Rigid bis-zinc(ii) salphen building blocks for the formation of template-assisted bidentate ligands and their application in catalysis

Article abstract of DOI:10.1039/b702375h

The template-induced formation of chelating bidentate ligands by the selective self-assembly of two monodentate pyridyl phosphorus ligands on a rigid bis-zinc(ii) salphen template with two identical binding sites was studied. Using UV-vis, NMR-spectroscopy and X-ray analysis the formed structures were unambiguously proven. The application of these templated bidentate ligands in transition metal catalysis showed, in most cases, typical bidentate character. Compared to previous work based on a more flexible bis-zinc(ii) porphyrin template, the current catalytic data suggest that the rigidity of the template is not an important factor for the improvement of the regio- and enantioselectivity under the applied reaction conditions. The Royal Society of Chemistry.

Full text of DOI:10.1039/b702375h

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Rigid bis-zinc(II) salphen building blocks for the formation of
template-assisted bidentate ligands and their application in catalysis†
Mark Kuil,a P. Elsbeth Goudriaan,a Arjan W. Kleij,a Duncan M. Tooke,b Anthony L. Spek,b
Piet W. N. M. van Leeuwena and Joost N. H. Reek*a
Received 15th February 2007, Accepted 30th March 2007
First published as an Advance Article on the web 26th April 2007
DOI: 10.1039/b702375h
The template-induced formation of chelating bidentate ligands by the selective self-assembly of two
monodentate pyridyl phosphorus ligands on a rigid bis-zinc(II) salphen template with two identical
binding sites was studied. Using UV-vis, NMR-spectroscopy and X-ray analysis the formed structures
were unambiguously proven. The application of these templated bidentate ligands in transition metal
catalysis showed, in most cases, typical bidentate character. Compared to previous work based on a
more flexible bis-zinc(II) porphyrin template, the current catalytic data suggest that the rigidity of the
template is not an important factor for the improvement of the regio- and enantioselectivity under the
applied reaction conditions.
The most powerful tool in homogeneous transition metal catalysis
is ligand design and optimization. Various ligand parameters
including steric properties, electronic properties, the bite angle
and chirality, have shown to be important.1,2 Systematic variation
of these properties is the general strategy for catalyst discovery
and optimization. Traditionally, monodentate3–6 and bidentate
ligands7,8 have been studied as important classes of ligands. For
several reactions (hetero)bidentate ligands are superior compared
to monodentate ligands,2,9 but synthetic routes towards these
ligands are generally more tedious.
tical binding sites (Fig. 1).20 A more rigid template (i.e. the rigid
bis-zinc(II) salphen versus the more flexible bis-zinc(II) porphyrin
template used in previous studies) was anticipated to give rise to
more selective catalyst systems. For the construction of templated
self-assembled bidentate ligands we used two identical monomeric
pyridyl phosphorus ligands. Transition metal complexes based
on these templated bidentate ligands were explored in various
catalytic transformations and they outperformed in most cases
their non-templated analogues.
An important new strategy comprises a supramolecular ap-
proach to form (hetero)bidentate ligands.10–19 In this approach
two monodentate ligands are brought together by a self-assembly
process using non-covalent interactions such as hydrogen bonds,
ionic or dynamic metal–ligand interactions. This class of ligands
combines the advantages of synthetic accessibility and the selective
formation of (hetero)bidentate ligands. Two monodentate ligands
can be assembled by using ligands with complementary binding
motifs,11–14 or alternatively, a template can be used that contains
binding sites for the assembly of two monodentate ligands.15 For
example, we reported the use of a flexible bis-zinc(II) porphyrin
template for the assembly of identical monodentate ligands,
which led to chelating bidentate ligands that showed increased
(enantio)selectivities in several reactions.15a
Fig. 1 Schematic representation of the formation of templated bidentate
ligands by self-assembly of two monomeric pyridyl phosphorus ligands on
a rigid bis-zinc(II) salphen template.
Results and discussion
We studied the formation of templated bidentate ligand assemblies
using monomeric pyridyl phosphorus ligands a–h in combination
with a bis-zinc(II) salphen template 1 (Scheme 1). The bis-zinc(II)
salphen building block was synthesized in a straightforward
two-step procedure.21 Pyridyl phosphorus ligands c–h were all
synthesized according to standard procedures (see Experimental).
Recently, we reported that pyridine units of pyridyl phosphorus
compounds coordinate selectively to bis-zinc(II) porphyrins15a and
zinc(II) salphen complexes.20–23 We found that the phosphorus
donor atom is still available for coordination to a transition metal
after coordination of the nitrogen donor atom to the zinc(II)
metal center. In this contribution we focus on the combination
of monomeric pyridyl phosphorus ligands and bis-zinc(II) salphen
template 1. UV-Vis titration experiments in toluene24 confirmed
the binding of two meta-pyridyldiphenylphosphine ligands a to the
bis-zinc(II) salphen building block 1, with high binding constants
Herein, we report the template-induced formation of chelating
bidentate ligands by the selective self-assembly of two monoden-
tate ligands on a rigid bis-zinc(II) salphen template with two iden-
aVan’t Hoff Institute for Molecular Sciences, University of Amsterdam,
Nieuwe Achtergracht 166, 1018 WV Amsterdam, The Netherlands. E-mail:
reek@science.uva.nl; Fax: +31 20 5255604; Tel: +31 20 5256437
bBijvoet Center for Biomolecular Research, Faculty of Science, Crystal and
Structural Chemistry, Utrecht University, Padualaan 8, 3584 CH Utrecht,
The Netherlands
† Electronic supplementary information (ESI) available: Crystallographic
Details. See DOI: 10.1039/b702375h
‡ Current address: Institute of Chemical Research of Catalonia (ICIQ),
Av. Paisos Catalans 16, 43007, Tarragona, Spain. Fax: +34 97 7920224;
Tel: +34 97 7920247
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Scheme 1 Zinc(II) salphen building blocks 1–3 and monomeric pyridyl phosphorus ligands a–h.
of K1 = 2.4 × 104 M−1 and K2 = 1.5 × 104 M−1 respectively
(Fig. 2).25 It is interesting to note that the binding constant is a
factor of 10 higher than that of a to zinc(II) porphyrins,11,15,22 but
lower than that of a to mono-zinc(II) salphens 2 and 3.21,23 This is
likely a consequence of electronic factors as shown previously.21,23
Fig. 2 Schematic representation of the selective formation of a templated
bidentate by self-assembly of two monomeric pyridyl phosphorus ligands
on a bis-zinc(II) salphen template.
Fig. 3 Molecular structure of two monomeric meta-pyridyldiphenyl-
phosphine ligands a assembled on a bis-zinc(II) salphen template 1 in the
solid state. The complex shown represents one of two crystallographically
independent structures in the asymmetric unit. Both are located on
a crystallographic inversion center. Hydrogen atoms, minor disordered
component and co-crystallized solvent molecules have been omitted for
clarity (grey = C, blue = N, red = O, yellow = P, green = Zn).
The coordination of two meta-pyridyldiphenylphosphine lig-
ands a to the bis-zinc(II) salphen template 1 was proven by X-ray
crystallography. Crystals were grown from a dichloromethane–
acetonitrile solvent mixture (Fig. 3, see Table 1 for selected bond
lengths and bond angles). The 1·(a)2 assembly consists of two
crystallographic independent structures in the solid state both
located on a crystallographic inversion centre. The two axial
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Table 1 Selected bond lengths and angles from the crystal structure of
1·(a)2.a Values for both independent complexes are given
of templated bidentate transition metal complexes and in this case
all assemblies gave trans-palladium complexes (see Experimental).
