Rhodocenium Monocarboxylic Acid Hexafluoridophosphate and Its Derivatives: Synthesis, Spectroscopy, Structure, and Electrochemistry

Article abstract of DOI:10.1002/ejic.202000071

As an extension of our continuing work in metallocenium chemistry, we report here on new functionalized rhodocenium salts. In contrast to isoelectronic cobaltocenium compounds, rhodium as a 4d metal allows synthetic routes via prefunctionalized cyclopentadienyl half-sandwich precursors, thereby facilitating access to monofunctionalized rhodocenium salts containing substituents comprising methyl, trimethylsilyl, carboxylate and carboxylate ester as well as amide derivatives. Synthetic aspects, scope and limitations, as well as spectroscopic (¹H/¹³C-NMR, IR, HR-MS), and structural (XRD) properties are reported. Like the parent rhodocenium ion, all new derivatives undergo two chemically consecutive reductions at potentials that depend on the respective ring substituent. The second reduction competes with a rapid reaction of the corresponding rhodocenes to their 18 VE dimers. Rate constants for this process range from 2 × 10³ s^–1 to 2.5 × 10⁵ s^–1 as estimated from digital simulations of experimental voltammograms. Rhodocenium carboxylic acid (8) constitutes a special case in that proton instead of metal reduction is observed at Pt or Au electrodes.

Full text of DOI:10.1002/ejic.202000071

A Journal of
Accepted Article
Title: Rhodocenium monocarboxylic acid hexafluoridophosphate
and its derivatives: synthesis, spectroscopy, structure and
electrochemistry
Authors: Markus Jochriem, Larissa A. Casper, Stefan Vanicek, Dirk
Petersen, Holger Kopacka, Klaus Wurst, Thomas Müller,
Rainer F. Winter, and Benno Bildstein
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To be cited as: Eur. J. Inorg. Chem. 10.1002/ejic.202000071
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10.1002/ejic.202000071
European Journal of Inorganic Chemistry
FULL PAPER
Rhodocenium monocarboxylic acid hexafluoridophosphate and
its derivatives: synthesis, spectroscopy, structure and
electrochemistry
Markus Jochriem,[a] Larissa A. Casper,[b] Stefan Vanicek,[a,c] Dirk Petersen,[c] Holger Kopacka,[a] Klaus
Wurst,[a] Thomas Müller,[d] Rainer F. Winter,*[b] and Benno Bildstein*[a]
Abstract: As an extension of our continuing work in metallocenium
chemistry, we report here on new functionalized rhodocenium salts.
In contrast to isoelectronic cobaltocenium compounds, rhodium as a
4d metal allows synthetic routes via prefunctionalized
cyclopentadienyl half-sandwich precursors, thereby facilitating access
to monofunctionalized rhodocenium salts containing substituents
comprising methyl, trimethylsilyl, carboxylate and carboxylate ester as
well as amide derivatives. Synthetic aspects, scope and limitations,
as well as spectroscopic (1H/13C-NMR, IR, HR-MS), and structural
(XRD) properties are reported. Like the parent rhodocenium ion, all
new derivatives undergo two chemically consecutive reductions at
potentials that depend on the respective ring substituent. The second
reduction competes with a rapid reaction of the corresponding
rhodocenes to their 18 VE dimers. Rate constants for this process
range from 2×103 s-1 to 2.5×105 s-1 as estimated from digital simulations
of experimental voltammograms. Rhodocenium carboxylic acid (8)
constitutes a special case in that proton instead of metal reduction is
observed at Pt or Au electrodes.
reason for the underdeveloped chemistry of these metallocenium
salts is their intrinsic cationic nature, making it quite difficult to
access functionalized derivatives by standard chemical reactions
developed for arenes. However, in recent years we were able to
introduce valuable new synthetic protocols in cobaltocenium
chemistry[1] that are currently used to access chemically
interesting cobaltocenium compounds.[2] Our motivation for
studying cobaltocenium compounds in general stems from their
unique combination of advantageous properties like air-stability,
reversible redox chemistry, high polarity with concomitant high
solubility in polar solvents, and a strongly electron-withdrawing
character, thereby opening up new applications in redox catalysis,
bioinorganic chemistry, ligand design and coordination
chemistry.[2]
In comparison to cobaltocenium chemistry, rhodocenium
chemistry is even less developed.[3] In fact, only three
monosubstituted rhodocenium compounds are presently known.
First, Sheats and coworkers[4] reported in 1983 the synthesis of
rhodocenium monocarboxylic acid hexafluoridophosphate in 20%
isolated yield by a non-chemoselective statistical approach,
taking advantage of the differences in solubility of unsubstituted
rhodocenium, rhodocenium mono- and rhodocenium dicarboxylic
acid salts. No further chemistry of rhodocenium monocarboxylic
acid hexafluoridophosphate was described. Second, Andre and
coworkers[5] published in 1990 the preparative HPLC separation
of ferrocenylrhodocenium from other metallocenylrhodocenium
salts. The latter were again obtained by a statistical protocol. Third,
Introduction
After almost 70 years of research in metallocene chemistry, still
relatively little is known about cobaltocenium, rhodocenium and
iridocenium salts that are isoelectronic and as stable and redox
responsive as their ubiquitous ferrocene congeners. The main
Yan
hydroxyethyltriazolylrhodocenium salt obtained from (h4-
ethynylcyclopentadiene)(h5-cyclopentadienyl)rhodium(I)
by
and
coworkers[6]
reported
in
2015
a
[a]
Markus Jochriem, MSc; Dr. Stevan Vanicek, Dr. Holger Kopacka,
Dr. Klaus Wurst, Prof. Dr. Benno Bildstein
Institute of General, Inorganic and Theoretical Chemistry, University
of Innsbruck
azide-alkyne cycloaddition followed by oxidation with ferrocenium
hexafluoridophosphate. This rhodocenium-functionalized alcohol
was converted to norbornene and methacrylate monomers and
subsequently polymerized to the corresponding side-chain
functionalized redox-responsive polymers.
Center for Chemistry and Biomedicine, Innrain 80-82
6020 Innsbruck, Austria
E-mail: benno.bildstein@uibk.ac.at
Homepage: https://www.uibk.ac.at/aatc/
In this contribution, we aim at developing rhodocenium chemistry
a bit further, in part based on our experience in cobaltocenium
chemistry,[1,2] and disclose here a first chemoselective synthesis
of rhodocenium monocarboxylic acid hexafluoridophosphate
which is a key synthon for other monosubstituted rhodocenium
salts accessible by standard carboxylic acid reactivity. The
structural, spectroscopic and electrochemical properties of these
new rhodocenium derivatives are compared with those of their
cobaltocenium congeners.
[b]
Larissa A. Casper, MSc; Prof. Dr. Rainer F. Winter
Department of Chemistry, University of Konstanz
Universitätsstrasse 10, 784557 Konstanz, Germany
E-mail: rainer.winter@uni-konstanz.de
Homepage: https://www.chemie.uni-konstanz.de/winter/
Dirk Petersen, Department of Chemistry, University of Oslo
Sem Sælands vei 26, 0315, Norway
[c]
[d]
Prof. Dr. Thomas Müller
Institute of Organic Chemistry, University of Innsbruck
Center for Chemistry and Biomedicine, Innrain 80-82
6020 Innsbruck, Austria
Supporting information and ORCIDs from the authors for this article
are available on the WWW under https://doi.org/10.1002/ejic.xxxx.
