Bis-(thiosemicarbazonato) Zn(II) complexes as building blocks for construction of supramolecular catalysts

Article abstract of DOI:10.1039/c2dt12096h

In this paper we report the application of bis-(thiosemicarbazonato) Zn(II) complexes as building blocks in the construction of supramolecular transition metal assemblies. We investigated their coordination behaviour towards pyridylphosphine molecules and found these systems comparable to those based on Zn(porphyrin) and Zn(salphen) complexes. Additionally, catalytic experiments and an in situ high-pressure FTIR study of the supramolecular rhodium hydroformylation catalysts, assembled using the bis-(thiosemicarbazonato) Zn(II) complexes, demonstrate their applicability in supramolecular catalysis and their potential for application in other areas of supramolecular chemistry. The Royal Society of Chemistry 2012.

Full text of DOI:10.1039/c2dt12096h

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PAPER
Bis-(thiosemicarbazonato) Zn(II) complexes as building blocks for
construction of supramolecular catalysts†
Vladica Bocokić,a Martin Lutz,b Anthony L. Spekb and Joost N. H. Reek*a
Received 3rd November 2011, Accepted 22nd December 2011
DOI: 10.1039/c2dt12096h
In this paper we report the application of bis-(thiosemicarbazonato) Zn(II) complexes as building blocks
in the construction of supramolecular transition metal assemblies. We investigated their coordination
behaviour towards pyridylphosphine molecules and found these systems comparable to those based on Zn
(porphyrin) and Zn(salphen) complexes. Additionally, catalytic experiments and an in situ high-pressure
FTIR study of the supramolecular rhodium hydroformylation catalysts, assembled using the bis-
(thiosemicarbazonato) Zn(II) complexes, demonstrate their applicability in supramolecular catalysis and
their potential for application in other areas of supramolecular chemistry.
blocks for the construction of all types of supramolecular assem-
blies and they are also frequently used for the construction of
porous solid materials.
1
Introduction
Supramolecular chemistry has evolved into a mature field of
science and a plethora of fascinating nano-sized structures have
been prepared by self-assembly. Whereas the initial focus was on
the generation of complex nano-sized structures and understand-
ing the principles involved in their formation, current research is
more often associated with the introduction of information (func-
tion) into these supramolecules.1,2 There is increasing interest in
the use of metal–ligand interactions as this strategy provides new
opportunities for the introduction of function. In addition,
metal–ligand interactions are ideal for the construction of supra-
molecular assemblies, since they are generally directional and
tunable in strength therefore facilitating the design of structures
with controlled geometries and dynamics. The supramolecular
structures constructed utilising these interactions include metal-
lodendrimers,3 molecular cages,4,5 catenanes and rotaxanes.6
The latter compounds were shown to be suited to the construc-
tion of molecular machines7 and such supramolecular structures
can also be of value in the area of light harvesting devices.8
An important research field that has attracted enormous inter-
est in recent years is that of controlled porous materials and
metal–organic frameworks (MOFs). The interest in these
materials is fuelled by (potential) application in separation tech-
niques, gas (hydrogen) storage, recognition, sensing, and
catalysis.9–23 Metalloporphyrins24 provide versatile building
In our group metalloporphyrins have been used as building
blocks to encapsulate transition metal catalysts.25–27 Recently we
also took advantage of the chromophoric character of porphyr-
ins: the assembly of Zn(II)porphyrins onto a functionalised bis
(thiolate)-bridged ([2Fe2S]) diiron-based hydrogenase catalyst
led to a supramolecular complex that displayed photo-activity
forming molecular hydrogen upon exposure to light in the pres-
ence of a proton source.28
Since the supramolecular approach to catalyst exploration is
very versatile we aimed at extending the concept to other build-
ing blocks. We demonstrated that the pyridyl analogues of classi-
cal BIAN-ligands could be used as the template ligand,29 and we
also showed that Zn(II)salphen building blocks can be used ana-
logously to Zn(II)porphyrins.30 These salphen building blocks
were much easier to prepare and were more structurally varied
than porphyrins, however, they were shown to be subject to
trans- and demetallation.31,32
We therefore continued our search for new building blocks
and looked into the chemistry of dithiocarbazones (Fig. 1) and
complexes thereof.33,34 Much to our surprise these building
blocks have not yet been studied as molecular building blocks in
supramolecular chemistry.
In this paper we report the application of the bis-(thiosemicar-
bazonato) Zn(II) (Zn(btsc)) complexes as building blocks in
supramolecular chemistry, i.e. their coordination chemistry with
respect to pyridine and pyridylphosphines, as compared, to the
Zn(II) building blocks we used previously. We show that bis-
(thiosemicarbazonato) Zn(II) complexes are stable building
blocks with coordination properties similar to Zn(II)porphyrins
and Zn(II)salphens. To demonstrate their applicability in the con-
struction of supramolecular transition metal catalysts, we applied
these complexes as building units of supramolecular rhodium
hydroformylation catalysts. With many heteroatoms in the
aUniversity of Amsterdam, van’t Hoff Institute for Molecular Sciences,
Science Park 904, 1098 XH Amsterdam, The Netherlands. E-mail:
J.N.H.Reek@uva.nl; Fax: (+31) (0)20 5255604;
Tel: (+31) (0)20 5256437
bUtrecht University, Faculty of Science, Bijvoet Center for Biomolecular
Research, Crystal and Structural Chemistry, Padualaan 8, 3584 CH
Utrecht, The Netherlands
†Electronic supplementary information (ESI) available. CCDC reference
numbers 799011–799012. For ESI and crystallographic data in CIF or
other electronic format see DOI: 10.1039/c2dt12096h
3740 | Dalton Trans., 2012, 41, 3740–3750
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Fig. 2 One-pot two-step protocol for the synthesis of dissymmetric Zn
(btsc) complexes.
