On the mechanism of palladium-catalyzed cross-coupling of diazonium salts with aryltrifluoroborates: A combined ESI-MS/NMR study

Article abstract of DOI:10.1002/ejic.200700532

A combined ESI-MS/NMR mechanistic study on the palladium-catalysed cross-coupling reaction between aryldiazonium salts and aryltrifluoroborates with bis(μ-acetato)bis(4,4′-difluoroazobenzene-C²,N) dipalladium(II) (4) as the precatalyst is reported. The reaction follows a Pd⁰/Pd^II cycle after reduction of 4 to a molecular Pd ⁰ species (I), which according to the combined ESI-MS and ¹⁹F NMR studies, bears an arylated azobenzene ligand. Oxidative addition by the diazonium salt generates the arylpalladium(II) intermediate (II), which was also detected in solution. The catalytic cycle is completed with a transmetallation between II and the organoborate, which is followed by fast reductive elimination of the cross-coupling product to restore the molecular Pd⁰ species I. A concurrent activation path was also observed which consists of the formation of (4,4′-difluoroazobenzene-C ²,N)dipalladium(II) tetrafluoroborate (7) by the reaction of 4 with the diazonium salt and subsequent reduction by the aryltrifluoroborate to give I. Wiley-VCH Verlag GmbH & Co. KGaA, 2007.

Full text of DOI:10.1002/ejic.200700532

FULL PAPER
DOI: 10.1002/ejic.200700532
On the Mechanism of Palladium-Catalyzed Cross-Coupling of Diazonium
Salts with Aryltrifluoroborates: A Combined ESI-MS/NMR Study
Nicola Taccardi,[a] Rossella Paolillo,[a] Vito Gallo,[a] Piero Mastrorilli,*[a]
Cosimo F. Nobile,[a] Minna Räisänen,[b] and Timo Repo[b]
Keywords: Homogeneous catalysis / Metallacycles / Mass spectrometry / Cross-coupling / Diazo compounds
A combined ESI-MS/NMR mechanistic study on the palla-
dium-catalysed cross-coupling reaction between aryldiazon-
ium salts and aryltrifluoroborates with bis(µ-acetato)bis(4,4Ј-
difluoroazobenzene-C2,N)dipalladium(II) (4) as the precata-
lyst is reported. The reaction follows a Pd0/PdII cycle after
reduction of 4 to a molecular Pd0 species (I), which according
completed with a transmetallation between II and the
organoborate, which is followed by fast reductive elimination
of the cross-coupling product to restore the molecular Pd0
species I. A concurrent activation path was also observed
which consists of the formation of (4,4Ј-difluoroazobenzene-
C2,N)dipalladium(II) tetrafluoroborate (7) by the reaction of
to the combined ESI-MS and 19F NMR studies, bears an aryl- 4 with the diazonium salt and subsequent reduction by the
ated azobenzene ligand. Oxidative addition by the diazo- aryltrifluoroborate to give I.
nium salt generates the arylpalladium(II) intermediate (II),
which was also detected in solution. The catalytic cycle is
(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim,
Germany, 2007)
acetato)bis(4,4Ј-difluoroazobenzene-C2,N)dipalladium(II)]
(4) as a precatalyst and methanol as solvent (Scheme 2).
The rationale for the use of the fluorinated substrate and
precatalyst resides in the possibility of monitoring the reac-
tion progress by 19F NMR spectroscopy, while methanol
was used instead of IL/MeOH mixtures in order to simplify
the experimental procedures.[4] ESI-MS, which is used here
as a complementary analytical method, is a rapidly emerg-
ing technique for mechanistic investigations due to the pos-
sibility of easily revealing all species present in an actual
catalyst solution.[5] The classical Suzuki cross-coupling[6]
and other palladium-catalyzed reactions,[7] for example,
have recently been studied by means of ESI-MS.[8]
Introduction
An innovative route to biaryls by palladium-catalysed
cross-coupling between diazonium salts and aryltrifluoro-
borates has been proposed recently.[1] This protocol, which
is an improved modification of the more popular Suzuki
cross-coupling reaction,[2] does not require either tempera-
tures higher than ambient or the addition of a base.
We have recently published a paper on the title reaction
carried out in IL/methanol (IL = ionic liquid) mixtures
where several (zero- and divalent) palladium complexes
were used as precatalysts.[3] In this framework it was found
that palladacycles of general formula [(CʝN)Pd(µ-OAc)]2
(see Scheme 1) were outstanding in terms of activity and
selectivity of the reaction.
