Directed syntheses and structural studies of mononuclear copper(II) and heterodinuclear copper(II)-iron(III) complexes from the same unsymmetrical dinucleating ligand

Article abstract of DOI:10.1039/a703366d

Mononuclear copper(II) [Cu(H₃L)(O₂CMe)]BPh₄ 1 and heterodinuclear copper(II)-iron(III) [FeCuL(μOEt)]ClO₄ 2 complexes from the same dinucleating ligand H₃L have been prepared and isolated {H₃L = 2-[bis(2-hydroxybenzyl)-aminomethyl]-4-methyl-6-[bis(2-pyridylmethyl) aminomethyl]phenol}. The crystal structure of the mononuclear complex 1 demonstrated the site-directed complexation with H₃L. For the heterodinuclear complex 2 the crystal structure reveals that the copper and iron atoms are bridged by the central phenoxo moiety and by an exogenous ethoxo group. In addition this structure showed that the iron atom is five-co-ordinated. Temperature-dependent magnetic susceptibility measurements revealed an antiferromagnetic interaction (ca. J = -58.1 cm^-1) between the copper and iron atoms in 2. Cyclic voltammograms in dichloromethane vs. Ag-AgNO₃ revealed a quasi-reversible redox behaviour for Cu^II-Cu^I in 1 and 2 (E_1/2 = -0.77 and -0.74 V respectively). The reduction of Fe^III in 2 is irreversible with E_pc = -1.69 V. The Cu^IFe^III complex is thermodynamically stable towards comproportionation.

Full text of DOI:10.1039/a703366d

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Directed syntheses and structural studies of mononuclear copper(II)
and heterodinuclear copper(II)–iron(III) complexes from the same
unsymmetrical dinucleating ligand‡
Catherine Belle,a Isabelle Gautier-Luneau,a Gisèle Gellon,a Jean-Louis Pierre,*,†,a
Irène Morgenstern-Badaraub and Eric Saint-Amanc
a
Laboratoire de Chimie Biomimétique, LEDSS, UMR CNRS 5616, Université J. Fourier, BP 53,
F-38041 Grenoble Cedex 9, France
b Laboratoire de Chimie Bioorganique et Bioinorganique, CNRS-URA 1384,
Institut de Chimie Moléculaire d’Orsay, Université Paris-Sud, Orsay, France
c Laboratoire d’Electrochimie Organique et de Photochimie Rédox, LEOPR, UMR CRNS 5630,
Université J. Fourier, Grenoble, France
Mononuclear copper() [Cu(H3L)(O2CMe)]BPh4 1 and heterodinuclear copper()–iron() [FeCuL(µOEt)]ClO4 2
complexes from the same dinucleating ligand H3L have been prepared and isolated {H3L = 2-[bis(2-hydroxybenzyl)-
aminomethyl]-4-methyl-6-[bis(2-pyridylmethyl)aminomethyl]phenol}. The crystal structure of the mononuclear
complex 1 demonstrated the site-directed complexation with H3L. For the heterodinuclear complex 2 the crystal
structure reveals that the copper and iron atoms are bridged by the central phenoxo moiety and by an exogenous
ethoxo group. In addition this structure showed that the iron atom is five-co-ordinated. Temperature-dependent
magnetic susceptibility measurements revealed an antiferromagnetic interaction (ca. J = Ϫ58.1 cmϪ1) between the
copper and iron atoms in 2. Cyclic voltammograms in dichloromethane vs. Ag–AgNO3 revealed a quasi-reversible
II
I
III
₁
redox behaviour for Cu ᎐Cu in 1 and 2 (E = Ϫ0.77 and Ϫ0.74 V respectively). The reduction of Fe in 2 is
₂
irreversible with Epc = Ϫ1.69 V. The CuIFeIII complex is thermodynamically stable towards comproportionation.
