Unsaturated Pd(0), Pd(I), and Pd(II) complexes of a new methoxy-substituted benzyl phosphine. Aryl-X (X = Cl, I) oxidative addition, C-O cleavage, and Suzuki-Miyaura coupling of aryl chlorides

Article abstract of DOI:10.1021/om0497588

The 14e^- Pd(0)L₂ complex 2 was prepared by reduction of [Pd(2-methylallyl)Cl]₂ in the presence of the new, electron-rich, bulky methoxy benzyl phosphine (dmobp) ligand 1. Structural characterization of this complex indicates that the methoxy groups are not coordinated to the metal center. Complex 2 undergoes oxidative addition of iodo- and chlorobenzene at room temperature to yield the monophosphine complexes LPd(Ph)X (4, X = I; 5, X = Cl) in which the methoxy group is coordinated to the Pd(II) center in the solid state, as indicated by the X-ray structure of 4. In solution there is no evidence for methoxy coordination, indicating the availability of a Pd(II) 14e^- complex. The Me-O bond in 4 is longer than the corresponding bond in 2, indicating that coordination of the methoxy group weakens the C-O bond. Reaction of complex 4 or 5 with the free ligand 1 results in nucleophilic attack and C-O cleavage, leading to the dimeric phenoxy-bridged complex 7, which was structurally characterized. Partial reduction of [Pd(2-methylallyl)Cl] ₂ in the presence of the ligand 1 leads to the Pd(I) dimer 3, which can be converted to the Pd(0) complex 2 by addition of ligand 1 and a base. This complex, which bears only one phosphine for each Pd atom, is a suitable precursor to a presumed catalytically active 12e^- Pd(0) catalyst, Complexes 2 and 3 catalyze the Suzuki-Miyaura cross-coupling of chlorobenzene with PhB(OH)₂ even at room temperature, albeit slowly, while the C-O cleaved phenoxy-bridged complex 7 is not catalytically active at 40°C, indicating that it is not an intermediate in the catalysis. The dmobp ligand 1 is more effective in Suzuki-Miyaura coupling than an analogous benzyl ligand lacking methoxy substituents.

Full text of DOI:10.1021/om0497588

Organometallics 2004, 23, 3931-3940
3931
Un sa tu r a ted P d (0), P d (I), a n d P d (II) Com p lexes of a New
Meth oxy-Su bstitu ted Ben zyl P h osp h in e. Ar yl-X
(X ) Cl, I) Oxid a tive Ad d ition , C-O Clea va ge, a n d
Su zu k i-Miya u r a Cou p lin g of Ar yl Ch lor id es
Haim Weissman,† Linda J . W. Shimon,‡ and David Milstein*,†
Department of Organic Chemistry and Unit of Chemical Research Support,
The Weizmann Institute of Science, Rehovot, 76100, Israel
Received April 4, 2004
The 14e- Pd(0)L2 complex 2 was prepared by reduction of [Pd(2-methylallyl)Cl]2 in the
presence of the new, electron-rich, bulky methoxy benzyl phosphine (dmobp) ligand 1.
Structural characterization of this complex indicates that the methoxy groups are not
coordinated to the metal center. Complex 2 undergoes oxidative addition of iodo- and
chlorobenzene at room temperature to yield the monophosphine complexes LPd(Ph)X (4, X
) I; 5, X ) Cl) in which the methoxy group is coordinated to the Pd(II) center in the solid
state, as indicated by the X-ray structure of 4. In solution there is no evidence for methoxy
coordination, indicating the availability of a Pd(II) 14e- complex. The Me-O bond in 4 is
longer than the corresponding bond in 2, indicating that coordination of the methoxy group
weakens the C-O bond. Reaction of complex 4 or 5 with the free ligand 1 results in
nucleophilic attack and C-O cleavage, leading to the dimeric phenoxy-bridged complex 7,
which was structurally characterized. Partial reduction of [Pd(2-methylallyl)Cl]2 in the
presence of the ligand 1 leads to the Pd(I) dimer 3, which can be converted to the Pd(0)
complex 2 by addition of ligand 1 and a base. This complex, which bears only one phosphine
for each Pd atom, is a suitable precursor to a presumed catalytically active 12e- Pd(0)
catalyst. Complexes 2 and 3 catalyze the Suzuki-Miyaura cross-coupling of chlorobenzene
with PhB(OH)2 even at room temperature, albeit slowly, while the C-O cleaved phenoxy-
bridged complex 7 is not catalytically active at 40 °C, indicating that it is not an intermediate
in the catalysis. The dmobp ligand 1 is more effective in Suzuki-Miyaura coupling than an
analogous benzyl ligand lacking methoxy substituents.
In tr od u ction
zylphosphine ligand, namely, di-tert-butyl(2,6-dimethoxy-
benzyl)phosphine (dmobp), 1. This ligand is analogous
to the previously reported di-tert-butyl-(2,4,6-trimeth-
ylbenzyl)phosphine (tmbp), lacking methoxy groups.4 It
was of interest to us to explore the possibility that the
methoxy substituent would be involved in hemilabile
coordination during catalysis, thus enhancing the sta-
bility of the active species without adversely affecting
its reactivity. The dmobp ligand 1 is a monodentate
analogue of the known PCP-type ligand 1,3-bis-[(di-tert-
butylphosphanyl)methyl]-2-methoxybenzene.5 We re-
port on the preparation and X-ray characterization of
unsaturated Pd(0), Pd(I), and Pd(II) complexes of this
ligand, including a monophosphine Pd(II) complex
resulting from aryl-X (X ) Cl, I) oxidative addition and
stabilized (in the solid state) by coordination of the
methoxy moiety. Unexpectedly, this complex was ob-
served to undergo ArO-Me activation of the ligand
under mild heating. Catalysis of Suzuki-Miyaura-type
coupling of chlorobenzene by the Pd(0)L2 complex has
Bulky trialkylphosphine ligands can promote the
generation of unsaturated Pd(0) and Pd(II) intermedi-
ates and thereby enhance the catalytic activity of Pd
complexes in coupling reactions.1,2 While 14-electron
three-coordinate Pd(II) complexes are proposed as in-
termediates in many of these reactions, such complexes
are rare, and the first example of an isolated monomeric
arylpalladium(II) monophosphine complex was reported
only recently by Hartwig.3 We describe here the prepa-
ration of a new, methoxy-substituted, di-tert-butylben-
* To whom correspondence should be addressed. Fax: 972-8-
9344142. E-mail: david.Milstein@weizmann.ac.il.
† Department of Organic Chemistry.
‡ Unit of Chemical Research Support.
(1) Reviews: (a) Riermeier, T. H.; Zapf, A.; Beller, M. Top. Catal.
1997, 4, 301. (b) Littke, A. F.; Fu, G. C. Angew. Chem., Int. Ed. 2002,
41, 4176. (c) Muci, A. R.; Buchwald, S. L. Top. Curr. Chem. 2002, 219,
131. (d) Culkin, D. A.; Hartwig, J . F. Acc. Chem. Res. 2003, 36, 234.
(e) Valentine, D. H.; Hillhouse, J . H. Synthesis 2003, 2437.
(2) For early studies demonstrating the importance of bulky alkyl
phosphines in Pd catalysis see: carbonylation of aryl chlorides: (a)
Ben-David, Y.; Portnoy, M.; Milstein, D. J . Am. Chem. Soc. 1989, 111,
8742. (b) Huser, M.; Youinou, M. T.; Osborn, J . A. Angew. Chem., Int.
Ed. 1989, 28, 1386. Heck reaction of aryl chlorides: (c) Ben-David, Y.;
Portnoy, M.; Gozin, M.; Milstein, D. Organometallics 1992, 11, 1995.
(3) (a) Stambuli, J . P.; Bu¨hl, M.; Hartwig, J . F. J . Am. Chem. Soc.
2002, 124, 9346. (b) Stambuli, J . P.; Incarvito, C. D.; Bu¨hl, M.; Hartwig,
J . F. J . Am. Chem. Soc. 2004, 126, 1184.
(4) Rybtchinski, B.; Konstantinovski, L.; Shimon, L. J . W.; Vigalok,
A.; Milstein, D. Chem. Eur. J . 2000, 6, 3287.
(5) van der Boom, M. E.; Liou, S.-Y.; Ben-David, Y.; Shimon, L. J .
W.; Milstein, D. J . Am. Chem. Soc. 1998, 120, 6531. (b) van der Boom,
M. E.; Liou, S.-Y.; Ben-David, Y.; Vigalok, A.; Milstein, D. Angew.
Chem., Int. Ed. Engl. 1997, 36, 625.
10.1021/om0497588 CCC: $27.50 © 2004 American Chemical Society
Publication on Web 07/09/2004

3932 Organometallics, Vol. 23, No. 16, 2004
Weissman et al.
Sch em e 1. P r ep a r a tion of Dm obp 1
Ta ble 1. Selected Bon d Len gth s (Å) a n d An gles
(d eg) for 2
been observed to proceed even at room temperature
(albeit slowly), and the possible involvement of the
complexes mentioned above in the catalysis is ad-
dressed.