In order to ascertain the effect of fluorine substitution of
the azobenzene moiety in complex 3 on the catalysis, we
Our preliminary mechanistic investigations of this reac-
tion demonstrated that the catalysis is molecular and is in-
fluenced by the ligand on palladium. Moreover, we found
that an excess of diazonium salt (but not of aryltrifluorob-
orate) allowed the catalytic solutions to be recycled.[3]
In this paper we report a combined ESI-MS and 19F
NMR mechanistic study on the title reaction using KPhBF3
and p-XC6H4N2BF4 (X = CH3 or F) as substrates, [bis(µ-
preliminarily checked the catalytic activity of
4 in
methanol using p-tolyldiazonium tetrafluoroborate (p-
CH3C6H4N2BF4) and KPhBF3. Precatalyst 4 (1% mol)
gave an 81% yield after 80 min while the non-fluorinated
analogue 3 gave a 55% yield after 15 min[3] (the reactions
were stopped when quantitative conversions were reached).
The effect of the fluorine substitution on the ligand further
confirms the molecular nature of the catalysis.
[a] Dipartimento di Ingegneria delle Acque e di Chimica del Po-
litecnico di Bari,
Results and Discussion
via Orabona 4, 70125 Bari, Italy
Fax: +39-080-596-3611
E-mail: p.mastrorilli@poliba.it
The commonly accepted catalytic cycle for a PdII-initi-
ated Suzuki cross-coupling is depicted in Scheme 3.[2] It
consists of a preliminary reduction of PdII to Pd0 followed
by the oxidative addition of the electrophile ArX (where X
is typically a halide or triflate), subsequent transmetallation
[b] Laboratory of Inorganic Chemistry, Department of Chemistry,
University of Helsinki,
P.O. Box 55, 00014 Helsinki, Finland
Supporting information for this article is available on the
WWW under http://www.eurjic.org or from the author.
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P. Mastrorilli et al.
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Scheme 1. Examples of highly active palladacycles for the cross-coupling reaction.[3] IL = 1-n-butyl-3-methylimidazolium tetrafluorobo-
rate.
Reaction of the Precatalyst with KPhBF3
The reaction mixture comprised of 4 and KPhBF3 in
methanol was monitored by 19F NMR spectroscopy at
room temperature (Figure 1). The lack of signals attribut-
able to 4 is due to its poor solubility in methanol.
Scheme 2. Cross-coupling reaction with a fluorinated substrate and
precatalyst 4.
of the organoborate and reductive elimination leading to
the cross-coupling product and regeneration of the Pd0 spe-
cies, which restarts the catalytic cycle.
Figure 1. 19F NMR spectroscopic monitoring of reaction mixtures
of 4 and KPhBF3 in methanol as a function of time (Pd:KPhBF3
= 1:15 mol/mol).
It is apparent that a new species I with 19F NMR reso-
nances at δ = –108.1 and –112.4 ppm forms immediately
upon mixing KPhBF3 with azapalladacycle 4. The signals
assigned to I progressively decrease in intensity over 5 h,
with concomitant appearance of two new signals at δ =
Scheme 3. General catalytic cycle for the Suzuki cross-coupling
leading to biaryls.
In order to verify whether such a cycle also holds when –111.7 and –111.9 ppm and formation of palladium black.
ArX is replaced by an aryldiazonium salt and the or- These latter signals belong to 4,4Ј-difluoro-2-phenylazoben-
ganoboron compound is changed to KPhBF3, every single zene (5), which was isolated and fully characterized. The
step shown in Scheme 3 was examined.
formation of Pd black and 5 indicates two possible struc-
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Eur. J. Inorg. Chem. 2007, 4645–4652

Palladium-Catalyzed Cross-Coupling
tures for I, one of which is a PdII complex in which the while that at δ = –112.4 ppm, which is compatible with a
bridging acetate of 4 has been replaced by a σ-bonded fluorine on a 2-phenylated aryl ring in 5, can be attributed
phenyl group and the other of which consists of already to F2.
formed ligand 5 bonded to Pd0 (Scheme 4; in this and in
the following structures the solvent molecules possibly
bound to palladium are omitted).
High resolution ESI-MS (HRMS) analysis of the solu-
tion showed an intense peak at m/z 419 (negative ion mode).
This peak represents the fluoride adduct of I (exact mass =
400.00 daltons), as confirmed by the simulation of its iso-
topic pattern (Figure 2). The presence of other anions at
m/z 399 [M – HF]– and 293 [M – Pd – HF]– in the MS/MS
spectrum provided the basis to discriminate between the
two structures depicted in Scheme 4. The latter ion, which
corresponds to the deprotonated form of 5, clearly indicates
Scheme 5. 19F NMR features of 4, I and 5. All the chemical shifts
that species I seen in the 19F NMR spectrum is a Pd0 com-
are given for the species in methanol.
plex of 5.
