Synthesis and molecular structure of [Pd{CH2CH2C(O)Me}(HL)][BAr′4] (HL = 6-Me-2,2′-bipyridine, Ar′ = 3,5-(CF3)2C6H3), an intermediate relevant to ethene/CO copolymerization

Article abstract of DOI:10.1021/om010278f

In the ethene / CO copolymerization catalyzed by palladium derivatives, species arising from ethene insertion into the palladium-acyl bond have been shown to play a key role. We report here the synthesis, spectroscopic characterization, and X-ray structure of the complex [Pd{CH₂CH₂C(O)Me}(HL)][BAr′₄] (HL = 6-Me-2,2′-bipyridine, Ar′ = 3,5-(CF₃)₂C₆H₃), 4 the first one having an ancillary N,N ligand to be characterized in the solid state.

Full text of DOI:10.1021/om010278f

Organometallics 2001, 20, 4111-4113
4111
Syn th esis a n d Molecu la r Str u ctu r e of
[P d {CH2CH2C(O)Me}(HL)][BAr ′4] (HL )
6-Me-2,2′-bip yr id in e, Ar ′ ) 3,5-(CF 3)2C6H3), a n
In ter m ed ia te Releva n t to Eth en e/CO Cop olym er iza tion
Sergio Stoccoro,† Giovanni Minghetti,†,* Maria Agostina Cinellu,†
Antonio Zucca,† and Mario Manassero‡
Dipartimento di Chimica, Universita` di Sassari, Via Vienna 2, 07100 Sassari, Italy, and
Dipartimento di Chimica Strutturale e Stereochimica Inorganica, Universita` di Milano,
Centro CNR, Via Venezian 21, 20133 Milano, Italy
Received April 3, 2001
Summary: In the ethene/ CO copolymerization catalyzed
by palladium derivatives, species arising from ethene
insertion into the palladium-acyl bond have been shown
to play a key role. We report here the synthesis, spectro-
scopic characterization, and X-ray structure of the
the solid state even at room temperature.3 The crystal
structure of the complex [Pd{CH2CH2C(O)Me}(NC6H4-
COOMe-2)(PPh3)][BF4] has been solved by X-ray dif-
fraction.4 The structure shows that, in addition to the
coordination of the five-membered C,O ring, of the
phosphorus atom, and of the nitrogen of the pyridine
ring, the stability is enhanced by a long-range interac-
tion involving the oxygen atom of the hybrid N,O ligand,
positioned in axial position. Very recently the structures
of two mononuclear species with a P,O5 and a P,N6
ancillary ligand, as well as that of a heterobimetallic
complex [Pd{CH2CH2C(O)Me}(HL)][BAr′4] (HL ) 6-Me-
2,2′-bipyridine, Ar′ ) 3,5-(CF3)2C6H3), 4, the first one
having an ancillary N,N ligand to be characterized in
the solid state.
Olefin insertion into a Pd-acyl bond is assumed to
be a crucial step in the palladium-catalyzed copolym-
erization of carbon monoxide and alkenes.1 The inser-
tion product is generally a complex of the type [(L)2Pd-
{CHRCHR′C(O)R′′}]+: the intramolecular coordination
of the carbonyl oxygen to palladium likely gives a
significant contribution to the stability of these inter-
mediates. Evidence for species of this type has been
achieved mostly by spectroscopic techniques. Some of
them have been isolated in the solid state: they usually
contain strong chelating ligands (N,N; P,P; N,P) and
concern insertion of strained or bulky alkenes.2 At
variance, in the case of ethene insertion, only a few
species have been isolated and shown to be stable in
derivative, [(OC)4Fe(µ-Ph2PNHPPh2)Pd{CH2CH2C(O)-
Me}][BF4],7 have been solved. This prompted us to
report that in the course of a study on the catalytic
activity in CO/ethene copolymerization of palladium-
(II) derivatives with 6-substituted-2,2′-bipyridines we
have been able to isolate complex 4 according to the
reaction Scheme 1 and to grow crystals suitable for
X-ray diffraction.
All three steps depicted in Scheme 1 occur under mild
conditions, i.e., at room temperature and atmospheric
pressure. The second step is likely to be dictated by the
insolubility of NaCl in dichloromethane and by the
absence of potential donors, either as solvents or anions.
