A functional model of extradiol-cleaving catechol dioxygenases: Mimicking the 2-His-1-carboxylate facial triad

Article abstract of DOI:10.1021/ic902462k

The synthesis and characterization of an iron-catecholate model complex of a tridentate 2-N-1-carboxylate ligand derived from l-proline are reported. The X-ray crystal structure of the complex [(L)₃Fe₃(DBC) ₃] (1) (where L is 1-(2-pyridylmethyl)pyrrolidine-2-carboxylate and DBC is the dianion of 3,5-di-tert-butyl catechol) reveals that the tridentate ligand binds to the iron center in a facial manner and mimics the 2-his-1-carboxylate facial triad motif observed in extradiol-cleaving catechol dioxygenases. The iron(III)-catecholate complex (1) reacts with dioxygen in acetonitrile in ambient conditions to cleave the C-C bond of catecholate. In the reaction, an equal amount of extra- and intradiol cleavage products are formed without any auto-oxidation product. The iron-catecholate complex is a potential functional model of extradiol-cleaving catechol dioxygenases.

Full text of DOI:10.1021/ic902462k

4518 Inorg. Chem. 2010, 49, 4518–4523
DOI: 10.1021/ic902462k
A Functional Model of Extradiol-Cleaving Catechol Dioxygenases:
Mimicking the 2-His-1-Carboxylate Facial Triad
Sayantan Paria, Partha Halder, and Tapan Kanti Paine*
Department of Inorganic Chemistry, Indian Association for the Cultivation of Science (IACS),
2A and 2B Raja S. C. Mullick Road, Jadavpur, Kolkata 700032, India
Received December 11, 2009
The synthesis and characterization of an iron-catecholate model complex of a tridentate 2-N-1-carboxylate ligand
derived from L-proline are reported. The X-ray crystal structure of the complex [(L)3Fe3(DBC)3] (1) (where L is
1-(2-pyridylmethyl)pyrrolidine-2-carboxylate and DBC is the dianion of 3,5-di-tert-butyl catechol) reveals that the
tridentate ligand binds to the iron center in a facial manner and mimics the 2-his-1-carboxylate facial triad motif
observed in extradiol-cleaving catechol dioxygenases. The iron(III)-catecholate complex (1) reacts with dioxygen in
acetonitrile in ambient conditions to cleave the C-C bond of catecholate. In the reaction, an equal amount of extra-
and intradiol cleavage products are formed without any auto-oxidation product. The iron-catecholate complex is a
potential functional model of extradiol-cleaving catechol dioxygenases.
Introduction
A large number of iron-catecholate model complexes are
reported where tetradentate N4,8-12 N3O,13,14 or tridentate
N2O15,16 donor ligands are employed. The model complexes
show a diverse C-C bond cleavage reactivity. A limited
number of iron-catecholate complexes of facial tridentate
N3 ligands, like 1,4,7-triazacyclononane (TACN),17,18 Me3-
TACN,19 and TpiPr2 20 are reported that selectively give
extradiol cleavage products. Among the reported models,
the iron complex of TACN has been shown to exhibit high
selectivity, and around 98% extradiol cleavage products were
obtained.17 Iron-catecholate model complexes that mimic
the 2-N-1-carboxylate facial coordination motif, as observed
in the active site of extradiol-cleaving catechol dioxygenases,
are rare. Gebbink et al.21 have recently reported iron-
catecholate complexes, where the iron is coordinated by
Catechol dioxygenases are nonheme iron enzymes that
catalyze the C-C bond cleavage of aromatic cis-diols by the
use of aerial oxygen as the oxidant.1,2 Catechol dioxygenases
are classified, depending on the position of the C-C bond
cleavage, into two categories.1 In the active site of intradiol
dioxygenases, iron(III) is ligated by two histidines and two
tyrosinates in the resting state.3 The extradiol-cleaving dioxy-
genases use an octahedral iron(II) center coordinated by two
histidines and one carboxylate ligand in one face, leaving the
other face for exogenous ligands.1,4
The 2-his-1-carboxylate facial triad5-7 is a common struc-
tural motif in the super family of nonheme iron enzymes.
