Effects of Ligand Halogenation on the Electron Localization, Geometry and Spin State of Low-Coordinate (β-Diketiminato)iron Complexes

Article abstract of DOI:10.1002/ejic.201600112

This contribution explores the influences of incorporating electron-withdrawing CF₃and halide groups into (β-diketiminato)iron complexes of tetrazene and isocyanide. The synthesis of a new halogenated β-diketimine (L^CF3,ClH) was accomplished by two different methods, including a novel microwave-assisted synthesis that improves the yield of the difficult condensation. Treatment of an iron(II) complex of this ligand with reductant and azide gives two diiron complexes with novel tetrazenes as bridging ligands. Structural and M?ssbauer data show that the bridging tetrazene is a radical anion. The halogenation of the supporting ligand also influences iron(I) complexes of the type [LFe(CNtBu)₂], which are low-spin and square-planar with alkyl substituents but high-spin and pseudotetrahedral with halogen substituents. DFT calculations suggest that the changes from halogenation come from a combination of steric and electronic effects, and that the electronic influence of ligand halogenation is minor.

Full text of DOI:10.1002/ejic.201600112

DOI: 10.1002/ejic.201600112
Full Paper
Halogenated Ligands
Effects of Ligand Halogenation on the Electron Localization,
Geometry and Spin State of Low-Coordinate
(
ꢀ-Diketiminato)iron Complexes
[a]
[a]
[a,b]
Sarina M. Bellows, William W. Brennessel, and Patrick L. Holland*
Abstract: This contribution explores the influences of incorpo- data show that the bridging tetrazene is a radical anion. The
rating electron-withdrawing CF3 and halide groups into (ꢀ- halogenation of the supporting ligand also influences iron(I)
diketiminato)iron complexes of tetrazene and isocyanide. The complexes of the type [LFe(CNtBu) ], which are low-spin and
2
CF3,Cl
synthesis of a new halogenated ꢀ-diketimine (L
H) was ac- square-planar with alkyl substituents but high-spin and pseu-
complished by two different methods, including a novel micro- dotetrahedral with halogen substituents. DFT calculations sug-
wave-assisted synthesis that improves the yield of the difficult gest that the changes from halogenation come from a combi-
condensation. Treatment of an iron(II) complex of this ligand nation of steric and electronic effects, and that the electronic
with reductant and azide gives two diiron complexes with novel influence of ligand halogenation is minor.
tetrazenes as bridging ligands. Structural and Mössbauer
Introduction
structure. For example, in tetrazene complexes (Figure 1), the
neutral tetrazene ligand can have two N=N double bonds
In coordination chemistry, the supporting ligand has several
roles: to limit the binding sites on the metal atom, to increase
the reactivity of the metal atom for a given reaction, and to
remain unreactive during synthesis and catalysis. One problem
with the popular N-aryl-ꢀ-diketiminato ligands with ortho sub-
stituents on the aryl rings is that they have weak C–H bonds
that are susceptible to abstraction by electrophilic oxo or imido
[7]
[
1.28(4) Å] and an N–N single bond (1.45 Å), a radical anionic
tetrazene with N–N bond lengths between single and double
bonds (ca. 1.35 Å), or a dianionic tetrazene ligand with two N–
N single bonds and one N=N double bond. Each reduction of
the tetrazene ligand results from an electron being donated
from the metal atom; therefore, the metal atom and ligand
oxidation levels are linked.
[
1]
ligands. One way to eliminate the reactivity of such C–H
bonds is to substitute them with oxidation-resistant C–F or C–
[
2]
Cl bonds. However, halogenation can influence the ꢀ-diket-
iminato ligands, affecting the electron density on the metal
[
3]
atom and its reactivity. Thus, it is important to evaluate the
influence of ligand halogenation on the properties of (ꢀ-diket-
[
4]
iminato)metal complexes.
Figure 1. Resonance structures of a tetrazene complex, where the oxidation
states of the tetrazene ligand and the metal atom change.
One potential influence of halogenation of the supporting
ligand is in the balance of electron density between the metal
atom and a “redox-active” co-ligand that can be reduced or
In a previous study, we evaluated the (ꢀ-diketiminato)-
[
5]
Me,iPr
Me,iPr
oxidized by the metal atom. Typically, the oxidation states of (tetrazene)iron complex [L
Fe(N Ad )], where L
= 2,4-
The Mössbauer
Fe(N Ad )] indicated a high-spin electronic
4
2
[
8]
the ligand and the metal atom are determined experimentally. bis[(2,6-diisopropylphenyl)imido]-3-pentyl.
Me,iPr
For iron complexes, Mössbauer spectroscopy is particularly use- spectrum of [L
4
2
ful for characterizing the metal atom's oxidation level,[6] while configuration at the iron(II) atom, and the N–N bond lengths
the oxidation level of the redox-active ligand can be deter- within the N Ad ligand were between single and double
4
2
mined using the bond lengths from a high-resolution crystal bonds [N1–N2 1.317(2), N2–N3 1.334(2), N3–N4 1.309(2) Å], cor-
responding to resonance structure B in Figure 1. Together, these
data led to a self-consistent picture of a high-spin iron(II) ion
[
a] Department of Chemistry, University of Rochester,
20 Trustee Rd, Rochester, NY 14627, USA
b] Department of Chemistry, Yale University,
25 Prospect St, New Haven, CT 06520, USA
1
(SFe = 2) with strong antiferromagnetic coupling to the radical
anion N Ad (S = 1/2) to give an overall S = 3/2 spin
[
4
2
tetrazene
2
Me,iPr
state. Cyclic voltammetry of [L
Fe(N Ad )] revealed a reversi-
4 2
E-mail: patrick.holland@yale.edu
http://holland.chem.yale.edu
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/ejic.201600112.
+
/0
ble, one-electron reduction at 1.84 V (vs. Cp Fe ) to give an
2
Me,iPr
[8]
isolable product, [K(cryptand-222)][L
Fe(N Ad )].
The
4
2
Mössbauer parameters remained consistent with a high-spin
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iron(II) ion, but the reduced compound had N–N bond lengths by the addition of 1. This mixture was heated by microwave
more consistent with a dianionic tetrazene (resonance structure radiation to 210 °C for 4 h to produce 2 in 52 % yield, with a
C of Figure 1); thus, the reduction had occurred at the ligand.
combined yield of 21 %. This two-step procedure takes a total
Here, we present a new ꢀ-diketiminato ligand with F and of 5 d and uses one-fifth of the amount of decane.
1
Cl substituents, and we evaluate the electronic effects on iron
The H NMR spectrum of 2 displays four resonances, with
complexes in two ways. First, we present several new (tetra- one downfield at δ = 11.5 ppm for the NH proton of the en-
zene)iron complexes and specify the redox level of the metal aminoimine tautomer of 2. This NH resonance is more upfield
atom and ligand. Second, we present the influence of the li- than that of the ꢀ-diketimines with ꢀ-Me groups by ca. 1 ppm,
gand fluorination on iron(I) isocyanide complexes, where the presumably due to deshielding by the electron-withdrawing tri-
[
9]
19
1
electron density at the iron site is probed using both the C–N fluoromethyl groups. The F{ H} NMR spectrum of 2 displays
stretching frequency and DFT calculations. Both facets of this a single resonance at δ = –68.9 ppm. The vibrant yellow color
work help to understand the relay effect of the supporting li- of 2 corresponds to a strong absorption at 330 nm with a molar
–
1
–1
gand on the electronics of the other ligands on the same iron absorptivity of 22000
M
cm . The color is reminiscent of that
Me,Ph3
center.
of L
H, which is yellow and has an absorption band at
–1
–1 [10]
3
77 nm (25000
M
cm ),
while other N-aryl-ꢀ-diketimines
are white.
Results and Discussion
Ligand Synthesis and Characterization
Synthesis and Characterization of a (ꢀ-Diketiminato)iron(II)
Complex of the Fluorinated Ligand
A new fluorinated ꢀ-diketimine, with trifluoromethyl groups on
the backbone and 2,6-dichlorophenyl groups on the nitrogen Deprotonation of
atoms, was synthesized by two different procedures (Scheme 1). [KN(SiMe ) ] produced a yellow-orange solution, which was
2
by potassium hexamethyldisilazide
3
2
The first method is a one-pot synthesis that begins by adding added to 1.2 equiv. of [FeCl (THF)1.5] in THF to produce a purple
2
TiCl to 2,6-dichloroaniline in decane, followed by addition of mixture. After drying and washing with pentane, a red powder
4
hexafluoroacetylacetone (HfacH) and 1,8-bis(dimethylamino)- was isolated in 80 % yield. Crystallization from concentrated
naphthalene (Proton Sponge®). After heating to 70 °C for 1 d, Et O solution at –40 °C gave red single crystals of
2
CF3,Cl
the temperature was raised to 140 °C for 1 d to afford ketoimine [L
FeCl(Et O)] (3·Et O). The molecular structure of 3·Et O is
2
2
2
CF3,Cl
1
in 57 % yield and L
H (2) in 14 % yield. This procedure shown in Figure 2. Diethyl ether is typically a weak ligand, and
could be repeated using 1 in place of HfacH to afford 2 in 40 % there are relatively few crystallographically characterized exam-
[
11]
yield, giving an overall yield of 38 %. Each one-pot procedure ples of it coordinating to iron atoms.
takes about 7 d to complete, including drying and vacuum dis- ether from 3·Et O was facile, as drying the crystals under vac-
Removal of diethyl
2
tillation of the large amount of decane needed.
uum gave a diethyl ether free chloride dimer [LFe(μ-Cl)] (4)
2
as shown by elemental analysis of the powder and by X-ray
crystallography of a crystal grown from a toluene solution.
