Versatile reagent Ph3As(OTf)2: One-pot synthesis of [P7(AsPh3)3][OTf]3 from PCl3

Article abstract of DOI:10.1002/chem.201405196

Compound Ph₃As(OTf)₂ as a pentacoordinated As^V Lewis acid readily forms dicationic Lewis acid/base ad-ducts upon addition of various Lewis bases. It also represents a stronger chloride-abstracting agent than Me₃SiOTf and facilitates the reductive coupling of PCl₃ in the presence of AsPh₃ to the unprecedented cation [P₇(AsPh₃)₃]³⁺ as triflate salt. This crystallographically characterized nor-tricyclane-type cation represents a P₇R₃-derivative with the most electron-withdrawing substituents, resulting in a pronounced effect on the structural parameters of the P₇ core.

Full text of DOI:10.1002/chem.201405196

DOI: 10.1002/chem.201405196
Communication
&
Polyphosphorus Chemistry
Versatile Reagent Ph3As(OTf)2: One-Pot Synthesis of
[P7(AsPh3)3][OTf]3 from PCl3
Maximilian Donath,[a] Michael Bodensteiner,[b] and Jan J. Weigand*[a]
In memory of Professor Piero Stoppioni
Abstract: Compound Ph3As(OTf)2 as a pentacoordinated
AsV Lewis acid readily forms dicationic Lewis acid/base ad-
ducts upon addition of various Lewis bases. It also repre-
sents a stronger chloride-abstracting agent than Me3SiOTf
and facilitates the reductive coupling of PCl3 in the pres-
ence of AsPh3 to the unprecedented cation [P7(AsPh3)3]3+
as triflate salt. This crystallographically characterized nor-
Scheme 1. Synthesis of 1[AlCl4]2 from reductive coupling of PCl3 with Ph3As
tricyclane-type cation represents a P7R3-derivative with the
most electron-withdrawing substituents, resulting in a pro-
nounced effect on the structural parameters of the P7
core.
in the presence of AlCl3.
Our chemistry often involves the triflate anion, TfOÀ, which
can be conveniently introduced by reagents, such as Me3SiOTf,
AgOTf, or TlOTf.[1,4] However, their use is sometimes limited,
because either Me3SiOTf is not reactive enough or the solubili-
ty of metal triflates in weakly coordinating solvents is quite
low. Alternatives are chloride-abstracting agents with WCAs,
such as halogenated carborane anions,[5] perfluorinated tetra-
phenylborate,[6] or perfluorinated tetraalkoxyaluminates.[7]
However, their use is often restricted due to laborious multi-
step syntheses with sometimes low overall yields. Although
there are cases, in which there is an essential need for those
WCAs, we believe that TfOÀ might be a suitable counterion in
many other cases, in which only a lack of an appropriate tri-
flate transfer reagent is present. In this light, we were interest-
ed to find a triflate-based chloride-abstracting agent that over-
comes the aforementioned problems.
Chloride-abstracting agents are crucial in cationic p-block
chemistry and find wide application in the synthesis of highly
electrophilic compounds. Their use often demands weakly co-
ordinating anions (WCAs) to prevent detrimental nucleophilic
attack of the anion followed by degradation. Often used re-
agents are, for example, Me3SiOTf, AgOTf, TlOTf, or the
Group 13 Lewis acids, such as AlCl3 and GaCl3, and they have
been used in the synthesis of several phosphanylpnictonium
cations.[1]
We previously reported the synthesis of the unusual butter-
fly-type
compound
[P4(AsPh3)2][AlCl4]2 (1[AlCl4]2)
and
[Ph3AsAsPh3][AlCl4]2 (2[AlCl4]2) from a reductive-coupling reac-
tion of PCl3 with AsPh3 in the presence of AlCl3 (Scheme 1).[2] In
recent studies, we have realized that its follow-up chemistry is
hampered by the presence of the [AlCl4]À anion. In solution,
12+ slowly degrades due to nucleophilic attack of liberated ClÀ
ions from the [AlCl4]À anion.[3] This observation motivated us to
search for alternative, less reactive counterions, that allow us
to explore the chemistry of the [P4(AsPh3)2]2+ dication, for ex-
ample, as a P4-transfer reagent.
