Processes involved in the reduction of a cyclometalated palladium(II) complex

Article abstract of DOI:10.1021/om7008815

Reaction of [Pd(TFA)₂] (1; TFA = trifluoroacetate) with 2 equiv of benzyldiisopropylphosphine resulted in formation of the metalated complex [Pd {C₆H₄(CH₂PⁱPr₂)}{(C ₆H₅CH₂)PⁱPr₂ }(TFA)] (2). The dinuclear trifluoroacetate complex [Pd(C₆H₄(CH ₂PⁱPr₂)}[TFA)]₂ (3) was formed when the reaction was performed with an equimolar amount of the phosphine. Both complexes were structurally characterized. Reduction of the cyclometalated palladium complex 2 with sodium metal in THF gave a mixture of cis and trans isomers of the dimetalated bis(o-benzyldiisopropylphosphine)palladium(II) (6a,b) and bis(benzyldiisopropylphosphine)palladium(0) (7). A mechanism for the reduction process is presented. Treatment of the reaction mixture of 6a,b and 7 with an equimolar amount of hydrochloric acid led to a mixture of 7, [Pd(C ₆H₄(CH₂PⁱPr₂)Ji(C ₆H₅CH₂)PⁱPr₂}(Cl)] (9), [Pd{(C₆H₅CH₂)PⁱPr₂J ₂(H)(Cl)] (8), [Pd((C₆H₅CH₂)-P ⁱPr₂}₂(Cl)₂] (4), and both isomeric forms of [Pd(C₆H₄(CH₂PⁱPr ₂)}₂] (6a,b). All complexes were independently prepared and fully characterized. The addition of another 1 equiv of hydrochloric acid to this reaction mixture resulted in the exclusive formation of 4. Reduction of 4 with sodium metal in THF cleanly yielded the Pd⁰ complex 7 in high yields, offering a new, facile, and high-yield route toward the synthesis of dicoordinated bis(phosphine) Pd⁰ complexes. Reaction of 7 with an equimolar amount of HCl led to the clean formation of [PdJ(C₆H ₅CH₂)PⁱPr₂}₂(H)(Cl)] (8). The addition of another 1 equiv of HCl led to the quantitative formation of 2 and H₂. Reactions of 9 and an equimolar amount of NaBHEt3 cleanly yielded complex 7, which was also exclusively formed by treatment of 4 with 2 equiv of NaBHEt3. Mixtures of the cis and trans isomers 6a,b were formed by the addition of 2 equiv of (lithiobenzyl)diisopropylphosphine to benzene solutions of bis(diethyl sulfide)palladium dichloride (5) at room temperature.

Full text of DOI:10.1021/om7008815

894
Organometallics 2008, 27, 894–899
Processes Involved in the Reduction of a Cyclometalated
Palladium(II) Complex
Christian M. Frech,*,† Gregory Leitus,‡ and David Milstein*,§
Department of Inorganic Chemistry, UniVersity of Zurich, 8057 Zurich, Switzerland, and Department of
Organic Chemistry and Department of Chemical SerVices, The Weizmann Institute of Science,
RehoVot 76100, Israel
ReceiVed September 3, 2007
Reaction of [Pd(TFA)2] (1; TFA ) trifluoroacetate) with 2 equiv of benzyldiisopropylphosphine resulted
in formation of the metalated complex [Pd{C6H4(CH2PiPr2)}{(C6H5CH2)PiPr2}(TFA)] (2). The dinuclear
trifluoroacetate complex [Pd{C6H4(CH2PiPr2)}(TFA)]2 (3) was formed when the reaction was performed
with an equimolar amount of the phosphine. Both complexes were structurally characterized. Reduction
of the cyclometalated palladium complex 2 with sodium metal in THF gave a mixture of cis and trans
isomers of the dimetalated bis(o-benzyldiisopropylphosphine)palladium(II) (6a,b) and bis(benzyldiiso-
propylphosphine)palladium(0) (7). A mechanism for the reduction process is presented. Treatment of the
reaction mixture of 6a,b and 7 with an equimolar amount of hydrochloric acid led to a mixture of 7,
[Pd{C6H4(CH2PiPr2)}{(C6H5CH2)PiPr2}(Cl)] (9), [Pd{(C6H5CH2)PiPr2}2(H)(Cl)] (8), [Pd{(C6H5CH2)-
PiPr2}2(Cl)2] (4), and both isomeric forms of [Pd{C6H4(CH2PiPr2)}2] (6a,b). All complexes were
independently prepared and fully characterized. The addition of another 1 equiv of hydrochloric acid to
this reaction mixture resulted in the exclusive formation of 4. Reduction of 4 with sodium metal in THF
cleanly yielded the Pd0 complex 7 in high yields, offering a new, facile, and high-yield route toward the
synthesis of dicoordinated bis(phosphine) Pd0 complexes. Reaction of 7 with an equimolar amount of
HCl led to the clean formation of [Pd{(C6H5CH2)PiPr2}2(H)(Cl)] (8). The addition of another 1 equiv of
HCl led to the quantitative formation of 2 and H2. Reactions of 9 and an equimolar amount of NaBHEt3
cleanly yielded complex 7, which was also exclusively formed by treatment of 4 with 2 equiv of NaBHEt3.
Mixtures of the cis and trans isomers 6a,b were formed by the addition of 2 equiv of (o-
lithiobenzyl)diisopropylphosphine to benzene solutions of bis(diethyl sulfide)palladium dichloride (5) at
room temperature.
was observed, whereas preformed [Pd{P(o-tol)3}2] was active.3
We have previously observed that reduction of the pincer-type
Introduction
palladium complexes [Pd{C6H3(CH2PiPr2)2}(X)] (X ) TFA,
Cl)2a results in collapse of the pincer framework, leading to a
novel binuclear Pd0/PdII complex incorporating a 14e linear Pd0
center and a nonplanar, “butterfly”-type PdII 16e center.4
We report here the reactivity pattern of a cyclopalladated
Pd(II) complex under strongly reducing conditions in the
absence of substrate molecules that could stabilize the formed
products or induce further transformations.
