Stabilized Carbenium Ions as Latent, Z-type Ligands

Article abstract of DOI:10.1002/anie.201911662

Controlling the reactivity of transition metals using secondary, σ-accepting ligands is an active area of investigation that is impacting molecular catalysis. Herein we describe the phosphine gold complexes [(o-Ph₂P(C₆H₄)Acr)AuCl]⁺ ([3]⁺; Acr=9-N-methylacridinium) and [(o-Ph₂P(C₆H₄)Xan)AuCl]⁺ ([4]⁺; Xan=9-xanthylium) where the electrophilic carbenium moiety is juxtaposed with the metal atom. While only weak interactions occur between the gold atom and the carbenium moiety of these complexes, the more Lewis acidic complex [4]⁺ readily reacts with chloride to afford a trivalent phosphine gold dichloride derivative (7) in which the metal atom is covalently bound to the former carbocationic center. This anion-induced Au^I/Au^III oxidation is accompanied by a conversion of the Lewis acidic carbocationic center in [4]⁺ into an X-type ligand in 7. We conclude that the carbenium moiety of this complex acts as a latent Z-type ligand poised to increase the Lewis acidity of the gold center, a notion supported by the carbophilic reactivity of these complexes.

Full text of DOI:10.1002/anie.201911662

A Journal of the Gesellschaft Deutscher Chemiker
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Accepted Article
Title: Stabilized carbenium ions as latent, Z-type ligands
Authors: Lewis Wilkins, Youngmin Kim, Elishua Litle, and Francois
Pierre Gabbai
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To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.201911662
Angew. Chem. 10.1002/ange.201911662
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Stabilized carbenium ions as latent, Z-type ligands
Lewis C. Wilkins, Youngmin Kim, Elishua D. Litle and François P. Gabbaï*[a]
Abstract: Controlling the reactivity of transition metals using
secondary, σ-accepting ligands is an active area of investigation that
is impacting molecular catalysis. Here we describe the phosphine
Carbenium ions, which have already received interest as ligands
for transition metals,[9] may also present a higher chemical
resilience, in particular to oxidative conditions. With the view to
capitalize on these attributes, we have thus decided to reengage
in carbenium chemistry[10] and explore the synthesis and
properties of carbenium-based analogs[11] of known L/Z ambiphilic
boron-containing ligands (Figure 1C).[3a] The carbenium-based
systems described herein bear similarities with phosphine ligands
functionalized by carbon-based redox non-innocent moieties[12]
while also adding to ongoing efforts in the chemistry of cationic
phosphines as ligands for late transition metal catalysts.[13]
gold complexes [(o-Ph2P(C6H4)Acr)AuCl]+ ([3]+, Acr
=
9-N-
methylacridinium) and [(o-Ph2P(C6H4)Xan)AuCl]+ ([4]+, Xan = 9-
xanthylium) where the electrophilic carbenium moiety is juxtaposed
with the metal atom. While only weak interactions occur between the
gold atom and the carbenium moiety of these complexes, the more
Lewis acidic complex [4]+ readily reacts with chloride to afford a
trivalent phosphine gold dichloride derivative (7) in which the metal
atom is covalently bound to the former carbocationic center. This
anion-induced Au(I)/Au(III) oxidation is accompanied by a conversion
of the Lewis acidic carbocationic center in [4]+ into an X-type ligand in
7. We conclude that the carbenium moiety of this complex acts as a
latent Z-type ligand poised to increase the Lewis acidity of the gold
center, a notion supported by the carbophilic reactivity of these
complexes.
