Iron(iii) complexes of tripodal tetradentate 4N ligands as functional models for catechol dioxygenases: The electronic vs. steric effect on extradiol cleavage

Article abstract of DOI:10.1039/c3dt52145a

A few mononuclear iron(iii) complexes of the type [Fe(L)Cl₂]Cl 1-6, where L is a tetradentate tripodal 4N ligand such as N,N-dimethyl-N′,N′-bis(pyrid-2-ylmethyl)ethane-1,2-diamine (L1), N,N-diethyl-N′,N′-bis(pyrid-2-ylmethyl)ethane-1,2-diamine (L2), N,N-dimethyl-N′,N′-bis-(6-methylpyrid-2-ylmethyl)ethane-1,2-diamine (L3), N,N-dimethyl-N′-(pyrid-2-ylmethyl)-N′-(1-methyl-1H-imidazol-2-ylmethyl)ethane-1,2-diamine (L4), N,N-dimethyl-N′,N′-bis(1-methyl-1H-imidazol-2-ylmethyl)ethane-1,2-diamine (L5) and N,N-dimethyl-N′,N′-bis(quinolin-2-ylmethyl)ethane-1,2-diamine (L6), have been isolated and characterized by CHN analysis, UV-Visible spectroscopy and electrochemical methods. The complex cation [Fe(HL1)Cl₃]⁺1a possesses a distorted octahedral geometry in which iron is coordinated by the monoprotonated 4N ligand in a tridentate fashion and the remaining three sites of the octahedron are occupied by chloride ions. The DFT optimized octahedral geometries of 1, 5 and 6 contain iron(iii) with a high-spin (S = 5/2) ground state. The catecholate adducts [Fe(L)(DBC)]⁺, where H₂DBC is 3,5-di-tert-butylcatechol, of all the complexes have been generated in situ in acetonitrile solution and their spectral and redox properties and dioxygenase activities have been studied. The DFT optimized geometries of the catecholate adducts [Fe(L1)(DBC)]⁺, [Fe(L5)(DBC)]⁺ and [Fe(L6)(DBC)]⁺ have also been generated to illustrate the ability of the complexes to cleave H₂DBC in the presence of molecular oxygen to afford varying amounts of intra- (I) and extradiol (E) cleavage products. The extradiol to intradiol product selectivity (E/I, 0.1-2.0) depends upon the asymmetry in bidentate coordination of catecholate, as determined by the stereoelectronic properties of the ligand donor functionalities. While the higher E/I value obtained for [Fe(L6)(DBC)]⁺ is on account of the steric hindrance of the quinolyl moiety to coordination the lower value observed for [Fe(L4)(DBC)]⁺ and [Fe(L6)(DBC)]⁺ is on account of the electron-releasing effect of the N-methylimidazolyl moiety. Based on the data obtained it is proposed that the detachment of the -NMe₂ group from the coordination sphere in the semiquinone intermediate is followed for dioxygen binding and activation to yield the extradiol cleavage product. This journal is

Full text of DOI:10.1039/c3dt52145a

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Iron(III) complexes of tripodal tetradentate 4N
ligands as functional models for catechol
dioxygenases: the electronic vs. steric effect on
extradiol cleavage†
Cite this: Dalton Trans., 2014, 43,
14653
Mani Balamurugan,a Prabha Vadivelub and Mallayan Palaniandavar *a,c
A few mononuclear iron(III) complexes of the type [Fe(L)Cl2]Cl 1–6, where L is a tetradentate tripodal 4N
ligand such as N,N-dimethyl-N’,N’-bis(pyrid-2-ylmethyl)ethane-1,2-diamine (L1), N,N-diethyl-N’,N’-bis-
(pyrid-2-ylmethyl)ethane-1,2-diamine (L2), N,N-dimethyl-N’,N’-bis-(6-methylpyrid-2-ylmethyl)ethane-
1,2-diamine (L3), N,N-dimethyl-N’-(pyrid-2-ylmethyl)-N’-(1-methyl-1H-imidazol-2-ylmethyl)ethane-1,2-
diamine (L4), N,N-dimethyl-N’,N’-bis(1-methyl-1H-imidazol-2-ylmethyl)ethane-1,2-diamine (L5) and
N,N-dimethyl-N’,N’-bis(quinolin-2-ylmethyl)ethane-1,2-diamine (L6), have been isolated and character-
ized by CHN analysis, UV-Visible spectroscopy and electrochemical methods. The complex cation
[Fe(HL1)Cl3]+ 1a possesses a distorted octahedral geometry in which iron is coordinated by the mono-
protonated 4N ligand in a tridentate fashion and the remaining three sites of the octahedron are occupied
by chloride ions. The DFT optimized octahedral geometries of 1, 5 and 6 contain iron(III) with a high-spin
(S = 5/2) ground state. The catecholate adducts [Fe(L)(DBC)]+, where H2DBC is 3,5-di-tert-butylcatechol,
of all the complexes have been generated in situ in acetonitrile solution and their spectral and redox pro-
perties and dioxygenase activities have been studied. The DFT optimized geometries of the catecholate
adducts [Fe(L1)(DBC)]+, [Fe(L5)(DBC)]+ and [Fe(L6)(DBC)]+ have also been generated to illustrate the ability
of the complexes to cleave H2DBC in the presence of molecular oxygen to afford varying amounts of
intra- (I) and extradiol (E) cleavage products. The extradiol to intradiol product selectivity (E/I, 0.1–2.0)
depends upon the asymmetry in bidentate coordination of catecholate, as determined by the stereoelec-
tronic properties of the ligand donor functionalities. While the higher E/I value obtained for [Fe(L6)(DBC)]+
is on account of the steric hindrance of the quinolyl moiety to coordination the lower value observed for
[Fe(L4)(DBC)]+ and [Fe(L6)(DBC)]+ is on account of the electron-releasing effect of the N-methylimid-
azolyl moiety. Based on the data obtained it is proposed that the detachment of the –NMe2 group from
the coordination sphere in the semiquinone intermediate is followed for dioxygen binding and activation
to yield the extradiol cleavage product.
Received 6th August 2013,
Accepted 4th August 2014
DOI: 10.1039/c3dt52145a
www.rsc.org/dalton
carbon cycle. Biodegradation of naturally occurring aromatic
molecules by soil bacteria involves the oxidative cleavage of
Introduction
In nature, mononuclear non-heme iron enzymes perform a catechol and other dihydroxy aromatics, which are the carbon
variety of important biological functions to maintain the sources for their growth.1–5 Among them is the widely distribu-
ted mononuclear non-heme family of catechol dioxygenase
enzymes, which particularly catalyze the oxidative cleavage6 of
catechol or its derivatives with insertion of both oxygen atoms
of molecular oxygen into the aromatic ring of the substrate
and convert the aromatic compounds into aliphatic com-
pounds. Catechol dioxygenase enzymes can be classified into
two types (Scheme 1), the intradiol-cleaving catechol dioxy-
genases, which use a non-heme iron(III) centre to catalyze the
cleavage of the carbon–carbon bond between the two hydroxyl
groups to give muconic anhydride, and the extradiol-cleaving
catechol dioxygenases, which use a non-heme iron(II) centre to
aSchool of Basic and Applied Sciences, Department of Chemistry, Central University
of Tamil Nadu, Thiruvarur – 610 004, Tamil Nadu, India.
E-mail: palanim51@yahoo.com, palaniandavarm@gmail.com,
palaniandavar@cutn.ac.in
bChemical Sciences and Technology Division, National Institute for Interdisciplinary
Science and Technology, CSIR, Trivandrum 695019, India
cSchool of Chemistry, Bharathidasan University, Tiruchirappalli – 620 024,
Tamil Nadu, India
†Electronic supplementary information (ESI) available. CCDC 949988. For
ESI and crystallographic data in CIF or other electronic format see DOI:
10.1039/c3dt52145a
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Scheme 1 Mode of cleavage of intradiol and extradiol dioxygenases: active site structures of intradiol- (A) and extradiol-cleaving (B) catechol dioxy-
genase enzymes.
