Synthesis, structure, spectra and reactivity of iron(iii) complexes of imidazole and pyrazole containing ligands as functional models for catechol dioxygenases

Article abstract of DOI:10.1039/b903602d

A series of new 1:1 iron(iii) complexes of the type [Fe(L)Cl₃], where L is a tridentate 3N donor ligand, has been isolated and studied as functional models for catechol dioxygenases. The ligands (1-methyl-1H-imidazol- 2-ylmethyl)pyrid-2-ylmethyl-amine (L1), N,N-dimethyl-N′-(1-methyl-1H- imidazol-2-ylmethyl)ethane-1,2-diamine (L2) and N-(1-methyl-1H-imidazol-2- ylmethyl)-N′-phenylethane-1,2-diamine (L3) are linear while the ligands tris(1-pyrazolyl)methane (L4), tris(3,5-dimethyl-1-pyrazolyl)methane (L5) and tris(3-iso-propylpyrazolyl)methane (L6) are tripodal ones. All the complexes have been characterized by spectral and electrochemical methods. The X-ray crystal structure of the dinuclear catecholate adduct [Fe(L2)(TCC)] ₂O, where TCC^2- is a tetrachlorocatecholate dianion, has been successfully determined. In this complex both the iron(iii) atoms are bridged by a μ-oxo group and each iron(iii) center possesses a distorted octahedral coordination geometry in which the ligand L2 is facially coordinated and the remaining coordination sites are occupied by the TCC^2- dianion. Spectral studies suggest that addition of a base like Et₃N induces the mononuclear complex species [Fe(L2)(TCC)Cl] to dimerize forming a μ-oxo-bridged complex. The spectral and electrochemical properties of the catecholate adducts of the complexes generated in situ reveal that a systematic variation in the ligand donor atom type significantly influences the Lewis acidity of the iron(iii) center and hence the interaction of the complexes with simple and substituted catechols. The 3,5-di-tert-butylcatecholate (DBC ^2-) adducts of the type [Fe(L)(DBC)Cl], where L is a linear tridentate ligand (L1-L3), undergo mainly oxidative intradiol cleavage of the catechol in the presence of dioxygen. Also, the extradiol-to-intradiol product selectivity (E:I) is enhanced upon removal of the coordinated chloride ion in these adducts to obtain [Fe(L)(DBC)(Sol)]⁺ and upon incorporating coordinated N-methylimidazolyl nitrogen in them. In contrast to the iron(iii) complexes of imidazole-based ligands, those of the tripodal pyrazole-based ligands L4-L6 yield major amounts of the oxidized product benzoquinone and small amounts of both intra- and extradiol products. One of the pyrazole arms coordinated in the equatorial plane of these sterically constrained complexes is substituted by a solvent molecule upon adduct formation with DBC^2-, which encourages molecular oxygen to attack this site leading to benzoquinone formation. The DBSQ/DBC^2- redox potentials of both the imidazole- and pyrazole-based complexes fall in the narrow range of -0.186 to -0.214 V supporting this proposal. The Royal Society of Chemistry 2009.

Full text of DOI:10.1039/b903602d

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PAPER
www.rsc.org/dalton | Dalton Transactions
Synthesis, structure, spectra and reactivity of iron(III) complexes of imidazole
and pyrazole containing ligands as functional models for catechol
dioxygenases†
Thirumanasekaran Dhanalakshmi,a Eringathodi Sureshb and Mallayan Palaniandavar*a
Received 20th February 2009, Accepted 15th July 2009
First published as an Advance Article on the web 17th August 2009
DOI: 10.1039/b903602d
A series of new 1 : 1 iron(III) complexes of the type [Fe(L)Cl3], where L is a tridentate 3N donor ligand,
has been isolated and studied as functional models for catechol dioxygenases. The ligands
(1-methyl-1H-imidazol-2-ylmethyl)pyrid-2-ylmethyl-amine (L1), N,N -dimethyl-N¢-
(1-methyl-1H-imidazol-2-ylmethyl)ethane-1,2-diamine (L2) and N-(1-methyl-1H-imidazol-2-ylmethyl)-
N¢-phenylethane-1,2-diamine (L3) are linear while the ligands tris(1-pyrazolyl)methane (L4),
tris(3,5-dimethyl-1-pyrazolyl)methane (L5) and tris(3-iso-propylpyrazolyl)methane (L6) are tripodal
ones. All the complexes have been characterized by spectral and electrochemical methods. The X-ray
crystal structure of the dinuclear catecholate adduct [Fe(L2)(TCC)]2O, where TCC2- is a
tetrachlorocatecholate dianion, has been successfully determined. In this complex both the iron(III)
atoms are bridged by a m-oxo group and each iron(III) center possesses a distorted octahedral
coordination geometry in which the ligand L2 is facially coordinated and the remaining coordination
sites are occupied by the TCC2- dianion. Spectral studies suggest that addition of a base like Et3N
induces the mononuclear complex species [Fe(L2)(TCC)Cl] to dimerize forming a m-oxo-bridged
complex. The spectral and electrochemical properties of the catecholate adducts of the complexes
generated in situ reveal that a systematic variation in the ligand donor atom type significantly influences
the Lewis acidity of the iron(III) center and hence the interaction of the complexes with simple and
substituted catechols. The 3,5-di-tert-butylcatecholate (DBC2-) adducts of the type [Fe(L)(DBC)Cl],
where L is a linear tridentate ligand (L1–L3), undergo mainly oxidative intradiol cleavage of the
catechol in the presence of dioxygen. Also, the extradiol-to-intradiol product selectivity (E : I) is
enhanced upon removal of the coordinated chloride ion in these adducts to obtain [Fe(L)(DBC)(Sol)]+
and upon incorporating coordinated N-methylimidazolyl nitrogen in them. In contrast to the iron(III)
complexes of imidazole-based ligands, those of the tripodal pyrazole-based ligands L4–L6 yield major
amounts of the oxidized product benzoquinone and small amounts of both intra- and extradiol
products. One of the pyrazole arms coordinated in the equatorial plane of these sterically constrained
complexes is substituted by a solvent molecule upon adduct formation with DBC2-, which encourages
molecular oxygen to attack this site leading to benzoquinone formation. The DBSQ/DBC2- redox
potentials of both the imidazole- and pyrazole-based complexes fall in the narrow range of -0.186 to
-0.214 V supporting this proposal.
