A Ferrocene-Based Dicationic Iron(IV) Carbonyl Complex

Article abstract of DOI:10.1002/anie.201809464

The 16-valence electron species [Cp*₂Fe]²⁺ (Cp=η-C₅Me₅), formally featuring a tetravalent iron ion, quantitatively binds CO in HF solution to form the stable, diamagnetic carbonyl species [Cp*₂Fe(CO)]²⁺. This dication forms salts in the presence of AsF₆^? and SbF₆^? that were crystallographically characterized. The molecular structure in crystals of [Cp*₂Fe(CO)](AsF₆)₂ displays cyclopentadienyl rings that are clearly not parallel and an equatorially bound η¹-CO ligand. The formal oxidation state +IV of iron was investigated by ⁵⁷Fe M?ssbauer spectroscopy and is supported by DFT computational analysis. A detailed spectroscopic characterization of the hitherto unprecedented high-valent iron carbonyl compounds is reported.

Full text of DOI:10.1002/anie.201809464

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Accepted Article
Title: A Ferrocene-Based Dicationic Fe(IV) Carbonyl Complex
Authors: Moritz Malischewski, Konrad Seppelt, Jörg Sutter, Dominik
Munz, and Karsten Meyer
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A Ferrocene-Based Dicationic Fe(IV) Carbonyl Complex
Moritz Malischewski,* Konrad Seppelt, Jörg Sutter, Dominik Munz and Karsten Meyer*
Dedicated to the 100th anniversary of Professor Ernst Otto Fischer
The 16-valence electron species [Cp*2Fe]2+, formally featuring a
In our efforts to explore and expand the classical organometallic
chemistry of cyclopentadienyl iron complexes,[9] we employed
the paramagnetic complex [Cp*2Fe](A)2 (A = AsF6 or SbF6 ) for
the envisioned CO chemistry. If this starting material is
tetravalent iron ion, quantitatively binds CO in HF solution to form
the stable, diamagnetic carbonyl species [Cp*2Fe(CO)]2+. This
–
–
–
dication forms salts in the presence of AsF6– and SbF6 that were
crystallographically characterized.
The molecular structure in
dissolved in anhydrous HF, it quantitatively reacts with CO gas
resulting in a color change from dark brown to blood-red to yield
the diamagnetic target compound [Cp*2Fe(CO)]2+. Although the
crystals of [Cp*2Fe(CO)](AsF6)2 displays significantly bent cyclo-
pentadienyl rings and an equatorially bound h1-CO ligand. The
formal oxidation state +IV of iron was investigated by 57Fe Mößbauer
–
–
AsF6 and SbF6 salts of the dication swiftly crystallize upon
spectroscopy and is supported by DFT computational analysis.
A
cooling of the HF reaction solution, multiple attempts to obtain
detailed spectroscopic characterization of the hitherto
unprecedented high-valent iron carbonyl compounds is reported.
–
high-quality single crystals of the SbF6 salt for X-ray diffraction
analyses were unsuccessful. Attempts to re-crystallize this salt
from anhydrous SO2 were also unsuccessful due to the
compound’s high solubility. Still, the data quality of the well
reproducible structural analyses of the AsF6– salt, supplemented
by 1H/13C NMR and IR vibrational spectroscopic characterization
and DFT computation, allowed for the unambiguous
identification of both dications; namely, the [Cp*2Fe(CO)](A)2 (A
The carbonyl ligand, CO, is an ubiquitous ligand in
organometallic chemistry,[1] due to its ability to interact with metal
ions synergistically via σ-donation as wells as π*-backbonding.
The latter property allows the isolation of complexes with metals
in unusually low and even formally negative oxidation states,[2]
such as Cr(–IV) in tetraanionic [Cr(CO)4]4–.[3] Examples of iron
carbonyl complexes are known in oxidation states ranging from
(–II), in e.g. Collman’s reagent Na2[Fe(CO)4], to (+II), as in
[Fe(CO)6]2+.[4] Carbonyl complexes featuring the versatile and
archetypal organometallic ferrocene ligand form two types of CO
complexes both fulfilling the 18-valence electron rule. These
complexes are the dimeric Fe(I) complex [CpFe(CO)2]2[5] and the
ferrous [Cp2Fe(CO)2],[6] comprising one η5 and one η1-bound Cp
ring (Scheme 1). Encouraged by the recent success in pre-
–
= AsF6– and SbF6 ) complexes.
paring tetravalent ferrocene derivatives, namely dicationic
[7]
[Cp*2Fe]2+
and [Cp2Fe(H)]+ species,[8] the preparation of a
formally high-valent CO adduct [Cp*2Fe(CO)]2+ seemed to be a
worthwhile synthetic target.
