Synthesis, structure and catecholase activity study of dinuclear copper(II) complexes

Article abstract of DOI:10.1039/a704245k

A series of dinuclear copper(II) complexes as models for the type 3 copper protein catechol oxidase were synthesised. They were characterised by spectroscopic, electrochemical, and in some cases, by single-crystal X-ray diffraction studies: [Cu₂(L¹)(OH)(EtOH)(H₂O)][ClO₄] ₂·H₂O 1, [Cu₂(L²)(OH)][NO₃]₂ 2, [Cu₂(L³)(OMe)(MeOH)-(ClO₄)]ClO₄ 3, [Cu₂(L⁴)(OH)(MeOH)₂][BF₄] ₂ 4, [Cu₂(L⁵)(OMe)][ClO₄]₂·2MeOH 5, [Cu₂(L⁶)(OMe)(MeOH)-(ClO₄)]ClO₄ 6 and [Cu₂(L⁷)(OMe)(MeOH)(ClO₄)]ClO₄ 7 (HL¹ = 4-bromo-2,6-bis(4-methylpiperazin-1-ylmethyl)-phenol, HL² = 4-bromo-2,6-bis[(2-pyridylmethyl)aminomethyl]phenol, HL³ = 4-bromo-2,6-bis{[2-(2-pyridyl)-ethyl]aminomethyl}phenol, HL⁴ = 4-bromo-2,6-bis{[2-(1-methyl-2-imidazolyl)ethyl]aminomethyl}phenol, HL⁵ = 4-bromo-2-(4-methylpiperazin-1-ylmethyl)-6-[(2-pyridylmethyl)aminomethyl]phenol, HL⁶ = 4-bromo-2-(4-methylpiperazin-1-ylmethyl)-6-{[2-(2-pyridyl)ethyl]aminomethyl} phenol and HL⁷ = 4-bromo-2-(4-methyl-piperazin-1-ylmethyl)-6-{[2-(1-methyl-2-imidazolyl)ethyl] aminomethyl}phenol). The copper centres are μ-phenoxo bridged by the pentadentate dinucleating ligand and exogenously μ-hydroxo or μ-methanolato bridged. Complex 1 crystallises in P2₁/c with a = 16.913(3), b = 11.046(2), c = 16.617(3) A, β = 94.06(3)°, U = 3097 A³ and Z = 4. Complex 3 crystallises in P2¹/n with a = 10.592(2), b = 12.123(2), c = 24.482(5) A, β = 92.97(3)°, U = 3139 A³ and Z = 4. Complex 4 crystallises in Pbcn with a = 16.043(3), b = 12.689(3), c = 15.810(3) A, U = 3218 A³ and Z = 4. Complex 6 crystallises in P2₁/c with a = 17.889(4), b = 10.401(2), c = 16.269(4) A, β = 92.94(2)°, U = 3023 A³ and Z = 4. Complex 7 crystallises in P2₁/c with a = 17.349(3), b = 8.828(2), c = 19.797(4) A, β = 94.04(3)°, U = 3025 A³ and Z = 4. A catecholase activity study revealed that only complexes 1, 5, 6 and 7 have significant catalytic activity with respect to the aerial oxidation of 3,5-di-tert-butylcatechol. A kinetic treatment on the basis of the Michaelis-Menten model was applied.

Full text of DOI:10.1039/a704245k

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Synthesis, structure and catecholase activity study of dinuclear
copper(II) complexes†
Jörg Reim and Bernt Krebs*
Anorganisch-Chemisches Institut der Universität Münster, Wilhelm-Klemm-Strasse 8,
D-48149 Münster, Germany
A series of dinuclear copper() complexes as models for the type 3 copper protein catechol oxidase were
synthesised. They were characterised by spectroscopic, electrochemical, and in some cases, by single-crystal X-ray
diffraction studies: [Cu2(L1)(OH)(EtOH)(H2O)][ClO4]2ؒH2O 1, [Cu2(L2)(OH)][NO3]2 2, [Cu2(L3)(OMe)(MeOH)-
(ClO4)]ClO4 3, [Cu2(L4)(OH)(MeOH)2][BF4]2 4, [Cu2(L5)(OMe)][ClO4]2ؒ2MeOH 5, [Cu2(L6)(OMe)(MeOH)-
(ClO4)]ClO4 6 and [Cu2(L7)(OMe)(MeOH)(ClO4)]ClO4 7 (HL1 = 4-bromo-2,6-bis(4-methylpiperazin-1-ylmethyl)-
phenol, HL2 = 4-bromo-2,6-bis[(2-pyridylmethyl)aminomethyl]phenol, HL3 = 4-bromo-2,6-bis{[2-(2-pyridyl)-
ethyl]aminomethyl}phenol, HL4 = 4-bromo-2,6-bis{[2-(1-methyl-2-imidazolyl)ethyl]aminomethyl}phenol,
HL5 = 4-bromo-2-(4-methylpiperazin-1-ylmethyl)-6-[(2-pyridylmethyl)aminomethyl]phenol, HL6 = 4-bromo-2-
(4-methylpiperazin-1-ylmethyl)-6-{[2-(2-pyridyl)ethyl]aminomethyl}phenol and HL7 = 4-bromo-2-(4-methyl-
piperazin-1-ylmethyl)-6-{[2-(1-methyl-2-imidazolyl)ethyl]aminomethyl}phenol). The copper centres are µ-phenoxo
bridged by the pentadentate dinucleating ligand and exogenously µ-hydroxo or µ-methanolato bridged. Complex
1 crystallises in P21/c with a = 16.913(3), b = 11.046(2), c = 16.617(3) Å, β = 94.06(3)Њ, U = 3097 Å3 and Z = 4.
Complex 3 crystallises in P21/n with a = 10.592(2), b = 12.123(2), c = 24.482(5) Å, β = 92.97(3)Њ, U = 3139 Å3 and
Z = 4. Complex 4 crystallises in Pbcn with a = 16.043(3), b = 12.689(3), c = 15.810(3) Å, U = 3218 Å3 and Z = 4.
Complex 6 crystallises in P21/c with a = 17.889(4), b = 10.401(2), c = 16.269(4) Å, β = 92.94(2)Њ, U = 3023 Å3 and
Z = 4. Complex 7 crystallises in P21/c with a = 17.349(3), b = 8.828(2), c = 19.797(4) Å, β = 94.04(3)Њ, U = 3025 Å3
and Z = 4. A catecholase activity study revealed that only complexes 1, 5, 6 and 7 have significant catalytic activity
with respect to the aerial oxidation of 3,5-di-tert-butylcatechol. A kinetic treatment on the basis of the Michaelis–
Menten model was applied.
The oxidation of organic substrates with molecular oxygen
under mild conditions is of great interest for industrial and syn-
thetic processes both from an economical and environmental
point of view.1 Although the reaction of organic substances
with dioxygen is thermodynamically favoured, it is kinetically
unfavoured due to the triplet ground state of O2. In biological
systems this problem is overcome by the use of copper- or iron-
containing metalloproteins which serve as highly efficient oxid-
ation catalysts.2 The synthesis and investigation of functional
model complexes for metalloenzymes with oxidase or oxygenase
activity is therefore of great promise for the development of new
and efficient catalysts for oxidation reactions.
Type 3 copper proteins are characterised by a dinuclear cop-
per active site which is antiferromagnetically coupled in the
oxidised state and therefore EPR silent.2,3 Well known repre-
sentatives of type 3 copper proteins are hemocyanin,4 the
dioxygen carrier for arthropods and mollusks, and tyrosinase,5
which catalyses the hydroxylation of tyrosine to dopa [3-(3,4-
dihydroxyphenyl)alanine] (cresolase activity) and the oxidation
of dopa to dopaquinone (catecholase activity). In contrast to
tyrosinase, other copper-containing polyphenol oxidases only
catalyse the aerial oxidation of o-diphenols (catechols) to the
corresponding o-quinones without acting on tyrosine.6 This
reaction is of great importance in medical diagnosis for the
determination of the hormonally active catecholamines adren-
aline, noradrenaline and dopa.7 X-Ray absorption spectroscopy
investigations on the native met forms of catechol oxidases (EC
1.10.3.1) from Lycopus europaeus and Ipomoea batatas have
revealed that the active site consists of a dicopper() centre,
where the metal atoms are co-ordinated by four N/O donor
ligands.8 Multiple scattering calculations have shown a high
significance for one or two co-ordinating histidine residues.8c
The short metal–metal distance of 2.9 Å and the results of EPR
investigations indicate a di-µ-hydroxo bridged dicopper()
active site in the met forms of the proteins.8
The catecholase activity of copper co-ordination compounds
with different structural parameters has been investigated to
some extent.9 In these studies mono- or multi-nuclear com-
plexes have been employed and the properties of the chelating
ligands with respect to architecture, number and nature of the
donor atoms have been varied. Nishida and co-workers9a have
found that square-planar mononuclear copper() complexes
exhibit only little catalytic activity while non-planar mono-
nuclear copper() complexes show a high catalytic activity.
Dinuclear complexes also catalyse the oxidation process if the
Cu ؒ ؒ ؒ Cu distance is less than 5 Å. A steric match between
substrate and complex is believed to be the determining factor:
two metal centres have to be located in close proximity to facili-
tate binding of the two hydroxyl oxygen atoms of catechol prior
to the electron transfer.9a This view is supported by the observ-
ation that dinuclear copper complexes are generally more
reactive towards the oxidation of catechols than are corre-
sponding mononuclear species.9b,c But so far only one crystal
structure of a catalytically active dinuclear copper() complex
with a co-ordinating catecholato ligand has been reported.9d No
direct correlation between the rates of reaction and the redox
potentials of complexes have been determined. Malachowski et
al.9e have pointed out that a window of redox potentials for
effective catalysis exists with a balance between ease of reduc-
tion by the substrate and subsequent reoxidation by molecular
oxygen. Although some general correlations of structure–
reactivity patterns have been found, the exploration of the
oxidation chemistry of structurally well characterised copper
complexes is still needed to fully understand the parameters
affecting their catecholase activity.
