A Highly Lewis Acidic Strontium ansa-Arene Complex for Lewis Acid Catalysis and Isobutylene Polymerization

Article abstract of DOI:10.1002/anie.202010019

The potential of a dicationic strontium ansa-arene complex for Lewis acid catalysis has been explored. The key to its synthesis was a simple salt metathesis from SrI₂ and 2 Ag[Al(OR^F)₄], giving the base-free strontium-perf

Full text of DOI:10.1002/anie.202010019

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Accepted Article
Title: A highly Lewis-acidic Strontium ansa-Arene Complex for Lewis-
acid Catalysis and Isobutylene Polymerization
Authors: Philipp Dabringhaus, Marcel Schorpp, Harald Scherer, and
Ingo Krossing
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10.1002/anie.202010019
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A highly Lewis-acidic Strontium ansa-Arene Complex for Lewis-
acid Catalysis and Isobutylene Polymerization
Philipp Dabringhaus, Marcel Schorpp, Harald Scherer and Ingo Krossing*
[*]
P. Dabringhaus, M. Schorpp, Dr. Harald Scherer, Prof. Dr. I. Krossing
Institut für anorganische und analytische Chemie
Albert-Ludwigs-Universität Freiburg
Albertstraße 21, 79104 Freiburg i.Br. (Germany)
E-mail: krossing@uni-freiburg.de
Supporting information for this article is given via a link at the end of the document.
Abstract: The potential of a dicationic strontium ansa-arene complex
for Lewis-acid catalysis has been explored. The key to its synthesis
was a simple salt metathesis from SrI2 and 2 Ag[Al(ORF)4] giving the
neutral complexes including Ae2+ ions are significantly less Lewis
acidic than Al3+ or B3+ based compounds, the Lewis acidity can be
enhanced by synthesis of cationic Ae2+ complexes as salts of
weakly coordinating anions (WCAs). Recently, Okuda
demonstrated for such a mono-cationic calcium hydride complex
bound to a neutral tetraamine ligand the increase of catalytic
activity.[13] As a first example for a cationic Ae-based Lewis-acid
catalyst, Parkin reported the cationic Mg-complex 1+[14] stabilized
base-free strontium-perfluoroalkoxyaluminate Sr[Al(ORF)4]2 (ORF
=
OC(CF3)3). Addition of the ansa-arene yielded the highly Lewis-acidic,
dicationic strontium ansa-arene complex. In preliminary experiments,
it was successfully applied as catalyst in a CO2-reduction to CH4 and
a surprisingly controlled isobutylene polymerization.
Lewis acidity plays a decisive role in modern main-group
catalysis. In the last decade, frustrated Lewis pairs (FLPs) have
emerged as flagship of main-group catalysis.[1] The reactivity of
FLPs originates from the synergistic reactivity of a Lewis acid and
Lewis base, which are sterically hindered from a quantitative
adduct formation.[2] The most widely used Lewis-acid for FLP
catalysts represents the soft Lewis-superacid B(C6F5)3.[3] Yet,
recent reports suggest the importance of more pronounced hard
Lewis acids to broaden the scope of FLP catalysts and enhance
their activity, e.g. for CO2 activation.[4] Exceptionally strong p-
block Lewis acids, e.g. Al(C6F5)3, have been shown to effectively
catalyze FLP-type hydrosilylations of 1-olefins,[5] CO2 reduction,[6]
and FLP-polymerizations.[7] Similarly to FLPs, the emerging class
of alkaline-earth metal-catalysts of the type LMR (L = innocent
ligand) depend on the cooperative electrophilicity of the metal
atom (M) and the nucleophilicity of the reactive, anionic ligand (R),
e.g. a hydride or formal carbanion. In recent years, Harder and
Hill impressively reinvented the field of organometallic alkaline-
earth (Ae) metal-catalysis.[8,9] Ae catalysis evolved around -bond
metathesis and polarized insertion as principal mechanistic steps.
