Synthesis and reactivity of the phosphinoboranes R2PB(C 6F5)2

Article abstract of DOI:10.1021/ic102003x

The phosphinoboranes [R₂PB(C₆F₅) ₂]₂ (R = Et 1, Ph 2) and R₂PB(C ₆F₅)₂ (R = tBu 3, Cy 4, Mes 5) were synthesized from the reaction of (C₆F_5<

Full text of DOI:10.1021/ic102003x

336 Inorg. Chem. 2011, 50, 336–344
DOI: 10.1021/ic102003x
Synthesis and Reactivity of the Phosphinoboranes R2PB(C6F5)2
Stephen J. Geier,† Thomas M. Gilbert,‡ and Douglas W. Stephan*,†
†Department of Chemistry, University of Toronto, Toronto, Ontario M5S 3H6, Canada, and
‡Department of Chemistry & Biochemistry, Northern Illinois University, DeKalb, Illinois 60115, United States
Received September 30, 2010
The phosphinoboranes [R2PB(C6F5)2]2 (R = Et 1, Ph 2) and R2PB(C6F5)2 (R = tBu 3, Cy 4, Mes 5) were synthesized
from the reaction of (C6F5)2BCl and the corresponding lithium phosphide. The relationships between B-P distance,
P pyramidality, and the extent of BP multiple bonding were further explored computationally. Natural Bond Order
(NBO) analyses of 3 and 4 showed that the π-bonding highest occupied molecular orbitals (HOMOs) were highly
polarized. In addition the Lewis acid-base adducts, R2(H)P B(H)(C6F5)2 (R = Et 6; Ph 7; tBu 8; Cy 9; Mes 10) were
3
prepared via the reaction of the phosphines R2PH with the borane HB(C6F5)2. Compounds 1 and 2 showed no signs of
reaction with H2; however, reaction of compounds 3 and 4 with H2 was observed to give 8 and 9. In a related set of
reactions compounds 3 and 4 were reacted with H3NBH3 or Me2(H)NBH3 also led to the generation of 8 and 9,
respectively. The reaction profile of the reaction of (CF3)2BPR2 with H2 was examined computationally and shown to
be exothermic. Efforts to effect the reverse reaction, that is, dehydrogenation of adducts 6-10 were unsuccessful.
Compound 4 was also shown to react with 4-tert-butylpyridine to give Cy2PB(C6F5)2(4-tBuC5H4N) 11 while reactions
of 3 and 4 with the Lewis acid BCl3 gave the dimers (R2PBCl2)2 (R = t Bu 12, Cy 13) and the byproduct ClB(C6F5)2.
Introduction
experimental2-16 and theoretical studies17-22 have focused
on main group compounds and amino-boranes in particular.
The dehydrogenation of ammonia-borane and related amine-
borane or phosphine-borane adducts have been shown to
occur under thermal duress23 or catalytically, using transition
metals,2,10-12,14-16,24-30 Lewis acid4 or Lewis base catalysts.31
The resulting products contain group13-group 15 multiple
bonds, and the nature of these materials has also generated
interest in the fundamental nature of the bonding modes in
these products. Moreover these dehydrogenation products
are thermodynamically downhill from the precursors, and thus
hydrogenation of such materials is strongly endothermic.
Combinations of group 13 Lewis acids and group 15 Lewis
bases are of significant interest as a resultof their potential for
use as hydrogen storage materials.1 To that end many recent
*To whom correspondence should be addressed. E-mail: dstephan@
chem.utoronto.ca.
(1) Grochala, W.; Edwards, P. P. Chem. Rev. 2004, 104, 1283.
(2) Keaton, R. J.; Blacquiere, J. M.; Baker, R. T. J. Am. Chem. Soc. 2007,
129, 1844.
(3) Stephens, F. H.; Baker, R. T.; Matus, M. H.; Grant, D. J.; Dixon,
D. A. Angew. Chem., Int. Ed. 2007, 46, 746.
(4) Stephens, F. H.; Pons, V.; Baker, R. T. Dalton Trans. 2007, 2613.
(5) Marder, T. B. Angew. Chem., Int. Ed. 2007, 46, 8116.
(6) Cheng, F.; Ma, H.; Li, Y.; Chen, J. Inorg. Chem. 2007, 46, 788.
(7) Pun, D.; Lobkovsky, E.; Chirik, P. J. Chem. Commun. 2007, 3297.
(8) Jiang, Y.; Berke, H. Chem. Commun. 2007, 3571.
(9) Denney, M. C.; Pons, V.; Hebden, T. J.; Heinekey, D. M.; Goldberg,
K. I. J. Am. Chem. Soc. 2006, 128, 12048.
(10) Clark, T. J.; Russell, C. A.; Manners, I. J. Am. Chem. Soc. 2006, 128,
9582.
(11) Jaska, C. A.; Bartole-Scott, A.; Manners, I. Phosphorus, Sulfur
(21) Nguyen, M. T.; Nguyen, V. S.; Matus, M. H.; Gopakumar, G.;
Dixon, D. A. J. Phys. Chem. A 2007, 111, 679.
(22) Paul, A.; Musgrave, C. B. Angew. Chem., Int. Ed. 2007, 46, 8153.
(23) Niedenzu, K.; Dawson, J. W. Boron-Nitrogen Compounds; Academic
Press, Inc: New York, 1965.
(24) Dorn, H.; Jaska, C. A.; Singh, R. A.; Lough, A. J.; Manners, I. Chem.
Commun. 2000, 1041.
(25) Dorn, H.; Singh, R. A.; Massey, J. A.; Lough, A. J.; Manners, I.
Angew. Chem., Int. Ed. 1999, 38, 3321.
(26) Dorn, H.; Vejzovic, E.; Lough, A. J.; Manners, I. Inorg. Chem. 2001,
40, 4327.
(27) Bartole-Scott, A.; Jaska, C. A.; Manners, I. Pure Appl. Chem. 2005,
77, 1991.
(28) Clark, T. J.; Jaska, C. A.; Turak, A.; Lough, A. J.; Lu, Z. H.;
Silicon Relat. Elem. 2004, 179, 685.
(12) Jaska, C. A.; Dorn, H.; Lough, A. J.; Manners, I. Chem. Eur. J.
2003, 9, 271.
;
(13) Jaska, C. A.; Manners, I. J. Am. Chem. Soc. 2004, 126, 1334.
(14) Jaska, C. A.; Manners, I. J. Am. Chem. Soc. 2004, 126, 2698.
(15) Jaska, C. A.; Temple, K.; Lough, A. J.; Manners, I. Chem. Commun.
2001, 962.
(16) Jaska, C. A.; Temple, K.; Lough, A. J.; Manners, I. J. Am. Chem.
Soc. 2003, 125, 9424.
(17) Chen, Y.; Fulton, J. L.; Linehan, J. C.; Autrey, T. J. Am. Chem. Soc.
2005, 127, 3254.
Manners, I. Inorg. Chem. 2007, 46, 7394.
(29) Clark, T. J.; Lee, K.; Manners, I. Chem. Eur. J. 2006, 12, 8634.
;
(30) Clark, T. L.; Rodezno, J. M.; Clendenning, S. B.; Aouba, S.;
Brodersen, P. M.; Lough, A. J.; Ruda, H. E.; Manners, I. Chem. Eur. J.
;
2005, 11, 4526.
(18) Luo, Y.; Ohno, K. Organometallics 2007, 26, 3597.
(19) Yang, X.; Hall, M. B. J. Am. Chem. Soc. 2008, 130, 1798.
(20) Li, Q. S.; Zhang, J.; Zhang, S. Chem. Phys. Lett. 2005, 404, 100.
