Reactivity control in iron(III) amino triphenolate complexes: Comparison of monomeric and dimeric complexes

Article abstract of DOI:10.1021/ic3008624

Iron(III) amino triphenolate complexes with different substituents in the ortho-position of the phenolate moiety (R = H, Me, tBu, or Ph) have been synthesized by the reaction of iron(III) chloride and the sodium salt (Na ₃L^R) of the requisite ligand. The complexes have been shown to be of either monomeric ([FeL^R(THF)]) or dimeric ([FeL ^R]₂) nature by a combination of X-ray diffraction, ¹H NMR, solution magnetic susceptibility, and cyclic voltammetry studies. These analytical studies have shown that the monomeric and dimeric [FeL^R] complexes behave distinctively, and that the dimer stability is a function of the ortho-positioned groups. Both the dimeric as well as monomeric complexes were tested as catalysts for the catalytic cycloaddition of carbon dioxide to oxiranes, and the data show that the monomeric complexes are able to mediate this conversion with significantly higher activities than the dimeric complexes. This difference in reactivity is controlled by the substitution pattern on the ligand L^R, and is in line with the catalytic requisite of binding of the epoxide substrate by the iron(III) center.

Full text of DOI:10.1021/ic3008624

Article
pubs.acs.org/IC
Reactivity Control in Iron(III) Amino Triphenolate Complexes:
Comparison of Monomeric and Dimeric Complexes
Christopher J. Whiteoak,† Blerina Gjoka,‡ Eddy Martin,† Marta Martínez Belmonte,†
Eduardo C. Escudero-Adan,† Cristiano Zonta,‡ Giulia Licini,*,‡ and Arjan. W. Kleij*,†,§
́
†Institute of Chemical Research of Catalonia (ICIQ), Av. Països Catalans 16, 43007 Tarragona, Spain
‡Dipartimento di Scienze Chimiche, Universita
̀
di Padova, Via Marzolo 1, 35131, Padova, Italy
§Catalan Institute of Research and Advanced Studies (ICREA), Pg. Lluís Companys 23, 08010 Barcelona, Spain
S
* Supporting Information
ABSTRACT: Iron(III) amino triphenolate complexes with different substituents
in the ortho-position of the phenolate moiety (R = H, Me, tBu, or Ph) have been
synthesized by the reaction of iron(III) chloride and the sodium salt (Na3LR) of
the requisite ligand. The complexes have been shown to be of either monomeric
([FeLR(THF)]) or dimeric ([FeLR]2) nature by a combination of X-ray
1
diffraction, H NMR, solution magnetic susceptibility, and cyclic voltammetry
studies. These analytical studies have shown that the monomeric and dimeric
[FeLR] complexes behave distinctively, and that the dimer stability is a function of
the ortho-positioned groups. Both the dimeric as well as monomeric complexes
were tested as catalysts for the catalytic cycloaddition of carbon dioxide to
oxiranes, and the data show that the monomeric complexes are able to mediate
this conversion with significantly higher activities than the dimeric complexes. This difference in reactivity is controlled by the
substitution pattern on the ligand LR, and is in line with the catalytic requisite of binding of the epoxide substrate by the iron(III)
center.
differences in activity and selectivity during catalysis. A
representative example in iron catalysis of these differences in
activities for different ligand topologies has been reported using
an iron(II) bis-aminopyridyl complex (N2Py2 donor ligand),
where it has been shown that for oxidation catalysis the cis-α
ligand topology leads to the most active catalyst over the cis-β
and trans topologies.17 The amino triphenol ligand offers
similar donor moieties to the salan ligand, with the benefit of a
controlled coordination topology similar to the TPA ligand and
to this end there have also been reports of “intermediate”
NPy2O and NPyO2 type donor ligands.18−24 Another
interesting advantage of the NO3 donor ligand over the other
types mentioned above, is the potential to realize neutral
M(III) complexes.
With regards to catalytic activity, there have been relatively
limited studies with metal complexes bearing the amino
triphenol ligand. It has been reported that amino triphenolate
complexes of titanium, zirconium and hafnium are active
catalysts for lactide polymerization.25−27 Efficient oxidation
catalysis results have also been achieved using titanium,
vanadium and molybdenum amino triphenolate com-
plexes.28−30 Iron(III) complexes bearing amino triphenolate
ligands were originally reported by Koch and co-workers more
than 10 years ago, but the catalytic potential of these complexes
INTRODUCTION
■
Iron mediated catalysis has and continues to receive
considerable attention as a result of the relatively low toxicity
and price of iron when compared to late transition metals.1−4
In addition, iron is one of the most abundant metals in nature
and plays a key role in many essential enzymatic conversions,5
which continues to inspire the use of iron in synthetic catalysis.
Simple iron salts have been shown to be capable of mediating
some catalytic conversions, for example, in Fenton chemistry.6
To attain greater control of the products and increase activities,
ligand systems are generally included in synthetic catalysts. The
ligands present in these synthetic catalysts significantly affect
reactivity by a combination of electronic and steric effects.
