Tetraphosphabenzenes obtained via a triphosphacyclobutadiene intermediate

Article abstract of DOI:10.1002/anie.200804432

(Chemical Equation Presented) An acyl triphosphirene ligand transfers an O atom to Nb to liberate the putative triphosphacyclobutadiene intermediate [RCP₃{W(CO)₅}₂], which engages in [2+4]-cycloaddition reactions with an o

Full text of DOI:10.1002/anie.200804432

Communications
DOI: 10.1002/anie.200804432
Phosphorus Chemistry
Tetraphosphabenzenes Obtained via a Triphosphacyclobutadiene
Intermediate**
Nicholas A. Piro and Christopher C. Cummins*
The ability to replace “RC” units in organic motifs with
isolobal phosphorus atoms has led to phosphorus being
dubbed the “carbon copy.”[1] Yet while the all-phosphorus
analogues of benzene and cyclobutadiene (the epitomes of
aromaticity and antiaromaticity) have been considered the-
oretically,[2–5] they remain elusive. What do exist are a few
examples of reduced P6 and P4 ligands complexed to one or
more metal centers.[6–9] Diphosphacyclobutadiene and
valence isomers of triphosphabenzene have been accessed
through metal-mediated oligomerization of phosphaal-
Scheme 1. Oxygen-atom transfer from an acyl triphosphirene ligand to
kynes,[10–12] and two related reactions generate triphosphacy-
the niobium center liberates the putative triphosphacyclobutadiene
intermediate 3R. One of several possible linkage isomers of 3R is
clobutadiene ligands p-complexed to reducing metal cen-
ters.[13,14] There is also one example of a highly distorted
P4(CtBu)2 ligand complexed between two rhodium centers,[15]
and a diradical valence isomer of a tetraphosphabenzene has
been reported to contain multiple one-electron bonds.[16]
Exploiting the chemistry of P2-derived cyclo-P3 complexes,[17]
we now describe neutral triphosphacyclobutadiene inter-
mediates and isomers of tetraphosphabenzene that are
stabilized only by mild {W(CO)5} substituents.[18]
Acylation of the {W(CO)5}-coordinated cyclo-P3 complex
[{(OC)5W}2P3Nb{N(CH2tBu)Ar}3]À with either pivaloyl chlo-
ride or 1-adamantanecarbonyl chloride affords the triphos-
phirene complexes [{(OC)5W}2RC(O)P3Nb{N(CH2tBu)Ar}3],
1R (Ar= 3,5-Me2C6H3, R = 1-adamantyl (1Ad), tert-butyl
(1tBu)).[17] These complexes are thermally unstable toward
deoxygenation of the acyl triphosphirene ligand to form
[ONb{N(CH2tBu)Ar}3] (2) and red precipitate of empirical
formula [RCP3{W(CO)5}2] (Scheme 1). As a monomer, the
species [RCP3{W(CO)5}2] (3R) could possess an RCP3 core
with either tetrahedrane or butadiene-like structure; the
latter structure would be one RC unit away from the elusive
planar P4.
drawn.
variant 1tBu produces a coproduct with increased solubility
in various solvents, allowing for a more thorough character-
ization. 31P NMR spectroscopic data acquired on the crude
reaction mixture formed by stirring 1tBu for several hours at
228C indicated that a mixture of phosphorus-containing
products had formed, though one major product was present.
Single-crystal X-ray diffraction analyses performed on several
small, red-orange crystals that were grown from CH2Cl2
solutions of the product mixture revealed three structurally
distinct isomers, 4a–c (Scheme 2).[19] All of these compounds
The poor solubility of the precipitate generated during the
formation of 2 from 1Ad proved a hindrance to identifying the
phosphorus-containing products. However, the tert-butyl
Scheme 2. Structures of identified dimerization products 4a–c.
