Synthesis and structure of the dimethyl sulfide adducts of mono- and bis(pentafluorophenyl)borane

Article abstract of DOI:10.1021/om100152v

The borane dimethyl sulfide adduct H₃BuSMe₂ and the diethyl ether adduct of tris(pentafluorophenyl)borane, (C₆F ₅)₃BuOEt₂, undergo facile exchange of hydride and pentafluorophenyl ligands,

Full text of DOI:10.1021/om100152v

2194 Organometallics 2010, 29, 2194–2197
DOI: 10.1021/om100152v
Synthesis and Structure of the Dimethyl Sulfide Adducts of Mono- and
Bis(pentafluorophenyl)borane
Anna-Marie Fuller, David L. Hughes, Simon J. Lancaster,* and Callum M. White
Wolfson Materials and Catalysis Centre, School of Chemistry, University of East Anglia, Norwich,
NR4 7TJ U.K.
Received February 25, 2010
Summary: The borane dimethyl sulfide adduct H3B SMe2 and
3
Scheme 1
the diethyl ether adduct of tris(pentafluorophenyl)borane,
(C6F5)3B OEt2, undergo facile exchange of hydride and penta-
3
fluorophenyl ligands, yielding (C6F5)2HB SMe2 (1) and
3
(C6F5)H2B SMe2 (2) depending upon the ratio of reagents
3
used. In the presence of excess dimethyl sulfide, both com-
pounds can be isolated as colorless crystals, which have been
structurally characterized.
Hoshi et al. have recently shown that the reagent generated
in situ from a 1:1 mixture of (C6F5)3B and H3B SMe2 was
Introduction
3
effective at promoting the hydroboration of an alkyne with
pinacolborane.9 The solution phase was characterized by 11
Bis(pentafluorophenyl)borane [Piers’ reagent, (C6F5)2HB]
was first reported in 1995.1,2 In the solid state the structure is
dimeric, but significant dissociation to the monomeric spe-
cies occurs in solution. Piers’ reagent is highly effective and
regioselective for the hydroboration reactions of alkenes,
alkynes,3 vinyl silanes,4 or allyl phosphines.5
The original preparation of [(C6F5)2HB] employed synthe-
tically demanding (C6F5)2ClB as the immediate precursor,
which is then converted to the borane using Me2SiHCl
(Scheme 1).1 An alternative synthetic method starting with
commercially available (C6F5)3B and Et3SiH was reported
shortly afterward.2
B
spectroscopy, which indicated the presence of (C6F5)3B,
(C6F5)2HB, and (C6F5)H2B as their dimethyl sulfide adducts.9
Evidently there is facile exchange between hydride and penta-
fluorophenyl groups in these boranes, in contrast to the lack
of reactivity between tris(pentafluorophenyl)borane and
boron trihalides.10 The observation of a mixture of pro-
ducts is not surprising given the stoichiometry and dimethyl
sulfide deficiency in a 1:1 reaction.11
The facile nature of the hydride and pentafluorophenyl
ligand exchange and the labile nature of the dimethyl sulfide
ligand suggested that Hoshi’s procedure might be refined to
provideanalternative toPiers’ syntheses. The dimethyl sulfide
adduct of bis(pentafluorophenyl)borane should retain much
of the utility of Piers’ borane, particularly for hydroboration
and the preparation of Lewis basic adducts. Furthermore,
providing the appropriate stoichiometry, this approach might
be amenable to the preparation of the dimethyl sulfide adduct
of the mono(pentafluorophenyl)borane. Herein we report
the fast and convenient syntheses of the dimethyl sulfide
adducts of mono- and bis(pentafluorophenyl)borane from tris-
(pentafluorophenyl)borane and the borane-dimethyl sulfide
adduct.
