Phosphinopnictinophosphonium frameworks

Article abstract of DOI:10.1039/b924918d

Reactions of the cyclic tetraphosphine (CyP)₄ with arsenium or stibenium cations (R₂Pn⁺, Pn = As, Sb and R = Cl, Ph) give salts of the new cationic frameworks [(CyP)₄(AN)]⁺, [(CyP)₄(SbCl₂)]⁺ and [{(CyP)₄} ₂(Sb₂Cl₂)]²⁺. The cations can be viewed as coordination complexes of a cyclophosphine ligand on a pnictenium cation, or a cyclo-2,3,4-triphosphino-1-pnictino-1-phosphonium cation, representing the first branched cyclic pnictogen framework, and examples of tertiary phosphonium centers with As-P or Sb-P bonds. The Royal Society of Chemistry.

Full text of DOI:10.1039/b924918d

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Phosphinopnictinophosphonium frameworksw
Eamonn Conrad,a Neil Burford,*a Ulrike Werner-Zwanziger,a Robert McDonaldb and
Michael J. Fergusonb
Received (in Berkeley, CA, USA) 26th November 2009, Accepted 15th February 2010
First published as an Advance Article on the web 2nd March 2010
DOI: 10.1039/b924918d
Reactions of the cyclic tetraphosphine (CyP)4 with arsenium or
stibenium cations (R2Pn+, Pn = As, Sb and R = Cl, Ph) give
salts of the new cationic frameworks [(CyP)4(AN)]+,
[(CyP)4(SbCl2)]+ and [{(CyP)4}2(Sb2Cl2)]2+. The cations can
be viewed as coordination complexes of a cyclophosphine ligand
on a pnictenium cation, or a cyclo-2,3,4-triphosphino-1-pnictino-
1-phosphonium cation, representing the first branched cyclic
pnictogen framework, and examples of tertiary phosphonium
centers with As–P or Sb–P bonds.
Interpnictogen compounds have promise in the search for
materials exhibiting new properties.1 Bonds between the heavy
pnictogen elements (P, As, Sb, Bi) are viable, however, in
contrast to compounds containing N–Pn bonds, compounds
containing Pn–Pn0 bonds (Pn = P, As, Sb, Bi) are rare
and are limited to primary (i.e. Pn–Pn0) and secondary
(i.e. Pn0–Pn–Pn0) heteropnictogen connectivities. Here we
describe the application of interpnictogen coordination chemistry
to prepare the first examples of compounds containing a
tertiary interpnictogen center (a branched phosphonium
center with three P–Pn bonds [Pn = P, As, Sb], and one
alkyl substituent). This new dimension in the topological
diversification of interpnictogen frameworks is inevitable
in light of the ability of the pnictogen elements to adopt tri-,
tetra- and pentacoordinate geometries.
While compounds containing P–P homoatomic bonds are
well known2–5 and examples of compounds containing
homoatomic bonds are known for arsenic,6–11 antimony12–17
and bismuth,18–22 as few as four examples of compounds
containing As–Sb or As–Bi bonds have been characterized,23,24
compounds containing P–Pn0 (Pn = As,9,25–31 Sb,23,31–34 Bi33)
We now describe reactions of tetracyclohexyl-cyclotetra-
are rare and only one example of a compound containing a
Sb–Bi bond has been reported.35
phosphine with arsenium and stibenium cations [reactions
(3)–(5)], which result in P–Pn bond formation with contrasting
outcomes. The new compounds represent the first examples
containing branched phosphonium centers with As–P or Sb–P
bonds. Reaction mixtures of (CyP)4, 10-chloro-5-hydro-
phenarsazine (ANCl) and Me3SiOSO2CF3 or GaCl3 in
CH2Cl2 exhibit a broadening of the 31P{1H} NMR signal
corresponding to (CyP)4 (ꢀ68 ppm) (ESIw) implicating a weak
interaction (reaction (3)). Crystalline solids isolated from
these mixtures were characterized by X-ray crystallography as
[(CyP)4(AN)][OSO2CF3] and [(CyP)4(AN)][GaCl4]ꢁCH2Cl2,
respectively. Consistent with the solid state structures, the
solid state 31P CP-MAS NMR spectra (ESIw) exhibit four
distinct chemical shifts for [(CyP)4(AN)][OSO2CF3], and five
distinct chemical shifts for [(CyP)4(AN)][GaCl4] corresponding to
two crystallographically independent formula units. Analogous
reaction mixtures of (CyP)4, SbCl3 and AlCl3 (reaction (4))
show solution 31P{1H} NMR spectra that are tentatively
We have developed synthetic procedures for Pn–Pn bond
formation that exploit the influence of a molecular cationic
charge to steer reaction pathways and provide high yield
reactions. As illustrated in reaction (1), a coordinate bond is
formed between pnictogen centers when a Pn–Cl bond is
activated by a halide abstracting agent (Abs = AlCl3, GaCl3
or Me3SiOSO2CF3).9,24,36 In addition, phosphenium cations
(PR2+) are observed to insert into P–P bonds, as illustrated in
reaction (2).37
a Department of Chemistry, Dalhousie University, Halifax,
Nova Scotia, Canada B3H 4J3. E-mail: neil.burford@dal.ca
b X-Ray Crystallography Laboratory, Department of Chemistry,
University of Alberta, Edmonton, Alberta, Canada T6G 2G2
