Heterolytic cleavage of disulfides by frustrated lewis pairs

Article abstract of DOI:10.1021/ic901590s

The addition of diphenyl disulfide (PhSSPh) to tBu₂P(C ₆F₄)B(C₆F₅)₂ (1) affords the zwitterionic phosphonium borate [tBu₂P(SPh)(C₆F ₄)B(SPh)(C₆F₅)₂] (2), while the addition of a base or donor solvent to 2 effected the liberation of disulfide and the formation of [tBu₂P(C₆F₄)B(donor) (C₆F₅)₂]. The reaction of 1 with S₈ gave tBu₂P(S)(C₆F₄)B(C₆F ₅)₂ (3). In a similar fashion, the frustrated Lewis pair of tBu₃P/B(C₆F₅)₃ reacts with RSSR to give [tBu₃P(SR)]-[(RS)B(C₆F₅)₃] (R = Ph (4), p-tolyl (5), iPr (6)). In contrast, the corresponding reaction of BnSSBn yields a 1:1:1 mixture of tBu₃P=S, Bn₂S, and B(C₆F₅)₃. Species 4 reacts with p-tolylSSp-tolyl to give a mixture of 4,5, PhSSPh, and p-tolyISS p-tolyl, while treatment of 5 with PhSSPh afforded a similar mixture. To probe this, a crossover experiment between [tBu₃P(SPh)][B(C₆F ₅)₄] (7) and [NBu₄][(p-tolylS)B(C ₆F₅)₃] (9) was performed. The former species was prepared by a reaction of 4 with [Ph₃C][B(C₆F ₅)₄], while cation exchange of [(Et₂O) ₂Li(p-tolylS)B(C₆F₅)₃] (8) with [NBu₄]Br gave 9. The reaction of compounds 7 and 9 gave a statistical mixture of the cations [tBu₃P(SR)]⁺ and anions [(RS)B(C₆F₅)₃]^-, R = Ph, Sp-tolyl. The mechanism of this exchange process was probed and is proposed to be an equilibrium involving disulfide and the frustrated Lewis pair. Crystallography data are reported for compounds 4-8, and the natures of the P-S cations are examined via DFT calculations.

Full text of DOI:10.1021/ic901590s

9910 Inorg. Chem. 2009, 48, 9910–9917
DOI: 10.1021/ic901590s
Heterolytic Cleavage of Disulfides by Frustrated Lewis Pairs
Meghan A. Dureen,†,§ Gregory C. Welch,† Thomas M. Gilbert,‡ and Douglas W. Stephan*,†,§
†Department of Chemistry and Biochemistry, University of Windsor, ON, Canada, N9B3P4, and
‡Department of Chemistry and Biochemistry, Northern Illinois University, Dekalb, Illinois, 60115.
§Current Address: Department of Chemistry, University of Toronto, 80 St. George St., Toronto,
ON, Canada, M5S3H6.
Received August 9, 2009
The addition of diphenyl disulfide (PhSSPh) to tBu2P(C6F4)B(C6F5)2 (1) affords the zwitterionic phosphonium borate
[tBu2P(SPh)(C6F4)B(SPh)(C6F5)2] (2), while the addition of a base or donor solvent to 2 effected the liberation of
disulfide and the formation of [tBu2P(C6F4)B(donor)(C6F5)2]. The reaction of 1 with S8 gave tBu2P(S)(C6F4)B(C6F5)2
(3). In a similar fashion, the frustrated Lewis pair of tBu3P/B(C6F5)3 reacts with RSSR to give [tBu3P(SR)]-
[(RS)B(C6F5)3] (R = Ph (4), p-tolyl (5), iPr (6)). In contrast, the corresponding reaction of BnSSBn yields a 1:1:1
mixture of tBu3PdS, Bn2S, and B(C6F5)3. Species 4 reacts with p-tolylSSp-tolyl to give a mixture of 4, 5, PhSSPh, and
p-tolylSS p-tolyl, while treatment of 5 with PhSSPh afforded a similar mixture. To probe this, a crossover experiment
between [tBu3P(SPh)][B(C6F5)4] (7) and [NBu4][(p-tolylS)B(C6F5)3] (9) was performed. The former species was
prepared by a reaction of 4 with [Ph3C][B(C6F5) 4], while cation exchange of [(Et2O)2Li( p-tolylS)B(C6F5)3] (8) with
[NBu4]Br gave 9. The reaction of compounds 7 and 9 gave a statistical mixture of the cations [tBu3P(SR)]þ and anions
[(RS)B(C6F5)3]-, R = Ph, Sp-tolyl. The mechanism of this exchange process was probed and is proposed to be an
equilibrium involving disulfide and the frustrated Lewis pair. Crystallographic data are reported for compounds 4-8,
and the natures of the P-S cations are examined via DFT calculations.
Introduction
GaCl3,10,11 FeCl3,9 and ZnCl212 as catalysts. In each case, it is
proposed that the Lewis acid initially interacts with a disul-
fide, generating a partial sulfenium cation which then reacts
with the unsaturated carbon-carbon bond. Nucleophilic
attack of the intermediate by the thiolate anion gives the
desired dithiolated product. On the other hand, phosphines
have been shown to promote the desulfurization of di- and
trisulfides,13 while the conversion of disulfides to thiols by
phosphines in the presence of H2O is widely known.14-18
Additionally, a recent computation study has suggested that
phosphines react with disulfides via an SN2 mechanism,
generating phosphonium cation-thiolate anion salts of
the form [R3PSR][SR],19 which is thought to be involved in
the phosphine-catalyzed metathesis of disulfides.20 While
The activation of E-E and E-H bonds (E=B, Si, S, Sn)
and their transfer to unsaturated carbon molecules are im-
portant processes in organic synthesis.1-4 Specifically, car-
bon-sulfur-bond-forming reactions are important for drug
development, and many organosulfur compounds are biolo-
gically active.5 The cleavage of S-H and S-S bonds is well-
known and can be accomplished using transition metals4,6
and main group nucleophiles and electrophiles.7 Recently,
several papers have described the Lewis acid catalyzed dis-
ulfidation of alkenes and alkynes employing BF3,8AlCl3,9
*To whom correspondence should be addressed. E-mail: dstephan@
chem.utoronto.ca.
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r
pubs.acs.org/IC
Published on Web 09/16/2009
2009 American Chemical Society

Article
Inorganic Chemistry, Vol. 48, No. 20, 2009 9911
these intermediates have not been isolated, thiophosphonium
cations [R3PSR]þ are known to be stable species21-23 and
have been prepared by electrochemical reduction of disul-
fides21 in the presence of phosphines or by the reduction of
phosphine sulfides.22Related reports of sulfenium cations
[RS]þ are rare, with the elusive species only being observed
in the gas phase24 or via stabilization with one25or two26
nitrogen donors.
