Structural diversity for phosphine complexes of stibenium and stibinidenium cations

Article abstract of DOI:10.1039/c1cc15693d

Reactions of trimethylphosphine or diphosphines with SbCl₃ in the presence of AlCl₃ or Me₃SiSO₃CF₃ give ligand stabilized stibenium and stibinidenium cations. The geometry at each antimony center reveals a variety of environments for antimony that describes new bonding and highlights new directions in the chemistry of the pnictogen elements.

Full text of DOI:10.1039/c1cc15693d

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COMMUNICATION
Structural diversity for phosphine complexes of stibenium and
stibinidenium cationsw
Saurabh S. Chitnis,ab Brendan Peters,a Eamonn Conrad,a Neil Burford,*ab Robert McDonaldc
and Michael J. Fergusonc
Received 15th September 2011, Accepted 5th October 2011
DOI: 10.1039/c1cc15693d
Reactions of trimethylphosphine or diphosphines with SbCl3 in
the presence of AlCl3 or Me3SiSO3CF3 give ligand stabilized
stibenium and stibinidenium cations. The geometry at each
antimony center reveals a variety of environments for antimony
that describes new bonding and highlights new directions in the
chemistry of the pnictogen elements.
phosphonium environment, although the structure of 1b is
distinguished from those of 2a and 2b by the non-symmetric
P–Sb–P interaction [P–Sb = 2.6011(4) and 3.0272(4) A].
Therefore, 1b is best considered as a stibinophosphonium,
analogous to 3,4,6 with a weak secondary P–Sb contact that
imposes only minor distortion on the trigonal pyramidal
environment of antimony. Consistently, the shorter P(1)–Sb(1)
distance in 1b is similar to the P–Sb bonds in 2a, 2b and 3, and
the distinct geometry at antimony in 1b is due to a frustrated
Sb–P Lewis pair imposed by the three atom P–C–P restriction
of dppm. In contrast to 1b, 2a and 2b exhibit a tetra-coordinate,
disphenoidal geometry at antimony, with chlorine atoms in the
axial positions and a symmetric P–Sb–P interaction, which is
unusual for an interpnictogen framework and reveals a flexible
new bonding environment for antimony. The structural
features observed for derivatives of 2 and 4 are analogous to
the features observed for isoelectronic neutral diphosphine-
GeX2 adducts.9
Electron-rich pnictogen centers can behave as Lewis acceptor
sites despite the presence of a lone pair of electrons,
and coordination chemistry involving non-metal donor sites
provides a versatile synthetic approach to bonds between non-
metal elements, as extensively demonstrated for phosphorus.1
Cationic centers are the most effective acceptors, and have
been recently applied to obtain a variety of rare bonds
including P–As,2–4 P–Sb,3–6 P–Bi,4 As–Sb,7 As–Bi7,8 and
Sb–Bi.8 We have now exploited this approach to obtain a
series of new P–Sb bonded compounds that illustrate a variety
of structural arrangements for antimony and highlight new
directions in the chemistry of the pnictogen elements.
Reactions of PMe3, dmpm, dppm, dmpe or dppe, with
SbCl3 in the presence of AlCl3 or Me3SiSO3CF3 occur rapidly
at room temperature. Compounds [dppmSbCl2] 1b [AlCl4],
[dmpeSbCl2] 2a [SO3CF3], [dppeSbCl2] 2b [AlCl4] and
[(Me3P)2SbCl2] 4 [SO3CF3] have been isolated from the respective
reaction mixture and comprehensively characterized (Table 1).
The 31P NMR spectrum of each reaction mixture shows the
isolated compound to be the quantitative product (1 and 2) or the
dominant species (4). Solid state structures of the cations 1b, 2a,
2b and 4, as determined by X-ray crystallography, are shown in
Fig. 1, and selected structural parameters are listed in Table 1.
The cations 1b, 2a and 2b all adopt a cyclic structure
in which each phosphorus center can be considered as a
The geometry at antimony (P–Sb–P and Cl–Sb–Cl angles) is
very similar in compounds 2a, 2b, and 4, indicating that the
cyclic structures of 2a and 2b impose only minor restriction on
the geometry at antimony. Cation 4 accommodates the phos-
phorus centers in the equatorial positions of a distorted
disphenoidal geometry at the antimony, in contrast to
[(Ph3P)2SbPh2] 5 [PF6],4 in which the phosphorus centers
occupy the axial positions. Consequently, the trans influence
of the phosphine ligands in 5 imposes substantially longer
[2.8694(8) A and 2.8426(9) A] Sb–P bonds relative to the
equatorial P–Sb bonds in 4 [2.5890(4) A and 2.5805(4) A].
The differences between the 31P NMR chemical shifts of the
a Department of Chemistry, Dalhousie University, Halifax,
NS B3H4J3, Canada
b Department of Chemistry, University of Victoria, Victoria,
BC V8W3V6, Canada. E-mail: nburford@uvic.ca;
Fax: +1 250 721 7147; Tel: +1 250 721 7150
c X-ray Crystallography Laboratory, Department of Chemistry,
University of Alberta, Edmonton, Alberta T6G 2G2, Canada
w Electronic supplementary information (ESI) available: Cif files with
crystallographic data and experimental information are presented
as Supporting Information. CCDC 844313–844318. For ESI and
crystallographic data in CIF or other electronic format see DOI:
10.1039/c1cc15693d
c
This journal is The Royal Society of Chemistry 2011
Chem. Commun., 2011, 47, 12331–12333 12331