The formation of I and its subsequent decomposition, as
shown in Figure 1, are depicted in Scheme 6. Such a process
is not unexpected for a Pd0 species such as I and has already
been suggested for analogous palladacycles.[11]
Figure 2. Experimental (solid line) and simulated (dotted line)
HRMS(–) spectrum of I as its fluoride adduct. The error between
simulated and observed isotopic patterns is –1.72 ppm.
Scheme 6. Reaction of 4 with phenyltrifluoroborate generates Pd0
complex I, which slowly decomposes to 5.
Scheme 4. Two possible structures for species I.
Reaction of I with Diazonium Salts
Although different structures with the same molecular
formula (e.g. coordination of the other N atom, η2 coordi-
The addition of a slight excess of diazonium salt to the
nation of the N=N moiety or different coordination modes catalyst solution caused the disappearance of I and con-
for the dangling phenyl) cannot be definitively ruled out,[9] comitant formation of coupling products (GLC analysis)
the 19F NMR resonances of I are in agreement with the and 5 (δ = –111.7 and –111.9 ppm). Depending on the ap-
structure depicted in Scheme 5. As suggested by a compari- plied diazonium salts (in this case p-FC6H4N2BF4 and p-
son of the 19F NMR signals of I with those of 4[10] and 5, CH3C6H4N2BF4), two new species were also formed (see
the signal at δ = –108.1 ppm in I can be attributed to F1 parts B in Figures 3 and 4).
Eur. J. Inorg. Chem. 2007, 4645–4652
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P. Mastrorilli et al.
FULL PAPER
Scheme 7. PdII species detected by HRMS(+) in the reaction mix-
ture of 4/KPhBF3 after the addition of p-FC6H4N2BF4.
Figure 3. 19F NMR monitoring of the reaction mixture of 4/
KPhBF3 and p-FC6H4N2BF4 in methanol. A: 4/KPhBF3; B: imme-
diately after addition of p-FC6H4N2BF4; C and D: after 25 and
100 min, respectively. ᭿ = 4,4Ј-difluorobiphenyl; o = 4-fluorobi-
phenyl. Pd/diazonium/KPhBF3 molar ratio of 1:18:15.
As already mentioned, the 19F NMR signals of interme-
diate II-F at δ = –106.4, –107.2 and –121.8 ppm integrate
in a 1:1:1 ratio and can therefore be assigned to the three
fluorine atoms of the structure with m/z 495 (see Support-
ing Information),[12] with the downfield ones being due to
the fluorine atoms of coordinated 5 and the upfield one
being due to the fluorine of the σ-bonded p-fluorophenyl
group.[13]
A structure similar to II-F, but with a p-tolyl ligand in
the place of p-fluorophenyl, can be attributed to II-CH3.
Complexes II-F and II-CH3 are the products of oxidative
addition of p-fluorophenyldiazonium and p-tolyldiazonium
salts, respectively, to I.
When the reaction between I and p-FC6H4N2BF4 was
carried out in the presence of an excess of KPhBF3 (Pd/
KPhBF3/diazonium molar ratio of 1:18:15) the same inter-
mediates as in Figure 3 were detected (Figure 5). However,
addition of the diazonium salt did not result in the immedi-
ate disappearance of I, as shown in part B of Figure 5, and
Figure 4. 19F NMR monitoring of the reaction mixture of 4/
KPhBF3 and p-CH3C6H4N2BF4 in methanol. A: 4/KPhBF3; B: im-
mediately after addition of p-CH3C6H4N2BF4; C and D: after 30
and 110 min, respectively. Pd/diazonium/KPhBF3 molar ratio of
1:18:15.
The new species formed in the case of p-fluorobenzenedi-
azonium (II-F) shows three broad signals at δ = –106.4,
–107.2 and –121.8 ppm that integrate in a 1:1:1 ratio (Fig-
ure 3), while that formed with p-tolyldiazonium (II-CH3)
shows only two broad signals at δ = –106.7 and –107.8 ppm
that integrate in a 1:1 ratio (Figure 4).
HRMS(+) analysis of a solution containing II-F (that of
Figure 3, B) revealed the presence of species with m/z values
of 399, 417, 495 and 789. Analysis of the isotopic pattern of
Figure 5. Reaction of 4 with excess KPhBF3 and p-FC6H4N2BF4.
