Synthesis and reactivity of cyclo-tetra(stibinophosphonium) tetracations: Redox and coordination chemistry of phosphine-antimony complexes

Article abstract of DOI:10.1039/c4sc03939d

Reductive elimination of [R₃PPR₃]²⁺, [11(R)]²⁺, from the highly electrophilic Sb^III centres in [(R₃P)₃Sb]³⁺, [8(R)]³⁺, gives Sb^I containing cations [(R₃P)Sb]¹⁺, [9(R)]¹⁺, which assemble into frameworks identified as cyclo-tetra(stibinophosphonium) tetracations, [(R₃P)₄Sb₄]⁴⁺, [10(R)]⁴⁺. A phosphine catalyzed mechanism is proposed for conversion of fluoroantimony complexes [(R₃P)₂SbF]²⁺, [7(R)]²⁺, to [10(R)]⁴⁺, and the characterization of key intermediates is presented. The results constitute evidence of a novel ligand activation pathway for phosphines in the coordination sphere of hard, electron deficient acceptors. Characterization of the associated reactants and products supports earlier, albeit less definitive, detection of analogous phosphine ligand activation in Cu^III and Tl^III complexes, demonstrating that these prototypical ligands can behave simultaneously as reducing agents and σ donors towards a variety of hard acceptors. The reactivity of the parent cyclo-tetra(stibinophosphonium) tetracation, [10(Me)]⁴⁺, is directed by high charge concentration and strong polarization of the P-Sb bonds. The former explains the observed facility for reductive elimination to yield elemental antimony and the latter enabled activation of P-Cl and P-H bonds to give phosphinophosphonium cations, [Me₃PPR′2]¹⁺, including the first example of an H-phosphinophosphonium, [(Me₃P)P(H)R′]¹⁺, and 2-phosphino-1,3-diphosphonium cations, [(Me₃P)₂PR′]²⁺. Exchange of a phosphine ligand in [10(Me)]⁴⁺ with [nacnac]^1- gives [(Me₃P)₃Sb₄(nacnac)]³⁺, [15(Me)]³⁺, and with dmap gives [(Me₃P)₃Sb₄(dmap)]⁴⁺, [16]⁴⁺. The lability of P-Sb or Sb-Sb interactions in [10(Me)]⁴⁺ has also been illustrated by characterization of heteroleptically substituted derivatives featuring PMe₃ and PEt₃ ligands. This journal is

Full text of DOI:10.1039/c4sc03939d

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Synthesis and reactivity of cyclo-
tetra(stibinophosphonium) tetracations: redox and
coordination chemistry of phosphine–antimony
complexes†
Cite this: Chem. Sci., 2015, 6, 2559
a
a
a
b
*
*
Saurabh S. Chitnis, Alasdair P. M. Robertson, Neil Burford, Jan J. Weigand
and Roland Fischerc
Reductive elimination of [R3PPR3]2+, [11(R)]2+, from the highly electrophilic SbIII centres in [(R3P)3Sb]3+
,
[8(R)]3+, gives SbI containing cations [(R3P)Sb]1+, [9(R)]1+, which assemble into frameworks identified as
cyclo-tetra(stibinophosphonium) tetracations, [(R3P)4Sb4]4+, [10(R)]4+. A phosphine catalyzed mechanism
is proposed for conversion of fluoroantimony complexes [(R3P)2SbF]2+, [7(R)]2+, to [10(R)]4+, and the
characterization of key intermediates is presented. The results constitute evidence of a novel ligand
activation pathway for phosphines in the coordination sphere of hard, electron deficient acceptors.
Characterization of the associated reactants and products supports earlier, albeit less definitive,
detection of analogous phosphine ligand activation in CuIII and TlIII complexes, demonstrating that these
prototypical ligands can behave simultaneously as reducing agents and s donors towards a variety of
hard acceptors. The reactivity of the parent cyclo-tetra(stibinophosphonium) tetracation, [10(Me)]4+, is
directed by high charge concentration and strong polarization of the P–Sb bonds. The former explains
the observed facility for reductive elimination to yield elemental antimony and the latter enabled
activation of P–Cl and P–H bonds to give phosphinophosphonium cations, [Me3PPR20 ]1+, including the
first example of an H-phosphinophosphonium, [(Me3P)P(H)R0]1+, and 2-phosphino-1,3-diphosphonium
Received 19th December 2014
Accepted 3rd February 2015
cations, [(Me3P)2PR0]2+
. Exchange of a
phosphine ligand in [10(Me)]4+ with [nacnac]1ꢀ gives
[(Me3P)3Sb4(nacnac)]3+, [15(Me)]3+, and with dmap gives [(Me3P)3Sb4(dmap)]4+, [16]4+. The lability of P–Sb
or Sb–Sb interactions in [10(Me)]4+ has also been illustrated by characterization of heteroleptically
substituted derivatives featuring PMe3 and PEt3 ligands.
DOI: 10.1039/c4sc03939d
www.rsc.org/chemicalscience
(Scheme 1b),8 or P–R bonds (Scheme 1c)9 are all known path-
ways of tertiary phosphine activation in transition metal
chemistry. One report10 hints at the reductive elimination of a
Introduction
Phosphines are prototypical ligands in the coordination
chemistry of d-block metals. While the chemistry of p-block
elements is primarily dened by covalent bonding as typied by
organic frameworks, an array of phosphine adducts has also
been characterized for main group element acceptors.1–5 Beyond
their versatile ligand properties as neutral, two-electron donors
(L-type),6 phosphines also exhibit redox reactivity within the
coordination sphere of an acceptor. For example, reductive
elimination of tetraorgano- or halotriorganophosphonium
cations (Scheme 1a),7 and oxidative addition of PR–X bonds
diphosphonium dication from a phosphine–metal complex
(Scheme 1d). In this instance, spectroscopic studies indicate
that the reaction of excess PMe3 with [Cu(MeCN)x][PF6]2 or
[Tl(MeCN)x][UF6]3 yields [Me3PPMe3]2+, and the reduced metal
complexes [Cu(PMe3)4][PF6] and [Tl(PMe3)2][UF6], respectively.10
However, neither the high oxidation state reactants nor the
reduced products have been structurally veried and
three different 31P NMR chemical shis were ascribed to
[Me3PPMe3]2+ (depending upon the counterion: +65.0 ppm,
+46.3 ppm, or +27.8 ppm). As reductive elimination is observed
for both a transition metal (CuII) and a main group metal (TlIII)
acceptor, phosphine activation may be broadly applicable to
complexes exhibiting a mismatch between hard (high oxidation
state/charge) acceptors and so phosphine donors. Indeed
phosphines are considered poor donors for hard acceptors and
coordination to such centres generally requires enforcement by
chelate or pincer ligands.11–13
aDepartment of Chemistry, University of Victoria, Victoria, BC V8W 3V6, Canada.
