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Chemistry Letters Vol.35, No.6 (2006)
Synthesis and Structural Features of a Series of ꢀ3SNS Pincer Complexes
of Group 10 Metals ꢁ-Bonded to Centered Pyrrolate Unit
Ken Okamoto, Takaki Kanbara,ꢀ and Takakazu Yamamotoꢀ
Chemical Resources Laboratory, Tokyo Institute of Technology, Nagatsuta 4259, Midori-ku, Yokohama 226-8503
(Received February 10, 2006; CL-060175; E-mail: tkanbara@res.titech.ac.jp)
N-Metalation of N,N0-dibenzyl-1H-pyrrole-2,5-dicarbo-
NiCl2 or
K2MCl4
thioamide afforded new pincer complexes of group 10 metals
with ꢀ3SNS-type coordination. Molecular structures of the
complexes were characterized by X-ray crystallography.
H
H
N
N
N
H
S
S
1
H
H
Pincer complexes of group 10 metals have recently attract-
ed much attention in catalysis and material science.1 Such
complexes with two five-membered metallacycle rings and a
metal–C(phenyl) ꢁ bond ((a) in Chart 1) have actively been
studied. These pincer complexes are generally formed by the
ortho,ortho-cyclometalation reaction of the starting ligand by
the transition metals or transmetalation of the corresponding
lithiated ligand. The complexes are stabilized by the ancillary
chelating donor atom E.
N
N
N
S
S
2a: M = Ni 83%
2b: M = Pd 98%
2c: M = Pt 78%
M
Cl
Scheme 1.
tion and the molecular structures of the ꢀ3SNS pincer com-
plexes.
4
The pincer ligand 1 was prepared by the Willgerodt–Kindler
reaction of pyrrole-2,5-dicarboxyaldehyde with benzylamine
in the presence of sulfur according to the previous reports.6–8
3
4
5
6
3
2
2
5
N
M
1N
.
Reaction of 1 with NiCl2 6H2O readily led to deprotonation of
1
E
E
the pyrrole nitrogen even in the absence of base at room temper-
ature, and afforded the ꢀ3SNS pincer Ni complex (Ni(1-H)Cl,
2a). Similar cyclometalation of 1 with K2MCl4 (M = Pd and
Pt) yielded the corresponding ꢀ3SNS pincer complexes (M(1-
H)Cl, 2b: M = Pd; 2c: M = Pt) in good yields (Scheme 1).8
The complexes 2a–2c were fairly stable to air and water, and
showed good solubility in organic solvents such as methanol,
acetone, THF, and DMSO. Structures of 2a–2c were confirmed
by NMR, IR, and FAB-MS spectroscopy and elemental analy-
M
E
M
E
E
E
(b)
(c)
(a)
Chart 1. Pincer complexes: E = chelating donor atom.
Pyrrole is a building block of porphyrin, phthalocyanine,
and related ligands.2 N-Metalation of pyrrole to give the pincer
type complex (b) in Chart 1 is intriguing because, in contrast to
the case of the complex (a), the 2- and 5-positions in the complex
(b) is connected with a ꢂ-conjugated system which would render
interesting electronic properties to the complex. However, ex-
amples of such square-planar pincer complexes containing the
centered pyrrole ring have been limited.3–5 For group 10 metal
complexes, only a few ꢀ3ENE-type pincer palladium complexes
with the centered pyrrolate unit have been reported. The isolated
ꢀ3ENE–Pd pincer complexes, so far reported, have the structure
(c) depicted in Chart 1, with two six-membered rings.4 Prepara-
tion of ꢀ3ENE-type pincer complexes of Ni and Pt has no prece-
dent.
1
sis.8 In the H NMR spectra of 2a–2c, the NH signal of pyrrole
ring disappeared, and a downfield shift (by 0.43–1.17 ppm) of
the NH signal of the thioamide groups was observed. The
ꢃ(C–N) IR peak of 1 (1541 cmꢁ1) shifted to a higher frequency
(1568, 1554, and 1563 cmꢁ1 for 2a, 2b, and 2c, respectively)
according to the S-coordination of the group.6c,6d,7,9
The ORTEP drawing of 2b is presented in Figure 1.10 The
complex 2b has a ꢀ3SNS-type coordination with a distorted
square-planar geometry. The S–Pd–S bond angle is smaller
and the Pd–S bond lengths are larger than those observed with
related phenyl-bis(thioamide)-based ꢀ3SCS-type pincer palladi-
um complexes (cf. Table S1 in Electronic Supporting Informa-
tion).6 The pyrrolate nitrogen atom is ꢁ-bonded to palladium
Preparation of phenyl-bis(thioamide)-based ꢀ3SCS pincer
Pd and Pt complexes of the type (a) shown in Chart 1 has previ-
ously been reported.6 In the previous studies, it was found that
the phenyl-bis(thioamide) pincer ligands served as a flexible
and good ligand for various pincer complexes. Nonoyama et
al. also reported that cyclopalladation of N–H bond in pyrrole
could occur for N,N-dimethyl-1H-pyrrole-2-carbothioamide.7
On these bases, the present work is concerned with cyclometala-
tion of the following pyrrole derivative 1 with two thioamide
side groups to form a new series of the ꢀ3ENE-type pincer com-
plexes of Ni, Pd, and Pt. The newly prepared complexes have the
type (b) structure exhibited in Chart 1. We here report prepara-
˚
with a Pd–N bond length of 1.932(3) A, which is comparable
to that observed for the reported ꢀ3NNN pincer Pd complexes
of the type (c) in Chart 1,4a whereas the centered pyrrole ring
is slightly tilted from the planes spanned by the four donor atoms
with dihedral angle of 1.1ꢂ. The C–C and C–N bond lengths of
the centered pyrrole ring are reminiscent of those observed for
Pd(II)–porphyrin complexes,11 and the pyrrole ring is almost
co-planar to the thioamide units as revealed by torsion angles
of C2–C1–C5–N1 and C2–C1–C5–S1 in Figure 1. It indicates
Copyright Ó 2006 The Chemical Society of Japan
Okamoto, Ken
