Isolation and reactivity of palladium hydrido complexes: Intermediates in the hydrodefluorination of pentafluoropyridine

Article abstract of DOI:10.1039/c0dt00086h

The hydrido complexes trans-[Pd(H)(4-C₅NF₄)(PiPr ₃)₂] (3) and trans-[Pd(H)(4-C₅NF ₄)(PCy₃)₂] (5) can be prepared by reaction of trans-[Pd(F)(4-C₅NF₄)(PiPr₃)₂] (2) or trans-[Pd(F)(4-C₅NF₄)(PCy₃)₂] (4) with HBpin (HBpin = 4,4,5,5-tetramethyl-1,3,2-dioxaborolane, pinacolborane). The iodo and triflato complexes trans-[Pd(I)(4-C₅NF ₄)(PiPr₃)₂] (7) and trans-[Pd(OTf)(4-C ₅NF₄)(PiPr₃)₂] (9) are generated on treatment of complex 3 with MeI or ethyltrifluoromethanesulfonate (EtOTf), respectively. Treatment of 3 with Ph₃CPF₆ in MeCN results in the formation of trans-[Pd(4-C₅NF₄)(NCMe)(PiPr ₃)₂]PF₆ (6a). Heating 3 to 60°C gives the products of reductive elimination 2,3,5,6-tetrafluoropyridine as well as [Pd(PiPr₃)₂] (1). In the presence of pentafluoropyridine [Pd(PiPr₃)₂] (1) affords the oxidative addition product 2. In a catalytic experiment, pentafluoropyridine can be converted into 2,3,5,6-tetrafluoropyridine in the presence of HBpin with 44% yield when 10% of 3 is employed as catalyst.

Full text of DOI:10.1039/c0dt00086h

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PAPER
www.rsc.org/dalton | Dalton Transactions
Isolation and reactivity of palladium hydrido complexes: intermediates in the
hydrodefluorination of pentafluoropyridine†‡
David Breyer, Thomas Braun* and Anna Penner
Received 6th March 2010, Accepted 6th May 2010
First published as an Advance Article on the web 15th June 2010
DOI: 10.1039/c0dt00086h
The hydrido complexes trans-[Pd(H)(4-C5NF4)(PiPr3)2] (3) and trans-[Pd(H)(4-C5NF4)(PCy3)2] (5) can
be prepared by reaction of trans-[Pd(F)(4-C5NF4)(PiPr3)2] (2) or trans-[Pd(F)(4-C5NF4)(PCy3)2] (4)
with HBpin (HBpin = 4,4,5,5-tetramethyl-1,3,2-dioxaborolane, pinacolborane). The iodo and triflato
complexes trans-[Pd(I)(4-C5NF4)(PiPr3)2] (7) and trans-[Pd(OTf)(4-C5NF4)(PiPr3)2] (9) are generated on
treatment of complex 3 with MeI or ethyltrifluoromethanesulfonate (EtOTf), respectively. Treatment of
3 with Ph3CPF6 in MeCN results in the formation of trans-[Pd(4-C5NF4)(NCMe)(PiPr3)2]PF6 (6a).
Heating 3 to 60 ◦C gives the products of reductive elimination 2,3,5,6-tetrafluoropyridine as well as
[Pd(PiPr3)2] (1). In the presence of pentafluoropyridine [Pd(PiPr3)2] (1) affords the oxidative addition
product 2. In a catalytic experiment, pentafluoropyridine can be converted into
2,3,5,6-tetrafluoropyridine in the presence of HBpin with 44% yield when 10% of 3 is employed as
catalyst.
the presence of iBu2AlH.13 We developed a selective and cat-
Introduction
alytic hydrodefluorination of pentafluoropyridine to give 2,3,5,6-
There is an ongoing interest to develop new routes to access
tetrafluoropyridine in the coordination sphere of rhodium using
fluoroorganic compounds, because of their versatile applications.1
dihydrogen as hydrogen source and [Rh(4-C5NF4)(PEt3)3] as
catalyst.14 [Rh(H)(PEt3)3] serves presumably as an intermediate.
The hydrodefluorination of pentafluoropyridine at [Pd(PiPr3)2]
(1) is based on the selective oxidative addition of a C–F bond
to give trans-[Pd(F)(4-C5NF4)(PiPr3)2] (2), which can be isolated.4
On reaction of the latter with Ph3SiH 2,3,5,6-tetrafluoropyridine
was generated. The existence of a Pd(II)-hydrido intermediate has
been postulated, but such a compound was not detected.5
In this work we succeeded in the isolation and characterisa-
tion of trans-[Pd(H)(4-C5NF4)(PiPr3)2] (3) and trans-[Pd(H)(4-
C5NF4)(PCy3)2] (5). Both complexes are unusually stable.15-18
Compound 3 gives at 60 ◦C the reductive elimination product
2,3,5,6-tetrafluoropyridine. Further studies on the reactivity of 3
and 5 are also reported.
