Synthesis of a water-stable 2, 4-bis(trifluoromethyl)phenyl-substituted phosphonous acid palladium complex and its catalytic activity in cross-coupling reactions

Article abstract of DOI:10.1002/zaac.201100344

This manuscript describes the selective synthesis of2, 4- bis(trifluoromethyl)phenylphosphinic acid, (R_f)P(O)(H)OH.Reaction of chlorobis(diethylamino)phosphane with a mixture of 2, 4-(CF₃) ₂C₆H₃Li and 2, 6-(CF₃) ₂C₆H₃Li results in the formation of 2, 4-bis(trifluoromethyl)phenylbis(diethylamino)phosphane, (R_f) P(NEt₂)₂. The following in situ reaction with gaseous HBr leads to the cleavage of the phosphorus nitrogen bonds resulting in the selective formation of the corresponding dibromophosphane (R_f) PBr₂.2, 4-Bis(trifluoromethyl)phenylphosphinic acid is available via two different synthetic strategies starting from the dibromophosphane (R _f)PBr₂. Reduction of the dibromophosphane with tributyltin hydride affords selectively the phosphane (R_f)PH₂. The following oxidation to the phosphinic acid (R_f)P(O)(H)OH, however, only proceeds in moderate yields. A more efficient synthesis is achieved via the hydrolysis of the dibromophosphane (R_f)PBr₂ in dichloromethane solution. 2, 4-bis(trifluoromethyl)phenylphosphinic acid, (R_f)P(O)(H)OH, is surprisingly stable towards air and water. The water soluble phosphinic acid reacts sluggishly with a 30 % H₂O ₂ solution, yielding only 23 % of the corresponding phosphonic acid (R_f)P(O)(OH)₂ after seven days at ambient temperature.In principle a tautomerization of the phosphinic acid (R_f)P(O)(H)OH to the corresponding phosphonous acid (R_f)P(OH)₂ - analogously to the phosphane oxide (R_f)₂P(O)H and the phosphinous acid (R_f)₂POH - is conceivable. However, the phosphonous acid could not be detected with spectroscopic methods. The energy difference between the phosphinic acid and the phosphonous acid is calculated by quantum chemical calculations at the B3LYP/6-311+G(2df, p) level of theory to 30.1 kJ·mol^-1. This high value explains why the phosphonous acid tautomer could not be detected by spectroscopic methods so far.As transition metal complexes featuring phosphonous acid ligands are of significant interest for the application as homogeneous catalysts we studied the coordination chemistry of the related phosphinic acid in more detail. The preligand 2, 4-bis(trifluoromethyl)phenylphosphinic acid reacts smoothly with palladium dichloride in THF to the dinuclear complex [Pd₂(μ-Cl) ₂{[2, 4-(CF₃)₂C₆H ₃P(OH)O]₂H}₂], featuring a phosphonous acid phosphonato unit, with a strong hydrogen bridge substantiated by an O...O distance of 247.7(3) pm. The dinuclear phosphonous acid palladium complex exhibits a high catalytic activity in the Suzuki cross-coupling. Alternatively, the phosphinic acid can be used as preligand for the in situ formation of a phosphonous acid palladium complex. These compounds combine catalytic activity with a very high stability towards air and water. Copyright

Full text of DOI:10.1002/zaac.201100344

ARTICLE
DOI: 10.1002/zaac.201100344
Synthesis of a Water-Stable 2,4-Bis(trifluoromethyl)phenyl-Substituted
Phosphonous Acid Palladium Complex and its Catalytic Activity in Cross-
Coupling Reactions
Boris Kurscheid,[a] Hans-Georg Stammler,[a] Beate Neumann,[a] and Berthold Hoge*[a]
Dedicated to Professor Helge Willner on the Occasion of His 65th Birthday
Keywords: Phosphinic acid; Phosphonous acid metal complex; Platinum; Palladium; Phosphorus
Abstract. This manuscript describes the selective synthesis of
2,4-bis(trifluoromethyl)phenylphosphinic acid, (Rf)P(O)(H)OH.
the phosphane oxide (Rf)2P(O)H and the phosphinous acid (Rf)2POH –
is conceivable. However, the phosphonous acid could not be detected
with spectroscopic methods. The energy difference between the phos-
phinic acid and the phosphonous acid is calculated by quantum chemi-
cal calculations at the B3LYP/6-311+G(2df,p) level of theory to
30.1 kJ·mol–1. This high value explains why the phosphonous acid tau-
tomer could not be detected by spectroscopic methods so far.
Reaction of chlorobis(diethylamino)phosphane with a mixture of 2,4-
(CF3)2C6H3Li and 2,6-(CF3)2C6H3Li results in the formation of 2,4-
bis(trifluoromethyl)phenylbis(diethylamino)phosphane, (Rf)P(NEt2)2.
The following in situ reaction with gaseous HBr leads to the cleavage
of the phosphorus nitrogen bonds resulting in the selective formation
of the corresponding dibromophosphane (Rf)PBr2.
