Synthesis, structure, and catalytic studies of palladium and platinum bis-sulfoxide complexes

Article abstract of DOI:10.1021/om4000067

The bis-sulfoxide ligand p-tolyl-binaso and its derivatives can be used to synthesize neutral and cationic palladium and platinum complexes. Various Pt bis-sulfoxide complexes could be made by using the precursor Pt{(M,S _S,S_S)-p-toly

Full text of DOI:10.1021/om4000067

Article
pubs.acs.org/Organometallics
Synthesis, Structure, and Catalytic Studies of Palladium and Platinum
Bis-Sulfoxide Complexes
Emma E. Drinkel,† Linglin Wu, Anthony Linden, and Reto Dorta*,‡
Institute of Organic Chemistry, University of Zurich, Winterthurerstrasse 190, 8057 Zurich, Switzerland
S
* Supporting Information
ABSTRACT: The bis-sulfoxide ligand p-tolyl-binaso and its
derivatives can be used to synthesize neutral and cationic
palladium and platinum complexes. Various Pt bis-sulfoxide
complexes could be made by using the precursor Pt{(M,SS,SS)-p-
tolyl-binaso}Cl2 (5a) and abstracting the chlorides with silver
salts. The Pd analogue of this compound (4) could not be used
in the same way. In this case, the sulfoxides appear to be very
weakly coordinated and different methods to make stable Pd bis-
sulfoxide complexes had to be found. The nature of the bonding
in the synthesized compounds was studied through analysis of
their IR and NMR spectra and X-ray crystal structures. These
studies revealed that the trans effect of other ligands in the
complex is quite significant in determining the lability of the
metal−sulfoxide bond. Some of the more stable platinum structures were then tested for their activity as catalysts in
hydroboration and diboration reactions.
binding of these types of ligands and also to see how the
presence of other ligands in the complex affects the binding of
bis-sulfoxides to these metals.35
INTRODUCTION
■
The binding of sulfoxides with transition metals has been of
interest for many years,1−3 and numerous complexes with
sulfoxide ligands, particularly dimethyl sulfoxide (DMSO), are
known and have been well characterized. It was observed very
early on that sulfoxides were ambidentate in nature4 and,
depending on the metal and the steric environment, could bind
through the lone pairs at sulfur or oxygen. Another interesting
feature of sulfoxides is their chirality when dissymmetrically
substituted. This makes them good candidates for the design of
chiral ligands for enantioselective catalysis. The synthesis of
enantiopure sulfoxides has been well studied,5−14 and they
generally show high optical stability. In contrast, only a few
studies have demonstrated the use of chiral sulfoxide ligands in
enantioselective catalysis, with excellent results in some
cases.15−20 Successful examples where the only chelating
moieties are sulfoxides are still sporadic.21−28
The ligand used for most of the following studies was p-tolyl-
binaso (1), previously developed and based on the well-known
diphosphine ligand binap developed by Noyori (Chart 1).36
A
Chart 1. p-tolyl-binaso (1) and cyclohexyl-binaso (2) Used
in This Study
Complexes of bis-sulfoxide ligands with palladium and
platinum are known,22,29−34 but there are not many reports
of them being used successfully in catalysis. The only examples
reported are from the groups of Shibasaki22 and White,33 using
palladium bis-sulfoxides. Following the success of bis-sulfoxide
ligands developed in our laboratory in Rh catalysis,23−25 we
decided to study complexes of these ligands with palladium and
platinum, with the goal of using them in catalysis. Herein, these
complexes are presented along with some preliminary catalytic
results.
related ligand, cyclohexyl-binaso (2), was also used in the
studies with palladium and platinum reported here and was
synthesized as previously reported by us using DAG-(S)-
cyclohexylsulfinate 3 (DAG = 1,2:5,6-di-O-isopropylidene-α-D-
glucofuranosyl).27
For several of the complexes, it was possible to grow crystals
suitable for X-ray crystallographic analysis. From these analyses
we were able to more thoroughly understand the nature of the
Received: January 7, 2013
© XXXX American Chemical Society
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RESULTS AND DISCUSSION
■
Our studies into palladium and platinum complexes with bis-
sulfoxide ligands began with the discovery that we could make
the monomeric dichloride complexes with 1 from M-
(PhCN)2Cl2 (where M = Pt, Pd). The two benzonitrile ligands
were displaced by 1 equiv of bis-sulfoxide ligand (Scheme 1).
Scheme 1. Syntheses of PdCl2·binaso and Pt(binaso)Cl2
Figure 1. Molecular structure of Pt{(M,SS,SS)-p-tolyl-binaso}Cl2 (5a)
(50% probability ellipsoids; hydrogen atoms have been omitted).
