Synthesis, characterization and spectroscopic studies of cyclometalated platinum(II) complexes containing meta-bis(2-pyridoxy)benzene

Article abstract of DOI:10.1016/j.jorganchem.2004.06.006

A series of mononuclear and binuclear cyclometalated platinum(II) complexes containing new terdentate meta -bis(2-pyridoxy)benzene ligands: 3,5-bis(2-pyridoxy)toluene (L₁H) and 3,5-bis(2-pyridoxy)-2-dodecylbenzene (L₂H): [Pt(L_1<

Full text of DOI:10.1016/j.jorganchem.2004.06.006

Journal of Organometallic Chemistry 689 (2004) 2888–2899
www.elsevier.com/locate/jorganchem
Synthesis, characterization and spectroscopic studies of
cyclometalated platinum(II) complexes containing
meta-bis(2-pyridoxy)benzene
a
a,
a
Brenda Ka-Wen Chiu , Michael Hon-Wah Lam *, Derek Yiu-Kin Lee ,
b
Wai-Yeung Wong
a
Department of Biology and Chemistry, City University of Hong Kong, 83 Tat Chee Avenue, Kowloon, Hong Kong, China
b
Department of Chemistry, Hong Kong Baptist University, Waterloo Road, Kowloon Tong, Hong Kong, China
Received 2 February 2004; accepted 3 June 2004
Available online 19 July 2004
Abstract
A series of mononuclear and binuclear cyclometalated platinum(II) complexes containing new terdentate meta-bis(2-pyri-
doxy)benzene ligands: 3,5-bis(2-pyridoxy)toluene (L1H) and 3,5-bis(2-pyridoxy)-2-dodecylbenzene (L2H): [Pt(L1)Cl] (1), [Pt(L2)Cl]
(2), [Pt(L1)(CH3CN)](ClO4) (3), {[Pt(L1)]2(l-dppm)}(ClO4)2 (4), {[Pt(L2)]2(l-dppm)}(ClO4)2 (5), {[Pt(L1)]2(l-pyrazole)}(ClO4) (6),
{[Pt(L2)]2(l-pyrazole)}(ClO4) (7), {[Pt(L1)]2(l-imidazole)}(ClO4) (8) and {[Pt(L2)]2(l-imidazole)}(ClO4) (9), have been synthesized
and characterized. These ligands are coordinated to platinum(II) in a ‘‘pincer’’-like manner and the presence of pyridyl donors en-
hances the availability of the ligand p* orbitals for electronic transition. Spectroscopic properties of these cyclometalated complexes
were studied. While the non-coplanar nature of the ligands hinders ligand–ligand and metal–metal interactions in these cyclometa-
lated complexes, the presence of long hydrocarbon side chain on ligand L2H seems to alleviate such hindrance. Intermolecular p–p,
and possibly Pt–Pt interactions were observed in complex 2 at high concentration.
ꢀ 2004 Elsevier B.V. All rights reserved.
Keywords: Platinum(II) complexes; Pincer ligands; meta-Bis(2-pyridoxy)benzene; Cyclometalated complexes; Pt–Pt interactions
1. Introduction
of [2,6-(ECH2)2C6H4] (where E is a neutral two-elec-
tron donor, e.g., ANR2, APR2 and ASR) [1c] and
(b) conjugated meridional g3-coordinating diiminophe-
nyl and diphenyliminoyl ligands with the general for-
mula of CANAN, NACAN and CANAC (where C
represents an aryl ring and N represents an imino
moiety) [4,5]. By varying the nature of the electron
donors, E, electronic properties and reactivity of the
metal centre cyclometalated by ‘‘pincer’’ ligands can
be fine-tuned. Stereochemistry around the metal centre
can also be adjusted by modifying substituents on E
and, sometimes, on the aryl ring. These features are
particularly desirable in the design of organometallic
catalysts. For the conjugated diiminophenyl and diph-
enyliminoyl ligands, the availability of the low lying p*
Cyclometalated platinum group metal complexes
containing terdentate ligands have aroused considera-
ble research interests in recent years owing to their
promising performance as catalysts [1], chemosensors
[2] and materials in opto-electronics and luminescent
devices [3]. The two most common classes of terden-
tate ligands for the cyclometalation of transition met-
als are: (a) ‘‘pincer’’ ligands with the general formula
*
Corresponding author. Tel.: +852-2788-7329; fax: +852-2788-
7406.
E-mail address: bhmhwlam@cityu.edu.hk (M.H.-W. Lam).
0022-328X/$ - see front matter ꢀ 2004 Elsevier B.V. All rights reserved.
doi:10.1016/j.jorganchem.2004.06.006

