Structure and catalytic activity of a new iron(ii) complex with a tetradentate carboxamide ligand: The effect of the outer-sphere donor on the chemoselectivity of the metal complex catalyst

Article abstract of DOI:10.1134/S0036023612100129

The interaction between Fe(ClO₄) or Fe(OTf)₂(MeCN) ₂ and 2 equiv of the potentially tetraden- tate ligand bis(2-pyridyl)methyl-2-pyridinecarboxamide (Py₂CHNHCOPy, tpcaH) yields the iron(II) com- plex [Fe^II(tpcaH)₂]X₂ (X = ClO₄, OTf), in which, according to X-ray crystallography data, tpcaH is facially coordinated to the iron atom as a tridentate ligand through the carbonyl group and two pyridyl donors of the bis(2-pyridyl)methylcarboxamide moiety and the third pyridyl group is uninvolved in coordination. The oxi- dation of saturated and unsaturated hydrocarbons with hydrogen peroxide involving this complex has been investigated. The presence of the uncoordinated nitrogen donor in the outer coordination sphere of the com- plex exerts a crucial effect on it catalytic properties. Pleiades Publishing, Ltd., 2012.

Full text of DOI:10.1134/S0036023612100129

ISSN 0036ꢀ0236, Russian Journal of Inorganic Chemistry, 2012, Vol. 57, No. 10, pp. 1328–1334. © Pleiades Publishing, Ltd., 2012.
Original Russian Text © A.A. Shteinman, 2012, published in Zhurnal Neorganicheskoi Khimii, 2012, Vol. 57, No. 10, pp. 1413–1419.
COORDINATION COMPOUNDS
Structure and Catalytic Activity of a New Iron(II) Complex
with a Tetradentate Carboxamide Ligand: The Effect
of the OuterꢀSphere Donor on the Chemoselectivity
of the Metal Complex Catalyst
A. A. Shteinman
Institute of Problems of Chemical Physics, Russian Academy of Sciences, Chernogolovka, Moscow oblast, 142432 Russia
eꢀmail: as237t@icp.ac.ru
Received February 10, 2011
Abstract—The interaction between Fe(ClO4)2 or Fe(OTf) (MeCN)2 and 2 equiv of the potentially tetradenꢀ
2
tate ligand bis(2ꢀpyridyl)methylꢀ2ꢀpyridinecarboxamide (Py CHNHCOPy, tpcaH) yields the iron(II) comꢀ
2
II
plex [Fe (tpcaH) ]X (X = ClO4, OTf), in which, according to Xꢀray crystallography data, tpcaH is facially
2
2
coordinated to the iron atom as a tridentate ligand through the carbonyl group and two pyridyl donors of the
bis(2ꢀpyridyl)methylcarboxamide moiety and the third pyridyl group is uninvolved in coordination. The oxiꢀ
dation of saturated and unsaturated hydrocarbons with hydrogen peroxide involving this complex has been
investigated. The presence of the uncoordinated nitrogen donor in the outer coordination sphere of the comꢀ
plex exerts a crucial effect on it catalytic properties.
DOI: 10.1134/S0036023612100129
The catalytic characteristics of a metal complex cent. The problem of finding more effective catalysts
depend considerably on its ligand structure, since the for preparative alkene cisꢀdihydroxylation was
ligand exposes particular properties of the metal ion. addressed by investigating the effect of the structure of
As a consequence, there is a wide diversity of catalysts carboxamide ligands on the catalytic activity of their
based on the same metal. The discovery and study of iron(II) complexes (Fig. 2, complexes 1–3) in the oxiꢀ
ironꢀcontaining oxygenases catalyzing, in biological dation of electron donor and electron acceptor alkꢀ
systems, a variety of selective oxidation processes, enes with hydrogen peroxide [5]. That study provided
II
IV
such as the hydroxylation of unactivated C–H bonds evidence that these reactions proceed via a Fe Fe
and the epoxidation and cisꢀdihydroxylation of double catalytic cycle involving these catalysts, as distinct
III
V
bonds, has stimulateda search for similar catalysts of from the Fe Fe catalytic cycle observed for iron
nonbiological origin [1]. The determination of the complexes with multidentate ligands containing only
structure of nonheme ironꢀcontaining oxygenases in nitrogen donors [6, 7].