An organometallic strategy involves the selective defluorination of
easily available highly fluorinated precursors.2,3 For instance, the
selective activation of carbon–fluorine bonds in the coordination
sphere of palladium or nickel followed by functionalisation of
the fluorinated ligands can lead to fluorinated compounds, which
are often not accessible otherwise.2,4-6 However, so far, most of
the routes to access fluoroorganic compounds via transition metal
mediated C–F activation comprise hydrodefluorination reactions,
e.g. the replacement of a C–F by a C–H moiety.2,7 Convenient hy-
drogen sources can be silanes, alanes, boranes or even dihydrogen
itself.2,5,8 Because of the strength of the E–F bonds (E = H, Si,
Al, B) which are formed the conversions are thermodynamically
favourable.9 Metal fluoro or hydrido complexes often play a crucial
role in hydrodefluorination reactions, either as catalysts or as
intermediates.2,5,8,10
The transition metal mediated activation and hydrodeflu-
orination of pentafluoropyridine has been investigated thor-
oughly. In a very recent example, Whittlesey et al. described
the use of a ruthenium hydrido catalyst in the presence of
alkylsilanes for the hydrodefluorination of hexafluorobenzene,
and pentafluoropyridine.11 Holland et al. reported the catalytic
conversion of pentafluoropyridine into 2,3,5,6-tetrafluoropyridine
in the presence of Et3SiH on using an iron(II) fluoro complex as a
catalyst.12 Rosenthal and Krossing achieved a significantly better
performance in the hydrodefluorination of pentafluoropyridine
on using zirconocene hydrides and zirconocene fluorides in
Results
Synthesis of trans-[Pd(H)(4-C5NF4)(PR3)2] (3: R = iPr; 5: R = Cy)
Complex trans-[Pd(F)(4-C5NF4)(PiPr3)2] (2) reacted at room
temperature with HBpin to give the hydrido compound trans-
[Pd(H)(4-C5NF4)(PiPr3)2] (3) and FBpin (Scheme 1).
Humboldt-Universita¨t zu Berlin, Institut fu¨r Chemie, Brook-Taylor-Str. 2,
12489, Berlin, Germany. E-mail: thomas.braun@chemie.hu-berlin.de
† CCDC reference numbers 768942–768944. For crystallographic data in
CIF or other electronic format see DOI: 10.1039/c0dt00086h
‡ This publication is part of the web themed issue on fluorine chemistry.
Scheme 1 Synthesis of the hydrido complexes 3 and 5.
Dalton Trans., 2010, 39, 7513–7520 | 7513
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◦
˚
Synthesis of a cationic palladium complex
Table 1 Selected bond lengths (A) and angles ( ) in trans-[Pd(H)(4-
C5NF4)(PCy3)2] (5) with estimated standard deviations in parenthesesa
The reaction of trans-[Pd(H)(4-C5NF4)(PiPr3)2] (3) with Ph3CPF6
in the presence of acetonitrile led to the formation of the
cationic complex trans-[Pd(4-C5NF4)(NCMe)(PiPr3)2]PF6 (6a) by
abstraction of the hydrido ligand and formation of Ph3CH.19
Compound 6a was characterised by its NMR spectroscopic data.
Pd(1)–P(1)
Pd(1)–P(1#)
Pd(1)–H(1)
Pd(1)–C(1)
C(1)–C(2)
C(2)–C(3)
C(4)–C(5)
P(1#)–Pd(1)–P(1)
P(1)–Pd(1)–H(1)
C(2)–C(1)–Pd(1)
C(1)–Pd(1)–H(1)
N(1)–C(3)–C(2)
C(1)–Pd(1)–P(1)
2.2956(6)
2.2955(6)
1.48(5)
C(1)–C(5)
C(2)–F(1)
C(3)–F(2)
C(4)–F(3)
C(5)–F(4)
C(3)–N(1)
C(4)–N(1)
C(5)–C(1)–Pd(1)
C(2)–C(1)–C(5)
C(1)–C(2)–C(3)
C(4)–C(5)–C(1)
N(1)–C(4)–C(5)
1.390(6)
1.367(4)
1.339(6)
1.366(5)
1.356(6)
1.337(6)
1.302(6)
125.6(3)
112.2(4)
123.1(4)
121.5(5)
125.7(4)
2.102(4)
1.373(6)
1.374(6)
1.372(6)
165.09(3)
83.29 (12)
122.3(3)
178.6(19)
123.5(4)
96.63(2)
The 31P NMR spectrum displays a signal at d 43.7 (t, JPF
=
3.7 Hz) for the phosphines in a mutually trans-position and a
septet at d -143.2 (JPF = 706.2 Hz) for the PF6 anion. The 19F
-
NMR spectrum reveals two signals at d -97.1 and d -114.4 for
the tetrafluoropyridyl ligand and a doublet at d -73.3 (JPF
=
706 Hz) which can be assigned to the anion. The cation in 6a
was also generated via an independent pathway starting from the
fluoride trans-[Pd(F)(4-C5NF4)(PiPr3)2] (2). Thus, a solution of 2
in acetonitrile reacts with NOBF4 to give complex trans-[Pd(4-
C5NF4)(NCMe)(PiPr3)2]BF4 (6b) (Scheme 2). The formation of
FNO was monitored via IR spectroscopy in MeCN. An absorption
band at 1871 cm-1 is compatible with the presence of FNO.20
a Symmetry transformations used to generate equivalent atoms: -x, y, z.
