Reductive routes to dinuclear d10-d10 palladium(0) complexes and their redistribution equilibria in solution

Article abstract of DOI:10.1021/om010281z

The reaction of hydrazine with (dcpe)PdCl₂ or (dippe)Pd(OAc)₂ leads to high yields of the isolated dinuclear complexes [(μ-dcpe)Pd]₂ and [(μ-dippe)Pd]₂, respectively. In solution, these dinuclear complexes undergo a monomer-dimer equilibrium with their corresponding 14-electron (P-P)Pd fragments. Solutions containing both [(μ-dcpe)Pd]₂ and [(μ-dippe)Pd]₂ give near statistical amounts of the novel mixed dimer (μ-dcpe)(μ-dippe)Pd₂.

Full text of DOI:10.1021/om010281z

Organometallics 2001, 20, 2959-2961
2959
Red u ctive Rou tes to Din u clea r d 10-d 10 P a lla d iu m (0)
Com p lexes a n d Th eir Red istr ibu tion Equ ilibr ia in
Solu tion
Steven M. Reid and Mark J . Fink*
Department of Chemistry, Tulane University, New Orleans, Louisiana 70118
Received April 5, 2001
Summary: The reaction of hydrazine with (dcpe)PdCl2
or (dippe)Pd(OAc)2 leads to high yields of the isolated
dinuclear complexes [(µ-dcpe)Pd]2 and [(µ-dippe)Pd]2,
respectively. In solution, these dinuclear complexes
undergo a monomer-dimer equilibrium with their cor-
responding 14-electron (P-P)Pd fragments. Solutions
containing both [(µ-dcpe)Pd]2 and [(µ-dippe)Pd]2 give
near statistical amounts of the novel “mixed” dimer (µ-
dcpe)(µ-dippe)Pd2.
Pd(II) species.9 A potentially cleaner and more direct
route to catalytically active (P-P)Pd fragments is of-
fered through the dissociation of the dinuclear com-
plexes [(µ-P-P)Pd]2. We have previously reported the
synthesis of the dinuclear complex [(µ-dcpe)Pd]2 (1) from
the photolysis of (dcpe)Pd(C2O4) (dcpe ) 1,2-bis(dicy-
clohexylphosphino)ethane).10 In solution, the dinuclear
complex 1 exists in a facile equilibrium with (dcpe)Pd,
thereby acting as a “bottled” source of this highly
reactive 14-electron intermediate. As a consequence, the
reaction of 1 with chlorobenzene was found to give
(dcpe)PdPhCl rapidly at room temperature.
Instances of these dinuclear complexes, however, are
relatively rare. A closely related derivative [(µ-dippe)-
Pd]2 (2) was tentatively identified by the groups of
Fryzuk4 and Po¨rschke11 in reaction mixtures but never
isolated in pure form. J olly et al. have structurally
characterized the complex [(µ-dippm)Pd]2, which was
isolated as a reaction side product.12 Very recently, we
have obtained the complexes [(µ-dcpm)Pd]2 and [(µ-
dtbpm)Pd]2 from the reductive elimination of ethane
from dimethylpalladium(II) complexes (dippm ) bis-
(diisopropylphosphino)methane; dcpm ) bis(dicyclo-
hexylphosphino)methane; dcpm ) bis(di-tert-butylphos-
phino)methane).13
The activation of aromatic carbon-halogen bonds by
coordinatively unsaturated complexes of palladium(0)
is a critical step in a number of important catalytic
reactions, including the Heck reaction1 and cross-
coupling reactions of aryl halides.2 Reactions involving
difficult bond activations such as the C-Cl bond of
chloroarenes often involve palladium(0) catalyst precur-
sors in the presence of chelating, and usually sterically
congested, bis-phosphine ligands. The C-Cl activation
step is believed to involve the extremely reactive 14-
electron dicoordinate intermediate (P-P)Pd. Milstein
and Portnoy, for example, demonstrated directly an
equilibrium giving rise to (dippp)Pd,3a the key interme-
diate in the catalytic carbonylation,3b formylation,3c and
reduction3d of aryl chlorides. Fryzuk and co-workers
have similarly identified (dippe)Pd as the putative and
highly reactive 14-electron fragment in the catalytic
reductive dechlorination of chlorobenzene by [(dippe)-
Pd]2(µ-dippe) (dippp ) 1,3-bis(diisopropylphosphino)-
propane; dippe ) 1,2-bis(diisopropylphosphino)ethane).4
(6) Pd(PPh3)n complexes (n ) 1-4): (a) Coulson, D. R. Inorg. Synth.