† Dedicated to Professor Gottfried Huttner on the occasion of his 60th
birthday.
J. Chem. Soc., Dalton Trans., 1997, Pages 3793–3804
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obtained by a Schiff-base condensation of 4-bromo-2,6-
Br
Br
diformylphenol with 2 equivalents of the appropriate primary
amine followed by reduction with sodium tetrahydroborate.
The asymmetric ligands were prepared using a route provided
by Fenton and co-workers.10 The asymmetric substituted
phenol 5-bromo-2-hydroxybenzaldehyde serves as the starting
material. The piperazine arm is introduced by a Mannich reac-
tion. The aldehyde function is then treated with 1 equivalent of
the appropriate primary amine and subsequently reduced to the
secondary amine.
N
N
NH
N
N
N
OH
HN
N
OH
HL1
HL2
Experimental
Br
Br
Materials and methods
All reagents were purchased from commercial sources and used
as received. If necessary solvents were dried by standard pro-
cedures. Proton NMR spectra were recorded on a Bruker WH
300 instrument; all chemical shifts are reported in parts per
million (ppm) relative to an internal standard of tetramethyl-
silane. Electronic spectra were recorded on a Shimadzu UV-
3100 spectrophotometer. Elemental analyses were performed
on a Heraeus CHN-O-RAPID instrument. Cyclovoltammetric
measurements were performed on a BAS CV-50W instrument
in acetonitrile solution that was 0.1 mol dmϪ3 in tetrabutyl-
ammonium perchlorate and 10Ϫ3 mol dmϪ3 in metal complex at
a glassy carbon electrode with an Ag–AgCl–3 mol dmϪ3 NaCl
reference electrode and a platinum wire as the auxiliary
electrode.
NH
NH
N
HN
N
OH
HN
N
OH
N
N
N
HL3
HL4
Br
Br
NH
N
NH
N
N
OH
N
N
OH
Ligand syntheses
4-Bromo-2,6-bis(hydroxymethyl)phenol,11
4-bromo-2,6-di-
N
formylphenol12 and 2-(2-aminoethyl)-1-methylimidazole di-
hydrochloride13 were prepared following published procedures.
The ligand HL1 is obtained analytically pure. The ligands HL2–
HL7 are isolated as crude products but are sufficiently pure for
the complex syntheses.
HL5
HL6
Br
4-Bromo-2,6-bis(4-methylpiperazin-1-ylmethyl)phenol HL1.
The ligand was prepared according to a literature procedure.14
4-Bromophenol (5.19 g, 30 mmol), 1-methylpiperazine (7.77
cm3, 70 mmol) and paraformaldehyde (2.10 g, 70 mmol) were
dissolved in ethanol (60 cm3) and refluxed for 24 h. After evap-
oration of the solvent the residue was treated with a 10%
aqueous solution of sodium carbonate (30 cm3) and sub-
sequently extracted twice with diethyl ether (30 cm3) and once
with dichloromethane (30 cm3). The combined organic phases
were dried over sodium sulfate. After evaporation of the solvent
a light yellow oil was obtained, which slowly crystallises upon
standing. Yield: 9.20 g (23.2 mmol, 77%), m.p. 65–69 ЊC
(Found: C, 54.07; H, 8.04; N, 14.05. C18H29BrN4O requires C,
NH
N
N
N
OH
N
HL7
Fig. 1 Pentadentate dinucleating ligands employed for the syntheses
of the complexes
1
54.40; H, 7.36; N, 14.10%). H NMR (CDCl3): δ 2.29 (s, 6 H),
In this work, we report on the synthesis of a series of dinu-
clear copper() complexes as potential structural and func-
tional models for the active site of catechol oxidase. The design
of the complexes is based (a) on the natural system, that means
the active site of catechol oxidase, and (b) on the known
structure–reactivity relationships. For this purpose, we syn-
thesised pentadentate dinucleating ligands with a N4O donor
set (Fig. 1). The copper centres were expected to be bridged by
the endogenous phenolate group of the ligand and by an
exogenous hydroxide or alcoholate anion from the solvent,
which should result in a Cu ؒ ؒ ؒ Cu distance of about 3 Å. In
order to easily identify parameters possibly affecting the activ-
ity, we only performed small structural variations in the ligands
within the series. These include chelate ring sizes, the nature of
the nitrogen donors and the symmetry/non-symmetry of the
pendant arms.
2.47 (br s, 8 H), 2.56 (br s, 8 H), 3.59 (s, 4 H), 7.17 (s, 2 H).
4-Bromo-2,6-bis[(2-pyridylmethyl)aminomethyl]phenol HL2.
4-Bromo-2,6-diformylphenol (2.29 g, 10 mmol) was dissolved
in warm ethanol (80 cm3). 2-(Aminomethyl)pyridine (2.16 g, 20
mmol) was added and the solution was heated under reflux for
3 h. The resulting red solution was cooled in an ice–water bath,
and sodium tetrahydroborate (1.01 g, 26.7 mmol) added in
small portions. The suspension was stirred overnight and
refluxed for 1 h. After cooling to room temperature the solution
was treated with 12 mol dmϪ3 hydrochloric acid (4 cm3) and
vigorously stirred for 30 min. A pH of 8–9 was adjusted by
addition of about 8 cm3 of 5 mol dmϪ3 sodium hydroxide. Pre-
cipitated sodium borate was filtered off, and the volume of the
filtrate reduced by evaporation. The residue was extracted with
chloroform and dried over sodium sulfate. Evaporation of the
solvent yielded an orange-red oil. Yield: 3.41 g (8.3 mmol, 83%)
(Found: C, 54.99; H, 4.98; N, 12.68. C20H21BrN4O requires C,
The symmetric ligand HL1 is easily accessible in one step in a
Mannich reaction. The other symmetric ligands HL2–HL4 were
3794
J. Chem. Soc., Dalton Trans., 1997, Pages 3793–3804

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1
1
58.12; H, 5.12; N, 13.56%). H NMR (CDCl3): δ 3.89 (s, 4 H),
57.28; H, 6.49; N, 13.36%). H NMR (CDCl3): δ 2.30 (s, 3 H),
2.47 (br s, 4 H), 2.57 (br s, 4 H), 3.02 (m, 4 H), 3.62 (s, 2 H), 3.83
(s, 2 H), 7.07 (d, 1 H), 7.11 (m, 1 H), 7.17 (m, 1 H), 7.20 (d, 1 H),
7.59 (m, 1 H), 8.53 (m, 1 H).
3.92 (s, 4 H), 7.13 (s, 2 H), 7.17 (m, 2 H), 7.27 (m, 2 H), 7.63 (m,
2 H), 8.56 (m, 2 H).
4-Bromo-2,6-bis{[2-(2-pyridyl)ethyl]aminomethyl}phenol
HL3. The procedure for the synthesis was analogous to the syn-
thesis of HL2 using 2-(2-aminoethyl)pyridine (2.45 g, 20 mmol).
Yield: 3.56 g (8.1 mmol, 81%) of an orange oil (Found: C,
58.65; H, 5.83; N, 12.28. C22H25BrN4O requires C, 59.87; H,
5.71; N, 12.69%). 1H NMR (CDCl3): δ 3.21 (m, 8 H), 4.06 (s, 4
H), 7.13 (m, 2 H), 7.16 (m, 2 H), 7.24 (s, 2 H), 7.62 (m, 2 H),
8.43 (m, 2 H).
4-Bromo-2-(4-methylpiperazin-1-ylmethyl)-6-{[2-(1-methyl-2-
imidazolyl)ethyl]aminomethyl}phenol HL7. 2-(2-Aminoethyl)-1-
methylimidazole dihydrochloride (3.96 g, 20 mmol) and
potassium hydroxide (2.24 g, 40 mmol) were dissolved in hot
methanol (100 cm3). Then 4-bromo-2-formyl-6-(4-methyl-
piperazin-1-ylmethyl)phenol (6.26 g, 20 mmol) was added to
the suspension (precipitated potassium chloride) and refluxed
for 3 h. Further synthesis and work-up was analogous to the
described synthesis of HL4. Yield: 7.01 g (16.6 mmol, 83%) of a
yellow viscous oil (Found: C, 50.62; H, 6.26; N, 14.44.
C19H28BrN5O requires C, 54.03; H, 6.68; N, 16.58%). 1H NMR
(CDCl3): δ 2.31 (s, 3 H), 2.50 (br s, 4 H), 2.60 (br s, 4 H), 2.95 (t,
2 H), 3.14 (t, 2 H), 3.56 (s, 2 H), 3.64 (s, 2 H), 3.92 (s, 2 H), 6.79
(d, 1 H), 7.09 (d, 1 H), 7.25 (d, 1 H).
4-Bromo-2,6-bis{[2-(1-methyl-2-imidazolyl)ethyl]amino-
methyl}phenol HL4. 2-(2-Aminoethyl)-1-methylimidazole di-
hydrochloride (3.96 g, 20 mmol) and potassium hydroxide (2.24
g, 40 mmol) were dissolved in hot methanol (100 cm3). Then
4-bromo-2,6-diformylphenol (2.29 g, 10 mmol) was added to
the suspension (precipitated potassium chloride) and refluxed
for 3 h. The residue which was obtained after evaporation of
the solvent was taken up in dichloromethane (50 cm3) and dried
over sodium sulfate. After removal of the solvent under reduced
pressure, the residue was dissolved in ethanol (50 cm3). The
Schiff base was reduced with sodium tetrahydroborate as
described for the synthesis of HL2. Yield: 3.62 g (8.1 mmol,
81%) of an orange oil (Found: C, 52.88; H, 6.29; N, 18.23.