Like (C6F5)3B,6 Piers’ reagent forms “frustrated Lewis
pairs”7 when combined with bulky Lewis bases such as PtBu3
or PMes3. Heterolytic cleavage of hydrogen by these systems
generates [R3PH][(C6F5)2H2B].8 However, this anion is not
stable, and under air- and moisture-free conditions dispro-
portionation can occur to give [R3PH][(C6F5)3HB] and
[R3PH][(C6F5)H3B], while subsequent release of hydrogen
yields dimeric [H2B(C6F5)]2.8 To the best of our knowledge,
this is the only report in the open literature of the base-free
monopentafluorophenylborane.
*To whom correspondence should be addressed. Tel: þ44 1603
592009. Fax: þ44 1603 592003. E-mail: S.Lancaster@uea.ac.uk.
(1) Parks, D. J.; Spence, R. E. von H.; Piers, W. E. Angew. Chem., Int.
Ed. Engl. 1995, 34, 809.
(2) Parks, D. J.; Piers, W. E.; Yap, G. P. A. Organometallics 1998,
17, 5492.
(3) Piers, W. E.; Chivers, T. Chem. Soc. Rev. 1997, 26, 345.
(4) Parks, D. J.; Piers, W. E. Tetrahedron 1998, 54, 15469.
€
(5) Spies, P.; Kehr, G.; Bergander, K.; Wibbeling, B.; Frohlich, R.;
Results and Discussion
Treatment of a light petroleum solution containing two
equivalents of (C6F5)3B OEt2 with one equivalent of
3
H3B SMe2 resulted in the precipitation of oily droplets on
3
the sides of the reaction vessel. The subsequent addition of an
excess of dimethyl sulfide resulted in a portion of the oil
Erker, G. Dalton Trans. 2009, 1534.
(6) For reviews of the chemistry of tris(pentafluorophenyl)borane
see: ref 3 and (a) Chen, E. Y.-X.; Marks, T. J. Chem. Rev. 2000, 100,
1391. (b) Erker, G. Dalton Trans. 2005, 1883. (c) Piers, W. E. Adv.
Organomet. Chem. 2005, 52, 1.
(7) For recent reviews on “frustrated Lewis pairs”, see: (a) Stephan,
D. W. Dalton Trans. 2009, 3129. (b) Stephan, D. W.; Erker, G. Angew.
Chem., Int. Ed. 2010, 49, 46.
(9) Hoshi, M.; Shirakawa, K.; Okimoto, M. Tetrahedron Lett. 2007,
48, 8475.
(10) Duchateau, R.; Lancaster, S. J.; Thornton-Pett, M.; Bochmann,
M. Organometallics 1997, 16, 4995.
(11) The active species for hydroboration is believed to be
[(C6F5)2HB], resulting from dissociation of the weakly bound dimethyl
sulfide ligand.
(8) Jiang, C.; Blacque, O.; Berke, H. Organometallics 2009, 28, 5233.
r
pubs.acs.org/Organometallics
Published on Web 04/14/2010
2010 American Chemical Society

Note
Organometallics, Vol. 29, No. 9, 2010 2195
Scheme 2
Table 1. Multinuclear NMR Data for Solutions in C6D6 at 23 °C
1H
11B
19F
BH
Me
o-F
m-F
p-F
(C6F5)3B SMe2
1
2
H3B SMe2
3
0.94
-3.2 -129.6 -162.1 -153.6
3
3.60 1.09 -11.9 -131.5 -162.8 -155.9
2.84 1.20 -17.2 -130.4 -163.8 -157.6
2.11 1.46 -19.4
dissolving while the remainder was separated by filtration.12
The desired product proved to be moderately soluble in light
petroleum and was purified by recrystallization from light
petroleum solution at 5 °C to yield a colorless crystalline solid
(1) (Scheme 2). Compound 1 was formulated as (C6F5)2HB
3
SMe2 on the basis of the multinuclear NMR characterization
(Table 1). The 1H NMR spectrum consisted of a broad
multiplet at δ = 3.60 ppm corresponding to the BH signal
and a sharp singlet for the dimethyl sulfide groups in the
expected 1:6 ratio. Most significantly, the 11B NMR spectrum
consisted of a doublet (1JBH = 81 Hz) at significantly lower
13
frequency than that observed for (C6F5)3B SMe2 but at
3
higher frequency than that found for H3B SMe2. These
3
assignments are consistent with those reported by Hoshi
et al. The bulk composition of 1 was confirmed by ele-
mental analysis.