1972, 13, 121. (b) Malatesta, L.; Angolette, M. J . Chem. Soc. 1957,
1186. (c) Fischer, E. O.; Werner, H. Chem. Ber. 1962, 95, 703. (d)
Kuran, W.; Musco, A. J . Organomet. Chem. 1972, 40, C47. (e) Kuran,
W.; Musco, A. Inorg. Chim. Acta 1975, 12, 187. (f) Mann, B. E.; Musco,
A. J . Chem. Soc., Dalton Trans. 1975, 1673. (g) Urata, H.; Suzuki, H.;
Moro-oka, Y.; Ikawa, T. J . Organomet. Chem. 1989, 364, 235. (h)
Grushin, V.; Alper, H. Organometallics 1993, 12, 1890. (i) Grushin,
V.; Bensimon, C.; Alper, H. Organometallics 1995, 14, 3259.
(7) Pd[P(o-tolyl)3]n complexes (n ) 1, 2): (a) Hartwig, J . F.; Paul, F.
J . Am. Chem. Soc. 1995, 117, 5373. (b) Paul, F.; Patt, J .; Hartwig, J .
F. J . Am. Chem. Soc. 1994, 116, 5969. (c) Paul, F.; Patt, J .; Hartwig,
J . F. Organometallics 1995, 14, 3030. (d) Hartwig, J . F. Synlett 1997,
329. (e) Louie, J .; Paul, F.; Hartwig, J . F. Organometallics 1996, 15,
2794.
(8) Other proposed monophosphine “L-M(0)” intermediates: (a)
Wolfe, J . P.; Singer, R. A.; Yang, B. H.; Buchwald, S. L. J . Am. Chem.
Soc. 1999, 121, 1. (b) Wolfe, J . P.; Tomori, H.; Sadighi, J . P.; Yin, J .;
Buchwald, S. L. J . Org. Chem. 2000, 65, 1158. (c) Krause, J .; Cestaric,
G.; Haack, K. J .; Seevogel, K.; Storm, W.; Po¨rschke, K. R. J . Am. Chem.
Soc. 1999, 121, 9807.
Sources of catalytically active palladium(0) typically
arise from ligand dissociation from coordinatively more
saturated Pd(0) complexes2b,5-8 or from reduction of a
* To whom correspondence should be addressed. E-mail:
fink@tulane.edu.
(1) (a) Heck, R. F. Palladium Reagents in Organic Synthesis;
Academic Press: San Diego, CA, 1985. (b) Hegedus, L. S. In Organo-
metallics in Synthesis; Schlosser, M., Ed.; Wiley: New York, 1994; pp
383-459. (c) Hermann, W. A. In Applied Homogeneous Catalysis with
Organometallic Compounds. A Comprehensive Handbook in Two
Volumes; Cornils, B., Hermann, W. A., Eds.; VCH: New York, 1996;
Vol. 2, pp 712-732.
(2) Aryl-amination reactions: (a) Wolfe, J . P.; Wagaw, S.; Marcoux,
J .-F.; Buchwald, S. L. Acc. Chem. Res. 1998, 31, 805. (b) Hartwig, J .
F. Angew. Chem., Int. Ed. 1998, 37, 2046. (c) Hartwig, J . F. Acc. Chem.
Res. 1998, 31, 852. (d) Stille couplings: Farina, V.; Krishnamurthy,
V.; Scott, W. J . Org. React. 1997, 50, 1.
(9) (a) Amatore, C.; J utand, A. J . Organomet. Chem. 1999, 576, 254.
(b) Amatore, C.; J utand, A. Acc. Chem. Res. 2000, 33, 314.
(10) Pan, Y.; Mague, J . T.; Fink, M. J . J . Am. Chem. Soc. 1993, 115,
3842.
(3) (a) Portnoy, M.; Milstein, D. Organometallics 1993, 12, 1665. (b)
Ben-David, Y.; Portnoy, M.; Milstein, D. J . Am. Chem. Soc. 1989, 111,
8742. (c) Ben-David, Y.; Portnoy, M.; Milstein, D. J . Chem. Soc., Chem.
Commun. 1989, 1816. (d) Ben-David, Y.; Gozin, M.; Portnoy, M.;
Milstein, D. J . Mol. Catal. 1992, 73, 173.
(4) Fryzuk, M. D.; Clentsmith, G. K. B.; Rettig, S. J . Organometallics
1996, 15, 2083.