Figure 1. Molecular structure of the [Cp*2Fe(CO)]2+ dication in crystals of
[Cp*2Fe(CO)](AsF6)2 • 6 HF (space group: I-42d), the two anions and co-
crystallized HF molecules are omitted for clarity, thermal ellipsoids are shown
at 50% probability.
Scheme 1. Previously reported cyclopentadienyl-iron carbonyl complexes
(left, middle) and compound reported herein (right).
–
The molecular structure of needles of the AsF6 salt (Form 1,
tetragonal space group I-42d, No. 122) was determined to
include six solvent HF molecules; the structure of the
simultaneously crystallizing platelets (Form 2, orthorhombic
space group Pbca, No. 61) includes two molecules of HF. Both
structures display significantly bent metallocene moieties with
Cp* tilt angles of 23.1° (Form 1) and 26.7° (Form 2).
Due to severely disordered HF solvent molecules, the XRD data
possess moderate quality; and thus, the estimated standard
deviations of the bond lengths are relatively large. Regardless,
the refined structures allow for a comparison with literature-
reported iron-carbonyl complexes. The Fe–CO distances of
1.80(2) Å (Form 1) and 1.785(9) Å (Form 2) as well as the C–O
distances of 1.17(2) Å (Form 1) and 1.16(1) Å (Form 2) are
similar to the corresponding metrics determined for the Fe0
complex [Fe(CO)5] (Fe–C 1.80–1.81 Å, C–O 1.12-1.13 Å)[10] and
distinctly different to those of the FeII complex [Fe(CO)6]2+ (Fe–C
Dr. Moritz Malischewski, Prof. Dr. Konrad Seppelt
Freie Universität Berlin - Institut für Chemie und Biochemie -
Anorganische Chemie
Fabeckstraße 34-36, 14195 Berlin
moritz.malischewski@fu-berlin.de
Dr. Jörg Sutter, Dr. Dominik Munz, Prof. Dr. Karsten Meyer
Friedrich-Alexander-Universität Erlangen-Nürnberg, Department für
Chemie und Pharmazie, Anorganische Chemie, Egerlandstr. 1,
91058 Erlangen, karsten.meyer@fau.de
Supporting information for this article is given via a link at the end of
the document.((Please delete this text if not appropriate))
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1.90-1.92 Å, C–O 1.10 Å).[4b] The Fe–CCp* distances of 2.13(1)–
2.22(1) Å (Form 1) and 2.142(8)–2.217(7) Å (Form 2) resemble
the values of [Cp*2Fe]2+ (2.12–2.15 Å),[7] rather than those of the
parent [Cp*2Fe] complex (2.05 Å).[11]
Crystal Form 1
23.1°
Crystal Form 2
26.7°
Tilt angle
Cp*–Fe–Cp*
153.2°
151.3°
centroid angle
Cp*(centroid)
–Fe–
1.786 Å
1.788 – 1.795 Å
Cp*(centroid)
distance
Fe–CO distance
C–O distance
1.80(2) Å
1.17(2) Å
1.785(9) Å
1.16(1) Å
Figure 2. Zero-field 57Fe Mößbauer spectrum of a solid sample of 57Fe-
enriched (approx. 30%) [Cp*2Fe(CO)](SbF6)2, recorded at 77 K.
While ferrocene derivatives of high-valent iron are rare,[7-8, 18]
a
Fe–C (cp)
distance
2.13(1) – 2.22(1) Å
2.142(8) – 2.217(7) Å
number of low-valent iron carbonyl[19] and also iron cyclopenta-
dienyl carbonyl complexes[20] have been studied by 57Fe Möß-
bauer spectroscopy in the past. However, for better comparison
of the here presented [Cp*2Fe(CO)]2+ with our previously
reported complex [Cp*2Fe]2+, we re-synthesized ferrous [Cp*2Fe]
and ferric [Cp*2Fe](PF6) to characterize the complexes by single-
crystal X-ray diffraction and 57Fe Mößbauer spectroscopy under
Table 1. Selected geometrical structure parameters of [Cp*2Fe(CO)]2+
.
