Unsubstituted quinoidal pyrrole and its reaction with oxygen, charge transfer and palladium(ii) complexes via DDQ oxidation

Article abstract of DOI:10.1039/c4ra07808j

The dehydrogenation reaction of 2,5-bis(3,5-dimethylpyrazolylmethyl)pyrrole with 2,3-dichloro-5,6-dicyano-1,4-benzoquinone (DDQ) followed by treatment with sodium bicarbonate afforded two new products containing oxygen atoms, 2,5-bis(3,5-dimethylpyrazolylcarbonyl)pyrrole (2) and 5-{bis-(3,5-dimethylpyrazolyl)methyl}pyrrole-2-carbaldehyde (3). Interestingly, the products are different, when the reaction mixture is not treated with base; they are 2,2-tri(3,5-dimethylpyrazolylmethyl)-5-(3,5-dimethylpyrazolylcarbonyl)pyrrole (4) and the adduct 5, the 2,3-dichloro-5,6-dicyanohydroquinone (DDQH₂) adduct of 2,5-bis{di(3,5-dimethylpyrazolyl)methene}-2,5-dihydropyrrole (6) isolated in 22% yield, in which 6 acts as an electron acceptor and DDQH₂as an electron donor. The treatment of 5 with NaBH₄or basic alumina afforded the free base 6 in good yield, representing a discrete unsubstituted quinoidal structure of pyrrole. 6 is fluorescent, reactive and could have a biradical resonance structure, which is demonstrated by its reaction with dioxygen only under the irradiation of sunlight, not reacting in darkness, giving the oxygen containing products: 2, bis-(3,5-dimethylpyrazolyl)methanone (7) and 2-(3,5-dimethylpyrazolylcarbonyl)-5-(3,5-dimethylpyrazolyl)pyrrole (8). Moreover, the quinoidal molecule loses its fluorescence property on coordinating with palladium metal atom and gives a binuclear complex, [Pd₂Cl₄{μ-C₄H₃N-2,5-(C(Me₂pz)₂)₂-N,N,N,N}], in good yield. The structures of most of the compounds, including this palladium complex, were determined by single crystal X-ray diffraction studies.

Full text of DOI:10.1039/c4ra07808j

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Unsubstituted quinoidal pyrrole and its reaction
with oxygen, charge transfer and palladium(II)
complexes via DDQ oxidation†
Cite this: RSC Adv., 2014, 4, 45603
Debasish Ghorai and Ganesan Mani*
The dehydrogenation reaction of 2,5-bis(3,5-dimethylpyrazolylmethyl)pyrrole with 2,3-dichloro-5,6-
dicyano-1,4-benzoquinone (DDQ) followed by treatment with sodium bicarbonate afforded two new
products containing oxygen atoms, 2,5-bis(3,5-dimethylpyrazolylcarbonyl)pyrrole (2) and 5-{bis-(3,5-
dimethylpyrazolyl)methyl}pyrrole-2-carbaldehyde (3). Interestingly, the products are different, when the
reaction mixture is not treated with base; they are 2,2-tri(3,5-dimethylpyrazolylmethyl)-5-(3,5-
dimethylpyrazolylcarbonyl)pyrrole (4) and the adduct 5, the 2,3-dichloro-5,6-dicyanohydroquinone
(
DDQH
yield, in which 6 acts as an electron acceptor and DDQH
NaBH or basic alumina afforded the free base 6 in good yield, representing a discrete unsubstituted
2
) adduct of 2,5-bis{di(3,5-dimethylpyrazolyl)methene}-2,5-dihydropyrrole (6) isolated in 22%
2
as an electron donor. The treatment of 5 with
4
quinoidal structure of pyrrole. 6 is fluorescent, reactive and could have a biradical resonance structure,
which is demonstrated by its reaction with dioxygen only under the irradiation of sunlight, not reacting in
darkness, giving the oxygen containing products: 2, bis-(3,5-dimethylpyrazolyl)methanone (7) and 2-(3,5-
dimethylpyrazolylcarbonyl)-5-(3,5-dimethylpyrazolyl)pyrrole (8). Moreover, the quinoidal molecule loses
its fluorescence property on coordinating with palladium metal atom and gives a binuclear complex,
Received 30th July 2014
Accepted 1st September 2014
DOI: 10.1039/c4ra07808j
2 4 4 3 2 2 2
[Pd Cl {m-C H N-2,5-(C(Me pz) ) -N,N,N,N}], in good yield. The structures of most of the compounds,
www.rsc.org/advances
including this palladium complex, were determined by single crystal X-ray diffraction studies.
rearranged oxidation products along with a binuclear palla-
dium complex; their structures were determined by X-ray
Introduction
2
,3-Dichloro-5,6-dicyano-1,4-benzoquinone (DDQ) is a known, diffraction.
powerful oxidizing agent used in organic synthesis; it is mainly
1
used for dehydrogenation of hydroaromatic compounds. In
Results and discussion
addition, DDQ has been used to cause the dehydrogenative C–C
2
coupling leading to interesting products, photocatalytic As shown in Scheme 1, the oxidation reaction of 2,5-bis(3,5-
3
oxygenation of benzene to phenol, and for preparing expanded dimethylpyrazolylmethyl)pyrrole 1 with three equivalents of
4
5
porphyrins and other p-conjugated molecules. Dolphin and DDQ followed by treatment with a base (NaHCO ) afforded the
3
6
co-workers have reported DDQ oxidation of tripyrrane and two new products 2 and 3 in 11% and 20% yields, respectively,
dipyrromethane to give unexpected, interesting products that aer silica gel column chromatography separation. Whereas,
have extended p-conjugation. As part of our ongoing activity in the same reaction without the treatment with a base afforded
7
8
developing pyrrole-based anion receptors and ligand systems, 2,3-dichloro-5,6-dicyanohydroquinone (DDQH ) adduct 5 in
2
we were interested in oxidizing 2,5-bis(3,5-dimethylpyr- 22% yield, in addition to 2 formed in 11% yield and 4 in very low
9
azolylmethyl)pyrrole 1 with DDQ to give conjugated molecules yield as yellow crystals aer silica gel column chromatographic
because 1 possesses two methylene groups. Herein, we report separation. Presumably, in the presence of sodium bicarbonate,
the reactive, uorescent and p-conjugated quinoidal form of a the adduct 5 disintegrates into its components and the liberated
simple pyrrole, which forms a donor–acceptor complex and quinoidal pyrrole molecule, 2,5-bis{di(3,5-dimethylpyrazolyl)
reacts with oxygen only under sunlight, and the other methene}-2,5-dihydropyrrole 6, then undergoes further reac-
tions with air, probably during the work-up procedure, to give
2
ꢀ4 containing oxygen atoms. Therefore, it is likely that in both
Department of Chemistry, Indian Institute of Technology, Kharagpur 721 302, India.
