Di-tert-butyldiphosphatetrahedrane: Catalytic Synthesis of the Elusive Phosphaalkyne Dimer

Article abstract of DOI:10.1002/anie.201910505

While tetrahedranes as a family are scarce, neutral heteroatomic species are all but unknown, with the only reported example being AsP₃. Herein, we describe the isolation of a neutral heteroatomic X₂Y₂ molecular tetrahedron (X, Y=p-block elements), which also is the long-sought-after free phosphaalkyne dimer. Di-tert-butyldiphosphatetrahedrane, (tBuCP)₂, is formed from the monomer tBuCP in a nickel-catalyzed dimerization reaction using [(NHC)Ni(CO)₃] (NHC=1,3-bis(2,4,6-trimethylphenyl)imidazolin-2-ylidene (IMes) and 1,3-bis(2,6-diisopropylphenyl)imidazolin-2-ylidene (IPr)). Single-crystal X-ray structure determination of a silver(I) complex confirms the structure of (tBuCP)₂. The influence of the N-heterocyclic carbene ligand on the catalytic reaction was investigated, and a mechanism was elucidated using a combination of synthetic and kinetic studies and quantum chemical calculations.

Full text of DOI:10.1002/anie.201910505

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Accepted Article
Title: Di-tert-butyldiphosphatetrahedrane: Catalytic Synthesis of the
Elusive Phosphaalkyne Dimer
Authors: Gabriele Hierlmeier, Peter Coburger, Michael Bodensteiner,
and Robert Wolf
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10.1002/anie.201910505
Angewandte Chemie International Edition
Di-tert-butyldiphosphatetrahedrane: Catalytic Synthesis of the
Elusive Phosphaalkyne Dimer
Gabriele Hierlmeier,[a] Peter Coburger,[a] Michael Bodensteiner,[a] and Robert Wolf*[a]
Abstract: While tetrahedranes as a family are scarce, neutral
heteroatomic species are all but unknown, with the only reported
example being AsP3. Herein, we describe the isolation of a neutral
heteroatomic X2Y2 molecular tetrahedron (X, Y = p-block
elements), which also is the long-sought-after free phosphaalkyne
dimer. Di-tert-butyldiphosphatetrahedrane, (tBuCP)2, is formed
from the monomer tBuCP in a nickel-catalyzed dimerization
while 1,2-diphosphatriafulvene (IV) is predicted to be the
preferred isomer of (HCP)2, the diphosphatetrahedrane (I) is the
most stable isomer of (tBuCP)2 (Figure 1b).[3,5] Related
diphosphacyclobutadienes II and III are considerably higher in
energy in both cases.
We reasoned that the dimerization of phosphaalkynes,
R−C≡P, could present an elegant avenue toward elusive
reaction using [(NHC)Ni(CO)3] (NHC
=
1,3-bis(2,4,6-
diphosphatetrahedranes.
phosphaalkyne dimers
diphosphacyclobutadienes,[23] but also other isomers) commonly
result from transition metal-mediated phosphaalkyne
Indeed,
transition
metal-bound
trimethylphenyl)imidazolin-2-ylidene (IMes) and 1,3-bis(2,6-
diisopropylphenyl)imidazolin-2-ylidene (IPr)). Single-crystal X-ray
structure determination of a silver(I) complex confirms the
structure of (tBuCP)2. The influence of the N-heterocyclic carbene
(most
frequently
1,3-
oligomerization reactions.[24] Free diphosphatetrahedranes have
also been proposed as key intermediates in thermal and
photochemical oligomerization reactions of phosphaalkynes,
which typically lead to higher phosphaalkyne oligomers (RCP)n
(n = 3–6).[25–30] However, an uncomplexed phosphaalkyne dimer
has never been observed.
ligand on the catalytic reaction was investigated, and
a
mechanism was elucidated using a combination of synthetic and
kinetic studies and quantum chemical calculations.
Tetrahedranes (tricyclo[1.1.0.02,4]butanes) have considerable
practical and theoretical significance because of their high energy
content, large bond strain and ensuing high reactivity.[1] While
theoretical chemists have endeavored to determine the electronic
structure and the thermodynamic stability of tetrahedranes with
ever increasing accuracy,[2–5] synthetic chemists have striven to
develop effective protocols for their preparation. The isolation by
Maier and co-workers of the first organic tetrahedrane, (tBuC)4,
was a milestone in organic synthesis (Figure 1a).[6] Nevertheless,
the number of well-characterized tetrahedranes remains small,
even more than four decades later.[7–13] Some heavier congeners,
e.g. (RE)4 (E = Si and Ge, R = SitBu3) and related group 13
element compounds, are also known,[14–21] as are the structures
adopted by white phosphorus (P4) and yellow arsenic (As4).
