Iron and cobalt complexes of 4,4,9,9-tetramethyl-5,8-diazadodecane-2,11- dione dioxime ligand: Synthesis, characterization and reactivity studies

Article abstract of DOI:10.1007/s12039-011-0171-7

Two oximate bridged dinuclear complexes [Co^II₂ (HL)₂](ClO₂)₂ (1) and [Fe^II ₂ (HL)₂](ClO₂)₂ (2), and a biomimetic iron(III)-catecholate complex [Fe^III(HL)(DBC)] (3) of a dioxime ligand (H₂L = 4,4,9,9- tetramethyl-5,8-diazadodecane-2,11- dione dioxime and DBCH₂ = 3,5-di-tert-butylcatechol) were synthesized and characterized. X-ray single-crystal structures of both the dinuclear complexes exhibit an out-of-plane oximate bridge where the six-membered M ₂(NO)₂ ring adopt a boat conformation with the metal ions in a fivecoordinate distorted trigonal bipyramidal geometry. Complexes 1 and 2 react with dioxygen at ambient condition to form the corresponding hydroxo- or oxo-bridged dinuclear cobalt(III) or iron(III) complexes. On the other hand, the iron(III)-catecholate complex (3) activate dioxygen to undergo oxidative C-C bond cleavage of catechol. The selective formation of extradiol catechol cleavage products in the reaction of 3 with dioxygen mimics the functional aspect of extradiol-cleaving catechol dioxygenases. The flexibility of ligand backbone is proposed to control the dioxygen reactivity of metal complexes. Indian Academy of Sciences.

Full text of DOI:10.1007/s12039-011-0171-7

J. Chem. Sci. Vol. 123, No. 6, November 2011, pp. 839–846. ꢀc Indian Academy of Sciences.
Iron and cobalt complexes of 4,4,9,9-tetramethyl-5,8-diazadodecane-2,
1
1-dione dioxime ligand: Synthesis, characterization
and reactivity studies
∗
OINDRILA DAS, SAYANTI CHATTERJEE and TAPAN KANTI PAINE
Department of Inorganic Chemistry, Indian Association for the Cultivation of Science, 2A and 2B,
Raja S C Mullick Road, Jadavpur, Kolkata 700032, India
e-mail: ictkp@iacs.res.in
II
2
II
2
Abstract. Two oximate bridged dinuclear complexes [Co (HL)2](ClO4)2 (1) and [Fe (HL)2](ClO4)2 (2),
III
and a biomimetic iron(III)-catecholate complex [Fe (HL)(DBC)] (3) of a dioxime ligand (H2L = 4,4,9,9-
tetramethyl-5,8-diazadodecane-2,11-dione dioxime and DBCH2 = 3,5-di-tert-butylcatechol) were synthesized
and characterized. X-ray single-crystal structures of both the dinuclear complexes exhibit an out-of-plane oxi-
mate bridge where the six-membered M2(NO)2 ring adopt a boat conformation with the metal ions in a five-
coordinate distorted trigonal bipyramidal geometry. Complexes 1 and 2 react with dioxygen at ambient con-
dition to form the corresponding hydroxo- or oxo-bridged dinuclear cobalt(III) or iron(III) complexes. On the
other hand, the iron(III)-catecholate complex (3) activate dioxygen to undergo oxidative C–C bond cleavage
of catechol. The selective formation of extradiol catechol cleavage products in the reaction of 3 with dioxygen
mimics the functional aspect of extradiol-cleaving catechol dioxygenases. The flexibility of ligand backbone is
proposed to control the dioxygen reactivity of metal complexes.
Keywords. Iron; cobalt; oximate ligand; oxidation; catechol cleavage; solid-state structure.
1
. Introduction
would therefore find application in biomimetic chem-
istry for designing model complexes. As a part of our
ongoing study in this direction, we report in this paper
the synthesis and structures of two oximate bridged
Oximes and dioximes are extensively used for the
synthesis of exchange coupled polynuclear metal
complexes relevant to molecular magnetic materials.
