Synthesis and structural characterization of iron(III) salen complexes possessing appended anionic oxygen donor ligands

Article abstract of DOI:10.1016/j.ica.2003.12.019

The synthesis and solid-state characterization of iron(III) salen complexes bearing monodentate and bidentate anionic oxygen donor ligands are reported. The synthesis of Fe(salen)OPh (OPh=2,6-diphenylphenoxide) (1) was accomplished using Fe[N(Si(CH₃)₃)₂]₃ with one equivalent of H₂salen and the corresponding phenol. The complex was isolated as a crystalline sample by the slow diffusion of pentane into a concentrated solution of 1 at -20°C. The solid-state structure of complex 1 reveals distorted square pyramidal geometry about the iron metal center. Alternatively the preparation of Fe(salen) complex 2 bearing the anionic, bidentate acetylacetonate ligand in the axial position was accomplished by thermally replacing two acetylacetonate ligands in Fe(acac)₃ with one equivalent of H₂salen. The complex was isolated and crystallographically characterized using X-ray quality crystals formed via the aforementioned protocol. The metal center adopts a very distorted octahedral geometry, indicating a preference for the square pyramidal species which is typical of other iron salen derivatives. Unfortunately, 1 and 2 were found to be inactive in the copolymerization of CO₂ and cyclohexene oxide to afford polycarbonate in the presence of a Lewis base (i.e., N-MeIm and PCy ₃). The μ-[Fe(salen)]₂O (3) derivative was also isolated via the protonation of Fe(salen)(N(Si(CH₃)₃) ₂). The complex is relatively air stable, forming large, dark crystals over a short period of time at -20°C. The solid-state structure reveals a nearly linear Fe-O-Fe bond angle, which is due to the electronic repulsions associated with the sterically encumbering tert-butyl groups on the salen framework.

Full text of DOI:10.1016/j.ica.2003.12.019

Inorganica Chimica Acta 357 (2004) 2143–2149
www.elsevier.com/locate/ica
Synthesis and structural characterization of iron(III) salen
complexes possessing appended anionic oxygen donor ligands
*
Donald J. Darensbourg , Cesar G. Ortiz, Damon R. Billodeaux
Department of Chemistry, Texas A&M University, P.O. Box 30012, College Station, TX 77842-3012, USA
Received 10 December 2003; accepted 18 December 2003
Abstract
The synthesis and solid-state characterization of iron(III) salen complexes bearing monodentate and bidentate anionic oxygen
donor ligands are reported. The synthesis of Fe(salen)OPh (OPh ¼ 2,6–diphenylphenoxide) (1) was accomplished using
Fe[N(Si(CH3)3)2]3 with one equivalent of H2salen and the corresponding phenol. The complex was isolated as a crystalline sample by
the slow diffusion of pentane into a concentrated solution of 1 at )20 °C. The solid-state structure of complex 1 reveals distorted square
pyramidal geometry about the iron metal center. Alternatively the preparation of Fe(salen) complex 2 bearing the anionic, bidentate
acetylacetonate ligand in the axial position was accomplished by thermally replacing two acetylacetonate ligands in Fe(acac)3 with one
equivalent of H2salen. The complex was isolated and crystallographically characterized using X-ray quality crystals formed via the
aforementioned protocol. The metal center adopts a very distorted octahedral geometry, indicating a preference for the square py-
ramidal species which is typical of other iron salen derivatives. Unfortunately, 1 and 2 were found to be inactive in the copolymerization
of CO2 and cyclohexene oxide to afford polycarbonate in the presence of a Lewis base (i.e., N-MeIm and PCy3). The l-[Fe(salen)]2O (3)
derivative was also isolated via the protonation of Fe(salen)(N(Si(CH3)3)2). The complex is relatively air stable, forming large, dark
crystals over a short period of time at )20 °C. The solid-state structure reveals a nearly linear Fe–O–Fe bond angle, which is due to the
electronic repulsions associated with the sterically encumbering tert-butyl groups on the salen framework.
Ó 2003 Elsevier B.V. All rights reserved.
Keywords: Iron(salen) complexes; l-oxoiron dimer; CO2/epoxide copolymerization
1. Introduction
(ARO) of epoxides by a Cr(salen)Cl catalyst which was
developed by Jacobsen and workers in the mid-1990s [2].
