Facile alkene insertion into a rhodium(III)-acetyl bond: Potential catalysts for CO/alkene copolymerization

Article abstract of DOI:10.1021/om049258e

Cationic Rh(III) methyl complexes, [(η⁵-ligand)-Rh(CO) ₂Me]⁺ (η⁵-ligand = C₅Me ₅, C₉Me₇, C₉Me₃H ₄), undergo facile reactions with strained alkenes (e.g. norbornadiene) to give products resulting from insertion of the alkene into a Rh-acetyl bond. Kinetic, mechanistic, and structural results are presented, along with evidence of catalytic activity for CO/alkene copolymerization.

Full text of DOI:10.1021/om049258e

Organometallics 2004, 23, 5907-5909
5907
Facile Alkene Insertion into a Rhodium(III)-Acetyl
Bond: Potential Catalysts for CO/Alkene
Copolymerization
Anthony Haynes,*,† Claire E. Haslam,† Kevin J. Bonnington,† Louise Parish,†
Harry Adams,† Sharon E. Spey,† Todd B. Marder,‡ and David N. Coventry‡
Departments of Chemistry, University of Sheffield, Brook Hill, Sheffield, U.K. S3 7HF, and
University of Durham, Durham, U.K. DH1 3LE
Received September 23, 2004
Summary: Cationic Rh(III) methyl complexes, [(η5-ligand)-
Rh(CO)2Me]+ (η5-ligand ) C5Me5, C9Me7, C9Me3H4),
undergo facile reactions with strained alkenes (e.g.
norbornadiene) to give products resulting from insertion
of the alkene into a Rh-acetyl bond. Kinetic, mechanis-
tic, and structural results are presented, along with
evidence of catalytic activity for CO/alkene copolymer-
ization.
Scheme 1. Insertion Steps for Pd-Catalyzed
CO/Alkene Copolymerization
The catalytic alternating copolymerization of CO and
alkenes, to give polyketones, has attracted substantial
interest in recent years, from both industrial and
academic researchers.1 The process is normally achieved
using Pd(II) complexes containing bidentate P,P or N,N
ligands. The propagation steps, illustrated in Scheme
1, are CO insertion into a Pd-alkyl bond and alkene
insertion into a Pd-acyl bond.
Scheme 2. Formation of 1[BF4]
Other d8 metals have also been tested as catalysts,
notably Ni(II)2 and Rh(I).3 The Rh(I) systems, which are
closely related to alkene hydroformylation catalysts,
have generally resulted in formation of relatively low
molecular weight co-oligomers. Migration of alkyl onto
CO is often facile in d6 Rh(III) complexes, and we sought
to investigate the reactivity of such systems toward
alkenes. In particular, we noted that [Cp*Rh(CO)2Me]+
(1) is proposed as a reactive intermediate in the oxida-
tive addition of MeI to [Cp*Rh(CO)2] (Scheme 2).4
Migration of methyl onto CO in this complex would
generate a vacant site cis to acetyl, ideally set up for
coordination and insertion of an alkene.
Scheme 3. Reactions of 1 with Strained Alkenes
in agreement, and the IR spectrum shows ν(CO) bands
at 2118 and 2087 cm-1, which are 10-20 cm-1 higher
in frequency than those of the known Ir analogue.4
Reaction of 1[BF4] with strained alkenes (norborna-
diene (nbd), norbornene (nbe), and dicyclopentadiene)
at room temperature results in the stable products 2a-c
(Scheme 3), which were isolated as solids and fully
characterized. The IR spectra showed, in addition to the
terminal ν(CO) band at ca. 2060 cm-1, an acetyl ν(CO)
absorption near 1620 cm-1. This is significantly lower
in frequency than normally found for a Rh(III) acetyl
(e.g. 1685 cm-1 for [Cp*Rh(CO)(COMe)I]) and close to
that observed for similar Pd chelates (e.g. [Pd(bipy)(κ2-
C7H8COMe)]+).6,7 High-frequency carbonyl 13C reso-
nances near δ 240 for 2a-c provided further evidence
that the carbonyl oxygen is coordinated to Rh.
We found that 1[BF4] can easily be generated in
solution by abstraction of iodide from [Cp*Rh(CO)-
(COMe)I] (Scheme 2). 1H NMR evidence for 1[OTf] has
been noted previously.5 Our NMR data for 1[BF4] are
* To whom correspondence should be addressed. E-mail: a.haynes@
sheffield.ac.uk.
† University of Sheffield.
‡ University of Durham.
(1) Sen, A. Acc. Chem. Res. 1993, 26, 303. Drent, E.; Budzelaar, P.
