Activation of H2 using P/Al based frustrated Lewis pairs and reactions with olefins

Article abstract of DOI:10.1039/c3dt51739j

The activation of H₂ using Mes₃P-AlX₃ (X = Cl, Br, I) FLPs was investigated. Heating a 1:2 mixture of Mes ₃P-AlI₃ under H₂ afforded [Mes ₃PH][H(AlI₃)₂] 1. The alane Al(p-C ₆F₄H)₃2 was prepared and a 1:2 solution of (otol)₃P and 2 under H₂ generated the salt [(otol) ₃PH][(μ-H)(Al(p-C₆F₄H)₃) ₂] which could not be isolated. However, the bulky anion salt [Li(OEt₂)][FAl(C₁₂F₉)₃] was readily converted to [Et₄N][FAl(C₁₂F₉)₃] 3 and subsequently to the hydride salt [Et₄N][HAl(C₁₂F ₉)₃] 4. Further reaction of 4 with [CPh ₃][B(C₆F₅)₄] generated the free alane, Al(C₁₂F₉)₃ which in the presence of Mes₃P and exposure to H₂ resulted in the formation of [Mes₃PH][(μ-H)(Al(C₁₂F₉)₃) ₂] 5. Exposing a solution of 1 in C₆D₅Br with ethylene, propylene or cyclohexene followed by quenching with water resulted in liberation of ethane, propane, or cyclohexane, respectively. In contrast, exposure of 5 to an atmosphere of ethylene showed no evidence of reaction. In related chemistry, protonation of [Et₄N][EtAl(C₆F ₅)₃] by the phosphonium salts [R₃PH][Al(C ₆F₅)₄] (R = otol (6), Mes (7), tBu (8)) and [(2,6-Cl₂C₆H₃)₃PH][AlCl ₄] (9) was also examined. In the case of the reaction of 6 and 9 ethane was liberated whereas with 7 and 8, no reaction occurred. The implications for transition metal-free hydrogenation catalysis are considered.

Full text of DOI:10.1039/c3dt51739j

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Activation of H2 using P/Al based frustrated Lewis pairs
and reactions with olefins†
Cite this: Dalton Trans., 2013, 42, 13685
Gabriel Ménard, Lina Tran and Douglas W. Stephan*
The activation of H2 using Mes3P–AlX3 (X = Cl, Br, I) FLPs was investigated. Heating a 1 : 2 mixture of
Mes3P–AlI3 under H2 afforded [Mes3PH][H(AlI3)2] 1. The alane Al(p-C6F4H)3 2 was prepared and a 1 : 2
solution of (otol)3P and 2 under H2 generated the salt [(otol)3PH][(µ-H)(Al(p-C6F4H)3)2] which could not
be isolated. However, the bulky anion salt [Li(OEt2)][FAl(C12F9)3] was readily converted to [Et4N]-
[FAl(C12F9)3] 3 and subsequently to the hydride salt [Et4N][HAl(C12F9)3] 4. Further reaction of 4 with
[CPh3][B(C6F5)4] generated the free alane, Al(C12F9)3 which in the presence of Mes3P and exposure to H2
resulted in the formation of [Mes3PH][(µ-H)(Al(C12F9)3)2] 5. Exposing a solution of 1 in C6D5Br with ethylene,
propylene or cyclohexene followed by quenching with water resulted in liberation of ethane, propane, or
cyclohexane, respectively. In contrast, exposure of 5 to an atmosphere of ethylene showed no evidence
of reaction. In related chemistry, protonation of [Et4N][EtAl(C6F5)3] by the phosphonium salts [R3PH]-
[Al(C6F5)4] (R = otol (6), Mes (7), tBu (8)) and [(2,6-Cl2C6H3)3PH][AlCl4] (9) was also examined. In the case
of the reaction of 6 and 9 ethane was liberated whereas with 7 and 8, no reaction occurred. The impli-
cations for transition metal-free hydrogenation catalysis are considered.
Received 28th June 2013,
Accepted 22nd July 2013
DOI: 10.1039/c3dt51739j
www.rsc.org/dalton
Introduction
Catalytic hydrogenation of olefins is most often achieved with
transition metal catalysts. Among the best known systems are
the Ru and Rh-based catalysts from the groups of Wilkinson1,2
and Noyori3–5 although a large number of others have been
explored and evaluated. In such reductions, the catalysts effect
the cleavage of the H–H bond via oxidative addition or by
heterolytic cleavage yielding a protonated ligand and a metal
hydride. In either case, subsequent olefin reduction occurs via
initial hydride delivery. In the last five years, a number of
groups have probed the ability of non-metal systems in hydro-
genation catalysis. Recent and highly innovative examples have
been described by the groups of Harder and Okuda who
reported Ca-based systems that act to hydrogenate olefins such
as 1,1-diphenylethylene.6–8
Other metal-free hydrogenation catalysts have been develo-
ped based on “frustrated Lewis pairs” (FLPs), in which combi-
nations of Lewis acids and bases act in concert to activate
hydrogen9 and then deliver proton and hydride to sub-
strates,10 such as imines, nitriles, aziridines, enamines and
silyl enol ethers.11–14 More recently this has been extended
Scheme 1 Catalytic hydrogenation of activated olefins using FLPs.
to activated olefins, such as 1,1-diphenylethylene.15 Poorly
Lewis basic triarylphosphines in combination with the
Lewis acidic borane, B(C6F5)3 generates a highly acidic phos-
phonium cation that readily protonates 1,1-disubstituted
olefins. The borohydride anion delivers hydride to the
carbocation (Scheme 1) regenerating the FLP for further H2
activation.
