Oxygenation of nitrogen-coordinated palladium(0): Synthetic, structural, and mechanistic studies and implications for aerobic oxidation catalysis [15]

Full text of DOI:10.1021/ja015683c

7188
J. Am. Chem. Soc. 2001, 123, 7188-7189
Scheme 1
Oxygenation of Nitrogen-Coordinated Palladium(0):
Synthetic, Structural, and Mechanistic Studies and
Implications for Aerobic Oxidation Catalysis
Shannon S. Stahl,* Joseph L. Thorman, Ryan C. Nelson, and
Michael A. Kozee
Department of Chemistry
UniVersity of Wisconsin-Madison
Madison, Wisconsin 53706
ReceiVed February 16, 2001
ReVised Manuscript ReceiVed June 6, 2001
The palladium-catalyzed Wacker process (eq 1) was developed
more than forty years ago1 and remains one of the most successful
aerobic oxidation reactions in the chemical industry. This process
and numerous related reactions require cocatalysts such as copper
salts to mediate dioxygen-coupled oxidation of reduced palladium
during catalysis.2 In contrast, a series of recent reports reveal that
CH2dCH2 + 1/2O2 Pd/Cu cat.8 CH3CHO
(1)
cocatalysts are not always necessary to achieve efficient catalytic
turnover.3 Several of these new oxidation reactions employ a
palladium catalyst with pyridine or bidentate nitrogen ligands,3a-c
suggesting that such ligands promote the reaction between reduced
palladium and dioxygen. A possible catalytic mechanism is shown
in Scheme 1.3b,4,5 We present herein the reaction between dioxygen
and a bathocuproine-palladium(0) complex to form a structurally
characterized peroxopalladium(II) species (step II, Scheme 1).
This molecule reacts with Brønsted acids to release hydrogen
peroxide (step III, Scheme 1). Bathocuproine (bc, 2,9-dimethyl-
4,7-diphenyl-1,10-phenanthroline) has been used successfully in
recent aerobic oxidation reactions.3b Mechanistic insights into the
oxygenation of palladium(0) are also presented.
methyl groups (δ 2.97, 3.21) and two doublets for the coordinated
3
olefin protons (δ 4.47, 4.24, J ) 9.5 Hz).
The brown bathocuproine peroxopalladium(II) complex, 2, was
synthesized by stirring 1 in dichloromethane under an atmosphere
of dry dioxygen for 20-30 min at ambient temperature (eq 3).
Oxygenation of 1 is irreversible, revealed by the ability to isolate
complex 2 by removal of the reaction solvent under vacuum. The
1H NMR spectrum of 2 exhibits a single resonance for the bc
methyl groups (δ 2.89), consistent with a symmetrical structure.
The reaction was monitored by UV-visible spectroscopy (Figure
1) and revealed an isosbestic point at 393 nm suggesting no
intermediates build up during the reaction.7 The charge-transfer
band for 1 in the 420 nm region is similar to that of other diimine
palladium(0)-olefin complexes.6d
The presence of a peroxo moiety is supported by infrared
spectroscopic data, which reveal a strong absorption band at 891
cm-1, similar to other late-transition-metal peroxo complexes.8
The preparation of 2 with 18O2 results in a new band at 839 cm-1
(Figure S1), corresponding to an isotopic shift of 52 cm-1, equal
to the value predicted by a simple diatomic oscillator model for
an O-O stretch. The η2-peroxo coordination mode was estab-
lished by single-crystal X-ray crystallography (Figure 2).9 Only
one other η2-peroxopalladium complex, [Ph(t-Bu)2P]2Pd(O2), 3,
has been structurally characterized.8b Unlike (bc)Pd(O2), 3 binds
Due to the preference of palladium(0) for soft ligands, relatively
few nitrogen-coordinated palladium(0) complexes have been
reported.6 Nevertheless, addition of 2 equiv of bathocuproine to
tris(dibenzylideneacetone)dipalladium(0) in dichloromethane readily
affords the orange, three-coordinate complex, (bc)Pd(η2-dba), 1
1
(eq 2), which was characterized by H and 13C NMR, IR, and
UV-visible spectroscopies and single-crystal X-ray crystal-
lography. The η2-olefin ligand in 1 does not rotate or exchange
on the NMR time scale; the 1H NMR spectrum of 1 features two
sharp singlets corresponding to the inequivalent bathocuproine
(1) Smidt, J.; Hafner, W.; Jira, R.; Sedlmeier, J.; Sieber, R.; Ru¨ttinger, R.;
Kojer, H. Angew. Chem. 1959, 71, 176-182.
