Pd(II) trifluoroacetate complexes containing chelating phosphine ligands: Synthesis, structures, and catalysis

Full text of DOI:10.1002/bkcs.10590

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DOI: 10.1002/bkcs.10590
BULLETIN OF THE
K.-W. Kim et al.
KOREAN CHEMICAL SOCIETY
Pd(II) Trifluoroacetate Complexes Containing Chelating Phosphine
Ligands: Synthesis, Structures, and Catalysis
‡
‡
Kun-Woo Kim,† Yong-Joo Kim,†, Hye Jin Lim, and Soon W. Lee
*
†Department of Chemistry, Kangnung-Wonju National University, Gangneung 210-702, Korea.
*E-mail: yjkim@kangnung.ac.kr
‡Department of Chemistry, Sungkyunkwan University, Suwon 440-746, Korea
Received July 3, 2015, Accepted August 27, 2015, Published online November 27, 2015
Keywords: Palladium, Trifluoroacetate, Chelating phosphine, Suzuki–Miyaura, C C coupling
Carboxylates coordinate totransitionmetals inaterminal, che-
lating, or bridging mode through their oxygen atoms, and
serve as active ligands in various catalytic reactions.1–3
Transition-metal complexes having a trifluoroacetate (TFA)
ligand are well known to have higher solubility and crystallin-
ity than the corresponding acetate counterparts depending on
the metal identity. In particular, TFA complexes of late tran-
sitionmetalsareefficientcatalysts inseveralorganicreactions,
Scheme 1. Syntheses of Pd(II) trifluoroacetates.
including polymerization of isocyanide or butadiene and
copolymerization of CO and olefin.4–7 Moreover, such com-
plexes exhibit higher efficiency than the corresponding acetate
complexes in the aerobic oxidation of organic substrates,
enamide formation by vinyl transfer, or oxidative cyclization
of organic substrates in the presence of Pd catalysts.8–11
Although many catalytic Suzuki–Miyaura C C coupling
reactions have been reported, those involving late-transi-
tion-metal–TFA complexes are still rare.12–15 For this reason,
we attempted to develop a new synthetic route to Pd–TFA
complexes possessing chelating phosphine ligands for use
in such coupling reactions. In this work, we prepared a series
of Pd(II)–TFA–(chelating diphosphine) complexes and
examined their catalytic C C coupling reactions of aryl
halides and arylboronic acids.
We first prepared a Pd(II)–TFA–TMEDA complex
(TMEDA = N,N,N0,N0-dimethyl ethylene diamine), [Pd
(TMEDA)(TFA)2](1), by themetathesis of [PdCl2(TMEDA)]
and 2 equiv of [Ag(TFA)]. Subsequently, the ligand replace-
ment of complex 1 was investigated with one or two equiva-
lents of chelating phosphine, including DMPE, DEPE, 1,2-
Ph2P(CH2)nPPh2 (n = 2, DPPE; n = 3, DPPP; n =
4, DPPB), and the ferrocene-bridged phosphines DPPF and
DIPPF (1,10-bis(diphenyl or diisoporpyl)phosphine ferro-
cene). These reactions afforded ionic bis(diphosphine)
Pd(II) complexes [Pd(P~P)2](TFA)2 (type A, 2–6) or neutral
mono(diphosphine) Pd(II)–TFA complexes [Pd(P~P)
(TFA)2] (type B, 7 and 8) in high yields (Scheme 1).
All products were characterized by spectroscopy (IR and
NMR) and elemental analysis, together with X-ray crystallog-
raphy for complexes 2 and 7. The IR spectra display charac-
teristic stretching C O and C F bands at 1671–1711 and
at 1105–1190 cm−1, respectively. Details on crystal data of
complexes are summarized in Supporting Information (see
Table S1). Figure 1 shows the cationic part of complex
2ꢀH2O, in which two CF3COO counterions and one lattice
water are omitted for clarity.
The molecular structure of 7 is given in Figure 2, which
shows a Pd(II) metal, a DPPF ligand, and two CF3COO
ligands. TwoCpringsintheDPPFligandare notperfectlypar-
allel, withadihedralangleof4.4(4)ꢁ. ThePdꢀꢀꢀFeseparationof
4.1932(8) Å indicates no direct interactions between these two
metals.
We examined the ligand exchange reaction of complex
8 with 2 equiv of NaN3 in CH2Cl2 (Scheme 2). The reaction
readily proceeded to give a Pd(II) azide complex, [(DIPPF)
Pd(N3)2] (9), as an orange crystal in quantitative yield, which
was characterized by spectroscopic data and elemental analy-
sis. The IR spectrum of 9 shows a strong absorption band at
2045 cm−1, a characteristic peak of the N3 group.