(11) (a) Schager, F.; Bonrath, W.; Po¨rschke, K. R.; Kessler, M.;
Kru¨ger, C.; Seevogel, K. Organometallics 1997, 16, 4276. (b) Schager,
F.; Haack, K. J .; Mynott, R.; Rufinska, A.; Po¨rschke, K. R. Organo-
metallics 1998, 17, 807. (c) Trebbe, R.; Goddard, R.; Rufinska, A.;
Seevogel, K.; Po¨rschke, K. R. Organometallics 1999, 18, 2466.
(12) Doehring, A.; Goddard, R.; Hopp, G.; J olly, P. W.; Kokel, N.;
Krueger, C. Inorg. Chim. Acta 1994, 222(1-2), 179.
(5) Equilibria involving mixtures of phosphines and “Pd(dba)2”:
Amatore, C.; J utand, A. Coord. Chem. Rev. 1998, 178-180, 511.
(13) Reid, S. M.; Mague, J . T.; Fink, M. J . J . Am. Chem. Soc. 2001,
123, 4081.
10.1021/om010281z CCC: $20.00 © 2001 American Chemical Society
Publication on Web 06/12/2001

2960 Organometallics, Vol. 20, No. 14, 2001
Communications
Detailed investigations of these dinuclear complexes
have hereto been limited by the availability of general
and scalable synthetic routes. For example, the pho-
tolysis of the oxalate, (dcpe)Pd(C2O4), could not be
successfully scaled to generate more than ∼50 mg of
pure 1. On the other hand, the synthesis of complexes
from reductive elimination of ethane is highly depend-
ent on the bis-phosphine ligand.14 We now report a
much more efficient and improved synthesis that allows
1 and 2 to be isolated in multigram quantities. The
preliminary reaction chemistry of 2 indicates that it also
dissociates in solution to give highly reactive (dippe)Pd
fragments, a finding that is corroborated by the direct
spectroscopic observation of a novel “mixed” dimer that
exists in equilibrium with mixtures of both 1 and 2.
combustion analysis, and mass spectrometry.19 The 31P
chemical shift of 33.8 ppm is identical with that ascribed
to it by Fryzuk and Po¨rschke on the basis of reaction
mixtures.4,11 The high-resolution mass spectrum and
elemental analysis of 2 are in full accord with a
dinuclear structure. The mass spectrum (EI, 70 eV)
shows a molecular ion at 738 amu and a high-mass
fragment (695 amu, 2.4%) arising from the loss of one
isopropyl group. A small fragment ion at 368 amu (3.4%)
corresponding to the monomer (dippe)Pd is also ob-
served. The intense red color of complex 2 arises from
a long-wavelength absorption band at λmax ) 455 nm,
which is attributed to a dσ*fpσ transition20 typical of
bimetallic bonding interactions in d10-d10 model sys-
tems (M ) Pd, Pt).21 The long-wavelength absorption
for 2 is virtually identical with that observed for 1,
which suggests a high degree of structural and bonding
similarity between the two complexes.
The preliminary reaction chemistry of 2 suggests the
intermediacy of a highly reactive mononuclear complex,
(dippe)Pd, in equilibrium with 2 in solution (Scheme 1).
For example, the activation of the carbon-chlorine bond
in PhCl by 2 in toluene proceeded smoothly at room
temperature to furnish the mononuclear insertion prod-
uct (dippe)Pd(Ph)Cl (3) in excellent isolated yield. Not
surprisingly, these mild conditions contrast sharply with
those reported previously (100 °C, overnight) by Portnoy
and Milstein for the synthesis of 3 from the coordina-
tively saturated (dippe)2Pd.22 Addition of PhSiH3 to 2
in benzene led to the high-yield formation of the white
crystalline solid (dippe)Pd(SiH2Ph)2 (4), a very rare
example23 of an isolated Pd(II) cis-bis(silyl) complex.24
In contrast to the related air-stable (dcpe)Pd(SiH2-
Ph)2,23f 4 is reactive toward oxygen and is stable in air
for only brief periods of time.
15
Rapid addition of excess N2H4 to suspensions of
16
either (dcpe)PdCl2 in THF or (dippe)Pd(OAc)2 in
pentane quickly gave rise to bright red solutions con-
taining essentially pure 1 or 2, respectively (eq 1).17,18
After filtration of the hydrazinium chloride and removal
of solvent, high yields of both 1 and 2 were obtained as
highly air-sensitive orange-red microcrystalline solids.
