White phosphorus as single source of P in the synthesis of nickel phosphide

Article abstract of DOI:10.1039/b802454e

White phosphorus, P₄, was reacted in a stoichiometric manner with Ni(0) complexes or particles to produce nanoparticles of nickel phosphide. The Royal Society of Chemistry.

Full text of DOI:10.1039/b802454e

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White phosphorus as single source of ‘‘P’’ in the synthesis of nickel
phosphidew
Sophie Carenco, Irene Resa, Xavier Le Goff, Pascal Le Floch* and
´
Nicolas Mezailles*
Received (in Cambridge, UK) 12th February 2008, Accepted 17th March 2008
First published as an Advance Article on the web 29th April 2008
DOI: 10.1039/b802454e
White phosphorus, P4, was reacted in a stoichiometric manner
with Ni(0) complexes or particles to produce nanoparticles of
nickel phosphide.
this latter method requires the use of large amounts, well over
stoichiometry, of the TOP solvant/ligand in an uncontrolled
process.
Taking into account the known chemistry of P4, we postu-
lated that the reaction between a metal center in a formal
‘‘zero’’ oxidation state, bearing labile ligands, and P4 would
result in successive metal-insertions in P–P bonds to lead to the
desired ‘‘metal phosphide’’ in a stoichiometric manner. As a
proof of the validity of our concept, we present here the first
results in the synthesis of a model species, Ni2P. Two different
strategies were envisioned for the source of Ni(0): the first
relies on monometallic organometallic precursors (route A,
Scheme 1) and the second uses Ni(0) nanoparticles (route B,
Scheme 1).
White phosphorus, P4, is the most widely employed starting
precursor for all the chemistry pertaining to phosphorus
derivatives. It serves as a phosphorus carrier for the elabora-
tion of most phosphines via the intermediacy of either PCl3 or
PH3. However, apart from these well-known basic transfor-
mations, P4 has only rarely been used in more elaborate
processes. Its reactivity towards transition metals has been
studied with the ultimate goal of devising metal catalyzed
transformations to phosphorus derivatives, without having to
handle highly toxic PH3 gas for instance. The development of
such processes has been hampered by the often uncontrollable
reactivity of P4 with metal precursors. Indeed, coordination of
one to four phosphorus centers and insertions into one to
several of the P–P bonds of P4 are highly competitive pro-
cesses. In some instances however, using carefully designed
metal complexes, with strongly bound ligands, one of these
processes may be favored.1
Before trying to stabilize nanoparticles of Ni2P made by
either one of these routes, it had to be verified that the reaction
could indeed be carried out in the bulk (route A, without
stabilizing ligands L). Thus, in a first attempt, [Ni(COD)2]
complex dissolved in toluene, was reacted at room tempera-
ture with a stoichiometric amount of P4 (also in toluene).z It
resulted in the instantaneous precipitation of a black solid,
insoluble in all common solvents (Scheme 2). NMR character-
ization of the supernatant solution showed the absence of both
the starting compounds, (P4 in the 31P NMR spectrum and
From another standpoint, some synthetic approaches to-
ward nanocrystalline transition metal phosphides have been
developed very recently. This interest is driven by the wide
range of applications found for these species (catalytic, elec-
tronic, or magnetic applications).2 Three different routes stand
out as the most reliable. Firstly, the solvothermal reaction
starting from a phosphate precursor may lead to different
derivatives depending on the decomposition temperature.3
This procedure typically requires very high temperatures.
Secondly, the highly sensitive phosphine derivative P(SiMe3)3
may be hydrolyzed in situ to form highly toxic PH3 which in
turns reacts with a metal precursor.4 This method was used in
particular for the synthesis of the highly desirable semi-con-
ductor InP, albeit only in mg quantities.5 Lastly, several
groups have used both the coordinating behavior of TOP
(tri-n-octylphosphine) and its ability to act as a ‘‘P atom’’
donor, through thermal decomposition at around 330 1C, to
form several metal phosphides such as Ni2P or FeP.6 However
1
[Ni(COD)2] in the H NMR spectrum) indicating a quantita-
tive reaction. A thermal treatment (300 1C, 1 h) of this solid
suspended in TOPO (tri-n-octylphosphine oxide), resulted in
the formation of highly insoluble nanocrystalline Ni2P as
shown both by X-ray powder diffraction (see Fig. 1) and
TEM, although the TEM pictures showed a binomial type
population (from a drop of the soluble part). Most impor-
tantly, this first experiment established the feasibility of our
approach.
We then turned our attention to obtaining soluble nano-
particles of Ni2P, which requires, as a prerequisite, the selec-
tion of a ligand that allows the kinetic stabilisation of the
growing species and prevents their aggregation. TOP, which
already proved to be a competent ligand for the stabilization
of nanoparticles of Ni2P synthesized at high temperature, was
Laboratoire ) He´te´roe´le´ments et Coordination *, Ecole
´
Polytechnique, CNRS, 91128 Palaiseau Cedex, France.