Many efforts were made to mimic the structural and func-
tional aspects of this enzymes family over the last decades.1
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*Corresponding author. Phone: þ91-33-2473-4971. Fax: þ91-33-2473-2805.
E-mail: ictkp@iacs.res.in.
(1) Costas, M.; Mehn, M. P.; Jensen, M. P.; Que, L., Jr. Chem. Rev. 2004,
104, 939–986.
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403–405.
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r
pubs.acs.org/IC
Published on Web 04/14/2010
2010 American Chemical Society

Article
Inorganic Chemistry, Vol. 49, No. 10, 2010 4519
Chart 1. Ligands
times with deionized water to remove the salts, and finally dried
under vacuum to give a pure compound. Yield: 0.22 g (92%).
X-ray quality single crystals of 1 NaBPh4 were grown by slow
3
diffusion of hexane into an acetonitrile solution of 1 in the
presence of sodium tetraphenylborate. Elemental analysis calcd
(%) for C75H99Fe3N6O12 (1444.16 g/mol): C 62.38, H 6.91,
N 5.82; Found: C 61.9, H 6.7, N 5.8. IR (KBr): υ = 3430(br),
2954(vs), 1737(w), 1636-1612(br), 1470(s), 1446(s), 1414(s),
1360(s), 1247(s), 1114(s), 982(m), 761(s), 727(m), 678(s) cm-1
.
ESI-MS (positive ion mode, CH3CN): m/z = 1465.10 (10%,
[{L3Fe3(DBC)3} þ Na]þ), 984.95 (50%, [{L2Fe2(DBC)2} þ
Na]þ), 962.98 (10%, [{L2Fe2(DBC)2} þ H]þ), 504.01 (100%,
[{LFe(DBC)} þ Na]þ), 482.03 (60%, [{LFe(DBC)} þ H]þ),
243.05 (70%, [DBC þ Na]þ), 221.06 (25%, [DBC þ H]þ).
UV-vis (in MeCN): λ, nm (ε, M-1cm-1): 715 (4100), 500
(3450), 325 (sh).
facial N,N,O donor ligands, closely mimicking the active site
of extradiol-cleaving catechol dioxygenases. The model com-
plexes react with dioxygen to give mainly the auto-oxidation
product of catechol along with a maximum of 38% extradiol
cleavage products. This work stems from our interest in
designing structural and functional models of nonheme iron
enzymes with the 2-his-1-carboxylate facial triad. In this
paper, we report the syntheses and characterization of iron-
catecholate complexes, [(L)3Fe3(DBC)3] (1) and [(L)3Fe3-
(cat)3] (2), of a proline-based flexible tridentate facial ligand
(HL = 1-(2-pyridylmethyl)pyrrolidine-2-carboxylic acid) con-
taining two nitrogen and one carboxylate oxygen donors
(Chart 1). The molecular structure of 1 was obtained by the
single crystal X-ray diffraction studies. We also report herein
the reactivity of iron-catecholate complexes toward dioxy-
gen and the selectivity of the C-C bond cleavage of catechol.
[(L)3Fe3(cat)3] (2). To a warm mixture of HL (0.206 g,
1 mmol), sodium hydroxide (0.04 g, 1 mmol), and iron(III)
perchlorate (0.354 g, 1 mmol) in 5 mL of methanol was dopwise
added a methanolic solution (5 mL) of a mixture of catechol
(0.10 g, 1 mmol) and sodium hydroxide (0.08 g, 2 mmol).
The color of the solution immediately turned to purple-blue.