A
Cl
Au
PiPr2
n-
Ph
B
B
B
PiPr2
i
iPr2P
P Pr2
Ph2P [Au] PPh2
[Fe]
III
I
II
B
C
[M]
B
[M]
C
A new paradigm has emerged in catalysis whereby the positioning
of an electron-deficient moiety next to a transition metal has led
to remarkable changes in catalytic activity for a number of
reactions.[1] This strategy has seen its most notable success in
the chemistry of transition metal complexes bearing boron-
containing Lewis acidic Z-type ligands[2] such as those in I, II and
III (Figure 1A).[3] The boron atoms of such ligands directly interact
with the transition metal center by drawing electron density from
filled n d-orbitals. This interaction, which is accompanied by a
lowering of the vacant n+1 px metal orbital, results in a more
electron deficient and Lewis acidic transition metal center (Figure
1B).[4] Such effects, which have been observed with other σ-
accepting ligands,[5] present substantial benefits in catalysis.[3b,3c]
However, because the boron px orbitals are high in energy, the
resulting orbital diagram resembles that of an inverted ligand field
complex,[6] limiting the extent of stabilization provided to the metal
orbitals. Carbenium ions are isolobal with boranes; yet, owing to
nuclear and overall charge effects, their accepting orbitals are
significantly lower leading to enhanced Lewis acidity[7] and
oxidizing properties.[8] It follows that the accepting orbital of a
carbocation should be better matched with those of the metal,
decreasing the ligand field inversion and strengthening the M→Z
interaction (Figure 1b). In other words, a carbenium ion should
R
[M]
PR'2
R
C
px
px
n+1 px
n dx2-y2
n+1 px
This work
Stabilization
n dx2-y2
Figure 1. A) Selected examples of complexes featuring boron-based Lewis
acidic Z-type ligands. B) Simplified metal-ligand molecular orbital diagram for
the interaction of a metal with a boron-based or carbenium-based Z-type ligand.
C) Graphical summary of the objective of this paper.
Bearing in mind that the high reactivity of carbenium ions may
complicate these studies, we decided to focus on the
incorporation of stabilized examples of such moieties. To this end,
1-lithio-2-diphenylphosphino-phenylene was quenched with N-
methyl acridone or xanthone leading to the formation of the
phosphinocarbinols
1
and 2, respectively (Scheme 1).[14]
Subsequently, the acridinium gold(I) chloride derivative [3]BF4
was generated via the sequential addition of (tht)AuCl to the
carbinol 1 in a 1:1 molar ratio followed by addition of 1 equiv. of
HBF4. This method afforded the target monocation as an air
stable yellow/green solid in 97% yield. The analogous reaction
was carried out with the xanthenol precursor 2, generating the
xanthylium product [4]BF4 as an orange solid in equally good yield
of 93% (Scheme 1). This transformation could be easily tracked
using multinuclear NMR spectroscopy. The presence of the
carbinol proton is diagnostic in the 1H NMR spectrum giving a
resonance at δ = 2.24 ppm for 1 and 2.49 ppm for 2 which
disappears upon dehydration. Another prominent resonance that
aids in identification is the N-methyl protons of 1 which appear at
δ = 3.64 ppm which, upon treatment with HBF4, undergo a down-
field shift to δ = 4.90 ppm, concomitant with formation of the
acridininium moiety.
be more potent σ-acceptor ligands, leading to
a greater
enhancement in the hardness and Lewis acidity of the metal atom.
[a]
Dr. Lewis C. Wilkins, Youngmin Kim, Elishua Litle and Prof. Dr. F. P.
Gabbaï
Department of Chemistry, Texas A&M University
College Station, TX 77843 (USA)
E-mail: francois@tamu.edu
Homepage: http://www.chem.tamu.edu/rgroup/gabbai/
Supporting information for this article is given via a link at the end of
the document.
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10.1002/anie.201911662
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only 10.0(9)° for [3]BF4 and 1.36(18)° for [4]BF4 (Figure 2).[16]
Furthermore, a relatively short interatomic distance is noted
between the gold atom and the carbenium center of 3.168(9) Å in
[3]BF4 and 3.131(3) Å for [4]BF4, indicating the possibility of a
weak interaction (ΣVDW(Au-C) = 3.36 Å). In order to probe this
interaction, an atoms-in-molecules (AIM) analysis[17] was
implemented at the geometry optimized structures of [3]+ and [4]+.
