Possible side reactions due to water in emulsion polymerization by late transition metal complexes. 1. Water complexation and hydrolysis of the growing chain

Article abstract of DOI:10.1021/ic050501r

The transition metal catalyzed ethylene polymerization in aqueous emulsion has been increasingly successful in the last couple of years. Water however adversely affects the polymerization process by (a) competing with ethylene for the binding site at the metal and (b) hydrolyzing the growing chain. Neutral salicylaldiminato and cationic diimine complexes of Ni and Pd with different substituent patterns are studied here by density functional theory to determine their propensity toward water complexation and hydrolysis of the growing chain. Experimental NMR studies have also been carried out on the protonolysis of the Ni(II)-based Grubbs catalyst. It is found that in general that (a) ethylene coordination is preferred over water coordination for both Ni and Pd catalysts and (b) hydrolysis of the metal alkyl bond is competitive to ethylene insertion.

Full text of DOI:10.1021/ic050501r

Inorg. Chem. 2005, 44, 7806−7818
Possible Side Reactions Due to Water in Emulsion Polymerization by
Late Transition Metal Complexes. 1. Water Complexation and Hydrolysis
of the Growing Chain
Iordan H. Hristov,† Roger L. DeKock,‡ Grant D. W. Anderson,† Inigo Go
1
ttker-Schnetmann,§
Stefan Mecking,§ and Tom Ziegler*,†
Department of Chemistry, UniVersity of Calgary, 2500 UniVersity DriVe NW, Calgary,
Alberta, Canada T2N 1N4, Department of Chemistry and Biochemistry, CalVin College,
3201 Burton Street SE, Grand Rapids, Michigan 49546-4388, and Faculty of Science,
UniVersity of Konstanz, UniVersita¨tsstr. 10, D-78464 Konstanz, Germany
Received April 4, 2005
The transition metal catalyzed ethylene polymerization in aqueous emulsion has been increasingly successful in
the last couple of years. Water however adversely affects the polymerization process by (a) competing with ethylene
for the binding site at the metal and (b) hydrolyzing the growing chain. Neutral salicylaldiminato and cationic diimine
complexes of Ni and Pd with different substituent patterns are studied here by density functional theory to determine
their propensity toward water complexation and hydrolysis of the growing chain. Experimental NMR studies have
also been carried out on the protonolysis of the Ni(II)-based Grubbs catalyst. It is found that in general that (a)
ethylene coordination is preferred over water coordination for both Ni and Pd catalysts and (b) hydrolysis of the
metal alkyl bond is competitive to ethylene insertion.
Introduction
production are extremely sensitive to moisture. Carrying out
such reactions in water is a highly attractive goal, as many
polymer microstructures are not available by other means
than catalytic coordination polymerization. In view of a
possible potential for catalytic coordination polymerization
in aqueous emulsion, the reactivity toward water of metal
complexes already known to polymerize olefins to higher
molecular weight products in nonaqueous systems is of
interest.
Considering catalytic transformations in aqueous media
in general, complexes of late transition metals are good
candidates due to their reduced oxophilicity. This is il-
lustrated by the commercial application of aqueous two-phase
catalysis in the Ruhrchemie-Rhone-Poulenc process for
propylene hydroformylation.3 It has long been assumed that
coordination polymerizations in aqueous media are possible
in principle.4 However, the coordination polymerization of
Emulsion polymerization of olefinic monomers is an
important and versatile process.1 Polymer lattices are ob-
tained, that is, aqueous dispersions of surfactant-stabilized
polymer microparticles with a size range of 50-1000 nm.
Important applications of lattices involve film formation upon
evaporation of the dispersing medium (e.g., in coatings), and
here the environmental friendliness and nonflammability of
water is particularly advantageous. To date, polymer lattices
are produced industrially by free radical processes exclu-
sively. Transition metal catalyzed coordination polymeriza-
tion has received comparatively little attention, as the early
transition metal catalysts2 used commercially for polyolefin
* To whom correspondence should be addressed. E-mail: ziegler@
ucalgary.ca.
† University of Calgary.
‡ Calvin College.
§ University of Konstanz.
(1) (a) Lovell, P. A.; El-Aasser, M. S. Emulsion Polymerization and
Emulsion Polymers; Wiley: Chichester, 1997. (b) Wa¨ârige Polymer-
dispersionen; Distler, D., Ed.; VCH: Weinheim, 1999. (c) Lagaly,
G.; Schulz, O.; Zimehl, R. Dispersionen und Emulsionen; Stein-
(3) (a) Ziegler Catalysts; Fink, G., Mu¨lhaupt, R., Brintzinger, H. H., Eds.;
Springer: Berlin, 1995. (b) Brintzinger, H. H.; Fischer, D.; Mu¨lhaupt,
R.; Rieger, B.; Waymouth, R. Angew. Chem. 1995, 107, 1255; Angew.
Chem., Int. Ed. Engl. 1995, 34, 1143. (c) Britovsek, G. J. P.; Gibson,
V. C.; Wass, D. F. Angew. Chem. 1999, 111, 448; Angew. Chem.,
Int. Ed. 1999, 38, 428. (d) Kaminsky, W.; Arndt, M. AdV. Polym. Sci.
1997, 127, 143.
kopff: Darmstadt, 1997. (d) Fitch, R. M. Polymer Colloids:
ComprehensiVe Introduction; Academic Press: San Diego, 1997.
a
(2) Fink, G.; Mu¨lhaupt, R.; Brintzinger, H. H. Ziegler Catalysts;
Springer: Berlin, 1995.
(4) Reppe, W.; Magin, A. U.S. Patent 2,577,208, 1948.
7806 Inorganic Chemistry, Vol. 44, No. 22, 2005
10.1021/ic050501r CCC: $30.25
© 2005 American Chemical Society
Published on Web 10/04/2005

