Synthesis and structure of iron(III) complexes of amine-bis(phenolate) ligands

Article abstract of DOI:10.1139/cjc-2014-0043

The synthesis and structures of four new iron(III) amine-bis(phenolate) complexes are reported. Reaction of anhydrous FeCl₃ with the diprotonated tridentate ligand isopropyl-N,N-bis(2-methylene-4-t-butyl-6- methylphenol) (H₂L1) and NEt₃ produces the trigonal bipyramidal iron(III) complex [NEt₃H]⁺ [FeCl ₂L1]^- (1). The reaction of FeBr₃ with the sodium or lithium salts, Na₂L1 and Li₂L2, results in the formation of FeBr₂L1H (2) and FeBr₂L2H (3), tetrahedral iron(III) complexes possessing two bromide ligands and quaternized ammonium fragments. A trigonal bipyramidal Fe^III hydroxido-bridged dimer, [Fe(μ-OH)L2]₂ (4), was also isolated during the synthesis of 3. Single-crystal X-ray molecular structures have been determined for complexes 1-4 and H₂L2.

Full text of DOI:10.1139/cjc-2014-0043

758
ARTICLE
Synthesis and structure of iron(III) complexes of
amine-bis(phenolate) ligands
Elliott F. Chard, John R. Thompson, Louise N. Dawe, and Christopher M. Kozak
Abstract: The synthesis and structures of four new iron(III) amine-bis(phenolate) complexes are reported. Reaction of anhydrous
FeCl3 with the diprotonated tridentate ligand isopropyl-N,N-bis(2-methylene-4-t-butyl-6-methylphenol) (H2L1) and NEt3 produces
the trigonal bipyramidal iron(III) complex [NEt3H]+ [FeCl2L1]– (1). The reaction of FeBr3 with the sodium or lithium salts, Na2L1
and Li2L2, results in the formation of FeBr2L1H (2) and FeBr2L2H (3), tetrahedral iron(III) complexes possessing two bromide
ligands and quaternized ammonium fragments. A trigonal bipyramidal FeIII hydroxido-bridged dimer, [Fe(␮-OH)L2]2 (4), was also
isolated during the synthesis of 3. Single-crystal X-ray molecular structures have been determined for complexes 1–4 and H2L2.
Key words: iron, amine-bis(phenolate), crystallography, ligand design, coordination chemistry.
Résumé : La synthèse et les structures de quatre nouveaux complexes de fer(III) et d’amine-bis(phénolate) sont décrites. La réaction
du FeCl3 anhydre avec le ligand tridendate diprotoné l’isopropyl-N,N-bis(2–methylène-4-t-butyl-6-méthylphénol) (H2L1) et le composé
NEt3 produit le complexe de fer(III) trigonal bipyramidal [NEt3H]+[FeCl2L1]– (1). La réaction du FeBr3 avec les sels de sodium ou de
lithium, Na2L1 et Li2L2, conduit a` la formation des composés FeBr2L1H (2) et FeBr2L2H (3), complexes de fer(III) tétraédriques qui
possèdent deux ligands bromures et des fragments d’ammonium quaternaire. Un dimère de fer(III) trigonal bipyramidal avec pont
hydroxyde, [Fe(␮-OH)L2]2 (4), a également été isolé au cours de la synthèse du composé 3. Les structures moléculaires des complexes
1 a` 4 et du complexe H2L2 ont été déterminées par cristallographie aux rayons X. [Traduit par la Rédaction]
Mots-clés : fer, amine-bis(phénolate), cristallographie, conception de ligands, chimie de la coordination.
transition metal amine-bis(phenolate) complexes. Several structur-
ally characterized examples of alkali metal compounds of amine-
Introduction
Recently, the use of chelating amine-bis(phenolates) as ligands
has played a very important role in transition metal catalyst de-
sign. These dianionic ligands are extremely diverse and have been
used primarily with high-valent early transition metals for which
the “hard” acid and base properties are well matched.1 Numerous
FeIII complexes supported by these ligands have been investigat-
ed,2–23 many of them as a result of their close relationship with
phenol-containing ligands found in non-heme-iron-containing
metalloenzymes.
We are interested in the development of new inexpensive iron-
based catalysts for organic synthesis. Previously, we reported the
catalytic potential of iron complexes supported by various triden-
tate and tetradentate amine-bis(phenolate) ligands (Fig. 1).24–31 For
example, we have investigated the cross-coupling of aromatic Gri-
gnard reagents with alkyl halides using iron(III) catalysts bearing
amine-bis(phenolate) ligands.24,26–28,31 In an attempt to generate
more reactive catalysts while still maintaining the robust nature
of the catalyst precursor, we turned our attention to tridentate
amine-bis(phenolate) ligands. However, the reduced coordination
ability and ligand field strength of these tridentate variations can
lead to a number of unexpected products upon metallation.
