Synthesis and catalytic applications of stable palladium dioxygen complexes

Article abstract of DOI:10.1021/om1003418

The synthesis and characterization of air-stable palladium dioxygen complexes Pd(O₂)L₂ of basic phosphines L = P(1-Ad) ₂ⁿBu, P^tBu₂ⁿBu is described. Contrary to general belief, these dioxygen complexes are stable catalyst precursors for palladium-catalyzed coupling reactions. Specifically, palladium-catalyzed formylations and alkoxy carbonylations of aryl bromides proceed in high yield in air using commercial non-degassed solvents. It is shown that Pd(O₂)L₂ complexes are reduced back to the catalytically active palladium(0) species in the reductive atmosphere of CO or H₂ at the onset of the carbonylation reaction.

Full text of DOI:10.1021/om1003418

3368 Organometallics 2010, 29, 3368–3373
DOI: 10.1021/om1003418
Synthesis and Catalytic Applications of Stable Palladium Dioxygen
Complexes
Alexey G. Sergeev, Helfried Neumann, Anke Spannenberg, and Matthias Beller*
€
Leibniz-Institut fu€r Katalyse e.V. an der Universitat Rostock, Albert-Einstein-Strasse 29a,
18059 Rostock, Germany
Received April 23, 2010
The synthesis and characterization of air-stable palladium dioxygen complexes Pd(O2)L2 of basic
phosphines L = P(1-Ad)2 Bu, PtBu2 Bu is described. Contrary to general belief, these dioxygen
complexes are stable catalyst precursors for palladium-catalyzed coupling reactions. Specifically,
palladium-catalyzed formylations and alkoxy carbonylations of aryl bromides proceed in high yield
in air using commercial non-degassed solvents. It is shown that Pd(O2)L2 complexes are reduced back
to the catalytically active palladium(0) species in the reductive atmosphere of CO or H2 at the onset of
the carbonylation reaction.
n
n
Palladium(0)-catalyzed transformations have become an
essential tool in the contemporary arsenal of organic che-
mists due to their versatility, functional group tolerance,
broad scope, and mild reaction conditions.1,2 Generally,
catalysts for these transformations are comprised of a palla-
dium precursor and phosphine3 or heterocyclic carbene
ligands or, in the latter case, their salts.4 Most of the known
precatalysts are fairly air stable and can be conveniently
weighted and handled in air as solids without taking special
precautions. However, being dissolved in organic solvents
during preparation of the reaction mixture, these precata-
lysts form highly air sensitive palladium(0) phosphine or
carbene complexes. To avoid rapid catalyst decomposition,
reaction mixtures for cross-coupling reactions should be
prepared under rigorously air free conditions using either
Schlenk or glovebox techniques. Obviously, the develop-
ment of more convenient air-friendly protocols would
simplify synthetic applications of palladium-catalyzed reac-
tions, making them more straightforward and practical.
Herein, we describe a simple and efficient air-tolerant
protocol for the palladium-catalyzed carbonylation of aryl
bromides based on the application of novel stable palladium
dioxygen complexes of di-1-adamantyl-n-butylphosphine and
di-tert-butyl-n-butylphosphine. The method tolerates prepara-
tion of the reaction mixtures in air using non-degassed solvents,
thus providing a considerable improvement in simplicity over
previously reported carbonylation methods.
Results and Discussion
Recently, we reported improved protocols for Pd-catalyzed
alkoxycarbonylations and formylations of aryl halides.5,6
Crucial for the formation of the active catalyst was the use of
n
bulky, unsymmetrical alkyl phosphines: either P(1-Ad)2 Bu or
n
PtBu2 Bu as ligand. On the basis of our studies, the formyla-
tion with synthesis gas (CO/H2) in the presence of Pd/P(1-
n
Ad)2 Bu is also performed in industry on a scale of tons,
demonstrating the practical value of these transformations.7
Stoichiometric studies of the formylation of aryl bromides
n
catalyzed by Pd/P(1-Ad)2 Bu demonstrated the correspond-
ing hydrobromide palladium complex as the resting state of
the catalyst, even in the presence of a large excess of base.
Generation of the catalytically active Pd(0) species in small
concentration upon reversible deprotonation of this complex
was found to be responsible for the longevity of the catalyst.8
Intrigued by these results, we continued to investigate the
organometallic chemistry of palladium complexes bearing
*To whom correspondence should be addressed. E-mail: matthias.
beller@catalysis.de.
