Iron(III) Half-Sandwich Complexes of the Two-Legged Piano-Stool [CpFe(aryl)Cl] Type from the Corresponding Aryliron(II) Precursors

Article abstract of DOI:10.1002/ejic.201601026

Instead of reacting with the σ-mesityliron(II) complex [Cp′′′Fe(σ-C₆H₂Me₃-2,4,6)] (1; Cp′′′ = 1,2,4-tri-tert-butylcyclopentadienyl) to form a cyclohexadienyl(cyclopentadienyl)iron(II) sandwich complex with palladium coordination to the ipso carbon atom of the six-membered ring, palladium(II) chloride oxidized 1 to the mesityliron(III) complex [Cp′′′Fe(σ-C₆H₂Me₃-2,4,6)Cl] (5) with a two-legged piano-stool geometry. The oxidation of [⁴CpFe(σ-C₆H₂Me₃-2,4,6)] (4;⁴Cp = tetraisopropylcyclopentadienyl) to [⁴CpFe(σ-C₆H₂Me₃-2,4,6)Cl] (6) was accomplished with hexachloroethane. Two other derivatives, [⁴CpFe(σ-C₆H₃iPr₂-2,6)Cl] (7) and [⁵CpFe(σ-C₆H₃iPr₂-2,6)Cl] (8;⁵Cp = pentaisopropylcyclopentadienyl), could also be obtained by the same method. The molecular structures of 5 and 6 are compared with the structure of the (tetraisopropylcyclopentadienyl)iron(III) dibromide [⁴CpFeBr₂] (9), which was formed from the iron(II) analogue [⁴CpFe(μ-Br)]₂during attempted nucleophilic substitution with an arylmagnesium bromide.

Full text of DOI:10.1002/ejic.201601026

DOI: 10.1002/ejic.201601026
Full Paper
Paramagnetic Organometallics
Iron(III) Half-Sandwich Complexes of the Two-Legged Piano-
Stool [CpFe(aryl)Cl] Type from the Corresponding Aryliron(II)
Precursors
Heiko Bauer,[a] Mark W. Wallasch,[a] Gotthelf Wolmershäuser,[a] Yu Sun,[a] and
Helmut Sitzmann*[a]
Abstract: Instead of reacting with the σ-mesityliron(II) complex plished with hexachloroethane. Two other derivatives, [4CpFe-
[Cp′′′Fe(σ-C6H2Me3-2,4,6)] (1; Cp′′′ = 1,2,4-tri-tert-butylcyclo- (σ-C6H3iPr2-2,6)Cl] (7) and [5CpFe(σ-C6H3iPr2-2,6)Cl] (8; 5Cp =
pentadienyl) to form a cyclohexadienyl(cyclopentadienyl)iron(II) pentaisopropylcyclopentadienyl), could also be obtained by the
sandwich complex with palladium coordination to the ipso same method. The molecular structures of 5 and 6 are com-
carbon atom of the six-membered ring, palladium(II) chloride pared with the structure of the (tetraisopropylcyclopentadi-
oxidized 1 to the mesityliron(III) complex [Cp′′′Fe(σ-C6H2Me3- enyl)iron(III) dibromide [4CpFeBr2] (9), which was formed from
2,4,6)Cl] (5) with a two-legged piano-stool geometry. The oxid- the iron(II) analogue [4CpFe(μ-Br)]2 during attempted nucleo-
4
ation of [4CpFe(σ-C6H2Me3-2,4,6)] (4; Cp = tetraisopropylcyclo- philic substitution with an arylmagnesium bromide.
