The Arene-Stabilized η5-Pentamethylcyclopentadienyl Arsenic Dication [(η5-Cp*)As(toluene)]2+

Article abstract of DOI:10.1002/anie.201902039

Double chloride abstraction of Cp*AsCl₂ gives the dicationic arsenic species [(η⁵-Cp*)As(tol)][B(C₆F₅)₄]₂ (2) (tol=toluene). This species is shown to exhibit Lewis super acidity by the Gutmann–Beckett test and by fluoride abstraction from [NBu₄][SbF₆]. Species 2 participates in the FLP activation of THF affording [(η²-Cp*)AsO(CH₂)₄(THF)][B(C₆F₅)₄]₂ (5). The reaction of 2 with PMe₃ or dppe generates [(Me₃P)₂As][B(C₆F₅)₄] (6) and [(σ-Cp*)PMe₃][B(C₆F₅)₄] (7), or [(dppe)As][B(C₆F₅)₄] (8) and [(dppe)(σ-Cp*)₂][B(C₆F₅)₄]₂ (9), respectively, through a facile cleavage of C?As bonds, thus showcasing unusual reactivity of this unique As-containing compound.

Full text of DOI:10.1002/anie.201902039

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Accepted Article
Title: The Arene-Stabilized η5-Pentamethylcyclopentadienyl Arsenic
Dication [(η5-Cp*)As(toluene)]2+
Authors: Jiliang Zhou, Liu Liu, Levy Cao, and Douglas Wade Stephan
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To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.201902039
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Angewandte Chemie International Edition
10.1002/anie.201902039
COMMUNICATION
5
The Arene-Stabilized η -Pentamethylcyclopentadienyl Arsenic
5
2+
Dication [(η -Cp*)As(toluene)]
Jiliang Zhou, Liu Leo Liu, Levy L. Cao, and Douglas W. Stephan*[a]
Dedication ((optional))
Abstract: Double chloride abstraction of Cp*AsCl
2
gives the
(2) (tol =
namely As(OTeF
5
)
5
. However, this species cannot be handled
5
[13]
dicationic arsenic species [(η -Cp*)As(tol)][B(C
6
F
5
)
4
]
2
with commonly used solvents due to the low stability in solution.
On the other hand, the chemistry of group 15 elements is
dominated by compounds in which the element adopts the +3
oxidation state and acts as an electron pair donor. Nevertheless,
the Lewis acidity of neutral compounds AsX (X = F, OTeF5,[15]
OTf[16]) and cationic arsenic(III) salts has been indicated by
toluene). This species is shown to exhibit Lewis super acidity by
the Gutmann-Beckett test and by fluoride abstraction from
[
[
NBu
(η -Cp*)AsO(CH
4
][SbF
6
]. 2 participates in the FLP activation of THF affording
(THF)][B(C (5). The reaction of 2 with
or dppe generates [(Me As][B(C ] (6) and [(σ-
Cp*)PMe ][B(C ] (7) or [(dppe)As][B(C ] (8) and [(dppe)(σ-
Cp*) ][B(C (9), respectively, via a facile cleavage of C-As
2
[14]
2
)
4
6 5 4 2
F ) ]
3
PMe
3
3
P)
2
6
F
5
)
4
F
5
)
4
6
F
5
)
4
binding with electron donors,[17] while Burford and co-workers
recently described highly Lewis acidic arsenic(III) trications
3
6
2
6 5 4 2
F ) ]
bonds, thus showcasing unusual reactivity of this unique As-
containing compound.
[
(L)
2
As][OTf]
3
(L = 2,2’-bipyridine or 4,4’-di-tbutyl-2,2’-bipyridine;
[17f]
OTf = triflate).
