Supramolecular bidentate phosphorus ligands based on bis-zinc(ii) and bis-tin(iv) porphyrin building blocks

Article abstract of DOI:10.1039/b702462m

Selective metal-ligand interactions have been used to prepare supramolecular bidentate ligands by mixing monodentate ligands with a suitable template. For these assemblies pyridine phosphorus ligands and a zinc(II) porphyrin dimer were used. In the rhodium-catalysed hydroformylation of 1-octene and styrene improved selectivities have been obtained for some of the assembled bidentate ligand systems. In the palladium catalysed asymmetric allylic alkylation similar effects were observed; the enantioselectivity increased by using a bisporphyrin template. The preparation of supramolecular catalyst systems was also explored using tin-oxygen interactions. Dihydroxotin(iv) porphyrin and carboxylic phosphorus ligands assemble into supramolecular ligands and the phosphorus donor atom coordinates to transition metals. The stronger oxygen-tin bond, compared to pyridine-zinc does not result in a better performance of the catalyst. The Royal Society of Chemistry.

Full text of DOI:10.1039/b702462m

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Supramolecular bidentate phosphorus ligands based on bis-zinc(II) and
bis-tin(IV) porphyrin building blocks†
Vincent F. Slagt, Piet W. N. M. van Leeuwen and Joost N. H. Reek*
Received 15th February 2007, Accepted 21st March 2007
First published as an Advance Article on the web 26th April 2007
DOI: 10.1039/b702462m
Selective metal–ligand interactions have been used to prepare supramolecular bidentate ligands by
mixing monodentate ligands with a suitable template. For these assemblies pyridine phosphorus ligands
and a zinc(II) porphyrin dimer were used. In the rhodium-catalysed hydroformylation of 1-octene and
styrene improved selectivities have been obtained for some of the assembled bidentate ligand systems.
In the palladium catalysed asymmetric allylic alkylation similar effects were observed; the
enantioselectivity increased by using a bisporphyrin template. The preparation of supramolecular
catalyst systems was also explored using tin–oxygen interactions. Dihydroxotin(IV) porphyrin and
carboxylic phosphorus ligands assemble into supramolecular ligands and the phosphorus donor atom
coordinates to transition metals. The stronger oxygen–tin bond, compared to pyridine–zinc does not
result in a better performance of the catalyst.
Introduction
the phosphorus donor atom is still available for transition metal
complexation. We have used these properties to make a library of
supramolecular ligands in which the assembly process was based
For more than half a century ligand effects have been studied in
transition metal catalysis, and ligand variation and optimisation
1
11
on the zinc–pyridine interaction. In addition, we described a
is still the most important tool for catalyst development. Key
features of transition metal catalysts such as activity, selectiv-
ity and stability can be optimised by variation of steric and
novel supramolecular strategy to prepare a chelating bidentate
ligand based on a multi-component assembly.
11
Here we describe an approach to prepare bidentate chelating
ligands that involves the assembly of monodentate ligands on a
2,3
electronic properties of ligands. Initially, mostly monodentate
phosphorus ligands have been studied, yielding transition metal
complexes that are active catalysts for various reactions including
13
bis-porphyrin template (Fig. 1). This strategy combines the easy
access of monodentate ligands with the properties of chelating
systems. For the formation of these assemblies pyridine appended
phosphorus ligands (a–d) and a bis-zinc(II) porphyrin building
block (3) were used. In addition, we explored the use of selective
tin–oxygen interactions to form similar catalyst assemblies. For
this purpose a bis-tin(II) porphyrin building block (4) was prepared
and combined with carboxylate appended phosphine ligands (e–
g). The new class of supramolecular bidentate ligands gives rise to
active supramolecular catalyst systems.
4
hydrocyanation, hydrogenation and hydroformylation. In the
early 1970’s chelating bidentate ligands were found to yield
5
very selective catalysts for asymmetric hydrogenation and many
examples of active catalysts based on bidentate ligands have
6
appeared ever since. Recently, there is a renewed interest in
7
the use of monodentate ligands, which is partly motivated by
the relative ease of preparation compared to bidentate ones,
especially when sophisticated chiral entities are required. However,
for several reactions chelating bidentate ligands are required to
3,6
obtain catalysts with appreciable selectivity and activity, which
is ascribed to the formation of a more rigid microenvironment in
8
the transition metal’s coordination sphere.
A novel class of ligands that has the advantage of the synthetic
accessibility of the monodentate ligands but behave as bidentate
ligands, comprises the class of supramolecular bidentate chelat-
ing ligands formed by a self-assembly process of monodentate
Fig. 1 Schematic representation of the formation of templated bidentate
ligands by self-assembly of two monomeric pyridyl phosphorus ligands on
a template.
9–11
ligands. This supramolecular strategy is in principle well-suited
for a combinatorial approach because the number of ligands
that become accessible grows exponentially with the number of
building blocks that are used. We recently reported that the
pyridine moiety of pyridyl appended phosphine ligands selectively
Results
Synthesis of the building blocks
12
coordinate to zinc(II) porphyrins. Due to selective complexation
To study these novel supramolecular ligand systems several
building blocks were prepared. The phosphorus ligands (b–
g) were prepared according to standard literature procedures
van ‘t Hoff Institute of Molecular Sciences, University of Amsterdam,
Nieuwe Achtergracht 166, 1018 WV, Amsterdam, The Netherlands. E-mail:
reek@science.uva.nl; Fax: +31-20-5256422; Tel: +31-20-5256437
14–16
(Scheme 1).
Template molecules 1 and 3 were prepared as
17,18
†
The HTML version of this article has been enhanced with colour images.
described earlier.
Dihydroxotin(IV) porphyrins 2 and 4 were
2
302 | Dalton Trans., 2007, 2302–2310
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2 2
Scheme 2 Transition metal catalyst [HRh(3(b) )(CO) ] formed by assem-
bly of 4-pyridyldiphenylphosphine b on bis-zinc(II) porphyrin 3 in the
presence of a rhodium precursor and under 20 bar of syn-gas.
(or in the presence of monomeric zinc(II) porphyrin 1) resulted in
a mixture of rhodium–hydride signals demonstrating that ligand
exchange takes place in the non-templated complex.
