Beyond Takai's Olefination Reagent: Persistent Dehalogenation Emerges in a Chromium(III)-μ3-Methylidyne Complex

Article abstract of DOI:10.1002/anie.202106608

Reaction of CHI₃ with six equivalents of CrCl₂ in THF at low temperatures affords [Cr₃Cl₃(μ₂-Cl)₃(μ₃-CH)(thf)₆] as the first isolable high-yield Cr^III μ₃-methylidyne complex. Substitution of the terminal chlorido ligands via salt metathesis with alkali-metal cyclopentadienides generates isostructural half-sandwich chromium(III)-μ₃-methylidynes [Cp^R₃Cr₃(μ₂-Cl)₃(μ₃-CH)] (Cp^R=C₅H₅, C₅Me₅, C₅H₄SiMe₃). Side and decomposition products of the Cl/Cp^R exchange reactions were identified and structurally characterized for [Cr₄(μ₂-Cl)₄(μ₂-I)₂(μ₄-O)(thf)₄] and [(η⁵-C₅H₄SiMe₃)CrCl(μ₂-Cl)₂Li(thf)₂]. The Cl/Cp^R exchange drastically changed the ambient-temperature effective magnetic moment μ_eff from 9.30/9.11 μ_B (solution/solid) to 3.63/4.32 μ_B (Cp^R=C₅Me₅). Reactions of [Cr₃Cl₃(μ₂-Cl)₃(μ₃-CH)(thf)₆] with aldehydes and ketones produce intricate mixtures of species through oxy/methylidyne exchange, which were partially identified as radical recombination products through GC/MS analysis and ¹H NMR spectroscopy.

Full text of DOI:10.1002/anie.202106608

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Coordination Chemistry
Beyond Takaiꢀs Olefination Reagent: Persistent Dehalogenation
Emerges in a Chromium(III)-m3-Methylidyne Complex
Simon Trzmiel, Jan Langmann, Daniel Werner, Cꢀcilia Maichle-Mçssmer, Wolfgang Scherer,*
and Reiner Anwander*
Abstract: Reaction of CHI3 with six equivalents of CrCl2 in
THF at low temperatures affords [Cr3Cl3(m2-Cl)3(m3-CH)(thf)6]
as the first isolable high-yield CrIII m3-methylidyne complex.
Substitution of the terminal chlorido ligands via salt metathesis
with alkali-metal cyclopentadienides generates isostructural
observed in an argon matrix (8 K),[5] heteroatom-substituted
ꢀ
carbyne derivatives such as [(C5Me5)Cr( CNiPr2)(CNtBu)3]-
[PF6]2 and the trimetallic cluster [Cp3Cr3(m2-Cl)3(m3-CH)]
[6]
(A)[7] display crystalline compounds.[7] Purple trivalent A has
remained the only structurally characterized methylidyne
complex of chromium.[7,8] On the other hand, the M3(m3-CH)
entity exhibits a common structural motif detected through-
out d-transition metal chemistry (Ti, Fe, Co, Ru, Re and
Os).[9] Prominent examples of the m3-alkylidyne compound
class are tricobalt nonacarbonyl clusters, which were inves-
tigated comprehensively by Seyferth et al.[10] Methylidyne
complexes structurally related to A comprise [{Cp*Ti(m2-
O)}3(m3-CH)][11] and [Cp*3Mo3(m2-O)2(m2-CH2)(m3-CH)][12] the
reactivity of which has been investigated as well.
half-sandwich chromium(III)-m3-methylidynes [CpR Cr3(m2-
3
Cl)3(m3-CH)] (CpR = C5H5, C5Me5, C5H4SiMe3). Side and
decomposition products of the Cl/CpR exchange reactions
were identified and structurally characterized for [Cr4(m2-
Cl)4(m2-I)2(m4-O)(thf)4] and [(h5-C5H4SiMe3)CrCl(m2-Cl)2Li-
(thf)2]. The Cl/CpR exchange drastically changed the ambi-
ent-temperature effective magnetic moment meff from 9.30/
9.11 mB (solution/solid) to 3.63/4.32 mB (CpR = C5Me5). Reac-
tions of [Cr3Cl3(m2-Cl)3(m3-CH)(thf)6] with aldehydes and
ketones produce intricate mixtures of species through oxy/
methylidyne exchange, which were partially identified as
radical recombination products through GC/MS analysis and
1H NMR spectroscopy.
