Phosphonium-borate zwitterions, anionic phosphines, and dianionic phosphonium-dialkoxides via tetrahydrofuran ring-opening reactions

Article abstract of DOI:10.1021/ic051713r

Sterically demanding secondary phosphines and phosphides react with (THF)B(C₆F₅)₃ (THF = tetrahydrofuran) to give the THF ring-opened compounds [R₂PHC₄H ₈OB(C₆F₅)

Full text of DOI:10.1021/ic051713r

Inorg. Chem. 2006, 45, 478−480
Phosphonium
−
Borate Zwitterions, Anionic Phosphines, and Dianionic
Dialkoxides via Tetrahydrofuran Ring-Opening Reactions
Phosphonium
−
Gregory C. Welch, Jason D. Masuda, and Douglas W. Stephan*
Department of Chemistry and Biochemistry, UniVersity of Windsor,
Windsor, Ontario, Canada N9B 3P4
Received October 4, 2005
Sterically demanding secondary phosphines and phosphides react
with (THF)B(C6F5)3 (THF tetrahydrofuran) to give the THF ring-
opened compounds [R2PHC4H8OB(C6F5)3] and [Mes2PC4H8OB-
(C6F5)3Li(THF)2] (Mes C6H2Me-2,4,6). These reactions also occur
consecutively to give the double THF ring-opened compounds
[Mes2P(C4H8OB(C6F5)3)2][Li(THF)4] and [t-Bu2P(C4H8OB(C6F5)3)2-
Li].
a contrived manner. Our interest in phosphorus-based ligands
prompted us to consider the reactions of THF and phosphorus-
based nucleophiles in the presence of a Lewis acid. To that
end, we have probed reactions of the Lewis acid-base adduct
(THF)B(C6F5)3 with sterically demanding phosphines and
phosphides. Herein, we demonstrate that such reactions affect
the facile and quantitative ring opening of THF, affording
synthetic routes to zwitterionic phosphonium borates or the
lithium salt of a phosphine borate. Alternatively, these
reactions can be done in tandem to provide lithium salts of
phosphonium diborate ligands.
In the selection of a suitable phosphorus-based nucleophile,
we noted that the reaction of (THF)B(C6F5)3 with Ph2PH
leads to the expected and known ligand-exchange product
(Ph2PH)B(C6F5)3,12 liberating THF. However, the analogous
reaction employing the sterically demanding phosphine Mes2-
PH (Mes ) C6H2Me-2,4,6) does not proceed in this fashion.
Rather, the reaction of B(C6F5)3, THF, and Mes2PH afforded
a tan solid 1 in 79% yield after a 72-h reaction at 25 °C and
appropriate workup.13 The 11B and 31P NMR spectra revealed
singlet resonances at -2.8 and -12.0 ppm, respectively. 1H
NMR data reveal methylene resonances at 3.48, 2.85, 1.62,
and 1.23 ppm as well as resonances attributable to the mesityl
groups. These spectroscopic data confirm the presence of
the B(C6F5)3 and PMes2 fragments in 1 as well as a ring-
opened THF molecule. Recrystallization afforded X-ray-
quality crystals that provided confirmation of the formulation
of 1 as the zwitterionic phosphonium borate [Mes2PHC4H8-
OB(C6F5)3] (Figure 1a and Scheme 1).14 The ring-opened
THF results in a O-B bond of 1.459(4) Å and a P-C bond
of 1.804(3) Å. These parameters are typical, while the
remaining metric parameters within the molecule are unex-
ceptional. In a similar fashion, reaction of t-Bu2PH with
(THF)B(C6F5)3 afforded the analogous zwitterionic ring-
opened product [t-Bu2PHC4H8OB(C6F5)3] (2), as confirmed
by both spectroscopic13 and crystallographic data (Figure 1b
and Scheme 1).14
)
)
Ring-opening reactions of tetrahydrofuran (THF) mediated
by Lewis acidic metal centers including complexes of U,1,2
Sm,3 Ti,4 and Zr5-7 have been described in the literature. In
contrast, ring-opening reactions with main-group Lewis acids
are less common. A boron center in a manganese carborane
complex was shown to prompt THF ring opening in reactions
with either PPh3 or NEt3.8 Campbell and Gladfelter reported
ring opening in reactions of an amine-alane adduct in THF,9
while Kunnari et al. reported the ring opening of THF upon
treatment of TeBr4 with PPh3 in THF.10 More recently,
Chivers and Schatte have reported that THF ring opening
occurs in the reaction of a tellurium diimide dimer with
B(C6F5)3.11 While often the isolation of these ring-opened
products was unexpected, the now numerous examples
suggest the possibility that such reactions could be used in
* To whom correspondence should be addressed. E-mail: stephan@
uwindsor.ca.
