Probing substituent effects on the activation of H2 by phosphorus and boron frustrated Lewis pairs

Article abstract of DOI:10.1039/c001133a

The impact of substituent changes on phosphorus and boron-containing frustrated Lewis pairs (FLPs) has been examined. The phosphites (RO) ₃P R = Me, Ph form classical Lewis acid-base adducts of the formula (RO)₃PB(C₆F₅)₃ R = Me 1; R = Ph 2, whereas P(O-2,4-^tBu₂C₆H₃) ₃ and P(O-2,6-Me₂C₆H₃)₃ generate FLPs. Nonetheless, these latter combinations do not react with H ₂. The more basic phosphinite tBu₂POR, R = tBu 3 reacts with B(C₆F₅)₃ to give (tBu₂(H)PO) B(C₆F₅)₃6. The related species tBu ₂POR, R = Ph 4; 2,6-Me₂C₆H₃5 showed no reaction with B(C₆F₅)₃ but the FLPs react under H₂ (4 atm) to give [tBu₂P(OR)H][HB(C ₆F₅)₃] R = Ph 7 and 2,6-Me₂C ₆H₃8. Similarly, tBu₂PCl in combination with B(C₆F₅)₃, generates an FLP that upon addition of H₂, gives [tBu₂PH₂][ClB(C₆F ₅)₃] 9 albeit in low yield. The diborane 1,4-(C ₆F₅)₂B(C₆F₄)B(C ₆F₅)₂ in combination with either tBu ₃P or (C₆H₂Me₃)₃P generates FLPs that react with H₂ to give [R₃PH] ₂[1,4-(C₆F₅)₂HB(C₆F ₄)BH(C₆F₅)₂] (R = tBu 11, C ₆H₂Me₃12). Similarly PhB(C₆F ₅)₂ and tBu₃P react with H₂ giving [tBu₃PH][HBPh(C₆F₅)₂] 13. The combination of B(OC₆F₅)₃ and PtBu₃ also generate an FLP which reacts with H₂ to give [HPtBu ₃][B(OC₆F₅)₄] 14, the product of substituent redistribution. The boronic esters, (C₆H ₄O₂)BC₆F₅15, (C₆H ₃FO₂)BC₆F₅16 and (C ₆F₄O₂)BC₆F₅17, and the borate esters B(OC₆H₃(CF₃)₂) ₃18, B(OC₆H₂F₃)₃19 and B(OC₆H₄CF₃)₃20 were prepared and shown to generate FLPs with tBu₃P or (C₆H ₂Me₃)₃P. Nonetheless, no reaction with H ₂ was observed for 15-17. Collectively these data suggest that there is a threshold of combined Lewis acidity and basicity that is required to effect the splitting of H₂.

Full text of DOI:10.1039/c001133a

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www.rsc.org/dalton | Dalton Transactions
Probing substituent effects on the activation of H2 by phosphorus and boron
frustrated Lewis pairs†
Rebecca C. Neu, Eva Y. Ouyang, Stephen J. Geier, Xiaoxi Zhao, Alberto Ramos and Douglas W. Stephan*
Received 19th January 2010, Accepted 5th March 2010
First published as an Advance Article on the web 25th March 2010
DOI: 10.1039/c001133a
The impact of substituent changes on phosphorus and boron-containing frustrated Lewis pairs (FLPs)
has been examined. The phosphites (RO)3P R = Me, Ph form classical Lewis acid–base adducts of the
formula (RO)3PB(C6F5)3 R = Me 1; R = Ph 2, whereas P(O-2,4-tBu2C6H3)3 and P(O-2,6-Me2C6H3)3
generate FLPs. Nonetheless, these latter combinations do not react with H2. The more basic phosphinite
tBu2POR, R = tBu 3 reacts with B(C6F5)3 to give (tBu2(H)PO)B(C6F5)3 6. The related species tBu2POR,
R = Ph 4; 2,6-Me2C6H3 5 showed no reaction with B(C6F5)3 but the FLPs react under H2 (4 atm) to
give [tBu2P(OR)H][HB(C6F5)3] R = Ph 7 and 2,6-Me2C6H3 8. Similarly, tBu2PCl in combination with
B(C6F5)3, generates an FLP that upon addition of H2, gives [tBu2PH2][ClB(C6F5)3] 9 albeit in low yield.
The diborane 1,4-(C6F5)2B(C6F4)B(C6F5)2 in combination with either tBu3P or (C6H2Me3)3P generates
FLPs that react with H2 to give [R3PH]2[1,4-(C6F5)2HB(C6F4)BH(C6F5)2] (R = tBu 11, C6H2Me3 12).
Similarly PhB(C6F5)2 and tBu3P react with H2 giving [tBu3PH][HBPh(C6F5)2] 13. The combination of
B(OC6F5)3 and PtBu3 also generate an FLP which reacts with H2 to give [HPtBu3][B(OC6F5)4] 14, the
product of substituent redistribution. The boronic esters, (C6H4O2)BC6F5 15, (C6H3FO2)BC6F5 16 and
(C6F4O2)BC6F5 17, and the borate esters B(OC6H3(CF3)2)3 18, B(OC6H2F3)3 19 and B(OC6H4CF3)3 20
were prepared and shown to generate FLPs with tBu3P or (C6H2Me3)3P. Nonetheless, no reaction with
H2 was observed for 15-17. Collectively these data suggest that there is a threshold of combined Lewis
acidity and basicity that is required to effect the splitting of H2.
silylenol ethers,9-11 while Repo et al.12,13 have examined N/B based
FLPs for imine hydrogenation. The activation of H2 by other
Introduction
FLPs derived from the combinations of phosphines,2 pyridines,14
imines,7 amines7,12,13 and carbenes15,16 with the Lewis acid B(C6F5)3
have also been documented (Scheme 1). Although the variation of
the Lewis basic partner has been probed, variations of the Lewis
acid have garnered lesser attention.
The activation of H2 was generally considered to require the
participation of a transition metal. However, recently we described
the reversible activation of H2 by (C6H2Me3)2PC6F4B(C6F5)2
(Scheme 1).1 Subsequently we demonstrated that simple combi-
nations of sterically encumbered phosphines and boranes such as
tBu3P or (C6H2Me3)3P with B(C6F5)3 also effect the heterolytic ac-
tivation of H2.2 These electron donor and acceptor pairs are unique
in that they fail to generate classical Lewis acid–base adducts3 as
this is precluded or frustrated by steric demands. Such combina-
tions provide sites of unquenched Lewis acidity and basicity that
can further react with H2 and have been termed “frustrated Lewis
pairs” (FLPs).4-6 Activation of H2 although less effective, was also
achieved using an FLP comprised of the significantly less Lewis
acidic species BPh3 in conjunction with tBu3P. 2 Investigations of
systems where the Lewis basicity and acidity are simultaneously
reduced, such as BPh3/P(C6H2Me3)3, resulted in no reaction with
H2.2 This concept of H2 activation by FLPs was exploited for
the development of metal-free hydrogenation catalysts employed
for the reduction of imines, aziridines and nitriles.7,8 Erker and
coworkers have extended this basic concept and developed highly
effective FLP catalysts for the hydrogenation of enamines and
B(C6F4H)3 in combination with tBu3P has been shown to afford
reversible uptake and release of H2.17 While B(C6F5)3 has been
used principally as the Lewis acid in the FLP activation of
a variety of other small molecules including olefins,18 dienes,19
acetylenes,20,21 disulfides,22 CO223 and N2O,24,25 the Al-based Lewis
acid Al(C6F5)3,20 and B(C6H4F)3 have also been proven to be
25
suitable options. Herein we probe the impact of varying the
substituents on B and P and the subsequent ability to form
FLPs and activate H2. Specifically, FLPs derived from phosphites,
phosphinites and a chlorophosphine with B(C6F5)3 as well as FLPs
derived from tBu3P and (C6H2Me3)3P with a bis-borane and a
series of boronic and borate ester derivatives are studied. The
implications of these results are discussed.