(1) (a) Li, J. J.; Limberakis, C.; Pflum., D. A. Modern Organic
Synthesis in the Laboratory. A Collection of Standard Experimental
Procedures; Oxford University Press: New York, 2007. (b) Miyaura, N.,
Ed. Cross Coupling Reactions: A Practical Guide; Springer-Verlag:
Heidelberg, Germany, 2002; Topics in Current Chemistry Series, Vol. 219.
(2) (a) Tsuji, J., Palladium Reagents and Catalysts: New Perspectives
for the 21st Century, 2nd ed.; Wiley: Chichester, U.K., 2004. (b) Negishi,
E.-i., Ed. Handbookof Organopalladium Chemistryfor Organic Synthesis;
Wiley: New York, 2002; Vols. 1 and 2.
€
(5) (a) Neumann, H.; Brennfuhrer, A.; Gross, P.; Riermeier, T.;
Almena, J.; Beller, M. Adv. Synth. Catal. 2006, 348, 1255. (b) Brennf€uhrer,
A.; Neumann, H.; Pews-Davtyan, A.; Beller, M. Eur. J. Org. Chem. 2009,
38. (c) Neumann, H.; Brennf€uhrer, A.; Beller, M. Adv. Synth. Catal. 2008,
350, 2437.
(3) For general reviews see, for example, ref 2 and: (a) Fleckenstein,
C. A.; Plenio, H. Chem. Soc. Rev. 2010, 39, 694. (b) Martin, R.; Buchwald,
S. L. Acc. Chem. Res. 2008, 41, 1461. (c) Surry, D. S.; Buchwald, S. L.
Angew. Chem., Int. Ed. 2008, 47, 6338. (d) Mauger, C. C.; Mignani, G. A.
Aldrichim. Acta 2006, 39, 17. (e) Schlummer, B.; Scholz, U. Adv. Synth.
Catal. 2004, 346, 1599.
€
(6) (a) Brennfuhrer, A.; Neumann, H.; Klaus, S.; Riermeier, T.;
Almena, J.; Beller, M. Tetrahedron 2007, 65, 6252. (b) Klaus, S.;
Neumann, H.; Zapf, A.; Str€ubing, D.; H€ubner, S.; Almena, J.; Riermeier,
T.; Gross, P.; Sarich, M.; Krahnert, W.-R.; Rossen, K.; Beller, M. Angew.
Chem., Int. Ed. 2006, 45, 154.
(7) (a) Zapf, A.; Beller, M. Chem. Commun. 2005, 431. (b) Torborg, C.;
Beller, M. Adv. Synth. Catal. 2009, 351, 3027.
(8) Sergeev, A. G.; Spannenberg, A.; Beller, M. J. Am. Chem. Soc.
ꢀ
(4) For general reviews, see: (a) Dıez-Gonzalez, S.; Marion, N.;
ꢀ
Nolan, S. P. Chem. Rev. 2009, 109, 3612. (b) Díez-Gonzalez, S.; Nolan,
S. P. Top. Organomet. Chem. 2007, 21, 47. (c) Kantchev, E. A.; O'Brien,
C. J.; Organ, M. G. Angew. Chem., Int. Ed. 2007, 46, 2768.
2008, 130, 15549.
r
pubs.acs.org/Organometallics
Published on Web 07/07/2010
2010 American Chemical Society

Article
Organometallics, Vol. 29, No. 15, 2010 3369
was described when crystalline Pd(PPhtBu2)2 was reacted
with air.10,13,14
Not surprisingly, palladium peroxo complexes 3 and 4 are
both stable in air, differing only in thermal stability. Com-
plex 4 can be stored at room temperature for at least 6
months, whereas 3 is stable only for several days at ambient
temperature but can be kept for an unlimited time at subzero
temperatures. Peroxo complex 4, bearing di-1-adamantyl-
n-butylphosphine, was characterized by X-ray data (see
Figure 1). Emerald green crystals of 4, suitable for a crystal
structure analysis, were obtained by slow diffusion of air into
a benzene solution of 2.
