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Coordination- and Redox-Noninnocent Behavior of Ambiphilic
Ligands Containing Antimony
J. Stuart Jones and François P. Gabbaï*
Department of Chemistry, Texas A&M University, College Station, Texas 77843-3255, United States
CONSPECTUS: Stimulated by applications in catalysis, the
chemistry of ambiphilic ligands featuring both donor and acceptor
functionalities has experienced substantial growth in the past several
years. The unique opportunities in catalysis offered by ambiphilic
ligands stem from the ability of their acceptor functionalities to play
key roles via metal−ligand cooperation or modulation of the
reactivity of the metal center. Ligands featuring group 13 centers,
most notably boranes, as their acceptor functionalities have
undoubtedly spearheaded these developments, with remarkable
results having been achieved in catalytic hydrogenation and
hydrosilylation. Motivated by these developments as well as by our
fundamental interest in the chemistry of heavy group 15 elements, we
became fascinated by the possibility of employing antimony centers
as Lewis acids within ambiphilic ligands.
The chemistry of antimony-based ligands, most often encountered as trivalent stibines, has historically been considered to mirror
that of their lighter phosphorus-based congeners. There is growing evidence, however, that antimony-based ligands may display
unique coordination behavior and reactivity. Additionally, despite the diverse Lewis acid and redox chemistry that antimony
exhibits, there have been only limited efforts to explore this chemistry within the coordination sphere of a transition metal. By
incorporation of antimony into the framework of polydentate ligands in order to enforce the main group metal−transition metal
interaction, the effect of redox and coordination events at the antimony center on the structure, electronics, and reactivity of the
metal complex may be investigated. This Account describes our group’s continuing efforts to probe the coordination behavior,
reactivity, and application of ambiphilic ligands incorporating antimony centers.
Structural and theoretical studies have established that both Sb(III) and Sb(V) centers in polydentate ligands may act as Z-type
ligands toward late transition metals. Although coordinated to a metal, the antimony centers in these complexes retain residual
Lewis acidity, as evidenced by their ability to participate in anion binding. Anion binding events at the antimony center have been
shown by structural, spectroscopic, and theoretical studies to perturb the antimony−transition metal interaction and in some
cases to trigger reactivity at the metal center. Coordinated Sb(III) centers in polydentate ligands have also been found to readily
undergo two-electron oxidation, generating strongly Lewis acidic Sb(V) centers in the coordination sphere of the metal.
Theoretical studies suggest that oxidation of the coordinated antimony center induces an umpolung of the antimony−metal
bond, resulting in depletion of electron density at the metal center.
In addition to elucidating the fundamental coordination and redox chemistry of antimony-containing ambiphilic ligands, our
work has demonstrated that these unusual behaviors show promise for use in a variety of applications. The ability of coordinated
antimony centers to bind anions has been exploited for sensing applications, in which anion coordination at antimony leads to a
colorimetric response via a change in the geometry about the metal center. In addition, the capacity of antimony Lewis acids to
modulate the electron density of coordinated metals has proved to be key in facilitating photochemical activation of M−X bonds
as well as antimony-centered redox-controlled catalysis.
■
INTRODUCTION AND BACKGROUND
Over the past decade, the field of Z-ligand chemistry1 has
experienced a renaissance prompted by the development of
new ambiphilic ligand platforms combining both L-type and Ztype ligands.2−9 These platforms have been used to generate
complexes in which the L-type ligands serve to position a metal
ion or atom in close proximity to the Lewis acidic Z-type ligand
site (Scheme 1). The success of this approach is illustrated by a
diverse series of complexes that incorporate a group 13 or
group 14 element as a Lewis acidic site. When appropriately
designed, these molecules display a strong donor−acceptor
© 2016 American Chemical Society
Scheme 1. Known Types of Ambiphilic Complexes
interaction involving the metal as the donor and the Z-type
ligand as the acceptor.
Received: December 15, 2015
Published: April 19, 2016
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Scheme 2. Syntheses of 1 and [2][PF6]
The most developed class of such complexes are those
containing a tricoordinate boron atom as the Z-type
ligand.5,7,8,10 In addition to displaying unique metal → boron
donor−acceptor interactions, complexes supported by such
ligands are also emerging in the domain of catalysis,10 where
the boron center can promote heterolytic bond activation
processes,11 facilitate redox processes at the adjacent transition
metal center,12 or increase the Lewis acidity of the metal center
by σ-inductive effects.13 Fascinated by these advances, we
questioned whether a related chemistry based on ambiphilic
ligands featuring a heavy group 15 element as the Lewis acidic
site could be developed.14 Our interest in this class of ligands
was reinforced by the rich coordination and redox chemistry
that heavy group 15 elements display.15,16
In this Account, we describe the results that we have
obtained using polydentate ligands incorporating antimony.
While stibines are usually regarded as weaker σ-donating
phosphine analogues,15 our contributions to this chemistry
underscore the noninnocence of these ligands. Stibines
supported by donor frameworks are readily converted into Ztype ligands by oxidation and participate in coordination events
without dissociation of the adjacent metal center. In addition to
investigating how this noninnocent behavior affects the manner
in which the antimony center interacts with coordinated
transition metals, we have also exploited the noninnocence of
these ligands in the context of anion sensing and redoxcontrolled catalysis.