As transition metal complexes featuring phosphonous acid ligands are
of significant interest for the application as homogeneous catalysts we
studied the coordination chemistry of the related phosphinic acid in
more detail. The preligand 2,4-bis(trifluoromethyl)phenylphosphinic
acid reacts smoothly with palladium dichloride in THF to the dinuclear
2,4-Bis(trifluoromethyl)phenylphosphinic acid is available via two dif-
ferent synthetic strategies starting from the dibromophosphane (Rf)
PBr2. Reduction of the dibromophosphane with tributyltin hydride af-
fords selectively the phosphane (Rf)PH2. The following oxidation to
the phosphinic acid (Rf)P(O)(H)OH, however, only proceeds in moder-
ate yields. A more efficient synthesis is achieved via the hydrolysis
of the dibromophosphane (Rf)PBr2 in dichloromethane solution. 2,4-
bis(trifluoromethyl)phenylphosphinic acid, (Rf)P(O)(H)OH, is surpris-
ingly stable towards air and water. The water soluble phosphinic acid
reacts sluggishly with a 30% H2O2 solution, yielding only 23% of
the corresponding phosphonic acid (Rf)P(O)(OH)2 after seven days at
ambient temperature.
complex [Pd2(μ-Cl)2{[2,4-(CF3)2C6H3P(OH)O]2H}2], featuring
a
phosphonous acid phosphonato unit, with a strong hydrogen bridge
substantiated by an O···O distance of 247.7(3) pm. The dinuclear phos-
phonous acid palladium complex exhibits a high catalytic activity in
the Suzuki cross-coupling. Alternatively, the phosphinic acid can be
used as preligand for the in situ formation of a phosphonous acid palla-
dium complex. These compounds combine catalytic activity with a
very high stability towards air and water.
In principle a tautomerization of the phosphinic acid (Rf)P(O)(H)OH
to the corresponding phosphonous acid (Rf)P(OH)2 – analogously to
preligands,[2] is getting more and more popular. Pentavalent
phosphane oxides are involved in tautomerization processes
with the corresponding trivalent phosphinous acid form. For
electron donating aryl- and alkyl substituents, this tautomeric
equilibrium (cf. Equation (1)) is completely shifted to the side
of the phosphane oxide tautomer.
Introduction
Although there is a variety of phosphorus ligands avail-
able,[1,2] the motivation to generate new ligands strongly de-
pend on the industrial demand for economical and robust li-
gands with a high catalytic activity and a high stability towards
ambient air and moisture. In contrast, common complexes,
such as [Pd(PPh3)4], are highly air and moisture sensitive. The
phosphane ligands, like tri-tert-butylphosphane, P(tBu)3, show
as the corresponding transition metal complex a amazing cata-
lytic activity in many cross-coupling reactions, despite their air
sensitivity.[1,3] Due to the sensitivity of phosphane ligands, the
employment of secondary phosphane oxides (SPO), so called
(1)
Introducing electron withdrawing substituents, such as tri-
fluoromethyl- and pentafluoroethyl-groups, allows the isola-
tion of the corresponding phosphinous acids.[4,5] The less elec-
tron withdrawing p-tetrafluoropyridyl-, pentafluorophenyl-,
2,4-bis(trifluoromethyl)phenyl- and 2,6-bis(trifluoromethyl)-
* Prof. Dr. B. Hoge
E-Mail: b.hoge@uni-bielefeld.de
[a] Fakultät für Chemie, Anorganische Chemie
Universität Bielefeld
Universitätsstr. 25
33615 Bielefeld, Germany
534
© 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
Z. Anorg. Allg. Chem. 2012, 638, (3-4), 534–542

2,4-Bis(trifluoromethyl)phenyl-Substituted Phosphonous Acid Pd Complex
phenyl groups favor the phosphane oxide tautomer in the solid
Results and Discussion
state, while in solution a solvent-dependent equilibrium is ob-
served. An increasing donor number[6] of the chosen solvent
leads to an increasing value of the corresponding phosphinous
acid tautomer.[7–10]
Metalation of commercially available 1,3-bis(trifluoro-
methyl)benzene at 0 °C with n-butyllithium in the presence of
TMEDA (tetramethylethylenediamine) leads to the selective
formation
of
2,6-bis(trifluoromethyl)phenyllithium.[13,14]
Chatt and Heaton demonstrated in 1968 that phosphinous
acids can be trapped by coordination to transition metals (cf.
Equation (1)).[11] In the recent years the development of new
secondary phosphane oxides has progressed rapidly, exhibiting
catalytic activity in many cross-coupling reactions.[2,7]
Phosphinic acids RP(O)(H)OH and phosphonous acids
RP(OH)2 are related by an equilibrium. However, free phos-
phonous acids could not be detected by spectroscopic methods
so far. But like phosphinous acids, they can be stabilized in
the coordination sphere of suitable transition metals (cf. Equa-
tion (2)).[12]
Quenching the mixture with chlorobis(diethylamino)phos-
phane, ClP(NEt2)2, afforded the corresponding bis(diethyl-
amino)phosphane in a 30% yield (cf. Equation (3)).[13]
In the absence of an auxiliary ligand such as TMEDA, the
reaction of n-butyllithium and 1,3-(CF3)2C6H4 results in the
formation of a mixture of 2,4- and 2,6-bis(trifluoromethyl)-
phenyllithium (cf. Scheme 1).[15–19] Treatment of PCl3 with an
excess of the lithiated species leads to the formation of a com-
plex product mixture (cf. Scheme 1). The study of the chemis-
try of mono-substituted 2,4-bis(trifluoromethyl)phenylphos-
phane derivatives, however, requires a more selective synthe-
sis.