(1) Avens, L. R.; Barnhart, D. M.; Burns, C. J.; McKee, S. D. Inorg.
Chem. 1996, 35, 537-539.
(2) Campello, M. P. C.; Domingos, A.; Santos, I. J. Organomet. Chem.
1994, 484, 37-46.
(3) Evans, W. J.; Leman, J. T.; Ziller, J. W.; Khan, S. I. Inorg. Chem.
1996, 35, 4283-4291.
(4) Mommertz, A.; Leo, R.; Massa, W.; Harms, K.; Dehnicke, K. Z. Anorg.
Allg. Chem. 1998, 624, 1647-1652.
(5) Polamo, M.; Mutikainen, I.; Leskela, M. Acta Crystallogr., Sect. C
1997, C53, 1036-1037.
(6) Breen, T. L.; Stephan, D. W. Inorg. Chem. 1992, 31, 4019-4022.
(7) Guo, Z. Y.; Bradley, P. K.; Jordan, R. F. Organometallics 1992, 11,
2690-2693.
(8) Gomez-Saso, M.; Mullica, D. F.; Sappenfield, E.; Stone, F. G. A.
Polyhedron 1996, 15, 793-801.
The related anionic phosphine borate is readily formed
via the reaction of a solution of (THF)B(C6F5)3 with [PMes2]-
(9) Campbell, J. P.; Gladfelter, W. L. Inorg. Chem. 1997, 36, 4094-
4098.
(10) Kunnari, S. M.; Oilunkaniemi, R.; Laitinen, R. S.; Ahlgren, M. Dalton
Trans. 2001, 3417-3418.
(11) Chivers, T.; Schatte, G. Eur. J. Inorg. Chem. 2003, 3314-3317.
(12) Lancaster, S. J.; Mountford, A. J.; Hughes, D. L.; Schormann, M.;
Bochmann, M. J. Organomet. Chem. 2003, 680, 193-205.
478 Inorganic Chemistry, Vol. 45, No. 2, 2006
10.1021/ic051713r CCC: $33.50
© 2006 American Chemical Society
Published on Web 12/23/2005

COMMUNICATION
Figure 2. POV-ray depiction of 3: carbon, black; phosphorus, orange;
fluorine, pink; boron, yellow-green; oxygen, red; lithium, gray.
of 2:1. Following a similar workup procedure, the product
4 was isolated in 80% yield.13 While the 11B NMR chemical
shift of -2.99 ppm is similar to that seen in 3, the 31P NMR
chemical shift of 30.9 ppm observed for 4 is markedly upfield
of the corresponding resonance for 3. This latter observation
Figure 1. POV-ray depiction of (a) 1 and (b) 2: carbon, black; phosphorus,
orange; fluorine, pink; boron, yellow-green; oxygen, red; hydrogen, light
gray.
