A new tripodal iron(III) monophenolate complex: Effects of ligand basicity, steric hindrance, and solvent on regioselective extradiol cleavage

Article abstract of DOI:10.1021/ic700646m

The new iron(III) complex [Fe(L3)Cl₂], where H(L3) is the tripodal monophenolate ligand N,N′-dimethyl-N′-(pyrid-2-ylmethyl)- N′-(2-hydroxy-3,5-dimethylbenzyl)ethylenediamine, has been isolated and studied as a structural and functional model for catechol dioxygenase enzymes. The complex possesses a distorted octahedral iron(III) coordination geometry constituted by the phenolate oxygen, pyridine nitrogen and two amine nitrogens of the tetradentate ligand, and two cis-coordinated chloride ions. The Fe-O-C bond angle (134.0°) and Fe-O bond length (1.889 A) are very close to those (Fe-O-C, 133° and 148°, Fe-O(tyrosinate), 1.81 and 1.91 A) of protocatechuate 3,4-dioxygenase enzymes. When the complex is treated with AgNO₃, the ligand-to-metal charge transfer (LMCT) band around 650 nm (ε, 2390 M^-1 cm^-1) is red shifted to 665 nm with an increase in absorptivity (ε, 2630 M^-1 cm^-1) and the Fe^III/Fe^II redox couple is shifted to a slightly more positive potential (-0.329 to -0.276 V), suggesting an increase in the Lewis acidity of the iron(III) center upon the removal of coordinated chloride ions. Furthermore, when 3,5-di-tert-butylcatechol (H₂DBC) pretreated with 2 mol of Et₃N is added to the complex [Fe(L3)Cl₂] treated with 2 equiv of AgNO₃, two intense catecholate-to-iron(III) LMCT bands (719 nm, ε, 3150 M^-1 cm^-1; 494 nm, ε, 3510 M^-1 cm^-1) are observed. Similar observations are made when H₂DBC pretreated with 2 mol of piperidine is added to [Fe(L3)Cl₂], suggesting the formation of [Fe(L3)(DBC)] with bidentate coordination of DBC^2-. On the other hand, when H₂DBC pretreated with 2 mol of Et₃N is added to [Fe(L3)Cl₂], only one catecholate-to-iron(III) LMCT band (617 nm; ε, 4380 M^-1 cm^-1) is observed, revealing the formation of [Fe(L3)(HDBC)(Cl)] involving monodentate coordination of the catecholate. The appearance of the DBSQ/H₂DBC couple for [Fe(L3)(DBC)] at a potential (-0.083 V) more positive than that (-0.125 V) for [Fe(L3)(HDBC)(Cl)] reveals that chelated DBC^2- in the former is stabilized toward oxidation more than the coordinated HDBC^-. It is remarkable that the complex [Fe(L3)(HDBC)(Cl)] undergoes slow selective extradiol cleavage (17.3%) of H ₂DBC in the presence of O₂, unlike the iron(III)-phenolate complexes known to yield only intradiol products. It is probable that the weakly coordinated (2.310 A) -NMe₂ group rather than chloride in the substrate-bound complex is displaced, facilitating O₂ attack on the iron(III) center and, hence, the extradiol cleavage. In contrast, when the cleavage reaction was performed in the presence of a stronger base-like piperidine before and after the removal of the coordinated chloride ions, a faster intradiol cleavage was favored over extradiol cleavage, suggesting the importance of the bidentate coordination of the catecholate substrate in facilitating intradiol cleavage. Also, intradiol cleavage is favored in dimethylformamide and acetonitrile solvents, with enhanced intradiol cleavage yields of 94 and 40%, respectively.

Full text of DOI:10.1021/ic700646m

Inorg. Chem. 2007, 46, 6038−6049
A New Tripodal Iron(III) Monophenolate Complex: Effects of Ligand
Basicity, Steric Hindrance, and Solvent on Regioselective Extradiol
Cleavage
Ramasamy Mayilmurugan,† Eringathodi Suresh,‡ and Mallayan Palaniandavar*,†
School of Chemistry, Bharathidasan UniVersity, Tiruchirapalli 620 024, India, and Analytical Science
Discipline, Central Salt and Marine Chemicals Research Institute, BhaVnagar 364 002, India
Received April 4, 2007
The new iron(III) complex [Fe(L3)Cl2], where H(L3) is the tripodal monophenolate ligand N,N-dimethyl-N′-(pyrid-
2-ylmethyl)-N′-(2-hydroxy-3,5-dimethylbenzyl)ethylenediamine, has been isolated and studied as a structural and
functional model for catechol dioxygenase enzymes. The complex possesses a distorted octahedral iron(III)
coordination geometry constituted by the phenolate oxygen, pyridine nitrogen and two amine nitrogens of the
tetradentate ligand, and two cis-coordinated chloride ions. The Fe
−O
−C bond angle (134.0°) and Fe−O bond
length (1.889 Å) are very close to those (Fe C, 133 and 148
−
O
−
°
°
, Fe O(tyrosinate), 1.81 and 1.91 Å) of
−
protocatechuate 3,4-dioxygenase enzymes. When the complex is treated with AgNO3, the ligand-to-metal charge
1
transfer (LMCT) band around 650 nm (
ꢀ
, 2390 M- cm-1) is red shifted to 665 nm with an increase in absorptivity
1
(
ꢀ
, 2630 M- cm-1) and the FeIII/FeII redox couple is shifted to a slightly more positive potential (
−0.329 to
−0.276
V), suggesting an increase in the Lewis acidity of the iron(III) center upon the removal of coordinated chloride ions.
Furthermore, when 3,5-di-tert-butylcatechol (H2DBC) pretreated with 2 mol of Et3N is added to the complex [Fe-
1
(L3)Cl2] treated with 2 equiv of AgNO3, two intense catecholate-to-iron(III) LMCT bands (719 nm,
ꢀ
, 3150 M-
1
cm-1; 494 nm,
ꢀ
, 3510 M- cm-1) are observed. Similar observations are made when H2DBC pretreated with 2
mol of piperidine is added to [Fe(L3)Cl2], suggesting the formation of [Fe(L3)(DBC)] with bidentate coordination of
DBC2-. On the other hand, when H2DBC pretreated with 2 mol of Et3N is added to [Fe(L3)Cl2], only one catecholate-
1
to-iron(III) LMCT band (617 nm;
ꢀ
, 4380 M- cm-1) is observed, revealing the formation of [Fe(L3)(HDBC)(Cl)]
involving monodentate coordination of the catecholate. The appearance of the DBSQ/H2DBC couple for [Fe(L3)-
(DBC)] at a potential (
−
0.083 V) more positive than that (
−
0.125 V) for [Fe(L3)(HDBC)(Cl)] reveals that chelated
-
DBC2 in the former is stabilized toward oxidation more than the coordinated HDBC-. It is remarkable that the
complex [Fe(L3)(HDBC)(Cl)] undergoes slow selective extradiol cleavage (17.3%) of H2DBC in the presence of O2,
unlike the iron(III)−phenolate complexes known to yield only intradiol products. It is probable that the weakly
coordinated (2.310 Å)
−NMe2 group rather than chloride in the substrate-bound complex is displaced, facilitating
O2 attack on the iron(III) center and, hence, the extradiol cleavage. In contrast, when the cleavage reaction was
performed in the presence of a stronger base-like piperidine before and after the removal of the coordinated
chloride ions, a faster intradiol cleavage was favored over extradiol cleavage, suggesting the importance of the
bidentate coordination of the catecholate substrate in facilitating intradiol cleavage. Also, intradiol cleavage is favored
in dimethylformamide and acetonitrile solvents, with enhanced intradiol cleavage yields of 94 and 40%, respectively.
Introduction
ring cleavage reactions are catalyzed by two classes of
dioxygenases, distinguished on the basis of regiospecific-
ity: the intradiol dioxygenases cleave the C-C bond between
the hydroxyl groups of catechol on coordination to the non-
The aerobic degradation of aromatic compounds typically
proceeds via a catecholic intermediate that is cleaved into a
product1 with the incorporation of molecular oxygen. The
(1) (a) Que, L., Jr. In Bioinorganic Catalysis, 2nd ed.; Reedijk, J., Ed.;
Marcel Dekker: New York, 1993; pp 347-393. (b) Kruger, H.-J.
Biomimetic Oxidations Catalyzed by Transition Metal Complexes;
Meunier, B., Ed.; Imperial College Press: London, 2000; pp 363-
413.
* Towhomcorrespondenceshouldbeaddressed.E-mail: palani51@sify.com
or palanim51@yahoo.com.
† Bharathidasan University.
‡ Central Salt and Marine Chemicals Research Institute.
6038 Inorganic Chemistry, Vol. 46, No. 15, 2007
10.1021/ic700646m CCC: $37.00
© 2007 American Chemical Society
Published on Web 06/23/2007