The formation of a templated chelating bidentate ligand in
the presence of a transition metal precursor was unambigu-
ously proven by X-ray crystallography. Crystals of Pt(1·(a)2)Cl2
were grown from a dichloromethane–acetonitrile solvent mixture
(Fig. 4). The molecular structure clearly shows the formation of
a chelating bidentate ligand; the meta-pyridyldiphenylphosphine
ligands a are coordinated via the nitrogen donor atoms to the axial
positions of the bis-zinc(II) salphen template and the phosphorus
donor atoms are coordinated to platinum(II) in a cis-fashion, i.e.
the minor component of the solution studies crystallized.27
˚
Bond lengths/A
Zn1–O1
Zn1–O2
Zn1–N1
Zn1–N2
Zn1–N3
1.962(3)
1.964(3)
2.107(4)
2.070(3)
2.102(3)
1.951(3)
1.964(3)
2.089(4)
2.086(3)
2.072(3)
Bond angles/◦
O1–Zn1–O2
O1–Zn1–N1
O1–Zn1–N2
O1–Zn1–N3
O2–Zn1–N1
O2–Zn1–N2
O2–Zn1–N3
N1–Zn1–N2
N1–Zn1–N3
N2–Zn1–N3
94.53(11)
102.02(13)
90.04(12)
157.76(13)
96.30(13)
153.40(12)
87.49(11)
108.36(13)
99.76(13)
78.83(13)
93.38(12)
105.46(13)
88.00(12)
151.64(13)
98.12(13)
158.07(13)
89.15(12)
102.60(13)
102.11(13)
79.62(12)
a The su’s are given in parentheses.
phosphine ligands coordinate at opposite sites of the bis-zinc(II)
salphen molecule. Unfortunately, it is not possible to determine
whether this coordination mode is also present in solution, or if
crystal packing effects account for it. We assume that in solution it
will be close to a statistical 1 : 1 mixture of the CS and C2 symmetric
assemblies as there is neither steric nor electronic interactions
between the two phosphines.26
Fig. 4 Molecular structure of [Pt(1·(a)2)Cl2] in the solid state. Minor
disordered components and co-crystallized solvent molecules have been
omitted for clarity (grey = C, white = H, orange = Cl, blue = N, red = O,
purple = P, green = Pt, yellow = Zn).
We were curious to discover if a templated chelating bidentate
ligand (the CS form) would form in the presence of a transition
metal precursor. Upon addition of one equivalent of the bis-
zinc(II) salphen template 1 to a CD2Cl2 solution of cis-[Pt(b)2Cl2],
a templated bidentate cis-platinum complex was formed as was
evidenced by 31P NMR spectroscopy (d = 17.0 ppm, 1JPt–P = 3628
Hz). Alternatively, bis-zinc(II) salphen 1 (11.8 mg, 0.0104 mmol)
was added to a toluene-d8 solution of cis-[Pt(b)2Cl2], after which
the solution was stirred for 13 h at 80 ◦C to allow formation of the
Crystals of Pd(1·(b)2)MeCl grown from a dichloromethane–
acetonitrile solvent mixture were also analyzed by X-ray crys-
tallography (Fig. 5). A preliminary solid state structure clearly
shows the formation of a chelating bidentate ligand.28 In contrast
to the above structure, the phosphorus donor atoms are coordi-
nated to palladium(II) in trans-disposition and as expected the
para-pyridyldiphenylphosphine ligands b are coordinated via the
thermodynamic most stable metal complex. Interestingly, the 31
P
NMR spectrum showed the presence of two platinum complexes
in solution: the cis-complex (d = 16.5 ppm, 1JPt–P = 3640 Hz) and
1
a trans-complex (d = 21.5 ppm, JPt–P = 2706 Hz). The ratio of
these complexes based on NMR integration was approximately 15
(cis) to 85 (trans) in solution.
31P NMR spectroscopy studies on in situ formed [Pt(1·(a)2)Cl2]
(conditions: 13 h at 80 ◦C in toluene-d8) also showed the
presence of two different templated bidentate platinum complexes.
However, the ratio between these two complexes was different:
1
25% of the cis-complex (d = 11.5 ppm, JPt–P = 3712 Hz)
1
and 75% of the trans-complex (d = 15.5 ppm, JPt–P = 2683
Hz) in solution. Interestingly, we observed a higher preference
for the formation of a templated trans-platinum complex for
the para-pyridyldiphenylphosphine ligand b compared to meta-
pyridyldiphenylphosphine ligand a. This shows that the position
of the nitrogen donor atom in the monomeric pyridyl phosphorus
ligand influences the geometry of the metal complex, although the
effect is rather small. 31P NMR spectroscopy with corresponding
palladium complexes (e.g. palladium(II) dichloride and methyl pal-
ladium(II) chloride complexes) also demonstrated the formation
Fig. 5 Preliminary crystal structure of assembly [Pd(1·(b)2)MeX] in the
solid state. The nature of X (not shown) connected in a disordered arrange-
ment to Pd is unclear along with the included solvent of crystallization
(grey = C, white = H, blue = N, red = O, orange = P, green = Pd,
yellow = Zn).
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Table 2 Hydroformylation of 1-octene catalyzed by non-templated and
nitrogen donor atoms to the axial positions of the bis-zinc(II)
salphen template.
templated rhodium complexesa
Entry Ligand
Conv (%)b l : bc Isomers (%)d Linear (%)e
1f
2f
(a)2
97
80
98
97
17
25
18
11
13
48
2.8
3.1
2.9
3.2
13.0 12
15.5 12
18.8 13
22.7 13
3.2
2.4
9.2
1.5
1.3
1.4
2
2
2
2
72
74
73
74
81
82
83
84
74
69
88
59
57
58
Application in transition metal catalysis
1·(a + a)
3f
(b)2
Hydroformylation of 1-octene. After having confirmed the
formation of transition metal complexes based on templated
bidentate ligands, we explored these complexes in various catalytic
reactions: (asymmetric) hydroformylation and asymmetric hydro-
genation. First, the self-assembled bidentate ligand assemblies
based on a rigid bis-zinc(II) salphen template were studied in the
rhodium-catalyzed hydroformylation of 1-octene under 20 bar of
syngas (CO : H2 = 1 : 1) at 80 ◦C (Scheme 2). The monomeric
meta-pyridyldiphenylphosphine ligand a gave a rhodium catalyst
that showed a high conversion and a moderate selectivity for the
linear aldehyde, which is typical of monodentate ligands in the
hydroformylation of 1-octene (Table 2, entry 1).9 The presence of
a rigid bis-zinc(II) salphen template afforded a templated chelating
bidentate rhodium catalyst that gave a small drop in conversion
along with a slight increase in regioselectivity (Table 2, entry 2).
Rhodium complexes based on ligand b afforded less pronounced
results. The non-templated rhodium catalyst gave high conversion
together with a moderate regioselectivity (Table 2, entry 3). The
templated rhodium complex afforded a similar conversion accom-
panied with a small increase in the selectivity for the formation
of the linear aldehyde (Table 2, entry 4). The typical catalytic be-
haviour for bidentate phosphine ligands in the rhodium-catalyzed
hydroformylation of 1-octene (e.g. lower reaction rate and higher
selectivity for the linear aldehyde in the case of large bite angle
ligands) was not observed for the templated chelating bidentate
ligands under the applied conditions. Previously, similar results
have been reported using self-assembled bidentate ligands based
on a flexible bis-zinc(II) porphyrin template.15 A comparison of the
catalytic results suggests that the rigidity of the template is not an
important factor for the improvement of the regioselectivity under
the applied reaction conditions. Apparently, these self-assembled
bidentate ligands using dynamic metal–ligand interactions are too
flexible under the applied conditions. This issue can be addressed
by using multiple component assemblies that fix the ligands more
rigidly in space, as we have demonstrated previously.15b
4f
1·(b + b)
(c)2
5g
6g
(2·c)2
(3·c)2
1·(c + c)
(f)2
7g
8g
9h
3.0
10h
11h
12h
13h
14h
(2·f)2
2.3
2.9
0.4
0.3
0.2
1·(f + f)
(g)2
3.9
>99
>99
>99
(2·g)2
1·(g + g)
a Reaction conditions: [Rh] = 1.00 mmol l−1, [pyridylphosphorus ligand] =
10.0 mmol l−1, [mono-zinc(II) salphen] = 10.0 mmol l−1 or [bis-zinc(II)
salphen] = 5.00 mmol l−1, Pressure = 20 bar (CO : H2 = 1 : 1).
b Percentage conversion of 1-octene to aldehydes and isomers. c l : b =
linear : branched. d Percentage isomerization to 2-, 3- and 4-octene based
on converted 1-octene. e Percentage linear aldehyde based on converted 1-
octene. f Rhodium : 1-octene = 1 : 640, reaction temperature = 80 ◦C, the
reaction was stopped after 1 h. g Rhodium : 1-octene = 1 : 1000, reaction
temperature = 80 ◦C, the reaction was stopped after 1 h. h Rhodium : 1-
octene = 1 : 1000, reaction temperature = 40 ◦C, the reaction was stopped
after 24 h.
an increased selectivity for the linear aldehyde (Table 2, entries 6
and 7). The templated bidentate ligand showed some bidentate
behaviour in catalysis: a small increase in selectivity (a linear :
branched ratio of almost 23) and a decrease in conversion (Table 2,
entry 8).
