One-pot syntheses of cationic polyphosphorus frameworks with two-, three-, and four-coordinate phosphorus atoms by one-pot multiple P-P bond formations from a P1 source

Article abstract of DOI:10.1002/anie.201201414

Accessible complexity: Polyphosphorus frameworks [R₄P ₄pyr]⁺ and [R₆P₇]⁺ (R=Cy, Ph; pyr=3,5-dimethylpyrazolyl) were prepared from a P₁ source, R₂PH. In a one-pot reaction, eight P-P bonds are formed via a unique combination of substitution and base-induced reductive P-P coupling. Copyright

Full text of DOI:10.1002/anie.201201414

Angewandte
Chemie
DOI: 10.1002/anie.201201414
Phosphorus Compounds
One-Pot Syntheses of Cationic Polyphosphorus Frameworks with Two-,
Three-, and Four-Coordinate Phosphorus Atoms by One-Pot Multiple
ꢀ
P P Bond Formations from a P1 Source**
Kai-Oliver Feldmann and Jan J. Weigand*
The structural diversity of neutral and anionic cyclic and cage-
reported trication 73+[9] (Scheme 1) as a P1 synthon with
secondary phosphanes in an unprecedented combination of
like polyphosphorus frameworks (for example 1,[1]
2
2ꢀ[2]) is
based on the two bonding motifs A and B.[3] These comprise
comproportionative and base-induced reductive P P bond
formations.
ꢀ
ꢀ
Scheme 1. P O bond formation by hydrolysis of 7[OTf]3. a) 2H2O,
CH3CN, ꢀ2 9[OTf] (3,5-dimethylpyrazolium triflate).
Established synthetic methods are inadequate for the
preparation of cations displaying bonding motive E. Key to
the success for the preparation of cations 5+ and 6a,b+ was the
realization that multiple-charged PIII-centered cations show
a propensity to reductively form PI species in the presence of
a Lewis base.[10] In contrast, we recently demonstrated that P-
centered trication 73+ yields the unprecedented cation 82+
(Scheme 1) upon careful hydrolysis.[9,11] The + 3 oxidation
state of the P atoms is retained in this reaction.
two- and three-coordinate phosphorus atoms that feature at
ꢀ
least two P P bonds. Only one compound featuring two- and
four-coordinate P atoms (C, 3[4]) was reported. In the realm of
cationic polyphosphorus frameworks this series is extended
by an array of cyclo-phosphinophosphonium ion frame-
works[5] (e.g. D, 42+[6]) and phosphorus-rich cationic cage
compounds[7,8] that are composed of three- and four-coordi-
nate P atoms. Polyphosphorus frameworks featuring a combi-
nation of two-, three-, and four-coordinate P atoms (E) have
been elusive to date. Herein we present the first examples of
cyclic (5+) and bicyclic (6a,b+, a: R = Cy, b: R = Ph) cationic
polyphosphorus frameworks displaying bonding motif E.
These cations result from the reaction of our recently
We were therefore interested to establish whether cation
73+ can also be reduced to PI compounds upon reactions with
Lewis bases and whether this can be exploited for base-
ꢀ
induced reductive P P coupling reactions. This would con-
ꢀ
stitute a novel synthetic approach towards P P bond for-
mation and contrast the previously reported protolysis
reaction.
The addition of 3.5 equivalents of the Lewis basic
phosphane Cy2PH to a suspension of 7[OTf]3 in CH2Cl2 at
room temperature yielded an orange solution (Scheme 2).
The 31P{1H} NMR spectrum[12] of the reaction mixture
revealed the clean formation of two phosphorus-containing
products indicated by the presence of an AMNN’XZZ’ spin
[*] K.-O. Feldmann, Dr. J. J. Weigand
Institut fꢀr Anorganische und Analytische Chemie and
Graduate School of Chemistry, WWU Mꢀnster
Corrensstrasse 30, 48149 Mꢀnster (Germany)
E-mail: jweigand@uni-muenster.de
[**] We gratefully acknowledge the FCI (fellowship for K.-O.F.), the
European Phosphorus Science Network (PhoSciNet CM0802), and
the DFG (WE 4621/2-1). J.J.W. thanks Prof. F. Ekkehardt Hahn
(WWU Mꢀnster) for his generous support, one of the referees for
very helpful comments, and Prof. Robert Wolf (Universitꢁt
Regensburg) for helpful discussions.
Scheme 2. Reaction of 7[OTf]3 with a secondary phosphane R2PH.
a) CH2Cl2, ꢀ9[OTf], ꢀ11; 6a[OTf]/10a[OTf]: R=Cy, 12 h, RT, 45%;
6b[OTf]/10b[OTf]: R=Ph, 3 h, RT, not isolated.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201201414.
