Regioselective and regiospecific C(naphthyl)-H bond activation: Isolation, characterization, crystal structure and TDDFT study of isomeric cyclopalladates

Article abstract of DOI:10.1016/j.jorganchem.2014.03.023

The C2(naphthyl)-H, C3(naphthyl)-H and C8(naphthyl)-H bonds of the naphthyl group present in a group of naphthylazo-2′-hydroxyarenes (H₂L) have been activated at room temperature by palladium(II) and stable cyclopalladates of the type [PdL(B)] have been isolated in presence of neutral Lewis bases (B). The activation of C2(naphthyl)-H and C8(naphthyl)-H bonds of 1-(2′-hydroxynaphthylazo) naphthalene (H₂L¹) lead to the formation of isomeric cyclopalladates 2a & 2b respectively. The single crystal X-ray structures of both the isomers show the naphthylazonaphtholate is coordinated to palladium(II) as a dianionic terdentate C,N,O-donor and Lewis base B occupies the fourth position in the coordination sphere. The ortho-palladate (2a) contains both five-membered carbopalladacycle and azonaphtholato chelate ring whereas a five-membered carbopalladacycle and a six-membered azonaphtholato chelate ring are present in peri-palladate (2b). On the other hand, only C3(naphthyl)-H bond of 2-(2′-hydroxyarylazo) naphthalene (H₂L² & H₂L³) has been found to be regiospecifically activated by palladium(II). The role of auxiliary donors on the regioselective and regiospecific C(naphthyl)-H bond activation and the rationale behind the formation of isomeric cyclopalladates have been discussed. All of the cyclopalladates absorb strongly in the ultraviolet and visible region. The Time-dependent density functional theory (TDDFT) calculations reveal that the high energy absorptions are predominantly due to intraligand π-π* and the low energy absorptions originate from intraligand π-π* with a small admixture of metal-to-ligand charge-transfer transitions.

Full text of DOI:10.1016/j.jorganchem.2014.03.023

Journal of Organometallic Chemistry 761 (2014) 147e155
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Journal of Organometallic Chemistry
journal homepage: www.elsevier.com/locate/jorganchem
Regioselective and regiospecific C(naphthyl)eH bond activation:
Isolation, characterization, crystal structure and TDDFT study of
isomeric cyclopalladates
Achintesh Narayan Biswas a, Debatra Narayan Neogi b, Purak Das b, Amitava Choudhury c,
,
Pinaki Bandyopadhyay b
*
a Department of Chemistry, Siliguri College, Siliguri 734001, India
b Department of Chemistry, University of North Bengal, Siliguri 734013, India
c Department of Chemistry, Missouri University of Science and Technology, Rolla, MO 65409-0010, USA
a r t i c l e i n f o
a b s t r a c t
Article history:
The C2(naphthyl)eH, C3(naphthyl)eH and C8(naphthyl)eH bonds of the naphthyl group present in a
group of naphthylazo-20-hydroxyarenes (H2L) have been activated at room temperature by palladium(II)
and stable cyclopalladates of the type [PdL(B)] have been isolated in presence of neutral Lewis bases (B).
The activation of C2(naphthyl)eH and C8(naphthyl)eH bonds of 1-(20-hydroxynaphthylazo) naphthalene
(H2L1) lead to the formation of isomeric cyclopalladates 2a & 2b respectively. The single crystal X-ray
structures of both the isomers show the naphthylazonaphtholate is coordinated to palladium(II) as a
dianionic terdentate C,N,O-donor and Lewis base B occupies the fourth position in the coordination
sphere. The ortho-palladate (2a) contains both five-membered carbopalladacycle and azonaphtholato
chelate ring whereas a five-membered carbopalladacycle and a six-membered azonaphtholato chelate
ring are present in peri-palladate (2b). On the other hand, only C3(naphthyl)eH bond of 2-(20-hydrox-
yarylazo)naphthalene (H2L2 & H2L3) has been found to be regiospecifically activated by palladium(II). The
role of auxiliary donors on the regioselective and regiospecific C(naphthyl)eH bond activation and the
rationale behind the formation of isomeric cyclopalladates have been discussed. All of the cyclopalladates
absorb strongly in the ultraviolet and visible region. The Time-dependent density functional theory
Received 25 January 2014
Received in revised form
15 March 2014
Accepted 26 March 2014
Keywords:
CeH activation
Cyclometallation
Palladium
(TDDFT) calculations reveal that the high energy absorptions are predominantly due to intraligand pep*
and the low energy absorptions originate from intraligand
ligand charge-transfer transitions.
p
e
p
* with a small admixture of metal-to-
Ó 2014 Elsevier B.V. All rights reserved.
Introduction
established tool for the functionalization of organic molecules [25e
28]. However, selective activation of a particular CeH bond within a
complex molecule is a challenging task. In such cases, the metal-
lation is usually directed to a particular CeH bond by introducing a
coordinating group (primary donor) at a suitable position within
the substrate, which ultimately leads to the formation of metale
carbon bond [29,30]. On the other hand, several organic molecules
having primary donor at a given position may possess more than
one non-equivalent CeH bonds as potential metallation sites. For
example, the cyclometallation of naphthyl group or fused ring
systems having N-donor poses interesting questions regarding the
regioselectivity of the CeH bond activation, due to the presence of
more than one non-equivalent CeH activation sites. The naphthyl
moiety with diazene function, a well-known directing group [31e
35], at C1 can offer the C2 (I) or C8 (II) positions for metallation,
similarly C3 (III) and C1 (IV) are the probable sites for metallation
Selective CeH activation and subsequent functionalization of
organic molecules under mild conditions, using either catalytic or
stoichiometric processes constitutes one of the most active areas of
research [1e10]. Cyclometallation reactions have emerged as an
attractive route to achieve selective activation of inert CeH or CeC
bonds in organic molecules [11e24] by incorporation of soft metal
centre into the organic skeleton. Moreover, this particular route of
CeH activation is most intriguing in terms of efficiency and atom
economy. In cyclometallation chemistry, cyclopalladation enjoys a
special status for its wide application potential, specially as a well-
* Corresponding author.
