Facile synthesis of new, stable, palladium-ethyl derivatives containing nitrogen-donor ligands

Article abstract of DOI:10.1021/om030517u

A study of the insertion reaction of ethylene into the Pd-C bond on complexes of general formula [PcK(CH₃)(N-N)₂][OTf] and [Pd(CH₃)(phen)(L)][OTf) led to the development of a facile procedure for the synthesis of new, stable, Pd-ethyl derivatives. The rate of this insertion reaction is affected by the nature of the nitrogen-donor ligand, N-N or L.

Full text of DOI:10.1021/om030517u

1974
Organometallics 2004, 23, 1974-1977
F a cile Syn th esis of New , Sta ble, P a lla d iu m -Eth yl
Der iva tives Con ta in in g Nitr ogen -Don or Liga n d s
Barbara Milani,* Angelica Marson,† Alessandro Scarel, and Giovanni Mestroni
Dipartimento di Scienze Chimiche, Universita` di Trieste, Via Licio Gorgieri 1,
34127 Trieste Italy
J an Meine Ernsting and Cornelis J . Elsevier
Institute of Molecular Chemistry, Organometallic and Coordination Chemistry,
University of Amsterdam, Nieuwe Achtergracht 166, 1018 WV Amsterdam, The Netherlands
Received J uly 2, 2003
Sch em e 1. Syn th esis of P d -Eth yl Com p lexes
Summary: A study of the insertion reaction of ethylene
into the Pd-C bond on complexes of general formula
[Pd(CH3)(N-N)2][OTf] and [Pd(CH3)(phen)(L)][OTf] led
to the development of a facile procedure for the synthesis
of new, stable, Pd-ethyl derivatives. The rate of this
insertion reaction is affected by the nature of the
nitrogen-donor ligand, N-N or L.
1b-7b
During the last 10 years there has been an increasing
interest in the development of catalytic systems, based
on late-transition metals, for polymerization of ethyl-
ene1,2 or copolymerization of ethylene, or R-olefins in
general, with carbon monoxide.3 The insertion of eth-
ylene into the metal-carbon bond represents a key step
for both polymerization reactions. This insertion has
been thoroughly investigated on palladium and nickel
complexes with nitrogen-donor chelating ligands, using
both theoretical and experimental approaches.4 In par-
ticular, for complexes such as [Pd(CH3)(OEt2)(phen)]-
[B(Ar′)4] (phen ) 1,10-phenanthroline; B(Ar′)4 ) B[3,5-
(CF3)2C6H3]4) the main product of the insertion reaction
was shown to be a mixture of cis- and trans-2-butene.
The Pd-ethyl-ethylene species [Pd(CH2CH3)(CH2CH2)-
(phen)]+, which has been detected only at low temper-
ature, is the resting state both for the ethylene dimer-
ization and for its polymerization catalyzed by Pd(II)-
R-dimine complexes.
For several years we have studied the catalytic
behavior of Pd(II) complexes in CO/olefin copolymeri-
zation reaction.5 More recently, we have also investi-
gated the insertion reaction of carbon monoxide into the
Pd-CH3 bond of complexes of general formula [Pd(CH3)-
(N-N)2][OTf] (N-N ) phen, 1a ; 4,7-dimethyl-1,10-
phenanthroline (dm-phen), 2a ; 3,4,7,8-tetramethyl-1,10-
phenanthroline (tm-phen), 3a ) and [Pd(CH3)(phen)-
(L)][OTf] (L ) pyridine (py), 4a ; 2-phenylpyridine (2-
Ph-py), 5a ; 2-picoline (2-pic), 6a ; 4-picoline (4-pic), 7a ;
OTf ) triflate).6 The insertion of CO into the pal-
ladium-alkyl bond led in all cases to the isolation of
the respective Pd-acyl species, [Pd(COCH3)(N-N)2][OTf]
and [Pd(COCH3)(phen)(L)][OTf]. The investigation of
this reaction by means of in situ NMR spectroscopy
revealed that the reaction is rather fast and the inser-
tion rate is not affected by the nature of N-N or L.
