Reactivity studies on [Cp′FeI]2: Monomeric amido, phenoxo, and alkyl complexes

Article abstract of DOI:10.1021/ic301770f

A series of monomeric mono(cyclopentadienyl) iron amido, phenoxo, and alkyl complexes were synthesized, and their structure and reactivity are presented. The iron(II) centers in these 14VE one-legged piano stool complexes are high spin (S = 2) in solid state and solution independent of solvent. The silylamide compound [Cp′FeN(SiMe₃)₂] (2a, Cp′ = 1,2,4-(Me₃C)₃C₅H₂) is an excellent starting material for the reaction with more acidic substrates such as phenols. Sterically encumbered phenols 2,6-(Me₃C)₂(4-R)C ₆H₂OH (R = H, Me, and tBu) were investigated. In all cases monomeric iron phenoxo half-sandwich complexes [Cp′FeOR′] (4-R) are initially formed. Rearrangement of 4-R to the diamagnetic oxocyclohexadienyl complex [Cp′Fe(η⁵-O= C₆H₂R′₂R″)] (5-R) is observed for 2,6-(Me₃C)₂(4-R)C₆H₂OH (R = H and Me) and the Gibbs free enthalpy of activation (ΔG^?) was determined. In contrast this rearrangement is inhibited when the 4-position is blocked by a tBu group. Removing the steric bulk from the 2,6-positions leads to the formation of a μ-phenoxo dimer, [Cp′Fe(μ-OC₆H ₃tBu₂-3,5)]₂ (5). Density functional theory (DFT) was used to further elucidate the structure-reactivity relationship in these molecules. The one-legged piano stool anilido complex [Cp′ Fe(NHC₆H₂tBu₃-2,4,6)] (7) is not accessible via acid-base reaction between 2a and H₂NC₆H ₂tBu₃-2,4,6, but can be prepared by conventional salt metathesis reaction from [Cp′FeI]₂ and [Li(NHC ₆H₂tBu₃-2,4,6)(OEt₂)]₂. In contrast, reaction of 2a with Ph₂NH yields the bimetallic [Cp′Fe(N,C-κ¹,η⁵-C₆H ₅NPh)Fe(N-κ¹-NPh₂)Cp′] (8) which combines two iron centers in the same oxidation state (+2), but different spin-states (S = 0 and S = 2) which is reflected in very different Cp(cent)-Fe distances of 1.68 and 2.04 A, respectively. A monomeric iron alkyl half-sandwich complex [Cp′FeCH(SiMe₃)₂] (9) was prepared that exhibits no reactivity toward H₂, C₂H ₄ or N₂O. This behavior might be rationalized by a spin-state induced reaction barrier. However, 9 reacts in the presence of CO to the iron acyl-complex [Cp′Fe(CO)₂(C(O)CH(SiMe₃) ₂)] (10) and with a CO/H₂ mixture [Cp′Fe(CO) ₂]₂ (11) and CH₂(SiMe₃)₂ are formed.

Full text of DOI:10.1021/ic301770f

Article
pubs.acs.org/IC
Reactivity Studies on [Cp′FeI]2: Monomeric Amido, Phenoxo, and
Alkyl Complexes
Marc D. Walter*,† and Peter S. White‡
†Institut fur Anorganische und Analytische Chemie, Technische Universitat Braunschweig, Hagenring 30, 38106 Braunschweig,
̈
̈
Germany
‡Department of Chemistry, University of North Carolina at Chapel Hill, Chapel Hill, North Carolina 27599-3290, United States
S
* Supporting Information
ABSTRACT: A series of monomeric mono(cyclopentadienyl)
iron amido, phenoxo, and alkyl complexes were synthesized,
and their structure and reactivity are presented. The iron(II)
centers in these 14VE one-legged piano stool complexes are
high spin (S = 2) in solid state and solution independent of
solvent. The silylamide compound [Cp′FeN(SiMe3)2] (2a, Cp′
= 1,2,4-(Me3C)3C5H2) is an excellent starting material for the
reaction with more acidic substrates such as phenols. Sterically
encumbered phenols 2,6-(Me3C)2(4-R)C6H2OH (R = H, Me,
and tBu) were investigated. In all cases monomeric iron phenoxo half-sandwich complexes [Cp′FeOR′] (4-R) are initially
formed. Rearrangement of 4-R to the diamagnetic oxocyclohexadienyl complex [Cp′Fe(η5-OC6H2R′2R″)] (5-R) is observed
for 2,6-(Me3C)2(4-R)C6H2OH (R = H and Me) and the Gibbs free enthalpy of activation (ΔG⧧) was determined. In contrast
this rearrangement is inhibited when the 4-position is blocked by a tBu group. Removing the steric bulk from the 2,6-positions
leads to the formation of a μ-phenoxo dimer, [Cp′Fe(μ-OC6H3tBu2-3,5)]2 (5). Density functional theory (DFT) was used to
further elucidate the structure−reactivity relationship in these molecules. The one-legged piano stool anilido complex
[Cp′Fe(NHC6H2tBu3-2,4,6)] (7) is not accessible via acid−base reaction between 2a and H2NC6H2tBu3-2,4,6, but can be
prepared by conventional salt metathesis reaction from [Cp′FeI]2 and [Li(NHC6H2tBu3-2,4,6)(OEt2)]2. In contrast, reaction of
2a with Ph2NH yields the bimetallic [Cp′Fe(N,C-κ1,η5-C6H5NPh)Fe(N-κ1-NPh2)Cp′] (8) which combines two iron centers in
the same oxidation state (+2), but different spin-states (S = 0 and S = 2) which is reflected in very different Cp(cent)−Fe
distances of 1.68 and 2.04 Å, respectively. A monomeric iron alkyl half-sandwich complex [Cp′FeCH(SiMe3)2] (9) was prepared
that exhibits no reactivity toward H2, C2H4 or N2O. This behavior might be rationalized by a spin-state induced reaction barrier.
However, 9 reacts in the presence of CO to the iron acyl-complex [Cp′Fe(CO)2(C(O)CH(SiMe3)2)] (10) and with a CO/H2
mixture [Cp′Fe(CO)2]2 (11) and CH2(SiMe3)2 are formed.
these complexes stems from the effective blocking of the
dismutation pathway. Sitzmann and co-workers reported the
synthesis of [(nCp)FeBr]2 (5Cp = C5(CHMe2)5, 4Cp =
C5H(CHMe2)4 and Cp′ = 1,2,4-(Me3C)3C5H2) without
additional donor ligands several years ago.25,26 These
complexes can be isolated as stable compounds, and they
react with substituted phenolates to yield monomeric and
dimeric complexes depending on the steric bulk of the
phenolate ligand.26 Aryl ligands connected to the Cp′Fe-
fragment have been reported to undergo σ/π-rearrange-
ments,27−29 whereas the related [5CpFe(C5H3iPr2-2,6)] com-
plex behaves as a single-molecule magnet.30 [(C5Me5)FeN-
(SiMe3)2] belongs to the same class of iron one-legged piano
stools31 and exhibits promising synthetic potential.32,33 In
general, silylamide metal complexes can readily deprotonate
more acidic substrates, and they have been used to generate
homo- and heteroleptic metal complexes.34−44
INTRODUCTION
■
Low-coordinate iron complexes have been explored as possible
models systems for the active sites in metalloenzymes, and
many groups have made important contributions in this
area.1−4 Sterically demanding ligands are a common motive
in these investigations since they allow highly unusual
molecules to be synthesized and their chemistry to be
explored.3,5−15 Ligands of choice have been based on
scorpionates or Schiff-base ligands because of their excellent
donor capabilities and highly modular ligand architectures
which allow for easy variations of the steric and electronic
properties.
However, in contrast to the rich and versatile ruthenium half-
sandwich chemistry,16−24 low-coordinate, open-shell mono-
(cyclopentadienyl) iron complexes have not been explored
extensively. The competitive formation of thermodynamically
favored ferrocene, [(C5H5)2Fe], required the use of sterically
demanding alkylcyclopentadienyl ligands for the isolation and
stabilization of low-coordinate high spin mono-
(cyclopentadienyl) iron(II) complexes. The kinetic stability of
Received: August 13, 2012
Published: October 15, 2012
© 2012 American Chemical Society
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Scheme 1
On the basis of this precedence, we have recently initiated an
active research program focusing on the reactivity of [Cp′FeI]2
(1) and its potential in the synthesis of synthetically attractive
target molecules. During these investigations we have shown
that 1 is a good synthon for [Cp′Fe]+ transfer to Ir pincer
complexes,45 and for the synthesis of iron polyhydrides,
[Cp′2Fe2H3] and [Cp′FeH2]2. In addition, [Cp′FeH2]2 acts
as Fe(I) synthon which efficiently activate white phosphorus
(P4).46 Furthermore, 1 can be used for the synthesis of
[Cp′Fe{N(SiMe3)2}] (2a) which reacts with water in an acid−
base reaction to yield the trimeric iron hydroxo complex,
[Cp′Fe(μ-OH)]3.47 In this contribution, we focus on the
synthesis of additional one-legged piano stools of the type
“Cp′FeX”, where X = phenolato, anilido, and alkyl groups, and
discuss their structure and reactivity.