The structure which is proposed for 3 is supported by the
1H, 19F, 31P NMR data. In the H NMR spectrum a signal at
1
d -9.30, which can be assigned to the hydride, appears as a
multiplet. Fluorine decoupling experiments reveal a hydrogen–
phosphorus coupling of JPH = 0.7 Hz. The assignment of 3 as a
4-tetrafluoropyridyl palladium derivative is based on the presence
of two signals at d -110.5 and d -115.3 in the 19F NMR spectrum.
1
The trans geometry is indicated by one signal in the 31P{ H}
NMR spectrum at d 57.4 (t, JPF = 1.5 Hz). We were also able
to synthesise trans-[Pd(H)(4-C5NF4)(PCy3)2] (5) on treatment of
trans-[Pd(F)(4-C5NF4)(PCy3)2] (4) with HBpin. The NMR data
of 5 are comparable to these of 3. The molecular structure of
5 was confirmed by X-ray crystallography (Fig. 1). Complex 5
was crystallised from toluene at room temperature. Selected bond
lengths and angles are summarised in Table 1.
Scheme 2 Synthesis of 6a and 6b.
The structure of 6b was confirmed by X-ray diffraction analysis
at 100 K (Fig. 2, Table 2). The cation reveals a square pla-
nar configuration at palladium with a trans disposition of the
phosphine ligands. The palladium–carbon distance to the pyridyl
˚
ring is 2.005(3) A and is similar to the Pd–C separation in 2,
◦
˚
Table 2 Selected bond lengths (A) and angles ( ) in trans-[Pd(4-
C5NF4)(NCMe)(PiPr3)2]BF4 (6b) with estimated standard deviations in
parentheses
Pd(1)–P(1)
Pd(1)–P(2)
Pd(1)–N(2)
Pd(1)–C(1)
C(24)–N(2)
C(24)–C(25)
C(1)–C(2)
C(2)–C(3)
P(1)–Pd(1)–P(2)
P(1)–Pd(1)–N(2)
P(2)–Pd(1)–N(2)
C(1)–Pd(1)–N(2)
C(1)–Pd(1)–P(1)
C(1)–Pd(1)–P(2)
C(2)–C(1)–Pd(1)
C(24)–N(2)–Pd(1)
2.3715(8)
2.3661(8)
2.062(2)
2.005(3)
1.132(4)
1.444(5)
1.381(4)
1.371(4)
173.68(3)
89.86 (8)
88.27(8)
173.11(13)
92.03(9)
90.50(9)
118.5(2)
171.2(3)
C(4)–C(5)
C(1)-C(5)
C(2)–F(1)
C(3)–F(2)
C(4)–F(3)
C(5)–F(4)
C(3)–N(1)
1.386(5)
1.377(4)
1.347(4)
1.345(4)
1.348(4)
1.351(4)
1.314(5)
1.309(5)
127.7(2)
113.8(3)
121.4(3)
124.5(3)
120.6(3)
124.8(3)
114.8(3)
178.5(4)
Fig. 1 An ORTEP diagram of 5; ellipsoids are drawn at the 50%
probability level; a toluene molecule and hydrogen atoms are omitted for
clarity, except for H(1).
The structure shows the expected trans disposition of the phos-
phine ligands with an approximately square-planar coordination
geometry at the metal centre. The palladium–carbon distance in
C(4)–N(1)
C(5)–C(1)–Pd(1)
C(5)–C(1)–C(2)
C(3)–C(2)–C(1)
N(1)–C(3)–C(2)
C(1)–C(5)–C(4)
N(1)–C(4)–C(5)
C(4)–N(1)–C(3)
N(2)–C(24)–C(25)
˚
5 is 2.102(4) A. For comparison, in 2 the Pd–C separation of
4
˚
1.988(3) A is shorter. The hydride at the palladium centre has
˚
been located. The palladium–hydrogen distance is 1.48(5) A. The
˚
Pd–H length in trans-[Pd(H)(OC6F5·C6F5OH)(PCy3)2] is 1.46(2) A
15
˚
and in trans-[Pd(H)(OC6H5·C6H5OH)(PCy3)2] it is 1.57(2) A.
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Fig. 2 An ORTEP diagram of the cation in 6b; ellipsoids are drawn at
the 50% probability level; hydrogen atoms have been omitted for clarity.
Fig. 3 An ORTEP diagram of 7; ellipsoids are drawn at the 50%
probability level; hydrogen atoms have been omitted for clarity.
4
˚
1.988(3) A. The metal–nitrogen bond length to the acetonitrile
A solution of 3 was reacted with EtOTf to produce the triflate
complex trans-[Pd(OTf)(4-C5NF4)(PiPr3)2] (9) and ethane. The 31
P
˚
ligand is 2.062(2) A.