Dinuclear centres containing transition metals such as iron and
copper have attracted great interest due to their relevance to
biological systems. Although numerous dinuclear complexes
from dinucleating ligands have been described to date, few of
them are heterodinuclear. This is, in part, due to the sym-
metrical nature of the ligands in which the two co-ordination
sites are equivalent. A limited number of synthetic dinucleating
ligands provide two chemically distinct environments. They
have been used to prepare homodinuclear complexes exhibiting
distinct properties of the two metal centres.1 Heterodinuclear
complexes are important in investigating their physical proper-
ties which may serve, in some cases, as models for hetero-
dimetallic metalloenzyme active sites. Our interest has been
focused on copper–iron complexes. Few non-haem complexes
involving dinucleating ligands have been described and charac-
terized by X-ray diffraction.2
We present here a monocopper() complex [Cu(H3L)-
(O2CMe)]BPh4 1 and a copper()–iron() complex [FeCuL-
(µ-OEt)]ClO4 2 from the compound H3L which possesses two
sets of ion-bridging groups:3 a hard set provided by the bis-
(hydroxyphenyl)methylamino moiety and a softer set provided
by the bis(2-pyridylmethyl)amino moiety. The unsymmetrical
nature of this compound allows a site-directed synthesis of the
heterodinuclear complex. The mononuclear complex 1 provides
a well suited precursor for the introduction of a second metal
into the other co-ordination site under mild conditions (Scheme
1).
Me
N
OH
N
OH
N
OH
N
H3L
Elemental analyses were performed by the CNRS Micro-
analysis Laboratory of Lyon, France. Magnetization data were
collected on microcrystalline powders using a Quantum Design
SQUID magnetometer over the temperature range 2–300 K, at
1 T field strength. Electronic spectra were obtained using a
Uvikon 930 spectrometer with quartz cells. Electrochemical
experiments were carried out using a PAR model 273 potentio-
stat equipped with a Kipp-Zonen x-y recorder, run under an
argon atmosphere at room temperature, in dichloromethane
(previously refluxed over CaH2 and distilled under an argon
atmosphere) with 0.1 mol dmϪ3 tetra-n-butylammonium per-
chlorate as supporting electrolyte. A Ag–0.01 mol dmϪ3 AgNO3
reference electrode and a glassy carbon disc as working elec-
trode (diameter: 5 mm) were used. A glassy carbon plate (2 × 2
cm) was employed for exhaustive electrolysis.
Experimental
Preparations
All chemicals were obtained from commercial sources and used
as received. Compound H3L was synthesized according to the
published method.3
[Cu(H3L)(O2CMe)]BPh4 1. To a methanolic solution of H3L
(5 cm3, 0.089 mmol) was added 1 equivalent of Cu(O2CMe)2ؒ
H2O in methanol (5 cm3). After 45 min, NaBPh4 dissolved in
methanol was added and the mixture was stirred for 45
min. Upon cooling, the precipitate was collected, dissolved in
dichloromethane and vapour-phase diffusion of methanol in
† E-Mail: Jean-Louis Pierre@ujf-grenoble.fr
‡ Non-SI unit employed: µB ≈ 9.27 × 10Ϫ24 J TϪ1
.
J. Chem. Soc., Dalton Trans., 1997, Pages 3543–3546
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+
Me
O
Cu(O2CMe)2,
MeOH
HO
N+H
N
H3L
Cu
N
MeCO2
OH
N
Fe(ClO4)2,
NEt3
ROH
+
Me
N
O
N
Cu
Fe
Fig. 1 Structure of the mononuclear cation of complex 1 in the
O
O
O
N
crystal. Selected distances (Å) and angles (Њ): O(3) ؒ ؒ ؒ O(4) 2.660(9),
N(2) ؒ ؒ ؒ O(1) 2.64(1), O(2) ؒ ؒ ؒ O(5) 2.76(1), Cu᎐O(1) 2.240(6),
Cu᎐O(4) 1.968(5), Cu᎐N(1) 2.054(7), Cu᎐N(3) 1.977(8), Cu᎐N(4)
1.982(8) and N(2)᎐O(1) 2.64(1); O(4)᎐Cu᎐O(1) 105.9(2), O(1)᎐Cu᎐N(3)
90.2(3), N(3)᎐Cu᎐N(1) 83.3(3), N(1)᎐Cu᎐N(4) 82.8(3) and
N(4)᎐Cu᎐O(4) 95.1(3)
R
N
Scheme 1 Stepwise complexation of Cu(O2CMe)2 and Fe(ClO4)3 by
unsymmetrical dinucleating ligand H3L
this solution afforded a blue crystalline material with a small
amount of a brown product. The crystals were filtered off,
washed with methanol and a small amount of dichloromethane
to remove the brown impurities. Successive vapour-phase diffu-
sion of methanol into the dichloromethane solution furnished
crystals of complex 1 used for X-ray diffraction study (Found:
C, 73.0; H, 5.9; Cu, 6.35; N, 5.6. Calc. for C61H59BCuN4O5: C,
72.95; H, 5.9; Cu, 6.2; N, 5.3%).
atoms were refined with anisotropic thermal parameters.