(31) Himmelberger, D. W.; Yoon, C. W.; Bluhm, M. E.; Carroll, P. J.;
Sneddon, L. G. J. Am. Chem. Soc. 2009, 131, 14101.
r
pubs.acs.org/IC
Published on Web 12/02/2010
2010 American Chemical Society

Article
Inorganic Chemistry, Vol. 50, No. 1, 2011 337
Indeed the regeneration of ammonia borane has proven to be
challenging.4,32-34
methods unless otherwise noted. 1H, 13C, 11B, 19F, and 31P nuclear
magnetic resonance (NMR) spectra were recorded on a Bruker
Avance-300 or Avance-400 spectrometer at 300 K unless other-
More generally, while early studies described the inter-
actions of H2 with main group species in an argon matrix,35 it
was not until the work of Power and co-workers36 that a main
group species was shown to react with H2. In that case,
germynes were hydrogenated to give a mixture of digermene,
digermane, and primary germane products. More recently
we have developed the concept of “frustrated Lewis pairs”
(FLPs) and shown that such systems are capable of a number
of small molecule activation reactions including heterolytic
cleave of H2.37-39 This reactivity has been extended to effect
metal-free catalytic hydrogenations of a variety of polar sub-
strates.37-46 This unprecedented reactivity prompted us to
probe the chemistry of sterically encumbered and electron
deficient phosphino-boranes. Previous computational and
experimental studies on directly bound phosphinoboranes
suggest that there is limited interaction between the lone pair
at phosphorus and the vacant p-orbital on boron because of a
mismatch of donor and acceptor orbital energies.47,48 Thus,
such systems might exhibit FLP-like behavior as these systems
should have residual donor and acceptor ability despite being
adjacent. In this manuscript we probe this possibility. Phos-
phino-boranes are prepared and characterized, the nature of
the bonding is studied computationally, and the reactivity of
these species is examined. We note that a preliminary com-
munication of a small portion of this work has appeared
previously.49
1
wise noted. H and 13C NMR spectra are referenced to SiMe4
using the residual solvent peak impurity of the given solvent. 11
B
and 19F NMR experiments were referenced to 15% BF3-Et2O in
CDCl3 and 31P NMR experiments were referenced to 85% H3PO4.
Chemical shifts are reported in parts per million (ppm) and
coupling constants in hertz (Hz) as absolute values. Combustion
analyses were performed in house employing a Perkin-Elmer
CHN Analyzer. Despite repeated attempts, elemental analyses
gave low C values for several compounds reported herein. This
was attributable to the formation of boron-carbide during com-
bustion. H2 was passed through a drierite column prior to use.
R2PLi (R = Et, Ph, tBu, Cy, Mes) were prepared by treating the
corresponding phosphine with 1.1 equiv of tBuLi in toluene and
collecting the precipitate. (C6F5)2BCl was prepared by the
literature method.50
Synthesis of [R2PB(C6F5)2]2 (R=Et 1, Ph 2) and R2PB(C6F5)2
(R=tBu 3, Cy 4, Mes 5). These compounds were prepared in a
similar fashion, and thus only one preparation is detailed. To a
slurry of Et2PLi (51 mg, 0.53 mmol) in toluene (5 mL) was added
a solution of (C6F5)2BCl (200 mg, 0.53 mmol) in toluene (5 mL)
at -35 °C. The mixture was allowed to stir overnight and was
then run through Celite. The filtrate was concentrated to ∼2 mL
and stored at -35 °C overnight. The solution was dried in vacuo
and washed with cold pentane (2 ꢀ 2 mL). 1 was isolated as a
colorless polycrystalline solid. Yield: 192 mg (84%).
1: Yield: 192 mg (84%). Anal. Calcd. for C16H10BF10P: C,
44.28; H, 2.32; Found: C, 44.66; H, 2.64. Crystals of 1 were
grown from a pentane solution at room temperature. 1H NMR
(CD2Cl2): 1.07 (6H, dt, 3JP-H=16 Hz,3JH-H=8 Hz, CH3), 2.16
(4H, m, CH2). 19F NMR (CD2Cl2): -125.0 (d, 3JF-F = 20 Hz,
4F, o-C6F5), -153.5 (t, 3JF-F = 23 Hz, 2F, p-C6F5), -160.3 (t,
3JF-F=20 Hz, 4F, m-C6F5). 31P NMR (CD2Cl2): -23.4 (br m).
11B NMR (CD2Cl2): -12.9 (t, 1J P-B =72 Hz). 13C{1H} NMR
Experimental Section
General Considerations. All preparations were done under an
atmosphere of dry, O2-free N2 employing both Schlenk line tech-
niques and an Innovative Technologies or Vacuum Atmospheres
inert atmosphere glovebox. Solvents (pentane, hexanes, toluene,
and methylene chloride) were purified employing a Grubbs’ type
column systems manufactured by Innovative Technology and
(CD2Cl2) partial: 8.3 (CP), 16.2 (m, CH3), 137.4 (dm, 1JC-F
248 Hz, CF), 140.4 (dm, 1JC-F=209 Hz, CF), 147.3 (dm, 1JC-F
227 Hz, CF).
=
=
2: Yield: 184 mg (65%). Anal. Calcd. for C24H10BF10P: C,
54.38; H, 1.90; Found: C, 55.32; H, 2.30. Crystals were grown by
slow evaporation from a 1:1 dichloromethane:pentane solution.
1H NMR (CD2Cl2): 7.26 (4H, t, 3JH-H = 8 Hz, m-C6H5), 7.41
(6H, m, o-C6H5, p-C6H5). 19F NMR (CD2Cl2): -121.3 (m, 4F,
o-C6F5), -156.3 (t, 3JF-F =20 Hz, 2F, p-C6F5), -164.2 (m, 4F,
m-C6F5). 31P NMR (CD2Cl2): -0.8 (s). 11B NMR (CD2Cl2):
-2.2 (t, 1J P-B=66 Hz). 13C{1H} NMR (CD2Cl2) partial: 127.8
˚
stored over molecular sieves (4 A), purchased from Aldrich
Chemical Co. and dried at 140 °C under vacuum for 24 h prior to
use. Deuterated solvents were dried over Na/benzophenone
(C6D6, C7D8) or CaH2 (CD2Cl2, CDCl3) and distilled prior to
use. All common organic reagents were purified by conventional
(32) Hausdorf, S.; Baitalow, F.; Wolf, G.; Mertens, F. O. R. L. Int. J.
Hydrogen Energy 2008, 33, 608.
1
(m, C6H5), 129.2 (d, JP-C = 32 Hz, PC), 130.8 (C6H5), 134.3
1
(C6H5), 137.0 (dm, JC-F = 228 Hz, CF), 140.3 (dm, JC-F
1
(33) Ramachandran, P. V.; Gagare, P. D. Inorg. Chem. 2007, 46, 7810.
(34) Davis, B. L.; Dixon, D. A.; Garner, E. B.; Gordon, J. C.; Matus,
M. H.; Scott, B.; Stephens, F. H. Angew. Chem., Int. Ed. 2009, 48, 6812.
(35) Spies, P.; Erker, G.; Kehr, G.; Bergander, K.; Froehlich, R.;
Grimme, S.; Stephan, D. W. Chem. Commun. 2007, 5072.
(36) Spikes, G. H.; Fettinger, J. C.; Power, P. P. J. Am. Chem. Soc. 2005,
127, 12232.
=
242 Hz, CF), 146.8 (dm, 1JC-F =239 Hz, CF).
3: Yield: 160 mg (61%). Anal. Calcd. for C20H18BF10P: C,
49.01; H,3.70; Found: C, 48.16; H, 3.54. Crystals were grown
3
from hexanes at -35 °C.1H NMR (CD2Cl2): 1.40 (d, JP-H
=
15 Hz). 19F NMR (CD2Cl2): -130.7 (m, 4F, o-C6F5), -156.0 (t,
F-F =23 Hz, 2F, p-C6F5), -163.4 (m, 4F, m-C6F5). 31P NMR
(CD2Cl2): 120.7 (br m). 11B NMR (CD2Cl2): 41.8 (d, 1J P-B
150 Hz). 13C{1H} NMR (CD2Cl2) partial: 32.9 (CH3), 39.6
(37) Stephan, D. W. Org. Biomol. Chem. 2008, 6, 1535.
(38) Stephan, D. W. Dalton Trans. 2009, 3129.
J
=
(39) Stephan, D. W.; Erker, G. Angew. Chem., Int. Ed. 2010, 49, 46.
€
(40) Axenov, K. V.; Kehr, G.; Frohlich, R.; Erker, G. J. Am. Chem. Soc.
1
1
2009, 131, 3454.