During our literature research, we came across Verma’s
report on the Ph3E(OTf)2 (E=Sb, Bi) derivatives, which seemed
to us to be suitable chloride-abstraction reagents.[8] Very re-
cently, Burford and co-workers used these compounds in the
synthesis of interesting donor/acceptor complexes.[9] The light-
er As and P derivatives were not reported. However, it is
known that attempts to generate these derivatives from the
reaction of Ph3PO or Ph3AsO (3) with Tf2O or (FSO2)2O resulted
in the formation of the cationic anhydrides [(Ph3P)2O]2+ or
[(Ph3As)2O]2+ (42+) as [OTf]À or [FSO3]À salts.[10] However, com-
parison of pnictogen–chlorine bond strengths (AsÀCl: 107 kcal
molÀ1; SbÀCl: (86Æ12) kcalmolÀ1)[11] implies that Ph3As(OTf)2
(5) should be a very promising chloride-abstracting reagent.
We found that the reaction of Ph3AsO (3) with 0.5 equiva-
lents of Tf2O in CH2Cl2 proceeds quantitatively and gives the
previously reported cationic anhydride [(Ph3As)2O]2+ (42+) as
triflate salt. In contrast, the reaction in a 1:1 stoichiometry re-
sults in the clean formation of the unreported Ph3As(OTf)2 (5)
derivative (Scheme 2). Compound 5 was conveniently synthe-
[a] M. Donath, Prof. Dr. J. J. Weigand
Fakultꢀt Chemie und Lebensmittelchemie
Technische Universitꢀt Dresden, 01062 Dresden (Germany)
Fax: (+49)351-463-31478
E-mail: jan.weigand@tu-dresden.de
[b] Dr. M. Bodensteiner
Rçntgenstrukturanalyse, Fakultꢀt fꢁr Chemie und Pharmazie
Universitꢀt Regensburg, 93040 Regensburg (Germany)
Supporting information for this article, which contains the experimental de-
tails, is available on the WWW under http://dx.doi.org/10.1002/
chem.201405196.
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Scheme 2. Synthesis of Ph3As(OTf)2 (5) from the reaction of Ph3AsO (3) with
triflic anhydride (Tf2O).
sized on a multigram scale (>100 g) and was obtained in high
yields (91%) as a colorless, highly moisture-sensitive crystalline
solid (for details, see the Supporting Information).
Single crystals of 4[OTf]2 and 5 suitable for X-ray single-crys-
tal diffraction analysis were obtained by slow diffusion of n-
hexane into a solution of 4[OTf]2 or 5 in CH2Cl2 at À308C. The
molecular structure of compound 5 revealed loosely coordinat-
ed triflate anions in the axial positions of the trigonal bipyrami-
dal sphere of the As atom (av AsÀO bond lengths in 42+: 1.756
vs. 5: 2.031 ꢁ; Figure 1).
Similarly to the related Sb and Bi derivatives,[9] 5 was expect-
ed to act as a highly reactive pentacoordinated AsV Lewis acid
giving the corresponding donor/acceptor complexes in the re-
action with appropriate Lewis bases. Thus, 5 was allowed to
react with the Lewis bases AsPh3, 4-dimethylaminopyridine
(DMAP) and the N-heterocyclic carbene (NHC) 1,3-bis(2,6-diiso-
propylphenyl)imidazole-2-ylidene (IDipp) to cleanly give the ex-
pected dicationic Lewis acid/base adducts 6–8[OTf]2
(Scheme 3). In all cases, the formation of the adducts proceeds
through displacement of the triflate anions accompanied with
the change of the coordination number at the arsenic atom
from five to four (Scheme 3; for experimental details, including
molecular structure, see the Supporting Information).
Scheme 3. Formation of triflate salts of dicationic Lewis acid/base adducts
6–8[OTf]2 between Ph3As(OTf)2 (5) and neutral Lewis bases AsPh3, DMAP,
and IDipp.
Figure 1. Molecular structures of a) the dication 42+ and b) 5. All hydrogen
atoms (a+b) and triflate anions (a) are omitted for clarity. Selected bond
lengths [ꢁ] and angles [8]: a) As1ÀO1 1.755(2), As2ÀO1 1.757(1); As1-O1-As2
149.33(8); b) As1ÀO1 2.038(1), As1ÀO4 2.023(1); O1-As1-O4 177.04(7).
To test the chloride-abstracting ability of 5, a solution of the
ditriflate 5 was reacted with Me3SiCl in various stoichiometries
(Scheme 4, for detailed discussion, see the Supporting Informa-
tion). The 1:1 reaction led to the clean formation of 9[OTf] and
Me3SiOTf. In the 1:2 reaction, one equivalent of Me3SiCl re-
mained unreacted, indicating that 9[OTf] was not capable of
abstracting chloride ions from the chlorosilane.