These ligand precursors can be coordinated to transition metals
by several routes. For example, alkali metal compounds of these
ligands can be first prepared using n-BuLi or NaH to generate M2L
salts where M = Li or Na, respectively. Salt metathesis of transition
metal halides with these M2L compounds generates the desired
bis(phenolate) compounds have recently been reported,32–43
A
protonolysis route may also be employed for labile transition metal
halides. For example, FeCl3 reacts with amine-bis(phenolate) ligands
in the presence of NEt3 resulting in complexes of the formula FeClL.
This gives compounds that are monomeric in the solid state when
tetradentate ligands are used24,25,30 but leads to dimeric structures for
tridentate ligands.27 The dimeric complexes exist as halide-bridged
species possessing distorted trigonal bipyramidal iron(III) ions. If
bulky groups such a t-pentyl substituents are present on the phenolate
donors, then monomeric FeIII compounds may also be obtained using
tridentate ligands.31 These previous tridentate amine-bis(phenolate) li-
gands contained benzyl or n-propyl amine groups. However, we found
that other FeIII complexes could be generated depending on the nature
of the halide ligands present, the purification procedures used, and the
steric requirements of the amine-bis(phenolate) backbone. Herein we
report the synthesis and structure of four new structurally authenti-
cated iron(III) complexes supported by tridentate amine-bis(phenolate)
ligands.
Results and discussion
The tridentate amine-bis(phenol) ligand precursors H2L1 and
H2L2 (Fig. 1) were readily synthesized by a modified Mannich con-
densation reaction in which the 2,4-disubstituted phenol, amine,
and formaldehyde were heated to reflux in water for 12 h. Single
crystals of H2L2 suitable for X-ray diffraction were obtained from
Received 26 January 2014. Accepted 3 April 2014.
E.F. Chard and C.M. Kozak. Department of Chemistry, Memorial University of Newfoundland, St. John’s, NL A1B 3X7, Canada.
J.R. Thompson. Department of Chemistry, Simon Fraser University, Burnaby, BC V5A 1S6, Canada.
L.N. Dawe. Department of Chemistry, Memorial University of Newfoundland, St. John’s, NL A1B 3X7, Canada; X-Ray Diffraction Laboratory, Centre for
Chemical Analysis, Research and Training, Memorial University of Newfoundland, St. John’s, NL A1B 3X7, Canada.
Corresponding author: Christopher M. Kozak (e-mail: ckozak@mun.ca).
This article is part of a Special Issue dedicated to Professor Barry Lever in recognition of his contributions to inorganic chemistry across Canada and beyond.
Can. J. Chem. 92: 758–764 (2014) dx.doi.org/10.1139/cjc-2014-0043
Published at www.nrcresearchpress.com/cjc on 10 April 2014.

Chard et al.
759
Fig. 1. Proligands previously used with FeIII by the authors and
those used in this study (H2L1 and H2L2).
Fig. 2. Single-crystal X-ray structure of H2L2. Hydrogen atoms
(except those on O(1) and O(2)) omitted for clarity. Ellipsoids are
shown at 50% probability. Bond lengths and angles are given in the
supplementary data.
a saturated methanol solution at –20 °C. The solid-state molecular
structure and the crystallographic data of H2L2 can be found in
Fig. 2 and Table 1, respectively. Bond lengths and angles can be
found in the supplementary data (see “Supplementary material”
section).
very similar to the Fe–N bond lengths reported in FeCl[O2NN=]Py
(2.2706(15) Å) and FeCl[O2NN=]NMe2 (2.248(2) Å). The phenolate oxygen
atoms in 1 exhibit bond distances of 1.855(2) and 1.848(2) Å for Fe(1)–
O(1) and Fe(1)–O(2), respectively. These interactions are only slightly
shorter than those observed in the FeCl[O2NN=] complexes where
average Fe–O distances of 1.86 Å are observed.
The reaction of FeCl3 with H2L1 followed by addition of NEt3 to
sequester the HCl produced (Scheme 1) was expected to yield a
compound formulated as FeClL, as was observed in previous work
from our group involving the n-propylamino-containing ligand.
Instead, the solid-state molecular structure of 1 (shown in Fig. 3)
reveals a trigonal bipyramidal iron(III) centre with a formal nega-
tive charge (hence, an “ate” complex) and a triethylammonium
counterion. Elemental analysis performed on a recrystallized sample
of 1 confirms this formulation for the bulk sample. 1H NMR spectros-
copy revealed only broad and inconclusively assignable peaks due to
the paramagnetism of the compound. Indeed, no clearly assignable
peaks were observed in the 1H NMR spectra of any of the FeIII com-
pounds reported herein. Selected bond lengths and angles for com-
pound 1 (as well as for 2 and 3) are given in Table 2. The equatorial
plane of the FeIII ion in 1 consists of two phenolate oxygens, O(1)
and O(2), and a chloride ion, Cl(2), where the sum of bond angles
is 359.69°, indicating near perfect planarity. The iron atom is dis-
placed 0.06 Å above the equatorial plane. The amine nitrogen donor
N(1) and the chloride ion Cl(1) occupy the apical sites, giving a
Cl(1)−Fe(1)−N(1) bond angle of 178.85(7)°, which is close to the ideal
linear geometry. The cis-oriented chloride ligands are nearly orthog-
onal with a Cl(1)−Fe(1)−Cl(2) bond angle of 91.42(5)°. The distorted
trigonal bipyramidal coordination environment of the FeIII ion pos-
sesses a trigonality index parameter, ␶, value of 0.837 as defined by
Addison and Reedijk.44 ␶ = (␤ – ␣)/60, where ␤ represents the largest
angle about the metal centre and ␣ represents the second largest
angle about the metal centre. For perfect trigonal bipyramidal and
square pyramidal geometries, the ␶ values are 1.0 and 0, respectively.