Complex 4 contains the side-on-coordinated dioxygen,
which is located in one plane with two coordinated phos-
phorus atoms and the metal center, as exemplified by the
P1-Pd1-O1A-O1 dihedral angle of 0.5(3)°. The O-O
Figure 1. Molecular structure of 4 (benzene). The thermal
3
˚
bond in the coordinated dioxygen molecule of 1.395(4) A is
ellipsoids correspond to 30% probability. Hydrogen atoms
are omitted for clarity.
considerably longer than that in the free oxygen molecule
15
˚
(1.2074 A) and slightly shorter than the standard O-O
n
n
P(1-Ad)2 Bu and PtBu2 Bu ligands. During these studies, we
noticed that bis phosphine palladium(0) complexes of P(1-
˚
single bond (e.g., 1.475 A in hydrogen peroxide).15 Coordi-
nation of dioxygen to palladium(0) complex 2 causes a con-
siderable decrease in the P-Pd-P angle from 172.18(5)° in 2
to 114.71(4)° in 4 and leads to only a slight elongation of the
n
n
Ad)2 Bu and PtBu2 Bu are easily oxidized in solution, lead-
ing to clean formation of the dioxygen complexes Pd(O2)L2.
n
Thus, excposure of a heptane solution of Pd(PtBu2 Bu)2 (1)
˚
Pd-P bonds from 2.286(2) to 2.3402(8) A, respectively.
n
and a benzene solution of Pd{P(1-Ad)2 Bu}2 (2)9 to air at
Interestingly, dioxygen in complexes 3 and 4 is coordi-
nated irreversibly;16 however, the peroxo complexes can be
reduced back to Pd(0) complexes 1 and 2 by reaction with
hydrogen under mild conditions. Thus, hydrogenation of
complex 3 with hydrogen at 5 bar and 40 °C in benzene-d8 led
to the formation of palladium(0) complex 1, as evidenced by
the appearance of the singlet of 1 at δ 57.6 ppm in the 31P
NMR spectrum of the reaction mixture (eq 3). Due to poor
solubility we did not perform the corresponding reaction
with 4. However, we obtained indirect proof that this com-
plex converted to the palladium(0) species under a reduc-
tive atmosphere (CO/H2) at elevated temperatures as well
(vide infra).
room temperature results in precipitation of the corresponding
peroxo complexes 3 and 4 in 60% and 94% yields as green
crystals (eq 1).
n
Remarkably, the PtBu2 Bu-ligated palladium(0) complex
1 reacts with air even in the solid state at room temperature,
yielding the corresponding peroxo complex 3 in quantitative
yield as mint-colored crystals (eq 2). In contrast, a complex
possessing isomeric tri-tert-butylphosphine, Pd(PtBu3)2,
does not react with oxygen in the solid state and does not
produce any well-characterized peroxo complex in solution.10,11
Apart from hydrogen, complex 3 can also be reduced with
CO (20 bar, 16 h) even in the solid state at room temperature
to give the palladium(0) carbonyl cluster Pdn(CO)m-
n
(PtBu2 Bu)n (5) as the major palladium-containing product.17
So far, only a few palladium(0) phosphine complexes
produce stable peroxo complexes on exposure to air in
solution.10,12 Formation of the corresponding dioxygen
palladium complexes in the solid state is even more rare.
To the best of our knowledge, only one such example
(13) Another example may include Pd(PCy3)2, but its reactivity to air
in the solid state is controversial: in ref 10b, Pd(PCy3)2 is claimed to form
the peroxo complex, whereas in ref 11, the same complex was found to be
inert to air.
(14) For solid-state reactions of palladium(0) carbene complexes with
air, see ref 11 and: Konnick, M. M.; Guzei, I. A.; Stahl, S. S. J. Am.
Chem. Soc. 2004, 126, 10212.
(15) Lide, D. R.., Ed. CRC Handbook of Chemistry and Physics, 90th
ed. (Internet Version 2010); CRC Press/Taylor and Francis: Boca Raton, FL,
2010.
(9) When the reaction was carried out in toluene, the solution turned
brown and no peroxo complex precipitated.
(10) (a) Matsumoto, M.; Yoshioka, K.; Nakatsu, K.; Yoshida, T.;
Otsuka, S. J. Am. Chem. Soc. 1974, 96, 3322. (b) Yoshida, T.; Otsuka, S.
J. Am. Chem. Soc. 1977, 99, 2134.
(11) Yamashita, M.; Goto, K.; Kawashima, T. J. Am. Chem. Soc.
2005, 127, 7294.
(12) (a) Wilke, G.; Schott, H.; Heimbach, P. Angew. Chem., Int. Ed.
1967, 6, 92. (b) Urata, H.; Suzuki, H.; Morooka, Y.; Ikawa, T. J. Organomet.
Chem. 1989, 364, 235. (c) Amatore, C.; Aziz, S.; Jutand, A.; Meyer, G.;
Cocolios, P. New J. Chem. 1995, 19, 1047. (d) Clegg, W.; Eastham, G. R.;
Elsegood, M. R. J.; Heaton, B. T.; Iggo, J. A.; Tooze, R. P.; Whyman, R.;
Zacchini, S. Dalton Trans. 2002, 3300.