Other building blocks that have been prepared for the assembly
ꢀ
of bidentate ligands are the chiral (S)- and (R)-(1,1 -binaphthyl-
ꢀ
2
,2 -diyl)-(3-pyridyl) phosphite c and d. The assemblies based
on in situ formed Rh(acac)(c)
2
and zinc(II) porphyrin building
blocks 1 and 3 were studied with NMR-spectroscopy. Mixing two
Scheme 1 Building blocks used in this study.
equivalents of meso-phenyl zinc(II) porphyrin 1 and Rh(acac)(c)
2
1
resulted in up-field shifts in the H-NMR spectrum of the protons
obtained in high yield from their dichlorotin(IV) porphyrin using
procedures reported by Crossley and co-workers.
on the pyridyl ring of c, which indicates the formation of the
19
31
assembled complex Rh(acac)(1(c)) . In addition, the P-NMR-
2
spectrum of this complex shows a large shift for the phosphite
P
The assembly of monomeric compounds on a bis-porphyrin
template
donor atom (Dd = 6.8 ppm) (Fig. 2). The addition of one
equivalent of bis zinc(II) porphyrin 3 to Rh(acac)(c)
2
results in
P
even larger shifts of the phosphorus resonance (Dd = 10.8 ppm)
Previously, we showed that the pyridine moiety of pyridyl–
phosphine type ligands selectively coordinates to zinc(II) por-
as a result of the formation of the bidentate ligand assembly
3
(c) . These large shifts are attributed to a combination of effects:
2
11,12
phyrins with high-binding constants.
In addition, we found
changes in the electronic and steric properties of the coordinating
phosphite ligands and the ring current of the two porphyrins that
are close to the phosphorus ligands in the assembly.
that after complexation of the zinc(II) porphyrin the phosphorus
donor atom is still available for transition metal complexation.
We anticipated that mixtures of bis(zinc) porphyrin templates
and pyridyl appended phosphorus ligands should give rise to
the formation of 3-component bidentate ligand assemblies. UV-
vis spectroscopy titrations in toluene showed that bis-zinc(II)
porphyrin 3 coordinates two pyridylphosphine building blocks b,
3
−1
with corresponding binding constants of K
1
= 5.1 × 10 M and
3
−1 20
K
2
= 1.4 × 10 M . This shows that bidentate ligands can be
prepared by only mixing two equivalents of monodentate ligand
building blocks with the bis-porphyrin template.
The coordination behaviour of these systems to transition
metals was studied using high-pressure NMR-spectroscopy in
8
21
toluene-d under 20 bars of syn-gas (H
Rh(acac)(CO) was used as a metal precursor and 4-pyridyl-
diphenylphosphine b as the phosphorus ligand. In the absence
of a porphyrin template HRh(b) (CO) formed as was evident
2
–CO = 1 : 1).
2
2
2
from the typical rhodium hydride signal at −9.5 ppm. The
hydride signal was shifted upfield (−11.1 ppm) upon addition
of template 3. The shift is caused by the shielding effect of the
porphyrins that embrace the rhodium catalyst, indicating that
complex [HRh(3(b)
2
)(CO) ] had formed (Scheme 2).
2
Addition of triphenylphosphine a to this mixture did not change
1
the hydride signal in the H NMR, showing the chelating effect of
31
the bidentate ligand assembly in complex [HRh(3(b)
2
)(CO)
2
]. In
Fig. 2
P-NMR spectra of several assembled rhodium acetylacetonate
contrast, mixing a with HRh(b) (CO) in the absence of template
2
2
bisphosphite complexes, JRh–P = 303 Hz for all complexes.
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a
Supramolecular bidentate ligands in catalysis
Table 2 Rhodium-catalysed hydroformylation of styrene
b
◦
c
d
e
Hydroformylation of 1-octene. The template-based supra-
Ligand
Temp./ C
T.O.F.
b/l
ee (%)
molecular bidentate ligands were studied in the rhodium-catalysed
c
25
25
25
25
25
25
0.01
0.02
0.15
0.01
0.02
0.14
> 100
> 100
> 100
> 100
> 100
> 100
7(S)
6(S)
33(S)
7(R)
6(R)
33(R)
hydroformylation of 1-octene under 20 bar of syn-gas (H –
2
1
3
d
(c)
(c)
CO = 1 : 1) in toluene (Scheme 3). The rhodium complex based
2
on monodentate 4-pyridyldiphenylphosphine b showed a high
◦
1(d)
(d)
activity and moderate selectivity for the linear aldehyde at 80 C
3
2
14,22
(
Table 1), typical of these monodentate ligands.
The application
a
−1
of the ligand based on the assembly of zinc(II) porphyrin 1 to
this complex resulted only in a small change in activity, while the
selectivity remained the same. In contrast, use of the assembled
[Rh(acac)(CO)
2
] = 0.084 mmol l , pressure = 20 bar (CO–H
2
= 1 :
b
−1
−1
−1
1
). [Phosphite] = 2.1 mmol l , [1] = 2.1 mmol l , [3] = 1.1 mmol l
,
c
styrene/rhodium = 5200. T.O.F. = turn over frequency = (mol aldehyde)
−1
−1
◦
d
(
mol Rh)
h
, the reaction was stopped after 64 h (25 C). b/l =
e
bidentate ligand 3(b)
2
resulted in a decrease of the reaction rate
branched/linear. ee = enantiomeric excess (%).
along with a slight increase in selectivity and lower isomerisation.
This catalytic behaviour is typical of bidentate phosphine ligands
in the rhodium-catalysed hydroformylation of 1-octene and shows
that the assembled bidentates are stable under catalytic conditions.
14,23
applied the ligands in the rhodium-catalysed hydroformylation
of styrene (Table 2). Rhodium complexes based on monodentates
c and d showed both a low but distinct enantiomeric excess
(
approximately 7%), which is in line with previous results of
24
monodentate ligands. The rhodium complexes based on 1(c)
and 1(d), have ligands with increased steric bulk that give similar
results. Interestingly, the use of 3 as a template molecule for the
assembly of c and d significantly improved the enantioselectivity
to 33%, along with an increase in activity. Although only moderate
enantioselectivities in the rhodium-catalysed hydroformylation of
styrene are observed, these results are very promising considering
the challenge that is involved. So far only a few covalently linked
chelating ligands gave high enantioselectivity in this reaction.
Next we applied the chiral bidentate ligand assemblies in
the rhodium-catalysed hydrogenation of dimethyl itaconate
Scheme 3 Rhodium-catalysed hydroformylation.