B.Ka-Wen Chiu et al. / Journal of Organometallic Chemistry 689 (2004) 2888–2899
2889
orbitals of the ligands for participation in electronic
2. Results and discussion
transition has given rise to many interesting spectro-
scopic and luminescent properties of the resultant cy-
clometalated complexes [6]. It would be interesting to
study the properties of cyclometalated metal com-
plexes containing terdentate chelating ligands with
both features of ‘‘pincer’’-type ligands and conjugated
CNN/NCN/CNC ligands. In this work, we report the
synthesis of two substituted meta-bis(2-pyridoxy)ben-
zene (L1H and L2H) which are able to coordinate,
in ‘‘pincer’’-like manner, with platinum(II) to form a
series of mononuclear and binuclear cyclometalated
NCN Pt(II) complexes (1–9) (Scheme 1). Special atten-
tion is given to the spectroscopic properties of these
new cyclometalated Pt(II) complexes as the presence
of pyridyl donors enhances the availability of the lig-
and p* orbitals for electronic transition while the ‘‘pin-
cer’’-like arrangement of the ligands de-conjugates the
three aromatic rings by ether spacers and hinders in-
tramolecular p–p and metal–metal interactions in the
binuclear cyclometalated NCN Pt(II) complexes.
2.1. Synthesis and characterization of ligands and cyclo-
metalated complexes
The substituted m-bis(2-pyridoxy)benzenes L1H and
L2H were obtained in gram-scale by a one-pot synthetic
approach [7] from substituted resorcinol and 2-bromo-
pyridine. Mononuclear cyclometalated NCN Pt(II)
complexes [Pt(L1)Cl] (1) and [Pt(L2)Cl] (2) were ob-
tained by reacting the corresponding m-bis(2-pyri-
doxy)benzene with one equivalent of K2[PtCl4] in
refluxing acetic acid. Lability of chloride trans- to the
PtAC r bond was demonstrated by its ease of substitu-
tion by acetonitrile to generate [Pt(L1)(CH3CN)]+ (3).
This is attributable to the strong trans-effect of the cyclo-
metalated aryl ring [8]. The binuclear complexes 4–9
were synthesized by reacting the mononuclear cyclo-
metalated NCN Pt(II) complexes with either dppm or
deprotonated pyrazole and imidazole (by potassium
tert-butoxide) in CH3CN/CH3OH mixture under an
Scheme 1. The meta-bis(2-pyridoxy)benzene ligands and cyclometalated Pt(II) complexes synthesized.