In targeting related systems that might be capable of cata-
lyzing the hydrogenation of simple olefins, we have recently
studied the reactions of aluminum-based FLPs with H2 and
unactivated olefins affording phosphonium alkylaluminates,
[R′Al(C6F5)3]−, which undergo substituent redistribution to the
anion [Al(C6F5)4]− and the alkyl alane, R′Al(C6F5)2 (Scheme 2).16
In the present study, we further probe steric and electronic
properties on the reactions of P/Al FLPs with H2. The hydrogen
activation products are characterized and the subsequent
reactivity with olefins is also probed.
Department of Chemistry, University of Toronto, 80 St George St., Toronto, Ontario,
Canada, M5S3H6. E-mail: dstephan@chem.utoronto.ca; Tel: +1-416-946-3294
†CCDC 947825–947827. For crystallographic data in CIF or other electronic
format see DOI: 10.1039/c3dt51739j
This journal is © The Royal Society of Chemistry 2013
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values of the coupling constants are in Hz. NMR assignments
are supported by additional 2D experiments. Elemental ana-
lyses (C, H, N) were performed in house.
Synthesis of [Mes3PH][(μ-H)(AlI3)2] (1)
A 50 mL Schlenk bomb equipped with a Teflon screw cap and
a
magnetic stirbar was charged with Mes3P (300 mg,
0.77 mmol), AlI3 (630 mg, 1.54 mmol) and fluorobenzene (ca.
10 mL). The bomb was transferred to the Schlenk line
equipped with a H2 outlet. The bomb was immersed in a
liquid N2 bath, degassed, filled with H2, and warmed to 25 °C.
The mixture was heated to 75 °C with rapid stirring on an oil
bath behind a blast shield for 4 h yielding a pale yellow solu-
tion. The solution was cooled to 25 °C and filtered through
Celite. The solvent was removed in vacuo and hexanes (ca.
Scheme 2 Reactions of Al-based FLPs with H2 and ethylene.
Experimental section
General considerations
All manipulations were performed under an atmosphere of 10 mL) were added to the residue and the mixtures stirred
dry, oxygen-free N2 by means of standard Schlenk or glovebox rapidly for 2–3 h. The resulting white powder was filtered,
techniques (Innovative Technology glovebox equipped with a washed with hexanes, and dried (680 mg, 0.56 mmol, 73%). 1H
1
−38 °C freezer). Hexanes, pentane, dichloromethane, and NMR (C6D5Br): 8.10 (d, JH–P = 480 Hz, 1H, PH), 6.79 (bs, 3H,
diethyl ether (Aldrich) were dried using an Innovative Techno- m-Mes), 6.73 (bs, 3H, m-Mes), 6.26 (bs, 1H, Al-H), 2.11 (s, 9H,
logies solvent system. Fluorobenzene and bromobenzene (–H5 p-CH3Mes), 2.10 (bs, 9H, o-CH3Mes), 1.79 (bs, 9H, o-CH3Mes). 31
P
and –D5) were purchased from Aldrich and dried on P2O5 for {1H} NMR (161 MHz, C6D5Br): −26 (s). 27Al NMR (C6D5Br): 36
4
several days and vacuum distilled onto 4 Å molecular sieves (bs, ν1/2 = ca. 800 Hz). 13C{1H} NMR (C6D5Br): 146.9 (d, JC–P
=
prior to use. Cyclohexene (inhibitor free) was purchased from 3 Hz, p-C6H2), 143.7 (bs, o-C6H2), 142.7 (bs, o-C6H2), 133.1 (bs,
2
Alfa, dried over CaH2 and distilled. tBu3P, (otol)3P, Mes3P, and m-C6H2), 131.9 (bs, m-C6H2), 111.3 (d, JC–P = 848 Hz, i-C6H2),
5
AlMe3 were purchased from Strem and used without further 22.3 (bs, o-CH3Mes), 22.0 (bs, o-CH3Mes), 21.6 (d, JC–P = 2 Hz,
purification. LiAlH4 was purchased from Strem and purified p-CH3Mes). Anal. Calc. for C27H35Al2I6P: C, 26.89; H, 2.93.
according to literature procedure.17 AlI3 was purchased from Found: C, 26.48; H, 3.01.