The new compound 2 was fully characterized by NMR,
Further support for the existence of facile monomer-
dimer equilibria for complexes 1 and 2 is given from
redistribution processes involving mixtures of both
species in solution (Scheme 2). Separate solutions of 1
and 2 each exhibit a sharp singlet in 31P{1H} NMR
spectra at δ 23.40 and 33.80, respectively. Dissolution
of approximately equimolar amounts of 1 and 2 in
toluene-d8 gave, after 3 days at 20 °C, an equilibrium
mixture of 1, 2, and the “mixed dimer” (µ-dcpe)(µ-dippe)-
(14) The reductive elimination of ethane from [R2P(CH2)nPR2]PdMe2
to give dipalladium complexes is limited to cases where n ) 1.13
(15) To our knowledge, only two other reports describe the synthesis
of Pd(0) complexes via hydrazine reduction of Pd(II) starting materi-
als: that for (Ph3P)4Pd (Malatesta, L.; Angoletta, M. J . Chem. Soc.
1957, 1186) and that for (1,2-(diphenylphosphinoethynyl)benzene)2Pd
(Coalter, N. L.; Concolino, T. E.; Streib, W. E.; Hughes, C. G.;
Rheingold, A. L.; Zaleski, J . M. J . Am. Chem. Soc. 2000, 122, 3112).
(16) Diversi, P.; Ingrosso, G.; Lucherini, A.; Lumini, T.; Marchetti,
F.; Adovasio, V.; Nardelli, M. J . Chem. Soc., Dalton Trans. 1988, 133.
(17) Synthesis of 1: A vigorously stirred suspension of (dcpe)PdCl2
(3.225 g, 5.376 mmol) in 28 mL of THF was treated at once with N2H4
(3.446 g, 107.520 mmol). Within 30 s, the color began to change to
orange-red. After 20 min, 30 mL of pentane was added and the dark
orange-red mixture was filtered through a bed of Celite. Volatiles were
removed completely from the filtrate by prolonged exposure to vacuum
(16 h) to leave a dark orange-red microcrystalline solid that was pure,
as judged by 1H and 31P{1H} NMR: yield 1.921 g (68%). 1H and 31P-
{1H} NMR match the reported data.10 Synthesis of 2: A vigorously
stirred suspension of (dippe)Pd(OAc)2 (5.649 g, 11.603 mmol) in 40 mL
of pentane was treated at once with N2H4 (5.578 g, 174.045 mmol),
whereupon immediate rapid gas evolution and a color change to deep
orange-red was observed. After 30 min, the mixture was filtered
through a bed of Celite. Volatiles were removed completely from the
filtrate by prolonged exposure to vacuum (16 h) to leave a dark orange-
red solid; yield 3.868 g (90%). 31P{1H} NMR (C6D6) of this crude product
showed that it contained 2-3% [(dippe)Pd]2(µ-dippe). Crystallization
from Et2O at -78 °C afforded the pure product as red-orange crystals
that were dried in vacuo: 1H NMR (C6D6) δ 1.33 (m, 24, CH(CH3)2,
1.61 (m, 4, PCH2CH2P), 1.81 (m, 4, CH(CH3)2); 13C{1H} NMR (C6D6) δ
20.93 (br, CH(CH3)2), 22.60 (m, PCH2CH2P), 26.64 (m, CH(CH3)2); 31P-
{1H} NMR (C6D6) δ 33.80 (s); UV-vis (hexane) λmax 455 nm; HRMS
(EI, 70 eV) m/z calcd for C28H64P4106Pd108Pd 738.2032, found 738.2011
(M+); Anal. Calcd for C28H64P4Pd2: C, 45.60; H, 8.75. Found: C, 45.57;
H, 8.80.
(19) A preliminary X-ray crystallographic analysis of 2 has been
performed and supports the dinuclear structure shown in eq 1. A
successful refinement could not be obtained due to chemical disorder
within the crystal.
(20) Harvey, P. D.; Gray, H. B. J . Am. Chem. Soc. 1988, 110, 2145.
(21) (a) Dedieu, A.; Hoffmann, R. J . Am. Chem. Soc. 1978, 100, 2074.
(b) Sakaki, S.; Ogawa, M.; Musashi, Y. J . Phys. Chem. 1995, 99, 17134.
(22) Portnoy, M.; Milstein, D. Organometallics 1993, 12, 1655.
(23) (a) Woo, T. K.; Pioda, G.; Rothlisberger, U.; Togni, A. Organo-
metallics 2000, 19, 2144. (b) Suginome, M.; Oike, H.; Shuff, P. H.; Ito,
Y. Organometallics 1996, 15, 2170. (c) Suginome, M.; Oike, H.; Park,
S.-S.; Ito, Y. Bull. Chem. Soc. J pn. 1996, 69, 289. (d) Suginome, M.;
Kato, Y.; Takeda, N.; Oike, H.; Ito, Y. Organometallics 1998, 17, 495.
(e) Murakami, M.; Yoshida, T.; Ito, Y. Organometallics 1994, 13, 2900.