were dimers of the [tBuCP3{W(CO)5}2] unit, and their
structures can be compared to those known for phosphaal-
kyne tetramers.[20,21] One fraction was isolated in approx-
imately 30% yield as a red powder that was insoluble in Et2O
and showed sparing solubility in CH2Cl2, and this red powder
was subjected to an X-ray powder diffraction study. Powder
diffraction patterns for 4a–c were simulated on the basis of
the single-crystal data, and these patterns were compared to
the experimental patterns for the isolated compound. While
two of the simulations showed a poor match to experiment,
the simulated pattern for 4a was in very good agreement with
the diffraction pattern of the isolated powder (Figure 1; see
Figure S2 in the Supporting Information for simulated
[*] N. A. Piro, Prof. Dr. C. C. Cummins
Department of Chemistry, Massachusetts Institute of Technology
77 Massachusetts Avenue, Room 6-435
Cambridge, MA 02139-4307 (USA)
Fax: (+1)617-258-5700
E-mail: ccummins@mit.edu
Homepage: http://web.mit.edu/ccclab/
[**] We gratefully acknowledge the U.S. National Science Foundation for
support through grant CHE-719157 and a predoctoral fellowship to
N.A.P. We also thank Thermphos International for a donation of
funds and supplies and Prof. David B. Collum (Cornell) for bringing
spiro[2.4]-hepta-4,6-diene to our attention.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.200804432.
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fraction study revealed the stereochemistry of the product to
be consistent with the prediction arrived at through consid-
eration of steric and secondary orbital interactions: the
p bonds of the diene/alkene interact with the four-membered
ring, while the sterically protruding spiro group aligns
opposite the ring (Figure 2).[18] This product demonstrates
clean diphosphene-like reactivity for the triphosphacyclobu-
tadiene intermediate.
Figure 1. X-ray powder diffraction of 4a confirmed its structural
identity by comparison of simulated and experimental data.
patterns of 4b and 4c). Comparing the 31P NMR spectrum of
the crude reaction mixture to that of isolated 4a further
confirmed this species as the major product. Furthermore, the
complex splitting pattern in the 31P NMR spectrum was
successfully simulated using the connectivity determined by
X-ray diffraction (Figure S4, Supporting Information).[22]
While the structural identity of the product originating from
the fragmentation of 1Ad could not be definitively confirmed,
on the basis of the data available from 1tBu it is assigned as one
or more isomers of the dimer [(AdCP3{W(CO)5}2)2].
The structures of 4a–c suggest the intermediacy of the
triphosphacyclobutadiene molecule 3R, which is depicted as
one of several possible linkage isomers in Scheme 1. To probe
the chemistry of such an intermediate, we attempted to trap
the reactive species 3Ad with suitable substrates. The platinum
complex [(Ph3P)2Pt(C2H4)] has been used successfully to trap
Figure 2. X-ray crystal structure of complex 6. Thermal ellipsoids are
set at the 50% probability level; hydrogen atoms are omitted for
clarity. Selected bond lengths [ꢀ] and angles [8]: P1–C40 1.690(3), P1–
P2 2.1969(11), P2–P3 2.2173(11), P3–C40 1.843(3), P1–W2 2.4515(8),
P2–W1 2.5044(8); C40-P1-P2 89.02(11), P1-P2-P3 79.82(4), C40-P3-P2
84.66(11), P1-C40-P3 106.48(17).
The dimerization reactions of triphosphacyclobutadiene
3R that are observed when no trap is present in solution
suggest that this intermediate can participate as both a
dieneophile and as a diene partner in [4+2]-cycloaddition
reactions, much as does cyclobutadiene itself.[18,34,35] If this is
the case then it should be possible to trap 3R with a molecule
containing a reactive p bond. 1-Adamantylphosphaalkyne
was employed in this role because of its potential to yield the
Dewar isomer of tetraphosphabenzene as a trapping prod-
uct.[12,36] Gentle thermolysis of 1Ad and AdCP for 4 h at 358C
in benzene affords one major product, as assayed by 31P NMR
spectroscopy. This product exhibits two coupled resonances in
the 31P NMR spectrum at d = 248 and À13 ppm. These data
are consistent with the C2-symmetric tetraphosphadewarben-
zene [(AdC)2P4{W(CO)5}2] (7, Figure 3a). This structure was
confirmed by a single crystal X-ray diffraction study on an
orange crystal of 7 grown from benzene solution (Figure 3b).
