Published on the web September 8, 2012
967
Synthesis, Structure, and Catalytic Activity of Palladium Complexes Bearing a Tridentate
PXP-Pincer Ligand of Heavier Group 14 Element (X = Ge, Sn)
Jun Takaya, Shuhei Nakamura, and Nobuharu Iwasawa*
Department of Chemistry, Tokyo Institute of Technology, O-okayama, Meguro-ku, Tokyo 152-8551
(Received July 7, 2012; CL-120725; E-mail: niwasawa@chem.titech.ac.jp)
Table 1. Synthesis of ligand precursors
An efficient method for the synthesis of tridentate PGeP-
and PSnP-palladium complexes is developed. Structural anal-
ysis revealed that PSiP-ligand exerts the strongest trans influence
and electron donation and that PGeP- and PSnP-ligands provide
wider coordination sphere around the palladium. Preliminary
studies demonstrated that both PGeP- and PSnP-palladium
complexes work as an efficient catalyst for reductive aldol-type
reaction, indicating promising utility in synthetic organic
chemistry.
Entry
1a
2
X
R1
R2
H
Reagent
Yield/%
Ge Me
Ph
1.5 equiv LiAlH4
2a
2b
75
89
Transition-metal complexes bearing a tridentate PSiP-pincer
ligand derived from bis(o-phosphinophenyl)silane have been
attracting much attention in organometallic chemistry.1-3 The
features of this type of PSiP-pincer ligand are 1) the strong trans
influence of the silicon which enhances nucleophilicity of the
trans substituent and 2) distorted square-planar structure due
to the sp3-Si atom which induces facile structural change.2b
Contrary to the well-developed chemistry of Si-containing
multidentate ligands,4 multidentate ligands containing a heavier
group 14 element such as Ge and Sn have rarely been developed
despite their unique characteristics different from Si.5,6 Recently
Nakazawa reported synthesis and structural analysis of new
rhodium and iridium complexes bearing a tetradentate P3Ge- or
P3Sn-ligand and their unique reactivity in ligand dissociation
and substitution reaction.7 However, there has been no report on
the synthesis and reactivity of metal complexes bearing an
anionic, tridentate PXP-pincer type ligand (X = Ge, Sn). Such
tridentate pincer complexes of divalent group 10 metals are
expected to be an active catalyst for molecular transformation
since similar PCP-palladium(II) complexes have been widely
utilized in synthetic organic chemistry.8 Herein we report the
synthesis and structural analysis of palladium complexes bearing
a tridentate PGeP- or PSnP-pincer ligand and their catalytic
activity in reductive aldol-type reaction between ¡,¢-unsat-
urated esters and aldehydes.
Based on the synthesis of the PSiP-pincer palladium(II)
complex,1a,2a bis[o-(diphenylphosphino)phenyl]methylgermane
(2a) was synthesized as a ligand precursor for the synthesis
of the corresponding PGeP-palladium complex. Treatment of
commercially available methyltrichlorogermane with 2 equiv of
o-(diphenylphosphino)phenyllithium (1) afforded diarylmethyl-
chlorogermane, which was successively reduced in one pot by
LiAlH4 to give desired 2a in 75% yield (Table 1, Entry 1).
Germane 2b bearing a phenyl group instead of methyl on
germanium was also obtained in good yield by the same
procedure using phenyltrichlorogermane as a starting material
(Entry 2). However, the synthesis of the corresponding stannane
derivative (Ar2MeSnH) by the same procedure was unsuccessful
probably due to its instability under the reaction conditions.
Therefore, the possibility of using allylstannane derivative 3 as a
3b
4b
Sn Me allyl 2 equiv allylMgCl 3a
Ph 3b
aThe reaction was carried out in Et2O-toluene (2:1). 1 was
added at 0 °C and the mixture was stirred for 1 h at rt.
16
40
b
Scheme 1. Synthesis of PXP-palladium complexes (X = Ge,
Sn).
more stable ligand precursor was examined with the expectation
that the reactive C-Sn bond of allylstannane would be cleaved
easily by palladium to form a Pd-Sn bond.9 Allylstannane
derivatives 3a (R = Me) and 3b (R = Ph) were synthesized
from commercially available methyl- or phenyltrichlorostannane
1 and allylmagnesium chloride (Entries 3 and 4).
Complexation of 2 or 3 with [Pd(C3H5)Cl]2 proceeded
smoothly at room temperature to give PXP-palladium(II)
chloride complexes 4 or 5 (X = Ge, Sn) in high yield
(Scheme 1). The reaction of the Ph-substituted germane and
stannane derivatives also proceeded without problem. It should
be noted that the allylstannanes 3 successfully worked as a
ligand precursor via C-Sn bond cleavage. Triflate complexes 6a
and 7a were also prepared by treatment of 4a or 5a with AgOTf.
Thus, an efficient method for the synthesis of tridentate PGeP-
and PSnP-pincer type palladium complexes was realized for the
first time.
X-ray analyses were performed for all palladium chloride
complexes10 4a, 4b, 5a, 5b, 8a, and 8b and ORTEP diagrams of
Chem. Lett. 2012, 41, 967-969
© 2012 The Chemical Society of Japan
Takaya, Jun