Palladacyclic compounds are among of the most active
precatalysts for C-C and C-heteroatom bond formation.1 They
are generally believed to undergo a reduction process which
leads to a catalytically active Pd0 species.2 For instance,
reactions of the dinuclear PdII complex [Pd{C7H6P(o-tol)2}-
(OAc)]2 with HNEt2 yielded the amine adduct, which was
deprotonated in the presence of NaOC(CH3)3 to give the bis(tri-
o-tolylphosphine) Pd0 complex [Pd{P(o-tol)3}2] via ꢀ-elimina-
tion of the amide ligand. When ꢀ-elimination was not possible,
i.e. when diphenylamine was used as a base, no catalytic activity
Results and Discussion
* To whom correspondence should be addressed. E-mail: chfrech@
aci.uzh.ch (C.M.F.); david.milstein@weizmann.ac.il (D.M.).
† University of Zurich.
When [Pd(TFA)2] (1; TFA ) trifluoroacetate) was treated
with 2 equiv of benzyldiisopropylphosphine in THF at 70 °C
overnight, cyclometalation took place, leading to the bis(pho-
sphine) trifluoroacetate complex [Pd{C6H4(CH2PiPr2)}-
{(C6H5CH2)PiPr2}(TFA)] (2) in high yields (Scheme 1). The
31P{1H} NMR spectrum of 2 exhibits two sets of doublet
resonances (AX) centered at δ 74.91 and 32.13 ppm with
coupling constants of 2JPP ) 371.8 Hz. The 1H NMR spectrum
displays all the signals due to the phosphine units, suggesting
a “free” and an ortho-metalated benzyl unit. The 13C{1H} NMR
‡ Department of Chemical Services, The Weizmann Institute of Science.
§ Department of Organic Chemistry, The Weizmann Institute of Science.
(1) See, for example: (a) Bedford, R. B. Chem. Commun. 2003, 1787.
(b) Herrmann, W. A.; Brossmer, C.; Öfele, K.; Reisinger, C.-P.; Priermeier,
T.; Beller, M.; Fischer, H. Angew. Chem., Int. Ed. 1995, 34, 1844. (c) Ohff,
M.; Ohff, A.; Milstein, D. Chem. Commun. 1999, 357. (d) Weissmann, H.;
Milstein, D. Chem. Commun. 1999, 1901. (e) Alonso, D. A.; Najera, C.;
Pacheco, M. C. AdV. Synth. Catal. 2002, 344, 172.
(2) (a) Beletskaya, I. P.; Cheprakov, A. V. Chem. ReV. 2000, 100, 3009.
(b) Herrman, W. A.; Böhm, V. P. W.; Reisinger, C.-P. J. Organomet. Chem.
1999, 576, 23. (c) Albrecht, M.; van Koten, G. Angew. Chem., Int. Ed.
2001, 40, 3750. (d) Dupont, J.; Pfeffer, M.; Spencer, J. Eur. J. Inorg. Chem.
2001, 1917. (e) Bedford, R. B. Chem. Commun. 2003, 1787. (f) Nowotny,
M.; Hanefeld, U.; van Koningsveld, H.; Maschmeyer, T. Chem. Commun.
2000, 1877.
(3) Louie, J.; Hartwig, J. F. Angew. Chem., Int. Ed. 1996, 35, 2359.
(4) Frech, C. M.; Shimon, L. J. W.; Milstein, D. Angew. Chem., Int.
Ed. 2005, 44, 2.
10.1021/om7008815 CCC: $40.75
2008 American Chemical Society
Publication on Web 02/14/2008

Reduction of a Cyclometalated PdII Complex
Organometallics, Vol. 27, No. 5, 2008 895
Scheme 1. Syntheses of Complexes 2 and 3
spectrum exhibits a signal centered at δ 147.87 ppm with a dd
defined doublet resonance at δ 148.14 ppm (2JPC ) 15.4 Hz),
which is assigned to the Pd-C carbon. The doublet pattern of
that signal indicates coordination of only one phosphine ligand
to the metal center. Quartet resonances centered at δ 160.56
(2JCF ) 34.4 Hz) and at δ 117.32 ppm (1JCF ) 291.8 Hz) are
attributed to the trifluoroacetate anions. Single crystals of 3 were
grown by slow evaporation of a concentrated THF solution at
–30 °C. The X-ray structure of 3 is shown in Figure 2.
2
pattern (2JPC ) 18.9 Hz and JPC ) 6.1 Hz) attributable to a
σ-bound Pd-Cipso carbon atom. In addition, two quartet signals
at δ 160.56 ppm (2JFC ) 34.4 Hz) and at δ 117.32 (1JFC
)
291.8 Hz) due to the trifluoroacetate ligand were detected. The
presence of the trifluoroacetate ligand was further confirmed
by 19F{1H} NMR spectroscopy, giving rise to a sharp singlet
at δ –74.11 ppm.
The molecular structure of 2 was determined by an X-ray
diffraction study (see Figure 1). Well-diffracting crystals were
grown by slow evaporation of a concentrated diethyl ether
solution of 2 at -30 °C.