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system for compound 6a[OTf] and a singlet (d = 84.4 ppm)
hexane into a solution of 6a[OTf] in fluorobenzene. The
molecular structure reveals a bicyclic P7 framework com-
posed of an envelope shaped five-membered ring and a three-
membered ring. These are annulated in a way that P1 and P6
are located above the plane spanned by P2, P3, P4, and P5.
This results in a rather short distance between the phosphorus
atoms P1 and P6 (3.500(8) ꢀ). Similar conformations were
observed in the neutral bicyclic hexaphosphanes tBu4P6 (1)[1]
1
for compound 10a[OTf].[12] Furthermore, the H NMR spec-
trum indicated the formation of 3,5-dimethylpyrazolium
triflate (9[OTf]) and 3,5-dimethylpyrazole (11). Compounds
9[OTf], 10a[OTf], and 11 can be conveniently separated by
evaporation of all volatiles from the reaction mixture under
reduced pressure and extraction with Et2O. Compound
6a[OTf] remains as analytically pure orange solid in moder-
ate yield (45%). Compound 10a[OTf] was obtained by
fractional crystallization from the Et2O extract (54%).
Figure 1 shows the 31P{1H} NMR spectrum of 6a[OTf] and
the molecular structure of 6a+ in 6a[OTf]·0.5C6H5F. Crystals
suitable for X-ray diffraction were obtained by diffusion of n-
and Cp*4P6.[13] The shortest P P bonds (P1 P2 2.120(1) ꢀ,
ꢀ
ꢀ
+
ꢀ
P1 P3 2.121(1) ꢀ) in cation 6a are observed between the
two- and three-coordinate P atoms. Similar bond lengths have
been observed for structurally related cations (for example,
2.137(6) ꢀ in [Ph3P P PPh3][AlCl4][10a]). According to an
ꢀ ꢀ
NBO[14] (natural bond orbital) analysis on the DFT (B3LYP/
6-311G(2d)) optimized structure of 6a+,[12] this shortening is
a result of donation of electron density from the p-type lone
ꢀ
pair of electrons on P1 into the s* orbitals of the adjacent P P
(LP(p)P1!s*P2–P4 7.24 kcalmolꢀ1, LP(p)P1!s*P3–P5 7.19 kcal
ꢀ1
mol ) and P C (LP(p)P1!s*P2–C 10.61 kcalmolꢀ1, LP(p)P1!
ꢀ
s*P3–C 10.51 kcalmolꢀ1) bonds. These secondary interactions
ꢀ
also result in a slight elongation of the P2 P4 (2.230(1) ꢀ)
ꢀ
and P3 P5 (2.235(1) ꢀ) bonds in comparison to related bonds
between three- and four-coordinate P atoms (for example
2.1952(6) ꢀ in 1,2,3,4-tetracyclohexyl-1-methylcyclotetra-
phosphan-1-ium triflate).[15]
The 31P{1H} NMR spectrum of 6a[OTf] shows several
remarkable features that can be attributed to its unique
bicyclic structure. The signals for the phosphorus atoms of the
three-membered ring of 6a+ (PN d = ꢀ180.0 ppm, PM d =
ꢀ190.9 ppm) are shifted upfield compared to phosphanyl-
substituted cyclo-triphosphanes (e.g. d = ꢀ157.2 ppm for 2,3-
bis(tert-butyl)-1-(tert-butylchlorophosphanyl)-cyclo-triphos-
phane).[16] The chemical shifts of the bridgehead (PN) and
four-coordinate phosphorus atoms (PZ: d = 108.6 ppm) are
best compared to the structurally related dication [Ph4P6]2+
(d = ꢀ174.4 and 80.5 ppm, respectively).[8a] The signal of the
two-coordinate P atom (PA, d = ꢀ257.2 ppm) is shifted upfield
compared to related acyclic triphosphenium ions (d =
+
[10a]
ꢀ ꢀ
ꢀ229 ppm for (nBu)3P P P(nBu)3 ).
