Tetranuclear iron(III) complexes of an octadentate pyridine-carboxylate ligand and their catalytic activity in alkane oxidation by hydrogen peroxide

Article abstract of DOI:10.1039/b512069a

Reaction of the octadentate ligand 2,6-bis{3-N,N-di(2-pyridylmethyl) aminopropoxy}benzoic acid (LH) with Fe(ClO₄)₃ leads to the formation of the tetranuclear complexes Fe₄(-O)₂(LH) ₂(ClCH₂-CO₂)₄(ClO₄) ₄ (1), {Fe₂(-O)L(R-CO₂)}₂(ClO ₄)₄ (2 R = C₆H₅-, 3 R = CH ₃-, 4, R = ClCH₂-). The crystal structures of complexes 1 and 2 reveal that they consist of two Fe^III₂(-O)(-RCO ₂)₂ cores that are linked via the two LH/L ligands to give a "dimer of dimers" structure. Complex 1 assumes a helical shape, with protonated carboxylic acid moieties of the two ligands forming a hydrogen-bonded pair at the center of the cation. In complexes 2, 3 and 4, central carboxylates of the two ligands bridge the iron ions in each of the two Fe₂O units, with an interdimer iron-iron separation of approximately 10 A and an intradimer separation of approximately 3.1 A. The second carboxylate bridge within the Fe₂O units is defined by exogenous benzoate (2), acetate (3) or chloroacetate (4) ligands. The aqua complex {Fe₂(-O)L(H₂O)₂}₂(ClO ₄)₆ (5) is proposed to have a similar structure, but with the exogenous bridging carboxylates replaced by two terminal water ligands. These complexes exhibit electronic and Moessbauer spectral features that are similar to those of (-oxo)diiron(iii) proteins as well as other related (-oxo)bis(-carboxylato)diiron(iii) complexes. This similarity shows that these properties are not significantly affected by the nature of the bridging exogenous carboxylate, and that the octadentate framework ligand is essential in stabilizing the "dimer of dimers" structure. This structural feature remains in highly diluted solution (10^-5 M) as evidenced by electrospray ionization mass-spectroscopy (ES MS). Cyclic voltammetric studies of complexes 2 and 5 showed two irreversible two-electron reductions, indicating that the two Fe₂O units of the tetranuclear complexes behave as distinct redox entities. Complexes 2, 3 and, especially, the aqua complex 5 are active alkane oxidation catalysts. Catalytic reactions carried out with alkane substrate molecules and hydrogen peroxide predominantly gave alcohols. High stereospecificity in the oxidation of cis-1,2-dimethylcyclohexane supports the metal-based molecular mechanism of O-insertion into C-H bonds postulated for non-heme iron enzymes such as methane monooxygenase. The Royal Society of Chemistry 2006.

Full text of DOI:10.1039/b512069a

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www.rsc.org/dalton | Dalton Transactions
Tetranuclear iron(III) complexes of an octadentate pyridine-carboxylate
ligand and their catalytic activity in alkane oxidation by hydrogen peroxide†
Elena A. Gutkina,a Vladimir M. Trukhan,a,b Cortlandt G. Pierpont,*c Shaen Mkoyan,a Vladimir V. Strelets,a
Ebbe Nordlander*b and Albert A. Shteinman*a
Received 25th August 2005, Accepted 2nd September 2005
First published as an Advance Article on the web 10th October 2005
DOI: 10.1039/b512069a
Reaction of the octadentate ligand 2,6-bis{3-[N,N-di(2-pyridylmethyl)amino]propoxy}benzoic acid
(LH) with Fe(ClO4)3 leads to the formation of the tetranuclear complexes [Fe4(l-O)2(LH)2(ClCH2-
CO2)4](ClO4)4 (1), [{Fe2(l-O)L(R-CO2)}2](ClO4)4 (2 R = C6H5-, 3 R = CH3-, 4, R = ClCH2-). The
crystal structures of complexes 1 and 2 reveal that they consist of two FeIII2(l-O)(l-RCO2)2 cores that
are linked via the two LH/L ligands to give a “dimer of dimers” structure. Complex 1 assumes a helical
shape, with protonated carboxylic acid moieties of the two ligands forming a hydrogen-bonded pair at
the center of the cation. In complexes 2, 3 and 4, central carboxylates of the two ligands bridge the iron
˚
ions in each of the two Fe2O units, with an interdimer iron–iron separation of approximately 10 A and
˚
an intradimer separation of approximately 3.1 A. The second carboxylate bridge within the Fe2O units
is defined by exogenous benzoate (2), acetate (3) or chloroacetate (4) ligands. The aqua complex
[{Fe2(l-O)L(H2O)2}2](ClO4)6 (5) is proposed to have a similar structure, but with the exogenous
bridging carboxylates replaced by two terminal water ligands. These complexes exhibit electronic
and Mo¨ssbauer spectral features that are similar to those of (l-oxo)diiron(III) proteins as well as other
related (l-oxo)bis(l-carboxylato)diiron(III) complexes. This similarity shows that these properties are
not significantly affected by the nature of the bridging exogenous carboxylate, and that the octadentate
framework ligand is essential in stabilizing the “dimer of dimers” structure. This structural feature
remains in highly diluted solution (10−5 M) as evidenced by electrospray ionization mass-spectroscopy
(ES MS). Cyclic voltammetric studies of complexes 2 and 5 showed two irreversible two-electron
reductions, indicating that the two Fe2O units of the tetranuclear complexes behave as distinct redox
entities. Complexes 2, 3 and, especially, the aqua complex 5 are active alkane oxidation catalysts.
Catalytic reactions carried out with alkane substrate molecules and hydrogen peroxide predominantly
gave alcohols. High stereospecificity in the oxidation of cis-1,2-dimethylcyclohexane supports the
metal-based molecular mechanism of O-insertion into C–H bonds postulated for non-heme iron
enzymes such as methane monooxygenase.