Hydrogen atoms of the ligand (except for ammonium and
phenol hydrogen which were located on the Fourier-difference
map and refined) and BPh4 counter ion were generated in
idealized positions.
Ϫ
Crystal data for complex 2. C37H38ClCuFeN4O8, M = 821.55,
dark platelet crystal 0.15 × 0.20 × 0.25 mm, monoclinic, space
group P21/c, a = 18.113(2), b = 13.903(2), c = 16.426(2) Å, β =
115.85(1)Њ, U = 3723(1) Å3, Z = 4, Dc = 1.466 g cmϪ3, µ = 1.089
mmϪ1. 7099 Measured reflections in the range 3 р 2θ р 48Њ;
4146 independent reflections. All non-hydrogen atoms were
refined with anisotropic thermal parameters. Three disordered
oxygen atoms of the perchlorate anion were refined on two sites
with a site occupation factor of 0.5. Hydrogen atoms of the
ligand were generated in idealized positions and refined, riding
on the carrier atoms, with isotropic thermal parameters
U(H) = 1.5Ueq(C) for the methyl hydrogen atoms and U(H) =
1.2Ueq(C) for the aromatic hydrogen atoms. The final cycle of
refinement including 496 parameters converged to R(F) = 0.072
[for 4146 F > 4σ(F)], R(F 2) = 0.191 for all 7099 F 2 (∆ρmax = 0.51,
∆ρmin = Ϫ0.54 e ÅϪ3).
[FeCuL(ì-OEt)]ClO4 2. To a methanolic solution of H3L (5
cm3, 0.178 mmol) was added 1 equivalent of Cu(O2CMe)2ؒH2O
in methanol (4 cm3). The solution was stirred for 2 h and
treated sequentially with 2.5 equivalents of triethylamine in
methanol (2 cm3), 1 equivalent of Fe(ClO4)3ؒ9H2O in methanol
(2 cm3), and 1 equivalent of NaOEt in methanol (2 cm3) leading
to a solution which was stirred for 2 h. After cooling, the brown
powder obtained was recrystallized from dichloromethane–
ethanol to yield crystals of complex 2 used for the X-ray dif-
fraction study (Found: C, 54.1; H, 4.7; Cl, 4.25; Cu, 7.25; Fe,
6.2; N, 6.65. Calc. for C37H38ClCuFeN4O8: C, 54.1; H, 4.65; Cl,
4.3; Cu, 7.7; Fe, 6.8; N, 6.8%).
CAUTION: although no problems were encountered during
the preparation of the perchlorate salts, suitable care should be
taken when handling such potentially hazardous compounds.
CCDC reference number 186/649.
Results and Discussion
X-Ray crystallography
Crystal structures
Crystals of compounds 1 and 2 were mounted on a Nicolet
XRD four-circle diffractometer using a graphite-crystal mono-
chromator [λ(Mo-Kα) = 0.710 73 Å]. The temperature of
measurement was 293 K. The reflections were corrected for
Lorentz-polarization effects but not for absorption. The struc-
tures were solved using an automatic Patterson procedure with
the SHELXS 86 program 4 and refined against all F2 (SHELXL
93).5
The crystal structure of complex 1 reveals a mononuclear
species (Fig. 1). The copper is five-co-ordinated (square
pyramidal) with three nitrogen atoms, one axial oxygen atom
[O(1)] from the cresol moiety and one oxygen atom [O(4)] from
the exogenous acetate group. The Cu᎐O(1) distance is 2.240(6)
Å, the Cu᎐O(4) distance 1.968(5) Å and the Cu᎐N distances
range from 1.977(8) to 2.054(7) Å. The two pyridine ligands are
trans to each other. Furthermore the unco-ordinated nitrogen
N(2) is protonated. Two intramolecular hydrogen bonds are
observed: one between N(2)᎐H(2n) and O(1) and the other
between O(3)᎐H(3p) of the phenol group and O(4) of the
acetate ligand.
