Synthesis and Characterization of Sterically Encumbered β-Ketoiminate Complexes of Iron(ii) and Zinc(ii)

Article abstract of DOI:10.1039/c1dt10024f

The synthesis, structure, and spectroscopic signatures of a series of four-coordinate iron(ii) complexes of β-ketoiminates and their zinc(ii) analogues are presented. An unusual five-coordinate iron(ii) triflate with three oxygen bound protonated β-ketoim

Full text of DOI:10.1039/c1dt10024f

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PAPER
Synthesis and Characterization of Sterically Encumbered b-Ketoiminate
Complexes of Iron(II) and Zinc(II)‡
David M. Granum,†a Paul J. Riedel,†a Joshua A. Crawford,b Thomas K. Mahle,a Chelsea M. Wyss,a
Anastasia K. Begej,a Navamoney Arulsamy,a Brad S. Pierceb and Mark P. Mehn*a
Received 7th January 2011, Accepted 30th March 2011
DOI: 10.1039/c1dt10024f
The synthesis, structure, and spectroscopic signatures of a series of four-coordinate iron(II) complexes
of b-ketoiminates and their zinc(II) analogues are presented. An unusual five-coordinate iron(II) triflate
with three oxygen bound protonated b-ketoimines is also synthesized and structurally characterized.
Single-crystal X-ray crystallographic analysis reveals that the deprotonated bis(chelate)metal complexes
are four-coordinate with various degrees of distortion depending on the degree of steric bulk and the
electronics of the metal center. Each of the high-spin iron(II) centers exhibits multiple electronic
transitions including ligand p to p*, metal-to-ligand charge transfer, and spin-forbidden d-d bands. The
1H NMR spectra of the paramagnetic high-spin iron(II) centers are assigned on the basis of chemical
shifts, longitudinal relaxation times (T1), relative integrations, and substitution of the ligands. The
electrochemical studies support variations in the ligand strength. Parallel mode EPR measurements for
the isopropyl substituted ligand complex of iron(II) show low-field resonances (g > 9.5) indicative of
complex aggregation or crystallite formation. No suitable solvent system or glassing mixture was found
to remedy this phenomenon. However, the bulkier diisopropylphenyl substituted ligand exhibits an
integer spin signal consistent with an isolated iron(II) center [S = 2; D = -7.1 0.8 cm-1; E/D = 0.1]. A
tentative molecular orbital diagram is assembled.
unusual magnetic and physical properties.34–37 Ligands that impose
a cis-divacant octahedral (or sawhorse) geometry have also been
invoked as key intermediates for cis-dihydroxylation catalysts for
Introduction
Iron is an attractive metal for catalysis due to its low cost and
higher formal oxidation states,38–40 but are usually found in clusters
for iron(II).41,42 Highly distorted four-coordinate high-spin iron(II)
centers exhibit unique physical properties and chemical reactivity.
Seminal work by Holm and coworkers demonstrates that
deprotonated a,b-unsaturated-b-ketoamines (or b-ketoiminates)
support four-coordinate metal geometries,43,44 resist coordination
polymer formation,44–47 and allow facile tuning of both steric and
electronic constraints. Furthermore, the stereochemical prefer-
ences of these bidentate monoanionic ligands in their reactions
with cobalt, nickel, iron, chromium, copper, and zinc have been
described.43,44,47–51 Due to our interest in generating catalysts of
earth abundant metals for small molecule activation featuring
sterically encumbered b-ketoiminate ligands, we have revisited
this classic coordination chemistry with bulkier ligand scaffolds.
In this work, the role of the steric bulk of these ligands in
modulating the physical properties of divalent metal centers is
explored. The full assignment of the 1H NMR spectra of the
paramagnetic [FeII(LR)2] complexes (where LR = a deprotonated
a,b-unsaturated-b-ketoamine or b-ketoiminate) is provided as
well as the electronic spectra into the near-infrared. Additionally,
electrochemical studies of the iron(II) and zinc(II) complexes
are presented. Interestingly, while the solid state structure of
relatively low toxicity. In particular, imine ligand scaffolds support
iron catalysts for ethylene polymerization,1–6 support unusual
three coordinate iron centers with unique electronic structures
giving rise to unusual reactivity with small molecules,7–12 and in
certain instances show redox non-innocent behavior.13,14 High-spin
four-coordinate iron(II) complexes are dominated by tetrahedral
geometries.15–21 Multiple examples of square planar iron(II) com-
plexes have been reported with sterically encumbered or macro-
cyclic ligands.22–33 Recent reports of trigonal monopyramidal high-
spin iron(II) complexes supported by multidentate ligands have
aDepartment of Chemistry, University of Wyoming, Laramie, Wyoming,
82071, USA. E-mail: mmehn@uwyo.edu; Fax: (307) 766 2807; Tel: (307)
766 5666
bDepartment of Chemistry & Biochemistry, Box 19065, 700 Planetar-
ium Place, Arlington, Texas, 76019, USA. E-mail: bspierce@uta.edu;
Fax: (817) 272 3808; Tel: (817) 272 9066
† These two authors contributed equally to this research endeavor
‡ Electronic supplementary information (ESI) available: Tables comparing
NMR features and bond distances of ligands, schemes of prevalent
tautomers and resonance contributors for the ligands, and figures showing
preliminary molecular orbital diagrams, zinc cyclic voltammograms and
electronic changes under dioxygen. CCDC reference numbers 807200–
807204. For ESI and crystallographic data in CIF or other electronic
format see DOI: 10.1039/c1dt10024f
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[FeII(LiPr)2] reflects a monometallic complex, the frozen solvent-
glass EPR spectra of this complex indicate formation of molecular
aggregates (or microcrystallites) in solution. These studies provide
a basis for ongoing work on the catalysts for the activation of small
molecules.
plex [{PhC(O)CHC(NiPr)CH3}2FeII] reported by Holm and
coworkers.44 Sodium hydride deprotonation is the preferred syn-
thetic method due to the limited introduction of byproducts, which
are potential metal center ligands, and the increased yield of the
product. Additionally, the bis(chelate)metal complex also forms
(albeit in lower amounts) when one equivalent of deprotonated
ligand is added to a single equivalent of iron(II). The reaction of
HLiPr and NaH with zinc(II) triflate affords colorless crystals of
the analogous [Zn(LiPr)2].
