Rigid, fluoroarene-containing phosphonium borates and boranes: Syntheses and reactivity studies

Article abstract of DOI:10.1021/om100776k

The reaction of (C₆F₅)₂B(OEt) with ^tBu₂P(CH₂Li) yields the cyclic phosphonium ethoxyborate (C₆F₅)(OEt)B(CH₂)(C ₆F₄)P^tBu

Full text of DOI:10.1021/om100776k

6012 Organometallics 2010, 29, 6012–6019
DOI: 10.1021/om100776k
Rigid, Fluoroarene-Containing Phosphonium Borates and Boranes:
Syntheses and Reactivity Studies
Anna Schnurr, Hannes Vitze, Michael Bolte, Hans-Wolfram Lerner, and
Matthias Wagner*
€
Institut fu€r Anorganische und Analytische Chemie, J. W. Goethe-Universitat Frankfurt,
Max-von-Laue-Strasse 7, D-60438 Frankfurt (Main), Germany
Received August 10, 2010
t
The reaction of (C6F5)2B(OEt) with Bu2P(CH2Li) yields the cyclic phosphonium ethoxyborate
(C6F5)(OEt)B(CH2)(C6F4)PtBu2 ([1]OEt) via B-C adduct formation and ortho-fluoride substitu-
tion. Treatment of [1]OEt with HCl in Et2O gives the chloroborate [1]Cl in almost quantitative yield.
The reaction of [1]Cl with Li[AlH4] leads not only to the reduction of the B-Cl bond but also to the
selective substitution of one fluorine atom on the tetrafluorophenylene bridge ([4]H). [1]Cl is readily
transformed into [1]F, [1]OAc, and [1]OTf upon reaction with Tl[PF6], AgOAc, and AgOTf, respec-
tively (OAc=acetate; OTf=triflate). [1]OTf represents a convenient storage form of [1]þ, because its
B-O bond is highly labile. The free Lewis acid [1]þ was prepared in the form of its aluminate salt
[1]þ[Al(OtBuF)4]-. The phenyl derivative of [1]OEt, (C6H5)(OEt)B(CH2)(C6F4)PtBu2 ([5]OEt), is
also accessible and serves as starting material for the preparation of [5]Cl and [5]þ[Al(OtBuF)4]-.
Introduction
(Figure 1) as an efficient fluoride and azide scavenger,
possessing an exceptionally high Lewis acidity.1,13,14 Related
para-configured phosphonium boranes have been described
by Stephan et al.15 With regard to organic synthetic trans-
formations, Corey et al. reported cationic oxazaborolidi-
nium triflates ([III]OTf; Figure 1) and triflimides ([III]NTf2)
to have an extraordinarily broad application profile as
catalysts in (enantioselective) [4þ2] and [3þ2] cycloaddition
reactions.16-18 Stephan et al. coined the term “frustrated
Lewis pair” (FLP) for electron-pair donor (e.g., PtBu3) and
acceptor (e.g., B(C6F5)3) molecules that are sterically incap-
able of adduct formation.9,10 As a result, such systems
possess a unique reactivity, which can be exploited for the
activation of added molecules, the most prominent example
being heterolytic H2 splitting.19 On the basis of this discovery,
Erker et al. designed the ethylene-bridged compound
(C6F5)2B-CH2CH2-PMes2 (Mes = mesityl), which turned
out to be one of the most active metal-free hydrogenation
catalysts described to date.20
Boron-based Lewis acids, in particular aryl boranes, are
applied as anion sensors1-4 and homogeneous (co)catalysts.5-10
Recent developments in the field have shown that the acidity
and thus the reactivity of these compounds can be improved
by (i) the use of perfluorinated aryl substituents or (ii) the
incorporation of a positive charge into the molecular frame-
work. For example, Marder and Piers introduced the ditopic
borane [I] (Figure 1) for use as an inverse chelator of F- and
OH- ions.11 In combination with Ph3C(OR) (R =Me, C6F5),
[I] acts as an initiator for zirconocene-mediated ethylene poly-
merization reactions.12 The Bourissou, Gabbaı, andKawashima
groups have employed the phosphonium borane [II]þ
*To whom correspondence should be addressed. Fax: þ49 69 798
29260. E-mail: Matthias.Wagner@chemie.uni-frankfurt.de.
(1) Kim, Y.; Hudnall, T. W.; Bouhadir, G.; Bourissou, D.; Gabbaı,
F. P. Chem. Commun. 2009, 3729–3731.
(2) Kim, Y.; Zhao, H.; Gabbaı, F. P. Angew. Chem., Int. Ed. 2009, 48,
4957–4960.
(3) Hudnall, T. W.; Chiu, C.-W.; Gabbaı, F. P. Acc. Chem. Res. 2009,
42, 388–397.
(4) Wade, C. R.; Broomsgrove, A. E. J.; Aldridge, S.; Gabbaı, F. P.
Originally aiming at the preparation of the methylene-
bridged FLP (C6F5)2B-CH2-PtBu2, we have recently obtained
the cyclic phosphonium borate [1]OEt (Scheme 1), which can
Chem. Rev. 2010, 110, 3958–3984.
(5) Ishihara, K.; Yamamoto, H. Eur. J. Org. Chem. 1999, 527–538.
(6) Chen, E. Y.-X.; Marks, T. J. Chem. Rev. 2000, 100, 1391–1434.
(7) Corey, E. J. Angew. Chem., Int. Ed. 2002, 41, 1650–1667.
(14) Hudnall, T. W.; Kim, Y.-M.; Bebbington, M. W. P.; Bourissou,
D.; Gabbaı, F. P. J. Am. Chem. Soc. 2008, 130, 10890–10891.
(15) Welch, G. C.; Cabrera, L.; Chase, P. A.; Hollink, E.; Masuda,
J. D.; Wei, P.; Stephan, D. W. Dalton Trans. 2007, 3407–3414.
(16) Corey, E. J.; Shibata, T.; Lee, T. W. J. Am. Chem. Soc. 2002, 124,
3808–3809.
€
(8) Butenschon, H. Angew. Chem., Int. Ed. 2008, 47, 3492–3495.
(9) Stephan, D. W. Dalton Trans. 2009, 3129–3136.
(10) Stephan, D. W.; Erker, G. Angew. Chem., Int. Ed. 2010, 49,
46–76.
(17) Ryu, D. H.; Lee, T. W.; Corey, E. J. J. Am. Chem. Soc. 2002, 124,
9992–9993.
(18) Zhou, G.; Corey, E. J. J. Am. Chem. Soc. 2005, 127, 11958–
11959.
(19) Welch, G. C.; Stephan, D. W. J. Am. Chem. Soc. 2007, 129, 1880–
1881.
€
(20) Spies, P.; Schwendemann, S.; Lange, S.; Kehr, G.; Frohlich, R.;
(11) Williams, V. C.; Piers, W. E.; Clegg, W.; Elsegood, M. R. J.; Collins,
S.; Marder, T. B. J. Am. Chem. Soc. 1999, 121, 3244–3245.
(12) Williams, V. C.; Irvine, G. J.; Piers, W. E.; Li, Z.; Collins, S.;
Clegg, W.; Elsegood, M. R. J.; Marder, T. B. Organometallics 2000, 19,
1619–1621.
(13) Lee, M. H.; Agou, T.; Kobayashi, J.; Kawashima, T.; Gabbaı,
F. P. Chem. Commun. 2007, 1133–1135.
Erker, G. Angew. Chem., Int. Ed. 2008, 47, 7543–7546.
r
pubs.acs.org/Organometallics
Published on Web 10/15/2010
2010 American Chemical Society

Article
Organometallics, Vol. 29, No. 22, 2010 6013
Figure 1. Boron-based Lewis acids can be applied as anion sca-
vengers (e.g., [I], [II]þ) and homogeneous catalysts (e.g., [III]þ).
