New PC(sp3)P pincer complexes of platinum and palladium

Article abstract of DOI:10.1016/j.jorganchem.2011.11.007

We describe the synthesis, characterization and properties of the new electrophilic palladium and platinum complexes bearing electron-rich PC(sp ³)P pincer ligands. We found that Lewis acidity of the compounds is low due to the strong σ-donation of the sp³-carbometalated ligand backbone.

Full text of DOI:10.1016/j.jorganchem.2011.11.007

Journal of Organometallic Chemistry 699 (2012) 92e95
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Note
New PC(sp3)P pincer complexes of platinum and palladiumq
*
Sanaa Musa, Alina Shpruhman, Dmitri Gelman
Institute of Chemistry, The Hebrew University of Jerusalem, Givat Ram, Jerusalem 91904, Israel
a r t i c l e i n f o
a b s t r a c t
Article history:
We describe the synthesis, characterization and properties of the new electrophilic palladium and
platinum complexes bearing electron-rich PC(sp3)P pincer ligands. We found that Lewis acidity of the
Received 14 September 2011
Received in revised form
25 October 2011
compounds is low due to the strong s
-donation of the sp3-carbometalated ligand backbone.
Ó 2011 Elsevier B.V. All rights reserved.
Accepted 3 November 2011
Keywords:
sp3-Metalated
Pincer complexes
Triptycene
Platinum
1. Introduction
modification highlight the C(sp2)-metalated pincer complexes as
ideal candidates for systematic studies.
Carbometalated pincer-type complexes were discovered over
the past 35 years (Fig. 1) [1,2]. Since then, these compounds have
been studied extensively and became very popular in many areas of
chemistry. For instance, they are widely used as homogeneous
catalysts and stoichiometric promoters in a variety of chemical
transformations [3], as building blocks for the construction of
advanced functional materials [4] and as components in supra-
molecular systems [4,5]. Furthermore, carbometalated pincer
complexes were found relevant to biological applications [6].
Carbometalated pincer complexes are organometallic compounds
bearing tridentate monocarbanionic ligands that coordinate metal in
In opposite, reports on the chemistry and practical applications
of aliphatic C(sp3)-metalated pincer complexes (Fig. 1, right) are far
more occasional [10], arguably, because the stronger s-donating
nature of the sp3ehybridized ligand significantly increases electron
density at the metal center turning them into less thermally stable
[1] and more difficult-to-manipulate compounds [11].
Recently, we demonstrated the utilization of the barrelene-
type scaffold, as a platform for the construction of the first
representative of a new class of sp3-carbometalated complexes
[12e14]. The desired compounds can be synthesized via either the
CeH activation or cycloaddition strategies (Scheme 1). The tradi-
tional CeH activation of bis-phosphinated barrelenes is somewhat
limited to large electrophilic metals (e.g. iridium, platinum and
ruthenium) due to a relatively low acidity of the methine
hydrogen [15], however the cycloaddition route perfectly
complements it, giving raise to diverse libraries of the target
compounds.
a h
3-mer fashion. This coordination mode gives rise to the formation
of the thermodynamically favored CeM bond-sharing bicyclic
structures [7] that are usually translated into robust materials
featuring exceptional thermal stability [8]. On the other hand, the
ligand scaffold and the coordinating groups can be tailored to control
reactivity of the carbon-metal bond by controlling steric and elec-
tronic environment of the metal center [3,9]. The combination of
these factors makes pincer complexes very popular in many areas of
chemistry.
In contrast to the most of known all-aliphatic C(sp3)-metalated
compounds, the novel ones exhibited excellent thermal and
conformational stability even under very harsh and non-inert
By far the most popular scaffold used to design DCD pincer
ligands relies on aromatic moieties (Fig. 1, left). This is not surprising
because availability of the synthetic methods for their facile
conditions, likely, due to the lack of labile a- or b-hydrogens.
Moreover, they proved themself as highly active catalysts for the
transfer hydrogenation of ketones [12], as well as transfer [16] and
acceptorless dehydrogenation of alcohols [17].
In this article, we wish to report on the synthesis and charac-
terization of new derivatives of PC(sp3)Ps addressing a possible
effect of
properties of the complexes with more electrophilic metal centers.
q
The paper is dedicated to Dr. Shmuel Cohen on the occasion of his retirement.
s
-donating nature of the sp3ehybridized ligands on the
* Corresponding author. Tel.: þ972 2 6584588; fax: þ972 2 6585279.
E-mail address: dgelman@chem.ch.huji.ac.il (D. Gelman).
0022-328X/$ e see front matter Ó 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.jorganchem.2011.11.007