Selected bond lengths (Å) and angles (deg): Pt1−S1, 2.2496(9); Pt1−
S2, 2.2527(9); Pt1−Cl1, 2.304(1); Pt1−Cl2, 2.315(1); S1−O1,
1.463(3); S2−O2, 1.467(3); Cl1−Pt1−Cl2, 88.72(4); S1−Pt1−S2,
98.46(3); Cl1−Pt1−S1, 88.62(4); Cl2−Pt1−S2, 85.48(3); S1−Pt1−
Cl2, 169.41(4); S2−Pt1−Cl1, 170.21(4).
and the nearby sulfoxide O atoms could be a contributing factor
(Cl···O = 3.063(3) and 3.153(3) Å; van der Waals radii sum
3.27 Å). The conformational constraints imposed by the seven-
membered metallacycle created by the cis-S,S-bidentate p-tolyl-
binaso ligand would prevent the SO groups rotating to a
more congenial orientation. The dihedral angle between the
mean planes through the two binaphthyl rings is 76.9(1)°, and
the preference for the rings in binapthyl systems to be nearly
perpendicular to one another might impose further conforma-
tional strain on the metallacycle. The large bite angle of
98.46(3)° for the bis-sulfoxide ligand may also be contributing
to the distortions, as it compresses the remaining angles around
the Pt atom and the smaller of the two S−Pt−Cl angles
corresponds with the longer of the two Cl···O distances.
It was found that palladium complexes with p-tolyl-binaso,
where the ligand was clearly bound, could be formed using
Pd(PhCN)2Cl2 in combination with silver salts AgX (Scheme
2). When 2 equiv of AgTFA (TFA = trifluoroacetate) was
added, Pd(p-tolyl-binaso)TFA2 (6) was formed. The synthesis
was possible with both diastereoisomers of the ligand. Crystals
of the complex 6·2Et2O, with the cis-S,S-bidentate (P,SS,SS)-p-
tolyl-binaso ligand, were grown, and an X-ray analysis was
performed (Figure 2). The C2-symmetric complex shows the
expected square-planar coordination geometry, with no atom
deviating from the plane defined by the four coordinated atoms
and the Pd atom by more than 0.008(4) Å and τ4 = 0.06.
Nonetheless, some small angular distortions within the square
plane are evident, again resulting from the large bite angle of
97.07(5)° for the bis-sulfoxide ligand necessitated by strain
within the seven-membered metallacycle. The dihedral angle
between the mean planes through the two binaphthyl rings is
70.5(2)°. The absence of significant tetrahedral distortion of
the square-planar geometry in this case could be due to weaker
steric interactions between the sulfoxide and triflate O atoms
(O···O = 2.870(5) Å) than between the Cl atom and the
sulfoxide O atom in 5a. Overall, the trifluoroacetate ligands
modulate the electronic environment at the metal in such a way
as to allow binaso to bind relatively tightly to palladium.
When AgBF4 was added to the reaction mixture instead of
AgTFA, an interesting transformation occurred. Initially the
reaction was carried out with 1 equiv of Pd(PhCN)2Cl2, 1 equiv
of (P,SS,SS)-p-tolyl-binaso, and 2 equiv of AgBF4. It was
observed that the quantity of white solid, at the time believed to
be AgCl, precipitating out of the reaction was greater than
The first results were achieved when the palladium complex
Pd(PhCN)2Cl2 (palladium(II) bis(benzonitrile)dichloride) was
treated with a solution of the ligand in CH2Cl2. The solution
changed color from yellow to red. However, when the solvent
1
was removed, the H NMR spectrum of the resulting red solid
was the same as that of the free ligand. Nevertheless, the
elemental analysis matched that predicted for Pd{(M,SS,SS)-p-
tolyl-binaso}Cl2 (4), and no benzonitrile was observed in the
1H NMR spectrum. We postulate that, in this case, the ligand
forms weak interactions with the palladium but is not properly
bound. This was also observed by White and co-workers, where
a bis-sulfoxide ligand appeared to form a complex with
1
Pd(OAc)2, but the H NMR of the “complex” and the free
ligand were the same.37
On the other hand, Pt{(M,SS,SS)-p-tolyl-binaso}Cl2 (5a) and
Pt{(M,SS,SS)-cyclohexyl-binaso}Cl2 (5b) were unequivocally
formed. The ligand was heated with the Pt precursor in toluene,
1
and a yellow solid precipitated out of solution. The H NMR
spectra of this solid showed that the signals of the bound ligand
were shifted in comparison to the free ligand. The complex is
very stable under N2 and can be kept for months without any
decomposition being observed. For further confirmation of its
structure, an X-ray crystal structure of 5a as its THF
monosolvate was obtained (Figure 1).
