)
1988.5(4)
2
9.931(4)
16.66(2)
17.64(2)
61.26(5)
87.52(5)
87.59(5)
2557(4)
4
/°
/°
V/Å3
Z
Dc/g cm−3
1.752
1.814
1.524
1.265
1.618
2.002
/mm−1
Reflections measured
Unique reflections, Rint
Refined parameters
Goodness-of-fit on F2
R(F, F2 > 2)
Rw(F2, all data)
9128
5370, 0.1048
384
1.029
0.0836
51968
8770, 0.0952
507
1.049
0.0447
7019
6583, 0.0987
609
1.031
0.0964
0.2358
0.0926
0.3169
the methyl group (or more generally an alkyl group) needs to
be attached to the secondary amine nitrogen donor atoms for
intramolecular ligand hydroxylation to be observed.
Unfortunately we were unsuccessful in isolating the copper(I)
complex of L1 as a pure solid material. Therefore, solutions of this
complex employed in the oxygenation experiments were prepared
in situ by mixing [Cu(CH3CN)4]ClO4 with L1 in methanol (or
dichloromethane) under an inert atmosphere. The yellow complex
[Cu2L1(CH3CN)2x](ClO4)2 (x = 1 or 2) is most likely formed, where
one or two acetonitrile molecules are coordinated additionally as
co-ligands to each copper(I) ion. After the reaction with dioxygen
the green phenolate-bridged product [Cu2(L1–O)(OH)(ClO4)]ClO4
(1) was isolated in good yield, clearly demonstrating again that
intramolecular ligand hydroxylation had occurred. Crystals of
complex 1 suitable for a single-crystal X-ray structure analysis
were obtained by slow diffusion of Et2O into a MeOH solution
containing 1. The quality of the crystals was poor and could not be
improved by varying the crystallization conditions. This seems to
be a general problem when six-membered chelate rings are present
in this type of ligands (see below) while in contrast it was much
easier obtaining crystals of high quality for diffraction studies if
only five-membered chelate rings are present.37 This might be a
consequence of the fact that copper(II) ions usually prefer five-
membered chelate rings in their complexes. We obtained acceptable
diffraction data for [Cu2(L1–O)(OH)(ClO4)]ClO4 (1) at 183(2) K. A
summary of the crystallographic data, bond lengths and angles for
1 can be found in Tables 1 and 2. An ORTEP42 view of the cation of
1 is shown in Fig. 3.
Scheme 1 Ligands used in this study.
Results and discussion
L1,L3 andMe–L3–OHwerereadilypreparedusingstandardsynthetic
procedures (see experimental section; however, alternative methods
for the syntheses of L3 and Me–L3–OH have been used as well: e.g.
L3 can be prepared from L1 using 2-picolyl chloride and base or by
an in situ reductive alkylation38,39 with 2-pyridinecarbaldehyde and
NaBH(OAc)3). The crude oils were purified by chromatography.
As described above it is well known that intramolecular ligand
hydroxylation occurs when [Cu2(R–XYL–H)]2+ is reacted with
dioxygen (Fig. 1). The different reaction pathways of related
xylyl-bridged copper complexes during oxidation raised the
question about the basic essential requirements for the occur-
rence of an intramolecular ligand hydroxylation. That intramo-
lecular ligand hydroxylation was observed when the Schiff base
complex [Cu2(H–BPB–H)S2]2+ (Fig. 2) was reacted with dioxygen
demonstrated that only two of the four “ethyl-pyridine arms” in
[Cu2(R–XYL–H)]2+ are required for this kind of oxidation.
However, it was not clear from the above finding if the imine
donor atoms present in [Cu2(H–BPB–H)S2]2+ are essential. It had
been demonstrated by some of us earlier that imine donor atoms
are not prerequisite by analyzing the oxidation of the dinuclear
copper(I) complex with L2.40,41 Once again intramolecular ligand
hydroxylation was observed and therefore suggesting that only
two nitrogen donor atoms of the ligand (per copper ion) are
sufficient for intramolecular ligand hydroxylation reactions.
Our findings were confirmed furthermore by the observations
of Tolman and coworkers who observed ligand hydroxylations
using bidentate ligands with nitrogen donor atoms.12 However,
the question remained and is addressed herein, as to whether
Fig. 3 ORTEP42 representation (50% probability displacement ellipsoids)
of the cation of 1. Hydrogen atoms omitted for clarity.