We studied non-templated and templated rhodium complexes
based on two other monomeric pyridyl phosphorus ligands f and
g in the hydroformylation of 1-octene under 20 bar of syngas (CO :
H2 = 1 : 1) at 40 ◦C (Table 2). The rhodium complex based on
ligand f, a phosphoramidite ligand, showed a low conversion and
a moderate selectivity for the linear aldehyde (Table 2, entry 9).
The rhodium catalyst based on supramolecular ligand assembly
2·f resulted in a decrease in selectivity to a l : b ratio of 2.4 along
with an almost fourfold increase in conversion (Table 2, entry 10).
Interestingly, the templated bidentate ligand provided a rhodium
complex that gave a much higher selectivity for the linear aldehyde,
although the conversion dropped to 3.9% (Table 2, entry 11). These
catalytic data are very typical of bidentate phosphorus ligands in
the rhodium-catalyzed hydroformylation of 1-octene (e.g. lower
reaction rate and higher selectivity for the linear aldehyde). The
use of a bulky phosphite ligand g showed a totally different
catalytic behaviour in the hydroformylation of 1-octene. Both
non-templated and templated rhodium complexes showed full
conversion and similar selectivities for the linear aldehyde (Table 2,
entries 12–14). These results indicate that a templated bidentate
Ligand assemblies based on monomeric pyridyl phosphite
ligand c, (S)-(1,1ꢀ-binaphtyl-2,2ꢀ-diyl)-3-pyridyl phosphite, were
also studied in the hydroformylation of 1-octene. The rhodium-
catalyst formed by ligand c showed a moderate catalytic activity
and a good regioselectivity for the linear product (Table 2, entry
5), which is common for catalysts based on bis-phosphite rhodium
complexes.9c Noteworthy, the isomerization side-reaction is sub-
stantial in this case. The use of supramolecular assemblies based
on ligand c and mono-zinc(II) salphens 2 and 3 afforded rhodium
complexes that showed higher conversions in combination with
Scheme 2 Rhodium-catalyzed hydroformylation of 1-octene.
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ligand is not formed under these conditions, most probably due
to steric hindrance caused by the bulky phosphite ligand, and that
the catalysis is dominated by bulky mono-phosphite rhodium-
complexes.29
enantiomeric excess. Similar to the hydroformylation reactions
with 1-octene (Table 2, entries 12–14), the catalysis is dominated
by bulky mono-phosphite rhodium complexes. This is also the
case for the templated bidentate catalyst assembly based on bulky
phosphoramidite ligand h (Table 3, entry 13).
Summarizing, the above catalytic results confirm that the
formation of bidentate ligands via self-assembly of two identical
monomeric pyridyl phosphorus ligands on a rigid bis-zinc(II)
salphen template is indeed successful and that the activity
and selectivity in the rhodium-catalyzed hydroformylation can
be steered, in most cases, using these supramolecular catalyst
assemblies. The concept can be easily extended by using other
monomeric pyridyl phosphorus ligands and by using mixtures
thereof,30 which might lead to improved ee’s.
Asymmetric hydroformylation of styrene
We were also interested to determine if these templated bidentate
ligands can give enhanced performance in asymmetric hydro-
formylation and therefore we explored these supramolecular
ligand assemblies in the rhodium-catalyzed asymmetric hydro-
formylation of styrene (Scheme 3). Previously, our group reported
the use of a flexible bis-zinc(II) porphyrin template for the
assembly of two monodentate phosphite ligands c, which led
to chelating bidentate ligands that induced higher activities and
enantioselectivities (33% ee) in the asymmetric hydroformylation
of styrene than any of the non-templated ligands (up to 7%
ee).15a In contrast, the rigid bis-zinc(II) salphen templated bidentate
ligand (1·(c + c)) provided a rhodium catalyst that gave the product
with low enantioselectivity (8% ee, Table 3, entry 3), whereas the
non-templated catalysts based on this ligand (Table 3, entries 1 and
2) provide the product almost in racemic form. A comparison of
both catalytic results suggests that the rigidity of the template does
not lead to an improvement of the enantioselectivity. Rhodium
complexes based on ligand e (Table 3, entries 4–6), templated or
not, all showed a low conversion and the enantiomeric excess
could not be determined. The rhodium catalyst assemblies based
on ligand f (a chiral phosphoramidite ligand with a methylated
nitrogen), templated or not, all afforded low enantioselectivity
(Table 3, entries 7–9). The rhodium complexes based on ligand
g (Table 3, entries 10–12) all gave full conversion with low
Asymmetric hydrogenation
We also examined the templated bidentate ligands in the asymmet-
ric rhodium-catalyzed hydrogenation of a-methylcinnamic acid
(Scheme 4). The rhodium-catalyst formed by ligand c showed
a low catalytic activity and a low enantioselectivity (7% ee,
Table 4, entry 1). The supramolecular catalyst assembly based
on 2·c showed a small increase in conversion as well as in
enantioselectivity (Table 4, entry 2). Although the templated
bidentate ligand assembly (1·(c + c)) afforded a slight increase
in conversion, the enantioselectivity dropped to only 4% ee
(Table 4, entry 3). Rhodium complexes based on ligand e (Table 4,
entries 4–6), templated or not, all showed a low conversion
and the enantiomeric excess could therefore not be determined.
The rhodium complex based on ligand g provided a catalyst
that still gave low conversion, but a good enantioselectivity of
79% was obtained (Table 4, entry 7). The use of supramolecular
assemblies based on ligand g and mono-zinc(II) salphen 2 afforded
rhodium complexes that showed a higher enantioselectivity (88%
Scheme 3 Rhodium-catalyzed hydroformylation of styrene.
Table 3 Hydroformylation of styrene catalyzed by non-templated and
templated rhodium complexesa
Scheme
4
Rhodium-catalyzed
asymmetric
hydrogenation
of
Conv (%)b
b : lc
ee (%)d
a-methylcinnamic acid.
Entry
Ligand
T/h
Table 4 Asymmetric hydrogenation of a-methylcinnamic acid catalyzed
1
2
3
4
5
6
7
8
9
10
11
12
13
(c)2
87
87
87
87
87
87
24
24
48
16
16
16
20
1.3
45
93
<1
<1
<1
8.2
59
9
>99
>99
>99
>99
3.6
4.8
5.2
—
—
—
8.2
12.7
5.15
12.2
12.5
13.5
19.0
2
3
8
—
—
—
3
by non-templated and templated rhodium complexesa
(3·c)2
1·(c + c)
Entry
Ligand
Conv (%)
ee (%)
(e)2
(3·e)2
1·(e + e)
(f)2
1
2
3
4
5
6
7
8
9
(c)2
0.3
0.7
1.2
<0.3
<0.3
<0.3
1.7
6.6 (S)
12 (S)
4.4 (S)
n. d.b
(2·c)2
1·(c + c)
(e)2
(2·f)2
4
1·(f + f)
(g)2
10
11
11
13
6
(2·e)2
1·(e + e)
(g)2
n. d.b
(3·g)2
1·(g + g)
1·(h + h)
n. d.b
79 (S)
88 (S)
89 (S)
(2·g)2
1.5
2.0
1·(g + g)
a Reaction conditions: [Rh] = 1.00 mmol l−1, [pyridylphosphorus ligand] =
10.0 mmol l−1, [mono-zinc(II) salphen] = 10.0 mmol l−1or [bis-zinc(II)
salphen] = 5.00 mmol l−1, rhodium : styrene = 1 : 1000. Pressure = 20 bar
(CO : H2 = 1 : 1), reaction temperature = 40 ◦C. b Percentage conversion
of styrene to aldehydes. c b : l = branched : linear. d In all cases the S
enantiomer of the product was formed.
a Reaction conditions: [Rh] = 1.00 mmol l−1, [pyridylphosphorus ligand] =
2.20 mmol l−1, [mono-zinc(II) salphen] = 2.20 mmol l−1or [bis-zinc(II)
salphen] = 1.10 mmol l−1, rhodium : a-methylcinnamic acid = 1 : 200.