pentadienyl) to [4CpFe(σ-C6H2Me3-2,4,6)Cl] (6) was accom-
Introduction
Aryliron(II)–cyclopentadienyl (Cp) complexes with carbonyl li-
gands of the [CpFe(CO)2(aryl)] type have been known for
60 years, and the first example was derived from the reaction
of the iodo complex [(C5H5)Fe(CO)2I] with phenylmagnesium
bromide.[1] More recently, aryltin or arylzinc compounds have
also been used as sources for aryl ligands,[2–4] and the catalytic
activity of such complexes for the hydrogenation of 2-pyridyl-
methanol has been demonstrated.[5] The first aryl(cyclopentadi-
enyl)iron(II) complex without additional donor ligands was the
paramagnetic and highly reactive (alkylcyclopentadienyl)-
(mesityl)iron complex [Cp′′′Fe(σ-C6H2Me3-2,4,6)][6] (1; Cp′′′ =
1,2,4-tri-tert-butylcyclopentadienyl; Scheme 1). Homo- and
heterodinuclear half-open ferrocene derivatives are available
from 1 by the addition of a Lewis acid MLn (MLn = CuCl, AlEt3,
or other reactive metal complexes with low coordination num-
bers and valence-electron counts). Examples of the general
formula [Cp′′′Fe(μ,η5:η1-C6H2Me3-2,4,6)MLn] with a bridging tri-
methylcyclohexadienyl-ylidene ligand, which can also be re-
garded as a metal-substituted π-arene ligand, are shown in
Scheme 1. When a phenyl ligand was introduced instead of a
mesityl ligand, the dimerization of the aryliron intermediate to
the diiron complex [(Cp′′′Fe)2(μ,η5:η5-H5C6=C6H5)] with
a
bridging bis(cyclohexadienyl-ylidene) ligand was observed
(Scheme 1).
[a] Fachbereich Chemie, TU Kaiserslautern
Scheme 1. Top: σ/π rearrangement of the aryl ligand of the cyclopentadienyl-
iron(II) complex 1. In pentane, 1 adds a Cp′′′NiBr fragment. In toluene, the
same reaction proceeds with the replacement of the mesityl ligand with an
ortho-, meta-, or para-tolyl ligand.[6] Lewis acids such as the Cp′′′FeBr frag-
ment[6] or copper(I) chloride[7] are also added with aryl σ/π rearrangement.
Bottom: dimerization of an unsubstituted phenyl anion.[8]
Erwin-Schrödinger-Str. 54, 67663 Kaiserslautern, Germany
E-mail: sitzmann@chemie.uni-kl.de
https://www.chemie.uni-kl.de/sitzmann/ag-sitzmann/
ORCID(s) from the author(s) for this article is/are available on the WWW
under http://dx.doi.org/10.1002/ejic.201601026.
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Two facets of the reactivity of (σ-aryl)(cyclopentadienyl)iron
complexes are illustrated in Scheme 1, both of which result in
σ/π rearrangement: addition of Lewis acids or aryl dimerization.
In this manuscript, we describe another reaction pathway avail-
able to these aryliron(II) complexes: in addition to nucleophilic
aryl anion reactivity, in which the [CpFe(aryl)] species resembles
a Grignard reagent, the oxidation to novel iron(III) half-sandwich
compounds of the [RCpFe(aryl)X] type with a two-legged piano-
stool geometry was encountered and will be discussed below.
The starting compounds 1–4 used in this study are depicted in
Scheme 2.
Scheme 2. Four paramagnetic (alkylcyclopentadienyl)(aryl)iron(II) complexes
used as starting compounds: [Cp′′′Fe(σ-C6H2Me3-2,4,6)] (1),[6] [4CpFe-
(σ-C6H2Me3-2,4,6)] (2; this work), [4CpFe(σ-C6H3iPr2-2,6)] (3),[9] [5CpFe-
(σ-C6H3iPr2-2,6)] (4).[10]
Figure 1. Molecular structure of 2. Selected distances [Å] and angles [°]: Fe1–
Cpcent 1.896, Fe2–Cpcent 1.893, Fe3–Cpcent 1.888, Fe1–C6 2.030(5), Fe2–C6a
1.993(6), Fe3–C6b 2.012(8); Cpcent–Fe1–C6 174.6, Cpcent–Fe2–C6a 174.2,
Cpcent–Fe3–C6b 175.2.
As the σ-aryl complex 2 has not been described previously,
we report its synthesis and discuss its molecular structure be-
fore we proceed to the oxidation of these iron(II) compounds.