NMR spectrum of 9 exhibits a triplet at d 42.5 (JPF = 4.6 Hz) which
can be assigned to the phosphines. The presence of three signals in
the 19F NMR spectrum results from the tetrafluoropyridyl ligand
(d -96.4, d -113.9) and the triflate ligand (d -77.2).
Whereas 3 did not react with acetic acid, treatment of 3 with HCl
or trifluoroacetic acid in C6H6 affords the known compound trans-
[Pd(Cl)(4-C5NF4)(PiPr3)2] (8)5 and complex trans-[Pd(O2CCF3)(4-
C5NF4)(PiPr3)2] (10), respectively.
Synthesis of trans-[Pd(X)(4-C5NF4)(PiPr3)2] (7: X = I; 8: X = Cl;
9: X = OTf; 10: X = O2CCF3)
To estimate the reactivity of the hydride trans-[Pd(H)(4-
C5NF4)(PiPr3)2] (3), it has been treated with several electrophiles.
A reaction with MeI in C6D6 afforded after 26 days at room
temperature the iodo complex trans-[Pd(I)(4-C5NF4)(PiPr3)2] (7).
An NMR experiment revealed the evolution of methane. Complex
7 was also prepared via an alternative synthesis. NaI was added to a
THF solution of trans-[Pd(F)(4-C5NF4)(PiPr3)2] (2) to yield 7 after
stirring overnight. 7 was characterized by its 1H, 19F and 31P NMR
spectra. The 19F NMR spectrum shows two multiplets at d -97.9
and d -113.6, which indicate the presence of the tetrafluoropyridyl
ligand with the palladium at the 4-position. The 31P NMR
spectrum exhibits a triplet at d 38.6 (JPF = 3.5 Hz). Complex 7
was crystallised from a hexane solution by slow evaporation of the
solvent at room temperature. The colourless crystals were suitable
for X-ray diffraction analysis (Fig. 3). Selected bond lengths and
angles are summarised in Table 3. Compound 7 exhibits a square-
planar geometry with the pyridyl group coordinated in the trans
position to the iodo ligand. The angles about the palladium atom
are distorted from an ideal square-planar geometry and vary
from 88.08(4) to 92.56(19)◦. The palladium–carbon bond in 7
˚
of 2.019(6) A is similar to the corresponding distance in trans-
5
˚
[Pd(Cl)(4-C5NF4)(PiPr3)2] (8) (2.012(2) A).
Scheme 3 Reactivity of 3 towards electrophilic compounds.
◦
˚
Table 3 Selected bond lengths (A) and angles ( ) in trans-[Pd(I)(4-
C5NF4)(PiPr3)2] (7) with estimated standard deviations in parentheses
1
The structure of complex 10 is supported by the H, 19F, 31P
NMR data. The assignment as a 4-tetrafluoropyridyl palladium
derivative is based on the presence of two signals in the 19F
NMR spectrum at d -97.1 and d -114.9 . The signal for the
trifluoroacetato ligand appears at d -74.4. The trans geometry is
Pd(1)–P(1)
Pd(1)–P(2)
Pd(1)–I(1)
Pd(1)–C(1)
C(1)–C(2)
C(2)–C(3)
C(4)–C(5)
P(1)–Pd(1)–P(2)
P(1)–Pd(1)–I(1)
P(2)–Pd(1)–I(1)
C(1)–Pd(1)–I(1)
C(1)–Pd(1)–P(1)
C(1)–Pd(1)–P(2)
C(2)–C(1)–Pd(1)
2.3642(16)
2.3670(16)
2.6945(7)
2.019(6)
1.406(9)
1.354(9)
1.362(10)
176.20(6)
88.08 (4)
88.26(4)
176.09(18)
92.56(19)
91.16(19)
122.3(5)
C(1)–C(5)
C(2)–F(1)
C(3)–F(2)
C(4)–F(3)
C(5)–F(4)
C(3)–N(1)
C(4)–N(1)
C(5)–C(1)–Pd(1)
C(5)–C(1)–C(2)
C(3)–C(2)–C(1)
N(1)–C(3)–C(2)
C(4)–C(5)–C(1)
N(1)–C(4)–C(5)
1.364(9)
1.354(7)
1.354(7)
1.360(8)
1.366(7)
1.297(9)
1.302(9)
124.5(5)
113.2(6)
120.3(6)
125.3(6)
121.5(6)
124.6(6)
indicated by a triplet in the 31P NMR spectrum at d 38.3 (JPF
=
3.7 Hz).
Catalytic hydrodefluorination of pentafluoropyridine
Heating a solution of trans-[Pd(H)(4-C5NF4)(PiPr3)2] (3) in
THF at 60 ◦C led to the reductive elimination of 2,3,5,6-
tetrafluoropyridine and the formation of [Pd(PiPr3)2] (1).21 In the
presence of 10 equiv. PiPr3 the reductive elimination is inhibited.