Side Reactions due to Water in Emulsion Polymerization
Figure 1. Schematic structures of the catalyst species investigated.
simple olefins in emulsion has only very recently begun to
attract interest. We5 and Claverie and Spitz et al.6,7 have
reported coordination polymerization of ethylene in aqueous
emulsion or suspension by neutral nickel(II) salicyladiminato
complexes (introduced as polymerization catalysts by Grubbs
et al. and Johnson et al.8), by cationic palladium(II) diimine
complexes (introduced as olefin polymerization catalysts by
Brookhart et al.9), as well as the well-known P∧O-chelated
neutral nickel(II) complexes,10 Figure 1.
In preliminary experimental investigations, low-tempera-
ture NMR studies revealed that the water ligand in the
palladium diimine complex (R ) Pr, L ) H2O, see Figure
i
1) is displaced completely upon addition of ethylene.5a Since
the binding of water in the first coordination sphere can be
the initial step toward the hydrolysis of the growing chain,
we have looked at the relative complexation energies of water
vs ethylene for all three catalyst systems shown in Figure 1
as a function of the substituent groups. The results are
presented in the first part of our discussion.
The polymerization chemistry of the salicylaldiminato and
diimine systems in organic solvents has been extensively
studied by theoretical methods.11 In this paper we are only
going to investigate the catalysts’ stability toward water.
Hydrolysis of the M-R′′ bond (see Figure 1) to release a
hydrocarbon H-R′′ and form a M-OH hydroxy link is
another possible side reaction. This is a similar process to
the reverse of the C-H activation process that has been
observed for some palladium(II) complexes.12 This type of
hydrolysis process is considered in part two of our discussion
for all three catalyst types shown in Figure 1. To date
experiments seem to indicate that catalysts of the Brookhart
type are stable in aqueous solution at room temperature for
days.5b
Experiment5b has shown that the addition of ethylene to
the Pd(II)-based Brookhart catalyst in aqueous solution leads
to immediate decomposition to palladium black; the reason
for this decomposition has not been clarified. Thus, only a
suspension-type ethylene polymerization has been reported,
(5) (a) Held, A.; Bauers, F. M.; Mecking, S. Chem. Commun. 2000, 301.
(b) Held, A.; Mecking, S. Chem.sEur. J. 2000, 6, 4623. (c) Bauers,
F. M.; Mecking, S. Macromolecules 2001, 34, 1165. (d) Bauers, F.
M.; Mecking, S. Angew. Chem. 2001, 113, 3112; Angew. Chem., Int.
Ed. 2001, 40, 3020. (e) Bauers, F. M.; Chowdhry, M. M.; Mecking,
S. Macromolecules 2003, 36, 6711. (f) Bauers, F. M.; Thomann, R.;
Mecking, S. J. Am. Chem. Soc. 2003, 125, 8838. (g) Zuideveld, M.
A.; Wehrmann, P.; Ro¨hr, C.; Mecking, S. Angew. Chem. 2004, 116,
887; Angew. Chem., Int. Ed. 2004, 43, 869.
(6) (a) Tomov, A.; Broyer, J.-P.; Spitz, R. Macromol. Symp. 2000, 150,
53. (b) Soula, R.; Novat, C.; Tomov, A.; Spitz, R.; Claverie, J.; Drujon,
X.; Malinge, J.; Saudemont, T. Macromolecules 2001, 34, 2022. (c)
Soula, R.; Saillard, B.; Spitz, R.; Claverie, J.; Llaurro, M. F.; Monnet,
C. Macromolecules 2002, 35, 1513.
(7) For a very slow (1 turnover/day) Rh-catalyzed reaction of ethylene to
low-molecular-weight material, cf. Wang, L.; Lu, R. S.; Bau, R.; Flood,
T. C. J. Am. Chem. Soc 1993, 115, 6999.
(8) (a) Wang, C.; Friedrich, S.; Younkin, T. R.; Li, R. T.; Grubbs, R. H.;
Bansleben, D. A.; Day, M. W. Organometallics 1998, 17, 3149. (b)
Younkin, T. R.; Connor, E. F.; Henderson, J. I.; Friedrich, S. K.;
Grubbs, R. H.; Bansleben, D. A. Science 2000, 287, 460. (c) Connor,
E. F.; Younkin, T. R.; Henderson, J. I.;Waltman, A. W.; Grubbs, R.
H. Chem. Commun. 2003, 2272. (d) Johnson, L. K.; Bennett, A. M.
A.; Ittel, S. D.; Wang, L.; Parthasarathy, A.; Hauptman, E.; Simpson,
R. D.; Feldman, J.; Coughlin, E. B. (DuPont) WO98/30609, 1998.
(9) Johnson, L. K.; Killian, C. M.; Brookhart, M. J. Am. Chem. Soc. 1995,
117, 6414.
(10) (a) Keim, W.; Kowaldt, F. H.; Goddard, R.; Krueger, C. Angew. Chem.,
Int. Ed. Engl. 1978, 17, 466; Angew. Chem. 1978, 90, 493. (b)
Klabunde, U.; Ittel, S. D. J. Mol. Catal. 1987, 41, 123. (c) Ostoja-
Starzewski, K. A.; Witte, J. Angew. Chem. 1987, 99, 76; Angew.
Chem., Int. Ed. Engl. 1987, 26, 63.
(11) (a) Woo, T. K.; Margl, P. M.; Deng, L.; Cavallo, L.; Ziegler, T. Catal.
Today 1999, 50, 479 (b) Michalak, A.; Ziegler, T. Organometallics
2003, 22, 2069. (c) Deng, L.; Woo, T. K.; Cavallo, L.; Margl, P. M.;
Ziegler, T. J. Am. Chem. Soc. 1997, 119, 6177. (d) Michalak, A.;
Ziegler, T. Organometallics 2001, 20, 1521. (e) Deubel, D.; Ziegler,
T. Organometallics 2002, 21, 1603. (f) Milano, G.; Guerra, G.;
Pellecchia, C.; Cavallo, L. Organometallics 2000, 19, 1343. (g)
Svensson, M.; Matsubara, T.; Morokuma, K. Organometallics 1996,
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2000, 19, 2741.
(12) (a) Biswas, B.; Sugimoto, M.; Sakaki, S. Organometallics 2000, 19,
3895. (b) Fang, X.; Scott, B. L.; Watkin, J. G.; Kubas, G. J.
Organometallics 2000, 19, 4193. (c) Tempel, D. J.; Johnson, L. K.;
Huff, R. L.; White, P. S.; Brookhart, M. J. Am. Chem. Soc. 2000,
122, 6686.
Inorganic Chemistry, Vol. 44, No. 22, 2005 7807