E-mail address: pbchem@rediffmail.com (P. Bandyopadhyay).
http://dx.doi.org/10.1016/j.jorganchem.2014.03.023
0022-328X/Ó 2014 Elsevier B.V. All rights reserved.

148
A.N. Biswas et al. / Journal of Organometallic Chemistry 761 (2014) 147e155
where diazene group is at C2. Rys et al. showed that the metallation
of 1-naphthylazoarenes with palladium(II) takes place at the ortho-
position (C2) of the naphthyl ring, whereas peri-palladation can
only be achieved by blocking all the ortho-positions by suitable
substituents [36e38]. However, regiospecific activation of
C8(napthyl)eH bond by incorporating suitable auxiliary donor in
the pendant aryl ring has been reported [39,40] (Fig. 1).
Here, we wish to report a detailed study on the role of additional
(auxiliary) donors in the selective activation of non-equivalent
C(naphthyl)eH bonds of naphthylazoarenes using palladium(II).
The effect of the position of the primary donor (diazene group) on
the selectivity of C(naphthyl)eH activation has also been examined.
The isolation and characterization of the resulting cyclopalladates
and rationalization of their structural and spectroscopic properties
have been discussed. The experimental results are complemented
by theoretical calculations employing TD-DFT to provide insight
into their electronic structures. So far, only qualitative EHMO cal-
culations have been performed on computer-generated models of
related azo-palladium complexes [41]. Thus attempts have been
made to provide a more comprehensive theoretical investigation
regarding the electronic properties of palladium (II) complexes.
O) is five-membered in the ortho-isomer (2a) but six-membered in
the peri-isomer (2b). The structure of the ortho-palladate 3a has
also been confirmed by X-ray crystallography. The molecular
structure of 3a (Fig. 4) shows a mononuclear compound in which
palladium is located in a slightly distorted square planar environ-
ment. The PdeC(naphthyl), PdeN(diazene), PdeO, PdeN (4-
picoline) and PdeP are all quite normal [45,46]. The lengthening
ꢀ
of PdeN(diazene) bond length 2.002(2) A in (3a) with respect to
ꢀ
PdeN(diazene) bond length 1.967(2) A of (2a) originates from the
strong trans-influence of phosphine. The shortening of C(11)-N(2)
ꢀ
bond length 1.353(3) & 1.385(5) A with respect to the C(1)eN(1)
ꢀ
bond 1.426(3) & 1.412(5) A reveals the presence of more
p-electron
density in azonaphtholato chelate ring than that present in the five
membered carbopalladacycles. In contrast, C(11)eN(2) bond length
ꢀ
ꢀ
1.438(3) A of (2a) is longer than C(1)eN(1) bond 1.392(3) A, which
ꢀ
is part of carbopalladacyle. The PdeC8 bond length of 1.987(4) A is
in agreement with the reported palladium(II)eC8(naphthyl) bond
lengths [39,40].
Following the regioselective activation of C2(naphthyl)eH and
C8(naphthyl)eH in 1-(20-hydroxynaphthylazo)naphthalene(H2L1)
by palladium(II), attempts have been made to study the effect of the
position of the primary donor on the sites of cyclopalladation. The
H2L2 and H2L3 having the primary donor at C2 were chosen as the
substrates. The reactions between Na2[PdCl4] and H2L2 or H2L3 in
aqueous ethanol in presence of the Lewis base (triphenyl phos-
phine or 4-picoline) were found to be regiospecific in nature and
green cyclopalladates (4e7) were isolated (Scheme 2). In each case
single isomer has been obtained. X-ray crystallographic analyses of
two cyclopalladates (4 and 6) revealed exclusive activation of
C3(naphthyl)eH bonds. The molecular structures of the isomers 4
and 6 are shown in Figs. 5 and 6 respectively.
The structure of green cyclopalladate 4 (Fig. 5) shows that the
palladium is bonded to C3, N2 of diazene group, O1 of phenolato
function and N(3) of 4-picoline. Thus 2-(20-hydroxyphenylazo)
naphthalene binds palladium(II) as a dianionic terdentate [C, N, O]
ligand. Similarly, the structure of green compound 6 (Fig. 6) shows
that the palladium is bonded to C3, N2 of diazene group, O1 of
naphtholato function and N(3) of 4-picoline. Thus, cyclopalladation
is found to occur in a regiospecific manner leading to the
C3(naphthyl)eH bond activation for both H2L2 and H2L3 leading to
the formation of products 4 & 6 respectively.
Results and discussion
Cyclopalladation via CeH bond activation
The naphthylazo-20-hydroxyarenes (H2L) have been prepared as
air-stable solids following reported methods [42e44]. The sub-
strates differ in the position of the primary donor, i.e., diazene
function (at either C1 or C2 of the naphthalene fragment) and in the
nature of 20-hydroxyarenes (either 20-hydroxyphenol or 20-
hydroxynaphthol). The reactivity of the naphthylazo-20-hydrox-
yarenes (H2L) toward disodium tetrachloropalladate has been
investigated. The treatment of Na2[PdCl4] with 1-(20-hydrox-
ynaphthylazo)naphthalene (H2L1) (1:1 M ratio) in presence of
Lewis bases (D: either 4-picoline or PPh3) in aqueous ethanol gives
isomeric cyclopalladates [Pd(L)(4-picoline)] (2a and 2b) or
[Pd(L)(PPh3)] (3a and 3b) (Scheme 1).