Experimental section
General data
Department of Chemistry, University of Toronto, 80 St. George St. Toronto,
Ontario, Canada M5S 3H6. E-mail: dstephan@chem.utoronto.ca; Tel: 416-
946-3294
† CCDC reference numbers 762375, 762376, 762377, 762378, 762379,
762380, 762381 and 762382. For crystallographic data in CIF or other
electronic format see DOI: 10.1039/c001133a
All preparations were done under an atmosphere of dry, O2-free
N2 employing both Schlenk line techniques and an Innovative
Technologie, Vacuum Atmospheres or a MBraun inert atmosphere
glove box. Solvents (pentane, hexanes, toluene, and CH2Cl2) were
purified employing a Grubbs’ type column systems manufactured
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0.19 mmol) and P(OCH3)3 (0.024 g, 0.19 mmol) in CH2Cl2 (2 mL)
was prepared. The solvent was removed in vacuo. To the remnants
was added pentane (2 mL) resulting in a clear supernatant and
a white solid layer. The supernatant was removed by pipette and
the solid was dried under vacuum. Yield: 77% (96 mg). X-Ray
quality crystals were grown by slow evaporation from a CH2Cl2
1
3
solution. H NMR (CDCl3): 3.80 (d, JP–H = 10 Hz, 9H, CH3);
19F NMR (CDCl3): -131.1 (d, JF–F = 22 Hz, 6F, o-C6F5), -156.6
3
3
3
(t, JF–F = 20 Hz, 3F, p-C6F5), -164.3 (td, JF–F = 20 Hz, J =
6 Hz, 6F, m-C6F5); 13C{ H} NMR (CDCl3) partial: 55.7 (d, JP–C
=
1
12 Hz); 11B NMR (CDCl3): -15.5 (d, JP–B = 141 Hz); 31P NMR
(CDCl3): 75.5 (br m). Anal. Calcd. for C21H9F15BPO3: C, 39.65;
H, 1.43. Found: C, 39.71; H, 1.24. 2, Yield: 93% (75 mg). 1H NMR
(CD2Cl2): 6.90 (dd, 3JH–H = 8 Hz, 4JH–P = 1 Hz, 6H, o-C6H6), 7.20
(t, 3JH–H = 7 Hz, 3H, p-C6H5), 7.23 (t, 3JH–H = 7 Hz, 6H, m-C6H5);
19F NMR (CD2Cl2): -129.8 (s, 6F, o-C6F5), -156.2 (s, 3F, p-C6F5),
1
-164.3 (s, 6F, m-C6F5); 13C{ H} NMR (CD2Cl2) partial: 150.9 (d,
1
JP–C = 14 Hz), 148.8 (d, 1JF–C = 248 Hz, o-C6F5), 140.7 (d, 1JF–C
=
1
256 Hz, p-C6F5), 137.3 (d, JF–C = 252 Hz, m-C6F5), 130.2 (d,
JP–C = 1 Hz), 126.2 (d, JP–C = 1 Hz), 120.3 (d, JP–C = 4 Hz); 11B
NMR (CD2Cl2): -8.9 (br s); 31P NMR (CD2Cl2): 62.0 (br s).Anal.
Calcd. for C36H15F15BPO3: C, 52.58; H, 1.84. Found: C, 52.08; H,
2.07.
Synthesis of tBu2POR, R = tBu 3; Ph 4; 2,6-Me2C6H3 5
These compounds were prepared by reaction of tBu2PCl with the
appropriate potassium alkoxide or aryloxide (generated in situ
by reaction of the corresponding alcohol with KH) over 4 h in
THF, followed by removal of the solvent, extraction into pentane
and filtration through celite (similar to the method previously
described for 4.30 Careful removal of the solvent in vacuo left
products 3-5 as colourless oils. If excess alcohol remained, the
product was redissolved in pentane and run through a plug of
alumina. 3: Yield: 76%. 1H NMR (C6D6): 1.14 (d, 3JP–H = 11 Hz,
Scheme 1 Examples of FLP Activation of H2.
by Innovative Technology and stored over molecular sieves (4 A).
˚
˚
Molecular sieves (4 A) were purchased from Aldrich Chemical
Company and dried at 140 ◦C under vacuum for 24 h prior
to use. Uninhibited THF was purchased from Caledon and
distilled from sodium/benzophenone. Deuterated solvents were
dried over Na/benzophenone (C6D6, C7D8, THF-d8) or CaH2
(CD2Cl2, C6D5Br). All common organic reagents were purified
4
1
18H, tBu), 1.26 (d, JP–H = 1 Hz, OtBu, 9H); 31P{ H} NMR
(C6D6): 136.7 (s); 13C{ H} NMR (C6D6): 28.7 (d, JP–C = 16 Hz,
1
1
by conventional methods unless otherwise noted. H, 13C, 11B,
19F and 31P NMR spectra were recorded on a Bruker Avance-
300 spectrometer at 300 K, a Varian NMR System 400 MHz or
P–C(CH3)3), 31.4 (d, JP–C = 7 Hz, OC(CH3)3), 34.6 (d, JP–C =
27 Hz, PCMe3), 75.5 (d, JP–C = 11 Hz, OCMe3). 4: Yield: 83%. 1H
1
a Bruker Avance III 400 MHz spectrometer at 298 K. H and
NMR (C6D6): 1.08 (d, 3JP–H = 12 Hz, 18H, tBu), 6.78 (t, 3JH–H
=
3
13C NMR spectra were referenced to SiMe4 using the residual
solvent peak impurity of the given solvent. 31P, 11B and 19F NMR
spectra are referenced to 85% H3PO4, BF3(OEt2), and CFCl3,
7 Hz, 1H, p-C6H5), 7.07 (t, JH–H = 8 Hz, 2H, m-C6H5), 7.25
3
1
(dm, JH–H = 8 Hz, 2H, o-C6H5); 31P{ H} NMR (C6D6): 153.5
(s); 13C{ H} NMR (C6D6): 27.4 (d, JP–C = 16 Hz), 35.6 (d, JPC
=
1
7
respectively. Li NMR spectra were referenced to LiCl in D2O.
26 Hz), 118.6 (d, JP–C = 11 Hz), 121.6 (d, JP–C = 1 Hz), 129.6 (d,
1
Chemical shifts are reported in ppm and coupling constants
in Hz as absolute values. DEPT and 2-D 1H/13C correlation
experiments were completed for assignment of the carbon atoms.
Combustion analyses were performed in house employing a Perkin
Elmer CHN Analyzer. B(C6F5)3 was generously donated by NOVA
Chemicals Corporation. P(OCH3)3 and P(OPh)3 were purchased
from Aldrich Chemicals, P(O-2,4-tBu2C6H3)3 was purchased from
Strem Chemicals and used as received. P(O-2,6-Me2C6H3)3,26
PhB(C6F5)2,27 1,4-(C6F5)2B(C6F4)B(C6F5)228 and B(OC6F5)329 were
prepared according to literature procedures.