2
39
catalyze the cleavage of the carbon–carbon bond adjacent to propyl-1-pyrazolyl)borate (Tpi-pr
)
have been studied as func-
the two hydroxyl groups to yield 2-hydroxymuconic semi- tional models for extradiol-cleaving catechol dioxygenase
aldehyde as the product.7–12 The X-ray crystal structure of the enzymes and up to 97% of extradiol cleavage product for-
intradiol-cleaving protocatechuate 3,4-dioxygenase (3,4-PCD) mation has been observed. Also, very recently, Gebbink et al.
reveals that the enzyme possesses trigonal bipyramidal geome- isolated iron(II)-catecholate complexes of 3,3-bis(1-alkylimid-
try in which the iron(III) centre is coordinated by four protein azol-2-yl) propionate-derived ligands40 as structural and func-
ligands (Tyr408, Tyr447, His460 and His462) and the fifth posi- tional models for extradiol-dioxygenases. Palaniandavar et al.
tion is occupied by a solvent derived ligand.8–10 Upon substrate also successfully isolated iron(III) complexes of several linear
(protocatechuic acid, H2PCA) binding, the active site is con- 3N44 and 3NO45 ligands as functional models for the extradiol
verted into a square-pyramidal geometry and the axial Tyr447 dioxygenases and correlated the dioxygenation reaction rate as
and equatorial –OH are displaced by the substrate. In contrast well as extradiol cleavage yields with the ligand steric and elec-
to intradiol dioxygenases, extradiol dioxygenases contain an tronic factors. Interestingly, many iron(III) complexes of facially
iron(II) centre and possess a 2-his-1-carboxylate facial triad coordinated tridentate 3N ligands elicit mostly extradiol-clea-
structural motif ligated by two histidine and one glutamate vage as they have a vacant/labile coordination site for dioxygen
amino acid residue and two water molecules in the active site binding for extradiol cleavage to occur.35–43
forming a square pyramidal coordination geometry. In nature
Although several model complexes of 3N ligands have been
the biodegradation of aromatic molecules in the soil by extra- reported to exhibit extradiol cleavage, functional models with
diol cleavage exhibited by the extradiol-dioxygenase enzymes is 4N ligand donors exhibiting extradiol cleavage are very few in
the more common pathway and typically requires the iron(II) number as they have no vacant coordination site for dioxygen
centre to utilize an oxygen activation mechanism.8–17
binding. Jang and coworkers reported the first iron(II) complex
So, the development of synthetic model systems to investi- [Fe(BLPA)(DBCH)]BPh4, where BLPA is bis(6-methyl-2-pyridyl-
gate the mechanism of ring cleavage by extradiol-cleaving cate- methyl)(2-pyridylmethyl)amine and DBCH is 3,5-di-tert-butyl-
chol dioxygenases has gained importance. Several iron(II/III) catecholate monoanion, which elicits 65% intradiol cleavage
complexes have been successfully isolated as structural and products along with 20% of extradiol products upon reaction
functional models for both intradiol-18–34 and extradiol- with dioxygen.22 The iron(III) complex [Fe(L-N4H2)(DBC)]+,
dioxygenases.34–46 Earlier Que and co-workers synthesized where L-N4H2 is 2,11-diaza[3,3](2,6)pyridinophane, affords
several iron complexes of phenolate,18 carboxylate19 and nitro- both extra- and intradiol cleavage products.27 Even though the
gen20 donors as structural and functional models for these adduct [Fe(TPA)(DBC)]+, where TPA is the tripodal 4N ligand
enzymes. Funabiki et al. also modeled the catechol dioxygen- tris(pyrid-2-yl)methyl)amine, reacts with dioxygen to give 98%
ase enzymes using nitrogen and phenolate ligand donors.21 of the intradiol product,20 a few iron(III) complexes of modified
Krüger23 and Krebs25 reported several iron(III) complexes of 4N tetradentate 4N ligands such as [Fe(6-Me3TPA)(DBC)]+ and [Fe-
ligands and observed the predominant formation of intradiol (6-Me2BPMCN)(DBC)]+ do not have a vacant coordination site
rather than extradiol cleavage products. Several other research for dioxygen binding, and also affords measurable amounts
groups also used different donor functionalities for making (3–12%) of extradiol cleavage products upon exposure to dioxy-
both the structural and functional mimics of catechol gen.43 Very recently, we have isolated the iron(III) complex [Fe-
dioxygenases.22,24,26–33 Palaniandavar et al. also successfully (L)Cl2]+, where L is 1,4-bis(2-quinolylmethyl)-1,4-diazepane,
isolated several iron(III) complexes with pyridine/imidazole/ which yields the highest amount of extradiol cleavage products
benzimidazole and mono- and bis-phenolate ligand donor (85%) in the presence of one equivalent of triethylamine and
functionalities,34 which are found to yield intradiol cleavage dioxygen.46 However, the factors which determine the selec-
products exclusively or along with small amounts of extradiol tivity of the cleavage pathway for the complexes with 4N
cleavage products. Also, a few synthetic iron(III)-catecholate ligands are still unclear and need more studies. This prompted
complexes of tridentate cis-facially coordinating 3N ligands us to study the cleavage of catechols by iron(III) complexes of
like triazacyclononane (TACN)35–38 and hydridotris(3,5-di-iso- tripodal tetradentate 4N ligands (Scheme 2) as functional
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Scheme 2 Structures of 4N ligands used in the study.
1
models for catechol dioxygenase enzymes, with an aim to (92%). H NMR (400 MHz, CDCl3): δ 1.12 (t, 6H), 2.28 (q, 4H),
obtain extradiol cleavage exclusively and to collect evidence for 2.49 (t, 2H), 2.58 (t, 2H), 3.83 (s, 4H), 7.18 (t, 2H), 7.49 (d, 2H),
+
the mechanistic pathway of the substrate-bound complex to 7.68 (t, 2H), 8.61 (d, 2H). EI-MS m/z = 298.21 C18H26N4 .
facilitate dioxygen attack on the iron(III) centre to yield extra-
N,N-Dimethyl-N′,N′-bis(6-methylpyrid-2-ylmethyl)ethane-1,2-
diol cleavage products. Also, we have already shown that incor- diamine (L3). The procedure employed for L1 was used for the
poration of the sterically demanding –NMe2 donor group in preparation of L2. 6-Methylpyridine-2-carboxaldehyde was
iron(III)-phenolate complexes45 increases the Fe–O–C bond used in place of pyridine-2-carboxaldehyde. The colourless oil
angle (133°, 148°) and hence the dioxygenase activity.
formed was used without further purification for complex
preparation. Yield: 1.25 g (84%). 1H NMR (400 MHz, CDCl3):
δ 2.18 (s, 6H), 2.41 (t, 2H), 2.59 (s, 6H), 2.65 (t, 2H), 3.85 (s,
4H), 7.11 (d, 2H), 7.46 (d, 2H), 7.89 (t, 2H). EI-MS m/z = 298.2
Experimental section
Materials
+
C18H26N4 .
N,N-Dimethyl-N′-(1-methyl-1H-imidazol-2-ylmethyl)-N′-(pyrid-
2-ylmethyl)ethane-1,2-diamine (L4). The ligand was prepared
Pyridine-2-carboxaldehyde,
N,N-dimethylethylenediamine,
as reported49 elsewhere. Yield: 1.12
g
(82%). 1H NMR
N,N-diethylethylenediamine, sodium triacetoxyborohydride,
sodium borohydride, 1-methylimidazole-2-carboxaldehyde,
3,5-di-tert-butylcatechol (H2DBC), 4-tert-butylcatechol (H2TBC)
(Aldrich), 6-methylpyridine-2-carboxaldehyde, quinoline-2-carb-
oxaldehyde (Alfa Aesar), 3,4,5,6-tetrachlorocatechol (H2TCC)
(Lancaster), protocatechuic acid (3,4-dihydroxybenzoic acid,
H2PCA) (Loba, India) and iron(III) chloride (anhydrous) (Merck,
India) were used as received. H2DBC was recrystallized from
hexane before use. The supporting electrolyte tetrabutyl-
ammonium perchlorate (Bu4ClO4, G. F. Smith, USA) was pre-
pared using the procedure already reported47 and recrystallized
twice from aqueous ethanol. Methanol (Sisco Research Labora-
tory, Mumbai), acetonitrile, dichloromethane, diethylether and
tetrahydrofuran (Merck, India) were distilled before use.
(400 MHz, CDCl3): δ 2.13 (s, 6H), 2.44 (t, 2H), 2.63 (t, 2H), 3.67
(s, 3H), 3.58 (s, 2H), 3.78 (s, 2H), 6.78 (s, 1H), 6.86 (s, 1H), 7.17
(t, 1H), 7.46 (d, 1H), 7.63 (t, 1H), 8.58 (d, 1H). EI-MS m/z =
+
273.2 C15H23N5 .
N,N-Dimethyl-N′,N′-bis(1-methyl-1H-imidazol-2-ylmethyl)-
ethane-1,2-diamine (L5). The ligand was prepared as
1
reported49 elsewhere. Yield: 1.05 g (76%). H NMR (400 MHz,
CDCl3): δ 2.14 (s, 6H), 2.41 (t, 2H), 2.59 (t, 2H), 3.68 (s, 6H),
3.59 (s, 4H), 6.80 (s, 2H), 6.87 (s, 2H). EI-MS m/z = 276.2
+
C14H24N6 .
