Aging and ball-milling as low-energy and environmentally friendly methods for the synthesis of Pd(II) photosensitizers

Article abstract of DOI:10.1021/om500008v

Simple and efficient solid-state synthetic methods, aging and ball milling (liquid-assisted grinding, LAG), have been employed to achieve reactions of triphenylphosphine (tpp) and 1,1′-bis(diphenylphosphino)ferrocene (dppf) with dicyclopalladated azobenzenes (DMF)PdCl(μ-R¹C ₆H₃N=NC₆H₃R²)PdCl(DMF), R¹ = H, OCH₃, N(CH₃)₂ and R ² = H, NO₂. For the first time the aging processes have been applied in the formation of Pd(II) heteroleptic complexes. Both synthetic procedures lead to four types of tetra- or pentacoordinated complexes (tpp)PdCl{(μ-Cl)(μ-R¹C₆H₃N=NC ₆H₃R²)}Pd(tpp) 1A-3A, (tpp) ₂PdCl(μ-R¹C₆H₃N=NC ₆H₃R²)PdCl(tpp)₂ 1B-3B, PdCl{(μ-Cl)(μ-R¹C₆H₃N=NC ₆H₃R²)}Pd(dppf) 1C-3C, and (dppf)PdCl(μ- R¹C₆H₃N=NC₆H₃R ²)PdCl(dppf) 1D-3D, in which azobenzenes simultaneously act as monodentate C- and bidentate C,N-donors (A, B, and C complexes) or, for the first time, only as double C-donors (D complexes). Although their formation requires complex intramolecular transformations, aging and ball milling have been proved to be efficient synthetic methods. All products have been fully characterized in solid state and solution. Solid-state structures of complexes have been resolved by a single crystal or powder X-ray diffraction (PXRD) and solid-state NMR spectroscopy. Their electronic structures have been rationalized by the quantum-chemical calculations. The structural features of complexes and their stability in solutions have been evaluated by ¹H and ³¹P NMR spectroscopy along with the ESI-MS spectrometry. The absorption and emission properties of complexes have been studied, and the nature of their electronic transitions in the low energy region has been examined by TD-DFT calculations. Significant fluorescence observed for the complexes with 4-dimethylamino-4′-nitroazobenzene ligand at room temperature in solid state and/or in the solution makes them interesting for potential applications as photoactive units in optical devices.

Full text of DOI:10.1021/om500008v

Article
pubs.acs.org/Organometallics
Aging and Ball-Milling as Low-Energy and Environmentally Friendly
Methods for the Synthesis of Pd(II) Photosensitizers
†
†
†
‡
§
,†
́
Marina Juribasic, Ivan Halasz, Darko Babic, Dominik Cincic, Janez Plavec, and Manda Curic*
̌
́
́
̌
́
́
†Ruđer Bosk
̌
ovic
́
Institute, Bijenick
̌
a 54, HR-10002 Zagreb, Croatia
‡Department of Chemistry, Faculty of Science, University of Zagreb, Horvatovac 102a, HR-10000 Zagreb, Croatia
§Slovenian NMR Centre, National Institute of Chemistry, Hajdrihova 19, SI-1000 Ljubljana, Slovenia
S
* Supporting Information
ABSTRACT: Simple and efficient solid-state synthetic
methods, aging and ball milling (liquid-assisted grinding,
LAG), have been employed to achieve reactions of
triphenylphosphine (tpp) and 1,1′-bis(diphenylphosphino)-
ferrocene (dppf) with dicyclopalladated azobenzenes (DMF)-
PdCl(μ-R1C6H3NNC6H3R2)PdCl(DMF), R1 = H, OCH3,
N(CH3)2 and R2 = H, NO2. For the first time the aging
processes have been applied in the formation of Pd(II)
heteroleptic complexes. Both synthetic procedures lead to four
types of tetra- or pentacoordinated complexes (tpp)PdCl{(μ-
Cl)(μ-R1C6H3NNC6H3R2)}Pd(tpp) 1A−3A, (tpp)2PdCl-
(μ-R1C6H3NNC6H3R2)PdCl(tpp)2 1B−3B, PdCl{(μ-
Cl)(μ-R1C6H3NNC6H3R2)}Pd(dppf) 1C−3C, and (dppf)-
PdCl(μ-R1C6H3NNC6H3R2)PdCl(dppf) 1D−3D, in which azobenzenes simultaneously act as monodentate C- and bidentate
C,N-donors (A, B, and C complexes) or, for the first time, only as double C-donors (D complexes). Although their formation
requires complex intramolecular transformations, aging and ball milling have been proved to be efficient synthetic methods. All
products have been fully characterized in solid state and solution. Solid-state structures of complexes have been resolved by a
single crystal or powder X-ray diffraction (PXRD) and solid-state NMR spectroscopy. Their electronic structures have been
rationalized by the quantum-chemical calculations. The structural features of complexes and their stability in solutions have been
1
evaluated by H and 31P NMR spectroscopy along with the ESI-MS spectrometry. The absorption and emission properties of
complexes have been studied, and the nature of their electronic transitions in the low energy region has been examined by TD-
DFT calculations. Significant fluorescence observed for the complexes with 4-dimethylamino-4′-nitroazobenzene ligand at room
temperature in solid state and/or in the solution makes them interesting for potential applications as photoactive units in optical
devices.
accelerated aging and vapor digestion5 are extremely simple and
require minor energy input, while they may be quantitative.
Despite these advantages, preparations of compounds by aging
are still rare.