(d, JP-C = 23 Hz, PC), 115.9 (m, BC), 137.5 (dm, JC-F
253 Hz, CF), 140.9 (dm, 1JC-F=253 Hz, CF), 143.0 (dm, 1JC-F
239 Hz, CF).
=
=
(41) Chase, P. A.; Jurca, T.; Stephan, D. W. Chem. Commun. 2008, 1701.
(42) Chase, P. A.; Welch, G. C.; Jurca, T.; Stephan, D. W. Angew. Chem.,
Int. Ed. 2007, 46, 8050.
4: Yield: 235 mg (83%). Anal. Calcd. for C24H22BF10P: C,
53.16; H, 4.09; Found: C, 52.28; H, 4.20. Crystals were grown
from hexanes at -35 °C. 1H NMR (CD2Cl2): 1.15 (tt, J=13, J=3,
(43) Geier, S. J.; Chase, P. A.; Stephan, D. W. Chem. Commun. 2010, 46, 4884.
€
(44) Spies, P.; Schwendemann, S.; Lange, S.; Kehr, G.; Frohlich, R.;
Erker, G. Angew. Chem., Int. Ed. 2008, 47, 7543.
(45) Sumerin, V.; Schulz, F.; Atsumi, M.; Wang, C.; Nieger, M.; Leskela,
M.; Repo, T.; Pyykko, P.; Rieger, B. J. Am. Chem. Soc. 2008, 130, 14117.
(46) Wang, H. D.; Frohlich, R.; Kehr, G.; Erker, G. Chem. Commun.
2008, 5966.
2H), 1.25 (m, J=13 Hz, J=3 Hz, 4H), 1.49 (m, J=13 Hz, JP-H
=
5 Hz, J=3 Hz, 4H), 1.64 (d, J=13 Hz, 2H), 1.75 (dd, J=13 Hz,
JP-H=3 Hz, 4H), 1.99 (d, J=13 Hz, 4H), 2.33 (dtt, J=13 Hz,
(47) Gilbert, T. M.; Bachrach, S. M. Organometallics 2007, 26, 2672.
(48) Pestana, D. C.; Power, P. P. J. Am. Chem. Soc. 1991, 113, 8426.
(49) Geier, S. J.; Gilbert, T. M.; Stephan, D. W. J. Am. Chem. Soc. 2008,
130, 12632.
(50) Parks, D. J.; Piers, W. E.; Yap, G. P. A. Organometallics 1998, 17,
5492.

338 Inorganic Chemistry, Vol. 50, No. 1, 2011
Geier et al.
19F NMR (CD2Cl2): -131.0 (m, 4F, o-C6F5), -159.4 (t, 3JF-F
JP-H =9 Hz,, J=3 Hz, 2H). 19F NMR (CD2Cl2): -130.78 (m,
4F, o-C6F5), -155.46 (t, 3JF-F = 20 Hz, 2F, p-C6F5), -163.51
(m, 4F, m-C6F5). 31P NMR (CD2Cl2): 92.1 (br m). 11B NMR
(CD2Cl2): 39.5 (d, 1JB-P = 142 Hz). 13C{1H} NMR (CD2Cl2):
=
P
20 Hz, 2F, p-C6F5), -164.7 (t, 3JF-F =17 Hz, 4F, m-C6F5). 31
NMR (CD2Cl2): 7.1 (br m). 11BNMR (CD2Cl2): -28.1 (d,1JP-B
=
68 Hz). 13C{1H} NMR (CD2Cl2) partial: 25.4(CH2), 26.7 (m,
CH2), 29.2, (d, J=17 Hz, CH2), 29.7 (d, J=35 Hz, CH), 136.6
(m, 1JC-F =185 Hz, CF), 148.0 (dm, 1JC-F =237 Hz, CF).
10: Yield: 70 mg (78%). Anal. Calcd. for C30H24BF10P: C,
25.4 (C6H11), 26.8 (d, 2JC-P = 34 Hz, C6H11), 33.7 (d, 3JC-P
=
4 Hz, C6H11), 35.0 (d, JC-P = 27 Hz, PC), 113.1 (BC), 137.6
1
1
(dm, JC-F = 260 Hz, CF), 141.0 (dm,1JC-F = 264 Hz, CF),
1
145.2 (dm, 1JC-F =247 Hz, CF).
5: Yield: 220 mg (68%). Anal. Calcd. for C30H22BF10P: C,
58.47; H, 3.93; C, 57.82; H, 3.91. H NMR (CD2Cl2): 2.24 (s,
12H, o-CH3), 2.26 (s, 6H, p-CH3), 6.88 (s, 4H, CH), 7.06 (dd,
1JH-P=402 Hz, 3JH-H=13 Hz, 1H, PH). 19F NMR (CD2Cl2):
-131.4 (m, 4F, o-C6F5), -158.7 (t, 3JF-F =20 Hz, 2F, p-C6F5),
1
58.66; H, 3.61; Found: C, 57.51; H, 3.53. H NMR (CD2Cl2):
2.25 (s, 6H, p-CH3), 2.29 (s, 12H, o-CH3), 6.89 (d, 4JP-H=6 Hz,
4H, CH). 19F NMR (CD2Cl2)δ: -131.2 (m, 4F, o-C6F5), -154.6
-164.8 (m, 4F, m-C6F5). 31P NMR (CD2Cl2): -39.0 (br m). 11
B
1
NMR (CD2Cl2): -25.4 (d, JP-B = 48 Hz). 13C{1H} NMR
(CD2Cl2) partial: 20.7 (CH3), 21.6 (CH3), 118.0 (d, 1JC-P=58 Hz,
CP), 130.5 (d, J=9 Hz, CH), 136.9 (dm, 1JC-F =245 Hz, CF),
3
(t, JF-F = 20 Hz, 2F, p-C6F5), -163.5 (m, 4F, m-C6F5). 31P
NMR (CD2Cl2, 121 MHz): 29.3 (br m). 11B NMR (CD2Cl2):
40.1 (br m). 13C{1H} NMR (CD2Cl2) partial: 20.9 (p-CH3), 22.6
(d, 3JC-P =7.7 Hz, o-CH3), 123.1 (d, JC-P =72 Hz, PC), 129.3
(d,3 JC-P =11 Hz, CH), 137.2 (1JC-F =246 Hz, CF), 141.2 (d,
1JC-F = 252 Hz, CF), 141.4 (d, 4JC-P = 3 Hz, p-CCH3), 143.6
(d,2JC-P = 7 Hz, o-CCH3), 146.0 (d, 1JC-F = 248 Hz, CF).
Synthesis of R2(H)PB(H)(C6F5)2 (R=Et 6, Ph 7, tBu 8, Cy 9,
Mes 10). These compounds were prepared in a similar fashion
and thus only one preparation is detailed. A solution of Et2PH
(12 mg, 0.15 mmol) in toluene (1 mL) was added to (C6F5)2BH
(50 mg, 0.15 mmol) in hexanes (2 mL). The mixture was stirred
for 1 h, then stored at -35 °C for 2 h. The solution was decanted
and the white precipitate 6 was dried in vacuo.
142.4 (p-C-CH3), 143.0 (d, J=8 Hz, o-C-CH3), 148.4 (dm, 1JC-F
240 Hz, CF).
=
Alternate Generation of 8 and 9. These compounds were
prepared in a similar fashion and thus only one preparation is
detailed. (A) A resealable NMR tube was charged with 3 (20 mg,
0.041 mmol) and tol-d8 (0.75 mL); the solution was subjected to
3 freeze-pump-thaw cycles, and 1 atm of H2 was added at 77 K
(∼4 atm at room temperature). 80% conversion to 8 was achieved
in 4 weeks at 25 °C, while quantitative conversion was achieved
over 48 h at 60 °C. (B) An NMR tube was charged with 3 (20 mg,
0.041 mmol), Me2NH-BH3 (2 mg, 0.04 mmol), and CD2Cl2
1
(0.75 mL) After 15 min at 25 °C, H, 11B, 31P, and 19F NMR
6: Yield: 48 mg (77%). Anal. Calcd. for C16H12BF10P: C,
44.07; H, 2.77; Found: C, 43.80; H, 2.73. Crystals were grown
spectroscopy revealed quantitative conversion to 8 and 0.5
(Me2NBH2)2.
from hexanes at -35 °C. 1H NMR (CD2Cl2): 1.17 (dt, 3JH-P
=
=
17 Hz, 2JH-H=8 Hz, 6H, CH3), 1.84 (dq, 2JP-H=24 Hz, 2JP-H
Synthesis of Cy2PB(C6F5)2(4-tBuC5H4N) 11. A solution of 3
(25 mg) in CDCl3 (0.75 mL) was added to 4-tert-butylpyridine
(1 equiv). The solution was monitored by multinuclear NMR
spectroscopy.