Pressure = 5 bar H2, reaction temperature = 35 ◦C, the reaction was
stopped after 16 h. b n. d. = not determined.
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for the S-product, Table 4, entry 8). The templated bidentate
ligand (1·(g + g)) provided a rhodium complex that gave a good
enantioselectivity of 89% for the S-product, albeit the conversion
was still low (Table 4, entry 9).
zinc(II) salphen template. A bidentate ligand is formed by self-
assembly of two monomeric pyridyl phosphorus ligands on a bis-
zinc(II) salphen template molecule by selective coordination to
the template as proven by UV-vis, NMR-spectroscopy and X-ray
analysis. For the current self-assembled ligands, we used pyridyl
phosphorus ligands a–h and a rigid bis-zinc(II) salphen template
1. In the rhodium-catalyzed hydroformylation of 1-octene the
templated ligand assemblies showed, in some cases, typical
bidentate behaviour. The same strategy was applied in asymmetric
catalyzed transformations. In asymmetric hydroformylation and
hydrogenation reactions templated bidentate ligands showed en-
hanced selectivities compared to their non-templated analogues.
In analogy to previous work based on a more flexible bis-zinc(II)
porphyrin template, the current catalytic data show that a more
rigid template does not necessarily lead to an improvement in
catalytic performance. So far, we have only used a limited set of
building blocks, but extension of the self-assembled catalyst library
will afford a range of new catalyst systems that can be studied in
a combinatorial fashion for various asymmetric transformations.
The application of chiral templates is one of the extensions we are
currently investigating.
We also studied the application of the templated bidentate
ligands in the rhodium-catalyzed hydrogenation of dimethyl
itaconate to dimethyl succinate (Scheme 5). The rhodium catalyst
based on ligand e, a chiral phosphoramidite ligand,31 gives low
conversion and low enantioselectivity in this reaction (Table 5,
entry 1). The supramolecular catalyst assembly 3·e shows almost
a two-fold increase in conversion and an enantioselectivity which
is three times as high (Table 5, entry 2). Interestingly, the rhodium
catalyst based on the templated bidentate gives the product with
much higher enantioselectivity (ee 37%) at a slightly lower conver-
sion (entry 3). The use of a chiral monodentate pyridyl phosphite
ligand c results in low conversion and low enantioselectivity in
the hydrogenation of dimethyl itaconate (Table 5, entry 4). The
supramolecular assembly 2·c yields similar results (Table 5, entry
5). On the other hand, the templated bidentate ligand provides a
rhodium catalyst that significantly improves the ee to 52% (Table 5,
entry 6). The current hydrogenation results are in contrast to
previous work of our group, in which a bis-zinc(II) porphyrin
template in combination with monomeric pyridyl phosphorus
ligands gave rise to rhodium catalysts that showed a decrease in
ee in the hydrogenation of dimethyl itaconate compared to the
non-templated catalysts.32 The hydrogenation results show that
the formation of supramolecular bidentate ligands has a marked
effect on the outcome of the catalysis and that this approach can
be applied successfully in asymmetric catalysis.
Experimental
General procedures
Unless stated otherwise, reactions were carried out under an at-
mosphere of nitrogen or argon using standard Schlenk techniques.
Hexane was distilled from sodium benzophenone ketyl, CH2Cl2
was distilled from CaH2, and toluene was distilled from sodium un-
1
1
der nitrogen. NMR spectra (1H, 31P{ H} and 13C{ H}) were mea-
sured on a Varian Mercury 300 MHz; chemical shifts are reported
in ppm and are given relative to TMS (1H and 13C{ H}) or H3PO4
1
(31P{ H}) as external standards. UV-Vis spectroscopy experiments
1
Scheme 5 Rhodium-catalyzed asymmetric hydrogenation of dimethyl
were performed on a HP 8453 UV/Visible System. Gas chro-
matographic analyses were run on a Shimadzu GC-17A apparatus
(split/splitless, equipped with a FID detector and a BPX35 column
(internal diameter of 0.22 mm, film thickness 0.25 lm, carrier gas
70 kPa He)) or on an Interscience HR GC Mega 2 apparatus
(split/splitless injector, J & W Scientific, DB-1 J&W 30 m column,
film thickness 3.0 lm, carrier gas 70 kPa He, FID detector)
equipped with a Hewlett-Packard Data system (Chrom-Card).
Chiral GC separations were conducted with a Supelco BETA
DEX column (0.25 mm × 30 m), Chirasil-L-Val capillary column
(0.25 mm × 25 m) or a Cp-Chirasil-Dex CB column (0.32 mm ×
25 m). Alternatively, chiral GC separations were conducted on an
Interscience Trace GC Ultra (FID detector) with a pH Megadex
column (internal diameter 0.1 mm, 5 m column, film thickness
0.1 lm). Chiral HPLC analyses for determination of enantiomeric
excesses were carried out on a Gilson apparatus with a Dyna-
max UV-1 absorbance detector. High Resolution Mass Spectra
were recorded at the Department of Mass Spectrometry at the
University of Amsterdam using FAB+ ionization on a JEOL JMS
SX/SX102A four sector mass spectrometer with 3-nitrobenzyl
alcohol as the matrix. Elemental analyses were carried out by
Mikroanalytisch Laboratorium Dornis und Kolbe, Mu¨lheim an
der Ruhr (Germany).
itaconate.
Conclusions
In conclusion, we have reported the selective formation of
chelating self-assembled bidentate ligands based on a rigid bis-
Table 5 Asymmetric hydrogenation of dimethyl itaconate catalyzed by
non-templated and templated rhodium complexesa
Entry
Ligand
Conv (%)
ee (%)
1
2
3
4
5
6
(e)2
2.9
5.2
1.2
1.6
1.2
4.7
4.9 (S)
14 (S)
37 (S)
13 (S)
14 (S)
52 (S)
(3·e)2
1·(e + e)
(c)2
(2·c)2
1·(c + c)
a Reaction conditions: [Rh] = 1.00 mmol l−1, [pyridylphosphorus ligand] =
2.20 mmol l−1, [mono-zinc(II) salphen] = 2.20 mmol l−1 or [bis-zinc(II)
salphen] = 1.10 mmol l−1, rhodium : dimethyl itaconate = 1 : 200.
Pressure = 5 bar H2, reaction temperature = 35 ◦C, the reaction was
stopped after 16 h.
2316 | Dalton Trans., 2007, 2311–2320
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Materials
thermodynamic most stable bidentate metal complex. 31P NMR
1
(121.5 MHz): d = 15.5 ppm (s, JPt–P = 2683 Hz, trans-complex
With the exception of the compounds given below, all
reagents were purchased from commercial suppliers and
used without further purification. Diisopropylethylamine and
triethylamine were distilled from CaH2 under nitrogen. The
following compounds were synthesized according to published
procedures: bis-zinc(II) salphen 1,21 mono-zinc(II) salphen
2,21 mono-zinc(II) salphen 3,20 (1S,7S)-4-chloro-9,9-dimethyl-
2,2,6,6-tetra(3,5-dimethylphenyl)-3,5,8,10-tetraoxa-4-phosphabi-
cyclo[5.3.0]decane,31c pyridyl phosphorus ligands a and b,34
pyridyl phosphite ligands c,11a–c,15a,33 pyridyl phosphoramidite
ligands e, f, and h,20 and pyridyl phosphite ligand g.11a–c,33
(75%)) and d = 11.5 ppm (s, 1JPt–P = 3712 Hz, cis-complex (25%)).