Compound 5 proved to be a very strong chloride-abstract-
ing agent, and we were interested, whether it could be used
as an appropriate surrogate for AlCl3 in the synthesis of cation
12+ as more stable triflate salt. Thus, we reacted PCl3, AsPh3,
and 5 in CH2Cl2 (Scheme 5). The 31P{1H} NMR spectrum of the
Scheme 4. Chloride abstraction from Me3SiCl by Ph3As(OTf)2 (5) generating
Me3SiOTf and 9[OTf].
Scheme 5. Idealized equation for the reductive coupling of PCl3 with AsPh3
and Ph3As(OTf)2 (5) to give [P7(AsPh3)3][OTf]3 (10[OTf]3).
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Figure 2. 31P{1H} NMR spectrum of the PCl3/Ph3As(OTf)2/AsPh3 reaction mixture in CH2Cl2 (optimized 1:2:3 stoichiometry) after two days. Inlets (103+)X, (103+)B,
and (103+
)
A show the multiplet resonances of the experimental (top) and iteratively fitted (bottom) 31P{1H} NMR spectrum of 10[OTf]3.[12] Inlets (12+
)
a
n
d
X
(12+ show the A2X2 spin system of 12+
)
.
X
reaction mixture is depicted in Figure 2, indicating a different
course of this reaction than expected.
The spectrum of this reaction mixture shows the formation
of small amounts of dication 12+ according to the appearance
of the expected A2X2 spin system (d(12+)A =À325.4 ppm,
d(12+)X =À175.3 ppm, 1JAX =162.2 Hz).[2] However, the main
product (ca. 70%) showed three complicated multiplet reso-
nances
(d(103+)A =À139.5 ppm,
d(103+)B =À130.9 ppm,
d(103+)X =69.6 ppm; Figure 2) that are integrating in a ratio of
3:1:3 and, according to 31P31P{1H} COSY NMR spectroscopy,
belong to the same highly symmetric species 103+
.
The multiplet resonances were iteratively fitted to an
AA’A’’BXX’X’’ spin system, which is in accordance with the typi-
cal pattern of a heptaphosphanortricyclane cage and is indica-
tive of the formation of the C3-symmetric stereoisomer of the
trication 103+ (1JAX =À355.7(4) Hz, 1JAA’ =À199.7(9) Hz,
2
2
2
1JBX =À323.7(3) Hz, JAB =44.62(11) Hz, JAX’ =4.77(18) Hz, JXX’
=
À6.7(12) Hz; Figure 2, inset). Careful adjustment of the stoichi-
ometry and the reaction conditions leads to a convenient syn-
thesis of compound 10[OTf]3, which was comprehensively
characterized. Compound 10[OTf]3 forms pale yellow crystals
that begin to decompose visibly only above 1408C. Although
the work-up procedure turned out to be extremely tedious,
this compound can be obtained in amounts of several hun-
dred milligrams in an overall yield of 13% based on PCl3.
Single crystals suitable for X-ray diffraction single-crystal
analysis of 10[OTf]3 were obtained by slow diffusion of Et2O
into a propionitrile solution of the salt at À308C (Figure 3). The
molecular structure of the cation 103+ revealed a heptaphos-
phanortricyclane cage with three triphenylarsonio substituents
at the bridging phosphorus atoms. One of the three triflate
anions shows close contacts between P3ÀO5 (2.905(2) ꢁ),
P6ÀO5 (2.922(2) ꢁ), and As2ÀO5 (2.990(2) ꢁ) with distances
well below the sum of the van der Waals radii (SvdW(AsÀO)
3.5 ꢁ; SvdW(PÀO)=3.4 ꢁ),[13] underlining the high electrophilicity
of the cation.
Figure 3. Molecular structure of the trication 103+. a) Only the phosphorus,
arsenic, and ipso-carbon atoms of the phenyl ring are shown for clarity.
b) View illustrating the close contact between one triflate anion and the
cation. Selected bond lengths [ꢁ]: P2ÀAs1 2.338(1), P5ÀP7 2.215(1), P3ÀP6
2.222(1), P1ÀP4 2.186(1), As2ÀO5 2.990(2), P3ÀO5 2.905(2), P6ÀO5 2.922(2).
The observed PÀP and AsÀP bond lengths are in the typical
range of single bonds and are comparable to those observed
in cation 12+ [2]
.
There are several crystallographically character-
ized nortricyclane-type P7R3 systems with varying substituents.
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Table 1. Selected 31P NMR shifts, bond lengths, and angles of selected C3- and C3v-symmetric stereoisomers of heptaphosphanortricyclane derivatives. En-
tries 4–6 and 8–10 refer to the average bond lengths (4–6), angle sums (8 and 9), and angle differences (10).