Recently, several Suzuki–Miyaura C C coupling reac-
tions employing Pd(II)–TFA complexes containing C–N
donor or pincer-type (PCP) ligands were reported.14,15 How-
ever, the corresponding reactions with bis(chelating phos-
phine) analogs are relatively rare. In this context, we
evaluated the catalytic activity of several complexes (1, 2,
6–8, and 9) for the coupling reactions (catalysts in Chart 1
and Scheme 3). In order to find the optimum conditions for
Chemical compositions of complexes 2–8 strongly indicate
that sterically hindered and less basic chelating phosphines
such as DPPF or DIPPF prefer to form mono(diphosphine)
compounds. Isolated bis(chelating phosphine)–Pd(II)–TFA
complexes are poorly soluble in organic solvents.
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Chart 1. Catalysts for Suzuki–Miyaura C C coupling
Figure 1. ORTEP drawing of the cationic part of complex
2ꢀH2O. Selected bond lengths (Å) and bond angles (ꢁ) in complex
2ꢀH2O: Pd1 P4 2.312(2), Pd1 P1 2.315(2), Pd1 P3 2.315(2),
Pd1 P2 2.316(2); P4 Pd1 P1175.87(5), P4 Pd1 P3 83.97(8),
P1 Pd1 P3 95.80(4), P1 Pd1 P2 84.50(8).
Scheme 3. Suzuki–Miyaura C C couplings.
activity whenp-chloroacetophenone(entries1–15)is thesub-
strate. Furthermore, even in the case of p-bromo acetophenone
(entries 16–21), the activity (entry 20) of complex 8 is higher
than that of the other complexes. Slightly higher activity of
8 compared to 7 may be due to the fact that complex 8 having
the more basic DIPPF ligand than the DPPF possesses a more
electron-rich metal center to stabilize an M–C(sp2) bond in the
oxidative addition product of aryl halides. According to
entries 1–21 in Table 1, p-bromo acetophenone is more reac-
tive than p-chloro acetophenone, andmono(diphosphine) Pd–
TFA complexes are more efficient catalysts than
bis(diphosphine) analogs. On the basis of the above catalytic
activities, we utilized complex 8 for further catalytic C C
coupling reactions.
Entries 1–13 in Table 2 show the results of the C C cou-
pling reactions of various aryl bromides and phenylboronic
acid in ethanol/H2O at 50 ꢁC in air. As expected, the coupling
products were obtained in moderate to good yields, which are
comparable to those for other known Pd-catalyzed reactions.
In addition, the same coupling reactions using varied aryl
boronic acids exhibited good activities. However, entries
12 and 13 gave poor yields, probably due to the deactivation
of the phenyl ring by the organic substituents or aryl moieties.
Entry 17 also gave very low yield, but we cannot give a
clear explanation at this moment. Therefore, our results
suggest that Pd(II)–TFA complexes having a single
chelating phosphine ligand can serve as efficient catalysts
for the C C coupling reactions involving aryl bromide with
organoboronic acid.
Figure 2. ORTEP drawing of complex 7. Selected bond lengths (Å)
and bond angles (ꢁ): Pd1 O3 2.071(3), Pd1 O1 2.088(3), Pd1 P1
2.244(1), Pd1 P2 2.261(1); O3 Pd1 O1 86.9(1), O3 Pd1 P1
91.30(9), O3 Pd1 P2 170.80(9), P1 Pd1 P2 97.38(5).
Scheme 2. Reaction of 8 with NaN3.
the reactions, we first examined the reaction of p-chloro
(or bromo) acetophenone, phenylboronic acid, and various
bases in the mole ratio of 1:1.2:1.2 in several solvents in air.
The reaction conditions and product yields are listed in
Table 1, in which entry 5 (complex 8) exhibits relatively high
In summary, we prepared several Pd(II)–TFA complexes
having one or two chelating phosphines, and one of them
showed a high catalytic activity in the Suzuki–Miyaura
C C coupling reactions of aryl bromide and aryl boronic acid
at 50 ꢁC in air.
Bull. Korean Chem. Soc. 2015, Vol. 36, 2952–2955
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Table 1. Optimization for Suzuki–Miyaura cross-coupling reactions of 4-chloro (or bromo) acetophenone with phenylboronic acid in air.