Hristov et al.
Scheme 1. QM/MM Partition of the Molecule Structures
basis. Non-hydrogen atoms were assigned a relativistic frozen-core
potential, treating as core shells up to and including 3d for Pd; 2p
for S, P, and Ni; 1s for C, N, and O. A set of auxiliary s, p, d, f,
and g functions, centered on all nuclei, was used to fit the molecular
density and to represent the Coulomb and exchange potentials
accurately in each SCF cycle. The counterions have not been
included in the model for simplicity. Solvation energies were
calculated from gas-phase structures by using the Conductor-like
Screening Model (COSMO)23 that has been implemented24a recently
into the ADF program.24b The solvation calculations were performed
with a dielectric constant of 2.38 for toluene. The radii used for
the atoms (in Å) are as follows: H 1.16, S 1.7, C 2.3, O 1.3, N
1.4, P 2.4, Ni 1.25, Pd 1.38. Some of these values were obtained
previously by optimization using least-squares fitting to experi-
mental solvation energies.25 Transition states were fully optimized
using the algorithm of Banerjee et al.26 starting from the structures
obtained by linear transit calculations. The Brookhart and the O-P
chelate systems were calculated using the QM/MM method.27 An
augmented AMBER95 force field28 was utilized to describe the
molecular mechanics potential. The separation of the catalyst
molecule into a QM and MM part is illustrated in Scheme 1. For
the Grubbs catalyst the complete system was described by QM.
in which the catalyst is protected from the access of water
in the initial stages of the reaction by being employed as a
water-insoluble solid of relatively large particle size, and
latterly by being encapsulated in the hydrophobic amorphous
polyethylene formed. Although no organic products were
isolated in the catalyst decomposition process, it is possible
that the decomposition occurs by a Wacker-type reaction.13
A theoretical discussion of this deactivation will be presented
in a forthcoming paper of possible side reactions that might
lead to catalyst decomposition in aqueous solution.14
Computational Method
All calculations were carried out using the Amsterdam Density
Functional (ADF 2.3.3) program15 developed by Baerends et al.16
and vectorized by Ravenek.17 The numerical integration scheme
applied for the calculations was developed by Velde and co-
workers.18 The geometry optimization procedure was based on the
method of Versluis and Ziegler.19 Geometry optimizations were
carried out and energy differences determined using the local density
approximation of Vosko, Wilk, and Nusair (LDA VWN)20 augu-
mented with the nonlocal gradient correction PW91 from Perdew
and Wang.21 Relativistic corrections were added using a scalar-
relativistic Pauli Hamiltonian.22 All atoms except Ni were described
by a core double-ú, valence triple-ú, polarized basis set, and Ni
was described by a core double-ú, valence triple-ú, doubly polarized
The bond lengths for C-N, C-C, and C-P that link the QM
and MM parts have been calculated for the generic compounds
with a full QM calculation. The capping atoms used were
hydrogens. Each link bond has a constant parameter R associated
with it,29 which was 1.41 for the N-C bond in the Grubbs systems,
1.29 for the C-P bonds, and 1.36 for the C-C bond in the O-P
chelate system.
(13) (a)Siegbahn, P. E. M. J. Am. Chem. Soc. 1995, 117, 5409. (b) Siegbahn,
P. E. M. J. Phys. Chem. 1996, 100, 14762.
(14) DeKock, R. L.; Hristov, I. H.; Anderson, G. D. W.; Gottker-
Schnetmann, I.; Mecking, S.; Ziegler, T. Organometallics 2005, 24,
2679.
(15) Amsterdam Density Functional program, Division of Theoretical
Chemistry, Vrije Universiteit, De Boelelaan 1083, 1081 HV Amster-
dam, The Netherlands; www.scm.com.
(23) Klamt, A.; Schuurmann, G. J. Chem. Soc.-Perkin Trans. 2 1993, 5,
799.
(24) (a) Pye, C. C.; Ziegler, T. Theor. Chem. Acc. 1999, 101, 396. (b)
Velde, G. T.; Bickelhaupt, F. M.; Baerends, E. J.; Guerra, C. F.; Van
Gisbergen, S. J. A.; Snijders, J. G.; Ziegler, T. J. Comput. Chem. 2001,
22, 931.
(16) Baerends, E. J.; Ellis, D. E.; Ros, P. Chem. Phys. 1973, 2, 41.
(17) Ravenek, W. In Algorithms and Applications on Vector and Parallel
Computers; te Riele, H. J. J., Dekker, T. J., van de Horst, H. A., Eds.;
Elsevier: Amsterdam, 1987.
(18) Boerrigter, P. M.; Velde, G. T.; Baerends, E. J. Int. J. Quantum Chem.
1988, 33, 87.
(19) Versluis, L.; Ziegler, T. J. Chem. Phys. 1988, 88, 322.
(20) Vosko, S. H.; Wilk, L.; Nusair, M. Can. J. Phys. 1980, 58, 1200.
(21) Perdew, J. P.; Wang, Y. Phys. ReV. B 1986, 33, 8800.
(22) Snijders, J. G.; Baerends, E. J.; Ros, P. Mol. Phys. 1979, 38, 1909.
(25) Marcus, Y. J. Chem. Soc.-Faraday Trans. 1991, 87, 2995.
(26) (a) Banerjee, A.; Adams, N.; Simons, J.; Shepard, R. J. Phys. Chem.
1985, 89, 52. (b) Fan, L.; Ziegler, T. J. Chem. Phys. 1990, 92, 3645.
(27) (a) Woo, T. K.; Cavallo, L.; Ziegler, T. Theor. Chem. Acc. 1998, 100,
307. (b) Reference 11a.
(28) Cornell, W. D.; Cieplak, P.; Bayly, C. I.; Gould, I. R.; Merz, K. M.;
Ferguson, D. M.; Spellmeyer, D. C.; Fox, T.; Caldwell, J. W.; Kollman,
P. A. J. Am. Chem. Soc. 1995, 117, 5179.
(29) (a) Reference 27a. (b) Woo, T. K. Ph.D. Thesis, University of Calgary,
Canada, 1998.
7808 Inorganic Chemistry, Vol. 44, No. 22, 2005