C20H27BrN6O requires C, 53.69; H, 6.08; N, 18.79%). 1H NMR
(CDCl3): δ 2.85 (t, 4 H), 3.07 (t, 4 H), 3.56 (s, 6 H), 3.89 (s, 4 H),
6.79 (d, 2 H), 6.90 (d, 2 H), 7.13 (s, 2 H).
Complex syntheses
CAUTION: In general, perchlorate salts of metal complexes
with organic ligands are potentially explosive. While none of
the present perchlorate complexes has proved to be shock
sensitive, care is recommended.
[Cu2(L1)(OH)(H2O)(EtOH)][ClO4]2ؒH2O 1. Copper() per-
chlorate hexahydrate (185 mg, 0.50 mmol) and HL1 (99 mg,
0.25 mmol) were dissolved in ethanol (10 cm3). Addition of a
few drops of triethylamine led to a colour change of the solu-
tion from green to green-blue. Upon standing for a few hours
green-blue prismatic crystals precipitated, which were suitable
for X-ray diffraction analysis. Yield: 110 mg (0.134 mmol, 54%)
(Found: C, 29.00; H, 4.71; N, 6.87. C20H39BrCl2Cu2N4O13
4-Bromo-2-(4-methylpiperazin-1-ylmethyl)-6-[(2-pyridyl-
methyl)aminomethyl]phenol HL5. (a) 4-Bromo-2-formyl-6-(4-
methylpiperazin-1-ylmethyl)phenol. The described procedure is
a modification of a literature method.10b To a solution of
5-bromo-2-hydroxybenzaldehyde (24.12 g, 120 mmol) in warm
ethanol (200 cm3) was added 1-methylpiperazine (15.03 g, 150
mmol) and paraformaldehyde (4.50 g, 150 mmol). The reaction
mixture was refluxed for 24 h. The volume of the solution was
reduced by evaporation and the solution treated with a 5%
aqueous solution of sodium carbonate (30 cm3). The mixture
was extracted four times with dichloromethane (25 cm3) and the
combined organic fractions dried over sodium sulfate. Evapor-
ation of the solvent resulted in a red oil which was taken up in
the minimum amount of hot dry diethyl ether. Upon cooling,
the product crystallised as yellow needles. Yield: 18.84 g (60
mmol, 50%), m.p. 83–84 ЊC (Found: C, 49.81; H, 5.63; N, 8.87.
Calc. for C13H17BrN2O2: C, 49.85; H, 5.47; N, 8.94%). 1H NMR
(CDCl3): δ 2.32 (s, 3 H), 2.52 (br s, 4 H), 3.72 (s, 2 H), 7.37 (d, 1
H), 7.76 (d, 1 H), 10.29 (s, 1 H).
(b) 4-Bromo-2-(4-methylpiperazin-1-ylmethyl)-6-[(2-pyridyl-
methyl)aminomethyl]phenol HL5. To a solution of 4-bromo-2-
formyl-6-(4-methylpiperazin-1-ylmethyl)phenol (18.79 g, 60
mmol) in ethanol (100 cm3) was added 2-(aminomethyl)-
pyridine (6.49 g, 60 mmol). The reaction mixture was refluxed
for 3 h. Further synthesis and work-up was analogous to the
described synthesis of HL2. Yield: 19.21 g (48 mmol, 79%) of
an orange-red oil (Found: C, 53.94; H, 6.06; N, 12.94.
C19H25BrN4O requires C, 56.35; H, 6.22; N, 13.82%). 1H NMR
(CDCl3): δ 2.30 (s, 3 H), 2.48 (br s, 4 H), 2.52 (br s, 4 H), 3.64 (s,
2 H), 3.84 (s, 2 H), 3.93 (s, 2 H), 7.06 (d, 1 H), 7.15 (m, 1 H),
7.26 (d, 1 H), 7.32 (m, 1 H), 7.63 (m, 1 H), 8.55 (m, 1 H).
requires C, 29.24; H, 4.79; N, 6.82%). UV/VIS (methanol): λmax
/
nm (ε/dm3 molϪ1 cmϪ1) 355 (2070), 633 (368).
[Cu2(L2)(OH)][NO3]2 2. To a solution of copper() nitrate
trihydrate (121 mg, 0.50 mmol) and HL2 (103 mg, 0.25 mmol) in
methanol (10 cm3) was added isopropanol (20 cm3). The com-
plex was obtained as a green powder. Yield: 30 mg (0.044 mmol,
18%) (Found: C, 34.94; H, 3.24; N, 12.50. C20H21BrCu2N6O8
requires C, 35.30; H, 3.11; N, 12.35%). UV/VIS (methanol):
λmax/nm (ε/dm3 molϪ1 cmϪ1) 380–500 (sh), 643 (190).
[Cu2(L3)(OMe)(MeOH)(ClO4)]ClO4 3. Copper() perchlor-
ate hexahydrate (185 mg, 0.50 mmol) and HL3 (110 mg, 0.25
mmol) were dissolved in methanol (20 cm3). After the solution
had been allowed to stand for a few hours, dark green rod-like
crystals suitable for X-ray diffraction analysis were deposited.
Yield: 98 mg (0.188 mmol, 47%) (Found: C, 33.71; H, 3.58; N,
6.65. C24H31BrCl2Cu2N4O11 requires C, 34.75; H, 3.77; N,
6.76%). UV/VIS (methanol): λmax/nm (ε/dm3 molϪ1 cmϪ1) 366
(3470), 613 (204).
[Cu2(L4)(OH)(MeOH)2][BF4]2 4. Copper() tetrafluoro-
borate hydrate (128 mg, 0.50 mmol) and HL4 (112 mg, 0.25
mmol) were dissolved in methanol (15 cm3). After a few hours
X-ray quality green-blue needles were deposited. Yield: 110 mg
(0.133 mmol, 53%) (Found: C, 31.41; H, 4.13; N, 10.07.
C22H35B2BrCu2F8N6O4 requires C, 31.91; H, 4.26; N, 10.15%).
UV/VIS (methanol): λmax/nm (ε/dm3 molϪ1 cmϪ1) 320–420 (sh),
646 (132).
4-Bromo-2-(4-methylpiperazin-1-ylmethyl)-6-{[2-(2-pyridyl)-
ethyl]aminomethyl}phenol HL6. To a solution of 4-bromo-
2-formyl-6-(4-methylpiperazin-1-ylmethyl)phenol (6.28 g, 20
mmol) in ethanol (65 cm3) was added 2-(2-aminoethyl)pyridine
(2.44 g, 20 mmol). The reaction mixture was refluxed for 3 h.
Further synthesis and work-up was analogous to the described
synthesis of HL2. Yield: 6.21 g (14.8 mmol, 74%) of a yellow oil
(Found: C, 56.65; H, 6.57; N, 13.19. C20H27BrN4O requires C,
[Cu2(L5)(OMe)][ClO4]2ؒ2MeOH 5. Copper() perchlorate
hexahydrate (185 mg, 0.50 mmol) and HL5 (101 mg, 0.25 mmol)
were dissolved in methanol (10 cm3). A few drops of triethyl-
amine were added, accompanied by a colour change from green
to green-blue. After 1 d, small amounts of precipitate were
filtered off. The product was obtained as a green-blue micro-
J. Chem. Soc., Dalton Trans., 1997, Pages 3793–3804
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crystalline solid by gas-phase diffusion of diethyl ether into the
solution. Yield: 37 mg (0.045 mmol, 18%) (Found: C, 31.63; H,
4.17; N, 6.79. C22H35BrCl2Cu2N4O12 requires C, 32.01; H, 4.27;
N, 6.79%). UV/VIS (methanol): λmax/nm (ε/dm3 molϪ1 cmϪ1
)
330–440 (sh), 627 (266).
[Cu2(L6)(OMe)(MeOH)(ClO4)]ClO4 6. To a solution of
copper() perchlorate hexahydrate (93 mg, 0.25 mmol) and HL6
(52 mg, 0.125 mmol) in methanol (10 cm3) was added a solution
of sodium acetate trihydrate (34 mg, 0.25 mmol) in methanol
(5 cm3). After 1 d, small amounts of precipitate were filtered off.
The product was obtained as dark green block-shaped crystals
suitable for X-ray diffraction analysis by gas-phase diffusion of
diethyl ether into the solution. Yield: 61 mg (0.075 mmol, 60%)
(Found: C, 30.51; H, 3.59; N, 6.97. C22H33BrCl2Cu2N4O11
requires C, 32.73; H, 4.12; N, 6.94%). UV/VIS (methanol):
λmax/nm (ε/dm3 molϪ1 cmϪ1) 355 (2700), 630 (280).
Fig.
2
Molecular structure of the [Cu2(L1)(OH)(H2O)(EtOH)]2ϩ
cation in 1 with the atomic numbering scheme. Hydrogen atoms are
omitted for clarity
[Cu2(L7)(OMe)(MeOH)(ClO4)]ClO4 7. Copper() perchlor-
ate hexahydrate (185 mg, 0.50 mmol) and HL7 (106 mg, 0.25
mmol) were dissolved in methanol (10 cm3). A few drops of
triethylamine were added, upon which the colour of the solu-
tion turned from green to green-blue. After 1 d, small amounts
of precipitate were filtered off. Gas-phase diffusion of diethyl
ether into the reaction mixture led to green-blue plate-like crys-
tals of the product suitable for crystal-structure determination.
Yield: 61 mg (0.075 mmol, 60%) (Found: C, 29.92; H, 3.86; N,
8.43. C21H34BrCl2Cu2N5O11 requires C, 31.12; H, 4.23; N,
8.64%). UV/VIS (methanol): λmax/nm (ε/dm3 molϪ1 cmϪ1) 342
(2090), 631 (252).