Complexes 1 and 2 have been isolated and fully char-
acterized, whereas 3 was detected only in solution.8
Complex 3 and the last step in Scheme 1 are the same
described by Brookhart in the case of the analogous
† Universita` di Sassari.
‡ Universita` di Milano.
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S.-M.; Chen, J .-T.; Liu, S.-T. Organometallics 1999, 18, 2574. (f) Scott
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(8) Selected IR and NMR data for 3: IR (CH2Cl2,νmax/cm-1) 2118;
1H NMR (CD2Cl2) δ 1.57 (s, 3H, Pd-Me), 2.85 (s, 3H, bipy-6-Me), 7.58-
8.20 (6H, aromatics), 8.62 (d, 1H, H6′). We did not attempt to isolate
3 at low temperature.
10.1021/om010278f CCC: $20.00 © 2001 American Chemical Society
Publication on Web 08/17/2001

4112 Organometallics, Vol. 20, No. 19, 2001
Notes
Sch em e 1
derivative with 1,10-phenantroline (phen).3a,b Of the two
possible geometric isomers, only one is formed for
complexes 1-4. For 1 and 2 evidence for the isomer
having the chloride in proximity of the methyl substitu-
F igu r e 1. ORTEP view of the cation in compound 4.
Ellipsoids are drawn at the 30% probability level. Principal
bond (Å) and angle (deg) parameters: Pd-O 2.069(2), Pd-
N1 2.022(2), Pd-N2 2.198(3), Pd-C12 2.046(4), C12-C13
1.525(5), C13-C14 1.459(5), C14-O 1.241(5), C14-C15
1.536(4), O-Pd-N1 177.3(1), O-Pd-N2 102.9(1), O-Pd-
C12 81.8(1), N1-Pd-N2 79.9(1), N1-Pd-C12 95.6(1), N2-
Pd-C12 174.9(1).
1
ent is given by the H NMR spectrum. The assessment
relies on the chemical shift of the H6′ proton which is
known to be remarkably influenced by the neighboring
ligands. The values observed for 1 and 2 indicate that
the methyl group is close to H6′. It is worth noting that
the formation of the same isomer has been recently
described for a series of analogous methyl chloride
derivatives of platinum(II) with 6-alkyl-substituted-2,2′-
bipyridines.9 Several factors have been considered to
interpret the stereoselectivity observed, the most likely
being the H bond formed between H-C(R) of the
substituent and the chloride ion bound to the Pt center.
Evidence for the intermediate 3 rests on the IR
spectrum, which shows a strong absorption in the region
of a terminal carbonyl (ν(CO) ) 2118 cm-1) as well as
upon an NMR NOE difference experiment.10
view of the cation is shown in Figure 1. Selected bond
lengths and angles are listed in the Figure 1 caption.
The metal atom is in a square planar coordination with
a very slight tetrahedral distortion. The Pd-O and
Pd-C12 lengths can be compared with corresponding
interactions in the recently reported cation [Pd{CH2-
CH2C(O)Me}(P,O)]+ (P,O ) Ph2PNHC(O)Me), com-
pound 5.5 The Pd-O distance found here, 2.069(2) Å, is
shorter than that found in 5, 2.141(2) Å; the latter is
however elongated by the influence of the trans phos-
phorus atom. The Pd-C distance, 2.046(4) Å, is longer
than in 5, 2.002(3) Å, thus suggesting that the trans-
influence of nitrogen is slightly larger than that of
oxygen. The Pd-N2 bond, 2.198(3) Å, is rather long,
even when considering the influence of the trans alkyl
group: compare for instance Pd-N ) 2.146(2) Å in [Pd-
(Ltb)Cl], HLtb ) N2C10H7CMe3, and Pd-N ) 2.138(2) Å
in [Pd(Lnp)Cl], HLnp ) N2C10H7CH2CMe3, compound
6.13 The Pd-N1 distance, 2.022(2) Å, is normal; see
for instance Pd-N ) 2.046(2) Å in compound 6, Pt-N
Complex 4 exhibits in the IR spectrum a strong
ν(CO) absorption at 1615 cm-1 (CH2Cl2), supporting
1
coordination of the carbonyl group.2a,c In the H NMR
spectrum the methylene protons appear as two triplets
at δ 2.44 (CH2-Pd) and δ 2.75 (CH2CO). A NOESY
spectrum11 helped in the assignments and established
that the CH2 group bound to palladium is close to the
bipy H6′ proton.12 In the 13C{1H} spectrum the reso-
nances at δ 25.04 and 49.54 can be ascribed to Pd-CH2
and Pd-CH2-CH2, respectively, the latter being shifted
downfield owing to the proximity of the carbonyl group.