8 Hz, 4H, CH2), 3.43 (br m, 1H, BH), 4.95 (dm, 1JP-H=388 Hz,
1H, PH). 19F NMR (CD2Cl2): -131.7 (m, 4F, o-C6F5), -159.2
3
(t, JF-F = 20 Hz, 2F, p-C6F5), -164.7 (m, 4F, m-C6F5).
1
31P NMR (CD2Cl2): -4.6 (br m). 11B NMR (CD2Cl2): -30.0
11: H NMR (CDCl3): 0.41-2.00 (m, 22H, C6H11), 1.41 (s,
9H, C(CH3)3), 7.32 (d, 3JH-H =7 Hz, 2H, CH), 9.05 (br s, 2H,
CH). 19F NMR (CDCl3): -128.0 (br m, 4F, o-C6F5), -157.2 (br
s, 2F, p-C6F5), -162.2 (m, m-C6F5). 13C NMR (CDCl3, partial):
26.5, 27.6 (d, JP-C= 6 Hz), 28.2 (JP-C=13 Hz), 30.1, 31.7 (ov),
35.8, 120.8, 122.6, 146.5 (d, JP-C = 20 Hz), 167.6. 31P NMR
(CDCl3): -28.3 (br s). 11B NMR (CDCl3): -1.3. Using toluene
(1 mL) as the solvent gave X-ray quality crystals. Rapid
decomposition precluded characterization by elemental analysis
and 13C NMR spectroscopy.
(d, 1JB-P=65Hz). 13C{1H} NMR (CD2Cl2) partial: 8.5 (d, JC-P
=
6 Hz), 10.6 (d, JC-P=38.5 Hz), 137.1 (dm, 1JC-F=208 Hz, CF),
140.3 (dm, 1JC-F=235 Hz, CF), 148.0 (dm, 1JC-F=233 Hz, CF).
7: Yield: 58 mg (74%). Anal. Calcd. for C2 4H12BF10P: C,
54.17; H, 2.27; Found: C, 53.82; H, 2.25. Crystals were grown
from 1:1 dichloromethane:hexanes at -35 °C. 1H NMR (CD2Cl2):
1
3
3.94 (br m, 1H, BH), 6.81 (ddm, JP-H = 409 Hz, JH-H
15 Hz, 1H, PH), 7.44 (ddd, 3JH-H=8 Hz, 3JH-H=6 Hz, 4JH-P
=
=
1 Hz, 4H, m-C6H5), 7.54 (tt, 3JH-H =6 Hz, 4JH-H =2 Hz, 2H,
p-C6H5), 7.60 (ddd, 3JH-P=12 Hz, 3JH-H=8 Hz, 4JH-H=2 Hz,
3H, o-C6H5). 19F NMR (CD2Cl2): -131.2 (m, 4F, o-C6F5),
-158.8 (t, 2F, 3JF-F J=20 Hz, p-C6F5), -164.7 (m, 4F, m-C6F5).
31P NMR (CD2Cl2,): -1.5 (br m). 11B NMR (CD2Cl2): -28.5
Generation of (R2PBCl2)2 (R = tBu 12, Cy 13). These com-
pounds were prepared in a similar fashion and thus only one
preparation is detailed. BCl3 (1 equiv) is added to a solution of 3
(25 mg) in toluene. The solution was allowed to stir for 3 h.
Volatiles were then removed in vacuo, and the residue was taken
up in CDCl3 for NMR spectroscopy. Resonances attributed to
ClB(C6F5)2 and species 12 were observed. The former resonance
1
(d, JP-B = 71 Hz). 13C{1H} NMR (CD2Cl2) partial: 122.9
1
2
(d, JC-P = 65 Hz, CH), 129.5 (dm, JC-P = 164 Hz, CH),
133.7(CH), 135.3 (CH) 136.9 (dm, 1JC-F = 248 Hz, CF), 140.7
(dm, 1JC-F =254 Hz, CF), 148.0 (dm, 1JC-F = 235 Hz, CF).
8: Yield: 48 mg (65%). Anal. Calcd. for C20H20BF10P: C,
48.81; H, 4.10; Found: C, 48.42; H, 3.90. Crystals were grown
50,51
corresponded to those previously reported ClB(C6F5)2
Although crystals of compound 12 were obtained for X-ray
diffraction, efforts to obtain bulk samples of 12 and 13 from
these reactions for elemental analysis proved problematic as sepa-
ration from ClB(C6F5)2 was not possible.
from the hexane wash layer. 1H NMR (CD2Cl2): 1.27 (d, 3JH-P
=
14 Hz, 18H, CH3), 3.48 (br m, 1H, BH), 4.84 (dd, 1JH-P=375 Hz,
3JH-H = 11 Hz, 1H, PH). 19F NMR (CD2Cl2): -129.7 (m, 4F,
o-C6F5), -159.4 (t, 3JF-F =20 Hz, 2F, p-C6F5), -164.7 (m, 4F,
m-C6F5). 31P NMR (CD2Cl2): 32.0 (br m). 11B NMR (CD2Cl2):
-30.0 (d, 1JP-B=48 Hz). 13C{1H} NMR (CD2Cl2): 29.2 (CH3),
(12) 1H NMR (CDCl3): 1.33 (d, 3JP-H=15 Hz, 9H, C(CH3)3).
1
31P NMR (CDCl3): -14.8 (sept, JP-B = 90 Hz). 11B NMR
=
(CDCl3): 4.2 (t, 1JP-B = 90 Hz, (tBu2PBCl2)2), 2.7 (d, 1JP-B
135 Hz, tBu2PBCl2).
1
(13) H NMR (CDCl3): 1.11-2.58 (m, 22H, Cy). 31P NMR
(CDCl3): -14.5 (sept, 1JP-B=99 Hz). 11B NMR (CDCl3): -0.5
(t, 1JP-B =99 Hz).
33.0 (d, 1JC-P=29 Hz, CP), 117.9 (br m, BC), 137.1 (dm, 1JC-F
255 Hz, CF), 139.6 (dm, 1JC-F=250 Hz, CF), 148.0 (dm, 1JC-F
239 Hz, CF).
=
=
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 an N2 stream, thus maintaining a dry,
O2-free environment for each crystal. The data were collected on
9: Yield: 61 mg (77%). Anal. Calcd. for C24H24BF10P: C,
52.97; H, 4.45; Found: C, 52.50; H, 4.56. Crystals were grown
from hexanes at -35 °C. 1H NMR (CD2Cl2): 1.15 (m, 2H,
PC6H11), 1.20-1.29 (br m, 6H, PC6H11), 1.37 (m, 2H, PC6H11),
1.68 (br d, 2JH-H =13 Hz, 2H, PC6H11), 1.75-1.84 (br m, 6H,
PC6H11), 1.89 (m, 2H, PC6H11), 2.00 (m, 2H, PC6H11), 3.33 (1H,
br m, BH), 4.78 (ddm, 1JP-H=381 Hz, 3JH-H=13 Hz,1H, PH).
(51) Parks, D. J.; Spence, R. E. V. H.; Piers, W. E. Angew. Chem., Int. Ed.
Engl. 1995, 34, 809.