Catecholase activity study
The reaction of complexes 1–7 with 3,5-di-tert-butylcatechol
(3,5-DTBC) was monitored as follows: 2 cm3 of a 10Ϫ4 mol
dmϪ3 solution of complex in methanol were treated with 0.1
cm3 of a 0.1 mol dmϪ3 solution of 3,5-DTBC in methanol. The
UV/VIS spectra of the original solution, of the solution directly
after the addition and after 5, 10, 15 and 20 min were recorded
and corrected for volume changes.
The titration of complexes 1, 5, 6 and 7 with tetrachlorocate-
chol (TCC) was carried out by adding 0.02 cm3 aliquots of a
0.02 mol dmϪ3 solution of TCC to 2 cm3 of a 2 × 10Ϫ4 mol dmϪ3
solution of complex in methanol. The thermodynamic equi-
librium was reached immediately. The recorded spectra were
corrected for volume changes.
Initial rates of catechol oxidation were determined by the
following procedure. To 2 cm3 of a methanolic solution of the
complexes 1, 5, 6 and 7, respectively, was added 0.1 cm3 of a
methanolic solution of 3,5-DTBC. The absorption at 400 nm
was measured as a function of time over the first 2 min. The
slope at t = 0 was used to calculate the rate. The mean value of
three experiments was determined. The maximum deviation
from the mean value did not exceed 10%.
X-Ray crystallography
Intensity data for complexes 1 and 6 were collected on a Syntex
P21 and a Siemens P3 four-cycle diffractometer, respectively
(Mo-Kα, λ = 0.710 73 Å, graphite monochromator), by using
the ω-scan technique with a variable scan rate of 2.93–29.30Њ
minϪ1. The intensities of two reflections were monitored and no
significant crystal deterioration was observed. An empirical
absorption correction was applied (ψ scan). Intensity data for
complexes 3, 4 and 7 were collected on a STOE IPDS diffract-
ometer (Mo-Kα, λ = 0.710 73 Å, graphite monochromator) with
a sample-to-plate distance of 60 mm and a scan range from 0 to
180Њ with an exposure time of 4 min per 1.5Њ increment for 3,
5 min per 2.5Њ increment for 4, and 3 min per 2.0Њ increment for
7. A combined absorption and decay correction was applied
(program DECAY15). Further data collection parameters are
summarised in Table 1.
The structures were solved by using direct methods with the
exception of 7, which was solved by a Patterson synthesis (pro-
gram XS16). A series of full-matrix least-squares refinement
cycles on F 2 (program SHELXL 9317) followed by Fourier syn-
theses gave all remaining atoms. The hydrogen atoms were
placed at calculated positions, except for the positions of the
hydrogen atoms bonded to the oxygen atoms O(2), O(3) and
O(4) of the co-ordinated solvent molecules in 1, which were
taken from the Fourier map. The hydrogen atoms were con-
strained to ‘ride’ on the atom to which they are attached. The
isotropic thermal parameters for the methyl and hydroxyl pro-
tons were refined with 1.5 times, and for all other hydrogen
atoms with 1.2 times, the Ueq value of the corresponding atom.
The non-hydrogen atoms were refined with anisotropic thermal
parameters with the following exceptions of disordered atoms
in the counter ions: the fluorine atoms F(5), F(6), F(7) and F(8)
of the tetrafluoroborate in 4 and the oxygen atoms O(24A) and
O(24B) of a perchlorate in 6. The occupancy factors for the
disordered atoms are 0.75(2) for F(1), F(2), F(3), F(4) and
0.25(2) for F(5), F(6), F(7), F(8) in 4, and 0.48(1) for O(24A)
and 0.52(1) for O(24B) in 6.
Results and Discussion
Description of the crystal structures
The crystal structures of complexes 1, 3, 4, 6 and 7 were deter-
mined. In general, all complexes consist of dinuclear cations in
which the copper centres are bridged by the phenolate group
[O(1)] of the respective deprotonated pentadentate ligand and
an exogenous hydroxide or methanolate anion [O(2)]. The
metal–metal distances vary between 2.874(1) and 3.002(1) Å.
Each of the copper atoms has two nitrogen donors [N(1)–N(4)]
of the ligand as terminal equatorial ligands. The axial positions
are occupied by one or two solvent molecules or counter anions
with elongated distances typical for Jahn–Teller distortion. The
structures of the cations are shown in Figs. 2–6, selected inter-
atomic distances and angles are given in Tables 2–6. Within the
common structural motif, each of the complexes shows its own
substantial characteristic features which is best documented in
Fig. 7 showing projections of the cations along their O(2)᎐O(1)
vectors.
[Cu2(L1)(OH)(H2O)(EtOH)][ClO4]2ؒH2O 1. The piperazine
rings of [L1]Ϫ are present in a boat conformation which is neces-
sary to co-ordinate via both nitrogen atoms. Owing to the steric
demand of this part of the ligand framework the angles
N(1)᎐Cu(1)᎐N(2) and N(3)᎐Cu(2)᎐N(4) are unusually small
[73.5(1) and 73.9(1)Њ, respectively] and deviate considerably
CCDC reference number 186/689.
3796
J. Chem. Soc., Dalton Trans., 1997, Pages 3793–3804

Table 1 Crystallographic data and experimental details
1
3
4
6
7
Formula
M
C20H39BrCl2Cu2N4O13
821.44
C24H31BrCl2Cu2N4O11
829.42
C22H35B2BrCu2F8N6O4
828.17
C22H33BrCl2Cu2N4O11
807.41
C21H34BrCl2Cu2N5O11
810.42
Crystal system
Space group
Monoclinic
P21/c
Monoclinic
P21/n
Orthorhombic
Pbcn
Monoclinic
P21/c
Monoclinic
P21/c
a/Å
b/Å
c/Å
16.913(3)
11.046(2)
16.617(3)
94.06(3)
10.592(2)
12.123(2)
24.482(5)
92.97(3)
16.043(3)
12.689(3)
15.810(3)
90
17.889(4)
10.401(2)
16.269(4)
92.94(2)
17.349(3)
8.828(2)
19.797(4)
94.04(3)
β/Њ
U/Å3
3097
3139
3218
3023
3025
2θ Range/Њ
Lattice segment
Z
4.40–54.04
10.22–53.99
9.96–54.00
8.80–52.31
ϩh, ϩk,
4
l
h, k,
l
h, k,
l
h, k, l
4
4
4
T/K
150
1.762
1672
213
1.755
1672
213
1.709
1664
170
1.774
1632
213
1.780
1640
Dc/g cmϪ3
F(000)
Crystal size/mm
Crystal shape and colour
µ/mmϪ1
0.38 × 0.25 × 0.10
Green-blue prisms
2.90
6703
3699
0.25 × 0.10 × 0.08
Dark green rods
2.86
6694
4006
400
0.0730
0.1457
0.1397
0.25 × 0.08 × 0.06
Green-blue needles
2.65
3492
1748
0.08 × 0.25 × 0.22
Dark green blocks
2.97
5831
3072
0.30 × 0.25 × 0.10
Green-blue plates
2.97
5785
4434
384
0.0562
0.1360
0.0796
Unique data
Observed data [I > 2σ(I)]
Number of parameters
R [I > 2σ(I)], a R1
wR2
400
226
383
0.0375
0.0672
0.0814
0.0734
0.0742
0.1195
0.1831
0.1650
0.0612
0.1431
0.1241
0.1630
R (all data),a R1
wR2
0.1868
0.1585
Weighting scheme, wϪ1 b
Goodness of fit on F 2
[σ2(Fo ) ϩ (0.0233P)2]
[σ2(Fo ) ϩ (0.0422P)2 ϩ 15.6529P]
[σ2(Fo ) ϩ (0.0319P)2 ϩ 16.28P]
[σ2(Fo ) ϩ (0.0891P)2]
[σ2(Fo ) ϩ (0.0594P)2 ϩ 9.2565P]
2
2
2
2
2
0.753
1.079
1.065
0.896
1.079
¹
a R1 = ͚ |Fo| Ϫ |Fc| /͚|Fo|, wR2 = [͚w(Fo2 Ϫ Fc2)2/͚w(Fo )2]². b P = (Fo2 ϩ 2Fc2)/3.