Recently, a tripodal ligand system bearing a central amino
donor and three phenoxide donors (NO3 donor ligand) has
attracted great interest (amino triphenol, Figure 1).7 We
consider this ligand to be a combination of the well exploited
N2O2 donor ligand, bis-aminophenolate (salan) and the NPy3
donor ligand, tripyridyl amine (TPA) (Figure 1), which have
both given rise to a variety of metal complexes with interesting
catalytic properties.8−15 A major drawback of the salan ligand is
its ability to form three principal ligand topologies around the
metal center when the complex adopts an octahedral
coordination mode; cis-α, cis-β, and trans (Figure 1), and this
results in limited potential for asymmetric catalysis compared
with the corresponding more rigid Schiff bases.16 Further, it is
likely that these different ligand topologies will impart distinct
Received: April 27, 2012
© XXXX American Chemical Society
A
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Figure 1. Structures of amino phenol, amino pyridyl phenol, and amino pyridyl ligands with representations of the possible ligand topologies
adopted upon coordination to a metal.
was not exploited at the time.31 More recently we have
Optics FTIR Alpha spectrometer equipped with a DTGS detector,
KBr beamsplitter at 4 cm−1 resolution. UV−vis measurements were
demonstrated that an iron(III) amino triphenolate complex is
able to catalyze the cycloaddition of carbon dioxide to a wide
variety of oxiranes and oxetanes to yield the corresponding
cyclic carbonate products under energetically favorable reaction
carried out on a Shimadzu UV-1800 spectrophotometer.
[FeLH]2. To a suspension of sodium hydride (75.2 mg, 3.13 mmol)
in tetrahydrofuran (10 mL) was slowly added a solution of H3LH (350
mg, 1.04 mmol) in tetrahydrofuran (10 mL). The mixture was stirred
conditions, further demonstrating the applicability of these
for 18 h and after this time it was added to a solution of anhydrous
amino triphenol ligands.32 Herein we will describe an extension
iron(III) chloride (167.7 mg, 1.04 mmol) in tetrahydrofuran (10 mL).
of our previously reported work with respect to a refinement of
the ligand structure by steric variation at the ortho-position of
the phenoxide moiety (H, Me, tBu, and Ph). The work will
show characterization of these complexes by X-ray diffraction,
The mixture was stirred for a further 4 h and then filtered through a
path of Celite, followed by removal of the solvent to yield a brown
residue, which was subsequently dissolved in dichloromethane, filtered
and the solvent then removed to yield a brown powder. Yield: 397 mg
(88%). Anal. Calcd for C42H36Fe2N2O6: C, 64.97, H, 4.67; N, 3.61.
Found: C, 64.92; H, 4.61; N, 3.38. MALDI(+)−MS (pyrene): m/z =
388 [M]+ (calcd. 388), 777 [2M+H]+ (calcd 777). UV−vis (CH2Cl2,
0.1 mM, 25 °C, ε = L·mol−1·cm−1): 314 nm (ε = 7970), 358 nm (ε =
5290, sh), 430 nm (ε = 3510). Magnetic moment (298 K) μeff = 4.14
μB. The remaining complexes were prepared by using the same
methodology.
1
cyclic voltammetry, H NMR studies, and in addition, the
reactivity of the complexes will be evaluated in terms of their
ability to mediate the catalytic cycloaddition of carbon dioxide
to oxiranes.
EXPERIMENTAL SECTION
■
[FeLMe]2. Yield: 231 mg (89%). Anal. Calcd for C48H48Fe2N2O6: C,
66.99; H, 5.62; N, 3.26. Found: C, 67.21; H, 5.82; N, 3.02.
MALDI(+)−MS (pyrene): m/z = 430 [M]+ (calcd. 430). UV−vis
(CH2Cl2, 0.1 mM, 25 °C, ε = L·mol−1·cm−1): 329 (ε = 4065), 416 (ε
= 3625). Magnetic moment (298 K) μeff = 6.74 μB.
General Procedures. The ligands, H3LH, H3LMe, H3LtBu, and
H3LPh were prepared by known literature procedures.33 The mono- and
di-[iron(III) amino triphenolate] complexes were prepared using
standard Schlenk and glovebox techniques under a nitrogen
atmosphere. Once prepared, the complexes were no longer considered
as air/moisture sensitive and were stored on the bench. Tetrahy-
drofuran used during the synthesis of the complexes was dried using a
Solvent Purification System (SPS), and all other solvents were of
reagent grade and used without any further purification. All other
reagents and carbon dioxide (purchased from PRAXAIR) were used as
received with no further purification. Elemental analysis was
[FeLtBu(THF)]. Yield: 408 mg (90%). Anal. Calcd for C37H50FeNO4:
C, 70.69; H, 8.02; N, 2.23. Found: C, 70.60; H, 8.34; N, 2.12.
MALDI(+)−MS (dithranol): m/z = 557 [M+H−THF]+ (calcd. 557).
UV−vis (CH2Cl2, 0.2 mM, 25 °C, ε = L·mol−1·cm−1): 329 nm (ε =
3570), 410 nm (ε = 3135). Magnetic moment (298 K) μeff = 5.49 μB.
[FeLPh(THF)]. Yield: 266 mg (61%). Anal. Calcd for C43H38FeNO4:
C, 75.00; H, 5.56; N, 2.03. Found: C, 74.75; H, 5.80; N, 2.12.
MALDI(+)−MS (pyrene): m/z = 616 [M−THF]+ (calcd. 616). UV−
vis (CH2Cl2, 0.2 mM, 25 °C, ε = L·mol−1·cm−1): 357 nm (ε = 5460),
426 nm (ε = 5045). Magnetic moment (298 K) μeff = 5.53 μB.