The solid-state structure of 3 exhibits two cyclometalated
square-planar palladium centers, bridged by two cis-arranged
trifluoroacetate ligands. The distance between the metal centers
of 3.066(8) Å seems too long for Pd · · · Pd interactions, although
there are instances where Pd · · · Pd separations of 3.0 Å are
treated as bonding.6 Nevertheless, weak interactions of the
palladium centers may be the reason for the line broadening in
the NMR spectra. On the other hand, no spectroscopic evidence
for Pd · · · Pd interactions was observed in a recently reported
Interestingly, when 1 was treated with an equimolar amount
of benzyldiisopropylphosphine, formation of the dinuclear
trifluoroacetate complex [Pd{C6H4(CH2PiPr2)}(TFA)]2 (3) was
observed, which was isolated as a colorless solid in 72% yield.5
Addition of another 1 equiv of benzyldiisopropylphosphine to
THF solutions of 3 led to the immediate, quantitative formation
of 2. The 31P{1H} NMR spectrum of 3 exhibits a singlet
resonance at δ 83.00 ppm. Cyclopalladation was confirmed by
NMR spectroscopy. For instance, the 1H NMR spectrum exhibits
in the aromatic region signals corresponding to eight hydrogen
atoms. In addition, the 13C{1H} NMR spectrum exhibits a well-
Figure 2. ORTEP diagram of a molecule of 3, showing the atom-
labeling scheme (50% probability). Hydrogen atoms are omitted
for clarity. Selected bond distances (Å) and angles (deg): Pd1 · · · Pd2
) 3.066(8), Pd1-P3 ) 2.2169(13), Pd1-O1 ) 2.170(3), Pd1-O4
) 2.143(4), Pd1-C18 ) 1.983(5), Pd2-P4 ) 2.2208(15), Pd2-O2
)2.143(4),Pd2-O3)2.157(4),Pd2-C5)1.996(5);C18-Pd1-O1
) 95.49(17), O1-Pd1-O4 ) 83.18(14), O4-Pd1-P3 ) 100.24(10),
P3-Pd1-C18)80.99(14),C5-Pd2-O3)93.75(19),O3-Pd2-O2
) 83.69(15), O2-Pd2-P4 ) 100.91(11).
Figure 1. ORTEP diagram of a molecule of 2, showing the atom-
labeling scheme (50% probability). Hydrogen atoms are omitted
for clarity. Selected bond distances (Å) and angles (deg): Pd1-P1
) 2.3783(19), Pd1-P2 ) 2.2702(19), Pd1-O1 ) 2.148(5),
Pd1-C14 ) 2.015(7); P1-Pd1-C14 ) 99.0(2), C14-Pd1-P2 )
81.1(2), P2-Pd1-O1 ) 90.37(15), O1-Pd1-P1 ) 90.21(15).

896 Organometallics, Vol. 27, No. 5, 2008
Scheme 2. Reduction of 2 and Related Reactions
Frech et al.
dinuclear palladium pincer complex bearing a Pd · · · Pd separa-
tion of 3.0389(11) Å.4
displayed two sharp singlets at δ 70.28 and 65.95 ppm and a
broad signal at δ 33.18 ppm, the latter corresponding to
approximately 50% of the products (by NMR). Since all the
resonance signals showed different intensities, the formation of
dinuclear compounds is unlikely. The generation of monophos-
phine complexes can be excluded, since no signals attributable
to free phosphines were spectroscopically detected. The presence
of cyclopalladated bis(phosphine) complexes in the reaction
mixture of reduced 2 was indicated by 13C{1H} NMR spec-
troscopy, where signals with triplet and doublet of doublets
patterns in the chemical shift range characteristic of Pd-C
σ-bonds were detected. In order to elucidate the identity of the
products formed, an equimolar amount (based on 2) of
hydrochloric acid (∼4 M in dioxane) was added to a THF
solution of the reaction mixture of reduced 2 (see Scheme 2).
Monitoring the reaction by 31P{1H} NMR spectroscopy showed
Reduction of the PdII complex 2 with a large excess (∼100
equiv) of sodium metal in THF under N2 at room temperature
overnight led to an inseparable mixture of three phosphorus-
containing compounds. None of the formed complexes contained
a trifluoroacetate ligand, as indicated by 19F{1H} NMR spec-
troscopy, and no hydride signals were detected in the 1H NMR
spectrum. The 31P{1H} NMR spectrum of this reaction mixture
(5) For the synthesis of strongly related palladacycles, see for example:
(a) Abicht, H. P.; Issleib, K. Z. Antimicrob. Antineoplast. Chemother. 1983,
500, 31. (b) Hiraki, K.; Fuchita, Y.; Uchiyama, T. Inorg. Chim. Acta 1983,
69, 187. (c) Ryabov, A. D.; Yatsimirskii, A. K.; Abicht, H. P. Polyhedron
1987, 6, 1619. (d) Pereira, M. M.; Muller, G.; Ordinas, J. I.; Azenha, M. E.;
Arnaut, L. G. J. Chem. Soc., Perkin Trans. 2 2002, 1583.
(6) (a) Murahashi, T.; Kurosawa, H. Coord. Chem. ReV. 2002, 231, 207.
(b) Yamamoto, Y.; Ohno, T.; Itoh, K. Organometallics 2003, 22, 2267.

II
Reduction of a Cyclometalated Pd Complex
Organometallics, Vol. 27, No. 5, 2008 897
formation of several compounds. The growth of two new
singlets at δ 47.26 and 29.43 ppm was observed, with a
concomitant decrease in the intensity of the signals at δ 70.28,
65.95, and 33.18 ppm. In addition, two sets of doublet
resonances (AX) with the centers of the signals at δ 76.86 and
30.77 ppm (2JPP ) 376.3 Hz), similar to the chemical shifts of
complex 2, were detected. The 1H NMR spectrum of this
mixture displayed one sharp triplet at δ –14.01 ppm with a
behavior that may be caused by weak interactions of the metal
center with the solvent or the π-system of the benzyl units of
the bis(benzyldiisopropylphosphine) ligands. Changing the NMR
solvent from THF to benzene did not have any influence on
the line broadening, indicating that interactions of the metal
center with the solvent can be excluded. Comparing the IR
spectra of 2 and of reduced 2 suggests the formation of a
palladium complex with “free” benzyl groups, such as
[Pd{(C6H5CH2)PiPr2}2] (7), since in both cases an intense
absorption band at 700 cm-1 was detected, which is charac-
teristic for C-H out-of-plane bending vibrations of unsubstituted
benzyl groups. Further evidence regarding the identity of the
product formed was gained from treatment of THF solutions
of 4 with sodium metal overnight. Remarkably, the bis(ben-
zyldiisopropylphosphine)palladium complex 7 was exclusively
formed, offering a facile and high-yield route toward the
synthesis of dicoordinated bis(phosphine) Pd0 complexes.
