New insight into solid-state molecular dynamics: Mechanochemical synthesis of azobenzene/triphenylphosphine palladacycles

Article abstract of DOI:10.1039/c1cc15377c

Solid-state reactions of dicyclopalladated azobenzenes and triphenylphosphine lead to the thermodynamically favorable bridged complexes. It was demonstrated for the first time that very complex molecular dynamics involving a series of structural transformations is also feasible in the solid state.

Full text of DOI:10.1039/c1cc15377c

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New insight into solid-state molecular dynamics: mechanochemical
synthesis of azobenzene/triphenylphosphine palladacyclesw
a
b
b
b
c
ˇ
ˇ
ˇ
ˇ
ˇ
ˇ
´
´
´
Dominik Cincic, Marina Juribasic, Darko Babic, Kresimir Molcanov, Primoz Sket,
c
b
´
Janez Plavec and Manda Curic*
´
Received 29th August 2011, Accepted 8th September 2011
DOI: 10.1039/c1cc15377c
Solid-state reactions of dicyclopalladated azobenzenes and
triphenylphosphine lead to the thermodynamically favorable
bridged complexes. It was demonstrated for the first time that
very complex molecular dynamics involving a series of structural
transformations is also feasible in the solid state.
Continuous interest in the development of the mechanochemical
synthetic methods has led to novel molecular and supramolecular
materials.1 The main advantages of these alternative preparative
routes are the absence of solvents and a reduction in environ-
mental pollution, as well as simplicity in processing and handling.2
Furthermore, solid-state reactions are regularly much faster, more
efficient and more selective than the analogous reactions in
solutions.3 Consequently, many organic, inorganic and
coordination compounds have been prepared by solid-state
reactions.4 In this regard, particularly interesting are dicyclo-
palladated azobenzenes bearing two highly labile ligands (1–4,
Scheme 1) which can be easily displaced via ligand substitution.5
The presence of such ligands and their position in the crystal
lattice (Fig. S1, ESIw) make dicyclopalladated azobenzenes ideal
building blocks for mechanochemical synthesis of new species
ranging from coordination compounds to organometallic
coordination polymers or aggregates. These products, due to
their structural and photophysical properties, can have broad
application in organic synthesis, catalysis and design of differential
chromogenic and fluorescent chemosensors.6 Derivatives of
dicyclopalladated azobenzenes suitable for different purposes
may be designed by appropriate selection of azobenzene
ligands as well as ancillary ligands.
Scheme 1
structural properties of new complexes as well as the results
of quantum-chemical calculations used in order to explain
isomerism in isolated complexes and examine trans influence
of the phosphorous and the carbon ligands.
Two types of the complexes, A and B (Scheme 1), could be
obtained by reactions of complexes 1–4 and triphenylphosphine
(PPh3) in a 1 : 2 molar ratio. They both have one molecule of
PPh3 attached to each Pd atom and differ in the coordination
mode of azobenzenes. In A complexes, azobenzenes act
simultaneously as monodentate C- and chelating C,N-donors
as in azobenzene/2,20-bypiridine palladacycles recently reported
by us,6a while in B complexes, azobenzenes act as double
chelating C,N-donors. Since two nitrogen atoms, denoted as
Na and Nb, in unsymmetrical substituted azobenzenes are not
equivalent, two isomers (alpha and beta) of A complexes 2a–4a
can be formed by breaking either Pd–Na or Pd–Nb bond.
Furthermore, depending on the trans or cis orientation of
phosphine ligand towards the Pd–C bond, two isomers are
possible for both types of complexes.
Liquid-assisted grinding (LAG)7 of initial compounds with
two molar equivalents of PPh3 in the presence of a small quantity
of nitromethane at room temperature resulted in new crystalline
products 1a–4a as revealed by powder X-ray diffraction
(PXRD), Fig. 1. These reactions followed by a drastic colour
change were complete in the range of 30–45 min depending on
Herein, we present the first example of mechanochemical
synthesis of novel azobenzene/triphenylphosphine palladacycles
which reveals very complex molecular dynamics induced by
grinding of solid reactants, Scheme 1. We also present the
a Department of Chemistry, Faculty of Science, University of Zagreb,
Horvatovac 102a, HR-10000 Zagreb, Croatia
b
´
ˇ
Division of Physical Chemistry, Ru:er Bosˇkovic Institute, Bijenicka 54,
HR-10000 Zagreb, Croatia. E-mail: curic@irb.hr;
Fax: +385 1-4680245; Tel: +385 1-4680097
c Slovenian NMR Centre, National Institute of Chemistry, Hajdrihova 19,
SI-1000 Ljubljana, Slovenia
w Electronic supplementary information (ESI) available: Experimental
and computational details. CCDC 831717–831720. For ESI and
crystallographic data in CIF or other electronic format see DOI:
10.1039/c1cc15377c.
c
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Chem. Commun., 2011, 47, 11543–11545 11543

