Isolation of a neutral P8 cluster by [2+2] cycloaddition of a diphosphene facilitated by carbene activation of white phosphorus

Article abstract of DOI:10.1002/anie.201301137

P-cing it together: Carbonyl-decorated carbenes readily activate white phosphorus to afford carbene-stabilized P₄ and P₈ clusters. Mechanistic evidence suggests the P₈ cluster (see structure from X-ray diffraction: C gray, P orange, N blue, O red) forms by a [2+2] cycloaddition of a transient diphosphene species. Copyright

Full text of DOI:10.1002/anie.201301137

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DOI: 10.1002/anie.201301137
Phosphorus Clusters
Isolation of a Neutral P8 Cluster by [2+2] Cycloaddition of
a Diphosphene Facilitated by Carbene Activation of
White Phosphorus**
Christopher L. Dorsey, Brian M. Squires, and Todd W. Hudnall*
The industrial preparations of organophosphorus compounds
typically utilize chlorinated phosphorus starting materials,
which are derived from readily available white phosphorus
(P4).[1,2] Ideally, more sustainable and environmentally benign
processes would directly convert P4 into useful organophos-
phorus products.[1,2] For this reason, the activation of P4 as
opposed its chlorination and subsequent functionalization has
become a topical objective.
Two primary areas of research include the activation of P4
with transition metals,[3,4] and more recently main-group
species,[5] as a means to eliminate the use of chlorinated
phosphorus compounds. The former area has been spear-
headed by Cummins,[3a–c,6] who has demonstrated the ability
to functionalize P4, affording remarkable molecules such as
AsP3.[6a] For the latter, stable singlet carbenes have been
utilized to activate P4 as well as to stabilize P1, P2, P4, and even
P12 fragments (Scheme 1).[7,8] When the steric demands of the
chosen carbene ligand are low, small P1 and P2 species such as
A, B, and C can be isolated.[7a,8] For the larger P4 and P12
clusters (D–G), a common unstable monocarbene adduct (H)
has been proposed through mechanistic studies.[7a,b]
Herein we report that carbenes decorated with carbonyl
moieties can activate white phosphorus in a manner analo-
gous to the more nucleophilic N-heterocyclic carbenes
(NHCs) and cyclic alkyl amino carbenes (CAACs). Carbon-
yl-decorated carbenes (CDCs) have already been shown to
exhibit a rich and versatile reactivity, motivating our inves-
tigation.[9–13] For this study, we investigated carbenes 1–5
(Scheme 2), which differ in the number and location of
carbonyl groups along with additional functionality in the
backbone; however, only clean reactions with 1 and 2 were
observed.
Scheme 2. CDCs investigated in this study. Mes=2,4,6-(CH3)3C6H2.
Gratifyingly, treatment of P4 with three equivalents of
carbenes 1 or 2 resulted in rapid activation at room temper-
ature (Scheme 3). For 1, addition of the carbene to a suspen-
sion of P4 in diethyl ether resulted in a dark red-orange
Scheme 1. Phosphorus fragments supported by stable carbenes.
Dipp=2,6-(iPr)2C6H3; E=NR or CR2.
[*] Dr. C. L. Dorsey, B. M. Squires, Prof. T. W. Hudnall
Department of Chemistry and Biochemistry
Texas State University
601 University Drive, San Marcos, TX 78666 (USA)
E-mail: hudnall@txstate.edu
Scheme 3. Reaction of white phosphorus with carbonyl-decorated
carbenes 1 and 2.
[**] We are grateful to the Research Corporation for Science Advance-
ments (20092), the National Science Foundation for MRI awards
(CHE-0821254, CHE-0946998), the Welch Foundation for a Depart-
ment Research Grant (AI-0045), and Texas State University for their
generous support.
solution followed by the precipitation of a red-orange solid
after five minutes. Isolation of the resulting solid afforded the
triscarbene-P4 complex 6 as determined by NMR spectrosco-
py and single-crystal X-ray diffraction (Figure 1).
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201301137.
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Figure 1. ORTEP drawing (ellipsoids set at 50% probability) for 6. H
atoms are omitted and two carbene moieties are represented by thin
lines for clarity.[16]
The 31P NMR spectrum of 6 exhibits a doublet centered at
d =+ 103.6 ppm (C6D6) and a quartet at d = À20.0 ppm (1JP-
P = 283 Hz), which integrate to 3 and 1, respectively. These
chemical shifts are similar to what Bertrand observed for F,
albeit the three equivalent phosphorus atoms are shifted
downfield considerably (+ 103.6 ppm for 6 versus + 68.1 ppm
for F),[7a] indicating that carbene 1 is more electrophilic than
the CAAC.[12]
Figure 2. A) ORTEP drawing (ellipsoids set at 50% probability) for 7.
H atoms are omitted and two carbene moieties are represented by
thin lines for clarity. B) Side view of the P8 cluster in 7. C) Top view of
the P8 cluster in 7.[16]
Surprisingly, the 1H NMR of 6 displayed inequivalent
mesityl substitutents as well as inequivalent a-methyl groups
in the carbene backbone. This indicated a lack of free rotation
a phosphaalkene moiety. The structure of 7 is to our
knowledge the first example of a neutral P8 cluster that is
supported by a carbene.[6b] Compound 7 is quite similar to that
of the tetraanionic P8 cluster [Na(DME)]4[(tBu3Si)4P8] first
prepared by Wiberg and co-workers[15] with the central P4
atoms adopting distorted-tetrahedral geometries (average SP-
À
about the C P bond. A single-crystal X-ray diffraction study
(Figure 1) not only confirmed the structure of 6 shown in
1
Scheme 3, but corroborated our assessment from H NMR
À
that there is significant multiple C P bond character in 6.
À
Indeed, the C2 P2 distance was found to be 1.728(6) ꢀ which