Scheme 3. Coordination Chemistry at the Mercury Center in
[2]+
■
ANTIMONY ONIUM SPECIES AS σ-ACCEPTORS
Our investigation into the coordination behavior of ambiphilic
antimony ligands was initiated by a series of studies on
compounds featuring Sb(V) centers tethered to mercury or
gold centers by rigid 1,8-naphthalenediyl linkers.17−19 Having
previously employed the 1,8-naphthalenediyl platform to
enforce the proximity of Lewis acidic centers in bidentate
Lewis acids,20 we questioned whether tethering a late, electronrich d-block metal to a Lewis acidic Sb(V) center using this
platform would promote the formation of otherwise unstable
metal → Sb interactions. The gold− and mercury−stibonium
species 1 and [2][PF6] were prepared in one-pot syntheses by
stepwise reaction of 1,8-dilithionaphthalene with Ph3SbBr2 and
AuI or HgII synthons (Scheme 2).17,18 Structural studies
showed that the Au−Sb distance in 1 (avg. 2.7616(8) Å) is
significantly shorter than the Hg−Sb distance in [2][PF6]
(3.0601(7) Å), supporting the greater metallobasicity of gold(I)
versus mercury(II). The juxtaposition of an electron-deficient
tetraarylstibonium with an electron-rich, heavy d-block center
results in an enhancement of the Lewis acidity of the d-block
center.19 The organomercury−stibonium compound [2]+, in
contrast to typical organomercury species, is able to engage
anionic (Cl−, Br−, I−) and neutral ligands (N,N-dimethylaminopyridine (DMAP), tetrahydrofuran (THF)) at the mercury
center (Scheme 3 and Figure 1).19
Figure 1. Structures of [2]+ (left) and 2-I (right).
Although electrostatic effects figure to be a strong
component in the Sb−Au and Sb−Hg bonding in these
compounds, we sought to investigate the extent of charge
transfer between the antimony and metal centers. Electron
localization function (ELF) studies on 118 and derivatives of
[2]+ 17 suggest that the stibonium moiety in these species
engages the proximal metal center via orbital-based interactions
(M → σ*(Sb−CPh)), which are enhanced in the case of 2-I
upon binding of I− trans to the antimony center (Figure 2).
Despite computational evidence for bonding interactions
between the antimony and metal centers in these compounds,
the Au and Hg L3-edge X-ray absorption near-edge structure
(XANES) spectra of 1 and 2-I show no discernible oxidation of
their respective metal centers relative to reference compounds
(Figure 3).18,19 A similar lack of significant charge transfer has
been observed in the XANES spectra of metal-only Lewis pairs
between zero-valent group 10 isocyanides and Tl+ cations.21
In addition to their heavy d-block centers, the tetraarylstibonium groups in compounds 1 and [2]+ also readily engage
Lewis basic substrates, including fluoride anions. Coordination
of the latter affords the fluorostiborane complexes [1-F]− and
2-F, respectively (Scheme 4 and Figure 4).18,19 The ability of
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Scheme 4. Reactions of 1 and [2]+ with F−
Figure 2. ELF plots for 1, [2]+, and 2-I. The ELF plot for 1 is
reproduced from ref 18. Copyright 2011 American Chemical Society.
The ELF plots for [2]+ and 2-I are reproduced with permission from
ref 17. Copyright 2010 Wiley-VCH.
the stibonium moiety to coordinate additional ligands suggests
that despite being in the coordination sphere of the d-block
center, the antimony center retains considerable Lewis acidity.
It is also noteworthy that fluoride coordination does not
strongly affect the length of the Au−Sb bond (2.7694(8) Å in
[1-F]−) or the Hg−Sb bond (3.0495(18) Å in 2-F), which are
almost equal to those in 1 (avg. 2.7616(8) Å) and [2][PF6]
(3.0601(7) Å), respectively.
Taken together, these seminal studies demonstrated two
important concepts: (1) incorporation of a Lewis acidic
antimony center into the coordination sphere of a d-block
center modifies the behavior of the d-block metal by polarizing
its electron density, and (2) the antimony center itself may
engage neutral and anionic Lewis bases, potentially allowing for
modification of the main group−transition metal interaction.
Figure 4. Structures of 1 (left) and its fluoride adduct [1-F]− (right).
Scheme 5. Coordination of 3-R to Gold
■
TRIVALENT ANTIMONY SPECIES AS σ-ACCEPTORS
In addition to oxidized heavy group 15 species acting as σacceptors toward metal centers, we have also demonstrated that
Sb(III) moieties may act as Lewis acids toward late transition
metal centers.22,23 Reaction of (tht)AuCl (tht = tetrahydrothiophene) with the bis(phosphinyl)stibine ligands 3-Ph and 3Cl affords complexes 4 and 5 (Scheme 5).22 The structures of 4
and 5 show that the antimony centers in both complexes adopt
disphenoid rather than tetrahedral geometries, indicating that
they are not strongly engaged with the gold center as Lewis
bases (Figure 5).15 Despite the overall structural similarity
between 4 and 5, replacement of a phenyl ligand on antimony
with chloride results in a significant distortion of the geometry
of the gold center, as indicated by their respective Sb−Au−Cl1
angles of 115.09(2)° (4) and 141.73(4)° (5). Such distortions
toward square-planar geometry in tetracoordinate gold(I)
complexes are associated with the incorporation of a Z-type
ligand in the gold coordination sphere, as seen in ClAu[(o-
Figure 3. (a) Solid-state XANES spectrum of the Au L3-edge of 1 compared to those of Au(III) and Au(I) reference compounds. Reproduced from
from ref 18. Copyright 2011 American Chemical Society. (b) Solid-state XANES spectrum of the Hg L3-edge of 2-I compared to a Hg(II) reference.
Reproduced with permission from ref 19. Copyright 2011 Royal Society of Chemistry.
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Scheme 7. Synthesis of [8]2+, [9]+, and 10
presence of cyclohexyl isocyanide to afford [8]2+ as the
hexafluoroantimonate salt. Dication [8]2+ readily binds fluoride
under biphasic (H2O/CH2Cl2) conditions to give fluorostiboranyl cation [9]+ as a hexafluoroantimonate salt. Finally,
treatment of 7-Cl with 2 equiv of TlF in the presence of
cyclohexyl isocyanide yields difluorostiborane 10. Compounds
[8]2+, [9]+, and 10 constitute a series separated by formal
stepwise addition of two fluoride anions to the antimony center
with retention of coordination geometry at the platinum center.