[d]
[d]
[e]
[f]
[g]
[h]
[h]
Entry/parameter[a]
[P7(AsPh3)3]3+[b]
[Ph6P7]3+[c]
P7(GePh3)3
P7(SiPh3)3
P7(SitBu3)3
P7(Si(SiMe3)3)3
Li3(tmeda)3P7
P7Br3
P7I3
1 d31P(Pa) [ppm]
2 d31P(Pe) [ppm]
3 d31P(Pb) [ppm]
4 PaÀPe [ꢁ]
À130.9
69.6
À157.1
112.6
À94
À105
À113
À100.6
À7.3
À57
À70.7
À64.5
12
4
À31.2
À103
177.7
126.0
À139.5
2.185(5)
2.224(4)
2.218(7)
287.2(2)
311(3)
À163.7
2.220(4)
2.226(3)
2.218(4)
274.99(12)
–
À158
À156
À175.6
2.182(10)
2.186(4)
2.226(4)
293.8(2)
331.3(18)
267.5(7)
6.9(3)
À161.6
2.183(3)
2.198(5)
2.218(4)
292.56(17)
318.1(6)
267.4(4)
6.5(3)
À162
À138.8
À136.1
2.184(5)
2.196(5)
2.215(4)
293.5(3)
309.9(5)
268.3(5)
3.0(3)
2.178(10)
2.183(9)
2.207(10)
294.8(8)
313.2(16)
268.5(9)
4.5(10)
2.204(15)
2.150(16)
2.255(12)
305.4(10)
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
5 PeÀPb [ꢁ]
6 PbÀPb [ꢁ]
7 Y(Pa) [8]
8 Y(Pe) [8]
9 Y(Pb) [8]
263.5(12)
13.5(2)
264.9(4)
3.13(16)
271.5(19)
0.0(13)
10 b2Àb1 [8]
[a] For numbering scheme and definition of b, see Figure 4. [b] This work. [c] Ref. [17]. [d] Ref. [14a]. [e] Ref. [14b]. [f] Ref. [14c]. [g] Refs. [14d,17].
[h] Ref. [16]. No crystal structures were reported.
Interestingly, the observed PÀP bond lengths in those com-
pounds are comparable, indicating only a minor influence of
the substituents (Table 1, compare entries 4–6).[14] The PÀP
bond lengths of 103+ (PbÀPb (av) 2.218(7) ꢁ) within the basal
P3 ring (Table 1, entry 6) are slightly longer than between the
apical and the equatorial P atoms (PaÀPe(av) 2.185(5) ꢁ; Table 1,
entry 4).
entry 10). A comparison to other C3-symmetric P7R3 nor-
triclyclane derivatives[14] shows that the cage twist is usually
much lower within a narrow range up to Ø(b2Àb1)=6.9(3)8 for
R=tBu3Si.[14b] The angle sum at the equatorial phosphorus
atoms (Table 1, entry 8) in 103+ (Y(Pe)=311(3)8) is comparable
to those of P7(GePh3)3 Y(Pe)=309.9(5)8) and P7(SiPh3)3 (Y(Pe)=
313.2(16)8),[14a] but smaller than those found for P7R3 deriva-
tives with sterically more demanding substituents (R=SitBu3:
Y(Pe)=331.3(18)8; R=Si(SiMe3)3: Y(Pe)=318.1(6)8).[14b,c] This
leads us to the assumption that the cage twist angle is more
influenced by the electronic properties of the substituent than
by its steric demand. The angle sum at the basal phosphorus
atoms Y(Pb) (Table 1, entry 9) instead decreases with the de-
crease of electron density inside the P7 cage from Y(Pb)=
271.5(19)8 (R=Li(tmeda))[14d] to Y(Pb)=263.5(12)8 (R=
(AsPh3)+). The angle sum at the apical phosphorus atom Y(Pb)
(Table 1, entry 7) follows the same trend from Y(Pa)=
305.4(10)8 (R=Li(tmeda)) to Y(Pa)=287.2(2)8 (R=(AsPh3)+).[14]
Both of the observed, relatively small angle sums Y(Pb) and
Y(Pa) are reflected in the 31P NMR shifts of the basal and apical
phosphorus atoms (d(Pb)=À139.5 ppm, d(Pa)=À130.9 ppm)
of 103+, which appear at relatively low and high field, respec-
tively.