The carbon resonance of the C(O) group is observed at
δ 238.95, significantly shifted with respect to organic
ketones.2f,g
) 2.013(2) Å in [Pt(HLEt)(Me)(MeCN)]+, HLEt
)
N2C10H7Et, and Pt-N ) 2.022(4) Å in [Pt(HLEt)-
(Me)Cl].9
As far as we know, in the case of species having N,N
chelating ligands, complex 4 is the first one arising from
ethene insertion into a palladium-acyl bond to be
characterized in the solid state.
In conclusion we have shown that with an unsym-
metric 6-substituted-2,2′-bipyridine, at least in the
presence of the noncoordinating Brookhart’s anion, bis-
chelated complexes such as 4 are selectively formed and
are stable even in the solid state. For comparison, it
must be underlined that, although acetyl migration to
ethene occurs also in the case of the analogous complex
with the unsubstituted 2,2′-bipyridine, the correspond-
The crystal structure of compound 4 confirms that the
preferred isomer has the σ Pd-C bond trans to the
nitrogen atom of the substituted pyridine. An ORTEP
(9) Doppiu, A.; Cinellu, M. A.; Minghetti, G.; Stoccoro, S.; Zucca,
A.; Manassero, M.; Sansoni, M. Eur. J . Inorg. Chem. 2000, 2555.
(10) Irradiation, in the 1H NMR spectrum of 3, of the resonance at
δ 1.57 (s, Pd-CH3) results in enhancement of the doublet at δ 8.62 (H6′
bipy-6-Me).
(11) Selected 1H-1H NOESYcorrelations: δ 2.44 (Pd-CH2) with δ
8.12 (bipy H6′) and δ 2.75 (CH2CO); δ 2.75 with 2.44 and 2.36 (MeCO);
δ 2.71 (bipy-6-Me) with δ 7.40 (bipy H5).
(12) In the 1H NMR spectra a second set of signals of low intensity
(ca. 10%) is likely to be due to a palladium enolate3d (1H NMR, CDCl3,
δ 1.21 Me, d, 6.4 Hz; δ 4.32, CH, q, 6.4 Hz; 2.32 s MeCO, 2.76 s bipy-
6-Me).
(13) Zucca, A.; Cinellu, M. A.; Pinna, M. V.; Stoccoro, S.; Minghetti
G.; Manassero, M.; Sansoni, M. Organometallics 2000, 19, 4295.

Notes
Organometallics, Vol. 20, No. 19, 2001 4113
7.71. Selected IR and NMR data: IR (Nujol, νmax/cm-1) 1687
m, 1594 m, 1562 m; (CH2Cl2,νmax/cm-1) 1697 s, 1597 m; 1H
NMR (CDCl3) δ 2.94 (br, 3H, bipy-6-Me), 2.69 (s, 3H, MeC-
(O)-Pd), 7.30-8.09 (6H, aromatics), 8.42 (br, 1H, H6′). MS-
FAB: m/z (%) 319 (30) [M - Cl]+, 291(35) [M - CO - Cl]+,
276 (100) [M - C(O)Me - Cl]+.
ing bis-chelated complex could not be isolated, being
unstable even at low temperature. 2g Thus it seems that
in the case of 2,2′-bipy the 6-substituent has an influ-
ence on the stabilization of species such as 4 which are
relevant to alkenes/CO copolymerization. Their role is
very complicated: they can affect the chain propagation
in a number of ways.
At present it is not easy to find a rationale for the
peculiar stability of complex 4. Even the X-ray data,
which give evidence, inter alia, for a rather long Pd-
N(2) distanceselongation likely essential to relieve the
steric effects of the substituentsdo not allow a clear-
cut conclusion owing to the lack of data for molecules
having comparable N,N and C-O rings bound to pal-
ladium.
