Insight into luminescent bisazoaromatic CNN pincer palladacycle: Synthesis, structure, electrochemistry and some catalytic applications in C-C coupling

Article abstract of DOI:10.1039/c4ra16584e

Chelating behavior of a potential flexidentate bisazoaromatic molecule 2-(phenylazo)azobenzene (PAAB) toward palladium have been examined. Activation of one instead of the two C(Ph)-H bonds of PAAB (HL) in the course of the metallation furnishes an exclus

Full text of DOI:10.1039/c4ra16584e

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Insight into luminescent bisazoaromatic CNN
pincer palladacycle: synthesis, structure,
electrochemistry and some catalytic applications in
C–C coupling†
Cite this: RSC Adv., 2015, 5, 22544
*
Sima Roy, Shuvam Pramanik, Tapas Ghorui and Kausikisankar Pramanik
Chelating behavior of a potential flexidentate bisazoaromatic molecule 2-(phenylazo)azobenzene (PAAB)
toward palladium have been examined. Activation of one instead of the two C(Ph)–H bonds of PAAB
(HL) in the course of the metallation furnishes an exclusive unsymmetrical mode of ligation.
Consequently, another member of the important class of CNN pincer palladacycles is achieved in almost
quantitative yields. Crystallographic analysis authenticates the monoanionic terdentate ligation of the
bisazoaromatic molecule and neutral [PdII(L)X] (X ¼ acetate and halides) complexes with planar
palladacycle and out-of-plane pendant phenyl ring. Unlike the N-donor pincers incorporating imine
moieties, the introduction of two azo chromophores not only makes the PAAB ligand strongly p-acidic
but also significantly modulates the FMOs in the palladacycles. An appreciable lowering of ligand-
centered (LC) p* orbitals is apparent upon complexation which is in accordance with the
electrochemical responses where large anodic shifts of LC LUMO and LUMO + 1 by 0.8–0.9 V have
been noted. The significant modification of FMOs leads to interesting optoelectronic properties in the
palladacycles. The cyclopalladated compounds are luminescent in solution at room temperature. This
property is plausibly a result of the LC emissive states (azo p*). The optoelectronic features are
interpreted by the Time-Dependant (TD) density functional theory (DFT) and natural transition orbital
(NTO) analyses. In addition, scrutiny of Suzuki–Miyaura and Heck-type reactions indicates that the pincer
palladacycles may be employed in the development of useful catalysts for C–C coupling reactions.
Received 17th December 2014
Accepted 16th February 2015
DOI: 10.1039/c4ra16584e
www.rsc.org/advances
photoconductivity, photoisomerization in addition to electro-
chemical and catalytic activities.6 Notably, the [C,N] palladacycles
Introduction
reported so far contain a single azo chromophore. Introduction
of a higher homologue of azobenzene with multiple azo func-
tionalities may appear as an attractive route to synthesize
interesting azo compounds. We have employed a ligand incor-
porating two azo chromophores in conjugation with phenyl
rings, 2-(phenylazo)azobenzene (PAAB) (Scheme 2). A compar-
ison of frontier molecular orbitals (FMOs) of PAAB with that of
azobenzene signies superior s-donor as well as p-acceptor
behavior of the former (Scheme 2). The reason behind the
choice of such multidentate azoaromatic compound lies in its
orthometallating nature and azo-based enhanced p-acceptor
Aer the discovery of a [C,N] palladacycle via activation of
the ortho C–H bond of azobenzene by Cope and coworkers1 in
1965, orthometallated palladium compounds have received
increasing attention with azo, imine and other functionalities
over the decades in coordination and organometallic chem-
istry.2 Among azobenzene complexes, two types of coordination
modes namely monocyclopalladated3 and doubly cyclopalladated4
species have been documented (Scheme 1). Recently, another
form of double cyclopalladation has been reported with 4-(phe-
nylazo)azobenzene (Scheme 1).5 Transition metal complexes with
the azo chromophore have been found to display interesting
physicochemical properties such as photoluminescence (PL),
Department of Chemistry, Inorganic Chemistry Section, Jadavpur University, Kolkata –
700032, India. E-mail: kpramanik@hotmail.com; Tel: +91 33 2457 2781
† Electronic supplementary information (ESI) available: NMR, absorption,
emission spectra and photoluminescence decay of ligand, Tables and gures
for TD-DFT of complexes, crystallographic data and structure renement
parameters, coordinates of optimized geometries of complexes. CCDC
1031475–1031479. For ESI and crystallographic data in CIF or other electronic
format see DOI: 10.1039/c4ra16584e
Scheme 1 Mono- and doubly cyclopalladation of azobenzene and
4-(phenylazo)azobenzene.
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Scheme 2 Comparison of FMOs between azobenzene and 2-(phenylazo)azobenzene at B3LYP/(6-311+G(d,p) level.
ability, which causes strong d-orbital splitting and leads to another member of the important class of unsymmetrical CNN
signicant stabilization of lower valence states of the metal. pincer. The CNN pincer complexes incorporating Pd(II) are less
Substantial lowering of ligand-centered (LC) virtual FMOs in the common compared to that of Pt(II).7a,8e,12 Notably, pincer type
bisazoaromatic system relative to that of metal-centered (MC) palladacycles are one of the most well-known and scrutinized
level may offer an easy way to modify the optoelectronic prop- classes of orthometallated compounds.13 These attractive
erties of the palladacycles. The complexes display luminescence classes of non-phosphine palladacycles disclose a challenge to
in uid solution at room temperature. The reports on lumi- chemists not only in terms of their synthesis but also in terms of
nescent mononuclear nonporphyrin palladacycles at room their structure, design, and the mode of coordination of the
temperature are sparse and they usually display luminescence ligand to the metal. Signicantly, unlike conjugated aromatic
only in a rigid matrix at 77 K.7,8 Recently, Che and coworkers polyimine molecules, the polyazo ligands are sparse in the
reported the most luminescent Pd(II) complexes till date in uid pincer chemistry and there are only two recent reports14
solution using tetradentate ONCN ligand.9 The lack of room of bisazoaromatic ligands viz. 3-(phenylazo)azobenzene and
temperature luminescence for cyclopalladated compounds in 2,6-bis(phenylazo)pyridine with Rh(III) and Fe(II) respectively. It
solution is likely due to population of thermally accessible low- is worth mentioning that the variation of coordinating atoms in
lying MC state (4dx2ꢀy2) which leads to effective non-radiative
decay through severe excited-state structural distortion.10
Though a report of isolation and in part characterization of
the ligand has been previously documented,11 we present a
modied synthetic procedure with comprehensive character-
ization including crystal structure determination. We observe
an exclusive pincer CPhNazoNazo type ligation of PAAB toward
Pd(II) (coordination mode A), nonetheless the free ligand
possesses four potential donors following the activation of
two C–H bonds (Scheme 3). Accordingly, the ligand provides
Scheme 3 Probable coordination modes of PAAB.
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Table 1 Electrochemical Dataa
the anionic six-electron donor ligand may alter the nature of the
metal center radically and make the palladacycles useful for
much wider application in the organic synthesis and catalysis.15
In this context, deployment of superior p-accepting pincers are
most enviable since the majority of catalytic cycles of Pd-
catalyzed reactions are generally based on PdII/Pd0 shuttles.16
In this work, we report the high-yield synthesis and character-
ization of a new set of unsymmetrical CNN pincer palladacycles
with 2-(phenylazo)azobenzene, as well as the discussion and
rationalization of their structural and optoelectronic properties.
