Controlled scrambling reactions to polyphosphanes: Via bond metathesis reactions

Article abstract of DOI:10.1039/c9sc04501e

Triphosphanes R′₂PP(R)PR′₂ (9a,c: R = Py; 9b R = BTz), 1,3-diphenyl-2-pyridyl-triphospholane 9d and pentaphospholanes (RP)₅ (13: R = Py; 18: R = BTz) are obtained in high yield of up to 98% from the reaction of dipyrazolylphosphanes RPpyr₂ (5: R = Py; 6: R = BTz; pyr = 1,3-dimethylpyrazolyl) and the respective secondary phosphane (R′₂PH, R′ = Cy (9a,b), ^tBu (9c); PhPH(CH₂)₂PHPh (9d)). The formation of derivatives 9a-d proceeds via a condensation reaction while the formation of 13 and 18 can only be explained by a selective scrambling reaction. We realized that the reaction outcome is strongly solvent dependent as outlined by the controlled scrambling reaction pathway towards pentaphospholane 13. In our further investigations to apply these compounds as ligands we first confined ourselves to the coordination chemistry of triphosphane 9a with respect to coinage metal salts and discussed the observation of different syn- and anti-isomeric metal complexes based on NMR and X-ray analyses as well as quantum chemical calculations. Methylation reactions of 9a with MeOTf yield triphosphan-1-ium Cy₂MePP(Py)PCy₂⁺ (10⁺) and triphosphane-1,3-diium Cy₂MePP(Py)PMeCy₂²⁺ (11²⁺) cations as triflate salts. Salt 11[OTf]₂ reacts with pentaphospholane 13 in an unprecedented chain growth reaction to give the tetraphosphane-1,4-diium triflate salt Cy₂MePP(Py)P(Py)PMeCy₂²⁺ (19[OTf]₂) via a P-P/P-P bond metathesis reaction. The latter salt is unstable in solution and rearranges via a rare [1,2]-migration of the Cy₂MeP-group followed by the elimination of the triphosph-2-en-1-ium cation [Cy₂MePPPMeCy₂]⁺ (20⁺) to yield a novel 1,4,2-diazaphospholium salt (21[OTf]).

Full text of DOI:10.1039/c9sc04501e

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Controlled scrambling reactions to
polyphosphanes via bond metathesis reactions†
Cite this: Chem. Sci., 2019, 10, 11054
b
Robin Schoemaker,a Kai Schwedtmann,a Antonio Franconetti,
All publication charges for this article
have been paid for by the Royal Society
of Chemistry
b
a
a
*
Antonio Frontera,
Felix Hennersdorf
and Jan J. Weigand
Triphosphanes R02PP(R)PR02 (9a,c: R ¼ Py; 9b R ¼ BTz), 1,3-diphenyl-2-pyridyl-triphospholane 9d and
pentaphospholanes (RP)5 (13: R ¼ Py; 18: R ¼ BTz) are obtained in high yield of up to 98% from the
reaction of dipyrazolylphosphanes RPpyr2 (5: R ¼ Py; 6: R ¼ BTz; pyr ¼ 1,3-dimethylpyrazolyl) and the
t
respective secondary phosphane (R02PH, R0 ¼ Cy (9a,b), Bu (9c); PhPH(CH2)2PHPh (9d)). The formation
of derivatives 9a–d proceeds via a condensation reaction while the formation of 13 and 18 can only be
explained by a selective scrambling reaction. We realized that the reaction outcome is strongly solvent
dependent as outlined by the controlled scrambling reaction pathway towards pentaphospholane 13. In
our further investigations to apply these compounds as ligands we first confined ourselves to the
coordination chemistry of triphosphane 9a with respect to coinage metal salts and discussed the
observation of different syn- and anti-isomeric metal complexes based on NMR and X-ray analyses as
well as quantum chemical calculations. Methylation reactions of 9a with MeOTf yield triphosphan-1-ium
Cy2MePP(Py)PCy2 (10+) and triphosphane-1,3-diium Cy2MePP(Py)PMeCy2
(112+) cations as triflate
+
2+
salts. Salt 11[OTf]2 reacts with pentaphospholane 13 in an unprecedented chain growth reaction to give
2+
Received 5th September 2019
Accepted 16th October 2019
the tetraphosphane-1,4-diium triflate salt Cy2MePP(Py)P(Py)PMeCy2 (19[OTf]2) via a P–P/P–P bond
metathesis reaction. The latter salt is unstable in solution and rearranges via a rare [1,2]-migration of the
Cy2MeP-group followed by the elimination of the triphosph-2-en-1-ium cation [Cy2MePPPMeCy2]+
(20+) to yield a novel 1,4,2-diazaphospholium salt (21[OTf]).
