Synthesis and resolution of planar-chiral ruthenium-palladium complexes with ECE' pincer ligands

Article abstract of DOI:10.1002/chem.200900117

The synthesis and chiral resolution of the first planar-chiral ECE'-metal complexes of type e was reported. n¹-coordination of the metal (M) was carried out first, followed by n⁶ of a second metal fragment (M'). The ECE'-palladium co

Full text of DOI:10.1002/chem.200900117

DOI: 10.1002/chem.200900117
Synthesis and Resolution of Planar–Chiral Ruthenium–Palladium Complexes
with ECE’ Pincer Ligands
Sylvestre Bonnet,[a] Jie Li,[a] Maxime A. Siegler,[b] Lars S. von Chrzanowski,[b]
Anthony L. Spek,[b] Gerard van Koten,[a] and Robertus J. M. Klein Gebbink*[a]
Metal complexes with ECE pincer ligands have attracted
much attention in the last decade because of their stability
and tunability.[1–4] More precisely the terdentate framework
of the ECE pincer ligand stabilizes the s metal–carbon
bond, which makes the ECE–metal unit an excellent build-
ing block in, for example, supramolecular and bioorganome-
tallic chemistry. Up to now, four approaches have led to
chiral ECE–metal complexes 1) substitution at the benzylic
position (a),[5,6] 2) introduction of substituents with a chiral
center on their organic backbone (b),[7–11] 3) introduction of
axially chiral organic substituents (c),[12] and 4) introduction
of stereogenic sulfur[13] or phosphorus[14,15] atoms (d).
We now report the synthesis and chiral resolution of the
first planar–chiral ECE’–metal complexes of type e. Planar–
chiral complexes rely on the ability of transition metals to
bind the p electrons of dissymmetrically substituted five-
[16–18]
or six-membered[16,19–21] aromatic rings. In this paper
we propose to use dissymmetric ECE’ pincer ligands with
two different heteroatoms (i.e., E¼E’). h1-Coordination to
the metal (M) is carried out first, followed by h6-coordina-
tion of a second metal fragment (M’). As h6-coordination
can take place from either side of the arene plane of the
ECE’ pincer complex, two enantiomers of the heterobime-
tallic h6,h1-complex can form (just one enantiomer of an e-
type complex is shown).
We have recently shown that h6-coordination of the frag-
ment M’=[Ru
(C5R5)]+ (R=H or Me)[22,23] to NCN–metal
AHCTUNGTRENNUNG
complexes can be achieved without affecting the h1-carbon–
metal bond.[24] The application of this methodology to dis-
symmetric PCN– and PCS–palladium complexes led to the
corresponding cationic ECE’–ruthenium–palladium com-
pounds (see Scheme 1). In the case of PCN, a solid-state
structure of the racemate 3-[BF4] was obtained, clearly dem-
onstrating the planar–chiral nature of the h6,h1-structure. In
the case of PCS, anion exchange was undertaken by using
[D-TRISPHAT]ꢀ, an enantiomerically pure chiral counter-
ion.[25] The diastereoisomeric mixtures 4-[D-TRISPHAT]
and 5-[D-TRISPHAT] were analyzed by NMR spectroscopy,
and the crystal structure of 5-[D-TRISPHAT], which con-
tained only one enantiomer of the planar–chiral 5+ ion, was
determined.
[a] Dr. S. Bonnet, J. Li, Prof. Dr. G. van Koten,
Prof. Dr. R. J. M. Klein Gebbink
Chemical Biology & Organic Chemistry
Debye Institute for Nanomaterials Science
Faculty of Science, Utrecht University
Padualaan 8, 3584CH Utrecht (The Netherlands)
Fax : (+31)30-252-3615.