Entry
Catalyst
Solvent
Base
T (ꢁC)
t (h)
Isolated yield (%)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16a
17a
18a
19a
20a
21a
1
2
6
7
8
9
8
8
8
8
8
8
8
8
8
1
2
6
7
8
9
EtOH/H2O
EtOH/H2O
EtOH/H2O
EtOH/H2O
EtOH/H2O
EtOH/H2O
EtOH/H2O
EtOH/H2O
EtOH/H2O
THF
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
K2CO3
50
50
50
50
50
50
50
50
50
50
50
50
50
50
r-t
50
50
50
50
50
50
1
1
1
1
1
1
1
1
1
1
1
1
1
1
6
1
1
1
1
1
1
36
35
41
46
61
34
47
39
38
42
42
36
32
18
14
87
83
Na2CO3
KOtBu
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
MeOH
MeOH/H2O
EtOH
DMF
EtOH/H2O
EtOH/H2O
EtOH/H2O
EtOH/H2O
EtOH/H2O
EtOH/H2O
EtOH/H2O
89
99(88)b
99(97) b
95
a 4-Bromoacetophenone was used.
b 0.5 h stirring.
Experimental
Preparation of [Pd(P~P)2](TFA)2 (P~P = DMPE, DEPE,
DPPE, DPPB, 2–6) and [(P~P)Pd(TFA)2] (P~P = DPPF,
7;DIPPF, 8). DMPE(102μL, 0.616mmol) wasslowly added
to a yellow suspension of [Pd(TFA)2(TMEDA)] (0.138 g,
0.31 mmol) in CH2Cl2 (3 mL). An initial suspension turned
gray. After stirring the reaction mixture for 2 h at room tem-
perature, the solvent was removed under vacuum, and washed
with n-hexane. The crude solid was recrystallized from
CH2Cl2/hexane to give [Pd(P~P)2](TFA)2 (P~P = DMPE),
2 (0.192 g, 99%). Complexes 3–8 were analogously prepared.
Analytical and spectroscopic data are available as Supporting
Information.
All manipulations of air-sensitive compounds were performed
under N2 or Ar using Schlenk-line techniques. THF and
diethyl ether were distilled from sodium benzophenone. Ele-
mental analyses were performed at the analytical laboratories
at Kangnung-Wonju National University. IR spectra were
recorded using a Perkin-Elmer BX spectrophotometer
(Waltham, MA, USA). NMR (1H, 13C{1H}, and 31P{1H})
spectra were obtained in CDCl3 using a JEOL Lamda
300 MHz spectrometer (Tokyo, Japan). Chemical shifts were
referenced to internal Me4Si and to external 85% H3PO4. X-
ray reflection data were obtained at either the Korea Basic Sci-
ence Institute (Seoul Center) and the Cooperative Center for
Research Facilities at Sungkyunkwan University.
[PdCl2(TMEDA)] was prepared by the literature method.16
Preparation of [Pd(TFA)2(TMEDA)] (1). A solution of
AgOCOCF3 (0.331 g, 1.51 mmol) dissolved in H2O (2 mL)
was added to a Schlenk flask containing [PdCl2(TMEDA)]
(0.200 g, 0.69 mmol) in CH2Cl2 (30 mL). After stirring the
reaction mixture for 2 h at room temperature, white solids pre-
cipitated. The mixture was filtered to remove the salts and
dried over CaCl2. The solvent was completely removed under
vacuum. The resulting residue was solidified with hexane. The
solids were washed with n-hexane (2 mL × 2) to give crude
solids, which were recrystallized from CH2Cl2/hexane to
give compound 1. The complex 1 was also prepared from
[Pd(TFA)2] and TMEDA (N,N,N0,N0-tetramethylethylenedia-
mine) under reflux.9
Preparation of [(DIPPF)Pd(N3)2] (9). NaN3 (0.011g,
0. 16 mmol) dissolved in H2O (1 mL) was slowly added to
a red solution of 8 (0.059 g, 0.08 mmol) in CH2Cl2 (3 mL).
After stirring the reaction mixture for 1 h at room temperature,
the solvent was removed under vacuum and extracted with
CH2Cl2. The collected solution was evaporated to give a crude
product, which was recrystallized from CH2Cl2/hexane to
give orange crystals of compound 9 (0.046 g, 96%).
General Procedure for Suzuki–Miyaura Cross-Coupling
Reactions. Experimental details and spectral data for the
organic compounds are described in Supporting information.
X-ray Structure Determination. Details on X-ray structure
determination are described in Supporting information.
Acknowledgments. This work was supported by the
Basic Science Research Program through the National
Research Foundation of Korea (NRF) funded by the
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Table 2. Cross-coupling reactions of arylbromide and arylboronic
acid catalyzed by complex 8.
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Supporting Information. Analytical, spectroscopic, and
crystallographic data are available from the supporting
information.
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