The mesityl ligands of 2 are oriented such that one methyl
group in an ortho position lies underneath the substitution gap
of the cyclopentadienyl ligand, and the other is underneath the
two adjacent isopropyl substituents pointing away from each
other. With a deviation of 4.8–5.4° from the ideal 180° angle
between the Cp centroids and the ipso-carbon atom, complex
2 can be considered to have a structure similar to an umbrella,
comparable to those of other (σ-aryl)(cyclopentadienyl)iron(II)
complexes such as 1,[6] [4CpFe(σ-C6H3iPr2-2,6)],[9] and [5CpFe-
(σ-C6H3iPr2-2,6)] (5Cp = pentaisopropylcyclopentadienyl).[10]
In an orienting experiment aiming at σ/π rearrangement
with a palladium-based Lewis acid, the mesityl complex 1 was
treated with palladium(II) chloride. From the black reaction so-
lution, the redox product [Cp′′′Fe(σ-C6H2Me3-2,4,6)Cl] was ob-
tained as a black oil, which was sparingly soluble in pentane
and highly soluble in toluene or tetrahydrofuran (THF;
Scheme 3).
Results and Discussion
During the reaction of the half-sandwich complex [4CpFe(μ-Br)]2
and mesitylmagnesium bromide the color of the reaction mix-
ture changed from orange-red to orange-yellow. The superna-
tant solution was separated from the solid, and the solvent was
evaporated under vacuum to yield 2 in nearly quantitative
yield. If further purification is needed, 2 can be recrystallized
from small amounts of pentane.
The σ-aryl complex 2 is extremely sensitive towards air and
moisture, and even solutions in noncoordinating solvents such
as toluene or pentane show decomposition products after sev-
eral hours without exposure to oxygen or moisture. The 1H NMR
spectrum shows the expected paramagnetic behavior of 2 with
broad signals in the range δ = 204 to –120 ppm, comparable
with the data obtained for the σ-aryl complex 1.[6] The mass
spectra and elemental analysis as well as the X-ray diffraction
data confirmed the formation of the expected product 2. The
concentration of a saturated pentane solution yielded yellow
crystals, which allowed for the determination of the molecular
structure (Figure 1). Complex 2 crystallizes in the unusual space
From a concentrated toluene solution layered with pentane,
a few black crystals were obtained. In the unit cell (space group
P21/c), one molecule of the chlorido(mesityl)iron(III) complex 5
was found with a distorted trigonal-planar geometry of the
iron–Cp centroid axis, the Fe–Cl bond, and the Fe–Cipso bond
(Figure 2).
The distance between the cyclopentadienide centroid and
group Im. Two molecules occupy general positions, and a third the iron atom is 1.871 Å. The Fe–C12 (Cipso) bond [1.969(4) Å]
molecule is located on a crystallographic mirror plane. The ge- is shorter than that (2.043 Å) of the aryliron(II) derivative 4[10]
ometries of the three molecules are almost identical, and pack- (Scheme 2), as expected from the different ionic radii of high-
ing effects may be responsible for the small structural varia- spin FeII and FeIII ions.[11] The FeIII–C bonds in N-heterocyclic
tions.
carbene complexes are even longer, for example, the Fe–CH3
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for the molecular ion, signals for large fragments of 5 could be
detected, for example, those derived from the loss of one
mesityl or one chlorido ligand in addition to signals of ligand
fragments such as [Cp′′′]+, [MesH]+, and [tBu]+.
The reducing properties of iron(II) salts in aqueous solution
are well known, and the facile oxidation to 5 can be understood
as a consequence of its high-spin d6 electron configuration,
which displays unpaired electrons in nonbonding or even anti-
bonding iron d orbitals. From this perspective, there should be
no need for a precious oxidizing agent. Complex 2, obtained
in situ from [4CpFe(μ-Br)]2 with mesitylmagnesium bromide in
pentane, could be oxidized by the addition of hexachloro-
ethane to the filtered reaction solution. Black crystals of the
chlorido(mesityl)iron(III) half-sandwich complex 6 were ob-
tained in good yield upon cooling (Scheme 4, Figure 3).