Results and discussion
Following standard synthetic routes for the preparation of a,b-
unsaturated-b-ketoamines, condensation of a b-diketone with a
primary amine and a catalytic amount of acid afforded the desired
ligands in high yield (Scheme 1).52–55 Although three tautomeric
forms are possible (i.e.; Schiff base, ketamine, and enimine), the
1H NMR spectra for each of the reaction products exhibits a
singlet between 4.9 and 5.2 ppm with an integration of one proton.
This observation is consistent with a methine proton (See ESI,
Schemes S1 and S2, and Tables S1 and S2‡). Furthermore, each
product exhibits a highly deshielded broad singlet between 10.7
and 12.5 ppm with an integration of one proton. This suggests
either the presence of an intramolecular hydrogen bond between
the amine and the adjacent ketone or the presence of an iminium
proton. Collectively, these observations are inconsistent with the
Schiff base or ketoimine tautomer, which would have a methylene
feature. Furthermore, the 13C NMR chemical shift for the a-
carbon is observed between 90 and 100 ppm, which is indicative
of some alkene character. The insensitivity of the acid dissociation
constants to aniline substitution led Martin and coworkers to
suggest that the enimine tautomer may predominate for aromatic
amines.56 However, the feature between 194–196 ppm in the 13C
NMR spectra of each ligand is consistent with the ketone formu-
lation. The highly deshielded feature in the 1H NMR (10.7 to 12.5
ppm) is not as highly shifted as the enol proton of acetylacetone,
which appears at 15.5 ppm. Furthermore, this feature can be split
by benzylic amines (e.g., when R = CH2C6H5) further emphasizing
the prevalence of the ketamine tautomer.54 Yet, the 13C NMR
spectra of each product exhibits a peak between 160 and 163 ppm,
which is consistent with significant C–N multiple bond character.
Zwitterionic resonance forms with iminium cation character are
undoubtedly resonance contributors in this delocalized p-system.
Therefore, departing from the literature precedent naming these
species as a,b-unsaturated-b-ketoamines,54,55 we will describe these
species as protonated b-ketoiminates.
One surprising experimental observation is that mixing HLdipp
and sodium hydride does not immediately evolve hydrogen gas.
Initially, upon addition of iron(II) triflate, a deep red complex is
formed which, after a few minutes, evolves a gas resulting in a deep
emeraldgreen [Fe(Ldipp)2] complex. The identity of the transient red
species remains unknown, and attempts at the production of the
anionic tris-chelate complex have not been successful. The reaction
of three equivalents of HLdipp with iron(II) triflate in the absence
of base affords the pale yellow [Fe(HLdipp)3(OTf)2]. Titration
of this complex with sodium hydride yields the emerald green
bis(chelate) metal complex without observation of the deep red
transient species. The treatment of HLdipp with Li(NiPr2) followed
by the addition of iron(II) triflate proceeds immediately to the
emerald green product, therefore on the basis of limited literature
precedent,57–59 we speculate that a sodium complex forms prior to
the formation of the emerald green [Fe(Ldipp)2]. Few other examples
of sodium and iron(II) heterobimetallic dimers are known.57,60
The structures of the [Fe(LiPr)2] and [Zn(LiPr)2] reveal isomor-
phous four-coordinate divalent metal centers each supported by
two bidentate b-ketoiminate ligands (Fig. 1, Tables 1 and 2).
Whether using Alvarez’s continuous symmetry measurements61,62
or Hauser’s qualitative t4 measurements (Table 2),63 the [MII(LiPr)2]
complexes all show similar degrees of distortion from tetrahedral.
The t4 values indicate a distorted trigonal pyramidal geometry
about each metal center. The bond distances and angles com-
pare well with those reported previously for high-spin iron(II)
complexes64–68 and comparable zinc complexes.69–71 The nearest
˚
Fe ◊ ◊ ◊ H–C contact is at 2.72 A and no evidence supports agostic
interactions with the isopropyl groups. The chelate bite angles are
similar (average N–M–O ~ 96◦) and are typical of those observed
for other b-ketoiminates with various divalent metals.53,71–73 The
ligand backbones (O1–C2–C3–C4–N1) of both complexes have
˚
bond distances with the range of 1.293–1.433 A, consistent with
delocalized p-systems (Table S3‡). The ionic radii of the iron and
Scheme 1
One equivalent of metal(II) triflate reacts with two equiv-
alents of protonated b-ketoiminate and base to afford the
mononuclear bis(chelate)metal(II) complexes. We have utilized
a number of different iron sources (i.e.; [FeCl4](NEt4)2, FeCl2,
Fe(OTf)2, or [Fe(Mes)2]2) and bases (i.e., KOtBu, NaH, or
Li(NiPr2)) to obtain intensely-colored yellow crystals of [Fe(LiPr)2]
in high yield, which are related to the deep red com-
Fig. 1 ORTEP plot of one of the two unique molecules in the crystals of
[Fe(LiPr)2] and [Zn(LiPr)2] showing 50% probability thermal ellipsoids and
the labeling scheme for unique atoms. All hydrogen atoms are omitted for
clarity.
5882 | Dalton Trans., 2011, 40, 5881–5890
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Table 1 Summary of the crystallographic data for [Fe(LiPr)2] (1), [Zn(LiPr)2] (2), [Fe(HLdipp)3(OTf)2]·C6H6 (3a), [Fe(Ldipp)2] (3b), and [Zn(Ldipp)2] (4)
1
2
3a
3b
4
chemical formula
Formula weight/g mol-1
Space group
C16H28FeN2O2
336.25
Iba2
10.6628(2)
15.7409(3)
20.5025(4)
90
90
90
3441.2(1)
8
1.298
C16H28N2O2Zn
345.77
Iba2
10.6260(2)
15.6935(3)
20.6601(4)
90
90
90
3445.3(1)
8
1.333
C59H81F6FeN3O9S2
1210.24
P1
C34H48FeN2O2
572.59
C2/c
23.7223(2)
12.0690(1)
11.7799(1)
90
107.93(1)
90
3208.93(5)
4
1.185
150
5.00
C34H48ZnN2O2
582.11
P1
9.9587(1)
10.9893(1)
16.4658(1)
84.78
76.17
65.85
1596.52(2)
2
1.211
¯
¯
˚
a/A
10.9902(9)
15.5889(13)
18.9999(15)
76.706(4)
86.343(4)
83.931(4)
3147.6(4)
2
1.277
150
3.78
0.0392/0.0937
0.0572/0.1037
˚
b/A
˚
c/A
a/◦
b/◦
g /◦
3
˚
V/A
Z
Dc/g cm-3
T/K
150
8.82
0.0283/0.0710
0.0356/0.0747
150
1.432
0.0256/0.0616
0.0364/0.0656
150
8.00
0.0262/0.0702
0.0305/0.0727
Absorption/cm-1
R1, wR2 (I > 2s(I))a
R1, wR2 (all data)
0.0376/0.0888
0.0537/0.0965
a R1 = R ꢀFo| - |Fcꢀ/R |Fo|. wR2 = [R[w(Fo - Fc2)2]/R[w(Fo2)2]]1/2 where w = q/s (Fo2) + (a*P)2 + b*P.