(13) All NMR data were recorded in C6D6 unless otherwise stated. J values
Scheme 1. Ring-Opening Reactions of THF with Sterically
Demanding Phosphines and Phosphides
1
3
are given in Hz. 1: 1H NMR δ 7.33 (dt, JH-P ) 531, JH-H ) 7,
4
1H), 6.40 (d, JH-P ) 4, 4H), 3.48 (m, 2H), 2.85 (m, 2H), 1.90 (s,
6H), 1.87 (s, 12H), 1.62 (m, 2H), 1.23 (m, 2H); 11B NMR δ -2.8;
13C{1H} NMR δ 149.0 (dm, 1JC-F ) 246), 146.1 (s), 143.4 (d, 2JC-P
) 11), 139.3 (dm, 1JC-F ) 252), 137.5 (dm, 1JC-F ) 257), 132.2 (d,
2JC-P ) 11), 125.0 (s), 112.1 (d, 1JC-P ) 80), 65.6 (s), 30.9 (s), 24.9
1
3
(d, JC-P ) 58), 23.9 (s), 21.8 (d, JC-P ) 8), 21.1 (s); 19F NMR δ
-133.9 (d, JF-F ) 23), -162.2 (s), -165.9 (s); 31P NMR δ -12.0.
3
2: 1H NMR (C6D6) δ 5.60 (dt, 1JH-P ) 453, 3JH-H ) 4, 1H), 3.24 (t,
3JH-H ) 5, 2H), 2.64 (m, 2H), 1.99 (m, 2H), 1.67 (m, 2H), 1.45 (d,
3JH-P ) 16, 18H); 11B NMR δ -2.9; 13C{1H} NMR δ 149.2 (dm,
1JC-F ) 230), 139.3 (dm, JC-F ) 244), 137.4 (dm, JC-F ) 237),
1
1
1
3
125.7 (s), 64.4 (s), 33.6 (d, JC-P ) 36), 32.8 (d, JC-P ) 11), 27.1
(s), 26.9 (m), 15.0 (d, JC-P ) 39); 19F NMR δ -133.9 (d, JF-F
)
1
3
23), -164.8 (t, 3JF-F ) 11), -168.0 (t, 3JF-F ) 20); 31P NMR δ 50.9.
3: 1H NMR (CD2Cl2) δ 6.75 (s, 4H), 3.72 (s, 8H), 3.22 (m, 2H), 2.33
(m, 2H), 2.20 (s, 18H), 1.85 (s, 8H), 1.49 (m, 2H), 1.14 (m, 2H,); 11
B
NMR (CD2Cl2) δ -3.1; 13C{1H} NMR (CD2Cl2) δ 148.4 (dm, 1JC-F
2
1
) 240), 142.2 (d, JC-P ) 13), 139.7 (dm, JC-F ) 240), 138.1 (s),
1
137.3 (dm, JC-F ) 240), 130.4 (s), 122.5, 117.9, 68.9 (s), 65.6 (s),
33.4 (d, 2JC-P ) 14), 28.2 (d, 1JC-P ) 14), 25.9 (s), 23.9 (s), 23.2 (d,
3JC-P ) 13.6), 21.0 (s); 19F NMR (CD2Cl2) δ -137.6 (d, JF-F
)
3
Li. Subsequent workup yields a tan solid 3 in 80%.13
Recrystallization afforded X-ray-quality crystals that pro-
vided confirmation of the formulation of 3 as [Mes2PC4H8-
OB(C6F5)3Li(THF)2] (Figure 2 and Scheme 1).14 Again, as
with 1 and 2, THF ring opening has resulted in the formation
of a four-coordinate boron center in 3, with the formation
of a B-alkoxide bond of 1.484(5) Å. However, the concur-
rently formed P-C bond affords a neutral and pendant
diarylalkylphosphine moiety. Countering the anionic charge
of the borate is a lithium cation. The lithium center is also
pseudotetrahedral, being coordinated to two THF molecules,
the alkoxide oxygen, and an o-fluorine atom from one of
the arene groups on boron. The Li-O distances range
between 1.891(10) and 1.967(10) Å, while the Li-F ap-
proach is 2.015(9) Å.
