Cationic four- and five-coordinate nickel(II) complexes: Insights into the nickel(II)-catalyzed copolymerization of ethylene and carbon monoxide

Article abstract of DOI:10.1021/om0008023

The four and five-coordinate intermediates with respect to the copolymerization of ethylene and carbon monoxide were synthesized using the dppp-derived nickel (II) catalyst systems. The activation barriers corresponding to the chain growth were determined

Full text of DOI:10.1021/om0008023

16
Organometallics 2001, 20, 16-18
Ca tion ic F ou r - a n d F ive-Coor d in a te Nick el(II)
Com p lexes: In sigh ts in to th e Nick el(II)-Ca ta lyzed
Cop olym er iza tion of Eth ylen e a n d Ca r bon Mon oxid e
C. Scott Shultz, J oseph M. DeSimone, and Maurice Brookhart*
Department of Chemistry, University of North Carolina at Chapel Hill,
Chapel Hill, North Carolina 27599-3290
Received September 18, 2000
Sch em e 1
Summary: Using the dppp-derived Ni(II) catalyst system
2, several four- and five-coordinate intermediates rel-
evant to the alternating copolymerization of ethylene and
carbon monoxide have been characterized. In addition,
the activation barriers for the migratory insertion steps
corresponding to chain growth have been determined to
be at or below ca. 10 kcal/ mol, indicating that a strongly
stabilized catalyst resting state is most likely responsible
for the low activity of 2 for copolymerization.
Cationic palladium(II) complexes containing bidentate
phosphine and nitrogen ligands emerged nearly two
decades ago as excellent catalysts for the alternating
copolymerization of olefins and carbon monoxide, and
extensive mechanistic and theoretical studies have been
carried out on these systems.1-8 Nickel(II) complexes
for the copolymerization9-15 have found little success,
despite the expectation of lower insertion barriers for
both CO and olefins.4,16 SHOP-type catalysts have been
examined for the copolymerization of ethylene and
carbon monoxide,10,11 as have a Ni(II) catalyst contain-
ing an S,O-chelate12 and the well-defined catalyst Ni-
(o-tolyl)(PPh3)(TpPh).13 All of these systems exhibit only
modest activity. Initial attempts of Drent et al. using
simple aryl-substituted bidentate phosphorus ligands
similarly suffered from low productivity;14 however, the
use of the modified ligand 1,2-bis(bis(2-methoxyphenyl)-
phosphino)ethane effected a 10-fold increase in catalyst
activity.15
In this communication we describe the synthesis and
study of a series of nickel complexes containing the 1,3-
bis(diphenylphosphino)propane (dppp) ligand which
yield insight into the structures of intermediates and
the migratory insertion processes in the alternating
copolymerization of CO and ethylene using dppp-derived
nickel catalysts. This work illustrates significant dif-
ferences between the nickel system and the highly active
dpppPdII catalyst system as well as uncovering a
number of mechanistic features characteristic of the
nickel(II)-based catalyst systems.
(1) For an excellent review see: Drent, E.; Budzelaar, P. H. M.
Chem. Rev. 1996, 96, 663 and references therein.
(2) Margl, P.; Ziegler, T. J . Am. Chem. Soc. 1996, 118, 7337.
(3) Margl, P.; Ziegler, T. Organometallics 1996, 15, 5519.
(4) Svensson, M.; Matsubara, T.; Morokuma, K. Organometallics
1996, 15, 5568.
(5) Koga, M.; Morokuma, K. J . Am. Chem. Soc. 1986, 108, 6136.
(6) Rix, F. C.; Brookhart, M. J . Am. Chem. Soc. 1995, 117, 1137.
(7) Rix, F. C.; Brookhart, M.; White, P. S. J . Am. Chem. Soc. 1996,
118, 4746.
(8) For more recent references on olefin/CO copolymerization sys-
tems see: Milani, B.; Corso, G.; Mestroni, G.; Carfagna, C.; Formica,
M.; Seraglia, R. Organometallics 2000, 19, 3435. Nozaki, K.; Sato, N.;
Tonomura, Y.; Yasutomi, M.; Takaya, H.; Hiyama, T.; Matsubara, T.;
Koga, N. J . Am. Chem. Soc. 1997, 119, 12779 and references therein.
