An Acyclic Arsenium Cation Stabilised by a Single P–As π-Interaction and a Cyclic Diphosphinophosphonium Salt

Article abstract of DOI:10.1002/anie.201905922

Stable acyclic arsenium cations R₂As⁺, isoelectronic analogues of germylenes, are rare in comparison to the corresponding phosphenium cations. The first example of a diphosphaarsenium salt, [{(Dipp)₂P}₂As][Al{OC(CF₃)₃}₄]?1 (Formula presented.) PhMe, is described. This salt exhibits remarkable stability due to the delocalisation of a lone pair from a planar phosphorus centre into the vacant p-orbital at arsenic; the bonding in 2 has been probed by DFT calculations. An attempt to synthesise an analogous diphosphaphosphenium salt unexpectedly generated the cyclic phosphonium salt [cyclo-{(Mes)P}₂P(Mes)₂][BAr^F₄]?CyMe through the cyclisation of a putative phosphine-substituted diphosphene cation intermediate.

Full text of DOI:10.1002/anie.201905922

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Accepted Article
Title: An Acyclic Arsenium Cation Stabilised by a Single P-As π-
Interaction and a Cyclic Diphosphinophosphonium Salt
Authors: Keith Izod, Peter Evans, and Paul G. Waddell
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An Acyclic Arsenium Cation Stabilised by a Single P-As π-
Interaction and a Cyclic Diphosphinophosphonium Salt
Keith Izod,*[a] Peter Evans,[a] Paul G. Waddell[a]
Dedication ((optional))
Abstract: Stable acyclic arsenium cations R2As+, isoelectronic
[(Mes*NH)2As][X] (V) [X = AsF6, BF4, AlCl4, GaCl4].[7b] In addition,
analogues of germylenes, are rare in comparison to the
a very small number of related acyclic stibenium and
corresponding phosphenium cations.
diphosphaarsenium salt [{(Dipp)2P}2As][Al{OC(CF3)3}4].1½PhMe (2)
is described. This salt exhibits remarkable stability due to
The first example of a
bismuthenium cations have been reported.[8]
It is notable that, while three-coordinate, electron-precise
arsino-phosphonium cations of the form [R2As-PR3]+ are known,[9]
no examples of two-coordinate phosphorus-substituted arsenium
cations have been reported to date.[10] The diaminoarsenium
cations I, IV and V benefit from significant stabilisation due to
delocalisation of the nitrogen lone pair(s) into the vacant p-orbital
at the arsenic centre. Although calculations indicate that the
inherent π-donor capacity of phosphorus is similar to that of
nitrogen,[11] P-As π-interactions should be disfavoured in
diphosphaarsenium cations [(R2P)2As]+, due to the high barrier to
planarisation of phosphorus.
delocalisation of a lone pair from a planar phosphorus centre into the
vacant p-orbital at arsenic; the bonding in 2 has been probed using
DFT calculations. An attempt to synthesise an analogous
diphosphaphosphenium salt unexpectedly generated the cyclic
phosphonium salt [cyclo-{(Mes)P}2P(Mes)2][BArF ].CyMe (5.CyMe),
4
via cyclisation of a putative phosphine-substituted diphosphene cation
intermediate.
Since the discovery of the first two-coordinate phosphorus
cation over 50 years ago, there has been continued interest in
these species and their heavier congeners, both as isolated
cations and as ligands to transition metals.[1-3] A significant
underlying reason for this is the isoelectronic relationship between
pnictenium cations R2E+ (E = P, As, Sb, Bi) and the corresponding
tetrylenes R2E (E = Si, Ge, Sn, Pb). Like singlet tetrylenes,
pnictenium cations possess a lone pair and a vacant p-orbital at
the pnictogen centre. This confers both electrophilic and
nucleophilic character on pnictenium cations, although the
positive charge at the pnictogen centre renders these compounds
substantially less nucleophilic and more electrophilic than their
tetrylene analogues, making them poor -donors, but good π-
acceptor ligands.
We recently demonstrated that the appropriate choice of
substituents enables the synthesis of diphosphagermylenes (VI,
VII) which are stabilised by pπ-pπ interactions between a single
planar phosphorus centre and the germanium atom.[12] Since
compounds VI and VII are isoelectronic with the corresponding
diphosphaarsenium cations, we speculated that a P-As π-
stabilised diphosphaarsenium cation might be accessible. We
detail below our attempts to prepare such a cation, its solid-state
structure, its solution behaviour and an analysis of its bonding. In
addition, we report the contrasting behaviour of a putative
diphosphaphosphenium cation.