Strem and sublimed three times prior to use under vacuum
Synthesis of Al(C6F4H)3·tol (2)
using a −78 °C cold finger and a 150 °C bath. PCl3 was pur-
chased from Aldrich and distilled prior to use. Me2SiHCl, 1,3- This compound was synthesized in an analogous fashion to
dichlorobenzene, BF3(OEt2), BuLi (1.6 M in hexanes), and HCl Al(C6F5)3·tol.18 A round bottom flask in the glovebox was
(2.0 M in Et2O) were purchased from Aldrich and used without charged with B(C6F4H)3 (350 mg, 0.76 mmol) and AlMe3
further purification. Et4NCl was purchased from Aldrich and (55 mg, 0.76 mmol) in toluene (ca. 10 mL). The solution was
dried under vacuum for several days. B(C6F5)3 was purchased allowed to stir rapidly overnight. The next morning, the solvent
from Boulder Scientific, sublimed under vacuum, then treated volume was reduced to 7–8 mL and the hot solution put in the
with excess Me2SiHCl for 4 h and re-sublimed after removal of −38 °C freezer. The crystals obtained (suitable for X-ray diffrac-
volatiles. [Ph3C][B(C6F5)4] was purchased from Boulder Scienti- tion) were filtered on a frit and washed with minimal hexanes.
fic and purified by precipitation from a saturated dichloro- The solid was lightly dried in vacuo (300 mg, 0.53 mmol, 70%).
methane solution with pentane. C6BrF5 and C6BrF4H were 1H NMR (C6D5Br): 7.18–6.98 (m, 5H, C6H5CH3), 6.62 (tt, 3JH–F
=
purchased from Apollo Scientific and used without further 9 Hz, JH–F = 7.1 Hz, 3H, C6F4H), 2.16 (s, C6H5CH3). 27Al NMR
purification. Al(C6F5)3·tol was prepared from B(C6F5)3 and (C6D5Br): blank. 19F{1H} NMR (376 MHz, C6D5Br): −121.6 (m,
AlMe3,18 (2,6-Cl2C6H3)3P was prepared from PCl3 and 1,3- o-C6F4H, 6F), −137.5 (m, m-C6F4H, 6F). 13C{1H} NMR (C6D5Br):
dichlorobenzene,19 B(C6F4H)3 was prepared from C6BrF4H 149.7 (dm, 1JC–F = 236 Hz), 145.4 (dm, 1JC–F = 250 Hz), 137.5 (s,
and BF3,20 C12BrF9 was prepared from C6F5Br and [Li(OEt2)]- C6H5CH3), 129.1 (s, C6H5CH3), 128.3 (s, C6H5CH3), 125.4 (s,
4
21
[FAl(C12F9)3] was prepared from C12BrF9 and AlCl3 and 8 was C6H5CH3), 117.9 (m, i-C6F4H), 108.3 (t, 2JC–F = 23 Hz, p-C6F4H),
prepared from tBu3P, Al(C6F5)3, and propylene,22 all according 21.5 (s, C6H5CH3). Anal. Calc. for Al(C6F4H)3·0.8(C7H8): C,
to literature procedures. Ethylene (grade 3.0) and propylene 51.74; H, 1.73. Found: C, 52.14; H, 1.74.
(grade 2.5) were purchased from Linde and passed through a
Synthesis of [Et4N][FAl(C12F9)3] (3)
Restek oxygen scrubber and a Restek moisture trap prior to
use. H2 (grade 5.0) was purchased from Linde and dried [Li(OEt2)][FAl(C12F9)3] (1.5 g, 1.40 mmol) was dissolved in
through a Nanochem WeldAssure purifier column prior to use. dichloromethane (15 mL) in a 50 mL flask in the glovebox. A
NMR spectra were obtained on a Bruker Avance 400 MHz or a solution of Et4NCl (232 mg, 1.40 mmol) in dichloromethane
Varian 400 MHz and spectra were referenced to residual (5 mL) was added dropwise to the rapidly stirring solution. Pre-
solvent of C6D5Br or externally (27Al: Al(NO3)3, 31P: 85% H3PO4, cipitation occurs immediately. The mixture was allowed to
19F: CFCl3). Chemical shifts listed are in ppm and absolute stir for an additional 2 h. The solvent was thoroughly removed
