Iron(III) complexes of N2O and N3O donor ligands as functional models for catechol dioxygenase enzymes: Ether oxygen coordination tunes the regioselectivity and reactivity

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Iron(III) complexes of N₂O and N₃O donor ligands as functional models for

catechol dioxygenase enzymes: ether oxygen coordination tunes the

regioselectivity and reactivity†

Karuppasamy Sundaravel,^aEringathodi Suresh,^bKolandaivel Saminathan^cand Mallayan Palaniandavar*^a

Received 17th November 2010, Accepted 5th May 2011

DOI: 10.1039/c0dt01598a

A series of mononuclear iron(III) complexes of the type [Fe(L)Cl₃], where L is a systematically modiﬁed

N₂O or N₃O ligand with a methoxyethyl/tetrahydrofuryl ether oxygen donor atom, have been isolated

and studied as models for catechol dioxygenases. The X-ray crystal structures of [Fe(L2)Cl₃] 2,

[Fe(L6)Cl₃] 6, [Fe(L5)(TCC)Cl] 5a, where H₂TCC = tetrachlorocatechol, [Fe(L6)(TCC)Br] 6a, and the

m-oxo dimer [{Fe(L6)Cl}₂O](ClO₄)₂6b have been successfully determined. In [Fe(L2)Cl₃] 2 the N₂O

ligand is facially coordinated to iron(III) through the pyridine and secondary amine nitrogen atoms and

the tetrahydrofuryl oxygen atom. In [Fe(L6)Cl₃] 6, [Fe(L5)(TCC)Cl] 5a and [Fe(L6)(TCC)Br] 6a the

N₃O donor ligands L5 and L6 act as a tridentate N3 donor ligand coordinated through two pyridine

and one secondary amine nitrogen atoms, whereas the ether oxygen is not coordinated. The spectral

and electrochemical properties of the adducts [Fe(L)(DBC)Cl] of 1–8, where H₂DBC =

3,5-di-tert-butylcatechol, in DMF and their solvated adduct species [Fe(L)(DBC)(Sol)]⁺, where Sol =

DMF/H₂O, generated in situ in dichloromethane, respectively, have been investigated. The product

analysis demonstrates that the adducts [Fe(L)(DBC)Cl] effect cleavage of catechol in the presence of O₂

in DMF to give mainly the intradiol (I) product with a small amount of the extradiol (E) product (E/I,

0.2:1–0.7:1). Interestingly, the solvated species [Fe(L)(DBC)(Sol)]⁺derived from 1–4 cleave H₂DBC to

provide mainly the extradiol cleavage products with lower amounts of intradiol products (E/I,

2.3:1–4.3:1) in dichloromethane. In contrast, the solvated species [Fe(L)(DBC)(Sol)]⁺derived from 5–8

cleave H₂DBC to provide both extradiol and intradiol products (E/I, 0.6:1–2.3:1) due to the

involvement of the ether oxygen donor of the methoxyethyl/tetrahydrofuryl arm in the coordination to

iron(III) upon removal of a chloride ion.

substrates dihydroxybenzenes, resulting in ring cleavage. Based on

the position of ring opening they can be divided into two main

Introduction

classes^5,9(Scheme 1). The intradiol dioxygenase^4,8protocatechuate

3,4-dioxygenase (3,4-PCD)^10,11contains a trigonal bipyramidal,

high-spin ferric center coordinated by 2-His-2-Tyr amino acid

residue and a solvent derived hydroxide ligand, which facilitates

the cleavage of the C–C bond between the enediol moiety.^9,10,12

In contrast, the extradiol dioxygenase 2,3-dihydroxybiphenyl 1,2-

dioxygenase (BphC) employs a square pyramidal high-spin ferrous

center (rarely manganese(II)) ligated by 2-His-1-carboxylate facial

triad motif and two water molecules, to regiospeciﬁcally cleave the

C–C bond^4,10,13–19adjacent (either the distal or proximal side) to

the enediol moiety. The intradiol-cleaving dioxygenases are well-

studied compared to the extradiol counterparts.

Dioxygen-activating enzymes with mononuclear non-heme iron

active sites perform a diverse range of important biological

functions.^1–5The bacterial catechol dioxygenase enzymes play a

key role in the degradation of all naturally occurring and xenobi-

otic aromatic pollutants^4,6–9into water soluble aliphatic products.

They catalyze the activation of molecular oxygen with concomi-

tant insertion of both oxygen atoms into the aromatic ring of the

^aCentre for Bioinorganic Chemistry, School of Chemistry,

Bharathidasan University, Tiruchirappalli, 620 024, Tamilnadu, India.

E-mail: palanim51@yahoo.com, palaniandavarm@gmail.com; Tel: +91-

431-2407053; Tel: +91-431-2407125

^bAnalytical Science Discipline, Central Salt and Marine Chemicals Research

Institute, Bhavnagar, 364 002, India

^cDepartment of Physics, Anna University, Tirunelveli, 627 007, Tamilnadu,

India

† Electronic supplementary information (ESI) available: Table S1, Table

S2 and crystallographic data in CIF format for 2, 5a, 6, 6a and 6b. CCDC

reference numbers 791683–791687. For ESI and crystallographic data in

CIF or other electronic format see DOI: 10.1039/c0dt01598a

Scheme 1 Mode of cleavage of extradiol and intradiol dioxygenases.

8092 | Dalton Trans., 2011, 40, 8092–8107

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Scheme 2 Structure of ligands used in this study.

Many of the reported mimics of intradiol-^20–36and extradiol-

cleaving^17,37–46catechol dioxygenases make use of linear/tripodal

tri- and tetradentate nitrogen, phenolate and carboxylate oxy-

gen donor ligands, which closely duplicate the structure and

function of the enzymes. The ﬁrst functional mimic for ex-

tradiol dioxygenases was reported³⁸by Funabiki et al. using

a mixture of FeCl₂and FeCl₃with py/bipy, elicit a major

amount of the auto-oxidation product benzoquinone (53%) and

a trace amount of extradiol cleavage product. With the help of

[Fe(Me₃tacn)(DBC)Cl], where Me₃tacn = 1,4,7-trimethyl-1,4,7-

triazacyclononane, Que et al. achieved⁴⁰a nearly quantitative yield

(97%) of extradiol-cleavage of catechol. Moro-Oka et al. studied⁴¹

the iron(III) complexes of facially capping and sterically hindering

tridentate trispyrazolyl borate ligands as models for extradiol-

cleaving enzymes and explored the essentiality of vacant site at

iron(III)-catecholate adduct. Recently, Gebbink et al. isolated⁴²

the iron(II) and iron(III) complexes based on bis(1-alkylimidazol-

2-yl)propionate ligands as structural and functional mimics of

the extradiol-cleaving catechol dioxygenase enzymes and obtained

equal amounts of both extra- (18–30%) and intradiol cleavage

(22–29%) products. All these studies accentuated the importance

of facial coordination of tridentate ligands and the presence

of a vacant coordination site at the metal center is necessary

for dioxygen binding to elicit extradiol cleavage. Very recently,

Limberg et al.⁴³reported the iron(III) complexes of py/im/pz

containing a tripodal tridentate N₃ligand, which furnish mainly

the extradiol cleavage products (66%) in the presence of NaBPh₄

as the additive in dichloromethane solvent.

that the iron(III) complexes⁴⁶of imidazole-based linear N₃ligands,

interestingly, elicit major amounts of intradiol products (25–

43%) and minor amounts of extradiol cleavage products (6–12%),

whereas those of pyrazole-based tripodal N₃ligands yield mainly

the oxidized product benzoquinone (16–40%). Continuing our

interest in developing new synthetic models for extradiol-cleaving

catechol dioxygenases, we have now isolated mononuclear 1 : 1

iron(III) complexes of newly designed tri- (N₂O) and tetradentate

(N₃O) ligands (Scheme 2). The ligands L1–L4 are regarded as

derivatives of the linear tridentate bis(pyrid-2-ylmethyl)amine

ligand in which one of the pyridyl arms is replaced by a N-

methylimidazolyl arm or ether oxygen donor arm. On the other

hand, the ligands L5–L8 are considered as derivatives of the well-

known tripodal ligands with three pyridyl⁴⁷or imidazolyl^23,29,48

pendants with one of the arms replaced by a methoxyethyl or

tetrahydrofurylmethyl arm. The ligand donor moieties are system-

atically tailored to better understand the role of ligand steric and

electronic properties on the iron(III) coordination geometry and

hence determine the dioxygenase activity of the complexes. All the

complexes have interacted with differently substituted catechols

to study their ability to form catecholate adducts. Remarkably,

the complexes 1–4 of tridentate N₂O donor ligands elicit extradiol

cleavage with yields higher than those 5–8 of the N₃O ligands with

N-ether substituents in dichloromethane solvent.

Experimental section

Materials

Pyridine-2-carboxaldehyde, 1-methylimidazole-2-carboxaldehy-

de, silver perchlorate monohydrate, tetrahydrofurfurylamine,

iron(III) perchlorate hydrate, sodium borohydride, sodium

We have already reported several 1 : 1 iron(III) complexes of a

number of linear N₃

and N₂O²²ligands with an aim to

22,27,44–46

mimic the function of catechol dioxygenase enzymes and corre-

lated the reaction rate as well as cleavage product yields with the

iron(III) ligand environment. We recently found that the iron(III)

complexes of linear tridentate N-alkyl substituted bis(pyrid-2-

ylmethyl)amine ligands as models for extradiol-cleaving enzymes

and correlated the extradiol cleavage yield (46–68%) with the

steric bulk of the N-alkyl substituents, which controls the re-

gioselectivity of catechol cleavage in dichloromethane.⁴⁴Also, by

using the iron(III) complexes of both central and terminal N-alkyl

substituted (pyrid-2-ylmethyl)ethylenediamine ligands we have

achieved⁴⁵higher extradiol-to-intradiol product selectivity (E/I,

7.2 : 1–18.5 : 1) in dichloromethane solvent. Also, we have found

triacetoxyborohydride,

3,5-di-tert-butylcatechol

(H₂DBC),

3-methylcatechol (3-MeH₂CAT) (Aldrich), 3,4,5,6-tetrachlo-

rocatechol (H₂TCC) (Lancaster), 2-methoxyethylamine (Merck,

Germany), catechol (H₂CAT) (Loba, India), iron(III) chloride

(anhydrous) (Merck, India) were used as received. The supporting

electrolyte tetra-N-butylammonium perchlorate (Aldrich) was

prepared by the procedure reported already.^49,50

Synthesis of ligands

The ligands L1–L8 were synthesized through an adapted literature

procedure.^27,51

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2-Methoxyethyl(pyrid-2-ylmethyl)amine

(L1). Pyridine-2-

L5 except that the amine used here was tetrahydrofurfurylamine

(0.51 g, 5 mmol). Yield 1.20 g (85%). ¹H NMR (400 MHz, CDCl₃):

d = 8.53–7.10 (m, 8H), 4.13 (m, 1H), 3.92 (s, 4H), 3.77 (t, 2H), 2.56

(m, 2H), 1.85 (m, 4H) ppm. EI-MS m/z = 283 (C₁₇H₂₁N₃O⁺).

carboxaldehyde (0.54 g, 5 mmol) and 2-methoxyethylamine

(0.38 g, 5 mmol) were mixed in methanol (20 mL) and stirred well

for one day. Sodium borohydride (0.28 g, 7.5 mmol) was added to

the above solution and the reaction mixture was stirred overnight

at room temperature. The reaction mixture was rotaevaporated

to dryness and the residue was dissolved in water (15 mL) and

extracted with dichloromethane (3 ¥ 15 mL). The organic layer

was dried and the solvent was rotaevaporated to give the ligand

as a yellow oil. The product obtained was used as such for the

(2-Methoxyethyl)bis(1-methyl-1H -imidazol-2-ylmethyl)amine

(L7). The general procedure described for L5 was followed. The

aldehyde 1-methylimidazole-2-carboxaldehyde (1.10 g, 10 mmol)

was used to obtain L6 as a pale yellow oil, which was used

as such for complex isolation. Yield 1.03 g (78%). ¹H NMR

(400 MHz, CDCl₃): d = 6.81–6.68 (m, 4H), 3.65 (s, 6H), 3.61

(s, 4H), 3.48 (t, 2H), 3.23 (s, 3H), 2.49 (t, 2H) ppm. EI-MS m/z =

263 (C₁₃H₂₁N₅O⁺).

