Solid-state and solution structure of a hypervalent AX5 compound: Sb(C6F5)5

Article abstract of DOI:10.1002/anie.201108858

Eliminating restraints: A trigonal-bipyramidal structure has been found to be the energetically favored geometry of the hypervalent AX₅ molecule Sb(C₆F₅)₅ in the solid state and also in fluid solution, where molecules move freely and no crystal packing effects operate (see picture). Copyright

Full text of DOI:10.1002/anie.201108858

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DOI: 10.1002/anie.201108858
Organometallic Chemistry
Solid-State and Solution Structure of a Hypervalent AX5 Compound:
Sb(C6F5)5**
M. Angeles Garcꢀa-Monforte, Pablo J. Alonso, Irene Ara, Babil Menjꢁn,* and Pilar Romero
Dedicated to Professor Gerhard Erker on the occasion of his 65th birthday
“There is no more basic enterprise in chemistry than the
determination of the geometrical structure of a molecule”.[1]
Detailed knowledge of the molecular structure of a chemical
species in the solid state—usually established by X-ray and/or
neutron diffraction techniques—should be complemented,
wherever possible, with information on its structure in
solution.[2] The fulfilment of this two-fold task is particularly
difficult in the case of pentasubstituted compounds, since they
are especially prone to undergo intramolecular stereomuta-
tion processes in solution.[3] This fluxional behavior is mainly
due to the low energy separation between the most common
geometries: trigonal bipyramidal (TBPY-5) and square pyr-
amidal (SPY-5, Scheme 1).[4] Paradigmatic examples of this
estimate the energy barrier (Ea = 7(1) kJmolÀ1) associated to
the CO-scrambling process in liquid Fe(CO)5 by careful
analysis of ultrafast two-dimensional IR spectroscopic
results.[7] Here we report on the molecular structure of
a hypervalent[8] AX5 compound, Sb(C6F5)5, both in the solid
state and in solution together with a detailed analysis of its
fluxional behavior in solution.
Sb(C6F5)5 (1) has been obtained in reasonable yield by
low-temperature treatment of SbCl5 with LiC6F5 in Et2O/
n-hexane [Eq. (1)]. Compound 1 is an air-stable, white solid
that sublimes at 958C and 10À1 Torr. At this point it is
appropriate to comment on a report that has recently
appeared in patent literature claiming the synthesis of 1 by
reaction of SbCl5 with Zn(C6F5)2.[9] Very few synthetic details
and no physical, analytic, or spectroscopic properties of the
final product are given therein. However, it is stated that
“unreacted bis(pentafluorophenyl)zinc may be removed
through sublimation at a pressure of 10À2 Torr and 1108C.”
This statement alone would raise doubts about the result,
since compound 1 also sublimes under the given conditions
and thus, if really formed, would be lost together with
Zn(C6F5)2 in the sublimate.
Scheme 1. Berry pseudorotation mechanism by which identical sub-
stituents located in axial (ax) and equatorial (eq) positions of a trigonal
bipyramid (TBPY-5) exchange their positions through a square pyrami-
dal (SPY-5) transition state.
behavior are the well-known PF5 and Fe(CO)5 molecules, the
NMR spectra of which show in solution a single signal for all
five substituents (19F[5] or 13CO,[6] respectively) even at the
lowest temperatures attainable. The activation energy asso-
ciated with the stereomutation process operating in these
model species is so low that it is very difficult to determine
experimentally. In fact, it has only recently been possible to
Compound 1 belongs to the class of neutral, homoleptic
organopnictogen(V) derivatives, ER5 (E = P, As, Sb, Bi). The
solid-state molecular structures of the whole phenyl series
(R = Ph) are known and the observed group trend is
especially intriguing. TBPY-5 structures were found for the
phenyl-derivatives of the lighter elements P[10] and As,[11] in
keeping with the VSEPR model.[12] In contrast, a SPY-5
geometry was found for the 6th period element Bi.[13]
Antimony occupies the inflection point in the series, since
SPY-5 and TBPY-5 structures were respectively established
[*] Dr. M. A. Garcꢀa-Monforte, Dr. I. Ara, Dr. B. Menjꢁn
Instituto de Sꢀntesis Quꢀmica y Catꢂlisis Homogꢃnea (ISQCH)
Universidad de Zaragoza-C.S.I.C.
for SbPh5 and the solvate SbPh5·0.5CyH.[15] The fact that
[14]
C/Pedro Cerbuna 12, 50009 Zaragoza (Spain)
E-mail: menjon@unizar.es
simple lattice effects are able to decide between the two
SbPh5 polytopes, suggests a small energy difference between
them. This observation is in keeping with the highly fluxional
behavior of all these ER5 molecules in solution.[16]
Prof. Dr. P. J. Alonso, Dr. P. Romero
Instituto de Ciencia de Materiales de Aragꢁn (ICMA)
Universidad de Zaragoza-C.S.I.C.