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B.Ka-Wen Chiu et al. / Journal of Organometallic Chemistry 689 (2004) 2888–2899
inert atmosphere. All the ligands and cyclometalated
NCN Pt(II) complexes were characterized by ES-MS
and 1H NMR spectrometry. X-ray crystal structures
were determined for complexes 1, 3, 6 and 8. Selected
bond lengths and angles as well as dihedral angles be-
tween aromatic rings of the complexes are tabulated in
Tables 1 and 2.
2. The cationic complex 3 was obtained by the substitu-
tion of chloride trans- to the PtAC r bond in complex 1
by acetonitrile. The Pt(II) to pyridyl-N and Pt(II) to
aryl-C bond distances are comparable to those in com-
˚
˚
plex 1 (Pt(1)–N(1): 2.027(7) A; Pt(1)–N(2): 2.030(7) A;
˚
Pt(1)–C(11): 1.965(9) A). The bond distance between
˚
Pt(II) and acetonitrile-N is 2.087(7) A. The aryl ring
Perspective view of the crystal structure of the neutral
complex 1, with atom and ring labelling, is shown in
Fig. 1. The 3,5-bis(2-pyridoxy)toluene ligand L1 is coor-
dinated to the platinum(II) centre via its two pyridyl-N
and one aryl-C in a terdentate manner. The Pt(II) centre
adopts a square planar coordination geometry with
bond angles N(1)–Pt(1)–N(2) and C(6)–Pt(1)–Cl(1) of
175.6(4)ꢁ and 179.5(4)ꢁ, respectively. The bond distances
between Pt(II) and the pyridyl-N, Pt(1)–N(1) and Pt(1)–
makes dihedral angles of 35.8ꢁ and 34.2ꢁ with the
N(1)- and N(2)-pyridyl rings, respectively.
Perspective view of the crystal structure of the cati-
onic binuclear complex 6, with atom and ring labelling,
is shown in Fig. 3. The two cyclometalated [Pt(L1)] units
are bridged by a deprotonated pyrazole via N(3) and
N(4). Within each cyclometalated [Pt(L1)] unit, the
Pt(II) centre adopts a square planar coordination geom-
etry with the pyridyl-NAPt(II)Apyridyl-N0 bond angle
of 176.54(16)–175.81(16)ꢁ and the aryl-CAPt(II)Apyraz-
pyrazolyl-N bond angle of 177.57(18)–178.68(18)ꢁ. The
Pt(II) to pyridyl-N and Pt(II) to aryl-C bond distances
˚
N(2), were 2.032(9) and 2.033(10) A, respectively. These
are shorter than those observed in typical Pt(II) com-
plexes of ‘‘NCN’’ pincer-type ligands with either
N(sp3) or N(sp2) donors [4c,9], but are very similar to
those observed in a related cyclometalated Pt(II) com-
plex containing a conjugated 1,3-bis(2-pyridyl)benzene