1

complex preparation. Yield 0.62 g (75%). H NMR (400 MHz,

CDCl₃): d = 8.48–7.06 (m, 4H), 3.88 (s, 2H), 3.47 (t, 2H), 3.28

(s, 3H), 2.78 (t, 2H), 2.65 (br s, 1H) ppm. EI-MS m/z = 166

(C₉H₁₄N₂O⁺).

Bis(1-methyl-1H -imidazol-2-ylmethyl)(tetrahhydrofuran-2-yl-

methyl)amine (L8). The ligand L8 was prepared using the same

procedure adopted for L5 except that the aldehyde used was 1-

methylimidazole-2-carboxaldehyde (1.10 g, 10 mmol). Yield 0.95 g

(66%). ¹H NMR (400 MHz, CDCl₃): d = 6.85–6.69 (m, 4H), 4.02

(m, 1H), 3.77 (t, 2H), 3.65 (s, 6H), 3.59 (s, 4H), 1.88 (m, 4H), 2.51

(m, 2H) ppm. EI-MS m/z = 289 (C₁₅H₂₃N₅O⁺).

Pyrid-2-ylmethyl(tetrahydrofuran-2-ylmethyl)amine (L2). The

ligand L2 was prepared by using a procedure similar to that

adopted for the preparation of L1. The amine tetrahydrofurfury-

lamine (0.51 g, 5 mmol) was used to obtain L2 as a brown oil,

which was used as such for the complex preparation. Yield 0.82 g

(85%). ¹H NMR (400 MHz, CDCl₃): d = 8.47–7.04 (m, 4H), 4.00

(m, 1H), 3.88 (s, 2H), 3.76 (t, 2H), 2.67 (d, 2H), 2.37 (br s, 1H),

1.92 (m, 4H) ppm. EI-MS m/z = 192 (C₁₁H₁₆N₂O⁺).

Preparation of iron(III) complexes

2-Methoxyethyl(1-methyl-1H-imidazol-2-ylmethyl)amine (L3).

The procedure described for L1 was adopted and N-

methylimidazole-2-carboxaldehyde (0.55 g, 5 mmol) was used

to obtain L3 as a pale yellow oil, which was used for complex

[Fe(L1)Cl₃] (1). A solution of L1 (0.17 g, 1 mmol) in methanol

(10 mL) was added dropwise to a stirred solution of anhydrous

FeCl₃(0.16 g, 1 mmol) in methanol (10 mL) and then cooled. The

pale yellow colored product was ﬁltered off, washed with small

amounts of cold methanol and then dried. Yield 0.24 g (73%).

Anal. Calcd for C₉H₁₄N₂OFe₁Cl₃(328.40): C, 32.91; H, 4.30; N,

8.53. Found: C, 32.94; H, 4.32; N, 8.57. MS m/z = 293 [Fe(L1)Cl₂]⁺.

1

preparation without further puriﬁcation. Yield 0.53 g (63%). H

NMR (400 MHz, CDCl₃): d = 6.82–6.67 (m, 2H), 3.85 (s, 2H),

3.67 (s, 3H), 3.55 (t, 2H), 3.31(s, 3H), 2.82 (t, 2H), 2.55 (br s, 1H)

ppm. EI-MS m/z = 169 (C₈H₁₅N₃O⁺).

[Fe(L2)Cl₃] (2). A procedure analogous to that used to prepare

1 was followed, and [Fe(L2)Cl₃] was obtained as an yellow colored

precipitate. Yield 0.28 g (79%). Anal. Calcd for C₁₁H₁₆N₂OFe₁Cl₃

(354.46): C, 37.27; H, 4.55; N, 7.90. Found: C, 37.29; H, 4.58; N,

7.94. Upon vapour diffusion of diethyl ether into the methanol–

acetonitrile solution of the product, yellow crystals were deposited

after 2 days.

1-Methyl-1H-imidazol-2-ylmethyl(tetrahydrofuran-2-ylmethyl)-

amine (L4). The ligand L4 was synthezised by using a procedure

similar to that followed for preparation of L3. The amine

tetrahydrofurfurylamine (0.51 g, 5 mmol) was used to obtain L4

as a pale yellow oil, which was used without further puriﬁcation

for isolating the complex. Yield 0.59 g (61%). ¹H NMR (400 MHz,

CDCl₃): d = 6.79–6.62 (m, 2H), 4.20 (m, 1H), 3.85 (s, 2H), 3.72

(t, 2H), 3.64 (s, 3H), 2.10 (br s, 2H), 1.85 (m, 4H) ppm. EI-MS

m/z = 195 (C₁₀H₁₇N₃O⁺).

[Fe(L3)Cl₃] (3). To a solution of L3 (0.17 g, 1 mmol) in

methanol (10 mL) was added anhydrous FeCl₃(0.16 g, 1 mmol)

in methanol (10 mL), stirred well and then cooled. The pure

complex was precipitated as a yellow colored powder. It was

ﬁltered off, washed with diethyl ether and dried. Yield 0.22 g

(69%). Anal. Calcd for C₈H₁₅N₃OFe₁Cl₃(331.40): C, 28.99; H,

4.56; N, 12.68. Found: C, 29.04; H, 4.60; N, 12.71. ESI-MS m/z =

295 [Fe(L3)Cl₂]⁺.

(2-Methoxyethyl)bis(pyrid-2-ylmethyl)amine (L5). Pyridine-2-

carboxaldehyde (1.07 g, 10 mmol) and 2-methoxyethylamine

(0.38 g, 5 mmol) were mixed in dry THF (30 mL). Sodium

triacetoxyborohydride (3.18 g, 15 mmol) and glacial acetic acid

(0.60 g, 10 mmol) was added to the above solution and the reaction

mixture was stirred well at room temperature for 48 h under a

nitrogen atmosphere. The solvent was removed under vacuum

and the residue was dissolved in dichloromethane (50 mL) and

then neutralised by the addition of saturated aqueous sodium

bicarbonate solution. The dichloromethane extract was dried and

the solvent was rotaevaporated to give a yellow oil. The product

obtained was used without further puriﬁcation for isolating the

[Fe(L4)Cl₃] (4). This complex was prepared using the proce-

dure employed for [Fe(L1)Cl₃] and was isolated as a yellow colored

powder. Yield 0.25 g (70%). Anal. Calcd for C₁₀H₁₇N₃OFe₁Cl₃

(357.47): C, 33.60; H, 4.79; N, 11.76. Found: C, 33.64; H, 4.82; N,

11.78. ESI-MS m/z = 322 [Fe(L4)Cl₂]⁺.

[Fe(L5)Cl₃] (5). The complex [Fe(L5)Cl₃] was prepared by

adding anhydrous FeCl₃(0.16 g, 1 mmol) in methanol (10 mL)

to a solution of ligand L5 (0.26 g, 1 mmol) in methanol

(10 mL) with constant stirring. After the addition of diethyl

ether (15 mL) into the solution the pure product was isolated

as yellow microcrystalline solid. Yield 0.35 g (83%). Anal. Calcd

1

complex. Yield 1.13 g (88%). H NMR (400 MHz, CDCl₃): d =

8.65–7.32 (m, 8H), 4.10 (s, 4H), 3.51 (t, 2H), 3.27 (s, 3H), 2.51 (t,

2H) ppm. EI-MS m/z = 257 (C₁₅H₁₉N₃O⁺).

Bis(pyrid-2-ylmethyl)(tetrahhydrofuran-2-ylmethyl)amine (L6).

The ligand L6 was prepared by the same procedure employed for

8094 | Dalton Trans., 2011, 40, 8092–8107

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for C₁₅H₁₉N₃OFe₁Cl₃(419.53): C, 42.94; H, 4.56; N, 10.02. Found:

range 1100–190 nm). 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)/AgNO₃were used as working, auxiliary and reference

electrodes respectively. The supporting electrolyte used was

NBu₄ClO₄. The platinum sphere electrode was sonicated for two

minutes 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). The solutions were deoxygenated

by bubbling research grade nitrogen and an atmosphere of nitrogen

was maintained over the solution during measurements. The E_1/2

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

C, 42.96; H, 4.60; N, 10. ESI-MS m/z = 384 [Fe(L5)Cl₂]⁺.

[Fe(L5)(TCC)Cl] (5a). Anhydrous FeCl₃(0.16 g, 1 mmol) and

tetrachlorocatechol (0.25 g, 1 mmol) were dissolved in ethanol

(5 mL) + acetone (5 mL) and triethylamine (27 mL, 2 mmol) was

added dropwise with constant stirring. The dark purple solution

was reﬂuxed, ﬁltered and upon layering of diethyl ether into

the solution, dark brown crystals suitable for X-ray diffraction

analysis were obtained. Yield 0.37 g (62%). Anal. Calcd for

C₂₁H₁₉N₃O₃Fe₁Cl₅(594.50): C, 42.43; H, 3.22; N, 7.07. Found:

C, 42.45; H, 3.25; N, 7.09.

[Fe(L6)Cl₃] (6). The complex [Fe(L6)Cl₃] was prepared by

using the procedure employed for 5. Anhydrous FeCl₃(0.16 g,

1 mmol) in methanol (10 mL) was added to a solution of ligand

L6 (0.28 g, 1 mmol) in methanol (10 mL) with constant stirring.