E-mail: gmani@chem.iitkgp.ernet.in
the reactions, the free quinoidal pyrrole molecule 6 (see below)
might have initially formed and then reacted with the by-
†
Electronic supplementary information (ESI) available. CCDC 998866–998870,
product of the reaction, DDQH
can act as an electron acceptor and DDQH
2
, to give an adduct 5, where 6
as an electron
1
011058. For ESI and crystallographic data in CIF or other electronic format see
DOI: 10.1039/c4ra07808j
2
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rearranged pyrazolate groups attached to the a-positions of the
pyrrole ring. The presence of carbonyl group indicates that
these products could have formed by the reaction of the initially
formed quinoidal pyrrole molecule 6 with air. A complete
planar conformation was observed for 2, whereas in the cases of
3
and 4, the planarity is maintained in the carbonyl group side
Fig. 1 Molecular structure of 2 (30% thermal ellipsoids); dotted lines
indicate hydrogen bonds. Selected bond lengths ( A˚ ) and angles (deg):
N1–N2 1.376(5), N2–C6 1.407(5), C6–O1 1.220(5), C6–C7 1.458(6),
N3–C7 1.361(5), C7–C8 1.388(6), C8–C9 1.383(7), C9–C10 1.376(6),
N3–C10 1.356(5), C10–C11 1.470(6), C11–O2 1.212(5), N4–C11
1
.410(5), N4–N5 1.370(5), O1–C6–N2 119.0(4), O1–C6–C7 120.2(4),
Scheme 1 The DDQ oxidation products of 2,5-bis(3,5-dimethylpyr-
azolylmethyl)pyrrole 1.
O2–C11–N4 120.1(4), O2–C11–C10 120.4(4), N1–N2–C6 120.8(3),
C7–N3–C10 109.0(4), N5–N4–C11 121.3(3), N2–C6–C7 120.7(4), N3–
C10–C11 128.3(4), C10–C11–N4 119.5(4), N3/N1 2.738(5), H1/N1
2
.03(5), N3–H1/N1 118(3), N3/N5 2.706(5), H1/N5 2.11(4), N3–H1/
donor. However, this reaction did not give the expected mole-
cule, that is, a molecule containing one pyrrole ring anked by
two pyrazolylmethylene groups with p conjugation, probably
because of the reaction conditions resulting from the presence
of three equivalents of DDQ and the considerably reactive
nature of the free base 6 with molecular oxygen (see below).
Consistent with the reaction products obtained with sodium
N5 110(3), C5/O1 2.824(6), H5a/O1 2.09, C5–H5a/O1 131.8, C12/
O2 2.819(6), H12a/O2 2.09, C12–H12a/O2 131.6.
bicarbonate, the treatment of the DDQH
base, NaBH , afforded the free base 6 in 67% yield. In addition,
was also obtained aer passing the adduct 5 through a basic
2
adduct 5 with another
4
6
alumina column (Scheme 2). All these oxidized products are
soluble in common organic solvents such as diethyl ether,
toluene and chloroform, and their structures were conrmed by
X-ray diffraction studies.
The X-ray structures of 2–4 (Fig. 1, 2 and 3, respectively)
revealed the presence of carbonyl group along with the
Fig. 2 Molecular structure of 3 (30% thermal ellipsoids); dotted lines
indicate hydrogen bonds. Selected bond lengths ( A˚ ) and angles (deg):
O1–C17 1.2207(19), C16–C17 1.445(2), C14–C16 1.381(2), C13–C14
1
.398(2), C12–C13 1.380(2), N5–C16 1.378(2), N5–C12 1.358(2), C1–
C12 1.500(2), N1–C1 1.442(2), N3–C1 1.466(2), O1–C17–C16 123.7(2),
N5–C16–C17 121.8(2), C12–N5–C16 109.0(1), N5–C12–C1 125.4(2),
Scheme 2 The generation of the free quinoidal pyrrole molecule 6 N1–C1–C12 115.3(1), N3–C1–C12 111.4(1), N5/N2 2.921(2), H5/N2
from the adduct 5. 2.49(2), N5–H5/N2 111(1).
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of the free base 6 in DMSO-d
6
(see ESI, Fig. S12†). This suggests
that the charge transfer complex 5 dissociates into its compo-
nent molecules in the Lewis base solvent DMSO. This is also
conmed by the UV-vis spectrum of a dilute solution of 5 in
DMSO, which appears to be very close to that of the free base 6.
Conversely, the same charge transfer complex in toluene (see
ESI, Fig. S26†) displays a discrete absorption band lmax at 353
nm, in addition to the absorbances at 424 nm and 444 nm. The
ꢀ1
absorption at 353 nm (80.1 kcal mol ) can be regarded as the
charge transfer band for this complex, which is in the order of
shorter wavelength charge transfer band values reported for
12
pyridinium iodides. A dichloromethane solution of 5 also
gives a similar UV-vis spectrum. The observation of the charge
transfer band in toluene or dichloromethane indicates that the
complex 5 is not dissociated in these solvents. In addition, the
solid state UV-vis spectrum of 5 (see ESI, Fig. S28†) exhibits
rather a broad band in the same region, where the absorptions
Fig. 3 Molecular structure of 4; dotted lines indicate hydrogen bonds.
Selected bond lengths ( A˚ ) and angles (deg): C6–O1 1.204(7), C6–N2
1
.415(8), C6–C7 1.464(8), C7–C8 1.365(8), C7–N3 1.369(6), C8–C9
1
.385(7), C9–C10 1.382(7), C10–N3 1.359(6), C10–C11 1.505(7), C11– are observed in the solution state spectra. Furthermore, to
N4 1.462(6), C11–N6 1.471(6), C11–N8 1.472(6), O1–C6–N2 119.7(6), support the presence of the benzenoid system of DDQH in 5,
2
O1–C6–C7 121.2(7), N2–C6–C7 119.1(6), N3–C7–C6 127.5(6),
N3–C10–C11 123.3(4), N4–C11–N6 107.5(4), N4–C11–N8 107.1(4),
N6–C11–N8 110.2(4), N4–C11–C10 113.2(4), N6–C11–C10 107.8(4),
N8–C11–C10 111.0(4), N3/N1 2.682(7), H2/N1 2.13(5), N3–H2/N1
ꢀ1
ꢀ1
the FTIR spectrum contains bands at 3222 cm and 2251 cm
due to the n(OH) and n(CN) stretching frequencies, respectively,
which is conrmed by X-ray structure.