New Tripodal Iron(III) Monophenolate Complex
Scheme 1. Structures of Sterically hindered Mono- and Bisphenolate Ligands
heme iron(III) center2-14 by a novel substrate activation
mechanism,2 while the extradiol dioxygenases generally
utilize non-heme iron(II) to cleave the C-C bond adjacent
to hydroxyl groups through an oxygen activation mecha-
nism.3 The X-ray crystal structure of the intradiol-cleaving
protocatechuate 3,4-dioxygenase (3,4-PCD) from Pseudomo-
nas putida reveals a trigonal-bipyramidal iron(III) site with
four endogenous protein ligands (Tyr408, Tyr447, His460,
and His462) and a solvent-derived ligand.4-9,15 Furthermore,
recent crystallographic and spectral studies have shown that
upon bidentate coordination of a catecholate substrate10,11
the axial Tyr447 residue and the equatorial hydroxide are
displaced from the iron(III) center, inducing the iron active-
site geometry to alter from trigonal-bipyramidal to square-
pyramidal. In contrast to intradiol dioxygenases, extradiol
dioxygenases contain iron(II) ligated to two histidines, a
glutamate ligand, and two water molecules in the active site,
forming a square-pyramidal coordination geometry.13-15
Also, they represent the more common cleavage pathway,
but insight into these enzymes has lagged behind that of
intradiol-cleaving enzymes.13
Mimicking the function of dioxygenases through the
synthesis and study of small molecule active-site analogues
can provide useful data to improve our understanding of
structure-function correlations for the protein metal sites.
In earlier studies, several biomimetic iron complexes were
constructed as structural and functional models for catechol
dioxygenases.12,13,16-25 Several structurally well-defined oc-
tahedral iron(III) complexes12,13,16-25 were reacted with 3,5-
di-tert-butylcatechol (H2DBC) in the presence of O2 to afford
intradiol cleavage products. The ligand-donor functionalities
in them exhibit different stereoelectronic effects in influenc-
ing the Lewis acidity of the iron(III) center and hence its
catalytic intradiol cleavage activity. Very recently, we have
observed20 that the incorporation of a sterically hindering
-NMe2 group in the iron(III)-phenolate complex [Fe(L1)-
Cl2] (1), where H(L1) is N,N-dimethyl-N′-(pyrid-2-ylmethyl)-
N′-(2-hydroxy-4-nitrobenzyl)ethylenediamine (Scheme 1),
imposes a relatively high Fe-O-C bond angle of 136.1°.
Furthermore, interestingly, the incorporation of two 3,5-
dimethylphenolate arms in the iron(III)-phenolate complex
[Fe(L2)Cl] (2), where H2(L2) is N,N-dimethyl-N′,N′-bis(2-
hydroxy-3,5-dimethylbenzyl)ethylenediamine (Scheme 1),
confers a trigonal-bipyramidal coordination geometry on
iron(III),19 but the complex fails to elicit catechol dioxyge-
nase activity. In the present study, we have extended our
work on iron(III) complexes of sterically hindered ligands
to understand the effect of incorporating only one 3,5-di-
methylphenolate arm, as in [Fe(L3)Cl2] (3), where H(L3) is
N,N-dimethyl-N′-(2-hydroxy-3,5-dimethylbenzyl)-N′-(pyrid-
2-ylmethyl)ethylenediamine (Scheme 1). The H2DBC binds
in a monodentate mode to iron(III) in this octahedral complex
despite having two cis-coordinated chloride ions, and it is
cleaved regiospecifically to yield extradiol-cleavage products
in major quantities and intradiol-cleavage products in smaller
quantities. This is interesting, because to date, only a few
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Inorganic Chemistry, Vol. 46, No. 15, 2007 6039

Mayilmurugan et al.
synthetic iron(III) model complexes have been reported to
serve as functional models for extradiol-cleaving catechol
dioxygenases. Funabiki et al. found that FeCl2/FeCl3 with
pyridine/bipyridine in THF/H2O cleaves H2DBC to produce
extradiol products in 40% yield.26 The complex [FeIII-
(TACN)(DBC)(Cl)], in which TACN (1,4,9-triazacyclononane)
is cis-facially coordinated, afforded27 the extradiol product
(2-pyrone, 35%) upon exposure to O2, and removal of the
coordinated chloride ion in the complex provides an almost
quantitative yield28 of the extradiol-cleavage product. Iron(III)
complexes of certain cis-facially coordinating tridentate
ligands29 elicited extradiol cleavage up to 97% (pyrone) and
also the complex [FeIII(Tpi-Pr )(DBC)],30 where Tpi-Pr is the
cis-facially coordinating tripodal ligand hydridotris(3,5-di-
isopropyl-1-pyrazolyl)borate, gave major amounts of extra-
diol-cleavage products (2-pyrone). The iron(III) 4N ligand
complexes [Fe(6-Me3-TPA)(DBC)]+, where 6-Me3-TPA
) tris(6-methylpyrid-2-ylmethyl)amine, and [Fe(BLPA)-
(HDBC)]2+, where BLPA ) bis((6-methylpyrid-2-ylmethyl)-
(pyrid-2-yl-methyl)amine, and [Fe(6-Me2-BPMCN)(D-
BC)]+, where 6-Me2-BPMCN ) N,N′-dimethyl-N,N′-bis(6-
methylpyrid-2-ylmethyl)-1,2-cyclohexadiamine, elicited meas-
urable amounts of extradiol-cleavage products (3, 20, and
12%, respectively).31 Also, both FeIII and FeII complexes of
a variety of macrocyclic ligands yielded the authentic
extradiol product 2-hydroxymuconic semialdehyde.32 Thus,
the iron(III) complex [Fe(L-N4H2)(DBC)]+, where L-N4H2
) 2,11-diaza[3,3](2,6)pyridinophane, reacts with O2 to afford
a roughly 1:1 mixture of extradiol- and intradiol-cleavage
products.33 Furthermore, no iron(III)-phenolate complex has
been shown so far to yield extradiol-cleavage products, and
so observation of the present iron(III)-phenolate complex
3 yielding regioselective extradiol-cleavage products is
remarkable. However, intradiol-cleavage products have been
obtained in higher quantities on removal of the coordinated
chloride ions in 3 in methanol and exclusively in dimeth-
ylformamide (DMF) and CH3CN solvents.
Experimental Section
2
2
Materials. N,N-Dimethylethylenediamine, 3,4,5,6-tetrachloro-
catechol (H2TCC, Lancaster), 3,5-di-tert-butylcatechol (H2DBC),
4-tert-butylcatechol (H2TBC), 2,4-dimethylphenol, 2-pyridinecar-
boxaldehyde, catechol (H2CAT), 4-nitrocatechol (H2NCAT, Ald-
rich), iron(III) chloride (anhydrous) (Merck, India), 3-methylcat-
echol (3-H2MCAT), and 4-methylcatechol (4-H2MCAT) (Acros
Organics) were used as received, unless noted otherwise, and H2-
DBC was recrystallized from hexane before use. The supporting
electrolyte tetrabutylammonium perchlorate (NBu4ClO4, G. F.
Smith, USA) was recrystallized twice from aqueous ethanol.
Physical Measurements. Elemental analyses were performed
1
on a Perkin-Elmer Series II CHNS/O Analyzer 2400. H NMR
spectra were recorded on a Bruker 200 MHz NMR spectrometer.
The electronic spectra were recorded on Varian Cary 300 double-
beam UV-vis and Agilent 8453 diode array UV-vis spectropho-
tometers. The EPR spectra were recorded on a JEOL JES-TE 100
X-band spectrometer. Cyclic voltammetry (CV) and differential
pulse voltammetry (DPV) were performed using a three-electrode
cell configuration. A platinum sphere, a platinum plate, and Ag-
(s)/Ag+were used as working, auxiliary, and reference electrodes,
respectively. The supporting electrolyte used was NBu4ClO4
(TBAP). The temperature of the electrochemical cell was maintained
at 25 ( 0.2 °C by a cryocirculator (HAAKE D8 G). By the bubbling
of research-grade nitrogen, the solutions were deoxygenated and
an atmosphere of nitrogen was maintained over the solutions during
measurements. The E1/2 values were observed under identical
conditions for various scan rates. The instruments utilized included
an EG&G PAR 273 potentiostat/galvanostat and a Pentium-IV
computer along with EG&G M270 software to carry out the
experiments and to acquire the data. The product analyses were
performed using a HP 6890 GC series gas chromatograph equipped
with a flame ionization detector (FID) and an HP-5 capillary column
(30 m × 0.32 mm × 2.5 µm), and the GC-MS analysis was
performed on a Perkin-Elmer Clarus 500 GC-MS instrument using
a PE-5 with the previously20 reported temperature program.
Synthesis of Ligand. N,N-Dimethyl-N′-(pyrid-2-ylmethyl)-N′-
(2-hydroxy-3,5-dimethylbenzyl)ethylenediamine (H(L3)). The
ligand H(L3) was synthesized in two steps. The first step in the
synthesis was the preparation of N,N-dimethyl-N′-(pyrid-2-ylmeth-
yl)ethylenediamine, which was then reacted with 2,4-dimethylphe-
nol and 37% formalin solution in the second step to obtain H(L3).
Step 1. N,N-Dimethyl-N′-(pyrid-2-ylmethyl)ethylenediamine was
prepared as reported19 elsewhere. 1H NMR (200 MHz, CDCl3): δ
2.20 (s, N(CH3)2), 2.4 (t, CH2), 2.7 (t, CH2), 3.90 (s, py-CH2), 3.0
(sb, NH), 7.1-8.5 (pyH).
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Step 2. N,N-Dimethyl-N′-(pyrid-2-ylmethyl)ethylenediamine (1.94
g, 10.8 mmol) was added to 37% formalin (1.22 g, 16.2 mmol) in
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6040 Inorganic Chemistry, Vol. 46, No. 15, 2007