INTRODUCTION
■
During the past decade mechanochemical reactions are
becoming increasingly popular preparative methods with
applications in the synthesis of inorganic, organic, coordination,
and supramolecular compounds.1 Consequently, a large
number of papers and reviews has been published over the
past several years dealing with synthetic and mechanistic
aspects of these reactions.2 Mechanochemical reactions induced
by grinding (neat grinding, NG; liquid-assisted grinding, LAG;
or ion and liquid-assisted grinding, ILAG) of the solid reactants
are often faster and more selective than solvent-based or
solvothermal reactions.3 Furthermore, the complete absence of
solvent or its use in only catalytic amounts results in reduced
environmental pollution and renders mechanochemical prepa-
rative methods relevant for industrial and pharmaceutical
applications.4 However, solid-state reactions can also occur
without an input of mechanical energy.5 The mixture of solid
reactants will undergo a chemical reaction in particular if
exposed to solvent vapors. These processes known as
We have decided to investigate aging reactions in the
synthesis of heteroleptic Pd(II) complexes because of their
specific advantages described above. Recently, we have
mechanochemically prepared the first examples of Pd(II)
heteroleptic complexes with triphenylphosphine and azoben-
zenes.6 The solid-state synthetic route produced monochloride-
bridged complexes and demonstrated that complex structural
changes are feasible during the synthesis in solid state.6 As an
extension of this work, here we report on the synthesis of a
series of new organopalladium complexes containing azoben-
zenes and triphenylphosphine (tpp) or 1,1′-bis-
Received: January 7, 2014
Published: February 24, 2014
© 2014 American Chemical Society
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(diphenylphosphino)ferrocene (dppf) by two solid-state
synthetic routes: aging and ball milling, Scheme 1.
activation. PXRD of such treated reactants did not detect the
presence of new products. All solid-state reactions are
characterized by a drastic color change and almost quantitative
yields. Compared to mechanochemical reactions, which were
complete within 30−90 min and used 30 μL of solvent, aging
reactions lasted up to 72 h and required about 1 mL of solvent.
Both methods yield pure products; however collecting of the
products from reaction vessels introduces small mass losses,
lowering the isolated yields to about 88 and 98% for
mechanochemical and aging reactions, respectively.
Scheme 1
The purity of the bulk products was determined by PXRD.
PXRD patterns of aging and LAG products were compared,
and for complexes 1A, 3A, 1C, and 1D−3D, these pairs are in
good agreement, Figures 1 and S1−S12, Supporting
In addition to the synthetic objectives, we have also focused
on the structural as well as the photophysical properties of the
new complexes because of their potential application in light-
emitting devices and as photochemical catalysts.7 Although
phosphine ligands are among the most frequently used ligands
in the palladium chemistry,8 investigations of the photophysical
properties of Pd(II) complexes containing both azobenzene
and phosphine moieties have not been reported yet. Our
previous studies of dicyclopalladated azobenzenes have
demonstrated a strong effect of 4,4′-substituents in azobenzene
unit on their photophysical properties, which may be
additionally enhanced by a suitable ancillary ligand.9 The
influence of the triphenylphosphine and 1,1′-bis-
(diphenylphosphino)ferrocene as ancillary ligands on the
photophysical properties of the dipalladated azobenzenes was
examined by absorption and emission spectroscopy as well as
by theoretical calculations.
Figure 1. PXRD patterns of complexes (a) 1A and (b) 3A involving
reactants tpp, 1 and 3.
Information. Differences between the remaining pairs of
PXRD patterns were rationalized in terms of a large number
of different possible conformers as indicated by the quantum
chemical calculations.
In order to identify the aging and LAG species, the single
crystals of 1A−3A and 3B were isolated from acetone or
acetone−ethanol mixture by recrystallization, and analogus
solvent-based reactions were carried out in acetone; see the
Supporting Information. The results of X-ray structural analysis,
solid-state and solution 31P NMR spectra as well as ESI mass
spectra and PXRD have confirmed that the aging and LAG
reactions lead to the same types of complexes.
RESULTS AND DISCUSSION
■
Synthesis and Characterization. The synthetic routes to
four series of heteroleptic Pd(II) complexes are described in
Scheme 1. Reactions of tpp and dppf with initial compounds in
molar ratio 2:1 and 1:1 lead to the Cl-bridged 1A−3A and 1C−
3C complexes, respectively, in which azobenzenes simulta-
neously act as monodentate C- and bidentate C,N-donors.
When the molar ratio of phosphine ligand and dicyclopalla-
dated azobenzenes was 4:1 (tpp) or 2:1 (dppf), the reactions
resulted in 1B−3B or 1D−3D complexes, respectively. For the
first time, in D complexes azobenzene acts as the double C-
donor, while in B complexes it has the same coordination mode
as in A and C complexes. All products are air-stable solids, A
and C complexes are soluble in solvents with poor coordinating
ability, i.e., CHCl3, CH2Cl2, or (CH3)2CO, while B and D
complexes are only sparingly soluble. Except 3A and C
complexes, all the others are insoluble in solvents with
coordinating ability, i.e., DMSO, DMF, or CH3CN.
The formation of A and C complexes requires the
substitution of DMF with tpp or dppf, cis−trans isomerization
of donor atoms with respect to Pd−C bond and breaking of
only one Pd−N bond, which allows the rotation of one phenyl
ring and positioning of both palladium atoms on the same side
of azobenzene in order to form a Cl-bridge. Prerequisites for
the formation of B and D complexes are also the exchange of
DMF with tpp or dppf, cis−trans isomerization, and in the cases
of D products, the breaking of both Pd−N bonds and the
rotation of both phenyl rings with bulky substituents, Scheme
1.