We recently synthesized
pentamethylcyclopentadienyl
Cp*)P(tol)]2+ featuring Lewis super acidity owing to low-lying
LUMOs on the dicationic P center (Figure 1c).[18] Seeking to
develop related LSAs, in the present work, we report the synthesis,
characterization and reactivity of an arene-stabilized η -
pentamethylcyclopentadienyl (Cp*) arsenic dication [(η -
Cp*)As(tol)]2+ (Figure 1c). This species is the first example in
a
toluene coordinated η5-
phosphorus dication
[(η5-
The advances in Lewis acid catalysis[1] and the articulation of the
concept of frustrated Lewis pairs (FLPs)[2] has garnered
tremendous interest in the last decade. For these applications
Lewis acidity must be balanced so as to be sufficient for reactivity
and yet mild enough to provide functional group tolerance. For
this reason, the studies of Lewis super acids (LSAs) which are
5
5
5
those with higher Lewis acidity than that of monomeric SbF , are
5
much less encountered.[3] The chemistry of the LSAs is dominated
by group 13 element-based compounds. For example, aluminum
which a Cp ligand is bound to arsenic in an η coordinating mode
and of an arsenic(III) LSA. This species is shown to partcipate in
FLP chemistry with THF. The C-As bonds are completely splitted
in reactions with phosphines.
3
halides AlX (X = Cl, Br, I) are LSAs in gas phase, whereas they
aggregate in solid state to compensate their intrinsic electron
deficiency.[3] Recently, the utilization of electron withdrawing and
bulky perfluorinated substituents has rendered the isolation of a
handful of discrete molecular LSAs, such as tris(p-
(a)
Group 13
F
F2
C
CF3
F
F
F
Al
F
CF3
CF3
F
F
F
O
O
F
O
[
4]
[5]
perfluorotolyl)borane and tris(perfluorophenyl)alane (Figure
a). In addition, Krossing et al. have described several aluminum
Lewis super acids, exemplified by Al[OC(CF
(Figure 1a).[3a, 6]
Al
F3C
F
F
F
CF3
1
C
FF
F3C
F
F
F
2
CF3
3
)
3
]
3
F
In the case of group 14 LSAs, carbocations have been considered
as LSAs in the gas phase;[7] however, in the condensed phase
Cl
Cl
Cl
(
b) Group 14 R
Cl
Cl
O
O
O
Cl
Cl
[
8]
Si
their Lewis super acidity is still doubtful. Silylium cations
Si
R
R
aryl
functionalize as LSAs (Figure 1b),[7] which has found a variety of
O
R =
alkyl,
applications in catalysis.[9] Intriguingly, Greb et al. reported the
Cl
this work
Group 15
first example of
bis(perchlorocatecholato)silane (Figure 1b).
a
neutral silicon LSA, namely
a
(c)
[
10]
Pnicogen(V) halides are conventional Lewis acids, while
recent development of tetracoordinated pnicogen(V) cations with
strong Lewis acidity has emerged,[11] some of which are viewed
as LSAs.[12] There is hitherto only one example of an arsenic LSA,
2+
2+
P
As
Lewis super acid
As(III)
Figure 1. Representative examples of (a) group 13 and (b) group
14 LSAs. (c) A representative example of a group 15 LSA and
present work.
[a]
Dr. J. Zhou, Dr. L. L. Liu, L. L. Cao, Prof. Dr. D. W. Stephan
Department of Chemistry,
University of Toronto,
8
0 St. George St., Toronto, Ontario, Canada M5S3H6
Chloride abstraction of Cp*AsCl [
of silylium salt [Et Si(tol)][B(C
solid in 95% yield, while double chloride abstraction with 2
equivalents of [Et Si(tol)][B(C ] produced 2 as a pale-yellow
19]
with an equimolar portion
E-mail: dstephan@chem.utoronto.ca
2
Supporting information and the ORCID identification number(s) for
the author(s) of this article can be found under:
3
6 5 4
F ) ] afforded 1 as a pale-yellow
https://doi.org/XXXXX
3
6 5 4
F )
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Angewandte Chemie International Edition
10.1002/anie.201902039
COMMUNICATION
solid in 94% yield (Figure 2a). The H, 13C, 19F and 11B NMR data
1