Substrate activation via coordination to the metal is widely
accepted to critically rely on the Ae metal`s Lewis acidity.[10] Yet,
the Lewis acidity of common Ae complexes is limited by the need
for kinetic stabilization of the extremely reactive ‘naked’ Ae2+
cations by large, anionic ligands, e.g. encumbered beta-diket-
iminate ligands (NacNac). This represents a significant drawback
compared to FLP catalysts with strong p-block acids. Hence, the
opening of the field of Ae catalysis to FLP-type chemistry
demands for the development of Ae salts with highly Lewis acidic,
cationic Ae moieties. In the literature, FLP systems incorporating
Ae metals are scarce. In 2017, the group of Harder used a
Mg-based Lewis acid of type (NacNacMgBr)2 in combination with
PPh3 for substrate activation of polar substrates, e.g. acetone.[11]
Moreover, the same group published a Ca(II)/Al(I)-FLP for
stoichiometric benzene reduction, which heavily relied on the
substantial reactivity of the Al(I) species.[12] Although the overall
Dipp
N
Ae
N
Dipp
N
Pri
N
Sr
F
Mg
Si
Si
N
N
N
F
Pri
N
Si
4
Pri
1+[HB(C6F5)3]-
2+[Al(ORF)4]-
32+[Al(ORF)4]2
-
with a bulky tetradentate ligand to prevent coordination of the
hydridic counter ion [HB(C6F5)3]– [15] as catalyst for an FLP-type
hydrosilylation of CO2. Harder and Hill independently develloped
several complex salts of the type [Ae(NacNac)]+[WCA]– ([WCA]–
= [B(C6F5)4]-, [Al(ORF)4]- with ORF = C(CF3)3; Ae = Mg, Ca).[16,17]
Their pronounced Lewis acidity was unambiguously shown by
isolation of the respective metal-arene complexes upon addition
of benzene (2+), toluene, m-xylene and mesitylene.[18] Inspired by
the surprisingly persistent coordination between the hard Ae2+
ions and soft arenes, our group reported a first synthesis of
unsupported, dicationic alkaline-earth metal arene-complexes via
oxidation of the Ae metals (Ae = Ca, Sr (32+), Ba) with the
hexamethylbenzene-radical cation salt [HMB][al-f-al] as oxidant
([al-f-al]– = [(FRO)3Al-F-Al(ORF)3]–).[19] On the basis of the
computed fluoride-ion affinities (FIA) of 32+ in CH2Cl2 solution,
Lewis acidities similar to the hard acids Al(C6F5)3 or Al(ORF)3 were
derived for the Ae2+ complexes making them promising Lewis acid
catalysts. Unfortunately, the fleeting stability of these salts, in
particular the Ca and Ba variants, as well as inseparable radical
impurities encumbered the application of these salts in catalysis.
With the aim to develop an improved synthetic route towards
highly pure dicationic Ae complexes with neutral ligands, we
investigated the salt metathesis between Ag[Al(ORF)4] and
excess SrI2 in 1,2-difluorobenzene (o-DFB) promoted by
ultrasonic irradiation as in Eq. (1). Here, the solvent-free
crystalline Sr[Al(ORF)4]2 4 was isolated in a high yield of 92 %.
The bulk purity of 4 was confirmed by X-ray powder diffraction
(ESI, Fig. S7).
1
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10.1002/anie.202010019
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2+
o-DFB, rt, ))));
I2(s) + 2 Ag[Al(ORF)4]
[Al(OR )4]2
F
o-DFB, rt, 2d, ))));
(1)
Sr
Sr
- 2 AgI(s)
F
F
2 AgI(s)
I2(s) + 2 Ag[Al(ORF)4]
[Al(ORF)4]
2
(2)
Sr
Sr
4
x
+ DXE, 1d, ))))
The molecular structure of 4 reveals an extraordinary 3-O,O’,O’’-
coordination of the cation to one of the anions, which induces an
52+
elongation of the respective AlO bonds to dAlO avg.
,
= 1.772(4) Å
In the molecular structure, the metal cation is in addition to the
DXE ligand coordinated by two 2-bound o-DFB molecules with
and short Sr–O distances of dSrO avg. = 2.655(5) Å (Figure 1).