Formation of palladium black was not observed in any
of these cases.
Here we describe the reactivity of the complexes [Pd-
(CH3)(N-N)2][OTf] and [Pd(CH3)(phen)(L)][OTf] with
ethylene. This reaction allowed us to develop a facile
procedure for the synthesis of the corresponding Pd-
ethyl derivatives, [Pd(CH2CH3)(N-N)2][OTf] (N-N )
phen, 1b; dm-phen, 2b; tm-phen 3b) and [Pd(CH2CH3)-
(phen)(L)][OTf] (L ) py, 4b; 2-Ph-py, 5b; 2-pic, 6b; 4-pic,
7b), which, notably, are thermally stable at room
temperature and can be readily isolated.
* E-mail: milani@dsch.univ.trieste.it.
† Current address: Institute of Molecular Chemistry, Homogeneous
Catalysis, University of Amsterdam, Nieuwe Achtergracht 166, 1018
WV Amsterdam, The Netherlands.
(1) (a) Ittel, S. D.; J ohnson, L. K.; Brookhart, M. Chem. Rev. 2000,
100, 1169. (b) J ohnson, L. K.; Killian, C. M.; Brookhart, M. J . Am.
Chem. Soc. 1995, 117, 6414. (c) J ohnson, L. K.; Mecking, S.; Brookhart,
M. J . Am. Chem. Soc. 1996, 118, 267. (d) Killian, C. M.; Tempel, D. J .;
J ohnson, L. K.; Brookhart, M. J . Am. Chem. Soc. 1996, 118, 11664. (e)
Small, B. L.; Brookhart, M.; Bennett, A. M. A. J . Am. Chem. Soc. 1998,
120, 4049. (f) Small, B. L.; Brookhart, M. J . Am. Chem. Soc. 1998,
120, 7143. (g) Gottfried, A. C.; Brookhart, M. Macromolecules 2001,
34, 1140.
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(b) Britovsek, G. J . P.; Gibson, V. C.; Kimberly, B. S.; Maddox, P. J .;
McTavish, S. J .; Solan, G. A.; White, A. J . P.; Williams, D. J . Chem.
Commun. 1998, 849. (c) Britovsek, G. J . P.; Bruce, M.; Gibson, V. C.;
Kimberly, B. S.; Maddox, P. J .; Mastroianni, S.; McTavish, S. J .;
Redshaw, C.; Solan, G. A.; Stro¨mberg, S.; White, A. J . P.; Williams, D.
J . J . Am. Chem. Soc. 1999, 121, 8728.
(3) (a) Drent, E.; Budzelaar, P. H. M. Chem. Rev. 1996, 96, 3435.
(b) Nozaki, K.; Hiyama, T. J . Organomet. Chem. 1999, 576, 248. (c)
Bianchini, C.; Meli, A. Coord. Chem. Rev. 2002, 225, 35.
(4) (a) Rix, F. C.; Brookhart, M. J . Am. Chem. Soc. 1995, 117, 1137.
(b) Rix, F. C.; Brookhart, M.; White, P. S. J . Am. Chem. Soc. 1996,
118, 4746. (c) Shultz, L. H.; Brookhart, M. Organometallics 2001, 20,
3975. (d) Shultz, L. H.; Tempel, D. J .; Brookhart, M. J . Am. Chem.
Soc. 2001, 123, 11539. (e) Margl, P. M.; Deng, L.; Ziegler, T. J . Am.
Chem. Soc. 1999, 121, 154. (f) Deng, L.; Margl, P. M.; Ziegler, T. J .
Am. Chem. Soc. 1997, 119, 1094. (g) Deng, L.; Woo, T. K.; Cavallo, L.;
Margl, P. M.; Ziegler, T. J . Am. Chem. Soc. 1997, 119, 6177. (h)
Michalak, A.; Ziegler, T. Macromolecules 2003, 36, 928.