ꢀ
1
18(4), N3/N5 2.739(6), H2/N5 2.18(5), N3–H2/N5 119(4).
Complex 5 crystallizes in the triclinic space group P1. The free
2
base 6 and one molecule of DDQH constitute the asymmetric unit
representing the adduct formation as shown in Fig. 4. The DDQH
2
˚
˚
˚
of the molecule which consists of the pyrrole ring and the C–C (1.381(6) to 1.420(6) A) and C–O (1.339(5) A and 1.345(5) A)
aldehyde (3) or 3,5-dimethylpyrazolylcarbonyl group (4). This bond lengths suggest the presence of partial double bond char-
planarity indicates the presence of extended p-conjugation acter and a benzenoid system, not quinoid. These distances are
among these groups. Moreover, in the structures of 2 and 4, the similar to the benzenoid systems in other reported charge transfer
10
13
acidic pyrrolic NH group forms a three centre hydrogen bond
with the pyrazolyl nitrogen atoms. Probably because of these DDQH
hydrogen bonds, the two carbonyl groups in 2 are oriented in In contrast to the free base structure 6, which has all pyr-
complexes. Thus, the free base 6 acts as an electron acceptor, and
2
acts as an electron donor in complex 5.
one direction. This is in contrast to the thiophene based azolate rings twisted with respect to the pyrrole ring plane (see
molecule, which exhibits its carbonyl groups oriented opposite below), 5 has two of the pyrazole rings, one from each side almost
11
1
to each other. The H NMR spectra of 2 and 4 in CDCl
resonances in a fairly deshielded region at d 13.00 ppm and formed by DDQH
2.04 ppm, respectively, for their pyrrolic NH protons, probably planar (C/C) distances indicate that there can be some p–p
3
showed coplanar to the pyrrole ring plane, which is parallel to that
. This parallel arrangement and the inter
2
1
because of the three centre hydrogen bonding interactions in interaction. Whereas, the nitrogen atoms of the other two
these molecules. Conversely, the structure of 3 showed one perpendicularly oriented pyrazole rings with the dihedral angle
intramolecular hydrogen bond between the pyrrolic NH and the N1–N2–C16–C15 ¼ 93.5(5)ꢁ are hydrogen-bonded to DDQH
.
2
adjacent pyrazolyl nitrogen atom; the NH group resonates at However, all the pyrazolate nitrogen–methylene carbon bond
1
1
0.74 ppm in the H NMR spectrum.
distances are almost the same and range from 1.409(5) to
˚
The adduct 5 is a uorescent yellow solid that gives 1.416(5) A. Further, these distances are shorter than those single
˚
˚
absorption maxima at 421 nm and 442 nm and the emission bond distances [1.473(6) A and 1.466(5) A] found in the structure
maximum at 490 nm upon excitation carried out at 420 nm in of 2,5-bis(3,5-dimethylpyrazolylmethyl)pyrrole 1, indicating the
DMSO (see ESI, Fig. S25a†). Its uorescence property originated involvement of pyrazolate groups in the extended p-conjugation.
1
3
˚
from the p-electrons involved in conjugation. The C NMR The bond distances, C14–C15 ¼ 1.448(5) A and C12–C13 ¼
˚
spectrum of 5 in CDCl
nances due to DDQH
3
shows 17 peaks including the reso- 1.448(6) A, refer to single bonds that are longer than the alter-
˚
2
, which is expected for this complex. natively located double bond distances (C11–C12 ¼ 1.353(6) A,
˚
˚
These chemical shi values shied only slightly to both the C13–C14 ¼ 1.339(6) A and C15–C16 ¼ 1.345(5) A), supporting the
deshielded and shielded regions with respect to those of the quinoidal pyrrole structure. On comparing to the free base 6, a
1
free base 6 (see below). Whereas, the H NMR spectrum of 5 noticeable change is observed for the C–C double bond distance
˚
˚
shows noticeable changes for the resonances due to the methyl in the pyrrole ring (1.339(6) A (5) and 1.363(10) A (6)). These
and the pyrrolic NH protons, suggesting that the complex is not metric parameter changes are attributed to the interaction
dissociated and that the solid state structure of 5 (see below) is between the quinoidal pyrrole and DDQH
2
. The pyrrolic NH
solution. While, the H NMR of 5 in DMSO- proton forms three-centred hydrogen bonds with the nitrogen
6
(see ESI, Fig. S10†) shows a spectral pattern very close to that atoms of the pyrazolate rings, located in front of it.
1
preserved in CDCl
d
3
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Fig. 5 UV-vis and emission (490 nm) spectra of the quinoidal pyrrole
ꢀ5
molecule 6 in DMSO (10 M).
Fig. 4 Molecular structure of 5 (30% thermal ellipsoids). Most of the
hydrogen atoms are omitted for clarity. Dotted lines indicate hydrogen
bonds. Selected bond lengths ( A˚ ) and angles (deg): N6–N7 1.373(4),
C11–N7 1.409(5), C11–N8 1.411(5), N8–N9 1.379(4), C11–C12 1.353(6), adduct structure 5, and the structure represents a novel discrete
C12–N5 1.387(5), C12–C13 1.448(6), C13–C14 1.339(6), C14–C15
quinoidal form of pyrrole with the NH proton retained, in
contrast to N-substituted and fused quinoidal pyrrole ring
molecules.
1.448(5), C15–N5 1.380(5), C15–C16 1.345(5), C16–N2 1.416(5), C16–N3
1.411(5), N1–N2 1.379(5), N3–N4 1.373(5), C31–O1 1.345(5), C28–O2
1.339(5), C31–C32 1.407(6), C30–C31 1.383(6), C27–C32 1.381(6), C27–
14
15
C28 1.394(6), C28–C29 1.389(5), C29–C30 1.420(6), C32–Cl1 1.717(4),
Molecule 6 is analogous to thiophene-based quinoidal
C27–Cl2 1.723(4), C29–C34 1.440(6), C34–N11 1.143(5), C30–C33 molecules such as 2,5-dimethylene-2,5-dihydrothiophene and
1
1
1
1
.447(6), C33–N10 1.132(5), N7–C11–N8 116.4(4), C12–C11–N7
24.8(4), C12–C11–N8 118.4(4), C11–C12–N5 128.1(4), C15–N5–C12
10.9(3), C16–C15–N5 128.6(4), C15–C16–N3 126.4(4), C15–C16–N2
17.8(4), N5/N6 2.730(5), H5n/N6 2.21(4), N5–H5n/N6 120(4), N5/
its furan and N-methylpyrrole derivatives, which have p-conju-
16
17
gations.