After the addition of diethyl ether (15 mL) into the solution yielded

yellow crystals of [Fe(L6)Cl₃] suitable for X-ray diffraction. Yield

0.36 g (80%). Anal. Calcd for C₁₇H₂₁N₃OFe₁Cl₃(445.57): C,45.82;

H, 4.75; N, 9.43. Found: C, 45.84; H, 4.78; N, 9.45.

1

acquire the data. H NMR spectra were recorded on a Bruker

400 MHz spectrometer. The cleavage products were analyzed using

a Hewlett Packard (HP) 6890 Gas Chromatograph (GC) series

equipped with a Flame Ionization Detector (FID) 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 (Electron

Ionization) instrument using a PE-5 (HP-5 equivalent) capillary

column.

[Fe(L6)(TCC)Br] (6a). To a solution of FeBr₃(0.15 g,

0.5 mmol) in ethanol (5 mL), L6 (0.14 g, 0.5 mmol) in acetonitrile-

acetone (5 mL, v/v) was added. To this stirred solution 3,4,5,6-

tetrachlorocatechol (0.12 g, 0.5 mmol) in ethanol (10 mL) pre-

treated with triethylamine (0,10 g, 1 mmol) was then added. The

mixture was heated to reﬂux for few minutes and ﬁltered. Upon

layering of diethyl ether into the solution dark purple crystals of

complex 6a suitable for X-ray analysis were deposited. Yield 0.48 g

(72%). Anal. Calcd for C₂₃H₂₁N₃O₃Fe₁Br₁Cl₄(664.99): C, 41.54;

H, 3.18; N, 6.32. Found: C, 41.56; H, 3.21; N, 6.34.

Crystallographic reﬁnement and structure solution

Single-crystal X-ray diffraction data for all the complexes were

collected on a Bruker SMART Apex diffractometer equipped

with a CCD area detector at 100 and 293 K, with Mo-Ka

˚

radiation (l, 0.71073 A). A crystal of suitable size was immersed

in parafﬁn oil and then mounted on the tip of a glass ﬁber

and cemented using epoxy resin. The crystallographic data and

experimental parameters were listed in Table 1. For all the

complexes, the SMART⁵²program was used for collecting frames

of data, indexing the reﬂections, and determination of lattice

parameters; SAINT⁵²program for integration of the intensity

of reﬂections and scaling; SADABS⁵³program for absorption

correction, and the SHELXTL⁵⁴program for space group and

structure determination, and least-squares reﬁnements on F².

The structure was solved by the direct method. Hydrogen atoms

attached to the ligand moieties are either located from the

difference Fourier map or stereochemically ﬁxed except on the

disordered carbon atom C14 of the ligand moiety.

[{Fe(L6)Cl}₂O](ClO₄)₂(6b). Anhydrous FeCl₃(0.11 g,

0.66 mmol), Fe(ClO₄)₃·xH₂O (0.12 g, 0.33 mmol) and L6 (0.28 g,

1 mmol) were dissolved in methanol (15 mL). The mixture was

stirred for 30 min, ﬁltered and then layered with diethyl ether.

Upon standing for 3 days dark red crystals of compound 6b

suitable for X-ray analysis were deposited. Yield 0.74 g (77%).

Anal. Calcd for C₃₄H₄₀N₆O₁₁Fe₂Cl₄(962.22): C, 42.44; H, 4.19; N,

8.73. Found: C, 42.46; H, 4.22; N, 8.78.

[Fe(L7)Cl₃] (7). To a solution of L7 (0.26 g, 1 mmol) in

methanol (10 mL), anhydrous FeCl₃(0.16 g, 1 mmol) in methanol

(10 mL) was added, stirred well and then cooled. The pale yellow

colored precipitate was ﬁltered off, washed with small amounts

of cold methanol and dried under vacuum. Yield 0.32 g (75%).

Anal. Calcd for C₁₃H₂₁N₅OFe₁Cl₃(425.54): C, 36.69; H, 4.97; N,

16.46. Found: C, 36.72; H, 4.99; N, 16.50. ESI-MS m/z = 390

[Fe(L7)Cl₂]⁺.

After locating the neutral complex 5a, interlinked diffused

-3

˚

peaks with residual electron density ranging from 2.3 A to

-3

˚

2.0 A were observed in the difference Fourier map, which can

be attributed to disordered solvent molecules (diethylether, used

for crystallization) present in the crystal lattice. Attempts were

made to model these residual electron density peaks, but were

unsuccessful since the peaks obtained were diffused and there was

no obvious major site occupations for the solvent molecules. The

PLATON/SQUEEZE⁵⁵option has been used to correct the data

[Fe(L8)Cl₃] (8). This complex was prepared using the proce-

dure employed for [Fe(L1)Cl₃] and were isolated as red-orange

microcrystalline solid. Yield 0.29 g (65%). Anal. Calcd for

C₁₅H₂₃N₅OFe₁Cl₃(451.58): C, 39.90; H, 5.13; N, 15.51. Found:

C, 39.92; H, 5.17; N, 15.54. ESI-MS m/z = 416 [Fe(L8)Cl₂]⁺.

for the presence of the disordered solvent. A potential solvent

3

˚

accessible volume of 434.0 A was found. A total of 88 electrons

Instrumental methods

per unit cell worth of scattering were located in the void. This

electron count corresponds to approximately two molecules of

diethylether present in the unit cell. Thus, the structure of the

compound 5a revealed that in addition to the reﬁned complex,

Elemental analyses were performed on a Perkin Elmer Series II

CHNS/O analyzer 2400. The electronic spectra were recorded

on a Agilent 8453 diode array spectrophotometer (wavelength

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Table 1 Crystal data and structure reﬁnement details for [Fe(L2)Cl₃] 2, [Fe(L6)Cl₃] 6, [Fe(L5)(TCC)Cl] 5a, [Fe(L6)(TCC)Br] 6a and

[{Fe(L6)Cl}₂O](ClO₄)₂6b

2

6

5a

6a

6b

Empirical formula

Formula weight

Crystal system

Crystal size (mm)

Space group

C₁₁H₁₆Cl₃FeN₂O

354.46

Monoclinic

0.12 ¥ 0.22 ¥ 0.26

P21/c

7.991(3)

11.870(4)

17.249(5)

90

117.599(12)

90

1450.0(9)

4

1.624

293

0.71073

1.093

5212

2021

163

0.075

724

C₁₇H₂₁N₃OFeCl₃

445.58

Monoclinic

0.30 ¥ 0.35 ¥ 0.70

P21/c

8.6152(13)

14.906(2)

15.224(2)

90

95.299(2)

90

1946.7(5)

4

1.520

293

0.71073

1.04

11238

4475

226

0.019

916

C₂₃H₂₄Cl₅FeN₃O_3.5

631.55

Monoclinic

0.16 ¥ 0.22 ¥ 0.26

P21/n

12.1080(4)

16.7012(5)

14.2209(5)

90

108.476(2)

90

2727.50(16)

4

1.538

293

0.71073

0.946

C₂₃H₂₁N₃O₃FeBrCl₄

664.99

Monoclinic

0.12 ¥ 0.33 ¥ 0.35

P21/n

12.1129(9)

15.7217(11)

14.6905(11)

90

108.7610(10)

90

2649.0(3)

4

1.667

100

0.71073

1.069

C₃₄H₄₂Cl₄Fe₂N₆O₁₁

964.24

Monoclinic

0.12 ¥ 0.28 ¥ 0.34

P21/c

8.2392(6)

10.8799(8)

22.3047(17)

90

99.5360(10)

90

1971.8(3)

2

1.624

100

0.71073

1.086

˚

a (A)

˚

b (A)

˚

c (A)

a (^◦)

b (^◦)

g (^◦)

3

˚

V (A )

Z

r (calc) (g cm^-3)

Temperature (K)

˚

Radiation Mo-Ka (l, A)

Goodness of ﬁt on F²

Number of reﬂections measured

Number of reﬂections used

Number of reﬁned parameters

R(int)

24306

4872

299

0.052

15628

6121

316

0.036

11440

4570

269

0.024

F(000)

1288

1332

992

Final R indices [I>2s(I)]

R^a

wR₂

0.0931

0.2225

0.0367

0.0955

0.0643

0.1641

0.0534

0.1506

0.0662

0.1647

b

^aR₁= [R(ꢀF_o| - |F_cꢀ)/R|F_o|] ^bwR₂= {[R(w(F_o- F_c²)²)/R(wF_o⁴)]^1/2

}

2

tentatively two disordered diethylether molecules may be present

in the unit cell corresponding to 84 electrons per unit cell, whose

contribution was removed from the modiﬁed reﬂection data by

the SQUEEZ program. Full-matrix least-squares reﬁnement of

all non-hydrogen atoms with anisotropic temperature factors

were carried out until convergence was reached. All the hydro-

gen atoms were positioned geometrically and treated as riding

atoms. Final cycles of least-squares reﬁnements with the modiﬁed

data set improved both the R-values and the Goodness of Fit

signiﬁcantly.

[Fe(L)(Sol)₃]³⁺(0.1 mmol), H₂DBC (0.1 mmol) and triethylamine

(0.2 mmol) in dichloromethane (20 mL) solvent under molecular

oxygen over 4 h at room temperature. After completion of the

reaction, 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 complex and the product extracted with diethylether (3 ¥

10 mL). The combined extracts were dried over Na₂SO₄and then

concentrated. All the products were quantiﬁed using GC (FID)

with the following temperature program: injector temperature

◦

130 ^◦C; initial temperature 60 ^◦C, heating rate 10 C min^-1to

◦

130 ^◦C, then increasing at a rate 2 C min^-1to 160 ^◦C and

Reactivity studies

◦

then increasing at a rate 5 C min^-1to 260 C; FID temperature

280 ^◦C. The products were analyzed in GC three times and the

average yields are listed in Table 2. GC-MS analysis was performed

under conditions identical to those used for GC analysis and

the oxygenation products identiﬁed by comparing the observed

retention times with those reported already.^43,44

The catechol cleavage activity of all the complexes toward

H₂DBC was examined by exposing a solution of iron(III)-DBC^2-

adduct generated in situ in N,N-dimethylformamide (DMF) and

dichloromethane (DCM) to molecular oxygen. Kinetic analyses of

the catechol cleavage reactions were carried out by time dependent

measurement of the disappearance of the low energy DBC^2--

to-iron(III) LMCT band in presence and absence of chloride

ions. The complex [Fe(L)(Sol)₃]³⁺was prepared in solution by

treatment of [Fe(L)Cl₃] with three equivalents of AgClO₄·H₂O

dissolved in DMF and centrifuging the solution to remove AgCl.

Stock solutions (6.0 ¥ 10^-3M) 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 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^-3M) by syringe to the O₂-saturated

solvent (2.8 mL). The solubility of O₂in DMF^29,30,44,56and in

DCM⁵⁷at 25 ^◦C were taken as 4.86 and 5.80 mM respectively.