(16) When a solution of 3 in benzene-d6 was subjected to four freeze-
pump-thaw cycles, no changes in 1H and 31P{1H } NMR spectra were
observed. Passing of argon via a benzene solution of 3 at 40 °C for
1 h caused formation of only traces of 1 and accumulation of a dark-
colored product of decomposition. Heating of the less soluble complex
4 under vacuum at 60 °C for 1 h also did not lead to appreciable
deoxygenation.
(17) According to a 31P{1H} NMR spectrum in benzene-d6 three
products formed: 5 free PtBu2nBu, and an unidentified complex ex-
hibiting three signals at 56.7 (s), 28.9 (br s), and 51.3 (br s) ppm in a ca.
2.8:1:0.6 ratio. Reaction conditions were not optimized.

3370 Organometallics, Vol. 29, No. 15, 2010
Sergeev et al.
n
Indeed, use of 0.25 mol % of Pd(OAc)2/3P(1-Ad)2 Bu under
The same complex 5 is obtained when 1 is carbonylated in
the solid state.18 Recently, we showed that such clusters
air-tolerant conditions and subsequent application in the
reductive carbonylation of aryl bromides with synthesis gas
at 100 °C and 5 bar provided a convenient and easy-to-apply
catalyst system. The catalyst showed high activity and proved
to be as efficient;in some cases even more efficient;as in the
experiments with complete exclusion of air (Table 1). All aryl
bromides shown in Table 1 were formylated in good to
excellent yields. Moreover, formylation of 4-bromobenzoni-
trile gave a higher yield (84%) under the air-friendly condi-
tions, compared to the reaction employing the standard
protocol (57%) (Table 1, entry 4). Under air-friendly condi-
n
n
Pdn(CO)m(L)n (L = PtBu2 Bu, P(1-Ad)2 Bu) can be the
resting state of the catalyst in the reductive carbonylation of
n
n
aryl halides catalyzed by Pd/P(1-Ad)2 Bu or PtBu2 Bu.8
Clearly, the ability of the peroxo complexes 3 and 4 to
generate catalytically active palladium(0) complexes under
an atmosphere of hydrogen and carbon monoxide provides
an opportunity to use 3 and 4 as convenient substitutes for
their air-sensitive analogues 1 and 2: e.g., as precatalysts in
various coupling reactions. To demonstrate the viability of 3
and 4 as palladium precursors, we employed them initially in
the reductive carbonylation of aryl halides with synthesis gas
(CO/H2) under the previously optimized reaction conditions
(toluene, TMEDA as a base, 5 bar of CO/H2, 100 °C). We
were pleased to find that complexes 3 and 4 indeed catalyzed
the formylation of a wide scope of aryl bromides at low
catalyst loadings (0.25 mol %). As can be seen from Table 1,
both electron-rich and electron-poor aryl halides are trans-
formed into the corresponding benzaldehydes in moderate to
excellent yields (42-99%). Functional groups such as CN
and C(O)Me are well tolerated, and halogen substituents (Cl
and F) remain intact toward hydrogenolysis as well (Table 1,
entries 1, 2 and 4, 5). In general, reactions catalyzed by 4
proceed with better yields compared to those with complex 3,
as illustrated by entries 1, 3, 7, 8, and 10-12. Notably, the
peroxo complex 4 was in most cases as efficient as the
n
tions, Pd(OAc)2/P(1-Ad)2 Bu gave also good results with
problematic substrates such as 2-bromoanisole and 5-bromo-
m-xylene (64% and 67% yields, respectively; Table 1, entries 9
and 12). In these cases the peroxo complex 4 performed less
efficiently.21 This difference in the catalytic behavior of the
peroxo complex 4 and the in situ system Pd(OAc)2/3P(1-
n
Ad)2 Bu might be explained by the presence of acetate ions
coming from palladium acetate, which is known to affect the
catalytic performance of palladium(0) species.20
As we have shown above, 3 and 4 are reduced to palladium-
(0) speciesnotonlywithhydrogenbutalsowithcarbon mono-
xide. This makes these complexes promising catalyst precur-
sors for all kinds of palladium-catalyzed carbonylations.