C/Pedro Cerbuna 12, 50009 Zaragoza (Spain)
The crystal and molecular structures of 1 were established
by single-crystal X-ray diffraction methods. The local envi-
ronment of the Sb center (Figure 1) can be described as
TBPY-5 according to the low continuous shape measure
(CShM) value[17] obtained for that geometry: S(TBPY-5) =
0.40.[18] The axial positions are virtually in line with the central
[**] This work was supported by the Spanish MICINN (DGPTC)/FEDER
(project number CTQ2008-06669-C02-01/BQU) and the Gobierno
de Aragꢁn (Grupo Consolidado E21: Quꢀmica Inorgꢁnica y de los
Compuestos Organometꢁlicos).
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201108858.
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Figure 1. Thermal ellipsoid diagram (50% probability) of Sb(C6F5)5 as
found in single crystals of 1·0.25C6H14. Selected bond lengths [pm]
Figure 2. 19F NMR spectra of 1 in CD2Cl2 solution evidencing rapid
(313 K) and slow (183 K) limiting exchange conditions. The 3:2
integrated ratio for each kind of F substituent in the later spectrum
evidences a TBPY-5 structure in the slow exchange limit.
À
and angles [8] with estimated standard deviations: average Sb Ceq
À
À
À
À À
214.7(3), average Sb Cax 222.7(3), C(1) Sb C(7) 113.9(1), C(1) Sb
À
À
À À
C(13) 124.5(1), C(7) Sb C(13) 121.6(1), C(19) Sb C(25) 178.8(1).
atom, but a slight Y distortion is observed in the equatorial
replacement of just one of the C6F5 substituents in 1 by the
less sterically demanding n-butyl group resulted in the
heteroleptic species Sb(C6F5)4 Bu (2), which is highly flux-
ional in CD2Cl2 solution even at 183 K. The solid-state
structure of 2 (Figure 4) also approaches a TBPY-5 geometry
with a CShM value[17] of S(TBPY-5) = 0.46.[18] The less
electronegative n-butyl group is located in an equatorial
position, as is usually found in pentasubstituted hypervalent
compounds with TBPY-5 geometry.[22] As in SbPh3Me2,[23]
À
plane. The axial Sb C bonds are significantly longer than the
n
equatorial ones (222.7(3) vs. 214.7(3) pm average values),
probably as the result of a 3-center/4-electron hypervalent
bond in the axial direction.[8] The difference between axial
À
and equatorial E C bond lengths is less pronounced in the
related open-shell, paramagnetic organotransition-metal
derivatives [Miii(C6F5)5]2À (Miii = Tiiii (d1),[19] Viii (d2)[20]),
which also show roughly similar TBPY-5 geometries.
The 19F NMR spectrum of 1 in CD2Cl2 solution at room
temperature (313 K) displays a single set of signals, one for
each kind of ortho-, meta-, and para-F substituents (Figure 2).
At low temperature (183 K), however, each signal
splits into pairs with 3:2 relative integrated ratio
(Figure 2). This spectroscopic behavior allows us to
conclude that compound 1 has a TBPY-5 geometry
also in solution and that it readily undergoes stereo-
mutation. The activation energy, Ea, associated with
this dynamic process was determined by line-shape
analysis of the thermal dependence of the 19F NMR
spectrum of 1 between the slow and the rapid
exchange limits (Figure 3). A common value of Ea =
24.4(4) kJmolÀ1 was obtained from the analysis of all
three nonindependent sets of signals corresponding
to the ortho-, meta-, and para-F substituents (see the
Supporting Information for details). The result
obtained can thus be considered to be highly reliable.
As far as we know, all previous attempts to detect
the limiting slow-exchange spectrum in homoleptic
organopnictogen(V) derivatives, ER5, have failed.[16]
Efforts to widen the frequency range of the observed
nucleus by introducing fluorinated substituents as in
À
À
there is no difference between the Sb C(alkyl) and Sb
C(aryl) bond lengths, but there is a difference between the
À
À
À
Sb C(ax) and Sb C(eq) ones. The average Sb C(ax) bond
Figure 3. 19F NMR spectrum of 1 in CD2Cl2 solution at variable temperature
showing the thermal evolution of the ortho-F signals. Vertical scale adjustment
has been applied to each individual spectrum. The marked signal corresponds
to a tiny impurity not involved in the fluxional process. For the thermal evolution
of the meta-F and the para-F signals together with a line-shape analysis of all of
these spectra see the Supporting Information.