Scheme 1. Synthetic routes to a new halogenated ꢀ-diketimine.
The second method uses two steps. The first step is acid-
catalyzed condensation of 2,6-dichloroaniline and HfacH with
p-toluenesulfonic acid in toluene heated by microwave radia-
tion to 210 °C for 30 min to produce 1 in 40 % yield. The second
2
Figure 2. Molecular structure of 3·Et O, showing 50 % thermal ellipsoids.
Hydrogen atoms omitted for clarity. Selected bond lengths [Å] and angles [°]:
Fe(1)–N(11) 2.012(1), Fe(1)–N(21) 2.017(1), Fe(1)–Cl(1) 2.2425(5), Fe(1)–O(14)
2.092(1); N(11)–Fe(1)–N(21) 92.97(5), O(14)–Fe(1)–Cl(1) 101.60(4).
step is the addition of TiCl to excess aniline in decane followed
4
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Zero-field Mössbauer spectroscopy of solid 4 at 80 K revealed
a doublet at δ = 0.92(2) mm/s with a quadrupole splitting of
|
ΔE | = 2.30(2) mm/s. The Mössbauer parameters of the related
Q
Me Me,Me
iron(II) complex [
L
Fe(μ-Cl)Li(Et O) ] (δ = 0.90 mm/s,
2 2
[
12]
and [Me
Me,Me
|
ΔE | = 2.40 mm/s)
L
Fe(μ-Cl)] (δ = 0.91 mm/s,
2
Q
|
ΔE | = 2.38 mm/s), which have methyl groups on the back-
Q
bone and the ortho-aryl positions, are very similar to the param-
Me Me,Me
–
eters of 4 {
L
is MeC[C(Me)N(2,6-Me C H )] }. The similar-
2 6 3 2
ity of the isomer shifts suggests that the halogenation does not
have a marked effect on the electron density at the iron nucleus
in this tetrahedral geometry.
A Mössbauer spectrum of 4 dissolved in THF had a similar
isomer shift of δ = 0.88(2) mm/s with a significantly different
quadrupole splitting of |ΔE | = 2.06(2) mm/s. This change in
Q
Mössbauer parameters between the solid state and the THF
solution of 4 suggests that dimeric 4 breaks up to give a mono-
CF3,Cl
1
meric complex [L
FeCl(THF)]. The H NMR spectrum of 4 in
C D exhibits three sharp, paramagnetically shifted resonances
6
6
Figure 3. Molecular structure of 4, showing 50 % thermal ellipsoids. Hydrogen
atoms omitted for clarity. Selected bond lengths [Å] and angles [°]: Fe(1)–
N(11) 1.9947(15), Fe(1)–N(21) 2.0034(14), Fe(1)–Cl(1) 2.3201(5); N(11)–Fe(1)–
N(21) 93.01(6).
having integrations consistent with one symmetric ligand envi-
ronment with C2v local symmetry. Its 19F{ H} NMR spectrum has
a single resonance at δ = –165.0 ppm that is also consistent
with one symmetric ligand environment. The red hue of 4 is
different from the yellow color of analogues of 4 that have ꢀ-
1
Synthesis and Characterization of (Tetrazene)iron- and
-diiron Complexes
CH instead of ꢀ-CF .
3
3
The crystal structure of 4 gives an opportunity to compare
bond lengths and angles to other four-coordinate (ꢀ-diketimin-
ato)iron(II) complexes (Table 1), in order to gain insight into the
steric and electronic effects of ligand halogenation. Increased
steric bulk of the ꢀ-substituent typically increases the C –N –
One of our initial goals was to create an (imido)iron(III) complex,
since we have shown two examples of these complexes, and
they are capable of abstracting hydrogen atoms from hydro-
[
16]
carbons.
Because these are generated by addition of azides
ꢀ
L
to iron(I) species, we desired a source of iron(I) analogous to
Caryl angles because of nonbonded interactions between the ꢀ-
Me,iPr
[
13,14]
L
Fe(N Ad ). However, adding KC to 4 under various condi-
4 2 8
substituent and the aryl rings.
With ꢀ-tBu groups, the aryl
tions gave intractable mixtures of products, probably from at-
tack of reducing agent (or reduced iron) on carbon–chlorine
bonds. Fortunately, it was possible to generate an (unidentified)
rings create enough steric hindrance around the iron atom to
inhibit dimerization, and no coordination of Et O was ob-
2
[
15]
served. The size of the ꢀ-CF substituent in 4 is between that
3
tBu,iPr
iron(I) species in situ using KC in THF at –108 °C. As will be
8
of the ꢀ-tBu groups on monomers like [L
Fe(Cl)(X)] (X =
shown below, this gave tetrazene products rather than an imido
complex. This may indicate the formation of a transient imido-
metal intermediate that undergoes an addition of a second
equivalent of azide, which is the most accepted mechanism for
3
[
-methylpyridine) and the ꢀ-Me groups on dimers like
Me Me,Me
[12,15]
L
FeCl]2.
The substituents decrease in size in the
order tBu > CF > CH and the C –N –C angles are 124°, 121°,
3
3
ꢀ
L
aryl
and 119°, respectively, indicating that the CF groups push the
3
[
17]
tetrazene formation.
aryl rings toward the metal atom, but not as much as tert-butyl
groups. Determining the electronic effects of ligands can be
In initial experiments with a 1:1 ratio of iron/azide, KC8
(2.3 equiv.) was added to the dimer 4 in THF at –108 °C, fol-
difficult to disentangle from the steric effects. Thus, we compare
Me Me,Me
lowed by 2 equiv. of adamantyl azide (N Ad) as shown in
3
the Fe–N and C =N bond lengths of 4 to that of [
L
FeCl]2,
ꢀ
Scheme 2. X-ray quality crystals were obtained from pentane in
47 % yield. The crystal structure revealed the diiron complex 5
with a bridging 1,4-bis(adamantyl)tetrazene ligand (Figure 4a).
The tetrazene ligand is coordinated to the Fe1 atom through
which has similarly sized ortho substituents (CF vs. CH ). The
3
3
Me Me,Me
average Fe–N bond lengths for 4 and [
L
FeCl]2 are
1.999(2) Å and 1.973(2) Å, respectively, and the average Cꢀ=
N bond lengths are 1.327(2) Å and 1.336(4) Å. Thus, the CF3
substituents appear to slightly lengthen the Fe–N bonds,
primarily from a steric effect (Figure 3).
the N1 and N4 atoms to form a five-membered FeN ring. The
4
other iron atom (Fe2) is coordinated by the N2 and N3 atoms
2
in an η -binding mode, and lies very close to the iron–tetrazene
plane. This “edge-on” coordination to Fe2 is reminiscent of a
known diketiminato-supported (pyrazolato)iron(II) complex.
Each iron atom has a tetrahedral geometry, when considering
the side-on coordination of N2 and N3 as two single bonds to
Fe2. Since the two iron atoms and the bridging tetrazene ligand
Table 1. Bond lengths [Å] and angles [°] of tetrahedral (ꢀ-diketiminato)iron(II)
complexes. Averages were taken when applicable.
[18]
Complex
C
ꢀ
–N–CAr [°]
C
ꢀ
=N [Å]
Fe–N [Å]
[Me
Me,MeFeCl]
[a]
L
2
119
121
124
1.336(4)
1.327(2)
1.327(3)
1.973(2)
1.999(2)
2.012(2)
CF3,Cl
[
L
L
FeCl]
2
(4)
are almost coplanar, the idealized geometry of the FeN Fe core
4
tBu,iPr
[b]
[
Fe(Cl)(L)]
is C . However, Fe1 is above the plane of its ꢀ-diketiminato
2
v
[
a] Ref.[12] [b] Ref.[15]; L = 3-methylpyridine.
ligand by 0.82 Å, breaking the horizontal mirror plane that
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would otherwise be perpendicular to the FeN4 ring. The 1
H
intensity, consistent with two unique iron environments that
NMR spectrum of 5 was complicated, and solution samples each give a doublet. Unfortunately, the presence of two equally
were always contaminated with variable amounts of 6 (see be- populated iron environments inevitably leads to ambiguity in
low), which prevented us from drawing reliable conclusions fitting. Figure 5 presents the data fit with two possibilities. Be-
about its solution structure.
low, we will present evidence that fit A is more likely to be
correct.