Results and Discussion
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10.1002/ejic.202000071
European Journal of Inorganic Chemistry
FULL PAPER
Synthesis
analogue,[1a] its purification from side products and isolation
proved not feasible despite many efforts. Similarly, direct
amination of rhodocenium salts by a vicarious nucleophilic
substitution gave only inseparable product mixtures, in contrast to
corresponding cobaltocenium salts.[1c] Therefore alternative
synthetic strategies are needed for new monofunctionalized
rhodocenium compounds. In this context, an advantageous
feature of rhodium chemistry is the much higher stability of half-
sandwich rhodium(III) halide complexes[7] in comparison to their
labile cobalt(III) analogues.[8] As we will see in the following, these
rhodium(III) species are valuable synthons for our purposes
(Scheme 1).
In general, rhodocenium chemistry is somewhat different from
cobaltocenium chemistry, as is often observed when comparing
the reactivities and stabilities of analogous compounds of 3d
versus 4d/5d metals in a homologous group of elements.
Successful synthetic routes to cobaltocenium compounds cannot
simply be applied for rhodocenium compounds, due to differences
in either reactivity or solubility of cobaltocenium/rhodocenium
salts. Specifically, while the main target of this study,
rhodocenium monocarboxylic acid hexafluoridophosphate, could
be generated by a nucleophilic addition/hydride removal/side-
chain oxidation sequence that was recently developed in our
group for the successful synthesis of its cobaltocenium
I
I
I
I2
Tl
NaCp
HPF6
Rh
Rh
Rh
Rh
Rh
Rh
PF6
I
Cl
Cl
2
3
4
7
Rh
Rh
1
I
I
I
SiMe3
SiMe3
TlCp
Me3Si
Rh
Rh
SiMe3
I2
AgPF6
PF6
SiMe3
I
Li
5
6
O
O
– H+
KMnO4
Rh
OH
PF6
O
Rh
Rh
PF6
4
8
9
SOCl2
O
O
O
NH3
CH3OH
OCH3
PF6
Cl
NH2
PF6
Rh
Rh
Rh
PF6
11
10
12
NaN3
O
O
H2SO4
NH2
PF6
Ac2O / AlCl3
N
N3
PF6
13
Rh
H
Rh
Rh
Cl
14
15
Scheme 1. Synthesis of compounds.
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10.1002/ejic.202000071
European Journal of Inorganic Chemistry
FULL PAPER
Freshly prepared thallium methylcyclopentadienide (Cp’Tl)
was reacted in dry tetrahydrofuran under an atmosphere of
argon with chlorido(cyclooctadiene)rhodium(I) dimer (1) to
afford (cyclooctadiene)(methylcyclopentadienyl)Rh(I) (2) as an
air-stable yellow oil. Without further purification, oxidation of 2
with diiodine gave di(methylcyclopentadienyl)-dirhodium(III)
tetraiodide (3) in a satisfactory overall yield of 67%. Reaction
of 3 with two equivalents of sodium cyclopentadienide (NaCp)
followed by precipitation of the product with aqueous
hexafluorophosphoric acid afforded methylrhodocenium
hexafluoridophosphate (4) in 77% isolated yield as an air-
hexafluoridophosphate (15) met so far only with limited
success (appr. 40% conversion according to NMR analysis)
and no methods to isolate 15 from these reaction mixtures
proved possible.
Physical, spectroscopic and structural properties
Rhodocenium salts 4, 7, 8, 11, 12, and 14 are highly stable with
melting points in the range of 173°-280° C whereas 9, 10 and
13 are hydrolytically unstable. In general, rhodocenium salts
are less colored than their usually yellow to orange cobalt
analogues,[1,2] due to stronger metal-ligand bonding of 4d
elements. Rhodocenium salts are highly polar compounds,
soluble in polar solvents like acetonitrile, dimethylsulfoxide,
methanol, acetone and water, an advantageous property for
potential applications in green chemistry or for
bioorganometallic chemistry. Hexafluoridophosphate is the
most convenient and preferred counterion, allowing
precipitation of products from aqueous solutions in the workup
procedure. Spectroscopically, the hexafluoridophosphate ion
was easily identified by its strong IR absorptions at ≈ 820 and
550 cm-1. In addition, diagnostic IR signals for the functional
groups of rhodocenium salts 8-14 were observed (compare
experimental section and spectra in the Supporting
Information). High resolution mass spectrometry gave signals
of the cations in excellent agreement with theoretical values.
1H- and 13C-NMR spectroscopy allowed for structural
characterization of these monosubstituted rhodocenium
compounds in solution. 1H-NMR spectra showed in the region
of 5.8-6.5 ppm the common pattern of monosubstituted
metallocenes (1 signal corresponding to 5H, two pseudotriplets
corresponding to 2H,) with additional two-bond scalar
couplings [2J(1H-103Rh) ≈ 1 Hz] to 103Rh, a 100% naturally
abundant spin ½ nucleus (see Experimental Section and
spectra in the Supporting Information). Additional appropriate
resonances were observed for the respective substituents.
stable
trimethylsilylrhodocenium hexafluoridophosphate (7) was
obtained from by treatment with lithium
white
powder.
In
a
similar
manner,
1
trimethylsilylcyclopentadenide via the silylated Rh(I)/Rh(III)
complexes 5/6 in 66% overall yield. Starting from lithium
trimethylstannylcyclopentadienide, an analogous reaction
sequence
trimethylstannylrhodocenium
was
performed
aiming
at
a
hexafluoridophosphate,
potentially useful synthon for Stille cross-coupling reactions.
However, due to the weaker carbon-tin bond, undesired redox
processes prevented a successful reaction.
Methylrhodocenium hexafluoridophosphate (4) served in the
following as precursor for further monosubstituted
rhodocenium salts. In analogy to early work by Sheats and
coworkers,[4] aromatic side-chain oxidation of 4 with basic
potassium permanganate afforded white rhodocenium-
monocarboxylic acid hexafluoridophosphate (8) in 90%
isolated yield. Being an unusual cationic carboxylic acid, 8
could be deprotonated on an anion-exchange resin to provide
the zwitterionic rhodocenium carboxylate (9). 9 constitutes an
interesting new neutral carboxylato ligand, similar to its
analogous cobalt congener.[2d] Derivatives of monocarboxylic
acid hexafluoridophosphate (8) were synthesized by standard
organic transformations. Quantitative conversion of 8 to
rhodoceniumcarboxylic acid chloride hexafluoridophosphate
(10) was performed in refluxing thionyl chloride, followed by
esterification with methanol to methoxycarbonylrhodocenium
hexafluoridophosphate (11) or amide formation with aqueous
Figure
1
shows
the
1H-NMR
spectrum
of
rhodoceniumcarboxylic acid hexafluoridophosphate (8) as a
representative example.
ammonia
hexafluoridophosphate (12), respectively. A major target
compound of this work was aminorhodocenium
to
rhodoceniumcarboxylic
acid
amide
hexafluoridophosphate (15), due to the high synthetic potential
of the amino functionality. In analogy to cobaltocenium
chemistry,[1b,4] the azide derivative 13 was prepared from 10
with sodium azide. Conversion of compound 13 to 15 in
refluxing concentrated sulfuric acid by classic Curtius
rearrangement worked out in part according to NMR analysis
of the crude product mixture, but despite all our efforts at
optimization, separation of the product mixture and isolation of
pure 15 proved so far impossible. For “difficult” organic
substrates,
a
modified Lewis acid-assisted Curtius
rearrangement[9] to acyl-protected amines followed by
solvolysis to the free amino derivative was developed. By
applying this methodology, acetamidorhodocenium chloride
(14) was synthesized in 60% yield, but unfortunately its acidic
or basic hydrolysis to the desired aminorhodocenium
Figure 1. 1H-NMR spectrum of rhodoceniumcarboxylic acid
hexafluoridophosphate (8) in methanol-d4 solution.