2 3 2 2
D a l t o n T r a n s . , 2 0 0 4 , 2 3 2 1 – 2 3 2 8

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Table 2 Selected bond lengths (Å) and interbond angles (°) for 1, 3 and 7
1
Cu(1)–N(1)
Cu(1)–O(2)
Cu(2)–N(4)
Cu(2)–O(4)
2.026(8)
1.918(7)
1.941(8)
2.499(7)
Cu(1)–N(2)
Cu(1)–O(3)
Cu(2)–O(1)
Cu(1)Cu(2)
1.946(8)
2.528(8)
1.916(6)
3.006(2)
Cu(1)–O(1)
Cu(2)–N(3)
Cu(2)–O(2)
1.973(7)
2.028(8)
1.943(7)
N(1)–Cu(1)–N(2)
N(2)–Cu(1)–O(1)
N(3)–Cu(2)–N(4)
N(4)–Cu(2)–O(1)
96.3(3)
90.1(3)
94.7(3)
171.4(3)
N(1)–Cu(1)–O(1)
N(2)–Cu(1)–O(2)
N(3)–Cu(2)–O(1)
N(4)–Cu(2)–O(2)
172.3(3)
165.3(3)
93.9(3)
94.4(3)
N(1)–Cu(1)–O(2)
O(1)–Cu(1)–O(2)
N(3)–Cu(2)–O(2)
O(1)–Cu(2)–O(2)
97.6(3)
76.4(3)
165.1(3)
77.1(3)
3
Cu(1)–N(1)
Cu(1)–O(40)
Cu(2)–N(6)
2.334(2)
2.835(2)
1.916(2)
Cu(1)–N(2)
Cu(2)–N(4)
Cu(2)–O(24)
1.909(2)
2.308(2)
3.087(2)
Cu(1)–N(3)
Cu(2)–N(5)
1.921(2)
1.902(2)
N(1)–Cu(1)–N(2)
N(1)–Cu(1)–O(40)
N(4)–Cu(2)–N(5)
N(4)–Cu(2)–O(24)
96.40(9)
123.55(8)
97.2(1)
N(1)–Cu(1)–N(3)
N(2)–Cu(1)–O(40)
N(4)–Cu(2)–N(6)
N(5)–Cu(2)–O(24)
83.50(9)
92.14(9)
83.74(9)
93.42(9)
N(2)–Cu(1)–N(3)
N(3)–Cu(1)–O(40)
N(5)–Cu(2)–N(6)
N(6)–Cu(2)–O(24)
172.8(1)
93.92(9)
171.2(2)
91.80(9)
136.90(8)
7
Cu(1)–N(1)
Cu(1)–Cl(1)
Cu(2)–N(4)
Cu(2)–Cl(3)
2.03(2)
2.339(5)
2.00(2)
2.421(5)
Cu(1)–N(2)
Cu(1)–Cl(2)
Cu(2)–O(1)
Cu(1)Cu(2)
2.06(2)
2.457(6)
1.95(2)
3.293(5)
Cu(1)–O(1)
Cu(2)–N(3)
Cu(2)–Cl(2)
1.92(2)
2.03(2)
2.426(5)
N(1)–Cu(1)–O(1)
N(2)–Cu(1)–O(1)
N(1)–Cu(1)–N(2)
O(1)–Cu(1)–Cl(1)
N(3)–Cu(2)–N(4)
N(3)–Cu(2)–Cl(3)
N(3)–Cu(2)–Cl(2)
168.2(5)
91.2(6)
93.6(6)
92.7(4)
93.1(6)
110.0(4)
142.8(4)
N(1)–Cu(1)–Cl(1)
N(2)–Cu(1)–Cl(1)
O(1)–Cu(1)–Cl(2)
N(3)–Cu(2)–O(1)
O(1)–Cu(2)–Cl(3)
O(1)–Cu(2)–Cl(2)
Cl(2)–Cu(2)–Cl(3)
94.5(5)
118.5(4)
79.1(4)
90.3(6)
94.6(4)
79.4(4)
106.4(2)
N(2)–Cu(1)–Cl(2)
Cl(1)–Cu(1)–Cl(2)
N(1)–Cu(1)–Cl(2)
N(4)–Cu(2)–O(1)
N(4)–Cu(2)–Cl(3)
N(4)–Cu(2)–Cl(2)
126.9(4)
114.0(2)
89.3(4)
167.6(5)
95.5(4)
90.7(4)
The two copper(II) centres (intramolecular separation
Cu(1)Cu(2) 3.006(2) Å) in 1 are both penta-coordinate; Cu(1)
is ligated by pyridyl nitrogen atom N(4), aliphatic amine N(3),
phenolate oxygen O(1) and oxygen atoms O(2) and O(3) of the
respective bridging hydroxo and perchlorate moieties. The bond
angle between Cu(1), the -phenolate oxygen and Cu(2) [Cu(1)–
O(1)–Cu(2)] = 101.1(3)°; the bond angle between Cu(1), the -
hydroxo moiety and Cu(2) [Cu(1)–O(2)–Cu(2)] = 102.2(3)°. The
coordination geometry about Cu(1) and Cu(2) is best described as
close to square pyramidal with values of the trigonality index43 ()
equal to 0.12 for Cu(1) and 0.11 for Cu(2) (where  = ( − )/60,
with  and  being the two largest coordination angles around
the metal centre:  = 0 for square pyramidal geometry and  = 1
for trigonal bipyramidal geometry). The basal plane of the square
pyramid around Cu(1) contains O(1), O(2), N(3) and N(4) with
O(3) occupying the axial coordination site. Metric parameters
around Cu(2) are similar to Cu(1).Anon-coordinating water solvent
molecule and perchlorate anion (not shown in Fig. 3) complete the
structure of 1.
Phenolate bridged complexes similar to 1 are well known and
their properties have been studied extensively (a few examples are
provided in the references).17,18,33,35,37 A structurally related complex
to 1 has been characterized by Grzybowski et al.34 The ligand
employed differed from L1–OH in that a para-methyl group was
present on the central aromatic ring (Scheme 1, L1–OH, however
with R = CH3 instead of H). Although the authors presented
detailed physicochemical studies no crystal structure of the
complex was described probably due to the same difficulties we
had with obtaining crystals of 1 suitable for X-ray crystal structure
determination.
with an analogous complex described earlier by some of us, where
the chelate ring sizes are smaller.37 There are significant differences
in bond lengths and bond angles between the two complexes. The
most striking effect of the smaller chelate ring size is reflected in the
bond angles N(1)–Cu(1)–N(2) and N(3)–Cu(2)–N(4): these values
are 84.8(4) and 84.1(4)° in the structure with the smaller chelate
rings present and 96.3(3) and 94.7(3)° for 1.