Presence of low-lying LC virtual orbitals is appraised by means
of electrochemical study (cyclic voltammetry). The experimental
results are complemented by theoretical calculations in order to
explain the nature of the electronic transitions in the pallada-
cycles. Preliminary catalytic activity of the non-phosphine Pd(II)
complexes has been explored toward C–C coupling reaction.
Compound
E1/2/V (DEp/mV)
PAAB
ꢀ1.21(160), ꢀ1.51(160)
ꢀ0.30(81), ꢀ0.76(76)
ꢀ0.28(80), ꢀ0.73(76)
ꢀ0.26(82), ꢀ0.74(78)
ꢀ0.26(82), ꢀ0.73(77)
[Pd(L)(OAc)] 1
[Pd(L)Cl] 2
[Pd(L)Br] 3
[Pd(L)I] 4
a
Solute concentration z10ꢀ3 mol dmꢀ3 scan rate, 50 mV sꢀ1; E1/2
¼
0.5(Epa ꢀ Epc), where Epa and Epc are the anodic and cathodic peak
potentials, respectively; DEp ¼ Epa ꢀ Epc
.
displays a peak at m/z 287 amu for [L + H]+ (ESI, Fig. S3†). The
analogous spectra of complexes show the peaks for [M ꢀ X]+ and
[2M ꢀ X]+ with the expected isotopic distribution pattern (ESI,
Fig. S4†), indicating their substantial stability in solution. The
NMR spectra of ligand and complexes are presented in ESI
(Fig. S5 and S6†).
Results and discussion
Synthesis, characterization and electrochemical study
The redox behavior of the uncoordinated ligand and the
cyclopalladated compounds has been scrutinized by cyclic
voltammetry; the data collected in Table 1. The free ligand
exhibits two successive one-electron quasi-reversible reductive
responses, at ꢀ1.21 and ꢀ1.51 V (vs. Ag/AgCl) (Fig. 1), indicating
the chronological lling of p* orbitals. The facile reductive
nature of PAAB is consistent with its superior p-acceptor ability.
A signicant anodic shi of these redox couples, by ꢂ0.9 and
ꢂ0.8 V respectively, was observed upon palladation (Fig. 1). The
reductive responses near ꢀ0.25 and ꢀ0.75 V vs. Ag/AgCl may be
attributed to the azo reductions in the complexes and this trend
can be ascribed to remarkable stabilization of ligand-centered
LUMO and LUMO + 1 in presence of electro-positive metal
center.
To gain insight into the nature of the redox orbitals, we have
performed DFT calculation of the free ligand and complexes.
The observed redox responses nicely substantiated with the
theoretical study. Illustrations of the selected FMOs of the
complexes are presented in Fig. 2. Calculation indicates that
LUMO and LUMO + 1 of PAAB encompass exclusively the azo as
well as phenyl moieties and remain symmetric along the C2 axis.
The PAAB ligand was synthesized by Baeyer–Mills coupling
reaction17 of 2-(amino)azobenzene with nitrosobenzene in
glacial acetic acid at elevated temperature followed by the
extraction with n-hexane. The reaction of Pd(OAc)2 and PAAB in
glacial acetic acid afforded [Pd(L)OAc] 1 in almost quantitative
yield. The chloro derivative [Pd(L)Cl] 2 was obtained from
K2[PdCl4] under similar reaction conditions in near stoichio-
metric yield. The ligand furnishes unsymmetrical terdentate
CNN mode of coordination in the palladacycles following the
activation of one C(Ph)–H bond despite of the presence of two
such C–H bonds in the free ligand. Coordination mode A is
favored over coordination mode B plausibly due to get rid of the
structural strain within the rigid PAAB skeleton. The anionic six-
electron donor PAAB furnishes two contiguous ve-membered
chelates and provides robust metal–organic conjugates, which
limits their dissociation from the metal center. The other
halo derivatives [Pd(L)Br] 3 and [Pd(L)I] 4 are obtained by
substituting the OAcꢀ of 1 with the respective excess of Xꢀ (X ¼
Br, I) in acetone. All the complexes are soluble both in polar and
nonpolar solvents such as toluene, dichloromethane, acetoni-
trile, ethanol, ether and furnishing a red solution. The acetato
complex was obtained as H-bonded dimer, where the free O
atoms of acetato groups are linked with one water molecule via
H bonding. The presence of water of hydration was corrobo-
rated with thermogravimetric analysis (TGA). Thermogram (ESI,
Fig. S1†) shows two decomposition steps within the tempera-
ꢁ
ture range 40–300 C. The rst step of decomposition, within
the temperature range 40–150 ꢁC, corresponds to the loss of
water molecule of hydration with a mass loss of 2.2% (calcd
2.0%). The second step (150–300 ꢁC) corresponds to the removal
of the acetate group with a mass loss of 12.7% (calcd 13.1%).
Sharp vibrations around 1486–1439 cmꢀ1 in the infrared
spectra of the free ligand were assigned to the nN]N (ESI,
Fig. S2†). The observed lowering of nN]N values in complexes
(1445–1430 cmꢀ1) as compared to that of free ligand is consis-
tent with the Pd(II) / p*(azo) back-bonding. An electrospray
ionization (ESI) mass spectrum (MS) of the ligand in acetonitrile Fig. 1 Voltammogram of PAAB and 2 in acetonitrile at 50 mV sꢀ1
.
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Fig. 2 Isodensity surface plots of some selected FMOs (HOMO ꢀ 1 to LUMO + 2) for the complexes 1, 2, 3 and 4 at their optimized S0 geometry
in gas phase (isodensity value 0.05 e Bohrꢀ3).
A sizeable stabilization of both LUMO and LUMO +1 by ꢂ0.60 these FMOs in the palladacycles remain almost insensitive with
and ꢂ0.75 eV respectively upon complexation has been the nature of coligands, excluding their involvement in the
computed (ESI, Tables S1–S5†). In these complexes, a slight redox processes. The components of LUMO and LUMO + 1 of all
polarization between the coordinated and pendant phenyl rings the complexes are essentially similar in nature and comprise
has been observed for these virtual orbitals. The energies of exclusively azo-p* (50%) and phenyl-p* (50%) of coordinated
Table 2 Selected Metrical Parameters for PAAB, 1, 2, 3 and 4
Bond parameter
PAAB
1
2
3
4
Pd1–C1
Pd1–N2
Pd1–N3
Pd1–O1
1.974(2)
1.945(2)
2.182(2)
2.053(2)
1.971(3)
1.961(2)
2.229(2)
1.978(3)
1.971(2)
2.227(2)
1.984(5)
1.973(4)
2.207(4)
Pd1–Cl1
2.289(1)
Pd1–Br1
2.412(1)
Pd1–I1
N1–N2
N3–N4
C1–Pd1–N2
N2–Pd1–N3
O1S/O2A (H1SB/O2A)
O1S/O4A (H1SB/O4A)
2.590(1)
1.272(5)
1.258(5)
79.39(17)
78.75(14)
1.244(1)
1.243(1)
1.274(2)
1.258(3)
79.27(9)
80.69(7)
2.868 (2.036)
2.869 (2.086)
1.282(3)
1.242(3)
79.49(10)
78.90(9)
1.280(3)
1.252(3)
79.70(10)
78.70(9)
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bisazoaromatic ligand with practically no participation of metal in the corresponding dN–N distances. The lengthening of N1–N2
˚
orbitals (ESI, Tables S2–S5†). It is very likely that these electro- as compared to N3–N4 bonds by ꢂ0.03 A is in accordance with
chemical processes correspond to the reduction of the bisa- the greater amount of Pd(d) / azo(p*) back donation in the
zoaromatic system, generating the metal-stabilized azo anion former azo linkage. A systematic variation in the Pd–N2 bond
radicals (eqn (1) and (2)). Redox noninnocence behavior of the was observed in the four complexes with the shortest length in 1
azo ligands is well documented in the literature.18
and the longest in 4. This observation is in accordance with the
trans effect order: Iꢀ > Brꢀ > Clꢀ > OAcꢀ.