This may be attrib-
uted to the rather acute angles in the five-membered ring.[17]
The magnitude of the coupling constants between the two-
and four-coordinate P atoms in 6a+ (1J(PAPZ) = ꢀ478.3 Hz) is
significantly larger than that between the three- and four-
coordinate nuclei (1J(PNPZ) = ꢀ334.0 Hz), which is in accord-
ance with previous observations.[18] Furthermore, they are
found to be smaller than in acyclic derivatives. This observa-
tion can be attributed to the lower s-orbital character of the
Figure 1. a) 31P{1H} NMR spectrum of 6a[OTf] (CD2Cl2, 300 K). Insets
show experimental (upwards) and fitted spectra (downwards); ?
indicates very small amounts of 10a[OTf]. AMNN’XZZ’ spin system
[ppm, Hz]: dA =ꢀ257.2 (1P), dM =ꢀ190.9 (1P), dN =ꢀ180.0 (2P),
dX =6.6 (1P), dZ =108.5 (2P); 1J(PAPZ)=1J(PAPZ’)=ꢀ478.3, 1J(PMPN)=
1J(PMPN’)=ꢀ169.6, 1J(PMPX)=ꢀ156.9, 1J(PNPZ)=1J(PN’PZ’)=ꢀ334.0,
1J(PNPN’)=ꢀ295.4, 2J(PAPN)=2J(PAPN’)=ꢀ2.3, 2J(PMPZ)=2J(PMPZ’)=
38.4, 2J(PNPX)=2J(PN’PX)=135.9, 2J(PNPZ’)=2J(PN’PZ)=ꢀ11.9,
2J(PZPZ’)=ꢀ5.2, 3J(PAPM)=46.1, 3J(PXPZ)=3J(PXPZ’)=7.2, 4J(PAPX)=
ꢀ3.5). b) Molecular structure of the cation in 6a[OTf]·0.5C6H5F
(hydrogen atoms omitted for clarity, ellipsoids set at 50% probability).
Selected bond lengths [ꢂ] and angles [8]: P2–P1 2.120(1), P2–P4
2.230(1), P4–P5 2.201(1), P4–P6 2.206(1), P6–P5 2.206(1), P6–P7
2.243(2), P3–P1 2.121(1), P3–P5 2.235(1), P1···P6 3.500(8); P2-P1-P3
93.19(5), P1-P2-P4 112.44(5), P1-P3-P5 112.61(5), P5-P4-P2 101.31(5),
P5-P4-P6 60.07(4), P6-P4-P2 96.79(5), P4-P5-P6 60.07(4), P4-P5-P3
101.41(5), P6-P5-P3 95.85(5), P4-P6-P5 59.86(4), P4-P6-P7 95.15(5),
P5-P6-P7 95.67(5); [P2,P3,P4,P5]-[P4,P5,P6] 89.2, [P2,P3,P4,P5]-
[P1,P2,P3] 141.4.
ꢀ
respective P P bonding orbitals in the cyclic structure of
6a+.[19]
To gather information on the reaction mechanism of the
formation of 6a[OTf], we investigated the reaction of 7[OTf]3
with various amounts of Cy2PH. The addition of three
equivalents of Cy2PH to a suspension of 7[OTf]3 in CH2Cl2
at room temperature again yielded an orange solution
(Scheme 3). The 31P{1H} NMR spectrum of the reaction
mixture[12] after 4 h revealed the clean formation of only two
phosphorus-containing products. A singlet (d = 84.4 ppm)
indicated the formation of the pyrazoliumyl-substituted
phosphane 10a[OTf], and the presence of an AMX2 spin
2
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Scheme 3. Reaction of 7[OTf]3 with Cy2PH in a 1:3 ratio. a) ꢀ9[OTf],
CH2Cl2, 4 h, RT.
Figure 3. 31P{1H} NMR spectrum of 5[OTf] (CD2Cl2, 300 K). Insets
show experimental (upwards) and fitted spectra (downwards). AMX2
system is indicative of the formation of the four-membered
ring compound 5[OTf]. The 1H NMR spectrum indicated the
formation of 9[OTf] as the only other by-product. The
isolation of compound 5[OTf] (yield of isolated product:
40%) followed the same procedure as for 6a[OTf].
spin system [ppm, Hz]: dA =ꢀ245.7 (1P), dM =ꢀ26.7 (1P), dX =53.2
1
(2P); J(PAPX)=ꢀ382.8, 1J(PMPX)=ꢀ227.3, 2J(PAPM)=ꢀ26.9.
The molecular structure of 5+ is depicted in Figure 2. The
bond lengths follow the same trends as already discussed for
6a+, and the angles are largely dictated by the four-membered
The formation of 5+ can be understood mechanistically in
terms of several subsequent substitution steps, followed by
a base-induced reduction (Scheme 4). Step 1 most likely
Scheme 4. Proposed mechanism for the formation of 5+.