20), 161.0 (t, JF-F ) 20), -165.6 (t, JF-F ) 20); 31P NMR (CD2-
3
3
Cl2) δ -21.4. 4: 1H NMR (CD2Cl2) δ 6.99 (d, JH-P ) 4), 3.69 (s,
4
16H), 3.15 (m, 4H), 2.75 (m, 4H), 2.32 (s, 6H), 2.17 (s, 12H), 1.85
(s, 16H), 1.48 (m, 4H), 1.36 (m, 4H); 11B NMR (CD2Cl2) δ -3.0;
13C{1H} NMR (CD2Cl2) δ 148.4 (dm, 1JC-F ) 240), 145.8 (s), 142.2
2
1
1
(d, JC-P ) 10), 139.3 (dm, JC-F ) 240), 137.3 (dm, JC-F ) 240),
133.5 (d, 3JC-P ) 10), 123.4, 117.1 (d, 1JC-P ) 88), 68.7 (s), 64.8 (s),
32.3 (d, 2JC-P ) 13), 27.2 (d, 1JC-P ) 45), 25.9 (s), 23.5 (d, 3JC-P
)
14), 21.2 (s), 21.0 (s); 19F NMR (CD2Cl2) δ -136.1 (s), -161.8 (s),
-166.0 (m); 31P NMR (CD2Cl2) δ 30.9. 5: 1H NMR (THF-d8) δ 3.22
(t, 3JH-H ) 5, 4H), 2.51 (m, 4H), 1.92 (m, 4H), 1.65 (m, 2H), 1.35 (d,
3JH-P ) 14, 18H); 11B NMR δ -2.9; 13C{1H} NMR δ 149.2 (dm,
1JC-F ) 246), 139.2 (dm, JC-F ) 244), 137.5 (dm, JC-F ) 245),
1
1
1
3
126.2, 64.5 (s), 35.3 (d, JC-P ) 38), 33.4 (d, JC-P ) 12), 27.5 (s),
26.6 (m), 17.8 (d, JC-P ) 40); 19F NMR δ -133.9 (d, JF-F ) 23),
1
3
-165.2 (t, JF-F ) 11), -168.2 (t, JF-F ) 20); 31P NMR δ 45.3.
(14) X-ray data. 1: space group P21/c, a ) 16.3017(14) Å, b ) 15.3950-
(13) Å, c ) 19.5528(17) Å, â ) 103.1340(10)°, V ) 4778.7(7) Å3, R
) 0.0577, Rw ) 0.1364, GOF ) 0.978. 2: space group C2/c, a )
15.488(3) Å, b ) 21.574(3) Å, c ) 19.979(3) Å, â ) 111.543(3)°, V
) 6209.7(18) Å3, R ) 0.0423, Rw ) 0.1102, GOF ) 1.011. 3: space
group P21/c, a ) 21.187(5) Å, b ) 12.700(3) Å, c ) 21.530(5) Å, â
) 109.807(3)°, V ) 5451(2) Å3, R ) 0.0691, Rw ) 0.1697, GOF )
0.988. 5: space group P21/n, a ) 12.841(9) Å, b ) 15.686(11) Å, c
3
3
Given the formation of 1-3, it was proposed that the
phosphine formed in 3 should be capable of initiating a
second ring opening. Thus, the reaction of (THF)B(C6F5)3
and Mes2PLi was repeated with the adjusted stoichiometry
) 29.97(2) Å, â ) 94.046(12)°, V ) 6022(7) Å3, R ) 0.1266, Rw
0.3381, GOF ) 0.990.
)
Inorganic Chemistry, Vol. 45, No. 2, 2006 479

COMMUNICATION
Two butoxide chains link the cationic phosphonium center
to two anionic borate fragments. The lithium counterion is
coordinated to the two oxygen atoms and interacts with four
fluorine atoms on the boron-bound aryl rings. This results
in a distorted octahedral coordination sphere for the lithium
atom. The Li-O distances average 2.052(19) Å, while the
Li-F interactions range from 2.02(2) to 2.240(19) Å.