(9) For a very early report of catalysis using a Ni(II) source and a
Lewis acid see: Shryne, T. M.; Holler, H. V. U.S. Patent 3,984,388,
1976; Chem. Abstr. 1976, 85, 178219w.
(10) Klabunde, U.; Ittel, S. D. J . Mol. Catal. 1987, 41, 123.
(11) Desjardins, S. Y.; Cavell, K. J .; Hoare, J . L.; Skelton, B. W.;
Sobolev, A. N.; White, A. H.; Keim, W. J . Organomet. Chem. 1997,
544, 163.
Treatment of the dimethyl complex 1 with the oxo-
nium acid H(OEt2)2BAr′4 (Ar′ ) 3,5-(CF3)2C6H3) in CD2-
Cl2 or CDCl2F liberates 1 equiv of methane and gener-
ates the cationic methyl complex 2 in situ (Scheme 1).
Complex 2 was too unstable to be isolated; however, it
1
was completely characterized by H, 31P{1H}, and 13C-
{1H} NMR spectroscopy at low temperatures in CD2-
Cl2.17 The nickel-bound methyl group in 2[OEt2] was
1
observed in the H NMR spectrum at δ -0.08, and the
coordinated ether resonances appear slightly upfield of
free diethyl ether. Two 31P{1H} NMR signals were
observed at δ 27.2 (d) and 0.95 (d) with J PP ) 30 Hz.
Upon treatment of 2 with CO at -140 °C (CDCl2F)
an acetyl complex is rapidly generated (Scheme 2); no
intermediate methyl carbonyl complex is observed,
indicating that the barrier to CO insertion in such a
species must be less than ca. 10 kcal/mol. NMR and IR
(12) Driessen, B.; Green, J . J .; Keim, W. U.S. Patent 5,214,126, 1993;
Chem. Abstr. 1992, 116, 152623g.
(13) Domho¨ver, B.; Kla¨ui, W.; Kremer-Aach, A.; Bell, R.; Mootz, D.
Angew. Chem., Int. Ed. 1998, 37, 3050.
(14) Drent, E. U.S. Patent 4,835,250, 1989.
(15) Drent, E.; Catharina, M.; De Kock, T. U.S. Patent 5,688,909,
1997.
(16) Bernardi, F.; Bottoni, A.; Nicastro, M.; Rossi, I.; Novoa, J .; Prat,
X. Organometallics 2000, 19, 2170.
(17) See the Supporting Information for complete spectroscopic
details.
10.1021/om0008023 CCC: $20.00 © 2001 American Chemical Society
Publication on Web 12/09/2000

Communications
Organometallics, Vol. 20, No. 1, 2001 17
Sch em e 2
analysis reveals that the acetyl complex formed is a five-
coordinate dicarbonyl complex, 5, in marked contrast
to the reaction of the analogous palladium complex, in
which a four-coordinate acyl monocarbonyl complex is
observed.18 On the basis of spectral data the trigonal-
bipyramidal structure 5, shown in Scheme 2, is pro-
posed.17 The 13C{1H} NMR spectrum of 5 generated
from 13CO shows equivalent CO resonances at δ 184.5
(dd) coupled to two inequivalent 31P nuclei (J P-CO ) 24,
22 Hz), while the acyl 13CO appears at δ 221.3 as a
broad doublet (J P-CO ) 45 Hz). Unlabeled 5 exhibits two
31P{1H} NMR signals at δ 11.65 and -0.19 (J PP ) 85
Hz) and a 1H CH3 resonance at δ 2.20. The low-
temperature (-80 °C) IR spectrum of 5 exhibits two νCO
bands at 2094 and 2059 cm-1, and from the relative
intensities the estimated angle between the CO ligands
is 130°, consistent with the proposed structure.17 Com-
plex 5 is in equilibrium with its four-coordinate precur-
sor 4, but under 1 atm of CO in CD2Cl2 at -80 °C
complex 5 is heavily favored. Purging this solution with
argon at -20 °C liberates 1 equiv of carbon monoxide
to generate the four-coordinate acyl monocarbonyl
complex 4 (Scheme 2). The bound 13CO ligand in 4 is
Sch em e 3
acyl chelate complex 6 (Scheme 3). If the stoichiometry
is kept close to 1:1, quantitative formation of 6 is
observed. Occasionally, a small amount of a second-
generation five-membered chelate, 7(2n d ), could be
observed. The νCO absorption for 6 was observed at 2045
cm-1. Under excess ethylene a mixture of five-membered
chelates 7(2n d ), 7(3r d ), etc. is formed. The coordinated
δ-keto moiety of 6 is observed as a singlet at δ 223.1 in
the 13C{1H} NMR spectrum, approximately 15 ppm
downfield from the expected resonance of a free keto
