Reductive elimination of C6F5-C6F 5 in the reaction of bis(pentafluorophenyl)palladium(ii) complexes with protic acids

Article abstract of DOI:10.1039/b802408a

Reductive elimination of C₆F₅-C₆F ₅ from cis-[Pd(C₆F₅)₂L] (L = cod, bpy, and dppb) was promoted by Bronsted acids. HNO₃ is a convenient acid for the formation of C₆F₅-C ₆F₅ from [Pd(C₆F₅) ₂(cod)]. The products are controlled by the auxiliary ligand.

Full text of DOI:10.1039/b802408a

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Reductive elimination of C6F5–C6F5 in the reaction of
bis(pentafluorophenyl)palladium(II) complexes with protic acids†
Take-aki Koizumi, Atsuko Yamazaki and Takakazu Yamamoto*
Received 12th February 2008, Accepted 9th May 2008
First published as an Advance Article on the web 3rd June 2008
DOI: 10.1039/b802408a
Reductive elimination of C6F5–C6F5 from cis-[Pd(C6F5)2L]
(L = cod, bpy, and dppb) was promoted by Brønsted acids.
HNO3 is a convenient acid for the formation of C6F5–C6F5
from [Pd(C6F5)2(cod)]. The products are controlled by the
auxiliary ligand.
Scheme 1. The structures of 2 and 3 were established by X-ray
crystallography and NMR spectroscopy.
Reductive elimination of organic compounds, R–R (R = alkyl or
aryl group), from M–R2 complexes (M = Ni, Pd) is a key step in
Ni- and Pd-catalyzed C–C coupling reactions.1–5 There have been
many reports on basic reductive elimination of R–R from the M–
R2 complexes.2,4–6 Palladium-catalyzed C–C coupling reactions are
used for the preparation of aromatic compounds from aryl halide
ArX and Arꢀm (m = B(ORꢀ)2, SnRꢀ3, etc.). However, when Ar
and Arꢀ groups are electron-withdrawing groups, ArX and Arꢀm
have low reactivity for the C–C coupling reactions. Use of these
aromatic compounds often requires accelerating reagents such
as Ag2O.7 Effective Pd-catalyzed coupling of C6F5X and C6F5m
(m = B(ORꢀ)2, SnRꢀ3, etc.) has not been achieved.7–9 Reductive
elimination of C6F5–C6F5 from an intermediate Pd(C6F5)2Lm
complex does not seem to take place easily, due to the formation
of strong Pd–carbon bonds. In fact, cis-[Pd(C6F5)2L] (L = diphos-
phines, etc.)10,11 complexes are highly stable, although usual cis-
diarylpalladium complexes are difficult to isolate because of facile
reductive elimination of biaryl compounds from the complex.
Recently, polyfluorinated aromatic p-conjugated molecules have
attracted strong interest for applications in electronic devices
such as field-effect transistors,12 and development of effective Pd-
catalyzed C–C coupling reactions of electron withdrawing ArX
and Arꢀm is demanded.
Scheme 1 Synthesis of [Pd(C6F5)2(bpy)] (2) and [Pd(C6F5)2(dppe)] (3).
The molecular structures of 1,‡ 2‡ and 3‡ are depicted in
Fig. S1, S2 and S3 in the ESI,† respectively. Selected coordination
parameters of the complexes are summarized in Table 1. The
Pd–Cipso(C6F5) bond length is in the order 2 < 1 < 3, and the
Cipso(C6F5)–Pd–Cipso(C6F5) bond angle becomes wider in the order
2 < 1 < 3. The dihedral angles formed by two C6F5 rings in 1 and
2 are 88.9 and 89.9◦, respectively. On the other hand, two C6F5
rings in 3 show a larger dihedral angle of 104◦ (cf . Fig. S4†).
1 is soluble in THF and CHCl3. Addition of HCl, HNO3,
1
and H2SO4 (excess) to 1 in THF led to changes of the 19F{ H}
1
NMR spectrum shown in Fig. S5.† HNO3 gave only 19F{ H}
NMR signals of the reductive elimination product, C6F5–C6F5
(Scheme 2) and intact 1 as shown in Fig. S5b.†
In order to obtain basic information on the reductive elimina-
tion from Pd(Ar)(Arꢀ)Ln with electron-withdrawing Ar and Arꢀ
groups, we have prepared Pd(C6F5)2Ln type complexes and found
new results about the reductive elimination of C6F5–C6F5 from the
complex.