All compounds were obtained by aging in acetone vapors and
by liquid-assisted grinding (LAG) using catalytic amount of
acetone or nitromethane at room temperature. The mixing of
solid reactants in the aging reactions was done by gentle
grinding in an agate mortar in order to avoid mechanical
NMR Spectra. 31P NMR spectra of A and C complexes
recorded in CHCl3 contain two signals (two sets for 2A and
2C), confirming that two phosphorus atoms of each phosphine
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ligand (tpp and dppf) are nonequivalent, Figure 2. A double set
of signals in the ratio 3:1 observed in the 31P NMR as well as in
Figure 2. 31P NMR spectra of (a) 3A and 3B (MAS); (b) 3C and 3D
(CDCl3); (c) 3C and 3D (MAS).
Figure 3. (+)ESI mass spectra of 1A and 1B in CHCl3/MeOH
recorded 3 min after dissolving. Insets: Experimental (black) and
calculated (blue) isotopic pattern of peaks assigned to (a) [Pd2Cl(L1-
2H)(tpp)2]+ (C48H38N2P2Pd2Cl) and (b) [Pd2Cl(L1-2H)(tpp)3]+
(C66H53N2P3Pd2Cl).
1H NMR spectra of 2A and 2C complexes is due to the
presence of alpha and beta isomers formed by breaking either
the Pd−Nα or the Pd−Nβ bond,6 Scheme 1, Table S1, and
Figure S13 (Supporting Information). The signals in the
spectra of C complexes are split into doublets due to the P−P
coupling (2JPP about 30 Hz), indicating chelate binding mode of
dppf ligand.10 The solid-state 31P NMR spectra of the A and C
complexes contain two broad signals, and consequently, it is
not possible to see the difference between the alpha and beta
isomers as well as the splitting of the phosphorus signals of the
C complexes.
The 31P NMR spectra confirmed the same coordination
mode of dppf ligands in C and D complexes. The spectrum of
complex 1D contains two doublets with 2JPP about 30 Hz, while
the complexes of asymmetrically substituted azobenzenes (2D
and 3D) with two nonequivalent phenyl rings contain four
signals, Figure 2 and Table S1 (Supporting Information). These
results indicate that each Pd atom in D complexes is bonded to
one dppf molecule. The spectra of complexes 2D and 3D
recorded in the solid state contain two broad signals due to
small differences between chemical shifts of atoms P1 and P2 as
well as P3 and P4, Scheme 1.
The solid-state 31P NMR spectra of the B complexes contain
only one phosphorus signal suggesting very similar environ-
ment around four phosphorus atoms, Figure 2 and Table S1
(Supporting Information). However, the spectra recorded in
CHCl3 have revealed that in the solution the B complexes
transform into the thermodynamically more favorable A
complexes, Figure S14 (Supporting Information). Their spectra
recorded immediately after dissolving in CHCl3 contain one
singlet corresponding to B complex at about 29 ppm, two
singlets of A complex, and one broad signal of free tpp ligand.
The B complexes transform completely into A complexes
within 24 h in solution, Figure S14 (Supporting Information).
Such transformations are also supported by the quantum-
chemical calculations as well as by ESI-MS and UV−vis spectra.
The A complexes transform back to B during evaporation of
the solvent accompanied by precipitation.
Mass Spectra. The ESI-MS spectra of all complexes were
recorded shortly after their dissolving. Base peaks of fresh
solutions of A, C, and D complexes corresponded to [M-Cl]+
ions (Figures 3 and 4). The assignment was additionally
supported by comparing the experimental peaks with the
calculated isotopic patterns, thus confirming the formulation of
these complexes.
Figure 4. (+)ESI mass spectra of 1C and 1D in CHCl3/MeOH
recorded 3 min after dissolving. Insets: Experimental (black) and
calculated (blue) isotopic pattern of peaks assigned to [Pd2Cl(L1-
2H)(dppf)2]+ (C46H36N2P2Pd2FeCl) and [Pd2Cl(L1-2H)(dppf)]+
(C80H64N2P4Pd2Fe2Cl).
In the spectra of B complexes, peaks assigned to [M-Cl-tpp]+
ions were the highest in mass, indicating that these complexes
readily decompose in solution, Figure 3. Species [M-Cl-2tpp]+
that correspond in mass to [M-Cl]+ ions of A complexes were
assigned as the base peaks, Figure 3. After one day, the spectra
of the solution of B complexes contained many unaccountable
peaks that were not observed in the spectrum of the fresh
solution, Figure S14 (Supporting Information). It is interesting
to note here that after collisional activation the fragmentation
of the A (and B) complexes yields almost exclusively PPh4+ ion
(m/z 339), Scheme S1 (Supporting Information). This can be
explained by the breaking of the P−C bond in one tpp ligand,
phenyl group migration from one palladium atom to another,
+
bonding to the second tpp and elimination of the PPh4 ions.
Similar behavior was observed for the mononuclear Pd
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complexes with tpp11 but was not reported using compounds
with two closely positioned Pd atoms that can communicate
and exchange ligands. The interchange between phosphorus
bound aryl groups is a pitfall of many palladium-mediated
catalytic reactions, and more information about these processes
is needed.11 Fragmentation of C and D complexes leads to the
breaking of the bond with dppf ligand and to the elimination of
the ferrocenyl or phenyl moieties, Scheme S2 (Supporting
Information).
Two tpp molecules are cis with respect to Pd−C bond, while
both chlorides are oriented trans to Pd−C bond in contrast to
their cis position in the initial compound 3. Molecular structure
of 3B complex is characterized by the planarity of the
azobenzene skeleton, Pd and Cl atoms. Pd centers are
pentacoordinated with the Pd−N distances of 2.608(13) and
2.630(11) Å. This complex is one of the rare examples of
pentacoordinated Pd(II) compounds.12 Complex 3B packs in
layers in the xy-plane with numerous CH···Cl interactions,
Figure S15 (Supporting Information). Weak CH···ON
interactions between dimethylamino- and nitro-groups connect
these layers in the [001] direction.