Karlin and coworkers and some of us observed that reducing the
chelate ring sizes in [Cu2(R–XYL–H)]2+ by substituting the “ethyl-
pyridine arms” with “methylpyridine arms” completely suppressed
intramolecular ligand hydroxylation.41,44 Therefore, it was an
obvious question to address as to whether partial substitution, i.e.
the substitution of only two “arms”, would support or suppress the
intramolecular ligand hydroxylation reaction.
Reaction of L3 (Scheme 1) and [Cu(CH3CN)4]ClO4 in acetone
led to the formation of [Cu2L3](ClO4)2 (3) that could be crystallo-
graphically characterized. A summary of the crystallographic data,
bond lengths and angles for 3 is presented in Tables 1 and 2. An
ORTEP42 view of the cation of 3 (including the weak interaction
of an acetone molecule and a perchlorate anion) is shown in Fig. 4.
Similar to the crystal structure of [Cu2(R–XYL–H)]2+
acetonitrile molecules are not coordinated to the copper(I) ions as
additional ligands.7 However, bond distances and angles are clearly
different due to the “replacement” of a six-membered chelate ring
with a five-membered chelate ring [e.g. Cu–N(amine) distances:
Cu(1)–N(1) 2.334(2) and Cu(2)–N(4) 2.308(2) Å are longer in 3
compared with Cu(1)–N(1) 2.121(8) and Cu(2)–N(4) 2.196(7) Å in
[Cu2(R–XYL–H)]2+].7
Oxidation of the solution-generated dicopper(I) complex of
the m-xylyl-based dinucleating ligand L3 and [Cu(CH3CN)4]ClO4
in MeOH at 298 K with O2 again led to an intramolecular ligand
hydroxylation. This was proved by isolation and characterization
of the hydroxylated ligand L3–OH according to the published
procedure for the hydroxylation of [Cu2(R–XYL–H)]2+.7 No crystals
of the product complex suitable for an X-ray crystal structure
Comparison of the crystal structure of 1 with the complex
[Cu2(L2–O)(OH)](ClO4)2 (2) described previously40 shows that
bond lengths and angles around the copper(II) centres are similar,
with the distance between the two copper(II) ions being close to 3 Å.
The situation is different if we compare the crystal structure of 1
D a l t o n T r a n s . , 2 0 0 4 , 2 3 2 1 – 2 3 2 8
2 3 2 3

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Efforts in obtaining better quality single-crystals have also been
unsuccessful. However, we accurately determined a pKa value of
4.76(2) for the deprotonation of [Cu2(Me–L3–O)(H2O)2](ClO4)3
leading to [Cu2(Me–L3–O)(OH)](ClO4)2. The pKa value we
determined is very close to the one obtained for the acid–base
equilibrium between [Cu2(Me–L4–O)(H2O)2](ClO4)3 and [Cu2-
(Me–L4–O)(OH)](ClO4)2 (pKa = 4.95).46
Treating complex 2 with an excess of chloride ions led to the
formation of [Cu2(L2–O)Cl3] (7). It was also possible to regenerate
2 from 7 by adding water to a solution of 7 in acetonitrile, with both
exchangereactionsbeingreadilymonitoredbyUV-visspectroscopy.
Confirmation of the composition of 7 was obtained from a single-
crystal X-ray structure determination. The asymmetric unit contains
two crystallographically independent molecules of complex 7. Both
molecules have essentially identical coordination geometries, but
the corresponding bond lengths and bond angles are different. A
summary of the crystallographic data, bond lengths and angles for
7 can be found in Tables 1 and 2. An ORTEP42 view of one of the
crystallographically independent molecules of 7 is shown below in
Fig. 5.
Fig. 4 ORTEP42 representation (50% probability displacement ellipsoids)
of the cation of 3 (including the weakly coordinated acetone molecule and
perchlorate anion). Hydrogen atoms omitted for clarity.
analysis were obtained (however, see below: independent synthesis
of phenolate-bridged complexes derived from Me–L3–OH).
As described above, it was possible to observe a peroxo
intermediate complex spectrophotometrically when [Cu2(R–
XYL–H)]2+ was oxidized with dioxygen using low temperature
stopped-flow techniques.8–10 However, our efforts to detect such a
transient “dioxygen adduct” with the dinuclear copper complexes
of the ligands L1, L2 and L3 using stopped-flow techniques were
unsuccessful. Even at low temperatures only the formation of the
final products was observed spectroscopically. This finding is in
agreement with our earlier results on the related imine complex
[Cu2(H–BPB–H)S2]2+ described above (Fig. 2) and a dinuclear
macrocyclic copper Schiff base compound; in both cases no
“dioxygen adduct” was observed.29–31 Furthermore, Murthy et al.
could not detect spectroscopically such an intermediate during the
analysis of the reaction of dioxygen with [Cu2(UN2–H)]2+ where
the unsymmetric ligand UN2–H consists of one half of R–XYL–H
and one half of L2.45 The probable reason in all these cases most
likely is based on the kinetics of the reaction: the rate of formation
of any “dioxygen adduct” is slower than its consecutive reactions
and therefore cannot be detected.
Fig. 5 ORTEP42 representation (50% probability displacement ellipsoids)
of 7. Hydrogen atoms omitted for clarity.