2
2
◦
˚
Table 2 Selected bond lengths (A) and angles ( ) for 1, 2, 3, and 4
1
2
3
4
[Fe(LiPr)2]
[Zn(LiPr)2]
[Fe(Ldipp)2]
[Zn(Ldipp)2]
M–O#
M–N#
1.9407(8)
2.0447(8)
93.92(3)
120.32(5)
113.72(3)
123.54(5)
2.914
81.9
2.08
23.96
0.82
1.9419(8)
2.0373(9)
94.31(3)
122.52(5)
112.80(3)
122.50(5)
2.918
80.7
2.06
24.53
0.81
1.9479(8)
2.0000(9)
97.67(3)
122.00(6)
111.98(4)
116.87(5)
2.972
82.61(2)
1.27
25.94
1.9481(8)
1.9915(9)
98.09(3)
123.82(6)
111.23(4)
115.44(5)
2.975
81.86(2)
1.27
25.72
1.9270(8)
2.0508(8)
91.38(3)
139.80(5)
100.39(3)
145.40(6)
2.848
59.41(3)
8.29
10.56
1.9617(9), 1.9570(8)
1.9818(9), 1.9792(9)
96.69(4), 95.91(4)
134.11(4)
96.69(4), 114.24(4)
105.65(4)
2.923, 2.947
88.90(3)
9.02
N#–M–O#
N#–M–N#A
N#–M–O#A
O#–M–O#A
N# ◊ ◊ ◊ O#
Ring dihedrals
S(Td)61,87
S(D4h)61,87
14.89
0.79
63
t4
0.86
0.86
0.53
zinc are similar and few meaningful differences can be found in
these similar structures.
Crystallographic analysis of the yellow crystals of
[Fe(HLdipp)3(OTf)2] reveals a mononuclear five-coordinate iron(II)
center with an oxygen rich coordination sphere (Fig. 2, and Tables
1 and 3). The distorted square pyramidal iron(II) center (t5
=
0.23)74 is supported by two trans triflates and three oxygen bound
protonated ligands. The triflate ligands have the longest Fe–O
bonds whilst the Fe–O bonds to the protonated ligand are shorter.
This type of monodentate oxygen-coordination for a protonated
HLdipp has been previously observed in [Mg(HLdipp)(Ldipp)2].73 The
bond distances of the protonated b-ketoiminate backbone are
indicative of delocalization across the p-system (Table S3‡). The
aromatic p-system of the phenyl ring is orthogonal to the p-system
of the b-ketoiminate backbone. Hydrogen-bonding interactions
are evident in each of the protonated ligands.
Fig. 2 ORTEP plot of [Fe(HLdipp)3(OTf)2] showing 50% probability
thermal ellipsoids and the labeling scheme for unique atoms. All hydrogen
atoms and the benzene solvate are omitted for clarity. The exceptions
are the three hydrogens associated with protonated ligands. Dashed lines
indicate hydrogen bonding interactions. No agostic interactions were
found.
The structures of the iron(II) and zinc(II) complexes of Ldipp-
,
deviate significantly from one another (Fig. 3, Tables 1 and 2).
While both show four-coordinate metal centers with bidentate
b-ketoiminates and similar degrees of distortion on the basis of
continuous symmetry measurements, the mean planes determined
by the six-membered chelate rings intersect at 88.90(3)◦ for the
zinc complex, while the same metric in the iron(II) complex is
59.41(3)◦. While the continuous symmetry measurements place
both complexes along the D2d spread distortion pathway, a t4
value of 0.53 for [Fe(Ldipp)2] is indicative of a seesaw or sawhorse
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Table 3 Selected Bond Lengths (A) and Angles (◦) for
˚
[Fe(HLdipp)3(OTf)2]·benzene
3a
[Fe(HLdipp)3(OTf)2]·benzene
Fe–O1
Fe–O2
Fe–O3
Fe–O4
2.0258(11)
2.0132(11)
2.0460(10)
2.1266(12)
2.1883(12)
148.58(4)
104.73(4)
106.57(4)
162.08(5)
0.23
Fe–O7
O1–Fe–O2
O1–Fe–O3
O2–Fe–O3
O4–Fe–O7
74
t5
Fig. 4 Electronic spectra of [Fe(LiPr)2] (—) and [Fe(Ldipp)2] (---) in THF at
room temperature. The extinction coefficients for the high energy features
are on the left, while the extinction coefficients for the low energy features
are on the right.
by approximately 2000 cm-1.44 This observation is more in line
with six-coordinate ferrous centers than distorted tetrahedral
complexes, which generally exhibit two bands in the 4000–7000
cm-1 region.75,76 These near-infrared features suggest the solid
state structures do not accurately convey the solution constitution
of these iron(II) complexes. Notably, [Fe(Ldipp)2] shows a similar
charge transfer band at 420 nm (i.e., 23 800 cm-1 with e = 1400 M-1
cm-1) and two weak low energy features at 680 and 1470 nm
(i.e., 14 700 and 6800 cm-1, respectively). The splitting in the d-
d bands is consistent with the higher degree of distortion from an
ideal tetrahedral geometry that is observed crystallographically
for [Fe(Ldipp)2]. The mineral gillespite is an example of a square
planar ferrous site which exhibits two transitions at 8000 and
20 000 cm-1.75 However, the near-infrared spectrum of [Fe(Ldipp)2] is
not consistent with a square planar geometry for the iron(II) center.