the last decade has revealed the general structural
Here, we consider the structure and catalytic activꢀ
motif of their active sites, specifically, the facially
ity of an iron(II) complex with the potentially tetꢀ
coordinated triad of two histidines and one carboxyꢀ
radentate ligand bis(2ꢀpyridyl)methylꢀ2ꢀpyridinecarꢀ
late constituting an N,N,Oꢀdonor set [2] (Fig. 1). A
boxamide (Py CHNHCOPy, tpcaH, 4). This ligand
2
complex between iron(II) and bis(2ꢀpyridyl)methylꢀ
benzamide, [Fe(Py CHNHCOPh) ](OTf) (Fig. 2,
2
2
2
complex 1), was synthesized earlier in order to model
the oxygenases [3]. This complex contains a facial
N,N,Oꢀdonor set and efficiently catalyzes the selecꢀ
tive cisꢀdihydroxylation of various alkenes, thus proꢀ
viding a model for the family of mononuclear oxygenꢀ
H O
2
OH2
N
H O
2
O
FeII
ase, which are called Rieske oxygenases [4]. The cis
ꢀ
O
dihydroxylation selectivity of this catalyst (diol :
epoxide ratio) is 80% for electron donor alkenes and
His
N
Asp/Glu
NH
1
00% for electron acceptor alkenes. A fairly efficient
NH
olefin oxidation catalyst, this complex is almost incaꢀ
pable of catalyzing C–H bond oxidation. For examꢀ
ple, cyclohexene hydroxylation at its weakest C–H (in
the allylic position) occurs to the extent of a few perꢀ
His
Fig. 1. Active site of ironꢀcontaining N,N,Oꢀoxygenases.
1328

STRUCTURE AND CATALYTIC ACTIVITY OF A NEW IRON(II) COMPLEX
1329
N
N
N NH
R
C
O
N
N
N
O
O
Fe
N
Fe
N
N
N
N
O
C
NH
R
N
(a)
(b)
Fig. 2. (a) Facial and (b) meridional configurations of the carboxamide ligands in complexes
C H CF ), and 2ꢀC H N ).
1–
4
. R = C6H5 (1), 4ꢀC H OMe (2), 4ꢀ
6 4
(3
(4
6
4
3
5 4
differs from the above tridentate ligands in that it has a refined with the SHELXRꢀ97 program package. All
pyridyl radical in place of phenyl at the carbonyl group nonꢀhydrogen atoms were located geometrically and
(
Fig. 2a) [8, 9]. The outerꢀsphere donor effect on the
refined anisotropically. All hydrogen atoms were
placed in their ideal position and were refined isotopiꢀ
cally. The lengths of the bonds between the iron atoms
chemoselectivity of the metal complex catalyst was
discovered.
EXPERIMENTAL
Table 1. Crystallographic data and Xꢀray structure determiꢀ
All solvents were 99.5% or purer. They were used as
received or were predried over molecular sieve
nation parameters
3 Å.
Hydrocarbons were distilled from sodium metal. The Formula
other chemicals were commercialꢀgrade (99%, Aldꢀ
C H Cl N O S F Fe
38 32 4 8 8 2 6
FW
1104.49
Monoclinic
II
rich). Iron(II) triflate Fe (OTf) (MeCN) was synꢀ
2
2
thesized via a standard procedure [10]. The synthesis System
was carried out in an argon atmosphere using a gloveꢀ
Space group
P
2 /
n
1
box or a Schlenk line.
Unit cell parameters
a
b
c
= 12.652(3) Å
= 15.815(4) Å
= 12.744(3) Å
α
β
γ
= 90°
Elemental analyses were carried out in the
Microanalysis Laboratory, Institute of Problems of
Chemical Physics, Russian Academy of Sciences.
UV–vis absorption spectra were recorded on Specord
= 110.969(5)°
= 90°
7
5ꢀIR and Mꢀ82 spectrophotometers. Electrospray
2381(1) Å3
V
Z
ionization mass spectra (ESIꢀMS) were obtained on a
highꢀresolution timeꢀofꢀflight mass spectrometer at
the Institute for Energy Problems of Chemical Physꢀ
ics, Russian Academy of Sciences [11]. H NMR
spectra were taken from solutions in acetonitrileꢀd3 on
a Varian UNITY spectrometer operating at 300 MHz,
with chemical shifts measured relative to the resoꢀ
nance of the residual protons of the solvent.