In other work, we have recently described in a series of
papers the activation of small molecules by frustrated Lewis
pairs (FLPs).27,28 These systems exploit the combination of
unquenched Lewis acidity and basicity to react with H2,29-39
olefins,40 dienes,41 B-H bonds,42 alkynes,43 CO2,44 and
N2O.45 In this report, we exploitFLPs to effect the heterolytic
activation of disulfides and stabilize the resulting cleavage
products as thiophosphonium thioborate zwitterions and
salts.
prior to use. All common organic reagents were purified by
conventional methods unless otherwise noted. 1H, 13C, 11B,
19F, and 31P nuclear magnetic resonance (NMR) spectrosco-
py spectra were recorded on a Bruker Avance-300 spectro-
1
meter at 300 K unless otherwise noted. H and 13C NMR
spectra are referenced to SiMe4 using the residual solvent
peak impurity of the given solvent. 31P, 11B, and 19F NMR
experiments were referenced to 85% H3PO4, BF3(OEt2), and
CFCl3, respectively. Chemical shifts are reported in parts per
million and coupling constants in hertz as absolute vaules.
1
DEPT and 2-D H/13C correlation experiments were com-
pleted for assignment of the carbon atoms. Combustion
analyses were performed in-house employing a Perkin-Elmer
CHN Analyzer. B(C6F5)3 was generously donated by NOVA
Chemicals Corporation. tBu3P was purchased from Strem
Chemicals and used as-received. tBu2PH(C6F4)BH(C6F5)2
and tBu2P(C6F4)B(C6F5)2 (1) were prepared as previously
published.46
Synthesis of [tBu2(PhS)P(C6F4)B(SPh)(C6F5)2] (2). A 20 mL
vial was charged with 1 (0.200 g, 0.31 mmol) and toluene (5 mL),
forming an orange solution. To this solution was added PhSSPh
(0.070 g, 0.32 mmol) in toluene (5 mL) dropwise at room
temperature. An immediate color change from orange to faint
yellow was observed. The reaction was stirred at room tempera-
ture for 1 h. All volatile materials were removed in vacuo to give
an off-yellow solid. Yield: 205 mg (76%). 1H NMR (C6D5Br):
7.86 (d, 2H, 3JH-H = 7 Hz, Ph), 7.78 (d, 2H, 3JH-H = 7 Hz, Ph),
7.54 (d, 2H, 3JH-H = 8 Hz, Ph), 7.43 (m, 2H, Ph), 7.21 (t, 2H,
Experimental Section
All preparations were done under an atmosphere of dry,
O2-free N2 employing both Schlenk line techniques and an
Innovative Technologies or Vacuum Atmospheres inert at-
mosphere glovebox. Solvents (pentane, hexanes, toluene, and
methylene chloride) were purified employing a Grubbs’ type
column system manufactured by Innovative Technology and
˚
were stored over molecular sieves (4 A). Molecular sieves
3
3JH-H = 8 Hz, Ph), 1.34 (d, 18H, JH-P = 19 Hz, PtBu).
˚
(4 A) were purchased fromAldrich ChemicalCo. and dried at
11B{1H} NMR (C6D5Br): -9.76. 13C{1H} NMR (C6D5Br)
=
140 °C under a vacuum for 24 h prior to use. Deuterated
solvents were dried over Na/benzophenone (C6D6, C7D8,
THF-d8) or CaH2 (CD2Cl2, C6D5Br) and vacuum-distilled
partial: 149.84 (dm, 1JC-F=254 Hz, CF), 148.56 (dm, 1JC-F
240 Hz, CF), 146.32 (dm, JC-F = 250 Hz, CF), 142.07 (s,
=
1
PSPh), 139.35 (dm, 1JC-F = 250 Hz, CF), 137.41 (dm, 1JC-F
250 Hz, CF), 135.17 (s, PSPh), 133.67 (s, BSPh), 131.72 (s,
(21) Ohmori, H.; Maeda, H.; Konomoto, K.; Sakai, K.; Masui, M. Chem.
Pharm. Bull. 1987, 35, 4473–4481.
2
PSPh), 128.71 (d, JC-P = 120 Hz, quat, PSPh), 127.91 (s,
(22) Omelanczuk, J.; Mikolajczyk, M. J. Am. Chem. Soc. 1979, 101, 7292–
7295.
(23) Peppe, C.; Nobrega, J. A.; Hernandes, M. Z.; Longo, R. L.; Tuck, D.
G. J. Organomet. Chem. 2001, 626, 68–75.
(24) Zheng, X. B.; Tao, W. A.; Cooks, R. G. J. Chem. Soc., Perkin Trans.
2001, 350–355.
(25) Takeuchi, H.; Tateiwa, J.; Moriguchi, S. Molecules 2003, 8, 392–400.
(26) Kobayashi, K.; Sato, S.; Horn, E.; Furukawa, N. Tetrahedron Lett.
1998, 39, 2593–2596.
BSPh), 124.64 (s, BSPh), 45.16 (d, 1JC-P = 22 Hz, tBu), 28.58 (s,
tBu). 19F NMR (C6D5Br): δ -123.93 (br s, 3F, C6F4), -130.35
(br s, 1F, C6F4), -130.89 (m, 4F, 3JF-F = 20 Hz, o-C6F5), -161.04
(m, 2F, 3JF-F = 17 Hz, p-C6F5), -165.36 (m, 4F, 3JF-F = 20 Hz,
m-C6F5). 31P{1H} NMR (C6D5Br): δ 76.2 (s). Anal. Calcd for
C38H28BF14PS2: C, 53.29; H, 3.29. Found: C, 54.05; H, 3.85%.
Synthesis of tBu2P(S)(C6F4)B(C6F5)2 (3). Method A. A 20 mL
vial was charged with 1 (0.132 g, 0.21 mmol), S8 (0.007 g, 0.22
mmol), and toluene (10 mL), forming a yellow slurry. The reac-
tion was stirred at room temperature for 12 h. The reaction mix-
ture was filtered through Celite, and all volatile materials were
removed in vacuo to give a sticky yellow solid. The product was
slurried in hexanes and stirred for 30 min, and the solvent removed
in vacuo to give an off-yellow powder. Yield: 120 mg (87%).
Method B. A Teflon cap-sealable J. Young NMR tube was
charged with tBu2PH(C6F4)BH(C6F5)2, the precursor to 1
(0.029 g, 0.045 mmol), S8 (0.010 mg, 0.31 mmol), and C6D5Br
(0.75 mL), forming a slurry. The NMR tube was sealed and
heated to 150 °C for 10 min. During this time, vigorous bubbling
was observed (H2 elimination), all solids dissolved, and the
reaction became intense yellow in color. The reaction was
cooled, and NMR confirmed 100% product formation and no
coordination of the excess S8 to boron.
(27) Stephan, D. W. Org. Biomol. Chem. 2008, 6, 1535–1539.
(28) Stephan, D. W. Dalton Trans. 2009, 3129–3136.
(29) Welch, G. C.; Stephan, D. W. J. Am. Chem. Soc. 2007, 129, 1880.
(30) Welch, G. C.; San Juan, R. R.; Masuda, J. D.; Stephan, D. W.
Science 2006, 314, 1124–1126.