The mixture was then stirred under warm conditions for 1 h. The
solution was concentrated under vacuum, and then diethylether
(5 mL) was added to the mixture. A blue precipitate was sepa-
rated upon cooling, which was isolated by filtation and dried
under vacuum. Yield: 0.28 g (68%). Elemental analysis calcd
(%) for C51H51Fe3N6O12 NaClO4(1229.96 g/mol): C 49.80,
3
H 4.18, N 6.83; Found: C 49.3, H 4.3, N 6.4. IR (KBr): υ =
3423(br), 2926(vs), 1640-1616(br), 1462(s), 1390(s), 1252(s),
1095(vs), 1028(s), 870(m), 787(s), 741(s), 671(s), 648(s), 623(s)
cm-1. ESI-MS (positive ion mode, CH3CN): m/z (%): 1128.84
(15%, [{L3Fe3(cat)3} þ Na]þ), 760.32 (50%, [{L2Fe2(cat)2} þ
Na]þ), 652.41 (25%, [{L2Fe2(cat)} þ Na]þ), 391.64 (100%,
[{LFe(cat)} þ Na]þ), 369.69 (10%, [{LFe(cat)} þ H]þ). UV-vis
(in MeCN): λ, nm (ε, M-1cm-1): 600 (3050), 400 (2900), 330 (sh).
Analyses of 3,5-Di-tert-butyl Catechol Cleavage Products.
60 mg (0.04 mmol) of complex 1 was dissolved in an organic
solvent (15 mL). Dioxygen was bubbled through the solution for
two minutes and then allowed to stir for 12 h at RT under
oxygen atmosphere. The purple-blue solution slowly turned to
green. To the solution, one equivalent of internal standard
(1,3,5-tribromobenzene) was added, and the solvent was re-
moved under reduced pressure. The residue was then treated
with 10 mL of 3 M HCl, the organic products were extracted
with diethylether (3 ꢀ 15 mL) and dried over sodium sulfate.
Removal of solvent gave the catechol cleavage products. The
organic products were analyzed by ESI-MS without further
purification and were quantified by 1H NMR spectroscopy.
Authentic compounds were prepared according to the literature
procedure or compared the spectral data with the reported
Experimental Section
General. All chemicals and solvents were purchased from
commercial sources and were used without further purifica-
tion unless otherwise noted. Solvents were distilled, dried, and
deoxygenated before use. 2-Methoxycarbonyl-1-(2-pyridyl-
methyl)pyrrolidine (L1) was prepared by a modification of the
literature procedure.22 Caution: Although no problems were
encountered during the synthesis of the complex, perchlorate
salts are potentially explosive and should be handled with care!23
Preparation and handling of air-sensitive materials were carried
out under an inert atmosphere either in a glovebox or in a
Schlenk line.
Synthesis of HL. The ester L1 (0.11 g, 0.5 mmol) was dissolved
in 1 mL of 1 M sodium hydroxide solution at 0 °C on an ice bath.
The ice bath was then removed, and the mixture was allowed to stir
for 6 h at room temperature (RT). The solution was maintained at
pH = 7, and aqueous layer was extracted with dichloromethane.
The aqueous phase was removed under vacuum to give a yellow
residue. The residue was treated with absolute alcohol, and sodium
chloride was separated by filtration. Removal of ethanol gave the
pure ligand HL as light-yellow oil. Yield: 0.08 g (78%). ESI-MS
(positive ion mode, CH3CN): m/z = 229.00 (100%, [(HL) þ
Na]þ), 207.03 (25%, [(HL) þ H]þ), 161.04 (10%, [(HL) -
COOH]þ). 1H NMR (300 MHz, CD2Cl2): δ = 8.51 (d, 1H, J =
4.2 Hz), 7.64 (m, 1H), 7.37 (d, 1H, J = 7.7 Hz), 7.20 (m, 1H), 4.26
(d, 1H, J = 14.2 Hz), 4.21 (d, 1H, J = 14.1 Hz), 3.92 (m, 1H), 3.50
(m, 1H), 2.92 (m, 1H), 2.23 (m, 2H), 1.89 ppm (m, 2H).