A: 4/KPhBF3; B: immediately after addition of p-FC6H4N2BF4; C
after 25 min. ᭿ = 4,4Ј-difluorobiphenyl; o = 4-fluorobiphenyl. Pd/
these peaks indicated the structures depicted in Scheme 7. diazonium/KPhBF3 molar ratio of 1:15:18.
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Palladium-Catalyzed Cross-Coupling
the signals of II-F appeared only after consumption of I served when following the reaction between 7 and KPhBF3
(Figure 5, C). Accordingly, HRMS analysis of the solution by 19F NMR spectroscopy. Thus, complex 7 can contribute
resulting from the addition of p-FC6H4N2BF4 (Figure 5, B) to the formation of I and accumulates only when the aryl-
did not show the cations with m/z 399, 417, 495 and 789 trifluoroborate has been consumed.
whereas anions with m/z 419 ([I + F]–), 399 ([I – H]–), 487
The fact that 7 survived the work-up procedures suggests
([I + BF4]–) and 499 ([I + BF3OMe]–) were detected in nega- that it could be responsible for the aforementioned recycl-
tive ion mode.
ability of the catalytic solutions for reactions carried out
in IL/MeOH mixtures in the presence of excess diazonium
salt.[3]
Concurrent Activation of the Precatalyst
19F NMR analysis of the reaction solutions prepared
from 4, KPhBF3 and diazonium salt in excess showed the
formation of a new Pd complex (7; δ = –101.3 and
–109.5 ppm) which after 1.5 h is the main Pd species in
solution (see parts D in Figures 3 and 4).
Catalytic Cycle
On the basis of the experimental data described above,
the catalytic cycle shown in Scheme 9 can be proposed for
palladium-catalyzed cross-coupling of diazonium salts with
aryltrifluoroborates.
Complex 7 was characterized as (4,4Ј-difluoroazoben-
zene-C2,N)palladium(II) tetrafluoroborate, which arises
from the reaction of p-fluorobenzenediazonium tetrafluo-
roborate with residual 4 (Scheme 8). Pure 7 was isolated in
80% yield from the reaction between 4 and p-FC6H4N2BF4
(1:1 Pd:diazonium salt molar ratio) in methanol at room
temperature. The formation of 7 results from the abstrac-
tion of the bridging acetate of 4 by the diazonium salt[14]
rather than simple metathesis of the acetate ligands with
tetrafluoroborate, as demonstrated by the lack of reaction
between 4 and NaBF4 or [bmim]BF4 (bmim = 1-n-butyl-3-
methylimidazolium). Related (azobenzene-C2,N)palladi-
um(II) tetrafluoroborates have been previously synthesized
by Heck starting from bis(µ-chloro)bis(azobenzene-C2,N)-
dipalladium(II) and AgBF4.[15]
Scheme 9. Proposed catalytic cycle for the palladium-catalyzed
cross-coupling of diazonium salts with aryltrifluoroborates.
Scheme 8. Formation of complex 7.
Complex 7 was found to be catalytically active. Thus,
The catalyst precursor 4 reacts with the trifluorophenyl-
when 7 was used as precatalyst in the cross-coupling reac- borate to give the Pd0 species I, which is responsible for the
tion between p-CH3C6H4N2BF4 and KPhBF3 in methanol observed catalytic activity. This species undergoes oxidative
an 88% yield of 4-methylbiphenyl was obtained in 15 min. addition of the aryldiazonium salt to give the cationic aryl-
A similar yield was obtained with the parental complex 4 palladium(II) complex II-F, which bears the azobenzene 5
only after 80 min. The higher activity of 7 compared with as a ligand. The catalytic cycle is completed by a transme-
that of 4 can be explained by two considerations: i) complex tallation reaction between II-F and the organoborate to
4 is poorly soluble in the reaction medium, while 7 is com- give the neutral compound III, which is followed by re-
pletely soluble, and ii) complex 7 is a coordinatively unsatu- ductive elimination of the cross-coupling product. Com-
rated species and therefore highly reactive towards pre- pound I can also be formed from the reaction between 7
–
reduction reaction with KPhBF3. In fact, the quantitative (formed from 4 and the aryldiazonium salt) and PhBF3 .
conversion of 7 into I and the subsequent slow decomposi-
No transmetallation intermediates were detected, which
tion of the latter into 5 and palladium black could be ob- indicates that the reductive elimination step from III is very
Eur. J. Inorg. Chem. 2007, 4645–4652
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P. Mastrorilli et al.
FULL PAPER
fast.[16] The 19F NMR spectroscopic data (Figure 4) al-
lowed us to extend this mechanism to the reaction with a
p-tolyldiazonium salt, where an intermediate analogous to
II-F was detected.