Scheme 1. Synthesis of the Zwitterionic Compound [1]OEt and
Its Transformation into the Chloro Derivative [1]Cla
Figure 2. Molecular structure and numbering scheme of compound
[1]OEtA. H-atoms have been omitted for clarity. Displacement
ellipsoids are drawn at the 30% probability level. Selected bond
˚
lengths (A), bond angles (deg), and torsion angles (deg): B(1)-
O(1) = 1.458(4), B(1)-C(1) = 1.659(5), B(1)-C(11) = 1.649(5),
B(1)-C(21)=1.651(4), P(1)-C(1)=1.751(3), P(1)-C(12)=
1.804(3); B(1)-C(1)-P(1) = 109.4(2), C(1)-B(1)-C(11) =
104.0(3), C(1)-P(1)-C(12) = 98.5(2); C(1)-B(1)-C(11)-
C(12)= -11.7(3), C(1)-P(1)-C(12)-C(11)= 0.5(2).
in toluene (Scheme 1) and recrystallized from a hot Et2O-
EtOH mixture (15:1). The compound is stable toward air and
moisture for extended periods of time.
[1]OEt crystallizes with two crystallographically indepen-
dent molecules in the asymmetric unit ([1]OEtA, [1]OEtB), of
which only [1]OEtA is discussed further (Figure 2). In addi-
23
˚
tion to a B-C adduct bond (B(1)-C(1) = 1.659(5) A),
A
˚
[1]OEt contains a P-aryl bond (P(1)-C(12)=1.804(3) A).
The OEt group is still present in the molecule (B(1)-O(1)=
˚
1.458(4) A), thus rendering the boron atom a four-coordinate
chiral center and [1]OEtA a phosphonium borate zwitterion.
The B(1)-C(1)-P(1) bond angle of 109.4(2)° matches the ideal
tetrahedral angle; the C(1)-B(1)-C(11) and C(1)-P(1)-C(12)
angles possess values of 104.0(3)° and 98.5(2)°, respectively,
thereby indicating a rather strain-free cyclic framework. [1]OEt
adopts a slight envelope conformation in the solid state
(dihedral angle C(11)C(12)P(1)C(1)//C(1)B(1)C(11) =11.0°).
In [1]OEt, a five-membered heterocycle has been formed
via the intramolecular nucleophilic substitution of an ortho-F
atom by the phosphine moiety. It is reasonable to assume
that the new B-C bond is created first (cf. the proposed inter-
mediate in Scheme 1) and that the LiF liberation provides a
driving force for the subsequent ring-closure reaction. Par-
allel to us, Stephan et al. investigated the reaction between
a Conditions: (i) toluene, -78 °C f rt, 12 h. (ii) Et2O, rt, 48 h.
be viewed as the ethoxy adduct of a cationic Lewis acid [1]þ.
[1]þ combines perfluoroaryl substituents as well as a phos-
phonium group in the same molecule and is therefore likely
to act as a particularly strong electron-pair acceptor for appli-
cations in the contexts mentioned above.
Given this background, we have investigated more closely
the reactivity of [1]OEt, with a particular emphasis on the
question of how the free cationic Lewis acid [1]þ can be generated.
[1]þ should possess a comparably rigid and chemically inert
framework and, with a view on [III]þ, can easily be turned into
a chiral molecule by introducing two different substituents at
the P atom.
t
(C6F5)2B(Cl) and Bu2P(CH2Li) and obtained a mixture of
the fluoro- and chloroborate analogues of [1]OEt (i.e., [1]F,
[1]Cl).24 On the basis of DFT calculations, they concluded that
the cyclization provides about 65 kcal mol-1 of thermodynamic
Results and Discussion
(23) [1]OEt-type compounds can be viewed as neutral intramolecular
adducts between a borane and a phosphorus ylide. Related intermole-
cular borane-ylide adducts have been described, i.e., H3B-C(R1)-
Synthesis and Characterization of [1]OEt and [1]Cl. [1]OEt
was synthesized from (C6F5)2B(OEt)21 and tBu2P(CH2Li)22
(R2)-PPh3 (Bestmann, H. J.; Suhs, K.; Roder, T. Angew. Chem., Int.
€
€
€
Ed. Engl. 1981, 20, 1038–1039) and (C6F5)3B-CH2-PPh3 (Doring, S.;
€
(21) Duchateau, R.; Lancaster, S. J.; Thornton-Pett, M.; Bochmann, M.
Organometallics 1997, 16, 4995–5005.
Erker, G.; Frohlich, R.; Meyer, O.; Bergander, K. Organometallics 1998, 17,
2183–2187). In (C6F5)3B-CH2-PPh3, the B-C and P-C bond lengths
amount to 1.675(2) and 1.792(1) Å, respectively; the B-C-P bond angle is
123.4(1)°.
€
(22) Eisentrager, F.; Gothlich, A.; Gruber, I.; Heiss, H.; Kiener,
C. A.; Kruger, C.; Notheis, J. U.; Rominger, F.; Scherhag, G.; Schultz,
€
€
M.; Straub, B. F.; Volland, M. A. O.; Hofmann, P. New J. Chem. 2003,
27, 540–550.
(24) Zhao, X.; Gilbert, T. M.; Stephan, D. W. Chem. Eur. J. 2010,
;
16, 10304–10308.

6014 Organometallics, Vol. 29, No. 22, 2010
Schnurr et al.
stabilization, which explains why the open-chain anion [(C6F5)2-
B(OEt)-CH2-PtBu2]- remains elusive.
Scheme 2. Attempts at the Deprotonation of [1]Cl; Cl/H
Substitution at the Boron Center with Formation of [1]H and
Subsequent F/tBu Substitution at the ortho-Phenylene Bridge
Yielding [3]H; F/H Substitution at the ortho-Phenylene Bridge
of [1]OEt with Formation of [4]OEt; Preparation of the Boron-
and ortho-Phenylene-Substituted Compound [4]Ha
In the 11B{1H} NMR spectrum, [1]OEt gives rise to a signal
at 1.9 ppm, typical of tetracoordinate boron nuclei.25 The
19F{1H} NMR spectrum reveals seven multiplet resonances.
Two of them (-164.6, -134.8 ppm) possess an integral value
of 2F, while each of the other signals integrates to 1F. This
finding is in accord with the presence of one C6F5 substituent
and one unsymmetrically substituted ortho-C6F4 ring in the
molecule. The tBu substituents are not magnetically equiva-
lent (δ(1H)=0.60, 0.83), and the same is true for the BCH2P
protons as well as the OCH2CH3 protons. We therefore con-
clude that the chiral ethoxy adduct remains intact in solution
(C6D6) on the NMR time scale.
The most significant peak in the ESIþ mass spectrum
corresponds to the [1]þ cation (m/z = 485.2).
Since the latter observation indicated that it should indeed
be possible to generate the free phosphonium borane without
degradation of the molecular framework, we next decided to
replace the ethoxy substituent of [1]OEt by a better leaving
group. To this end, [1]OEt was treated with 4 equiv of HCl in
Et2O at rt. After 48 h, the chloro adduct [1]Cl had precipitated
from the reaction mixture in 97% yield. The compound,
which was characterized by an X-ray crystal structure analysis
(see the Supporting Information (SI) for details) and by NMR
spectroscopy, served as the starting material for most sub-
sequent attempts at the preparation of free [1]þ.
Reaction of [1]OEt and [1]Cl with Brønsted Bases and Hydride
Donors. At first, we tried to deprotonate [1]Cl at the methylene
bridge in order to temporarily neutralize the positive charge of
the Lewis acid and thereby to facilitate the abstraction of the
chloride anion (Scheme 2).