13686 | Dalton Trans., 2013, 42, 13685–13691
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in vacuo and fluorobenzene (30 mL) was added to the residue. stirbar. The bomb was transferred to the Schlenk line
The mixture was filtered on Celite. Pentane (10–15 mL) was equipped with a H2 outlet. The bomb was immersed in a
added dropwise to the rapidly stirring filtrate to yield a white liquid N2 bath, degassed, filled with H2, and slowly warmed to
precipitate which was filtered, washed with pentane, and dried 25 °C. The solution was stirred for 12 h in the glovebox after
3
(1.1 g, 0.98 mmol, 70%). 1H NMR (C6D5Br): 2.28 (q, JH–H
=
which time a precipitate had formed. Hexanes (ca. 5–7 mL)
3
3
7 Hz, 8H, N(CH2CH3)4), 0.57 (tt, JH–H = 7 Hz, JH–N = 2 Hz, were added dropwise to the stirring mixture and the precipi-
12H, N(CH2CH3)4). 27Al NMR (C6D5Br): 112 (bs, ν1/2 = ca. tate was filtered on a glass frit, washed with hexanes and dried
900 Hz). 19F{1H} NMR (376 MHz, C6D5Br): −121.6 (m, 3F), −139.0 in vacuo (120 mg, 0.051 mmol, 38%). Vapour diffusion of a bro-
(m, 3F), −141.6 (m, 6F), −156.8 (m, 3F), −157.3 (m, 3F), −157.9 mobenzene solution with hexanes yielded single crystals suit-
(m, 3F), −165.2 (m, 6F), −175.7 (bs, 1F, Al–F). 13C{1H} NMR able for X-ray crystallography. H NMR (C6D5Br), partial: 8.02
1
1
1
1
(C6D5Br), partial: 151.7 (dm, JC–F = 234 Hz), 145.3 (dm, JC–F
=
(d, JH–P = 478 Hz, 1H, PH), 6.78 (bs, 3H, m-Mes), 6.73 (bs, 3H,
247 Hz), 144.8 (dm, 1JC–F = 242 Hz), 141.3 (dm, 1JC–F = 252 Hz), m-Mes), 2.10 (s, 9H, p-CH3Mes), 2.01 (s, 9H, o-CH3Mes), 1.78 (s,
139.6 (dm, JC–F = 245 Hz), 137.3 (dm, JC–F = 229 Hz), 116.3 9H, o-CH3Mes). 31P{1H} NMR (161 MHz, C6D5Br): −26 (s). 27Al
1
1
(bs), 110.7 (bm), 52.1 (t, JC–N = 3 Hz, N(CH2CH3)4), 6.6 NMR (C6D5Br): blank. 19F{1H} NMR (376 MHz, C6D5Br): −112
1
(s, N(CH2CH3)4). Anal. Calc. for C44H20AlF28N: C, 47.12; H, (bm, 4F), −134.7 (bm, 4F), −135.3 (bm, 2F), −136.8 (bm, 6F),
1.80; N, 1.25. Found: C, 46.64; H, 2.06; N, 1.69.
−137.7 (bm, 2F), −138.0 (bm, 4F), −154.6 (bm, 6F), −155.0
(bm, 6F), −156.0 (bm, 2F), −156.9 (bm, 4F), −162.1 (bm, 4F),
−162.3 (bm, 4F), −162.7 (bm, 6F). 13C{1H} NMR (C6D5Br): due
Synthesis of [Et4N][HAl(C12F9)3] (4)
In a 50 mL flask in the glovebox was dissolved 3 (900 mg, to the extremely poor solubility of this compound could not be
0.80 mmol) in fluorobenzene (10 mL). A solution of LiAlH4 recorded. Anal. Calc. for C99H35Al2F54P: C, 50.92; H, 1.51.
(37 mg, 0.97 mmol) in diethyl ether (3 mL) was added drop- Found: C, 50.78; H, 1.85.
wise to the rapidly stirring solution. The solution was allowed
to stir overnight (12 h) during which time precipitation
Synthesis of [(otol)3PH][Al(C6F5)4] (6)
occurred. The solvent was thoroughly removed in vacuo and A 50 mL Schlenk bomb equipped with a Teflon screw cap and
fluorobenzene (20 mL) was added to the residue. The mixture
a magnetic stirbar was charged with (otol)3P (98 mg,
was filtered on Celite and the filtrate concentrated to 10 mL. 0.32 mmol), Al(C6F5)3·tol (400 mg, 0.64 mmol), ca. 50 equiv.
Pentane (10 mL) was added dropwise to the rapidly stirring cyclohexene (1.60 mL, 15.8 mmol), and fluorobenzene (ca.