Scheme 3. Palladium(II) chloride did not rearrange the mesityl ligand to form
a heterodinuclear species but resulted in the oxidized aryliron complex 5.
Scheme 4. In situ preparation of the mesityl(tetraisopropylcyclopentadi-
enyl)iron(II) derivative 2 and subsequent oxidation with hexachloroethane to
form the iron(III) half-sandwich complex 6.
Figure 2. Molecular structure of 5. Selected distances [Å] and angles [°]:
Fe–Cpcent 1.871, Fe–Cl 2.1756(12), Fe–C12 1.969(4); Cpcent–Fe–Cl 135.1,
Cpcent–Fe–C12 127.2, Cl–Fe–C12 97.52(12).
distances are 2.060(11) Å in the 17-valence-electron iron(III)
complex [Cp*Fe(η3-C3H5)(CH3)][12] and 2.090(2) Å in the 1,3-
diisopropylimidazol-2-ylidene complex of iron(III) chloride.[13]
The tert-butyl substituent of C-4 is located directly above the
mesityl moiety and bent out of the plane of the cyclopentadi-
Figure 3. Molecular structure of the chlorido(mesityl)(tetraisopropylcyclo-
pentadienyl)iron(III) derivative 6. Selected distances [Å] and angles [°]:
Fe–Cpcent 1.898, Fe–Cl 2.1864(9), Fe–C6 2.012(3); Cpcent–Fe–Cl 133.86,
Cpcent–Fe–C6 127.92, Cl–Fe–C6 97.83(9).
1
enyl ring by ca. 14.4° for steric reasons. The H NMR spectrum
shows broad signals between δ = 121.30 and –19.37 ppm, in
agreement with the expected paramagnetic behavior of the
The Fe–Cpcent distance of 1.898 Å is slightly longer than that
iron(III) complex 5 with a 15-valence-electron count. Although for the Cp′′′ derivative 5 (1.871 Å; cf. Table 1) and indicates the
4
the electron ionization mass spectrum did not display a signal higher steric demand of the Cp ligand compared with that of
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Table 1. Comparison of the distances [Å] and angles [°] in 5, 6, and 9.
Distance/angle
5 (X1 = Cl, X2 = mesityl)
6 (X1 = Cl, X2 = mesityl)
9 (X1 = Br1, X2 = Br2)
Fe–Cpcent
Fe–X1
Fe–X2
Cpcent–Fe–X1
Cpcent–Fe–X2
X1–Fe–X2
1.871
2.1756(12)
1.969(4)
135.1
127.2
97.52(12)
1.898
2.1864(9)
2.012(3)
133.86
127.92
97.83(9)
1.883
2.3204(16)
2.3246(17)
131.30
122.91
105.73(6)
the Cp′′′ ligand. The two structures illustrate the cone-angle has only four owing to the higher symmetry of the cyclopenta-
concept for cyclopentadienyl ligands;[14] the bulk of 4Cp was dienyl ligand.
From the reaction of [4CpFe(μ-Br)]2 with biphenyl-4,4′-diyl-
associated with an average cone angle of 146.4°, and that for
Cp′′′ was 132°. Although the individual Cp substituents are the
bis(magnesium bromide), the expected dinuclear biaryldiiron(II)
compound could not be detected. Instead, only a few red crys-
tals could be isolated and characterized by X-ray diffraction. The
crystallographic data revealed mononuclear dibromido(tetraiso-
propylcyclopentadienyl)iron(III) (9; Figure 4).
4
bulkier tert-butyl groups for Cp′′′ and isopropyl groups for Cp,
the higher number of substituents is important here. The Cp′′′
ligand with two substitution gaps offers more space for the two
ortho methyl groups of the mesityl ligand and, hence, suffers
less from steric crowding (Figure 2). The tetraisopropylcyclo-
pentadienyl ligand offers one gap above one mesityl methyl
substituent. The second methyl group points between two iso-
propyl substituents. These are rotated away from each other to
provide as much space as possible, but this is not as effective
4
as a second ring CH position. Therefore, the Cp ligand is bulk-
ier than Cp′′′ in this example and in other cases.