Undoubtedly, P4 is the most industrially significant tetrahedrane.
Moreover, neutral tetrahedranes containing two different
heteroatoms in their skeleton are almost unknown, the only
example to have been isolated so far being AsP3, which was
synthesized by reaction of a niobium cyclotriphosphido complex
with AsCl3.[22]
Figure 1. a) The tetrahedrane (tBuC)4 in equilibrium with the cyclobutadiene
isomer and DFT structure of (tBuCP)2;[6] b) calculated relative electronic
energies (E in kcal∙mol-1) for (RCP)2 with R = H (data from ref. [3]) and R = tBu
(see the SI).
Diphosphatetrahedranes, (RCP)2, represent a particularly
attractive target in this area, potentially providing a hybrid
between the two most famous tetrahedral molecules, P4 and
(tBuC)4. However, high level quantum chemical studies indicate
that, similar to pure carbon-based tetrahedranes, such a species
must be stabilized by bulky alkyl substituents (Figure 1b). Thus,
Building on previous work on iron(–I)- and cobalt(–I)-mediated
phosphaalkyne dimerizations,[31–33] we recently began studying
the analogous reactivity of phosphaalkynes with nickel(0) species.
Unexpectedly, the 31P{1H} NMR spectrum of the reaction of
[Ni(CO)4] with an excess of tBuCP (50 equivalents) exhibited a
high field-shifted singlet at −468.2 ppm in addition to the signal of
free tBuCP at −68.1 ppm. It was anticipated that such an upfield
shift could be consistent with formation of a P2C2 tetrahedron (cf.
P4, δ = −521 ppm), through dimerization of tBuCP. This
assumption was later confirmed through isolation of the pure
product 1a (vide infra). A subsequent screening of various nickel
tricarbonyl complexes [(NHC)Ni(CO)3] (NHC = IMes, IPr, iPr2ImMe
(= 1,3-di(isopropyl)-4,5-di(methyl)imidazolin-2-ylidene)) for this
[a]
G. Hierlmeier, Dr. P. Coburger, Dr. M. Bodensteiner, Prof. Dr. R.
Wolf
University of Regensburg, Institute of Inorganic Chemistry, 93040
Regensburg, Germany, E-mail: robert.wolf@ur.de
Supporting information for this article is given via a link at the end of
the document.
This article is protected by copyright. All rights reserved.

10.1002/anie.201910505
Angewandte Chemie International Edition
dimerization reaction of tBuCP revealed that the bulky NHC
ligands IPr and IMes gave optimal results (see the Supporting
Information for details), while the use of the smaller isopropyl-
substituted ligand iPr2ImMe resulted in only a low yield of 1a. Using
[(IMes)Ni(CO)3], 1a can be isolated in up to 55% yield on a
500 mg scale using just 2 mol% of the nickel catalyst in n-hexane
for 18 h (Figure 2). Fractional condensation of the raw product
affords pure 1a as a pyrophoric, yellow oil with a melting point of
−32 °C. Above the melting point, neat 1a dimerizes to the known
ladderane-type tetramer tetraphosphatricyclo[4.2.0.02,5]octadiene
(2a, Figure 2) within several hours.[25] However, 1a is stable at
−80 °C for weeks without noticeable decomposition as evidenced
by 31P{1H} NMR. Dimerization of 1a to 2a is significantly slower in
dilute solutions (e.g. 0.2 M in toluene). The use of 1-
adamantylphosphalkyne under similar conditions results in the
analogous formation of diadamantyldiphosphatetrahedrane (1b),
as indicated by a resonance at −479.8 ppm in 31P{1H} NMR
spectra. However, attempts to isolate 1b in pure form have thus
far been hampered by decomposition to higher phosphaalkyne
oligomers (e.g. the ladderane (AdCP)4 (2b) analogous to 2a).
(εmax =1200 L∙mol─1∙cm─1) tailing into the visible region with a
shoulder at 350 nm accounting for the yellow color. Analysis of 1a
by EI-MS mass spectrometry revealed a molecular ion peak at
m/z = 200.0879 in good agreement with the calculated molecular
ion peak (m/z = 200.0878) and additionally showed fragmentation
pathways via loss of P2 units (e.g. M+−CH3−P2: 123.1172, calcd.
123.1173).