The isomeric cyclopalladates (2a & 2b) have been characterized
by spectral and single crystal X-ray diffraction data. Single crystals
of 2a and 2b were obtained from CH2Cl2-hexane solution at 25 ꢀC
and the ORTEP diagrams are shown in Figs. 2 and 3 respectively. In
the cyclopalladates (2a and 2b), 1-(20-hydroxynaphthylazo) naph-
thalene binds palladium via dianionic terdentate [C, N, O] fashion.
The fourth coordination site is occupied by the Lewis base (4-
picoline), thereby providing a quasi-ideal square planar arrange-
ment around palladium. The most noticeable structural difference
between the cyclopalladates 2a and 2b is the attachment of azo (N)
with palladium. In the ortho-palladate (2a), palladium is bonded to
N2(diazene) whereas in the peri-palladate (2b), N1(diazene) binds
palladium(II). This difference is reflected in the size of their chelate
rings. Thus, both the cyclopalladates contain five-membered car-
bopalladacycle (C, N) whereas the azonaphtholato chelate ring (N,
Regioselectivity vs. regiospecificity: plausible mechanism
The formation of isomeric cyclopalladates (2a & 2b) in reaction
of PdCl24ꢁ with substrate H2L1 deserves special mention from a
mechanistic viewpoint. The exact mechanism behind the formation
of two isomeric cyclopalladates is not completely clear to us.
However, the sequence shown in Scheme 3 seems probable. Ary-
lazonaphthols are known [47e49] to exist as a mixture of azoenol
(1a) and hydrazoketo (1b) forms in ethanolic medium. It is gener-
ally accepted [50,51] that cyclopalladation is initiated by coordi-
nation of a heteroatom to the metal, which is followed by ligand
Fig. 1. Non-equivalent C(naphthyl)eH bonds for metal chelation.

A.N. Biswas et al. / Journal of Organometallic Chemistry 761 (2014) 147e155
149
Scheme 1. Isolation of the isomeric cyclopalladates (2 & 3).
dissociation from the square planar species giving rise to highly
reactive intermediate which activates a CeH bond in kinetically
controlled electrophilic step with a marked preference for the
formation a five-membered palladacycle. The azoenol form (1a) is
known [52e54] to form six-membered chelate ring with transition
metal ions rapidly leaving only option of palladation at C8 of the
pendant naphthyl ring, which is actually observed in the case of
(2b). The hydrazoketo form (1b) can only offer N2 for metal binding
which leads to the formation of a five-membered palladacycle (C2,
N2) followed by chelation with naphtholato function forming five
membered chelate ring in (2a). There is precedence for such
cycopalladation of hydrazoketo form followed by enolisation sub-
sequently PdeOeAr (Ar ¼ aryl) bond formation [45].
naphthalene (HLOMe) undergoes facile cyclopalladation reaction
with disodium tetrachloropalladate in ethanol. The deep red col-
oured cyclopalladate [Pd2(LOMe)2Cl2] (8) was isolated.
The structure of [Pd2(LOMe)2Cl2] (8), has been determined by X-
ray crystallography. The molecular structure is shown in Fig. 7.
Cyclopalladate 8 contains two palladium atoms bridged by two
chloride atoms and the azonaphthalene ligand acting as a chelating
C,N donor. Consequently two five membered chelate rings are
formed. The bond lengths and angles are within the range of values
observed in analogues compounds [54]. In contrast to other
cyclopalladates, 8 has Ci symmetry, the crystallographic inversion
centre lies at the midpoint of the line joining the two-palladium.
For steric reasons, the azonaphthalene moiety can no longer be
planar with the non-metallated naphthalene ring turning out of the
plane formed by the cyclopalladated fragment by an angle of
70.78ꢀ.
The proposed sequence can further be verified by incorporating
an auxiliary donor instead of eOH group, which excludes the
possibility of any azo-hydrazo equilibrium. Further, it appears that
the presence of a weak auxiliary donor would prevent the forma-
tion of six membered N,O-chelate ring at first step leading to
exclusive C2(naphthyl)eH bond activation. Driven by this antici-
pation, the eOH (naphtholato) in H2L1 is replaced by neutral
methoxy group and the resulting 1-(20-methoxynaphthylazo)
Expectedly in this ligand where the eOH function of H2L1 is
replaced by eOMe group, palladium(II) regiospecifically activates
C(2)eH bond of HLOMe under the same mild reaction condition.
Here, the diazene function being a stronger donor than alkoxy
group towards electrophilic metal ions, preferentially binds
Fig. 2. Molecular structure of 2a with ellipsoids drawn at the 50% probability level.
Selected bond lengths (A) and angles (deg): Pd(1)eN(2) 1.972(8), Pd(1)eC(2) 1.970(11),
Fig. 3. Molecular structure of 2b with ellipsoids drawn at the 50% probability level.
Selected bond lengths (A) and angles (deg): Pd(1)eN(1) 1.967(2), Pd(1)eC(8) 1.983(3),
ꢀ
ꢀ
Pd(1)eN(3) 2.070(9), Pd(1)eO(1), 2.123(7), N(1)eN(2) 1.291(11), N(1)eC(1), 1.405(12),
C(11)eN(2) 1.394(12) C(12)eO(1), 1.288(12), C(2)ePd(1)eN(3) 102.6(4), N(3)ePd(1)e
O(1) 96.3(3), N(2)ePd(1)eO(1) 81.0(3), N(2)ePd(1)eC(2) 80.6(4).