Inspection of the 31P NMR spectra of the series found that
the 1JPt−P coupling constants increase across the series,
consistent with the platinum center becoming more reduced
(Table 1).31 The increase in the 1JPt−P coupling constant
Figure 5. Front and side views of the structures of (a) 4 and (b) 5.
(iPr2P)C6H4)2BPh] (∠Cl−Au−B = 168.7°).24 Natural bond
orbital (NBO) analysis of 4 breaks the Sb−Au bonding into
distinct lp(Sb) → 6p(Au) and lp(Au) → σ*(Sb−CPh)
interactions. In contrast to the “confused” σ-donor/acceptor
behavior displayed by the stibine ligand in 4, NBO analysis of 5
finds only an lp(Au) → σ*(Sb−Cl2) interaction, indicating that
the chlorostibine moiety acts as a pure Z-type ligand. Similar σacceptor behavior has also been documented in related
bis(phosphinyl)bismuthine complexes.25,26
Table 1. Selected Spectroscopic Data for [8]2+, [9]+, and 10
■
complex
COORDINATION-NONINNOCENCE IN
ANTIMONY−TRANSITION METAL COMPLEXES
As observed in the gold and mercury stibonium complexes 1
and [2]+, organoantimony(V) species may retain residual Lewis
acidity despite being in the coordination sphere of a metal. In
addition to Sb(V) centers, coordinated Sb(III) centers have
also been shown to engage in secondary interactions with Lewis
basic substrates.14,27 This phenomenon, which we term
coordination-noninnocence, is typically manifested through
the presence of low-lying Sb−X or Sb−C σ* orbitals. Drawing
an analogy between four-coordinate stibonium species, which
are well-known Lewis acids,28,29 and four-coordinate, cationic
transition-metal-coordinated stibines, we questioned whether
an Sb−M σ* orbital could also impart Lewis acidic properties
in a direction trans to the metal site (Scheme 6).
In a fundamental effort to investigate this behavior, we
prepared and characterized a series of platinum complexes
featuring the tris(phosphinyl)stibine ligand 6 (Scheme 7).30
Reaction of 6 with (Et2S)PtCl2 afforded compound 7-Cl, which
was subsequently treated with 2 equivalents of AgSbF6 in the
2+
[8]
[9]+
10
JPt−P (Hz)
ν(CN) (cm−1)
dSb−Pt (Å)
2330
2964
2888, 3351
2214
2196
2181
2.4706(5)
2.6236(3)
2.6568(6)
1
observed across the series is not monotonic, with the largest
increase occurring upon introduction of the first fluoride ligand
at antimony. This difference can be rationalized by noting that
the first fluoride equivalent binds trans to the Sb−Pt bond,
whereas the second equivalent binds trans to an Sb−C bond.
Similarly, the solid-state CyNC CN stretching frequency
diminishes across the series, further suggesting an increase in
electron density at the platinum center.
The structures of [8]2+, [9]+, and 10 provide further insight
into the perturbation of the Sb−Pt bond by anion coordination
at antimony (Figure 6). Notably, the Sb−Pt separation
increases dramatically upon binding of fluoride to the antimony
center, indicating that fluoride binding weakens the Sb−Pt
interaction (Table 1). This view is supported by NBO
calculations, which show a marked increase in the polarization
of the Sb−Pt bonding pair toward Pt upon fluoride anion
coordination.
Taken together, these experimental and computational
results suggest that anion binding at a coordinationnoninnocent antimony center results in a net push of the
bonding pair toward the metal center, increasing the electron
density at the metal center and weakening the Sb−M
interaction (Scheme 8).
With anion coordination at a noninnocent stibine ligand
being shown to affect the antimony−metal bond, coordinationnoninnocence may be envisioned as a means to induce
Scheme 6. Illustration of the Isolobal Relationship Between
[R3SbX]+ and [(R3Sb)M]+
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Figure 6. Structures of [8]2+ (left), [9]+ (center), and 10 (right).
two-electron oxidant, we surmised the possibility of three
distinct modes of oxidation (A−C), with B and C highlighting
the potential redox-noninnocence of the antimony ligand
(Scheme 10).
Scheme 8. Effect of Anion Binding on the Sb−M Bond
Scheme 10. Possible Two-Electron Oxidation Products of
(R3Sb)[M]
reactivity or otherwise alter the electronic properties of the
complex. This possibility is supported by the reaction of 11-Cl,
the nickel analogue of 7-Cl, with in situ-generated catecholate
dianion (Scheme 9).32 In the absence of an auxiliary ligand, 11Scheme 9. Reactions of [o-C6H4O2]2− with 11-Cl
In a first attempt to access such species, we targeted a gold
complex featuring the tris(phosphinyl)stibine ligand 6.34
Reaction of (tht)AuCl with 6 afforded compound 15-Cl
(Scheme 11), which underwent a clean antimony-centered
Scheme 11. Synthesis and Subsequent Oxidation of 15-Cl
oxidation upon treatment with PhICl2, leading to complex 16.
The formation of 15-Cl was found to be reversible by treatment
of 16 with excess NaI as the reductant to afford the gold iodide
15-I. The structure of 16 confirms that PhICl2 oxidizes the
stibine ligand to produce a dichlorostiborane moiety in the
coordination sphere of gold. Although this oxidation is
antimony-centered, the gold atom shows a noticeable response
in its coordination geometry, which changes from distorted
tetrahedral in 15-Cl to square-planar in 16. This geometry
change occurs concomitant with a shortening of the Sb−Au
separation from 2.8374(4) to 2.7086(9) Å. The square-planar
geometry of the gold center in 16, which is typically associated
with the trivalent state, suggests that oxidation of the
coordinated stibine effects a significant electronic perturbation
of the gold center. As indicated by NBO calculations,
conversion of 15-Cl into 16 is accompanied by an umpolung
of the Au−Sb donor−acceptor interaction, which switches from
Sb → Au in 15-Cl to Sb ← Au in 16 (Figure 7).
