Four- and five-coordinate CO insertion mechanisms in d8-nickel(II) complexes [1]

Full text of DOI:10.1021/ja0160896

9172
J. Am. Chem. Soc. 2001, 123, 9172-9173
which a four-coordinate acyl monocarbonyl complex is formed.1b
The five-coordinate structures are supported by low-temperature
Four- and Five-Coordinate CO Insertion
Mechanisms in d8-Nickel(II) Complexes
1
13C{1H}, 31P{1H}, H, and IR spectroscopic data.4,5 The relative
C. Scott Shultz, Joseph M. DeSimone, and
Maurice Brookhart*
intensities of the νCO bands for 5a,b (2094 and 2059 cm-1 for
5a, 2096 and 2061 cm-1 for 5b), indicate OC-Ni-CO bond
angles of 130° and 106°, respectively. The reduction in bond angle
suggests more steric crowding in 5b which is likely responsible
for the higher lability of the CO ligands in this complex (vide
infra).
Department of Chemistry
UniVersity of North Carolina at Chapel Hill
Chapel Hill, North Carolina 27599-3290
ReceiVed April 26, 2001
Complexes 5a,b are in equilibrium with their four-coordinate
precursors 4a,b, but under 1 atm CO (CD2Cl2, -80 °C) 5a,b are
heavily favored. Purging a solution of the o-MeO-dppe-derived
5b with argon at -80 °C for ca. 30 min liberates an equivalent
of CO to generate the four-coordinate acyl monocarbonyl complex
4b.4 In contrast, the CO ligands in dppp-based 5a are not as labile,
and the argon purge must be carried out at -20 °C to drive the
reaction to 4a.4
Both monocarbonyl complexes 4a,b can be further decar-
bonylated by warming to 25 °C under an argon purge (eq 2).
Unexpectedly, in the case of the dppp complex 4a, liberation of
1 equiv CO yields an equilibrium mixture (40:60) of two
complexes. The minor one is readily assigned to the methyl
carbonyl complex 3a(mono) (Ni-CH3, 1H, 0.56 ppm).4 The acetyl
group is still present in the major complex, 6a (Ni-COCH3, 1H,
2.14 ppm),4 and we tentatively assign the structure as an η2-acyl
complex as shown, on the basis of the following observations:6
(1) A (dppp)Ni(COCH3)(solv)+ structure is ruled out by the fact
that the ratio of 3a(mono) to 6a is unchanged on addition of
excess ether or water. (2) Cooling the solution to -130 °C results
in no significant line broadening of the CH3 resonance in 6a and
suggests that a â-agostic structure, (dppp)NiCOCH2-µ-H+, is
unlikely.7 (3) A reasonable analogue of 6a, a Ni(II) η2-iminoacyl
complex has been structurally characterized by Carmona et al.8
Monocationic Pd(II) complexes containing bidentate ligands
have been exploited extensively as catalysts for olefin/CO
copolymerizations. Mechanistic work employing well-defined
systems has established that the carbonylation step occurs via
migratory insertion of four-coordinate (L-L)Pd(alkyl)(CO)+
+
species.1 The migratory insertion rate of (L-L)Pd(CO)CH3
complexes is not accelerated by external CO and, for L-L )
1,10-phenanthroline and 1,3-bis(diphenylphosphino)propane, is
virtually the same in methylene chloride and the strong donor
solvent, acetone.1a,2 In short, all available evidence suggests that
five-coordinate species are not involved either as transition states
or as intermediates in the carbonylation step. We have been
investigating ethylene/CO copolymerizations by Ni(II) analogues
and report here that, in contrast to Pd analogues, carbonylation
can occur via a four- or five-coordinate species but the five-
coordinate pathway is preferred.
Investigations have focused on Ni(II) complexes of 1,3-bis-
(diphenylphosphino)propane (dppp) and 1,2-bis(bis(2-methoxy-
phenyl)phosphino)ethane (o-MeO-dppe). Both systems are known
to be active for copolymerization, but the latter is far more
productive.3 Protonation of (dppp)NiMe2, 1a, or (o-MeO-dppe)-
NiMe2, 1b, with H(OEt2)2BAr′4 in halogenated solvents such as
CH2Cl2 at -80 °C yields (dppp)NiMe(solv)+, 2a, and (o-MeO-
dppe)NiMe(solv)+, 2b, (solv ) OEt2, OH2),4 which are highly
reactive precursors to carbonylated species.
At higher temperatures, both 3a(mono) and 6a exhibit a pattern
of 1H and 31P NMR line broadening, which establishes that they
are rapidly interconverting. Applying the slow-exchange ap-
proximation to the broadening of the CH3 signal of 3a(mono) at
Exposure of 2a to CO at -130 °C (CDCl2F) results in
immediate formation of the five-coordinate acetyl dicarbonyl
complex 5a (eq 1).5 No intermediate methyl carbonyl complexes,
3a(mono) or 3a(bis), were observed during this transformation.