In an analogous experiment, the (diisopropylphenyl)iron(II)
analogue [4CpFe(σ-C6H3iPr2-2,6)] (3) was generated in situ and
oxidized with hexachloroethane to afford black cubes from the
pentane reaction solution at –30 °C. The 1H NMR spectrum
showed many broad and partially overlapping signals in the
range between δ = 175.50 and –145.73 ppm. The elemental
analysis agrees with the expected conversion to the iron(III)
complex [4CpFe(σ-C6H3iPr2-2,6)Cl] (7). The poor-quality diffrac-
tion data allow the identification of 7 as a monomeric aryl-
iron(III) complex like 5 and 6 but do not warrant a discussion
of structural details. Compared to its mesityl analogue 6, the
diisopropylphenyl derivative 7 appears more stable and shows
a melting point with decomposition at 150 °C. At 50 °C, 7 de-
composes within several days to a brown solid. Crystalline 7 is
air-stable at room temperature for approximately 1 h but in
Figure 4. Molecular structure of 9. Selected distances [Å] and angles [°]:
Fe–Cpcent 1.883, Fe–Br1 2.3204(16), Fe–Br2 2.3246(17); Cpcent–Fe–Br1 131.30,
Cpcent–Fe–Br2 122.91, Br1–Fe–Br2 105.73(6).
The distance of 1.883 Å between the iron atom and the cen-
solution for less than 5 min with a color change from black to troid of the cyclopentadienyl ligand is between the values
brown.
found for the mesityl derivatives 5 and 6 (Table 1). Both Fe–Br
bonds of 9 show nearly the same length and are 2.3204(16)
and 2.3246(17) Å, respectively. In contrast to the bond lengths,
the Cpcent–Fe–Br angles are different. The angle to Br1 is
131.30°, whereas the angle to Br2 is significantly smaller
(122.91°). The Br–Fe–Br angle is 105.74°. The unsymmetrical ar-
rangement of the bromido ligands with Br1 directly under an
isopropyl substituent and Br2 in the gap between two isopropyl
substituents orienting their methyl groups away from each
other results in a smaller Cp–Fe–Br2 angle.
A pentane solution of the aryliron(II) derivative [5CpFe-
(σ-C6H3iPr2-2,6)] (4) turned black within minutes after hexa-
chloroethane addition. The paramagnetic iron(III) complex
[5CpFe(σ-C6H3iPr2-2,6)Cl] (8) shows broad 1H NMR signals be-
tween δ = 175.05 and –126.50 ppm. Compared to the tetraiso-
propylcyclopentadienyl derivative 7, compound 8 shows fewer
NMR signals, which agrees with the higher symmetry of the
pentaalkylcyclopentadienyl ligand of 8. The chemical shifts for
7 and 8 are similar, as expected for similar compounds. The
NMR signals of the mesityl derivative 6 differ markedly from
Further characterization could not be performed owing to
those of 7 and 8. For example, the diisopropylphenyl deriva- the small amount of crystals. The trace amounts of an iron(III)
tives 7 and 8 have only one signal downfield from δ = 100 ppm complex formed during the reaction of an iron(II) precursor
(δ = 175.50 ppm for 7, δ = 175.07 ppm for 8), whereas complex with an organomagnesium compound raises questions regard-
6 has four different signals between δ = 183 and 100 ppm (δ = ing the oxidizing agent. If traces of atmospheric oxygen were
109.43, 113.96, 120.86, 182.16 ppm). Furthermore, 4Cp complex responsible, we would expect an oxido bridge, which is not
5
7 has ten signals with negative shifts, and the Cp derivative 8 likely to be replaced readily by bromide ions. Another hypothe-
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sis assumes the reduction of the [4CpFe(μ-Br)]2 precursor by
the organomagnesium component to form an FeI intermediate,
followed by oxidative addition of unreacted aryl bromide ad-
mitted together with the solution of the Grignard compound.
The conversion of a putative [4CpFe(aryl)Br] intermediate to the
dibromide 9 could then follow by Schlenk equilibria involving
heterodinuclear iron(III)–magnesium(II) complexes. The unin-
tended formation of the iron(III) half-sandwich complex 9 un-
derscores the outlook in an earlier report[15] on the possible
Finnigan MAT 90 instrument with an ionization energy of 70 eV.