Pd(1)eN(3) 2.058(2), Pd(1)eO(1) 2.0605(18), N(1)eN(2) 1.284(3), N(1)eC(1) 1.426(3),
C(11)eN(2) 1.353(3), C(12)eO(1) 1.290(3), C(8)ePd(1)eN(3) 95.66(9), N(3)ePd(1)e
O(1) 90.27(8), N(1)ePd(1)eO(1) 90.84(7), N(1)ePd(1)eC(8) 83.23(9).

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A.N. Biswas et al. / Journal of Organometallic Chemistry 761 (2014) 147e155
those in the visible region are due to metal-to-ligand charge-
transfer transition [55]. In order to confirm the nature of the ab-
sorptions we proceeded to perform the time-dependent DFT (TD-
DFT) calculations of the representative cyclopalladates, viz., 2a and
2b in dichloromethane. The coordinates of these cyclopalladates
are directly imported from their crystal data. It is well known that
an experimentally used model of an excited state corresponds to
excitation of an electron from an occupied orbital to a virtual
orbital. Assignments of the character of each excited states are
based on the compositions of the occupied and virtual orbitals of
the dominant configurations for that excited state. The TDDFT re-
sults do not provide information on triplet-singlet absorption in-
tensities since spin-orbit coupling effects are not included in the
current TDDFT methods. Thus we have only calculated the singlet
excited states and only the singlet states with oscillator strengths
greater than 0.05 are listed. The frontier molecular orbital com-
positions, excitation energies and oscillator strengths for the
various absorption bands are reported in the supporting informa-
tion (Tables S1eS4), together with the composition of the solution
vectors in terms of most relevant transitions. Isodensity surface plot
of the relevant MO’s is presented in Fig. 8. The simulated spectra of
2a and 2b have been presented in Fig. S1. Detailed analyses of the
highest occupied and lowest unoccupied molecular orbitals of 2a
and 2b are presented in Table S1 and S2 respectively, where orbital
energies and composition in terms of atomic contributions are
reported.
The electronic structure of the complex 2a (Table S1) shows that
the HOMO (77a) is largely based on the naphtholato fragment of
the ligand with a small contribution (9.53%) from the Pd-d orbitals.
The HOMO ꢁ 1 (76a) orbital, w0.82 eV lower than 77a, is localized
mostly on palladium (65%). The next four lowest highest occupied
molecular orbitals (75a, 74a, 73a and 72a) have 8.59%, 34%, 19% and
16% metal character respectively. LUMO þ 1 (79a) orbital is largely
based on 4-picoline. The HOMOeLUMO gap is computed to be
w1.27 eV. In case of HOMO (77a), the largest orbital contributions
arise from the naphtholato orbitals with a small percentage of
metal d orbitals. HOMO ꢁ 1, HOMO ꢁ 2 and HOMO ꢁ 3 show a
sizeable Pd-d character (23%, 66% and 34% respectively). The lowest
unoccupied molecular orbital (78a) is mostly concentrated on the
Lewis base, 4-picoline resulting from the combination of the p or-
bitals of nitrogen and carbon, which mix in an antibonding fashion.
The other LUMO’s have predominant ligand character. The HOMOe
LUMO gap is computed to be w1.76 eV, which is 0.49 eV greater
Fig. 4. Molecular structure of 3a with ellipsoids drawn at the 50% probability level.
Solvent molecule has been omitted for clarity. Selected bond lengths (A) and angles
(deg): Pd(1)eN(2) 2.002(2), Pd(1)eC(2) 2.001(3), Pd(1)eP(1) 2.2690(8), Pd(1)eO(1),
2.094(2), N(1)eN(2) 1.290(4), N(1)eC(1), 1.412(4), C(11)eN(2) 1.385(5) C(12)eO(1),
1.305(5), C(2)ePd(1)eP(1) 100.89(10), P(1)ePd(1)eO(1) 99.03(7), N(2)ePd(1)eO(1)
80.23(10), N(2)ePd(1)eC(2) 79.85(12).
ꢀ
palladium(II), followed by palladation at ortho-position of the
naphthyl ring resulting in the formation of five-membered palla-
dacycle like palladates of other arylazonaphthalenes (Scheme 3).
UVevis spectra and excited singlet state calculations
All the cyclopalladates are soluble in common organic solvents.
The electronic spectra of all cyclopalladates, recorded in dichloro-
methane, show several intense absorptions in the visible and ul-
traviolet regions (Table 1). The bands observed in the ultraviolet
region with high molar extinction co-efficient are generally
thought to be arisen from intraligand charge transfer transition and
Scheme 2. Isolation of the cyclopalladates (4e7).

A.N. Biswas et al. / Journal of Organometallic Chemistry 761 (2014) 147e155
151
The reduction of the HOMOeLUMO gap in 2a is due to the raising of
the HOMO energy computed in the ortho-isomer (2a) with respect
to the peri-isomer (2b) (ꢁ4.885 eV versus ꢁ5.221 eV). For 2b, the
first absorption band at ca. 532 nm has a multitransition character
and involves excitation from the highest occupied to the lowest
unoccupied molecular orbitals (Table S4). The absorption band,
with an onset at 2.33 eV, involves multitransitions from 77a
(HOMO) and 76a (HOMO ꢁ 1) to the lowest unoccupied
delocalized on the ligand. The transition has dominant intraligand
* character and small admixture of MLCT. The other absorp-
p* orbital
pep
tions in the visible region in 2b have common density redistribu-
tion features with a significant amount of metal to ligand charge
transfer and more or less
The most intense transition observed in 3b in UV region (ca.
265 nm) involves excitation from 74a (HOMO 3), 71a
pep* intraligand density redistribution.