E-mail: r.j.m.kleingebbink@uu.nl
[b] Dr. M. A. Siegler, Dr. L. S. von Chrzanowski, Prof. Dr. A. L. Spek
Crystal and Structural Chemistry
Bijvoet Center for Biomolecular Research
Faculty of Science, Utrecht University
Padualaan 8, 3584CH Utrecht (The Netherlands)
To selectively address the pincer arene ring with the are-
(MeCN)3]+ (R=H or Me) the ECE’–pal-
nophile [RuACTHNUTRGENNU(G C5R5)ACHTUGNTRENNUGN
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.200900117.
ladium complexes have to carry N, S, and P substituents
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crystal structure, the two enantiotopic faces of the PCN–pal-
ladium complex are differentiated by h6-coordination to the
ruthenium center on one side only, that is, the bimetallic
complex 3+ is planar–chiral. However, both enantiomers of
the 3+ ion are found in the crystal, that is, 3-[BF4] crystalli-
zes as a racemate.
To demonstrate the planar–chiral nature of 4-[PF6] and 5-
ꢀ
ꢀ
[BF4], we exchanged the achiral PF6 and BF4 counterions
for an enantiomerically pure chiral counterion, namely [D-
TRISPHAT]ꢀ.[25] This anion has been used as chiral shift
agent for complexes of the [MACHTNUGRTNEUNG
(bpy)3]2+ family (M=Fe or
Ru, bpy=2,2’-bipyridine),[30–33] as well as for planar–chiral
ꢀ
chromium complexes.[34] In a first step, the lipophilic PF6 or
ꢀ
BF4 counterions were exchanged for Clꢀ on Amberlite.
The resulting 4-[Cl] and 5-[Cl] complexes are soluble both
in water and in organic solvents. In a second step, one equiv-
alent of [cinchonidium][D-TRISPHAT][35] enables a second
anion exchange to take place in water/dichloromethane, as
[cinchonidium]Cl is hydrophilic and 4-[D-TRISPHAT] and
5-[D-TRISPHAT] are hydrophobic (see Scheme 2 and Sup-
porting Information). This procedure affords 4-[D-TRIS-
Scheme 1. Synthesis of racemic planar–chiral complexes 3+–5+. Only the
enantiomers of pS configuration (according to the extended Cahn–
Ingold–Prelog nomenclature)[27] are shown.
AHCTUNGTREGPNNUN HAT] and 5-[D-TRISPHAT] in 46 and 63% yield, respec-
tively, as white amorphous powders.
with aliphatic groups.[26] PCN- and PCS–palladium com-
plexes 1 and 2 (see Figure S2 in the Supporting Information)
were subjected to h6-coordination by using 1.1 equivalents
of [RuACHTUNGTRENNUNG(C5R5)ACHTUNGTRENNUNG
(MeCN)3]+ in dry dichloromethane at room
temperature (see Scheme 1). Upon completion of the reac-
tion and after purification by chromatography on silica gel,
complexes 3-[BF4], 4-[PF6], and 5-[BF4] were obtained as
brown (3+) or white (4+ and 5+) powders that are stable in
air for months.
ACTHNUTRGNEUNG
h6-Coordination of the [Ru(C5R5)]+ fragment is unambig-
1
uously shown by the large shift in H NMR signals of the ar-
omatic protons towards higher fields (Dd>0.5 ppm) in the
cations 3+–5+, compared to 1 and 2. In addition, in 4+ and
5+ facial differentiation induced by the coordination of the
ruthenium fragment splits the S- and P-benzylic protons into
two well-resolved AB resonance patterns. Similarly in 3+–5+
the two P–alkyl substituents are different due to planar dis-
symmetrization by the ruthenium fragment. Variable-tem-
perature NMR spectra were measured for 2, 4+, and 5+ be-
tween 25 and ꢀ908C in acetone. Neither 4+ nor 5+ showed
any dynamic behavior, whereas the well-known inversion of
configuration of the sulfur ligand, already studied for SCS–
palladium complexes,[28,29] occurs for PCS–palladium precur-
sor 2 (coalescence temperature: ꢀ508C, see Figure S1). We
interpret the lack in dynamic behavior of 4+ and 5+ to be
the result of intramolecular steric hindrance between the cy-
clopentadienyl moiety and the alkyl substituents of the PCS
pincer ligand, whereas the arene ring in 2 can freely wag.
