Syntheses, crystallographic/computational characterizations, and reactions of the first 10-vertex arachno-and nido-phosphamonocarbaboranes

Article abstract of DOI:10.1021/ja020944j

A synthetic sequence involving the initial reaction of a substituted phosphorus dihalide (RPCI₂, R = CH₃, C₆H₅) with the arachno-CB₈H₁₃^- (1^-) monoanion followed by an in situ dehydrohalogenation reaction initiated by Proton Sponge, resulted in phosphorus cage insertion to yield the first 10-vertex arachno-and nido-phosphamonocarbaboranes, exo-6-R-arachno-6,7-PCB₈H₁₂ (2a, 2b) and PSH⁺6-R-nido-6,9-PCB₈H₉^- (PSH⁺3a^-, PSH⁺3b^-) (R = C₆H₅ (a), CH₃ (b)). Alternatively, 2a and 2b were synthesized in high yield as the sole product of the reaction of the arachno-4-CB₈H₁₂^2- (1^2-) dianion with RPCI₂. Crystallographic determinations of PSH⁺3a^- and PSH⁺3b^- in conjunction with DFT/GIAO computational studies of the anions have confirmed the expected nido cage framework based on an octadecahedron missing the six-coordinate vertex. DFT/GIAO computational studies have also shown that while the gross cage geometries of the exo-6-R-arachno-6,7-PCB₈H₁₂ compounds 2a and 2b resemble the known isoelectronic arachno-6,9-SCB₈H₁₂, the phosphorus and carbon atoms are in thermodynamically unfavorable adjacent positions on the six-membered puckered face. They also each have an endo-hydrogen at the P6-position arising from proton transfer to the basic phosphorus during the cage-insertion reaction. Possible stepwise reaction pathways that can account for the formation of both the arachno and nido products are discussed. Deprotonation of 2a and 2b resulted in the formation of their corresponding conjugate monoanions, 6-R-arachno-6,7-PCB₈H₁₁^- (2a^-, 2b^-), in which the proton that had been attached to the P6 atom was removed. Reactions of 2a- with O₂, S₈, BH₃THF, or Br₂ further demonstrated the basicity of the P6-phosphorus yielding the new arachno-substituted compounds, endo-6-O-exo-6-(C₆H₅)-arachno-6,7- PCB₈H₁₁^- (4a^-), endo-6-S-exo-6-(C₆H₅)- arachno-6,7-PCB₈H₁₁^- (5a^-), endo-6-BH₃-exo-6- (C₆H₅)-arachno-6,7- PCB₈H₁₁^- (6a^-), and endo-6-Br-exo-6-(C₆H₅)- arachno-6,7-PCB₈H₁₁ (7a), respectively, in which the O, S, BH₃, and Br substituents are bound to the phosphorus at the endo position.

Full text of DOI:10.1021/ja020944j

Published on Web 12/04/2002
Syntheses, Crystallographic/Computational Characterizations,
and Reactions of the First 10-Vertex arachno- and
nido-Phosphamonocarbaboranes
Alexandra M. Shedlow, Daniel E. Kadlecek, Jude C. Clapper, Scott E. Rathmill,
Patrick J. Carroll, and Larry G. Sneddon*
Contribution from the Department of Chemistry, UniVersity of PennsylVania,
Philadelphia, PennsylVania 19104-6323
Received July 9, 2002
Abstract: A synthetic sequence involving the initial reaction of a substituted phosphorus dihalide (RPCl2,
R ) CH3, C6H5) with the arachno-CB8H13 (1-) monoanion followed by an in situ dehydrohalogenation
-
reaction initiated by Proton Sponge, resulted in phosphorus cage insertion to yield the first 10-vertex arachno-
and nido-phosphamonocarbaboranes, exo-6-R-arachno-6,7-PCB8H12 (2a, 2b) and PSH+6-R-nido-6,9-
PCB8H9- (PSH+3a-, PSH+3b-) (R ) C6H5 (a), CH3 (b)). Alternatively, 2a and 2b were synthesized in high
yield as the sole product of the reaction of the arachno-4-CB8H122- (12-) dianion with RPCl2. Crystallographic
determinations of PSH+3a- and PSH+3b- in conjunction with DFT/GIAO computational studies of the anions
have confirmed the expected nido cage framework based on an octadecahedron missing the six-coordinate
vertex. DFT/GIAO computational studies have also shown that while the gross cage geometries of the
exo-6-R-arachno-6,7-PCB8H12 compounds 2a and 2b resemble the known isoelectronic arachno-6,9-
SCB8H12, the phosphorus and carbon atoms are in thermodynamically unfavorable adjacent positions on
the six-membered puckered face. They also each have an endo-hydrogen at the P6-position arising from
proton transfer to the basic phosphorus during the cage-insertion reaction. Possible stepwise reaction
pathways that can account for the formation of both the arachno and nido products are discussed.
Deprotonation of 2a and 2b resulted in the formation of their corresponding conjugate monoanions, 6-R-
arachno-6,7-PCB8H11- (2a-, 2b-), in which the proton that had been attached to the P6 atom was removed.
Reactions of 2a- with O2, S8, BH3‚THF, or Br2 further demonstrated the basicity of the P6-phosphorus
yielding the new arachno-substituted compounds, endo-6-O-exo-6-(C6H5)-arachno-6,7-PCB8H11 (4a-),
-
endo-6-S-exo-6-(C6H5)-arachno-6,7-PCB8H11- (5a-), endo-6-BH3-exo-6-(C6H5)-arachno-6,7-PCB8H11- (6a-),
and endo-6-Br-exo-6-(C6H5)-arachno-6,7-PCB8H11 (7a), respectively, in which the O, S, BH3, and Br
substituents are bound to the phosphorus at the endo position.
Introduction
lone pair localized on the phosphorus. We further demonstrate
in the following paper, that these 10-vertex phosphamonocar-
We recently reported high-yield routes for heteroatom-cage
insertions into polyborane clusters that have allowed the
systematic syntheses of a range of thia- and phospha-boranes,
thia- and phospha-dicarbaboranes, and the first thiaphosphabo-
rane cluster.1,2 We describe here the rational high-yield syntheses
of the first 10-vertex arachno- and nido-phosphamonocarbabo-
ranes,3 exo-6-R-arachno-6,7-PCB8H12, and 6-R-nido-6,9-PCB8H9-
along with their structural characterizations using combined
crystallographic and DFT/GIAO/NMR methods. We also report
systematic studies of the reactions of the exo-6-R-arachno-6,7-
PCB8H11- anion with Lewis acids that have demonstrated that
this anion, in contrast to the other known phosphaborane
clusters, exhibits strong donor properties arising from an electron
boranes are an important new class of highly versatile cluster
ligands that can stabilize transition metals in an unprecedented
variety of coordination geometries and formal oxidation states.4
Experimental Section
All manipulations were carried out using standard high-vacuum or
inert-atmosphere techniques as described by Shriver.5
Materials. Proton Sponge (1,8-bis(dimethylamino)naphthalene, PS),
BH3‚THF (1.0 M solution in THF), HCl‚OEt2 dichlorophenylphosphine,
and dichloromethylphosphine were purchased from Aldrich and used
as received. Bromine was purchased from Alfa Aesar and used as
received. Oil-dispersed sodium hydride was purchased from Aldrich,
washed with dry hexanes under a N2 atmosphere, and then dried under
vacuum. Tetrahydrofuran was dried over sodium/benzophenone ketyl
* Address correspondence to this author. E-mail: lsneddon@sas.upenn.edu.
(1) Shedlow, A. M.; Sneddon, L. G. Inorg. Chem. 1998, 37, 5269-5277.
(2) Hong, D.; Rathmill, S. E.; Kadlecek, D. E.; Sneddon, L. G. Inorg. Chem.
2000, 39, 4996-4997.
(3) The first 10-vertex closo-phosphamonocarboranes were recently reported,
see: Holub, J.; Bakardjiev, M.; St´ıbr, B.; Hnyk, D.; Tok, O. L.;
Wrackmeyer, B. Inorg. Chem. 2002, 41, 2817-2819.
(4) Kadlecek, D. E.; Shedlow, A. M.; Kang, S. O.; Carroll, P. J.; Sneddon, L.
G. J. Am. Chem. Soc. 2002, 124, 212-220.
(5) Shriver, D. F.; Drezdzon, M. A. Manipulation of Air-SensitiVe Compounds,
2nd ed.; Wiley: New York, 1986.
9
200
J. AM. CHEM. SOC. 2003, 125, 200-211
10.1021/ja020944j CCC: $25.00 © 2003 American Chemical Society