enzymes exemplified by catechol 1,2-dioxygenase (CTD) and
protocatechuate 3,4-dioxygenase (3,4-PCD) possess a high-spin
Introduction
iron(III) center6 bound to the protein backbone primarily by two
histidine imidazole nitrogen and two phenolate oxygen (tyrosinate)
atoms in a five-coordinate trigonal bipyramidal geometry.7,8 In
contrast, the extradiol dioxygenases utilize an iron(II) cofactor
ligated by two histidine nitrogens and one glutamic acid in a
square-pyramidal geometry.9
Several bioinorganic modeling studies10–21 have focused on the
structural and spectroscopic characterization of iron(III) com-
plexes of linear11,12,20 and tripodal tetradentate14–16,19,20 and tetraaza
macrocyclic17,18 ligands as structural and functional models for the
catecholate-iron(III) form of catechol dioxygenases. In a vast ma-
jority of these studies the oxygenation products reported are con-
sistent with intradiol cleavage. The significance of Lewis acidity of
iron(III) center in the dioxygenase reaction has been highlighted10–14
and a substrate activation mechanism has been postulated for
The catechol dioxygenases are mononuclear non-heme iron en-
zymes isolated from soil bacteria.1,2 They have attracted much in-
terest because they convert aromatic pollutants in the environment
to water-soluble aliphatic products by oxidative ring cleavage3,4
with the incorporation of both atoms of dioxygen into the aromatic
ring. They are subdivided into intradiol- and extradiol-cleaving
enzymes depending on the catalysis of ring cleavage between or
outside the two ortho-hydroxyl groups.5 The intradiol-cleaving
aSchool of Chemistry, Bharathidasan University, Tiruchirappallli, 620
024, India. E-mail: palanim51@yahoo.com, palaniandavarm@gmail.com;
Tel: +91-431-2407125
bAnalytical Science Discipline, Central Salt and Marine Chemicals Research
Institute, Bhavnagar, 364 002, India
† CCDC reference number 721218. For crystallographic data in CIF or
other electronic format see DOI: 10.1039/b903602d
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intradiol-cleaving enzymes.5,6 In contrast, model complexes that
elicit extradiol cleavage activity are fewer in number22–34 and a
five-coordinate iron center as in the active sites of the extradiol-
cleaving enzymes has been suggested9 to be necessary for the
extradiol cleavage to occur. Also, the model complexes so far
reported have emphasized the importance of facial coordination of
tridentate ligands and a vacant coordination position for oxygen
binding as well in order to elicit extradiol cleavage.25–28 Thus,
iron(III) complexes of meridionally coordinating tridentate ligands
like terpyridine yield quinone in a large amount and intradiol
cleavage products, while those of facially coordinating macro-
cyclic 3N ligands like triazacyclononane (tacn), 1,4,7-trimethyl-
1,4,7-triazacyclononane (Me3TACN), and N-benzyl-N,N ¢-bis(2-
pyridylmethyl)amine elicit extradiol cleavage.28 In our laboratory,
several mononuclear iron(III) complexes of tridentate 3N21,30,31 and
2NO14,21 and tetradentate 4N14,34 and 3NO14,20,33 ligands have been
studied as functional models for catechol dioxygenases. Gebbink
et al. have isolated the catecholate adducts of iron(II) and iron(III)
complexes of a new family of substituted 3,3-bis(1-alkylimidazol-
2-yl)propionate ligands, which elicit both the extradiol and
intradiol cleavage products in comparable amounts.32
From our laboratory we have reported31 iron(III) complexes of
linear tridentate N-alkyl substituted bis(pyrid-2-ylmethyl)amine
(BPA) ligands as models for extradiol-cleaving enzymes. The
nature of the N-alkyl substituents in these complexes controls the
regioselectivity of catechol cleavage and enhances the extradiol
cleavage yield to 46–68% in dichloromethane. Very recently, we
also found30 that the iron(III) complexes with both centrally and
terminally N-alkyl substituted (pyrid-2-ylmethyl)ethylenediamine
3N ligands exhibit higher extradiol-to-intradiol product selectivity
in dichloromethane solvent. As the imidazole group of the
histidine amino acid acts as the nitrogen donor in the enzyme
active sites, we have now became interested in the study of 1 : 1
iron(III) complexes of imidazole-containing linear ligands (L1–L3,
Scheme 1) as models for catechol dioxygenase enzymes and the
effect of the imidazolyl moiety on the dioxygenase activity of
the iron(III) complexes has been probed. Indeed, it is remarkable
that the N-Me-imidazolyl moiety in the iron(III) complex of a
tripodal tetradentate monophenolate ligand enhances the reaction
rate with efficient conversion (77.1%) of the substrate to intradiol
cleavage products.20 Incorporation of N-alkyl substituents on
the terminal amine nitrogen atom in the iron(III) complexes30
of (pyrid-2-ylmethyl)ethylenediamine ligands leads to confer ex-
tradiol cleavage with yields relatively higher than those on the
central nitrogen atom in the BPA-based complexes.31 The effect
of varying the steric bulk of the terminal donor group in the
present imidazolyl-based ligands has been studied. The results
indicate that mainly intradiol cleavage products are obtained but
with the extradiol-to-intradiol product selectivity being lower
than those for the corresponding pyridyl-based complexes.21
The dinuclear m-oxo-bridged tetrachlorocatecholate adduct of
one of the complexes of the imidazolyl-based ligands has been
structurally characterized to show the ability of these ligands to
engage in facial coordination.
Also, Moro-Oka et al.27 have reported the iron(III) com-
plexes of the sterically hindered tris(pyrazolyl)borate ligands
hydrotris(3-tert-butyl-5-iso-propyl-1-pyrazolyl)borate (TptBu,iPr
)
and hydrotris(3,5-di-iso-propyl-1-pyrazolyl)borate (TpiPr
) as
2
structural and functional models for the catechol dioxygenases
and found that the complex with the ligand TpiPr alone reacts with
2
O2 to cause extradiol cleavage. In continuation of our work30,31 to
explore the necessity of facial coordination of ligands to elicit ex-
tradiol cleavage, simple tripodal tridentate tris(pyrazolyl)methane
ligands with different substituents (L4–L6, Scheme 1) have also
been used, and the reactivity of iron(III) complexes of these
ligands towards cleavage of H2DBC has been investigated. Also,
it is expected that the differently substituted pyrazoles in the
ligands L4–L6 would impose varying steric and electronic effects
on the iron(III) coordination geometry, tune the Lewis acidity of
the iron(III) center and hence affect the dioxygenase activity of the
complexes. Interestingly, all the pyrazolyl-based complexes yield
the oxidized product benzoquinone in major amounts and both
intra- and extradiol cleavage products in smaller amounts. The
interaction of all the present complexes with a series of catechols
(Scheme 2) has been monitored using electronic absorption
spectral and electrochemical methods.
Scheme 2 Structures of catechols used.
Results and discussion
Syntheses of ligands and iron(III) complexes
The linear tridentate ligands L1–L3 were synthesized by the con-
densation of 1-methylimidazole-2-carboxaldehyde with the
corresponding amines to form a Schiff base followed by the
reduction of the latter with sodium borohydride.21,30 The ligands
L4–L6 were synthesized according to known procedures,35,36 which
involve the treatment of chloroform with the corresponding
pyrazole in the presence of sodium carbonate and a phase transfer
catalyst like tetra-n-butylammonium bromide. They were reacted
with equimolar amounts of iron(III) chloride in methanol to obtain
mononuclear 1 : 1 iron(III) complexes. The differently substituted
amine functions in the ligands are expected to impose steric and
electronic effects on the iron(III) coordination geometry, tune the
Scheme 1 Structures of ligands used.
8318 | Dalton Trans., 2009, 8317–8328
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Table 1 Crystal data and structure refinement details for 7
Lewis acidity of the iron(III) center and hence determine the
dioxygenase activity of the complexes. On the basis of the results of
elemental analyses the present iron(III) complexes are formulated
as [FeLCl3], which is supported by the ESI-MS data. Conductivity
studies of the 1 : 1 complexes in DMF solution (KM, 290–
320 ohm-1 cm2 mol-1) reveal that solvent molecules replace all the
three coordinated chloride ions. This suggests that the bidentate
catechols would bind to the coordination positions occupied by
chloride ions. The present iron(III) complexes 1–6 have magnetic
moments in the range 5.95–6.05 BM at room temperature, which
is characteristic of a high-spin iron(III) center.
7
Empirical formula
Formula weight
Crystal system
Crystal size
C30H34Cl8Fe2N8O5·2(C3H6O)·O
1114.12
Triclinic
0.08 ¥ 0.14 ¥ 0.24
¯
Space group
P1 (No. 2)
˚
a/A
10.696(5)
10.942(5)
12.328(6)
89.689(8)
75.395(8)
60.989(8)
1209.3(10)
1
0.71073
1.530
293
0.99
7160
˚
b/A
˚
c/A
a/◦
b/◦
g /◦
V/A
Z
3
˚
Description of the structure of
[(Fe(L2)(TCC))2O]·2(CH3)2CO·H2O 7
˚
l(Mo Ka)/A
D
calc/g cm-3
An ORTEP view of the molecular structure of complex 7 including
the atom numbering scheme is given in Fig. 1 (crystal data and
structure refinement details can be found in Table 1). Selected
bond distances and bond angles relevant to the iron coordination
sphere are listed in Table 2. The unit cell contains one dinuclear
iron(III) complex with a m-oxo bridge and also one water and
two acetone molecules of crystallization. The coordination sphere
of each iron(III) center has a distorted octahedral geometry.
Since the bridging oxygen atom O(3) is located on the centre of
symmetry, the second half of the dinuclear complex is generated
Temperature/K
Goodness-of-fit on F2
Number of reflections measured
Number of reflections used
Number of L.S. restraints
Number of refined parameters
R(int)
5301
0
291
0.033
Final R indices [I > 2s(I)]
a
R1
0.0634
0.1601
b
wR2
ꢀ
ꢀ
ꢀ
a R1
=
[
(ꢀFo|
-
|Fcꢀ)/ |Fo|]. b wR2
=
{[ (w(Fo
-
Fc2)2)/
2
ꢀ
(wFo4)]1/2}.