Syn th esis of [P d {CH2CH2C(O)Me}(HL)][BAr ′4] (4). To
a solution of 2 (379 mg, 1.06 mmol) in dichloromethane (50
mL), under stirring, at room temperature, were added 946 mg
of Na[BAr′4] (1.06 mmol). The color of the solution fades, and
instantaneously a precipitate of NaCl is formed. The reaction
vessel was satured with ethene for 10 min. The solution was
filtered on Celite, then evaporated to dryness. The solid residue
was washed with pentane and filtered to obtain the analytical
sample as a pale orange solid. Yield: 81%; mp 103-5 °C. Anal.
Calcd for
C47H29BF24N2OPd: C, 46.61; H, 2.40; N, 2.31.
Found: C, 46.25; H, 2.34; N, 2.46. Selected IR and NMR
The influence of the nature of the 6-substituent on
the stability of species such as 1-4 is hardly predictable.
As we have recently reported, the behavior of these
ligands (R alkyl, benzyl, aryl), particularly in palladium
chemistry, is rather erratic.13
data: IR (Nujol, νmax/cm-1) 1622 m; 1606 w, 1277 s; 1125 br;
1
(CH2Cl2,νmax/cm-1), 1615 s; 1566 m; H NMR (CDCl3) δ 2.36
(s, 3H, COMe), 2.44 (t, 2H, Pd-CH2), 2.71 (s, 3H, bipy-6-Me),
2.75 (t, 2H, Pd-CH2-CH2), 7.34-7.95 (18H, aromatics), 8.12 (d,
1H, H6′); 13C{1H} NMR (CDCl3) δ 24.38 (MeCO); 25.04
(CH2-Pd); 27.77 (Me-bipy); 49.54 (CH2-CH2-Pd); 119.42, 123.24,
126.64, 128.38, 139.70, 140.10 (aromatic C-H bipy); 149.98
(C6′ bipy); 152.12, 157.60, 161.54 (aromatic C bipy); 238.95
Exp er im en ta l Section
3
(CO-Pd); 117.49 (Cpara [BAr′4]- J C-F ) 4.0 Hz); 124.98 (CF3
All solvents were purified by standard techniques. [Pd(Me)-
Cl(COD)]14 (COD ) Z,Z-1,5-cyclooctadiene), 6-methyl-2,2′-
bipyridine (HL),15 and Na[BAr′4]16 were synthesized via the
published routes. IR spectra were recorded using a Perkin-
Elmer spectrophotometer 983. NMR spectra were recorded
with a Varian VXR300 spectrometer, operating at 299.9 MHz
for 1H and at 75 MHz for 13C. Chemical shifts, in ppm, are
relative to TMS for 1H and 13C. The mass spectrum was
obtained with a VG 7070EQ instrument operating under FAB
conditions with 3-nitrobenzyl alcohol as a supporting matrix.
Elemental analyses (C, H, N) were performed by Mr. Antonello
Canu in the Microanalytical Laboratory of our Department.
Syn th esis of [P d (Me)Cl(HL)] (1). To a solution of HL (386
mg, 2.27 mmol) in 20 mL of diethyl ether was added (under
stirring at 20 °C) [Pd(Me)Cl(COD)] (541 mg, 2.04 mmol)
dissolved in 10 mL of benzene. A pale yellow precipitate formed
immediately, and the suspension was stirred for 2 h. The
product was filtered off, washed with diethyl ether, and dried
in vacuo, yielding 641 mg of the analytical sample. Yield )
96%, mp 173-174 °C. Anal. Calcd for C12H13ClN2Pd: C, 44.05;
H, 3.98; N, 8.56. Found: C, 43.79; H, 4.02; N, 8.39. Selected
[BAr′4]- 1J C-F ) 272.6 Hz); 128.98 (Cmeta [BAr′4]- 2J C-F ) 31.4
Hz, 3J C-B ) 2.8 Hz); 134.76 (Cortho [BAr′4]- broad); 161.71 (Cipso
1
[BAr′4]- J C-B ) 50.0 Hz). An APT experiment allowed us to
distinguish between the methyl and methylene groups. MS-
FAB: m/z (%): 347 (100) [M]+, 291(ca. 20) [M - CH2
-
CH2CO]+, 276(96) [M - CH2 - CH2 - CO - Me]+.