E-mail: nburford@uvic.ca; Fax: +1 250 721 7147; Tel: +1 250 721 7150
bDepartment of Chemistry and Food Chemistry, TU Dresden, 01062, Dresden,
Germany. E-mail: jan.weigand@tu-dresden.de; Tel: +49 351 46842800
cInstitute for Inorganic Chemistry, TU Graz, 98010, Graz, Austria. E-mail: roland.
scher@tugraz.at; Tel: +43 316 87332109
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c4sc03939d
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phosphine ligand activation pathway in the coordination
sphere of polycationic SbIII centres. Specically, reductive
elimination of diphosphonium dications (Scheme 1e) from
trialkylphosphine complexes of SbIII has been demonstrated,
comprehensively dening a fundamental P–P bond forming
redox process. The reduction products are the unusual cyclo-
tetra(stibinophosphonium) tetracations [10(R)]4+, representing
a new catena-homocyclic framework.14 Examples of cationic
homocycles for p-block metalloids are limited to unsupported
selenium and tellurium dications15 and heavily substituted
silicon16 or germanium17 monocations. For antimony,
a
18–20
number of acyclic catenated monocations ([1]1+ and [2]1+
)
and dications ([3]2+, [4]2+, [5]2+ and [6]2+)19,21–23 have recently been
isolated (Chart 1), but generally on small scales, precluding further
reactivity studies of these interesting species. Enabled by a rational
and large scale synthetic protocol for cations [10(R)]4+, we now
report the reaction chemistry of the prototypical derivative,
[10(Me)]4+, debuting the coordination chemistry of a new catena-
Scheme 1 Activation of phosphine ligands in the coordination sphere element framework.
of a Lewis acceptor. Numerals in red denote formal oxidation states for
the element.
Results and discussion
Reactions of PR3 with FSb(OTf)2 and Sb(OTf)3
Combinations of FSb(OTf)2 or Sb(OTf)3 with PR3 (R ¼ Me, Et, Pr,
or Bu) in MeCN solvent at the optimized stoichiometries given
1
in Scheme 2 have been investigated. The 31P, 13C, 19F and H
NMR spectra of reaction mixtures indicate quantitative forma-
tion of cyclo-tetra(stibinophosphonium) triate salts
[10(R)][OTf]4 (R ¼ Me, Et, Pr, Bu) together with derivatives of
[11(R)][OTf]2 (Scheme 2a) or [12(R)][OTf] (Scheme 2b). Large
lattice enthalpy differences permit separation of the mono-
cationic salts [12(R)][OTf] from the tetracationic salts
[10(R)][OTf]4 by fractional crystallization, whereas pure salts
cannot be isolated from mixtures of dicationic [11(R)][OTf]2 and
[10(R)][OTf]4.
Four derivatives of [10(R)][OTf]4 (R ¼ Me, Et, Pr, Bu) have
been characterized spectroscopically by solution NMR spec-
troscopy, and two derivatives, [10(Me)][OTf]4 and [10(Et)][OTf]4,
comprehensively characterized. The solid-state structures of
these two salts have been determined by X-ray crystallography to
conrm formulae involving a tetracation with a folded Sb4-
cyclic core with four exocyclic PR3 units and four triate anions
(Fig. 1 and Table 1). The Sb–Sb bond lengths are very similar for
Chart 1 Structurally confirmed cations featuring Sb–Sb bonds. See
text for references.
As part of a systematic evolution of p-block element phos-
phine complexes, we have sought derivatives featuring multiply-
charged, hard acceptors and now report evidence of a new
4+
[10(Me)]4+ [2.8354(6)–2.8797(5) A] and [10(Et)]
[2.838(2)–
˚
˚
2.884(2) A] and their values are marginally longer than those
Scheme 2 Formations of cations [10(R)]4+, [11(R)]2+, and [12(R)]1+ as triflate salts in reaction mixtures containing trialkylphosphines and (a)
Sb(OTf)3 or (b) FSb(OTf)2.
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Scheme 2b describes a similar redox process that involves
formation of [11(R)]2+ as transients, which are converted to the
corresponding uorophosphonium cations, [12(R)]1+, in the
presence of the uoride ion, as envisaged in the mechanism
outlined in Scheme 3 (le). The key feature in both processes is
reductive elimination of a diphosphonium unit from a hard,
tricationic SbIII centre to give a so, monocationic SbI centre,
representing a novel mode of phosphine ligand activation in the
coordination sphere of metals (Scheme 1e).
31P NMR spectra (Fig. 2) of reaction mixtures containing
PR3 and FSb(OTf)2 in a 2 : 1 stoichiometry show a broad
doublet in the +20 to +40 ppm range and the signal due to the
free phosphine (ꢀ60 to ꢀ20 ppm) is not observed. The 19F
NMR spectra of these mixtures show a broad triplet in the
range ꢀ170 to ꢀ175 ppm and no evidence of FSb(OTf)2. The
broadness of peaks in the 31P and 19F NMR spectra is consis-
tent with the connectivity of these nuclides to a quadrupolar
antimony center [I ¼ 5/2 for 121Sb (57%), 7/2 for 123Sb (43%)],40
and we assign these signals to the dicationic bis-phosphine
cations [7(R)]2+, which are stable as MeCN solutions (Scheme
3a). Upon addition of ca. 5 mol% of phosphine to these solu-
tions, the 31P NMR signals due to cations [7(R)]2+ are replaced
over 16 hours by doublets corresponding to [12(R)]1+ (d31P:
+140 to +150 ppm, 1JPF ¼ 950–1000 Hz) and a singlet in the ꢀ25
to 0 ppm range, corresponding to [10(R)]4+. Addition of ca. 15
mol% of phosphine increases the rate of the reaction and
effects complete conversion of [7(R)]2+ to [12(R)]1+ and [10(R)]4+
within an hour.
Fig.
1 Solid-state molecular structure of the cation in
[10(Et)][OTf]4$(MeCN). Hydrogen atoms, anions and solvent mole-
cules have been omitted for clarity. Thermal ellipsoids are drawn at
30% probability level. Metric parameters are given in Table 1.
Table 1 Selected bond lengths (A˚) and angles (ꢁ) in the solid-state
structures of [10(Me)][OTf]4$(MeCN)3 (ref. 26) and [10(Et)][OTf]4$MeCN
[10(Me)][OTf]4$(MeCN)3
[10(Et)][OTf]4$(MeCN)
d(Sb–Sb)
d(Sb/Sb)
d(P–Sb)
dmin(Sb/OOTf
<(Sb–Sb–Sb)
<(P–Sb–Sb)
2.8354(6)–2.8797(5)
3.7471(5)–3.7792(5)
2.552(2)–2.564(2)
3.210(4)
2.838(2)–2.884(2)
3.646(2)–3.804(2)
2.553(3)–2.578(2)
2.871(8)
)
82.09(2)–83.36(2)
93.74(4)–99.60(4)
38.85(2)–39.27(2)
78.62(3)–82.88(4)
91.91(7)–98.68(7)
43.30(2)–44.56(3)
<(Sb–Sb–Sb–Sb)
We propose that displacement of uoride from [7(R)]2+ by
added phosphine yields the highly-charged trications [8(R)]3+
(Scheme 3b), which undergo reductive elimination of [11(R)]2+
and [9]1+ (Scheme 3c). Subsequent tetramerization of the six-
valence electron cations [9(R)]1+ to [10(R)]4+ (Scheme 3d), and
displacement of PR3 from [11(R)]2+ by uoride gives [12(R)]1+,
regenerating the phosphine catalyst (Scheme 3e). Cyclization of
transients [9(R)]1+ is analogous to the formation of tetrameric
(Mes-E)4 (Mes ¼ 2,4,6-trimethylphenyl, E ¼ As or Sb) via cata-
lytic extrusion of Mes-AsI from a zirconium complex29 or Mes-
SbI from a hafnium complex.30 Nucleophilic displacement of
PMe3 has been reported31 in reaction mixtures of
[11(Me)][ClO4]2 and [NEt4][F], and we have further conrmed
that the equimolar reaction of [11(Me)][OTf]2 with CsF (Fig. S1,
ESI†) yields a 1 : 1 mixture of PMe3 and [12(Me)]1+. Trications
[8(R)]3+ are also implicated in the formation of [10(R)]4+ from
Sb(OTf)3 (Scheme 3f) and we have previously reported26 the
structure of the ternary salt [8(Me)][11(Me)][OTf]5 from a 1 : 3
mixture of Sb(OTf)3 and PMe3 at ꢀ30 ꢁC (vide infra). Consistent
with the role of [8(Me)]3+ as an intermediate, the same reaction
stoichiometry yields only [10(Me)][OTf]4 and [11(Me)][OTf]2 at
ambient temperature.