Therefore, we tested 3 and 4 in the alkoxycarbonylation of
aryl bromides under air-tolerant conditions using 0.5 mol %
of the catalyst in neat n-butyl alcohol at 5 bar of CO in the
presence of TMEDA as a base (Table 2). In line with our
assumptions, peroxo complexes 3 and 4 both proved to be
good catalysts under these conditions, affording the esters in
58-98% and 89-99% yields, respectively. The scope of the
carbonylation encompassed both electron-rich and electron-
deficient aryl bromides as well as heterocyclic 3-bromo-
n
original precatalyst Pd(OAc)2/P(1-Ad)2 Bu.6
Having established for the first time the catalytic activity of
palladium peroxo complexes in non-oxidative coupling reac-
tions, we decided to exploit their tolerance to air and applied 4to
an air-tolerant formylation, including preparation of the reac-
tion mixture in air. To further simplify the procedure, we used
non-degassed and non-dried commercial toluene (Table 1). To
our delight, under these conditions 4 was surprisingly efficient,
even at catalyst loadings of 0.25 mol %! Thus, formylation of
electron-deficient 4-halobromobenzenes and 4-bromobenzotri-
fluoride, as well as electron-rich 3- and 4-bromoanisoles and
4-bromo-N,N-dimethylaniline gave the corresponding benzal-
dehydes in good to excellent yields (80-97%) (Table 1, entries
1-3, 8, 10, and 11). However, in the case of some problematic
substrates such as 3-bromothiophene and 2-bromoanisole,
yields decreased by 20-25% as compared to the reactions
under standard conditions (see Table 1, entries 5, 7, 9, and 12).
Next, we tested the original catalyst system comprised of
n
pyridine. As in the formylation reaction, the P(1-Ad)2 Bu-
ligated peroxo complex 4 was a more general catalyst than its
n
PtBu2 Bu analogue 3. This is illustrated by the results of the
carbonylation of challenging electron-rich substrates such as
4-bromoanisole and 4-bromo-N,N-dimethylaniline, where 4
gave the products in 89 and 99% yields, whereas the yields
with 3 did not exceed 71% (Table 2, entries 2 and 4).22 It is
worth noting that under air-friendly conditions the peroxo
complex 4 works as well as the original catalyst Pd(OAc)2/
n
P(1-Ad)2 Bu does under an inert atmosphere (Table 2).
Additionally, the latter catalyst also gave excellent results
in the presence of air (Table 2). To the best of our knowledge,
no similar air-tolerant alkoxy or reductive carbonylations
have been reported before.23
n
Pd(OAc)2 and air-stable crystalline P(1-Ad)2 Bu under the
air-tolerant reaction conditions. It is known that the combina-
tion ofPd(OAc)2 and phosphine ligands L in a ratioof 1:N can
generate PdLN-1 complexes.19,20 We assumed that mixing
n
Pd(OAc)2 and 3 equiv of P(1-Ad)2 Bu should similarly lead
n
to the formation of Pd{P(1-Ad)2 Bu}2 (2) species. When the
(21) To check if a different Pd:L ratio is responsible for the observed
difference in reactivity of 4 (the ratio is 1:2) and Pd(OAc)2/3P(1-Ad)2nBu
(the ratio is 1:3), we performed formylation of 1-bromonaphthalene,
4-bromoacetophenone and 4-bromobenzonitrile using a combination of
0.25 mol % of 4 and 0.25 mol % of added P(1-Ad)2nBu ligand. Whereas
for the first two aryl bromides the yields (89% for both substrates) were
comparable to those with Pd(OAc)2/3P(1-Ad)2nBu (92 and 89% respec-
tively), 4-bromobenzonitrile still gave the lower yield (59%) compared
to Pd(OAc)2/3P(1-Ad)2nBu (84%).
mixing is done in air, this species should be easily oxidized to
give 4, which is a suitable catalyst for the air-friendly
formylation, as shown above.
(18) Carbonylation of solid 1 at 20 bar and room temperature for 16 h
leads to the formation of 5 and PtBu2nBu as major products in a 1:1 ratio,
according to the 31P{1H} NMR spectrum in benzene-d6.
(22) With regard to the limitation of this protocol, di-ortho-substi-
tuted 2-bromomesitylene gave only low yields of 15 and 30% in the
reactions with 3 and 4, respectively.
(19) See for example: (a) Amatore, C.; Jutand, A.; Khalil, F. Arkivoc
ꢀ
2006, IV, 38. (b) Amatore, C.; Carre, E.; Jutand, A.; M'Barki, M. A.