1
compounds.24-27 RT H NMR data for 3,5-di-tert-butyl cate-
chol cleavage products (300 MHz, CDCl3): δ = 4,6-di-tert-
butyl-2-pyrone (A): 1.22 (s, 9H), 1.36 (s, 9H), 6.05 (m, 2H); 3,5-
di-tert-butyl-2-pyrone (B): δ = 1.22 (s, 9H), 1.40 (s, 9H), 7.21 (d,
1H), 7.24 (d, 1H); cis,cis-3,5-di-tert-butyl-2-hydroxymuconic
semialdehyde (C): δ = 1.11 (s, 9H), 1.19 (s, 1H), 5.90 (s, 1H),
9.60 (s, 1H); 3,5-di-tert-butyl-1-oxacyclohepta-3,5-diene-2,
7-dione (D): δ=1.16 (s, 9H), 1.28 (s, 1H), 6.14 (d, 1H), 6.44
(d, 1H); 3,5-di-tert-butyl-5-(carboxymethyl)-2-furanone (E):
δ=0.98 (s, 9H), 1.24 (s, 9H), 2.91 (d, 1H), 2.81 (d, 1H), 6.96
(s, 1H), 9.72 (s, 1H); 3-tert-butylfuran-2,5-dione (F): 1.33(s, 9H),
[(L)3Fe3(DBC)3] (1). Ligand HL (0.103 g, 0.5 mmol) was
dissolved in methanol (5 mL). To that, a methanolic solution
(5 mL) of iron(III) perchlorate hydrate (0.18 g, 0.5 mmol) was
added with stirring. The color of the solution changed to red-
dish brown. A mixture of 3,5-di-tert-butyl catechol (0.11 g,
0.5 mmol), and triethylamine (140 μL, 1 mmol) in methanol
(2 mL) was added to the reaction mixture. The color of the mix-
ture immediately took deep purple-blue. The mixture was allowed
to stir for 2 h and solvent was removed under vacuum to give a
deep purple-blue solid. This crude solid was then washed several
(24) Demmin, T. R.; Rogic, M. M. J. Org. Chem. 1980, 45, 1153–1156.
(25) Matsumoto, M.; Kuroda, K. J. Am. Chem. Soc. 1982, 104, 1433–
1434.
(26) Weiner, H.; Hayashi, Y.; Finke, R. G. Inorg. Chim. Acta 1999, 291,
(22) Chelucci, G.; Falorni, M.; Giacomelli, G. Tetrahedron: Asymmetry
1990, 1, 843–849.
(23) Wolsey, W. C. J. Chem. Educ. 1973, 50, A335–A337.
426–437.
(27) Mayilmurugan, R.; Suresh, E.; Palaniandavar, M. Inorg. Chem.
2007, 46, 6038–6049.

4520 Inorganic Chemistry, Vol. 49, No. 10, 2010
Paria et al.
Table 1. Crystallographic Data for Complex 1 NaBPh4
Scheme 1. Synthesis of Ligand and Iron-Catecholate Complexes
3
1 NaBPh4
3
empirical formula
formula weight
crystal system
space group
C99H119BFe3N6NaO12
1786.35
trigonal
R3
˚
a, A
17.4081(4)
17.4081(4)
27.0084(14)
90.00°
90.00°
120.00°
7088.1(4)
3
˚
b, A
˚
c, A
R, °
β, °
γ, °
methanol (Scheme 1). Complex 1 can also be synthesized by
reacting L1 with an iron(III) salt and 3,5-di-tert-butyl cate-
chol in the presence of a base. The ester gets hydrolyzed
during the reaction with concomitant formation of the
catecholate complex. Electro-spray ionization mass spectro-
metry (ESI-MS) of 1 inacetonitrileshowspredominant peaks
at m/z 1465.10, 984.95, 504.00, and 482.03, with isotope
3
˚
volume, A
Z
Dcalcd., Mg/m3
μ Mo KR, mm-1
F(000)
1.255
0.521
2835
θ range, °
1.55-31.81
29 657
8930
0.0238
7576
343
distribution patterns calculated for [{(L)3Fe3(DBC)3
þ
reflections collected
reflns unique
R(int)
Na}]þ, [{(L)2Fe2(DBC)2} þ Na]þ, [{(L)Fe(DBC)} þ Na]þ
and [{(L)Fe(DBC)} þ H]þ, respectively. ESI-MS (positive
ion mode) of 2 in acetonitrile shows peaks at m/z 1128.84,
760.32, 652.41, 391.64, and 369.69, with expected isotope
distribution patterns calculated for [{L3Fe3(cat)3} þ Na]þ,
[{L2Fe2(cat)2} þ Na]þ, [{L2Fe2(cat)} þ Na]þ, [{LFe(cat)} þ
Na]þ, and [{LFe(cat)} þ H]þ ions, respectively. This indi-
cates that trinuclear species dissociate in solution. It is also
possible that the trinuclear species get decomposed in the
experimental condition of mass spectrometry. Interestingly,
the ESI-MS of an acetonitrile solution containing both 1 and
2 shows additional peaks at m/z 1395.99, 1354.19, 1134.37,