Since treatment of [1]Cl with LiN(SiMe3)2 in THF gave no
t
reaction, the next experiment was conducted with BuLi-
pentane. After addition of approximately 1 equiv of the organo-
lithium reagent, only one major product signal was visible in
the 11B{1H} NMR spectrum (-17.1 ppm; C6D6); the signal
split into a doublet in the proton-coupled 11B NMR spec-
trum. The resonance of a corresponding B-H proton ap-
peared as a broad 1:1:1:1 quartet at 3.71 ppm (1JBH=95 Hz;
1
1H). Apart from that, the general signal pattern in the H
NMR spectrum, as well as in the 19F{1H} NMR spectrum,
remained qualitatively the same. Thus, tBuLi had obviously
not reacted as a base to give the phosphonium borane 2, but
as a hydride donor, producing the phosphonium hydrido-
borate [1]H (Scheme 2). Further addition of tBuLi-pentane
to the reaction mixture, which still contained some starting
material ([1]Cl), led not only to complete Cl/H exchange (11B
NMR spectroscopic control) but also to the formation of a
second phosphonium hydridoborate species lacking one of
the fluoride substituents at the phenylene bridge (19F{1H}
NMR spectroscopic control). The latter compound gave rise
to a new doublet of doublets at δ(1H) = 1.39, thereby
suggesting the introduction of a tBu group into the phenylene
fragment. Our proposal regarding the specific site of sub-
stitution (cf. [3]H; Scheme 2) is based on steric as well as
electronic considerations and on the results of the reaction of
[1]Cl with Li[AlH4] (see below).
a Conditions: (i) benzene-pentane, 5 °C f rt, 3 h. (ii) Et2O, rt, 72 h.
(iii) Et2O-THF, rt, 14 d.
0.5 equiv of Li[AlH4] in Et2O (Scheme 2). The transformation
took 72 h to arrive at completion. According to NMR spectros-
copy, the reaction product (87% yield) still contained the
ethoxy substituent and a four-coordinate boron center. In
the aromatic region of the 1H NMR spectrum, a new multi-
plet appeared at 6.57 ppm (1H). Corresponding to that, one
of the original seven resonances in the 19F{1H} NMR spec-
trum disappeared, thus pointing toward an F/H rather than
an OEt/H exchange. X-ray crystallography (see the SI for
details) finally revealed that the F atom in para-position to
the phosphonium group on the bridging phenylene ring had
been substituted by a hydrogen atom (cf. [4]OEt; Scheme 2).
Both the reaction time and the product remained the same
when 1 equiv of Li[AlH4] was employed.
With the aim to prepare an authentic sample of the B-H
product [1]H, we next studied the reaction of [1]OEt with
For its higher reactivity, [1]Cl was chosen as the next
starting material. Its reaction with 0.5 equiv of Li[AlH4] in
Et2O-THF initially gave three different species together with
some unconsumed starting material. The amount of added reduc-
ing agent was therefore increased to 1.2 equiv of Li[AlH4].
€
(25) Noth, H.; Wrackmeyer, B. Nuclear Magnetic Resonance Spec-
troscopy of Boron Compounds. In NMR Basic Principles and Progress;
Diehl, P.; Fluck, E.; Kosfeld, R., Eds.; Springer: Berlin, 1978.

Article
Organometallics, Vol. 29, No. 22, 2010 6015
Scheme 3. Substitution Reactions at the Boron Atom of [1]Cl
with Formation of [1]F, [1]OAc, and [1]OTf; Preparation of the
Free Phosphonium Borane in the Form of Its [Al(OtBuF)4]- Salta
Figure 3. Molecular structure and numbering scheme of com-
pound [4]H. H-atoms (except the atom on B(1)) have been omitted
for clarity. Displacement ellipsoids are drawn at the 50% prob-
˚
ability level. Selected bond lengths (A), bond angles (deg), and
torsion angles (deg): B(1)-C(1)=1.669(4), B(1)-C(11)=1.626(5),
B(1)-C(21)=1.625(5), P(1)-C(1)=1.790(3), P(1)-C(12)=
1.806(3); B(1)-C(1)-P(1) = 107.0(2), C(1)-B(1)-C(11) =
103.3(2), C(1)-P(1)-C(12) = 97.4(1); C(1)-B(1)-C(11)-
C(12)= -16.6(4), C(1)-P(1)-C(12)-C(11) = 11.8(3).
After 14 days, the reaction was finished and the double-
substitution product [4]H (Scheme 2) could be isolated in
53% yield (the actual proportion of [4]H in the reaction
mixture prior to workup was higher).
An NMR analysis of the stoichiometric ratio of the reac-
tion intermediates (i.e., compounds where either the B-Cl or
the C-F bond had been reduced) showed that the derivati-
zation at the boron atom is significantly faster than that of
the tetrafluorophenylene bridge. The presence of a B-H
group in [4]H is evidenced by a sharp doublet in the 11B
NMR spectrum (δ(11B)=-17.4; 1JBH =95 Hz). Moreover,
the 19F{1H} NMR resonance pattern is very similar to that of
[4]OEt (and [3]H), but distinctly different from that of [1]Cl.
X-ray crystallography on [4]H confirmed the proposed
molecular structure (Figure 3). The boron center indeed
bears a hydrogen atom, and the site of tetrafluorophenylene
substitution is the same as in [4]OEt. With regard to the selec-
tivity of the fluoride substitution reaction, a directing effect
of the OEt group during hydride attack (for example by com-
plexation of the Liþ/Al3þ counterion) can thus safely be excluded.
We rather suggest that nucleophilic attack of a hydride ion at
the para-position to the phosphonium substituent shifts
π-electron density to the P-Cipso carbon atom in the transi-
tion state. This accumulation of negative charge is then
stabilized by the π-accepting properties of the phosphonium
moiety by mechanisms similar to those operative in phos-
phonium ylides. For the same electronic reasons, we believe
a Conditions: (i) [1]F: benzene, rt/ultrasonication, 48 h; [1]OAc:
benzene, rt/ultrasonication, 36 h. (ii) benzene, rt, 30 d. (iii) Et2O, rt, 16 h.
(iv) CH2Cl2, rt, 1 h.
All other NMR data of the reaction product are closely
similar to those of the starting material. Thus, after abstrac-
tion of a chloride ion from [1]Cl, the resulting strong Lewis
acid [1]þ has obviously pulled off fluoride from the [PF6]-
ion with formation of the derivative [1]F (for an X-ray crystal
structure analysis of [1]F see the SI).26
Treatment of [1]Cl with the silver salts led to the exchange
of chloride by coordinated acetate ([1]OAc) and triflate ([1]OTf;
Scheme 3). Both compounds have been characterized by X-ray
crystallography. A plot of the molecular structure of [1]OTf
is presented in Figure 4 (see the SI for details of the solid-state
structure of [1]OAc).
˚
TheB(1)-O(1) bond length of [1]OTf amounts to 1.589(3) A,
˚
˚
which is 0.075(3) A longer than in [1]OAc and even 0.131(3) A
longer than in [1]OEt. This increase in bond length is paralleled
by a decrease in the hydrolytic stability of the three derivatives
(note: the molecular framework of [1]þ remains unaffected by
hydrolysis).
t
that the Bu substituent of [3]H is also located in the para-
position of the phosphorus atom.
The last experiment of the series was carried out using silver
tetra(perfluoro-tert-butoxy)aluminate (Ag[Al(OtBuF)4];27,28
Scheme 3). Addition of this compound to [1]Cl in C6D6 led to
Reaction of [1]Cl with Tlþ and Agþ Salts of Weakly Coordi-
nating Anions. In an attempt to prepare ionic species of the
form [1]þ[anion]-, [1]Cl was treated with Tl[PF6], AgOAc,
and AgOTf (Scheme 3).
(26) For other examples of fluoride abstraction reactions from the
[PF6]- anion by main group Lewis acids, see: (a) Kennedy, T.; Payne,
D. S. J. Chem. Soc. 1960, 4126–4130. (b) Jones, F. R.; Plesch, P. H.
J. Chem. Soc., Dalton Trans. 1979, 927–932. (c) Padma, D. K.; Kumar,
H. S. Synth. React. Inorg. Met.-Org. Chem. 1992, 22, 1533–1549.
(d) Gavritchev, K. S.; Sharpataya, G. A.; Smagin, A. A.; Malyi, E. N.;
Matyukha, V. A. J. Therm. Anal. Cal. 2003, 73, 71–83.
