Interaction of carbene and olefin donors with [Cl2PN] 3: Exploration of a reductive pathway toward (PN)3

Article abstract of DOI:10.1021/ic201812a

The iminophosphine-phosphazene [P^III-P^V] heterocyclic adduct [IPr?PN(PCl₂N)₂] was prepared via reduction of the cyclic phosphazene [Cl₂PN]₃ in the presence of the carbene donor IPr {IPr = [(HCNDipp)₂C:], where Dipp = 2,6- ⁱPr₂C₆H₃}. By contrast, the treatment of [Cl₂PN]₃ with the N-heterocyclic olefin IPr=CH₂ yielded the olefin-grafted phosphazene ring [(IPr=CH)P(Cl)N(PCl₂N)₂].

Full text of DOI:10.1021/ic201812a

COMMUNICATION
pubs.acs.org/IC
Interaction of Carbene and Olefin Donors with [Cl2PN]3:
Exploration of a Reductive Pathway toward (PN)3
S. M. Ibrahim Al-Rafia, Michael J. Ferguson, and Eric Rivard*
Department of Chemistry, University of Alberta, 11227 Saskatchewan Drive, Edmonton, Alberta, Canada T6G 2G2
S Supporting Information
b
Scheme 1. Synthesis of the IminophosphineꢀPhosphazene
ABSTRACT: The iminophosphineꢀphosphazene [PIIIꢀPV]
Adduct 1 and Representative Chemistry
heterocyclic adduct [IPr PN(PCl2N)2] was prepared via re-
3
duction of the cyclic phosphazene [Cl2PN]3 in the presence of
the carbene donor IPr {IPr = [(HCNDipp)2C:], where Dipp =
2,6-iPr2C6H3}. By contrast, the treatment of [Cl2PN]3 with the
N-heterocyclic olefin IPrdCH2 yielded the olefin-grafted phos-
phazene ring [(IPrdCH)P(Cl)N(PCl2N)2].
he use of N-heterocyclic carbenes (NHCs) as supporting
T
ligands to isolate/stabilize inorganic species that were either
unknown or inaccessible using conventional methods is a rapidly
developing avenue of research.1 In this regard, the synthesis of
NHC adducts featuring reactive entities, such as :BH, :SiX2 (X = Cl
and Br), :SidSi:, P2, and PH, represent particularly noteworthy
achievements.2 These breakthroughs have substantially expanded
our general knowledge of bonding in inorganic chemistry and have
facilitated the discovery of a number of useful chemical transforma-
tions involving once elusive inorganic species as reagents.3
δX 6.1 (d, J = 86.9 Hz)]. The disparate nature of the observed
chemical shifts suggested the presence of a single product
with two phosphorus environments in different oxidation states.
Single-crystal X-ray crystallography11 later identified this species
as the novel iminophosphineꢀphosphazene [PIIIꢀPV] adduct
Attempts to further reduce the remaining PV centers in 1 with
additional equivalents of IPr and sodium yielded no discernible
reaction. Compound 1 could also be obtained in low isolated
yield (13%) when [Cl2PN]3 was directly combined with 2 equiv
of IPr in the absence of sodium; the low yield of 1 stems from the
formation of a number of unidentified side products during the
reaction. Importantly, this latter transformation reveals that IPr
can also serve as a dehalogenation/reducing agent.8
Our recent syntheses of stable inorganic methylene and
ethylene adducts (:EH2 and H2EE0H2, where E and E0 = Si,
Ge, and/or Sn),4,5 have provided an impetus to explore the
preparation of NHC-supported complexes of phosphorus mono-
nitride (PN) and/or its oligomers (PN)x. Molecular PN was
originally generated and studied via matrix-isolation techniques6
and was later identified as a component of interstellar space.7 In
addition, PN represents a heavier analogue of N2 and is thus an
attractive species from a fundamental standpoint. In this Com-
munication, we report a potential route toward isolating an
adduct of (PN)3 under ambient conditions. Our strategy relies
upon reduction of the readily available cyclic precursor [Cl2PN]3
in the presence of carbon-based donors to yield a stable complex
of (PN)3 (eq 1).8,9 The present study is conceptually linked with
the elegant synthesis of a formal bis(carbene) adduct of PN by
Bertrand and co-workers in 2010.10
[IPr PN(PCl2N)2] (1; Scheme 1 and Figure 1).
3
As shown in Figure 1, 1 contains a carbene-ligated P center
with a CIPrꢀP distance [C(1)ꢀP(1)] of 1.8791(13) Å. This value
is elongated compared to the CꢀP distances in the cationic
i
i
phosphorus bisadduct [(ImMe2 Pr2) P (ImMe2 Pr2)]Cl (where
3 3
ImMe2 Pr2 = [(MeCNiPr)2]C:) [1.824(3) Å ave]8b and is much
i
longer than the CIPrꢀP linkages within IPr P2 IPr [1.7504(17) Å],
3
3
wherein significant PꢀCIPr π bonding is present.2e The P3N3
heterocycle in 1 adopts an envelope conformation with pyramidal
geometry about the apical P(1) atom in the ring [angle sum =
307.3(1)°]. Compound 1 also features considerable intraring PꢀN
bond-length variation, with long PꢀN bonds of 1.6770(12) and
1.6845(13) Å involving the three-coordinate P(1) center, while the
remaining PꢀN distances vary from 1.5423(12) to 1.5923(13) Å;
The interaction of the hindered carbene IPr {IPr =
[(HCNDipp)2C:], where Dipp = 2,6-iPr2C6H3]} with
[Cl2PN]3 in the presence of sodium metal as a reductant yielded
a new crystalline product, which exhibited an AX2 splitting
pattern in the 31P NMR spectrum [δA 101.4 (t, J = 86.9 Hz);
Received: August 18, 2011
Published: October 07, 2011
r
2011 American Chemical Society
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|