Complex 7 was isolated in 87% yield.12,13 The 31P{1H} NMR
spectrum of 7 exhibits a broad singlet at δ 33.18 ppm, which is
identical with one of the signals detected in the reaction mixture
of reduced 2. The 1H NMR and 13C{1H} NMR spectra display
all the signals due to the phosphine ligands. No carbon
resonances were found in the chemical shift range characteristic
of Pd-C atoms, in line with the proposed structure. Further
information about the identity of 7 was obtained from reactions
with hydrochloric acid. The addition of an equimolar amount
of hydrochloric acid (∼4 M in dioxane) to THF solutions of 7
led to the clean and quantitative formation of [Pd{(C6-
H5CH2)PiPr2}2(H)(Cl)] (8). The 31P{1H} NMR spectrum of 8
3
coupling constant of JPH ) 9.3 Hz, suggesting the formation
of a palladium hydride complex.7 Remarkably, when another 1
equiv of hydrochloric acid was added, evolution of dihydrogen
gas (detected by 1H NMR spectroscopy and GC equipped with
a TCD detector) and exclusive formation of [Pd{(C6H5-
CH2)PiPr2}2(Cl)2] (4) within 1 h was observed. Complex 4 was
isolated in almost quantitative yield (based on 2). The 31P{1H}
NMR spectrum of 4 displays a sharp singlet at δ 29.43 ppm.
The 1H NMR spectrum exhibits a doublet corresponding to four
hydrogen atoms centered at δ 7.45 ppm (3JHH ) 7.3 Hz) and a
multiplet due to six protons at δ 7.04 ppm. The 13C{1H} NMR
spectrum exhibits all the signals due to the phosphine ligands,
but none in the chemical shift range characteristic of Pd-C
bonds. The structure of 4 was additionally confirmed by its
independent synthesis.8 Further information about the identity
of the products formed upon reduction of 2 was obtained by
the independent synthesis of both isomeric forms of bis(o-
benzyldiisopropylphosphine)palladium complexes. The addition
of 2 equiv of (o-lithiobenzyl)diisopropylphosphine to benzene
solutions of bis(diethyl sulfide)palladium dichloride (5) at room
temperature led to a mixture of cis and trans isomers of bis(o-
benzyldiisopropylphosphine)palladium (6a,b) in a ratio of
approximately 2:1.9–11 Importantly, their chemical shifts in the
31P{1H} NMR and 13C{1H} NMR spectra were identical with
those found in the reaction mixtures of reduced 2, proving their
formation in the reduction process.
1
shows a sharp singlet at δ 47.26 ppm. The H NMR spectrum
exhibits, in addition to all the signals due to the phosphine
ligands, a sharp triplet at δ -14.01 ppm with a coupling constant
2
of JPH ) 9.3 Hz, which is characteristic for a bis(phosphine)
hydrido PdII complex. When another 1 equiv of hydrochloric
1
The signal at δ 33.18 ppm in the 31P{1H} NMR of the
reaction mixture of reduced 2 is broad, indicating dynamic
acid was added, evolution of H2 gas (detected by H NMR
spectroscopy and by GC equipped with a TCD detector) and
quantitative formation of 4 were observed.
(7) It finally turned out that this reaction mixture contained bis(benzyl-
diisopropylphosphine)palladium, (benzyldiisopropylphosphine)(o-benzyl-
diisopropylphosphine)chloropalladium, bis(benzyldiisopropylphosphine)-
chlorohydridopalladium, bis(benzyldiisopropylphosphine)dichloropalladium,
and both isomeric forms of bis(o-benzyldiisopropylphosphine)palladium.
(8) See the Experimental Section.
(9) (a) For examples of bis-cyclopalladated complexes containing
phosphine ligands see: Longoni, G.; Fantucci, P.; Chini, P.; Canziani, F. J.
Organomet. Chem. 1972, 39, 413. (b) Fantucci, P.; Chini, P.; Canziani, F.
Gazz. Chim. Ital. 1974, 104, 249. (c) Abicht, H. P.; Issleib, K. Z. Antimicrob.
Antineoplast. Chemother. 1976, 422, 237. (d) Abicht, H. P.; Issleib, K. J.
Organomet. Chem. 1978, 149, 209. (e) Abicht, H. P.; Issleib, K. J.
Organomet. Chem. 1980, 185, 265. (f) Abicht, H. P.; Lehniger, P.; Issleib,
K. J. Organomet. Chem. 1983, 250, 609.
A possible scenario leading to the products formed in the
reduction process includes single electron transfer from sodium
to the PdII centers with precipitation of NaTFA and formation
of a tricoordinate PdI intermediate which is likely to undergo
dimerization,14,15 followed by disproportionation to the cationic
PdII and anionic Pd0 intermediates. The Pd0 system can undergo
intramolecular C-H activation16 followed by a hydride transfer
to the cationic PdII complex to yield complexes 6a,b together
(12) It was reported that attempts to form [Pd0(PR3)2] (R ) bulky tertiary
phosphine) by direct reduction of the corresponding dichlorides with Na/
Hg were not successful, palladium metal being formed: Otsuka, S. J.
Organomet. Chem. 1980, 200, 191.
(10) For examples of bis-cyclopalladated complexes containing amine
ligands, see: (a) Kasahara, A.; Izumi, T. Bull. Chem. Soc. Jpn. 1969, 42,
1765. (b) Trofimenko, S. Inorg. Chem. 1973, 12, 1215. (c) Garber, A. R.;
Garrou, P. E.; Hartwell, G. E.; Smas, M. J.; Wilkinson, J. R.; Todd, L. J.
J. Organomet. Chem. 1975, 86, 219. (d) Murahashi, S.; Tamba, Y.;
Yamamura, M.; Yoshimura, N. J. Org. Chem. 1978, 43, 4099. (e) van der
Ploeg, A. F. M. J.; van Koten, G.; Vrieze, K. J. Organomet. Chem. 1981,
222, 155. (f) Van der Ploeg, A. F. M. J.; van Koten, G.; Schmitz, J. E. J.;
Van der Linden, J. G. M. Inorg. Chim. Acta 1982, 58, 53. (g) Arlen, C.;
Pfeffer, M.; Bars, O.; Grandjean, D. J. Chem. Soc., Dalton Trans. 1983, 8,
1535. (h) Janecki, T.; Jeffreys, J. A. D.; Pauson, P. L.; Pietrzykowski, A.;
McCullough, K. J. Organometallics 1987, 6, 1553. (i) lsters, P. L.;
Teunissen, H. T.; Boersma, J.; Spek, A. L.; van Koten, G. Organometallics
1993, 12, 4691. (j) Valk, J.-M.; Maassarani, F.; van der Sluis, P.; Spek,
A. L.; Boersma, J.; van Koten, G. Organometallics 1994, 13, 2320. (k)
Valk, J.-M.; Boersma, J.; van Koten, G. Organometallics 1996, 15, 4366.