,
Overall, the best formulation describing the composition of 4 and
considering the structure would be [Sr{Al(ORF)4}]+[Al(ORF)4]–. The
second anion in 4 is weakly coordinated to [Sr{Al(ORF)4}]+ over
three fluorine atoms. In the IR spectrum, this 3-coordination
induced band splitting of degenerate modes, due to the reduction
of the typical S4-symmetry of the free anion, e.g. in the undistorted
salt [NEt4]+[Al(ORF)4]–.[20] A CCDC search revealed, only cation-
2-O,O’-coordination to [Al(ORF)4]– was previously reported.[21]
SrF distances between
(Figure 2).[19,22] Moreover, both aromatic rings adopt an
6-coordination
with SrC distances ranging from
2.945(3) - 3.117(3) Å. These values coincide with previously
reported distances for the strontium HMB complex.[19]
2.534(2) Å
and
2.669(2) Å
Negligible
contamination
by
the
dimeric
compound
[Sr2(ORF)2(o-dfb)6]2+([Al(ORF)4]–)2 was observed by single
crystal X-ray analysis (cf. ESI, Fig S8). The high Lewis acidity of
the strontium atom, already visible from the unusual tripodal anion
coordination, is further underlined by coordination of trace
acetone and water – present in the mass spectrometer as impu-
rities – to the generated [Sr{Al(ORF)4}]+ cation (ESI, Fig. S11). Yet,
the narrow peak widths of the 19F and 27Al NMR resonances (19F
NMR: 1/2 = 2.2 Hz, 27Al NMR: 1/2 = 22.9 Hz; ESI Fig. S5 and
S6) indicate that the anions are presumably quantitatively
exchanged for solvent molecules in o-DFB solution. Hence, 4 is
anticipated to in situ generate a highly reactive, o-DFB solvated
strontium dication, which can be trapped by neutral Lewis bases
with a stronger coordination ability than o-DFB.
Figure 2. Molecular structure of the dicationic moiety in [Sr(DXE)(o-DFB)2]2+
([Al(ORF)4]–)2 5. Thermal displacement of ellipsoids set at 50 % probability.
Protons and counterions were omitted for clarity. Selected interatomic distances
for 52+ in Å given as ranges and averages due to multiple cationic moieties in
the asymmetric unit: Sr─F1/F3}range = 2.636(2)-2.669(2), Sr─F1/F3}avg.
=
=
2.651(2),
Sr─F2/F4}range = 2.534(2)-2.559(2),
Sr─C[Ar]range = 2.945(3)-3.117(3),
Sr─F2/F4}avg.
Sr─C[Ar]avg.
2.541(2),
=
3.008(3),
Sr─{Ct1/Ct2}range = 2.655-2.681, Sr─{Ct1/Ct2}avg. = 2.667.
The cation-arene interaction was analyzed by means of the
quantum theory of atom in molecules (QTAIM). The calculated
electronic charge densities  and Laplacians of the electron
density  are summarized along with results of QTAIM and NBO
population analyses in Figure 3a. In analogy to AIM studies on
known Ae arene complexes,[16,19] the partial positive charge on the
arene ligand indicates a ligand-to-metal bonding. Computed
electron densities at the cage and bond critical points (CCP &
BCP) match  values obtained for the strontium HMB complex
32+.[19] This implies an interaction energy per arene moiety of DXE
comparable to HMB. The positive Laplacian of electron density
 at the BCPs and CCPs illustrates the depletion of electron
density in the interatomic surface, strongly indicating
non-covalent interactions. In contrast, covalent cation-arene
interactions, e.g. in a proton-benzene complex, are characterized
by a negative Laplacian.[23] Nonetheless, calculated bonding MOs
unambiguously display an interaction of strontium d-orbitals with
the -system of both arene moieties as a ligand-()-to-metal-(d)
bonding (Figure 3b).[24] Hence, a covalent contribution to the
predominantly ionic bonding can be assumed.