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10.1002/ejic.202000071
European Journal of Inorganic Chemistry
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In the13C-NMR spectra of rhodocenium salts the signals of the
carbon atoms of the Cp ligands were observed in the region of
85-95 ppm as doublets with 1J(13C-103Rh) ≈ 7 Hz, together with
103Rh-uncoupled signals of the carbon atoms due to the
respective functional group (see Experimental Section and
spectra in the Supporting Information).
In general, characterization of rhodium compounds by 103Rh-
NMR spectroscopy[10] is rarely done, simply because most
NMR spectrometers are not adequately equipped for 103Rh
measurements. Although the element rhodium has a 103Rh
nucleus of spin of ½ with 100% natural abundance, direct
observation of 103Rh signals is quite difficult due to the
extremely low gyromagnetic ratio (g = -0.8420´107 radT-1s-1),
the low receptivity (≈ 0.2% of that of 13C), and the extremely
large chemical shift range (≈ 13000 ppm). However,
diamagnetic Rh(I) or Rh(III) compounds with scalar couplings
to other nuclei (1H, 13C, 31P) allow for application of
magnetization transfer pulse sequences like INEPT
(insensitive nuclei enhanced by polarization transfer) that
increase the sensitivity to approximately 20% of that of 13C.
Rhodocenium salts 4, 7, 8, 11, 12, and 14 with their
heteronuclear coupling to 1H and 13C [2J(1H-103Rh) ≈ 1 Hz,
1J(13C-103Rh) ≈ 7 Hz] are therefore good candidates for such
measurements. However, whereas detection of the
corresponding signal for the unsubstituted parent compound
Figure 2. Molecular structure of 3 with thermal ellipsoids at a 50% probability.
Selected bond distances (Å) and angles (°): Rh(1)-I(1) = 2.7001(3), Rh(1)-
I(2) = 2.6817(3); Rh(1)-I(1)-I(1A) = 94.402(8), I(1)-Rh(1)-I(2) = 92.024(9),
I(2)-Rh(1)-I(1A) = 91.976(9), I(1)-Rh(1)-I(1A) = 85.598(8).
–
rhodocenium hexafluoridophosphate, Cp2Rh+PF6 , proved
possible (d = –10122.80 ppm vs Rh(acac)3), repeated attempts
on substituted rhodocenium salts 4, 7, 8, 11, 12, and 14 met
so far without conclusive results, even when applying carefully
optimized spectrometer settings.
Figure 3. Molecular structure of 7 with thermal ellipsoids at a 50% probability,
counterions PF6 (65% occupancy) and I (35% occupancy) omitted for clarity.
Selected bond distance (Å: C(10)-Si(1) = 1.876(5).
Single crystal structures are available for compounds 3, 7, 8,
11, 12 and 14 (Figures 2-7). Rhodium(III) halide precursor 3
shows the expected dimeric structure with bridging and
terminal iodide ligands, similar as in derivatives with
cyclopentadienyl ligands other than Cp’.[7] Rhodocenium salts
7, 8, 11 and 12 display overall regular, undistorted metallocene
moieties with rhodium-carbon bond lengths in the range of
2.147 to 2.188 Å (see Supporting Information). In the structure
of 14, the bond distance of Rh to the substituted carbon atom
is slightly elongated [Rh(1)-C(10) = 2.201(3) Å] with a slightly
shortened C(10)-N(1) bond of 1.388(4) Å. This indicates some
contributions of an imine fulvenoid resonance structure, a
common feature of N-substituted metallocenium salts.[1b] In the
structure of 8, hydrogen-bonded dimers of the carboxylic
functionality are observed with anti-oriented rhodocenium
moieties (see Supporting Information), quite analogous to the
Figure 4. Molecular structure of 8 with thermal ellipsoids at a 50% probability,
counterion PF6 omitted for clarity. Selected bond distances (Å): C(3)-C(4) =
1.403(7), C(4)-O(1) = C(4)-O(1A) = 1.258(4).
structure
of
cobaltocenium
carboxylic
acid
hexafluoridophosphate.[1a]
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10.1002/ejic.202000071
European Journal of Inorganic Chemistry
FULL PAPER
agreement with the much higher propensity of complexes of 4d
and 5d metal ions to obey the 18 valence electron rule. Parent
rhodocene, generated by the chemical or electrochemical
reduction of the rhodocenium ion, has a half-life of only about
2
seconds at r. t.[11] It rapidly evolves into
dihydrofulvalene-bridged dimer
[{(h5-C5H5)Rh}2(µ,h4:h4-
a stable
C10H10)], where the bridging bis(diene)-type ligand donates
four electrons to every rhodium atom such that the Rh atoms
maintain their 18 VE count (Scheme 2). Further reduction of
unstable rhodocene to the associated, even more reactive
anion can nevertheless be observed as
a chemically
irreversible process and occurs at a ca. 770 mV lower (more
negative) potential (Table 1). Ring substitution at one or both
cyclopentadienyl ligands exerts a strong influence on the
reduction potentials of the +/0 and 0/- processes and the
chemical stabilities of the reduced formally 19 or 20 valence
electron rhodocenes or rhodocene anions. Permethylation of
Figure 5. Molecular structure of 11 with thermal ellipsoids at a 50%
probability, counterion PF6 omitted for clarity. Selected bond distances (Å):
C(10)-C(11) = 1.480(3), C(11)-O(1) = 1.201(2), C(11)-O(2) = 1.327(2), O(2)-
C(12) = 1.449(2).
one
Cp
ligand
as
in
[Cp*CpRh]+
(Cp*
=
pentamethylcyclopentadienyl) shifts the two consecutive
reductions by 260 mV or 480 mV to more negative potentials
but fails to stabilize the corresponding rhodocene against
dimerization; instead an unsymmetrical dimer with a new C-C
bond between a former Cp and Cp* ligand results.[12]
Methylation of the second Cp ring as in [Cp*2Rh]+ or
[Cp*CpMe4Rh]+ (CpMe4
=
h5-tetramethlycyclopentadienide)
decreases the reduction potential further but, while preventing
dimerization, still fails to chemically stabilize the corresponding
neutral rhodocenes against other follow-reactions.[12,13] Higher
stabilities of reduced rhodocenium ions were reported for
congeners
with
the
Cp’’’
(Cp’’’
=
1,2,3-
Figure 6. Molecular structure of 12 with thermal ellipsoids at a 50%
probability, counterion PF6 omitted for clarity. Selected bond distances (Å)
and angles (°): C(10)-C(11) = 1.431(4), C(11)-O(1) = 1.226(4), C(11)-N(1) =
1.335(4).
tritertbutylcyclopentadienide) ligand, culminating in the
observation of two chemically reversible reductions of the
[Cp’’’Cp*Rh]+ cation.[14] Again, sizable cathodic shifts of the
reduction potentials on replacing the Cp ligand by Cp* were
noted (Table 1). The only stable rhodocenes with a genuine
sandwich structure reported to date seem to be [(h5-
C5Ph4H)2Rh],[15] which could even be crystallized and
investigated by X-ray crystallography, and [Cp*(h5-
indenyl)Rh].[12] In the former complex, the bulky C5Ph4H ligand
prevents dimerization and other follow reactions by blocking
access to the reactive sites, while the indenyl ligand of the latter
complex can adapt to the decreasing electron demand of the
rhodium ion by a hapticity change from h5 to h3. Such ring-
slippage is particularly facile for indenyl ligands.[16]
Figure 7. Molecular structure of 14 with thermal ellipsoids at a 50%
probability, counterion Cl and H2O solvent molecule omitted for clarity
Selected bond distances (Å) and angles (°): C(10)-N(1) = 1.388(4), N(1)-
C(11) = 1.365(4), C(11)-O(1) = 1.219(3), C(11)-C(12) = 1.499(4).