10-Vertex arachno- and nido-Phosphamonocarbaboranes
A R T I C L E S
and distilled prior to use. Anhydrous diethyl ether was purchased from
Fisher and used as received. The arachno-4-CB8H14 was prepared
according to the literature procedure.6
Synthesis of exo-6-CH3-arachno-6,7-PCB8H12 (2b) and PSH+6-
CH3-nido-6,9-PCB8H9 (PSH+3b-) from arachno-4-CB8H13 (1-).
A 0.225-g (2.0 mmol) sample of 1 was dissolved in 40 mL of dry
THF under a N2 atmosphere. To this stirred solution was added 1.07 g
(5.0 mmol) of Proton Sponge. Then, 0.18 mL (2.0 mmol) of dichlo-
romethylphosphine was injected dropwise at room temperature. After
18 h, the solution was filtered to remove PSH+Cl-. The solvent was
removed under reduced pressure and the oily residue extracted with
toluene. The toluene was then vacuum evaporated and the resulting
yellow solid was dried under vacuum to give 0.294 g (1.88 mmol,
94.0% yield) of exo-6-CH3-arachno-6,7-PCB8H12 (2b) as a pale yellow
solid. For 2b: Anal. calcd for C2H15B8P1: C, 15.34; H, 9.65; found:
-
-
Physical Measurements. 1H NMR spectra at 500.4 MHz, 11B NMR
spectra at 160.5 MHz, 13C NMR at 125.8 MHz, and 31P NMR spectra
at 202.6 MHz were obtained on a Bruker AM-500 spectrometer
equipped with the appropriate decoupling accessories. The 11B NMR
1
at 64.2 MHz and H NMR at 200 MHz were obtained on a Bruker
AC-200 spectrometer. The 31P NMR spectra at 145.8 MHz were
obtained on a Bruker AM-360 spectrometer. All 11B chemical shifts
are referenced to external BF3‚O(C2H5)2 (0.00 ppm) with a negative
1
sign indicating an upfield shift. All H and 13C chemical shifts were
C, 15.17; H, 9.45; HRMS calcd for 12C2 H1511B831P1 m/e 158.1656;
1
measured relative to internal residual protons or carbons in the lock
solvent and are referenced to Me4Si (0.00 ppm). All 31P chemical shifts
are referenced to external 85% H3PO4 (0.0 ppm) with a negative sign
indicating an upfield shift. Two-dimensional COSY 11B-11B NMR
experiments and two-dimensional HETCORR 11B-1H experiments were
performed at 160.5 MHz using the procedures described previously.7
High- and low-resolution mass spectra (HRMS and LRMS) were
recorded on a Micromass Autospec spectrometer. Infrared spectra were
recorded on a Perkin-Elmer 1430 spectrophotometer. FT and diffuse-
reflectance (DRIFT) infrared spectra were obtained on a Perkin-Elmer
System 2000 FTIR spectrophotometer. Elemental analyses were
performed at the University of Pennsylvania microanalysis facility.
Melting points were obtained on a standard melting point apparatus
and are uncorrected.
found, 158.1648; mp 140-141 °C (dec); IR (NaCl, CH2Cl2, cm-1):
3231 (w), 3050 (s), 2600 (s), 2560 (vs), 2541 (s), 2360 (s), 2260 (s),
1471 (m), 1438 (s), 1419 (m), 1395 (m), 1310 (m), 1194 (w), 1110
(m), 1015 (w), 930 (m), 836 (m), 776 (m), 738 (m), 682 (m), 645 (s),
543 (s), 484 (m, br).
Only very small amounts of PSH+3b- were observed to have been
formed in reactions carried out with the conditions described above.
However, when the reactions were carried out with twice the concentra-
tion of 1 and at lower temperatures, higher yields of PSH+3b- were
obtained. Thus, in a typical reaction, 0.225 g (2.0 mmol) of 1, dissolved
in 20 mL of dry THF, was reacted at 0 °C with 1.07 g (5.0 mmol) of
Proton Sponge, followed by the dropwise addition of 0.18 mL (2.0
mmol) of dichloromethylphosphine. The mixture was then allowed to
stir for 1 h at which time 11B NMR analysis of the reaction mixture
indicated nearly quantitative formation of PSH+6-CH3-nido-6,9-
PCB8H9- (PSH+3b-). The reaction mixture was filtered in the glovebag
to remove precipitated PSH+Cl- and the resulting solid washed with 2
× 20 mL portions of dry THF. Drying of the solid overnight under
high vacuum afforded an oily yellow solid. This solid was then extracted
with 4 × 20 mL portions of toluene, followed by 4 × 20 mL portions
of diethyl ether until a bright, yellow powder resulted. The product
was then dried under high vacuum to yield 0.679 g (1.84 mmol, 92%)
Synthesis of exo-6-(C6H5)-arachno-6,7-PCB8H12 (2a) and PSH+6-
(C6H5)-nido-6,9-PCB8H9- (PSH+3a-) from arachno-4-CB8H13- (1-).
To a 250-mL, two-neck, round-bottom flask fitted with a vacuum
adapter, magnetic stirbar, and septum was added 1.13 g (10.0 mmol)
of arachno-4-CB8H14 (1). The flask was then attached to the high-
vacuum line and 100 mL of dry THF was condensed into the flask at
-196 °C. The flask was then attached to the Schlenk line and, after
equilibrating the solution at 0 °C, 4.3 g (20.0 mmol) of Proton Sponge
was added through one neck of the flask under a N2 flow. Then, 1.80
g (10.1 mmol) of dichlorophenylphosphine was added dropwise via
syringe. The solution was then stirred for 3 h at which point 11B NMR
analysis indicated nearly quantitative formation of exo-6-(C6H5)-
arachno-6,7-PCB8H12 (2a). The solution was then taken into the
glovebag, filtered, and the volatiles removed from the filtrate under
reduced pressure. Extraction of the yellow residue with 5 × 50 mL
portions of diethyl ether followed by concentration of the combined
extracts under reduced pressure and drying of the residue overnight
under high-vacuum afforded 2.1 g (9.6 mmol, 96%) of exo-6-(C6H5)-
arachno-6,7-PCB8H12 (2a): yellow, oily solid; mp 76 °C (dec); anal.
calcd for C7H17B8P: C, 38.45; H, 7.84; found C, 38.39; H, 7.76; HRMS
-
of PSH+6-CH3-nido-6,9-PCB8H9 (PSH+3b-). For PSH+3b-: Anal.
calcd for C16H31N2B8P1: C, 52.10; H, 8.47; N, 7.59; found: C, 52.07;
H, 8.42; N, 7.60; mp 265-267 °C (dec); IR (NaCl, CH2Cl2, cm-1):
3520 (w), 3225 (s), 3045 (w), 2960 (s), 2860 (w), 2500 (vs), 2350 (w),
1595 (m), 1509 (m), 1452 (m), 1402 (m), 1295 (w), 1260 (m), 1216
(m), 1155 (w), 1100 (w), 1030 (m), 900 (m), 825 (w), 764 (m), 728
(s).
Synthesis of exo-6-R-arachno-6,7-PCB8H12 (2a and 2b) from
arachno-4-CB8H122- (12-). To a 250-mL, two-neck, round-bottom flask
fitted with a vacuum adapter, magnetic stirbar, and septum was added
0.79 g (7.0 mmol) of 1 under a flow of N2. Approximately 40 mL of
THF was added by vacuum transfer. The stirring solution was
maintained at 0 °C while 0.49 g (21.0 mmol) of NaH was added. After
30 min, 11B NMR analysis of the reaction mixture indicated complete
formation of arachno-4-CB8H122- (12-).8 The solution was then taken
into the glovebag and filtered to remove excess NaH. After reattaching
the flask to the Schlenk line, the stirring solution was maintained at 0
°C, while 1.3 g (7.0 mmol) of dichlorophenylphosphine was added
dropwise via syringe. 11B NMR analysis of the reaction mixture after
12 h indicated the exclusive formation of exo-6-(C6H5)-arachno-6,7-
PCB8H12 (2a). The reaction was then filtered in the glovebag and the
THF removed from the filtrate under reduced pressure to give 1.45 g
(6.6 mmol, 94% yield) of 2a.
calcd for 12C7 H1711B831P1 (P) m/e 220.1812; found m/e 220.1800;
1
DRIFT (KBr, cm-1): 3225 (w), 3043 (s), 3032 (s), 2622 (s), 2578 (vs),
2516 (s), 2250 (s), 1965 (w), 1897 (w), 1812 (w), 1586 (m), 1484 (m),
1437 (s), 1313 (m), 1186 (w), 1137 (w), 1111 (m), 1028 (w), 985 (w),
952 (m), 854 (m), 739 (m), 690 (m), 611 (s), 517 (s), 425 (m, br). The
compound was normally stored as a ∼0.15 M stock solution, which
was prepared by dissolving the oily solid in an appropriate amount of
dry THF.
The yellow solid remaining after the diethyl ether extractions was
collected to give 0.1 g (0.23 mmol, 2.3%) of PSH+6-(C6H5)-nido-6,9-
PCB8H9 (PSH+3a-). Elemental analysis was performed on solid
-
PSH+3a- obtained from recrystallization in CH2Cl2/hexanes. Anal.
calcd for [PSH+6-(C6H5)-nido-6,9-PCB8H9
]1.00[CH2Cl2]0.77: C, 52.68;
-
An analogous reaction of 0.226 g (2.0 mmol) of 1, excess NaH and
0.24 mL (2.0 mmol) of dichloromethylphosphine yielded 0.253 g (1.6
mmol, 80% yield) of exo-6-CH3-arachno-6,7-PCB8H12 (2b).
Synthesis of exo-6-R-arachno-6,7-PCB8H11- (2a- and 2b-) from
exo-6-R-arachno-6,7-PCB8H12 (2a and 2b). To a 100-mL, two-neck,
round-bottom flask fitted with a vacuum adapter, magnetic stirbar, and
H, 7.01; N, 5.64; found: C, 52.64; H, 7.33; N, 5.51; IR (KBr plates,
CCl4, cm-1): 3210 (s), 2510 (s), 1390 (s), 1310 (m), 1020 (w), 930
(w).
(6) Wille, A. E.; Plesek, J.; Holub, J.; St´ıbr, B.; Carroll, P. J.; Sneddon, L. G.
Inorg. Chem. 1996, 35, 5342-5346.
(7) Kang, S. O.; Carroll, P. J.; Sneddon, L. G. Organometallics 1988, 7, 772-
776.
(8) Kadlecek, D. E.; Sneddon, L. G. Inorg. Chem. 2002, 41, 4239-4244.
9
J. AM. CHEM. SOC. VOL. 125, NO. 1, 2003 201