Pd(1)-P(2)
Pd(1)-P(3)
P(2)-C(21)
P(2)-C(22)
P(2)-C(26)
P(3)-C(41)
P(3)-C(43)
P(3)-C(42)
2.2860(19)
2.2965(19)
1.877(7)
1.903(7)
1.913(7)
1.868(7)
1.890(7)
1.896(7)
O(1)-C(32)
O(1)-C(37)
O(2)-C(36)
O(2)-C(38)
O(3)-C(56)
O(3)-C(57)
O(4)-C(52)
O(4)-C(58)
1.387(9)
1.424(9)
1.380(9)
1.421(9)
1.375(9)
1.435(9)
1.369(9)
1.419(9)
Resu lts a n d Discu ssion
Liga n d Syn th esis. The new dmobp ligand 1 was
synthesized in three steps from the commercially avail-
able 2,6-dimethoxybenzoic acid. Reduction of this com-
pound with LiAlH4 according to a literature procedure6
yielded 2,6-dimethoxybenzyl alcohol, which was con-
verted to 2,6-dimethoxybenzyl chloride in 92% yield by
its reaction with N-chlorosuccinimide-dimethyl sulfide
adduct (Scheme 1). Reaction of this compound with
lithium di-tert-butyl phosphide yielded the pure ligand
1 as a colorless oil in 25% yield after purification by
column chromatography.
P(2)-Pd(1)-P(3)
C(21)-P(2)-Pd(1)
C(22)-P(2)-Pd(1)
C(26)-P(2)-Pd(1)
C(41)-P(3)-Pd(1)
C(43)-P(3)-Pd(1)
C(42)-P(3)-Pd(1)
166.86(7) C(56)-O(3)-C(57) 116.4(6)
121.9(2)
110.6(2)
111.5(2)
122.8(2)
111.6(2)
109.4(2)
C(52)-O(4)-C(58) 118.1(6)
C(33)-C(32)-O(1) 123.5(7)
O(1)-C(32)-C(31) 114.1(7)
O(2)-C(36)-C(35) 123.5(7)
O(2)-C(36)-C(31) 114.7(7)
O(4)-C(52)-C(51) 116.4(7)
O(4)-C(52)-C(53) 122.5(7)
C(55)-C(56)-O(3) 123.6(7)
C(32)-O(1)-C(37) 117.0(6)
C(36)-O(2)-C(38) 118.0(6)
Syn th esis of a P d (0) Com p lex. The Pd(0) complex
2 was prepared in analogy with a literature procedure7
by reaction of [(2-methylallyl)PdCl]2 in MeOH with
NaOH in the presence of ligand 1. It precipitated as a
white powder in 85% yield (eq 1). The 31P{1H} NMR
spectrum of 2 exhibits a singlet at 60.89 ppm, indicating
that the two phosphorus atoms are equivalent and that
the complex is symmetric. The tert-butyl groups appear
in the 1H NMR spectrum at 1.49 ppm as a virtual
triplet, indicating that the phosphines are disposed in
a mutually trans arrangement.
F igu r e 1. ORTEP drawing of a molecule of 2 (50%
probability level). Hydrogen atoms are omitted for clarity.
the tert-butyl groups, and it is 10° smaller than the
analogous angle reported for ((tBu)2PhP)2Pd(0),8b indi-
cating a large steric effect induced by the benzylic
moiety.
In the X-ray of 2 as well as in NMR spectra there is
no evidence of methoxy group coordination, despite the
high unsaturation of the 14e- Pd center and the
potentially relatively stable six-membered ring that
would have been formed upon coordination of a methoxy
group. This is likely due to the “softness” of the Pd(0)
center, which renders the “hard” oxygen atom coordina-
tion unfavorable.
White crystals (plates) suitable for a single-crystal
X-ray diffraction study were obtained upon slow evapo-
ration of a pentane solution of 2 under nitrogen at room
temperature. The overall structural features (Figure 1,
Table 1) are similar to those reported for the very few
other structurally characterized 14e- Pd(0) complexes.8
Interestingly, the P-Pd-P angle (166.9°) is bent toward
(8) Immirzi, A.; Musco, A. Chem. Commun. 1974, 400. (b) Otsuka,
S.; Yoshida, T.; Matsumoto, M.; Nakatsu, K. J . Am. Chem. Soc. 1976,
98, 5850. (c) Paul, F.; Patt, J .; Hartwig, J . F. J . Am. Chem. Soc. 1994,
116, 5969. (d) Mann, G.; Incarvito, C.; Rheingold, A. L.; Hartwig, J . F.
J . Am. Chem. Soc. 1999, 121, 3224.
(6) Forrester, J .; J ones, R. V. H.; Newton, L.; Preston, P. N.
Tetrahedron 2001, 57, 2871.
(7) Kuran, W.; Musco, A. Inorg. Chim. Acta 1975, 12, 187.

Unsaturated Pd(0), Pd(I), and Pd(II) Complexes
Organometallics, Vol. 23, No. 16, 2004 3933
Ta ble 2. Selected Bon d Len gth s (Å) a n d An gles
(d eg) for 3
Pd(1)-C(102)
Pd(1)-P(2)
Pd(1)-Cl(1)
Pd(1)-Pd(2)
Pd(2)-C(101)
Pd(2)-P(1)
Pd(2)-Cl(1)
P(1)-C(18)
P(1)-C(31)
P(1)-C(27)
P(2)-C(1)
2.072(5)
C(1)-C(2)
1.510(6)
1.370(6)
1.399(7)
1.380(8)
1.374(7)
1.401(7)
1.371(6)
1.417(6)
1.420(6)
1.409(7)
1.401(7)
1.532(7)
2.3122(12)
2.4223(12)
2.6338(5)
2.053(5)
2.3145(12)
2.4222(12)
1.870(5)
1.895(4)
1.903(5)
1.881(4)
1.898(5)
C(3)-O(2)
C(3)-C(4)
C(4)-C(5)
C(5)-C(6)
C(6)-C(7)
C(7)-O(1)
O(1)-C(8)
O(2)-C(9)
C(101)-C(100)
C(100)-C(102)
C(100)-C(103)
P(2)-C(14)
P(2)-C(10)
1.901(5)
C(102)-Pd(1)-P(2) 101.36(14) Pd(2)-Cl(1)-Pd(1)
C(102)-Pd(1)-Cl(1) 145.43(15) C(2)-C(1)-P(2)
65.87(3)
117.1(3)
117.3(4)
120.2(4)
122.4(4)
122.4(4)
116.2(4)
121.3(5)
114.8(4)
123.1(4)
122.2(4)
118.9(4)
118.0(4)
88.1(3)
F igu r e 2. ORTEP drawing of a molecule of 3 (50%
probability level). Hydrogen atoms are omitted for clarity.
P(2)-Pd(1)-Cl(1)
112.81(4) C(7)-C(2)-C(3)
C(102)-Pd(1)-Pd(2) 88.44(14) C(7)-C(2)-C(1)
P(2)-Pd(1)-Pd(2)
Cl(1)-Pd(1)-Pd(2)
164.43(3) C(3)-C(2)-C(1)
57.06(3) O(2)-C(3)-C(4)
When the preparation of complex 2 lacked sufficient
excess of ligand and base, an additional signal appeared
in the 31P{1H} NMR spectrum at 64.01 ppm (singlet),
which could be washed away with a small amount of
diethyl ether. Suspecting that this compound was a
partially reduced Pd complex, we prepared and char-
acterized the complex by reacting [(2-methylallyl)PdCl]2
with only 1 equiv of dmobp.
Syn th esis of a P d (I) Com p lex. Addition of 2 molar
equiv of ligand 1 to a suspension of the dimer [(2-
methylallyl)PdCl]2 in MeOH (i.e., ratio 1/Pd ) 1) fol-
lowed immediately with 2 molar equiv of NaOH re-
sulted, after stirring overnight at room temperature, in
complex 3, which precipitated as a green powder.
Extraction with benzene and concentration under high
vacuum gave complex 3 as a yellow powder in 95% yield
(eq 2). The 31P{1H} NMR spectrum shows one singlet
at 64.01 ppm, indicating that the two phosphorus atoms
are equivalent and that the complex is symmetric.
However, the 1H NMR spectrum shows two different
P-C(CH3)3 groups at 1.48 and 1.53 ppm as virtual
triplets. In 13C{1H} NMR the methyl groups appear at
30.04 and 30.36 ppm as virtual triplets. The different
chemical shifts indicate asymmetry in the molecule.
C(101)-Pd(2)-P(1) 102.90(15) O(2)-C(3)-C(2)
C(101)-Pd(2)-Cl(1) 144.78(15) C(4)-C(3)-C(2)
P(1)-Pd(2)-Cl(1)
112.32(4) O(1)-C(7)-C(2)
C(101)-Pd(2)-Pd(1) 87.71(15) O(1)-C(7)-C(6)
P(1)-Pd(2)-Pd(1)
Cl(1)-Pd(2)-Pd(1)
C(1)-P(2)-C(14)
C(1)-P(2)-C(10)
C(14)-P(2)-C(10)
C(1)-P(2)-Pd(1)
C(14)-P(2)-Pd(1)
C(10)-P(2)-Pd(1)
169.23(3) C(2)-C(7)-C(6)
57.07(3) C(7)-O(1)-C(8)
101.5(2)
104.7(2)
108.8(2)
C(3)-O(2)-C(9)
C(100)-C(101)-Pd(2)
C(102)-C(100)-C(101) 125.3(5)
119.69(14) C(102)-C(100)-C(103) 116.4(5)
112.93(16) C(101)-C(100)-C(103) 117.0(5)
108.55(16) C(100)-C(102)-Pd(1)
87.5(3)
Sch em e 2. Rea ction of 2 w ith Ar yl Ha lid es
the allyl plane and the plane of the Pd-Pd-Cl atoms,
87.6°, is larger than the corresponding angle in other
allyl- and halide-bridged Pd(I) clusters, which are in the
range of 80-84°.9a The angle is smaller than 90° due to
back-bonding interaction in the dipalladium complex
involving overlap between the central carbon p orbital
and the dσ-dσ and dπ-dπ orbitals of Pd2.9a The larger
angle might be a result of the higher steric bulk relative
to other reported dimers that contained PPh3 or PiPr3
as ligands.9
Syn th esis a n d Rea ctivity of P d (II) Com p lexes.