Complex 7 is a dinuclear copper(II) complex of the same di-
nucleating ligand as in complex [Cu2(L2–O)(OH)](ClO4)2 (2).40
The two copper(II) centres in complex 7 (Cu(1)Cu(2) separa-
tion = 3.293(5) Å; for the other molecule 3.272(5) Å) are bridged
by an endogenous phenolate and an exogenous chloride ion, with
additional chloride coordination at each copper(II) centre. Both
Cu(1) and Cu(2) are penta-coordinate being ligated by one pyridyl
nitrogen, one aliphatic amine nitrogen, a phenolate oxygen and two
chloride ions. The phenolate oxygen and a chloride ion bridge the
copper(II) centres. The geometry about each copper(II) centre is
best described as distorted trigonal bipyramidal/square pyramidal:
Phenolate-bridged complexes
As described above, the phenolate-bridged complexes [Cu2(L1–O)-
(OH)(ClO4)]ClO4 (1) and [Cu2(L2–O)(OH)](ClO4)2 (2) could be
readily prepared in good yields by oxidizing the copper(I) complexes
of L1 and L2 with dioxygen, while this was not possible with the
ligand L3. Therefore, we prepared three copper(II) complexes
[Cu2(Me–L3–O)X](ClO4)2·nH2O
(Me–L3–OH = 2,6-bis[N-(2-

43 = 0.69 [Cu(1)] and  = 0.41 [Cu(2)], the corresponding values
pyridylethyl)-N-(2-pyridylmethyl)amino]-4-methylphenol)
for the other molecule are  = 0.74 [Cu(1a)] and  = 0.51 [Cu(2a)].
Cu(1) is displaced 0.09 Å and Cu(2) 0.11 Å out of the trigonal
plane, towards the pyridyl nitrogen atom N(1) and N(4), respec-
tively. The angles between the central phenolate ring and pyridyl
rings are 18.4(8) and 42.1(7)° for the molecule shown in Fig. 5. The
structural motif exhibited in complex 7 is rare; there are only three
other structurally characterized copper(II) complexes of nitrogen
donor-based ligands containing both a bridging phenolate anion
and a chloride anion.47–49
−
−
−
(Scheme 1) with X = C3H3N2 (prz) (4), MeCO2 (5) and N3 (6);
n = 1 for 4 and n = 2 for 5 and 6. Microanalytical data, IR spectra
and solution electrical conductivity measurements are in conformity
with our proposed formulations. The IR data demonstrates that in
complex 6 the azide group most likely is present in a -1,1-bridging
mode. So far we have been unsuccessful in determining the three-
dimensional X-ray crystal structures of the complexes 4–6 because
of the poor quality of crystals obtained. In contrast we obtained
crystals of [Cu2(Me–L3–O)(H2O)2](ClO4)3 by reacting the ligand
Me–L3–OH with [Cu(H2O)6](ClO4)2 in a mixture of water and
methanol. A crystal structure determination supported the above
formulation and showed that in the phenolate-bridged complex
an additional water molecule is coordinated to each copper(II) ion,
in a similar manner to the structurally characterized complexes
[Cu2(F–L4–O)(H2O)2](ClO4)3 and [Cu2(CF3–L4–O)(H2O)2](ClO4)3
where L4 = 2,6-bis[bis(2-pyridylmethyl)aminomethyl]-4-
R-phenol.46 The quality of the structural refinement of [Cu2-
(Me–L3–O)(H2O)2](ClO4)3 was not good enough for publication
due to disorder problems encountered with the perchlorate anions.
Magnetic characteristics
Due to the presence of the phenoxo-/hydroxo-bridge in 2 signifi-
cant antiferromagnetic exchange coupling is present (−440 cm−1).41
Variable-temperature (80–300 K) magnetic susceptibility analyses
for 4–7 were performed. Their magnetic properties are of inter-
est owing to the presence of an invariant phenoxide bridge and
variable exogenous bridges. At 300 K the eff/Cu values (in B) for
this set of compounds are: 1.73 (4), 1.76 (5), 1.77 (6) and 1.25 (7).
The corresponding values at 80 K are: 1.28, 1.33, 1.34 and 0.34,
2 3 2 4
D a l t o n T r a n s . , 2 0 0 4 , 2 3 2 1 – 2 3 2 8

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Table 3 Magnetic data for endogenously phenoxo-bridged dicopper(II) complexes
Complex
−2J/cm−1
 (%)
108R
Ref.
[Cu2(L2–O)(OH)](ClO4)2 (2)
[Cu2(L4–O)(-pyz)](ClO4)2·H2O (4)
[Cu2(L4–O)(-OAc)](ClO4)2·2H2O (5)
[Cu2(L4–O)(-1,1-N3)](ClO4)2·2H2O (6)
[Cu2(L1–O)Cl3] (7)
[Cu2(L–O)(OH)](PF6)2
[Cu2(L–O)Cl](BPh4)2·MeCOMe
[Cu2(L–O)(-1,1-N3)](PF6)2
−440
−92
−86
40
0.44
0.40
0.53
0.91
5.67
11.60
20.20
7.05
This work
This work
This work
This work
7,48
−88
−374
−600
−335
−440
48
48,51
L–OH = 2,6-Bis[N-(2-pyridylethyl)aminomethyl]phenol.
respectively. Plots of MT vs. T for two representative complexes
4 and 7 are shown in Fig. 6. The observed magnetic susceptibility
data were fitted to the modified Bleaney–Bowers equation (1)50 by
allowing for the presence of monomeric impurity, where  is the
mole-fraction of the non-coupled copper(II) impurity.
parameters of J and  using eqn. (1) were obtained by a nonlinear
least-squares fitting procedure. The quality of fit was estimated
by the R index defined as R = ∑(Mexpt − Mcalc)2/∑(Mexpt)2. The
parameters that were obtained are collected in Table 3.