H. M. Chem. Rev. 1996, 96, 663.
(2) Klabunde, U.; Ittel, S. D. J. Mol. Catal. 1987, 41, 123. Klabunde,
U.; Tulip, T. H.; Roe, D. C.; Ittel, S. D. J. Organomet. Chem. 1987,
334, 141.
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A.; Brumbaugh, J. S. J. Organomet. Chem. 1985, 279, C5. Consiglio,
G.; Studer, B.; Oldani, F.; Pino, P. J. Mol. Catal. 1990, 58, L9. Sen,
A.; Brumbaugh, J. S.; Lin, M. R. J. Mol. Catal. 1992, 73, 297.
Robertson, R. A. M.; Poole, A. D.; Payne, M. J.; Cole-Hamilton, D. J.
Dalton 2000, 1817. Robertson, R. A. M.; Poole, A. D.; Payne, M. J.;
Cole-Hamilton, D. J. Chem. Commun. 2001, 47. Robertson, R. A. M.;
Cole-Hamilton, D. J. Coord. Chem. Rev. 2002, 225, 67.
(4) Kang, J. W.; Maitlis, P. M. J. Organomet. Chem. 1971, 26, 393.
(5) Stang, P. J.; Huang, Y.-H. J. Organomet. Chem. 1992, 435, 185.
(6) van Asselt, R.; Gielens, E. E. C. G.; Ru¨lke, R. E.; Vrieze, K.;
Elsevier, C. J. J. Am. Chem. Soc. 1994, 116, 977.
(7) Markies, B. A.; Kruis, D.; Rietveld, M. H. P.; Ververk, K. A. N.;
Boersma, J.; Kooijman, H.; Lakin, M. T.; Spek, A. L.; van Koten, G. J.
Am. Chem. Soc. 1995, 117, 5263.
10.1021/om049258e CCC: $27.50 © 2004 American Chemical Society
Publication on Web 11/10/2004

5908 Organometallics, Vol. 23, No. 25, 2004
Communications
Scheme 4. Mechanism of Reactions of 1 with
Alkenes and Phosphinesa
a L ) nbd (a), nbe (b), dicyclopentadiene (c), PPh3 (d),
PPh2Me (e), PPhMe2 (f).
Figure 1. X-ray structure of 2a (one of two independent
molecules in the unit cell). Hydrogens and counterion are
omitted for clarity. Selected bond lengths (Å) and angles
(deg): Rh-C(1) ) 1.852(12), Rh-O(2) ) 2.130(5), Rh-C(13)
) 2.108(9), C(1)-O(1) ) 1.111(14); C(1)-Rh-C(13) ) 90.7-
(5), O(2)-Rh-C(1) ) 99.1(5), O(2)-Rh-C(13) ) 79.5(3),
Rh-C(1)-O(1) ) 170.9(12).
The structures of 2a,c were confirmed by X-ray
crystallography. The structure of 2a8 is illustrated in
Figure 1, and details of the structure of 2c are provided
as Supporting Information. The principal geometrical
features of these five-membered metallacycles are very
similar to those found in related Pd(II) systems.7 The
Rh-acetyl moiety adds to the exo face of the alkene
double bond (and in the case of dicyclopentadiene,
selectively at the nbd-like double bond). These observa-
tions represent the first well-characterized products of
alkene insertion into a Rh-acyl bond. As well as being
required for CO/alkene copolymerization, this process
has been suggested as a possible step in catalytic or
stoichiometric hydroacylation.9,10 To our knowledge, the
only other non-palladium system where analogous
insertions have been observed is [RMn(CO)5], for which
high pressures were required.11 Reactions of 1 with
simple alkenes such as ethylene and 1-hexene were
attempted but have so far not yielded isolable prod-
ucts.12
Figure 2. Plots of 1/kobs vs 1/[L] for reactions of 1 (CH2-
Cl2, 25 °C).
structures, which will be reported elsewhere. Since
typical reaction time scales were on the order of seconds,
stopped-flow IR spectroscopy was employed for reaction
monitoring. Pseudo-first-order conditions were em-
ployed, with the reactant ligand (alkene or phosphine)
in at least a 10-fold excess over [1]. In each experiment,
clean exponential decay of the reactant ν(CO) bands was
observed, indicating a first-order dependence on 1. Plots
of kobs vs [L] displayed saturation behavior, rising
toward a limiting rate at high [L]. This is consistent
with the mechanism shown in Scheme 4, with a rate-
limiting, reversible methyl migration step (k1).