Another (tetrazene)diiron complex crystallized as 5·THF,
where a THF molecule is coordinated to Fe2 of 5. In the molec-
ular structure of 5·THF (Figure 4b) both iron atoms are out of
their ꢀ-diketiminato planes. In 5·THF, five-coordinate Fe2 is still
very close to the Fe1–tetrazene plane. Coordination of THF to
Fe2 causes a lengthening of the bonds from Fe2 to the N atoms
of the tetrazene from 2.00–2.02 Å to 2.04–2.08 Å. The 19F{ H}
NMR spectrum of 5·THF displays two resonances at δ = –116.3
and –124.4 ppm. The chemical shifts of these resonances are
very similar to those of 5, which suggests that 5 and 5·THF are
in equilibrium. To determine the binding strength of the THF
1
1
9
1
ligand to 5, the F{ H} NMR spectrum of 5 in C D was moni-
6
6
tored with varying amounts of added THF, and the peaks
shifted as expected for a THF-binding equilibrium that is fast
on the NMR time scale. A plot of the change in chemical shift
Scheme 2. Preparation of (tetrazene)iron complexes.
1
–1
vs. [THF] (see Supporting Information) yielded Keq = 15(2)
M
The 19F{ H} NMR spectrum of 5 exhibits two resonances at for binding at 298 K.
δ = –117.4 and –122.3 ppm, corresponding to the supporting
In order to force the formation of a mononuclear complex,
ligands on the two inequivalent iron atoms. The solution mag- the KC reduction of 4 at –65 °C was followed by addition of
8
netic moment of 5 is 5.2(1) BM at room temperature. The Möss- 4 equiv. of N Ad, and then the mixture was stirred at room
3
1
9
1
bauer spectrum of 5 at 80 K displays four peaks of nearly equal temperature for 18 h. The F{ H} NMR spectrum showed a mix-
Figure 4. (a) Molecular structure of 5, showing 50 % thermal ellipsoids. Selected bond lengths [Å] and angles [°]: Fe(1)–N(11) 2.032(4), Fe(1)–N(21) 2.033(4),
Fe(2)–N(14) 1.987(4), Fe(2)–N(24) 1.988(4), Fe(1)–N(1) 1.975(4), Fe(1)–N(4) 2.004(4), Fe(2)–N(2) 2.019(4), Fe(2)–N(3) 1.983(4), N(1)–N(2) 1.331(5), N(2)–N(3) 1.323(6),
N(3)–N(4) 1.316(5); N(11)–Fe(1)–N(21) 90.20(17), N(24)–Fe(2)–N(14) 93.75(17). (b) Molecular structure of 5·THF, showing 30 % thermal ellipsoids. Selected bond
lengths [Å] and angles [°]: Fe(1)–N(14) 2.046(2), Fe(1)–N(24) 2.039(2), Fe(2)–N(11) 2.064(2), Fe(2)–N(21) 2.077(2); Fe(2)–O(19) 2.208(2), Fe(1)–N(1) 2.008(2), Fe(1)–
N(4) 1.962(2), Fe(2)–N(2) 2.081(2), Fe(2)–N(3) 2.043(2), N(1)–N(2) 1.323(3), N(2)–N(3) 1.311(3), N(3)–N(4) 1.341(3); N(24)–Fe(1)–N(14) 89.26(9), N(11)–Fe(2)–N(21)
8
7.91(9). (c) Molecular structure of 6, showing 50 % thermal ellipsoids. Selected bond lengths [Å] and angles [°]: Fe(1)–N(11) 2.028(4), Fe(1)–N(21) 2.012(4),
Fe(1)–N(1) 2.002(4), Fe(1)–N(4) 1.972(4), N(1)–N(2) 1.316(6), N(2)–N(3) 1.332(6), N(3)–N(4) 1.309(6); N(11)–Fe(1)–N(21) 90.42(17). In each, hydrogen atoms are
omitted for clarity.
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The Mössbauer spectrum of 6 (Figure 6) exhibits two dou-
blets, one with δ = 0.75(4) mm/s and |ΔE | = 1.55(4) mm/s
Q
(82 %), and an apparent impurity with δ = 0.42(8) mm/s and
|
ΔE | = 0.80(10) mm/s (18 %). [We have been unable to purify
Q
6
to a level where this apparent iron(III) impurity is not ob-
served by Mössbauer spectroscopy.] The major species has
Me,iPr
parameters similar to those of [L
Fe(N Ad )] [δ =
4 2
[
8]
0
.69(2) mm/s, |ΔE | = 1.32(4) mm/s], and is thus consistent
Q
with the tetrazene complex 6. The spectrum of 6 is useful in
distinguishing the two possible assignments of the Mössbauer
data of 5 (Figure 5b). In fit A, the doublet with larger intensity
is the same within error as the Mössbauer parameters of 6,
which agrees with the very similar environment of one of the
iron atoms in 5. The unequal intensities of the two doublets in
fit A for 5 is consistent with the presence of 6 as a 30 % impu-
rity, which is not observed as a separate signal because of its
1
very similar parameters. Impurities were also evident in the H
NMR spectrum of 5 that are consistent with the presence of an
impurity.
Figure 5. Two fits of the zero-field Mössbauer spectrum of solid 5 at 80 K.
The black circles are the data points. (a) Fit A: the blue and green solid lines
show fitted components with δ = 0.73(2) mm/s, |ΔE
δ = 1.21(2) mm/s, |ΔE | = 2.08(2) mm/s in a 1.3:1 ratio, and the solid red line
is the sum of the two. (b) Fit B: the solid lines show fitted components with
δ = 0.80(2) mm/s, |ΔE | = 1.30(2) mm/s (blue) and δ = 1.11(2) mm/s, |ΔE | =
.27(2) mm/s (47 %, green) in a 1.1:1 ratio, and the solid red line is the sum
of the two components.
Q
| = 1.45(2) mm/s and
Q
Q
Q
2
ture of products, and red-orange crystals could be isolated from
pentane in 13 % yield. These were crystallographically charac-
CF3,Cl
terized as [L
Fe(N Ad )] (6), which is shown in Figure 4c. The
4 2
Me,iPr
[8]
structure of 6 is similar to that of [L
Fe(N Ad )] with N–N
4 2
distances within 0.002 Å (see Table 2). The Fe atom of 6 is out
of the ꢀ-diketiminato plane by 0.815 Å, whereas the Fe atom in
Me,iPr
1
[
L
Fe(N Ad )] is out of the plane by 0.855 Å. The H NMR
4 2
spectrum of 6 has only five detectable signals (the other two
expected signals may be broadened). This can be compared to
Figure 6. Zero-field Mössbauer spectrum of solid 6 at 80 K. The black circles
are the data points. The blue and green solid line show fitted components
for a major (82 %) species with δ = 0.75(4) mm/s and |ΔE | = 1.55(6) mm/s
Q
Me,iPr
[8]
the 19 signals for [L
Fe(N Ad )]. The relative simplicity of
4 2
1
the H NMR spectrum of 6 is attributed to flipping of the iron
atom to both sides of the tetrazene plane (like in 5·THF above),
which is not possible in the more crowded literature complex.
and a minor (18 %) species with δ = 0.42(8) mm/s and |ΔE | = 0.80(10) mm/
Q
s. The solid red line is the sum of the two components.
1
9
1
The F{ H} NMR spectrum of crystallized 6 has a single reso- Determination of Ligand and Metal Oxidation States in
nance at δ = –158.2 ppm.
Tetrazene Complexes
By X-ray crystallography, Mössbauer spectroscopy, EPR spectro-
Me,iPr
Table 2. The N–N bond lengths [Å] and Mössbauer parameters of 5, 6, and
scopy, and magnetometry studies, [L
Fe(N Ad )] was previ-
4 2
[
8]
[
LM
e,iPrFe(N
4
Ad
2
)].
ously determined to have an electronic structure in which there
is a high-spin iron(II) ion antiferromagnetically coupled to a rad-
ical monoanion tetrazene ligand. The N–N bond lengths and
N–N [Å]
1.331(5),
Mössbauer parameters
δ [mm/s], |ΔE | [mm/s]
[
8]
Q
Mössbauer parameters of 5 and 6 are compared to those of
5
6
Fe1: 0.73(2), 1.45(2)
Fe2: 1.21(2), 2.08(2)
Me,iPr
1
1
.323(6),
.316(5)
[L
Fe(N Ad )] in Table 2, in an effort to assign metal and
4 2
ligand oxidation states.
1.316(6),
.332(6),
.309(6)
0.75(4), 1.55(6)
0.69(2), 1.32(4)
The Fe1–Ndiketiminate bond lengths of 5 are 2.03 Å, consistent
with the average Fe –N bond length of four-coordinate (ꢀ-
diketiminato)iron(II) complexes of 2.03 Å.
lengths of 5 are 1.331(5), 1.323(6), and 1.316(5) Å and the N–N
bond lengths of 6 are 1.316(6), 1.332(6), and 1.309(6) Å. These
are similar to one another, and are similar to those of
2
+
1
L
1
[19]
The N–N bond
[
LMe,iPrFe(N
4
Ad
2
)][8]
1.317(2),
.334(2),
.309(2)
1
1
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[
LMe,iPrFe(N Ad )] (Table 2). These N–N bond lengths are be- cyanide complexes of the LCF3,ClFe system were synthesized to
4
2
tween those expected for an N–N single (1.45 Å) and a N=N compare to the known diketiminato-supported iron(I) com-
[
20]
Me Me,Me
Me,iPr
Me,iPr
double (1.25 Å) bond.
nance form B (Figure 1).