group. The acyl carbon and the carbonyl carbon are
observed at δ 241.2 (d) and 180.1 (dd), respectively, and
the CH3 group is observed at δ 2.35 in the 1H NMR
spectrum.17
Treating complex 6 with CO at -80 °C resulted in
immediate formation of the second-generation acyl
dicarbonyl complex 5(2n d ) (Scheme 4). Significantly,
the first-generation five-membered chelate complex 7
could also be generated from complex 6 by purging a
solution of 6 with argon and warming to ∼-20 °C over
30 min (Scheme 4). The carbon of the coordinated keto
moiety appears at δ 239.6 (d, J CP ) 14 Hz) in the 13C-
{1H} NMR spectrum, and the 1H signal of the CH3 group
is at δ 2.24.17 This experiment confirms that for this
catalyst system CO insertion is a reversible process.
Complex 7 was surprisingly stable (up to 40 °C) in CD2-
Cl2 solution.
now observed at δ 180.6 as a doublet of doublets (J CP
)
52, 15 Hz) and νCO 2082 cm-1. In addition, the acetyl
CH3 group shifts to δ 1.87 and a new set of doublets is
observed in the 31P{1H} NMR spectrum at δ 3.13 and
-1.28 (J PP ) 57 Hz).17
Since no intermediate methyl carbonyl species can be
detected in the conversion of 2 to 5, we have been unable
to determine whether migratory insertion occurs via the
four-coordinate species, 3(m on o), or the five-coordinate
species, 3(bis). The extremely low insertion barrier is
consistent with theoretical predictions. Bottoni et al.
have calculated the insertion barriers for the neutral
four- and five-coordinate complexes (CH3)Ni(CO)2Cl and
(CH3)Ni(CO)3Cl to be 6.8 and 4.4 kcal/mol, respectively,
suggesting possible insertion from a five-coordinate
intermediate.16 Morokuma et al. have calculated an
insertion barrier of 9.8 kcal/mol for the four-coordinate
diimine complex (NHdCH-CHdNH)Ni(CH3)(CO)+.4
The measured barrier for the four-coordinate (dppp)-
Pd(CH3)(CO)+ complex is 14.8 kcal/mol,18 consistent
with calculations.2
To assess the barrier to insertion of ethylene into the
Ni-CH3 bond, the methyl ethylene complex 8 was
generated and studied (Scheme 5). The addition of 1-5
equiv of ethylene to 2 at -130 °C effected little displace-
ment of diethyl ether; however, addition of a large excess
Treating the dicarbonyl complex 5 with ethylene gave
a new acyl derivative identified as the six-membered
(18) Shultz, C. S.; Ledford, J .; DeSimone, J . M.; Brookhart, M. J .
Am. Chem. Soc. 2000, 122, 6351.

18 Organometallics, Vol. 20, No. 1, 2001
Communications
Sch em e 4
ppm; however, the low-temperature limiting spectrum
could not be obtained. To better characterize complex
10, the above experiment was repeated using doubly 13
C
labeled ethylene. Upon loss of propylene (13CH2d13CH2-
CH3 and CH2d13CH213CH3) the doubly 13C labeled 10
was observed. The R-methylene 13C resonance appears
at 30.0 ppm (J CH ) 160 Hz), while the â-methyl
resonance was observed at 3.17 ppm (1J CC ) 32 Hz, 1J CH
) 124 Hz). The NMR spectral characteristics of 10 are
completely consistent with other nickel(II) ethyl agostic
complexes.21 The migratory insertion of 8 follows clean
first-order kinetics with k ) 9.3 × 10-4 s-1 at -95.7 °C;
∆Gq ) 12.7(1) kcal/mol. The barrier in the palladium
analogue is 16.6(1) kcal/mol, while the barrier in a
related nickel aryl diimine complex, (diimine)Ni(CH3)-
(C2H4)+, is 13.3(1) kcal/mol, close to that of 8.22 We have
not been able to access the acyl ethylene complex,
(dppp)Ni(COCH3)(C2H4)+. We expect the migratory
insertion barrier to be small (ca. 9 kcal/mol), as the
barrier for the acyl ethylene migratory insertion reac-
tion in the analogous series of palladium complexes
((dppp)Pd(R)(C2H4)+, R ) CH3, COCH3) is ca. 4 kcal/
mol lower than the alkyl ethylene migratory insertion
barrier.
