Facile Synthesis of AsP3
Brandi M. Cossairt,1 Mariam-Céline Diawara,2 Christopher C. Cummins1*
hite phosphorus consists of tetrahedral
P4 molecules and is the key industrial intermediate for most phosphorus compounds of commercial importance (1).
In contrast, in condensed phase the corresponding arsenic molecule, As4 (yellow arsenic), is both
thermally and photochemically unstable, reverting readily to the stable gray allotrope with an
infinite sheet structure (2). To date, essentially
all that is known of the binary molecules AsnP4–n
(n = 1 to 3) has come from gas-phase studies
wherein hot (660°C) vapors of phosphorus and
arsenic mixtures under equilibrium reaction conditions were subjected to Raman spectroscopic
analysis (3). To determine the properties of AsP3
as a pure and isolated substance, we set out to
synthesize it by using a method hinging on
transition-metal chemistry.
Two salts of the [P3Nb(ODipp)3]− anion,
of interest as a P33− synthon, were synthesized
by P4 activation with dichloride precursor
Cl2Nb(ODipp)3(THF) in the presence of reducing agents (Fig. 1A), where Dipp indicates 2,6C6H3[CH(CH3)2]2 and THF, tetrahydrofuran (4).
The anion [P3Nb(ODipp)3]− was characterized by
x-ray crystallography as its sodium salt and found
to contain chemically equivalent P atoms and a
structure in which a Nb(ODipp)3 unit replaces
one vertex of the P4 tetrahedron (5). Solution nuclear magnetic resonance (NMR) spectroscopy
(31P, 13C, and 1H) studies of [P3Nb(ODipp)3]− salts
are consistent with the structural assignment, with
the 31P NMR data [singlets at a chemical shift, d,
of −206 and −170 parts per million (ppm) for the
sodium and cobaltocenium salts, respectively] serving as spectroscopic signatures of this system (5).
AsP3 itself was obtained from the reaction in
THF solvent of either [P3Nb(ODipp)3]− salt with
AsCl3; dichloride Cl2Nb(ODipp)3(THF) is regenerated in the process (Fig. 1A). Purification of
AsP3 after workup is achieved either by crystallization from ether at −35°C or by sublimation. In
a typical synthesis, ~100 mg of bright white solid
AsP3 was obtained, representing a 75% yield (5).
AsP3 is identified with a single high-field 31P NMR
resonance at −484 ppm, in good agreement with
the prediction given by our theoretical calculations
[supporting online material (SOM) text]. Furthermore, gas chromatography–mass spectrometry
(GC-MS) analysis of a toluene solution of the white
powder revealed a single band of retention time of
9.1 min, corresponding to the molecular weight
of AsP3 at 168 mass/charge (m/z) (Fig. 1B). Highresolution mass spectrometry data obtained by
using the electron impact technique featured a
parent ion signal at 167.842 m/z (5).
We acquired Raman spectra of microcrystalline AsP3 by using a solid state laser at 785 nm
(Fig. 1D) (5). The four expected bands are prominent and compare well with theory (SOM text)
and with those bands observed by Ozin for the
molecule as one component of a hot gas (3).
W
Cl
A
C
ODipp
DippO
DippO
P
P
i or ii
P
iii
P
P1
557 cm-1
Nb
40000
ODipp
ODipp
DippO
35000
GCMS of AsP3 in toluene solvent
retention time: 9.1 minutes
200000
150000
100000
62
P 2+
75
As+
50000
106
AsP+
93
P 3+
137
AsP2+
www.sciencemag.org/cgi/content/full/323/5914/602/DC1
Materials and Methods
SOM Text
Figs. S1 to S5
References
30000
Intensity (a.u.)
168
AsP3+
345 cm-1
25000
428 cm-1
20000
313
cm-1
15000
10000
200
5000
300
400
500
600
0
80
100
120
140
160
0
500
1000
1500
2000
2500
3000
3500
Raman shift (cm-1)
m/z
Fig. 1. (A) Synthetic scheme: (i) 0.5% Na/Hg, THF. (ii) 1. SmI2, THF; 2. 2 CoCp2 (M is Na or CoCp2).