These bond lengths suggest reso- pounds [
L
Fe(CNtBu) ], [L
Fe(CNtBu) ], and [L
Fe-
3
3
[
22–24]
(CNtBu)2].
Complex 4 was reduced with KC at –65 °C, and
8
The similarity in the Mössbauer parameters of one of the an excess of tert-butyl isocyanide was added to produce a dark
doublets in 5, the doublet of 6, and the doublet of red mixture. The excess tBuNC was removed under vacuum to
Me,iPr
L
Fe(N Ad ) suggest that all three complexes have an iron form a dark green powder in 86 % yield. A single crystal was
4 2
site with the same oxidation state [high-spin iron(II)] and same obtained from a 1:1 toluene/pentane solution and was crystal-
geometry (pseudotetrahedral). The crystal structures corro- lographically characterized as [LCF3,ClFe(CNtBu)
] (7, Figure 7).
borate the similar geometries. Since a combination of spectro- The Fe–C bonds are distinctly longer in this complex than those
3
Me Me,Me
scopic and computational studies showed that iron(II) is the in the analogues with the smaller
L
ligand or the larger
ligand (Table 3), indicating a potential electronic effect
data of 5 and 6 suggest that Fe1 is in the +2 oxidation state, of the halogenated ligand. The Mössbauer spectrum of 7
Me,iPr
tBu,iPr
best description of [L
Fe(N Ad )], the similar Mössbauer
L
4
2
and therefore resonance structure B is most appropriate (Fig- showed a doublet at δ = 0.25(2) mm/s and ΔE = 0.63(2) mm/s,
Q
ure 1). Thus, the electron-withdrawing groups on the support- which are similar to the parameters of LMe,MeFe(CNXyl)
Me
of
3
ing ligands have little effect on the electron localization in the
tetrazene. It is notable that 5 and 5·THF are the first examples
2
2
of a tetrazene ligand bridging two metal atoms in an η :κ -
binding mode. In these complexes, the much higher isomer
shift of 1.21 mm/s suggests that Fe2 in 5 is high-spin iron(I).
The Fe2–N bond lengths are 1.99 Å and consistent with the
L
1+
average of 1.97 Å for Fe –N bond lengths of (ꢀ-diketimi-
nato)metal complexes with π-bound ligands. Thus, 5 has one
L
[
21]
iron(I) site and one iron(II) site, with a radical tetrazene ligand
between the iron atoms.
Me,iPr
The reduction potential of [L
Fe(N Ad )] was reported to
4 2
+
/0
be –1.84 V (vs. Cp Fe ), and the reduction was assigned to
2
tetrazene since the iron atom stayed as iron(II).[8] Cyclic vol-
·/–
tammetry of its halogenated analogue 6 in THF gives a one-
+/0
electron quasireversible reduction at –1.85 V (vs. Cp Fe ).
2
Thus, the halogen atoms on the ꢀ-diketiminato ligand do not
affect the redox potential. This supports the idea that reduction
occurs at the tetrazene ligand, which is fairly unresponsive to
substitution on the diketiminate.
Synthesis and Characterization of (ꢀ-Diketiminato)iron(I)
Isocyanide Complexes
Figure 7. Molecular structure of 7, showing 50 % thermal ellipsoids. Hydrogen
atoms omitted for clarity. Selected bond lengths [Å] and angles [°]: Fe(1)–
N(11) 1.9940(18), Fe(1)–N(21) 2.0045(18), Fe(1)–C(14) 1.952(2), Fe(1)–C(15)
To query the electronic effects of the electron-withdrawing ꢀ-
diketiminato ligand on other coordinated ligands, tert-butyl iso-
1
.846(2), Fe(1)–C(16) 1.843(2), C14(1)–N(14) 1.166(3), C(15)–N(15) 1.169(3),
C(16)–N(16) 1.169(3); N(11)–Fe(1)–N(21) 91.11(7).
Table 3. Key bond lengths [Å] and Mössbauer parameters of isocyanide complexes for comparison to those of 7. The axial isocyanides are listed first.
L
Fe–C [Å]
C–N [Å]
Mössbauer parameters
δ [mm/s], |ΔE | [mm/s]
Q
MeLMe,Me (tris[22]
)
1.885(1),
1.170(2),
1.185(2),
1.173(2)
0.17, 0.81
1
.798(1),
.822(1)
1
LtBu,iPr (tris[21]
n/a[a]
)
1.912(4), 1.824(4),
.833(4)
1.168(4),
1.164(5),
1
1
.168(5)
LCF3,Cl (tris, 7)
1.952(2), 1.846(2),
.843(2)
1.166(3),
1.169(3),
0.25, 0.63
1
1
.169(3)
LMe,iPr (bis[20]
n/a[a]
)
1.817(1),
.821(1)
1.222(9),
1.153(9)
1
LCF3,Cl (bis, 8)
a] n/a: not available.
1.988(6)
1.158(8)
0.64, 2.36
[
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δ = 0.17 mm/s and ΔE = 0.81 mm/s. The larger isomer shift in s and a minor species (18 %) at δ = 0.14(4) mm/s, |ΔE | =
Q
Q
7
corroborates the longer Fe–C distances, because the bond 0.58(10) mm/s (Figure 9b). The doublet of the minor species is
[
25]
CF3,Cl
lengths and isomer shift are typically correlated.
The CN similar to that of the tris(isocyanido) complex [L
Fe(CNtBu)3]
stretching vibrations of the CNtBu ligands for 7 have a large (7) (discussed above), which has Mössbauer parameters of δ =
–
1
frequency range of 1991–2122 cm compared to the range for 0.25(2) mm/s, |ΔE | = 0.63(2) mm/s (Figure 9a), suggesting that
Q
tBu,iPr
–1
[
L
Fe(CNtBu) ] of 2012–2096 cm , but the comparison is it is the small impurity in this sample. The putative presence of
3
difficult because of the overlap between an indeterminate 7 as an impurity could not be confirmed by IR spectroscopy,
number of peaks. The EPR spectrum of 7 as a frozen toluene because the bands for 7 (see below) overlap with those in the
solution at 10 K exhibits a nearly rhombic signal with g = [2.067, IR spectrum of 8. Note that the compound passed elemental
2
.076, 1.998], which is similar to those of other (ꢀ-diketimin- analysis, suggesting that elemental analysis is not sufficiently
[
20–22]
ato)iron(I) tris(isocyanide) complexes.
sensitive to establish the purity of 8. The UV/Vis spectrum of 8
It was also possible to prepare a bis(isocyanido)iron(II) com- exhibits a prominent absorbance at 710 nm with an extinction
–1
–1
plex with the halogenated ligand. Compound 4 was reduced coefficient of ca. 3000
M
cm . The solution magnetic moment
with KC at –108 °C, and only 2 equiv. of CNtBu were added to of 7 is 3.6(2) BM, consistent with an S = 3/2 spin state and high-
8
the cold solution. Warming and filtration gave a quantitative spin iron(I). Table 3 compares the CN stretching frequencies in
Me,iPr
[27]
yield of a blue-green powder of 8 that was used without further the IR spectra of 8 to those of [L
Fe(CNtBu)
]. The lowest-
2
Me,iPr
purification. Compound 8 was identified by elemental analysis energy stretching bands of 8 and [L
Fe(CNtBu) ] are 2108
2
and X-ray diffraction (see below) as [LCF3,ClFe(CNtBu) ], and even and 2050 cm , respectively, with a significant difference of
–1
2
though it had consistent elemental analysis, the spectra showed 58 cm– that indicates that backbonding is less in 8. It is not
some impurities (see below). This material was too sensitive for easy to relate this change to the changes in the supporting
us to successfully purify it; attempts to crystallize 8 typically ligand, because the change in spin state is expected to cause a
resulted in the formation of an unidentified red oil, though on large perturbation on backbonding.
one occasion we obtained a single crystal of 8 for structural
1
characterization (see below) (Figure 8).
Figure 8. Molecular structure of 8, showing 50 % thermal ellipsoids. Hydrogen
atoms omitted for clarity. Selected bond lengths [Å] and angles [°]: Fe(1)–
Figure 9. (a) Zero-field Mössbauer spectrum of solid 7 at 80 K. The black
N(11) 1.937(5), Fe(1)–C(14) 1.988(6); N(11)–Fe(1)–N(11A) 97.31(3), C(14)–N(14)
circles are the data points, and the solid red line is the total fit. The blue and
1
.158(8).
green solid lines show fits to the data for a major species [δ = 0.25(2) mm/s,
ΔE | = 0.63(2) mm/s, 88 %, green] and a minor species [δ = 0.59(3) mm/s,
|
Q
The IR spectrum of crude 8 exhibits three bands at 2180,
126, and 2108 cm in the region of CN stretches. Decomposi-
tion of 7 in the presence of air resulted in a red powder with
|ΔE | = 2.35(7) mm/s, 12(3)%, blue]. (b) Zero-field Mössbauer spectrum of
solid 8 at 80 K. The black circles are the data points, and the solid red line is
the total fit. The blue and green solid lines show fits to the data for a major
Q
–
1
2
species [δ = 0.64(2) mm/s, |ΔE
species [δ = 0.14(4) mm/s, |ΔE
Q
| = 2.36(3) mm/s, 82(3)%, blue] and a minor
| = 0.58(10) mm/s, 18 %, green].