Figure 1. Chemical shift (δ) vs T−1 plot for 2a and 2b.
RESULTS AND DISCUSSION
■
1
rotation along the Cp′(cent)−Fe and Fe−N axis the H NMR
spectrum of 2b is consistent with a time-averaged Cs symmetry
at ambient temperature (Chart 1). Decreasing the temperature
Iron Amides. Complex 1 is an excellent starting material for
monomeric 14 valence electron (VE) iron amide complexes
such as [Cp′Fe{N(SiMe3)(R)}] (R = SiMe3 (2a),47 CMe3
(2b)) which are readily prepared by salt metathesis between 1
and [Li{N(SiMe3)(R)}(OEt2)]2 in aliphatic solvents such as
pentane (Scheme 1). They are very soluble in all common
organic solvents, but can be obtained as yellow crystals by
sublimation or crystallization from concentrated (Me3Si)2O
solutions at −30 °C.
Chart 1
Complexes 2a and 2b are thermally quite robust and melt at
122−124 °C and 112−114 °C, respectively. These compounds
are monomeric in solid state and gas phase and can be sublimed
1
in diffusion pump vacuum at 60−70 °C. However, the H
NMR spectra of 2a and 2b in C6D6 and C4D8O are distinctly
different from those of [(C5Me5)FeN(SiMe3)2], for which a
spin state change has been observed in solution depending on
the solvent.31 Complexes 2a and 2b contain high spin iron(II)
centers with four unpaired electrons (S = 2) as determined by
solution magnetic susceptibility studies yielding magnetic
moments of 5.4(2) μB and 5.3(2) μB, respectively, at 290 K
independent of solvent, and exhibit well-resolved paramagnetic
chemical shifts in the range +100 to −40 ppm (see Supporting
Information, Table S1, and Experimental Section for details). In
general, 1H NMR spectroscopy of paramagnetic mono-
(cyclopentadienyl) iron complexes is a very useful tool to
distinguish monomeric from dimeric/trimeric species since
they exhibit very distinct chemical shift patterns (Supporting
Information, Table S1).
As expected for isostructural molecules 2a and 2b exhibit
very similar temperature dependences of the chemical shifts
(Figure 1). However, there is a slight curvature to the
temperature dependence of the chemical shifts corresponding
to the Cp′-tBu groups especially at low temperatures. The
nonlinearity of the δ vs T−1 (Figure 1) might be caused by a
hindered rotation of the Cp′-ring because of the sterically
demanding N(SiMe3)(R) groups, and 2b is ideal to evaluate
this hypothesis. As a result of fast (on the NMR times scale)
slows this rotation, and the two tBu groups (A and B) undergo
decoalescence (Figure 1) with a barrier of ΔG⧧(Tc = −55 °C)
= 8 kcal/mol is derived for this process.
In an attempt to evaluate the relative binding strength of
[N(SiMe3)(CMe3)]− and [N(SiMe3)2]− to the [Cp′Fe]+
fragment a C6D6 solution of 2a, 2b, [Li{N(SiMe3)(CMe3)}-
(OEt2)]2 and [Li{N(SiMe3)2}(OEt2)]2 was transferred in an
NMR tube. No exchange on the NMR time scale was observed,
but when this mixture was heated at 65 °C for 25 days and
equilibrium was established with Keq = 1.6(4) (Scheme 2).
Hence there is a slight thermodynamic preference of the
[Cp′Fe]+ fragment for the more nucleophilic [N(SiMe3)-
(CMe3)]− moiety.
However, complexes 2a and 2b are exceedingly sensitive
toward oxygen and moisture, and the reaction of 2a with
stoichiometric quantities of H2O yielded the trimeric iron
hydroxo complex [Cp′Fe(μ-OH)]3 (3) (Scheme 3).47
Iron Phenoxides. The reaction of 2a with H2O was quite
encouraging and motivated us to investigate other Brønsted
acids such as sterically demanding phenol (pKa ca. 18) and
amine derivatives (PhNH2, pKa ca. 30; Ph2NH, pKa ca. 25).48
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Scheme 2. Chemical Exchange between 2a, 2b, [Li{N(SiMe3)(CMe3)}(OEt2)]2, and [Li{N(SiMe3)2}(OEt2)]2
Scheme 3. Reactivity of 2a with Various Brønsted Acids
Scheme 4. Rearrangement of 4-R to 5-R
Complexes 2a and 2b should react with these more acidic
substrates in an acid−base reaction, and this approach
represents a synthetic alternative to a salt metathesis.
Furthermore the starting materials and reaction products are
(in most cases) soluble in C6D6, so the reaction progress can
conveniently be monitored by NMR spectroscopy and the
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Table 1. Crystallographic Data
compound
4-CMe3
C35H59FeO
551.67
monoclinic
21.309(4)
16.214(3)
9.5786(16)
90.00
5-Me
6
8
9
chemical formula
formula mass
C32H52FeO
508.59
C62H100Fe2O2
989.12
C58H78Fe2N2
914.92
C24H48FeSi2
448.65
crystal system
monoclinic
12.6864(4)
15.0289(4)
15.2002(4)
90.00
monoclinic
11.1695(5)
14.3936(6)
19.9752(8)
90.00
monoclinic
10.9446(2)
38.0888(7)
13.1366(2)
90.00
monoclinic
9.8466(10)
14.7897(16)
18.8919(19)
90.00
a/Å
b/Å
c/Å
α/deg
β/deg
92.013(5)
90.00
97.573(2)
90.00
93.7720(10)
90.00
113.7010(10)
90.00
94.933(6)
90.00
γ/deg
unit cell volume/Å3
3307.4(10)
100(2)
C2/m
2872.83(14)
100(2)
P2(1)/n
4
3204.4(2)
100(2)
P2(1)/n
2
5014.32(15)
100(2)
P2(1)/c
4
2741.0(5)
100(2)
P2(1)/c
4
temperature/K
space group
no. of formula units per unit cell, Z
4
radiation type
MoKα
MoKα
MoKα
CuKα
CuKα
absorption coefficient, μ/mm−1
no. of reflections measured
no. of independent reflections
Rint
0.479
0.546
0.488
4.915
5.272
9845
40816
18861
72431
35321
3060
8759
9281
9264
5039
0.0666
0.0603
0.1561
0.0924
0.1694
1.081
0.0662
0.0325
0.0499
0.0540
final R1 values (I > 2σ(I))
final wR(F2) values (I > 2σ(I))
final R1 values (all data)
final wR(F2) values (all data)
goodness of fit on F2
0.0425
0.0491
0.0443
0.0340
0.0913
0.1307
0.1035
0.1048
0.0674
0.0708
0.0495
0.0415
0.1022
0.1399
0.1057
0.1193
1.025
1.052
1.142
1.061
stability of in situ generated species can be evaluated. The
reaction of 2a with substituted phenols is instantaneous, and
the solution color changes immediately. NMR spectroscopy
indicates the formation of monomeric or dimeric phenoxo
complexes depending on the substitution pattern of the
phenols (Scheme 3). Monomeric κ1-O 14VE species are
initially formed when the 2,6-positions of the phenols are
blocked by sterically demanding CMe3-groups that prevent
dimer formation. However, the stability of these monomeric
[Cp′FeOR] complexes is very sensitive toward the para-
substitution at the phenol (Scheme 4): While in all cases, R =
H, Me, and CMe3, the monomeric 14VE [Cp′FeOR]
complexes are initially formed (as confirmed by a similar
solution VT-NMR behavior; see Supporting Information,
Figures S1−S4 for details), only in the case of R = CMe3 can
the one-legged piano stool actually be successfully isolated. It is
noteworthy that no significant spin-density is transferred to the
para-position of the phenoxo ligand, since p-H and p-Me
substituted derivatives exhibit the same temperature depend-
ence, that is, the dipolar contributions to the chemical shift
dominate the Fermi-contact terms.49
order kinetics with similar activation barriers ΔG⧧(312 K) =
23.4(1) (R = H) and 23.7(1) (R = Me) kcal mol−1 (see
Experimental Section and Supporting Information for details).
The slightly increased barrier for 4-Me compared to 4-H may
be caused by steric rather than electronic effects.51
The monomeric 4-CMe3 and the rearranged oxocyclohex-
adienyl sandwich complexes, 5-H/Me (see Experimental
Section for details) were isolated, and the molecular structures
of 4-CMe3 and 5-Me were determined by single crystal X-ray
diffraction (Table 1, Figures 2 and 3). Selected bond distances
and angles are given in the Figure captions. The long Fe-
Cp(cent) distance of 1.88 Å in 4-CMe3 is comparable to the
ones observed for 1 (1.93 Å),45 2a-DMAP (2.05 Å),47 3 (1.97
Å),47 and [(C5Me5)FeN(SiMe3)2] (1.90 Å).31 It is indicative of
a high spin Fe(II) center (S = 2) and can be compared to ca.
