Synthesis, Structure, and Reductive Elimination Chemistry of Three-Coordinate Arylpalladium Amido Complexes

Article abstract of DOI:10.1021/ja0315107

Four three-coordinate arylpalladium amido complexes with a single hindered phosphine were isolated and structurally characterized. Each possessed a T-shaped geometry. Several of these complexes possessed true three-coordinate structures that lacked any additional coordination by ligand C-H bonds. All of the three-coordinate complexes underwent reductive elimination to form the corresponding triarylamine. A comparison of the rate of reaction of the three-coordinate compounds demonstrated that the rate of elimination from the pentaphenylferrocenyl di-tert-butylphosphine complex were the fastest. A comparison of the rates of reactions between three-coordinate and four-coordinate complexes showed that the rates were much faster from the three-coordinate complexes. Copyright

Full text of DOI:10.1021/ja0315107

Published on Web 04/13/2004
Synthesis, Structure, and Reductive Elimination Chemistry of
Three-Coordinate Arylpalladium Amido Complexes
Makoto Yamashita and John F. Hartwig*
Department of Chemistry, Yale UniVersity, P.O. Box 208107, New HaVen, Connecticut 06520-8107
Received December 4, 2003; E-mail: john.hartwig@yale.edu
Scheme 1. Syntheses of Three-Coordinate Arylpalladium Amides
2a-2c
Three-coordinate organopalladium(II) complexes are often claimed
to be the species that forms the carbon-carbon or carbon-hetero-
atom bond in the product during reductive eliminations of palladium
complexes with monodentate ligands.1-3 The intermediacy of such
complexes has been deduced primarily from kinetic and compu-
tational studies. Three-coordinate organopalladium alkyl, aryl, al-
koxo, or amido complexes have never been isolated or observed
directly. The existence of true three-coordinate palladium(II) com-
pounds without a donor in the open coordination site has even been
called into question.4 We recently isolated arylpalladium halide
complexes with a single phosphine,5 but each of these complexes
contained an agostic interaction at the open site of a T-shaped
geometry.
We now report the generation, isolation, structural characteriza-
tion, and reductive elimination of three-coordinate arylpalladium
amido complexes that form the arylamine in catalytic C-N coup-
lings.6-8 These amido complexes are monomeric, with a single
phosphine ligand, and several display true T-shaped structures. All
of the complexes undergo direct reductive elimination. These re-
ductive eliminations allow a direct comparison of the rates of elimi-
nation from three- and four-coordinate arylpalladium amido com-
plexes and the rates and mechanism for elimination from arylpal-
ladium halide9 and amide complexes with the same dative ligand.
To isolate three-coordinate arylpalladium amido complexes, we
focused on the preparation of complexes with an electron-donating
substituent on the palladium-bound aryl group and electron-
withdrawing substituents on the amide. These groups on the
respective anionic ligands retard reductive elimination.3a,10 Ani-
sylpalladium bromide complexes 1a-1c were synthesized from
Pd(dba)2, anisyl bromide and the hindered alkylphosphines P(t-
Bu)3, FcP(t-Bu)2 (Fc ) ferrocenyl) and Ph5FcP(t-Bu)2 (Ph5Fc )
1′,2′,3′,4′,5′-pentaphenyl-1-ferrocenyl) by methods related to those
published recently.11 Treatment of these complexes with potassium
bis(3,5-bis(trifluoromethyl)phenyl)amide in ether or THF generated
three-coordinate arylpalladium amides 2a-2c (see Scheme 1) that
were stable at room temperature and were isolated as crystalline
solids, albeit in modest yields due to their high solubility.
The palladium amido complex 3 containing a heteroaromatic
thienyl group was isolated by a modified method (eq 1) because
Table 1. Selected Bond Lengths (Å) and Angles (deg) of
Three-Coordinate and Four-Coordinate Arylpalladium Amido
Complexes 2a-2c, 3, and 4a
Pd−P(1)
Pd−N
Pd−C
P(1)−Pd−N
C−Pd−N
amide
(Å)
(Å)
(Å)
(deg)
(deg)
2a
2a′
2b
2c
3
2.2809(11)
2.2849(12)
2.2965(10)
2.2637(10)
2.2741(12)
2.2786(12)
2.078(3)
2.080(3)
2.070(3)
2.091(2)
2.073(2)
2.107(3)
1.987(4)
1.984(4)
1.965(3)
1.988(3)
1.970(3)
2.041(4)
168.23(10)
166.00(9)
167.23(7)
168.52(7)
170.66(7)
168.32(8)
89.09(15)
93.60(14)
93.46(22)
91.12(22)
88.75(11)
83.86(13)
4a
Figure 1. ORTEP drawing of one of two independent molecules of 2a
(30% thermal ellipsoid, hydrogen atoms are omitted for clarity).
to those of the corresponding four-coordinate (DPPF)Pd(anisyl)-
[N(3,5-(CF3)2C6H3)2] (4a). An ORTEP diagram of P(t-Bu)3-complex
2a is shown in Figure 1; ORTEP diagrams of 2b, 2c, 3, and 4a are
provided as Supporting Information. The phosphine was located
trans to the amido group, which has a weaker trans influence than
an aryl group,13 in all the complexes with monodentate phosphines.
The Pd-N and Pd-P bond distances in 2a-2c and 3 were similar
to those in DPPF-ligated 4a. However, the absence of a ligand trans
to the aryl group in three-coordinate 2a-2c and 3 causes the Pd-C
bonds in 2a-2c and 3 to be shorter than that in 4a.
[(PtBu3)Pd(2-thienyl)(Br)] is unstable toward formation of the
Pd(I) dimer (PtBu3PdBr)2.12 Reaction of 2-thienyl bromide with
Pd(PtBu3)2 in the presence of potassium diarylamide to trap the
transient thienylpalladium bromide formed the heteroarylpalladium
amide 3. Complex 3 was isolated as red crystals.
Single-crystal X-ray diffraction of 2a-2c and 3 showed the
complexes to possess three-coordinate T-shaped structures. Table
1 provides a comparison of structural parameters of these complexes
Complex 2a crystallized with two independent molecules in the
asymmetric unit. One of these molecules and thienyl complex 3
9
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J. AM. CHEM. SOC. 2004, 126, 5344-5345
10.1021/ja0315107 CCC: $27.50 © 2004 American Chemical Society

