Inverse-electron-demand ligand substitution in palladium(0)-olefin complexes

Article abstract of DOI:10.1021/ja028738z

Ligand substitution reactions are ubiquitous in transition-metal chemistry and catalysis. Investigation of ligand substitution reactions for a series of electron-rich palladium(0)-olefin complexes, (bathocuproine)Pd(nitrostyrene) reveals an unprecedented mechanism in which the metal serves as the nucleophilic partner in an associative substitution pathway. Copyright

Full text of DOI:10.1021/ja028738z

Published on Web 12/05/2002
“Inverse-Electron-Demand” Ligand Substitution in Palladium(0)-Olefin
Complexes
Shannon S. Stahl,* Joseph L. Thorman, Namal de Silva, Ilia A. Guzei, and Robert W. Clark
Department of Chemistry, UniVersity of Wisconsin-Madison, 1101 UniVersity AVenue, Madison, Wisconsin 53706
Received September 28, 2002; E-mail: stahl@chem.wisc.edu
Palladium(0) participates in numerous catalytic reactions, includ-
ing carbon-carbon and carbon-heteroatom cross-coupling, alkyne
reductive coupling, and carbonylation reactions.1 Our interest in
the reactivity of palladium(0) derives from its proposed role in
dioxygen-coupled oxidation reactions.2,3 Mechanistic studies of the
reaction between molecular oxygen and a palladium(0) species, (bc)-
Pd(dba),3a (bc ) bathocuproine, i.e., 2,9-dimethyl-4,7-diphenyl-
1,10-phenanthroline; dba ) dibenzylidene acetone) indicated that
this reaction proceeds by an associative mechanism, resembling
Figure 1. (A) ORTEP drawing of the p-bromo-â-nitrostyrene adduct of
associative olefin substitution at palladium(0) (eqs 1 and 2).4 The
palladium(0). (B) Frontal view reflecting the nearly planar palladium(0)
comparison between these two related reactions belies the distinct
coordination environment. Thermal ellipsoids are shown at the 30%
electronic structures of the substrates: olefins have well-defined
donor (filled π) and acceptor (empty π*) orbitals, whereas dioxygen
has two singly occupied π* orbitals resulting in a triplet electronic
ground-state. In this context, we have undertaken an investigation
of olefin substitution reactions at bathocuproine-palladium(0)
complexes, (bc)Pd(nsX) (nsX ) trans-â-nitrostyrene derivative) to
probe their mechanistic details. We describe here the initial results
of this study that contradict the general model for ligand substitution
reactions, specifically the description of the incoming ligand and
the metal as nucleophilic and electrophilic reaction partners,
respectively. The associative substitution reactions described below
proceed by an “inverse-electron-demand” pathway involving nu-
cleophilic attack of the metal on the external ligand. The principles
revealed in this study should apply to numerous other electron-
rich transition-metal systems, which are finding growing application
in homogeneous catalysis.
probablility level.
Figure 2. Olefin concentration dependence of the intermolecular exchange
rate between (bc)Pd(nsCF ) and ns based on 1H NMR line-broadening
CF3
3
kinetics. Reaction conditions: [Pd] ) 10 mM, [nsCF ] ) 24-91 mM,
3
CD2Cl2, 286 K.
to achieve an equilibrium mixture of the palladium(0)-olefin
complexes (eq 3). Temperature-dependent equilibrium constants
were obtained for individual pairs of olefins. The relative binding
constants for the entire series of olefins yields a linear Hammett
correlation with a positive slope (F ) 3.9), that is, derivatives with
more electron-withdrawing substituents exhibit greater stability.6
The self-exchange reactions between (bc)Pd(nsX) and the cor-
responding nsX derivative are amenable to study by dynamic NMR
Previous studies of zerovalent, group 10 transition metals have
revealed a relatively flat energy landscape for two-, three-, and four-
coordinate complexes.5 Consequently, both associative and dis-
sociative ligand substitution mechanisms have been characterized.
To permit a direct comparison between oxygenation and olefin
substitution, a systematic series of palladium(0)-olefin complexes
was prepared by addition of 1.2 equiv of a para-substituted trans-
â-nitrostyrene derivative (nsX), XC6H4CHdCHNO2 (X ) CH3O,
CH3, H, Br, CF3), to (bc)Pd(dba) in dichloromethane. Single crystals
suitable for X-ray diffraction were obtained for several of the
analogues (X ) H, Br, CH3), and the molecular geometry exhibits
the trigonal planar structure expected for palladium(0)-olefin
complexes (Figure 1).
1
spectroscopy. Line-widths of H NMR resonances of the bound
olefin were monitored as a function of temperature (nsCF ,OCH ,CH
)
3
3
3
and the concentration of added olefin (nsCF ). Eyring plots derived
from the temperature-dependence data reveal similar activation
parameters for each of the three olefins: ∆Hq ) 7.3, 8.2, 7.2 kcal/
3
OCH3
mol and ∆Sq ) -21, -20, and -24 eu for nsCF , ns
, and nsCH3
,
3
respectively. The [olefin]-dependence study with nsCF (Figure 2)
revealed a first-order fit that intersects the y-axis at the origin.
Together, these data support an associative ligand substitution
pathway with essentially no contribution from a parallel dissociative
pathway.
3
In solution at room temperature, the (bc)Pd(nsX) complexes are
nonfluxional on the NMR time scale; however, upon addition of a
nitrostyrene derivative to this solution, rapid exchange takes place
9
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J. AM. CHEM. SOC. 2003, 125, 12-13
10.1021/ja028738z CCC: $25.00 © 2003 American Chemical Society