Previously, we reported mononuclear trigonal bipyramidal
iron(III) complexes of related tetradentate diamine-bis(phenolate)
ligands (abbreviated [O2NN=], where N= represents a pendant dimethyl-
aminoethyl, [O2NN=]NMe2, or pyridyl, [O2NN=]Py, arm).38 The Fe–Cl
bond lengths in FeCl[O2NN=]Py and FeCl[O2NN=]NMe2 were found to be
2.3051(10) and 2.2894(5) Å respectively, which are very similar to the
Fe(1)–Cl(2) interaction observed in the equatorial plane of 1. The Fe(1)–
Cl(1) bond length (2.3618(13) Å) in 1, where Cl(1) is trans to a nitrogen
donor, is slightly longer than the Fe–Cl bond lengths observed in
FeCl[O2NN=]Py and FeCl[O2NN=]NMe2 (where Cl is also trans to an ami-
no nitrogen donor). 1 has a Fe(1)–N(1) distance of 2.255(3) Å, which is
The coordination geometry around iron(III) in 1 is very closely
related to a pair of iron(III) chloride-bridged dimers bearing n-propyl
and benzyl amines, [Fe(␮–Cl)[ONO]nPr]2 (A) and [Fe(␮–Cl)[ONO]Bn]2 (B),
previously reported in our group.27 Like 1, the five-coordinate
trigonal bipyramidal iron(III) centres in A and B are composed of
two chloride ions along with two phenolate oxygen donor at-
oms and a central amine nitrogen atom originating from a tri-
dentate amine-bis(phenolate) backbone. The axial Fe–Cl bond
length in 1 (2.3618(4) Å) is slightly shorter than the axial Fe–Cl
bond lengths found in A (2.4911(8) Å) and B (2.5025(3) Å). The
equatorial Fe(1)–Cl(2) bond length in 1 (2.3038(14) Å) is intermedi-
ate to the equatorial Fe–Cl bond lengths reported in A (2.298(2) Å)
and B (2.3290(4) Å). The Fe(1)–N(1) distance of 2.255(3) Å observed
in 1 is longer than the observed Fe–N distances found in both
chloride-bridged dimers of 2.183(4) Å in A and 1.1819(10) Å in B. The
Fe–O distances in 1 are 1.855(2) and 1.848(3) Å for Fe(1)–O(1) and
Fe(1)–O(2), respectively, which are longer than the distances re-
ported between iron and the phenolate oxygen atoms in A (aver-
age distance of 1.818 Å) and B (average distance of 1.825 Å). Since
the iron centre in 1 has a formal negative charge, the anionic
oxygen donors may be slightly repelled by the metal centre. From
an electronic perspective, this may account for the longer Fe–O
distances observed in 1. Of course, in the case of both A and B,
steric hindrance originating from the presence of two large
amine-bis(phenolate) ligands about the two iron(III) centres may
also be a major contributor. The amine nitrogen donor and a
bridging chloride ion take up the axial positions, giving a Cl(1)*–
Fe(1)–N(1) bond angle of 178.32(9)° in A and a Cl(2)–Fe(1)–N(1)
bond angle of 177.28(3)° in B. Complex 1 has a Cl(1)–Fe(1)–N(1) bond
angle of 178.85(7)°, which is closer to the ideal linear geometry.
The cis-oriented chloride ligands are nearly orthogonal with a
Cl–−Fe–Cl bond angle of 87.36(4)° in A and 84.341(14)° in B. The
Cl–−Fe–Cl bond angle in 1 is 91.42(5)°, which is closer to the perfect
orthogonal angle of 90°.
In 2002, Leznoff and co-workers reported a five-coordinate
iron(III) chloride-bridged dimer with a distorted trigonal bipyra-
midal geometry.45 Unlike the coordination environment of 1,
which contains two anionic oxygen donor atoms and a central
Published by NRC Research Press

760
Can. J. Chem. Vol. 92, 2014
Table 1. Crystallographic and structure refinement data for H2L2 and 1−4.