A R T I C L E S
Shedlow et al.
septum was added under a flow of N2 a solution of 0.604 g (2.76 mmol)
of 2a in diethyl ether. An excess of Proton Sponge (or LiH or NaH) in
diethyl ether was then added to the flask. After 30 min, NMR analysis
of the reaction mixture indicated complete formation of the air-sensitive
Synthesis of endo-6-Br-exo-6-(C6H5)-arachno-6,7-PCB8H11 (7a).
To a 100-mL, two-neck, round-bottom flask fitted with a vacuum
adapter, magnetic stirbar, and septum was added, via syringe, 6.1 mL
(1.0 mmol) of a 0.16 M solution of 2a in THF. After cooling the
solution to 0 °C, 0.21 g (1.0 mmol) of Proton Sponge was added through
one neck of the flask under a N2 flow to generate PSH+6-(C6H5)-
-
salt exo-6-(C6H5)-arachno-6,7-PCB8H11 (PSH+2a-).
An analogous reaction of 0.156 g (1.0 mmol) of 2b and 0.214 g
arachno-6,7-PCB8H11 (PSH+2a-). With the solution at 0 °C, 0.05
-
(1.0 mmol) of Proton Sponge in THF afforded the air-sensitive salt,
exo-6-CH3-arachno-6,7-PCB8H11 (PSH+2b-).
-
mL (1.0 mmol) of bromine was added dropwise via syringe. After
stirring for 1 h, 11B NMR analysis of the reaction mixture indicated
near quantitative formation of endo-6-Br-exo-6-(C6H5)-arachno-6,7-
PCB8H11 (7a). The reaction was filtered in the glovebag to remove the
precipitated PSH+Br- and the filtrate then concentrated under reduced
pressure to afford a yellow residue. The residue was then extracted
with diethyl ether and the volatiles removed from the combined extracts
under reduced pressure. The resulting oil was sublimed at 60 °C under
vacuum to yield 0.14 g (0.47 mmol, 47%) of endo-6-Br-exo-6-(C6H5)-
arachno-6,7-PCB8H11 (7a) as a pale yellow oil. LRMS (CI-) (m/e) calcd
Reprotonation of a 0.109-g (0.5 mmol) sample of Na+2a- (prepared
from the reaction of NaH and 2a) in diethyl ether with 1.0 mmol of
HCl‚OEt2 resulted in the regeneration of 2a.
Synthesis of endo-6-O-exo-6-R-arachno-6,7-PCB8H11 (4a-). To
-
a 50-mL, two-neck, round-bottom flask fitted with a vacuum adapter,
magnetic stirbar, and septum was added via syringe 5.0 mL (1.0 mmol)
of a 0.20 M solution of exo-6-(C6H5)-arachno-6,7-PCB8H12 (2a) in THF.
After cooling the solution to 0 °C, 0.214 g (1.0 mmol) of Proton Sponge
was added through one neck of the flask to generate PSH+2a-. Once
the reaction was determined to be complete by 11B NMR, the solution
was opened to the air. Within several minutes, the 11B NMR indicated
that complete conversion to the new anion 4a- had occurred. The
volatiles were then removed under reduced pressure to leave an oily,
light yellow solid. The oily solid was then precipitated from THF/
pentane to afford 0.408 g (0.91 mmol, 91%) of PSH+endo-6-O-exo-
1
for 12C711B8 H1631P181Br1: 300, found 300 with the peaks in the parent
envelope having relative intensities in accord with the expected isotopic
composition.
Crystallographic Data for PSH+3a- (Upenn #3149), PSH+3b-
(Upenn #3158), and PSH+5a- (Upenn #3201). Yellow crystals of both
PSH+3a- and PSH+3b- were grown from hexanes/CH2Cl2 solutions
of the compounds at -25 °C inside a glovebox. Clear crystals of
PSH+5a- were grown from pentane/THF solutions at -10 °C inside
a refrigerator.
6-(C6H5)-arachno-6,7-PCB8H11 (PSH+4a-). For 4a-: HRMS calcd
-
for 12C7 H1611B831P116O1 m/e 235.1683; found, 235.1693; IR (NaCl
1
plates, CH2Cl2, cm-1): 2525 (vs), 2390 (w), 1450 (m), 1409 (s), 1300
(s), 1021 (m), 920 (m), 825 (w), 725 (s), 705(m).
Collection and Reduction of the Data. X-ray intensity data for
PSH+3a- and PSH+3b- were collected on a Rigaku R-AXIS IIc area
detector employing graphite-monochromated Mo-KR radiation at a
temperature of 200 K for PSH+3a- and 210 K for PSH+3b-. X-ray
intensity data for PSH+5a- were collected on a Rigaku Mercury CCD
area detector employing graphite-monochromated Mo-KR radiation at
a temperature of 143 K. Indexing for PSH+3a- and PSH+3b- was
performed from a series of 1° oscillation images with exposures of 4
min per frame for PSH+3a- and 5 min per frame for PSH+3b-. A
hemisphere of data was collected using 5° oscillation angles with
exposures of 5 min per frame for PSH+3a- and 500 s for PSH+3b- at
a crystal-to-detector distance of 82 mm. Indexing for PSH+5a- was
performed from a series of four 0.5° oscillation images with exposures
of 20 s per frame. A hemisphere of data was collected using 30 s
exposures and a crystal-to-detector distance of 35 mm. Oscillation
images were processed using bioteX9 (PSH+3a- and PSH+3b-) or
CrystalClear10 (PSH+5a-) producing a listing of unaveraged F2 and
σ(F2) values which were then passed to the teXsan11 program package
for further processing and structure solution on Silicon Graphics Indigo
R4000 or O2 computers. The intensity data were corrected for Lorentz
and polarization effects. The intensity data for PSH+5a- were also
corrected for absorption using REQAB12 (minimum and maximum
transmission 0.720, 1.000).
Synthesis of endo-6-S-exo-6-R-arachno-6,7-PCB8H11 (5a-). To
-
a 100-mL, three-neck, round-bottom flask fitted with a vacuum adapter,
magnetic stirbar, septum, and sidearm addition tube containing 0.048
g (1.5 mmol) of elemental sulfur was added via syringe 8.3 mL (1.0
mmol) of a 0.12 M solution of exo-6-(C6H5)-arachno-6,7-PCB8H12 (2a)
in THF. The volatiles were then vacuum evaporated to leave an oily,
yellow solid. A solution of 0.214 g (1.0 mmol) of Proton Sponge in 20
mL of CHCl3 was then added to generate PSH+2a-. After cooling the
solution to 0 °C, the elemental sulfur was added by tipping the sidearm
attachment. Within several minutes, 11B NMR indicated that complete
conversion to the new compound (5a-) had occurred. The solution was
filtered and the volatiles were removed under reduced pressure to leave
an oily residue, which was extracted with THF. The THF was then
removed in vacuo to leave an oily, light yellow solid. The solid was
recrystallized from THF/pentane to afford 0.414 g (0.89 mmol, 89%)
of PSH+endo-6-S-exo-6-(C6H5)-arachno-6,7-PCB8H11- (PSH+5a-). For
5a-: Anal. calcd for (C21H35N2B8P1S1)1.00 (THF)0.90: C, 55.77; H, 8.03;
N, 5.28; found: C, 55.38; H, 8.04; N, 4.88; HRMS calcd for
12C7 H1611B831P132S1 m/e 251.1455; found, 251.1465; IR (NaCl plates,
1
CH2Cl2, cm-1): 3300 (m), 2530 (vs), 1605 (w), 1458 (m), 1435 (m),
1220 (w), 1160 (w), 1100 (w), 1027 (m), 945 (w), 825 (m), 809 (w),
729 (s), 695 (s).
Solution and Refinement of the Structure. The structures were
solved by direct methods (SIR92).13 Refinement was by full-matrix
least squares based on F2 using SHELXL-93.14 All reflections were
used during refinement (F2’s that were experimentally negative were
Synthesis of endo-6-BH3-exo-6-(C6H5)-arachno-6,7-PCB8H11- (6a-).
To a 100-mL, two-neck, round-bottom flask fitted with a vacuum
adapter, magnetic stirbar, and septum was added via syringe 4.3 mL
(1.0 mmol) of a 0.23 M solution of exo-6-(C6H5)-arachno-6,7-PCB8H12
(2a) in THF. After cooling the solution to 0 °C, 0.21 g (1.0 mmol) of
Proton Sponge was added through one neck of the flask to generate
PSH+2a-. Then, 1.5 mL (1.5 mmol) of 1.0 M BH3‚THF was added
dropwise via syringe. After stirring for 2 h, 11B NMR analysis of the
reaction mixture indicated near quantitative formation of PSH+endo-
6-BH3-exo-6-(C6H5)-arachno-6,7-PCB8H11- (PSH+6a-). The volatiles
were then removed under reduced pressure and the resulting oily solid
washed with diethyl ether to afford 0.23 g (0.51 mmol, 51%) of
PSH+endo-6-BH3-exo-6-(C6H5)-arachno-6,7-PCB8H11- (PSH+6a-) as
a pale yellow solid. IR (NaCl, CCl4, cm-1): 2500 (vs), 2390 (s), 2305
(s), 1450 (s), 1430 (s), 1410 (w), 1405 (w), 1375 (m), 1265 (w), 1220
(m), 1180 (w), 1150 (w), 1090 (w), 1030 (s), 1010 (w), 940 (m), 890
(w), 690 (m), 605 (w), 580 (m), 490 (m), 460 (m).
replaced by F2 ) 0). The weighting scheme used was w ) 1/[σ2(Fo )
2
2
+ aP2 + bP] where P ) (Fo + 2Fc2)/3. Non-hydrogen atoms were
refined anisotropically and hydrogen atoms were refined isotropically,
except for the THF hydrogen atoms in the structure of PSH+5a- which
were instead refined using a “riding” model.
(9) bioteX: A suite of Programs for the Collection, Reduction, and Interpretation
of Imaging Plate Data, Molecular Structure Corporation, 1995.
(10) CrystalClear: Rigaku Corporation, 1999.
(11) teXsan: Crystal Structure Analysis Package, Molecular Structure Corpora-
tion, 1985 & 1992.
(12) REQAB4: Jacobsen, R. A. Private Communication, 1994.
(13) SIR92: Altomare, A.; Burla, M. C.; Camalli, M.; Cascarano, M.; Giaco-
vazzo, C.; Guagliardi, A.; Polidoro, G. J. Appl. Cryst. 1994, 27, 435.
(14) Sheldrick, G. M. SHELXL-93: Program for the Refinement of Crystal
Structures; University of Go¨ttingen: Go¨ttingen, Germany, 1993.
9
202 J. AM. CHEM. SOC. VOL. 125, NO. 1, 2003