The product analysis was carried out by stirring the complex

Results and discussion

Syntheses of ligands and iron(III) complexes

The ligands L1–L8 were synthesized according to known pro-

cedures with suitable modiﬁcations. The tridentate N₂O donor

ligands L1–L4 were obtained^27,45as oils in good yields by the

reduction of the corresponding Schiff bases by using NaBH₄and

the Schiff bases were obtained by the condensation of pyridine-2-

carboxaldehyde (L1, L2) or 1-methyl-2-imidazolecarboxaldehyde

(L3, L4) with the corresponding amines. The ligands L5–L8 were

synthesized⁵¹by the reductive amination of 2-methoxyethylamine

and tetrahydrofurfurylamine with the corresponding aldehydes in

8096 | Dalton Trans., 2011, 40, 8092–8107

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Table 2 Cleavage products 9–15 (Scheme 3) obtained upon oxygenation and rate of oxygenation^areactions of iron(III)-catecholate adduct generated in

situ in dichloromethane solution

Reaction rate

Complexes

Intradiol (%)

Extradiol (%)

Other products (%)

E/I ratio

(¥ 10^-1M^-1s^-1)

[Fe(L1)(Sol)₃]³⁺

[Fe(L2)(Sol)₃]³⁺

[Fe(L3)(Sol)₃]³⁺

[Fe(L4)(Sol)₃]³⁺

[Fe(L5)(Sol)₃]³⁺

[Fe(L6)(Sol)₃]³⁺

[Fe(L7)(Sol)₃]³⁺

[Fe(L8)(Sol)₃]³⁺

25.0 (12, 13)

28.2 (12, 13)

13.4 (12, 13)

22.7 (12, 13)

33.4 (12, 13)

56.5 (12, 13)

19.9 (12, 13)

18.2 (12, 13)

71.0 (9, 10, 11)

71.5 (9, 10, 11)

57.3 (9, 10, 11)

52.3 (9, 10, 11)

35.9 (9, 10, 11)

31.2 (9, 10, 11)

46.7 (9, 10, 11)

28.1 (9, 10, 11)

2.0 (15)

—

2.8

2.5

4.3

2.3

1.1

0.6

2.3

1.5

5.73 0.03

5.71 0.04

2.04 0.01

4.69 0.02

22.26 0.01

48.04 0.03

14.37 0.05

21.85 0.06

7.9 (15)

12.4 (15)

5.4 (15)

8.3 (15)

7.9 (15)

11.4 (15)

◦

^ak_O= k_obs/[O₂]. The solubility of O₂in dichloromethane is taken to be 5.80 mM at 25 C.^25,26The kinetic data were obtained by monitoring the

2

disappearance of the lower energy DBC^2--to-iron(III) LMCT band. ^bCoordinated chloride ions are removed by treatment with three equivalents of silver

perchlorate monohydrate in DMF solution.

the presence of glacial acetic acid using sodium triacetoxyboro-

hydride as a reducing agent. The pyridyl and imidazolyl donors

of the ligands mimic the imidazole moiety of histidine amino

acid residue in catechol dioxygenase enzymes. The stoichiometric

reaction of FeCl₃with these ligands results in the formation of 1 : 1

mononuclear iron(III) complexes. All the complexes are formulated

as [Fe(L)Cl₃] based on ESI-MS and elemental analysis, which is

supported by the X-ray crystal structures of complexes 2 and 6.

The (m-oxo)diiron(III) complex 6b is prepared by the reaction

of FeCl₃and Fe(ClO₄)₃·xH₂O (2 : 1 ratio) with one equivalent

of the ligand L6 in methanol. Conductivity experiments reveal

that all the 1 : 1 complexes [Fe(L1)Cl₃]–[Fe(L8)Cl₃]: (K_M, 82–130

ohm^-1cm²mol^-1) behave as 1 : 1 electrolytes in DMF solution.

The present iron(III) complexes 1–8 have magnetic moments in the

range 5.45–5.87 BM at room temperature, which is characteristic

of high-spin iron(III) center.

by three chloride ions (Cl1, Cl2, Cl3). The distortion in the

coordination geometry^27,29,44,45from an ideal octahedron is revealed

by the deviation in the Cl–Fe–N (163.70(14)–162.40(3)^◦), O–Fe–

N (166.90(2)^◦) and Cl–Fe–Cl (96.57(12)–100.29(11)^◦) bond angles

from the ideal angles respectively of 180^◦and 90^◦. The Fe–O_thf

˚

bond (2.146(8) A) is longer than that (2.074(3) A) observed in the

iron(III) complex of a tripodal tetradentate bis(phenolate) ligand

reported previously.⁵⁸The molecular structure of [Fe(L6)Cl₃] 6

is depicted in Fig. 2 and selected bond distances and angles are

listed in Table 3. In the distorted octahedral complex molecule

the tetradentate N₃O ligand L6 act as a tridentate donor ligand,

coordination facially to the iron(III) through three nitrogen atoms

(N1, N2, N3), and the remaining octahedral sites are occupied

by three chloride ions and interestingly, the tetrahydrofurylmethyl

arm is not coordinated. Also, the three Fe–Cl (2.2621(7)–2.3055(7)

˚

A) bond distances are different and the Cl–Fe–Cl (97.21(2)–

100.89(7)^◦) bond angles are greater than 90^◦indicating that

the octahedral iron(III) coordination geometry is rhombically

distorted.

Description of crystal structures of [Fe(L2)Cl₃] 2, [Fe(L6)Cl₃] 6,

[Fe(L5)(TCC)Cl] 5a, [Fe(L6)(TCC)Br] 6a and

[{Fe(L6)Cl}₂O](ClO₄)₂6b

The molecular structure of [Fe(L2)Cl₃] 2 is shown in Fig. 1 along

with atom numbering scheme and the relevant bond lengths and

bond angles are given in Table 3. The geometry around the iron(III)

center is best visualized as a distorted octahedral with one face

of the octahedron constituted by the facially coordinating linear

tridentate ligand (N1, N2, O1) and the opposite face occupied

Fig. 1 Thermal ellipsoid plot of the complex [Fe(L2)Cl₃] 2 (50%

Fig. 2 Thermal ellipsoid plot of the complex [Fe(L6)Cl₃] 6 (50%

probability; hydrogen atoms omitted for clarity).

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◦

sp²hybridizations respectively of the amine and pyridyl nitrogen

atoms.^{27,29,44,45,59,60}The Fe–N_pyand Fe–N_aminebonds are shorter than

all three Fe–Cl bonds in 2 and 6 suggesting that the chloride ions

are probably substituted by the catecholate dianion (cf. below).

Like 2 and 6, the complexes of the other ligands are expected to

display a similar facial coordination, based on the similarities in

their spectral properties (cf. below).

The X-ray structures of the catecholate adducts

[Fe(L5)(TCC)Cl] 5a and [Fe(L6)(TCC)Br] 6a including atom

numbering scheme are given respectively in Fig. 3 and 4 and the

selected bond lengths and bond angles are collected in Table 3.

The asymmetric unit for 5a has one complex molecule and the

equivalent of half a symmetrically disposed diethylether molecule

of solvation. The tetradentate N₃O donor ligands in both 5a and

6a act as a tridentate N₃donor and the ether/tetrahydrofuryl

oxygen atom of the ligands are not coordinated. The Fe–N_py

˚

Table 3 Selected bond lengths (A) and bond angles ( ) for [Fe(L2)Cl₃]

2, [Fe(L6)Cl₃] 6, [Fe(L5)(TCC)Cl] 5a, [Fe(L6)(TCC)Br] 6a and

[{Fe(L6)Cl}₂O](ClO₄)₂6b

[Fe(L2)Cl₃] 2

Fe(1)–Cl(1)

Fe(1)–Cl(2)

Fe(1)–Cl(3)

2.290(3)

2.276(3)

2.302(4)

100.29(11)

96.57(12)

162.40(3)

89.00(2)

90.70(2)

100.08(13)

94.40(2)

163.70(3)

Fe(1)–N(1)

Fe(1)–N(2)

Fe(1)–O(1)

2.201(8)

2.218(9)

2.146(8)

89.30(2)

90.30(2)

92.10(3)

166.90(2)

74.60(3)

79.70(3)

77.10(3)

Cl(1)–Fe(1)–Cl(2)

Cl(1)–Fe(1)–Cl(3)

Cl(1)–Fe(1)–N(1)

Cl(1)–Fe(1)–N(2)

Cl(1)–Fe(1)–O(1)

Cl(2)–Fe(1)–Cl(3)

Cl(2)–Fe(1)–N(1)

Cl(2)–Fe(1)–N(2)

Cl(2)–Fe(1)–O(1)

Cl(3)–Fe(1)–N(1)

Cl(3)–Fe(1)–N(2)

Cl(3)–Fe(1)–O(1)

N(1)–Fe(1)–N(2)

O(1)–Fe(1)–N(1)

O(1)–Fe(1)–N(2)

[Fe(L6)Cl₃] 6

Fe(1)–Cl(1)

2.3055(7)

2.2889(7)

2.2621(7)

97.21(2)

100.89(2)

162.09(5)

90.38(4)

87.95(5)

98.74(2)

92.27(5)

166.68(4)

Fe(1)–N(1)

Fe(1)–N(2)

Fe(1)–N(3)

2.2001(17)

2.2666(16)

2.2121(18)

92.54(5)

92.59(5)

90.50(4)

164.61(5)

77.67(6)

76.45(6)

Fe(1)–Cl(2)

Fe(1)–Cl(3)

bond distances in both the adducts (5a: 2.162(4), 2.193(4); 6a:

III

˚

2.1460(3), 2.1690(3) A) are in the typical range for Fe –N_py

Cl(1)–Fe(1)–Cl(2)

Cl(1)–Fe(1)–Cl(3)

Cl(1)–Fe(1)–N(1)

Cl(1)–Fe(1)–N(2)

Cl(1)–Fe(1)–N(3)

Cl(2)–Fe(1)–Cl(3)

Cl(2)–Fe(1)–N(1)

Cl(2)–Fe(1)–N(2)

Cl(2)–Fe(1)–N(3)

Cl(3)–Fe(1)–N(1)

Cl(3)–Fe(1)–N(2)

Cl(3)–Fe(1)–N(3)

N(1)–Fe(1)–N(2)

N(1)–Fe(1)–N(3)

N(2)–Fe(1)–N(3)

bonds in the chloride complexes like 6 and other such catecholate

adducts.^29,44,45They are shorter than the Fe–N_aminebond distance

˚

(5a: 2.253(4); 6a: 2.266(3) A), as found for 6. The difference

in Fe–N_pybond distances in the adducts confer an asymmetry

˚

(0.02 A) in the Fe–O_catbond lengths (5a:1.970(3), 1.950(3); 6a:

76.75(6)

˚

1.969(3), 1.950(3) A), which is a consequence of orientation of the

non-coordinating methoxyethyl (5a)/tetrahydrofuryl (6) arm),

which is similar to those found in certain iron(III)-catecholate

adducts.^{29,33,40,41,44}The structures of the adducts 5a and 6a are

analogous to that of the adduct [Fe(L11)(TCC)(NO₃)], where

L11 is bis(pyrid-2-ylmethyl)-iso-propylamine.⁴⁴Like the active

site of the Bphc enzyme-substrate adduct⁶¹the iron(III) center in

this and the present adducts is facially coordinated by the ligand.