2
1541 g/cm3
ρ
calc
1
μ
θ
0.0710 cm–1
1.29°–25.09°
20 914
Total number of reꢀ
flections
The structure of complex
4 was determined by
Xꢀray diffraction in the Xꢀray Crystallographic Laboꢀ Independent reflecꢀ
4218 [
R
(int) = 0.0665]
3190
ratory, Department of Chemistry, University of Minꢀ tions
nesota, on a Siemens SMART Platform CCD diffracꢀ
Observed reflections
α
tometer (173(2) K, Mo
chromator). The crystal (~0.32
K radiation, graphite monoꢀ
Refinement method
Fullꢀmatrix leastꢀsquares
×
0.28 0.21 mm) was
×
2
refinement on
F
glued onto the end of a glass capillary 0.1 mm in diamꢀ
eter. The structure determination conditions and crysꢀ
tallographic data are presented in Table 1. The strucꢀ
ture was solved using the Sir97 software and was
GOOF on F2
0.989
R
[I
> 2σ(I
)]
R1 = 0.1110, wR2 = 0.2455
RUSSIAN JOURNAL OF INORGANIC CHEMISTRY Vol. 57 No. 10 2012

1330
SHTEINMAN
II
Table 2. Comparison of
r(Fe –donor atom) distances (Å) for complexes 1–4, 6* and 7** and EDO mononuclear N,N,Oꢀ
oxygenases (averaged values for 11 structures [13])
Bond
Fe–O
1
2
3
4
6
*
7
**
EDO
2.043
2.171
2.181
0.010
2.043
2.167
2.187
0.020
2.047
2.174
2.178
0.004
2.043
2.091
2.214
0.123
2.121
2.147
2.173
0.026
2.228
2.100
2.122
0.022
2.04
2.20
Fe–N(2)
Fe–N(3)
2.27
Δ
(FeN(2–3))
0.070
Notes: * [L Fe] · 2D O (6), where L = 3,3ꢀbis(1ꢀmethylimidazolꢀ2ꢀyl)propionate [13].
2
2
1
1
*
* [L Fe](BPh ) (7), where L = propyl 3,3ꢀbis(1ꢀmethylimidazolꢀ2ꢀyl)propionate [12].
2 4 2
and the donors of the inner coordination sphere are
listed in Table 2.
For C H N O S F Fe anal. calcd. (%): C, 46.26;
36 28 8 8 2 6
H, 3.02; N, 11.99; S, 6.86. Found (%): C, 45.98; H,
.07; N, 11.92; S, 7.05.
3
The oxidation of saturated hydrocarbons in the presꢀ
ence of complex
4
was carried out at 20 С in 10ꢀmL glass
°
vessels [9]. A syringe technique [3] was used in cycloꢀ
hexene oxidation. The oxidation products were anaꢀ
lyzed on a Hewlett Packard 5880A chromatograph
with a flameꢀionization detector and Carbowax 20M
or ATꢀ1 capillary column.
RESULTS AND DISCUSSION
The known model complexes imitating nonheme
ironꢀcontaining oxygenases have bidentate [12], triꢀ
dentate [3, 5, 13], tetradentate [14], or pentadentate
[
15] ligands. The multidentate ligand tpcaH can be
The tpcaH ligand was synthesized via an earlier
described procedure [9] and was purified by recrystalꢀ
tetradentate, tridentate, or bidentate. In the case of
the tetradentate or tridentate coordination mode,
there can be different sets of donor atoms involved in
the inner coordination sphere of the complex [16]. In
addition, owing to the presence of the acidic H atom
in the carboxamide group, the ligand can coordinate
to the iron atom either as a neutral molecule or as an
anion (in deprotonated form) [16]. Yellow complexes
–1
lization from ethanol; mp 146
°
С. IR (KBr),
ν
, cm
:
3350, 1670 (amide band 1), 1504 (amide band 2).
II
Synthesis of [Fe (tpcaН) ](ClO ) (4ꢀClO4)
.
2
4 2
(
Attention! Perchlorates are potentially explosive and
should be handled with care.) A solution of Fe(ClO4)2
⋅
6
H O (0.095 g, 0.26 mmol) in MeCN (1 mL) was careꢀ
2
fully added to a solution of tpcaH (0.145 g, 0.5 mmol) in
4
ꢀClO4 and
ligand ratio of 1 : 2. As this ratio is increased to 1 : 1,
the earlier described orange complex [9] forms as
4ꢀOTf were synthesized at an iron saltꢀtoꢀ
3
mL of MeCN under stirring, and stirring was continꢀ
ued for 2 h. Diethyl ether was added dropwise to the
resulting solution until slight opacity, and the solution
was left standing overnight in a refrigerator. The next
day, the yellow precipitate of 4ꢀClO4 was filtered and
vacuumꢀdried. UVꢀvis (MeCN),
cm ): 270 (25000), 360 (3000), 415 (2400), 800 (100).
