NHC-mediated aggregation of P4: Isolation of a P12 cluster

Article abstract of DOI:10.1021/ja077296u

Just like transition metal complexes, N-heterocyclic carbenes (NHCs) promote the aggregation of white phosphorus (P₄) as shown by the high yield synthesis of a P₁₂ cluster capped by two NHCs. The nuclearity observed is equal to that

Full text of DOI:10.1021/ja077296u

Published on Web 10/31/2007
NHC-Mediated Aggregation of P4: Isolation of a P12 Cluster
Jason D. Masuda, Wolfgang W. Schoeller,* Bruno Donnadieu, and Guy Bertrand*
UCR-CNRS Joint Research Chemistry Laboratory (UMI 2957), Department of Chemistry, UniVersity of California
RiVerside, RiVerside, California 92521-0403
Received September 24, 2007; E-mail: guy.bertrand@ucr.edu
Scheme 1
Since the discovery by Ginsberg and Lindsell of the first P4
transition metal complex,1 metal-mediated degradation of white
phosphorus (P4) has been widely studied, and a variety of complexes
featuring P1-P4 fragments have been isolated.2 Not only do
transition metals induce the cleavage of P4 but they also promote
the reaggregation of the small fragments, although complexes with
nuclearity higher than six are usually obtained as byproducts in
low yields.2b Complex A is the only example of P12-containing
complex3 and is the largest cluster obtained from P4;4 its structure
has been established by 31P NMR spectroscopy (Scheme 1). There
are also a few studies concerning the reaction of P4 with main group
element derivatives;5,6 however, the reactions usually proceed
without inducing the fragmentation or aggregation of P4. The only
notable exceptions are the reactions of white phosphorus with tBu3-
SiNa7 and (Me3Si)3SiK,8 which lead to P8-containing molecules B
and C, respectively.
Scheme 2
We have recently shown that stable cyclic (alkyl)(amino)carbenes
(CAACs)9 also react with white phosphorus without inducing
fragmentation or aggregation of P4; indeed, 2,3,4,5-tetraphospha-
trienes D were obtained in good yields.10 Since the chemical
reactivity of CAACs is often different11 than that of N-heterocyclic
carbenes (NHCs),12 these results prompted us to investigate the
reactivity of the latter with P4. Here we report that NHC 113 not
only reacts with white phosphorus but also allows for the high yield
synthesis of a P12-containing compound. The mechanism of the
formation of this cluster has been studied theoretically, and some
of the fragments involved have either been characterized by
spectroscopy or trapped chemically.
three-membered rings, and one six-membered ring with P-P bond
lengths between 2.176 and 2.233 Å, typical of P-P single bonds.
The architecture of the P12 core of 3 is very different from that
found in the P12 transition metal complex A3 and is obviously
unprecedented, although the nonaphosphane core (P3-P7 and P9-
P12) is reminiscent of the organosubstituted polyphosphines and
polyphosphorus anions investigated by Baudler.15
In order to gain insight into the mechanism of the reaction leading
to cluster 3, trapping experiments of possible intermediates were
attempted. Thus, NHC 1 was reacted with 0.5 equiv of P4 in the
presence of 2,3-dimethylbutadiene. Analysis of the reaction mixture
by 31P NMR revealed the presence of two products in a 1/5 ratio.
Both compounds were isolated and fully characterized including
by single-crystal X-ray diffraction studies. The isolated minor and
major products, 4 (9% yield) and 5 (79% yield), were identified as
the [4 + 2] cycloadducts of dimethylbutadiene with the PP double
bond of triphosphirene 616 and (E)-tetraphosphatriene 2b, respec-
tively (Figure 2).
Calculations (B3LYP/6-311G**, ZPE corrected)17a using the
parent NHC (H instead of Dipp) show that 6 and 2 are able to
undergo, without energy barrier, an exothermic [3 + 2] cycload-
dition (∆E ) -13.3 kcal/mol) to give an intermediate 7 (Figure
2). The latter rearranges, with loss of two NHCs, into heptaphos-
phanorbornadiene 8,17b which is known in the coordination sphere
of transition metals.3,18 Last, derivative 8 can undergo, without
energy barrier, a [π2 + π2 + π2] reaction19 with another triphos-
Upon mixing P4 with 2 equiv of NHC 1 in C6D6, a blue solution
immediately formed. The 31P NMR spectrum of the crude reaction
mixture revealed two sets of signals similar to those observed for
D,10 suggesting the formation of (Z)- and (E)-tetraphosphatriene
isomers 2a and 2b (2a: major,14 396.7 and 69.4 ppm, JAA′
)
-523.2, JAX ) -420.8, JAX′ ) -9.3, JXX′ ) -331.5 Hz; 2b: minor,
506.5 and 63.0 ppm, JAA′ ) -647.5, JAX ) -302.3, JAX′ ) -9.2,
JXX′ ) -25.8 Hz) (Scheme 2). However, over time, the signals
attributed to 2 decreased and a series of 10 very broad peaks
appeared from +120 to -160 ppm, precluding the isolation of 2.
A mixture of 1 and P4 was then heated to 70 °C overnight, yielding
an air-sensitive, thermally stable yellow precipitate (81% isolated
yield), in addition to NHC 1. Analysis of the precipitate by 31P-
{1H} NMR spectroscopy revealed exclusively the same series of
10 peaks as noted previously, in an approximately 1:1:1:1:1:1:1:
3:1:1 ratio, suggesting the presence of a species 3 containing 12
phosphorus atoms. The 13C{1H} NMR spectrum indicated the
presence of two different PdC fragments, as shown by two doublets
at 191 (1JPC ) 128 Hz) and 189 ppm (1JPC ) 131 Hz).
Recrystallization from hot CD3CN gave yellow single crystals
that upon analysis by X-ray diffraction revealed a system containing
12 phosphorus atoms capped by two NHC ligands (Figure 1). The
polycyclic structure consists of three five-membered rings, two
9
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J. AM. CHEM. SOC. 2007, 129, 14180-14181
10.1021/ja077296u CCC: $37.00 © 2007 American Chemical Society

C O M M U N I C A T I O N S
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Figure 1. Structure of 3 (50% thermal ellipsoids are shown). Hydrogen
atoms and CH3 fragments of the aryl-iPr groups have been omitted for
clarity. Selected bond lengths (Å): P1-C1 1.756(3), P8-C29 1.777(4),
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Figure 2. Products 4 and 5, resulting from the trapping of (E)-tetraphos-
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This work demonstrates that N-heterocyclic carbenes are at least
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aggregation of P4. The understanding of the reaction pathway
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Acknowledgment. We are grateful to the NSF (CHE 0518675)
and Rhodia Inc. for financial support of this work, and the Natural
Sciences and Engineering Research Council of Canada for a
Postdoctoral Fellowship (J.D.M.). Thanks are due to D. Borchardt
for high-field NMR experiments.
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Supporting Information Available: Full experimental and spec-
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