The near-infrared features of a trigonal monopyramidal iron(II)
center supported by a trianionic ligand exhibits two features at
1636 nm and 1911 nm (e = 250 and 180 M-1 cm-1, respectively).37
Regardless of the geometry, these low energy features are likely
spin-forbidden d-d bands.
Fig. 3 ORTEP plot of [Fe(Ldipp)2] and [Zn(Ldipp)2] showing 50% proba-
bility thermal ellipsoids and the labeling scheme for unique atoms. All
hydrogen atoms are omitted for clarity.
geometry for the iron(II) center (with C2v symmetry). Similar bond
distances are observed in both complexes and the iron-nitrogen
and iron-oxygen distances are typical of high-spin iron(II) centers.
˚
The closest Fe ◊ ◊ ◊ H–C distance is 3.341 A in the iron complex
[Fe(Ldipp)2] with no evident agostic interactions. Due to the similar
ionic radii of the divalent metal centers and the structure of the
zinc complex, steric bulk must not be the sole factor governing
the distortion in the iron complex. The orthogonality of the
diisopropylphenyl substituent relative to the N,O-chelate p-system
negates any electron-withdrawing resonance effect from the phenyl
ring leaving only the electron-releasing inductive effect.
Each iron complex exhibits paramagnetically shifted 1H NMR
peaks providing further support for the paramagnetic nature of
these species in benzene solution (Fig. 5, Table 4). The spectra show
little deviation in THF, chloroform, or benzene. The assignments
have been made on the basis of chemical shifts, longitudinal relax-
ation measurements (T1), relative integration, and substitution
studies. [Fe(LiPr)2] exhibits several paramagnetically broadened
and shifted peaks (Fig. 5A). The proximity of the hydrogen
atoms observed in the solid state structures is consistent with
the short relaxation times (< 1 ms) observed in solution. While
the overlapping nature of the peaks complicates the assignments,
the relatively low number of peaks is consistent with the ligands
appearing in symmetric environments on the NMR time scale
at room temperature. This could also be due to aggregation in
solution to form dimers, trimers, or oligomers, as has often been
observed in acetylacetonate chemistry.46,77–80 If the interaction of
the trigonal monopyramidal iron centers with the oxygen or
nitrogen of another complex results in dimers,66,67 most of the
postulated equilibria would remove the magnetic equivalence
of the chelated ligands. Therefore, an equilibrium between the
Both the iron and zinc complexes undergo no noticeable color
changes between the solid state and solution. Furthermore, the
electronic spectra have been collected in a variety of solvents
(toluene, THF, benzene) and over a wide temperature range
without observing any dramatic change in the appearance or
the position of the spectral features. All of the b-ketoiminate
complexes exhibit an intense feature at approximately 310 nm
(e ª 20 000 M-1 cm-1), which we have attributed to a ligand
p–p* transition. The zinc complexes are spectroscopically silent
above 400 nm. The electronic spectrum of [Fe(LiPr)2] exhibits an
intense group of features centered at 410 nm (i.e., 24 400 cm-1
with e = 2600 M-1 cm-1) and a much weaker set of near-IR
bands at 920 nm and 1120 nm (i.e., 11 000 and 9000 cm-1 with
e = 30 and 20 M-1 cm-1, respectively, Fig. 4). On the basis of
the extinction coefficients, the high energy features are proposed
to be charge-transfer bands whereas the lower energy features
are d-d bands. Holm’s high-spin iron(II) b-ketoiminate complexes
each exhibit near-IR features near 10 000 cm-1, which are split
5884 | Dalton Trans., 2011, 40, 5881–5890
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Table 4 Summary of 1H NMR Parametersa
C(O)-CH3
d/ppm
-CH-
d/ppm
-21
C(N)-CH3
d/ppm
iPr-
ortho-H
meta-H
para-H
d/ppm
d/ppm
d/ppm
d/ppm
[Fe(LiPr)2]
-64.5 (1.4)
-54 (1.0)
-19 (0.3, -CH)
-24 (0.4, -CH3)
—
0.28(5.3, 6H),
-16 (0.9, 6H),
-24(3.6)
—
—
[Fe(LPh)2]
[Fe(Ldipp)2]
-72 (0.9)
-28(0.7)
-17 (0.5)
-55 (0.9)
-22 (0.9)
-63 (0.3)
—
15 (6.3)
15 (9.3)
-38 (6.1)
-43 (1.9)
b
[Fe(L3,5-Me )2]
-71.8
-55.5
-8.9
-12.5
-55.9
-52.8
-63.4
36.8 (-CH3)
-15.5 (-CH3)
13.3
-37.6
1.5 (-CH3)
2
[Fe(LMes)2]
a All spectra were recorded in benzene-d6 at room temperature. Numbers in parentheses are relaxation times (T1) in milliseconds. b Not uniquely identified.
The change in paramagnetic shift is consistent with a dominant
p-delocalization pathway for the arene ring. The peak at -63
ppm has a very short relaxation time (0.3 ms) indicative of close
proximity to the metal center and has an integration of nearly two
protons. These properties are consistent with the ortho-protons
of the phenyl ring. This feature is absent in both [Fe(Ldipp)2] and
[Fe(LMes)2] (Fig. 5C and E, respectively). In [Fe(LPh)2], the feature
at -38 ppm has an integration of one proton and the similar
relaxation time leads us to assign this peak to the para-position
of the phenyl ring. The remaining feature at -17 ppm must be
the methine at the a-position of the b-ketoiminate. Unfortunately,
attempts to obtain correlated spectra have been unsuccessful due
to the rapid relaxation processes. Collectively, the 1H spectra of the
iron(II) b-ketoiminates are consistent with the ligands appearing
in identical magnetic environments on the NMR time scale.
Electrochemical investigation of the bis(b-ketoiminate)metal
complexes has been carried out in THF (Fig. 6). Both iron
complexes exhibit a quasi-reversible one-electron oxidative wave
(Fig. 6A and B). The position of the anodic peak potentials differ
with the substitution of the b-ketoiminate ligand. For [Fe(LiPr)2],
this feature is centered at -185 mV vs. Fc+/0 or +345 mV vs. SCE,
while for [Fe(Ldipp)2] the wave is centered at -245 mV vs. Fc+/0 (DE =
120 mV) or +285 mV vs. SCE. The positive potential shift (60 mV)
of the FeII/III couple indicates that the [Fe(Ldipp)2] is more easily
oxidized. This shift in potential suggests a stronger ligand field
effect for the Ldipp anion than for the LiPr anion- if the complexes
have identical constitutions in solution. The orthogonality of the
diisopropylphenyl p-system to the ligand backbone removes the
electron-withdrawing resonance effect and leaves the electron-
releasing inductive effect resulting in a stronger ligand.