DOI: 10.1039/c9sc04501e
rsc.li/chemical-science
metathesis reactions (Scheme 1).5,6 In this regard, the reaction
of 1 with Cy2PH to give triphosphane 2 or iso-tetraphosphane 3
depending on the applied stoichiometry has been reported.5
The reaction of 1 with 1,2-bis(phenylphosphanyl)ethane yields
hexaphosphane 4 in a sequence of condensation and P–N/P–P
bond metathesis reactions, showing an example of a controlled
scrambling reaction.6 In our effort to further control these
metathesis reactions and driven by our general interest in azole-
Introduction
A wide range of polyphosphorus compounds, comprising
diverse bonding motifs, have been synthesised in the last few
decades.1,2 Many of the observed compounds are oen found to
be labile in solution and their rearrangement or decomposition
reactions are discussed to proceed via intermolecular scram-
bling reactions.3 Even though there are early reports on
controlling such scrambling reactions,4 the selective and high
yielding synthesis of certain polyphosphanes is still difficult,
hampering their subsequent chemistry. Besides classical
approaches for the generation of P–P bonds such as salt
metathesis reactions,2a,b dehalosilylation reactions2c or
condensation reactions,2d our group focusses on the develop-
ment of methodologies using tripyrazolylphosphane 1 or
derivatives thereof (vide infra) as a P1-building unit for P–P bond
forming reactions via condensation or P–N/P–P bond
aFaculty of Chemistry and Food Chemistry, TU Dresden, Chair of Inorganic Molecular
Chemistry, 01062 Dresden, Germany. E-mail: jan.weigand@tu-dresden.de
bDepartment of Chemistry, Universitat de Illes Balears, 07122 Palma de Mallorca,
Spain
Scheme
1 Application of tripyrazolylphosphane 1 in P–P bond
† Electronic supplementary information (ESI) available. CCDC 1950469–1950486.
For ESI and crystallographic data in CIF or other electronic format see DOI:
10.1039/c9sc04501e
formation reactions; (i) 2 Cy2PH, –2 pyrH, CH3CN; (ii) 3 Cy2PH, –3
pyrH, and CH3CN; (iii) 5 PhPH(CH2)2PHPh, 4 Ppyr3, –PhP(pyr)(CH2)2-
P(pyr)Ph, –10 pyrH, CH3CN.
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and azine-substituted phosphanes, we explored the synthesis of
phosphanes with a pyridyl- or benzothiazolyl-substituent bound
to phosphorus via the carbon atom. This limits the number of
P–N bound pyrazolyl-substituents and therefore the possible
reaction pathways by P–N/P–P bond metathesis. For a better
overview and in thought of a systematic approach we organised
our general types of phosphanes with respect to their number of
P–N bound substituents as type I to type IV (Scheme 2). In type I
phosphanes all three substituents are bound via P–N bonds
whereas a type II phosphane is dened by two P–N bonds and
one P–C bond. This order is maintained in type III phosphanes
with one P–N and two P–C bonds and is nalised in type IV
phosphanes featuring only P–C bonds. Importantly, all
substituents at the phosphorus atom have an additional sp2
hybridised donor atom such as nitrogen, being essential as
a further reaction or coordination site and for the possible
formation of resonance stabilised reaction intermediates. In
this contribution, we report the synthesis of the hitherto
unknown type II phosphanes carrying either a pyridyl- or
benzothiazolyl-substituent and their use for the generation of
triphosphanes, a triphospholane and pentaphospholanes and
selected one triphosphane to explore its coordination chemistry
with respect to coinage metal salts.
Scheme 3 Synthesis of dipyrazolylphosphanes 7 and 8; R ¼ Py; (i) 2 eq.
pyrSiMe3, neat, –2 Me3SiCl; R ¼ BTz; (i) 2 eq. pyrSiMe3, Et2O, –2
Me3SiCl.
8. Their molecular structures are depicted in Fig. 1. The P–C
˚
˚
bond lengths (7: P1–C1 1.827(1) A; 8: P1–C1 1.828(2) A) are in
10
˚
the expected range of a P–C single bond (1.83 A), involving
a tri-coordinate phosphorus atom. For both phosphanes the
˚
˚
P–N bond lengths (7: P1–N4 1.721(1) A, P1–N2 1.724(1) A; 8 P1–
0
˚
N2/N2 1.719(1) A) are in accordance with those reported for
tri(1H-pyrazol-1-yl)phosphane (1.714(4) A). In continuation,
11
˚
we carried out the reaction of dipyrazolylphosphanes 7 and 8
with secondary phosphanes R02PH (R0 ¼ Cy, tBu or R02PH ¼
PhP(H)C2H4P(H)Ph) to give triphosphanes 9a–c and triphos-
pholane 9d with concomitant release of two equivalents of 3,5-
dimethylpyrazole (Scheme 4). Compounds 9a–d readily precip-
itate from the reaction mixture and are isolated by ltration.