The formation of either κ1-O or η5-oxocyclohexadienyl
coordination has previously been observed for iron half-
sandwich complexes,26 but it was not clear from these studies if
a monomeric κ1-O complex precedes the formation of the η5-
oxocyclohexadienyl compound, for example, for ruthenium it is
postulated that the dimeric complex [(C5Me5)Ru(μ-OR)]2 is
involved in the rearrangement.19,50 However, in our case the
one-legged piano stools 4-H/Me rearrange in solution and in
solid state to form the diamagnetic oxocyclohexadienyl
complexes 5-H/Me (Scheme 4). While the rearrangement in
solid state is not very clean and also significant amounts of
insoluble material are obtained, this transformation is clean in
solution and accompanied by a color change from orange-red
to purple. The conversions of 4-H/Me → 5-H/Me were also
Figure 2. ORTEP diagram of 4-CMe3 (ellipsoids at 50% probability
level). Selected bond distances (Å) and angles (deg): Cp(cent)−Fe1
1.88, Fe1−O1 1.811(4), O1−C6 1.345(6), Cp(Cent)−Fe1−O1 175.1,
Fe1−O1−C6 144.0(3).
1
followed by H NMR spectroscopy and found to obey first-
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Figure 3. ORTEP diagram of 5-Me (ellipsoids at 50% probability
level). Selected bond distances (Å) and angles (deg): Cp(cent)−Fe1
1.72, Fe1−Cp(plane) 1.717, Fe1−dienyl(plane) 1.635, Fe1−C61
2.4421(15), Fe1−C62 2.2098(14), Fe1−C63 2.1029(14), Fe1−C64
2.1221(16), Fe1−C65 2.0988(16), Fe1−C66 2.2115(16), C61−O1
1.2478(19), C61−C62 1.469(2), C62−C63 1.414(2), C63−C64
1.407(2), C64−C65 1.411(2), C61−C66 1.472(2), ∠ Cp(plane)−
dienyl(plane) 0.71.
Figure 4. ORTEP diagram of 6 (ellipsoids at 50% probability level).
Selected bond distances (Å) and angles (deg): Cp(cent)−Fe(1) 1.94,
Fe(1)−O(1) 1.9881(12), Fe(1)−O(1a) 2.0115(13), Fe(1)···Fe(1a)
3.06, O(1)−Fe(1)−O(1a) 80.03(5), Fe(1)−O(1)−Fe(1a) 99.97(5).
1.7 Å for closed-shell (S = 0) iron complexes such as 5-Me,
[Cp′2Fe],45 and other iron half-sandwich complexes.52 This
notion is further supported by the solution magnetic
susceptibility data for 4-CMe3 of 5.3(2) μB (at 300 K).
However, when the CMe3-groups are moved from the ortho-
(2,6)- to the meta-(3,5)-positions the steric hindrance is no
longer sufficient to stabilize a monomeric species, and the green
dimer 6 is formed in good yield (Scheme 3). Because of the low
solubility of 6 in aromatic and aliphatic solvents, the formation
is rapid, and no intermediate (such as a monomeric [Cp′FeOR]
species) was observed by NMR spectroscopy. Complex 6 is
thermally quite robust (mp 218−221 °C (rev.)) and shows a
molecular ion in the EI-MS spectrum. The solution magnetic
moment of 6 was determined with the Evans method yielding a
magnetic moment (per iron center at 290 K) of 5.5(2) μB (cf.
[Cp′FeI]2: 5.3(2) μB).45 No rearrangement at elevated
temperature to an oxocyclohexadienyl complex or exchange
with free phenol was observed in solution in contrast to
[(C5Me5)Ru(μ-OR)]2.19,50 Possible explanations for this
reactivity difference may include the more sterically hindered
environment at the Fe atom in 6, the different spin states in
both complexes and the reduced arenophilicity of the [Cp′Fe]+
fragment45 when compared to [(C5Me5)Ru]+. The δ vs T−1
plot is linear in the temperature range −100 °C to +100 °C and
suggests that electron exchange coupling between the two
Fe(II) (S = 2) centers is small and that no dissociation of the
Fe2O2 core into monomeric fragments occurs in solution (see
Supporting Information, Figures S6 and S7 for details). Strong
Fe−O bonds have also been observed for [Fe(OC6H2tBu3-
2,4,6)2]2.35
available from 2a, the analogue anilido species 7 must be
synthesized by salt metathesis from 1 and [Li(NHC6H2tBu3-
2,4,6)(OEt2)]2. The origin of this reactivity difference is mainly
due to steric reasons, since less encumbered aniline derivatives
(such as p-toluidine) with similar pKa values yield dimeric
complexes of two different isomers.53 It is interesting to note
that no reaction was observed between 2a or 2b with a bulky β-
diketimine (Hnacnac = H2C{C(Me)(2,6-i-Pr2C6H3N)}2) even
at elevated temperatures (65 °C) for a prolonged period of
time. In contrast, [(nacnac)FeN(SiMe3)2] is accessible via
acid−base reaction between Hnacnac and [Fe{N(SiMe3)2}2]2
at 110−120 °C under solvent-free conditions.39 Unfortunately,
no suitable single crystals for an X-ray diffraction experiment
were obtained, but the VT-NMR study for [Cp′FeNH-
(C6H2(CMe3)3-2,4,6)] (7) reveals a similar temperature
dependence as the phenolate complex 4-CMe3 which is
expected for structurally related complexes (see Supporting
Information, Figure S8 for details). The solution magnetic
susceptibility data for 7 (5.3(2) μB at 298 K) further supports
this notion. No resonances corresponding to the anilido NH
1
and the Cp′ ring-CH protons were found in the H NMR
spectrum presumably because of extreme line broadening.
However, an N−H stretching vibration was observed at 3380
cm−1 in the IR spectrum of 7.
In general, amido transition metal complexes can be used for
the synthesis of terminal imido complexes and two different
strategies have been employed in this context: (a) concerted
proton-coupled electron transfer (PCET) and (b) stepwise
oxidation and proton abstraction. The PCET approach has
precedence in the work by Hillhouse54 and Smith.55 Attempts
to deprotonate 7 with [LiN(SiMe3)2(OEt2)]2 failed, and
instead an equilibrium mixture of 2a, 7, and [Li-
(NHC6H2(CMe3)3-2,4,6)(OEt2)]2, and [LiN(SiMe3)2(OEt2)]2
is established in C6D6 solution, Keq(298 K) = 34(2) (Scheme
5). This shows a slight thermodynamic preference of the
[Cp′Fe]+ fragment for the [N(SiMe3)2]− group compared to
the [NHC6H2(CMe3)3-2,4,6)]− moiety. For comparison, no
The solid state structure of 6 is shown in Figure 4, and
selected bond distances and angles are given in the Figure
caption. The two halves of the dimer are related by a center of
symmetry. Thus the Fe2O2 core is required to be planar. The
internal ring angle at the iron atom is 80.03(5)° and 99.97(5)°
at the oxygen atom. Overall the structural features are related to
[Fe{N(SiMe3)2}{μ-OC6H2tBu3-2,4,6}]2.35 The high spin con-
figuration at the iron atom is reflected in the Cp(cent)−Fe
distance of 1.94 Å.
reaction was observed for [(nacnacMe)Fe(NHtol)] (nacnacMe
=
Iron Anilido. Interestingly, whereas monomeric phenoxo
systems using sterically encumbered phenols are readily
HC{C(Me)(2,6-i-Pr2C6H3N)}2) and strong bases such as
LiN(SiMe3)2 and NaOtBu.56 The lack of reactivity might
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Scheme 5. Synthesis and Reactivity of 7
again be related to steric hindrance in the transition state of
proton transfer. However, on addition of HOC6H2(CMe3)3-
2,4,6 to 7 the monomeric iron phenoxo complex 4-CMe3 was
formed. Addition of C2H4I2 leads to the formation of the
unusual 15VE iron(III) half-sandwich complex [Cp′FeI2].57
Since this phenol can participate in a proton transfer reaction it
is likely to assume that the stable supermesitylene phenoxo
radical, [2,4,6-(Me3C)3C6H2O]·, might be suitable for a
concerted PCET, and attempts in this directions are ongoing.