Electrochemistry
The electrochemical properties of the parent rhodocenium ion
and of several ring-substituted derivatives have been studied
on several occasions. Neutral rhodocenes generally lack the
chemical stability of their lighter cobaltocene homologues in
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10.1002/ejic.202000071
European Journal of Inorganic Chemistry
FULL PAPER
R
R
R
+ e-
- e-
+ e-
- e-
Rh
Rh
Rh
Kdimerization
R
Rh
H
R
H
or other isomers
Rh
Scheme 2. The reduction of rhodocenium ions.
All of the above studies focused on rhodocenium ions with
multiply substituted Cp rings. The present series of complexes
provides us with the opportunity to probe the impact of a single
ring substituent on the redox potential of a rhodocenium ion
and the rate constant for dimerization following their initial one-
electron reduction. Cyclic and square wave voltammograms of
the various rhodocenium derivatives were measured in
THF/NBu4PF6 and DMF/NBu4PF6 as the supporting
electrolytes. In most cases the THF-based electrolyte
produced somewhat better results in terms of the accessible
potential range and the degree of chemical reversibility. Only
in the cases of the N-acetylamide derivative 14 and of
carboxylate 9 strong adsorption of the complex to the working
electrode led to distortions of the reduction wave or additional
features, which were absent in DMF.
Figure 8 compares representative cyclic voltammograms of the
various rhodocenium derivatives at a sweep rate v of 0.1 V/s in
THF (or, for 9 and 14 in DMF) at r. t.; relevant data are collected
in Table 1. Voltammograms in DMF are shown as Figures S33
to S41 in the Supporting Information. All rhodocenium
derivatives display a well-defined, chemically only partially
reversible wave or, in the case of amide derivative 12, an
irreversible cathodic peak, whose potential corresponds with
the electronic properties of the respective substituent.
Figure 8. Cyclic voltammograms of the rhodocenium salts on a Pt working
electrode (except for 8, see text) in either DMF/nBu4PF6 (0.1 M) or THF/
nBu4PF6 (0.1 M) at v = 0.1 V/s and at r.t. F and F’ denote small additional
peaks/waves that most likely result from reduction at the rhodocenium
substituent.
In principle, a smaller peak current for the second reduction
peak would also be in line with the reduction of other
electroactive follow product(s) formed by the corresponding
rhodocene. The assignment of this peak to the second
reduction of the respective rhodocenium ion is therefore only
tentative but in line with the potential separations of 770 to 990
mV for the two consecutive reductions observed for other
rhodocenium ions (Table 1). In fact, additional features that are
obviously due to other follow products formed after the first
reduction are observed (marked as peaks F/F’ in Figure 8) in
the case of derivatives 11 and 12, which feature a reducible
ester or amide substituent at the Cp ring.
Table 1. Electrochemistry Data for the Investigated Rhodocenium
Salts.[a]
E1/2+/0 [V]
(ΔEp [mV])
Ep0/- [V][b]
(ΔEp [mV])
ΔE[V][c]
E1/2ox (V)[d]
(ΔEp [mV])
Reduction potentials thus follow the ordering COOCH3
COOH > CONH2 > NHCOMe > SiMe3 > Me > COO-, in good
qualitative match with the substituents´ Hammett
»
4-Me
-1.830
(88)
-2.820
-2.720
n.a.[f]
-0.990
-0.955
n.a.[f]
-0.72
-0.53
-0.60
-0.81
-0.51[e]
parameters.[17] For most derivatives, a second, completely
irreversible reduction with an appreciably smaller peak height
as compared to that of the +/0 couple was also observed. This
decrease in peak height is due to competing dimerization of the
rhodocene produced during the first reduction at the working
electrode.
7-SiMe3
8-COOH
9[e]-COO-
11-COOMe
-1.765
(71)
-1.535
(82)
-1.900
(65)
n.a.[f]
n.a.[f]
-1.500
(91)
-2.615
-1.115
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10.1002/ejic.202000071
European Journal of Inorganic Chemistry
FULL PAPER
ferrocenes.[19] Table S38 of the Supporting Information
provides such a comparison.
12-CONH2
-1.645
(90)
-2.510
-2.230
-2.58
-0.865
-0.515
-0.77
-0.99
-0.96
n.a.
-0.790
-0.800
-0.90[f]
-0.5[f]
n.a.
14[e]-NHCOMe
Cp2Rh+[g]
-1.715
(-)[f]
Table 2. Peak currents ip for the reduction (ipc) of 4 and the first and
second anodic oxidation peaks of the follow products (ipa,fp1, ipa,fp2) from
concentration-dependent cyclic voltammograms in THF at r. t.[a]
-1.81
(-)
Cp*CpRh+[h]
Cp*2Rh+[h]
-2.07
(-)
c(mM)
6.4
ipc [μA]
43
ipa,fp,1 [μA]
4.7
ipa,fp,2 [μA]
4.2
ipa,fp1 / ipc
0.109
-3.06
-2.38
(-)
-3.34
n.a.
4.4
30
2.5
3.0
0.083
Cp’’’CpRh+[i]
Cp’’’Cp*Rh+[i]
-1.83
(-)
3.4
23
1.7
1.6
0.074
n.a.
1.6
11
0.3
0.3
0.023
-2.04
(-)
n.a.
n.a.
n.a.
[a] Data was recorded in dry, degassed THF with nBu4PF6 (40 mM) under
argon atmosphere, v = 0.1 V/s.
[a] If not noted otherwise, data in dry, degassed THF/nBu4PF6 (0.1M)
under argon atmosphere, v = 0.1 V/s. Potentials are given on the
ferrocene/ferrocenium scale (E1/2 (Cp2Fe0/+) = 0.000 V). [b] Cathodic
+/0
There is, however, one notable exception, namely carboxylic
acid 8, when measured at a platinum or a gold working
peak potential of an irreversible process. [c] Potential difference E1/2
-
0/-
Ep between first and second reductions. [d] Peak potential of the
anodic follow wave. [e] Data in DMF as solvent under otherwise identical
conditions as in [a]. [f] No second reduction was observed within the
available potential range. [g] Data in CH3CN from ref. [11a,15] [h] Data in
THF from ref.[12,15] [i] Data in THF from ref.[14b]
electrode. Based on the comparison of the half-wave potentials
0/+
for the first oxidation of ferrocene carboxylic acid (E1/2
=
0.19 V) and of its methyl ester derivative (E1/20/+ = 0.24 V, both
in CH3CN/NBu4PF6)[20] and on the differences of half-wave
potentials between the free carboxylic acid and its
deprotonated form, ferrocenecarboxylate, of 220 mV
(CH3CN/H2O 4:1[21] to 293 mV in the aprotic solvent PhCN,[22]
one would expect that the first reduction of rhodocenium
carboxylic acid 8 occurs at ca. -1.55 to -1.60 V. Instead, an
unusually broad reduction peak at -1.235 V (THF) or -1.260 V
(DMF) is observed as a chemically partially reversible (THF) or
irreversible (DMF) process with a large potential splitting
between the cathodic forward and the anodic return peak. The
initial reduction of 8 produces another considerably more ideal
(in an electrochemical, i. e. kinetic, sense), chemically partially
reversible reduction feature, whose potential matches within
experimental error that of the zwitterionic carboxylate
derivative 9. The reduction of rhodocenium carboxylic acid thus
seems to follow a different pathway in that the COOH proton is
reduced rather than the rhodocenium core, thus generating
carboxylate 9 and molecular hydrogen (Equation 1).