[Pd(C6F5)2(cod)] (1) was prepared according to the literature.10
A bpy complex 211 and a dppe complex 311 were synthesized by
the reaction of 1 with bpy and dppe, respectively, as shown in
Scheme 2 Reaction of [Pd(C6F5)2(cod)] (1) with HNO3.
The reaction of 1 with HCl gave both C6F5–C6F5 (50% yield)
and the protonolysis product C6F5H (86 mol% per molecule
of 1).13 For this reaction, a p-allyl complex 4 is also formed as
judged from the 1H NMR spectrum (Scheme 3).14 4 is considered
to be formed via [PdCl(C6F5)(cod)].14 In the case of the reaction
Chemical Resources Laboratory, Tokyo Institute of Technology, 4259
Nagatsuta, Midori-ku, Yokohama, 226-8503, Japan. E-mail: tyamamot@
res.titech.ac.jp; Fax: +81 45-924-5976; Tel: +81 45-924-5221
† Electronic supplementary information (ESI) available: Experimental
details including synthetic procedures and physical measurements of 5,
19F NMR spectra of 1 with protic acids (Fig. S5), X-ray diffraction studies
and selected bond lengths and angles of 1, 2, 3, and 5 (Fig. S1, S2,
S3, and S6, respectively; Tables S1, S2, S3, and S4, respectively). CCDC
reference numbers 653704 (1), 653705 (2), 653706 (3), and 653707 (5). For
ESI or crystallographic data in CIF or other electronic format see DOI:
10.1039/b802408a
Scheme 3 Reaction of [Pd(C6F5)2(cod)] (1) with HCl.
Dalton Trans., 2008, 3949–3952 | 3949
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Table 1 Pd–Cipso bond length (A), Cipso–Cipso distance (A), Cipso–Pd–Cipso bond angle (◦), and dihedral angle (◦) of the two C6F5 planes of 1, 2, 3, and 5
˚
˚
1
2
3
5
Pd–Cipso
2.032(4)
2.842(6)
88.75(19)
88.9
2.009(6)(av.)
2.747(8)
86.3(2)
89.9
2.087(2)(av.)
3.026(3)
92.90(8)
2.062(4)
2.829(5)
86.65(16)
86.6
Cipso · · · Cipso
Cipso–Pd–Cipso
plane(C6F5)–plane(C6F5)
104.1
with H2SO4, a complex mixture including a trace amount of C6F5–
C6F5 was obtained.
Although similar reductive elimination of C6F5–C6F5 from
Ni(C6F5)2(bpy) in reactions of protic acids was reported,2e the
occurrence of such a protic acid-induced reductive elimination in
Pd complexes has no precedent.
The difference in the reactivity between 1 and 2 may be due
to a stronger Pd–Cipso(C6F5) bond in 2 than in 1, as suggested by
the shorter Pd–Cipso(C6F5) bond in 2 than in 1. Reaction of 3 with
HNO3 gave the protonolysis product C6F5H, and C6F5–C6F5 was
not formed.
In the case of Ni(C6F5)2(bpy), CF3CO2H can also cause
reductive elimination of C6F5–C6F5.2e However, the corresponding
Pd complex, 2, was inert to organic acids such as CF3CO2H and
CH3SO3H.
Use
of
THF,
chloroform,
and
conc.
HNO3
(1 : 3 :1.3 vol/vol/vol) in the reaction gave better results for
the reductive elimination of C6F5–C6F5, although the reaction
system became heterogeneous and the reaction was followed in
the organic layer by 19F NMR spectroscopy. Fig. 1 shows changes
in the 19F NMR spectrum of the reaction system of 1 and HNO3
in the mixed system. After 4 h, the 19F NMR spectrum showed
only peaks of C6F5–C6F5.