Interestingly, quinoidal thiophene molecules,
18
Thiele's p-xylylene and Chichibabin's hydrocarbons and
19
N4 2.801(5), H5n/N4 2.28(4), N5–H5n/N4 121(4), O1/N9 2.635(5), several other conjugated molecules have been reported to
H1o/N9 1.65(5), O1–H1o/N9 168(4), O2/N1 2.688(5), H2o/N1 form charge transfer complexes, which have been shown to
1
3
.77(5), O2–H2o/N1 173(4), C12/C31 ¼ 3.5930(2), C13/C30 ¼
exhibit biradical structures and react with molecular oxygen.
Having mentioned these, 6 could have the biradical canonical
.4644(2), C14/C29 ¼ 3.4721(2), C15/C28 ¼ 3.5923(2).
Compound 6 is a yellow coloured solid that is stable in air,
but decomposes in solution under sunlight. Its emission
490 nm) and absorption (421 nm and 443 nm) spectra are very
close to those of 5 in DMSO with a quantum yield of 0.023
(
1
(
Fig. 5). The H NMR spectrum of 6 in CDCl features a pattern
3
of signals resembling that of 5. However, the characteristic
signal is the resonance due to the pyrrolic NH proton at d 9.42
ppm, which is in the slightly shielded region in comparison to
that of the adduct 5 (d 9.64 ppm), among other chemical shi
1
3
changes. The C NMR spectrum shows only thirteen peaks,
which is in agreement with the X-ray structure.
Compound 6 crystallizes in the orthorhombic space group
Pna2 and two molecules of 6 constitute the asymmetric unit.
1
These two independent molecules have almost identical metric
parameters and an ORTEP view of one of the molecules is
shown in Fig. 6. In contrast to the structure of 5, the X-ray
structure of the free base 6 adopts a different conformation that
contains none of the pyrazole rings coplanar to the pyrrole ring
Fig. 6 Molecular structure of 6 (30% thermal ellipsoids); most of the
hydrogen atoms are omitted for clarity. Selected bond lengths ( A˚ ) and
angles (deg): C11–C12 1.335(10), C11–N3 1.413(9), C11–N2 1.425(9),
C12–N5 1.392(9), C12–C13 1.451(10), C13–C14 1.363(10), C14–C15
plane, however the pyrazolate nitrogen atoms (N2, N3, N6 and 1.426(9), C15–C16 1.362(10), C15–N5 1.387(8), C16–N6 1.414(9), C16–
N8 1.407(9), C12–C11–N3 122.6(7), C12–C11–N2 121.5(7), N3–C11–
N2 115.9(7), C11–C12–N5 125.0(7), C11–C12–C13 129.6(7), C16–C15–
N5 125.3(6), C16–C15–C14 128.5(7), C15–C16–N6 120.2(6), C15–
C16–N8 122.3(7), N6–C16–N8 117.4(7), N5/N9 2.885(5), H2/N9
N8) bonded to the methylene carbons are in the plane dened
by the pyrrole ring along with the methylene carbon atoms (C16
and C11). This suggests that these pyrazolate nitrogen atoms
are involved in the p-conjugation, similar to the case in 5. The
2.30(7), N5–H2/N9 121(5), N14/N15 2.873(8), H1/N15 2.33(7), N14–
other structural features are close to those observed in the H1/N15 123(6).
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structure, as depicted in Chart 1, contributing to the ground
state structure. However, the alkenyl–pyrrole carbon distances
˚
˚
(
C11–C12 ¼ 1.335(10) A and C15–C16 ¼ 1.362(10) A) are not
considerably elongated; elongation is expected if this biradical
canonical structure contributes to the ground state. However,
the biradical form of 6 could be transient and very reactive.
Accordingly, a toluene solution of 6 readily reacts with molec-
2
ular oxygen O under sunlight irradiation (Scheme 3) to give 7
and 8 along with the dicarbonyl compound 2, which were
separated by silica gel column chromatography. Interestingly,
no reaction was observed when the toluene solution was
exposed to sunlight (visible light) for hours under nitrogen
atmosphere, suggesting that the observed products 2–4, 7 and 8
are the results of the reaction of 6 with air. Importantly, the
Scheme 3 The reaction of the quinoidal pyrrole molecule 6 with
dioxygen under sunlight irradiation and the synthesis of its binuclear
palladium(II) complex.
2
reaction between 6 and O did not proceed to completion in the
absence of exposure to visible light, that is, in darkness. These
observations led us to propose that probably the transient and
reactive biradical species or the excited state structure of 6 could
have formed in situ upon exposure to visible light, which then
reacted with O or air and cleaved the double bond of O to
in the form of the solvents of crystallization, formulated as 9
CH CN$1.5H O. The X-ray structure revealed that the pyrazolyl
3 2
nitrogen atoms on both the sides of the molecule are chelated to
the palladium metal atom, thus forming two six-membered
palladacycles adopting a boat conformation. This type of boat
conformation has been reported for several other palladium
2
2
obtain an oxygen containing products as shown in Schemes 1
and 3. All compounds, except 7, are new compounds. The
structure of 8 is based on the spectroscopic data, including
20
complexes containing pyrazolyl groups. The molecule can also
+
HRMS (ESI+), which showed the molecular ion [M + H ] peak at
be regarded to be a compartmental complex, which contains the
two distorted square planar palladium(II) atoms located above
the plane formed by the pyrrole ring and the methylene carbons
and pyrazolyl nitrogen atoms are facing each other, which can
be regarded to be the cis orientation. This is in contrast to the
structure of the palladium(II) complex of 2,3,4,5-tetrakis(3,5-
1
m/z 284.1581 (calc. 284.1511). Its H NMR spectrum showed a
broad resonance at d 8.85 ppm for the pyrrolic NH protons.
Each of the b-CH protons of the pyrrole ring give one doublet of
doublets and two different pyrazole CH protons give two sepa-
rate resonances, which are consistent with the structure. The
presence of a carbonyl group is conrmed by the IR spectrum,
ꢀ1
which displays the n(CO) stretching frequency at 1700 cm
.