Upon prolonged light-exposure of solutions of 7, isomer-
ization takes place to provide 8, which at 208C displays two
broad resonances in the 31P NMR spectrum at d = + 19 and
À108 ppm. This conversion can be accelerated by photolysis
of 7 in THF with high-intensity broadband light to afford
complete conversion to 8 in less than 20 min. An X-ray
diffraction study performed on crystals grown from mixtures
of 7 that had been exposed to ambient light revealed a
benzvalene structure with a diphosphene moiety h2-coordi-
nated to one {W(CO)5} unit and h1-coordinated to the second
{W(CO)5} (Figure 3c). The two broad signals in the 31P NMR
spectrum of 8 can be attributed to a dynamic process in which
the exchange of {W(CO)5} units between faces and termini of
À
units of P P unsaturation with simple displacement of the
ethylene molecule, but such reactivity was not found with
intermediate 3Ad [23–27]
Instead, a product incorporating the
.
ethylene unit into the CP3 framework, [(Ph3P)(OC)Pt{P3C-
(C2H4)Ad}{W(CO)5}2] (5), was isolated in low yield and
characterized by NMR spectroscopy and X-ray crystal
structure determination.[28]
Seeking to trap the putative triphosphacyclobutadiene
intermediate in a simple one-to-one reaction, we employed
organic dienes that are known to react by [4 + 2]-cyclo-
additions with diphosphenes.[29–31] When 1Ad was allowed to
fragment in the presence of either 1,3-cyclohexadiene or 2,3-
dimethylbutadiene, mixtures of products were observed by
31P NMR spectroscopy. Spiro[2.4]hepta-4,6-diene is known to
be a particularly active Diels–Alder reagent,[32] and when 1
was allowed to fragment in the presence of this diene, one
major product was observed by 31P NMR spectroscopy. This
product, [C7H8(P3CAd){W(CO)5}2] (6), has resonances con-
sistent with the desired trapping of the diphosphene func-
tional group, while the phosphaalkene moiety (31P NMR d =
295 ppm) remains intact. Extraction of coproduct 2 with n-
pentane and recrystallization of the remaining fraction from
toluene affords compound 6.[33] A single-crystal X-ray dif-
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Figure 3. a) Synthesis of two valence isomers of tetraphosphabenzene supported by 2 equiv {W(CO)5}. b),c) X-ray crystal structures of 7 and 8.
Thermal ellipsoids are set at the 50% probability level; hydrogen atoms are omitted for clarity. Selected bond lengths [ꢀ] and angles [8]: 7: P1–C1
1.71(1), P1–P2 2.197(5), P2–P3 2.220(5), P3–C1 1.83(1); P4-P3-C1 101.4(5), P1-C1-P3 104.8(6), P2-P1-C1 90.0(5). 8: P1–P2 2.1047(15), P3–P4
2.1559(17), W(1)–P(1) 2.6269(11), W(1)–P(2) 2.6704(10), P1–W2 2.5807(11); C3-P3-C4 82.27(19), P3-C4-P4 70.15(15), C4-P1-P2 96.93(14), C3-P2-
P1 95.16(14).
the diphosphene can equate pairs of P atoms. Upon cooling to
À1008C these two resonances resolve into four (d = 37 (d), 10
(br), À21 (d), À232 ppm (br)) with a doublet pair displaying
strong coupling (1JPP = 450 Hz) that is consistent with P P
À
multiple bonding. This product can be seen as arising from
intramolecular [2+2]-cycloaddition of the phosphaalkene
moieties of 7 and subsequent radical rearrangements, and it
is consistent with the “rule of five.”[37] Such rearrangements
have also been observed for monophosphadewarbenzenes.[38]
À
The Z-diphosphene of 8 shows a short P P distance of
2.1047(15) ꢀ and angles of 96.93(14) and 95.16(14)8. It is
remarkable that such a species is stabilized by only one
p complexation and one s complexation from the relatively
mild {W(CO)5} unit.[39,40]
To investigate the mechanism by which 2 and 3Ad are
generated from 1Ad, the kinetics of this reaction were
Scheme 3. Proposed mechanism for formation of transient triphospha-
cyclobutadiene 3Ad proceeding via the observed intermediate 1’.