Side Reactions due to Water in Emulsion Polymerization
Experimental Conditions
General Considerations for NMR Protonolysis Studies of Ni-
(II)-Based Grubbs Complexes Gexp14a,b-(L) [a: R ) 3,5-(CF3)-
C6H3, b: 3,5-Xylyl; R′ ) X ) I: R′′ ) Me; L ) Me2NCH2CH2-
NMe2 (tmeda), Pyridine, PPh3] as Shown in Scheme 1. All
samples were prepared in a nitrogen atmosphere glovebox. Benzene-
d6 was thoroughly degassed by several freeze-pump-thaw cycles
and vacuum-transferred from finely dispersed sodium metal prior
to use. Methanol-d4 and deuterium oxide were degassed. Complexes
Figure 2. cis and trans convention: cis, active binding site is cis to O;
trans, active binding site is trans to O.
Table 1. Calculated π-Complexation Energies for Water and Ethylene
in the Grubbs-Type Complexes
Gexp14a,b-(L) (L ) tmeda, pyridine) were prepared according to
known procedures.5g Gexp14a,b-(PPh3) were prepared in analogy
to the pyridine complexes by reacting Gexp14a,b-(tmeda) with PPh3
and removal of excess PPh3. Complexes Gexp14a,b-(tmeda) are
dynamic mixtures of several isomers in benzene-d6 solution.
However, upon addition of CD3OD to these samples, one major
compound is formed and signals for free tmeda were observed.
NMR samples for protonolysis experiments were prepared by
dissolving 2.8 µmol of the respective complexes Gexp14a,b-(L) in
a J. Young tube in 560 mg of a stock solution prepared from 3.650
g of benzene-d6 and 5.000 g of methanol-d4. The sample was then
transferred out of the glovebox and inserted into the preheated (313
K) probe of a Varian Inova 400 spectrometer. Decay of the Ni-
Me resonances of tmeda-complexes Gexp14a,b-(tmeda) as well as
generation of CH3D and the assumed (N∩O)2Ni chelate were
monitored by 1H NMR experiments over 4-5 half-life times (t1/2).
Protonolysis of Gexp14a,b-(pyridine) was monitored over ca. 1 half-
life while only traces of protonolysis products were observed for
∆E(M-L)c-e ∆E(M-L)
name Ma Xb
R′ R′′
R
L ) H2O
L ) C2H4 ∆∆Ef
G1
G2
G3
G4
G5
G6
G7
G8
G9
Ni
Ni
Ni NO2
Ni
Ni
Ni
Ni NO2
Ni
Ni
H
H
H
CF3
Ph
H
H
H
H
Me
H
H
H
H
-25.92
-23.25
-24.64
-5.86
-17.15
-22.95
-26.29
-25.28
-24.29
-16.11
-24.06
-18.79
-25.18
-25.11
-26.03
-27.48
-9.09
-20.65
-27.46
-31.41
-31.05
-31.03
-23.02
-31.60
-27.03
-36.76
-0.81
2.78
2.84
3.23
3.5
4.51
4.82
5.77
6.74
6.91
7.54
8.24
11.58
H
H
H
H
Ph
H
H
H
iPr
Ph
tBu Ph
H
H
Me
Me
H
H
H
H
H
H
tBu Me
G10 Ni
G11 Ni
G12 Ni
G13 Pd
H
H
H
H
Me Me
Me iPr
Ph
Me
iPr
H
a Metal. b For labeling of the substituents on the Grubbs catalyst see
Figure 1a. c Energies in kcal mol-1 d Complexation energies. e Toluene was
.
used as the solvent. f ∆∆E ) ∆E(M-H2O) - ∆E(M-C2H4).
G
exp14a,b-(PPh3) after 5 d at 313 K.
mentally determined solid-state structures of complexes
R′′ ) Me and L ) acetonitrile8c or pyridine.5g For water the
cis isomer is more stable by around 8 kcal mol-1, while in
the case of ethylene the difference is less than 2 kcal mol-1.
This is to be expected since the trans labilizing ability of
H2O is much smaller than ethylene. The reported results are
for the cis structures. The complexation energies are calcu-
lated relative to the uncoordinated activated complex of
lowest energy with one empty ligand site and collected in
Table 1.
We shall start our discussion of the Grubbs system by
considering the substitution of R′′ ) CH3 (G8) by other
groups while X, R′, and R of Figure 1a are represented by
hydrogen and M ) Ni. The replacement of CH3 by CF3 in
the R′′ position (G1) results in a substantial decrease in
-∆E(M-L) for L) C2H4 and a smaller increase for L )
H2O. The replacement of CH3 by CF3 is the only substitution
where the -∆E(M-L) binding energy was calculated to be
larger for H2O. The group CF3 has a strong electron-
withdrawing effect that will make the metal harder by
accepting more of its d-electron density. This will decrease
the repulsive interaction of the lone pair of oxygen in H2O
with the metal and thus stabilize the M-H2O bond. On the
other hand, the electron-withdrawing ability of CF3 will
disfavor the back-donation of electron density from the metal
to the π* orbital on ethylene and destabilize the M-C2H4
bond.
Results and Discussion
Ethylene vs Water Complexation. We shall now discuss
how the complexation energy of water and ethylene is
influenced by the substituents on the catalysts shown in
Figure 1. The complexation energies ∆E(M-L) of water and
ethylene have been found as the negative of the energy
required to remove L from the most stable form of the
complexes in Figure 1 to produce L ) H2O, C2H4, and the
coordinativly unsaturated complex without L in its most
stable form. A prime after the name of the activated complex
will indicate an aqua complex, a double prime an olefin
complex.
Grubbs’ Systems. The binding of ethylene in the Grubbs
and Brookhart systems has been investigated previously as
part of the polymerization process, where the emphasis was
on the effect of the growing alkyl chain on the ethylene
binding.11b,c We have also studied the binding of ethylene
and R-substituted olefins to Grubbs- and Brookhart-type
complexes in their copolymerization reactions.11d,e
The small size of the Grubbs system allowed us to carry
out full quantum mechanical (QM) calculations based on
DFT for all systems discussed, without resorting to partial
use of molecular mechanics (MM). In this case the electronic
effects in the phenyl group at the nitrogen are treated
explicitly. For the structures involving only one functional
group we examined the cis and trans isomers for both the
water and ethylene complex, Figure 2.
Phenyl complexes (R′′ ) Ph) have been employed as
catalyst precursors, also for emulsion polymerization. Having
a phenyl group in the R′′ position (G2) reduces the
-∆E(M-L) bond energy for L ) H2O and especially L )
C2H4. This is understandable since R′′ ) C5H6 is less
Our study revealed that the cis structures, in which either
water or ethylene are in the position cis to the oxygen, are
more stable. These findings are in accordance with experi-
Inorganic Chemistry, Vol. 44, No. 22, 2005 7809