◦
We envisioned that at 80 C the ligand assembly might be too
dynamic to create the catalyst environment that leads to high
selectivity for the linear aldehyde. Therefore the reaction was
◦
also studied at 25 C. At this temperature the rhodium catalyst
based on the monodentate ligand 1(b) resulted in low but still
reasonable activity and a small increase in selectivity was observed
for this ligand system, compared to the results at 80 C. Also
◦
(
Scheme 4). The monodendate ligands c and d both give rise to
for the bidentate supramolecular ligand 3(b) a small increase in
2
moderate enantioselectivity in this reaction (ee = 36%), accom-
panied with low activity (Table 3). The conversion considerably
increased upon using the monodentate assemblies 1(c) and 1(d)
at the cost of a small decrease in enantiomeric excess. With
selectivity to l/b = 3.3 was observed, at the cost of a small decrease
in activity. The application of various phosphite ligands (c, 1(c) and
3(c)
2
) was also studied at this temperature. The supramolecular
provided a catalyst that produced the linear
bidentate ligand 3(c)
2
the use of chiral bidentate ligand assemblies 3(c)
2
and 3(d) the
2
adduct in high selectivity (94%), which is an increase compared
to that based on the monodentate 1(c) (83%). These results
clearly show that the supramolecular bidentate ligand systems
can improve the catalytic properties of transition metal catalysts
with respect to their non-templated analogues.
reaction rate was even higher, but the enantioselectivity decreased
dramatically. In this example the supramolecular ligands do not
provide catalysts that outperform the parent complexes based on
the monodentate ligands.
Supramolecular bidentate ligands in asymmetric catalysis. We
also studied the application of the supramolecular bidentate
ligands in asymmetric catalysis applying chiral building blocks
c and d in combination with bis-zinc(II) porphyrin 3. Initially we
Scheme 4 Rhodium-catalysed hydrogenation of dimethyl itaconate.
a
Table 1 Rhodium catalysed hydroformylation of 1-octene
b
◦
c
d
e
f
f
Ligand
Temp./ C
Conversion (%)
T.O.F.
l/b
Isomers (%)
Linear (%)
b
80
80
80
25
25
25
25
93
89
33
6.2
4.7
1.1
0.8
2250
2100
727
2.7
2.7
3.0
2.9
3.3
5.0
16.4
1.8
1.8
0.5
0.1
0.1
0.0
0.0
72
72
75
74
77
83
94
1
3
1
3
1
3
(b)
(b)
(b)
(b)
(c)
2
2
7.4
5.6
1.3
0.9
(c)
2
a
−1
b
−1
−1
−1
[
Rh] = 0.084 mmol l in toluene, pressure = 20 bar (CO–H
2
= 1 : 1), 1-octene/rhodium = 5200. [Phosphorus] = 2.1 mmol l , [1] = 2.1 mmol l , [3] =
−
1
c
d
−1
1
.1 mmol l
.
Percent total conversion of 1-octene to aldehydes and 2-octene. T.O.F. = average turn over frequency = (mol aldehyde) (mol Rh)
h ,
◦
◦
e
f
the reaction was stopped after 2 h (80 C) or 43 h (25 C). l/b = linear/branched. Percent 2-octene and percent selectivity for linear aldehyde based on
converted 1-octene, typical error 0.5%.
2
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Table 3 Rhodium-catalysed hydrogenation of dimethyl itaconate using
ligands as they displace the hydroxo groups of dihydroxotin(IV)
a
chiral assemblies of pyridyl phosphite and zinc(II) porphyrin templates
31,32
porphyrins quantitatively.
The ligand exchange proceeds via
a hydrogen bonded intermediate, after which carboxylatotin(IV)
b
◦
c
d
Ligand
Temp./ C
Conversion (%)
ee (%)
27
porphyrin complexes form with the loss of water. Once formed,
these carboxylatotin(IV) porphyrin complexes are kinetically stable
and, in the absence of acid, ligand exchange occurs over a period of
c
40
40
40
40
40
40
2.2
7.5
9.5
2.0
6.9
36(S)
31(S)
8(S)
36(R)
32(R)
7(R)
1
3
d
1
3
(c)
(c)
2
33
weeks. We anticipated that supramolecular ligands based on the
carboxylatotin(IV) porphyrin complexes would be ideal to study
the effect of the dynamic character in supramolecular ligands.
(d)
(d)
2
10.2
a
b
−1
[
[
Rh(nbd)
2
BPh
4
]
=
0.10 mmol
l
,
substrate/rhodium
=
200.
−1
−1
−1
The assembly of carboxylic phosphorus ligands on dihydroxotin(IV)
porphyrin templates
Phosphite] = 0.60 mmol l , [1] = 0.60 mmol l , [3] = 0.30 mmol l in
c
d
toluene. The reaction was stopped after 15 h. ee = enantiomeric excess
%).
(
The coordination behaviour of carboxylic phosphine ligand e to
meso-phenyl dihydroxotin(IV) porphyrin 2 was studied using NMR
spectroscopy. The addition of two equivalents of e to a solution
of 2 resulted in large upfield shifts of the protons on the aryl ring
The scope of the ligand assemblies was extended to the
palladium-catalysed allylic alkylation (Scheme 5). For this reaction
the palladium complexes based on monomeric pyridyl phosphite
ligands c and d yielded low enantioselectivity (18%) (Table 4).
The monodentate assemblies 1(c) and 1(d) changed the ligand
properties to such an extent that the enantiomeric excess was
increased to 32%. Further improvement of the enantioselectivity in
this reaction was obtained by using the bidentate ligand assemblies
1
H1
of e in the H-NMR spectrum (CDCl
3
, Dd = 3.15 ppm and
H2
Dd = 1.12 ppm), caused by the shielding effect of the porphyrin
1
ring (Fig. 3). The typical signal in the H-NMR spectrum at
−
7.5 ppm which is from the hydroxyl group of 2 disappeared. In
addition, the broad resonances of the ‘free’ (unbound) carboxylic
acid resonance were no longer observed in the mixture, which
corroborates the formation of dicarboxylatotin(IV) porphyrin
3
(c)
2
and 3(d) . The obtained results confirm that bidentate
2
assembly of chiral monomeric ligands on a bis-porphyrin template
is successful and amends the activity and enantioselectivity in
asymmetric transition metal catalysis.