Subsequent washing with CH3CN and drying in vacuo give 9a–
d as colourless solids in good to excellent yield (96% (9a); 93%
(9b); 77% (9c); 69% (9d)). The 31P NMR spectra of all
compounds show AX2 spin systems which are characteristic of
triphosphanes and 1,2,3-triphospholanes (Fig. 2). The NMR
spectroscopic parameters are listed in Table 1. Single crystals
for X-ray analysis are obtained by slow vapour diffusion of n-
pentane into saturated CH2Cl2 solutions of 9a–d. The molecular
structures of 9a–d are depicted in Fig. 3. Their structural
parameters compare very well with those of similar triphos-
phanes12 and are thus not discussed in detail. Only the bond
angle of the P–P–P moiety in compound 9c is approximately 10ꢀ
wider which can be explained by the sterically demanding tert-
butyl substituents. In recent years numerous examples of the
coordination chemistry of anionic13 and zwitterionic14 oligo-
phosphorus compounds towards transition metals have been
Results and discussion
The targeted type II phosphanes are synthesised by the reaction
of the dichlorophosphanes RPCl2 (R ¼ Py (5); R ¼ BTz (6))7,8 with
two equivalents of 3,5-dimethyl-1-(trimethylsilyl)pyrazole
(Scheme 3).9 The released chloro(trimethyl)silane can be
removed under reduced pressure aer 16 hours and no further
purication is necessary. Both phosphanes 7 and 8 are conve-
niently prepared on a multi gram scale in excellent yields (97%
(7); 96% (8)). The 31P NMR spectra of the dipyrazolylphosphanes
show a singlet resonance in each case (7: d(P) ¼ 44.8 ppm; 8: d(P)
¼ 38.0 ppm) being signicantly shied to a higher eld
compared to the corresponding dichlorophosphanes (5: d(P) ¼
138.7 ppm; 6: d(P) ¼ 132.0 ppm). Suitable crystals for X-ray
analysis of phosphanes 7 and 8 are obtained by slow vapour
diffusion of n-pentane into a saturated CH2Cl2 solution of 7 and
Fig. 1 Molecular structures of 7 (left) and 8 (right) (hydrogen atoms are
omitted for clarity and thermal ellipsoids are displayed at 50% proba-
bility). Selected bond lengths (A˚) and angles (ꢀ): (7) P1–C1 1.827(1), P1–
Scheme 2 Classification of N-heterocyclic substituted phosphanes N4 1.721(1), P1–N2 1.724(1), C1–P1–N4 101.28(5), N4–P1–N2
into type I to type IV phosphanes (n is the number of P–N bonds) with 103.26(5), N2–P1–C1 100.18(5); (8) P1–C1 1.828(2), P1–N2 1.719(1),
an emphasis on the type II phosphanes presented in this contribution. C1–P1–N2 101.26(5), N2–P1–N20 103.84(7).
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metal centre.15 Starting from stable triphosphanes only two
complexes coordinating to metal carbonyls have been struc-
turally characterised (Scheme 5, top),5,16 while another example
is described in solution.17 In complex I, the triphosphane ligand
chelates the W(CO)4 moiety while in complex II the coordina-
tion of the Fe(CO)4 moiety proceeds only via the free electron
pair of one of the dicylcohexylphosphanyl groups. Featuring an
additional nitrogen based donor site we envisioned the forma-
tion of multi-dentate coinage metal complexes using triphos-
phane 9a, exemplarily. This yielded the rst triphosphane
coordination complexes of Cu(I), Ag(I) and Au(I) (Scheme 5).
Reacting 9a with an equimolar amount of Ag[OTf] in uo-
robenzene at room temperature leads to the formation of
a colourless precipitate which is isolated by ltration to yield
the dinuclear complex [(9a*Ag)2][OTf]2 in essentially quantita-
tive yield. The corresponding reaction of 9a with [(MeCN)4Cu]
[OTf] in CH2Cl2 gives colourless plates upon slow vapour
diffusion of n-pentane at ꢁ30 ꢀC which can be isolated as
[(9a*Cu)2][OTf]2 up to a yield of 94%. Further transmetallation
reaction of [(9a*Ag)2][OTf]2 with (tht)AuCl in MeCN gives
[(9a*Au)2][OTf]2 in 64% yield by the concomitant precipitation
of AgCl. Slow diffusion of n-pentane into a CH2Cl2 solution of 9a
and [(MeCN)4Cu][OTf] in a 2 : 1 ratio gives crystalline [(9a)2*Cu]
[OTf] in 93% yield. Formation of (9a*CuBr)2 is achieved by slow
diffusion of n-pentane into a THF solution of equimolar
amounts of 9a and (tht)CuBr. The molecular structures of the
aforementioned coinage metal complexes are shown in Fig. 4
and the structural parameters are summarised in Table 2.
Different from the known coordination motifs for I and II, the
molecular structures of [(9a*Cu)2][OTf]2, [(9a*Ag)2][OTf]2 and
[(9a*Au)2][OTf]2 reveal dinuclear metal complexes featuring two
triphosphane ligands. In addition to the terminal coordinating
phosphorus atoms, the nitrogen atoms of the pyridyl-
substituents are involved in the coordination resulting in an
approximately tetrahedral coordination environment of the
respective metal centre. While the molecular structures of the
dinuclear complexes [(9a*Cu)2][OTf]2 and [(9a*Ag)2][OTf]2
reveal an anti-arrangement with inversion symmetry, the gold
complex [(9a*Au)2][OTf]2 crystallises as a centro-symmetrical
syn-isomer. The inversion process within the complexes has
Scheme 4 Synthesis of triphosphanes 9a–c: (i) +2 eq. R02PH, CH3CN,
–2 pyrH; and triphospholane 9d: (ii) 1 eq. PhPH(CH2)2PHPh, CH3CN,
–2 pyrH.
Fig. 2 31P NMR spectra of compounds 9a–d.