Figure 1. ORTEP of the major component of 1.14 Displacement
ellipsoids are drawn at the 50% probability level. Selected bond
lengths [A] and angles [deg]: B(1)-S(2) 1.963(4), B(1)-C(1)
1.612(6), B(1)-C(11) 1.628(6); C(1)-B(1)-S(2) 112.3(3), C(11)-
B(1)-S(2) 105.0(3).
The solid-state structure was elucidated by X-ray diffrac-
tion techniques (Figure 1), and the molecule was found to be
disordered over two resolved orientations.14 The boron
center displays near tetrahedral geometry, with the B-S
˚
bond lengths slightly shorter than observed in (C6F5)3B
3
unnecessary. Any residual H3B SMe2 or [BH3]2 was easily
SMe2.13b The B-C bond length is slightly longer than
3
removed under reduced pressure. Proton-coupled 11B NMR
spectroscopy provides an excellent means with which to
monitor the reaction, and the generation of a triplet signal
at δ = -17.2 ppm (1JBH = 105 Hz) is indicative of a four-
coordinate dihydride (Table 1). The frequency of the 11B
observed in the donor-free dimer.2
Treatment of (C6F5)3B OEt2 with two equivalents of
3
H3B SMe2 in light petroleum at room temperature resulted
3
in partial conversion to (C6F5)H2B SMe2. Intriguingly,
3
under these conditions H3B SMe2 and (C6F5)2HB SMe2
3
3
NMR signal for 2 is intermediate between that for H3B
SMe2 and 1, reflecting the systematic variation in electron
3
undergo only very slow ligand exchange, in contrast to the
rapid reaction between (C6F5)3B SMe2 and H3B SMe2. If a
modest excess of H3B SMe2 is employed and the reaction is
3
3
density, as hydride ligands are replaced by the more electron-
1
withdrawing pentafluorophenyl group. The H NMR con-
3
conducted in toluene at 80 °C, complete conversion to
(C6F5)H2B SMe2 (2) occurs (Scheme 2). In the presence of
an excess of H3B SMe2 the addition of further Me2S is
sisted of a sharp singlet at δ = 1.20 ppm corresponding to the
methyl groups and a broad multiplet at δ = 2.84 ppm for the
BH2. Recrystallization of the crude material from dichloro-
methane-light petroleum solution yielded the pure com-
pound as a colorless crystalline solid, the composition of
which was confirmed by elemental analysis.
3
3
(12) Comparison of the spectroscopic data for this oily material
with an independently prepared reference sample suggests that it is
(C6F5)3B SMe2.
3
The solid-state structure of 2 was confirmed by X-ray
diffraction methods with all hydrogen atoms being located in
difference maps and refined freely (Figure 2). The boron adopts
a pseudotetrahedral geometry with the B-S bond length
(13) NMR data for (C6F5)3B SMe2 have previously been reported in
3
CDCl3 and toluene, respectively: (a) Lancaster, S. J.; Hughes, D. L.
Dalton Trans. 2003, 1779. (b) Denis, J.; Forintos, H.; Szelke, H.; Toupet, L.;
Pham, T.; Madec, P.; Gaumont, A. Chem. Commun. 2003, 54. NMR data in
C6D6 were collected from a standard sample.
only slightly shorter than that observed in (C6F5)3B SMe2.13b
(14) The structure of 1 exhibited significant disorder of the boron and
sulfur atoms over two positions, the methyl carbon atoms being
common to both components. Full refinement of the C6F5 rings of the
minor component was not possible since not all atoms could be located
in difference maps: the located atoms were refined isotropically.
3
The substituents on the boron and sulfur centers adopt a
staggered conformation about the B-S bond, and the mole-
cules pack together in pairs through an offset face-to-face

2196 Organometallics, Vol. 29, No. 9, 2010
Fuller et al.