For a selective synthesis of bis[2,4-bis(trifluoromethyl)-
phenyl]phosphane derivatives we utilized the steric demand of
the diethylamino group to discriminate between the two lith-
ium aryls.[19] As a consequence the reaction of chlorobis(di-
ethylamino)phosphane with an excess (1:2,6) of 2,4-and 2,6-
(2)
To investigate the influence of electron withdrawing groups bis(trifluoromethyl)phenyllithium leads to a selective forma-
on the catalytic activity of phosphonous acids palladium tion of 2,4-bis(trifluoromethyl)phenylbis(diethylamino)phos-
complexes, this paper describes the selective synthesis of 2,4- phane (1). The 31P NMR spectrum of the aminophosphane 1
bis(trifluoromethyl)phenylphosphinic acid, 2,4-(CF3)2C6H3- reveals a multiplet at δ = 92.7 with a 4J(P,F) coupling of 55 Hz.
P(O)(H)OH, and the thorough study of the coordination The subsequent treatment of the aminophosphane 1 with gas-
properties of the corresponding phosphonous acid eous HBr at –80 °C results in a cleavage of the two phosphorus
2,4-(CF3)2C6H3P(OH)2 towards catalytically relevant metals, nitrogen bonds and in the selective formation of the corre-
such as palladium.
sponding dibromophosphane 2 (Scheme 2). Product 2 is ob-
(3)
Scheme 1. Reaction of PCl3 with lithiated 1,3-bis(trifluoromethyl)benzene.
Z. Anorg. Allg. Chem. 2012, 534–542
© 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
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535

B. Kurscheid, H.-G. Stammler, B. Neumann, B. Hoge
ARTICLE
Scheme 2. Reaction of chlorobis(diethyl)aminophosphane with lithiated 1,3-bis(trifluoro-methyl)benzene.
4
tained by distillation as a colorless liquid in an overall yield 1J(P,H) and J(P,F) couplings of 208 Hz and 85 Hz, respec-
of 59%.
tively (cf. Table 1 and Table 2). The subsequent oxidation to
The synthesis of 2,4-bis(trifluoromethyl)phenylphosphinic the phosphinic acid 4 proceeds through contact with ambient
acid (4) is realized by two different methods starting from the air or with an excess of NO2 gas in CH2Cl2.
dibromophosphane 2. In the low-volatile solvent 1,6-dibromo-
A more efficient access to the phosphinic acid 4 is the hy-
hexane the reduction of 2 with a slight excess of tributyltin drolysis of the dibromophosphane 2 in CH2Cl2 within 30 min-
hydride selectively affords 2,4-(CF3)2C6H3PH2 (3), which can utes at room temperature (cf. Scheme 3). Removal of all vola-
be separated from the reaction mixture via a fractional conden- tiles in vacuo yielded 95% of 2,4-bis(trifluoromethyl)phenyl-
sation in a 80% yield. In the 31P NMR spectrum the product phosphinic acid (4) as a colorless solid, which melts in the
reveals a triplet of quartets at δ = –121.0 with characteristic range of 77–79 °C. The 31P NMR spectrum of Phosphinic acid
Table 1. Compilation of selected 31P and 19F NMR spectroscopic data of 2,4-bis(trifluoromethyl)phenyl derivatives.a).
No.
δ(31P)
4J(P,F)
δ(19Fo)
δ(19Fp)
2,4-(CF3)2C6H3P(NEt2)2
2,4-(CF3)2C6H3PH2
1
3
2
4
5
7
8
92.7
–121.0
140.6
17.5
7.8
55
29
85
8
n. r.
n. r.
n. r.
–
–61.7
–56.5
–57.6
–58.6
–
–
–63.0
–63.5
–63.3
–63.3
–
b)
2,4-(CF3)2C6H3PBr2
2,4-(CF3)2C6H3P(O)(H)OHc)
d)
2,4-(CF3)2C6H3P(O)(OH)2
[PdCl2{(2,4-(CF3)2C6H3P(OH)2}2]
[Pd2(μ-Cl)2{[2,4-(CF3)2C6H3P(OH)O]2H}]
94.6
77.4
–56.2
–63.7
a) Coupling constants in Hz. CDCl3; room temp.; n. r. = not resolved; b) C6D6, δ(31P) –121.0 (t, q, 1J(P,H) 208 Hz); c): δ = (31P) 17.5 (d,
3
1J(P,H) 615 Hz); d) D2O, (d, J(P,H) 14.3 Hz, ArH).
Table 2. 13C{1H} NMR spectroscopic data of 2,4-bis(trifluoromethyl)phenyl derivatives (R = 2,4-(CF3)2C6H3)a) Chemical Shifts.
δ(13C1) δ(13C2) δ(13C3) δ(13C4) δ(13C5) δ(13C6) δ(13C7) δ(13C8)
RPH2 (3)b)
RPBr2 (2)
135.8
141.4
133.8
131.1
132.9
132.4
122.9
121.8
123.9
123.1
130.2
133.6
134.9
132.7
n. r.b)
129.4
128.7
128.2
136.8
137.2
134.0
136.6
123.8
123.1
123.0
123.0
122.9
122.8
122.6
123.4
RP(O)(H)OH (4) 133.3
RP(O)(OH)2 (6)c) 136.7
Coupling Const. 1J(P,C1) 2J(F,C2) 2J(P,C2) 3J(F,C3) 2J(F,C4) 4J(P,C4) 3J(F,C5) 3J(P,C5) 2J(P,C6) 1J(F,C7) 3J(P,C7) 1J(F,C8)
RPH2 (3)
RPBr2 (2)
RP(O)(H)OH (4) 124.0
RP(O)(OH)2 (6) 179.0
24.7
82.3
31.7
32.5
34.2
33.4
10.4
32.4
9.9
5.3
33.5
33.7
33.5
33.1
n. r.
n. r.