́ ́
performed by the Unidad de Analisis Elemental at the Universidad
de Santiago de Compostela (Spain). X-ray diffraction studies and mass
spectroscopy measurements were carried out by the Research Support
Group at ICIQ. FT-IR measurements were carried out on a Bruker
B
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Table 1. X-ray Structure Data Collection and Refinement Parameters for All Reported Structures
[FeLPh(trans-2,3-
epoxybutane)]
[FeLH]2
[FeLMe(THF)]
[FeLtBu(H2O)]
[FeLPh(THF)]
[FeLPh(Py)]
formula
Fw (g·mol−1
T (K)
C105H96Fe4N4O2
1829.26
C28H32FeNO4
502.40
C33H46FeNO5
592.56
C43H34FeNO4
684.56
C44H35FeN2O3
695.59
C43H38FeNO4
688.59
)
100(2)
100(2)
100(2)
100(2)
100(2)
100(2)
λ (Å)
0.71073
0.71073
triclinic
0.71073
0.71073
0.71073
monoclinic
P21/c
0.71073
crystal system
space group
a (Å)
triclinic
monoclinic
C2/c
monoclinic
P21/c
monoclinic
P21/c
P1
̅
P1
̅
13.5240(13)
17.1679(17)
19.236(2)
77.392(3)
79.290(3)
78.116(4)
4218.4(7), 2
1.440
10.4445(6)
10.9051(6)
11.2722(6)
73.886(2)
75.873(2)
79.199(2)
1186.23(11), 2
1.407
25.102(2)
10.9015(11)
25.252(3)
90.00
16.0348(19)
16.1658(18)
13.0742(14)
90.00
16.140(2)
16.580(2)
12.8255(16)
90.00
16.0716(16)
16.2018(17)
13.2037(12)
90.00
b (Å)
c (Å)
α (deg)
β (deg)
112.938(3)
90.00
92.126(4)
90.00
91.774(4)
90.00
94.218(3)
90.00
γ (deg)
volume (Å3), Z
ρ(calcd) (mg·m−3
6363.8(11), 8
1.237
3386.7(7), 4
1.343
3430.3(8), 4
1.347
3428.8(6), 4
1.334
)
μ (mm−1
)
0.744
0.671
0.513
0.491
0.484
0.485
absorption correction
empirical
empirical
empirical
empirical
empirical
empirical
refinement method
full-matrix least-
full-matrix least-
full-matrix least-
full-matrix least-
full-matrix least-
full-matrix least-
squares on F2
squares on F2
squares on F2
squares on F2
squares on F2
squares on F2
data/restraints/
parameters
GOF on F2
26557/0/1129
6294/0/311
5817/4/382
8248/641/586
8392/204/496
8457/2746/953
1.043
1.041
1.161
1.066
1.050
1.056
final R indices
[I > 2σ(I)]
R1 = 0.0546
R1 = 0.0359
R1 = 0.0833
R1 = 0.0979
R1 = 0.0770
R1 = 0.0678
wR2 = 0.1238
R1 = 0.1003
wR2 = 0.0944
R1 = 0.0402
wR2 = 0.2054
R1 = 0.1002
wR2 = 0.2416
R1 = 0.1453
wR2 = 0.1640
R1 = 0.1346
wR2 = 0.1624
R1 = 0.0944
R indices (all data)
largest diff. peak and
wR2 = 0.1438
0.833 and −0.889
wR2 = 0.0970
0.822 and −0.257
wR2 = 0.2144
1.267 and −0.671
wR2 = 0.2524
1.161 and −1.170
wR2 = 0.1824
0.426 and −1.032
wR2 = 0.1769
0.798 and −0.972
hole (e Å−3
)
Catalytic Experiments. In a typical catalytic experiment the
oxirane substrate (2.0 mmol), tetra-butyl-ammonium bromide (32.6
mg, 0.1 mmol), the respective iron(III) amino triphenolate complex
(0.01 mmol of complex [FeLH]2 or [FeLMe]2, and 0.02 mmol
[FeLtBu(THF)] or [FeLPh(THF)]) and mesitylene (278 μL, 2.0
mmol) were dissolved in the respective solvent (5 mL). The reaction
mixture was then transferred to a stainless steel autoclave and three
cycles of pressurization and depressurization with carbon dioxide were
applied (pCO2 = 0.5 MPa). The final pressure was then adjusted to 1.0
MPa, and the reaction was left stirring at the required temperature for
X-ray Crystallography. Crystals suitable for single crystal X-ray
analysis were obtained by slow evaporation of tetrahydrofuran
solutions of [FeLMe(THF)], [FeLtBu(THF)], and [FeLPh(THF)] or
in the case of [FeLH]2, crystals were obtained from a concentrated
toluene solution after three weeks. The structure obtained for [FeLtBu
]
contains a molecule of water instead of the expected molecule of
tetrahydrofuran as the coligand and is denoted as [FeLtBu(H2O)]. The
crystals of [FeLPh(Py)] were grown by slow evaporation of a
concentrated solution of the complex in tetrahydrofuran/pyridine
(50:1) and the crystals of [FeLPh(trans-2,3-epoxybutane)] were
obtained in a similar manner from a solution of tetrahydrofuran/
trans-2,3-epoxybutane (25:1).
1
18 h. After this time, the yield was calculated using the H NMR
spectrum (d6-DMSO) of an aliquot of the reaction mixture and
The measured crystals were stable under atmospheric conditions;
nevertheless they were treated under inert conditions immersed in
perfluoropoly ether as protecting oil for manipulation. Data collection
measurements were made on a Bruker-Nonius diffractometer
equipped with an APPEX 2 4 K CCD area detector, an FR591
rotating anode with MoKα radiation, Montel mirrors, and a Kryoflex
low temperature device (T = −173 °C). Full-sphere data collection
was used with ω and ϕ scans. Programs used: data collection Apex2
V2011.3 (Bruker-Nonius 2008), data reduction Saint+ Version 7.60 A
(Bruker AXS 2008) and absorption correction SADABS V. 2008-1
(2008). For structure solution SHELXTLVersion 6.10 (Sheldrick,
2000)36 was used (Table 1). Structure refinement: SHELXTL-97-
UNIX version.
mesitylene as the internal standard.