observed in rare examples of catena-antimony cations [cf. [1]1+
¼
¼
2.8205(12) A, 2.8278(3) A, [2]1+ ¼ 2.8203(4) A, [3]2+
18
19
20
˚
˚
˚
2+
2.7624(11) A and 2.7867(12) A,21 [4] ¼ 2.811(1) A and 2.830(1)
˚
˚
˚
A, and [5]2+ ¼ 2.8484(12) A and 2.8353(12) A 22]. Consistent
19
˚
˚
˚
with the high Lewis acidity of the Sb4 core, the P–Sb distances
˚
[2.552(2)–2.578(2) A] are similar to those observed in other tri-
˚
ate salts such as [(Me3P)2SbCl][OTf]2 [2.5950(4) A and 2.5834(4)
32
˚
˚
A] and [(Me3P)SbPh2][OTf] [2.5584(4) A].
A number of Sb–OOTf contacts are also observed, the shortest
4+
˚
˚
of which measure 3.210(4) A for [10(Me)] and 2.871(8) A for
[10(Et)]4+. Given the high molecular charge, these values are
expectedly smaller than the sum of the van der Waals radii
P
24
˚
(
¼ 3.61 A) but nevertheless signicantly longer than
r,vdW
P
˚
the sum of the single bond covalent radii (
¼ 2.05 A)25 for
r,cov
the two elements. For [10(Me)]4+, a gas-phase optimization26 of
the cation at the MP2 level in the absence of the triate anions
produced a geometry that is essentially identically to that
observed experimentally, and we therefore infer that the anion
contacts do not distort the structural features to a measurable
extent.
The reactions in Scheme 2a represent a two electron reduc-
tion of each antimony(III) center and collectively, an eight elec-
tron reductive coupling of four antimony centers to form
derivatives of [10(R)]4+. In Scheme 2a, eight of the twelve
equivalents of phosphine are involved in the redox process,
being oxidatively coupled to give four diphosphonium cations,
[11(R)]2+,27,28,31 and the remaining four equivalents represent
ligands on the reduced antimony(I) centers of [10(R)]4+.
The 31P NMR spectra of reaction mixtures containing
[(Me3P)2SbCl]2+ and 20 mol% PMe3 show only partial conver-
sion to [10(Me)][OTf]4 and [11(Me)][OTf]2 aer 48 hours. Addi-
tionally, a broad signal at +10.4 ppm is also observed (Fig. S2,
ESI†), which is close to the average for the values in [(Me3P)2-
SbCl]2+ (+15.8 ppm) and [(Me3P)2SbCl2]1+ (+6.2 ppm),32 sug-
gesting that the free chloride ion is sequestered in an
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Scheme 3 (Left) Proposed catalytic mechanism for the formation of derivatives of cations [7(R)]2+, [8(R)]3+, [9(R)]1+, [10(R)]4+, [11(R)]2+, and
[12(R)]1+. See text for descriptions of a–f. (Right) Non-catalytic formation of [10(Me)]4+ from the reaction of [(Me3P)2SbCl]2+ with PMe3.
equilibrium between the starting material and [(Me3P)2SbCl2]1+. mixtures and neither [Me3PCl]1+ nor free phosphine are detec-
Consequently, nucleophilic attack by chloride to liberate free ted by 31P NMR spectroscopy.
phosphine from [11(Me)]2+ is precluded in these reaction
Signifying the role of free phosphine as a catalyst, formation
of [10(Me)]4+ does not occur catalytically in the chloride system
because the reaction is arrested upon formation of [11(Me)]2+,
which is the spectroscopically detected oxidation product.
Generation of free phosphine from diphosphonium, the turn-
over limiting step, does not take place (Scheme 3, right). In
contrast, no diphosphonium is detected in reactions involving
the uoroantimony complexes [7(R)]2+ (Scheme 3, le), where,
due to nucleophilic attack by uoride anions on [11(R)]2+, only
the uorophosphoniums [12(R)]1+ are detected as the oxidation
product and the formation of [10(R)]4+ occurs catalytically in the
presence of free PR3. Differences in the reactivity of homologous
Sb–X (X ¼ Cl, F) complexes towards Lewis acids have been noted
previously.33
Solution NMR data for derivatives of [7(R)]2+, [8(R)]3+,
[10(R)]4+, [11(R)]2+, and [12(R)]1+ are summarized in Table 2,
with evidence for the assignments discussed below. It has not
been possible to detect or isolate derivatives of [9(R)]1+.
Attempts to trap these cations, or radical intermediates arising
from one-electron processes, in the presence of a twenty-fold
excess of 2,3-dimethyl-1,3-butadiene were unsuccessful.
Fig. 2 31P{1H} NMR spectra (202.5 MHz, 298 K, CD3CN) of reaction
mixtures containing FSb(OTf)2 and PR3 leading to the formation of
[7(R)]2+ (blue) and, upon addition of 15 mol% PR3, to [12(R)]1+, and
[10(R)]4+ (red). See Table 2 for chemical shift data.
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Table 2 Solution NMR data (CD3CN, 298 K) for derivatives of [7(R)]2+
,
pnictogen oxides34,39 have been reported. The 2JPF couplings for
[7(Me)]2+ and [7(Pr)]2+ are resolved as a doublet in the 31P NMR
spectra and as a triplet in the 19F NMR spectra, consistent with
an AX2 spin system. Fine structure could not be resolved for
[7(Et)]2+ and [7(Bu)]2+ even under the dilute conditions and low
[8(R)]3+, [10(R)]4+, [11(R)]2+, and [13]2+. Values in parentheses indicate
literature values for chemical shifts of known compounds. Values in
square brackets denote peak width at half-maximum where the
expected 2JPF coupling was not observed (n.o.)
31P (ppm)
d
19F (ppm)
nJPF (Hz)
ꢁ
d
temperature (ꢀ30 C) employed to mitigate broadening due to
exchange.
[7(Me)]2+
[7(Et)]2+
+15.9
+38.0 [23]
+29.3
+32.4 [62]
+41.3
+21.3
ꢀ178.2
44
n.o.
41
n.o.
39
—
Although [7(Me)][OTf]2 and [7(Et)][OTf]2 have both been
isolated as analytically pure substances and spectroscopically
characterized, we were unable to obtain X-ray quality crystals.
Moreover, to the best of our knowledge, there are no known
ꢀ174.2 [73]
[7(Pr)]2+
ꢀ173.4
[7(Bu)]2+
[13]2+
ꢀ173.9 [52]
ꢀ187.1
—
[8(Me)]3+
[8(Et)]3+
2
examples of JPF coupling constants through an antimony
+27.8
+22.1
—
—
—
—
centre for direct comparison with our assigned NMR data. For
this reason, we prepared and isolated the analogous [(dmpe)
SbF][OTf]2, [13][OTf]2, from an equimolar mixture of 1,2-bis-
(dimethylphosphino)ethane (dmpe) and FSb(OTf)2 in MeCN.