JP–C = 1 Hz), 160.3 (d, JP–C = 10 Hz). 5, Yield: 78%. H NMR
(C6D6): 1.08 (d, 3JP–H = 11 Hz, 18 H, tBu), 2.29 (s, 3H, CH3), 2.59
(s, 3H, CH3), 6.76 (t, 3JH–H = 7 Hz, 1H, p-C6H3), 6.88 (d, 3JH–H
=
1
7 Hz, 2H, m-C6H3);z)) 31P NMR (C6D6): 162.3 (s); 13C{ H} NMR
(C6D6): 27.5 (d, JP–C = 16 Hz), 36.4 (d, JP–C = 33 Hz), 122.0, 154.9
(d, JP–C = 2 Hz).
Synthesis of tBu2(H)POB(C6F5)3 6
A clear solution of B(C6F5)3 (0.050 g, 0.10 mmol) and 3 (0.030 g,
0.10 mmol) in CH2Cl2 (2mL) was prepared. The solvent was
removed in vacuo. To the remnants was added pentane (2mL)
resulting in a clear supernatant and a white solid layer. The
supernatant was removed by pipette and the solid was dried
under vacuum. Yield: 82% (55 mg) 1H NMR (C6D6): 0.57
Synthesis of (R3O)3PB(C6F5)3 R = Me 1; R = Ph 2
These compounds were prepared in a similar fashion and thus only
one preparation is reported. A clear solution of B(C6F5)3 (0.100 g,
4286 | Dalton Trans., 2010, 39, 4285–4294
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3
1
(d, JP–H = 16 Hz, 18H, tBu), 5.33 (d, JP–H = 452 Hz, 1H, PH);
19F NMR (C6D6): -132.3 (d, 3JF–F = 23 Hz, 6F, o-C6F5), -157.2 (t,
3JF–F = 21 Hz, 3F, p-C6F5), -163.8 (tm, 3JF–F = 23 Hz, 6F, m-C6F5);
237 Hz, m-C6F5), 32.6 (d, 1JCP = 32 Hz, quat-tBu), 27.9 (d, 2JCP
=
2 Hz, tBu). 19F NMR (CD2Cl2): -132.7 (m, 6F, o-C6F5), -161.4 (t,
3F, 3JF–F = 21 Hz, p-C6F5), -166.6 (m, 6F, m-C6F5). 31P (CD2Cl2):
27.1 (t, 1JP–H = 463 Hz). Anal. Calcd. for C26H20BClF15P: C, 44.96;
H, 2.90. Found: C, 45.12; H, 3.04%.
1
1
31P{ H} NMR (C6D6): 77.0 (s); 11B{ H} NMR (C6D6): -0.30 (br
s); 13C{ H} NMR (C6D6) partial: 24.6 (br s, C(CH3)3), 33.7 (d,
1
JP–C = 57 Hz, C(CH3)3), 137.5 (dm, JC–F = 256 Hz, CF), 14 8.2
(dm, JC–F = 242 Hz, CF). Anal. Calcd. for C26H19BF15OP (%) C:
46.32, H: 2.84; found: C: 46.23, H: 2.97.
Synthesis of [tBu2PD2][ClB(C6F5)3] 10-d2
This species was prepared via the method used to prepare 9 with
the substitution of D2 for H2. 2D NMR (CH2Cl2): 5.89 (d, 1JDP
=
Synthesis of [tBu2P(OR)H][HB(C6F5)3] R = Ph 7; R =
1
70 Hz). 31P{ H} (CD2Cl2): 24.7 (quintet, 1JPD = 70 Hz).
2,6Me2C6H3 8
Synthesis of [R3PH]2[1,4-(C6F5)2HB(C6F4)BH(C6F5)2] (R = tBu
These compounds were prepared in a similar fashion and thus only
one preparation is reported. Compound 4 (47 mg, 0.20 mmol) was
added to a solution of B(C6F5)3 (100 mg, 0.20 mmol) in 5 mL
of toluene in a small solvent bomb. The solution was subjected
to three freeze-pump-thaw cycles and subsequently exposed to an
atmosphere of H2 at 78 K (~4 atm). The solution was allowed
to stir overnight at room temperature. The solution was removed
and the bomb was washed with CH2Cl2 (2 ¥ 2 mL). Volatiles
were removed in vacuo and pentane was added to the resulting
thick oil. The product crystallized over 3 days, producing X-ray
quality crystals. Yield: 142 mg (97%). 1H NMR (CD2Cl2): 1.43 (d,
3JP–H = 19 Hz, 18H, tBu), 3.55 (q, 1JB–H = 93 Hz, 1H, BH), 6.98
(d, 1JP–H = 474 Hz, 1H, PH), 7.08 (d, 3JH–H = 8 Hz, 2H, o-C6H5),
7.26 (1H, br s, 1H, p-C6H5), 7.37 (t, 3JH–H = 8 Hz, 2H, m-C6H5);z))
11, C6H2Me3 12)
These compounds were prepared in a similar fashion and thus only
one preparation is detailed. A 100 mL thick walled tube bomb was
charged with 1,4-(C6F5)2B(C6F4)B(C6F5)2 (0.040 g, 0.048 mmol)
and tBu3P (0.019 g, 0.094 mmol) in bromobenzene (5 mL). The
clear and colourless solution was degassed using three freeze-
pump-thaw cycles. The reaction vessel was exposed to a continuous
flow of H2 for a period of one minute and then sealed and allowed
to stir for a period of 24 h. At this time, pentane (20 mL) was added
to the opaque solution precipitating a microcrystalline solid. The
solvent was decanted and the product washed with pentane (3 ¥
1
5 mL) and dried in vacuo for 2 h. Yield: 0.051 g (93%). 11: H
NMR (CD2Cl2): 5.25 (br d, 2H, 1JH–P = 437 Hz, PH), 3.69 (br q,
2H, 1JH–B = 86 Hz, BH), 1.56 (d, 54H, 3JH–P = 16 Hz, P{C(CH3)3}).
