Synthesis, chemical properties, and electrochemical behavior of a Pt(II) complex bearing a benzenediamide-derived ligand

Article abstract of DOI:10.1016/j.inoche.2010.11.018

The reaction of [PtCl₂(cod)] (cod = 1,5-cyclooctadiene) with Ph-Ph′-Ph′(2,3-NH₂)-Ph′-Ph (LH₂) (Ph = phenyl; Ph′ = p-phenylene; Ph′(2,3-NH₂) = p-phenylene-2,3-diamine) gave a new platinum(II) complex bearing a π-

Full text of DOI:10.1016/j.inoche.2010.11.018

Inorganic Chemistry Communications 14 (2011) 292–295
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Inorganic Chemistry Communications
journal homepage: www.elsevier.com/locate/inoche
Synthesis, chemical properties, and electrochemical behavior of a Pt(II) complex
bearing a benzenediamide-derived ligand
⁎
Take-aki Koizumi , Takuya Teratani, Takakazu Yamamoto
Chemical Resources Laboratory, Tokyo Institute of Technology, 4259 Nagatsuta, Midori-ku, Yokohama 226-8503, Japan
a r t i c l e i n f o
a b s t r a c t
Article history:
The reaction of [PtCl2(cod)] (cod=1,5-cyclooctadiene) with Ph-Ph′-Ph′(2,3-NH2)-Ph′-Ph (LH2) (Ph=phenyl;
Ph′=p-phenylene; Ph′(2,3-NH2)=p-phenylene-2,3-diamine) gave a new platinum(II) complex bearing a
π-conjugated Ph-Ph′-Ph′(2,3-NH)-Ph′-Ph (L, Ph′(2,3-NH) = p-phenylene-2,3-diamide) ligand, [Pt(L)(cod)],
with the liberation of 2 HCl. The X-ray structure determination of the complex confirmed the coordination of the
ligand to the platinum center in an o-benzenediamide form. The cyclic voltammogram of the complex showed
reversible two-step [Pt-bda]0/[Pt-sqdi]+ and [Pt-sqdi]+/[Pt-bqdi]2+ redox couples (bda=o-benzenediamide,
sqdi=o-semiquinone diimine, bqdi = o-benzoquinone diimine).
Received 25 September 2010
Accepted 17 November 2010
Available online 24 November 2010
Keywords:
Platinum complex
Benzenediamide
X-ray structure
Electrochemistry
DFT calculation
© 2010 Elsevier B.V. All rights reserved.
Introduction
The presence of an expanded π-conjugation system in L lowered
the LUMO level of the complex. π-Conjugated oligomers and polymers
1,2-Phenylenediamines (pdas) serve as chelating ligands, and many
reports have been published about pda-coordinated transition-metal
complexes [1–10]. Quinonoid ligands, such as o-benzoquinone and
o-benzoquinone diimine ligands, are called “non-innocent ligands”, and
the interesting chemical properties of metal complexes with these
ligands have attracted much attention [11]. As shown in Scheme 1, pda
undergoes the following acid–base reaction and reversible two-step
redox electrochemical reactions, both as a free compound and as a
ligand in metal complexes.
Such acid–base and electrochemical reactions have been investigated
in detail using Ru complexes with this ligand [12,13]. However,
electrochemical studies of Pt–pda complexes have received less attention
[2–9]. Two examples of Pt–bda-type complexes have been reported
[3a,10]; however, the electrochemical properties of the Pt–bda complexes
have not been investigated.
have been the subject of many studies, and transition-metal
complexes with ligands having large π-conjugated systems are now
attracting interest [14].
In this paper, we report the synthesis of a new platinum(II) complex,
[Pt(L)(cod)] (cod=1,5-cyclooctadiene), where L serves as a bda-type
ligand in contrast to the bqdi-type coordination of L in [Ru(L)(bpy)2].
Differences in electronic structures between [Ru(bpy)2] and [Pt(cod)]
seem to bring about such differences. The crystal structure of [Pt(L)
(cod)] confirms the bda-type coordination mode of L, and optical and
electrochemical properties of the complex have been revealed.
[Pt(L)(cod)] was prepared by the reaction of [PtCl2(cod)] with LH2 in
the presence of 1,8-diazabicyclo-[5.4.0]undec-7-ene (DBU) as a base in
high yield (cf. Scheme 2). The reaction of AgPF6-treated [PtCl2(cod)]
with diamine LH2 in 2-methoxyethanol gave [Pt(L)(cod)] in low yield.
Crystals of [PtL(cod)] suitable for X-ray diffraction analysis were
obtained from a CH2Cl2–hexane solution. Fig. 1 shows the molecular
structure of [Pt(L)(cod)] determined by X-ray crystallography [15].
[Pt(L)(cod)] has a distorted square-planar geometry around the metal
center. The bond lengths of N1–C1 and N2–C2 are 1.377(2) and 1.389
(2) Å, respectively, which are longer than those in Pt-sqdi-type
complexes (1.331(9)–1.368(7) Å) [4b,4d,5,8b], and shorter than those
in [Pt(pda)2]Cl2 (1.450(2) Å) [8a], indicating that the ligand in [Pt(L)
(cod)] coordinates to Pt as a bda-type dianionic ligand, and the
complex is a Pt(II) complex (not a Pt(0) complex). The Pt1–N1 and
Pt1–N2 lengths are 1.974(2) and 1.9652(19) Å, respectively, which
are slightly shorter than those of [Pt(pda)2]Cl2 (2.0402(17) Å) [8a].
The five benzene rings are twisted, and the dihedral angles are similar
to those of [Ru(L)(bpy)2]2+ [13].
Previously, we reported the synthesis of a new pda-type π-conjugated
LH2 (cf. eq. 1), and the reaction of [RuCl2(bpy)2] with LH2 gave an Ru
complex with a bqdi-type neutral ligand, Ph-Ph′-Ph′(NH)2-Ph′-Ph (L)
[13].
⁎
Corresponding author. Tel.: +81 45 924 5222; fax: +81 45 924 5976.
E-mail address: tkoizumi@res.titech.ac.jp (T. Koizumi).
1387-7003/$ – see front matter © 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.inoche.2010.11.018