The solid state structure of [13][OTf]2, as determined by X-ray
crystallography, shows a dimeric arrangement with the cations
bridged by O–S–O contacts from the triate anions, and addi-
tional interactions with two non-bridging triate anions, as
shown in Fig. S3 (ESI).† The pyramidal geometry at Sb in the
cation is retained in solution, as demonstrated by the two non-
equivalent methyl group resonances in the 13C (6.1 and 7.2
[8(Pr)]3+
[8(Bu)]3+
[10(Me)]4+
[10(Et)]4+
[10(Pr)]4+
[10(Bu)]4+
[11(Me)]2+
[11(Et)]2+
[11(Pr)]2+
[11(Bu)]2+
[12(Me)]1+
[12(Et)]1+
[12(Pr)]1+
[12(Bu)]1+
+22.4
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
ꢀ24.4
+9.3
+0.6
+0.5
+28.5 (28.4)27
+38.5 (21)31
+31.4 (32)31
+31.0 (32)31
+148.0
+150.2
+145.6
+145.6
—
—
ꢀ138.0
ꢀ159.3
ꢀ154.7
ꢀ155.4
948
973
971
971
1
ppm) and H NMR (1.86 and 2.10 ppm) spectra. Crucially, the
2
expected JPF coupling was unambiguously observed (Fig. S4,
ESI†) in signals due to [13]2+, and the chemical shi and
coupling constants are comparable to those assigned to deriv-
atives of [7(R)]2+ (Table 2).
It was not possible to isolate salts of [8(R)]3+ due to their high
reactivity, consistent with their disproportionation to [11(R)]2+
and [10(R)]4+ in solution as proposed above. The 31P NMR
signals assigned to derivatives of [8(R)]3+ are singlets and
broadened (Dn1/2 ¼ 90–500 Hz), presumably due to a combi-
nation of the quadrupolar antimony nuclides40 and dynamic
ligand exchange. Nevertheless, [8(Me)][OTf]3 has been detected
as a co-crystallate with [11(Me)][OTf]2 in a 3 : 1 reaction mixture
Scheme 4 Synthesis of [11(Et)][OTf]2 via oxidative coupling of PEt3
with [Ph3Sb][OTf]2.
of PMe3 and Sb(OTf)3 at ꢀ30 C.26 The molecular structure of
ꢁ
[8(Me)]3+ (Fig. S5, ESI†) in the salt [8(Me)][11(Me)][OTf]5 shows a
pyramidal arrangement around the Sb atom with three P–Sb
˚
lengths in the range 2.5974(8)–2.6115(7) A and P–Sb–P angles in
the range 101.33(3)–102.40(2)ꢁ. In addition, three interion Sb–O
˚
contacts are observed in the 2.791(2)–2.960(2) A range (cf.
P
¼ 3.61 A),24 with each contact appearing trans to a P–Sb
˚
r,vdW
bond, illustrating a triple displacement of triate anions from
Sb(OTf)3 by three PMe3 ligands.
Signals attributed to derivatives of [11(R)]2+ are assigned by
comparison with previously reported 31P chemical shis for
their triate or perchlorate salts in MeCN for [11(Me)]2+,
[11(Pr)]2+, and [11(Bu)]2+.27,31 Isolation of [11(Pr)][OTf]2 enabled
comprehensive characterization, including X-ray structural
determination and we have reported this data elsewhere.41 The
salt [11(Et)][OTf]2 has been prepared independently from a 2 : 1
reaction of PEt3 with in situ generated Ph3Sb(OTf)2, according to
Scheme 4,42 and the structure of the cation is shown in Fig. 3.
Fig. 3 Molecular structure of the cation in [11(Et)][OTf]2 in the solid
state. Hydrogen atoms and triflate anions have been omitted for
clarity. Thermal ellipsoids are drawn at 30% probability level. Bond
lengths (A˚) and angles (ꢁ) are as follows: P1–P2 ¼ 2.2209(8), P1–C3 ¼
1.800(2), P1–C5 ¼ 1.802(2), P1–C7 ¼ 1.806(2), P2–C9 ¼ 1.796(2), P2–
C11 ¼ 1.809(2), P2–C13 ¼ 1.798(2), C3–P1–P2–C9 ¼ 28.7 (1).
Derivatives of [7(R)]2+ represent the rst examples of phos-
phine complexes of uoroantimony acceptors although
numerous uoroantimony complexes with hard, oxidatively-
resistant donors such as pyridines,33,34 ethers,35–38 and
˚
The P–P bond length [2.2209(8) A] is comparable to that in rare
examples of acyclic diphosphonium dications such as
[11(Me)]2+ [2.198(2) A] or [Me3PPEt3]2+ [2.216(1) A], and a
27
27
˚
˚
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the reaction mixture containing PiPr3 and FSb(OTf)2 in a 2 : 1
ratio displayed numerous uorine containing products as
indicated by the observation of spin system with P–F couplings
but no pure compounds could be isolated. A 3 : 1 mixture of
PiPr3 with Sb(OTf)3 also gave a complex mixture of products at
room temperature which could not be separated. Deprotona-
tion of MeCN solvent was observed upon reuxing the reaction
mixture for short periods or stirring at room temperature for 16
hours. We conclude that steric bulk at the a-carbon of the
phosphine hinders the coordination required for clean trans-
formation of bis-phosphine cations [7(R)]2+ to tris-phosphine
cations [8(R)]3+.
While the initial isolation of [10(Me)][OTf]4 as a pure
substance was achieved on a 150 mg scale (ca. 0.1 mmol),
making it unamenable to reactivity studies, reaction conditions
have now been optimized for a one-pot, three-step reaction (ESI)
to give reproducible yields of analytically pure [10(Me)][OTf]4
and [10(Et)][OTf]4 on a scale up to 10 g. Consistent with the
exquisite sensitivity of these compounds towards hydrolysis and
oxidation, particularly in solution, the key determinant of purity
and reactions yields is the rigorous drying and deoxygenation of
the solvent and careful application of dynamic vacuum (ca. 10ꢀ1
mbar) in the latter stages of the reaction to avoid free phos-
phine-catalyzed decomposition (vide infra).
Fig. 4 Molecular structure of [12(Me)][OTf] in the solid state. Thermal
ellipsoids are drawn at 30% probability level. Bond lengths (A˚) and
angles (ꢁ) are as follows: P1–F2 ¼ 1.551 (1), P1–C ¼ 1.757(2), 1.755 (2),
1.755 (2), P1–O3 ¼ 3.301(2), F2–P1–O3 ¼ 177.34(8), F2–P1–C ¼
106.38(6), 105.2(1), 106.38(6).
Thermolysis and photolysis of [10(Me)][OTf]4
Scheme 5 Thermolysis of [10(Me)][OTf]4 to yield [11(Me)][OTf]2 and
elemental antimony.
The four-membered ring of [10(Me)]4+ contains four of the six
Sb–Sb bonds required to make neutral, tetrahedral Sb4,
which is directly analogous to P4 and As4. Moreover,
[10(Me)]4+ also contains four phosphine ligands which may
be susceptible to further reductive elimination of two
diphosphonium dications, [11(Me)]2+, to yield neutral Sb4.