Fig. 1 Structure of bis-thiosemicarbazone proligand (top left) and its
metal complex (top right). ZnTPP (bottom left) and Zn(salphen) (bottom
right) are commonly used building blocks in supramolecular chemistry.
2.1 Synthesis
The preparation of M(II)(btsc) complexes is rapid and facile. A
typical synthesis protocol consists of two steps: preparation of
the (pro)ligand by condensation of a dicarbonyl compound with
two thiosemicarbazide molecules, followed by metallation with
an appropriate metal salt.43,44 Condensation of two different thio-
semicarbazides to a dicarbonyl (leading eventually, to dissym-
metric M(btsc) complexes), can also be performed, but a good
control of reaction conditions during synthesis, is required to
avoid formation of unwanted side products.45
For our syntheses we used a slightly modified synthetic pro-
cedure as we did not isolate any of the intermediates. The con-
densation and metallation were performed in a one-pot two step
protocol under very mild conditions, as shown in Fig. 2. For this
initial study three exemplary dissymmetric Zn(btsc) complexes
9–11 with different peripheral substituent sizes were prepared in
good to high yields (70–80%) and purity, (≥95% by NMR).
molecule, most notably sulfur, which may act as a catalyst
poison, it is important to demonstrate the compatibility of the Zn
(btsc) complexes with the transition metal atom. Formation of
the catalyst and confirmation of its activity in hydroformylations
are therefore important goals of this paper. By using in situ high-
pressure infrared spectroscopy we show the formation of the bis-
phosphine ligated rhodiumhydrido species, commonly formed
under hydroformylation conditions. The catalytic activity of this
species, was evaluated using a showcase set of linear terminal
and internal alkenes as substrates in catalysis. The results of our
investigations suggest a great potential for application of these
Zn(II)btsc complexes in the area of supramolecular chemistry,
which herein is demonstrated with examples in the area of supra-
molecular catalysis.
2
Results and discussion
Tetradentate bis-thiosemicarbazone (btsc) (pro)ligands and their
transition metal complexes have been known for over 50 years.34
The exceptional stability of these complexes, manifested in
reversible protonation of the amido side arms but not demetalla-
tion upon treatment with concentrated sulfuric (phosphoric and
perchloric as well) acid, was exploited by using these protonated
metal complexes as cations in synthesis of complex salts.35 In
our stability experiment, we exposed it to conditions under
which demetallation of the Zn(II)salphen, occurred easily.32 Con-
trary to Zn(II)salphens,‡ no demetallation was observed when 3
equivalents of benzimidazole and a Zn(btsc) complex were
mixed, neither upon mixing, nor after 18 h of heating at 75 °C
(see Experimental section). Besides being highly stable, their
(transition) metal complexes are biologically active and many
studies have shown that they display antitumor, antibacterial and
antiviral properties.36–38 Potential medical application could be
found in the affinity of Cu(II)(btsc) complexes towards oxygen–
poor (cancer) tissue, and applications of M(II)(btsc) complexes
(M = Zn, Cu) as tumour marker agents have already been
demonstrated.39–42
2.2 Coordination studies
2.2.1 Assembly in solution. The essential element of the
ligand–template approach for encapsulation of transition metal
catalysts is the coordination of a pyridyl moiety to the Zn(II)
centre.46 The strength of this association is therefore crucial for
successful assembly formation, and for low (catalytic) concen-
trations the association constant should be high.
The association constant of pyridine to a Zn(II) building block
is often used as a guideline value for the family of similar
systems. By performing a UV-Vis titration of the complex 11
with pyridine (see Fig. 3) we found that their association con-
stant falls roughly in the order of magnitude of the Zn(II)
salphen–pyridine interaction, with K = 1.6 × 105 l mol−1 26,47–49
.
This high K value makes the Zn(btsc) complexes interesting for
the application as supramolecular building units.50,51
2.2.2 Assemblies in the solid state. To further confirm the
coordination of the phosphine–pyridyl ligands§ to these Zn(II)
building blocks, the coordination complexes were crystallized
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of, the plane defined by the two complexes. This 2 : 2 assembly,
is strikingly similar to a 1 : 2 assembly Kuil et al. reported,
where two L2 molecules coordinate to Zn centres on opposite
sides of a bis-Zn(II)–salphen complex.49 The Zn⋯Zn distances
in the current structures are 8.7803(8) Å (9·L3) and 9.185(2) Å
(9·L2) which is similar to the Zn⋯Zn distance in the 1 : 2 as-
sembly. It would be interesting to use these self-assembled
dimers as template to bring two pyridylphosphines together.
However there is no evidence yet that such structures form N–
H⋯S bonds in solution, as we did not observe any effect typical
for formation of a supramolecular bidentate ligand on a bis-Zn
platform in asymmetric hydroformylation of styrene, as was
described previously.49
2.3 In situ high-pressure infrared spectroscopy and catalysis
The assemblies of Zn(btsc) complexes with L2–L4 are essen-
tially monodentate ligands, with unconventional steric bulk
remote from the phosphorus atom and the transition metal. They
are also electronically different from a typical monodentate
ligand PPh3 (L1) and it was anticipated that the combination of
the unusual steric and electronic properties of these ligands
would have an impact on catalysis.
In a typical hydroformylation experiment Rh(acac)(CO)2 and
the ligands are mixed in situ to form the active species.¶ We
monitored the formation of rhodium supramolecular catalyst
in situ from Rh(acac)(CO)2, tris-(m-pyridyl)phosphine L4, and
the Zn(btsc) complex 9,k using infrared spectroscopy.57,58 The
IR spectrum we obtained is similar to that of the triphenylpho-
sphine analogue. Four bands of similar intensities at 2071, 2056,
2005 and 1970 cm−1 were observed, indicating formation of the
ee and ea isomers (Fig. 6) in approximately equimolar
amounts.59
Fig. 3 UV-Vis changes during titration are small, but sufficient to
produce a well-behaved titration curve (insert), which was used to calcu-
late the association constant.