Hristov et al.
Figure 3. Activated coordinatively unsaturated precursor complexes for Grubbs catalysts with â-agostic interactions.
electron-donating and sterically more demanding. The for-
mation of the cis isomers of the water and ethylene
complexes would also require that the R′′ phenyl is closer
to the N-phenyl ring. Our calculations however do not show
any steric effects, for example, the cis-G2′ aqua complex is
more stable by 7.7 kcal mol-1 over the trans complex.
Isopropyl complexes are of interest as models for the
growing chain in ethylene polymerization in which branching
We shall now turn to the substitution of R ) H on the
aryl ring attached to the nitrogen with other groups such as
R ) CH3 and iPr. The complexation energies we get for the
ethylene complex are several kcal mol-1 lower for R ) CH3
than when R ) H. This reduction is very pronounced with
the methyl groups, where two of the methyl hydrogens form
agostic bonds with the metal center in the coordinatively
unsaturated precursor complex, Figure 3. When R ) Me this
led to the rotation of the associated phenyl ring into the plane
of the complex, decreasing the dihedral angle from around
40° (G8) to 9° (precursor complex G10), see Figure 3. In
i
occurs via “chain running”. When R′′ ) Pr (G4), both
binding energies are very small due to the agostic interactions
between â-hydrogens on the iPr group and the metal center
in the activated coordinatively unsaturated complex, Figure
3. The â-agostic interaction leads to the stretching of the
C-H bond (R ) 1.21 Å) on the â carbon atom and the
formation of a Ni-H link (R ) 1.60 Å). This activated
complex is not very susceptible toward ligand binding with
complexation energies of only -9.1 kcal mol-1 for ethylene
and -5.9 kcal mol-1 for water, Table 1. Another contribution
that can explain the low binding energies is the steric
i
the case of Pr groups the phenyl ring is raised above the
plane of the complex and the dihedral angle with the plane
remains at 40° (G12). Thus, the complex accomplishes the
formation of Ni-H bonds without the increase in energy
associated with the torsion around the dihedral angle. The
length of the Ni-H bonds is between 1.69 (G10) and
1.80 Å (G12).
The two Ni-H bonds (when R ) Me) stabilize the
coordinatively unsaturated precursor complex G10 and
reduce the binding energies for water and ethylene. When
i
repulsion between the Pr group and the phenyl ring on the
N atom. For the trans isomer of the ethylene complex G4′′
or in the case when the phenyl ring is substituted by H, the
bonding energies are around 10 kcal mol-1 higher.11b,h
i
R ) Pr, the â-agostic bond is somewhat weaker and the
∆E(M-L) bonding energies are not reduced to the same
7810 Inorganic Chemistry, Vol. 44, No. 22, 2005

Side Reactions due to Water in Emulsion Polymerization
Table 2. Calculated Complexation Energies for Water and Ethylene in
the Brookhart Type Complexes
∆E(M-L)c-e
L ) H2O
∆E(M-L)
L ) C2H4
name
Ma
R′′b
R
∆∆Ef
B1
B2
B3
Pd
Pd
Ni
Me
Me
Me
Me
iPr
Me
-28.92
-24.05
-31.15
-35.22
-34.74
-34.58
6.30
8.42
3.43
a Metal. b For labeling of the substituents on the Brookhart catalyst see
Figure 1b. c Energies in kcal mol-1 d Complexation energies. e Toluene was
.
used as the solvent. f ∆∆E ) ∆E(M-H2O) - ∆E(M-C2H4).
degree as for R ) CH3. In fact, ethylene binds slightly
stronger
to
the
i
R ) Pr systems (G11, G12) than the generic systems with
R ) H, while water always forms weaker bonds. This could
be due to the positive induction effect of the iPr groups that
increases the ability of the metal center to back-donate
electron density to the olefin, which also makes the electron
donation from water less favorable. Out of all compounds
considered based on nickel, complex G12 is the most
promising in terms of olefin preference with ∆Eπ ) 8.24
kcal mol-1.
We discuss finally the substitution of the X ) H and
R′ ) H groups of the aryl on oxygen with X ) NO2 (G3
and G7) or R′ ) tBu (G6 and G9). The acceptor NO2 group
increases the complexation energies for both water and
ethylene, while the difference between them depends on the
R′′ ligand. The tBu group has a very small positive effects
it disfavors the complexation with water and increases the
difference in the complexation energies.
In the last example of the monosubstituted Grubbs systems
we considered replacing Ni for Pd. In this case the com-
plexation energy for water is almost unchanged, whereas
ethylene binds more strongly by about 5.7 kcal mol-1. This
is due to the easier back-donation to ethylene by Pd(II). This
result is in qualitative agreement with the theoretical study30
of ethylene binding to M(NH3)xCl3-x systems (M ) Ni2+,
Pd2+, Pt2+) where the binding energy for the neutral M(NH3)-
Cl2 was 8.9 kcal mol-1 more favorable for Pd. The Pd-based
Grubbs system exhibits the highest olefin preference with
∆Eπ of about 11.6 kcal mol-1, which is even slightly higher
than the values observed for the Brookhart system (see
below).
Figure 4. Activated coordinatively unsaturated precursor complexes for
Brookhart catalysts.
The size of the Brookhart system barred us from conduct-
ing a complete investigation based on full QM calculations.
Instead, use was made of a combined quantum mechanical
and molecular mechanical method (QM/MM); see Compu-
tational Method section. This method does not allow us to
model the formation of a metal-H agostic bond when the
hydrogen atom is part of the N-phenyl group, since N-phenyl
is described by MM. Thus, to explore the feasibility for this
kind of bonding, we performed a full QM calculation on
the precursor complexes (B1, B2, and B3) (Figure 4). We
were not able to locate any minima with a M-H agostic
bond. We contribute this to steric factorssthat is, the
presence of the Me groups on the far side of the N-C-
C-N ring will hinder the rotation of the phenyl ring and
keep the 2,6-Me groups out of the plane of the complex and
away from the metal. It is perhaps surprising that the 2,6-
iPr groups in B2 as well are unable to interact with the metal
given that in the Grubbs system G12 agostic bonds are
The Brookhart System. The olefin complexation in the
Brookhart system has been studied extensively in a recent
paper.31 Here we concentrated on the complexation energies
of the cationic compounds in which all R groups are the
i
formed between R ) Pr and the metal.
i
It is worthwhile to mention two recent experimental
studies12b,c where the activation of a C-H bond on a side
chain in the Brookhart system leads to the elimination of
the methyl ligand as methane accompanied by the formation
of a cyclic structure. This is a type of metalation process.
However, we shall not discuss it any further since it is not
specific for polymerization in an aqueous system.
same (i.e., either Me or Pr) and R′′ ) Me for both Ni an
Pd. The positive charge on the complex affects most strongly
the ability of the metal center to accept electron density from
either water or ethylene, while the back-donation ability of
the metal is somewhat reduced by the charge. In addition,
positive charge on the metal will interact favorably with the
water dipole. The complexation energies for the Brookhart
systems are collected in Table 2.
As there is no agostic bond formation here, any difference
i
in the binding energies for Me and Pr substituents can be
(30) Stromberg, S.; Svensson, M.; Zetterberg, K. Organometallics 1997,
explained as the combination of sterics and (inductive)
16, 3165.
i
electronic effects. In the case of R ) Pr (B2), water is
(31) Woo, T. K.; Blochl, P. E.; Ziegler, T. J. Phys. Chem. A 2000, 104,
121.
particularly destabilized due to the strong electron donation
Inorganic Chemistry, Vol. 44, No. 22, 2005 7811