10-Vertex arachno- and nido-Phosphamonocarbaboranes
A R T I C L E S
Table 1. NMR Data
compounds
nucleus
11Ba,b
δ(multiplicity, intensity, assignment, J(Hz))
2a
1.7 (d, B2, JBH 166), -4.2 (d, B10, JBH 144), -5.1 (d, B8, JBH 148), -19.8 (d, B1, JBH 148), -22.9 (d, B9, JBH 157),
-24.8 (d, B3, JBH 165), -42.7 (d, B4, JBH 161), -44.1 (dd, B5, JBH 162, JPH 77)
11B(calcd)c 1.1 (B2), -4.0 (B10), -7.8 (B8), -19.3 (B1), -24.5 (B3), -25.7 (B9), -45.5 (B4), -46.5 (B1)
11B-11Ba,b Observed cross-peaks: B1-B2, -B3, -B4, -B5, -B10; B2-B3, -B5; B3-B4, -B8; B4-B8, -B9, -B10; B5-B10;
B8-B9
1H{11B}b,d 8.0-7.4 (m, Ph), 4.0 (d, PH, JPH 425) 3.4 (s, BH), 2.8 (s, BH), 2.6 (s, BH), 2.1 (s, 2BH), 1.2 (s, cage CH), 0.7 (s, BH),
0.2 (s, BH), -3.0 (BHB, JPH 26), -3.9 (BHB, JPH 18)
11B-1Ha,b,d Observed cross-peaks: BHB (-3.0 ppm)-B8 (-5.1 ppm), -B9 (-22.9 ppm); BHB (-3.9 ppm) -B10 (-4.2 ppm),
-B9 (-22.9 ppm)
13Cb,e
(RT) 134.2-123.9 (m, Ph), 6.0 (d, C7, JCH 177); (-83 °C) 134.3-122.0 (m, Ph), 5.2 (dd, C7, JCP 26.7, JCH 168)
13C(calcd)c 12.1 (C7)
31Pb,f
-31.5 (d, P6, JPH 425)
31P(calcd)c -23.1 (P6)
2b
11Ba,b
3.0 (d, B2, JBH 161), -4.8 (d, B8,10, JBH 148), -19.0 (d, B1, JBH 149), -23.0 (d, B9, JBH 149), -25.0 (d, B3, JBH 164),
-43.1 (d, B4, JBH 155), -43.9 (dd, B5, JBH 155, JBP 76)
11B(calcd)c 3.0 (B2), -4.1 (B10), -7.3 (B8), -18.3 (B1), -24.5 (B3), -25.4 (B9), -45.8 (B4), -45.9 (B5)
11B-11Ba,b observed cross-peaks: B1-B2, -B3, -B4, -B5, -B(8,10); B2-B3, -B5; B3-B4, -B(8,10); B5-B(8,10);
B(8,10)-B9
1H{11B}b,d 4.50 (d, PH, JPH 415), 3.23 (d, PCH3, JPH 4.5) 2.00 (s, cage CH), -2.00 (BHB, JPH 27), -2.98 (BHB, JPH unresolved)
11B-1Ha,b,d selected observed cross-peaks: BHB (-2.00 ppm)-B8 (-4.8 ppm), -B9 (-23.0 ppm); BHB (-2.98 ppm)-B10 (-4.8 ppm),
-B9 (-23.0 ppm)
13Cb,e
13C(calcd)c 14.8 (CH3), 7.2 (C7)
31Pb,f
-46.8 (d, P6, JPH 415)
31P(calcd)c -45.6 (P6)
PSH+2a- 11Ba,b
7.7 (dd, CH3, JCH 56, JCP 14), 3.5 (d, C7, JCH 174)
-2.8 (d, B8, JBH 146), -6.3 (d, B2, JBH 141), -23.2 (d, B10, JBH br), -32.8 (d, B1, JBH 139), -36.0 (d, B5, JBH 141),
-37.4 (d, B4, JBH 141), -39.5 (d, B9, JBH 118), -44.2 (d, B3, JBH 152)
11B(calcd)c -1.6 (B2), -3.2 (B8), -23.6 (B10), -31.2 (B1), -37.5 (B4), -40.4 (B5), -42.9 (B9), -44.7 (B3)
1H{11B}b,d 7.97-6.91 (m, Ph, PSH+ and cage Ph-P), 2.73 (s, CH3, 12, PSH+), 1.97 (s, cage CH), -2.41 (s, BHB), -4.39 (s, BHB)
13Cb,e
147.6-118.2 (m, Ph, PSH+), 44.4 (d, PSH+), 10.5 (dd, C7, JCP 43, JCH 160); (-83 °C) 134.3-122.0 (m, Ph),
10.6 (dd, C7, JCP 11, JCH 69)
13C(calcd)c 17.6 (C6)
31Pb,f
-136.4 (s, P6)
31P(calcd)c -119.4 (P6)
PSH+2b- 11Ba,g
-2.0 (d, B8, JBH ∼170), -4.1 (d, B2, br), -22.0 (d, B10, br), -31.4 (d, B1, JBH 170), -33.6 (d, B5, JBH ∼180),
-34.9 (d, B4, JBH ∼200), -38.6 (d, B9, JBH ∼110), -44.1 (d, B3, JBH 197)
11B(calcd)c 2.2 (B8), -3.8 (B2), -28.6 (B10), -28.7 (B1), -31.4 (B5), -31.7 (B4), -42.0 (B9), -48.6 (B3)
PSH+3a- 11Ba,h
8.4 (d, B5,7, JBH 145), -3.8 (d, B1,4)i, -4.4 (d, B8,10)i, -23.2 (d, B3, JBH 150), -30.0 (d, B2, JBH 161)
11B(calcd)c 12.6 (B5,7), -3.5 (B1,4), -6.1 (B8,10), -19.5 (B3), -27.5 (B2)
1H{11B}d,h 8.27-7.55 (m, Ph, PSH+ and cage Ph-P), 6.54 (s, cage CH), 4.47 (2, BH), 3.50 (2, BH), 3.35 (2, BH), 3.17 (s, CH3, 12, PSH+),
1.44 (1, BH), 1.24 (1, BH)
31Pf,h
31P(calcd)c -41.5 (P6)
-25.0 (s, P6)
PSH+3b- 11Ba,b
8.9 (d, B5,7, JBH 145), -4.8 (d, B1,4; B8,10), -25.6 (d, B3, JBH 153), -32.1 (d, B2, JBH 153)
11B(calcd)c 16.6 (B5,7), -3.8 (B1,4), -5.8 (B8,10), -22.0 (B3), -30.9 (B2)
1H{11B}b,j 18.7 (PSH+), 8.04-7.75 (m, Ph, PSH+), 5.65 (s, cage CH), 3.55 (1, BH), 3.23 (s, CH3, 12, PSH+), 2.75, (2, BH), 2.58 (2, BH),
1.56 (P-CH3), 1.66 (1, BH), 0.74 (1 BH), 0.65 (1 BH), 0.45 (1, BH)
31Pb,k
31P(calcd)c -67.3 (P6)
PSH+4a- 11Ba,g
0.3 (d, B2, JBH ∼150), -6.9 (d, B8,10, br), -11.7 (d, B1, JBH 145), -14.8 (d, B3, JBH 148), -19.4 (d, B9, JBH 148),
-37.4 (dd, B5, JBH 130, JBP 115), -45.8 (d, B4, JBH 152)
-33.3 (s, P6)
11B(calcd)c -1.7 (B2), -8.9 (B10), -9.7 (B8), -12.6 (B1), -17.7 (B3), -24.8 (B9), -39.8 (B5), -49.1 (B4)
1Hd,g
31Pg,k
18.7 (PSH+), 7.98-7.31 (m, Ph, PSH+ and cage Ph-P), 3.16 (s, CH3, 12, PSH+), 1.60 (s, cage CH), -1.57 (1, BHB), -2.55 (1, BHB)
-4.2 (q, P6, JPB ∼114)
31P(calcd)c -9.5 (P6)
PSH+5a- 11Ba,g
1.2 (d, B2, JBH 158), -8.7 (d, B10, br), -9.3 (d, B8, JBH br), -14.2 (d, B1, JBH 139), -17.3 (d, B3, JBH ∼130),
-18.3 (d, B9, JBH ∼150), -38.1 (dd, B5, JBH 113, JBP 93), -44.4 (d, B4, JBH 142)
11B(calcd)c 0.3 (B2), -10.2 (B10), -10.5 (B8), -13.6 (B1), -18.1 (B3), -21.0 (B9), -39.8 (B5), -47.2 (B4)
1H{11B}d,g 18.7 (PSH+), 8.16-7.25 (m, Ph, PSH+ and cage Ph-P), 3.16 (s, CH3, 12, PSH+), 2.80 (1, BH), 2.47 (1, BH), 2.26 (1, BH),
2.08 (1, BH), 2.01 (1, BH), 1.93 (1, BH), 1.30 (1, BH), 1.17 (s, cage CH), 0.88 (1, BH), -1.19 (1, BHB), -2.91 (1, BHB)
31Pg,k
31P(calcd)c 10.1 (P6)
PSH+6a- 11Ba,b
6.0 (d, B2, JBH 138), -7.8 (d, B8, JBH 122), -10.7 (d, B10, JBH 134), -20.5 (d, B1, JBH 139), -24.4 (d, B9, JBH 135),
-9.9 (q, P6, JPB 94)
-27.4 (d, B3, JBH 146), -40.7 (dd, B5, JBH >100, JBP 67), -43.6 (d, B4, JBH ∼149), -44.5 (m, B33, JBH ∼164, JBP ∼49)
11B(calcd)c 5.0 (B2), -10.4 (B8), -12.4 (B10), -21.0 (B1), -28.7 (B9), -28.8 (B3), -43.8 (B33), -46.7 (B4), -47.3 (B5)
1H{11B}b,d 19.15 (s, 1, PSH+), 8.03-7.30 (m, Ph, PSH+ and cage Ph-P), 3.13 (s, CH3, 12, PSH+), 2.95 (1, BH), 2.43 (1, BH), 2.08 (1, BH), 1.86 (1, BH),
1.56 (2, BH), 0.67 (s, cage CH), 0.55 (d, BH3, JHP 11), 0.47 (1, BH), -0.07 (1, BH), -2.27 (1, BHB), -3.41 (1, BHB)
31Pb,f
31P(calcd)c 58.6 (P6)
11Ba,b
-2.4 (d, B10, JBH 147), -4.9 (d, B2, JBH 177), -6.9 (d, B9, JBH 147), -10.0 (d, B1,3,8, JBH ∼150),
-65.8 (P6)
7a
-29.3 (dd, B5, JBH 147, JBP 89), -42.2 (d, B4, JBH 151)
11B(calcd)c -1.9 (B10), -6.9 (B3), -8.1 (B2), -9.0 (B1), -9.9 (B8), -11.1 (B9), -26.0 (B5), -44.1 (B4)
11B-11Ba,b observed cross-peaks: B1-B2; B1-B4; B1-B5; B1-B10; B2-B3; B2-B5; B3-B4; B4-B8; B4-B9; B4-B10; B5-B10
1H{11B}b,d 8.31-7.88 (m, Ph), 3.04 (1, BH), 2.96 (s, cage CH), 2.92 (1, BH), 2.77 (2, BH), 2.69 (1, BH), 2.61 (1, BH), 2.12 (1, BH),
1.51 (1, BH), -1.71 (1, BHB), -2.81 (1, BHB)
13C{1H}b,e (RT) 24.5 (s, C7); (-83 °C) 24.5 (dd, C7, JCH 171, JCP 23)
13C(calcd)c 38.2 (C7)
31Pb,f
16.3 (P6)
31P(calcd)c 58.6 (P6)
a 160.5 MHz. b CD2Cl2. c B3LYP/6-311G*//B3LYP-6-311G*. d 500.4 MHz. e 125.8 MHz. f 202.6 MHz. g THF-d8. h CDCl3. i Coupling not resolved. j 200.1
MHz. k 145.8 MHz.
9
J. AM. CHEM. SOC. VOL. 125, NO. 1, 2003 203