11
complex 2(e) . A similar behaviour has been observed previously
2
for the complexation of benzoic acid to phenyl dihydroxotin(IV)
1
porphyrin 2, yielding comparable upfield shifts in H-NMR
spectroscopy.
Scheme 5 Palladium-catalysed allylic alkylation.
Dihydroxotin(IV) porphyrins as molecular building blocks
We wanted to study the effect of the dynamic character of the
supramolecular ligands by simply changing the metal center in the
bis-porphyrin template. For the construction of supramolecular
assemblies several metallo porphyrins have been applied including
2
5,26
ruthenium, palladium and magnesium porphyrins.
cently, dihydroxotin(IV) porphyrins
More re-
have been used as building
blocks utilising the selective coordination of ligands with oxygen
19,27
28–30
donor atoms to arrive at novel supramolecular assemblies.
Car-
boxylate, alkoxide and phenoxide have been shown to be suitable
Table 4 Palladium-catalysed allylic alkylation using chiral assemblies of
a
pyridyl phosphite and zinc(II) porphyrin templates
1
2
b
◦
c
d
Fig. 3 Porphyrin complex induced shifts (Dd (ppm)) for H an H after
Ligand
Temp./ C
Conversion (%)
ee (%)
formation of the dicarboxylatotin(IV) porphyrin complex 2(e)
2
. In each
c
25
25
25
25
25
25
> 99
> 99
> 99
> 99
> 99
> 99
18 (S)
31 (S)
45 (S)
18 (R)
32 (R)
44 (R)
case Dd = d (complex) − d (free acid).
1
3
d
1
3
(c)
(c)
3
1
2
In the P-NMR-spectra the coordination of e to dihydrox-
otin(IV) porphyrin 2 resulted also in a chemical shift of the
(d)
(d)
P(e)
phosphorus resonance (Dd = 1.01 ppm). Gradual addition of
2
porphyrin 2 to a solution of e in CDCl
3
(2/e = 0.3) yielded two
a
b
c
−1
[
[
[Pd(allyl)Cl]
2
] = 0.10 mmol l , 3-diphenyl-allyl acetate/rhodium = 100.
31
P
distinct signals in the P-NMR spectra (d (e) = −4.45 ppm and
−1
−1
−1
Phosphite] = 0.60 mmol l , [1] = 0.60 mmol l , [2] = 0.30 mmol l
.
P
d
d (2(e)
2
) = −5.46 ppm), which shows that dicarboxylatotin(IV)
2
The reaction was stopped after 63 h. ee = enantiomeric excess (%).
porphyrin complex 2(e)
is in slow exchange on the NMR
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spectroscopy time scale (300 MHz, 298 K) with the excess ‘free’
carboxylic e which is present (Fig. 4).
to the solid complex 6 resulted in instantaneous dissolution of
the platinum complex. NMR spectra of the dissolved material
indicated that the carboxylic groups coordinated to the dihydrox-
3
1
otin(IV) porphyrin 2 and the P-NMR-spectra demonstrated the
formation of a cis-platinum complex (dPt–P = 13.82 ppm) with
an identical platinum–phosphorus coupling constant as found
for complex 5 (JPt–P = 3674 Hz). Subsequent addition of two
equivalents of e to the in situ formed cis-[Pt(2(e))
2
Cl
] complex 5. This shows that
] initially formed leads to very soluble complexes
and that the hydroxyl group on the tin template can still be
2
], resulted
in the formation of cis-[Pt(2(e)
cis-[Pt(2(e)) Cl
2
)
2
Cl
2
2
2
35
substituted.
Bis-porphyrin 4 consists of two dihydroxotin(IV) metal ions,
enabling the coordination of four carboxylic phosphorus ligands
(
e–g). The complexation of two or more of such ligands on
a bis-tin(IV) porphyrin template 4, yields an in situ assembled
multidentate phosphorus ligand that potentially can act as a
chelating ligand system, similar to those based on bis-zinc(II)
porphyrin 3 and 4-pyridyldiphenylphosphines b. The coordination
behaviour of carboxylic phosphine ligand e to bis-dihydroxotin(IV)
porphyrin 4 was studied. The addition of four equivalents of
e to a solution of 4 resulted in comparable chemical shifts in
3
1
Fig. 4
0
P-NMR-spectra of carboxylic phosphine a in the presence of 0,
.3 and 0.5 equivalents of dihydroxotin(IV) porphyrin 2.
Increasing the amount of 2 (2/e = 0.5) yielded exclusively
1
31
P
the formation of complex 2(e)
2
, which indicates that two
the H- and P-NMR spectroscopy (d (4(e) ) = −5.46 ppm)
4
molecules of e are coordinated to the dihydroxotin(IV) porphyrin
template 2. Larger 2/e ratios did not affect the phosphorus
signal in P-NMR spectroscopy. The addition of dihydrox-
as found for the assembly based on dihydroxotin(IV) porphyrin
2. In addition, the hydroxotin(IV) signal of 4 at −7.4 ppm
3
1
1
in the H-NMR spectrum disappeared as well as that of the
otin(IV) porphyrin to 3-carboxyphenyldiphenylphosphine f and
carboxylic acid resonance of e. In a separate experiment one
equivalent of bis-porphyrin 4 dissolved in chloroform-d was added
to ‘insoluble’ solid cis-[Pt(e) Cl ] complex 6, which resulted in the
2
-carboxyphenyldiphenylphosphine g resulted in a upfield shift
3
1
P(f)
of the phosphorus signal in the P-NMR spectrum of Dd
0
=
2
2
P(g)
.95 ppm and Dd = 1.34 ppm, respectively (Table 5).
formation of highly soluble cis-[Pt(4(e) )Cl ] complex 7 (Fig. 5).