The shoulder at about 1920 cm−1 might be the rhodium-
hydride (Rh–H) stretch, which is very rarely observed due to its
weak intensity.60
For comparison, application of ZnTPP instead of Zn(btsc)
leads exclusively to the formation of the mono-ligated rhodium
complex, since the assembly L4·(ZnTPP)3 is so large that only
one phosphorus can coordinate to rhodium.25
This indicates that the significant difference in size between
the Zn(btsc) and ZnTPP is reflected in the phosphine coordi-
nation mode allowing formation of the bis-phosphine rhodium
species with Zn(btsc) as building block.
The molecular modelling figure of the ea and the ee isomers,
Fig. 7, shows that there is no significant steric hindrances
between the various building blocks in the bisphosphine
rhodium species. Numerous phosphine conformations and large
rotational freedom around the N–Zn coordinative bond convey
rather considerable flexibility to the assembly allowing facile
avoidance of steric crowding.
That these rhodium species were also active catalysts for
hydroformylation was confirmed by addition of substrate to the
IR autoclave. Injection of 1-octene into the autoclave triggered
the immediate start of catalysis, with an initial turnover fre-
quency (TOF) of about 60 h−1, which is significantly faster than
observed with triphenylphosphine under similar conditions
(4–15 h−1).26,30,61 The four carbonyl bands of the rhodium
from toluene solution by hexane diffusion via the gas-phase. The
crystal structures of assemblies of complex 9 with 3- and 4-
(diphenylphosphino)-pyridine (L2 and L3 Fig. 3a) show axial
coordination of the pyridyl moiety to the Zn(II) centre, which
adopts a square-pyramidal geometry bending slightly out of the
SNNS–plane (see Fig. 4). Importantly, the lone pair of either
phosphorus atom is not involved in coordination, and therefore
could be used for coordination to (soft) transition metals in the
construction of supramolecular catalysts. An interesting feature
of both assemblies is formation of dimeric structures via two N–
H⋯S bonds per dimeric unit, similar to Zn(btsc) dimers Alsop
et al. reported (Fig. 4).52 The N⋯S distances of atoms involved
in hydrogen bonding are typical for such structures (3.702(11) Å
in 9·L2 and 3.463(4) Å in 9·L3).52 Small inserts in Fig. 4
display stacking of the Zn(btsc) complexes with distance
between the Zn–N–CC–N planes of the two stacking units
around 3.5 Å (3.560(4) Å in 9·L2 and 3.5295(18) Å in 9·L3),
suggesting existence of “π–π” interactions, not unusual between
layers of flat aromatic molecules.53,54 In these dimers the Zn
(btsc) complexes are almost perfectly coplanar while the pyridyl-
phosphine moieties coordinate to Zn centres on opposite sides
3742 | Dalton Trans., 2012, 41, 3740–3750
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Fig. 4 Solid state structures of the extended assemblies. Thermal motion ellipsoids drawn at 50% probability. H atoms and co-crystallised solvent
molecule (toluene in 9·L3) have been removed for clarity. Colour legend: C = white, N = blue, S = yellow, P = orange and Zn = magenta. Dimeric
super-structures are formed via a pair of N–H–S hydrogen bonds. Pyridylphosphines coordinate on opposite sides of the [Zn(btsc)]2-plane. π –π stack-
ing shown in small inserts.
Fig. 5 Generally accepted rhodium hydroformylation catalyst acti-
vation under syngas atmosphere.55
Fig. 7 Molecular models (PM3, Spartan) of the ea and the ee rhodium-
hydrido complexes. Although bulky, the supramolecular ligand allows
formation of bis-ligated rhodium species. Colour legend: C = white (in
9) or grey, H = white ball, O = red, N = blue, P = orange, Zn = magenta
and Rh = turquoise; for clarity, Zn(btsc) molecules were rendered partly
as wire.
species did not change during catalysis, showing that the
rhodium hydride complexes were the resting state of the catalyst.
This, together with the rate order in respect to the substrate being
1 (see insert in Fig. 8), indicated that the hydroformylation
follows type I kinetics.**
A more extensive set of catalytic experiments was performed
using Zn(btsc) complexes 9–11, ligands L1–L4, and substrates
shown in Fig. 9. As expected, the conversions of internal
alkenes were lower than those of terminal alkenes for all catalytic
systems. Additionally, the PPh3-based rhodium catalysts dis-
played lower conversions than those with pyridyl–Zn(btsc) moi-
eties. The conversion was higher because the phenyl groups on
P were replaced by pyridyl–Zn(btsc) motifs see Fig. 10, reflect-
Fig. 6 Carbonyl region of the in situ IR spectrum showing the four
bands of the ee and ea rhodium isomer mixture. The weak band at
2135 cm−1 is the free CO, and the shoulder at around 1920 cm−1 might
ing the increasing withdrawal of the electronic density from
be the Rh–H stretch, p = 20 bar, CO/H2 = 1 : 1,T = 40 °C.
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rhodium. Unexpectedly, the presence of three equivalents of Zn
(btsc) 9 in hydroformylation experiment using L1, (entries 2, 12,
and 24 in Table 1), which has no positions for Zn(btsc) coordi-
nation, has also led to conversion higher than achieved only with
L1. Importantly, these experiments ultimately show that not only
the Zn(btsc) complexes do not poison the catalyst, but that their
presence is actually beneficial for catalysis, even if there is no
ligand-provided coordination site as in L2–L4. The origin of this
effect is not clear yet, however, it is possible that the Zn(btsc)
complexes contribute to electron-depletion of the rhodium
centre. The Zn(II) centre may coordinate to the O of a Rh-associ-
ated carbonyl functionality,†† withdrawing electron density via
the σ bonds. In experiments with Zn(btsc)·L2–L4, the pyridyl-
phosphine itself is electron-poorer than PPh3 due to the electro-
negative nitrogen atom in the ligand; additionally, coordination
of Zn(btsc) complexes to N leads to further electron-depletion of
the ligand and rhodium centre. In consequence, conversion
numbers in experiments with the supramolecular ligands are
higher than those achieved using PPh3 or PPh3/Zn(btsc) system.