and 873.40, attributable to [{L3Fe3(DBC)2(cat)(MeCN)} þ
Na]þ, [{L3Fe3(DBC)2(cat)} þ Na]þ, [{L3Fe3(DBC)(cat)} þ
Na]þ, and [{L2Fe2(DBC)(cat)} þ Na]þ, respectively. The
detection of peaks corresponding to mixed catecholate
species establishes the existence of an equilibrium among
mono-, di- and trinuclear species in solution. The deep-blue
complex 1 shows two strong bands at 715 and 500 nm in
acetonitrile typical for catecholate-to-iron(III) charge-trans-
fer transitions.13 The charge-transfer bands for complex 2,
on the other hand, are shifted to higher energy at 600 and
400 nm. The shifting of the charge-transfer band to higher
energy for 2 with respect to 1 is due to the absence of electron-
donating substituent on the catecholate ring in 2.30
data (I > 2σ(I))
parameters refined
goodness-of-fit on F2
R1 [I > 2σ(I)]
wR2
1.048
0.0637
0.1748
0.733-2.875
-3
˚
residuals e.A
6.52(s, 1H); and 3,5-di-tert-butylbenzoquinone (Q): δ = 1.22
(s, 9H), 1.28 (s, 9H), 6.21 (d, 1H), 6.93 (d, 1H).
Physical Measurements. Fourier transform infrared spectros-
copy on KBr pellets was performed on a Shimadzu FT-IR 8400S
instrument. Elemental analyses were performed on a Perkin-
Elmer 2400 series II CHN series. Solution electronic spectra
were measured on an Agilent 8453 diode array spectrophoto-
meter. Electro-spray mass spectra were recorded with a Waters
QTOF Micro YA263 instrument. All RT 1H NMR spectra were
collected on a Bruker DPX-300 spectrometer.
X-ray Crystal Structure Determination. Crystallographic data
for 1 are provided in Table 1. X-ray single crystal data for
1 NaBPh4 were collected at 100 K using Mo KR (λ = 0.7107 A)
˚
3
radiation on a SMART APEX diffractometer equipped with
CCD area detector. Data collection and reduction and structure
solution/refinement were carried out using the software package
of APEX II.28 All structures were solved by direct method and
refined in a routine manner. The non-hydrogen atoms were
treated anisotropically. The disordered carbons were treated
isotropically. The hydrogen atoms were geometrically fixed,
except those that are attached with disordered carbon atoms.
The tetraphenyl borate counteranion is found to be disordered
on a three-fold axis. The isotropic thermal parameters of
disorder carbon atoms of the phenyl ring which lie on three-
fold axis were fixed, and the other three phenyl rings and one
tertiary butyl group were refined isotropically using the second
free variable facility (FVAR) provided with SHELXTL,29 so the
total site occupancy factor (SOFs) of the disordered atom is 1.
Crystal Structure. X-ray quality single crystals of
1 NaBPh4, crystallized in a chiral space group trigonal
3
R3, were grown by slow diffusion of hexane into an aceto-
nitrile solution in the presence of sodium tetraphenyl borate.
It is important to note that the complex could not be
crystallized in the absence of sodium salt. The role of sodium
ion in the crystallization process can be explained from the
structure of 1 (Figure 1), which exhibits a cyclic trinuclear
complex with three 3,5-di-tert-butyl catecholates, three
irons, and three ligands forming a sodium ion encapsulated
12-MC-3 metallacoronate.31 In the monocationic C3 sym-
metric trinuclear complex, each iron center is in octahedral
environment coordinated by the tridentate N,N,O ligand
occupying one face of the octahedron and a bidentate
catecholate. The remaining sixth coordination site is occu-
pied by a bridging carboxylate oxygen atom from another
ligand.