5
well. Reducing this ratio to 1 : 3 does not lead to the
formation of any other complexes. The perchlorate
complexes were synthesized in acetonitrile. The best
results for the triflate complexes were obtained with
–1
λ
max, nm, ( , M
ε
–1
CH Cl2. Since the properties of 4ꢀClO4 and 4ꢀOTf
2
II
Synthesis of [Fe (tpcaН) ](OTf) (4ꢀOTf)
.
Yellow were indistinguishable, both complexes will hereafter
2
2
fine crystals of
4
ꢀOTf were precipitated by stirring be referred to as
4.
II
Fe (OTf) (MeCN) (0.25 mmol) and tpcaH (0.50 mmol)
2
2
The elemental analysis and mass spectrometric
in 5 mL dichloromethane overnight. The precipitate
II
2+
data are consistent with the formula [Fe (tpcaН) ]
for the complex cation. The H NMR peaks are narꢀ
row and lie within the 9–90 ppm range. Therefore,
complex 4 is a highꢀspin complex of iron(II) [14]. The
IR spectrum of the ligand shows characteristic bands
of the carboxamide group, namely, a narrow strong
2
filtered, washed with a small amount of CH Cl2, and
2
1
vacuumꢀdried under pumping. Crystals for Xꢀray crysꢀ
tallography were grown at –25
°С
using slow pentane
diffusion into a solution of
4
ꢀOTf in CH Cl /MeCN
2
2
(
10 : 1).
UV–vis (MeCN),
λ
ε
, M–1 cm–1): 265
max, nm (
–
1
–1
band at 3350 cm
( NH) and strong bands at 1670 cm
ν
–1
(
20000), 358 (3000), 412 (2500), 800 (100).
, cm–1: 1639, 1602, 1560, 1475, 1445,
IR (KBr),
375, 1257, 1163, 1031, 998, 759, 701, 638, 584, 517.
1H NMR (300 MHz, Ме СО, 25
С), , ppm: 9.0,
4.0, 22.5, 35.2, 39.0, 43.2, 60.1, 67.1, 77.0, 80.9. 90.0.
(ν
, amide band 1) and 1504 cm (
ν
NH, amide band 2)
CO
[17]. The formation of complex
and broadening of the 3350 cm band and shifts
amide band 1 to a lower frequency of 1639 cm . This
4 causes a weakening
ν
–1
1
1
2
–1
°
δ
–1
2
shift,
observed for other carboxamide complexes [5, 17].
ESIꢀMS, fragment (X = FeLH, %): [X(MeCN)] , The complexationꢀinduced decrease in indicates
Δ
= 31 cm , is in agreement with the shift values
2+
ν
CO
2
+
2+
+
4; [X(MeCN) ] , 72; [X(LH)] , 100; [X(L)] , 43; that the carboxamide group of the ligand is coordiꢀ
2
+
[
X(LH)(OTf)] , 35
.
nated to iron through its carbonyl oxygen atom, since
RUSSIAN JOURNAL OF INORGANIC CHEMISTRY Vol. 57 No. 10 2012

STRUCTURE AND CATALYTIC ACTIVITY OF A NEW IRON(II) COMPLEX
1331
coordination through the nitrogen atom would
increase because of the disruption of conjugation
ν
CO
С(17A)
C(16A)
[
18]. In mononuclear complexes, this coordination
N(4A)
mode of the carboxamide group renders impossible
the coordination of the neighboring pyridyl to the
same iron atom. According to spectroscopic data,
C(13A)
N(3A)
C(6A)
C(15A)
C(14A)
C(4A)
C(8A)
complex 4 is in the highꢀspin state. This is quite natuꢀ
ral since the carboxamide group is a weakꢀfield ligand.