Assuming a generous half-life of 15 min at -130 °C indicates a
maximum barrier for insertion, ∆Gq < 10 kcal/mol. In contrast,
exposure of 2b to CO at -130 °C resulted in the formation of
the methyl dicarbonyl complex 3b(bis)4 which undergoes insertion
at -127 °C (k ) 3.4 × 10-4 s-1, ∆Gq ) 10.6(2) kcal/mol) to
give 5b.
-25 °C yields a rate constant for migratory insertion of 50 s-1
,
∆Gq ) 12.6(1) kcal/mol.9 This barrier is at least 2.6 kcal/mol
greater than that observed for carbonylation of 2a and thus
indicates conversion of 2a to 5a/4a cannot occur via migratory
insertion of 3a(mono).
Treating the methyl carbonyl complex 3b(mono) and the
mixture of 3a(mono) and 6a with ethylene yields the alkyl chelate
complexes 7a,b4 that must arise via formation of an acyl complex
The five-coordinate acetyl complexes formed in these reactions
are in marked contrast to reaction of the dpppPd(II) analogue in
(6) Theoretical analysis suggests L2M(acyl)+ (M ) Pd, Ni) complexes are
stabilized by an η2 interaction, see: Margl, P.; Ziegler, T. J. Am. Chem. Soc.
1996, 118, 7337. Svensson, M.; Matsubara, T.; Morokuma, K. Organometallics
1996, 15, 5568.
(1) (a) Rix, F. C.; Brookhart, M.; White, P. S. J. Am. Chem. Soc. 1996,
118, 4746. (b) Shultz, C. S.; Ledford, J.; DeSimone, J. M.; Brookhart, M. J.
Am. Chem. Soc. 2000, 122, 6351. (c) Ledford, J. S.; Shultz, C. S.; Gates, D.
P.; White, P. S.; DeSimone, J. S.; Brookhart, M. Organometallics 2001, in
press.
(7) For an example of a â-agosic acyl complex of molybdenum see:
Carmona, E.; Sa´nchez, L.; Mar´ın, J. M.; Poveda, M. L.; Atwood, J. L.; Priester,
R. D.; Rogers, R. D. J. Am. Chem. Soc. 1984, 106, 3214.
(2) Ledford, J. S., Brookhart, M. Unpublished results.
(3) Drent, E.; Catharina, M.; De Kock, T. WO 9700127, 1997.
(4) See Supporting Information for complete spectroscopic details.
(5) For a preliminary report of formation of 5a from 2a see: Shultz, C. S.;
DeSimone, J. M.; Brookhart, M. Organometallics 2001, 20, 16.
(8) Belderra´ın, T. R.; Paneque, M.; Poveda, M. L.; Sernau, V.; Carmona,
E.; Gutie´rrez, E.; Monge, A. Polyhedron 1995, 14, 323.
(9) Calculated using the standard equation for slow-exchange: k1 ) π(∆ω)
where ∆ω ) 15 Hz. k1 ) 50 s-1
.
10.1021/ja0160896 CCC: $20.00 © 2001 American Chemical Society
Published on Web 08/24/2001

Communications to the Editor
J. Am. Chem. Soc., Vol. 123, No. 37, 2001 9173
followed by insertion of ethylene and coordination of the carbonyl
group (eq 3). For the dppp-derived complexes 3a(mono) and 6a,
the chelate, 7a, is formed immediately at -120 °C (t1/2 < 15
min, ∆Gq < 11 kcal/mol), and no intermediates are detected. Since
this rate is significantly greater than the rate of interconversion
of 3a(mono) and 6a, kinetic trapping of the equilibrium mixture
has occurred, and product formation via migratory insertion of
3a(mono) can be ruled out. A likely scenario is that ethylene
binds to both 3a(mono) and 6a to give different intermediates.
(The barrier for subsequent insertion of the acyl ethylene complex,
(dppp)Ni(COMe)(C2H4)+, must also be less than 11 kcal/mol since
this species is unobserved.)
such that 8b does not build up under reaction conditions). An
alternative to eq 5 is concerted ethylene attack and insertion.
Regardless of the precise mechanistic scenario, CO insertion
occurs via both four-and five-coordinate pathways for conversion
of 3b(mono) to 7b, with the five-coordinate pathway dominating
at high [C2H4]. As indicated before for the dppp system,
conversion of 3a(mono) to the chelate 7a must also have access
to a five-coordinate pathway since the rate exceeds the rate for
migratory insertion of 3a(mono). This is clearly reasonable since
the dppp system is less hindered than the o-MeO-dppe system
and should react more rapidly with ethylene to form a five-
coordinate species. These findings are consistent with the earlier
results in the carbonylation of 2b to 4b/5b, where the reaction
proceeds through the intermediate 3b(bis). The dppp-derived
complex 2a is also expected to proceed through the analogous
dicarbonyl intermediate 3a(bis) which is supported by the fact
that the rate of carbonylation exceeds the migratory insertion rate
of 3a(mono).