31P NMR spectroscopy study of [PtCl2(1(b + b))]. 5.50 mg
(0.0209 mmol) of para-pyridyldiphenylphosphine b and 3.92 mg
of dichloro(1,5-cyclooctadiene)platinum(II) (0.0104 mmol) were
dissolved in CD2Cl2 (0.80 ml) and stirred for 30 min at room tem-
perature. 31P NMR (121.5 MHz): d = 15.5 ppm (s, 1JPt–P = 3641 Hz,
cis-complex). Bis-zinc(II) salphen 1 (11.8 mg, 0.0104 mmol) was
added to the reaction mixture and the solution was stirred for
15 min at room temperature. 31P NMR (121.5 MHz): d = 17.0 ppm
(s, 1JPt–P = 3628 Hz, cis-complex).
Alternatively, 5.50 mg (0.0209 mmol) of para-pyridyl-
Synthesis of (S,S)-3-[4,4,8,8-tetrakis-(3,5-dimethyl-phenyl)-2,2-
dimethyl-tetrahydro-[1,3]dioxolo[4,5-e][1,3,2]dioxaphosphepin-6-
yloxy]-pyridine d. 3-Hydroxypyridine (0.583 g, 6.13 mmol),
azeotropically dried with toluene (3 × 5 ml), and triethylamine
(0.94 ml, 6.7 mmol) were dissolved in dichloromethane (20 ml)
and the solution was cooled to −50 ◦C. Freshly prepared (1S,7S)-
4-chloro-9,9-dimethyl-2,2,6,6-tetra(3,5-dimethylphenyl)-3,5,8,10-
tetraoxa-4-phosphabicyclo[5.3.0]decane (3.94 g, 6.13 mmol),
dissolved in dichloromethane (20 ml) was added dropwise. The
cooling bath was removed and the solution was allowed to warm
to room temperature, stirring was continued for 2 h. The solvent
was evaporated, after which the product was extracted with a
mixture of 20 ml toluene and 60 ml hexanes. After filtration the
solvent was removed in vacuo, giving d (3.31 g, 4.72 mmol, 77%)
as a white foam. 1H NMR (300 MHz, CDCl3): d = 8.26 (dd, 1H,
J = 4.65 Hz, 1.50 Hz), 7.90 (br d, 1H, J = 2.70 Hz), 7.28-7.26
(m, 1H), 7.21-7.15 (m, 5H), 7.10-7.02 (m, 5H), 6.97-6.91 (m, 3H),
6.84 (br d, 2H, J = 9.60 Hz), 5.45 (d, 1H, J = 7.80 Hz), 5.06 (dd,
J = 8.25 Hz, 0.60 Hz), 2.32 (d, 15H, J = 11.4 Hz), 2.24 (s, 9H),
0.85 (s, 3H), 0.73 (s, 3H); 31P NMR (121.5 MHz, CDCl3): d =
126.5; 13C NMR (75.5 MHz, CDCl3): d = 149.9, 149.2, 149.1,
145.9, 144.3, 142.5, 142.4, 141.1, 141.0, 137.6, 137.4, 136.7, 136.6,
129.7, 129.3, 129.2, 129.1, 128.5, 127.1, 127.0, 126.9, 126.7, 125.6,
125.2, 125.1, 123.5, 113.2, 86.9, 86.8, 85.9, 85.8, 82.6, 82.5, 80.1,
27.0, 26.7, 21.9, 21.8; Anal. calcd. for C44H48NO5P: C 75.30, H
6.89, N 2.00; found: C 75.18, H 6.77, N 1.96.
diphenylphosphine
octadiene)platinum(II) (0.0104 mmol) were dissolved in toluene-d8
(0.80 ml) and stirred for 30 min at room temperature. 31P NMR
b and 3.92 mg of dichloro(1,5-cyclo-
1
(121.5 MHz): d = 15.0 ppm (s, JPt–P = 3641 Hz, cis-complex).
Bis-zinc(II) salphen 1 (11.8 mg, 0.0104 mmol) was added to the
reaction mixture and the solution was stirred for 13 h at 80 ◦C
to allow formation of the thermodynamic most stable bidentate
metal complex. 31P NMR (121.5 MHz): d = 21.5 ppm (s, 1JPt–P
=
1
2706 Hz, trans-complex (85%)) and d = 16.5 ppm (s, JPt–P
=
3640 Hz, cis-complex (15%)).
NMR spectroscopy study of [PdCl2(1(a + a))]. 11.0 mg
(0.0418 mmol) of meta-pyridyldiphenylphosphine a and 5.42 mg
of dichloro(bisacetonitrile)palladium(II) (0.0209 mmol) were dis-
solved in CD2Cl2 (1.60 ml) and stirred for 5 min at room
1
temperature. H NMR (300 MHz): d = 8.70 (m, 2H), 8.64 (m,
2H), 7.97-7.94 (s, 2H), 7.80-7.73 (m, 8H), 7.54-7.46 (m, 12H),
7.35-7.31 (m, 2H); 31P NMR (121.5 MHz): d = 21.5 ppm (s). Bis-
zinc(II) salphen 1 (23.6 mg, 0.0209 mmol) was added to the reaction
mixture and the solution was stirred for another 5 min to allow
formation of the bidentate metal complex. 1H NMR (300 MHz):
d = 8.79 (s, 4H), 8.57 (d, 2H, J = 4.5 Hz), 8.39 (br s, 2H), 7.66-7.59
(m, 10H), 7.49 (d, 4H, J = 2.7 Hz), 7.49-7.44 (m, 4H), 7.30-7.26
(m, 12H), 7.16 (s, 4H, J = 2.4 Hz), 1.51 (s, 36H), 1.41 (s, 36H); 31
NMR (121.5 MHz): d = 19.5 ppm (s).
P
NMR spectroscopy study of [PdCl(CH3)(1(a
+
a))].
NMR spectroscopy study of 1·(a)2. 5.50 mg (0.0209 mmol)
of meta-pyridyldiphenylphosphine a and 11.8 mg of bis-zinc(II)
salphen 1 (0.0104 mmol) were dissolved in CD2Cl2 (0.80 ml) and
stirred for 30 min at room temperature. 1H NMR (300 MHz): d =
8.80 (s, 4H), 8.43 (d, 2H, J = 4.8 Hz), 8.25 (br. s, 2H), 7.65-7.59
(m, 4H), 7.46 (d, 4H, J = 2.4 Hz), 7.37-7.33 (m, 6H), 7.29-7.24 (m,
10H), 7.18-7.11 (m, 10H), 1.49 (s, 36H), 1.38 (s, 36H); 31P NMR
(121.5 MHz): d = −11.0; 13C NMR (75.5 MHz): d = 171.7, 161.6,
148.7, 143.1, 142.9, 142.3, 139.0, 135.5, 135.4, 134.6, 133.8 (d, J =
19.6 Hz), 130.0, 129.4, 129.3, 129.0 (d, J = 7.32 Hz), 124.5, 124.4,
118.5, 35.8, 34.1, 31.4, 29.6.
7.14 mg (0.0104 mmol) of methyl palladium(II) bis(meta-
pyridyldiphenylphosphine) chloride was dissolved in CD2Cl2
(0.80 ml). 1H NMR (300 MHz): d = 8.78 (s, 2H), 8.63 (d, 2H, J =
3.9 Hz), 8.07 (s, 2H), 7.70 (br s, 8H), 7.51-7.44 (m, 12H), 7.37-7.32
(m, 2H), 0.01 (br s, 3H); 31P NMR (121.5 MHz): d = 28.5 ppm (s).