The first, only partially reversible reduction gives also rise to
one or two anodic follow peaks, which are solely observed after
sweeping past the +/0 wave. In keeping with the well-
established, general sequence of reaction steps in
rhodocenium electrochemistry of Scheme 2, we assign this
peak/these peaks to the oxidation of
a
neutral
dihydrofulvalene-bridged dirhodium complex formed by
dimerization of the 19 VE rhodocene. Peak potentials for this
process are also provided in Table 1. The observation of two
such peaks in the case of 4 probably indicates that two different
isomers of this dimer are formed that are distinguished by
which of the two different Cp rings couple in the C-C bond
forming process. Such a situation was already reported for the
mixed iridocene [Cp*CpIr].[18]
To further probe the dimerization hypothesis, we measured the
concentration dependence and the peak current ratios for the
first reduction wave and the first and second anodic counter
peak for the methylrhodocenium ion 4. A clear increase of the
relative peak heights of the anodic follow peaks assignable to
the oxidation of a dimer with increasing concentration levels
was noted. This is in line with a higher order reaction involving
the electrogenerated reduction product (Figure S42 of the
Supporting Information). Thus, the ratio ipa,fp/ipc for the first
anodic peak (fp) increases from 0.023 to 0.109 on a fourfold
increase of the initial concentration (Table 2). Moreover, the
ratios of the peak currents of the two anodic follow peaks
proved to be invariant to concentration within the experimental
error limits, indicating that the underlying species most
probably form in a parallel process. In general, the potential
shifts of the +/0 wave of the monosubstituted rhodocenium ions
with respect to the parent rhodocenium ion match qualitatively
well with those reported for the 0/+ couple of the corresponding
[CpRh(h5-C5H4-COOH]+ + e- ® [CpRh(h5-C5H4-COO] + ½ H2
Equation 1
In order to further probe this hypothesis we investigated the
behavior of 8 at different working electrodes (note that
reduction overpotentials for proton reduction depend on the
electrode material) and in the presence of various amounts of
tetramethylpiperidine as a strong, redox non-innocent and non-
nucleophilic base. In the case of a metallocene-based
reduction, the base would simply serve to alter the ratio of the
free acid and its conjugate base, and hence the relative heights
of the corresponding peaks/waves of these compounds, just as
it was observed in the ferrocene case.[21] In the event of proton
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10.1002/ejic.202000071
European Journal of Inorganic Chemistry
FULL PAPER
reduction, however, increasing the pH will strongly diminish the
proton activity a(H+) and therefore induce sizable cathodic
shifts of the corresponding reduction peak. Both these
anticipated effects were experimentally observed. Thus, the
position of the first reduction wave differs strongly on Pt and
Au working electrodes (Table 3), while that of the second one,
which is assigned to the reduction of the associated
carboxylate 9, varies very little. Moreover, the proton reduction
peak is completely absent at a glassy carbon electrode (see
Figure 9, left; for square wave voltammograms see Figure S43
of the Supporting Information). Furthermore, incremental
addition of tetramethylpiperidine base caused strong cathodic
shifts of the broad reduction peak (Figure 9, right).
CH3, NH(C=O)Me < SiMe3, C(=O)NH2 < COOH, COO-,
COOMe. In particular, there are rather minor differences
between the free carboxylic acid 8 and the corresponding
carboxylate 9 in spite of the vastly different Hammett
parameters of these two substituents and Coulombic repulsion
between the two negatively charged monomers of reduced 9
as opposed to neutral rhodocene 8.
Conclusions
A first chemoselective synthesis of rhodocenium carboxylic
acid hexafluoridophosphate (8) has been developed via half-
sandwich Rh(III) halide precursors. Common carboxylic acid
derivatives like esters and amides are easily prepared, opening
thereby new opportunities in bioorganometallic chemistry, e.g.
peptide conjugates containing rhodocenium moieties, or in
macromolecular and dendrimer chemistry, e.g. redox
responsive polymers or dendrimers with rhodocenium groups.
Unfortunately, aminorhodocenium is not yet accessible via
Curtius rearrangement of the corresponding rhodocenium
carboxylic azide, due to difficulties in separation of the various
rhodocenium species formed. Spectroscopic and structural
characterization of these first monosubstituted rhodocenium
salts comprises 1H/13C-NMR, IR, HR-MS and single crystal
structure analyses on several derivatives. Attempted 103Rh-
NMR measurements of these monosubstituted rhodocenium
salts met so far with limited success, due to the unfavorable
magnetic properties of the 103Rh nucleus. Like the parent
rhodocenium ion and other substituted congeners, all new
monosubstituted rhodocenium ions can be reduced to reactive
rhodocenes. The reduction potentials follow the electronic
substituent parameters of the corresponding substituent and
shift to more negative (cathodic) values for stronger donors.
The corresponding rhodocenes undergo a rapid (k = 2×103 s-1
to 2.5×105 s-1 as estimated from digital simulations of the cyclic
voltammograms) chemical follow reaction, which competes
with the second reduction process to the corresponding, even
more reactive rhodocene anions. In agreement with an
underlying dimerization process as it was established for other
rhodocenes, the chemical reversibility of the first reduction
decreases with increasing concentration of the analyte as was
exemplarily shown for the methyl derivative 4. Rhodocene
carboxylic acid hexafluoridophosphate (8) constitutes a special
case as it preferably undergoes reduction of the acidic COOH
proton to generate zwitterionic rhodoceniumcarboxylate 9 plus
dihydrogen at Pt or Au electrodes, but rhodocenium reduction
on glassy carbon, where the overpotential for proton reduction
is sufficiently large.
Table 3. Cyclic voltammetric experiments of the reduction of carbonic
acid 8.
Electrode material
Pt
Ep,f [mV] [a]
-1300
-1555
-1535
-
E1/2+/0 [mV] in DMF[b]
-1900
-1870
-
Au
Glassy C
Platinum (9)
Pt (8+0.1 eq of tmp)
Pt (8+0.2 eq of tmp)
-1900
-1900
-1900
-1730
-1790
[a] Peak potential for proton reduction. [b] Half-wave potential of the
follow wave assigned to the reduction of zwitterionic carboxylate 9.
Figure 9. Left: Cyclic voltammograms of 8 in DMF/nBu4PF6 at v = 0.1 V/s at
r.t with a platinum (Pt), a gold (Au), or a glassy carbon (glassy C) electrode.
Right: Voltammograms with a platinum electrode in the presence of 0, 0.1 or
0.2 equivalents of 2,2,6,6-tetramethylpiperidine base.
In order to elucidate the substituent´s effect on the dimerization
rate, we estimated the dimerization rate constant kdim by
digitally simulating voltammograms of every complex. Figures
S48 to S54 of the Supporting Information provide exemplary
comparisons between experimental and digitally computed
voltammograms. The derived rate constants kf and kb as well
as the equilibrium constant K = kf / kb are provided in Table S39
of the Supporting Information. The forward rate constants for
the rhodocene dimerization spread over two orders of
magnitude and range from ca. 2×103 to 2.5×105 with an ordering
Experimental Section
General
procedures:
Synthetic
work
and
spectroscopic
characterization methods, including X-ray crystallography, were
performed as recently described.[23] 103Rh NMR spectra were measured
at the University of Oslo on a Bruker BBO 500MHz S2 10mm (BBLR-
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10.1002/ejic.202000071
European Journal of Inorganic Chemistry
FULL PAPER
H-D) with Z-gradient spectrometer. Starting materials were obtained
commercially and used as received.
a rotary evaporator and extracting the residue with acetone, filtering,
evaporating the acetone and drying the now white product in vacuo.