The PCN–palladium unit in 1 and 3+ is almost planar due to
conjugation of the imine with the aromatic ring, and no dy-
namic behavior was observed.
Scheme 2. Anion exchange for the 4+ and 5+ ions.
The 31P NMR spectra of 4-[D-TRISPHAT] and 5-[D-
TRISACTHNUTRGENUPGN HAT] in dichloromethane show two well-resolved
singlets in a 1:1 ratio for the coordinated P ligand, whereas
in the same conditions 4-[Cl] and 5-[Cl] give a singlet (see
Figure 1): as the [D-TRISPHAT]ꢀ counterion is chiral and
enantiomerically pure, the racemic complexes 4-[Cl] and 5-
[Cl] are turned into the corresponding diastereoisomeric 1:1
mixtures 4-[D-TRISPHAT] and 5-[D-TRISPHAT] after
anion exchange. In the 1H NMR spectra only two of the
three aromatic signals are doubled after the Clꢀ$[D-TRIS-
AHCTUNGTRENNUNG
PHAT]ꢀ exchange, which suggests that [D-TRISPHAT]ꢀ sits
dissymmetrically on the PCS–palladium complex. These
chiral interactions should not be taken for granted, as anion
exchange with (+)-camphor sulfonate did not allow differ-
entiation of the two enantiomers of 5+ by NMR spectrosco-
py (see Figure S4).
Single crystals of 3-[BF4] suitable for X-ray diffraction
were obtained by vapor diffusion (see Figure S3). In the
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R. J. M. Klein Gebbink et al.
chiral complexes,[27] C1 has the (pR) configuration. The C4–
C42 contact (~3.63 ꢁ) suggests p–p stacking interactions be-
tween one tetrachlorocatecholate moiety of [D-TRIS-
AHCTUNGTRENNUNG
PHAT]ꢀ and the h6,h1-coordinated arene ring, which might
explain why [D-TRISPHAT]ꢀ is a good chiral-shift reagent
for the 4+ and 5+ ions. As in the crystal structure of 2 (see
Figure S2 in the Supporting Information), the palladacycles
of the PCS pincer moiety are puckered and the S-isopropyl
group lies in equatorial position (the configuration of the
sulfur atom is (R)). The crystal structure of 5-[D-TRIS-
AHCTUNGTREGPNNNU HAT] nicely shows the possibility to separate the two en-
antiomers of planar–chiral ions of type 3+–5+ by crystalliza-
tion of their [D-TRISPHAT]ꢀ salts.
ACTHNUTRGNEUNG
In conclusion, h6-coordination of a [Ru(C5R5)]+ fragment
(R=H or Me) to dissymmetric PCN– or PCS–palladium
complexes affords in one step racemic planar–chiral Ru–Pd
complexes. These h6,h1-organometallic complexes are highly
stable in the solid state and in solution. Among the well-
documented family of planar–chiral five-membered metalla-
cycles,[16] the 3+ and 5+ ions are, to our knowledge, the first
examples based on the ECE’ pincer architecture. Compound
3-[BF4] crystallized as a racemate, but 5-[D-TRISPHAT]
contains a single enantiomer of the 5+ ion, which seems to
result from complementary p–p stacking between the [D-
TRISPHAT]ꢀ counterion and the h6,h1-arene ring of the
cation. These results open the door to the chiral resolution
of complexes of type 3+–5+ on a larger scale. Earlier studies
in the field of homogeneous catalysis have shown that dis-
symmetric PCS–palladium complexes have a good catalytic
activity in tandem processes,[36] and that h6-coordination of
Figure 1. 1H NMR (400 MHz, aromatic region, 5.6–6.6 ppm) and
31P NMR (162 MHz, coordinated P, 68–72 ppm) spectra of 4-[Cl] (above)
and 4-[D-TRISPHAT] (below) in CD2Cl2.