Additionally, the oxidation of alkenes via M(salen)Cl
(M ¼ Mn, Cr) catalysts in the presence of an oxygen
donor also spurred intense research in the area of Schiff
base chemistry [3]. A very important reaction in organic
synthesis which involves the use of predominantly
chromium based salen complexes is the Diels–Alder re-
action. Indeed, there are numerous reports where these
catalysts have been employed as part of a long synthetic
strategy to afford complex natural products [4].
One of the most widely used and applicable synthetic
methodologies is the carbon–carbon coupling reaction,
which is dominated by palladium based catalysts [5].
Although unprecedented results have been obtained
using this metal through the years, the high cost of many
of these catalysts is an immense drawback. An alterna-
tive and highly under explored option is the use of iron
catalysts which possess many advantages over
Optimization of catalytic activities for a given process
is typically achieved through the methodical tailoring of
the metalÕs ligand environment. Therefore, ligand
frameworks such as the Schiff base containing salen li-
gand, that can be sterically and electronically modified
1
with ease, are very attractive [1]. The use of the salen
ligand framework in catalytic reactions has been re-
ceiving increasing interest due to the aforementioned
advantage and its success in many newly discovered
processes. Most notable is the asymmetric ring opening
* Corresponding author. Tel.: +1-9798455417; fax: +1-9798450158.
E-mail address: djdarens@mail.chem.tamu.edu (D.J. Darens-
bourg).
1
Typically, the term salen refers to the symmetrical and unsubsti-
tuted Schiff base ligand framework. However, herein, this term is used
to specifically represent the N,N0-bis(3,5-di-tert-butylsalicylidene)-1,2-
ethyldiimine derivative.
0020-1693/$ - see front matter Ó 2003 Elsevier B.V. All rights reserved.
doi:10.1016/j.ica.2003.12.019

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D.J. Darensbourg et al. / Inorganica Chimica Acta 357 (2004) 2143–2149
Ph
Ph
O
O
Fe
O
= acac
N
O
N
O
O
N
O
N
O
O
Fe
t-Bu
t-Bu
t-Bu
t-Bu
t-Bu
t-Bu
t-Bu
t-Bu
1
2
Fig. 1. Fe(salen)X complexes incorporating monodentate and bidentate anionic ligands.
traditional catalyst due to ironÕs copious, non-toxic, and
inexpensive nature. The use of Fe(salen)Cl as a pre-
catalyst in the cross-coupling of aryl chlorides and
Grignard alkyls has been recently investigated and
found to be highly effective [6]. Such report opens a new
avenue for the utilization of iron salen complexes in
organometallic catalysis.
In this context, few investigations incorporating iron
as the metal center in the copolymerization of CO2 and
epoxides to afford polycarbonate have been conducted.
Due to the success associated with the Cr(salen)Cl cat-
alyst for this purpose, the search for new catalysts
bearing the salen ligand framework with a variety of
metals is of interest [7]. Herein, we wish to report the
synthesis of a novel Fe(salen)(OPh) (1) complex, incor-
porating 2,6-diphenylphenoxy moiety in the axial site
(Fig. 1). The synthetic strategy for the preparation of
these types of complexes is discussed as well the solid
state characterization of 1. The Fe(salen)(acac) (2)
complex has also been prepared through the use of a
previously reported synthetic route (Fig. 1) [8]. The co-
ordination mode of a bidentate ligand such as acetyl-
acetonate induces dramatic changes in the geometry at
the metal center.
In addition, the solid-state characterization of the l-
oxo dimer, 3, is presented. Such species are of impor-
tance due to the many bioinorganic processes involving
dioxygen activation [9]. For example, methane mono-
oxygenase effectively cleaves the O2 bond to produce
one molecule of both water and methanol. Although it is
not the purpose of this manuscript to fully compare the
oxo species to relevant bioinorganic models, many im-
portant physical attributes such as selected bond
distances and angles are provided. Additionally, l-
[Fe(salen)]2O complexes of this type have also been used
in the catalytic olefin cyclopropanation reaction to af-
ford many useful organic substrates [10].
Schlenk and drybox techniques. Prior to their use, all
solvents were distilled using standard techniques. The
synthetic precursors, FeCl3, Fe(acac)3, 2,6-diphenyl-
phenol, and 1,2-ethyldiamine were purchased from Al-
drich Chemical Co. 2,6-Diphenylphenol was purified by
sublimination prior to its use. The preparations of
Fe[N(Si(CH3)3)2]3 [11] and 3,5-di-tert-butyl-2-hydroxy-
benzaldehyde [12] were accomplished by the literature
procedures. The synthesis of N,N0-bis(3,5-di-tert-butyl-
salicylidene)-1,2-ethyldiimine(salen) was accomplished
by the typical protocol of refluxing 1,2-ethyldiamine and
3,5-di-tert-butyl-2-hydroxybenzaldehyde in methanol
over a period of 5 h in the presence of molecular sieves.