New Tripodal Iron(III) Monophenolate Complex
methanol (10 mL) at 0 °C under nitrogen, and then 2,4-dimeth-
ylphenol (1.32 g, 10.8 mmol) in methanol (8 mL) was added to
this dropwise. The solution was allowed to warm to room
temperature and then treated with acetic acid (3 mL). After the
mixture was stirred for 48 h, the solvent was removed under
vacuum, neutralized with dilute NaOH, and extracted three times
with CH2C12. The organic layer was dried over Na2SO4, and CH2-
C12 was removed under vacuum to get the ligand H(L3). The crude
product was then purified by passing it over a silica column using
CH2C12 as the eluent. The yield was 60% as a viscous oil. 1H NMR
(200 MHz, CDCl3): δ 2.25 (s, 6H, CH3), 2.35 (s, 6H, N-CH3),
2.55 (s, 4H, CH2CH2), 3.55 (s, 2H, Ph-CH2), 3.70 (s, py-CH2),
6.60, 6.75 (s, aryl), 7.05-7.9 (s, py-H).
Syntheses of Complexes. The complex [Fe(L3)Cl2] (3) was
prepared by the reaction of a solution of ferric chloride (0.16 g,
1.0 mmol) in methanol (5 mL) to a methanolic solution (10 mL)
of an equivalent amount of the ligand H(L3) (0.31 g, 1.0 mmol) in
the presence of an equivalent amount of triethylamine (Et3N) (0.10
g, 140 µL, 1.0 mmol) in stoichiometric combination. The solution
was stirred and then refluxed for an hour. The blue-colored
precipitate obtained was filtered off, washed with small amounts
of cold methanol, and dried in vacuo over P4O10. The yield was
0.33 g (75%). Anal. Calcd. for C19H26N3OFeCl2: C, 51.96; H, 5.97;
N, 9.57. Found: C, 51.91; H, 6.01; N, 9.64. X-ray quality crystals
were obtained by the slow evaporation of a methanolic solution of
the complex. The complex [Fe(L2)Cl] (2) was prepared by using
the reported procedure.19
Kinetics and Product Analysis. The kinetic analysis17,19-25,34
of the catechol-cleavage reactions was carried out by a time-
dependent measurement of the disappearance of the lower-energy
catecholate-to-iron(III) ligand-to-metal charge transfer (LMCT)
band at ambient temperature (25 °C) by exposing the catecholate
adducts generated in situ to molecular oxygen. The solvents were
equilibrated at the atmospheric pressure of O2 at 25 °C, and the
solubility of O2 at 25 °C is as follows: methanol, 2.12 × 10-3
M;24,35 acetonitrile, 8.1 × 10-3 M;25,35 and DMF,19,20,35 4.86 × 10-3
M. A stock solution of the adduct [Fe(L3)(HDBC)(Cl)]33,36 was
prepared by treating the complex 3 (1.0 × 10-2 M) with an
equivalent amount of H2DBC pretreated with 2 equiv of Et3N. The
adduct [Fe(L3)(DBC)] was generated in situ by treating 3 (1.0 ×
10-2 M) with an equivalent amount of H2DBC pretreated with 2
equiv of piperidine. The adduct [Fe(L3)(DBC)] was generated in
methanol in situ also by treating 3 (1.0 × 10-2 M) with 2 equiv of
AgNO3, centrifuging the solution to remove AgCl, and then adding
to it 1 equiv of H2DBC pretreated with 2 equiv of Et3N. The adduct
[Fe(L3)(DBC)] was generated in acetonitrile or DMF in situ by
treating 3 (1.0 × 10-2 M) with an equivalent amount of H2DBC
pretreated with 2 equiv of Et3N. Oxygenation was started by rapid
delivery of a stock solution (0.1 mL) of the catecholate adducts
(1.0 × 10-2 M) by syringe to O2-saturated solvent (4.9 mL).
The products of dioxygenation of catecholate adducts of 3 were
determined by using a known20,26,37,38 procedure with modifications.
The complex 3 (44 mg, 0.1 mmol), H2DBC (22 mg, 0.1 mmol),
and Et3N (20 mg, 28 µL, 0.2 mmol) or piperidine (17 mg, 20 µL,
0.2 mmol) were dissolved in methanol/DMF/CH3CN (10 mL) and
stirred under an oxygen atmosphere. For reactions in methanol,
DMF, and CH3CN, piperidine was used as the base. After 24
(piperidine) or 48 h (Et3N), the oxygenation reaction was quenched
by the addition of 6 M HCl solution (25 mL). The products were
extracted from the aqueous solution with CHCl3 (3 × 50 mL). The
clear yellow CHCl3 layer was separated, washed twice, first with
2 M HCl (20 mL) and then with water, and then extracted with a
saturated NaHCO3 solution (10 mL) to separate the acid products.
The CHCl3 layer was saved for further manipulation to obtain
nonacid products. The aqueous solution was acidified with 6 M
HCl and then extracted twice with 5 mL of CHCl3. The CHCl3
extracts were combined, dried over anhydrous Na2SO4, filtered, and
the solvent was rotaevaporated to yield an acid product. The CHCl3
layer remaining after NaHCO3 extraction was dried over 5 g of
anhydrous Na2SO4 at room temperature. The Na2SO4 was then
filtered off, rinsed twice with 5 mL of fresh CHCl3, and the
combined filtrate was evaporated in vacuo at 40 °C, which yields
nonacid products. Further purification of the products was ac-
complished by column chromatography using silica gel and 200
mL of CHCl3 as eluent. The major products were separated and
1
dissolved in CDCl3 and analyzed by GC, GC-MS, and H NMR
techniques. The other minor products were analyzed as a mixture,
detected by GC-MS (EI) (EI ) electron ionization), and quantified
using GC (FID) with the following temperature program: initial
temperature, 50 °C; heating rate 5 °C min-1; final temperature 250
°C; injector temperature 150 °C; and FID temperature 280 °C. GC-
MS analysis was performed under conditions identical to those used
for GC analysis.
cis,cis-3,5-Di-tert-butyl-2-hydroxymuconic semialdehyde (9).
1
GC-MS (EI) data found for C14H20O4: m/z 252. H NMR (200
MHz, CDCl3): δ 1.10 (s, 9H), 1.17 (s, 9H), 5.90 (s, 1H), 9.85 (s,
1H). The 1H NMR data were in agreement with literature
data.26,32,37,38
3,5-Di-tert-butyl-5-(2-oxo-2-piperidinylethyl)-5H-furanone (7).
1
GC-MS (EI) data found for C19H31NO3: m/z 321. H NMR (200
MHz, CDCl3): δ 1.10 (s, 9H), 1.21 (s, 9H), 1.54 (m, 6H), 2.90
1
and 3.06 (2H), 3.40 (m, 4H), 7.14 (s, 1H). The GC-MS and H
NMR data were in agreement with literature data.12e,20,21
Single-Crystal X-ray Data Collection and Structure Solution.
A crystal of suitable size was selected from the mother liquor,
immersed in paraffin oil, mounted on the tip of a glass fiber, and
cemented using epoxy resin. Intensity data for the crystal were
collected using Mo KR (λ ) 0.71073 Å) radiation on a Bruker
SMART APEX diffractometer equipped with a CCD area detector
at 293 K. The crystallographic data are collected in Table 1. The
SMART39 program was used for collecting frames of data, indexing
the reflections, and the determination of lattice parameters; the
SAINT39 program was used for the integration of the intensity of
reflections and scaling; the SADABS40 program was used for
absorption correction; and the SHELXTL41 program was used for
the space group and structure determination and least-squares
refinements on F2. The structure was solved by the heavy-atom
method. Other non-hydrogen atoms were located in successive
difference Fourier syntheses. The final refinement was performed
by a full-matrix least-squares analysis. Hydrogen atoms attached
(34) Cox, D. D.; Que, L., Jr. J. Am. Chem. Soc. 1988, 110, 8085-8092.
(35) (a) Japan Chemical Society, Kagaku-Binran Basic Part II, 2nd ed.;
Maruzen: Tokyo, 1975. (b) Sawyer, D. T. Oxygen Chemistry; Oxford
University Press: New York, 1991.
(36) Zirong, D.; Battacharya, S.; McCusker, J. M.; Hagen, P. M.;
Hendrickson, D. N.; Pierpont, C. G. Inorg. Chem. 1992, 31, 870-
877.
(37) Weiner, H.; Finke, R. G. J. Am. Chem. Soc. 1999, 121, 9831-9842.
(38) Weiner, H.; Hayashi, Y.; Finke, R. G. Inorg. Chim. Acta 1999, 291,
426-437.
(39) SMART and SAINT Software Reference Manuals, version 5.0; Bruker
AXS, Inc.: Madison, WI, 1998.
(40) Sheldrick, G. M. SADABS Software for Empirical Absorption Cor-
rection; University of Go¨ttingen: Go¨ttingen, Germany, 2000.
(41) SHELXTL Reference Manual, version 5.1; Bruker AXS, Inc.: Madison,
WI, 1998.
Inorganic Chemistry, Vol. 46, No. 15, 2007 6041