[PdII(LꢀI)X] + eꢀ 4 [PdII(LꢀII)X]ꢀ
(1)
(2)
The Pd1–N3 bond length remains almost unaltered in the
˚
halo compounds while a slight decrease of ꢂ0.04 A in 1 occurs
[PdII(LꢀII)X]ꢀ + eꢀ 4 [PdII(LꢀIII)X]2ꢀ
possibly due to less steric crowding. The pendant phenyl
rings (C13–C18) do not lie on the molecular plane in the
complexes and makes an angle varying in the range 41–68ꢁ with
the rest. Crystallographic analysis reveals that the two acetato
complexes are joined via intervening water molecule with
Crystal structures
˚
The structures of the free ligand and all four complexes have
been authenticated by single-crystal X-ray diffraction. Crystal-
lographic details are given in ESI (Table S6†), signicant
metrical parameters in Table 2. Rod-shaped crystals suitable for
X-ray crystallography of the free ligand have been grown by
slow evaporation of hexane solution of the compound. Fig. 3
shows the ORTEP and atom numbering scheme of PAAB. The X-
strong H-bonding (Oacetato/Owater 2.87(1) A) as shown in the
inset of Fig. 4. The chelate bite angle C1–Pd1–N2 is marginally
larger than that of N2–Pd1–N3 by ꢂ1ꢁ.
Ground state geometries and frontier molecular orbital
compositions
ray crystallographic analysis reveals a symmetrical (C2) and All the complexes are diamagnetic at room temperature indi-
˚
almost planar structure (md ¼ 0.165 A), with practically iden- cating their singlet ground state. The geometry optimizations of
˚
tical dN–N values of 1.244(1) and 1.243(1) A for two azo groups, the complexes were performed using their crystallographic
respectively. Two phenylazo groups are lying opposite to each coordinates at (R)B3LYP levels in gas phase in their singlet spin
other with respect to the plane passing through the central state without any ligand simplication. The optimized geome-
benzene ring (inset of Fig. 3).
tries of the synthesized complexes and the signicant metrical
Single crystals of complexes 1–4 suitable for X-ray diffraction parameters are given in ESI (Fig. S7 and S8 and Table S7†
analysis were obtained by slow diffusion of their respective respectively). The optimized structural parameters of 1, 2, 3 and 4
toluene solution into hexane solvent. The ORTEP and atom- are in general agreement with the experimental values and the
numbering scheme are shown in Fig. 4. The bis azo ligand slight discrepancy arises due to the crystal lattice distortion
acts as monoanionic tridentate donor toward Pd(II) using one existing in real molecules. The calculated N–N(azo) distance of
˚
CPh and two Nazo atoms with the formation of two contiguous the free ligand (dN–N 1.253 A) is found to be shorter, as compared
˚
ve-membered chelates, albeit it possesses four potential donor to their coordinated forms (average dN–N 1.276 and 1.263 A,
atoms; furnishing an unsymmetrical coordination mode. The respectively), indicating reasonable population of ligand p* in
coordination geometry of all the complexes is described as a complexes. Isodensity plots of some selected FMOs of the Pd(II)
square planar with two dissimilar Pd–N(azo) bond lengths differ complexes and energy levels (H ꢀ 1 to L + 2) are depicted in Fig. 2.
˚
by ꢂ0.25 A. The Nazo atom closer to orthometallated CPh forms a The partial frontier molecular orbital compositions of the PAAB
stronger bond with the metal relative to the other Nazo atom. ligand and the Pd(II) complexes along with the HOMO–LUMO
The appreciable difference in Pd–Nazo bond lengths is reected energy gap, are listed in ESI (Tables S1–S5†).
Fig. 3 ORTEP of PAAB with ellipsoids at the 50% probability level. Hydrogen atoms are omitted for clarity. Inset: Molecular view along the plane
passing through the central benzene ring.
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Fig. 4 ORTEP of 1–4 with ellipsoids at the 50% probability level. Hydrogen atoms are omitted for clarity. Inset: H-bonding interaction in acetato
complex.
In the ground state, the HOMO of 1 mainly consists of characteristics of the spectra for these complexes (Table 3) and
2
2p(OAc) with little 4dz (Pd) character, while that of 2 is consti- the excitations are attributed primarily to the charge-transfer
tuted largely of 3p(Cl) with minor participation of p(Ph) and transitions within the tri-coordinated bisazoaromatic ligand
p(azo). The more stabilized HOMO – 1 in both the complexes with varying amounts of anionic coligand-p orbital and metal-d
are constituted of comparable metal participation (ꢂ25%), orbital participation. To gain deeper insight into the electronic
whereas the involvement of the respective coligand OAc/Cl properties of the transitions involved in the optical absorption
differs appreciably. The HOMO and HOMO ꢀ 1 are mostly processes, we investigated all the synthesized complexes by
4p(Br)/5p(I) coligand character for 3 and 4. The LUMO and means of time-dependent density functional theory (TD-DFT) in
LUMO + 1 are essentially similar in all four complexes and CH2Cl2 solvent using CPCM model. The most relevant transi-
composed of ꢂ50% p*(azo) and ꢂ50% p*(Ph) of the PAAB tions involved, along with their energy, character, oscillator
ligand. These two FMOs are substantially stabilized upon strengths of all the compounds, are listed in ESI (Tables S8–
complexation. It is worth noting that the LUMO + 2 in these S11†). In order to analyze the nature of absorption, we per-
complexes are basically analogous in nature and alike to 4dx2ꢀy2 formed an NTO analysis based on the calculated transition
of Pd (ꢂ45%) with substantial contribution from p*(Ph) density matrices.19 This method offers the most compact
(ꢂ35%). Computation, therefore, reveals that LUMO and LUMO representation of the transition density between the ground and
+ 1 (LC states) are appreciably stabilized relative to the LUMO + excited states in terms of an expansion into single-particle
2 (MC level) by ꢂ1.85 and ꢂ1.45 eV, respectively.
transitions (hole and electron states for each given excitation).