Generally accepted62 effects of reduced back-donation from
Rh to the coordinated CO or alkene are:
• weaker back-donation from Rh to the coordinated alkene,
facilitating rotation around Rh–alkene σ-bond and lowering the
hydride migration barrier.
Such effects directly cause higher catalytic activity, but also
higher alkene isomerisation rate. This is well documented in the
case of phosphites, which display both effects (increase of iso-
merisation and hydroformylation rate).63–68 In our case, the elec-
tronic effects are much smaller than with the phosphites, and we
only see a slight increase in conversion. Decreased back-
donation to the antibonding CO-orbitals leads to stronger CO
bonds of the coordinated CO molecules and as a consequence to
higher frequencies of the CO vibrations in the IR spectrum.69
Indeed we find this experimentally: the HP-IR spectrum of the
PPh3-based rhodium catalyst displays bands 15–30 cm−1 lower
than the supramolecular catalyst (PPh3: 2055, 2030, 1989 and
1941 cm−1; L4·93: 2071, 2056, 2005 and 1970 cm−1, see com-
parison of spectra in Fig. 12, Experimental section).
Similar wavenumber differences were previously reported
with systems using xantphos ligands bearing substituents with
various electron-withdrawing properties.56
The selectivity of the current encapsulated catalyst is virtually
identical to PPh3-based rhodium catalysts. Thus, the application
of Zn(btsc) building blocks for supramolecular rhodium catalysts
• facilitated CO dissociation from the rhodiumhydrido resting
state and entry of the catalyst into the catalytic cycle
Fig. 10 Comparison of octene hydroformylation with various ligands.
Conversions are higher in systems containing more [Zn(btsc)–pyridyl]
moieties.
Fig. 8 Hydroformylation of 1-octene is typical for type I kinetics, with
the rate order in respect to the substrate being 1 (here 0.97, see insert).
Fig. 9 Hydroformylation of 1-, trans-2- and trans-3-octene.
3744 | Dalton Trans., 2012, 41, 3740–3750
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Table 1 Hydroformylation of terminal and internal octenes
Entry
Alkene
Ligand
Zn(btsc)
[Zn]/[L]
Conv. (%)
Iso. (%)
PI
P2
P3
P4
Pn/P(n+1)
1
2
3
4
5
6
7
8
1-octene
1-octene
1-octene
1-octene
1-octene
1-octene
1-octene
1-octene
1-octene
1-octene
2-octene
2-octene
2-octene
2-octene
2-octene
2-octene
2-octene
2-octene
2-octene
2-octene
2-octene
2-octene
3-octene
3-octene
3-octene
3-octene
3-octene
3-octene
3-octene
3-octene
LI
LI
L2
L2
L3
L3
L4
L4
L4
L4
LI
LI
L2
L2
L3
L3
L4
L4
L4
L2
L3
L4
LI
LI
L2
L3
L4
L4
L4
L4
—
9
9
—
9
—
—
9
10
11
—
9
—
3
1
—
1
—
—
3
3
3
—
48
60
69
55
71
57
63
91
84
89
11
17
18
13
19
12
21
35
27
23
22
34
13
19
30
32
24
46
38
39
1.1
1
≤0.1
1
≤0.1
1
1.2
0.4
0.4
74
75
69
73
70
74
75
69
73
70
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
25
24
31
26
30
25
24
30
27
29
56
55
56
56
57
56
56
59
56
56
56
56
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
44
45
44
44
43
44
44
41
44
44
44
44
50
50
50
50
50
50
50
50
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
50
50
50
50
50
50
50
50
3.0
3.1
2.2
2.8
2.3
2.9
3.1
2.3
9
2.7
2.4
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
0.5
≤0.1
≤0.1
≤0.1
≤0.1
≤0.1
≤0.1
≤0.1
≤0.1
≤0.1
≤0.1
≤0.1
0.2
≤0.1
≤0.1
≤0.1
≤0.1
≤0.1
≤0.1
≤0.1
≤0.1
1.27
1.27
1.27
1.27
1.32
1.27
1.27
1.44
1.27
1.27
1.27
1.27
1.00
1.00
1.00
1.00
1.00
1.00
1.00
1.00
3
1
—
9
—
9
1
—
—
9
10
11
11
11
—
9
9
9
—
9
10
11
—
—
3
3
1
1
3
—
3
1
1
—
3
3
3
Conditions: [Rh] = 0.70 mmol l−1, [P]/[Rh] = 5.0, [Zn]/[equiv. pyridyl group] = 1.0, [1-octene]/[Rh] = 1000, 16 h; [trans-2-octene]/[Rh] = [trans-3-
octene]/[Rh] = 500, 24 h; T = 40 °C, p = 20 bar, CO/H2 = 1 : 1. No base added.
leads to increased conversions in hydroformylation of terminal
and internal octenes, while preserving the selectivity typical for
PPh3-based catalysts.
observed with PPh3. The differences in reactivity are likely due
to electronic properties, as the supramolecular ligands are more
electron-deficient than PPh3.
We have thus successfully demonstrated the applicability of
Zn(btsc) complexes as building blocks for the construction of
supramolecular ligand assemblies and rhodium hydroformylation
catalysts thereof. A particularly interesting aspect, and a great
potential of these complexes, is their structural variability and
possibility for post-synthetic modifications. The opportunity to
access a stable building block that is easy to modify makes these
compounds very attractive for further exploration in supramole-
cular chemistry, also including applications other than catalysis.