(31) Sumerin, V.; Schulz, F.; Nieger, M.; Leskela, M.; Repo, T.; Rieger, B.
Angew. Chem., Int. Ed. 2008, 47, 6001–6003.
::
(32) Spies, P.; Kehr, G.; Bergander, K.; Wibbeling, B.; Frohlich, R.;
Erker, G. Dalton Trans. 2009, 1534–1541.
::
(33) Spies, P.; Erker, G.; Kehr, G.; Bergander, K.; Frohlich, R.; Grimme,
S.; Stephan, D. W. Chem. Commun. 2007, 5072–4.
(34) Holschumacher, D.; Bannenberg, T.; Hrib, C. G.; Jones, P. G.;
Tamm, M. Angew. Chem., Int. Ed. 2008, 47, 7428–7432.
(35) Geier, S. J.; Gilbert, T. M.; Stephan, D. W. J. Am. Chem. Soc. 2008,
130, 12632–12633.
(36) Chase, P. A.; Stephan, D. W. Angew. Chem., Int. Ed. 2008, 47, 7433.
(37) Chase, P. A.; Jurca, T.; Stephan, D. W. Chem. Commun. 2008, 1701–
1703.
(38) Ullrich, M.; Lough, A. J.; Stephan, D. W. J. Am. Chem. Soc. 2009,
131, 52–53.
(39) Geier, S. J.; Stephan, D. W. J. Am. Chem. Soc. 2009, 131, 3476–3477.
(40) McCahill, J. S. J.; Welch, G. C.; Stephan, D. W. Angew. Chem., Int.
Ed. 2007, 46, 4968–71.
1
1H NMR (C6D5Br): 1.31 (d, 18H, JH-P = 17 Hz, PtBu.
11B{1H} NMR (C6D5Br): No signal observed. 13C{1H} NMR
(C6D5Br) partial: 148.87 (dm, 1JC-F=251 Hz, CF), 144.89 (dm,
1JC-F=257 Hz, CF), 137.72 (dm, 1JC-F = 257 Hz, CF), 113.82
1
(quat) 33.62 (d, JC-P=39 Hz, PtBu), 27.67 (s, C(CH3)3). 19F
(41) Ullrich, M.; Seto, K. S.-H.; Lough, A. J.; Stephan, D. W. Chem.
Commun. 2009, 2335–2337.
NMR (C6D5Br): -119.44 (s, 1F, C6F4), -125.38 (s, 1F, C6F4),
-126.74 (s, 1F, C6F4), 127.78 (s, 4F, o-C6F5), -130.01 (s,
1F, C6F4), -144.49 (s, 2F, p-C6F5), -160.46 (s, 4F, m-C6F5).
(42) Dureen, M., A.; Lough, A.; Gilbert, T. M.; Stephan, D. W. Chem.
Commun. 2008, 4303–5.
(43) Dureen, M. A.; Stephan, D. W. J. Am. Chem. Soc. 2009, 131, 8396–7.
::
::
(44) Momming, C. M.; Otten, E.; Kehr, G.; Frohlich, R.; Grimme, S.;
Stephan, D. W.; Erker, G. Angew. Chem., Int. Ed. 2009, 48, 0000.
(45) Otten, E.; Neu, R. C.; Stephan, D. W. J. Am. Chem. Soc. 2009, in
press.
(46) Welch, G. C.; Cabrera, L.; Chase, P. A.; Hollink, E.; Masuda, J. D.;
Wei, P. R.; Stephan, D. W. Dalton Trans. 2007, 3407–3414.

9912 Inorganic Chemistry, Vol. 48, No. 20, 2009
Dureen et al.
31P{1H} NMR (C6D5Br): 85.9 (s). Anal. Calcd for C26H18-
BF14PS: C, 46.59; H, 2.71.
Synthesis of [tBu3P(SPh)][B(C6F5)4] (7). In one portion,
[Ph3C][B(C6F5)4] (112 mg, 0.12 mmol) was added to a solution
of 4 (110 mg, 0.12 mmol) in dichloromethane (5 mL). The
reaction mixture was shaken until it turned pale yellow. The sol-
vent was removed under reduced pressure to afford pale yellow
crystals, which were washed with pentane (5 mL) and dried
further, affording 92 mg of product (79%). 1H NMR (CD2Cl2):
Synthesis of [tBu3P(SPh)][(PhS)B(C6F5)3] (4), [tBu3P(Sp-
tolyl)][(p-tolylS)B(C6F5)3] (5), and [tBu3P(SiPr)][(iPrS)B(C6F5)3]
(6). These compounds were prepared in a similar fashion,
and thus only one preparation is detailed. A solution of PhSSPh
(106 mg, 0.4 mg) and B(C6F5)3 (249 mg, 0.5 mmol) in toluene
(10 mL) was cooled to -35 °C, and tBu3P (98 mg, 0.4 mmol) in
toluene (1 mL) was added in one portion. A colorless immiscible
oil formed immediately, and the toluene layer was decanted and
the oil washed with toluene (2 ꢀ 10 mL). The oil was dried under
reduced pressure and subsequently triturated with pentane until
a white solid formed. This solid was washed with pentane (3 ꢀ
10 mL) and dried in vacuo (399 mg, 88%). 1H NMR (CD2Cl2):
7.89 (m, 2H, 3JH-H=8 Hz, Ph), 7.57 (m, 1H, 3JH-H=8 Hz, Ph),
7.49 (m, 2H, 3JH-H = 8 Hz, Ph), 7.09 (m, 2H, 3JH-H=8 Hz, Ph),
6.88 (m, 3H, 3JH-H = 8 Hz, Ph), 1.67 (d, 27H, 3JH-P=16 Hz,
PtBu). 11B{1H} NMR (CD2Cl2): -9.95 (s). 13C{1H} NMR
3
3
7.86 (d, 2H, JH-H=8 Hz, o-Ph), 7.62 (t, 1H, JH-H = 8 Hz,
p-Ph), 7.51 (t, 2H, 3JH-H = 8 Hz, m-Ph), 1.68 (d, 27H, 3JH-P
=
16 Hz, PtBu).11B{1H} NMR (CD2Cl2): -16.67 (s). 13C{1H}
NMR (CD2Cl2) partial: 148.15 (dm, 1JC-F = 243 Hz, o-C6F5),
138.16 (dm, 1JC-F = 245 Hz, p-C6F5), 137.80 (d, JP-C = 3 Hz),
136.36 (dm, 1JC-F = 243 Hz, m-C6F5), 132.54 (d, JP-C=3 Hz),
130.72 (d, JP-C=2 Hz), 121.23 (d, JP-C=10 Hz, ipso-C6H5),
45.86 (d, 1JC-P=16 Hz, tBu), 30.38 (s, tBu). 19F NMR (CD2-
Cl2): -133.46 (s, br, 6F, o-C6F5), -164.15 (t, 3F, 3JF-F = 21 Hz,
p-C6F5), -167.97 (t, 6F, JF-F = 19 Hz, m-C6F5). 31P NMR
3
(CD2Cl2): 86.0 (s). Anal. Calcd for C42H32BF20PS (990.536): C,
50.93; H, 3.26. Found: C, 50.80; H, 3.57. X-ray-quality crys-
tals were grown from the slow cooling of a saturated solution
in CH2Cl2. The crystalline product was suitable for X-ray
diffraction.