Sb{C6H4(CF3)-4}5 did not yield any improvement.[16a]
No better results were attained when the methyl
group in SbMe5 was replaced by the sterically
demanding CH2SiMe3, as Sb(CH2SiMe3)5 also gave
a single type of ligand down to 153 K.[21] In contrast,
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19F NMR (470.385 MHz, CD2Cl2, 313 K): d = À128.2 (2F, o-F),
À148.4 (1F, p-F), À158.2 ppm (2F, m-F). 19F NMR (470.385 MHz,
CD2Cl2, 183 K): d = À125.2 (6F, o-F(eq)), À132.8 (4F, o-F(ax)),
À145.6 (3F, p-F(eq)), À151.2 (2F, p-F(ax)), À156.7 (6F, m-F(eq)),
À158.5 ppm (4F, m-F(ax)). MS (MALDI + , DCTB) m/z = 937
[Sb(C6F5)4(C6F4)]+, 789 [Sb(C6F5)4]+.
Crystal data for 1·0.25C6H14: Single crystals suitable for X-ray
diffraction purposes were obtained by slow evaporation at 48C of
a CHCl3–n-hexane solution of 1. C31.5H3.5F25Sb; triclinic; space group
ꢀ
P1; a = 1104.20(5), b = 1133.00(5), c = 1417.28(6) pm, a = 76.135(1),
b = 69.095(1), g = 84.210(1)8, V= 16.0789(12) nm3; Z = 2; T=
100(2) K; l = 71.073 pm; absorption coefficient 1.029 mmÀ1, range
for data collection 1.97 < q < 28.758; reflections collected/unique:
15303/7549 (Rint = 0.0206); Bruker Smart CCD diffractometer. The
crystallographic data were corrected for absorption with SADABS.[26]
The structure was solved by direct methods and refinement against F2
with SHELXL-97[27] converged to final residual indices of R1 = 0.0341,
wR2 = 0.0868 [I > 2s(I)] and R1 = 0.0388, wR2 = 0.0890 (all data).
GoF = 1.034.
2: A freshly prepared solution of SbCl5 (1.18 g, 3.94 mmol) in n-
hexane (20 cm3) was added dropwise to a n-hexane (30 cm3) solution
containing LiC6F5 (19.7 mmol) and LinBu (3.9 mmol) at À788C. The
mixture was allowed to slowly reach room temperature and, after
a further 20 h of stirring, it was filtered. The filtrate was concentrated
to dryness yielding an oil which was extracted in Et2O (5 cm3). The
extract was cooled to À508C and n-hexane (15 cm3) was added
dropwise under vigorous stirring until a white solid separated. The
solid was filtered, washed at 08C with iPrOH, (3 ꢀ 1 cm3) and vacuum
dried. By allowing the mother liquor to stand at À308C for 3 days,
a second crop was obtained (2: 0.5 g, 0.6 mmol, 15% joint yield).
Figure 4. Thermal ellipsoid diagram (50% probability) of 2. Selected
bond lengths [pm] and angles [8] with estimated standard deviations:
À
À
À
À
Sb C(1) 227.7(2), Sb C(7) 212.3(2), Sb C(13) 227.4(2), Sb C(19)
À
À
À
À À
213.7(2), Sb C(25) 214.3(2), C(1) Sb C(13) 175.4(1), C(7) Sb C(19)
À
À
À À
118.0(1), C(7) Sb C(25) 114.5(1), C(19) Sb C(25) 127.4(1).