This is in line with the PÀP bond lengths of the neutral, iso-
electronic derivative P7(GePh3)3.[14a] However, the PÀP bonds of
103+ between the basal and equatorial phosphorus atoms
(PbÀPe (av) 2.224(1) ꢁ) are significantly longer than in
P7(GePh3)3 (PbÀPe (av) 2.195(2) ꢁ), which is most likely due to
electronic effects (Table 1, entry 5). The inner angles a, g, and
d have very similar values compared to known P7R3 derivatives,
whereas for the angles b1 and b2, large deviations were found
(Figure 4).[14] With the difference between these angles the
twisting of the cage around the threefold axis can be quanti-
fied. The trication 103+ (Ø(b2Àb1)=13.5(2)8) shows a measured
value, which is four times as high as was found for its isoelec-
tronic derivative P7(GePh3)3 (Ø(b2Àb1)=3.0(3)8; Table 1,
If the angle sum Y(Pb) decreases (Table 1, compare entries 3
and 9) the s character of the lone pair of electrons decreases
resulting in a deshielding of the phosphorus nucleus and thus
in a downfield shift.[15] The same effect can be observed for the
apical phosphorus atom Pa (Table 1, compare entries 1 and 7).
Herein, the decrease of the angle sum Y(Pa) towards 2708 re-
sults in the increase of the s character of the lone pair of elec-
trons. Therefore, the phosphorus nucleus is more shielded,
which causes a highfield shift. The resonance for the equatorial
phosphorus atoms Pe appears at relatively low field (d=
69.6 ppm). Similar 31P NMR shifts are usually observed for C3-
symmetric P7R3 cage compounds with electronegative substitu-
ents (R=Br, I; Table 1, entry 2).[16] Due to the lack of crystal
structures of P7Br3 and P7I3, 10[OTf]3 represents the heptaphos-
phanortricyclane derivative with the most electron-withdraw-
ing substituents, in which these effects could be observed to
date.
Figure 4. Denotations of the chemically different phosphorus atoms and the
most important bond angles in heptaphosphanortricyclane systems.
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[9] A. P. M. Robertson, N. Burford, R. McDonald, M. J. Ferguson, Angew.
Chem. Int. Ed. 2014, 53, 3480–3483; Angew. Chem. 2014, 126, 3548–
3551.
[10] a) J. B. Hendrickson, M. S. Hussoin, J. Org. Chem. 1989, 54, 1144–1149;
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1897.
In summary, we presented the synthesis of Ph3As(OTf)2
through the facile one-pot reaction of Ph3AsO with Tf2O. Com-
pound Ph3As(OTf)2 readily forms dicationic Lewis acid/base ad-
ducts upon addition of Lewis bases under displacement of the
triflate anions. We could demonstrate its great potential as
a strong chloride-abstracting agent with a high solubility in
weakly coordinating solvents. Initial application of Ph3As(OTf)2
as chloride-abstracting Lewis acid could be demonstrated in
the synthesis of the first C3 symmetric P7R3 tri-arsonium trica-
tion [P7(AsPh3)3]3+ (103+) with a nortricyclane type structure of
the P7 core.
[11] Y.-R. Luo, Comprehensive handbook of chemical bond energies, CRC
Press, Boca Raton, 2007, pp. 488–489.
[12] a) For reasons of clarity, the depicted inlets (103+)A, (103+
) )
and (103+
B
X
are taken from the resolution enhanced 31P{1H} NMR spectrum of pure
10[OTf]3 in CD3CN, which was iteratively fitted. The apical phosphorus
atom (103+
) shows a strong dependency on the used solvent. Relative
B
to the basal phosphorus atoms (103+)A, the resonance (103+
) appears
B
at higher field in CH2Cl2 (reaction mixture, formally, ABB’B’’XX’X’’ spin
system) and at lower field in CD3CN (formally, AA’A’’BXX’X’’ spin
system). For the enlargements of the respective multiplets of the
31P{1H} NMR spectrum of the reaction mixture see Figure S7 in the Sup-
porting Information. b) Attempts to isolate or identify the compound
attributed to the singlet at d= +100.3 ppm was unsuccessful due to
its high reactivity. This resonance is observed during the initial stage of
the reaction and is believed to be cation [(Ph3As)PCl2]+. All other ob-
servable resonances in the 31P NMR spectrum that are not explained in
the manuscript are either unidentified byproducts and/or intermedi-
ates.
Acknowledgements
We gratefully acknowledge financial support from the Fonds
der Chemischen Industrie (fellowship to M.D.), the DFG (WE
4621/2–1), and the ERC (SynPhos307616).
Keywords: 1,2-dications
phosphorus · polycations
· Lewis acids · nortricyclane ·
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Received: September 9, 2014
Published online on October 21, 2014
Chem. Eur. J. 2014, 20, 17306 – 17310
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17310
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