Gallium Methylene

Article abstract of DOI:10.1002/anie.201902063

Despite the eminent importance of metal alkylidene species for organic synthesis and industrial catalytic processes, molecular homoleptic metal methylene compounds [M(CH₂)_n] as the simplest representatives, have remained elusive. Reports on this topic date back to 1955 when polymeric [Li₂(CH₂)]_n and [Mg(CH₂)]_n were accessed by pyrolysis of methyllithium and dimethylmagnesium, respectively. However, the insoluble salt-like composition of these compounds has impeded their application as valuable reagents. We report that rare-earth metallocene methyl complexes [(C₅Me₅)₂Ln{(μ-Me)₂GaMe₂}] (Ln=Lu, Y) trigger the formation of homoleptic gallium methylene [Ga₈(μ-CH₂)₁₂] from trimethylgallium [GaMe₃] (Me=methyl) via a cascade C?H bond activation involving the dodecametallic clusters [(C₅Me₅)₆Ln₃(μ₃-CH₂)₆Ga₉(μ-CH₂)₉] as crucial intermediates. Such gallium methylene compounds feature a reversible [Ga₈(μ-CH₂)₁₂]/[Ga₆(μ-CH₂)₉] oligomer switch in donor solvents and act as Schrock-type methylene-transfer reagents.

Full text of DOI:10.1002/anie.201902063

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Accepted Article
Title: Gallium Methylene
Authors: Reiner Anwander, Peter Sirsch, Cäcilia Maichle-Mössmer,
and Martin Bonath
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To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.201902063
Angew. Chem. 10.1002/ange.201902063
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10.1002/anie.201902063
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Gallium Methylene
Martin Bonath, Cäcilia Maichle-Mössmer, Peter Sirsch,* and Reiner Anwander*
Abstract: Despite the eminent importance of metal alkylidene species
for organic synthesis and industrial catalytic processes, molecular
homoleptic metal methylene compounds [M(CH2)n] as the simplest
representatives, have remained elusive. Reports on this topic date
back to 1955 when polymeric [Li2(CH2)]n and [Mg(CH2)]n were
accessed by pyrolysis of methyllithium and dimethylmagnesium,
respectively. However, the insoluble salt-like composition of these
compounds has impeded their application as valuable reagents. We
report that rare-earth metallocene methyl complexes [(C5Me5)2Ln({µ-
Me}2GaMe2)] (Ln = Lu, Y) trigger the formation of homoleptic gallium
and neutron powder diffraction techniques.[9] Methylene-bridged
aluminum centers of the type R2Al–CH2–AlR2 (IV)[10] have been
made accessible via derivatization of the halogenido derivatives
X2Al–CH2–AlX2 (available from Al and CH2X2)[11] and
degradation/deprotonation of methyl and tetramethylaluminato
ligands as found in, e.g., [(tBu3PN)2TiAl3(CH2)2(CH3)7][12] or
[La4Al8(C)(CH)2(CH2)2(CH3)22(toluene)].[13]
CO
CH2
CH2
CO
Ta
Mn
CO
Mn
methylene [Ga8(µ-CH2)12] from trimethylgallium [GaMe3] (Me
=
CH3
OC
methyl) via a cascade C–H bond activation with dodecametallic
[(C5Me5)6Ln3(µ3-CH2)6Ga9(µ-CH2)9] as crucial intermediates. These
I
II
Schrock (1975)
Herrmann (1975)
gallium methylene compounds feature
a
reversible [Ga8(µ-
CH2
CH3
CH2)12]/[Ga6(µ-CH2)9] oligomer switch in donor solvents and act as
Schrock-type methylene transfer reagents.
CH(SiMe3)2
CH(SiMe3)2
CH2
(SiMe3)2HC
(SiMe3)2HC
Ti
Al
Al
Al
Cl
CH3
III
IV
Tebbe (1975)
Uhl (1990)
Metal methylene/methylidene moieties display key components of
carbonyl olefination reactions for natural product synthesis[1] and
major industrial catalytic processes such as olefin metathesis[2] or
Fischer-Tropsch synthesis alike.[3] In particular the discovery and
structural elucidation of transition metal methylidene species has
triggered immense research in these emerging fields[4]. Seminal
Figure 1. Milestones in metal methylene/methylidene chemistry.