M.p: 280 °C (darkening/decomp. from 230 °C). 1H-NMR (300 MHz,
acetone-d6): δ 2.25 (s, 3H, CH3), 5.92 (br s, 2H, 2,5-Cp‘), 6.03 (d, 5H,
2J(1H-103Rh) = 0.9 Hz, Cp), 6.06 (br s, 2H, 3,4-Cp‘) ppm. 13C{1H}-NMR
(75 MHz, acetone-d6): δ 13.77 (s, CH3), 86.90 (d, 1J(13C-103Rh) = 7.15
Hz, 2,5-Cp‘), 88.29 (d, 1J(13C-103Rh) = 6.86 Hz, 3,4-Cp‘ and Cp), 108.39
(d, 1J(13C-103Rh) = 6.9 Hz, 1-Cp‘) ppm. MS (MALDI pos): C11H9RhPF6,
calcd. most abundant isotope peak m/z = 246.9994 (M – PF6)+, found
m/z = 247.0055. IR (ATR): 3128 (w, νC-H), 1477 (w), 1454 (w), 1417 (w),
1396 (w), 1378 (w), 1033 (w), 1009 (w), 900 (w), 878 (w), 810 (s, νP-F),
628 (w), 553 (s, νP-F), 413 (m) cm-1.
Electrochemical Measurements: All voltammetric measurements
were conducted in a custom-made one-compartment cell with a spiral-
shaped Pt wire and an Ag wire as the counter and reference electrodes
respectively. As working electrode, a platinum (gold, glassy-carbon)
electrode is used, that is polished with 1 µm and 0.25 µm diamond
paste from Buehler-Wirtz consecutively prior to measurement. The cell
is attachable to a conventional Schlenk line and all experiments were
conducted under Argon atmosphere. The supporting electrolyte
NBu4+PF6- was dissolved in THF or DMF respectively to yield 5 mL of
degassed analyte solution. Voltammograms were referenced by the
addition of an internal reference of ferrocene. The data was acquired
with a computer-controlled BASi potentiostat. Any additives were
added to the cell under Argon (eg. 2,2,6,6-tetramethylpiperidine as a
base). All simulations of cyclic voltammograms were conducted by
using DigiSim[24] with the following fixed parameters: electrode surface
Di(trimethylsilylcyclopentadienyl)dirhodium(III) tetraiodide (6): A
Schlenk vessel was charged with trimethylsilylcyclopentadiene (69.0 µl,
0.42 mmol, 2.1 eq) and 15 mL anhydrous THF. The mixture was cooled
to -80 °C and 168 µl of a 2.5 M butyllithium hexane solution (0.42 mmol,
2.1 eq) was added to effect deprotonation. The solution was allowed to
warm to approximately 0 °C and chlorido(cyclooctadiene)rhodium(I)
dimer, [(cod)RhCl]2 (1) (97.6 mg, 0.20 mmol) was added. After stirring
for two hours the solvent was evaporated, the residue extracted with
pentane, filtered and the solvent was evaporated, affording a yellow oil
of (cyclooctadiene)(trimethylsilylcyclopentadienyl)Rh(I) (5). Without
protection from air, this material was dissolved in dichloromethane and
iodine (101.5 mg, 0.40 mmol, 2.0 eq) was added. After stirring overnight,
heptane was added, dichloromethane was evaporated and the product
was filtered off, washed with pentane and dried in vacuo, affording
149.6 mg (0.15 mmol, 76.2 yield%) 6. 1H-NMR (300 MHz, CD2Cl2): δ
0.36 (s, 9H, TMS), 5.59 (m, 2H, Cp), 5.66 (m, 2H, Cp) ppm. 13C{1H}-
NMR (75 MHz, CD2Cl2): δ 0.26 (s, TMS), 85.61 (d, 1J(13C-103Rh) = 6.79
Hz, Cp), 86.39 (d, 1J(13C-103Rh) = 6.81 Hz, Cp), 92.20 (d, 1J(13C-103Rh)
= 6.46 Hz, Cp) ppm.
area = 0.0256 cm2, T = 298 K and non-Faradaic currents = 3.2 x 10-7
F
and other variable parameters shown in Table S38 of the Supporting
Information. Iteration of experimental cyclic voltammograms recorded
over a scan rate range of 400-2000 mV/s, the program adjusted these
variable parameters to best universal fits of the CV shapes at any given
scan rate.
Di(methylcyclopentadienyl)dirhodium(III) tetraiodide (3): Methyl-
cyclopentadienide thallium (Cp’Tl) was prepared by reaction of 0.94 ml
(9.33 mmol, 2.1 eq) of freshly cracked methylcyclopentadiene with 2.46
g (9.33 mmol, 2.1 eq) of TlOAc and 0.52g (9.33 mmol, 2.1 eq) KOH in
anhydrous ethanol under Ar-atmosphere. A white precipate was formed
and the reaction was carried on for one hour. Volatiles were removed
in vacuo, the residue was dispersed in anhydrous THF, 2.19 g (4.44
mmol, 1 eq) of chlorido(cyclooctadiene)rhodium(I) dimer, [(cod)RhCl]2
(1), was added and the reaction mixture was stirred for 3 hours. Workup
under ambient conditions without protection from air: Volatiles were
removed on a rotary evaporator, the residue was extracted with
pentanes and, after removal of pentane on a rotary evaporator, yellow,
oily (cyclooctadiene)(methylcyclopentadienyl)Rh(I) (2) was obtained.
Without further purification and without protection from air, 2 was
dissolved in dichloromethane and iodine (2.25 g, 8.88 mmol, 2 eq) was
added. After 18 hours of stirring, heptane was added to the
solution/dispersion, the dichloromethane was evaporated on a rotary
evaporator, the precipitated product was filtered off on a fine glass frit,
washed with pentane and dried in vacuo, affording 3.45 g (3.96 mmol,
89%) of [Cp’RhI2]2 (3) as a dark brown powder. 1H-NMR (300 MHz,
CD2Cl2): δ 2.23 (s, 3H, CH3), 5.45 (d, 4H, 2J(1H-103Rh) = 1.32 Hz, CpH)
ppm. 13C{1H}-NMR (75 MHz, CD2Cl2): δ 13.77 (CH3), 83.4 (Cp), 85.7
(Cp) ppm. Single crystal structure analysis: Figure 2, supporting
information.
Trimethylsilylrhodocenium hexafluroidophosphate (7): A Schlenk
vessel was charged with 6 (149.6 mg, 0.15 mmol, 1 eq) and 10 mL
anhydrous acetone. AgPF6 (80.6 mg, 0.32 mmol, 2.1 eq) was added
and the mixture was stirred under protection from light for one hour,
resulting in red suspension. Thallium cyclopentadienide (89.6 mg, 0.33
mmol, 2.2 eq) was added and the mixture was stirred overnight. Then
the solution was concentrated to about a tenth of its volume and diethyl
ether was added. Solids were filtered off and washed with
acetone/ether (v:v = 1:5). The solvents were evaporated and the
residue dried in vacuo to yield 118.3 mg 7 (0.26 mmol, 87%) as a
greyish crystalline powder. M.p: 250 °C (decomp.). 1H-NMR (300 MHz,
CD2Cl2): δ 0.30 (s, 9H, TMS), 5.77 (m, 2H, CpTMS), 5.86 (d, 2J(1H-103Rh)
= 0.97 Hz, 5H, Cp), 6.01 (m, 2H, CpTMS) ppm. 13C{1H}-NMR (75 MHz,
CD2Cl2): δ 0.45 (s, TMS), 87.52 (d, 1J(13C-103Rh) = 6.78 Hz, Cp), 89.77
(d, 1J(13C-103Rh) = 7.31 Hz, CpTMS), 91.70 (d, 1J(13C-103Rh) = 7.20 Hz,
CpTMS) ppm. MS (FTMS + pESI Full MS): C13H18SiRhPF6, calcd. most
abundant isotope peak m/z = 305.0233 (M – PF6)+, found m/z =
305.0217. IR (ATR): 3124 (w, νC-H), 2960 (w), 1417 (m), 1388 (m), 1259
(m), 1248 (m), 1160 (m), 1043 (m), 1011 (w), 897 (m), 883 (m), 818 (s,
νP-F), 761 (s), 701 (w), 629 (m), 555 (s, νP-F), 461 (m), 428 (m) cm-1.