While P4 and As4 are well characterized, Sb4 has not been
isolated as a bulk solid, and only one solid-state structural
determination has been made using a scanning tunnelling
microscope to characterize a thin lm of Sb4 under ultra-
high-vacuum conditions.44 In this context, we envisioned the
thermal or photochemical decomposition of [10(Me)][OTf]4
as a route to bulk solid Sb4.
A sample of solid [10(Me)][OTf]4 (yellow-colored) heated
under argon at 120 ꢁC for 16 hours turned black, consistent
with the formation of elemental antimony (Scheme 5). A
CD3CN extract of the black product showed 31P, 1H and 13C
NMR signals corresponding exclusively to [11(Me)]2+ as the
sole oxidation product. A Raman spectrum of the black solid
partially eclipsed conformation is observed between the six
ethyl groups. In contrast to a previously assigned 31P NMR
chemical shi (+21 ppm)31 for [11(Et)][ClO4]2, which was not
structurally authenticated, [11(Et)][OTf]2 exhibits a 31P NMR
chemical shi of +38.5 ppm.
The 31P and 19F NMR resonances attributed to uo-
rophosphonium cations [12(R)]1+ were conrmed by compar-
ison to literature values or independent synthesis of triate salts
from small-scale equimolar mixtures of the appropriate phos-
phine with XeF2 followed by treatment with one equivalent of
TMSOTf. To the best of our knowledge, the solid-state structure
of [12(Me)][OTf] (Fig. 4) represents the rst structural charac-
terization of a trialkyluorophosphonium salt, and involves
three hydrogen bonds with the triate anion in addition to one
˚
weak contact [3.301(2) A] between a triate oxygen atom and the
phosphorus atom, which is marginally shorter than Sr,vdw for
the two elements (3.320 A).24 The O–P–F angle generated by this
˚
(Fig. S7, ESI†) matched that of the amorphous a-phase (110
cmꢀ1, 150 cmꢀ1
)
of antimony rather than the reported
contact is 177.34(8)ꢁ, representing adjustment of the D3h
structure of Me3PF2.43
45
Raman spectrum of tetrahedral Sb4 in argon matrix (138 cmꢀ1
,
The 31P NMR chemical shis for species in Table 2 over a 150
ppm range but within each class of cations, generally decrease
in the order d(R ¼ Et) > d(R ¼ Pr) z d(R ¼ Bu) > d(R ¼ Me).
Notably, the chemical shis of the free phosphines (range of 30
ppm) also show the same order, providing additional support
for the proposed assignments (Fig. S6, ESI†).
179 cmꢀ1, 242 cmꢀ1).46 Identical results were obtained when
heating was carried out in the dark, under vacuum, or in
solution (toluene). Irradiating solid [10(Me)][OTf]4 or as a
solution in MeCN at 256 nm for 3 hours at room temperature
had no measurable effect. It should be noted that in the gas
phase tetrahedral Sb4 is the preferred allotrope of the element
up to 1050 K.47 It is possible that despite its gaseous stability,
tetrahedral Sb4 is thermodynamically unstable with respect to
Attempts to isolate cations [7(R)]2+ or [10(R)]4+ with bulky
phosphines such as PiPr3 were unsuccessful. A 31P NMR assay of
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its amorphous phases in the condensed state, preventing its spectrum of the reaction involving CyPH2 shows (Fig. 5a) both
1
isolation as a solid and is, in this context, analogous to tetra- 1JPP and JHP couplings for the phosphinic signal centered at
hedral As4 (yellow arsenic) which spontaneously decomposes ꢀ83.6 ppm. The Pa–Pb(Hb)Cy connectivity is also conrmed in
1
to
a
hexagonal allotrope (grey arsenic, a-As) at room the H NMR spectrum of the reaction mixture (Fig. S8, ESI†),
1
2
temperature.48
where Hb resonates at +3.65 ppm exhibiting JHbPb, JHbPa, and
The thermolysis described above must be carried out in 3JHbHg couplings, the last of these arising from coupling to the
rigorously dried glassware, the surface of which has been ipso proton (Hg) of the cyclohexyl ring. The methyl protons (Ha)
2
3
treated with Me3SiCl to silanize terminal –OH groups. Samples around Pa also show the expected JHaPa and JHaPb couplings,
heated without prior passivation of glassware produced indicating a P–P bond. Finally, a two-dimensional 31P/1H HSQC
elemental antimony and [11(Me)][OTf]2, but also showed reso- (Fig. 5b) spectrum, which was optimized to show one-bond
nances due to [Me3PH]1+ and a singlet at +115.6 ppm in the 31
P
couplings, shows coupling between Hb and Pb but no coupling
{1H} NMR spectrum (CD3CN) of the reaction mixture, consistent involving Hb and Pa. The corollary two-dimensional HMBC
with formation of [Me3POPMe3]2+. This assignment is sup- experiment (Fig. 5c), optimized to exclude one-bond couplings,
ported by an independent synthesis from a 2 : 1 mixture of shows coupling between Hb and Pa, but no coupling involving
Me3PO and triic anhydride, using a well-established protocol Hb and Pb. Despite numerous attempts, it was not possible to
for these reagents.49 We interpret the formation of these by- separate [Me3PP(H)Cy][OTf] from [Me3PH][OTf], precluding
products as being due to the reaction of the extremely moisture elemental analysis or structural determination by X-ray
sensitive [10(Me)][OTf]4 with surface hydroxyl groups in non- diffraction. Nevertheless, to the best of our knowledge this is
silanized glassware.
the rst spectroscopic detection of an H-phosphinophospho-
nium cation.
The formation of [Me3PH]1+ and the phosphinophospho-
nium salts is understood in broad terms as a metathesis step
followed by a reductive elimination step as outlined in Scheme
6. We speculate that coordination of Cy2PH to one of the anti-
mony centres in [10(Me)][OTf]4 is followed by intramolecular
deprotonation by PMe3 to yield the observed [Me3PH]1+ cation
and a tricationic intermediate, [A]3+. This trication can undergo
rapid intramolecular reductive elimination of the rst equiva-
lent of the phosphinophosphonium cation to give dication
[B]2+. A second round of coordination, deprotonation and
reductive elimination completes the reduction of antimony to
its elemental form and furnishes the observed distribution of
products. Unfortunately, the partially reduced species were not
observed and appear to be eeting intermediates. Nevertheless,
formation of [Me3PP(H)R]1+ from reactions involving primary
phosphines (Scheme 7) is consistent with the proposed mech-
anism, although it is unclear why the second deprotonation
does not occur to yield the corresponding dication
[(Me3P)2PR]2+. As before, Raman analysis of the black
Reactions of [10(Me)][OTf]4 with RnPX(3ꢀn); X ¼ H, Cl; n ¼ 1, 2
Addition of a solution of R2PH (R ¼ C6H11 (Cy), Ph) to a clear
yellow-colored solution of [10(Me)][OTf]4 in MeCN results in
immediate deposition of a ne black precipitate and loss of the
yellow coloration. The 31P{1H} NMR spectra of reaction super-
natants show a singlet due to [Me3PH]1+ and two doublets
characteristic of phosphinophosphonium cations [Me3PPR2]1+
1
(R ¼ Cy, Ph) with typical JPP values in the 300–350 Hz range
(Scheme 6).50 Cation [Me3PPPh2]1+ is known51 and the assign-
ment of [Me3PPCy2]1+ was conrmed by comparison of chem-
ical shis and coupling constants with literature values50 for
phosphinophosphonium salts and by elemental analysis.