(NCN) ligand [4c]. On the other hand, the bond distance
˚
are 2.023(4)–2.038(4) and 1.954(4)–1.974(5) A, respec-
tively. These bond distances are comparable to those ob-
served in the mononuclear complex 1 and 3. The Pt(II)
˚
to pyrazolyl-N bond distance is 2.098(4)–2.113(4) A
˚
of 1.959(14) A between the Pt(II) centre and the aryl-C,
Pt(1)–C(6), is longer than the corresponding bond dis-
and is in accordance with that observed in {[Pt(L)]2(l-
py-
razole)}+ (L=6-phenyl-2,20-bipyridine), where the
˚
tance of 1.907(8) A in the cyclometalated Pt(II) 1,3-
˚
bis(2-pyridyl)benzene complex [4c], but lies in a similar
range of most of the reported Pt(II) complexes of
‘‘NCN’’ pincer-type ligands. The Pt(1)–Cl(1) bond dis-
Pt(II)Apyrazolyl-N bond distance is 2.009(4) A [6d].
The aryl and pyridyl rings of the cyclometalating ligands
in the binuclear complex are also deviated from co-plan-
arity as in the mononuclear complex 1. Dihedral angles
between the pyridyl rings lie in the range 63.5–63.6ꢁ
whereas those between the pyridyl rings and the aryl
ring in the range 35.4–48.9ꢁ. The intramolecular PtAPt
˚
tance is 2.419(3) A. As the three aromatic rings of the
ligand are de-conjugated, the three rings in complex 1
are not co-planar with each other. Dihedral angles be-
tween the pyridyl rings and the aryl ring are 42.7ꢁ and
32.0ꢁ. Perspective view of the crystal structure of com-
plex 3, with atom and ring labelling, is shown in Fig.
˚
distance in the binuclear complex is 3.876 A, which is
much longer than those in the related binuclear Pt(II)
Table 1
˚
Selected bond distances (A), bond angles (ꢁ) and dihedral angles between aromatic rings (ꢁ) of [Pt(L1)Cl] (1) and [Pt(L1)(CH3CN)](ClO4) (3)
[Pt(L1)Cl] (1)
Pt(1)–C(6)
Pt(1)–N(1)
1.959 (14)
2.032 (9)
Pt(1)–Cl(1)
Pt(1)–N(2)
2.419 (3)
2.033 (10)
C(6)–Pt(1)–Cl(1)
C(6)–Pt(1)–N(1)
C(1)–O(1)–C(11)
179.5 (4)
88.5 (5)
119.1 (11)
N(1)–Pt(1)–N(2)
C(6)–Pt(1)–N(2)
C(13)–O(2)–C(7)
175.6 (4)
87.7 (6)
123.5 (11)
Ring A–Ring B
42.7
Ring B–Ring C
32.0
[Pt(L1)(CH3CN)](ClO4) (3)
Pt(1)–C(11)
Pt(1)–N(1)
1.965 (9)
2.027 (7)
Pt(1)–N(2)
Pt(1)–N(3)
2.030 (7)
2.087 (7)
C(11)–Pt(1)–N(3)
C(11)–Pt(1)–N(1)
C(5)–O(1)–C(6)
176.8 (3)
88.6 (3)
121.6 (7)
N(1)–Pt(1)–N(2)
C(11)–Pt(1)–N(2)
C(13)–O(2)–C(10)
177.6 (3)
89.6 (3)
124.6 (7)
Ring A–Ring B
35.8
Ring B–Ring C
34.2