The reaction with Tl[PF6] in benzene resulted in a clean
product showing a doublet resonance in the 11B{1H} NMR
spectrum (δ(11B) = 4.7). The associated coupling constant
of 72 Hz is in good agreement with literature-reported 1JBF
values of various fluoroborates.25 In line with that, a broad,
poorly resolved resonance was visible at δ(19F) = -176.8,
which can be assigned to a boron-coordinated fluorine atom.
(27) Krossing, I. Chem. Eur. J. 2001, 7, 490–502.
;
(28) Krossing, I.; Reisinger, A. Coord. Chem. Rev. 2006, 250, 2721–
2744.

6016 Organometallics, Vol. 29, No. 22, 2010
Schnurr et al.
Scheme 4. Synthesis of the Phenyl Derivative [5]OEt and Its
Transformation into [5]Cl and [5]þ[Al(OtBuF)4]-a
Figure 4. Molecular structure and numbering scheme of com-
pound [1]OTf. H-atoms have been omitted for clarity. Displace-
ment ellipsoids are drawn at the 30% probability level. Selected
˚
bond lengths (A), bond angles (deg), and torsion angles (deg):
B(1)-O(1) = 1.589(3), B(1)-C(1) = 1.646(4), B(1)-C(11) =
1.622(3), B(1)-C(21)=1.625(4), P(1)-C(1)=1.794(2), P(1)-
C(12) = 1.807(2); B(1)-C(1)-P(1) = 107.9(2), C(1)-B(1)-
C(11) = 105.5(2), C(1)-P(1)-C(12) = 96.9(1); C(1)-B(1)-
C(11)-C(12) =-5.0(3), C(1)-P(1)-C(12)-C(11) =12.7(2).
the formation of an insoluble sticky solid. An NMR spectro-
scopic investigation of the mother liquor showed no signals
except the solvent signal, thereby confirming that the pre-
cipitation was quantitative. Subsequent addition of [Ph4P]Cl
(EtOH) to the precipitate regenerated [1]Cl ([1]OEt) (NMR
spectroscopic control). The reaction was repeated in CH2Cl2
on a preparative scale. After mixing of [1]Cl and Ag[Al-
(OtBuF)4] (1:1) at rt, an off-white, voluminous precipitate
immediately formed, which was removed by filtration. After
the insoluble material had been dried in vacuo, its mass
corresponded to 109% of the theoretically expected mass
of AgCl. The clear yellow filtrate was evaporated to dryness
under reduced pressure, redissolved in CD2Cl2, and investi-
gated by NMR spectroscopy. The 11B{1H} NMR spectrum
of the product showed only one signal; its chemical shift
value (64.7 ppm) and large width at half-height (950 Hz) are
characteristic of a three-coordinate boron compound (cf.
(C6F5)3B: δ(11B) = 59.429 vs [1]Cl: δ(11B) = -2.1 (h1/2 = 135
Hz)). The tBu groups give rise to only one doublet in the 1H
a Conditions: (i) toluene, -78 °Cfrt,48h.(ii) Et2O, rt, 16 h. (iii) CD2Cl2,
rt/ultrasonication, 5 min.
3
NMR spectrum (1.43 ppm, JPH = 17.7 Hz), and the same
Figure 5. Molecular structure and numbering scheme of com-
pound [5]Cl. H-atoms have been omitted for clarity. Displace-
ment ellipsoids are drawn at the 50% probability level. Selected
applies to the BCH2P protons (2.56 ppm, 2JPH=8.3 Hz). We
therefore conclude that the boron atom of [1]þ[Al(OtBuF)4]-
is no longer a chiral center and that the molecule possesses an
average Cs symmetry. Further evidence for the presence of
free [1]þ stems from the chemical shift value of the para-
fluoro substituent in the C6F5 ring: The tetracoordinate
starting material [1]Cl shows a corresponding signal at
δ(19F) =-158.8, significantly more shielded than in the three-
coordinate compound (C6F5)3B (δ(19F) = -143.029). In com-
parison, the chemical shift of the para-fluorine atom in the
C6F5 group of [1]þ[Al(OtBuF)4]- (δ(19F) =-141.1) fits almost
perfectly to the latter value.30
˚
bond lengths (A), bond angles (deg), and torsion angles (deg):
B(1)-Cl(1) = 1.944(2), B(1)-C(1) = 1.659(2), B(1)-C(11) =
1.620(2), B(1)-C(21) = 1.608(2), P(1)-C(1) = 1.788(1), P(1)-
C(12) = 1.805(1); B(1)-C(1)-P(1) = 107.9(1), C(1)-B(1)-
C(11) = 105.5(1), C(1)-P(1)-C(12) = 98.3(1); C(1)-B(1)-
C(11)-C(12) = -3.9(2), C(1)-P(1)-C(12)-C(11) = 6.5(1).
boranes, it is mandatory to exchange the C6F5 group for less
electron-withdrawing substituents. For this reason, we prepared
the C6H5 (Ph) derivative [5]OEt (Scheme 4). The required
starting material, (C6F5)(Ph)B(OEt), is readily accessible
from (Ph)(Cl)B(OEt) (see the SI) and C6F5MgBr in Et2O.
The reaction of (C6F5)(Ph)B(OEt) with tBu2P(CH2Li) leads
to [5]OEt, which can subsequently be transformed into [5]Cl
using HCl in Et2O.
Synthesis of a Less Lewis Acidic Analogue of [1]þ. In order
to modulate the Lewis acidity of [1]þ-type phosphonium
€
(29) Sundararaman, A.; Jakle, F. J. Organomet. Chem. 2003, 681,
134–142.
(30) [1]þ[Al(OtBuF)4]- can be viewed as a phosphorus ylide adduct of
The molecular structure of [5]Cl in the solid state was con-
firmed by X-ray crystallography (Figure 5). All key structural
parameters are virtually the same as in [1]Cl with the exceptions
€
a borenium ion. For reviews on related compounds, see: (a) Kolle, P.;
€
Noth, H. Chem. Rev. 1985, 85, 399–418. (b) Piers, W. E.; Bourke, S. C.;
Conroy, K. D. Angew. Chem., Int. Ed. 2005, 44, 5016–5036.

Article
Organometallics, Vol. 29, No. 22, 2010 6017
˚
of the B(1)-Cl(1) bond, which is shorter by 0.015(2) A, and
Table 1. Selected Crystallographic Data for [1]OEt and [4]H
˚
the B(1)-C(21) bond, which is longer by 0.024(2) A in the
[1]OEt
[4]H
C6F5 derivative.
Similar to [1]Cl, [5]Cl can cleanly be converted into [5]þ[Al-
(OtBuF)4]- using Ag[Al(OtBuF)4] in CD2Cl2 (Scheme 4; see
the SI for the NMR data of [5]þ[Al(OtBuF)4]-).
formula
fw
C23H25BF9OP
530.21
C21H22BF8P
468.17
color, shape
temp (K)
radiation
cryst syst
space group
colorless, plate
173(2)
Mo KR, 0.71073 A
orthorhombic
Pbca
10.2630(6)
20.5092(9)
45.270(2)
90
90
90
colorless, plate
173(2)
Mo KR, 0.71073 A
monoclinic
P21/n
8.4845(13)
15.6453(15)
16.591(2)
90
101.627(12)
90
˚
˚
Conclusion
˚
a (A)
Decoration of an aryl borane with (i) fluorine substituents
and (ii) peripheral cationic groups enhances its Lewis acidity,
which can be exploited for anion sensing and Lewis acid
catalysis. In this paper, we report on the facile synthesis of
the cyclic phosphonium chloroborate (C6F5)(Cl)B(CH2)-
(C6F4)PtBu2 ([1]Cl) and its transformation into the derivatives
[1]OTf and [1]þ[Al(OtBuF)4]-, both of them representing con-
venient storage forms of the free fluorinated phosphonium
borane [1]þ.