Methane activation via solvent-free rare-earth metallocene
derivatives [(C5Me5)2LnMe] (Ln = Sc, Lu) features one of the
major discoveries in organolanthanide chemistry.[14] We have
shown that such highly reactive Ln–CH3 moieties can engage in
multiple C–H bond activation, preferably in the presence of AlMe3
even at ambient temperature.[13] Crucially, such methyl group
degradation can be controlled in the presence of sterically
demanding ancillary ligands, affording, e.g., methylidene complex
works
comprise
Schrock´s
terminal
methylidene
Cp2Ta(=CH2)(CH3) (I, Figure 1),[4a,4b] Herrmann´s bridging
methylene Cp(CO)2Mn(µ-CH2)MnCp(CO)2 (II),[4c,4d] and Tebbe´s
reagent Cp2Ti(µ-CH2)(µ-Cl)Al(CH3)2 (II),[4e-g] having crucially
contributed to a fundamental understanding of the processes
involved as well as successful further development. d/f-Transition
metal methylidene/alkylidene chemistry is still a current hot topic,
e.g., unveiling terminal methylidene complexes of the group 4
metals, and more specifically complexes (PNP)M=CH2(OAr) (M =
Zr, Hf, PNP = N[2-P(iPr)2-4-CH3-C6H3]2, Ar = C6H3iPr2-2,6)[5] and
(PN)Ti=CH2 (PN = N[2-P(iPr)2-4-CH3-C6H3]2(C6H2Me3-2,4,6)[6].
The latter progress has been made possible mainly through tailor-
made ligand environments and the emergence of new synthesis
strategies. In stark contrast, and despite the enormous
[(C5Me5)3Y3(µ-Cl)3(µ3-Cl)(µ3-CH2)(thf)3][15]
[(TptBu,Me)La[(µ-CH2){(µ-Me)AlMe2}2]] (TptBu,Me
and
Tebbe-like
=
hydrotris(3-tert-
butyl-5-methylpyrazolyl)borato).[16] By studying the reactivity of
[(C5Me5)2LnMe] (Ln = Y, Lu) toward an excess of trimethylgallium
we now obtained oligomeric gallium methylene [Ga8(µ-CH2)12].
Herein we describe this organorare-earth metal-promoted
formation and full characterization of [Ga8(µ-CH2)12], displaying
the first molecular homoleptic metal methylene complex.
importance of Wittig´s reagent Ph3PCH2 for olefination
7]
chemistry,[1,
main group metal methylene chemistry has
Treatment of [(C5Me5)2Ln({µ-Me}2GaMe2)] (Ln = Lu (1Lu),[17]
Y
(1Y);[18] cf., supplementary material) with an eightfold excess of
[GaMe3] at 130 °C in aromatic solvents like C6D6 generates
methane and provides access to [Ga8(µ-CH2)12] (2) as a pale
yellow crystalline material in 80% yields (Scheme 1).[19] The solid-
state structure of oligomeric compound 2 was determined by
single-crystal X-ray diffraction (XRD) analysis (Scheme 1),
remained underdeveloped. Remarkably, the syntheses of
dilithiummethylene Li2CH2 and magnesium methylene MgCH2
were described by Ziegler already in 1955 via pyrolysis of the
respective methyl compounds,[8] but it was only in 1990 that the
solid-state structure of salt-like Li2CD2 was examined by X-ray
revealing
a cage-like structural motif closely related to
[*]
M. Bonath, Dr. C. Maichle-Mössmer, Dr. P. Sirsch,
Prof. Dr. R. Anwander
Institut für Anorganische Chemie
Eberhard Karls Universität Tübingen
Auf der Morgenstelle 18, 72076 Tübingen (Germany)
E-mail: reiner.anwander@uni-tuebingen.de
silsesquioxanes of the general formula [Si8(µ-O)12]R8.[20] Each
gallium atom of the “[Ga8] cube” binds to three methylene bridges
(“edges”), thus adopting a trigonal planar geometry (sum of
angles about Ga 359.2°) with bond lengths (1.960(2) Å, 1.961(1)
Å, 1.972(2) Å) in the same range as reported for [GaMe3].[21]
Supporting information for this article is given via a link at the end of
the document.
