Preparation of the [(DippNP)2(P4)2] 2+-dication by the reaction of [DippNPCl]2 and a Lewis acid with P4

Article abstract of DOI:10.1021/ja906878q

(Chemical Equation Presented) The controlled activation of white phosphorus, P₄, provides a key entry point into many aspects of phosphorus chemistry. The activation and functionalization of P₄ by N-heterocyclic carbenes and carbene-

Full text of DOI:10.1021/ja906878q

Published on Web 09/23/2009
Preparation of the [(DippNP)2(P4)2]2+-Dication by the Reaction of [DippNPCl]2
and a Lewis Acid with P4
Michael H. Holthausen and Jan J. Weigand*
Institut fu¨r Anorganische und Analytische Chemie, Westfa¨lische Wilhelms-UniVersita¨t Mu¨nster,
Corrensstrasse 30, 48149 Mu¨nster, Germany
Received August 21, 2009; E-mail: jweigand@uni-muenster.de
The activation and functionalization of white phosphorus by
N-heterocyclic carbenes1 and carbene-like main group element
fragments2 is of considerable current interest. Carbene-analogous
phosphenium cations display a pronounced ambiphilic nature which
renders their behavior both Lewis acidic and Lewis basic. Their
electrophilic character comes to the fore in element-element bond
insertion reactions,3 making them interesting species for the
systematic investigation of P4 functionalization. Only recently, the
first structurally characterized inorganic cationic P5 cluster [P5Br2]+
was obtained Via phosphenium insertion by Krossing and co-
workers.4 Our solvent-free approach to consecutively insert the
phosphenium cation [Ph2P]+ into P-P5a,b bonds of P4 resulted in
the formation of unprecedented phosphorus-rich cationic clusters
5c
[Ph2P5]+, [Ph4P6]2+, and [Ph6P7]3+
.
In this communication, we
Figure 1. 31P NMR spectrum of cation 3 (in C6H5F, C6D6-capillary, 25
°C; 161.94 MHz). Full spectrum (bottom) and expansions (inset) showing
the experimental (up) and fitted (down) spectra;9 a very small amount of
an unidentified side-product is indicated by an asterisk.8b
report on the functionalization of P4 formally through the cationic
2+
bifunctional Lewis acid [DippNP]2 obtained from cyclo-1,3-
diphospha-2,4-diazane [DippNPCl]2 (1). This has enabled the
targeted preparation of novel mono- and dicationic phosphorus-
rich clusters 3[GaCl4]•C6H5F and 4[Ga2Cl7]2 (Scheme 1).
A single-crystal X-ray study of [3][GaCl4]•C6H5F (Figure 2)
confirmed the insertion of 2 into one of the P-P bonds of P4. The
structural features of the P5 core are comparable to [Ph2P5]+.5c The
P-P bonds involving the four-coordinate phosphorus center P1 and
the P4-P5 bond (2.1462(7) - 2.164(1) Å) are significantly shorter
than the remaining P-P bonds (2.2461(8) - 2.2535(8) Å).
Comparably short P-P distances have been observed in other
compounds with cationic four-coordinate phosphorus centers and
strained phosphorus cages.2i,11 The P-N bonds between the tri-
coordinate phosphorus atom P6 and N1 or N2 (1.733(2), 1.732(2)
Å) are typical for neutral diphosphadiazanes.12 In contrast, the P-N
bonds involving the phosphonium center P1 are significantly shorter
(1.662(2), 1.669(2) Å).
a
Scheme 1. Reaction of 1 with GaCl3 and P4
a (i) 1 eq. GaCl3, C6H5F, rt, 10 min, (ii) 1 eq. P4, C6H5F, rt, 2 h; (iii) 3
eq. GaCl3, P4 C6H5F, rt, 6 h.
Cyclic phosphenium cation 2 can be generated from 1 in the
presence of the Lewis acid GaCl3.6 The addition of one eq. GaCl3
to a solution of 1 in C6H5F afforded an instant color change of the
initially colorless solution to deep red. 31P{1H} NMR investigation
of the reaction mixture showed a new broad signal (C6D6 capillary,
rt, δ ) 242.3 ppm, ∆ν1/2 ) 104 Hz) shifted downfield compared
to the sharp signal of 1 (cis isomer, δ ) 210.5 ppm, ∆ν1/2 ) 10
Hz),7 indicating the formation of cation 2.8a The subsequent addition
of one eq. P4 gave rise to a clear, pale orange solution within 2 h
(Scheme 1). The 31P NMR spectrum of the reaction mixture revealed
the exclusive formation of monocation 3, which showed an
A2MVXZ spin system (δA ) -346.7 ppm, δM ) 85.1 ppm, δV )
152.7 ppm, δX ) 168.2 ppm, δZ ) 197.5 ppm; 1JAV ) -139.7 Hz,
1JAX ) -139.9 Hz, JMV ) -282.5 Hz, JMX ) -293.4 Hz, JAM
) 16.4 Hz, 2JVX ) 57.0 Hz, Figure 1).8b,9 The resonances for this
spin system are in an approximate ratio of 2:1:1:1:1. Similar to
other N2P2 systems,7,10 the two ring phosphorus atoms of 3 do not
couple, resulting in the observation of a singlet for the tri-coordinate
phosphorus atom (s, δZ ) 197.5 ppm). The very air- and moisture-
sensitive material was isolated as the [GaCl4]- salt as fluorobenzene
solvate.
1
1
2
Figure 2. ORTEP plot of the molecular structure of the cation 3 in
3[GaCl4] · C6H5F. Thermal ellipsoids at 50% probability (hydrogen atoms,
counteranion and C6H5F omitted for clarity). Selected bond lengths (Å):
P6-Cl1 2.0788(7), P6-N1 1.733(2), P6-N2 1.732 (2), P1-N1 1.669(2),
P1-N2 1.662(2), P1-P2 2.1518(7), P1-P3 2.1462(7), P2-P4 2.2535(8),
P2-P5 2.2461(8), P3-P4 2.2484(8), P3-P5 2.2472(8), P4-P5
2.164(1).
9
14210 J. AM. CHEM. SOC. 2009, 131, 14210–14211
10.1021/ja906878q CCC: $40.75  2009 American Chemical Society