Similar two-electron oxidations of gold-coordinated stibines
were observed upon treatment of bis(phosphinyl)stibine−gold
complexes 4 and 5 with tetrachloro-1,2-benzoquinone (o-
Cl reacts with catecholate dianion to produce the orthometalated species 13. Compound 13 is speculated to stem from
nickel insertion into an Sb−C bond from a putative
catecholatostiborane species (12), which features a nominal
Ni0 center as a result of the polarization of the Sb−Ni bonding
pair upon coordination of the catecholate dianion. Repeating
the reaction in the presence of cyclohexyl isocyanide as a
secondary, π-acidic ligand afforded catecholatostiborane complex 14, which is valence isoelectronic with 10.
■
REDOX CHEMISTRY OF ANTIMONY−TRANSITION
METAL COMPLEXES
In addition to exploring the Lewis acidity of coordinated
antimony centers, we have undertaken an investigation of the
redox properties of heterobimetallic antimony−transition metal
complexes. While oxidations of metal−stibido compounds with
halogens had been previously reported,33 oxidations of
coordinated stibines were unknown prior to our efforts.
Upon treatment of a transition metal−stibine complex with a
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Figure 7. (a) Front and side views of the structures of 15-Cl (top) and 16 (bottom). (b) NBO plots of the principal Sb−Au interactions in 15-Cl
(top) and 16 (bottom).
chloranil) to form catecholatostiborane complexes 17 and 18,
respectively (Scheme 12).22 Congruent with the structural
Scheme 13. Synthesis and Subsequent Oxidation of 19
Scheme 12. Oxidation of 4 and 5 by o-Chloranil
changes associated with the conversion of 15-Cl into 16,
comparison of the structures of 4 and 17 finds a significant
shortening of the Au−Sb bond (2.8669(4) Å in 4 to 2.6833(3)
Å in 17) as well as a change in the coordination geometry of
the gold center from trigonal-pyramidal to distorted squareplanar. NBO analyses performed on 17 and 18 describe the
Sb−Au bond in both cases as a natural localized molecular
orbital (NLMO) with elevated orbital contributions from the
gold atom (16.2% Sb/83.8% Au for 17, 16.3% Sb/83.7% Au for
18), consistent with the presence of a polar covalent Au → Sb
bond.
Having demonstrated that polyphosphinylstibine−gold complexes may undergo two-electron oxidation at their antimony
centers, we next sought to investigate whether other transition
metal−stibine complexes might display similar reactivity. With
this goal in mind, we turned our attention to the synthesis of
zero-valent group 10−stibine complexes, which would provide
a valence isoelectronic comparison to the gold complexes
previously investigated. Reaction of 6 with Ni(PPh3)4 in THF
afforded the Ni0−stibine complex 19 as an orange solid
(Scheme 13).32
In contrast to the reactivity of 15-Cl, treatment of 19 with
PhICl2 produced the chlorostiboranyl complex 11-Cl, which
results from formal addition of a Cl2 equivalent across the Sb−
Ni heterobimetallic core (Figure 8). Despite the increase in the
Figure 8. (a) Structures of 19 (top) and 11-Cl (bottom). (b) NBO
plots of the principal Sb−Ni interaction in 19 (top) and the Sb−Ni
bond in 11-Cl (bottom).
oxidation state of the Ni−Sb heterobimetallic core, the Ni−Sb
distances observed in the structures of 19 (2.4575(10) Å) and
11-Cl (2.4852(5) Å) show little variation. Although the Ni−Sb
bonding in complex 19 can be described in similar terms as that
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Inspection of the structures of [21]+ and [22]+ shows that in
both cases the palladium center adopts a square-planar
geometry. Although the 31P NMR spectrum of [22]+ contains
a single resonance, its structure shows that the third phosphine
arm is not coordinated in the solid state, suggesting that the
coordination of the phosphine arms is fluxional in solution.
Treatment of CH2Cl2 solutions of both [21][BPh4] and
[22][BPh4] with tetra-n-butylammonium fluoride (TBAF)
results in the formation of fluoride complexes 21-F and 22-F,
as indicated by the appearance of resonances featuring coupling
to phosphorus in their 19F NMR spectra (21-F: −121.61 ppm,
3
JF−P = 26.7 Hz; 22-F: −129.72 ppm, 3JF−P = 20.7 Hz). In the
case of [22]+, formation of the fluoride adduct is accompanied
by a rapid color change from pale yellow to deep orange
(Figure 9).
Comparison of the structures of 21-F and 22-F reveals that
while the geometry of the palladium center in 21-F remains
square-planar, the palladium center of 22-F attains a trigonalbipyramidal geometry via coordination of the third phosphine
arm. It is this change in the palladium coordination geometry,
along with the corresponding change in ligand-field transitions,
that is responsible for the colorimetric turn-on response to
fluoride anion binding. The coordination-noninnocent antimony center in [22]+ thus acts as an allosteric site, with fluoride
coordination at antimony triggering coordination of the
pendant third phosphine arm to the palladium center. While
the insolubility of [22]+ in water limits its use in pure water,
CH2Cl2 solutions layered with aqueous solutions having
fluoride concentrations as low as 4 ppm display fluoride
binding, as monitored by UV−vis and 31P NMR spectroscopy.
In addition to utilizing coordination-noninnocent stibine
complexes as anion sensors, we have also explored the use of
transition metal complexes featuring Sb(V) centers in the same
capacity.37 Treatment of (Et2S)2PtCl2 with bis(phosphinyl)stibine ligand 20 afforded complex 23 (Scheme 15), which is
characterized by a singlet 31P NMR resonance flanked by 195Pt
satellites (1JPt−P = 2706 Hz). Subsequent oxidation of 23 with
in classical stibine complexes, the presence of an antimonybound chloride in 11-Cl (Sb−Cl: 2.6835(9) Å) makes the
classification of the Ni−Sb bonding more ambiguous. While
NBO analysis finds an Sb → Ni donor−acceptor interaction for
19, the Ni−Sb bond in 11-Cl is described as an NLMO with
distinct polarization toward the nickel center (Sb 36.3%/Ni
57.7%). Thus, while the antimony fragment in 19 can be
considered to act as an L-type ligand, oxidation across the Ni−
Sb core causes the antimony fragment in 11-Cl to act as an Xtype ligand, which forms a polar covalent bond with the Ni
atom.