Oxid a tive Ad d ition of Ar yl Ha lid es. The reactivity
of the Pd(0) complex 2 toward oxidative addition of aryl
halides was studied. Reaction with 1 equiv of iodoben-
zene in C6D6 at room temperature proceeded smoothly,
quantitatively yielding complex 4 after 2 days (Scheme
2). During the follow-up of the reaction in 31P{1H} NMR,
the peak corresponding to complex 2 disappeared, giving
rise to two new peaks in a 1:1 ratio at 64.45 and 35.25
ppm, the latter corresponding to the free ligand 1. After
evaporation and successive washing with pentane, the
Yellow crystals were obtained upon slow evaporation
of a benzene solution of 3 under nitrogen at room
temperature. The overall features of the structure of 3
(Figure 2, Table 2) are similar to those observed with
other Pd(I) allyl dimer complexes.9 The angle between
(9) (a) Review: Murahashi, T.; Kurosawa, H. Coord. Chem. Rev.
2002, 231, 207. For example: (b) Werner, H.; A Ku¨hn, J . Organomet.
Chem. 1979, 179, 439. (c) Krause, J .; Goddard, R.; Mynott, R.; Po¨rschke,
K.-R. Organometallics 2001, 20, 1992.

3934 Organometallics, Vol. 23, No. 16, 2004
Weissman et al.
oxidative addition complex (dmobp)Pd(Ph)I 4 was ob-
tained in 60% isolated yield as a white powder. 1H NMR
showed a new set of aromatic peaks at 6.70, 6.83, and
7.37 ppm. 13C{1H} NMR exhibited three doublets at
116.29, 126.54, and 139.19 ppm, with J CP ) 1.2, 1.2,
and 3.4 Hz, respectively. In 1H NMR the tert-butyls
appear as a doublet (J HP ) 13.5 Hz), indicating that only
one phosphine is bound to the Pd center.
Crystals of complex 4 were obtained by slow evapora-
tion of a C6H6/THF solution. The X-ray structure of 4
(Figure 3, Table 3) shows a square planar coordination
around the Pd center, the methoxy group occupying one
of the corners. Coordination of the methoxy group is not
observed in solution by NMR, probably due to its
hemilabile character. Another case of oxidative addition
that results in an alkoxy group coordinated to the Pd
(II) center was reported recently.10 The phenyl ring is
located trans to the methoxy group, and the iodide
ligand is positioned trans to the phosphorus atom. This
configuration is probably the thermodynamically stable
one, the ligands with the high trans influence being
located opposite the weak trans influence ligands.
Hartwig has recently reported the monomeric, 14e-
F igu r e 3. ORTEP drawing of a molecule of 4 (50%
probability level). Hydrogen atoms are omitted for clarity.
Ta ble 3. Selected Bon d Len gth s (Å) a n d An gles
(d eg) for 4
t
t
Pd(II) complexes Bu2(adamantyl)PPd(Ph)Br and Bu3-
PPd(2,4-xylyl)I, formed by aryl halide oxidative addition
I(1)-Pd(1)
Pd(1)-C(10)
Pd(1)-O(2)
Pd(1)-P(1)
P(1)-C(1)
2.6481(9)
1.977(5)
2.276(3)
2.3172(15)
1.870(4)
1.882(4)
P(1)-C(16)
O(1)-C(7)
O(1)-C(8)
O(2)-C(3)
O(2)-C(9)
1.888(5)
1.372(6)
1.429(6)
1.395(5)
1.449(5)
t
to PdL2.3 With Bu2(adamantyl)PPd(Ph)Br, an agostic
interaction between the Pd center and an adamantyl-H
bond was observed, the hydrogen atom occupying one
of the corners of the twisted square plane around the
Pd center.
P(1)-C(17)
C(10)-Pd(1)-O(2) 176.44(16) C(9)-O(2)-Pd(1)
121.4(3)
116.3(3)
122.7(4)
114.3(4)
123.5(4)
115.4(4)
Another fact worth noting about the structure of
complex 4 is the elongation of the Ar-O (O(2)-C(3)
1.395 Å) and O-Me (O(2)-C(9) 1.449 Å) bonds of the
coordinated methoxy group relative to the Ar-O (O(1)-
C(7) 1.372 Å) and the noncoordinated O-Me (O(1)-C(8)
1.429 Å). This elongation suggests the weakening of the
C-O bond, making its activation possible. Indeed, C-O
activation in our system was observed, as will be
detailed later.
C(10)-Pd(1)-P(1)
O(2)-Pd(1)-P(1)
C(10)-Pd(1)-I(1)
O(2)-Pd(1)-I(1)
P(1)-Pd(1)-I(1)
C(1)-P(1)-C(17)
C(1)-P(1)-C(16)
94.42(13) C(2)-C(1)-P(1)
86.03(9) C(4)-C(3)-O(2)
89.08(13) C(2)-C(3)-O(2)
90.91(8)
172.14(3)
100.9(2)
104.5(2)
O(1)-C(7)-C(6)
O(1)-C(7)-C(2)
C(11)-C(10)-Pd(1) 122.1(4)
C(15)-C(10)-Pd(1) 119.4(4)
C(17)-P(1)-C(16) 113.4(2)
C(18)-C(16)-P(1)
113.3(3)
105.8(3)
111.8(3)
114.2(3)
105.2(3)
111.2(3)
C(1)-P(1)-Pd(1)
C(17)-P(1)-Pd(1) 113.38(16) C(20)-C(16)-P(1)
C(16)-P(1)-Pd(1) 118.58(16) C(21)-C(17)-P(1)
103.34(15) C(19)-C(16)-P(1)
As expected, there is a large difference in the reactiv-
ity of iodobenzene and chlorobenzene with complex 2.
When 0.02 mmol of iodobenzene was reacted with 0.02
mmol of complex 2 in 500 µL of C6D6 at room temper-
ature, ca. 88% yield was obtained after 4 h. On the other
hand, reaction of 10 equiv of chlorobenzene with com-
plex 2 under the same conditions resulted in ca. 2% yield
after 4 h. Thus, the reaction of complex 2 with iodoben-
zene is more than 2 orders of magnitude faster than
with chlorobenzene (roughly 400 times faster in case of
first-order dependence in the complex and the aryl
halide).
C(7)-O(1)-C(8)
C(3)-O(2)-C(9)
C(3)-O(2)-Pd(1)
117.3(4)
116.9(3)
107.0(2)
C(23)-C(17)-P(1)
C(22)-C(17)-P(1)
at 115.77, 126.93, and 137.91 ppm, with J CP ) 1.6, 1.2,
and 3.2 Hz, respectively. In 1H NMR the tert-butyls
appear as a doublet with J HP ) 13.5 Hz, indicating that
only one phosphine is bound to the Pd center.
C-O Bon d Activa tion of a n Ar om a tic Meth yl
Eth er by a Mon op h osp h in e P d Com p lex. Complex
4 was stable for days at 45 °C and for at least 1.5 h at
65 °C, but when a solution of complex 4 was heated in
C6D6 at 80 °C, it decomposed to unidentified products.
However, upon prolonged heating of complex 4 or 5 in
C6D6 at 80 °C in the presence of 1 equiv of ligand 1,
these complexes and the ligand disappeared and a white
precipitate, identified as the phosphonium salt 6 (see
below), appeared. 31P{1H} NMR of the solution exhibited
in both cases a new singlet at 92.08 ppm, corresponding
to complex 7 (see below) and an AX system at 70.91 and
40.23 ppm (J PP ) 367.1 Hz). Additional peaks were
observed at 49.07 and 49.80 ppm in the thermolysis of
complexes 4 and 5, respectively. Complex 5 decomposed
faster than complex 4. Follow-up of the reaction revealed
that after 20 h ca. 30% of complex 4 converted to
The complex (dmobp)Pd(Ph)Cl, 5, was prepared by
reaction of complex 2 with 10 equiv of chlorobenzene in
C6D6 at 40 °C. The reaction proceeded smoothly and
quantitatively yielded complex 5 after 2 days. Reaction
follow-up by 31P{1H} NMR revealed that the peak
corresponding to complex 2 disappeared, giving rise to
two new peaks in a 1:1 ratio at 71.01 and 35.25 ppm,
the latter corresponding to the free ligand 1. After
evaporation and successive washing with pentane the
oxidative addition product, (dmobp)Pd(Ph)Cl, 5, was
1
obtained in 70% isolated yield as a white powder. H
NMR showed a new set of aromatic peaks at 6.80, 6.89,
and 7.43 ppm. 13C{1H} NMR exhibited three doublets

Unsaturated Pd(0), Pd(I), and Pd(II) Complexes
Organometallics, Vol. 23, No. 16, 2004 3935
Sch em e 3. Dea lk yla tion of η2-Coor d in a ted Dm obp by a Secon d Dm obp Liga n d
complex 7 and 17% to the peaks at 70.91 and 40.23 ppm.