Crystal Structures. Molecular structures of complexes
resolved by single-crystal (2A and 3B) or powder X-ray
diffraction (1D) are shown in Figures 5, 6, and S17
Crystal and molecular structure of 1D complex was solved by
simulated annealing in direct space by using various candidate
molecular structures provided by quantum-chemical calcula-
tions. The calculations involved both isomers, with and without
Pd−N coordination. In this crystal structure, the molecule was
found to have the center of symmetry bisecting the NN
bond of the azobenzene fragment. Palladium atoms are in
square-planar coordination environment with two phosphorus
atoms, the chloride and the azobenzene carbon as donors,
confirming the breaking of both Pd−N bonds in initial
compound 1, Figure 6a. Two phosphorus atoms of dppf
molecule are mutually cis, since dppf acts as a bidentate ligand
in D complexes. Consequently, one of the phosphorus atoms is
oriented trans to the Pd−C bond, Scheme 1 and Figure 6a. The
azobenzene skeleton and Pd atoms also have almost planar
arrangement in 1D complex. Furthermore, measured and
calculated PXRD patterns show a very good agreement, Figure
6b. The molecular structure of this compound is probably
among the most complex structures of coordination com-
pounds solved by PXRD.
Despite the crystal’s low quality, the single crystal X-ray
diffraction data of 2A were sufficient to identify it as the Cl-
bridged complex in which 4-methoxyazobenzene simultane-
ously acts as the monodentate C- and bidentate C,N-donor. Its
molecular structure, which is analogus to recently reported
structures 1A and 3A,6 has confirmed breaking of only one Pd−
N bond in the initial compound 2, Figure S16 (Supporting
Information). Pd centers in A complexes are bridged by the
azobenzene and additionally by the Cl-bridge. Pd(1) atom of A
complexes is coordinated by carbon, azo-nitrogen, bridged
chloride, and phosphorus. Pd(2) atom is bonded to carbon,
terminal chloride, bridged chloride, and phosphorus. Both tpp
ligands are oriented cis to the Pd−C bond as in complex 3B.
The structures of A complexes are characterized by the
intramolecular metal−metal interactions. The Pd···Pd distances
in A complexes are slightly shorter (about 0.15 Å) than the sum
of their van der Waals radii. Furthermore, despite the low
quality of the X-ray data, two molecules of 2A, alpha and beta
isomers, were unambiguously found in the asymmetric unit,
Figure S16 (Supporting Information), confirming the presence
of both isomers. This is in a good agreement with NMR
studies. In addition, ethanol molecules were found in two
accessible voids in the crystal packing. Each of two isomeric
molecules of 2A complex forms a layer in xy plane, Figure S17
(Supporting Information). These two layers are repeated along
the [001] direction.
Figure 5. Molecular structure of 3B. Displacement ellipsoids are
drawn at 30% probability level. Hydrogen atoms are omitted, tpp
ligands are labeled A−D and phenyl rings are numbered 1 (rings with
atoms C1a−C6a, C1b−C6b, C1c−C6c, C1d−C6d), 2 (rings with
atoms C7a−C12a, C7b−C12b, C7c−C12c, C7d−C12d) and 3 (rings
with atoms C13a−C18a, C13b−C18b, C13c−C18c, C13d−C18d) for
clarity.
Figure 6. (a) Molecular structure of 1D. Hydrogen atoms are omitted
for clarity. Sym. op. i is [1 − x, 1 − y, 2 − z]. (b) Final Rietveld
refinement plot for 1D. The high-angle region above approximately
28° is enlarged to reveal more detail.
(Supporting Information), and the selected bonds and angles
are presented in Table S2 (Supporting Information). All
attempts to obtain single crystals of C and D complexes were
unsuccessful, and these complexes were structurally charac-
terized by PXRD. The results of the X-ray structural analysis
have confirmed the coordination mode of azobenzenes and
phosphines as well as approximately square planar coordination
of Pd atoms in A and D complexes and pseudotrigonal-
bipyramidal coordination in B complexes.
We also attempted to solve the crystal structure of 1C but
were not successful. The sample was of poorer crystallinity than
that of 1D leading to less well resolved peaks and weak
scattering at higher diffraction angles, which hindered structure
solution, Figure S18 (Supporting Information). However, we
Two palladium centers in the complex 3B are bridged by the
azobenzene ligand, and each of them is pentacoordinated by
the carbon atom of the phenyl ring, the azo-nitrogen, the
chloride ion and two phosphorus atoms of tpp ligands, which
are mutually in trans position, Figure 5.
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were able to index the powder diffraction pattern (Table S3,
Supporting Information) and found that one-quarter of the unit
cell volume corresponded very well to the estimated volume of
one molecule of 1C. This strongly indicated that the assumed
molecular structure, as given in Scheme 1, is correct. On the
basis of the presented results, spectral data and recently
reported molecular structures of the complexes of dipalladated
azobenzenes with bidentate 2,2-bipyridine as well as the results
of quantum-chemical calculations, the structures of C
complexes can be presumed with a great certainty to be similar
to those of A complexes as given in Scheme 1.
Finally, the results of the X-ray structural analysis have
revealed cis−trans isomerism in isolated complexes and also
that the bond lengths between Pd atom and coordinated atoms
in trans position are significantly longer than could be predicted
on the basis of their covalent radii because of a strong trans
influence of C and P donors, which is also confirmed by
quantum-chemical calculations.13
Quantum-Chemical Calculations. Because of two labile
bonds around Pd atoms in the initial compounds, the
considered complexes may exist in several isomeric forms. As
we recently reported,6 the reactions of initial complexes 1−3
and tpp ligand in 1:2 molar ratio could result in two isomers
(denoted as A and A′, Scheme S4, Supporting Information),
which differ in the coordination mode of azobenzenes. Besides,
each isomer can have cis or trans orientation of tpp ligand with
respect to the Pd−C bond. Furthermore, A isomers of
asymmetrically substituted azobenzenes may exist as alpha
and beta isomers formed by breaking either the Pd−Nα or the
Pd−Nβ bond (see NMR section). However, the relative Gibbs
energies have shown that isolated isomers 1A−3A are the most
stable, Table S5 (Supporting Information).