a carboxylate moiety should be even more relevant for modeling
Introduction
l-oxo-l-carboxylate diiron sites since the carboxylate bridges in
these enzymes are part of the protein.5,6 Two successful7,8 and two
unsuccessful9,l0 attempts have been reported for the preparation
of dinuclear iron complexes containing similar ligands; in these
ligands, the carboxylate is one of the “peripheral” donors at the ter-
minus of a polydentate ligand. Successful structural and functional
modeling of sMMO constitutes a special challenge, requiring the
preparation of diiron complexes that not only model structural
features of the sMMO active site but that are also capable of
effecting alkane oxidation by a metal based mechanism involving
peroxide and high valent iron oxo intermediates.1a,11 The significant
criteria of this mechanism are stereospecific alkane oxidation12 and
incorporation of l8O from H2l8O in product alcohol.13
A number of Fe2O complexes has been synthesized to model the
spectroscopic and structural properties of l-oxo-l-carboxylato
diiron sites in proteins such as soluble methane monooxygenase
(sMMO), ribonucleotide reductase (RNR) and hemerythrin.1 In
many cases, such biomimetic complexes have been found to be
relatively unstable.1,2 Efforts have therefore been made to prepare
framework polydentate ligands that may stabilize complexes of this
type.3 Examples include ligands in which multidentate nitrogen-
donors that bind to metals in a capping manner are linked to
alkoxo or phenoxo groups capable of bridging two metals.4 Binu-
cleating polydentate ligands containing nitrogen donors linked to
aInstitute of Problems of Chemical Physics, 142432, Chernogolovka, Moscow
district, Russia. E-mail: ast@icp.ac.ru
We recently described the synthesis of the ligand 2,6-bis{3-
[N,N-di(2-pyridylmethyl)amino]propoxy}benzoic acid (LH), con-
taining a potentially bridging carboxylate moiety included as part
of a spacer that links two di(picolyl)amine groups, as well as the
synthesis and structure determination of the tetranuclear iron(III)
complex [Fe4(l-O)2(LH)2(ClCH2CO2)4](ClO4)4 (1).14 Complex
1, containing two distinct (l-oxo)bis(l-chloroacetato)diiron(III)
bInorganic Chemistry, Chemical Center, Lund University, Box 124, SE-
221 00, Lund, Sweden. E-mail: Ebbe.Nordlander@inorg.lu.se
cDepartment of Chemistry and Biochemistry, University of Colorado,
Boulder, CO, 80309-0215, USA. E-mail: Pierpont@Colorado.edu
† Electronic supplementary information (ESI) available: NMR spectra of
complexes 2, 3 and 5. See DOI: 10.1039/b512069a
492 | Dalton Trans., 2006, 492–501
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cores, was shown to have a helical shape formed by bridging
LH ligands joined by hydrogen-bonded carboxylic acid moieties
(Fig. 1). We have since found that different reaction conditions
permit the isolation of tetranuclear iron complexes in which the
carboxylate of L bridges adjacent diiron cores. Here, we report the
synthesis and characterization of a series of complexes, [{Fe2(l-
O)L(R-CO2)}2](ClO4)4 (R = Ph 2, CH3 3, CH2Cl 4), in which the
two iron ions in each Fe2O unit are coordinated by the carboxylate
of L as well as an exogenous carboxylate anion. In addition, we
present the syntheses and characterization of the related aquo
complex [{Fe2(l-O)L(H2O)2}2](ClO4)6 5. Comparative studies of
1H-NMR, Mo¨ssbauer spectra and the electrochemical behavior
of complexes 1–5 are described, as well as the results of an
investigation into their activity as alkane oxidation catalysts.
The cyclic voltammetry studies were carried out over a potential
range of +1 to −2 V (vs. SCE) using a scan rate of 0.2 V s−1 at
15 ◦C.
Methyl 2,6-di(3-bromopropoxy)benzoate
Powdered K2CO3 (26.5 g 184 mmol) and 1,3-dibromopropane
(50 ml, 247 mmol) were added to a solution of methyl resorcylate
(8 g, 47.6 mmol) in dry acetone (500 ml). The mixture was refluxed
for 3 days under N2. Then the solution was filtered, and the
acetone solution containing an excess of 1,3-dibromopropane was
evaporated under reduced pressure. The remaining residue was
redissolved in benzene and chromatographed on a silica-gel col-
umn (230–400 mesh, elution with benzene) to yield 19.0 g (94.4%)
of colorless crystals of methyl 2,6-di(3-bromopropoxy)benzoate
with mp 34–38 ◦C. Anal. Calcd for C14H18Br2O4, (%): C, 41.00;
1
H, 4.42; Br, 38.97. Found (%): C, 41.32; H, 4.45; Br, 38.90. H
NMR (300 MHz, CDCl3, ppm): 7.26 t, (1 H, C6H3); 6.57 d, (2 H,
C6H3); 4.13 t, (4 H, Br–CH2); 3.87 s, (3 H, COOCH3); 3.54 t, (4 H,
O–CH2); 2.27 p, (4 H, CH2).
Methyl 2,6-di(3-(di(2-pyridylmethyl)amino)propoxybenzoate
(LMe)
Dipicolylamine (1.467 g, 7.38 mmol), triethylamine (1.12 g,
11.06 mmol) and methyl 2,6-di(3-bromopropoxy)benzoate (1.5 g,
3.65 mmol) were dissolved in absolute THF (50 ml). The solution
was refluxed for four days under a nitrogen atmosphere. Then
the solid material was filtered off and the solvent was removed by
vacuum evaporation. The residue was dissolved in 200 ml of diethyl
ether, washed three times with 100 ml portions of water, and dried
over MgSO4. After evaporation, the residue was chromatographed
on a silica-gel column (230–400 mesh, elution with ethyl acetate–
methanol, 9 : 1) to yield a product with Rf 0.57 in methanol.
Yellow oil, 1.81 g (76.7%). Anal. Calcd for C38H42N6O4, (%): C,
70.59; H, 6.50; N, 13.00 Found (%): C, 70.56; H, 6.55; N, 13.00. 1H
Fig. 1 An ORTEP drawing of the molecular structure of the [Fe4O2-
(LH)2(l-O2CCH2Cl)4]4+ cation 114 showing the atom labeling scheme.
Thermal ellipsoids are drawn at the 30% level.
Experimental
=
NMR (300 MHz, CDCl3, ppm): 8.5 d, (4 H, CH N); 7.58 t, (4 H,
C5H4N); 7.46 d, (4 H, C5H4N); 7.18 t, (1 H, C6H3); 7.10 t, (4 H,
C5H4N); 6.45 d, (2 H, C6H3); 3.97 t, (4 H, O–CH2); 3.82 s, (8 H,
N–CH2–C5H4N); 3.61 s, (3 H, COOCH3); 2.7 t, (4 H, N–CH2);
1.96 t, (4 H, CH2).
General considerations
All reagents and solvents were of commercial reagent grade and
were used without further purification except where noted. Dry
methanol was obtained by distillation over CaH2. Solvents used for
spectroscopic measurements were all spectroscopic grade. Prepar-
ative separations were performed by silica gel flash column chro-
matography (Merck Kieselgel 6). N,N-bis(2-pyridylmethyl)amine
(dipicolylamine) was synthesized in 78% yield according to a
literature procedure.15
2,6-Di(3-(di(2-pyridylmethyl)amino)propoxybenzoic acid (LH)
The methyl ester LMe (1 g, 1.54 mmol) was dissolved in 50 ml of
a 5% ethanol solution of KOH and refluxed gently for 36 h. After
the solvent was evaporated, the residue was dissolved in 10 ml of
water, acidified with concentrated hydrochloric acid to pH 7 and
extracted with ethyl acetate. The combined ethyl acetate solution
was then dried over MgSO4 and the solvent was removed under
reduced pressure to give 840 mg (86.6%) of an amorphous yellow
solid. Anal. Calcd for C37H40N6O4, %: C, 70.23; H, 6.37; N, 13.28.
Found%: C, 70.57; H, 6.54; N, 12.99. 1H NMR (300 MHz, CDCl3,
Physical methods
1H NMR spectra were recorded in CDCl3 or acetonitrile-d3
solutions on a Varian UNITY 300 MHz spectrometer; chemical
shifts are given as values in ppm relative to the solvent resonances.
Electrospray mass spectra in CH3CN were recorded on a prototype
of a time-of flight mass spectrometer equipped with an ion source
with ES ionization. The instrument and its principle of operation
have been described in detail.16 UV-visible spectra (200 to 900 nm)
were recorded on a Milton Roy 3000 spectrophotometer. Electro-
chemical measurements were performed in acetonitrile solution
using a PAR 173 potentiostat with a PAR 175 signal generator.