2
2
3
1
These experiments demonstrate that, as expected, the assembly
The P-NMR-spectra indicated the formation of a cis-platinum
complex 7 (dPt–P = 13.88 ppm, J = 3694 Hz). The addition
processes are based on oxygen–tin interactions. The phosphorus
atom is not involved and therefore is available for coordination
to catalytically active transition metals. Indeed, upon mixing
Pt–P
of a stoichiometric amount triphenylphosphine a to the in situ
3
1
formed cis-[Pt(4(e) )Cl ] 7 resulted in several peaks in the P-
2
2
two equivalents of in situ prepared 2(e)
an assembly was formed with the transition metal sandwiched
2
with [Pt(CH
3
CN)
2
Cl
2
]
NMR spectra indicative of the formation of new platinum–
phosphorus complexes. The addition of 20 equivalents of a yielded
predominantly thecis-[Pt(a) Cl ] complex. This demonstrates that,
3
1
between the two porphyrin building blocks. The P-NMR-spectra
in CDCl showed the formation of a cis-[Pt(2(e) Cl ] complex 5,
2
2
3
2
)
2
2
in contrast to 3(b) , the supramolecular bidentate ligand 4(e) is
2
2
yielding two distinct phosphorus signals of the non-coordinated
and coordinated phosphorus (dPt–P = 13.82 ppm, JPt–P = 3674 Hz
and d
P
= −5.47 ppm). At these concentrations the formation of
polymers was not observed.
Complex 6 (cis-[Pt(e) Cl ]), like many other carboxylic phospho-
2
2
rus transition metal complexes, is practically insoluble in the most
common organic solvents. Highly polar solvents, like methanol,
have been shown to be suitable solvents for these transition metal
14,34
complexes.
Indeed complex 6 did not dissolve in CDCl
3
, but
the addition of two equivalents of dihydroxotin(IV) porphyrin 2
3
1
Table 5
P-NMR spectroscopy data of the assembly of carboxylic
phosphorus ligands and dihydroxotin(IV) porphyrins
31
Complex
d( P)-phosphine (ppm)
a
−4.45
−5.46
−5.46
−4.79
−5.74
−3.38
−4.72
2
4
f
(e)
(e)
2
4
2
g
2
(f)
2
Fig. 5 The assembly bis-porphyrin 4 and complex 6 leads to the assembled
complex cis-[Pt(2(e) )Cl ] 7.
(g)
2
2
2
2
306 | Dalton Trans., 2007, 2302–2310
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not a strong chelating ligand. Apparently, the chelate ring formed
is too large to form proper bidentate ligands (four atoms more
Table 7 Hydroformylation of 1-octene using different rhodium catalyst
a
assemblies: variation of phosphine and porphyrin building blocks
than in 3(b)
making triphenylphosphine a stronger coordinating ligand than
the phosphorus in ligand 4(e)
2
). In addition, electronic effects might play a role in
b
c
d
e
e
Ligand
T.O.F.
l/b
Isomerisation (%)
Linear (%)
3
3
3
3
3
3
3
3
3
3
2
.
a
2
4
4(e)4
f
1.7 × 10
0.5 × 10
2.4 × 10
2.3 × 10
0.7 × 10
2.0 × 10
2.0 × 10
0.01 × 10
0.03 × 10
0.3 × 10
2.7
2.7
2.1
2.1
2.8
2.6
2.5
2.7
2.4
2.1
1.5
1.3
0.8
1.0
1.1
0.8
0.4
2.6
2.4
1.7
72
72
67
66
73
71
71
71
68
67
+ a
(e)
2
Supramolecular ligands based on dihydroxotin(IV) porphyrins and
carboxylic phosphorus in rhodium catalysed hydroformylation
2
4
g
2
(f)
(f)
2
4
The supramolecular ligand assemblies based on dihydroxotin(IV)
porphyrins 2 and 4 and carboxylic phosphorus ligands e–g were
applied in the rhodium-catalysed hydroformylation of 1-octene.
(g)
4(g)4
2
The hydroformylation experiments were carried out under 20
a
−1
◦
[Rh(acac)(CO)
2
] = 0.084 mmol l in toluene, pressure = 20 bar (CO–
= 1 : 1), 1-octene/rhodium = 5160, in none of the reactions was
bar of syn-gas (H
2
–CO = 1 : 1) at 80 C in toluene and the
H
2
b
−1 c
−1 −1
results are depicted in Table 6. The rhodium complex based on
ligand e showed a low catalytic activity in rhodium-catalysed
hydroformylation in toluene (T.O.F. = 50), which is most likely
due to the non-innocence of the carboxylic acid functions. The
formation of inactive rhodium carboxylate complexes results in
decreased concentration of the active hydrido rhodium complex,
with explains the low activity. In addition, the active rhodium
complex is poorly soluble in most organic solvents. Previously
this low solubility was explained by the formation of polymeric
rhodium structures from the intermolecular reaction of the
hydrogenation observed. [Phosphine] = 0.84 mmol l . T.O.F. = average
turn over frequency = (mol aldehyde) (mol Rh)
h , the reaction
e
◦
d
was stopped after 1 h (80 C). l/b = linear/branched ratio. Product
distribution: percent isomerisation to 2-, and 3- and 4-octene and percent
selectivity for linear aldehyde (percent branched = 100 − (%isomerisation
+ %branched)).
36
acidic to protonate the rhodium hydride or it may coordinate to the
rhodium complex, resulting in the formation of inactive rhodium
species.
The use of multidentate ligand assemblies 4(e)2 and 4(e)4
resulted also in highly active rhodium catalysts, giving similar
14
carboxylic group and the rhodium hydride.
In contrast, the assembled ligand system 2(e)
2
and 2(e) resulted
results to those found for the complexes based on 2(e) and 2(e).
2
in highly active catalysts and the catalytic behaviour of the
rhodium complexes based on these assembled ligand systems is
comparable with those based on other monodentate phosphine
This implies that 4(e) and 4(e) do not show a typical bidentate
2
4
ligand behaviour as found for the assembled ligand systems based
on biszinc(II) porphyrin 3 and 4-pyridyl-diphenylphosphine b, a
difference that was also observed in the NMR experiments.