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H2
C
H2
C
H2C
Ga
CH2
CH2
Ga
Ga
Ga
24 [GaMe3]
3 [(C5Me5)2Ln({µ-Me}2GaMe2)]
1Ln (Ln = Lu, Y)
H2C
CH2
H2C
Ga
toluene, 130 °C, 5 d
- exc. CH4
- [6(GaMe3)n]
Ga
Ga
Ga
methylcyclohexane
6 [GaMe3]
C
C
130 °C, 2 d
- 15 CH4
H2C
CH2
H2
H2
2
5 [GaMe3]
toluene
130 °C, 3 d
- exc. CH4
- [6(GaMe3)n]
thf
25 °C, 18 h
1. 80 °C
2. vacuum
H2 H2
CH2
C
C
Ga Ga
Ga
thf
H2 H2
CH2
CH2
Ga
CH2
H2C
C
C
H2
thf
C
Ga Ga
Ln
thf
Ga
Ga
H2C
Ln
CH2
Ln
Ga
H2C
Ga
H2C
H2C
CH
2 CH2
CH2
CH2
Ga
Ga
Ga
thf
C
C
H2
Ga
H2
Ga
thf
C
CH2
C
H2
H2
6Ln (Ln = Lu, Y)
thf
3
Scheme 1. Synthesis of compounds 2, 3 and 6, including the crystal structures of gallium methylenes 2 and 3. Atomic displacement parameters set at 50%
probability. Hydrogen atoms and the disorder in coordinated thf molecules in 3 have been omitted for clarity. For selected interatomic distances and angles, see
supporting information.
For comparison, galloxane clusters like [Ga12tBu12(µ3-O)8(µ-
O)2(µ-OH)4] obtained via alkyl hydrolysis tend to adopt an
icosahedral arrangement of the alkylgallium(III) moieties,[22] while
silicon methylene derivatives favor ring structures.[23]
confirmed by 1H NMR spectroscopy. Single-crystal XRD analysis
corroborates a rather weak Ga–thf interaction since the solid-
state structures of [Ga8(µ-CH2)12(thf)4] (4) and [Ga8(µ-CH2)12(thf)5]
(5) display only partial thf coordination (Figure 2).
Analysis of 2 by means of nuclear magnetic resonance (NMR)
spectroscopy at ambient temperature in d8-thf solutions provided
data consistent with homoleptic gallium methylene 2. The 1H NMR
spectrum of 2 shows a singlet resonance at δ 0.19 ppm
assignable to equivalent methylene protons. However, at ambient
temperature in d8-thf solutions compound 2 converted within 24
hours
to
the
smaller
gallium
methylene
oligomer
[Ga6(µ-CH2)9(thf)6] (3), giving rise to a set of signals at 0.12 ppm
(d, 2JH,H = 9.1 Hz, 6H), 0.10 ppm (s, 6H), and –0.49 ppm (d, 2JH,H
= 8.8 Hz, 6H) consistent with three magnetically inequivalent
protons. Tracking this process at variable temperatures by NMR
spectroscopy revealed that the [Ga8(µ-CH2)12]/[Ga6(µ-CH2)9]
oligomer switch in thf solutions is suppressed at temperatures
below –40 °C and pushed back at temperatures above 80 °C
[Figures S2 and S3]. Single crystals of compound 3 could be
harvested from saturated thf solutions at –40 °C and were
subjected to XRD analysis. The solid-state structure of
hexagallium methylene 3 (Scheme 1) features two six-membered
[Ga3(µ-CH2)3] rings in chair conformation. The two [Ga3(µ-CH2)3]
subunits are co-facially oriented and connected by three Ga–
CH2–Ga bridges, overall being reminiscent of silsesquioxanes of
the general formula [Si6(µ-O)9]R6.[24] All gallium atoms in
compound 3 are coordinated additionally by a thf molecule and
therefore display distorted tetrahedral geometry with slightly
elongated average Ga–C bond lengths (av. 1.982 Å) compared to
2.