1.0
n. r.
3.5
n. r.
13.2
n. r.
n. r.
n. r.
3.5
8.8
1.6
7.4
7.6
274.4
275.6
275.5
273.6
n. r.
2.3
3.0
4.5
272.3
270.1
273.1
272.2
n. r.
n. r.
n. r.
7.2
3.1
a) Coupling constants in Hz. CDCl3; room temp.; n. r. = not resolved; b) C6D6; resonance overlaid with [D6]benzene; c) solvent [D8]THF.
536 www.zaac.wiley-vch.de
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2,4-Bis(trifluoromethyl)phenyl-Substituted Phosphonous Acid Pd Complex
Scheme 3. Synthesis of 2,4-bis(trifluoromethyl)phenylphosphane (3) and 2,4-bis(trifluoromethyl)phenylphosphinic acid (4).
Scheme 4. Syntheses of 2,4-bis(trifluoromethyl)phenylphosphonic acid (5).
4 is characterized by a multiplet signal at δ = 17.5 with charac- tion.[20] The tautomer 2,4-(CF3)2C6H3P(OH)2 (6) could not be
1
3
teristic couplings of J(P,H) = 615 Hz, J(P,H) = 14 Hz and detected even in highly donating solvents, such as DMA (di-
4J(P,F) = 8 Hz (cf. Table 1 and Table 2). The phosphinic acid
methylacetamide), DMF (dimethylformamide) or HMPA
4 is stable towards hydrolysis and not easy to oxidize. A solu-
tion of 4 in a 30% hydrogen peroxide solution shows a conver-
sion of only 23% to the corresponding phosphonic acid 5 after
7 days.
A more efficient way to oxidize 4 to 5 is the reaction with
conc. HNO3 (Scheme 4). Alternatively 2,4-(CF3)2C6H3P(O)-
(OH)2 (5) is synthesized by in situ hydrolysis and oxidation
of the dibromophosphane 3 with conc. HNO3. The 31P NMR
spectrum of 2,4-(CF3)2C6H3P(O)(OH)2 (5) is characterized by
a singlet signal at δ = 7.8.
Similar to the tautomerization between a phosphinous acid
and a phosphane oxide (cf. Equation (1)), for the phosphinic
acid 4 an equilibrium with the corresponding phosphonous
acid, 2,4-(CF3)2C6H3P(OH)2 (6) (cf. Equation (4)) can be pos-
tulated.
(hexamethylphosphoric acid triamide). The energy difference
between the phosphinic acid 4 and the phosphonous acid 6 is
calculated by quantum chemical calculation at the B3LYP/6-
311+G(2df,p) level of theory to 30.1 kJ·mol–1. This large value
may rationalize why the phosphonous acid tautomer is still
elusive.
Like phosphinous acids, phosphonous acids also can be sta-
bilized or trapped in the coordination sphere of suitable transi-
tion metals (cf. Equation (1) and (2), respectively). However,
only few examples of phosphonous acid transition metal com-
plexes are known so far.[12] With regard to a possible applica-
tion in cross-coupling reactions, we investigated the coordina-
tion properties of 2,4-bis(trifluoromethyl)phenylphosphonous
acid (6) in more detail.
The addition of solid palladium dichloride to a THF solution
of 2,4-bis(trifluoromethyl)phenylphosphinic acid (4) leads to a
mixture of a mononuclear (7) and a dinuclear palladium com-
plex (8) (cf. Scheme 5) after six days of stirring at room tem-
perature. The reversible transformation between the mono- and
the dinuclear complex mirrors the equilibrium observed for
phosphinous acid derivatives bearing 2,4-bis(trifluoromethyl)-
phenyl groups.[7] In keeping with this, exposure of a THF solu-
tion of the dinuclear complex 8 in an atmosphere of HCl selec-
tively furnished the mononuclear complex 7 (cf. Scheme 5).
The reverse reaction occurs in the presence of a base, such
as pyridine, or by the removal of all volatiles in vacuo. The
constitution and configuration of complex 8 in the solid state
was elucidated by a single crystal structure analysis. Colorless
crystals of 8 were obtained from a THF solution. Complex 8
(4)
To the best of our knowledge there is no example of a free
phosphonous acid described in the literature so far and the
postulated equilibrium between phosphinic acids and their cor-
responding phosphonous acids could not be substantiated by
spectroscopic evidence. However, the existence of the parent
phosphonous acid HP(OH)2 is postulated from kinetic studies
and it is estimated that the ratio HP(OH)2/H2P(O)(OH) in the
equilibrium mixture does not exceed 10–12 in an aqueous solu- crystallizes with two disordered solvent molecules (thf) in the
Z. Anorg. Allg. Chem. 2012, 534–542
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B. Kurscheid, H.-G. Stammler, B. Neumann, B. Hoge
ARTICLE
Scheme 5. Coordination of the phosphonous acid 6 and equilibrium between the mononuclear 7 and dinuclear palladium complex 8.