There is an appreciable drop in the extent of antiferromagnetic
coupling, comparing complexes 2 and 7 (Table 3). A similar trend
was observed by Karlin and co-workers, for closely related systems
(the geometry around the copper(II) centres, however, remained
invariant).48 Thus the extent of antiferromagnetic exchange
coupling is much higher in 7 than that present in complexes 3–6.
Within the similar class of complexes 4–6, temperature-dependent
magnetic susceptibility studies reveal that (i) there is medium
antiferromagnetic exchange coupling between pairs of copper(II)
ions in each case and (ii) the magnitude of the antiferromagnetic
exchange depends on the identity of the exogenous bridge
(prz > azide > acetate). It is worth noting that the present work
complements the ones of several authors who investigated the effect
of exogenous bridges in the transmission of magnetic exchange
between two copper(II) centres.52
−1

2Nβ2g2
3kT
1
−2J
Nβ2g2ρ
2kT








(1−ρ) +
χM
=
1+ exp
+ 2Nα (1)







3
kT

The effective magnetic moments for 4–6 in MeCN solution
(300 K) were determined by using the NMR method to examine
whether or not the solid state structures of the complexes are
retained in solution. The eff/Cu values for 4 and 5 are in reasonable
agreement with solid-state values. However, for 6 the solution-state
value (1.55 B) is slightly less than the solid-state value (1.77 B).
This behaviour could be due to a relaxed geometry in the solution
state, allowing a better pathway for magnetic coupling.
Conclusions
Considering the large number of studies that have been performed
on xylyl-bridged dicopper(I) complexes it is surprising that at
present we do not have a detailed understanding of their reaction
pathways when oxidized with dioxygen. Sorrell commented earlier
that attempts to find a correlation between the physical properties
of the complexes and their ability to support intramolecular ligand
hydroxylation reactions have been unsuccessful.17 Chelate ring
sizes in the complexes seem to play an important role; for imine
complexes such as [Cu2(H–BPB–H)S2]2+ (Fig. 2) or for the amine
complex [Cu2(R–XYL–H)]2+ (Fig. 1) a decrease in the chelate ring
size from six to five completely suppressed the intramolecular
ligand hydroxylation. In our work presented herein we also observed
intramolecular ligand hydroxylation when dioxygen was reacted
with the copper(I) complex of L1 with only six-membered chelate
rings present. Furthermore, intramolecular ligand hydroxylation
was observed with the copper(I) complex of L3, where two five-
membered chelate rings were introduced additionally. Several
other copper(I) complexes containing only five-membered chelate
rings are known, which also show ligand hydroxylation reactions.
Therefore, the occurrence of intramolecular ligand hydroxylation
cannot be a result of chelate ring size alone. Based on the results/
observations available it is more probable that the overall geometry,
which the ligand enforces on the copper centres, plays an important
role in determining the fate of the metal-bound “activated dioxygen
adduct”. If the ligand backbone allows the approach of the “activated
oxygen adduct” close to the aromatic C–H bond to be activated then
intramolecular ligand hydroxylation is observed. However, if the
Fig. 6 mT vs. temperature T plots for 4 and 7 (from top to bottom). Circles
represent experimental data, the solid lines represent the fit.
In this expression, N, g and k have their usual meaning; 2J
is the energy difference between the singlet and triplet states;
M is the molar susceptibility per dimer. The values of g and
temperature-independent paramagnetic susceptibility (N) were
kept fixed at g = 2.1 [typical value for a tetragonal Cu(II)] and
60 × 10−6 cm3 mol−1, respectively, during the fitting procedure.
To get a better control of g values the EPR spectra of 4–6 were
recorded. In fact, in the polycrystalline state at 300 K each complex
exhibits an almost isotropic signal. The g values are: 2.08 for 4,
2.07 for 5, 2.11 (gav value from a weak axial spectrum). The best-fit
D a l t o n T r a n s . , 2 0 0 4 , 2 3 2 1 – 2 3 2 8
2 3 2 5

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steric demands of the ligand enforces a larger distance between
the metal-bound “dioxygen adduct” and the aromatic ring then the
hydroxylation reaction is suppressed and “normal” intermolecular
oxidation reactions are observed.
10 °C with magnetic stirring. A solution of -′-dibromo-m-xylene
(0.62 g, 2.35 mmol) in dry MeOH (40 cm3), over a period of 10 min
was then added. The reaction mixture was stirred for a further 2.5
h and then was kept at ~30 °C overnight. The solvent was removed
in vacuo to obtain the crude product as a brown–yellow semi-solid.
The desired ligand was obtained by exhaustive extraction of the
aqueous phase with CHCl3 (a little water was also added at this
stage) until the aqueous layer was colourless. The organic fractions
were combined and then dried over anhydrous Na2SO4. Filtration
and removal of the solvent yielded the crude product as a red–brown
oil which was chromatographed on neutral aluminium oxide with
ethyl acetate as eluent (Rf = 0.40) yielding L3 as a yellow oil (0.9 g,
60%). H (300 MHz; solvent CDCl3; standard SiMe4) 8.46 [4 H, d,
py–H], 7.60–7.04 [16 H, m, Ar–H], 3.79 [4 H, s, pyCH2N–], 3.69
[4 H, s, ArCH2N–], 2.98 [8 H, m, pyCH2CH2N–]. C (75 MHz;
solvent CDCl3; standard SiMe4) 160.6, 160.2, 149.1, 148.7, 139.2,
136.2, 136.0, 129.2, 128.1, 127.4, 123.4, 122.7, 121.7, 121.0, 60.1,
58.5, 54.1, 36.0.