Interestingly, the zinc complexes show both irreversible oxida-
tive and quasi-reversible reductive waves, which must be ascribed
to ligand oxidation and reduction (Fig. S1‡). Similar features are
observed in the iron complexes and are assigned to the oxidation
and reduction of the ligand anion. The ligand oxidation and
reduction features become less reversible at low scan rates, which is
consistent with rapid chemical steps following the electrochemical
process.
Fig. 5 1H NMR spectrum of (A) [Fe(LiPr)2], (B) [Fe(LPh)2], (C) [Fe(Ldipp)2],
(D) [Fe(L3,5-Me )2], and (E) [Fe(LMes)2] in benzene-d6 at room temperature.
2
Spectra are referenced to the residual protic solvent peak at 7.16 ppm.
Dashed lines identify the peak associated with benzene.
aggregated and monometallic species would have to be rapid to be
consistent with the spectra reported.
[Fe(LPh)2] reveals five well resolved resonances and the assign-
ment of these features will be discussed as an example of the
process used for the other complexes (Fig. 5B). The very broad
peak at -72 ppm with the rapid relaxation time (1.0 ms) integrates
to three hydrogens and is consistent with the methyl group next
to the ketone functionality of the b-ketoiminate complex. The
other broad resonance at -55 ppm with a similarly short T1 and
integration of 3 protons is assigned to the other methyl group
of the LPh anion. The peak with the largest downfield shift (+15
ppm) has a relative long T1 and integrates for two protons. This is
consistent with the meta-position of the phenyl group. In further
support of the assignment of the meta proton, the spectrum of
All EPR data were collected using a variety of solvents and
glassing-solvent mixtures to ensure homogeneous samples (Fig.
7). The 7 K X-band perpendicular (1a) and parallel (1b) mode
spectra of 1 show more resonances than are theoretically possible
for an isolated S = 2 paramagnetic species. Moreover, the signals
[Fe(L3,5-Me )2] (Fig. 5D) displays a new feature at -15 ppm with
2
an integration of six protons and loss of the feature at +15 ppm.
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Fig. 7 7 K perpendicular mode (a) and parallel mode (b) X-band
EPR spectra of 1.3 mM [Fe(LiPr)2] (1) and 1.7 mM [Fe(Ldipp)2] (2) in
CH2Cl2–toluene. All signals are normalized for concentration. Instru-
mental parameters: microwave frequency, (a) 9.64 GHz, (b) 9.38 GHz;
microwave power, (1a) 0.02 mW, (1b) 0.06 mW, (2a) 0.02 mW, (2b)
0.63 mW; temperature, 7 K; modulation amplitude, 0.9 mT. The small
g ~ 4 feature observed in parallel mode is from trace O2 (S = 1) condensing
on top of the frozen sample and can be ignored.
Fig. 6 Cyclic voltammogram of (A) 1.0 mM [Fe(LiPr)2] and (B) 1.0 mM
[Fe(Ldipp)2] in THF at room temperature under nitrogen with 0.4 M
(nBu4N)(ClO4) as the supporting electrolyte at a scan rate of 200 mV
s-1.
observed at g > 9.5 (1b) shift upon rotation of the sample within
the EPR cavity thus indicating that these signals can be attributed
to molecular aggregates within the sample. This behavior was
observed for all solvents and glassing solvent mixtures attempted.
Therefore, no meaningful interpretation of these spectra can be
made. However, the increased steric bulk of the Ldipp ligand to
produce the [Fe(Ldipp)2] complex (2) adequately prevents molecular
crowding as indicated by EPR spectroscopy. The X-band EPR
spectra of 2 taken under identical conditions as shown for 1
show a strong parallel mode (2b) resonance and a weak signal
in perpendicular mode (2a). Both of these can be attributed to
an isolated high-spin ferrous iron (S = 2) species. Spectra 2b
exhibits homogeneous saturation behavior with a power at half-
saturation (P1/2) of 490 mW at 10 K. When normalized for Curie
law dependence, the intensity of 2b decreases with increasing
temperature thus indicating that this signal originates from within
A descent in symmetry from the ideal tetrahedral configuration
(for instance, D2d spread as suggested by continuous symmetry
measurements) is expected for the high-spin d6 iron(II) complexes.
However, positive zero-field splitting values are observed for
square planar and flattened tetrahedral complexes.75,76 Only elon-
gated tetrahedral,75 a C3-symmetric tripodal thiolate rich iron(II)
methyl,81 and a trigonal monopyramidal iron(II) complex36 are
known to exhibit negative zero field splittings. While the sawhorse
geometry is unusual for iron(II), a precedent for this geometry
exists in a cluster and the complex has a negative zero-field splitting
value.41
Preliminary oxygenation studies have been undertaken and both
high-spin iron(II) complexes react readily with dioxygen even at
low temperature to give brick red or orange products (Fig. S3‡).
The electronic spectra of the product(s) are dominated by a broad
band, which tails into the visible. Holm and coworkers reported
a brick red iron(II) complex, [{PhC(O)CHC(NiPr)CH3}2FeII].44
The comparable extinction coefficient (e ~ 40 M-1 cm-1) in the
near-infrared bands would not be obtained if the complex was
partially oxidized. We attribute the difference in color to the
electronic tuning of the b-ketoiminate backbone by replacement of
a methyl group with a phenyl group. Preliminary studies indicate
multi-step kinetic processes in the formation of the red oxidation
products, which we are attempting to unravel. Mechanistic studies
are ongoing and will be the topic of a separate manuscript
along with the reactivity of these complexes with other small
molecules.
the ground (ms
=
2) doublet. By fitting the temperature-
normalized intensity of this signal to a Boltzman population
distribution for a three level system (S = 2), both the sign and
magnitude for the axial zero-field splitting term was determined,
D = -7.1 0.8 cm-1 (refer to Fig. S2 and Eqn S1‡). Simulations
for spectra 2a and 2b are provided in Fig. 8.