monoclinic space group P21/c (no. 14) (a = 1359.0(1) pm, b =
The successful synthesis of 2,4-bis(trifluoromethyl)phenyl-
1120.7(1) pm, c = 1842.0(1) pm, β = 109.87(1)°) and reveals phosphinic acid (4) and the corresponding phosphonous acid
a quasi-chelating phosphonous acid phosphonato unit with an
O1···O3 separation of 247.7(2) pm, indicating a strong intra-
molecular hydrogen bridge. The oxygen-bonded hydrogen
atoms were refined isotropically revealing an O1–H1 distance
of 108(4) pm and a H1···O3 distance of 1.41(4) pm, hence the
P1–O1 distance is significantly longer than the P2–O3 distance
(155.6(2) pm resp. 153.4(2) pm). The oxygen atoms O2 and
O4 build intermolecular hydrogen bridges (Figure 1).
palladium complex 8 are the basis for the investigations in
terms of a possible catalytic activity in Heck cross-coupling
reactions between n-butyl acrylate and 1-bromo-2-fluorobenz-
ene (cf. Table 3). The advantage of the fluorinated benzene is
the possibility to determine the conversion rate by 19F NMR
spectroscopy. Using a catalyst loading of 0.01 mol-% palla-
dium, which parallels 0.005 mol-% of the dinuclear complex
8, leads to a conversion of 61.5% after 48 hours at 135 °C.
This underlines the high catalytic activity of the air stable com-
plex 8. The known[1a–e] air sensitive catalyst designed from
Pd2(dba)3 (dba = dibenzylideneacetone) and tri-tert-butylphos-
phane gave a slightly higher conversion rate of 69.0% under
the same conditions.
The phosphinic acid 4 can be successfully used as a so
called preligand for the in situ formation of catalytically active
phosphonous acid complexes. The catalytic system Pd2(dba)3 /
2,4-(CF3)2C6H3P(O)(H)OH (4) (Pd atom ligand ratio 1:2) re-
sults within the chosen test system in a conversion rate of
51.0% (cf. Table 3). Using Pd2(dba)3 without the preligand 4
leads to a conversion rate of only 23.5%, which proves the
beneficial influence of the phosphinic acid 4 to the catalytic
activity.
The Suzuki cross-coupling is one of the most popular and
industrially used reactions and therefore a widely used bench-
mark for new synthesized catalysts.[1,2] According to a former
study of the catalytic activity of the phosphinous acid palla-
dium complex [Pd2(μ-Cl)2{{[2,4-(CF3)2C6H3]2PO}2H}2}] in
the reaction of 1-bromo-3-fluorobenzene with phenyl boronic
acid clearly proves 2-propanole as solvent and potassium phos-
phate as the most efficient base.[7] Based on these observa-
tions, the catalytic activity of the phosphonous acid palladium
complex 8 and the phosphinic acid 4 as a preligand in combi-
nation with Pd2(dba)3 are investigated under identical condi-
tions (cf. Table 4).
Using a catalyst loading of 0.01 mol-% palladium (0.005
mol-% of the dinuclear complex 8) leads after 20 hours at
room temperature to a conversion rate of 80.0%. The competi-
tion with the air sensitive catalyst[1a–e] Pd2(dba)3 / P(tBu)3 re-
sults under identical conditions in a conversion of only 55.0%.
Using three equivalents of the phosphinic acid 4 as a preligand
Figure 1. Molecular structure and numbering scheme of
[Pd2(μ-Cl)2{[2,4-(CF3)2C6H3P(OH)O]2H}2]·2THF (8); the solvent
molecules (thf) and all carbon-bonded hydrogen atoms are omitted for
clarity; 50% probability amplitude displacement ellipsoids are shown;
selected bond lengths /pm: P1–Pd1 221.8(1), P2–Pd1 223.9(1), P1–C1
182.6(2), P2–C9 183.0(2), P1–O1 155.6(2), P2–O3 153.4(2), O1–O3
247.7(3), Pd1–Cl1 239.9(1), Pd–Cl1# 243.6(1).
538
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2,4-Bis(trifluoromethyl)phenyl-Substituted Phosphonous Acid Pd Complex
Table 3. Results of the Heck cross-coupling reactions.
Catalyst /0.01 mol-% palladium
M:La)
1:2
t /h
Yield /%b)
TON
TOF
Pd2(dba)3 / 2,4-(CF3)2C6H3P(O)(H)OH (4)
24
48
24
48
24
48
24
48
24
48
42.0
51.0
45.0
61.5
58.0
69.0
13.0
23.5
19.5
24.5
4200
5100
4500
6150
5800
6900
1300
2350
1950
2450
175
106
187
128
241
143
54
49
81
51
[Pd2(μ-Cl)2{[2,4-(CF3)2C6H3P(OH)O]2H}2] (8)
Pd2(dba)3 / P(tBu)3
Pd2(dba)3
1:2
PdCl2
a) M:L = palladium ligand ratio. b) Determined via integration of the 19F NMR resonances.
Table 4. Results of the Suzuki cross-coupling reaction.
Catalyst /0.01 mol-% palladium
M:La)
Yield /%b)
TON
TOF
Pd2(dba)3 / 2,4-(CF3)2C6H3P(O)(H)OH (4)
Pd2(dba)3 / 2,4-(CF3)2C6H3P(O)(H)OH (4)
Pd2(dba)3 / 2,4-(CF3)2C6H3P(O)(H)OH (4)
[Pd2(μ-Cl)2{[2,4-(CF3)2C6H3P(OH)O]2H}2] (8)
Pd2(dba)3 / P(tBu)3
1:1
1:2
1:3
88.0
90.5
73.1
80.0
55.0
78.0
7.5
8800
9050
7310
8000
5500
7800
750
440
452
365
400
229
325
31
1:2
Pd2(dba)3
PdCl2
a) M:L = palladium ligand ratio. b) Determined via integration of the 19F NMR resonances.