This analysis reveals that a bond path is indeed present between
the C9 atom and the gold atom with electron density ρ(r) of 1.4 ×
10-2 e/bohr3 and 1.5 × 10-2 e/bohr3 at the localized bond critical
points (BCP) for [3]+ and [4]+ respectively (Figure 2). The low BCP
ρ(r) values suggest that these contacts are extremely weak. This
conclusion is supported by the results of Natural Bond Orbital
calculations that show only weak donor-acceptor bonding
between the gold atom and the adjacent acridinium or xanthylium
units.
Li
PPh2
THF, -78 °C
RT, 18 h
X
1
X = NMe (83%)
HO
2
PPh2
+
X = O (80%)
X
1) (tht)AuCl; 2) HBF4
CH2Cl2, RT
O
X
-
BF4
X
3
[ ]BF4
Cl
Cl
Au
P
X = NMe (97%)
Au
P
[4]BF4
Ph
Ph
Ph
Ph
X = O (93%)
a
b
resonance form
resonance form
Scheme 1. Synthetic overview of carbinols 1 and 2 and monocations [3]BF4 and
[4]BF4. Isolated yields shown.
The weakness of the Au"C interaction in the structures of [3]BF4
and [4]BF4 is consistent with the observation of the 13C NMR
signals at δ = 159.7 and 173.8 ppm corresponding to the
Additionally, the 31P NMR signals undergo a shift from δ ~ -19
ppm for both carbinol precursors 1 and 2, to δ = 22.4 and 25.8
ppm for [3]BF4 and [4]BF4, respectively. While the central ring of
the acridinium and xanthylium moieties are aromatic, their
reactivity and the structure of their carbinol precursors show that
the C9 position is electrophilic, as accounted by resonance
structure b. In the case of xanthylium cations, the importance of
this resonance structure was noted by Bayer in 1905 who
proposed that such species should be considered as carbenium
rather than oxonium ions.[15]
carbenium centers, respectively.
These structural and
spectroscopic findings indicate that the carbenium centers
present in these complexes are uncompromised and thus
potentially poised for engaging a Lewis basic partner. Intrigued by
this possibility, we decided to interrogate the complexes with a
simple Lewis base, a chloride anion. As a prelude to these
studies, we first studied the interaction of N-methyl-phenyl-
acridinium [5]+ and phenyl-xanthylium [6]+ with chloride anions.
UV-vis. titration experiments carried out by addition of TBACl to
solutions of the cation (10-4 M) in CH2Cl2 indicate that chloride
anion binding by [6]+ is essentially quantitative upon addition of 1
equivalent, even under dilute conditions (K > 5 x 106 M-1) (Scheme
2). By contrast, [5]+ shows no affinity for chloride anions. We also
tested the interaction of Ph3PAuCl with chloride and confirmed by
31P NMR the absence of any changes.
-
-
X
BF4
BF4
X
X
K
+ Cl-
Ph
Cl
CH2Cl2
not observed
for X = NMe
Ph
Ph
K > 5 x 106 for X = O
5
[ ]BF4 (X = NMe)
[6]BF4 (X = O)
Scheme 2. Quenching of carbenium center by chloride with experimental rate
constant
With this reactivity baseline established, we moved to studying
the behavior of the gold(I) complexes upon addition of chloride,
using UV-vis spectroscopy. While the spectrum of [3]BF4
remained unchanged, addition of chloride to the more Lewis
acidic xanthylium derivative [4]BF4 led to the formation of a new
species, referred to as 7, characterized by a strong absorption
band centered at 328 and 432 nm. The appearance of this
atypical feature prompted us to also study the reaction by NMR
spectroscopy. Addition of 1 equiv. of TBACl gave rise to a new
Figure 2. Structure of [3]BF4 (left) and [4]BF4 (right). BF4 counterions and
hydrogen atoms omitted for clarity. Thermal ellipsoids drawn at 50% probability.
Result of AIM calculations showing gold-carbon bond path with BCPs (bottom).