In contrast, 2a reacts with HNPh2 at elevated temperatures
to a red crystalline compound that is nearly insoluble in
aliphatic and aromatic hydrocarbons, but decomposes in
CH2Cl2. Although [PhB(CH2PiPr2)3]− and [Cp′]− are
isoelectronic, their reactivity with the [Ph2N]− ligand differs
distinctly. In the case of [PhB(CH2PiPr2)3]− the monomeric
high spin (S = 2) iron amide complex [{PhB(CH2PiPr2)3}Fe-
(NPh2)] was isolated,58 whereas [Cp′Fe(N,C-κ1,η5-C6H5NPh)-
Fe(N-κ1-NPh2)Cp′] (8) is formed with Cp′ in which a cationic
[Cp′Fe]+ fragment coordinates to an anionic N,N-phenyl-
cyclohexadienyl-imine ligand, and a neutral [Cp′FeNPh2]
fragment is coordinated to the imino functionality (Scheme 3
and Figure 5). For comparison the homoleptic dimer
[Fe(NPh2)2]2 shows Fe−N distances of 1.895(3) and
2.036(3) Å for the terminal and bridging NPh2 groups,
respectively.59 More importantly, 8 combines in one molecule
two iron centers in the same oxidation state (+2), but different
spin-states (S = 0 and S = 2) which is nicely reflected in very
different Cp(cent)−Fe distances of 1.68 and 2.04 Å,
respectively. Only one other complex was reported that
contains a comparable structural feature. In this case a lithium
ion is coordinated to the O-donor atoms of a bis(μ,κ-O-2,6-
dimethylphenolato)(tetraisopropylcyclopentadienyl)ferrate(II)-
ion and the oxocyclohexadienyl of an iron sandwich complex.26
Figure 5. ORTEP diagram of 8 (ellipsoids at 50% probability level).
Selected bond distances (Å) and angles (deg): Cp(cent)−Fe1 2.04,
Cp(plane)−Fe1 2.036, Cp(cent)−Fe2 1.68, Cp(plane)−Fe2 1.683,
Fe1−N1 2.1494(18), N1−C200 1.436(3), N1−C206 1.333(3), Fe1−
N2 2.047(2), N2−C100 1.387(3), N2−C106 1.398(3), C206−C207
1.441(3), C207−C208 1.414(3), C208−C209 1.406(3), C210−C211
1.409(3), C211−C206 1.434(3), Fe2-dienyl(plane) 1.563, Fe2−C206
2.427(2), Fe2−C207 2.124(2), Fe2−C208 2.054(2), Fe2−C209
2.076(2), Fe2−C210 2.060(2), Fe2−C211 2.128(2), ∠ Cp(plane)−
dienyl(plane) 1.89.
However, a similar rearrangement from an amido to an imino
ligand was observed by Hillhouse for a [(NHC)Ni-
(NHC6H3(iPr)2-2,6)] (NHC = N-hetereocyclic carbene)
upon oxidation.60 Unfortunately, the slow reaction and the
lack of solubility of 8 make it difficult to elucidate the reaction
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Scheme 6. Synthesis and Reactivity of 9
mechanism, for example, to confirm the initial formation of a
one-legged piano stool vs dimeric complex or to obtain any
kinetic information on this rearrangement process.
sensitive to trace amounts of moisture and oxygen. The Evans′
moment in solution of μeff(292 K) = 5.0(2) μB confirms the
high spin configuration with 4 unpaired spins (S = 2). Variable
1
temperature H NMR studies for 9 were undertaken and a
Iron Alkyls. Two alternative entries into the synthesis of
“Cp′FeH” complexes can be considered either by hydro-
genation of a “Cp′FeR” precursor or salt methathesis of 1 with
KHBEt3. Both approaches have precedents in the work by
Peters and Holland for the synthesis of low-coordinate Fe−H
molecules. Peters and co-workers demonstrated that a
[{PhB(CH2PiPr2)3}FeMe] complex can react in the presence
of a suitable phosphine trap to generate [{PhB(CH2PiPr2)3}-
Fe(H)(PMe3)] and [{PhB(CH2PiPr2)3}Fe(H)3(PMe3)], re-
spectively, while in the absence of such traps the iron hydride
reacts with aromatic solvents to form cyclohexadienyl-capped
Fe(II) complexes.61,62 In contrast, Holland and co-workers
showed that despite the fact that [(nacnac)Fe-R] species
(nacnac = HC{C(CMe3)(2,6-i-Pr2C6H3N)}2) are accessible
and have also a S = 2 ground state like the phosphinoborate
complex, the nacnac complexes do not react with H2, but under
an atmosphere of CO insertion into the Fe-R bond occurs.63
However, we have previously reported on the reactivity of 1
with KHBEt3 and showed that initially formed “Cp′FeH”
species is unstable and degrades to [Cp′2Fe2H3] and
[Cp′FeH2]2 under an argon atmosphere, while in the presence
of H2 the formation of [Cp′FeH2]2 becomes more favorable.46
Here, we want to investigate mono(cyclopentadienyl) iron alkyl
complexes and their reactivity toward small molecules. Several
attempts to react 1 and MeLi to prepare a stable “[Cp′FeMe]”
derivative failed and only the cleavage of the Cp′Fe-fragment
accompanied by the formation of [Li(OEt2)][Cp′] was
detected. This is in agreement with previous reports that
showed that Cp′ is not inert to substitution when reacted with
organolithium reagents.64,65
deviation from linearity in the δ vs T−1 plot was noted (Figure
6). We attribute this observation to a hindered rotation of the
Figure 6. Chemical shift (δ) vs T−1 plot for 7.
Cp′ fragment relative to the bulky alkyl group changing the
dipolar contribution to the chemical shift.49 However, the
barrier is relatively low, and no decoalescence behavior was
observed. An ORTEP diagram of 9 is shown in Figure 7, and
selected bond distances and angles are given in the figure
caption. The molecular structures of [nCpFe(σ-C6H3iPr2-2,6)]
(nCp = 4Cp29 and 5Cp30) were reported, and the metric
parameters agree well with those presented in this work.
Despite the high reactivity of 9 toward trace amounts of
oxygen and moisture, it proved to be surprisingly unreactive.
No reaction was observed with N2O (1 atm), C2H4 (1 atm), or
H2 (1 atm) at ambient temperature even after 7 days. Since the
reaction of 9 with H2 is formally spin-forbidden several
attempts to induce a spin-state change at room temperature
have been undertaken, for example, on addition of a good σ-
donating ligand such as 4-N,N-dimethylaminopyridine
(DMAP) in the presence of H2. While in the presence of an
However, the use of a sterically more demanding alkyl group
such as LiCH(SiMe3)2 allowed the isolation of the monomeric
14VE complex, [Cp′FeCH(SiMe3)2] (9) (Scheme 6). Complex
9 is soluble in all common organic solvents, and thermally quite
robust. No degradation occurred when 9 was heated in C6D6
solution at 65 °C for 30 days, while its stability in solid state
was exemplified by reversible melting point behavior, and on
sublimation under diffusion pump vacuum at 60−65 °C. In
addition, a molecular ion of 9 is observed in the EI-MS
spectrum ([M+] = 448 amu). As expected, 9 is exceedingly
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iron formyl-complex are subject to further investigations at low
temperature and will be reported in due course.
COMPUTATIONAL RESULTS
■
Density functional theory (DFT) calculations in concert with
experimental data have become valuable tools in organometallic
chemistry. Especially for larger molecules and clusters of low
symmetry, the low computational expense compared to post-
Hartree−Fock methods makes DFT very often the only option
when the electronic structure of complex systems needs to be
evaluated. However, the exact functional of the electron density
is unknown and only approximate expressions are available;
hence an exhaustive evaluation of different Density Functionals
(DFs) is required. We have previously evaluated a series of
modern hybrid and hybride meta DFs and found that the best
agreement between experimental and calculated structures have
been achieved with the dispersion-corrected B97 functional,
B97D71 in combination with the 6-311G(d,p) basis set.46,52
Encouraged by this previous success we investigated computa-
tionally selected complexes for which structural and magnetic
data were available at the B97D/6-311G(d,p) level of theory
and found that the computed geometries of these complexes
agree very well with the experimental X-ray diffraction data (see
Supporting Information, Table S2 for details). We also
extended our computational study to molecules for which no
structural data were available and addressed the question of
spin states and relative energy of these spin states (Figure 8 and
Figure 7. ORTEP diagram of [Cp′FeCH(SiMe3)2] (ellipsoids at 50%
probability level). The hydrogen atom on C60 was located in the
Fourier difference map and refined isotropically. Selected bond
distances (Å) and angles (deg): Cp(cent)−Fe(1) 1.91, C(60)−Si(1)
1.8680(19), C(60)−Si(2) 1.8651(18), Fe(1)−C(60) 2.035(2),
C(60)−H(100) 0.93(3), Cp(cent)−Fe(1)−C(60) 172.8. No short
contacts or agostic bonds are observed in the crystal structure.
excess of DMAP a mono-DMAP adduct, 9*(DMAP) is
formed, rapid exchange between free and bound DMAP was
observed on the NMR time scale. This behavior is reminiscent
of the weak DMAP adduct to 2a, 2a*(DMAP).47 However, no
reaction with H2 was detected suggesting that DMAP binding is
weak and does not induce the necessary spin-state change to
facilitate the hydrogenolysis of the Fe-alkyl bond. Overall, 9
behaves analogous to the 12VE [(nacnac)FeR] systems.63 The
influence of spin-states on reaction mechanisms and rate
constants has been subject to intense debates over the years.