3-Carboxyphenyldiphenylphosphine f resulted in catalysts that
gave moderate activity in the rhodium-catalysed hydroformylation
14
ligands. These results show that the assembly of carboxylic phos-
phorus ligands on dihyroxotin(IV) porphyrins efficiently block the
acid groups preventing the formation of rhodium carboxylates and
the formation of insoluble polymeric structures. The assemblies
are stable under catalytic conditions and polymer formation via
intermolecular interaction between the carboxylate and rhodium
is not observed. The presence of an excess of dihydroxotin(IV)
porphyrin (2/e = 2) retards the reaction rate; a T.O.F. of 800 was
obtained compared to 2100 using 2/e = 1. In a control experiment
using triphenylphosphine a as the ligand in the absence and
presence of dihydroxotin(IV) porphyrin 2 a similar drop in activity
was observed (T.O.F. dropped from 1700 to 500, Table 7). It can
be anticipated that the dihydroxotin(IV) porphyrin 2 is sufficiently
3
of 1-octene (T.O.F. = 0.7 × 10 , which is 14 times higher than
found for ligand e). The corresponding supramolecular ligand
assemblies 2(f) and 4(f) resulted in higher activities, comparable
2
4
with those obtained for the assemblies based on e. Again, the
efficient blocking of the acid group of f by dihydroxotin(IV)
porphyrins 2 and 4 prevent polymer formation and lead to soluble
active catalysts based on the assemblies 2(f) and 4(f) .
2
4
The catalyst formed by 2-carboxyphenyldiphenylphosphine g
showed hardly any activity in the hydroformylation of 1-octene
and was found to be 5 times slower than the catalyst formed
by e (T.O.F. = 10). In contrast to the ligands e and f the
addition of dihydroxotin(IV) porphyrin 2 to g did not result in
a large increase in catalytic activity (T.O.F. = 30). A possible
Table 6 Hydroformylation of 1-octene using different rhodium catalysts
and their porphyrin assemblies: variation of the dihydroxotin(IV) por-
a
phyrin : phosphine ratio
explanation for the poor activity of 2(g) in the hydroformylation
2
b
c
d
e
e
of 1-octene is the formation of less active P–O coordinated
rhodium complexes. The catalytic reaction mixture based on
Ligand
2/e ratio T.O.F.
l/b Isomerisation (%) Linear (%)
37
3
3
3
3
e
0
0.5
1.0
0.05 × 10
2.2 × 10
2.1 × 10
0.8 × 10
2.8
0.9
73
67
67
72
ligand 2(g) was homogeneous indicating that the solubilising
2
2
2
2
(e)
(e)
2
2.1 1.1
effect of the porphyrin was similar to that of the other examples.
2.1
2.6
1.1
0.6
Surprisingly, the catalyst formed by multidentate 4(g) did show
4
(e) + 2 2.0
much higher activity (T.O.F. = 300), for which we do not have a
a
−1
[
Rh(acac)(CO)
2
] = 0.084 mmol l in toluene, pressure = 20 bar (CO–
clear explanation yet.
H
2
= 1 : 1), 1-octene/rhodium = 5160, in none of the reactions was
b
−1 c
hydrogenation observed. [Phosphine] = 0.84 mmol l . T.O.F. = average
−
1
−1
turn over frequency = (mol aldehyde) (mol Rh)
h , the reaction
e
Conclusion
◦
d
was stopped after 1 h (80 C). l/b = linear/branched ratio. Product
distribution: percent isomerisation to 2-, and 3- and 4-octene and percent
selectivity for linear aldehyde (percent branched = 100 − (%isomerisation
A new effective strategy to prepare bidentate ligands using
supramolecular interactions is presented. The bidentate ligands
are easily prepared by mixing monodentate ligands with a
+
%branched)).
This journal is © The Royal Society of Chemistry 2007
Dalton Trans., 2007, 2302–2310 | 2307

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template, forming novel chelating ligands by selective coordination
to the template as proven by NMR-spectroscopy. Some of the
assemblies based on pyridine–phosphorus ligands b–d and bis-
zinc(II) porphyrins 3 formed catalysts that exhibited high selec-
tivities in the rhodium-catalysed hydroformylation of 1-octene.
This strategy is also applicable in asymmetric transition metal
catalysis. The enantioselectivity in the hydroformylation of styrene
is improved significantly upon using the assembled bidentate
ligands (33%) compared to the monodentate assemblies (7%).
In the palladium catalysed allylic alkylation similar effects were
observed as the enantioselectivity increased from 18 to 44% ee.
We have also shown that supramolecular ligands can be
prepared via the assembly of carboxylic phosphorus ligands
on dihydroxotin(IV) porphyrin building blocks. The assembled
structures are based on selective oxygen coordination to tin(IV)
porphyrins and yield a new class of supramolecular phosphorus
ligand systems. We observed a large change in ligand properties
of the carboxylic phosphines, caused by the assembly on the
porphyrin template. In the rhodium-catalysed hydroformylation
of 1-octene in toluene the addition of dihydroxotin(IV) porphyrins
to carboxylic phosphorus rhodium complexes yielded catalyst
systems that are up to forty times more active. The assembly
of carboxylic phosphorus ligands on dihyroxotin(IV) porphyrins
efficiently blocks the acid groups, which prevents the formation
of inactive rhodium carboxylate complexes and insoluble poly-
meric structures, explaining the higher activity of the catalysts
assemblies. This shows that both the dihydroxotin(IV) porphyrin
template and the carboxylic phosphorus building block determine
the activity of the rhodium catalyst.
Materials
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 CaH
pounds were synthesised according to published procedures:
2
under argon. The following com-
14
4
e–g,
-pyridyldiphenylphosphine b, carboxylic phosphorus ligands
14,15
34
phosphorus platinum complexes, 5,10,15,20-tetrakis-
phenylporphyrin zinc(II) 1, 5,10,15,20-tetrakis-phenylporphyrin
dihydroxotin(IV) 2 and bis-zinc(II) porphyrin 3.
13c
13,18
Syntheses
5
,10,15,20-Tetrakisphenylporphyrin dichlorotin(IV). 5,10,15,
2
2
0-Tetrakisphenylporphyrin (1.0 g, 1.64 mmol) was dissolved in
00 ml pyridine and tin(II) chloride dihydrate (1.0 g, 4.4 mmol)
was added and the mixture heated at reflux for 3 h. Excess water
was added to precipitate the product which was then filtered
off, washed with water, 1 mol l hydrochloric acid solution,
water and air dried to give 5,10,15,20-tetrakisphenylporphyrin
dichlorotin(IV) (1.1 g, 83%) as a purple solid, mp >300 C. The
−1
◦
1
compound had identical H NMR properties to those reported
31
1
by Arnold and co-workers: H NMR (300 MHz): d 9.22 (s, 8 H,
4
satellites JH–Sn = 15.3 Hz), 8.31–8.34 (m, 8 H), 7.80–7.84 (m,
1
2 H).
5
,10,15,20-Tetrakisphenylporphyrin dihydroxotin(IV)2. Potas-
sium carbonate (800 mg, 5.8 mmol) and 5,10,15,20-
tetrakisphenylporphyrin dichlorotin(IV) (280 mg, 0.348 mmol)
were dissolved in a mixture of 150 ml tetrahydrofuran and
With the current series of supramolecular ligands we wanted to
study the effect of the dynamic character of this class of ligands, by
comparing the zinc-based systems with those of the tin-based ones.