solution to yield a white precipitate which was filtered, washed 5 mL). The bomb was transferred to the Schlenk line equipped
with pentane, and dried (770 mg, 0.70 mmol, 87%). Crystals with a H2 outlet. The bomb was immersed in a liquid N2 bath,
suitable for X-ray diffraction were obtained from a cooled degassed, filled with H2, and slowly warmed to 25 °C. The
1
(−38 °C) toluene solution. H NMR (C6D5Br): 2.50 (bs, 1H, Al– solution was stirred on a 60 °C oil bath for 72 h. The bomb
3
3
H), 2.32 (q, JH–H = 7 Hz, 8H, N(CH2CH3)4), 0.62 (tt, JH–H
=
was cooled, depressurized and ca. 1.5 mL Et2O was added to
7 Hz, JH–N = 2 Hz, 12H, N(CH2CH3)4). 27Al NMR (C6D5Br): 124 the stirring solution. Hexanes (ca. 10 mL) were then added
(bs, ν1/2 = ca. 1100 Hz). 19F{1H} NMR (376 MHz, C6D5Br): dropwise with rapid stirring. The precipitate that forms was fil-
3
1
−118.2 (m, 3F), −138.8 (m, 3F), −140.5 (m, 6F), −155.7 (m, 3F), tered and dried (190 mg, 0.19 mmol, 59%). H NMR (C6D5Br):
−156.6 (m, 3F), −157.8 (m, 3F), −163.8 (m, 6F). 13C{1H} NMR 8.04 (d, JH–P = 482 Hz, 1H, PH), 7.33 (t, J = 8.0 Hz, 3H),
1
1
1
(C6D5Br), partial: 151.6 (dm, JC–F = 226 Hz), 145.2 (dm, JC–F
=
7.07–7.01 (m, 6H), 6.84–6.78 (m, 3H), 1.99 (s, 9H, o-CH3). 31P
253 Hz), 144.6 (dm, 1JC–F = 248 Hz), 141.3 (dm, 1JC–F = 253 Hz), {1H} NMR (161 MHz, C6D5Br): −13.2 (s). 27Al NMR (C6D5Br):
139.7 (dm, JC–F = 246 Hz), 139.2 (dm, JC–F = 250 Hz), 137.5 116 (s). 19F{1H} NMR (376 MHz, C6D5Br): −122.3 (bd, JF–F
=
1
1
3
1
1
3
(dm, JC–F = 250 Hz), 116.4 (bm), 110.9 (bm), 52.1 (t, JC–N
=
18.8 Hz, 8F, o-C6F5), −157.7 (t, JF–F = 18.8 Hz, 4F, p-C6F5),
3.0 Hz, N(CH2CH3)4), 6.6 (s, N(CH2CH3)4). Anal. Calc. for −163.9 (m, 8F, m-C6F5). 13C{1H} NMR (C6D5Br), partial: 150.2
1
1
C44H21AlF27N: C, 47.89; H, 1.92; N, 1.27. Found: C, 47.82; H, (dm, JC–F = 231 Hz), 143.0 (d, JC–P = 9 Hz), 140.3 (dm, JC–F
2.16; N, 1.50.
=
250 Hz), 136.5 (dm, 1JC–F = 251 Hz), 136.4 (d, JC–P = 3 Hz), 134.2
(d, JC–P = 13 Hz), 132.9 (d, JC–P = 11 Hz), 128.2 (d, JC–P = 14 Hz),
Synthesis of [Mes3PH][(µ-H)(Al(C12F9)3)2] (5)
1
3
111.2 (d, JC–P = 87 Hz, i-C6H4), 20.4 (d, JC–P = 9 Hz, o-CH3).
In a vial in the glovebox was dissolved [Ph3C][B(C6F5)4] Anal. Calc. for C45H22AlF20P: C, 54.02; H, 2.22. Found: C,
(251 mg, 0.27 mmol) in fluorobenzene (3–4 mL). To this stir- 53.64; H, 2.49.
ring solution was added a solution of 4 (300 mg, 0.27 mmol)
dropwise. The initial orange solution turned pale yellow and
Synthesis of [Mes3PH][Al(C6F5)4] (7)
was allowed to stir rapidly for 10 min. The solvent was This compound could be isolated in a similar fashion to the
thoroughly removed and the residue was dissolved in hexanes salt [tBu3PH][Al(C6F5)4] (8).22 A 50 mL Schlenk bomb equipped
and filtered on Celite. The filter cake was washed with copious with a Teflon screw cap and a magnetic stirbar was charged
amounts of hexanes. The filtrate solvent was removed and with Mes3P (100 mg, 0.26 mmol), Al(C6F5)3·tol (319 mg,
Mes3P (53 mg, 0.14 mmol) in bromobenzene (10 mL) was 0.51 mmol), and fluorobenzene (ca. 5 mL). The bomb was
added to this. The solution was transferred to a 50 mL Schlenk transferred to the Schlenk line equipped with a propylene
bomb equipped with a Teflon screw cap and a magnetic outlet. The bomb was degassed and filled with propylene
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(1 atm). The solution was stirred on a 50 °C oil bath overnight. data. In the latter cases atoms were treated isotropically. C–H
The bomb was cooled and the solvent removed in vacuo. atom positions were calculated and allowed to ride on the
Hexanes (ca. 10 mL) were added to the residue and the carbon to which they are bonded assuming a C–H bond length
mixture was stirred rapidly until a white precipitate appeared. of 0.95 Å. H-atom temperature factors were fixed at 1.20 times
The precipitate was filtered and dried (160 mg, 0.15 mmol, the isotropic temperature factor of the C-atom to which they
1
1
57%). H NMR (C6D5Br): 7.99 (d, JH–P = 478 Hz, 1H, PH), 6.75 are bonded. The H-atom contributions were calculated, but
(bs, 3H, m-Mes), 6.70 (bs, 3H, m-Mes), 2.08 (s, 9H, p-CH3Mes), not refined. The locations of the largest peaks in the final
1.98 (s, 9H, o-CH3Mes), 1.75 (s, 9H, o-CH3Mes). 31P{1H} NMR difference Fourier map calculation as well as the magnitude of
(161 MHz, C6D5Br): −26 (s). 27Al NMR (C6D5Br): 116 (s). 19F the residual electron densities in each case were of no chemi-
3
{1H} NMR (376 MHz, C6D5Br): −122.3 (bd, JF–F = 19 Hz, 8F, cal significance.