Further, the chloride and bromide ion coordination in 5a and 6a

is possibly replaced by dioxygen during the course of oxygenation.

[Fe(L5)(TCC)Cl] 5a

Fe(1)–Cl(1)

Fe(1)–O(1)

2.284(17)

1.970(3)

1.950(4)

Fe(1)–N(1)

Fe(1)–N(2)

Fe(1)–N(3)

O(1)–Fe(1)–N(3)

O(2)–Fe(1)–N(1)

O(2)–Fe(1)–N(2)

O(2)–Fe(1)–N(3)

N(1)–Fe(1)–N(2)

N(1)–Fe(1)–N(3)

N(2)–Fe(1)–N(3)

2.162(4)

2.253(4)

2.193(4)

167.17(17)

163.69(17)

91.32(16)

87.72(16)

76.64(17)

99.34(16)

74.40(17)

Fe(1)–O(2)

Cl(1)–Fe(1)–O(1)

Cl(1)–Fe(1)–O(2)

Cl(1)–Fe(1)–N(1)

Cl(1)–Fe(1)–N(2)

Cl(1)–Fe(1)–N(3)

O(1)–Fe(1)–O(2)

O(1)–Fe(1)–N(1)

O(1)–Fe(1)–N(2)

97.79(12)

101.70(13)

92.71(13)

161.29(13)

92.57(13)

82.79(14)

87.77(15)

97.12(16)

[Fe(L6)(TCC)Br] 6a

Fe(1)–Br(1)

Fe(1)–O(2)

2.4648(7)

1.9690(3)

1.9500(3)

95.25(8)

100.12(9)

97.02(9)

165.31(9)

92.56(9)

82.99(11)

88.65(11)

98.01(12)

Fe(1)–N(1)

Fe(1)–N(2)

Fe(1)–N(3)

O(2)–Fe(1)–N(3)

O(3)–Fe(1)–N(1)

O(3)–Fe(1)–N(2)

O(3)–Fe(1)–N(3)

N(1)–Fe(1)–N(2)

N(1)–Fe(1)–N(3)

N(2)–Fe(1)–N(3)

2.1460(3)

2.2660(3)

2.1690(3)

170.33(11)

161.53(13)

87.80(13)

90.04(11)

77.05(13)

96.06(12)

74.95(12)

Fe(1)–O(3)

Br(1)–Fe(1)–O(2)

Br(1)–Fe(1)–O(3)

Br(1)–Fe(1)–N(1)

Br(1)–Fe(1)–N(2)

Br(1)–Fe(1)–N(3)

O(2)–Fe(1)–O(3)

O(2)–Fe(1)–N(1)

O(2)–Fe(1)–N(2)

[{Fe(L6)Cl}₂O](ClO₄)₂6b

Fe(1)–Cl(1)

2.2793(10)

Fe(1)–N(1)

Fe(1)–N(2)

Fe(1)–N(3)

O(1)–Fe(1)–N(3)

O(2)–Fe(1)–N(1)

O(2)–Fe(1)–N(2)

O(2)–Fe(1)–N(3)

N(1)–Fe(1)–N(2)

N(1)–Fe(1)–N(3)

N(2)–Fe(1)–N(3)

2.1150(3)

2.2140(3)

2.1230(3)

88.40(12)

91.42(8)

92.17(8)

91.63(9)

77.96(12)

154.62(12)

76.75(12)

Fe(1)–O(1)

Fe(1)–O(2)

2.2410(3)

1.7868(5)

87.42(7)

Cl(1)–Fe(1)–O(1)

Cl(1)–Fe(1)–O(2)

Cl(1)–Fe(1)–N(1)

Cl(1)–Fe(1)–N(2)

Cl(1)–Fe(1)–N(3)

O(1)–Fe(1)–O(2)

O(1)–Fe(1)–N(1)

O(1)–Fe(1)–N(2)

104.69(3)

102.89(9)

163.06(9)

100.68(9)

167.65(7)

83.40(11)

75.82(11)

Fig. 3 Thermal ellipsoid plot of the adduct [Fe(L5)(TCC)Cl] 5a (50%

probability; hydrogen atoms omitted for clarity).

The molecular structure of the complex cation [{Fe(L6)Cl}₂O]²⁺

6b including atom numbering scheme is given in Fig. 5 and selected

bond lengths and bond angles are listed in Table 3. The molecular

structure is composed of the centrosymmetric dimeric cations

with a linear Fe–O–Fe (Fe(1)–O(2)–Fe(1_a), 180^◦) unit.²⁹Each

iron(III) center is in a distorted octahedral environment ligated

In 2 and 6 the Fe–N_pybond distance (2: 2.201(8); 6: Fe–N1,

˚

2.2001(17); Fe–N3, 2.2121(18) A) is shorter than the Fe–N_amine

3

˚

distance (2: 2.218(9); 6: 2.2666(16) A), which is expected of sp and

8098 | Dalton Trans., 2011, 40, 8092–8107

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Fig.

5

Thermal ellipsoid plot of the dinuclear complex

[{Fe(L6)Cl}₂O](ClO₄)₂6b (50% probability; hydrogen atoms omitted for

clarity, “_a” characters in the Fe1_a label indicates that this atom is at

(-x, 1 - y, -z).

Fig. 4 Thermal ellipsoid plot of adduct [Fe(L6)(TCC)Br] 6a (50%

probability; hydrogen atoms omitted for clarity).

may be assigned to Cl^-→ iron(III) ligand-to-metal charge transfer

(LMCT) transition, the more intense bands in the range 240–

298 nm to p → p* transition(s) within the ligands.^27,44–46The

catecholate adducts of complexes 1–8 were generated in situ in

DMF solution by the addition of two equivalents of triethylamine

to catechol. Two visible bands observed (450–520, 590–855 nm,

Table 4, Fig. 6) are assignable to catecholate-to-iron(III) charge-

transfer transitions (LMCT) involving two different catecholate

frontier orbitals and the dp* orbital of iron(III).^{20,27,29,31–33,37,44–46}

The positions of both the DBC^2--to-iron(III) charge-transfer

(LMCT) bands of the adducts [Fe(L)(DBC)Cl] of 1–8, show

by the N₃O donor set of the tetradentate ligand L6, one chloride

ion and the oxygen atom of m-oxo bridge, which is also bound

to the second iron(III) center. The Fe–O bond distance (Fe–O2,

˚

1.7868(5) A) falls in the range (1.73–1.82 A) reported for oxo-

bridged iron(III) complexes.²⁹The Fe–O_thfbond (2.2410(3) A) is

˚

much longer than the Fe–O_thfbond (2.146(8) A) in [Fe(L2)Cl₃]

2 and that in the mononuclear iron(III) complex of a tripodal

58

˚

tetradentate bis(phenolate) ligand (2.074(3) A), and this arises

from the presence of stronger donors in the dimeric structure. It

is now clear that the tetrahydrofuryl oxygen donor in [Fe(L6)Cl₃]

6 is not coordinated obviously because the neutral ether oxygen

coordination to iron(III) weaker than that of anionic chloride ions.

a

remarkable dependence on the nature of the primary

ligand,^{20,27,29,31–33,37,44–46}6 > 5 > 8 > 7 > 2 > 1 > 4 > 3 and, a

similar variation in band energies are observed for the adducts of

other catechols also. The increase in energies of the bands along

the above series correlates with the decrease in Lewis acidity of

the iron(III) center as modiﬁed by the ligand donor functions.

Thus, upon replacing the pyridylmethyl arm in [Fe(L1)(DBC)Cl]

Electronic absorption spectra

The electronic absorption spectral data of the iron(III) complexes

1–8 and their adducts with differently substituted catechols are

collected in Table 4. The spectral features observed near 300 nm

Table 4 Electronic absorption spectral data for iron(III) complexes and their adducts^bin DMF solution

_max, nm (e, M^-1cm^-1)

l

Added Ligand

None^a

[Fe(L1)Cl₃]

[Fe(L2)Cl₃]

[Fe(L3)Cl₃]

[Fe(L4)Cl₃]

[Fe(L5)Cl₃]

[Fe(L6)Cl₃]

[Fe(L7)Cl₃]

[Fe(L8)Cl₃]

367 (3211)

300 (4283)

246 (7287)

796 (2097)

505 (1795)

822 (2218)

549 (1943)

740 (2071)

497 (1806)

731 (1853)

464 (1503)

593 (2311)

—

380 (4097)

290 (4992)

254 (7923)

800 (2415)

510 (1909)

852 (2223)

547 (1878)

756 (2377)

503 (1964)

733 (1998)

465 (1560)

601 (2275)

—

375 (3568)

290 (4739)

243 (7528)

772 (2166)

500 (1739)

785 (2303)

530 (1602)

739 (2092)

491 (1752)

723 (1954)

456 (1610)

605 (2196)

—

378 (3746)

296 (3860)

240 (4049)

784 (2289)

498 (1773)

795 (2408)

535 (1803)

752 (2391)

489 (1887)

732 (2126)

459 (1640)

642 (2242)

470 (1521)

389 (3905)

299 (5767)

255(11 864)

850 (2158)

517 (1429)

905 (3015)

572 (2105)

807 (2605)

512 (1891)

790 (2227)

477 (1628)

702 (2493)

484 (1752)

385 (3888)

298 (5890)

255 (12725)

855 (2066)

519 (1757)

938 (3155)

576 (2063)

802 (1923)

516 (1577)

785 (1808)

477 (1523)

655 (2002)

486 (1524)

366 (3995)

317 sh

365 (3934)

311 sh

281 (9500)

806 (1847)

515 (1623)

869 (2728)

549 (1988)

771 (2215)

494 (1901)

753 (2038)

464 (1711)

606 (2145)

468 (1763)

298 (10 450)

810 (2122)

505 (1738)

892 (2588)

551 (1823)

767 (2018)

497 (1835)

757 (1994)

463 (1610)

617 (1954)

463 (1517)

H₂DBC^b

H₂DBC^c

3-MeH₂CAT

H₂CAT

H₂TCC

^aConcentration of iron(III) complexes, 1.0 ¥ 10^-4M. ^bConcentration of catecholate adducts, 2.0 ¥ 10^-4M. The ratio of added ligands to iron(III) complexes,

1 : 1. The anions were generated by adding two equivalents of triethylamine in DMF. H₂DBC = 3,5-di-tert-butylcatechol, 3-MeH₂CAT = 3-methylcatechol,

H₂CAT = catechol, H₂TCC = tetrachlorocatechol. ^cCoordinated chloride ions are removed by treatment with silver perchlorate monohydrate in DMF

solution. The DBC^2-adduct of the solvated adduct species [Fe(L)(DBC)(Sol]⁺are generated in dichloromethane solvent.