Compound
H2L2
1
2
3
4
Chemical formula
Colour
Habit
Formula mass
Crystal system
a (Å)
b (Å)
c (Å)
␣ (°)
␤ (°)
C33H53NO2
Colourless
Prism
496.76
Mononclinic Triclinic
15.185(4)
11.707(3)
18.596(4)
90
108.298(4)
90
3138.7(12)
158(1)
P21/c (#14)
4
C33H55Cl2FeN2O2·C7H8 C27H40Br2FeNO2·2.5(C7H8) C33H52Br2FeNO2·C7H8 C66H104Fe2N2O6·2(C7H8)
Dark red
Prism
730.70
Black
Dark red
Prism
Red
Prism
856.62
Monoclinic
14.816(4)
16.729(4)
18.202(5)
90
107.386(3)
90
4305.4(20)
163(1)
Prism
1317.53
Orthorhombic
24.3216(17)
18.1938(11)
18.8654(12)
90
90
90
8348.0(9)
193(1)
802.57
Triclinic
10.5127(10)
13.6960(14)
15.3413(15)
68.421(5)
79.693(6)
77.951(6)
1996.1(3)
163(1)
11.162(3)
11.397(4)
17.686(6)
83.003(14)
75.944(13)
69.005(11)
2036.1(11)
163(1)
␥ (°)
Unit cell V (Å3)
Temperature (K)
Space group
Z
P−1 (#2)
2
P21/c (#14)
4
P−1 (#2)
2
Pccn (#56)
4
Dc (g/cm3)
Radiation type
Absorption, ␮ (mm−1
F(000)
1.049
MoK␣
0.630
1.192
1.321
1.335
1.048
MoK␣
0.535
MoK␣
2.246
MoK␣
2.412
MoK␣
0.393
)
1096
786
1784
838
2856
Reflections measured
40638
17721
54221
14400
24717
Independent reflections 6483
8338
8910
6952
5448
Rint
R1 (all)
wR(F2) (all)
0.0369
0.0574
0.0902
0.1905
0.0634
0.1577
1.093
0.0588
0.0701
0.1483
0.0565
0.1386
1.104
0.0962
0.1683
0.2578
0.0991
0.2159
1.035
0.1590
0.2272
0.4243
0.1586
0.3791
1.177
0.0880
0.2384
0.0830
0.2337
1.157
R1 (I > 2␴(I))a
wR(F2) (I > 2␴(I))b
Goodness of fit on F2
CCDC reference
982453
982454
982455
982456
982457
aR1 = ͚ (|Fo| − |Fc|)/͚|Fo|).
bwR2 = [͚ (w(Fo2 − Fc2)2)/͚w(Fo2)2]1/2
.
Scheme 1. Synthesis of 1.
Table 2. Selected bond lengths (Å) and bond angles (°)
of 1−3.
1
2
3
Fe(1)−O(1)
1.855 (2)
1.848 (3)
2.3618 (13) 2.3596 (9)
2.3038 (14) 2.3491 (8) 2.3697 (19)
2.255 (3)
1.822 (2)
1.832 (3)
1.843 (6)
1.851 (6)
2.355 (2)
Fe(1)−O(2)
Fe(1)−X(1)a
Fe(1)−X(2)
Fe(1)−N(1)
Fe(1)···N(1)
O(1)−Fe(1)−O(2)
O(1)−Fe(1)−X(1)
3.439 (4)
114.58 (12) 106.38 (13)
3.429 (7)
105.9 (3)
108.9 (2)
110.8 (2)
107.5 (3)
112.0 (2)
111.53 (7)
91.49 (8)
108.43 (8)
O(1)−Fe(1)−X(2) 128.60 (10) 110.90 (9)
O(2)−Fe(1)−X(1)
O(2)−Fe(1)−X(2)
X(1)−Fe(1)−X(2)
92.71 (9)
116.51 (8)
91.42 (5)
110.71 (9)
110.08 (9)
110.26 (3)
Fig. 3. Molecular structure (ORTEP) and partial atom labeling of 1.
Hydrogen atoms (except at N(2)) and cocrystallized solvent
molecules omitted for clarity. Ellipsoids are shown at 50%
probability.
Cl(1)−Fe(1)−N(1) 178.85 (7)
Cl(2)−Fe(1)−N(1)
O(1)−Fe(1)−N(1)
O(2)−Fe(1)−N(1)
89.35 (9)
87.35 (10)
87.71 (11)
aX = Cl for compound 1 and Br for compounds 2 and 3.
nitrogen donor, the iron(III) centre in {Fe(␮-Cl)[tBuN(SiMe2)]2O}2 is
composed of two anionic nitrogen donor atoms and a neutral central
oxygen donor. The Fe–Cl bond lengths in {Fe(␮-Cl)[tBuN(SiMe2)]2O}2
are 2.3181(19) and 2.4652(17) Å whereas the corresponding dis-
tances in 1 are 2.3618(14) and 2.3038(14) Å. The Cl−Fe–Cl bond
angle in {FeCl[tBuN(SiMe2)]2O}2 is 86.75(6)°, which is lower than
the Cl−Fe–Cl angle observed in 1 (91.42(5)°) and intermediate to
those observed in A (87.36(6)°) and B (84.341(14)°).
When the sodium and lithium salts prepared from the reaction
of NaH with H2L1 and n-BuLi with H2L2 in THF at −78 °C were
reacted with anhydrous FeBr3, zwitterionic tetrahedral iron(III)
complexes 2 and 3 bearing two bromide ligands and quaternized
ammonium fragments were generated (Scheme 2). A similar com-
plex was previously reported by us using NEt3 as the base.27 The
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Chard et al.