CCDC 1499981 (for 2), 1499980 (for 5), 1499981 (for 6), and 1499982
(for 9) contain the supplementary crystallographic data for this pa-
per. These data can be obtained free of charge from The Cambridge
Crystallographic Data Centre. [4CpFe(μ-Br)]2,[9] [5CpFeBr(dme)]
(dme = 1,2-dimethoxyethane),[10] mesitylmagnesium bromide,[16]
mesityl(tri-tert-butylcyclopentadienyl)iron (1),[6] (2,6-diisopropyl-
phenyl)magnesium bromide,[10] (2,6-diisopropylphenyl)(tetraiso-
propylcyclopentadienyl)iron (3),[8] and (2,6-diisopropylphenyl)-
(pentaisopropylcyclopentadienyl)iron (4)[10] were synthesized ac-
oxidation of cyclopentadienyliron(II) bromides to the corre- cording to literature procedures. Hexachloroethane (Acros, 99 %)
was used as purchased.
sponding iron(III) dibromides and calls for the development of
[4CpFe(σ-C6H2-2,4,6-Me3)] (2): A solution of mesitylmagnesium
bromide–diethyl ether (173 mg, 0.82 mmol) in THF (5 mL) was
added to a solution of [4CpFe(μ-Br)]2 (300 mg, 0.41 mmol) in THF
(5 mL). After the reaction mixture had turned dark green, the sol-
vent was evaporated, and the residue was extracted with pentane
(10 mL) and centrifuged. The yellow solution was concentrated, and
the solvent was allowed to evaporate slowly at room temperature.
After 1 d, yellow crystals were obtained (221 mg, 0.55 mmol, 67 %).
1H NMR (400 MHz, C6D6, 298 K) δ = 203.95, 196.91, 87.18, 73.97,
–23.93 (Δν1/2 = 556 Hz), –119.73 (Δν1/2 = 1062 Hz) ppm. C26H40Fe
(408.45): calcd. C 76.45, H 9.87; found C 75.42, H 9.55.
a procedure for the rational synthesis of 9 and its derivatives.
Conclusions
Starting from paramagnetic alkylcyclopentadienyl(o-dialkyl-
phenyl)iron(II) complexes of the [CpFe(σ-aryl)] type, a previously
unknown reaction pathway was encountered in reactions with
palladium(II) chloride and hexachloroethane.
Palladium(II) chloride is too strongly oxidizing to give the
Lewis acid reaction with aryliron(II)–cyclopentadienides as pre-
viously encountered with copper(I) chloride to form heterodi- [Cp′′′Fe(σ-C6H2-2,4,6-Me3)Cl] (5):
A solution of 1 (100 mg,
nuclear [CpFe(μ,η5:η1-aryl)CuCl] complexes. The facile oxidation
by PdCl2 to form aryliron(III) chloride complexes of the
[CpFe(aryl)Cl] type occurs owing to the presence of unpaired
electrons in high-lying d orbitals of the high-spin d6 FeII ion.
This interpretation is supported by the hexachloroethane oxid-
ation of aryliron(II) precursors. It remains to be seen whether a
palladium(0) starting compound will be able to undergo the
rearrangement reaction, which failed with palladium(II) chlor-
ide.
0.25 mmol) in THF (5 mL) was added to a suspension of palladium
dichloride (22 mg, 0.13 mmol) in THF (5 mL). The reaction mixture
turned deep black after a few minutes. The reaction mixture was
stirred overnight, and the solvent was removed in vacuo. The resi-
due was extracted with toluene (5 mL) and centrifuged. The solu-
tion was separated from the black residue, and the solvent was
removed in vacuo. A viscous black oil was isolated (81 mg,
0.18 mmol, 73 %). 1H NMR (400 MHz, C6D6, 298 K) δ = 121.30
(Δν1/2 = 144 Hz), 72.61 (Δν1/2 = 368 Hz), –13.45 (Δν1/2 = 322 Hz),
–19.37 (Δν1/2 = 381 Hz) ppm. Satisfactory elemental analysis could
not be obtained. MS: m/z = 408.2 [M – Cl]+, 324.2 [M – Mes]+, 234.2
[Cp′′′H]+, 120.1 [MesH]+, 105.0 [Mes – CH3]+, 57.1 [tBu]+.