C O M M U N I C A T I O N S
the results described here appear to be the first demonstration of
an inverted electronic dependence in ligand substitution.10,11
Mechanistically, this reactivity is closely related to oxidative
addition reactions that proceed by nucleophilic attack of the metal
on the substrate, as in the addition of methyl iodide to iridium(I)
in Vaska’s complex.12 We have initiated computational studies to
probe details of the orbital interactions in these reactions, in
particular to compare the reactivity between olefins and dioxygen
with palladium(0). The data described herein, in addition to their
fundamental value, have implications for the design and develop-
ment of electron-rich transition-metal catalysts, which find wide-
spread application in catalytic transformations.
Figure 3. Approach-to-equilibrium absorbance data for the intermolecular
H
exchange between (bc)Pd(nsCH ) and ns . Reaction conditions: [(bc)Pd-
3
H
(nsCH )] ) 100 µmol, [ns ] ) 140 µmol, 2.5 mL CH2Cl2, 300 K.
3
Acknowledgment. This work was supported by the Camille
and Henry Dreyfus Foundation (New Faculty Award), Merck
Research Laboratories, Research Corporation (Innovation Award),
and the NSF (CAREER Award, CHE-0094344). We thank Profes-
sor Clark Landis for training in numerical data fitting methods.
Supporting Information Available: Synthesis and characterization
details (including crystallographic data) for (bc)Pd(nsX) derivatives and
description of kinetics experiments (PDF). This material is available
free of charge via the Internet at http://pubs.acs.org.
Figure 4. Hammett plot reflecting electronic effects on ligand substitution
of nsCH from (bc)Pd(nsCH ) by p-substituted nitrostyrene derivatives.
3
3
References
(1) (a) Heck, R. F. Palladium Reagents in Organic Syntheses; Academic
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(4) (a) Ozawa, F.; Ito, T.; Nakamura, Y.; Yamamoto, A. J. Organomet. Chem.
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Figure 5. Qualitative molecular orbital representations of normal- and
inverse electron demand “associative” ligand substitution reactions.
(5) See ref 4 and Halpern, J.; Weil, T. A. J. Chem. Soc., Chem. Commun.
1973, 631-632.
The nondegenerate exchange between (bc)Pd(nsX) and different
(6) For leading references describing similar equilibrium electronic effects
for other zerovalent, group 10 olefin complexes, see: (a) Otsuka, S.;
Yoshida, T.; Tatsuno, Y. J. Am. Chem. Soc. 1971, 93, 6462-6469. (b)
Tolman, C. A. J. Am. Chem. Soc. 1974, 96, 2780-2789. (c) Ittel, S. D.;
Ibers, J. A. AdV. Organomet. Chem. 1976, 14, 33. (d) Ittel, S. D. Inorg.
Chem. 1977, 16, 2589-2597.
(7) This nomenclature is borrowed from the frontier-orbital description of
Diels-Alder and related cycloaddition reactions: Fleming, I. Frontier
Orbitals and Organic Chemical Reactions; Wiley: New York, 1976.
(8) (a) Langford, C. H.; Gray, H. B. Ligand Substitution Processes; W. A.
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of Inorganic Reactions; 2nd ed.; Wiley: New York, 1967.