However, there has been a growing number of examples and
computational studies supporting the notion that spin-states
can indeed affect the reaction rate.66,67 This was exemplified by
the slow addition of H2 to [W{N(CH2CH2NSiMe3)3}H] which
is a “spin-blocked” reaction with a high reaction barrier due to
crossing between the reactant triplet and the product singlet
potential energy surfaces.68
Hence, the reactivity of 9 toward a good π-acceptor ligand
such as CO and a CO/H2 mixture (ratio 30:70) was
investigated. The reaction of 9 in pentane solution under CO
(1 atm) proceeds smoothly to give the iron acyl-complex
[Cp′Fe(CO)2(C(O)CH(SiMe3)2] (10). The presence of a κ1-
acyl coordination mode (instead of η2-acyl) is verified by the
13C{1H} NMR data. Crystals of moderate quality were grown
from concentrated hexamethyldisiloxane solution at −38 °C.
Although the quality of the structure precludes a detailed
discussion of bond distances and angles, the connectivity was
clearly established and a ball-stick representation of 10 is shown
in Supporting Information, Figure S11. Complex 10 is a rare
example of CO insertion into a M-CH(SiMe3)2 moiety, but it
has been observed for group 4 transition metal complexes.69
The reaction of 10 with a mixture of CO and H2 (30:70) at
ambient temperature proceeds differently. While CH2(SiMe3)2
was detected as one of the side products of this transformation,
the only isolated product was the known carbonyl complex
[Cp′Fe(CO)2]2 (11).70 This implies that the hydrogenolysis of
the Fe-R bond is feasible in the presence of a good π-acceptor
ligand such as CO which induces the required spin-state change
for the reaction with H2 to proceed. However, the reaction
mechanism and the intermediacy of other species such as an
Figure 8. Calculated structures for several iron one-legged piano
stools.
see Supporting Information for details). For [(C5Me5)FeN-
(SiMe3)2] a low and high spin state was observed depending on
the solvent.31 In contrast for 2a the high spin state prevails in
solid state and solution independent of solvent.47 Calculations
at the B97D/6-311G(d,p) level of theory also favor the high
spin state for 2a by ΔG(298 K) = 12.1 kcal/mol in agreement
with the experimental data. The related alkyl 9 and phenoxo
complexes 4-CMe3 exhibit a similar stabilization of the high
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spin state with ΔG(298 K) = 17.3 and 9.8 kcal/mol,
respectively.
The next question of interest was the relative bond strength
of Fe−N(SiMe3)2 vs Fe-NHR and Fe-OR bonds. From the
experiments discussed above the following relative order is
expected Fe-OR ≫ Fe−N(SiMe3)2 ≥ Fe-NHR and DFT
calculations predict the following free enthalpies (ΔG):
2a + HOC6H2tBu3‐2,4,6 → 4‐CMe3 + HN(SiMe3)2 ;
ΔG(298 K) = −16.7 kcal/mol
2b + HOC6H2tBu3‐2,4,6 → 4‐CMe3 + HN(SiMe3)(CMe3);
ΔG(298 K) = −18.1 kcal/mol
9 + HOC6H2tBu3‐2,4,6 → 4‐CMe3 + H2C(SiMe3)2 ;
ΔG(298 K) = −28.6 kcal/mol
Figure 9. Free energy diagram, ΔG0(298 K), for the rearrangement 4-
R → 5-R.
2a + H2NC6H2tBu3‐2,4,6 → 7 + HN(SiMe3)2 ;
ΔG(298 K) = −4.5 kcal/mol
thermoneutrality for R = CMe3 whereas the reaction is
exergonic for R = H and Me substitution.72
2b + H2NC6H2tBu3‐2,4,6 → 7 + HN(SiMe3)(CMe3);
ΔG(298 K) = −5.9 kcal/mol
CONCLUSIONS
■
9 + H2NC6H2tBu3‐2,4,6 → 7 + H2C(SiMe3)2 ;
ΔG(298 K) = −16.4 kcal/mol
The one-legged piano stool iron amido complexes [Cp′Fe{N-
(SiMe3)(R)}] (R = SiMe3 (2a) and CMe3 (2b)) are excellent
starting materials for salt-free metathesis reactions with more
acidic substrates. However, the outcome of the reaction is
determined by the steric demand of the phenols, and
monomeric and dimeric phenoxo complexes are formed. In
the case of 2,6-di-t-butyl substituted phenols paramagnetic
monomeric complexes (4-R) are initially observed which
rearrange to diamagnetic oxocyclohexadienyl complexes (5-R)
when the 4-position carries an H or a Me group. However, tBu-
substitution in 4-position stabilizes the one-legged piano stool
fragment. Alternatively, 3,5-di-t-butyl substituted phenol yields
a paramagnetic (monocyclopentadienyl) iron phenoxo dimer
[Cp′Fe(μ-OC6H3tBu-3,5)]2 (6). With aniline derivatives the
reaction is less straightforward and the one-legged piano stool
anilido complex [Cp′Fe(NHC6H2tBu3-2,4,6)] (7) is not
accessible via acid−base reaction, but can be prepared by
conventional salt metathesis. In contrast, reaction of 2a with
Ph2NH yields the bimetallic [Cp′Fe(N,C-κ1,η5-C6H5NPh)Fe-
(N-κ1-NPh2)Cp′] (8) which combines two iron centers in the
same oxidation state (+2), but different spin-states (S = 0 and S
= 2). An alkyl iron one-legged piano stool [Cp′FeCH(SiMe3)2]
(9) can be prepared, but despite being low-coordinate and
possessing only 14VE no reactivity was observed with ethylene,
N2O or H2 even at elevated temperatures. This behavior is
most likely traced to a spin-state introduced reaction barrier
which precludes the reaction between the high-spin iron center
and the closed shell substrate. In the presence of strong field
ligands such as CO or CO/H2 mixtures the iron(II) acyl
complex [Cp′Fe(CO)2(C(O)CH(SiMe3)2)] (10) and the
iron(I) carbonyl [Cp′Fe(CO)2]2 (11) are formed, respectively.
Additional reactivity studies and the quest for iron multiple-
bonded species are ongoing and will be reported in due course.
In addition we are currently investigating the issue of spin-state
induced reaction barriers in these mono(cyclopentadienyl) iron
complexes and the consequences thereof.
7 + HOC6H2tBu3‐2,4,6 → 4‐CMe3 + H2NC6H2tBu3‐2,4,6;
ΔG(298 K) = −12.2 kcal/mol
On the basis of these results the reaction of 2a/2b with
H2NC6H2(CMe3)3-2,4,6 is slightly exergonic and should
proceed at room temperature. While there is a methodological
uncertainty associated with DFT methods and open-shell
systems, which is usually on the order of a few kcal/mol, the
predicted trends are generally correct. In addition, we attribute
the experimentally observed lack of reactivity to the steric
hindrance at the metal center which prevents efficient proton
transfer in the transition state. This is consistent with the
experimental fact that the reaction proceeds with less
substituted aniline derivatives. In these cases an additional
driving force is provided by the formation of dimeric μ-anilido
complexes. For [Cp′Fe(μ-OC6H3tBu2-3,5)]2 (6) the dimeriza-
tion of two monomeric [Cp′Fe(μ-OC6H3tBu2-3,5)] (S = 2)
fragments is quite exergonic with ΔG0(298 K) = −34.5 kcal/
mol (see Supporting Information for details).
We also evaluated the energetics for the transformation 4-R
→ 5-R in which sterically encumbered groups in the para-
position of the phenolate ligand prevent this rearrangement.
Figure 9 shows the Gibbs free energy profile. The calculations
are in good qualitative agreement with the experiments, for
example, the transition states connecting 4-R (S = 2) → 5-R (S
= 2) react sensitive to the para-substitution. The transition state
converting 5-R (S = 2) → 5-R (S = 0) has not been located, but
the additional intersystem crossing barrier is most likely
responsible for the larger barrier (ΔG⧧ = ca. 23 kcal/mol)
observed experimentally. However, as a results of steric
repulsion the final products 5-R (S = 0) differ significantly in
their thermodynamic stability compared to the starting
materials 4-R (S = 2), for example, the reaction is close to
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(9H, SiMe3, ν1/2 = 1100 Hz), −27.4 (18H, Cp′-CMe3, ν1/2 = 810 Hz),
−40.0 (9H, Cp′-CMe3, ν1/2 = 630 Hz). IR (Nujol mull; CsI windows;
cm−1): 1365s, 1240s, 1200s, 1110w, 1038m, 1005s, 960m, 865vs,
830brs, 780s, 750m, 722w, 680m, 670m, 632w, 600vw, 545m, 530m,
490m, 450m, 430w, 380s, 350vw, 320br.w, 260m, 230vbr.w. Anal.
Calcd. for C24H47NSiFe: C, 66.49; H, 10.93; N, 3.23. Found: C, 66.03;
H, 11.03; N, 3.15.
EXPERIMENTAL SECTION
■
General Considerations. All reactions and product manipulations
were carried out under an atmosphere of dry, oxygen free argon or
dinitrogen using standard high-vacuum, Schlenk, or drybox techniques.