Oximate ligands show different binding modes to stabi-
lize structurally and magnetically interesting transi-
tion metal complexes.1 Additionally, studies on the
1
II
dinuclear metal complexes [Co (HL) ](ClO ) (1) and
2
2
4 2
II
2
[
Fe (HL) ](ClO ) (2). We also report the isolation and
2 4 2
characterization of a biomimetic iron(III)-catecholate
–11
III
complex, [Fe (HL)(DBC)] (3) as model for catechol-
reactivity of metal-coordinated oximates have attracted
cleaving dioxygenases. The reactivity of metal com-
plexes of oximate ligand towards dioxygen and the
much attention in coordination chemistry.1
2–16
We have
recently reported the structure and magnetic properties
of dinickel(II) and dicopper(II) complexes derived from
a β-aminoketoxime ligand, 4,4,9,9-tetramethyl-5,8-
17
diazadodecane-2,11-dione dioxime (H L) (chart 1).
2
The nickel(II) complexes have been shown to undergo
a proton-assisted coupling of oxime with nitriles result-
ing in the iminoacylation of oxime H L. The dioxime
2
ligand forms complexes with other metal ions like
iron(II), cobalt(II), and zinc(II) ions.18–20 The flexible
N4 ligand is expected to stabilize mononuclear comp-
lexes in the presence of a bidentate coligand. This
∗
For correspondence
Chart 1. Ligands.
839

840
Oindrila Das et al.
II
2
nature of oxidative catechol cleavage products from the 2.1b [Fe (HL) ](ClO ) (2): Complex 2 was syn-
2
4 2
iron(III)-catecholate complex are discussed.
thesized according to a protocol described for comp-
lex 1 except that iron(II) perchlorate was used instead
of cobalt(II) perchlorate. A white solid was isolated
at the end of the reaction. The white solid was dis-
solved in a solvent mixture of dichloromethane and
2. Experimental
Commercial grade chemicals were used for synthetic acetonitrile (1:1) mixture and kept for layer diffusion
purposes. Solvents were distilled, dried and deoxy- with diethyl ether to obtain colourless crystals of 2
genated before use. Preparation and handling of air- suitable for X-ray diffraction. Yield: 0.093 g (70%).
sensitive compounds were carried out under an inert Anal. Calcd for C28
H58Cl Fe N O12 (881.42 g/mol):
2 2 8
atmosphere in a glove box. Although no problem was C, 38.16; H, 6.63; N, 12.71. Found: C, 38.04; H,
−1
encountered during the synthesis of the complexes, per- 6.46; N, 12.43%. IR (KBr, cm ): 3416(br), 3248(s),
chlorate salts are potentially explosive and should be 3088(w), 2974–2878(s), 2735(w), 1663(w), 1626(w)
handled with care!21 The ligand, 4,4,9,9-tetramethyl- 1468(m), 1420(m) 1371(s), 1280–1232(w), 1163–
5
,8-diazadodecane-2,11-dione dioxime (H L), was syn- 1094(vs), 1018(w), 995(m), 943(m), 851(m), 777(w),
2
22
thesized according to a literature procedure. Fourier 696(m), 625(s). ESI-MS (positive ion mode, MeCN):
+
transform infrared spectroscopy on KBr pellets was per- m/z = 287.10 (100%, [H
L+H] ). UV-Vis in MeCN:
2
−1
−1
formed on a Shimadzu FT-IR 8400S instrument. Ele- λ, nm; (ε, M cm ): 380 (sh). Magnetic moment
mental analyses were performed on a Perkin Elmer (298 K): μeff = 6.81 μ
400 series II CHN analyzer. Electro-spray ioniza-
.