8.70 eV) in [(η -Cp*)As(tol)] are slightly lower than those (-8.60
5
2+
5
2+
of both 1 and 2 were consistent with the products containing a
dynamically bound Cp* ligand and a [B(C F )
6 5 4
]− anion, but gave
and -8.48 eV) of [(η -Cp*)P(tol)]
electronegativity of P over As. The As atom of [(η -Cp*)As(tol)]
due to the higher
5
2+
little further structural insight. Single crystals of 1 and 2 were
obtained by layering pentane on a dichloromethane (DCM)
solution and by layering pentane on a toluene/difluorobenzene
is positively charged (NBO charge: +1.20 a.u.). Recent studies
suggest that IBOs offer a definitive and intuitively accessible
description of the bonding for Lewis structures.[28] Four IBOs were
located and shown to engage in bonding with the As atom (Figure
3b) consistent with π-type bonding between the As atom and Cp*
ring, and one lone pair of electrons at the As atom. The former π
(
DFB) solution at room temperature, respectively. The X-ray data
2
] and 2 is [(η5-
confirmed that 1 is [(η -Cp*)AsCl][B(C
6
F
5
)
4
Cp*)As(tol)][B(C
to [ClAl(OC{CF
features the Cp* ligand η -bonded to the central As with the C−As
6
F
5
)
4
−
]
2
(Figure 2). While the former is analogous
[20]
5
2+ [29]
3
}
3
)
3
] salt reported by Krossing,
compound 2
2 3
interactions are similar to those seen for [(η -Cp*)Ir(OH ) ] ,
5
5
2+
suggesting that the Lewis structure of [(η -Cp*)As] is most
bond lengths ranging from 2.190(3) to 2.246(3) Å which compare
plausibly described by the coordination of an aromatically anionic
+
[21]
−
3+
to the shortest C−As distance in the [(Cp*)
2
As] cation (2.186
[Cp*] ligand to the [As] . To compensate the electron deficiency
of the cationic As center, one of the three π-type IBOs in the
toluene polarizes toward the As atom (Figure S42). Indeed, the
Å). The electron deficient nature of the As center results in the
interaction with a toluene molecule in the solid state. The distance
of toluene-ring carbons to the As ranges from 2.823(3) to 3.041(5)
Å, which is much shorter than the sum of Van der Waals radii of
C and As (3.55 Å).[22] This indicates the π-donation of the aromatic
toluene to As.[23] 2 did not lose toluene at room temperature under
dynamic vacuum and decomposed at evaluated temperature. The
5
2+
free energy for the dissociation of toluene from [(η -Cp*)As(tol)]
to generate [(η -Cp*)As] is computed to be 21.7 or -2.1 kcal mol-
in the gas or solution phase, respectively (M06-2X/Def2-
5
2+
1
TZVP//M06-2X/Def2-SVP). This indicates that such dissociation
is slightly favorable in solution.
attempt to synthesize toluene-free [Cp*As][B(C
CpSn]+[24] by performing the reaction of Cp*AsCl
equivalents of [Et Si(tol)][B(C ] in pentane gave 1 as the only
As-containing product. Similar interactions to aromatic solvents in
6
F
5
)
4
]
2
akin to the
[
2
with 2
(a)
3
6 5 4
F )
the solid state have been reported for (C
6
F
5
)
3
Al(tol),[5b]
,[18]
][25]
and
[(η -Cp*)P(tol)][B(C
5
F
)
]
[
Et
3
Si(tol)][B(C
6
5
F )
4
6
5
4
2
consistent with the high Lewis acidity of 2. It is noteworthy that 2
represents the first structurally characterized arsenic compound
5
with a true η -Cp-type ligand, although a computational study
demonstrating that [(Cp)(Cp*)As]+ cation contains η -Cp-type
5
LUMO
8.91 eV
LUMO+1
-8.70 eV
-
ligands has been reported.[26]
(b)
F
(
a)
[Et3Si][B(C6 5)4]
Et SiCl
(η2-Cp*
F
[
)AsCl][B(C6 5)4]
-
3
1
Cp*AsCl2
2
F
[
Et3Si][B(C6 5)4]
2 Et SiCl
(η5-Cp*
)As(tol)][B(C6 5)4]2
F
[
-
3
2
(c)
(b)
Figure 2. (a) Synthesis of 1 and 2. POV-ray depictions of the
cations of (b) 1 and (c) 2; C: black; Cl: green; As: pale-pink. All H
6 5 4
F )
]− anions are omitted for
atoms, solvent residue and the [B(C
clarity.