■
APPLICATION OF
COORDINATION-NONINNOCENT STIBINE
COMPLEXES IN ANION SENSING
As part of our ongoing interest in anion sensing,29,35 we
investigated the use of coordination-noninnocent ligands for
sensing of aqueous fluoride. As binding of hard anions such as
fluoride at coordination-noninnocent antimony centers affects
the electronics of the transition metals to which they are
coordinated,30 we questioned whether this property could be
harnessed to provide a “turn-on” response to anion binding.
Our first effort toward this goal was the investigation of cationic
bis- and tris(phosphinyl)stibine−palladium species [21]+ and
[22]+, prepared by complexation of (cod)PdCl2 (cod = 1,5cyclooctadiene) with the corresponding polyphosphinylstibine
ligands 20, 6, and followed by subsequent chloride abstraction
with NaBPh4 (Scheme 14).36
Scheme 14. Synthesis of [22][BPh4] and [22][BPh4]
Figure 9. (a) Colorimetric and geometric changes in [22]+ upon binding of fluoride to form 22-F. (b) Changes in the 31P and 19F NMR upon
formation of 22-F. (c) Changes in the absorption spectrum of [22]+ upon incremental addition of TBAF.
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relevance to light-driven HX splitting reactions. In this context,
a number of binuclear transition metal species have been
investigated because of their ability to sustain reversible twoelectron oxidation.38 As heavy main group elements readily
undergo two-electron redox processes,39,40 we have investigated
the possibility that their incorporation into heterobimetallic
transition metal−main group complexes may provide platforms
that also sustain reversible two-electron redox processes at their
dinuclear cores.41,42 Complexation of (Et2S)2PtCl2 with the
bis(phosphinyl)chlorostibine ligand 20-Cl leads to the
formation of platinum pincer complex 26 (Scheme 16).43
Complex 26 undergoes oxidation by PhICl2 to afford the
corresponding tetravalent platinum species 27. We surmised
that with five electron-withdrawing chlorine atoms decorating
the central Sb−Pt core, the complex may be destabilized and
thus prone to photoreduction. Upon irradiation at 320 nm in
the presence of 2,3-dimethyl-1,3-butadiene (DMBD) as a
radical trap, 27 undergoes clean elimination of a Cl2 equivalent
to regenerate the reduced species, as indicated by both UV−vis
and 31P NMR spectroscopy (Figure 11). Upon optimization of
the DMBD concentration, a quantum yield of 13.8% was
obtained for 27, which is lower than the value of 38% obtained
for a related Pt2 complex.44 Remarkably, complex 27 also
evolves chlorine when irradiated in the solid state at
atmospheric pressure in the absence of a trap. Evolution of
chlorine or chlorine radicals was confirmed using sodium metal
as a trapping agent, accounting for ∼70% of the predicted
chlorine released.
Having demonstrated that a change in the redox state of an
antimony center held in proximity to a transition metal center
affects the electronics of the heterobimetallic bond, we
speculated that redox changes in antimony-containing ligands
may provide the means to control the Lewis acidity and
possibly the catalytic properties of the adjoining transition
metal. Such a concept is illustrated by the report of a gold−
borane complex featuring a [L2Au → BAr3]+ core that acts as a
catalyst for enyne cyclization.13 As [L2Au]+ species are typically
not active in catalysis, we questioned whether a redox-active
antimony fragment may provide similar activation of an
otherwise inactive [L2Au]+ fragment (Scheme 17). To this
end, we chose to examine gold complexes featuring the
bis(phosphinyl)chlorostibine ligand 20-Cl, which because of its
electron withdrawing chloride substituent, features a Lewis
acidic antimony center even in the Sb(III) state.23
Complexation of (tht)AuCl with 20-Cl followed by chloride
abstraction with AgSbF6 afforded the cationic gold species
[28][SbF6] (Scheme 18). To investigate whether the mildly
Scheme 15. Oxidative Synthesis of 24 and Its Reaction with
F−
o-chloranil resulted in the formation of complex 24, whose
diminished 1JPt−P value (2192 Hz) suggested that the platinum
center had undergone oxidation. Inspection of the structure of
24 shows that oxidation of the antimony center induces a
rearrangement in which the phenyl and chloride ligands are
transferred from antimony to platinum. Hypothesizing that the
metallastiborane fragment of complex 24 may function as a
unique Lewis acid,35 we next investigated its reactivity toward
fluoride anions. Treatment of 24 with an acidified aqueous
solution of KF under biphasic conditions resulted in the rapid
formation of complex 25, characterized by a singlet 31P NMR
resonance (1JPt−P = 2855 Hz) and a 19F resonance at −77.1
ppm. The elevated 1JPt−P value of 25 relative to 24 suggested
that the platinum center underwent reduction, an assignment
that was borne out by the square-planar platinum coordination
geometry in the structure of 25. As a result of an exchange of a
platinum-bound chloride for an antimony-bound fluoride, the
Sb−Pt bond in 24 is cleaved, as indicated by the large increase
in Sb−Pt separation (24, 2.5906(5) Å; 25, 3.0868(11) Å)
(Figure 10). This structural change can be reversed by addition
of fluoride scavengers such as AlCl3 to the aqueous phase. The
observed structural rearrangement is also accompanied by a
change in solution color from yellow to orange, which can be
used as a colorimetric turn-on response to indicate the presence
of aqueous fluoride anions.