=
ppm): 8.4 d, (4 H, CH N); 7.55 t, (4 H, C5H4N); 7.38 d, (4 H,
C5H4N); 7.18 t, (1 H, C6H3); 7.05 t, (4 H, C5H4N); 6.51 d, (2 H,
C6H3); 4.02 t, (4 H, O–CH2); 3.85 s, (8 H, N–CH2–C5H4N); 2.85
t, (4 H, N–CH2); 1.96 t, (4 H, CH2).
[Fe4O2(LH)2(l-O2CCH2Cl)4](ClO4)4 (1). Chloroacetic acid
(200 mg, 2.128 mmol) was dissolved in 5 ml of acetonitrile and
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the mixture was added dropwise to a solution of Fe(ClO4)3·9H2O
(160 mg, 0.31 mmol) in 3 ml of acetonitrile. To this solution, the
acid LH (98 mg, 0.155 mmol) in 2 ml of methanol was added.
Brown crystals of 1 precipitated within two days giving 120 mg of
the complex in 65% yield as an unstable mixed water/methanol
solvate. UV-Vis [MeCN; kmax/nm (eM(Fe)/M−1 cm−1): 211 (23 000),
245 sh (15 000), 344 (4200), 380 sh (3100), 472 (840), 510 (730),
555 sh (220), 721 (155). IR (KBr), m/cm−l: 1687 (CO2H), 1607
(Py), 1570 (masCOO−), 1463, 1418 (msCOO−), 536 (msFe–O–Fe). 1H
NMR (CD3CN, 25 ◦C): 28 (o-H(Py)), 15.6 (Py–CH2–N), 12.0
(m-H(Py)), 11.5 (O2CCH2Cl), 8.5 (m-(H)Ph), 7.8 (p-(H)Ph), 7.4
(p-(H)Py), 6.7, 5.4, 4.1 (CH2); ESMS—see text.
N, 7.50; Found%: C, 42.3; H, 3.95; Cl, 9.4; N, 7.6. UV-Vis
[MeCN; kmax/nm (eM(Fe)/M−1 cm−1): 210 (80 500), 241 (61 700),
344 (19 100), 472 (3200), 510 (2647), 550 sh (800), 725 (470). IR
(KBr), m/cm−l: 3417; 1608; 1579 (masCOO−); 1463; 1416 (msCOO−);
1
1265; 1088 (ClO4−); 767; 625 (ClO4−); 530 (msFe–O–Fe); 415. H
NMR (CD3CN, 25 ◦C): 29 (o-H(Py)), 15.68 (N–CH2–Py), 12.0
(m-H(Py)), 11.6 (COOCH2Cl), 8,5 (m-H(Ph)), 7.76 (p-H(Ph)),
6.7, 5.44, 4.1 (CH2); ESMS—see text.
[{Fe2O(L)(H2O)2}2](ClO4)6 (5). The acid LH (50 mg,
0.08 mmol), dissolved in 3 ml of ethanol was mixed with
Fe(ClO4)3·9H2O (83 mg, 0.16 mmol) dissolved in 2 ml of ethanol.
A brown/green powder precipitated. The complex was isolated
by filtration and washed with ethanol. It was obtained as an
unstable hydrate in 71.7% yield (62 mg). UV-Vis [MeCN; kmax/nm
(eM(Fe) M−1 cm−1)]: 220 (35 000), 259 (20 000), 330 (4000), 474
(659), 520 sh (535), 724 (97). IR (KBr), v/cm−1:1608, 1541
(masCOO−), 1463, 1419 (msCOO−), 537 (msFe–O–Fe), 473. 1H NMR
(CD3CN, 25 ◦C): d 26 (o-py), 15.4, 14.0 (pyCH2N), 11.6, 11.3 (m-
py), 8.9, 8.6 (m-Ph), 8.0 (p-Ph), 7.3 (p-Py), 6.7, 4.4, 3.9, 3.6 (CH2).
Cf. ref. 14 for microanalytical data.
[{Fe2(l-O)(L)(l-O2CC6H5)}2](ClO4)4 (2). Sodium benzoate
(45.5 mg, 0.316 mmol) in 2 ml of methanol was added to
an acetonitrile solution containing Fe(ClO4)3·9H2O (163 mg,
0.316 mmol) whereupon the mixture turned pale brown/green.
Then the acid LH (100 mg, 0.158 mmol) in 5 ml of acetonitrile was
added to the solution and the mixture turned deep green/brown.
The solution was left for several days as brown crystals of complex
2 formed. The complex was obtained in 44.7% yield (80 mg)
as an unstable mixed water/acetonitrile solvate. UV-Vis [MeCN;
kmax/nm (eM(Fe)/M−1 cm−1): 235 (78 000), 340 (17 600), 470 (2700),
505 (2100), 550 sh (750), 725 (450). IR (KBr), m/cm−l: 3420; 1607;
1539 (masCOO−); 1463; 1405 (msCOO−); 1265; 1088 (ClO4−); 763;
715; 625 (ClO4−); 539 (msFe–O–Fe); 475; 418. 1H NMR (CD3CN,
25 ◦C): 28.4 (o-H(Py)), 15.49 (N–CH2–Py), 12.14 (m-H(Py)),
11.62 (m-H(Py)), 8,5 (m-H(Ph)), 7.9 (p-H(Ph)), 7.7 (p-H(Py)),
6.83, 6.28, 5.45, 3.95 (CH2); ESMS (m/z, amu) 979(M+), 619
Crystallographic studies
Crystals of 1 and 2 suitable for X-ray diffraction studies were
grown by slow evaporation of acetonitrile/methanol solutions.
Crystals of both complexes were observed to lose solvent within
seconds upon separation from the recrystallization solution.
Crystallographic data are summarized in Table 1.
3+
({[Fe2OL(OBz)]2(ClO4)} ), 440 ([Fe2OL(OBz)]24+).
[Fe4O2(LH)2(l-O2CCH2Cl)4](ClO4)4 (1). A crystal of 1 was
mounted on a glass fiber and coated with an amorphous resin to
retard crystal deterioration. Data were collected with a Siemens
SMART CCD area detector, using graphite-monochromated Mo-
[{Fe2(l-O)(L)(l-O2CCH3)}2](ClO4)4 (3). Sodium acetate
(26 mg, 0.316 mmol) in 2 ml of methanol was added to an
acetonitrile solution containing Fe(ClO4)3·9H2O (163 mg,
0.316 mmol), whereupon the mixture turned pale brown/green.