Figure 2. Crystal structures 4 (A) and 5 (B). Atomic displacement parameters
set at 50% probability. Hydrogen atoms and the disorder in coordinated thf
molecules have been omitted for clarity. For selected interatomic distances and
angles, see supporting information.
Crystallization of gallium methylene 2 from thf solutions at
ambient temperature by slowly condensing n-pentane into the
solution triggers the rearrangement of the [Ga8(µ-CH2)12] cage as
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Thf coordination leads to structural distortions of the cage motif
disclosed for unsolvated compound 2. The coordinated thf
molecules in 3, 4, and 5 get slowly displaced at glovebox
atmosphere and can be evaporated completely under reduced
pressure [Figure S13].
Compound 6Lu was identified as a crucial intermediate in the
formation of 2, since it converts excessive [GaMe3] to 2 at 130 °C
within 3 days (87%, Scheme 1). Even at ambient temperature
compound 6Lu slowly extrudes 2 in the presence of [GaMe3],
accompanied
by
the
release
of
starting
material
[(C5Me5)2Lu({µ-Me}2GaMe2)] (1Lu), as evidenced by NMR
spectroscopy [Figure S6]. Monitoring compound 6Lu in the
presence of excessive [GaMe3] by NMR spectroscopy revealed
spectral data very similar to those achieved when examining the
The ease of donor-mediated interconversion of Ga6 and Ga8
methylene cages involving Ga–C bond disruption and formation,
respectively, seems remarkable. To gain further insights into this
process and the relative stabilities of the different oligomers in
dependence of donor solvent coordination, density functional
theory (DFT) calculations were carried out. They revealed that in
the absence of thf, the larger Ga8 oligomers are clearly more
stable than the Ga6 cages: ΔE = +226 kJ/mol and ΔG(298 K) =
+175 kJ/mol for the conversion of three Ga8 into four Ga6
oligomers. However, the smaller cages should become more
favorable in the case of thf coordination, as more donor molecules
in total can now be bound by gallium atoms: Whereas in 3 all
gallium atoms were coordinated by thf, only partial coordination
was observed for the larger cages 4 and 5. Interestingly, the
additional bonds formed and the concomitant loss in entropy
balance each other out, and the computed value of ΔG (+4 kJ/mol
at 298 K) for the conversion of 5 into 3 (for comparison, ΔE = −400
kJ/mol) points to an equilibrium at room temperature. At lower
temperature, the equilibrium shifts towards the smaller cage (e.g.,
ΔG = −129 kJ/mol at 200 K), which is in-line with the low-
temperature NMR results. Replacing thf by the stronger donor
pyridine renders the Ga6 cage the preferred species at room
temperature (cf., NMR spectra and solid-state structures of the
pyridine adduct [Ga6(µ-CH2)9(pyr)6] (7) and its partial
decomposition product [Ga5(µ-CH2)7(CH3)(pyr)5] (8) in the
supplementary material). It is noteworthy, that the calculated
thermochemical data (ΔE = −464 kJ/mol, ΔG(298 K) = −111
kJ/mol) suggest, that this is only partly due to stronger bonds
forming between the solvent molecules and the gallium atoms. To
a larger extent, it is a consequence of a lower entropy contribution
of the free pyridine ligands, which are more rigid than their thf
counterparts. Finally, in order to understand the observed partial
coordination of the Ga8 oligomer, we computed the changes in
energy and free enthalpy for the successive coordination of 2 by
thf. The values for ΔE range from −70 to −45 kJ/mol (coordination
of the first and last thf, respectively). Due to entropy losses, the
differences in free energy at 298 K, however, are considerably
lower (−27 and +11 kJ/mol, respectively). ΔG for binding a sixth
or seventh thf donor are less than −5 kJ/mol and explain the
partial coordination pattern observed for the crystal structures of
4 and 5. Packing effects can therefore be ruled out as a potential
cause and partial coordination might also be prevalent in solution.
transformation of [GaMe3] to compound
2 promoted by
[(C5Me5)2Lu({µ-Me}2GaMe2)] (1Lu). This suggests that the reactive
intermediate is in fact an addition product of 6Lu and [GaMe3]
(Scheme 1).