T. Koizumi et al. / Inorganic Chemistry Communications 14 (2011) 292–295
293
acid-base
reaction
1st
oxidation
2nd
oxidation
-
NH2
NH2
NH
NH
NH
NH
-
-
-
-2H+
+2H+
-e
+e
-e
_
-
+e
-
NH
NH
o-phenylenediamine
o-benzenediamide
o-semiquinone
o-benzoquinone
diimine
diimine
pda
bda
sqdi
bqdi
Scheme 1. Redox behavior of o-phenylenediamine.
Numbering system of the benzene rings
Method A (82%)
DBU
3
2
1
4
5
CH2Cl2 /r.t.
Cl
Cl
+
Pt
H N
N H
H2N NH2
Pt
2 AgPF6
LH2
MeOCH2CH2OH
70 oC
[Pt(L)(cod)]with
a bda-type ligand
Method B (4%)
Scheme 2.
The 1H NMR spectrum of [Pt(L)(cod)] shows the expected signals
for the cod and aromatic ligands; six independent signals were
observed in the aromatic region, and two cod-originated olefinic and
aliphatic peaks were observed at δ 3.81 and 2.48, respectively.
Fig. 2 shows the UV–vis spectrum of [Pt(L)(cod)] in CH2Cl2; the
complex shows three absorption bands with peaks at λmax =261
(ε=46,500), 353 (ε=28,500), and 427 nm (ε=17,500). The
absorption band at λmax =261 nm is assigned to an intraligand π–π*
transition of L. The other absorption bands were assigned by time-
dependent density functional theory (TD-DFT) calculations [16].
Table 1 shows a summary of selected calculated singlet excited-
state transitions for [Pt(L)(cod)]. Three-dimensional plots of the
frontier orbitals of [Pt(L)(cod)] and its molecular orbitals (MO) are
shown in Fig. 3 using GaussView 4.1 [17]. HOMO, HOMO-1, and
HOMO-3 possess contributions of the d orbitals of Pt, π-orbitals of the
bda (HN–C=C–NH) unit, and the π-orbitals of p-quinquephenyl in L.
The HOMO-2 and LUMO-2 orbitals are primarily localized on L, and
LUMO has some metal character. The TD-DFT calculation of [Pt(L)
Fig. 2. UV–vis spectrum of [Pt(L)(cod)] in CH2Cl2.
Fig. 1. ORTEP drawing of [PtL(cod)] at 50% ellipsoidal level. Benzene ring 4 is disordered.
H atoms except H1 and H2 are omitted for simplicity. Selected bond lengths (Å) and dihedral
angles (°): Pt1–N1 1.974(2); Pt1–N2 1.9652(19); Pt1–C35 2.165(2); Pt1–C36 2.151(2);
Pt1–C39 2.142(2); Pt1–C40 2.143(2); N1–C1 1.377(2); N2–C2 1.389(2); C1–C2 1.416(3);
C2–C3 1.406(2); C3–C4 1.396(3); C4–C5 1.377(3); C5–C6 1.401(3); and C1–C6 1.397
(2); Plane1(C1–C6)–Plane2(C7–C12) 49.97(8); Plane1(C1–C6)–Plane3(C13–C18) 17.43(10); Plane1
(C1–C6)–Plane4(C19–C24) 75.02(13); Plane1(C1–C6)–Plane4′(C19,C22,C25–28) 52.89(16);
Plane1(C1–C6)–Plane5(C25–C30) 70.09(10); Plane2(C7–C12)–Plane3(C13–C18) 33.39(10);
Table 1
Selected calculated singlet excited-state transitions for [PtL(cod)].
Wavelength/nm
Oscillator strength
Assignment (% of major transition
contributing to the band)
453
376
0.1685
0.5416
HOMO→LUMO (71%)
HOMO-1→LUMO (85%)
HOMO→LUMO+2 (4%)
HOMO-3→LUMO (82%)
HOMO-3→LUMO+1 (3%)
HOMO-2→LUMO+2 (4%)
HOMO-1→LUMO+9 (44%)
Plane2(C7–C12)–Plane4(C19–C24) 26.82(13); Plane2(C7–C12)
– Plane4′
(C19,C22,C25–28)
277
259
0.3269
0.2010
102.45(17); Plane2(C7–C12)–Plane5(C25–C30) 119.30(11); Plane3(C13–C18)–Plane4(C19–C24)
59.58(10); Plane3(C13–C18)–Plane5(C25–C30) 85.95(12); Plane3(C13–C18)– Plane4′ (C19,C22,C25–28)
69.07(17); Plane4(C19–C24)–Plane5(C25–C30) 145.10(15); Plane4′ (C19,C22,C25–28)–Plane5(C25–C30)
17.25(18).