The relatively short Fe–CCO and long C–O bond lengths
intuitively suggest a significant amount of backbonding from the
iron d into the π* orbitals of the CO ligand. While backbonding
into the d orbitals is also in agreement with the observed C–O
identical experimental conditions (SI).[21]
In general, all
vibrational stretch at 2042 cm–1 (AsF6 salt) and 2034 cm–1
–
investigated Cp* FeII and FeIV compounds give rise to doublets,
while Cp*2Fe+ only gives one broad signal. The quadrupole
splitting ΔEQ of [Cp*2Fe(CO)](SbF6)2 is 3.22 mm s–1 and 1.89 mm
s–1 for [Cp*2Fe](SbF6)2. The significant increase of ΔEQ is
indicative of an increased d-electron anisotropy due to the larger
tilt angle induced by CO coordination (Table 2).
–
(SbF6 salt) in the IR spectra, this observation poses the
question of how a formally high-valent, electron-poor ion is able
to engage in Fe to CO π-backbonding. An explanation is given
by calculations (see also below). The calculation, applying the
TPSSh functional and the def2-TZVPP basis set, correctly
predicts the observed stretching frequencies of the IR spectrum
(n!calc = 2020 cm–1). We therefore conclude that the experiment-
tally determined short C–O bond distance and C–O stretching
frequency below the value of free CO (2143 cm–1) identify the
Quadrupole
Isomer shift
in mm s–1
δ
Cp tilt angle in
°
splitting ΔEQ in
mm s–1
newly synthesized [Cp*2Fe(CO)]2+ species as
a
classical
0.42±0.01
(calc.: 0.45)
0.59±0.01
(calc.: 0.62)
0.59±0.01
(calc.: 0.58)
0.57±0.01
(calc.: 0.54)
3.22±0.01
(calc.: 3.18)
1.89±0.01
(calc.: 2.28)
2.00±0.01
(calc.: 2.40)
0
[Cp*2Fe(CO)](SbF6)2
(S = 0)
26.7 (for AsF6)
carbonyl complex.[12] Accordingly, the experimentally found C–O
stretching frequency of [Cp*2Fe(CO)]2+ is more akin to the
corresponding values of the well-known [Fe(CO)5] (2014 and
2034 cm–1) than to those of [Fe(CO)6]2+ (2204 cm–1)[4b] or
[Fe(CO)4]2– (1730 cm–1).[13]
[Cp*2Fe](SbF6)2
(S = 1)
16.6
[Cp*2Fe](Sb2F11
)
2
–
–
0
0
0
Room temperature NMR spectra of the AsF6 and SbF6 salts
[Cp*2Fe(CO)](AsF6)2 and [Cp*2Fe(CO)](SbF6)2 were recorded in
anhydrous HF, the only available solvent with sufficient complex
solubility and stability. In the 1H NMR spectrum the ten
equivalent methyl groups of the Cp* ligand give rise to the
expected singlet signal (0.96 ppm of the unlocked spectrum of
[Cp*2Fe(CO)](AsF6)2). In the 13C NMR spectrum, the Cp* groups
give raise to two intense signals at 7.1 ppm (CCH3) and at 110.9
ppm (CCp_ring). These signals are shifted compared to the
corresponding carbonyl free [Cp*2Fe], with chemical shifts of
9.5 ppm (CCH3) and 79.0 ppm (CCp ring).[14] Another absorption at
215.4 ppm, with considerably less intensity, was assigned to the
terminally bound carbonyl C atom.[15] This value is very close to
the chemical shift of [Fe(CO)5], observed at 213 ppm,[16] and
other known iron carbonyl complexes.[17]
(S = 1)
[Cp*2Fe](PF6)
(S = 1/2)
(calc. 0.32)
0.51±0.01
(calc.: 0.54)
2.50±0.01
(calc.: 2.64)
[Cp*2Fe]
(S = 0)
Table 2. Comparison of 57Fe Mößbauer parameters, measured as a solid
sample at 77 K with computational predictions. As part of this study, [Cp*2Fe]
and [Cp*2Fe](PF6) were re-synthesized, crystallographically, and spectros-
copically characterized (see SI).