Bis-zinc(II) salphen 1 (11.8 mg, 0.0104 mmol) was added to the
reaction mixture and the solution was stirred for another 5 min
1
to allow formation of the bidentate metal complex. H NMR
(300 MHz): d = 8.75 (s, 4H), 8.66 (d, 2H, J = 5.4 Hz), 8.34 (d,
2H, J = 2.1 Hz), 7.57-7.43 (m, 22H), 7.33-7.29 (m, 8H), 7.17 (d,
4H, J = 2.7 Hz), 1.54 (s, 36H), 1.41 (s, 36H), −0.31 (t, 3H, J =
6.0 Hz); 31P NMR (121.5 MHz): d = 27.5 ppm (s); trans-complex.
31P NMR spectroscopy study of [PtCl2(1(a + a))]. 5.50 mg
(0.0209 mmol) of meta-pyridyldiphenylphosphine a and 3.92 mg
of dichloro(1,5-cyclooctadiene)platinum(II) (0.0104 mmol) were
dissolved in toluene-d8 (0.80 ml) and stirred for 30 min at
room temperature. 31P NMR (121.5 MHz): d = 12.5 ppm (s,
1JPt–P = 3710 Hz, cis-complex). Bis-zinc(II) salphen 1 (11.8 mg,
0.0104 mmol) was added to the reaction mixture and the so-
lution was stirred for 13 h at 80 ◦C to allow formation of the
NMR spectroscopy study of [PdCl2(1(b + b))]. 11.0 mg
(0.0418 mmol) of para-pyridyldiphenylphosphine b and 5.42 mg
of dichloro(bisacetonitrile)palladium(II) (0.0209 mmol) were dis-
solved in CD2Cl2 (1.60 ml) and stirred for 5 min at room
temperature. 1H NMR (300 MHz): d = 8.62 (br s, 4H),
7.81-7.75 (m, 8H), 7.59-7.41 (m, 16H); 31P NMR (121.5 MHz):
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d = 25.0 ppm (s). Bis-zinc(II) salphen 1 (23.6 mg, 0.0209 mmol)
was added to the reaction mixture and the solution was stirred for
another 5 min to allow formation of the bidentate metal complex.
1H NMR (300 MHz): d = 8.92 (s, 4H), 8.17-8.14 (m, 4H), 7.86-
7.79 (m, 8H), 7.75 (s, 2H), 7.59-7.44 (m, 12H), 7.47 (d, 4H, J =
2.4 Hz), 7.23 (d, 4H, J = 2.7 Hz), 7.16-7.11 (m, 4H), 1.51 (s, 36H),
1.38 (s, 36H); 31P NMR (121.5 MHz): d = 25.5 ppm (s).
reaction mixtures were stirred at 40 ◦C for the appropriate reaction
time (see main text in this paper). After catalysis the autoclave was
cooled to 0 ◦C, the pressure was reduced to 1.0 bar and a few drops
of tri-n-butyl-phosphite were added to all the reaction vessels to
prevent any further reaction. Please note that the reaction mixtures
were not filtered over silica–alumina (to remove catalyst residues),
because filtration may cause retention of the aldehyde products
and thus influence the GC-result! Next, a sample was taken and
the conversion was measured by GC of the crude product. The
enantiomeric purity was determined by chiral GC (pH Megadex
column; initial temperature = 40 C and DT = 25 C min−1; tR
(R) = 5.59 min and tR (S) = 5.67 min). Alternatively, the crude
product mixture was subjected to reduction with NaBH4 (0.2 g)
by stirring in 5.0 ml methanol for 30 min. Quenching with water,
extraction with a solution of ethyl acetate–hexane = 1 : 1, drying of
the organic layer with MgSO4, filtration and removal of the solvent
gave the corresponding alcohol, for which the enantiomeric purity
was determined by chiral GC (Cyclosil-B, isothermal at T = 95 ◦C,
tR (R) = 65.6 min and tR (S) = 69.1 min). Both GC measurements
of the enantiomeric purity of the product gave similar results. All
reactions were performed in duplo. Further details can be found
in the main text.
NMR spectroscopy study of [PdCl(CH3)(1(b
+
b))].
7.14 mg (0.0104 mmol) of methyl palladium(II) bis(para-
pyridyldiphenylphosphine) chloride was dissolved in CD2Cl2
◦
◦
1
(0.80 ml). H NMR (300 MHz): d = 8.61 (br s, 4H), 7.74 (br s,
8H), 7.53-7.45 (m, 16H), 0.01 (br s, 3H); 31P NMR (121.5 MHz):
d = 32.0 ppm (s). Bis-zinc(II) salphen 1 (11.8 mg, 0.0104 mmol)
was added to the reaction mixture and the solution was stirred for
another 5 min to allow formation of the bidentate metal complex.
1H NMR (300 MHz): d = 8.91 (s, 4H), 8.21-8.19 (m, 4H), 7.77
(s, 2H), 7.70-7.64 (m, 8H), 7.52-7.36 (m, 20H), 7.21 (d, 4H, J =
2.4 Hz), 1.53 (s, 36H), 1.37 (s, 36H), −0.46 (t, 3H, J = 6.0 Hz);
31P NMR (121.5 MHz): d = 32.5 ppm (s); trans-complex.
General procedure for the hydroformylation of 1-octene. The
hydroformylation experiments were carried out in a stainless
steel autoclave (volume 150 ml) charged with an insert suitable
for 14 reaction vessels (including Teflon mini stirring bars) for
conducting parallel reactions. The substrate 1-octene was filtered
freshly over basic alumina to remove possible peroxide impurities.
In a typical experiment, the autoclave was charged with 0.50 lmol
of [Rh(acac)(CO)2], 5.00 lmol of phosphorus ligand, if applicable
5.00 lmol of mono-zinc(II) salphen complex 2 or 3, or 2.50 lmol of
bis-zinc(II) salphen complex 1, 4.4 ll of diisopropylethylamine as a
base, 78.5 ll of 1-octene and 29.2 ll of decane in 0.50 ml of toluene.
Before starting the catalytic reactions, the charged autoclave was
purged three times with 10 bar of syngas (CO : H2 = 1 : 1), then
pressurized to 20 bar (CO : H2 = 1 : 1) and the reaction mixtures
were stirred (see main text in this paper for appropriate reaction
temperature and reaction time). After catalysis the autoclave was
cooled to 0 ◦C, the pressure was reduced to 1.0 bar and a few drops
of tri-n-butyl-phosphite were added to all the reaction vessels to
prevent any further reaction. Please note that the reaction mixtures
were not filtered over silica–alumina (to remove catalyst residues),
because filtration may cause retention of the aldehyde products
and thus influence the GC-result! The reaction mixtures were
diluted with dichloromethane for GC-analysis. All reactions were
performed in duplo. Further details can be found in the main text.
General procedure for the asymmetric hydrogenation of a-
methylcinnamic acid. The hydrogenation experiments were car-
ried out in a stainless steel autoclave (volume 150 ml) charged
with an insert suitable for 14 reaction vessels (including Teflon
mini stirring bars) for conducting parallel reactions. In a typ-
ical experiment, the autoclave was charged with 0.50 lmol of
[Rh(nbd)2(BF4)], 1.10 lmol of phosphorus ligand, if applicable
1.10 lmol of mono-zinc(II) salphen complex 2 or 3, or 0.55 lmol
of bis-zinc(II) salphen complex 1, 1 ll of diisopropylethylamine as
a base, 100 lmol of a-methylcinnamic acid and 50 lmol of decane
in 0.50 ml of CH2Cl2. Before starting the catalytic reactions, the
charged autoclave was purged five times with 5 bar of hydrogen
and then pressurized to 5 bar H2. The reaction mixtures were
stirred at 35 ◦C for 16 h. After catalysis the autoclave was cooled
◦
to 0 C, the pressure was reduced to 1.0 bar. Methanol (1.5 ml)
and (trimethylsilyl)diazomethane (0.5 ml of a 2.0 M solution in
ether) was added to all the reaction mixtures at RT. The reaction
mixtures were stirred for 30 min at RT. The conversion and the
enantiomeric purity were checked by chiral GC measurement (Cp-
Chirasil-Dex CB; isothermal at T = 80 ◦C for 10 min, then DT =
3 ◦C min−1 until T = 180 ◦C; tR (R) = 27.1 min, tR (S) = 27.3 min
and tR (sub) = 31.1 min). All reactions were performed in duplo.
General procedure for the asymmetric hydroformylation of
styrene. The hydroformylation experiments were carried out in
a stainless steel autoclave (volume 150 ml) charged with an insert
suitable for 14 reaction vessels (including Teflon mini stirring bars)
for conducting parallel reactions. The substrate styrene was filtered
freshly over basic alumina to remove possible peroxide impurities.