294
T. Koizumi et al. / Inorganic Chemistry Communications 14 (2011) 292–295
Fig. 3. Energy level diagram and graphic representation of frontier rbitals of [PtL(cod)]. HOMO-1, HOMO-2, and LUMO+1 are primarily localized in the L ligand.
[Pt(L)(cod)] shows a weak luminescence at λEM =466 nm when
excited at λEX =293 nm in CH2Cl2, though LH2 shows a strong
luminescence at λEM =391 nm (λEX =314 nm) [18]. For [Pt(L)(cod)],
the low energy band predicted to arise from the HOMO-to-LUMO
(453 nm) transition matches the experimentally observed emission
maximum.
The cyclic voltammogram (CV) of [Pt(L)(cod)] displays two quasi-
reversible one-electron redox processes as shown in Fig. 4. The redox
potentials for E11/2 and E12/2 are −0.260 V and +0.341 V (vs. Fc+/Fc),
respectively. No other peaks are observed in the range of +1.2 V to
−2.5 V. These two reversible redox couples are based on the [Pt(sqdi)]+/
[Pt(bda)]0 and [Pt(bqdi)]2+/[Pt(sqdi)]+ redox processes, as shown in
Scheme 3. There has been no evidence of the generation of Pt(I) or Pt(III)
species in the redox process, and the central Pt ion is thought to be
divalent in all neutral, cationic, and dicationic forms. The E11/2 of [PtL(cod)]
Fig. 4. Cyclic voltammogram of [Pt(L)(cod)] in CH2Cl2.
is more negative than those of Pt–catecholate (cat) complexes with a COD
ligand [19]. This result indicates that the electron density of the bda ligand
(cod)] indicates that a strong absorption at 353 nm, which is related
to the 376 nm transition obtained by calculation (cf. Table 1), is
dominated by an HOMO-1-to-LUMO transition, and a weaker peak at
427, which is related to the calculated 453 nm transition, originates
from an HOMO-to-LUMO transition.
is higher than that of the cat ligand.
In summary, we have demonstrated the synthesis and electrochemical
properties of a new Pt complex bearing the p-quinquephenyl-2″,3″-
diamide ligand. The complex [PtL(cod)] shows two-step reversible redox
waves, which correspond to the [Pt(sqdi)]+/[Pt(bda)]0 and [Pt(bqdi)]2+
/
+
2+
-e-
-e-
Ar
Ar
Ar
Ar
Ar
Ar
+e -
+e-
HN NH
HN NH
HN NH
[Pt]
[Pt]
[Pt]
E 1/2 =-0.260V
E 1/2 =+0.341V
Pt-bda
Pt-sqdi
Pt-bqdi
Ar:
[Pt]:Pt(cod)
Scheme 3. Redox processes of [PtL(cod)]. The negative charge on the L ligand is considered to be delocalized in the π-conjugated ligand.