(palladium ligand ratio 1:3) leads to a decreased catalytic ac- romethyl)phenylphosphinic acid (4) is surprisingly stable
tivity (cf. Table 2). Test reactions without any palladium spe- towards air and water. The water soluble acid 4 is extremely
cies show no conversion at all.
resistant vs. oxidation. Dissolved in a 30% H2O2 solution after
seven days at room temperature the corresponding phosphonic
acid 5 is detected in only 23%.
Analogously to the phosphinous acid and phosphane oxide
tautomerization, an equilibrium between the phosphinic acid 4
and the corresponding phosphonous acid 6 may be possible.
In contrast to the phosphinous acid derivative, however, there
is no spectroscopic evidence of the phosphonous acid 6 even
in strongly donating solvents such as HMPA.
The preligand 2,4-bis(trifluoromethyl)phenylphosphinic
acid (4) reacts smoothly with palladium dichloride in THF to
the dinuclear complex [Pd2(μ-Cl)2{[2,4-(CF3)2C6H3P-
(OH)O]2H}2] (8) featuring a phosphonous acid phosphonato
unit. The strong hydrogen bridge is identified by an O···O dis-
tance of 247.7(3) pm.
The phosphonous acid palladium complex 8 combines cata-
lytic activity with a remarkably high stability towards air and
water. The phosphinic acid 4 also proved useful for catalysis
Conclusion
The selective synthesis of 2,4-bis(trifluoromethyl)phenyl-
phosphinic acid (4) was achieved by the reaction of
chlorobis(diethylamino)phosphane with
a
mixture of
2,4-(CF3)2C6H3Li and 2,6-(CF3)2C6H3Li, resulting in the for-
mation of 2,4-bis(trifluoromethyl)phenylbis(diethylamino)-
phosphane (1). The subsequent in situ reaction with gaseous
HBr leads to the cleavage of the two phosphorus nitrogen
bonds yielding the corresponding dibromophosphane 2. The
target product 2,4-bis(trifluoromethyl)phenylphosphinic acid
(4) can be synthesized by two different routes. Reduction of
the dibromophosphane 2 with tributyltin hydride yields 2,4-
(CF3)2C6H3PH2 (3). The following air oxidation to the phos-
phinic acid 4 gave only moderate yields. A more efficient syn-
thesis is achieved by hydrolysis of the dibromophosphane 2 in
dichloromethane solution at room temperature. 2,4-Bis(trifluo- as a preligand for the in situ formation of a transition metal
Z. Anorg. Allg. Chem. 2012, 534–542
© 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
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B. Kurscheid, H.-G. Stammler, B. Neumann, B. Hoge
ARTICLE
complex enhancing the catalytic activity of the palladium Table 5. Crystal data and refinement characteristics for 8.
source, Pd2(dba)3. These remarkable attributes enable the pos-
sibility for the employment in green chemistry.
8
Crystallographic Section
empirical formula
F(000)
C32H18Cl2F24O8P4Pd2, 2·(C4H8O)
1512
a /pm
b /pm
c /pm
1359.0(2)
1120.7(1)
1842.0(1)
109.87(1)
2638.5(1)
2
Experimental Section
θ /°
Materials and Apparatus
V /106·pm3
Z
Chlorobis(diethylamino)phosphane was prepared according to the pro-
cedure of Burg and Slota treating PCl3 with two equivalents of HNEt2
in a hexane solution.[21] All other chemicals were obtained from com-
mercial sources and used without further purification. Standard high-
vacuum techniques were employed throughout all preparative pro-
Dx /g·cm–3
crystal system
space group
shape / color
crystal size /mm
1.936
monoclinic
P21/c (no. 14)
colorless fragment
0.20 ϫ 0.16 ϫ 0.10
cedures. Non-volatile compounds were handled under dry nitrogen by Data collection
diffractometer
radiation
Bruker Nonius Kappa CCD
Mo-Kα (graphite monochromator, λ =
using Schlenk techniques.
71.073 pm)
100(2)
The NMR spectra were recorded with a Bruker Model Avance III 300
T /K
theta range
index range
1
spectrometer (31P 111.92 MHz; 19F 282.40 MHz; 13C 75.47 MHz, H
2.92–30.00
–19 Յ h Յ 19
–15 Յ k Յ 15
–25 Յ l Յ 25
79079
7673
6406
417
1.043
300.13 MHz) with positive shifts being downfield from the external
standards (85% orthophosphoric acid (31P), CCl3F (19F) and TMS
(1H)). EI mass spectra were recorded with a Finnigan MAT 95 spec-
trometer (20 eV). Intensities are referenced to the most intense peak
of a group. Isotope patterns for comparison were calculated with the
program Isopro.[22] ESI mass spectra were recorded using an Esquire
3000 ion trap mass spectrometer (Bruker Daltonik GmbH, Bremen,
Germany) equipped with a standard ESI/APCI source. Samples were
introduced by direct infusion with a yringe pump. Nitrogen served both
as nebulizer gas and dry gas. Nitrogen was generated by a Bruker
nitrogen generator NGM 11. Helium served as cooling gas for the ion
trap and collision gas for MSn experiments. The spectra were recorded
with the Bruker Daltonik esquireNT 5.2 esquireControl software by the
accumulation and averaging of several single spectra. DataAnalysis™
software 3.4 was used for processing the spectra.
total data collected
unique data
observed data (I Ͼ 2 σ)
parameters
µ /mm–1 (numerical)
absorption correction
T min / max
multi scan
0.8186 / 0.9029
SHELX-97[24]
R1 / wR2 [I Ͼ 2σ(I)]
R1 / wR2 (all data)
goodness of fit
0.031 / 0.077
0.042 / 0.081
1.042
0.052
0.931 / –0.671
833102
R(int) / R(σ)
Δρmax/min / 106 e·pm–3
CCDC number
Melting and visible decomposition points were determined using a
HWS Mainz 2000 apparatus. C, H and N analyses were carried out
with a HEKAtech Euro EA 3000 apparatus.