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Fig. 6 Electronic absorption spectra of adducts of complex 4 (2 ¥ 10^-4M) generated in situ by adding equimolar amounts of various catecholate dianions

in DMF solution: [Fe(L4)(DBC)Cl] (a), [Fe(L4)(3-MeCAT)Cl] (b), [Fe(L4)(CAT)Cl] (c), [Fe(L4)(TCC)Cl] (d).

and [Fe(L2)(DBC)Cl] by the more strongly s-bonding

N-methylimidazolylmethyl arm to obtain [Fe(L3)(DBC)Cl] and

[Fe(L4)(DBC)Cl] respectively, the negative charge built on iron(III)

increases and as a result the dp* orbitals of iron(III) are destabilized

leading to an increase in energy^{20,25,28,29,33,37,44–46}gap between the dp*

orbitals and catecholate orbitals and hence the higher energies of

the LMCT bands for the latter adducts. A similar trend observed

for 5–8 demonstrating that the presence of two imidazolyl nitrogen

donors²¹in the adducts of 7 and 8 renders the Lewis acidity

of the iron(III) core in them lower than that in 5 and 6. On

replacing the methoxy oxygen in [Fe(L1)(DBC)Cl] by the sterically

hindering tetrahydrofuryl oxygen as in [Fe(L2)(DBC)Cl], the

LMCT bands are shifted to lower energies because the lone-

pair orbital on the oxygen atom of the cyclic ether donor is not

oriented properly towards the iron(III) orbital. A similar variation

in LMCT band energy is observed going from [Fe(L3)(DBC)Cl]

to [Fe(L4)(DBC)Cl]. However, very small changes are observed

going from 5 to 6 and 7 to 8 as the methoxy and tetrahydrofuryl

oxygen donors are not coordinated (cf. above). On substituting the

central N–H hydrogen in [Fe(L1)(DBC)Cl] and [Fe(L2)(DBC)Cl]

by one more pyridylmethyl arm to obtain [Fe(L5)(DBC)Cl]

and [Fe(L6)(DBC)Cl] respectively, the low-energy bands are red-

shifted and are in the range 848–859 nm reported for the analo-

gous iron(III) complexes^29,44with N-alkyl substituted bis(pyrid-

2-ylmethyl)amine ligands. A similarly substitution with one

more electron-releasing N-methylimidazolyl arm in the adducts

[Fe(L3)(DBC)Cl] and [Fe(L4)(DBC)Cl] to obtain respectively

the adducts [Fe(L7)(DBC)Cl] and [Fe(L8)(DBC)Cl], the iron(III)-

catecholate charge transfer (LMCT) bands are red-shifted.

Interestingly, when 1–8 in DCM are treated with three equiv-

alents of AgClO₄·H₂O (DMF) to generate [Fe(L)(Sol)₃]³⁺species

and then with an equivalent amount of DBC^2-the low energy

LMCT bands are observed ([Fe(L)(DBC)(Sol)]⁺: 530–576, 785–

938 nm) at energies lower than those of the corresponding

[Fe(L)(DBC)Cl] adducts in DMF solution. This is expected

because removal of the coordinated chloride anion from the

iron(III) coordination sphere would decrease the negative charge

built on iron(III) causing stabilization of the d-orbitals and hence

decrease the catecholate-to-iron(III) LMCT band energy.^44–46Also,

for the [Fe(L)(DBC)(Sol)]⁺species, the low-energy LMCT bands

vary (10–83 nm) depending upon the ligand donor moieties, as

observed for [Fe(L)(DBC)Cl] species (cf. above) supporting the

importance of both the Lewis basicity and steric effect of ligand

donor groups.^44–46

Electrochemical behavior

The redox features of the iron(III) complexes and their catecholate

adducts generated in situ were studied in DMF solution by

employing cyclic (CV) and differential pulse voltammetry (DPV)

on a stationary platinum sphere as a working electrode. In DMF

solution all the complexes 1–8 exhibit a quasi-reversible redox wave

with cathodic (-0.218– -0.468 V) as well as anodic (-0.120– -0.286

V) peaks at negative potentials, which are assigned^{23–26,28,29,44–46}to

Fe^III/Fe^IIcouple (Table S1, ESI†). The cathodic current functions

(D, 0.42–1.60 ¥ 10^-6cm²s^-1) calculated by using Randles–Sevciks’

equation^49,50are of the same order as those observed for other

iron(III) complexes undergoing a diffusion-controlled one-electron

reduction process.^{23–26,28,29,44–46}The values of peak current ratio

(i_pa/i_pc), which is far from unity, and the peak potential separation

(DE_p, 92–182 mV) suggest that the redox process is far from

reversible. The E_1/2values of Fe^III/Fe^II(Table S1, ESI†; Fig. 7A

& 7B, DPV) redox potentials (-0.160– -0.381 V) of the present

complexes follow the trend 6 < 5 < 8 ~ 7 < 2 < 1 < 4 < 3

in DMF solvent, which represents the decrease in Lewis acidity

of the iron(III) center, and is consistent with the above spectral

results. The Lewis basicity of imidazolyl (strongly s-bonding)

moiety in 3 and 4, which is higher than that of the pyridyl

moiety in 1 and 2, leads to enhanced stabilization of the iron(III)

oxidation state in the former complexes, rendering the Fe^III/Fe^II

redox potentials of 3 and 4 more negative than those of 1 and

2 respectively.^25,28,29Also, on replacing the linear methoxyethyl

donor in 1 and 3 by the bulky tetrahydrofuryl moiety to obtain 2

and 4 respectively, the lone pair orbital of the oxygen atom in the

latter becomes improperly oriented towards iron(III) orbital (cf.

above) leading to a higher positive charge on iron(III) and hence

a higher Fe^III/Fe^IIredox potential for 2 and 4. On introducing

one more pyridylmethyl arm in 1 and 2 respectively to obtain

5 and 6 the E_1/2values are shifted to more positive values, and

are in the same range reported for the iron(III) complexes of N-

alkyl substituted bis(pyrid-2-ylmethyl)amine ligands.⁴⁴Similarly,

upon incorporation of one more N-methylimidazolyl donor in

complexes 3 and 4 to obtain 7 and 8 respectively, an increase in

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Fig. 8 Differential pulse voltammograms of 1 mM [Fe(L5)(Sol)₃]³⁺(a),

with 1 mM H₂DBC added (b) and 1 mM H₂DBC and 2 mM triethylamine

added (c) in dichloromethane solution at 25

electrolyte: 0.1 M TBAP. Scan rate: 5 mV s^-1.

0.2 ^◦C. Supporting

Fig. 7 Differential pulse voltammograms of 1 mM (A) [Fe(L4)Cl₃] 4 and

(B) [Fe(L6)Cl₃] 6 (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: 5 mV s^-1.

Fe^III/Fe^IIredox potential is observed. A similar trend in the redox

potentials of Fe^III/Fe^IIcouple (E_1/2, +0.070– +0.130 V; D, 0.86–

4.24 ¥ 10^-6cm²s^-1; Fig. 8; Table 5) has been observed for the

solvated complexes [Fe(L)(Sol)₃]³⁺generated in dichloromethane

solution by treating 1–8 with three equivalents of AgClO₄·H₂O

(DMF) to remove the coordinated chloride ions. Also, the redox

potentials of the solvated species are more positive than those

of the chloro complexes [Fe(L)Cl₃] in DMF solution, which

is expected^44–46of the increase in Lewis acidity of the iron(III)

center upon solvation. Interestingly, a plot of the Fe^III/Fe^IIredox

potential of the complexes against the DBC^2--to-iron(III) LMCT

band energy for both chloro and solvated complexes are linear

(Fig. 9A & 9B) illustrating that spectral and electrochemical

parameters can be used to illustrate the dioxygenase activity of

the complexes.^23,29,44,45

Upon adding one equivalent of H₂DBC to 1–4 in DMF a

new wave appears in both the CV and DPV responses, which

corresponds to DBSQ/H₂DBC redox couple (-0.259– -0.291 V,

Fig. 7A, DPV) of the adduct species generated. On adding two

equivalents of Et₃N to deprotonate catechol the DBSQ/DBC^2-

couple appears in the range -0.243– -0.281 V and the Fe^III/Fe^II

redox wave vanishes completely and is expected^{22,26,27,44–46}to be

shifted to a more negative potential due to chelation of DBC^2-. In

contrast to 1–4, the DBSQ/H₂DBC redox wave of 5–8 is overlaid

Fig. 9 Plot of E_1/2(Fe^III/Fe^IIredox couple of chloro complexes (A)

and solvated species [Fe(L)(Sol)₃]³⁺(B) derived from 1–8, respectively)

vs. the energy of the lower energy DBC^2--to-iron(III) LMCT band of

respectively, adducts [Fe(L)(DBC)Cl] in DMF and [Fe(L)(DBC)(Sol)]⁺

1–8 in dichloromethane solution. (R²= 0.966 (A), 0.983 (B)].

on the Fe^III/Fe^IIredox wave (-0.160– -0.302 V, Fig. 7B, DPV)

and interestingly, upon addition of two equivalents of Et₃N, it is

not shifted (-0.181– -0.285 V) but a small increase in reduction

current is observed. The Fe^III/Fe^IIredox wave is expected to be

shifted to a more negative potential as discussed above. The

DBSQ/DBC^2-redox couple is observed at potentials more positive

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Table 5 Electrochemical data^afor [Fe(L)(Sol)₃]^3+band [Fe(L)(Sol)(DBC)]^+cin dichloromethane solution at 25.0 0.2 ^◦C at a scan rate of 50 mV s^-1(CV)

and 5 mV s^-1(DPV)

E_1/2(V)

Complex

E_pc(V)

E_pa(V)