N,N-Dimethyl-N′,N′-bis(quinolin-2-ylmethyl)ethane-1,2-diamine
(L6). The ligand was prepared as reported49 elsewhere. Yield:
1
1.28 g (69%). H NMR (400 MHz, CDCl3): δ 2.13 (s, 6H), 2.42
(t, 2H), 2.62 (t, 2H), 3.80 (s, 4H), 7.15 (d, 2H), 7.42 (t, 2H), 7.57
Syntheses of ligands
(t, 2H), 7.67 (d, 2H), 7.89 (d, 2H), 8.13 (d, 2H). EI-MS m/z =
+
370.2 C24H26N4 .
N,N-Dimethyl-N′,N′-bis(pyrid-2-ylmethyl)ethane-1,2-diamine
(L1). The ligand was prepared as reported48 elsewhere. Yield:
1
Preparation of iron(III) complexes
1.19 g (88%). H NMR (400 MHz, CDCl3): δ 2.18 (s, 6H), 2.41
(t, 2H), 2.65 (t, 2H), 3.87 (s, 4H), 7.15 (t, 2H), 7.53 (d, 2H), 7.65
(t, 2H), 8.51 (d, 2H). EI-MS m/z = 270.1 C16H22N4 .
[Fe(L1)Cl2]Cl 1. A methanol solution (5 mL) of anhydrous
FeCl3 (0.162 g, 1 mmol) was added to a methanol solution
+
N,N-Diethyl-N′,N′-bis(pyrid-2-ylmethyl)ethane-1,2-diamine (L2). (5 mL) of L1 (0.27 g, 1 mmol) with stirring at room tempera-
The ligand was prepared as reported49 elsewhere. Yield: 1.38 g ture for 30 min. The yellow complex (0.30 g, 82%) was filtered
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off, washed with small amounts of cold methanol and diethyl- analysis was performed on an Agilent GC-MS equipped with
ether and dried under vacuum. Yield, 0.33 g, 78%. ESI-MS m/z 7890A GC series (HP-5 capillary column) and 5975C inert
= 396 [Fe(L1)Cl2]+. Anal. Calcd C16H22N4Cl3Fe: C, 44.42; H, MSD. The Density Functional Theory (DFT) calculations were
5.13; N, 12.95. Found: C, 44.40; H, 5.14; N, 12.89.
performed with the g09 program56 using the B3LYP basis func-
[Fe(HL1)Cl2]Cl2 1a. A methanol solution of one equiv. of tion. While the metal center (Fe) was described by the
HCl was added to the acetonitrile solution (5 mL) of 1 with LANL2DZ basis set along with the associated RECPs, the rest
stirring at room temperature and stirring was continued for of the atoms were described by the 6-31G** basis set.
30 min. The slow evaporation of the methanol–acetonitrile
Crystallographic refinement and structure solution
solution of the complex gave yellow crystals, which are suitable
for X-ray crystallographic analysis. Yield, 0.38 g, 82%. Anal. The diffraction experiments were carried out on a Bruker
Calcd C16H23N4Cl4Fe: C, 40.97; H, 4.94; N, 11.95. Found: C, SMART APEX diffractometer equipped with a CCD area detec-
40.93; H, 4.88; N, 11.92.
tor. High quality crystals, suitable for X-ray diffraction, were
[Fe(L2)Cl2]Cl 2. The complex 2 was prepared using the pro- chosen after careful examination under an optical microscope.
cedure employed for isolating 1. Yield, 0.33 g, 72%. ESI-MS Intensity data for the crystal was collected using MoKα (λ =
m/z = 424 [Fe(L2)Cl2]+. Anal. Calcd C18H26N4Cl3Fe: C, 46.93; H, 0.71073 Å) radiation on a Bruker SMART APEX diffractometer
5.69; N, 12.16. Found: C, 46.89; H, 5.68; N, 12.14.
equipped with a CCD area detector at 293 K. The SMART50
[Fe(L3)Cl2]Cl 3. The complex 3 was prepared using the pro- program was used for collecting frames of data, indexing the
cedure employed for isolating 1. Yield, 0.35 g, 76%. ESI-MS reflections, and determining the lattice parameters; the
m/z = 424 [Fe(L3)Cl2]+. Anal. Calcd C18H26N4Cl3Fe: C, 46.93; H, SAINT50 program was used for integration of the intensity of
5.69; N, 12.16. Found: C, 46.92; H, 5.65; N, 12.13.
reflections and scaling. An empirical absorption correction
[Fe(L4)Cl2]Cl 4. The complex 4 was prepared using the pro- was applied to the collected reflections with SADABS.51 The
cedure employed for isolating 1. Yield, 0.30 g, 71%. ESI-MS structure was solved by direct methods using SHELXTL52 and
m/z = 399 [Fe(L4)Cl2]+. Anal. Calcd C15H23N5Cl3Fe: C, 41.36; H, refined on F2 by the full-matrix least-squares technique using
5.32; N, 16.08. Found: C, 41.39; H, 5.37; N, 16.05.
the SHELXL-9752 package. The structure was solved by the
[Fe(L5)Cl2]Cl 5. The complex 5 was prepared using the pro- heavy atom method and other non-hydrogen atoms were
cedure employed for isolating 1. Yield, 0.29 g, 67%. ESI-MS located in successive difference Fourier syntheses. Crystal data
m/z = 402 [Fe(L5)Cl2]+. Anal. Calcd C14H24N6Cl3Fe: C, 38.34; H, and additional details of the data collection and refinement of
5.52; N, 19.16. Found: C, 38.33; H, 5.46; N, 19.11.
the structure are presented in Table 1. The selected bond
[Fe(L6)Cl2]Cl 6. The complex 6 was prepared using the pro- lengths and bond angles are listed in Table 2.
cedure employed for isolating 1. Yield, 0.37 g, 70%. ESI-MS
Reactivity studies
m/z = 496 [Fe(L6)Cl2]+. Anal. Calcd C24H26N4Cl3Fe: C, 54.11; H,
4.92; N, 10.52. Found: C, 54.08; H, 4.86; N, 10.49.
The catechol cleavage activity of all of the complexes toward
H2DBC was examined by exposing a solution of an iron(III)-
Physical measurements
DBC2− adduct generated in situ in acetonitrile (ACN) to mole-
Elemental analyses were performed on a Perkin Elmer Series II cular oxygen. Kinetic analyses of the catechol cleavage reac-
CHNS/O Analyzer 2400. 1H NMR spectra were recorded on a tions were carried out by time-dependent measurement of the
Bruker 400 MHz NMR spectrometer. The electronic spectra disappearance of the lower-energy DBC2−-to-iron(III) LMCT
were recorded on an Agilent-8453 diode array spectrophoto- band. The solvents were equilibrated at the atmospheric
meter. Mass spectrometry was performed on a QTOF ESI-MS pressure of O2 at 25 °C and the solubility of O2 in acetonitrile
spectrometer. Cyclic voltammetry (CV) and differential pulse at 25 °C is 8.1 × 10−3 M.53 A stock solution of the adducts
voltammetry (DPV) were performed using a three electrode cell [Fe(L)(DBC)]+ was prepared by treating the complexes 1–6
configuration. A platinum sphere, a platinum plate and Ag(s)/ (6 × 10−3 M) with an equivalent amount of H2DBC pretreated
Ag+ were used as working, auxiliary and reference electrodes with two equivalents of Et3N. Oxygenation was started by rapid
respectively. The supporting electrolyte used was Bu4ClO4 delivery of a stock solution (0.2 mL) of the catecholate adducts
(TBAP). The temperature of the electrochemical cell was main- using a syringe to the O2-saturated solvent (2.8 mL). The
tained at 25.0 0.2 °C using a cryocirculator (HAAKE D8 G). By product analysis was carried out by adding the [Fe(L)Cl2]+
bubbling research grade nitrogen the solutions were deoxyge- (0.1 mmol), H2DBC (0.1 mmol) and triethylamine (0.2 mmol)
nated and an atmosphere of nitrogen was maintained over the to acetonitrile (15 mL) solvent under molecular oxygen and
solutions during measurements. The E1/2 values were observed stirring for 24 h at room temperature. The oxygenation reac-
under identical conditions for various scan rates. The instru- tion was quenched by the addition of 6 M HCl (5 mL) and
ments utilized included an EG & G PAR 273 Potentiostat/ the products were extracted from the aqueous solution with
Galvanostat and a Pentium-IV computer along with the EG & G diethylether (3 × 10 mL). The clear yellow organic layer was
M270 software to carry out the experiments and to acquire the separated and dried over anhydrous Na2SO4 at room tempera-
data. The product analysis was performed using a HP 6890 GC ture. All the products were quantified using a GC (FID) with
series Gas Chromatograph equipped with a FID detector and a the following temperature program: injector temperature
HP-5 capillary column (30 m × 0.32 mm × 2.5 μm) and GC-MS 130 °C; initial temperature 60 °C, heating rate 10 °C min−1 to
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Table 1 Crystallographic data for 1a
Results and discussion
1a
Synthesis and characterization of ligands and iron(III)
complexes
Empirical formula
C
16H23N4Cl4Fe
Formula weight/g mol−1
469.03
The tripodal tetradentate 4N ligands L1–L6 (Scheme 2) were
synthesized according to known procedures,48,49 which involve
Schiff base condensation and reductive amination. All the
ligands, except for L4, were synthesized by reductive amination
of N,N-dimethylethylenediamine or N,N-diethylethylenedi-
amine with two moles of pyridine-2-carboxaldehyde (L1, L2) or
6-methyl- pyridine-2-carboxaldehyde (L3) or 1-methylimid-
azole-2-carboxaldehyde (L5) or quinoline-2-carboxaldehyde
(L6) using sodium triacetoxyborohydride as a reducing agent
and were characterized by 1H NMR spectroscopy and mass
spectrometry. The ligand L4 was prepared by condensation of
N,N-dimethylethylenediamine with pyridine-2-carboxaldehyde,
followed by reduction of the resulting Schiff base, and reduc-
tive amination with 1-methylimidazole-2-carboxaldehyde. The
iron(III) complexes [Fe(L)Cl2]Cl 1–6 were isolated by treating
anhydrous FeCl3 with the corresponding ligands L1–L6 in
methanol with stirring. The complex 1a was isolated by adding
one equivalent of hydrochloric acid to an acetonitrile solution
of 1. All the complexes were characterized by elemental ana-
lysis and electronic spectroscopy and were formulated as
[Fe(L)Cl2]Cl based on elemental analysis. Conductivity
measurements in acetonitrile solution (ΛM, 80–125 Ω−1 cm2
mol−1) show that all the complexes behave as 1 : 1 electrolytes,
and that both the chloride ligands remain coordinated. So, the
octahedral complexes are formulated as [Fe(L)Cl2]Cl. The X-ray
crystal structure confirms the formulation of 1a as [Fe(HL1)-
Cl3]Cl. The tripodal ligands L1–L6 with different donor func-
tions are expected to display varying stereoelectronic effects
and determine the pathway of catechol cleavage.