Kinetic studies have been conducted in order to probe
the mechanistic details of the reactions of 1 with alkenes
and with phosphines. The reactions with phosphines
gave the expected products [Cp*Rh(CO)L(COMe)]+ (L
) PPh3 (3d), PPh2Me (3e), PPhMe2 (3f); Scheme 4)
resulting from methyl migration onto CO and coordina-
tion of phosphine. Complexes 3d-f were fully charac-
For such a mechanism, a plot of 1/kobs vs 1/[L] should
be linear with y intercept 1/k1 and slope k-1/k1k2. The
reciprocal plots shown in Figure 2 demonstrate clearly
that the linear relationship holds with the same y
intercept for each ligand, L. Thus, k1 is independent of
the nature of L for both phosphines and alkenes. An
Eyring plot of variable-temperature data gave activation
parameters ∆Hq ) 72 ( 2 kJ mol-1 and ∆Sq ) -16 ( 6
J mol-1 K-1 for methyl migration. The activation barrier
is therefore much lower than for methyl migration in
the isoelectronic group 8 analogue [CpRu(CO)2Me], for
which reaction with PPh3 occurred at 137 °C with a rate
constant of ca. 10-5 s-1, equating to ∆Gq ) 141 kJ
-
terized (as their BF4 salts), including X-ray crystal
(8) Crystallographic data for for 2a[BF4]: C20H26BF4O2Rh, Mr
)
488.13, orthorhombic, a ) 32.942(17) Å, b ) 15.150(8) Å, c ) 8.584(5)
Å, Pna21 (C2,9 No. 33), Z ) 8, T ) 298(2) K, R1 ) 0.0624 (on F2) (wR2
) 0.1652 for all 9844 data).
(9) Marder, T. B.; Roe, D. C.; Milstein, D. Organometallics 1988, 7,
1451.
(10) Jones, W. D.; Bergman, R. G. J. Am. Chem. Soc. 1979, 101,
5447.
mol-1 13
This can be explained by the decreased back-
.
(11) DeShong, P.; Slough, G. A. Organometallics 1984, 3, 636.
DeShong, P.; Sidler, D. R.; Rybczynski, P. J.; Slough, G. A.; Rheingold,
A. L. J. Am. Chem. Soc. 1988, 110, 2575.
(12) Two examples of alkyne insertion into Rh-acyls have recently
been reported. Kataoka, Y.; Iwato, Y.; Shibahara, A.; Yamagata, T.;
Tani, K. Chem. Commun. 2000, 841. Ogata, K.; Kuge, K.; Tatsumi,
K.; Yamamoto, Y. Chem. Commun. 2002, 128.
bonding in the cationic complex 1, resulting in more
electrophilic CO ligands.
(13) George, R.; Andersen, J. M.; Moss, J. R. J. Organomet. Chem.
1995, 505, 131.

Communications
Organometallics, Vol. 23, No. 25, 2004 5909
Scheme 5. Reaction of Indenyl Complexes with
nbd
The slopes of the plots in Figure 2 give a measure of
the ratio k-1/k2. Assuming that k-1 is independent of L,
this gives an order of nucleophilicity toward the Rh
intermediate I1 of nbe < nbd < PPh3 < PPh2Me <
PPhMe2, in broad agreement with expectation. The
presumed intermediate species 3a-c with an η2-alkene
coordinated cis to the acetyl ligand were not detected
in these experiments, indicating that the subsequent
alkene insertion step is fast (however, direct evidence
for similar intermediates was obtained in a related
system; see below).
For these Rh(III) systems to be active as CO/alkene
copolymerization catalysts, chelate complexes of the
type 2a-c must undergo CO insertion. In tests on 2a,
however, we found no detectable reaction with CO at
either atmospheric or elevated pressure (9 bar). Despite
this, when 2a was treated with 100 equiv of nbd in CH2-
Cl2, under 10 bar of CO at 40 °C, ca. 50% of the alkene
was consumed after 5 days, as judged by the IR band
of nbd at 1542 cm-1. A strong broad product band at
ca. 1720 cm-1 was observed in the region associated
with organic keto functionalities.
and acetyl ligands: i.e., prior to insertion of the alkene
into the Rh-acetyl bond. We assign this species as the
η2-alkene complex 6, which isomerizes to the insertion
product 8 (Scheme 5). The buildup of detectable quanti-
ties of 6 can be explained if methyl migration is
accelerated more than alkene insertion in the Ind*
system. The half-life for alkene insertion in 6 is esti-
mated as 0.6 s (15 °C), corresponding to ∆Gq ) ca. 70
kJ mol-1. This value is almost identical with that
measured (at -46 °C) for ethene insertion in [Pd(phen)-
(η2-C2H4)(COMe)]+.19 In preliminary kinetic experi-
ments, the Ind′ complex 5 behaved similarly, with a rate
constant for methyl migration ca. double that for 4. An
intermediate 7 (ν(CO) 2071, 1738 cm-1) was again
detected in the reaction of 5 with nbd.