Under similar conditions, after 11 h ca. 42% of complex
5 converted to complex 7 and 17% to the peaks at 70.91
and 40.23 ppm. 31P{1H} NMR of complex 4 after 20 h
of heating at 80 °C revealed an integration ratio of
complex 7/complex 4/AX system of 0.6:1.0:0.6. Upon
addition of ligand 1 (2.5 equiv relative to the amount of
complex 4 prior to the thermolysis) at room temperature
to the reaction mixture, the ratio of the peaks changed
to 0.2:1.0:1.0, respectively. In other words, complex 7
diminished and the peaks of the AX system increased
at its expense relative to the peak of complex 4. It seems
that the compound, which is responsible for the AX
system, is the product of the reaction between ligand 1
and 7 in the presence of 1 equiv of ligand 1 (see below).
The white solid was characterized as the phospho-
nium salt of the ligand dmobp. It was independently
prepared by reaction of dmobp with MeI and had
identical spectroscopic properties. The chemical shifts
in 31P{1H} NMR of the phosphonium salts dmobp(Me)I,
6, and dmobp(Me)Cl are 49.07 and 49.86 ppm, respec-
tively.11
complex containing the demethylated ligand, [(η2-dmobp-
µ-O)PdPh]2, complex 7, which has a dimeric form in the
solid state (Figure 4; Scheme 3). Complex 7 was
prepared by reaction of complex 2 with 10 equiv of
chlorobenzene in C6D6 at 80 °C. The reaction proceeded
smoothly and quantitatively. Follow-up of the reaction
by 31P{1H} NMR revealed that the concentration of
complex 2 and later complex 5 and ligand 1 diminish,
giving rise to a new singlet at 92.08 ppm corresponding
to complex 7, in addition to a small amount of the AX
system mentioned earlier. After evaporation and suc-
cessive washing with pentane, the phenoxide complex,
[(η2-dmobp-µ-O)PdPh]2, 7, was obtained in 61% isolated
1
yield as a white powder and was characterized by H
NMR and 13C{1H} NMR. The aromatic carbon, para to
the phenoxide oxygen, appears in the 13C{1H} NMR at
99.61 ppm. The carbon para to the methoxy moves from
106.78 ppm in complex 5 to 114.63 ppm. The chemical
shifts of the phenyl ring carbons almost do not shift
relative to the corresponding ones of complexes 4 and
5.
The compound responsible for the AX system in the
31P{1H} NMR at 70.91 and 40.23 ppm, although not
isolated, may belong to the monomer (η2-dmobp-O)Pd-
(dmobp)Ph, which might form by bridge cleavage of the
dimer 7 by dmobp. Indeed, addition of dmobp to 7
resulted in appearance of the AX system in the 31P{1H}
NMR. The analogous (η2-TMMP-O)Pd(TMMP)Cl was
reported by Dunbar.15f The peak at 40.23 ppm is broad,
indicating dynamic behavior of the bound monodentate
dmobp. (η2-dmobp-O)Pd(dmobp)Ph might be in an equi-
librium with complex 7, which did not disappear even
after prolonged heating at 80 °C in the presence of more
than 2 equiv of dmobp. This fact is compatible with the
dynamic character of the bound dmobp.
We confirmed the C-O bond activation by the prepa-
ration, isolation, and characterization of the new Pd
(10) Alami, M.; Amatore, C.; Bensalem, S.; Choukchou-Brahim, A.;
J utand, A. Eur. J . Inorg. Chem. 2001, 2675.
(11) The dmobp(Me)Cl salt yielded single crystals that were suitable
for X-ray crystallography.
(12) (a) Albert, J .; Granell, J .; Moragas, R.; Font-Bardia, M.; Solans,
X. J . Organomet. Chem. 1996, 522, 59. (b) Kim, J . S.; Sen, A.; Guzei,
I. A.; Liable-Sands, L. M.; Rheingold, A. L. J . Chem. Soc., Dalton Trans.
2002, 24, 4726. (c) Sembiring, S. B.; Colbran, S. B.; Craig, D. C.;
Scrudder, M. L. J . Chem. Soc., Dalton Trans. 1995, 22, 3731.
(13) (a) Mann, G.; Incarvito, C.; Rheingold, A. L.; Hartwig, J . F. J .
Am. Chem. Soc. 1999, 121, 3224. (b) Shelby, Q.; Kataoka, N.; Mann,
G.; Hartwig, J . J . Am. Chem. Soc. 2000, 122, 10718.
(14) Yamamoto, Y.; Sato, R.; Ohshima, M.; Mysuo, F.; Sudoh, C. J .
Orgnomet. Chem. 1995, 489, C68-C70. (b) Yamamoto, Y.; Sato, R.;
Mysuo, F.; Sudoh, C.; Igoshi, T. Inorg. Chem. 1996, 35, 2329. (c) Han,
X.-H.; Yamamoto, Y. J . Orgnomet. Chem. 1998, 561, 156. (d) Yama-
moto, Y.; Sato, R.; Ohshima, M.; Mysuo, F.; Sudoh, C. J . Orgnomet.
Chem. 2002, 616, 149.
F igu r e 4. ORTEP drawing of complex 7 (50% probability
level). Hydrogen atoms are omitted for clarity.

3936 Organometallics, Vol. 23, No. 16, 2004
Weissman et al.
Ta ble 4. Selected Bon d Len gth s (Å) a n d An gles
(d eg) for 7 (a ver a ges of va lu es for tw o
in d ep en d en t m olecu les)
explained by the smaller bulk and the constraint
induced by the chelating character of the (dmobp-O) in
complex 7. The Pd(1)-Pd(2) distance in complex 7 is
shorter by ca. 0.06 Å than the corresponding distances
in Pd(dppmp-µ-O)(Cl)]2 and [Pd(dippmp-µ-O)(Cl)]2.12b
However, it is still ca. 0.53 Å longer than the distance
Pd(1)-Pd(2) of complex 3 (Table 2), corresponding to
an antibonding interaction between the typical bridged
Pd(II) dimers.9a
Pd(1)-C(21)
Pd(1)-O(3)
Pd(1)-O(1)
Pd(1)-P(2)
Pd(1)-Pd(2)
Pd(2)-C(31)
Pd(2)-O(1)
Pd(2)-O(3)
Pd(2)-P(1)
P(1)-C(18)
P(1)-C(52)
1.954(3)
2.097(2)
2.119(2)
2.2334(8)
3.1615(4)
1.952(3)
2.099(2)
2.117(2)
2.2308(8)
1.821(3)
1.859(3)
P(1)-C(51)
P(2)-C(8)
P(2)-C(41)
P(2)-C(42)
O(1)-C(1)
C(3)-O(2)
O(2)-C(7)
O(3)-C(11)
C(15)-O(4)
O(4)-C(17)
1.872(3)
1.836(3)
1.858(3)
1.864(3)
1.340(3)
1.369(4)
1.418(4)
1.322(4)
1.370(4)
1.405(5)
Phosphine complexes bearing ortho methoxy-arene
groups14,15 and a methoxy-substituted pincer ligand5
were reported to undergo ArO-Me cleavage.
Aromatic ethers, such as anisole, undergo exclusive
sp3-sp3 C-O cleavage upon reaction with HI or HBr
at elevated temperatures to give an alkyl halide and
an aromatic alcohol (eq 4).16 Lewis acids, such as AlCl3
or BF3, are also effective. The reaction proceeds by
protonation of (or Lewis acid binding to) the oxygen
followed by an external nucleophilic attack of the anion
on the alkyl group (SN2 mechanism).