˚
by inversion of the first. The Fe ◊ ◊ ◊ Fe distance is 3.5612 A and
both the iron atoms are surrounded by a N3O3 donor set, each
constituted by the three nitrogen (N1, N3, N4) atoms of the
facially coordinated ligand L2, two oxygen atoms (O1, O3) of
the dianionic tetrachlorocatecholate and one oxygen atom of the
m-oxo bridge. The facial coordination of L2 is similar to that of
the 3 N ligands in mononuclear octahedral iron(III) complexes.28,30
◦
˚
Table 2 Selected bond lengths (A) and angles ( ) for 7
Bond lengths
Bond angles
Fe(1)–O(1)
Fe(1)–O(2)
Fe(1)–O(3)
Fe(1)–N(1)
Fe(1)–N(3)
Fe(1)–N(4)
2.035(4)
2.018(4)
1.7806(12)
2.117(3)
2.337(5)
2.232(4)
O(1)–Fe(1)–O(2)
O(1)–Fe(1)–O(3)
O(1)–Fe(1)–N(1)
O(1)–Fe(1)–N(3)
O(1)–Fe(1)–N(4)
O(2)–Fe(1)–O(3)
O(2)–Fe(1)–N(1)
O(2)–Fe(1)–N(3)
O(2)–Fe(1)–N(4)
O(3)–Fe(1)–N(1)
O(3)–Fe(1)–N(3)
O(3)–Fe(1)–N(4)
N(1)–Fe(1)–N(3)
N(1)–Fe(1)–N(4)
N(3)–Fe(1)–N(4)
Fe(1)–O(3)–Fe(1_a)
80.57(12)
102.91(11)
90.10(13)
88.52(14)
161.02(16)
104.28(12)
159.36(18)
87.44(15)
86.37(13)
95.71(13)
164.70(11)
93.60(13)
73.86(16)
97.61(14)
77.12(15)
180.00
˚
The Fe–Namine bonds (Fe–N3, 2.337(5); Fe–N4, 2.232(4) A) are
longer than the Fe–Nim bond (2.117(3)A) due to sp and sp2
3
˚
hybridization respectively of the amine and imidazole nitrogen
The symmetry operation that generates Fe(1_a) is 2 - x, 1 - y, -z.
atoms. The Fe–N3amine bond is longer than the Fe–N4amine bond due
to the strong trans effect imposed by the m-oxo bridge leading to the
lengthening of Fe(1)–N(3) bond and hence the stronger binding
of the other two nitrogen atoms (N1 and N4). Consequently, the
˚
Fe–Ocat bonds [Fe–O1, 2.035(4); Fe–O2, 2.018(4) A] trans to the
nitrogen atoms also become different in length and are longer than
those in the TCC2- adducts of iron(III) complexes of tri-27,28,31,32
and tetradentate16,34,37,38 ligands. The structure is similar to that
of the complex37 [{Fe(L7)(tcc)}2O] 8, where L7 is the tridentate
ligand N¢-acetyl-N,N-bis[(2-pyridyl)methyl]ethylenediamine, and
Fig. 1 ORTEP diagram of complex 7 showing 50% probability thermal
ellipsoids and the labelling scheme for selected atoms. All hydrogen atoms
are omitted for clarity. The symmetry operation 2 - x, 1 - y, -z is used to
generate Fe(1_a).
˚
the Fe(1)–O(3) bond (1.7806(12) A) is shorter than that in 8
˚
(1.7929(5) A) suggesting the presence of a higher negative charge
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Table 3 Electronic spectral data (lmax in nm; emax in M-1 cm-1 in parenthesis) for iron(III) complexesa and their adducts in dimethylformamide solution
l
max/nm (e, M-1cm-1)
Added ligandb
None
[Fe(L1)Cl3]
[Fe(L2)Cl3]
[Fe(L3)Cl3]
[Fe(L4)Cl3]
[Fe(L5)Cl3]
[Fe(L6)Cl3]
325 sh
310 (2430)
260 (5850)
780 (1920)
485 (1575)
750 (1935)
475 (1625)
735 (2010)
445 (1670)
665 (2075)
450 (1530)
301 (6930)
267 (6570)
718 (1598)
503 (1590)
697 (1396)
467 (1474)
710 (1546)
451 (1484)
620 (1908)
434 (1379)
342 (2730)
280 (74 100)
660 (1710)
530 (1390)
625 (1900)
502 (1560)
600 (1700)
450 (1520)
590 (1821)
—
328 (3490)
276 (72 570)
655 (1670)
528 (1390)
605 (1770)
510 (1645)
585 (1645)
—
336 (2570)
272 (78 260)
670 (1920)
546 (1620)
625 (1940)
490 (1590)
600 (1740)
440 (1610)
596 (1990)
—
275 (11 332)
795 (2505)
490 (1730)
765 (2490)
480 (1805)
745 (2490)
455 (1770)
677 (2252)
455 (1585)
DBC2-c
3-MeCAT2-c
CAT2-c
TCC2-c
588 (1950)
—
a Concentration of iron(III) complexes, 2 ¥ 10-4 M. b The ratio of added ligand to iron(III) complexes was 1 : 1; the anions were generated by
adding 2 equivalents of triethylamine. c H2DBC = 3,5-di-tert-butylcatechol; H2-3-MeCAT = 3-methylcatechol; H2CAT = catechol; H2TCC = 3,4,5,6-
tetrachlorocatechol.
˚
LMCT bands exhibit remarkable dependence on the nature of
the tridentate ligand11,17 and the magnitude of energy of this
band reflects the Lewis acidity of the iron(III) center as modified
by the ligands. The LMCT band energies increase in the order:
1 < 2 < 3 and a similar variation in band energies are observed
for the other catecholate adducts also. On replacing one of the
pyridyl moieties31 in [Fe(L8)(DBC)Cl], where L8 is bis(pyrid-
2-ylmethyl)amine, by N-methylimidazolyl group to obtain
[Fe(L1)(DBC)Cl] and the pyridyl moiety30 in [Fe(L9)(DBC)Cl],
where L9 is N¢-(pyrid-2-ylmethyl)-N,N-dimethylethylenediamine,
by N-Me-imidazolyl moiety to obtain [Fe(L2)(DBC)Cl], the
catecholate-to-iron(III) LMCT bands are shifted to a higher
energy. Incorporation of the electron-releasing N-Me-imidazolyl
moiety destabilizes16,20 the dp* orbital on iron(III) leading to
an increase in dp*-catecholate orbital energy gap and hence a
shift in the LMCT band to a higher energy. Also, interestingly,
upon replacing the pyridylmethyl arm in [Fe(L1)(DBC)Cl] by
the –CH2–NMe2 and –CH2–NPh arms to get [Fe(L2)(DBC)Cl]
and [Fe(L3)(DBC)Cl] respectively, the catecholate-to-iron(III)
LMCT band is shifted to higher energies. It is clear that the
electron-releasing effect rather than the steric bulk of –NMe2 and
–NPh groups is important in dictating the electronic environment
around iron(III) in the catecholate adducts. Also, it is interesting
that in DMF the low energy LMCT band of [Fe(L5)(DBC)Cl]
is lower in energy than that of [Fe(L4)(DBC)Cl] while the low
energy band of [Fe(L6)(DBC)Cl] is higher in energy than that
of [Fe(L4)(DBC)Cl]. It is obvious that incorporation of the
sterically demanding iso-propyl substituent in [Fe(L6)(DBC)Cl]
would hinder the coordination of the pyrazole nitrogen and
concomitantly stabilize the dp* orbital leading to a decrease in
dp*–catecholate orbital energy gap and hence the shift in the
LMCT band to low energy. On the other hand, incorporation
of electron-releasing methyl groups on the pyrazole ring in
[Fe(L5)(DBC)Cl] adduct would enhance the electron density
on dp* orbital and destabilize it leading to an increase in
dp*–catecholate orbital energy gap and hence the shift in the
LMCT band to a higher energy. As both steric and electronic
effects of the ligand donor atoms are significant in affecting
the energies of the LMCT bands they should also affect the
electronic environment around iron(III) and hence the dioxygenase
activity of the complexes. It is interesting to note that the visible
bands for the adducts [Fe(TptBu,iPr)(DBC)Cl] (672, 830 nm) and
on the oxo group. Furthermore, the difference (0.017 A) in
FeIII–Ocat bonds in 7 with dianionic catechol binding is much
lower than that (0.2–0.4 A) observed in the FeIII–Ocat bonds
39
˚
of BphC enzyme-substrate complex reflecting the monoanionic
coordination of catechol in the latter.