(2) Tsuji, J. Palladium Reagents and Catalysts; Wiley: New York, 1995.
(3) For leading references, see: (a) ten Brink, G.-J.; Arends, I. W. C. E.;
Sheldon, R. A. Science 2000, 287, 1636-1639. (b) Bianchi, D.; Bortolo, R.;
D’Aloisio, R.; Ricci, M. Angew. Chem., Int. Ed. 1999, 38, 706-708. (c)
Nishimura, T.; Onoue, T.; Ohe, K.; Uemura, S. J. Org. Chem. 1999, 64, 6750-
6755. (d) Peterson, K. P.; Larock, R. C. J. Org. Chem. 1998, 63, 3185-
3189. (e) Ro¨nn, M.; Andersson, P. G.; Ba¨ckvall, J.-E. Acta Chem. Scand.
1997, 51, 773-777. (f) van Benthem, R. A. T. M.; Hiemstra, H.; van Leeuwen,
P. W. N. M.; Geus, J. W.; Speckamp, W. N. Angew. Chem., Int. Ed. Engl.
1995, 34, 457-460.
(7) The isosbestic behavior shown in Figure 1 arises a few minutes after
initially placing the cuvette in the sample holder. Initial nonisosbestic behavior
(not shown) has been traced to self-association of the (bc)Pd(dba) starting
material; a nonlinear Beer’s law plot is obtained for this species. The products
of the oxygenation reaction, (bc)Pd(O2) and dba, appear to inhibit this self-
association, evidenced by the subsequent isosbestic behavior. The lack of an
1
intermediate in eq 3 was confirmed by monitoring the reaction by H NMR
(4) Thiel, W. R. Angew. Chem., Int. Ed. 1999, 38, 3157-3158.
spectroscopy.
(5) Alternative mechanisms have been proposed. These include dioxygen
insertion into a palladium(II) hydride (ref 3c) and binuclear activation of
dioxygen by 2 equiv of palladium(0) (ref 3a). We consider these possibilities
less likely, but the present work does not necessarily exclude them.
(6) (a) Ito, T.; Hasegawa, S.; Takahashi, Y.; Ishii, Y. J. Organomet. Chem.
1974, 73, 401-409. (b) Pierpont, C. G.; Buchanan, R. M.; Downs, H. H. J.
Organomet. Chem. 1977, 124, 103-112. (c) Sustmann, R.; Lau, J.; Zipp, M.
Recl. TraV. Chim. Pays-Bas 1986, 105, 356-359. (d) van Asselt, R.; Elsevier,
C. J.; Smeets, W. J. J.; Spek, A. L. Inorg. Chem. 1994, 33, 1521-1531. (e)
Klein, R. A.; Elsevier, C. J.; Hartl, F. Organometallics 1997, 16, 1284-1291.
(f) Milani, B.; Anzilutti, A.; Vicentini, L.; Sessanta o Santi, A.; Zangrando,
E.; Geremia, S.; Mestroni, G. Organometallics 1997, 16, 5064-5075. (g)
Elsevier, C. J. Coord. Chem. ReV. 1999, 185-186, 809-822.
(8) For peroxopalladium(II) complexes bearing soft donor ligands, see: (a)
Valentine, J. S. Chem. ReV. 1973, 73, 235-245. (b) Yoshida, T.; Tatsumi,
K.; Matsumoto, M.; Nakatsu, K.; Nakamura, A.; Fueno, T.; Otsuka, S. NouV.