Slow evaporation of a solution of 5-[D-TRISPHAT] in
acetone/water led to single crystals suitable for X-ray dif-
fraction (see Experimental Section and Supporting Informa-
tion). The crystal structure includes only one enantiomer of
the 5+ ion with the [D-TRISPHAT]ꢀ counterion
(Figure 2).[25] The absolute configuration was established by
anomalous dispersion effects in diffraction measurements on
the crystal; according to the extended CIP rules for planar–
the [RuACHTUNGTRNEUNG
(C5R5)]+ fragment to ECE–palladium complexes en-
hances the catalytic properties of palladium.[37] Our new ap-
proach towards planar–chiral ECE’–ruthenium–palladium
complexes might hence lead to new and efficient asymmet-
ric catalysts.
Experimental Section
Racemic cationic complexes 3+–5+: In a typical synthesis, either com-
pound 1 or 2 (200 mmol) was put in a dry Schlenk flask under nitrogen,
ꢀ
and a solution containing [Ru
(C5R5)
N
R=Me, Y=BF4ꢀ; 1.1 equiv) in dry dichloromethane (5 mL) was added
at room temperature under nitrogen. The black reaction mixture was
stirred under nitrogen for 1–3 days and deposited on silica gel. Elution
with dichloromethane/MeOH (99:1) afforded 3-Y–5-Y in good to excel-
lent yields.
The synthesis and variable-temperature NMR spectroscopy of complex
2; the full characterization of complexes 3+–5+; the X-ray structures of 2,
3-[BF4], and the X-ray crystal structure refinement method; and the
anion exchange experimental procedures, are given as Supporting Infor-
mation.
X-ray crystallographic data for 5-[D-TRISPHAT]: C51H51Cl13O6P2PdRuS,
Mr =1522.24, colorless irregular block, 0.25ꢂ0.15ꢂ0.14 mm3, triclinic, P1
(no. 1), a=9.8733(2), b=11.3092(3), c=13.7873(5) ꢁ, a=99.090(1), b=
Figure 2. Displacement ellipsoid plots (50% probability level) of 5-[D-
TRISPHAT] at 150(2) K. The minor component of one disordered cyclo-
hexane group and the H atoms have been omitted for clarity. Character-
90.518(1), g=96.097(2)8, V=1511.02(7) ꢁ3, Z=1, 1calcd =1.673 gcmꢀ3
,
m=1.26 mmꢀ1
; number of reflections (measured/unique): 45178/13826
ꢀ
ꢀ
ꢀ
istic bond lengths (ꢁ): Pd1 C1=1.985(3), Pd1 P1=2.2481(7), Pd1 S1=
(Rint =0.024); number of reflections [I>2s(I)]: 12687; number of params/
restraints: 732/183; R1/wR2 [I>2s(I)]: 0.026/0.051; R1/wR2 (all data):
ꢀ
ꢀ
ꢀ
2.3462(6), Pd1 Cl1=2.3483(7), Ru1 C1=2.276(3), Ru1 C4=2.216(3).
3342
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Pincer Complexes
COMMUNICATION
0.032/0.053; S=1.03; 1min/max
ꢀ0.038(12).
CCDC-710332 (2), -710333 (3-[BF4]), and -710334 (5-[D-TRISPHAT])
contain the supplementary crystallographic data for this paper. These
data can be obtained free of charge from The Cambridge Crystallograph-
ic Data Centre via www.ccdc.cam.ac.uk/data_request/cif.
:
ꢀ0.44/0.62 eꢁꢀ3
;
Flack parameter[38]
=
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Acknowledgements
We are grateful to Utrecht University for financial support. We also
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Keywords: arenes · chiral resolution · palladium · pincer
complexes · planar chirality · ruthenium
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Received: January 16, 2009
Published online: February 19, 2009
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