C(21)-Pd(1)-O(3)
93.16(11) O(1)-Pd(2)-O(3)
80.45(8)
96.87(9)
169.24(6)
89.99(6)
C(21)-Pd(1)-O(1) 173.52(11) C(31)-Pd(2)-P(1)
O(3)-Pd(1)-O(1)
C(21)-Pd(1)-P(2)
O(3)-Pd(1)-P(2)
O(1)-Pd(1)-P(2)
80.45(8) O(1)-Pd(2)-P(1)
96.09(9) O(3)-Pd(2)-P(1)
169.80(7) C(1)-O(1)-Pd(2) 126.65(19)
90.37(6) C(1)-O(1)-Pd(1) 108.63(19)
C(21)-Pd(1)-Pd(2) 132.91(9) Pd(2)-O(1)-Pd(1) 97.12(9)
O(3)-Pd(1)-Pd(2)
O(1)-Pd(1)-Pd(2)
P(2)-Pd(1)-Pd(2)
C(31)-Pd(2)-O(1)
41.62(6) C(3)-O(2)-C(7)
117.2(3)
41.20(6) C(11)-O(3)-Pd(1) 127.25(19)
128.30(3) C(11)-O(3)-Pd(2) 106.61(18)
92.62(11) Pd(1)-O(3)-Pd(2) 97.23(9)
ArO-CH3 + HX f ArOH + CH3X
(4)
C(31)-Pd(2)-O(3) 173.06(11)
Examples of H3C-O bond cleavage of aryl alkyl ethers
with soluble metal complexes were reported.5,15 A Lewis
acid-type interaction with a metal could promote the loss
of the alkyl group by a subsequent internal interaction
with a coordinated anion, as suggested by Shaw15a,b and
Tolman,15c,d or by an external nucleophilic attack.14,15e-g
H3C-OAr bond cleavage with late transition metal
complexes and analogous ligands includes Rh(III) with
(2,6-dimethoxyphenyl)diphenylphosphine,14c Ru(II) and
Pd(II) with tris(2,6-dimethoxyphenyl)phosphine,14a,b,d
Rh(I), Pd(II), and Pt(II) with tris(2,4,6-trimethoxyphe-
nyl)phosphine, and TMPP15e,f (Scheme 4) and Ru(II)
with 2-(ethoxyphenyl)diphenylphosphine.15g
As mentioned earlier, C-O bond cleavage proceeds
via nucleophilic attack of dmobp on the methyl group
of the coordinated methoxy group. This bond is activated
toward the attack by coordination to Pd, as evident from
the elongated C-O bond of the coordinated methoxy
group in the X-ray structure of complex 4 (Table 3).
Demethylation of an aromatic methoxy group by a
diphosphine-chelated Pd complex was reported (Scheme
5).5 It is likely that the electrophilic Pd(II) center formed
after the oxidative addition of chloro- or iodobenzene
acts as a Lewis acid and upon coordination to the
methoxy group generates an electrophilic methyl group.
The thermal decomposition of complex 4 in the absence
of free dmobp confirms the need for an external nucleo-
phile for clean C-O activation. An exception in this
series of metal complexes is the diphosphine-chelated
Rh(I) complex (Scheme 5),5 which activates the O-aryl
bond and not the O-Me bond.
The molecular structure of 7 was confirmed by an
X-ray diffraction study of colorless single crystals ob-
tained by slow evaporation of its benzene/pentane
solution (Figure 4, Table 4). The crystal contained two
independent, structurally analogous molecules in the
asymmetric unit. Complex 7 exhibits a distorted square
planar geometry around the Pd centers, with the sum
of the four angles at the metal being equal to 360.09°.
A similar geometry with almost no deviation from
planarity was observed for [Pd(dppmp-µ-O)(Cl)]2 (dppmp
) 2-diphenylphosphino-4-methylphenoxide) and [Pd-
(dippmp-µ-O)(Cl)]2 (dippmp ) 2-diisopropylphosphino-
4-methyl phenoxide).12b The length of the Pd(1)-O(3)
bond of 2.097 Å is shorter by ca. 0.02 Å than that of
Pd(1)-O(1). However, in [Pd(dppmp-µ-O)(Cl)]2 and [Pd-
(dippmp-µ-O)(Cl)]212b the differences in lengths between
the analogous Pd(1)-O(1) and Pd(1)-O(3) are 0.12-0.14
Å, while for both complexes the length of the bond Pd-
(1)-O(1), 2.022 Å, is the shorter one. The difference
between these complexes and complex 7 may be ex-
plained by the significant differences in bulk and ring
strain. Analogous dimeric Pd complexes with an in-
tramolecular bridging phenoxide have been reported,12
including an imine-based system, [Pd(3,5-dimethoxy-2-
12a
phenyliminomethylphenoxide-µ-O)(OAc)]2,
and the
tetramer [Pd(2-diphenyl phosphanyl-4-hydroxyphenox-
ide-µ-O)(Br)]4.12c A bulky Pd complex similar to 7 with
an intermolecular bridging phenoxide, [Pd[P(ferrocenyl)-
(tert-Bu)2](o-MeC6H4)(µ-O-p-C6H4OMe)]2, was reported
recently.13 Its structure exhibits a distorted square
planar geometry around the Pd centers with a smaller
Pd-O-Pd-O torsion angle of 33.2°. Complex 7 has a
Pd-O-Pd-O torsion angle of 16.4°, which can be
Notably, there is a halide ligand effect on the rate of
the thermal decomposition of complexes 4 and 5, the
chloro complex 5 decomposing faster. The two-step
decomposition suggested by Shaw15a,b seems unlikely,
because in that case the decomposion should proceed
even in the absence of free ligand. It is also unlikely
that the decomposition proceeds through a three-step
mechanism proposed by Dunbar et al. (Scheme 4)15e or
(15) J ones, C. E.; Shaw, B. L.; Turtle, B. L. J . Chem. Soc., Dalton
Trans. 1974, 992. (b) Empsall, H. D.; Hyde, E. M.; J ones, C. E.; Shaw,
B. L. J . Chem. Soc., Dalton Trans. 1974, 1980. (c) Tolman, C. A.; Ittel,
S. D.; English, A. D.; J esson, J . P. J . Am. Chem. Soc. 1979, 101, 1742.
(d) Ittel, S. D.; Tolman, C. A.; English, A. D.; J esson, J . P. J . Am. Chem.
Soc. 1978, 100, 7577. (e) Dunbar, K. R.; Haefner, S. C.; Ezelmeier, C.;
Howard, A. Inorg. Chem. Acta 1995, 240, 527. (f) Sun, J .-S.; Uzelmeier,
C. E.; Ward, D. L.; Dunbar, K. R. Polyhedron 1998, 17, 2049. (g) Rogers,
C. W.; Patrick, B. O.; Rettig, S. J .; Wolf, M. O. J . Chem. Soc., Dalton
Trans. 2001, 1278.
(16) Smith, M. B.; March, J . Advanced Organic Chemistry-Reac-
tions, Mechanisms, and Structures, 5th ed.; J ohn Weily & Sons: New
York, NY, 2001; p 520.

Unsaturated Pd(0), Pd(I), and Pd(II) Complexes
Organometallics, Vol. 23, No. 16, 2004 3937
Sch em e 4. Dea lk yla tion of η2-Coor d in a ted TMP P by a Secon d TMP P Liga n d
Sch em e 5. C-O Activa tion in a Bisp h osp h in e Ch ela tin g Liga n d
Ta ble 5. Su zu k i-Miya u r a Rea ction of Ar yl
Ch lor id es Ca ta lyzed by P d (0) Ca ta lystsa
Wolf et al.,15g involving halide dissociation followed by
its nucleophilic attack on the methyl group and reaction
of the product methyl halide with free phosphine. Due
to the stronger bond of the chloride to the Pd atom
relative to the iodide, such a mechanism is expected to
result in faster decomposition of the iodo complex 4,
contrary to what we observed. The faster decomposition
with the chloro complex 5 as compared with 4 might be
explained by the higher electrophilicity of the Pd center
when the more electronegative chloride is bound to it,
facilitating nucleophilic attack on the methyl group
(Scheme 3).
Ca ta lysis w ith Dm obp P d Com p lexes. The new
Pd complexes were tested as catalysts for the Suzuki-
Miyaura coupling of chlorobenzene with phenyl boronic
acid. While reaction conditions were not optimized, some
trends emerge. Complex 2 catalyzes this reaction even
at 25 °C, although the reaction is slow (Table 5).
Comparing the reactions at 40 °C, it is interesting to
note that the Pd(I) complex 3 is more active than 2,
whereas the phenoxy Pd(II) complex 7 is completely
inactive at this temperature. Thus, we can conclude that
7 does take part in the catalysis by 2 or 3. This may
indicate that (a) complex 7 cannot undergo reduction
to Pd(0) under the reaction conditions or (b) a Pd(II)/
Pd(IV) mechanism with complex 7 is impossible. The
issue of Pd(II)/Pd(IV) versus Pd(0)/Pd(II) mechanisms
in the catalytic activation of organic halides is of
considerable current interest.17
temp timeb yieldc
entry
catalyst
aryl chloride
PhCl
PhCl
PhCl
PhCl
PhCl
PhCl
3-chloro-toluene
PhCl
(°C)
(h)
(%)
1
2
3
4
5
6
7
8
9
2
2
2
2
3
3
3
7
25
25
40
40
40
40
40
40
27
92
22
46
22
46
46
22
20
2
14
25
16
31
32
41
43
0
42
83
10
88
Pd(OAc)2 + 2dmobp PhCl
130
130
130
10 Pd(OAc)2 + 4dmobp PhCl
11
Pd(OAc)2 + 2dmbp PhCl
25
2
12 Pd(OAc)2 + 4dmobp 3-chloro-toluene 130
a
Typical reaction conditions: into a 25 mL round-bottom flask
fitted with a condenser were added under Ar atmosphere 13 mL
of o-xylene, 2.5 mmol of aryl chloride, 3.75 mmol of PhB(OH)2,
5.5 mmol of CsF, and 0.025 mmol of catalyst. The reaction mixture
was stirred at the specified temperature. Reaction times were not
b
optimized and are given for the purpose of comparison. The yield
is based on the aryl chloride and was determined by GC. The rest
of the product solution is unreacted aryl chloride.
(dmobp)Pd, which is stabilized by the dmobp ligand bulk
and the hemilabile coordination of the methoxy groups.