The electronic spectrum of the complex shows two
C(7A)
C(9A)
C(12)
C(3A)
strong bands at 360 and 415 nm, which are consistent
C(5A)
N(1A)
O(1A)
6
with the
d
electron configuration. The weak band at
N(2A)
5
5
830 nm is assigned to the T2g
– Eg transition, which is
C(10A)
C(2A)
the only spinꢀallowed transition for the highꢀspin,
C(1)
C(11A)
6
2+
C(1A)
octahedrally coordinated
d
Fe ion [19]. The Xꢀray
N(1)
C(11)
C(2)
N(2)
crystallographic data for 4 confirm the inferences from
the spectroscopic studies.
Fe(1)
O(1)
C(10)
The crystal structure of complex
fers significantly from the structure of [Fe (tpca)2
synthesized by reacting tpcaH with [Fe (MeCOO)2
4
ꢀOTf (Fig. 3) difꢀ
C(5)
C(3)
II
]
C(9)
II
C(4)
C(12)
C(7)
]
C(6)
N(3)
C(8)
[
[
16]. The deprotonated carboxamide ligand in the
Fe (tpca)2] complex acts as a tridentate meridional
C(14)
II
ligand (Fig. 2b). Either of the two ligands in this comꢀ
plex is coordinated to iron only through its nitrogen
atoms, namely two nitrogen atoms from the structurꢀ
ally nonequivalent pyridyls and one carboxamide
nitrogen atom, while one of the pyridyl donors
remains uncoordinated. The observed meridional
coordination suggests that a planar configuration is
favorable for the anionic ligand; however, this configꢀ
uration prevents the coordination of the third pyridyl.
According to Xꢀray crystallographic data, tpcaH in
C(13)
C(15)
C(16)
N(4)
C(17)
Fig. 3. Xꢀray structure of complex cation 4.
complex 4
is also a tridentate ligand, but with another donors in complex 4 differ insignificantly from the
set of donor atoms: iron is coordinated with two facial same parameters of the other model complexes. Note
N,N,Oꢀligands (Fig. 1a), and one of the pyridyls in that the difference between Fe–N(2) and Fe–N(3) in
either ligand is again uncoordinated. Thus, the strucꢀ
ture of is similar to the structure of complexes
and differs from the latter in that it has two uncoordiꢀ the active site of the EDO enzyme [13].
4
is larger than the same difference in the other model
4
1–3
complexes and is close to the difference observed in
nated pyridyls.
The catalytic activity of complex
4 was studied in
The complex cation
4 has an almost ideal octaheꢀ the oxidation of cyclohexane, 1,2ꢀdimethylcyclohexꢀ
dral configuration with an iron atom in the inversion ane, and cyclohexene (Table 3, Fig. 4). It follows from
center. Its two carboxamide oxygen atoms are trans to the data presented in Table 3 that the introduction of
one another. The relatively short Fe–Oamide bond the electron donor pyridyl into the outer sphere (by
length (2.043 Å) is evidence of strong interaction substituting pyridyl for phenyl in the carboxamide
between the carboxamide oxygen atom and the metal moiety of the ligand) radically changes the chemoseꢀ
atom, and this is in agreement with the observed comꢀ lectivity of the metal complex catalyst. While complex
plexationꢀinduced decrease in CO. The Fe–N bond is a very selective catalyst for olefin cisꢀdihydroxylaꢀ
lengths (Table 2) confirm the highꢀspin state of iron in tion and is practically inactive even in the oxidation of
. For comparison, we present, un the same table, the weak C–H bond in the allylic position of olefins,
Xꢀray crystallographic data for complexes and for the new complex does not catalyze olefin dihydroxylaꢀ
the related complexes and with N,N,Oꢀfacial set of tion but is capable of abstracting an H atom from
1
ν
4
1–4
6
7
donor atoms and averaged bond lengths for 11 crystal unactivated C–H bonds in stereospecific alkane
structures of the catechol dioxygenase family (EDO), hydroxylation (Fig. 4). The alcohol : ketone ratio in
II
whose active site contains Fe coordinated with two the catalytic oxidation of cyclohexene with hydrogen
histidines and one glutamate (Fig. 1). It can be seen peroxide is 2.6, In the hydroxylation of the tertiary
from the data presented in Table 2 that the structural bond in cisꢀ1,2ꢀdimethylcyclohexane, the resulting
characteristics of 1–3 depend only slightly on the alcohol retains the cis configuration to the extent of
presence of electronꢀdonating of electronꢀwithdrawꢀ 62%, indicating that the metal complex is involved in
ing substituents in the ligand. On the whole the bond oxidation. As is clear from Fig. 5, conversion (product
distances between the iron atom and the nearest yield on the oxidizer consumption basis) for complex
4
RUSSIAN JOURNAL OF INORGANIC CHEMISTRY Vol. 57 No. 10 2012

1332
SHTEINMAN
Table 3. Cyclohexene oxygenation with hydrogen peroxide in an Ar atmosphere catalyzed by complexes
Fe : H O : RH = 1 : X : 1000)
1 and 4
(
2
2
Double bond
Allylic
α
ꢀC–H bond
Complex,
conditions
H O
2 2
conversion, %
Reference
epoxide, TN diol, TN
cyclohexenol, TN cyclohexenone, TN
1
, X = 10, 25
°
C, 1 h
0
5.6
0.1
0.5
14
0.4
7
65
51
[5]
4, X = 50, 20
°
C, 2 h
4.5
This work
is only 2 times lower than the conversion for complex
5,
The involvement of the cisꢀdihydroxyferryl interꢀ
which is an efficient catalyst for alkane oxidation with mediate in olefin cisꢀdihydroxylation was postulated
hydrogen peroxide [9].