ꢁ
(HOMO ꢁ 6), 64a (HOMO ꢁ 11) and 70a (HOMO ꢁ 7) to 83a
(LUMO þ 5), 80a (LUMO þ 2), 78a (LUMO) and 80a (LUMO þ 2)
respectively with predominantly intraligand
pep* character. In
case of the corresponding ortho isomer (2b) the lowest energy
band having an onset at 1.67 eV, mainly involves transition from
77a (HOMO) to 78a (LUMO). This transition has predominant
intraligand pep* character.
Conclusion
In this study we have explored how the non-equivalent
C(naphthyl)eH bonds can be selectively activated by palladiu-
m(II) at room temperature with incorporation of different auxiliary
donors along with variation of the position of primary donor.
Regioselective activation of C2(naphthyl)eH and C8(naphthyl)eH
bonds has been achieved by palladium(II) where the primary donor
is at C1 and the auxiliary donor is 20-naphthol. The formation of
isomer having C2epalladium(II) bond can be explained in terms of
C,N-cyclopalladation with the keto form of 1-(20-hydroxynaph-
thylazo)-naphthalene followed by five membered N,O-chelation.
On the other hand, the six membered N,O-chelation with the enol
form of 1-(20-hydroxynaphthylazo) naphthalene followed by C,N-
cyclopalladation results in the formation of isomer having C8-
palladium(II) bond. Exclusive activation of C2(naphthyl)eH bond
Fig. 5. Molecular structure of 4 with ellipsoids drawn at the 50% probability level.
Selected bond lengths (A) and angles (deg): Pd(1)eN(2) 1.983(16), Pd(1)eC(3) 1.97(2),
Pd(1)eN(3) 2.027(15), Pd(1)eO(1) 2.124(14), N(1)eN(2) 1.26(2), N(1)eC(2) 1.47(3),
C(3)ePd(1)eN(3) 102.8(10), N(3)ePd(1)eO(1) 96.2(8), N(2)ePd(1)eO(1) 82.7(9),
N(2)ePd(1)eC(3) 78.4(11).
ꢀ
than the corresponding computed value for the ortho-palladate
(2a). Interestingly, this value nicely corresponds to the experi-
mental red shift observed for the first visible absorption band in the
cyclopalladates (722 nm in 2a versus 547 nm in 2b) (Table S3 & S4).
Fig. 6. Molecular structure of 6 with ellipsoids drawn at the 50% probability level.
ꢀ
Possible intramolecular hydrogen bond C18eH18/N1 2.28 A. Selected bond lengths
ꢀ
(A) and angles (deg): Pd(1)eN(2) 1.971(5), Pd(1)eC(3) 1.990(7), Pd(1)eN(3) 2.057(4),
Fig. 7. Molecular structure of 8 with ellipsoids drawn at the 50% probability level.
Selected bond lengths (A) and angles (deg): Pd(1)eN(2) 2.001(2), Pd(1)eC(2) 1.956(3),
Pd(1)eCl(1) 2.4484(9), N(1)eN(2) 1.276(3), N(2)eC(11) 1.438(3), C(2)ePd(1)eN(2)
78.74(10), C(2)ePd(1)eCl(1) 95.65(8), N(2)ePd(1)eCl(1) 99.60(6).
ꢀ
Pd(1)eO(1) 2.109(5), N(1)eN(2) 1.285(6), N(1)eC(2) 1.401(8), C(3)ePd(1)eN(3)
101.7(2), N(3)ePd(1)eO(1) 95.3(2), N(2)ePd(1)eO(1) 81.73(19), N(2)ePd(1)eC(3)
81.3(2).

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A.N. Biswas et al. / Journal of Organometallic Chemistry 761 (2014) 147e155
Fig. 8. Partial molecular orbital diagram of the cyclopalladates 2a and 2b.
group with phenol or naphthol as auxiliary donor. The electronic
structures of representative cyclopalladates have been studied us-
ing time-dependent density function theory (TD-DFT). The simu-
lated electronic spectra of the cyclopalladates are in close
agreement with the experimental spectra. The low energy ab-
sorptions are attributed to intraligand
pep* transitions having a
small admixture of metal-to-ligand charge-transfer transitions. The
high energy absorptions are primarily due to intraligand
transitions.
pep*
Experimental section
General procedures
All reagents were obtained from commercial sources and used
without purification, unless otherwise stated. The ligands (H2L1,
H2L2 and H2L3) were prepared following the reported method [42e
44]. 1-(20-Methoxynaphthylazo)naphthalene (H2L4) was prepared
by methylation of 1-(20-hydroxynaphthylazo)naphthalene [56].
Elemental microanalyses (C, H and N) were done by either Perkine
Elmer (Model 240C) or Heraeus Carlo Erba 1108 elemental analyzer.
The IR and Electronic spectra were recorded on Jasco 5300 FT-IR
spectrophotometer and JASCO V-500 spectrophotometer respec-
tively. NMR spectra were obtained by using Bruker DPX 300 NMR
Spectrometer.
Scheme 3. Probable steps in the formation of isomeric cyclopalladates, 2 and 3.
by palladium(II) has been achieved with the change of auxiliary e
OH (naphtholato) donor by neutral alkoxy group. Due to poor donor
ability of the alkoxy group, formation of six membered N,O-chelate
at first step is least probable, which prevents the formation of peri-
palladate. Therefore, only C,N-cyclometallation is operative result-
ing in ortho-palladate as exclusive product. Moreover, regiospecific
C3(naphthyl)eH bond activation by palladium(II) has been ach-
ieved, when the primary diazene donor is at C2 of the naphthyl
Isolation of cyclopalladates
Table 1
Isolation of [PdL1(4-picoline)] (2a and 2b)
Electronic spectral data of the cyclopalladates.