2,4-Bis(trifluoromethyl)phenylphosphane (3)
A degassed solution of 2,4-bis(trifluoromethyl)phenyldibromophos-
phane (2) (1.96 g, 4.85 mmol) in 1,6-dibromohexane (25 mL) was co-
oled to 0 °C and tributyltin hydride (4.59 g, 15.8 mmol) was added
within 2 hours. After 30 minutes of stirring at 0 °C, the temperature
was slowly raised to room temperature. The colorless liquid 3 was
isolated by fractional condensation in a yield of 0.95 g (3.86 mmol).
Single-crystal X-ray diffraction data of 8 were determined with a
Bruker Nonius Kappa CCD diffractometer (Mo-Kα radiation). The
crystal details and refinement characteristics are shown in Table 5.
MS (EI; 20 eV) {m/z (%) [assignment]}: 490 (20) [2,4-
(CF3)2C6H3P(H)-P(H)2,4-(CF3)2C6H3]+; 246 (100) [M]+; 195 (35);
175 (20) [M-CF3-2H]+; 157 (35) [M-CF3-HF]+.
2,4-Bis(trifluoromethyl)phenyldibromophosphane (2)
A 1.6 m n-butyllithium solution in hexane (100 mL, 160.0 mmol) was
added to
a solution of 1,3-bis(trifluoromethyl)benzene (35.0 g,
163.0 mmol) in diethyl ether (200 mL) at –78 °C. The temperature was
slowly raised to room temperature. Stirring the mixture overnight gave
a black solution. After the addition of chlorobis(diethylamino)phos-
phane, ClP(NEt2)2 (12.6 g, 59.9 mmol) at –78 °C, the mixture was
warmed to room temperature and stirred overnight. The diethyl ether
was removed under reduced pressure, n-hexane (700 mL) was added
and the solution was stirred for 60 minutes at –60 °C in an atmosphere
of HBr (248 mmol, 4.1 equiv.). The temperature was slowly raised to
room temperature and stirring was continued for 60 minutes. The pre-
cipitate was filtered off. After evaporation of the solvent, compound 2
was obtained by vacuum distillation at 33–36 °C as colorless liquid in
59% yield (14.4 g, 35.6 mmol).
2,4-Bis(trifluoromethyl)phenylphosphinic Acid (4)
A solution of 2,4-bis(trifluoromethyl)phenyldibromophosphane (2)
(1.65 g, 4.10 mmol) in CH2Cl2 (15 mL) was combined with H2O
(0.5 mL) and stirred for 30 minutes at room temperature. After removal
of all volatiles in vacuo and recrystallization from CH2Cl2 the phos-
phinic acid 4 was obtained in 95% yield (1.08 g, 3.88 mmol) as a
colorless solid (mp. 77–79 °C).
Elemental analysis calcd. (%) for C8H5F6PO2 (MW = 277.9 g·mol–1):
C 34.53, H 1.81; found: C 33.73, H 1.79. MS (EI; 20 eV) {m/z (%)
540
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© 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
Z. Anorg. Allg. Chem. 2012, 534–542

2,4-Bis(trifluoromethyl)phenyl-Substituted Phosphonous Acid Pd Complex
[assignment]}: 474 (8) [{2,4-(CF3)2C6H3}2PHO]+; 278 (100) [M]+; (qt, J(H,H) = 7.3, J(H,H) = 6.8 Hz, 2 H, C2-H), 0.92 (t, J(H,H) =
3
3
3
259 (10) [M-F]+; 230 (56) [M-2F]+; 214 (14); 194 (30).
7.3 Hz, 3 H, C1-H).
2,4-Bis(trifluoromethyl)phenylphosphonic Acid (5) (Route 1)
A solution of 2,4-bis(trifluoromethyl)phenyldibromophosphane (2)
(0.66 g, 1.63 mmol) in CH2Cl2 (30 mL) was treated with conc. HNO3
(2 mL) and stirred for one day at room temperature. The aqueous layer
was separated and the organic phase was extracted twice with water
(5 mL). The aqueous layers were combined and all volatiles were re-
moved in vacuo. The product was recrystallized from a CH2Cl2/THF
solution at –28 °C to give 5 as a colorless solid in 50% yield (0.24 g,
0.81 mmol).
13C{1H} NMR (75.47 MHz CDCl3): δ = 166.7 (s, C5), 161.3 (d,
1J(C,F) = 254.3 Hz, C9), 137.1 (d, 3J(C,F) = 3.0 Hz, C7), 131.5 (d,
1J(C,F) = 8.8 Hz) 128.9 (d, 1J(C,F) = 3.0 Hz), 124.3 (d, 1J(C,F) =
3.6 Hz), 122.5 (d, J(C,F) = 11.6 Hz, C8), 120.8 (d, J(C,F) = 6.5 Hz,
C6), 116.1 (d, 2J(C,F) = 22.9 Hz, C10), 64.5 (s, C4), 30.7 (s, C3), 19.1
(s, C2), 13.9 (s, C1); an unambiguous assignment for 13C atoms (C11,
C12, C13) could not be given. 19F NMR (282.40 MHz, CDCl3): δ =
–116.5 (s).