In a typical experiment, the autoclave was charged with 0.50 lmol
of [Rh(acac)(CO)2], 5.00 lmol of phosphorus ligand, if applicable
5.00 lmol of mono-zinc(II) salphen complex 2 or 3, or 2.50 lmol
of bis-zinc(II) salphen complex 1, 4.4 ll of diisopropylethylamine
as a base, 57.3 ll of styrene and 29.2 ll of decane in 0.50 ml
of toluene. Before starting the catalytic reactions, the charged
autoclave was purged three times with 10 bar of syngas (CO :
H2 = 1 : 1) and then pressurized to 20 bar (CO : H2 = 1 : 1). The
General procedure for the asymmetric hydrogenation of dimethyl
itaconate. The hydrogenation experiments were carried out in a
stainless steel autoclave (volume 150 ml) charged with an insert
suitable for 14 reaction vessels (including Teflon mini stirring
bars) for conducting parallel reactions. In a typical experiment,
the autoclave was charged with 0.50 lmol of [Rh(nbd)2(BF4)],
1.10 lmol of phosphorus ligand, if applicable 1.10 lmol of mono-
zinc(II) salphen complex 2 or 3, or 0.55 lmol of bis-zinc(II) salphen
complex 1, 1 ll of diisopropylethylamine as a base, 100 lmol of
dimethyl itaconate and 50 lmol of decane in 0.50 ml of CH2Cl2.
Before starting the catalytic reactions, the charged autoclave was
purged five times with 5 bar of dihydrogen and then pressurized
to 5 bar H2. The reaction mixtures were stirred at 35 ◦C for 16 h.
After catalysis the autoclave was cooled to 0 ◦C, the pressure was
2318 | Dalton Trans., 2007, 2311–2320
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The Royal Society of Chemistry 2007
©

View Online
−3
reduced to 1.0 bar. The reaction mixtures were filtered over a plug
of silica. The conversion was checked by GC measurement and
the enantiomeric purity was determined by chiral GC (Supelco
BETA DEX; isothermal at T = 70 ◦C; tR (R) = 36.7 min and tR
(S) = 37.8 min). All reactions were performed in duplo.
˚
1.20 e A . Structure calculations and checking for higher sym-
metry were performed with the PLATON package.36
28
X-Ray crystal structure determination of PdMe(X)(1)·(b)2
.
Data for: C66H86N4O4Zn2 + 2C17H14NP + CH3PdX + disordered
solvent, Mr = 1813.54§ 0.06 × 0.07 × 0.20 mm3; monoclinic, C2/c
X-Ray crystal structure determination of (1)·(a)2. Data for:
˚
(no. 15); a = 32.6639(10), b = 12.4434(10), c = 25.3741(10) A, b =
◦
3
−3
C66H86N4O4Zn2 + 2C17H14NP + 0.8 CH2Cl2 + disordered solvent,
˚
97.3120(10) , V = 10229.4(10) A ; Z = 4, q = 1.178 g cm §, l =
0.745 mm−1§; X-ray data were collected on a Nonius KappaCCD
diffractometer with rotating anode (graphite monochromator,
3
¯
Mr = 1724.59§, 0.12 × 0.18 × 0.30 mm ; triclinic, P1(no. 2);
˚
a = 16.799(2), b = 18.411(3), c = 20.4292(18) A, a = 110.621(8),
◦
3
˚
b = 107.510(10), c = 96.557(12) , V = 5465.4(14) A ; Z = 2,
˚
k = 0.71073 A) at a temperature of 150 K. 80 925 reflections
q = 1.048 g cm−3§, l = 0.553 mm−1§; X-ray data were collected
−1
˚
were measured up to a resolution of (sinh/k)max = 0.54 A .
on a Nonius Kappa CCD diffractometer with rotating anode
An absorption correction based on multiple measured reflections
was applied (correction range 0.72–0.96); 6678 reflections were
unique (Rint = 0.0741). The structure was solved with automated
Patterson methods (DIRDIF99).37 and refined with SHELXL-
9738 against F2 of all reflections to R = 0.0859. The exact nature
of the disordered ligand(s) in the plane perpendicular to the line
P–Pd–P remains unresolved. The crystal structure contained dis-
ordered solvent molecules that was accounted for with PLATON-
SQUEEZE.35,36 Structure calculations and checking for higher
symmetry were performed with the PLATON package.36
CCDC reference numbers 638013 and 638014.
˚
(graphite monochromator, k = 0.71073 A) at a temperature of
150 K. 123 114 Reflections were measured up to a resolution
−1
˚
of (sinh/k)max = 0.54 A . An absorption correction based on
multiple measured reflections was applied (correction range 0.78–
0.94); 14 272 Reflections were unique (Rint = 0.0642). The structure
was solved with automated Patterson methods (DIRDIF99).37
and refined with SHELXL-9738 against F2 of all reflections. The
crystal structure also contained disordered solvent molecules,
amounting to 180 electrons per unit cell. Their contribution to
the structure factors was secured by back-Fourier transformation
with PLATON-SQUEEZE.35,36 Some of the tert-butyl groups were
found to be conformationally disordered and were refined with a
disorder model. Hydrogen atoms were introduced at calculated
positions and refined riding on their carrier atoms. 1168 Refined
parameters, 207 restraints; R (10808 reflections F > 4r(F)): R1 =
0.0515, wR2 = 0.1358; R (all data): R1 = 0.0740, wR2 = 0.1448;
For crystallographic data in CIF or other electronic format see
DOI: 10.1039/b702375h.
Acknowledgements
We gratefully acknowledge NWO-CW for financial support.
GOF = 1.085; residual electron density between −0.57 and 0.78
−3
˚
e A . Structure calculations and checking for higher symmetry
References
were performed with the PLATON package.36
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X-Ray crystal structure determination of PtCl2(1)·(a)2. Data
for: C66H86N4O4Zn2 + 2C17H14NP + Cl2Pt + disordered solvent,
3
¯
Mr = 1922.64§, 0.16 × 0.20 × 0.36 mm ; triclinic, P1(no. 2); a =
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˚
17.390(2), b = 18.1344(15), c = 19.6577(19) A, a = 115.297(7),
◦
3
˚
b = 96.581(11), c = 92.182(9) , V = 5541.6(10) A ; Z = 2,
q = 1.152 g cm−3§, l = 1.810 mm−1§; X-ray data were collected
on a Nonius Kappa CCD diffractometer with rotating anode
˚
(graphite monochromator, k = 0.71073 A) at a temperature of
150 K. 121 700 Reflections were measured up to a resolution
−1
˚
of (sinh/k)max = 0.62 A . An absorption correction based on
multiple measured reflections was applied (correction range 0.42–
0.75); 21 810 Reflections were unique (Rint = 0.0574). The structure
was solved with automated Patterson methods (DIRDIF99).37 and
refined with SHELXL-9738 against F2 of all reflections. Some of
the tert-butyl groups were found to be orientationally disordered
and were refined with a disorder model. The crystal structure
also contained disordered solvent molecules, amounting to 192
electrons per unit cell. Their contribution to the structure factors
was secured by back-Fourier transformation with PLATON-
SQUEEZE.35,36 Hydrogen atoms were introduced at calculated
positions and refined riding on their carrier atoms. 1171 Refined
parameters, 237 restraints; R(17 290 reflections F > 4r(F)): R1 =
0.0347, wR2 = 0.792; R (all data): R1 = 0.0459, wR2 = 0.830;
GOF = 1.049; residual electron density between −0.71 and
§ Crystallographic data denotes the values without the contribution of the
7 See for example:(a) K. Nozaki, N. Sakai, T. Nanno, T. Higashijima, S.
Mano, T. Horiuchi and H. Takaya, J. Am. Chem. Soc., 1997, 119, 4413;
disordered solvent molecules.
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The Royal Society of Chemistry 2007
Dalton Trans., 2007, 2311–2320 | 2319
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9 See for example: (a) A. Buhling, P. C. J. Kamer and P. W. N. M.
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Dalton Trans., 2006, 3385.
11 Chelating diphosphine ligand systems through metal–ligand interac-
tions, reported by our group: (a) J. N. H. Reek, R. Chen, P. C. J.