1
(CD2Cl2) partial: 148.56 (dm, JC-F=245 Hz, CF), 142.72 (s,
1
PSPh), 138.96 (dm, JC-F = 240 Hz, CF), 138.48 (s, PSPh),
1
137.11 (dm, JC-F = 245 Hz, CF), 133.43 (s, BSPh), 131.30
2
(s, PSPh), 128.94 (d, JC-P =110 Hz, quat, PSPh), 127.72 (s,
BSPh), 124.02 (s, BSPh), 46.46 (d, 1JC-P=15 Hz, tBu), 30.96 (s,
Synthesis of [(Et2O)2Li(p-tolylS)B(C6F5)3] (8). A solution of
B(C6F5)3 (180 mg, 0.35 mmol) in diethyl ether (5 mL) was cooled
to -35 °C, and a slurry of LiSC6H4Me (35 mg, 0.35 mmol,
formed from the 1:1 reaction of MeC6H4SH and nBuLi) in
diethyl ether (2 mL) was added in one portion. The reaction was
warmed to room temperature, and the solution was concen-
trated to one-half its original volume, layered with pentane
(4 mL), and cooled to -35 °C overnight to afford white crystals
(187 mg, 79%). 1H NMR (d8-THF): 6.90 (d, 2H, 3JH-H = 8 Hz),
3
tBu). 19F NMR (CD2Cl2): -131.70 (d, 6F, JF-F = 23 Hz,
o-C6F5), -163.65 (t, 3F, 3JF-F = 20 Hz, p-C6F5), -167.56 (m,
6F, 3JF-F = 20 Hz, m-C6F5). 31P NMR (CD2Cl2): 85.7 (s). Anal.
Calcd for C42H37BF15PS2 (932.648): C, 54.09; H, 4.00. Found:
C, 54.19; H, 4.20.
Compound 5. White crystalline solid (304 mg, 90%). 1H
3
NMR (C6D5Br): 7.49 (m, 4H, m-Ar), 7.11 (d, 2H, JH-H
=
8 Hz, o-BSAr), 6.88 (d, 2H, 3JH-H = 8 Hz, o-PSAr), 2.25 (s, 3H,
PSC6H4Me), 2.17 (s, 3H, BSC6H4Me), 1.28 (d, 27H, 3JH-P = 16
Hz, PtBu). 11B{1H} NMR (C6D5Br): -9.13 (s). 13C{1H} NMR
(C6D5Br) partial: 148.8 (dm, 1JC-F = 240 Hz, o-C6F5), 143.8 (s),
139.5(s), 138.7 (dm, 1JC-F = 250 Hz, p-C6F5), 137.9 (s), 137.8
3
3
6.62 (d, 2H, JH-H = 8 Hz), 3.38 (q, 8H, JH-H = 7 Hz,
3
OCH2CH3), 2.09 (s, 3H, BSC6H4Me), 1.11 (m, 8H, JH-H
=
7 Hz, OCH2CH3). 11B{1H} NMR (d8-THF): -11.73 (s).
13C{1H} NMR (d8-THF), partial: 146.3 (dm, 1JC-F = 238 Hz,
o-C6F5), 137.3 (s), 136.2 (dm, 1JC-F = 241 Hz, p-C6F5), 134.3
(dm, 1JC-F=257 Hz, m-C6F5), 131.2 (s), 130.2 (s), 125.3 (s), 63.5
(s, OCH2CH3), 18.06 (s, BSC6H4CH33), 12.82 (s, OCH2CH3).
19F NMR (C6D5Br): -132.73 (d, 6F, JF-F=24 Hz, o-C6F5),
1
(s), 137.2 (dm, JC-F = 242 Hz, m-C6F5), 133.7 (s), 133.1 (s),
131.6 (s), 128.6 (s), 117.5 (d, JP-C =14 Hz), 45.4 (d, JC-P =
1
16 Hz, tBu), 30.2 (s, tBu), 21.5 (s, br, PSC6H4Me), 21.2 (s,
=
3
BSC6H4Me). 19F NMR (CD2Cl2): -131.68 (d, 6F, JF-F
3
3
-167.34 (t, 3F, JF-F = 20 Hz, p-C6F5), -170.69 (t, 6F,
23 Hz, o-C6F5), -163.89 (t, 3F, JF-F = 21 Hz, p-C6F5),
3JF-F = 19 Hz, m-C6F5). Anal. Calcd for C33H27LiBF15O2S
(790.371): C, 50.15; H, 3.44. Found: C, 48.04; H, 3.29. The
crystalline product was suitable for X-ray diffraction.
3
-167.76 (t, 6F, JF-F=19 Hz, m-C6F5). 31P NMR (C6D5Br):
84.9 (s). Anal. Calcd for C44H41BF15PS2 (960.702): C, 55.01; H,
4.30. Found: C, 54.82; H, 4.42. X-ray quality crystals were
grown from the slow cooling of a saturated solution in PhCl.
Compound 6. White crystalline solid (126 mg, 93%). 1H
Synthesis of [NBu4][(p-tolylS)B(C6F5)3] (9). To a solution of 8
(334 mg, 0.5 mmol) in diethyl ether (5 mL) was added tetra-
butylammonium bromide (161 mg, 0.5 mmol) in one portion. The
solvent was removed under reduced pressure and the white solid
recrystallized from a 1:1 mixture of chlorobenzene and pentane to
afford a white crystalline solid (156 mg, 36%). 1H NMR (C6D5Br):
7.23 (d, 2H, 3JH-H = 8 Hz), 6.72 (d, 2H, 3JH-H = 8 Hz), 2.70 (m,
8H, NCH2), 2.02 (s, 3H, BSC6H4Me), 1.21 (m, 8H, NCH2CH2),
NMR (CD2Cl2): 3.91 (m, 1H, PSiPr), 2.12 (sept, 1H, 3JH-H
=
7 Hz, BSiPr), 1.67 (m, 33H, PtBu þ PSiPr), 1.02 (d, 6H, BSiPr).
11B{1H} NMR (C6D5Br): -11.04 (s). 13C{1H} NMR (CD2Cl2)
partial: 148.72 (dm, JC-F = 241 Hz, o-C6F5), 138.65 (dm,
1
1
1JC-F = 245 Hz, p-C6F5), 137.17 (dm, JC-F = 240 Hz,
m-C6F5), 45.68 (d, 1JC-P = 18 Hz, PtBu), 42.02 (d, 2JC-P = 7 Hz,
PSiPr), 33.44 (s, BSiPr), 30.69 (s, C(CH3)3), 27.25 (s, BSiPr),
27.14 (d, 3JC-P = 3 Hz, PSiPr). 19F NMR (CD2Cl2): -131.13
(d, 6F, 3JF-F = 21 Hz, o-C6F5), -163.91 (t, 3F, 3JF-F = 21 Hz,
3
1.11 (m, 8H, CH2CH3), 0.79 (t, 12H, JH-H =8 Hz CH2CH3).