Crystal habit, colour
Crystal system
Crystal size
Space group
a/Å
b/Å
c/Å
α/°
Yellow
Monoclinic
0.20 × 0.16 × 0.16 mm
P21/c
12.9185(4)
7.2101(2)
23.6891(6)
90.000
102.369(2)
90
2155.27(10)
4
1.445
964.0
293
18 279
5173
0.71073
1.040
227
0.0457/0.1099
0.0639/0.1198
β/°
γ/°
V/Å3
Z
ρcalcd/g cm−3
F(000)
T/K
No. of reflections collected
No. of unique reflections
Radiation(MoKα)/Å
Goodness-of-fit on F2
Number of refined parameters
R1/wR2[I > 2s(I)]a
R1/wR2 (all data)
a R1 = [∑(kFo| − |Fck)/∑|Fo|]; wR2 = {[∑(w(Fo2 − Fc2)2)/∑(wFo4)]1/2}.
Table 2 Selected bond lengths [Å] and bond angles [°] for 1a
1a
Bond lengths/Å
Fe(1)–N(1)
Fe(1)–N(2)
Fe(1)–N(3)
Fe(1)–Cl(1)
Fe(1)–Cl(2)
Fe(1)–Cl(3)
2.177(3)
2.271(3)
2.200(7)
2.2769(11)
2.2822(12)
2.3010(11)
Description of the crystal structure of [Fe(HL1)Cl3]Cl 1a
Bond angles/°
The molecular structure of the complex cation [Fe(HL1)Cl3]+ of
1a is shown in Fig. 1 together with the atom numbering
scheme and the selected bond lengths and bond angles are
presented in Table 2. The complex cation possesses a distorted
octahedral coordination geometry around iron(III) in which the
two pyridyl nitrogen atoms (N1 and N3) and the central tertiary
amine nitrogen atom (N2) of the tetradentate 4N ligand are
facially coordinated to iron(III) and the remaining three coordi-
nation sites of the octahedron are occupied by chloride ions.
The terminal tertiary amine nitrogen (N4) is protonated and so
the tetradentate 4N ligand acts as a 3N donor. The Fe–Npy
(2.177(3), 2.200(7) Å) and Fe–Namine bond distances (2.271(3)
Å) fall respectively in the ranges 2.143(4)–2.227(4) Å and
N(1)–Fe(1)–N(3)
N(1)–Fe(1)–N(2)
N(3)–Fe(1)–N(2)
N(1)–Fe(1)–Cl(1)
N(3)–Fe(1)–Cl(1)
N(2)–Fe(1)–Cl(1)
N(1)–Fe(1)–Cl(2)
N(3)–Fe(1)–Cl(2)
N(2)–Fe(1)–Cl(2)
Cl(1)–Fe(1)–Cl(2)
N(1)–Fe(1)–Cl(3)
N(3)–Fe(1)–Cl(3)
N(2)–Fe(1)–Cl(3)
Cl(1)–Fe(1)–Cl(3)
Cl(2)–Fe(1)–Cl(3)
78.38(11)
77.69(12)
76.85(12)
90.29(9)
165.74(9)
92.41(8)
93.52(10)
91.30(9)
166.38(8)
98.08(4)
165.20(10)
91.09(9)
89.88(8)
98.33(4)
97.13(5)
130 °C, then increasing at a rate 2 °C min−1 to 160 °C and 2.160(5)–2.334(7) Å observed for mononuclear iron(III) com-
then increasing at a rate 5 °C min−1 to 260 °C; FID tempera- plexes reported in the literature.34,44 The Fe–Npy bonds are
ture 280 °C. The products were analyzed by GC three times shorter than the Fe–Namine bond due to sp2 and sp3 hybridiz-
and the average yields are listed in Table 2. GC-MS analysis ations respectively of the pyridyl and tertiary amine nitrogen
was performed under conditions identical to those used for atoms.25,44,54,55 Also, the lone-pair orbital on the central amine
GC analysis and the oxygenation products were identified by nitrogen atom is not oriented exactly toward the iron(III)
comparing the observed retention times with those reported orbital, rendering the Fe–Namine bond longer. Furthermore, all
already.41,44
the Fe–Cl bonds (2.2769(11), 2.2822(12), 2.3010(11) Å) are
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amine nitrogen atoms. However, the other Fe–Npy bond
(Fe–N3, 2.28 Å) is longer than the Fe–Namine bond (Fe–N4,
2.27 Å) due to the trans effect exerted by the chloride ion trans
to the pyridyl N3 nitrogen. The optimized structure of [Fe(L5)-
Cl2]+ 5 is similar to that of 1 and the Fe–N4 (1, 2.27; 5, 2.28 Å),
Fe–Cl1 (1, 2.32; 5, 2.33 Å) and Fe–Cl2 (1, 2.26; 5, 2.25 Å) bond
lengths are nearly equal to the respective bonds in 1. The
Fe–Nim bond in 5 (Fe–N3, 2.17 Å) is shorter than the Fe–Npy
bond in 1 (Fe–N3, 2.28 Å) revealing that the imdazolyl nitrogen
in 5 is coordinated more strongly than the pyridyl nitrogen in
1 (pKa, BH+: ImH+, 7.2; pyH+, 5.6) and concomitantly, the
Fe–Namine (Fe–N2, 2.43 Å) and Fe–N1 (1, 2.18; 5, 2.16 Å) bonds
are weaker than those in 1 (Fe–N2, 2.31 Å). The stronger
coordination of the imidazolyl ligand in 5 is expected to stabil-
ize its HOMO, which is reflected in the larger SOMO–LUMO
gap in 5 (0.23 a.u) than that in 1 (0.20 a.u). The optimized
structure of [Fe(L6)Cl2]+ 6 is also similar to that of 1 and the
Fe–N4amine (2.28 Å), Fe–Cl1 (2.34 Å) and Fe–Cl2 (2.25 Å) bond
lengths are nearly equal to those in 1. The Fe–N3amine bond in
6 (Fe–N3, 2.26 Å) is shorter than the Fe–N3amine bond in 1 due
to weaker coordination of one of the quinolyl nitrogen donors
(Fe–N2, 2.53 Å), revealing that the steric effect of the quinolyl
donor is more important than its electronic effect.
The optimized geometry of 1 is helpful in discussing the
effect of protonation of complex 1. Upon protonation of the
–NMe2 donor in 1, the Fe–NMe2 coordinate bond is broken
and concomitantly the Fe–Namine (1, 2.31; 1a, 2.27 Å) and
Fe–Cl1 (1, 2.32; 1a, 2.27 Å) bonds become stronger and the
Fe–N3 bond (1, 2.28; 1a, 2.20 Å) becomes weaker with the
Fe–Npy bond strength (Fe–N1, 1, 2.18; 1a, 2.17 Å) remaining
constant.