Results and Discussion
The ligand (HL) was synthesized, according to a modifica-
tion of the reported procedure,22 by reductive amination of
pridine-2-carboxaldehyde with L-proline methyl ester hydro-
chloride using sodium cyanoborohydride in methanol fol-
lowed by hydrolysis of the ester (Scheme 1). The iron(III)-
catecholate complexes were synthesized by reacting the
ligand, iron(III) perchlorate, catechol, and base (1:1:1:3) in
(30) Cox, D. D.; Benkovic, S. J.; Bloom, L. M.; Bradley, F. C.; Nelson,
M. J.; Que, L., Jr.; Wallick, D. E. J. Am. Chem. Soc. 1988, 110, 2026–2032.
(31) Mezei, G.; Zaleski, C. M.; Pecoraro, V. L. Chem. Rev. 2007, 107,
4933–5003.
(28) Bruker, version 2.1-0; Bruker AXS, Inc.: Madison, WI, 2006.
(29) Sheldrick, G. M. SHELXL-97, Program for crystal structure refine-
€
€
ment; University of Gottingen: Gottingen, Germany, 1997.

Article
Inorganic Chemistry, Vol. 49, No. 10, 2010 4521
Figure 1. Crystal structure of sodium encapsulated [(L)3Fe3(DBC)3]. The
hydrogen atoms and tert-butyl groups (on catechols) are omitted for clarity.
˚
Table 2. Selected Bond Lengths (A) and Angles (°) for 1 NaBPh4
3
Fe(1)-N(1)
2.228(3)
2.145(3)
2.039(2)
1.908(2)
1.951(2)
2.068(3)
2.443(3)
2.250(2)
162.26(11)
163.95(11)
96.18(11)
96.18(11)
83.43(10)
99.47(11)
96.28(12)
C(12)-C(25)
1.403(4)
1.414(5)
1.385(6)
1.395(6)
1.391(5)
1.386(4)
1.344(4)
1.350(4)
176.01(13)
83.43(10)
94.41(11)
76.58(12)
87.61(12)
96.38(10)
95.25(11)
Figure 2. UV-vis spectral changes of complex 1 (conc. = 0.25 mM) in
MeCN upon exposure to dioxygen at ambient condition.
Fe(1)-N(2)
C(12)-C(13)
Fe(1)-O(1)
C(13)-C(18)
Fe(1)-O(3)
C(18)-C(19)
solution. During the reaction, the catecholate-to-iron(III)
charge-transfer bands at 715 and 500 nm slowly disappear
with time at a rate of 5.15 ꢀ 10-4 s-1. The disappearance of
the charge-transfer bands (Figure 2) indicates the oxidation
of bound catecholate. The rate of the reaction becomes
slighter slower in dichloromethane (kobs =3.31ꢀ 10-4 s-1).
Further, the oxidized products from 3,5-di-tert-butyl
catechol were identified by 1H NMR spectroscopy and by
comparing with independently synthesized compounds
or their known spectra. The products were quantified by
1H NMR spectroscopy using 1,3,5-tribromobenzene as
an internal standard. The distribution of catechol-derived
products was found to be solvent dependent. In the
reaction of 1 with O2 in acetonitrile, 42% of extradiol
cleavage products were obtained along with the forma-
tion of same percentage of intradiol cleavage products
(Table 3). In this reaction, a small amount (4%) of a five-
membered ring, 3-tert-butylfuran-2,5-dione (F), was
formed as a side product. The amount of organic product
from catechol cleavage accounts for 88% of 3,5-di-tert-
butyl catechol. The remaining 12% was accounted as
unreacted substrate. The percentage of auto-oxidation
product, 3,5-di-tert-butylbenzoquinone, was found to be
negligible. In contrast, in the work of Gebbink et al.21
with a model iron-catecholate complex of a facial N,N,O
donor ligand, the major product was the quinone. There-
fore, the model complex reported here is an improvement
over the reported catecholate complexes supported by
N,N,O donor ligands. The product distribution changed
dramatically in dichloromethane with a decrease in extra-
and intradiol cleavage products. In this case, about 13%
auto-oxidation product of catechol was observed.