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This suggests that prior to the reductive elimination step a
dissociation of a phosphine ligand occurs. Heating a solution of
3 with pentafluoropyridine affords the C–F activation product
trans-[Pd(F)(4-C5NF4)(PiPr3)2] (2) and 2,3,5,6-tetrafluoropyridine
(Scheme 4).4
hydrido ligand at a palladium centre have been reported before.19,24
The Reaction of 2 with NOBF4 leads to the generation of trans-
[Pd(4-C5NF4)(NCMe)(PiPr3)2]BF4 (6b) and FNO. This reactivity
pattern is unprecedented. FNO can be applied as a fluorinating
reagent.25 NOBF4 has been used for the synthesis of nitrosyl
compounds.26
Treatment of 3 with electrophiles such as MeI or EtOTf
yielded the compounds trans-[Pd(I)(4-C5NF4)(PiPr3)2] (7) or trans-
[Pd(OTf)(4-C5NF4)(PiPr3)2] (9), respectively. An SN2 type oxida-
tive addition followed by a C–H reductive elimination may be
more likely for MeI than a radical mechanism, which is although
conceivable.27 A comparable reaction for the generation of a triflate
complex was reported by Ozerov et al.18,28 We did not observe any
C–C coupling reactions, as has been found on treatment of [Rh(4-
C5NF4)(PEt3)2] with MeI.29
Complex trans-[Pd(H)(4-C5NF4)(PiPr3)2] (3) reacts with
Brønsted acids HX (X
=
O2CCF3, Cl) to give trans-
[Pd(O2CCF3)(4-C5NF4)(PiPr3)2] (10) and trans-[Pd(Cl)-(4-
C5NF4)(PiPr3)2] (8).5 We did not observe any generation
of 2,3,5,6-tetrafluoropyridine. Note that the fluoro complex
trans-[Ni(F)(2-C5NF4)(PEt3)2] reacts with HCl to give 3,4,5,6-
tetrafluoropyridine.30
Scheme 4 Formation of 2,3,5,6-tetrafluoropyridine from 3.
Therefore, we turned our attention to catalytic experiments for
the catalytic hydrodefluorination of pentafluoropyridine to give
2,3,5,6-tetrafluoropyridine with HBpin on using 3 or 5 as catalysts.
At 60 ◦C in THF as a solvent 2,3,5,6-tetrafluoropyridine has
been obtained in 44% and 30% yield, respectively, on employing
10 mol% catalyst (Scheme 5).
However, heating of a solution of 3 at 60 ◦C gave the
products of reductive elimination 2,3,5,6-tetrafluoropyridine and
[Pd(PiPr3)2] (1). Treatment of 1 with pentafluoropyridine affords
the regeneration of the C–F activation product trans-[Pd(F)-(4-
C5NF4)(PiPr3)2] (2), which completes a cyclic process for a selec-
tive hydrodefluorination of pentafluoropyridine at the 4-position
(Scheme 6). Our studies show that 2,3,5,6-tetrafluoropyridine can
also be generated catalytically starting from pentafluoropyridine
and HBpin with the hydrido complexes 3 (TON = 4) or 5 (TON =
3) as catalysts.31 However, the yields are very low; possibly in part
because of the formation of HF and F2Bpin- salts. The F2Bpin-
anion could be identified by 19F NMR spectroscopy.32 We did not
observe the formation of fluorinated phosphoranes.4,33
Scheme 5 Catalytic hydrodefluorination of pentafluoropyridine.
Discussion
The synthesis of the palladium hydrido complexes trans-[Pd(H)(4-
C5NF4)(PiPr3)2] (3) and trans-[Pd(H)(4-C5NF4)(PCy3)2] (5) are
shown in Scheme 1. Both compounds are stable in solution at
room temperature and can be prepared on treatment of the
fluoro precursors trans-[Pd(F)(4-C5NF4)(PiPr3)2] (2) or trans-
[Pd(F)(4-C5NF4)(PCy3)2] (4) with HBpin. A comparable reaction
with the corresponding chloro compounds is not possible. It
has been shown before that fluoro complexes often exhibit a
higher reactivity than their chloro or bromo counterparts.5,6,10
There are only a few mononuclear palladium hydrido complexes
known that exhibit an additional palladium carbon bond. All of
them are pincer complexes.16,18 Note, that complexes that bear
a tetrafluoropyridyl ligand often exhibit an unusual stability.22
The formation of 3 could only be achieved on using HBpin as a
“hydride source”. Complex 3 does not react with NaH, LiEt3BH
or LiAlH4. We have no evidence for a reaction of 3 with HBpin to
produce H2 and a boryl complex. Generation of H2 and pinBBpin
from two equivalents of HBpin by a dehydrodimerization pathway
would also be conceivable in the presence of 3, but such a
transformation, which is endothermic, was not observed.23
Scheme
pentafluoropyridine.