With complex 2, an additional phosphine is present,
which may retard the catalysis by competition with the
substrate for free binding sites of the Pd center. Pd(0)
complexes that are possible precursors to 12e- Pd-
(0)9c,18,19 catalytic species were reported.
Reactions were also carried out at 130 °C with Pd-
(OAc)2 and the ligand, generating the active Pd(0)
We believe that complex 3 is reduced under the
reaction conditions to give the active 12e- species
(17) (a) Ohff, M.; Ohff, A.; van der Boom, M. E.; Milstein, D. J . Am.
Chem. Soc. 1997, 119, 11687. (b) Shaw, B. L. New J . Chem. 1998, 77.
(c) Loch. J . A.; Albrecht, M.; Peris, E.; Mata, J .; Faller, J . W.; Crabtree,
R. H. Organometallics 2002, 21, 700.
(18) Andreu, M. G.; Zapf, A.; Beller, M. Chem. Commun. 2000, 2475.
(19) Viciu, M. S.; Navarro, O.; Germaneau, R. F.; Kelly, R. A., III;
Sommer, W.; Marion, N.; Stevens, E. D.; Cavallo, L.; Nolan, S. P.
Organometallics 2004, 23, 1629.

3938 Organometallics, Vol. 23, No. 16, 2004
Weissman et al.
AVANCE 400 spectrometer at 400 MHz (1H), 162 MHz (31P),
100 MHz (13C) or 61 MHz (2D). All spectra were recorded at
23 °C. 1H NMR and 13C{1H} NMR chemical shifts are reported
catalyst in situ under the reaction conditions. Signifi-
cantly higher rates were obtained, leading to an 83%
yield after 2 h with the dmobp ligand, although excess
ligand is required for stabilization of the complex at the
elevated temperature (compare entries 9 and 10). Use
of the analogous ligand, which lacks methoxy groups,
di-tert-butyl-(2,4,6-trimethylbenzyl)phosphine (tmbp),
resulted in a much lower yield (compare entries 9 and
11), perhaps a reflection of the absence of the stabiliza-
tion effect of the active catalyst by the methoxy groups.
Several systems for efficient Suzuki-Miyaura catalysis
of aryl chlorides have been reported.1a-c,e,18,20
1
in ppm downfield from tetramethylsilane. H NMR chemical
shifts were referenced to the residual hydrogen signal of the
deuterated solvents (7.15 ppm, benzene; 7.24 ppm, chloroform;
5.32 ppm, dichloromethane). In 13C{1H} NMR measurements
the signals of C6D6 (128.0 ppm), CDCl3 (77.0 ppm), and CD2-
Cl2 (53.8 ppm) were used as a reference. 31P NMR chemical
shifts are reported in ppm downfield from H3PO4 and refer-
enced to an external 85% solution of phosphoric acid in D2O.
Screwcap 5 mm NMR tubes were used in the NMR follow-up
experiments. Abbreviations used in the description of NMR
data are as follows: b, broad; s, singlet; d, doublet; t, triplet;
q, quartet, m, multiplet, v, virtual. Elemental analyses were
performed by The Microanalysis Laboratory of The Hebrew
University, Israel. GC analysis was performed on a HP6890
chromatograph with FID detector using capillary columns HP5
(30 m × 0.32 mm × 0.25 µm). ES-MS were preformed on a
MicroMass-LSZ4000 instrument.
Su m m a r y
Pd(0), Pd(I), and Pd(II) complexes of the new electron-
rich methoxy benzyl phosphine ligand dmobp were
prepared and structurally characterized. The 14e- PdL2
complex 2, in which the methoxy groups are not
coordinated, undergoes facile oxidative addition of iodo-
and chlorobenzene to yield the monophosphine com-
plexes LPd(Ph)X 4 (X ) I) and 5 (X ) Cl) in which the
methoxy group is coordinated to the Pd(II) center in the
solid state, although in solution there is no evidence for
its coordination, probably because of its hemilabile
nature, indicating the availability of a Pd(II) 14e-
complex. Coordination of the methoxy group weakens
the C-O bond, as observed in the X-ray structure, and
results in its cleavage upon nucleophilic attack by the
free ligand, leading to the dimeric phenoxy-bridged
complex 7. Partial reduction of [Pd(2-methylallyl)Cl]2
leads to the Pd(I) dimeric complex 3, which can be
converted to the Pd(0) complex 2 by addition of the
dmobp ligand and a base. Complexes 2 and 3 catalyzed
the Suzuki-Miyaura cross-coupling of chloroarenes
with PhB(OH)2. The Pd(I) dimer 3 is a possible precur-
sor to a catalytically active monophosphine 12e- Pd(0)
catalyst. The phenoxy-bridged complex 7, obtained by
C-O cleavage, was catalytically inactive at 40 °C, and
hence it is not an intermediate in the catalysis, probably
indicating that Pd(0) is essential for the Suzuki-
Miyaura catalysis in this system. The effect of the
dmobp ligand in Suzuki-Miyaura coupling was found
to be superior to that of an analogous ligand that does
not bear methoxy substituents.
Syn th esis of 2,6-Dim eth oxyben zyl Ch lor id e. A solution
of N-chlorosuccinimide (6.2 g, 0.046 mmol) in 200 mL of
(alumina dried HPLC grade) CH2Cl2 was cooled to 0 °C under
argon, and 3.7 mL of Me2S was added during a few minutes,
resulting in a white precipitate. The reaction mixture was
cooled to -20 °C, and a solution of 2,6-dimethoxybenzyl alcohol
(7.04 g, 0.043 mmol) in 25 mL of CH2Cl2 was added dropwise.
The solution was stirred for 2 h, allowing the temperature to
reach 0 °C, during which time all the solid precipitate had
redissolved, giving a clear solution. The solution was poured
over cold brine and extracted twice with diethyl ether. The
combined organic layers were washed twice with cold brine
(neutral pH), dried over Na2SO4, filtered, and concentrated in
a vacuum. The resulting solid was washed with cold pentane,
decanted, and dried under high vacuum. Yield: 7.2 g (92.4%).
1HNMR (CDCl3): δ 3.99 (s, OCH3, 6H), 4.90 (s, CH2Ar, 2H),
6.67 (d, J HH ) 8.4 Hz, Ar-H, 2H), 7.37 (t, J HH ) 8.4 Hz, Ar-H,
1H). 13C{1H}NMR (CDCl3): δ 35.68 (s, CH2Ar), 55.90 (s, 2
OCH3), 103.68 (s, 2 ArCH2-P), 114.17 (s, ArCH2-P), 130.21 (s,
ArCH2-P), 158.54 (s, 2 ArCH2-P). The compound is known, but
neither preparation procedure nor spectral data are available.
Syn th esis of th e Dm obp Liga n d 1. A solution of di-tert-
butylphosphine (5.548 g, 38 mmol) in 50 mL of freshly distilled
degassed ether was placed in a 500 mL Schlenk flask and
n
cooled to -40 °C under argon. BuLi in hexanes (30 mL, 1.43
M) was added dropwise, and the solution was stirred for 2-3
h, allowing the temperature to reach 0 °C. The reaction
mixture was cooled to -78 °C, and a solution of 2,6-dimethoxy-
benzyl chloride (5.763 g, 31 mmol) in 60 mL of freshly distilled,
degassed ether was added dropwise. The reaction mixture was
stirred at room temperature for 48 h and decomposed with 50
mL of double distilled, degassed water. The organic layer was
separated (under argon pressure) via cannula, and the aqueous
layer was extracted with ether (2 × 150 mL). The combined
organic layers were dried on sodium sulfate, filtered under
argon pressure with a sintered cannula, and concentrated
under high vacuum. The residue was introduced to a drybox
and chromatographed on dry silica eluted with pentane,
pentane/ether 2%, and finally pentane/ether 5%. The main
Exp er im en ta l Section
Gen er al P r ocedu r es. All procedures with air- and moisture-
sensitive compounds were performed in
a nitrogen-filled
glovebox (Vacuum Atmospheres, equipped with a “Nexus One”
purification system) or on a high-vacuum line using Schlenk
techniques. All solvents were reagent grade or better. All
nondeuterated solvents were refluxed over sodium/benzophe-
none ketyl and distilled under argon atmosphere. Deuterated
solvents were used as received. All the solvents were degassed
with argon and kept in the glovebox over 4 Å molecular sieves,
except MeOH, which was kept over 3 Å molecular sieves.
Commercially available reagents were used as received. [(2-
1
fractions gave 2.1 g (25%) of pure ligand. H NMR (C6D6): δ
1.26 (d, J PH ) 10.1 Hz, C(CH3)3, 18H), 3.05 (d, J HH ) 3.8 Hz,
CH2Ar, 2H), 3.43 (s, OCH3, 6H), 6.35 (d, J HH ) 8.2 Hz, Ar-H,
2H), 7.02 (td, J HH ) 8.4 Hz, J PH ) 1.3 Hz, Ar-H, 1H), 13C{1H}
NMR (C6D6): δ 16.84 (d, J CP ) 26.8 Hz, ArCH2-P) 29.70 (d,
J CP ) 14.2 Hz, P6 C(CH3)3), 32.23 (d, J CP ) 14.2 Hz, (H3C)3-
C-P), 55.07 (s, 2 OCH3), 104.27 (d, J CP ) 1.2 Hz, 2 ArCH2-P),
119.01 (d, J CP ) 2.2 Hz, ArCH2-P), 126.40 (d, J CP ) 1.9 Hz,
ArCH2-P), 158.54 (d, J CP ) 2.8 Hz, 2 ArCH2-P). 31P{1H} NMR
(C6D6): δ 35.25.
methylallyl)PdCl]2 and 2,6-dimethoxybenzyl alcohol6 were
21
prepared according to literature procedures. NMR measure-
ments were performed on
a Bruker AVANCE APX 250
spectrometer at 250 MHz (1H) and 101 MHz (31P) or a Bruker
(20) Zapf, A.; J ackstell, R.; Rataboul, F.; Riermeier, T.; Monsees,
A.; Fuhrmann, C.; Shaikh, N.; Dingerdissen, U.; Beller, M. Chem.