B
2
tion (ESI) mass spectra were recorded with a Waters
QTOF Micro YA263 instrument. Solution electronic
spectra were measured on an Agilent 8453 diode array
III
2
(
.1c [Fe (HL)(DBC)] (3): To a solution of ligand
0.143 g, 0.5 mmol) in methanol (5 mL), was added
a methanolic solution (10 mL) of Fe(ClO ) ·xH O
1
spectrophotometer. Room temperature H NMR spec-
4
3
2
tra were collected on a Bruker DPX-500 spectrome-
ter. Room temperature magnetic data were collected
on a Gouy balance (Sherwood Scientific, Cambridge,
UK). Diamagnetic contributions were estimated for
each compound by using Pascal’s constants.
(
0.177 g, 0.5 mmol). The resulting orange solution
was treated with a mixture of 3,5-di-tert-butylcatechol
0.11 g, 0.5 mmol) and triethylamine (128 μL,
mmol) in methanol (2 mL). The solution was imme-
diately turned to deep blue. The clear reaction
mixture was stirred for 6 h and then solvent was
removed to dryness. The solid residue was washed sev-
eral times with distilled water and finally with diethyl
ether to obtain a purple-blue powder. Yield: 0.26 g
(
1
2.1 Synthesis of the complexes
II
2
2
.1a [Co (HL) ](ClO ) (1): To a methanolic solu-
2
4 2
(93%). Anal. Calcd for C28
H49FeN O (561.56 g/mol):
4 4
tion (10 mL) of Co(ClO ) ·6H O (0.11 g, 0.3 mmol),
4
2
2
C, 59.89; H, 8.80; N, 9.98. Found: C, 59.64; H, 8.72;
N, 9.79%. IR (KBr, cm ): 3430(br), 2953–2868 (vs),
−1
ligand (0.086 g, 0.3 mmol) was added. A pink solution
was formed immediately which was stirred at room
temperature for 3 h under nitrogen to precipitate a pink
solid. The pink precipitate was isolated by filtration, re-
dissolved in methanol and kept for layer diffusion with
diethyl ether. Pink plate-shaped crystals suitable for
X-ray diffraction were isolated after a few days. Yield:
1
1
1
7
5
624(w), 1585(w), 1550(w), 1464(s), 1414(s),
360(m), 1310(w), 1277(w), 1247(s), 1205(w),
173(w), 1115(s), 1028(w), 980(s), 858(w), 829(w),
46, 679. ESI-MS (positive ion mode, MeCN): m/z =
+
62.89 (10%, [Fe(HL)(DBC)+H] ), 287.10 (100%,
+
−1
−1
[
H L+H] ). UV-Vis in MeCN: λ, nm (ε, M cm ):
2
0
.073 g (55%). Anal. Calcd for C H Cl Co N O
28 58 2 2 8 12
665 (1920). Magnetic moment (298): μ = 5.85 μ .
eff
B
(
887.58 g/mol): C, 37.89; H, 6.59; N, 12.62. Found:
−1
C, 38.02; H, 6.27; N, 12.58%. IR (KBr, cm ):
3
2
1
7
433(br), 3262(m), 32110(m), 3075(w), 2970(w),
731(w), 1663(w), 1626(w), 1468(m), 1371(s), 1277–
202(w), 1146–1090(vs), 1003(m), 947(m), 855(m),
2
.2 Analysis of the catechol cleavage products
The iron-catecholate complex (0.05 mmol) was dis-
solved in oxygen saturated acetonitrile (15 mL) and
stirred for 12 h at room temperature. The blue solution
was changed slowly to light green. The solvent was
then removed under vacuum and the residue was treated
73(w), 696(m), 625(s). ESI-MS (positive ion mode,
+
MeCN): m/z = 343.13 (100%, [L+Co] ). UV-Vis in
−1
−1
MeCN: λ, nm (ε, M cm ): 550 (44). Magnetic
moment (298 K): μ = 4.25 μ .
eff
B

Synthesis and reactivity of iron and cobalt complexes
841
with 3 M hydrochloric acid (10 mL). The organic prod- 1. Data collection, data reduction, and structure solu-
ucts were extracted with diethyl ether (3 × 15 mL) and tion/refinement were carried out using the software
28
the organic fraction was dried over anhydrous sodium package of APEX II. All structures were solved by
sulphate. The catechol derived products were analysed direct method and refined in a routine manner. All the
1
by H-NMR spectroscopy without further purification. non-hydrogen atoms were refined anisotropically and
The spectral data were compared with the reported all the hydrogen atoms were fixed geometrically. In
data for authentic extradiol and intradiol cleavage prod- both the complexes (1 and 2), some disordered electron
23–27 1
ucts.