In addition, a preliminary study of the coordination behav-
iour of 6 was carried out with palladium metal. The reaction of
the quinoidal pyrrole 6 with [Pd(PhCN) Cl ] gave the bis-chelate
2
2
complex 9 in 72% yield (Scheme 3). Interestingly, upon
complexation the uorescence of 6 is quenched, suggesting that
the pyrazole p-electrons are involved in the origin of the uo-
rescence. The UV-vis absorption spectrum of 9 in acetonitrile
ꢀ
1
ꢀ1
showed a blue shied lmax at 371 nm (3 ¼ 32 400 M cm
)
compared to that of 6 (Fig. S32†).
The X-ray structure of 9 is given in Fig. 7 along with the
selected bond lengths and angles; its renement data are given
in Table 1. Complex 9 crystallizes in the monoclinic P2 /n space
1
group and the asymmetric unit contains one molecule of 9, one
molecule of acetonitrile, and one and a half molecules of water
Fig. 7 Molecular structure of 9 (30% probability ellipsoids); most of the
hydrogen atoms, acetonitrile and water molecules are omitted for
clarity. Selected bond lengths ( A˚ ) and angles (deg): N1–Pd1 2.020(11),
N3–Pd1 2.031(13), Cl1–Pd1 2.284(5), Cl2–Pd1 2.281(4), N6–Pd2
2
.039(11), N8–Pd2 2.025(12), Cl3–Pd2 2.273(4), Cl4–Pd2 2.285(4),
C11–C12 1.360(18), C11–N2 1.410(17), C11–N4 1.418(18), C12–N5
.382(17), C12–C13 1.455(18), C13–C14 1.317(19), C14–C15 1.446(18),
C15–C16 1.297(18), C15–N5 1.404(16), C16–N9 1.425(16), C16–N7
.431(16), N1–Pd1–N3 86.5(5), N1–Pd1–Cl2 177.8(4), N3–Pd1–Cl2
1
1
91.6(3), N1–Pd1–Cl1 91.9(4), N3–Pd1–Cl1 177.5(4), Cl2–Pd1–Cl1
89.99(18), N8–Pd2–N6 86.9(5), N8–Pd2–Cl3 177.0(4), N6–Pd2–Cl3
91.9(4), N8–Pd2–Cl4 91.3(3), N6–Pd2–Cl4 174.4(4), Cl3–Pd2–Cl4
89.72(16), C12–C11–N2 124.0(13), C12–C11–N4 119.9(13), N2–C11–
N4 116.0(11), C11–C12–N5 126.0(13), C11–C12–C13 128.1(14), C16–
Chart 1 The proposed biradical resonance structure for the quinoidal C15–C14 128.5(13), N5–C15–C14 105.6(12), C15–C16–N9 122.2(13),
pyrrole molecule 6.
C15–C16–N7 125.1(13), N9–C16–N7 112.8(12).
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Table 1 Crystallographic data for compounds 2–6 and 9
2
3
4
5
6
9
Empirical
formula
Formula
weight
Wavelength (A)
Temperature (K)
Crystal
C
16
H
17
N
5
O
2
C
16
H
19
N
5
O
C
26
H
31
N
9
O
C
34
H
33Cl
2
N
11
O
2
C
26
H
31
N
9
C
28 4 10 2
H37Cl N O1.5Pd
311.35
297.36
485.60
698.61
469.60
892.28
˚
0.71073
293(2)
Monoclinic
1.54178
110(2)
Triclinic
0.71073
293(2)
Monoclinic
0.71073
293(2)
Triclinic
0.71073
293(2)
Orthorhombic
0.71073
293(2)
Monoclinic
system
Space group
Cc
P1ꢀ
8.6780(9)
P2 /c
1
P1ꢀ
8.1453(7)
Pna2
1
1
P2 /n
˚
a/A
19.167(6)
4.5557(15)
18.226(6)
90.00
100.720(10)
90.00
1563.7(9)
4
1.322
19.389(3)
13.8717(17)
9.8796(12)
90.00
104.655(4)
90.00
2570.7(6)
4
1.255
23.991(4)
8.3841(13)
25.825(4)
90.00
13.058(3)
15.125(3)
20.479(4)
90.00
˚
b/A
10.1309(12)
10.3932(11)
104.171(6)
108.391(6)
110.259(6)
746.32(14)
2
12.1246(10)
18.4077(16)
106.647(3)
99.820(3)
94.826(3)
1699.1(3)
2
˚
c/A
a/degree
b/degree
g/degree
Volume (A )
Z
90.00
90.00
106.625(7)
90.00
3
˚
5194.6(13)
8
3875.6(13)
4
ꢀ3
D
calcd, g cm
1.323
0.702
1.366
0.241
1.201
1.529
ꢀ
1
m/mm
0.091
0.082
0.076
1.241
F (000)
q range
656
2.16 to 25.38
316
4.89 to 59.08
1032
1.09 to 25.34
728
1.18 to 25.00
2000
1.58 to 25.00
1788
1.66 to 25.00
(degree)
Limiting
indices
ꢀ22 # h # 22,
ꢀ5 # k # 5,
ꢀ9 # h # 9,
ꢀ11 # k # 10,
ꢀ11 # l # 11
8494/2031
ꢀ23 # h # 23,
ꢀ16 # k # 16,
ꢀ10 # l # 11
30 038/4528
ꢀ9 # h # 9,
ꢀ28 # h # 28,
ꢀ9 # k # 9,
ꢀ29 # l # 30
59 470/8778
ꢀ14 # h # 15,
ꢀ17 # k # 17,
ꢀ24 # l # 24
44 801/6824
ꢀ14 # k # 14,
ꢀ21 # l # 21
20 611/5978
ꢀ
21 # l # 21
Total/unique
8729/2449
no. of rens
R
int
0.0404
0.0985
0.1460
0.0825
0.2162
0.2343
Data/restr./
params.