B.Ka-Wen Chiu et al. / Journal of Organometallic Chemistry 689 (2004) 2888–2899
2891
Table 2
˚
Selected bond distances (A), bond angles (ꢁ) and dihedral angles between aromatic rings (ꢁ) of {[Pt(L1)]2(l-pyrazole)}(ClO4) (6) and {[Pt(L1)]2(l-
imidazole)}(ClO4) (8)
{[Pt(L1)]2(l-pyrazole)}(ClO4) (6)
C(7)–Pt(1)
N(1)–Pt(1)
N(2)–Pt(1)
N(3)–Pt(1)
1.954 (4)
2.037 (4)
2.027 (4)
2.098 (4)
C(27)–Pt(2)
N(5)–Pt(2)
N(6)–Pt(2)
N(4)–Pt(2)
1.974 (5)
2.023 (4)
2.038 (4)
2.113 (4)
C(7)–Pt(1)–N(3)
N(1)–Pt(1)–N(2)
C(7)–Pt(1)–N(1)
C(7)–Pt(1)–N(2)
N(1)–Pt(1)–N(3)
N(2)–Pt(1)–N(3)
C(5)–O(1)–C(6)
C(8)–O(2)–C(13)
177.57 (18)
176.54 (16)
88.80 (18)
88.13 (18)
91.61 (15)
91.53 (15)
120.9 (4)
C(27)–Pt(2)–N(4)
N(5)–Pt(2)–N(6)
C(27)–Pt(2)–N(5)
C(27)–Pt(2)–N(6)
N(5)–Pt(2)–N(4)
N(6)–Pt(2)–N(4)
C(26)–O(3)–C(25)
C(28)–O(4)–C(33)
178.68 (18)
175.81 (16)
87.90 (17)
87.94 (18)
92.67 (15)
91.48 (16)
122.1 (4)
122.1 (4)
117.0 (4)
Ring A–Ring B
Ring A–Ring D
Ring B–Ring D
Ring D–Ring E
Ring D–Ring G
Ring E–Ring G
Ring C–Ring E
39.9
50.2
38.0
70.4
65.1
63.5
16.7
Ring A–Ring C
Ring B–Ring C
Ring C–Ring D
Ring D–Ring F
Ring E–Ring F
Ring F–Ring G
63.6
35.4
71.5
34.9
37.5
48.9
{[Pt(L1)]2(l-imidazole)}(ClO4) (8)
C(11)–Pt(1)
N(1)–Pt(1)
N(2)–Pt(1)
N(3)–Pt(1)
1.986 (10)
2.043 (9)
2.023 (8)
2.082 (8)
C(31)–Pt(2)
N(5)–Pt(2)
N(6)–Pt(2)
N(4)–Pt(2)
1.976 (11)
2.030 (9)
2.039 (10)
2.097 (8)
C(11)–Pt(1)–N(3)
N(1)–Pt(1)–N(2)
C(11)–Pt(1)–N(1)
C(11)–Pt(1)–N(2)
N(1)–Pt(1)–N(3)
N(2)–Pt(1)–N(3)
C(6)–O(1)–C(5)
C(10)–O(2)–C(13)
178.8 (4)
177.5 (3)
88.9 (4)
88.6 (4)
90.1 (3)
92.4 (3)
121.9 (9)
121.7 (8)
C(31)–Pt(2)–N(4)
N(5)–Pt(2)–N(6)
C(31)–Pt(2)–N(5)
C(31)–Pt(2)–N(6)
N(5)–Pt(2)–N(4)
N(6)–Pt(2)–N(4)
C(26)–O(3)–C(25)
C(30)–O(4)–C(33)
178.9 (4)
177.9 (3)
89.0 (4)
89.0 (4)
90.5 (3)
91.4 (3)
122.8 (8)
121.3 (9)
Ring A–Ring B
Ring A–Ring D
Ring B–Ring D
Ring D–Ring E
Ring D–Ring G
Ring E–Ring G
40.1
59.8
33.3
75.0
60.4
60.2
Ring A–Ring C
Ring B–Ring C
Ring C–Ring D
Ring D–Ring F
Ring E–Ring F
Ring F–Ring G
58.8
31.2
58.7
44.2
34.9
38.0
˚
cyclometalated complexes, e.g., 3.081 A in {[Pt(tpy)]2(l-
cussed later. The torsion angle about the Pt(1)–Pt(2) axis
is 14.3ꢁ.
guanidine)}3+ [10], 3.270 A in {[Pt(L)]2(l-dppm)}2+ [11],
˚
3+
˚
˚
3.432 A in {[Pt(tpy)]2(l-pyrazole)} [12] and 3.612 A in
{[Pt(L)]2(l-pyrazole)}+ [6d] (where tpy=terpyridine;
L=6-phenyl-2,20-bipyridine). Such a long intramolecu-
lar PtAPt distance is probably caused by the steric hin-
drance of the two non-coplanar cyclometalated ligands.
This also implies the absence of any metal–metal inter-
action in the binuclear complex as it has been proposed
that the upper limit of intramolecular PtAPt distance for
Perspective view of the crystal structure of the cationic
binuclear complex 8, with atom and ring labelling,
is shown in Fig. 4. The two cyclometalated [Pt(L1)] units
are bridged by a deprotonated imidazole. Bridging via
the imidazolium-N (N(3) and N(4)) or imidazolium-C
(C(18) and C(20)) are possible and X-ray crystallography
may not be able to distinguish the two possibilities. Nev-
ertheless, 1H NMR of 8 did not contain any highly
downfield protons in the range 12–14 ppm correspond-
ing to imidazolium N–H [13]. Thus, it is reasonable to as-
sign the structure of 8 as l-imidazolyl cyclometalated
Pt(II) dimer bridged via N(3) and N(4) of the imidazo-
˚
metal–metal interaction is ꢀ3.6 A [6d]. The lack of met-
al–metal interaction in the binuclear complex 6 has also
been demonstrated spectroscopically and will be dis-

