First example of pyrrole-2-carbaldehyde thiosemicarbazone as tridentate dianion in [Pd(η3-N4,N3,S-ptsc)(PPh3)] complex

Article abstract of DOI:10.1016/j.inoche.2006.12.024

Reaction of pyrrole-2-carbaldehyde thiosemicarbazone (H₂ptsc) with PdCl₂(PPh₃)₂ in 1:1 mol ratio in presence of Et₃N leads to complete dechlorination to generate square planar complex, [Pd(η³

Full text of DOI:10.1016/j.inoche.2006.12.024

Inorganic Chemistry Communications 10 (2007) 506–509
www.elsevier.com/locate/inoche
First example of pyrrole-2-carbaldehyde thiosemicarbazone
as tridentate dianion in [Pd(g3-N4,N3,S-ptsc)(PPh3)] complex
a,
a
b
c
Tarlok S. Lobana *, Gagandeep Bawa , Alfonso Castineiras , Ray J. Butcher
a
Department of Chemistry, Guru Nanak Dev University, Amritsar 143 005, India
Departamento de Quimica Inorganica, Facultad de Farmacia, Universidad de Santiago, 15782 Santiago, Spain
b
c
Department of Chemistry, Howard University, Washington, DC 20059, USA
Received 16 November 2006; accepted 20 December 2006
Available online 12 January 2007
Abstract
Reaction of pyrrole-2-carbaldehyde thiosemicarbazone (H2ptsc) with PdCl2(PPh3)2 in 1:1 mol ratio in presence of Et3N leads to com-
plete dechlorination to generate square planar complex, [Pd(g3-N4,N3,S-ptsc)(PPh3)] (1), existing as three independent molecules in the
same unit cell, and H2ptsc behaves in an unusual dinegative tridentate mode. Salicylaldehyde thiosemicarbazone (H2stsc) formed similar
compound, [Pd(g3-O,N3,S-stsc)(PPh3)] (3).
Ó 2007 Elsevier B.V. All rights reserved.
Keywords: Pyrrole-2-carbaldehyde thiosemicarbazone; Palladium(II); Triphenyl phosphine; Dechlorination
Pyrrole-2-carbaldehyde thiosemicarbazone (H2ptsc) has
several potential donor sites, and thus binds to metals as a
neutral ligand in g1-S terminal [1,2], or g2-N3,S-chelating
bonding modes [3]. Further, it acts as N3,S-chelating anio-
nic ligand after loss of hydrazinic (–N2H) proton [3–5]. In
all the cases reported, pyrrole ring at C2 carbon remains
pendant, and there is no example to date in which a metal
activates pyrrole (–NH) hydrogen to form M–N bonds [1–
5], except removal of this hydrogen by a strongly basic
alkyl group of a metal alkyl [6]. On the other hand, pyri-
dine substituent at C2 carbon is well known to coordinate
to metals, such as in [Pd(g3-N4,N3,S-pytsc)Cl] (2)
strong tendency of pytscÀ ligand to act as a tridentate with
removal of both phosphine ligands. These observations
increased our curiosity to study the comparative behaviour
of pyrrole-2-carbaldehyde thiosemicarbazone, which has
pendant (–N4H) nitrogen, and could be a N4-donor, if
deprotonation occurs at this center. Interestingly, reaction
of [PdCl2(PPh3)2] with H2ptsc in toluene in presence of
Et3N in 1:1 and 1:2 mol ratios, invariably formed com-
pound of stoichiometry, [Pd(ptsc)(PPh3)] (1). This is an
unusual reaction in which both halogens and one PPh3
ligand are removed along with deprotonation of hydrazinic
(–N2H) and pyrrole ring (–N4H) protons, the comparative
behaviour of salicylaldehyde thiosemicarbazone is also
described and these findings are reported in this communi-
cation. In literature, pyrrole ring is known to undergo
deprotonation and subsequently bind to a metal center in
porphyrins and some other class of ligands [8].