■
APPLICATIONS OF REDOX-ACTIVE
ANTIMONY−TRANSITION METAL PLATFORMS
Complexes that support the photoreductive elimination of
halogens are attracting increasing interest because of their
Figure 10. Colorimetric and geometric changes in 24 upon undergoing F−/Cl− exchange to form 25.
864
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Scheme 16. Synthesis and Photoredox Chemistry of 27
Figure 11. (a) Absorption spectra obtained during the photolysis of 27. The inset shows a plot of quantum yield vs DMBD concentration. (b) 31P
NMR spectra of the photolysis of 27.
Scheme 17. Proposed Activation of an [L2Au]+ Fragment by
Oxidation at Antimony
center, we questioned whether oxidation of the antimony
center might allow for stronger activation of the gold center
and thus improve the catalytic performance.
Treatment of 28-Cl with PhICl2 cleanly afforded 29-Cl,
which was then converted into the more soluble 30-Cl with
TBAF (Scheme 19). Subsequent chloride abstraction with
Scheme 19. Synthesis of [30][SbF6]
Scheme 18. Synthesis of [28][SbF6]
Lewis acidic Sb(III) center present in [28][SbF6] is sufficient to
activate the bis(phosphinyl)gold cation fragment for catalysis,
its ability to catalyze the hydroamination of phenylacetylene
with p-toluidine was tested (Table 2).45 The salt [28][SbF6]
displays poor but non-negligible activity in this hydroamination
reaction relative to [(Ph3P)2Au][SbF6] (no conversion), with
only 2.7% conversion into the imine product after 3 h.
Although the catalytic activity of [28][SbF6] demonstrates that
a chlorostibine ligand may activate an otherwise inert gold
AgSbF6 afforded the cationic bisphosphinyl gold species
[30][SbF6]. Comparison of the structures of [28][SbF6] and
[30][SbF6] (Figure 12) reveals that oxidation of the antimony
center results in a notable shortening of the Au−Sb separation
from 2.9318(5) to 2.8196(4) Å, indicating the stronger Au →
Sb interaction present in [30][SbF6]. In line with the
strengthening of the Au−Sb interaction, the acidity of the
gold center in [30][SbF6] increases, as evidenced by its ability
to form adducts with adventitious water, a trait typically
associated with trivalent gold species.46
In an attempt to quantify the acidities of these cations
relative to typical bis(phosphine)gold(I) species, we carried out
Gutmann−Beckett-type measurements on [28][SbF6], [30][SbF6], and [Au(PPh3)2][SbF6] using triphenylphosphine
oxide (Ph3PO) (31P NMR (CH2Cl2): δ = 27.3 ppm) as the
Lewis base. While no change in the chemical shift of Ph3PO
Table 2. Activities of Selected Gold Catalysts for
Hydroamination
[Au]
reaction time
conversion (%)
[(Ph3P)2Au][SbF6]
[28][SbF6]
[30][SbF6]
3h
3h
40 min
0
2.7
98
865
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Accounts of Chemical Research
such species may also display coordination-noninnocent
behavior, engaging in secondary interactions while in the
coordination sphere of a metal. Additionally, we have shown
that the antimony centers of these ambiphilic ligands are redoxactive, which allows electronic tuning of their metal complexes.
These developments constitute a significant advance in the
fundamental understanding of antimony coordination chemistry as well as the entry of antimony-containing ambiphilic
ligands into the growing catalog of Z-type ligands.
Having established the fundamental coordination behavior
and reactivity of these tethered antimony ligands, we have
begun to exploit their properties in a variety of applications. On
the basis of our results, we envision that the coordinationnoninnocent properties of antimony ligands may be used to aid
in the activation of transition metal−halide bonds, generating
active sites for catalysis in situ. Furthermore, we anticipate that
the Lewis acidity enhancement provided by positioning a highly
acidic Sb(V) center in the coordination sphere of a metal may
be utilized in a variety of reactions catalyzed by electrophilic
metal complexes. The ability of the antimony centers in such
complexes to sustain reversible two-electron redox chemistry
may provide the means to modulate their catalytic activities.
Figure 12. Structures of [28][SbF6] (left) and [30][SbF6] (right).
occurs upon mixing with [Au(PPh3)2][SbF6], mixing of Ph3PO
with [28][SbF6] and [30][SbF6] causes the Ph3PO resonance
to shift downfield to 30.6 and 32.9 ppm, respectively. Taken
collectively, these results suggest that the otherwise poorly
acidic gold atoms are activated by their proximity to the Sb(III)
and Sb(V) centers in [28][SbF6] and [30][SbF6], respectively.
With the Sb(V) species [30][SbF6] in hand, we next
investigated its catalytic activity using the same model reaction
employed for [28][SbF6] (Table 2). In sharp contrast to the
activity of [28][SbF6], hydroamination of phenylacetylene with
p-toluidine by [30][SbF6] affords virtually complete conversion
after only 40 min of reaction time at room temperature. As
many transition metal-catalyzed hydroaminations require an
inert atmosphere, elevated temperatures, and extended reaction
times, the ability of [30][SbF6] to catalyze this reaction in air
under mild conditions is noteworthy. Although [30][SbF6]catalyzed hydroaminations cannot be extended to alkylamines,
the reactions proceed with a variety of arylamines as well as
phenylhydrazine (Figure 13).
■
AUTHOR INFORMATION
Corresponding Author
*E-mail: [email protected].
Funding
This work was supported by the National Science Foundation
(CHE-0952912 and CHE-1300371), the Welch Foundation
(A-1423), and Texas A&M University (Arthur E. Martell Chair
of Chemistry).
Notes
The authors declare no competing financial interest.
Biographies
J. Stuart Jones was born in 1988 in Richmond, Virginia. After
receiving a B.S. degree from the College of William & Mary in 2010,
he began his Ph.D. studies at Texas A&M, where he works on
heterobimetallic complexes featuring main group centers.