(l) Fuchita, Y.; Yoshinaga, K.; Hanaki, T.; Kawano, H.; Kinoshita-Nagaoka,
J. J. Organomet. Chem. 1999, 580, 273. (m) Berger, A.; Djukic, J.-P.;
Pfeffer, M.; Lacour, J.; Vial, L.; De Cian, A.; Kyritsakas-Gruber, N.
Organometallics 2003, 22, 5243.
(13) (a) Burrows, A. D.; Mahon, M. F.; Varrone, M. Dalton Trans. 2004,
3321. (b) Grotjahn, D. B.; Gong, Y.; Zakharov, L.; Golen, J. A.; Rheingold,
A. L. J. Am. Chem. Soc. 2006, 128, 438. (c) Littke, A. F.; Fu, G. C. Org.
Synth. 2005, 81, 63. (d) Weng, Z.; Teo, S.; Koh, L. L.; Hor, T. S. A.
Organometallics 2004, 23, 4342. (e) Netherton, M. R.; Fu, G. C. Angew.
Chem., Int. Ed. 2002, 41, 3910. (f) Kirchhoff, J. H.; Netherton, M. R.; Hills,
I. D.; Fu, G. C. J. Am. Chem. Soc. 2002, 124, 13662. (g) Bohm, V. P. W.;
Herrmann, W. A. Chem. Eur. J. 2001, 7, 4191. (h) Holzhey, N.; Pitter, S.
J. Mol. Catal. A: Chem. 1999, 146, 25. (I) Paul, F.; Patt, J.; Hartwig, J. F.
Organometallics 1995, 14, 3030. (j) Mann, G.; Shelby, Q.; Roy, A. H.;
Hartwig, J. F. Organometallics 2003, 22, 2775 and ref 14b.
(14) For examples of PdI dimer formation upon reduction of PdII, see:
(a) Portnoy, M.; Milstein, D. Organometallics 1994, 13, 600. (b) Weissman,
H.; Shimon, L. J. W.; Milstein, D. Organometallics 2004, 23, 3931.
(15) For PdI dimer formation by reactions between 2 equiv of
[Pd(NCCH3)4]2+ and [Pd2. (dba)3], see for example: Han, X.; Weng, Z.;
Hor, T. S. A. J. Organomet. Chem. 2007, 692, 5690.
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(11) Mixtures of 6a and 6b were synthesized in analogy to related
derivatives (see ref 9c).

898 Organometallics, Vol. 27, No. 5, 2008
Frech et al.
from a concentrated diethyl ether solution at -30 °C gave the
with the neutral (benzyldiisopropylphosphine)(o-benzyldiiso-
propylphosphine)hydridopalladium(II) complex, which subse-
quently can undergo reductive elimination of the benzyl unit to
give 7.17 Significantly, according to the proposed mechanism
an overall yield of 50% of 7 was expected, which was
experimentally confirmed. Furthermore, treatment of THF
solutions of [Pd{C6H4(CH2PiPr2)}{(C6H5CH2)PiPr2}(Cl)] (9)
with an equimolar amount of NaBHEt3 (∼1.0 M in toluene)
cleanly yielded 7, further supporting the proposed reaction
mechanism. Similarly, treatment of THF solutions of 4 with 2
equiv or an excess (∼5 equiv) of NaBHEt3 (∼1.0 M in toluene)
quantitatively yielded complex 7 within a few hours.
complex as a colorless solid (148.6 mg, 0.234 mmol, 78% yield).
31P{1H} NMR (C6D6; δ, ppm): 74.91 (d (left part of an AX
system), J ) 371.8 Hz, C6H4(CH2PiPr2)), 32.13 (d (right part of
an AX system), J ) 371.8 Hz, (C6H5CH2)PiPr2). 1H NMR (δ, ppm):
7.72 (d, J ) 7.2 Hz, 2H, Ar), 7.27 (t, J ) 7.3 Hz, 2H, Ar), 7.14 (t,
J ) 7.3 Hz, 1H, Ar), 7.01 (broad t, J ) 7.3 Hz, 2H, Ar), 6.84
(broad s, 2H, Ar), 3.24 (d, J ) 9.8 Hz, 2H, CH2P), 2.82 (d, J ) 9.8
Hz, 2H, CH2P), 2.18 (m, 2H, PCH(CH3)2), 2.10 (m, 2H,
PCH(CH3)2), 1.37 (dist q, J ) 4.9 Hz, 6H, PCH(CH3)2), 1.07 (m,
12H, PCH(CH3)2), 0.88 (dist q, J ) 4.9 Hz, 6H, PCH(CH3)2).
13C{1H} NMR (δ, ppm): 160.56 (q, J ) 34.4 Hz, OC(dO)CF3),
147.87 (dd, J ) 18.9 Hz, J ) 6.1 Hz, Cipso), 138.93 (dd, J ) 11.2
Hz, J ) 1.3 Hz, Ar), 135.90 (d, J ) 2.3, Ar), 130.47 (d, J ) 4.6,
Ar), 128.45 (d, J ) 4.6, Ar), 128.06 (s, Ar), 126.54 (d, J ) 2.3,
Ar), 124.76 (s, Ar), 124.49 (s, Ar), 124.10 (d, J ) 17.6 Hz, Ar),
117.32 (q, J ) 291.8 Hz, OC(dO)CF3), 32.77 (d, J ) 13.8 Hz,
CH2P), 27.49 (d, J ) 11.3 Hz, CH2P), 25.04 (dd, J ) 17.6 Hz, J
) 3.7 Hz, PCH(CH3)2), 22.79 (dd, J ) 15.1 Hz, J ) 1.3 Hz,
PCH(CH3)2), 19.44 (s, PCH(CH3)2), 18.88 (s, PCH(CH3)2), 18.30
(s, PCH(CH3)2), 17.82 (s, PCH(CH3)2). 19F{1H} NMR (δ, ppm):
-74.11 (OC(dO)CF3). IR (ATR; cm-1): 700 (C-Harom of 1,2-
disubstituted phenyl groups). Anal. Calcd for C28H41F3O2P2Pd: C,
52.96; H, 6.51. Found: C, 52.93; H, 6.56.