3
Summary and conclusion
We have studied the application of a small set of bis-(thiosemi-
carbazonato) Zn(II) complexes as building blocks for construc-
tion of supramolecular complexes; in particular as building
blocks that associate to ligand-templates to encapsulate transition
metal complexes for catalysis. The coordination behaviour of
these complexes is similar to that of Zn(II)–salphens, with the
pyridine–Zn(btsc) association constant of K = 1.6 × 105 l mol−1
,
two orders of magnitude higher than those displayed by related
Zn(II)porphyrin–pyridine systems. In addition, the current com-
plexes are significantly more stable than the Zn(II)–salphen com-
plexes, especially concerning demetallation under acidic
conditions.
Under syngas atmosphere, the supramolecular ligands formed
by these bis-(thiosemicarbazonato) Zn(II) complexes and tris-
pyridyl-phosphine ligand building blocks react with a rhodium
precursor giving active hydroformylation catalysts that display
type I kinetics. According to HP-IR the encapsulation does not
lead to phosphine dissociation as bisphosphine complexes are
formed. The selectivity obtained in the hydroformylation of
linear alkenes is typical for the PPh3-based rhodium catalysts,
however, the activity is about four times higher than that
4
Experimental
Solvents used for the syntheses of Zn(btsc) complexes were used
as obtained from supplier. Solvents used in catalytic, spectro-
scopic (HP-FTIR, UV-Vis) and crystallisation experiments were
dried and freshly distilled prior to use: toluene and hexane were
distilled over Na, CH2Cl2 over CaH2. All deuterated solvents
except DMSO were prepared by freeze–pump–thaw technique
and kept under 3–4 Å molecular sieves. Reagents 1–3, 7, 8, L1
and Zn(CH3COO)2·2H2O were purchased from Aldrich and used
without further purification. Substrates for catalysis and high-
pressure IR were degassed by dinitrogen bubbling and filtered
over a short plug of alumina prior to use. Phosphine ligands
were prepared according to the previously published
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procedures.70 NMR spectra were acquired on the Varian
Mercury-VX 300, Bruker DRX300 (1H at 300 MHz, 31P at
100 MHz, and 13C at 75 MHz), and Bruker ARX400 (1H at
400 MHz, 31P at 125 MHz, and 13C at 100 MHz). The reson-
ances are referenced to solvent itself as an internal standard and
are reported in parts per million (ppm). Mass spectrometry (MS)
measurements were performed using a fast atom bombardment
(FAB) ionisation mode. IR spectra were recorded on the Nicolet
Nexus 670 FT-IR spectrometer operated by Omnic 6.2 Software;
UV-Vis, measurements were performed on the Hewlett–Packard
8453 spectrometer; gas chromatography (GC) analysis were
done on the Shimadzu GC-17A chromatograph equipped with
an FID detector using a 30 mm long column with 0.32 mm
diameter and dimethylsiloxane cross-linked phase of 3 μm thick-
ness. Catalytic experiments were performed in mini-4-autoclaves
connected to the same high-pressure line, allowing all four reac-
tions to be ran under the same pressure. Before each run the
autoclaves were evacuated, flushed with nitrogen, and tested for
leaks at ca. 35 bar syngas. Molecular graphics images were pro-
duced using the UCSF Chimera package from the Resource for
Biocomputing, Visualization, and Informatics at the University
of California, San Francisco.71
25 °C): δ 175.7 (CvO), 152.1 (C), 138.4 (C), 127.5 (2C),
127.4, 124.3 (3C), 24.5 (CH3), 17.4 (CH3). MS (FAB+): m/z =
235.0663 found, calculated: 235.080.
4.1.3 Compound 7. This compound was prepared as
described by Bost and Smith.72 Benzil, 2, (2.00 g, 9.51 mmol),
and thiosemicarbazide 4 (1.60 g, 9.51 mmol) were treated with
1 ml of glacial acetic acid in ethanol under reflux for 2 h. A
creamy precipitate formed upon cooling which was collected, by
filtration, washed with ice-cold ethanol (3 × 10 ml) and dried
in vacuo. Yield: 2.33 g (86%) of fine white powder. NMR analy-
sis showed ≥95% purity. The product was used further without
1
additional purification. H NMR (300 MHz, DMSO-d6, 25 °C):
δ 12.23 (s, 1H, NH), 8.45 (s, 1H, NH), 7.62 (d, 3 J = 7.4 Hz,
3
2H, HAr), 7.39–7.14 (m, 12H, HAr), 6.11 (d, J = 7.4 Hz, 1H,
HAr). 13C{1H} NMR (75 MHz, DMSO-d6, 25 °C): δ 170.8
(CvO), 146.1, 140.9, 138.8, 134.3, 132.1. 131.5, 130.4, 129.6,
129.5, 128.7, 128.1, 127.8 (2C), 127.7 (4C), 127.6, 127.1, 126.5
(C). MS (FAB+): m/z = 359.1096 found, calculated: 359.1100.