3 seems to have a weaker Pd–Cipso(C6F5) bond because the
Pd–Cipso(C6F5) bond length in 3 is longer than those in 1 and
2. This may be advantageous for the reductive elimination of
C6F5–C6F5 from 3. However, the wider C–Pd–C bond angle in
˚
3 gives a longer Cipso · · · Cipso length of 3.026(3) A in 3 than
˚
˚
those (2.842(6) A and 2.747(8) A) in 1 and 2. The longer
Cipso · · · Cipso bond length in 3 seems to prevent the Cipso–Cipso
interaction thus retarding the reductive elimination and leading
to protonolysis. The reductive elimination and protonolysis may
take place through an intermediate as shown in Scheme 4. Such an
interaction between Pd and H–Y may be related to the basicity of
transition metal complexes.15 Intra- and intermolecular M · · · (H–
X) bond formation have been observed in some transition metal
complexes.16
Scheme 4 Postulated reaction pathway of [Pd(C6F5)2(cod)] (1) with HY.
Coordination of a highly electron-withdrawing protic acid (H-
Y) seems to facilitate the reductive elimination of C6F5–C6F5.
The present results suggest that weaker Pd–C bond and
shorter Cipso · · · Cipso length are important for effective reductive
elimination.
To compare the steric effect of the auxiliary ligand for the
reaction, dppb-coordinated complex, [Pd(C6F5)2(dppb)] 5, was
synthesized, and its reactivity in HNO3 acidic solution was investi-
gated. The molecular structure of 5‡ is depicted in Fig. S6.†
1
Fig. 1 Time-dependent 19F{ H} NMR spectra for the reaction of 1 with
HNO3 in the mixed system (THF-d8–CDCl3–conc. HNO3 = 1 : 3 : 1.3) at
room temperature.
A similar reaction of 2 with HNO3 in a 1 : 3 mixture of THF
and CHCl3 gave C6F5–C6F5 as the single product, however, at a
slower reaction rate, giving C6F5–C6F5 in 20% yield after 3 days.
3950 | Dalton Trans., 2008, 3949–3952
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The Pd–Cipso bond length is 2.062(4) A, which is shorter than
that of 3.17 The dihedral angle formed by the two C6F5 rings and
the Cipso · · · Cipso distance are 86.6 and 2.829 A, respectively, which
are more acute and shorter than those of 3, respectively, and are
similar to those of 1.
5 was stable under the conditions described above. However, 5
underwent reductive elimination in quantitative yield when treated
with a mixture of toluene and conc. HNO3, and no C6F5H was
observed (Scheme 5) at 80 ◦C.18
◦
˚
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and G. Wu, J. Am. Chem. Soc., 1989, 111, 3089.
Scheme 5 Reaction of [Pd(C6F5)2(dppb)] (5) with HNO3.
These results suggest that the Cipso · · · Cipso distance in
[Pd(C6F5)2L] is an important factor in favoring the C–C coupling
reaction. However, the reductive elimination of C6F5–C6F5 from
5 proceeded under conditions different from those applied for
other Pd(C6F5)2 complexes and a more detailed comparison of the
reactivity of 1, 2, 3 and 5 will be necessary.
As described above, reductive elimination of C6F5–C6F5 from
cis-[Pd(C6F5)2L] (L = cod, bpy and dppb) was promoted by
Brønsted acids.19,20 HNO3 is a convenient acid for the formation of
C6F5–C6F5 from [Pd(C6F5)2(cod)] (1). The products are controlled
by the auxiliary ligand.
6 For Pt complexes, see: (a) S. Shekhar and J. F. Hartwig, J. Am. Chem.
Soc., 2004, 126, 13016; (b) Y. Suzaki and K. Osakada, Bull. Chem. Soc.
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group-substituted aromatic halide have been reported; for examples,
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(b) T. Korenaga, T. Kosaki, Y. Kawauchi, T. Ema and T. Sakai,
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Transition Met. Chem., 1983, 8, 131.
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Li, J. Am. Chem. Soc., 2000, 122, 7787; (b) H.-Z. Chen, M.-M. Shi, T.
