Ligand-stabilized [P4]2+ cations

Article abstract of DOI:10.1002/anie.201109010

The bis(triphenylarsane) complex of the [P₄]²⁺ dication has been formed in a high-yielding one-pot synthesis. X-ray crystallography reveals a butterfly structure of the bicyclo[1.1.0]- tetraphosphane-1,4-diium core, with two triphenylarsane ligands in an exo,exo configuration (see picture). The reaction of [(Ph₃As) ₂P₄]²⁺ with Ph₃P results in the quantitative formation of [(Ph₃P)₂P₄] ²⁺ and Ph₃As. Copyright

Full text of DOI:10.1002/anie.201109010

.
Angewandte
Communications
DOI: 10.1002/anie.201109010
Cationic Phosphorus
Ligand-Stabilized [P4]2+ Cations**
Maximilian Donath, Eamonn Conrad, Paul Jerabek, Gernot Frenking, Roland Frçhlich,
Neil Burford,* and Jan J. Weigand*
Ligand stabilization has been exploited to access novel,
soluble allotropes of main-group elements.[1] A series of
neutral, homoatomic units (Si2, C1, P2, P4, P12) in an oxidation
state of zero have been recently isolated using carbenes as
supporting ligands.[2–8] Homoatomic cages and clusters of
Group 15 elements are known, and derivatives of P4, poly-
phosphorus anions (for example Zintl anions), and polyphos-
phanes have been extensively developed;[9,10] however, exam-
ples of polyphosphorus cations have not been reported.[11]
Cations based on silicon[12] or germanium[13,14] have been
reported, including complexes such as [Cp*Si][B(C6F5)4]
(Cp* = pentamethylcyclopentadienyl),[15–17] which mimic the
coordination chemistry of transition metals. Furthermore, the
triflate salt of a cryptand-encapsulated germanium(II) dicat-
ion [Ge(cryptand[2.2.2])]2+ was recently reported.[18] The
coordination chemistry of electron-rich (lone-pair-bearing)
phosphorus centers as Lewis acceptors has been extensively
utilized in recent years to realize a facile synthetic method to
form bonds between phosphorus and a variety of donor atoms
(for example C, N, O, P, S, Se, Ga).[19–21]
in the formation of the cationic cages [Ph2P5]+, [Ph4P6]2+, and
+
[Ph6P7]3+ by the consecutive insertion of [Ph2P] into the P P
ꢀ
bonds of the P4 tetrahedron[24] and is a general method for the
formation of [R2P5]+ cations, which are polyphosphorus
cationic frameworks involving organic substituents.[25,26]
Recognizing the isolobal relationship between the organic
substituent and a Group 15 ligand, we have now utilized the
Lewis acceptor behavior of phosphorus centers as a means to
stabilize the bicyclo[1.1.0]-tetraphosphane-1,4-diium dication
([P4]2+), analogues of neutral and anionic derivatives.[27]
Our experience with reductive coupling reactions of
monocationic
chlorophosphanylphosphonium
cations
[R3PP(R)Cl]+ with Ph3P or Ph3Sb to give dications
[R3PP(R)P(R)PR3]2+ reveals a powerful method for P P
ꢀ
bond formation[28,29] that we have applied to PCl3 to give the
ligand-stabilized dication 12+. Moreover, the redox system
PCl3/SnCl2 has been used as a source for the in situ generation
ꢀ
of chlorophosphandiyl [P Cl], which, in the presence of the
cyclophosphane cyclo-tBu3P3, gives the cyclotetraphosphane
cyclo-tBu3P4Cl.[30] On this basis, we anticipated that the
appropriate combination of PCl3, Lewis basic reducing
agent, and halide abstracting agent should result in the
formation of phosphorus cations with the potential for ligand
stabilization. In this context we examined reaction mixtures
of PCl3, Ph3As, and AlCl3 in CH2Cl2 and observed the
formation of the highly reactive Cl2P-PCl2 (d = 155.1 ppm)[31]
and [Ph3AsCl]+ (2) (Scheme 1a), indicating the in situ
formation of 2-Cl resulting from oxidation of the arsane and
reduction of PCl3.[32] When PCl3, Ph3As, and AlCl3 are
combined in a 4:12:12 stoichiometry in CH2Cl2 (RT, dark,
12 h; Scheme 1b), quantitative formation of 1[AlCl4]2 and
3[AlCl4]2 is observed, where 12+ is the reduction product and
The first examples of cationic polyphosphorus cages
[P5X2]+ were obtained by Krossing et al.[22,23] from the
reaction of Ag[Al{OC(CF3)3}4] with PX3 (X = Cl, Br, I) in
the presence of P4. We obtained analogous results by applying
a stoichiometric melt at elevated temperatures (60 to 1008C)
and extended this chemistry to the more stable diorgano-
phosphenium cation [Ph2P]+ with P4. This approach resulted
[*] M. Donath, R. Frçhlich, Dr. J. J. Weigand
Department of Inorganic and Analytical Chemistry, WWU Mꢀnster
Corrensstrasse 30, 48149 Mꢀnster (Germany)
E-mail: jweigand@uni-muenster.de
E. Conrad, Prof. N. Burford
Department of Chemistry, Dalhousie University
Halifax, NS, B3H 4J3 (Canada)
and
University of Victoria, Victoria, BC, V8W 3V6 (Canada)
E-mail: nburford@uvic.ca
P. Jerabek, Prof. G. Frenking
Department of Chemistry, Philipps University Marburg
35032 Marburg (Germany)
[**] This work was supported by the European Phosphorus Science
Network (PhoSciNet CM0802), the FCI (Liebig scholarship for
J.J.W.) and the German Science Foundation (DFG, WE 4621/2-1).