761
Scheme 2. Synthesis of 2–4.
Fig. 4. Molecular structures (ORTEP) and partial atom labeling of 2
(top) and 3 (bottom). Hydrogen atoms (except for H(1)) and
cocrystallized solvent molecules omitted for clarity. Ellipsoids are
shown at 50% probability.
mass spectra of 2 and 3 show peaks corresponding to the loss of
one bromide ligand from the parent ion along with the proton of
the central nitrogen atom ([M–Br]+). There also exist peaks corre-
sponding to the loss of both bromide ligands and the proton of the
central nitrogen atom ([M–2Br]+) from the parent ion.
Single crystals of 2 and 3 suitable for X-ray diffraction were
obtained from saturated toluene solutions at –35 °C inside a
nitrogen-filled glove box. The solid-state molecular structures of 2
and 3 are shown in Fig. 4. In the solid state, complexes 2 and 3
exhibit monomeric structures having tetrahedral iron(III) centres.
Unlike complex 1, and also unlike the previously reported iron(III)
complexes of amine-bis(phenolate) ligands,27 the bis(phenolate)
ligands in 2 and 3 bind in a bidentate fashion. The central nitrogen
donors in 2 and 3 are protonated, giving quaternized ammonium
groups. The oxygen donors of the phenolate groups remain anionic,
giving net monoanionic ammonium-bis(phenolate) ligands. Two
bromide ions and the phenolate oxygen donor atoms make up the
tetrahedral coordination environment about the iron(III) centres
in 2 and 3. The four-coordinate tetrahedral iron(III) centres are
thereby formally anionic, resulting in zwitterionic iron(III) com-
plexes. The bond angles around the metals range from 106.38 (13)°
to 110.90(9)° in 2 and from 105.9(3)° to 112.0(2)° in 3, which are only
moderately distorted from the ideal tetrahedral angle of 109.5°.
The bond lengths of Fe(1)–Br(1) and Fe(1)–Br(2) are slightly asym-
metrical in 2 and 3. The Fe–Br distances in 2 are 2.3596(9) and
2.3491(8) Å for Fe(1)–Br(1) and Fe(1)–Br(2), respectively, while the
Fe–Br distances in 3 are 2.355(2) and 2.3697(19) Å for Fe(1)–Br(1) and
Fe(1)–Br(2), respectively. The Fe–Br distances observed in 2 and 3
are similar to the terminally bonded Fe–Br bond length (2.3683(11) Å)
found in a previously reported mononuclear square pyramidal
iron(III) bromide complex (FeBr[O2N2]) containing a salan ligand.25 In
2, the phenolate oxygen atoms exhibit bond distances to iron of
1.822(2) and 1.832(3) Å for Fe(1)–O(1) and Fe(1)–O(2), respectively. The
Fe(1)–O(1) and Fe(1)–O(2) bond lengths observed in the related com-
plex 3 (containing bulkier 2,4-di-t-butylphenolate groups) are slightly
longer, with distances of 1.843(6) and 1.851(6) Å, respectively. The
Fe–O interactions observed in 2 and 3 are similar to those observed in
FeBr[O2N2], where average Fe–O distances of 1.837 Å are observed.25
The coordination geometries of 2 and 3 are similar to a tetrahe-
dral iron(III) complex that we previously reported, abbreviated
FeBr2[O2NH]BuMenPr 27 Like 2, FeBr2[O2NH]BuMenPr contains 2-t-butyl-4-
.
methylphenolate groups. However, unlike 2 and 3, which possess
isopropyl alkyl groups on the central nitrogen donors, the nitrogen
donor of FeBr2[O2NH]BuMenPr contains a n-propyl alkyl substituent.
The Fe–O bond lengths of 1.828 (3) and 1.836 (3) Å observed in
FeBr2[O2NH]BuMenPr are slightly shorter than the corresponding
Fe–O distances found in 3. However, the Fe–O bond lengths are
very similar to those observed in 2. Similarly, as found in both
2 and 3, the Fe–Br bond lengths observed in FeBr2[O2NH]BuMenPr are
slightly asymmetrical. The Fe(1)–Br(2) bond length (2.3723(7) Å) ob-
served in FeBr2[O2NH]BuMenPr is slightly longer than the Fe–Br dis-
tances found in 2 and 3. The bond angles around the iron centre
range from 105.24(15)° to 112.87(10)° in FeBr2[O2NH]BuMenPr and
from 106.38(13)° to 110.90(9)° in 2. Since both complexes share the
same substituents on the phenolate rings, the differences in bond
angles observed may be attributed to the differences in sterics
originating from the alkyl substituents on the central nitrogen
donor.
During the preparation of 3, crystals were obtained that upon
diffraction revealed that a second species was formed. The struc-
ture is shown in Fig. 5 and selected bond lengths and angles are
given in Table 3. Complex 4 was isolated in good yield instead of
the expected iron bromide compound. As reported by Attia and
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762
Can. J. Chem. Vol. 92, 2014
Fig. 5. Molecular structure (ORTEP) and partial atom labeling of 4.