˚
Figure 2. ORTEP of 2. Displacement ellipsoids are drawn at the 50% probability level. Selected bond lengths [A] and angles [deg]:
B(1)-S(2) 1.9577(13), B(1)-C(1) 1.604(2); C(1)-B(1)-S(2) 106.66(8).
Table 2. Crystallographic Data
pentafluorophenyl-pentafluorophenyl interaction about a
crystallographic inversion center.
1
2
The perfectly atom-efficient synthesis of 1 implied by
Scheme 2 is not achieved in practice, at least in part because
of the oily precipitate, and the isolated yield is typically just
over 50%. An attempt to increase the yield of 1 by conducting
the ligand exchange in toluene at 80 °C did not result in an
improvement. The preparation of 2 is more efficient, and
quantitative conversion was observed by monitoring the
reaction mixture by 11B NMR while isolated yields were in
excessof 70%.Substitutingthe diethyl etheradduct forfreshly
sublimed donor-free tris(pentafluorophenyl)borane does not
significantly influence the outcome of these reactions.
chem formula
M
C
408.07
monoclinic
P21/n
10.2570(2)
13.0169(3)
12.2595(3)
109.639(2)
1541.60(6)
140(1)
4
27 438
2710, 0.046
2216
0.089, 0.048
0.096, 0.070
0.19
14H7BF10
S
C8H8BF5S
242.01
monoclinic
P21/n
12.1602(6)
6.0157(3)
13.4785(7)
99.859(5)
971.42(8)
140(1)
4
18 598
2828, 0.041
2313
0.070, 0.027
0.072, 0.038
0.38
cryst syst
space group
˚
a/A
˚
b/A
˚
c/A
β/deg
3
˚
V/A
T/K
Z
reflns measd
unique reflns, Rint
obsd data
wR2, R1 (obsd data)
wR2, R1 (all data)
largest diff peak
Preliminary results indicate that 2undergoes facile hydrobora-
tion with two equivalents of alkenes. Compounds 1 and 2 serve
as convenient precursors for Lewis adducts between mono- and
bis(pentafluorophenyl)borane and donors such as ammonia.
University. The synthesis of (C6F5)3B OEt2 was conducted
3
according to the literature procedure.15
(C6F5)2HB SMe2 (1). To a solution of (C6F5)3B OEt2 (6.46 g,
11.0 mmol) in light petroleum (300 mL) was added H3B SMe2
Conclusion
3
3
Treatment of (C6F5)3B OEt2 with H3B SMe2 in the appro-
3
3
3
priate stoichiometry and where necessary with the addition of
Me2S yields the mono- and bis(pentafluorophenyl)boranes
as their dimethyl sulfide adducts (C6F5)2BH SMe2 (1) and
(0.52 mL, 5.5 mmol). There was an immediate precipitation of
an oil, which aggregated into droplets after stirring for a few
minutes. An excess of Me2S (0.50 mL, 6.8 mmol) was added
before the oil was separated by filtration and the filtrate cooled
to 0 °C, precipitating an unidentified compound and a small
3
(C6F5)H2B SMe2 (2). These compounds can be prepared
3
quickly in good yield from commercially available starting
materials.
quantity of (C6F5)2HB SMe2, which was separated by filtra-
3
tion. The filtrate was reduced to approximately half volume
and cooled to -25 °C to yield the pure product as a colorless
crystalline solid (3.64 g, 54%). X-ray quality crystals were
obtained by recrystallization from light petroleum solution at
5 °C. Anal. Calcd (found): C, 41.2 (41.2); H, 1.7 (1.9). 1H NMR
(C6D6): δ 3.60 (1H, br s, BH), 1.09 (6H, s, CH3). 13C NMR
(C6D6):δ21.8 (CH3).11BNMR(C6D6):δ-11.9 (1B, d, J=81Hz).
19F NMR (C6D6): δ -131.5 (4F, d, JFF = 20 Hz, o-F), -155.9 (2F,
t, JFF = 20 Hz, p-F), -162.8 (4F, m, m-F).