Arsenium cations are less numerous than their phosphorus
analogues and the vast majority of isolated two-coordinate
arsenium ions are cyclic, N-substituted species (I),[4] analogues of
N-heterocyclic
carbenes.
By
comparison,
structurally
characterised acyclic arsenium ions are limited to just a few
examples. The formally two-coordinate arsacenium cation II was
reported in 1983 by Jutzi and co-workers,[5] while Krossing and
co-workers subsequently reported the chloroarsacenium cation
III.[6] The poorly stable diaminoarsenium cation IV was reported
by Shulz and co-workers[7a] and these authors have very recently
isolated the room temperature-stable diaminoarsenium salts
[a]
Dr K. Izod, Mr P. Evans, Dr P. G. Waddell
Main Group Chemistry Laboratories, School of Chemistry
Bedson Building, Newcastle University
Newcastle upon Tyne, NE1 7RU, UK
The
reaction
between
two
equivalents
of
[13]
[(Dipp)2P]Na(THF)1.5
and AsCl3 in cold THF yields the
E-mail: keith.izod@ncl.ac.uk
chloroarsane {(Dipp)2P}2AsCl (1), along with a small quantity of
the diphosphane (Dipp)2P-P(Dipp)2 and a very small amount of
the secondary phosphane (Dipp)2PH [Dipp = 2,6-iPr2C6H3]
(Scheme 1). Storage of a solution of this material in a mixture of
MeCN and Et2O at -25 °C for 12 h leads to deposition of the co-
Supporting information for this article is given via a link at the end of
the document. CCDC 1815449, 1815450, 1913170 and 1913171
contain the supplementary crystallographic data for this paper.
These data are provided free of charge by The Cambridge
Crystallographic Data Centre.
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crystals {(Dipp)2P}2AsCl.(Dipp)2P-P(Dipp)2 (1a, see Supporting
Information). Subsequent removal of solvent from the supernatant
gives 1 as a yellow powder in reasonable yield, although the
product remains contaminated with a small amount of Dipp2PH,
which could not be removed.
22maj [2.1501(9) and 2.1482(9) Å, respectively] are approximately
6% (ca. 0.15 Å) shorter than the distances between the pyramidal
phosphorus centres and the arsenic atoms [2.2925(7) and
2.3028(11) Å for 21
and 22maj, respectively], suggesting a
maj
significant degree of multiple bond character in the former bonds.
The short P-As distances in 21maj and 22maj are at the longer end
of the range of P=As distances in the few crystallographically
characterised phosphaarsenes RP=AsR, in which there is a true
P=As double bond; for example, the P-As distances in
(Mes*)P=As{CH(SiMe3)2} and (2,6-Trip2C6H3)As=P(Mes) are
2.125(1) and 2.134(2) Å, respectively [Mes = 2,4,6-Me3C6H2,
Mes* = 2,4,6-tBu3C6H2, Trip = 2,4,6-iPr3C6H2].[15,16] However, the
As-P(planar) distances in 21 and 22 are similar to the As-P
Treatment of 1 with one equivalent of either GaCl3 or
Na[BArF ] in fluorobenzene gave deep green solutions, which,
4
unexpectedly, exhibited no discernible peaks in their room
temperature 31P{1H} NMR spectra, other than those due to small
quantities of (Dipp)2PH and (Dipp)2P-P(Dipp)2 [ArF = C6H3-3,5-
(CF3)2]. Although we were able to grow small green crystals of
these supposed arsenium salts [{(Dipp)2P}2As]+[M]─ ([M]─
=
maj
maj
GaCl4─, BArF ─), these were not suitable for X-ray crystallography
4
distances
in
the
1,3-diphospha-2-arsaallyl
anion
in
and so the identities of these compounds could not be verified.