An ethanolic solution (10 cm3) of sodium tetrachloropalladate
(0.05 g, 0.170 mmol) was slowly added to H2L1 (0.04 g, 0.134 mmol)
in ethanol (10 cm3). The mixture was stirred for 8 h at room tem-
perature and the colour of the mixture gradually changed to deep
brownish red. A solution of 4-methylpyridine (0.05 g, 0.191 mmol)
in benzene (5 cm3) was slowly added to the above mixture. The
mixture was further stirred for 2 h at room temperature and deep
pink red colour appeared. The mixture was kept for overnight. The
solid residue was collected after the removal of solvent followed by
washing with water and ethanol. The residue was dissolved in
dichloromethane and chromatographed on silica gel (60e120 mesh
size). A red violet band of H2L1 followed by a pink red band of (3a)
was eluted by mixtures of petroleum ether and benzene in 9:1 (v/v)
and 3:1 (v/v) respectively. Finally a green band of (3b) was eluted by
a
Compound lmax/nm (ε/Mꢁ1 cmꢁ1
)
2a
2b
3a
3b
272 (39,500), 400 (8800), 664 (7950), 725 (8200)
267 (10,300), 475 (2100), 571 (2850)
275 (53,000), 402 (11,300), 672 (11,050), 733 (11,500)
265 (38,150), 366 (5650), 429sh (4900), 455sh (6400),
518 (16,600), 549 (20,300).
280 (21,650), 334 (13,000), 482 (5250), 615 (5700)
238 (45,600), 282sh (18,000), 292 (17,800), 336 (20,650),
482 (7350), 625 (8900), 665 (7850)
242 (40,500), 285 (19,700), 455 (4900), 630 (7400), 665 (5900).
241 (38,500), 275 (15,600), 330 (21,100), 620 (5200), 652 (6300).
416 (5075), 540sh (2700), 582 (3900), 622 (4200)
268 (45,600), 466 (6900), 551 (7800)
4
5
6
7
8
9
shShoulder.
a
In dichloromethane.

A.N. Biswas et al. / Journal of Organometallic Chemistry 761 (2014) 147e155
153
pure benzene. Red coloured compound (3a) and green coloured
isomeric compound (3b) were collected from the pink red band and
green band respectively.
Yield: 52%. Anal. Calc.: C, 60.07; H, 4.16; N, 9.14. Found: C, 60.27;
H, 4.22; N, 8.85. 1H NMR (300 MHz; CDCl3; standard SiMe4):
d 2.18
(s, 3H, 50-Me), 2.49 (s, 3H, Me protons of 4-picoline), 6.50e8.80
2a, Yield: 0.017 g, 25%. Anal. Calc.: C, 62.98; H, 3.86; N, 8.47.
(aromatic protons).
Found: C, 63.23; H, 4.23; N, 7.55. 1H NMR (300 MHz; CDCl3; stan-
dard SiMe4):
d
2.52 (s, 3H, 50-Me), 6.84 (d, 1H), 7.05 (d, 1H,
Isolation of [PdL2(PPh3)] (5)
J ¼ 9.24 Hz), 7.22e7.55 (m, 5H), 7.57e7.74 (m, 7H), 8.38 (d, 1H,
Compound (5) was synthesized following the procedure out-
lined previously using 2-(20-hydroxyphenylazo)naphthalene as the
ligand and triphenyl phosphine as the donor molecule.
Yield: 42%. Anal. Calc.: C, 66.83; H, 4.33; N, 4.45. Found: C, 67.03;
J ¼ 7.41 Hz), 8.78 (d, 1H), 8.90 (d, 2H).
2b, Yield: 0.010 g, 15%. Anal. Calc.: C, 62.98; H, 3.86; N, 8.47.
Found: C, 62.75; H, 3.62; N, 7.98. 1H NMR (300 MHz; CDCl3; stan-
dard SiMe4):
d
2.39 (s, 3H, 50-Me), 6.56 (d, 1H, J ¼ 8.22 Hz), 6.69 (d,
H, 4.26; N, 4.85. 1H NMR (300 MHz; CDCl3; standard SiMe4):
d 2.26
1H, J ¼ 9.11 Hz), 7.21e7.36 (m, 7H), 7.43e7.57 (m, 4H), 7.64 (d, 1H,
(s, 3H, 50-Me), 5.97 (d, 1H), 6.53 (d, 1H, J ¼ 8.78 Hz), 6.71 (d, 1H,
J ¼ 7.64 Hz), 6.95 (d, 1H, J ¼ 8.79), 7.11e7.21 (m, 2H), 7.39e7.48 (m,
PPh3 signals), 7.87 (s, 1H), 8.46 (d, 1H), 9.19 (d, 1H, J ¼ 9.00 Hz).
J ¼ 8.23), 8.44 (d, 1H, J ¼ 8.49), 9.17 (d, 1H, J ¼ 8.34 Hz).
Isolation of [PdL1(PPh3)] (3a and 3b)
Compounds 3a and 3b were isolated following the above pro-
cedure using triphenylphosphine instead of 4-methylpyridine.
3a, Yield: 0.025 g, 30%. Anal. Calc.: C, 68.63; H, 4.09; N, 4.21.
Found: C, 68.82; H, 4.20; N, 4.11. 1H NMR (300 MHz; CDCl3; stan-
Isolation of [PdL3(4-picoline)] (6)
Compound (6) was isolated following the procedure outlined
previously using 2-(20-hydroxyphenylazo)naphthalene as the
ligand and 4-picoline as the donor molecule.
dard SiMe4):
d
6.41 (d, 1H, J ¼ 9.0 Hz), 6.78 (m, 2H), 7.37 (m, 2H),
Yield: 18%. Anal. Calc.: C, 62.98; H, 3.86; N, 8.47. Found: C, 63.23;
7.43e7.53 (m, 15H, PPh3), 7.73e7.80 (m, 6H), 8.45 (d, 1H, J ¼ 9.0 Hz),
H, 4.23; N, 8.15. 1H NMR (300 MHz; CDCl3; standard SiMe4):
d 2.52
8.81 (d, 1H, J ¼ 9.0 Hz).