Here we refer to the unoccupied and occupied NTOs as “elec-
tron” and “hole” transition orbitals. The NTOs for 1 are illus-
trated in Fig. 6 and that of 2, 3 and 4 are given in ESI (Fig. S10–
S12†). Based on TD-DFT NTO analysis for 1, lower energy
transition around 545 nm is computed at 580 nm (2.1354 eV, f ¼
0.0730) and 504 nm (2.4585 eV, f ¼ 0.0459) as the 4dzx(Pd) +
2p(OAc) + p(Ph) / p*(azo + Ph) 1MLCT/1LLCT/1ILCT transi-
Photophysical studies
a. UV-Vis absorption spectra. Electronic spectra of
compounds 2 and 4 were recorded in dichloromethane at room
temperature and are depicted in Fig. 5 along with their
respective theoretical spectra and that of compounds 1, 3 and
PAAB are shown in ESI (Fig. S9†). More structured prole
observed for palladacycles may result from the increased elec-
tronic asymmetry caused by the presence of the electron-
accepting PAAB ligand. Multiple transitions are the
tion. The transition near 475 nm (2.6842 eV, f ¼ 0.0673, ltheo
¼
462 nm) is computed as the 2p(OAc) + p(Ph) + 4dxy(Pd) /
p*(azo + Ph) 1LLCT/1ILCT/1MLCT transitions. For 2, lower
energy transitions around 530 nm and 480 nm are computed at
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(2.4631 eV, f ¼ 0.0921). The rst one can be exclusively ascribed
to 4dz2(Pd) / p*(azo + Ph) 1MLCT transition and the latter one
1
is assigned to p(Ph) + 4dzx(Pd) / p*(azo + Ph) ILCT/1MLCT
transition. In the UV region two distinct excitations near 380
and 325 nm are observed. The former excitation can be assigned
exclusively as p(Ph) / p*(azo + Ph) 1ILCT transition for 1 while
incorporation of some 4dz2(Pd) / p*(azo + Ph) 1MLCT char-
acter is found in the halo analogues 2–4. In contrast, higher
energy UV absorption at 316 nm of 1 is described mostly as
p(Ph) / p*(azo) 1ILCT transition mixed with small 1MLCT
nature. For 2, the corresponding peak at 323 nm is assigned as
p(azo + Ph) + 4dz2(Pd) / p*(azo + Ph) 1ILCT transition admixed
with small 1MLCT character. The analogous transition at
318 nm (3.7788 eV, f ¼ 0.0296, ltheo ¼ 312 nm) in 3 is computed
as p(Ph + azo) + 4p(Br) / p*(azo + Ph) and can be attributed
1
to ILCT/1LLCT. In 4, the related band (lexp ¼ 334 nm, ltheo
¼
348 nm) can be ascribed largely as p(Ph) / p*(azo + Ph)
1ILCT transition admixed with some 1LLCT character originated
from 5p(I).
The lowest energy transitions in the complexes are affected
largely by the nature of anionic coligand. Each of these excita-
tions originates from energetically dissimilar occupied orbitals
i.e., exclusive 5p orbital of iodine in 4, while the involvement of
metal t2 becomes the dominating feature on going from 3 to 1.
In contrast, almost similar unoccupied orbitals are found in all
complexes and composed of mainly azo-p* admixed with little
phenyl-p*. The bathochromic shi of longest wavelength
transition from 1 to 4 is associated with the increasing order of
polarizability of the anionic coligand. The major features in the
300–400 nm region in all the complexes can be attributed
largely due to p–p* within the terdentate ligand.
Fig. 5 Experimental (red) and theoretical (green) absorption spectra of
2 (top) and 4 (bottom).
523 nm (2.3803 eV, f ¼ 0.0700) and 488 nm (2.5421 eV, f ¼
0.0669) and both are attributed to the 4dzx(Pd) + 3p(Cl) /
1
p*(azo + Ph) MLCT/1LLCT transition. For 3, the lower energy
b. Luminescence spectra. The complexes exhibit broad
luminescent maxima near 355 nm (Fig. 7) and these remain
unaffected with the energy of excitation wavelengths. Time-
resolved uorescent spectra were recorded to understand the
decay process and the emissive nature of the compounds
(Fig. S13, ESI†). These are found to be moderate emitters
(quantum yields (F) ¼ 3.0–9.0 ꢃ 10ꢀ3). The luminescence
spectrum and time-resolved uorescence decay plot of PAAB are
depicted in ESI (Fig. S14†). Table 3 summarizes the emission
maxima (lem), quantum yield (F), lifetime (s), radiative (kr) and
nonradiative (knr) decay rate constants.
transition around 560 nm is computed at 590 nm (2.1002 eV, f ¼
0.0380) as 4p(Br) + 4dxy(Pd) + p(azo) / p*(azo + Ph) 1LLCT
admixed with some 1MLCT/1ILCT character and at 524 nm
2
(2.3660 eV, f ¼ 0.0353) as 4dz (Pd) / p*(azo + Ph) 1MLCT
transition. The shoulder near 490 nm is calculated at 503 nm
(2.4642 eV, f ¼ 0.0685) as 4p(Br) + 4dz2(Pd) + p(Ph) / p*(azo +
1
Ph) LLCT/1MLCT/1ILCT transition. For 4, lowest energy tran-
sition around 600 nm is computed at 653 nm (1.8970 eV,
f ¼ 0.0257) as predominantly 5p(I) / p*(azo + Ph) 1LLCT
transition. The next lower energy transition near 500 nm
is computed at 507 nm (2.4414 eV, f ¼ 0.0347) and 503 nm
Table 3 UV-vis and luminescence spectral dataa for 1, 2, 3 and 4 in dichloromethane at room temperature
Compound
lmax [nm] (3 [Mꢀ1 cmꢀ1])
lem (nm)
s (ns)
F (ꢃ10ꢀ3
)
kr, sꢀ1 (ꢃ105)
knr, sꢀ1 (ꢃ108)
PAAB
1
306 (26 500), 343b (19 900), 446b (1000)
316 (23 700), 375 (20 000), 475b (5700),
545b (2900)
384
369
7.06
9.52
4.507
4.531
6.384
4.759
1.410
1.046
2
3
4
323 (22 300), 381 (19 000), 480b (5600),
530b (3200)
352
352
352
9.13
8.71
9.23
3.018
7.546
9.358
3.306
8.664
1.092
1.139
1.073
318 (25 500), 377 (21 000), 490b (6000),
560b (2500)
334 (22 400), 382 (19 400), 500b (5500),
600b (1300)
10.139
a
The compounds PAAB, 1, 2, 3 and 4 are excited at 324 nm. b Shoulder.
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Fig. 6 Natural transition orbitals (NTOs) for complex 1 illustrating the nature of singlet excited states in the absorption bands in the range 250–
700 nm. For each state, the respective number of the state, transition energy (eV), and the oscillator strength (in parentheses) are listed. Shown
are only occupied (holes) and unoccupied (electrons) NTO pairs that contribute more than 25% to each excited state.