The crystal structure of 5a·THF revealed discrete molecules
with a cis-S,S-bidentate p-tolyl-binaso ligand and uniform Pt−Cl
and Pt−S distances. The Pt atom has a square-planar
coordination geometry with a small tetrahedral distortion.
The coordinating atoms deviate by up to 0.19 Å from the mean
plane defined by the four coordinated atoms and the Pt atom
and τ4 = 0.14. The τ4 parameter has been used as a measure of
the degree of distortion in square-planar and tetrahedral
geometries; τ4 = 1 for an ideal tetrahedron and τ4 = 0 for an
ideal square-planar geometry,38 although distortions of a
square-planar geometry in the direction of a square pyramid
will also yield values of τ4 > 0. The exact cause of the distortion
is unclear, although steric interactions between the Cl atoms
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Scheme 2. Synthesis of Palladium p-tolyl-binaso Complexes 6 and 7 via Abstraction of Chlorides
Figure 2. Molecular structure of Pd((P,SS,SS)-p-tolyl-binaso)TFA (6)
Figure 3. Structure of the cation of [Pd((P,SS,SS)-p-tol-binaso)2]-
[BF4]2 (7) (50% probability ellipsoids; hydrogen atoms have been
omitted). Selected bond lengths (Å) and angles (deg): Pd1−S1,
2.238(1); Pd1−S3, 2.2374(9); Pd1−O2, 2.086(3); Pd1−O4,
2.088(3); S1−O1, 1.457(3); S2−O2, 1.548(3); S3−O3, 1.451(3);
S4−O4, 1.551(3); S1−Pd1−S3, 89.63(3); O2−Pd1−O4, 90.1(1);
S1−Pd1−O2, 90.38(8); S3−Pd1−O4, 90.74(8); O2−Pd1−S3,
173.39(8); O4−Pd1−S1, 173.13(8).
(50% probability ellipsoids; hydrogen atoms have been omitted).
Selected bond lengths (Å) and angles (deg): Pd1−S3, 2.243(1); Pd1−
O1, 2.045(3); S3−O3, 1.460(3); S3−Pd1−S3′, 97.07(5); O1−Pd1−
O1′, 88.6(2); S3−Pd1−O1, 87.19(9); O1−Pd1−S3′, 175.72(9).
1
expected. The solid was filtered off, and an H NMR analysis
revealed it to be a mixture of AgCl and Pd(PhCN)4(BF4)2. The
remaining red solution was also dried and was speculated to be
Pd((P,SS,SS)-p-tolyl-binaso)2(BF4)2 (7). 1H NMR spectra
showed the ligand to be desymmetrized and supported this
theory. The complex is fairly stable; when it was stored in the
glovebox, no decomposition was observed after 1 year.
Confirmation of the structure came from X-ray crystallographic
analysis of crystals of the complex salt as its CH2Cl2
monosolvate, grown by layering a concentrated CH2Cl2
solution with pentane (Figure 3).
Each p-tolyl-binaso ligand in the complex cation of 7·CH2Cl2
coordinates in a cis-S,O-bidentate fashion to the Pd atom, and
the coordinating S atoms from the two ligands are also cis-
positioned. To the best of our knowledge, this is the first
example of a bis-sulfoxide ligand chelating simultaneously
through S and O with a late transition metal. The Pd−S bond
distances in 7 are not significantly different from those in 6.
The ambidentate binding modes of the p-tolyl-binaso ligands
leads to expected differences in the SO bond lengths, with an
average of 1.454(4) Å for the S-bound sulfoxide group and a
substantially longer 1.550(4) Å for the O-bound sulfoxide
group. For comparison, the SO bond length in the free
ligand is intermediate at 1.491(2) Å.39
The Pd atom has a slightly tetrahedrally distorted square
planar coordination geometry, τ4 = 0.10, with the coordinating
atoms deviating by up to 0.13 Å from the mean plane defined
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by the four coordinated atoms and the Pd atom. This is despite
the fact that p-tolyl-binaso forms an eight-membered metalla-
cycle in this complex, which gives the ligands more flexibility to
form the preferred 90° bite angles. The dihedral angles between
the mean planes through the two pairs of binaphthyl rings are
83.1(2) and 85.7(1)°. These angles are closer to 90° than has
been observed in all of the crystal structures reported here
involving S,S-coordinated binaso ligands, which may further
reflect the reduced strain inherent in the eight-membered
metallacycle.