w Electronic supplementary information (ESI) available: Experimental
details and characterization data, including NMR data. CCDC
747678–747681. For ESI and crystallographic data in CIF or other
electronic format see DOI: 10.1039/b924918d
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Table 1 P–Pn bond lengths (A) in phosphinophosphonium salts,
arsinophosphonium salts, phosphinoarsonium salts, stibinophosphonium
salts, and in [{(CyP)4}2(Sb2Cl2)][AlCl4]2
Compound
P–Pn
Ref.
b
b
[(CyP)4(AN)][OSO2CF3]
[(CyP)4(AN)][GaCl4]ꢁCH2Cl2
2.4660(7)
2.4501(8)
2.4545(7)
2.3239(7)
2.3267(6)
2.3283(6)
2.3579(12)
2.3555(6)
2.5950(12)
2.8694(8)
2.8426(9)
2.6178(7)
2.8484(12)
2.8353(12)
a
[Me3AsPPh2][OSO2CF3]
[Me3AsP(Ph)AsMe3][OSO2CF3]
9
9
Fig.
1
Crystallographic views of the cations in [(CyP)4(AN)]-
[Ph2(2-MeOCH2C6H4)PAsMePh][PF6]
[Ph3PAsPh2][PF6]
25
26
26
26
[OSO2CF3] (reaction (3)) and in [(CyP)4(SbCl2)][Al2Cl7] (reaction
(4)). Ellipsoids presented at 50% probability, hydrogen atoms and
counteranion removed for clarity. The analogous structure of
[(CyP)4(AN)][GaCl4] (reaction (4)) is presented in ESIw.
[Ph3PSbPh2][PF6]ꢁCH2Cl2
[(Ph3P)2SbPh2][PF6]
b
b
[(CyP)4(SbCl2)][Al2Cl7]
[{(CyP)4}2(Sb2Cl2)][AlCl4]2
interpreted as an A2BX spin system (ESIw), consistent with the
cystallographically determined solid state structure of formula
[(CyP)4(SbCl2)][Al2Cl7]. Mixtures of (CyP)4, BiCl3 and AlCl3
also exhibit patterns tentatively assigned as a A2BX spin
system (ESIw).
a
b
Two crystallographically distinct cations. This work.
Sb–Sb reductive coupling. In this context, [{(CyP)4}2(Sb2Cl2)]2+
represents a reductively coupled dimer of [(CyP)4(SbCl2)]+.
The structures illustrated in Fig. 1 and 2 all include a typical
folded conformation for the cyclotetraphosphorus framework.
The arsenic and antimony centers in [(CyP)4(AN)]+ and
[(CyP)4(SbCl2)]+ adopt a pyramidal geometry, while the
antimony centers in the Sb2P8 framework are in a disphenoidal
environment consistent with the presence of four bonds and a
lone pair. The Sb–P bond in [(CyP)4(SbCl2)]+ is slightly longer
than that in the stibinophosphonium cation, and is substantially
longer in [{(CyP)4}2(Sb2Cl2)]2+ possibly due to the trans
influence of two (CyP)4 donors about each antimony center.
In summary, the first compounds containing tertiary
interpnictogen phosphonium centers have been prepared using
P - Pn coordination chemistry. The dimeric ten pnictogen
assembly [{(CyP)4}2(Sb2Cl2)]2+ involves four P - Sb bonds,
and the preparation bodes well for the discovery and development
of other and larger polypnictogen topologies.
The solid state structures of the cations [(CyP)4(AN)]+ and
[(CyP)4(SbCl2)]+ are shown in Fig. 1 and P–Pn distances are
listed in comparison with related compounds in Table 1. The
cyclo-2,3,4-triphosphino-1-pnictino-1-phosphonium cations
can also be viewed as coordination complexes of a cyclo-
phosphine ligand on a pnictenium cation 1, and represent the
first branched cyclic interpnictogen framework. The reactions
described by reactions (3) and (4) contrast the P–P insertion
reaction observed for phosphenium cations, described by
reaction (2).37 We rationalize this reactivity difference in terms
of the relative stability of the pnictine (arsine or stibine) center
over the pnictonium center that would result from P–P bond
insertion.
We thank the Natural Sciences and Engineering Research
Council of Canada, the Canada Research Chairs Program, the
Canada Foundation for Innovation, the Walter C. Sumner
In the presence of excess SbCl3 the combination of (CyP)4
and AlCl3 in a concentrated solution (reaction (5)) gave the
salt [{(CyP)4}2(Sb2Cl2)][AlCl4]2 in moderate yield. The solid
state structure is shown in Fig. 2, and the cation can
be described as two bidentate tetraphosphine ligands m2-
coordinated to a dichlorodistibenium dication 2, or two
cyclodiphosphino–diphosphonium dications bound to
a
central dichlorodistibide dianion (reaction (5)). Formation of
the Cl–Sb–Sb–Cl fragment implies the occurrence of a redox
reaction, which may involve disproportionation of SbCl3 to
give SbCl5 (or [SbCl4][AlCl4]) and Sb2Cl4, with subsequent
chloride abstraction by AlCl3 to form the [Sb2Cl2]2+ dication.
Alternatively, the dichlorostibenium [SbCl2]+ cation may be
formed by chloride ion abstraction from SbCl3 followed by
Fig.
2 Crystallographic view of the solid state structure of
[{(CyP)4}2(Sb2Cl2)][AlCl4]2 (reaction (5)). Ellipsoids presented at
50% probability, hydrogen atoms removed for clarity.
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Foundation and the Nova Scotia Research and Innovation
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