The acid LH (100 mg, 0.158 mmol) in 2 ml of acetonitrile
was added to the solution and the mixture turned deep
green/brown. Slow evaporation gave brown crystals of complex
3 in 55.4% yield (98 mg) as a water/methanol solvate, [{Fe2(l-
O)(L)(l-O2CCH3)}2](ClO4)4·6H2O·2CH3OH. Anal. Calcd for
C80H104Cl4Fe4N12O40, %: C, 43.54; H, 4.75; Cl, 6.43; Fe, 10.12; N,
7.62. Found%: C, 43.3; H, 4.1; Cl, 6.3; N, 7.7. UV-Vis [MeCN;
kmax/nm (eM(Fe)/M−1 cm−1): 212 (60 000), 255 sh, 344 (12 400),
467 (2650), 505 (2200), 550 sh (800), 723 (700). IR (KBr), m/cm−l:
3422; 1607; 1547 (masCOO−); 1463; 1439 (msCOO−); 1265; 1090
(ClO4−); 768; 625 (ClO4−); 535 (msFe–O–Fe); 473. 1H NMR
(CD3CN, 25 ◦C): 27.5 (o-H(Py)), 15.47 (N–CH2–Py), 12.06
(m-H(Py)), 11.56 (COOCH3), 8,49 (m-H(Ph)), 7.34 (p-H(Ph)),
6.73, 6.3, 5.45, 3.9 (CH2); ESMS (m/z, amu) 917 (M+), 578
˚
Karadiation (k = 0.71069 A) from a Rigaku rotating anode X-ray
generator. Intensity data were corrected for Lorentz, polarization
and absorption effects. The positions of the iron atoms were
determined using direct methods, and all the non-hydrogen atoms
were located from difference Fourier syntheses. The hydrogen
Table 1 Crystallographic data for [{Fe2O(LH)(ClCH2CO2)2}2](ClO4)4·2
CH3OH·H2O (1) and [{Fe2O(L)(C6H5CO2)}2](ClO4)4 2 CH3CN 2 H2O (2)
(1)
(2)
Formula
FW
Fe4Cl8O37N12C84H98
2372.6
Triclinic
P-1
2
16.4575(3)
17.8346(2)
19.3790(3)
77.521(1)
89.245(1)
86.432(1)
5542.9(2)
293(2)
Fe4Cl4O32N14C92H92
2271.0
Triclinic
P-1
1
13.685(3)
14.468(3)
15.670(3)
72.65(3)
68.65(3)
64.34(3)
2567.9(9)
293(2)
Crystal system
Space group
Z
˚
a/A
˚
b/A
˚
c/A
3+
2+
({[Fe2OL(OAc)]2(ClO4)} ), 409 {[Fe2OL(OAc)]2(ClO4)2} .
a/◦
b/◦
c /◦
[{Fe2(l-O)(L)(l-O2CCH2Cl)}2](ClO4)4 (4). Chloroacetic acid
(29.74 mg, 0.316 mmol) in 5 ml of acetonitrile was added to
an acetonitrile solution containing Fe(ClO4)3·9H2O (163 mg,
0.316 mmol). The mixture turned green. The acid LH (100 mg,
0.158 mmol) in 2 ml of acetonitrile was added to the green
solution and the mixture turned green/brown. The solution was
left overnight to precipitate brown crystals of complex 4 as a
water/methanol solvate. Yield 103 mg (58.2%). Anal. Calcd for
C80H98Cl6Fe4N12O36, %: C, 42.90; H, 4.41; Cl, 9.50; Fe, 9.97;
3
˚
V/A
T/K
d/g cm−3
l/mm−1
1.410
0.788
0.097, 0.205
0.854
1.494
0.746
0.098, 0.202
0.773
a
R, Rw
GOF
a Discrepancy indices are defined as: R = RꢀFo| − |F|/R|Fo| and Rw
=
[Rw(|Fo| − |Fc|)2/Rw(Fo)2]1/2
.
494 | Dalton Trans., 2006, 492–501
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atoms were placed in calculated positions. Rotational disorder
in the orientations of the CH2Cl groups of two chloroacetates
was detected and successfully modeled using fractional occupancy
factors; the conformations shown in Figs. 1 and 2 are for chloride
locations with highest occupancy. The final refinement was carried
out by full-matrix least-squares calculations on F2 and with
anisotropic thermal parameters for all non-hydrogen atoms.
addition was controlled by a Cambridge syringe. The solution was
stirred for an additional 5 min after H2O2 addition was complete.
The concentration of catalyst was typically 0.7 0 mM and the H2O2
and alkane concentrations were adjusted by addition of MeCN to
give a catalyst : peroxide : alkane ratio of 1 : 420 : 1000. The
iron complex was removed by passing the solution through a silica
gel column followed by elution with 3 ml of MeCN. An internal
standard (chlorobenzene) was added at this point and oxidation
products were analyzed on a Hewlett-Packard 5880A chromato-
graph with a flame-ionization detector and capillary OV-5 (60 m ×
0.20 mm) column. The H2O2 concentrations in catalytic solutions
before and after the experiment were determined by iodometric
titration and yields of oxidation products were calculated per mole
of H2O2 consumed. Blank control experiments in the absence of
catalyst(s) resulted in no detectable oxidation under the conditions
used. All reactions were run at least in triplicate, and the data
reported below are the average of these reactions. In the one-
pot experiments, samples of the catalytic solution were withdrawn
during the development of the reaction, while in the syringe–pump
experiments only the final product mix was analysed.
Results and discussion
Fig. 2 An ORTEP plot of the [Fe4O2(LH)2(l-O2CCH2Cl)4]4+ cation 1
showing the hydrogen bonded carboxylic acid moieties of LH at the center
of the dimer.
Ligand synthesis
The new ligand, 2,6-bis{3-[N,N-di(2-pyridylmethyl) amino]pro-
poxy}benzoic acid (LH), containing a potentially bridging car-
boxylate moiety which is a part of the spacer that links two
dipicolylamine groups has been prepared in a three-step synthesis
with a general overall yield of ca. 60% (Scheme 1).
[{Fe2(l-O)(L)(l-O2CC6H5)}2](ClO4)4 (2). A crystal of 2 was
placed in a glass capillary with a drop of mother liquor for
data collection. Data were collected as before with a Siemens
SMART CCD area detector, using graphite-monochromated Mo-
˚
Karadiation (k = 0.71069 A) from a Rigaku rotating anode X-ray
generator. Intensity data were corrected for Lorentz, polarization
and absorption effects. The locations of the iron atoms and many
of the atoms from within the inner coordination sphere were
determined from a sharpened Patterson map. The “dimer of
dimers” was found to be located about a crystallographic center
of inversion symmetry in the triclinic space group P-1 (no. 2).
After atoms of the cation and perchlorate anions were located, the
atoms of water and acetonitrile solvent molecules appeared in the
difference Fourier and were included in the refinement.
CCDC reference numbers 127873 (1)14 and 255394 (2).
See http://dx.doi.org/10.1039/b512069a for crystallographic
data in CIF or other electronic format.
Investigation of catalytic alkane oxidation reactions catalyzed by
complexes 2, 3 and 5
Experiments were carried out at 20 ◦C in glass vials (10 ml)
closed by rubber septa. Solutions of alkane and catalyst in MeCN
were placed in a vial and the reaction was initiated by addition
of hydrogen peroxide with vigorous stirring. The volume of the
catalytic solution was approximately 3 ml. Addition of a 30%
aqueous solution of H2O2 was carried out in two different ways:
as a single portion together with other reagents at the beginning
of the reaction (“one-pot” experiment) or by slow addition during
reaction through a polyethylene capillary tube. The tube was
inserted into the catalytic solution through a septum cap and
connected to a glass syringe filled with H2O2 solution. The rate of
Scheme 1
The ligand was designed on the basis of relative ease of synthesis
and potential fit to a dinuclear oxo-bridged metal (iron) core as
determined by molecular modeling. The products LMe and LH
have been identified by 1H NMR spectroscopy, microanalysis, and,
in the case of LMe, FAB mass spectrometry. The 1H NMR spectra
of LMe and LH are identical except for the methyl resonance at d
3.61 that is present in the spectrum of LMe (the carboxylate proton
of LH could not be detected due to broadening by exchange).