All CNN pincer complexes in this account are emissive emission spectrum (lem ¼ 352 nm) of 2 is computed at 352 nm
in solution at room temperature which is an uncommon (3.5172 eV, f ¼ 0.0662) and tentatively assigned to as p*(azo +
phenomenon for Pd(II) compounds.7 This is due to the presence Ph) / p(Ph) + 3p(Cl) 1ILCT/1LLCT transition. The emission
of thermal accessible metal-centred (MC) excited states, the spectrum (lem ¼ 352 nm) of 3 is computed at 348 nm (3.5598 eV,
geometry of which is strongly distorted with respect to that of f ¼ 0.0553) and can be ascribed to as p*(azo) / p(Ph) + 4p(Br)
the ground state, leading to the rapid deactivation of the excited 1ILCT/1ILCT transition. Similarly, the observed band at 352 nm
luminescent state.10 The emission spectra of 1–4 are very similar for 4 is computed at 348 nm (3.5619 eV, f ¼ 0.2726) and can be
and comparable to the spectrum of free ligand. In order to tentatively assigned to as mostly p*(azo) / p(Ph) + 5p(I)
analyze the nature of emission, NTO analysis was performed 1ILCT/1LLCT admixed with p*(azo) / dyz(Pd) 1LMCT transi-
based on the calculated transition density matrices in their tion. The photoluminescence property mainly originates from
singlet states. The NTOs are illustrated in Fig. 8. The broad singlet state charge transfer transitions revealing the ligand-
luminescent band of the 1 in the experimental spectrum at 360 centered 1IL nature of the emissive state for all complexes.
nm is computed at 356 nm (3.4853 eV, f ¼ 0.1492) with signif- Our investigation has shown that the nature of the luminescent
icant transition having p*(azo + Ph) / 2p(OAc)+ p(Ph) + dyz(Pd) occupied (hole) transition orbitals of 1–4 and PAAB remains
and attributed to 1ILCT/1LLCT/1LMCT. The experimental virtually similar and emerged as conjugated p* orbital
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and iodo analogues. The NTO analysis, therefore, suggest a
ligand-dominated emissive states p*(azo + Ph) by virtue of their
enhanced stability. The structurally similar luminescence
spectra can be assigned as intraligand p–p* transitions within
the planar bisazo chromophore perturbed by the anionic col-
igands and metal in varying extent. Notably, no metal contri-
bution (dx2ꢀy2) was found computationally in the emissive state
of the present palladacycles. It should be noted that the reports
of luminescent of azo complexes are less documented in the
literarture.20
Catalysis
Many of the N-donor ligands are inexpensive and stable in air
unlike phosphine ligands. Facile synthesis, stability and easy
handling of non-phosphine cyclometalated Pd complexes are
accountable for their popularity and growing applications in
catalysis. Since the C–C coupling reactions are of vital impor-
tance in organic synthesis and the palladacycles are a class of
typical catalysts in these reactions,12a,15d,e,l the Suzuki–Miyaura
cross-coupling reaction of aryl halides with phenylboronic acid
was carried out to explore the catalytic activity of the non-
phosphine pincers 1 and 4. Upon heating a mixture of aryl-
bromide, phenylboronic acid and K2CO3 in toluene at 70 ꢁC for
Fig. 7 Luminescence spectra of complexes.
encompassed over both the azo chromophores (Fig. 8). The
luminescent unoccupied (electron) transition orbital appears
exclusively as p(Ph) in the free ligand while that in the palla-
dacycles is dependent upon the nature of anionic coligands.
The corresponding orbital appears as the combination of p(Ph)
with p(coligand) in chloro and bromo complexes whereas
different amounts of metal-d participation is found in acetato
Fig. 8 Natural transition orbitals (NTOs) for the complexes 1, 2, 3 and 4 illustrating the nature of the singlet excited state in the emission band in
the region 340–380 nm. For each state the transition energy (eV) and the oscillator strength (in parentheses) are listed. Shown are only occupied
(holes) and unoccupied (electrons) NTO pairs that contribute more than 30% to each excited state.
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Table 4 Suzuki–Miyaura coupling reaction of 1 and 4 with aryl halides and phenylboronic acida
Amount of cat.
(mol%)
t
(h)
Entry
Catalyst
R
X
T (ꢁC)
Product
Yieldb (%)
TONc
1
2
3
4
5
6
7
8
9
1
4
1
4
1
4
1
1
1
COCH3
COCH3
NO2
NO2
CN
CN
COCH3
NO2
Br
Br
Br
Br
Br
Br
Cl
Cl
Cl
0.001
0.001
0.001
0.001
0.001
0.001
0.01
70
70
70
70
70
3
3
3
3
3
3
6
6
6
5
5
6
6
7
7
5
6
7
83
81
89
88
93
92
10
63
60
83 000
81 000
89 000
88 000
93 000
92 000
1000
70
110
110
110
0.01
0.01
6300
6000
CN
a
Reaction conditions: aryl halide (1.0 mmol), phenyl boronic acid (1.3 mmol), K2CO3 (2 mmol), Pd catalyst, toluene (5 mL). b Isolated yield. c TON,
turnover number {(mol of product)/(mol of catalyst)}.
3 h in the presence of Pd complexes, the biaryls were isolated in boronic acid had also been performed. Aryl chlorides provide
excellent yields (Table 4). In general, the electron rich palladium the corresponding biaryls in moderate to low yields (entries 7–9)
center is benecial toward Suzuki–Miyaura coupling reactions. with 0.01 mol % catalyst loading. Examples of Suzuki–Miyaura
The reported electron decient palladacycles 1 and 4 show high coupling with aryl chlorides are scarce with CN2 coordinated
turnover numbers with good yields for certain substituted pincer palladacycles.15d
bromobenzenes. As shown in Table 4, only the strong electron-
The report of the C–C cross coupling by Heck-type reactions
withdrawing group on phenyl bromides can generate the cor- catalyzed by the non-NHC CNN pincer Palladacycles are sparsely
responding products. Notably, we were unable to isolate any known.12e,g,h The high stability of the present pincer complexes
coupling product upon varying the substituents on phenyl may make themselves as promising candidates to achieve the
bromides from strong electron-withdrawing to moderate high turnover numbers (TON's) under the harsh conditions of
electron-withdrawing groups e.g., carbaldehyde or electron- the Heck coupling reactions. In this context, investigation into
donating groups e.g., methoxy derivative under similar reac- the ability of easily obtainable complexes 1 and 4 to act as
tion conditions. To explore the versatility of the reported pal- precatalysts has been performed for Heck-type coupling reac-
ladacycles, the coupling between aryl chlorides and phenyl tions. The results obtained are summarized in Table 5. Though
Table 5 Heck coupling of 1 and 4 with functionalized aryl halides and styrenea
Amount of cat.
(mol%)
Entry
Catalyst
R
X
Product
Yieldb (%)
TONc
1
2
3
4
5
6
7
8
1
4
1
4
4
1
4
1
4
1
4
H
H
I
I
0.01
0.01
0.01
0.01
0.001
0.01
0.01
0.01
0.01
0.01
0.01
8
8
9
9
74
70
84
86
83
72
68
79
76
63
58
7400
7000
8400
8600
83 000
7200
6800
7900
7600
6300
5800
CHO
CHO
CHO
COCH3
COCH3
NO2
NO2
OCH3
OCH3
Br
Br
Br
Br
Br
Br
Br
Br
Br
9
10
10
11
11
12
12
9
10
11
a
Reaction conditions: aryl halide (1.0 mmol), styrene (1.5 mmol), K2CO3 (1.8 mmol), Pd catalyst, dimethylformamide (7 mL), 135 ꢁC, 18 h. b Isolated
yield. c TON, turnover number {(mol of product)/(mol of catalyst)}.
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catalysis was observed in both the cases, the acetato complex 1 Miyaura and Heck-type reactions. Novel functionalized bisa-
shows somewhat better activity compared to that of the iodo zoaromatic ligands and their cyclometalated complexes are
analogue 4. All the reactions have been performed under argon currently under investigation.