Exposure of a CH2Cl2 solution of 8 to 1 atm of CO
generated weak ν(CO) bands at 2129 (2118 sh) and 2102
(2089 sh) cm-1, possibly indicative of a dicarbonyl
product, [Ind*Rh(CO)2(COC7H8COMe)]+. Treatment of
8 with 100 equiv of nbd in CH2Cl2, under 10 bar of CO,
resulted in consumption of ca. 50% of the alkene after
3 days at 22 °C. After a further 2 days at 40 °C, all the
alkene had reacted. On addition of a further 100 equiv
of nbd, ca. 50% was consumed in 1 day at 40 °C, again
leading to a strong broad ketone ν(CO) band at ca. 1720
cm-1. Thus, catalytic activity appears to be significantly
higher for the Ind* system relative to Cp*. Further
experiments have indicated that the product of these
catalytic reactions is polymeric (or oligomeric) in nature.
Work is ongoing to determine the structural features
of this polymer.
One potential way to enhance the reactivity of chelate
complexes such as 2a is to replace Cp* by an indenyl
ligand. Indenyl ligands are known to give dramatic
accelerations (relative to their Cp analogues) of CO
substitution,14 and they also increase the rate of alkyl
migration15,16 and some catalytic reactions.9,17 A number
of Rh(I) complexes, [(η5-C9MenH7-n)Rh(CO)2], have been
reported previously.18 We found that oxidative addition
of MeI to the η5-C9Me7 (Ind*) and η5-1,2,3-Me3C9H4
(Ind′) complexes gave the expected iodo acetyl products.
Iodide abstraction using AgBF4 gave cations 4 and 5,
analogous to 1. Kinetic measurements on the reaction
of 4 with PPh3 again showed saturation behavior, but
the limiting rate constant at high [PPh3] was found to
be ca. 24 times faster (at 15 °C) than for 1. This
enhancement in the rate of methyl migration is very
similar to that reported for [IndMo(CO)3Me] relative to
[CpMo(CO)3Me].15 Activation parameters for methyl
migration in 4 are ∆Hq ) 64 ( 1 kJ mol-1 and ∆Sq )
-18 ( 5 J mol-1 K-1, representing a lowering of ∆Hq
In summary, we have demonstrated that both of the
key propagation steps for CO/alkene copolymerization
are facile on Rh(III), leading to structurally character-
ized products. Intermediate Rh(η2-alkene)(acetyl) com-
plexes have been directly observed for indenyl sytems,
and catalytic transformation of nbd and CO into poly-
meric products occurs under mild conditions.
relative to that for 1 of 8 kJ mol-1
.
Notably, in the reaction of 4 with nbd, evidence for
an intermediate species was obtained. As the two
terminal ν(CO) absorptions of 4 decayed, two new bands
grew at 2061 and 1731 cm-1. The intensity of these
bands reached a maximum and then decayed, while
absorptions of the final product 8 appeared at 2050 and
1625 cm-1. The IR spectrum of the intermediate is
consistent with a complex containing both terminal CO
Acknowledgment. We thank the University of
Sheffield, The Royal Society, and the EPSRC for sup-
porting this research.
Supporting Information Available: Tables, text, and
figures giving kinetic data, experimental details, spectroscopic
and analytical data, and ORTEP plots and a CIF file giving
crystallographic data. This material is available free of charge
via the Internet at http://pubs.acs.org.
(14) Basolo, F. Polyhedron 1990, 9, 1503.
(15) Hart-Davis, A. J.; Mawby, R. J. J. Chem. Soc. A 1969, 2403.
(16) Allevi, M.; Bassetti, M.; Lo Sterzo, C.; Monti, D. J. Chem. Soc.,
Dalton Trans. 1996, 3527.
(17) Hauptman, E.; Sabo-Etienne, S.; White, P. S.; Brookhart, M.;
Garner, J. M.; Fagan, P. J.; Calabrese, J. C. J. Am. Chem. Soc. 1994,
116, 8038.
OM049258E
(18) Kakkar, A. K.; Taylor, M. J.; Marder, T. B.; Shen, J. K.;
Hallinan, N.; Basolo, F. Inorg. Chim. Acta 1992, 198-200, 219.
(19) Rix, F. C.; Brookhart, M.; White, P. S. J. Am. Chem. Soc. 1996,
118, 4746.