Encapsulated transition metal catalysts comprising peripheral Zn(II)salen building blocks: Template-controlled reactivity and selectivity in hydroformylation catalysis

Article abstract of DOI:10.1039/b503708e

Encapsulated phosphane ligands can be easily constructed through coordinative interactions between Zn(n)-salphen complexes and pyridylphosphane templates; the template has a pronounced impact on the catalyst structure and consequently on the performance in the hydroformylation of 1-octene. The Royal Society of Chemistry 2005.

Full text of DOI:10.1039/b503708e

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Encapsulated transition metal catalysts comprising peripheral
Zn(II)salen building blocks: template-controlled reactivity and
selectivity in hydroformylation catalysis{
Arjan W. Kleij,a Martin Lutz,b Anthony L. Spek,b Piet W. N. M. van Leeuwena and Joost N. H. Reek*a
Received (in Cambridge, UK) 14th March 2005, Accepted 13th May 2005
First published as an Advance Article on the web 10th June 2005
DOI: 10.1039/b503708e
As a model study, the interaction between diphenyl 4-pyridyl-
phosphane (P1) and Zn(II)–salphen 1 was studied by NMR and
UV-vis spectroscopy (Scheme 1). UV-vis titration experiments in
toluene revealed a very high association constant for the assembly
1?P1 (Kass 5 4.0 6 105 M21), whereas PPh3 showed no
measurable association to 1. This implies that the phosphorus
centre in 1?P1 is still available for selective coordination to other
metal fragments. Importantly, in contrast to similar porphyrin
assemblies the Zn–pyridine interaction is still of substantial
magnitude in more polar solvents, and NMR-titrations in
d6-acetone gave a binding constant of Kass 5 2.3 6 103 M21
for 1?P1. NMR analysis (d6-acetone) revealed a significant up-field
shift (Dd 5 0.26 ppm) for the pyr-Hortho proton of P1 upon
complexation to 2. The 31P{1H} resonance shifted only slightly
(Dd 5 1.50 ppm), which supported a selective Zn–pyridine
coordination. Single and sharp resonance patterns were observed,
which corroborate fast exchange processes on the NMR time
scale.
Encapsulated phosphane ligands can be easily constructed
through coordinative interactions between Zn(II)–salphen
complexes and pyridylphosphane templates; the template has
a pronounced impact on the catalyst structure and conse-
quently on the performance in the hydroformylation of
1-octene.
Coordination chemistry constitutes a highly useful tool in
supramolecular chemistry,1 and various assemblies with appealing
3D-structures based on various building blocks such as calixarene/
resorcinarene,2 porphyrin,3 or other components4 have been
reported. Molecular capsules represent an interesting class of
assemblies since the internal cavities can accommodate small
organic molecules. Chemical conversions applied within these
cavities is particularly fascinating, and molecular encapsulation has
ambiguously opened up new opportunities to control reactivity
and selectivity behavior.5
Previously, we have introduced a templated approach towards
encapsulated homogeneous catalysts using Zn(II)porphyrins.
These supramolecular assemblies comprised highly selective
Zn(II)–pyridine coordinative binding motifs.6 The variation of
the porphyrin building block in the supramolecular P-assembly
using tris(meta-pyridyl)phosphane as template was effectively used
to steer the reactivity and selectivity of the catalyst.6a,6b Here, we
introduce a new class of encapsulated catalysts based on the
assembly of different pyridyl–phosphane templates and Zn(II)–
salphen complexes through selective pyridine–Zn coordination
that has a higher stability constant than the porphyrin analogue.7
The structural features of these Zn–salphen building blocks can be
altered easily and therefore fine-tuning of the desired supramole-
cular assemblies is more straightforward as compared to the
porphyrin-based assemblies. More importantly, we will demon-
strate that the pyridylphosphane template structure (P2 vs. P3,
Scheme 1) has a large influence on the structural features of the
formed assembly and consequently on the catalytic performance in
the hydroformylation of 1-octene.