Analogously, addition of a solution of RPH2 (R ¼ Cy, tbutyl)
to a solution of [10(Me)][OTf]4 results in immediate precipita-
tion of elemental antimony. The 31P NMR spectra of these
reaction mixtures show complete consumption of RPH2 and
[10(Me)][OTf]4, and formation of a singlet due to [Me3PH]1+ and
a pair of doublets assigned to [Me3PP(H)R]1+ (Scheme 7).
Consistent with this formulation, the 31P–1H coupled NMR
Scheme 6 Formation of [Me3PH]1+ and [Me3PPCy2]1+ from the reaction of [10(Me)][OTf]4 with Cy2PH.
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Scheme 7 Reaction of CyPH2 with [10(Me)][OTf]4.
Fig. 5 (a) 31P NMR spectrum of the crude reaction mixture containing CyPH2 and [10(Me)][OTf]4 in a 2 : 1 ratio. Insets show detailed views of the
31P{1H} and 31P NMR resonances assigned to the Me3P- (left) and -P(H)Cy (right) fragments in [Me3PP(H)Cy]1+. A list of coupling constants
deduced from the combination of 31P and 1H NMR is also given. (b) Sections of the 31P/1H HSQC spectrum showing a 1JPH coupling between Pb
2
and Hb. (c) Sections of the 31P/1H HMBC spectrum showing a JPH coupling between Pa and Hb.
precipitate matches the amorphous a-phase of metallic anti- propose formation of chloroantimony species as transients that
mony rather than pyramidal Sb4.
undergo loss of chlorine gas to yield elemental antimony as
The observation that [10(Me)][OTf]4 serves as a source of there is no evidence of Sb–Cl bond stretching modes in the
PMe3, which deprotonates added primary and secondary Raman spectra of the insoluble black solid isolated from these
phosphines, implies a labile and polarized P–Sb bond that reactions. However, since no products expected from reactions
undergoes facile heterolytic cleavage. Consistently, addition of of dissolved Cl2 could be detected, the fate of the chlorine atoms
Cy2PCl or CyPCl2 to a solution of [10(Me)][OTf]4 results in cannot yet be denitively described. When intermediate stoi-
quantitative formation of [Me3PPCy2]1+ or [(Me3P)2PCy]2+
,
chiometries of CyPCl2 are employed, formation of the known
52
respectively, concomitant with deposition of elemental anti- [Me3PP(Cl)Cy]1+ cation53 is also observed, indicating a single
mony (Scheme 8). In these cases, [10(Me)]4+ behaves overall as a chloride abstraction event, that is analogous to the formation of
chloride abstractor and phosphine donor. We tentatively [Me3PP(H)Cy]1+ in reactions with CyPH2. 31P NMR data for
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Scheme 8 Formation of [Me3PPCy2]1+, [Me3PP(Cl)Cy]1+, and [(Me3P)2PCy]2+ from the reaction of [10(Me)][OTf]4 with Cy2PCl or CyPCl2.
[Me3PPCy2][OTf], [Me3PP(H)Cy][OTf], [(Me3P)2PCy][OTf]2, and
[Me3PP(Cl)Cy][OTf] are given in Table 3.
Table 3 31P NMR (CD3CN, 298 K) chemical shifts and coupling
constants for products obtained from the reaction of Cy2PH, CyPH2,
Reaction of [10(Me)][OTf]4 with PMe3
The 31P NMR spectrum of a reaction mixture containing 15
Cy2PCl, and CyPCl2 with [10(Me)][OTf]4
mol% of PMe3 and [10(Me)][OTf]4 shows slow disappearance of
31P (ppm)
1JPP (Hz)
Reference
the signal due to the latter and evolution of broadened signals
due to [11(Me)]2+ and free PMe3. Concomitantly, a mirror of
antimony is deposited in the reaction vessel. Within 12 hours at
298 K, there is no evidence of [10(Me)]4+, while signals due to
[11(Me)]2+ and free PMe3 persist, consistent with complete
decomposition of the tetracation, catalyzed by PMe3. The
proposed mechanisms (Scheme 9) involve nucleophilic attack
by the added phosphine at either the antimony or the
[Me3PPCy2][OTf]
+12.8, ꢀ5.1
+14.8, ꢀ83.6
+22.7, ꢀ30.8
+23.0, +78.4
327
252
307, 326
326
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52
[Me3PP(H)Cy][OTf]a
[(Me3P)2PCy][OTf]2
[Me3PP(Cl)Cy][OTf]
53
a 1
J
¼ 214 Hz.
PH
Scheme 9 Catalytic decomposition of [10(Me)][OTf]4 by PMe3 via nucleophilic attack at Sb (upper half) or P (lower half).
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phosphorus centres. Attack at a stibine should yield inter- Experimental section). However, if a dynamic vacuum is
mediate [A]4+ (Scheme 9), featuring a hypercoordinate anti- applied to the dark orange solution to remove the volatile
ꢁ
mony centre. Several examples of such hypervalent P–Sb PMe3 (b.p. ¼ 38 C), the solution maintains a yellow colour,
complexes have been reported.32,3 Due to its high charge leading to the formation of [10(Me)][OTf]4. Moreover reactions
concentration, this complex is predicted to be strongly with Lewis bases that displace PMe3 must be carried out with
oxidizing, and, in a process analogous to reductive elimina- explicit steps to remove the liberated phosphine in order to
tion from [8(Me)]3+, an equivalent each of [11(Me)]2+ and avoid decomposition (vide infra).
intermediate [B]2+ (Scheme 9) can be generated, enabling
dissociation of PMe3. Alternatively, attack at one of the
Reaction of [10(Me)][OTf]4 with [Li][nacnac(dipp)
]
phosphorus centres of [10(Me)]4+ directly generates interme-
diate [B]2+ together with [11(Me)]2+ and PMe3. The liberated
phosphine can further reduce [B]2+ by a second nucleophilic
attack either at Sb or P to evolve the second equivalent of
[11(Me)]2+ and yield fully reduced antimony. Nucleophilic
attack by a neutral two-electron ligand at tetracoordinate tri-
methylchlorophosphonium, trimethylphosphonium and
dimethylthiophosphonium cations has been demonstrated
previously.54–56 The broadness of signals for [11(Me)]2+ and
PMe3 in these reaction mixtures is attributed to an exchange
process that is also detected when free PMe3 is added to a
solution of [11(Me)][OTf]2.