However, while the reaction between 1 and Li[Al{OC(CF3)3}4][14] in
fluorobenzene initially gave a dark green oil on attempted
crystallisation from a mixture of toluene and fluorobenzene,
storage of this oil for approximately one week at -25 °C resulted
in gradual crystallisation of the material. An X-ray crystallographic
study of the resulting dark green crystals confirmed these to be of
(Mes*P)2AsLi(TMEDA)(Me2NH) (3), recently reported by Wright
and co-workers [As-P 2.176(1) and 2.170(1) Å; TMEDA =
N,N,N',N'-tetramethylethylenediamine].[17] The P-As-P angles in
21
and 22
are 105.45(3) and 109.95(4)°. Overall, the
maj
maj
structures of the cations 21maj and 22maj are remarkably similar to
that of the isoelectronic diphosphagermylene VI [sum of angles at
P = 358.35 and 311.53°, P-Ge-P angle 107.4(4)°].[12]
the
diphosphaarsenium
salt
[{(Dipp)2P}2As][Al{OC(CF3)3}4].1½PhMe (2). Compound
2
decomposes only very slowly in solution at room temperature and
is stable in the solid state for long periods.
(a)
Scheme 1. Synthesis of 1 and 2.
Compound
2 crystallises with two crystallographically
independent cation/anion pairs in the asymmetric unit, along with
three molecules of toluene (Figure 1), all of which are disordered;
the cations and anions are well separated (closest As…F contact
>5 Å).
(b)
Figure 1. Solid-state structures of (a) the major (21maj) and (b) the minor (21
disorder component of independent cation 1 of 2 with H atoms (except one
)
min
methine H atom) omitted for clarity. Selected bond lengths (Å) and angles (°)
The two independent cations in 2 are subject to different
modes of disorder: cation 1 (21) exhibits disorder of the arsenic
atom and one phosphorus centre, along with its associated
substituents, over two positions, while cation 2 (22) exhibits
disorder of the two phosphorus atoms and the arsenic centre over
two positions (see the Supporting Information). In each cation,
the major disorder components (21maj and 22maj, with 94 and 92%
occupancy, respectively) possess one planar and one pyramidal
phosphorus centre [sum of angles at P(1A) = 359.75, P(2A) =
320.16, P(3A) = 359.87, P(4A) = 319.49°]. The distances between
the planar phosphorus centres and the arsenic atoms in 21maj and
for 21
: P(1A)-As(1A) 2.1501(9), P(2)-As(1A) 2.2925(7), P(1A)-C(25A)
maj
1.834(3), P(1A)-C(37A) 1.809(3), P(2)-C(1) 1.849(3), P(2)-C(13) 1.845(3),
P(1A)-As(1A)-P(2) 105.45(3), As(1A)-P(1A)-C(25A) 135.75(11), As(1A)-P(1A)-
C(37A) 111.28(10), C(25A)-P(1A)-C(37A) 112.72(14), As(1A)-P(2)-C(1)
95.72(8), As(1A)-P(2)-C(13) 118.30(10), C(1)-P(2)-C(13) 106.15(13). See main
text for details of 21
.
min
The minor disorder components of the two independent
cations (21min and 22min) have rather low occupancies (6% and 8%,
respectively), and so it is not appropriate to discuss their
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structures in detail; however, the gross structural features of 21
min
and 22 are clear. Disorder component 22 adopts the same
min
min
structural motif as 21
and 22maj, with one planar and one
maj
pyramidal phosphorus centre. In contrast, 21
adopts a
min
structure with two pyramidal phosphorus centres [sum of angles
at P(1B) = 324.5, P(2) = 339.97°]. Unexpectedly, this leads to a
close approach of the methine hydrogen atom of one isopropyl
group to the arsenic centre [H(1B)As(1B) 2.02(1) Å], such that
there appears to be an agostic-type C-HAs interaction (the
agostic-type nature of this interaction is supported by DFT
calculations, see below). We have previously reported related B-
HE agostic-type interactions (E = Sn, Pb) in phosphine-borane-
stabilised dialkylstannylenes and –plumbylenes,[18] but, to the
best of our knowledge, the C-HAs interaction in 21min is the first
crystallographically observed C-HE agostic-type interaction in a
heavier main group carbene analogue.
The solvent of crystallisation in 2 is held only weakly; the 1H
NMR spectrum of a sample which had been subjected to vacuum
for 10 mins is consistent with the alternative solvate
[{(Dipp)2P}2As][Al{OC(CF3)3}4].½PhMe (2a). At room temperature
Figure 2. Variable-temperature 31P{1H} NMR spectra of 2a in CD2Cl2.
1
the H and 13C{1H} NMR spectra of 2a in CD2Cl2 exhibit broad
signals which are consistent with a single ligand environment. As
the temperature is reduced the 1H NMR spectra become
increasingly broad, with overlapping signals which are difficult to
interpret.