3
19F NMR (CD2Cl2): -134.2 (d, JF–F = 24 Hz, o-C6F5), -164.8 (t,
1
3JF–F = 23 Hz, p-C6F5), -163.7 (tm, 3JF–F = 24 Hz, m-C6F5). 31P{ H}
1
1
11B{ H} NMR (CD2Cl2): -24.9 (d, 1JB–H = 86 Hz). 13C{ H} NMR
NMR (CD2Cl2) d: 98.5 (s); 11B NMR (CD2Cl2): -25.6 (d, 1J B–H
=
1
(CD2Cl2): 149.38 (br dm, JC–F = 234 Hz, o-C6F5), 149.03 (br d,
1
93 Hz); 13C{ H} NMR (CD2Cl2) partial: 25.4 (d, JP–C = 2 Hz,
1JC–F = 239 Hz, C6F4), 138.32 (br d, 1JC–F = 243 Hz, p-C6F5), 137.18
(br dm, 1JC–F = 248 Hz, m-C6F5), 128.74 (br s, ipso-C6F5), 125.16
(br s, ipso-C6F4), 37.94 (d, 1JC-P = 28 Hz, P{C(CH3)3}), 30.10 (s,
P{C(CH3)3}). 19F NMR (CD2Cl2): -132.95 (d, 8F, 3JF–F = 22 Hz,
C(CH3)3), 36.4 (d, JP–C = 42 Hz, C(CH3)3), 115.4, 117.6 (d, JP–C
=
6 Hz), 131.5. Anal. Calcd. for C32H25BF15PO (%) C: 51.09, H:
3.35; found: C: 51.21; H: 3.59. 8, R = 2,6-Me2C6H3: Yield: 147 mg
(96%). 1H NMR (CD2Cl2): 1.44 (d, 3JP–H = 20 Hz, 18 H, tBu), 2.28
o-C6F5), -137.30 (s, 4F, C6F4), -165.24 (t, 4F, 3JF–F = 22 Hz, p-C6F5),
1
1
(s, 6H, CH3), 3.51 (q, JB–H = 93 Hz, 1H, BH), 7.02 (d, JP–H
=
-167.74 (m, 8F, m-C6F5). 31P{ H} NMR (CD2Cl2): 59.37 (d, 1JP–H
=
1
483 Hz, 1H, PH), 7.02-7.13 (m, 3H, Ar-H);z)) 19F NMR (CD2Cl2):
-134.3 (d, 3JF–F = 23 Hz, o-C6F5), -165.0 (t, 3JF–F = 20 Hz, p-C6F5),
437 Hz). Anal. Calcd. for C54H56B2F24P2: C, 52.04; H, 4.69. Found:
C, 51.90; H, 4.87%. 12: Yield: 0.103 g (88%). 1H NMR (CD2Cl2):
-163.7 (tm, JF–F = 20 Hz, m-C6F5). 31P{ H} NMR (CD2Cl2) d:
3
1
1
8.26 (br d, 2H, JH–P = 480 Hz, PH), 7.12 (s, 6H, C6H2(CH3)3),
97.5 (s); 11B NMR (CD2Cl2): -25.3 (d, J
= 93 Hz); 13C{ H}
1
1
1
B–H
7. 03 (s, 6H, C6H2(CH3)3), 3.58 (br q, 2H, JH–B = 86 Hz, BH),
NMR (CD2Cl2): 25.5 (d, JP–C = 2 Hz, C(CH3)3) 36.8 (d, JP–C
=
2.34 (s, 18H, p-CH3), 2.24 (s, 18H, o-CH3), 1.98 (s, 18H, o-CH3).
37 Hz, C(CH3)3), 126.5, 127.6, 128.6, 131.2 (d, JP–C = 6 Hz). Anal.
Calcd. for C34H29BF15PO (%) C: 52.33, H: 3.75; found: C: 52.33;
H: 3.98.
1
1
11B{ H} NMR (CD2Cl2): -25.18 (d, 1JB–H = 86 Hz).13C{ H} NMR
(CD2Cl2) partial: 148.76 (br d, 1JC–F = 239 Hz, o-C6F5), 147.83 (br
d, 1JC–F = 245 Hz, C6F4), 147.59 (d, 4JC–F = 2.8 Hz, p-C6H2Me3),
144.64 (d, 3JC–P = 11 Hz, m-C6H2Me3), 143.45 (d, 3JC–P = 11 Hz,
m-C6H2Me3), 137.30 (br d, 1JC–F = 249 Hz, p-C6F5), 136.15 (br d,
Synthesis of [tBu2PH2][ClB(C6F5)3] 9
1JC–F = 245 Hz, m-C6F5), 133.58 (d, JC–P = 11 Hz, o-C6H2Me3),
2
A 100 mL Schlenk bomb charged with a solution of ClPtBu2
(30 mL, 0.158 mmol) and B(C6F5)3 (81 mg, 0.158 mmol) in toluene
(5 mL) was filled with approximately 4 bars of H2 through 3 freeze-
pump-thaw cycles. The reaction was stirred for a period of 16 days
at RT. Toluene was pumped down and pentane was added to
precipitate the product, which was filtered and further washed
with pentane. Upon drying in vacuo, a white powder was obtained.
Yield: 64 mg, 58%. Single crystals suitable for X-ray diffraction
were grown by layering a CH2Cl2 solution of the product with
pentane at 25 ◦C. 1H NMR (CD2Cl2): 5.87 (d, 1JH–P = 454 Hz, 2H,
2
1
132.24 (d, JC–F = 11 Hz, o-C6H2Me3), 112.02 (d, JC–P = 83 Hz,
ipso-C6H2Me3), 22.41 (br s, o-CH3), 21.45 (br s, p-CH3), 21.34
(br s, o-CH3). 19F NMR (CD2Cl2): -133.14 (d, 8F, 3JF–F = 19 Hz,
3
o-C6F5), -138.28 (s, 4F, C6F4), -166.02 (t, JF–F = 20 Hz, 4F,
p-C6F5), -168.07 (m, 8F, m-C6F5). 31P{ H} NMR (CD2Cl2): -26.38
1
1
(d, JP–H = 482 Hz). Anal. Calcd. for C84H70B2F24P2·CH2Cl2: C,
59.88; H, 4.26. Found: C, 59.83; H, 4.67%.
Synthesis of [tBu3PH][HBPh(C6F5)2] 13
1
PH2), 1.55 (d, 3JH–P = 19 Hz, 18H, tBu). 11B{ H} NMR (CD2Cl2):
A 100 mL thick walled tube bomb was charged with PhB(C6F5)2
(0.183 g, 0.434 mmol) and tBu3P (0.088 g, 0.435 mmol) in toluene
(10 mL). The clear and colourless solution was degassed using
1
1
-6.46 (br).13C{ H} NMR (CD2Cl2): 148.6 (dm, JC–F = 241 Hz,
1
1
o-C6F5), 139.7 (dm, JC–F = 241 Hz, p-C6F5), 137.3 (dm, JC–F
=
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three freeze-pump-thaw cycles. The reaction bomb was exposed
to a continuous flow of H2 for a period of one minute and then
sealed and allowed to stir for a period of 24 h. Pentane (20 mL)
was added precipitating a clear and colourless oil. The solvent was
decanted, the residue redissolved in CH2Cl2 (3 mL) and triturated
with pentane (40 mL), affording a white powder. The solvent was
again decanted, the product washed with pentane (3 ¥ 5 mL) and
dried in vacuo for 2 h. Crystals suitable for X-ray diffraction were
grown from a layered solution of CH2Cl2–pentane at 25 ◦C. Yield:
purified by crystallization at -35 ◦C while 17 was sublimed at 80 ◦C
and the residue was then taken up in pentane, filtered and purified
by crystallization at -35 ◦C. Crystals suitable for X-ray diffraction
were grown from a mixed 1 : 5 CH2Cl2–pentane solution at -35 ◦C.
15: Yield: 1.01 g (78%). 1H NMR (CD2Cl2): 7.40 (dd, 2H, 3JH–H
=
4
3
4
6 Hz, JH–H = 3 Hz, ArH), 7.21 (dd, 2H, JH–H = 6 Hz, JH–H
=
3 Hz, ArH). 11B{ H} NMR (CD2Cl2): 29.78 (s). 13C{ H} NMR
1
1
1
(CD2Cl2): 150.45 (br d, JC–F = 254 Hz, o-C6F5), 147.78 (s, ipso-
C6F5), 144.36 (br d, 1JC–F = 258 Hz, p-C6F5), 137.86 (br d, 1JC–F
=
1
3
0.202 g (74%). H NMR (CD2Cl2): 7.16 (d, 2H, JH–H = 6.8 Hz,
253 Hz, m-C6F5), 124.20 (s, m-O2C6H4), 113.52 (s, o-O2C6H4). 19
F
o-C6H5), 7.04 (t, 2H, 3JH–H = 6.8 Hz, m-C6H5), 6.94 (t, 1H, 3JH–H
=
NMR (CD2Cl2): -129.30 (dt, 2F, JF–F = 19 Hz, JF–F = 8 Hz,
3 4
3
4
7.2 Hz, p-C6H5), 5.12 (br d, 1H, 1JH–P = 427 Hz, PH), 3.66 (br q,
o-C6F5), -147.80 (tt, 1F, JF–F = 20 Hz, JF–F = 6 Hz, p-C6F5),
3 4
3
1JH–B = 89 Hz, BH), 1.58 (d, 27H, JH–P = 16 Hz, P{C(CH3)3}).