is similar to F (1.733 ꢀ) and known phosphaalkenes derived
angles 2768). However, close inspection of the central P4
P-P
from carbenes (ca. 1.73–1.76 ꢀ).[14]
ring reveals a more puckered ring in 7 as evidenced by the
As described previously by Bertrand,[7a] the formation of 6
most likely results from the addition of two CDCs 1 to an
initially formed species of type H. Curiously, when the
carbene is changed to the less electrophilic carbene 2,[10,11b]
the product obtained is quite different (Scheme 3). Indeed,
when three equivalents of 2 (featuring single carbonyl) are
added to P4 in Et2O, an instantaneous dark green suspension
is observed. The dark green color rapidly changes to a bright
orange within one minute. Isolation and characterization of
the resulting orange precipitate revealed the formation of the
tetrakiscarbene-P8 complex 7.
shorter intraannular P P distances (average diagonal P P
distance 2.78 ꢀ for 7 versus 3.19 ꢀ for Wibergꢁs compound)
and the larger displacement of P1 from a plane defined by P2-
P3-P4 (displacement distance 1.754 ꢀ for 7 versus 0.270 ꢀ for
Wibergꢁs compound).[15]
À
À
Wiberg delineated the mechanism for the formation of the
tetraanionic P8 cluster, which occurs by a [2+2] cycloaddition
dimerization of a linear P4-diphosphene similar to compound
D reported by Bertrand.[7b,15] We therefore hypothesized that
7 also formed by an in situ [2+2] cycloaddition reaction of
a
transient diphosphene (8, Scheme 4A). Apart from
The 31P NMR of 7 is reminiscent of an AA’XX’ spin
system with signals centered d =+ 41.6 ppm and À45.2 ppm.
In the 1H NMR the carbene peaks were all shifted downfield
by about 0.2 ppm when compared to the free ligand 2.[10]
Based on these data alone, we were unable to conclusively
assign the structure of 7. To fully elucidate the structure,
single crystals of 7 were subjected to an X-ray diffraction
analysis. 7 is comprised of a central P8 cluster that is
surrounded by four carbene ligands (Figure 2A). Close
inspection of the P8 cluster reveals a central P4 butterfly
(Figure 2B,C) that is capped at each phosphorus atom with
Wibergꢁs studies, the appearance of the dark green color
upon addition of carbene 2 to P4 also serves as evidence for
the formation of the transient diphosphene 8. As the P4-
diphosphene D from Bertrand was isolated as a dark blue
solid,[7b] we believe that the dark green color observed
corresponds to the formation of 8. To verify this proposed
mechanism, we performed a Diels–Alder experiment to trap
the putative diphosphene 8 (Scheme 4B).[7b]
Addition of 2 to a suspension of P4 in hexanes saturated
with 2,3-dimethyl-1,3-butadiene (2/P4 ratio 2:1; 1 mL buta-
diene dissolved in hexanes) resulted in an instantly observed
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Scheme 4. A) Proposed mechanism for the formation of 7. B) Diels–
Alder experiment to trap the putative diphosphene 8.
Scheme 5. Proposed reaction pathways for the formation of 6 and 7.
yellow solution followed by the precipitation of a yellow solid.
After 12 h, the solid was isolated and confirmed to be
and F,[7b] it became apparent how the electrophilicity of the
chosen carbene can dictate the identity of the final product.
The reaction mechanisms in Scheme 5 describe how both
6 and diphosphene 8 can be obtained from a monocarbene-P4
intermediate similar to H. Initial attack on H by a carbene
would result in the zwitterionic intermediate I, which has
a formal negative charge on a b-phosphorus atom. For
a weakly electrophilic carbene such as 2, the reaction
proceeds by a ring-opening mechanism to afford the linear
P4-diphosphene as described by Path 1.[7b] However, when the
strongly electrophilic carbene 1 is used, the reaction follows
Path 2, and the phosphorus anion in I attacks the carbene
empty p-orbital, eventually leading to the P4 cluster observed
in 6.
1
compound 9 by H and 31P NMR spectroscopy as well as X-
ray diffraction, thus confirming our hypothesized mechanism.
Interestingly, by using hexanes as the solvent, only a single
diastereomer of 9 precipitates, facilitating characterization by
NMR spectroscopy. The 31P NMR spectrum exhibits two
signals centered at À42.44 and + 23.33 ppm ([D8]THF). These
signals are characteristic of the expected AA’MM’ spin
system for 9. The crystal structure of 9 (Figure 3) reveals
a central linear P4 chain in which the phosphaalkenes
[14]
À
(average P C distances 1.73 ꢀ)
are oriented in an E
configuration (P-P-P-P dihedral angle 132.58).
In conclusion, we have shown that CDCs are capable of P4
activation and aggregation. Unique to the second reaction
type is the ability to rapidly prepare and isolate a neutral P8
cluster, which, to our knowledge, is the first neutral P8 cluster
of its kind. We have also shown that the electrophilicity of the
chosen CDC greatly impacts the resultant product. Future
efforts will be geared toward investigating compounds 6 and 7
as potential phosphorus transfer reagents for the preparation
of useful organophosphorus compounds. Such advances will
help aid in the development of more sustainable and
environmentally benign methods for the preparation of
phosphorus derivatives.
Received: February 7, 2013
Published online: March 12, 2013
Keywords: carbenes · clusters · cycloaddition · diphosphenes ·
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Figure 3. ORTEP drawing (ellipsoids set at 50% probability) for 9. H
atoms are omitted and mesityl substituents on carbene ligands are
represented by thin lines for clarity.[16]
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