(5) (a) Milani, B.; Vincentini, L.; Sommazzi, A.; Garbassi, F.;
Chiarparin, E.; Zangrando, E.; Mestroni, G. J . Chem. Soc., Dalton
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10.1021/om030517u CCC: $27.50 © 2004 American Chemical Society
Publication on Web 03/27/2004

Communications
Ta ble 1. Releva n t 1H a n d 15N NMR Da ta for
Organometallics, Vol. 23, No. 9, 2004 1975
Com p lexes [P d (CH2CH3)(N-N)2][OTf], 1b-3ba
N-N
H2,9
CH2
CH3
15N NMR
phen 1b
dm-phen 2b
tm-phen 3b
8.86 (dd)
8.68 (dd)
8.58 (dd)
1.92 (q)
1.86 (q)
1.79 (q)
0.63 (t)
0.60 (t)
0.56 (t)
-120.3b
n.d.
n.d.
a
Spectra recorded in CD2Cl2 at 295 K; dd ) double doublet, t
) triplet, q ) quartet. H2,9 for free phen: 9.10 ppm. Average (see
b
text) recorded in CD2Cl2 at 295 K. [Pd(CH3)(phen)2][OTf], 1a ,
δ(15N) -123.6.6a
When the Pd-methyl complexes 1a -7a were reacted
with ethylene, at room temperature, in methylene
chloride, the Pd-ethyl derivatives [Pd(CH2CH3)(N-N)2]-
[OTf] 1b-3b and [Pd(CH2CH3)(phen)(L)][OTf] 4b-7b
1
were quantitatively isolated (Scheme 1).7 Their H NMR
spectra in CD2Cl2 exhibit the characteristic quartet and
triplet pattern in the region of the aliphatic protons,
assigned to the Pd-ethyl fragment (Tables 1 and 2). The
number of signals observed in the aromatic region of
the spectra is different depending on whether the
complexes of the type [Pd(CH2CH3)(N-N)2][OTf] 1b-
3b or [Pd(CH2CH3)(phen)(L)][OTf] 4b-7b are consid-
ered. Indeed, for complexes 1b-3b the number of
resonances and their integration confirm the presence
of two equivalent molecules of N-N ligand in the
coordination sphere of palladium (Table 1). Therefore,
we assume the occurrence of a dynamic process by which
the two N-N ligands exchange, which is in agreement
with the behavior of the Pd-methyl precursors 1a -3a
1
in solution.6a This process is fast on the H NMR time
scale. On the other hand, the resonances observed in
the spectra of complexes 4b-7b clearly indicate the
coordination of phenanthroline in a nonsymmetrical
chemical environment, and no dynamic process is ap-
parent in this case (Table 2). We note that the signal of
one of the two “probe-protons” of phen (H9) has remark-
ably shifted upfield (by more than 1 ppm) with respect
to the free ligand, which is in agreement with the fact
that H9 resides within the shielding cone of the aromatic
nitrogen ligand (L) in cis position.
For three exponents of the Pd-ethyl complexes, 1b,
4b, and 5b, the 15N NMR spectra were recorded by PFG
HMQC H{15N} experiments at natural abundance of
1
F igu r e 1. 1H,15N PFG HMQC spectrum of complex 4b,
the 15N isotope.6a,8 Correlations due to 4J (15N,1H) in
15N-Pd-C-CH3 (Figure 1 for 4b) were observed for all
15N nuclei of each compound, further substantiating the
coordination of N atoms. For complex 1b, one average
recorded at natural abundance of 15N in CD2Cl2 at 295 K.
signal at -120 ppm was observed instead of four,
confirming the fast exchange of all N atoms, even on
the 15N NMR time scale (Table 1). The observed chemi-
cal shift is very close to the estimated value obtained
as the average of the expected 15N shifts of the four N
atoms belonging to one bidentate and one monodentate
coordinated phenanthroline (-152, -117, -152, and
-75 ppm). For 4b and 5b , three distinct 15N signals
were observed (Table 2); the nitrogen atoms in mutual
trans positions have 15N chemical shifts around -137
(monodentate ligand) and -152 ppm (phen), whereas
the resonance of N (phen) in trans position relative to
the ethyl group is found at approximately -117 ppm.