We also acknowledge the Natural Sciences and Engineering
Research Council of Canada, the Killam Foundation, and the Canada
Research Chairs Program for funding. We thank R. McDonald and
M. J. Ferguson (University of Alberta, Edmonton) for initial X-ray
investigations. J.J.W. thanks Prof. F. Ekkehardt Hahn (WWU
Mꢀnster) for his generous support and advice.
Scheme 1. a) Idealized reaction of PCl3, Ph3As, and AlCl3 showing the
in situ formation of Cl2P PCl2 and 22+; b) formation of the bicyclo-
ꢀ
[1.1.0]-tetraphosphane-1,4-diium dication 12+ and hexaphenyldiarsane-
1,2-diium dication 32+ as tetrachloroaluminate salts 1[AlCl4]2 and
3[AlCl4]2, respectively.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201109010.
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the hexaphenyldiarsane-1,2-diium dication 32+ (Supporting
Information, Figure S4)[33,35] is the oxidation product. The
31P{1H} NMR spectrum of 12+ reveals an A2X2 spin system
(dA = ꢀ325.9 ppm, dX = ꢀ174.9 ppm; 1JAX = ꢀ160 Hz; Sup-
porting Information, Figure S1),[33] which is significantly
upfield-shifted with respect to that reported for (Mes*)2P4
(Mes* = 2,4,6-tBu3C6H4, dA = ꢀ272 and dX = ꢀ130 ppm).[34]
Compound 1[AlCl4]2 was isolated by fractional crystal-
lization as a CH2Cl2 solvate (1[AlCl4]2·3CH2Cl2) in good yield
(84%). The dichloromethane molecules can be completely
removed from 1[AlCl4]2·3CH2Cl2 in vacuo to give analytically
pure 1[AlCl4]2 as an amorphous, colorless powder, which is
stable for longer periods when stored in the dark at ꢀ358C.[36]
Decomposition associated with melting was observed at
temperatures higher than 408C. The geometrical parameters
for the [P4]2+ core of the molecular structure of cation 12+
(Figure 1) are typical of butterfly structures showing a dis-
2
Figure 2. Laplacian contour diagram 5 1(r) of 12+ in the As-P1-P3-As’
2
plane. Solid lines indicate areas of charge concentration (5 1(r)<0);
2
dashed lines show areas of charge depletion (5 1(r)>0). The thick
solid straight lines connecting the atomic nuclei are the bond paths.
The thick solid lines separating the atomic basins indicate the zero-flux
surfaces crossing the molecular plane.
ꢀ
tinctly shorter P P bond between the bridgehead phosphorus
2+
atoms (P2 P4 2.184(2) ꢀ). In comparison to P4 (P P distance
2.719 ꢀ). The P4 moiety in 12+ is significantly deformed
ꢀ
ꢀ
!
of 2.21 ꢀ),[37] the remaining P P bonds are typical P P single
bonds. The Ph3As ligands are arranged in an exo,exo fashion
from its equilibrium structure as the result of the P As
ꢀ
ꢀ
donor–acceptor interactions. The optimized geometry of free
and the P As bonds (2.332(2); P4 As2 2.335(2) ꢀ) are in the
same range as found in phosphanylarsonium cations.[38] To
gain further insights into the bonding in 12+, we optimized the
geometry of the molecule using density functional theory
(DFT) at the BP86/TZVPP level of theory.[39]
P42+ has D2d symmetry with four short (2.168 ꢀ) and two long
ꢀ
ꢀ
[42]
ꢀ
(2.692 ꢀ) P P bonds (Figure 1). NBO calculations agree
with the description of the bonding situation in 12+. According
ꢀ
to the NBO results, there are five P P single bonds and four
electron lone pairs at the P atoms of the P42+ moiety and two
Figure 1 shows that the calculated bond lengths of 12+
agree well with the experiment, except for the P As
P As bonds. The NBO partial charges suggest that the P4
fragment carries a positive charge of only + 0.12e while the
AsPh3 ligands have a total charge of + 1.88e.
ꢀ
ꢀ
interatomic distances, which are about 0.07 ꢀ longer than
the observed values. We think that the difference probably
comes from interatomic interactions in the solid state, which
are known to shorten donor–acceptor bonds.[40] Analysis of
the electronic structure using the atoms-in-molecules (AIM)
!