Figure 1. Molecular structure of Sr[Al(ORF)4]2. Thermal displacement of the
ellipsoids was set at 50 % probability. Selected interatomic distances for 4 in Å
given as ranges and averages due to multiple molecules in the asymmetric unit:
Al─Orange = 1.763(6)─1.787(6),
Al─Oavg. = 1.772(4),
Sr─Orange = 2.620(5)-
2.681(5), Sr─Oavg. = 2.655(5), Sr─F1range = 2.519(6)-2.578(6), Sr─F1avg.
= 2.551, Sr─F2range = 2.514(5)-2.653(5), Sr─F2avg. = 2.568(5).
Arenes represent a promising ligand class due their tunability
and soft Lewis basicity allowing for access to highly Lewis acidic
Ae complexes with an open coordination sphere compared to
ligands commonly used in Ae complexation chemistry. Chelating
ansa-arenes – related to the ansa-metallocenes – would add
entropic stability to the corresponding complexes and allow for an
empty coordination space opposite to the ansa-bridge. Hence, to
probe the synthetic potential of the salt metathesis,
dixylylethylene (DXE) was added to 4, prepared as an
intermediate in the reaction of SrI2 and Ag[Al(ORF)4] as in Eq. (2).
The molecular structure of the in 96 % yield isolated Ae ansa-
arene complex 5 is displayed in Figure 2. The high bulk purity of
5 was confirmed by powder X-ray diffraction (ESI, Fig. S20).
Figure 3. a) Population analyses by QTAIM and NBO as well as electron
densities and Laplacian at bond and cluster critical points. b) HOMO-6 of the
optimized structure of 52+ (BP86/D3(BJ)/def-SVP).
To evaluate the Lewis acidity of 5, the fluoride and hydride ion
affinities (FIA/HIA) were calculated in the gas phase and in the
condensed phase in dichloromethane DCM ((DCM) = 9.08[25]).
2
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The DCM-values were shown to successfully level the influence
of the charges upon comparing Lewis acidities.[26,27] FIA and HIA
+
values were referenced to SiMe3
calculated at the
CCDS(T)/CBS[27] level and G3 level[28] respectively (Table 1). As
desired by enhancing the soft-hard mismatch interaction with the
chelating ansa-arene ligand, 5 reaches an even higher FIA value
than the known strontium HMB complex in gas phase and
condensed phase. Although DCM-solvation of the cation leads to
significant dampening of the FIA, 5 is expected to retain a similar
Lewis acidity to strong p-block Lewis acids (Table 1).
Table 1. Summary of calculated XIA values for various Ae complexes and ex-
ample p-block Lewis acids (BP86/D3(BJ)/def2-SVP/CPCM; (DCM) = 9.08[25]).
Figure 4. a) Endothermic and endergonic calculated thermodynamics of the
isodesmic solvent exchange reactions at [Sr(o-DFB)]2+ with TFB yielding two
isomers of [Sr(TFB)]2+ (BP86-D3(BJ)/def-SVP). b) Comparison of the chemical
shift differences and peak widths observed in 1H NMR spectra of 5 and free
dixylylethylene in o-DFB and TFB.
FIAGas
FIADCM
HIADCM
Lewis acid
HIAGas
[a]
SbF5
484
548
544
458
935
871
867
699
303
308
316
232
274
232
240
233
-
-
Al(C6F5)3
494
491
488
827
751
753
593
178
187
182
104
56
Hence, although NMR-spectroscopic analysis reveals the
dissociation of DXE from 5 in o-DFB, the analysis for 5 in TFB
strongly indicates coordination of the DXE ligand as well as a
stronger interaction of the strontium ion to the anion.
Al(ORF)3
B(C6F5)3
[Sr(DXE)(2-o-dfb)2]2+
[Sr(o-dfb)7]2+ [b]
[Sr(HMB)(2-o-dfb)4]2+
[Sr(NacNac)]+
To study the reactivity of the highly Lewis acidic strontium
ansa-arene complex preliminarily, 5 was applied as catalyst in
FLP-type catalysis typically achieved with strong p-block Lewis
acids. Here, the results of the solution structure studies on 5 were
confirmed experimentally by altering its reactivity, since 5 was
found unreactive for catalysis in o-DFB-solution. Thus,
subsequent reactions were exclusively performed in TFB.