Kamer, V. F. Slagt and P. W. N. M. van Leeuwen, EP 1479439, 2004;
(b) V. F. Slagt, M. Ro¨der, P. C. J. Kamer, P. W. N. M. van Leeuwen and
J. N. H. Reek, J. Am. Chem. Soc., 2004, 126, 4056; (c) J. N. H. Reek, M.
Ro¨der, P. E. Goudriaan, P. C. J. Kamer, P. W. N. M. van Leeuwen and
V. F. S l ag t , J. Organomet. Chem., 2005, 690, 4505; (d) X.-B. Jiang, L.
Lefort, P. E. Goudriaan, A. H. M. de Vries, P. W. N. M. van Leeuwen,
J. G. de Vries and J. N. H. Reek, Angew. Chem., Int. Ed., 2006, 45, 1223.
12 Chelating diphosphine ligand systems through anion binding: (a) P. A.
Duckmanton, A. J. Blake and J. B. Love, Inorg. Chem., 2005, 44, 7708;
(b) L. K. Knight, Z. Freixa, P. W. N. M. van Leeuwen and J. N. H.
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J. Durand, X. Morise, A. Tiripicchio and F. Ugozzoli, Organometallics,
2000, 19, 444; (b) P. Braunstein, G. Clerc, X. Morise, R. Welter and G.
Mantovani, Dalton Trans., 2003, 1601.
19 Supramolecular approaches to allosteric catalysts, some examples:
(a) L. Kovbasyuk, H. Pritzkow, R. Kramer and I. O. Fritsky, Chem.
Commun., 2004, 880; (b) N. C. Gianneschi, P. A. Bertin, S. T. Nguyen,
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C. A. Mirkin, Angew. Chem., Int. Ed., 2004, 43, 5503.
20 M. Kuil, P. E. Goudriaan, P. W. N. M. van Leeuwen and J. N. H. Reek,
Chem. Commun., 2006, 4679.
21 A. W. Kleij, M. Kuil, D. M. Tooke, M. Lutz, A. L. Spek and J. N. H.
Reek, Chem.–Eur. J., 2005, 11, 4743.
22 (a) V. F. Slagt, J. N. H. Reek, P. C. J. Kamer and P. W. N. M. van
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Kamer, P. W. N. M. van Leeuwen and J. N. H. Reek, J. Am. Chem. Soc.,
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23 (a) A. W. Kleij, M. Lutz, A. L. Spek, P. W. N. M. van Leeuwen and
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Chim. Acta, 2006, 359, 1807.
24 Note that the zinc(II) salphen complexes proved to be unstable in the
presence of relatively acidic solvents such as CDCl3.
25 We examined the binding curve in the present study with software
developed by Prof. C. A. Hunter, (Sheffield University, UK). See also:
A. P. Bisson, C. A. Hunter, J. C. Morales and K. Young, Chem.–Eur. J.,
1998, 4, 845.
26 It is important to note that the molecular structure of 1·(a)2 is a Ci
symmetric assembly.
27 The crystallization of [Pt(1·(a)2)Cl2] from
a dichloromethane–
acetonitrile solvent mixture was performed via slow solvent evaporation
at room temperature, and apparently under these conditions the kinetic
cis-complex was formed.
28 The configuration about the palladium metal is unclear. There are
˚
˚
multiple density maxima at a distance of 2 A and 2.8 A from Pd,
respectively. The first ones are clearly Pd–C distances, the latter ones
have no obvious atom type assignment. Apart from this and the fact
that the contents of the disordered solvent area is also unclear, the rest
of the reported crystal structure is certain.
13 Chelating diphosphine ligand systems through hydrogen bonding:
(a) B. Breit and W. Seiche, J. Am. Chem. Soc., 2003, 125, 6608; (b) B.
Breit and W. Seiche, Angew. Chem., Int. Ed., 2005, 44, 1640; (c) C. Ja¨kel,
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Breit, Angew. Chem., Int. Ed., 2006, 45, 1599; (h) A. J. Sandee, A. M.
van der Burg and J. N. H. Reek, Chem. Commun., 2007, 864.
14 Chelating diphosphine ligand systems through metal–ligand interac-
tions reported by the group of Takacs et al.: (a) J. M. Takacs, D. S.
Reddy, S. A. Moteki, D. Wiu and H. Palencia, J. Am. Chem. Soc., 2004,
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and J. L. Clark, New J. Chem., 2005, 29, 263; (c) K. Chaiseeda, S. A.
Moteki, D. S. Reddy, D. Wu, K. Chandra and J. M. Takacs, Fudan
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Takacs, Org. Lett., 2006, 8, 2759.
29 (a) Bulky mono-phosphite Rh-complexes are formed, see:P. W. N. M.
van Leeuwen and C. F. Roobeek, J. Organomet. Chem., 1983, 258, 343;
(b) T. Jongsma, G. Challa and P. W. N. M. van Leeuwen, J. Organomet.
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P. W. N. M. van Leeuwen, Organometallics, 1995, 14, 34.
30 See for example ref. 20. A full paper of this work will be submitted:M.
Kuil, P. E. Goudriaan, P. W. N. M. van Leeuwen and J. N. H. Reek,
submitted.
31 See for some successful examples of monodentate phosphoramidite
ligands in catalysis: (a) B. L. Feringa, Acc. Chem. Res., 2000, 33, 346;
(b) M. van den Berg, A. J. Minnaard, E. P. Schudde, J. van Esch,
A. H. M. de Vries, J. G. de Vries and B. L. Feringa, J. Am. Chem.
Soc., 2000, 122, 11539; (c) M. D. K. Boele, P. C. J. Kamer, M. Lutz,
A. L. Spek, J. G. de Vries, P. W. N. M. van Leeuwen and G. P. F. van
Strijdonck, Chem.–Eur. J., 2004, 10, 6232.
32 V. F. Slagt, Template Directed Assembly of Transition Metal Catalysts,
PhD Thesis, University of Amsterdam, The Netherlands, 2003, ch. 4.
33 G. J. H. Buisman, L. A. van der Veen, A. Klootwijk, W. G. J. de Lange,
P. C. J. Kamer, P. W. N. M. van Leeuwen and D. Vogt, Organometallics,
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15 (a) V. F. Slagt, P. W. N. M. van Leeuwen and J. N. H. Reek, Chem.
Commun., 2003, 2474; (b) V. F. Slagt, P. W. N. M. van Leeuwen and
J. N. H. Reek, Angew. Chem., Int. Ed., 2003, 42, 5619.
16 Many (supramolecular) approaches to heterobimetallic catalysts have
been reported,17–19 but in these contributions bidentate ligands are not
created by using one of the metals as an assembly motif.
34 Dr. P. H. M. Budzelaar, (Koninklijke/Shell Laboratorium Amsterdam,
(present address University of Manitoba, Winnipeg, Canada) is
gratefully acknowledged for the donation of the pyridylphosphines a
and b.
35 P. van der Sluis and A. L. Spek, Acta Crystallogr., Sect. A, 1990, A46,
194.
17 Heterobimetallic complexes equipped with both hard and soft metals,
some examples: (a) D. A. Wrobleski and T. B. Rauchfuss, J. Am. Chem.
Soc., 1982, 104, 2314; (b) D. A. Wrobleski, C. S. Day, B. A. Goodman
and T. B. Rauchfuss, J. Am. Chem. Soc., 1984, 106, 5464; (c) M.
Quirmbach, A. Kless, J. Holz, V. Tararov and A. Borner, Tetrahedron:
Asymmetry, 1999, 10, 1803.
36 A. L. Spek, J. Appl. Crystallogr., 2003, 36, 7.
37 P. T. Beurskens, G. Beurskens, R. de Gelder, S. Garcia-Granda, R. O.
Gould, R. Israel and J. M. M. Smits, The DIRDIF99 program system,
Technical Report of the Crystallographic Laboratory, University of
Nijmegen, The Netherlands, 1999.
38 G. M. Sheldrick, SHELXL97, University of Go¨ttingen, Germany,
18 Approaches to dinuclear catalysis, some examples: (a) P. Braunstein,
1997.
2320 | Dalton Trans., 2007, 2311–2320
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The Royal Society of Chemistry 2007
©