Organometallics 1995, 14, 1818. (c) Mandai, T.; Matsumoto, T.; Tsuji, J.;
Saito, S. Tetrahedron Lett. 1993, 34, 2513. (d) Amatore, C.; Juntand, A.;
M'Barki, M. A. Organometallics 1992, 11, 3009. (e) Ozawa, F.; Kubo, A.;
Hayashi, T. Chem. Lett. 1992, 2177.
(23) (a) For a recent review on palladium-catalyzed carbonylations of
€
Angew. Chem., Int. Ed. 2009, 48, 4114 and references therein. (b) Note
added in proof: recently Lei et al. reported Pd-catalyzed oxidative carbonyla-
tion of arylboronic acids via formation of Pd(O2)L2 intermediates: Liu, Q.; Li,
G.; He, J.; Liu, J.; Lei, A. Angew. Chem., Int. Ed. 2010, 49, 3371.
aryl halides, see for example: Brennfuhrer, A.; Neumann, H.; Beller, M.
(20) For the role of [Pd(OAc)L2]- intermediates in cross-coupling reac-
tions, see for example: Amatore, C.; Jutand, A. Acc. Chem. Res. 2000,33, 314.

Article
Organometallics, Vol. 29, No. 15, 2010 3371
n
Table 1. Reductive Carbonylation of Aryl Bromides Catalyzed by Peroxo Complexes 3 and 4 and Pd(OAc)2/P(1-Ad)2 Bu
under Inert and Air-Tolerant Conditionsa
a Reaction conditions: 2 mmol of aryl bromide, 1.5 mmol of TMEDA, 0.4 mmol of hexadecane (internal standard for GC), 0.25 mol % of complex
3 or 4, 2 mL of toluene, 5 bar of CO/H2 (1:1), 100 °C, 16 h. b Determined by GC. c Reaction mixtures were prepared in argon. d Reaction mixtures
were prepared in air using non-degassed commercial toluene. e See ref 6. f 0.33 mol % of palladium was used.
In summary, we have described an efficient air-tolerant
protocol for palladium-catalyzed formylation and alkoxycar-
bonylation of aryl bromides at low catalyst loadings
(0.25-0.5%). Our protocol features simple preparation of
the reaction mixtures in air using commercial non-degassed
solvents and simplifies the existing synthetic procedures. The
success of thismethod is basedonthe remarkableability of the
n
n
palladium(0) complexes PdL2 (L = P(1-Ad)2 Bu, PtBu2 Bu)
to react with air, yielding the well-defined and stable peroxo
complexes Pd(O2)L2, which are easily converted back to the

3372 Organometallics, Vol. 29, No. 15, 2010
Sergeev et al.
n
Table 2. Alkoxycarbonylation Catalyzed by Peroxo Complexes 3 and 4 and Pd(OAc)2/P(1-Ad)2 Bu under Inert and
Air-Tolerant Conditionsa
a Reaction conditions: 2 mmol of aryl bromide, 1.5 mmol of TMEDA, 0.4 mmol of hexadecane (internal standard for GC), 0.5 mol % of 3 or 4, 2 mL
of n-butyl alcohol, 5 bar of CO, 115 °C, 16 h. b Determined by GC. c The reaction mixture was prepared in air using non-degassed commercial n-butyl
alcohol. d See ref 5a.
catalytically active palladium(0) complexes under carbonyla-
tion conditions.
protiated solvents, respectively. 31P NMR chemical shifts are
reported relative to 85% H3PO4. Gas chromatographic
analysis was performed on a Agilent HP-5890 instrument
with a FID detector and HP-5 capillary column (poly-
dimethylsiloxane with 5% phenyl groups, 30 m, 0.32 mm
i.d., 0.25 μm film using argon as carrier gas). Gas chromato-
graphic-mass spectrometric analysis was carried out on
an Agilent HP-5890 instrument with an Agilent HP-5973
mass selective detector (EI) and HP-5 capillary column (poly-
dimethylsiloxane with 5% phenyl groups, 30 m, 0.25 mm
i.d., 0.25 μm film thickness) using helium carrier gas.
Experimental Section
General Comments. Aryl halides and hexadecane were
purchased from Aldrich and used as received. Benzadehydes
and n-butyl benzoates (standards for GC analysis) were
purchased from Aldrich or prepared according to literature
procedures.5a,6 Toluene and n-butyl alcohol were purchased
from Merck. For the air-friendly carbonylations, solvents
(toluene, n-butyl alcohol) were used straight from the bottle.