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Table 1 Selected interatomic distances (A), bond angles (1) and 31P NMR shifts (ppm) for title compounds and 31P NMR shifts for free ligands
1b [AlCl4]
2a [SO3CF3]
2b [AlCl4]
4 [SO3CF3]
6b [AlCl4]2
7b [Al2Cl7]2
Sb(1)–P(1)
Sb(1)–P(2)
Sb(1)–Cl(1)
Sb(1)–Cl(2)
Shortest
2.6011(4)
3.0272(4)
2.4088(4)
2.3718(4)
Sb–Cl
3.5596(6)
61.215(12)
96.328(17)
5.2
2.5655(4)
2.5658(4)
2.5729(4)
2.5520(4)
Sb–O
2.9436(14)
81.190(11)
165.527(14)
34.1
2.6121(5)
2.6164(5)
2.6791(5)
2.4640(5)
Sb–Cl
3.1255(5)
80.933(16)
162.004(19)
17.5
2.5890(4)
2.5805(4)
2.6707(4)
2.5079(4)
Sb–O
2.996(1)
101.754(13)
158.504(14)
6.2
2.6270(11)
2.6101(11)
2.3397(11)
—
Sb–Cl
3.086(1)
65.77(3)
—
2.6191(6)
2.6074(6)
2.3187(9)
—
Sb–Cl
3.3415(7)
80.60(2)
—
Cation–Anion
P–Sb–P
Cl–Sb–Cl
Dd (31P)
À2.3
49.1
À13.1
Dd (31P) ligand
DDd (31P)a
À22.8
À48.4
À13.1
À55
À22.8
28.0
82.5
30.6
61.2
20.5
62.2
a
DDd (31P) = Dd (31Pcomplexed phosphine) À Dd (31Pfree phosphine).
Fig. 1 Views of the cations in (i) 1b [AlCl4], (ii) 2b [AlCl4], (iii) 2a [SO3CF3], (iv) 4 [SO3CF3], (v) 6b [AlCl4]2, and (vi) 7b [Al2Cl7]2. Thermal
ellipsoids correspond to 50% probability, and hydrogen atoms are omitted for clarity.
complexed and free forms of the diphosphines [DDd(31P) in ppm,
Table 1] for 1b, 2b, and 5 [DDd(31P) = 28.0, 30.6, and 1.5,
respectively] are uniformly smaller than those for 1a, 2a and
4 [DDd(31P) = 55.9, 82.5, and 61.2, respectively], which
correlates with the relative length of the P–Sb bonds, and
the relative donor strength of the phosphine centers.
model 9 for frameworks 6b and 7b, in which the acceptor is a
stibinidenium cation, [Sb–Cl]2+ (Sb–R = stibinidene). In this
context, cations 2a and 2b are examples of 8, chelate complexes
of a stibenium cation [SbCl2]+. Consistently, 31P NMR spectra
of reaction mixtures containing 1b or 2b with either PMe3 or
dmpe show quantitative formation of 4 and 2a, respectively,
together with free dppm or dppe, illustrating the coordinative
nature of the interaction. Similarly, the 31P NMR spectrum of a
reaction mixture of dppm with 7b shows broadened signals due
to 7b, 6b, free dppm and free dppe, demonstrating facile ligand
redistribution.
Reactions of dppm or dppe with SbCl3 in the presence of
excess AlCl3 occur rapidly at room temperature and the 31P
NMR spectrum of each reaction mixture shows a single
phosphorus containing species (Table 1), assigned as
[dppmSbCl] 6b [AlCl4]2 and [dppeSbCl] 7b [Al2Cl7]2, respec-
tively. The solid state structures of the cations 6b and 7b,
shown in Fig. 1, reveal symmetric P–Sb–P interactions in
both cases with acute P–Sb–P angles [65.77(3) and 80.60(2)1,
respectively]. The structural adjustment from 1b to 6b upon
abstraction of the chloride ion is more dramatic than observed
from 2b to 7b, and is borne out in the observed 31P NMR
shifts, which for 7b [49.1 ppm] is at a higher frequency than
that for 2b [17.5 ppm], but the shift for 6b [À2.3 ppm] is at a
lower frequency than that for 1b [5.2 ppm].
The P–Sb–P frameworks of 6 and 7 represent the first
examples of stibinodiphosphonium cations, analogous to the rare
examples of 2-phosphino-1,3-diphosphonium cations.10–12
The central Sb–Cl bonds in 6b [2.3397(11) A] and 7b
[2.3187(9) A] are very short compared to those in 1b
[2.4088(4) A], 2a [2.5520(4) A], 2b [2.4640(5) A] and 4
[2.5079(4) A], and the Sb–P bonds in 6b and 7b are essentially
identical to those in the monocationic analogues 1b and 2b.
The Sb–Cl unit can be considered as accommodating the
dicationic charge, invoking a chelate coordination complex