Fe(1)-O(4)
C(19)-C(24)
Fe(1)-O(2)
C(24)-C(25)
Na(1)-O(1)
O(3)-C(12)
Na(1)-O(4)
O(4)-C(25)
O(2)-Fe(1)-N(1)
O(1)-Fe(1)-O(3)
O(1)-Fe(1)-N(2)
O(1)-Fe(1)-O(2)
O(4)-Fe(1)-O(1)
O(4)-Fe(1)-N(1)
O(3)-Fe(1)-N(2)
O(4)-Fe(1)-N(2)
O(3)-Fe(1)-O(4)
O(3)-Fe(1)-O(2)
N(2)-Fe(1)-N(1)
O(2)-Fe(1)-N(2)
O(4)-Fe(1)-O(2)
O(3)-Fe(1)-N(1)
In each of the [(L)Fe(DBC)] unit of 1, one catecholate
oxygen atom (O4) and the pyridine nitrogen atom (N2)
occupy the axial position and are trans to each other
(O4-Fe1-N2 angle at 176.01(13)°) with the Fe1-O4 and
˚
Fe1-N2 distances of 1.951(2) and 2.145(3) A, respec-
tively. The other catecholate oxygen atom (O3) occupies a
position in the equatorial plane trans to the carboxylate
oxygen atom (O1) from the ligand with the Fe1-O3 and
˚
Fe1-O1 distances of 1.908(2) and 2.039(2) A, respec-
tively. The C-O and C-C bond lengths of the catecho-
late ring indicate the binding of dianionic catecholate
(Table 2). It is important to note that the catecholate
oxygens bind to the iron center trans to the histidine
nitrogen atoms leaving the site trans to carboxylate donor
for the activation of dioxygen, as observed in the structure
of extradiol dioxygenase.4 In complex 1, two other posi-
tions in the equatorial plane are occupied by the amine
˚
nitrogen N1 (Fe1-N1 at 2.228(3) A) and a bridging
carboxylate oxygen atom O2 (Fe1-O2 at 2.068(3) A)
˚
from another ligand with the O2-Fe1-N1 angle of
162.26(11)°. The Fe-N and Fe-O (catecholate) bond
distances are closely comparable to the reported [(TPA)-
Fe(DBC)]BPh4 complex,8 while the Fe-O(carboxylate)
distances are slightly longer than those reported in related
complexes.13,21 The encapsulated sodium ion is in an
octahedral coordination environment ligated by catecho-
late oxygen (O4) and carboxylate oxygen (O1) atoms.
Reactivity of Catecholate Complexes toward Dioxygen.
The reactivity of 1 toward dioxygen was investigated to
determine the catechol cleavage products. The deep blue
solution of 1 reacts with dioxygen in acetonitrile at
ambient conditions over a period of 2 h to give a green
While complex 1 reacts with oxygen to form 3,5-di-tert-
butyl catechol cleavage products, complex 2 is unreactive
toward O2 under similar conditions and does not give any
catechol cleavage product. This may be due the fact that
the unsubstituted catecholate ring is not sufficiently
electron rich to form iron(II)-semiquinone species for
further reaction with dioxygen.
To understand the role of metal valency on the selecti-
vity of C-C bond cleavage, the reactivity of iron(II)-
catecholate complex toward dioxygen was studied.

4522 Inorganic Chemistry, Vol. 49, No. 10, 2010
Paria et al.
Table 3. Catechol Cleavage Products upon Reaction of 1 with O2
extradiol cleavage pathway. However, product C was ob-
served only during the reaction with in situ generated
complexes but not with 1. The use of iron(II) did not change
the selectivity of extradiol products.