earlier from quantum chemical calculations for
another iron(II) complex [22]. The precursor of this
The epoxide : diol ratio is commonly correlated
with the donor set of the complex [20] and with the
spin state of the hydroperoxide intermediate [21]. The
cisꢀdiol selectivity of the catalyst is attributed to the
presence of donating oxygen atoms in the coordinaꢀ
II
intermediate is
η
2ꢀFe
·
H O2, whose formation needs
2
two labile donor to be located nearby in the coordinaꢀ
tion sphere of the metal.
At the same time, the catalysis of the hydroxylation
tion sphere of the metal and/or to the highꢀspin state of unactivated C–H bonds and epoxidation of double
of the complex. The catalytic activity and selectivity of bonds by the model complexes with electronꢀdonating
complexes 1–3 is almost independent on the presence nitrogen atoms is conventionally attributed to the
III
V
of electronꢀdonating or electronꢀwithdrawing substitꢀ involvement of perferryl intermediates in the Fe Fe
uents in the ligand [5]. Why does the replacement of catalytic cycle [6, 7, 14]:
phenyl with pyridyl, which leaves the nucleus of the
III
III
V
Fe + H O
→
Fe OOH
→
Fe =O + RH
2
2
complex intact and has no significant effect on the
spectral and structural characteristics of the complex,
changes the chemoselectivity of the metal complex
catalyst so greatly?
III
→
Fe + ROH.
The FeIII complex forms at the initial stage of the
reaction via the oxidation of the original complex. The
formation of the hydroperoxide intermediate at –40 С
An isotopic study of the mechanism of catalysis of
°
alkene dihydroxylation by complexes
1–3 [5] demꢀ
in MeCN was reliably established by a number of
III
onstrated that the reaction proceeds via the following
methods. Because the Fe OOH intermediate cannot
II
IV
Fe Fe catalytic cycle:
activate strong C–H bonds, it was hypothesized that
its heterolytic dissociation yields a perferryl intermeꢀ
diate and the latter is the active oxidizer. The oxoperꢀ
II
II
IV
Fe + H O → η ꢀFe
2
·
Fe + C(OH)–C(OH).
H O
2
→
cisꢀFe (OH)2
2
2
2
II
+
C=C
→
V
ferryl radical Fe =O has recently been identified by
4
5
1
35
30
25
20
15
10
5
0
1
2
3
Time, h
4
5
Fig. 5. Hypothetical stabilization of the linear configuration
of the peroxide intermediate
η
ꢀFe(tpcaH)(H O )(S) ·
1 2 2 2
n
H O by a chain of hydrogen bonds as a result of the incluꢀ
sion of several water molecules between the peroxide and
2
Fig. 4. Kinetics of cyclohexane oxidation catalyzed by
complexes , and
1
,
4
5.
the outerꢀsphere pyridyl donor of the ligand.