A R T I C L E S
Shedlow et al.
Table 2. Crystallographic Data Collection and Structural Refinement Information
PSH+3a-
PSH+3b-
PSH+5a-
C25B8H43N2PSO
empirical formula
formula weight
C21B8H33N2P
430.94
C16B8H31N2P
368.88
537.12
crystal class
space group
Z
monoclinic
P21/c (#14)
4
monoclinic
P21/n (#14)
4
monoclinic
P21/c (#14)
4
cell constants
a
b
c
â
V
µ
10.7136(1) Å
16.1166(2) Å
14.4975(1) Å
90.555(1)°
13.9467(2) Å
15.8609(2) Å
10.3485(1) Å
101.898(1)°
11.445(2) Å
10.723(2) Å
25.458(5) Å
104.249(4)°
3028.2(9) Å3
1.82 cm-1
0.30 × 0.20 × 0.18
1.178 g/cm3
1144
2503.12(4) Å3
1.22 cm-1
2239.98(5) Å3
1.26 cm-1
0.42 × 0.30 × 0.08
1.094 g/cm3
crystal size, mm
Dcalc
F(000)
0.35 × 0.20 × 0.18
1.144 g/cm3
912
784
radiation
2θ range
hkl collected:
Mo-KR (λ ) 0.71069 Å)
5.06-50.68 °
-12 e h e 12; -19 e k e 19;
-17e l e 17
19744
4547 (Rint ) 0.0272)
4203 (F > 4σ)
4547
Mo-KR (λ ) 0.71069 Å)
5.14-50.7 °
-16 e h e 16; -19 e k e 18;
-12 e l e 12
16686
3968 (Rint ) 0.0426)
3634 (F > 4σ)
3968
Mo-KR (λ ) 0.71069 Å)
5.04-54.96 °
-14 e h e 12; -11 e k e 13;
-32 e l e 33
19957
6720 (Rint ) 0.0441)
5293 (F > 4σ)
6720
no. reflections measured
no. unique reflections
no. observed reflections
no. reflections used in refinement
no. parameters
421
280
483
R1 ) 0.0728
R indices (F > 4σ)a
R1 ) 0.0503
wR 2 ) 0.1120
R1 ) 0.0560
wR2 ) 0.1150
1.145
R1 ) 0.0710
wR2 ) 0.1391
R1 ) 0.0804
wR2 ) 0.1433
1.272
wR2 ) 0.1339
R1 ) 0.1009
R indices (all data)
wR2 ) 0.1449
GOF
1.119
final difference peaks, e/Å3
+0.213, -0.247
+0.202, -0.322
+0.659, -0.434
a R1 ) ∑||Fo| - |Fc||/∑|Fo|; wR2 ) {∑w(F2o - Fc2)2/∑w(Fo2)2}1/2
.
Computational Studies. The DFT/GIAO/NMR method,15 using the
Gaussian 94 program,16 was used in a manner similar to that previously
described.1,8,17 The geometries were fully optimized at the B3LYP/6-
311G* level within the specified symmetry constraints (using the
standard basis sets included) on a (4)-processor Origin 2000 computer
running IRIX 6.5.5 or a (6)-processor Power Challenge XL computer
running IRIX 6.5.6. A vibrational frequency analysis was carried out
on each optimized geometry at the B3LYP/6-311G* level with a true
minimum found for each structure (i.e., possessing no imaginary
frequencies). The NMR chemical shifts were calculated at the B3LYP/
6-311G* level using the GIAO option within Gaussian 94. 11B NMR
GIAO chemical shifts are referenced to BF3‚O(C2H5)2 using an absolute
shielding constant of 102.24.18,19 The 13C NMR GIAO chemical shifts
were referenced to TMS using an absolute shielding constant of 184.38
and were corrected according to the method described by Schleyer.20
The 31P NMR GIAO chemical shifts were first referenced to PH3 using
an absolute shielding constant of 557.2396 ppm and then converted to
the H3PO4 reference scale using the experimental value of δ(PH3) )
-240 ppm.21
Results and Discussion
As shown in eq 1, reaction of arachno-4-CB8H14 (1) with an
excess of Proton Sponge and an equivalent of RPCl2 (R ) C6H5
(a), CH3 (b)) yielded a mixture of the new 10-vertex phospha-
monocarbaboranes exo-6-R-arachno-6,7-PCB8H12 (2a and 2b)
and PSH+6-R-nido-6,9-PCB8H9 (PSH+3a- and PSH+3b-).
-
+exs PS
arachno-4-CB8H14
1 R ) C6H5 (a), CH3 (b)
+ RPCl2
8
-2 PSH+Cl-
1 - x exo-6-R-arachno-6,7-PCB8H12
2a, 2b
+
xPSH+6-R-nido-6,9-PCB8H9- + xH2
PSH+3a-, PSH+3b-
(1)
The reactions with C6H5PCl2 gave relative yields of 2a and
PSH+3a- that were largely independent of reaction conditions,
with the major product 2a being obtained in greater than 95%
isolated yield. However, the relative amounts of 2b and
PSH+3b- that were obtained from the reactions with CH3PCl2
varied according to the reaction temperature and initial con-
centration of 1. As described in the Experimental Section, room-
temperature reactions with lower concentrations of 1 (2.0 mmol
of 1 in 40 mL of THF) gave high yields (>90%) of 2b and
only small amounts of PSH+3b-, while reactions performed at
0 °C with a higher concentration of 1 (2.0 mmol of 1 in 20 mL
of THF) afforded up to 92% PSH+3b-. Separation of the
products could easily be accomplished because of their differ-
ences in solubility. Thus, the neutral products 2a and 2b were
soluble in toluene, while the ionic products PSH+3a- and
PSH+3b- were insoluble. PSH+3a- and PSH+3b- were also
soluble in THF and could thus be easily separated from the
insoluble reaction byproduct, PSH+Cl-.
(15) Yang, X.; Jiao, H.; Schleyer, P. v. R. Inorg. Chem. 1997, 36, 4897-4899
and references therein.
(16) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Gill, P. M. W.; Johnson, B.
G.; Robb, M. A.; Cheeseman, J. R.; Keith, T.; Peterson, G. A.; Montgomery,
J. A.; Raghavachari, K.; Al-Laham, M. A.; Zakrzewski, V. G.; Ortiz, J.
V.; Foresman, J. B.; Cioslowski, J.; Stefanov, B. B.; Nanayakkara, A.;
Challacombe, M.; Peng, C. Y.; Ayala, P. T.; Chen, W.; Wong, M. W.;
Andres, J. L.; Replogle, E. S.; Gomperts, R.; Martin, R. L.; Fox, D. J.;
Binkley, J. S.; Defrees, D. J.; Baker, J.; Stewart, J. P.; Head-Gordon, M.;
Gonzalez, C.; Pople, J. A. Gaussian 94, revision E.2; Gaussian, Inc.:
Pittsburgh, PA, 1995.
(17) (a) Kadlecek, D. E.; Carroll, P. J.; Sneddon, L. G. J. Am. Chem. Soc. 2000,
122, 10868-10877. (b) Shedlow, A. M.; Sneddon, L. G. Collect. Czech.
Chem. Commun. 1999, 64, 865-882. (c) Bausch, J. W.; Rizzo, R. C.;
Sneddon, L. G.; Wille, A. E.; Williams, R. E. Inorg. Chem. 1996, 35, 131-
135.
(18) Tebben, A. J. Master’s Thesis, Villanova University, 1997.
(19) Tebben, A. J.; Bausch, J. W. private communication.
(20) Maerker, C.; Schleyer, P. v. R.; Salahub, D. R.; Malkina, O. L.; Malkin,
V. G. private communication.
(21) Crutchfield, M. M.; Dungan, C. H.; Letcher, L. H.; Mark, V.; Van Wazer,
J. R. Top. Phosphorus Chem. 1967, 5, 1-487.
9
204 J. AM. CHEM. SOC. VOL. 125, NO. 1, 2003