Informative kinetic results came from examination of the
trapping of 3b(mono) (eq 4). Formation of 7b was monitored at
-80 °C in the presence of varying concentrations of excess
ethylene.
These results have a direct bearing on the propagation step
involving carbonylation in olefin/CO copolymerization. For a
Pd(II) chelate complex (P-P)Pd(CH2CH2COCH3)+ to insert CO,
the chelate must convert to the four-coordinate (P-P)Pd(CO)-
CH2CH2COCH3+, an energetically unfavorable process under
moderate CO pressures.1b,10 In contrast, the Ni(II) chelates, 7a,b,
identified as the resting states of the copolymerization,11 are very
rapidly carbonylated at -120 °C to yield acyl derivatives 12a,b
(eq 6). The availability of low-energy five-coordinate insertion
pathways for migratory insertion in Ni(II) complexes could have
broad implications with respect to energetics of polymer chain
growth in Ni(II)-based catalyst systems.12
A plot of the observed first-order rate constants versus ethylene
concentration is shown in Figure 1. This plot shows a classical
case of competing first- and second-order processes where
kobs ) k + k′[C2H4] The second-order rate constant, k′, determined
from the slope is 4.7(3) × 10-3 M-1 s-1, while the first-order
rate constant obtained from the intercept is 6.4(6) × 10-4 s-1
,
∆Gq(-80 °C) ) 14.0(1) kcal/mol.
Acknowledgment. This work was supported by the National Science
Foundation (CHE-9710380). C.S.S. thanks the Sloan Foundation for a
graduate fellowship. We also thank Dr. Chi-Duen Poon and Dr. David
Harris for NMR technical assistance.
Supporting Information Available: Synthesis and characterization
of complexes 1-7 and kinetic data for the insertion reactions of
3a,b(mono), and 3b(bis) (PDF). This material is available free of charge
via the Internet at http://pubs.acs.org.
Figure 1. kobs vs [ethylene] in the conversion of 3b(mono) to 7b.
The first-order process is ascribed to rate-determining migratory
insertion of 3b(mono) to give 10b, followed by rapid trapping
of the acyl intermediate (eq 5). (10b is assumed to be an η2-acyl
complex based on 6a.) The barrier of 14.0 kcal/mol for the
migratory insertion of 3b(mono) rules out the possibility that
formation of 5b from 3b(bis) (eq 1) occurs via 3b(mono). The
14.0 kcal/mol barrier is consistent with the 12.6 kcal/mol barrier
determined for 3a(mono) via line broadening. The second-order
process must involve coordination of ethylene in the transition
state for CO insertion. Several mechanistic possibilities are
consistent with these kinetics (eq 5). A five-coordinate intermedi-
ate 8b is likely formed. Its formation could be rate-determining
or 3b(mono) and 8b could be in rapid preequilibrium (with Keq
JA0160896
(10) (a) Zuideveld, M. A.; Kamer, P. C. J.; van Leeuwen, P. W. N. M.;
Klusener, P. A. A.; Stil, H. A.; Roobeek, C. F. J. Am. Chem. Soc. 1998, 120,
7977. (b) Drent, E.; Budzelaar, P. H. M. Chem. ReV. 1996, 96, 663. (c) Mul,
W. P.; Oosterbeek, H.; Beitel, G. A.; Kramer, G.-J.; Drent, E. Angew. Chem.,
Int. Ed. 2000, 39, 1848. (d) Luo, H.-K.; Kou, Y.; Wang, X.-W.; Li, D.-G. J.
Mol. Catal. A. 2000, 151, 91. (e) Dekker, G. P. C. M.; Elsevier, C. J.; Vrieze,
K.; van Leeuwen, P. W. N. M.; Roobeek, C. F. J. Organomet. Chem. 1992,
430, 357. (f) Nozaki, K.; Sato, N.; Tonomura, Y.; Yasutomi, M.; Takaya, H.;
Hiyama, T.; Matsubara, T.; Koga, N. J. Am. Chem. Soc. 1997, 119, 12779.
(11) Shultz, C. S., Ph.D. Thesis, University of North Carolina, Chapel Hill,
NC, 2001.
(12) In the copolymerization system where CO is in excess, insertion of
ethylene into the acyl bond may occur via a five-coordinate (P-P)Ni(acyl)-
(C2H4)(CO)+ complex rather than a four-coordinate acyl ethylene complex.