Hristov et al.
Table 3. Calculated Complexation Energies for Water and Ethylene in
the O-P Chelate Complexes
The results here are comparable to the ones for the Grubbs
system without agostic bonds.
∆E(M-L)c-e
L ) H2O
∆E(M-L)
L ) C2H4
O-P Chelate System. Finally, we briefly examined the
anionic O-P chelate system for both Ni and Pd, Table 3.
Here too the system was separated into QM and MM regions,
see computational details. Shown below in Figure 5 is the
activated anionic complex with Pd.
The high complexation energies of more than 30 kcal
mol-1 for water and 40 kcal mol-1 for ethylene were also
observed in the gas phase, as the solvent effects accounted
for only about 3 kcal mol-1 in the binding energies, Table
3. The negative charge is concentrated on the terminal O
atoms, which allows the metal center to act as a good electron
acceptor. Thus, we observe an increase in the complexation
energies for both water and ethylene. Despite these high
energies the selectivity toward ethylene binding is similar
to the other two systems.
name
Ma
R′′b
∆∆Ef
P1
P2
Pd
Ni
Ph
Ph
-38.22
-41.56
-44.33
-46.78
6.11
5.22
a Metal. b For labeling of the substituents on the O-P chelate catalyst,
see Figure 1c. c Energies in kcal mol-1 d Complexation energies. e Toluene
.
was used as the solvent. f ∆∆E ) ∆E(M-H2O) - ∆E(M-C2H4).
Catalyst Hydrolysis. Hydrolysis resulting in irreversible
loss of the metal-bound methyl or phenyl group (R′′ ) Me
or Ph) can affect the catalyst precursor during the handling
prior to the polymerization reaction, e.g., during preparation
of catalyst precursor miniemulsions.5d,6b Equally, in the active
species during polymerization, hydrolysis of the growing
chain (R′′ ) alkyl) can result in irreversible deactivation.
Here we have examined two mechanisms for the hydrolysis
and looked at the factors that influence the barrier for that
process.
Grubbs Catalyst Hydrolysis. In the first hydrolysis
mechanism which we studied for the Grubbs system, Figure
6a, water directly attacks the ethylene complex 1 by first
forming a weak associate 2 where H2O occupies the axial
position. The energy diagram for the mechanism is shown
in Figure 7.
Solvent effects disfavor complexation of H2O perpendicu-
lar to the coordination plane of the complex and the
complexation energy for the axial water molecule is zero,
Figure 7. In the transition state structure TS[2-3], Figure
8a, the C-H bond length is 1.25 Å and the ethylene moiety
sits below the plane of the complex. The barrier for this
process is 26.3 kcal mol-1 in solution. In a modification of
this mechanism we considered a bridge of two water
Figure 5. Activated coordinatively unsaturated precursor complexes for
the O-P chelate system.
from the iPr groups and the steric repulsion. Despite the more
favorable back-donation to ethylene the steric effect also
leads to a slightly lower complexation energy for this ligand.
In the case of the methyl complex B1 there is less steric
repulsion and less electron donation to the metal. This serves
to bring the complexation energies of water and ethylene
closer, thus reducing the binding selectivity.
Changing the metal from M ) Pd in B1 to M ) Ni in B3
increases the water binding energy by more than 2 kcal mol-1
as nickel is more oxophilic since the occupied d-orbital
overlaps less with the occupied oxygen lone pair. At the same
time the ethylene complexation energy is reduced slightly.
Figure 6. Hydrolysis mechanisms for the Grubbs system (a) external water attack and (b) in the plane attack.
7812 Inorganic Chemistry, Vol. 44, No. 22, 2005

Side Reactions due to Water in Emulsion Polymerization
hydrogen transfer takes place in the same plane, Figure 6b.
This pathway seems plausible in view of the small energy
required for water to replace ethylene (see Table 1). The
energy diagram for the generic Grubbs catalyst is shown in
Figure 9a.
The barrier that corresponds to the transition state structure
TS[4-5], Figure 8b, is 16.8 kcal mol-1 in solution. Here
the bond length of the C-H linkage is 1.30 Å which is very
similar to the one observed in TS[2-3]. The product of the
reaction is the tri-coordinated hydroxy complex 6, which
could dimerize to a µ-hydroxo bridged dimer.32
The mechanism outlined in Figure 6b is similar to the
reverse of methane C-H activation by σ-bond metathesis.
The hydrogen atom being transferred to the methyl group
does not stay in the plane of the complex during the entire
transition. For C-H bond distances below 1.30 Å the C-H
bond is normal to the plane of the complex with Ni forming
a σ-adduct with the C-H bond, Figure 10.
Preliminary experimental investigations33 toward the hy-
drolysis process were performed with salicylaldiminato
complexes Gexp14a,b-(L) [a: R ) 3,5-(CF3)C6H3, b: 3,5-
xylyl; R′ ) X ) I: R′′ ) Me; L ) Me2NCH2CH2NMe2
(tmeda)]5g (Figure 11) which in benzene-d6 solution exist as
Figure 7. Energy diagram for the external hydrolysis of the generic Grubbs
catalysts. Energies in kcal mol-1 in toluene.
molecules for the hydrolysis in which the first water supplies
the OH group to the metal and the second water the H to
the alkyl group. We obtained a barrier of 24.1 kcal mol-1
for this process. Thus, the use of a water bridge is not likely
to lower the barrier for this mechanism when one takes into
account possible entropic costs in forming the bridge (see
below).
In the second mechanism that we investigated, water first
replaces ethylene in the plane of the complex and the
Figure 8. Geometry of the transition state structures (a) for external hydrolysis and (b) for hydrolysis in the plane, for the generic Grubbs catalyst.
Inorganic Chemistry, Vol. 44, No. 22, 2005 7813

Hristov et al.
Figure 9. Energy diagram for the hydrolysis of the Grubbs catalysts. Energies in kcal mol-1 in toluene.
Figure 10. Intermediate structure with R(C-H) ) 1.20 Å for the in the plane hydrolysis of the generic Grubbs system.
isomeric mixtures. As these complexes are water-insoluble
and only sparingly soluble in methanol, protonolysis of
Gexp14a,b-(tmeda) was studied by 1H NMR experiments in
ca. 5 mM solutions of benzene-d6/CD3OD mixtures (ca 2.6
M in benzene-d6/8.4 M in CD3OD) at initially 298 K. Under
these conditions one isomer in each case accounts for more
than 90% of the sample on the basis of the integration of
the Ni-Me and the resonances of the backbone ligand while
the signals of tmeda fit that of free tmeda. We therefore
assume that CD3OD displaces tmeda on the Ni center in an
equilibrium reaction. Over time gradual evolution of CH3D
is observed in these samples as evidenced by a 1:1:1 triplet
2
with JH-D ) ca. 1.9-2.0 Hz at 0.18 ppm while the reso-
nances corresponding to the Ni-Me complexes disappear
and one new set of resonances for the salicylaldiminato
ligand bond to Ni grows in. We have assigned the newly
formed Ni complexes as (N∩O)2Ni but cannot rule out a
methoxy brigded dimer (N∩O)Ni(µ-OCD3)2Ni(N∩O) (Scheme
2).34
(32) Svejda, S. A.; Johnson, L. K.; Brookhart, M. J. Am. Chem. Soc. 1999,
121, 10634.
(33) DeKock, R. L.; Hristov, I. H.; Andersen, G. D. W.; Go¨ttker-
Schnetmann, I.; Mecking, S.; Ziegler, T. Organometallics 2005, 24,
2679-2687.
(34) X-ray diffraction analysis of crystals obtained from the NMR tube
experiments of Gexp1a-(tmeda) support the formation of (N∩O)2Ni;
however, the quality of the crystals was poor.
7814 Inorganic Chemistry, Vol. 44, No. 22, 2005