10-Vertex arachno- and nido-Phosphamonocarbaboranes
A R T I C L E S
While the above in situ dehydrohalogenation sequence yielded
a mixture of both the 2 and 3- products, it was found that
reactions of RPCl2 employing the arachno-4-CB8H12 (12-
)
2-
dianion8 gave only the exo-6-R-arachno-6,7-PCB8H12 products
in 94% (2a) and 80% (2b) isolated yields.
2-
Na2[arachno-4-CB8H12
]
+ RPCl2 f
Na212-
exo-6-R-arachno-6,7-PCB8H12 + 2 NaCl
2a, 2b
(2)
The 11B NMR spectra of PSH+3a- and PSH+3b- are
consistent with CS symmetry, each showing five doublets of
intensities 2:2:2:1:1. Likewise, their boron-decoupled 1H NMR
spectra show, in addition to the PSH+ and the Me or Ph
resonances, five singlets corresponding to the terminal BH
protons and one singlet attributed to the cage CH proton.
Given their 24 skeletal electron counts (n + 2 skeletal electron
pairs), the structures of the 3a- and 3b- anions would be
expected to be similar to those found for other 10-vertex, nido-
- 22
clusters, such as the thiamonocarbaborane nido-6,9-SCB8H9
and tricarbadecaborane 6-CH3-nido-5,6,9-C3B7H9- 23 monoan-
ions, with a cage structure based on an octadecahedron missing
its six-coordinate vertex. Single-crystal, X-ray structural deter-
minations of PSH+6-(C6H5)-nido-6,9-PCB8H9- (PSH+3a-) and
PSH+6-CH3-nido-6,9-PCB8H9- (PSH+3b-), in fact, confirmed
this geometry. Since there were no significant differences in
the structures of the two anions, only the structure of 3a- is
shown in Figure 1, where it can be seen that the anion has a
puckered six-membered open face with the phosphorus and
carbon atoms adopting the three-coordinate 6 and 9 positions
that are above the plane of the other four borons (B5, B7, B8,
B10) on the face. Such low-coordinate sites are known to be
favored by more electron-rich elements,17c,24 and the sulfur and
carbon atoms occupy the same positions in the known isoelec-
Figure 1. Comparison of the crystallographically determined structure of
the 6-C6H5 -nido-6,9-PCB8H9- anion in PSH+3a- (a) with that of the DFT
calculated geometry IIIa- (b). Selected bond distances (Å) and angles (°)
in PSH+3a-: P6-B7, 1.891(2); B7-B8, 1.861(3); B8-C9, 1.529(4); B10-
C9, 1.523(4); B5-B10, 1.871(4); B5-P6, 1.897(3); C9-B4, 1.681(3); P6-
B2, 2.016(2); P6-C11, 1.821(2); C11-P6-B2, 104.07(9); C11-P6-B5,
116.8(1); C11-P6-B7, 116.3(1); B7-P6-B5, 98.5(1); B10-C9-B8,
119.1(2). Selected calculated bond distances (Å) and angles (°) in IIIa-:
P6-B7, 1.938; B7-B8, 1.868; B8-C9, 1.536; B10-C9, 1.536; B5-B10,
1.868; B5-P6, 1.938; C9-B4, 1.708; P6-B2, 2.046; P6-C11, 1.850; C11-
P6-B2, 100.2; C11-P6-B5, 114.4; C11-P6-B7, 114.5; B7-P6-B5,
96.7; B10-C9-B8, 120.0.
spectra of 2a and 2b each showed eight separate resonances,
indicating that they have C1 symmetry rather than the CS
symmetry that has been confirmed for arachno-6,9-SCB8H12.22
tronic nido-6,9-SCB8H9 anion.22 Selected intracage distances
-
and angles are given in the Figure 1 caption and are all in the
normal ranges.
1
The H NMR spectra of 2a (Figure 3) and 2b also indicate C1
DFT calculations at the B3LYP/6-311G* level on the
structures of 3a- and 3b- yielded optimized geometries IIIa-
and IIIb- that are in excellent agreement with the crystallo-
graphically determined structures, as can be seen in Figure 1
where the optimized geometry IIIa- is compared to that of the
crystallographically determined structure of 3a-. Likewise, the
GIAO calculated chemical shifts and resonance assignments
given in Table 1 are in accord with the experimental values
obtained from the 1 and 2D NMR studies.
Given the similarity of the structures of 3a- and 3b- to that
of nido-6,9-SCB8H9-, it might be expected that the structure
for the arachno-compounds 2a and 2b should likewise be similar
to that found for their isoelectronic thiamonocarbaborane
analogue, arachno-6,9-SCB8H12.22 However, as illustrated by
the 11B NMR spectra of 2a presented in Figure 2, the 11B NMR
symmetry, showing eight terminal BH and two different bridge
hydrogen resonances. Furthermore, while the 1H NMR spectrum
of arachno-6,9-SCB8H12 contains a CH2 resonance, the spectra
of 2a and 2b exhibit only a single CH resonance. The most
unique feature of the 1H NMR spectra of 2a and 2b is that they
both contain a PH resonance (2a, JPH ) 425; 2b, JPH ) 415
Hz). The presence of a PH group is also supported by the 31P
NMR spectra which show a doublet at -31.7 ppm for 2a (JPH
) 425) and at -45.6 ppm for 2b (JPH ) 415 Hz). The observed
PH coupling constants are consistent with previously reported
values for clusters containing a P(R)H moiety. For example,
the bridging phosphaborane cluster, [B5H7P(H)(CH(OSiMe3)-
(tBu))]1- has a JPH ) 421 Hz, while the neutral clusters, B5H7P-
(H)(CH(OSiMe3)(tBu)) and B5H8P(H)(CH(OSiMe3)(adamantyl)
display coupling constants of JPH ) 410 and JPH ) 406 Hz,
respectively.25
(22) (a) Holub, J.; Kennedy, J. D.; Jel´ınek, T.; St´ıbr, B. J. Chem. Soc., Dalton
Trans. 1994, 1317-1323. (b) Holub, J.; Jel´ınek, T.; Plesek, J.; St´ıbr, B.;
Hermanek, S.; Kennedy, J. D. J. Chem. Soc., Chem. Commun. 1991, 1389-
1390.
The 125.7 MHz 13C NMR spectra of 2a (Figure 4) and 2b
1
support the H NMR data, each showing a broad doublet cage
CH resonance (Figure 4b) at room temperature and not the CH2
(23) (a) Kang, S. O.; Furst, G. T.; Sneddon, L. G. Inorg. Chem. 1989, 28, 2339-
2347. (b) Wasczcak, M. D.; Wang, Y.; Garg, A.; Geiger, W. E.; Kang, S.
O.; Carroll, P. J.; Sneddon, L. G. J. Am. Chem. Soc. 2001, 123, 2783-
2790.
(25) Miller, R. W.; Donaghy, K. J.; Spencer, J. T. Organometallics 1991, 10,
1161-1172.
(24) Williams, R. E. AdV. Inorg. Chem. Radiochem. 1976, 18, 67-142.
9
J. AM. CHEM. SOC. VOL. 125, NO. 1, 2003 205

A R T I C L E S
Shedlow et al.
Figure 4. 125.8 MHz 13C NMR spectra of 2a: (a) proton-decoupled
spectrum at room temperature; (b) proton coupled spectrum at room
temperature; (c) proton-decoupled spectrum at -83 °C; (d) proton-coupled
spectrum at -83 °C.
Figure 2. 160.5 MHz 11B NMR spectra of 2a: (a) proton-coupled, (b)
proton-decoupled.
isomeric structures were investigated, some of which are shown
in Figure 5. The CS symmetry structure IIb1 in which the
phosphorus and carbon atoms, like in the arachno-6,9-SCB8H12
cluster, are situated in the low-coordinate 6, 9 positions proved
to be of lowest energy. However, both the symmetry and GIAO
calculated chemical shifts for this isomer are inconsistent with
those observed for 2a and 2b. Among the C1 symmetry isomers,
the IIb2 structure with the carbon in one of the three-coordinate
vertexes (9-position), but with the phosphorus in the higher
coordinate 7-position, was next lowest in energy, being only
2.1 kcal/mol higher than the CS structure. Higher still in energy
were the exo and endo structures IIb3 and IIb4 with the
phosphorus in the favored three-coordinate 6-vertex, but with
the carbon in the higher coordinate 7-vertex adjacent to the
phosphorus. In the highest energy isomer IIb5, the carbon and
phosphorus are in the nonadjacent 6 and 8 positions but have
the same coordination numbers as in IIb3 and IIb4. Given the
tendency of electron-rich elements to separate, it is perhaps
surprising that isomer IIb5 proved to be of higher energy than
IIb3 and IIb4.
Examination of the GIAO calculated 11B, 31P, and 13C NMR
shifts and 11B assignments for each of the above isomers
demonstrated that only structure IIb3 (and that of the calculated
structure, IIa3, for the phenyl analog) is in complete agreement
with the observed NMR data for 2b (and 2a). As shown in
Figure 6a for IIa3, in this isomer the phosphorus and carbon
atoms are on the six-membered open face of the cluster with
the phosphorus atom situated in one of the three-coordinate
Figure 3. 500.4 MHz 1H NMR spectra of 2a: (a) boron-decoupled, (b)
boron-coupled.
resonance found for arachno-6,9-SCB8H12. At -83 °C, the
doublet resonance sharpens (Figure 4d) because of quenching
of carbon-boron scalar coupling26 and shows additional fine
phosphorus coupling (JCP ) 26.7 Hz) which strongly supports
a cage framework in which the phosphorus and carbon occupy
adjacent positions.
Unfortunately, both 2a and 2b are oily solids, thus crystal-
lographic determinations of their structures were not possible.
Therefore, DFT/GIAO computational studies were undertaken
to confirm their structures. For compound 2b, numerous
(26) (a) Wrackmeyer, B. In Progress in NMR Spectroscopy; Emsley, J. W.,
Feeney, J., Sutcliffe, L. H., Eds.; Pergamon: New York, 1979; Vol. 12,
pp 227-259. (b) Gragg, B. R.; Layton, W. J.; Niedenzu, K. J. Organomet.
Chem. 1977, 132, 29-36.
9
206 J. AM. CHEM. SOC. VOL. 125, NO. 1, 2003