(s, 3H, Me protons of 4-picoline), 6.25e8.90 (aromatic protons).
3b, Yield: 0.015 g, 17%. Anal. Calc.: C, 68.63; H, 4.09; N, 4.21.
Found: C, 68.71; H, 4.15; N, 4.09. 1H NMR (300 MHz; CDCl3; stan-
dard SiMe4):
d
6.17 (s, 1H), 6.55 (d, 2H, J ¼ 9.1 Hz), 6.84 (d, 1H,
Isolation of [PdL3(PPh3)] (7)
J ¼ 8.0 Hz), 7.25e7.67 (m, PPh3 and other naphthyl ring protons),
8.48 (d, 1H, J ¼ 7.8 Hz), 9.18 (d, 1H, J ¼ 8.0 Hz).
Compound (7) was isolated following the procedure outlined
previously using 2-(20-hydroxynaphthylazo)naphthalene as the
ligand and triphenyl phosphine as the donor molecule.
Yield: 15%. Anal. Calc.: C, 68.63; H, 4.09; N, 4.21. Found: C, 68.54;
Isolation of [PdL2(4-picoline)] (4)
Compound (4) was isolated following the procedure outlined
previously using 2-(20-hydroxyphenylazo)naphthalene as the
ligand and 4-picoline as the donor molecule.
H, 3.97; N, 4.55. 1H NMR (300 MHz; CDCl3; standard SiMe4):
d 6.70
(d, 1H), 7.10 (d, 1H, J ¼ 9.24 Hz), 7.22e7.95 (m, 15H), 8.10e8.25 (m,
7H), 8.42 (d, 1H, J ¼ 7.41 Hz), 8.80 (d, 1H), 9.10 (d, 2H).
Table 2
Crystal data and data collection parameters of the cyclopalladates.
Identification code 2a
2b
3a
4
6
8
Empirical formula C26H19N3OPd
C26H19N3OPd
495.86
298(2) K
C38H29N2O2PPd
683.06
293(2) K
C23H19N3OPd
459.85
298(2) K
C26H19N3OPd
495.84
298(2) K
Orthorhombic,
P212121
C42H30Cl2N4O2Pd2Cl2
906.40
298(2) K
Formula weight
Temperature
Crystal system,
space group
Unit cell
495.84
293(2) K
Monoclinic, P2(1)/c
Monoclinic, P2(1)/n
Triclinic, P-1
Monoclinic, P2(1)
Monoclinic, P2(1)/n
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
a ¼ 15.246(4) A
a ¼ 10.880(3) A
a ¼ 9.0278(12) A
a ¼ 10.2088(7) A
a ¼ 7.3923(14) A
a ¼ 13.148(3) A
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
dimensions
b ¼ 7.451(2) A
b ¼ 17.184(4) A
b ¼ 12.6561(17) A
b ¼ 7.4438(4) A
b ¼ 13.848(3) A
b ¼ 12.008(3) A
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
c ¼ 18.482(5) A
c ¼ 11.164(3)) A
c ¼ 14.4336(19) A
c ¼ 13.2692(9) A
c ¼ 20.149(4) A
c ¼ 13.215(3) A
a
b
g
¼ 90ꢀ
a
b
g
¼ 90ꢀ
a
b
g
¼ 77.426(2)ꢀ
¼ 82.567(2)ꢀ
¼ 74.423(2)ꢀ
a
b
g
¼ 90ꢀ
a
b
g
¼ 90ꢀ
¼ 90ꢀ
¼ 90ꢀ
a
b
g
¼ 90ꢀ
¼ 101.615(6)ꢀ
¼ 90ꢀ
¼ 100.375(4)ꢀ
¼ 90ꢀ
¼ 102.436(2)ꢀ
¼ 90ꢀ
¼ 119.245(3)
¼ 90ꢀ
ꢀ
Volume
2056.6(10) A3
4, 1.601
2053.1(9) A3
4, 1.604
1546.0(4) A3
2, 1.467
984.70(11) A3
2, 1.551
2062.6(7) A3
4, 1.597
1820.5(7) A3
2, 1.654
Z, Calculated
density (Mg/m3)
Absorption
0.926 mmꢁ1
0.928 mmꢁ1
0.689 mmꢁ1
0.960 mmꢁ1
0.924 mmꢁ1
1.178 mmꢁ1
co-efficient
F(000)
1000
1000
696
464
1000
904
Crystal size
Theta range for
data collection
Limiting indices
0.25 ꢂ 0.12 ꢂ 0.10 mm 0.40 ꢂ 0.22 ꢂ 0.10 mm 0.35 ꢂ 0.26 ꢂ 0.12 mm 0.21 ꢂ 0.16 ꢂ 0.08 mm 0.22 ꢂ 0.18 ꢂ 0.09 mm 0.33 ꢂ 0.22 ꢂ 0.11 mm
1.36e25.00 deg.
2.24e25.02 deg.
2.66e25.01 deg.
1.57e20.99 deg.
2.50e24.98 deg.
1.79e25.13 deg.