5
2+
Figure 3. (a) LUMO and LUMO+1 of [(η -Cp*)As(tol)] . (b) IBOs
5
2+
of [(η -Cp*)As(tol)] with enclosing 80% of the orbital electron’s
density.
To experimentally gauge the Lewis acidity of 2, the Gutmann-
Beckett test was performed in DFB at room temperature using an
equimolar ratio of Et PO. The 31P NMR resonance of Et PO
The bonding in [(η -Cp*)As(tol)]2+ was probed with density
functional theory (DFT) calculations, coupled with natural bond
orbital (NBO) (M06-2X/Def2-TZVP) and intrinsic bond orbital
5
(
IBO)[27] (PBE/Def2-TZVP) analyses. The LUMO and LUMO+1
3
3
moved downfield with a shift change of Δδ = 52.0 ppm (Figure S9),
which is much larger than the values seen for
bis(perchlorocatecholato)silane (Δδ = 35.0 ppm)[10] and tris(p-
primarily involve the vacant p-orbitals of the As atom (Figure 3a),
which is similar to observations made for [(η -Cp*)P(tol)] (Figure
S41), whereas the energies of LUMO (-8.91 eV) and LUMO+1 (-
5
2+
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Angewandte Chemie International Edition
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COMMUNICATION
[
4]
perfluorotolyl)borane (Δδ = 29.0 ppm) . The displayed Lewis
Treatment of 2 with excess tetrahydrofuran (THF) in DFB gave
product 5 which contains a THF motif as suggested the H NMR
spectrum (Scheme 1). To our surprise, single crystal X-ray
diffraction analysis unveiled the formulation of 5 as [(η2-
1
super acidity of 2 was unambiguously confirmed by reaction with
2
[
[
NBu
NBu
4
][SbF
][B(C
6
], which led to [(η -Cp*)AsF][B(C
6
F
5
)
4
]
3
and
4
6
F
5
)
4
]. Compound
Me ][B(C
][30] as evidenced by 1H, 19F and
C{ H} NMR spectroscopy (Scheme 1, Figure S14-S17) and
3
proved thermally unstable,
affording [C
5
5
H
2
6
5
F )
4
2 4 6 5 4 2
Cp*)AsO(CH ) (THF)][B(C F ) ] (Figure 4c). 5 can be regarded
1
3
1
as structural interception of the first steps in THF polymerization
induced by Lewis/Brønsted acids.[31]
confirmed by X-ray diffraction analysis (Figure 4a, S40c). It is
reasoned that the high fluoride affinity of 2 is attributed to charge
neutralization and the high Lewis acidity of 2. The latter was also
The mechanism leading to 5 was further investigated by DFT
calculations at the SMD-M06-2X/Def2-TZVP//M06-2X/Def2-SVP
5
demonstrated by its capacity to abstract chloride from Ph
3
CCl to
level of theory (Figure 5). The approach of THF toward [(η -
give 1 and trityl salt [Ph
3
C][B(C F
6 5
)
4
] (Scheme 1).
Cp*)As(tol)]2+ gives rise to the ligand exchange at the As, coupled
4
with the haptotropic shift of the Cp* ring, which affords [(η -
1
+
Cp*)As(THF)]2+ (IN1) (-7.4 kcal mol−1). Although double
coordination of THF to the As atom is slightly thermodynamically
[
Ph
3
C][B(C
F
]
6
5 4
)
favorable to give [(η -Cp*)As(THF)
2
]2+ (IN2) (-8.7 kcal mol ), the
3
−1
second THF molecule is also able to form a Van der Waals
complex IN3 (-5.3 kcal mol ) upon interacting of O(2) with C(1).