3
o-C6F5), −157.9 (t, JF–F = 19 Hz, 4F, p-C6F5), −163.9 (m, 8F,
1
m-C6F5). 13C{1H} NMR (C6D5Br), partial: 150.2 (dm, JC–F
=
236 Hz), 147.2 (d, 4JC–P = 3 Hz, p-C6H2), 143.8 (bs, o-C6H2), 142.3
Results and discussion
1
1
(bs, o-C6H2), 140.3 (dm, JC–F = 246 Hz), 136.6 (dm, JC–F
=
=
251 Hz), 133.1 (bs, m-C6H2), 131.7 (bs, m-C6H2), 111.1 (d, 2JC–P
The activation of H2 using Mes3P–AlX3 (X = Cl, Br, I) FLPs
83 Hz, i-C6H2), 21.8 (bs, o-CH3Mes), 21.3 (d, JC–P = 1 Hz, was investigated by exposing 1 : 2 solutions of Mes3P–AlX3 in
p-CH3Mes), 20.9 (bs, o-CH3Mes). Anal. Calc. for C51H34AlF20P: C, bromobenzene to H2 at 25 °C. This resulted in no reaction over
5
56.47; H, 3.16. Found: C, 56.17; H, 3.10.
24 h as evidenced by NMR spectroscopy. The 31P NMR spec-
trum displayed only signals attributable to the weak adducts,
Mes3P–AlX3.23 In order to favour dissociation, the reaction mix-
Synthesis of [(2,6-Cl2C6H3)3PH][AlCl4] (9)
A 50 mL Schlenk flask was charged with (2,6-Cl2C6H3)3P tures were heated under H2. While the reactions with AlCl3
(200 mg, 0.43 mmol), AlCl3 (57 mg, 0.43 mmol), and fluoro- and AlBr3 were sluggish, heating to 75 °C a 1 : 2 mixture of
benzene (ca. 5 mL). The flask was transferred to the Schlenk Mes3P–AlI3 in fluorobenzene under 4 atm of H2 for 4 h
line and HCl (2.0 M in Et2O; 0.23 mL, 0.46 mmol) was added resulted in the isolation of a pure white powder (1) after
dropwise with rapid stirring. A precipitate formed which was solvent removal and trituration with hexanes. The 27Al NMR
filtered on a swivel frit and washed with hexanes (160 mg, spectrum displayed a single broad resonance at 36 ppm, while
1
0.25 mmol, 58%). 1H NMR (C6D5Br): 9.33 (d, JH–P = 561 Hz, the 1H and 31P NMR spectra displayed signals indicative of the
3
1H, PH), 7.21 (t, JH–H = 8 Hz, 3H, p-Cl2C6H3), 7.09–7.06 presence of the [Mes3PH]+ cation.16 In addition to these 1H reso-
(m, 6H). 31P{1H} NMR (161 MHz, C6D5Br): −19 (s). 27Al NMR nances, an additional broad peak integrating to 1 proton was
(C6D5Br): 104 (s). Anal. Calc. for C18H10AlCl10P: C, 33.85; H, observed at 6.26 ppm, suggesting the presence of an Al–H
1.58. Found: C, 33.71; H, 1.63.
bond. The isotopologue was synthesized using D2 and the
product (1-d2) displayed the expected 1 : 1 : 1 triplet at
−26 ppm in the 31P NMR spectrum, replacing the P–H reson-
X-ray crystallography
Crystals were coated in Paratone-N oil in the glovebox, ance. In addition, the peak assigned to the Al–H fragment was
1
mounted on a MiTegen Micromount and placed under an N2 not observed in the H NMR spectrum. Combined with com-
stream, thus maintaining a dry, O2-free environment for each bustion analysis, these data are consistent with a formulation
crystal. The data were collected on a Bruker Apex II diffracto- for 1 of [Mes3PH][H(AlI3)2]. Repeated attempts to obtain crys-
meter employing Mo Kα radiation (λ = 0.71073 Å). Data collec- tals suitable for X-ray diffraction were unsuccessful and thus
tion strategies were determined using Bruker Apex software this assignment could not be unambiguously confirmed.