Mayilmurugan et al.
Table 1. Crystal Data and Structure Refinement for 3
empirical formula
fw
cryst syst
space group
a (Å)
C19H26Cl2Fe N3O
439.18
monoclinic
C2/c
16.524(4)
7.1673(15)
35.346(7)
103.317(8)
4073.6(15)
293(2)
b (Å)
c (Å)
â (deg)
V (Å3)
T (K)
Mo KR λ, (Å)
0.71073
1.432
density (Mg m-3
)
Z
8
µ (mm-1
F(000)
)
1.016
1832
Figure 1. Molecular structure of 3 (40% probability factor for the thermal
ellipsoid). Hydrogen atoms have been omitted for clarity.
no. of reflections
collected
GOF on F2
R1a
9617
1.058
0.0652
0.1310
Table 2. Selected Bond Lengths (Å) and Bond Angles (deg) for 3
wR2b
Fe(1)-O(1)
Fe(1)-Cl(1)
Fe(1)-Cl(2)
1.889(3)
2.3667(16) Fe(1)-N(2)
2.2999(16) Fe(1)-N(3)
Fe(1)-N(1)
2.227(4)
2.214(4)
2.312(4)
a R1 ) ∑||Fo| - |Fc||/∑|Fo|. b wR2 ) ∑w[(Fo - Fc )2/∑w[(Fo )2]}1/2
.
2
2
2
O(1)-Fe(1)-N(2)
O(1)-Fe(1)-N(1)
N(2)-Fe(1)-N(1)
O(1)-Fe(1)-Cl(2) 103.32(12)
N(2)-Fe(1)-Cl(2) 163.21(11)
N(1)-Fe(1)-Cl(2)
O(1)-Fe(1)-N(3)
N(2)-Fe(1)-N(3)
88.74(15)
163.12(16)
74.38(15)
N(1)-Fe(1)-N(3)
87.87(16)
89.41(12)
94.85(12)
92.30(12)
85.91(12)
98.17(6)
to the ligand moiety were located from the difference Fourier map
and refined isotropically.
Cl(2)-Fe(1)-N(3)
O(1)-Fe(1)-Cl(1)
N(2)-Fe(1)-Cl(1)
N(1)-Fe(1)-Cl(1)
Cl(2)-Fe(1)-Cl(1)
N(3)-Fe(1)-Cl(1)
Results and Discussion
93.23(12)
89.01(16)
79.02(16)
The ligand N,N-dimethyl-N′-(pyrid-2-ylmethyl)-N′-(2-hy-
droxy-3,5-dimethylbenzyl)ethylenediamine (H(L3)) was syn-
thesized according to a known procedure,20,34 which involves
a simple substitution reaction, with suitable modifications.
The stoichiometric reaction of FeCl3 and H(L3) in the
presence of Et3N results in the formation of the complex
[Fe(L3)Cl2] (3) in 75% yield. The formulation of the complex
is based on elemental analysis and is supported by its X-ray
crystal structure. The presence of two cis-coordinated
chloride ions in the complex would facilitate the bidentate
coordination of the 3,5-di-tert-butylcatechol (H2DBC) to
iron(III). The conductivity results reveal quite certainly that
only one chloride ion is coordinated in methanol solution
(ΛM, 123 Ω-1 cm2 mol-1). However, both chloride ions are
coordinated in acetonitrile (ΛM, 24 Ω-1 cm2 mol-1) and DMF
(ΛM, 38 Ω-1 cm2 mol-1) solutions. The complex has a
magnetic moment of 5.96 µB at room temperature, which is
characteristic of high-spin iron(III).18-22,24
Description of Crystal Structure of [Fe(L3)Cl2] (3). The
molecular structure of complex 3 is shown in Figure 1
together with the atom-numbering scheme, and the selected
bond lengths and bond angles are collected in Table 2. The
molecule contains a FeN3OCl2 coordination sphere with
distorted octahedral geometry constituted by the phenolate
oxygen atom, the pyridine nitrogen atom and two tertiary
amine nitrogen atoms of the tripodal ligand occupying four
coordination sites of the octahedron, and two chloride ions
occupying the remaining two cis-coordination sites. The 3,5-
dimethylphenolate oxygen atom occupies a coordination site
trans to the pyridine nitrogen and cis to the sterically hindered
-NMe2 group. The chloride ions are located trans to the
tertiary amine nitrogens at longer distances. The bond angles
of N3-Fe1-Cl1 (170.44°), O1-Fe1-N1 (163.12°), and
N2-Fe1-Cl2 (163.21°) deviate markedly from that of an
ideal octahedron (180°), suggesting distortion in the octa-
170.44(12)
C(13)-O(1)-Fe(1) 134.0(3)
hedral coordination geometry. The Fe-O-C bond angle in
3 (134.0°) is higher than those observed for octahedral
iron(III)-phenolate complexes,18,19 suggesting the importance
of the sterically demanding -NMe2 group20 as in 1. Interest-
ingly, it is close to that (Fe-O-C, 133° (equatorial), 148°
(axial)) of the 3,4-PCD enzyme.10,15 The higher Fe-O-C
bond angles are possibly imposed by the stereochemical
constraints of the enzyme and lead to the quite different
equatorial and axial Fe-O(tyrosinate) bond distances (1.81
and 1.91 Å), which illustrates the function of the enzyme.15
The replacement of the electron-withdrawing p-nitropheno-
late moiety20 in the analogous complex [Fe(L1)Cl2] (1) by
the electron-releasing 3,5-dimethylphenolate moiety (Scheme
2) to obtain 3 leads to the shortening of both Fe-Ophenolate
(1, 1.929 Å; 3, 1.889 Å) and Fe-NMe2 bonds (1, 2.334 Å;
3, 2.312 Å) with a concomitant increase in Fe-Npy (1, 2.180
Å; 3, 2.227 Å) and Fe-Cl (1, 2.285 and 2.339 Å; 3, 2.2999
and 2.3667 Å) bond lengths, with virtually no change in the
Fe-Namine bond distance (1, 2.213 Å; 3, 2.214 Å) and a
decrease in the Fe-O-C bond angle (1, 136.1°; 3, 134.0°)
as well. The Fe-N and Fe-O bond lengths are typical of
high-spin octahedral iron(III) complexes.18-20,22,24,29,42 The
shorter Fe-O bond distance in 3 suggests that the iron-
oxygen overlap is stronger than that in the analogous complex
120 on account of the more Lewis-basic 3,5-dimethylpheno-
late donor and that the Lewis acidity of the iron(III) center
in 3 is lower. The replacement of the pyridylmethyl arm in
(42) (a) Fujii, H.; Funabashi, Y. Angew. Chem., Int. Ed. 2002, 41, 3638-
3641. (b) Kurahashi, T.; Kobayashi, Y.; Nagatomo, S.; Tosha, T.;
Kitagawa, T.; Fujii, H. Inorg. Chem. 2005, 44, 8156-8166. (c)
Kurahashi, T.; Oda, K.; Sugimoto, M.; Ogura, T.; Fujii, H. Inorg.
Chem. 2006, 45, 7709-7721.
6042 Inorganic Chemistry, Vol. 46, No. 15, 2007

New Tripodal Iron(III) Monophenolate Complex
Scheme 2. Effect of Incorporating Electron-Releasing 3,5-dimethylphenolate Donors on Iron(III) Coordination Geometry, Fe-O Bond Length, and
Fe-O-C Bond Angle
Table 3. Electronic Spectral Dataa for Iron(III) Complexes and Their
Adducts in Methanol
3 by one more electron-releasing 3,5-dimethylphenolate arm
(Scheme 2) as in the analogous complex [Fe(L2)Cl] (2)
λ
max, nm (ꢀ, M-1 cm-1
)
confers, interestingly, the unusual trigonal-bipyramidal co-
ordination geometry on iron(III),19 which closely resembles
that of the 3,4-PCD enzyme active site, but with relatively
low Fe-O-C bond angles (122.05° and 122.15°). Also, it
leads to the shortening of the Fe-Ophenolate bond lengths
(1.868 and 1.855 Å), which suggests a lower Lewis acidity
for iron(III) center in 2, and a concomitant decrease in Fe-
O-C bond angles (122.05° and 122.15°, Supporting Infor-
mation, Table S1), which are much less than that in 3, with
only one 3,5-dimethylphenolate arm, and also that in 1.
Furthermore, the presence of two 3,5-dimethylphenolate
moieties with enhanced Lewis basicity is insufficient, and
the presence of the sterically hindering -NMe2 arm is
essential to confer a trigonal-bipyramidal coordination
geometry on iron(III), emphasizing the importance of steric
as well as electronic effects in iron(III) on coordination
geometry. Thus, very recently, Fujii et al. have shown42 that
the binding of water as an external ligand to the iron(III)
center in a sterically hindered complex leads to a structural
change from a preferred square-pyramidal to a distorted
trigonal-bipyramidal geometry.
[Fe(L3)Cl2]b
+ 2 AgNO3
added ligand
none
[Fe(L3)Cl2]
[Fe(L1)Cl2]c
650 (2390)
361 (4690)
288 (7 220)
665 (2630)
341 (6180)
279 (12 200)
545 (3755)
430 (3195)
360 (17 405)
250 (14 035)
790 (3260)
485 (3320)
380 (14 770)
295 (8795)
760 (2825)
485 (3375)
370 (14 800)
295 (9345)
730 (2670)
480 (3025)
365 (14 695)
295 (8920)
650 (3365)
480 (3755)
355 (15 570)
310 (10 575)
DBC2-
617 (4360)
337 (8050)
280 (14 800)
243 (15 820)
611 (6760)
288 (42 020)
239 (45 460)
719 (3150)
494 (3510)
288 (17 130)
TBC2-
667 (2630)
482 (2890)
288 (12 710)
CAT2-
564 (5470)
588 (2670)
475 (sh)
286 (14 170)
320 (12 520)
283 (29 750)
238 (38 630)
558 (7110)
287 (29 100)
235 (51 340)
TCC2-
569 (3100)
438 (sh)
291 (11 040)
3-MCAT2-
4-MCAT2-
4-NCAT2-
569 (6740)
627 (2880)
478 (3370)
286 (13 920)
320 (12 760)
285 (37 800)
235 (44 390)
578 (7320)
323 (13 270)
289 (39 340)
239 (49 490)
517 (13 600)
399 (35 350)
320 (25 740)
265 (57 820)
236 (55 470)
674 (2740)
495 (sh)
290 (12 990)
Electronic Absorption and EPR Spectra. In methanol
solution, 3 exhibits two bands (Figure 2 and Table 3) around
360 (ꢀ, 4690 M-1 cm-1) and 650 nm (ꢀ, 2390 M-1 cm-1),
553 (sh)
398 (9870)
263 (1760)
a Concentration of iron(III) complexes ) 2.0 × 10-4 M. The ratio of
added ligands to iron(III) complex is 1:1. The catecholate anions were
generated by adding 2 equiv of Et3N. b Chloride ions were removed by
adding AgNO3. c Data taken from ref 20.
which are assigned17-19,43 respectively to phenolate (pπ) f
FeIII (dσ*) and phenolate (pπ) f FeIII (dπ*) LMCT transi-
tion, and the ligand-based bands around 288 nm (ꢀ, 7220
M-1 cm-1). Interestingly, when it is treated with AgNO3 (to
remove coordinated chloride ions), the low-energy LMCT
band experiences a red shift to 665 nm with an increase in
absorptivity (ꢀ, 2630 M-1 cm-1). This is expected because
the removal of the coordinated chloride anions from the
iron(III) coordination sphere would decrease the negative
Figure 2. Electronic absorption spectra of 3 (2.0 × 10-4 M) before (a)
and after (b) removal of Cl- ions by adding 2 equiv of AgNO3 and its
catecholate adduct generated in situ before (c) and after (d) the removal of
Cl- ion by adding 2 equiv of AgNO3.
Inorganic Chemistry, Vol. 46, No. 15, 2007 6043