Magnetic Susceptibility Measurements. Evans NMR technique
was used for the determination of the magnetic susceptibilities of the
complexes.34,35 The solvent system used was CDCl3:cyclohexane
(95:5 v/v), and the diamagnetic susceptibility was neglected during the
calculation. To each of the samples, pyridine was added to give 10% v/
v of the solvent system where the magnetic susceptibility in the
presence of pyridine is reported.
UV−vis Titration Studies. A 0.1 mM dichloromethane solution of
[FeLH]2 was prepared and the UV−vis spectrum recorded at 25 °C
using a quartz cell with a 1 cm path length. This solution was titrated
with propylene oxide or pyridine, with the UV−vis spectrum measured
immediately after each addition.
Cyclic Voltammetry Studies. Cyclic voltammetric measurements
were carried out with a Princeton Applied Research PARSTAT 2273
electrochemical analyzer. A three electrode assembly, comprising a
glassy carbon working electrode, a platinum wire auxiliary electrode,
and Ag−AgCl (sat. NaCl) reference electrode was used. The
experimental data were obtained under an argon atmosphere in
dichloromethane solvent using NBu4PF6 as supporting electrolyte.
Results involving the presence of pyridine were obtained by using a
dichloromethane:pyridine (90:10 v/v) solvent system.
RESULTS AND DISCUSSION
■
Complex Synthesis and Characterization. Synthetic
routes to all the ligands used in this investigation have been
published previously, and the ligands have been synthesized as
described in these reports.26 The reaction of H3LR (R = H,
methyl, tert-butyl, or phenyl) with 3 equiv of sodium hydride in
tetrahydrofuran and subsequent addition of 1 equivt of
iron(III) chloride yields analytically pure mono- or di-[iron(III)
C
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Scheme 1. Synthesis of Mono- and Di-[Iron(III) Amino Triphenolate] Complexes
Figure 2. Solid state structures of [FeLH]2 (top left), [FeLMe(THF)] (top right), [FeLtBu(H2O)] (bottom left), and [FeLPh(THF)] (bottom right).
H-atoms, cocrystallized solvent and disorder have been omitted for clarity, and only a partial numbering scheme is provided.
amino triphenolate] complexes after filtration of the reaction
mixture and removal of the solvent (Scheme 1). All complexes
have been initially characterized by MALDI(+) mass
spectrometry and elemental analysis, and the obtained data is
consistent with the proposed structures. Further to this
characterization data, it has been possible to obtain solid state
structures of all the complexes, which confirm the presence of
iron(III) amino triphenolate units.
tetrahydrofuran solutions of the complexes, except for
[FeLH]2 where crystals were obtained from a concentrated
toluene solution. The solid state structures of the complexes
obtained are shown in Figure 2, with selected bond lengths and
angles shown in Table 2. The molecular structure of the
complex obtained from H3LH consists of two iron(III) amino
triphenolate complexes forming a dimeric assembly, [FeLH]2,
with one phenoxide O-atom from each individual complex
acting as a bridging ligand, resulting in the formation of a
central Fe2O2 motif. The solid state structures of the complexes
Solid State Structures. Crystals suitable for X-ray
diffraction studies were obtained by slow evaporation of
D
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Table 2. Selected Bond Lengths (Å) and Angles (deg) in [FeLH]2, [FeLMe(THF)], [FeLtBu(H2O)], and [FeLPh(THF)]
[FeLH]2
[FeLMe(THF)]
[FeLtBu(H2O)]
[FeLPh(THF)]
Bond Lengths (Å)
1.8643(12)
1.8555(12)
1.8685(11)
2.1063(11)
2.1795(13)
Bond Angles (deg)
121.15(5)
Fe(1)−O(1)
Fe(1)−O(2)
Fe(1)−O(3)
Fe(1)−O(4)
Fe(1)−N(1)
1.9713(18)
1.8611(18)
1.8519(19)
2.0642(17)
2.1440(20)
1.885(4)
1.853(4)
1.861(4)
2.097(4)
2.178(4)
1.856(3)
1.859(3)
1.878(3)
2.055(5)
2.163(4)
O(1)−Fe(1)−O(2)
O(1)−Fe(1)−O(3)
O(2)−Fe(1)−O(3)
O(1)−Fe(1)−O(4)
O(1)−Fe(1)−N(1)
N(1)−Fe(1)−O(4)
120.76(8)
116.00(8)
123.14(9)
77.44(7)
91.15(8)
168.33(8)
113.79(17)
130.41(17)
115.77(17)
92.16(16)
89.75(16)
172.98(17)
116.22(14)
121.94(14)
121.82(14)
93.41(18)
90.11(13)
173.95(19)
119.08(6)
119.58(5)
89.28(5)
91.71(5)
178.24(4)
obtained from H3LMe, H3LtBu and H3LPh display complexes of a
monomeric form containing an apical ligand originating from
the solvent medium. The molecular structure obtained for
recrystallized [FeLtBu(THF)] contains a water ligand in place of
the expected tetrahydrofuran ligand, [FeLtBu(H2O)], which is
likely to be a consequence of ligand exchange with water
present in the solvent used during crystallization, promoted by
the steric constraints imposed on the coordination sphere of
the iron center by the bulky tert-butyl groups.
Similar but less significant distortions from perfect C3 symmetry
are also observed in the case of [FeLPh(THF)] compared with
[FeLtBu(H2O)], indicating that the phenyl groups contribute
less to the steric constraints immediately around the iron(III)
center. The resulting angles being 116.22(14)°, 121.82(14)°,
and 121.94(14)° for O(1)−Fe(1)−O(2), O(1)−Fe(1)−O(3),
and O(1)−Fe(1)−O(3), respectively.