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Scheme 2
◦
˚
Table 2 Selected bond lengths (A) and angles ( ) for the [{Fe2O(LH)-
Synthesis of complexes
(ClCH2CO2)2}2]4+ cation in complex 1
Reactions of LH with an iron(III) salt followed by addition of a
carboxylate salt leads to the formation of two kinds of tetranuclear
complexes, depending on the nature of the added carboxylate
(Scheme 2). Both types of tetranuclear complexes consist of linked
FeIII2(l-O) dimers, but the nature of the linkage between the
dimers, and hence the structures of the complexes, differ.
Fe1–O1
Fe1–N1
Fe1–N2
Fe1–N3
Fe1–O3
Fe1–O5
Fe2–O1
Fe2–N7
Fe2–N8
Fe2–N9
Fe2–O4
Fe2–O6
Fe1–Fe2
Fe3–Fe4
1.776(6)
2.196(8)
2.215(9)
2.100(10)
2.028(8)
2.013(7)
1.787(6)
2.209(9)
2.140(8)
2.220(10)
2.023(9)
2.036(7)
3.102(4)
3.138(4)
Fe3–O2
Fe3–N4
Fe3–N5
Fe3–N6
Fe3–O7
Fe3–O9
Fe4–O2
Fe4–N10
Fe4–N11
Fe4–N12
Fe4–O8
Fe4–O10
Fe1–O1–Fe2
Fe3–O2–Fe4
1.794(6)
2.216(8)
2.198(10)
2.152(9)
2.020(8)
2.054(7)
1.786(6)
2.226(8)
2.216(9)
2.104(10)
2.051(8)
2.013(7)
Reaction of a seven-fold excess of chloroacetic acid with iron(III)
perchlorate followed by addition of L to the mixture led to the
formation of complex 1, which was identified as [Fe4O2(HL)2(l-
O2CCH2Cl)4](ClO4)4 on the basis of mass spectrometry. It was
possible to grow crystals of 1 from a MeOH/MeCN solution
and its crystal structure was determined in order to confirm
the proposed tetranuclear structure. Complexes 2, 3 and 4 were
prepared by addition of benzoate, acetate or chloroacetate anions
to the reaction mixture. The stoichiometric formulas of these
complexes were established on the basis of their mass spectra, and
these formulas indicate that 2–4 possess two less “non-ligand”
carboxylates than 1. However, the UV-Vis spectra of 2–4 (vide
infra) suggest that the complexes contain Fe2O clusters with
irons bridged by two carboxylate moieties. One of these must
be the deprotonated carboxylate associated with the framework
ligand (L) and the determination of the crystal structure of 2
(vide infra) confirms this feature. Finally, reaction of LH with
two equivalents of Fe(ClO4)3 in methanol leads to the formation
of a green–brown product which has been assigned the formula
[Fe2OL(H2O)2](ClO4)3 5 on the basis of UV-Vis, IR, and 1H NMR
spectroscopy.
121.1(4)
122.5(4)
metric triclinic unit cell consists of a complete cation with no
imposed symmetry. Within the independent l-oxodiiron units,
iron atoms are bridged by two chloroacetate ligands while the
non-bridging facial positions are filled by two di(picolyl)amine
moieties from two molecules of LH, so that the two LH lig-
ands link the adjacent l-oxodiiron units. Sessler and coworkers
found a similar extended bridge linking two Fe2O units for the
[Fe2L2(l-O)(l-HCO2)2(HCO2)2]2 dimer, where L2 = 1,3-bis((bis(1-
methylimidazol-2-yl)phenylmethoxy)methyl)-benzene.17 Within
˚
complex 1 the average Fe-l-oxo distance is 1.79 A and the average
Fe–O–Fe angle is 121.8◦; the intra-dimer Fe–Fe distances are
˚
˚
3.102(4) A [Fe(1)–Fe(2)] and 3.138(4) A [Fe(3)–Fe(4)] while the
˚
˚
interdimer Fe–Fe distances vary from ca. 10.5 A to 10.8 A. The
conformation of the linking ligands is such that the whole molecule
acquires a helical shape, with a dihedral angle between the Fe–O–
Fe planes in 1 of 42.5◦. Detailed features within the inner coordina-
tion spheres of the four iron atoms are similar and compare well
with the metrical dimensions of other [L2Fe2O(carboxylate)2]+n
cations.17–19 Iron lengths to the picolyl nitrogen trans to the Fe–oxo
bond are roughly equal to the lengths to the tertiary amine nitro-
gens as a result of the trans influence of the strong Fe–oxo bond.
Structural characterization on complex 2 has shown that the
composition and structure of the complex cation are fundamen-
tally different from that of 1. A view of the [{Fe2(l-O)(L)(l-
O2CC6H5)}2]4+ cation is shown in Fig. 3 and selected bond lengths
Structural features of complexes 1 and 2. The cationic product
of the reaction with chloroacetic acid, described above, consists
of two Fe2O dimers linked by protonated LH ligands to give
the helical structure shown in Fig. 1. Selected bond lengths
and angles are listed in Table 2. At the center of the “dimer
of dimers” the carboxylic regions of the two LH ligands are
hydrogen bonded in the head-to-head structure found com-
monly for crystallized carboxylic acids in the solid state. This
interaction is shown in Fig. 2 with hydrogen atoms placed in
idealized positions. The asymmetric region of the centrosym-
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Table 3 Selected bond lengths (A) and angles (◦) for the
˚
Spectroscopic properties
[{Fe2O(L)(PhCO2)2}2]4+ cation in complex 2
Spectroscopic analyses indicate that the Fe2O environments in 3, 4
and 5 are similar to those found in 2, and the spectroscopic prop-
erties are entirely consistent with those observed for (l-oxo)bis(l-
carboxylato)diiron centers of other complexes.1,19 Characteristic
spectral features include high intensity of d–d transitions, low-
energy shifts in charge-transfer absorptions compared with high-
spin mononuclear iron complexes, and significant quadrupole
splitting in the Mo¨ssbauer spectra. These unique features result
from the considerable strength and, hence, the unusually short
length, of the highly covalent Fe–oxo bond in these centers. The
rather strong antiferromagnetic interaction (−J = 100–130 cm−l)
between the high-spin iron ions in such complexes is a consequence
of significant overlap of partially occupied iron d orbitals with
occupied p and s orbitals of the bridging oxygen. This interaction
results in an increase in the intensity of d–d transitions. Because
orbital overlap depends on the Fe–O distance and the Fe–O–Fe
bond angle, the spectroscopic and magnetic characteristics of Fe2O
complexes are also correlated with these structural parameters.
The positions of the absorption maxima in the UV-Vis spectra
and molar absorption coefficients of complexes 1–5 are listed in
Table 4. The electronic spectra of Fe2O complexes are usually
subdivided into three regions.1b Region 1 (300–400 nm) contains
two intense (e/Fe = 3–6 × 103 M−1 cm−1) absorptions due
to LMCT transitions from the bridging oxo ligand to Fe(III)
which are not always clearly resolved. Region 2 (400–550 nm)
contains two or three absorptions whose intensities are lower
than those of absorptions in region 1 by a factor of 5–10. These
absorptions correspond to ligand field (LF) transitions enhanced
by mixing with LMCT transitions from region l. Region 3 (550–
800 nm) contains a characteristically wide low-intensity LF (6A1 →
4T2)/LMCT band whose position is very sensitive to the Fe–O–
Fe bond angle.1 The Fe–O–Fe angle in 2 is ∼121.0(4)◦, and the
proximity of the wide absorptions at 724 nm observed for 2 to
analogous absorptions in 3–5 suggests that similar angles apply to
the Fe2O units in the latter complexes.