4.1.4 Complex 9. This complex was prepared according to
43
procedure described by Cowley et al. Compound 8 (253 mg,
2.12 mmol) was dissolved in tetrahydrofurane (THF) and treated
with glacial acetic acid (0.2 ml) for ca. 20 min. 6 (500 mg,
2.12 mmol) was added to the solution and the mixture was
stirred at room temperature next 3 h. Zinc(II) acetate dihydrate
(559 mg, 2.54 mmol) was dissolved in 15 ml methanol and then
added to the reaction mixture, followed by triethylamine
(0.74 ml, 5.30 mmol). After stirring overnight at room tempera-
ture the solvents were evaporated and the residue suspended in
ca. 10 ml methanol. The yellow solid was filtered off, washed
with methanol (3 × 5 ml) and dried in vacuo. Yield: 647 mg
(76%) of yellow powder, whose purity was estimated ≥95% by
4.1 Syntheses
4.1.1 Compound 5. It was prepared as described by Cowley
et al.43 Thiosemicarbazide 3 (1.00 g, 9.51 mmol) was dissolved
in water (17 ml) and treated with 5.5 ml conc. HCl at 10 °C in a
100 ml round-bottom flask. A cooled solution of diketone 1
(1.0 ml, 11.85 mmol) in water (20 ml) was added dropwise
under vigorous stirring, with white bulky precipitate forming
immediately. After 2 h of stirring under ice cooling, the product
was collected by filtration, washed twice with 20 ml cold water,
once with 15 ml of cold diethyl ether and dried in air. Yield:
1.33 g (81%) of white powder. NMR analysis of the product
showed ≥95% purity and it was used further without additional
1
NMR. H NMR (300 MHz, DMSO-d6, 25 °C): δ 9.35 (s, 1H,
3
5
NH), 7.57 (dd, J = 8.2 Hz, J = 1.2 Hz, 2HAr), 7.31 (t, 3J = 8.0
3
5
Hz, 2H, 2HAr), 7.04 (dt, J = 7.6 Hz, J = 1.2 Hz, HAr), 3.23 (s,
6H, N(CH3)2), 2.31 (s, 6H, CH3), 2.25 (s, 6H, CH3). 13C{1H}
NMR (75 MHz, DMSO-d6, 25 °C): δ 178.2, 172.4, 149.1,
144.0, 141.2, 128.3 (2C) 121.2, 119.7 (2C), 39.6 (2C), 14.7,
13.8. MS (FAB+): m/z = 399.0396 found, calculated: 399.0404.
1
purification. H NMR (300 MHz, DMSO-d6 25 °C): δ 10.63 (s,
1H, NH), 8.61 (s, br, 1H, CH3NH), 3.04 (s, br, 3H, CH3NH),
2.40 (s, 3H, CH3CO), 1.94 (s, 3H, CH3CN). 13C{1H} NMR
(75 MHz, DMSO-d6, 25 °C): δ 197.2 (CvO), 178.6 (C), 145.2
(C), 31.1 (CH3), 24.5 (CH3), 9.7 (CH3). MS (FAB+): m/z =
173.0491 found, calculated: 173.0600.
4.1.5 Complex 10. This complex was prepared analogously
to complex 9 from 8 (119 mg, 1.00 mmol), 5 (173 mg,
1.00 mmol), and Zn(II) acetate dihydrate (231 mg, 1.05 mmol);
glacial acetic acid and triethylamine were added in correspond-
ing amounts as described above. Yield: 260 mg (77%) of yellow
4.1.2 Compound 6. Prepared analogously to compound 5.
Thiosemicarbazide 4 (2.51 g, 15.00 mmol) was partially dis-
solved in water (60 ml) and treated with 0.5 ml 6 M HCl at 10°
C in a 200 ml round-bottom flask. A cooled solution of 1
(1.65 ml, 18.75 mmol) in water (30 ml) was added dropwise to
the solution of 4 under vigorous stirring. White bulky precipitate
formed immediately. More water (50 ml) was added to facilitate
stirring. After 1.5 h of stirring under ice cooling, the product was
collected by filtration, washed twice with 30 ml cold water, once
with 25 ml of cold diethyl ether and dried in air. Yield: 3.23 g
(91%) of white powder. NMR analysis showed the product
purity ≥95%. It was used further without additional purification.
1H NMR (300 MHz, DMSO-d6, 25 °C): δ 10.34 (s, 1H, NH),
1
powder, NMR purity ≥95%. H NMR (300 MHz, DMSO-d6,
25 °C): δ 7.20 (s br, 1H, NH), 3.17 (s, 6H, N(CH3)2), 2.81 (s,
3H, NHCH3), 2.17 (s, 6H, 2 × CH3). 13C{1H} NMR (100 MHz,
DMSO-d6, 25 °C): δ 178.0 (2C) 145.0 (2C), 48.9, 29.6 (2C),
14.2, 14.1. MS (FAB+): m/z = 337.6256 found, calculated:
337.6300.
4.1.6 Complex 11. The reactants 8 (50 mg, 0.42 mmol) and
7 (151 mg, 0.42 mmol) were stirred in THF (ca. 7 ml) at 50 °C
for 2 h. The reaction mixture was cooled to room temperature,
then Zn(II) acetate dihydrate (92 mg, 0.46 mmol) dissolved in
5 ml methanol was added. After stirring over night at room
temperature the solvents were evaporated, the red residue was
suspended in ca. 5 ml cold methanol, filtered off, washed twice
3
3
9.82 (s, 1H, NH), 7.59 (d, J = 7.6 Hz, 2H, HAr), 7. 36 (t, J =
3
7.4 Hz, 2H, HAr), 7.15 (t, J = 7.4 Hz, 1H, HAr), 1.99 (s, 3H,
CH3), 1.07 (s, 3H, CH3). 13C{1H} NMR (75 MHz, DMSO-d6,
3746 | Dalton Trans., 2012, 41, 3740–3750
This journal is © The Royal Society of Chemistry 2012

View Article Online
with stirring bars, and set under a dinitrogen atmosphere. To
them ca. 3 ml toluene was added and the mixture was warmed to
50 °C under stirring. Additional toluene was added untill every-
thing was fully dissolved. The hot solution was quickly filtered
through an HPLC filter into a Pasteur pipette‡‡ (ca 1.2 ml), in a
tall Schlenk-tube, all under dinitrogen. Hexane was added (ca.
15 ml) into the bottom of the Schlenk-tube (outside of the
pipette), which was then tightly closed. It was kept at ambient
temperature and protected from light. After two weeks first crys-
tals appeared on inner wall of the pipette (the upper part), which
grew to sufficient size in the course of next three weeks.