Article
Inorganic Chemistry, Vol. 50, No. 1, 2011 339
Table 1. Crystallographic Data for 1-4, 6-8, 12, and 14a
1d
2 CH2Cl2
3
3d
4d
6d
d
formula
C32H20P2F20B2
868.04
triclinic
P1
9.699(2)
9.703(2)
10.404
67.072(2)
80.770(3)
67.852(2)
835.1(3)
1
1.726
0.269
2933
2274
253
C50H22P2F20B2Cl2
1131.10
triclinic
P1
10.3197(16)
12.0982(19)
20.926(3)
74.492(2)
76.700(2)
68.458(2)
2316.1(6)
2
1.622
0.327
8128
3801
676
0.0573
0.1347
0.919
C20H18BF10
490.12
orthorhombic
Pbca
12.194(9)
18.331(13)
19.774(14)
90.00
90.00
90.00
P
C24H22BF10
P
C16H12BF10
436.04
triclinic
P1
9.198(6)
12.989(8)
15.681(9)
83.854(6)
87.955(7)
75.462(6)
1803.0(19)
4
1.606
0.249
6329
5186
521
P
formula weight
crystal system
space group
542.20
triclinic
P1
9.4503(13)
10.4530(14)
13.7545(19)
110.6870(10)
99.036(2)
95.566(2)
1238.1(3)
2
1.454
0.197
4354
2870
325
0.0631
0.2154
1.043
˚
a (A)
˚
b (A)
˚
c (A)
R (deg)
β (deg)
γ (deg)
3
˚
V (A )
4420(5)
8
1.473
0.212
3891
2640
289
0.0553
0.1889
1.059
Z
d(calc) g cm-1
abs coeff, μ, cm-1
data collected
data Fo2 > 3σ(Fo
variables
Rb
2
)
0.0390
0.0942
1.035
0.0493
0.1449
1.050
c
Rw
goodness of fit
7e
8d
11e
13e
formula
C24H22BF15
P
C20H20BF10
492.14
triclinic
P1
9.5420(17)
12.872(2)
19.447(3)
96.656(2)
95.583(2)
109.144(2)
2217.9(2)
4
1.474
0.212
7778
2777
594
P
C40H43BF10NP
769.53
triclinic
P1
9.8554(5)
12.3799(6)
16.4730(8)
72.596(2)
78.921(2)
86.032(2)
1881.95(16)
2
1.358
0.153
19836
13490
478
0.0441
0.1333
1.038
C16H36B2Cl4P2
453.81
monoclinic
P21/n
8.8289(3)
14.3784(5)
9.0538(3)
90.00
formula weight
crystal system
space group
542.20
triclinic
P1
˚
a (A)
8.9820(6)
10.3690(6)
12.4150(6)
106.381(3)
94.690(3)
96.226(3)
1095.11(1)
2
1.644
0.223
4947
3199
333
˚
b (A)
˚
c (A)
R (deg)
β (deg)
γ (deg)
92.991(1)
90.00
3
˚
V (A )
1147.77(7)
2
1.313
0.654
2633
2416
115
0.0277
0.0770
1.050
Z
d(calc) g cm-1
abs coeff, μ, cm-1
data collected
data Fo2 > 3σ(Fo
variables
Rb
2
)
0.0573
0.1834
1.078
0.0949
0.2839
0.952
c
Rw
goodness of fit
P
P
temperature factor of the C-atom to which they are bonded. The
P
P
All data were collected with Mo KR radiation (λ = 0.71069 A). b R = ||Fo| - |Fc||/ |Fo|. c Rw = { w(Fo2 - Fc2)2/ [w(Fo)2]}1/2
.
d These data
a
˚
were collected at 293 K. e These data were collected at 150 K.
a Bruker Apex II diffractometer (Table 1). The data were col-
lected at 150((2) K for all crystals. The frames were integrated
with the Bruker SAINT software package using a narrow-frame
algorithm. Data were corrected for absorption effects using the
empirical multiscan method (SADABS).
H-atom contributions were calculated, but not refined. The
locations of the largest peaks in the final difference Fourier
map calculation as well as the magnitude of residual electron
densities in each case were of no chemical significance. Addi-
tional details are provided in the Supporting Information (Table 1).
Computational Methods. Optimizations were performed with
the GAUSSIAN (G98) suite.54 Phosphinoboranes and phosphine-
boranes were first optimized without constraints at modest com-
putational levels, typically HF/3-21G or HF/BS0, where the
BS0 basis set uses the 6-31þG(d) basis set on non-hydrogen atoms
and the 3-21G basis set on hydrogens. Examination of the optimized
Structure Solution and Refinement. Non-hydrogen atomic
scattering factors were taken from the literature tabulations.52
The heavy atom positions were determined using direct methods
employing the SHELXTL direct methods routine.53 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 refine-
ment, all non-hydrogen atoms were assigned anisotropic tem-
perature 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
(54) 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. Gaussian 98, Revision A.11.4; Gaussian, Inc: Pittsburgh, PA, 1998.
˚
which they are bonded assuming a C-H bond length of 0.95 A.
H-atom temperature factors were fixed at 1.10 times the isotropic
(52) Cromer, D. T.; Waber, J. T. Int. Tables X-Ray Crystallogr. 1974, 4,
71.
(53) Sheldrick, G. M. Acta Crystallogr. 2008, A64, 112.

340 Inorganic Chemistry, Vol. 50, No. 1, 2011
Scheme 1. Synthesis of 1-10
Geier et al.
structures by analytical frequency analyses at these levels dem-
onstrated them to be minima (no imaginary frequencies). The
structures were reoptimized at higher levels, either at the density
functional theory (DFT)-based MPW1K55 /6-31þG(d) level, or
using a two-layer ONIOM56,57 approach (denoted ONIOM-
(MPW1K/6-31þG(d)), using the MPW1K/6-31þG(d) model
for high layers, and the MPW1K/3-21G model for low layers.
Partitioning of the layers for B(C6F5)3 and PR3 moieties was
described previously.58 In some cases, to assess the dependence
of structural parameters on model and basis set, different ONIOM
combinations were employed for further optimizations, includ-
ing MPW1K/6-311þG(d):MPW1K/3-21G (denoted ONIOM-
(MPW1K/6-311þG(d)), PBE1PBE/6-311þþG(d, p):HF/3-21G
(denoted ONIOM(PBE1/6-311þþG(d, p), and PBE1PBE/
6-311þþG(2df, p):HF/3-21G (denoted ONIOM(PBE1/
6-311þþG(2df, p)). Natural Bond Order (NBO) calculations
were performed using a upgraded version of the NBO subroutine
in the Gaussian98 program,59,60 using the MPW1K/BS0-optimized
structures and wave functions.
Figure 1. POV-Ray depictions of phosphinoborane dimers (a) 1 and
(b) 2. C, black; B, yellow-green; F, deep pink; P, orange. Solvent and
hydrogen atoms are omitted for clarity. Selected metrical parameters
˚
(distance A, angle deg) 1: B(1)-P(1) 2.056(3), B(1)-P(1a) 2.058(3),
B(1)-P(1)-B(1a) 92.30(10), P(1)-B(1)-P(2) 87.70(10). 2: P(1)-B(1)
2.096(5), P(1)-B(2) 2.088(5), P(2)-B(1) 2.022(5), P(2)-B(2) 2.080(5),
B(1)-P(1)-B(2) 92.65(19), B(1)-P(2)-B(2) 95.05(18), P(1)-B(1)-P(2)
86.57(18), P(1)-B(2)-P(2) 85.32(18).
7.82 ppm for 1 and 2, respectively. These latter data are typical
of neutral, 4 coordinate boron centers, consistent with the
postulate of the dimeric formulations, [R2PB(C6F5)2]2 (R =
Et 1, Ph 2).61,62 These structural assignments were addition-
ally supported by X-ray crystallography (Figure 1).
The crystal structure of 1 reveals a dimer with crystal-
lographic 2-fold symmetry. The P-B distances are 2.056(3)
Scans of the potential energy surface for H2 loss from phosphine-
boranes (F5C6)2(H)BP(H)(R)2 employed a slightly different
ONIOM(MPW1K/6-311þþG(d, p)) approach, with the high layer
encompassing the B and P atoms and the H atoms bound to
them. The 6-311þþG(d, p) basis set was used in the high layer,
the 3-21G basis set in the low layer. Relative energies from the scan
steps were corrected using unscaled zero point energies (ZPEs)
from the frequency analyses. Optimized Cartesian coordinates
of molecules studied are available as Supporting Information.