The vibrational characteristics of the Fe–O–Fe units in 1–5 were
studied by IR spectroscopy. The ms(Fe–O–Fe) absorptions were
observed at 530–540 cm−l while the mas absorptions are obscured by
other ligand absorptions in this region. Comparison of the IRspec-
tra of complexes 1–5 (cf. Table 4) and [{Fe2O(dpab)}2](C1O4)8,20
in which the same terminal dipicolylamine ligands are present
but carboxylate bridges are absent, suggests that the strong
absorptions of approximately equal intensity in the IR spectra of
complexes 1–5 can be identified as the absorptions corresponding
to asymmetric and symmetric vibrations of both carboxylates. The
Dm value, which characterizes the difference in the positions of
these absorptions, provides support for bridging bidentate coordi-
nation of both carboxylates in complexes 1–5 (Table 4); this value
is consistent with the range Dm = 110–170 cm−1, which is typical
of bridging carboxylate groups in diiron complexes of this type.
Mo¨ssbauer DEQ values of ∼1.6 mm s−1 are found for short
Fe–O bonds in doubly or triply bridged Fe2O clusters, whereas
significantly lower quadrupole splitting is indicative of the elonga-
tion of this bond in mononuclear, hydroxo-bridged, or trinuclear
(or polynuclear) iron complexes. The spectrum of complex 2
(Fig. 4) with the parameters d = 0.49, DEQ1 = 1.47, and
DEQ2 = 1.82 mm s−1, is similar to the spectrum of complex 5
Fe1–O1
Fe1–N1
Fe1–N2
Fe1–N3
Fe1–O2
Fe1–O4
C1–O2
1.801(6)
2.162(9)
2.138(11)
2.144(8)
1.979(7)
2.025(8)
1.302(12)
1.203(11)
3.111(4)
Fe2–O1
Fe2–N4
Fe2–N5
Fe2–N6
Fe2–O3
Fe2–O5
C8––O4
C8–O5
1.774(6)
2.252(8)
2.195(9)
2.209(11)
2.037(7)
2.048(7)
1.279(11)
1.279(10)
C1–O3
Fe1–Fe2
Fe1–O1–Fe2
121.0(4)
Fig. 3 An ORTEP drawing of the molecular structure of the [{Fe2(l-O)-
(L)(l-O2CC6H5)}2]4+ cation 2. Each Fe2O unit of the centrosymmetric
cation is bridged by carboxylate groups from one L and from an exogenous
benzoate anion. Thermal ellipsoids are drawn at the 30% level.
and angles are listed in Table 3. The cation is located about a
center of inversion symmetry relating the two Fe2O regions. As in
the cation of 1, the two Fe2O units are linked by bridging L (LH)
ligands. However, the carboxylic acid moieties of these ligands
are deprotonated and bridge Fe2O units to give a structure that is
somewhat more strained than the open structure of 1. Carboxylate
groups of the benzoate ligands complete the bridged structure of
the [Fe2O(carboxylate)2]2+ regions of the dimer. Detailed features
of the two Fe atoms of the crystallographically independent
Fe2O unit are significantly different (Table 3). The oxo bridge is
unsymmetrical with the shorter Fe–O length to Fe2 (1.774(6) A).
The trans influence of this bond results in a rather long bond length
to amine nitrogen N4 (2.252(8) A), and generally longer bond
lengths to all other atoms within the inner coordination sphere of
Fe2, relative to equivalent lengths to Fe1. As a consequence of the
different electronic properties of the two metals, the double bond
of the benzoate carboxylate group is localized at C1–O3 (1.208(11)
A), with a significantly longer Fe2–O3 length (2.036(7) A) relative
to the long C1–O2 (1.297(12) A) bond and shorter Fe1–O2 length
˚
˚
˚
˚
˚
˚
(1.983(7) A). This relatively subtle feature is significant in the
carboxylate detachment mechanism for peroxide coordination in
catalytic alkane oxidation reactions (vide infra). Other carboxylate
groups of both structures have equivalent C–O lengths. This
dissimilarity between the Fe centers results from the disposition of
the di(picolyl)amine groups of L, folded on one side to coordinate
with Fe1 at the dimer including the bridging carboxylate, and
extended to bridge to the outer Fe2O unit at Fe2 (Fig. 3).
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Table 4 Spectroscopic and geometric properties of the dinuclear complexes [Fe4O2(L)2(l-O2CCH2Cl)4](ClO4)4 1, [{Fe2(l-O)(L)(l-O2CC6H5)}2](ClO4)4 2,
[{Fe2(l-O)(L)(l-O2CCH3)}2](ClO4)4 3, [{Fe2(l-O)(L)(l-O2CCH2Cl)}2](ClO4)4 4, [Fe2O(L)(H2O)2](ClO4)3 5, and the azido derivative of met-hemerythrin
(metN3Hr)
1
2
3
4
5
metN3Hr
kmax/nm (e/M−1 cm−1
)
LMCT
LMCT
344 (17000)
380 (12400)
472 (3380)
510 (2650)
555 (sh)
721 (622)
537
340 (17600)
344 (12400)
344 (19100)
330 (4000)
326 (3375)
380 (2150)
446 (1850)
497 (325)
530 (sh)
680 (95)
507
4
6A1 → T2
467 (2650)
510 (1820)
550 (sh)
725 (450)
539
467 (2650)
505 (2100)
550 (sh)
723 (700)
535
472 (3200)
505 (2200)
550 (sh)
725 (470)
530
474 (649)
505 (590)
555 (sh)
724 (97)
537
122
0.49;0.45
1.68;0.95
O2− → Fe CT
6A1 → A1, 4E
4
6A1 → T2/LMCT
4
ms(Fe–O–Fe)/cm−1
Dm(COO)/cm−1
d/mm s−1
152
134
108
163
0.49;0.48
1.47;1.82
3.111(4)
121.0(4)
0.50
1.90
3.23
130
DEQ/mm s−1
˚
Fe–Fe/A
3.102(4); 3.138(4)
121.1(4); 122.5(4)
Fe–O–Fe/◦
Table 5 1H NMR data for the complexes of LH/L and the structurally
similar [Fe2O((PyCH2)2NCH2CO2)2(BzO)]3+ (a);19 see ESI† for NMR
spectra
H
1
2
3
4
5
a
o-Py
28
28.4
27.5
15.47
12.06
11.56a
8.49
7.7
29
26
15.4 14.0
11.6 11.3
24
16.2
12.4
N-CH2-Py
m-Py
OAc
15.6 15.49
12.0 12.14 11.62
11.5
8.5 8.5
7.8 7.9
15.7
12.0
11.6
8.5
7.76 8.0
7.3
m-Ph
8.9 8.6
8.5
6.8
6.7
p-Ph
p-Py
7.4 7.7
7.2
a The signal disappears when acetate is replaced with deuteroacetate.
The weak paramagnetism of the Fe2O complexes, resulting
from strong antiferromagnetic coupling between the Fe(III) atoms,
1
leads to relatively well-resolved signals in their H NMR spectra
and narrow spectral widths (up to 40 ppm). Assignments of the
1H NMR spectra of the complexes in this study are based on
their similarities to the spectra of oxo-bridged diiron complexes
of related ligands, viz. {PyCH2}2N(CH2)nCO2 (n = 1,2).19 The
−
signals in the 1H NMR spectra of complexes 1–5 (Table 5) are all
located in the region 0–28 ppm, suggesting similar J values (and
Fe2O (sub)structures) for all complexes. There are broad features
above d 20 (PyCH2 and o-Py protons), sharper signals at d 15–18
(m-Py protons), and a very sharp signal at d ca. 6 (p-Py protons).