These 15N resonances fall within the expected chemical
shift ranges and are 3-5 ppm higher in frequency
compared to those of the corresponding methyl com-
plexes 4a and 5a .6a The values of the coordination
chemical shifts are comparable to previously obtained
values for, for example, [Pd(CH3)(N-N-N)]+ (N-N-N
(6) (a) Milani, B.; Marson, A.; Zangrando, E.; Mestroni, G.; Ernsting,
J . M.; Elsevier, C. J . Inorg. Chim. Acta 2002, 327, 188. (b) Milani, B.;
Scarel, A.; Zangrando, E.; Mestroni, G.; Carfagna, C.; Binotti, B. Inorg.
Chim. Acta 2003, 350, 592.
(7) The complexes 1a -7a were synthesized starting from Pd(CH3-
COO)2 and accordingly to the procedure previously reported.6 All
manipulations were carried out in argon atmosphere by using standard
Schlenk technique. Dichloromethane was previously distilled over
CaCl2. Synthesis of complexes [Pd(CH2CH3)(N-N)2][OTf] 1b-3b and
[Pd(CH2CH3)(phen)(L)][OTf] 4b-7b: A solution of [Pd(CH3)(N-N)2]-
[OTf] or [Pd(CH3)(phen)(L)][OTf] in dichloromethane (0.05 mmol of
complex in 10 mL of CH2Cl2) is kept under ethylene atmosphere for
the proper reaction time (see below). The volume of the solution is
reduced to half and some diethyl ether is added to complete the
precipitation of the product as an orange (1b-3b) or yellow (4b-7b)
solid. The solid is filtered, washed with diethyl ether, and vacuum-
dried. It is stored at 4 °C. Reaction time and yields: 1b 1 h, 90%; 2b
2 h, 90%; 3b overnight, 85%; 4b 3 h, 74%; 5b 3 h, 65%; 6b 4 h, 72%;
7b 3 h, 72%. The alternative synthetic procedure based on the reaction
of [Pd(Cl)2(COD)] with [Sn(C2H5)4] led to the isolation of [Pd(C2H5)-
(Cl)(COD)] in very low yield (<20%) due to formation of palladium
metal.

1976 Organometallics, Vol. 23, No. 9, 2004
Communications
Ta ble 2. Releva n t 1H a n d 15N NMR Da ta for Com p lexes [P d (CH2CH3)(p h en )(L)][OTf], 4b-7ba
phen
15N NMRb
L
H2
H9
CH2
CH3
N(phen)-trans-C
N(phen)-trans-N
N(L)
py 4b
2-Ph-py 5b
2-pic 6b
9.05 (dd)
8.90 (dd)
9.03 (dd)
9.05 (dd)
7.97 (dd)
2.04 (q)
1.85 (m)
2.01 (m)
2.01 (q)
0.95 (t)
0.70 (t)
0.88 (t)
0.94 (t)
-117.3
-117.6
n.d.
-151.6
-151.5
n.d.
-137.0
-136.6
n.d.
7.98-8.03 (m)c
7.82-7.78 (m)d
7.98 (dd)
4-pic 7b
n.d.
n.d.
n.d.
a
Spectra recorded in CD2Cl2 at 295 K; dd ) double doublet, t ) triplet, q ) quartet, m ) multiplet. H2,9 for free phen: 9.10 ppm.