It was noted that 1B−3B complexes dissolved in CHCl3
transform to 1A−3A complexes by releasing two tpp molecules.
Calculated Gibbs energies of 1A−3A complexes are lower than
those of 1B−3B complexes by 11−12 kcal/mol, which is in
agreement with the experimental findings. Since 1B complex
may have an inversion center as the symmetry element, we have
optimized Ci geometry to compare its Gibbs energy with the
conformer determined by the X-ray diffraction. To our surprise,
the conformer with Ci symmetry was calculated to have lower
Gibbs energy than the experimental conformer by 5.6 kcal/mol.
Similar findings were obtained with 2B and 3B complexes (6.9
and 6.3 kcal/mol). The fact that seemingly less stable
conformers were isolated can be attributed to the unaccounted
solid state effects. Since the crystal and molecular structures of
1C−3C complexes were not resolved by single-crystal or
powder X-ray diffraction, their geometries were tentatively
assigned from the most stable calculated conformers. Eight
conformers were found for each of the three species, with the
Gibbs energies of those less stable being 0.6−9 kcal/mol higher
than of the most stable conformer (geometries and energies of
all conformers are listed in Supporting Information). The
geometries of the most stable conformers were used for the
calculation of UV−vis and NMR spectra.
Figure 7. UV−vis spectra of 3A and 3C recorded in CHCl3. Vertical
lines denote calculated transitions from the ground to the excited
singlet states; the size of vertical lines is proportional to the calculated
oscillator strengths.
calculated excited states represented by the electronic transition
with the largest contribution are listed in Tables S6−S8
(Supporting Information). The calculated transition intensity is
well while the calculated wavelengths are slightly red or blue-
shifted relative to experimental values for these transitions.
According to calculations, the most intense bands in the
complexes 1A−3A, 1C−3C, and 1D−3D are assigned as
intraligand (IL) π* ← π transitions localized on the azobenzene
ligands. As expected, these transitions have higher intensity and
are significantly shifted to the lower energies in complexes with
4-dimethylamino-4′-nitroazobenzene 3A, 3C, and 3D than
their analogues with azobenzene and 4-methoxyazobenzene,
Figures 7 and S20−S25 (Supporting Information). Further-
more, the less intense transitions in A and C complexes have
metal to ligand (MLCT), metal-chloride to ligand (MXLCT),
metal−metal to ligand (MMLCT) and long-range ligand to
ligand (L′LCT) charge transfer character, where M = Pd, L =
azobenzene ligand, X = terminal or bridging chloride and L′ =
tpp or dppf, Tables S6−S8 (Supporting Information).
Compared to D complexes, the majority of the transitions in
A and C complexes have mixed character involving one or
more above-described charge transfers with varying contribu-
tion π* ← π intraligand transitions.
The complexes with 4-dimethylamino-4′-nitroazobenzene
fluoresce at room temperature in the low energy region in
solid state (3A, 3B, 3C, and 3D) and in the solution (3A, 3C,
and 3D), confirming the combined influence of the electron-
donating and electron-withdrawing 4,4′-substituents on the
fluorescence of dipalladated azobenzenes, Table 1, Figures 8
and S25 (Supporting Information). In addition, the presence of
phosphine ligands significantly increases intensity of their
fluorescence, which is the most pronounced for the bridged
complex 3A. Its emission is even much more intense than that
Table 1. Fluorescence Data
λexc (nm)
λem (nm)
3A
3B
3C
3D
580
543
633
−
594
633
570
594
633
580
594
633
615
CHCl3
solid
610−660
610−660
−
610−600
650−700
635
CHCl3
solid
Absorption and Emission Spectra. The UV−vis spectral
data for complexes 1A−3A, 1C−3C, and 1D−3D recorded in
CHCl3 at room temperature are summarized in the Supporting
Information. The spectra of B complexes could not be obtained
because of their transformation into A complexes. The
experimental absorption bands corresponding to the singlet
excited states are assigned on the basis of TD-DFT calculations,
Figures 7 and S19−S25 (Supporting Information). The first 10
CHCl3
solid
630−660
640−680
632
CHCl3
solid
640−610
640−610
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°C by confocal microscope using nujol mull of samples with
appropriate excitation wavelengths.
1
NMR Measurements. The H and 31P NMR spectra in solution
1
were recorded at 25 °C in CDCl3 at 600.13 MHz for the H and
242.92 MHz for the 31P resonances. The 31P NMR spectra were
externally referenced using 85% H3PO4, Table S1 (Supporting
1
Information). H signal assignment was based on the chemical shifts
and quantum chemical calculations of the chemical shifts.
31P MAS NMR spectra of solid samples were recorded on 300 MHz
NMR spectrometer equipped with 5 mm Magic Angle Probe, Table S1
(Supporting Information). Larmor frequency of phosphorus nuclei
was 122.65 MHz. The 31P MAS NMR spectra were externally
referenced using AlPO4, which was set to −29.5 ppm corresponding to
85% H3PO4 (δ 0.0 ppm). All samples were spun at the magic angle
with ca. 5 kHz spinning frequency. Repetition delay in all experiments
was 30 s; the number of scans was between 168 and 3760.