Hydrogen atoms (except for H(1)), t-butyl methyl groups, and
cocrystallized toluene molecules omitted for clarity. Ellipsoids are
shown at 50% probability. Symmetry operations used to generate
equivalent atoms (asterisks): –x + 1, –y + 1, –z.
nolate oxygen donor atoms and a bridging hydroxido oxygen
atom occupy the equatorial plane around each iron ion in 4. The
amine nitrogen and the bridging hydroxido oxygen atom O(3)*
take up the axial positions in 4 and [Fe(␮–OH)[ONO]BuMeMe
]
2. The O
are
(3)*–Fe(1)–N(1) bond angles in 4 and [Fe(␮–OH)[ONO]BuMeMe
]
2
considerably distorted from the ideal linear geometry.
Conclusions
A series of iron(III) complexes supported by amine-bis(phenolate)
ligands has been prepared. Complex 1 exhibits mildly distorted
trigonal bipyramidal coordination geometry whereas 2 and 3 con-
tain distorted tetrahedral FeIII centres. Unlike complexes 1–3, com-
plex 4 was determined to be dimeric in the solid state, giving
distorted trigonal bipyramidal iron(III) ions bridged by hydroxido
ligands. The central nitrogen donors of 2 and 3 are protonated to give
a quaternized ammonium fragment. Complexes 1–4and ligand H2L2
have been structurally characterized by single-crystal X-ray diffrac-
tion. Additionally, all paramagnetic FeIII complexes have been
analytically verified by elemental analysis and MALDI–TOF mass spec-
trometry. Ligand precursors H2L1 and H2L2 have been characterized by
1H and 13C NMR spectroscopy and high-resolution mass spectrometry
(HRMS).
Experimental
General methods and materials
All iron complexes were synthesized under an atmosphere of
dry nitrogen by means of standard Schlenk techniques or by using
an MBraun LabmasterDP glove box. THF was stored over 4 Å mo-
lecular sieves and distilled from sodium benzophenone ketyl un-
der nitrogen. Anhydrous toluene was purified using an MBraun
solvent purification system. Anhydrous FeCl3 (97%) and FeBr3
(99%) were obtained from Strem Chemicals. Other reagents were
purchased from Strem, Sigma-Aldrich, or Alfa Aesar and used
without further purification.
NMR spectra were recorded using CDCl3 solutions on a Bruker
Avance III 300 MHz instrument with a 5 mm multinuclear broad-
band observe (BBFO) probe. MALDI–TOF MS spectra were per-
formed using an ABI QSTAR XL Applied Biosystems/MDS hybrid
quadrupole TOF MS/MS system equipped with an oMALDI-2 ion
source. Samples were prepared at a concentration of 10.0 mg/mL
in toluene. Anthracene was used as the matrix, which was mixed
at a concentration of 10.0 mg/mL. Infrared spectra were recorded
on a Bruker Alpha FTIR spectrometer equipped with diamond
ATR sampling. HRMS spectra were recorded using a High Resolu-
tion MSD Waters Micromass GCT Premier spectrometer equipped
with an electron impact ion source and a time-of-flight (TOF) mass
analyzer. Melting point data were collected on a MPA100 OptiMelt
automated melting point system. Elemental analyses were carried
out by Canadian Micro-analytical Services Ltd., Delta, British Co-
lumbia.
Table 3. Selected bond lengths (Å) and bond angles (°) of 4.
Fe(1)−Fe(1)a
3.13645 (17)
1.870 (8)
2.010 (8)
106.8 (2)
38.5 (2)
133.3 (2)
110.2 (4)
91.4 (3)
Fe(1)−O(1)
Fe(1)−O(3)
Fe(1)−N(1)
1.857 (8)
1.980 (8)
2.214 (10)
117.8 (2)
37.9 (2)
116.9 (4)
96.4 (3)
132.6 (4)
88.6 (3)
95.0 (3)
Fe(1)−O(2)
Fe(1)−O(3)a
Fe(1)a−Fe(1)−O(1)
Fe(1)a−Fe(1)−O(3)
Fe(1)a−Fe(1)−N(1)
O(1)−Fe(1)−O(3)
O(1)−Fe(1)−N(1)
O(2)−Fe(1)−O(3)a
O(3)−Fe(1)−O(3)a
Fe(1)a−Fe(1)−O(2)
Fe(1)a−Fe(1)−O(3)a
O(1)−Fe(1)−O(2)
O(1)−Fe(1)−O(3)a
O(2)−Fe(1)−O(3)
O(2)−Fe(1)−N(1)
O(3)−Fe(1)−N(1)
93.5 (3)
76.38 (5)
aSymmetry operations used to generate equivalent atoms: −x + 1, −y + 1, −z.
co-workers, treatment of a monomeric FeIII species (with coordi-
nated monoanionic ligands) with a strong base (such as KOH) at
room temperature leads to a ␮-dihydroxido FeIII species.46 If LiOH
was present as an impurity in the n-BuLi used for the reaction, it
may have led to formation of the observed hydroxido-bridged
dimer. Indeed, when the reaction is repeated with freshly ob-
tained and titrated n-BuLi, compound 3 is exclusively obtained. In
the mass spectrum of 4, a molecular ion peak ([M]+) is evident. An
intense [M–OH]+ peak was also observed in the mass spectrum.