GOF (F )
2449/2/212
2031/0/207
4528/0/328
5978/0/451
8778/1/638
6824/0/415
2
1.007
0.942
1.005
1.002
1.106
1.002
R1, wR2
0.0553, 0.1353
0.0362, 0.0886
0.0428, 0.0909
0.0726, 0.1659
0.1961, 0.2341
0.0625, 0.1393
0.1418, 0.1806
0.0945, 0.1616
0.2114, 0.2085
0.0924, 0.2214
0.2083, 0.2836
R1, wR2 (all data) 0.0829, 0.1532
Peak and
hole (e A˚
0.197 and ꢀ0.170 0.159 and ꢀ0.236 0.296 and ꢀ0.283 0.346 and ꢀ0.329 0.200 and ꢀ0.281 1.685 and ꢀ0.603
ꢀ3
)
2
dimethylpyrazol-1-ylmethyl)pyrrole that contains two palladium characterised molecule of its type. The cleavage of O under
21
atoms above and below the pyrrole ring plane. Compared to sunlight demonstrates the considerable reactive nature of
the free molecule 6, upon coordination a signicant decrease in this pyrrole and gives insight into its possible biradical
the bond distance is observed in 9 for the pyrrole ring b C–C resonance structure. The isolated DDQ oxidation products
˚
˚
distance: C13–C14 ¼ 1.317(19) A against 1.363(10) A in 6. are observed to correlate with the isolated products of the
Further, the Pd–N(pz) and Pd–Cl bond distances fall within the reaction of the quinoidal pyrrole with oxygen. The structures
range reported for the pyrazole nitrogen coordinated palladium for most of the compounds, including a binuclear palladiu-
22
complexes. The molecule appears to be rigid, which is shown m(II) complex, are unambiguously established by X-ray
1
by the H NMR spectrum displaying one separate singlet for diffraction methods. Further reactions for adduct and metal
each pyrazolyl CH proton and methyl group, probably because complex synthesis are in progress.
of the conjugation to be maintained. The pyrrolic NH group is
hydrogen-bonded to one of the co-crystallized water molecules
1
and gives a broad singlet at 8.52 ppm in the H NMR spectrum. Experimental section
Experimental section, general
All reactions were carried out under a nitrogen atmosphere
using standard Schlenk-line techniques. Work-up proce-
Conclusions
In conclusion, a new class of uorescent quinoidal pyrrole dures including NaHCO
3
treatment were carried out under
ꢁ
with four high electron affinity nitrogen atoms attached to aerobic conditions. Petroleum ether (bp 40–60 C) and other
the methylene carbons causing it to act as an electron solvents were distilled according to the standard proce-
acceptor was developed through the dehydrogenation reac- dures. Other chemicals were obtained from commercial
1
tion of 1 with DDQ; this represents the rst structurally sources and used without further purication. H NMR (200
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3
MHz and 400 MHz) and C NMR (50.3 MHz and 100.6 MHz) Synthesis of 2-tri(3,5-dimethylpyrazolylmethyl)-5-(3,5-
spectra were recorded on a Bruker ACF200 spectrometer. dimethylpyrazolylcarbonyl)pyrrole, 4, and the DDQH adduct
2
Chemical shis are referenced with respect to the chemical of 2,5-bis{di(3,5-dimethylpyrazolyl)methene}-2,5-
shi of the residual protons present in the deuterated dihydropyrrole, 5
solvents. FTIR spectra were recorded using Perkin Elmer
To a dichloromethane (ꢂ50 mL) solution of 1 (1.0 g, 3.13 mmol)
Spectrum Rx. High Resolution Mass Spectra (ESI) were
recorded using the Xevo G2 Tof mass spectrometer (Waters).
Elemental analyses were carried out using a Perkin Elmer
ꢁ
at 0 C was added solid DDQ (2.13 g, 9.40 mmol). The colour of
the solution immediately changed to dark green and stirring
was continued for an additional 2 h at room temperature. The
solution was concentrated and loaded onto a silica gel column.
Elution using ethyl acetate–petroleum ether (1 : 3, v/v) afforded
the rst fraction, from which the solvent was removed to obtain
2
400 CHN analyzer. Melting points were determined in open
capillaries and are corrected using benzophenone as a
reference. UV-vis spectra were recorded using a Varian Cary
5
000 UV-vis-NIR spectrophotometer.
2
as a colourless solid (0.11 g, 0.35 mmol, 11%). Removal of the
solvent from the second orange coloured fraction obtained
compound 5 as a yellow solid (0.5 g, 0.72 mmol, 22%). Slow
evaporation of the reaction mixture in acetonitrile gave a few
single crystals of 4. Suitable single crystals of 5 were obtained by
the slow evaporation of a solution of 5 in toluene at room
temperature.
Synthesis of 2,5-bis(3,5-dimethylpyrazolylcarbonyl)pyrrole, 2,
and 5-{bis(3,5-dimethylpyrazolyl)methyl}pyrrole-2-
carbaldehyde, 3
1
ꢁ
For 4: H NMR (CDCl
NH), 5.99 (s, 1H, pyrazole CH), 5.91 (s, 3H, pyrazole CH), 5.79
3
, 200 MHz, 25 C): d ¼ 12.04 (br s, 1H,
To a dichloromethane (ꢂ50 mL) solution of 1 (1.0 g, 3.13
ꢁ
mmol) at 0 C was added solid DDQ (2.13 g, 9.4 mmol). The
(
dd, J ¼ 4.0, 2.4 Hz, 2H, pyrrole CH), 2.62 (s, 3H, CH
3
), 2.27
colour of the solution immediately changed to dark green.
Aer stirring at room temperature for 2 h, a saturated
(s, 3H, CH ), 2.17 (s, 9H, CH ), 1.69 (s, 9H, CH ). Further
3
3
3
characterization could not be carried out because of the very low
yield.
aqueous NaHCO
solution and stirred under aerobic conditions. The organic
layer was separated and dried over anhydrous Na SO . The
3
solution (2 ꢃ 50 mL) was added to the
ꢁ
1
For 5: mp 140 C (decomposed). H NMR (CDCl , 200 MHz,
3
2
4
ꢁ
2
(
2
(
1
1
2
5 C): d ¼ 9.64 (br s, 1H, NH), 6.00 (s, 2H, pyrrole CH), 5.88
solvent was removed from the organic layer and the residue
was loaded onto a silica gel column. Elution using ethyl
acetate–petroleum ether (1 : 3, v/v) afforded the rst fraction
from which the solvent was removed to give 2 as a colourless
solid (0.11 g, 0.35 mmol, 11%). Further elution using the
same solvent gave the second fraction from which the solvent
was removed under vacuum to obtain compound 3 as a col-
ourless solid (0.19 g, 0.64 mmol, 20%). Single crystals of 2
were obtained by the slow evaporation of a solution of 2 in
methanol. 3 was crystallized by layering petroleum ether
s, 2H, pyrazole CH), 5.79 (s, 2H, pyrazole CH), 2.44 (s, 6H, CH
3
),
13
.34 (s, 6H, CH
3
), 1.91 (s, 6H, CH
3
), 1.75 (s, 6H, CH
3
). C NMR
ꢁ
CDCl
3
, 51.3 MHz, 25 C): d ¼ 151.8, 149.8, 149.7, 144.0, 141.0,
37.5, 133.4, 126.0, 113.6, 109.0, 107.6, 106.8, 106.2, 14.0, 13.5,
ꢀ1
1.0, 10.6. IR (KBr, cm ): n ¼ 3222 (br, s), 2925 (w), 2526 (w),
364 (w), 2251 (m), 1661 (w), 1564 (m), 1450 (s), 1414 (m), 1355
(
(
s), 1274 (m), 1193 (s), 1078 (w), 1039 (w), 979 (w), 891 (m), 778
w), 692 (w), 623 (w). HRMS (+ESI): calcd m/z for [M + Na]
+
C
34
H
33Cl
2
N
11NaO
2
: 720.2093, found: 720.2114. UV-vis (DMSO):
ꢀ1
ꢀ1
l
max, nm (3, M cm ) ¼ 421 (35 000), 442 (32 200).