The dimeric complex, [FeLH]2, displays two Fe−O bonds
with lengths of 1.8611(18) Å and 1.8519(19) Å corresponding
to the nonbridging phenoxide moieties and two longer Fe−O
bonds with lengths of 1.9713(18) Å and 2.0642(17) Å for the
bonds involving the bridging phenoxide moieties. These bond
lengths are similar to those previously reported for a similar
dimeric structure.25
The molecular structure obtained for [FeLMe(THF)], as has
been mentioned, shows the monomeric form of the complex,
but further investigations (vide infra) indicate that a dimeric
structure may be easily formed as in the case of [FeLH]2. We
therefore propose that this complex forms a weak dimeric
structure and that the crystals obtained are as a result of
preferential crystallization of the monomer. No evidence of
dimeric complexes in the cases of [FeLtBu(THF)] or
[FeLPh(THF)] are proposed.
Magnetic Susceptibility Measurements. The Evans
method was used to calculate the solution magnetic
susceptibility values, μeff, for all complexes (Table 3).34,35 In
The solid state structure of [FeLMe(THF)] displays idealized
C3 symmetry, with the angles between the oxygen atoms of the
phenoxide moieties being 121.15(5)°, 119.08(6)°, and
119.58(5)° for O(1)−Fe(1)−O(2), O(1)−Fe(1)−O(3), and
O(2)−Fe(1)−O(3), respectively. The corresponding angle
between the amine and the oxygen atom of the tetrahydrofuran
ligand (178.24(4)° is close to the ideal value of 180° expected
in a perfectly C3 symmetrical molecule. The structure also
displays similar bond length values for all the Fe−O bonds with
the ligand and an elongated Fe−O bond length for the bond
between the tetrahydrofuran and metal.
The addition of tert-butyl groups impacts severely on the C3
symmetry of the complex in [FeLtBu(H2O)] compared with
[FeLMe(THF)]. It appears that the tert-butyl groups have a
significant steric effect on the structure. The angles between of
O(1)−Fe(1)−O(2), O(1)−Fe(1)−O(3), and O(1)−Fe(1)−
O(3) are significantly different from those expected in a perfect
C3 symmetrical molecule and have values of 113.79(17)°,
115.77(17)°, and 130.41(17)°, respectively. These distortions
are similar in value to those observed by Safaei and co-workers
for a similar complex containing a methanol ligand in the apical
position.37 A further difference in this molecule compared to
[FeLMe(THF)] is that not all the Fe−O bond lengths between
the metal and the ligand have the same length, with one of
them being clearly elongated. The observation of these
differences in structure adds weight to the proposal that the
original tetrahydrofuran ligand has exchanged with the water
ligand during crystallization to form a more stable complex.
Table 3. Solution Magnetic Suceptibility Data (μeff) for
a b
,
[FeLH]2, [FeLMe]2, [FeLtBu(THF)], and [FeLPh(THF)]
complex
complex + pyridine
[FeLH]2
[FeLMe
2.12 μB
3.37 μB
5.49 μB
5.53 μB
5.72 μB
5.43 μB
5.50 μB
5.95 μB
]
2
[FeLtBu(THF)]
[FeLPh(THF)]
a
b
In CDCl3 at 298 K. Values are on a per iron atom basis.
addition to this we have also calculated the μeff values for all the
complexes in the presence of pyridine for comparison. Both
[FeLtBu(THF)] and [FeLPh(THF)] give rise to μeff values
consistent with those expected for high-spin iron(III)
complexes (μeff = 5.49 μB and 5.53 μB respectively). In the
presence of pyridine these complexes also display μeff values
consistent with high-spin iron(III) complexes (μeff = 5.50 μB
and μeff = 5.95 μB respectively). We propose that in both these
cases the iron(III) complexes are in the monomeric form, both
before and after addition of pyridine.
Calculation of the μeff values for the dimeric complexes
[FeLH]2 and [FeLMe]2 gives rise to μeff values which are lower
than expected for noncoupled dimeric high-spin iron(III)
complexes (μeff = 2.12 μB and 3.37 μB on a per iron atom basis,
respectively). The decrease in the measured μeff values of these
complexes can be attributed to antiferromagnetic coupling
between the two iron(III) atoms. Other antiferromagnetically
coupled iron(III) phenolate bridged dimeric complexes have
E
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Figure 3. 1H NMR spectra (500 MHz, CDCl3, 298 K) of [FeLH]2, [FeLMe]2, [FeLtBu(THF)], and [FeLPh(THF)]; with/without the addition of d5-
Pyridine. For clarity 15 to −5 ppm region omitted.
Scheme 2. Schematic Representation of the Proposed Reaction of Pyridine with [FeLH]2
been reported previously and suppression of the μeff value is
observed as a result.38−41 The values also imply that the
antiferromagnetic coupling in [FeLH]2 is significantly stronger
than in [FeLMe]2, suggesting a stronger dimer in the former
complex. Upon addition of pyridine to both of these dimeric
complexes, μeff values consistent with monomeric high-spin
iron(III) complexes are obtained (μeff = 5.72 μB and 5.43 μB
respectively).
presence of a paramagnetic species, most likely as a result of
strong antiferromagnetic coupling between the two iron(III)
centers resulting in very broad signals. Upon addition of
pyridine to both of these complexes, it is possible to observe
the appearance of new resonances in the 90 to −80 ppm range,
indicating the formation of paramagnetic species. In the case of
[FeLH]2/pyridine, we propose that the observed resonances,
shown in Figure 3, are those of the aromatic protons of the
ligand. Three of the aromatic protons appear as a pair of
resonances (Ha′ = Ha″, Hb′ = Hb″, and Hc′ = Hc″), with the
resonance corresponding to the remaining ortho-proton of the
aromatic moiety being likely present in the 0−12 ppm range
(see Supporting Information). We propose that a pair of
resonances is observed for each proton because of the
possibility of either one or two pyridine ligands coordinating
to the iron(III) center, and this is further confirmed by the
presence of only a single set of resonances when the experiment
is run in neat d5-pyridine (see Supporting Information). In the
case of a single coordinated pyridine, the complex displays
trigonal bipyramidal geometry, whereas upon coordination of a
second pyridine ligand the complex becomes octahedral in
geometry, resulting in different orbital overlap and therefore
displaying slightly different chemical shifts (Scheme 2). The
proposed ability of [FeLH]2 to bind two external ligands and
form an octahedral complex is a result of lower steric
1H NMR Spectroscopy Studies. Paramagnetic complexes
generally exhibit broad resonances with large chemical shifts.