For other reactions benzene and toluene were distilled
over benzophenone ketyl; n-butyl alcohol, heptanes, and
TMEDA were distilled from calcium hydride. Solvents were
stored under argon in Aldrich Sure Stor flasks. Palladium
acetate was purchased from Strem. Complexes 1 and 2 were
prepared according to the literature procedures.8 Di-1-ada-
mantyl-n-butylphosphine (cataCXium A) was donated by
Evonik-Degussa, and n-butyl-di-tert-butylphosphine24 was
synthesized according to the literature procedure.
n
n
Pd(O2){P(1-Ad)2 Bu}2 C6H6 (4). Pd{P(1-Ad)2 Bu}2 (41mg,
3
4.98 Â 10-2 mmol) was dissolved in 4 mL of benzene, and air
was bubbled through the solution for 1 h. The resulting green
precipitate was filtered and washed with benzene to give 44 mg
(94%) of the product. Large crystals of 4 can be obtained by
1
exposure of the benzene solution to air overnight. H NMR
(300 MHz, CDCl3): δ, 7.36 (s, benzene), 2.16-2.34 (apparent
bs, 24H), 1.93-2.09 (bs, 12H), 1.59-1.88 (m, 32H), 1.29-1.47
(m, 4H), 0.97(t, J= 7 Hz, 6H). 13C{1H} NMR (75 Hz, CDCl3):
δ 128.3 (benzene), 40.9 (m, overlapping, C), 40.8 (overlapping,
CH2), 36.7 (CH2), 29.3, 28.7 (t, J = 4 Hz, CH2), 25.9 (t, J = 4.5
Hz, CH2), 21.4 (t, J = 6 Hz, CH2) 14.0 (CH3). 31P{1H} NMR
(121 MHz, C6D6): δ 60.3. Anal. Calcd for C54H86O2P2Pd: C,
69.47; H, 9.07. Found: C, 69.29; H, 9.55.
NMR spectra were recorded on Bruker ARX 300 and
1
Bruker ARX 400 spectrometers. 13C and H NMR spectra
were referenced to signals of deuterio solvents and residual
X-ray Crystal Structure Analysis of 4. Single crystals of 4
were grown by the slow diffusion of air into a solution of
(24) Cooper, J. W.; Roberts, B. P. J. J. Chem. Soc. Perkin Trans. 2
1976, 808.

Article
Organometallics, Vol. 29, No. 15, 2010 3373
complex 2 in benzene at room temperature. Data were
collected on a STOE IPDS II diffractometer using gra-
phite-monochromated Mo KR radiation. The structure
was solved by direct methods (SHELXS-97)25 and refined
by full-matrix least-squares techniques on F2 (SHELXL-97).25
XP (Bruker AXS) was used for the graphical representation.
Crystal data: C60H90O2P2Pd, Mr = 1011.66, monoclinic,
J = 6 Hz, 36H), 1.01 (t, J = 7.5 Hz, 6 H). 31P{1H} NMR
(121 MHz, C6D6; spectrum of 1): δ 57.6.8
General Procedure for the Air-Friendly Reductive Formyla-
tion of Aryl Bromides using 3 and 4. In air, a 4 mL screw-cap
vial was charged with 5 Â 10-3 mmol of 3 or 4, 2 mmol of aryl
bromide, 1.5 mmol of TMEDA, 0.4 mmol of hexadecane
(internal standard), 2 mL of toluene, and a magnetic stir bar.
The vial was closed with a screw cap equipped with a septum
and inlet needle and placed in an alloy plate, which was
transferred to a 300 mL autoclave from Parr Instruments
(Model 4561) under an argon atmosphere. The autoclave
was flushed with synthesis gas (a 1:1 CO/H2 mixture) and
then pressurized to 5 bar at room temperature and heated at
100 °C for 16 h. The conversion and product yield were
determined by GC using an internal standard and authentic
compounds.
˚
˚
space group C2/c, a = 15.864(3) A, b = 14.489(3) A, c =
3
˚
˚
22.849(5) A, β = 95.02(3)°, V = 5231.7(18) A , Z = 4,
calcd = 1.284 g cm-3, μ = 0.458 mm-1, T = 293 K, 33 863
reflections collected, 5150 independent reflections (Rint
F
=
0.0688), of which 3768 were observed (I > 2σ(I)), final R indices
(I > 2σ(I)) R1 = 0.0361 and wR2 = 0.0712, R indices (all data)
R1 = 0.0572 and wR2 = 0.0742, 265 refined parameters.