(P,SS,SS)-p-tolyl-binaso could only be used to make neutral
complexes with palladium; no neutral platinum complexes were
made cleanly with this ligand. In addition, only the P,SS,SS
ligand could be used to make complex 7. When the reaction
was carried out using 1 equiv of Pd(PhCN)2Cl2, 1 equiv of
(P,SS,SS)-p-tolyl-binaso, and 1 equiv of AgBF4, a complicated
Figure 4. Molecular structure of Pt((M,SS,SS)-p-tol-binaso)(TFA)2 (8)
(50% probability ellipsoids; hydrogen atoms and the minor positions
of the atoms of the disordered triflate ligand (involving O3B) have
been omitted). Selected bond lengths (Å) and angles (deg): Pt−S1,
2.197(2); Pt1−S2, 2.239(1); Pt1−O3A, 2.063(8); Pt1−O3B,
2.063(9); Pt1−O5, 2.041(4); S1−O1, 1.465(5); S2−O2, 1.475(4);
S1−Pt1−S2, 99.03(7); O3A−Pt1−O5, 82.7(3); O3B−Pt1−O5,
83.2(6); S1−Pt1−O5, 91.1(2); S2−Pt1−O5, 169.6(2); S1−Pt1−
O3A, 163.9(3); S2−Pt1−O3A, 88.1(3); S1−Pt1−O3B, 168.7(5); S2−
Pt1−O3B, 86.5(6).
1
mixture of products was seen in the H NMR spectrum. We
had hoped that the chloro-bridged dimer, with one ligand
coordinated to each palladium, would form, but this does not
seem to be the case.
Returning to the investigation into the coordination of
binaso to platinum, we were able to use complex 5a as an entry
point to a variety of other complexes (Scheme 3). The
Scheme 3. Platinum Complexes Formed Using 5a as a
Precursor
which is disordered, yielding two molecular conformations.
One conformation, adopted by about 66% of the molecules, has
a significant tetrahedral distortion of the Pt coordination
environment with τ4 = 0.19 and the coordinating atoms
deviating by up to 0.21 Å from the mean plane defined by the
four coordinated atoms and the Pd atom. The remaining
molecules actually have a slightly square pyramidal coordina-
tion geometry with τ4 = 0.15 and the Pt atom 0.104(1) Å from
the plane defined by the four coordinating atoms. The bite
angle of the bis-sulfoxide is slightly larger than in the Pd
analogue (99.03(7)° for 8, compared with 97.07(5)° for 6) as a
natural consequence of the slightly smaller covalent radius of
Pt. The dihedral angle between the mean planes through the
two binaphthyl rings is 79.7(3)°.
When 5a was treated with only 1 equiv of AgTFA, only one
chloride was abstracted and replaced by a TFA ligand to form
complex 9, which could be stored for months at room
1
temperature under nitrogen. The H NMR spectrum of this
complex shows that the binaso ligand is desymmetrized by this
transformation, and we assume the structure to be that shown
in Scheme 3. Complex 9 could be reacted with another 1 equiv
of AgTFA to cleanly give 8.
Reacting 5a with NaI in an acetone solution substituted the
chloride ligands for iodides, giving an orange precipitate in less
than 10 min. The orange solid was redissolved in CH2Cl2 and
layered with THF to give crystals of 10·CH2Cl2 suitable for X-
ray analysis (Figure 5). Unlike complexes 8 and 9, 10
decomposed in a matter of weeks when stored at room
temperature under an inert atmosphere. Although only the
halogen atoms have been exchanged, the crystal structures of 5a
and 10 are not isostructural, possibly because the two structures
incorporate different solvent molecules. The molecules of 10
have crystallographic C2 symmetry. The Pt−I bond distance in
10 is 2.5877(5) Å; this is almost 0.3 Å longer than the Pt−Cl
bonds in 5a. However, the Pt−I distance is not unusually long;
in fact, it is marginally shorter than the distances found in a
similar coordination environment. The Cambridge Structural
Database40 contains details for 14 structures with a square-
dichloride complex 5a could be treated with AgTFA in CH2Cl2
to abstract both chlorides, resulting in complex 8, which was
very stable at room temperature inside the glovebox. Crystals
suitable for X-ray analysis were grown by diffusion of THF into
a CH2Cl2 solution of the complex (Figure 4). The crystal
structure of 8 shows complex molecules with a cis-S,S-bidentate
(M,SS,SS)-p-tolyl-binaso ligand and two triflate ligands, one of
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oxides 1 and 2 were formed cleanly as shown in Scheme 4.
Crystals of the racemic dimethyl complex 11b as its CH2Cl2
solvate were grown, and an X-ray analysis was performed
(Figure 6). The crystal structure revealed Pt−S bond lengths of
Figure 5. Molecular structure of Pt((M,SS,SS)-p-tol-binaso)I2 (10)
(50% probability ellipsoids; hydrogen atoms have been omitted).