Electronic absorption spectra
The results obtained from the electronic absorption spectral
studies on the iron(III) complexes and their adducts with different
substituted catechols are provided in Table 3. While the spectral
features observed in the range 300–375 nm may be assigned to
Cl- → Fe(III) charge transfer transition,20 the intense absorption
band observed in the range 260–280 nm is ascribed to the p → p*
transition within the imidazole moiety of the ligands. The interac-
tion of the iron(III) complexes with a series of catechols were stud-
ied by using electronic spectroscopy to understand the electronic
environment around iron(III) upon catecholate adduct formation.
Two new visible bands (434–503 nm, 620–795 nm, Fig. 2) appear
on adding catechols pre-treated with two equivalents of Et3N to the
iron(III) complexes 1–6 in DMF solution. They are assignable to
catecholate-to-iron(III) LMCT transitions involving two different
catecholate ligand orbitals and dp* orbital of iron(III).14,16
Fig. 2 Electronic absorption spectra of adducts of complex 1 (4 ¥ 10-4
M) generated in situ by adding equimolar amounts of various catecholate
dianions in DMF solution: [Fe(L1)(DBC)Cl] (a), [Fe(L1)(3-MeCAT)Cl]
(b), [Fe(L1)(CAT)Cl] (c), [Fe(L1)(TCC)Cl] (d).
For the 3,5-di-tert-butylcatecholate (DBC2-) adducts
[Fe(L)(DBC)Cl] generated in DMF both the low- and high-energy
8320 | Dalton Trans., 2009, 8317–8328
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27
[Fe(TpiPr )(DBC)Cl] (574, 1046 nm) appear at wavelengths
higher than the present complexes. The iron(III) center in these
adducts of the anionic tris(pyrazolyl)borate ligands are more
Lewis acidic than those of tris(pyrazolyl)methane ligands.
appears around 560 nm with the spectrum obtained being very
similar to that obtained for the dinuclear complex 7 (cf. above).
This suggests the spontaneous formation of the dinuclear complex
from the mononuclear species and water impurity in solution upon
adding the third equivalent of base (Scheme 3). The catecholate
dianion chelates with iron(III) symmetrically as seen from the
observation of only one iron(III)-to-catecholate LMCT band.
2
The position of the low energy catecholate → Fe(III) LMCT
band depends also upon the substituents11 on the catecholate ring.
Thus the energy of the band is found to shift to higher energy as
the substituents on the catecholate ring are varied from electron-
donating to electron-withdrawing: H2DBC (795 nm) > H23-
MeCAT (765 nm) > H2CAT (745 nm) > H2TCC (677 nm) for com-
plex 1. A similar variation is also observed for other complexes.
Electron-donating substituents would be expected to raise the
energy of the catecholate frontier orbitals and thus minimize the
ligand-to-metal energy gap. Thus the 3,5-di-tert-butylcatecholate
(DBC2-) adducts of the iron(III) complexes absorb at energies lower
than the other catecholate adducts. Similar spectral trends have
been observed for the catecholate-bound iron(III) complexes of
tridentate 3N21,30,31 and tetradentate ligands.20
Scheme 3 Proposed pathway for the formation of m-oxo dimeric complex.
Electrochemical behavior
Spectrophotometric titrations
The spectrophotometric titrations37 were carried out to investigate
the ability of 2 to form the m-oxo bridged complex 7 and also
to gain an insight into the species distribution in solution upon
adding an external base to a mixture of the iron(III) complex
The electrochemical features of the iron(III) complexes and their
DBC2- adducts generated in situ in DMF solution were investi-
gated by employing cyclic (CV) and differential pulse voltammetry
(DPV) on a stationary platinum sphere electrode. The iron(III)
complexes 1–6 exhibit a coupled pair of redox waves at negative
potentials, which are assigned14,21,30,31 to FeIII/FeII couple (Fig. 4;
Table 4). The FeIII/FeII redox potentials (-0.306 to -0.372 V) of
and the substrate tetrachlorocatechol (H2TCC). To a 4 ¥ 10-4
M
M
methanolic solution of complex 2 was added a 4 ¥ 10-4
solution of H2TCC and the resulting mixture was titrated with
a methanolic solution of piperidine (0.3 M) and the results are
presented in Fig. 3. Similar results were obtained when Et3N was
used as the base. Upon adding 0.0–0.5 equivalents of piperidine,
the two catecholate-to-iron(III) LMCT bands appear around 475
and 745 nm and then rise in absorbance (1.0–2.0 equivalents of
base) dramatically suggesting the formation of the mononuclear
substrate adduct. Upon adding a further amount of base (3.0
equivalents), the two bands start disappearing and a new band
Fig. 4 (top) Cyclic voltammograms of 1 mM [Fe(L2)Cl3] 2 (a), with 1 mM
H2DBC added (b) and 1 mM H2DBC and 2 mM triethylamine added (c)
in DMF solution at 25 ◦C. Supporting electrolyte: 0.1 M TBAP. Scan
rate: 0.050 V s-1. (bottom) Differential pulse voltammograms of 1 mM
[Fe(L4)Cl3] 4 without (a), and with 1 mM of H2DBC added (b) and 1 mM of
H2DBC and 2 mM triethylamine added (c) in DMF solution at 25 ◦C. Scan
rate 0.005 V s-1.
Fig. 3 Spectrophotometric titration of a solution of [Fe(L2)Cl3] 2 (4 ¥
10-4 M) and H2TCC (4 ¥ 10-4 M) against piperidine (0.3 M) (a: 0.0, b: 0.5,
c: 2, d: 2.25, e: 3 equivalents). Inset: plot of absorbance vs. base equivalents
at 745 nm.
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Table 4 Electrochemical dataa for [Fe(L)Cl3] and [Fe(L)(DBC)Cl]b in dimethylformamide at 25.0 0.2 ◦C at a scan rate of 0.05 V s-1 (CV) and 0.005 V s-1
(DPV)
E
1/2/V
E1/2/V
Complexes
CV
DPV
Complexes
CV
DPV
Redox process
[Fe(L1)Cl3]
-0.343
-0.306
[Fe(L4)Cl3]
-0.422
—
-0.203
-0.433
—
—
—
—
-0.517
—
—
-0.361
—
-0.217
-0.503
—
-0.432
-0.290c
-0.182
-0.438
-0.208
—
FeIII → FeII
+ H2DBC
-0.239
-0.318
-0.228
—
-0.336
-0.219
-0.339
-0.219
—
-0.256
-0.306
-0.214
—
-0.318
-0.218
-0.338
-0.188
—
+ H2DBC
DBSQ → H2DBC
FeIII → FeII
[Fe(L1)(DBC)Cl]
[Fe(L4)(DBC)Cl]
DBSQ → DBC2-
FeIII → FeII
[Fe(L2)Cl3]
+ H2DBC
[Fe(L5)Cl3]
+ H2DBC
-0.444
—
FeIII → FeII
DBSQ → H2DBC
FeIII → FeII
-0.492
-0.186
—
-0.398
-0.265c
-0.174
-0.482
-0.186
—
[Fe(L2)(DBC)Cl]
[Fe(L3)Cl3]
[Fe(L5)(DBC)Cl]
[Fe(L6)Cl3]
DBSQ → DBC2-
FeIII → FeII
—
-0.372
FeIII → FeII
+ H2DBC
—
—
—
—
—
—
—
—
+ H2DBC
DBSQ → H2DBC
FeIII → FeII
[Fe(L3)(DBC)Cl]
[Fe(L6)(DBC)Cl]
DBSQ → DBC2-
FeIII → FeII
—
a Potential measured vs. Ag(s)/AgNO3 (0.01 M, 0.10 M TBAP); add 0.544 V to convert to NHE. b Generated by adding one equivalent of H2DBC and
two equivalents of triethylamine to complex [Fe(L)Cl3]. c Additional cathodic wave corresponding to [Fe(L)(Sol)Cl2]+ species.14d
the complexes of the imidazole ligands follow the trend 1 > 2 > 3
reflecting a decrease in Lewis acidity of iron(III) center along
this series. With the replacement of the pyridyl moiety in 1 by
the electron-releasing groups as in 2 and 3, the Lewis acidity of
iron(III) center decreases. The FeIII/FeII redox potentials of the
pyrazole-based complexes 4–6 (-0.398 to -0.444 V) are more
negative than those for the pyridyl-based complexes 1–3 clearly
suggesting that the tripodal ligands are more strongly coordinated
than the linear ligands. The FeIII/FeII redox potentials of the
tripodal ligand complexes follow the trend 6 > 4 > 5, which
represents a decrease in Lewis acidity of the iron(III) center along
this series. On introducing the iso-propyl group on the 3-position
of pyrazole rings in 4 to give 6, the sterically hindering iso-propyl
group weakens the coordination of N2 nitrogen of pyrazolyl ring
leading to confer an enhanced Lewis acidity of the iron(III) center.