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B.Ka-Wen Chiu et al. / Journal of Organometallic Chemistry 689 (2004) 2888–2899
Fig. 1. Perspective view of [Pt(L1)Cl] (1).
Fig. 2. Perspective view of [Pt(L1)(CH3CN)](ClO4) (3).
lium ring. As in complex 6, the Pt(II) centres are also in
square planar coordination geometry with the pyridyl-
NAPt(II)Apyridyl-N0 bond angle of 177.5(3)–177.9(3)ꢁ
and the aryl-CAPt(II)Aimidazolium-N bond angle of
178.8(4)–178.9(4)ꢁ. The Pt(II) to pyridyl-N and Pt(II)
to aryl-C bond distances are 2.023(8)–2.043(9) and
˚
1.976(11)–1.986(10) A, respectively. The Pt(II) to imida-
˚
zolium-N bond distance is 2.082(8)–2.097(8) A. These
bond distances are comparable to those observed in the
l-pyrazole bridged binuclear complex 6. Within each

B.Ka-Wen Chiu et al. / Journal of Organometallic Chemistry 689 (2004) 2888–2899
2893
Fig. 3. Perspective view of the cation of {[Pt(L1)]2(l-pyrazole)}(ClO4) (6).
1
those in the region 300–340 nm are likely to be MLCT
[Pt(L1)] unit, the dihedral angles between the two pyridyl
rings range from 58.8ꢁ to 60.2ꢁ. The dihedral angles be-
tween the pyridyl rings and the aryl ring are 31.2–40.1ꢁ.
Structures of the mononuclear and binuclear cyclo-
metalated Pt(II) complexes containing the 3,5-bis(2-py-
ridoxy)-2-dodecylbenzene ligand L2 are expected to
resemble those of the related complex 1, 3, 6 and 8.
However, all attempts to obtain X-ray quality crystals
of complexes 2, 5, 7 and 9 were not successful.
Pt(dp)fiL(p*) in nature. The latter are of much higher
energy compared to those MLCT transitions in cyclo-
1
metalated Pt(II) complexes with conjugated terdentate
ligands (kmax in the range 400–440 nm) [11]. This is most
probably caused by the de-conjugated nature of the pre-
sent NCN ligands where L(p*) are expected to be of
higher energy than those in conjugated terdentate lig-
ands. Room temperature solution emission spectra of
most of the mononuclear and binuclear cyclometalated
NCN Pt(II) complexes are dominated by ligand centred
p* fip transition at 440–460 nm. At higher concentra-
tion (1·10ꢁ3 M), ligand L2H shows an additional lower
energy emission peak at 530 nm. This is probably caused
by intermolecular p–p interaction at high concentration,
aided by the dodecyl sidearm on L2H, which lowers the
pp* energy gap. No such intermolecular interaction is
observed in ligand L1H up to a concentration of
1·10ꢁ2 M. Solution emission properties of mononu-
clear complex 2 also show evidence of ligand–ligand
p–p and possibly d8–d8 interaction at high concentra-
tion. At 1·10ꢁ2 M in CHCl3, intensity of the original
ligand centred p* fip emission is diminished and an in-
2.2. Spectroscopic and luminescent properties of the
cyclometalated complexes
Table 3 summarizes the electronic transition and lu-
minescent properties of the ligands as well as the mono-
nuclear and binuclear cyclometalated NCN Pt(II)
complexes. By comparing with the UV–Vis absorption
characteristics of ligands L1H and L2H as well as those
of related cyclometalated Pt(II) complexes with conju-
gated 6-phenyl-2,20-bipyridine (CNN) and 2,6-diphenyl-
pyridine (CNC) ligands [6d,14], the high energy
transitions (k<300 nm) of the cyclometalated complexes
are attributable to intraligand pfip* transitions while