Commun. 2004, 38, and references therein.
Syn th esis of 2. A solution of dmobp (120 mg, 0.405 mmol)
in 2 mL of MeOH was added to a stirred suspension of [(2-
(21) Dent, W. T.; Long, R.; Wilkinson, A. J . J . Chem. Soc. 1964, 1585.

Unsaturated Pd(0), Pd(I), and Pd(II) Complexes
Organometallics, Vol. 23, No. 16, 2004 3939
Ta ble 6. Exp er im en ta l Cr ysta llogr a p h ic Da ta for Com p lexes 2, 3, 4, a n d 7
2
4
3
7
color, shape
empirical formula
fw
colorless, plate
colorless, prism
C23H34O2PIPd + C4H8O
678.88
colorless, prism
C38H65O4P2ClPd2
869.09
colorless, prism
C44H62O4P2Pd2
929.68
C
34H58O4P2Pd
699.14
radiation
cryst syst
space group
unit cell
a/Å
b/Å
c/Å
R/deg
â/deg
Mo KR (0.710)
triclinic
P1h
Mo KR (0.710)
Mo KR (0.710)
monoclinic
P21/c
Mo KR (0.710)
triclinic
triclinic
P1h
P1h
10.8540(3)
12.31120(3)
13.7990(4)
89.479(2)
93.469(1)
71.771(2)
1.333
10.755(2)
11.280(2)
14.160(3)
67.82(3)
79.31(3)
62.50(3)
1.598
12.9240(3)
16.4440(5)
20.1180(6)
13.022(1)
18.466(1)
18.605(1)
111.130(2)
89.660(2)
97.270(3)
1.493
107.63(1)
ø/deg
D
calcd/g cm-3
1.461
1.063
0.05 × 0.02 × 0.02
49 523, 10540
0.087
µ(Mo KR)/mm-1
cryst size/mm
0.659
1.833
0.988
0.1 × 0.1 × 0.05
14 198, 7926
0.052
0.1 × 0.05 × 0.05
15 354, 5779
0.052
0.05 × 0.05 x 0.03
36 890,15419
0.045
total/unique no. of reflns
Rint
no. of params, restraints
R1, wR2
386, 0
0.0498, 0.1334
2.348
306, 0
0.0302, 0.0639
0.581
520, 0
0.0434, 0.0915
1.247
965, 0
0.0331, 0.0759
0.687
resid density/e Å3
methylallyl)PdCl]2 (34.4 mg, 0.087 mmol) in 4 mL of MeOH
under N2, immediately followed by an addition of NaOH (8.2
mg, 0.405 mmol) in 2 mL of methanol. The solution turned
clear yellow and was stirred at room temperature overnight.
Pd(dmobp)2 was obtained as a white precipitate. The solids
were collected on a sinter no. 4 and washed with a small
amount of MeOH three times. The solids left on the sinter were
dissolved in benzene. After evaporation of the solvent, 100 mg
(85% yield) of pure complex 2 as a white powder was obtained.
Single crystals suitable for X-ray diffraction were obtained by
slow evaporation of a pentane solution. 1H NMR (C6D6): δ 1.49
(vt, J HP ) 5.9 Hz, C(CH3)3, 36H), 3.10 (vt, J HP ) 3.1 Hz, ArCH2,
4H), 3.36 (s, OCH3,12H), 6.34 (d, J HH ) 8.2 Hz, ArH, 4H), 7.04
(t, J HH ) 8.2 Hz, 2H, ArH). 13C{1H} NMR (C6D6): δ 18.44 (vt,
J CP ) 1.6 Hz, 2 ArCH2-P), 31.03 (vt, J CP ) 3.1 Hz, 12 (H3C)3-
C-P), 35.94 (vt, 3J CP ) 3.1 Hz, 4 (H3C)3C-P), 55.26 (s, 4 OCH3),
104.57 (bs, 2 ArCH2-P), 126.27 (bs, 2 ArCH2-P), 158.63 (bs, 4
ArCH2-P). 31P{1H} NMR (C6D6): δ 60.89. ES-MS 699 m/z+,
699.2 is the calculated mass. Anal. Calcd for C34H58O4P2Pd:
C, 58.41; H, 8.36. Found: C, 58.57; H, 8.30.
Anal. Calcd for C38H65O4P2ClPd2: C, 50.93; H, 7.31. Found:
C, 50.85; H, 7.40.
Syn th esis of 4 a n d 5. X ) I, 4. Iodobenzene (21.2 µL, 0.19
mmol) was added to a solution of 13.2 mg (0.019 mmol) of Pd-
(dmobp)2 in 500 µL of C6D6 in a screw-cap NMR tube. After 2
days at room temperature the reaction was complete. The
solvent was evaporated, and the residue was washed five times
with pentane and dried under high vacuum. Pure (dmobp)-
Pd(Ph)I, 4 (6.8 mg, 60% yield), was obtained. Single crystals
suitable for X-ray diffraction were obtained by slow evapora-
1
tion of a benzene solution. H NMR (C6D6): δ 1.02 (d, J HP
)
13.5 Hz, C(CH3)3, 18H), 2.63 (d, J HP ) 10.3 Hz, PhCH2P, 2H),
3.63 (s, OCH3, 6H), 6.46 (d, J HH ) 8.2 Hz, PCH2Ar-H, 2H),
6.70 (t, J HH ) 7.05 Hz, PdPh-H, 1H), 6.83 (t, J HH ) 7.8 Hz,
PdPh-H, 2H), 6.91 (td, J HH ) 8.34 Hz, J HP ) 1.6 Hz, PCH2-
Ar-H, 1H), 7.38 (ddd, J HH ) 8.2 Hz, J HH ) 1.6 Hz, J HP ) 1.2
Hz, PdPh-H, 2H). 13C{1H} NMR (C6D6): δ 14.56 (d, J CP ) 14.3
Hz, ArCH2-P), 29.16 (d, J CP ) 4.6 Hz, 6 C(CH3)3), 36.71 (d,
J CP ) 9.9 Hz, C(CH3)3), 59.76 (s, 2 OCH3), 107.58 (d, J CP ) 1.8
Hz, 2 ArCH2-P), 116.29 (d, J CP ) 1.2 Hz, PdPh), 123.04 (s,
PdPh), 126.54 (d, J CP ) 1.2 Hz, PdPh), 128.53 (d, J CP ) 2.3
Hz, ArCH2-P), 129.51 (s, ArCH2-P), 139.19 (d, J CP ) 3.4 Hz,
PdPh), 157.44 (d, J CP ) 3.7 Hz, 2 ArCH2-P). 31P NMR {1H}
(C6D6): δ 64.45. ES-MS: m/z+ 646 (M + K+), 629 (M + Na+),
479 (M+ - I), m/z- 127 (I-). MALDI-TOF-MS: m/z+ 629 (M +
Na+). Anal. Calcd for C23H34O2PIPd‚C4H8O: C, 58.41; H, 8.36.
Found: C, 58.57; H, 8.30.
Syn th esis of Com p lex 3. A solution of dmobp (22.2 mg,
0.072 mmol) in 2 mL of MeOH was added to a stirred
suspension of [(2-methylallyl)PdCl]2 (14.8 mg, 0.036 mmol) in
4 mL of MeOH under N2, immediately followed by addition of
NaOH solution (2 mL, of 0.018 M). The solution turned yellow
and was stirred at room temperature overnight. (dmobp)Pd-
(µ-Cl)(µ-2-methylallyl)Pd(dmobp) was obtained as a green
precipitate. The methanol solution was decanted, and the
solids were extracted three times with benzene. Some Pd black
was left as residue. After evaporation of the solvent 31 mg
(95% yield) of pure complex 3 as a yellow powder was obtained.