The electron-releasing methyl groups on the pyrazole ring in 5
increases the electron density on pyrazole N2 nitrogen and hence
decreases the Lewis acidity of the iron(III) center.
On adding one equivalent of H2DBC to 1–6 in DMF so-
lution a new wave appears in the CV and DPV responses
(Fig. 4), which corresponds to the DBSQ/DBC2- couple20,21,30,31
of [Fe(L)(DBC)Cl] species generated. On adding two equivalents
of Et3N to deprotonate the catechol the reduction current of this
couple increases and the FeIII/FeII redox wave tends to vanish
completely, which is expected10,14,21,30,31 to be shifted to a more
negative potential due to chelation of DBC2-. The redox potential
of DBSQ/DBC2- couple of [Fe(L)(DBC)Cl] species is observed in
the narrow range of -0.188 to -0.214 V (Table 4), which is less
negative than that of free DBSQ/DBC2- couple (Epc, -1.34 V vs.
SCE).41 Thus the extent of stabilization of the semiquinone radical
of DBC2- towards oxidation is found to be the same for both the
linear and tripodal tridentate ligand complexes in spite of the
lower Lewis acidity of the iron(III) center observed for the latter
complexes (cf. above). It is possible that upon DBC2- coordination
one of the tripodal arms in 4–6 dissociate leading to an increase in
Lewis acidity of the iron(III) center and hence the same extent of
stabilization of the semiquinone radical towards oxidation for both
the imidazole- and pyrazole-based complexes. In this regard it is
interesting to note that the E1/2 values of DBSQ/DBC2- couple for
the adducts of terminal amine substituted 3 N ligand complexes31
are more negative (-0.209 to -0.223 V) than those (-0.045 to
+0.025 V) for the central amine substituted BPA complexes.30 This
reveals that the DBSQ/DBC2- couple of the catecholate adducts
are sensitive to the iron(III) environment as modified by the ligand
substituents. It is obvious that the electronic environment around
iron(III) in both the imidazole- and pyrazole-based complexes is
the same, which is possibly achieved by dissociation of one of the
three arms of the tripodal ligand complexes from the coordination
sphere upon DBC2- coordination. This is consistent with the small
differences in LMCT spectral band positions between the two
types of complexes.
Dioxygenase activity
The catecholate adducts [Fe(L)(DBC)Cl] and [Fe(L)(DBC)(Sol)]+
were generated in situ by treating respectively the complexes
[Fe(L)Cl3] and [Fe(L)(Sol)3]3+ (obtained30,31 in situ by treating
[Fe(L)Cl3] with three equivalents of AgClO4·H2O) with equimolar
quantities of H2DBC and two equivalents of Et3N in DMF solvent
and then exposed to molecular oxygen. The disappearance of
the low energy catecholate-to-iron(III) LMCT band (Fig. 5) on
oxygenation of the catecholate adducts exhibits pseudo-first order
kinetics as judged from the linearity of the [1 + log(absorbance)]
vs. time plot (Fig. 6) and the values of kobs were obtained from
the slopes of these plots. The second order rate constants were
calculated17,18 (Table 5) by using the equation,
kO2 = kobs/[O2]
The products of the cleavage of H2DBC in DMF solvent
1
(Table 5) were identified (a–e, Fig. 7) by GC-MS and H NMR
techniques and quantified by GC analysis. The complexes
1–3 with imidazole nitrogen donors afford major amounts
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Fig. 5 Progress of the reaction of adduct [Fe(L2)(DBC)(Sol)]+ with O2 in
DMF solution. The disappearance of the DBC2--to-iron(III) charge-trans-
fer band is monitored.
Fig.
7
Products of catechol cleavage of 3,5-di-tert-butylcatechol
(H2DBC) mediated by the iron(III) complexes: 3,5-di-tert-butyl-5-(N,N -
dimethylamidomethyl)-2-furanone (a), 3,5-di-tert-butyl-1-oxacyclohepta-
3,5-diene-2,7-dione (b), 3,5-di-tert-butyl-2-pyrone (c), 4,6-di-tert-butyl-2-
pyrone (d), and 3,5-di-tert-butyl-2-hydroxy-1,4-benzoquinone (e).
38% yield of extradiol products. The second order reaction rate
constants (kO2 , 0.79–4.02 ¥ 10-2 M-1 s-1, Table 5) for the adducts
[Fe(L)(DBC)(Sol)]+ derived from 1–3 in DMF are higher than
those (kO2 , 0.59–2.40 ¥ 10-2 M-1 s-1) for the respective adducts
[Fe(L)(DBC)Cl] in the same solvent. In contrast to the iron(III)
complexes 1–3 of imidazole-based ligands, those of the tripodal
pyrazole-based ligands (4–6) effect the oxidation of H2DBC
on exposure to O2 to afford mainly the benzoquinone product
(e); also, small amounts of extradiol cleavage products (c and
d) and very small amounts of intradiol cleavage products (a
and b) are also obtained. So, the second order reaction rate
constants observed (kO2 , 1.85–2.39 ¥ 10-2 M-1 s-1, Table 5) for
the [Fe(L)(DBC)Cl] adducts of 4–6 with pyrazolyl ligands in DMF
(Table 3) correspond mainly to two-electron oxidation of catechol.
The dioxygenase activity of 1–3 can be illustrated by invoking
the substrate activation mechanism proposed8 for intradiol-
cleaving enzymes. Thus, O2 attacks the iron(III)-bound catecholate
Fig. 6 Plots of [1 + log(absorbance)] vs. time for the reaction of
Fe(L)(DBC)(Sol)]+ with O2 at 25 ◦C in DMF solution. Concentration of
complexes, 4 ¥ 10-4 M (a): [Fe(L2)(DBC)(Sol)]+, (b): [Fe(L1)(DBC)(Sol)]+,
(c): [Fe(L3)(DBC)(Sol)]+.
of the intradiol cleavage product (I) 3,5-di-tert-butyl-5-(N,N-
dimethylamidomethyl)-2-furanone (a) and smaller amounts of
the extradiol cleavage products (E) 3,5-di-tert-butyl-2-pyrone
(c) and 4,6-di-tert-butyl-2-pyrone (d) in DMF solvent with the
extradiol to intradiol product selectivity (E : I) varying from
0.14 : 1 to 0.45 : 1. Interestingly, when the coordinated chloride
ion in [Fe(L)(DBC)Cl] is removed to generate [Fe(L)(DBC)(Sol)]+
species in DMF solvent, the product selectivity (E : I) increases
to 0.6 : 1–1.8 : 1, with the adduct [Fe(L2)(DBC)(Sol)]+ giving
Table 5 Kinetic dataa for the catalytic reaction and quantification of cleavage products (% yield)
Complex
Intradiol (%)
Extradiol (%)
E : I ratio
Quinone (%)
Reaction rate (¥ 10-2 M-1s-1)
[Fe(L1)Cl3]
43.1 (a)
26.0 (a)
12.0 (c)
9.7 (c)
5.5 (d)
11.1 (c)
27.9 (c)
11.0 (f)
6.0 (c)
20.4 (c)
—
0.27 : 1
0.58 : 1
—
—
0.59 0.03
2.25 0.01
[Fe(L1)(Sol)]3+
[Fe(L2)Cl3]
24.7 (a)
21.3 (a)
0.45 : 1
1.83 : 1
—
—
0.21 0.01
4.02 0.04
[Fe(L2)(Sol)]3+
[Fe(L3)Cl3]
40.7 (a)
18.2 (a)
a (2.0)
b (3.0)
b (1.8)
0.14 : 1
1.09 : 1
—
—
—
—
—
—
—
—
e (40.0)
—
e (22.3)
—
e (16.3)
—
2.40 0.02
0.79 0.001
1.85 0.02
[Fe(L3)(Sol)]3+
[Fe(L4)Cl3] 4
—
[Fe(L5)Cl3] 5
[Fe(L6)Cl3] 6
c (11.8)
d (1.5)
c (2.0)
d (4.0)
1.90 0.01
2.39 0.01
b (2.9)
a kO = kobs/[O2]. The solubility of O2 in DMF at 25 ◦C is 4.86 mM. The kinetic data were obtained by monitoring the disappearance of the lower energy
2
LMCT band.