H-NMR data for 3,5-di-tert-butylcatechol densities were located in the asymmetric unit at the end
cleavage products (500 MHz, CDCl , 295 K): 4,6-di- of the refinement cycles. SQUEEZE calculation indi-
3
tert-butyl-2-pyrone (A): δ = 1.22 (s, 9H), 1.36 (s, 9H), cated the presence of 80 and 127 electron densities per
6
.05 (m, 2H); 3,5-di-tert-butyl-2-pyrone (B): δ = 1.22 unit cell for 1 and 2, respectively. The final full matrix
(
s, 9H), 1.40 (s, 9H), 7.21 (d, 1H), 7.24 (d, 1H) ; 3,5-di- least squares refinement converged to R = 0.0473 and
1
tert-butyl-5-(carboxymethyl)-2-furanone (C): δ = 0.98 wR = 0.1218 for 1, and R = 0.0475 and wR =
2
1
2
2
(
s, 9H), 1.24 (s, 9H), 2.91 (d, 1H), 2.81 (d, 1H) , 6.96 (s, 0.1321 (F , all data) for 2.
H), 9.72 (s, 1H); 3,5-di-tert-butylbenzoquinone (D):
δ = 1.22 (s, 9H), 1.28 (s, 9H), 6.21 (d, 1H), 6.93 (d, 1H).
1
3
. Results and discussion
Reactions of ligand H L and equimolar amounts
2
of cobalt(II) perchlorate or iron(II) perchlorate in
methanol at ambient conditions under nitrogen
2.3 X-ray crystallographic studies
X-ray single crystal data were collected at 298 K using atmosphere yield the respective metal complexes,
II
II
Mo-K (λ = 0.7107 Å) radiation on a SMART APEX-II [Co (HL) ](ClO ) (1) and [Fe (HL) ](ClO ) (2). The
α
2
2
4 2
2
2
4 2
III
diffractometer equipped with CCD area detector. Crys- iron(III)-catecholate complex, [Fe (HL)(DBC)] (3) is
tallographic data for 1 and 2 are provided in table synthesized in high yield by reacting the ligand with
Table 1. Crystallographic data for complexes 1 and 2.
1
2
Empirical formula
Formula weight
Crystal system
Space group
a/Å
C28H58Cl2Co2N8O12
887.58
C28H58Cl2Fe2N8O12
881.42
Monoclinic
C2/c
22.01(3)
12.689(17)
16.82(2)
90.00
108.81(3)
90.00
4445(11)
4
1.326
Monoclinic
C2/c
20.831(6)
13.670(4)
16.562(7)
90.00
106.129(9)
90.00
4531(3)
b/Å
c/Å
α/
0
0
β/
γ/
0
Volume/Å3
Z
4
−3
Dcalc/gcm
F(000)
1.292
1856
0.816
1864
0.925
−1
μ MoKα/mm
Temperature/K
Rint
298(2)
0.0809
298(2)
0.0893
Range of h, k, l
−19/19, −11/11, −15/15
1.88/18.72
7783/1694/1251
1694/0/242
0.881
−21/21, −14/14, −17/16
1.80/22.13
15499 /2761/2073
2761/0/250
0.867
◦
θmin/max/
Reflections collected/unique/observed [I > 2σ(I)]
Data/restraints/ Parameters
Goodness of fit on F2
a
Final R indices [I > 2σ(I)]
R1 = 0.0473
R1 = 0.0475
wR2 = 0.1218
wR2 = 0.1321
1
a
2
2 2
2 2 /2
R1 = ꢀ||Fo| − |Fc||/ꢀ|Fo|, wR2 = [ꢀw(Fo − Fc ) /ꢀw(Fo ) ]

842
Oindrila Das et al.