Crystal data for complex 1. C61H59BCuN4O5, M = 1002.47,
blue platelet crystal 0.15 × 0.25 × 0.30 mm, monoclinic, space
group P21/n, a = 10.664(5), b = 25.662(9), c = 19.232(7) Å,
β = 102.29(3)Њ, U = 5167(4) Å3, Z = 4, Dm = 1.289, Dc = 1.289 g
cmϪ3, µ = 0.477 mmϪ1. 9418 Measured reflections in the range
3 р 2θ р 50Њ; 8911 independent reflections. All non-hydrogen
The crystal structure of complex 2 reveals a heterodinuclear
CuIIFeIII species (Fig. 2). The iron is five-co-ordinated to a
3544
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Fig. 2 Structure of the dinuclear complex 2 in the crystal. Selected
distances (Å) and angles (Њ): Fe ؒ ؒ ؒ Cu 3.122(2), Cu ؒ ؒ ؒ O(5) 2.84(2),
Cu᎐O(1) 2.322(6), Cu᎐O(4) 1.930(7), Cu᎐N(1) 2.012(8), Cu᎐N(3)
1.984(10), Cu᎐N(4) 1.956(9), Fe᎐O(1) 1.945(6), Fe᎐O(2) 1.847(8),
Fe᎐O(3) 1.858(7), Fe᎐O(4) 1.971(6) and Fe᎐N(2) 2.184(8); Cu᎐O(1)᎐Fe
93.6(2), Cu᎐O(4)᎐Fe 106.3(3), O(1)᎐Fe᎐O(4) 83.8(3) and O(1)᎐Cu᎐
O(4) 75.3(3)
nitrogen atom [N(2)] and four oxygen atoms: the two phenolate
oxygens [O(2) and O(3)], a bridging oxygen from the cresol
moiety and a bridging oxygen from an exogenous ethanolate
ligand. An interaction is observed between an oxygen [O(5)] of
the counter ion and the copper: O(5) is the only oxygen of
ClO4Ϫ which is not disordered. Thus, the copper centre could be
considered as octahedral. The elongation of 0.082 Å along the
Cu᎐O(1) bond compared to Cu᎐O(1) in 1 evidences the Jahn–
Teller distortion. The structure of 2 is very similar to the struc-
ture of the iron() complex [Fe2L(µ-OMe)(OMe)]BPh4.1k In 2
the phenoxo and the ethoxo bridges are non-symmetric and
exhibited a shorter Fe ؒ ؒ ؒ Cu [3.122(2) Å] distance compared to
a reported CuIIFeIII complex.2b It has been emphasized that the
complexation with a second metal atom (i.e. FeIII from 1)
requires a major conformational change only around the
C(22)᎐C(23) bond. This may be regarded as a minor structural
reorganization.
Fig. 3 Temperature dependence of the effective magnetic moments
µeff is calculated as 2.828 (χmT)². The solid line represents the best fit of
the theoretical susceptibility values to the experimental data
¹
Magnetic properties
Magnetic susceptibility studies of microcrystalline powders
clearly indicate that the two metal ions of the dinuclear unit are
antiferromagnetically coupled. The temperature dependence of
the effective magnetic moment is shown in Fig. 3. The moment
decreases from room temperature to 35 K. Between 35 and 15
K, a plateau is observed, indicating a Curie-law behaviour with
a constant value of the magnetic moment equal to 4.4 µB. This
confirms that the S = 2 state is the ground state and that the
S = 3 state is no longer populated below 35 K, both S = 2 and 3
5
–
states arising from coupling of the two spin states SFe
=
and
2
SCu = ¹. Zero-field splitting of the S = 2 state leads to further
decrease of the magnetic moment when cooling to 2 K. The
spin Hamiltonian (1) and the derived expression of the Zeeman
¯
²
Fig. 4 The electrochemical behaviour of complexes 1 and 2 in
CH2Cl2 ϩ NBun ClO4 (0.1 mol dmϪ3) using a Ag–0.01 mol dmϪ3
4
AgNO3 reference electrode and a glassy carbon disc as working
electrode (diameter: 5 mm). Curves: A, a 2.3 mmol dmϪ3 solution of 1;
B, a 2.2 mmol dmϪ3 solution of 2, ν = 0.1 V sϪ1
2
2
1
–
H = ϪJSFeSCu ϩ D(S = 2)(Sz Ϫ 3S ) ϩ gβHS
(1)
main features at 444 (ε = 1091) and 690 nm (ε = 276 dm3 molϪ1
cmϪ1). The band at 444 nm may be assigned to a phenolate to
copper()6 charge-transfer transition (LMCT), and that of 690
nm to a d–d transition. Two bands are observed for 2 at 320 (sh)
and 418 nm (ε = 5374 dm3 molϪ1 cmϪ1). That at 418 nm arises
probably from the overlapping of the LMCT transitions from
the phenolate ligands to FeIII and CuII.