Complex A has been obtained by thermal treatment
(608C) of [CpCr(CH3)(m2-Cl)]2 via multiple abstraction of
hydrogen from a methyl ligand,[13] while its reactivity was not
commented on.[7] Dehalogenation of organic halides features
another viable pathway to alkylidyne/methylidyne com-
plexes.[14] Both pathways can proceed via intermediate
alkylidene/methylidene species. For chromium, well-defined
alkylidene species are just as rare as alkylidyne com-
plexes.[15,16] The most prominent chromium alkylidene com-
plex is Takaiꢀs olefination/cyclopropanation reagent [Cr2Cl2-
(m2-Cl)2(m2-CHI)(thf)4] (B, shown in Scheme 1).[16] The active
reagent is routinely generated in situ applying the dehaloge-
nation protocol, with CrCl2 and CHX3 (X = Cl, Br, I) as the
main components in varying ratios.[17] Recently, we succeeded
in determining the solid-state structure of the Takai haloal-
kylidene complex B,[18] only confirming the connectivity
originally proposed by Takai. By taking a closer look at the
formation of the Takai olefination reagent, we have now
uncovered the chromium(III) methylidyne species [Cr3Cl3(m2-
Introduction
Carbyne or alkylidyne moieties display archetypal ligands
in organo(transition)metal chemistry.[1] In particular, alkyli-
dyne complexes of the high-oxidation-state heavier group 6
metals molybdenum and tungsten emerged as eminent alkyne
metathesis catalysts.[2] Such discrete complexes feature multi-
ꢀ
ply bonded terminal moieties of the type M CR (M = Mo, W)
and have been studied comprehensively.[3,4] On the other
hand, derivatives of the first-row homologue chromium are
ꢀ
very rare. While molecules [X3Cr CH] (X = F, Cl) have been
[*] S. Trzmiel, Dr. D. Werner, Dr. C. Maichle-Mçssmer,
Prof. Dr. R. Anwander
Institut fꢀr Anorganische Chemie, Eberhard-Karls-Universitꢁt Tꢀbin-
gen
Auf der Morgenstelle 18, 72076 Tꢀbingen (Germany)
E-mail: reiner.anwander@uni-tuebingen.de
J. Langmann, Prof. Dr. W. Scherer
Institut fꢀr Physik, Universitꢁt Augsburg
Universitꢁtsstr. 1, 86159 Augsburg (Germany)
E-mail: wolfgang.scherer@physik.uni-augsburg.de
Supporting information and the ORCID identification number(s) for
the author(s) of this article can be found under:
https://doi.org/10.1002/anie.202106608.
ꢂ 2021 The Authors. Angewandte Chemie International Edition
published by Wiley-VCH GmbH. This is an open access article under
the terms of the Creative Commons Attribution License, which
permits use, distribution and reproduction in any medium, provided
the original work is properly cited.
Scheme 1. Synthesis of methylidyne complex 1, directly (upper path)
or via the Takai olefination reagent B (lower path).
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Cl)3(m3-CH)(thf)6] (1) as the ultimate product of the CrCl2/
molecules each complete the slightly distorted octahedral
CHI3 reaction. Interestingly, complex 1 engages in selective
halogenido exchange reactions, preserving the M3(m3-CH)
entity. Preliminary conversions of aldehydes or ketones
revealed reaction pathways involving radical intermediates.
coordination of the CrIII atoms.[19]
À
The Cr (m3-CH) distances in 1 of 2.018(3)/2.019(3)/2.022-
(3) ꢁ appear slightly longer than those in Theopoldꢂs com-
pound [Cp3Cr3(m2-Cl)3(m3-CH)] (A; 1.935(10) and 1.949-
À
(14) ꢁ), as are the bridging Cr Cl distances (2.3328(7) to
2.4186(7) ꢁ versus 2.348(4) to 2.360(4) ꢁ).[7] The average
Cr—Cr distance of 3.167 ꢁ is also considerably longer than in
Results and Discussion
A (2.82 ꢁ), which has been referred to as an unusually short
[7]
À
Formation of Methylidyne Complex [Cr3Cl3(m2-Cl)3(m3-CH)(thf)6]
(1) in the Reaction of Chromium(II) Chloride with Iodoform
contact (range for Cr Cr single bonds: 2.65–2.97 ꢁ).