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B.Ka-Wen Chiu et al. / Journal of Organometallic Chemistry 689 (2004) 2888–2899
Fig. 4. Perspective view of the cation of {[Pt(L1)]2(l-imidazole)}(ClO4) (8).
Table 3
Electronic transition and luminescent properties of all the mononuclear and binuclear cyclometalated NCN Pt(II) complexes
Ligand/complex
Solvent
UV–Vis absorption spectraa
max/nm (e/dm3 molꢁ1 cmꢁ1
Solution emission spectrab
kmax/nm (kex at 360 nm)
k
)
L1H
L2H
1
CHCl3
CHCl3
251 (38,000), 276 (30,890)
252 (37,410), 275 (31,140)
241 (32,501), 311 (7704)
450a
440a, 530c
440a
CHCl3
CH3 CN
CHCl3
282 (5974), 301 (4378), 330 (1629)
241 (17,921), 307 (1974)
231 (41,055), 290 (9889)
445a
2
3
455a, 555c
385a
CH2 Cl2
CH3 CN
CHCl3
220 (13,120), 245 (8 014), 300 (1798)
241 (31,556), 313 (11,170)
444a
4
450a
CH3 CN
CHCl3
CHCl3
221 (25,705), 272 (11,976), 310 (4080) sh
241 (34,037), 268 (18,241) sh, 312 (4589) sh
243 (42,218), 292 (12,926) sh, 335 (6720) sh
221 (31,625), 242 (24,847) sh, 286 (3328), 321 (5541)
275 (40,529), 405 (1059)
445a
5
6
460a, 530c
450a
CH3 CN
CHCl3
CHCl3
450a
7
8
450a
241 (38,816), 287 (8894),303 (7054) sh, 332 (2814) sh
249 (47,570), 272 (47,402),332 (4075) sh
240 (38,178), 277 (19,938),336 (2852) sh
465a
CH3 CN
CHCl3
466a
9
445a
Concentration=1.0·10ꢁ5 mol dmꢁ3
At 298 K.
.
a
b
c
Concentration=1.0·10ꢁ2 moldmꢁ3
.
tense new lower energy emission peak at kmax 555 nm is
observed (Fig. 5). The red-shift of the emission peak
from 530 to 555 nm indicates extra interaction between
mononuclear units of 2 at high concentration in addi-
tion to the intermolecular ligand–ligand p–p interaction.
Metal–metal interaction similar to those observed in
square planar Pt(II) complexes with conjugated copla-
nar terdentate ligands [15] is possible. Again, the long
carbon chain on ligand L2 seems to facilitate these lig-
and–ligand and metal–metal interactions as no lower en-
ergy emission peak is observed in complex 1 (an analog
of 2 without the long carbon chain on the ligand) even at
high concentration. Unlike many binuclear cyclometa-
lated Pt(II) complexes with conjugated CNN and

B.Ka-Wen Chiu et al. / Journal of Organometallic Chemistry 689 (2004) 2888–2899
2895
1 × 10-4
1 × 10-3
M
M
M
   
1 × 10-2
.
.
.



400
450
500
550
600
650
700
λ / nm
Fig. 5. Emission spectra of complex 2 at different concentrations in CHCl3 at 298 K.
Complex 5 (1 ⋅ 10-5M)
Ligand L2H (1 ⋅ 10-2 M)
   
Ligand L2H (1 ⋅ 10-5 M)
.
.
.