Single-crystal XRD analysis of complex 6Lu disclosed its solid-
state structure (Figure 3), which can be described as a highly
aggregated anionic [(µ3-CH2)6Ga9(µ-CH2)9]3- core stabilized by
three cationic [(C5Me5)2Lu]+ entities. The [(µ3-CH2)6Ga9(µ-CH2)9]3-
core unit comprises three staggered six membered [Ga3(µ-CH2)3]
rings interconnected by µ3-Ga2Lu-bridging methylene units
involving the flanking rare-earth metallocene units. Comparably to
3, the outer rings adopt a chair conformation with gallium atoms
in distorted trigonal planar geometry. In contrast, the central ring
system exhibits a planar arrangement with gallium atoms in a
distorted tetrahedral coordination environment. Consequently, the
3-coordinate gallium atoms display Ga–C bond lengths (1.961(6),
1.973(6), 1.972(6) Å) comparable to compound 2 whereas all 4-
coordinate gallium atoms exhibit longer Ga–C distances (Ga–(µ-
CH2) = 2.008(7), Ga–(µ3-CH2) = 2.125(6) Å) similar to those
reported for [GaMe4]- entities.[25] The Lu–(methylene) distances of
2.510(6) Å are considerably shorter than the Lu–Me bond lengths
of the starting material [(C5Me5)2Lu({µ-Me}2GaMe2)] (1Lu) (av.
2.614 Å) [Figure S35] which is attributable to the increased
negative charge at the methylene carbon atoms.
Treating the supernatant of the initial reaction mixture with
another eight equivalents of [GaMe3] at 130 °C generated
additional 2 in about the same yields. Since this procedure can be
repeated several times, an elusive intermediate capable of
promoting the “pseudocatalytic” transformation of [GaMe3] into 2
persists after each cycle. Fortunately, prolonged thermal
treatment of the supernatant without addition of [GaMe3] afforded
reproducibly colorless crystals of the bimetallic rare-earth
metallocene gallium methylene complexes [(C5Me5)6Ln3(µ3-
CH2)6Ga9(µ-CH2)9] (Ln = Lu (6Lu); Y (6Y); Figure 3; compound 6Y
behaves in complete analogy to 6Lu, cf., supplementary material).
Figure 3. Crystal structure of 6Lu. Atomic displacement parameters set at 50%
probability in metal methylene/methylidene chemistry. For selected interatomic
distances and angles, see supporting information.
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In contrast to homoleptic gallium methylene 2, compound 6Lu is
insoluble in d8-thf but dissolved to a minor extent when adding
4-dimethylaminopyridine (DMAP) to the corresponding
suspension in d8-thf. The data achieved by NMR spectroscopy
analysis of 6Lu in d8-thf are consistent with the composition in the
solid state [Figure S14]. The [(µ3-CH2)6Ga9(µ-CH2)9] core
structure of 6Lu is evidenced by 1H NMR resonances at δ 0.25 (d,
The present findings on oligomeric gallium methylenes show that
rare-earth metal alkyl complexes display efficient promotors for
methyl group deprotonation being capable of both stabilizing and
transferring methylene moieties. The ease of Ga–C(methylene)
bond disruption and formation might contribute to a better
understanding of the occurrence of multiple methylene insertions
on gallium-rich GaAs(100) surfaces and open new avenues for
gallium as a promotor metal in Fischer-Tropsch catalysis.[30]
2
2JH,H = 9.3 Hz, 6H), –0.75 (d, JH,H = 9.3 Hz, 6H), –0.99 (s, 12H)
and –1.35 ppm (s, 6H). Similarly as found for compound 2, the
equatorial and axial arrangements of the protons at C13 give rise
to doublet resonance signals while the protons at C14 and C15
perceive a symmetrical environment and show singlet resonance
signals. However, compound 6Lu is not stable in solution and
decomposes slowly by forming 3 and other products [Figure S15].