X-r a y Str u ctu r e Deter m in a tion . Crystal data for 4:
47H29BF24N2OPd, M ) 1210.94, triclinic space group P1h (no.
C
2), a ) 12.810(1) Å, b ) 12.969(2) Å, c ) 16.842(2) Å, R )
108.84(1)°, â ) 101.58(1)°, γ ) 105.54(1)°, U ) 2421.4(6) Å3, Z
) 2, Dc ) 1.661 g cm-3, µ ) 5.0 cm-1, F(000) ) 1200, T ) 200
K; reflections measured 29 323, independent 11 295 with Rint
) 0.021. Empirical absorption correction, SADABS17 (Tmax
)
)
1.00, Tmin ) 0.92). Final R2 (F2, all reflections) ) 0.074, R2w
0.113, conventional R1 ) 0.044 for 778 parameters; Bruker
SMART CCD area-detector, Mo KR radiation (λ ) 0.71073 Å),
ω scan mode, θmin ) 3°, θmax ) 26°. The structure was resolved
by Patterson and Fourier methods and refined by full-matrix
least squares with anisotropic thermal parameters for all non-
hydrogen atoms with the exception of three fluorine atoms
having occupancy factors of 0.20, which were refined isotro-
pically. Many fluorine atoms are disordered, as often found
for the same anion.18 The six methylic hydrogen atoms were
located in the final Fourier maps and not refined; the remain-
ing hydrogen atoms were placed in calculated positions and
also not refined. The program used was Personal SDP19 on a
Pentium III PC.
1
IR and NMR data: IR (Nujol, νmax/cm-1) 1595 m, 1562 m; H
NMR (CDCl3) δ 1.28 (s, 3H, Me-Pd), 3.07 (s, 3H, bipy-6-Me),
7.32-8.07 (6H, aromatics), 8.62 (d, 1H, H6′). MS-FAB: m/z
(%): 311(15) [M - Me]+, 291(42) [M - Cl]+, 276 (100) [M -
Me - Cl]+.
Syn th esis of [P d (MeCO)Cl(HL)] (2). A yellow solution
of 1 (402 mg, 1.23 mmol) in 10 mL of dichloromethane was
placed in a 100 mL flask connected to a vacuum line. The flask
was evacuated and subsequently filled with carbon monoxide
to a pressure of 1 bar. This sequence was repeated twice, and
then the mixture was stirred 10 min, at 20 °C, until the
solution became yellow-green. The flask was opened, the
solution filtered through Celite, and the filter washed with
dichloromethane (2 × 5 mL). The combined filtrates were
evaporated to small volume, and diethyl ether was added. A
yellow precipitate was filtered off and washed with diethyl
ether (2 × 5 mL) to obtain 398 mg of the analytical sample.
Yield ) 91%; mp 134-136 °C. Anal. Calcd for C13H13ClN2-
OPd: C, 44.14; H, 3.68; N, 7.92. Found: C, 43.89; H, 3.59; N,
Ack n ow led gm en t. Financial support from the Uni-
versity of Sassari and Consiglio Nazionale delle Ricerche
(CNR) is gratefully acknowledged.
Su p p or tin g In for m a tion Ava ila ble: X-ray structure
information for compound 4. This material is available free of
charge via the Internet at http://pubs.acs.org.
OM010278F
(17) Sheldrick, G. M. SADABS, Empirical Absorption Correction
Program; University of Gottingen, 1997.
(18) (a) Gatti, G.; Lopez, J . A.; Mealli, C., Musco, A. J . Organomet.
Chem. 1994, 483, 77. (b) Cooper, A. C.; Clot, E.; Huffman, J . C.; Streib,
W. E.; Maseras, F.; Eisenstein, O.; Caulton, K. G. J . Am. Chem. Soc.
1999, 121, 97.
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Crystallographic Computing 5; Oxford University Press: Oxford, U.K.,
1991; Chapter 11, p 126.
(14) Rulke, R. E.; Ernsting, J . M.; Spek, A. L.; Elsevier, C. J .; Van
Leeuwen, P. W. N. M.; Vrieze, K. Inorg. Chem. 1993, 32, 5769.
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