2

View Article Online
Table 2 Selected interatomic distances (Å) and angles (Њ) for complex
1
Cu(1)᎐O(1)
Cu(1)᎐O(2)
Cu(1)᎐O(3)
Cu(1)᎐N(1)
Cu(1)᎐N(2)
Cu(1) ؒ ؒ ؒ Cu(2)
1.996(2)
1.927(3)
2.227(3)
1.979(3)
2.063(3)
2.902(1)
Cu(2)᎐O(1)
Cu(2)᎐O(2)
Cu(2)᎐O(4)
Cu(2)᎐N(3)
Cu(2)᎐N(4)
2.011(3)
1.932(3)
2.166(3)
1.982(3)
2.048(3)
Cu(1)᎐O(1)᎐Cu(2)
O(1)᎐Cu(1)᎐O(2)
O(1)᎐Cu(1)᎐O(3)
O(1)᎐Cu(1)᎐N(1)
O(1)᎐Cu(1)᎐N(2)
O(2)᎐Cu(1)᎐O(3)
O(2)᎐Cu(1)᎐N(1)
O(2)᎐Cu(1)᎐N(2)
O(3)᎐Cu(1)᎐N(1)
O(3)᎐Cu(1)᎐N(2)
N(1)᎐Cu(1)᎐N(2)
92.8(1)
84.9(1)
90.9(1)
94.7(1)
162.5(1)
99.1(1)
167.1(1)
103.7(1)
93.9(1)
102.6(1)
73.5(1)
Cu(1)᎐O(2)᎐Cu(2)
O(1)᎐Cu(2)᎐O(2)
O(1)᎐Cu(2)᎐O(4)
O(1)᎐Cu(2)᎐N(3)
O(1)᎐Cu(2)᎐N(4)
O(2)᎐Cu(2)᎐O(4)
O(2)᎐Cu(2)᎐N(3)
O(2)᎐Cu(2)᎐N(4)
O(4)᎐Cu(2)᎐N(3)
O(4)᎐Cu(2)᎐N(4)
N(3)᎐Cu(2)᎐N(4)
97.5(1)
84.3(1)
89.4(1)
94.6(1)
164.0(1)
94.2(1)
164.8(1)
103.9(1)
101.1(1)
103.5(1)
73.9(1)
Fig. 3 Structure of [Cu2(L3)(OMe)(MeOH)(ClO4)]ClO4 3 with the
atomic numbering scheme. Hydrogen atoms are omitted for clarity
Table 3 Selected interatomic distances (Å) and angles (Њ) for complex
3
Cu(1)᎐O(1)
Cu(1)᎐O(2)
Cu(1)᎐O(3)
Cu(1)᎐O(10)
Cu(1)᎐N(1)
Cu(1)᎐N(2)
Cu(1) ؒ ؒ ؒ Cu(2)
1.967(5)
1.936(5)
2.392(6)
2.704(7)
1.978(6)
1.995(6)
3.002(1)
Cu(2)᎐O(1)
Cu(2)᎐O(2)
Cu(2)᎐O(11)
Cu(2) ؒ ؒ ؒ O(20)
Cu(2)᎐N(3)
Cu(2)᎐N(4)
1.931(5)
1.925(5)
2.640(5)
3.207(8)
2.009(6)
1.961(6)
Cu(1)᎐O(1)᎐Cu(2)
O(1)᎐Cu(1)᎐O(2)
O(1)᎐Cu(1)᎐O(3)
O(1)᎐Cu(1)᎐N(1)
O(1)᎐Cu(1)᎐N(2)
O(1)᎐Cu(1)᎐O(10)
O(2)᎐Cu(1)᎐O(3)
O(2)᎐Cu(1)᎐O(10)
O(2)᎐Cu(1)᎐N(1)
O(2)᎐Cu(1)᎐N(2)
O(3)᎐Cu(1)᎐O(10)
O(3)᎐Cu(1)᎐N(1)
O(3)᎐Cu(1)᎐N(2)
O(10)᎐Cu(1)᎐N(1)
O(10)᎐Cu(1)᎐N(2)
N(1)᎐Cu(1)᎐N(2)
100.7(2)
77.6(2)
87.9(2)
89.1(2)
169.7(2)
88.4(2)
93.3(2)
89.6(2)
166.7(2)
96.8(2)
174.7(2)
85.4(3)
101.2(3)
90.8(2)
82.8(2)
96.4(2)
Cu(1)᎐O(2)᎐Cu(2)
O(1)᎐Cu(2)᎐O(2)
O(1)᎐Cu(2)᎐O(11)
O(1)᎐Cu(2)᎐N(3)
O(1)᎐Cu(2)᎐N(4)
O(2)᎐Cu(2)᎐O(11)
O(2)᎐Cu(2)᎐N(3)
O(2)᎐Cu(2)᎐N(4)
O(11)᎐Cu(2)᎐N(3)
O(11)᎐Cu(2)᎐N(4)
N(3)᎐Cu(2)᎐N(4)
102.1(2)
78.7(2)
82.2(2)
93.0(2)
169.8(2)
89.9(2)
166.5(3)
95.0(2)
99.6(2)
89.9(2)
94.6(2)
Fig. 4 Structure of [Cu2(L4)(OH)(MeOH)2][BF4]2 4 with the atomic
numbering scheme. Hydrogen atoms are omitted for clarity
Fig. 6 Molecular structure of the [Cu2(L7)(OMe)(MeOH)(ClO4)]ϩ
cation in 7 with the atomic numbering scheme. Hydrogen atoms are
omitted for clarity
Fig. 5 Molecular structure of the [Cu2(L6)(OMe)(MeOH)(ClO4)]ϩ
cation in 6 with the atomic numbering scheme. Hydrogen atoms are
omitted for clarity
The calculation of the least-squares planes defined by the
atoms of the square-planar co-ordination plane including
the central atom shows the copper ions to be displaced from the
basal planes towards the axial donors by 0.188(1) [Cu(1)] and
from the ideal 90Њ. This is compensated by the corresponding
large angles O(2)᎐Cu(1)᎐N(2) and O(2)᎐Cu(2)᎐N(4) showing
values of 103.7(1) and 103.9(1)Њ, respectively.
3798
J. Chem. Soc., Dalton Trans., 1997, Pages 3793–3804

View Article Online
Table 4 Selected interatomic distances (Å) and angles (Њ) for complex
4*
Cu᎐O(1)
Cu᎐O(3)
Cu᎐N(2)
Cu ؒ ؒ ؒ Cu(a)*
1.933(4)
2.324(6)
1.916(6)
2.995(2)
Cu᎐O(2)
Cu᎐N(1)
Cu ؒ ؒ ؒ F(1)
1.929(3)
1.974(6)
2.875(8)
Cu᎐O(1)᎐Cu(a)* 101.6(3)
Cu᎐O(2)᎐Cu(a)* 101.8(2)
O(1)᎐Cu᎐O(2)
O(1)᎐Cu᎐N(1)
O(2)᎐Cu᎐O(3)
O(2)᎐Cu᎐N(2)
O(3)᎐Cu᎐N(2)
78.3(2)
92.8(2)
89.7(2)
92.5(2)
94.2(3)
O(1)᎐Cu᎐O(3)
O(1)᎐Cu᎐N(2)
O(2)᎐Cu᎐N(1)
O(3)᎐Cu᎐N(1)
N(1)᎐Cu᎐N(2)
91.8(2)
169.0(2)
169.0(2)
97.2(3)
95.5(2)
* Symmetry transformation: Ϫx, y, 0.5 Ϫ z.
Table 5 Selected interatomic distances (Å) and angles (Њ) for complex
6
Cu(1)᎐O(1)
Cu(1)᎐O(2)
Cu(1)᎐O(3)
Cu(1)᎐N(1)
Cu(1)᎐N(2)
Cu(1) ؒ ؒ ؒ Cu(2)
1.959(4)
1.891(5)
2.431(6)
1.963(6)
2.012(6)
2.938(1)
Cu(2)᎐O(1)
Cu(2)᎐O(2)
Cu(2)᎐O(11)
Cu(2)᎐N(3)
Cu(2)᎐N(4)
Cu(2) ؒ ؒ ؒ O(21)
1.958(4)
1.911(5)
2.615(7)
1.980(7)
1.999(6)
3.031(16)
Cu(1)᎐O(1)᎐Cu(2)
O(1)᎐Cu(1)᎐O(2)
O(1)᎐Cu(1)᎐O(3)
O(1)᎐Cu(1)᎐N(1)
O(1)᎐Cu(1)᎐N(2)
O(2)᎐Cu(1)᎐O(3)
O(2)᎐Cu(1)᎐N(1)
O(2)᎐Cu(1)᎐N(2)
O(3)᎐Cu(1)᎐N(1)
O(3)᎐Cu(1)᎐N(2)
N(1)᎐Cu(1)᎐N(2)
97.2(2)
80.5(2)
87.0(2)
95.0(2)
156.2(2)
91.5(2)
175.1(2)
108.2(2)
90.1(2)
114.3(3)
75.3(3)
Cu(1)᎐O(2)᎐Cu(2) 101.2(2)
O(1)᎐Cu(2)᎐O(2)
O(1)᎐Cu(2)᎐O(11)
O(1)᎐Cu(2)᎐N(3)
O(1)᎐Cu(2)᎐N(4)
O(2)᎐Cu(2)᎐O(11)
O(2)᎐Cu(2)᎐N(3)
O(2)᎐Cu(2)᎐N(4)
O(11)᎐Cu(2)᎐N(3)
O(11)᎐Cu(2)᎐N(4)
N(3)᎐Cu(2)᎐N(4)
80.0(2)
86.5(3)
89.6(2)
171.6(2)
84.8(2)
168.6(2)
94.3(2)
89.8(3)
99.3(3)
96.5(3)
Table 6 Selected interatomic distances (Å) and angles (Њ) for complex
7
Cu(1)᎐O(1)
Cu(1)᎐O(2)
Cu(1)᎐O(3)
Cu(1)᎐N(1)
Cu(1)᎐N(2)
Cu(1) ؒ ؒ ؒ Cu(2)
1.975(4)
1.885(4)
2.274(5)
1.984(5)
2.043(5)
2.874(1)
Cu(2)᎐O(1)
Cu(2)᎐O(2)
Cu(2)᎐O(10)
Cu(2)᎐N(3)
Cu(2)᎐N(4)
1.995(4)
1.935(4)
2.532(5)
1.990(4)
1.960(5)
Cu(1)᎐O(1)᎐Cu(2)
O(1)᎐Cu(1)᎐O(2)
O(1)᎐Cu(1)᎐O(3)
O(1)᎐Cu(1)᎐N(1)
O(1)᎐Cu(1)᎐N(2)
O(2)᎐Cu(1)᎐O(3)
O(2)᎐Cu(1)᎐N(1)
O(2)᎐Cu(1)᎐N(2)
O(3)᎐Cu(1)᎐N(1)
O(3)᎐Cu(1)᎐N(2)
N(1)᎐Cu(1)᎐N(2)
92.7(2)
81.9(2)
95.4(2)
96.2(2)
154.9(2)
101.3(2)
166.7(2)
102.1(2)
92.0(2)
107.9(2)
74.2(2)
Cu(1)᎐O(2)᎐Cu(2)
O(1)᎐Cu(2)᎐O(2)
O(1)᎐Cu(2)᎐O(10)
O(1)᎐Cu(2)᎐N(3)
O(1)᎐Cu(2)᎐N(4)
O(2)᎐Cu(2)᎐O(10)
O(2)᎐Cu(2)᎐N(3)
O(2)᎐Cu(2)᎐N(4)
O(10)᎐Cu(2)᎐N(3)
O(10)᎐Cu(2)᎐N(4)
N(3)᎐Cu(2)᎐N(4)
97.6(2)
80.2(2)
84.5(2)
93.4(2)
170.4(2)
92.9(2)
173.1(2)
93.5(2)
89.0(2)
88.6(2)
93.2(2)
Fig. 7 Projection of the cations of 1, 3, 4, 6 and 7 along their
O(2)᎐O(1) vectors (co-ordinating molecules or anions in axial positions
are omitted)
which can be attributed to the conformation of the ligand and
so to the co-ordination to the metal centres: the methylene
carbon atoms bound to the phenolate group are not in plane
with the aromatic ring, they show unusually large displace-
ments of 0.238(6) [C(7)] and 0.372(6) Å [C(13)].