Correspondingly, both the Cr-Cl-Cr and Cr-C-Cr angles are
more flat in 1 (81.88(2)–82.96(2)8; 102.72(12)–103.66(12)8)
than in A (73.0(1) and 73.9(1)8; 92.4(6) and 93.8(5)8).
The Takai olefination reagent is routinely generated in
situ via a 3:1 mixture of CrCl2 and CHX3 (Scheme 1). The
original report also mentioned the use of a 4:1 ratio in case of
bromoform which did not significantly affect the yield and E/
Z ratio of the alkenyl halide product.[17] Therefore we
pondered about whether use of excessive CrCl2 would affect,
if at all, the formation of Takai reagent B. Surprisingly, the
reaction of CrCl2 with CHI3 in a 6:1 molar ratio at À358C in
THF afforded the red methylidyne complex [Cr3Cl3(m2-Cl)3-
(m3-CH)(thf)6] (1) in up to 70% yield (Scheme 1, Figure 1)
along with the precipitation of three equivalents of CrCl2I-
(thf)3.
The 1H NMR spectroscopic investigation of 1 in [D8]THF
did not reveal any signal for the m3-CH proton in the range of
À500 to 500 ppm, presumably caused by paramagnetic
broadening.[20] Also, any distinct m3-CH vibration band was
not detectable by IR-spectroscopy (ESI, Figure S26). The
effective magnetic moment of 1 in dissolved and solid form
was determined by the Evans method[21] and SQUID
magnetic measurements, respectively. Both methods consis-
tently point to ferro- or ferrimagnetic coupling between the
individual CrIII centers already at ambient temperature. The
derived values of meff (Evans method: 9.30 mB; SQUID:
9.11 mB) are significantly larger than those expected for three
uncoupled CrIII centers (6.71 mB). Notably, the effective
moment of solid 1 is nearly temperature independent down
to 2 K (Figure 2, Figure S30). A fit of the field-dependent
molar magnetization Mmol(H) at 2 K with a Brillouin function
(the Landꢃ g-factor was assumed to be 2.0) yields a spin
quantum number of S = 4.45(4) which is in line with a S = 9/2
Figure 1. Crystal structure of 1, ellipsoids shown at 50% probability,
THF lattice solvent and hydrogen atoms are omitted for clarity.
Selected interatomic distances/angles are listed in the Supporting
Information (ESI, Figure S17).[32]
Compound 1 is also accessible via B and addition of
another two equivalents of CrCl2 (Scheme 1). Crystallization
from the THF solution at À358C yielded compound 1 as
a microcrystalline solid. Repeated crystallization increased
the overall yield, but to the expense of co-crystallizing
CrCl2I(thf)3. Crystallization from less concentrated solutions
gave red plates suitable for X-ray diffraction (XRD) analysis.
The crystal structure of 1 shows the known tetrahedral M3(m3-
CH) structural motif, with three chromium atoms forming
a nearly equilateral triangle (Figure 1). Three m2-bridging
chlorido ligands complement the cluster core, resembling
a truncated cube. One terminal chlorido and two THF
Figure 2. Temperature-dependent molar magnetic susceptibility cmol(T)
(black open symbols; left ordinate) and effective magnetic moment
meff(T) (red filled symbols; right ordinate) as obtained by SQUID
magnetic measurements on crystalline powders of 1 and 3 in applied
fields H=3 kOe and 10 kOe, respectively. The cmol(T) data were
corrected for diamagnetic contributions (1: À4.243ꢃ10À4 emumolÀ1
3: À3.071ꢃ10À4 emumolÀ1; calculated from Pascal’s constants),[23]
and a spin-only g factor of 2.0 was assumed in the calculation of
;
meff(T). Note, that 1 contains an additional THF solvent molecule per
formula unit in the crystal packing.