6 Cyclic process for the hydrodefluorination of
Conclusions
Stoichiometric reactions of 3 are summarised in Schemes 2–
4. The abstraction of the hydrido ligand on using Ph3CPF6
in MeCN gave the cationic complex trans-[Pd(4-C5NF4)-
(NCMe)(PiPr3)2]PF6 (6a). Comparable reactions to abstract a
In conclusion, we presented studies on the synthesis and reactivity
of palladium hydrido complexes trans-Pd(H)(4-C5NF4)(PiPr3)2]
(3) and trans-[Pd(H)(4-C5NF4)(PCy3)2] (5) Both compounds ex-
hibit an unexpected stability. These hydrido complexes as well
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as the fluorides trans-[Pd(F)(4-C5NF4)(PiPr3)2] (2) and trans-
[Pd(F)(4-C5NF4)(PCy3)2] (4) might serve as intermediates in the
hydrodefluorination of pentafluoropyridine.
(121.5 MHz, [D8]THF): d 43.7 (t, JPF = 3.7 Hz, PdP), -143.4 (sep,
-
JPF = 706.2 Hz, PF6 ).
Synthesis of trans-[Pd(4-C5NF4)(NCMe)(PiPr3)2]BF4 (6b)
Experimental
NOBF4 (12 mg, 0.103 mmol) was added to a solution of 2 (77 mg,
0.129 mmol) in MeCN (4 mL). The reaction mixture was stirred
for 5 h at room temperature and the volatiles were removed under
vacuum. The remaining yellow solid was washed with hexane to
give a white powder. Yield 48 mg (66%). (Found: C, 42.19; H, 5.98;
N, 3.77%. C25H45BF8NP2Pd requires C, 42.60; H, 6.44; N 3.97%);
1H NMR (300.1 MHz, [D8]THF): d 2.38 (s, 3 H, NCMe), 1.98 (m,
br, 6 H, CH), 0.98 (dd, JHH = 7.2, JPH = 14.9 Hz, 36 H, CHCH3);
19F NMR (282.4 MHz, [D8]THF): d -92.2 (m, 2 F, CF), -114.4
The synthetic work was carried out on a Schlenk line. All solvents
were purified and dried by conventional methods and distilled
under argon before use. [D6]Benzene and [D8]THF were dried by
stirring over Na/K and then distilled. Complexes trans-[Pd(F)(4-
C5NF4)(PiPr3)2] (2) and trans-[Pd(F)(4-C5NF4)(PCy3)2] (4) were
prepared according to the literature procedures.4
The NMR spectra were recorded on a Bruker DPX 300 NMR
spectrometer at 300 K. The 1H NMR chemical shifts were
referenced to residual C6D5H at d = 7.15 or [D8]THF at d =
1.8. The 19F NMR spectra were referenced to external C6F6 at d =
1
(m, 2 F, CF), -152.7 (s, 4 F, BF4); 31P{ H} NMR (121.5 MHz,
[D8]THF): d 43.9 (t, JPF = 3.7 Hz).
–162.9. The 31P{ H} NMR spectra were referenced externally to
1
Synthesis of trans-[Pd(I)(4-C5NF4)(PiPr3)2] (7)
H3PO4 at d 0.0. Infrared spectra were recorded on a Bruker Vector
22 spectrometer that was equipped with an ATR unit (diamond).
(a) A solution of 2 (55 mg, 0.092 mmol) in THF (5 mL) was
treated with NaI (80 mg, 0.534 mmol). After stirring for 24 h
the volatiles were removed under vacuum. The yellow solid was
washed with hexane (5 mL) and dried under vacuum to afford a
yellow solid. Yield 56 mg (87%). (Found: C, 38.87; H, 6.00; N,
1.60%. C23H42F4INP2Pd requires C, 39.26; H, 6.02; N 1.99%); 1H
Synthesis of trans-[Pd(H)(4-C5NF4)(PiPr3)2] (3)
A
solution of trans-[Pd(F)(4-C5NF4)(PiPr3)2] (2) (43 mg,
0.07 mmol) in C6H6 (4 mL) was treated with HBpin (13 mL,
0.087 mmol). The reaction mixture was stirred for 2 h at room
temperature. The volatiles were then removed in vacuo to give a
white solid. Yield 41 mg (98%). (Found: C, 47.86; H, 7.45; N,
NMR (300.1 MHz, C6D6): d 2.39 (m, br, 6 H, CH), 1.02 (dd, JHH
=
7.1, JPH = 14.3 Hz, 36 H, CHCH3); 19F NMR (282.4 MHz, C6D6):
1
d -97.9 (m, 2 F), -113.6 (m, 2 F). 31P{ H} NMR (121.5 MHz,
˜
1.94%. C23H43F4NP2Pd requires C, 47.82; H, 7.51; N 2.43%); n
C6D6): d 38.6 (t, JPF = 3.5 Hz).
(ATR, diamond)/cm-1 1911 (PdH); 1H NMR (300.1 MHz, C6D6):
d 1.72 (m, br, 6 H, CH), 0.96 (dd, JHH = 7.1, JPH = 14.3 Hz,
36 H, CH3), -9.06 (m, JPH = 0.74 Hz, 1 H, PdH); 19F NMR
(b) In an NMR tube CH3I (0.04 ml, 0.66 mmol) was added to a
solution of 3 (74 mg, 0.128 mmol) in C6D6 (0.6 mL). The reaction
mixture was stirred for 26 days at room temperature. The yield
of 7 was determined by measuring NMR spectra of the reaction
solution on using an internal standard. Yield 77%. The formation
of methane was observed by 1H NMR spectroscopy.