̂
François P. Gabbai ̈ received a Maitrise
from the University of
Bordeaux, a Ph.D. from the University of Texas at Austin under Alan
Cowley and a Habilitation from TU Munich under Hubert
Schmidbaur. He moved to Texas A&M in 1998, where he now
holds the Arthur E. Martell Chair of Chemistry. His research deals
with the chemistry of late transition metal and p-block elements.
■
REFERENCES
(1) Green, M. L. H. A new approach to the formal classification of
covalent compounds of the elements. J. Organomet. Chem. 1995, 500,
127−148.
(2) Shriver, D. F. Transition metal basicity. Acc. Chem. Res. 1970, 3,
231−238.
(3) Parkin, G. A simple description of the bonding in transition-metal
borane complexes. Organometallics 2006, 25, 4744−4747.
(4) Hill, A. F. An unambiguous electron-counting notation for
metallaboratranes. Organometallics 2006, 25, 4741−4743.
(5) Bouhadir, G.; Amgoune, A.; Bourissou, D. Phosphine-boranes
and related ambiphilic compounds: Synthesis, structure, and
coordination to transition metals. Adv. Organomet. Chem. 2010, 58,
1−107.
(6) Amgoune, A.; Bourissou, D. σ-acceptor, Z-type ligands for
transition metals. Chem. Commun. 2011, 47, 859−871.
Figure 13. Scope of alkyne hydroaminations catalyzed by [30][SbF6].
The reaction time, conversion percentage, and (in parentheses)
isolated yield are listed under each entry.
■
CONCLUSION AND OUTLOOK
Our work demonstrates that the utilization of donor buttresses
to enforce antimony−transition metal interactions enables the
observation of unique redox and coordination chemistry of the
tethered antimony ligand. We have established that tethered
Sb(III) and Sb(V) centers are capable of acting as Z-type
ligands toward late transition metals. The antimony centers of
866
DOI: 10.1021/acs.accounts.5b00543
Acc. Chem. Res. 2016, 49, 857−867
Article
Accounts of Chemical Research
(7) Braunschweig, H.; Dewhurst, R. D. Transition metals as Lewis
bases: “Z-type” boron ligands and metal-to-boron dative bonding.
Dalton Trans. 2011, 40, 549−558.
(8) Kameo, H.; Nakazawa, H. Recent developments in the
coordination chemistry of multidentate ligands featuring a boron
moiety. Chem. - Asian J. 2013, 8, 1720−1734.
(9) Rudd, P. A.; Liu, S.; Gagliardi, L.; Young, V. G.; Lu, C. C. Metal−
alane adducts with zero-valent nickel, cobalt, and iron. J. Am. Chem.
Soc. 2011, 133, 20724−20727.
(10) Bouhadir, G.; Bourissou, D. Complexes of ambiphilic ligands:
Reactivity and catalytic applications. Chem. Soc. Rev. 2016, 45, 1065−
1079.
(11) Harman, W. H.; Peters, J. C. Reversible H2 addition across a
nickel−borane unit as a promising strategy for catalysis. J. Am. Chem.
Soc. 2012, 134, 5080−5082.
(12) Anderson, J. S.; Rittle, J.; Peters, J. C. Catalytic conversion of
nitrogen to ammonia by an iron model complex. Nature 2013, 501,
84−87.
(13) Inagaki, F.; Matsumoto, C.; Okada, Y.; Maruyama, N.; Mukai, C.
Air-stable cationic gold(I) catalyst featuring a Z-type ligand:
Promoting enyne cyclizations. Angew. Chem., Int. Ed. 2015, 54, 818−
822.
(14) Benjamin, S. L.; Levason, W.; Reid, G.; Warr, R. P. Halostibines
SbMeX2 and SbMe2X: Lewis acids or Lewis bases? Organometallics
2012, 31, 1025−1034.
(15) Levason, W.; Reid, G. Developments in the coordination
chemistry of stibine ligands. Coord. Chem. Rev. 2006, 250, 2565−2594.
(16) Burton, J. W. Product class 2: Antimony compounds. Sci. Synth.
2002, 4, 53−75.
(17) Lin, T.-P.; Wade, C. R.; Pérez, L. M.; Gabbaï, F. P. A mercury →
antimony interaction. Angew. Chem., Int. Ed. 2010, 49, 6357−6360.
(18) Wade, C. R.; Lin, T.-P.; Nelson, R. C.; Mader, E. A.; Miller, J.
T.; Gabbaï, F. P. Synthesis, structure and properties of a T-shaped 14electron stiboranyl-gold complex. J. Am. Chem. Soc. 2011, 133, 8948−
8955.
(19) Lin, T.-P.; Nelson, R. C.; Wu, T.; Miller, J. T.; Gabbaï, F. P.
Lewis acid enhancement by juxtaposition with an onium ion: The case
of a mercury stibonium complex. Chem. Sci. 2012, 3, 1128−1136.
(20) Hudnall, T. W.; Chiu, C.-W.; Gabbaï, F. P. Fluoride ion
recognition by chelating and cationic boranes. Acc. Chem. Res. 2009,
42, 388−397.
(21) Barnett, B. R.; Moore, C. E.; Chandrasekaran, P.; Sproules, S.;
Rheingold, A. L.; DeBeer, S.; Figueroa, J. S. Metal-only Lewis pairs
between group 10 metals and Tl(I) or Ag(I): Insights into the
electronic consequences of Z-type ligand binding. Chem. Sci. 2015, 6,
7169−7178.
(22) Ke, I.-S.; Gabbaï, F. P. σ-donor/acceptor-confused ligands: The
case of a chlorostibine. Inorg. Chem. 2013, 52, 7145−7151.
(23) Yang, H.; Gabbaï, F. P. Activation of a hydroamination gold
catalyst by oxidation of a redox-noninnocent chlorostibine Z-ligand. J.
Am. Chem. Soc. 2015, 137, 13425−3432.