energy levels were used in order to simulate the experimental
data, the parameters J, D, and g having their usual meanings.
The best fit of the theoretical values to the experimental data is
shown as a solid line in Fig. 3. The following values of the spin-
Hamiltonian parameters have been obtained: J = Ϫ58.1 cmϪ1
,
D(S = 2) = 0.39 cmϪ1, g = 1.85. This antiferromagnetic exchange
interaction is comparable to those found in similar complexes
formed from dinucleating ligands.2b
Electrochemistry
Electronic spectra
The cyclic voltammogram of complex 1 (Fig. 4, curve A) dis-
₁
The electronic spectrum of complex 1 in acetonitrile displays
plays a quasi-reversible wave at E = Ϫ0.77 V (∆Ep = 0.18 V,
₂
J. Chem. Soc., Dalton Trans., 1997, Pages 3543–3546
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ν = 0.1 V sϪ1) which corresponds to the CuII᎐CuI redox couple.
Coulometric reduction confirms that it is a one-electron pro-
cess. Further reduction occurs at Epc = Ϫ1.85 V and leads to
decomposition of the complex and deposition of Cu0 onto the
electrode surface.
have thus been evidenced. The compound H3L had been
designed with two sets of ion-binding groups suited to selective
complexation. This strategy may be extended to the preparation
of other heterodimetallic complexes with the view of avoiding a
statistical distribution. Such complexes are relevant to the
development of models for the active sites of dinuclear
metalloproteins.
The heterodimetallic complex 2 exhibits two electrochemical
responses (Fig. 4, curve B) at Ϫ0.74 V (∆Ep = 0.11 V, ν = 0.1 V
sϪ1) and Epc = Ϫ1.69 V (Epa = Ϫ1.01 V, ν = 0.1 V sϪ1) respect-
ively. The first appears quasi-reversible. Exhaustive potentio-
static electrolysis at Ϫ0.9 V gives a n = 1 reduction process and
reoxidation of this solution fully restores the initial solution as
judged by UV/VIS and electrochemical measurements. We
ascribe the first one-electron transfer to the CuII᎐CuI redox
couple. This assignment is consistent with redox potential
values for CuII᎐CuI 6 in related complexes. In addition, the CuII᎐
CuI redox potentials for 1 and 2 are similar. The second electro-
chemical process leads to an iron() species. The voltammo-
gram of the [Fe2L(µ-OMe)(OMe)]BPh4 complex1k showed for
the second reduction process of the FeIII᎐FeII couple associated
with the five-co-ordinated iron bonded to two phenolate ter-
minal groups a shift to more cathodic potentials (Epc = Ϫ1.34 V
in CH2Cl2) compared to the iron bonded to two pyridyl groups
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₁
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₂
CuIFeII species seems to be associated with reorganization of
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₁
ated by the corresponding constant k [∆E = (RT/F)ln k]. From
₂
₁
the separation of redox potentials (∆E = 0.61) we have deter-
₂
mined k to be 1.48 × 1010 indicating that 2 has a stable CuIFeIII
form.
Conclusion
The use of an unsymmetrical dinucleating ligand has allowed
the synthesis of an heterodinuclear CuIIFeIII complex. The
copper has been placed first in the ‘nitrogen box’. The crystal
structure of the mononuclear copper complex has evidenced
that the ‘oxygen box’ remained free for the complexation of
another metal atom. In a second step, an FeIII has been placed
in the ‘oxygen box’. The regiocontrolled syntheses of mono-
and heterodi-nuclear complexes of the dinucleating ligand H3L
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Received 15th May 1997; Paper 7/03366D
3546
J. Chem. Soc., Dalton Trans., 1997, Pages 3543–3546