J. Chim. 1979, 3, 761-774.
(9) Crystal data for 2: monoclinic space group C2/c, a ) 17.671(3) Å,
b ) 21.326(5) Å, c ) 27.367(6) Å, â ) 90.510(4)°, V ) 10313(4) Å3, Z )
16, T ) 133 K, µ(Mo K) ) 0.749 mm-1, 9034 unique reflections, wR )
0.3192, R1[I>2σ] ) 0.1054. The structure of 3 was redetermined at 173 K
(cf. ref 8b, 248 K) in order to obtain improved structural resolution. Crystal
data for 3: orthorhombic space group P212121, a ) 25.6846(8) Å, b ) 12.2033-
(4) Å, c ) 11.0834(3) Å, V ) 3473.95(18) Å3, Z ) 4, T ) 173 K,
µ(Mo K) ) 0.654 mm-1, 7011 unique reflections, wR2[I>2σ] ) 0.0756,
R1[I>2σ] ) 0.0355.
10.1021/ja015683c CCC: $20.00 © 2001 American Chemical Society
Published on Web 06/29/2001

Communications to the Editor
J. Am. Chem. Soc., Vol. 123, No. 29, 2001 7189
Figure 3. pO2 dependence on the formation of 2 from 1 and O2. The
nonzero y-intercept reflects a slow background decomposition of 1 in
CH2Cl2.
Figure 1. UV-visible absorption spectra obtained during the conversion
of 1 to 2 in CH2Cl2 (23 °C, pO2 ) 1 atm, [1] ) 220 µM). Inset: Kinetic
data (b) at 375 and 430 nm with first-order fits to the data (t1/2 ) 8.2(4)
min).
activation, ∆Sq ) -43(7) eu. Together, these data support an
associative oxygenation mechanism. The lack of inhibition by
added dba rules out preequilibrium or steady-state loss of dba
prior to dioxygen addition. Thus, although oxygenation of 1
represents a formal oxidative addition step, the reaction is
mechanistically more closely related to olefin substitution.
As suggested in Scheme 1, substrate oxidation (step I) requires
labile coordination sites on the palladium(II) catalyst. These sites
may be created by protonolysis of the basic peroxo ligand in 2.
Addition of CH3CO2H or H2SO4 to 2 quantitatively forms
(bc)PdX2 [X2 ) (OAc)2, SO4] based on 1H NMR analysis of the
palladium species.13a A 73% yield of hydrogen peroxide was
obtained via a colorimetric assay.13b,14
The chemistry described above demonstrates the chemical
viability of steps II and III of the proposed mechanism in Scheme
1. Initial calculations suggest these steps are also kinetically
competent to participate in the catalytic reactions;3b,15 however,
the reaction conditions are sufficiently different to warrant further
investigation, especially in light of alternative mechanistic propos-
als.5
Figure 2. Molecular structure of (bc)Pd(O2), 2. Thermal ellipsoids are
drawn at 50% probability.
dioxygen reversibly, yet despite this reactivity difference, both
complexes exhibit similar structural and spectroscopic features.10
It is reasonable to expect that the oxygenation of 1 should
exhibit mechanistic similarities to reactions of other substrates
with palladium(0) such as oxidative addition of aryl halides11 or
olefin substitution.6d,12 These two reactions generally proceed by
different mechanisms. Aryl halides typically add to a 14-electron
L2Pd(0) fragment that arises from endergonic ligand dissociation
from a 16-electron species such as L2Pd(dba) (L ) phosphine),
whereas olefin substitution reactions, e.g., by another olefin, are
dominated by associative mechanisms in which tetrahedral, 18-
electron intermediates are proposed.