–
1
–1
one band at 2176 cm , suggesting that the peak at 2180 cm
in 8 is due to the presence of a decomposition product. Thus,
appears to have two CN stretching vibrations (2126 and
Q
8
2
A blue-green single crystal of 8 was obtained from Et O and
2
–
1
108 cm ) as expected for symmetric and antisymmetric com- characterized by X-ray crystallography. There is a large differ-
binations.
ence in the Fe–C bond lengths, which are 0.17 Å longer in 8
The Mössbauer spectrum of 8 displays two doublets with a than in [LMe,iPrFe(CNtBu) ]. This large change correlates with a
2
major species (82 %) at δ = 0.64(2) mm/s, |ΔE | = 2.36(3) mm/ change in geometry: the structure of 8 is tetrahedral (τ = 0.94)
Q
4
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at the iron atom and has linear CNtBu ligands, whereas structures (particularly the Fe–C and C–N bond lengths; see
Me,iPr
[
L
Fe(CNtBu) ] is square-planar (τ = 0.10) with bent CNtBu Supporting Information) with the crystal structure of the litera-
2 4
[
32,26]
Me,iPr
ligands.
The change in geometry begs the question of ture precedent [L
Fe(CNtBu) ] and with 8. The doublet spin
2
whether this is a steric or an electronic effect. There are two states of each complex optimized to a pseudo-square-planar
Me,iPr
CF3,Cl
steric differences between L
and L
: (a) the increase in geometry. This is consistent with the square-planar
Me,iPr
size in the ꢀ-substituent from ꢀ-Me to ꢀ-CF and (b) the de- [L
crease in size of the N-aryl ortho substituents from iPr to Cl. The EPR spectroscopy.
Fe(CNtBu) ] having a doublet spin state as determined by
3
2
[
22]
The quartet spin states of each complex
change of the ꢀ-substituent could push the N-aryl rings toward optimized to a tetrahedral geometry consistent with the S =
the iron atom and destabilize the square-planar geometry. 3/2 spin state of 8, supporting the magnetic susceptibility re-
However, this must not be a major destabilizing factor, because sults shown above and indicating that 8 indeed has an S = 3/2
tBu,iPr
[
L
Fe(CO) ] is also square-planar, even though tBu groups ground state.
2
[
27]
are much larger than CF groups. With respect to the second
The difference in electronic (E), enthalpic (H), and free (G)
3
steric change, the N-aryl rings are perpendicular to the ꢀ-diket- energies between the doublet and quartet states are compared
iminato plane causing the ortho substituents to lie above and in Figure 10 for each ligand variant. The high-spin state is
below the N Fe plane. It is possible for the large out-of-plane clearly overstabilized by the B3LYP functional, but we can still
2
iPr groups to push the CNtBu ligands into a square-planar ge- interpret the trends upon ligand variation. Each CF substitution
3
ometry. Of course, an electronic effect is also possible. Thus, (right side of the graph) lowers the electronic energy of the
density functional theory calculations were performed to evalu- quartet state relative to the doublet state by ca. 1.2 kcal/mol.
ate the electronic effects of the CF groups on the geometry of This result indicates that the CF groups make the tetrahedral
3
3
the metal atom by a stepwise replacement of a CF group with geometry more favored over the square-planar geometry, but
3
a methyl group for each spin state.
not enough to account for the geometry difference between 8
Calculations utilized the B3LYP level of theory with triple- and [LMe,iPrFe(CNtBu) ]. Instead, the influence of the ortho sub-
2
zeta basis sets (def2-TZVP) on the iron atom and atoms coordi- stituents appears to be the most important (left side of the
nated to the iron center, and double-zeta basis sets on all other graph), where changing from isopropyl to methyl to chloro
atoms, as in earlier calculations on (ꢀ-diketiminato)iron com- gives much greater changes. Since methyl and isopropyl are
[
28]
plexes.
The B3LYP functional was chosen over BP86 and electronically similar but give a drastic change, it is most rea-
revPBE based on the superior agreement of the optimized sonable to attribute the difference to the relative size of the
substituents.
Summary and Conclusions
New, heavily halogenated ꢀ-diketiminato supporting ligands
can be synthesized by a one-pot synthesis or by a 2-step micro-
wave synthesis, where the latter method produces less waste.
Use of this ligand afforded an iron(II) chloride complex with a
coordinated diethyl ether ligand. Diethyl ether can be removed
CF3,Cl
easily under vacuum to form the dimer, [L
FeCl] (4).
2
Reduction of complex 4 is difficult, presumably because the
halogenated substituents are susceptible to attack by reducing
agents. We solved this issue by reducing at low temperature
and adding substrates to the cold solution; however, in many
1
cases the products were difficult to purify well, and the H NMR
spectra indicate impurities. Despite these difficulties, single
crystals could be obtained on rare occasions, which elucidated
the nature of the products. When N Ad is added to reduced 4,
3
the azides couple to form a bis(adamantyl)tetrazene, and this is
·–
reduced by one electron to give N Ad in iron(II) complexes.
4
2
These tetrazene radical anion complexes can be coordinated to
one iron atom, or bridge between two iron ions [one iron(I) and
one iron(II)] in a new binding mode for dialkyltetrazene ligands.
The crystallographic and spectroscopic evidence indicate that
the influence of ligand halogenation is small, because the sin-
gle-electron reduction of the tetrazene is very similar between
the halogenated tetrazene complex and its alkylated analogues.
When iron(I) complexes with two or three isocyanido ligands
were synthesized with the new halogenated ligand, interest-
ingly the bis(isocyanido) complex is high-spin and tetrahedral in
Figure 10. (top) Computed favorability of tetrahedral to square-planar inter-
conversion with varying R, R′, and R′′ groups. (bottom) Plot of energy differ-
ences (in kcal/mol) between tetrahedral and square-planar complexes with
varying R, R′, and R′′ groups.
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contrast to the square-planar, low-spin [LMe,iPrFe(CNtBu) ]. DFT ments were referenced to the Cp
calculations suggest that these structural differences and
Fe /Cp
+
Fe couple using an inter-
2
2
2
nal ferrocene standard.
change in spin state are mostly from steric influences, and the
electronic differences are again relatively small. This is consist- the ORCA program package, version 2.8.0. Calculations reported
ent with more general observations on complexes with ꢀ-diket- here employed the B3LYP functional. The Ahlrichs triple-ꢁ-quality
Computational Methods: DFT calculations were performed with
[
31]
[
32]
iminato ligands, where steric effects are greater than electronic basis set with one set of polarization functions, def2-TZVP, was used
[
3]
for the iron atom, atoms directly coordinated to it, and all atoms of
effects. Therefore, substitution of halogens for the purposes
of ligand protection is likely to be a useful strategy that will not
cause major electronic changes at the metal atom, but it will
be important to make the supporting ligand isosteric for closer
concurrence between the spin state, structure, and nuclearity
of analogous complexes.
[
33]
the alkyl ligand.
The smaller polarized Ahlrichs double-ꢁ-quality
basis set, def2-SVP, was used on all other atoms. The auxiliary basis
set def2-SVP/J was used to speed the calculations through the reso-
[
34]
lution of identity (RIJCOSx) approximation.
An empirical
van der Waals correction was applied to the DFT energy
[35]
(
VDW10). The SCF calculations were tightly converged (TightSCF)
with unrestricted spin (UKS), and geometry optimizations con-
verged normally (Opt). Numerical frequency (NumFreq) calculations
were performed on each complex to obtain the Gibbs free energy
(in kcal/mol at STP). Ground states had no imaginary frequencies.
Experimental Section
General Considerations: Unless otherwise specified, all manipula-
tions were performed under an inert gas using standard Schlenk
X-ray Crystallography: Single crystals were placed onto the tip of
a 0.1 mm diameter glass capillary tube or fiber and mounted on a
Bruker SMART APEX II CCD Platform diffractometer for a data collec-
tion at 100.0(1) K. A preliminary set of cell constants and an orienta-
tion matrix were calculated from reflections harvested from three
orthogonal wedges of reciprocal space. The full data collection was
carried out using Mo-Kα radiation (graphite monochromator) with
appropriate frame times ranging from 25 to 120 s with a detector
distance of approximately 4.00 cm. The structures 4 and 4·THF were
techniques or in an M. Braun Unilab N -filled glovebox maintained
2
at or below 1 ppm O2 and H O. Glassware was dried at 150 °C
2
overnight. Celite was dried at 250 °C under vacuum overnight. Tolu-
ene, diethyl ether and pentane were purified by passage through
activated alumina and Q5 columns from Glass Contour Co. (Laguna
Beach, CA). THF was dried by distilling from Na/benzophenone.