ꢁ18 ꢃ h ꢃ 15,
ꢁ8 ꢃ k ꢃ 8,
ꢁ12 ꢃ h ꢃ 12,
ꢁ20 ꢃ k ꢃ 20,
ꢁ13 ꢃ l ꢃ 13
19,095/3622
[R(int) ¼ 0.0264]
1.156
ꢁ10 ꢃ h ꢃ 10,
ꢁ15 ꢃ k ꢃ 15,
ꢁ17 ꢃ l ꢃ 17
13,925/5230
[R(int) ¼ 0.0277]
1.140
ꢁ9 ꢃ h ꢃ 10,
ꢁ7 ꢃ k ꢃ 7,
ꢁ13 ꢃ l ꢃ 11
3310/2004
ꢁ8 ꢃ h ꢃ 8,
ꢁ16 ꢃ k ꢃ 16,
ꢁ23 ꢃ l ꢃ 23
18,902/3504
[R(int) ¼ 0.0685]
1.189
ꢁ15 ꢃ h ꢃ 15,
ꢁ14 ꢃ k ꢃ 14,
ꢁ15 ꢃ l ꢃ 15
16,063/3212
[R(int) ¼ 0.0276]
1.213
ꢁ20 ꢃ l ꢃ 21
Reflections
14,245/3607
collected/unique [R(int) ¼ 0.0963]
[R(int) ¼ 0.1221]
Goodness-of-fit
on F2
1.000
0.878
Final R indices
[I > 2sigma(I)]
R indices (all data) R1 ¼ 0.1381,
wR2 ¼ 0.2523
R1 ¼ 0.0941,
wR2 ¼ 0.2173
R1 ¼ 0.0293,
wR2 ¼ 0.0744
R1 ¼ 0.0312,
wR2 ¼ 0.0755
R1 ¼ 0.0399,
wR2 ¼ 0.1009
R1 ¼ 0.0419,
wR2 ¼ 0.1024
R1 ¼ 0.0714,
wR2 ¼ 0.1662
R1 ¼ 0.1187,
wR2 ¼ 0.1873
R1 ¼ 0.0556,
wR2 ¼ 0.1072
R1 ¼ 0.0592,
wR2 ¼ 0.1087
R1 ¼ 0.0249,
wR2 ¼ 0.0696
R1 ¼ 0.0277,
wR2 ¼ 0.0813

154
A.N. Biswas et al. / Journal of Organometallic Chemistry 761 (2014) 147e155
Synthesis of di(
m
-chloro)bis(1-(10-napthylazo)-2-methoxy-C(2),Nb)
uncontracted triple-STO basis set with a polarization function for
dipalladium(II), [Pd(L2)Cl]2, (8)
all atoms. In the calculation of the optical spectra, 70 lowest spin-
allowed singletesinglet transitions have been taken into account.
Transition energies and oscillator strengths have been interpolated
An ethanolic solution (10 cm3) of 1-(10-napthylazo)-2-
methoxynapthalene (0.075 g, 0.24 mmol) was added to an etha-
nolic solution (20 cm3) of sodium tetrachloropalladate (0.070 g,
0.24 mmol). The solution was stirred magnetically at room tem-
perature for six hours. The resulting precipitate was filtered,
washed with aqueous ethanol (4 ꢂ 5 cm3) and dried and recrys-
tallized from dichloromethane/ethanol. Yield: 0.095 g 82%. Anal.
Calc.: C, 55.65; H, 3.34; N, 6.18. Found: C, 55.86; H, 3.52; N, 5.97. 1H
by a Gaussian convolution with a
s of 0.2 eV. Solvent effects were
modelled by the “Conductor-like Screening Model” (COSMO)
[65,66] of solvation as implemented in ADF.
Acknowledgement
NMR (300 MHz; CDCl3; standard SiMe4):
(m, 4H), 7.43e7.61 (m, 6H), 7.87e7.94 (m, 2H).
d 4.07 (s, 3H, OMe), 7.25
This work has been supported by Science and Engineering
Research Board (SR/S1/IC-53/2010), Department of Science and
Technology, New Delhi and Council of Scientific and Industrial
Research [1(2498)/11/EMR-II], New Delhi.
Isolation of (h5-cyclopentadienyl)(1-(10-napthylazo)-2-methoxy-
C(2),Nb)dipalladium(II) [Pd(L2)Cp], (9)
To a suspension of [Pd(L2)Cl]2 (8) (0.05 g, 0.055 mmol) in ben-
zene (25 cm3) was added solid TlCp (0.036 g, 0.1 mmol) with stir-
ring. The mixture was further stirred for four hours at room
temperature. The colour of the solution became dark red. The
precipitate of TlCl was removed by filtration through a glass sin-
tered (G-4 frit) funnel. The filtrate was evaporated under reduced
pressure. The solid residue was dissolved in CH2Cl2 and chroma-
tographed on a cellulose column. The compound (9) was eluted as
deep red band using benzene as eluent. Removal of the solvent
afforded the compound as dark red microcrystals. Yield: 0.04 g,
75%. Anal. Calc.: C, 65.26; H, 4.46; N, 5.64. Found: C, 64.90; H, 4.58;
Appendix A. Supplementary material
CCDC 618786e618788, 650028 & 650029 contain the supple-
mentary crystallographic data for this paper. These data can be
obtained free of charge from The Cambridge Crystallographic Data
Centre via www.ccdc.cam.ac.uk/data_request/cif.
Appendix B. Supplementary material
Supplementary data related to this article can be found at http://
dx.doi.org/10.1016/j.jorganchem.2014.03.023.
N, 5.21. 1H NMR (300 MHz; CDCl3; standard SiMe4):
OMe), 5.62 (s, 5H, Cp-protons), 7.39 (s, 6H), 7.81 (s, 6H), 8.13 (s, 1H),
8.73 (s, 1H). 13C NMR (300 MHz; CDCl3; standard SiMe4):
57.08
d 4.06 (s, 3H,
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