Mayilmurugan et al.
Table 4. Electronic Spectral and FeIII/FeII Redox Potentials for 3 in
Acetonitrile and DMF
displays two DBC2--to-iron(III) LMCT bands around 742
(ꢀ, 3510 M-1 cm-1) and 483 nm (ꢀ, 3710 M-1 cm-1). Also,
when 3 is treated with AgNO3 (to remove coordinated
chloride ions) and then DBC2- (Et3N), two new DBC2--to-
iron(III) CT bands (719 nm, ꢀ, 3150 M-1 cm-1; 494 nm,
3510 M-1 cm-1, Figure 2 and Table 3) are observed,
suggesting the substitution of two solvent molecules (or one
solvent molecule and the weakly coordinated -NMe2 group)
by chelating DBC2-. This is in sharp contrast to the
observation of a single blue-shifted DBC2--to-iron(III)
LMCT band (540-503 nm, Supporting Information, Table
S1) when 2 is treated with AgNO3 followed by DBC2- (Et3N)
in the present study. Furthermore, 3 exhibits two catecholate-
to-iron(III) LMCT bands in acetonitrile (690 nm, 2660 M-1
cm-1; 480 nm, 2360 M-1 cm-1) and DMF (694 nm, 3560
M-1 cm-1; 484 nm, 3520 M-1 cm-1) solutions, suggesting
the bidentate coordination of DBC2- (Et3N) by displacing
both the coordinated chloride ions (cf. above, Table 4). Thus,
it is clear that the decreased Lewis acidity of the iron(III)
center in 3 on account of the coordination of the 3,5-
dimethylphenolate donor with enhanced Lewis basicity leads
to the displacement of only one of the weakly coordinated
chloride ions (cf. conductivity studies) by methanol to form
[Fe(L3)(MeOH)(Cl)]+ species. The latter reacts with H2DBC
pretreated with the weaker base Et3N (pKa (BH+) ) 10.64)
to form [Fe(L3)(HDBC)(Cl)] and with H2DBC pretreated
with a stronger base like piperidine (pKa (BH+) ) 11.22) to
form [Fe(L3)(DBC)]. The monodentate coordination of
HDBC- in the iron(III)36 and iron(II)31 complexes has been
demonstrated previously by X-ray crystal structures. Upon
E1/2 (V)c
LMCT band
λ
max, nm
solvent
complex
(ꢀ, M-1 cm-1
)
CV
DPV
acetonitrile
[Fe(L3)Cl2]
342 (2750)
618 (1420)
480 (2360)
690 (2660)
476 (3530)
717 (3700)
341 (2750)
612 (1420)
484 (3520)
694 (3560)
473 (3830)
724 (3340)
-0.256
-0.257
[Fe(L3)Cl2]
+ DBC2- a
[Fe(L3)Cl2]
+ DBC2- b
[Fe(L3)Cl2]
-0.767
-
-0.765d
-
DMF
-0.604
-0.714
-0.587
-0.718d
[Fe(L3)Cl2]
+ DBC2- a
[Fe(L3)Cl2]
+ DBC2- b
a DBC2- generated by treating H2DBC with 2 equiv of triethylamine.
b DBC2- generated by treating H2DBC with 2 equiv of piperidine. c Scan
rate of 50 mV/s (CV) and 1 mV/s (DPV). d FeIII f FeII + ligand reduction.
charge built on iron(III) causing the stabilization18 of the d
orbitals and, hence, the decrease of the phenolate-to-iron(III)
LMCT band energy. The LMCT band observed around 650
nm in MeOH solution is blue shifted to 618 (ꢀ, 1420 M-1
cm-1, Table 4) and 612 nm (ꢀ, 1420 M-1 cm-1) in acetonitrile
and DMF solutions, respectively, which is consistent with
both the chloride ions of 3 remaining coordinated in solution
(cf. above).
When 3 is interacted with a series of catecholate anions
generated in methanol solution by treating the catechols with
2 equiv of Et3N as the base, a new band in the range of
517-617 nm (H2DBC, 617 nm; ꢀ, 4380 M-1 cm-1) assign-
able to the catecholate-to-iron(III) LMCT transition18-22,24,44
is observed (Figure 2 and Table 3). This is similar to that
observed19 previously for the five-coordinate complex 2
(Supporting Information, Table S1), which shows only one
catecholate-to-iron(III) LMCT band, suggesting the mono-
dentate coordination of catecholate ion-to-iron(III) in 3 also.
This is in contrast to the observation of two catecholate-to-
iron(III) LMCT bands (790, 485 nm) for 1 on treatment with
DBC2- (Et3N) and also for [Fe(HDP)(DBC)] (4) (726, 476
nm) (H(HDP) ) 2-[(bis(2-pyridylmethyl)-aminomethyl]-4,6-
dimethylphenol).34 The two bands originate from charge
transfer from two different catecholate orbitals to iron(III),
corresponding to the bidentate coordination20 of DBC2-,
possibly by displacing the two coordinated chloride ions or
the weakly coordinated -NMe2 donor (2.334 Å) and one of
the coordinated chloride ions in 1. Interestingly, the cat-
echolate adduct generated in situ by treating 3 with H2DBC
pretreated with 2 equiv of piperidine in methanol solution
[Fe(L3)Cl2] + MeOH h [Fe(L3)(MeOH)(Cl)]+ + Cl-
[Fe(L3)(MeOH)(Cl)]+ + HDBC- h
[Fe(L3)(HDBC)(Cl)] + MeOH
[Fe(L3)(MeOH)(Cl)]+ + DBC2-
h
[Fe(L3)(DBC)] + Cl- + MeOH
removal of the coordinated chloride in [Fe(L3)(Cl)(MeOH)]+
by treatment with AgNO3, the Lewis acidity of the iron(III)
center is enhanced, encouraging the bidentate coordination
of DBC2- to form [Fe(L3)(DBC)]. A further decrease in the
Lewis acidity of the iron(III) center on moving from 3 to 2
with two 3,5-dimethylphenolate donors of enhanced basicity
leads to the reluctance of 2 to expand its coordination sphere.
All these observations illustrate the critical role of the Lewis
acidity of the iron(III) center, the base used for the depro-
tonation of H2DBC, and the solvents in dictating the mode
of coordination of DBC2-.
The position of the low-energy catecholate f FeIII LMCT
band for complex-substrate adducts [Fe(L3)(catecholate)]
generated from 3 after removal of coordinated chloride is
found to be shifted to higher energies as the substituents18-22,44
on the catecholate ring are varied from electron-releasing to
electron-withdrawing, as observed20 previously for 1: DBC2-
(719 nm) > 4-MCAT2- (674 nm) > TBC2- (667 nm) >
3-MCAT2- (627 nm) > CAT2- (588 nm) > TCC2- (569
(43) (a) Casella, L.; Gullotti, M.; Pintar, A.; Messouri, L.; Rockenbauer,
A.; Gyor, M. Inorg. Chem. 1987, 26, 1031-1038. (b) Wang, S.; Wang,
L.; Wang, X.; Luo, Q. Inorg. Chim. Acta 1997, 254, 71-77. (c) Krebs,
B.; Schepers, K.; Bremer, B.; Henkel, G.; Althus, E.; Muller-Warmuth,
W.; Griesar, K.; Haase, W. Inorg. Chem. 1996, 35, 2360-2368. (d)
Gaber, B. P.; Miskowski, V.; Spiro, T. G. J. Am. Chem. Soc. 1974,
96, 6868. (e) Shongwe, M. S.; Kaschula, C. H.; Adsetts, M. S.;
Ainscough, E. W.; Brodie, A. M.; Morris, M. J. Inorg. Chem. 2005,
44, 3070. (f) Imbert, C.; Hratchian, H. P.; Lanznaster, M.; Heeg, M.
J.; Hryhorczuk, L. M.; McGarvey, B. R.; Schlegel, H. B.; Verani, C.
Inorg. Chem. 2005, 44, 7414.
(44) 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.
6044 Inorganic Chemistry, Vol. 46, No. 15, 2007