˚
b (A)
˚
c (A)
R (deg)
β (deg)
γ (deg)
3
˚
V (A )
Z
9528.7(8)
16
1.478
4352
0.200
2157.2(5)
4
1.442
960
0.201
Dcalcd (g cm-3
F(000)
)
μ (mm-1
)
cryst size (mm3)
0.34 ꢀ 0.27 ꢀ 0.11
0.24 ꢀ 0.19 ꢀ 0.08
[1]þ is able to abstract a fluoride ion from [PF6]- and forms
a covalent bond to the triflate ion, which testifies to an extra-
ordinarily high Lewis acidity. Thus, from our experiences
with the various counterions employed in this work, we come
to the conclusion that only the most weakly coordinating
specimen allows the successful preparation of the free Lewis
acid [1]þ.
no. of rflns collected
no. of indep rflns (Rint
data/restraints/params
GOOF on F2
35 845
9021
)
8398 (0.0648)
8398/0/632
0.952
4000 (0.0628)
4000/0/280
0.862
R1, wR2 (I > 2σ(I))
R1, wR2 (all data)
largest diff peak
0.0529, 0.1266
0.0866, 0.1398
0.817, -0.354
0.0508, 0.1033
0.1018, 0.1176
0.503, -0.310
-3
˚
and hole (e A
)
We have shown that the phenyl analogue of [1]þ, i.e.,
[(C6H5)B(CH2)(C6F4)PtBu2]þ ([5]þ), can also be synthesized.
This provides an important set-screw for a fine-tuning of the
Lewis acidity of this class of compounds. Moreover, the
tetrafluorophenylene bridge of [1]þ undergoes nucleophilic
substitution in para-position to the phosphorus atom when
treated with hydride or alkyl sources, which offers another
way of derivatizing the basic phosphonium borane framework.
Finally, the use of mixed-substituted phosphinomethanides
tBu(R)P(CH2Li) instead of tBu2P(CH2Li) in combination with
(C6F5)2B(OEt) should give rise to chiral [1]þ-type Lewis acids.
mixture of Et2O (15 mL) and EtOH (1 mL). X-ray quality crys-
tals of [1]OEt grew slowly upon storage of the solution at 5 °C
for 14 days. Yield: 0.80 g (1.52 mmol, 54%).
1H NMR (300.0 MHz, C6D6): δ 0.60 (d, 9H, 3JPH =15.2 Hz;
C(CH3)3), 0.81 (n.r., 1H; BCH2P), 0.83 (d, 9H, 3JPH =15.6 Hz;
C(CH3)3), 1.29 (n.r., 1H; BCH2P), 1.37 (tr, 3H, 3JHH =6.9 Hz;
OCH2CH3), 3.42 (m, 1H; OCH2CH3), 3.82 (m, 1H; OCH2CH3).
11B{1H} NMR (96.3 MHz, C6D6): δ 1.9 (h1/2=20 Hz). 13C{1H}
NMR (62.9 MHz, CDCl3): δ 12.5 (m; BCH2P), 18.0 (OCH2CH3),
27.6 (m; C(CH3)3), 28.0 (m; C(CH3)3), 35.0 (m; C(CH3)3), 35.6
(m; C(CH3)3), 59.4 (m; OCH2CH3), n.o. (CF). 19F{1H} NMR
(282.3 MHz, C6D6): δ -164.6 (m, 2F; C6F5), -160.2 (tr, 1F,
3JFF = 21 Hz; C6F5), -155.6 (m, 1F), -146.4 (m, 1F), -134.8
(m, 2F; C6F5), -125.2 (m, 1F), -123.9 (m, 1F). 31P{1H} NMR
(121.5 MHz, C6D6): δ 84.8 (m). MS (ESIþ): m/z (%) 485.2 (100)
[M - OEt]þ. Anal. Calcd for C23H25BF9OP [530.21]: C, 52.10;
H, 4.75. Found: C, 52.34; H, 4.66.
Experimental Section
General Remarks. All reactions were carried out under an N2
atmosphere using Schlenk tube techniques and carefully dried
solvents. NMR: Bruker AM 250, Avance 300, and Avance 400.
Chemical shifts are referenced to residual solvent signals (1H,
13C{1H}) or external BF3 Et2O (11B{1H}), CFCl3 (19F{1H}),
Synthesis of [1]Cl. A solution (0.91 M) of HCl in Et2O (4.9 mL,
4.46 mmol) was added at rt via syringe to a stirred solution of
[1]OEt (0.59 g, 1.12 mmol) in Et2O (30 mL). Stirring was con-
tinued for 48 h, whereupon [1]Cl gradually precipitated. All
volatiles were driven off from the reaction mixture under
reduced pressure to give [1]Cl as a colorless solid in analytically
pure form. Yield: 0.56 g (1.08 mmol, 97%). Single crystals suit-
able for X-ray crystallography were grown by slow evaporation
of a CH2Cl2 solution of [1]Cl at rt.
3
and 85% H3PO4 (31P{1H}). Abbreviations: s=singlet, d=doublet,
tr = triplet, q = quartet, vtr = virtual triplet, m = multiplet,
br=broad, n.r.=multiplet expected in the 1H NMR spectrum
but not resolved, n.o. = signal not observed. The compounds
(C6F5)2B(OEt)21,tBu2P(CH2Li),22 and Ag[Al(OtBuF)4] CH2Cl227,28
3
were synthesized according to literature procedures. Compound
(Ph)(Cl)B(OEt) is also known in the literature;31,32 an optimized
synthesis and full NMR characterization are described in the SI.
1H NMR (300.0 MHz, C6D6): δ 0.43 (d, 9H, 3JPH =15.6 Hz;
C(CH3)3), 0.70 (d, 9H, 3JPH =15.9 Hz; C(CH3)3), 1.30 (dd, 1H,
t
Synthesis of [1]OEt. Bu2P(CH2Li) (0.47 g, 2.83 mmol) was
suspended in toluene (25 mL) and the resulting slurry cooled to
-78 °C. A solution of (C6F5)2B(OEt) (1.10 g, 2.83 mmol) in
toluene (10 mL) was added dropwise with stirring. The reaction
mixture was gradually warmed to rt and stirred for 12 h. After
addition of deionized H2O, the resulting two phases were separated
and the aqueous layer was extracted with CHCl3 (3 ꢀ 10 mL).
The combined organic layers were dried over Na2SO4 and
filtered, and the solvent was removed from the filtrate in vacuo.
The slightly yellow crude product was dissolved in a refluxing
2JHH =16.4 Hz, 2JPH =8.8 Hz; BCH2P), 1.86 (dd, 1H, 2JHH
=
16.4 Hz, 2JPH = 11.9 Hz; BCH2P). 11B{1H} NMR (96.3 MHz,
C6D6): δ -2.1 (h1/2 = 135 Hz). 13C{1H} NMR (62.9 MHz,
CDCl3):δ14.8 (m; BCH2P), 26.8 (m; C(CH3)3), 27.8 (m; C(CH3)3),
35.1 (m; C(CH3)3), 35.6 (m; C(CH3)3), n.o. (CF). 19F{1H} NMR
(282.3 MHz, C6D6):δ-164.4 (m, 2F; C6F5), -158.8 (tr, 1F, 3JFF
=
21 Hz; C6F5), -154.4 (m, 1F), -145.3 (m, 1F), -132.0 (m, 2F;
C6F5),-125.6 (m, 1F), -123.7 (m, 1F). 31P{1H} NMR (121.5 MHz,
C6D6): δ 88.3 (m). MS (ESIþ): m/z (%) 485.2 (100) [M - Cl]þ.
Anal. Calcd for C21H20BClF9P [520.60]: C, 48.45; H, 3.87.
Found: C, 48.51; H, 3.67.
(31) Dandegaonker, S. H.; Gerrard, W.; Lappert, M. F. J. Chem. Soc.
1957, 2872–2877.
(32) Dandegaonker, S. H.; Gerrard, W.; Lappert, M. F. J. Chem. Soc.
t
Synthesis of [3]H. A solution (1.9 M) of BuLi in pentane
(0.21 mL, 0.40 mmol) was added with stirring at 5 °C to a solution
1957, 2893–2897.