˚
and 2.058(3) A, while the B-P-B and P-B-P bond angles
are 92.30(10)° and 87.70(10)° respectively. The structure of 2
shows similar metrical parameters, with average P-B bond
˚
lengths of 2.072 A, average P-B-P angle of 85.95° and
average B-P-B angle of 93.87°. The data for the related
63
ꢀ
˚
ꢀ
˚
species [(Et2P)2B(μ-PEt2)]2 (2.012(6) A, 2.033(5) A), [iBu-
(H)B(μ-P(tBu)2)]2 (2.004(4), 2.022(4) A)64 and 1,1-ferrocene-
˚
65
ꢀ
˚
ꢀ
˚
[B(CH3)(μ-PPh2)]2 (2.036(4) A, 2.033(4) A) show P-B
bond lengths that are slightly shorter than those reported for
the present compounds. This is presumably due to crowding
resulting from the larger substituents at boron.
Results and Discussion
The phosphinoboranes R2PB(C6F5)2 (R=Et 1, Ph 2) were
synthesized from the reaction of (C6F5)2BCl and the corre-
sponding lithium phosphide (Scheme 1). The products were
isolated in 84 and 65% yield, respectively. Compounds 1 and
Reaction of the more sterically demanding lithium phos-
phides R2PLi (R=tBu, Cy and Mes) with (C6F5)2BCl resulted
in the generation of phosphinoborane monomers R2PB(C6F5)2
(R=tBu 3, Cy 4, Mes 5) (Scheme 1). These formulations are
consistent with NMR spectroscopic data. In particular, the
11B NMR spectra show upfield doublet resonances, typical of
3-coordinate boranes, that exhibit coupling to phosphorus
2 are characterized by triplet resonances at -12.9 (1JP-B
=
72 Hz) and -2.2 (1JP-B=68 Hz) ppm in the 11B NMR spectra,
respectively. In addition, the 19F NMR spectra show gaps
between the meta- and para-fluorine signals of 6.85 and
(55) Lynch, B. J.; Truhlar, D. G. J. Phys. Chem. A 2001, 105, 2936.
(56) Vreven, T.; Morokuma, K. J. Comput. Chem. 2000, 21, 1419.
ꢀ
(57) Dapprich, S.; Komaromi, I.; Byun, K. S.; Morokuma, K.; Frisch,
(61) Horton, A. D.; deWith, J. Organometallics 1997, 16, 5424.
M. J. J. Mol. Struc. (Theochem) 1999, 461-462, 1.
(62) Blackwell, J. M.; Piers, W. E.; Parvez, M. Org. Lett. 2000, 2, 695.
€
(58) Gille, A. L.; Gilbert, T. M. J. Chem. Theory Comput. 2008, 3, 1681.
(59) Glendening, E. D.; Badenhoop, J. K.; Reed, A. E.; Carpenter, J. E.;
Bohmann, J. A.; Morales, C. M.; Weinhold, F. NBO 5.0.; Theoretical
Chemistry Institute, University of Wisconsin: Madison, WI, 2001.
(60) Reed, A. E.; Curtiss, L. A.; Weinhold, F. Chem. Rev. 1988, 88, 899.
(63) Noth, H. Z. Anorg. Allg. Chem. 1987, 555, 79.
(64) Karsch, H. H.; Hanika, G.; Huber, B.; Riede, J.; Mueller, G.
J. Organomet. Chem. 1989, 361, C25.
(65) Jakle, F.; Mattner, M.; Priermeier, T.; Wagner, M. J. Organomet.
Chem. 1995, 502, 123.

Article
Inorganic Chemistry, Vol. 50, No. 1, 2011 341
(1JB-P∼150 Hz). The 31P NMR resonances are also upfield
and significantly broadened because of coupling to the quad-
rupolar nucleus of the boron atom. Interestingly the gaps
between para- and meta-fluorine resonances in the 19F NMR
spectra lie closer to the range for typical neutral 4-coordinate
boranes, suggesting that there is substantial electron density
being donated into the vacant p-orbital on boron from the
lone pair at phosphorus.
The structures of 3 and 4 were confirmed by X-ray crystal-
lography (Figure 2). In compound 3 both phosphorus and
boron centers are essentially planar, with sums of angles at
359.1° and 360.0°, although these planes are not coplanar,
but canted with respect to each other with C-P-B-C torsion
angles of 21.6° and 7.4°. Compound 3 shows a significantly
˚
shorter P-B bond length (1.786(4) A) than the sterically
˚
similar compound Mes2BPtBu2 (1.841 A) suggesting the
electron-withdrawing effect of the C6F5 groups result in a
contraction of the P-B bond length. Compound 4 shows an
˚
even shorter P-B bond length of 1.762(4) A, presumably
a result of a stronger P-B π-bond. This is reflected in
the smaller C-P-B-C torsion angles 1.1° and 5.6°. The
P-B bonds in the present compounds are much shorter than
those observed for the analogous compounds, tBu2PBMes2
48
48
˚
˚
(1.839(8), 1843(8) A), Mes2PBMes2 (1.839(8) A),
Figure 2. POV-Ray depictions of phosphinoboranes (a) 3 and (b) 4. C,
black; B, yellow-green; F, deep pink; P, orange. Hydrogen atoms are
48,66
˚
Ph2PBMes2 (1.859(3) A),
consistent with significant
˚
omitted for clarity. Selected metrical parameters (distance A, angle deg).
P-B π-bonding. Literature data formulate P-B double
˚
3: P(1)-B(1) 1.786(4), P(1)-C(13) 1.862(3), P(1)-C(17) 1.865(3), B(1)-
C(1) 1.580(4), B(1)-C(7) 1.590(4), C(13)-P(1)-C(17) 117.08, B(1)-
P(1)-C(13) 120.85(14), B(1)-P(1)-C(17) 121.14(15), C(1)-B(1)-C(7)
113.3(2), C(1)-B(1)-P(1) 124.0(2), C(7)-B(1)-P(1) 122.7(2). 4: P(1)-
B(1) 1.762(4), P(1)-C(13) 1.820(4), P(1)-C(19) 1.835(4), B(1)-C(1)
1.585(5), B(1)-C(7) 1.596(5), C(13)-P(1)-C(19) 112.6(2), B(1)-P(1)-
C(13) 126.4(2), B(1)-P(1)-C(19) 120.88(19), C(1)-B(1)-C(7) 117.0(3),
C(1)-B(1)-P(1) 119.9(3), C(7)-B(1)-P(1) 123.1(3).
bonds with P-B distances in the range 1.79-1.84 A, and
P-B single bonds from 1.90 to 2.00 A.67 This infers that 3 and
˚
4 have significant P-B multiple bonding character. Thus
it appears that the steric bulk about P and B precludes
dimerization of 3-5 and encourages planarity and thus
P-B π-bonding.
The relationships between B-P distance, P pyramidality,
and the extent of BP multiple bonding were further explored
computationally. The ONIOM(MPW1K/6-31þG(d))-
of the molecule.69,58,70-72 This in turn implies that the solid
state B-P distance cannot be finely correlated with B-P
bond order. Thus the seemingly notable changes in structural
parameters reflect small energy differences. It is likely that the
structural differences in B-P bond lengths and P planarities
between experiment and theory for 3 are due to solid state
effects such as crystal packing forces distorting the molecules
from the gas phase ideal.