New Tripodal Iron(III) Monophenolate Complex
nm) > 4-NCAT2- (553 nm). This is expected, as the
electron-releasing substituents on the catecholate ring would
lower the position, while the electron-withdrawing substit-
uents would raise the position of the low-energy band,18-22,24
thus reflecting the importance of the electronic effects pro-
vided by the substituents on catechols. The order of energy
of the catecholate-to-iron(III) LMCT band observed (Table
3) when 3 is treated with various catecholates (Et3N) to form
[Fe(L3)(catecholate)(Cl)], HDBC- (617 nm) > HTBC- (611
nm) > H(4-MCAT)- (578 nm) > H(3-MCAT)- (569 nm)
> HCAT- (564 nm) > HTCC- (558 nm) > H(NCAT)- (517
nm), is almost the same as that observed above for the low-
energy band upon bidentate coordination of DBC2- to 3 after
the removal of coordinated chloride ions. This implies that
the electronic effects in iron(III) in [Fe(L3)(HDBC)(Cl)] are
very similar to those in [Fe(L3)(DBC)].
The frozen solution EPR spectrum of 3 (Figure 3 and
Supporting Information Table S2) displays high-spin (S )
5/2) rhombic ferric signals42,44 at g ) 8.5, 4.3, and 2.1 (E/D,
0.088) associated with | /2, -1/2 f | /2, +1/2 transition and
also signals corresponding to rhombic low-spin45,46 (S ) 1/2)
iron(III) species (g ) 2.04, 1.99, 1.87). Upon removal of
the coordinated chloride ions by treatment with AgClO4‚
H2O, 3 exhibits similar high-spin ferric signals but with small
shifts in g values (8.2, 4.1, 2.2; E/D, 0.086). The small
decrease in E/D value reveals a small decrease in rhombicity
upon the increase in the Lewis acidity of the iron(III) center
on the substitution of chloride ions by solvent molecules.
When interacted with DBC2- (Et3N), 3 exhibits appreciable
shifts in g values of the rhombic signals (8.0, 4.1, 2.1; E/D,
0.081) with a decrease in intensity of the low-spin iron(III)
signals, and this is consistent with monodentate coordination
of catecholate (cf. above). Furthermore, an intense signal
appears at g ) 2.03, which is assignable to low-spin iron(III)-
bound semiquinonate species,46 which has been invoked in
both intra- and extradiol-cleavage mechanisms.13 On the other
hand, when DBC2- is added to 3 pretreated with 2 equiv of
AgClO4‚H2O, the high-spin rhombic features are unaffected
(8.1, 4.2, 2.15; E/D, 0.081), the signals due to low-spin
iron(III) species disappear, and a highly intense semiquinone
signal appears at 2.04, suggesting the strong bidentate coor-
dination of DBC2- to form [Fe(L3)(DBC)] adducts. In con-
trast to 3, the five-coordinate complex 2 exhibits19 an intense
signal around g ) 5.5 and a weak signal around 2.0 signi-
fying the absence of any rhombicity in the iron(III) coordina-
tion geometry. However, after treatment with AgClO4·H2O,
2 exhibits a rhombic EPR spectrum (Supporting Information,
Figure S5) with high-spin ferric signals (8.6, 4.2, 2.1; E/D,
0.092) and low-spin iron(III) signals (2.02, 1.95, 1.88). When
DBC2- is added to 2 pretreated with AgClO4‚H2O, no
appreciable shift in the rhombic spectral features (8.5, 4.2,
2.11, E/D, 0.090) is observed and, in contrast to 3, no strong
signal around g ) 2.0 characteristic of semiquinone radical
5
5
Figure 3. EPR spectra of 3 before (a) and after removal of coordinated
chloride ions by adding AgClO4‚H2O (b), [Fe(L3)(HDBC)(Cl)] (c), and
[Fe(L3)(DBC)] (d) in frozen methanol/acetone solution at 77 K.
is observed, which is consistent with the inactivity of the
complex toward dioxygen. The E/D values of both 2 and its
DBC2- adduct are almost the same, suggesting similar five-
coordinate geometries for both of them.
Redox Properties. The electrochemical properties of 3
and its H2DBC adduct generated in situ were studied in
(45) (a) Simaan, A. J.; Biollot, M.-L.; Riviere, E.; Boussac, A.; Girerd,
J.-J. Angew. Chem., Int. Ed. 2000, 39, 196-198. (b) Bukowshi, M.
R; Comba, P.; Limberg, C.; Merz, M.; Que. L., Jr.; Wistuba, T. Angew.
Chem., Int. Ed. 2004, 43, 1283-1287.
(46) Yoon, S.; Lee, H.-J. Lee, K.-B.; Jang, H. G. Bull. Korean Chem. Soc.
2000, 21, 923-928.
Inorganic Chemistry, Vol. 46, No. 15, 2007 6045

Mayilmurugan et al.
Table 5. Electrochemical Dataa for 3 and Its DBC2- Adductc in Methanol at 25 ( 0.2 °C Using a Scan Rate of 50 mV/s (CV) and 1 mV/s (DPV)
E1/2 (V)
compound
[Fe(L3)Cl2]
Epc (V)
Epa (V)
∆E p (mV)
CV
DPV
redox process
-0.375
-0.384
-0.644
-0.160
-0.309
-0.322
-0.190
-0.285
-0.274
-0.534
-0.072
-0.257
-0.280
-0.084
90
110
110
88
52
42
-0.330
-0.328
-0.589
-0.116
-0.283
-0.301
-0.137
-0.329
-0.327
FeIII f FeII
+ H2DBC
FeIII f FeII
+ DBC2-
FeIII f FeII + ligand reduction
DBSQ f H2DBC
FeIII f FeII
-0.125
-0.276
-0.283
-0.071
-0.629
-0.083
0.623
[Fe(L3)(MeOH)2]b
+ H2DBC
FeIII f FeII
106
DBSQ f H2DBC
FeIII f FeII + ligand reduction
DBSQ f H2DBC
DBQ f DBSQ
+ DBC2-
-0.040
0.162
a Potential measured vs Ag/AgNO3 (0.01 M, 0.1 M TBAP); add 0.544 V to convert to NHE. b Chloride ions removed by adding AgClO4‚H2O. c Generated
by adding 1 equiv of H2DBC and 2 equiv of triethylamine to complex 3.
methanol solution by CV and DPV. The complex shows both
cathodic and anodic waves (E1/2, - 0.329 V, Supporting
Information Figures S2 and S3 and Table 5). The value of
the diffusion coefficient (D, 1.1 × 10-6 cm2/s) calculated
by substituting the slope obtained from the linear ipc vs ν1/2
(ν < 0.05 V s-1) plot in the Randles-Sevcik equation47 is
of the same order as those observed previously18-21 for a
one-electron reduction process in similar20 iron(III) com-
plexes. The ipc vs ν1/2 plot is linear, but the values of peak
current ratio ipa/ipc (0.75) and peak potential separation ∆Ep
(90 mV) suggest a fairly reversible to irreversible redox
process. On removing the coordinated chloride ions by
Figure 4. Differential pulse voltammograms of 1 mM 3 before (a) and
after the addition of 1 mM H2DBC (b) and 2 mM Et3N (c) in methanol at
treating the complex 3 with AgClO4‚H2O, the FeIII/FeII redox
25 °C. Conditions: supporting electrolyte, 0.1 M TBAP; scan rate, 1 mV
potential is shifted to a more positive value (E1/2, -0.276
s-1; reference electrode, Ag/Ag+; working electrode, Pt sphere.
V), which is expected18-21 of the increase in Lewis acidity
of the ferric center upon replacing the coordinated chloride
by solvent molecules. On replacing the p-nitrophenolate arm
3 after treatment with AgClO4‚H2O to remove the coordi-
in 1 by the electron-releasing 3,5-dimethylphenolate arm, as
in 3, the FeIII/FeII redox potential is shifted to a more negative
value (1, -0.266; 3, -0.329 V) suggesting a decreased Lewis
acidity of the iron(III) center in 3, which is consistent with
the shorter Fe-O bond in 3 (cf. above). It may be noted
that the FeIII/FeII redox potential of 3 is shifted to a more
negative value (E1/2, -0.587 V, Table 4) in DMF solution
and to a more positive value (E1/2, -0.257 V, Table 4) in
acetonitrile solution, illustrating the effect of solvent.
nated Cl- ions, the reduction current of the FeIII f FeII
process (E1/2, -0.276 V) decreases and a new redox wave
corresponding to the DBSQ/H2DBC couple (E1/2, -0.071 V,
Supporting Information Figure S4 and Table 5) is observed.
Further, on adding 2 equiv of Et3N, the reduction current of
the DBSQ/H2DBC couple (E1/2, -0.083 V, Supporting
Information, Figure S4) increases, with the FeIII f FeII
reduction wave disappearing and possibly merging with the
ligand reduction responses (-0.629 V). The appearance of
the DBSQ/H2DBC redox wave even in the absence of added
base is on account of the higher Lewis acidity of the iron(III)
center on removing the coordinated chloride ions. Further-
more, the appearance of the DBSQ/H2DBC couple for [Fe-
(L3)(DBC)] at a potential (-0.083 V) more positive than
that (-0.125 V) for [Fe(L3)(HDBC)(Cl)] reveals that DBC2-
coordinated in a bidentate mode in the former is stabilized
toward oxidation more than HDBC-, which is coordinated
in a monodentate fashion. In other words, the higher Lewis
acidity of the iron(III) center in [Fe(L3)(MeOH)2]2+ enhances
the covalency of the iron-catecholate interaction and
increases the semiquinone character of the bound DBC2-.
Kinetics and Reactivity Studies. The oxygenation reac-
tions of the complex-H2DBC adducts prepared in situ (using
Et3N or piperidine as base) were investigated in various
solvents. When a methanolic solution of the substrate adduct
[Fe(L3)(HDBC)(Cl)], generated in situ by reacting 3 with
H2DBC pretreated with 2 equiv of Et3N (cf. above), is
On adding 1 equiv of H2DBC to 3 in methanol solution,
no appreciable shift in the redox wave is observed (Figure 4
and Table 5). However, on adding 2 equiv of Et3N, a new
redox wave appears (E1/2, -0.125 V), corresponding to the
DBSQ/H2DBC couple.18-21 The FeIII f FeII reduction
process, which is expected19,20 to be shifted to more negative
potentials on incorporating two catecholate hydroxyl groups
in the coordination sphere, is difficult to discern, as it would
have merged with the ligand reduction waves (-0.589 V).
The redox potential of the DBSQ/H2DBC couple48 of
coordinated HDBC- observed at -0.125 V is less negative
than that of the free DBSQ/H2DBC couple (Epc, -1.34 V vs
SCE)48, suggesting the stabilization of the coordinated
HDBC- anion. Upon the addition of 1 equiv of H2DBC to
(47) Bard, A. J.; Faulkner, L. R. Electrochemical Methods: Fundamentals
and Applications; John Wiley & Sons: New York, 1980; pp 218.
(48) Nanni, E. J.; Stalling, M. D.; Sawyer, D. T. J. Am. Chem. Soc. 1980,
102, 4481-4485.
6046 Inorganic Chemistry, Vol. 46, No. 15, 2007