6018 Organometallics, Vol. 29, No. 22, 2010
Schnurr et al.
of [1]Cl (0.095 g, 0.18 mmol) in benzene (11 mL). The mixture
was allowed to warm to rt, and stirring was continued for 3 h. An
aqueous solution of NaOH (10%, 1 mL) was added in one
portion, and the resulting mixture was filtered through Na2SO4.
The solid was washed with benzene (1ꢀ5 mL, 3ꢀ3 mL), and the
combined organic phases were evaporated to dryness in vacuo.
The crude product thus obtained formed a pale yellow micro-
crystalline solid.
Table 2. Selected Crystallographic Data for [1]OTf and [5]Cl
[1]OTf
[5]Cl
formula
fw
C22H20BF12O3PS
634.22
C21H25BClF4P
430.64
color, shape
temp (K)
radiation
cryst syst
space group
colorless, block
173(2)
Mo KR, 0.71073 A
monoclinic
P21/n
10.8970(7)
14.7165(7)
16.2526(10)
90
92.259(5)
90
colorless, block
173(2)
Mo KR, 0.71073 A
orthorhombic
Pbca
12.3167(5)
17.5802(6)
19.4216(7)
90
90
90
˚
˚
1H NMR (300.0 MHz, C6D6): δ 0.74 (d, 9H, 3JPH =15.2 Hz;
PC(CH3)3), 0.81 (d, 9H, 3JPH =15.2 Hz; PC(CH3)3), 0.80-0.89
(m, 1H; BCH2P), 1.13-1.19 (m, 1H; BCH2P), 1.39 (dd, 9H;
C(CH3)3), 3.77 (br q, 1H, 1JBH = 95 Hz; BH). 11B{1H} NMR
(96.3 MHz, C6D6): δ -17.0 (h1/2=50 Hz). 11B NMR (96.3 MHz,
C6D6): δ -17.0 (d, 1JBH =95 Hz). 19F{1H} NMR (282.3 MHz,
C6D6): δ -165.3 (br, 2F; C6F5), -161.9 (tr, 1F, 3JFF = 21 Hz;
C6F5), -135.4 (m, 1F), -133.1 (br, 2F; C6F5), -129.3 (m, 1F),
-97.2 (m, 1F). 31P{1H} NMR (121.5 MHz, C6D6): δ 89.0 (m).
MS (ESIþ): m/z (%) 523.7 (100) [M - H]þ.
˚
a (A)
˚
b (A)
˚
c (A)
R (deg)
β (deg)
γ (deg)
3
˚
V (A )
Z
2604.3(3)
4
1.618
1280
0.296
4205.4(3)
8
1.360
1792
0.296
Dcalcd (g cm-3
F(000)
)
Synthesis of [1]F. Neat Tl[PF6] (0.13 g, 0.38 mmol) was added
in one portion at rt to a stirred solution of [1]Cl (0.10 g, 0.19 mmol)
in benzene (20 mL). The resulting suspension was agitated in an
ultrasonic bath for 48 h, after which time all starting material
had been consumed (NMR spectroscopic control). After filtra-
tion, the insoluble material was washed with benzene (3ꢀ3 mL),
and the organic phases were combined and freeze-dried in
vacuo. The obtained colorless product was further purified by
column chromatography (silica gel, hexane-EtOAc, 4.5:1) to
obtain [1]F as a colorless solid. Yield: 0.044 g (0.087 mmol,
45%). Single crystals suitable for X-ray crystallography were
grown by slow evaporation of an Et2O solution of [1]F at rt.
1H NMR (300.0 MHz, C6D6): δ 0.64 (d, 9H, 3JPH =15.5 Hz;
C(CH3)3), 0.75 (d, 9H, 3JPH=15.7 Hz; C(CH3)3), 0.85-0.98 (m,
1H; BCH2P), 1.17-1.35 (m, 1H; BCH2P). 11B{1H} NMR (96.3
μ (mm-1
)
cryst size (mm3)
0.22 ꢀ 0.21 ꢀ 0.17
0.32 ꢀ 0.25 ꢀ 0.13
no. of rflns collected
no. of indep rflns (Rint
data/restraints/params
GOOF on F2
28 528
55 248
)
4883 (0.0855)
4883/0/362
0.948
4302 (0.0818)
4302/0/253
1.055
R1, wR2 (I > 2σ(I))
R1, wR2 (all data)
largest diff peak
0.0401, 0.0821
0.0695, 0.0903
0.569, -0.305
0.0332, 0.0797
0.0445, 0.0841
0.293, -0.328
-3
˚
and hole (e A
)
0.25 g (0.39 mmol, 98%). For the synthesis of [1]OTf from [1]Cl
and AgOTf, see the SI.
1H NMR (300.0 MHz, C6D6): δ 0.34 (d, 9H, 3JPH =15.8 Hz;
C(CH3)3), 0.81 (d, 9H, 3JPH =16.3 Hz; C(CH3)3), 1.23 (dd, 1H,
=
1
MHz, C6D6): δ 4.7 (d, JBF = 72 Hz). 19F{1H} NMR (282.3
2JHH =16.3 Hz, 2JPH =7.0 Hz; BCH2P), 2.27 (vtr, 1H, 2JHH
2JPH = 16.3 Hz; BCH2P). 11B{1H} NMR (96.3 MHz, C6D6):
δ 4.4 (h1/2 = 300 Hz). 19F{1H} NMR (282.3 MHz, C6D6):
MHz, C6D6): δ -176.8 (m, 1F; BF), -164.6 (m, 2F; C6F5),
-159.8 (tr, 1F, 3JFF=21 Hz; C6F5), -154.5 (m, 1F), -145.4 (m,
1F), -134.4 (m, 2F; C6F5), -128.8 (m, 1F), -124.5 (m, 1F).
31P{1H} NMR (121.5 MHz, C6D6): δ 86.8 (m). MS (ESIþ): m/z
(%) 485.2 (100) [M - F]þ. Anal. Calcd for C21H20BF10P [504.15]:
C, 50.03; H, 4.00. Found: C, 49.92; H, 3.91.
3
δ -163.3 (m, 2F; C6F5), -156.0 (tr, 1F, JFF = 21 Hz; C6F5),
-150.8 (m, 1F), -144.2 (m, 1F), -133.7 (m, 2F; C6F5), -125.2
(m, 1F), -124.8 (m, 1F), -78.2 (s, 3F; OSO2CF3). 31P{1H} NMR
(121.5 MHz, C6D6): δ 84.3 (m).
Synthesis of [1]þ[Al(OtBuF)4]-. CH2Cl2 (6 mL) was added at rt
to a solid mixture of [1]Cl (0.050 g, 0.096 mmol) and Ag[Al-
Synthesis of [1]OAc. A mixture of [1]Cl (0.10 g, 0.19 mmol)
and AgOAc (0.045 g, 0.27 mmol) was suspended in benzene
(15 mL) at rt. Under exclusion of light, the slurry was agitated in
an ultrasonic bath for 36 h, after which time all starting material
had been consumed (NMR spectroscopic control). After filtra-
tion, the insoluble material was washed with benzene (5 mL),
and the organic phases were combined and freeze-dried in vacuo.
[1]OAc was obtained as a colorless flaked solid. Yield: 0.093 g
(0.17 mmol, 89%). Single crystals suitable for X-ray crystal-
lography were grown by slow evaporation of an Et2O solution of
[1]OAc at rt.
(OtBuF)4] CH2Cl2 (0.111 g, 0.096 mmol). The resulting suspen-
3
sion was stirred for 1 h. All insolubles were collected on a frit and
washed with CH2Cl2 (2ꢀ1 mL). The combined organic phases
were evaporated to dryness in vacuo to obtain [1]þ[Al(OtBuF)4]-
as a colorless solid. Yield: 0.14 g (0.096 mmol, 100%).