Scheme 1. Synthesis of complexes.
iron(III) perchlorate and 3,5-di-tert-butylcatechol in
ESI-mass spectra (positive ion mode in acetonitrile)
methanol in the presence of triethylamine (scheme 1). exhibit ion peaks at m/z = 343.13 for 1 and at m/z =
+
+
The presence of strong and broad bands due to perchlo- 287.10 for 2 attributable to [L+Co] and [H L+H] ,
2
−1
−1
rate ions at around 1165–1085 cm and at 625 cm
respectively. ESI-MS data indicate the decomposition
in the IR spectra suggest the cationic nature of 1 and of dimeric complexes in the experimental condition
. Complexes 1 and 2 show characteristic sharp N–H of mass spectrometry. On the other hand, ESI-mass
2
−1
bands of the ligand in the region 3260–3210 cm indi- spectrum of 3 shows ion peak at m/z = 562.89
cating the binding of ligand with the metal ions. The with expected isotope distribution pattern calculated for
+
bands for perchlorate ions are absent in the IR spec- [Fe(HL)(DBC)+H] . This further supports the neutral
trum of 3, indicating a neutral complex. The ligation of nature of the complex. All the complexes are stable
3
,5-di-tert-butylcatechol anion to the metal centre in 3 in the solid state under nitrogen atmosphere. Room
is supported by the presence of strong and sharp bands temperature magnetic moment values of 4.25 μ and
B
−1
in the region of 2955-2868 cm .
6.81 μ for 1 and 2, respectively, suggest the presence
B
Figure 1. ORTEP plots (50% thermal ellipsoids) of the complex cations of 1 and 2.

Synthesis and reactivity of iron and cobalt complexes
843
◦
Table 2. Selected bond lengths (Å) and angles ( ) for 1 and 2.
1
(M = Co)
2 (M = Fe)
M(1)–N(1)
M(1)–N(2)
M(1)–N(3)
M(1)–N(4)
M(1)–O(2)
N(1)–O(1)
N(4)–O(2)
N(1)–M(1)–N(2)
N(2)–M(1)–N(3)
N(3)–M(1)–N(4)
N(4)–M(1)–N(1)
N(4)–M(1)–O(2)
N(1)–M(1)–N(3)
O(2)–M(1)–N(2)
N(2)–M(1)–N(4)
O(2)–M(1)–N(3)
2.054(7)
2.175(6)
2.076(6)
2.058(7)
1.941(5)
1.393(7)
1.406(7)
92.1(3)
83.0(2)
90.8(3)
91.5(3)
96.3(2)
2.087(4)
2.134(4)
2.123(4)
2.072(4)
1.940(3)
1.390(4)
1.386(4)
90.00(15)
82.41(15)
87.39(15)
91.23(14)
96.35(12)
119.23(16)
93.48(15)
168.91(15)
129.43(16)
124.8(2)
87.1(2)
173.7(3)
123.2(2)
of moderate to weak exchange coupling between the mag- the apical positions with the N2–M1–N4 angle of
netic centres in dimeric complexes. The room tempe- 173.7(3) for 1 and 168.91(15) for 2 (table 2). A similar
rature magnetic moment of 3 is found to be 5.85 μB, coordination geometry is observed in oximate bridged
a value very close to that calculated for a mono- dinickel(II) and dicopper(II) complexes of the oxime
17
nuclear high-spin iron(III) complex. Complex 3 ligand (H L).