2 2
upon a solution of 3 in CH Cl .
ꢁ
1
ꢁ
For 2: mp 134 C. H NMR (CDCl , 200 MHz, 25 C): d ¼ 13.00
3
Synthesis of 2,5-bis{di(3,5-dimethylpyrazolyl)methene}-2,5-
dihydropyrrole 6 from 5
(
br s, 1H, NH), 7.38 (d, 2H, pyrrole CH), 6.06 (s, 2H, pyrazole
1
3
CH), 2.65 (s, 6H, CH
3
), 2.38 (s, 6H, CH
3
). C NMR (CDCl
3
, 51.3
ꢁ
MHz, 25 C): d ¼ 157.7, 152.9, 146.1, 129.2, 120.2, 111.0, 15.0,
Method A. To a solution of 5 (1.0 g, 1.43 mmol) in dry MeOH
ꢀ
1
ꢁ
1
4.3. FT-IR (KBr, cm ): n ¼ 3437 (br, s), 3310 (s), 3112 (w), 2976 (ꢂ50 mL) at 0 C was slowly added solid NaBH
4
(2.7 g, 71.50
(
w), 2923 (w), 1676 (vs), 1585 (m), 1518 (m), 1486 (m), 1411 (m), mmol). The solution was stirred at room temperature for 16 h.
1
365 (s), 1340 (s), 1312 (m), 1275 (m), 1195 (m), 1148 (w), 1106 Aer adding water (ꢂ150 mL) to the solution, the solution was
(m), 1028 (m), 968 (w), 870 (m), 816 (m), 768 (m), 731 (w), 627 extracted for three times with dichloromethane (50 mL). The
+
(w). HRMS (+ESI): calcd m/z for [M + H ] C H N O : 312.1455, dichloromethane solution was dried over anhydrous Na SO4
1
6
18
5
2
2
found: 312.1458.
and ltered. The ltrate was concentrated and loaded onto a
, 200 MHz, 25 C): d ¼ 10.74 silica gel column. Elution using ethyl acetate–petroleum ether
ꢁ
1
ꢁ
For 3: mp 146 C. H NMR (CDCl
3
(br s, 1H, NH), 9.54 (s, 1H, CHO), 7.60 (s, 1H, CH), 6.93 (s, 1H, (1 : 2, v/v) gave the rst fraction, from which the solvent was
pyrrole CH), 6.25 (s, 1H, pyrrole CH), 5.80 (s, 2H, pyrazole CH), removed to obtain 6 as a yellow solid (0.45 g, 0.96 mmol, 67%).
1
3
ꢁ
ꢁ
1
ꢁ
2
1
(
1
.22 (s, 12H, CH
3
). C NMR (CDCl
3
, 51.3 MHz, 25 C): d ¼ 179.5, mp 147 C. H NMR (CDCl
3
, 200 MHz, 25 C): d ¼ 9.42 (br s, 1H,
49.2, 141.1, 134.2, 132.7, 121.1, 111.5, 107.5, 67.9, 13.8, 11.5. IR NH), 6.22 (d, 2H, pyrrole CH), 5.90 (s, 2H, pyrazole CH), 5.82 (s,
ꢀ1
KBr, cm ): n ¼ 3299 (s), 2923 (w), 2865 (w), 1666 (s), 1555 (m), 2H, pyrazole CH), 2.28 (s, 6H, CH
3
), 2.25 (s, 6H, CH
3
), 2.00 (s,
1
3
ꢁ
468 (m), 1418 (s), 1380 (m), 1316 (m), 1268 (w), 1223 (m), 1168 6H, CH ), 1.69 (s, 6H, CH ). C NMR (CDCl , 51.3 MHz, 25 C): d
3
3
3
(
6
C
s), 1116 (w), 1035 (m), 976 (w), 875 (w), 786 (s), 757 (m), 719 (m), ¼ 149.7, 149.4, 143.2, 140.9, 137.8, 126.7, 109.5, 107.2, 106.4,
+
ꢀ1
50 (w), 516 (w). HRMS (+ESI): calcd m/z for [M + Na]
14.0, 13.8, 11.2, 10.5. IR (KBr, cm ): n ¼ 3357 (m), 2956 (w),
16
H
19
N
5
ONa: 320.1482, found: 320.1485.
2925 (w), 2861 (w), 1676 (w), 1647 (w), 1562 (m), 1454 (m), 1415
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(
(
s), 1358 (s), 1204 (w), 1117 (w), 1027 (w), 974 (w), 909 (w), 838 vacuum. Yield: 72% (0.035 g, 0.042 mmol). A suitable single
w), 792 (m), 724 (w), 656 (w), 629 (w), 581 (w). HRMS (+ESI): crystal of 9 was grown from an acetonitrile solution of 9 by slow
+
1
ꢁ
calcd m/z for [M + H] C H N : 470.2775, found: 470.2797. evaporation. H NMR (CDCl , 200 MHz, 25 C): d ¼ 8.52 (br s,
2
6
32
9
ꢀ
3
1
ꢀ1
UV-vis (DMSO): lmax, nm (3, M cm ) ¼ 420 (23 700), 443 1H, NH), 6.90 (s, 2H, pyrrole CH), 6.03 (s, 2H, pyrazole CH), 5.98
(
21 900) and 259 (16 500).
Method B. A solution of 5 in ethyl acetate–petroleum ether 6H, CH
1/1, v/v) was passed through a basic alumina column. The rst (w), 2961 (w), 2930 (w), 1656 (m), 1625 (m), 1561 (m), 1464 (m),
(s, 2H, pyrazole CH), 2.58 (s, 6H, CH
3 3
), 2.56 (s, 6H, CH ), 2.42 (s,
ꢀ
1
3
), 2.38 (s, 6H, CH
3
). IR (KBr, cm ): n ¼ 3454 (br), 3126
(
fraction was collected and the solvent was removed under 1416 (m), 1393 (m), 1261 (m), 1098 (s), 803 (m), 654 (w), 471 (w).