(Hpytsc = pyridine-2-carbaldehyde
thiosemicarbazone),
prepared by the reaction of Li2PdCl4 with Hpytsc in meth-
anol [7].
Reaction of [PdCl2(PPh3)2] with Hpytsc in 1:1 and 1:2
ratios in presence of Et3N, designed to prepare stepwise
[PdCl(pytsc)(PPh3)], and [Pd(pytsc)2] complexes, invariably
formed compound [Pd(g3-N4,N3,S-pytsc)Cl] 2. It showed
Scheme 1 depicts the formation of compound 1, along-
with formation of compounds 2 and 3. Reaction of
PdCl2(PPh3)2 [9] with one mole of H2ptsc, in presence of
Et3N base, in toluene gave complex [Pd(ptsc)(PPh3)] (1);
same product resulted with two moles of H2ptsc ligand.
*
Corresponding author. Fax: +91 183 2 258820.
E-mail address: tarlokslobana@yahoo.co.in (T.S. Lobana).
1387-7003/$ - see front matter Ó 2007 Elsevier B.V. All rights reserved.
doi:10.1016/j.inoche.2006.12.024

T.S. Lobana et al. / Inorganic Chemistry Communications 10 (2007) 506–509
507
Table 1
N4
PPh3
PdCl2(PPh3)
i
˚
Bond parameters {bond distances (A) and bond angles (°)} of three
molecules (A, B, C) of compound 1
Pd
Pd
1
N3
N4
S
A
B
C
Pd–N12, Pd– 2.005(4), 2.029(4) 2.009(4), 2.037(4) 2.016(4), 2.039(4)
N11
Pd–P, Pd–S
Cl
PdCl2(PPh3)
ii
S
H
N2 1
2.2364(13),
2.2566(13)
2.2596(13),
2.2577(13)
80.13(16)
177.37(12)
102.09(11)
83.34(12)
163.46(11)
94.45(5)
2.2617(11),
2.2543(14)
80.10(17)
177.90(11)
98.36(12)
83.30(13)
163.39(12)
98.23(5)
2
3
C
1
C2
R
N3
O
NH2
N
S
H
N12–Pd–N11 80.79(16)
N12–Pd–P
N11–Pd–P
N12–Pd–S
N11–Pd–S
P–Pd–S
176.53(11)
99.90(12)
83.44(11)
164.22(12)
95.82(4)
PPh3
PdCl2(PPh3)
iii
Pd
3
N3
S
4
OH
5
3
(1)
(2)
(3)
R
=
6
4
N
H
˚
N
2.005(4) (Pd–N12azomethine) and 2.029(4) A (Pd–N11pyrrole),
4
˚
with Pd–P bond distance of 2.2364(13) A. In molecules
(B) and (C), the corresponding distances are marginally
longer; however, Pd–S distances are almost identical for
the three molecules. These distances are comparable with
analogous PdII complexes with tridentate pyridine-2-carbal-
dehyde thiosemicarbazones {N4,N3, S-donors} [10,7]. The
bite angles N12–Pd–N11 and N12–Pd–S marginally
decrease from molecules A to C. The trans N12–Pd–P bond
angles, ca. 177–178° are close to linearity, while N11–Pd–S
angles ca. 164° significantly deviate from linearity. The dif-
ference in bond parameters is attributed to different hydro-
gen bond interactions. For example, molecule A forms
centrosymmetric dimers with two identical N–HÁ Á ÁN hydro-
gen bonds, whereas molecules B and C interact mutually and
form different types of hydrogen bonds.
Scheme 1. (i–iii) Toluene Et3N.
The crystal structure of complex 1 (Fig. 1) reveals that Pd is
coordinated to one thiosemicarbazone ligand via N4,N3
and S donor atoms after deprotonation of –N2H and
–N4H protons. The reaction appears to occur in a system-
atic manner in presence of Et3N base, resulting in removal
of both ClÀ ions as insoluble Et3NH+ClÀ salt in toluene.