1H NMR (300.0 MHz, CD2Cl2): δ 1.43 (d, 18H, 3JPH =17.7
2
Hz; C(CH3)3), 2.56 (d, 2H, JPH = 8.3 Hz; BCH2P). 11B{1H}
NMR (96.3 MHz, CD2Cl2): δ 64.7 (h1/2 = 950 Hz). 13C{1H}
NMR (100.6 MHz, CD2Cl2): δ 18.3 (very br; BCH2P), 27.2 (m;
C(CH3)3), 36.5 (d, 1JPC =29 Hz; C(CH3)3), n.o. (CF). 19F{1H}
NMR (282.3 MHz, CD2Cl2): δ -159.3 (m, 2F; C6F5), -141.1
(tr, 1F; C6F5), -138.4 (m, 1F), -133.7 (m, 1F), -126.6 (m, 2F;
C6F5),-117.7 (m, 1F), -114.7 (m, 1F), -75.8 (s, 36F; Al(OtBuF)4).
31P{1H} NMR (121.5 MHz, CD2Cl2): δ 82.5 (m).
1H NMR (300.0 MHz, C6D6): δ 0.20 (d, 9H, 3JPH =15.3 Hz;
C(CH3)3), 0.66 (d, 9H, 3JPH =15.8 Hz; C(CH3)3), 0.92 (dd, 1H,
2JHH=15.5 Hz, 2JPH=6.9 Hz; BCH2P), 1.76 (s, 3H; (CO)CH3),
1.95 (vtr, 1H, 2JHH =2JPH =15.5 Hz; BCH2P). 11B{1H} NMR
(96.3 MHz, C6D6): δ 0.5 (h1/2 = 75 Hz). 19F{1H} NMR (282.3
3
Synthesis of [4]OEt. A solution (1.0 M) of Li[AlH4] in Et2O
(0.19 mL, 0.19 mmol) was added via syringe at rt to a stirred
solution of [1]OEt (0.20 g, 0.38 mmol) in Et2O (15 mL). The
reaction mixture was stirred for 72 h, during which time the
initially clear colorless solution turned slightly turbid. Deio-
nized H2O was added dropwise under an inert atmosphere until
the hydrogen evolution ceased. A voluminous colorless micro-
crystalline solid precipitated, which was transformed into a
more coarse-grained material by the addition of a saturated aque-
ous solution of Na2SO4 (2 mL). The reaction mixture was filtered
through Na2SO4. The solid was washed with a toluene-Et2O
mixture (1:1, 3ꢀ5 mL), and the combined organic phases were
evaporated to dryness in vacuo to obtain analytically pure
[4]OEt. Yield: 0.17 g (0.33 mmol, 87%). Single crystals suitable
MHz, C6D6): δ -164.5 (m, 2F; C6F5), -159.3 (tr, 1F, JFF
=
21 Hz; C6F5), -155.0 (m, 1F), -146.7 (m, 1F), -133.6 (m, 2F;
C6F5), -129.7 (m, 1F), -126.1 (m, 1F). 31P{1H} NMR (121.5
MHz, C6D6): δ 84.1 (m). MS (ESIþ): m/z (%) 485.2 (70) [M -
OAc]þ. Anal. Calcd for C23H23BF9O2P [544.19]: C, 50.76; H,
4.26. Found: C, 50.84; H, 4.24.
Synthesis of [1]OTf. [1]OEt (0.21 g, 0.40 mmol) was placed in a
Schlenk vessel and cooled to -196 °C. Et2O (10 mL), dried over
potassium, was vacuum-transferred into the reaction flask. The
vessel was purged with N2 and then allowed to warm to rt. Neat
Me3SiOTf (0.077 mL, 0.094 g, 0.42 mmol) was added, and the
resulting clear solution was stirred at rt for 16 h, whereupon a
colorless crystalline precipitate formed. All volatiles were driven
off under reduced pressure to obtain [1]OTf in pure form. Yield:

Article
Organometallics, Vol. 29, No. 22, 2010 6019
for X-ray crystallography were obtained by slow evaporation of
a CH2Cl2 solution of [4]OEt at rt.
evaporated to dryness in vacuo. The residue was extracted with
hexane (1ꢀ10 mL, 1ꢀ5 mL). All volatiles were removed from the
combined extracts under reduced pressure to obtain crude [5]OEt
(0.14 g, 54%) as a yellow oil. Subsequent column chromatography
(silica gel, hexane-EtOAc, 1:2) gave analytically pure [5]OEt as a
colorless solid. Yield: 0.074 g (0.17 mmol, 28%). Note: [5]OEt
partly decomposes on silica gel.
1H NMR (300.0 MHz, C6D6): δ 0.63 (d, 9H, 3JPH = 15.1 Hz;
C(CH3)3), 0.87 (d, 10H, 3JPH=15.5 Hz; C(CH3)3, BCH2P), 1.30-
1.39 (m, 1H; BCH2P), 1.40 (tr, 3H, JHH = 6.9 Hz; OCH2CH3),
3
3.46 -3.57 (m, 1H; OCH2CH3), 3.84 -3.93 (m, 1H; OCH2CH3),
6.57 (m, 1H; Ar-H). 11B{1H} NMR (96.3 MHz, C6D6): δ 1.6
(h1/2 = 20 Hz). 13C{1H} NMR (62.9 MHz, CDCl3): δ 12.5 (m;
BCH2P), 18.0 (OCH2CH3), 27.6 (m; C(CH3)3), 28.1 (m; C(CH3)3),
34.9 (m; C(CH3)3), 35.5 (m; C(CH3)3), 59.2 (m; OCH2CH3), 109.8
(m;Ar-C), n.o. (CF). 19F{1H} NMR (282.3 MHz, C6D6):δ-164.9
(m, 2F; C6F5), -160.7 (tr, 1F, 3JFF=21 Hz; C6F5), -136.2 (m, 1F),
-134.6 (m, 2F; C6F5), -129.7 (m, 1F), -100.2 (m, 1F). 31P{1H}
NMR (121.5 MHz, C6D6): δ 84.1 (m). MS (ESIþ): m/z (%) 467.4
(100) [M - OEt]þ. Anal. Calcd for C23H26BF8OP [512.22]: C, 53.93;
H, 5.12. Found: C, 53.79; H, 4.98.
1H NMR (300.0 MHz, C6D6): δ 0.59 (d, 9H, 3JPH =15.0 Hz;
C(CH3)3), 0.78-0.83 (m, 1H; BCH2P), 0.88 (d, 9H, 3JPH =15.3
=
Hz; C(CH3)3), 1.18-1.26 (m, 1H; BCH2P), 1.42 (tr, 3H, 3JHH
6.9 Hz; OCH2CH3), 3.47-3.58 (m, 1H; OCH2CH3), 4.01-4.12
(m, 1H; OCH2CH3), 7.33 (m, 1H; Ph-Hp), 7.51 (vtr, 2H; Ph-Hm),
7.74 (d, 2H, 3JHH = 7.2 Hz; Ph-Ho). 11B{1H} NMR (96.3 MHz,
C6D6): δ 5.0 (h1/2=50 Hz). 19F{1H} NMR (282.3 MHz, C6D6):
δ -156.3 (m, 1F), -146.4 (m, 1F), -123.7 (m, 1F), -121.8 (m, 1F).
31P{1H} NMR (121.5 MHz, C6D6): δ 85.7 (m). MS (ESIþ): m/z
(%) 395.4 (100) [M - OEt]þ. Anal. Calcd for C23H30BF4OP
[440.26]: C, 62.75; H, 6.87. Found: C, 62.61; H, 6.86.
Synthesis of [4]H. [1]Cl (0.15 g, 0.29 mmol) was dissolved in a
mixture of Et2O (10 mL) and THF (6 mL). A solution (1.0 M) of
Li[AlH4] in Et2O (0.36 mL, 0.36 mmol) was added quickly via
syringe. The resulting mixture was stirred at rt and continuously
monitored by NMR spectroscopy. After 14 days the reaction was
complete. An alkaline (NaOH) saturated aqueous Na2SO4 solu-
tion was purged with argon and then added dropwise under an
inert atmosphere to the reaction vessel until hydrogen evolution
ceased. The resulting suspension was filtered through Na2SO4. The
solid was washed with a toluene-Et2O mixture (1:1, 3ꢀ5 mL), and
the combined organic phases were evaporated to dryness in vacuo
to obtain [4]H as a colorless solid. Yield: 0.072 g (0.15 mmol, 53%).