1
(282.4 MHz, C6D6): d -100.5 (m, 2 F), -115.3 (m, 2 F). 31P{ H}
NMR (121.5 MHz, C6D6): d 57.4 (t, JPF = 1.5 Hz).
Synthesis of trans-[Pd(H)(4-C5NF4)(PCy3)2] (5)
Synthesis of trans-[Pd(OTf)(4-C5NF4)(PiPr3)2] (9)
A
solution of trans-[Pd(F)(4-C5NF4)(PCy3)2] (4) (38 mg,
0.045 mmol) in toluene (5 mL) was treated with HBpin (8 mL,
0.054 mmol). The reaction mixture was stirred for 2 h at room
temperature. The volatiles were removed under vacuum to give
a white solid. Yield 36 mg (98%). (Found: C, 63.37; H, 8.20; N,
1.33%. C41H66F4NP2PdC7H8 requires C, 63.34; H, 8.31; N 1.54%);
A solution of 3 (31 mg, 0.054 mmol) in C6H6 (4 mL) was treated
with EtOTf (38 mL, 0.54 mmol). After stirring for 24 h the reaction
mixture was filtered and the volatiles were removed under vacuum.
A brown solid was obtained. Yield 32 mg (81%). (Found: C, 39.97;
H, 6.09; N, 1.58%. C24H42F7NO3P2PdS requires C, 39.72; H, 5.84;
N 1.93%); 1H NMR (300.1 MHz, C6D6): d 2.22 (m, br, 6 H, CH),
0.96 (dd, JHH = 7.2, JPH = 14.6 Hz, 36 H, CHCH3; 19F NMR
(282.4 MHz, C6D6): d -77.2 (s, 3 F, CF3), -96.4 (m, 2 F, CF),
n (ATR, diamond)/cm-1 1886 (PdH); 1H NMR (300 MHz, C6D6):
˜
d 1.01–1.93 (m, 66 H, Cy), -9.06 (s, br, 1 H, PdH); 19F NMR
1
(282.4 MHz, C6D6): d -100.6 (m, 2 F), -115.5 (m, 2 F); 31P{ H}
NMR (121.5 MHz, C6D6): d 45.5 (s).
1
-113.9 (m, 2 F, CF); 31P{ H} NMR (121.5 MHz, C6D6): d 42.5 (t,
JPF = 4.6 Hz).
Synthesis of trans-[Pd(4-C5NF4)(NCMe)(PiPr3)2]PF6 (6a)
Formation of trans-[Pd(Cl)(4-C5NF4)(PiPr3)2] (8)
Ph3CPF6 (23 mg, 0.058 mmol) was added to a solution of trans-
Pd(H)(4-C5NF4)(PiPr3)2] (3) (42 mg, 0.073 mmol) in CH3CN
(4 mL). The reaction mixture was stirred for 24 h at room
temperature and the volatiles were removed under vacuum. The
remaining yellow solid was washed with hexane to give a white
powder. Yield 31 mg (85%). (Found: C, 39.85; H, 5.68; N, 3.40%.
A solution of 3 (44 mg, 0.076 mmol) in 3 mL THF was treated
with HCl (0.02 ml, 0.152 mmol). After stirring for 24 h the reaction
mixture was filtered, the volatiles were removed under vacuum. A
white solid was obtained which consisted of 8 according to the
NMR spectra.4 Yield 38 mg (82%).
1
C25H45F10NP3Pd requires C, 39.36; H, 5.95; N 3.67%); H NMR
(300.1 MHz, [D8]THF): d 2.38 (s, 3 H, NCMe), 1.94 (m, br, 6
Synthesis of trans-[Pd(O2CCF3)(4-C5NF4)(PiPr3)2] (10)
H, CH), 0.98 (dd, JHH = 7.3, JPH = 14.9 Hz, 36 H, CHCH3); 19
F
NMR (282.4 MHz, [D8]THF): d -73.3 (d, JPF = 706 Hz, 6 F,
A solution of 3 (31 mg, 0.054 mmol) in C6H6 (4 mL) was treated
with CF3COOH (42 mL, 0.54 mmol). After stirring for 24 h the
-
1
PF6 ), -97.1 (m, 2 F, CF), -114.4 (m, 2 F, CF); 31P{ H} NMR
This journal is
The Royal Society of Chemistry 2010
Dalton Trans., 2010, 39, 7513–7520 | 7517
©

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Table 4 Crystallographic data
Compound
5·C7H8
6b
7
Crystal dimensions/mm3
Empirical formula
Formula weight
Crystal system
0.36 ¥ 0.10 ¥ 0.08
C48H75F4NP2Pd
910.43
Orthorhombic
Pmn21
13.9664(9)
16.9951(9)
9.6839(6)
—
—
—
2298.6(2)
2
1.315
0.522
2.40 to 29.02
22 622
6337
0.0676
1.031
0.40 ¥ 0.26 ¥ 0.11
C25H45BF8N2P2Pd
704.78
Monoclinic
P21/n
10.3812(4)
8.1047(2)
37.4697(16)
—
97.560(3)
—
3125.2(2)
4
1.498
0.24 ¥ 0.16 ¥ 0.16
C23H42F4INP2Pd
703.82
Triclinic
¯
P1
Space group
˚
a/A
10.1655(8)
10.3275(9)
14.6934(14)
103.329(7)
100.118(7)
108.091(7)
1374.9(2)
2
1.700
1.953
3.17 to 25.50
9749
5100
˚
b/A
˚
c/A
a/◦
b/◦
g /◦
V/A
Z
3
˚
Density (calcd.)/Mg m-3