New Tripodal Iron(III) Monophenolate Complex
Table 6. Kinetic Data for the Oxidative Cleavage of H2DBC Catalyzed by 3 in Different Solvents and the Cleavage Products Analyzed after 24 h (see
Scheme 3)
Cleavage products (%)b
c
complex
solvent/base
intradiol
extradiol
kobs (10-5 s-1
)
kO (10-2 M-1 s-1
)
t1/2 (h)d
2
[Fe(L3)(HDBC)(Cl)]
[Fe(L3)(DBC)]
CH3OH/Et3N
CH3OH/Pip
CH3OH/Pip
CH3CN/Pip
CH3CN/Et3N
DMF/ Pip
0.5 (6)
22 (5, 6, 7)
56 (7)
17.3 (9, 10, 13)e
42 (9, 10, 11)
30 (9, 11)
1.19 ( 0.21
4.29 ( 0.56
135.97 ( 1.62
8.07 ( 0.84
8.84 ( 0.45
19.34 ( 1.27
6.36 ( 0.36
0.56 ( 0.01
2.02 ( 0.03
64.14 ( 0.76
1.00 ( 0.10
1.09 ( 0.06
3.98 ( 0.26
1.31 ( 0.07
16.1
4.5
0.1
2.4
2.2
1.0
3.0
[Fe(L3)(DBC)]a
[Fe(L3)(DBC)]
40 (7)
<1 (11, 12)
[Fe(L3)(DBC)]
94 (7, 8)
<1 (11, 12)
DMF/Et3N
d
a [Fe(L3)(MeOH)2]2+ species generated from [Fe(L3)Cl2] by adding 2 equiv of AgNO3. b Based on H2DBC, piperidine (Pip). c kO ) kobs/[O2]. t1/2
)
2
0.693/kobs
.
e 48 h.
Scheme 3. Cleavage Products of H2DBC Catalyzed by Complex 3 in the Presence of O2
exposed to O2, the absorbance of the HDBC--to-iron(III)
LMCT band (λmax, 617 nm; ꢀ, 4360 M-1 cm-1) decreases
very slowly (Supporting Information, Figure S6) with
pseudo-first-order kinetics due to an excess of dioxygen. The
electronic spectrum of the reaction mixture after 96 h shows
the disappearance of the 617 and 339 nm bands and the
appearance of bands around 568 (ꢀ, 5020 M-1 cm-1) and
326 nm (ꢀ, 14 890 M-1 cm-1) indicating the completion of
the oxygenation reaction. The rate constant (kobs, 1.19 × 10-5
s-1, Table 6) was calculated by fitting the decrease in
intensity of the band into the following equation49 applicable
for relatively slow reactions
(17.3%) and a trace amount (0.5%) of intradiol products
(Scheme 3and Table 6). The oxygenation products were
identified by GC-MS (EI) and H NMR and quantified by
1
GC (FID) techniques. The extradiol ring-cleavage prod-
ucts26,32,37,38 C14H20O4, m/z 252, cis,cis-3,5-di-tert-butyl-2-
hydroxymuconic semialdehyde (9) (15%), C14H22O5, m/z 268,
3,5-di-tert-butyl-2-hydroxymuconic semialdehyde methyl
ester (10) (1.1%), and 3,5-di-tert-butyl-5-(formyl)-2-furanone
(13) (1.2%) and the intradiol-cleavage product C15H24O4, m/z
268, (2,4-di-tert-butyl-5-oxo-2,5-dihydrofuran-2-yl)acetic acid
methyl ester (6) (0.5%), are formed. Similar very slow rate
of cleavage and lower cleavage yield for [Fe(TACN)(Cat)-
(Cl)] when Et3N was used as an external base have been
noted by Bugg et al.32 Molecular oxygen replaces the
coordinated chloride ion in [Fe(L3)(HDBC)Cl], and on
activation it attacks the semiquinone form of HDBC-
coordinated to iron(III) in a monodentate fashion.
A ) A∞ + (A0 - A∞) exp(-kobst)
(1)
where t is time, A, A0, and A∞ are the absorbances at time t,
0 and ∞, respectively, and kobs is the pseudo-first-order rate
constant. The second-order rate constant kO () kobs/[O2])
2
The reactivity of the adduct [Fe(L3)(DBC)] generated in
situ in methanol solution by treating 3 with H2DBC pretreated
with 2 equiv of piperidine was determined by monitoring
the decay of the DBC2--to-iron(III) LMCT band around 742
nm on exposure to an excess of dioxygen (Supporting
Information, Figure S7). The decay of the band is faster than
that for the complex species [Fe(L3)(HDBC)(Cl)] in metha-
was then calculated18-22,50,51 as 5.6 × 10-3 M-1 s-1. The
complex [Fe(L3)(HDBC)(Cl)] was reacted with dioxygen
over 48 h (t1/2, 16.1 h) to afford almost exclusively extradiol
(49) Hitomi, Y.; Yoshida, M.; Higuchi, M.; Minami, H.; Tanaka, T.;
Funabiki, T. J. Inorg. Biochem. 2005, 99, 755-763.
(50) (a) Mialane, P.; Tehertanov, L.; Banse, F.; Sainton, J.; Girerd, J. Inorg.
Chem. 2000, 39, 2440-2444.
(51) Yamahara, R.; Ogo, S.; Watanabe, Y.; Funabiki, T.; Jitsukawa, K.;
Masuda, H.; Einaga, H. Inorg. Chim. Acta 2000, 300-302, 587-
596.
nol (cf. above). The second-order rate constant (kO , 2.0 ×
2
10-2 M-1 s-1) for the reaction was calculated using eq 1.
Inorganic Chemistry, Vol. 46, No. 15, 2007 6047