S. Musa et al. / Journal of Organometallic Chemistry 699 (2012) 92e95
93
D
D
TM
D
TM
D
D = O, S, N, P or As
TM = transition or main group metals
Fig. 1. Carbometalated pincer complexes.
2. Results and discussion
Scheme 2.
Cationic complexes were prepared by reacting the previously
reported precursors 1e2 with silver tetrafluoroborate (Schemes 2
and 3) [14]. The target complex 3 was obtained as pale yellow
powder in 75% yield, while 4 formed as white solid in 69% yield.
Both were found completely stable toward air and moisture in both
solid state and solution.
case of 4 and 6e9 the difference between them was summed up
less than 1 ppm.
To get further clue into the level of Lewis acidity of the metal
centers, the complexes 3e4 were dissolved and stirred in 1:1
CH3CN/pyridine mixture. NMR analysis of the product mixture
taken after evaporation of the solvent showed almost equal
distribution of the acetonitrile and pyridine adducts (based on the
integration of the signal assigned to the methyl carboxylate
groups). Similarly, full conversion of 6 into to the parent 2 by
treatment with excess of LiCl proceeds slowly and requires long
reaction time of 48 h. This behavior, apparently, suggests that
positive charge on the metal centers is almost fully compensated by
All new complexes have been characterized by NMR spectros-
copy using samples prepared in CDCl3. Observation of one signal in
the 31P{1H} NMR spectra in these complexes confirmed the
equivalence of the two phosphorus nuclei, while the magnitude of
the coupling constant measured for the platinum complex
(J ¼ 1518 Hz) supported their mutually trans disposition [18]. The
presence of a coordinated acetonitrile molecule in 3 and 4 was
indicated by a singlet resonance in their 1H NMR spectra (CDCl3).
Despite quite an unambiguous characterization of the
complexes by NMR, x-ray crystallographic analysis of the species
was undertaken (Fig. 2). As one would expect for the complexes
bearing strongly bent dibenzobarrelene-based ligands, the palla-
dium center in 3 is distorted from the square planar geometry
toward a butterfly-like environment. For example, the observed
interplanar angle between the C(1)ePd(1)eP(1) and C(1)ePd(1)e
P(2) planes is 157.2ꢀ, while the C(1)ePt(1)eN(1) angle is 173.34(2)ꢀ.
Another series of complexes bearing labile carboxylate ligands
has been prepared via interaction of 2 and 5 with silver tri-
fluoroacetate and silver 2-ethylheptanoate according to Scheme 2.
The reactions were virtually quantitative according to the 31P NMR
monitoring and the products 6e9 were isolated in 75e90% yield as
white solids.
Single crystals grown from the acetonitrile/chloroform solution
of 8 were analyzed using x-ray crystallography. We found that
platinum center is strongly distorted from the square planar
geometry toward a butterfly-like environment (the ORTEP illus-
trations are shown on Fig. 3). For example, the observed P(1)e
PteP(2), C(1)ePteO(1) angles for 8 are 156.99(4)ꢀ and 177.44(4)ꢀ,
respectively. Other bonds angles and distances fall into the usual
range. The trifluoroacetate ligand is clearly bound in a h1 fashion.
Due to the lability of the carboxylate ligands, their complexes
are often seen as neutral analogs of the cationic ones. However,
a greater electrophilicity of the metal centers in both the naked
(3e4) and “masked” (6e9) cationic complexes bearing the
electron-rich PC(sp3)P ligands was not pronounced at all, as re-
flected in their very slight downfield 31P chemical shifts compared
to those of the corresponding parent compounds (1, 2 and 5), as
well as in the M.NCMe M.O2CR bond lengths that fall within the
average range [19]. For example, the palladium-based 3 demon-
the
Furthermore, attempted employment of the new complexes in
electrophilic metal-catalyzed reactions met a limited success. So
aldol addition of methyl -isocyanoacetate to benzaldehyde that is
s
-donation from the sp3-hybridized backbone [20].
a
known to be promoted by cationic pincer complexes failed
completely using 3 and 4 supporting a very low Lewis acidity of the
cationic complexes bearing the new ligands [21].
Complexes 6e9 were found moderately active in Pt-catalyzed H/
D exchange at aromatic rings, a benchmark reaction often used to
evaluate the reactivity of the carbometalated compounds. A brief
experimentation showed that H/D exchange of benzene does take
place in the presence of 2 mol% of 6e8 in CD3CO2D, CF3CO2D,
CD3OD or D2O although complete deuterium incorporation was
achieved only in TFA-d using 6 and 8 (Table 1).
In a typical run, a mixture of benzene, 2 mol% catalyst, and
deuterium source were sealed in a 20 ml pressure tube and heated
at 100 ꢀC for 24 h. The obtained mixture was extracted with hexane
after cooling to room temperature. The extracts were concentrated
and purified using short silica-gel column chromatography. The
product was analyzed by GCeMS and 1H NMR.
Compared to the known catalytic systems [22], the activity is
modest; it was shown that 6 and 8 promote complete deuteration
CO2Me
MeO2C
CO2Me
MeO2C
Pt
Ph2P
PPh2
Pt
RCO2
Ph2P
PPh2
6 R = CF3
Cl
2
7
R = C8H17
Ag(O2CR)
strated a shift of only ca. 6 ppm (
d
47.8 vs 41.9 for 1), while in the
R
R
R
R
Diels-Alder
C-H activation
Pt
Ph2P
TM
PPh2
( -Pr) P
i
Pt
2
P( -Pr)
i
Cl
( -Pr) P
L
2
i
TM
Ph2P
2 RCO2
P( -Pr)
PPh2
i
Ph2P
2
PPh2
L
5
8 R = CF3
TM = Ir, Ru, Pd, Pt, Ni
R = Ar, CO2R, Ph
9
R = C8H17
Scheme 1.
Scheme 3.