In addition, there are sharp signals from the polymethylene chain
of L at d 3–9. The paramagnetically shifted and broadened
signals due to the terminally coordinated (PyCH2)2N group in
1–5 (see Fig. S1, ESI†) are very similar to the signal pattern
observed for the complex [Fe2O((PyCH2)2NCH2CO2)2(BzO)]3+,
which contains a similar di(picolyl)amine ligand moiety.19 This
fact suggests that there is some structural similarity between 1–5
and [Fe2O((PyCH2)2NCH2CO2)2(BzO)]3+.
Fig. 4 Mo¨ssbauer spectra recorded on [{Fe2(l-O)(L)(l-O2CC6H5)}2]-
(ClO4)4 2 (top) and [Fe2O(L)(H2O)2](ClO4)3 5; see text for experimental
details.
(DEQ = 1.68 mm s−1) and spectra that are characteristic of struc-
tures in which two high-spin Fe(III) ions are antiferromagnetically
coupled in the Fe2O(l-RCO2)2 system. The inequivalence of iron
atoms in 2 is detected in the Mo¨ssbauer spectrum by the presence
of two overlapping quadrupole doublets in a 1 : 1 ratio. The
large DEQ2 = 1.82 mm s−1 suggests a significant distortion from
octahedral symmetry for one of the iron centers as noted in
the X-ray structure of 2 (vide supra). The superposition of two
overlapping quadrupole doublets with equal intensity leads to
broadening of the Mo¨ssbauer spectra for all of the complexes
(1–5, average DEQ ∼ 1.65) in agreement with their tetranuclear
structures. The appearance of an additional component with
DEQ < 1 in the Mo¨ssbauer spectrum of complex 5 is probably
due to the admixture of an Fe(OH)2Fe complex.
Electrospray (ES) ionization mass spectrometry
The mass spectra of CH3CN solutions of 1–5 were measured on
a prototype time-of flight mass spectrometer equipped with an
ion source with ES ionization. Attempts to detect the molecular
ion of 1 were unsuccessful but prominent ions were detected at
m/z 952, 601 and 426; these ions are consistent with the molec-
x+
ular formulation(s) {[Fe4L2(ClCH2CO2)2](ClO4)4−x
}
(x = 2–4,
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Fig. 5 An electrospray (ES) ionization mass spectrum of [{Fe2(l-O)(L)(l-O2CC6H5)}2]4+ 2 in acetonitrile. Inserts show the fine structure of the molecular
ion signals and the theoretical isotope distributions.
Table 6 Potentials of peaks of reduction of complexes 2 and 5 (C = 1 ×
10−3 M) in MeCN/0.1 M Bu4NPF6 on a glassy carbon electrode (v =
0.2 V s−1) at +15 ◦C. Numbers of transferred electrons are specified in
brackets.
respectively), corresponding to complex 4 (vide infra). We have
found that complex 4, [Fe4O2(L)2(l-O2CCH2Cl)2](ClO4)4, is in
equilibrium with complex 1, [Fe4O2(LH)2(l-O2CCH2Cl)4](ClO4)4,
in solution, but that the equilibrium can be shifted towards
the formation of 1 by addition of a relatively large excess of a
carboxylic acid. The mass spectrum suggests that 1 readily loses
two chloroacetic acid molecules in the gas phase.
Complex
Ep/V (SCE)
2
5
–0.39 (2e)
–0.28 (2e)
–1.75 (2e)
–1.48 (2e)
At a concentration of 3 × 10−4 M of complex 2, the mass
spectrum (Fig. 5) exhibits three ions of m/z 440, 619 and 979,
3+
which correspond to [Fe2OL(OBz)]24+, {[Fe2OL(OBz)]2(ClO4)}
as well as several other complexes.1b The values of Ep obtained
in these experiments are in agreement with the observation that
reduction potentials of Fe2O complexes depend on the basicity
and charge of the bridging anionic ligand.21
2+
and {[Fe2OL(OBz)]2(ClO4)2} . Similarly, complex 3 gives ions
4+
at m/z: 409, 578 and 917 corresponding to [Fe2OL(OAc)]2
,
3+
2+
{[Fe2OL(OAc)]2(ClO4)} and {[Fe2OL(OAc)]2(ClO4)2} . The
mass spectrum of 4 was found to be identical to that of 1 (vide
supra) and in full agreement with the proposed formula for 4.
On the other hand, complex 5 appears to fragment even under
electrospray ionization conditions, and no ions attributable to the
complex could be detected.
As the electrochemically active binuclear moieties in the tetranu-
˚
clear complex are separated by relatively large distances (∼10 A)
and show no evidence for interdimer electronic coupling, they
are reduced to isolated binuclear entities at identical potentials
and undergo two synchronous one-electron transfers. Thus, the
investigated complexes are reduced from the {Fe(III)Fe(III)}2
oxidation state to the fully reduced {Fe(II)Fe(II)}2 oxidation state
in two two-electron (1 e− + 1 e−) reductions. For comparison,
the tetranuclear complex [LFe2OA]2(PF6)4, (A = the dianion of
glutaric acid), shows two analogous consecutive peaks at −0.500
and 0.635 V (vs. Ag/AgNO3).22
Electrochemical properties
For complexes 2 and 5, two irreversible two-electron reduction
waves could be detected, the potentials of which are listed in
Table 6. A quantitative estimate of the number of electrons trans-
ferred was achieved by comparing the height of reduction peak
currents to that of the first (one-electron) reduction of the trinu-
clear complex [(MeOH)(H2O)2Fe3(l3-O)(PBAH+)6]7+ (PBA = 2-
(pyrid-2-ylmethoxy)benzoate) under the same experimental con-
ditions of concentration and scan rate.10 This complex was chosen
as a standard because of its comparable size, and, hence, close
value of diffusion coefficient, to 2 and 5. For complex 3 only one
irreversible reduction wave is seen at –0.41 V (vs SCE, not listed
in Table 6). A similar instability with respect to electrochemical
reduction has been observed for [Fe2O(MeCO2)2(HBpz3)2, for
which an irreversible reduction occurs at –0.76 V (vs. SCE),18b
The electrochemical behavior of 2 and 5 suggests the possibility
of preparing mixed valence analogues of these complexes. The
difference between the first and second reduction potentials for
complexes 2 (1.36 V) and 5 (1.20 V) specifies the thermodynamic
stability of their mixed valence states. As to kinetic stability, the
irreversible character of the peaks (which remains even at −30 ◦C)
indicates that the mixed valence (Fe(III)Fe(II)) and fully reduced
all-ferrous forms of the complexes are unstable on the timescale
of cyclic voltammetry, and/or that a specific stage of the electron
transfer process is slow due to structural reorganization induced
by the transfer of an electron. It is likely that such structural
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reorganization only consists of relatively minor changes in the
coordination spheres of the irons and not a change in the nuclearity
of the complex, at least in the case of electrochemically generated
mixed valence species. If the tetranuclear composition of com-
plexes 2 and 5 was not retained and the complexes were to fragment
upon reduction, then a substantially larger current should be
observed for the second reduction wave because diffusion of
mononuclear complexes should be much higher than for di- or
tetranuclear complexes. As expected, the addition of benzoate to
a solution of complex 5 resulted in a negative shift in reduction
potential, in agreement with the formation of cation 2 (Table 6).