According to textbook knowledge, for structurally and electro-
nically closely related iron complexes of a given series, the
isomer shift (d) typically decreases with increasing oxidation
state. That said, in the series of Cp* Fe-complexes, d increases
upon oxidation from divalent [Cp*2Fe] (d = 0.51 mm s–1)[22] to
trivalent [Cp*2Fe]+ (d = 0.57 mm s–1) and tetravalent [Cp*2Fe]2+ (d
= 0.59 mm s–1). Notably, the calculated 57Fe Mößbauer
parameters (Table 2) reproduce these experimental results
surprisingly well.[23]
The purity and iron oxidation state of the newly synthesized
carbonyl complex was screened by zero-field 57Fe Mößbauer
spectroscopy. The Mößbauer spectrum of [Cp*2Fe(CO)](SbF6)2
exhibits a single quadrupole doublet, indicative of clean and
quantitative conversion to the carbonyl species (Figure 2).
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In order to assess the electronic structure of these compounds
in more detail, we calculated Knizia’s intrinsic bond orbitals
(IBOs).[24] This method allows to localize orbitals, and hence, is
an excellent tool for the intuitive analysis of electronic structures.
The evaluation of the IBOs of [Cp*2Fe(CO)]2+ with more than
75% metal character, corroborates a d6 electron configuration for
[Cp*2Fe] (i.e., FeII), a d 5 for [Cp*2Fe]+ (i.e., FeIII) and d 4 (i.e.,
FeIV) for [Cp*2Fe]2+ and [Cp*2Fe(CO)]2+. However, especially for
the latter Fe(IV) compounds, the calculations show that all
molecular orbitals associated with the Cp*–Fe interaction are
considerably covalent; more so than for the analogous lower
valent species (SI). A simplified canonical molecular orbital
Å) to [Cp*2Fe](PF6) (1.708 Å) and [Cp*2Fe](SbF6)2 (1.764 Å), in
accord with the observed increasing isomer shifts.
As expected, coordination of strongly π-acidic CO to [Cp*2Fe]2+
results in
a significantly smaller d
of 0.42 mm s–1 for
[Cp*2Fe(CO)](SbF6)2 compared to 0.59 mm s–1 for [Cp*2Fe]-
(SbF6)2. Overall, we conclude that the synthesized CO complex
indeed qualifies as an FeIV compound, although the concept of
assigning formal oxidation states to transition metals is difficult
for complexes with highly covalent metal–ligand bonds. Hence,
another concept of assessing the electronic structure of iron
carbonyl complexes has recently been reported by Wolczanski,
offering an alternative interpretation based on ligand-centered
oxidations.[28]
diagram of [Cp*2Fe(CO)]+ (Figure 3) reveals a fully occupied dz
2
While the oxidation state +IV for non-biomimetic iron is rare,[29] it
is even less common for complexes with carbonyls, which are
generally good ligands for metals in low oxidation states. Hence
high-valent metal carbonyl complexes such as [PtCl5(CO)]–,[30]
[OsF5(CO]–,[31] and [OsCl5(CO)]–,[32] are exceedingly rare and
(highest occupied molecular orbital, HOMO) and dxz (HOMO-2)
2
orbitals rendering the complex’s electron configuration {(dz
)
2
(dxz)2}. Also notable, the π-backbonding does not involve iron d
orbitals, but occurs between the two Cp* and the CO ligands as
seen in the HOMO-1 (Figure 3, inset). This ligand-to-ligand
charge transfer is reminiscent of the interaction observed in the
seminal [(C5Me4H)3U(CO)],[25] and explains the observed,
surprisingly similar, stretching frequency also found for low
valent iron carbonyl complexes. Similar π-backbonding also
was proposed for d0 metal carbonyl complexes of ZrIV and HfIV;
namely [Cp*2M(H)2(CO)].[26]
only the latter one is crystallographically characterized.