Single crystal structure analysis: Figure 3, supporting information.
Methylrhodocenium hexafluoridophosphate (4): A Schlenk vessel
was charged under an atmosphere of Ar with 3.45 g (3.96 mmol, 1 eq)
3 and 50 mL of dry dichloromethane. To the resulting mixture 4.2 ml
(8.32 mmol, 2.1 eq) of sodium cyclopentadienide solution in THF was
added under vigorous stirring. A greyish precipitate formed and the
suspension was stirred overnight. Workup under ambient conditions:
The mixture was filtered, water was added and the dichloromethane
was evaporated. The aqueous solution was filtered again and the
product was precipitated by addition of aqueous hexafluorophosphoric
acid (1.23 ml, 8.32 mmol, 2.1 eq). The precipitate was filtered off,
washed with cold water and dried in vacuo affording 2.39 g (6.1 mmol,
77%) 4 as a white powder. If the product is still colored from residual
iodine, the product can be purified by dissolving it in a saturated
aqueous solution of sodium thiosulfate, then evaporating the water on
Rhodoceniumcarboxylic acid hexafluoridophosphate (8):
A
solution of 4 (2.39 g, 6.1 mmol, 1 eq), KMnO4 (2.41 g, 6.1 mmol, 2.5
eq) and 1.83 ml (18.3 mmol, 3 eq) 10 M NaOH in water was refluxed
for 18h. After cooling to room temperature, MnO2 was filtered off and
washed with five portions of acetonitrile. After the organic fractions
were evaporated and the remaining aqueous solution was
concentrated on a rotary evaporator, 4.5 ml (27.5 mmol, 4.5 eq)
aqueous HPF6 was then added at 0 °C. The white precipitate was
filtered off and washed with cold water. The product (2.32 g, 5.5 mmol,
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10.1002/ejic.202000071
European Journal of Inorganic Chemistry
FULL PAPER
90.0%) was dissolved in acetone and filtered, the solution was
evaporated and the white powder dried in vacuo. If traces of
methylrhodocenium (from incomplete oxidation) or rhodocenium (from
cyclopentadiene-containing methylcyclopentadiene) are found,
recrystallization from an acetone/dichloromethane/diethylether (4:2:1)
mixture afforded rhodeniumcarboxylic acid hexafluorophosphate (8).
Crystals suitable for x-ray analysis were obtained by slow evaporation
of a solution in acetone. M.p.: 240 – 250 °C (decomp.). 1H-NMR (300
MHz, CD3CN): δ 5.93 (d, 5H, 2J(1H-103Rh) = 0.75 Hz, Cp), 5.96 (m, 2H,
3,4-CpCOOH), 6.27 (m, 2H, 2,5- CpCOOH) ppm. 13C{1H}-NMR (75 MHz,
CD3CN): δ 89.02 (d, 1J(13C-103Rh) = 6.80 Hz, 3,4- CpCOOH ), 89.44 (d,
1J(13C-103Rh) = 6.99 Hz, Cp), 90.16 (d, 1J(13C-103Rh) = 6.76 Hz, 2,5-
CpCOOH), 93.77 (d, J = 8.75 Hz, 1-CpCOOH), 163.76 (s, COOH) ppm. MS
(MALDI pos, [m/z]): C11H10O2RhPF6, calcd. most abundant isotope
peak m/z = 276.9736 (M – PF6)+, found m/z = 277.0050. IR (ATR): 3129
(w, νC-H), 1709 (m, νC=O), 1584 (w), 1491 (m), 1415 (m), 1298 (m), 1178
(m), 1031 (m), 807 (s, νP-F), 751 (s), 553 (s, νP-F), 454 (m) cm-1. Single
crystal structure analysis: Figure 4, supporting information.
(from 601 mg, 1.42 mmol rhodocenium carboxylic acid
hexafluoridophosphate (8)) was dissolved in 10 ml dry acetonitrile and
0.53 ml (7.12 mmol, 5 eq) of 25% ammonia in water was added. The
solution was stirred for two hours, filtered and evaporated to dryness.
Drying in vacuo yielded 57% (344 mg, 0.817 mmol) 12 as slightly
greyish crystalline powder. Single crystals suitable for x-ray analysis
were grown by slow concentration of a solution in acetonitrile. M.p.:
173 °C. 1H-NMR (300 MHz, CD3CN): δ 5.90 (d, 2J(1H-103Rh) = 1.00 Hz,
5H, Cp), 5.91 (m, 2H, 2,5-CpCONH2), 6.25 (m, 2H, 3,4- CpCONH2), 6.73
(br s, 2H, NH2) ppm. 13C{1H}-NMR (75 MHz, CD3CN): δ 88.00 (d,
1J(13C-103Rh) = 6.93 Hz, 2,5-CpCONH2), 89.59 (m, Cp and 3,4- CpCONH2),
163.52 (s, CONH2) ppm. MS (MALDI pos,): C11H11NORhPF6, calcd.
most abundant isotope peak m/z = 275.9896 (M – PF6)+, found m/z =
275.9996. IR (ATR): 3460 (w), 3315 (w), 3128 (w, νC-H), 2923 (w), 2853
(w), 1687 (m, amide I), 1618 (w, amid II), 1474 (w), 1419 (m), 1396 (m),
1334 (w), 1249 (w), 1175 (w), 1137 (w), 1071 (w), 1029 (w), 955 (w),
904 (w), 809 (s, νP-F), 778 (s), 686 (m), 642 (w), 598 (w), 553 (s, νP-F),
523 (m), 442 (m) cm-1. Single crystal structure analysis: Figure 6,
supporting information.
Rhodoceniumcarboxylate (9): A concentrated solution of 8 (280 mg,
0.60 mmol) in 8 ml water/acetonitrile 3:1 was poured upon a column
filled with DOWEX Marathon ATM anion-exchange resin and slowly
eluated. After evaporating of solvents the yellowish product was dried
in vacuo, it is slightly hygroscopic. Yield: 93% (153 mg, 0.55 mmol).
M.p.: 240 °C (decomp.). 1H-NMR (300 MHz, methanol-d4): δ 5.97 (m,
2H, CpCOO-), 5.99 (d, 2J(1H-103Rh) = 0.97 Hz, 5H, Cp), 6.22 (m, 2H,
CpCOO-) ppm. 13C{1H}-NMR (75 MHz, methanol-d4): δ 88.80 (d, 1J(13C-
103Rh) = 2.55 Hz, CpCOO-), 88.89 (d, 1J(13C-103Rh) = 2.85 Hz, CpCOO-),
88.98 (d, 1J(13C-103Rh) = 7.08 Hz, Cp) ppm. IR (ATR): 3367 (broad, m),
3099 (m), 3068 (m), 1598 (s), 1455 (m), 1415 (m), 1398 (m), 1382 (m),
1356 (m), 1331 (m), 1029 (m), 1006 (m), 897 (m), 857 (m), 807 (m),
777 (m), 699 (m), 576 (m), 530 (m), 452 (m) cm-1.