The crystal structures of [3]BF4 and [4]BF4 are similar in terms of
the eclipsing of the P(1)-Au(1)-Cl(1) motif with the C(9)-C(14)-
C(15) plane showing an Au(1)-P(1)-C(14)-C(15) dihedral angle of
.
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Figure 3. (Left) Solid-state structure of 7. Thermal ellipsoids drawn at 50% probability. Hydrogen atoms omitted for clarity. (Center) NBO depictions of the LUMO,
HOMO and HOMO-1 orbitals. Isosurface value = 0.04. (Right) UV-vis. spectrum where solid trace = experimental, dashed trace = simulated.
BF4
silico comparison of 7 with its boron analog shows that the
carbenium ion of 7 engages the gold atom more effectively
(Figure 4). These computational results illustrate the greater σ-
accepting properties of carbenium-based Z-type ligand as
depicted in Figure 1B.
O
O
O
Cl
Cl
(tht)AuCl
HCl
Au Cl
TBACl
Au
HO
P
P
P
Ph
CH2Cl2
RT
CH2Cl2
RT
Ph
Ph
Ph
Ph
Ph
4
7
2
[ ]BF4
(88%)
O
Cl
C
Au Cl
Scheme 3. Oxidative addition of gold upon chloride coordination. Yield shown
from alternative synthesis using HCl.
2.158
Å
2.405 Å
P
ρBCP
Ph
ρBCP
Ph
0.116 e/bohr3
0.084 e/bohr3
7
broad singlet resonance in the 31P NMR spectrum at δ = 58.3 ppm
which resolved upon cooling to -30 °C to a much sharper singlet
centered at δ = 60.2 ppm. Furthermore, the resonance of the
carbenium center was no longer present in the 13C NMR spectrum
with a quaternary carbon being observed at δ = 77.7 ppm.[18]
Formation of 7 is also observed when the carbinol precursor (2)
is combined with (tht)AuCl and ethereal HCl simultaneously.
Finally, formation of 7 is reversible since treatment with AgBF4
leads to regeneration of [4]+.
O
Cl
B
Au Cl
2.355 Å
2.646 Å
P
ρBCP
Ph
ρBCP
Ph
0.064 e/bohr3
0.050 e/bohr3
7
boron analog of
Figure 4. Comparison of 7 with its boron analogue showing selected interatomic
separation obtained from DFT calculations. The corresponding bond critical
points electron density (ρBCP) obtained from AIM calculations are also provided.
We reasoned that 7 could be the results of chloride addition either
to the carbenium ion or to the gold center. A structural assay
showed that the latter had taken place, in concert with the
formation of a covalent bond between the gold atom and the
former carbenium ion (Scheme 3). Indeed the structure of 7
displays a square planar gold atom.[16] The two Au-Cl bonds have
similar lengths (2.3746(2) Å for Au(1)-Cl(1) and 2.3943(3) Å for
the Au(1)-Cl(2)) and the Au(1)-C(9) bond of 2.135(6) Å is in line
with a covalent bond between an sp3 carbon and gold atom.[18]
TD-DFT calculations afford two dominant vertical excitations at
340 and 420 nm which satisfactorily coincided with those
experimentally observed at 328 and 432 nm. Both absorptions,
which have ligand to metal charge transfer character, involve the
xanthene-based HOMO and HOMO-1 as the ligand-based
orbitals, respectively. For both transitions, the metal-based orbital,
which is the LUMO of the complex, has dx2-y2 character as
expected in the case of a d8 electronic configuration (Figure 3). In
The structure of 7 shows that the arrested Au"Ccarbenium
coordination seen in [4]BF4 may be bolstered by the addition of a
chloride anion which enhances the metallobasic character of the
gold atom, promoting its coordination to the Lewis acidic
carbenium ion. The result is a net anion-induced oxidation of the
gold center which transitions from a d10 electronic configuration in
[4]BF4 into a d8 electronic configuration in 7. This anion induced
oxidation bears a parallel with the susceptibility of aurates towards
oxidation as documented in the seminal work of Kochi.[19] The
reaction of [4]+ with chloride could be viewed as providing a
snapshot of a hypothetical, SN1-like, Csp3-X bond activation
reaction where halide dissociation from the carbon electrophile
would have occurred first. Such a pathway has not been
proposed for the insertion of gold(I) fragments into Csp3-X bonds
which are generally assumed to occur via an SN2 mechanism,[20]