Selected bond lengths (Å) and angles (deg): Pt1−S1, 2.261(1); Pt1−
I1, 2.5877(5); S1−O1, 1.466(4); S1−Pt1−S1′, 97.78(7); I1−Pt1−I1′,
88.75(3); S1−Pt1−I1, 89.09(4); S1−Pt1−I1′, 162.91(3).
planar PtI2S2 core, and the range of Pt−I bond lengths for those
complexes is 2.60−2.63 Å. The Pt atom has a significantly
tetrahedrally distorted square planar coordination geometry, τ4
= 0.24, with the coordinating atoms deviating by up to 0.36 Å
from the mean plane defined by the four coordinated atoms
and the Pt atom. The distortions appear to be clearly a
consequence of steric interactions between the I atoms and the
nearby sulfoxide O atoms. The I···O distance of 3.353(5) Å is
shorter than the van der Waals radii sum of 3.50 Å by an
amount similar to that observed for the Cl···O separation in 5a,
but were the coordination geometry in 10 strictly square planar,
the I···O distance would be even shorter. The dihedral angle
between the mean planes through the two binaphthyl rings is
70.5(3)°, and the bite angle of the S,S-bidentate ligand is
97.78(7)°. Both of these values, particularly the dihedral angle,
are slightly smaller than the corresponding angles in 5a and 8
and may be related to the Pt−S bond in 10 being slightly longer
than in the analogous chloride and TFA complexes (Table 1)
as a consequence of the trans effect, which will be discussed
below.
Figure 6. Molecular structure of Pt(cyclohexyl-binaso)(Me)2 (11b)
(50% probability ellipsoids; hydrogen atoms have been omitted).
Selected bond lengths (Å) and angles (deg): Pt1−S1, 2.3075(6); Pt1−
S2, 2.3107(6); Pt1−C33, 2.071(3); Pt1−C34, 2.067(2); S1−O1,
1.4793(17); S2−O2, 1.4809(17); S1−Pt1−S2, 97.41(2); C33−Pt1−
C34, 84.39(11); S1−Pt1−C33, 88.77(8); S2−Pt1−C34, 89.53(8);
C33−Pt1−S2, 173.63(8); C34−Pt1−S1, 172.47(8).
2.3075(6) and 2.3107(6) Å. These are the longest Pt−S bonds
observed for any of the complexes described herein but are
consistent with three other structures containing the PtS2Me2
core listed in the Cambridge Structural Database (range 2.305−
3.363 Å). Complex 11b was also the least stable and had to be
stored in the refrigerator, inside the glovebox, to avoid
decomposition. Similar to the case for 6, the tetrahedral
distortion of the square-planar coordination geometry is small.
The coordinating atoms deviate by only up to 0.05 Å from the
mean plane defined by the four coordinated atoms and the Pt
atom, while τ4 = 0.09. Nonetheless, there are significant angular
distortions within the square plane induced by the bite angle of
the S,S-bidentate ligand of 97.41(2)°. The dihedral angle
between the mean planes through the two binaphthyl rings is
72.47(7)°. The smaller degree of tetrahedral distortion of the
coordination geometry correlates with a weaker steric
interaction between the sulfoxide O atoms and the methyl C
atoms than in complexes 5a and 10. The O···C distances are
3.096(3) and 3.164(3) Å, which are only 0.12 and 0.06 Å,
respectively, shorter than the sum of the van der Waals radii of
these atoms.
Table 1. Comparison of Pt−S Bond Lengths and IR
Frequencies of SO Bonds
compd
trans ligand av Pt−S bond distance (Å) ν(SO) (cm−1
)
free binaso
N/A
−TFA
−Cl
N/A
1055.84
1177.33
1136.83
1129.12
1093.44
8
2.218(2)
2.251(1)
2.261(1)
2.3091(8)
5a
10
11b
−I
−CH3
It has long been known that there are two phenomena at play
in square-planar Pt complexes, where a ligand X, trans to
another ligand A, affects the stability of the Pt−A bond (trans
Starting from a different Pt precursor, namely [Pt(μ-
SMe2)Me2], the dimethyl complexes incorporating bis-sulf-
Scheme 4. Synthesis of Complexes 11a,b
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Scheme 5. Cationic Platinum Complexes Made from 5a
influence) or the ease of substitution of A (kinetic trans
effect).41,42 It was verified experimentally that the labilizing
effect (kinetic trans effect) decreases in the order
This is in agreement with what is stated in the literature for
square-planar Pt complexes41,42 and our own observations as to
the stability of these complexes. The trans influence and kinetic
trans effect are important to bear in mind when designing new
complexes to synthesize or when predicting the behavior of
these complexes in catalysis. Ligands with a strong trans
influence in the position trans to the sulfoxides might labilize
the bis-sulfoxide during catalysis. If this were the case, the chiral
ligand might not be chelating to the metal throughout the
catalytic cycle.