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ground state (Figure S32). A similar large meff value of 9.61
was found for the related chromium chlorocarbyne complex
[Cr3Cl3(m2-Cl)3(m3-CCl)(thf)6] assuming an S = 9/2 ground
state.[8] We note, that also the tetranuclear CrIII/CrII complex
[CpR4Cr4(m2-H)5(m3-H)2] (CpR = h5-tetramethyl-ethyl-cyclo-
pentadienyl) displays a temperature-independent high meff of
8.1 mB with S = 3.4(2) which is due to intramolecular ferri-
magnetic couplings and in line with a S = 7/2 ground state.[22]
Compound 1 was found to be infinitely stable in the solid
state, while high purity samples showed minor decomposition
in THF at À358C over several weeks. Thermal decomposition
of 1 in THF occurred rapidly above 408C (as indicated by
a gradual color change from red to yellow). Similarly,
progressive decomposition of 1 was observed in non-coordi-
nating solvents like toluene as indicated by the formation of
a precipitate as well as decoloration. Utilization of high-purity
reactants is crucial for the successful synthesis of 1, as water-
containing solvents or oxygen-containing impurities of iodo-
form or CrCl2 (99.99% trace metal basis, anhydrous CrCl2)
led to partially inseparable decomposition/side products, as
evidenced for the serendipitous identification of [Cr4(m2-
Cl)4(m2-I)2(m4-O)(thf)4] (2a) and mixed-valent [Cr4Cl(m2-Cl)4-
(m2-I)2(m4-O)(thp)4] (2b, thp = tetrahydropyran) by XRD
analysis. As complexes 2 were obtained in minute amounts
and the crystals were of poor quality, the crystal structures
represent only connectivities (Figures 3/S18/S19). The molec-
ular structure of 2a is isostructural to [Cr4(m2-Cl)6(m4-O)(thf)4]
reported by Cotton et al. with two iodido ligands instead of
chloridos.[24] The core of complexes 2 features a [M4O]6+
entity with the central oxygen tetrahedrally coordinated by
the metal atoms. Each 5-coordinate chromium(II) engages
further in two chlorido and one iodido bridge, and the
coordination of a THF molecule. The single 6-coordinate
chromium(III) in 2b is additionally coordinated by a terminal
chlorido ligand. Compounds 2 probably formed at an early
stage of the reaction, most likely due to solvent water
impurities. However, neither 2a nor 2b could be reproduced
by the admittance of deliberate amounts of water to the
reaction mixture.
Formation of Half-Sandwich Methylidyne Complexes [CpR Cr3(m2-
3
Cl)3(m3-CH)(thf)6] via Selective Cl/CpR Exchange
With compound 1 accessible in decent yields, we targeted
the selective exchangeability of the terminal chlorido ligands.
As the cyclopentadienyl ligand proved a stabilizing ligand for
the M3(m3-CH) entity in general, and specifically for com-
pound A, we probed the reactivity of 1 toward NaCp (Cp =
C5H5) and the substituted cyclopentadienides LiCp* and
LiCp’ (Cp* = C5Me5, Cp’ = [C5H4(SiMe3)] (Scheme 2).
Scheme 2. Synthesis of trimetallic half-sandwich methylidyne com-
plexes; representation of the suggested equilibrium of ate complex 8
in solution.
Reaction of 1 with three equivalents of NaCp in THF at
À508C yielded a clear dark purple solution. Crystallization
from n-pentane gave purple needles of [Cp3Cr3(m2-Cl)3(m3-
CH)] (A) in good yield (78%). Lower yields at elevated
temperatures and absence of metathesis salt most likely result
from the instability of 1 dissolved in THF and its partial
decomposition during the reaction, through unknown reduc-
tion pathways. Moreover, chromocene [Cp2Cr] could be
identified as a major impurity by 1H NMR spectroscopy
([D8]THF, d = 319.4 ppm) being separable by sublimation at
408C. Other CrII and CrIII species present in reaction
mixtures, even at À508C, were not assignable by NMR
spectroscopy (distinct signals in the range 10 to 50 ppm), but
1
could be removed by crystallization. The H NMR spectrum
Figure 3. Connectivity of [Cr4(m2-Cl)4(m2-I)2(m4-O)(thf)4] (2a, left), ellipsoids shown at 30% probability, lattice solvent and hydrogen atoms are
omitted for clarity. Crystal structures of [(h5-Cp*)3Cr3(m2-Cl)3(m3-CH)] (3, middle) and [(h5-Cp’)3Cr3(m2-Cl)3(m3-CH)] (6, right), ellipsoids shown at
50% probability, lattice solvent and hydrogen atoms (except for the methylidyne hydrogen atom in 6) are omitted for clarity. Selected interatomic
distances/angles are listed in the Supporting Information (Figures S18/S20/S23).[32]
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of A measured in [D8]THF at ambient temperature shows
tallization of 7 from n-hexane produced orange crystals
a broad singlet at d = 30.27 ppm (in CDCl3 at d = 31.05 ppm),
suitable for an X-ray diffraction study (Figure S24). The
1H NMR spectrum of 7 measured in [D8]THF at ambient
temperature displays signals at d = 322.33 ppm, 249.42 ppm,
and À3.32 ppm (Figure S5). Half-sandwich ate complex [(h5-
Cp’)CrCl(m2-Cl)2Li(thf)2] (8) could be crystallized as another
side product from reactions in THF. The crystal structure of
deep blue 8 proved the existence of an intramolecular ate
complex (Figure S25). Compound 8 provides further evidence
for the equilibrium theory proposed by Rojas et al.