4.2.2 X-Ray crystal structure determinations. X-Ray inten-
sities were measured on a Nonius KappaCCD diffractometer
with a rotating anode and a graphite monochromator (λ =
0.71073 Å) at a temperature of 150(2) K. Intensity data were
integrated with the Evall473 (assembly 9·L2) or HKL200074
(assembly 9·L3) software. Absorption correction and scaling
was performed with SADABS.75 The structures were solved
with Direct Methods using the programs SHELXS-9776 (assem-
bly 9·L2) or SIR-9777 (assembly 9·L3). Both structures were
refined with SHELXL-9776 against F2 of all reflections. Hydro-
gen atoms were introduced in calculated positions and refined
with a riding model. Geometry calculations and checking for
higher symmetry was performed with the PLATON program.78
4.2.2.1 Assembly 9·L2. C31H32N7PS2Zn, Mw
= 663.10,
3
ˉ
yellow needle, 0.20 × 0.08 × 0.06 mm , triclinic, P1, a =
11.1142(8), b = 11.1774(9), c = 13.0986(8 Å, α = 81.434(3), β =
94.669(4), γ = 78.433(2)°, V = 1573.1(2) Å3, Z = 2, Dc
=
1.400 g cm−3, μ = 1.00 mm−3. 13454 reflections were measured
up to a resolution of (sinθ/λ)max = 0.50 Å−1. A large anisotropic
mosaicity of 1.6° was used for the integration of this weakly dif-
fracting crystal. 3373 Reflections were unique (Rint = 0.056), of
which 2429 were observed (I > 2σ(I)). 383 parameters were
refined with no restraints. R1/wR2 (I > 2σ(I)): 0.0867/0.1990.R1/
wR2 (all refl.): 0.1219/0.2191. S = 1.071. Residual electron
Fig. 11 NMR spectra showing that neither demetallation nor decompo-
sition occur when benzimidazole is heated with the Zn(btsc) complex
10.
with methanol and dried under vacuum. Yield: 150 mg (68%) of
a bright red solid, NMR purity ≥95%. 1H NMR (300 MHz,
DMSO-d6, 25 °C): δ 9.47 (s, 1H, NH), 7.60 (d, 3 J = 8.0 Hz,
density between 0.56 and 2.23 e Å−3
.
3
2H, HAr), 7.34–7.19 (m, 10H, HAr), 7.08 (t, J = 7.8 Hz, 2H,
4.2.2.2 Assembly 9·L3. C31H32N7PS2Zn·C7H8, Mw = 755.23,
3
HAr), 6.83 (t, J = 7.7 Hz, 2H, HAr), 3.17 (s, 6H, N(CH3)2). 13C
3
ˉ
orange triangular prism, 0.36 × 0.27 × 0.21 mm , triclinic, P1, a
{1H} NMR (75 MHz, DMSO-d6, 25 °C, δ): 180.4, 173.7, 150.2,
143.8, 141.4 (2C), 134.8 (2C), 134.0 (2C), 130.8 (2C), 130.5
(2C), 129.1, 128.8 (2C), 128.0 (2C), 127.8, 122.2, 120.6, 39.6
(2C). MS (FAB+): m/z = 522.0586 found, calculated: 522.0600.
= 12.9582(3), b = 13.0266(2), c = 13.1409(3) Å, α = 115.9892
(8), β = 93.8687(10), γ = 107.0963(9)°, V = 1855.38(7) Å3, Z =
2, Dx = 1.352 g cm−3, μ = 0.85 mm−1. 21422 Reflections were
measured up to a resolution of (sinθ/λ)max = 0.57 Å−1. 5715
Reflections were unique (Rint = 0.045), of which 4719 were
observed (I > 2σ(I)). 447 Parameters were refined with no
restraints. R1/wR2 (I > 2σ(7)): 0.0616/0.1526. R1/wR2 (all refl.):
0.0758/0.1615. S = 1.037. Residual electron density between
4.1.7 Stability test. The complex 10 (0.075 mmol, 25.3 mg)
was mixed with benzimidazole (4 equiv. (35.44 mg) in DMSO-
d6, and 5 equiv. (44.30 mg) in acenotinitrile-d3). NMR spectra
were recorded immediately at room temperature, and after 18 h
at 75 °C.
0.56 and 1.18 e Å−3
.
No colour change, precipitation or appearance of the free
ligand signals in NMR (Fig. 11) were observed in these exper-
iments, indicating that no demetallation or other decomposition,
reactions took place.
4.3 UV-Vis titration
To 2.500 ml 50.0 × 10−6mol l−1 solution of complex 11 in a
quartz cuvette, aliquots (5 μl for first 25 points, 10 μl for all fol-
lowing points) of 2.50 × 10−3mol l−1 solution of pyridine were
added. After each addition, the solution was shaken, left to equi-
librate for 1 min and a UV-Vis spectrum was recorded. A blue-
shift of the band at 487 nm and of the valley at 420 nm, as well
as the appearance of the new absorption band at 343 nm were
observed upon addition of pyridine. Absorbance values at
4.2 Crystallographic analysis
4.2.1 Crystalisation procedure. The procedure was the same
for both assemblies. Equimolar amounts of Zn(btsc) 9 and
ligand (L2 or L3) were weighed in 5 ml Schlenk-tubes equipped
This journal is © The Royal Society of Chemistry 2012
Dalton Trans., 2012, 41, 3740–3750 | 3747

View Article Online
Table 2 Concentrations and amounts of compounds used in catalytic
experiments
completed incubation, indicating slow transformation of the
hydride species into the inactive rhodium dimeric species. There-
fore, in separate experiment the substrate was injected 60 min
after Rh(acac)(CO)2 was added, and the reaction monitored. The
band of the aldehyde-CO at 1722 cm−1 started growing immedi-
ately, and the band of the CvC bond (1639 cm−1) diminished.
During the catalysis (18 h) a weak shoulder at 1986 cm−1 could
be observed, possibly the vibration of the the CO-bridged di-
rhodium species, whose corresponding signal at ca. 1790 cm−1
was observed before the aldehyde band covered it. The signal of
the dinuclear rhodium species stayed constant at low intensity all
the time during catalysis, suggesting that it was in the equili-
brium with the catalytically active rhodium species.