Argon was purified by passage through BASF R3-11 catalyst
(Chemalog) and 4 Å molecular sieves. Dry, oxygen-free solvents
were employed throughout. NMR spectra were recorded on Bruker
DRX 500 MHz, a Bruker DRX 400 MHz, or a Bruker 400 MHz
AVANCE spectrometer. All chemical shifts are reported in δ units with
reference to the residual protons of the deuterated solvents, which are
internal standards, for proton chemical shifts. Probe temperatures were
calibrated using ethylene glycol and methanol as previously
described.73 The elemental analyses were performed by the analytical
facilities at the University of California at Berkeley or at the TU
Braunschweig. Crystallographic data were also deposited with
Cambridge Crystallographic Data Centre. Copies of the data
(CCDC 893606−893610) can be obtained free of charge via http://
www.ccdc.cam.ac.uk/data_request/cif by e-mailing data_request@
ccdc.cam.ac.uk, or by contacting The Cambridge Crystallographic
Data Centre, 12, Union Road, Cambridge CB 1EZ, U.K.; fax +44 1223
336033.
Materials. All solvents were deoxygenated and dried by passage
over columns of activated alumina.74 Tetrahydrofuran was dried over
sodium/benzophenone and freshly distilled prior to use. Deuterated
solvents, CD2Cl2, C6D6, C7D8, cyclohexane-d12 and methylcyclohex-
ane-d14, were purchased from Cambridge Laboratories, Inc., refluxed
for 3 days over sodium metal (with the exception of CD2Cl2 which was
dried over CaH2 for 4 days), vacuum transferred to a Teflon sealable
Schlenk flask containing 4 Å molecular sieves, and degassed via three
freeze−pump−thaw cycles. [Cp′FeI]2, [Li(NHC6H2tBu3-2,4,6)-
(OEt2)]2, [LiCH(SiMe3)2], and [Li{N(SiMe3)(CMe3)}(OEt2)]2
were prepared according to literature procedures.45,75−77
[Cp′Fe(O-κ1-OC6H2tBu3-2,4,6)] (4-CMe3). A mixture of 2a (0.18 g,
0.4 mmol) and freshly sublimed HOC6H2tBu3-2,4,6 (0.104 g, 0.4
mmol) was dissolved in n-hexane (ca. 15 mL) and stirred at room
temperature 20 min to form an orange-red solution. The volume was
reduced to about 3 mL, and slow cooling to −25 °C yielded red
1
crystals of 4-CMe3 (0.09 g, 0.16 mmol, 41%). Mp 165−170 °C. H
NMR (C7D8, 290 K): δ 118.5 (2H, m-CH, ν1/2 = 360 Hz), 27.6 (9H,
p-CMe3, ν1/2 = 50 Hz), −13.8 (18H, o-CMe3, ν1/2 ∼ 1800 Hz), −30.7
(9H, Cp′-CMe3, ν1/2 ∼ 800 Hz), −33.3 (18H, Cp′-CMe3, ν1/2 ∼ 1060
Hz) (the two Cp-CMe3 resonances overlap). IR (Nujol mull; CsI
windows; cm−1): 1430w, 1300vw, 1270br.s, 1245s, 1200m, 1170vw,
1160vw, 1127w, 1025vw, 1005vw, 895w, 880w, 858m, 830m, 780w,
760w, 680w, 650w, 580s, 490br.w, 475br.w. Anal. Calcd. for
C35H58OFe: C, 76.34; H, 10.62. Found: C, 75.98; H, 10.42.
NMR Experiments of 2a with HOC6H3tBu2-2,6 and
HOC6H2tBu2Me. In a screw-cap NMR tube 2a (22.0 mg, 0.05
mmol) and HOC6H2tBu2R (10.3 mg (R = H) or 11.0 mg (R = Me))
were dissolved in C7D8. A color change from yellow to orange-red was
observed on mixing, and the 1H NMR spectra of 4-H and 4-Me were
recorded.
4-H: 1H NMR (C7D8, 289 K): δ 116.6 (2H, m-CH, ν1/2 ∼ 400 Hz),
−8.3 (9H, p-H, ν1/2 = 115 Hz), −12.7 (18H, o-CMe3, ν1/2 ∼ 2130
Hz), −29.9 (9H, Cp′-CMe3, ν1/2 ∼ 980 Hz), −32.1 (18H, Cp′-CMe3,
ν1/2 ∼ 1060 Hz) (the two Cp-CMe3 resonances overlap).
1
4-Me: H NMR (C7D8, 301 K): δ 118.5 (2H, m-CH, ν1/2 = 440
Hz), 103.4 (3H, p-Me, ν1/2 = 230 Hz), −15.8 (18H, o-CMe3, ν1/2
∼
All other chemicals were purchased from Acros Organics or Sigma
Aldrich. The substituted phenols and aniline derivatives were sublimed
prior to use. The solution magnetic susceptibility was determined
using a modified78 Evans method.79
2240 Hz), −30.5 (9H, Cp′-CMe3, ν1/2 ∼ 920 Hz), −34.2 (18H, Cp′-
CMe3, ν1/2 ∼ 1200 Hz) (the two Cp-CMe3 resonances overlap).
[Cp′Fe(η5-OC6H3tBu2-2,6)] (5-H). A mixture of 2a (0.18 g, 0.4
mmol) and freshly sublimed HOC6H3tBu2-2,6 (0.082 g, 0.4 mmol)
was dissolved in n-hexane (ca. 15 mL) and stirred at room temperature
to form an orange-red solution of 4-H. The reaction mixture was
heated to 45 °C for 16 h, and during this time the color changed to
deep purple. The reaction mixture was filtered, concentrated, and
cooled to yield deep purple crystals of 5-H. Yield: 0.15 g (0.3 mmol,
Synthesis. [Cp′FeN(SiMe3)2] (2a).47 A mixture of [Cp′FeI]2 (1)
(1.41 g, 1.7 mmol) and [Li{N(SiMe3)2}(OEt2)]2 (0.81 g, 3.4 mmol)
was dissolved in about 20 mL of pentane, and the reaction mixture
turned immediately bright yellow. The solution was stirred for 10 min
and filtered. The solvent was removed under reduced pressure, and the
yellow residue was sublimed in diffusion pump vacuum at 60−70 °C
1
3
76%). H NMR (C6D6, RT): δ 5.46 (2 H, d, JHH = 5.8 Hz, m-CH),
to give yellow crystalline material (1.06 g, 2.36 mmol, 69%). Mp 122−
3
1
5.29 (1H, t, JHH = 5.8 Hz, p-CH), 4.17 (2H, Cp′-CH), 1.65 (18H, s,
124 °C (rev.). H NMR (C6D6, 292K): δ 44.9 (18H, SiMe3, ν1/2
=
o-CMe3), 1.43 (9H, s, Cp′-CMe3), 1.21 (18H, s, Cp′-CMe3). 13C{1H}
NMR (C6D6, RT): δ 157.8 (CO), 104.9 (Cp′-C(4)), 102.2 (o-C),
93.4 (Cp-C(1,2)), 84.4 (m-CH), 68.6 (Cp′-CH), 66.5 (p-CH), 35.8 (o-
CMe3), 33.5 (6C, CH3), 32.6 (2C, CMe3), 32.4 (3C, CH3), 31.9 (1C,
CMe3), 31.3 (CH3). IR (Nujol mull; KBr windows; cm−1): 1552s,
1513m, 1486m, 1316w, 1251br.m, 1240br.m, 1197vw, 1170w, 1110m,
1090m, 1023m, 995w, 945w, 919vw, 870s, 853m, 824vw, 806vbr.m,
741m. Anal. Calcd. for C31H50OFe: C, 75.28; H, 10.19. Found: C,
74.95; H, 10.15.
1100 Hz), −30.3 (18H, CMe3, ν1/2 = 1050 Hz), −42.6 (9H, CMe3,
1
ν1/2 = 700 Hz). H NMR (thf-d8, 292K): δ 43.8 (18H, SiMe3, ν1/2
=
690 Hz), −29.0 (18H, CMe3, ν1/2 = 700 Hz), −41.6 (9H, CMe3, ν1/2
1
= 570 Hz). H NMR (py-d5, 292K): δ 18.3 (18H, SiMe3, ν1/2 = 640
Hz), −13.6 (18H, CMe3, ν1/2 = 430 Hz), −24.1 (9H, CMe3, ν1/2
=
1
540 Hz). H NMR (CD2Cl2, 292K): δ 43.5 (18H, SiMe3, ν1/2 = 980
Hz), −29.2 ppm (18 H, Cp′-CMe3, ν1/2 = 900 Hz), −42.0 ppm (9 H,
Cp′-CMe3, ν1/2 = 630 Hz). IR (Nujol mull; CsI windows; cm−1):
1365m, 1260m, 1249vs, 1205vw, 1160vw, 1100vbr.vw., 1005sh,
970brvs, 875vs, 852m, 830brs, 798sh, 752w, 723w, 670s, 637vw,
615w, 545vw, 450br.w, 362vs, 270vw, 238s. Anal. Calcd. for
C23H47NSi2Fe: C, 61.43; H, 10.53; N, 3.12. Found: C, 61.17; H,
10.21; N, 2.95. The E.I. mass spectrum showed a molecular ion at m/e
= 449 amu. The parent ion isotopic cluster was simulated: (calcd. %,
observd. %): 447(6, 6), 448(2, 3), 449(100, 100), 450(38, 39),
451(14, 13), 452(3, 2).