10-Vertex arachno- and nido-Phosphamonocarbaboranes
A R T I C L E S
Figure 6. Optimized geometries and selected calculated bond distances
(Å) and angles (°) for structures: (a) IIa3, P6-H, 1.435; P6-B2, 2.232;
P6-B5, 1.884; P6-C7, 1.819, C7-B8, 1.677; B8-B9, 1.815; B9-B10,
1.825; B5-B10, 1.845; P6-C22, 1.817; C22-P6-B2, 105.3; C22-P6-
B5, 119.6; C22-P6-C7, 113.3; C7-B6-B5, 93.9; B10-B9-B8, 101.1;
P6H-P6-C22, 100.7; P6H-P6-C7, 107.0; P6H-P6-B5, 122.2. (b)
IIa-2, P6-B2, 2.204; P6-B5, 1.984; P6-C7, 1.925, C7-B8, 1.606; B8-
B9, 1.852; B9-B10, 1.877; B5-B10, 1.770; P6-C22, 1.881; C22-P6-
B2, 90.0; C22-P6-C7, 106.2; C22-P6-B5, 109.2; C7-B6-B5, 89.7;
B10-B9-B8, 99.8.
Figure 5. Energy comparisons for DFT optimized isomeric structures of
2b.
alternate exo and endo positioning of the metal and R substit-
uents at the phosphorus, thus illustrating that both conformations
are possible. For 2a and 2b, the DFT/GIAO calculations in
conjunction with the NMR data indicate that the Me and Ph
groups are substituted at the exo position of the phosphorus
with the hydrogen in the endo position. However, as discussed
in the following paper,4 the endo-Me isomer IIb4 proved to be
only 2.5 kcal/mol less stable than the exo-isomer IIb3.
vertexes (6-position), while the carbon atom occupies the
adjacent higher energy four-coordinate 7-vertex. Bridge protons
span the B8-B9 and B9-B10 edges. The NMR data for 2a
and 2b and structures IIa3 and IIb3 are compared in Table 1,
where it can be seen that there is excellent agreement between
the observed and calculated NMR chemical shifts and assign-
ments. The phosphorus fine-coupling found for the resonance
near -44 ppm in the 11B NMR spectra of compounds 2a (JPB
∼ 77 Hz) and 2b (JPB ∼ 76 Hz) is likewise in agreement with
the GIAO assignment of this resonance to the B5 boron adjacent
to P6. The fact that the in situ dehydrohalogenation reactions
yield compounds 2a and 2b having the adjacent phosphorus-
carbon cage structure IIb3 instead of the energically favored
IIb1 structure, is undoubtedly a result of the mild reaction
conditions which allow the isolation of kinetic rather than
thermodynamic products.
Final confirmation of the cage framework was obtained, as
reported in the following paper,4 by the crystallographic
characterizations of two transition-metal derivatives of 2a, endo-
6-[(η5-C5H5)Fe(CO)2]-exo-6-Ph-arachno-6,7-PCB8H11 and exo-
6-[(Mn(CO)5]-endo-6-Ph-arachno-6,7-PCB8H11 complexes. In
both complexes, the phosphacarbaborane ligand has, as proposed
in 2a and 2b, a six-membered puckered open face containing
two bridge hydrogens and a 6-position PR group adjacent to
the cage CH in the 7-position. The two structures differ in their
The formation of exo-6-R-arachno-6,7-PCB8H12 (2a and 2b)
2-
via the arachno-4-CB8H12 (12-) dianion probably follows a
pathway similar to that given in Figure 7. The arachno-4-
2-
CB8H12 anion has been shown to have the structure shown
in the figure, resulting from deprotonation of one bridging and
the endo-CH hydrogen from the parent arachno-4-CB8H14.8
Reaction of this dianion with RPCl2 in the manner shown in
the figure with insertion of the phosphorus between the vacant
B5-B6 edge and endo-carbon position (structure A), ac-
companied by proton transfer to the basic phosphorus endo
position then leads in a straightforward manner to the structures
observed for 2a and 2b.
The formation of the exo-6-R-arachno-6,7-PCB8H12 (2) and
PSH+6-R-nido-6,9-PCB8H9 (3-) products from the reactions
-
of the arachno-4-CB8H13 (1-) monoanion with RPCl2 in the
-
presence of Proton Sponge are more complex. Attempts to
thermally induce molecular hydrogen loss from either 2 or 2-
to form 3- were unsuccessful, thus indicating that the formation
9
J. AM. CHEM. SOC. VOL. 125, NO. 1, 2003 207

A R T I C L E S
Shedlow et al.
by loss of molecular hydrogen and deprotonation would instead
generate 3-. At this point, it is not clear why the reactions with
C6H5PCl2 yield almost exclusively 2b, while the reactions with
CH3PCl2 give, depending upon reaction conditions, high yields
of either 2a or 3a-. However, it may be that the more electron-
donating CH3P unit helps facilitate the loss of molecular
hydrogen that is a key step (from either B′ or C) that is needed
to form the 3- structure.
Attempts to form the neutral compound, 6-R-nido-6,9-
PCB8H10, by protonation of 3- with strong acids such as
concentrated H2SO4 gave no reaction. However, as shown in
eq 3, 2a and 2b underwent reversible deprotonation/protonation
reactions. Thus, deprotonation of 2a and 2b by NaH, BuLi, or
Proton Sponge resulted in the formation of the 2a- and 2b-
monoanions. Reprotonation of the Na+ or Li+ salts with HCl‚
OEt2, or the Proton Sponge salt with concentrated H2SO4,
resulted in the regeneration of the neutral exo-6-R-arachno-
6,7-PCB8H12 compounds.
+
-H
exo-6-R-arachno-6,7-PCB8H
2a, 2b
y
z
12
+H+
-
exo-6-R-arachno-6,7-PCB8H11
2a-, 2b-
(3)
As discussed below, the 2- monoanions are very air sensitive
and it was not possible to isolate them in pure form; however,
their structures can be confidently assigned on the basis of the
spectroscopic and computational studies. The 11B NMR spectra
of the 2- monoanions each showed eight separate doublet
resonances indicative of C1 symmetry. The 125.7 MHz 13C
NMR spectra of 2a- contained a CH resonance and the 1H NMR
spectra of both anions showed, in addition to the Me or Ph
resonances, two different bridge hydrogen resonances and a CH
resonance but no PH resonance. Deprotonation at the phosphorus
atom was further confirmed by their 31P NMR spectra, which
showed a singlet resonance for both 2a- and 2b- rather than
doublet resonances found in the spectra of 2a and 2b.
DFT/GIAO calculations of the possible structures of the 2a-
and 2b- anions yielded optimized geometries IIa-2 and IIb-2
with calculated 11B NMR shifts and assignments in good
agreement with the experimental values. As shown in Figure
6b for structure IIa-2, these structures result from deprotonation
at phosphorus, with the phosphorus and carbon remaining
adjacent and the phenyl (and methyl) still in the phosphorus
exo position. As depicted in Figure 8, structure IIb-2 was, in
fact, the lowest energy C1 symmetry methyl isomer in which
the carbon and phosphorus remained in adjacent positions but
is 3.1 kcal/mol less stable than the CS symmetry isomer IIb-1
in which both the phosphorus and carbon occupy the favored
low-coordinate 6 and 9 positions.
Figure 7. Possible reaction steps leading to the formation of 2 and 3-.
of 3- does not follow from 2 or 2-. The formation of the 2 and
3- products are most likely a consequence of competing
substitution/dehydrohalogenation steps, such as those depicted
in Figure 7. The 1- monoanion is initially generated by
deprotonation of the B6-B7 bridge hydrogen of 1 by Proton
Sponge.6 The reaction of 1- with RPCl2 could then result in
substitution of the RClP- group at either the terminal B6
(structure B) or the B6-B7 bridging (structure B′) cage-position.
A Proton Sponge initiated dehydrohalogenation of B′, involving
the bridging hydrogen at the B7-B8 edge, accompanied by both
loss of molecular hydrogen and deprotonation then leads to the
structure observed for the 3- anions. On the other hand, if B is
formed instead of B′, then proton transfer from a bridge to the
terminal RClP- group could occur as indicated to generate the
base adduct, exo-6-(RPHCl)-arachno-4-CB8H12 (structure C),
with C being another example of the known series of exo-6-
L-arachno-4-CB8H12 compounds.27 Such a migration of a cluster
proton to an exopolyhedral Ph2P group has previously been
observed by Zakharkin et al.28 during the reaction between
diphenylchlorophosphine and nido-7,8-C2B9H12- to yield 9-Ph2-
PH-nido-7,8-C2B9H11. A Proton Sponge initiated dehydrohalo-
genation from C involving the acidic endo-CH hydrogen would
then result in the formation of 2 where the phosphorus and
carbon atoms are in adjacent positions, while a dehydrohalo-
genation involving the B7-B8 bridging proton accompanied
Upon exposure to air, the 2a- anion readily reacted to form
the oxygen adduct 4a-.
- + 1/2 O2 f
PSH+6-(C6H5)-arachno-6,7-PCB8H11
PSH+2a-
PSH+endo-6-O-exo-6-(C6H5)-arachno-6,7-PCB8H11
-
(4)
PSH+4a-
(27) Plesek, J.; St´ıbr, B.; Fontaine, X. L. R.; Jel´ınek, T.; Thornton-Pett, M.;
Hermanek, S.; Kennedy, J. D. Inorg. Chem. 1994, 33, 2994-3002.
(28) Zakharkin, L. I.; Ol’shevskaya, V. A.; Zhigareva, G. G.; Antonovich, V.
A.; Petrovskii, P. V.; Yanovskii, A. I.; Polyakov, A. V.; Struchkov, Y. T.
Metalloorg. Khim. 1989, 2, 1274-1282.
As shown in eq 5 and 6, the corresponding anionic sulfur
(PSH+5a-) and borane adducts (PSH+6a-) were also formed
9
208 J. AM. CHEM. SOC. VOL. 125, NO. 1, 2003