Table 1 31P NMR parameters of compounds 9a–d
Compound
Chemical shis (ppm)
1JPP (Hz)
9a
9b
9c
9d
ꢁ57.3
ꢁ58.1
ꢁ44.1
ꢁ12.1
ꢁ7.7
ꢁ251
ꢁ268
ꢁ307
ꢁ260
ꢁ0.7
37.0
14.5
reported. Also, few examples of neutral triphosphanes coordi-
nated to transition metals are known, however, the triphos-
phane moiety was assembled while coordinated to at least one
Fig. 3 Molecular structures of triphosphanes 9a–c and triphospholane 9d (hydrogen atoms are omitted for clarity and thermal ellipsoids are
displayed at 50% probability). Selected bond lengths (A˚) and angles (ꢀ) (9a): P1–P2 2.2301(3), P1–P3 2.2371(3), P3–P1–P2 97.38(1) (9b); P1–P2
2.2377(5), P2–P3 2.2234(5), P1–P2–P3 98.11(2) (9c); P1–P2 2.2181(3), P1–P3 2.2485(3), P2–P1–P3 107.64(1) (9d); P1–P2 2.270(2), P2–P3 2.197(2),
P1–P2–P3 97.16(6).
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been investigated by variable temperature NMR experiments
and theoretical calculations (vide infra). The difference in the
arrangement in the solid state causes a widening of the P–M–P
angles in the copper complex [(9a*Cu)2][OTf]2 (P30-Cu1-P1
136.52(2)ꢀ) compared to the silver complex [(9a*Ag)2][OTf]2
(P1–Ag1–P30 143.96(3)ꢀ) to an almost linear geometry around
the gold atoms in [(9a*Au)2][OTf]2 (P1–Au1–P30 171.52(3)ꢀ). The
distances between the silver atoms in [(9a*Ag)2][OTf]2 and the
gold atoms in [(9a*Au)2][OTf]2 indicate the presence of argen-
tophilic and aurophilic interactions, respectively.18 As the
distance between the copper atoms in [(9a*Cu)2][OTf]2 is
slightly larger than the sum of the van der Waals radii of two
19
˚
copper atoms (2.80 A), signicant van der Waals interactions
can be excluded. The observed P–M bond lengths in [(9a*M)2]
[OTf]2 as well as in [(9a)2*Cu][OTf] and (9a*CuBr)2 are in good
agreement with bond lengths reported for the structurally
similar [M2(dcpm)2]2+ cations (M ¼ Cu(I), Ag(I) or Au(I); dcpm ¼
bis(dicylcohexylphosphanyl)methane).20 For (9a*CuBr)2 the
formation of the typical Cu2X2 (X ¼ halogen) geometry is found,
only marginally differing from the typical planar arrangement
reported for many Cu2X2-type structures.21 Compared to free
ligand 9a, the P–P bond lengths and the P–P–P angles in the
coinage metal complexes are only marginally altered. The room
temperature NMR spectra of the dinuclear complexes [(9a*Au)2]
[OTf]2 and [(9a*Ag)2][OTf]2 show broadened resonances due to
dynamic processes in solution.22 The low temperature 31P NMR
spectrum of [(9a*Au)2][OTf]2 shows two separate AA0XX0X00X000
spin systems at d(PA) ¼ ꢁ34.2 ppm, d(PX) ¼ 52.6 ppm and d(PA)
¼ ꢁ37.8 ppm, d(PX) ¼ 50.3 ppm corresponding to the respective
syn- and anti-isomers (Fig. 5). Details of the coupling constants
are included in the ESI.† For [(9a*Ag)2][OTf]2 two isomers with
two separate AA0XX0X00X000 spin systems at d(PA) ¼ ꢁ48.5 ppm,
d(PX) ¼ 36.1 ppm and d(PA) ¼ ꢁ51.5 ppm, d(PX) ¼ 36.1 ppm are
Scheme 5 Known transition metal complexes featuring a neutral tri-
phosphane ligand (top); synthesis of coinage metal coordination
complexes starting from triphosphane 9a (bottom); (i) 2 eq. Ag[OTf],
PhF (M ¼ Ag); 2 eq. [(MeCN)4Cu][OTf], CH2Cl2, –8 MeCN (M ¼ Cu); (ii) 2
eq. (tht)AuCl, MeCN, –2 tht, –2 AgCl; (iii) 1 eq. [(MeCN)4Cu][OTf],
CH2Cl2, –4 MeCN; (iv) 2 eq. (tht)CuBr, THF, –2 tht.
Fig. 4 Molecular structures of the coinage metal complexes [(9a*Cu)2][OTf]2, [(9a*Ag)2][OTf]2, [(9a*Au)2][OTf]2, [(9a)2*Cu][OTf] and (9a*CuBr)2
(hydrogen atoms, anions and solvate molecules are omitted for clarity, cyclohexyl groups are represented only by their phosphorus bound
carbon atoms, and thermal ellipsoids are displayed at 50% probability; left). Selected geometrical parameters are given in Table 2; Optimised
geometries of [(9a*M)2][OTf]2 using the BP86-D3/def2-TZVP functional, showing the different coordination arrangement of the pyridyl-moieties
towards the metal atoms (M ¼ Cu, Ag, and Au; right).