After each CO2/epoxide copolymerization experi-
ment, the resulting reaction mixture was analyzed by IR
spectroscopy using a Mattson 6021 FTIR spectrometer
with DTGS and MCT detectors. X-ray data were re-
corded using a Bruker Smart 1000 CCD diffractometer.
2.2. Synthesis of Fe(salen)(OPh) (1)
To
a
100-mL Schlenk flask was added
Fe[N(Si(CH3)3)2]3 (0.150 g, 0.28 mmol) which was dis-
solved in approximately 15 mL of toluene. To the re-
sulting green solution was added the H2salen ligand
(0.138 g, 0.28 mmol) in 15 mL of toluene via cannula
technique, and the dark solution was stirred for ap-
proximately 2 h. Freshly sublimed 2,6-diphenylphenol
(0.069 g, 0.28 mmol), in 10 mL of toluene, was cannu-
lated into the reaction solution to afford a deep red so-
lution which was stirred overnight. Subsequently, the
solvent was removed under vacuum, and the remaining
solid was dried. Yield: 92.8%. The complex was char-
acterized by X-ray crystallography via the use of dark,
plate-like crystals formed by the slow diffusion of pen-
tane in a methylene chloride solution of 1 at )20 °C.
2.3. Synthesis of Fe(Salen)(acac) (2) [8]
2. Experimental
The synthetic methodology employed in the synthesis
of this derivative was identical to that reported by
Cheng and coworkers [8]. To a 50-mL Schlenk flask
equipped with a condenser was added Fe(acac)3 (0.300
g, 0.85 mmol) and the H2salen ligand (0.415 g, 0.85
2.1. Materials and methods
Unless otherwise indicated, all reactions were carried
out under an inert argon atmosphere using standard

D.J. Darensbourg et al. / Inorganica Chimica Acta 357 (2004) 2143–2149
2145
mmol) in approximately 30 mL of acetonitrile. The re-
sulting red solution was refluxed for 2 h, causing the
formation of a dark purple solution. The solution was
filtered and the solvent was removed under vacuum.
Complex 2 was characterized by X-ray crystallography
using purple crystals formed by the diffusion of pentane
into a toluene solution of 2 at )20 °C.
polymer with methylene chloride. An infrared spectrum
of the reaction mixture was immediately taken in
CH2Cl2 to determine the extent of polycarbonate and
cyclohexene carbonate formation.
3. Results and discussion
2.4. Copolymerization of CO2 and epoxides
The preparation of complexes 1–2 was achieved via
two independent synthetic routes. Complex 1 was syn-
thesized using the Fe[N(Si(CH3)3)2]3 precursor with one
equivalent of H2salen and the corresponding phenol
(Scheme 1). Immediately, after the addition of the
H2salen ligand to the Fe[N(Si(CH3)3)2]3 [11] green pre-
cursor, a deep red solution was observed. Character-
ization of 1 was achieved by growing dark, plate-like
crystals via the slow introduction of pentane into a
toluene solution of 1 at )20 °C. An X-ray analysis of the
bulk crystals resulted in the solid-state characterization
of 1. Crystallographic data, and selected bond distances
and angles are tabulated in Tables 1 and 2, respectively.
A thermal ellipsoid representation of 1 showing 50%
probability is provided in Fig. 2. The iron center is
found to adopt a distorted square pyramidal geometry
A typical reaction was carried out using the following
protocol. A 35-mL glass vial with a septum was charged
with 1 (50 mg, 0.063 mmol) followed by the addition of
20 mL of cyclohexene oxide. To this solution was added
via microliter syringe N-MeIm (11.7 mg, 0.142 mmol,
11.3 lL). The solution was then introduced into the
stainless steel autoclave via an injection port followed by
pressurizing the reactor to approximately 550 psi CO2
pressure at room temperature. The temperature was
raised to 80 °C, at which time the CO2 pressure in-
creased to 800 psi. The reaction was carried out for 24 h,
after which the heat was turned off and the reactor was
allowed to cool to room temperature. After venting the
excess CO2, the reactor was cleaned by dissolving any
Ph
Ph
2,6-Diphenylphenol
Toluene, r.t.
O
N
O
N
O
Fe[N(Si(CH3)3)2]3
+ H2salen
Fe
t-Bu
t-Bu
t-Bu
t-Bu
1
Scheme 1.