-162.13 (tt, 2F, JF–F = 20 Hz, JF–F = 6 Hz, m-C6F5). Anal.
1
1
11B{ H} NMR (CD2Cl2): -19.71 (d, 1JB–H = 89 Hz). 13C{ H} NMR
(CD2Cl2) partial: 148.70 (br d, 1JC–F = 249 Hz, o-C6F5), 138.18 (br
Calcd. for C12H4BF5O2: C, 50.40; H, 1.41. Found: C, 50.20; H,
1
1.73%. 16: Yield: 0.382 g (75%). H NMR (CD2Cl2): 7.24 (m,
1
1
d, JC–F = 253 Hz, p-C6F5), 137.11 (br d, JC–F = 242 Hz, m-
C6F5), 134.00 (s, p-C6H5), 126.91 (s, o-C6H5), 123.44 (s, m-C6H5),
1H, o-C6H3F); 7.19 (m, 1H, m-C6H3F); 7.04 (m, 1H, m-C6H3F).
1
11B{ H} NMR (CD2Cl2): 29.92 (s). 13C NMR (CD2Cl2) partial:
38.06 (d, JC–P = 27 Hz, P{C(CH3)3}), 30.47 (s, CH3).19F NMR
150.95 (br d, 1JC–F = 244 Hz, o-C6F5); 150.60 (d, 3JC–F = 4.9 Hz,
1
3
1
(CD2Cl2): -131.68 (d, 4F, JF–F = 23 Hz, o-C6F5), -165.48 (t, 2F,
ipso-C6H3F), 149.73 (d, JC–F = 250 Hz, o-C6H3F), 145.05 (br d,
3JF–F = 20 Hz, p-C6F5), -167.80 (t, 4F, JF–F = 24 Hz, m-C6F5).
272 Hz, p-C6F5), 138.30 (br d, 253 Hz, m-C6F5), 135.36 (d, 2JC–F
=
3
1
31P{ H} NMR (CD2Cl2): 59.55 (d, 1JP–H = 427 Hz). Anal. Calcd.
14 Hz, ipso-C6H3F), 124.49 (d, 3JC–F = 7.0 Hz, m-C6H3F), 111.95
(d, 2JC–F = 17 Hz, m-C6H3F), 109.60 (d, 4JC–F = 3.7 Hz, o-C6H3F).
19F NMR (CD2Cl2): -128.89 (m, o-C6F5), -135.94 (m, 2F, C6H3F),
-146.96 (tt, 1F, 3JF–F = 20 Hz, 4JF–F = 6 Hz, p-C6F5), -161.83 (m, 2F,
m-C6F5). Anal. Cacld. for C12H3BF6O2: C, 47.37 H, 0.99. Found:
C, 47.28; H, 1.14%. 17: Yield: 0.340 g (42%). 11B NMR (CD2Cl2):
for C30H34BF10P: C, 57.53; H, 5.47. Found: C, 57.25; H, 5.19%.
[tBu3PH][B(OC6F5)4] 14
A 100 mL tube bomb was charged B(OC6F5)3 (0.116 g,
0.207 mmol) and tBu3P (0.042 g, 0.208 mmol) in toluene (10 mL).
The reaction was degassed using three freeze-pump-thaw cycles.
The reaction bomb was exposed to a continuous flow of H2
for a period of one minute and then sealed and allowed to stir
for a period of 24 h. At this time, pentane (20 mL) was added
precipitating a colourless oil. The resulting oil was triturated with
pentane (40 mL) affording a white solid. Yield: 40 mg (25%). 1H
1
1
30.29 (s). 13C{ H} NMR (CD2Cl2) partial: 151.06 (br d, JC–F
=
256 Hz, o-C6F5), 145.66 (br d, 1JC–F = 264 Hz, p-C6F5), 139.11 (br
d, 1JC–F = 250 Hz, C6F4), 138.35 (br d, 1JC–F = 250 Hz, m-C6F5),
135.29 (br d, 1JC–F = 255 Hz, C6F4). 19F NMR (CD2Cl2): -127.76
3
4
(m, 2F, o-C6F5), -144.79 (tt, 1F, JF–F = 20 Hz, JF–F = 7 Hz, p-
C6F5), -160.12 (m, 2F, o-C6F4), -160.81 (m, 2F, m-C6F5), -163.41
(m, 2F, meta-C6F4). Anal. Calcd. for C12BF9O2: C 40.23; Found:
C 40.22%.
1
NMR (CD2Cl2): 5.08 (br d, 1H, JH–P = 428 Hz, HP), 1.64 (d,
3
27H, JH–P = 16 Hz, P{C(CH3)3}). 11B NMR (CD2Cl2): -1.21 (s).
1
13C{ H} NMR (CD2Cl2) partial: 142.78 (br d, 1JC–F = 247 Hz, o-
Synthesis of B(OC6H3(CF3)2)3 18, B(OC6H2F3)3 19,
B(OC6H4CF3)3 20
C6F5), 138.35 (br d, 1JC–F = 246 Hz, p-C6F5), 135.11 (br d, 1JC–F
=
242 Hz, m-C6F5), 38.25 (d, 1JC–P = 27 Hz, P{C(CH3)3}), 30.55 (s,
P{C(CH3)3}). 19F NMR (CD2Cl2): -158.28 (d, 8F, 3JF–F = 19 Hz,
These compounds were prepared in a similar manner using the
appropriate reagents and thus only one preparation is detailed.
A 250 mL schlenk was charged with 3,5-trifluoromethylphenol
(2 mL, 13.1 mmol) in CH2Cl2 (15 mL) while a second 250 mL
schlenk was charged with 1.0 M BCl3 in hexanes (4.5 mL,
4.50◦mmol) in CH2Cl2 (20 mL). The BCl3 solution was cooled to
-60 C and the solution of 3,5-trifluoromethylphenol was added
dropwise via cannula. The reaction was maintained at -60 ◦C,
with stirring, for four hours and was then allowed to warm to
room temperature. The solvent was removed in vacuo and left
under active vacuum for an additional 30 min to remove the excess
BCl3. The resulting off-white solid was purified by recrystallization
o-OC6F5), -168.95 (t, 8F, 3JF–F = 21 Hz, m-OC6F5), (t, 4F, 3JF–F
=
22 Hz, p-OC6F5). 31P NMR (CD2Cl2): 61.34 (d, 1JP–H = 428 Hz).
Anal. Calcd. for C36H28BF20O4P: C, 45.69; H, 2.98. Found: C,
45.30; H, 2.83%.
Synthesis of C6H4O2BC6F5 15, (C6H3FO2)BC6F5 16,
C6F4O2BC6F5 17
These compounds were prepared in a similar manner using the
appropriate reagents and thus only one preparation is detailed.
A 250 mL schlenk was charged with 1,2-benzenediol (0.500 g,
4.54 mmol) and pentafluoroboronic acid (0.968 g, 4.54 mmol)
in toluene (60 mL) and equipped with a Dean–Stark apparatus.
The reaction mixture was heated to reflux for a period of 12 h.