Side Reactions due to Water in Emulsion Polymerization
Figure 11. Structures of Grubbs-type complexes used in protonolysis studies.
1
Scheme 2. Protonolysis of Grubbs-Type Complexes in CD3OD/Benzene-d6 Mixtures Observed by H NMR Experiments
The half-life time (t1/2) of the reaction is ca. 23 min for
complex Gexp14a-(tmeda) and 18 min for complex Gexp14b-
(tmeda) at 313 K. Substitution of L ) tmeda in complexes
Gexp14a,b-(L) by pyridine or PPh3 results in improved
stability toward protonolysis with t1/2 ) ca. 6 h [Gexp14a-
(pyridine)], ca. 4 h [Gexp14b-(pyridine)], and .5 d
[Gexp14a,b-(PPh3)] under identical reaction conditions. Other
than with complexes Gexp14a,b-(tmeda), the respective
pyridine or PPh3 complexes Gexp14a,b-(pyridine/PPh3) are
the only observed ground states of Ni-Me-containing species
in these reaction mixtures. As for hydrolysis, addition of 10
vol % of deuterated water, e.g., to benzene-d6/CD3OD
solutions of Gexp14a,b-(tmeda), did not influence the reaction
rates substantially. The increasing stability of complexes
Gexp14a,b-(L) toward protonolysis on going from L ) tmeda
to PPh3 mirrors the expected stabilization of the coordina-
tively unsaturated (N∩O)Ni-Me fragments by L. Likewise,
we assume that this stabilization is higher for the more
electron-deficient Ni center in complexes Gexp14a-(L) when
compared to Gexp14b-(L). NMR assignments are given in
the Supporting Information.
The aforementioned theoretical studies indicate that pro-
tonolysis is likely to occur, but will not be an extremely fast
reaction. Also, comparison of the calculated insertion barrier
from previous theoretical studies on salicylaldiminato com-
plexes with the calculated barrier to hydrolysis indicated that
protonolysis is likely to occur in polymerization experiments,
but will not outrun polymerization entirely as chain growth
is sufficiently rapid. In this sense, the preliminary experi-
mental data is in good agreement with the theoretical results.
Since the barrier for the in-the-plane water attack was
smaller than the barrier for the external water attack, at least
for the generic Grubbs system, we decided to investigate
only the former mechanism for the other structures. The
substitution of the methyl ligand for phenyl (Figure 9c) leads
to an increase in the barrier of hydrolysis by about 2 kcal
mol-1, which is due to the stronger M-phenyl bond and the
steric repulsion accompanying the formation of benzene.
Here we also looked at the hydrolysis of the G12 complex
(Figure 9d) which exhibited the highest selectivity toward
ethylene binding from all Ni-based Grubbs complexes. The
barrier increased marginally, which is probably due to a
Inorganic Chemistry, Vol. 44, No. 22, 2005 7815

Hristov et al.
Figure 12. Energy diagram for the hydrolysis of the Brookhart catalysts. Energies in kcal mol-1 in toluene.
Figure 13. Solvent-assisted hydrolysis mechanism for the Brookhart system, internal water attack.
further increase in the M-phenyl bond strength as the 2,6-
iPr groups increase the back-donation ability of the metal.
The steric requirements for the reaction make it impossible
for a product complex of benzene (5 in Figure 6b) to be
observed.
The highest barrier of 21.7 kcal mol-1 was obtained for
the Pd-based system (G13), Figure 9b. A similar hydrolysis
process has been observed before for the formic acid
hydrolysis of a Pd(II)-CH3 complex where the barrier was
28.5 kcal mol-1.35 The results suggest that the Pd-based
Grubbs catalyst that was shown to have the highest prefer-
ence for ethylene complexation will also have a high
hydrolysis barrier, making it the most stable species toward
water of all Grubbs systems studied here. It must be noted,
however, that experimental studies with other neutral pal-
ladium complexes to date have found them to be rather
sluggish polymerization catalysts.36
the complex. The energy diagrams for the Brookhart systems
are shown in Figure 12.
Here the effect of the substituent R group is smallsthe
i
introduction of the electron donating Pr groups in place of
R ) Me results in more stable hydroxo complexes 5 but the
barrier in TS[4-5] is virtually unaffected. The change in
the metal has a stronger influence. The highest barrier is
exhibited by the Pd-based systems (Figure 12a,c) since they
are less oxophilic compared to the Ni-based systems. The
complex shown in Figure 12a (B1) has the highest barrier
of hydrolysis out of all studied compounds. For this reason
a slight modification to the hydrolysis mechanism, involving
the inclusion of an additional two solvent water molecules
in the active site, shown in Figure 13, was studied for this
particular complex. It was reported by Siegbahn13 that water
chains significantly lower the calculated barriers for the
(36) (a) Britovsek, G. J. P.; Keim, W.; Mecking, S.; Sainz, D.; Wagner,
T.; Hemilabile P, O-Ligands in Palladium Catalysed C-C Linkages,
J. Chem. Soc., Chem. Commun. 1993, 1632. (b) Drent, E.; van Dijk,
R.; van Ginkel, R.; van Oort, B.; Pugh, R. I. Chem. Commun. 2002,
964.
Brookhart Catalyst Hydrolysis. For the Brookhart system
we again considered only the water attack in the plane of
(35) Biswas, B.; Sugimoto, M.; Sakaki, S. Organometallics 2000, 19, 3895.
7816 Inorganic Chemistry, Vol. 44, No. 22, 2005