The data obtained from single crystals of 4 were consistently of
poor quality and led to very weak diffraction at high angles (low
cutoff in 2␪ (45°)); however, we include the results for connectivity
purposes.47 In the solid state, 4 exhibits a dimeric structure result-
ing in trigonal bipyramidal iron(III) centres bridged by hydroxide
ligands. A similar compound ([Fe(␮–OH)[ONO]BuMeMe]2) has been
previously reported by Chaudhuri and co-workers.48 However,
unlike 4, which contains 2,4-di-t-butylphenolate groups, [Fe(␮–
Crystallographic data were solved on an AFC8-Saturn 70 single
crystal X-ray diffractometer from Rigaku/MSC, equipped with an
X-stream 2000 low-temperature system and a SHINE optic. The
data were processed using CrystalClear47 software and corrected
for Lorentz and polarization effects and absorption.48 Neutral
atom-scattering factors for all nonhydrogen atoms were taken
from the “International Tables for X-ray Crystallography”.49 The
structures were solved by direct methods using SIR9250 and ex-
panded using Fourier techniques (DIRDIF99).51 All nonhydrogen
atoms were refined anisotropically. Hydrogen atoms were refined
OH)[ONO]BuMeMe
] possesses 2-t-butyl-4-methylphenolate groups,
using the riding model. Anomalous dispersion effects were in-
2
52
cluded in Fcalc
;
the values for ⌬f= and ⌬f== were those of Creagh
which may impart more of an electronic rather than a steric dif-
ference at the metal centre. Perhaps more significant is that the
central nitrogen donor of [Fe(␮-OH)[ONO]BuMeMe]2 contains a meth-
yl substituent, while the central nitrogen donor in 4 possesses a
bulkier isopropyl group. Like [Fe(␮–OH)[ONO]BuMeMe]2, two phe-
and McAuley.53 The values for the mass attenuation coefficients
are those of Creagh and Hubbell.54 All calculations were per-
formed using the CrystalStructure55 crystallographic software
package except for refinement, which was performed using
Published by NRC Research Press

Chard et al.
763
SHELXL-97.56 Structural illustrations were created using ORTEP-III
(v. 2013.1) for Windows.57
FeBr2L2H (3)
A 1.6 mol/L hexane solution of n-butyllithium (5.50 mL,
8.87 mmol) was added via syringe to a stirred solution of H2L2
(2.00 g, 4.03 mmol) in THF (50 mL) at –78 °C. Upon return to room
temperature, the lithiated ligand (clear pale yellow solution) was
transferred via cannula to a solution of anhydrous FeBr3 (1.19 g,
4.03 mmol) in THF (30 mL) at –78 °C. After stirring for 2 h, the
solvent was removed via vacuum to give a dark purple powder.
The dark purple solid was then extracted with minimal toluene
and the resulting dark purple solution was filtered through Celite.
The solution was then placed in a freezer at –35 °C for recrystalli-
zation whereupon dark red crystals suitable for X-ray diffraction
were obtained (2.156 g, 76%). Anal. calcd. for C33H52Br2FeNO2: C
55.79, H 7.38, N 1.97; found: C 55.61, H 7.19, N, 2.11. MALDI–TOF m/z
(%, ion): 710.468 (2, [M]+), 549.260 (10, [M–2Br]+), 492.320 (100, [M–
Fe–2Br]+).
H2[O2N]BuMeiPr (H2L1)
To a stirred mixture of 2-t-butyl-4-methylphenol (20.398 g,
0.1232 mol) in 100 mL of deionized water was added 37% aqueous
formaldehyde (10 mL, 0.1232 mol) followed by slow addition of
isopropylamine (3.55 g, 0.0615 mol). The reaction was heated to
reflux for 12 h. Upon cooling, the reaction mixture separated into
two phases. The upper phase was decanted and the remaining oily
residue was triturated with cold methanol to give an analytically
pure, white powder (16.25 g, 64%); mp range 130.2–131.7 °C.
1H NMR (300 MHz, CDCl3) ␦: 7.00 (s, ArH, 2H), 6.73 (s, ArH, 2H), 3.65
(s, CH2, 4H), 3.16 (septet,3J = 5 Hz, CH, 1H), 2.24 (s, CH3, 6H), 1.39 (s,
CH3, 18H), 1.17 (d,3J = 5 Hz, CH3, 6H). 13C{1H} NMR (75 MHz, 298 K,
CDCl3) ␦: 152.68 (Ar), 136.80 (Ar), 128.93 (Ar), 128.03 (Ar), 127.20 (Ar),
122.36 (Ar), 51.64 (CH2), 48.33 (CH), 34.59 (C(CH3)3), 29.64 (C(CH3)3),
20.80 (ArCH3), 16.64 (CH(CH3)2). HRMS (TOF MS EI+): m/z [M]+ calcd.
for H2L1: 411.3137; found: 411.3143.