Compound
equiv.
c/(10−3mol l−1
)
m mg−1 or V
Rh(acac)(CO)2
LI
L2 or L3
L4
1-Octene
2- or 3-Octene
Zn(btsc) 9
Zn(btsc) 10
Zn(btsc) 11
1.00
5.00
5.00
5.00
1000
500
5(15)
5(15)
5(15)
0.70
3.50
3.50
3.50
700
350
3.50(10.5)
3.50(10.5)
3.50(10.5)
0.903
4.59
4.61
4.64
550/μl
275 μl
6.98 (20.9)
5.91 (17.7)
9.80 (29.4)
529 nm were taken to create plot (A0 − A) vs. Cpyridine, shown in
insert in Fig. 3. The binding constant was calculated using the
curve fitting procedure for the 1 : 1 complexation case, devel-
oped in the Hunter Group50 by having Aend and K as unknowns,
what resulted with A0 − Aend = 0.0826771 and K = (1.4 0.2)
105 lmol−1. The fitting using in-house developed curve-fitting
scripts gave value of (1.6 0.1) ×105, which was more accurate
and therefore given in the text.
4.4.2 HP-IR using triphenylphosphine as ligand. This
experiment was performed analogously to the previously
described one. No Zn complex was added. The amounts of
ligand and rhodium precursor used: triphenylphosphine 25 mg
(95.3 μmol, 5 equiv.), Rh(acac)(CO)2 4.920 mg (19.10 μmol, 1
equiv.). The substrate was, however, not added, only the for-
mation of the hydride species was monitored. Like in the above
case, four main bands were observed, however, at lower frequen-
cies (2056, 2030, 1989 and 1941 cm−1). In addition to them, the
signals of the dinuclear rhodium species start appearing soon
after rhodium injection and are present in significant amount (we
estimate up to 30% of all rhodium is in form of dimers, based on
the relative intensities of the bands in IR).
4.4 High-pressure infrared spectroscopy
The autoclave built for in situ infrared spectroscopy (described
in detail by van Leeuwen et al.),57 was cleaned, dried, tested for
leaks (pressurised with 40 bar hydrogen for 16 h), and flushed
(3 × 15 bar) with syngas prior to use. All handling and manipu-
lations were performed under oxygen and water-free atmosphere
(Ar of 1 bar 1 : 1 syngas). Only freshly dried and degassed
liquids were used. 1-Octene was additionally purified by fil-
tration over a short plug of alumina.
4.4.1 HP-IR experiment with the supramolecular ligand.
The Zn(btsc) complex 9 was weighed (93.85 mg, 235.2 μmol,
14.2 equiv.) in air, transferred into a Schlenk-tube, and repeat-
edly evacuated and flushed with Ar. 10.0 ml dichloromefhane
(DCM) and a solution of L4 (20.8 mg, 78.4 μmol, 4.7 equiv.) in
1.0 ml DCM were added to it. The mixture was stirred at room
temperature for 30 min, and then transferred into the IR-auto-
clave. The Schlenk-tube was washed with 3.0 ml DCM, which
was added to the mixture in the autoclave, which was then again
flushed (3 × 15 bar) with syngas and pressurised to 19 bar. The
injection chamber of the autoclave was charged with solution of
Rh(acac)(CO)2 (4.283 mg, 16.6 μmol, 1 equiv.) in 1 ml DCM
and pressurised to 30 bar. The autoclave was heated to 40 °C
(the pressure increased to about 20.5 bar). Background spectrum
was taken ca. 1 h after the temperature reached 40 °C, and then
the solution of rhodium precursor was injected. The injection
chamber was cleaned, dried, charged with 1.0 ml 1-octene sol-
ution (521 μl, 3.32 mmol, 200 equiv.) and pressurised again with
ca. 30 bar syngas. The incubation was followed all the while.
The frequencies around 2000 cm−1 stopped changing ca. 45 min
after rhodium precursor injection. Four bands (2071, 2056, 2005
and 1970 cm−1) were observed, indicating mixture of two (eq–
eq and eq–ap, see main text) rhodium hydride species. In the
region of the CO-bridged Rh-dimers no signal appears initially,
however, small signals start appearing about two hours after the
Fig. 12 Comparison of the HP-FTIR spectra of the rhodium species
formed with the supramolecular ligand L4·93 (black line) and triphenyl-
phosphine (LI, magenta line). Use of only L4 or LI with 3 equivalents
of complex 9 leads to spectra (not shown here) identical to the one
obtained with LI.
4.5 Catalysis
The Zn(btsc) complexes were weighed directly into the glass
inlays before the autoclaves were assembled. After closing, the
autoclaves were charged respectively with solutions of the ligand
(s), Rh(acac)(CO)2, and the substrate (for the exact amounts see
Table 2). Toluene was added at the end to fill the total reaction
volume to 5 ml. The charged autoclave was flushed three times
with 30–35 bar syngas (CO/H2 = 1 : 1), pressurized to 20 bar
and lowered into the previously warmed (40 °C) oil bath. After
16 or 24 h, the pressure was released, autoclave cooled, rinsed
with nitrogen and the reaction quenched with tributylphosphite
(0.5 ml in each reactor). The raw reaction mixture was diluted
3748 | Dalton Trans., 2012, 41, 3740–3750
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Acknowledgements
This research was financially supported by the EU - Marie Curie
Research and Training Network RevCat, and Dutch National
School Combination Catalysis Controlled by Chemical Design
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‡In order to prevent demetallation of the Zn(II)(salphens) and Zn(II)popr-
hyrins during hydroformylation, an excess of a non-coordinating nitro-
gen base, diisopropyl-ethylamin (DIPEA), is routinely added to the
reaction mixture.
§The ligands L2–L4 (Fig. 3a) are routinely used as templates for con-
struction of supramolecular transition metal catalysts. The position of
nitrogen donor in L2 and L3 allows creation of qualitatively different
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