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Table 2 Selected geometrical parameters of crystallographically characterised coinage metal complexes [(9a*Cu)2][OTf]2, [(9a*Ag)2][OTf]2,
[(9a*Au)2][OTf]2, [(9a)2*Cu][OTf] and (9a*CuBr)2
[(9a)2*Cu][OTf]a
(9a*CuBr)2
a
[(9a*Cu)2][OTf]2
[(9a*Ag)2][OTf]2
[(9a*Au)2][OTf]2
0
˚
M1–M1 in A
2.8225(5)
2.2755(4)
2.2176(5)
2.048(1)
2.225
2.9013(4)
2.3818(8)
2.4468(7)
2.404(3)
2.224
2.880(3)
2.3033(8)
2.3198(8)
2.948(3)
2.226
—
2.243
2.942
2.191
˚
M1–P10 in A
˚
M1–P3 in A
˚
M–N in A
2.106
2.212
—
2.116
2.214
—
a
˚
P–P in A
P1-M1-P3ꢀin ꢀ0
136.52(2)
98.26(2)
143.96(3)
100.16(4)
171.52(3)
98.29(5)
P–P–P in
104.43
102.43
a
Average bond lengths and angles are given.
[(9a*Au)2]2+ is favored by 1.7 kcal molꢁ1 compared to its anti-
isomer. The optimised geometries also show the different
coordination behaviour of the pyridyl-moieties in the three
metal complexes [(9a*M)2]2+. While the free electron pair of the
pyridyl nitrogen atom is pointing towards the metal centre in
[(9a*Cu)2]2+ and [(9a*Ag)2]2+ it is pointing towards the middle of
the Au–Au bond in [(9a*Au)2]2+ (Fig. 4, right). Our calculations
furthermore indicate that the isomerisation is not driven by
rotation but proceeds via dissociation of the M(I)–N coordina-
tion bond or pseudo-coordination in the case of [(9a*Au)2]2+ and
subsequent association of the ligand on the opposite side
(Fig. 6). This interconversion is the highest in energy for
[(9a*Cu)2]2+ (14.7 kcal molꢁ1), followed by [(9a*Ag)2]2+
(13.2 kcal molꢁ1) and nally [(9a*Au)2]2+ (9.6 kcal molꢁ1).23
Further reactivity studies on triphosphane 9a revealed that
this compound readily rearranges in solution by means of a P–
P/P–P bond metathesis reaction. Aer 24 h the CH2Cl2 solution
of 9a shows an additional singlet resonance at d(P) ¼ 21.0 ppm
which we assign to tetracyclohexyldiphosphane (12)24 and an
AA0BB0C spin system at d(P) ¼ 14.0–25.2 ppm being charac-
teristic of pentaphospholanes of type (RP)5.24,25 As this scram-
bling reaction is in a state of equilibrium the isolation of 13 is
hampered. However, the reaction of pure 13 with Cy4P2
yielding 9a illustrates that pentaphospholane 13 might be used
as a PyP-synthon as it inserts into the P–P bond of Cy4P2. The
selective synthesis of pentaphospholane 13 is achieved by the
equimolar reaction of 7 with Cy2PH in Et2O as a non-polar
solvent (Scheme 6). The formed precipitate is ltered and
Fig. 5 31P NMR spectrum of [(9a*Au)2][OTf]2 (CD2Cl2, 243 K, insets
show the two AA0XX0X00X000 spin systems of the experimental (upwards)
and fitted spectra (downwards)).
also observed. However, further line splitting can be observed as
a result of the complexation with the Ag107/Ag109 nuclei, which
made us refrain from iteratively tting the corresponding 31P
NMR spectrum. The 31P NMR spectrum of the dinuclear copper
complex [(9a*Cu)2][OTf]2 shows only the resonances of one
AA0XX0X00X000 spin system at d(PA) ¼ ꢁ39.1 ppm and d(PX) ¼
15.4 ppm. An additional broadened AX2 spin system (d(PA) ¼
ꢁ37.5 ppm and d(PX) ¼ 15.3 ppm (1J(PAPX) ¼ ꢁ168 Hz)) gives
rise to the formation of a monomeric species in solution which
was not further investigated. A comparable AX2 spin system is
observed for [(9a)2*Cu][OTf] (d(PA) ¼ ꢁ44.8 ppm and d(PX) ¼
ꢁ8.9 ppm (1J(PAPX) ¼ ꢁ275 Hz)) as well as for (9a*CuBr)2 (d(PA)
¼ ꢁ46.6 ppm and d(PX) ¼ ꢁ6.2 ppm (1J(PAPX) ¼ ꢁ253 Hz)). For
both mononuclear copper complexes the resonances in the 31
P
NMR spectrum are noticeably broadened due to the fast quad-
63
rupole relaxation of the Cu nucleus.22,23 Theoretical calcula-
tions performed at the BP86-D3/def2-TZVP level of theory
support the isomerisation in the dinuclear metal complexes
[(9a*M)2]2+. For [(9a*Cu)2]2+ we found a relative energy differ-
ence between the syn- and the anti-isomer of 5.0 kcal molꢁ1
which explains that only one isomer is observed in the 31P NMR
spectrum. While the two isomers of [(9a*Ag)2]2+ are isoenergetic
with a difference of 0.1 kcal molꢁ1, the syn-isomer of
Fig. 6 Interconversion mechanism of the syn- and anti-isomers of
[(9a*M)2]2+; (M ¼ Cu, Ag, and Au).