ꢁ
with 0.01 mol% of catalysts in DMF at 135 C with K2CO3 (1.8
equiv. of base). The use of 0.001 mol % catalyst loading merely
affects the efficiency (entry 5). The complexes under investiga-
tion are thermally stable and inert toward air and moisture in
Experimental section
General information
the solid state. These properties allowed for catalytic experi-
ments under aerobic conditions as well. The use of K2CO3 as
base furnished better yield compared to that of NaOAc for the
coupling reactions. Different para-functionalized aryl halides
are used in the catalytic experiments. The palladacycles have
shown good catalytic activity over 18 h at elevated temperature
with activated substrates such as 4-bromobenzaldehyde, 4-bro-
monitrobenzene and 4-bromoacetophenone. A slightly lower
activity was observed with the deactivated substrates like 4-bro-
moanisole. The coupling of bromobenzene with styrene pro-
ceeded as well but with low yield.
All manipulations were performed under an atmosphere of
argon except where mentioned. Solvents were dried by standard
methods and distilled under argon prior to use. Palladium
chloride, palladium acetate and styrene were purchased from
Aldrich Chemical Company. Other chemicals and solvents were
purchased from Merck, India and dried before use according to
standard methods.
Synthesis of ligand
2-(Phenylazo)azobenzene (PAAB). 2-Amino-azobenzene21
(5 g, 0.025 mol) was dissolved in 20 mL glacial acetic acid with
heating. Solid nitrosobenzene (2.9 g, 0.027 mol) was added and
the mixture was stirred for 24 h at 50 ꢁC until the precipitation
of a dark mass was obtained. The precipitate was dried in
vacuo and subsequently dissolved in minimum volume of hot
n-hexane. The deep red crystals were obtained from the hot
hexane solution. An n-hexane extract was made with the ltrate,
giving an additional crop of red crystals. Yield: 6.24 g, 86%. mp.
107–108 ꢁC (lit. mp. 106ꢀ108 ꢁC);11 FT-IR (cmꢀ1): 1486–1439
(nN]N); 1H NMR (300 MHz, CDCl3): d 7.96–7.98 (m, 4H), 7.76 (q,
2H, J ¼ 3.3 Hz), 7.51–7.58 (m, 8H). 13C NMR (300 MHz, CDCl3): d
153.1, 148.2, 139.3, 130.9, 129.1, 123.3, 118.2. Elem. anal. calcd
(%) for C18H14N4: C, 75.50; H, 4.93; N, 19.57. Found: C, 75.44; H,
4.87; N, 19.42. HRMS (ESI): m/z calcd for C18H15N4 287.1297;
found 287.1293 (M + H)+.
Conclusion
We have introduced a typical bisazobenzene, 2-(phenylazo)
azobenzene, as the novel chelating ligand and have presented a
high-yield preparation of a new class of unsymmetrical CPh
-
N
azoNazo pincer palladacycles with the symmetrical ligand. The
reported palladacycles represent the rst member of pincer
compounds containing two Nazo atoms. A noteworthy feature of
the ligand is the presence of two conjugated p-acidic azo
chromophores, which makes it both a superior s-donor as well
as p-acceptor relative to azobenzene. Remarkable stabilization
of azo-based virtual orbitals is also apparent upon palladation,
where LC (LUMO and LUMO + 1) states are computed to be the
lower energy orbitals relative to MC (LUMO + 2) level. Low-lying
nature of LUMO and LUMO +1 in the free ligand as well as in all
the palladacycles is nicely substantiated with the electro-
chemical study. In consequence, modication of FMOs leads to
Synthesis of complexes
[Pd(L)(OAc)] 1. To a solution of PAAB (317 mg, 1.1 mmol) in
interesting optoelectronic property in the palladacycles. The AcOH (25 mL) was added Pd(OAc)2 (226 mg, 1.0 mmol) under an
cyclopalladated compounds exhibit luminescence in solution at argon atmosphere. The mixture was stirred at room tempera-
room temperature. This property is plausibly as a result of the ture for 80 h. The solvent was removed under reduced pressure.
enhanced stability of LC emissive states (azo p*) which are Crystallization by slow diffusion of toluene solution of the
hardly mixed by non-emissive MC state (dx2ꢀy2). The similarity in complex into hexane afforded 1 as rod shaped crystals (445 mg,
luminescence spectra of the cyclometalated complexes suggest 98% yield). FT-IR (cmꢀ1): 1446, 1430 (nN]N), 3524 (nO–H). 1H
a ligand-dominated emissive state, which can be assigned NMR (300 MHz, CDCl3): d 8.31 (d, 1H, J ¼ 7.9 Hz), 8.10 (d, 1H,
essentially as intraligand p–p* transitions admixed with little J ¼ 6.0 Hz), 7.95 (d, 1H, J ¼ 6.1 Hz), 7.82 (d, 1H, J ¼ 7.0 Hz), 7.76–
coligands and/or metal orbitals. Among the palladacycles, the 7.67 (m, 5H), 7.56 (d, 3H, J ¼ 6.6 Hz), 7.28 (s, 1H), 1.72 (s, 3H).
iodo complex exhibits the longest wave-length visible transition 13C NMR (300 MHz, CDCl3): d 177.1, 165.6, 154.9, 152.3, 151.5,
near 600 nm. The observed bathochromic shi of the lowest 134.3, 133.3, 132.1, 129.1, 128.7, 128.3, 126.9, 122.4, 117.9, 23.4.
energy transition in chloro to iodo compounds is caused by the Elem. anal. calcd (%) for C20H16N4O2Pd: C, 53.29; H, 3.58; N,
incorporation of p(halide) / p*(PAAB) LLCT character to a 12.43. Found: C, 53.21; H, 3.62; N, 12.36. HRMS (ESI): m/z calcd
greater extent, consistent with the polarizability order of the for C18H13N4Pd 391.0182; found 391.0180 (M ꢀ OAc)+.
halides. In contrast, analogous excitation in the chloro and
[Pd(L)Cl] 2. To a solution of PAAB (193 mg, 0.674 mmol) in
acetato derivatives is dominated by 4dzx(Pd) / p*(PAAB) EtOH (25 mL) was added K2[PdCl4] (200 mg, 0.612 mmol) under
MLCT. The transitions in the UV region are almost entirely an argon atmosphere. The mixture was stirred at room
bisazoaromatic-based p–p* admixed with little participation of temperature for 80 h, until a dark red solution was formed. The
metal or coligand orbitals. In addition, the results of C–C cross- solvent was removed under reduced pressure. Crystallization by
coupling reactions show that the CNN palladacycles may be slow diffusion of toluene solution of the complex into hexane
employed in the advancement of effective catalysts for Suzuki– afforded 2 as rod shaped crystals (251 mg, 96% yield). FT-IR
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(cmꢀ1): 1446, 1430 (nN]N). 1H NMR (300 MHz, CDCl3): d 8.32 (d,
1H, J ¼ 7.0 Hz), 8.11 (d, 1H, J ¼ 6.9 Hz), 7.97 (d, 1H, J ¼ 5.8 Hz),
7.83 (d, 1H, J ¼ 7.0 Hz), 7.77–7.68 (m, 5H), 7.57 (d, 3H, J ¼ 7.2
Hz), 7.29 (s, 1H). 13C NMR (300 MHz, CDCl3): d 165.9, 152.7,
137.1, 133.7, 133.4, 132.6, 129.4, 128.7, 123.1, 118.4. Elem. anal.
calcd (%) for C18H13N4ClPd: C, 50.61; H, 3.07; N, 13.12. Found:
C, 50.53; H, 3.18; N, 12.97. HRMS (ESI): m/z for C18H13N4Pd
391.0182; found 391.0188 (M ꢀ Cl)+, calcd 819.0052; found
819.0059 (2M ꢀ Cl)+.