mechanism for alkane oxidation requires initial coordination of
H2O2 at one metal of the Fe2O dimer. In cases where there is a
dicarboxylate bridge between irons, this requires prior detachment
of a carboxylate oxygen to create a vacant coordination site.27
Structural features of the benzoate ligand of 2 show a localized
=
C O double bond for the carboxylate and lengths to the metals
that suggest that it is poised for detachment from Fe2 to create
a site for H2O2 addition. However, during the catalytic cycle
there is constant competition between the detached carboxylate
oxygen and peroxide for the vacant coordination site. Complete
dissociation of the carboxylate appears unlikely since the activity
of the complex remains less than that of 5, and there is a correlation
between catalytic activity and redox potential (Table 6). Further,
the activity of 5 is expected to be higher since it contains twice
the number of labile coordination sites than the carboxylate-
detached Fe2O dimer. The high stereospecificity observed for all
three complexes is a clear indication that the oxidation reaction
occurs at one (or both) of the iron centers, and, to our knowledge,
these are the first observations of stereospecific alkane oxidation
catalyzed by (l-carboxo)diiron complexes.
Catalytic activity of complexes 2, 3 and 5 in alkane oxidation with
hydrogen peroxide
An acetonitrile solution containing one of the iron complexes (2,
3 or 5; 0.70 mM), hydrogen peroxide and cyclohexane, in a molar
ratio of 1 : 420 : 1000, was stirred for 2.5 h at room temperature
in air. Cyclohexanol and cyclohexanone were observed to form
at a turnover range of 4 to 7 (Table 7). Complex 1 is not
active under these conditions and complex 4 was not checked.
The yield per H2O2 consumed is not large (7–13%) due to
disproportionation of H2O2 as a side reaction. It was possible
to increase the conversion of oxidant into organic products
when the oxidant was delivered by a syringe pump to suppress
disproportionation.23 This is especially the case for the aqua
complex 5 (see Table 7), where the yield is increased substantially
by the use of a syringe pump. The high stereospecificity in
the oxidation of cis-1,2-dimethylcyclohexane points to a metal-
based molecular mechanism24 similar to the mechanism of oxo
transfer suggested for enzymes such as cytochrome P-45025 and
sMMO.26 Catalytic oxidation of cyclohexane by complex 2 was
also performed under a vigorous argon purge in order to remove
any traces of dioxygen and to check its influence on the observed
results.24b In order to exclude a diminution of the concentration
of the relatively volatile cyclohexane in the catalytic solution, the
argon gas was saturated with cyclohexane vapors. No differences
could be detected between the experiments carried out under
argon or air, indicating that there is no involvement of O2 in this
oxidation.
In general, stereospecific alkane oxidation reactions catalyzed
by non-heme iron complexes are rare,12 and every new example is
interesting. The ligand composition of iron in complexes 2, 3 and
5 is similar to that in the iron tripicolylamine (TPA) complexes29
that also catalyze stereospecific alkane oxidation. The ligand
environments in 2, 3, and 5 differ from the TPA complexes in
the respect that one pyridine nitrogen (from the TPA ligand) has
been replaced by an oxygen of a carboxylate. In comparison with
Fe–TPA complexes, the rate of oxidation by complexes 2, 3 and
5 is slower, perhaps due to a geometric constraint created by the
framework of ligand L. At the same time these complexes are
stable at high H2O2 concentration allowing them to be used in
“one-pot” experiments. It has been shown in earlier papers28,29
that mononuclear iron(II) complexes used in such reactions are
transformed into dinuclear Fe2O complexes a short time after
the beginning of the reaction. However, the data in these papers
does not point uniquely to either a mono- or dinuclear complex
as the active catalyst in oxo transfer and usually a mechanism
involving a mononuclear center is suggested.29 According to mass
spectrometry data, the dinuclear structure of complex 2 is stable up
to micromolar concentrations in MeCN due to the encapsulation
of the iron ions by the framework polydentate ligand. Therefore
The catalytic activity of complexes 2 and 3 is lower than that of
the more short lived complex 5 (see Table 7). The most plausible
Table 7 Alkane oxidation with H2O2 catalyzed by iron complexesa
Cyclohexane, one-pot
Cyclohexane, syringe pump
Complex
TN
A/K
2.5
Y (%)
TN
A/K
Y (%)
cis-DMCH RC (%)
Ref.
19
[Fe2O(phenNO2)4(H2O)2](ClO4)4
14.0
10
72b
82c
100
94
93
93
[FeII(tpa)(MeCN)2](ClO4)2
5
2
3
3.0
4.3
3.6
3.5
5.0
3.7
2.3
3.0
32
52
13
12
23
6.5
4.5
4.0
2.4
2.3
2.5
13
9
6.5
This work
This work
This work
a Introduction of H2O2: “one-pot”—as one portion at the beginning of the reaction together with other reagents, “syringe pump”—delivered dropwise
by syringe pump over the time of the reaction. TN—Average turnover number, mole of products (A + K)/mole of catalyst, A—cyclohexanol, K—
cyclohexanone, Y —yield of products (A + K) per H2O2 consumed, cis-DMCH—cis-1,2-dimethylcyclohexane, RC—retention of configuration in the
oxidation of the tertiary C–H bonds of cis-DMCH, expressed as the ratio of the corresponding tertiary alcohols: (cis − trans)/(cis + trans). b Value
obtained for the oxidation of trans-1,4-dimethylcyclohexane. c E. A. Gutkina, O. N. Gritsenko, unpublished data.
500 | Dalton Trans., 2006, 492–501
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it is possible to conclude in our case that the Fe2O centers with
labile coordination sites are active in oxo-transfer catalysis.
The framework octadentate ligand, 2,6-bis{3-[N,N-di(2-
pyridylmethyl)-amino]propoxy}benzoic acid (LH) prepared in
this project forms tetranuclear iron complexes 1–5 in which
separated Fe2O cores are linked via two molecules of LH or its
corresponding anion L. The structural and spectroscopic results
obtained provide evidence for the “dimer of dimers” structure of
complexes 2–5. When the carboxylate group of L bridges iron ions
inside the Fe2O core, the resulting complexes (2, 3 and 5) are active
in the catalysis of stereospecific alkane oxidation by hydrogen
peroxide, thus mimicking the corresponding diiron centres of the
MMO enzymes. These complexes provide new insights for further
structural development of chemical models for MMO.
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Acknowledgements
This work was supported by the Russian Foundation for Basic
Research (project no. 97-03-32253 and 00-15-97367), the Swedish
Research Council (VR), the Royal Swedish Academy of Sciences,
CRDF (grant # RC1-2058) and INTAS (grant # 97–1289). We
thank Prof. A. F. Dodonov, Dr V. I. Kozlovski and I. V. Souli-
menkov, Institute of Energetic Problems of Chemical Physics,
Chernogolovka, Russia and Dr Kenneth B. Jensen, Department
of Chemistry, Odense University, Denmark for electrospray mass
spectrometry of the iron complexes. We are also grateful to Dr
N. S. Ovanesyan for Mo¨ssbauer spectroscopy measurements, Prof.
˚
Ake Oskarsson for assistance with X-ray data collection and Prof.
Bernt Krebs for useful discussions.
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