(f) Pan, Y.; Mague, J . T.; Fink, M. J . Organometallics 1992, 11, 3495.
(24) Synthesis of 4: A stirred orange-red solution of 2 (300 mg, 0.407
mmol) in 10 mL of benzene was treated with PhSiH3 (528 mg, 4.881
mmol), whereupon a rapid color change to pale amber occurred. After
45 min, the product had crystallized from the reaction mixture as fine
white needles. Pentane (20 mL) was added to induce further crystal-
lization. The mother liquor was siphoned from the crystals, which were
then suspended in pentane, collected by filtration, washed with 3 × 5
mL of pentane, and dried in vacuo; yield 432 mg (91%): 1H NMR (CD2-
Cl2) 1.03 (m, 24, CH(CH3)2), 1.80 (m, 4, PCH2CH2P), 2.28 (m, 4,
CH(CH3)2), 4.46 (m, 4, Si-H, J SiH ) 82), 7.17 (m, 6, Ph), 7.59 (m, 4,
Ph); 31P{1H} NMR (CD2Cl2) δ 65.25 (s). Anal. Calcd for C26H46P2-
PdSi2: C, 53.55; H, 7.95. Found: C, 53.48; H, 8.20.
(18) Attempted reductions with Mg sand led to no reactions, and
those with KC8 gave irreproducible and low yields of the products.

Communications
Organometallics, Vol. 20, No. 14, 2001 2961
Sch em e 1. Rea ctivity of [(d ip p e)P d ]2 (2)
Sch em e 2. F or m a tion of “Mixed Dim er ” 5
Pd2 (5), in an approximate 1.0:3.0:3.7 composition. The
31P{1H} NMR spectrum of this mixture is presented in
Figure 1 along with a computer simulation of resonances
for 5 that were analyzed as an AA′BB′ spin system.25
The derived chemical shift values of 25.1 and 33.3 ppm
are nearly the same as the values of the pure complexes
1 and 2, respectively. A large coupling constant of 267.45
Hz for |J AB| is also consistent with structure 5, in which
the trans phosphorus nuclei are inequivalent. Additional
coupling constants listed below Figure 1 are in general
agreement with those found for AA′BB′ spin systems of
other dipalladium complexes that contain bridging bis-
phosphine ligands.26
We have disclosed a highly efficient synthesis of the
dinuclear palladium complexes 1 and 2 in which multi-
gram quantities of these complexes can now be prepared
and investigated. The formation of the “mixed dimer” 5
unequivocally establishes a monomer-dimer equilibri-
um of 1 and 2 in solution, a finding that illustrates the
propensity of these dimers to serve as “bottled” sources
of the highly reactive 14-electron fragments (P-P)Pd.
We are currently exploring the generality of hydrazine
reduction routes to other related dinuclear palladium
complexes which contain chelating bis-phosphine ligands
as well as the relevance of these complexes to catalytic
processes.
F igu r e 1. (Bottom) 161.96 MHz 31P{H} NMR spectrum
of an equilibrium mixture containing 1, 2, and 5 in toluene-
d8. The resonances for 1 and 2 appear as singlets and are
labeled as such. The remaining AA′BB′ multiplet is at-
tributed to complex 5. (Top) Simulated and fitted AA′BB′
pattern for 5 with δA 33.25, δB 25.10, J AB ) (267.45 Hz,
J AB′ ) (3.37 Hz, J AA′ ) (35.11 Hz, and J BB′ ) (40.95 Hz.
Ack n ow led gm en t. We thank Dow Corning Corp.
and the Tulane Center for Bioenvironmental Research
(DSWA) for financial support of this work. We also
thank David Bostwick and Sarah Shealy of the Georgia
Institute of Technology for their assistance with mass
spectrometry.
(25) After subspectral analysis, starting data and the experimental
spectrum for 5 were used for iterative, least-squares optimization of
chemical shifts and coupling constants using the gNMR 4.1 software
package (Budzelaar, P. H. M. Cherwell Scientific Limited, 1999).
(26) (a) Balch, A. L.; Hunt, C. T.; Lee, C. L.; Olmstead, M. M.; Farr,
J . P. J . Am. Chem. Soc. 1981, 103, 3764. (b) Lee, C.-L.; Yang, Y.-P.;
Rettig, S. J .; J ames, B. R.; Nelson, D. A.; Lilga, M. A. Organometallics
1986, 5, 2220.
Su p p or tin g In for m a tion Ava ila ble: Detailed experi-
mental procedures for (dippe)Pd(OAc)2 and 4. This material
is available free of charge via the Internet at http://pubs.acs.org.
OM010281Z