11B{1H} NMR (C6D5Br): -8.95 (s). 13C{1H} NMR (C6D5Br)
partial: 148.3 (dm, 1JC-F=244 Hz, o-C6F5), 138.5 (dm, 1JC-F
=
3
p-C6F5), -167.76 (t, 6F, JF-F = 20 Hz, m-C6F5). 31P NMR
230 Hz, p-C6F5), 136.9 (dm, 1JC-F=255 Hz, m-C6F5), 133.7 (s),
133.6 (s), 128.3 (s), 58.4 (s, NCH2), 23.3 (s, NCH2CH2), 20.7 (s,
BSC6H4CH3), 19.5 (s, CH2CH3), 13.5 (s, CH2CH3). 19F NMR
(C6D5Br): -130.17 (d, 6F, 3JF-F = 22 Hz, o-C6F5), -161.64 (t,
(C6D5Br): δ 88.0 (s). Anal. Calcd for C36H41BF15PS2 (864.614):
C, 50.01; H, 4.78. Found: C, 49.77; H, 4.72. X-ray-quality
crystals were grown from the slow cooling of a saturated
solution in CH2Cl2.
3
3
3F, JF-F=21 Hz, p-C6F5), -165.80 (t, 6F, JF-F=20 Hz, m-
C6F5). Anal. Calcd for C41H43BNF15S (877.654): C, 56.11; H, 4.94;
N, 1.60. Found: C, 56.04; H, 5.04; N, 1.59.
Reaction of tBu3P, B(C6F5)3 and BnSSBn. Dibenzyl disulfide
(47 mg, 0.19 mmol) was added in one portion to a solution of
tBu3P (one portion to a solution of tBu3P (50 mg, 0.19 mol) and
B(C6F5)3 (98 mg, 0.19 mmol) in toluene (3 mL) and the reaction
mixture shaken until dissolution was complete. The solution
remained colorless, and an immediate phase separation was
observed. However, within 5 min of the initial mixing, the reaction
mixture became monophasic. The solvent was removed under
reduced pressure to afford a white solid that was spectroscopically
identical to a 1:1:1 mixture of tBu3PdS, Bn2S, and B(C6F5)3.
Reaction of 7 and 9. To an intimate mixture of 7 (8.8 mg,
0.01 mmol) and 9 (9.9 mg, 0.01 mmol) was added CD2Cl2 (ca.
1 mL). 1H, 11B, 19F, and 31P{1H} NMR spectroscopy, 15 min post-
mixing, showed no evidence of disulfide scrambling. However,
24 h later, a resonance attributable to [tBu3P(Sp-tolyl)]þ was
observed in the 31P{1H} spectrum at 84.9 ppm, and a triplet at
-163.7 ppm corresponding to the para-fluorines in [(PhS)B-
(C6F5)3]- was observed in the 19F spectrum.

Article
Inorganic Chemistry, Vol. 48, No. 20, 2009 9913
Table 1. Crystallographic Data
4
5
6
7
8
formula
fw
C
42H10BF15PS2
C44H41BF15PS2
960.67
triclinic
P1
11.968(2)
13.989(3)
14.243(3)
70.79(3)
73.62(3)
73.93(3)
2115.4(7)
2
C36H41BF15PS2
864.59
monoclinic
P21/n
14.9509(4)
16.9921(4)
15.2385(4)
90
98.3890(10)
90
3829.88(17)
4
150(2)
1.499
0.281
62433
0.0257
13250
496
0.0370
0.0982
1.015
C36H33BF20PS
990.53
triclinic
P1
10.9180(5)
12.5771(5)
15.3755(8)
101.684(3)
91.286(3)
98.070(2)
2044.27(17)
2
150(2)
1.492
0.236
474844
0.0495
8389
586
0.0506
0.1148
1.013
C33H27BF15LiO2S
790.36
triclinic
P1
10.6303(2)
13.4360(4)
13.9523(5)
89.076(2)
67.826(2)
71.165(2)
1733.84(9)
2
150(2)
1.514
0.205
58806
0.0241
11252
478
0.0385
905.40
orthorhombic
Pbca
17.1871(8)
20.1089(10)
23.8650(10)
90
90
90
8248.1(7)
8
150(2)
1.458
0.266
60605
0.0564
5731
cryst syst
space group
˚
a (A)
˚
b (A)
˚
c (A)
R (deg)
β (deg)
γ (deg)
3
˚
V (A )
Z
T (K)
150(2)
1.508
0.263
d (calcd) g cm-3
Abs coeff, μ, mm-1
data collecteda
Rint
27926
0.0748
9621
568
0.0540
0.1318
1.048
data used
variables
R (>2σ)
wR2 (>2σ)
GOF
604
0.0680
0.1767
1.105
0.1012
1.037
a
˚
Data collected with Mo KR radiation (λ = 0.71069 A).
Table 2. Computation of Natural Bond Orders
basis set compd
S
HF/ca. 6-31þg(d) P/N/H
C(-S)
S
B3LYP/6-311þþg(d,p) P/N/H
C(-S)
[tBu3P(SPh)]þ
[H3N(SPh)]þ
PhSH
0.02
0.56
0.01
0.13
1.51
-0.98
-0.23
-0.24
-0.35
0.13
0.03
0.47
-0.01
0.11
1.49
-0.83
0.12
-0.21
-0.30
-0.19
-0.20
PhSSPh
-0.26
X-Ray Data Collection and Reduction. Crystals were coated in
Paratone-N oil in the glovebox, mounted on a MiTegen Micro-
mount, and placed under a N2 stream, thus maintaining a dry,
O2-free environment for each crystal. The data for crystals of
5 were collected on a Nonius Kappa-CCD diffractometer; for
crystals of 4, 6, 7, and 8, data were collected on a Bruker Apex
II diffractometer. The data were collected at 150((2) K for all
crystals. For crystals of 5, data were processed with the DEN-
ZO-SMN package. For crystals of 4, 6, 7, and 8, the frames were
integrated with the Bruker SAINT software package using a
narrow-frame algorithm. Data were corrected for absorp-
tion effects using the empirical multiscan method (SADABS).
Table 1 provides crystallographic data for 4-8.
difference Fourier map calculation as well as the magnitude of
the residual electron densities in each case were of no chemical
significance. Additional details are provided in the Supporting
Information.