+
vacuum to obtain an oily residue, which was washed with HRMS (+ESI): calcd m/z for [M ꢀ 2Cl + Na] C26
H
31Cl
2
N
9
2
NaPd :
1
13
hexane to obtain 6 in the form of a yellow solid. The H and
C
774.0041, found: 774.0657. UV-vis (acetonitrile): l
max
(3) ¼ 371
ꢀ1
ꢀ1
NMR spectra of 6 obtained by this method were identical to nm (32 400 M cm ).
the spectra obtained from the above mentioned procedure,
Method A.
X-ray crystallography
Suitable single crystals of 2–6 and 9 were obtained from solvents
mentioned in their respective synthetic procedures. Single
crystal X-ray diffraction data collections for these crystals were
performed using a Bruker-APEX-II CCD diffractometer with
The sunlight irradiation of 6
A solution of 6 (0.10 g, 0.21 mmol) in toluene (20 mL) was
connected to a bladder containing oxygen gas and stirred under
sunlight irradiation for 6 h at room temperature. The solvent
was removed under vacuum and the residue was loaded onto a
silica gel column. Elution using ethyl acetate–petroleum ether
graphite monochromated molybdenum Ka radiation (l ¼
˚
0
.71073 A). The structures were solved by SIR-92 (ref. 24) avail-
able in the WinGX program which successfully located most of
the non-hydrogen atoms. Subsequently, least-squares rene-
ments were carried out on F2 using SHELXL-97 (ref. 25) (WinGX
version) to locate the remaining non-hydrogen atoms. Typically
for all the structures, hydrogen atoms attached to carbon atoms
were xed in calculated positions. The pyrrolic NH or OH
hydrogen atoms were located from the difference Fourier map
and freely rened isotropically with their thermal parameters
set as equivalent to 1.2 times that of their parent atoms. In case
of the structure of 9, water hydrogens did not appear and, thus
are not located; the pyrrolic NH hydrogens are xed. Crystallo-
graphic renement data are given in Table 1.
(
1 : 5 v/v) gave the rst fraction. Elution using ethyl acetate–
petroleum ether (1 : 1 v/v) gave the second and the third frac-
tions. Removal of solvents from these fractions under vacuum
gave compound 2 as a colourless solid (0.01 g, 0.032 mmol,
1
1
5
5%), compound 7 as a colourless oil (0.008 g, 0.037 mmol,
7%) and compound 8 as a colourless solid (0.03 g, 0.11 mmol,
0%), respectively. The identity of 2 obtained by this method
was conrmed aer comparing the NMR and IR data with those
of 2 obtained from the above-described reaction of 1 with DDQ.
2
3
1
For bis-(3,5-dimethylpyrazolyl)methanone.
7
H NMR
ꢁ
(
1
CDCl , 200 MHz, 25 C): d ¼ 5.85 (s, 2H, pyrazole CH), 2.28 (s,
3
ꢀ1
2H, CH ). IR (KBr, cm ): n ¼ 3440 (s), 2925 (m), 2852 (m), 1740
3
(
m), 1699 (s), 1650 (s), 1560 (m), 1541 (m), 1513 (w), 1460 (m),
1
6
2
418 (w), 1393 (w), 1261 (w), 1189 (w), 1105 (s), 1029 (w), 800 (w),
97 (w), 629 (w). HRMS (+ESI): calcd m/z for [M + H] C11H N O:
15 4
19.1246, found: 219.1292.
Acknowledgements
+
We thank the CSIR and DST for support and for the X-ray and
For
azolyl)pyrrole 8. H NMR (CDCl
2-(3,5-dimethylpyrazolylcarbonyl)-5-(3,5-dimethylpyr- NMR facilities. We also thank Dr Joseph H. Reibenspies,
1
ꢁ
, 200 MHz, 25 C): d ¼ 8.85 (br s, Department of Chemistry, Texas A&M University for one data
3
1
5
H, NH), 6.84 (dd, J ¼ 5.6, 1.6 Hz, 1H, pyrrole CH), 6.13 (dd, J ¼ collection and structure renement.
.6, 1.6 Hz, 1H, pyrrole CH), 5.99 (s, 1H, pyrazole CH), 5.90 (s,
1
CH
1
1
3
H, pyrazole CH), 2.27 (s, 3H, CH ), 2.26 (s, 3H, CH ) 1.97 (s, 3H,
3
3
1
3
ꢁ
Notes and references
, 51.3 MHz, 25 C): d ¼
3
), 1.60 (s, 3H, CH
71.3, 152.4, 151.1, 143.6, 142.4, 136.3, 129.4, 124.8, 116.8,
3
). C NMR (CDCl
3
1
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W.-P. Deng, J. Org. Chem., 2014, 79, 1156.
ꢀ1
09.3, 107.9, 13.9, 13.7, 11.0, 10.3. IR (KBr, cm ): n ¼ 3384 (m),
250 (m), 2925 (s), 2856 (m), 1700 (vs), 1567 (s), 1450 (s), 1393
(s), 1352 (s), 1235 (w), 1192 (m), 1119 (m), 1073 (m), 1033 (m),
9
75 (m), 915 (m), 807 (s), 732 (s), 799 (m), 632 (w), 590 (w), 513
+
(w). HRMS (+ESI): calcd m/z for [M + H] C H N O: 284.1506,
15 18 5
2
found: 284.1581.
Synthesis of [Pd Cl {m-C H N-2,5-(C(Me pz) ) -N,N,N,N}], 9
2
4
4
3
2
2 2
To a solution of [Pd(PhCN)
2 2
Cl ] (0.045 g, 0.12 mmol) in toluene
(
10 mL) was added 6 (0.028 g, 0.059 mmol). The solution was
3 K. Ohkubo, A. Fujimoto and S. Fukuzumi, J. Am. Chem. Soc.,
2013, 135, 5368.
4 (a) J.-Y. Shin, H. Furuta, K. Yoza, S. Igarashi and A. Osuka, J.
Am. Chem. Soc., 2001, 123, 7190; (b) S. Ramakrishnan,
stirred at room temperature for 16 h to obtain a brown precip-
itation of 9. The solution was ltered and the precipitate was
washed with diethyl ether (3 ꢃ 10 mL) and then dried under
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