(Scheme 2) and explains the virtually non-existent (non-
separable) amount of metathesis salt in the reaction mixtures
of 1 and compounds MCpR.[27] Nearly identical solubilities of
the side products clearly counteract the isolation of these
compounds. In general, the proneness of 1 to reduction (and
the formation of CpR2CrII) can be minimized by performing
the reactions at low temperatures in less polar solvents. In
THF, the reactions proceeded with minor impurities only at
À508C, while in n-hexane and n-pentane acceptable results
were obtained at À358C. Toluene is unsuitable as a solvent,
since decomposition of 1 was significant within minutes, even
at low temperatures.
in agreement with the literature.[7]
The 3-equivalent reaction of 1 with LiCp* in THF at
À508C led to an instant color change from dark red to dark
green. Crystallization from concentrated toluene/n-hexane
mixtures gave dark green crystals of [(h5-Cp*)3Cr3(m2-Cl)3(m3-
CH)] (3) featuring a structural motif similar to 1 (Figure 1)
and A. Compound 3 crystallizes in the trigonal space group
R3 and displays a local symmetry of C3 with Cr—Cr distances
À
of 2.9103(5) ꢁ, slightly longer than in A. The Cr Cl distances
of 2.3416(5) to 2.3615(5) ꢁ as well as the Cr-C-Cr angles
involving the central m3-CH moiety (92.71(11)8) match those
1
in A. The H NMR spectrum of crystalline 3 in [D8]THF at
ambient temperature shows a slightly broadened singlet at
d = À5.8 ppm assignable to C5(CH3)5. The ambient-temper-
ature magnetic moment drastically changed upon Cl/CpR
exchange as evidenced by the Evans method in solution
(meff = 3.63 mB) and in the solid state by SQUID measurements
(meff = 4.32 mB). These values are in accordance with the
results obtained for dissolved A (Evans method: meff
=
3.55 mB)[7] and substantially below the effective magnetic
moment expected in case of three uncoupled CrIII centers
(meff = 6.71 mB). A possible explanation may be the establish-
ment of antiferromagnetic interactions causing the observed
gradual decrease of meff for solid 3 upon cooling (Figure 2,
Figures S31/S33).[7] A similar temperature-dependent de-
crease of the effective magnetic moment upon cooling has
been observed earlier in the related complex A and consid-
ered as an evidence for antiferromagnetic couplings between
the chromium ions.[7] A further analogy to A is that reaction
mixtures of 3 show a multitude of paramagnetically shifted
proton signals, due to partial reduction and decomposition of
complex 1. Identified side products comprise Cp*2Cr (d =
À6.2 ppm, [D8]THF)[25] and [Cp*CrCl2]2 (d = À71.5 ppm,
CDCl3).[26] Overall, the synthesis of such half-sandwich
complexes is extremely sensitive toward change of reaction
conditions and choice of precursor. While switching the
solvent from THF to toluene led to the isolation of trivalent
[Cp*CrCl2(thf)] (4), probing the direct synthesis of 3 from
[Cp*Cr(m2-Cl)]2/CHI3 gave only partial halogenido exchange
in [(Cp*Cr)2(m2-Cl)(m2-I)] (5) (synthesis details and crystal
structures, see Supporting Information).