Side Reactions due to Water in Emulsion Polymerization
Figure 14. Energy diagram for the solvent-assisted hydrolysis of the Pd
Brookhart catalyst where R ) Me. Energies in kcal mol-1 in toluene.
Wacker process, and we wished to ascertain whether using
a water chain model would have a similar effect on barrier
for the Brookhart system.
In this mechanism two water molecules bind to 4 to give
a three water molecule chain bound to the complex, with
the tail hydrogen atom of the chain being weakly bound to
the Pd dz2 orbital, to give complex 7. This step is exothermic
by 11.1 kcal mol-1. The first water molecule then provides
the hydroxyl group to the metal, and the third water molecule
provides the proton to the methyl group, with the central
water molecule providing the bridge for the “Proton
Shuttle”37-43 transfer of the proton. The methyl group is thus
hydrolyzed to give the agostic methane-bound structure 8,
which then evolves methane to give structure 9 with the tail
end of the water chain bound to the palladium coordination
site vacated by the methane ligand. Two water molecules
are then lost back to solvent as an ethene molecule binds to
the palladium to give structure 3.
Figure 15. Transition state structure for the solvent-assisted internal
hydrolysis of the Pd Brookhart catalyst B1.
preference is not large enough to compensate for the loss of
the hydrogen bond in the chain, coupled with the greater
nucleophilicity of the water molecule when one of its
hydrogen atoms is involved in hydrogen bonding, leading
to a stronger palladium-oxygen interaction.
However, the entropic cost of binding the additional two
water molecules must be taken into account when considering
whether this solvent-assisted mechanism provides a better
model than the unassisted pathway. Literature figures give
the entropic penalties for ligand binding to a square planar
complex between 9 and 17.2 kcal mol-1, dependent on the
system.44-48 This entropic penalty eclipses the energetic
advantage of the solvent-assisted mechanism, making it an
unlikely pathway.
The energy diagram for this process is shown in Figure
14.
The transition state for the hydrolysis, shown in Figure
15, is distinctly product-like, as would be expected from an
endothermic process, with the two additional water molecules
sited at long hydrogen bonding distances from the hydroxyl
and (nearly fully formed) agostically bound methane ligand.
This solvent-assisted mechanism lowers the energy of the
hydrolysis transition state to 21.7 kcal mol-1, 6.6 kcal mol-1
lower than the barrier found without the solvent assistance,
whereas the endothermicity of the reaction remains the same
at 19.4 kcal mol-1. Loss of the methane molecule to give
the hydroxyl/water chain ligated complex 9 is exothermic
by 17.2 kcal mol-1, giving a structure only 2.0 kcal mol-1
higher in energy than 7. Displacement of the two water
molecules by an ethene molecule to give 3 is endothermic
by a further 4.0 kcal mol-1, making the total process
endothermic by 6.0 kcal mol-1. Although it has been shown
(Table 2, compound B1) that the preference for ethene
binding over water binding is 6.3 kcal mol-1, this energetic
O-P Catalyst Hydrolysis. The energy diagram for the
O-P chelate system is shown in Figure 16.
Here the Ni compounds (Figure 16b,d) exhibit the same
trend as the neutral Grubbs system, while the profiles for
the Pd compounds (Figure 16a,c) are like the ones observed
in the Brookhart complexes. This difference can be explained
by examining the charges on the metal centers in the two
cases. The positive charge on Ni in the O-P chelate system
is very small (0.303), while for Pd it is 0.613. Thus, the Pd
system will have the features of the cationic Brookhart
system, while the Ni system will have features which are
closer to the neutral Grubbs system.
Conclusions
All three catalyst systems investigated here (Figure 1)
show similar trends with regard to the preference of binding
(37) Olsen, L.; Antony, J.; Ryde, U.; Adolph, H. W.; Hemmingsen, L. J.
Phys. Chem. B 2003, 107, 2366.
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Biochemistry 2002, 41, 15429.
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(43) Marx, D.; Tuckerman, M. E.; Hutter, J.; Parrinello, J. Nature 1999,
397, 601.
(44) Mecking, S.; Johnson, L. K.; Wang, L.; Brookhart, M. J. Am. Chem.
Soc. 1998, 120, 888.
(45) Michalak, A.; Ziegler, T. Organometallics 2003, 22, 2069.
(46) Moullet, B.; Zwahlen, C.; Frey, U.; Gervasio, G.; Merbach, A. E. Inorg.
Chim. Acta 1997, 261, 67.
(47) Hallinan, N.; Besancon, V.; Forster, M.; Elbaze, G.; Ducommun, Y.;
Merbach, A. E. Inorg. Chem. 1991, 30, 1112.
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1453.
Inorganic Chemistry, Vol. 44, No. 22, 2005 7817

Hristov et al.
Figure 16. Energy diagram for the hydrolysis of the O-P chelate catalysts. Energies in kcal mol-1 in toluene.
ethene over water, with the greatest preference for ethene
found for palladium rather than nickel. This is understandable
since the more diffuse d-orbitals have better bonding
π-overlaps with the olefin. At the same time, the more diffuse
occupied orbitals of palladium interact more repulsively with
the occupied oxygen lone pairs, leading to a weaker binding
of water. The nature of the substituent groups was also
important in that more electron-donating substituents lead
to a greater preference for ethene over water, though these
effects did not appear to be directly cumulative in that
multiple substitutions did not give rise to significantly higher
preference than single ligand substitutions. The catalyst with
the greatest preference for ethene over water was the Grubbs
catalyst G13, with Pd and unsubstituted ligands.
Hydrolysis of the three catalyst systems (Figure 1)
proceeded in a similar mannerssubstitution of the ethene
ligand with water proved to be a more favorable approach
than axial attack of water on the palladium center. The
structures that showed greater preference for ethene binding
over water also had higher barriers to hydrolysis, with the
Brookhart catalysts showing the greatest resistance to hy-
drolysis. While solvent assistance in the hydrolysis lowered
the energetic barrier, it was not sufficient to offset the
entropic penalty of binding additional water molecules. It is
found that in general that (a) ethylene coordination is
preferred over water coordination for both Ni and Pd catalysts
and (b) hydrolysis of the metal alkyl bond is competitive to
ethylene insertion.
Acknowledgment. This work has been supported by the
National Sciences and Engineering Research Council of
Canada (NSERC). R.D.K. wishes to acknowledge the donors
of the American Chemical Society Petroleum Research Fund
for partial support of this work. T.Z. thanks the Canadian
Government for a Canada Research Chair. G-S. and S.M.
thank the DFG for financial support (AM2Net: Me 1388-
3).
Supporting Information Available: Individual NMR assign-
ments for the experimental investigations into hydrolysis of the
Grubbs catalyst. This material is available free of charge via the
Internet at http://pubs.acs.org.
IC050501R
7818 Inorganic Chemistry, Vol. 44, No. 22, 2005