Single-Crystal X-ray Measurements. Measurements were
performed on X-ray diffractometers with microfocus Cu or Mo
tubes at room temperature. Program package CrysAlisPRO15 was used
for data reduction. The structures were solved using SHELXS9716 and
refined with SHELXL97.16 Models were refined using the full-matrix
least-squares refinement. Hydrogen atoms were treated as riding
entities using the command AFIX in SHELXL-97.16 Molecular
geometry calculations were performed by PLATON,17 and molecular
graphics were prepared using ORTEP-318 and CCDC-Mercury.19
Crystallographic and refinement data for the structures reported in this
paper are shown in Table S2 (Supporting Information).
Powder X-ray Diffraction (PXRD) Experiments. Experiments
were performed on X-ray diffractometer with Cu Kα1 (1.54056 Å)
radiation at 40 mA and 40 kV. The scattered intensities were measured
with a scintillation counter. The angular range was from 3° to 50° (2θ)
with steps of 0.02°, and the measuring time was 1 s per step. The data
collection and analysis was performed using the program package
Philips X’Pert. Details about the solving and the refinement of the
structure for 1C and 1D are given in the Supporting Information.
ESI Mass Spectrometry. ESI mass spectra were recorded on a
mass spectrometer working in the positive mode. The compounds
were dissolved in CHCl3 to obtain a concentration of approximately
10−4 mol/dm3. Stock solution was diluted with MeOH to
approximately 10−5 mol/dm3 and injected into the ESI source.
Helium was used as a collision gas. Expected natural abundance
isotope cluster patterns for various ion clusters were calculated with
the ICR-2LS program.20
Computational Methods. The computations were performed
with Gaussian09,21 using B3LYP functional22 and two basis sets.
Geometry optimization was done with SDD pseudopotential and
accompanying basis23 on Pd atoms and 6-31G(d,p) basis on all other
atoms. NMR shifts and excited states were calculated with the
extended SDD basis on Pd by additional s, p, d and f functions,24 while
6-311+G(d,p) basis was used on all other atoms. In the latter
calculations solvated state was accounted by the PCM model25 with
chloroform as a solvent. NMR shifts were calculated by the GIAO
formalism26 and the lowest 32 excited singlet states by the time
dependent DFT method (TD-DFT).
Figure 8. Fluorescence of 4-dimethylamino-4′-nitroazobenzene
complexes in (a) CHCl3 (λexc = 580 nm for 3A, 3D, 3E and λexc
=
570 nm for 3C) and (b) the solid state (λexc = 633 nm). The emission
intensity is normalized to the same concentration.
of the complex with 4-dimethylamino-4′-nitroazobenzene and
2,2′-bipyridine reported recently (denoted as 3E in Figure 8).
The complexes 3A, 3C, and 3D exhibit fluorescence in the
CHCl3 solution with λmax at 615, 635, and 632 nm upon
excitation at 580, 570, and 580 nm, respectively. Spectra of
these complexes are similar in terms of their band shapes and
its position, indicating that the same emissive state is involved
in each case. According to the literature observed emission can
be attributed to IL π* → π, MLCT, or MMLCT transitions.14
The fluorescence emission was also observed in the solid-state
with a confocal microscope for complexes 3A, 3B, 3C, and 3D
in the wavelength range 600−700 nm upon excitation at 543
and 633 nm for 3A complex (Figure S29, Supporting
Information) and 594 and 633 nm for 3B, 3C, and 3D
complexes, Table 1.
CONCLUSIONS
■
We have successfully applied accelerated aging and ball milling
for the synthesis of several types of heteroleptic complexes of
Pd(II) containing azobenzenes and phosphines, despite the fact
that their formation requires a series of complex molecular
modifications during the reaction. The high efficiency, use of
solvent in catalytic amounts, low-energy input as well as
simplicity in processing and handling reactants and products
render these two solid-state synthetic methods as excellent
alternatives to solvent-based synthesis. Described results, which
are consistent with the objectives of green chemistry, open up
the possibilities to much wider application of accelerated aging
and ball milling for synthesis of elaborate molecular systems.
This preliminary investigation clearly demonstrated a strong
influence of triphenylphosphine and 1,1′-bis-
(diphenylphosphino)ferrocene as ancillary ligands on the
photophysical properties of dipalladated azobenzenes. In
comparison with initial dipalladated 4-dimethylamino-4′-nitro-
azobenzene complex 3, the presence of phosphines in new
complexes coupled with appropriate 4,4′-substituents on the
azobenzene ligand and Pd···Pd interactions in the bridged
complexes significantly enhance fluorescence at room temper-
ature in solid state for complexes 3A, 3B, 3C, and 3D or in
solution for complexes 3A, 3C, and 3D. Thus, the Pd(II)
heteroleptic complexes with azobenzenes and phosphines could
be utilized in designing new emitters.
Aging Reactions. In all aging reactions the mixture of solid
reactants prepared by gentle grinding in an agate mortar and put in a
small test tube underwent a chemical reaction upon exposure to
acetone vapors at room temperature. The reactions were carried out in
a sealed vessel (100 mL volume) filled with air and acetone vapors.
The mixture of solid reactants was kept in acetone vapors for 48 (1A−
3A, 1B−3B) or 72 (1C−3C, 1D−3D) hours. Analysis of the products
using PXRD revealed complete conversion of reactants into the
products. Initial complexes (DMF)PdCl(μ-R1C6H3NNC6H3R2)-
PdCl(DMF) 1−3 were prepared as described previously.9a,27
1A. The reaction was carried out with 55.52 mg (0.091 mmol) of 1
and 47.73 mg (0.182 mmol) of tpp. Isolated yield: 98.0% (88.15 mg).
Found: C 57.98, H 4.02, N 2.78. Calcd. for C48H38N2P2Pd2Cl2: C
58.32, H 3.87, N 2.83. UV−vis (CHCl3) λmax/nm (ε/104 M−1 cm−1):
305 (1.40), 345 (1.25), 395 (0.78), 466 (0.30). (+)ESI-MS m/z (rel.