1
The H NMR spectra of all the complexes were obtained in
CDCl3 at 298 K and are shown in Figure 3, with the region
between 15 and −5 ppm omitted for clarity (this region
principally displays resonances for the protio-impurities of the
deuterated solvents, a broad resonance for the alkyl protons of
the ligand and in the case of [FeLtBu(THF)], a broad resonance
1
for the protons of the tert-butyl group). Additionally, the H
NMR spectra of all complexes have been obtained in a
CDCl3:d5-Pyridine (90:10 v/v) mixture to allow for compar-
ison with the species obtained in the solution magnetic
susceptibility measurements. We have tentatively assigned the
paramagnetically shifted resonances, as efforts to acquire 2D
NMR spectra have proved to be unsuccessful.
In the 1H NMR spectra of both [FeLH]2 and [FeLMe]2, there
are no observable strongly shifted resonances to indicate the
F
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constraints imposed by this ligand compared with the other
ligand structures and this complex would also be similar in
structure to the first iron(III) amino triphenolate complex
reported by Koch and co-workers, [FeLH(bipy)], containing a
bipyridine ligand.31 In the case of [FeLMe]2, the steric
constraints imparted by the methyl groups of the ligand
prevent the coordination of a second pyridine ligand, and hence
only a single set of resonances (Ha, Hb, and Hc) are observed
for each proton. The protons from the methyl group of
[FeLMe]2 are observed as the second resonance (CH3) at
around 60 ppm.
Information). A ligand-to-metal charge transfer band
(LMCT) is observed in all the complexes around 400−450
nm. A second LMCT band is observed for all complexes
around 310−360 nm and in addition, [FeLH]2 displays a third
LMCT band in this region as a shoulder at 358 nm which is
most likely as a result of the bridging phenoxide moieties.
It has been previously reported that a complex similar to
[FeLMe]2 can be disrupted by titration with propylene oxide
(i.e., a substrate used in cycloaddition catalysis, vide infra).25 All
the complexes presented in this work have also been titrated in
a similar fashion (see Supporting Information). Upon titrating
[FeLMe]2 with propylene oxide we also observed a similar
change in spectrum suggesting disruption of a dimeric
structure. Titration of the remaining complexes with the same
titrant does not give rise to any significant changes in the
spectra, and therefore we propose that this is a result of
[FeLtBu(THF)] and [FeLPh(THF)] already being in the
monomeric forms and that propylene oxide is not a strong
enough ligand to disrupt the dimeric structure of [FeLH]2.
When the dimeric complex [FeLH]2 is titrated with pyridine (a
stronger competitive ligand), a significant change in spectra is
observed (Figure 5). For comparison [FeLPh(THF)] was also
titrated with an excess of pyridine and in this case only a small
change in spectrum was observed. It is therefore suggested that
[FeLH]2 forms a stronger dimeric structure than [FeLMe]2 and
as a result [FeLH]2 requires a stronger ligand to disrupt its
dimeric structure.
Both the spectra of [FeLtBu(THF)] and [FeLPh(THF)]
display the paramagnetically shifted resonances for the aromatic
protons (Ha, Hb, and Hc) in the absence of pyridine, further
adding to the evidence that these two complexes exist in their
monomeric form. Upon addition of pyridine to [FeLtBu(THF)]
there is no change in the spectrum, as the pyridine is only
substituting the tetrahydrofuran ligand to yield [FeLtBu(Py)] in
situ. The same behavior is not true for [FeLPh(THF)], as after
addition of pyridine the resonances which we propose are
assigned to the protons of the ortho-phenyl group disappear
(Hd, He, and Hf). The solid state structure of [FeLPh(Py)] has
been obtained (Figure 4, Table 4) and indicates that pyridine
can coordinate and act as a ligand though resulting in a reduced
rotational freedom of the Ph groups of the amino triphenolate
ligand.
Electrochemical Studies. Cyclic voltammetry studies have
been carried out in dichloromethane for all the complexes
(Figure 6 and Table 5). Data has also been obtained for all
complexes in dichloromethane with pyridine (10% v/v). The
monomeric complexes [FeLtBu(THF)] and [FeLPh(THF)]
exhibit quasi-reversible waves at −1.008 and −0.825 V,
respectively. Both of these complexes exhibit changes in E1/2
upon addition of pyridine, although the nature of these waves
remains quasi-reversible.