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in [Fe(L)(DBC)Cl] to form the substrate–alkylperoxo–Fe3+ in-
termediate [(L)(DBSQ)Fe(III)O2]-, which gives intradiol cleavage
products upon acyl migration and extradiol cleavage products
upon alkenyl migration (Scheme 4). The formation of extradiol
cleavage products can also be explained by invoking the extradiol
cleavage mechanism,39,40 which involves the attack of molecular
oxygen on iron(III). This mechanism has been used to illustrate
the nearly quantitative yield of the oxidative extradiol cleavage
products28 observed for the reaction of molecular oxygen with
[Fe(Me3tacn)(DBC)Cl], which contains facially coordinated lig-
ands. Similarly, for the adduct [Fe(BBA)(DBC)]ClO4, where BBA
is bis(benzimidazol-2-ylmethyl)amine, a large amount (60%) of
the extradiol cleavage products c and d has been observed26
upon oxygenation. Also, the six-coordinate octahedral complex
Scheme 5 Facial (a) and meridional (b) coordination of 3 N ligands
leading to different cleavage products.
[Fe(TpiPr )(DBC)(CH3CN)] gives the extradiol cleavage products
2
in 70% yield.27 The facial coordination of ligands in all these
adducts allow both O2 and substrate to occupy the opposite face
to form the intermediate22 [(L)(DBSQ)Fe(III)–O2]-, which is the
same as that obtained in the above intradiol cleavage mechanism
(Scheme 5(a)). Very recently, the key peroxide intermediate, along
with the O2 adduct precursor and the product complex successor to
the intermediate, has been found to be present in the X-ray crystal
structures of different subunits of the homoprotocatechuate 2,3-
dioxygenase enzyme reacted with 4-nitrocatechol as substrate.40
By invoking the same alkylperoxo intermediate with iron(III)
oxidation state we have previously explained the formation of
varying amounts of intradiol and extradiol cleavage products for
the iron(III) complexes of the pyridyl analogues of L1–L3 ligands,21
differently substituted bis(pyrid-2-ylmethyl)amine (BPA) ligands31
and sterically constrained 4N34 ligands has been also explained.
On the other hand, meridional coordination of a 3N ligand
like TPY in [Fe(TPY)(DBC)]+ yields major amounts of quinone
product (78%) in addition to intradiol cleavage product (20%)
(Scheme 5(b)).28 The O2 molecule bound to iron(III) is constrained
to be in the same plane as that of the catecholate substrate, which
it cannot attack, but transfers two electrons through the iron(III)
center to the substrate leading to the oxidation to catechol to give
the quinone product. So, it is expected that upon oxygenation
the DBC2- adducts of 1–3 also would yield extradiol products
since the ligands are facially coordinated and a coordination
position is vacant for dioxygen binding. The increase in product
selectivity (E : I) upon removal of the coordinated chloride ion
in these adducts [Fe(L)(DBC)Cl] to generate [Fe(L)(DBC)(Sol)]+
species in DMF solvent clearly indicates that the presence of
a vacant or solvent-coordinated site on iron(III) center of the
catecholate adducts is essential for dioxygen attack to achieve
extradiol cleavage products and that a coordinated chloride ion in
the adduct [Fe(L)(DBC)Cl] is difficult to be replaced by dioxygen
to afford extradiol cleavage products.30,31 Also, the observation
of a higher E : I value for [Fe(L2)(DBC)Cl] reveals that the
sterically hindering terminal –NMe2 group of the ligand appears
to be oriented away from the sp3 hybridized carbon atom of
the catecholate substrate in the above alkylperoxo intermediate
and facilitates alkenyl migration to give enhanced regioselective
extradiol cleavage products.30 Also, upon replacement of one of
the pyridyl moieties in the 1 : 1 iron(III) complex30 of N¢-(pyrid-
2-ylmethyl)-N,N-dimethylethylenediamine by N-Me-imidazolyl
moiety to get [Fe(L2)(DBC)Cl] a dramatic decrease in product
selectivity (E : I) from 18.5 : 1.0 to 1.83 : 1.0 is observed. Obviously,
the sterically less hindering N-Me-imidazolyl moiety favours the
attack of molecular oxygen and also facilitates acyl rather than
alkenyl migration to give intradiol cleavage products.
The second order reaction rate constants (kO2 , 0.79–4.02 ¥
10-2 M-1 s-1, Table 5) for the adducts [Fe(L)(DBC)(Sol)]+ derived
from 1–3 in DMF are higher than those (kO2 , 0.59–2.40 ¥
10-2 M-1 s-1) for the respective adducts [Fe(L)(DBC)Cl] in the
same solvent. This is expected of the higher Lewis acidity10 of the
iron(III) center in [Fe(L)(DBC)(Sol)]+ adducts, which favours both
catecholate and dioxygen binding. As the sterically demanding
N-Ph substituent on the primary ligand in [Fe(L3)(DBC)Cl] would
hinder the fast approach of dioxygen a decreased rate is expected
for the complex.16,27 However, interestingly, this adduct shows
the highest rate of oxygenation among the present complexes
obviously because the sterically demanding N-Ph substituent
Scheme 4 Proposed mechanism for oxidative cleavage of H2DBC by
dioxygen catalysed by 1–3.
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strengthens the iron(III)–catecholate interaction in the alkylperoxo
intermediate [(L3)(DBSQ)Fe(III)O2]- by increasing the Lewis acid-
ity of the iron(III) center and thus enhances the rate of formation
of both the intra- and extradiol cleavage products (cf. above).31
Interestingly, the rate of cleavage of [Fe(L2)(DBC)(Sol)]+ species
is higher than the other two solvated adducts suggesting that the
sterically less hindering N-Me-imidazolyl moiety, supported by
the higher Lewis acidity of the iron(III) center, favours the attack
of molecular oxygen. Thus, the trend in the observed yields of
dioxygenase reaction of complexes could be illustrated solely on
the basis of the steric hindrance to substrate and O2 binding while
the trend in the observed values of kO2 on the basis of Lewis acidity
of the iron(III) center, better by a delicate balance between these
two factors.
As illustrated above for 1–3, the DBC2- adducts of 4–6 are also
expected to yield extradiol cleavage products upon oxygenation
since they contain the tripodal ligands facially coordinated
and also a coordination position vacant for dioxygen binding.
However, interestingly, all of them yield mainly the oxidized
product benzoquinone. It is clear that they are not prone
to form the intermediate [(L)(DBSQ)Fe(III)O2]- responsible for
catechol cleavage. Funabiki et al. have found13 that the complex
[Fe(BPY)(DBC)(H2O)2], where BPY is 2,2¢-bipyridine, yields
mainly the oxidized product benzoquinone upon oxygenation.
They have also reported13 the formation of benzoquinone when
FeCl3/BPY/Py was used for dioxygenation of catechol in THF
solution. Thus, the formation of the oxidized products for 4–6
can be explained by assuming equatorial attack of dioxygen on
the DBC2- adducts of the complexes (Scheme 6(c)) in which
one of the arms of the tripodal ligand (trans to one of the
strong Fe–O(catecholate) bonds) is displaced from the octahedral
coordination sphere due to the sterically constrained 6,6,6-chelate
ring system. The equatorially bound dioxygen is unable to attack
the semiquinone radical located in the same plane and hence the
oxidation product benzoquinone is formed in larger amounts as
for the BPY and TERPY complexes (Scheme 5(b)). The extradiol
cleavage products are obtained when dioxygen attacks the axial
position of the adduct with (Scheme 6(b)) or without (Scheme 6(a))
dissociation of one of the arms of the tripodal ligand (Scheme 6(c)),
while the intradiol cleavage products are formed (Scheme 6(a)) by
the concerted8 dioxygen attack on iron(III) and the carbon atom
of the bound catecholate.