[
D ]THF and C D were dried with CaH , then with Na/benzo-
8 6 6 2
phenone, vacuum-transferred, and stored over 3 Å molecular sieves.
Decane was dried by filtering through activated alumina and placed
over 3 Å molecular sieves for 24 h two times, and stored over mo-
lecular sieves. Adamantyl azide was purchased from Sigma Aldrich
and crystallized from pentane. Potassium hexamethyldisilazide,
titanium tetrachloride, p-toluenesulfonic acid monohydrate, and
[
36]
[37]
solved using SIR97
and refined using SHELXL-97.
The struc-
[
38]
tures of 3, 3·Et O, 5, 6, and 7 were solved using SIR2011
and
2
[
39]
refined using SHELXL-2013. A direct-methods solution was calcu-
lated, which provided most non-hydrogen atoms from the E-map.
Full-matrix least-squares/difference Fourier cycles were performed,
which located the remaining non-hydrogen atoms. All non-
hydrogen atoms were refined with anisotropic displacement param-
eters. All hydrogen atoms were placed in ideal positions and refined
as riding atoms with relative isotropic displacement parameters.
1
,8-bis(dimethylamino)naphthalene (Proton Sponge®), were pur-
chased from Sigma Aldrich and 2,6-dichloroaniline from Oakwood
Products, Inc. Hexafluoroacetylacetone (HfacH) was purchased from
Sigma Aldrich (colorless liquid) and dried by distillation from cal-
cium sulfate twice (yellow liquid). FeCl (THF) was synthesized by
2
1.5
CCDC 1477165 (for 3·Et O), 1477166 (for 4), 1477167 (for 5),
a literature procedure.[
29]
2
KC was prepared by heating potassium
8
1477168 (for 5·THF), 1477169 (for 6), 1477170 (for 8) and 1477171
and graphite at 150 °C under argon. Reactions at –65 °C were per-
formed in a glovebox with a cold well with an external Dewar filled
(for 7) contain the supplementary crystallographic data for this pa-
per. These data can be obtained free of charge from The Cambridge
Crystallographic Data Centre.
1
13
1
with dry ice and acetone. H and C{ H} NMR spectra were re-
corded with a Bruker Avance 500 MHz spectrometer at room tem-
perature; 19F{ H} NMR spectra were recorded with a Bruker Avance
1
One-Pot Synthesis of LCF3,ClH (2): Under N , a three-neck flask
2
4
00 MHz spectrometer. Chemical shifts (δ) are reported in ppm,
was loaded with 2,6-dichloroaniline (12.5 g, 77.2 mmol) and decane
(400 mL). A dropping funnel was loaded with a solution of 1,8-
bis(dimethylamino)naphthalene (16.5 g, 77.2 mmol) in decane
(100 mL) and attached to the three-neck flask. Another dropping
relative to residual protiated solvent in C D (δ = 7.15 ppm) or
6
6
1
9
[D ]THF (δ = 1.73, 3.58 ppm), and F chemical shifts are reported
8
in ppm relative to an added reference compound of trifluorotolu-
ene (δ = –63.7 ppm) or hexafluorobenzene (δ = –164.9 ppm). Peaks
were singlets unless otherwise noted. Overlapping peaks are re-
funnel loaded with TiCl (16.9 mL, 154 mmol) was attached to the
4
three-neck flask. The aniline solution was cooled to 0 °C, and the
ported together with one integration. Solution magnetic suscepti- TiCl was added dropwise. The resulting red mixture was warmed
4
bilities were determined by Evans' method.[ Microwave reactions
30]
to ambient temperature and stirred for 18 h. The empty dropping
funnel was replaced with a septum, the mixture was cooled to 0 °C,
were performed with a CEM Discover SP (2450 MHz) reactor in a
1
0 mL pressure vessel (note that the cap is not air-tight). Infrared and HfacH (2.80 mL, 19.9 mmol) was added by syringe followed
data were obtained with a Shimadzu FT-IR Prestige-21 spectrometer
by the dropwise addition of the 1,8-bis(dimethylamino)naphthalene
using KBr pellets. Electronic spectra were recorded between 400 solution. The septum was replaced with a condenser and the yellow
and 1000 nm with a Cary 50 UV/Vis spectrophotometer, using Tef-
lon-sealed air-free cuvettes of 0.1 cm optical path length. Elemental
mixture was warmed to 70 °C for 24 h, then 140 °C for 24 h. The
mixture was cooled to room temperature and filtered through Cel-
analyses were performed by the CENTC Elemental Analysis Facility ite to remove a dark brown solid and isolate a deep yellow solution.
at the University of Rochester. Electrochemical data were obtained The solid was washed with Et O until the wash was colorless. Et O
2
2
under Ar using a Cypress Systems 3100 potentiostat. The working
electrode was glassy carbon with a 1 mm diameter working area,
was removed under vacuum, and the decane was removed by vac-
uum distillation at 45 °C to produce an orange solid. Excess aniline
and 1 were removed by Kugelrohr sublimation at 75 °C under dy-
namic vacuum. The remaining orange solid was crystallized from
and Ag wires were used as the pseudoreference electrode and the
F
auxiliary electrode. All spectra were taken using a [Bu N][BAr ] elec-
4
trolyte solution in THF. The scan rate was 500 mV/s. All measure- hot hexane to afford 2 (1.39 g, 2.80 mmol, 14 %). The second frac-
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tion from the Kugelrohr sublimation was isolated as 1 (3.87 g,
1.0 mmol, 57 %). The above procedure was repeated using 1 in
place of HfacH to produce 2 (2.2 g, 4.44 mmol, 40 %) for an overall
yield of 38 %. Spectroscopic data for 1 and 2 are below.
der. Toluene (50 mL) was added to the red powder, and the suspen-
sion was stirred at 90 °C for 18 h. The dark red mixture was filtered
through Celite and dried under vacuum to produce a dark red solid,
which was washed with pentane (7 mL) three times. The resulting
bright red powder of 4 (380 mg, 0.324 mmol, 80 %) was collected.
1
Two-Step Microwave Synthesis of LCF3,ClH (2). Keto-Imine (1):
1
H NMR (500 MHz, C D , 25 °C): δ = 10.4 (8 H, m-Ar), –29.5 (2 H, ꢀ-
6
6
Under N , a 10 mL tube was loaded with 2,6-dichloroaniline
2
1
CH), –53.7 (4 H, p-Ar) ppm. H NMR (500 MHz, [D ]THF, 25 °C): δ =
8
(300 mg, 1.8 mmol), p-toluenesulfonic acid (180 mg, 0.92 mmol),
1
9
1
1
8.2 (4 H, m-Ar), –50.1 (2 H, p-Ar), –61.4 (1 H, ꢀ-CH) ppm. F{ H}
NMR (376.3 MHz, C D , 25 °C): δ = –63.7 (CF ) ppm. F{ H} NMR
HfacH (250 μL, 1.8 mmol), and toluene (3 mL) and heated to 210 °C
by microwave radiation (Pmax = 300 W) for 30 min. Under air, the
mixture was filtered, and the solid was washed with hexane until
the wash was colorless. Volatile materials were removed under vac-
uum, and excess aniline was removed by sublimation at 40 °C under
19
1
6
6
3
(
376.3 MHz, THF, 25 °C): δ = –165.0 (CF ) ppm. μ ([D ]THF, 25 °C) =
3 eff 8
5
1
.6(2) BM per Fe. IR (KBr): ν˜ = 1562 (m), 1541 (m), 1435 (s), 1420 (s),
321 (m), 1296 (m), 1234 (m), 1188 (s), 1159 (s), 1146 (s), 1109 (m),
–1
1
1096 (w), 955 (w), 887 (w), 806 (m), 787 (m), 772 (m), 775 (w) cm .
UV/Vis (THF): λ (ε) = 254 (21), 365 (36), 515 (1.9 m
C
4
dynamic vacuum to afford 1 (250 mg, 0.72 mmol, 41 %). H NMR
–
1
–1
M
cm ) nm.