4-Phenylbenzonitrile (7).22 1H NMR (300 MHz, CDCl3): d 7.72
(d, J ¼ 8.4 Hz, 2H), 7.66 (d, J ¼ 7.5 Hz, 2H), 7.59 (d, J ¼ 8.1 Hz,
2H), 7.53–7.42 (m, 3H). 13C NMR (300 MHz, CDCl3): d 145.6,
139.2, 132.6, 129.1, 128.7, 127.7, 127.2, 118.9, 110.9.
General procedure for Heck-coupling reactions
A mixture of K2CO3 (1.8 mmol), aryl halide (1 mmol) and
styrene (1.5 mmol) were added in a ask and put under an
atmosphere of argon followed by addition of 7 mL degassed
DMF. Aer thermostating at 135 ꢁC for 15 min the catalyst
(0.01 mol%) was added against a positive stream of argon. The
mixture was allowed to heat at 135 ꢁC for 18 h. Aer cooling to
ambient temperature, the solvent was removed under
vacuum and puried by column chromatography on silica gel,
using 20 : 1 hexane/ethyl acetate as eluent. Products were
identied by comparison of NMR data with those in the
literature.
[Pd(L)Br] 3. To a solution of 1 (200 mg, 0.444 mmol) in
acetone, KBr solution (10ꢀ2 molar solution in water) was added
dropwise and the resulting solution was stirred for 1 h at room
temperature. The dark residue was ltered and dried in vacuum
(191 mg, 91% yield). FT-IR (cmꢀ1): 1444, 1430 (nN]N). 1H NMR
(300 MHz, CDCl3): d 8.31 (d, 1H, J ¼ 6.9 Hz), 8.11 (d, 1H, J ¼ 6.2
Hz), 7.95 (d, 1H, J ¼ 5.6 Hz), 7.82 (d, 1H, J ¼ 6.9 Hz), 7.76–7.68
(m, 5H), 7.56 (d, 3H, J ¼ 7.5 Hz), 7.28 (s, 1H). 13C NMR
(300 MHz, CDCl3): d 162.3, 139.5, 133.7, 133.6, 133.4, 132.5,
129.5, 128.8, 126.8, 123.0, 118.5. Elem. anal. calcd (%) for C18-
H13N4BrPd: C, 45.84; H, 2.78; N, 11.88. Found: C, 45.71; H, 2.61;
N, 11.75. HRMS (ESI): m/z for C18H13N4Pd 391.0182;
found 391.0186 (M ꢀ Br)+, calcd 862.9543; found 862.9547
(2M ꢀ Br)+.
(E)-1,2-Diphenylethene (8).22 1H NMR (400 MHz, CDCl3): d
7.51 (d, 4H, J ¼ 7.6 Hz), 7.36 (t, 4H, J ¼ 8.0 Hz), 7.26 (t, 2H, J ¼
7.2 Hz), 7.11 (s, 2H). 13C NMR (300 MHz, CDCl3): d 137.4, 128.8,
127.9, 127.7, 126.6.
(E)-1-Formyl-4-styrylbenzene (9).22 1H NMR (400 MHz,
CDCl3): d 9.99 (s, 1H), 7.87 (d, 2H, J ¼ 8.4 Hz), 7.66 (d, 2H, J ¼
8.4 Hz), 7.55 (d, 2H, J ¼ 7.6 Hz), 7.39 (t, 2H, J ¼ 8.0 Hz), 7.32 (t,
1H, J ¼ 7.2 Hz), 7.27 (d, 1H, J ¼ 16.4 Hz), 7.15 (d, 1H, J ¼
16.4 Hz). 13C NMR (300 MHz, CDCl3): d 191.6, 143.4, 136.6,
135.4, 132.2, 130.2, 128.8, 128.5, 127.3, 126.9.
[Pd(L)I] 4. The complex 4 was prepared from 1 (200 mg,
0.444 mmol) with KI using similar procedure applied for the
synthesis of complex 3 (212 mg, 92% yield). FT-IR (cmꢀ1): 1435,
1383 (nN]N). 1H NMR (300 MHz, CDCl3): 8.48 (d, 1H, J ¼ 7.7 Hz),
8.36 (d, 1H, J ¼ 7.3 Hz), 8.16 (d, 1H, J ¼ 8.9 Hz), 8.01 (d, 1H, J ¼
7.4 Hz), 7.73–7.67 (m, 5H), 7.56 (d, 3H, J ¼ 7.5 Hz), 7.22 (s, 1H).
13C NMR (300 MHz, CDCl3): d 166.4, 144.0, 133.9, 133.8, 133.5,
132.6, 129.8, 129.0, 126.6, 123.2, 118.7. Elem. anal. calcd (%) for
(E)-1-Acetyl-4-styrylbenzene (10).22 1H NMR (400 MHz,
CDCl3): d 7.96 (d, 2H, J ¼ 8.4 Hz), 7.59 (d, 2H, J ¼ 8.4 Hz), 7.55 (d,
2H, J ¼ 7.2 Hz), 7.39 (t, 2H, J ¼ 8.0 Hz), 7.30 (t, 1H, J ¼ 7.2 Hz),
7.24 (d, 1H, J ¼ 17.2 Hz), 7.14 (d, 1H, J ¼ 16.4 Hz), 2.62 (s, 3H).
13C NMR (300 MHz, CDCl3): d 197.5, 142.0, 136.7, 136.0, 131.5,
128.9, 128.8, 128.3, 127.5, 126.8, 126.5, 26.6.
C
18H13N4IPd: C, 41.68; H, 2.53; N, 10.80. Found: C, 41.49; H,
2.39; N, 10.65. HRMS (ESI): m/z for C18H13N4Pd 391.0182; found
391.0181 (M ꢀ I)+, calcd 910.9414; found 910.9412 (2M ꢀ I)+.
(E)-1-Nitro-4-styrylbenzene (11).22 1H NMR (300 MHz, CDCl3):
d 8.22 (d, 2H, J ¼ 8.7 Hz), 7.64 (d, 2H, J ¼ 8.7 Hz), 7.55 (d, 2H, J ¼
7.1 Hz), 7.42–7.33 (m, 3H), 7.28 (d, 1H, J ¼ 16.3 Hz), 7.14 (d, 1H,
J ¼ 16.5 Hz). 13C NMR (300 MHz, CDCl3): d 146.8, 143.9, 136.2,
133.3, 128.9, 128.8, 127.0, 126.9, 126.3, 124.1.
General procedure for Suzuki–Miyaura coupling reactions
A mixture of K2CO3 (2 mmol), aryl halide (1 mol) and phenyl
boronic acid (1.3 mmol) were added in a ask and put under
an atmosphere of argon followed by addition of 5 mL
degassed toluene. Aer thermostating at an indicated
temperature (typically at 70 ꢁC) for 15 min, the catalyst was
added against a positive stream of argon. The mixture was
allowed to heat with stirring for the requisite time. Aer
cooling to ambient temperature, the solvent was removed
under vacuum and puried by column chromatography on
silica gel, using 20 : 1 hexane/ethyl acetate as eluent. Products
(E)-1-Methoxy-4-styrylbenzene (12).22 1H NMR (400 MHz,
CDCl3): d 7.49 (d, 2H, J ¼ 7.6 Hz), 7.46 (d, 2H, J ¼ 8.4 Hz), 7.35 (t,
2H, J ¼ 7.6 Hz), 7.23 (t, 1H, J ¼ 7.2 Hz), 7.07 (d, 1H, J ¼ 16 Hz),
6.97 (d, 1H, J ¼ 16.4 Hz), 6.90 (d, 2H, J ¼ 8.8 Hz), 3.83 (s, 3H). 13
C
NMR (300 MHz, CDCl3): d 159.4, 137.7, 130.2, 128.7, 128.3,
127.7, 127.2, 126.7, 126.3, 114.2, 55.3.