C O M M U N I C A T I O N S
Scheme 2. Reductive Elimination of Amine from 2a-2c and 3
with potassium di-p-tolylamide at -30 °C in THF to generate
(tBu3P)Pd(anisyl)N(p-tol)2 (2d). The trans relationship between
phosphorus and nitrogen atoms was confirmed by a large 31P-15N
coupling (2JPN ) 40 Hz) in the analogous complex that was
generated from K15NPh2.
P(t-Bu)3-ligated 2d underwent reductive elimination at -10 °C
to form the triarylamine product in 91% yield based on the starting
arylpalladium bromide 1a. The first-order decay of this complex
corresponded to a half-life of 20 min at -10 °C. This rate is much
faster than the rate of reaction of DPPF complex 4b, which was
shown previously to eliminate with a half-life of 55 min at the
much higher temperature of 75 °C.3c
Scheme 3
In summary, arylpalladium amido complexes with a single
hindered phosphine have been isolated, and several of these
complexes display true three-coordinate, T-shaped geometries.
Kinetic data are consistent with irreversible reductive elimination
directly from the three-coordinate species. Even complexes with
highly deactivating groups on both the palladium-bound aryl ring
and the nitrogen undergo reductive elimination, and a comparison
of the rates of reaction of the three-coordinate complexes to the
rates of reaction of well-studied four-coordinate complexes show
much faster rates for elimination from the three-coordinate species.20
possessed Pd-H distances of 2.09 and 2.14 Å that suggest the pres-
ence of agostic interactions and four-coordinate palladium centers.14
However, the other independent molecule of 2a and complexes
2b and 2c were three-coordinate. The hydrogen atoms nearest the
metal in the second independent molecule in 2a, of 2b, and in 2c
were 2.44, 2.65, and 2.52 Å, respectively, and these hydrogen atoms
lay out of the palladium square plane by 0.792, 1.410 and 1.683
Å. Moreover, the Pd-Fe distances in 2b and 2c of 4.47 and 3.54
Å are well beyond the sum of atomic radii (2.617 Å),15 and are
beyond the longest Pd-Fe distance (3.228 Å)16 for a Pd-Fe bond
in the Cambridge Crystallographic database. Thus, three-coordinate,
T-shaped d8 palladium(II) complexes can exist and can be observed
directly.
The three-coordinate arylpalladium amido complexes 2a-2c and
complex 3 underwent reductive elimination upon heating in toluene.
The yields of amine from 2a and 2b exceeded 81 and 79%,
respectively; the yields of amine from the less hindered 2c and
heteroaryl 3 were lower. The major side product from reaction of
2c and 3 was diarylamine by protonolysis. The DPPF complex 4a
with an electron-poor diarylamide did not undergo reductive
elimination. Diarylamine was the major product containing nitrogen
that was identified by GC/MS.
Acknowledgment. We thank the NIH (GM-58108) for support
of this work and Johnson-Matthey for a gift of PdCl2. M.Y. thanks
JSPS for a postdoctoral fellowship.
Supporting Information Available: All experimental procedures
and spectroscopic data of new complexes; X-ray structural data for
2a-2c, 3, and 4a in CIF format. This material is available free of charge
via the Internet at http://pubs.acs.org.
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The rates of reaction of 2a-c and 3 at 75 °C were monitored by
19F NMR spectroscopy (Scheme 2). The values of kobs for reaction
of 2a (see Supporting Information for data) were indistinguishable
in the presence of 0 mM, 1 mM, or 5 mM of added P(t-Bu)3. These
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the electron-poor amide are remarkably similar to those for
elimination of aryl halide from LPd(Ar)(X) (see Scheme 3).
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