2
exhibits a broad band at 665 nm typical for iron(III)-
Two metal ions along with two oximate oxygens (O2)
catecholate complex.29 The analytical and spectro- and two imine nitrogens (N4) form a six-membered
scopic data leave no doubt about the composition of M (NO) core in 1 and 2. The M (NO) unit adopts a
2
2
2
2
complexes, 1–3.
boat conformation as observed in the crystal structures
17
of other dimeric metal complexes of H L. This par-
2
ticular binding motif of the ligand forces the metal ions
to occupy the positions diagonally on the mean plane
formed by two metal ions and two bridging oximate
oxygens (O2) with the M· · · M distances of 3.739 Å in 1
3
.1 Crystal structures
For the structural verification of the complexes, sin-
gle crystals of 1 and 2 were grown by layer di-
ffusion of a methanol or acetonitrile solution of
complex with diethyl ether. All attempts to crystallize
3
were unsuccessful. Complexes 1 and 2, crystallized
in monoclinic space group, are structurally similar. In
both the oximate-bridged dimeric complexes, half of
+
the cationic molecule [M(HL)] is located on a centre
of inversion with a perchlorate anion. The metal ions
are coordinated by four nitrogen donors from HL and
one oximate oxygen (figure 1) giving rise to a distorted
trigonal bipyramidal coordination geometry (τ = 0.82
and 0.66 for 1 and 2, respectively).
In both the complexes, one amine nitrogen (N3),
one imine nitrogen (N1) and an oximate oxygen (O2)
+
from another ligand of the symmetry related [M(HL)]
unit occupy the equatorial plane with the M1–N3, M1–
N1 and M1–O2 distances of 2.076(6), 2.054(7) and
1
.941(5) Å, respectively for 1 and of 2.123(4), 2.087(4),
and 1.940(3) Å, respectively for 2. The other amine
nitrogen (N2) and the imine nitrogen (N4) occupy (0.5 mM) at room temperature.
ox
ox
Figure 2. Optical spectra of 1, 1 , 2 and 2 in acetonitrile

844
Oindrila Das et al.
oxo-bridged diiron(III) complexes, may be attributed to
III
2+
ox 30
charge-transfer bands of [Fe O(HL) ] (2 ). Unfor-
2
2
tunately, the final oxidized products could not be iso-
lated for further characterization.
The iron(III)-catecholate complex (3) reacts with
dioxygen in acetonitrile at ambient condition over a
period of 8–10 h during which the purple solution
slowly turns to a light green solution. The optical spec-
tral changes during the reaction reveal the disappear-
ance of the catecholate-to-iron(III) charge-transfer band
at 665 nm with time at a pseudo-first order rate of
−
4
−1
1
.20 × 10
s
(figure 3). The decay of charge-transfer
band at 665 nm suggests the oxidation of 3,5-di-tert-
butylcatechol in the reaction of 3 with dioxygen.
Analysis of organic products from the reaction solu-
1
tion by H-NMR spectroscopy after separation of metal
Figure 3. Optical spectral changes during the reaction of 3
0.5 mM in acetonitrile) with dioxygen at room temperature.
ion by acidic work-up reveals the formation of cate-
chol derived products. Three C–C bond cleavage
products from catechol: 4,6-di-tert-butyl-2-pyrone (A),
(
and 3.713 Å in 2. Apart from the intermolecular hydro-
gen bonding involving the complex cations and perchlo-
rate anions, intramolecular hydrogen bonding is also
observed between the oximate (O2) and oxime oxygen
3
(
,5-di-tert-butyl-2-pyrone (B) and 3,5-di-tert-butyl-5-
carboxymethyl)-2-furanone (C) are obtained. In the
reaction, 66% catechol is oxidized to afford about
8% extradiol cleavage products (A and B) and
% intradiol product (C). The autooxidation prod-
4
7
(O1) in the solid-state structure of both the complexes.
uct of catechol, 3,5-di-tert-butylbenzoquinone (D), is
obtained with 11% yield leaving 34% unreacted cate-
chol (scheme 2). It is important to mention here that the
selectivity of extradiol catechol cleavage is decreased
in the presence of an acid or a base. A large num-
ber of model complexes show catechol cleavage reac-
3
.2 Dioxygen reactivity
The dinuclear complexes (1 and 2) are oxygen sen-
sitive. A light pink solution of complex 1 in aceto-
nitrile reacts with dioxygen over a period of 2 h to
form an amber solution. The ESI-MS of the final reac-
tion solution shows peak at m/z = 702.22 with iso-
31–33
tivity,
but examples of biomimetic iron-catecholate
complexes that show selective extradiol cleavage are
33–35
+
limited.