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Scheme 6 Equilibrium reaction of 9a with 13 and 12 and selective
synthesis of 13 by the reaction of 7 with Cy2PH; (i) Et2O, –pyrH.
dried in vacuo giving 13 in 98% yield as a colourless powder
which shows the AA0BB0C spin system in the 31P NMR spectrum
(Fig. 7). Dicyclohexyl(3,5-dimethylpyrazolyl)phosphane (14) is
identied as a side product in the supernatant solution by its
singlet resonance in the 31P NMR spectrum (d(P) ¼ 58.4 ppm)
indicating a P–N/P–P bond metathesis reaction.6 To further
understand the formation of 13 we studied the reaction in
CD2Cl2 by means of 31P NMR spectroscopy (Fig. 8). Aer 3.5 h
compound 7 is still present in the solution while Cy2PH is fully
consumed by the condensation reaction with 7 to form tri-
phosphane 9a (vide supra) and diphosphane 15 which is
identied by an AX spin system (d(PA) ¼ 10.2 ppm and d(PX) ¼
33.4 ppm; 1J(PAPX) ¼ ꢁ281 Hz). The AMX spin system at d(PA) ¼
ꢁ35.6 ppm, d(PM) ¼ ꢁ7.2 ppm and d(P)X ¼ 29.9 ppm is
assigned to triphosphane 16 which is formed via a P–N/P–P
bond metathesis reaction of two equivalents of 15 and via a P–
N/P–P bond metathesis reaction of 7 and 9a (Scheme 7). In
both cases phosphane 14 is liberated. We propose a further
chain growth in which triphosphane 16 reacts with 15 with the
release of phosphane 14 in a P–N/P–P oligomerisation reaction
which ultimately yields pentaphospholane 13. We furthermore
propose the formation of tetraphosphetane 17 (d(P) ¼ ꢁ49.4
ppm) which can be formed via the reaction of two equivalents
16 with concomitant release of 14 (Scheme 7). Similar to the
known ring expansion reactions of certain cyclophosphanes,26
17 might react to form pentaphospholane 13, being the
Fig. 8 31P NMR spectrum of the reaction of 7 with Cy2PH in a 1 : 1 ratio
in CD2Cl2 after 3.5 h; unidentified products are marked with asterisks.
thermodynamically favoured product. In analogy to 13 we are
able to isolate the corresponding benzothiazolyl substituted
pentaphospholane 18.27 Crystals suitable for X-ray analysis are
obtained for both compounds by the slow diꢀffusion of n-
pentane into saturated CH2Cl2 solutions at ꢁ30 C (Fig. 9).
Both compounds reveal the typical envelope conformation of
the P5-ring featuring the substituents in the all-trans position.
The P–P bond lengths observed for both pentaphospholanes 13
˚
˚
(av. P–P 2.218 A) and 18 (av. P–P 2.246 A) are in a range known
˚
for pentaphospholanes such as (PhP)5 (av. 2.217 A); also the
P–P–P angles are in good accordance with the reported data for
(PhP)5.28 We subsequently envisioned the use of 13 as a PyP-
synthon for insertion reactions into P–P bonds which was
already indicated by the aforementioned scrambling reaction
between 9a, Cy4P2 and 13 (see Scheme 6).
As similar scrambling reactions might be expected for tri-
phosphanium and triphosphanediium salts, we methylated
triphosphane 9a with different amounts of MeOTf. The 31P
Fig. 7 31P NMR spectrum of pentaphospholane 13 (CD3CN, 300 K, the
inset shows the AA0BB0C spin system of the experimental (upwards) Scheme 7 Possible formation of 16 via P–N/P–P bond metathesis
and fitted spectra (downwards)).
reactions.
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Fig. 10 Molecular structures of 10+ in 10[OTf] (left) and 112+ in 11
[OTf]2 (hydrogen atoms and anions are omitted for clarity and thermal
ellipsoids are displayed at 50% probability). Selected bond lengths (A˚)
and angles (ꢀ) (10+): P1–P2 2.1956(5), P2–P3 2.2362(5), P1–P2–P3
97.89(2) (112+); P1–P2 2.2152(4), P2–P3 2.2182(4), and P1–P2–P3
107.43(1).
Fig. 9 Molecular structures of the pentaphospholanes 13 (left) and 18
(right) (hydrogen atoms and solvate molecules are omitted for clarity
and thermal ellipsoids are displayed at 50% probability). Selected bond
lengths (A˚) and angles (ꢀ) (13): P1–P2 2.2222(4), P2–P3 2.2200(5); P3–
P4 2.22200(5), P4–P5 2.2078(4), P1–P2–P3 97.25(2), P2–P3–P4
95.59(2), P3–P4–P5 102.27(2), P4–P5–P1 106.77(2), P5–P1–P2
97.14(2) (18); P1–P2 2.2336(6), P2–P3 2.2232(6), P3–P4 2.338(6), P4–
P5 2.2069(6), P5–P1 2.2288(6), P1–P2–P3 93.54(2), P2–P3–P4
100.29(2), P3–P4–P5 107.27(2), P4–P5–P1 96.32(2), P5–P1–P2
96.85(2).
+
2.230(1) A).29b The P–P–P bond angle in 10 (97.89(2)ꢀ) is similar
˚
to that in compound 9a (97.16(6)ꢀ). The P–P bond lengths in
112+ (P1–P2 2.2152(4) A and P2–P3 2.2182(4) A) are marginally
shorter compared to those in 9a but in accordance with bond
lengths reported for similar triphosphane-1,3-diium cations
˚
˚
2+
29b
˚
˚
(compare [Me3P–P(Cy)-PMe3] : 2.1979(5) A and 2.1976(6) A).