[Cp′Fe(η5-OC6H2MetBu2-2,6)] (5-Me). A mixture of 2a (0.18 g,
0.4 mmol) and freshly sublimed HOC6H2MetBu2 (0.088 g, 0.4 mmol)
was dissolved in n-hexane (ca. 15 mL) and stirred at room temperature
to form an orange-red solution of 4-Me. The reaction mixture was
heated to 45 °C for 16 h, and during this time the color changed to
deep purple. The reaction mixture was filtered, concentrated, and
cooled to yield deep purple crystals of 5-Me. Mp 140−144 °C. Yield:
1
0.16 g (0.31 mmol, 79%). H NMR (C7D8, RT): δ 5.33 (2 H, s, m-
[Cp′FeN(SiMe3)(CMe3)] (2b). A mixture of 1 (1.41 g, 1.7 mmol) and
[Li{N(SiMe3)(CMe3)}(OEt2)]2 (0.76 g, 1.7 mmol) was dissolved in
about 20 mL of pentane, and the reaction mixture turned immediately
bright yellow. The solution was stirred for 10 min and filtered. The
solvent was removed under reduced pressure, and the yellow residue
was crystallized from O(SiMe3)2 (ca. 5 mL) at −30 °C to yield bright
yellow blocks. Complex 2b was sublimed slowly in a sealed ampule at
60−70 °C. Yield: 0.92 g (2.12 mmol, 62%). Mp 112−114 °C (rev.).
1H NMR (C6D6, 292K): δ 96.0 (9H, CMe3, ν1/2 = 1840 Hz), 50.0
CH), 4.21 (2H, s, Cp′-CH), 2.18 (3H, s, p-CH3), 1.61 (18H, s, o-
CMe3), 1.41 (9H, s, Cp′-CMe3), 1.25 (18H, s, Cp′-CMe3). 13C{1H}
NMR (C6D6, RT): δ 157.7 (CO), 104.6 (Cp′-C(4)), 98.5 (o-C),
94.6 (Cp′-C(1,2)), 84.3 (m-CH), 82.6 (p-CMe), 68.4 (Cp′-CH), 35.8
(o-CMe3), 33.6 (6C, CH3), 32.8 (2C, CMe3), 32.4 (1C, CMe3), 32.2
(3C, CH3), 31.5 (CH3), 21.9 (p-CH3). IR (Nujol mull; KBr windows;
cm−1): 1563s, 1536s, 1482m, 1312w, 1258sh, 1247s, 1237sh, 1215m,
1201w, 1158s, 1119m, 1107m, 1030br.m, 995w, 946w, 926w, 865m,
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Inorganic Chemistry
Article
810sh, 799m, 774w, 741w. Anal. Calcd. for C32H52OFe: C, 75.57; H,
10.31. Found: C, 75.32; H, 10.22.
m/e = 448 amu. The parent ion isotopic cluster was simulated: (calcd.
%, observd. %): 446(6, 5), 447(2, 3), 448(100, 100), 449(39, 40),
450(14, 14), 451(3, 2).
[Cp′Fe(μ-OC6H3tBu2-3,5]2 (6). A mixture of 2a (0.18 g, 0.4 mmol)
and freshly sublimed HOC6H3tBu2-3,5 (0.082 g, 0.4 mmol) was
dissolved in n-hexane (ca. 15 mL) and stirred at room temperature 20
min to form a green solution. The volume was reduced to about 3 mL
and slow cooling to −25 °C yielded dark green crystals of 6*C6H12
(0.13 g, 0.12 mmol, 61%). The crystals contained one molecule of
hexane per dimer which was verified by 1H NMR and elemental
analysis. Mp 218−221 °C, accompanied with some decomposition. 1H
NMR (C7D8, 290K): δ 99.6 (2H, Cp-CH/o-H, ν1/2 = 1200 Hz), −3.4
(18H, Cp′-CMe3, ν1/2 = 220 Hz), −9.4 (9H, Cp′-CMe3, ν1/2 = 190
Hz), −19.1 (18H, m-CMe3, ν1/2 = 27 Hz), −49.7 (1H, p-H, ν1/2 = 30
Hz), −105.0 (2H, o-H Cp-CH, ν1/2 ∼ 500 Hz) IR (Nujol mull; CsI
windows; cm−1): 1600m, 1582m, 1425m, 1320sh, 1300m, 1260w,
1242m, 1235sh, 1215m, 1205m, 1160w, 1120m, 1030w, 1050w, 965
vs, 900w, 880w, 870vs, 822sh, 820vs, 692w, 685m, 642m, 605s, 580vw,
550vw, 465br.s, 405br.s, 365vw, 350br.w. Anal. Calcd. for
C68H114Fe2O2: C, 75.95; H, 10.69. Found: C, 75.59; H, 10.34. The
E.I. mass spectrum showed a molecular ion at m/e = 988 amu. The
parent ion isotopic cluster was simulated: (calcd. %, observd. %):
986(12, 11), 987(9, 9), 988(100, 100), 989(74, 72), 990(29, 28),
991(8, 7), 992 (2,1)
[Cp′Fe(CO)2(C(O)CH(SiMe3)2)] (10). [Cp′FeCH(SiMe3)2] (9) (0.12
g, 0.27 mmol) was dissolved in 10 mL of pentane. The headspace was
evacuated and replaced with CO (1 atm). The solution was stirred at
ambient temperature for 20 h, and the solvent was removed under
dynamic vacuum. The yellow residue was dissolved in hexamethyldi-
siloxane (ca. 4 mL) and filtered. The yellow-orange solution was
concentrated and cooled to −30 °C to give yellow blocks (85 mg, 0.16
1
mmol, 60%). H NMR (C6D6, RT): δ 4.56 (s, ring-CH, 2H), 3.22 (s,
CH(SiMe3)2, 1H), 1.32 (s, CMe3, 18H), 1.27 (s, CMe3, 9H), 0.32 (s,
SiMe3, 18H). 13C{1H} NMR (C6D6, 300 K): δ 250.4 (1C,
C(O)CH(SiMe3)2), 219.2 (2 C, CO), 110.7 (2C, ring-CCMe3),
110.4 (1C, ring-CCMe3), 94.2 (2C, ring-CH), 66.7 (1C, CH(SiMe3)2),
33.7 (6C, CH3), 32.8 (2C, CMe3), 31.7 (6C, CH3), 31.3 (1C, 6C,
CH3), 1.9 (3C, SiMe3). Anal. Calcd. for C23H48OSi2Fe: C, 61.03; H,
10.69. Found: C, 61.32; H, 10.76.
[{Cp′Fe(CO)2}2] (11). [Cp′FeCH(SiMe3)2] (9) (0.12 g, 0.27 mmol)
was dissolved in 10 mL of pentane. The headspace was evacuated and
replaced with a 7:3 mixture of H2 and CO (1 atm). The solution
changed color from yellow to red-violet and was stirred at ambient
temperature for 20 h. The solvent was removed under dynamic
vacuum. The dark red-violet residue was dissolved in pentane, filtered,
concentrated, and cooled to −30 °C to give red-violet crystals (0.04 g,
0.06 mmol, 42%). IR (toluene; KBr windows; cm−1): 1934 (s, CO),
1765 (s, CO). 1H NMR (C6D6, RT): δ 4.80 (s, ring-CH, 4H), 1.37 (s,
CMe3, 36H), 1.35 (s, CMe3, 9H). Anal. Calcd. for C38H58Fe2O4: C,
66.13; H, 8.47. Found: C, 66.33; H, 8.56. Complex 11 was previously
prepared by another synthetic route.80
[Cp′Fe(N-κ1-NHC6H2tBu3-2,4,6)] (7). A mixture of 1 (0.38 g, 0.46
mmol) and [Li(NHC6H2tBu3-2,4,6)(OEt2)]2 (0.32 g, 0.47 mmol) was
dissolved in about 20 mL of pentane. The reaction mixture turned
yellow-brown immediately, and a colorless precipitate was formed.