X = C2H4 ≈ CO > CN− > R3P ≈ H− > CH−3 > C6H−5
> I− > Br− > Cl− > NH3 > OH− > H2O
Parallels can be drawn between the trends observed for the
Pt complexes and those observed with the Pd complexes.
Above we have seen that Cl− appears to exert a stronger trans
influence on the bis-sulfoxide ligand than TFA in the neutral Pt
complexes discussed. Similarly, we saw earlier that in the Pd
bis-sulfoxide dichloride complex 4, the binaso ligand seemed to
coordinate weakly, whereas the Pd bis-sulfoxide TFA complex 6
was a stable, isolable compound.
It is believed that σ and/or π interactions are important for
ligands displaying a strong kinetic trans effect. Good σ bonding
between the p orbital of the ligand X and the 6p orbitals of Pt
stabilizes the transition state in the substitution of A for another
ligand.41 On the other hand, empty π* orbitals on the ligand X
may assist the substitution reaction by accommodating electron
density from the entering ligand built up on the metal center.
Studies of complexes of the general formula trans-PtCl2(L)-
(NH3), where L was various σ-bonding ligands, led to the
conclusion that the trans influence closely mirrors the kinetic
trans effect in most cases.41 Upon inspection of the Pt−S bond
lengths (from the X-ray crystal structures) and the SO bond
IR frequencies of the neutral Pt complexes described herein
(Table 1), we observed a trans influence in the order43
In an extension of the complexation chemistry of these bis-
sulfoxides with platinum, we synthesized various cationic Pt
complexes employing 5a as the starting material (Scheme 5).
When 5a was treated with 2 equiv of AgBF4 in a solvent
mixture of CH2Cl2 and CH3CN, the chlorides were abstracted
and replaced by two acetonitrile molecules to form complex 12.
1
The coordinated CH3CN was clearly seen in the H and 13C
CH−3 > I− > Cl− > TFA
NMR spectra.
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Scheme 6. Synthesis of [Pt(binaso)(COD)][BF4]2 Complexes 15a−c
Table 2. Screening of Pt Complexes in Hydroboration Reactions
a
bc
,
entry
catalyst
borane
temp (°C)
yield (%)
ratio
4:1
1
16
16
16
16
16
16
16
16
17
17
17
17
17
17
17
−20
−20
−20
−25
−25
23
0
2
5a
trace
0
d
3
5a
4
15a
15c
15a
15c
15b
15a
15c
15c
55
38
trace
trace
0
5
2.6:1
6
7
23
8
0
9
23
90
94
79
39
47
82
1.2:1
1.1:1
1:1
10
11
12
13
14
15
23
−5
23
e
15c
0.9:1
1.3:1
0.8:1
1.1:1
f
15c
23
g
15c
23
h
15c
23
60
a
b
c
Isolated yield after appropriate workup and purification by column chromatography. 18:19, determined by GC. Products containing stereocenters
d
were racemic mixtures according to HPLC on a chiral stationary phase. Pt complex was reacted with 10 mol % of AgOTf for 30 min and filtered
prior to catalytic reaction. Reaction carried out in THF. Reaction carried out in ether. Reaction carried out in acetone. Reaction carried out in
e
f
g
h
toluene.
This complex was stable inside the glovebox and could be
stored at room temperature. Similarly, complex 13 was formed
by reacting 5a with 2 equiv of AgBF4 and an extra 1 equiv of p-
tolyl-binaso in a CH2Cl2 solution. We were not able to obtain a
crystal of this complex to confirm the structure unambiguously,
but spectroscopic evidence points to a structure analogous to
that of the Pd complex 7 (Scheme 2). The 1H NMR spectrum
of 13 shows the binaso ligand to be desymmetrized, which
suggests formation of the proposed structure.
nate) to give the monocationic complex 14. This complex was
1
stable at room temperature under nitrogen, and H and 13C
NMR spectra revealed that both acac and binaso ligands were
coordinated.
Finally, another group of cationic platinum complexes were
made from the precursor Pt(COD)Cl2 (COD = cyclo-
octadiene). It was found that, upon treating the precursor
with 1 equiv of ligand and 2 equiv of AgBF4, the chlorides were
abstracted and the ligand was coordinated to Pt to produce the
1
When 5a was treated with only 1 equiv of AgBF4, unlike in
dicationic complexes 15a−c (Scheme 6). H NMR spectra
the Pd case, a chloro-bridged complex was formed. No crystals
confirmed that both the bis-sulfoxide ligand and cyclooctadiene
were coordinated in each case.