94
S. Musa et al. / Journal of Organometallic Chemistry 699 (2012) 92e95
Table 1
Representative results of Pt-catalyzed H/D exchange at arenes.
6-9 (2 mol%)
TFA-d, 24 h
100 oC
H
D
Run
Catalyst
Substrate
%D incorporation
1
2
3
4
5
6
7
6
7
8
9
6
8
6
Benzene
Benzene
Benzene
Benzene
Toluene
Toluene
Tetralin
99
79
99
81
32
26
50
of benzene (Table 1, runs 1 and 3), although deuterium incorpo-
ration into toluene and tetralin under the same conditions is only
32 and 50%, respectively.
To conclude, in this work we synthesized, characterized and
examined properties of the some new cationic complexes of
palladium and platinum bearing PC(sp3)P pincer ligands. It was
found that positive charge on the metal centers is compensated by
electron-rich backbone. These observations suggest that catalytic
applications requiring high Lewis acidity of the metal centers,
apparently, cannot be targeted using this, otherwise powerful,
family of compounds.
Fig. 2. ORTEP drawing (50% probability ellipsoids) of the structures 3. Hydrogen atoms
and solvent molecules are omitted for clarity. Selected bond lengths (Å) and angles
(deg.) for 3: Pd1eN1 (2.082(2)), Pd1eC1 (2.056(2)), Pd1eP1 (2.3032(7)), Pd1eP2
(2.2958(7)). C15eC1ePd1 (120.81(17)), C1ePd1eN1 (173.46(10)), P1ePt1eP2
(155.50(2)). Selected bond lengths (Å) and angles (deg.) for 4: Pt1eN1 (2.009(12)),
Pt1eC1 (1.924(11)), Pt1eP1 (2.304(3)), Pt1eP2 (2.305(3)). C15eC1ePt1 (128.0(8)),
C1ePt1eN1 (176.7(5)), P1ePt1eP2 (156.46(12)).
Acknowledgment
This research was supported by the Israel Science Foundation.
We especially thank Dr. Shmuel Cohen for solving X-Ray structures.
Appendix A. Supplementary material
CCDC 843593; 3, 843594; 4, 843595; 8 contain the supple-
mentary crystallographic data for this paper. These data can be
obtained free of charge from The Cambridge Crystallographic Data
Centre via www.ccdc.cam.ac.uk/data_request/cif.
Appendix. Supplementary material
Supplementary data related to this article can be found online at
doi:10.1016/j.jorganchem.2011.11.007.
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