The P–P–P bond angle in 112+ (107.43(1)ꢀ) is signicantly wider
compared to those in 9a and [Me3P–P(Cy)-PMe3]2+ (103.11(2)ꢀ).
In order to investigate the envisioned PyP-transfer into a P–P
bond, we reacted 13 with 5 equivalents of 11[OTf]2 in CH3CN
(Scheme 9). Upon addition of 13 to a colourless CH3CN solution
of 11[OTf]2 the reaction mixture turns from deep red to pale
yellow. The 31P NMR spectrum of the reaction mixture aer 24 h
shows two AA0XX0 spin systems which are attributed to two
isomers of tetraphosphane-1,4-diium cations 192+ (Fig. 11),
indicating the successful PyP-transfer via a P–P/P–P bond
metathesis reaction. The A part of the prominent AA0XX0 spin
system at d(PA) ¼ ꢁ55.1 ppm is assigned to the inner pyridyl-
substituted P nuclei and the X part at d(PX) ¼ 40.7 ppm to the
tetra-coordinate phosphorus atoms which is similar to known
tetraphosphane-1,4-diium salts.32 The minor spin system reso-
nates at d(PA) ¼ ꢁ40.1 ppm and d(PX) ¼ 39.8 ppm, respectively.
Aer work-up compound 19[OTf]2 can be isolated analytically
pure in 51% yield. X-ray analysis revealed that the centrosym-
metric meso-isomer 192+ crystallized showing a central torsion
angle of 180ꢀ (Fig. 11). The P–P bond lengths in 192+ (P1–P2
NMR spectrum of the 1 : 1 reaction of 9a with MeOTf shows an
AMX spin system (d(PA) ¼ ꢁ46.2 ppm, d(PM) ¼ ꢁ12.9 ppm and
d(PX) ¼ 34.2 ppm (1J(PAPM) ¼ ꢁ290 Hz, 1J(PAPX) ¼ ꢁ281 Hz, and
2J(PMPX) ¼ 58 Hz)), suggesting the methylation of the phos-
phorus atom rather than the nitrogen atom of the pyridyl-
substituent. Aer the removal of all volatiles in vacuo
triphosphan-1-ium triate salt 10[OTf] can be isolated in
quantitative yield as a colourless powder. A second methylation
is achieved when 9a is reacted with an excess of MeOTf under
solvent free conditions (Scheme 8). Similar to the aforemen-
tioned reaction, only the phosphorus atoms are methylated
giving triphosphane-1,3-diiumtriate salt 11[OTf]2. As both
dicyclohexylphosphanyl moieties are methylated, the 31P NMR
spectrum of 10[OTf]2 shows an AX2 spin system with resonances
at d(PA) ¼ ꢁ67.9 ppm and d(PX) ¼ 44.0 ppm (1J(PAPX) ¼ ꢁ315
Hz). The molecular structures of cation 10+ and dication 112+
are depicted in Fig. 10. Note that a small number of triphos-
phanediium salts are reported in the literature and compound
10[OTf] extends the library of cationic, catenated phosphorus
compounds as a structurally characterised triphosphan-1-ium
salt.29–31 The P1–P2 bond length in cation 10+ (P1–P2 2.1956(5)
0
˚
˚
2.234(1) A and P2–P2 2.232(2) A) are in good agreement with the
reported data for comparable tetraphosphane-1,4-diium cations
(compare [Ph3P–(PPh)2–PPh3]2+: P1–P2 2.258(1) A, P2–P20
˚
˚
˚
A) is shortened whereas the P2–P3 bond (P2–P3 2.2362(5) A) is
2.221(1) A).32 Similar to the reported PhP-transfer from (PhP)5 to
˚
˚
comparable to those in 9a (P1–P2 2.2301(3) A, P1–P3 2.2371(3)
a NHC,33 13 can be used as a PyP-synthon, thus featuring an
+
˚
A) or similar diphosphanium cations (compare [Ph2P–PPh3] :
Scheme 8 Methylation reactions of 9a; (i) 1 eq. MeOTf, Et2O; (ii) 5 eq. Scheme
9
Synthesis of tetraphosphane-1,4-diium triflate salt 19
MeOTf, neat.
[OTf]2; (i) MeCN.
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Fig. 13 Molecular structure of 1,4,2-diazaphospholium 21+ in 21[OTf]
(left) and 3l5-triphosph-2-en-1-ium 20+ in 20[OTf] (right) (hydrogen
atoms and anions are omitted for clarity and thermal ellipsoids are
displayed at 50% probability). Selected bond lengths (A˚) and angles (ꢀ)
(20+): P1–C1 1.733(1), P1–N2 1.741(1), C1–P1–N2 87.08(6) (20); P1–P2
2.134(4), P2–P3 2.1382(4), P1–P2–P3 103.85(2).