Computational Methods. Optimizations were performed with
the Gaussian (G98) suite.48 The species given in Table 2 were
fully optimized without constraints (save PhS-SPh, which was
initially fixed to Ci symmetry and optimized to C2h symmetry) at
the HF level using a mixed basis set applying the 3-21G basis set
to phenyl and t-butyl hydrogens, and the 6-31þg(d) basis set to
all other atoms. Examination of the optimized structures by
analytical frequency analysis at this level demonstrated them to
be minima (no imaginary frequencies). The structures were then
reoptimized at the B3LYP/6-311þþg(d,p) level using the HF
structures as starting points. Natural bond order (NBO) calcu-
lations49,50 were performed using an upgraded version of the
NBO subroutine in the G98 program, using structures and wave
Structure Solution and Refinement. Non-hydrogen atomic
scattering factors were taken from the literature tabulations.47
The heavy atom positions were determined using direct methods
employing the SHELXTL direct methods routine. The remain-
ing non-hydrogen atoms were located from successive difference
Fourier map calculations. The refinements were carried out by
using full-matrix least-squares techniques on F, minimizing the
function ω (Fo - Fc)2, where the weight ω is defined as 4Fo2/2σ
(Fo2) and Fo and Fc are the observed and calculated structure
factor amplitudes, respectively. In the final cycles of each
refinement, all non-hydrogen atoms were assigned anisotropic
temperature factors in the absence of disorder or insufficient
data. In the latter cases, atoms were treated isotropically. C-H
atom positions were calculated and allowed to ride on the
carbon to which they were bonded, assuming a C-H bond
(48) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb,
M. A.; Cheeseman, J. R.; Zakrzewski, V. G.; Montgomery, J. A., Jr.;
Stratmann, R. E.; Burant, J. C.; Dapprich, S.; Millam, J. M.; Daniels, A.
D.; Kudin, K. N.; Strain, M. C.; Farkas, O.; Tomasi, J.; Barone, V.; Cossi,
M.; Cammi, R.; Mennucci, B.; Pomelli, C.; Adamo, C.; Clifford, S.;
Ochterski, J.; Petersson, G. A.; Ayala, P. Y.; Cui, Q.; Morokuma, K.;
Malick, A. D.; Rabuck, K. D.; Raghavachari, K.; Foresman, J. B.;
Cioslowski, J.; Ortiz, J. V.; Baboul, A. G.; Stefanov, B. B.; Liu, G.;
Liashenko, A.; Piskorz, P.; Komaromi, I.; Gomperts, R.; Martin, R. L.;
Fox, D. J.; Keith, T.; Al-Laham, M. A.; Peng, C. Y.; Nanayakkara, A.;
Challacombe, M.; Gill, P. M. W.; Johnson, B.; Chen, W.; Wong, M. W.;
Andres, J. L.; Gonzalez, C.; Head-Gordon, M.; Replogle, E. S.; Pople, J. A.
Gaussian 98, revision A.11.4; Gaussian, Inc.: Pittsburgh, PA, 1998.
(49) Glendening, E. D.; Reed, A. E.; Carpenter, J. E.; Bohmann, J. A.;
Morales, C. M.; Weinhold, F. NBO 5.0; Gaussian NBO 3.1; Theoretical
Chemistry Institute, University of Wisconsin: Madison, WI, 2001.
˚
length of 0.95 A. H-atom temperature factors were fixed at 1.10
times the isotropic temperature factor of the C atom to which
they were bonded. The H-atom contributions were calculated,
but not refined. The locations of the largest peaks in the final
(47) Cromer, D. T.; Waber, J. T. Int. Tables X-Ray Crystallogr. 1974, 4,
71–147.
(50) Reed, A. E.; Curtiss, L. A.; Weinhold, F. Chem. Rev. 1988, 88, 899–
926.

9914 Inorganic Chemistry, Vol. 48, No. 20, 2009
Scheme 1. Synthesis of 1 and 2
Dureen et al.
Scheme 2. Reactions of Disulfides with FLPs
functions from both model chemistries. The values differed little
(Table 2). Optimized Cartesian coordinates for the compounds
investigated are available as Supporting Information.
Results and Discussion
The addition of diphenyl disulfide (PhSSPh) to an orange
toluene solution of the phosphino-borane tBu2P(C6F4)B-
(C6F5)2 (1) at room temperature resulted in an immediate
loss of color. After stirring for 1 h at 25 °C and removal of the
solvent, an off-yellow solid was recovered in 76% yield. The
31P NMR spectrum revealed a singlet resonance at 76.2 ppm,
which is about 51 ppm downfield from the parent phosphino-
fluorine resonance gap of 16 ppm is similar to that found for
1 and B(C6F5)3 (Δm,p=18)58 consistent with the presence of a
three-coordiante boron center. This phosphine-sulfide (3)
showed no sign of aggregation via phosphine-sulfide co-
ordination to boron at 25 °C consistent with the known
ability of such soft donors to form only weak adducts with
Lewis acids.59 However, upon cooling to -50 °C, the meta,
para fluorine chemical shift difference changes from 16 to
8 ppm, suggesting the presence of weak aggregation at low
temperatures.
1
borane. The H NMR spectrum showed resonances from
7.9 to 7.2 ppm and at 1.3 ppm attributable to Ph and
tBu groups, respectively. The 11B NMR chemical shift of
-9.8 ppm is comparable to that of a related thioborate anion,51
and the small gap between the meta and para 19F NMR
resonances (Δm,p = 4.4 ppm) is consistent with the formation
of a four-coordiante anionic borate center.52-57 On the basis
of these data, the product (2) was formulated as the zwitter-
ionic phosphonium borate [tBu2P(SPh)(C6F4)B(SPh)(C6F5)2]
(Scheme 1).
While 2 was a stable and isolable species, the heating of
trial reactions in the presence of a base such as PMe3 or a
donor solvent such as THF to temperatures in excess of
100 °C resulted in regeneration of the disulfide and the
base coordinated phosphino-borane [tBu2P(C6F4)B(donor)-
(C6F5)2] as evidenced by NMR data. These observations
suggest that the activation of the disulfide PhSSPh is rever-
sible. This is directly analogous to the heterolytic loss of
hydrogen from [(C6H2Me3)2P(C6F4)B(C6F5)2] upon thermo-
lysis of [(C6H2Me3)2P(H)(C6F4)B(H)(C6F5)2]. Nonetheless,
the corresponding thermolysis of [tBu2P(H)(C6F4)B(H)-
(C6F5)2] does not yield 1.
In a similar fashion, the FLP tBu3P/B(C6F5)3 readily
activates PhSSPh at 25 °C in toluene to give [tBu3P(SPh)]-
[(PhS)B(C6F5)3] (4) as a white solid in 84% yield (Scheme 2).
The NMR data show a characteristic 11B chemical shift at
-10.0 ppm. The 31P resonance for the cation appears at
85.7 ppm, downfield from the parent phosphine resonance
(57.8 ppm) and yet clearly distinct from the known shift of
tBu3PS (90 ppm).60 The formulation of 4 was also confirmed
crystallographically. The structural data for the pseudote-
trahedral thio-borate anion showed a B-S distance of
˚
1.966(4) A, while the B-S-C angle was found to be
103.90(19)°. This angle positions the phenyl ring at an
approximately 28.6° angle to one of the fluoro-arene rings
on B, suggesting that electron-rich, electron-poor π-π stack-
ing61 dictates the solid-state orientation of this thiolate
fragment. The corresponding cation of 4 exhibited a disorder
of the t-butyl groups. Nonetheless, the P-S bond distance
For comparative purposes, species 1 was oxidized with
elemental sulfur in toluene at room temperature, affording
the phosphine-sulfide tBu2P(S)(C6F4)B(C6F5)2 (3) as an off-
yellow solid (Scheme 1). The 31P NMR chemical shift of 3 is
observed at 85.9 ppm. The 19F NMR spectra of 3 exhibit
slightly broadened ortho, meta, and para fluorine resonances
for two equivalent C6F5 rings. Additionally, the meta-para
˚
was determined to be 2.0929(15) A.