A
transient tetravalent [Rh(O)2CO] complex has been postulated
on an Al2O3 surface at low temperatures,[33] and the tetravalent
Mo and W complexes [M(O)2(CO)4] have been detected by
matrix spectroscopy.[34] The complex [OsO2(CO)4]2+, formally
comprising Os(VI), was prepared in liquid SbF5 but was too
unstable to be isolated, purified, or crystallized.[35]
In summary, the isolation and characterization of [Cp*2Fe(CO)]2+,
including its solid state molecular structure, closes a gap for the
compound class of iron-cyclopentadienyl carbonyl complexes.
The C–O stretching frequency of the high-valent complex
[Cp*2Fe(CO)]2+ is reminiscent of classical low valent iron
carbonyl complexes, but originates from Cp*-to-CO π-
backbonding. It is the only crystallographically characterized
cationic transition metal complex containing
tetravalent transition metal ion.
a
formally
Experimental Section
Experimental, crystallographic and computational details are given in the
Supporting Information. CCDC-1860739 ([Cp*2(CO)Fe](AsF6)2 · 6 HF),
CCDC-1860740 ([Cp*2(CO)Fe](AsF6)2
([Cp*2Fe]), and CCDC-1860962
supplementary crystallographic data for this paper. These data can be
·
2
HF), CCDC-1860961
([Cp*2Fe]PF6) contain the
obtained
free
of
charge
from
via
The Cambridge Crystallographic Data Centre
www.ccdc.cam.ac.uk/data_request/cif.
Acknowledgements
This work was supported by Freie Universität Berlin, Verband
der Chemischen Industrie (VCI), and Deutsche Forschungs-
gemeinschaft (DFG, GRK 1582, “Fluorine as a Key Element”).
M. M. acknowledges a postdoctoral fellowship by DFG and
Leopoldina – Nationale Akademie der Wissenschaften. D. M.
Figure 3. Simplified and qualitative frontier molecular orbital diagram of the
[Cp*2Fe(CO)]2+ dication.
The calculation also corroborates strong mixing of ligand-
centered π-type with metal d-orbitals (Figure 3; e.g. HOMO-3 or
HOMO-4). As a result, the iron +IV oxidation state is partially
masked for the determination by Mößbauer spectroscopy due to
the charge transfer from the Cp* ligands to the iron ion. This
observation is perfectly in line with a more recent interpretation
by Neese et al., emphasizing that “perhaps the most decisive
factor for the isomer shift is the metal–ligand bond distance.”[27]
Indeed, the Fe-to-Cp* centroid distances increase with
increasing iron oxidation state when going from Cp*2Fe (1.652
thanks the Fonds der Chemischen Industrie for
a Liebig
fellowship. Computing time was made available by High-
Performance Computing at ZEDAT/FU Berlin and RRZE
Erlangen. Sascha Löffler (FAU) is acknowledged for the
preparation of [Cp*2Fe] and [Cp*2Fe](PF6). Friedrich-Alexander-
Universität Erlangen-Nürnberg (FAU) is acknowledged for
generous financial support.
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[24] a) G. Knizia, J. E. M. N. Klein, Angew. Chem., Int. Ed. 2015, 54, 5518-
5522; Angew. Chem. 2015, 127, 5609-5613; b) G. Knizia, J. Chem.
Theory Comput. 2013, 9, 4834-4843.
Keywords: carbonyl ligands • cyclopentadienyl ligands • iron •
metallocenes • x-ray diffraction • 57Fe Mössbauer
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the Mößbauer parameters follow a procedure developed by F. Neese
and coworkers: c) R. Bjornsson, F. Neese, S. DeBeer, Inorg. Chem.
2017, 56, 1470-1477; d) M. Römelt, S. Ye, F. Neese, Inorg. Chem.
2009, 48, 784-785. For further details, see the Supporting Information.
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10.1002/anie.201809464
Angewandte Chemie International Edition
COMMUNICATION
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COMMUNICATION
The classical iron carbonyl complex
[Cp*2Fe(CO)](AsF6)2, containing for-
mally tetravalent iron, has been pre-
pared by reacting [Cp*2Fe]2+ with
carbon monoxide in anhydrous HF.
The title complex was characterized
by various spectroscopic techniques,
including 57Fe Mössbauer spectros-
Moritz Malischewski,* Konrad Seppelt,
Jörg Sutter, Dominik Munz and Karsten
Meyer*
Page No. – Page No.
A Ferrocene-Based Dicationic Fe(IV)-
Carbonyl Complex
copy,
and
DFT
computational
analysis.
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