We have calculated the bond strength of the P As bonds
in 12+ according to reaction (1):
12þ ! P42þ þ 2 AsPh3
ð1Þ
!
method[41] suggests that the P As donor–acceptor bond is
polarized toward the phosphorus acceptor end. The Laplacian
distribution (Figure 2) exhibits an area of charge concentra-
The free energy of reaction (1) is DGR298 = 185.5 kcal
molꢀ1, which gives a very high average bond strength of
2
2+
tion (5 1(r) < 0, solid lines) along the P As axis, which is
92.75 kcalmolꢀ1 for the Ph3As!P4 donor–acceptor bond.
ꢀ
strongly localized at P. This result is in agreement with the
This can be explained with the very large acceptor strength of
the doubly charged P42+ cation.[43] We have also calculated the
geometry and bond energy of the related triphenylphosphane
complex [P4(Ph3P)2]2+ (42+). Figure 1 shows that the equilib-
NBO atomic partial charges, which give a value of + 1.31e at
ꢀ
As and ꢀ0.01e at P1/P3. Note that there is no P1 P3 bond
path in 12+, which has a rather long distance of 2.725 ꢀ (Expt.
Figure 1. Molecular view of cations 12+ and 42+ in 1[AlCl4]2·3CH2Cl2 and 4[AlCl4]2·3CH2Cl2 in the solid state (X-ray crystallography) together with
the calculated structure of free P42+. Tetrachloroaluminate anions, solvate molecules, and hydrogen atoms have been omitted for clarity. Selected
calculated bond lengths [ꢁ] and angles [8] are compared with the experimental values (in parentheses).
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2+
rium geometry of 42+ resembles that of 1 . The P P bond
ꢀ
[7] J. D. Masuda, W. W. Schoeller, B. Donnadieu, G. Bertrand, J.
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Wiley, Chichester, 1994, pp. 3106 – 3140.
lengths of the P4 core are in good agreement with experiment
!
while the calculated exocyclic P PPh3 donor bonds are again
about 0.07 ꢀ too long. The analysis of the bonding in 42+ gave
very similar results as for 12+ and thus it is not reported here
(Supporting Information, Figure S3).[39] The calculation of the
!
P
P donor strength according to reaction (2):
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H. G. Stammler, Science 2004, 305, 849.
42þ ! P42þ þ 2 PPh3
ð2Þ
gives a value of DGR298 = 201.2 kcalmolꢀ1. The bond strength
of one Ph3P!P4 donor–acceptor bond is thus 100.6 kcal
2+
molꢀ1.
On the basis of our calculations, the Ph3As ligands in 12+
should be easily displaced by a stronger ligand, such as Ph3P.
Consistently, the addition of 2 equiv of Ph3P to a solution of
1[OTf]2 in CH2Cl2 resulted in the formation of a pale yellow
solution. The 31P{1H} NMR spectrum of the reaction mixture
shows
a new set of resonances, which was fitted as
a
A2MM’XX’ spin system (dA = ꢀ317.7 ppm, dM =
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1
ꢀ180.2 ppm, dX = 29.8 ppm; 1JAM = 1JAM’ = ꢀ157 Hz, JMX
=
=
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2
2
3
3JM’X’ = ꢀ139 Hz, JXX’ = 39 Hz) in a ratio of 2:2:2, indicating
the formation of a new species (Supporting Information,
Figure S2).[33] The resonances for the A2MM’XX’ spin system
are consistent with the displacement of the Ph3As ligands in
4
12+ by Ph3P, suggesting the formation of dication 42+ and
2+
ꢀ
proving the coordinative nature of the P As bond in 1 . The
postulated structure was confirmed by X-ray diffraction
(Figure 1) and the bond lengths and angles in the [P4] core
of 42+ are reminiscent of that of cation 12+, including the
ꢀ
ꢀ
exocyclic P1 P5 and P3 P6 bonds (2.223(3) and 2.218(3) ꢀ).
In summary, cation 12+ results from the one-pot reduction
of PCl3 in the presence of Ph3As and AlCl3 and can be
considered as ligand-stabilized [P4]2+. The ease of this
reaction, together with the facile accessibility of 1[AlCl4]2
from cheap, commercially available starting materials, makes
this compound a potential precursor for a plethora of
interesting subsequent transformation reactions and may
give access to many new, hitherto-unknown polyphosphorus
moieties.
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Received: December 20, 2011
[31] B. W. Tattershall, N. L. Kendall, Polyhedron 1994, 13, 1517.
Published online: February 6, 2012
ꢀ
[32] The formation of Cl2P PCl2 was only observed when an excess
of AlCl3 was used. No reaction was observed when AlCl3 was
replaced with GaCl3. [Ph3AsCl][AlCl4] was isolated from the
reaction mixtures and fully characterized (Ref. [33]).
[33] Details of the synthesis, isolation and characterization of
1[AlCl4]2, 3[AlCl4]2, and 4[AlCl4]2 are available in the Support-
ing Information.
Keywords: cations · donor–acceptor complexes · phosphorus ·
structure elucidation
.
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[39] Details about the methods are given in Supporting Information.
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