Higher-order assemblies were created with pyridylphosphanes
P2 and P3 (Scheme 1). UV-vis titration and NMR spectroscopic
experiments with P2/P3 and complex 1 supported the formation of
the 3 : 1 assemblies (1)3?P2 and (1)3?P3. We were unable to obtain
reliable association constants for these assemblies, but from the
titration curves we can estimate that they are in the same range as
that of 1?P1. The 1H NMR spectra (d6-acetone) for (1)3?P2
displayed a similar up-field shift for the pyr-Hortho protons
(Dd 5 0.29 ppm) as compared to assembly 1?P1, and a
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: +3120-5256422; Tel: +3120-5256437
bDepartment of Crystal and Structural Chemistry, Bijvoet Center for
Biomolecular Research, Utrecht University, Padualaan 8, 3584 CH,
Utrecht, The Netherlands. Fax: +3130-2533940; Tel: +3130-2532538
{ Electronic supplementary information (ESI) available: Titration curves,
experimental details, molecular modelling studies and displacement
ellipsoid plot for assembly (1)3?P2. See http://www.rsc.org/suppdata/cc/
b5/b503708e/
Scheme 1 Assembly formation between pyridylphosphanes P1–P3 and
Zn–salphen complexes 1–4 and schematic drawing of the 3 : 1 assemblies
([Zn])3?P2 and ([Zn])3?P3.
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Chem. Commun., 2005, 3661–3663 | 3661

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comparable, relatively small shift of the 31P{1H} resonance
(Dd 5 1.62 ppm). Likewise, the observed changes for (1)3?P3
were rather similar (CD2Cl2, Dd(pyr-Hortho) 5 0.38 and 0.33 ppm,
Dd 31P{1H} NMR 5 0.56 ppm). Crystals of assembly (2)3?P2
suitable for X-ray analysis8 were obtained from CH2Cl2–CH3CN
and the structure is visualised in Fig. 1.{ Although the assembly is
rather bulky, the phosphorus centre is not completely encapsu-
lated. Interestingly, one of the three salphen units has a different
orientation, with the chloro atoms pointing in another direction.
Obviously, such a preferred frozen arrangement in the solid state is
lost in solution as a result of the dynamic behavior of the Zn–Npyr
Molecular modelling (PM3 calculations, see supporting infor-
mation{) clearly shows the effect of the template on the
encapsulation efficiency of the phosphorus ligand. A much more
accessible phosphorus centre is available in assembly (2)3?P2 as
compared to (2)3?P3. Consequently, the difference in steric
demands upon complexation to a metal precursor between P2
and P3 should lead to preferential mono-phosphane or diphos-
phane metal complexes, which will show different catalytic
properties.6b
A 2 : 1 mixture of (1)3?P3 and Rh(CO)2(acac) in CD2Cl2 was
examined by 31P{1H} NMR and showed a doublet at d 5 43.3
(1J[Rh–P] 5 182 Hz) and a singlet line at 220.9 ppm (i.e., free
encapsulated (1)3?P3)) in an approximate 1 : 1 integral ratio. This
result supports the formation of a mono-phosphane ligated Rh-
species, as expected on the basis of the modelling results. Although
NMR studies with (1)3?P2 revealed the presence of two dipho-
sphane metal species with resonances at d 5 31.4 ppm (1J[Rh–P] 5
147 Hz) and d 5 30.3 ppm (1J[Rh–P] 5 139 Hz), also other species
could be observed.9 Mono- and diphosphane assignments were
corroborated by separate NMR experiments (CD2Cl2) with PPh3
interaction. The unit cell of (2)3?P2 clearly illustrates that the two
…
phosphane centres are proximally orientated (the shortest P
P
˚
distance is 7.0965(16) A, see Fig. 2). Also, the Zn(II)-complex 2 and
phosphane template P2 are preorganised and present in a 6 : 2
ratio suggesting that the formation of a diphosphane metal
complex should be feasible.