Experimental
Reagents and materials
Reagents and solvents used, unless stated otherwise, were of
commercially available reagent grade quality. [Cu(CH3CN)4]PF6
and [Cu(CH3CN)4]ClO4 were synthesized according to literature
procedures.53
Physical measurements
Elemental analyses were obtained either from the University of
Erlangen-Nürnberg or the Facility for Ecological and Analytical
Testing (FEAT) laboratory, Indian Institute of Technology, Kanpur.
Solution electrical conductivity measurements were carried out
with an Elico (Hyderabad, India) Type M-82 T conductivity bridge
with a solute concentration of ≈1.0 × 10−3 M. Spectroscopic data
were obtained by using the following instruments: infrared, Bruker
2,6-Bis[N-(2-pyridylethyl)-N-(2-pyridylmethyl)amino]-4-
methylphenol (Me–L3–OH). A solution of 2,6-bis(chloromethyl)-
4-methylphenol47 in MeOH (20 cm3) was added dropwise to a
vigorously stirred solution of 2-pyridylmethyl(2-pyridylethyl)-
methylamine57 (0.50 g, 2.44 mmol) and Et3N (0.49 g, 4.88 mmol)
in MeOH (40 cm3) at 0 °C. The solution was then stirred at ~10 °C
for 3 h and then stirred overnight at ~30 °C. Solvent was removed
in vacuo and the ligand was extracted with CHCl3. The organic
layer was washed first with a saturated brine solution and then with
distilled water and dried over anhydrous Na2SO4. Removal of the
solvent in vacuo afforded Me–L3–OH as a brown oil (0.67 g, 88%)
that can be further purified by chromatography using basic alumina
oxide with ethyl acetate as eluent (Rf = 0.50). H (300 MHz; solvent
CDCl3; standard SiMe4) 8.54 [2 H, d, 3JHH = 4.4 Hz, py–H], 8.48 [2
H, d, 3JHH = 4.4 Hz, py–H], 7.54–7.44 [4 H, m, py–H], 7.25–7.01 [8
H, m, py–H], 6.84 [2 H, s, Ar–H], 3.86 [4 H, s, pyCH2N–], 3.76 [4
H, s, ArCH2N–], 3.00 [8 H, s, pyCH2CH2N–], 2.16 [3 H, s, –CH3].
C (75 MHz; solvent CDCl3; standard SiMe4) 160.3, 159.2, 153.5,
149.1, 148.8, 136.4, 136.2, 129.2, 127.5, 123.4, 121.9, 121.1, 59.9,
54.7, 53.9, 35.6, 20.6.
1
Vector 22; H NMR, Bruker DXP 300 AVANCE (300 MHz, Uni-
versity of Erlangen-Nürnberg); UV-vis, Agilent 8453 diode-array.
Variable-temperature solid-state magnetic susceptibility measure-
ments were performed either by the Faraday technique using a local
built magnetometer (at a fixed main field strength of ≈10 kG)50 or
a Quantum Design (Model MPMSXL-5) SQUID magnetic suscep-
tometer operating at a magnetic field of 0.5 T. Solution state mag-
netic susceptibility measurements were done by the NMR technique
of Evans54 in MeCN with a PMX-60 JEOL (60 MHz, IIT Kanpur)
NMR spectrometer. Susceptibilities were corrected by using appro-
priate diamagnetic corrections.55
Stopped-flow measurements at ambient and at low temperatures
were performed as described previously.10,56 Solutions of copper(I)
complexes were prepared by mixing stoichiometric amounts
of copper(I) salts with the appropriate ligand under argon in a
glove box (Braun, Garching, Germany; water and dioxygen less
than 1 ppm) and then transferred with gas-tight syringes to the
instrument.Adioxygen saturated solution was prepared by bubbling
dioxygen through the solvent for 20 min.
Preparation of [Cu2(L1–O)(OH)(ClO4)]ClO4 (1). L1 (0.173 g,
0.50 mmol) in MeOH (5 cm3) was added dropwise under nitrogen
to a suspension of [Cu(CH3CN)4]ClO4 (0.327 g, 1.00 mmol) in
MeOH (15 cm3). The solution turned yellow in colour and was
very sensitive towards oxidation by air. Exposure to air lead to the
formation of a deep green solution. After removal of the solvent in
vacuo a solid was obtained which was recrystallized by diffusion of
Et2O into a MeOH solution of complex 1 (0.26 g, 75%). Crystals
of 1 suitable for a single-crystal X-ray structure determination were
obtained in this manner. Found: C, 37.8; H, 3.7; N, 7.6. Calc. for
C22H26Cl2Cu2N4O10: C, 37.5; H, 3.7; N, 7.9%.
Ligand syntheses
,′-Bis[(2-pyridylethyl)aminomethyl]benzene (L1). 2-(2-
aminoethyl)pyridine 2.44 g (20 mmol) was added to a solution of
isophthalaldehyde 1.34 g (10 mmol) in MeOH (110 cm3) and the
solution was stirred for 2 h at 60 °C. NaBH4 1.00 g (26 mmol) was
added slowly to the solution and the cloudy solution was stirred
overnight. By careful addition of 10 M HCl the excess NaBH4
was destroyed and the solution was brought to a pH value of 2.