Spectra recorded in CD2Cl2 at 295 K. c Overlapped with H3. Overlapped with H8.
b
d
) 2-(2-((2′-pyridylmethylene)amino)ethyl)pyridine).10 No
agostic Pd-C-CH interactions are present, as was
1
apparent from variable-temperature H and 13C NMR
and T1(1H) studies concerning 1b and 4b. The 13C
resonances of the ethyl groups are found at normal
positions, e.g., δ/ppm 1b: 17.94 CH2, 17.24 CH3, 1J (C,H)
1
124 Hz; 4b: 20.60 CH2, 16.04 CH3; J (C,H) 125 Hz.
The reaction with ethylene was studied in more detail
1
by in situ H NMR experiments, by bubbling C2H4 into
a 10 mM solution of the complex in CD2Cl2 at room
temperature.11 In the spectrum of a solution of 1a
recorded after 15 min, the signals of the Pd-ethyl species
were evident, together with other peaks attributed to
propene (multiplets centered at 1.69, 4.95, and 5.75-
5.88 ppm), to a mixture of cis- and trans-2-butene
(multiplets at 1.58 and 5.45 ppm, close to the peak of
free ethylene), and the unreacted Pd-methyl precursor.
The reaction was followed in time without charging the
NMR tube with more ethylene. The variations observed
in the spectra, recorded after 40 and 60 min from the
treatment with C2H4, consisted of the decrease and the
subsequent disappearance of the signal due to the Pd-
methyl moiety and the enhancement of the signals of
all other compounds present in solution. In particular,
in the spectrum recorded after 60 min, the signals of
the unique palladium species 1b are shown. No signal
due to free phenanthroline was present.
F igu r e 2. Plot of ln[Pd-CH3] versus time for complexes
1a -4a . Conditions of pseudo-zero-order in ethylene are
assumed.
methyl species was transformed into 1b in 60 min, only
28% of [Pd(CH3)(tm-phen)2][OTf] was transformed into
the Pd-ethyl derivatives 3b after the same reaction time.
This effect could be related to the Lewis basicity of N-N,
which increases in the same order.12
Even the insertion rate of ethylene into the Pd-CH3
bond of complexes 4a -7a was affected by the nature of
the L ligand. When L was pyridine (complex 4a ), the
reaction rate was higher than that of 1a (Figure 2), and
for L ) 2-Ph-py (complex 5a ) the palladium-methyl
complex was completely transformed in the Pd-ethyl
derivative in 20 min. For complexes with picolines, 6a
and 7a , the reaction was slow, and in particular, for 6a
only 57% of the starting complex was transformed into
the Pd-ethyl derivative, 6b, in 60 min. Therefore, the
order of reactivity is 2-Ph-py > py > 4-pic > 2-pic, while
the order for the Lewis basicity is 2-Ph-py < py < 2-pic
≈ 4-pic. In agreement with the results obtained for
complexes 1a -3a , the insertion rate decreases on
increasing the electron-donor power of the ligand and
on increasing its steric hindrance, thus indicating that
the insertion reaction follows an associative mechanism.
For all complexes the conversion of ethylene into the
mixture of cis- and trans-2-butene proceeded, even after
the complete transformation of the Pd-methyl into the
Pd-ethyl species, thus indicating that, as expected, these
complexes are catalysts for ethylene dimerization. The
active species for this reaction is the Pd-hydride deriva-
tive, which should form after insertion of the first
molecule of ethylene into the Pd-CH3 bond, followed
by the â-hydrogen elimination of propene (detected in
the 1H NMR spectra; Scheme 2). The proposed catalytic
cycle is similar to that reported by Brookhart,4b the
main difference being the resting state, which is in this
case represented by the Pd-ethyl species, [Pd(CH2CH3)-
(N-N)2][OTf] or [Pd(CH2CH3)(N-N)(L)][OTf] (Scheme
Analogous reactivity was observed when complexes
2a and 3a were reacted with ethylene, the only differ-
ence being the reaction rate, which remarkably de-
creased on going from phen to dm-phen to tm-phen
(Figure 2). For instance, whereas for 1a 88% of Pd-
(8) 15N NMR spectra were recorded by a PFG HMQC sequence9 on
a Bruker DRX 300 spectrometer equipped with a 5 mm triple resonance
inverse probe with z-gradient, operating at 30.42 MHz 15N frequency
and a second 300 W X decoupler giving a 15N 90° pulse of 10 µs. Spectra
were recorded without decoupling 15N in f2, using a spectral width in
f2 (1H) of 10 ppm, an acquisition time of 0.4 s, giving a digital resolution
of 1.2 Hz per point. Two values for J {1H,15N} (2.5 and 6 Hz) were used.