A molecular orbital diagram can be hypothesized for [Fe(Ldipp)2]
(Fig. S3‡). The electronic spectra provide relative positions for the
p, d, and p* orbitals. Noting the energies of the MLCT band, we
postulate the relative positions for the lowest energy d-orbital. The
low energy spin-forbidden d-d transitions of [Fe(Ldipp)2] observed
are consistent with removal of the degeneracy of the eg orbitals.
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orange oil. Yield: 23.7 g, (0.117 mol, 80.1%). 1H NMR (CDCl3, 400
MHz) d (ppm): 12.42 (s, 1H), 6.82 (s, 1H), 6.73 (s, 1H), 5.15 (s, 1H),
2.29 (s, 6H), 2.08 (s, 3H), 1.98 (s, 3H). HLMes was prepared from
acetylacetone (15.4 g, 0.154 mol), 2,4,6-trimethylaniline (20.1 g,
0.149 mol), and a catalytic amount of p-toluenesulfonic acid in
toluene (125 mL). The solution was heated to reflux for 4 d under
N2, while H2O was removed using a Dean–Stark apparatus. The
solvent was removed under reduced pressure, leaving a dark red-
orange oil. Crystallization from the slow evaporation of hexane led
to the isolation of clear colorless crystals. Yield: 4.52 g (0.64 mmol,
14.0%). 1H NMR (CDCl3, 400 MHz) d (ppm): 11.88 (s, 1H), 6.92
(s, 2H), 5.21 (s, 1H), 2.30 (s, 1H), 2.18 (s, 6H), 2.12 (s, 3H), 1.64 (s,
3H).
Complexes. [Fe(LiPr)2], (1). NaH (0.0488 g, 2.03 mmol) was
added to a solution of HLiPr (0.285 g, 2.02 mmol) in THF and
stirred until the evolution of hydrogen ceased. Fe(OTf)2(CH3CN)2
(0.437 g, 1.00 mmol) was added and the mixture was stirred
overnight. A bright yellow solution was isolated via filtration of
the inorganic salts. Bright yellow blocks of 1 suitable for single-
crystal X-ray diffraction were obtained by slow evaporation of
the THF from the filtrate. This procedure led to the isolation of
0.300 g (0.893 mmol, 89.3%) of diffraction quality crystals. Anal.
calcd for 1, C16H28FeN2O2: C 57.15, H 8.39, N 8.33. Found: C
57.13, H 8.21, N 8.14. UV-vis [lmax, nm (e, M-1 cm-1) in THF] 410
(2600), 916 (30), 1120 sh (20). ESI/MS (THF, 200 ◦C): m/z = 336
([M]+, i.e., [Fe(LiPr)2]+).
Fig. 8 10 K X-band perpendicular (2a) and parallel (2b) mode spectra
of [Fe(Ldipp)2]. Quantitative simulations (dashed lines) are overlaid on the
spectra for comparison. Instrumental parameters: microwave frequency,
(a) 9.64 GHz, (b) 9.38 GHz; microwave power, (a) 0.06 mW, (b) 0.20 mW;
temperature, 10 K; modulation amplitude, 0.9 mT. Simulation parameters:
[Zn(LiPr)2], (2). NaH (0.0495 g, 2.06 mmol) was added to a
solution of HLiPr (0.291 g, 2.06 mmol) in THF and stirred until
the evolution of hydrogen ceased. Zn(OTf)2 (0.372 g, 1.02 mmol)
was added and the mixture was stirred overnight. A pale yellow
solution was isolated via filtration of the inorganic salts. Bright
yellow blocks of 2 suitable for single-crystal X-ray diffraction,
were obtained by slow evaporation of the THF from the filtrate.
This procedure led to the isolation of 0.116 g (0.335 mmol,
32.9%) of diffraction quality off-white crystals. Anal. calcd for
2, C16H28ZnN2O2: C 55.57, H 8.16, N 8.10. Found: C 55.74, H
8.03, N 7.93. ESI/MS (THF, 200 ◦C): m/z = 367 ([M + Na]+, i.e.,
[Zn(LiPr)2Na]+).
[Fe(HLdipp)3(OTf)2]·C6H6, (3a). Fe(OTf)2(CH3CN)2 (0.869 g,
1.99 mmol) was added to a suspension of HLdipp (1.559 g,
6.01 mmol) in THF. The solution was stirred vigorously for
90 min. The solvent was removed from the resultant dark yellow
solution and the residue dissolved in benzene. Filtration yielded
a dark yellow solution which was evaporated to dryness. Bright
yellow blocks of 1 suitable for single-crystal X-ray diffraction were
obtained from slow cooling of a concentrated benzene solution
of this compound. This procedure led to the isolation of 1.64 g
(1.35 mmol, 68.1%) of diffraction quality crystals. Anal. calcd for
3a, C59H81F6FeN3O9S2: C 60.24, H 8.44, N 3.47, F 9.42. Found:
C 60.24, H 8.44, N 3.38, F 9.3. UV-vis [lmax, nm (e, M-1 cm-1) in
benzene] 420 (480). ESI/MS (THF, 200 ◦C): m/z = 1232 ([M+
Na]+, i.e., [Fe(OTf)2(HLdipp)3]Na+).
S = 2; gx,y,z = 2.02 2.07 2.19; D = -7.1 0.8 cm-1; E/D = 0.096; sD = sE/D
0.01; sB = 0.9 mT.
=
Experimental
General considerations
Materials. All manipulations were carried out using standard
Schlenk or glove box techniques under a dinitrogen atmosphere
unless otherwise noted. All reagents and solvents were obtained
from commercial vendors and used as received unless otherwise
noted. THF, toluene, benzene, and diethyl ether were distilled
under nitrogen from Na/benzophenone and subsequently dried
over activated alumina. Diisopropylamine was vacuum distilled
under nitrogen prior to use. Acetonitrile was distilled from calcium
hydride under nitrogen. Non-halogenated solvents were typically
tested with a standard purple solution of sodium benzophenone
ketyl in tetrahydrofuran to confirm effective oxygen and mois-
-
ture removal. Fe(OTf)2· 2CH3CN (OTf = OSO2CF3) was pre-
pared according to literature precedent utilizing Me3Si(OTf).82,83
[Fe(Mes)2]2 was prepared according to literature precedent.84,85
All chemical reactions were performed at high altitude conditions
(~7200 feet or ~2200 m).