Mayilmurugan et al.
CN, 717 nm, Table 4) undergoes slower oxygenation on
exposure to excess of dioxygen (Supporting Information,
Figure S8), and the rate constants were calculated as above
(kO ) (DMF) 4.0 × 10-2 and (CH3CN) 1.0 × 10-3 M-1
2
s-1). The analysis of cleavage products after 24 h of reaction
(t1/2 ) (DMF) 1.0 and (CH3CN) 2.4 h) reveals that intradiol
cleavage is favored with an exclusive (DMF, 92%) or higher
amount of the intradiol-cleavage products 7 (CH3CN, 40%)
and 3,5-di-tert-butyl-5-(N,N-dimethylamidomethyl)-2-fura-
none (8) (DMF, 1.7%), traces of the extradiol-cleavage
products 11 (<1%) and 4,6-di-tert-butyl-2-pyrone 12 (<1%)
(Table 6), and 14 (DMF, 1.3%; CH3CN, 2.8%) as side
product. As illustrated above, the bidentate coordination of
catechol in [Fe(L3)(DBC)] favors intradiol-cleavage products
via a substrate activation mechanism.13,17 Also, the solvent
molecules can displace the weakly coordinated -NMe2
(2.312 Å, cf. above) group but may in turn be difficult to be
displaced by dioxygen for extradiol cleavage to occur.
It is interesting to note that the rate of cleavage for [Fe-
Figure 5. Plot of log(A0 - A∞) vs time observed at 719 nm (R2, 0.93) for
the reaction of [Fe(L3)(DBC)] (generated in situ after the removal of chloride
ions from [Fe(L3)Cl2] by adding 2 equiv of AgNO3) with O2 at 25 °C in
methanol solution. Concentration: [Fe(L3)(DBC)] ) 1.0 × 10-4 M. Inset:
progress of reaction.
(L3)(HDBC)(Cl)] (kO ) 5.6 × 10-3 M-1 s-1) to afford
2
Upon exposure of the adduct [Fe(L3)(DBC)] to an excess
of dioxygen over 24 h (t1/2, 4.5 h) in methanol, major amounts
of extradiol products (42%) [Scheme 3 and Table 6, 9 (33%)
and 10 (2%), both obtained by extradiol ring fission,26,32,33,37,38
and C13H20O2, m/z 208, 3,5-di-tert-butyl-2-pyrone (11) (7%)
with the insertion of an oxygen atom into H2DBC followed
by the loss26,29 of CO], lower amounts of intradiol16,20,21
products (22%) [C14H20O4, m/z 254, 3,5-di-tert-butyl-5-
(carboxymethyl)-2-furanone (5) (10%), 6 (3%), and C19H31-
NO3, m/z 321, 3,5-di-tert-butyl-5-(2-oxo-2-piperidinylethyl)-
5H-furanone (7) (trace)], and C13H19O3, m/z 223, 2,5-di-tert-
butyl-2H-pyran-3,6-dione (14) (5%) and C8H10O3, m/z 154,
3-tert-butylfuran-2,5-dione (15) (4%) as side products16,20,21
were obtained. It is clear that the bidentate coordination of
catechol (cf. above) favors intradiol-cleavage products via
the substrate activation mechanism.13,17 Also, the weakly
coordinated -NMe2 (2.312 Å, cf. above) group in [Fe(L3)-
(DBC)] is displaced to facilitate dioxygen binding,31 and the
bound dioxygen then attacks the semiquinone form of the
catechol, which is coordinated in a monodentate fashion,
leading to extradiol cleavage.28,32,33 As the bidentate coor-
dination of DBC2- by displacement of the coordinated
methanol or -NMe2 group will be inhibited by electron-
releasing 3,5-Me2 substituents on the phenolate ring in [Fe-
(L3)(DBC)], lower yields of intradiol-cleavage products are
obtained.
exclusively extradiol product is almost four times slower than
that (kO ) 2.0 × 10-2 M-1 s-1) for [Fe(L3)(DBC)] in the
2
same solvent (methanol) to yield both intra- and extradiol
products. This illustrates the importance of the base used to
deprotonate the catechol substrate and the slowness of the
extradiol-cleavage reaction due to the difficulty in generating
a vacant coordination site on the iron(III) center and the
presence of the iron(III) rather than the iron(II) oxidation
state as in the extradiol-cleaving enzyme. Similarly, the
inactivity of 2 toward catechol cleavage may be explained
on the basis of steric hindrance from the coordinated -NMe2
group and the decreased Lewis acidity of the iron(III) center
leading to the monodentate coordination of the substrate
(HDBC-) by replacing the axially coordinated chloride ion.
It is obvious that the bidentate coordination of DBC2- would
require structural changes from trigonal-bipyramidal19 to
octahedral geometry, which is more difficult to achieve than
simple substitution reactions in octahedral complexes like 1
and 3. So, we speculate that the -NMe2 arm in 3 is displaced
to create a vacant site, which is susceptible to attack by O2,
as suggested previously.31 No such vacant site is available
for 2, illustrating its inactivity toward dioxygen activation.
The rate of cleavage (kO ) 6.4 × 10-1 M-1 s-1) for [Fe-
2
(L3)(DBC)], generated in methanol using Et3N as the base
after the removal of the coordinated chloride ion, to yield
increased intra- and decreased extradiol products, is 32 times
faster than that for the same complex [Fe(L3)(DBC)] gener-
ated by piperidine in the presence of chloride ions. It is clear
that the chloride anions compete for the coordination sites
of the catecholate adduct. Also, the rate of catechol cleavage
for [Fe(L3)(DBC)] in methanol is 130 times faster than that
The adduct [Fe(L3)(DBC)] generated in situ by treating 3
with AgNO3 and then DBC2- (Et3N) in methanol solution
displays the immediate decay of the DBC2--to-iron(III)
LMCT band at 719 nm (cf. above) on exposure to an excess
of dioxygen (Figure 5), and the second-order rate constant
(kO , 6.4 × 10-1 M-1 s-1) was calculated by using eq 1. The
for [Fe(HDP)(DBC)] (4) (kO ) 4.9 × 10-3 M-1 s-1)34, with
2
2
product analysis after 24 h of reaction (t1/2, 0.14 h) using
piperidine as the base reveals an increase in the yield of the
intradiol product 7 (56%) and a decrease in the yield of the
extradiol-cleavage products 9 (30%) and 11 (2%). Further-
more, the adduct generated in DMF and CH3CN in situ by
treating 3 with DBC2- (piperidine) (DMF, 724 nm; CH3-
only one iron-phenolate bond, as in [Fe(L3)(DBC)]. On
replacing one of the pyridyl moieties in the latter by the
-NMe2 pendant to obtain the former, the sterically hindering
and weakly coordinated -NMe2 donor enhances the Lewis
acidity of iron(III) center and, hence, the binding affinity of
DBC2-, leading to the enhanced reaction rate. Also, the
6048 Inorganic Chemistry, Vol. 46, No. 15, 2007

New Tripodal Iron(III) Monophenolate Complex
ligand steric hindrance in [Fe(L3)(DBC)] would facilitate
the release of products from the reactive intermediate18-21
and hence the cleavage. Furthermore, the high Lewis acidity
of the iron(III) center in [Fe(L3)(DBC)] would enhance the
semiquinone character of the bound DBC2- (cf. above) and
render the adduct more prone to oxygen attack, thereby
accelerating the rate of oxidative cleavage.13 In nonaqueous
solvents, piperidine acts as a base to deprotonate H2DBC
much better than Et3N, facilitating the bidentate coordination
of the catecholate anion and, hence, the faster substrate
activation by molecular oxygen to yield intradiol-cleavage
products.
Remarkably, for the chloride complex [Fe(L3)Cl2], the
extradiol cleavage of H2DBC treated with Et3N base provides
an almost exclusively cis,cis-3,5-di-tert-butyl-2-hydroxymu-
conic semialdehyde rather than 3,5-di-tert-butyl-2-pyranone.
However, on removing the coordinated chloride ion or using
piperidine as the base, a major amount of intradiol-cleavage
product is observed, with considerable enhancement in the
cleavage rate. Thus, the cleavage activities of the model
complex depend strongly upon the solvent, the base used to
deprotonate the substrate, and the mode of coordinations
mono- or bidentatesof the catecholate substrate to the
iron(III) center, the Lewis acidity of which is tuned by a
novel combination of ligand donors. Attempts are in progress
to tune the environment around the iron(III) atom by using
various sterically hindering donor atoms to enhance the yield
of regioselective extradiol cleavage.
Conclusions
The iron(III) complex of a tripodal monophenolate ligand
also containing pyridylmethyl and sterically hindered -NMe2
arms possesses a distorted octahedral coordination geometry.
On account of the sterically demanding -NMe2 group and
the electron-releasing 3,5-dimethylphenolate pendant in the
complex, the monoanion HDBC- of the substrate H2DBC
(3,5-di-tert-butylcatechol), generated with 2 equiv of Et3N
as the base, interacts with the complex in a monodentate
fashion despite the presence of two cis-coordinated chloride
ions. On the other hand, interestingly, the removal of the
coordinated chloride ions or the use of a stronger baselike
piperidine results in the bidentate coordination of the DBC2--
to-iron(III) center, as revealed by the presence of two DBC2--
to-iron(III) LMCT bands and the observation of the DBSQ/
H2DBC couple at a more positive potential, which is facil-
itated by the enhanced Lewis acidity of the iron(III) center.
Acknowledgment. We sincerely thank the Department
of Science and Technology, New Delhi, for supporting this
research (Scheme No. SR/S1/IC-45/2003), and the Council
of Scientific and Industrial Research, New Delhi, for a Senior
Research Fellowship to R.M. We thank Professor P. Sam-
basiva Rao, Pondicherry University, for providing the EPR
facility.
Supporting Information Available: Electronic spectra, EPR
spectra and data, cyclic voltammograms, differential pulse
voltammograms, and crystallographic data in CIF format for 3.
This material is available free of charge via the Internet at
http://pubs/acs.org.
IC700646M
Inorganic Chemistry, Vol. 46, No. 15, 2007 6049