The identity of the species responsible for peak B is less
clear. We have previously observed similar variable-temperature
NMR behaviour for the isoelectronic germylene VI: at -95 °C the
31P{1H} NMR spectrum of VI consists of a pair of broad signals at
98.5 and -82.0 ppm, corresponding to the planar and pyramidal
phosphorus centres, respectively, along with two sharp singlets at
7.8 and -43.1 ppm, which we assigned to configurations
containing two pyramidal phosphorus centres.[12] It is therefore
likely that the low-temperature broadening of peak B is due to
decoalescence into two peaks arising from configurations of 2a
with two pyramidal phosphorus centres. Consistent with this, DFT
calculations suggest that an alternative configuration of 2, with
two pyramidal phosphorus centres, would give rise to a signal at
34 ppm.
While at room temperature no signal is observed in the 31P{1H}
NMR spectrum of 2a, as the temperature is reduced, the spectrum
resolves into three broad signals, A, B and C (Figure 2), which, at
-20 °C, are in the approximate ratio 1:3:1. As the temperature is
reduced further, signals A and C sharpen, while peak B sharpens
and then broadens, although the approximate ratio of A:B:C
remains 1:3:1. At -90 °C the spectrum consists of a pair of
doublets at 223.9 (A) and -34.0 ppm (C, JPP = 151.8 Hz), while
the signal at 95.2 ppm (B) has broadened to such an extent that
it is barely resolved, clearly suggesting further decoalescence of
this peak. Unfortunately, we were unable to obtain a temperature
low enough to observe this behaviour further. We were also
unable to observe complete coalescence of the peaks at higher
temperatures due to the instability of 2a above ca. 40 °C.
In order to investigate the bonding in 2 we have carried out a
DFT study. Local minima were found for configurations with one
planar and one pyramidal phosphorus centre (2plan),
corresponding to the solid-state structure of 21maj, and with two
pyramidal phosphorus centres (2pyr) (Figure 3). Attempts to
locate a minimum corresponding to 21min were unsuccessful, each
time converging to a geometry corresponding to 21maj. Geometry
The solid-state 31P{1H} CP-MAS NMR spectrum of 2a exhibits
two signals, a sharp singlet at -40.7 ppm with few spinning side
bands and a broad signal at 217.4 ppm exhibiting a large degree
of chemical shift anisotropy (see Supporting Information). These
signals are clearly consistent with peaks A and C in the low
temperature solution state 31P{1H} NMR spectrum. On the basis
of their chemical shifts and the large chemical shift anisotropy of
the low field signal in the solid-state spectrum, we attribute peaks
A and C to the planar and pyramidal phosphorus centres in 2a,
respectively (for 2 DFT calculations suggest chemical shifts of 219
and 19 ppm for the planar and pyramidal phosphorus centres,
respectively). For comparison, the chemical shifts of the two-
coordinate phosphorus centres in phosphaarsenes typically lie in
the range 530-670 ppm;[15,16] the chemical shift observed for the
planar phosphorus centre in 2a is closer to that observed for the
1,3-diphospha-2-arsaallyl anion in 3 (235.3 ppm for the E,E-
isomer).[17]
2plan is 25.7 kJ mol-1 lower in free energy than 2pyr
.
The calculated geometry of 2plan is very similar to the solid
state structures of 21 and 22 [P-As-P angle 105.748°, P-As
maj
maj
distances 2.1765 and 2.3003 Å]. Inspection of the molecular
orbitals reveals that the P-As π-orbital is the HOMO-5, although
this orbital also has large components on the aromatic rings, while
the P-As π*-orbital is the LUMO (see the Supporting Information).
NBO analysis yields Wiberg bond indices of 1.558 and 1.055 for
the As-P(planar) and As-P(pyramidal) bonds, respectively, clearly
indicating significant multiple bond character in the former. For
2pyr, which exhibits no As-P multiple bond character, the Wiberg
bond indices are both 0.936.
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analysis also suggests that there is a genuine C-HAs agostic-
type interaction, with significant delocalisation of electron density
from the C-H -bond into the P-As π*-orbital; this stabilises 2alt by
approximately 21 kJ mol-1.
Given the unusual structure and solution behaviour of 2, we
became interested in preparing the analogous phosphenium
cation [{(Dipp)2P}2P]+. However, the reaction between two
equivalents of [(Dipp)2P]Na(THF)1.5 and PCl3 in cold THF gave a
complex mixture of products, as judged by 31P{1H} NMR
spectroscopy; this is likely due to the bulky nature of the Dipp
substituents. In order to overcome this problem, we targeted the
less hindered {(Mes)2P}2PCl (4), which was successfully prepared
by the reaction of two equivalents of (Mes)2PLi with PCl3 in cold
diethyl ether.