Due to tridentate binding of the thiosemicarbazone ligand,
one of the –PPh3 group is also removed from the metal
center. Complex 1 is soluble in toluene, methanol, dichlo-
romethane and chloroform. Compound 3 with salicylalde-
hyde thiosemicarbazone (H2stc) was prepared similarly.
Compound 1 showed three types of molecules (A, B, C)
with different bond parameters (Table 1). Inmolecule(A), pal-
ladium(II) is bonded to two N atoms at bond distances of
In compound 3, PdII is coordinated to O, N3 and S
atoms of salicylaldehyde thiosemicarbazone ligand, and
the fourth site is occupied by PPh3 ligand (Fig. 2). The
˚
˚
Pd–N {2.0190(13) A}, Pd–S {2.2453(4) A} and Pd–P
˚
{2.2756(4) A} distances are comparable to those of com-
Fig. 1. Structure of complex 1 (molecule A) with atomic numbering
˚
scheme. Selected bond lengths (A) and angles (°): Pd(1A)–N(12A)
2.005(4), Pd(1A)–N(11A) 2.029(4), Pd(1A)–P(1A) 2.2364(13), Pd(1A)–
S(1A) 2.2566(13) and N(12A)–Pd(1A)–N(11A) 80.79(16), N(12A)–
Pd(1A)–P(1A) 176.53(11), N(11A)–Pd(1A)–P(1A) 99.90(12), N(12A)–
Pd(1A)–S(1A) 83.44(11), N(11A)–Pd(1A)–S(1A) 164.22(12), P(1A)–
Pd(1A)–S(1A) 95.82(4).
Fig. 2. Structure of complex 3 with atomic numbering scheme. Selected
˚
bond lengths (A) and angles (°) : Pd–O, 2.0170(13), Pd–N(1) 2.0190(13),
Pd–S 2.2453(4), Pd–P 2.2756(4) and O–Pd–N(1) 92.99(5), O–Pd–S
177.14(4), N(1)–Pd–S 84.16(4), O–Pd–P 91.08(4), N(1)–Pd–P 173.99(4),
S–Pd–P 173.99(4).

508
T.S. Lobana et al. / Inorganic Chemistry Communications 10 (2007) 506–509
plex 1. The N12A–Pd–S bite angle in complexes 1 and N1–
Pd–S angle of 3 are similar, whereas N12A–Pd–N11A
{80.79(16)°} bite angle of 1 and O–Pd–N1 {92.99(5)°}
angle of 3 are quite different. This may be attributed to
the fact that in 1 the N donor atom is part of the five mem-
bered pyrrole ring, whereas in complex 3 the O atom is exo-
cyclic. Thus system 1 is more strained as compared to 3,
this also explains the difference in trans angles N11A–
PdA–S1A {164.22(12)°} and O–Pd–S {177.14(4)°}.
d
ligand) = 30.08 ppm (31P NMR spectra were recorded by
taking TMP {(MeO)3P} as external reference taken at zero
position).
Crystallographic data for 1: C24H21N4PPdS, M =
˚
˚
534.88, triclinic, a = 10.307(2) A, b = 14.962(3) A, c =
˚
24.351(5) A, a = 100.908(4)°, b = 99.508(4)°, c = 107.255
3
˚
ꢀ
(3)°, V = 3422.0(12) A , T = 293(2) K, space group P1
(No. 2),
q , Z = 6, l(Mo Ka) =
calcd = 1.557 g cmÀ3
0.994 mmÀ1, 21,136 reflections measured on a Bruker
SMART CCD-1000 diffractometer unique 15,007
(Rint = 0.0382). The final R1 0.0490 was for 10,152 reflec-
tions [I > 2r(I)] and wR2 was 0.1233. Crystals 3.