O) (660.47): calcd. C 38.19, H 2.59, N 4.24;
: C H Cl F Fe N (1172.70): calcd. C 34.82, H 1.20, N 4.78; found
(
7
500 MHz, CDCl ): δ = 11.2 (s, 1 H, NH), 7.4 (d, J = 8 Hz, 4 H, m-Ar),
.3 (t, J = 8 Hz, 2 H, p-Ar), 6.1 (s, 1 H, ꢀ-CH) ppm. C{ H} NMR
3
1
3
1
21 5 6 2 2
H17Cl F FeN O (3·Et
34
14 10 12
2 4
(
125.7 MHz, CDCl ): δ = 181.6 (q, J = 36 Hz, O=CCF ), 154.0 (q, J =
3
3
C 35.02, H 1.18, N 4.55. Zero-field Mössbauer (solid, 80 K): δ =
3
3 Hz, N=CCF ), 135.8 (s, NC ), 132.0 (s, p-C ), 130.8 (s, o-C ), 128.8
3 Ar Ar Ar
0
.92(2) mm/s, |ΔE | = 2.30(2) mm/s; Zero-field Mössbauer (THF solu-
Q
(
s, m-C ), 117.7 (2 overlapping q, N=CCF and O=CCF ), 89.6 (s, ꢀ-
Ar 3 3
19
1
tion, 80 K): δ = 0.88(2) mm/s, |ΔE
Q
| = 2.06(2) mm/s.
solution of 4 (100 mg,
C) ppm. F{ H} NMR (376.3 MHz, CDCl ): δ = –68.5 (N=CCF ), –78.4
3
3
(
O=CCF ) ppm. IR (KBr): ν˜ = 1946 (w), 1871 (w), 1663 (s), 1601 (s),
[LCF3,ClFe(N Ad )FeLCF3,Cl] (5):
3
A
4
2
1
1
6
568 (m), 1508 (m), 1441 (m), 1381 (m), 1325 (m), 1267 (s), 1209 (s),
148 (s), 1090 (s), 935 (w), 849 (w), 820 (s), 783 (s), 748 (w), 727 (m),
0
.086 mmol) in THF (5 mL) was cooled to –108 °C, and KC (27 mg,
8
0.20 mmol) as a solid was added and the mixture stirred for 1 min.
–1
+
71 (w) cm . MS (EI ): m/z (%) = 351 (100), 352 (12), 353 (61), 354
N Ad (30 mg, 0.17 mmol) in THF (3 mL) was cooled to –108 °C and
3
(
7), 355 (10), 356 (1.2). C H Cl F NO (352.06): calcd. C 37.53, H 1.43,
11 5 2 6
added to the mixture, which was stirred for 10 min. Volatile materi-
als were removed under vacuum. The residue was dissolved in pent-
ane, filtered through Celite, concentrated to 5 mL (a dark red precip-
itate formed), filtered, and cooled to –40 °C to afford dark red-
CF3,Cl
N 3.98; found C 37.52, H 1.43, N 3.91. L
H (2): Under N , a 10 mL
2
tube was loaded with 2,6-dichloroaniline (300 mg, 1.8 mmol) and
decane (2 mL) and cooled to –30 °C. TiCl (33 μL, 0.3 mmol) was
4
added to form a red-orange mixture, which was stirred for 24 h. As
a solid, 1 was added and the vial was rinsed with decane (1 mL).
This procedure was performed with 6 10 mL reaction tubes. Each
1
orange crystals of 5 (60 mg, 0.040 mmol, 47 %). H NMR (500 Hz,
C D , 25 °C): δ = 11.5 (2 H,), 9.6 (2 H), 5.1 (3 H), –0.9 (2 H), –1.7 (4
6
6
H), –6.5 (1 H), –7.2 (3 H), –14.2/–14.5 (6 H), –24.5 (1 H), –24.9 (1 H),
–25.9 (1 H) ppm. The integrations given are rough estimates (see
tube was heated to 210 °C using variable microwave power (Pmax
=
3
00 W) for 4 h. Under air, each reaction tube was washed with
1
Supporting Information, section S-4). The H NMR spectrum showed
hexane (7 mL), toluene (5 mL), and hexane (5 mL) and combined.
The yellow mixture was filtered through Celite, and the volatile ma-
terials were removed under vacuum. The decane was vacuum-dis-
tilled under dynamic vacuum at 40 °C. The excess aniline and unre-
acted 1 were sublimed from the product under dynamic vacuum
at 65 °C. The orange and yellow solid was dissolved in hexane
some impurities including 6, consistent with the deviations in the
19
1
microanalysis. F{ H} NMR (376.3 MHz, C D , 25, °C): δ = –117.4,
6
6
–
2
1
7
122.5 ppm. μeff (C D , 25, °C) = 5.2(1) BM. IR (KBr): ν˜ = 2907, (m),
6
6
849, (m), 1558, (w), 1535, (w), 1435, 1312, (m), 1288, (m), 1234, (m),
186, (s), 1152, (s), 1103, (m), 1070, (w), 955, (w), 883, (w), 789, (m),
–1
71, (m), 746, (w), 698, (w) cm . C H Cl F Fe N (1428.29): calcd.
54
44
8
12
2 8
(15 mL) and filtered through a column of silica gel (2.7 cm × 5 cm)
C 45.41, H 3.10, N 7.84; found C 45.93, H 3.15, N 7.24. Zero-field
in hexane, and the yellow solution was dried under vacuum to af-
Mössbauer (solid, 80 K): δ = 0.73(2) mm/s, |ΔE | = 1.45(2) mm/s
Q
ford yellow crystals of 2 (463 mg, 0.930 mmol, 52 %). 1H NMR
(
57 %) and δ = 1.21(2) mm/s, |ΔE | = 2.08(2) mm/s (43 %).
Q
(
7
500 MHz, CDCl ): δ = 11.5 (s, 1 H, NH), 7.4 (d, J = 10 Hz, 4 H, p-Ar),
.1 (t, J = 10 Hz, 2 H, m-Ar), 6.0 (s, 1 H, ꢀ-CH) ppm. C{ H} NMR
3
[LCF3,ClFe(N Ad )] (6): A solution of 4 (90 mg, 0.15 mmol) in THF
1
3
1
4 2
(
15 mL) was cooled to –65 °C, and a suspension of KC8 (24 mg,
.18 mmol) in THF (1 mL) was added. The dark green mixture was
stirred at –65 °C for 15 min, and N Ad (0.38 mL, 0.80 in THF) was
(
100.5 MHz, CDCl ): δ = 152.3 (q, J = 31 Hz, CCF ), 138.2 (s, NC ),
3 3 Ar
0
1
30.8 (s, p-C ), 128.2 (s, m-C ), 127.6 (s, o-C ), 118.8 (q, J = 283 Hz,
Ar Ar Ar
1
9
1
M
CF ), 89.3 (s, ꢀ-C) ppm. F{ H} NMR (376.3 MHz, CDCl ): δ =
3
3
3
+
added, giving a color change to dark red. Stirring of the mixture
was continued at –65 °C for 30 min, followed by warming to room
temp. and stirring overnight. The mixture was filtered through Cel-
ite, and volatile materials were removed under vacuum. The residue
was dissolved in pentane, filtered through Celite, concentrated, and
–68.9 ppm. MS (EI ): m/z (%) = 494 (78), 495 (14), 496 (100), 497
(
(
(
19), 498 (48), 499 (10), 500 (9). IR (KBr): ν˜ = 2924 (w), 2855 (w), 1645
s), 1589 (s), 1564 (s), 1514 (s), 1495 (m), 1439 (s), 1406 (m), 1387
m), 1331 (m), 1283 (s), 1229 (m), 1186 (s), 1142 (s), 1109 (m), 1092
–
1
(m), 1065 (m), 943 (w), 874 (w), 791 (s), 773 (s), 745 (m) cm . UV/
–
1
–1
cooled to –40 °C to afford red crystals of 6 (17 mg, 0.020 mmol,
Vis (toluene): λ (ε) = 330 (22 m
M
cm ) nm. C H Cl F N (496.06):
17 8 4 6 2
1
1
3 %). H NMR (500 MHz, C D , 25 °C): δ = 4.5 (12 H), –0.9 (6 H),
calcd. C 41.16, H 1.63, N 5.65; found C 41.20, H 1.55, N 5.58.
6 6
–
6.4 (6 H), –23.9 (2 H), –24.4 (6 H) ppm. Two expected peaks are
[
LCF3,ClFeCl]2 (4): A solution of potassium hexamethyldisilazide
19
1
absent. F{ H} NMR (376.3 MHz, C D , 25 °C): δ = –158.2 (CF ) ppm.
6
6
3
(
161 mg, 0.808 mmol) in Et O (5 mL) was added to a solution of 2
2
μeff (C D , 25 °C) = 3.4(1) BM. IR (KBr): ν = 3063 (w), 2908 (s), 2851
6
6
˜
(
402 mg, 0.811 mmol) in Et O (10 mL) at a rate of 2–4 drops/s. The
2
(
(
(
m), 2088 (w), 1559 (w), 1532 (w), 1434 (s), 1410 (s), 1317 (w), 1283
m), 1234 (m), 1205 (m), 1183 (s), 1151 (s), 1131 (w), 1107 (w), 1097
dark yellow solution was stirred at room temp. for 2 h and com-
pletely dried under vacuum at 35 °C as determined by a vacuum
gauge. The dark yellow solid was dissolved in THF (10 mL) and
added to a mixture of FeCl (THF) in THF (5 mL) to produce a dark
–
1
w) cm . C H Cl F FeN (877.39): calcd. C 50.65, H 4.25, N 9.58;
37
37
4
6
6
found C 50.41, H 4.30, N 8.69. Zero-field Mössbauer (solid, 80 K):
2
1.5
δ = 0.75(2) mm/s, |ΔE | = 1.55(2) mm/s.