Both [FeLH]2 and [FeLMe]2 give rise to a more complex
electrochemical behavior. In the case of [FeLMe]2, it appears
that there are two sequential reductions (i.e., addition of an
electron to both iron(III) centers independently), as though
the iron(III) centers are coupled; the antiferromagnetic
coupling is not as strong as that observed in [FeLH]2. Upon
addition of pyridine to [FeLMe]2 only a single quasi-reversible
wave is observed with a similar E1/2 to the monomeric pyridine
containing species derived from [FeLPh(THF)]. The dimeric
complex [FeLH]2 exhibits a single quasi-reversible wave at
−0.288 V. This value is significantly lower than for the
monomeric species, most likely because upon addition of an
electron the electron density is completely delocalized between
two iron centers (Fe2O2 motif) rather than a single one, making
this reduction more facile. Upon addition of pyridine to
[FeLH]2 a complex cyclic voltammogram containing three
quasi-reversible waves is obtained, and we propose that this
results from a mixture of monomeric species having either one
or two pyridines associated to the complex as was discussed
Figure 4. Solid state structure of [FeLPh(Py)]. H-atoms and
cocrystallized solvent molecules, and rotational disorder are omitted
for clarity. A partial numbering scheme (around the Fe center) is
provided.
Table 4. Selected Bond Lengths (Å) and Angles (deg) in
[FeLPh(Py)]
bond lengths (Å)
bond angles (deg)
O(1)−Fe(1)−O(2)
O(1)−Fe(1)−O(3)
O(1)−Fe(1)−N(1)
O(1)−Fe(1)−N(2)
N(1)−Fe(1)−N(2)
Fe(1)−O(1)
1.856(3)
1.870(3)
1.860(3)
2.182(4)
2.135(4)
123.40(14)
117.45(13)
88.43(13)
89.76(13)
177.99(13)
1
previously in the H NMR study of this complex (Scheme 2).
Fe(1)−O(2)
Fe(1)−O(3)
Fe(1)−N(1)
Fe(1)−N(2)
Catalytic Studies. Previously, we reported that an iron(III)
amino triphenolate complex was able to successfully mediate
the catalytic cycloaddition of carbon dioxide to a range of
oxiranes and oxetanes.25 Having established that the sub-
stitution pattern on the amino triphenolate ligand L has a
pronounced effect on the electrochemical, spectroscopic, and
solid state features, we next examined the influence of the
UV−vis Studies. The electronic spectra of all the complexes
are very similar to one another (see the Supporting
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Figure 5. UV−vis spectra in DCM of [FeLH]2 (left), [FeLPh(THF)] (right), before and after addition of pyridine.
Figure 6. Cyclic voltammograms at 298 K in CH2Cl2 at 298 K (scan rate = 100 mV s−1) using 0.1 M NBu4PF6 as supporting electrolyte at a glassy
carbon working electrode, platinum wire auxiliary electrode, and Ag-AgCl (sat. NaCl) reference electrode of [FeLH]2, [FeLMe]2, [FeLtBu(THF)], and
[FeLPh(THF)]: before and after addition of 5% v/v pyridine.
Table 5. Electrochemical Data for [FeLH]2, [FeLMe]2,
monomer-to-dimer equilibrium in catalysis as the effect (and
a
[FeLtBu(THF)], and [FeLPh(THF)]
optimization) of varying the catalyst structure and the steric
bulk around the iron(III) center has not yet been established.
Table 6 shows the results obtained from the catalytic
cycloaddition of carbon dioxide (CO2) to various oxiranes
providing organic carbonates (Scheme 3) using the iron(III)
amino triphenolate complexes reported in this work.
When propylene oxide is studied as the substrate in
dichloromethane it can be observed that the monomeric
complexes are significantly more active than the dimeric
complexes. When the solvent is exchanged for methylethyl
ketone (MEK), which is considered to be a solvent with
coordinating potential and thus can disrupt the less active
dimeric structure, the use of both monomeric complexes results
in good yields of carbonate. In addition to this, both the
E1/2{complex}
E1/2{complex/pyridine}
complex
[FeLH]2
[FeLMe
[FeLtBu(THF)]
[FeLPh(THF)]
E
1/2(1)
E1/2(2)
E1/2(1)
E1/2(2)
−0.593
E1/2(3)
−0.160
−0.288
−1.013
−1.008
−0.825
−1.055
−0.732
−1.095
−0.715
b
]
−0.930
2
a
In CH2Cl2 at 298 K (scan rate = 100 mV s−1) using 0.1 M NBu4PF6
as supporting electrolyte at a glassy carbon working electrode,
platinum wire auxiliary electrode and Ag-AgCl (sat. NaCl) reference
b
electrode. Potential for irreversible reduction.
H
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a
Table 6. Yields (%) of Cyclic Carbonates Obtained Using [FeLH]2, [FeLMe]2, [FeLtBu(THF)], and [FeLPh(THF)] as Catalysts
for the Cycloaddition of Carbon Dioxide to Various Oxiranes
a
Yields (%) calculated using 1H NMR and mesitylene as internal standard. Conditions: 2.0 mmol epoxide, 0.01 mmol [FeLH]2 and [FeLMe]2 or 0.02
b
mmol [FeLtBu(THF)] and [FeLPh(THF)], 0.1 mmol nBu4NBr, 2.0 mmol mesitylene, 5 mL of solvent, 1.0 MPa CO2, 25 °C, 18 h. At 85 °C. MEK =
methylethyl ketone, DCM = dichloromethane.
possible to form the more active monomeric species from
[FeLH]2 and [FeLMe]2.
Scheme 3. Cycloaddition of Carbon Dioxide to Oxiranes to
Give Cyclic Organic Carbonates
We have been able to obtain crystals suitable for single crystal
X-ray diffraction studies from [FeLPh(THF)], whereby the
tetrahydrofuran ligand is replaced by a trans-2,3-epoxybutane
ligand (Figure 7, Table 7), indicating that the substrate is
dimeric complexes now show enhanced activities, as a probable
result of formation of a monomeric MEK-adduct prior to
epoxide coordination and conversion. Interestingly, the use of
MEK results in a slight increase in activity of [FeLMe]2 over
[FeLH]2, and we attribute this increase in activity to the fact
that MEK is a better competitive ligand for dimer dissociation
in the complex [FeLMe]2 compared with [FeLH]2 resulting in a
more active system.