The IR spectra were recorded with a Bruker Vector 22 FT-IR spec-
trometer. GLC analyses were performed on an HP5890 instrument
equipped
with
a
Supelco
SPB-1
capillary
column
(30 mϫ320 µmϫ0.25 µm). GC-MS analyses were performed on
an HP6890-HP5973MSD instrument equipped with an HP-5MS
capillary column (30 mϫ250 µmϫ0.25 µm).
Synthesis of 4,4Ј-Difluoroazobenzene[18]
Conclusions
We have demonstrated that the cross-coupling between
aryltrifluoroborates and diazonium salts facilitated by the
acetato(azobenzene-C2,N)palladium(II) complex 4 follows
a catalytic cycle involving Pd0 and PdII species. The active
species is a Pd0 complex (I) bearing an arylated azobenzene
as a ligand that can form by two different pathways, namely
direct reaction of 4 with the arylborate or by arylation of
(azobenzene-C2,N)Pd(BF4) (7), which is generated by ac-
tion of the diazonium salt on 4. The Pd0 complex I un-
dergoes addition of the diazonium salt to give the cationic
arylpalladium(II) complex II which, upon transmetallation
with organoborate and fast reductive elimination, restores
the active species I.
Sodium perborate (10.86 g, 0.109 mol) was added to a solution of
4-fluoroaniline (12.51 g, 0.113 mol) in 150 mL of glacial acetic acid.
The system was heated at 50 °C under vigorous stirring and the
reaction monitored by TLC. When the aniline has been completely
consumed the system was cooled and the reaction mixture poured
into a beaker containing 500 mL of cold water. The resulting sus-
pension was neutralized with sodium carbonate and extracted with
diethyl ether (3ϫ150 mL). The organic phases were collected and
washed with saturated sodium hydrogen carbonate (2ϫ150 mL),
water (2ϫ250 mL) and brine (2ϫ250 mL) then dried with anhy-
drous sodium sulfate, filtered and the solvents evaporated. The
crude material was purified by column chromatography with petro-
leum ether (boiling range 40–70 °C) as eluent. Yield 2.46 g (20%)
ppm. MS (70 eV, EI): m/z 218 (53) [M+], 123 (28), 95 (100), 75 (34).
C12H8F2N2 (218.20): calcd. C 66.05, H 3.70, N 12.84; found C
A combination of 19F NMR and ESI-MS data has there-
fore allowed us to detect both intermediates I and II from
the actual catalyst solutions.
Experimental Section
65.98, H 3.78, N 12.60. IR (KBr): ν = 430, 462, 540, 643, 668, 721,
˜
General: Unless otherwise specified, all manipulations were carried
out in air. Potassium phenyltrifluoroborate,[1] p-tolyldiazonium tet-
rafluoroborate[17] and p-fluorobenzenediazonium tetrafluorobo-
rate[17] were synthesized according to literature methods. All the
other reagents were commercial and used as received. All the sol-
vents were HPLC grade and were purified by standard techniques.
Flash chromatography was performed on SiO2 Kieselgel (230–400
mesh). The NMR spectra were recorded with a Bruker AX400
755, 840, 951, 1008, 1093, 1141, 1201, 1232, 1288, 1302, 1415, 1499,
1593, 1653, 1896, 3061 cm–1. 1H NMR (400 MHz, CDCl3): δ = 7.2
(m, 4 H, H3), 7.9 (m, 4 H, H2) ppm. 13C NMR (101 MHz, CDCl3):
2
3
δ = 116.1 (d, JC,F = 23 Hz, C3), 124.9 (d, JC,F = 9 Hz, C2), 149.0
(d, 4JC,F = 2 Hz, C1), 164.4 (d, 1JC,F = 252 Hz, C4) ppm. 19F NMR
(376 MHz, CD3OD): δ = –111.6 ppm (m).