Q
purple mixture, which was stirred at room temp. for 18 h. Volatile
materials were removed under vacuum, and the solid was heated
under dynamic vacuum at 60 °C for 6 h to afford a bright red pow-
[LCF3,ClFe(CNtBu) ] (7): A solution of 4 (47 mg, 0.080 mmol) in THF
3
(5 mL) was cooled to –65 °C. KC (12 mg, 0.089 mmol) suspended
8
Eur. J. Inorg. Chem. 0000, 0–0
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10
© 0000 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Full Paper
in THF (1 mL) was added dropwise while stirring. After stirring for
0 min, an excess of tBuNC (0.050 mL, 0.44 mmol) was added, and
[3] C. Chen, S. M. Bellows, P. L. Holland, Dalton Trans. 2015, 44, 16654–16670.
[
4] a) E. Said-Galiyev, L. Nikitin, R. Vinokur, M. Gallyamov, M. Kurykin, O.
1
Petrova, B. Lokshin, I. Volkov, A. Khokhlov, Ind. Eng. Chem. Res. 2000, 39,
the mixture was stirred at –65 °C for 30 min. The dark brown mix-
ture was warmed to ambient temperature and filtered through Cel-
ite. Volatile materials were removed under vacuum to give a dark
red-orange residue. The residue was dissolved in toluene (5 mL)
and placed under dynamic vacuum. As the solution became more
concentrated, the color changed from dark red to dark green and
resulted in a dark green residue that was taken up in pentane and
dried to form 7 as a dark green powder (55 mg, 0.069 mmol, 86 %).
4
891–4896; b) D. S. Laitar, C. J. N. Mathison, W. M. Davis, J. P. Sadighi,
Inorg. Chem. 2003, 42, 7354–7356; c) L. M. R. Hill, B. F. Gherman, N. W.
Aboelella, C. J. Cramer, W. B. Tolman, Dalton Trans. 2006, 4944–4953; d)
J. A. Flores, H. V. R. Dias, Inorg. Chem. 2008, 47, 4448–4450; e) N. Carrera,
N. Savjani, J. Simpson, D. L. Hughes, M. Bochmann, Dalton Trans. 2011,
4
0, 1016–1019; f) V. A. K. Adiraju, J. A. Flores, M. Yousufuddin, H. V. R.
Dias, Organometallics 2012, 31, 7926–7932; g) J. Liu, L. Vieille-Petit, A.
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1
19
1
H NMR (500 MHz, C D , 25 °C): silent. F{ H} NMR (376.3 MHz,
6
6
C D , 25 °C): silent. EPR (toluene, 10 K): g = 2.076, 2.067, 1.998. IR
6
6
eff
[
[
5] a) P. J. Chirik, Inorg. Chem. 2011, 50, 9737; b) W. Kaim, Inorg. Chem. 2011,
(
1
1
KBr): ν˜ = 2982 (m), 2112 (s), 2043 (s), 1991 (m), 1543 (w), 1431 (s),
5
0, 9752; c) W. Kaim, Eur. J. Inorg. Chem. 2012, 343.
6] a) K. Chlopek, E. Bill, T. W. Ueller, K. Wieghardt, Inorg. Chem. 2005, 44,
087–7098; b) N. Muresan, C. C. Lu, M. Ghosh, J. C. Peters, M. Abe, L. M.
368 (w), 1298 (m), 1231 (m), 1207 (m), 1171 (s), 1132 (s), 1069 (w),
–
1
036 (w), 959 (w), 889 (w), 783 (m), 743 (w) cm . C H Cl F FeN
3
2
34
4
6
5
7
(800.30): calcd. C 48.03, H 4.28, N 8.75; found C 48.37, H 4.32, N
Henling, T. Weyhermüller, E. Bill, K. Wieghardt, Inorg. Chem. 2008, 47,
4579–4590.
8
.58. Zero-field Mössbauer (solid, 80 K): δ = 0.25(2) mm/s, |ΔE | =
Q
0
.63(2) mm/s.
[7] The average N=N bond length of XN=NX (X = nonmetal) is 1.28(4) Å,
calculated from 3041 examples in the Cambridge Structural Database
LCF3,ClFe(CNtBu) ] (8): A solution of 4 (50 mg, 0.085 mmol) in THF
[
2
(
Aug 2008 update): F. H. Allen, Acta Crystallogr., Sect. B 2002, 58, 380–
88.
8] R. E. Cowley, E. Bill, F. Neese, W. W. Brennessel, P. L. Holland, Inorg. Chem.
009, 48, 4828–4836.
9] P. H. M. Budzelaar, A. B. van Oort, A. G. Orpen, Eur. J. Inorg. Chem. 1998,
485.
(3 mL) was cooled to –108 °C, and KC8 (14 mg, 0.10 mmol) was
3
added followed by tert-butyl isocyanide (19 μL, 0.17 mmol). The
bright blue-green mixture was stirred at ambient temperature for
[
[
2
3
0 min, filtered through Celite, and dried under vacuum to produce
1
a blue-green powder of 8 (61 mg, 0.085 mmol, 100 %). H NMR
1
(
(
(
500 MHz, C D , 25 °C): δ = 29.4 (4 H, m-Ar), –49.8 (2 H, p-Ar), –147.2
[10] R. E. Cowley, P. L. Holland, Inorg. Chem. 2012, 51, 8352–8361.
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968; b) E. A. Gregory, R. J. Lachicotte, P. L. Holland, Organometallics 2005,
6
6
19
1
1 H, ꢀ-CH) ppm. F{ H} NMR (376.3 MHz, C D , 25 °C): δ = –65.1
6
6
major), –67.5, –120.2 ppm. We assume that the latter two peaks
2
4, 1803; c) J. Vela, J. M. Smith, Y. Yu, N. A. Ketterer, C. J. Flaschenriem,
correspond to the small impurities observed in the Mössbauer spec-
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Bouwkamp, E. Lobkovsky, P. J. Chirik, J. Am. Chem. Soc. 2005, 127, 9660;
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N. M. Navarrete, M. J. Hettinga, A. Meetsma, B. Hessen, Eur. J. Inorg. Chem.
tra. μeff (C D , 25 °C) = 3.6(2) BM. IR (KBr): ν˜ = 2986 (m), 2938 (w),
6
6
2
870 (w), 2180 (m), 2126 (s), 2108 (s), 1578 (w), 1555 (w), 1516 (w),
433 (s), 1369 (m), 1296 (s), 1242 (m), 1169 (s), 1146 (s), 1113 (s),
1
1
–
1
067 (m), 950 (w), 887 (m), 808 (m), 773 (m), 743 (w), 719 (w) cm .
–
1
–1
UV/Vis (THF): λ (ε) = 255 (14), 285 (15), 710 (3 m
M
cm ) nm.
2011, 91; g) E. R. King, E. T. Hennessy, T. A. Betley, J. Am. Chem. Soc.
C H Cl F FeN (717.17): calcd. C 45.22, H 3.51, N 7.81; found C
2011, 133, 4917; h) A. M. Tondreau, S. Chantal, E. Stieber, C. Milsmann,
E. Lobkovsky, T. Weyhermuller, S. P. Semproni, P. J. Chirik, Inorg. Chem.
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Muller, W. Seidel, H. Gorls, Z. Anorg. Allg. Chem. 1997, 623, 155.
27
25
4
6
4
4
5.21, H 3.58, N 7.63. Zero-field Mössbauer (solid, 80 K): δ =
0
.64(2) mm/s, |ΔE | = 2.36(3) mm/s.
Q
Supporting Information (see footnote on the first page of this
article): Spectra and computational information.
[
[
[
[
12] M. M. Rodriguez, E. Bill, W. W. Brennessel, P. L. Holland, Science 2011,
3
34, 780–783.
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4, 1042.
1
Acknowledgments
This work was supported by the National Institutes of Health
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(GM065313). We thank Megan Reesbeck and Stephanie Daifuku
for assistance with EPR spectroscopy.
5
106.
[
16] a) R. E. Cowley, N. J. DeYonker, N. A. Eckert, T. R. Cundari, S. DeBeer, E.
Keywords: Diketiminates · Fluorinated ligands · Tetrazenes ·
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4
value is a measure of the four-coordinate geometry, which lies
Received: February 4, 2016
between 0 and 1 with τ
4
= 0 for a square-planar and τ
4
= 1 for a tetrahe-
Published Online: ■
Eur. J. Inorg. Chem. 0000, 0–0
www.eurjic.org
12
© 0000 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Full Paper
Halogenated Ligands
We present a new halogenated version
of a ꢀ-diketiminato ligand, which has
fluoro and chloro substituents. An
iron(II) complex can be reduced in situ
to react with adamantyl azide to give
tetrazene complexes and with isocyan-
ides to give organometallic complexes.
The electronic and steric impact of the
new ligand is elucidated using struc-
ture analysis, spectroscopy, and com-
putations.
S. M. Bellows, W. W. Brennessel,
P. L. Holland* .................................... 1–13
Effects of Ligand Halogenation on
the Electron Localization, Geometry
and Spin State of Low-Coordinate
(
ꢀ-Diketiminato)iron Complexes
DOI: 10.1002/ejic.201600112
Eur. J. Inorg. Chem. 0000, 0–0
www.eurjic.org
13
© 0000 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim