Prototypical phosphine complexes of antimony(III)

Article abstract of DOI:10.1021/ic500723y

Complexes of the generic formula [Cl_n(PR₃) _mSb]^(3-n)+ (n = 1, 2, 3, or 4 and m = 1 or 2) have been prepared featuring [ClSb]²⁺, [Cl₂Sb]¹⁺, Cl ₃Sb, or [Cl₄Sb]^1- as acceptors with one or two phosphine ligands {PMe₃, PPh₃, PCy₃ (Cy = C₆H₁₁)}. The solid-state structures of the complexes reveal foundational features that define the coordination chemistry of a lone pair bearing stibine acceptor site. The experimental observations are interpreted with dispersion-corrected density functional theory calculations to develop an understanding of the bonding and structural diversity.

Full text of DOI:10.1021/ic500723y

Article
pubs.acs.org/IC
Prototypical Phosphine Complexes of Antimony(III)
Saurabh S. Chitnis,† Neil Burford,*,† Robert McDonald,‡ and Michael J. Ferguson‡
†Department of Chemistry, University of Victoria, Victoria, British Columbia V8W 3V6, Canada
‡Department of Chemistry, University of Alberta, Edmonton, Alberta T6G 2G2, Canada
S
* Supporting Information
ABSTRACT: Complexes of the generic formula
[Cln(PR3)mSb](3−n)+ (n = 1, 2, 3, or 4 and m = 1 or 2) have
been prepared featuring [ClSb]2+, [Cl2Sb]1+, Cl3Sb, or
[Cl4Sb]1− as acceptors with one or two phosphine ligands
{PMe3, PPh3, PCy3 (Cy = C6H11)}. The solid-state structures
of the complexes reveal foundational features that define the
coordination chemistry of a lone pair bearing stibine acceptor
site. The experimental observations are interpreted with
dispersion-corrected density functional theory calculations to
develop an understanding of the bonding and structural
diversity.
rationalization of configurational preferences that define the
coordination chemistry of a lone pair bearing stibine acceptor
site, and this is confirmed by analogous observations for select
derivatives of Cl2PhSb and [Ph2Sb]1+ acceptors. The
experimental observations are interpreted with dispersion-
corrected density functonal theory (DFT) calculations to
develop an understanding of the bonding and structural
diversity. Together, the study debuts a coordination chemistry
for antimony that has the potential to be as extensive and
diverse as many transition metals.
INTRODUCTION
■
The vast array of transition metal coordination complexes
involving neutral two-electron donors is the foundation of
classical coordination chemistry, which usually involves multi-
ple ligands and is understood by invoking acceptor d-orbitals at
the metal center. The donor−acceptor (Lewis acid−base)
concept is extrapolated to describe adducts between p-block
elements, referring to an acceptor with a single or multiple
ligands [e.g., H3NBH3 and (THF)2AlH3, respectively]. For the
relatively rare examples of electron-rich centers behaving as
Lewis acceptors, the presence of a lone pair of electrons at the
acceptor site1 offers interesting structural and electronic
features and some demonstrated applications in catalysis.2
The most diverse array of complexes involving an electron-rich
acceptor site is reported for monoantimony acceptors with
phosphine ligands, including examples of neutral com-
pounds,3−5 anions,6 and a broad array of cations. Stibenium
cations can accommodate a single phosphine ligand to give the
general structure 1,7−9 chelating diphosphine ligands to impose
either structure 2 or 3,10 or two phosphine ligands in structural
types 48 and 5.10 In addition, two examples of the dication
chelate complex structure 610 and the unique tris-phosphine
tricationic complex 7 have been reported.11
The structural diversity demonstrated by complexes of the
type 1−7 prompted us to perform a systematic assessment of
PR3 complexes of chloro-antimony acceptors (R = Me, Ph, Cy).
The potentially broad configurational variety for such
compounds is illustrated in Chart 1 for the generic formula
[Cln(PR3)mSb](3−n)+ (n = 2, 3, or 4 and m = 1 or 2) using an
octahedral frame that includes a lone pair at the antimony
acceptor site and in some cases a vacant coordination site. Our
observations reveal substantial configurational versatility for
antimony including examples of 9 of the 12 configurations
listed in Chart 1. More importantly, the data provide for a
EXPERIMENTAL SECTION
■
Reagents and Apparatus. All reagents were handled in a MBraun
Labmaster 130 glovebox (dry N2) or on a grease-free dual manifold
argon and vacuum (base vacuum = 2 × 10−2 mbar) Schlenk line. Prior
to use, HPLC-grade CH3CN (Aldrich) was refluxed over CaH2 for at
least 48 h and distilled under an atmosphere of argon directly onto 3 Å
molecular sieves, which had been freshly activated at 300 °C under
dynamic vacuum for 24 h. All other solvents were obtained from an
MBraun Solvent purification system and stored for at least 48 h over
activated 4 Å molecular sieves prior to use. All reagents were
purchased from Sigma-Aldrich. Deuterated solvents were dried using
the same procedure as CH3CN. SbCl3 (99.99%) was purified by
sublimation and Sb(OTf)3 was prepared in situ from the metathesis of
SbF3 and Me3SiO3SCF3 (TMSOTf). SbF3 (99.99%), Ph3P (97%), and
Cy3P (97%) were used as received. PMe3 (97%) and TMSOTf (99%)
were distilled before use. Reactions were carried out inside the
glovebox in screw-cap glass vials that had been dried at 200 °C for at
least 1 h and placed under dynamic vacuum (glovebox antechamber)
while still hot. NMR tubes fitted with J-Young valves were charged and
sealed inside the glovebox. Elemental analyses were carried out by
Canadian Microanalytical Ltd. in Delta, British Columbia, Canada. All
Received: April 1, 2014
© XXXX American Chemical Society
A
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glass wool, and concentrated under a vacuum to 1 mL. Very pale
yellow block-like crystals were obtained over 48 h at −30 °C, isolated
by decantation, washed with 0.5 mL of cold CH2Cl2 and dried under a
vacuum. Yield: 0.21 g, 82%. Melting point: 142−145 °C (dec).
Elemental analysis for C18H33Cl3SbP (calcd/expt): C (42.51/42.73),
Chart 1. Potential Structural Variety (VSEPR-Inconsistent
Structures Are Not Considered) in an Octahedral Frame for
Antimony Complexes Composed of Four (Anion), Three
(Neutral) or Two (Monocation) Chloride Substituents and
a
One or Two Phosphine Ligands (P = PR3)
1
H (6.54/6.63). NMR (CD2Cl2, 298 K, 8.45 T field strength): H:
1.36−1.92 ppm (m, 10H), 3.23 ppm (tq, 12.9 Hz, 2.6 Hz, 1H),
3
2
13C{1H}: 26.3 ppm, (s), 27.82 (d, JC−P = 11.2 Hz), 29.65 (d, JC−P
=
4.8 Hz), 33.85 (d, JC−P = 15.0 Hz), 31P{1H}: 25.1 ppm (s).
1
[Cl3(PMe3)2Sb]. A solution of PMe3 (0.46 g, 6.0 mmol) in 4 mL of
CH3CN was added to SbCl3 (0.68 g, 3.0 mmol) dissolved in 4 mL of
CH3CN. Midway through the addition a large amount of white
precipitate appeared. By the end of the addition a clear, pale yellow
solution was obtained which was allowed to stir for 30 min,
concentrated under a vacuum to ca. 2 mL and filtered through glass
wool. Colorless blocks were obtained by diffusion of Et2O into the
concentrated CH3CN solution. The crystals were washed with cold
CH3CN and dried under a vacuum. Yield: 1.10 g, 97%. Melting point:
110−113 °C (dec). Elemental analysis for C6H18Cl3SbP2 (calcd/expt):
C (18.95/18.84), H (4.77/4.59). NMR (CD3CN, 298 K, 8.45 T field
strength): 1H: 1.76 ppm (d, 2JH−P = 11.33 Hz), 13C{1H}: 11.9 ppm (d,
2
1JC−P = 25.0 Hz), 31P: −10.2 ppm (m, JP−H = 11.2 Hz).
[Cl3(PPh3)2Sb]. PPh3 (0.52 g, 2.0 mmol) and SbCl3 (0.23 g, 1.0
mmol) were combined in 5 mL of CH2Cl2. The resulting clear yellow
solution was stirred for 1 h, filtered through glass wool, concentrated
under a vacuum to 2 mL, and recrystallized at −30 °C to obtain light
yellow translucent crystals. The crystals were isolated by decantation,
washed with 0.5 mL of cold CH2Cl2, and dried under a vacuum. Yield:
0.65 g 86%. Melting point: 115−118 °C. Elemental analysis for
C36H30Cl3SbP2 (calcd/expt): C (57.45/57.75), H (4.02/4.39). NMR
(CD2Cl2, 298 K, 7.0 T field strength): 1H: 7.1−7.9 (m, PPh3),
13C{1H}: 128.4 ppm (d, JC−P = 25.0 Hz), 31P{1H}: 5.6 ppm (s).
1
a
The labels denote the abbreviated generic formula and distinguish
[Cl3(PCy3)2Sb]. SbCl3 (0.11 g, 0.50 mmol) and PCy3 (0.28 g, 1.0
mmol) were combined in a 3 mL of CH3CN. The reaction mixture
was stirred for 16 h to get a light yellow suspension. The supernatant
was removed, and the precipitate was washed with 3 mL of cold
CH3CN and pumped dry. The supernatant was concentrated under a
vacuum to obtain yellow needle-shaped crystals. The crystals and bulk
powder exhibited identical NMR spectra, although trace amounts of
[Cl3(PCy3)Sb] was also found in the bulk and could not be separated
by recrystallization. Yield (impure): 0.66 g. Melting point: 135−138
°C. Elemental analysis for C36H66Cl3P2Sb (calcd/expt): C (54.80/
52.38), H (8.43/8.07). NMR (CD2Cl2, 298 K, 7.0 T field strength):
1H: 1.34−2.56 ppm (m), 31P{1H}: 15.7 ppm (s).
between each configuration of the chloride substituents and the
phosphine ligands. Bold line = lone pair; □ = vacant coordination site.
quantum chemical calculations were carried out using Gaussian 09. See
Supporting Information for complete citation.
Preparation Procedures and Characterization Data.
[Cl3(PMe3)Sb]. PMe3 (0.08 g, 1.0 mmol) and SbCl3 (0.23 g, 1.0
mmol) were combined in CH3CN. The resulting white suspension was
stirred for 1 h and then allowed to settle. Crystals were obtained from
the supernatant by slow evaporation. Yield: 0.27 g, 88%. Elemental
analysis for C3H9Cl3PSb (calcd./expt.): C (11.85/11.87), H (2.98/
3.22). Melting point: 188−190 °C. NMR (CD3CN, 298 K, 7.05 T
[Cl2(PMe3)Sb][OTf]. SbCl3 (0.11 g, 0.50 mmol) and TMSOTf (0.11
g, 0.50 mmol) were combined in 2.5 mL of CH3CN and stirred for 1
h. A solution of PMe3 (0.04 g, 0.50 mmol) in 2 mL of CH3CN was
added, and the reaction mixtures were stirred for 1 h to obtain a clear,
colorless solution. All volatiles were removed under a vacuum to
obtain a white powder (0.207 g, calcd. for product 0.209 g), which was
recrystallized from CH3CN at −30 °C to obtain colorless block-
shaped crystals, which were washed with cold CH3CN and dried under
a vacuum. Yield: 0.19 g, 91%. Melting point: 108−111 °C. Elemental
analysis for C4H9Cl2F3O3PSSb (calcd/expt): C (11.49/11.37), H
field strength): H: 1.98 ppm (d, JH−P = 14.3 Hz), 13C{1H}: 10.1
1
2
ppm, (d, JC−P = 35 Hz), 31P{1H}: 9.6 ppm (s).
1
[Cl3(PPh3)Sb]. SbCl3 (0.23 g, 1.0 mmol) and PPh3 (0.26 g, 1.0
mmol) were combined in CH2Cl2 (4 mL) to obtain a clear, golden
yellow solution, which was stirred for 1 h. The reaction mixture was
filtered and concentrated under a vacuum to ca. 2 mL and stored at
room temperature for 24 h, which led to the formation of small yellow
crystals. The reaction mixture was removed from vacuum and allowed
to sit at room temperature for 24 h, which led to the formation of
yellow block-shaped crystals that were washed with 0.5 mL of cold
CH2Cl2 and dried under a vacuum. Yield: 0.41 g, 84%. Melting point:
120−122 °C. Elemental analysis for C18H15Cl3PSb (calcd/expt): C
(44.09/44.97), H (3.08/3.23). NMR (CD2Cl2, 298 K, 7.0 T field
1
(2.17/2.19). NMR (CD3CN, 298 K, 8.45 T field strength): H: 1.99
2
1
ppm (d, JH−P = 14.7 Hz), 13C{1H}: 9.7 ppm (d, JC−P = 34.1 Hz),
31P{1H}: 14.6 ppm (s), 19F{1H}: −80.0 ppm (s).
[Ph2(PMe3)Sb][OTf]. Sb(OTf)3 (0.29 g, 0.50 mmol) and SbPh3
(0.35 g, 1.0 mmol) were combined in CH3CN (4 mL) and stirred for
1 h to get a pale yellow solution of Ph2SbOTf. Neat PMe3 (0.11 g, 1.5
mmol) was added in three portions to obtain a clear, light yellow
solution. This reaction mixture was stirred for 40 min, filtered through
glass wool, concentrated under a vacuum to approximately 2 mL.
strength): H: 7.1−7.9 (m, Ph3P), 13C{1H}: 128.4 ppm (d, JC−P
=
1
1
25.0 Hz), 31P{1H}: 5.6 ppm (s).
[Cl3(PCy3)Sb]. SbCl3 (0.11 g, 0.50 mmol) and PCy3 (0.14 g, 0.50
mmol) were combined in 3 mL of CH2Cl2 to obtain a light yellow
solution. The reaction mixture was stirred for 20 min, filtered through
B
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Table 1. Crystal Data for the Featured Compounds
[Cl3(PMe3)Sb]
[Cl3(PPh3)Sb]
[Cl3(PCy3)Sb]
[Cl3(PMe3)2Sb]
[Cl3(PPh3)2Sb]
[Cl3(PCy3)2Sb]
formula
fw
C3H9Cl3PSb
304.17
C18H15Cl3PSb
490.37
C18H33Cl3PSb
508.51
C6H18Cl3P2Sb
380.24
C36H30Cl3P2Sb
752.64
C36H66Cl3P2Sb
788.93
space grp
a /Å
P21/n
P1
P1
P21/n
I2/a
P1
̅
̅
̅
7.9261 (11)
19.089 (3)
13.1115 (18)
10.5374 (8)
10.5630 (8)
10.6579 (8)
92.9183 (8)
115.7836 (7)
113.9875 (7)
937.79 (12)
2
10.376 (3)
15.895 (4)
20.641 (5)
85.320 (4)
80.800 (4)
73.177 (4)
3214.2 (14)
6
7.6594 (4)
11.4178 (6)
16.0941 (8)
15.8017 (12)
9.5425 (8)
22.4802 (18)
9.7577 (7)
10.3383 (7)
11.2342 (8)
115.1065 (7)
104.6530 (7)
94.0515 (8)
972.13 (12)
1
b /Å
c /Å
α /deg
β/ deg
γ /deg
V /Å3
Z
100.8495 (17)
96.3001 (5)
101.4987 (9)
1948.3 (5)
8
1398.98 (12)
4
3321.7 (5)
4
ρ /g cm−3
2.074
1.737
1.576
1.805
1.505
1.348
λ /Å
0.710 73
173
0.710 73
173
0.710 73
173
0.710 73
173
0.710 73
173
0.710 73
173
T /K
GOF
R1
1.036
1.102
0.872
1.048
1.068
1.079
0.0259
0.0513
0.0190
0.0394
0.0123
0.0295
0.0204
0.0507
0.0250
wR2
0.0455
0.0776
0.0672
[Cl2(PMe3)Sb]
[Ph2(PMe3)Sb]
[Cl2(PPh3)2Sb]
[OTf]
[Ph2(PMe3)2Sb]
[OTf]
[Cl(PMe3)2Sb]
[OTf]2
[Mg(CH3CN)6]
[Cl4(PMe3)Sb]2
[OTf]
[OTf]
formula
C4H9Cl2F3O3PSSb
417.79
C16H19F3O3PSSb
501.09
C37H30Cl2F3O3P2SSb
866.26
C19H28F3O3P2SSb
577.16
C8H18ClF6O6P2S2Sb
607.48
C18H36Cl8MgN6P2Sb2
949.88
fw
space grp
a /Å
P21/c
P1
P1
P21/c
P21/n
P212121
̅
̅
10.798 (2)
11.059 (2)
12.214 (3)
8.2553 (2)
10.8326 (3)
11.1367 (3)
98.4361 (3)
97.6027 (3)
93.1231 (3)
973.70 (4)
2
9.5641 (7)
12.0248 (8)
17.5876 (12)
94.0493 (9)
96.8668 (10)
110.3615 (9)
1869.0 (2)
2
8.2974 (3)
11.1805 (4)
26.4962 (9)
13.5151 (8)
11.4915 (7)
13.9937 (8)
10.9776 (15)
10.9983 (15)
32.060 (4)
b /Å
c /Å
α /deg
β/ deg
γ /deg
V /Å3
Z
114.382 (2)
93.3876 (4)
105.7092 (6)
1328.5 (5)
4
2453.73 (15)
2092.2 (2)
4
3870.7 (9)
4
4
ρ /g cm−3
2.089
1.709
1.539
1.562
1.929
1.630
λ /Å
0.710 73
173
0.710 73
173
0.710 73
173
0.710 73
173
0.710 73
173
0.710 73
173
T /K
GOF
R1
1.081
1.063
1.062
1.050
1.037
1.063
0.0313
0.0810
0.0157
0.0289
0.0201
0.0175
0.0434
0.0367
0.0797
wR2
0.0413
0.0719
0.0506
[Cl2(Ph)(PMe3)Sb]
[Cl2(Ph)(PPh3)Sb]
[Cl2(Ph)(PPh3)2Sb]
formula
C9H14Cl2PSb
345.82
C24H20Cl2PSb
532.02
C42H35Cl2P2Sb
794.29
fw
space grp
a /Å
P21/n
P21/n
C2/c
6.8302 (2)
17.8515 (5)
10.5339 (3)
10.3232 (4)
11.6723 (4)
18.4318 (7)
12.9097 (2)
14.2568 (2)
19.9245 (3)
b /Å
c /Å
α /deg
β/ deg
γ/ deg
V /Å3
Z
93.8302 (3)
90.4966 (4)
96.6050 (7)
1281.52 (6)
4
2220.87 (14)
4
3642.78 (9)
4
ρ /g cm−3
λ /Å
1.792
1.591
1.448
0.710 73
173
0.710 73
173
0.710 73
173
T /K
GOF
R1
1.173
1.149
1.111
0.0163
0.0411
0.0230
0.0555
0.0185
0.0474
wR2
Colorless crystals were obtained upon storage −30 °C for 24 h,
isolated by removing the supernatant by pipet and washed with 0.5 mL
of cold CH3CN and dried under a vacuum. Yield: 0.42 g, 83%. Melting
point: 117−119 °C. Elemental analysis for C16H19F3O3PSSb (calcd/
expt): C (38.35/38.66) H (3.82/3.88). NMR (CD3CN, 298 K, 7.05 T
ppm (m, 6H), 7.67−7.72 (m, 4H), 13C{1H}: 10.3 ppm (d, 1JC−P = 31.9
Hz), 130.9 ppm (s), 131.6 ppm (s), 138.1 ppm (s), 31P{1H}: −21.2
ppm (s), 19F: ppm (s): −79.2 ppm (s).
[Cl2(PMe3)2Sb][OTf]. SbCl3 (0.23 g, 1.0 mmol) and TMSOTf (0.22
g, 1.0 mmol) were combined in CH2Cl2 (4 mL) and stirred for 30 min
to yield a clear and colorless solution. This was added dropwise to a
1
2
field strength): H: 1.60 ppm (d, JH−P = 13.5 Hz, 9H), 7.52−7.57
C
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Scheme 1. Synthetic Approaches to Prototypical Complexes of Antimony(III)
stirring solution of PMe3 (0.228 g, 3.00 mmol) in CH2Cl2 (4 mL) over
30 s to produce a cloudy white suspension. The reaction mixture was
allowed to stir for 1 h yielding a fine white powder and a clear and
colorless supernatant, the 31P{1H} NMR spectrum of which exhibited
two signals (5.8 ppm, 85% and −2.2 ppm, 15%). Removal of solvent
under dynamic vacuum at room temperature over 4 h yielded a fine
white powder (0.515 g), which was recrystallized in CH3CN (10 mL)
at −30 °C. Highly reflective colorless block-shaped crystals were
obtained after 5 days, isolated by decantation, washed with cold
CH3CN (3 × 3 mL); Yield: 0.40 g, 80%; melting point: 135 °C dec.;
elemental analysis calcd (found): C 17.02 (16.97), H 3.67 (3.41);
NMR: 1H (CD3CN, 500 MHz, 293 K): 2.08 (d, 18H, 2JPH = 14.1 Hz);
mixture was stirred for 16 h to obtain a white suspension. Neat PMe3
(0.16 g, 2.0 mmol) was added to obtain a clear solution, which became
cloudy upon stirring for an additional 1 h. The supernatant was
removed from the white precipitate and filtered. Slow removal of
CH3CN at room temperature yielded colorless crystalline blocks. The
1
dried precipitate and crystals gave identical 31P, H, and 13C NMR
spectra. Yield: 0.78 g, 74%. Melting point: 175−178 °C (dec). Samples
prepared for elemental analysis (dynamic vacuum for 24 h) show loss
of one molecule of CH3CN giving [Mg(CH3CN)5][Cl4(PMe3)Sb]2,
C16H33N5Sb2MgCl8P (calcd/expt): C (21.14/21.27), H (3.66/3.49),
N(7.71/7.84). NMR (CD3CN, 298 K, 8.45 T field strength): 1H: 1.95
2
ppm (d, 18H, JH−P = 14.2 Hz, PMe3); 1.99 ppm (s, 18H, CH3CN),
13C{1H}: 10.4 ppm (d, 1JC−P = 36.2 Hz), 31P{1H}: 7.7 ppm (s, broad).
[Cl2(Ph)(PMe3)Sb]. A solution of PMe3 (0.04 g, 0.50 mmol) in 1 mL
of CH3CN was added to PhSbCl2 (0.13 g, 0.50 mmol) dissolved in 3
mL of CH3CN. The clear colorless reaction mixture was allowed to stir
for 15 min, filtered, and concentrated under a vacuum at room
temperature to obtain clear colorless crystalline blocks that were
washed with 0.5 mL of cold CH3CN and dried under a vacuum. Yield:
0.30 g, 86%. Melting point: 147−149 °C. Elemental analysis for
C9H14Cl2PSb (calcd/expt): C (31.26/31.53), H (4.08/4.19). NMR
(CD3CN, 298 K, 7.05 T field strength): 1H: 1.75 ppm (d, 2JH−P = 13.9
Hz, 9H), 7.38−7.52 ppm (m, 3H), 8.29 ppm (m, 2H), 13C{1H}: 10.2
1
13C{1H} NMR (CD3CN, 125.76 MHz, 293 K): 11.6 (d, JPC = 34.4
Hz), 118.5 (s); 19F NMR (CD3CN, 282.44 MHz, 293 K): −79.1 (s);
31P{1H} NMR (CD3CN, 202.46 MHz, 293 K): 6.2 (s).
[Cl2(PPh3)2Sb][OTf]. To a solution of SbCl3 (0.09 g, 0.40 mmol) in
CH2Cl2 (1 mL) was added a solution of PPh3 (0.11 g, 0.40 mmol) in
CH2Cl2 (1 mL). Neat TMSOTf (0.09 g, 0.40 mmol) was added, and
the clear, bright yellow solution was stirred at room temperature.
Following filtration through glass wool and concentration under
reduced vacuum, crystalline yellow blocks were obtained after storage
at room temperature for 24 h. The crystals were washed with 0.5 mL
of cold CH2Cl2 and dried under a vacuum. Yield: 0.66 g, 76%. Melting
point: 105 °C (dec.). Elemental analysis for C37H30Cl2F3O3P2SSb
(calcd/expt): C (51.30/51.05), H (3.49/3.01). NMR (298 K, 11.7 T
field strength): 1H (CDCl3): 7.46−7.57 ppm (m, Ph), 13C{1H}
1
ppm, (d, JC−P = 35 Hz), 31P{1H}: −9.2 ppm (s).
[Cl2(Ph)(PPh3)Sb]. A solution of PPh3 (0.262 g, 1 mmol) in 2 mL of
CH2Cl2 was added to PhSbCl2 (0.260 g, 1 mmol) dissolved in 2 mL of
DCM. The clear golden yellow reaction mixture was allowed to stir for
15 min, filtered, and concentrated under vacuum to ca. 1 mL. Yellow
crystalline blocks were obtained by diffusion of pentane vapor into this
solution at −30 °C over 24 h and isolated by decanting the
supernatant. Yield: 0. 217 g, 42%. Melting point: 118 °C dec; NMR
(CD2Cl2, 298 K, 8.45 T field strength): 1H: 7.1−7.9 (m, Ph3P),
1
3
(CDCl3): 128.4 (d, JC−P = 24 Hz), 129.2 (d, JC−P = 10 Hz), 131.3
(s), 134.0 (d, 2JC−P = 13 Hz), 31P{1H}(CD2Cl2): −5.0 ppm (s, broad).
[Ph2(PMe3)2Sb][OTf]. A solution of Sb(OTf)3 (0.29 g, 0.50 mmol)
and SbPh3 (0.35 g, 1.0 mmol) were combined in CH3CN (4 mL) and
stirred for 1 h to get a pale yellow solution. All volatiles were removed
under a vacuum to yield an off-white powder, which was not isolated.
The powder was redissolved in CH2Cl2 (3 mL), and a solution of
PMe3 (0.34 g, 4.5 mmol) in CH2Cl2 (3 mL) was dropwise added to
get a clear, very light brown solution. This reaction mixture was stirred
for 10 min, filtered, concentrated under a vacuum to approximately 3
mL. Large, colorless blocks, obtained upon recrystallization at −30 °C
over 24 h, were isolated by removing the supernatant by pipet and
were washed with 0.5 mL cold CH2Cl2 and dried under a vacuum.
Yield: 0.69 g, 80%. Melting point: 98−100 °C. Elemental analysis for
C19H29F3O3P2SSb (calcd/expt): C (39.54/39.96), H (4.89/4.68).
1
13C{1H}: 128.4 ppm (d, JC−P = 25.0 Hz), 31P{1H}: 5.6 ppm (s) .
Structure Determination of [Cl2(Ph)(PPh3)2Sb]. This product could
not be isolated as a pure substance and was formed together with
[Cl2(Ph)(PPh3)Sb]. A solution of PPh3 (0.262 g, 1 mmol) in 2 mL of
CH3CN was added to PhSbCl2 (0.130 g, 1 mmol) to obtain a yellow
solution, which was allowed to stir for 40 min. Upon concentration to
1 mL at −30 °C for 72 h, a mixture of yellow and colorless crystals was
obtained, and the colorless crystals were identified as [Cl2(Ph)-
(PPh3)2Sb] by X-ray crystallography. It was not possible to manually
isolate a sufficient quantity of the colorless crystals to characterize the
compound as a pure substance.
1
NMR (CD2Cl2, 298 K, 8.45 T field strength): H: 1.19 ppm (d, 2JH−P
= 7.9 Hz, 18H), 7.53−7.59 ppm (m, 6H), 7.65−7.69 (m, 4H),
1
13C{1H}: 12.2 ppm (d, JC−P = 12.1 Hz), 130.6 ppm (s), 131.0 ppm
(s), 136.6 ppm (s), 31P{1H}: −41.3 ppm (s), 19F: −79.2 ppm (s).
[Cl(PMe3)2Sb][OTf]2. A solution of excess TMSOTf (0.67 g, 3.0
mmol) in 4 mL of CH3CN was added to solid [(PMe3)2Cl3Sb] (0.38
g, 1.0 mmol), and the clear, colorless solution was allowed to stir for 1
h. Volatiles were removed under a vacuum to yield a sticky powder,
which was redissolved in a minimum of CH3CN and recrystallized via
diffusion of Et2O at −30 °C to give colorless block-shaped crystals that
were washed with 0.5 mL of cold CH3CN. Yield: 0.50 g, 82%. Melting
point: 157−160 °C. Elemental analysis for C8H18ClF6O6P2S2Sb
(calcd/expt): C (15.81/15.45), H (2.99/2.83). NMR (CD3CN, 298
K, 8.45 T field strength): 1H: 2.0 ppm (d, 2JH−P = 14.6 Hz), 13C{1H}:
RESULTS AND DISCUSSION
■
Preparation of Phosphine Complexes of Antimony. A
series of PR3 (R = Me, Ph, Cy) complexes on [ClSb]2+,
[Cl2Sb]1+, Cl3Sb, or [Cl4Sb]1− acceptors have been prepared
and structurally characterized. As illustrated in Scheme 1a,
reactions of PR3 with SbCl3 yield the corresponding derivatives
of [Cl3(PR3)Sb]. In the presence of excess phosphine the bis-
phosphine complexes, [Cl3(PR3)2Sb], are formed. Analogous
reactions of Cl2(Ph)Sb with PR3 give the corresponding
derivatives of [Cl2(Ph)(PR3)Sb]. Attempts to prepare
[Cl2(Ph)(PMe3)2Sb] yielded only [Cl2(Ph)(PMe3)Sb] and
free PMe3 even in the presence of a 6-fold excess of phosphine.
1
9.6 ppm (d, JC−P = 34.0 Hz), 31P{1H}: 15.8 (s, broad).
[Mg(CH3CN)6][Cl4(PMe3)Sb]2. MgCl2 (0.10 g, 1.0 mmol) and SbCl3
(0.46 g, 2.0 mmol) were combined in 8 mL of CH3CN. The reaction
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Figure 1. Views of the solid-state structures for (a) [mer-Cl3(PMe3)Sb] (showing Sb−Cl intermolecular interactions that results in a polymeric
array), (b) [fac-Cl3(PPh3)Sb] (dimeric structure), (c) [mer-Cl3(PCy3)Sb], (d) [mer-Cl3-cis-(PMe3)2Sb], (e) [mer-Cl3-trans-(PPh3)2Sb], and (f)
[mer-Cl3-trans-(PCy3)2Sb].
We speculate that the Lewis acidity of the antimony center in
[Cl2(Ph)(PR3)Sb] is quenched by PMe3, but not by PPh3.
Reaction of PMe3 with SbCl3 in the presence of 1 equivalent
of TMSOTf gives [Cl2(PMe3)Sb][OTf] with elimination of
TMSCl as illustrated in Scheme 1b, while attempts to obtain
[Cl2(PPh3)Sb][OTf] were unsuccessful, giving only
[Cl2(PPh3)2Sb][OTf]. The corresponding [Cl2(PMe3)2Sb]-
[OTf] is formed in the presence of 2 equivalents or excess
PMe3. The analogous reaction of PMe3 with Ph2SbCl and
TMSOTf gives [Ph2(PMe3)Sb][OTf] according to Scheme 1c,
and [Ph2(PMe3)2Sb][OTf] is formed according to Scheme 1d
from Ph2(OTf)Sb [generated in situ via substituent exchange
between (OTf)3Sb and Ph3Sb].
In the presence of excess TMSOTf, mixtures of PMe3 with
SbCl3 yield [Cl(PMe3)2Sb][OTf]2, as in Scheme 1e. Attempts
to prepare the PPh3 derivative were unsuccessful, resulting in
formation of [Cl2(PPh3)2Sb][OTf] even in the presence of a
large excess of TMSOTf. Reaction of PMe3 with MgCl2 and
SbCl3 in CH3CN yields [Mg(CH3CN)6][Cl4(PMe3)Sb]2,
according to Scheme 1f. However, there is no evidence of an
analogous reaction occurring with PPh3.
Structure and Bonding in Neutral Phosphine Com-
plexes of Antimony. The structural features of the neutral
complexes [Cl3(PMe3)Sb], [Cl3(PPh3)Sb], [Cl3(PCy3)Sb],
[Cl3(PMe3)2Sb], [Cl3(PPh3)2Sb], and [Cl3(PCy3)2Sb] are
illustrated in Figure 1, and selected structural parameters are
listed in Table 2. In the solid state, [Cl3(PMe3)Sb] adopts a
polymeric arrangement in which antimony centers are bridged
by chlorine centers that impose a distorted square planar SbCl4
frame with a phosphine center in an axial position. The Sb−Cl
distances involving the bridging chlorine centers are predictably
longer than those involving the terminal chlorine centers.
Considering the shortest Sb−Cl interactions (Figure 2), we
envisage a lone pair of electrons occupying the site trans to the
phosphine ligand in the square-based pyramid, representing the
configuration [mer-Cl3PSb] A, illustrated in Chart 1.
urations [fac-Cl3PSb] and [mer-Cl3PSb] B, respectively (Chart
1). In each case, two chlorine atom bridges result from edge-
sharing, square-based pyramids with a chlorine substituent in
the axial position. Interestingly, [Cl3(PPh3)Sb] adopts a
centrosymmetric dimer, while [Cl3(PCy3)Sb] adopts a dimer
with C2 symmetry. The bridging Sb−Cl distances are longer
than those involving the terminal chlorine centers.
The bis-phosphine complexes [Cl3(PR3)2Sb] all adopt
monomeric structures in the solid state with meridionally
configured chlorine substituents. The phosphine ligands are cis-
configured in [mer-Cl3-cis-(PMe3)2Sb], while a trans config-
uration is observed for [mer-Cl3-trans-(PPh3)2Sb] and [mer-Cl3-
trans-(PCy3)2Sb]. We envisage that the sixth coordination site
at antimony in these complexes is occupied by a lone pair of
electrons. The alternative configuration [fac-Cl3-cis-(PR3)2Sb]
is not observed.
The P−Sb and Sb−Cl bond lengths in derivatives of
[Cl3(PR3)Sb] and [Cl3(PR3)2Sb] are compared graphically in
Figure 3, coded according to the substituent (or lone pair) that
is trans to the P−Sb and Sb−Cl bonds, respectively. Derivatives
of [Cl3(PR3)Sb] exhibit average P−Sb lengths (horizontal lines
in Figure 3 left) that are consistent with the relative Lewis
basicity of the phosphine. Longer P−Sb bonds are observed in
the analogous derivatives of [Cl3(PR3)2Sb], imposed in the
PPh3 and PCy3 derivatives by the trans configuration and
influence of the phosphines. The cis configuration of the
phosphines observed in [mer-Cl3-cis-(PMe3)2Sb] avoids the
trans influence of the phosphines with the greatest basicity,
resulting in the shortest average P−Sb bonds within the
[Cl3(PR3)2Sb] series. The significantly longer bonds in [mer-
Cl3-trans-(PCy3)2Sb] is due to a greater basicity and steric
presence of PCy3. The P−Sb or Sb−Cl bonds trans to a
chlorine atom (2, ○) are observed with intermediate length
between those trans to a phosphine and those trans to a lone
pair (Figure 3, ◊). Consequently, trans Cl−Sb−Cl arrange-
ments are preferred over trans P−Sb−Cl and trans P−Sb−P
arrangements. The trans P−Sb−Cl arrangement is tolerated by
the relatively low basicity of PPh3 in [fac-Cl3(PPh3)Sb] and is
Dimeric arrangements are observed for [Cl3(PPh3)Sb] and
[Cl3(PCy3)Sb] in the solid state, which represent config-
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Figure 2. Illustrating the local configuration at antimony for the dimeric [fac-Cl3(PPh3)Sb] (top) and polymeric [mer-Cl3(PCy3)Sb] (bottom).
Figure 3. P−Sb (left) and Sb−Cl (right) bond lengths in derivatives of [Cl3(PR3)Sb] and [Cl3(PR3)2Sb], coded according to the substituent trans to
that bond. Horizontal bars indicate the average value for the compound.
enforced in [mer-Cl3-cis-(PMe3)2Sb] to avoid the trans
configuration of the more basic PMe3 ligands. The trans P−
Sb−P arrangements observed for [mer-Cl3-trans-(PPh3)2Sb]
and [mer-Cl3-trans-(PCy3)2Sb] are unavoidable due to the
greater steric presence of PPh3 and PCy3, compared to PMe3.
On the basis of these structural comparisons, we conclude that
in the coordination sphere of antimony in neutral phosphine
complexes, the magnitude of trans influence has the trend: lone
pair < PPh3 ≤ Cl− < Cy3P ≈ PMe3.
Calculations on gas-phase models using dispersion-corrected
DFT (B3LYP-D12) were carried out to determine the reaction
enthalpies (ΔHr) and free energies (ΔGr) for the formation of
[Cl3(PR3)Sb] (R = Me, Ph) according to Scheme 1a, under
standard conditions. Geometry optimization revealed the global
minimum as a trigonal pyramidal C3v symmetry configuration,
[pyr-Cl3(PR3)Sb] 8, with the phosphine in the apical position
and the antimony below the basal plane, consistent with
previous theoretical investigations.31 However, there are no
experimental data for such a non-VSEPR geometry, and the
observed [mer-Cl3(PMe3)Sb] is a shallow minimum, observed
only when using the strictest convergence criterion and when
the angle between two chlorine substituents is restricted to
greater than 175 degrees. We conclude that the fac- and mer-
configurations observed for [Cl3(PR3)Sb] result from inter-
molecular interactions evidenced in the solid state.
The reaction enthalpies (Table 3) for complexes of PMe3 are
negative (exothermic) and small, as expected for closed-shell
interactions. However, the entropic costs of complexation are
sufficiently high that the overall free-energy changes for the
observed mer and fac isomers are positive (nonspontaneous).
Table 3. Calculated (B3LYP-D with Counterpoise
Correction) Enthalpies and Free Energies for Reaction of
Cl3Sb with PR3 to give [Cl3(PR3)Sb] in kJ mol−1
[Cl3(PMe3)Sb]
[Cl3(PPh3)Sb]
ΔHr
ΔGr
ΔHr
ΔGr
[fac-Cl3(PR3)Sb]
[mer-Cl3(PR3)Sb]
[pyr-Cl3(PR3)Sb] 8
−18.0
−9.2
−93.7
+21.8
+33.1
−11.8
+105.9
+140.6
−49.4
+151.9
+192.9
+10.5
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Figure 4. Variations in selected calculated bond lengths for [mer-Cl3-cis-(PMe3)2Sb] as a function of the dielectric constant (ε) of the applied PCM
solvent field.
Figure 5. Solid-state structures of the cation in (a) [fac-Cl2(PMe3)Sb][OTf], showing Sb---O contacts, (b) [trans-Cl2−cis-(PMe3)2Sb][OTf],
showing the dimeric interactions and Sb---O contacts, (c) [cis-Cl2-trans-(PPh3)2Sb][OTf], showing Sb---O contacts, (d) [fac-Ph2(PMe3)Sb][OTf],
(e) [cis-Ph2-trans-(PMe3)2Sb][OTf], showing Sb---O contact, (f) [fac-Ph2(PPh3)Sb][PF6], and (g) [cis-Ph2-trans-(PPh3)2Sb][PF6].
The polymeric solid-state structure of [Cl3(PMe3)Sb] results
from intermolecular coordination of a chlorine center from an
adjacent [Cl3(PMe3)Sb] complex. The gas-phase reaction free
energies predict that all three configurations of [Cl3(PPh3)Sb]
are unstable with respect to the starting reagents, consistent
with the importance of intermolecular lattice interactions in the
stability of the complex.
The calculated gas-phase structures for derivatives of
[Cl3(PR3)2Sb] failed to reproduce the experimental geometric
parameters. For instance, the calculated gas-phase (vacuum)
structure of [mer-Cl3-cis-(PMe3)2Sb] (Figure 4) shows good
agreement with the experimentally observed SbA−PA (Δ = 0.01
Å; Δ = calcd − expt), SbA−ClA (Δ = 0.01 Å), and SbA−ClB (Δ
= 0.05 Å) distances, but the calculations overestimate the SbA−
PB distance by 0.19 Å and underestimate the SbA−ClC distance
by 0.17 Å. In the gas phase, one ligand is bound only weakly,
affecting a relatively short SbA−ClC bond trans to the long PB−
SbA bond. One calculated imaginary frequency is associated
with SN2-type displacement of ClC from the metal center by PB.
Calculations in a simulated solvent field (PCM) of pentane
(dielectric constant, ε = 1.84), acetonitrile (ε = 37.5), or water
(ε = 80.1) show that greater solvent field polarity recovers the
experimentally observed structural parameters (Figure 4). We
conclude that calculated gas-phase SbA−PB and SbA−ClC
distances for [mer-Cl3-cis-(PMe3)2Sb] model the initial steps
of an SN2-type nucleophilic displacement of a chloride anion by
a phosphine ligand at the antimony center. In solvents of
greater polarity, the minimum energy structure is farther along
the reaction coordinate toward [Cl2(PMe3)2Sb]1+.
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Table 4. Selected Structural Parameters for [fac-Cl2(PMe3)Sb][OTf], [trans-Cl2-cis-(PMe3)2Sb][OTf], [cis-Cl2-trans-
(PPh3)2Sb][OTf], [fac-Ph2(PMe3)Sb][OTf], [cis-Ph2−trans-(PMe3)2Sb][OTf], [fac-Ph2(PPh3)Sb][PF6], [cis-Ph2-trans-
(PPh3)2Sb][PF6]
[Cl2(PMe3)Sb] [OTf]
[Ph2(PMe3)Sb] [OTf]
[Ph2(PPh3)Sb] [PF6]
[Ph2(PMe3)Sb]4 [Cl][PF6]3 [Ph2(PMe3)Sb]4 [Br][PF6]3
PA−Sb
P−C
Sb−XA
Sb−XB
Sb---OA
Sb---OB
PA−Sb−XA
PA−Sb−XB
XA−Sb−XB
ref
2.6043(9)
1.797(3)−1.809(3)
2.3719(9)
2.3926(8)
2.705(2)
2.5584(4)
2.5950(12)
1.794(4)−1.805(5)
2.152(5)
2.5906(14)
1.800(6)−1.804(6)
2.153(4)
2.5846(13)
1.7981(16)−1.8047(17)
2.1487(15)
2.1549(15)
3.5475(16)
3.6882(14)
89.55(4)
1.798(5)−1.799(4)
2.153(3)
2.153(5)
2.153(4)
2.153(3)
2.755(2)
90.71(3)
93.70(12)
97.61(12)
100.14(18)
8
89.08(9)
89.08(9)
89.43(8)
89.43(8)
90.62(3)
94.74(4)
92.78(3)
96.35(6)
107.64(19)
7
107.08(16)
7
this work
this work
[Cl2(PMe3)2Sb] [OTf]
[Cl2(PPh3)2Sb] [OTf]
[Ph2(PMe3)2Sb] [OTf]
[Ph2(PPh3)2Sb] [PF6]
PA−Sb
PB−Sb
P−C
Sb−XA
Sb−XB
Sb---OA
Sb---OB
2.5890(4)
2.8621(5)
2.8034(4)
2.8694(8)
2.5805(4)
2.8042(5)
2.7559(4)
2.8426(9)
1.7914(18)−1.8005(16)
2.6707(4)
1.810(2)−1.818(2)
2.3643(6)
1.8047(19)−1.812(2)
2.1594(17)
1.811(12)−1.833(11)
2.157(2)
2.5079(4)
2.4083(6)
2.1558(16)
2.157(3)
2.996(1)
2.6793(17)
3.2332(18)
160.901(17)
81.519(18)
84.404(19)
82.142(18)
88.230(19)
98.27(3)
3.4951(19)
PA−Sb−PB
101.754(13)
79.283(13)
87.027(14)
79.634(14)
87.122(14)
158.504(14)
this work
177.142(13)
86.64(4)
89.60(4)
90.51(4)
90.90(4)
95.06(6)
this work
170.11(8)
87.9(2)
86.9(3)
86.5(2)
86.2(3)
101.1(3)
8
PA−Sb−XA
PA−Sb−XB
PB−Sb−XA
PB−Sb−XB
XA−Sb−XB
ref
this work
Structure and Bonding in Cationic Phosphine Com-
plexes of Antimony. The solid-state structures of the cations
in [fac-Cl2(PMe3)Sb][OTf], [trans-Cl2-cis-(PMe3)2Sb][OTf],
[cis-Cl2-trans-(PPh3)2Sb][OTf], [fac-Ph2(PMe3)Sb][OTf], and
[cis-Ph2-trans-(PMe3)2Sb][OTf] are illustrated in Figure 5 and
are compared with the structures of the previously reported
[fac-Ph2(PPh3)Sb][PF6] and [cis-Ph2-trans-(PPh3)2Sb][PF6].8
Selected structural parameters for these compounds are listed
in Table 4. All triflate derivatives in Table 4 are considered to
be ionic, exhibiting Sb---O contacts that are substantially greater
than the sum of the covalent radii (ΣCR Sb−O = 2.05 Å). The
cation in [fac-Cl2(PMe3)Sb][OTf] (Figure 5a) involves two
interion Sb---O contacts imposing a square pyramidal geometry
at antimony. Similar structures are observed for the cations in
[fac-Ph2(PMe3)Sb][OTf] (Figure 5d) and [fac-Ph2(PMe3)-
Sb]4[Cl][PF6]3 (Figure 5f).7 The bisphosphine cations in
[trans-Cl2-cis-(PMe3)2Sb][OTf] (Figure 5b) and [cis-Cl2-trans-
(PPh3)2Sb][OTf] (Figure 5c) illustrate further structural
variety for antimony. The cis configuration of the more basic
PMe3 ligands in the structure of in the cation of [trans-Cl2-cis-
(PMe3)2Sb][OTf] is consistent with that of [mer-Cl3-cis-
(PMe3)2Sb] and contrasts the trans configuration of the
phosphines in the cation of [cis-Cl2-trans-(PPh3)2Sb][OTf],
made possible by the less basic PPh3 ligands and the steric
stress imposed by phenyl substituents. A trans configuration of
two PMe3 ligands is uniquely observed in the cation of [cis-Ph2-
trans-(PMe3)2Sb][OTf] (Figure 5e), suggesting that aryl
substituents are the most strongly trans-labilizing of the groups
discussed here, and the trend in the magnitude of trans
influence: lone pair < PPh3 ≤ Cl− < Cy3P ≈ PMe3 <Ph−.
Derivatives of [Ph2(PR3)2Sb]1+ (R = Me, Ph) adopt P−Sb
distances that are shorter by approximately 0.08 Å when R =
Me. The Sb−Cl distances are more sensitive (range of 0.21 Å)
to the substitution pattern around Sb. Complexes with shorter
P−Sb distances show longer Sb−Cl bond lengths. By
comparison, in complexes featuring Ph2Sb centers, the Sb−
Cphenyl distances are relatively unperturbed by the variation of
the number or type of phosphine ligand (range of 0.01 Å),
consistent with the greater elasticity in the more polarized
(more ionic) Sb−Cl bond than in the less polarized (more
covalent) Sb−C bond.
Structure and Bonding in a Dicationic Phosphine
Complex of Antimony. Two views of the dication in the
solid-state structure of [fac-Cl(PMe3)2Sb][OTf]2 are shown in
Figure 6, one illustrating the three closest contacts to oxygen
atoms of anions (a), and the other showing the dimeric
association of the formula units through bridging O−S−O units
of the anions (b). The definitive facial configuration of the
chlorine substituent and two phosphine ligands and the oxygen
centers of three separate triflate units impose a distorted
octahedral environment for antimony. The Sb---O contacts are
comparable in length to the shorter contacts in the
monocationic salts (Table 4) and are substantially greater
than the sum of the covalent radii (ΣCR Sb−O = 2.07 Å),
defining an ionic formulation. The P−Sb bond lengths in the
dication are predictably short but are in the range that is
observed for neutral and cationic phosphine complexes of
antimony. The very short Sb−Cl bond [2.3511(4) Å] is inside
the sum of the covalent radii for the two elements (ΣCR Sb−Cl
= 2.38 Å).
The trans Influence in Phosphine Complexes of
Antimony. The trend in the magnitude of trans influence at
the antimony acceptor site (lone pair < PPh3 ≤ Cl− < PCy3 ≈
PMe3 <Ph−) is further demonstrated in the neutral complexes
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The molecular structure of the [Cl4(PMe3)Sb]1− anion is
derived from [Cl5Sb]2− by replacing the apical chloride ion with
a PMe3 ligand (Figure 8). The Sb−Cl bonds [2.555(2)−
2.634(2) Å, average 2.593 Å] are on average shorter than the
basal Sb−Cl bonds in [Cl5Sb]2− [2.4775(12)−2.8847(12) Å,
average 2.646 Å]14 and those in [Cl4Sb]1− [2.4317(7)−
2.6395(11) Å, average 2.536 Å].15 The magnesium center is
not engaged by the phosphine due to encapsulation of the hard
[Mg]2+ atom with hard CH3CN ligands (solvent), leaving the
soft [Cl4Sb]1− anion as the sole coordination site for the soft
phosphine donor.
Coordination of a Lewis base to cationic or neutral
haloantimony centers is understood as donor occupation of
vacant p-orbitals or low-lying σ* Sb−X antibonding orbitals at
antimony. As shown in Figure 9, phosphine coordination to
anionic [Cl4Sb]1− also involves overlap of the Me3P lone pair
and a vacant p-orbital perpendicular to the SbCl4 plane
(HOMO−1). Bonding within the SbCl4 plane involves overlap
between the chlorine lone pairs and the two remaining p-
orbitals (HOMO-11, HOMO-12), a small degree of Cl---Cl
bonding via in-phase overlap of the chlorine p-orbitals in a ring
around Sb (HOMO-9), and delocalized π-bonding involving
the chlorine lone-pairs and the p-orbital that is used for P−Sb
bonding (HOMO-10). A nonbonding lone pair is evident as
the HOMO, thereby accounting for the five bonding pairs and
one nonbonding pair of electrons around Sb in six molecular
orbitals.
A Charge-Variant Series of PMe3 Complexes of
Chloroantimony Centers. The compounds described
above, together with [(PMe3)3Sb]3+,11 include the extensive
charge-variant series [Cl4(PMe3)Sb]1−, [Cl3(PMe3)Sb],
[Cl2(PMe3)Sb]1+, [Cl3(PMe3)2Sb], [Cl2(PMe3)2Sb]1+, [Cl-
(PMe3)2Sb]2+, and [(PMe3)3Sb]3+, for which selected bond
lengths are illustrated graphically in Figure 10. Notwithstanding
the outlier long P−Sb bond for [Cl3(PMe3)2Sb] corresponding
to the only P−Sb bond that is trans to a chloride substituent,
the Sb−Cl bonds adopt a substantially broader range than the
P−Sb bonds. A similar effect is evident for phosphine
complexes of tin16 and bismuth halides.17
Figure 6. Solid-state structural features in [fac-Cl(PMe3)2Sb][OTf]2,
showing (a) the cation with Sb---O contacts and (b) the Sb---OSO---
Sb interion contacts between adjacent cations. Thermal ellipsoids are
drawn at the 30% probability level. Hydrogen atoms have been
omitted for clarity. Select bond lengths (Å) and angles (deg): PA−SbA
= 2.5950(4), PB−SbA = 2.5843(4), SbA−ClA = 2.3511(4), SbA−OA =
2.7922(13), SbA−OB = 2.8459(14), SbA−OC = 2.7114(13), PA−SbA−
PB = 102.026(15), ClA−SbA−PA = 90.111(15), ClA−SbA−PB
91.812(15), OA−SbA−OB = 103.36(5), ClA−SbA−OC = 153.19(3).
=
featuring PR3, Cl−, and Ph− on the same antimony center. The
solid-state structures of [Cl2(Ph)(PMe3)Sb], [Cl2(Ph)(PPh3)-
Sb], and [Cl2(Ph)(PPh3)2Sb] are shown in Figure 7, and
selected structural parameters are listed in Table 5. The weakly
trans-labilizing chlorine substituents in the three compounds
allow for the observed trans Cl−Sb−Cl configurations in all
three structures. Consistently, the more trans-labilizing PMe3
ligand and Ph− substituent are cis configured. In this context,
the trans labilizing trend defined above enables rationalization
and prediction of the configurations even with the variety of
VSEPR-consistent configurational options available for deriva-
tives of [Cl2(Ph)(PR3)Sb] and [Cl2(Ph)(PR3)2Sb] illustrated
in Chart 2.
Structure and Bonding in an Anionic Phosphine
Complex of Antimony. Hypervalent centers such as those
in pnictogen pentafluorides are powerful Lewis acids and react
with halides to give weakly coordinating hexahalopnictogen
anions. Indeed anionic [SbCl4]1− is sufficiently Lewis acidic to
form the square-based pyramidal (AX4E) dianion [SbCl5]2− in
the presence of excess chloride anions.13 The anions
[dmpeSbI4]1− and [(Et3P)2Sb2Br7]1− have also been reported
as serendipitous products of a hydrolysis reaction.6 In this
context, preparation of [Mg(CH3CN)6][Cl4(PMe3)Sb]2 from
PMe3 and [Mg(CH3CN)6][Cl4Sb]2 represents a rational
synthesis of the complex anion. Formation of [Cl4(PPh3)Sb]1−
is precluded by the relatively low basicity of PPh3.
While a greater positive charge is expected to result in
stronger and shorter coordinate bonds, the P−Sb bonds are
slightly longer for complexes with greater charge. These results
are borne out by the calculations (MP2/Def2-TZVPP) of
electronic structure for select complexes (see Supporting
Information). For example, the calculated gas-phase geometry
of [Cl3(PMe3)Sb] shows a P−Sb bond (2.542 Å) that is slightly
shorter than that calculated for [Cl2(PMe3)Sb]1+ (2.561 Å).
The corresponding Sb−Cl distances for the netural (2.491 Å)
Figure 7. Solid state structures of (a) [Cl2(Ph)(PMe3)Sb], II (b) [Cl2(Ph)(PPh3)Sb], III and (c) [Cl2(Ph)(PPh3)2Sb], VI. The Roman numerals
refer to labels in Chart 2.
J
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Table 5. Selected Structural Parameters for [Cl2(Ph)(PMe3)Sb], [Cl2(Ph)(PPh3)Sb], and [Cl2(Ph)(PPh3)2Sb]
[Cl2(Ph)(PMe3)Sb]
2.5612(4)
[Cl2(Ph)(PPh3)Sb]
2.9656(5)
[Cl2(Ph)(PPh3)2Sb]
PA−Sb
PB−Sb
P−C
2.8741(4)
2.8741(4)
1.7971(17)−1.7992(19)
2.6286(5)
1.819(2)−1.822(2)
2.3965(6)
SbA−ClA
1.8174(15)−1.8204(15)
2.5989(4)
Sb−ClA
3.3152(6)
SbA−ClA′
Sb−ClB
Sb−CA
2.5566(5)
2.5463(6)
2.146(2)
2.5989(4)
2.163(2)
2.1640(16)
PA−Sb−PB
PA−Sb−ClA
169.098(16)
85.722(15)
78.952(15)
169.437(19)
PA−SbA−ClA′
100.920(15)
PA−SbA−ClA′
83.233(18)
84.04(6)
PA−Sb−ClB
PA−Sb−CA
PB−Sb−ClA
PB−Sb−ClB
PB−Sb−CA
ClA−Sb−ClB
87.297(15)
95.85(4)
94.419(15)
84.549(8)
94.418(15)
85.721(15)
84.548(8)
178.53(2)
165.789(16)
90.09(2)
a
Chart 2. VSEPR-Consistent Configurational Options for [Cl2(Ph)(PR3)Sb] and [Cl2(Ph)(PR3)2Sb]
a
P = phosphine; bold line = lone pair; □ = vacant coordination site.
bond with increasing charge, predicted by increased electro-
static attraction across the polar Sb−Cl bond. In addition,
electronic structure calculations on the model complex
[Cl2(PMe3)Sb]1+ also show (Figure 11) increased Sb−Cl
covalency via two π-type bonding MOs (HOMO-4, HOMO-5)
delocalized over the Cl−Sb−Cl fragment, evidencing a
competitive occupation of a p-orbital at Sb by the phosphine
and chlorine lone pairs. This π-type interaction is in addition to
two Sb−Cl σ-bonding MOs (HOMO-6, HOMO-8) and may
explain preferential shortening of the Sb−Cl bonds at the cost
of elongated P−Sb bonds.
The sum of bond angles around the trigonal pyramidal Sb
centers in [Cl2(PMe3)Sb]1+ (274.1°), [Cl(PMe3)2Sb]2+
(283.9°), and [(PMe3)3Sb]3+ (305.8°) increase drastically
with steric bulk upon stepwise substitution of chlorides with
phosphines. For comparison, the sum of the Cl−Sb−Cl bond
angle in solid and gaseous SbCl3 is 285.6° and 291.57°,
respectively.18,19 As expected, the extent of pyramidalization
around phosphorus decreases only slightly upon successive
removal of chlorides in [Cl4(PMe3)Sb]1− (319.1°),
[Cl3(PMe3)Sb] (320.5°), and [Cl2(PMe3)Sb]1+ (322.2°).
The 31P NMR spectra of derivatives of [Cln(PMe3)mSb](3−n)+
(n = 1−4, m = 1, 2) in CD3CN at 298 K are plotted in Figure
12 as a function of the formal charge on the stibine complex.
The phosphorus centers in [Cl2(PMe3)2Sb]1+ and [Cl-
Figure 8. Molecular structure of the anion in [Mg(CH3CN)6]-
[Cl4(PMe3)Sb]2. One of two asymmetric units is shown. Thermal
ellipsoids are drawn at 30% probability level. Hydrogen atoms have
been omitted for clarity. Select bond lengths (Å) and angles (deg):
PA−SbA = 2.582(2), SbA−ClA = 2.580(2), SbA−ClB = 2.634(2), SbA−
ClC = 2.601(2), SbA−ClD = 2.555(2), PA−SbA−ClA = 82.10(7), PA−
SbA−ClB = 85.53(8), PA−SbA−ClC = 88.89(7), PA−SbA−ClD
=
81.05(8), ClA−SbA−ClB = 90.93(8), ClA−SbA−ClC = 170.93(7), ClA−
SbA−ClD = 90.91(7), ClB−SbA−ClC = 89.45(8), ClB−SbA−ClD
166.08(7), ClC−SbA−ClD = 86.58(7).
=
and cationic (2.315 Å) complexes show the opposite trend.
Similarly, the P−Sb distance in the calculated structure of
[Cl2(PMe3)2Sb]1+ (2.555 Å) is shorter than in [Cl-
(PMe3)2Sb]2+ (2.561 Å), whereas the Sb−Cl distance is shorter
for the greater charge (2.507 and 2.317 Å, respectively). These
observations are consistent with strengthening of the Sb−Cl
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characterized by decoalescence and peak separation was not
reached at −90 °C, preventing a determination of the chemical
shifts for the two environments.
The 31P NMR results are consistent with greater deshielding
of the phosphorus nucleus for complexes of higher positive
charge. The monophosphine complexes show chemical shifts
that are further downfield than their bisphosphine analogues,
indicating a distribution of the charge over two phosphines.
The effect is most pronounced in complexes of [Ph2Sb]1+,
which show 31P chemical shifts of −21.2 ppm and −41.3 ppm
for the mono-PMe3 and bis-PMe3 complexes, respectively.
Similarly, the shift for [Cl3(PCy3)Sb] (+25.1 ppm) is 10 ppm
downfield of [Cl3(PCy3)2Sb] (+15.7 ppm). By comparison, the
31P chemical shifts of PPh3 complexes are essentially
independent of charge or geometry and are almost identical
to that of free PPh3 (−5.6 ppm). We conclude that even in
weakly polar solvents (e.g., CD2Cl2), solutions of the PPh3
complexes consist of dissociated phosphine ligands and
solvated SbCl3, Cl2Sb(OTf), or Ph2Sb(OTf). The solvation
enthalpies of these individual species are therefore approx-
imately equal to the strength of the phosphine-stibine
interaction, and in the absence of solvent, crystalline products
with P−Sb bonds are obtained in the solid state. Consistently,
all observed PPh3 complexes are soluble in CH2Cl2.
CONCLUSIONS
■
A series of cationic, neutral, and anionic prototypical phosphine
complexes of antimony(III) have been prepared and structur-
ally characterized. On the basis of the observed configurations
of substituents and ligands at antimony in this series, the
relative trans-labilizing influence exerted by a lone pair, a
substituent or a ligand is concluded to be lone pair < PPh3 ≤
Cl− < PCy3 ≈ PMe3 < Ar−. Consistent with this model, all
previously reported adducts of SbCl3 with Lewis bases weaker
than PMe3 adopt a facial configuration of chloride substitu-
ents,20,21 while complexes with the strong donors 2,2-
bipyridine, 1,3-dimethyl-2-imidazolethione,22 tetramethylth-
iourea,23 or PCy3 show the meridional configuration of chloride
substituents, and the distinction is also evident in the solid-state
Figure 9. Molecular orbitals (MP2/Def2-TZVPP) relevant to the
bonding in [Cl4(PMe3)Sb]1−. Hydrogen atoms have been omitted for
clarity.
(PMe3)2Sb]2+ are isochronous by symmetry and give rise to a
single chemical shift value. For [mer-Cl3-cis-(PMe3)2Sb], only
one signal is observed, despite the nonequivalence of the
phosphorus centers in its solid-state structure, indicating a
dynamic process that exchanges the two phosphines. However,
in a low-temperature NMR experiment, the slow-exchange limit
Figure 10. P−Sb (left) and Sb−Cl (right) bond lengths in PMe3 complexes of chloroantimony acceptors.
L
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broadly applicable to analogous complexes of other main-group
acceptor centers bearing a lone pair. We further predict that
carbene complexes of the type [Cl3(NHC)Sb] (NHC = N-
heterocyclic carbene) will feature a [mer-Cl3(NHC)Sb] A
configuration, since carbenes are stronger donors than the most
basic phosphines. By corollary, donors weaker than phosphines,
such as dialkylsulfides, will adopt a [fac-Cl3LSb] configuration.
Calculations on molecules in the gas phase suggest that the
1:1 neutral complexes are stable, but the 1:2 stibine/phosphine
complexes are unstable with respect to dissociation of one of
the phosphine ligands. Nevertheless, they are observed in the
solution and solid state due to additional stabilization from
intermolecular interactions. We therefore urge caution when
interpreting statements regarding the stability of such weakly
bound complexes in cases when such conclusions are drawn
exclusively from gas-phase modeling. Comparison of the
experimental structural data for a charge-variant series of
PMe3 complexes of haloantimony centers reveals counter-
intuitive differences in the P−Sb and Sb−Cl bond lengths,
which are explained with reference to increased ionic and
covalent Sb−Cl bonding, as borne out in computational
models. For PMe3 complexes, solution-phase 31P NMR
spectroscopy is a sensitive probe of the overall charge on the
complex, while PPh3 complexes dissociate in solution and show
only the free ligand.
There is renewed interest in the development of ligand-
stabilized main-group halides, with a view toward using their
reduction chemistry to access species featuring multiply-bonded
elements. While most efforts have been directed toward
carbene-stabilized main-group centers,25−28 we have success-
fully demonstrated analogous reductive coupling chemistry for
phosphine-stabilized cationic halophosphine29,30 and halosti-
bine11,31 centers. Moreover, a recent theoretical investigation
has made specific predictions about the viability of accessing
phosphine-stabilized group 15 element dimers (E-E triple-
bonded molecules) from phosphine-ECl3 adducts.32 The
complexes presented here represent ideal precursors to test
these predictions.
ASSOCIATED CONTENT
■
Figure 11. Molecular orbitals (MP2/Def2-TZVPP) relevant to the
bonding in [Cl2(PMe3)Sb]1+. Hydrogen atoms have been omitted for
clarity.
S
* Supporting Information
IR/Raman characterization of selected compounds; Cartesian
coordinates for calculated structures; crystallographic informa-
tion files. This material is available free of charge via the
Internet at http://pubs.acs.org.
AUTHOR INFORMATION
■
Corresponding Author
*Fax: +1 250 721 7147; tel: +1 250 721 7150; e-mail:
nburford@uvic.ca.
Notes
The authors declare no competing financial interest.
REFERENCES
■
Figure 12. 31P{1H} NMR chemical shifts in CD3CN for observed
complexes in the series [Cln(PMe3)mSb](3−n)+; n = 1−4, m = 1, 2
plotted as a function of the formal charge of the complex.
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