Fig. 1 Molecular structure of [Fe(HL1)Cl3]Cl 1a (50% probability factor
for the thermal ellipsoid). Hydrogen atoms have been omitted for clarity.
longer than the Fe–Npy and Fe–Namine bonds, as expected. The
N–Fe–N, N–Fe–Cl and Cl–Fe–Cl (76.85(12)–98.33(4)°) and
N–Fe–Cl (165.20(10)–166.38(8)°) bond angles deviate from the
ideal octahedral angles of 90° and 180°, respectively, revealing
the presence of significant distortion in the iron(III) coordi-
nation geometry.
Upon displacing the chloride ions in 1 by bidentate DBC2−
dianions to obtain the adduct [Fe(L1)(DBC)]+ (Fe–O1, 2.08; Fe–
O2, 2.01 Å), both the Fe–N1py (2.27 Å) and Fe–N4amine (2.35 Å)
bonds are weakened while the Fe–N2py (2.20 Å) and Fe–N3amine
bonds (2.30 Å) are strengthened. Similarly, upon displacing
the chloride ions in 5 by DBC2− to form [Fe(L5)(DBC)]+, the
Structures of 1, 5 and 6 and their DBC2− adducts: density
functional theory calculations
A DFT study was performed to investigate the geometries of 1, Fe–N4amine (2.37 Å) bond is weakened while the Fe–N3amine
5 and 6 and their DBC2− adducts. Initially, as a benchmark cal- (2.38 Å) bond is strengthened. As in 5, the Fe–N2im bond
culation, the crystal structure of 1a was optimized and the (2.16 Å) in the adduct is stronger than the corresponding
computed geometry is in good agreement with the experi- Fe–N2py bond in [Fe(L1)(DBC)]+; however, the Fe–N1im (2.27 Å)
mentally determined structure, except for slight elongations in bond is equal to that of the Fe–N1py (2.27 Å) bond. Also, as in
bond lengths ranging from 0.02 to 0.21 Å (Fig. S1†). So, the 5 and 1, the spin density of the iron(III) center in 5·DBC2−
same computational methodology was followed for optimizing (ρ, 3.77) is higher than that in 1·DBC2− (ρ, 3.75) and the
the geometries of 1, 5 and 6 and their DBC2− adducts. Like 1a, SOMO–LUMO gap is larger in 5·DBC2− (0.15 a.u) than that in
the complexes 1, 5 and 6 also exhibit distorted octahedral coordi- 1·DBC2− (0.12 a.u). Upon displacing the chloride ions in 6 by
nation geometries (Fig. 2) containing iron(III) with high-spin (S = DBC2− to give 6·DBC2−, the Fe–N4amine (2.37 Å) bond becomes
5/2) rather than the low-spin (S = 1/2) ground state (Table 3). weaker, as for 1·DBC2− and 5·DBC2−. Interestingly, the weakly
The optimized geometries of the catecholate adducts of these coordinated quinolyl moiety in 6 becomes coordinated more
complexes, namely [Fe(L1)(DBC)]+ (1·DBC2−), [Fe(L5)(DBC)]+ strongly (Fe–N2quin, 2.26 Å) and the other quinolyl nitrogen
(5·DBC2−) and [Fe(L6)(DBC)]+ (6·DBC2−), with high-spin state becomes weakly bound (Fe–N2quin, 2.33 Å). Also, the Fe–Nquin
(S = 5/2) are provided also in Fig. 2.
bonds are weaker than the corresponding Fe–Npy bonds in
The complex [Fe(L1)Cl2]+ 1 possesses a distorted octahedral 1·DBC2−, as expected (cf. above). Thus for 1, 5 and 6 the
geometry and the Fe–Npy bond (Fe–N1, 2.18 Å) is shorter than Fe–NMe2 bond is elongated upon adduct formation with
the Fe–Namine bond (Fe–N2, 2.31; Fe–N4, 2.27 Å) due to sp2 DBC2−. Also, a remarkable variation in asymmetry of the coor-
and sp3 hybridizations respectively of the pyridyl and tertiary dinate bonds of DBC2− (1, Fe–O1, 2.08; Fe–O2, 2.01: 5, Fe–O1,
14658 | Dalton Trans., 2014, 43, 14653–14668
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Fig. 2 The optimized geometries of [Fe(L1)Cl2]+ (1), [Fe(L5)Cl2]+ (5) and [Fe(L6)Cl2]+ (6) and their catecholate derivatives [Fe(L1)(DBC)]+ (1·DBC2−),
[Fe(L5)(DBC)]+ (5·DBC2−) and [Fe(L6)(DBC)]+ (6·DBC2−) in their S = 5/2 spin state. Selected bond lengths (Å) along with the key spin densities are
given.
2.04; Fe–O2, 2.02: 6, Fe–O1, 2.02; Fe–O2, 2.01 Å), as dictated by (Table 4), which is assigned to the Cl− → Fe(III) ligand to metal
the stereoelectronic factors of ligand donor functionalities, charge transfer (LMCT) transition.25 The more intense band in
has been observed (cf. below).
the range 240–250 nm is caused by π→π* transitions within
the ligands. When H2DBC pretreated with two equivalents of
Et3N is added to 1–6 in acetonitrile, two catecholate-to-iron(III)
LMCT bands appear in the ranges 500–570 and 850–920 nm
(Fig. 3, Table 4), which are assignable to DBC2−-to-Fe(III) LMCT
Electronic absorption spectral studies
The electronic absorption spectra of [Fe(L)Cl2]Cl 1–6 in aceto-
nitrile solution exhibit a band in the range 350–370 nm
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[Fe(L1)(DBC)]+ by one and two of the imidazolyl arms to get
respectively [Fe(L4)(DBC)]+ and [Fe(L5)(DBC)]+ the LMCT
bands are shifted to higher energies in both the adducts. The
stronger coordination of the imidazolyl group (cf. above) raises
the energy of the iron(III) dπ* orbital, leading to higher LMCT
band energies. However, upon replacing the pyridylmethyl
arms in [Fe(L1)(DBC)]+ by the sterically hindering quinolyl-
methyl arms the LMCT bands are shifted to lower energies as
observed for [Fe(L3)(DBC)]+ with sterically hindering 6-methyl-
pyridyl arms (cf. above). Thus the Lewis acidity of the iron(III)
Table 3 Relative stability of [Fe(L1/L5)(Cl2)]+ and [Fe(L1/L5)(DBC)]+
complexes in their high-spin (S = 5/2) and low-spin (S = 1/2) states. Free
energies in kJ mol−1
Relative stability in kJ mol−1
Complexes
High spin (S = 5/2)
Low spin (S = 1/2)
[Fe(L1)(Cl2)]+
[Fe(L1)(DBC)]+
[Fe(L5)(Cl2)]+
[Fe(L5)(DBC)]+
0.0
0.0
0.0
0.0
+53.2
+42.3
+64.8
+58.0
transitions22–34,44–46 involving two different catecholate orbi-
tals on the chelated DBC2−. When one mole of Et3N is added
to the catecholate adducts no change in absorptivity of the
bands is observed, suggesting that H2DBC is completely de-
protonated and is coordinated to iron(III) bound to all the four
nitrogen atoms of the tetradentate ligand (Scheme 3).20,25–27,46
The energies of both the DBC2−-to-Fe(III) LMCT bands of
the adducts [Fe(L)(DBC)]+ show a remarkable dependence on
the ligand donors of the primary ligands: L1 > L2 > L3 < L4 <
L5 > L6.20–34,43–46 Similar trends in band energies are observed
for the high energy band of all the adducts of other catechols.
Upon replacing the –NMe2 group in [Fe(L1)(DBC)]+ by the
–NEt2 group to obtain [Fe(L2)(DBC)]+ both the LMCT bands
are shifted to lower energies. The weaker coordination of the
sterically hindering –NEt2 group causes a decrease in the nega-
tive charge on the iron(III) centre, which stabilizes the dπ* orbi-
tals of iron(III) leading to a decrease in energy gap18 between
the dπ* orbital and the ligand catecholate orbitals, and hence
the observed decrease in the catecholate-to-iron(III) LMCT
band energy. Similarly, upon replacing both the pyridyl arms in
[Fe(L1)(DBC)]+ by 6-methylpyridyl arms to obtain [Fe(L3)(DBC)]+
the LMCT bands are shifted to lower energies. The steric bulk
of the 6-Me group renders the lone pair orbital on pyridyl
nitrogen not oriented exactly towards the iron(III) orbital and
causes the negative charge built on the iron(III) centre to
decrease. Upon replacing one and two of the pyridyl arms in
Fig. 3 Electronic absorption spectra of adducts of 2 (2 × 10−4 M) gen-
erated in situ by adding equimolar amounts of various catecholate di-
anions in acetonitrile solution: [Fe(L2)(DBC)]+ (a), [Fe(L2)(3-MCAT)]+ (b),
[Fe(L2)(CAT)]+ (c), [Fe(L2)(TCC)]+ (d).
Scheme 3 Schematic illustration of coordination of DBC2− to the
iron(III) centre.
Table 4 Electronic spectral data (λmax in nm, εmax in M−1 cm−1 in parenthesis) for iron(III) complexesa and their catecholateb adducts in acetonitrile
solution
Added catecholate
Nonea
[Fe(L1)Cl2]Cl
[Fe(L2)Cl2]Cl
[Fe(L3)Cl2]Cl
[Fe(L4)Cl2]Cl
[Fe(L5)Cl2]Cl
[Fe(L6)Cl2]Cl
348 (3700)
317 (7150)
250 (13 880)
870 (2045)
530 (1780)
804 (1835)
518 (1620)
778 (1845)
508 (1655)
685 (1915)
520 (2065)
359 (2850)
304 (2940)
250 (8040)
880 (2320)
534 (1665)
828 (2035)
530 (1650)
788 (2020)
478 (1660)
700 (2105)
493 (1880)
350 (4735)
290 (6305)
253 (11 050)
910 (2305)
570 (3165)
875 (1790)
545 (2810)
810 (1690)
508 (2490)
750 (sh)
362 (4500)
304 (7640)
250 (14 835)
855 (1990)
527 (1890)
814 (1800)
491 (1885)
774 (1815)
478 (1930)
710 (sh)
365 (3865)
304 (4855)
240 (7840)
780 (2250)
502 (2470)
755 (1515)
500 (1600)
735 (1655)
480 (1740)
720 (sh)
369 (4380)
296 (4770)
250 (9740)
920 (2480)
563 (3025)
866 (1830)
512 (2685)
818 (1745)
490 (2290)
765 (sh)
DBC2− b
3-MCAT2− b
CAT2− b
TCC2− b
521 (2555)
505 (2610)
522 (2500)
517 (2675)
a Concentration of the iron(III) complexes, 4 × 10−4 M. The ratio of the added ligand to iron(III) complexes was 1 : 1; the anions were generated by
adding 2 equiv. of triethylamine. b DBC2− = 3,5-di-tert-butylcatechol; 3-MCAT2− = 3-methylcatechol; CAT2− = catechol; TCC2− = 3,4,5,6-
tetrachlorocatechol.
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Table 5 Electrochemical data for [Fe(L)Cl2]+ a and [Fe(L)(DBC)]+ b in
acetonitrile at 25.0
5 mV s−1 (DPV)
center in the catecholate adducts is modified upon changing
the primary ligand environment. Also, the position of the low
energy catecholate-to-Fe(III) LMCT band in the complex–sub-
strate adducts [Fe(L)(catecholate)]+ generated from [Fe(L)Cl2]+
and two equivalents of Et3N is found to be shifted to higher
energies as the substituents20–34 on the catecholate ring are
varied from electron-releasing to electron-withdrawing as
0.2 °C at a scan rate of 50 mV s−1 (CV) and
E1/2 (V)
Redox
process
Complexes
Epc/V
Epa/V
CV
DPV
observed20,25,34,46 previously: DBC2− > 3Me-CAT2− > CAT2−
>
[Fe(L1)Cl2]Cl
−0.064
0.036 −0.014
0.045 0.013
0.022 FeIII → FeII
0.008 FeIII → FeII
0.345 DBSQ → DBC
−0.010 FeIII → FeII
TCC2− (Table 4). This is expected as the electron-releasing sub-
stituents on the catecholate ring would decrease the energy of
the low energy band while the electron-withdrawing substitu-
ents enhance it,20–34,46 thus reflecting the importance of elec-
tronic effects expressed by the substituents on catechols.
When 1a is treated with one equivalent of H2DBC no spec-
tral change is observed. On the addition of one equivalent of
Et3N no spectral change is observed, revealing that the added
base is used to abstract the proton from the –+N(H)Me2 moiety
rather than H2DBC. On adding a second equivalent of Et3N to
the reaction mixture two characteristic catecholate-to-iron(III)
LMCT bands are observed illustrating that the deprotonation
of H2DBC is followed by catecholate adduct formation.
Addition of a third equivalent of Et3N leads to an increase in
intensities of the LMCT bands and the spectra obtained are in
good agreement with that for [Fe(L1)(DBC)]+. The addition of a
fourth equivalent of Et3N fails to effect any significant change
in the spectral bands. All these observations reveal that three
equivalents of Et3N are required to effect the coordination of
the –NMe2 group as well as the DBC2− dianion to the iron(III)
centre bound to all the four nitrogens of the 4N ligand.
[Fe(L1)Cl2]Cl + H2DBC −0.072
[Fe(L1)(DBC)]+
[Fe(L2)Cl2]Cl
−0.125 −0.014 −0.069 −0.037 FeIII → FeII
[Fe(L2)Cl2]Cl + H2DBC −0.120 −0.012 −0.066 −0.043 FeIII → FeII
[Fe(L2)(DBC)]+
0.290 DBSQ → DBC
−0.048 FeIII → FeII
[Fe(L3)Cl2]Cl
−0.052
0.094
0.091
0.021
0.023
0.047 FeIII → FeII
0.018 FeIII → FeII
0.368 DBSQ → DBC
0.034 FeIII → FeII
[Fe(L3)Cl2]Cl + H2DBC −0.045
[Fe(L3)(DBC)]+
[Fe(L4)Cl2]Cl
−0.081
0.032 −0.024 −0.007 FeIII → FeII
0.029 −0.023 −0.003 FeIII → FeII
0.312 DBSQ → DBC
[Fe(L4)Cl2]Cl + H2DBC −0.076
[Fe(L4)(DBC)]+
−0.023 FeIII → FeII
[Fe(L5)Cl2]Cl
−0.225 −0.035 −0.130 −0.105 FeIII → FeII
[Fe(L5)Cl2]Cl + H2DBC −0.221 −0.036 −0.128 −0.110 FeIII → FeII
[Fe(L5)(DBC)]+
0.195 DBSQ → DBC
−0.132 FeIII → FeII
[Fe(L6)Cl2]Cl
−0.104
0.018 −0.043 −0.028 FeIII → FeII
0.012 −0.043 −0.032 FeIII → FeII
0.324 DBSQ → DBC
[Fe(L6)Cl2]Cl + H2DBC −0.098
[Fe(L6)(DBC)]+
−0.043 FeIII → FeII
a Potential measured vs. Ag(s)/Ag+ (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)Cl2]+.
Electrochemical behavior
FeIII/FeII redox couple of the complexes (DPV, Table 5) vary in
the order, [Fe(L1)Cl2]+ > [Fe(L2)Cl2]+ < [Fe(L3)Cl2]+ > [Fe(L4)-
Cl2]+ > [Fe(L5)Cl2]+ < [Fe(L6)Cl2]+ revealing that the Lewis
acidity of the iron(III) centre is determined by the donor func-
tionalities of the primary ligand. Thus upon replacing the
–NMe2 group in 1 by the –NEt2 group to give 2, the redox
potential is shifted to a negative value. The weaker coordi-
nation of the sterically hindered nitrogen of the –NEt2 group is
expected to increase the positive charge on iron(III) and raise
the FeIII/FeII redox potential; however the chloride ions
become coordinated more strongly and decrease the Lewis
acidity of the iron(III) centre.34,41,44–46 Upon replacing both the
pyridyl moieties in 1 by 6-methylpyridyl moieties to give 3, the
redox potential is shifted to a more positive value, which is on
account of the weaker coordination of the sterically hindering
6-methylpyridyl moiety making the iron(III) centre more Lewis
acidic. However, replacing one of the pyridyl moieties in 1 by
an imidazolyl moiety to obtain 4 the redox potential is shifted
to a negative value as the stronger coordination of the imid-
azolyl nitrogen atom makes the iron(III) centre less Lewis
acidic. The same trend is observed for 5 also, upon replacing
both the pyridylmethyl arms in 1 by imidazolylmethyl arms as
in 5 the redox potential is shifted to a negative value by render-
ing the iron(III) centre less Lewis acidic (cf. above). Also, upon
replacing both the pyridyl moieties in 1 by quinolyl moieties to
The redox behaviour of the iron(III) complexes and their cate-
cholate adducts generated in situ in acetonitrile solution was
studied on a stationary platinum sphere electrode by employ-
ing cyclic (CV) and differential pulse voltammetry (DPV). For
1–6, a cathodic (−0.052–−0.225 V) as well as an anodic wave
(0.094–−0.035 V, Fig. 4) are observed. The E1/2 values of the
Fig. 4 Cyclic (CV) and differential pulse voltammogram (DPV) of 5 in
acetonitrile solution at 25 °C. Supporting electrolyte: 0.1 M TBAP. Scan
rate: for CV 50 mV s−1, for DPV 5 mV s−1
.