All volatiles were removed under reduced pressure resulting in
a maroon, pearlized solid. The product was then dissolved in
CH2Cl2 (3 mL) and filtered through a celite plug. Pentane (15 mL)
was added to the solution and cooled to -35 ◦C, resulting in a
colourless product. By contrast 16 was isolated as an oil and
triturated at low temperature yielding a tan solid. It was later
◦
from pentane at -35 C. By contrast, 20 was isolated as a yellow
oil. An off-white solid was produced upon cooling of the oil to
-35 ◦C followed by gentle trituration with cold pentane (10 mL).
Purification was achieved by recrystallization of the crude product
from pentane at -35 ◦C. Crystals suitable for X-ray diffraction were
◦
grown from a pentane solution at -35 C. 18: Yield: 1.80 (58%).
1H NMR (CD2Cl2): 7.75 (s, 3H, p-C6H3(CF3)2), 7.64 (s, 6H, o-
1
1
C6H3(CF3)2). 11B{ H} NMR (CD2Cl2): 15.94 (s). 13C{ H} NMR
2
(CD2Cl2): 152.73 (s, ipso-C6H3(CF3)2), 133.20 (q, JC–F = 34 Hz,
4288 | Dalton Trans., 2010, 39, 4285–4294
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1
m-C6H3(CF3)2), 123.07 (br d, JC–F = 273 Hz, CF3), 121.14 (m,
placed in calculated positions using an appropriate riding model
and coupled isotropic temperature factors.
o-C6H3(CF3)2), 137.29 (m, p-C6H3(CF3)2). 19F NMR (376 MHz,
CD2Cl2): d -63.77 (s). Anal. Calcd. for C24H9BF18O3: C 41.29;
H 1.30. Found: C 41.07; H 1.24%. 19: Yield: 1.03 (75%). 1H
Results and discussion
NMR (CD2Cl2): d 6.84 (m, 6H, B(OC6H2F3)3). 11B{ H} NMR
1
(CD2Cl2): d15.64 (s). 13C{ H} NMR (CD2Cl2): 151.62 (dm, 1JC–F
=
=
1
Varying P-substituents in FLP activation of H2
249 Hz, p-C6H2F3), 147.62 (m, ipso-C6H2F3), 137.68 (dm, 1JC–F
The phosphites (RO)3P R = Me, Ph exhibit comparatively small
cone angles of 107◦ and 118◦, respectively.32 As expected these
species, upon reaction with B(C6F5)3, are shown to form classical
Lewis acid–base adducts of the formula (RO)3PB(C6F5)3 R =
Me 1; R = Ph 2, in isolated yields of 77 and 93%, respectively
(Scheme 2). Species 1 and 2 exhibit downfield 31P resonances at
75.5 and 62.0 ppm, and give rise to 19F signals at approximately
-130, -156 and -164 ppm with 11B NMR doublets at -15.5 and
-8.9 ppm respectively. These data support the original formulations
and an X-ray crystallographic study of 1, further confirmed the
assignment (Fig. 1). The metric parameters for 1 were found to be
247 Hz, m-C6H2F3), 105.89 (m, JC–F = 249 Hz, o-C6H2F3). 19F
NMR (CD2Cl2): -134.34 (dd, 6F, 3JF–F = 21 Hz, 3JF–H = 8 Hz, m-
C6H2F3), -167.33 (tt, 3F, 3JF–F = 21 Hz, 4JF–H = 6 Hz, p-C6H2F3).
Anal. Calcd. for C18H6BF9O3: C 47.83; H 1.34. Found: C 47.49;
1
1
H 1.41%. 20: Yield 1.00 g (92%). H NMR (CD2Cl2): 7.63 (d,
3
3
6H, JH–H = 9 Hz, m-C6H4CF3), 7.27 (d, 6H, JH–H = 9 Hz, o-
C6H4CF3). 11B{ H} NMR (CD2Cl2): 15.98 (s). 13C{ H} NMR
1
1
(CD2Cl2): 155.29 (s, ipso-C(CF3)), 127.10 (q, 3JC–F = 4 Hz, m-C6H4
CF3), 126.38 (q, 2JC–F = 33 Hz, p-C6H4CF3), 124.41 (br d,1JC–F
=
124 Hz, CF3), 120.82 (s, o-CF3C6H4); 19F NMR (CD2Cl2): -62.74
(s). Anal. Calcd. for C21H12BF9O3: C 51.05; H 2.45. Found: C
50.69; H 2.56%.
˚
typical. Of particular note is the B–P bond length of 2.0209(11) A,
which is significantly shorter than that seen in Me3PB(C6F5)3
33
˚
(2.061(4) A) despite the fact that PMe3 is a considerably more
basic donor. This is attributed to the reduced cone angle of the
phosphite.
X-ray data collection, reduction, solution and refinement
Single crystals were mounted in thin-walled capillaries either
under an atmosphere of dry N2 in a glove box and flame sealed
or coated in paratone-N oil. The data were collected using the
SMART software package on a Siemens SMART System CCD
diffractometer using a graphite monochromator with Mo-Ka
˚
radiation (l = 0.71073 A) (Table 1). A hemisphere of data was
collected in 1448 frames with 10 s exposure times unless otherwise
noted. Data reduction was performed using the SAINT software
package and an absorption correction applied using SADABS.
The structures were solved by direct methods using XS and
refined by full-matrix least-squares on F2 using XL as implemented
in the SHELXTL suite of programs.31 All non-hydrogen atoms
were refined anisotropically. Carbon-bound hydrogen atoms were
Scheme 2 Reactions of phosphites with B(C6F5)3.
The above results infer that the generation of a phosphite/
B(C6F5)3 based FLP requires the use of bulkier phosphites.
Table 1 Crystallographic data
Crystal
1
6
8
9
13
15
18
20
Formula
C21H9BF15O3P C26H19BF15OP C34H29BF15OP C26H20BClF15P C30H34BF10P
C12H4BF4O2 C24H9BF18O3 C21H12BF9O3
Formula weight
Crystal system
Space group
636.06
Triclinic
¯
P1
674.19
Monoclinic
Pc
780.35
Orthorhombic Triclinic
Pbca
694.65
626.35
Monoclinic
P21/c
285.96
Monoclinic Triclinic
P21/n
698.12
494.12
Monoclinic
C2/c
¯
P1
¯
P1
˚
a/A
10.1164(4)
10.9283(4)
10.9799(4)
70.753(2)
87.539(2)
81.346(2)
1132.96(8)
2
1.865
0.271
9906
8039
10.6660(5)
9.8862(4)
16.6408(6)
18.3248(13)
18.6811(13)
19.3253(15)
11.1779(4)
11.5522(4)
11.7279(5)
81.855(2)
70.451(2)
77.021(2)
1387.01(9)
2
1.663
0.314
6312
5260
9.508(2)
16.170(4)
19.191(4)
7.4706(8)
6.0946(6)
23.464(3)
8.9246(4)
8.9972(4)
16.6085(7)
86.400(2)
77.376(2)
88.971(2)
13.8692(7)
13.4263(6)
22.0941(11)
˚
b/A
˚
c/A
a (◦)
b (◦)
g (◦)
129.863(2)
91.518(10)
92.603(6)
94.773(3)
3
˚
V/A
1346.88(10)
2
1.662
0.347
5947
6615.6(8)
8
1.567
0.197
7578
2949.4(12)
4
1.411
0.176
25914
6830
1067.24(19) 1298.78(10)
4099.9(3)
8
Z
4
2
d(calc)/g cm-1
1.780
0.176
20781
4203
181
0.0402
0.0962
0.953
1.785
0.202
21720
5928
415
0.0980
0.2826
1.089
1.601
0.159
28124
3789
m/cm-1
Data: total
Data Fo2>2s(Fo
Variables
Ra
2
)
4386
5442
370
407
477
405
396
334
0.0362
0.1120
1.020
0.0325
0.0537
0.791
0.0393
0.1045
1.010
0.0399
0.0999
1.035
0.0672
0.1793
1.040
0.0442
0.1065
0.998
b
Rw
Goodness of Fit
ꢀ
ꢀ
ꢀ
ꢀ
1
2
a
2
2
2
This data were collected at 150 K with Mo-Ka radiation (l = 0.71069 A). R = (Fo - Fc)/ Fo. b Rw = ( [w(Fo - Fc 2]/ [w(Fo) ]) .