In corresponding reactions, FLP tBu3P/B(C6F5)3 readily
activates the disulfides RSSR (R=p-tolyl, iPr) to give [tBu3P-
(Sp-tolyl)][(p-tolylS)B(C6F5)3] (5) and [tBu3P(SiPr)][(iPrS)-
B(C6F5)3] (6) in yields of 90 and 93%, respectively
(Scheme 2). These products were also characterized crystal-
lographically (Figure 1). The B-S distances in the anions of
(51) Drewitt, M. J.; Niedermann, M.; Kumar, R.; Baird, M. C. Inorg.
Chim. Acta 2002, 335, 43–51.
(52) Blackwell, J. M.; Morrison, D. J.; Piers, W. E. Tetrahedron 2002, 58,
8247–8254.
(53) Blackwell, J. M.; Piers, W. E.; McDonald, R. J. Am. Chem. Soc. 2002,
124, 1295–1306.
(54) Blackwell, J. M.; Piers, W. E.; Parvez, M. Org. Lett. 2000, 2, 695–698.
(55) Blackwell, J. M.; Sonmor, E. R.; Scoccitti, T.; Piers, W. E. Org. Lett.
2000, 2, 3921–3923.
(58) Bergquist, C.; Bridgewater, B. M.; Harlan, C. J.; Norton, J. R.;
Friesner, R. A.; Parkin, G. J. Am. Chem. Soc. 2000, 122, 10581–10590.
(59) Britovsek, G. J. P.; Ugolotti, J.; White, A. J. P. Organometallics 2005,
24, 1685–1691.
(56) Horton, A. D.; de With, J.; van der Linden, A. J.; van de Weg, H.
Organometallics 1996, 15, 2672–2674.
(57) Horton, A. D.; deWith, J. Organometallics 1997, 16, 5424–5436.
(60) Steinberger, H. U.; Ziemer, B.; Meisel, M. Acta Crystallogr., Sect. C
2001, 57, 835–837.
(61) Williams, J. Acc. Chem. Res. 1993, 26, 593–598.

Article
Inorganic Chemistry, Vol. 48, No. 20, 2009 9915
Scheme 3. Resonance Forms of Thiophosphonium Cation, [tBu3P-
(SPh)]þ
independently. This reaction is thought to proceed via the
transient formation of the salt [tBu3P(SBn)][(BnS)B(C6F5)3]
(efforts to observe this intermediate spectroscopically were
futile), with a subsequent benzyl group transfer from the
cation to the S of the anion, yielding tBu3PdS and Bn2S B-
3
(C6F5)3 (Scheme 2). In contrast, thermolysis of 4 or 6 at
150 °C does not result in the formation of tBu3PdS, rather,
only a mixture of uncharacterized decomposition products.
The contrasting reactivity of BnSSBn suggests that the P-S
bond in the transient cation is significantly stronger than that
in the cation of 4, possibly due to the stability of the benzyl
cation,62 consistent with the trends in the P-S bond distances
observed in 4-6. In addition, in the case of BnSSBn,
diminished steric demands can permit the facile approach
of the transient cation to effect alkyl group transfer. Con-
sistent with the impact of steric effects, the combination of
tBu3P and B(C6F5)3 with tBuSStBu resulted in no reaction at
room temperature and multiple unidentified products at high
temperatures.
Figure 1. POV-ray depictions of (a) 5 and (b) 6. Hydrogen atoms are
omitted for clarity.
One can formally describe these species as phosphine-
stabilized sulfenium cations, consistent with the heterolytic
cleavage of the S-S bond. Alternatively, once formed, the
cations could be described simply as a thiolate-susbstituted
phosphonium salt, or for that matter as an alkylated phos-
phine-sulfide (Scheme 3). In order to assess the best descrip-
tion, calculations were employed to probe the NBO in the
cations in 4. For comparative purposes, [tBu3P(SPh)]þ,
[H3N(SPh)]þ, PhSH, and PhSSPh were examined at the
Hartree-Fock and DFT B3LYP levels (Table 2). The
NBO charges suggest that virtually all of the positive charge
in [tBu3P(SPh)]þ reside on the P atom. This reflects the
greater electronegativity of S versus P and the fact that in
the cation P is hypervalent four-coordinate while S is normal
two-coordinate. Support for this view comes from the data
for [H3N(SPh)]þ where the sulfur holds most of the positive
charge because the bound N is more electronegative and has
less of an ability to disperse the charge. In general, the S atom
in SPh fragments holds little charge, as shown by the data for
the neutral species PhSH and PhSSPh. These data support
the description of the cation in 4 as a thiolate-susbstituted
phosphonium salt.
Crossover Experiments. The treatment of 4 with
p-tolylSSp-tolyl was monitored by NMR spectroscopy,
revealing the form formation of a mixture of 4, 5,
PhSSPh, and p-tolylSSp-tolyl. Conversely, the treatment
of 5 with PhSSPh afforded a similar mixture. These data
suggest the possibility that the formation of the salts 4 and
5 is reversible, although this equilibrium goes undetected.
To confirm this hypothesis, a crossover experiment
was envisioned in which specifically labeled cations
and anions were combined. To this end, the species
[tBu3P(SPh)][B(C6F5)4] (7) was prepared via the reaction
of 4 with [Ph3C][B(C6F5)4] (Scheme 4). Compound 7 was
˚
5 and 6 were found to be 1.991(3) and 1.9536(13) A,
respectively, consistent with the stronger electron-donating
nature of the alkylthiolate fragment in 6. The corresponding
P-S distances in the cations of 5 and 6 were found to be
2.0916(13) and 2.0694(4) A, respectively. The longer P-S
bond length in 5 also reflects the poor electron-donating
character of the aryl substituent.
˚
In efforts to garner further mechanistic data on the
formation of the salts, independent combinations of tBu3P
and PhSSPh and B(C6F5)3 and PhSSPh were examined. In
the former case, no interaction was observed from -80 to
þ25 °C, and no desulfurization of the disulfide was observed
in a CD2Cl2 solution after a week from mixing, while in the
latter case, only a weak donor-acceptor interaction was
observed below -30 °C. Thus, the activation of the disulfide
required both phosphine and borane. Indeed, the addition of
PhSSPh to 1 in C6D5Br at -30 °C resulted in the rapid
formation of 2, and no intermediate was detected. In addi-
tion, it is noteworthy that an analogous combination of R3P
(2,4,6-C6H2Me3, 2-C6H4Me) and B(C6F5)3 with PhSSPh or
(C6H2Me3)2P(C6F4)B(C6F5)2 and PhSSPh resulted in no
reaction. This latter observation suggests that the disulfide
activation requires a more nucleophilic phosphine as a
constituent of the FLP. In a similar sense, the electronic
nature of the FLP has been shown to limit the reaction with
H2.27-30,38 Nonetheless, the combinations of R3P (2,4,6-
C6H2Me3, 2-C6H4Me) and B(C6F5)3 do react with H2.