monitored by both 1H and 31P NMR spectroscopy. The
decay of starting material 1 was found to follow clean first-
order kinetics, but the growth of an intermediate was found to
accompany production of 2. This intermediate (1’) displays
three 31P environments and two tBu environments of the
anilide ligands in its NMR spectra. The 31P resonances are a
triplet (JPP = 175 Hz) at d = À4 ppm and two broad resonan-
ces at d = À28 and À37 ppm. By introducing a 13C label at the
acyl carbon atom (C1) it was determined that in 1’, C1 is
attached only to the one phosphorus atom with a chemical
shift of d = À4 ppm (JCP = 108 Hz). Furthermore, the 13C
chemical shift for C1 in 1’ was found to shift downfield by
46 ppm relative to the starting material (from 213 to
259 ppm), consistent with Lewis acid activation of the
carbonyl group,[41] leading to the proposed structure for 1’
that is shown in Scheme 3.
The decay of starting material 1Ad follows first-order
exponential behavior, but the ratios of species 1Ad, 1’, and 2
did not fit the simple kinetic model shown in Scheme 4a.
Rather, the data were better suited by a model in which there
is a competitive process that proceeds without an observable
intermediate with a rate constant k1’ ꢀ k1, Scheme 4b (see the
Supporting Information for details). It is possible that the
coordination sites of the two {W(CO)5} units coupled to subtle
conformational differences lead to two rates of rearrange-
ment from structures analogous to 1’.
From 1’ we can envision attack by one of the niobium-
bound phosphorus atoms on the Lewis acid activated
carbonyl group to give 1’’ with a structure that is poised to
eliminate the triphosphacyclobutadiene 3Ad upon formation
of 2 through a six-electron rearrangement with formation of
the strong niobium oxo bond. Importantly, the same inter-
mediate 1’ was observed in the presence of either AdCP or
spiro[2.4]hepta-4,6-diene.
Scheme 4. Kinetic scheme (b) provides a better fit to the time-
dependent concentrations of 1Ad, 1’, and 2 than does the simpler
model (a).
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triphosphacyclobutadiene intermediate. The reactions
observed suggest that this species can dimerize or serve as
either a reactive Z-diphosphene or as a four-p-electron
cycloaddition partner, depending on the reagents present.
Such reactivity has led to the synthesis of the most phospho-
rus-rich congeners of benzene isomers reported, the Dewar
and benzvalene isomers of tetraphosphabenzene. This
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6: Solid red 1Ad (195 mg, 0.125 mmol, 1 equiv) was dissolved in a
THF solution (14 wt%, 2 g) of spiro[2.4]hepta-4,6-diene (280 mg,
24 equiv) and the solution was then diluted with THF to a total
volume of 5 mL. The solution was stirred at 228C for 5 h, after which
time the solution was bright yellow in color. The mixture was filtered
through celite, and the filtrate was concentrated to dryness under
dynamic vacuum. The resulting residue was suspended in pentane
(4 mL) and cooled to À358C for 24 h to precipitate the product as a
bright yellow powder, which was collected atop a sintered glass frit
and washed with cold pentane (65 mg, 53% yield). 31P NMR (C6D6,
208C, 121.5 MHz): d = 295.4 (dd, 1JPP = 125 Hz, 2JPP = 28 Hz, JPW
=
240 Hz, C P), 3.0 (dd, 1JPP = 156 Hz, 2JPP = 28 Hz, C P-P-P),
=
=
À24.9 ppm (dd, 1JPP = 156 Hz, 1JPP = 125 Hz, JPW = 210 Hz, C P-P-P).
=
7: Solid red 1Ad (360 mg, 0.230 mmol, 1 equiv) and solid colorless
AdCP (83 mg, 0.46 mmol, 2 equiv) were mixed and then dissolved in
benzene (10 mL) to give a deep red solution. The solution was heated
to 358C for 3.5 h, after which time the orange mixture was filtered
through celite and evaporated to dryness. Niobium oxo 2 was
extracted from the orange residue with pentane (ca. 10 mL) and the
remaining solid was collected on a frit and then washed with pentane
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31
=
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Received: September 8, 2008
Published online: December 19, 2008
Keywords: niobium · phosphabenzenes ·
.
phosphacyclobutadienes · phosphorus · triphosphirenes
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