Inorganic Chemistry
COMMUNICATION
Scheme 2. Synthesis of the Alkene-Substituted Phosphazene 4
Figure 1. Thermal ellipsoid plot (30% probability level) for 1 with
hydrogen atoms and toluene solvate omitted for clarity. Selected bond
lengths (Å) and angles (deg): C(1)ꢀP(1) 1.8791(13), P(1)ꢀN(1)
1.6770(12), P(1)ꢀN(3) 1.6845(13), P(2)ꢀN(1) 1.5423(12), P(2)ꢀ
N(2) 1.5900(13), P(3)ꢀN(2) 1.5923(13), P(3)ꢀN(3) 1.5543(13);
C(1)ꢀP(1)ꢀN(1) 99.05(6), C(1)ꢀP(1)ꢀN(3) 100.14(6), N(1)ꢀ
P(1)ꢀN(3) 108.12(6).
the latter bond lengths are in the range usually observed in
phosphazene heterocycles.12
The above-mentioned data support the presence of a ster-
ochemically active phosphorus lone pair in 1. Congruently, 1
reacted rapidly with sulfur to afford the novel phosphine sulfide
[IPr (S)PN(PCl2N)2] (2) as a colorless solid (Scheme 1 and
3
Figure S1).11 The NMR spectra for 2 were consistent with the
presence of phosphazene environments, and correspondingly
short PꢀN distances of 1.5595(17)ꢀ1.6256(16) Å were
observed by X-ray crystallography.11 Despite the presence of
a terminal sulfido group in 2, the dative CIPrꢀP(1) interac-
tion [1.8582(18) Å] was similar in length to the carbeneꢀ
phosphorus interaction within the reduced precursor 1
[1.8791(13) Å]. For comparison, the PdS bond distance in 2
[1.9361(7) Å] lies within the typical bond length values deter-
mined for phosphine sulfides, R3PdS [e.g., 1.950(3) Å within
Ph3PdS].13 Oxidation of the carbene-bound phosphorus cen-
ter in 1 with a chalcogen is reminiscent of prior work by Kuhn
and co-workers, who prepared the phosphonium selenide
Figure 2. Thermal ellipsoid plot (30% probability level) for 4 with
hydrogen atoms and solvate omitted. The N3P3Cl5 group was disor-
dered over two positions (70:30), and only the major orientation is
shown for clarity. Selected bond lengths (Å) and angles (deg) with
values due to the minor orientation of the N3P3Cl5 group in brackets:
C(1)ꢀC(4) 1.398(2), C(4)ꢀP(1A) 1.687(2) [1.708(3)], P(1A)ꢀ
N(3A) 1.6089(17), P(1A)ꢀN(5A) 1.610(2), P(2A)ꢀN(3A) 1.5617-
(16), P(2A)ꢀN(4A) 1.583(2), P(3A)ꢀN(4A) 1.583(2), P(3A)ꢀ
N(5A) 1.567(2); C(1)ꢀC(4)ꢀP(1A) 129.07(14) [132.52(17)].
i
complex [Ph2P(Se) ImMe2 Pr2]AlCl4 via the direct oxidation
3
of an NHC phosphenium (Ph2P+) adduct with selenium.9c Of
note, we were also able to coordinate BH3 to the phosphorus
IPrdCH group at phosphorus. The latter process yields the
insoluble imidazolium salt [IPrCH3]Cl, which was isolated in
pure form by filtration.11,15 Attempts to functionalize the remain-
ing PꢀCl bonds in 4 with excess IPrdCH2 failed; however, the
IPrdCH residue was readily cleavable from the phosphazene
ring by treatment with anhydrous HCl, regenerating [Cl2PN]3 in
the process (eq 2).
donor site in 1 to yield the stable adduct [IPr P(BH3)N(P
3
Cl2N)2] (3); however, attempts to obtain crystals suitable for
X-ray crystallographic analysis were unsuccessful.11
Inspired by the recent use of the N-heterocyclic olefin IPr=CH2
as a donor ligand in low-oxidation-state main-group chemistry,4c,14
we subsequently investigated the reaction of IPrdCH2 with
[Cl2PN]3. As illustrated in Scheme 2, the sole phosphorus-contain-
ing product in the reaction was the alkene-substituted heterocycle