0.192(1) Å [Cu(2)]. Although the angle between the two planes
is 26.0(1)Њ, the central Cu2O2 moiety is essentially planar with a
Cu(1)᎐O(1)᎐Cu(2)᎐O(2) dihedral angle of 5.1(1)Њ. The residues
bound at the phenolate group have a cis conformation, meaning
that the piperazine rings are located on the same side of the
aromatic ring (see also Fig. 7). As a consequence, all atoms of
the phenolate group lie on the same side of the Cu2O2 plane.
The angle between the Cu2O2 and the aromatic ring plane is
calculated to be 38.9(1)Њ. The phenolate oxygen O(1) adopts a
trigonal-pyramidal geometry as can be expressed by the sum of
the angles around O(1) which is 337.7(5)Њ and clearly smaller
than 360Њ.
[Cu2(L3)(OMe)(MeOH)(ClO4)]ClO4 3. A least-squares calcu-
lation of the CuN2O2 planes reveals only relatively minor dis-
tortions for the basal plane of Cu(1): the deviations of the four
donors lie in the range from 0.051(3) [N(1)] to 0.095(3) Å [O(1)],
while the displacement of Cu(1) towards O(3) is 0.063(3) Å, and
therefore in between these values and so of no significance.
More notable distortions are observed for the basal plane of
Cu(2): the donor atoms are displaced from the best plane by
values between 0.129(3) [N(4)] and 0.166(3) Å [O(2)], while the
central atom shows virtually no deviation [0.019(2) Å]. The
angle between the two basal planes is 11.3(3)Њ, the central
Cu2O2 unit is essentially planar as indicated by the Cu(1)᎐
There is another striking aspect within the ligand molecule
J. Chem. Soc., Dalton Trans., 1997, Pages 3793–3804
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O(1)᎐Cu(2)᎐O(2) dihedral angle of 6.8(2)Њ. In contrast to the
situation in 1, the residues bound at the bridging phenoxo group
have a trans conformation. One half of the phenyl ring lies
‘above’, the other half ‘beneath’ the Cu2O2 plane (see also Fig.
7). The phenolate oxygen atom O(1) is in a trigonal-planar
surrounding, the sum of angles is calculated to be 359(1)Њ.
A least-squares calculation shows a mean deviation of the four
equatorial ligands from the best plane of about 0.10 Å with a
displacement of the central atom towards the pyramid apex by
0.257(2) Å. The Cu(2) atom on the other hand shows much less
distortion. The displacement of the copper centre by 0.035(2)
Å is negligible in comparison to the mean deviation of the
equatorial ligands from the best plane of about 0.08 Å. The two
edge-shared co-ordination polyhedra are unusually strongly
tilted toward each other by 41.8(1)Њ. Thereby, the central Cu2O2
unit in 7 is not planar which is expressed by the Cu(1)᎐O(1)᎐
Cu(2)᎐O(2) dihedral angle of 20.0(2)Њ. The residues of the
µ-phenoxo bridge adopt a cis conformation with respect to the
phenol ring plane. All of the ring atoms lie on the same side of
the least-squares plane defined by the Cu2O2 unit (see also Fig.
7). The phenolate oxygen atom O(1) is in a trigonal-pyramidal
geometry with its neighbours C(1), Cu(1) and Cu(2), confirmed
by the sum of angles of 341.7(8)Њ.
[Cu2(L4)(OH)(MeOH)2][BF4]2 4. A two-fold crystallographic
axis passes through O(2), O(1), C(1), C(4) and Br; thus the co-
ordination geometries around the two copper centres are
identical. A least-squares calculation of the CuN2O2 plane
shows that Cu lies only slightly [0.092(2) Å] above its basal
plane in the direction of its axial ligand, O(3). The angle
between the two least-squares planes around the copper
centres is 0.5(2)Њ. For symmetry reasons, the central Cu2O2
unit forms an exact plane. The ligand possesses a trans con-
formation with respect to the aminomethyl groups and the
phenol ring plane. The phenolate oxygen atom O(1) is exactly
trigonal planar as indicated by the sum of angles which is
calculated to be 360.0(5)Њ. Similar to 3, one half of the phe-
nol ring is ‘above’, the other half ‘beneath’ the central Cu2O2
moiety (see also Fig. 7).
Discussion of the crystal structures
The compound [Cu2(L)(OH)(H2O)2][ClO4]2ؒH2O 1a is analo-
gous to 1 and has been described by Murray and co-
workers.18 The difference between the ligand HL and HL1 is
the substituent in the position para to the phenol group,
which is chloride in HL and bromide in HL1. The cations of 1
and 1a have essentially the same co-ordination geometries but
differ in details. In the crystal of 1a two crystallographically
independent molecules are found both containing a mirror
plane. Whereas in 1a water molecules are bound to each of
the copper centres, in 1 there is one water and one ethanol
molecule resulting in an asymmetric co-ordination. In the
compound studied in this work, the distances to these axial
donors are about 0.1 Å shorter than in 1a for which they are
reported as 2.323(8) and 2.268(8) Å. Also in comparison to
other copper() complexes, the axial bond lengths are very
short, indicative of strong bonding. The Cu ؒ ؒ ؒ Cu distances
in 1a were determined to be 2.872(2) and 2.857(3) Å, respect-
ively, and are therefore significantly shorter than in 1. Where-
as in 1a the dihedral angle between the phenol ring plane and
the central Cu2O2 plane is reported to be 48.4 and 42.7Њ,
respectively, in our compound it amounts to 38.9(1)Њ. Despite
the use of analogous ligands, all of the described differences
are significant and outside of estimated standard deviations.
This cannot be ascribed simply to the influence of the para
substituent of the phenolate bridge, because there are even
deviations between the crystallographically independent com-
plex molecules in the unit cell of 1a. It seems to be more likely,
that in the first line packing effects and the hydrogen-bonding
network are responsible for the observed differences. For the
synthesis of the complexes triethylamine was used as a base for
1 and sodium hydroxide for 1a. The higher concentration of
water in the reaction solution of 1a may have led to the crystal-
lisation of the complex molecule with two co-ordinated water
molecules whereas 1 crystallises with one water and one ethanol
molecule in the axial positions of the copper centres. Hence,
despite similar co-ordinations there is a relatively broad range
for bond distances and angles. It can be assumed, that the com-
plexes show an enhanced flexibility in solution.
[Cu2(L6)(OMe)(MeOH)(ClO4)]ClO4 6. The equatorial posi-
tions of each copper ion are occupied by two nitrogen donors
of the asymmetric ligand [L6]Ϫ arising in a non-equivalent
environment for the metal centres. For Cu(1) these are two
tertiary nitrogen atoms of the piperazine ring, for Cu(2) these
are a secondary amine nitrogen and an aromatic pyridyl nitro-
gen donor. While the piperazine nitrogen atoms and Cu(1) are
embedded in two five-membered chelate rings, Cu(2) and the
nitrogen atoms N(1) and N(4) are part of a six-membered
chelate ring. This results in significantly different N᎐Cu᎐N
angles. Owing to the small bite of the piperazine group the
angle N(1)᎐Cu(1)᎐N(2) is very small [75.3(3)Њ], in contrast to
the angle N(3)᎐Cu(2)᎐N(4) which has a quite normal value of
96.5(3)Њ.
The square-planar basal plane of Cu(1) is strongly distorted.
The plane defined by N(1), N(2) and Cu(1) is twisted toward
the plane defined by O(1), O(2) and Cu(1) by an angle of
22.0(3)Њ. Calculation of the CuN2O2 least-squares plane of
Cu(2) shows the basal plane of the pyramid also to be mark-
edly distorted. The mean deviation of the atoms is 0.08 Å, but
the central atom itself is only unessentially displaced in the
direction of the axial perchlorate donor by 0.013(3) Å. The two
CuN2O2 least-squares planes are tilted towards each other by
19.3(3)Њ. Nevertheless, the central Cu2O2 unit is nearly planar
indicated by the Cu(1)᎐O(1)᎐Cu(2)᎐O(2) dihedral angle of
7.7(2)Њ. While in complex 1 the residues bound at the central µ-
phenoxo bridge definitely assume a cis and in complex 3 a trans
conformation, the situation in 6 is a mixture of both.
Although, basically a trans conformation is found, the aro-
matic ring lies completely on one side of the central Cu2O2
plane which is characteristic for cis arrangements (see also Fig.
7). This is also expressed by the sum of angles around the
phenolate oxygen atom O(1) which is 354.8(10)Њ indicating
trigonal-pyramidal character.