Table 1
Crystallographic data and data collection parameters for complexes 1, 2, and 3
1
2
3
Crystal system
Space group
monoclinic
P21=n
4704(2)
monoclinic
C2=c
7475(3)
monoclinic
C2=c
6567(7)
3
ꢀ
V (A )
Z
4
8
6
ꢀ
a (A)
b (A)
c (A)
16.130(4)
16.123(5)
19.555(6)
24.801(5)
10.637(2)
28.336(6)
24.546(14)
16.707(10)
17.075(10)
ꢀ
ꢀ
a (°)
b (°)
112.320(6)
90.258(4)
110.315(10)
c (°)
T (K)
dcalc (g/cm3)
Absorption coefficient (mmꢀ1
Ra (%)
110
1.235
0.477
110
1.197
0.444
110
1.227
0.498
)
[I > 2rðIÞ]
9.73
27.07
6.64
10.25
5.37
5.85
a
Rw (%)
P
P
P
P
2
1=2
a R ¼ kFoj ꢀ jFck= Fo and RwF ¼ f½ wðFo ꢀ FcÞ ꢁ= wFo2g
.

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D.J. Darensbourg et al. / Inorganica Chimica Acta 357 (2004) 2143–2149
Table 2
Selected bond distances (A) and angles for complexes 1, 2, and 3
ꢀ
Complex 1
Fe(1)–N(1)
Fe(1)–N(2)
Fe(1)–O(1)
Fe(1)–O(2)
N(1)–C(2)
2.094(9)
2.091(9)
1.894(7)
1.887(6)
1.301(12)
N(2)–C(18)
Fe(1)–O(3)
N(1)–C(1)
N(2)–C(17)
1.249(12)
1.876(8)
1.448(13)
1.478(13)
O(1)–Fe(1)–N(2)
O(2)–Fe(1)–N(1)
N(1)–Fe(1)–O(3)
N(1)–Fe(1)–O(1)
N(2)–Fe(1)–O(3)
159.1(3)
131.3(3)
110.9(3)
86.8(3)
N(2)–Fe(1)–O(1)
N(2)–Fe(1)–O(2)
N(1)–Fe(1)–N(2)
O(1)–Fe(1)–O(2)
O(1)–Fe(1)–O(3)
159.1(3)
85.7(3)
77.3(4)
95.0(3)
105.5(3)
92.9(3)
Complex 2
Fe(1)–N(1)
Fe(1)–N(2)
Fe(1)–O(1)
N(2)–C(17)
Fe(1)–O(2)
N(1)–C(2)
2.154(4)
2.124(4)
1.898(3)
1.461(6)
1.934(3)
1.288(6)
N(2)–C(18)
Fe(1)–O(3)
Fe(1)–O(4)
N(1)–C(1)
1.287(6)
2.060(3)
2.006(3)
1.471(6)
O(1)–Fe(1)–N(2)
O(2)–Fe(1)–N(1)
N(1)–Fe(1)–O(3)
N(1)–Fe(1)–O(1)
N(2)–Fe(1)–O(3)
O(4)–Fe(1)–N(2)
O(4)–Fe(1)–N(1)
156.61(15)
111.00(14)
80.38(14)
84.52(14)
160.81(13)
109.78(14)
163.04(14)
N(2)–Fe(1)–O(1)
N(2)–Fe(1)–O(2)
N(1)–Fe(1)–N(2)
O(1)–Fe(1)–O(2)
O(1)–Fe(1)–O(3)
O(4)–Fe(1)–O(2)
O(4)–Fe(1)–O(3)
156.61(15)
82.30(14)
75.38(15)
94.12(13)
102.53(14)
85.88(13)
83.84(14)
Complex 3
Fe(1)–N(1)
Fe(1)–N(2)
Fe(1)–O(1)
Fe(1)–O(2)
N(1)–C(2)
2.116(3)
2.100(3)
1.917(2)
1.926(2)
1.267(4)
N(2)–C(18)
Fe(1)–O(3)
N(1)–C(1)
N(2)–C(17)
1.282(4)
1.7653(10)
1.453(4)
1.469(4)
O(1)–Fe(1)–N(2)
O(2)–Fe(1)–N(1)
N(1)–Fe(1)–O(3)
N(1)–Fe(1)–O(1)
N(2)–Fe(1)–O(3)
Fe(1)–O(3)–Fe(1A)
174.26(9)
156.18(9)
97. 50(10)
84.44(9)
N(2)–Fe(1)–O(1)
N(2)–Fe(1)–O(2)
N(1)–Fe(1)–N(2)
O(1)–Fe(1)–O(2)
O(1)–Fe(1)–O(3)
134.26(9)
86.30(9)
76.67(9)
96.10(9)
109.92(7)
113.58(8)
171.63(17)
Estimated standard deviations are shown in parenthesis.