Experimental Section
All reactions were carried out under a dry nitrogen atmo-
sphere using standard Schlenk techniques in predried glassware.
Solvents were dried using an appropriate drying agent and
distilled under nitrogen prior to use: dichloromethane (CaH2),
light petroleum (sodium/dyglyme/benzophenone), and toluene
(sodium). Dimethyl sulfide was dried over LiAlH4 and distilled
prior to use. Samples for NMR analysis were prepared using
(C6F5)H2B SMe2 (2). An excess of H3B SMe2 (2.40 mL,
3
3
25.3 mmol) was added to a solution of (C6F5)3B OEt2 (4.36 g,
3
˚
degassed deuterated solvents dried over activated 4 A molecular
7.4 mmol) in toluene (60 mL). The solution was heated to 80 °C
for 2 h before an aliquot was checked by 19F and 11B NMR to
ensure the reaction had gone to completion. The volatiles were
removed under reduced pressure to yield a colorless solid.
sieves. NMR spectra were obtained using a Bruker Avance
DPX300 spectrometer at 23 °C. Chemical shifts are reported
in parts per million and referenced to residual solvent reso-
nances (1H, 13C{1H}); 19F is relative to CFCl3; 11B is relative to
F3B OEt2. Elemental analyses were carried out at the Depart-
3
(15) (a) Lancaster, S. J. http://www.syntheticpages.org/pages/215.
(b) Pohlmann, J. L. W.; Brinckman, F. E. Z. Naturforsch. 1965, 20b, 5.
ment of Health and Human Sciences, London Metropolitan

Note
Organometallics, Vol. 29, No. 9, 2010 2197
Recrystallization from a dichloromethane-light petroleum
solution at -25 °C yielded colorless crystals of the pure product
(3.95 g, 73%). A further crop was obtained by reducing the
solvent volume under reduced pressure and cooling to -25 °C.
Anal. Calcd (found): C, 39.7 (39.7); H, 3.3 (2.9). 1H NMR
(C6D6): δ 2.84 (2H, m, BH2), 1.20 (6H, s, CH3). 13C NMR
processed using the CrysAlis-CCD and RED programs.16 The
structures were determined by direct method routines in the
SHELXS program and refined by full-matrix least-squares methods
on F2 in SHELXL17 within the WinGX program suite.18 The results
are collated in Table 2. Scattering factors for neutral atoms were
taken from literature values.19 All hydrogen atoms in 1were included
in idealized positions, and their Uiso values were set to ride on the Ueq
values of the parent boron and carbon atoms. All hydrogen atoms in
2 were located in difference maps and refined freely.
(C6D6): δ 22.2 (CH3). 11B NMR (C6D6): δ -17.2 (1B, t, JBH
=
105 Hz). 19F NMR (C6D6): δ -130.4 (2F, d, JFF = 17 Hz, o-F),
-157.6 (1F, t, JFF = 20 Hz, p-F), -163.8 (2F, m, m-F).
X-ray Crystallography. Diffraction data were collected at UEA
on an Oxford Diffraction Xcalibur-3 CCD diffractometer and
Acknowledgment. The authors are grateful to the Engi-
neering and Physical Sciences Research Council and the
University of East Anglia for support. We thank Dr. Joseph
Wright for helpful discussions.
(16) CrysAlis-CCD and -RED; Oxford Diffraction Ltd: Abingdon, UK,
2008.
(17) Sheldrick, G. M. Acta Crystallogr., Sect. A: Found. Crystallogr.
2008, 64, 112.
(18) Farrugia, L. J. J. Appl. Crystallogr. 1999, 32, 837.
(19) International Tables for X-Ray Crystallography; Wilson, A. J. C.,
Ed.; Kluwer Academic Publishers: Dordrecht, The Netherlands, 1992;
Vol. C, pp 193, 219, and 500.
Supporting Information Available: Crystallographic informa-
tion file (CIF). This material is available free of charge via the
Internet at http://pubs.acs.org.