[(IPrdCH)P(Cl)N(PCl2N)2] (4). Interestingly, the same product
is obtained when the reaction is conducted in the presence of
sodium metal as a potential reductant.
The formation of 4 likely involves the initial nucleophilic
displacement of a phosphorus-bound chloride in [Cl2PN]3 by
IPrdCH2, followed by deprotonation (HCl elimination) in
the presence of excess basic IPrdCH2 to generate an alkenyl
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Inorganic Chemistry
COMMUNICATION
Compound 4 was also characterized by single-crystal X-ray
crystallography (Figure 2). The phosphazene heterocycle 4
contains PꢀN bond lengths in the narrow range of 1.5617-
(6)ꢀ1.610(2) Å, while the exocyclic PꢀC interaction is sig-
nificantly shorter [P(1A)ꢀC(4) = 1.692(4) Å ave] than the
dative PꢀCIPr linkages within the heterocyclic adducts 1 and 2.
Furthermore, the short PꢀC distance in 4 is accompanied by the
substantial lengthening of the proximal P(1)ꢀCl(1) bond length
[2.088(2) Å ave] relative to the PꢀCl distance observed in
phenyl-substituted phosphazene [PhP(Cl)N(PCl2N)2] [2.021(2)
Å].16 These metrical parameters suggest that the IPrdCH sub-
stituent is strongly electron-releasing, thereby leading to a weaken-
ing of the adjacent PꢀCl interaction. Unfortunately, our attempts to
remove a chloride ion from 4 using the known halide abstractors
Ag[A] (A = O3SCF3ꢀ and SbF6ꢀ) led to inseparable product
mixtures in place of the desired cyclophosphazene cation
[(IPrdCH)PN(PCl2N)2]+ ([4]+).17
In summary, partial reductive dehalogenation of [Cl2PN]3 in
the presence of the carbene donor IPr affords the novel mixed
PIIIꢀPV heterocyclic adduct 1; this species was also reacted with
sulfur to give the sulfido adduct 2. A divergent reaction pathway
was observed between IPrdCH2 and [Cl2PN]3, leading to the
olefin-bound cyclophosphazene 4. Future work will focus on the
reduction of [Cl2PN]3 in the presence of less hindered NHC
coligands in order drive the system toward a fully dehalogenated
(PN)3 heterocycle. The ability to prepare metastable complexes
of (PN)3 should lead to the discovery of new binary PꢀN
materials upon controlled removal of the stabilizing ligands,3b,e
with potential applications in materials science envisioned.18
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’ ASSOCIATED CONTENT
S
Supporting Information. Full synthetic procedures and
b
X-ray crystallographic information files in CIF format. This material
is available free of charge via the Internet at http://pubs.acs.org.
’ AUTHOR INFORMATION
Corresponding Author
*E-mail: erivard@ualberta.ca.
(10) Kinjo, R.; Donnadieu, B.; Bertrand, G. Angew. Chem., Int. Ed.
2010, 49, 5930.
(11) See the Supporting Information for full details.
’ ACKNOWLEDGMENT
(12) Bartlett, S. W.; Coles, S. J.; Davies, D. B.; Hursthouse, M. B.;
€
We thank the NSERC of Canada, the Canada Foundation for
Innovation, Alberta Innovates (New Faculty Award), and Suncor
Energy Inc. (Petro-Canada Young Innovator Award) for finan-
cial support of this work. S.M.I.A. also thanks Alberta Innovates
for a graduate scholarship.
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Scheme 2 via the 1:1 reaction of [Cl2PN]3 and IPrdCH2 yielded only 4
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