In summary, reduction of [Pd{C6H4(CH2PiPr2)}{(C6H5CH2)-
PiPr2}(TFA)] (2) leads to an inseparable mixture of bis(o-
benzyldiisopropylphosphine) PdII complexes (6a,b) and the Pd0
complex [Pd{(C6H5CH2)PiPr2}2] (7). Thus, although cyclopal-
ladated compounds of the general type of κ2L,C (L ) donor)
are thermally very stable, they can undergo reduction to
bis(phosphine) Pd0 complexes, which are known to catalyze
various C-C and C-heteroatom coupling reactions, following
the traditional Pd0/PdII cycles. Reduction of [Pd{(C6H-
5CH2)PiPr2}2(Cl)2] (4) with sodium metal results in the bis-
(phosphine)palladium(0) complex 7 in high yield, offering a
facile, high-yield route that may be applicable to various other
bulky [Pd(PR3)2] complexes. A plausible mechanism was
proposed and supported by experimental observations.
Preparation of [Pd{C6H4(CH2PiPr2)}(TFA)]2 (3). To a THF
solution of [Pd(TFA)2] (100 mg, 0.301 mmol) was added 1 equiv
of (C6H5CH2)PiPr2 (62.7 mg, 0.301 mmol), and the solution was
refluxed overnight. After filtration of the reaction mixture through
a cotton pad at room temperature the solvent was removed under
reduced pressure. Complex 3 was washed with diethyl ether (3 ×
5 mL) and dried again under reduced pressure. The product was
crystallized from THF at -30 °C and obtained as a colorless solid
(92.2 mg, 0.108 mmol, 72%).
31P{1H} NMR (THF-d8; δ, ppm): 83.00 (s, C6H4(CH2PiPr2)).
1H NMR (δ, ppm): 7.35 (broad s, 2H, Ar), 7.08 (d, J ) 7.3 Hz,
2H, Ar), 7.00 (t, J ) 7.3 Hz, 2H, Ar), 6.93 (t, J ) 7.3 Hz, 2H, Ar),
3.00 (broad s, 4H, CH2P), 2.09 (m, 2H, PCH(CH3)2), 1.67 (m, 2H,
PCH(CH3)2), 1.34 (m, 6H, PCH(CH3)2), 0.98 (m, 12H, PCH(CH3)2),
0.92 (m, 6H, PCH(CH3)2). 13C{1H} NMR (δ, ppm): 160.56 (q, J
) 34.4 Hz, OC(dO)CF3), 148.14 (d, J ) 15.4 Hz, Cipso), 146.88
(s, Ar), 135.78 (broad s, Ar), 126.27 (d, J ) 17.4 Hz, Ar), 124.39
(d, J ) 2.3 Hz, Ar), 117.32 (q, J ) 291.8 Hz, OC(dO)CF3), 35.34
(d, J ) 33.9 Hz, CH2P), 26.38 (broad s, PCH(CH3)2), 21.56 (broad
s, PCH(CH3)2), 19.56 (broad s, PCH(CH3)2), 17.98 (broad s,
PCH(CH3)2). 19F{1H} NMR (δ, ppm): –75.32 (OC(dO)CF3). Anal.
Calcd for C30H40F6O4P2Pd2: C, 42.22; H, 4.72. Found: C, 42.31;
H, 4.79.
Experimental Section
General Procedures. All synthetic operations were carried out
in oven-dried glassware using a combination of glovebox (M. Braun
150B-G-II) and Schlenk techniques under a dinitrogen atmosphere.
Solvents were reagent grade or better, freshly distilled under an
N2 atmosphere by standard procedures, and were degassed by
freeze–thaw cycles before use. Deuterated solvents were purchased
from Armar, stored in a Schlenk tube (Teflon tap) over P4O10,
distilled, and degassed prior to use. All the chemicals were
purchased from Aldrich Chemical Co. or Fluka and used as
received.
Analysis. 1H, 13C{1H}, 31P{1H}, and 19F{1H} NMR data were
recorded at 500.13, 125.76, 202.46, and 235.40 MHz, respectively,
on a Bruker AMX-500 and a Bruker DRX-500 spectrometer.
Chemical shifts (δ) are expressed in parts per million (ppm), and
1
coupling constants (J) are in Hz. The H and 13C NMR chemical
shifts are relative to tetramethylsilane; the resonances of the residual
1
protons of the solvents were used as internal standards for H (δ
7.15 benzene; δ 3.58 and 1.73 tetrahydrofuran) and all-d solvent
peaks for 13C (δ 128.0 benzene; δ 67.4 and 25.2 tetrahydrofuran).
31P NMR chemical shifts are reported downfield relative to external
85% H3PO4 at in D2O at δ 0.0 ppm. 19F NMR chemical shifts
were referenced to C6F6 (δ -163 ppm). All measurements were
carried out at 298 K. Abbreviations used in the description of NMR
data are as follows: s, singlet; d, doublet; t, triplet; q, quartet; m,
multiplet; v, virtual. Elemental analyses were performed by H.
Kolbe Mikroanalytisches Laboratorium (Germany) and at the
University of Zurich.
Preparation of [Pd{(C6H5CH2)PiPr2}2(Cl)2] (4). To a THF
solution of [Pd(COD)(Cl)2] (50 mg, 0.175 mmol) was added 2 equiv
of (C6H5CH2)PiPr2 (36.5 mg, 0.301 mmol), and the solution was
stirred for 5 min at room temperature. After filtration of the reaction
mixture through a cotton pad the solvent was removed under
reduced pressure. Complex 4 was washed with pentane (2 × 5 mL)
and dried again under reduced pressure. The product was obtained
as a bright yellow solid in quantitative yield (103.8 mg, 0.174
mmol).