m(Mo-Ka)/mm-1
q range/◦
0.762
3.29 to 29.50
15 491
8573
0.0458
Reflections collected
Independent reflections
Rint
0.0596
1.003
0.0716, 0.1084
0.0483, 0.1013
3925
Goodness-of-fit on F2
R1, wR2 on all data
R1, wR2 [Io > 2s(Io)]
Reflect. with Io > 2s(Io)]
0.971
0.0459, 0.0827
0.0392, 0.0807
5765
0.0683, 0.1186
0.0482, 0.1116
6522
-3
˚
Max diff peak, hole e A
1.502 and -0.915
0.801 and -1.551
1.415 and -1.030
reaction mixture was filtered, the volatiles were removed under
vacuum. A white solid was obtained. Yield 29 mg (77%). (Found:
C, 43.72; H, 6.12; N, 1.81%. C25H42F7NO2P2Pd requires C, 43.53;
(b) In an NMR tube HBpin (44 mL, 0.29 mmol) was added
to a mixture of trans-[Pd(H)(4-C5NF4)(PCy3)2] (5) (24 mg,
0.029 mmol) and pentafluoropyridine (32 mL, 0.29 mmol) in THF
(0.4 mL). The reaction mixture was heated for 3 days at 60 ◦C.
The yield of 2,3,5,6-tetrafluoropyridine was determined by NMR
spectroscopy on using an external standard of fluorobenzene and
is 30% (TON = 3).
1
H, 6.14; N 2.03%); H NMR (300.1 MHz, C6D6): d 1.79 (m, br,
6 H, CH), 0.97 (dd, JHH = 7.2, JPH = 14.5 Hz, 36 H, CHCH3),
-9.06 (s, br, 1 H, PdH); 19F NMR (282.4 MHz, C6D6): d -74.4 (s,
1
3 F, CF3), -97.1 (m, 2 F, CF), -114.9 (m, 2 F, CF); 31P{ H} NMR
(121.5 MHz, C6D6): d 38.3 (t, JPF = 3.7 Hz).
Structure determinations for the complexes 5, 7 and 8
Formation of 2,3,5,6-tetrafluoropyridine
White crystals of 5·C7H8 and 7 and yellow crystals of 6b were
obtained by slow evaporation of the solvent (5, toluene; 6b, MeCN,
7, hexane) at 293 K. The diffraction data were collected on a
STOE IPDS 2T diffractometer at 100 K. Crystallographic data
are depicted in Table 4. The structures were solved by direct
methods and refined with the full matrix least squares method
on F2 (SHELX-97).34 For complex 5 the metal bound hydrogen
was located in the difference fourier map and refined isotropically.
All other hydrogen atoms were placed at calculated positions and
refined using a riding model.
A solution of 3 (13◦mg, 0.023 mmol) in THF (0.5 mL) was
heated for 5 h to 60 C. The formation of [Pd(PiPr3)2] (1)21 and
2,3,5,6-tetrafluoropyridine was observed. The yield of 2,3,5,6-
tetrafluoropyridine was determined by using a capillary which
contained fluorobenzene as an external standard. Yield 84%.
Formation of trans-[Pd(F)(4-C5NF4)(PiPr3)2] (2)
Pentafluoropyridine (0.01 ml, 0.097 mmol) was added to a solution
of 3 (56 mg, 0.097 mmol) in THF (3 mL). The reaction mixture
was heated for 4 h at 60 ◦C and was allowed to cool to room
temperature. The black mixture was filtered and the volatiles were
removed under vacuum from the filtrate to give a brown solid.
Yield 49 mg (85%).
Acknowledgements
We would like to acknowledge the Fonds der Chemischen In-
dustrie for financial support. We thank P. Kla¨ring for help with
the X-ray crystallography. The graduate school “fluorine as a key
element” is gratefully acknowledged. We also thank the BASF SE
for a gift of solvents.
Catalytic formation of 2,3,5,6-tetrafluoropyridine
(a) In an NMR tube HBpin (97 mL, 0.64 mmol) was added to a mix-
ture of trans-[Pd(H)(4-C5NF4)(PiPr3)2] (3) (37 mg, 0.064 mmol)
and pentafluoropyridine (70 mL, 0.64 mmol) in [D8]THF (0.4 mL).
The reaction mixture was heated for 3 days at 60 ◦C. The yield of
2,3,5,6-tetrafluoropyridine was determined on using an external
standard of fluorobenzene and is 44% (TON = 4).
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