After the mixture was stirred for 10 min, the solvent was removed
under dynamic vacuum. The residue was extracted with O(SiMe3)2
(ca. 4 mL), filtered, and the filtrate was concentrated to about 3 mL
and cooled to −25 °C. Two crops of dark orange-brown crystals were
Kinetic Study for the Conversion of 4-H/Me to 5-H/Me. In a
screw-cap NMR tube, 2a (22.0 mg, 0.05 mmol) and HOC6H2tBu2R
(10.3 mg (R = H) or 11.0 mg (R = Me)) were dissolved in C6D6 (500
μL). As internal standard 1,3,5-(F3C)3C5H3 (5 μL) was added to the
C6D6 solution. Complexes 4-H/Me formed rapidly upon mixing as
indicated by a color change from light yellow to orange-red. The NMR
sample was inserted into a temperature-calibrated probe, and the
1
isolated. Yield: 0.28 g (0.51 mmol, 55%). Mp 124−134 °C (dec.). H
NMR (C6D6, 290 K): δ 108.2 (2H, m-CH, ν1/2 = 420 Hz), 24.1 (9H,
p-CMe3, ν1/2 = 45 Hz), 15.1 (18H, o-CMe3, ν1/2 ∼ 2500 Hz), −24.7
(9H, Cp-CMe3, ν1/2 ∼ 740 Hz), −26.7 (18H, Cp-CMe3, ν1/2 ∼ 960
Hz) (the two Cp-CMe3 resonances overlap). IR (Nujol mull; CsI
windows; cm−1): 3380s (N−H), 1765br.vw., 1610br.vw., 1380br.s,
1365s, 1350sh, 1262m, 1245s, 1200m, 1165m, 1120w, 1035br.w,
1005w, 960w, 924br.w., 885sh, 880s, 840s, 825s, 820sh, 775m, 755m,
720br.w., 690vw, 678m, 640m, 600vw, 580s, 545br.w., 495br.w,
480br.w., 450w., 380br.w, 260vbr.vw. Anal. Calcd. for C35H59NFe: C,
76.47; H, 10.82. Found: C, 76.03; H, 10.92.
1
rearrangement of 4-H/Me to 5-H/Me was monitored by H NMR
spectroscopy over approximately 2 half-lives. The decay of the m-CH
resonance for 4-H/Me was converted to concentration and fitted to a
first-order plot of ln [4-H/Me] vs time, which gave observed rate
constants as the slope. Sample graphs of the kinetic data can be found
in the Supporting Information, Figures S9 and S10.
Computational Details. All calculations employed the long-range
dispersion-corrected Grimme′s functional (B97D).71 The calculations
were carried out with Gaussian 09 and no symmetry restrictions
imposed (C1). C, H, O, N, and Fe were represented by an all-electron
6-311G(d,p) basis set. Unrestricted calculations were performed for all
paramagnetic species studied. The nature of the extrema (minima and
transition states) was established with analytical frequencies
calculations. The zero point vibration energy (ZPE) and entropic
contributions were estimated within the harmonic potential
approximation. The Gibbs free energy, ΔG, was calculated for T =
298.15 K and 1 atm. Geometrical parameters were reported within an
accuracy of 10−3 Å and 10−1 degrees.
[Cp′Fe(N,C-κ1,η5-C6H5NPh)Fe(N-κ1-NPh2)Cp′] (8). A mixture of 2a
(0.18 g, 0.4 mmol) and freshly sublimed Ph2NH (0.068 g, 0.4 mmol)
was dissolved in toluene (ca. 5 mL) and heated to 65 °C for 20 h
(without stirring) to form a dark red solution from which small a
amount of a red crystalline material had already precipitated. The
solution was allowed to cool to ambient temperature, and the volume
of the solution was reduced to about 3 mL. Slow cooling to −25 °C
yielded additional dark red crystals of 8. Total yield: 0.09 g (0.10
mmol, 49%). Mp 162−182 °C (dec.). IR (Nujol mull; KBr windows;
cm−1): 1583m, 1532s, 1482m, 1343m, 1282s, 1241w, 1204w, 1173w,
1150vw, 1077vw, 1023w, 996w, 950vw, 915vw, 871w, 818m, 753m,
698m, 671w, 560vs, 548vs, 520s, 490m. Anal. Calcd. for C58H78N2Fe2:
C, 76.14; H, 8.59; N, 3.06. Found: C, 75.77; H, 8.34; N, 3.15.
[Cp′FeCH(SiMe3)2] (9). A mixture of 1 (0.7 g, 0.84 mmol) and
LiCH(SiMe3)2 (0.33 g, 2.0 mmol) was dissolved in about 60 mL of
pentane. The reaction mixture turned yellow-orange immediately.
After the mixture was stirred for 2 h, the solution was filtered and the
pentane was removed under reduced pressure. The yellow-orange
residue was sublimed in diffusion pump vacuum at 60−65 °C to give
golden, crystalline material (0.50 g, 1.1 mmol, 66%). Mp 110−112 °C
ASSOCIATED CONTENT
■
S
* Supporting Information
Crystallographic information files (CIF), VT-NMR data, details
of the DFT calculations and mol2-files of all calculated
structures, and comparison of experimental and computational
structural data. This material is available free of charge via the
Internet at http://pubs.acs.org.
1
(rev.). H NMR (C6D6, 290 K): δ 38.7 (18H, SiMe3, ν1/2 = 810 Hz),
−35.3 (27H, CMe3, ν1/2 = 1030 Hz, two resonances overlapped). IR
(Nujol mull; CsI windows; cm−1): 2815w, 1610br.vw., 1370sh, 1362s,
1348sh, 1285w, 1255m, 1245vs, 1205m, 1165sh, 1150w, 1110vw,
1035sh, 1010s, 955w, 920vw, 830vbr.vs, 775sh, 770s, 732s, 722sh,
680vs, 665vs, 610vs, 545m, 488s, 445m, 410vw, 370m, 340w, 280sh,
268m, 230s. Anal. Calcd. for C24H48Si2Fe: C, 64.25; H, 10.78. Found:
C, 63.99; H, 10.84. The E.I. mass spectrum showed a molecular ion at
AUTHOR INFORMATION
■
Corresponding Author
*E-mail: mwalter@tu-bs.de.
11870
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Article
(26) Wallasch, M.; Wolmershauser, G.; Sitzmann, H. Angew. Chem.
2005, 117, 2653.
̈
Notes
The authors declare no competing financial interest.
(27) Wallasch, M. W.; Rudolphi, F.; Wolmershauser, G.; Sitzmann,
̈
H. Z. Naturforsch. B 2009, 64, 11.
ACKNOWLEDGMENTS
(28) Wallasch, M. W.; Vollmer, G.; Kafiyatullina, A.; Wolmershauser,
̈
■
G.; Jones, P. G.; Mang, M.; Meyer, W.; Sitzmann, H. Z. Naturforsch. B
We thank the Alexander von Humboldt Foundation for a
Feodor-Lynen Fellowship (M.D.W.) and Prof. Maurice
Brookhart for providing financial support (through NSF
Grant CHE-0615704) and laboratory facilities (M.D.W.)
during the initial phase of this research program. M.D.W.
gratefully acknowledges the current financial support by the
Deutsche Forschungsgemeinschaft (DFG) through the Emmy-
Noether program (WA 2513/2-1).
2009, 64, 18.
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(72) There is a methodological uncertainty on the order of a few
kcal/mol associated with DFT methods and open-shell systems. The
dispersion-corrected Grimme B97D functional tends, in some cases, to
overestimate dispersion and noncovalent interactions. Therefore the
more crowded (complex) oxocyclohexadienyl structure might be
overly stabilized. To exclude this phenomenon we also computed the
energies of 4-CMe3 and 5-CMe3 with BP86 and B3LYP which does
not take dispersion effects into account (see Supporting Information
for details). The values obtained with BP86 are in complete
disagreement with the experiment, since the oxocyclohexadienyl 5-
CMe3 is predicted to be significantly more stable than 4-CMe3 by
about 10 kcal/mol. In contrast B3LYP is in agreement with the
experimental observation, but 4-CMe3 is definitely overstabilzed
compared to 5-CMe3 with about 35 kcal/mol. The energetics obtained
with B97D and B3LYP were also compared for the following
reactions:
2a + H2NC6H2tBu3‐2,4,6 → 7 + HN(SiMe3)2 ;
ΔGB97D(298 K) = −4.5 kcal/mol;
ΔGB3LYP(298 K) = −1.3 kcal/mol
2b + H2NC6H2tBu3‐2,4,6 → 7 + HN(SiMe3)(CMe3);
ΔGB97D(298 K) = −5.9 kcal/mol;
ΔGB3LYP(298 K) = −5.9 kcal/mol
2a + Li[N(SiMe3)(CMe3)] ⇌ 2b + Li[N(SiMe3)2];
ΔGcalc,B97D(338 K) = −5.9 kcal/mol;
ΔGcalc,B3LYP(338 K) = −1.9 kcal/mol;
ΔGexp(338 K) = −0.32 kcal/mol
7 + Li[N(SiMe3)2] ⇌ 2a + Li[NHC6H2tBu3‐2,4,6];
ΔGcalc,B97D(298 K) = −0.5 kcal/mol;
ΔGcalc,B3LYP(298 K) = −0.5 kcal/mol;
ΔGexp(298 K) = −2.1 kcal/mol
Overall while the absolute values predicted by B3LYP and B97D differ,
both DFT functionals predict the same trends and energetic
preferences.
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dx.doi.org/10.1021/ic301770f | Inorg. Chem. 2012, 51, 11860−11872