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give 6 the redox potential is shifted to a slightly negative value the plot of [1 + log(absorbance)] vs. time (Fig. 6). The second
due to the weaker coordination of the bulky quinolyl moiety, order rate constants (= kobs/[O2], Table 6) of the reactions were
as expected. Upon adding one equivalent of H2DBC to 1–6 in then calculated.25,41,44–46 The products of cleavage of H2DBC
acetonitrile solution, the DBSQ/H2DBC redox wave is overlaid in acetonitrile solvent (Table 6) were identified (7–14,
1
on the FeIII/FeII redox wave (0.018–0.110 V). When 2 equi- Scheme 4) using GC-MS and H NMR techniques and quanti-
valents of Et3N are added, the FeIII/FeII redox potential is fied by GC analysis. Interestingly, the adducts [Fe(L)(DBC)]+ of
slightly shifted to a more negative value due to bidentate 1–6 were reacted with dioxygen over 24 h to afford extradiol
coordination of DBC2− and the new redox wave in the more (8–55%) and intradiol cleavage products (23–74%, Table 6)
positive potential range corresponds to the DBSQ/DBC2− redox along with very small amounts of other products. The extradiol
couple.44–46
cleavage leads to two regio-isomeric products,36–38,44 namely
4,6-di-tert-butyloxepine-2,3-dione (7) and 5,7-di-tert-butyl-
oxepine-2,3-dione (9), which are obtained by insertion of an
oxygen atom into H2DBC and two more regio-isomeric pro-
ducts namely 3,5-di-tert-butyl-2-pyrones (8) and 4,6-di-tert-
butyl-2-pyrones (10) have been obtained from 7 and 9 respecti-
vely by the loss of CO.36–39,44 The intradiol cleavage results in
two products, namely 3,5-di-tert-butyl-1-oxacyclohepta-3,5-
diene-2,7-dione (11) and 3,5-di-tert-butyl-5-(carboxymethyl)-
2-furanone (12) and the side products are 3-tert-butylfuran-2,5-
dione (13) and 3,5-di-tert-butyl-1,2-benzoquinone (14). Interest-
Catechol dioxygenase activity of iron(III) complexes
When an acetonitrile solution of the DBC2− adducts [Fe(L)-
(DBC)]+ generated in situ by reacting 1–6 with H2DBC pre-
treated with two equivalents of Et3N was exposed to dioxygen,
the intensities of both the DBC2−-to-iron(III) LMCT bands
(850–920, 500–570 nm) decrease with time (Fig. 5, Table 6). All
the adducts were found to react with molecular oxygen and the
oxygenation reactions follow a pseudo-first order kinetics due
to the excess of dioxygen used, as judged from the linearity of
Fig. 5 Progress of the reaction of adduct [Fe(L4)(DBC)]+ with O2 in ACN
solution. The disappearance of the DBC2−-to-iron(III) charge transfer
band is monitored.
Fig. 6 Plots of [1 + log(absorbance)] vs. time for the reaction of [Fe(L4)-
(DBC))]+ (4 × 10−4 M) with O2 at 25 °C in acetonitrile solution.
Table 6 Kinetic dataa for oxidative cleavage of H2DBC catalyzed by iron(III) complexes in acetonitrile
Cleavage products in %
Complex
Extradiol
Intradiol
Others
E/I ratio
1.6
K (×10−2 M−1 s−1
1.12 0.02
0.87 0.04
1.20 0.03
2.47 0.03
4.83 0.01
1.34 0.04
)
[Fe(L1)Cl2]Cl
[Fe(L2)Cl2]Cl
[Fe(L3)Cl2]Cl
[Fe(L4)Cl2]Cl
[Fe(L5)Cl2]Cl
[Fe(L6)Cl2]Cl
4.2 (7), 20.6 (8)
6.4 (9), 23.8 (10)
3.7 (7), 15.6 (8)
5.2 (9), 17.5 (10)
19.9 (8)
5.6 (9) 19.5 (10)
10.2 (8)
2.3 (9), 12.1 (10)
2.1 (8)
5.7 (10)
34.0 (11, 12)
28.1(11, 12)
30.0 (11, 12)
48.0 (11, 12)
74.2 (11, 12)
23.6 (11, 12)
5.2 (13, 14)
4.1 (13, 14)
3.4 (13, 14)
8.0 (13, 14)
4.0 (13, 14)
5.3 (13, 14)
1.5
1.5
0.5
0.1
2.8 (7), 16.5 (8)
4.4 (9), 24.3 (10)
2.0
a kO = kobs/[O2]. The solubility of O2 in acetonitrile is accepted to be 8.1 mM at 25 °C. The kinetic data were obtained by monitoring the
2
disappearance of the lower-energy DBC2−-to-iron(III) LMCT band.
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Scheme 4 Products of catechol cleavage of H2DBC mediated by [Fe(L)Cl2]+ complexes using molecular oxygen: 4,6-di-tert-butyloxepine-2,3-
dione (7), 3,5-di-tert-butyl-2-pyrone (8), 5,7-di-tert-butyloxepine-2,3-dione (9), 4,6-di-tert-butyl-2-pyrone (10), 3,5-di-tert-butyl-1-oxacyclo-
hepta-3,5-diene-2,7-dione (11), 3,5-di-tert-butyl-5-(carboxymethyl)-2-furanone (12), 3-tert-butylfuran-2,5-dione (13), 3,5-di-tert-butyl-1,2-benzo-
quinone (14).
ingly, the trend in the E/I ratio follows the order 1 (1.6) ≥ goes either substrate activation or dioxygen activation.5,17,24,38
2 (1.5) ≈ 3 (1.5) > 4 (0.5) > 5 (0.1) < 6 (2.0), which reveals that In the substrate activation pathway, in the absence of a vacant
the stereoelectronic properties of 4N ligand donor functional- site, dioxygen attacks the catechol carbon in the iron(II)-DBSQ
ities dictate the regiospecificity of catechol cleavage.
intermediate to form the cyclohexadienyl peroxide intermedi-
Most of the iron(III) complexes of 3N ligands yield a major ate (Scheme 5) which undergoes acyl migration to yield intra-
amount of extradiol products along with intradiol cleavage diol products.22–27,36–46 On the other hand, oxygen activation
products35–42,44 due to the presence of a vacant coordination involves attack of dioxygen on the vacant site created by the
site in the catechol adduct for dioxygen binding. In contrast, displacement of the coordinated –NMe2 group in iron(II)-DBSQ
several iron(III) complexes of tetradentate tripodal or linear 4N species to form the same cyclohexadienyl peroxide intermedi-
ligands have been reported to cleave catechol to intradiol clea- ate (Scheme 5). Acyl migration in the intermediate gives intra-
vage products either exclusively or with small amounts of diol cleavage products36–46 while alkenyl migration yields extradiol
extradiol cleavage products.20–23,25–27,30,32,36,43 Interestingly, cleavage products.5,17,24,38,44 The latter involves cleavage of the
the adduct [Fe(L1)(DBC)]+ with no vacant coordination site for O–O bond in the intermediate, followed by insertion of one
oxygen binding affords higher amounts of extradiol (55%) and oxygen atom into the aromatic ring to form a lactone interme-
lower amounts of intradiol (34%) cleavage products (E/I, 1.6). diate, which proceeds to form the observed pyrone by loss of CO.
Also, the amount of extradiol cleavage products obtained for 1
In [Fe(L1)(DBC)]+, the coordinated –NMe2 rather than the
is higher than those for the previously reported iron(III) com- pyridyl nitrogen donor in the adduct (Fe–N4amine, 2.35;
plexes of 4N ligands.20,22,27,43 Thus the adduct20 [Fe(TPA)- Fe–N1py, 2.35 Å; cf. above) is preferably detached from the
(DBC)]+ reacts with dioxygen to yield 98% of intradiol cleavage coordination sphere during the formation of the cyclohexa-
products; however, when one of the pyridylmethyl arms in the dienyl peroxide intermediate. Furthermore, spectrophoto-
adduct is replaced by the –CH2–NMe2 arm to give [Fe(L1)- metric titration of [Fe(L1)Cl2]+ and H2DBC requires two
(DBC)]+, the extradiol cleavage product is obtained, illustrating equivalents of triethylamine for obtaining the maximum
that the –NMe2 donor plays a vital role in dictating the clea- absorption intensity for the catecholate-to-iron(III) LMCT band,
vage pathway. However, the yield of the extradiol product is indicating that the –NMe2 group does not act as an internal
lower than that for the adduct46 [Fe(L)(DBC)]+, where L is base and that both the detachment of the –NMe2 group and
a linear 4N ligand with the diazapane backbone, which binding of dioxygen on the iron(II) centre in the iron(II)-DBSQ
favours bidentate coordination of monoanionic catecholate. species occur simultaneously (Scheme 5). So, it is clear that a
This observation has been illustrated by invoking the involve- vacant or solvent-coordinated site in the catecholate adducts is
ment of both dioxygen and substrate activation mechanisms essential to form the cyclohexadienyl peroxide species. An ana-
(Scheme 5).22–27,36–46 The iron(II)-DBSQ species formed under- lysis of computed geometry of the catecholate adduct (Fig. 2)
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Scheme 5 Proposed oxidative cleavage mechanism for DBC2− of 1–6.