Also the ligand building blocks were adjusted, from pyridyl ap-
pended to carboxyl appended, which unfortunately made a direct
comparison impossible. Other changes to the system include the
use of more rigid templates, which will be discussed in the following
4
0 ml water and heated at reflux for 3 h. The organic solvent
was removed and the aqueous layer was extracted with 100 ml
dichloromethane. The organic layer was washed with water (2 ×
8
0 ml) and then dried over anhydrous sodium sulfate, filtered and
then the solvent was removed to give the crude product, which was
then recrystallised from hexane–dichloromethane (1 : 1) to give
38
38
5
9
,10,15,20-tetrakisphenylporphyrin dihydroxotin(IV) 2 (242 mg,
1%) as a metallic purple crystalline solid, mp >300 C. The
contribution, and the use of mixtures of ligand building blocks.
◦
1
compound had identical H NMR properties to those reported
Experimental
31 1
by Arnold and co-workers: H NMR (300 MHz): d 9.13 (s, 8 H,
4
satellites JH–Sn = 11.1 Hz), 8.30–8.34 (m, 8 H), 7.80–7.86 (m, 12 H),
General procedures
−7.50 (br s, 2 H, Sn–OH).
Unless stated otherwise, reactions were carried out under an
atmosphere of argon using standard Schlenk techniques. THF,
hexane and diethyl ether were distilled from sodium benzophe-
Bis-dihydroxotin(IV) porphyrin 4. This compound was pre-
pared as described for 2, using bis-porphyrin, yield (89%) as a
1
purple solid: H NMR (300 MHz): d 9.25 (d, 4 H, J = 4.8 Hz),
none, CH
CaH and toluene was distilled from sodium under nitrogen.
NMR spectra ( H, P and C) were measured on a Bruker DRX
00 MHz and Varian Mercury 300 MHz; CDCl was used as
2
2
Cl , isopropanol and methanol were distilled from
4
9
.16 (s, 12 H, satellites JH–Sn = 13.3 Hz), 8.36 (m, 16 H), 7.91 (dd, 2
2
H, J = 5.4 and 3.3 Hz), 7.87–7.80 (m, 18 H), 7.66 (dd, 2 H, J = 5.4
1
31
13
and 3.3 Hz), 7.60 (d, 4 H, J = 8.9 Hz), 5.75 (s, 4 H), −7.38 (br s, 2 H,
3
3
1
3
Sn–OH); C-ATP (75.465 MHz) d 159.28, 147.26, 146.97, 146.94,
a solvent, if not further specified. Mass spectra were recorded
on a JEOL JMS SX/SX102A four sector mass spectrometer; for
FAB-MS 3-nitrobenzyl alcohol was used as matrix. Elemental
analyses were obtained on an Elementar Vario EL apparatus. Gas
chromatographic analyses were run on an Interscience HR GC
Mega 2 apparatus (split/splitless injector, J & W Scientific, DB-
1
1
1
46.89, 141.58, 136.76, 136.18, 135.53, 135.40, 134.50, 133.06,
32.96, 129.91, 129.64, 128.50, 128.48, 128.47, 127.25, 127.23,
25.79, 121.48, 121.41, 121.35, 113.81. MS (FAB+): m/z calcd for
+
C
96
H
67
N
8
O
6
Sn
2
([MH ]): 1663.3; anal. calcd for C96
H
66
N
8
O
6
2
Sn :
C, 69.25; H, 4.00; N, 6.73. Found: 69.52; H, 4.23; N, 6.36%.
ꢀ
ꢀ
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). UV-vis spectroscopy experiments were performed
on a HP 8453 UV/Visible System.
(S)-(1,1 -Binaphthol-2,2 -diyl)-(3-pyridyl) phosphite c. -Hy-
droxypyridine (1.44 g, 15.1 mmol), azeotropically dried with
toluene (3 × 5 ml), and triethylamine (2.3 ml, 16.6 mmol)
were dissolved in THF (40 ml) and the solution was cooled to
(
2
308 | Dalton Trans., 2007, 2302–2310
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◦
ꢀ
◦
−
40 C. Freshly prepared (S)-2,2 -binaphthol phosphorochlo-
chiral GC (Cyclosil-B, isothermal; T = 90 C, t
R
(R) = 63.5 min
ridite (5.3 g, 15.1 mmol) was dissolved in THF (20 ml) and added
dropwise. The cooling bath was removed and the solution was
allowed to warm to room temperature, stirring was continued for
h. The reaction mixture was filtered and the solvent evaporated.
A mixture of toluene–hexane 1 : 3 (40 ml) was added to extract the
and t
S
(S) = 64.8 min).
Asymmetric hydrogenation. Asymmetric hydrogenation reac-
tions were performed as follows. A stainless steel 150 ml autoclave,
equipped with 15 vessels and Teflon stirring bars in each vessel
(BPh )], 1.5 lmol of
phosphine, 1.5 lmol of porphyrin, 1.0 ll of dipea, 100 lmol
dimethyl itaconate and 50 lmol of decane in 0.5 ml of toluene. The
pressure was adjusted to 5 bar, without incubation. The mixture
was stirred for 15 h at 40 C. Then the autoclave was cooled to
C and the pressure was reduced to 1.0 bar. The conversion was
checked by GC measurement of the crude product after filtration
over a plug of magnesium sulfate and silica. Enantiomeric purities
1
was charged with 0.5 lmol of [Rh(nbd)
2
4
product. After filtration the solvent was removed in vacuo, giving
c (5.4 g, 13.2 mmol, 87%) as a white solid: H NMR (300 MHz):
1
d 8.54 (d, 1 H, J = 2.4 Hz), 8.40 (d, 1 H, J = 4.2 Hz), 8.02 (d, 1 H,
H
2
J = 8.7 Hz), 7.95 (d, 1 H, J = 8.7 Hz), 7.92 (d, 2 H, J = 8.7 Hz),
◦
3
1
13
7
.56–7.16 (m, 10 H); P NMR (121.5 MHz): d 143.05; C (75.465
◦
0
MHz): 148.88 (d, Jcp = 6.1 Hz), 147.52 (d, Jcp = 4.8 Hz), 146.84 (d,
J
(
(
1
1
(
cp = 1,4 Hz), 145.80 (s), 142.71 (d, Jcp = 7.3 Hz), 133.02 (s), 132.75
s), 132.01 (s), 131.55 (s), 130.97 (s), 130.36 (s), 129.29 (s), 128.67
were determined by chiral GC (Chirasil-L-Val, isothermal; T =
d, Jcp = 4.8 Hz), 128.48 (s), 127.945 (s), 127.83 (s), 127.28 (s),
◦
7
0 C, t
R
(R) = 34.4 min and t (S) = 35.2 min).