20
and optimized to provide >99.5% complete data to a 2θ value
The alane Al(p-C6F4H)3 was prepared from B(p-C6F4H)3
of at least 55°. The data were collected at 150( 2) K for all crys- and AlMe3 via an established substituent exchange protocol.18
tals. The frames were integrated with the Bruker SAINT soft- In this fashion, Al(p-C6F4H)3 2 was isolated at −38 °C in 70%
ware package using a narrow-frame algorithm. Data were yield. The 19F NMR spectrum displayed two multiplets at
1
corrected for absorption effects using the empirical multi-scan −121.6 and −137.5 ppm and the H NMR spectrum displayed
method (SADABS). Non-hydrogen atomic scattering factors a triplet of triplets with H–F coupling at 6.62 ppm, as well as
were taken from the literature tabulations.24 The heavy atom the resonances attributable to one equivalent of toluene. The
positions were determined using direct methods employing structure was confirmed by X-ray crystallography as the η1
the SHELXTL direct methods routine. The remaining non- toluene adduct, Al(p-C6F4H)3·tol (2) (Fig. 1) similar to the
hydrogen atoms were located from successive difference reported Al(C6F5)3·tol adduct.18
Fourier map calculations. The refinements were carried out by
A 1 : 2 solution of (otol)3P and 2 in C6H5Br under H2 was
using full-matrix least squares techniques on F, minimizing monitored by NMR spectroscopy. Two new broad signals in
2
the function ω(Fo − Fc)2 where the weight ω is defined as 4Fo / the 19F NMR spectrum emerged concomitant with the appear-
2σ(Fo ) and Fo and Fc are the observed and calculated structure ance of the [(otol)3PH]+ cation as observed by 31P NMR spec-
2
1
factor amplitudes, respectively. In the final cycles of each troscopy. A broad signal at 4.47 ppm in the H NMR spectrum
refinement, all non-hydrogen atoms were assigned anisotropic was attributed to a bridging hydride in the anion, similar to
temperature factors in the absence of disorder or insufficient that reported for the [(µ-H)(Al(C6F5)3)2] anion.16 However
13688 | Dalton Trans., 2013, 42, 13685–13691
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Fig. 1 POV-Ray depiction of the molecular structure of 2. C: black; Al: teal;
F: pink; H atoms are omitted for clarity.
efforts to isolate the present salt, presumed to be [(otol)3PH]-
[(µ-H)(Al(p-C6F4H)3)2], were challenging due to the slow acti-
vation and reversible loss of H2. This is thought to result from
the diminished Lewis acidity of 2 in comparison to Al(C6F5)3.
In a related system, we also probed alanes with bulky sub-
stituents. In this regard we noted the work of Marks and co-
workers21 who had previously reported the isolation of the
bulky anion salt [Li(OEt2)][FAl(C12F9)3]. Efforts to effect trans-
Fig. 2 POV-Ray depiction of the anion of 4. C: black; Al: teal; F: pink; H: grey.
[B(C6F5)4] was separated from the alane by dissolving the reac-
tion products in hexanes. While the 19F and 27Al NMR data for
the solution were consistent with the presence of the free
alane, this product could not be isolated from the by-product,
Ph3CH. Nonetheless, addition of the phosphine, Mes3P and
exposure to H2 resulted in the generation of a new product 5
metallation reactions18 of the borane B(C12F9)3 or Zn-reagent
21,25
Zn(C12F9)2·tol with AlMe3
did not afford the alane Al-
1
in 38% isolated yield. The 31P and H NMR spectra were con-
(C12F9)3. However, reaction of [Li(OEt2)][FAl(C12F9)3] with
[Et4N]Cl in dichloromethane afforded the cation exchanged
salt [Et4N][FAl(C12F9)3] 3 in 70% yield. This product displayed
the expected 19F NMR spectrum for the biphenyl ring as well
as a broad singlet at −175 ppm, indicative of the Al–F bond.
Subsequent treatment with LiAlH4 in a mixed fluorobenzene–
Et2O solvent afforded clean conversion to the salt [Et4N]-
[HAl(C12F9)3] 4 in 87% yield (Scheme 3).16 The 27Al NMR signal
shifted from a broad singlet at 112 ppm to one at 124 ppm,
while the 1H NMR displayed the expected signals for the
cation and a very broad Al–H resonance at 2.50 ppm located by
a 1H{27Al} NMR experiment. An X-ray diffraction study of 4 con-
firmed its formulation (Fig. 2). The biphenyl rings on Al are all
oriented on the hydride side of the tetrahedral coordination
sphere, providing steric shielding of the Al–hydride.
sistent with the presence of the [Mes3PH]+ cation; however, no
resonance attributable Al–H fragment could be unambiguously
identified. The 19F NMR spectra showed 13 resonances with
some apparent overlapping signals.21 Nonetheless, X-ray cryst-
allography for 5 confirmed its formulation as [Mes3PH][(µ-H)-
(Al(C12F9)3)2] (Fig. 3, Scheme 3). The anion, in contrast to that
seen in 4 contains two bulky Al(C12F9)3 units bridged by a
hydride. The Al–H bond distances were found to be 1.717(1) and
1.778(1) Å with a Al–H–Al angle of 160.9(1)°. In this case, the
Reactions of 4 with [CPh3][B(C6F5)4] was undertaken to
generate the free alane, Al(C12F9)3. The salt by-product, [Et4N]-
Scheme 3 Synthesis of 4 and 5.
Fig. 3 POV-Ray depiction of the anion of 5. C: black; Al: teal; F: pink; H grey.