The unexpected formation of dibromido(tetraisopropylcyclo-
pentadienyl)iron(III) in low yield during the reaction of the
iron(II) bromide precursor [4CpFe(μ-Br)]2 with the difunctional
Grignard reagent biphenyl-4,4′-diylbis(magnesium bromide)
was observed as a side reaction and requires an oxidizing
agent. The reduction of iron(II) to iron(I) by the organomagne-
sium reagent, followed by the oxidative addition of residual aryl
bromide could produce iron(III). This hypothesis requires subse-
quent ligand exchange in analogy to the well-known Schlenk
equilibria to produce the observed dibromide.
[4CpFe(σ-C6H2-2,4,6-Me3)Cl] (6):
A
solution of [4CpFe(μ-Br)]2
(738 mg, 1.0 mmol) in pentane (10 mL) was added to a suspension
of mesitylmagnesium bromide (with 1 equiv. of Et2O, 595 mg,
2.0 mmol) in pentane (10 mL) at 0 °C with stirring. The reaction
mixture was allowed to warm slowly to room temperature and then
stirred at room temperature for 1 h. After centrifugation, the orange
solution was separated from the precipitate. Without further charac-
terization, the solution containing complex 2 was added to a solu-
tion of hexachloroethane (239 mg, 1.0 mmol) in pentane (5 mL).
After some minutes, the reaction mixture turned black and was
stirred at room temperature for 16 h. The mixture was separated
from insoluble compounds by centrifugation, concentrated, and
stored at –30 °C. After several days, complex 6 was obtained as
Experimental Section
General: For the preparation of air- and moisture-sensitive com-
pounds under an inert gas, Schlenk-line techniques and a glovebox
(MBraun) filled with argon were applied. The solvents were dried
rigorously and deoxygenated by distillation under an inert gas with
molten potassium (THF) or sodium/potassium alloy (pentane) be-
fore use. NMR spectra were obtained with a Bruker Avance 400
spectrometer and referenced to the residual proton signals of the
deuterated solvents. Half-widths Δν1/2 are given in Hz, and chemical
shifts δ are in ppm. Elemental analyses were performed in the ana-
lytical laboratory of the chemistry department of the TU Kaiserslaut-
ern with a Vario Micro Cube analyzer (Elementar Analysentechnik).
The crystal structures were obtained by XRD measurements with a
Stoe IPDS diffractometer (5) or an Oxford Diffraction Gemini Ultra
diffractometer (others). The mass spectra were recorded with a
1
black crystals (552 mg, 1.24 mmol, 62 %). H NMR (600 MHz, C6D6,
298 K): δ = 182.16 (Δν1/2 = 2663 Hz), 120.86 (Δν1/2 = 233 Hz), 113.96
(Δν1/2 = 118 Hz), 109.43 (Δν1/2 = 3977 Hz), 61.66 (Δν1/2 = 3852 Hz),
48.48, 18.88, –20.84 (Δν1/2 = 3419), –41.07 (Δν1/2 = 3378 Hz), –55.97
(Δν1/2 = 5300 Hz), –150.50 ppm. C26H40ClFe (443.88): calcd. C 70.35,
H 9.08; found C 69.86, H 9.19.
[4CpFe(σ-C6H3-2,6-iPr2)Cl] (7):
(369 mg, 0.5 mmol) in THF (10 mL) was added to a solution of (2,6-
diisopropylphenyl)magnesium bromide with equiv. of THF
A
solution of [4CpFe(μ-Br)]2
1
(338 mg, 1.0 mmol) in THF (10 mL) at room temperature with stir-
ring. The reaction mixture became lighter, and the solvent was re-
moved after 5 min. The residue was extracted with pentane (2 ×
10 mL), and the volume of the combined pentane extracts was
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reduced to 10 mL. This solution of the known complex 3[8] was
added to a solution of hexachloroethane (120 mg, 0.5 mmol) in
pentane (5 mL). After some minutes, the reaction mixture turned
black and was stirred at room temperature for 16 h. After centrifu-
gation, the black solution was concentrated and stored at –30 °C.