T. Koizumi et al. / Inorganic Chemistry Communications 14 (2011) 292–295
295
[Pt(sqdi)]+ reactions. This is the first example that clarifies the
electrochemical properties of a di-deprotonated phenylenediamine-
coordinated Pt complex.
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Acknowledgements
We gratefully acknowledge Prof. Koji Tanaka of the Institute for
Molecular Science for helpful discussions. This work was financially
supported by a Grant-in-Aid for Scientific Research for Young
Chemists (No. 20750045) from the Ministry of Education, Culture,
Sports, Science, and Technology, Japan.
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Appendix A. Supplementary material
CCDC 793964 contains the supplementary crystallographic data
for [PtL(cod)]. These data can be obtained free of charge from The
Cambridge Crystallographic Data Centre via http://www.ccdc.cam.
ac.uk/data_request/cif. Supplementary data associated with this
article can be found in its online version at doi:10.1016/j.
inoche.2010.11.018.
[13] T. Koizumi, B.-J. Choi, T. Yamamoto, Inorg. Chim. Acta 361 (2008) 2131.
[14] T. Yamamoto, T. Koizumi, Polymer 48 (2007) 5449.
[15] Crystal data for [PtL(cod)]: C38H34N2Pt, Molecular weight=713.79, monoclinic,
space group P21/n (No.14), a=9.9875(18) Å, b=10.6103(18) Å, c=28.273(5) Å,
μ=93.5513(9)°, V=2990.4(9) Å3, Z=4, μ=47.089 cm−1
, F(000)=1512.00,
Dcalc =1.585 g cm−3, R(R1)=0.0404 (0.0294), Rw =0.0302 for 20,763 unique
reflections and 517 parameters.
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