In contrast to the sterically unhindered and neutral base PMe3,
a bulky and anionic base is expected to yield products arising
from ligand substitution rather than from addition. Consis-
tently, the 31P{1H} NMR spectra of equimolar reaction
mixtures of [10(Me)][OTf]4 and Li[nacnac(dipp)] (dipp ¼ 2,6-
diisopropylphenyl), indicate quantitative formation of
[15(Me)][OTf]3 (Scheme 10). The 1,3-diketiminate anion
[nacnac ]
(dipp) 1ꢀ, abbreviated as nacnac, displaces one PMe3
ligand from [10(Me)]4+ to give [(Me3P)3Sb4(nacnac)]3+
([15(Me)]3+), which is an analogue of [(Me3P)3Sb4(PCy2)]3+
(intermediate [A]3+ in Scheme 6). The 31P{1H} NMR spectrum
(Fig. 6) of [15(Me)][OTf]3 shows the expected AX2 spin system
[ꢀ26.6 ppm (triplet), ꢀ33.6 ppm (doublet), 3JPP ¼ 32 Hz] and a
corresponding AX2 spin system [ꢀ6.3 ppm (triplet), ꢀ2.5 ppm
(doublet), 3JPP ¼ 23 Hz] is also observed for [15(Et)]3+, prepared
from the reaction of [10(Et)]4+ with [Li][nacnac(dipp)]. Isolation
of [15(Me)][OTf]3 is only possible when the reaction is per-
formed under a mild dynamic vacuum to remove the displaced
phosphine, which effects redox decomposition at high
concentrations, presumably via similar mechanisms as
The catalytic decomposition of [10(Me)][OTf]4 in the pres-
ence of PMe3 explains the difficulties encountered during
synthesis of this salt. For instance, if the addition rate of PMe3
to FSb(OTf)2 is too high, a dark orange solution is obtained
which rapidly deposits elemental antimony (see note in
described above for [10(Me)]4+
.
The solid-state structure of the cation in [15(Me)][OTf]3$MeCN
(Fig. 7) shows three phosphine ligands and the rare g-coordina-
tion mode for the nacnac substituent,57 which, to the best of our
knowledge, has not been observed for haloantimony centres
bound to this substituent.58 Heteroleptic substitution is very rare
in antimony homocycles59 and examples for cationic systems
have not been reported. The range of Sb–Sb [2.8209(5)–2.8612(5)
Scheme 10 Formation of [15(Me)]3+ by nucleophilic displacement of
PMe3 from [10(Me)]4+
.
A] and Sb–P [2.538(5)–2.604(9) A] distances are similar to those in
˚
˚
Fig. 6 31P{1H} NMR spectrum of [15(Me)][OTf]3 at 298 K in CD3CN. Inset shows the AX2 spin system due to 3JPP coupling between two equivalent
and one unique phosphorus environment in [15(Me)]3+
.
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[10(Me)]4+ indicating minimal distortion of the Sb4 ring upon
displacement of PMe3 with nacnac. While [15(Me)][OTf]3 is stable
in the solid state under inert atmosphere, 31P{1H} NMR spectra of
ꢁ
MeCN solutions show decomposition over ve days at 20 C to
elemental antimony, [10(Me)][OTf]4 and [11(Me)][OTf]2 (Fig. S9,
ESI†).
To assess whether or not bonding via the g carbon of nacnac
is a general feature of antimony compounds and because
nacnac functionalized antimony centers are rare in the litera-
ture, we also prepared (nacnac)Sb(OTf)2 by salt metathesis
between an equimolar mixture of in situ generated Sb(OTf)3
and [Li][nacnac(dipp)]. Upon removal of LiOTf, the compound
was isolated as a pure substance and comprehensively char-
acterized. The molecular structure of (nacnac)Sb(OTf)2, deter-
mined by X-ray diffraction, shows a see-saw geometry around
antimony with two strongly-interacting triate anions in axial
positions (Fig. S10, ESI†). In contrast to g-coordination
observed for [15(Me)]3+, N,N0-chelation is observed for (nacnac)
Sb(OTf)2, and we attribute the difference in bonding modes to
the different steric environments around antimony in the two
compounds, rather than intrinsic features of the nacnac-Sb
interaction.
Fig. 7 Molecular structure of the cation in [15(Me)][OTf]3$MeCN in the
solid state. Hydrogen atoms and triflate anions have been omitted for
clarity. Thermal ellipsoids are drawn at 30% probability level. Bond
lengths (A˚) and angles (ꢁ) are as follows: Sb1–Sb2 ¼ 2.8209(5), Sb2–
Sb3 ¼ 2.8457(5), Sb3–Sb4 ¼ 2.8501(5), Sb1–Sb4 ¼ 2.8612(5), P1–Sb4
¼ 2.538(2), P2A–Sb2 ¼ 2.548(5), P2B–Sb2 ¼ 2.604(9), P3–Sb3 ¼
2.541(1), C1–Sb1 ¼ 2.209(5), Sb1–Sb3 ¼ 3.7344(5), Sb2–Sb4 ¼
3.7003(5), Sb1–N3 ¼ 3.42(1), Sb3–N3 ¼ 3.19(1), Sb1–Sb2–Sb3 ¼
82.45(1), Sb2–Sb3–Sb4 ¼ 81.03(1), Sb3–Sb4–Sb1 ¼ 81.67(2), Sb4–
Sb1–Sb2 ¼ 81.26(1), Sb1–Sb2–Sb3–Sb4 ¼ ꢀ42.10(2).
Interestingly, the 19F resonances for the two triate CF3
groups in (nacnac)Sb(OTf)2 are different (ꢀ78.3 and ꢀ78.4
Fig. 8 Formation of [11(Me)]2+ (A), [16]4+ (ꢂ), [17]2+ (-), [18]1+ (C), and PMe3 (>) in the equimolar reaction of dmap with [10(Me)][OTf]4 (B).
Peaks labelled with a vertical line (|) correspond to an unidentified product. Insets show the spin systems observed for [16]4+ (ꢂ) and the
unidentified product (|).
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Scheme 11 Proposed pathways to formation of [16]4+ (a), [17]2+, [11(Me)]2+, elemental antimony (b and d), and [18]1+ (c) in reaction mixtures
containing [10(Me)][OTf]4 and dmap in a 1 : 1 stoichiometry.
ppm), implying a rigid ring system with non-equivalent displacement of one phosphine ligand by dmap (Scheme 11).
positions above and below the plane of the ring. Consistently, It was not possible to isolate the resulting products. Following
the isopropyl substituents show two unique resonances for ltration of the reaction mixture (black suspension), the
the Cipso protons. Furthermore, there is restricted rotation yellow-green ltrate shows the expected AX2 spin system
3
around the Cipso–Cphenyl bond giving rise to four unique (triplet at +66.9 ppm, doublet at +42.5 ppm, JPP ¼ 24 Hz),
signals for the methyl groups in the 1H NMR spectrum of the tentatively assigned to [(Me3P)3Sb4(dmap)]4+ ([16]4+), and
compound. We speculate that this is due to solution-phase broad signals due to PMe3 (ꢀ62 ppm), [Me3P(dmap)]2+ ([17]2+
,
persistence of the weak hydrogen bonding interactions +89.0 ppm)54 and [11(Me)]2+. Within hours, signals due to
between the nitrogen atoms and the isopropyl Cipso protons, [Me3PCH2PMe2]1+ ([18]1+, doublet at ꢀ53.9 ppm, doublet at
2
detected as short contacts in the solid state molecular struc- +26.0 ppm, JPP ¼ 58 Hz)60 appear in the 31P NMR spectrum
ture (Fig. S10, ESI†).
and a signicant amount of [dmapH]+ is observed by 1H NMR
spectroscopy. We propose that the latter two species arise
from deprotonation of the slightly acidic protons of [11(Me)]2+
by dmap and the subsequent rearrangement of [Me3PPMe2-
CH2]1+ (Scheme 11). Consistently, a 1 : 1 control reaction of
dmap and [11(Me)]2+ initially shows broad signals for [17]2+
and free PMe3 as the kinetic products, but within 4 hours
Reaction of [10(Me)][OTf]4 with dmap
The reaction of [10(Me)][OTf]4 with 4-dimethylaminopyridine
(dmap) has been examined by 31P NMR (Fig. 8) and shows
Scheme 12 Kinetic and thermodynamic pathways in the reaction
between [11(Me)]2+ and dmap.