E-mail: nicolas.mezailles@polytechnique.edu. E-mail:
pascal.lefloch@polytechnique.edu; Fax: (+33) 1 69 33 44 49;
Tel: (+33) 1 69 33 44 02
w Electronic supplementary information (ESI) available: TEM images
and crystallographic data. CCDC 654636. See DOI: 10.1039/b802454e
Scheme 1 Reactions of P4 towards Ni(0) precursors.
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Scheme 2 Bulk synthesis of Ni2P from [Ni(COD)2].
Scheme 3 Mechanistic study of the reaction of a Ni(0) complex with
P4 in the presence of excess phosphine: route A.
used in a mechanistic study of route A. The reaction of the
[Ni(COD)2] complex with 10 equivalents of TOP was carried
out at room temperature (Scheme 3), and followed by 31P{1H}
NMR spectroscopy. The spectrum of the crude mixture
showed the co-existence of three phosphorus containing spe-
cies: a sharp signal for the homoleptic [Ni(TOP)4] complex
reported in the literature that thermal decomposition of a
Ni(II) precursor, [Ni(acac)2], at 215 1C in the presence of large
amounts of either TOP, TOPO or OA (oleylamine) resulted in
the formation of Ni(0) nanoparticles.7 We reasoned that
starting from a more reactive, already in the ‘‘right’’ oxidation
state, Ni(0) precursor, the reaction would occur at much lower
temperature.8 Indeed, thermal decomposition of [Ni(COD)2]
at 100 1C in the presence of 10 equivalents of TOPO or OA led
to the formation of the desired monodisperse Ni(0) NPs
(Scheme 4, Fig. 2b and c). The same procedure using TOP
did not lead to the formation of Ni NPs but to the above
described mixture of complexes, despite a long heating period
at 100 1C. However, in order to test the second route, Ni(0)
particles were prepared from [Ni(acac)2] and TOP at 215 1C
(Scheme 4, Fig. 2a). It may be noted here that the analogous
reaction performed at 215 1C with the Ni(II) precursor
[Ni(acac)2], and 10 TOPO only led to agglomerated particles,
whereas with 10 TOP they remained isolated.
(dP = ꢁ4.2 ppm), a broad signal for the free TOP (dP
=
ꢁ33.3 ppm), and a sharp singlet (dP = 10.7 ppm) correspond-
ing to a previously unknown complex. The two complexes are
formed in an approximate 1 : 2 ratio, respectively (as attested
by integration of the respective signals). Suspecting that this
latter complex could be the mixed [Ni(COD)(TOP)2] complex,
we synthesized it quantitatively from the reaction of
[Ni(COD)2] with only two equivalents of TOP, and character-
1
ized it by 31P and H NMR spectroscopies.
Because of the long alkyl chains it was reluctant to crystal-
lize, but final proof of the structure was obtained with the
analogous [Ni(COD)(PBu3)2] complex (dP = 10.3 ppm) using
a similar procedure (see ESIw). The reaction of the mixture
with a stoichiometric amount of P4 at room temperature was
then followed by 31P{1H} NMR spectroscopy. Again, the
reaction is very fast, being complete within a few minutes.
The spectrum showed the lack of signal for P4 (at ꢁ524 ppm)
and the complete disappearance of the starting Ni complexes
with the concomitant release of TOP (signal at ꢁ33.3 ppm), as
well as a weak, very broad signal at around 0 ppm. However,
precipitation of Ni2P particles proved to be impossible, sug-
gesting the formation of very small species or soluble cluster
type complexes. A thermal process was then performed in
order to grow larger particles.
The isolated (or not) Ni(0) nanoparticles, stabilized by
either TOP or OA were reacted with stoichiometric amounts
of P4 (vs. starting Ni complex) at 100 1C for a few hours,
leading to the formation of the desired nanoparticles of Ni2P.
X-Ray powder diffraction of the as-synthesized black solid
precipitated from the solution showed only featureless peaks,
consistent with the formation of nanoparticles of a few nano-
metres in size (Fig. 2d). A thermal treatment of the crude
product at 100 1C for 4 h, allowed the crystallization of the
material which the XRD spectrum showed to be Ni2P, exempt
of residual Ni(0) (Fig. 1b). For example, starting from isolated
OA stabilized Ni nanoparticles of 6.5 nm (see ESIw), Ni2P
nanoparticles of 4.8 nm were obtained. Most interestingly, our
results show that the reaction performed at much lower
temperature than usual leads to the desired nanoparticles of
Ni2P.
A second route (route B, Scheme 1) was also devised,
starting from preformed nickel nanoparticles. It has been
In conclusion, we present here a new and simple synthesis,
as well as stoichiometric, of Ni2P at low temperature based on
the high reactivity of P4 toward reactive Ni(0) complexes. An
extension of this reaction to the reactivity of Ni(0)
Fig. 1 X-Ray powder diffraction: (a) Ni2P after thermal treatment at
300 1C for 1 h in TOPO. (b) Ni(0) nanoparticles + P4, after isolation
and thermal treatment at 100 1C for 4 h (solid). (c) Ni2P before thermal
treatment.