EXPERIMENTAL SECTION
■
General Measurements. All chemicals were used as supplied, and
the reactions were carried out under aerobic condition. Electronic
absorption spectra were recorded in CHCl3 at 25 °C with a
thermostatted cell compartment. Fluorescence spectra were recorded
in CHCl3 at 25 °C. Fluorescence in the solid-state was detected at 25
1232
dx.doi.org/10.1021/om500008v | Organometallics 2014, 33, 1227−1234

Organometallics
Article
int., %): [Pd2Cl(L1-2H)(tpp)2]+ 953.0 (100); [Pd2Cl2(L1-2H)-
(tpp)2+NaCl]+ 1044.9 (2).
UV−vis (CHCl3) λmax/nm (ε/104 M−1 cm−1): 307 (2.24), 541 (1.99).
(+)ESI-MS m/z (rel. int., %): [Pd2Cl(L3-2H)(dppf)2]+ 1625.2 (100);
[Pd2Cl(L3-2H)(dppf)(dppfOH)]+ 1641.2 (7); [Pd2Cl2(L3-2H)-
(dppfOH)2+Na]+ 1717.2 (5); [Pd4Cl2(L3-2H)2(dppf)3]2+ 1348.1
(45); [Pd4Cl4(L3-2H)2(dppf)3+CH3OH+2Na]2+ 1394.1 (7).
Ball-Milling Reactions. All complexes were obtained by analogus
ball-milling reactions with the same ratio of azobenzenes and the initial
Pd complex. Complexes 1A and 3A were prepared by liquid-assisted
grinding (LAG) as described previously.6
Grinding experiments were performed at room temperature in a 10
mL stainless steel jar using two 7 mm stainless steel grinding balls. A
grinder mill operating at 25 Hz frequency was used for the synthesis.
All LAG reactions were performed according to recently reported
procedure for complexes 1A and 3A6 using 30 μL of acetone (or
nitromethane) in each reaction. PXRD experiments revealed complete
conversion of reactants into the products after 30 (1A and 3A), 60
(2A, 2C and 3C), and 90 (1B−3B, 1D−3D) minutes of grinding.
Elemental analysis results of all complexes are in good agreement with
the calculated values (data given in the Supporting Information).
2A. 53.18 mg (0.083 mmol) of 2 and 43.58 mg (0.166 mmol) of
tpp. Isolated yield: 98.5% (83.35 mg). Found: C 57.77, H 4.11, N 2.52.
Calcd. for C49H40N2OP2Pd2Cl2: C 57.78, H 3.96, N 2.75. UV−vis
(CHCl3) λmax/nm (ε/104 M−1 cm−1): 349 (0.71), 393 (0.62), 460
(0.43). (+)ESI-MS m/z (rel. int., %): [Pd2Cl(L2-2H)(tpp)2]+ 983.1
(100); [Pd2Cl2(L2-2H)(tpp)2+NaCl]+ 1075.0 (13).
3A. 52.35 mg (0.075 mmol) of 3 and 39.33 mg (0.150 mmol) of
tpp. Isolated yield: 99.0% (79.91) Found: C 56.02, H 4.11, N 4.91.
Calcd. for C50H42N4O2P2Pd2Cl2: C 55.78, H 3.93, N 5.20. UV−vis
(CHCl3) λmax/nm (ε/104 M−1 cm−1): 300 (2.24), 384 (0.78), 542
(3.96). (+)ESI-MS m/z (rel. int., %): [Pd2Cl(L3-2H)(tpp)2]+ 1041.1
(100); [Pd2Cl2(L3-2H)(tpp)2+NaCl]+ 1133.0 (5).
1B. 33.25 mg (0.054 mmol) of 1 and 57.17 mg (0.218 mmol) of
tpp. Isolated yield: 99.0% (81.62 mg). Found: C 66.98. H 4.66, N 1.88.
Calcd. for C84H68N2P4Pd2Cl2: C 66.68, H 4.53, N 1.85. (+)ESI-MS m/
z (rel. int., %): [Pd2Cl(L1-2H)(tpp)2]+ 953.0 (100); [Pd2Cl2(L1-
2H)(tpp)2+NaCl]+ 1044.9 (17); [Pd2Cl(L1-2H)(tpp)3]+ 1214.9 (7).
2B. 31.22 mg (0.049 mmol) of 2 and 51.17 mg (0.195 mmol) of
tpp. Isolated yield: 98.6% (74.21 mg). Found: C 66.47, H 4.92, N 1.86.
Calcd. for C85H70N2OP4Pd2Cl2: C 66.16, H 4.57, N 1.82. (+)ESI-MS
m/z (rel. int., %): [Pd2Cl(L2-2H)(tpp)2]+ 983.1 (100); [Pd2Cl2(L2-
2H)(tpp)2+NaCl]+ 1075.1 (3).
3B. 30.55 mg (0.044 mmol) of 3 and 45.91 mg (0.175 mmol) of
tpp. Isolated yield: 99.4% (69.64 mg). Found: C 64.75, H 4.71, N 3.34.
Calcd. for C86H72N4O2P4Pd2Cl2: C 64.51, H 4.53, N 3.50. (+)ESI-MS
m/z (rel. int., %): [Pd2Cl(L3-2H)(tpp)2]+ 1041.1 (100); [Pd2Cl2(L3-
2H)(tpp)2+NaCl]+ 1133.0 (2).
1C. 54.25 mg (0.089 mmol) of 1 and 49.29 mg (0.089 mmol) of
dppf. Isolated yield: 98.8% (89.46 mg). Found: C 54.73, H 3.74, N
2.67. Calcd. for C46H36N2P2FePd2Cl2: C 54.25, H 3.56, N 2.75. UV−
vis (CHCl3) λmax/nm (ε/104 M−1 cm−1): 325 (1.54), 460 (0.40), 488
sh (0.36). (+)ESI-MS m/z (rel. int., %): [Pd2Cl(L1-2H)(dppf)]+
983.0 (100); [Pd2Cl(L1-2H)(dppfOH)]+ 1000.0 (17); [Pd4Cl2(L1-
2H)2(dppf)3]2+ 1260.0 (21).