(2plan
)
(2pyr)
Figure 3. Minimum energy geometries of 2plan and 2pyr (B97D/6-311G(2d,p)).
In addition to the P=As π interaction, NBO analysis reveals
that 2plan is further stabilised by delocalisation of the arsenic lone
pair into a P-C *-orbital associated with the planar phosphorus
centre. Second order perturbation theory analysis indicates that
the approximate stabilisation afforded by this interaction [the E(2)
energy] is 45 kJ mol-1. The stabilisation of 2plan may therefore be
characterised as an unusual version of the push-pull type, with
the push arising from π-donation of the lone pair of the planar
phosphorus atom into the vacant p-orbital at arsenic and the pull
arising from delocalisation of the arsenic lone pair into a P-C *-
orbital at the planar phosphorus centre.
Treatment of a solution of 4 in fluorobenzene with GaCl3 or
Li[Al{OC(CF3)3}4] gave dark purple solutions, which quickly faded
to pale yellow. Unfortunately, these reactions yielded only impure,
pale yellow oils, which proved difficult to characterize. Similarly,
the reaction between 4 and Na[BArF ] in fluorobenzene initially
4
gave a dark purple solution, which again faded to pale yellow over
the course of two mins. A 31P{1H} NMR spectrum of the crude
reaction solution indicated the formation of a single product which
gave rise to a doublet at 97.7 ppm and a triplet at -46.2 ppm (JPP
= 312 Hz), suggesting that a P2P’ unit persists in this compound.
Single crystals were obtained from a PhF solution layered with
methylcyclohexane and were shown by X-ray crystallography to
be the unexpected diphosphinophosphonium salt [cyclo-
In 2pyr two of the aromatic rings are significantly canted
towards the arsenic atom, such that the CipsoAs distances are
2.4801 Å. This lies well within the sum of the van der Waal’s radii
of C and As (3.55 Å)[19] and suggests a significant bonding
interaction between these atoms. Consistent with this, NBO
analysis of 2pyr reveals significant delocalisation of electron
density from the aromatic rings into the formally vacant p orbital
of the arsenic centre, such that the latter orbital has an occupancy
of 0.53 electrons and the AsCipso Wiberg bond indices are 0.267;
these interactions are calculated to stabilise 2pyr by 208 kJ mol-1.
These areneAs interactions are related to the areneE
interactions in Menshutkin complexes such as SbCl3.C6H6.[20,21]
Complexes with areneAs interactions are much less common
than similar complexes with Sb or Bi and there has been some
debate as to the exact nature of these interactions;[20a,22] however,
recent calculations suggest that, for arsenic-containing systems,
they are best described as arising from donation of the arene π-
electron density to the Lewis acid arsenic centre.[23] Nonetheless,
in Menshutkin complexes, the interaction energies are calculated
to be rather low (e.g. for AsCl3C6H6 the interaction energy is
calculated to be just 27.1 kJ mol-1),[22] suggesting a significantly
{(Mes)P}2P(Mes)2][BArF ].C6H11Me (5.CyMe, Figure 4).[24]
4
Figure 4. Solid-state structure of the cation of 5.CyMe, with H atoms omitted
for clarity. Selected bond lengths (Å) and angles (): P(1)-P(2) 2.2048(11), P(1)-
P(3) 2.1984(10), P(2)-P(3) 2.2645(11), P(1)-C(1) 1.798(3), P(1)-C(10) 1.807(3),
P(2)-C(19) 1.834(3), P(3)-C(28) 1.845(3), P(1)-P(2)-P(3) 58.91(3), P(1)-P(3)-
P(2) 59.19(3), P(2)-P(1)-P(3) 61.90(4).
weaker interaction than the CipsoAs interaction in 2pyr
.
The
strength of the calculated interaction in 2pyr may be attributed to
the more efficient overlap of the aromatic π-orbital and the vacant
p-orbital in 2pyr compared to the overlap of the aromatic π-orbital
and the As-X *-orbitals in Menshutkin complexes.