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mechanism of these transformations is not fully verified, as is
usually the case for mechanochemical reactions.9 However, IR
spectra confirmed that initial complexes do not spontaneously
lose DMF upon grinding (Fig. S3, ESIw) and the introduction of
PPh3 should be the first step in the reaction. Besides, it is obvious
that the step involving cis–trans isomerization at the Pd–Na bond
is prerequisite for the formation of Cl-bridge which also looks
energetically more demanding than cis–trans isomerization.
The products formation in the NG reactions was qualitatively
monitored by the PXRD. Fig. 2 shows that the reactants lost
their crystallinity and produced amorphous material followed
by the crystalline product formation. These results suggest a
process that involves fast molecular diffusion with crystal
disintegration and phase transformation which lead to products.
Furthermore, the PXRD experiments and the solid-state
31P NMR spectra confirmed that after 20 min of grinding, the
reaction continued in the jar even without further mechanical
treatment, and that after 24 hours the same products were
formed and only a small amount of impurities was present. This
is in agreement with the studies on chemical and mechanical
aspects of mechanochemical synthesis.10
Fig. 1 Comparison of PXRD patterns between products obtained by
grinding and simulated from the single crystals.
the initial complexes used, Fig. 1 and Fig. S2 (ESIw). The same
products can also be prepared in the absence of solvent by neat
grinding (NG) reactions but at a slower rate than in LAG
reactions, confirming that a small quantity of nitromethane
accelerates solid-state reactions, Fig. 2.8
In order to identify the ground species their single crystals
were isolated from acetone or acetone–ethanol mixture by
recrystallization, and analogous solvent-based reactions were
carried out in acetone, see ESI.w
Since PPh3 is a monodentate ligand it was reasonable to
expect the formation of B complexes in the solid-state reactions
of 1–4 and PPh3. However, the results of X-ray single-crystal
and powder-diffraction experiments and solid-state 31P NMR
spectra, supported by quantum-chemical calculations, have
confirmed that these reactions lead to the bridged complexes
1a–4a. Attempts to isolate complexes 1b–4b resulted in the
products 1a–4a.
Results of the single crystal X-ray analysis have clearly
demonstrated that crystallization of LAG species from acetone
or mixture of acetone and ethanol leads to A complexes, Fig. 3
and Fig. S4–S7 (ESIw). In all complexes two Pd atoms are
connected by a Cl-bridge and an azobenzene ligand. Single
crystals of alpha isomers were isolated for 3a and 4a, while beta
isomer was isolated only for 2a, Fig. 3 and Fig. S6 and S7
(ESIw). Furthermore, in isolated complexes phosphine ligands
are cis oriented with respect to the Pd–C bond. Formation of
trans isomers is not preferred due to the destabilizing effect of
carbon and phosphorus ligands in trans positions,11 as we have
also confirmed by quantum-chemical calculations, Table 1.
LAG products were structurally identified by comparing
their PXRD patterns with those simulated from the single
Formation of bridged complexes under mechanochemical
conditions is surprising since it requires very complex changes in
the molecular structure during the reaction. These changes
comprise substitution of DMF with PPh3, cis–trans isomerization,
breaking of one Pd–N bond, rotation of the phenyl ring with a
bulky substituent and positioning of both palladium atoms on
the same side of azobenzene and finally the formation of the
monochloride bridge, sketch C in Scheme 1. The step-by-step
Fig. 3 Molecular structure of the beta isomer of 2a. Displacement
ellipsoids are drawn at 30% probability level and hydrogen atoms are
omitted for clarity.
Fig. 2 PXRD patterns of NG reaction of complex 2 and PPh3.
11544 Chem. Commun., 2011, 47, 11543–11545
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Table 1 Calculated energies for possible isomers of complexes A and
B (in kcal molꢀ1) relative to the most stable isomer of A in the gas phase
In summary, we have presented the first examples of
triphenylphosphine derivatives of dicyclopalladated azobenzenes
obtained by solid-state synthesis. This synthetic route produced
bridged complexes and for the first time demonstrated that very
complex molecular dynamics involving a series of structural
transformations is also feasible in the solid state, Scheme 1.
The described results are important for understanding the
structural transformations occurring during the mechanical
processing of solid reactants and will have significant implication
for the future design and synthesis of the new generation of
coordination compounds.
A
B
cis
trans
cis
trans
Complexa
Alpha
Beta
Alpha
Beta
1a
2a
3a
4a
0.0b
0.0
0.0
0.0
7.6b
7.6
7.5
8.1
1b
2b
3b
4b
2.5
2.0
1.7
2.3
12.7
12.5
11.5
13.1
0.1
1.0
2.2
8.3
8.6
8.4
a
b
Only the complexes of type A were isolated. Alpha and beta
isomers are identical.
The authors thank the Ministry of Science, Educations and
Sports of the Republic of Croatia for financial support (grant nos.
098-0982915-2950, 098-1191344-2943 and 119-1193079-3069).
crystal structure determinations, Fig. 1 and Fig. S2 (ESIw).
In all cases, except for 4a, these pairs of PXRD patterns show
a very good agreement. Thus, the solid-state reactions of
1–3 with PPh3 resulted in A complexes. The formation of 4a
from precursor 4 was indicated by solid-state 31P NMR
spectrum of the ground product that contains two signals
(31.34 and 39.89 ppm) since two atoms of phosphorus in all
bridged complexes 1a–4a are not equivalent, Fig. 4b and Table
S1 (ESIw). In contrast to A complexes, two phosphorus atoms
in B complex with azobenzene 1b are equivalent (Scheme 1),
while in complexes with unsymmetrical substituted azobenzenes
2b–4b differences between them are too small to be visible in
NMR spectra. Consequently, only one phosphorus signal should
be present in spectra of B complex which was not the case here.
31P NMR spectra recorded in CDCl3 revealed that only the
structure of 2a in solution is not consistent with the observed
solid-state structure. In contrast to 1a, 3a and 4a whose
31P NMR spectra contain one set of two phosphorus signals
(Fig. 4a), the spectrum of 2a contains double set of signals due
to the simultaneous presence of alpha and beta isomers in the
solution, Fig. 4c and Table S1 (ESIw). Thus, dissolving 2a,
isolated as a beta isomer, in CDCl3 results in equilibrium
between alpha and beta isomers which is confirmed by
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Fig. 4 31P NMR spectra of 1a recorded in CDCl3 (a), 1a recorded in
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c
This journal is The Royal Society of Chemistry 2011
Chem. Commun., 2011, 47, 11543–11545 11545