˚
)
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Fig. 2 POV-ray depiction of 6. Hydrogen atoms, except for the P–H, are
omitted for clarity.
Fig. 1 POV-ray depiction of 1. Hydrogen atoms are omitted for clarity.
To this end the phosphites P(O-2,4-tBu2C6H3)3 and P(O-2,6-
acidity of the tert-butyl protons, prompting simultaneous proton
migration to phosphorus with concurrent elimination of iso-
butene (Scheme 3). Presumably this reaction is thermodynamically
downhill as a result of the formation of the strong BO and PO
bonds, a di-substituted alkene and a new P–H bond that easily
offsets the loss of the C–H and C–O bonds.
Me2C6H3)3 were examined. Using spectroscopic methods, 31P, 19
F
and 11B NMR, these bases demonstrated no evidence indicative
of interaction with B(C6F5)3 confirming that these combinations
of Lewis acids and bases generate FLPs. No reaction was evident
by NMR spectrscopy upon exposure of solutions of these pairs to
H2. This finding suggests that while these mixtures show no sign
of adduct formation, the combined Lewis basicity and acidity of
the phosphite and B(C6F5)3 does not meet the threshold required
to effect the activation of H2. In these cases, the lack of H2
activation is presumably attributable to the relatively low basicity
of the phosphite where the pKa of the conjugate acid of P(O-2,6-
Me2C6H3)3 is estimated to be -0.4.34
In an effort to increase the basicity of the phosphorus containing
species, while maintaining the steric bulk required to prevent
adduct formation, the phosphinites tBu2POR, R = tBu 3; Ph
4; 2,6-Me2C6H3 5 were easily synthesized by reaction of tBu2PCl
with the corresponding potassium alkyl- and aryloxides. Reaction
of compound 3 with B(C6F5)3 demonstrated evidence for the
quantitative formation of a new species, 6. Particularly diagnostic
Scheme 3 Proposed mechanism of formation of 6.
for this species is the new broad P–H peak at 5.33 ppm with 1JH–P
=
452 Hz observed in the 1H NMR spectrum. The 19F resonances at
In contrast, the bulky di-tert-butylphosphinites 4 and 5 showed
no reaction with B(C6F5)3. The derived FLP mixtures were found
to react under an atmosphere of H2 (4 atm) affording the salts, 7
and 8. In both cases new peaks for the P–H protons in the 1H NMR
spectra were observed at 6.98 ppm (1JP–H = 474 Hz) and 7.02 ppm
(1JP–H = 483 Hz), respectively. In addition, quartets attributable to
B–H protons were also observed at 3.55 ppm (1JB–H = 93 Hz) and
3.51 ppm (1JB–H = 93 Hz), respectively. These data were consistent
with the formulation of the products as [tBu2P(OR)H][HB(C6F5)3]
R = Ph 7 and 2,6-Me2C6H3 8. The structure of 8 was also
unambiguously confirmed crystallographically (Fig. 3).
-132.3, -157.2 and -163.8 ppm together with the 31P{ H} signal at
1
77.0 and the 11B{ H} peak at -0.30 ppm, suggest the formulation of
1
6 as the secondary phosphine-oxide adduct tBu2(H)POB(C6F5)3.
This was also crystallographically confirmed (Fig. 2). The metric
parameters for 6 are unexceptional, with a B–O bond length of
˚
1.524(2) A which is identical to those reported for other phosphine
oxide adducts of B(C6F5)3.35
In order to probe the details of the course of the formation
of 6, the reaction was monitored by NMR spectroscopy. After
1
1 h, H NMR peaks at 1.59 and 4.73 ppm in a 3 : 1 ratio, were
observed characteristic of the methyl and alkenyl resonances of iso-
butene. This product in addition to 6 was formed quantitatively.
These observations suggest that the interaction of the phosphinite
3 with B(C6F5)3 proceeds via initial coordination of boron to
the, more accessible oxygen center. This interaction increases the
The metrical parameters are similar to those reported for related
phosphonium borate ion pairs, with a relatively large P–H—H–B
˚
contact of 2.348 A. This distance is shorter than those observed
2
˚
for the pair [tBu3PH][HB(C6F5)3] (2.75 A) and the zwitterion
tBu2(H)PC6F4B(H)(C6F5)2 (2.60 A). This is likely due to the
1
˚
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In order to probe the mechanism of formation of 9 the reaction
was repeated using D2 in place of H2. The resulting product
exhibited a quintet31P NMR signal at 21.4 ppm exhibiting P-
D coupling of 71 Hz. These data in addition to other NMR
data confirmed the product as [tBu2PD2][ClB(C6F5)3] 10. This
observation supports the notion that the two protons on P are
derived from H2 gas. This suggests that the initial product resulting
from the activation of H2 by the FLP is [tBu2P(H)Cl][HB(C6F5)3],
and this subsequently undergoes chloride/hydride redistribution
to give the more thermodynamically favorable salt (Scheme 4).
Presumably this redistribution is facilitated by borane. Similar sub-
stituent resdistribution reactions have been previously observed.36
Scheme 4 Proposed mechanism for the formation of 9.
Fig. 3 POV-ray depiction of 8. Hydrogen atoms, except for the P–H and
B–H, are omitted for clarity.
Varying B-substituents in FLP activation of H2
Use of the diborane 1,4-(C6F5)2B(C6F4)B(C6F5)228 in combination
with either tBu3P or (C6H2Me3)3P led to FLP behavior and reac-
tivity. Addition of H2 afforded the double activation of H2 to give
products formulated as [R3PH]2[1,4-(C6F5)2HB(C6F4)BH(C6F5)2]
(R = tBu 11, C6H2Me3 12) (Scheme 5). These compounds were
isolated in high yield and gave the expected and diagnostic
resonances in the 1H, 11B, 19F and 31P NMR spectra. These
observations are perhaps not surprising given the similarity of the
Lewis acidity of the B centers in this bis-borane to that of B(C6F5)3.
Nonetheless, the present result demonstrates that formation of a
borate at a single B center of the bis-borane, does not impact on
the Lewis acidity of the remaining borane center sufficiently in
order to deter further reaction with H2.
lesser degree of steric congestion about the PH fragment in the
phosphonium cation of 8.
As phosphine chlorides are known to exhibit similar properties
to phosphinites, the analogous chemistry of tBu2PCl was explored.
In combination with B(C6F5)3, no reaction was observed at
room temperature demonstrating the formation of an FLP. Upon
addition of H2, very slow conversion to a single new species 9 was
observed over 16 days. The 11B NMR signal appears as a singlet
at -6.5 ppm and the 31P signal as a triplet at 27.1 ppm (1JP–H
463 Hz), suggesting the formulation of [tBu2PH2][ClB(C6F5)3].
This formulation was subsequently confirmed crystallographically
(Fig. 4). While the molecular metric parameters were typical, the
=
˚
closest P–H–Cl contact was found to be 2.796 A.