Although we were unable to locate a stable minimum
corresponding to the minor disorder component 21min, in order to
probe the potential C-HAs agostic-type interaction a single point
energy calculation was performed on the geometry corresponding
to the crystal structure atomic coordinates (2alt). NBO analysis
suggests that, although both P centres have a somewhat
pyramidal geometry in this structure, there remains a substantial
P-As π-interaction from the P atom which is closer to planar. NBO
A similar structural motif has been reported by Burford and co-
workers for a small series of diphosphinophosphonium cations,
which were prepared by the quaternisation of cyclic triphosphines
(RP)3 with MeOTf.[25] In the present case, we propose that the
initial reaction between 4 and Na[BArF ] yields the desired
4
phosphenium salt (VIII), as evidenced by the dark purple colour
of the solutions, which we suggest arises from n-*/-*
transitions in a diphosphaphosphenium cation possessing a
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planar terminal phosphorus atom. This cation undergoes a 1,2-
aryl shift to give the corresponding phosphine-substituted
diphosphene cation (IX), which rapidly cyclises to give 5 (Scheme
2). Attempts to observe the intermediates VIII and/or IX by low
temperature 31P{1H} NMR spectroscopy were unsuccessful.
cyclic diphosphinophosphonium salt (5) via
migration/cyclisation process.
a
1,2-aryl
Acknowledgements
This work was partly funded by the U.K. Engineering and Physical
Sciences Research Council (EPSRC; Grant EP/L5048281). The
authors are grateful to Dr N. Rees (Oxford, UK) for obtaining the
solid-state NMR spectra.
Keywords: arsenic • P-ligands • solid-state structure • density
functional calculations • pi-interactions
[1]
[2]
a) K. Dimroth, P. Hoffmann, Angew. Chem. Int. Ed. Engl. 1964, 3, 384;
b) K. Dimroth, Top. Curr. Chem. 1973, 38, 1-147.
For reviews on phosphenium cations see: a) A. H. Cowley, R. A. Kemp.
Chem. Rev. 1985, 85, 367-382; b) D. Gudat, Acc. Chem. Res. 2010, 43,
1307-1316; c) D. Gudat, Dalton Trans. 2016, 45, 5896-5907.
For reviews on transition metal complexes of phosphenium cations see:
a) H. Nakazawa, J. Organomet. Chem. 2000, 611, 349-363; b) D. Gudat,
Coord. Chem. Rev. 1997, 163, 71-106; c) H. Nakazawa, Adv. Organomet.
Chem. 2004, 50, 107-143; d) L. Rosenberg, Coord. Chem. Rev. 2012,
256, 606-626.
[3]
[4]
Scheme 2. Proposed pathway to 5 [calculated relative free energies (kJ mol-1)
in square brackets].
For selected examples see: a) J. Price, M. Lui, N. D. Jones, P. J.
Ragogna, Inorg. Chem. 2011, 50, 12810-12817; b) C. J. Carmalt, V.
Lomeli, B. G. McBurnett, A. H. Cowley, Chem. Commun. 1997, 2095-
2096; c) N. Burford, J. C. Landry, M. J. Ferguson, R. McDonald, Inorg.
Chem. 2005, 44, 5897-5902; d) A. Schulz, A. Villinger, Inorg. Chem.
2009, 48, 7359-7367; e) H. A. Spinney, I. Korobkov, G. DiLabio, G. P.
Yap, D. S. Richeson, Organometallics 2007, 26, 4972-4982.
a) P. Jutzi, T. Wippermann, C. Kruger, H.-J. Kraus, Angew. Chem. Int.
Ed. 1983, 22, 250; b) R. J. Wiacek, J. N. Jones, C. L. B. Macdonald, A.
H. Cowley, Can. J. Chem. 2002, 80, 1518-1523.
In support of this mechanism, DFT calculations indicate that
the cationic diphosphene IX is essentially isoergonic with the
putative diphosphaphosphenium salt VIIIplan and that this latter
compound is 28.9 kJ mol-1 higher in free energy than the cyclic
cation 5 (see Supporting Information). Similar room-temperature
1,2-migrations to that proposed have been observed in closely
related compounds; for example, the cyclic silylene X undergoes
[5]
[6]
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Entry for the Table of Contents (Please choose one layout)
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The first diphosphaarsenium cation
has been synthesised. This cation
exhibits remarkable stability due to
the delocalisation of a lone pair from
a planar phosphorus centre into the
vacant arsenic p-orbital.
K. Izod,* P. Evans, P. G. Waddell
Page No. – Page No.
An Acyclic Arsenium Cation
Stabilised by a Single P-As π-
Interaction and a Cyclic
Diphosphinophosphonium Salt
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