Reactivity of Methylidyne Complex 1 toward Aldehydes and
Ketones
The Takai and Takai-Utimoto olefination reagents engage
in (E)-selective olefinations of aldehydes, with high functional
group tolerance.[28,29] Later, reagent extensions involved the
formation of (heteroatom-)substituted cyclopropane prod-
ucts.[30] It was of interest how the methylidyne complex
1 would affect such olefination reactions. Direct NMR-scale
reactivity studies turned out difficult to interpret because of
paramagnetic shifting and broadening. However, filtration of
the reaction mixtures over aluminum oxide facilitated the
1
observation of organic products via H NMR spectroscopy.
The conversions of benzaldehyde and pivaldehyde with 1 in
[D8]THF were complete after 1 h at ambient temperature.[31]
During this period, the mixtures changed color from deep red
to turbid green brown, leading to a multitude of products as
detected by GC/MS analysis (see Figures S34 to S43). Most of
these compounds are suggested to be formed by radical
recombination, involving transient olefinic radical, as a result
from methylidyne/oxy exchange (Scheme 3). 1D and 2D
NMR spectroscopies could not resolve the observed over-
lapping signals of the product mixtures (Figures S6 to S12).
For example, the benzaldehyde reaction revealed the forma-
Salt metathesis of 1 with three equivalents of LiCp’ in
THF at À508C gave a dark red/violet solution. After several
extraction steps, crystallization from n-hexane yielded dark
purple needles of [(h5-Cp’)3Cr3(m2-Cl)3(m3-CH)] (6). The
crystal structure of 6 is isostructural to A and 3 (Figure 3),
À
with similar Cr Cl distances of 2.3243(4) ꢁ to 2.3519(4) ꢁ.
Not unexpectedly, the Cr—Cr distances of 2.8192(3) ꢁ to
1
2.8363(3) ꢁ match those in A. The H NMR spectrum of 6
recorded in [D8]THF at ambient temperature shows two
broadened signals at d = 35.35 and 30.39 ppm for the aromatic
protons of the Cp’ ligands, and one sharp singlet at d =
0.49 ppm for the SiCH3 protons (meff = 2.70 mB). Again, the
1H NMR spectrum of the reaction mixture of 6 shows
numerous other signals. To prove similar reaction/decompo-
sition behavior as found for A and 3, chromocene Cp’2Cr (7)
was synthesized independently from CrCl2 and LiCp’. Crys-
Scheme 3. Reactions of 1 with aldehydes and ketones.
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tion of styrene as the only component identifiable by
Conflict of Interest
1H NMR spectroscopy. Striking was the observation of trace
amounts of (2-iodoethenyl)-benzene by GC/MS analysis. As
the synthesis of 1 produces a substantial amount of the
iodinated side product CrCl2I(thf)3 (approximate solubility of
1 mgmLÀ1 in THF at 178C), product contamination with
iodine seems inevitable. ICP (Inductively Coupled Plasma)
analysis of recrystallized samples of compound 1 indicated
a persistent iodine content of roughly 3.3%.
The authors declare no conflict of interest.
Keywords: chromium · cylopentadienyl · magnetism ·
methylidyne · olefination
[1] C. Elschenbroich, Organometallics, Wiley-VCH, Weinheim,
2006.
Other causes for the iodine contamination could be the
presence of decomposition product 2 or non-reacted Takai
reagent [Cr2Cl2(m2-Cl)2(m2-CHI)(thf)4] (B), which are both
easily soluble in THF and hence difficult to separate via
crystallization.
[2] a) R. R. Schrock, Chem. Rev. 2002, 102, 145 – 179; b) A.
Fꢄrstner, P. W. Davies, Chem. Commun. 2005, 2307 – 2320;
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Taynton, W. Zhang, Acc. Chem. Res. 2014, 47, 1575 – 1586; e) H.
Ehrhorn, M. Tamm, Chem. Eur. J. 2019, 25, 3190 – 3208.
[3] For examples, see: a) J. H. Wengrovius, J. Sancho, R. R. Schrock,
J. Am. Chem. Soc. 1981, 103, 3932 – 3934; b) J. C. Peters, A. L.
Odom, C. C. Cummins, Chem. Commun. 1997, 118, 1995 – 1996;
c) A. Fꢄrstner, C. Mathes, C. W. Lehmann, J. Am. Chem. Soc.