10-Vertex arachno- and nido-Phosphamonocarbaboranes
A R T I C L E S
Figure 9. 160.5 MHz 11B NMR spectra of 4a-: (a) proton-coupled, (b)
proton-decoupled.
for these compounds. Thus, the single resonance observed in
the 31P NMR spectrum of each compound showed no JPH
coupling indicating that substitution at the phosphorus had
occurred. Their boron-decoupled 1H NMR spectra each showed,
in addition to the appropriate phenyl or methyl signals,
resonances attributable to eight terminal BH protons, a cage-
CH proton, and two inequivalent bridging protons. The spectrum
of 6a- also contained a doublet at 0.54 ppm with JHP ) 11 Hz
that can be assigned to the protons of the phosphorus-bonded
borane group. The 11B NMR spectra of the compounds again
indicate C1 symmetry and, as illustrated in the spectra of 4a-
shown in Figure 9, 4a-, 5a-, and 7a each have a resonance at
high field (-37.4, -38.1, and -29.3 ppm, respectively) showing
JBP fine-coupling characteristic of a boron (B5) adjacent to a
phosphorus. For 6a-, two of the nine resonances, -40.7 ppm
(JBP ) 67 Hz) and -44.5 ppm (JBP of 49 Hz), exhibit such
fine-coupling and can thus be assigned as arising from a
phosphorus-adjacent cage boron and to the phosphorus-bonded
borane, respectively.
Figure 8. Energy comparisons for DFT optimized isomeric structures of
2b-.
by the reaction of PSH+2a- with elemental sulfur and BH3‚
THF, respectively.
PSH+6-(C6H5)-arachno-6,7-PCB8H11
- + 1/8 S8 f
PSH+2a-
-
PSH+endo-6-S-exo-6-(C6H5)-arachno-6,7-PCB8H11
(5)
PSH+5a-
PSH+6-(C6H5)-arachno-6,7-PCB8H11
- + BH3‚THF f
PSH+2a-
-
PSH+endo-6-BH3-exo-6-(C6H5)-arachno-6,7-PCB8H11
PSH+6a-
(6)
DFT/GIAO computational investigations of the structures of
4a-, 5a- , 6a-, and 7a yielded the optimized geometries IVa-,
Va-, VIa-, and VIIa (Figures 10 and 11b) having calculated
11B, 13C, and 31P NMR chemical shifts (Table 1) in good
agreement with the experimental values. The calculations also
correctly assign the 11B NMR resonances that exhibit JPB
coupling to borons directly bonded to the phosphorus. Consistent
with the fact that the endo-hydrogen in 2a undergoes reversible
deprotonation/protonation, the DFT/GIAO calculations confirm
that the O, S, BH3, and Br-substituents are bonded at the
phosphorus endo position. While the calculated 31P NMR shifts
for 4a- , 5a-, and 6a- are in reasonable agreement with their
experimentally determined chemical shift values, the difference
between the calculated (58.6 ppm) and observed (16.3 ppm)
shifts of the phosphorus resonance in 7a is much larger. This
Reaction of PSH+2a- with bromine resulted in heterolytic
cleavage of the Br-Br bond with a formal bromonium ion
transfer to the cage phosphorus to generate the neutral Br-
substituted derivative endo-6-Br-exo-6-(C6H5)-arachno-6,7-
PCB8H11 (7a) as given in eq 7.
- + Br2 f
PSH+6-(C6H5)-arachno-6,7-PCB8H11
PSH+2a-
-
endo-6-Br-exo-6-(C6H5)-arachno-6,7-PCB8H11
+
PSH+Br-
(7)
7a
The NMR spectra of 4a-, 5a-, 6a-, and 7a have many
common features strongly supporting similar cage frameworks
9
J. AM. CHEM. SOC. VOL. 125, NO. 1, 2003 209

A R T I C L E S
Shedlow et al.
Figure 11. Comparison of the crystallographically determined structure
-
of the endo-6-S-exo-6-(C6H5)-arachno-6,7-PCB8H11 anion in PSH+5a-
(a) with that of the DFT calculated geometry Va- (b). Selected bond
distances (Å) and angles (°) in 5a-: P6-S33, 2.0042(10); P6-B2, 2.258-
(3); P6-B5, 1.898(3); P6-C7, 1.829(3), C7-B8, 1.677(5); B8-B9, 1.807-
(6); B9-B10, 1.810(6); B5-B10, 1.810(5); P6-C22, 1.817(3); C22-P6-
B2, 91.84(13); C22-P6-C7, 106.43(13); C22-P6-B5, 110.47(14); C7-
P6-B5, 90.89(14); B10-B9-B8, 100.9(2); S33-P6-C22, 105.48(8);
S33-P6-C7, 117.46(10); S33-P6-B5, 124.69(11). Selected calculated
bond distances (Å) and angles (°) for Va-: P6-S33, 2.030; P6-B2, 2.308;
P6-B5, 1.828; P6-C7, 1.871, C7-B8, 1.670; B8-B9, 1.802; B9-B10,
1.819; B5-B10, 1.834; P6-C22, 1.853; C22-P6-B2, 91.6; C22-P6-
C7, 104.5; C22-P6-B5, 111.5; C7-P6-B5, 88.9; B10-B9-B8, 101.3;
S33-P6-C22, 105.7; S33-P6-C7, 116.6; S33-P6-B5, 127.2.
Figure 10. DFT optimized geometries for (a) IVa-, selected calculated
bond distances (Å) and angles (°): P6-O33, 1.520; P6-B2, 2.318; P6-
B5, 1.913; P6-C7, 1.874, C7-B8, 1.675; B8-B9, 1.805; B9-B10, 1.824;
B5-B10, 1.841; P6-C22, 1.855; C22-P6-B2, 95.0; C22-P6-C7, 105.2;
C22-P6-B5, 113.1; C7-P6-B5, 89.4; B10-B9-B8, 101.1; O33-P6-
C22, 105.3; O33-P6-C7, 114.9; O33-P6-B5, 126.7. (b) VIa-, selected
calculated bond distances (Å) and angles (°): P6-B33, 1.988; P6-B2,
2.284; P6-B5, 1.947; P6-C7, 1.878, C7-B8, 1.650; B8-B9, 1.813; B9-
B10, 1.833; B5-B10, 1.803; P6-C22, 1.861; C22-P6-B2, 90.7; C22-
P6-C7, 105.8; C22-P6-B5, 110.3; C7-B6-B5, 89.3; B10-B9-B8,
100.9; B33-P6-C22, 103.9; B33-P6-C7, 116.8; B33-P6-B5, 128.8.
(c) VIIa, selected calculated bond distances (Å) and angles (°): P6-Br,
2.375; P6-B2, 2.170; P6-B5, 1.876; P6-C7, 1.806, C7-B8, 1.711; B8-
B9, 1.801; B9-B10, 1.800; B5-B10, 1.893; P6-C22, 1.822; C22-P6-
B2, 100.5; C22-P6-C7, 111.0; C22-P6-B5, 118.3; C7-P6-B5, 94.1;
B10-B9-B8, 102.1; Br-P6-C22, 98.9; Br-P6-C7, 111.1; Br-P6-B5,
123.7.
framework. The bridging hydrogens again span the B8-B9 and
B9-B10 edges. Consistent with the DFT calculations, the sulfur
is found substituted at the endo position of the phosphorus. The
observed P6-S bond distance of 2.0042(10) Å is substantially
longer than that found for the P-S length in SPPh3 (1.950(3)
Å),29 suggesting little multiple bond character to the P6-S bond
in 5a-.
In conclusion, the studies discussed above have resulted in
the development of rational high-yield routes to the first two
examples of 10-vertex arachno- and nido-phosphamonocarb-
aboranes, exo-6-R-arachno-6,7-PCB8H12 (2) and 6-R-nido-6,9-
PCB8H9 (3-), and should, therefore, now enable extensive
-
investigations of the chemical properties of this new cluster
class. However, of perhaps even greater significance, is the
discovery of the unique donor properties of the exo-6-R-
arachno-6,7-PCB8H11- (2-) anion that emerged from the studies
of its reactions (eqs 3-7) with Lewis acids. The insertion of
electron-rich elements, such as phosphorus, into a boron cluster
normally results in the formation of a compound having a formal
lone pair of electrons associated with the heteroatom.30 But,
discrepancy is most likely due to the limitation of the 6-311G*
basis set to accurately model the larger bromine atom.
The structure of compound PSH+5a- was further established
by means of an X-ray crystallographic determination. As shown
in the ORTEP plot in Figure 11a, the structure of the 5a- anion
retains the same framework as the parent compound 2a in which
the phosphorus and carbon atoms are in adjacent positions on
the puckered six-membered open face of the arachno cage
(29) Codding, P. W.; Kerr, K. A. Acta Crystallogr. 1978, B34, 3785-3787.
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210 J. AM. CHEM. SOC. VOL. 125, NO. 1, 2003

10-Vertex arachno- and nido-Phosphamonocarbaboranes
A R T I C L E S
because of the extensive electron delocalization in polyhedral
boron clusters, these heteroatom lone pairs are usually delo-
calized and have low Lewis basicity. As a result, they can neither
be protonated nor form adducts with Lewis acids. Such was
the case for 3-, where it was found that it was not possible to
protonate the anion even with concentrated H2SO4. In contrast,
the reactions of the exo-6-R-arachno-6,7-PCB8H11- (2-) anion
have demonstrated that it has a much different electronic
distribution. Both the reversible deprotonation/reprotonation of
2 and the fact that 2a- reacts with O2, S8, BH3, and Br2 to yield
the endo-substituted products 4a- , 5a- , 6a-, and 7a suggests
that in 2- a lone pair of electrons is localized at the phosphorus
endo position. This behavior appears to be unprecedented in
phosphaborane chemistry. These studies further suggested to
us that the 2- anions should be able to form adducts with an
even wider variety of Lewis acids and, indeed, the following
paper describes the syntheses and structural characterizations
of a series of new types of metallaphosphamonocarbaborane
complexes in which the metals adopt an unprecedented array
of coordination geometries, including not only η4, η5, and η6
metal-inserted bonding configurations, but also new types of
sigma-donor endo-η1 and exo-η1 complexes in which the 2-
anions are functioning, as has been previously observed for
diphenylphosphidoboratabenzene (PPh2(BC5H5))-,31 as an an-
ionic equivalent of a PR3 ligand.
Acknowledgment. This paper is dedicated to Professor
Russell N. Grimes on the occasion of his retirement. We thank
the National Science Foundation for support of this research.
Supporting Information Available: Tables listing Cartesian
coordinates and energies for DFT-optimized geometries. X-ray
crystallographic data for structure determinations of PSH+3a-,
PSH+3b-, and PSH+5a- (CIF). This material is available free
of charge via the Internet at http://pubs.acs.org.
(30) (a) Todd, L. J. In ComprehensiVe Organometallic Chemistry; Wilkinson,
G., Stone, F. G. A., Abel, E. W., Eds.; Pergamon Press: New York, 1982;
Vol. 1, pp 543-553. (b) Todd, L. J. In ComprehensiVe Organometallic
Chemistry; Wilkinson, G., Stone, F. G. A., Abel, E. W., Housecroft, C. E.,
Eds.; Pergamon Press: New York, 1995; Vol. 1, pp 257-273. (c) Binder,
H.; Hein, M. Coord. Chem. ReV. 1997, 158, 171-232.
JA020944J
(31) Hoic, D. A.; Davis, W. M.; Fu, G. C. J. Am. Chem. Soc. 1996, 118, 8176-
8177.
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