η2-Cp*
Ph CCl
3
[
(
)AsF][B(C
3
+
6F5
)
4
]
N]
−1
[
[
Bu
4
N
N
2+
As
SbF
6
]
2,2'-bipy
2
Such interaction allows for the cleavage of the C(1)−O(1) bond
-
SbF5
1-
via TS1 (15.8 kcal mol ) with an activation barrier of 23.2 kcal/mol
−1
F
]
η
[
Bu
4
N][B(C
6
5
)
4
Cp*
F
B(C ]
6
)
5 4 2
4
THF
(
IN1→TS1), leading to a THF-ring opening product IN4 (-12.3 kcal
[
−
1
2
mol ). Finally, rotation of the carbon chain in IN4 generates [(η -
Cp*)AsO(CH )
2 4
(THF)]2+ (-15.3 kcal mol ). This C−O σ bond
O
(η2-Cp*
−1
As
O
)
scission by the combination of the Lewis acidic As atom and Lewis
basic O atom hints at the possibility of other FLP reactions
involving 2.
[
B(C
6F5
]
)
4 2
5
Scheme 1. Reactions of 2.
The coordination chemistry of 2 with Lewis bases was probed by
mixing 2 and 2,2'-bipyridine (bipy) (1:1) in DFB at -35 C. This
immediately resulted in the formation of a clear yellow solution,
O(2)
TS1
o
C(1) 2.113
(15.8)
O(1) 1.923
from which a new species 4 was isolated along with a small
1
amount of [(bipy)H][B(C
6
F
5
)
4
] (Scheme 1). The H NMR data of 4
1.493
O(2)
2.704
C(1)
(
Figure S21) suggests that the Cp* ligand is in rapid dynamic
O(1)
η
5
-Cp*
2+
o
[(
)As(tol)]
THF
0.0)
circumambulation even at -80 C. Single crystal X-ray diffraction
analysis of
confirmed its formulation as [(η1-
Cp*)As(bipy)][B(C
(
4
tol
THF
IN3
1.469
O(2)
O(1)
IN1
F
5
)
4
]
2
(Figure 4b). The cation of 4 exhibits a
(-5.3)
6
(
-7.4)
1
“
slipped half sandwich” structure with an η -Cp* ligand, supported
IN2
-8.7)
(
IN4
C(1)
by the similar C−C bond lengths of the Cp*-ring (average 1.438
Å). The As−C bond length (1.98(1) Å) is shorter than the average
As−C bond length (2.214 Å) in 2, while the As−N bond lengths are
2.133O(1)
2.267
(-12.3)
2.202
(-15.3)
O
η
2
-Cp*
)As
(
O
2.026(6) and 2.040(6) Å.
−1
Figure 5. Free energy profile (kcal mol ) for the formation of 5.
In the 3D structures, bond lengths are given in angstroms.
(a)
(b)
3
Adding an excess of PMe (1M in THF) into DFB/toluene solution
of 2 resulted in the immediate consumption of 2 and formation of
two major products 6 and 7, along with a small amount of
3 6 5 4
HPMe ][B(C F )
] (Figure 6a). The 31P NMR of the crude product,
[
showed two singlets at 28.2 and 0.8 ppm and a small doublet at -
5.0 ppm. Crystallization of the products by layering pentane on
(c)
the
(Me
7) (Figure S40a), as well as [HPMe
Compound 6 corresponds to resonance at 28.2 ppm in the 31P
NMR spectrum comparable to that reported for [(Me P) As][OTf]
by Burford et al.[17f] In 6, the As atomis slightly negatively charged
while P atoms are positively charged (NBO charge: As, -0.22 a.u.;
P, +1.16 a.u.). Unfortunately, attempts to isolate bulk samples of
reaction
solution
] (6) (Figure 6b) and [(σ-Cp*)PMe
][B(C ] (Figure S40b).
gave
colorless
crystals
of
[
(
3
2
P) As][B(C
6
5
F )
4
3
][B(C
6 5 4
F ) ]
3
6 5 4
F )
3
2
Figure 4. POV-ray depictions of the cations of (a) 3, (b) 4 and (c)
; C: black; F: pink; N: blue; O: red; As: pale-pink. All H atoms,
5
6 5 4
solvent residue, disorder of 3 and the [B(C F )
]− anions are
omitted for clarity.
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Angewandte Chemie International Edition
10.1002/anie.201902039
COMMUNICATION
6
or 7 were unsuccessful, as these species proved highly
atoms (NBO charge: As, -0.15 a.u.; P, +1.17 and +1.19 a.u.).