˚
optimized structure of 3 exhibited a B-P distance of 1.806 A,
somewhat longer than that observed crystallographically,
but supporting the view that the electron withdrawing ability
of the fluoroarene substituents shortens the interaction. The
C-P-B-C torsion angles varied with the model, but were
nonetheless consistent with the canting observed crystallog-
raphically, averaging about 25° and 10° for the larger and
smaller torsions, respectively. The sum of the angles around
phosphorus was about 355.5°, in fair agreement with the
crystallographic result; however, the more direct measure of
phosphorus pyramidality, the “plane angle”,68 was predicted
to be about 151° whereas that determined crystallographi-
cally was 169.0°. For further comparison, the plane angles
for (F3C)2BdPtBu2 and Mes2B=PMes2 were predicted to
be 164-180°;47 and 179.9°, respectively. The plane angle
is known to be a “soft” parameter, capable of changing
significantly with only a modest energy cost. To probe the
effect of the plane angle on the B-P distance and the
energetics, 3 was optimized with the plane angle fixed at
180°. This structure was 0.9 kcal/mol less stable than the
unrestricted one and exhibited a much smaller B-P distance
Similar studies of 4 do not support the crystallographic
suggestion of a shorter, stronger P-B bond in this molecule;
the ONIOM(MPW1K/6-31þG(d)) approach predicts a dis-
˚
tance of 1.816 A, slightly longer than that in 3. Moreover,
computationally, it is not observed that the smaller substit-
uents on phosphorus in 4 allow greater planarity for the
C2BPC2 core atoms. The lowered bulk around phosphorus
causes its geometry to become notably pyramidal as the plane
angle in 4 is predicted to be about 137° while in the solid state,
the plane angle is observed to be 175°. As above, an opti-
mization of 4 with a fixed plane angle of 180° was performed
at the ONIOM(MPW1K/6-31þG(d)) level. It exhibited a
˚
B-P distance of 1.777 A, comparable to the solid state value,
and was only 2.3 kcal/mol less stable than the fully optimized
˚
of 1.785 A. This demonstrates that planarity at phosphorus
(69) Indeed, in phosphinoboranes with small substituents, the conformer
with planar phosphorus is a transition state between versions with pyramidal
phosphorus, with corresponding decreased stability.
affects the B-P distance, but without increasing the stability
(70) Vogel, U.; Hoemensch, P.; Schwan, K.-C.; Timoshkin, A. Y.; Scheer,
(66) Feng, X.; Olmstead, M. M.; Power, P. P. Inorg. Chem. 1986, 25, 4615.
M. Chem. Eur. J. 2003, 9, 515.
;
€
(67) Paine, R. T.; Noth, H. Chem. Rev. 1995, 95, 343.
(71) Allen, T. L.; Scheiner, A. C.; Schaefer, H. F., III Inorg. Chem. 1990,
29, 1930.
(68) The plane angle is the angle between the B-P axis and the bisector of
the angle formed by the P atom and the two C atoms attached to it.
(72) Coolidge, M. B.; Borden, W. T. J. Am. Chem. Soc. 1990, 112, 1704.

342 Inorganic Chemistry, Vol. 50, No. 1, 2011
Geier et al.
version. As above, the differences are on the order of crystal
packing energies, so the crystallographic observations cannot
be taken as clear evidence of increased or decreased BP bond
order. Nonetheless, the data indicate that the BP bond order
in (C6F5)2BdPR2 phosphinoboranes exceeds that in other
R2BdPR2 compounds, making their reactivities of interest.
The data also indicate that the presence of a perfectly planar
phosphorus atom is not a requirement for substantial B-P π
bonding, although greater planarity helps.
To probe the effect of less donating substituents on phos-
phine, the structure of 5 was also optimized. At the ONIOM-
(MPW1K/6-31þG(d)) level, the B-P distance was predicted
˚
to be 1.795 A, and the plane angle to be 151°. This bond
Figure 3. POV-Ray depiction one of two crystallographically indepen-
dent molecules of 6. C, black; H, white; B, yellow-green; F, deep pink; P,
orange. C-bound H-atoms are omitted for clarity. Selected metrical param-
eters (distance A, angle deg). P(1)-B(1) 1.950(3), H(1)-B(1)-P(1)-H(2)
176.76.
distance is shorter than in 3 despite the fact that the plane
angles are comparable. This points again to the energetic
softness of the latter parameter. Moreover, it appears phos-
phorus basicity plays a relatively small role in setting the B-P
distance, given that the PMes2 moiety should be significantly
less basic than the PtBu2 moiety.
˚
NBO analyses of 3 and 4 showed that the π-bonding
HOMOs were highly polarized, with 74% of the molecular
orbital derived from the phosphorus atom, with only 26%
from the boron atom. These values are similar to those
predicted for (F3C)2BdPtBu2, (70 and 30%),47 suggesting
that the C6F5 and CF3 substituents are close in their abilities
to inductively attract electrons from the phosphorus to the
boron. The Wiberg index in the natural atomic orbital (NAO)
basis for 3 is 1.62. This model predicts B-C bond orders of
about 0.85, and P-C bond orders of about 0.9. Bond order
determinations from other models within the NBO frame-
work give comparable predictions, implying that the π bond
in phosphinoboranes like 3-5 is similar to that in (F3C)2Bd
PtBu2; highly polarized toward phosphorus and best de-
scribed as about 70% of a perfectly covalent π bond.
Though the experimental data for compounds 3-5 sug-
gests significant interaction between the lone pair at phos-
phorus and the vacant orbital at boron, the computational
data suggest that the interaction is highly polarized, and thus
subject to chemical attack. To probe this, we examined the
ability of these compounds to participate in chemistry attrib-
uted to “frustrated Lewis pairs” (FLPs). Initially, we con-
sidered the potential for hydrogenation of the P-B bonds. In
preparation for these studies, we prepared and characterized
Figure 4. POV-Ray depictions of (a) 7 and (b) of two crystallographi-
cally independent molecules of 8. C, black; H, white; B, yellow-green; F,
deep pink; P, orange. C-bound H-atoms are omitted for clarity. Selected
metrical parameters (distance A, angle deg). 7: P(1)-B(1) 1.966(3), H(1)-
B(1)-P(1)-H(2) 178.98. 8: P(1)-B(1) 1.966(9); H(1)-B(1)-P(1)-H(2)
166.55.
the Lewis acid-base adducts, R2(H)P B(H)(C6F5)2 (R=Et 6;
3
˚
Ph 7; tBu 8; Cy 9; Mes 10) (Scheme 1) via the reaction of the
phosphines R2PH with the borane HB(C6F5)2. Crystal struc-
tures were obtained for compounds 6 (Figure 3), 7, and 8
(Figure 4). In each case the structures were as anticipated
with pseudotetrahedral P and B centers. Of these adducts, 6
the solid state exhibiting a typically staggered conformation
(H-P-B-H=176.76° and 178.98°, respectively). In contrast,
8 shows more twisting in the solid state from the staggered
conformation with a H-P-B-H torsion angle of 166.55°.
This twisting appears to be a solid-state effect as there is
no evidence of dissymmetric geometry observed by NMR
spectroscopy.
Hydrogenation of compounds 1-5 was explored via
exposure of these species to 4 atm of H2 at both room tem-
perature (25 °C) and at 60 °C. Compounds 1 and 2 showed no
signs of reaction. However at 25 °C reaction of compounds 3
and 4 was observed to give 8 and 9 in 2 and 4 weeks,
respectively (Scheme 1). These products were formed in only
2 days at 60 °C. Compound 5 showed only traces of a new
product after several weeks at room temperature.
˚
has the shortest P-B bond length (1.950(3) A), while 7 and 8
˚
show indistinguishable P-B bond lengths of 1.966(3) A and
˚
1.966(9) A, respectively. These lengths are significantly short-
er than those for the related R2(H)P B(C6F5)3 species (R =
3
73
˚
˚
˚
Cy, 2.0270(14) A, cyclopentyl 2.0243(3) A, Et 2.036(8) A,
74
˚
Ph 2.098(3) A), because of the reduced steric congestion
about B in HB(C6F5)2 compared to that in B(C6F5)3. Inter-
estingly, for 6 and 7, the P-H and B-H hydrogen atoms are
oriented in a trans disposition with respect to one another in
(73) Lancaster, S. J.; Mountford, A. J.; Hughes, D. L.; Schormann, M.;
Bochmann, M. J. Organomet. Chem. 2003, 680, 193.
(74) Welch, G. C.; Prieto, R.; Dureen, M. A.; Lough, A. J.; Labeodan,
O. A.; Holtrichter-Rossmann, T.; Stephan, D. W. Dalton Trans. 2009, 1559.

Article
Computations suggest that the reaction of (CF3)2BPR2
Inorganic Chemistry, Vol. 50, No. 1, 2011 343
species with H2 is exothermic.47 However, the slow reaction
of 5 infers there may be a relatively large kinetic barrier.
Efforts to effect the reverse reaction, dehydrogenation of the
adducts 6-10, were also undertaken. Heating these species
for 2 days at 140 °C showed no loss of H2. These observations
are in contrast to some primary or secondary phosphine
adducts of BH3 which can lose hydrogen under thermal
duress11,12,14-16,24-26 to produce B/P oligomers and polymers.
It appears that the presence of electron-withdrawing groups
at boron in the present case enhances the affinity for H2.