1
as ligand at 1 : 1 (d 5 50.4 ppm, J[Rh–P] 5 178 Hz) and 2 : 1
1
P : Rh ratios (d 5 33.3 ppm, J[Rh–P] 5 141 Hz).
In order to authenticate that diphosphane species based on P2
are feasible, NMR experiments with (COD)PtCl2 were carried out.
First, a 1 : 2 mixture of this salt with P2 was analysed showing the
formation of the bis-phosphane, cis-complex PtCl2(P2)2 with
1J(Pt–P) 5 3648 Hz. Subsequent addition of 6 equivalents of 1 to
this mixture gave rise to the exclusive formation of the assembly
1
PtCl2[(1)3?P2]2 with J(Pt–P) 5 2820 Hz, and no free phosphane
(1)3?P2 was observed. The coupling constant of 2820 Hz strongly
points to the presence of a trans-complex. Obviously, the steric
crowding induced by the encapsulation provokes this cis-to-trans
isomerism.
Fig. 1 Molecular structure of (2)3?P2 in the crystal shown from different
angles. Co-crystallized acetonitrile solvent molecules are omitted for
clarity. For numbering scheme and selected distances see supporting
information.{
The encapsulated phosphanes were used as ligands in the
rhodium-catalysed hydroformylation of 1-octene (20 bar syngas,
1 : 1 CO–H2) at ambient temperature (see Table 1).10 Entries 1–3
show that the monodentate ligands PPh3, P2 and P3 give rise to
the same expected selectivity and that the use of P2, the ligand with
the 4-pyridyl substituents, results in higher conversion. This is
consistent with our previous results11 and the higher reactivity is
explained by the electron-withdrawing effect of the 4-pyridyl
Table 1 Hydroformylation of 1-octene in toluene with ligand
assemblies based on P2 or P3 leading to linear (L) or branched (B)
aldehyde products. IS denotes isomerised internal olefin speciesa
Entry Ligand Conversion (%) IS (%)b L/B L (%) B (%)
1
2
3
4
5
6
7
8
9
a
PPh3
P2
P3
(1)3?P2 27
(3)3?P2 15
(1)3?P3 97
(2)3?P3 97
(3)3?P3 97
(4)3?P3 97
27
67
31
1.0
1.4
2.4
5.1
6.1
1.1
2.5
1.6
2.4
2.6
2.7
2.3
2.2
2.7
1.4
1.0
1.2
0.8
72
73
70
69
73
59
49
56
45
28
27
30
31
27
41
51
44
55
Reagents and conditions: Rh(acac)(CO)2 5 0.7 mM in toluene,
pressure 5 20 bar (CO : H2 5 1 : 1), 1-octene : rhodium 5 1052 : 1,
[P] 5 6.4 mM, reaction conditions: 65 h, 25 uC. Reactions were
performed in duplo. Total amount of isomerisation based on
Fig. 2 A plot showing the crystallographic unit cell containing two
template ligands (P2) encapsulated by six salphen building blocks (2 :
P2 5 6 : 2). The orange atoms represent the phosphane centres
(green 5 Zn, blue 5 N, red 5 O, gray 5 C, yellow 5 Cl).
b
converted 1-octene.
3662 | Chem. Commun., 2005, 3661–3663
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were kept fixed during refinement. All other H-atoms were refined as rigid
groups. 1270 refined parameters; no restraints. R (I . 2s(I)): R1 5 0.0421,
wR2 5 0.0794. R (all data): R1 5 0.0857, wR2 5 0.0958, S 5 1.031.
groups. The catalytic data show a number of interesting differences
between the use of ligand assemblies based on P2 and P3. The
assemblies based on P2 give rise to much slower rhodium catalysts
as compared to the parent phosphane P2, though the selectivity
characteristics of typical diphosphane-ligated hydroformylation
catalysts are preserved (cf. entries 1–5). These results clearly point
to the presence of a diphosphane species during catalysis. The
lower activity of the catalysts with the P2-assembly ligands as
compared to parent P2 is ascribed to the steric impact of the
Zn(II)–salphen complexes in the ‘second coordination sphere’ of
the rhodium metal centre. In contrast, the catalysts derived from
the P3-assemblies show much higher activity (at least a 4-fold
increase) than their non-assembled parent phosphane P3, PPh3
(entries 1 and 3 vs. 6–9) and the P2-based assemblies (entries 4–5).