1
could be grown to confirm the structure, but an in situ H
NMR spectrum after the first step of the reaction to form 14
showed the ligand to be still symmetric. Furthermore, there is a
precedence for this type of chloro-bridged Pt dimers in the
literature with phosphine ligands.44,45 This intermediate then
reacted cleanly with 1 equiv of Ag(acac) (acac = acetylaceto-
Hydroboration. To evaluate some of our platinum bis-
sulfoxide complexes as possible catalysts in asymmetric
transformations, we decided to initially focus on hydroboration
and diboration reactions. Hydroboration reactions can be
carried out noncatalytically, but the major product observed is
G
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Table 3. Screening of Pt Catalysts in the Diboration Reaction
ab
,
entry
1
catalyst
yield (%)
5a
18
39
65
74
45
80
0
c
2
5a
3
4
5
6
7
8
8
14
13
11a
15a
15c
0
a
b
Isolated yield after quenching with NaOH/H2O2 and appropriate workup and purification by column chromatography. Products containing
c
stereocenters were racemic mixtures according to HPLC on a chiral stationary phase. 5a was first reacted with AgOTf in THF and then filtered and
used in the catalytic reaction.
the anti-Markovnikov addition product;46 catalysts can be used
to invert this selectivity. The catalytic version of this reaction
has been most well-developed with rhodium catalysts.47−60
There is very little in the current literature regarding the use of
platinum in hydroboration reactions. There is only one example
of catecholborane being used in the hydroboration of a terminal
alkene with a platinum catalyst.61 Other examples include the
hydroboration of terminal alkenes with polyboranes62−64 and
the hydroboration of allenes with pinacolborane.65
As can be seen from entry 1 of Table 2, when no catalyst is
present there is no reaction. For this type of reaction the
noncatalytic version usually requires heating to high temper-
atures to obtain any product.47 5a was the first platinum
complex tested in the hydroboration of styrene with catechol
borane (18), but only a trace of product was observed.
Abstracting the chlorides in the complex with silver salt first
(entry 3 of Table 2) failed to give the desired product. The
cationic Pt complexes 15a−c were then tested in the reaction;
15a,c gave moderate yields at −25 °C.
Interestingly, when the temperature was raised to room
temperature, only traces of product were obtained. 15a gave the
best selectivity observed in this study, with a 4:1 ratio of
Markovnikov to anti-Markovnikov product being observed at
−25 °C (entry 4). 15b did not give any product (entry 8).
When the borane was changed to pinacolborane, the yields
were dramatically improved and the reaction could be run at
room temperature (entries 9 and 10), although the
regioselectivity was decreased. Running the reaction at lower
temperatures did not improve the selectivity with pinacolbor-
ane, and the yield was lower (entry 11). The reaction was
screened with different solvents (entries 12−15), and it was
found that the solvent affects both the yield and the selectivity
of the reaction. THF and acetone gave higher selectivity for the
anti-Markovnikov product.
diboration of styrene by B2(pin)2 (Table 3).66,67 Diborations
have been well developed with transition-metal catalysts.68−70
They are useful transformations for the introduction of new
functional groups into a molecule. Obviously, if the reaction can
be carried out enantioselectively, it is an efficient way to
introduce chiral centers.
Most examples of diboration with platinum catalysts have
been carried out with Pt(0),71−76 although there are some
examples using Pt(II).77 This reaction was more convenient
than the hydroboration reaction, as it could be run at room
temperature with good results. Interestingly, 15a,c were not
successful as catalysts in this reaction. This might indicate that
the diboration goes through a different mechanism than the
hydroboration reaction. Baker and co-workers showed that
Pt(COD)Cl2 was a good catalyst for diboration and proposed
that the first step of the mechanism was reduction of Pt(II) to
Pt(0) by the diborane, as they observed R2B−Cl by 11B
NMR.77 This could explain why 15a,c do not catalyze the
reaction. Both of these complexes do not have any ligand that
can be removed by transmetalation to the boron.
As can be seen in Table 3, relatively good yields can be
obtained with some of the platinum complexes in this reaction,
particularly 14 and 11a. We were hoping that the product
would show some enantiomeric excess. Unfortunately, when it
was analyzed by HPLC using a chiral stationary phase (chiralcel
OD-H), the product turned out to be racemic.
Summary. We have developed a family of new palladium
and platinum complexes with the bis-sulfoxide ligands p-tolyl-
binaso and cyclohexyl-binaso. These include a novel example of
a bis-sulfoxide ligand binding simultaneously through S and O.