S
27.19 (s), 126.80 (s), 126.67 (s), 125.68 (s), 125.49 (s), 124.49 (s)
Allylic alkylation experiments. The allylic alkylation experi-
ments were performed as follows. Under Schlenk conditions 0.50
lmol of [Pd(allyl)Cl] , 3.0 lmol phosphite and 3.0 lmol phosphine
were dissolved in 5.0 ml of CH Cl and stirred for 30 min. Re-
spectively, 50 lmol 1,3-diphenyl-allyl acetate, 150 lmol dimethyl-
malonate, 150 lmol BSA (N,O-bis(trimethylsilyl)acetamide) and
0 lmol decane and a pinch of KOAc were added. The mix-
ture was stirred for 64 h at 25 C and subsequently stopped
by adding a saturated ammonium chloride solution of water.
21.77 (s), 121.59 (s). HRMS (FAB+): m/z calcd for C25
H
17NO
3
P
+
[MH ]): 410.0946; obsd: 410.0952; anal. calcd for C25
H
16NO P:
3
C, 73.35; H, 3.94; N, 3.42. Found: C, 73.20; H, 4.16; N, 3.25%.
2
2
2
ꢀ
ꢀ
(
R)-(1,1 -Binaphthol-2,2 -diyl)-(3-pyridyl) phosphite d. This
compound was prepared as described for c, using 5-(3-hydro-
xyphenyl)-10,15,20-tris(phenyl)porphyrin-zinc(II) and freshly pre-
pared (R)-2,2 -binaphthol phosphorochloridite. Yield (78%) as a
5
ꢀ
◦
1
purple-red solid: H NMR (300 MHz): d 8.54 (d, 1 H, J = 2.4 Hz),
8
.40 (d, 1 H, J = 4.2 Hz), 8.02 (d, 1 H, J = 8.7 Hz), 7.95 (d, 1 H,
◦
Subsequently, 5.0 ml of petroleum ether (40–60 C) was added
3
1
J = 8.7 Hz), 7.92 (d, 2 H, J = 8.7 Hz), 7.56–7.16 (m, 10 H);
P
and the solution was washed once more with a saturated NH
solution. The organic phase was dried over Na SO , filtered and
the conversion was checked by GC measurement. The solution
; petroleum ether–CH
to give analytically pure products. Enantiomeric purities were
determined by chiral HPLC (OD column, eluent 0.5% isopropanol
4
Cl
1
3
NMR (121.5 MHz): d 143.05; C-ATP (75.465 MHz): d 148.88
2
4
(
1
(
4
1
1
4
d, Jcp = 6.1 Hz), 147.52 (d, Jcp = 4.8 Hz), 146.84 (d, Jcp = 1,4 Hz),
45.80 (s), 142.71 (d, Jcp = 7.3 Hz), 133.02 (s), 132.75 (s), 132.01
was chromatographed (SiO
2
2
Cl
2
= 1 : 1)
s), 131.55 (s), 130.97 (s), 130.36 (s), 129.29 (s), 128.67 (d, Jcp
=
40
.8 Hz), 128.48 (s), 127.945 (s), 127.83 (s), 127.28 (s), 127.19 (s),
26.80 (s), 126.67 (s), 125.68 (s), 125.49 (s), 124.49 (s) 121.77 (s),
21.59 (s). HRMS (FAB+): m/z calcd for C25
10.0946; obsd: 410.0938; anal. calcd for C25
H, 3.94; N, 3.42. Found: C, 73.64; H, 4.38; N, 3.05%.
in hexane t
High-pressure NMR-experiments
In a typical experiment the high pressure NMR tube was filled
R
(R) = 33.2 min and t
S
(S) = 34.9 min).
+
H
17NO
3
P ([MH ]):
H
16NO
3
P: C, 73.35;
21
Catalysis
with (30 lmol) of [Rh(acac)(CO) ], (75 lmol) of phosphite zinc(II)
porphyrin, (75 lmol) phosphine and 1.5 ml of toluene-d . The tube
(1 : 1), pressurised
2
8
Hydroformylation experiments. The hydroformylation exper-
iments were performed as follows. A stainless steel 25 ml
autoclave, equipped with a Teflon stirring bar, was charged with
was purged three times with 15 bar of CO–H
2
3
9
◦
to approximately 20 bar, heated to 80 C and incubated for 1 h.
◦
Measurements were performed at 25 C.
◦
0
.42 lmol of [Rh(acac)(CO)
2
] (80 C) and [HRh(PPh
3
)
3
(CO)]
◦
ꢀ
(
25 C), 10.4 lmol of phosphine and 0.017 ml of N,N -
References
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was incubated for 1 h under 20 bar CO–H (1 : 1). The pressure was
reduced to 1 bar and a mixture of 0.34 ml 1-octene (styrene) and
.17 ml of decane in 0.67 ml of toluene was added. Subsequently
the CO–H pressure was pressurised to 20 bar. The mixture was
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◦
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◦
1
.0 bar. The samples withdrawn from the autoclave were analysed
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styrene was subjected to reduction with NaBH
4
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, filtration and removal of solvent gave the corresponding
alcohols, for which the enantiomeric purities were determined by
This journal is © The Royal Society of Chemistry 2007
Dalton Trans., 2007, 2302–2310 | 2309

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7
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6 Ligands c and d were prepared using similar methods as described by:
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40 Product characterisation is in agreement with those reported in the
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310 | Dalton Trans., 2007, 2302–2310
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