Synthesis of Bis(µ-acetato)bis(4,4Ј-difluoroazobenzene-C2,N)dipalla-
dium(II) (4)
1
spectrometer (400 MHz for H) at 295.0 K; chemical shifts are re-
ported in ppm relative to SiMe4 for 1H and 13C, and CFCl3 for
1
19F; H and 13C signals were attributed by means of 13C{1H}APT,
1
1
1
13C{1H}DEPT, H COSY, H-13C HMQC and H-13C HMBC ex-
periments. Deuterated solvents were purchased from Aldrich or
Acros and used as received. Mass spectrometric analyses were per-
formed using an ion-trap mass spectrometer equipped with an elec-
trospray ion source (Bruker Esquire3000plus). All analyses were car-
ried out in both positive- and negative-ion modes. The sample solu-
tions were introduced by continuous infusion with the aid of a
syringe pump at a flow-rate of 180 µLmin–1. The instrument was
operated in the positive-ion mode (ESI+) with an end-plate offset
of –500 V and a capillary offset of –4000 V. The nebuliser pressure
was 0.8 bar (N2) and the drying gas (N2) flow 7 Lh–1. The capillary
exit and skimmer 1 voltages were 90 and 30 V, respectively. The
drying gas temperature was set at 200 °C. The mass spectrometric
parameters in the negative ion mode (ESI–) were as follows: end-
plate offset: –500 V; capillary: –4000 V; capillary exit: –120 V; skim-
mer 1: –50 V; nebulizer pressure: 0.4 bar; drying gas flow: 5 Lh–1;
drying gas temperature: 200 °C. All the samples for mechanistic
studies were prepared and handled in air using HPLC-grade meth-
anol. The software used for the simulations was Bruker Daltonics
DataAnalysis (version 3.3).
Palladium acetate (0.450 g, 0.0020 mol) and 4,4Ј-difluoroazoben-
zene (0.436 g, 0.0020 mol) were dissolved in glacial acetic acid
(50 mL) and the mixture was stirred at 50 °C. Once the 4,4Ј-difluo-
roazobenzene had been completely consumed (TLC monitoring)
the system was cooled down to room temperature and methanol
(200 mL) added to precipitate 4 as a black microcrystalline solid.
This solid was filtered off, washed with diethyl ether (10 mL) and
pentane (3ϫ10 mL) and air-dried. Yield 0.629 g (82%). ESI-MS
(CH2Cl2/CH3CN): exact mass calcd. for C28H20F4N4O4Pd2:
763.95; found 364.0 [M/2 – OAc + CH3CN]+. C28H20F4N4O4Pd2
(765.32): calcd. C 43.94, H 2.63, N 7.32, Pd 27.81; found C 43.69,
H 2.58, N 7.20, Pd 27.95. IR (KBr): ν = 436, 470, 534, 685, 779,
˜
810, 844, 877, 1101, 1153, 1184, 1234, 1246, 1320, 1348, 1416, 1501,
1
C, H, N elemental analyses were carried out with a Carlo Erba
EA1108 CHNS-O elemental analyzer. Pd analyses were performed
with a Perkin–Elmer SIMAA6000 atomic absorption spectrometer.
1560, 1597, 3077 cm–1. H NMR (400 MHz, CDCl3): δ = 2.1 (s, 6
H, H11), 6.1 (m, 2 H, H3), 6.8 (m, 2 H, H5), 7.0 (m, 4 H, H9), 7.4
(m, 4 H, H8), 7.7 (m, 2 H, H6) ppm. 13C NMR (101 MHz, CDCl3):
4650
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Palladium-Catalyzed Cross-Coupling
2
2
1
1
δ = 24.5 (s, C11), 113.2 (d, JC,F = 25 Hz, C5), 115.1 (d, JC,F
=
C), 158.9 (C), 163.8 (d, JC,F = 265 Hz, 1 C), 165.7 (d, JC,F
=
23 Hz, C9), 119.5 (d, JC,F = 21 Hz, C3), 125.4 (d, JC,F = 9 Hz,
252 Hz, 1 C) ppm. 19F NMR (376 MHz, CD3OD): δ = –109.9 (m,
2
3
C8), 130.5 (d, 3JC,F = 10 Hz, C6), 146.9 (d, 4JC,F = 3 Hz, C7), 158.2 F1), –100.4 (m, F2) ppm.
(d, JC,F = 7 Hz, C2), 159.9 (d, JC,F = 3 Hz, C1), 162.2 (d, JC,F
=
3
4
1
Synthesis of (4,4Ј-Difluoroazobenzene-C2,N)palladium(II) Tetrafluo-
264 Hz, C4), 164.1 (d, JC,F = 253 Hz, C10), 181.4 (s, COO) ppm.
19F NMR (376 MHz, CD3OD): δ = –110.5 (m, F1), –102.9 (m, F2)
ppm.
1
roborate (7)
Synthesis of 4,4Ј-Difluoro-2-phenylazobenzene (5)
Complex 4 (0.380 g, 0.50 mmol) and p-fluorobenzenediazonium
tetrafluoroborate (0.210 g, 1.0 mmol) were suspended in dry meth-
anol (10 mL) at room temperature under nitrogen. The suspension
was vigorously stirred and the gas (dinitrogen) evolved monitored
by means of a gas burette. Once the diazonium salt had completely
reacted (approx. 24 mL of gas) the methanol was evaporated. The
residue was treated with dry dichloromethane (3ϫ10 mL) and the
resulting suspension was filtered under N2 through a Celite® pad.