The combination of tBu3P and B(C6F5)3 with the disulfide
BnSSBn also results in a rapid reaction. However, in contrast
to the reactions above affording the salts 4-6, this reaction
does result in the formation of a clathrate but immediately
converts to a monophasic solution. Spectroscopic examina-
tion of the reaction mixture showed that the products were a
1:1:1 mixture of tBu3PdS, Bn2S, and B(C6F5)3. This was
confirmed by a comparison to an authentic mixture prepared
(62) Olah, G. A.; Schleyer, P. v. R. Carbonium Ions; Interscience Publish-
ers: New York, 1968.

9916 Inorganic Chemistry, Vol. 48, No. 20, 2009
Dureen et al.
Figure 2. POV-ray depiction of 7. Hydrogen atoms are omitted for
clarity.
Figure 3. POV-ray depiction of 8. Hydrogen atoms are omitted for clarity.
Scheme 4. Synthesis of 7-9
Scheme 5. Proposed Mechanism of RS Group Exchange in Reaction
of 7 and 9
isolated in 79% yield as pale yellow crystals. NMR data
were consistent with the formulation, and X-ray data
confirmed the nature of the salt 7 (Figure 2). The metric
parameters in the anion of 7 were unexceptional, while the
cation in 7 was not disordered as it was in 4. The P-S
˚
The combination of compounds 7 and 9 was monitored
by NMR spectroscopy; 24 h after mixing, integration
of the signals showed a statistical mixture of the ca-
tions [tBu3P(SR)]þ and anions [(RS)B(C6F5)3]- (R=Ph,
Sp-tolyl). Particular evidence of this scrambling process
was the observation of a 31P resonance at 84.9 ppm attri-
butable to [tBu3P(Sp-tolyl)]þ, and a triplet in the 19F
spectrum at -163.7 ppm corresponded to the para-fluor-
ines in [(PhS)B(C6F5)3]-. The mechanism of this scram-
bling of a formally thiolate and sulfenium fragment was
considered. Two possible mechanisms can be envisioned
given that neither the cation nor the anion react indepen-
dently with disulfides. Alkylation of the thioborate by the
cation could generate transient phosphine-sulfide and
thioether-borane species. The reverse of this reaction
would result in scrambling of the arene substituents.
However, independent combination of tBu3PS and Ph2S-
(B(C6F5)3 showed no evidence of alkylation to give 4,
suggesting that this mechanism is unlikely. A more likely
mechanism involves a small, apparently not discernible
(by NMR spectroscopy) equilibrium in which disulfides
and the FLP are in equilibrium with the salt products of
heterolytic disulfide activation (Scheme 5). An analogous
mechanism has been proposed for the heterolytic activa-
tion of H2 by FLPs.63,64 The reformation of disulfide
distance was found to be 2.0820(10) A. Treatment of
7 with excess p-tolylSSp-tolyl resulted in no spectrosco-
pically observable [tBu3P(Sp-tolyl)]þ, indicating that the
thiophosphonium cation alone does not exchange a
sulfenium cation with disulfides.
The crossover experiment required a thiolate-labeled
anion salt. It was accessed in a two-step synthesis. Initi-
ally, the reaction of B(C6F5)3 with LiSC6H4Me in diethyl
ether afforded the species [(Et2O)2Li(p-tolylS)B(C6F5)3]
(8) as white crystals in 79% isolated yield (Scheme 4).
1H NMR data confirmed the presence of 2 equiv of di-
ethyl ether while the 19F NMR resonances at -132.73,
-167.34, and -170.69 ppm and the 11B signal at -11.73
confirmed quaternization of B. The X-ray structure
of 8 (Figure 3) contains an interesting six-membered
Li-S-B-C-C-F ring, where the thiolate group, which
˚
is coordinated to a B-S distance of 2.0042(12) A, bridges
˚
to Li with a Li-S distance of 2.459(2) A and a B-S-Li
angle of 113.38(6)°. The coordination sphere of Li is
comprised of S, two coordinated diethyl ether molecules,
and a Li-F interaction to one of the ortho-F atoms on
B(C6F5)3. The Li F distance was found to be 2.043(2)
3 3 3
˚
A. In a cation exchange reaction of 8 with [NBu4]Br
(Scheme 4), the corresponding salt [NBu4][(p-tolylS)B-
(C6F5)3] (9) was isolated as a white crystalline solid in
36% yield. The treatment of 9 with excess PhSSPh
resulted in no spectroscopically observable [(p-tolylS)B-
(C6F5)3]-, indicating that the thioborate anion alone does
not exchange a thiolate anion with disulfides.
ꢀ
ꢀ
(63) Rokob, T. A.; Hamza, A.; Stirling, A.; Soos, T.; Papai, I. Angew.
Chem., Int. Ed. 2008, 47, 2435–2438.
ꢀ
(64) Rokob, T. A. S.; Hamza, A.; Stirling, A. S.; Papai, I. J. Am. Chem.
Soc. 2009, 131, 2029–2036.

Article
Inorganic Chemistry, Vol. 48, No. 20, 2009 9917
suggests that, despite the apparent localization of charge
on P in the thiophosphonium cations [tBu3P(SPh)]þ, this
cation reacts as a source of sulfenium cations to allow the
regeneration of disulfide by reaction with the thioborate
anion.
Summary. In conclusion, we have shown that FLPs
react with disulfide to effect the heterolytic cleavage of the
S-S bond affording formally sulfenium cations stabilized
by phosphine and the corresponding thio-borate anion.
This reactivity appears to be limited to sterically demand-
ing yet basic FLP systems and sterically demanding or
electron-poor disulfides. Such limitations are analogous
to that previously described for H2 activation by FLPs.
The activation of disulfides is reversible, as evidenced
by exchange and crossover experiments. Efforts are
underway to exploit this heterolytic cleavage for subse-
quent thiolation chemistry. Reports of this reactivity will
follow in due course.
Acknowledgment. We thank NSERC of Canada for
financial support. D.W.S. is grateful for the award of a
Canada Research Chair and a Killam Research Fellow-
ship. G.C.W. is grateful for an NSERC postgraduate
scholarship. M.A.D. is grateful for NSERC and OGS
postgraduate scholarships.
Supporting Information Available: Crystallographic data in
CIF format; table of optimized (B3LYP/6-311þþG(d,p)) Car-
tesian coordinates of various thiolate species. This material is
available free of charge via the Internet at http://pubs.acs.org.