Sch em e 5
In summary, we have characterized a number of
potential intermediates in the nickel(II) dppp catalyzed
copolymerization of C2H4 and CO as well as estimated
key barriers to migratory insertions. The identification
of several five-coordinate intermediates is in contrast
to the palladium system, where four-coordinate species
are the rule. Insertion barriers, as expected, fall well
below those of the palladium analogues. That the Ni
complex is a much less efficient catalyst is likely due to
a strongly stabilized catalyst resting state relative to
the transition state for the turnover-limiting step. In
future work we will examine the working catalyst
system by in situ spectroscopy in order to assess the
role of the intermediates observed here, the catalyst
resting state(s), and the kinetics of chain growth.
of ethylene (ca. 25 equiv) resulted in complete displace-
ment and formation of 8. The 31P{1H} NMR spectrum
2
of 8 exhibited resonances at δ 13.3 (d, J PP ) 45) and
2
1
2.09 (d, J PP ) 45), while the H signal of the Ni-CH3
group moved to -0.01 ppm. The bound ethylene in 8
rapidly exchanges with free ethylene, and only an
1
average H signal is observed.19
When the solution of 8 was warmed to -95.7 °C, the
migratory insertion reaction of complex 8 was followed
(Scheme 5). Propylene is released, and the â-agostic
ethyl complex 10 is formed. A transient intermediate
can be observed and is thought to be an agostic propyl
complex, 9.20 In the presence of excess ethylene at -50
°C, complex 10 is the catalyst resting state of an
ethylene dimerization cycle.6,18 The -CH3 group in
Ack n ow led gm en t. This work was supported by the
National Science Foundation (Grant No. CHE-9710380).
C.S.S. thanks the Sloan Foundation for support. We also
thank Dr. Chi-Duen Poon and Dr. David Harris for
NMR technical assistance.
1
complex 10 was observed at -1.0 ppm in the H NMR
spectrum. This resonance is indicative of an agostic
-CH3 group with an agostic hydrogen rapidly exchang-
ing with two nonagostic hydrogens. Cooling the sample
to -118 °C caused broadening of the resonance at -1.0
Su p p or tin g In for m a tion Ava ila ble: Text giving experi-
mental details for the synthesis and characterization of
complexes 1, 2, 4-8, and 10, representative spectra, and
kinetic data for the migratory insertion reaction of 8. This
material is available free of charge via the Internet at
http://pubs.acs.org.
(19) Generation of 8 by reaction of 1 with (C6F5)3B in the presence
of ca. 5 equiv of C2H4 allows observation of the bound ethylene
resonance at 5.2 ppm. That rapid exchange occurs with increasing
ethylene concentration implies an associative exchange mechanism.
See: Temple, D. J .; J ohnson, L. K.; Huff, R. L.; White, P. S.; Brookhart,
M. J . Am. Chem. Soc. 2000, 122, 6626.
(20) When a solution containing complex 9 is cooled to ca. -110 °C,
a broad resonance at ca. -5 ppm is observed, which we believe to be
that of an agostic proton attributable to complex 9.
OM0008023
(21) Conroy-Lewis, F. M.; Mole, L.; Redhouse, A. D.; Litster, A.;
Spencer, J . L. J . Chem. Soc., Chem. Commun. 1991, 1601.
(22) Svejda, S. A.; J ohnson, L. K.; Brookhart, M. J . Am. Chem. Soc.
1999, 121, 10635.