olefins, nsX′, for which X * X′, is too rapid to monitor by NMR
methods. Instead, the reaction between (bc)Pd(nsCH ) and four
3
different para-substituted nitrostyrene derivatives was studied by
UV-visible spectroscopy. Kinetics data for these bimolecular
exchange reactions (e.g., Figure 3) were analyzed by numerical
integration to obtain forward and backward rate constants. The
Hammett analysis (Figure 4) of the forward rate constants reveals
that exchange rates are faster with more electron-withdrawing
substituents on the incoming olefin, namely less nucleophilic
substrates react more readily in the associatiVe ligand substitution
reaction. This observation implicates an “inverse-electron-demand”
ligand substitution pathway in which the primary electronic
interaction in the transition state occurs between a filled palladium
orbital donating into the empty olefin π* orbital (Figure 5).7 Such
orbital energetics are favored by the use of palladium(0), which
has fully occupied 4d orbitals, together with an electron-deficient
olefin bearing a low-lying LUMO. This transition-state argument
is linked to thermodynamic considerations via the Hammond
postulate, namely ligands with enhanced π-acidity might be
expected to stabilize a high-energy tetrahedral intermediate and
thereby promote the substitution reaction.
(9) The biphilic, i.e., both nucleophilic and electrophilic, character of certain
ligands (NO2-, SeCN-, and thiourea) has been identified in ligand
substitution reactions based on (1) their relative reactivity toward platinum-
(II) complexes bearing different charges and (2) deviations from predicted
rates based on their nucleophilicity parameter, nPt. See, for example: (a)
Pearson, R. G.; Gray, H. B.; Basolo, F. J. Am. Chem. Soc. 1960, 82, 787-
792. (b) Gray, H. B. J. Am. Chem. Soc. 1962, 84, 1548-1552. (c) Belluco,
U.; Cattalini, L.; Turco, A. J. Am. Chem. Soc. 1964, 86, 226-229.
(10) The following reference states explicitly that such “electrophilic” ligand
substitution pathways are unknown, and we are unaware of any subsequent
precedent. Collman, J. P.; Hegedus, L. S.; Norton, J. R.; Finke, R. G.
Principles and Applications of Organotransition Metal Chemistry; 2nd
ed.; University Science Books: Mill Valley, CA, 1987; p 238.
(11) Anomalous electronic effects observed recently in ligand substitution
reactions of electrophilic platinum(II) and iridium(III) complexes have
been attributed to ground-state stabilization effects. The mechanism in
these cases remains consistent with attack of a nucleophilic ligand on an
electrophilic metal center. (a) Zhong, H. A.; Labinger, J. A.; Bercaw, J.
E. J. Am. Chem. Soc. 2002, 124, 1378-1399. (2) Tellers, D. M.; Yung,
C. M.; Arndtsen, B. A.; Adamson, D. R.; Bergman, R. G. J. Am. Chem.
Soc. 2002, 124, 1400-1410.
π-Acceptor contributions to ligand substitution reactions, for
example, in the trans-effect, have been recognized since the early
seminal studies of ligand substitution at platinum(II).8,9 However,
(12) Reference 10, Chapter 5.
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