The yield of quinone product for the [Fe(L)(DBC)Cl] adducts of
4–6 with pyrazolyl ligands in DMF decreases along the series 4 >
5 > 6 (Table 3) while their second order reaction rate constants
vary in the order 6 > 5 ~ 4. As substrate binding is the first
step in the mechanism (Scheme 6(c)), the highest Lewis acidity
of 6 (cf. above) would encourage its strongest binding with both
anionic DBC2- and dioxygen leading to the highest yield of the
benzoquinone product. However, only a low yield of quinone is
observed revealing that the bulky iso-propyl substituent on the
pyrazole ring in 6 sterically hinders the binding of DBC2-, which
is verified by building molecular models. Indeed, a decreased
dioxygenase activity has been found upon introducing bulky alkyl
groups near the donor atoms in the heterocyclic ring systems26
in [Fe(TpiPr,tBu)(DBC)]+ and [Fe(6-MeTPA)(DBC)]+. Similarly, the
catecholate adduct of 6 with a sterically hindering iso-propyl
substituent (cf. above) would be expected to show a decreased
rate of quinone formation but it exhibits the highest rate of
Scheme 6
reaction. Therefore, it appears that the bulky iso-propyl groups
on the pyrazolyl ring in 6 do not hinder the approach of DBC2-
and dioxygen and it is the highest Lewis acidity of 6 which
strengthens the iron(III)–catecholate interaction in the alkylperoxo
intermediate [(L6)(DBSQ)Fe(III)O2]- leading to facile dioxygen
attack and hence the highest rate of dioxygenation is observed.30,31
Also, the dissociation of one of the sterically hindering pyrazolyl
arms from the coordination sphere (cf. above) would lead to
an increase in Lewis acidity of the iron(III) center. Thus it is
remarkable that the iron(III) complexes with facially coordinated
tris(pyrazolyl)methane ligand yields only the quinone product
obviously due to bidentate coordination of the tripodal ligand.
Also, for complexes of both the imidazole- and pyrazole-based
3N ligands the steric constraint around iron(III) determines the
yield of dioxygenation products while the Lewis acidity of the
iron(III) center determines the observed rates of dioxygenation.
Experimental
Materials
The chemicals iron(III) chloride (anhydrous) (FeCl3),
1-methyl-2-imidazolecarboxaldehyde,
aminomethylpyridine,
N-phenylethylenediamine, N,N -dimethylethylenediamine, sodium
borohydride, 2-methyl-3-butanone, 3,5-di-tert-butylcatechol
(H2DBC), 3-methylcatechol (3-MeH2CAT), (Aldrich Chemicals),
3,4,5,6-tetrachlorocatechol (H2TCC) (Lancaster), catechol
(H2CAT) (Loba India), acetylacetone, hydrazine hydrochloride,
chloroform, ethyl formate, sodium methoxide, triethylamine
(Merck, India), and pyrazole (Fluka), were used as received.
Methanol, acetonitrile, dichloromethane, dimethylformamide and
diethylether were purchased from Merck, India. The supporting
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electrolyte tetra-N-butylammonium perchlorate (TBAP) was
0.21 g (55%). Anal. Calcd for C13H18Cl3FeN4 (392.51): C, 39.78;
H, 4.62; N, 14.27. Found C, 39.81; H, 4.69; N, 14.17. ESI-MS
m/z = 357 [Fe(L3)Cl2]+.
prepared by using a procedure reported41 already.
Preparation of the ligands L1–L6
[Fe(L4)Cl3] (4). A procedure analogous to that used to prepare
1 was followed to prepare this complex from L4. The pale yellow
colored microcrystalline product was filtered off, washed with
small amounts of cold methanol and then dried. Yield: 0.28 g
(75%). Anal. Calcd for C10H10N6FeCl3 (376.43): C, 31.91; H, 2.68;
N, 22.33. Found: C, 31.81; H, 2.73; N, 22.38. ESI-MS, m/z = 339
[Fe(L4)Cl2]+.
The ligands L1–L3 were synthesized using the method re-
ported in the literature.30 Pyrazole, 3,5-dimethylpyrazole and
3-iso-propylpyrazole required for the syntheses of ligands
L4–L6 were either purchased or prepared by the already reported
procedures.35,36
1-Methyl-1H-imidazol-2-ylmethyl-(pyrid-2-ylmethyl)amine
(L1). 1-Methyl-2-imidazolecarboxaldehyde (1.10 g, 10 mmol) in
methanol (20 mL) was added dropwise to aminomethylpyridine
(1.08 g, 10 mmol) in methanol (20 mL). The mixture was stirred
overnight and then NaBH4 (0.57 g, 15 mmol) was added and
the solution was stirred for another day and then rotaevaporated
to dryness. The resulting solid was dissolved in water and then
extracted with dichloromethane. The organic layer was dried with
anhydrous sodium sulfate and then rotaevaporated to obtain the
[Fe(L5)Cl3] (5). The complex was isolated as an orange colored
powder using the procedure employed for isolating 4. Yield: 0.28 g
(62%). Anal. Calcd for C16H22N6FeCl3 (460.59): C, 41.72; H, 4.81;
N, 18.25. Found: C, 41.60; H, 4.95; N, 18.23. ESI-MS, m/z = 424
[Fe(L5)Cl2]+.
[Fe(L6)Cl3] (6). The complex was prepared as dark yellow
colored powder using the procedure employed for 4. Yield: 0.25 g
(51%). Anal. Calcd for C19H28N6FeCl3 (502.67): C, 45.40; H, 5.61;
N, 16.72. Found: C, 45.45; H, 5.58; N, 16.70%. ESI-MS, m/z = 466
[Fe(L6)Cl2]+.
1
ligand as an oil. Yield: 1.32 g (65.3%); C11H14N4 (202.26). H
NMR (200 MHz, CDCl3): d 8.44–7.26 (m, 4H), 6.72 (s, 1H), 6.62
(s, 1H), 4.14 (s, 2H), 3.81 (s, 2H), 3.63 (s, 3H).
[Fe(L2)(TCC)]2O·2(CH3)2CO·H2O (7). To the ligand L2
(18.2 mg, 0.1 mmol) in acetonitrile, anhydrous FeCl3 in acetonitrile
(16.2 mg, 0.1 mmol) was added. To this solution was added 3,4,5,6-
tetrachlorocatechol (24.79 mg, 0.1 mmol) and then an excess
of triethylamine (5 mL, 0.4 mmol) dissolved in acetonitrile. The
mixture was stirred for a few minutes and then filtered. The filtrate
was layered with acetone. Dark purple crystals of the adduct were
formed within a few days upon slow cooling and standing. Yield:
0.05 g (55%). Anal Calcd for C36H50Cl8Fe2N8O8 (1114): C, 38.67;
H, 4.51; N, 10.02. Found: C, 38.61; H, 4.59; N, 10.05.
N,N-Dimethyl-N¢-(1-methyl-1H-imidazol-2-ylmethyl)ethane-
1,2-diamine (L2). The ligand L2 was prepared by the same
procedure employed for L1, except that the amine used here was
N,N -dimethylethylenediamine (0.88 g, 10 mmol). Yield: 1.23 g
(67.5%); C9H18N4 (182.27). H NMR (200 MHz, CDCl3): d 6.72
(s, 1H), 6.62 (s, 1H), 3.81 (s, 2H), 3.63 (s, 3H), 2.65–2.48 (m, 4H),
1
2.27 (s, 6H).
N-(1-Methyl-1H-imidazol-2-ylmethyl)-N¢-phenylethane-1,2-
diamine (L3). The ligand L3 was prepared by the same
procedure employed for L1, except that the amine used here was
N-phenylethylenediamine (1.36 g, 10 mmol). Yield: 0.88 g (73%);
C13H18N4 (230.31). 1H NMR (200 MHz, CDCl3): d 6.43–7.04 (m,
5H), 6.72 (s, 1H), 6.62 (s, 1H), 4.14 (s, 1H), 3.81 (s, 2H), 3.63
(s, 3H), 2.72–3.2 (m, 4H).