[Cu2(L7)(OMe)(MeOH)(ClO4)]ClO4 7. The Cu(1) atom is
co-ordinated by the tertiary aliphatic nitrogen atoms of the
piperazine ring of [L7]Ϫ, Cu(2) is co-ordinated by the secondary
aliphatic nitrogen function and the aromatic nitrogen donor of
the methylimidazole group of [L7]Ϫ. Similar to 6, this leads to a
non-equivalency in the environment of the copper centres.
Owing to the formation of two five-membered chelate rings, the
angle N(1)᎐Cu(1)᎐N(2) [74.2(2)Њ] is remarkably contracted. The
corresponding angle for Cu(2) N(3)᎐Cu(2)᎐N(4) is in a six-
membered chelate ring and is therefore allowed to adopt an
almost ideal value of 93.2(2)Њ.
Recently, the complex [Cu2LЈ(OH)]2ϩ 3a has been reported
as the product of a hydroxylation reaction (LЈ = 2,6-bis-
{N-methyl-[2-(2-pyridyl)ethyl]aminomethyl}phenolate).19 The
main differences between this and the analogous compound 3
are the tertiary aliphatic amine donor instead of a secondary
NH function and the hydroxo bridge instead of the meth-
anolato bridge. The co-ordination geometries in the cation of 3
and 3a are nearly identical considering the Cu ؒ ؒ ؒ Cu distances
[3.002(1) and 2.999(1) Å, respectively] and the dihedral angles
[6.8(2) and 7.2(2)Њ, respectively] of the central Cu2O2 unit. All
other bond lengths and angles differ only slightly. The dinuclear
metal centre in 3 is bridged by a perchlorate anion. This
The CuN2O2 basal plane of Cu(1) is considerably distorted.
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of 6 three reduction peaks are observed whereas for the other
complexes two peaks appear. The cyclic voltammogram of 2
contains particularly broad peaks, so that the first ‘peak’ is
observed as a weak shoulder in the range from about 0 to Ϫ0.3
V. These reduction peaks are assigned to the one-electron pro-
cesses CuIICuII → CuIICuI and CuIICuI → CuICuI. In the
case of 6 it has to be assumed that the third peak is due to a
further reduction to Cu0. All of the complexes exhibit a broad
anodic oxidation peak in the range from ϩ0.06 to ϩ0.18 V. The
breadth of this peak suggests a superposition of two close one-
electron processes, which are assigned to the reoxidations to the
CuIICuI and CuIICuII species, respectively.
Table 7 Cyclic voltammetric data of the complexes
Complex
Epc,1/V
Ϫ0.43
—
Ϫ0.51
Ϫ0.46
Ϫ0.38
Ϫ0.37
Ϫ0.41
Epc,2/V
Ϫ0.72
Epa/V
1
ϩ0.17
ϩ0.12
ϩ0.18
ϩ0.06
ϩ0.13
ϩ0.17
ϩ0.18
2
Ϫ0.83
Ϫ0.80
Ϫ0.72
Ϫ0.96
Ϫ0.71
Ϫ0.79
3
4
5
6*
7
* Epc,3 = Ϫ0.93 V.
These observations are typical for µ-hydroxo bridged dicop-
per() complexes and have been found in electroanalytical stud-
ies of similar or analogous compounds.18b,20h,23 Irreversibility of
redox processes in metal complexes in electrochemical experi-
ments are attributed to changes in the co-ordination geometry
or co-ordination number (e.g. solvent co-ordination/dis-
sociation) upon change of the oxidation state or even to the
expulsion of metal ions from the co-ordination sphere.24
Hydroxo bridges have a poor ability to bind to (electrogen-
erated) CuI centres,23c,25 which should also be the case for alkoxo
bridges. Therefore, the OHϪ and OMeϪ bridges, respectively,
are expected to dissociate from the co-ordination sphere upon
reduction. Due to the different preferred co-ordination poly-
hedra of CuI and CuII it is further expected that considerable
changes of the co-ordination geometry occur. Association of
acetonitrile is not unlikely because of its high binding tendency
towards CuI. On the whole, it can be postulated that the
structure of the CuICuI species (or even the CuICuII species)
differs notably from the structure of the original CuIICuII com-
plex. Considerable structural changes occur again upon reoxi-
dation. However, a reassociation to the original hydroxo or
methanolato bridged copper() complex is most unlikely: the
chance for a combination of the reoxidised CuIICuII species
with the previously dissociated oxo bridge within the cyclo-
voltammetric experiment is extremely low. This assumption is
confirmed by the fact that in a second scan the shape and pos-
ition of the reduction peaks change. The irreversible charge-
transfer processes in the cyclovoltammetric experiments do not
completely exclude the applicability of the complexes as oxid-
ation catalysts. The fact that the complexes are able to be reoxi-
dised is decisive.
structural motif is rather unusual and has so far been found in
only a few dinuclear copper() complexes.20
In the syntheses of complexes 5–7 asymmetric ligands were
employed. The crystal structures of 6 and 7 were determined.
So far only three dicopper() complexes also containing
pentadentate dinucleating ligands with µ-phenoxo bridge and a
N4O donor set have been characterised by X-ray crystal-
lography.10,21,22 These compounds are typical Schiff-base
complexes. Hence, complexes 5–7 are the first asymmetric com-
plexes within this series not containing an imine function.
The co-ordination geometries of the central atoms of the
asymmetric complexes 6 and 7 correspond to those which are
found for the metal centres in the corresponding symmetric
complexes 1, 3 and 4. This means, in 6 and 7 Cu(1) is in a
strongly distorted square-pyramidal environment dominated by
the rigid piperazine ligand and which is also found in 1. In 6 and
7 Cu(2) is surrounded by the more flexible [2-(2-pyridyl)ethyl]-
amine and [2-(1-methyl-2-imidazolyl)ethyl]amine arms, respect-
ively, forming six-membered chelate rings. The metals are
allowed to form relatively minor distorted co-ordination poly-
hedra similar to those observed in 3 and 4. In 1 the ligand has to
possess a cis conformation with respect to the piperazine arms
and the phenol ring plane to realise a µ-hydroxo bridged
dicopper() centre. Owing to the higher flexibility of the ligands
in 3 and 4, they are found in the more favourable trans con-
formation. It is interesting, that in 6 and 7 different compro-
mises were followed to balance the unlike steric constraints of
both ligand frameworks. In 6 the piperazine arm and the [2-(2-
pyridyl)ethyl]amine residue are arranged in a trans conform-
ation. This unfavourable solution for the piperazine part leads
to the observed twist within the basal plane of Cu(1). On the
contrary, the ligand in 7 adopts a cis conformation. As a con-
sequence, the basal planes of the copper atoms are extremely
tilted: the angle between both least-squares planes is calculated
to be 41.8(1)Њ, the dihedral angle within the Cu2O2 moiety is
20.0(2)Њ. This is the reason for the very short metal–metal
separation of 2.874(1) Å. It is difficult to understand why the
analogous ligands HL6 and HL7 have chosen these different co-
ordination patterns. The conformation observed in 6 could also
be realised in 7 and vice versa. Possibly, an equilibrium of two
(or more) conformers exists in solution.
Catecholase activity study and kinetics
In most of the catecholase activity studies of model complexes
3,5-di-tert-butylcatechol (3,5-DTBC) has been employed as
the substrate. The product, 3,5-di-tert-butyl-o-quinone (3,5-
DTBQ), is considerably stable and has a strong absorption at
λmax = 400 nm (ε = 1900 dm3 molϪ1 cmϪ1).9a Therefore, activities
and reaction rates, respectively, can be determined using
electronic spectroscopy by following the appearance of the
absorption maximum of the quinone. The reactivity studies
were performed in methanol solution because of the good solu-
bility of the complexes as well as of the substrate and of its
product.
Prior to a detailed kinetic study, it is necessary to get an
estimation of the ability of the complexes to oxidise catechol.
For this purpose, 10Ϫ4 mol dmϪ3 solutions of 1–7 in methanol
were treated with 50 equivalents of 3,5-DTBC in the presence
of air. The course of the reaction was followed by UV/VIS
spectroscopy over the first 20 min. The first apparent result is
that the reactivities of the complexes differ significantly from
each other. Whereas 1 and the unsymmetric complexes 5–7
show a comparatively high catecholase activity, the symmetric
complexes 2–4 show only little or no activity. Complex 1 has the
highest catecholase activity of all complexes in this study. After
5 min an increase in absorption at 400 nm of about 1.4 units is
observed, which means a turnover of more than seven equiv-
The dinuclear copper complexes 1, 3, 4, 6 and 7 represent
good structural models for the active sites of the met forms of
catechol oxidases. The complexes simulate the short copper–
copper distance of about 3 Å as well as the N2O2 donor set,
which is formed in the enzymes by two imidazoles of the amino
acid histidine and the di(µ-hydroxo) bridge.8
Electrochemical properties
Cyclic voltammograms were recorded in the potential range
from ϩ1.00 to Ϫ1.00 V vs. Ag–AgCl in acetonitrile (starting
value: ϩ1.00 V). The scan rate was 0.20 V sϪ1 in all cases, with
the exception of 2 and 4, which were measured at a scan rate of
0.30 V sϪ1. Table 7 gives the obtained peak potentials.
The cyclic voltammograms of all of the studied complexes
reveal irreversible and ill defined cathodic reduction peaks in
the range from Ϫ0.37 to Ϫ0.96 V. In the cyclic voltammogram
J. Chem. Soc., Dalton Trans., 1997, Pages 3793–3804
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Table 8 Kinetic parameters for the active complexesa
νmax/10Ϫ6
Complex kcat/hϪ1
KM/mol dmϪ3
mol dmϪ3 sϪ1
Rb
1
5
6
7
214
33
48
5
(2.4 0.2) × 10Ϫ4
(2.3 0.2) × 10Ϫ3
(3.1 0.2) × 10Ϫ4
(1.4 0.2) × 10Ϫ3
2.83 0.07
0.87 0.03
1.26 0.02
1.13 0.06
0.99 155
0.99 359
0.99 376
0.99 177
1
1
2
43
a
Standard deviations are taken from the Lineweaver–Burk plot.
b Discrepancy value of the Lineweaver–Burk plot.
transfer transition, the lower energy band is possibly due to a
catecholate → CuII charge transfer.9d The corresponding
titrations of 5, 6 and 7 differ considerably from the one just
described, but among themselves they show an analogous
behaviour towards TCC. Fig. 8(b) shows the UV/VIS spectra of
the titration of 6 with TCC. In this case the end of the titration
is reached after addition of 2 equivalents of TCC. The maxi-
mum of the charge-transfer band at 355 nm disappears, con-
comitantly a broad plateau-like band evolves between 400 and
500 nm (ε ≈ 1000 dm3 molϪ1 cmϪ1). This absorption can be
interpreted as a superposition of the phenolate and the catecho-
late → CuII charge-transfer transition. The intensity of the
d–d bands increases slightly and also a plateau-like maximum is
formed between 600 and 700 nm (ε ≈ 400 dm3 molϪ1 cmϪ1). On
the contrary, addition of TCC to solutions of the inactive com-
plexes 2, 3 and 4 leads to only insignificant changes of their UV/
VIS spectra. Thus, binding of TCC to the copper centres of the
active complexes is evident, whereas the inactive complexes
seem to be indifferent towards TCC.