2
4
2,4-Bis(trifluoromethyl)phenylphosphonic Acid (5) (Route
2)
A sample of 2,4-bis(trifluoromethyl)phenylphosphinic acid (0.25 g,
0.89 mmol) was solved in conc. HNO3 (6 mL) and the mixture was
stirred for one day at room temperature. After removal of all volatiles
in vacuo, 2,4-bis(trifluoromethyl)phenylphosphonic acid was obtained
in 98% yield (0.26 g, 0.88 mmol) as a colorless solid.
General Procedure for Suzuki Cross-Coupling Reaction
(e.g. for [Pd2(μ-Cl)2{[2,4-(CF3)2C6H3P(OH)O]2H}2] (8))
A 50 mL Schlenk flask was charged with phenylboronic acid (2.18 g,
17.88 mmol, 1.5 equiv.), bromo-3-fluorobenzene (2.06 g, 11.77 mmol,
1.0 equiv.), potassium phosphate (3.79 g, 17.85 mmol, 1.5 equiv.), and
2-propanole (20 mL). After stirring the mixture for three hours at room
temperature, the catalyst (1 mg, e.g. 8, 0.595 μmol, 0.01 mol-% palla-
dium) was added. The reaction mixture was stirred for 20 hours at
room temperature and the conversion was analyzed by 19F NMR spec-
troscopy. The amount of boronic acid, benzene and bases were calcu-
lated for 1 mg catalyst.
Elemental analysis calcd. (%) for C8H5F6PO3 (MW = 293.9 g·mol–1):
C 32.65, H 1.71; found: C 32.50, H 1.74. MS (EI; 70 eV) {m/z (%)
[assignment]}: 294 (100) [M]+; 275 (28) [M-F]+; 255 (16) [M – HF2]+;
230 (60); 226 (35) [M-CF2–OH2]+; 210 (82) [M-CF2–2OH]+; 194 (37);
182 (34); 144 (45).
[Pd2(μ-Cl)2{[2,4-(CF3)2C6H3P(OH)O]2H}2] (8)
1H NMR (300.13 MHz, CDCl3): δ = 7.67–7.61 (m, 2 H, CH), 7.55–
7.40 (m, 5 H, CH), 7.40–7.35 (m, 1 H, CH), 7.15–7.06 (m, 1 H, CH).
13C{1H} NMR (75.47 MHz, CDCl3): δ = 163.2 (d, 1J(C,F) =
245.4 Hz), 143.5 (d, 3J(C,F) = 7.6 Hz), 139.9 (d, 4J(C,F) = 2.2 Hz),
Solid palladium dichloride (49 mg, 0.278 mmol, 1.0 equiv.) was added
to
a solution of 2,4-bis(trifluoromethyl)phenylphosphinic acid
(155 mg, 0.557 mmol, 2.0 equiv.) in THF (15 mL)and the mixture was
stirred for six days at room temperature. The solid components were
filtered off and all volatiles were removed from the filtrate in vacuo.
The palladium(II) complex 8 was obtained in 85% yield (200 mg,
0.119 mmol) as a light yellow solid.
3
130.2 (d, J(C,F) = 8.3 Hz), 128.9 (s), 127.9 (s), 127.1 (s), 122.8 (d,
2
2
4J(C,F) = 3.4 Hz), 114.07 (d, J(C,F) = 21.8 Hz), 114.05 (d, J(C,F) =
20.9 Hz). 19F NMR (282.40 MHz, CDCl3): δ = –113.1 (s). Comparison
with literature gave identical 13C and 19F NMR spectroscopic data.[23]
MS (EI; 70 eV) {m/z (%) [assignment]}: 172 (100) [M]+; 154 (32)
[C12H10]+; 152 (12) [M – HF]+.
ESI MS in the negative range (neg.) {m/z (%) [assignment]}: 1394
(24) [M-H]–; 1356 (12) [M-2F]–; 880 (14); 852 (27); 696 (100) [M-
2H]2–.
Acknowledgments
General Procedure for Heck Cross-Coupling Reactions (e.g.
for [Pd2(μ-Cl)2{[2,4-(CF3)2C6H3P(OH)O]2H}2] (8))
The Merck KGaA (Darmstadt, Germany) is acknowledged for finan-
cial support. We thank Dr. N. Ignatiev (Merck KGaA), Prof. Dr. L.
A 150 mL Schlenk tube was charged with n-butyl acrylate (2.33 g, Weber and Dr. J. Bader for helpful discussions. We are grateful to
18.18 mmol, 1.5 equiv.), bromo-2-fluorobenzene (2.08 g, 11.89 mmol, Prof. Dr. D. Naumann for the generous support of this work.
1.0 equiv.), potassium carbonate (2.47 g, 17.87 mmol, 1.5 equiv.), and
DMF (20 mL). After stirring the mixture for 10 minutes at room tem-
perature, the catalyst (1 mg, e.g. 8, 0.595 μmol, 0.01 mol-% palladium)
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Z. Anorg. Allg. Chem. 2012, 534–542