The formation of intra- and extradiol products in a 1:1
ratio suggests an equal probability of acyl and alkenyl
migration rearrangements of the alkylperoxide inter-
mediate. Bugg et al.2 have proposed that the fate of the
alkylperoxide intermediate is governed by stereoelectro-
nic factors. Alkenyl migration gives rise to extradiol
cleavage as a result of a pseudoaxial arrangement of the
alkylperoxide intermediate. This has been supported by a
recent crystal structure of the alkylperoxide intermediate
of iron(II)-containing homoprotocatechuate 2,3-dioxy-
genase reported by Kovaleva et al.,4 where the peroxide
assumes a pseudoaxial orientation with respect to the
substrate ring. On the other hand, a pseudoequatorial
arrangement of the peroxide leads to an acyl migration to
give intradiol cleavage. The structure of the model com-
plex reported in this work indicates that both arrange-
ments of the alkylperoxide intermediate are possible as a
result of a vacant coordination site created by labile
carboxylate oxygens (O1 and O2) in solution. This is also
supported by the structural data of complex 1 with longer
Fe-O(carboxylate) distances. It has been shown that the
trimeric unit of complex 1 is in equilibrium with mono
and dinuclear species in solution. The ESI-MS data of the
solution immediately after reacting 1 with O2 show only a
peak at m/z 481.91 with isotope distribution patterns
calculated for [{(L)Fe(DBC)} þ H]þ ion. The ion peak
corresponding to the trinuclear unit could not be ob-
served. This suggests that a mononuclear complex reacts
with dioxygen. This also explains a 1:1 formation of extra-
and intradiol products and supports its mimicry relevance
of a mononuclear enzyme.
Figure 3. UV-vis spectral changes of iron(II)-catecholate complex
prepared in situ (conc. = 0.75 mM of Fe(ClO4) 6H2O, HL, and
H2DBC and 1.5 mM of Et3N) in MeCN upon exposure to dioxygen at
3
50 °C.
Several attempts to isolate an iron(II)-catechloate complex
of HL were failed. The iron(II)-catecholate complex of HL
was prepared in situ at 50 °C in acetonitrile. Initially a yellow
solution was formed which slowly changed to purple-blue in
the presence of dioxygen (Figure 3). The optical spectrum
of the purple-blue species was found to be exactly similar
to that of the iron(III)-catecholate complex 1. The purple-
blue iron(III)-catecholate species further changed to a
green solution as observed for 1. This supports the proposal
by Que et al.10 and Moro-oka et al.20 that the iron(II)-
catecholate complex is oxidized to iron(III)-catecholate
complex by an outer-sphere electron transfer followed by
the attack of O2 to form an iron(III)-peroxide intermedi-
ate. In this case, the isolated products from the reaction mix-
ture showed similar distribution of products with about 36%
extradiol, 34% intradiol cleavage, and no auto-oxidation
product (Table 3). It is worth mentioning here about the
formation of 9% of an extradiol cleavage product, cis,cis-
3,5-di-tert-butyl-2-hydroxy muconic semialdehyde (C),
which is rarely reported in model studies.26,27 This product
is formed as a result of hydrolysis of the lactone ring in the
Conclusions
In conclusion, we have isolated and structurally character-
ized a novel iron(III)-catecholate model complex of a
tridentate facial N,N,O-donor ligand derived from a natural

Article
Inorganic Chemistry, Vol. 49, No. 10, 2010 4523
amino acid,
L
-proline. The binding mode of ligand mimics
Acknowledgment. This work was supported by the
Department of Science and Technology (DST), Govern-
ment of India (Project SR/S1/IC-10/2006). S.P. and P.H.
are thankful to the Council of Scientific and Industrial
Research (CSIR), India for fellowship. Crystal struc-
ture determination was performed at the DST-funded
National Single Crystal Diffractometer Facility at the
Department of Inorganic Chemistry, IACS.
the 2-his-1-carboxylate facial triad motif observed in the
superfamily of nonheme iron enzymes. The extradiol clea-
vage in acetonitrile without the formation of any auto-
oxidation product is the first of this kind with a model
iron-catecholate complex of a 2-N-1-carboxylate ligand.
The iron-catecholate complex is a potential functional
model of extradiol-cleaving catechol dioxygenases. A de-
tailed study to model the other enzymes in the family of
2-his-1-carboxylate facial triad with this ligand is currently
under investigation.
Supporting Information Available: Crystallographic data in
CIF file format. This material is available free of charge via the
Internet at http://pubs.acs.org.