After some days, complex 7 was obtained as black crystals (150 mg,
0.31 mmol, 31 %). 1H NMR (600 MHz, C6D6, 298 K):
δ = 175.50 (Δν1/2 = 1262 Hz), 76.26 (Δν1/2 = 1454 Hz), 49.69
(Δν1/2 = 1439 Hz), 42.54 (Δν1/2 = 1185 Hz), 29.62 (Δν1/2 = 1064 Hz),
Keywords: Iron · Half-sandwich complexes · Arene ligands ·
Cyclopentadienyl ligands · Piano-stool complexes
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17.19 (Δν1/2 = 1643 Hz), –15.47 (Δν1/2 = 993 Hz), –22.08 (Δν1/2
=
961 Hz), –28.60, –39.45 (Δν1/2 = 1572 Hz), –46.55 (Δν1/2 = 1114 Hz),
–53.55, –56.15, –74.40 (Δν1/2 = 2772 Hz), –111.27 (Δν1/2 = 57 Hz),
–145.73 (Δν1/2 = 1628 Hz) ppm. C29H46ClFe (485.98): calcd. C 71.67,
H 9.54; found C 70.91, H 9.55.
[5CpFe(σ-C6H3-2,6-iPr2)Cl] (9):
(501 mg, 1.0 mmol) in THF (10 mL) was added to a solution of (2,6-
diisopropylphenyl)magnesium bromide with equiv. of THF
A
solution of [5CpFeBr(dme)]
1
(338 mg, 1.0 mmol) in THF (10 mL) and stirred at 75 °C for 3 h,
and the reaction mixture turned orange-red. After the mixture had
cooled to room temperature, the solvent was removed in vacuo.
The residue was extracted with pentane (2 × 10 mL), and the vol-
ume of the combined pentane extracts after centrifugation was re-
duced to 10 mL. This solution of the known complex 4[10] was
added to a solution of hexachloroethane (120 mg, 0.5 mmol) in
pentane (5 mL). After some minutes, the reaction mixture turned
black and was stirred at room temperature for 16 h. After centrifu-
gation to remove insoluble compounds, the black solution was con-
centrated and stored at –30 °C. After a few days, complex 9 was
obtained as black crystals (110 mg, 0.28 mmol, 28 %). 1H NMR
(600 MHz, C6D6, 298 K): δ = 175.07 (Δν1/2 = 725 Hz), 78.28 (Δν1/2
835 Hz), 56.22 (Δν1/2 = 136 Hz), 21.10 (Δν1/2 = 325 Hz), 17.29, 16.25,
–12.97 (Δν1/2 = 1673 Hz), –26.71 (Δν1/2 = 280 Hz), –54.05 (Δν1/2
=
=
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309 Hz), –126.50 (Δν1/2 = 1252 Hz) ppm. C32H52ClFe (528.06): calcd.
C 72.79, H 9.93; found C 72.75, H 9.86.
Acknowledgments
H. S. is grateful to Professor O. J. Scherer for many years of
friendly support.
Received: August 19, 2016
Published Online: ■
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Paramagnetic Organometallics
Paramagnetic iron(III) half-sandwich
complexes with bulky alkylcyclopenta-
dienyl ligands are available from high-
spin aryliron(II) precursors by oxidation
with hexachloroethane and are charac-
terized as 15-valence-electron mono-
mers with two-legged piano-stool
geometries.
H. Bauer, M. W. Wallasch,
G. Wolmershäuser, Y. Sun,
H. Sitzmann* ....................................... 1–7
Iron(III) Half-Sandwich Complexes of
the
Two-Legged
Piano-Stool
[CpFe(aryl)Cl] Type from the Corre-
sponding Aryliron(II) Precursors
DOI: 10.1002/ejic.201601026
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