Scheme 4 Synthesis of Ni2P nanoparticles from Ni nanoparticles:
route B.
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300 MHz, 25 1C, ppm): 10.27. X-Ray structure data, Mt = 285.75,
crystal dimensions 0.30 ꢂ 0.16 ꢂ 0.08 mm, monoclinic, C2/c, a =
21.777(1), b = 8.967(1), c = 17.218(1) A, a = 90.00, b = 90.769(1), g
= 90.001, V = 3361.9(5) A3, Z = 8, rcalcd = 1.129 g cmꢁ3, m = 0.690
cmꢁ1, F(000) = 1264, ymax = 30.031, hkl ranges: ꢁ25 30, ꢁ11 12, ꢁ20
24, 15 342 data collected, 4911 unique data (Rint = 0.0274), 4007 data
with I 4 2s(I), 163 parameters refined, GOF(F2) = 1.073, final R
indices (R1 = S||Fo| ꢁ |Fc||/S|Fo|, wR2 = [Sw(Fo2 ꢁ Fc2)2/
Sw(Fo2)2]1/2, R1 = 0.0371, wR2 = 0.0994, max/min residual electron
density 0.514(0.057)/ꢁ0.469(0.057) e Aꢁ3. CCDC 654636.
Synthesis of Ni nanoparticles. A mixture of [Ni(COD)2] (0.5 mmol)
and trioctylphosphine oxide (5 mmol) or oleylamine (5 mmol) was
stirred magnetically under nitrogen and heated at 100 1C for 2 h. The
color of the solution changed from orange to black. After cooling at
room temperature, 25 mL of acetone was added to the solution to give
a black precipitate, which was separated from the solution by cen-
trifugation (3500 rpm, 10 min), washed several times with acetone and
dried under vacuum. The recovered nanoparticles could be redispersed
in hexanes.
Synthesis of Ni2P nanoparticles. To a mixture of Ni nanoparticles
(0.5 mmol) and trioctylphosphine oxide (5 mmol) or oleylamine
(5 mmol), 0.5 mL of a 0.127 M solution of P4 in toluene (0.06 mmol)
was added. The mixture was stirred under nitrogen at 100 1C for 1 h.
After cooling at room temperature, 25 mL of acetone was added to
the solution to give a black precipitate, which was washed several
times with acetone and dried under vacuum. An XRD spectrum
is shown in Fig. 1. A second XRD spectrum is shown after thermal
treatment of the solid for 4 h at 100 1C. The X-ray powder diffraction
was carried out on the dried samples after a thermal treatment at
100 1C for several hours. The nanoparticles could be redispersed in
hexane.
Fig. 2 TEM images for Ni(0) particles stabilized with TOP (a),
TOPO (b) or oleylamine (c), and Ni2P particles stabilized with
oleylamine (d).
nanoparticles stabilized by labile ligands (alkylphosphines,
phosphine oxides or amines) led to the formation of nano-
particles of Ni2P. The application of this strategy for the
syntheses of several other ‘‘metal phosphide’’ derivatives is
currently underway in our laboratories, and results will be
reported in due course.
1 For a review see: M. Peruzzini, L. Gonsalvi and A. Romerosa,
Chem. Soc. Rev., 2005, 34, 1038–1047.
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S. L. Brock, S. C. Perera and K. L. Stamm, Chem.–Eur. J.,
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The authors thank the CNRS, the Ecole Polytechnique and
the DGA for financial support. I. R. thanks the DGA for a
post doctoral fellowship. The ‘‘Laboratoire des solides
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´
acknowledged for the generous access to their TEM.
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Notes and references
z Safety Note: white phosphorus is stable in water but highly flam-
mable and very toxic if swallowed or inhaled. It is incompatible with
strong oxidizing agents and strong bases. It is light and heat sensitive.
It should be handled accordingly.
Synthesis of [Ni(COD)(PBu3)2]. Over a solution of [Ni(COD)2]
(0.45 mmol) in THF (5 mL) tributylphosphine (0.9 mmol) was added.
The color of the solution changed from yellow to orange and the
solution was stirred 1 h at room temperature. The solvent was
removed under vacuum yielding a yellow solid which was crystallized
from hexanes. 1H NMR (C6D6, 300 MHz, 25 1C, ppm): 1.04 (6H), 1.49
(8H), 1.68 (4H), 2.40 (4H), 2.60 (4H), 4.55 (4H). 31P{1H} NMR (C6D6,
7 J. Park, E. Kang, S. U. Son, H. M. Park, M. K. Lee, J. Kim, K. W.
Kim, H. J. Noh, J. H. Park and T. Hyeon, Adv. Mater., 2005, 17,
429–434.
8 Decomposition of [Ni(COD)2] under H2 at room temperature has
been reported: N. Cordente, M. Respaud, F. Senocq, M.-J.
Casanove, C. Amiens and B. Chaudret, Nano Lett., 2001, 1,
565–568.
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2570 | Chem. Commun., 2008, 2568–2570