Ligand Synthesis. HLiPr (R = -iPr), HLPh (R = -C6H5) and
HLdipp (R = -2,6-iPr2C6H3) were prepared according to literature
methods.48,53 HL3,5Me2 (R = 3,5-Me2-C6H3) was prepared from
acetylacetone (14.6 g, 0.146 mol), 3,5-dimethylaniline (17.6 g,
0.146 mol), and a catalytic amount of p-toluenesulfonic acid in
toluene (150 mL). The solution was heated to reflux for 48 h
under N2, while H2O was removed using a Dean–Stark apparatus.
The solvent was removed under reduced pressure, leaving a red-
[Fe(Ldipp)2], (3b). NaH (0.0974 g, 4.06 mmol) was added to a
solution of HLdipp (1.04 g, 4.01 mmol) in THF and stirred for
over 1 h. Fe(OTf)2(CH3CN)2 (0.872 g, 2.01 mmol) was added and
the mixture was stirred overnight. The solvent was removed from
the resultant brown solution and the residue dissolved in toluene.
Filtration yields a dark green solution which was evaporated
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to dryness. Bright green blocks of 3 suitable for single-crystal
X-ray diffraction were obtained by layering acetonitrile on a
concentrated THF solution of this compound. This procedure
led to the isolation of 0.683 g (1.19 mmol, 59.2%) of diffraction
quality crystals. Anal. calcd for 3b, C34H48FeN2O2: C 71.32, H
8.45, N 4.89. Found: C 71.82, H 8.45, N 4.89. UV-vis [lmax, nm (e,
M-1 cm-1) in THF] 420 (1400), 680 (50), 1470 (7). ESI/MS (THF,
200 ◦C): m/z = 572 ([M]+, i.e., [Fe(Ldipp)2]+).
layered with CH3CN. No crystals have been obtained, however a
green/yellow precipitate formed in the bottom of the vial.
[Fe(LMes)2]. NaH (0.0557 g, 2.32 mmol) was added to a
solution of HLMes (0.433 g, 1.99 mmol) in THF and stirred,
resulting in a rapid formation of bubbles. Fe(OTf)2·2CH3CN
(0.429 g, 1.11 mmol) was added and the solution was allowed
to stir overnight. THF was removed under reduced pressure, the
remaining oil was dissolved in toluene, and filtered to remove
inorganic salts. Toluene was then removed under reduced pressure,
and the remaining oil/solid was dissolved in a minimal amount
of hot THF and layered with CH3CN, leading to the isolation of
pure, light green/yellow crystals.
Two additional synthetic methods were used to produce
[Fe(Ldipp)2]. In the first, HLdipp (0.519 g, 1.99 mmol) was added
to an excess of [Fe(Mes)2]2 (0.589 g, 1.00 mmol) in benzene.
Solvent and byproducts were removed under vacuum to yield
a green solid. NMR and UV-vis of the product are identical
n
to those listed above. The second method used 2.5 M BuLi in
Physical Methods. Elemental analyses were carried out at
Columbia Analytical Services, Inc., Tucson, AZ. Electronic spec-
tra were recorded in quartz cuvettes on an Agilent 8453 diode-
array spectrophotometer (250–1100 nm), which was equipped
with a liquid nitrogen cryostat (Unisoku). The near-IR data were
collected on a Perkin–Elmer Lamba 950 STD detector module (up
to 2500 nm). 1H and 13C NMR were collected on a Bruker Avance
DRX-400 NMR spectrometer at room temperature and referenced
to the residual protic solvent peak (at 7.16 ppm for benzene).
Special care was taken with paramagnetic samples to ensure that
the delay between pulses was greater than five times the longest
proton longitudinal relaxation time (T1) for proper integration
of the peaks. IR spectra were recorded as KBr pellets at room
temperature on a Varian 800 FTIR (Scimitar Series) set to 1 cm-1
resolution. Electrochemical measurements were carried out in a
drybox under N2 in THF solution with 0.4 M (Bu4N)(ClO4) as the
supporting electrolyte using a model ED401 computer controlled
potentiostat (eDAQ). A three-electrode configuration with a
glassy carbon working electrode, either a non-aqueous Ag/Ag+
reference electrode or a Ag wire quasi-reference electrode with a
Fc+/0 internal reference, and a platinum wire auxiliary electrode
was used. The potential values were referenced to an internal
ferrocenium/ferrocene couple which is reported to be +0.53 V
vs. SCE in [NBu4][ClO4] in THF.86 The peak separations are
reported with a scan rate of 200 mV s-1 (Fc+/0 peak separation was
90 mV under these conditions). The low resolution electrospray
ionization mass spectral data for the complexes were obtained
using an LCQ mass spectrometer (Finnigan MAT) on THF
solutions that were directly infused into the spectrometer via a
syringe pump. The heated capillary was set at 200 ◦C. X-band
(9 GHz) EPR spectra were recorded on a Bruker EMX Plus
spectrometer equipped with a bimodal resonator (Bruker model
4116DM). Low-temperature measurements were made using an
Oxford ESR900 cryostat and an Oxford ITC 503 temperature
controller. A modulation frequency of 100 kHz was used for all
EPR spectra. All experimental data used for spin-quantitation
were collected under non-saturating conditions. EPR spectra were
simulated and quantified using SpinCount (ver. 3.1.2), created
by Professor M.P. Hendrich at Carnegie Mellon University. The
simulations were generated with consideration of all intensity
factors, both theoretical and experimental, to allow concentration
determination of species. The only unknown factor relating the
spin concentration to signal intensity was an instrumental factor
that depended on the microwave detection system. However, this
was determined by the spin standard, Cu(EDTA), prepared from
a copper atomic absorption standard solution purchased from
hexane (0.99 mL, 2.48 mmol) added via syringe to a solution
of HNiPr2 (0.295 mL, 2.10 mmol) in 20 mL of THF chilled
with dry ice. A solution of HLdipp (0.488 g, 1.88 mmol) in
10 mL of THF was transferred via cannula to the LiNiPr2
solution and stirred for 2 h at dry ice temperature. Next, a
solution of Fe(OTf)2·2CH3CN (0.4090 g, 0.94 mmol) in 20 mL
of THF was cannula transferred to the deprotonated ligand
with immediate formation of a green solution. After stirring for
1 h the solution was evaporated to dryness. The residue was
dissolved in 10 mL of toluene and solids removed by filtration.