length in
2 (227.5(2) pm) is slightly longer than in
1 (222.7(3) pm), whereas the opposite applies for the average
À
Sb C(eq) value: 213.4(2) pm in 2 vs. 214.7(3) in 1. There is no
À
À
obvious reason for the rather different C(alkyl) Sb C(aryl)
angles formed between the nBu group and the equatorial C6F5
ones: 114.48(9)8 vs. 127.45(9)8. Nevertheless, the sum of all
~
Satisfactory elemental analysis. IR (KBr): nmax = 2968 (w), 2881 (w),
1640 (m), 1513 (s), 1489 (s), 1484 (s), 1458 (s), 1438 (m), 1385 (m),
1347 (m), 1281 (w), 1266 (m), 1183 (w), 1142 (w), 1085 (s), 1071 (s),
1015 (w), 972 (s; C–F), 959 (s; C–F), 895 (w), 799 (w; C6F5: X-
sensitive),[25] 748 (w), 720 (w), 705 (w), 622 (w), 608 (w), 582 (w), 480
(w), 388 (w), 369 (w), 361 cmÀ1 (w). 1H NMR (400 MHz, CD2Cl2,
298 K): d = 3.23 (tt, 2H, a-H, 3J(Ha,Hb) = 7.8 Hz, 4J(Ha,Hg) ꢀ 0.4 Hz),
1.79 (m, 2H, b-H, 3J(Hb,Hg) = 7.5 Hz, 4J(Hb,Hd) ꢀ 0.4 Hz), 1.44 (m,
2H, g-H, 3J(Hg,Hd) = 7.5 Hz), 0.90 ppm (tt, 3H, d-H). 19F NMR
(282.231 MHz, CD2Cl2, 298 K): d = À130.4 (2F, o-F), À152.6 (1F, p-
F), À161.2 ppm (2F, m-F). MS (MALDI + , DCTB) m/z = 789
[Sb(C6F5)4]+, 679 [Sb(C6F5)3(C4H9)]+, 569 [Sb(C6F5)2F3(C4H9)]+.
Crystal data for 2: Single crystals suitable for X-ray diffraction
purposes were obtained by slow diffusion of a n-hexane layer into
a Et2O solution of 2 at À308C. C28H9F20Sb; monoclinic; space group
P21/n; a = 1143.87(6), b = 1344.51(7), c = 1791.02(9) pm, b =
94.202(1)8, V= 27.471(2) nm3; Z = 4; T= 100(2) K; l = 71.073 pm;
absorption coefficient 1.162 mmÀ1, range for data collection 1.90 <
q < 28.818; reflections collected/unique: 24647/6658 (Rint = 0.0311);
Bruker Smart CCD diffractometer. The crystallographic data were
corrected for absorption with SADABS.[26] The structure was solved
by direct methods and refinement against F2 with SHELXL-97[27]
converged to final residual indices of R1 = 0.0296, wR2 = 0.0649 [I >
2s(I)] and R1 = 0.0371, wR2 = 0.0675(all data). GoF = 1.003.
À
À
equatorial C Sb C’ angles virtually amounts to 3608.
To summarize, in contrast to the structural dichotomy
shown by Sb(C6H5)5 in the solid state depending on the crystal
environment,[14,15] the perfluorinated derivative Sb(C6F5)5 (1)
has been found to favor the TBPY-5 geometry both in the
solid state and in solution. Even so, compound 1 is also
stereochemically nonrigid in solution. The activation energy
of the operating stereomutation process has been experimen-
tally determined. The value obtained, Ea = 24.4(4) kJmolÀ1,
sets the upper limit for the energy separation between the
TBPY-5 (lower-energy state) and SPY-5 (transition state)
geometries.
Experimental Section
General working techniques are described in ref. [24]. Additionally,
a Bruker AV 500 spectrometer was used to obtain the variable-
temperature 19F NMR spectroscopic data.
1: A freshly prepared solution of SbCl5 (1.18 g, 3.94 mmol) in
n-hexane (15 cm3) was added dropwise to a solution of LiC6F5
(13.81 mmol) in Et2O (60 cm3) at À788C. A white precipitate
formed while the temperature of the mixture was allowed to slowly
reach 08C. After 15 h of stirring at that temperature, the white solid in
suspension was separated by filtration and further extracted in
CH2Cl2 (30 cm3). The extract was concentrated to dryness and the
resulting residue was treated with n-hexane (5 cm3). Awhite solid was
eventually obtained, which was filtered, washed with n-hexane (3 ꢀ
2 cm3), and vacuum dried (1: 1.1 g, 1.15 mmol, 30% yield). Satisfac-
CCDC 856956 (1·0.25C6H14) and 856957 (2) contain the supple-
mentary crystallographic data for this paper. These data can be
obtained free of charge from The Cambridge Crystallographic Data
Centre via www.ccdc.cam.ac.uk/data_request/cif.
Received: December 15, 2011
Revised: January 18, 2012
Published online: February 3, 2012
~
tory elemental analysis. IR (KBr): nmax = 1637 (m), 1516 (s), 1487 (s),
1465 (s), 1379 (m), 1360 (m), 1284 (w), 1274 (w), 1148 (w), 1086 (s),
1074 (s), 1001 (w), 976 (s; C–F), 965 (s; C–F), 793 (w; C6F5: X-
sensitive),[25] 750 (w), 721 (w), 623 (w), 584 (w), 484 (w), 380 cmÀ1 (w).
Keywords: hypervalent compounds · organometallic chemistry ·
structure elucidation
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