Fig. 2. Thermal ellipsoid representation of Fe(Salen)(OPh) (1) showing 50% probability.

D.J. Darensbourg et al. / Inorganica Chimica Acta 357 (2004) 2143–2149
2147
with the N(1)–Fe(1)–O(2) and N(2)–Fe(1)–O(1) bond
angles having values of 131.3(3) and 159.1(3)°, respec-
tively. An expanded view of the metal center with its
immediate coordination sphere is presented in the inset
in Fig. 2. Such orientation is not surprising and is also
observed in other M(salen) complexes incorporating the
ethyl backbone fragment within the ligand framework
[13]. The Fe(1)–O(1) and Fe(1)–O(2) bond distances
selected bond distances and angles are presented in
Tables 1 and 2, respectively.
A thermal ellipsoid representation of 2 showing 50%
probability is given in Fig. 3. As has been previously
described in the analogous complexes, the use of the
bidentate acetylacetonate ligand demands the iron cen-
ter to adopt a distorted octahedral geometry (inset in
Fig. 3). For example, the N(1)–Fe(1)–O(2) and N(2)–
Fe(1)–O(1) bond angles are found to have values of
111.00(14) and 156.61(15)°, respectively. The large de-
viation from the typical square pyramidal geometry as-
sociated with M(salen)Cl (M ¼ Cr, Fe, Mn) complexes is
clearly evident by other bond angle data presented in
Table 2. The Fe(1)–O(1) and Fe(1)–O(2) bond distances
ꢀ
were found to be 1.894(7) and 1.887(6) A and are nearly
identical to the Fe(1)–O(3) bond distance (i.e., 1.876(8)
ꢀ
A) pertaining to the phenoxy axial ligand. The N(1)–
C(2) and N(1)–C(18) bond distance were found to differ
significantly from each other with values of 1.301(12)
ꢀ
and 1.249(12) A, respectively. Such differences in the
ꢀ
N@C bond distance within the same salen framework is
not typical.
are found to have values of 1.898(3) and 1.934(3) A,
respectively. Such deviation from one another is not
surprising due to the octahedral nature of the iron
center. As expected and observed in other Fe(sa-
len)(acac) complexes, the Fe(1)–O(4) bond distance is
approximately 0.06 A shorter than the Fe(1)–O(3) dis-
tance corresponding to the acetato ligand [8].
Unfortunately, using 1 as a catalyst in the copoly-
merization of CO2 and cyclohexene oxide was unsuc-
cessful. The addition of 2.5 equivalents of the Lewis
base, N-MeIm, to 1, in cyclohexene oxide, also resulted
in an inactive system [7]. Perhaps the use of electron
The acetylacetonate derivative, 2, was successfully
prepared thermally by reacting one equivalent of H2-
salen with Fe(acac)3 in acetonitrile under refluxing
conditions (Scheme 2). This protocol has been previ-
ously used in the preparation of chiral iron complexes
bearing binaphthyl Schiff base analogues [8]. The pro-
cedure leads to the production of the desired product in
good yields (>80%). Allowing the slow diffusion of
pentane into a toluene solution of 2 resulted in bulk X-
ray quality dark crystals. Crystallographic data, and
ꢀ
O
O
Fe
O
= acac
N
O
N
O
O
CH3CN,
reflux, 2 hr
t-Bu
Fe(acac)3
+ H2salen
t-Bu
t-Bu
t-Bu
2
Scheme 2.
Fig. 3. Thermal ellipsoid representation of Fe(Salen)(acac) (2) showing 50% probability.

2148
D.J. Darensbourg et al. / Inorganica Chimica Acta 357 (2004) 2143–2149
Fig. 4. Thermal ellipsoid representation of l-[Fe(Salen)]2O (3) showing 50% probability.
deficient OPh groups as initiators are not suitably nu-
cleophilic for either the ring opening of epoxides or the
CO2 insertion into the Fe–OPh bond even in the pres-
ence of an auxiliary Lewis base. Additionally, using 2
also resulted in no polycarbonate production and yiel-
ded only low amounts of cyclohexene carbonate with
PCy3 as the cocatalyst.