CV

DPV

Redox process

[Fe(L1)(Sol)₃]³⁺

H₂DBC

0.037

—

0.051

0.235

0.053

0.039

—

0.045

0.232

0.045

-0.054

—

0.096

0.175

-0.018

0.047

—

0.065

0.159

0.054

0.067

0.341

0.061

0.303

-0.014

0.126

0.336

-0.028

0.330

0.016

0.017

—

0.029

0.394

0.170

0.021

0.049

0.484

0.156

0.423

0.157

-0.064

0.147

—

0.159

0.307

0.129

0.141

—

0.159

0.334

0.135

0.190

—

0.152

0.331

0.132

0.149

—

0.209

0.265

0.078

0.153

0.465

0.139

0.427

0.160

0.206

0.504

0.206

0.464

0.180

0.137

—

0.139

0.484

0.248

0.065

0.175

0.688

0.248

0.584

0.325

0.034

0.092

—

0.105

0.271

0.091

0.090

—

0.102

0.283

0.090

0.053

—

0.124

0.253

0.057

0.098

—

0.137

0.212

0.066

0.110

0.403

0.100

0.365

0.074

0.166

0.420

0.089

0.397

0.098

0.077

—

0.084

0.439

0.209

0.043

0.112

0.586

0.202

0.487

0.241

-0.015

0.102

—

0.940

0.265

0.116

0.110

—

0.092

0.287

0.108

0.070

—

0.120

0.274

0.075

0.098

—

0.104

0.222

0.054

0.116

0.404

0.102

0.385

0.108

0.130

0.431

0.063

0.401

0.097

0.085

—

0.082

0.421

0.215

0.072

0.108

0.575

0.114

0.465

0.261

0.008

Fe^III–Fe^II

H₂DBC-DBSQ

Fe^III–Fe^II

[Fe(L1)(DBC)(Sol)]⁺

Fe^III–Fe^II

[Fe(L2)(Sol)₃]³⁺

H₂DBC

Fe^III–Fe^II

H₂DBC-DBSQ

Fe^III–Fe^II

[Fe(L2)(DBC)(Sol)]⁺

Fe^III–Fe^II

[Fe(L3)(Sol)₃]³⁺

H₂DBC

Fe^III–Fe^II

H₂DBC-DBSQ

Fe^III–Fe^II

[Fe(L3)(DBC)(Sol)]⁺

Fe^III–Fe^II

[Fe(L4)(Sol)₃]³⁺

H₂DBC

Fe^III–Fe^II

H₂DBC-DBSQ

Fe^III–Fe^II

[Fe(L4)(DBC)(Sol)]⁺

Fe^III–Fe^II

[Fe(L5)(Sol)₃]³⁺

H₂DBC

Fe^III–Fe^II

H₂DBC-DBSQ

Fe^III–Fe^II

[Fe(L5)(DBC)(Sol)]⁺

DBC^2--DBSQ

Fe^III–Fe^II

[Fe(L6)(Sol)₃]³⁺

H₂DBC

Fe^III–Fe^II

H₂DBC-DBSQ

Fe^III–Fe^II

[Fe(L6)(DBC)(Sol)]⁺

DBC^2--DBSQ

Fe^III–Fe^II

[Fe(L7)(Sol)₃]³⁺

H₂DBC

Fe^III–Fe^II

H₂DBC-DBSQ

Fe^III–Fe^II

[Fe(L7)(DBC)(Sol)]⁺

DBC^2--DBSQ

Fe^III–Fe^II

[Fe(L8)(Sol)₃]³⁺

H₂DBC

Fe^III–Fe^II

H₂DBC-DBSQ

Fe^III–Fe^II

[Fe(L8)(DBC)(Sol)]⁺

DBC^2--DBSQ

Fe^III-Fe^II

Fe^III–Fe^II

^aPotential measured vs. Ag(s)/AgNO₃(0.01 M, 0.10 M TBAP); add 0.544 V to convert to NHE. ^bCoordinated chloride ions are removed by treatment

with three equivalents of silver perchlorate monohydrate in DMF solution. ^cGenerated by adding one equivalent of H₂DBC and two equivalents of

triethylamine to complex [Fe(L)(Sol)₃]³⁺

.

than that of free DBSQ/DBC^2-couple (E_pc, -1.34 V vs. SCE).^49,50

This reﬂects the stabilization of DBC^2-towards oxidation upon

coordination to iron(III) and conﬁrms the enhanced covalency of

iron(III)-catecholate interaction in these adducts.

+0.385– +0.465 V revealing that the donor atoms do not exert

any strong inﬂuence on electronic properties of the adducts.

Also, they appear at potentials more positive than those of

the [Fe(L)(DBC)(Cl)] species revealing that the semiquinone

form of the coordinated DBC^2-in the former is much more

stabilized toward oxidation than that in the latter. Upon adding

one equivalent of H₂DBC to generate the [Fe(L)(H₂DBC)(Sol)]³⁺

species of 1–4, the DBSQ/H₂DBC redox wave is overlaid

on the Fe^III/Fe^IIredox wave, which is in contrast to 5–8.

Upon adding two equivalents of Et₃N to these adducts,

two waves (+0.054– +0.116 V; +0.222– +0.287 V) of equal

reduction current are observed in the same potential range of

Fe^III/Fe^IIcouple. This suggests that the redox behaviour of

the [Fe(L1)(DBC)(Sol)]⁺–[Fe(L4)(DBC)(Sol)]⁺adducts are

On adding one equivalent of H₂DBC to generate the

[Fe(L)(H₂DBC)(Sol)]³⁺species of 5–8,

a new redox wave

corresponding to DBSQ/H₂DBC couple is observed at more

positive potentials (superimposed on the Fe^III/Fe^IIredox wave

for [Fe(L7)(H₂DBC)(Sol)]³⁺) (Fig. 8). On adding two equivalents

of Et₃N to these species, the reduction current of the original

wave decreases and that of the new wave increases. The Fe^III/Fe^II

redox couple of the catecholate bound [Fe(L)(DBC)(Sol)]⁺

species appears to be shifted^22,44–46to more negative potentials.

The observed DBSQ/DBC^2-redox potentials are in the same

range as those observed for the DBC^2-adducts of iron(III)

complexes of N₃ligands,^44,45and fall in the narrow range of

different

from

those

of

the

[Fe(L5)(DBC)(Sol)]⁺–

[Fe(L8)(DBC)(Sol)]⁺adducts. It is likely that the former

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adducts containing the coordinated –NH group are unstable in

the presence of added Et₃N base and would possibly undergo

dimerization involving water impurity as reported previously.^44,45

The slight differences in redox potentials of the two waves

may correspond to two iron(III) centers with slightly different

coordination environments in solution.

The products of cleavage of H₂DBC (Table 2 & Table S2, ESI†)

1

were identiﬁed (9–13, Scheme 3) by GC-MS (EI) and H NMR

techniques and quantiﬁed by GC (FID) analysis.^{17,39,40,42,43,44}

The adducts [Fe(L1)(DBC)Cl] of 1–8 were reacted with dioxy-

gen to afford major amounts of intradiol cleavage products (I, 4.9–

16.7%) and minor amounts of extradiol (E, 1.8–5.9%) products

with low extradiol-to-intradiol cleavage product selectivity (E/I,

0.17 : 1–0.72 : 1) in DMF solvent. However, remarkably, upon

exposure of the solvated adducts [Fe(L)(DBC)(Sol)]⁺of 1–8 to

O₂in DCM solution a signiﬁcant enhancement in extradiol

cleavage product yield (28.1–71.5%; Table 2) with a higher product

selectivity (E/I, 1 : 1–4.3 : 1) is observed. A similar enhancement

in yields of extradiol cleavage products for the solvated complexes

[Fe(L)(DBC)(Sol)]⁺, where L is both central and terminal N-alkyl

substituted N-(pyrid-2-ylmethyl)ethyleneamine,⁴⁵and bis(pyrid-

2-ylmethyl)amine⁴⁴ligands has been observed. We have invoked

the involvement of a substrate-alkylperoxo-Fe³⁺key reaction

intermediate (Scheme 4)^{12,44,45,62–64}to illustrate these observations.

Upon alkenyl migration in the alkylperoxo intermediate extradiol

products are obtained and upon acyl migration intradiol cleavage

products are obtained.^18,19The key peroxide intermediate and

the O₂adduct precursor and product complex successor to the

substrate-alkylperoxo-Fe²⁺intermediate involved in the extradiol

cleavage reaction are remarkably found^62,63to be simultaneously

present in the X-ray crystal structures of different subunits of

the homoprotocatechuate 2,3-dioxygenase enzyme reacted with

the slow reacting substrate 4-nitrocatechol (4-NC). Also, by

using the less electron-withdrawing substrate 4-sulfonylcatechol

(4-SC) Lipscomb et al. have now isolated⁶⁴the new reactive

intermediate gem diol species that appears to occur between

Fe(III)-alkylperoxo intermediate and the product complex in the

catalytic cycle of extradiol-cleaving catechol dioxygenases. So,

the attack of molecular oxygen on iron(III) center with the

sixth coordination position remaining free in a non-coordinating

solvent such as dichloromethane is conducive leading to extradiol

cleavage products upon alkenyl migration and intradiol cleavage

products upon acyl migration.^18,19,44In the present adducts the

ligands L1–L8 with sterically hindering substituents occupy the

facial positions allowing both the substrate catechol and molecular

oxygen to occupy the opposite face of the octahedron thereby

bringing them in close proximity and in proper orientation to

form the substrate-alkylperoxo-Fe³⁺intermediate. A concerted

attack of O₂on both the activated carbon atom of catecholate

and the vacant coordination site on iron would lead to the same

Fe(III)-alkylperoxo intermediate.^12,62An analysis of the product

yields reveal that the complexes 1 and 2 both with a pendant

ether oxygen donor atom exhibit the highest extradiol cleavage

product yield (~72%). On moving from 1 to 3 and from 2 to 4 the

extradiol yield decreases (1: 71.0, 2: 71.5, 3: 57.3; 4: 52.3%) with

the extradiol-to-intradiol product selectivity (E/I) changing from

2.8 (1) to 4.3 (3) and remaining the same (E/I, 2.3) for 2 and 4.

This demonstrates that upon replacing the pyridyl donor in 1 and

2 by N-methylimidazolyl donor to obtain 3 and 4 respectively the

Lewis acidity of the iron(III) center decreases, which discourages

the O₂attack leading to a lower extradiol product yield. The

sterically less hindering N-methylimidazolylmethyl moiety does

not seem to encourage O₂attack but facilitates alkenyl migration

in the alkylperoxo intermediate to give regioselective extradiol

cleavage and hence a higher product selectivity. Also, upon

Kinetics and reactivity studies

The 3,5-di-tert-butylcatecholate (DBC^2-) adducts of [Fe(L1)Cl₃]

1-[Fe(L8)Cl₃] 8 and those of the solvated complex species

[Fe(L1)(Sol)₃]³⁺–[Fe(L8)(Sol)₃]³⁺(Sol = DMF/H₂O), which were

generated by the addition of three equivalents of AgClO₄·H₂O

(DMF) to the DMF solutions of the complexes, were prepared

in situ in DMF and DCM solvent and their reactivity towards

molecular oxygen was examined by monitoring the decay of

the low-energy DBC^2--to-iron(III) LMCT band (Fig. 10). The

reaction of all the adducts with O₂show pseudo-ﬁrst-order

kinetics due to excess of dioxygen and the pseudo-ﬁrst-order

rate constant (k_obs) was determined from the linear plot of [1

+ log(Absorbance)] vs. time (Fig. 11). The second order rate

constants were calculated^30,35,44,45(Table 2) from the values of k_obs

by using the equation,

k_O2= k_obs/[O₂]

Fig. 10 Progress of the reaction of adduct [Fe(L6)(DBC)(Sol)]⁺with O₂

in dichloromethane solution. The disappearance of the DBC^2--to-iron(III)

charge-transfer band is monitored.