reveals that the Fe–Ocatecholate bonds (2.01, 2.08 Å) are asym- imidazolyl nitrogen donor to give [Fe(L4)(DBC)]+, a decreased
metric, which would favour more the dissociation of the Fe–O1 regiospecificity (E, 25%; I, 48%; E/I, 0.5) is observed. In con-
bond (CvO1, 1.29, C–O2, 1.31 Å) and hence alkenyl migration trast, upon replacing both the pyridyl donors in [Fe(L1)(DBC)]+
to achieve extradiol cleavage more than the intradiol cleavage by quinolyl donors to give [Fe(L6)(DBC)]+, the regiospecificity
product. Upon replacing the –NMe2 group in [Fe(L1)(DBC)]+ by increases (E, 48%; I, 24%; E/I, 2.0). Obviously, it is the large
the –NEt2 group to give [Fe(L2)(DBC)]+, 42% of extradiol clea- asymmetry in binding of the DBC2− (Fe–O1, 2.11; Fe–O2,
vage products and 28% of intradiol cleavage products (E/I, 1.5) 2.02 Å) which favours alkenyl migration further and hence
are obtained. The detachment of the –NEt2 group is encour- enhances the extradiol cleavage yield. Interestingly, a plot of
aged by its bulkiness, which is higher than that of the –NMe2 E/I against the difference in the Fe–Ocatecholate bond lengths is
group, during oxygen binding and thereby enhances the clea- linear (Fig. 7). The bulkiness of the quinolyl moiety accounts
vage product yield. However, the yields of both extradiol and for the decrease in both the total and extradiol yield, lower
intradiol cleavage products decrease, revealing that the steric than that for [Fe(L1)(DBC)]+, by discouraging the dioxygen
bulk of the –NEt2 group does not favor O2 attack.
attack. Similarly, the replacement of both the pyridyl arms in
Interestingly, upon replacing both the pyridyl moieties in [Fe(L1)(DBC)]+ by 6-methylpyridyl arms to give [Fe(L3)(DBC)]+,
[Fe(L1)(DBC)]+ by imidazolyl moieties to give [Fe(L5)(DBC)]+, the steric effect of methyl groups on the pyridyl ring leads to
the intradiol cleavage product is obtained in higher yields lower yields of cleavage products, but does not affect the
(E, 8%; I, 74%; E/I, 0.1). The drastic decrease in regiospecificity regiospecificity (E, 45%; I, 30%; E/I, 1.5), as expected. Thus,
is obviously on account of stronger coordination of the imid- the pyridyl(1, 2)/6-methylpyridyl(3)/quinolyl(6) nitrogen donors
azolyl arm making the Fe–Ocatecholate bonds less asymmetric facilitate alkenyl migration leading to high extradiol cleavage
(Fe–O1, 204; Fe–O2, 2.02 Å), disfavouring alkenyl migration products while the imidazolyl nitrogen donor(s) (4, 5) favor(s)
and hence intradiol cleavage. It may be noted that the stronger acyl migration leading to higher intradiol cleavage products.
coordination of the imidazolyl nitrogen donor slightly The decrease in the Lewis acidity of the iron(III) centre due to
weakens the Fe–NMe2 bond (Fe–Namine, 2.37 Å, cf. above), coordination of a strong donor like imidazole does not facili-
however the extradiol yield is decreased. As expected, the repla- tate dioxygen attack leading to a decreased extradiol cleavage
cement of one of the pyridyl donors in [Fe(L1)(DBC)]+ by an product. So, the ligand donor functionalities strongly influ-
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Fig. 7 Plots of E/I vs. difference in the Fe–Ocatecholate bond length (Å).
Fig. 8 Variation of the product yield with no. of equivalents of TEA in
the oxidative cleavage of H2DBC by 1a.
ence the extradiol to intradiol product selectivity (E/I, 0.1–2.0)
by controlling the symmetry of the Fe–Ocatecholate bonds. Also,
interestingly, we have observed a correlation between the
ligand donor functionalities and the regio-isomer selectivity in
the extradiol cleavage (Fig. S2†), which can be illustrated using
the two pathways in Scheme S1.†
Conclusions
Iron(III) complexes of a few systematically varied tripodal tetra-
dentate 4N ligands containing a sterically hindering –NMe2
alkylamine arm have been isolated and characterized by
ESI-MS, electronic spectral and electrochemical techniques.
The complex cation of 1a shows a distorted octahedral iron(III)
coordination geometry with the ligand facially coordinated to
iron(III) and the protonated amine nitrogen atom of the 4N
ligand remaining not coordinated. The iron(III) complexes
interact with the substrate 3,5-di-tert-butylcatechol (H2DBC) in
the presence of two equivalents of Et3N as a base to form the
adducts of the type [Fe(L)(DBC)]+, as revealed by the obser-
vation of two catecholate-to-iron(III) LMCT bands in the visible
region. Remarkably, these adducts afford extradiol (8–55%)
and intradiol (74–23%) cleavage products and the amount of
extradiol cleavage products obtained depends strongly on the
ligand architecture. The ability of one of the parent complexes
to cleave H2DBC has been studied as a function of the amount
of base added to deprotonate the substrate and this study
reveals that displacement of the weakly coordinated –NMe2
group in the substrate-bound complex facilitates dioxygen
attack. The decrease in the Lewis acidity of the iron(III) centre
due to coordination of a strong donor like imidazole does not
facilitate dioxygen attack leading to a decreased extradiol clea-
vage product. The extradiol to intradiol product selectivity (E/I,
0.1–2.0) depends upon the asymmetry in bidentate coordi-
nation of catecholate, as determined by the stereoelectronic
properties of the ligand donor functionalities.
To study the effect of coordinated terminal alkyl amine
nitrogen on the catechol cleavage pathway, 1 was treated with
an equimolar amount of dil. HCl to obtain 1a in which the
amine nitrogen of the terminal –NMe2 group rather than the
pyridyl nitrogen is protonated (cf. above) and then detached
from the coordination sphere. When 1a is treated with one
equivalent of H2DBC and one equivalent of Et3N and then
exposed to dioxygen, 12% of extradiol and 15% of intradiol
cleavage products (E/I, 0.8) are observed (Table 7). When two
equivalents of Et3N are used, a higher amount of cleavage pro-
ducts (E, 25%; I, 32%; E/I, 0.8) is observed. Interestingly, when
three equivalents of triethylamine are added a higher amount
of extradiol cleavage products (E, 48%; E/I, 1.7) is observed
with the product distribution being almost the same as that
observed for [Fe(L1)(DBC)]+ (cf. above). So it is clear that cre-
ation of a vacant site on the semiquinone intermediate by
detachment of the coordinated –NMe2 group in the adduct
[Fe(L1)(DBC)]+ for molecular oxygen attack is vital to achieve
extradiol cleavage products (Fig. 8). Hence the addition of a
fourth equivalent of Et3N leads to a lower amount of extradiol
cleavage products (E, 39; I, 30%; E/I, 1.3). All these obser-
vations reveal that both ligand electronic and steric factors of
the ligand donor functionalities play a vital role in dictating
the cleavage pathway.
Acknowledgements
Table 7 Effect of the added Et3N base on oxidative cleavage products
of H2DBC mediated by [Fe(HL1)Cl3]Cl 1a using molecular oxygen
We sincerely thank the Department of Science and Technology,
New Delhi, for Research Associateship under DST Nano
Mission Project [Scheme no. SR/NM/NS-110/2010(G)] and the
Department of Science & Technology, SERB, for the award of
Fast track Young Scientist to P. V. (no. SB/FT/CS-071/2012). We
also thank the Director, National Institute for Interdisciplinary
Equivalents of Et3N Extradiol Intradiol Quinone + others E/I
1
2
3
4
12.0
25.0
48.0
39.0
15.0
32.0
28.0
30.0
10.0
12.0
07.0
16.0
0.80
0.78
1.71
1.30 Science and Technology (NIIST)-CSIR, Trivandrum for provid-
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ing the computational facility and Dr C. H. Suresh, CSIR-NIIST, 12 (a) M. I. Davis, E. C. Wasinger, A. Decker, M. Y. M. Pau,
Trivandrum for fruitful discussions. This work was also sup-
ported by Indo-French Centre (IFCPAR) [Scheme no. IFC/A/
4109-1/2009/993]. We thank the Department of Science and
Technology (FIST program), New Delhi for the use of the
diffractometer in School of Chemistry.
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