O(1)–Fe(2) ¼ 156.5(3)°) [14]. The Fe(1)–O(3) bond dis-
ꢀ
tance is approximately 0.11 A shorter than that found in
the phenoxy analogue, 1, while the Fe(1)–N(2) bond
distance was found to be slightly longer than those
found in 1.
An attempt to isolate the iron amido derivative,
Fe(salen)(N(Si(CH3)3)2), was unsuccessful. Due to the
highly unstable nature of this intermediate, the hydro-
lysis product was readily formed as indicated by the
formation of the l-oxo dimer, 3. This complex is air
stable and forms perfect rectangular black crystals in
large quantities. A solid-state structure of the complex
was obtained by X-ray crystallography. Crystallo-
graphic data, including selected bond distances and
angles, are tabulated in Tables 1 and 2, respectively. A
thermal ellipsoid drawing showing 50% probability is
shown in Fig. 4. Due to the sterically encumbering tert-
butyl groups on the ligand framework, the iron centers
are stacked in a 100.9° rotation with respect to one
another to afford the most stable conformation. The
4. Concluding remarks
Iron complexes, 1–2, bearing the Schiff base con-
taining a salen ligand were successfully synthesized using
two synthetic approaches. The use of Fe[N(Si(CH3)3)2]3
as a precursor with one equivalent of both the H2salen
and 2,6-diphenylphenol ligand has been found to be an
effective route for the preparation of 1 and is a feasible
synthetic route for the synthesis other iron complexes
bearing an array of OR groups in the axial position.
Yields are excellent due to the ease in removing the
HN(Si(CH3)3)2 byproduct under reduced pressure.
Characterization of 1 was achieved by X-ray analysis of
small plate-like crystals. The primary purpose behind
the preparation of 1 was to determine its catalytic ac-
tivity in the copolymerization of CO2 and epoxides.
Unfortunately, a completely inactive system was ob-
served using cyclohexene oxide as the epoxide, even in
the presence of a Lewis base cocatalyst. Perhaps, de-
rivatives incorporating alkoxy groups or more electron
rich phenoxides coordinated to the iron center may serve
as better nucleophiles for the CO2 or ring opening ini-
tiation step. Adding bidentate ligands, such as acetyl-
acetonate, has been found to cause dramatic
coordination changes. In 2, the iron adopts a very dis-
torted octahedral geometry, indicating a strong prefer-
ence for the square pyramidal analogues, as has been
observed in many other M(salen)X complexes, where X
represents an anionic monodentate ligand [13]. A lack of
catalytic activity was also observed using 2 as the cata-
ꢀ
Fe(1) atom is also found to lie 0.566 A above the plane
formed by the N(1), N(2), O(1), and O(2) atoms. Such
deviation is typical and observed in other [Fe(salen)]2O
species [14]. The two complexes, bridged by the lone
oxygen atom, O(3), form a nearly linear Fe(1)–O(3)–
Fe(1A) angle with a value of 171.63(17)°, similar to the
other salen Fe complexes and porphyrin derivatives such
as [Fe(TTP)]2O (TTP ¼ meso-tetrakis(p-tolyl)-porphyri-
nato) (i.e., 178.2(3)°) [15]. The linear characteristic of the
Fe–O–Fe unit is heavily dependent on the aryl substit-
uents as sterically encumbering groups aid in the for-
mation of a linear angle by imposing electronic and
steric repulsions between the ligands while those pos-
sessing less sterically encumbering groups cause a more
bent framework (i.e., no substitution on salen: Fe(1)–

D.J. Darensbourg et al. / Inorganica Chimica Acta 357 (2004) 2143–2149
2149
(d) A.M. Daly, C.T. Dalton, M.F. Renehan, D.G. Gilheany,
Tetrahedron Lett. 40 (1999) 3617;
lyst for the aforementioned copolymerization process
with the use of PCy3 as the cocatalyst. Furthermore, the
hydrolysis of two equivalents of Fe(salen)(N(Si
(CH3)3)2) with a proton source (e.g., water), yields the l-
oxo derivative, 3. Complex 3 was also characterized by
X-ray crystallography and found to be similar to other
analogues incorporating sterically encumbering groups
on the salen framework. This air stable derivative may
serve as an active catalyst in the olefin cyclopropanation
reaction due to the potential formation of a more stable
Fe(II) intermediate than those formed with related
complexes [10].
(e) A.M. Daly, M.F. Renehan, D.G. Gilheany, Org. Lett. 3 (2001)
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Financial support from the National Science Foun-
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