Through inspection of the X-ray crystal structures and IR
spectra of the neutral, square-planar Pt complexes, we were able
to establish how the trans influence of the other ligands affects
the Pt−S binding strength of our bis-sulfoxides. This is
important to bear in mind when designing Pt-binaso complexes
to be used as catalysts. When the other ligands in the complex
have a trans influence on the Pt−S bond, the chiral bis-sulfoxide
ligand may be made labile and not be fully chelating throughout
the catalytic cycle. This could explain the lack of enantiose-
lectivity we observed in the catalytic reactions tested. On
extrapolation of the trends seen for Pt to Pd, it appears likely
that binaso and other chelating bis-sulfoxide ligands will have to
For each of the reactions yielding enough isolable product,
the mixtures of primary and secondary alcohols were analyzed
by GC to give the ratio of these two alcohols. The ee value of
the secondary alcohol was determined using a GC column with
a chiral stationary phase (Lipodex E). In each case the
secondary alcohol was found to be racemic.
Diboration. Inspired by recent publications on Pt-catalyzed
diborations, we tested some of our Pt complexes in the
H
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Organometallics
Article
(7) Han, Z. X.; Krishnamurthy, D.; Grover, P.; Fang, Q. K.; Su, X. P.;
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more tightly to the palladium center.
It was also observed from the X-ray crystal structures that
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O atoms and the other coordinated ligands lead to various
degrees of distortion, usually toward tetrahedral, of the square-
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moiety plays an important role in determining the structures of
the sulfoxide−metal complexes is not surprising, as we have
seen similar results in rhodium complexes with bis-sulfoxide
ligands.28
Selected Pt complexes were tested in the hydroboration and
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Markovnikov product vs the anti-Markovnikov product with
catecholborane. Better yields were obtained with pinacolbor-
ane, but the selectivities were lower. In the diborations, 11a was
found to be the best catalyst, with 15a,c giving no product at all.
Work is ongoing to find new catalytic applications for these
complexes and to try to reach a more thorough understanding
of the nature of the binding of these ligands in respective
catalytic cycles with palladium and platinum catalysts.
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ASSOCIATED CONTENT
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(20) Han, F. Z.; Chen, J.; Zhang, X. Y.; Liu, J. B.; Cun, L. F.; Zhu, J.;
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(23) (a) Mariz, R.; Luan, X. J.; Gatti, M.; Linden, A.; Dorta, R. J. Am.
Chem. Soc. 2008, 130, 2172. For the original synthesis of one of the
diastereomeric pairs of binaso, see: (b) Clayden, J.; Kubinski, P. M.;
Sammiceli, F.; Helliwell, M.; Diorazio, L. Tetrahedron 2004, 60, 4387.
(24) Burgi, J. J.; Mariz, R.; Gatti, M.; Drinkel, E.; Luan, X. J.;
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S
* Supporting Information
Text and figures giving detailed descriptions of experimental
procedures and spectroscopic data of the compounds described
and CIF files giving crystal structure data for complexes 5a, 6,−
8, 10, and 11b. This material is available free of charge via the
Internet at http://pubs.acs.org.
AUTHOR INFORMATION
■
Corresponding Author
*R.D.: tel, +61 8 6488 3161; fax, +61 8 6488 7330; e-mail, reto.
dorta@uwa.edu.au.
(25) Mariz, R.; Burgi, J. J.; Gatti, M.; Drinkel, E.; Luan, X. J.; Dorta,
R. Chimia 2009, 63, 508.
Present Addresses
(26) Chen, J.; Chen, J.; Lang, F.; Zhang, X.; Cun, L.; Zhu, J.; Deng, J.;
Liao, J. J. Am. Chem. Soc. 2010, 132, 9219.
(27) Mariz, R.; Poater, A.; Gatti, M.; Drinkel, E.; Burgi, J. J.; Luan, X.
J.; Blumentritt, S.; Linden, A.; Cavallo, L.; Dorta, R. Chem. Eur. J.
2010, 16, 14335.
(28) Poater, A.; Ragone, F.; Mariz, R.; Dorta, R.; Cavallo, L. Chem.
Eur. J. 2010, 16, 14348.
(29) Cattalini, L.; Michelon, G.; Marangoni, G.; Pelizzi, G. J. Chem.
Soc., Dalton Trans. 1979, 96.
†Department of Chemistry, CFM, Federal University of Santa
Catarina, Campus Universitario Trindade-C.P. 476, 88040-900,
SC-Florianopolis, Brazil.
‡School of Chemistry and Biochemistry, University of Western
Australia, 35 Stirling Highway, 6009 Crawley, Australia.
Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS
■
We thank the Trace Element and Microanalysis laboratory at
ETH Zurich, Switzerland, for elemental analyses of compounds.
We acknowledge financial support of this work by the Alfred
Werner Foundation (R.D.), the SNF (L.W.), the Roche
Research Foundation (E.E.D.), the University of Zurich
(E.E.D.), and the University of Western Australia (R.D.).
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