The filtrate was concentrated to about one third and dry n-hexane
(50 mL) added to precipitate 7 as an orange powder. The product
was decanted, washed with small portions of n-hexane and dried
under vacuum. Yield 0.330 g (80%). ESI-MS (CH3CN): exact
mass calcd. for C12H7BF6N2Pd: 322.96; found 323.0 [M]+.
C12H7BF6N2Pd: 409.96; found 323.0 [M – BF4]+. C12H7BF6N2Pd
(410.42): calcd. C 35.12, H 1.72, N 6.83, Pd 26.93; found C 35.02,
Complex 4 (0.380 g, 0.50 mmol), potassium phenyltrifluoroborate
(0.250 g, 1.4 mmol) and potassium carbonate (0.225 g, 1.6 mmol)
were suspended in methanol (10 mL). The system was refluxed for
1 h, then cooled and the methanol evaporated. The residue was
extracted with diethyl ether (4ϫ25 mL), then the organic fractions
were collected and washed with water (2ϫ50 mL) and brine
(2ϫ50 mL), dried with anhydrous sodium sulfate, filtered and the
solvents evaporated. The crude product was purified by column
chromatography with petroleum ether (boiling range 40–70 °C) as
eluent. Yield 0.073 g (25%). MS (70 eV, EI): m/z 293 (99) [M+ – 1],
184 (14), 170 (100), 151 (12), 123 (12), 95 (66), 75 (18). C18H12F2N2
(294.30): calcd. C 73.46, H 4.11, N 9.52; found C 73.69, H 4.28, N
9.30. IR (KBr): ν = 461, 542, 698, 737, 762, 781, 843, 880, 893,
˜
1109, 1136, 1186, 1271, 1444, 1477, 1570, 1594 cm–1. 1H NMR
(400 MHz, CDCl3): δ = 6.9–7.0 (m, H9, H5), 7.1 (m, H3), 7.2 (m,
Ph), 7.5–7.6 (m, H8, H6) ppm. 13C NMR (101 MHz, CDCl3): δ =
115.2 (d, 2JC,F = 23 Hz, C5), 116.0 (d, 2JC,F = 22 Hz, C9), 117.2 (d,
H 1.69, N 6.49, Pd 26.81. IR (nujol): ν = 470, 593, 779, 845, 1064,
˜
1154, 1193, 1257, 1499, 1568, 1583, 2050 cm–1. 1H NMR
(400 MHz, [D6]dmso): δ = 7.1 (m, 1 H, H3), 7.3 (m, 1 H, H5), 7.5
(m, 2 H, H9), 7.9 (m, 2 H, H8), 8.1 (m, 1 H, H6) ppm. 13C NMR
2
(101 MHz, [D6]dmso): δ = 114.3 (d, JC,F = 24 Hz, C5), 116.1 (d,
3
3
2JC,F = 22 Hz, C3), 118.0 (d, JC,F = 9 Hz, C6), 125.1 (d, JC,F
=
2
3
2JC,F = 23 Hz, C9), 119.2 (d, JC,F = 23 Hz, C3), 126.0 (d, JC,F
=
9 Hz, C8), 127.7 (s, CH), 127.8 (s, CH), 130.7 (s, CH), 137.7 (d,
9 Hz, C8), 132.3 (d, 3JC,F = 10 Hz, C6), 147.1 (d, 4JC,F = 3 Hz, C7),
3
4
4JC,F = 2 Hz; 1 C), 143.6 (d, JC,F = 9 Hz, C2), 145.8 (d, JC,F
=
156.3 (d, JC,F = 7 Hz, C2), 160.5 (d, JC,F = 3 Hz, C1), 161.8 (d,
1JC,F = 263 Hz, C4), 163.6 (d, 1JC,F = 250 Hz, C10) ppm. 19F NMR
(376 MHz, CD3OD): δ = –154.6 (br. s, BF4), –109.6 (br. s, F1),
–101.6 (br. s, F2) ppm.
3
4
3 Hz, C1), 149.2 (d, JC,F = 3 Hz, C7), 164.0 (d, JC,F = 252 Hz,
4
1
C4), 164.2 (d, JC,F = 252 Hz, C10) ppm. 19F NMR (376 MHz,
1
CD3OD): δ = –112.0 (m, F2), –111.7 (m, F1) ppm.