Crystallization by layering pentane on a DCM solution of the
residue obtained by removing most of 8 from the crude product
allowed for the isolation of another product 9, and the X-ray
diffraction analysis showed the formulation of 9 as [(dppe)(σ-
sensitive, inevitably decomposing to [HPMe
3
][B(C
6
F
5
)
4
].
(a)
As
As
As
As
Cp*)
2 6 5 4 2
][B(C F ) ] (Figure 6c). The formation of 6/7 or 8/9
Ph
Ph
P
P
Ph
Ph
Ph
Ph
P
P
Ph
Ph
[B(C6F5)4]
P
P
P
P
demonstrate the facile splitting of the Cp*-As bonds in 2 into the
[As] cation and [Cp*] cation[34]. DFT calculations show that the
+
[33]
+
[
B(C6F5)4]
[B(C6F5)4]
PMe3
[B(C6F5)4]
6
8
5
2+
+
+
complexion of [(η -Cp*)As(tol)] with two PMe molecules leads
to the formation of IN6 (-33.8 kcal mol ) and a free toluene
Figure 7). In contrast to the η coordinating mode in IN2 (Figure
2
3
-
σ
dppe
Cp*
P
−1
Ph
Ph
3
(
Ph
Ph
P
-
PMe3
σ
5), IN6 features a σ-bonded Cp* ligand due to the strong electron-
Cp*
[
B(C6F5)4]
7
[B(C6F5)4]2
donating ability of PMe over THF. Subsequently, the third PMe3
3
9
molecule is able to attack a C atom of the Cp* ring, leading to a
−
1
S
N
2-type TS2 with an activation barrier of 22.7 kcal mol . As a
(b)
result, the charge redistribution affords two cationic species
−1
exothermically (-73.1 kcal mol ).
In conclusion, we have synthesized a Cp*-bonded arsenic
(c)
dicationic compound 2 by chloride abstraction of Cp*AsCl
is the first true η -Cp-bound arsenic species. Compound 2 is
2
, which
5
shown to exhibit Lewis super acidity by the Gutmann-Beckett test
and by fluoride abstraction from [NBu ][SbF ]. In addition, 2 has
4 6
shown to participate in the FLP activation of THF. The facile
cleavage of C-As bonds has been achieved by reactions of 2 with
3
PMe or dppe. This reactivity showcases the potential of 2 as a
Figure 6. (a) Reactions of 2. POV-ray depictions of the cations of
precursor for unusual As-containing compounds. Further
applications of 2 in Lewis acid catalysis and FLP chemistry are
currently under active investigation in our laboratory.
(
b) 6 and (c) 9; C: black; P: orange; As: pale-pink. All H atoms,
]− anions are omitted for clarity.
solvent residue and the [B(C F )
6 5 4
Acknowledgements
η
5
-Cp*
)As(tol)]
2+
[
(
2
.521 2.455
PMe3
(0.0)
D.W.S. gratefully acknowledges the financial support from
NSERC Canada and the award of Canada Research Chair. DWS
is also grateful for the award of an Einstein Fellowship at TU Berlin.
L.L.C. is grateful for the funding from NSERC Canada.
tol
TS2
2.878
(
-11.1)
IN5
PMe3
PMe3
(
-18.1)
Keywords: Cp ligand, Arsenic Dication, Lewis super acid, FLP
2.411
IN6
-33.8)
chemistry, Arsenic(I) cation transfer
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Angewandte Chemie International Edition
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COMMUNICATION
Entry for the Table of Contents
COMMUNICATION
O
THF
η2-Cp* As
O
(
)
Arsenic(III) Lewis super acid:
Jiliang Zhou, Liu Leo Liu, Levy L.
Cao, and Douglas W. Stephan*
An arsenic dicationic species
As2+
5
[
(η -Cp*)As(tol)][B(C
6
5
F )
4
]
2
As
PMe3
exhibits Lewis super acidity,
participating in FLP chemistry
P
P
+
Page No. – Page No.
PMe3
An Arsenic(III)
Lewis super acid
The Arene-Stabilized η5-
Pentamethylcyclopentadienyl
Arsenic Dication [(η5-
Cp*)As(toluene)]2+
+
+
and splitting into [As] /[Cp*]
cations in reactions with
phosphines.
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