Attempts to grow single crystals of 1a suitable for X-ray
crystallography have so far been unsuccessful. For this reason,
the preparation of a metal complex was attempted with
[Ag(CH2Cl2)2(pftb)] (pftb = Al{OC(CF3)3}4).[37,38] A clean reaction
was observed in toluene using two equivalents of 1a per silver
atom, and a species with a significantly downfield shifted 31P{1H}
NMR signal (─446.8 ppm, cf. ─468.2 ppm for 1a) was detected.
Further NMR monitoring also showed the slow formation of the
tetramer 2a. A single-crystal X-ray diffraction study on crystals
grown from CH2Cl2 revealed the formation of [{Ag(1a)(2a)}2][pftb]2
(3), where both 1a and 2a are incorporated in the same complex
(Figure 3).[39] Crucially, the X-ray diffraction experiment confirms
the tetrahedral structure of 1a. The P2C2 tetrahedron is bound to
the Ag atom in an η2 fashion via the P−P bond (P1−P2 2.308(3) Å).
The four P–C bond lengths in the tetrahedron range 1.821(9)-
1.836(9)Å, while the C−C bond length (C1-C2 1.462(12) Å) is
similar to that of (tBuC)4 (average: 1.485 Å).[40] Broadened singlet
resonances are observed in the 31P{1H} NMR spectrum at −19.8
and −446.8 ppm when crystals of 3 are dissolved in CD2Cl2, and
the 1H NMR data are also consistent with the molecular structure
obtained by X-ray crystallography.[41]
Figure 3. Molecular structure of 3 in the solid state. Thermal ellipsoids are set
at 50% probability level. Hydrogen atoms and the [pftb]─ counterions are omitted
for clarity. Selected bond lengths [Å] and angles [°]: P1−P2 2.308(3), P1−C1
1.836(9), P1−C2 1.835(9), P2−C1 1.821(9), P2−C2 1.820(8), C1−C2 1.462(12),
C1−P2−P1 51.2(3), C1−P2−C2 47.4(4), C2−P2−P1 51.1(3), C1−P1−P2 50.6(3),
C2−P1−P2 50.5(3), C2−P1−C1 46.9(4), P2−C1−P1 78.3(3), C2−C1−P2 66.3(5),
C2−C1−P1 66.5(5), P2−C2−P1 78.3(3), C1−C2−P2 66.4(5), C1−C2−P1 66.6(5).
Figure 2. a) Synthesis of 1a by [(NHC)Ni(CO)3] (NHC = IMes, IPr) catalyzed
dimerization of tBuCP, b) 31P{1H} and 13C{1H} NMR spectra for 1a at 300 K in
C6D6. The asterisk marks a trace of the tetramer (tBuCP)4 (2a, i.e. the
dimerization product of 1a).
In an attempt to identify possible intermediates in the
formation of 1a, the nickel tricarbonyl complexes [(NHC)Ni(CO)3]
(NHC = IMes, IPr, iPr2ImMe) were reacted with one equivalent of
phosphaalkyne RCP (R = tBu, Ad) in n-hexane at ambient
temperature. Each of these reactions led to an instant color
change from colorless to bright yellow and concomitant gas
evolution (liberation of CO gas). For the sterically more
demanding NHC ligands IPr and IMes, the phosphaalkyne
complexes [(NHC)Ni(CO)(PCR)] (NHC = IMes, R = tBu (4a), Ad
(4b), NHC = IPr; R = tBu (4c), Ad (4d)) featuring η2-bound
phosphaalkyne ligands were the sole P-containing products of
these reactions (Figure 4a). Complexes 4a–4d can be isolated as
crystalline solids in yields from 34% to 87%, and were
characterized by single crystal X-ray diffraction, multinuclear
Multinuclear NMR spectra of 1a are in agreement with the
tetrahedral structure with localized C2 symmetry. The 31P{1H}
v
NMR spectrum of 1a in C6D6 displays a singlet resonance at
−468.2 ppm similar to other tetrahedral phosphorus compounds,
e.g. P4 (δ(31P) = −520 ppm) and AsP3 (δ(31P) = −484 ppm).[34–36]
The 1H NMR spectrum shows a singlet resonance at 1.07 ppm for
the tBu group. In the 13C{1H} spectrum, a singlet resonance is
observed for the methyl groups, whereas the two other carbon
1
2
signals split into triplets with JP–C = 46.7 Hz and JP–C = 5.7 Hz
(Figure 2). 1a was further characterized by elemental analysis, IR,
UV/VIS spectroscopy and mass spectrometry. The UV/VIS
spectrum reveals
a weak absorption band at 275 nm
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10.1002/anie.201910505
Angewandte Chemie International Edition
NMR spectroscopy, IR spectroscopy and elemental analysis (see
the Supporting Information for details). The structural and
spectroscopic data compare well to related, isoelectronic
spectroscopic monitoring of this catalytic dimerization reaction
revealed the presence of 4a at a constant concentration
throughout the whole reaction (see the SI for further details).