(iii) AsCl3, THF. (B) GC-MS spectrum of AsP3. (C) Thermal ellipsoid plot of (AsP3)Mo(CO)3(PiPr3)2,
with 50% thermal ellipsoids and hydrogen atoms omitted for clarity. (D) Raman spectrum of AsP3.
602
14 October 2008; accepted 12 November 2008
10.1126/science.1168260
Raman shift (cm-1)
0
60
References and Notes
1. J. Emsley, The 13th Element: The Sordid Tale of Murder,
Fire, and Phosphorus (Wiley, New York, 2000).
2. N. N. Greenwood, A. Earnshaw, Chemistry of the Elements
(Butterworth-Heinemann, Oxford, ed. 2, 1997).
3. G. A. Ozin, J. Chem. Soc. A 1970, 2307 (1970).
4. J. R. Clark et al., Inorg. Chem. 36, 3623 (1997).
5. Materials and methods are detailed in supporting
material available on Science Online.
6. T. Groer, G. Baum, M. Scheer, Organometallics 17, 5916
(1998).
7. O. Manasreh, Semiconductor Heterojunctions and
Nanostructures (McGraw-Hill, New York, 2005).
8. We thank the NSF (grant CHE-719157), Thermphos
International, and the Massachusetts Institute of Technology for
funding and N. A. Piro and H. A. Spinney for helpful
discussions. Crystallographic parameters for (Na)[P3Nb(ODipp)3],
Cl2Nb(ODipp)3(THF), and (AsP3)Mo(CO)3(PiPr3)2 are available
free of charge from the Cambridge Crystallographic Data
Centre (CCDC 704466 to 704468).
Supporting Online Material
D
300000
250000
Mo1
P -M+
P
Abundance
P2
As
P
P
B
As1
P3
Cl
P
0.75 P
P32
P31
THF
Nb
AsP3 melts without decomposition at 71° to
73°C and is thermally quite stable, withstanding
temperatures of 120°C for more than 1 week in
a toluene solution. No precautions against irradiation were necessary when handling isolated
samples of AsP3 in ambient light; however, much
like P4, AsP3 must be handled anaerobically because it is pyrophoric.
For structural characterization, AsP3 was
derivatized by ligation to molybdenum in the
(AsP3)Mo(CO)3(PiPr3)2 coordination complex,
obtained as orange crystals in 63% yield (6).
Single-crystal x-ray diffraction (Fig. 1C) revealed
the AsP3 tetrahedron bound to the metal center
by a single phosphorus vertex at a distance of
2.487(1) Å. The three P–P bonds in the tetrahedron average to 2.177 Å, whereas the As–P
bonds are longer, at an average of 2.305 Å (5).
The observation of specific AsP3 coordination to
molybdenum at a phosphorus vertex is an initial
example of selectivity in a reaction of AsP3.
The synthetic strategy developed here for
P33− transfer with generation of an EP3 tetrahedron is not limited to E = As. When using SbCl3
in place of AsCl3 in the synthesis, we generated
the exotic SbP3 molecule and identified it with a
broadened resonance in the 31P NMR spectrum
at d of −461.8 ppm (5).
One possible application for AsP3 might be
as a stoichiometrically exact 1:3 source of arsenic and phosphorus atoms for the synthesis of
advanced materials (7).
30 JANUARY 2009
VOL 323
SCIENCE
1
Department of Chemistry, Massachusetts Institute of Technology, 77 Massachusetts Avenue, Room 6-435, Cambridge,
MA 02139, USA. 2École Normale Supérieure de Lyon, Université Claude Bernard Lyon 1, 46, Allée d‘Italie, 69364 Lyon
Cedex 07, France.
*To whom correspondence should be addressed. E-mail:
[email protected]
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