n
Pd(O2)(PtBu2 Bu)2 (3). Synthesis in solution, 3 was pre-
pared from 1 in 60% yield following the procedure for 4 using
heptane as a solvent. Synthesis in the solid state: off-white
crystals of 1 (51 mg, 0.1 mmol) were exposed to air overnight
(12 h) at room temperature to give 54 mg of 3 as pale mint-
colored crystals (99%). 1H NMR (300 MHz, C6D6): δ
1.45-1.72 (m, 8H, 1.35 (apparent d, J = 13 Hz, 36H),
1.15-1.29 (apparent sextet, J = 7.5 Hz, 4H), 0.82 (t, J = 7.5
Hz, 6H). 13C{1H} NMR (75 Hz, C6D6): δ 35.8 (dd, J = 7 Hz,
J=5.5Hz, C), 30.5(t, J=3Hz, CH3), 28.9(CH2), 25.9(t, J=
5.5 Hz, CH2), 24.1 (dd, J = 7 Hz, J = 5 Hz, CH2), 13.9 (CH3).
31P{1H} NMR (121 MHz, C6D6): δ 64.0. Anal. Calcd for
C24H54O2P2Pd: C, 53.08; H, 10.02. Found: C, 52.86; H, 9.77.
Reduction of 3 with Hydrogen To Give 1. A 4 mL screw-cap
vial was charged with 3 (27.6 mg, 5.08 Â 10-3 mmol) and a
magnetic stir bar and closed with a screw cap equipped with
septum and inlet needle. The vial vas evacuated and filled
with argon via the inlet needle, and 0.8 mL of degassed C6D6
was added via syringe. After brief stirring a green solution
formed. The vial was placed in an alloy plate, which was
transferred to a 300 mL autoclave from Parr Instruments
(Model 4561) under an argon atmosphere. The autoclave
was flushed with hydrogen and then pressurized to 5 bar at
room temperature and heated at 40 °C for 16 h. After it was
cooled to room temperature, the reaction vial was taken out
from the autoclave and the dark brown solution was trans-
ferred into a NMR tube under argon. 1H and 31P{1H} NMR
spectra of the reaction mixture showed complete consump-
tion of 3 and formation of 1 and a side product in an 8:1 ratio
(the peak ratio in the 31P{1H} NMR spectrum; see the
Supporting Information). 1H NMR (300 MHz, C6D6; spec-
trum of 1, selected signals: δ 1.84-2.02 (m, 4H), 1.57-1.74
(m, 4H), 1.43-1.46 (m, 4H), 1.36-1.43 (m, 4H), 1.33 (m,
General Procedure for Air-Friendly Reductive Formylation
n
of Aryl Bromides using Pd(OAc)2/P(1-Ad)2 Bu. The proce-
dure is similar to that described above, except 5 Â 10-3 mmol
n
of Pd(OAc)2 and 15 Â 10-3 mmol of P(1-Ad)2 Bu were used
instead of 4.
General Procedure for Air-Friendly Alkoxycarbonylation
of Aryl Bromides using 3 and 4. In air, a 4 mL screw-cap vial
was charged with 1 Â 10-2 mmol of 3 or 4, 2 mmol of aryl
bromide, 1.5 mmol of TMEDA, 0.4 mmol of hexadecane
(internal standard), 2 mL of n-butyl alcohol, and a magnetic
stir bar. The vial was closed with a screw cap equipped with a
septum and inlet needle and placed in an alloy plate, which
was transferred to a 300 mL autoclave from Parr Instru-
ments (Model 4561) under an argon atmosphere. The auto-
clave was flushed with carbon monoxide and then
pressurized to 5 bar at ambient temperature and heated at
115 °C for 16 h. The conversion and product yield were
determined by GC using an internal standard and authentic
compounds.
General Procedure for Air-Friendly Alkoxycarbonylation
n
of Aryl Bromides using Pd(OAc)2/P(1-Ad)2 Bu. The proce-
dure is similar to that described above, except 1 Â 10-2 mmol
n
of Pd(OAc)2 and 3 Â 10-2 mmol of P(1-Ad)2 Bu were used
instead of 4.
Acknowledgment. We thank Dipl. Ing. A. Koch, Mrs.
S. Leiminger, and Ms. A. Kammer (all LIKAT) for their
excellent analytical and technical support.
Supporting Information Available: Crystallographic data for
4 (CIF) NMR spectra of 3 and 4. This material is available free
of charge via the Internet at http://pubs.acs.org.
(25) Sheldrick, G. M. Acta Crystallogr. 2008, A64, 112–122.