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Brandhorst, J. Grunenberg, M. Tamm, Angew. Chem. Int. Ed.
2007, 46, 8890 – 8894; Angew. Chem. 2007, 119, 9047 – 9051;
e) E. F. van der Eide, W. E. Piers, M. Parvez, R. McDonald,
Inorg. Chem. 2007, 46, 14 – 21; f) R. R. Thompson, M. E. Rotella,
P. Du, X. Zhou, F. R. Fronczek, R. Kumar, O. Gutierrez, S. Lee,
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Leutzsch, A. Fꢄrstner, Angew. Chem. Int. Ed. 2019, 58, 15690 –
15696; Angew. Chem. 2019, 131, 15837 – 15843; h) A. Haack, J.
Hillenbrand, M. Leutzsch, M. van Gastel, F. Neese, A. Fꢄrstner,
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Compound 1 did not show any reactivity toward alkynes
ꢀ
ꢀ
ꢀ
HC CSiMe3, HC CPh and PhC CPh, neither alkyne meta-
thesis nor insertion/addition-type reactions. The latter inves-
tigations were carried out in [D8]THF and monitored by
1H NMR spectroscopy over several hours, also by heating to
the decomposition temperature of 1. Finally, the reactivity of
[(h5-Cp*)3Cr3(m2-Cl)3(m3-CH)] (3) toward benzaldehyde or
benzophenone was examined under similar conditions, but
1
the H NMR spectra were inconclusive and only indicated
decomposition of the methylidyne complex. Further research
is needed to elucidate the reactivity of the organometallic
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Conclusion
The chromium(III) m3-methylidyne complex [Cr3Cl3(m2-
Cl)3(m3-CH)(thf)6] features the ultimate C-X cleavage product
in the dehalogenation sequence of haloforms CHX3(here:
CrCl2/CHI3 mixture). The decent yields of the methylidyne
complex enabled a series of reactivity studies. The terminal
chlorido ligands can be selectively displaced via salt meta-
thesis with alkali-metal cyclopentadienides to afford rare
examples of half-sandwich chromium(III) methylidynes, [(h5-
CpR)3Cr3(m2-Cl)3(m3-CH)]. Despite the paramagnetic nature
of CrIII, these compounds exhibit only slightly broadened
signals in the 1H NMR spectra, facilitating the observation of
in situ derivatizations. Treatment of [Cr3Cl3(m2-Cl)3(m3-CH)-
(thf)6] with ketones and aldehydes led to olefination, entailing
the formation of various products probably formed by radical
recombination. The methylidyne complexes under study do
not promote alkyne metathesis reactions or insertions/addi-
tions with acetylenes, but display exceptional magnetic
behavior. Finally, our study underlines the importance of
complying with correct CrCl2/haloform ratios for efficient
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Acknowledgements
We thank the German Science Foundation for financial
support (Grant: AN 238/15-2). Open access funding enabled
and organized by Projekt DEAL.
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Angew. Chem. Int. Ed. 2021, 60, 2 – 8
These are not the final page numbers!

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Version of record online: && &&
,
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www.angewandte.org
&&&&
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Angewandte
Research Articles
Chemie
Research Articles
Coordination Chemistry
S. Trzmiel, J. Langmann, D. Werner,
C. Maichle-Mçssmer, W. Scherer,*
R. Anwander*
&&&& — &&&&
Beyond Takai’s Olefination Reagent:
Persistent Dehalogenation Emerges in
a Chromium(III)-m3-Methylidyne Complex
The methylidyne complex [CrIII3Cl3(m-Cl)3-
(m3-CH)(thf)6] displays the ultimate
product of the CrCl2/CHI3 redox reaction,
significantly affecting carbonylic group
transformation; the trimetallic com-
pound also exhibits unusual magnetic
behavior and provides ready access to
half-sandwich clusters.
&&&&
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ꢂ 2021 The Authors. Angewandte Chemie International Edition published by Wiley-VCH GmbH
Angew. Chem. Int. Ed. 2021, 60, 2 – 8
These are not the final page numbers!