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13 In the case of the reaction of 1 with HCl, the solvents were removed by
evaporation after the reaction (cf . Method B in ESI†) and the 19F NMR
spectrum was measured using the residue with CDCl3. Consequently, a
major part of C6F5H seemed to be lost during the removal of the solvent,
Notes and references
‡ Crystallographic data for 1 (CCDC number 653704): C20H12F10Pd, M =
547.70 g mol−1, monoclinic, space group C2/c (No. ◦15), a = 19.002(10) A,
˚
3
˚
˚
˚
b = 8.037(4) A, c = 13.386(7) A, b = 117.9746(17) , V = 1805.4(16) A ,
T = 113 K, Z = 4, qcalcd = 2.004 g cm−3, l = 11.328 cm−1, F(000) = 1056,
no. unique reflections = 3481, Rint = 0.023, no. variables = 147, R1 = 0.0461
(I > 2.0r(I)), wR2 = 0.0969 (all data). Crystallographic data for 2 (CCDC
number 653705): C22H8F10N2Pd, M = 596.70 g mol−1, monoclinic, space
˚
˚
group P21/c (No. 14), a = 13.450(4) A, b = 7.1639(19) A, c = 25.045(7)
◦
3
˚
˚
A, b = 90.3929(13) , V = 2413.2(11) A , T = 113 K, Z = 4, qcalcd
=
1.642 g cm−3, l = 8.578 cm−1, F(000) = 1160, no. unique reflections =
4966, Rint = 0.023, no. variables = 324, R1 = 0.0542 (I > 2.0r(I)), wR2 =
0.0983 (all data). Crystallographic data for 3 (CCDC number 653706):
C38H24F10P2Pd, M = 838.94 g mol−1, monoclinic, space group P21/n (No.
◦
˚
˚
˚
14), a = 13.599(3) A, b = 16.667(4) A, c = 15.613(7) A, b = 93.7837(11) ,
V = 3497.2(13) A , T = 113 K, Z = 4, qcalcd = 1.593 g cm−3, l = 7.033 cm−1
,
3
˚
F(000) = 1672, no. unique reflections = 7352, Rint = 0.020, no. variables =
484, R1 = 0.0316 (I > 2.0r(I)), wR2 = 0.0408 (all data). Crystallographic
data for 5 (CCDC number 653707): C40H28F10P2Pd, M = 866.99 g mol−1
,
˚
˚
monoclinic, space group C2 (No. 5), a = 16.622(6) A, b = 9.954(3) A, c =
◦
3
˚
˚
11.013(4) A, b = 109.1665(14) , V = 1721.1(10) A , T = 113 K, Z = 2,
qcalcd = 1.673 g cm−3, l = 7.175 cm−1, F(000) = 868, no. unique reflections =
3234, Rint = 0.016, no. variables = 296, R1 = 0.0160 (I > 2.0r(I)), wR2 =
0.0186 (all data).
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and its 19F NMR peaks observed are only small ones (Fig. S5(c)†). The
yield of C6F5H was calculated based on the area of the 19F NMR peak
of compound 4 (yield: 22mol% per molecule of 1) by assuming the
reaction shown in Scheme 3.
18 When the reaction of 1 or 2 with HNO3 was carried out in
toluene at 80 ◦C, C6F5–C6F5 was obtained. On the other hand, a
similar reaction using 3 produced C6F5H and no C6F5–C6F5 was
obtained.
14 A. C. Albe´niz, P. Espinet, Y. Jeannin, M. Philoche-Levisalles and B. E.
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5, Pt(C6F5)2(dppe) and Pt(C6F5)2(dppb); G. B. Deacon, P. W. Elliott,
A. P. Erven and G. Meyer, Z. Anorg. Allg. Chem., 2005, 631, 843.
19 1, 2, 3, and 5 are stable in toluene-d8 at 80 ◦C without HNO3.
20 It may be thought that the reductive elimination was enhanced by an
oxidant function of HNO3. However, the reaction of 1 with HNO3
under dark conditions also caused the reductive elimination reaction
in a similar fashion. In addition, reaction of 1 with H2O2 and I2 did
not lead to reductive elimination. These results suggest that HNO3
works as a Brønsted acid in the reductive elimination reaction of C6F5–
C6F5 from the cis-[Pd(C6F5)2L] complexes rather than as an oxdizing
reagent.
3952 | Dalton Trans., 2008, 3949–3952
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©