After concentration of the solution in vacuo, aqueous 5 M NaOH
was added to the residue until a pH value of 12 was reached. The
aqueous solution was extracted with CH2Cl2 (4 × 30 cm3 portions)
and the organic fractions were combined and dried over anhydrous
Na2SO4. Removal of the solvent yielded the crude product as a
yellow oil which was chromatographed on silica gel (60 Å pore
size, 70–230 mesh) with MeOH–Et3N (50:1) as eluent (Rf = 0.37)
yielding L1 as a pale-yellow oil (6.2 g, 90%). H (300 MHz; solvent
CDCl3; standard SiMe4) 8.42 [2 H, d, Ar–H], 7.49–6.93 [10 H, m,
Ar–H], 3.68 [4 H, s, ArCH2NH–], 2.98 [8 H, m, –NHCH2CH2py].
C (75 MHz; solvent CDCl3; standard SiMe4) 160.3, 149.5, 140.4,
128.9, 128.3, 127.5, 124.2, 121.4, 54.3, 49.1, 38.4.
Preparation of [Cu2(L3)(C3H6O)(ClO4)]ClO4 (3). Under inert
conditions [Cu(CH3CN)4]ClO4 (0.327 g, 1.00 mmol) was added
in small portions to a stirred solution of 0.264 g (0.5 mmol) L3 in
acetone (15 cm3). Diffusion of Et2O into this solution lead to the
formation of yellow crystals suitable for a single-crystal X-ray
structure determination.
Preparation of [Cu2(Me–L3–O)(C3H3N2)](ClO4)2·H2O (4). A
mixture of Me–L3–OH (0.10 g, 0.179 mmol), NaOMe (0.0193 g,
0.358 mmol) and pyrazole (0.0122 g, 0.179 mmol) in MeCN
(5 cm3) was stirred at 0 °C for 0.5 h.Asolution of [Cu(H2O)6](ClO4)2
(0.133 g, 0.358 mmol) in MeCN (5 cm3) was then added dropwise.
After 12 h the resulting greenish brown solution was filtered and
allowed to evaporate slowly at room temperature. A deep brownish
green microcrystalline product that deposited was filtered off,
washed with a MeCN–Et2O (1:4) mixture (5 cm3) and recrystal-
lized from a 2:1 (v/v) mixture of Et2O–MeCN (15 cm3) (0.1 g,
49%). Found: C, 47.6; H, 4.3; N, 11.6. Calc. for C38H42Cl2Cu2N8O10:
,′-Bis[N-methyl-N-(2-pyridylethyl)amino]-m-xylene (L2).
L2 was synthesized as described previously.41
,′-Bis[N-(2-pyridylethyl)-N-(2-pyridylmethyl)amino]-m-
xylene (L3). A solution of Et3N (0.475 g, 4.69 mmol) in dry MeOH
(20 cm3) was added dropwise to a solution of 2-pyridylethyl(2-
pyridylmethyl)amine57 (1.0 g, 4.69 mmol) in dry MeOH (40 cm3) at
2 3 2 6
D a l t o n T r a n s . , 2 0 0 4 , 2 3 2 1 – 2 3 2 8

View Article Online
m-xylyl spacers, N-methyl groups and one of the carbon atoms of
the ethylene spacer on each arm of the ligand in one of the mol-
ecules of 7 (the molecule shown in Fig. 5 did not show any disorder)
were disordered over two positions and were refined with isotropic
displacement parameters. The positions of hydrogen atoms in 7
were calculated assuming ideal geometries of the atoms concerned,
and their positions and thermal parameters were not refined. 3 was
refined by full-matrix least-squares methods on F2 using SHELXTL
NT 6.12.61 Absorption effects have been corrected on the basis of
multiple scans using SADABS (Tmin = 0.791, Tmax = 1.000).62 All
non-hydrogen atoms were refined with anisotropic displacement
parameters and all hydrogen atoms are geometrically positioned
with isotropic displacement parameters being 1.2 or 1.5 times Ueq
of the preceding C atom.
C, 47.1; H, 4.3; N, 11.6%. IR (KBr disc, selected peaks) ν max/cm−1:
−
3440br (OH); 1090 and 630 (ClO4 ). Molar conductance, M
(MeCN, 298 K) = 245 Ω−1 cm2 mol−1 (expected value for a 1:2
electrolyte:58 220–300 Ω−1 cm2 mol−1). UV/vis (MeCN) max/nm
(/M−1 cm−1): 660 (sh) (250), 470 (sh) (1000), 290 (sh) (9400) and
258 (19000). eff/Cu (in MeCN, 298 K) 1.70 B.
Preparation of [Cu2(Me–L3–O)(O2CMe)](ClO4)2·2H2O (5). A
mixture of Me–L3–OH (0.10 g, 0.179 mmol) and Et3N (0.018 g,
0.178 mmol) in MeCN (5 cm3) was stirred at 0 °C for 20 min.
[Cu(H2O)6](ClO4)2 (0.133 g, 0.358 mmol) was added and the
mixture was stirred for 5 min resulting in a colour change from
light brown to dark brown. A solution of NaO2CMe·3H2O (0.024 g,
0.179 mmol) in MeOH (2 cm3) under magnetic stirring was then
added. During the addition the colour changed from deep brown
to deep greenish brown. After 4 h of stirring the reaction mixture
was filtered through a Celite pad and the filtrate kept for slow
evaporation. The solid obtained was filtered off and recrystallized
from a 1:2 (v/v) mixture (15 cm3) of MeCN–Et2O (0.112 g, 58%).
Found: C, 45.4; H, 4.7; N, 8.8. Calc. for C37H44Cl2Cu2N6O13: C,
45.4; H, 4.5; N, 8.6%. IR (KBr disc, selected peaks) ν max/cm−1:
CCDC reference numbers 211842 (1), 238967 (3) and
211448 (7).