Examples of coordination complexes involving a phosphine
ligand on a stibenium cation4,6 represent part of a developing
series of interpnictogen compounds,2,3,5,7,8 however chelate
complexes are rare for any pnictenium acceptor. For example,
the phosphorus center in 10 represents an iminophosphenium
c
12332 Chem. Commun., 2011, 47, 12331–12333
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cation interacting with a chelating bipyridine ligand,13 the
arsenic center in 11 represents an arsenium cation in which
the amine substituent is tethered to an imine donor, and in 12,
an arsenidenium dication is supported by both a pendant
imine donor and achelating bipyridine ligand.14 Classification
of 6 and 7 as chelate complexes of [Sb–Cl]2+is consistent with
the structure of [SbCl(15-crown-5)][SbCl6]2, which also con-
tains [Sb–Cl]2+(Sb–Cl = 2.365 A) shrouded by a crown
ether.15 The new complexes provide a ready access to
[Sb–Cl]2+ as a synthon for the systematic development of
new antimony frameworks.
Notes and references
1 N. Burford and P. J. Ragogna, J. Chem. Soc., Dalton Trans., 2002,
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2 E. Conrad, N. Burford, R. McDonald and M. J. Ferguson, Inorg.
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4 N. L. Kilah, S. Petrie, R. Stranger, J. W. Wielandt, A. C. Willis
and S. B. Wild, Organometallics, 2007, 26, 6106–6113.
5 E. Conrad, N. Burford, R. McDonald and M. J. Ferguson, J. Am.
Chem. Soc., 2009, 131, 17000–17008.
6 J. W. Wielandt, N. L. Kilah, A. C. Willis and S. B. Wild, Chem.
Commun., 2006, 3679–3680.
7 E. Conrad, N. Burford, R. McDonald and M. J. Ferguson, J. Am.
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8 E. Conrad, N. Burford, R. McDonald and M. J. Ferguson, Chem.
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9 F. Cheng, A. L. Hector, W. Levason, G. Reid, M. Webster and
W. Zhang, Inorg. Chem., 2010, 49, 752–760.
10 A. Schmidpeter, S. Lochschmidt, K. Karaghiosoff and
W. S. Sheldrick, Chem. Commun., 1985, 1447–1448.
11 K. B. Dillon, A. E. Goeta, J. A. K. Howard, P. K. Monks,
H. J. Shepherd and A. L. Thompson, Dalton Trans., 2008,
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12 K. B. Dillon and R. J. Olivey, Heteroat. Chem., 2004, 15, 150–154.
13 N. Burford, A. D. Phillips, H. A. Spinney, M. D. Lumsden,
U. Werner-Zwanziger, M. J. Ferguson and R. McDonald,
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14 A. L. Brazeau, A. S. Nikouline and P. J. Ragogna, Chem. Commun.,
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15 M. Schafer, G. Frenzen, B. Neumuller and K. Dehnicke, Angew.
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In summary, we have prepared a series of diphosphine-
stabilized cations that are rare examples of the
[R3P–Sb(X2)–PR3]+ framework and the first examples of the
[R3P–Sb(X)–PR3]2+ frameworks including the first chelate
complexes featuring a stibenium or stibinidenium acceptor.
Ring strain and phosphine donor strength are determinants of
the geometry at the antimony centre. We are exploiting the
coordination chemistry of pnictinidenium cations and more
highly charged pnictogen centers as a means to systematically
develop new interpnictogen frameworks.
We thank the Natural Sciences and Engineering Research
Council of Canada, the Canada Research Chairs Program, the
Canada Foundation for Innovation and the Nova Scotia
Research and Innovation Trust Fund for funding and
NMR-3 for use of instrumentation.
c
This journal is The Royal Society of Chemistry 2011
Chem. Commun., 2011, 47, 12331–12333 12333