RUSSIAN JOURNAL OF INORGANIC CHEMISTRY Vol. 57 No. 10 2012

STRUCTURE AND CATALYTIC ACTIVITY OF A NEW IRON(II) COMPLEX
1333
EPR spectroscopy at –70
°
С
[7]. The observed
H
H
H
chemoselectivity of complex
4
is definitely consistent
O
OH
FeV
III
V
O
with the Fe Fe catalytic cycle. The change in
chemoselectivity caused by the replacement of phenyl
with pyridyl in the ligand means that the introduction
of an outerꢀsphere donor either hampers the formaꢀ
tion of the key intermediate of cisꢀdihydroxylation or
facilitates the formation of the key intermediate of
olefin epoxidation. Since the cisꢀdihydroxyferryl
FeIII
O
O
+ H O
2
(
a)
H
H
O
(
H O) Py
2
n
FeV
IV
+ (H O)n + 2Py
2
O
intermediate apparently stabilizes Fe and slows down
O H
FeIII
O
further iron oxidation, the exclusion of this intermediꢀ
ate from the catalytic cycle is expected to give way to
the formation of the more active, perferryl radical. The
inhibition of the formation of the cisꢀdihydroxyferryl
intermediate might be explained by the absence of
labile cis monodentate ligands S in the active complex
resulting from the solvolysis of the original complex in
H
(
b)
Fig. 6. Possible mechanisms of the heterolytic dissociation
of the peroxide bond: (a) mechanism involving a nearestꢀ
neighbor water molecule and (b) catalysis by the outerꢀ
sphere pyridyl donor involving associated water molecules.
1
the catalytic solution. However, according to H NMR
and mass spectrometric data, the following equilibꢀ
rium takes place in dilute solutions of complex
MeCH + H O: Fe (tpcaН) = Fe (tpcaН)S + tpcaH. and is apparently involved in the acid–base catalysis of
4
in not coordinated to iron acts as an outerꢀsphere donor
II
II
2
2
3
This equilibrium yields a complex in which one of the the heterolytic dissociation of the peroxide bond
two facial N,N,Oꢀdonor ligands is replaced by solvent yielding a reactive perferryl intermediate, simulating
molecules. The equilibrium is shifted to the leftꢀhand the same function of the outerꢀsphere amino acid resꢀ
side for complexes1
water, according to H NMR data for the similar comꢀ alytic mechanism is unknown, it is clear that the
plex L2Fe 2D O (6), where L is the N,N,Oꢀfacial outerꢀsphere donor radically changes the chemoselecꢀ
1–3 in dry MeCN [5], while in idues in enzymatic oxidation. Although the exact catꢀ
·
2
donor ligand 3,3ꢀbis(1ꢀmethylimidazolꢀ2ꢀyl)propiꢀ tivity of the metal complex catalyst, switching the oleꢀ
onate, the same equilibrium is shifted to the right [13]. fin oxygenation process from cisꢀdihydroxylation to
epoxidation. At the same time, the presence of an
outerꢀsphere donor markedly enhances the effectiveꢀ
ness of the catalyst, making it possible to hydroxylate
unactivated C–H bonds. The important role of outerꢀ
sphere noncovalent interactions in the modulation of
the strength of the active site is observed both in nature
and in biomimetic model systems [25, 26].
The solvolytic formation of the complex containꢀ
ing three solvent molecules cis to one another in the
case of 1–3 is confirmed by the observation of oxygen
isotope exchange between hydrogen peroxide and
IV
water [5], which is possible in the Fe (OH) (ОН )
2
2
intermediate via proton transfer. The hampering of the
II
formation of the
presence of two labile cis coordination sites may be due
to the stabilization of the isomeric intermediate 1ꢀFe
η
2ꢀFe · H O intermediate in the
2 2
η
·
ACKNOWLEDGMENTS
H O2, in which the peroxide occupies a single coordiꢀ
nation site. This stabilization is possible due to the forꢀ
2
I am grateful to Drs. W.W. Brennessel and
mation of a bridge consisting of several hydrogenꢀ V.G. Young, Jr., XꢀRay Crystallographic Laboratory,
bonded water molecules between
η1ꢀFe ·
H O and the Department of Chemistry, University of Minnesota,
2 2
outerꢀsphere pyridyl (Fig. 5). In this case, provided for determination of the structure of complex
4
and to
that there is a water molecule in the cis position, there Prof. L. Que, Jr. for giving me the opportunity to work
can be the heterolytic dissociation of the peroxide in his laboratory.
bond via proton transfer (Fig. 6a [6, 14]). At the same
time, the heterolytic dissociation of the peroxide O–O
bond in this structure may be more likely than the
homolytic dissociation owing to pyridyl catalyzing
proton transfer from the water pool (Fig. 6b). This
acid–base catalysis of the heterolytic dissociation of
the peroxide bond was observed in both biological [23]
and chemical [24] systems.
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