In addition, a different selectivity behavior is noted using these P3-
assemblies, and in general higher amounts of branched aldehyde
(B) (up to 55%, entry 9) are formed. These results agree with the
presence of a mono-phosphane rhodium catalyst during cataly-
sis.6a,b,12 Clearly, the encapsulation of phosphane template P3
upon addition of Zn(II)–salphen building blocks is far more
effective than for template P2.
Residual electron density between 20.33 and 0.40 e A23. CCDC 251478.
˚
See http://www.rsc.org/suppdata/cc/b5/b503708e/ for crystallographic data
in CIF or other electronic format.
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In summary, we have demonstrated that Zn(II)–salphen
complexes are excellent building blocks for the construction of
catalytically active supramolecular assemblies based on coordina-
tive Npyr–Zn patterns. More importantly, small differences in the
phosphane template structure can be used to modify the catalytic
properties of the metal centre upon complexation to these salphen
structures and fine-tuning is viable through a proper choice of the
supramolecular building blocks (i.e., salphen and/or porphyrin
complexes). Particularly in the case of P3, encapsulation results in
effective shielding of the phosphorus atom. From the X-ray
structure it is clear that the assemblies based on P2 are sufficiently
open to allow the formation of a bis-phosphane metal complex.
We have successfully used this template effect in the hydro-
formylation of 1-octene and a substantial difference in the
regioselectivity and activity was observed between the use of P2-
and P3-assemblies. This can be attributed to the structural
difference of the intermediate Rh(I)-species (diphosphane ligation
for P2 vs. mono-phosphane ligation for P3), which is supported by
the NMR spectroscopic, X-ray crystallographic and modelling
studies. Currently, our research program is focused on the full
exploitation of these new phosphine assemblies to make extended
catalyst libraries and the optimisation of various organic
transformations.
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8 The crystallisation of assemblies of Zn–salphen complex 2 proved to be
easier than 1.
9 When 2 was used as Zn(II)–salpen complex in these NMR experiments,
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exclusive formation of a diphosphane species based on this Rh-
precursor can be interpreted as the influence of the cis-coordinating
mode of the bidentate acac-ligand, which effectively blocks a trans-
spanning of the two phosphane ligands through isomerisation.
10 Under the conditions applied the salphen building blocks are not
affected, which was confirmed in a separate control experiment.
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Notes and references
{ Crystal structure determination: Structure (2)3?P2: C99H96Cl6N9O6PZn3?
5CH3CN, FW 5 2152.90, yellow needle, 0.36 6 0.12 6 0.09 mm3,
¯
˚
triclinic, P1 (No. 2), a 5 14.4818(3), b 5 17.0579(3), c 5 23.2334(4) A,
3
˚
a 5 78.2385(6), b 5 87.6536(6), c 5 71.2122(9)u, V 5 5317.55(17) A , Z 5 2,
r 5 1.345 g cm23, m 5 0.90 mm21, 57108 measured reflections, 15149
unique reflections (Rint 5 0.082). Absorption correction based on multiple
measured reflections (correction range 0.83–0.92). Non H-atoms were
refined freely with anisotropic displacement parameters. H-atoms were
introduced in calculated positions. H-atoms of the acetonitrile molecules
This journal is ß The Royal Society of Chemistry 2005
Chem. Commun., 2005, 3661–3663 | 3663