[Fe(␮–OH)L2]2 (4)
The reaction was performed as for the synthesis of 3; however,
upon recrystallization, compound 4 was obtained due to the
presence of hydroxide (see “Results and discussion” section). A
1.6 mol/L hexane solution of n-butyllithium (5.50 mL, 8.87 mmol)
was added via syringe to a stirred solution of H2L2 (2.00 g, 4. 03
mmol) in THF (50 mL) at –78 °C. Upon return to room temperature,
the lithiated ligand (clear pale yellow solution) was transferred via
cannula to a solution of anhydrous FeBr3 (1.19 g, 4.03 mmol) in
THF (30 mL) at –78 °C. After stirring for 2 h, the solvent was
removed via vacuum to give a dark purple powder. The dark pur-
ple solid was then extracted with minimal toluene and the result-
ing dark purple solution was filtered through Celite. The dark
purple solution was then placed in a freezer for recrystallization
whereupon dark brown crystals suitable for X-ray diffraction were
obtained (1.905 g, 83%). Anal. calcd. for C66H104Fe2N2O6: C 69.95, H
9.25, N 2.47; found: C 70.12, H 8.98, N 2.65. MALDI–TOF m/z (%, ion):
496.479 (100, [M–FeOH]+), 549.399 (10, [M–OH]+), 564.394 (7, [M]+).
H2[O2N]BuBuiPr (H2L2)
To
a stirred mixture of 2,4-di-t-butylphenol (26.491 g,
0.1232 mol) in 100 mL of deionized water was added 37% aqueous
formaldehyde (10 mL, 0.1232 mol) followed by slow addition of
isopropylamine (3.55 g, 0.0615 mol). The reaction was heated to
reflux for 12 h. Upon cooling, the reaction mixture separated into
two phases. The upper phase was decanted and the remaining
light orange solid was triturated with cold methanol to give an
analytically pure, white powder (17.32 g, 57%); mp range 142.5–
143.3 °C. 1H NMR (300 MHz, CDCl3) ␦: 7.21 (s, ArH, 2H), 6.92 (s, ArH,
2H), 3.71 (s, CH2, 4H), 3.17 (septet,3J = 5 Hz, CH, 1H), 1.39 (s, CH3,
18H), 1.28 (s, CH3, 18H), 1.18 (d,3J = 5 Hz, CH3, 6H). 13C{1H} NMR
(300 MHz, 298 K, CDCl3) ␦: 152.60 (ArCOH), 141.43 (Ar), 136.02 (Ar),
125.03 (Ar), 123.41 (Ar), 121.63 (Ar), 52.00 (NCH(CH3)2), 48.40
(ArCH2), 34.88 (C(CH3)3), 34.18 (C(CH3)3), 31.67 (C(CH3)3), 29.70
(C(CH3)3), 16.66 (CH(CH3)2). HRMS (TOF MS EI+): m/z [M]+ calcd. for
H2L2: 495.4076; found: 495.4063. IR (neat) ␯: 3196, 2958, 2905,
2865, 1606, 1476, 1451, 1391, 1362, 1290, 1225, 1207, 1157, 1123, 1078,
1027, 995, 967, 935, 879, 824, 792, 755, 722, 682, 653, 600, 540,
503 cm–1.
Supplementary material
Supplementary spectroscopic and crystal structure data are
available with the article through the journal web site at http://
nrcresearchpress.com/doi/suppl/10.1139/cjc-2014-0043. Crystal-
lographic data for CCDC 982453 (H2L2), CCDC 982454 (1), CCDC
982455 (2), and CCDC 982456 (3) (pdf and cif format) can be
obtained, free of charge, via http://www.ccdc.cam.ac.uk/products/
csd/request/ (or from the Cambridge Crystallographic Data Centre, 12
Union Road, Cambridge CB2 1EZ, UK (fax: 44-1223-336033 or e-mail:
deposit@ccdc.cam.ac.uk)).
[NEt3H] + [FeCl2L1]– (1)
To a THF solution (50 mL) of recrystallized H2L1 (2.00 g,
4.87 mmol) was added a solution of anhydrous FeCl3 (0.800 g,
4.93 mmol) in THF resulting in an intense purple solution. To this
solution was added triethylamine (1.00 g, 9.86 mmol) and the
resulting mixture was stirred for 2 h. After stirring, the dark pur-
ple solution was filtered through Celite. Removal of solvent under
vacuum yielded a dark purple solid. Crystals suitable for X-ray
diffraction were obtained by slow evaporation of a toluene solu-
tion (1.693 g, 55%). Anal. calcd. for C33H55Cl2FeN2O2·(C7H8)1.3: C
66.68, H 8.69, N 3.69; found: C 66.73, H 8.92, N 3.44.
Acknowledgements
Financial support was provided by the Canada Foundation for
Innovation, Newfoundland and Labrador Research Development
Corporation, the Natural Sciences and Engineering Research
Council of Canada, and Memorial University of Newfoundland.
FeBr2L1H (2)
References
A THF solution (50 mL) of recrystallized H2L1 (2.00 g, 4.86 mmol)
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