(24) Sircoglou, M.; Bontemps, S.; Mercy, M.; Saffon, N.; Takahashi,
M.; Bouhadir, G.; Maron, L.; Bourissou, D. Transition-metal
complexes featuring Z-type ligands: Agreement or discrepancy
between geometry and dn configuration? Angew. Chem., Int. Ed.
2007, 46, 8583−8586.
(25) Lin, T.-P.; Ke, I.-S.; Gabbaï, F. P. σ-accepting properties of a
chlorobismuthine ligand. Angew. Chem., Int. Ed. 2012, 51, 4985−4988.
(26) Tschersich, C.; Limberg, C.; Roggan, S.; Herwig, C.; Ernsting,
N.; Kovalenko, S.; Mebs, S. Gold− and platinum−bismuth donor−
acceptor interactions supported by an ambiphilic PBiP pincer ligand.
Angew. Chem., Int. Ed. 2012, 51, 4989−4992.
(27) Benjamin, S. L.; Reid, G. Neutral organoantimony(III) and
organobismuth(III) ligands as acceptors in transition metal complexes
− role of substituents and co-ligands. Coord. Chem. Rev. 2015, 297−
298, 168−180.
(28) Pan, B.; Gabbaï, F. P. [Sb(C6F5)4][B(C6F5)4]: An air stable,
Lewis acidic stibonium salt that activates strong element-fluorine
bonds. J. Am. Chem. Soc. 2014, 136, 9564−9567.
(29) Ke, I.-S.; Myahkostupov, M.; Castellano, F. N.; Gabbaï, F. P.
Stibonium ions for the fluorescence turn-on sensing of F− in drinking
water at parts per million concentrations. J. Am. Chem. Soc. 2012, 134,
15309−15311.
(30) Jones, J. S.; Wade, C. R.; Gabbaï, F. P. Guilty on two counts:
Stepwise coordination of two fluoride anions to the antimony atom of
a noninnocent stibine ligand. Organometallics 2015, 34, 2647−2654.
(31) Hope, E. G.; Levason, W.; Powell, N. A. Coordination chemistry
of higher oxidation states. Part 21. Platinum-195 NMR studies of
platinum(II) and platinum(IV) complexes of bi- and multi-dentate
phosphorus, arsenic and sulphur ligands. Inorg. Chim. Acta 1986, 115,
187−192.
(32) Jones, J. S.; Wade, C. R.; Gabbaï, F. P. Redox and anion
exchange chemistry of a stibine nickel complex: Writing the L, X, Z
ligand alphabet with a single element. Angew. Chem., Int. Ed. 2014, 53,
8876−8879.
(33) Malisch, W.; Panster, P. Antimony(V)-transition metalcompounds with covalent and ionic character. Angew. Chem., Int. Ed.
Engl. 1974, 13, 670−672.
(34) Wade, C. R.; Gabbaï, F. P. Two-electron redox chemistry and
reversible umpolung of a gold−antimony bond. Angew. Chem., Int. Ed.
2011, 50, 7369−7372.
(35) Hirai, M.; Gabbaï, F. P. Lewis acidic stiborafluorenes for the
fluorescence turn-on sensing of fluoride in drinking water at ppm
concentrations. Chem. Sci. 2014, 5, 1886−1893.
(36) Wade, C. R.; Ke, I.-S.; Gabbaï, F. P. Sensing of aqueous fluoride
anions by cationic stibine−palladium complexes. Angew. Chem., Int. Ed.
2012, 51, 478−481.
(37) Ke, I.-S.; Jones, J. S.; Gabbaï, F. P. Anion-controlled switching of
an X ligand into a Z ligand: Coordination non-innocence of a
stiboranyl ligand. Angew. Chem., Int. Ed. 2014, 53, 2633−2637.
(38) Esswein, A. J.; Nocera, D. G. Hydrogen production by molecular
photocatalysis. Chem. Rev. 2007, 107, 4022−4047.
(39) Carrera, E. I.; McCormick, T. M.; Kapp, M. J.; Lough, A. J.;
Seferos, D. S. Thermal and photoreductive elimination from the
tellurium center of π-conjugated tellurophenes. Inorg. Chem. 2013, 52,
13779−13790.
(40) Carrera, E. I.; Seferos, D. S. Efficient halogen photoelimination
from dibromo, dichloro and difluoro tellurophenes. Dalton Trans.
2015, 44, 2092−2096.
(41) Lin, T.-P.; Gabbaï, F. P. Two-electron redox chemistry at the
dinuclear core of a TePt platform: Chlorine photoreductive
elimination and isolation of a TeVPtI complex. J. Am. Chem. Soc.
2012, 134, 12230−12238.
(42) Yang, H.; Lin, T.-P.; Gabbaï, F. P. Telluroether to telluroxide
conversion in the coordination sphere of a metal: Oxidation-induced
umpolung of a Te−Au bond. Organometallics 2014, 33, 4368−4373.
(43) Yang, H.; Gabbaï, F. P. Solution and solid-state photoreductive
elimination of chlorine by irradiation of a [PtSb]VII complex. J. Am.
Chem. Soc. 2014, 136, 10866−10869.
(44) Cook, T. R.; Surendranath, Y.; Nocera, D. G. Chlorine
photoelimination from a diplatinum core: Circumventing the back
reaction. J. Am. Chem. Soc. 2009, 131, 28−29.
(45) Huang, L.; Arndt, M.; Gooßen, K.; Heydt, H.; Gooßen, L. J.
Late transition metal-catalyzed hydroamination and hydroamidation.
Chem. Rev. 2015, 115, 2596−2697.
(46) Wu, C.-Y.; Horibe, T.; Jacobsen, C. B.; Toste, F. D. Stable
gold(III) catalysts by oxidative addition of a carbon-carbon bond.
Nature 2015, 517, 449−454.
867
DOI: 10.1021/acs.accounts.5b00543
Acc. Chem. Res. 2016, 49, 857−867