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as typically seen in late transition metal chemistry.[21] The
structure of the L/Z ligand in [4]BF4 and its rigidity are certainly
conducive of the formation of a d8 gold center, an attribute that
has been exploited to promote Csp2-X bond activation by gold
using structurally related L2-type ligands.[20a,22] Finally, we will
note that the carbon ligand also changes from Z-type in [4]BF4 to
X-type in 7. As a result, 7 resembles complexes obtained by
intramolecular oxidative insertion of gold(I) halide moieties into C-
X bonds.[23] A parallel must also be drawn with a series of recently
reported Pd(II) and Rh(I) complexes featuring o-phosphino-
triarylmethyl ligands.[24]
carbenium ions as latent Z-type ligands, poised to increase the
Lewis acidity of the gold center, should a Lewis base become
available. This possibility is illustrated by the reaction of [4]+ with
chloride which results in the formation of the trivalent gold
derivative 7. This anion-induced oxidation is also accompanied
by a conversion of the carbenium Z-type ligand in [4]+ into an X-
type ligand in 7. Finally, the carbenium moiety also influences the
carbophilic reactivity of the gold center as illustrated by the activity
of [4]+ as a catalyst for the cyclization of propargyl amide 8.
Acknowledgements
The conversion of [4]+ into 7 is the result of a chloride push –
carbenium pull effect. We reasoned that the pull exerted by the
carbenium ion could also be assessed by probing the electrophilic
reactivity of the gold center. Following up on this idea, we
selected the cyclization of propargyl amide as a reporter
reaction.[25] Using a 2 mol% catalyst loading, cyclization of 8 into
9 progressed swiftly at 60 °C in CH2Cl2 in the case of [4]BF4 which
proved to be markedly more active than [3]BF4, in line with the
higher Lewis acidity of the xanthylium unit (Scheme 4).
Interestingly, no conversion was observed when Ph3PAuCl was
used in the presence or in the absence of either [5]BF4 or [6]BF4.
These results underscore the importance of having the Lewis
acidic carbenium ion integrated within the same molecular
platform, leading us to propose that substrate activation by [3]BF4
and [4]BF4 is the result of a substrate-push/carbenium-pull effect
as per the working model presented in Scheme 4. This working
model is conceptually related to that proposed to explain the
carbophilic reactivity of complexes of type II (Figure 1)[3b,26]
whereby donation of electron density from the gold center to the
Z-type ligand increases the electrophilic character of the former.
A more acute version of these effects have been observed with
more strongly Lewis acidic Z-type ligands,[5a] leading to reactivity
This work was supported by the National Science Foundation
(CHE-1856453), the Welch Foundation (A-1423), and Texas A&M
University (Arthur E. Martell Chair of Chemistry).
Conflict of interest
The authors declare no conflictof interest.
Keywords: carbocation • gold • Z-type ligand • oxidative addition
• alkynes
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F
F
1.5 h
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Conversion
52% for cat. = [3]BF4
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4
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97% for cat. = [ ]BF4
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10.1002/anie.201911662
Angewandte Chemie International Edition
COMMUNICATION
.
COMMUNICATION
Lewis C. Wilkins, Youngmin Kim,
Elishua Litle, and François P. Gabbaï
Page No. – Page No.
Stabilized carbenium ions as latent,
Z-type ligands
Like in a capacitor about to arc, the positive face of the xanthylium cation
installed in a new ambiphilic ligand architecture stands set to engage the adjacent
gold center in an Au→Ccarbenium interaction. The formation of this interaction shows
that stabilized carbenium ions may act as latent Z-type ligands which, in the present
case, can be used to increase the carbophilic reactivity of the gold center.
This article is protected by copyright. All rights reserved.