31P{1H} NMR (THF-d8; δ, ppm): 29.43 (s, (C6H5CH2)PiPr2)).
1H NMR (δ, ppm): 7.45 (d, J ) 7.3 Hz, 4H, Ar), 7.04 (m, 6H, Ar),
3.60 (t, J ) 4.2 Hz, 4H, CH2P), 2.05 (m, 4H, PCH(CH3)2), 1.25
(m, 24H, PCH(CH3)2). 13C{1H} NMR (δ, ppm): 137.10 (s, Ar),
131.52 (s, Ar), 129.31 (s, Ar), 127.23 (s, Ar), 32.12 (d, J ) 33.9
Hz, CH2P), 24.74 (s, PCH(CH3)2), 20.06 (s, PCH(CH3)2), 18.96
(s, PCH(CH3)2). Anal. Calcd for C26H42Cl2P2Pd: C, 52.58; H, 7.13.
Found: C, 52.39; H, 7.08.
Preparation of [Pd{C6H4(CH2PiPr2)}{(C6H5CH2)PiPr2}(TFA)]
(2). To a THF solution of [Pd(TFA)2] (100 mg, 0.301 mmol) was
added 2 equiv of (C6H5CH2)PiPr2 (125.4 mg, 0.605 mmol), and
the solution was refluxed overnight. The reaction mixture was
filtered through a cotton pad at room temperature, and the solvent
was removed under reduced pressure. The colorless solid was
extracted with diethyl ether (3 × 10 mL) and dried. Crystallization
Preparation of [Pd{C6H4(CH2PiPr2)}2] (6a,b). A diethyl ether
solution of 2 equiv of (o-lithiobenzyl)diisopropylphosphine was
added dropwise at room temperature to a benzene solution of 5
(17) The involvement of the solvent in this process can be excluded,
since no deuterium was incorporated in complex 7 when the reaction was
performed in THF-d8, as shown by 1H and 2D NMR spectroscopy.

Reduction of a Cyclometalated PdII Complex
Organometallics, Vol. 27, No. 5, 2008 899
1
(50 mg, 0.140 mmol), and the mixture was stirred at room
temperature for 3 h. After filtration of the reaction mixture through
a cotton pad the solvent was removed under reduced pressure. The
residue was washed with cold pentane (2 × 5 mL); extraction with
benzene gave a mixture of 6a (∼65%) and 6b (∼35%) in a yield
of 62% (45.2 mg, 0.086 mmol) as an off-white solid.
31P{1H} NMR (C6D6; δ, ppm): 47.26 (s, (C6H5CH2)PiPr2). H
NMR (δ, ppm): 7.72 (d, J ) 7.2 Hz, 4H, Ar), 7.17 (m, 4H, Ar),
7.05 (t, J ) 7.2 Hz, 2H, Ar), 3.37 (t, J ) 3.6 Hz, 4H, CH2P), 1.91
(m, 4H, PCH(CH3)2), 1.02 (dist q, J ) 7.2 Hz, 24H, PCH(CH3)2),
–14.01 (t, J ) 9.3 Hz, 1H, Pd-H). 13C{1H} NMR (δ, ppm): 137.11
(s, Ar), 130.78 (broad s, Ar), 128.49 (s, Ar), 126.49 (s, Ar), 29.61
(vt, J ) 9.0 Hz, CH2P), 25.47 (t, J ) 11.3 Hz, PCH(CH3)2), 19.52
(s, PCH(CH3)2), 19.38 (s, PCH(CH3)2). Anal. Calcd for
C26H43ClP2Pd: C, 55.82; H, 7.75. Found: C, 55.96; H, 7.79.
Preparation of [Pd{C6H4(CH2PiPr2)}{(C6H5CH2)PiPr2}(Cl)]
(9). To a THF solution of a mixture of 6a and 6b (30 mg, 0.057
mmol) was added an equimolar amount of hydrochloric acid (∼4
M in dioxane), and the solution was stirred for 1 h. After filtration
of the reaction mixture through a cotton pad the solvent was
removed under reduced pressure. The colorless solid was extracted
with pentane (10 mL), filtered, and dried under reduced pressure.
Complex 9 was obtained in almost quantitative yield (29.3 mg,
0.052 mmol, 92%).
31P{1H} NMR (C6D6; δ, ppm): 76.86 (d (left part of an AX
system), J ) 376.3 Hz, C6H4(CH2PiPr2)), 30.77 (d (right part of
an AX system), J ) 376.3 Hz, (C6H5CH2)PiPr2)). 1H NMR (δ,
ppm): 7.69 (d, J ) 7.2 Hz, 2H, Ar), 7.21 (t, J ) 7.2 Hz, 2H, Ar),
7.09 (t, J ) 7.3 Hz, 1H, Ar), 6.93 (broad t, J ) 7.3 Hz, 2H, Ar),
6.82 (broad s, 2H, Ar), 3.19 (d, J ) 9.7 Hz, 2H, CH2P), 2.82 (d, J
) 9.8 Hz, 2H, CH2P), 2.11 (m, 2H, PCH(CH3)2), 2.04 (m, 2H,
PCH(CH3)2), 1.33 (dist q, J ) 4.8 Hz, 6H, PCH(CH3)2), 1.07 (m,
12H, PCH(CH3)2), 0.85 (dist q, J ) 4.7 Hz, 6H, PCH(CH3)2).