These observations suggest that 4a is the resting state for the
catalytic cycle. Further reaction intermediates were not detected
by 31P{1H} NMR spectroscopy even upon monitoring the reaction
at –80 °C. Also noteworthy is that treatment of 4a with one
complexes
[(iPr2Im)2Ni(PCtBu)]
(iPr2Im
=
1,3-
di(isopropyl)imidazolin-2-ylidene) and [(trop2NMe)Ni-(PCPh3)}]
(trop = 5H-dibenzo[a,d]cyclohepten-5-yl).[42,43]
Conversely, the reaction of tBuCP with [(iPr2ImMe)Ni(CO)3]
equivalent
AdCP
affords
the
mixed-substituted
afforded
diphosphacyclobutadiene
P2C2tBu2)] (5), the dinuclear complex [{(iPr2ImMe)Ni(CO)}2(µ,η2:η2-
tBuCP)] (6) and tetranuclear cluster
a
mixture
of
the
mononuclear
1,3-
diphosphatetrahedrane (P2C2AdtBu, 1c), which can be identified
by a 31P{1H} NMR singlet at −473.8 ppm.
complex
[(iPr2ImMe)Ni(CO)(η4-
a
Kinetic analysis with 0.5 to 4 mol% of 4a indicates a first-order
dependence of the dimerization reaction in both catalyst and
phosphaalkyne. The proposed rate law is therefore
푑[ퟏ풂]
[{(iPr2ImMe)Ni2(CO)2(tBuCP)}2] (7, Figure 4b). The three different
species were identified in the 31P{1H} NMR spectrum and
structurally authenticated by X-ray diffraction experiments after
fractional crystallization. Treatment of [(iPr2ImMe)Ni(CO)3] with just
0.5 or two equivalents of tBuCP resulted in similar mixtures. Upon
addition of tBuCP to one equivalent of [Ni(CO)4], more than ten
different species were detected by 31P{1H} NMR spectroscopy.
The unselective nature of these reactions is in contrast to the
selective formation of the η2-bound phosphaalkyne complexes
4a-d and presumably accounts for the lower yields in the catalytic
formation of 1a.
[
] [
]
푟 =
= 푘 ∙ ퟒ풂 ∙ 푡BuCP .
푑푡
These results are in good agreement with DFT calculations
performed on the TPSS-D3BJ/def2-TZVP level, which suggest
that the reaction between the truncated model complex
[(IXy)Ni(CO)(tBuCP)]
(4′,
IXy
=
1,3-bis(2,6-
dimethylphenyl)imidazolin-2-ylidene) and a molecule of tBuCP
initially affords the 1,3-diphosphacyclobutadiene complex A
(Figure 5, cf. complex 5, which differs only in the identity of NHC
ligand; see the SI for more details).[44] However, A is not the global
minimum of the potential hypersurface and transforms into an
intermediate B showing an isomerized (tBuCP)2 ligand. In the
next step, a diphosphatetrahedrane complex C is formed. The
formation of
C
has
a
calculated activation barrier of
26.9 kcal∙mol–1 with respect to A. This is well in line with the
reaction temperature of +60 °C required for the reaction to
proceed at an appreciable rate (vide supra). Subsequent
replacement of the diphosphatetrahedrane 1a by another
phosphaalkyne molecule is a downhill process and re-forms the
resting state 4′ (cf. complex 4, which is the only species we could
identify by NMR spectroscopy in solution). Notably, a different
scenario has been calculated for a further truncated model system
consisting of Me−C≡P and [(IPh)Ni(CO)(PCMe)], (IPh = 1,3-
diphenylimidazolin-2-ylidene, see the SI for further details). In this
case, significant stabilization of the analogous 1,3-
diphosphacyclobutadiene complex (A') is observed. The high
activation barrier calculated for the transformation A'  C'
(49.8 kcal∙mol-1)
precludes
the
formation
of
the
diphosphatetrahedrane. It appears that the steric repulsion
between bulky substituents on the NHC such as Mes and Dipp
and the tBu groups has a destabilizing effect on A, and this
destabilization of the 1,3-diposphacyclobutadiene complex, which
is usually a thermodynamic sink in other reactions,[33] enables
catalytic turnover in this particular case.