Spectroscopic data was identical to those obtained by other
synthetic methods.
[Zn(Ldipp)2], (4). NaH (0.0965 g, 4.02 mmol) was added to a
solution of HLdipp (1.04 g, 4.00 mmol) in THF and stirred for over
1 h. Zn(OTf)2 (0.730 g, 2.01 mmol) was added and the mixture was
stirred overnight. A pale yellow solution was isolated via filtration
of the inorganic salts. Off-white blocks of 4 suitable for single-
crystal X-ray diffraction were obtained by slow evaporation of the
THF from the filtrate. This procedure led to the isolation of 0.405
g (0.696 mmol, 34.8%) of diffraction quality crystals. Anal. calcd
for 4, C34H48ZnN2O2: C 70.15, H 8.31, N 4.81. Found: C 70.51,
◦
H 8.51, N 5.01. ESI/MS (THF, 200 C): m/z = 580 ([M]+, i.e.,
[Zn(Ldipp)2]+).
[Fe(LPh)2]. NaH (0.048 g, 2.00 mmol) was added to a solution of
HLPh (0.350 g, 2.00 mmol) in THF and stirred until the evolution
of hydrogen ceased. Fe(OTf)2·2CH3CN (0.436 g, 1.00 mmol) was
added and the mixture was stirred overnight. The solvent was
removed from the resultant dark brown solution and the residue
dissolved in toluene. Filtration gave a dark orange solution,
which was evaporated to dryness. Modification of literature
procedures44,48 was used to produce this complex. NaH (0.096 g,
4.0 mmol) was added to a solution of HLPh (0.703 g, 4.01 mmol)
in THF and stirred until the evolution of hydrogen ceased. FeCl2
(0.252 g, 2.00 mmol) was added and the resulting suspension
was allowed to stir overnight. A brown solution was obtained
after filtration. Electronic and 1H NMR features of the products
obtained from the two syntheses were identical.
[Fe(L3,5-Me )2]. NaH (0.0543 g, 2.26 mmol) was added to a
2
solution of HL3,5-Me (0.411 g, 2.02 mmol) in THF and stirred.
2
Fe(OTf)2·2CH3CN (0.432 g, 0.991 mmol) was added, resulting
in the slow formation of bubbles, and the solution was allowed
to stir overnight. THF was removed under reduced pressure, the
remaining oil was dissolved in toluene, and filtered to remove
inorganic salts. Toluene was then removed under reduced pressure,
and the remaining oil/solid was dissolved in minimal THF and
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Sigma–Aldrich. Preliminary kinetic measurements were carried
out using distilled THF which was shaken over activated alumina
and stored over 3A molecular sieves under nitrogen. Crystalline
materials were dissolved in THF at room temperature. Oxygen was
bubbled into the solutions and oxygen saturation was assumed in
these experiments but was not directly measured.
us to better interpret the reactivity of these complexes with small
molecules.
Acknowledgements
M.P.M. and B.S.P. thank the Departments of Chemistry (and
Biochemistry) at the University of Wyoming and The University
of Texas at Arlington, respectively, for start-up funds supporting
this work. M.P.M also thanks Prof. Parkinson for use of the
Lamba 950 spectrophotometer and Prof. T. Brunold for helpful
discussions regarding interpretation of the electronic spectra.
We thank Prof. Santiago Alvarez for freely sharing his SHAPE
continuous symmetry measurement program. T.K.M. and D.G.M.
thank the Univ. of Wyoming School of Energy Research R.E.U.
program for funding. C.M.W. and A.K.B. were supported by NSF
REU funds (CHE 0851931). Financial support by the NSF (CHE
0619920) for the purchase of the Bruker Apex II Diffractometer is
gratefully acknowledged. B.S.P. thanks Professor M.P. Hendrich
(Carnegie Mellon University) for providing the EPR analysis
and simulation software (SpinCount) and the UTA Center for
Nanostructured Materials (http://www.uta.edu/cos/cnm/) for
the use of EPR instrumentation.
X-Ray
diffraction
analysis. [Fe(LiPr)2],
[Zn(LiPr)2],
[Fe(HLdipp)3(OTf)2]·C6H6, [Fe(Ldipp)2], and [Zn(Ldipp)2] were
characterized using single-crystal X-ray diffraction. A translucent
intense-yellow rectangular prismatic crystal of [Fe(LiPr)2],
pale white crystals of [Zn(LiPr)2], yellow rectangular prism of
[Fe(HLdipp)3(OTf)2]·C6H6, emerald green rectangular prism of
[Fe(Ldipp)2], and a colorless rectangular plate of [Zn(Ldipp)2]
were glued to either a MiTeGen micromount or a Hampton
Research CryoLoop using Paratone N oil and mounted on a
Bruker Smart Apex II CCD area detector for data collection at
˚
150 K using graphite-monochromated Mo-Ka (l = 0.71073 A)
radiation. A summary of the crystallographic details for 1,
2, 3a, 3b, and 4 is given in Table 1. All non-hydrogen atoms
were refined anisotropically, whereas the H-atoms were placed
in ideal positions and refined as riding atoms with relative
isotropic displacement parameters. Two of the crystals (1 and
2) are isomorphous. The asymmetric unit consists of two
crystallographically dissimilar half-molecules of M[L1]2. The
iron centers are located on two-fold symmetry axes. In 3a, the
asymmetric unit consists of a [Fe(HLdipp)3(OTf)2] molecule and a
solvated benzene molecule. Both molecules are located on general
positions and are well separated from one another. The protons
from the HLdipp lignad (i.e., N–H) were located in the Fourier
maps and refined isotropically. In 3b, the asymmetric unit consists
of half of the complex molecule with the iron center located on
a two-fold symmetry axis. In 4, the asymmetric unit consists of
a well ordered [Zn(Ldipp)2] molecule. The final full matrix least
squares refinement converged to R1 = 0.0283 and wR2 = 0.0709
for 1, R1 = 0.0256 and wR2 = 0.0656 for 2, R1 = 0.0392 and wR2 =
0.01037 for 3a, and R1 = 0.0376 and wR2 = 0.0965 for 3b, and R1 =
0.0262 and wR2 = 0.0727 for 4.
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