C O M M U N I C A T I O N S
of the disguised bifunctional Lewis acid [DippNP]22+ into the P-P
bonds of two P4 tetrahedra. The utilization of bifunctional phos-
phenium cations13 represents a rational and potentially versatile
synthetic method for the assembly of large clusters using P4 as a
building block. Due to the cationic charge such cluster may be
amenable to a host of subsequent transformations. Studies directed
at the synthesis of further cationic clusters and the reactivity of 3
and 4 are in progress.
Acknowledgment. We gratefully acknowledge the Alexander
von Humboldt-Foundation (Feodor Lynen Return Fellowship for
J.J.W.), the FCI (Liebig fellowship for J.J.W.), the European
Phosphorus Science Network (PhoSciNet CM0802), and the
International Research Training Group (IRTG 1444). J.J.W. thanks
Prof. F. Ekkehardt Hahn (WWU Mu¨nster) for his generous support
and advice. We thank Dr. Robert Wolf for helpful discussions.
Figure 3. 31P{1H} NMR spectrum of 3[GaCl4] and 4[Ga2Cl7]2 (in C6H5F,
C6D6-capillary, 25 °C; 161.94 MHz); very small amounts of unidentified
side-products are indicated by an asterisk.8b
The reaction of two equiv. of P4 with cation 2 and an excess of
Lewis acid (2, GaCl3 1:4) in C6H5F resulted in the formation of a
pale-yellow solution with small amounts of orange-yellow precipi-
tate (Scheme 1). The 31P{1H} NMR spectrum of the filtrate is
depicted in Figure 3. Beside the resonances of monocation 3, signals
of a new A2MX2 spin system (δA ) -341.8 ppm, δM ) 67.4 ppm,
Supporting Information Available: Full experimental and spec-
troscopic data for compounds 2[Ga2Cl7] and 3[GaCl4]·C6H5F and
selected data for 4[Ga2Cl7]2 (including 31P-31P DQF COSY), and X-ray
crystallographic data for 3[GaCl4] and 4[Ga2Cl7]2 (CCDC numbers
743848, 743849). This material is available free of charge via the
Internet at http://pubs.acs.org.
1
1
2
δX ) 142.3 ppm; JAX ) -320.3 Hz, JMX ) -135.2 Hz, JAM
)
21.9 Hz) in a ratio of 2:1:2 indicate the formation of a new species
in approximately 60% yield.8b The resonances for the A2MX2 spin
system are consistent with two C2v-symmetric P5 cages bridged by
two imido groups, suggesting the formation of dication 4 (Figure
3). 4[Ga2Cl7] crystallized as a conglomerate with 3[GaCl4]. The
postulated structure in solution was confirmed by X-ray diffraction
(Figure 4). To our knowledge, dication 4 represents the first
structurally characterized example of two homoatomic P5 cages
fused Via an imido bridge. In the solid state, dication 4 is
centrosymmetric, consistent with the A2MX2 pattern observed in
the 31P{1H} NMR spectrum. The bond lengths and angles in the
P5 cores of dication 4 follow a similar trend as observed for
monocation 3. The N2P2 ring is planar with a short P1-N1 bond
(1.684(2) Å). The pronounced short character of the P-N bonds
in the P2N2 core might account for an increased reactivity. In
solution, dication 4 is not very stable and readily decomposes to
an insoluble orange material at room temperature. The 31P{1H}
NMR spectrum of the reactions mixture after ∼12 h shows
an additional complex set of signals. This indicates the formation
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Figure 4. ORTEP plot of the molecular structure of the cation 4 in
4[Ga2Cl7]2. Thermal ellipsoids at 50% probability (hydrogen atoms and
counteranions omitted for clarity). Selected bond lengths (Å): N1-P1
1.684(2), P1-P2 2.1388(7), P1-P3 2.1380(7), P2-P4 2.2603(8), P2-P5
2.2450(8), P3-P4 2.2598(8), P3-P5 2.2425(8), P4-P5 2.1659(9); [sym-
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