C26H22N3OPPdS’, M = 561.90, Triclinic, a = 7.5551(2),
1H NMR spectrum of compounds 1 and 3 were recorded
in CDCl3. In the spectrum of complex 1, the absence of –
N2H and –N4H hydrogens, at 11.28 and 11.34 ppm respec-
tively for free ligands [4], confirmed their deprotonation,
followed by coordination to Pd center as a dianion. Other
characteristic peaks of the thiosemicarbazone ligand are:
C2H 5.65(s), C5H(q) 5.80, C4H(dd) 6.49, C6H(d)
7.18 ppm. In addition, PPh3 ring peaks also appear in the
spectra in the region, 7.48–7.67 ppm. The 31P NMR
spectrum shows a single peak at À79.2 ppm with a coordi-
nation shift, Dd(dcomplex–dligand) = 33.9 ppm, supporting
coordination of P atom to Pd. In the IR spectrum, the
m(N–H) band due to –N2H– moiety appears at 3152 cmÀ1
in the free ligand, which is absent in complex 1. Similarly
for complex 3, the spectrum showed the absence of –OH
and –N2H proton signals {cf. –OH, 9.87 and –N2H,
11.38 ppm, free ligand}, confirming double deprotonation.
31P NMR also showed a single peak with a coordination
shift of 30.08 ppm.
As depicted in Scheme 1, the deprotonation of acidic
hydrogen such as –N4H and –OH in rings at C2 carbon
facilitates tricoordination by the thiosemicarbazone as dia-
nions in 1 and 3, thus retaining one PPh3 ligand, while lack
of acidic hydrogen as in 2, retains one Cl and not PPh3 in
view of overall charge balance. The Cl in 2 is not appearing
as anion such as in anticipated, [Pd(g3-N4,N3,S-pytsc)
(PPh3)]Cl, thus presumably {Pd(g3-N4, N3, S-pytsc)} spe-
cies has strong affinity to bring Cl in coordination sphere,
leading to exit of PPh3.
˚
b = 10.2388(3), c = 17.0045(5) A, a = 77.04, b = 80.8980
3
˚
(10), c = 71.54°, V = 1210.52(6) A , T = 298(2) K, space
group P1 (No. 2), qcalcd = 1.542 g cmÀ3, Z = 2, l(Mo
ꢀ
Ka) = 0.943 mmÀ1, 13,980 reflections measured on a Bru-
ker SMART CCD-1000 diffractometer unique 6826
(Rint = 0.0146). The final R1 0.0246 was for 6203 reflections
[I > 2r(I)] and wR2 was 0.0639.
Acknowledgement
Financial support from CSIR, New Delhi, to one of us
(Gagandeep) is gratefully acknowledged.
Appendix A. Supplementary material
CCDC 627413 and 627414 contain the supplementary
crystallographic data for 1 and 3. These data can be
obtained free of charge via http://www.ccdc.cam.ac.uk/
conts/retrieving.htmlor from the Cambridge Crystallo-
graphic Data Centre, 12 Union Road, Cambridge CB2
1EZ, UK; fax: (+44) 1223-336-033; or e-mail: depos-
it@ccdc.cam.ac.uk. Supplementary data associated with
this article can be found, in the online version, at
doi:10.1016/j.inoche.2006.12.024.
In conclusion, an acidic hydrogen in the ring has impor-
tant role in determining coordination modes of thiosemic-
arbazones, and tricoordination by pyrrole-2-carbaldehyde
thiosemicarbazone (H2ptsc) is first example.
References
Compound 1: Mp. 240–242 °C (dec.), yield: 0.025 g,
62%. C, H, N, analysis for C24H21N4PPdS: C, 53.9; H,
3.92; N, 10.48; Found: C, 53.5; H, 3.64; N, 9.51. Main
IR Peaks (KBr, cmÀ1): m(N–H), 3463m, 3342m, 3280m
(–NH2); m(C–H), 3072w; d(NH2) + m(C@N) + m(C–C),
1600m, 1566s, 1517s; m(C@S) + m(C–N), 1033s, 1000w,
850w (thioamide moiety). 3: Mp. 230–232 °C, yield:
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(m, 1H, C5H), 7.40–7.78 (17H, Ph-H + C4,6) ppm. 31P
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