Fig. 11 31P NMR spectrum of dication 19[OTf]2 (CD2Cl2, 300 K, the
inset shows the AA0XX0 spin system of the experimental (upwards) and
fitted spectra (downwards)); molecular structure of 192+ in 19[OTf]2
(hydrogen atoms and anions are omitted for clarity; thermal ellipsoids
are displayed at 50% probability). Selected bond lengths (A˚) and angles
(ꢀ):P1–P2 2.234(1), P2–P20 2.232(2), and P1–P2–P20 97.60(6).
structural parameters of 20+ are similar to those observed for the
cation [Ph3P–P–PPh3]+, i.e. slightly shortened P–P bond lengths
(P1–P2 2.134(4), P2–P3 2.1382(4)) and a P–P–P bond angle of
103.85(2)ꢀ.34 By co-crystallisation, a further product was identied
as the triate salt of diazaphospholium cation 21+ (Fig. 13). We
assign this species to the pseudo triplet resonance observed in the
additional reaction or coordination site. When compound 19
[OTf]2 was kept in a CD3CN solution for 14 days, the 31P NMR
spectrum shows three rearrangement products (Fig. 12). Next to
an AX2 spin system (d(PA) ¼ ꢁ235.3 ppm and d(PX) ¼ 38.8 ppm
(1J(PAPX) ¼ ꢁ501 Hz)), a pseudo triplet resonance (d(P) ¼
153.1 ppm; 1J(PN) ¼ 50 Hz) and an AMXY spin system (d(PA) ¼
ꢁ208.2 ppm, d(PM) ¼ ꢁ30.0 ppm, d(PX) ¼ 31.1 ppm, and d(PY) ¼
39.8 ppm; 1J(PAPy) ¼ ꢁ490 Hz, 1J(PMPX) ¼ ꢁ342 Hz, 1J(PAPM) ¼
ꢁ298 Hz, 2J(PMPY) ¼ 91 Hz, 2J(PAPX) ¼ 45 Hz, 3J(PXPY) ¼ 34 Hz)
are observed. The AX2 spin system can be attributed to the 3l5-
triphosph-2-en-1-ium cation (20+), as the spectroscopic param-
eters are in the range of those observed for [Ph3P–P–PPh3]+
31
˚
P NMR spectrum. The P–C bond distance of 1.733(1) A slightly
exceeds the upper limit of typical P]C bond lengths (1.61–1.71
36
˚
˚
A), while the P–N bond length of P1–N2 1.741(1) A indicates
a P–N single bond (P–N: 1.78 A).37 Furthermore, the structure of
˚
21+ reveals an acute angle around the P atom (C1–P1–N2
87.08(6)ꢀ) and a planar arrangement, suggesting a delocalized p-
system. We assign the AMXY spin system to asymmetric tetra-
phosphanediium dication 222+, most likely formed by an intra-
molecular aromatic substitution reaction of 192+. This can be
attributed to be the rst step of the rearrangement reaction of
192+ to form 20+ and 21+. We further studied this rearrangement
reaction using the TURBOMOLE 7.0 program at the PB86-D3/
def2-TZVP level of theory (see ESI for details†) and taking into
consideration solvent effects by using the Conductor-like
Screening Model (COSMO). Our calculations started from 22+
and show that the cleavage of Cy2PMe is energetically quite costly.
This is in accordance with the 31P NMR spectrum as Cy2PMe is
not observed (d(P) ¼ ꢁ19.8 ppm).38 More likely and energetically
favoured is a [1,2]-shi of the Cy2PMe group to form iso-222+ with
a reaction barrier of +22.0 kcal molꢁ1. Fig. 14 shows the opti-
mised structures for this [1,2]-Cy2PMe shi reaction. It is note-
worthy that similar [1,2]- and [1,3]-PR3 shis have been
reported.39 Compared to 222+, iso-222+ is almost isoenergetic and
1
(d(PA) ¼ ꢁ174 ppm and d(PX) ¼ 30 ppm; J(PAPX) ¼ ꢁ500 Hz)
which was rst reported by Schmidpeter and co-workers.34
A
variety of similar derivatives has been synthesised and reported
by the group of Macdonald.35 Slow vapour diffusion of Et2O into
the CD3CN solution yielded crystals suitable for X-ray analysis
conrming the structural connectivity of 20+ (Fig. 13). The
Fig. 12 31P NMR spectrum of a solution of 19[OTf]2 in CD3CN after 14
days.
Fig. 14 Optimised structures of 222+, iso-222+ and the transition state.
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a controlled stepwise manner in order to further investigate
their fascinating reaction space.
Conflicts of interest
There are no conicts to declare.
Acknowledgements
We thank the German Science Foundation (DFG Grant number
WE 4621/3-1), the European Regional Development Fund, the
Free State of Saxony (ERDF-InfraPro, GEPARD-100326379) and
the European Research Council (ERC starting grand, SynPhos-
307616) for nancial support. A. F. thanks MINECO/AEI of
Spain (project CTQ2017-85821-R FEDER funds) for nancial
support. A. F. thanks the MINECO of Spain for a “Juan de la
Cierva” contract. We also thank Philipp Lange for experimental
assistance and EA measurements.
Scheme 10 Rearrangement reaction of 222+ to 20+ and 21+ calcu-
lated at the PB86-D3/def2-TZVP level of theory.
readily reacts in an exergonic step (ꢁ31.9 kcal molꢁ1) to form
cations 20+ and 21+ with a low barrier of only 1.9 kcal molꢁ1 (see
Scheme 10 and ESI for details†).
Notes and references
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In summary we have shown that phosphanes which we denote
as type II phosphanes (two P–N bonds and one P–C bond) can
be used to build polyphosphorus compounds featuring
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