Physical measurements
were identied by comparison of NMR data with those in the The elemental analyses (C, H, N) were performed with a Perkin-
literature. Elmer model 2400 series II elemental analyzer. FT-IR spectra
4-Acetylbiphenyl (5).22 1H NMR: (300 MHz, CDCl3): d 8.06 (d, were recorded on Perkin-Elmer L1600300 spectrometer. The
J ¼ 8.4 Hz, 2H), 7.71 (d, J ¼ 8.7 Hz, 2H), 7.66 (d, J ¼ 8.4 Hz, 2H), attenuated total reectance method was used. NMR spectral
7.53–7.42 (m, 3H), 2.66 (s, 3H). 13C NMR (300 MHz, CDCl3): d measurement was carried out on Bruker FT 300 MHz and
197.7, 145.8, 139.9, 135.9, 131.9, 129.9, 128.9, 128.2, 127.2, 26.6. Bruker DPX-400 MHz spectrometers with TMS as an internal
4-Nitro-biphenyl (6).22 1H NMR (300 MHz, CDCl3): d 8.23 (d, reference. The electrospray ionization mass spectra (ESI-MS
J ¼ 8.7 Hz, 2H), 7.76 (d, J ¼ 8.4 Hz, 2H), 7.66 (d, J ¼ 8.4 Hz, 2H), positive) were measured in acetonitrile on Waters HRMS spec-
7.47–7.55 (m, 3H). 13C NMR (300 MHz, CDCl3): d 147.6, 147.1, trometers (Model: QTOF Micro YA263). Thermogravimetric
138.7, 129.2, 128.9, 127.8, 127.4, 124.1.
analysis (TGA) was carried out using a SDT Q600 V8.2 Built 100
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thermal analyzer. The electronic spectra in dichloromethane Molecular structure plots were drawn using the Oak Ridge
solution were obtained using a Perkin-Elmer LAMDA 25 spec- thermal ellipsoid plot ORTEP.28
trophotometer with a solute concentration of about 10ꢀ4 M.
Emission spectra were recorded on PTI QM40 spectrometer
in deaerated dichloromethane solutions at room temperature.
Computational study
The molecular geometry of the singlet ground state (S0) of PAAB,
1, 2, 3 and 4 have been calculated by DFT method using the (R)
B3LYP29 hybrid functional approach incorporated in GAUSSIAN
09 program package.30 The geometries of the complexes were
fully optimized in gas phase without imposing any symmetry
constraints. The nature of all the stationary points was checked
by computing vibration frequencies, and all the species were
(3) found to be true potential energy minima, as no imaginary
frequency were obtained (NImag ¼ 0). The single crystal X-ray
coordinates have been used as the initial input in all calcula-
Emission quantum yields of the complexes were determined in
deaerated solutions of the complexes by a relative method using
anthracene in ethanol as the standard.23 The emission quantum
yield (Fr) and radiative (kr) and nonradiative (knr) decay rate
constants for complexes was calculated by the equations24 given
below:
2
Astd Ir hr
Fr ¼ Fstd
2
Ar Istd hstd
F
kr ¼
s
(4)
tions. On the basis of the optimized geometries, the absorption
and emission spectra properties in dichloromethane (CH2Cl2)
media were calculated by the time-dependent density functional
1 ꢀ F
knr
¼
(5)
s
theory (TD-DFT)31 approach associated with the conductor-like
polarizable continuum model (CPCM).32 We computed the
lowest 100 singlet–singlet transitions in absorption and emission
processes respectively. The electronic structure contributions to
the differences in observed optoelectronic properties are
compared by natural transition orbital (NTO) analysis. The
computed vertical transitions were calculated at the equilibrium
geometry of the S0 state and described in terms of one-electron
excitations of molecular orbitals of the corresponding S0 geom-
etry. The calculated transitions with moderate intensities (f $
0.02) can be envisaged going from the lower to the higher energy
region of the spectrum. The palladium atom and iodine were
described by a double-z basis set with the effective core potential
of Hay and Wadt (LANL2DZ)33 and the 6-311+G(d,p) basis set34
was used for the other elements present in the complexes to
optimize the ground state geometries. The calculated electronic
density plots for frontier molecular orbitals were prepared by
using the GaussView 5.0 soware. GaussSum program, version
2.2 (ref. 35) was used to calculate the molecular orbital contri-
butions from groups or atoms.
where Fr and Fstd are the quantum yields of unknown and
standard samples (Fstd ¼ 0.27 (at 298 K) in ethanol at lex ¼ 341
nm),25 Ar and Astd (<0.1) are the solution absorbances at the
excitation wavelength (lex), Ir and Istd are the integrated emis-
sion intensities, and hr and hstd are the refractive indices of the
solvents.26 For all measurements, the same slit widths for exci-
tation and emission were maintained. Time-correlated single
photon counting (TCSPC) measurements were carried out for
recording of the luminescence decay prole for the free ligand
and Pd(II) complexes in dichloromethane solution using a
picosecond diode laser (IBH Nanoled-07) in an IBH Fluorocube
apparatus. Air-equilibrated solutions were used for recording of
the luminescence lifetime. The uorescence decays were
collected on a Hamamatsu MCP photomultiplier (R3809). The
uorescence decays were analyzed using IBH DAS6 soware.
Electrochemical measurements were carried out at 27 ꢁC
with VersaStat II Princeton Applied Research potentiostat/
galvanostat under argon atmosphere. The cell contained a Pt
working electrode and a Pt wire auxiliary electrode. Tetraethy-
lammonium perchlorate (NEt4ClO4) was used as a supporting
electrolyte and the potentials are referenced to the Ag/AgCl
electrode without junction correction.
Acknowledgements
We thank the Department of Science and Technology, New
Delhi, for nancial assistance (grant SR/S1/IC-75/2010). We are
also thankful to the Department of Science and Technology,
New Delhi, India for the data collection on the CCD facility
setup (Jadavpur University). We also acknowledge CAS,
Department of Chemistry, Jadavpur University and the DST-
PURSE program for other facilities. Thanks to Dr S. Chakra-
barti of University of Calcutta, Kolkata for computational
assistance. Ms S. Roy thanks DST and Mr S. Pramanik and Mr T.
Ghorui thank UGC, New Delhi for the research fellowship.
Crystallographic studies
X-ray intensity data for all the compounds were measured at
296(2) K on Bruker AXS SMART APEX II equipped with a CCD
diffractometer using graphite-monochromator (Mo Ka, l ¼
˚
0.71073 A). Metal atoms were located by direct methods, and
the rest of the non-hydrogen atoms emerged from successive
Fourier synthesis. The structures were rened by full-matrix
least squares procedures on F2. All the H atoms were included
in calculated positions and treated as riding atoms using
SHELXL default parameters (except those of water molecule in 1
which were found and isotopically rened). Calculations were
performed using the SHELXTL v 6.14 program package.27
Thermal ellipsoids are drawn at the 30% probability level.
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