Figure 2. Molecular structures of the cations 5+ and 10a+ in 5[OTf]
and 10a[OTf] (hydrogen atoms omitted for clarity, ellipsoids set at
50% probability). Selected bond lengths [ꢂ] and angles [8]: 5+: P1–P2
2.1387(7), P1–P3 2.1356(7), P2–P4 2.2143(7), P3–P4 2.2162(7), P4–N1
1.716(2), P2–C1 1.839(2), P2–C7 1.834(2), P3–C13 1.845(2), P3–C19
1.831(2); P2-P1-P3 84.11(3), P1-P2-P4 92.58(3), P1-P3-P4 92.61(3),
P2-P4-P3 80.52(2). 10a+: P1–N1 1.775(1), P1–C1 1.850(2), P1–C7
1.845(2).
involves the formation of intermediates 12+ and 132+. This
step is a protolysis reaction, which is related to the reaction of
73+ with water.[9] Subsequently, monocation 12+ may substi-
tute one pyrazoliumyl substituent in dication 132+, forming
dicationic intermediate 142+ and 3,5-dimethylpyrazolium ion
(9+) after deprotonation (step 2). Similarly, an intramolecular
substitution reaction of intermediate 145+ yields dication 152+
via the loss of 3,5-dimethylpyrazole (11, step 3). These
reaction steps are related to our recent report that pyra-
+
ring geometry.[20] Interestingly, the P4 N1 bond in 5
ꢀ
(1.716(2) ꢀ; Wiberg bond index WBIP–N = 0.7752) is signifi-
+
ꢀ
ꢀ
cantly shorter than the respective P N bond in 10a (P1 N1
1.775(1) ꢀ, Figure 3; WBIP–N = 0.6551).[12] It has been pre-
viously observed that protonation of the sp2-type lone pair of
zolyl-substituted phosphanes react with secondary phos-
[21]
ꢀ
phanes to form P P bonds via the elimination of pyrazole
and constitute a variation of the classical preparation of
electrons of P-pyrazole moieties leads to a significant length-
phosphanylphosphonium moieties.[5] The lower covalent
[11]
2+
ening and weakening of the P N bond. The 31P{1H} NMR
bond order of the P P single bond in 15 as a result of
ꢀ
ꢀ
spectrum of dissolved 5[OTf] (CD2Cl2, 300 K) is depicted in
Figure 3. The chemical shift of the four-coordinate P atoms
(PX, d = 53.2 ppm) is in the expected range for phosphonium
moieties in four-membered rings.[6] The signal of the three-
coordinate phosphorus atom in 5+ is shifted to higher field
compared to the related acyclic derivative Cy2PP(pyr)PCy2
(d = 29.0 ppm; pyr= 3,5-dimethylpyrazolyl).[21] This may be
rationalized by the rather acute angles in the four-membered
ring, resulting in increased shielding similar to the observa-
tions made for 6a[OTf].
a substantial bond polarity (canonical structure 15b2+ vs.
15a2+) can be assumed by analogy with investigations on P-
phosphanyldiazaphopholenes.[22] In combination with a base-
induced redox reaction (pyrH, 11), this rationalizes the
formation of cation 5+ via the elimination of 10a+ in step 4.
We could confirm that compound 5+ is an intermediate in
the formation of cation 6a+ by reacting 5[OTf] with either
0.5 equiv of Cy2PH or 1 equiv of Ph2PH. The reaction of the
latter with 5[OTf] yields 6a[OTf], 10a[OTf], 3,5-dimethyl-
pyrazole (11), and remarkably diphosphane 16 (Scheme 5).[12]
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dynamically more favored owing to the presence of fused five-
and three-membered rings.[3b]
A side product of the synthesis of compound 6a[OTf] is
the pyrazoliumyl-substituted phosphane 10a+. This cation
results from the protonation of the formed Cy2Ppyr by
[pyrH][OTf] (9[OTf]). The latter is formed along with
diphosphane 16 from a comproportionation reaction of the
intermediately formed cation 10b+ and Ph2PH, which was
confirmed by a test reaction (Scheme 8).[12] Thus, all observed
products are accounted for by the proposed mechanism.
Scheme 5. Reaction of 5[OTf] with Ph2PH in a 1:1 ratio. a) ꢀ11,
CH2Cl2, 15 h, RT.
The [R2P] moiety of the secondary phosphane is not
incorporated into the framework of 6a+. This observation
has significant implications for the mechanism of the for-
mation of cation 6a+. We propose that the reaction of cation
5+ and Ph2PH yields the elusive neutral intermediate 17
(Scheme 6). A related tetraphosphete (3) has been previously
Scheme 8. Formation of 16 from 10b[OTf] and Ph2PH. a) CH2Cl2, 3 h,
RT.