Rhodoceniumcarboxylic acid azide hexafluoridophosphate (13):
Rhodoceniumcarboxylic acid chloride hexafluoridophosphate (10)
(from 380 mg, 0.900 mmol rhodocenium carboxylic acid
hexafluoridophosphate (8)) was dissolved in
a mixture of dry
nitromethane and t-BuOH (v/v = 1/1). Under stirring, NaN3 (1.17 g, 18.0
mmol, 20 eq) was added and after 4 hours solvents were evaporated,
the residue was extracted with acetone, filtered, the solvent was
evaporated and the residue was dried in vacuo. Quantitative
transformation is assumed, the azide is used immediately for further
reactions. IR (ATR): 3127 (w, νC-H), 2201 (w), 2156 (w), 1688 (m, νC=O),
1609 (w), 1462 (w), 1417 (w), 1398 (w), 1371 (w), 1265 (m), 1184 (m),
1054 (w), 1030 (w), 1003 (w), 899 (w), 810 (s, νP-F), 751 (s), 639 (w),
554 (s, νP-F), 490 (w), 434 (m), 407 (m) cm-1.
Rhodoceniumcarboxylic acid chloride hexafluoridophosphate
(10): Rhodoceniumcarboxylic acid hexafluoridophosphate (5) (380 mg,
0.900 mmol) was refluxed in 13 ml of thionyl chloride overnight. The
excess of thionyl chloride was then evaporated and the residue dried
in vacuo. The slightly orange, air-sensitive powder was then used
directly for further reactions. IR (ATR): 3126 (w, νC-H), 1768 (m, νC=O),
1718 (m), 1416 (w), 1401 (w), 1236 (m), 1049 (m), 942 (w), 823 (s),
790 (s, νP-F), 554 (s, νP-F), 452 (m), 409 (m) cm-1.
Acetamidorhodocenium chloride hydrate (14): Freshly prepared
rhodoceniumcarboxylic acid azide hexafluorophosphate (13) (from 380
mg, 0.900 mmol rhodoceniumcarboxylic acid hexafluorophosphate (8))
was dissolved in acetic acid anhydride and 240 mg (1.8 mmol, 2 eq)
AlCl3 was added. After reflux over night 5 ml of water was slowly added
under cooling and stirring for another 30 min. Then the solution was
evaporated to dryness and extracted with acetonitrile, filtered, again
evaporated to dryness and dried in vacuo, affording 174 mg 14 (0.54
mmol, 60%). Single crystals suitable for X-ray analysis were grown by
slow concentration of a solution in methanol. M.p.: 259 °C (loss of water
from 120 °C). 1H-NMR (300 MHz, methanol-d4): δ 2.08 (s, 3H, CH3),
5.80 (m, 2H, Cp’), 5.94 (d, 5H, 2J(1H-103Rh) = 0.7 Hz, Cp), 6.32 (m, 2H,
Cp’) ppm. 13C{1H}-NMR (75 MHz, methanol-d4): δ 23.63 (CH3), 77.90
(d, 1J(13C-103Rh) = 6.92 Hz, CpNHR), 83.69 (d, 1J(13C-103Rh) = 8.64 Hz,
CpNHR), 88.30 (d, 1J(13C-103Rh) = 7.72 Hz, Cp) ppm. MS (FTMS + pESI
Full MS): C12H13NORhPF6, calcd. most abundant isotope peak m/z =
290.0052 (M – PF6)+, found m/z = 290.0035. IR (ATR): 3481 (w), 3406
(w), 3151 (w), 3083 (w), 2991 (w), 2769 (w), 1719 (w), 1692 (m), 1609
(w), 1551 (m), 1490 (m), 1415 (w), 1390 (w), 1375 (m), 1336 (w), 1266
(m), 1245 (m), 1105 (w), 1042 (w), 1009 (w), 842 (m), 756 (w), 648 (w),
604 (w), 580 (w), 556 (w), 474 (w), 449 (w), 419 (m) cm-1. Single crystal
structure analysis: Figure 7, supporting information.
Methoxycarbonylrhodocenium
hexafluoridophosphate
(11):
Rhodoceniumcarboxylic acid hexafluoridophosphate (5) (100.2 mg,
0.237 mmol) was transformed to the acid chloride as described above,
dried and dissolved in 5 ml dry methanol. After stirring overnight and
evaporating to dryness the residue was extracted with acetone, the
solution filtered, evaporated to dryness and the product dried in vacuo.
Crystals suitable for x-ray analysis were obtained by slow evaporation
of a solution in methanol. Yield: 65.6 % (67.9 mg, 0.156 mmol). M.p.:
225°C (decomp.). 1H-NMR (300 MHz, acetone-d6): δ 3.87 (s, 3H,
OCH3), 6.23 (d, 5H, 2J(1H-103Rh) = 0.9 Hz, Cp), 6.26 (m, 2H, Cp‘), 6.53
(m, 2H, Cp‘) ppm. 13C{1H}-NMR (75 MHz, acetone-d6): δ 53.63 (s,
OCH3), 88.87 (d, 1J(13C-103Rh) = 6.65 Hz, CpCOOMe), 89.51 (d, 1J(13C-
103Rh) = 7.41 Hz, Cp), 90.26 (d, 1J(13C-103Rh) = 7.03 Hz, CpCOOMe) ppm.
MS (FTMS + pESI Full MS): C12H12O2RhPF6, calcd. most abundant
isotope peak m/z = 290.9892 (M – PF6)+, found m/z = 290.9879. IR
(ATR): 3115 (m, νC-H), 2953 (w), 1721 (s, νC=O), 1473 (m), 1416 (m),
1397 (w), 1285 (m), 1153 (m), 1031 (m), 1008 (m), 964 (m), 900 (w),
822 (s, νP-F), 777 (s), 554 (s, νP-F), 498 (m), 453 (m) cm-1. Single crystal
structure analysis: Figure 5, supporting information.
CCDC 1973357 (for 3), CCDC 1973358 (for 7), CCDC 1973359 (for 8),
CCDC 1973360 (for 11), CCDC 1973361 (for 12), CCDC 1973362 (for
14) contain the supplementary crystallographic data for this paper.
These data can be obtained free of charge from the Cambridge
Crystallographic Data Centre.
Rhodoceniumcarboxylic acid amide hexafluoridophosphate (12):
Rhodoceniumcarboxylic acid chloride hexafluoridophosphate (10)
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10.1002/ejic.202000071
European Journal of Inorganic Chemistry
FULL PAPER
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This work was supported by the Austrian Science Fund (FWF),
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10.1002/ejic.202000071
European Journal of Inorganic Chemistry
FULL PAPER
Entry for the Table of Contents
FULL PAPER
A first chemoselective route to
monofunctionalized rhodocenium salts
has been developed. Spectroscopic,
structural and electrochemical
properties are reported.
Metallocenes
Markus Jochriem, Larissa A. Casper,
Stefan Vanicek, Dirk Petersen, Holger
Kopacka, Klaus Wurst, Thomas Müller,
Rainer F. Winter* and Benno Bildstein*
Page No. – Page No.
Rhodocenium monocarboxylic acid
hexafluoridophosphate and its
derivatives: synthesis, spectroscopy,
structure and electrochemistry
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