66
71
[a] [SbF5H]- is unstable[28] and thus the HIAs were omitted. [b] [Sr(o-dfb)7]2+ is the
DFT calculated optimum structure of Sr2+ only solvated by o-dfb (cf. ESI).
In a first catalytic transformation, only 0.6 mol-% 5 suc-
cessfully catalyzed the reduction of CO2 straight to methane with
triethylsilane as hydrogen source (Scheme 1). Previously, Lewis
acid catalyzed hydrosilylations of CO2 to CH4 have only been
Since 5 was intended to be used as homogenous catalyst, it
is crucial that the ansa-arene coordination to the metal is retained
1
in solution. The comparison of H,13C HMBC spectra of the free
ligand and the strontium-DXE complex, both measured in o-DFB,
reveal only a shift of the resonances assigned to the aromatic
carbon atoms of C = 0.56 – 1.56 ppm. For comparison, in the
complex 32+ a downfield shift of 13C = 2.4 ppm was reported for
the aromatic carbon atoms upon coordination to the metal.[19] In
addition, NMR analysis revealed only a slight dampening of the
diffusion constant of DXE in a solution of 5 in o-DFB compared to
free DXE (ESI, Table S1 and S2). This observation indicates that
the majority of the ligand is displaced by o-DFB molecules.
reported
for
a
synergistic
catalyst
comprised
of
Al(C6F5)3/B(C6F5)[6] and for salts with the highly Lewis acidic,
cationic [AlR2]+ moiety (R = Et,[30] OAr[31]). A high selectivity for
CH4 was observed (23 % respective to silane) by monitoring the
reaction with NMR spectroscopy, whereas only diminished
amounts of the intermittently generated silyl formate and silyl
acetal complexes were detected (cf. ESI). Unfortunately, the
reduction was rather slow, thus, only 24 % of initially used silane
have reacted after 14 d. Nonetheless, the successful
hydrosilylation of thermodynamically highly stable CO2 clearly
demonstrates the catalytic potential of dicationic Ae-arene
complexes.
Therefore,
we
switched
the
solvent
o-DFB
for
1,2,3,4-tetrafluorobenzene (TFB). TFB possesses a considerably
diminished coordinating ability (cf. Figure 4a), owing to the lower
negative partial charge per fluorine atom, whereas the polarity
and the physical properties closely resemble those of o-DFB (cf.
[29] and ESI, Table S12). As anticipated, a significant shift of proton
and carbon resonances is observed for 5 compared to the free
ligand in TFB solution (cf. Figure 4b). In combination with a
pronounced peak broadening of the proton signals in the 1H NMR
spectrum (ESI Fig S16), the NMR spectra indicate coordination of
the ansa-arene to the electron withdrawing strontium ion in TFB
solution. These observations are supported by a significant
dampening of diffusion constants determined by means of NMR
spectroscopy (ESI, Table S1 and S2). The 27Al NMR resonance
attributed to the [Al(ORF)4]– anion is strongly broadened in TFB
(1/2 = 617 Hz, ESI Fig. S18), whereas the respective signal in
o-DFB is sharp (1/2 = 29 Hz, ESI Fig S14) and comparable to
free [Al(ORF)4]– in [NEt4]+[Al(ORF)4]– (1/2 = 12 Hz).[20]
0.6-mol % 5 in TFB, rt
CO2 + 4 HSiEt3
CH4 + 2 O(SiEt3)2
(0.18)
via
[LA]
+ HSiEt3
+ HSiEt3
{63.9}
+ HSiEt3
- O(SiEt3)2
[LA]
{46.2}
SiEt3
SiEt3
O
+ HSiEt
O
O
3
(8.54)
SiEt3
O
SiEt3
H
H
H
O
H
H
H
(4.67)
Silyl-
Formate
Silyl-
Acetal
(1H) and {29Si} in ppm
Methoxy-
silane
Scheme 1. Postulated pathway for stepwise hydrosilyation of CO2 to CH4. Lewis
acid-adducts (indicated by [LA]) of silyl-formate and silyl-acetal were observed
by NMR spectroscopy and the chemical shifts are given (ESI). Formation of
methoxy-silane is hypothetic as it could not be detected in the NMR experiment.