Synthesis of (4,4Ј-Difluoro-6-phenylazobenzene-C2,N)palladium(II)
Catalysis with Precatalyst 4: KPhBF3 (221 mg, 1.2 mmol) and p-
CH3C6H4N2BF4 (206 mg, 1.0 mmol) were added, with vigorous
stirring, to a suspension of 4 (7.65 mg, 0.010 mmol) in methanol
(4 mL) in a 10-mL Schlenk tube. At the end of the reaction
(80 min) the mixture was diluted with water (100 mL) and the prod-
ucts were extracted with diethyl ether (3ϫ25 mL); the organic
phases were collected and dried with Na2SO4. The products were
purified by flash chromatography and characterized by GC-MS.
The yield was determined by GLC using naphthalene as an internal
standard.
Tetrafluoroborate (6)
PdCl2 (36 mg, 0.2 mmol) and 5 (60 mg, 0.2 mmol) were placed in
a centrifuge tube and dmso (1 mL) was added. The system was
heated to 140 °C for 10 min, then cooled to room temperature.
Methanol (3 mL) was added to the resulting deep red solution to
precipitate an orange solid, which was centrifuged, washed with
methanol (3ϫ3 mL) and diethyl ether (3ϫ3 mL) and vacuum
dried. A 25-mg portion of this solid was placed in an NMR tube
and a solution of AgBF4 (12 mg, 0.06 mmol) in CD3OD (0.5 mL)
was added. The NMR tube was sonicated briefly and the resulting
Reaction of 4 with Potassium Phenyltrifluoroborate: HPLC-grade
methanol (0.5 mL) was added to a vial containing complex 4 (7 mg,
9 µmol) and potassium phenyltrifluoroborate (25 mg, 137 µmol)
and the mixture shaken for half a minute. The resulting suspension
was transferred into an NMR tube containing an internal insert
of CDCl3 for the deuterium lock. A 0.05-mL aliquot of the same
suspension was diluted to 5 mL with HPLC-grade methanol and
used in ESI-MS experiments.
1
suspension allowed to settle before analysis. H NMR (400 MHz,
CD3OD): δ = 6.8 (m, 1 H), 7.2 (m, 1 H), 7.3 (m, 2 H, H9), 7.5 (m,
Reaction of Complex 4 with Potassium Phenyltrifluoroborate and
Diazonium Salts: HPLC-grade methanol (0.5 mL) was added to a
vial containing complex 4 (7 mg, 9 µmol) and potassium phenyltri-
fluoroborate (25 mg, 137 µmol) and the mixture shaken for half a
3 H, Ph), 7.6 (m, 2 H, Ph), 7.7 (m, 2 H, H8) ppm. 13C NMR
2
(101 MHz, CD3OD): δ = 116.0 (d, JC,F = 24 Hz, CH), 117.1 (d,
2JC,F = 24 Hz, C9), 119.3 (d, JC,F = 23 Hz, CH), 126.5 (d, JC,F
=
2
3
9 Hz, C8), 129.3 (s, CH), 130.0 (s, CH), 131.2 (s, CH), 138.8 (C), minute. A solution of diazonium salt (165 µmol) in HPLC-grade
3
3
148.0 (d, JC,F = 10 Hz, 1 C), 148.9 (C), 157.9 (d, JC,F = 7 Hz, 1
methanol (0.5 mL) was added to the resulting suspension and the
Eur. J. Inorg. Chem. 2007, 4645–4652
© 2007 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjic.org
4651

P. Mastrorilli et al.
FULL PAPER
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to 5 mL with HPLC-grade methanol and used in ESI-MS experi-
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[8] A paper on the mechanism of Pd0-catalysed cross-coupling re-
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Supporting Information (see also the footnote on the first page of
this article): 19F, 1H and 13C NMR spectra of 4–7. HR mass spectra
of a methanol solution of complex 4, p-FC6H4N2BF4 and KPhBF3;
experimental and simulated HRMS(+) spectrum of II-F.
1
[9] The H NMR spectra of the reaction solution were poorly in-
formative due to the overlap of phenyl signals coming from the
ligand and the borate species.
[10] These signals were observed after 1024 scans when a 19F NMR
spectrum of a methanol suspension of 4 was recorded at 50 °C.
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Acknowledgments
Financial support from the European Cooperation in the field of
Scientific and Technical Research (COST, D30 action, fellowship
to N. T.) is gratefully acknowledged. We thank Ms. E. Andriani for
experimental assistance.
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Received: May 21, 2007
Published Online: August 9, 2007
4652
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Eur. J. Inorg. Chem. 2007, 4645–4652