See http://www.rsc.org/suppdata/dt/b4/b406329p/ for crystallo-
graphic data in CIF or other electronic format.
Acknowledgements
Financial support from the Volkswagen-Stiftung (Germany) is
gratefully acknowledged. F. T. thanks the Fonds der Chemischen
Industrie for providing him with a scholarship. Prof. Rudi van
Eldik (University of Erlangen-Nürnberg) is acknowledged for
his support of this work. The work of the group of R. M. was
supported by grants from the Department of Science & Technology
(DST) and the Council of Scientific & Industrial Research (CSIR),
Government of India. J. M. gratefully acknowledges the award of a
fellowship by CSIR. Furthermore, we are thankful for the valid and
important comments of the referees.
−
3436 (OH); 1570 and 1445 (OAc); 1084 and 626 (ClO4 ). Molar
conductance, M (MeCN, 298 K) = 260 Ω−1 cm2 mol−1. UV/vis
(MeCN) max/nm (/M−1 cm−1): 680 (sh) (250), 450 (sh) (1250),
290 (sh) (9400) and 258 (sh) (19400). eff/Cu (in MeCN, 298 K)
1.72 B.
Preparation of [Cu2(Me–L3–O)(N3)](ClO4)2·2H2O (6).
This compound was prepared in the same way as 4 using NaN3
(0.012 g, 0.179 mmol) as the bridging ligand; microcrystals of
6 were obtained and recrystallized from a 1:2 (v/v) mixture
(15 cm3) of MeCN–Et2O (0.121 g, 64%). Found: C, 44.0; H, 4.5;
N, 13.3. Calc. for C35H41Cl2Cu2N9O11: C, 43.7; H, 4.3; N, 13.1%.
References
−
IR (KBr disc, selected peaks) ν max/cm−1: 3440 (OH); 2076 (N3 );
1 A. Sánchez-Ferrer, J. N. Rodriguéz-López, F. García-Cánovas and
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−
1090 and 630 (ClO4 ). Molar conductance, M (MeCN, 298 K) =
290 Ω−1 cm2 mol−1. UV/vis (MeCN) max/nm (/M−1 cm−1): 660 (sh)
(300), 500 (sh) (800), 390 (sh) (1560), 290 (sh) (8580) and 258
(17870). eff/Cu (in MeCN, 298 K) 1.55 B.
Preparation of [Cu2(L2–O)Cl3] (7). Asolution of [Et4N]Cl·xH2O
(0.075 g, 0.402 mmol) in MeCN (5 cm3) was added dropwise
to a magnetically stirred MeCN (5 cm3) solution of [Cu2(L2–O)-
40
(OH)](ClO4)2 (0.060 g, 0.082 mmol). During the progress of
the reaction the colour of the solution changed from deep green to
reddish-brown. After an additional stirring for 4 h the solution was
concentrated in vacuo and Et2O was slowly allowed to diffuse into
the solution. Shiny red-brown crystals of 7 were obtained within
two days (0.020 g, 40%), which were suitable for a single-crystal X-
ray structure determination. Found: C, 45.8; H, 4.6; N, 8.7. Calc. for
C24H29Cl3Cu2N4O: C, 46.3; H, 4.7; N, 9.0%. Molar conductance, M
(DMF, 298 K) = 30 Ω−1 cm2 mol−1 (expected value for a 1:1 electro-
lyte:58 65–90 Ω−1 cm2 mol−1). UV/vis (DMF) max/nm (/M−1 cm−1):
980 (sh) (100), 805 (140), 460 (730) and 284 (sh) (2000).
13 T. N. Sorrell and M. L. Garrity, Inorg. Chem., 1991, 30, 210.
14 T. N. Sorrell, V. A. Vankai and M. L. Garrity, Inorg. Chem., 1991, 30,
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J. A. Halfen, V. G. Young, T. A. Kaden, L. Que, Jr., A. D. Zuberbühler
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18 K. D. Karlin, Z. Tyeklár andA. D. Zuberbühler, Bioinorganic Catalysis,
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19 S. Schindler, Eur. J. Inorg. Chem., 2000, 2311.
20 A. G. Blackman and W. B. Tolman, Struct. Bonding (Berlin), 2000, 97,
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Crystallography
Data collection and refinement details for 1, 3 and 7. Intensity
data for 1 and 7 were collected on an Enraf Nonius CAD-4-Mach
four-circle diffractometer (–2 scan technique) (University of
Erlangen-Nürnberg and IIT Kanpur, respectively) and for 3 on a
Nonius Kappa CCD instrument using graphite-monochromated
Mo-K radiation ( = 0.71073 Å). Intensity data for 1, 3 and 7
were corrected for Lorentz-polarization effects. The structures
were solved by direct methods for 1 and 3 and Patterson heavy-
atom method for 7. Complexes 1 and 7 were refined by full-matrix
least-squares methods on F2 using SHELXL-9759 which was incor-
porated in the WINGX 1.61 collective crystallographic package.60
All non-hydrogen atoms were refined with anisotropic thermal
parameters. Problems during the refinement procedure for 7 were
encountered; the problem was due to the poor diffracting nature
of the crystals. All carbon atoms of the methylene groups of the
21 G. Battaini, L. Casella, M. Gullotti, E. Monzani, G. Nardin, A. Perotti,
L. Randaccio, L. Santagostini, F. W. Heinemann and S. Schindler, Eur.
J. Inorg. Chem., 2003, 6, 1197.
22 M. G. B. Drew, J. Trocha-Grimshaw and K. P. McKillop, Polyhedron,
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