Single crystals suitable for X-ray diffraction were obtained by
slow evaporation of a benzene solution. 1H NMR (C6D6): δ 1.1
(br s, PdCCH3(CH2)2, 2H), 1.25 (vt, J H,P ) 2.8 Hz, PdCCH3-
(CH2)2, 3H), 1.48 (vt, J H,P ) 12.5 Hz, C(CH3)3, 18H), 1.53 (vt,
J H,P ) 12.3 Hz, C(CH3)3, 18H), 2.84 (vt, J H,P ) 5.5 Hz, PdCCH3-
(CH2)2, 2H), 3.31 (vt, J H,P ) 4.6 Hz, PCH2Ar, 2H), 3.32 (vt,
J H,P ) 3.3 Hz, PCH2Ar, 2H) 3.36 (s, OCH3, 12H), 6.25 (d, J H,H
) 8.15 Hz, PCH2Ar-H, 4H), 7.01 (t, J H,H ) 8.5 Hz, PCH2Ar-H,
2H). 13C{1H} NMR (C6D6): δ 18.87 (vt, J C,P ) 2.2 Hz, ArCH2-
X ) Cl, 5. Chlorobenzene (36.5 µL, 0.36 mmol) was added
to a solution of 25 mg (0.036 mmol) of Pd(dmobp)2 in 500 µL
of C6D6 in screw-cap NMR tube. After 2 days at 40 °C the
reaction was complete, the solvent was evaporated, and the
residue was washed five times with pentane and dried under
high vacuum. Pure (dmobp)Pd(Ph)Cl, 5 (13 mg, 70% yield),
was obtained. 1H NMR (C6D6): δ 1.03 (d, J HP ) 13.5 Hz,
C(CH3)3, 18H), 2.66 (d, J HP ) 10.5 Hz, PhCH2P, 2H), 3.64 (s,
OCH3, 6H), 6.42 (d, J HH ) 8.4 Hz, PCH2Ar-H, 2H), 6.80 (t,
J HH ) 7.05 Hz, PdPh-H, 1H), 6.89 (t, J HH ) 7.74 Hz, PdPh-H,
2H), 6.93 (td, J HH ) 8.3 Hz, J HP ) 1.6 Hz, PCH2Ar-H, 1H),
7.43 (ddd, J HH ) 8.3 Hz, J HH ) 2.2 Hz, J HP ) 1.2 Hz, PdPh-H,
2H). 13C NMR {1H} (C6D6): δ 15.05 (d, J CP ) 15.6 Hz, ArCH2-
P), 29.21 (d, J CP ) 4.1 Hz, C(CH3)3), 36.40 (d, J CP ) 13.6 Hz,
C(CH3)3), 58.19 (s, 2 OCH3), 106.78 (d, J CP ) 1.6 Hz, 2 ArCH2-
P), 115.77 (d, J CP ) 1.6 Hz, PdPh), 123.35 (s, PdPh), 126.93
(d, J CP ) 1.2 Hz, PdPh), 128.48 (d, J CP ) 2.1 Hz, ArCH2-P),
129.53 (s, ArCH2-P), 137.91 (d, J CP ) 3.2 Hz, PdPh), 157.40
(d, J CP ) 3.7 Hz, 2 ArCH2-P). 31P NMR {1H} (C6D6): δ 71.01.
ES-MS: m/z+ 479 (M+ - Cl). Anal. Calcd for C23H34O2PClPd:
C, 53.60; H, 6.65. Found: C, 53.95; H, 6.43
P), 26.08 (vt, J C,P ) 2.3 Hz, PdCCH3(CH2)2), 30.04 (vt, J C,P
)
3.7 Hz, 6 6 C(CH3)3), 30.36 (vt, J C,P ) 3.9 Hz, 6 6 C(CH3)3),
36.12 (vt, J C,P ) 3.5 Hz, C(CH3)3), 36.15 (vt, J C,P ) 3.2 Hz,
C(CH3)3), 38.92 (vt, J C,P ) 4.8 Hz, PdCCH3(CH2)2), 54.73 (s,
OCH3, 4C), 103.77 (s, 4 ArCH2-P), 116.63 (s, 2 ArCH2-P),
127.08 (s, 2 ArCH2-P), 158.62 (d, J C,P ) 1.5 Hz, PdPh). 31P NMR
{1H} (C6D6): δ 64.01. ES-MS m/z+ 934 (M + K+), 460
(crotylPd+(dmobp)). MALDI-TOF-MS: m/z+ 934 (M + K+).

3940 Organometallics, Vol. 23, No. 16, 2004
Weissman et al.
Syn th esis of Dm obp (Me)I, 6. A 100 µL sample of a 0.017
M solution of MeI in C6D6 was added to 500 µL of a solution
containing 5 mg (0.017 mmol) of dmobp in C6D6. A precipitate
of microcrystals formed immediately upon the addition of MeI.
After 2 days at room temperature, the reaction was complete.
The solvent was decanted, and the solids were dried under
high vacuum. The phosphonium salt (7 mg, 95%) was obtained.
1H NMR (CDCl3): δ 1.53 (d, J HP ) 15.1 Hz, C(CH3)3, 18H),
1.98 (d, J HP ) 11.9 Hz, PCH3, 3H), 3.60 (d, J HP ) 12.6 Hz,
PCH2Ar, 2H), 3.92 (s, OCH3, 6H), 6.63 (d, J HP ) 8.53 Hz, PCH2-
Ar-H, 2H), 7.35 (dt, J HH ) 8.4 Hz, J HP ) 1.9 Hz, P-CH2Ar-H,
1H). 13C{1H} NMR (CDCl3): δ 1.81 (d, J CP ) 46.5 Hz, P-CH3),
12.80 (d, J CP ) 42.8 Hz, ArH2C-P), 27.29 (bs, 6 C(CH3)3), 36.12
(d, J CP ) 36.4 Hz, C(CH3)3), 55.85 (s, 2 OCH3), 104.22 (s,
ArCH2-P), 130.6 (s, 2 ArCH2-P), 127.08 (s, 2 ArCH2-P), 157.6
(d, J CP ) 2.8 Hz, PdPh). 31P{1H} NMR (CDCl3): δ 49.07. ES-
MS: m/z+ 311 (M+ - I).
56.0 (s, OCH3), 99.61 (s, ArCH2-P), 114.63 (d, J CP ) 2.1 Hz,
PdPh), 115.29 (d, J CP ) 2.5 Hz, PdPh), 123.39 (s, PdPh), 126.58
(d, J CP ) 1.2 Hz, PdPh), 126.96 (d, J CP ) 1.6 Hz, ArCH2-P),
137.94 (d, J CP ) 2.5 Hz, PdPh), 144.24 (s, ArCH2-P), 157.07
(d, J CP ) 4.1 Hz, ArCH2-P), 163.82 (d, J CP ) 2.3 Hz, ArCH2-
P). 31P{1H} NMR (C6D6): δ 92.01. ES-MS: m/z+ 465 (M+).
X-r a y Str u ctu r e Deter m in a tion a n d Refin em en t of
Com p lexes 2, 3, 4, a n d 7. Colorless, platelike crystals were
mounted in a nylon loop and flash frozen in a cold nitrogen
stream (120 K) on a Nonius Kappa CCD with Mo KR radia-
tion (λ ) 0.71071 Å). Accurate unit cell dimensions were
obtained from 20° of data. The data were processed with the
Denzo-Scalepack package. The structure was solved by direct
methods (SHELXS-97) and refined by full-matrix least-squares
(SHELXL-97). Idealized hydrogen atoms were placed and
refined in the riding mode. Crystal data are given in Table 6.
Syn th esis of [(η2-d m obp -µ-O)P d P h ]2, 7. A 36.5 µL (0.36
mmol) sample of chlorobenzene was added to a solution of 25
mg (0.036 mmol) of Pd(dmobp)2 in 500 µL of C6D6 in a screw-
cap NMR tube. After 14 h at 80 °C the reaction was complete
and colorless crystals of dmobp(Me)Cl precipitated in the
bottom of the tube. The solvent was evaporated, and the
residue was washed six times with pentane and dried under
high vacuum. A total of 10 mg (61% yield) of (η2-dmobp-O)-
PdPh was obtained. 1H NMR (C6D6): δ 1.25 (d, J HP ) 13.6
Hz, C(CH3)3, 18H), 2.68 (d, J HP ) 10. 7 Hz, PhCH2P, 2H), 3.36
(s, OCH3, 3H), 6.10 (d, J HP ) 8.3 Hz, PCH2Ar-H, 1H), 6.72 (tt,
J HH ) 7.2 Hz, J HH ) 1.4 Hz, PdPh-H, 1H), 6.90 (t, J HH ) 7.5
Hz, PdPh-H, 2H), 7.08 (td, J HH ) 8.2 Hz, J HH ) 1.6 Hz, PCH2-
Ar-H, 1H), 7.46 (d, J HH ) 8.0 Hz, PdPh-H, 1H), 7.75 (ddd, J HH
) 8.1 Hz, J HH ) 2.2 Hz, J HH ) 1.2 Hz, PCH2Ar-H, 1H). 13C
NMR {1H} (C6D6): δ 14.78 (d, J CP ) 21.1 Hz, PCH2Ar), 29.63
(d, J CP ) 3.7 Hz, 6 C(CH3)3), 36.71 (d, J CP ) 15.9 Hz, C(CH3)3),
Ack n ow led gm en t. This work was supported by the
Israel Science Foundation, J erusalem, Israel, by the
MINERVA Foundation, Munich, Germany, and by the
Helen and Martin Kimmel Center for Molecular Design.
D.M. is the Israel Matz Professor of Organic Chemistry.
We thank Dr. M. E. van der Boom for fruitful discus-
sions.
Su p p or tin g In for m a tion Ava ila ble: NMR spectra of
compounds 6 and 7; tables of X-ray crystalographic data for
complexes 2, 3, 4, and 7 and the corresponding CIF files. This
material is available free of charge via the Internet at
http://pubs.acs.org.
OM0497588