2C. 51.20 mg (0.080 mmol) of 2 and 44.34 mg (0.080 mmol) of
dppf. Isolated yield: 99.2% (83.18 mg). Found: C 53.41, H 3.33, N
2.28. Calcd. for C47H38N2OP2FePd2Cl2: C 53.85, H 3.65, N 2.67. UV−
vis (CHCl3) λmax/nm (ε/104 M−1 cm−1): 361 (1.26), 474 (0.59).
(+)ESI-MS m/z (rel. int., %): [Pd2Cl(L2-2H)(dppf)]+ 1013.0 (51);
[Pd2Cl(L2-2H)(dppfOH)]+ 1030.0 (100); [Pd4Cl2(L2-
2H)2(dppf)3]2+ 1290.0 (4).
3C. 50.44 mg (0.072 mmol) of 3 and 40.05 mg (0.072 mmol) of
dppf. Isolated yield: 99.0% (79.13 mg). Found: C 52.49, H 3.16, N
4.97. Calcd. for C48H40N4O2P2FePd2Cl2: C 52.11, H 3.64, N 5.06.
UV−vis (CHCl3) λmax/nm (ε/104 M−1 cm−1): 300 (2.05), 347 sh
(1.18), 425 (0.80), 564 (2.78). (+)ESI-MS m/z (rel. int., %):
[Pd2Cl(L3-2H)(dppf)]+ 1071.1 (91); [Pd2Cl(L3-2H)(dppfOH)]+
1088.1 (100); [Pd4Cl2(L3-2H)2(dppf)3]2+ 1348.1 (11).
1D. 38.38 mg (0.063 mmol) of 1 and 69.75 mg (0.126 mmol) of
dppf. Isolated yield: 99.5% (98.44 mg). Found: C 60.97, H 4.36, N
1.69. Calcd. for C80H64N2P4Fe2Pd2Cl2: C 61.10, H 4.10, N 1.78. UV−
vis (CHCl3) λmax/nm (ε/104 M−1 cm−1): 364 (1.13), 380 sh (1.01),
455 (0.13). (+)ESI-MS m/z (rel. int., %): [Pd2Cl(L1-2H)(dppf)2]+
1537.1 (100); [Pd2Cl(L1-2H)(dppf)(dppfOH)]+ 1553.1 (4);
[Pd2Cl2(L1-2H)(dppfOH)2+Na]+ 1629.0 (14); [Pd4Cl2(L1-
2H)2(dppf)3]2+ 1260.1 (3); [Pd4Cl4(L1-2H)2(dppf)3+CH3OH
+2Na]2+ 1305.6 (1).
ASSOCIATED CONTENT
* Supporting Information
■
S
Procedures for the mechanochemical and solvent-based
reactions; PXRD patterns of reactants and products for all
experiments; X-ray crystallographic data (CIF) for CCDC
975112−975114; details of molecular and crystal structure of
2A and 3B; crystallographic data collection and structure
refinement data; details about the solving and the refinement of
the structure for 1C and 1D; 31P NMR spectra of all complexes
(in CHCl3 and MAS); calculated geometries and energies of
isomers of complexes; UV−vis spectra and the complete list of
the computed excited states. This material is available free of
charge via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
*E-mail: curic@irb.hr.
■
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
The Ministry of Science, Education and Sports of the Republic
of Croatia provided financial support for this research (Grant
No. 098-0982915-2950, 098-0982904-2953, and 119-1193079-
3069). Computations were done on the Isabella cluster at
SRCE, Zagreb. The authors thank the European Commission
for access to SLONMR (EU FP7 Project “East-NMR”, Grant
No. 228461), and ARRS (Grant No. P1-242). Authors would
like to acknowledge Prof. Robert E. Dinnebier and Dr.
̌
Dubravka Sisa
̌
k for support in the structure determinations
from powder diffraction data, Miha Friedrich and Aleksandar
for help with the NMR measurements, Dr. Primoz Sket
for valuable discussions, and Dr. Snjezana Kazazic
̌
Gace
̌
sa
̌
̌
for help with
̌
́
the ESI-MS measurements.
2D. 37.50 mg (0.058 mmol) of 2 and 64.95 mg (0.117 mmol) of
dppf. Isolated yield: 99.1% (93.04 mg). Found: C 60.33, H 4.49, N
1.74. Calcd. for C81H66N2OP4Fe2Pd2Cl2: C 60.70, H 4.15, N 1.75.
UV−vis (CHCl3) λmax/nm (ε/104 M−1 cm−1): 382 (1.30), 483 (0.23).
(+)ESI-MS m/z (rel. int., %): [Pd2Cl(L2-2H)(dppf)2]+ 1567.1 (85);
[Pd2Cl(L2-2H)(dppf)(dppfOH)]+ 1585.2 (14); [Pd2Cl2(L2-2H)-
(dppfOH)2+Na]+ 1659.0 (25); [Pd4Cl2(L2-2H)2(dppf)3]2+ 1290.1
(100); [Pd4Cl4(L2-2H)2(dppf)3+CH3OH+2Na]2+ 1335.6 (69).
3D. 35.60 mg (0.051 mmol) of 3 and 56.53 mg (0.102 mmol) of
dppf. Isolated yield: 98.9% (83.74 mg). Found: C 58.98, H 4.38, N
3.33. Calcd. for C82H68N4O2P4Fe2Pd2Cl2: C 59.30, H 4.13, N 3.37.
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