Scheme 5 Formation of 11-13.
27
The combination of PhB(C6F5)2 and tBu3P was shown spec-
troscopically to behave as a non-interacting FLP, and addition
of H2 prompts the generation of the salt, [tBu3PH][HBPh(C6F5)2]
13 as a clear oil which was subsequently isolated as crystals in
74% yield. This view is supported by the observation of a broad
doublet at 5.12 ppm in the 1H NMR with P–H coupling of 427 Hz
Fig. 4 POV-ray depiction of 9. Hydrogen atoms except for the P–H are
omitted for clarity.
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accompanied by a broad quartet at 3.66 ppm with B–H coupling
of 89 Hz. In addition, the narrowing of the gap between the meta
and para-fluorine 19F NMR signals and the 11B NMR signal at
-19.71 ppm are consistent with borate anion formation. This was
subsequently confirmed by crystallographic data (Fig. 5). The
metric parameters are unexceptional. This reactivity demonstrates
that the Lewis acidity of the borane can be reduced without
sacrificing the ability to activate H2.
The NMR data were as expected with 11B chemical shifts of 29.8,
29.9 and 29.9 ppm for 15, 16 and 17, respectively.
The species 15 was also crystallographically characterized
(Fig. 6). This boronic ester exhibited planar three coordinate
˚
geometry at B, with B–O distances averaging 1.3812(14) A, and
˚
B–C distance of 1.5509(16) A. The chelating catecholate O–B–O
angle was found to be 112.10(10)◦, which resulted in O–B–C angles
of 123.53(10)◦ and 124.36(10)◦.
Fig. 6 POV-ray depiction of 15. Hydrogen atoms omitted for clarity.
Fig. 5 POV-ray depiction of 13. Hydrogen atoms, except for the P–H and
B–H, are omitted for clarity.
In
a similar fashion the species B(OC6H3(CF3)2)3 18,
B(OC6H2F3)3 19 and B(OC6H4CF3)3 20 were prepared from
the reaction of the corresponding substituted phenol with BCl3
(Scheme 6). These species were obtained in isolated yield ranging
Incorporation of aryloxide substituents on B also reduces the
from 58-92%. 11B{ H} NMR resonances were observed for these
1
29
Lewis acidity at B. The combination of B(OC6F5)3 and PtBu3
borate esters between 15 and 16 ppm. The upfield shift is consistent
with the greater donor ability of the oxygen-based ligands.
Molecular structures of 18 (Fig. 7) and 20 (Fig. 8) confirmed
the expected three-fold symmetry of the planar B centers. B–
were found to generate an FLP in solution. This combination
subsequently reacts with H2 to give the product 14, which was
isolated in 25% yield. The NMR spectroscopic data of 14 were con-
sistent with the formulation, [tBu3PH][B(OC6F5)4]. Preliminary
crystallographic data confirmed this formula although the poor
data quality precluded publication.37 While the reaction is thought
to proceed via the initial generation of [tBu3PH][HB(OC6F5)3],
it appears that this species is unstable with respect to ligand
redistribution, prompting the formation of 14 in low isolated
yield. The precise details of this redistribution process are not
known although ligand scrambling of borate anions is thought to
be similar to that seen for the formation of 9.
˚
˚
O distances averaged 1.362(5) A and 1.360(3) A in 18 and 20,
respectively. The corresponding average O–B–O angles were found
to be 125.4(3)◦ and 128.3(3)◦.
To further probe the impact of Lewis acidity, the boronic esters,
C6H4O2BC6F5 15, (C6H3FO2)BC6F5 16 and C6F4O2BC6F5 17 were
prepared by a condensation reaction of the appropriate substituted
diol with pentafluoroboronic acid in toluene (Scheme 6). This
afforded 15, 16 and 17 in yields ranging from 42-78% (Scheme 6).
Fig. 7 POV-ray depiction of 18. Hydrogen atoms omitted for clarity.
Combinations of 15-17 with tBu3P or (C6H2Me3)3P were
Scheme 6 Synthesis of 15-20.
monitored by 11B, 19F and 31P NMR spectroscopy. In each case
4292 | Dalton Trans., 2010, 39, 4285–4294
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“soft” donor crotonaldehyde, are weaker than with the “harder”
phosphineoxide yielding the differing rankings. Nonetheless, it is
clear that FLP activation of H2 only proceeds for the cases where
the Lewis acidity of the Lewis acid is towards the higher end of
these scales. Furthermore, it is apparent that there is a threshold
of combined Lewis acidity and basicity that is required to effect
the splitting of H2. These experimental observations are in accord
with recent computational studies recently published44,45 In these
works, the researchers put forth the notion that the approach of
the constituents of an FLP generates an electric field that effects
the polarization of H2 prompting heterolytic cleavage of H2. The
present results suggest that the boranes 15-17 in combination with
the phosphine used above generate only a weak electric field upon
approach and thus are incapable of H2 activation. Indeed, the
FLP systems most reactive with H2 involve strong Lewis acids and
strong bases, combinations that would presumably generate the
strongest electric field gradient.
Conclusions
Variations in the substituents on phosphorus and boron of
frustrated Lewis pairs have been examined. Substituent variation
can result in unexpected C–H bond reactivity and substituent
scrambling reactions. While bulky substituents are necessary to
preclude the formation of classical Lewis acid–base adducts, the
generation of an FLP is not a sufficient condition to ensure
subsequent reaction with H2. For heterolytic activation of H2, it
appears that the Lewis acid partner of the FLP must incorporate
strongly electron withdrawning substituents, while basic and bulky
bases are desirable. The reactivity of new FLPs with other
substrate molecules continues to be a subject of investigation in
our laboratories.
Fig. 8 POV-ray depiction of 20. Hydrogen atoms omitted for clarity.
the spectral data were unchanged from those of the separate com-
ponents, inferring the potential for FLP reactivity. Nonetheless,
exposure of these mixtures to 4 atm of H2 at 25 ◦C resulted in no
apparent reactions while combinations of 18-20 with tBu3P under
4 atm of H2 at ambient temperature resulted in redistribution
products analogous to 14. These data clearly suggest that while
formation of an FLP is a necessary condition for H2 activation, it
is not a sufficient one.
In an attempt to correlate the Lewis acidity limitations on
FLP reactivity, the present boranes 15-20 were studied via
the Gutmann-Beckett39-42 and Childs43 methods. These methods
correlate Lewis acidity of an acceptor to NMR parameters
resulting from the interactions of a borane with Et3PO or
crotonaldehyde respectively (Table 2). The results of the Gutmann-
Beckett methods suggest increasing Lewis acidity is 20 <19 <17
<18 <16 <15 <BPh(C6F5)2 <B(C6F5)3 <B(OC6F5)3 whereas the
Childs methods ranks the same Lewis acids in the order 15ª16
<20 <19 <18 <17<B(OC6F5)3 <BPh(C6F5)2 <B(C6F5)3. As the
present Lewis acids are relatively “hard”, interactions with the
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Gutmann-Beckett
Childs
Dd
AN
Dd
CN
B(C6F5)3
BPh(C6F5)2
B(OC6F5)3
15
16
17
18
19
20
26.59
25.2
65.9
67.2
62.3
65.2
64.5
64.1
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63.5
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1.06
0.73
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0.31
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0.05
38
29
31.17
27.47
28.34
28.80
28.74
29.63
30.94
Dd = difference in chemical shift; AN = acceptor number; CN = Childs
number.
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