The X-ray crystal structure analysis was carried out with crystalline
material obtained by slow evaporation of an Et2O solution of [4]H
at rt under an N2 atmosphere.
Synthesis of [5]Cl. [5]OEt (0.090 g, 0.20 mmol) was dissolved
in Et2O (7 mL), and a solution (0.91 M) of HCl in Et2O (0.90 mL,
0.82 mmol) was added via syringe at rt. The reaction mixture
was stirred for 16 h, whereupon a colorless precipitate formed.
All volatiles were driven off under reduced pressure to obtain
analytically pure [5]Cl as a colorless solid. Yield: 0.084 g (0.19
mmol, 95%). Single crystals suitable for X-ray crystal structure
analysis were grown by slow evaporation of a CH2Cl2 solution
of [5]Cl at rt.
1H NMR (300.0 MHz, C6D6): δ 0.52 (d, 9H, 3JPH =15.4 Hz;
C(CH3)3), 0.82 (d, 9H, 3JPH =15.7 Hz; C(CH3)3), 1.22 (dd, 1H,
2JHH =16.1 Hz, 2JPH =9.7 Hz; BCH2P), 1.64 (dd, 1H, 2JHH
=
2
16.1 Hz, JPH = 10.5 Hz; BCH2P), 7.27 (m, 1H; Ph-Hp), 7.43
(vtr, 2H; Ph-Hm), 7.84 (d, 2H, 3JHH =7.7 Hz; Ph-Ho). 11B{1H}
NMR (96.3 MHz, C6D6): δ 2.5 (h1/2 =180 Hz). 19F{1H} NMR
(282.3 MHz, C6D6): δ -155.2 (m, 1F), -144.8 (m, 1F), -124.6
(m, 1F), -123.7 (m, 1F). 31P{1H} NMR (121.5 MHz, C6D6): δ
88.8 (m). MS (ESIþ): m/z (%) 395.4 (100) [M - Cl]þ. Anal.
Calcd for C21H25BClF4P [430.64]: C, 58.57; H, 5.85. Found: C,
58.63; H, 5.82.
1H NMR (300.0 MHz, C6D6): δ 0.70 (d, 9H, 3JPH =15.1 Hz;
C(CH3)3), 0.77 (d, 9H, 3JPH=15.2 Hz; C(CH3)3), 1.08-1.18 (m,
=
1H; BCH2P), 1.25-1.40 (m, 1H; BCH2P), 3.70 (br q, 1H, 1JBH
95 Hz; BH), 6.46 (m, 1H; Ar-H). 11B{1H} NMR (96.3 MHz,
C6D6): δ -17.4 (h1/2 = 50 Hz). 11B NMR (96.3 MHz, C6D6):
1
δ -17.4 (d, JBH = 95 Hz). 19F{1H} NMR (282.3 MHz, C6D6):
3
δ -165.2 (br, 2F; C6F5), -161.5 (tr, 1F, JFF = 21 Hz; C6F5),
-138.2 (m, 1F), -133.0 (br, 2F; C6F5), -131.3 (m, 1F), -103.5
(m, 1F). 31P{1H} NMR (121.5 MHz, C6D6): δ 89.7 (m). MS
(ESIþ): m/z (%) 467.4 (100) [M - H]þ. Anal. Calcd for C21H22-
BF8P [468.17]: C, 53.87; H, 4.74. Found: C, 53.98; H, 4.86.
Synthesis of (C6F5)(Ph)B(OEt). (Ph)(Cl)B(OEt) (1.51 g, 9.0
mmol) was dissolved in Et2O (20 mL), and the solution was
cooled to 0 °C. Freshly prepared C6F5MgBr (9.08 mmol) in
Et2O (20 mL) was added dropwise with stirring over a period of
30 min, whereupon a colorless solid precipitated from the light
brown solution. The reaction mixture was allowed to warm to rt
and stirred for 12 h. The solvent was removed in vacuo, and the
residue was extracted with benzene (1ꢀ20 mL, 2ꢀ10 mL). The
extracts were combined, and the solvent was removed by distilla-
tion at ambient pressure. (C6F5)(Ph)B(OEt) was obtained by
subsequent distillation of the oily residue at lower pressure
(0.1 Torr, 79-81 °C) as a slightly viscous colorless liquid. Yield:
1.89 g (6.30 mmol, 70%).
X-ray Crystal Structure Analysis of [1]OEt, [1]Cl, [1]F, [1]OAc,
[1]OTf, [4]H, [4]OEt, (C6F5)2B(OEt), and [5]Cl. Data were col-
lected on a Siemens CCD three-circle diffractometer ([1]Cl) and on
a STOE IPDS II two-circle diffractometer (all other structures) with
graphite-monochromated Mo KR radiation. Empirical absorption
corrections were performed for all structures except (C6F5)2B(OEt)
using the MULABS33 option in PLATON.34 The structures were
solved by direct methods using the program SHELXS35 and refined
against F2 with full-matrix least-squares techniques using the
program SHELXL-97.36
The compounds [1]OEt and [4]OEt crystallize with two crys-
tallographically independent molecules in the asymmetric unit.
CCDC reference numbers: 787432 ([1]OEt), 787433 ([1]Cl),
787435 ([1]F), 787436 ([1]OAc), 787437 ([1]OTf), 787434 ([4]H),
787438 ([4]OEt), 787439 ((C6F5)2B(OEt)), and 787440 ([5]Cl).
Acknowledgment. Financial support by the Chemetall
GmbH is gratefully acknowledged.
1H NMR (300.0 MHz, C6D6): δ 0.99 (tr, 3H, 3JHH =7.1 Hz;
OCH2CH3), 3.68 (q, 2H, 3JHH=7.1 Hz; OCH2CH3), 7.12-7.25
(m, 3H; Ph-H), 7.69-7.72 (m, 2H; Ph-H). 11B{1H} NMR (96.3
MHz, C6D6): δ 43.1 (h1/2=140 Hz). 13C{1H} NMR (75.5 MHz,
C6D6): δ 17.0 (OCH2CH3), 65.2 (OCH2CH3), 128.3 (Ph-C),
133.0 (Ph-C), 135.6 (Ph-C), n.o. (CF, BC). 19F{1H} NMR
(282.3 MHz, C6D6): δ -160.6 (m, 2F; Fm), -151.9 (tr, 1F,
3JFF =21 Hz; Fp), -131.9 (m, 2F; Fo). Anal. Calcd for C14H10-
BF5O [300.03]: C, 56.04; H, 3.36. Found: C, 55.86; H, 3.13.
Supporting Information Available: Synthesis of [1]OTf from
[1]Cl and AgOTf; synthesis details and NMR spectroscopic
characterization of (Ph)(Cl)B(OEt); plots of the NMR spectra
of [1]þ[Al(OtBuF)4]- and NMR data of [5]þ[Al(OtBuF)4]-. Crys-
tallographic data of [1]OEt, [1]Cl, [1]F, [1]OAc, [1]OTf, [4]H,
[4]OEt, (C6F5)2B(OEt), and [5]Cl in CIF format. This material is
available free of charge via the Internet at http://pubs.acs.org.
t
Synthesis of [5]OEt. Bu2P(CH2Li) (0.10 g, 0.60 mmol) was
(33) Blessing, R. H. Acta Crystallogr. 1995, A51, 33–38.
(34) Spek, A. L. J. Appl. Crystallogr. 2003, 36, 7–13.
(35) Sheldrick, G. M. Acta Crystallogr. 1990, A46, 467–473.
(36) Sheldrick, G. M. SHELXL-97, Program for the Refinement of
suspended in toluene (10 mL), and the slurry was cooled to -78 °C.
A solution of (C6F5)(Ph)B(OEt) (0.18 g, 0.60 mmol) in toluene
(5 mL) was added dropwise with stirring over 10 min. The reac-
tion mixture was allowed to warm to rt, stirred for 48 h, and
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Crystal Structures; Universitat Gottingen: Gottingen, Germany, 1997.