When styrene oxide is used as substrate and dichloromethane
as the solvent, the monomeric complexes are again observed to
be considerably more active than the dimeric complexes.
[FeLPh(THF)] is seen to be less active than [FeLtBu(THF)].
This result can be rationalized considering the accessibility of
the substrate upon coordination to the iron(III) center; in the
case of [FeLPh(THF)] the substrate is relatively more
embedded in the pseudocavity of the system and thus hinders
the bromide for nucleophilic attack of the coordinated epoxide.
When trans-2,3-epoxybutane is investigated as substrate and
dichloromethane as the solvent at 85 °C, poor activity is again
observed for the dimeric complexes. In contrast with the
previous substrates, [FeLtBu(THF)] also proves to be a poor
catalyst for this substrate compared with [FeLPh(THF)]. The
difference in activities of these two monomeric complexes in
dichloromethane can be rationalized by considering the steric
effect of an epoxide bearing substituents on both carbon atoms;
in this case, the steric restrictions near the coordination sphere
of the metal are more dominant thus showing better
performance for the less congested complex [FeLPh(trans-2,3-
epoxybutane)]. The steric constraints around the metal center
can also be observed by looking at the increased distortion in
the O−Fe−O bond angles in the solid state structure of
[FeLtBu(THF)] compared with [FeLPh(THF)]. Upon changing
the solvent to MEK, all the complexes (except [FeLtBu(THF)])
display similar activities. The more sterically congested
[FeLtBu(epoxide)] intermediate complex has lower activity
than the other complexes as it is probably more difficult to
form. Again, under these conditions (elevated temperature,
coordinating medium = MEK) it appears that it has been
Figure 7. X-ray Crystal Structure of [FeLPh(trans-2,3-epoxybutane)].
H-atoms, cocrystallized solvent molecules, and the disorder in the
position of the epoxide ligand and the Ph groups of the ligand L are
omitted for clarity. Only a partial numbering scheme is provided.
indeed able to bind to the iron(III) center. Thus, it seems that
the catalytic activity of these complexes is controlled by their
tendency to form dimeric structures, the solvent medium, the
reaction temperature, and the steric impediment upon
coordination of the substrate to the iron(III) center. Notably,
Table 7. Selected Bond Lengths (Å) and Angles (deg) in
[FeLPh(trans-2,3-epoxybutane)]
bond lengths (Å)
bond angles (deg)
O(1)−Fe(1)−O(2)
Fe(1)−O(1)
1.860(2)
1.860(2)
1.880(2)
2.145(10)
2.174(2)
116.23(10)
119.79(10)
90.66(9)
Fe(1)−O(2)
Fe(1)−O(3)
Fe(1)−O(4)
Fe(1)−N(2)
O(1)−Fe(1)−O(3)
O(1)−Fe(1)−N(1)
O(1)−Fe(1)−O(4)
O(4)−Fe(1)−N(1)
98.00(30)
171.10(30)
I
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Scheme 4. Proposed Mechanistic Cycle for Formation of Cyclic Organic Carbonate Products via the Monomeric Catalyst
the Fe−Oepoxide distance (2.145(10) Å) is significantly longer
than the Fe−OTHF distance in [FeLPh(THF)] which amounts
to 2.055(5) Å, see Table 2. This may be a result of the steric
requirements around the iron(III) center.
The proposed mechanistic cycle is shown in Scheme 4. It is
generally accepted42−44 that Lewis acid activation of the
coordinated substrate is followed by ring−opening of the
epoxide by the external nucleophile (here bromide) producing
an iron−alkoxide intermediate. The latter reacts with CO2 to
form a carbonate complex through insertion of CO2 into the
Fe−O bond, followed by ring-closure and regeneration of the
initial solvated complex.
ASSOCIATED CONTENT
* Supporting Information
Full crystallographic data in CIF format, UV−vis titration
studies, and 1H NMR spectra of all the complexes. This
material is available free of charge via the Internet at http://
pubs.acs.org.
■
S
AUTHOR INFORMATION
Corresponding Author
*E-mail: akleij@iciq.es (A.W.K.), giulia.licini@unipd.it (G.L.).
Notes
■
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
We thank ICIQ, ICREA, the Spanish Ministerio de Economıa y
Competitividad (MINECO) through CTQ2011-27385 and
COST CM1003 for support. B.G. thanks COST-D40 for a
STSM at ICIQ. We also thank Prof. Miquel Costas for his
helpful comments.
CONCLUSIONS
■
■
́
In summary, we have synthesized and fully characterized four
new iron(III) amino triphenolate complexes. These complexes
have been found to be dimeric (in the case of [FeLH]2 and
[FeLMe]2) or monomeric (in the case of [FeLtBu(THF)] and
[FeLPh(THF)]). The potential to form a dimeric structure is
dependent upon the substituent in the ortho-position of the
phenolate moiety. In the case of [FeLMe]2, though the solid
state structure reveals a monomeric complex, evidence from the
solution characterization methods indicates the presence of a
dimer, albeit weaker in nature than the dimeric structure of
[FeLH]2. Catalytic testing of the complexes for the cyclo-
addition of carbon dioxide to oxiranes has shown that the
monomeric form of these iron complexes is significantly more
active than the dimeric species. It has also been shown that by
changing the reaction conditions (higher temperatures, using a
solvent with coordinating potential and better CO2-dissolution
potential) the dimeric structure can be disrupted and a more
active form of the complex can be obtained. Our focus is now
on the implementation of these iron complexes in other types
of catalytic processes.
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