Figure 4. Synthesis of 4a-d, 5, 6 and 7; and structures of 4a and 5 in the solid
state. Thermal ellipsoids are set at 50% probability level. Hydrogen atoms and
the second crystallographically independent molecule (in case of 4a) are
omitted for clarity. Selected bond lengths [Å] and angles [°] for 4a: Ni1−C1
1.777(3), Ni1−C7 1.931(2), Ni1−P1 2.1793(9), Ni1−C2 1.898(3), C1−O1
1.137(4), P1−C2 1.636(3), C3−C2−P1 144.2(2), C7−Ni1−P1 102.89(7),
C2−Ni1−P1 46.67(8), C2−Ni1−C7 149.56(11), C2−P1−Ni1 57.59(10),
O1−C1−Ni1 171.4(4); 5: Ni1−P1 2.3114(3), Ni1−P2 2.3113(3), Ni1−C2
2.0898(11), Ni1−C3 2.0637(11), P1−C2 1.7966(11), P1−C3 1.8143(11), P2−C2
1.8121(11), P2−C3 1.7992(11), Ni1−C1 1.7538(13), C1−O1 1.1458(17),
Ni1−C12 1.9421(11), C1−Ni1−C12 94.29(5), O1−C1−Ni1 176.74(12),
C2−P1−C3 78.74(5), C3−P2−C2 78.73(5), P1−C2−P2 100.90(6), P2−C3−P1
100.71(6).
In conclusion, diphosphatetrahedranes (RCP)2 (R = tBu, Ad)
have been synthesized by an unprecedented nickel(0)-catalyzed
dimerization reaction of the corresponding phosphaalkynes RCP.
The tert-butyl-derivative (tBuCP)2 (1a) is stable enough to be
isolated and thoroughly characterized. The molecular structure of
the silver(I) complex 3 confirms the tetrahedral structure of the
molecule. 1a is a very rare ‘mixed’ tetrahedrane, which, moreover,
represents is the hitherto elusive free phosphaalkyne dimer. Its
synthesis therefore closes a significant gap in phosphaalkyne
oligomer chemistry. 1a is a metastable compound that slowly
converts to the ladderane 2a. This reaction shows that such
With a high-yielding protocol for the preparation of 4a in hand,
the reactivity of this species was investigated. 4a is the most
potent catalyst for the dimerization of tBuCP among all nickel
complexes investigated. Thus, a significantly shorter reaction time
for full conversion of the phosphaalkyne is required with 4a than
with [(IMes)Ni(CO)3]. High temperature 31P{1H} NMR
dimers
are
indeed
intermediates
in
phosphaalkyne
tetramerizations as proposed previously.[25,28] Synthetic, kinetic
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10.1002/anie.201910505
Angewandte Chemie International Edition
and computational investigations suggest that
a
1,3-
fellowship for G. H.) is gratefully acknowledged. We thank the
group of Prof. Manfred Scheer (Luis Dütsch and Martin Piesch)
for the donation of [Ni(CO)4)] and [Ag(CH2Cl2)2(pftb)]. We also
thank Jonas Strohmaier and Georgine Stühler for assistance, and
Dr. Daniel Scott and Dr. Sebastian Bestgen for helpful comments
on the manuscript.
diphosphacyclobutadiene complex is a key intermediate and that
destabilization of this complex by steric repulsion is a crucial
factor in achieving catalysis. We are currently exploring the further
reactivity of the remarkable small molecule 1a.
Keywords: phosphorus • nickel • phosphaalkynes • dimerization
• homogeneous catalysis
Acknowledgements
Financial support by the European Research Council
(CoG 772299) and the Fonds der Chemischen Industrie (Kekulé
Figure 5. Reaction profile calculated with DFT at the TPSS-D3BJ/def2-TZVP level for the dimerization of tBu−C≡P catalyzed by [(IXy)Ni(PCtBu)] (IXy = 1,3-bis(2,6-
dimethylphenyl)imidazolin-2-ylidene) (4′). Calculated Gibbs energies (in kcal∙mol−1at 298 K) and schematic drawings of intermediates and transition states are given.
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COMMUNICATION
Entry for the Table of Contents
COMMUNICATION
Free dimer at last: Di-tert-
G. Hierlmeier, P. Coburger, M.
butyldiphosphatetrahedrane is
accessible by the facile nickel-
catalyzed dimerization of tert-butyl
phosphaalkyne. This compound not
only represents an uncomplexed
phosphaalkyne dimer, but also is the
first example of a tetrahedral molecule
with carbon and phosphorus atoms in
its scaffold.
Bodensteiner, R. Wolf*
Page No. – Page No.
Di-tert-butyldiphosphatetrahedrane:
Catalytic Synthesis of the Elusive
Phosphaalkyne Dimer
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