Kinetic studies of the oxygenation reaction revealed a first-
order dependence on dioxygen pressure (Figure 3), whereas
addition of up to 10 equiv of dba had no effect on the rate (Figure
S3). Activation parameters obtained from a temperature-
dependence study revealed a substantial negative entropy of
Acknowledgment. We gratefully acknowledge financial support from
the University of Wisconsin-Madison, the Camille and Henry Dreyfus
Foundation, and Merck Research Laboratories, and a generous donation
of laboratory equipment by Abbott Laboratories. We appreciated crystal-
lographic assistance from Dr. Ilia A. Guzei. The National Science
Foundation provided financial support for NMR and X-ray instrumenta-
tion.
Supporting Information Available: Experimental procedures, spec-
troscopic data, kinetics data, and X-ray crystallographic tables for
complexes 1, 2, and 3 (PDF). This material is available free of charge
via the Internet at http://pubs.acs.org.
JA015683C
(13) (a) An NMR tube was charged with 2 (10.46 mmol), 1,3,5-(tBu)3-
C6H3 as internal standard (2.66 µmol), CH3CO2H (40.2 µmol), and CD2Cl2
(ca. 0.63 mL). The solution immediately turns light yellow. All (bc)Pd(O2)
had been consumed and (bc)Pd(OAc)2 (10.25 µmol, 98% yield) was observed
by 1H NMR after 5 min. (b) Literature procedures were utilized in the
spectrophotometric detection of H2O2 as peroxotitanyl species: (i) Eisenberg,
G. M. Ind. Eng. Chem. Anal. Ed. 1943, 15, 327-328. (ii) Werner, W. U.S.
Patent 4,908,323, 1990.
(14) This can be compared with other known Pt and Pd complexes: (a)
Muto, S.; Ogata, H.; Kamiya, Y. Chem. Lett. 1975, 809-812. (b) Muto, S.;
Kamiya, Y. Bull. Chem. Soc. Jpn. 1976, 49, 2587-2589. (c) Muto, S.; Tasaka,
K.; Kamiya, Y. Bull. Chem. Soc. Jpn. 1977, 50, 2493-2494.
(15) The temperature dependence of the oxygenation reaction (Figure S2)
can be used to determine its rate under conditions comparable to those in the
(bc)Pd(OAc)2-catalyzed oxidation of carbon monoxide (70 °C, pO2 ) 64 atm).
This rate, 0.79 s-1, compares favorably with the maximum catalytic turnover
frequency, 0.16 s-1 (ref 3b).
(10) Complex 2, νO-O ) 891 cm-1, dO-O ) 1.411(18) Å (average). Complex
3, νO-O ) 916 cm-1, dO-O ) 1.412(4) Å.
(11) (a) Amatore, C.; Jutand, A. Coord. Chem. ReV. 1998, 178-180, 511-
528. (b) Alcazar-Roman, L. M.; Hartwig, J. F.; Rheingold, A. L.; Liable-
Sands, L. M.; Guzei, I. A. J. Am. Chem. Soc. 2000, 122, 4618-4630. (c)
Portnoy, M.; Milstein, D. Organometallics 1993, 12, 1665-1673.
(12) See, for example: (a) Cheng, P.-T.; Cook, C. D.; Nyburg, S. C.; Wan,
K. Y. Inorg. Chem. 1971, 10, 2210-2213. (b) Ozawa, F.; Ito, T.; Nakamura,
Y.; Yamamoto, A. J. Organomet. Chem. 1979, 168, 375-391. (c) Go´mez-de
la Torre, F.; Jalo´n, F. A.; Lo´pez-Agenjo, A.; Manzano, B. R.; Rodr´ıguez, A.;
Sturm, T.; Weissensteiner, W.; Mart´ınez-Ripoll, M. Organometallics 1998,
17, 4634-4644. (d) Klein, R. A.; Witte, P.; van Belzen, R.; Fraanje, J.;
Goubitz, K.; Numan, M.; Schenk, H.; Ernsting, J. M.; Elsevier, C. J. Eur. J.
Inorg. Chem. 1998, 319-330. (e) Reid, S. M.; Mague, J. T.; Fink, M. J. J.
Organomet. Chem. 2000, 616, 10-18.