Fig. 11 Plots of [1 + log(Absorbance)] vs. time for the reaction of

[Fe(L)(DBC)(Sol)]⁺with O₂at 25 ^◦C in dichloromethane solution.

Concentration of the complexes, 4 ¥ 10^-4M (a): Fe(L6)(DBC)(Sol)]⁺

(

), (b): [Fe(L5)(DBC)(Sol)]⁺(ꢀ), (c): [Fe(L8)(DBC)(Sol)]⁺

( ), (d):

᭺

᭹

[Fe(L7)(DBC)(Sol)]⁺(᭡).

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Scheme 3 Products of catechol cleavage of H₂DBC mediated by [Fe(L)Cl₃)] complexes using molecular oxygen: 4,6-di-tert-butyl-2-pyrone

(9), 3,5-di-tert-butyl-2-pyrone (10), 5,7-di-tert-butyloxepine-2,3-dione (11), 3,5-di-tert-butyl-5-(N,N-dimethylamidomethyl)-2-furanone (12),

3,5-di-tert-butyl-1-oxacyclohepta-3,5-diene-2,7-dione (13), 3,5-di-tert-butyl-1,2-benzoquinone (14), 3-tert-butylfuran-2,5-dione (15).

Scheme 4 Proposed oxidative cleavage mechanism for DBC^2-adducts of 1–8.

introducing sterically less hindering terminal N-alkyl substituents

in the iron(III) complexes of (pyrid-2-ylmethyl)ethylenediamine

ligands the product selectivity increases.⁴⁵In contrast to the

catecholate adducts of 1–4, those of 5–8 show lower amounts

of extradiol cleavage products (28.1–46.7%) and lower product

selectivity (E/I, 0.6 : 1–2.3 : 1). Though the methoxyethyl and

tetrahydrofurylmethyl arms are not coordinated to iron(III) in the

chloro adducts 5a and 6a they may be coordinated (cf. above)

in the corresponding ‘solvated’ adducts in DCM at the vacant

position to a give a six-coordinate adduct rendering O₂attack

on iron(III) difﬁcult; instead O₂attacks the activated catecholate

leading to enhanced intradiol cleavage and decreased extradiol

cleavage products. Also, it is possible that the sterically hindering

N-ether substituents at the central amine nitrogen donor appear

to be very near the sp³hybridized carbon atom of the catecholate

substrate hindering the alkenyl migration and hence the formation

of extradiol products. Further, for the adducts of 5–8, the

change in ligand donor function does not exhibit a pronounced

effect on the extradiol product yield and product selectivity as

well.

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The second order reaction rate constants (k_O2, 2.04–48.04 ¥

10^-1M^-1s^-1, Table 2) for the adducts [Fe(L)(DBC)(Sol)]⁺derived

from 1–8 in DCM are much higher than those (k_O2, 0.92–2.81 ¥

10^-2M^-1s^-1, Table S2, ESI†) for the adducts [Fe(L)(DBC)Cl] in

DMF solvent. This is expected of the ease of displacement of

coordinated H₂O/DMF in solvated adducts by dioxygen in the

non-coordinating solvent DCM; however, it is difﬁcult to displace

coordinated chloride ions in the adducts of the chloro complex

in coordinating solvent DMF. The observed trends in the rate of

reaction of the solvated complexes [Fe(L)(DBC)(Sol)]⁺in DCM

solvent are 1 ~ 2 > 4 > 3; 6 > 5; 8 > 7, which is in accordance with

the increase in Lewis acidity of the adducts in the non-coordinating

solvent (cf. above). The complexes 5–8 with N₃O donor ligands

exhibit rates of dioxygenation higher than 1–4 with N₂O donor

ligands, which is in accordance with the steric demand of the

central N-ether substituent in the complex, which strengthens

the iron(III)-catecholate interaction and facilitates the electron-

transfer from the substrate to O₂in both the intra- and extradiol

cleavage mechanisms^{12,18,19,31,44,45,65}(cf. above). Thus, on replacing

the pyridylmethyl arm in 1 and 2 by N-methylimidazolylmethyl

arm in 3 and 4 reaction rate decreases due to the decrease in Lewis

acidity of the iron(III) center and a similar decrease in reaction

rate is observed on replacing the pyridyl donors in 5 and 6 by

N-methylimidazolyl donor as in 7 and 8, respectively. In fact,

a plot of the rate of dioxygenation against the Fe(III)/Fe(II)

redox potential, which is a measure of the Lewis acidity of

iron(III) center (cf. above), is linear (Fig. 12).^44,45Interestingly,

the complex species [Fe(L1)(Sol)₃]³⁺–[Fe(L4)(Sol)₃]³⁺fall on a

straight line while the species [Fe(L5)(Sol)₃]³⁺–[Fe(L8)(Sol)₃]³⁺

with higher dioxygenase activity fall on another straight line.

Obviously, in the former complexes the N₂O ligand acts as

a tridentate ligands and hence a slow extradiol mechanism is

preferred, while in the latter complexes the N₃O ligands act

as tetradentate ligands and hence a faster intradiol mechanism

is preferred. Among the latter complexes, the rate of catechol

cleavage of [Fe(L6)(DBC)(Sol)]⁺species is the highest obviously

due to the higher steric demand of the N-ether substituent on the

central amine nitrogen atom in the complex, which strengthens

the iron(III)-catecholate interaction and facilitates the electron-

transfer from the substrate to O₂in both the intra- and extradiol

cleavage mechanisms (cf. above).^{12,18,19,31,44,45,65}

Interestingly, a plot of log(k_O2) versus energy of the low

energy DBC^2--to-iron(III) LMCT band (Fig. 13) is linear. Similar

linear correlations have been observed by Funabiki et al.⁶⁶for

the reaction of the [Fe(TPA)(R-cat)]BPh₄adducts, where R-

Cat are differently substituted catechols, with O₂, as well as

by us for the catecholate adducts of iron(III) complexes of

bis(pyrid-2-ylmethyl)amine⁴⁴and N-alkyl substituted N-(pyrid-2-

ylmethyl)ethylenediamine.⁴⁵The LMCT energy-dependent dioxy-

genase activity can be illustrated by invoking the Lewis acidity

of the iron(III) center, which is higher for the complexes 5–8,

for example, with lower energy LMCT band energy (cf. above).

Also, a two-state reaction pathway, which involves a spin-inversion

process from the spin-quartet (S = 5/2) in (L)Fe^III-Cat^2-to

the upper-lying spin-singlet (S = 1/2) in (L)Fe^II-SQ, upon O₂

approaching the iron(III) center may be invoked to illustrate the

trend in reactivity. The spin-pairing and hence spin-inversion is

caused by the enhancement in ligand ﬁeld strength on O₂binding

to the catecholate adduct. In the case of [Fe(L6)(DBC)(Sol)]⁺

with N-R = N-tetrahydrofurylmethyl, the approach of O₂takes

place on the face opposite to the N-tetrahydrofurylmethyl group

and as the sterically hindering N-tetrahydrofuryl moiety leads

to the strongest iron(III)-catecholate interaction (cf. above) and

formation of the upper-lying (L)Fe^II-SQ state, which is ready to

react with O₂and hence the highest reaction rate observed. Also,

the energy needed for the spin-inversion⁶⁶from (L)Fe^III-Cat^2-(S =

5/2) to (L)Fe^II-SQ (S = 1/2) state, which corresponds to the

LMCT band energy, varies systematically along the present series

of complexes with the energy being the lowest for the catecholate

adduct of [Fe(L6)(Sol)₃]³⁺. Thus the present study the contribution

of both the Lewis acidity and the ligand steric demand to reaction

pathway and yield and, in addition, interestingly, the presence of an

additional donor can switch the reaction pathway from extradiol

to intradiol and enhance the reactivity of the catecholate adducts

toward dioxygenation.

Fig. 13 Plot of the logrithm of the second-order reaction rate constant

vs. the energy of the lower energy LMCT band of [Fe(L)(DBC)(Sol)]⁺1–8

in DCM solution.

Conclusions

A series of iron(III) complexes of the type [Fe(L)Cl₃], where L

is a linear tri- (N₂O) and tripodal tetradentate (N₃O) ligands

containing one or two pyridine/imidazole moieties have been

isolated and studied as functional models for catechol dioxygenase

Fig. 12 Plot of the logarithm of the second-order reaction rate constant

vs. E_1/2(Fe^III/Fe^IIcouple) of [Fe(L)(Sol)₃]³⁺derived from 1–8 in DCM

solution.

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enzymes. The X-ray crystal structures of two of the complexes

reveal that both the N₂O and N₃O ligands are facially coordinated

with the latter also acting as a tridentate ligand with no ether

oxygen coordination. In the tetracholorcatecholate adducts, two

N₃O ligands are facially coordinated, the tetrachlorocatecholate

bidentatively coordinated in the plane of the two pyridine ni-

trogens and the ether donor remains uncoordinated. All of the

present complexes elicit major amounts of extradiol cleavage

products with enhancement in the extradiol-to-intradiol product

selectivity upon aquation/solvation of the catecholate adducts in

the non-coordinating solvent dichloromethane. Interestingly, the

iron(III) complexes with N₂O donor ligands yield higher amounts

of extradiol cleavage products with a sterically less hindering

imidazolyl N₂O ligand exhibiting the highest extradiol-to-intradiol

product selectivity. In contrast, the catecholate adducts of com-

plexes of N₃O ligands provide higher amounts of intradiol cleavage

products and react more than ten times faster than those of

N₂O ligands revealing the importance of coordination of ether

oxygen donor upon solvation of the catecholate adduct in the

non-coordinating DCM solvent. Thus the present study reveals

that the Lewis acidity of iron(III) center and steric demand of the

ligands, as regulated by the ligand donor function and central

N-R substituents, determine both the cleavage yields and rate of

dioxygenation in dichloromethane and DMF solvents.

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Acknowledgements

We sincerely thank the Council of Scientiﬁc and Industrial

Research, New Delhi for Senior Research Fellowships to K.S. We

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]. Professor M. Palaniandavar

is the recipient of the Ramanna Fellowship [SR/S1/RFIC-

01/2010] from DST, New Delhi. This work was also supported

by the Indo-French Centre (IFCPAR) [Scheme No. IFC/A/4109-

1/2009/993]. We thank Department of Science and Technology

(FIST programme) diffractometer facility at School of Chemistry,

Bharathidasan University. We thank Professor K. Natarajan,

Bharathiar University, Coimbatore for CHN analysis.

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Iron(III) complexes of N2O and N3O donor ligands as functional models for catechol dioxygenase enzymes: Ether oxygen coordination tunes the regioselectivity and reactivity

DOI: 10.1039/c0dt01598a

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Article abstract of DOI:10.1039/c0dt01598a

Full text of DOI:10.1039/c0dt01598a

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