Extradiol products have been observed in
tope distribution pattern calculated for [Co O(L) ] .
2
2
iron-catecholate complexes of facial N3 or NNO donor
The optical spectral data (figure 2) along with ESI-MS
data indicate a possible formation of hydroxo-bridged
dicobalt(III) complex, [Co (OH)(L) ] (1 ). It is
likely that the bridging hydroxide group gets deproto-
nated in the experimental conditions of ESI-MS. Simi-
larly, the oximate-bridged diiron(II) complex (2) reacts
27,36–42
ligands.
The iron-catecholate complexes of N4
III
2
2+
ox
donor ligands have been shown to afford intradiol prod-
2
31
ucts. Complex 3, being an iron(III)-catecholate comp-
lex of a tetradentate nitrogen donor ligand, shows
extradiol selectivity and thereby mimics the extradiol-
cleaving catechol dioxygenases.
with O within 1 h. The featureless optical spectrum of
2
2
A lower conversion of catechol cleavage (66%) in 3
is replaced by a new spectrum with shoulders at 330,
4
28 and 555 nm. These peaks, by analogy with other may be attributed to the flexible nature of the oximate
Scheme 2. Catechol cleavage products in the reaction of 3 with dioxygen.

Synthesis and reactivity of iron and cobalt complexes
845
Scheme 3. The effect of conformational flexibility of HL on catechol-cleavage reactivity of 3.
ligand where isomers with both planar and non-planar role of ligand flexibility on the dioxygen reactivity of
conformations are expected to be present in solution metal complexes.
(scheme 3). The planar orientation of the ligand has
been observed in square-planar or octahedral transition
metal complexes.1
7,43,44
The isomer with non-planar Supplementary information
conformation of the ligand leads to dioxygen activation
with concomitant C–C bond cleavage of catechol. The
extradiol-selectivity clearly suggests that an alkylpe-
roxo intermediate, formed upon dioxygen activation at
the iron(II)-semiquinone species, leads to an alkenyl
migration. The isomer with planar conformation of
the ligand, on the other hand, binds catechol in a mono-
dentate fashion because it is not geometrically possi-
ble for two phenolate oxygens of catecholate ring to
occupy the axial positions. In this case the activation of
dioxygen does not form an alkylperoxo intermediate to
undergo C–C bond cleavage of catechol.
CCDC–833673 (1) and CCDC–833674 (2) contain
the supplementary crystallographic data for this
paper. The data can be obtained free of charge at
www.ccdc.cam.ac.uk/conts/retrieving.html or from the
Cambridge Crystallographic Data Center, 12 Union
Road, Cambridge CB2 1EZ, UK; Fax: +44–1223/336–
45
033; E-mail: deposit@ccdc.cam.ac.uk.
Acknowledgements
This research was supported by the Department of Sci-
ence and Technology (DST), Government of India. SC
is thankful to the Council of Scientific and Industrial
Research (CSIR), India for a fellowship. Crystal struc-
ture determination was performed at the DST-funded
National Single Crystal Diffractometer Facility at the
Department of Inorganic Chemistry, Indian Association
for the Cultivation of Science (IACS).
4. Conclusion
We have isolated and structurally characterized two
oximate bridged dinuclear metal complexes where the
metal centres are connected by out-of-plane oximate
bridge. The dimeric iron(II) and cobalt(II) complexes
are oxidized to hydroxo or oxo-bridged dimetallic References
complexes. A biomimetic iron(III)-catecholate complex
is described which activates dioxygen to show oxidative
C–C bond cleavage of catechol. The extradiol selec-
tivity in the iron(III)-catecholate complex is the first
example of a biomimetic system supported by an oxime
ligand. The results described in the work highlight the
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