13C{1H} NMR (δ, ppm): 147.66 (dd, J ) 18.6 Hz, J ) 6.2 Hz,
Cipso), 138.76 (dd, J ) 11.0 Hz, J ) 1.2 Hz, Ar), 135.79 (s, Ar),
130.32 (s, Ar), 128.36 (s, Ar), 128.07 (s, Ar), 126.53 (s, Ar), 124.68
(s, Ar), 124.54 (s, Ar), 124.13 (d, J ) 17.4 Hz, Ar), 32.64 (d, J )
13.8 Hz, CH2P), 27.54 (d, J ) 11.0 Hz, CH2P), 24.97 (dd, J )
17.8 Hz, J ) 3.8 Hz, PCH(CH3)2), 22.74 (dd, J ) 15.0 Hz, J )
1.2 Hz, PCH(CH3)2), 19.54 (s, PCH(CH3)2), 18.93 (s, PCH(CH3)2),
18.27 (s, PCH(CH3)2), 17.80 (s, PCH(CH3)2). IR (ATR; cm-1):
701 (C-Harom of 1,2-disubstituted phenyl groups). Anal. Calcd for
C26H41ClP2Pd: C, 56.02; H, 7.41. Found: C, 55.93; H, 7.56.
Data for 6a are as follows. 31P{1H} NMR (C6D6; δ, ppm): 65.95
1
(s, C6H4(CH2PiPr2)). H NMR (δ, ppm): 7.76 (d overlapped with
signals of 6b, J ) 11.6 Hz, 2H, Ar), 7.24 (m overlapped with signals
of 6b, 3H, Ar), 2.89 (s, 4H, CH2P), 1.93 (m, 4H, PCH(CH3)2), 1.16
(m overlapped with signals of 6b, 24H, PCH(CH3)2). 13C{1H} NMR
(δ, ppm): 173.94 (dd, JPC ) 119.54 Hz, JPC ) 13.1 Hz, Cipso),
148.06 (vt, JPC ) 11.1 Hz, Ar), 142.74 (vt, JPC ) 5.2 Hz, Ar),
133.21 (s, Ar), 132.84 (s, Ar), 128.08 (s, Ar), 127.91 (s, Ar), 125.59
(s, Ar), 124.58 (s, Ar), 124.44 (s, Ar), 36.43 (dd, JPC ) 12.4 Hz,
JPC ) 4.1 Hz, CH2P), 24.58 (vt, JPC ) 10.4 Hz, PCH(CH3)2), 22.43
(vt, JPC ) 10.3 Hz, PCH(CH3)2), 20.51 (vt, JPC ) 2.6 Hz,
PCH(CH3)2), 20.38 (vt, JPC ) 3.3 Hz, PCH(CH3)2), 18.41 (broad
s, PCH(CH3)2), 16.05 (broad s, PCH(CH3)2). Data for 6b are as
follows. 31P{1H} NMR (C6D6; δ, ppm): 70.28 (s, C6H4(CH2PiPr2)).
1H NMR (δ, ppm): 7.74 (d overlapped with signals of 6a, J )
11.4 Hz, 2H, Ar), 7.24 (m overlapped with signals of 6a, 3H, Ar),
2.93 (s, 4H, CH2P), 1.89 (m, 4H, PCH(CH3)2), 1.16 (m overlapped
with signals of 6a, 24H, PCH(CH3)2). 13C{1H} NMR (δ, ppm):
2
2
167.80 (vt, JPC ) 7.2 Hz, Cipso), 149.61 (vt, JPC ) 9.2 Hz, Ar),
131.03 (s, Ar), 128.58 (s, Ar), 126.04 (s, Ar), 124.01 (s, Ar), 34.82
(vt, JPC ) 12.4 Hz, CH2P), 31.82 (d, JPC ) 5.2 Hz, PCH(CH3)2),
20.09 (s, PCH(CH3)2), 18.96 (s, PCH(CH3)2). Anal. Calcd for
C26H40P2Pd: C, 59.95; H, 7.74. Found: C, 60.07; H, 7.88.
Preparation of [Pd{(C6H5CH2)PiPr2}2] (7). A THF solution of
4 (50 mg, 0.084 mmol) and a large excess of sodium metal (∼500
equiv) was stirred at room temperature overnight. After filtration
of the reaction mixture through a cotton pad the solvent was
removed under reduced pressure. 7 was extracted with pentane (15
mL), filtered, and dried again under reduced pressure. The product
was obtained in 87% yield (34.3 mg, 0.066 mmol) as an oily
substance, which solidified within a few days.
31P{1H} NMR (THF-d8; δ, ppm): 33.18 (broad s, (C6H5-
1
CH2)PiPr2)). H NMR (δ, ppm): 7.70 (broad d, J ) 11.7 Hz, 2H,
Acknowledgment. This work was supported by the Swiss
National Science Foundation (SNSF), the University of
Zurich, the program for German-Israeli Cooperation (DIP),
and the Helen and Martin Kimmel Center for Molecular
Design. D.M. is the holder of the Israel Matz Professorial
Chair of Organic Chemistry.
Ar), 7.12 (m, 3H, Ar), 2.95 (broad s, 4H, CH2P), 1.81 (m, 4H,
PCH(CH3)2), 1.16 (broad s, 24H, PCH(CH3)2). 13C{1H} NMR (δ,
ppm): 139.96 (s, Ar), 131.25 (s, Ar), 128.62 (s, Ar), 126.36 (s, Ar),
32.16 (broad s, CH2P), 26.14 (broad s, PCH(CH3)2), 20.64 (s,
PCH(CH3)2), 20.28 (s, PCH(CH3)2). Anal. Calcd for C26H42P2Pd:
C, 59.71; H, 8.09. Found: C, 59.62; H, 8.02.
Preparation of [Pd{(C6H5CH2)PiPr2}2(Cl)(H)] (8). To a THF
solution of 7 (20 mg, 0.038 mmol) was added an equimolar amount
of hydrochloric acid (∼4 M in dioxane) at room temperature, and
the mixture was stirred for a few minutes. After filtration of the
reaction mixture through a cotton pad the solvent was removed
under reduced pressure. Complex 8 was extracted with pentane (15
mL), filtered, and dried again under reduced pressure. The product
was obtained as an orange powder in almost quantitative yield (20.1
mg, 0.036 mmol).
Supporting Information Available: CIF files containing X-ray
crystallographic data for complexes 2 (CCDC-637692) and 3
(CCDC-637693). This material is available free of charge via the
Internet at http://pubs.acs.org. This material is also available free
of charge from the Cambridge Crystallographic Data Centre via
http://www.ccdc.cam.ac.uk/data_request/cif.
OM7008815