Further analysis of compounds 2 and 6Lu in the solid state was
performed by means of cross polarization magic angle spinning
(CP-MAS) NMR spectroscopy. The 13C CP/MAS NMR spectrum
of 2 exhibits a very broad singlet at δ 28 ppm assigned to the
methylene carbon atoms [Figure S12]. The line broadening may
originate from a strong quadrupolar coupling caused by both
NMR-active isotopes of gallium (69Ga and 71Ga).[26] In a similar
vein, the 13C CP-MAS NMR spectrum of 6Lu revealed very broad
methylene resonances at δ 18 ppm and –4 ppm in addition to the
signals of the C5Me5 ligands at δ 116 ppm and 12 ppm [Figure
S16]. The absence of gallium methyl moieties was verified by
deuterolysis experiments of compounds 2 and 6Lu [Figure S28].
Acknowledgements
We thank the German Science Foundation (Grant AN 238/15-2)
for funding.
Conflict of interest
The authors declare no conflict of interest.
Keywords: gallium • methylidene • C–H bond activation •
lutetium • yttrium
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The olefination of carbonyl moieties is a characteristic reaction of
Schrock-type metal alkylidene compounds. This prompted us to
study the reactivity of gallium methylene compounds 2 and 6Lu
toward 9-fluorenone. Treating 2 with two equivalents 9-fluorenone
at ambient temperatures in d8-thf solutions resulted in complete
conversion to 9-methylidene-fluorene within 5 days [Figure S29],
while upon heating to 80 °C the reaction was nearly complete after
two hours [Figure S30]. Due to the insolubility of compound 6Lu in
d8-thf olefination of 9-fluorenone required more forcing conditions.
Hence, 9-methylidene-fluorene could be detected in the
[3]
[4]
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1
corresponding H NMR spectra only after heating the sample to
130 °C for three days, besides other products [Figure S31].
Clearly, gallium methylene compounds are prone to olefination
reactions.
Encouraged by the existence of 2 we wondered whether the
original pyrolysis protocol described by Ziegler et al.[8] would also
be viable to transfer [GaMe3] into 2. Indeed, heating neat [GaMe3]
to 280 °C for 42 hours led to the formation of trackable amounts
of 2 (~1%) next to unreacted [GaMe3], according to 1H NMR
measurements [Figure S4]. In contrast, Ziegler et al. could not
isolate the analogous aluminum compound upon heating [AlMe3]2
to temperatures up to 235 °C. Instead mixed aluminum [methyl-
methylene-methine-carbide] compounds or aluminum carbide
[Al4C3] were achieved.[27] Certainly, [Al2(CH2)3]n would be a
reasonable intermediate but is prone to further degradation under
that harsh conditions.[28] Since gallium carbide is presumed to be
thermodynamically unstable[29] pyrolysis of [Ga8(µ-CH2)12] is,
therefore, much more disfavored. Additionally, formation of 2 may
benefit from the increased covalent character of the Ga–C bond
compared to the Al–C bond owing to a decreased difference of
the electronegativities.
[5]
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11093-11095.
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[12] J. E. Kickham, F. Guérin, D. W. Stephan, J. Am. Chem. Soc. 2002, 124,
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[13] L. C. H. Gerber, E. Le Roux, K. W. Törnroos, R. Anwander, Chem. Eur.
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Entry for the Table of Contents
COMMUNICATION
H2
C
thf
H2
C
H2 H2
Martin Bonath, Cäcilia Maichle-
Mössmer, Peter Sirsch,* and Reiner
Anwander*
CH2
H2
C
CH2
CH2
C
C
thf
Ga Ga
Ga
Oligomer Switch
Ga
Ga
Ga
thf
Ga
thf
H2
C
CH2
H2
Ga
C
H2
C
CH
2 CH2
Δ or
vacuum
Ga
Ga
Ga
Ga
Ga
thf
C
H2
Ga
thf
C
C
C
H2
C
CH2
CH2
H2
H2
Page No. – Page No.
H2
thf
Gallium Methylene
Rare-earth metal-mediated cascade C–H bond activation of trimethylgallium gives
access to the first molecular homoleptic metal methylene complex, engaging in a
reversible [Ga8(µ-CH2)12]/[Ga6(µ-CH2)9] oligomer switch.
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