Physical measurements
Electronic spectral measurements in dimethylformamide solu-
tions of the complexes were made with a Varian Cary 300
spectrophotometer. 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)/AgNO3 were used as working, auxiliary, and reference
electrodes, respectively. The supporting electrolyte used was
NBu4ClO4. The platinum sphere electrode was sonicated for
2 min in dilute nitric acid, dilute hydrazine hydrate, and then in
double-distilled water to remove the impurities. The temperature
of the electrochemical cell was maintained at 25 0.2 ◦C by a
cryocirculator (HAAKE D8 G). Bubbling research grade nitrogen
deoxygenated the solutions, and an atmosphere of nitrogen was
maintained over the solution 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 redox potential (E1/2) was calculated from the
anodic (Epa) and cathodic peak (Epc) potentials of CV traces as
(Epa + Epc)/2. The redox potentials were also estimated from the
DPV peak potentials (Ep) using the relation,
Preparation of iron(III) complexes
[Fe(L1)Cl3] (1). The complex [Fe(L1)Cl3] 1 was prepared by
adding anhydrous FeCl3 (0.16 g, 1 mmol) in methanol (10 mL) to
a solution of the ligand L1 (0.20 g, 1 mmol) in methanol (10 mL)
with stirring. The yellow colored complex was filtered off, washed
with cold methanol and diethylether and then dried under vacuum.
Yield: 0.24 g (75%). Anal. Calcd for C11H14Cl3FeN4 (364.46):
C, 36.25; H, 3.87; N, 15.37. Found: C, 36.23; H, 3.90; N, 15.36.
ESI-MS m/z = 329 [Fe(L1)Cl2]+.
[Fe(L2)Cl3] (2). This was isolated using a procedure analogous
to that used to prepare 1 using L2 (0.18 g, 1 mmol) instead of L1.
The pure product was isolated as an orange colored precipitate.
Yield: 0.17 g (50%). Anal. Calcd for C9H18Cl3FeN4 (344.47):
C, 31.38; H, 5.27; N, 16.26. Found: C, 31.35; H, 5.25; N, 16.31.
ESI-MS m/z = 308 [Fe(L2)Cl2]+.
[Fe(L3)Cl3] (3). A procedure analogous to that used to prepare
1 was followed to isolate this complex as a dark brown precipitate
and L3 (0.23 g, 1 mmol) instead of L1 was employed. The solution
was stirred and filtered which resulted in the formation of 3. Yield:
E1/2 = Ep + DE/2
8326 | Dalton Trans., 2009, 8317–8328
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©

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where E1/2 is the equivalent of the average of Epc and Epa in
CV experiments and DE is the pulse amplitude. GC analyses
were performed by HP (Hewlett Packard) 6890 GC series Gas
Chromatograph equipped with a FID detector and a HP-5
capillary column (30 m ¥ 0.32 mm ¥ 2.5 mm). GC-MS analysis
was performed on a Perkin Elmer Clarus 500 GC-MS instrument
using a PE-5 (HP-5 equivalent) capillary column.
the following temperature program: injec◦tor temperature◦130 ◦C,
initial temperature 60 ◦C, heating rate 10 C min-1 to 130 C, then
increasing at a rate of 2 ◦C min-1 to 160 ◦C and then increasing at
a rate of 5 ◦C min-1 to 260 ◦C; FID temperature 280 ◦C. GC-MS
analysis was performed under conditions identical to those used
for GC analysis and the oxygenation products were identified by
comparing the retention times already reported.30 The oxidative
cleavage products of H2DBC detected and identified (a–h) by
GC-MS technique are depicted in Scheme 3.
Crystallographic refinement and structure solution†
A crystal of a suitable size was selected from the mother liquor
and immersed in paraffin oil, then mounted on the tip of a glass
fiber and cemented using epoxy resin. Intensity data for the crystal
Conclusions
Mononuclear iron(III) complexes of both linear and tripodal
tridentate 3N ligands containing imidazolyl andpyrazolyl moieties
respectively, have been isolated and studied as functional models
for catechol dioxygenases. One of the complexes with a facially
coordinated imidazolyl ligand has been found to exhibit a
tendency to dimerize in the presence of base upon binding to
tetrachlorocatecholate. In DMF solution all the complexes of
imidazole-based linear ligands elicit intradiol cleavage of di-tert-
butylcatechol. Also, the extradiol-to-intradiol cleavage product
selectivity increases upon replacing the coordinated chloride ions
in these complexes by solvent, but it decreases upon incorporation
of an imidazolyl nitrogen donor. In sharp contrast to these
complexes, the pyrazole-containing tripodal ligand complexes
yield mainly the oxidized product benzoquinone although the
iron(III) electronic environments in the catecholate adducts of
complexes of both the linear and tripodal tridentate ligands are
almost the same, as revealed by the DBSQ/DBC2- redox potentials
falling in the same narrow range. One of the pyrazolyl arms in the
catecholate adducts is sterically constrained by the 6,6,6-chelate
ring system and appears to dissociate from the coordination sphere
upon binding to the catecholate substrate, which is followed by
dioxygen attack at the equatorial plane leading to the formation
of benzoquinone. Thus the present study reveals that a change in
ligand topology of 1 : 1 iron(III) complexes of 3N ligands would
lead to drastic changes in the dioxygenase reactivity pattern. Also,
the yield of the quinone/cleavage product is found to decrease
upon increasing the steric bulk of the ligands, while the rate of
oxygenation increases upon increasing the Lewis acidity of the
iron(III) center by modifying the ligand substituents.
˚
were collected using Mo Ka (l = 0.71073 A) radiation on a Bruker
SMART APEX diffractometer equipped with CCD area detector
at 293 K. The crystallographic data are collected in Table 1.
The SMART42 program was used for collecting frames of data,
indexing the reflections, and determination of lattice parameters;
the SAINT42 program was used for the integration of the intensity
of reflections and scaling; the SADABS43 program was used for
absorption correction, and the SHELXTL44 program for 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 full-
matrix least-squares analysis. Hydrogen atoms attached to the
ligand moiety were located from the difference Fourier map and
refined isotropically.
Dioxygenation studies
The catechol cleavage activity of all the complexes towards
H2DBC was examined by exposing a solution of iron(III) complex–
DBC2- adduct generated in situ in dimethylformamide (DMF)
to molecular oxygen. Kinetic analyses of the catechol cleavage
reactions were carried out by time dependent measurements
of the disappearance of the lower energy DBC2--to-iron(III)
LMCT band in the presence and absence of chloride ions. Silver
perchlorate monohydrate dissolved in DMF was used to remove
chloride ions from the complex. Stock solutions (6.0 ¥ 10-3 M)
of the adducts [Fe(L)(DBC)(Cl)] and [Fe(L)(DBC)(Sol)]+ were
prepared by treating [Fe(L)Cl3] and [Fe(L)(Sol)3]3+ complexes
([Fe(L)(Sol)3]3+ was generated in situ by treatment with three
equivalents of AgClO4·H2O in DMF and centrifuging the solution
to remove AgCl) with an equivalent amount of H2DBC pretreated
with two equivalents of Et3N. The oxygenation reaction was
started by rapid delivery of a stock solution (0.2 mL) of the
catecholate adducts (6.0 ¥ 10-3 M) by a syringe to O2-saturated
solvent (2.8 mL). The solubility of O2 in DMF at 25 ◦C is
4.86 mM.5 The product analysis was carried out by stirring
the complex [Fe(L)(Sol)3]3+ (0.1 mmol), H2DBC (0.1 mmol)
and triethylamine (0.2 mmol) in DMF (20 mL) solvent under
molecular oxygen over 4 h at room temperature. After the reaction
was completed, the reaction mixture was concentrated under
reduced pressure and extracted with diethylether (3 ¥ 5 mL). The
remaining residue was acidified with HCl to pH 3 to decompose the
metal complexes and extracted with diethylether (3 ¥ 5 mL). The
combined extracts were dried over Na2SO4, and then concentrated.
All the products were quantified using GC (FID) analysis with
Acknowledgements
We sincerely thank the Council of Scientific and Industrial
Research (CSIR), New Delhi for a Senior Research Fellow-
ship to T. D. We also thank the Department of Science and
Technology (DST), New Delhi for supporting this research
[Scheme no. SR/S1/IC/45/2003 and SR/S5/BC-05/2006]. Pro-
fessor M. Palaniandavar is the recipient of the Ramanna Fel-
lowship from DST, New Delhi. We thank Professor K. Natara-
jan, Bharathiar University, Coimbatore for CHN analysis and
Dr Balachandran Unni Nair, Central Leather Research Institute,
Chennai for ESI-MS spectra.
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©