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Al⋯Al separation is 3.448 Å, somewhat longer than that seen
in [tBu3PH][(µ-H)(Al(C6F5)3)2] (3.327(1) Å).16 This is consistent
with the increased steric congestion resulting from the per-
fluorinated biphenyl rings. It is also noteworthy that in the
anion of 5, the biphenyl groups on each Al are rotated,
oriented away from the bridging hydride atom.
Reactions with olefins
The reactions of 1 with ethylene, propylene or cyclohexene
were probed. Exposing a solution of 1 in C6D5Br to 1 atm of
ethylene in a J-Young NMR tube at 25 °C resulted in an
immediate reaction. This stands in contrast to the salts [R3PH]-
[(µ-H)(Al(C6F5)3)2] (R = tBu, Mes) which react very slowly with
ethylene at r.t.16 The 27Al NMR spectrum displays a broad
singlet at −17 ppm, downfield shifted from 1 (36 ppm). While
Fig. 4 POV-Ray space filling models of the anion of 4. Left: perpendicular to
the Al–H plane; right: down the Al–H bond. C: black; Al: teal; F: pink; H: white.
notion as they display an extremely congested environment
around the Al–H bond (Fig. 4).
In a similar fashion, the species [Et4N][EtAl(C6F5)3]16 was
prepared and the ability of phosphonium cations to protonate
the alkyl fragment were probed. To this end, the salts, [R3PH]-
[Al(C6F5)4] (R = otol (6), Mes (7), tBu (8)22) and [(2,6-
Cl2C6H3)3PH][AlCl4] (9)19 were prepared. Reactions with [Et4N]-
1
the 31P and H NMR spectra display resonances arising from
the [Mes3PH]+ cation, the signal attributable to the Al–H bond
is replaced by a triplet at 1.09 ppm and a quartet at 0.92 ppm,
attributable to the presence of an ethyl fragment. Using the
isotopologue 1-d2 reduces the quartet to a broadened triplet.
These data, together with the fact that the 27Al NMR shift is
1
[EtAl(C6F5)3] were monitored by H NMR spectroscopy. In the
26
case of the reaction of 6 the liberation of ethane was evident.
In addition the products included (o-tol)3P, [Et4N][Al(C6F5)4] as
similar to that observed for Mes3P(CH2CH2)(AlI3)2 suggests
the formulation of the product as [Mes3PH][(I3Al)(µ-I)(AlI2Et)]
although this could not be unambiguously confirmed. Similar
evidence of insertion of propylene and cyclohexene were also
observed although efforts to isolate and structurally character-
ize these products were unsuccessful. Heating the reaction
mixtures to 120 °C did not result in the protonation Al–alkyl by
[Mes3PH]+. Nonetheless, quenching of these reactions with
water resulted in the rapid liberation of ethane, propane,
or cyclohexane, respectively consistent with the presence of
Al–alkyl intermediates (Scheme 4).
Exposure of 5 to an atmosphere of ethylene was monitored
by NMR spectroscopy. Even upon extensive heating, no evi-
dence of insertion into the Al–hydride bond was observed.
This result stands in contrast to the previously reported obser-
vation that [R3PH][(µ-H)(Al(C6F5)3)2] reacts with olefin,16 as the
presence of olefin is thought to facilitate alane dissociation via
the equilibrium formation of an olefin adduct. In the present
case, the increased steric bulk of the biphenyl groups about Al
in 5, together with the increased Lewis acidity precludes such
dissociation from the anion, making the hydride inaccessible.
Space filling models of the anion of 4 further support this
1
well as EtAl(C6F5)2 as evidenced by 31P, 19F, and H NMR spec-
troscopy. This mixture suggests protonation of the alkyl group
of the anion [EtAl(C6F5)3] frees alane which reacts with [EtAl-
(C6F5)3] in a redistribution reaction to give the observed
Al-species [Al(C6F5)4] and EtAl(C6F5)2. In the case of 7 and 8, the
reaction with [Et4N][EtAl(C6F5)3] resulted in no reaction at
25 °C after 6 h or upon heating to 70 °C for 48 h. The corres-
ponding reactions of 9 with [Et4N][EtAl(C6F5)3] also generated
ethane, but also gave rise to a complex mixture of products
resulting from apparent Cl/C6F5 scrambling.
Conclusions
The above results illustrate the ability of olefin to react
with sterically accessible Al–hydrides to generate an Al–alkyl.
In addition, the more acidic phosphonium salts effect proto-
nation of alkyl–aluminates to liberate the alkane. While in
principle these steps suggest the potential for a catalytic
cycle of hydrogenation, scrambling of substituents on Al and
redistribution preclude such catalytic olefin hydrogenation.
We are continuing to probe new FLP systems with a view
to developing new avenues for transition metal-free catalytic
chemistry.
Acknowledgements
NSERC of Canada is thanked for financial support. GM grate-
fully acknowledges the financial support of a Queen Elizabeth
II Scholarship. DWS is grateful for the award of a Canada
Research Chair.
Scheme 4 Reactions of Mes3P/AlI3 with H2/olefin and H2O.
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14 H. D. Wang, R. Fröhlich, G. Kehr and G. Erker, Chem.
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