In a related set of reactions compounds 3 and 4 were
reacted with H3NBH3 or Me2(H)NBH3. This also led to the
generation of 8 and 9, respectively (Scheme 1). The nature of
the byproduct from the reaction with H3NBH3 was not clear
because of poor solubility, although they are thought to be
oligomers of formula (H2NBH2)n. Similarly the reactions with
Me2(H)NBH3 produced the known dimer (Me2N-BH2)2.15
These experiments demonstrate the greater affinity for H2
exhibited by compounds 3 and 4. This is due in part to the
enhanced Lewis acidity at boron in 3 and 4 compared to that
of the boron centers of amino-borane. The proton trans-
fer from N to P is aided by the enthalpically favorable for-
mation of BdN double bond, which subsequently oligomerize
or dimerize.
Figure 5. Computed reaction profile (ONIOM(MPW1K/6-311þþG-
(d,p)) for H2 addition/dissociation (energies are given in kcal/mol).
Computational studies at the ONIOM(MPW1K/6-311þþG-
(d, p)) level revealed that H2 activation by 3 to form 8 is
exothermic by 41.4 kcal/mol.49 In contrast, the formations of
the related phosphine-borane species by hydrogenation of
Mes2BPtBu2 and Mes2BPMes2 are exothermic by 23.3 and
11.8 kcal/mol, respectively. This implies that fluoroarene
substituents on B and alkyl groups on P increase the affinity
for hydrogen. Moreover, in this limited data set, it appears
that the former has a greater impact (ca. 18 kcal/mol) than
the latter (ca. 12 kcal/mol).
Figure 6. POV-Ray depiction of 11. C, black; B, yellow-green; F, deep
pink; N, blue; P, orange. H- and solvent atoms are omitted for clarity.
Selected metrical parameters (distance A, angle deg). B(1)-N(1) 1.6332(11),
B(1)-P(1) 2.0329(9), N(1)-B(1)-P(1) 106.04, N(1)-B(1)-C(1) 101.81(6),
N(1)-B(1)-C(7) 109.06(6), C(1)-B(1)-C(7) 113.53(7), P(1)-B(1)-C(1)
115.87(5), P(1)-B(1)-C(7) 109.77(5), B(1)-P(1)-C(13) 101.27(4), B(1)-
P(1)-C(19) 109.43(4), C(13)-P(1)-C(19) 104.20(4).
Kinetic aspects of H2 addition/dissociation were explored
using relaxed scans of the potential energy surfaces for the
processes. Typically a scan began with optimization of the struc-
ture of the phosphine-borane adducts, and involved shortening
the H-H distance until H2 “formed”, then “dissociated”
from the resulting phosphinoborane (Figure 5). Considering
the H2 addition reaction, climbing the barrier to the transi-
tion state is initiated by coordination of H2 to B, followed by
a slip of the coordinated H2 toward P and H-H bond
cleavage with concomitant P-H bond formation. These
steps do not appear to require individual transition states.
Indeed, the transition state region is rather flat and we were
unable to locate a stationary point in the area. Nonetheless,
the scan data give a value for the barrier of about 18 kcal/mol
at the ONIOM(MPW1K/6-311þþG(d, p)) level. This seems
reasonable, if slightly low, for a reaction that occurs very
slowly at 25 °C, but much faster at 60 °C. This barrier energy
for addition of H2 implies that the barrier for dissociation of
bound H2 is nearly 60 kcal/mol. Given the computed
exothermicities and associated barriers to hydrogenation, it
is not surprising that 8-10 do not release hydrogen, even on
heating for extended periods at elevated temperatures.
To probe the ambiphilic nature of the monomeric P-B
compounds, reactions with simple donors and acceptors were
conceived. Initially reactions with donors were considered.
For example, reaction of 4 with 4-tert-butylpyridine shows
evidence of the formation of a new species within 20 min of
mixing. The product 11 exhibits a 11B NMR resonance at
˚
Scheme 2. Synthesis of 11-13
-1.3 ppm while the 31P signal broadens and shifts to -28.3 ppm.
These data suggest B-N adduct formation. The reaction is
quantitative although the product was challenging to isolate,
and decomposition precluded characterization by elemental
analysis. Nonetheless, 11 was characterized crystallographically
(Figure 6, Scheme 2) and confirmed to be Cy2PB(C6F5)2(4-
tBuC5H4N). The crystal structure reveals a dramatic lengthening

344 Inorganic Chemistry, Vol. 50, No. 1, 2011
Geier et al.
these species gave rise to 31P septets at -14.8 and -14.5 ppm
with P-B couplings of 90 and 99 Hz, respectively. These data
were consistent with the formation of the dimers (R2PBCl2)2
(R=tBu 12, Cy 13) (Scheme 2). In addition to these products
the byproduct ClB(C6F5)2 was also observed spectroscopi-
cally. Compound 13 was characterized crystallographically
(Figure 7). Metrical parameters for 13 were similar to those of
1 and 2, with the P-B bond lengths again falling toward the
long end of the range for related P-B dimers63-65 because of
the bulky groups at phosphorus. The analogous combination
of 5 with BCl3 under similar conditions led to no reaction.
This suggests that the phosphorus center in 5 is too hindered
to initiate reaction by coordination to the incoming Lewis acid.
Figure 7. POV-Ray depiction of 13. C, black; B, yellow-green; Cl, aqua-
marine; P, orange; F, deep pink. H-atoms are omitted for clarity. Selected
metrical parameters (distance A, angle deg). P(1)-B(1) 2.0552(15), P(1)-
Conclusions
˚
Monomeric phosphinoboranes of the general formula
R2PB(C6F5)2 can be readily synthesized and characterized.
Large substituents on P prevent dimerization, and the mono-
meric species exhibit spectroscopic and structural evidence
of significant P-B π-bonding. Nonetheless, these bonds are
sufficiently polarizable for these species to react with H2,
ammonia-borane, a Lewis acid or base. Lewis acid-base
adducts of these ambiphilic phosphinoboranes are rather
unstable because of the steric repulsion and weakening of the
P-B bond caused by the loss of the π-bonding component of
the interaction. The reactivity of these species with hydrogen
under mild conditions without a catalyst is particularly
interesting in the context of efforts to illuminate new strate-
gies toward hydrogen storage materials that can be regener-
ated. It is this latter information that we are striving to exploit
in ongoing efforts.
B(1a) 2.0557(15), B(1)-Cl(1) 1.8409(15), B(1)-Cl(2) 1.8453(15), B(1)-
P(1)-B(1a) 89.01(6), P(1)-B(1)-P(1a) 90.99(6).
˚
of the P-B bond to 2.0329(9) A as a result of the loss of the π-
bonding interaction between the formerly vacant orbital on
boron and the lone pair on phosphorus. The B-N bond of
ꢀ
˚
1.6332(11) A is slightly longer than that reported for the anal-
75
˚
ogous B(C6F5)3 adduct of 4-tert-butyl pyridine (1.618(2) A).
This longer bond length isa result of diminished Lewis acidity
at boron and increased steric crowding caused by the adja-
cent phosphorus center. As expected, the boron center has
become pseudotetrahedral. The N-B-C angles average
105.44° while the P-B-C angles average 112.82°, consistent
with greater steric conflict between the B(C6F5)2 and the
PCy2 units. The phosphorus center has also pyramidalized
with the sum of angles totaling 314.9°. This is somewhat
surprising since the planarity of the precursor phosphino-
borane was attributed to steric conflict between substituents
on P and B. The pyramidalization at phosphorus in 11
suggests that electronic effects and π-bonding facilitates the
planarity observed in 4.
Acknowledgment. The authors 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. S.J.G. is grateful for an NSERC postgraduate
scholarship.
Considering the inverse reactions, the phosphinoboranes 3
and 4 were reacted with the Lewis acid BCl3. The new
products 12 and 13 (Scheme 2) were characterized by new
11B NMR signals at 4.2 and -0.5 ppm respectively. In addition
Supporting Information Available: Crystallographic data in
CIF format, and optimized cartesian coordinates of molecules
examined computationally. This material is available free of
charge via the Internet at http://pubs.acs.org.
(75) Geier, S. J.; Gille, A. L.; Gilbert, T. M.; Stephan, D. W. Inorg. Chem.
2009, 48, 10466.