The 15N spectral width was 60 ppm with 256 increments and 32-512
scans per increment depending on the concentration of the compound.
This provided, after linear prediction to 1024, a digital resolution of
1.8 Hz per point. The relaxation delay was 1 s. For experiments using
256 increments and 16 scans, data collection required 2 h. Chemical
shifts were referenced to external nitromethane ) 0 ppm, negative
chemical shift reported for lower frequencies. Spectra were recorded
at 295 K.
(9) (a) Hurs, R. E.; J ohn, B. K. J . Magn. Reson. 1991, 91 648. (b)
Ruiz-Cabello, J .; Vuister, G. W.; Moonen, C. T. W.; van Gelderen, P.;
Cohen, J . S.; van Zijl, P. C. M. J . Magn. Reson. 1992, 100, 282. (c)
Wilker, W.; Leibfritz, D.; Kerssebaum, R.; Bermel, W. Magn. Reson.
Chem. 1993, 31, 287.
(10) Ru¨lke, R. E.; Ernsting, J . M.; Spek, A. L.; Elsevier, C. J .; van
Leeuwen, P. W. N. M.; Vrieze, K. Inorg. Chem. 1993, 32, 5769.
(11) The in situ 1H NMR experiments were carried out as follows:
CD2Cl2 (0.7 mL) was added to a 5 mm NMR tube charged with the
complex (7 × 10-3 mmol). Ethylene was bubbled into the solution for
5 min, via a needle inserted through a rubber cap into the NMR tube.
The 1H NMR spectrum was recorded after 15 min. All manipulations
were done at room temperature. The values of chemical shifts reported
are referred to the CD2Cl2 peak versus TMS at 5.33 ppm.
(12) Berthelot, M.; Laurence, C.; Safar, M.; Besseau, F. J . Chem.
Soc., Perkin Trans. 2 1998, 283.

Communications
Organometallics, Vol. 23, No. 9, 2004 1977
Sch em e 2. P r op osed Ca ta lytic Cycle for Eth ylen e Dim er iza tion a
a
Step i consists of a series of â-hydrogen elimination and insertion reactions.
2). The reactions were monitored for at least 6 h, and
neither decomposition to palladium metal nor any
dissociation of N-N or L ligands was observed for the
complexes studied.
of CO into the Pd-CH3 bond, the insertion of ethylene
is slow (under the applied reaction conditions) and it is
affected by the nature of nitrogen-donor ligands bound
to palladium.
The reactivity with ethylene was also investigated on
1
the Pd-ethyl complexes 1b and 4b, by in situ H NMR
Ack n ow led gm en t. This work was supported by the
European Network “PALLADIUM” (Fifth Framework
Program, contract No. HPRN-CT-2002-00196). The
authors wish to thank J ohnson Matthey for a generous
loan of Pd(CH3COO)2. The authors thank COST Action
D17 for a STSM of B.M. at the University of Amster-
dam.
experiments in CD2Cl2, at room temperature. In both
cases the catalytic formation of cis- and trans-2-butene
was observed. The reaction was monitored during 2 h,
and neither decomposition of the Pd-ethyl species nor
dissociation of N ligands occurred. In agreement with
the data obtained on the Pd-methyl species, the reaction
catalyzed by 4b is faster than that catalyzed by 1b.
Summarizing, the reaction of cationic palladium-
methyl compounds with ethylene constitutes a facile,
general procedure for the quantitative synthesis of
stable, monocationic Pd-ethyl compounds without for-
mation of palladium metal. In contrast to the insertion
Su ppor tin g In for m ation Available: The elemental analy-
ses; detailed 1H NMR data. This material is available free of
charge via the Internet at http://pubs.acs.org.
OM030517U