The kinetics of the oxidation of 3,5-DTBC were determined
by the method of initial rates by monitoring the growth of the
400 nm band of the product 3,5-DTBQ. A linear relationship
for the initial rates and the complex concentration is obtained
for complexes 1, 5, 6 and 7, meaning a first-order dependence
on the catalyst concentration for the four systems. To determine
the dependence of the rates on the substrate concentration,
solutions of the complexes 1, 5, 6 and 7 were treated with
varying amounts of 3,5-DTBC. At low concentrations of 3,5-
DTBC (stoichiometric or slightly higher) a first-order depend-
ence of the substrate concentration was observed. At higher
concentrations, a saturation kinetics was found for all four
compounds. A treatment on the basis of the Michaelis–Menten
model, originally developed for enzyme kinetics, was applied.
In our case, we can also propose a pre-equilibrium of free com-
plex and substrate on the one hand, and a complex–substrate
adduct on the other hand. The irreversible conversion into
complex and quinone can be imagined as the rate-determining
step. Although a much more complicated mechanism may be
involved, the results show this simple model to be sufficient for
a kinetic description.
Fig. 8 (a) The UV/VIS spectra of the titration of 1 (c = 2 × 10Ϫ4 mol
dmϪ3) with 1, 2 and 3 equivalents of TCC; (b) UV/VIS spectra of the
titration of 6 (c = 2 × 10Ϫ4 mol dmϪ3) with 1 and 2 equivalents of TCC
alents of 3,5-DTBC. Complexes 6, 7 and 5 have significantly
lower activities and can be graduated in this order: the increase
of absorption at 400 nm after 20 min can be expressed in turn-
overs of 10.7, 7.0 and 5.3, respectively. Hence, these complexes
show catalytic activity. Complex 2 possesses a negligibly low
catecholase activity (turnover of 0.6 after 20 min). A detailed
kinetic analysis is therefore dispensable. The catecholase
activity of 3 is even lower, and 4 is essentially inactive.
It is remarkable, that the addition of 50 equivalents of 3,5-
DTBC to the inactive complexes has nearly no influence on
their UV/VIS spectra. The bands in the charge-transfer region
as well as the d–d bands remain unchanged with respect to
position and intensity. Therefore, binding of 3,5-DTBC to the
dinuclear copper() centre can be definitely excluded. Owing to
the developing product bands in the UV/VIS spectra of the
active complexes, it is difficult to make clear statements about
the co-ordination of catechol to the metal centre. Hence, the
complexes were titrated with tetrachlorocatechol (TCC). Owing
to its electron withdrawing substituents, this o-diphenol has a
high redox potential and is not oxidised by the copper com-
plexes. Fig. 8(a) shows the UV/VIS spectra of the titration of a
2 × 10Ϫ4 mol dmϪ3 solution of 1 with TCC. The original spec-
trum changes dramatically upon addition of TCC. After add-
ition of 3 equivalents, the end of the titration is reached. In the
charge-transfer region of about 340–450 nm the intensity
decreases, simultaneously a new weak band appears at about
500 nm. The maximum of the d–d bands shifts from 633 to 725
nm accompanied by a drastic decrease of the absorption coef-
ficient to 125 dm3 molϪ1 cmϪ1. The two bands at about 400 nm
For the determination of the kinetic parameters for 1 the
substrate concentration was varied in the range from
2.41 × 10Ϫ4 to 4.51 × 10Ϫ3 mol dmϪ3 at a complex concentration
of 4.76 × 10Ϫ5 mol dmϪ3. The concentration of 5, as well as of 6
and 7, was adjusted to 9.52 × 10Ϫ5 mol dmϪ3. The substrate
concentration was varied between 1.88 × 10Ϫ3 and 1.13 × 10Ϫ2
mol dmϪ3. For 6 a variation of substrate concentration between
4.77 × 10Ϫ4 and 9.34 × 10Ϫ3 mol dmϪ3 was chosen, and for 7 the
corresponding range was 9.24 × 10Ϫ4 to 9.39 × 10Ϫ3 mol dmϪ3
.
Table 8 contains the results evaluated from Lineweaver–Burk
plots.
Conclusion
A series of dinuclear copper() complexes has been synthesised.
The copper centres are µ-phenoxo bridged by a pentadentate
dinucleating ligand and further have an exogenous µ-hydroxo or
µ-methanolato bridge. They differ by the employed terminal
(ε ≈ 370 dm3 molϪ1 cmϪ1) and 500 nm (ε ≈ 170 dm3 molϪ1 cmϪ1
)
reveal unexpected low intensities. The higher energy band can
presumably be assigned to a phenolate → CuII charge-
3802
J. Chem. Soc., Dalton Trans., 1997, Pages 3793–3804

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nitrogen donors within the ligand. For the symmetric com-
plexes two piperazine units (1), two (2-pyridylmethyl)amine
units (2), two [2-(2-pyridyl)ethyl]amine units (3) and two [2-(1-
methyl-2-imidazolyl)ethyl]amine units (4) were used. For the
asymmetric complexes a combination of one piperazine unit
and one of the other three mentioned functional groups were
employed. The structures of the complexes 1, 3, 4, 6 and 7 could
be obtained by X-ray crystallography. They represent good
structural models for the active sites of the met forms of
catechol oxidases. They simulate the short Cu ؒ ؒ ؒ Cu distance
of about 3 Å as well as the N2O2 donor set and the Cu2O2
central unit.
The investigation of the catecholase activity of compounds
1–7 revealed that only the symmetric complex 1 and the asym-
metric complexes 5–7 have significant catalytic activity with
respect to the aerial oxidation of 3,5-DTBC to its correspond-
ing o-quinone. The decisive thermodynamic property determin-
ing an electron-transfer reaction is the redox potential of each
reactant. The electrochemical behaviour of the complexes was
investigated in acetonitrile solution revealing only irreversible
and ill defined reduction steps. The potentials for the oxidation
of 3,5-DTBC to its corresponding semiquinone and quinone
have been reported.26 These potentials are very sensitive to the
degree of protonation and/or the number of transferred elec-
trons. No clear relationship between the electrochemical prop-
erties of the complexes and the reported data for 3,5-DTBC
exists. It may be that the poorly defined redox chemistry of this
class of complexes will never allow such a correlation to be
established.
The common characteristic of the active complexes is the co-
ordinating piperazine group within their ligand framework.
This distinguishes the active complexes from the inactive ones.
Obviously, this structural unit is essential for catecholase activ-
ity of the studied series of complexes. The crystal structures
have revealed that the square-pyramidal co-ordination geom-
etries of the copper ions in 1, 5, 6 and 7 are strongly distorted,
and that for the dinuclear cations on the whole a strained
structure results. The symmetric complexes 2, 3 and 4 on the
contrary are present in a relaxed, energetically favoured con-
formation. This leads to the assumption that the differences in
reactivity are primarily based on geometric factors.
Complex 1 is definitely the most strained system of all of the
complexes synthesised in this work. This strain is caused by the
exogenous µ-hydroxo bridge forcing the rigid ligand framework
to assume an unfavoured conformation. In the presence of
alternative bridging co-ordination partners with a larger bite
distance, the complex will adopt a relaxed conformation. This is
shown by the crystal structure of a dinuclear bis(µ-acetato)-
bridged copper() complex of a ligand which is analogous to
HL1.18b Therefore, 1 is willing to give up the µ-hydroxo bridged
structural motif in favour of a bridging catechol co-ordination.
This is surely also valid for complexes 5–7, although in a
weakened sense. Binding of catechol was proven by spectro-
scopic titration with TCC. In contrast to this, complexes 2–4 are
completely indifferent towards catechol. Even in the presence
of a large excess of 3,5-DTBC no binding is observed. The
Acknowledgements
Financial support from the Deutsche Forschungsgemeinschaft
and the Fonds der Chemischen Industrie is gratefully
acknowledged.
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a µ-hydroxo and µ-alkoxo bridged
dicopper() centre, respectively, is obviously significantly more
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transfer to the copper centres.
The distinct gradation of oxidation rates of the active com-
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complexes. For the most strained complex 1 a turnover number
of 214 hϪ1 was determined, for the less strained systems of 5, 6
and 7 values between 33 and 48 hϪ1 were obtained. Also in
accord with this view is the correlation of kcat with the
Michaelis–Menten constant KM (see Table 8): higher activity
goes along with a lower KM value, i.e. a more stable complex–
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Received 17th June 1997; Paper 7/04245K
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