It should be noted that in the absence of B(C6F5)3 the
reaction of THF with [Et2P]Li was reported in 1959 to give
an uncharacterized product upon standing in a THF solution
for prolonged periods.15 Subsequently, in 1968, the ring
opening of THF by [Me2P]Li in refluxing THF was
reported.16 In related chemistry, PX3 and THF in the presence
of HgX2 has been shown to give moderate yields of ring-
opened phosphorus acid esters.17 However, the formation of
1-5 demonstrates that sterically demanding nucleophilic
phosphines and phosphides effect the ring opening of THF
in (THF)B(C6F5)3. This stands in contrast to the related
reactions of sterically less demanding phosphines such as
Cy2PH or Ph2PH, which form the simple adducts (R2PH)B-
(C6F5)3.12 Steric demand is thought to disfavor ligand-
exchange reactions and thus prompt ring-opening reactions.
The quantitative nature of these reactions affords the op-
portunity to examine the chemistry of phosphonium borate
zwitterions and the coordination chemistry of new anionic
phosphine and anionic phosphonium-dialkoxide ligands.
This work and the versatility of the reactions of Lewis acid
adducts and sterically demanding nucleophiles in ligand
synthesis are under investigation. The results of these studies
will be reported in due course.
Figure 3. POV-ray depiction of 5: carbon, black; phosphorus, orange;
fluorine, pink; boron, yellow-green; oxygen, red; lithium, gray.
suggests the formation of a phosphonium center. The 19F
NMR spectra give resonances at -136.1, -161.8, and
-166.0 ppm, typical of the anionic OB(C6F5)3 fragment. The
1H NMR data for 4 are consistent with the ring opening of
THF because methylene resonances are observed at 3.15,
2.75, 1.48, and 1.36 ppm, although the integration is
consistent with a 1:1 ratio of mesityl/methylene chain
fragments. In addition, resonances at 3.69 and 1.85 ppm were
attributed to THF coordinated to lithium. On the basis of
these data, 4 was formualated as [(C6H2Me-2,4,6)2P(C4H8-
OB(C6F5)3)2][Li(THF)4]. Unfortunately, attempts to obtain
X-ray-quality crystals of 4 were unsuccessful, and thus this
formulation was not confirmed crystallographically.
Following a similar procedure, the analogous reaction of
a 2:1 ratio of (THF)B(C6F5)3 and t-Bu2PLi was performed.
The resulting white solid 5 was isolated in 89% yield.13 The
NMR data for 5 were similar to those reported for 4, with a
11B NMR signal at -2.90 ppm and a 31P NMR resonance at
45.3 ppm consistent with the presence of both borate and
Acknowledgment. Financial support from NSERC of
Canada is gratefully acknowledged. J.D.M. is grateful for
the award of an Ontario Graduate Scholarship and a Bayer
Inc. Award from the Macromolecular Science and Engineer-
ing Division of the Canadian Institute for Chemistry.
1
phosphonium fragments. In a similar fashion, the H NMR
Supporting Information Available: Crystallographic data in
CIF format and preparative details and full spectroscopic and
analyical characterizations are provided. This material is available
free of charge via the Internet at http://pubs.acs.org.
data were consistent with THF ring opening, affording two
methylene chains on phosphorus. In contrast to 4, compound
5 does not contain THF. The resulting formulation for 5
based on these data is [t-Bu2P(C4H8OB(C6F5)3)2]Li. Crystals
of [t-Bu2P(C4H8OB(C6F5)3)2]Li‚0.5C6H6 were grown from
a THF/benzene/pentane solution at 25 °C (Figure 3 and
Scheme 1).
IC051713R
(15) Isleib, K.; Tzschach, A. Chem. Ber. 1959, 92, 118.
(16) Goldsberry, R. E.; Lewis, D. E.; Cohn, K. J. Organomet. Chem. 1968,
15, 491-493.
Although the crystal quality of 5 is not the best, the data
do confirm the formulation and establish the connectivity.14
(17) Peringer, P.; Winkler, P. P. Inorg. Chim. Acta 1986, 118, L1-L2.
480 Inorganic Chemistry, Vol. 45, No. 2, 2006