3
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COMMUNICATION
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(Scheme 2a). Only 0.01 mol-% 5 was sufficient and polymers
were isolated as viscous, rubber-like materials (Scheme 2b).
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n
b)
0.01 mol-% 5, TFB
(5)
2h; + iPrOH
n
IB
PIB
Scheme 2. a) Polymerization of 2.0…2.3 g isobutylene initiated by 0.01 mol-%
5 in 1,2,3,4-tetrafluorobenzene (10 mL). Initiator concentration refers to amount
of isobutylene used. b) Picture of rubber-like polyisobutylene isolated in
polymerization @ 12 °C.
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[4]
[5]
[6]
[7]
[8]
[9]
In contrast, common catalysts at similar conditions yield oily,
low-molecular weight polyisobutylenes (Mw < 2000 g mol–1),
whereas medium-to-high molecular weights (Mw > 1•105 g mol1)
and narrow dispersities (Đ < 2.0) are typically only achieved at
temperatures below 80 °C.[32] Excitingly, the polymer isolated
from the –12 °C polymerizations in TFB revealed in GPC analyses
a high molecular weight of Mw = 1.30•105 g mol1 in combination
with a narrow molecular mass distribution (Đ = 1.87) – similar to
the current record holding scandium catalysts.[33] These results
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and alkaline earth metals.[8] Recently, various cationic rare earth
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molecular weight polyisobutylene (Mw > 1 • 105 gmol1) with low
dispersities (Đ = 1.8) at elevated temperatures up to 15°C.[33,34]
In accordance with the typical characteristics of cationic polymeri-
zations, a higher reaction temperature of 0 °C resulted in our case
[10]
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to
a lower molecular weight of the isolated polymers
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[13]
In summary, we report on the isolation of the solvent free
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coordination of one of the [Al(ORF)4]–-anions. Due to the weakly
coordinating character of the anion, the WCAs can be easily
replaced by weak, neutral ligands, hence, opening a simple route
towards highly Lewis-acidic, dicationic alkaline earth metal
complexes. As a first example, the synthesis of a strontium
ansa-arene complex 5 was presented. The catalytic activities of
highly Lewis-acidic 5 in hydrosilylation of the thermodynamically
extremely stable CO2 represent promising studies for broadening
the scope of alkaline-earth metal-catalysis to FLP-type reactions
typically achieved with strong p-block Lewis acids such as
Al(C6F5) or B(C6F5). Moreover, 5 was able to initiate a remarkably
controlled polymerization of isobutylene at elevated temperatures.
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We thank Dr. Harald Scherer and F. Bitgül for performing NMR
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Keywords: Alkaline earth metals • Lewis acids •
Polymerizations • Main-group catalysis • CO2 reduction
4
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10.1002/anie.202010019
Angewandte Chemie International Edition
COMMUNICATION
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1,2,3,4-Tetrafluorobenzene possesses similar physical properties
and polarity (cf. ESI) compared to 1,2-difluorobenzene, since the
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1,2,3,4-tetrafluorobenzene. This behaviour of TFB as even less
coordinating solvent compared to o-DFB is reflected in the calculated
by 36 to 41 kJ mol–1 endothermic and endergonic thermodynamics
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5
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COMMUNICATION
Entry for the Table of Contents
Soft shell, hard core: The synthesis of a highly Lewis acidic, dicationic strontium ansa-arene complex as salt of weakly coordinating
[Al(ORF)4]-anion (ORF = C(CF3)3) is reported via a base-free strontium-perfluoroalkoxyaluminate Sr[Al(ORF)4]2 as source of a naked
strontium dication. The strontium ansa-arene complex was successfully applied as catalyst in a CO2 reduction and a highly controlled
isobutylene polymerization.
6
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