Table 4 Comparison of 31P NMR chemical shifts for some phosphorus
containing main-group cations stabilized by PMe3 or dmap. Values for
tetracoordinate phosphorus centers are given in parentheses, where
applicable
31P NMR
Reference
[Me2PPMe3]1+
[Me2P(dmap)]1+
[Ph2P(PMe3)]1+
[Ph2P(dmap)]1+
[Me2(S)PPMe3]1+
[Me2(S)P(dmap)]1+
[11(Me)]2+
+18 (ꢀ59)
+91
27
54
51
55
56
56
27
54
ꢀ23 (+15)
+88
+16 (+38)
+88
(+28.4)
[17]2+
(+89.0)
(ꢀ24.5)
[10(Me)]4+
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60
Fig. 9 31P{1H} NMR spectra (CD3CN, 298 K) of (a) [10(Me)]4+, (b)
[10(Et)]4+, (c) 1 : 1 mixture of [10(Me)]4+ and [10(Et)]4+, and (d) 1 : 1
mixture of [10(Me)]4+ and PEt3. Symbols denote tentative assignments
for [10(Me)3(Et)]4+ (ꢂ), cis-[10(Me)2(Et)2]4+ (B), trans-[10(Me)2(Et)2]4+
(C), and [10(Me) (Et)3]4+ (A).
[15(Me)]3+
(ꢀ26.6), (ꢀ33.6)
(ꢀ2.5), (ꢀ6.3)
(+66.9), (+42.5)
(+26.0), ꢀ53.9
[15(Et)]3+
[16]4+
[18]1+
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signals due to [18]1+ and [dmapH]1+ are observed, revealing
them to be the thermodynamic products (Scheme 12, Fig. S11
in ESI†). In addition to [18]1+, four unique and mutually
coupled phosphorus environments (by 31P NMR spectros-
copy) are also observed which could not been assigned
denitively.
Reaction of [10(Me)][OTf]4 with [10(Et)][OTf]4
Neutral catena-antimony rings are known to participate in ring–
ring equilibria unless bulky substituents or dilute solutions are
employed. For instance, solutions of hexaphenylcyclohex-
astibine (Ph6Sb6) equilibrate to give a mixture of four-, ve-, and
six-membered rings suggesting labile Sb–Sb bonds.61
3
The JPP coupling constant for the signal assigned to [16]4+
To assess the possibility of preparing heteroleptic derivatives of
[10(R)][OTf]4, pure samples of [10(Me)][OTf]4 and [10(Et)][OTf]4
were combined in a 1 : 1 stoichiometry. The 31P{1H} NMR spec-
trum (Fig. 9c) of the resulting mixture suggests formation of
multiple constitutional isomers of [(PMe3)x(PEt3)(4ꢀx)Sb4]4+, impli-
cating a scrambling process in the two ring systems via Sb–Sb or
P–Sb bond cleavage. A scrambling process involving Sb–Sb
cleavage has been described previously for distibines.62 However, a
control experiment, where free PEt3 was added to [10(Me)][OTf]4,
also showed (Fig. 9d) formation of these isomers. Therefore a
nucleophilic displacement pathway, where a bound PR3 ligand is
displaced by an added PR30 ligand, cannot be precluded. However
(24 Hz) is comparable to the values in [15(Me)]3+ (32 Hz) and
[15(Et)]3+ (23 Hz). However, the 31P{1H} NMR chemical shis
observed for [16]4+ (A: +66.9, X2: +42.5) are signicantly down-
eld from those of [15(Me)]3+ and [15(Et)]3+ (Table 4), and this
cannot be attributed solely to the different formal charges in the
species as the PMe3 groups in tetracationic [10(Me)]4+ resonate
at ꢀ24.5 ppm. We propose that dmap-stabilized main-group
cations generally show 31P NMR chemical shis that are
substantially downeld from their PMe3-stabilized homologues
(Table 4) due to the greater electronegativity of nitrogen relative
to phosphorus, supporting the assignment for [16]4+.
Scheme 13 Proposed formation of constitutional isomers from the equimolar reaction of [10(Me)]4+ and [10(Et)]4+
.
Scheme 14 Reactivity of a prototypical cyclo-tetra(stibinophosphonium) tetracation, [10(Me)]4+. See text for descriptions of a–i.
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this displacement route to heteroleptically substituted deriva-
Within the broader context of phosphines as ubiquitous
tives also yields signicant amounts of [11(Me)]2+ and elemental ligands in coordination chemistry, evidence of a novel ligand
antimony, presumably due to free PMe3 catalyzed decomposi- activation pathway has been presented and the associated
tion of [10(Me)][OTf]4 as described earlier. Although, it has not reactants and products characterized. Taken together with
yet been possible to purify these reaction mixtures and isolate previous, albeit less denitive, detection of such reactivity,10,42
the rst examples of heteroleptically substituted catena-anti- the observation of this reductive elimination pathway
mony rings, signal multiplicities consistent with AM2X, A2X2, conrms that these prototypical ligands can behave simulta-
and AA0XX0 spin systems are observed, as expected from a neously as reducing agents and stabilizing ligands, a feature
mixture of [10(Me)3(Et)]4+, cis/trans-[10(Me)2(Et)2]4+, and that may be generally applicable for phosphine complexes of
[10(Me)(Et)3]4+ (Scheme 13). Moreover, the coupling constants highly electrophilic acceptors across the periodic table.
3
lie in the 21–26 Hz range and are comparable to JPP coupling Diversication of this synthetic protocol may therefore provide
constants detected in [15(Me)]3+ (32 Hz), [15(Et)]3+ (23 Hz), and access to more extensively catenated systems for antimony as
[16]4+ (24 Hz). Collectively, these data enable a tentative well as other elements. As demonstrated for [10(Me)]4+, a
assignment of the spectral features observed in Fig. 9.
unique and rich reaction chemistry can be expected, in addi-
tion to the potential for valuable emergent properties such as
s-bond conjugation and cooperative catalysis due to metal
catenation.
Conclusions
The reductive elimination of diphosphonium dications
[11(R)]2+ from trialkylphosphine complexes of highly electro-
philic antimony(III) centres is reported. The reduced antimony(I)
Acknowledgements
fragments cyclize into frameworks identied as cyclo-tetra- We thank the Natural Sciences and Engineering Research
(stibinophosphonium) tetracations, [10(R)]4+. As outlined in Council (NSERC) of Canada and the Vanier Canada Graduate
Scheme 3, a phosphine catalyzed mechanism is proposed for Scholarships Program for funding. We gratefully acknowledge
uoroantimony complexes, and isolation or spectroscopic nancial support from the ERC (SynPhos 307616) for a six
characterization of key mechanistic intermediates is presented. month research stipend for S.S.C. at the TU Dresden. We also
The scope of this reductive assembly is dependent upon the thank Dipl.-Chem. Kai Schwedtmann for experimental assis-
steric bulk of the phosphine employed as demonstrated by non- tance and Prof. Lisa Rosenberg for valuable discussion.
productive reactions involving PiPr3. Formation of cyclic (R-Pn)n
or [L-Pn]n(n+) species (R ¼ aryl group, L ¼ alkylphosphine ligand,
References
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(stibinophosphonium) tetracation, [10(Me)][OTf]4, has enabled
reactivity studies that are summarized in Scheme 14.
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