Scheme 6. Formation of intermediate 17 from cation 5+ and Ph2PH.
The formation of a diphosphane as side product is
predominant in case of Ph2PH. However, the formation of
ꢀ
Cy2P PCy2 was not observed in reactions of 7[OTf]3 or
prepared.[4] The formation of 6a+ is then initiated by
a nucleophilic attack of 17 on the three-coordinate phospho-
rus atom of 5+ according to Scheme 7.[23] This attack most
5[OTf] with Cy2PH. Apparently, the formation of 10b+ is not
as favorable as the formation of 10a+. This explains why the
reaction of 7[OTf]3 and 3.5 equiv of Ph2PH proceeds less
selectively, preventing the isolation of 6b[OTf] (Scheme 2).[25]
Futhermore, we found that 6b[OTf] decomposes in solution
comparably fast to copious amounts of insoluble orange
residue and 16. The low stability of 6b+ is in line with previous
observations of substituent effects on polyphosphanes. Cy-
clohexyl substituents were found to yield products of higher
stability than those with phenyl substituents.[26] Nevertheless,
we obtained single crystals of 6b[OTf] co-crystallized with
9[OTf] as an n-hexane solvate from concentrated solutions of
the reaction mixture layered with n-hexane at ꢀ358C. The
structural parameters of the molecular structure of 6b+
(depicted in the Supporting Information) are very similar to
those of 6a+.
In summary, remarkable cationic polyphosphorus com-
pounds 6a,b+ were synthesized. These species represent the
first examples of polyphosphorus cations that feature two-,
three-, and four-coordinate phosphorus atoms in direct
Scheme 7. Plausible mechanism of the formation of cation 6a+.
ꢀ
ꢀ
likely results in the cleavage of the P P bond involving one of
connectivity. Overall, eight P P bonds are formed by
the four-coordinate P atoms and the formation of intermedi-
ate 18+. This mode of attack generates a Lewis basic
phosphanyl moiety, which can undergo intramolecular nucle-
ophilic attack, and furthermore results in an umpolung of the
initially two-coordinate phosphorus atom of compound 17.
Thus, the intramolecular attack may result in the formation of
intermediate 19+, in which the thermodynamically favored
five-membered ring[3b] of 6a+ is present. Intermediate 19+
may then eliminate Cy2Ppyr, yielding the fused bicyclo-
[3.2.0]heptaphosphane-1,3-ium cation 20+. This type of
framework is known for polyphosphanes.[24] However, inter-
a unique combination of protolysis and base-induced reduc-
+
ꢀ
tive P P coupling reactions. The syntheses of cations 6a,b is
an example of the distinct reactivity of phosphorus-centered
cations compared to neutral and anionic phosphorus-con-
taining compounds. We hope that further studies of the
reactivity of phosphorus-centered cations will make complex-
ity in phosphorus chemistry more accessible, broaden the
structural variety of polyphosphorus compounds, and may
also inspire the development of new synthetic methods for
related p-block-element compounds.
mediate
20+
may
rearrange
to
the
bicyclo-
Received: February 20, 2012
Revised: April 30, 2012
[3.1.0]hexaphosphane-1,3-ium cation 6a+, which is thermo-
Published online: && &&, &&&&
4
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ꢀ
polyphosphorus frameworks · P P bond formation ·
reductive coupling · structure elucidation
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ꢀ ꢀ
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[25] Comparative 31P{1H} NMR spectra of the reaction mixtures of
the preparation of 6a[OTf] and 6b[OTf] are depicted in the
Supporting Information.
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Angew. Chem. Int. Ed. 2012, 51, 1 – 6
ꢀ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
5
These are not the final page numbers!

.
Angewandte
Communications
Communications
Phosphorus Compounds
Accessible complexity: Polyphosphorus
frameworks [R4P4pyr]+ and [R6P7]+ (R=Cy,
Ph; pyr=3,5-dimethylpyrazolyl) were
prepared from a P1 source, R2PH. In
K.-O. Feldmann,
J. J. Weigand*
&&&& — &&&&
ꢀ
a one-pot reaction, eight P P bonds are
formed via a unique combination of
One-Pot Syntheses of Cationic
Polyphosphorus Frameworks with Two-,
Three-, and Four-Coordinate Phosphorus
substitution and base-induced reductive
ꢀ
P P coupling.
ꢀ
Atoms by One-Pot Multiple P P Bond
Formations from a P1 Source
6
www.angewandte.org
ꢀ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2012, 51, 1 – 6
These are not the final page numbers!