400
450
500
550
λ / nm
600
650
700
Fig. 6. Emission spectrum of binuclear complex 5 (1·10ꢁ5 M) compared with those of ligand 2 at the same and higher concentrations in CHCl3 at
298 K.
CNC ligands [6d,14], no intramolecular d8–d8 and/or p–
p interaction in the binuclear complexes 4 and 6–9 is ap-
parent. Long intramolecular PtAPt distance for the ab-
sence of any metal–metal interaction is observed in the
crystal structure of complex 6. Lower energy emission
peak likely to be caused by intramolecular ligand–ligand
p–p interaction is also absent. This is attributable to the
distortion of the ‘‘pincer’’-like ligands from co-planarity
which hinders any close interaction between the cyclo-
metalated NCN Pt(II) units in the binuclear complexes.
There is, on the other hand, spectroscopic evidence that
shows the existence of intramolecular p–p interaction
between the cyclometalated Pt(II) units in the l-dppm
bridged binuclear complex 5. At low concentration
(1·10ꢁ5 M) in CHCl3, a low energy emission peak at
530 nm similar to that appeared in the emission spec-
trum of ligand L2H at much higher concentration is ob-
served (Fig. 6). The analogous pyrazole-bridged
binuclear complex 7 does not show any lower energy
emission peak even at high concentration. This may be
due to the coordination geometry of the pyrazole-bridge
which is unable to bring the two cyclometalated NCN
Pt(II) units close enough for interaction as in the l-
dppm bridged complex 5.
3. Conclusion
A series of mononuclear and binuclear cyclometa-
lated Pt(II) complexes containing m-bis(2-pyridoxy)ben-

2896
B.Ka-Wen Chiu et al. / Journal of Organometallic Chemistry 689 (2004) 2888–2899
zenes as terdentate cyclometalation NCN ligands have
been synthesized and characterized. The m-bis(2-pyri-
doxy)benzenes were able to cyclometalate Pt(II) in a
‘‘pincer’’-like manner. The non-coplanar nature of the
cyclometalation ligands hinders inter- and intramolecu-
lar ligand–ligand and metal–metal interactions in mono-
nuclear and binuclear complexes. The presence of long
carbon side chain on the ligand (as in ligand L2) seems
to alleviate such hindrance. Intermolecular p–p, and pos-
sibly d8–d8, interactions are observed in the mononu-
1H, tolyl-H), 6.80 (d, 2H, pyridoxy-H), 6.74 (s, 1H, to-
lyl-H), 2.36 (s, 3H, tolyl-CH3). ES-MS (CH2Cl2, +ve
mode): 279 [M]+.
4.3. Synthesis of 3,5-bis(2-pyridoxy)-2-dodecylbenzene
(L2H)
3,5-Bis(2-pyridoxy)-2-dodecylbenzene (L2H) was ob-
tained from 4-dodecylresorcinol (6.74 g, 24.2 mmol) in
a similar way as L1H. Yield 6.0 g, 57%. 1H NMR
(CDCl3) d: 8.16 (m, 2H, pyridoxy-H), 7.64 (td, 2H, py-
ridoxyl-H), 7.24 (d, 2H, pyridoxyl-H), 6.95 (m, 2H, py-
ridoxyl-H), 6.90 (s, 1H, aryl-H), 6.87 (d, 1H, aryl-H),
6.83 (d, 1H, aryl-H), 1.24 (m, 25H, AC12H25). ES-MS
(Ether/THF, +ve mode): 433 [M]+.
clear cyclometalated complex
2 at high solution
concentration. X-ray structure and spectroscopic char-
acteristics of the binuclear complexes with different
bridging moieties (l-imidazolyl, l-pyrazolyl and
l-dppm) indicate the absence of intramolecular interac-
tion between the cyclometalated NCN Pt(II) mononu-
clear units, except in complex 5 where the combination
of the l-dppm bridge which is known to be able to effect
close proximity of mononuclear units and the presence
of the dodecyl side chain on the ligand is likely to bring
about intramolecular p–p interaction.
4.4. Synthesis of [Pt(L1)Cl] (1)
K2[PtCl4] (1.66 g, 4.0 mmol) was suspended in a gla-
cial acetic acid solution (30 ml) of 3,5-bis(2-pyri-
doxy)toluene (L1H) (1.11 g, 4.0 mmol) and the mixture
was refluxed for 24 h. The resultant mixture was cooled
to room temperature, treated with 200 ml of deionized
water and extracted with 3·30 ml of chloroform. The
organic extracts were combined, dried over anhydrous
MgSO4, and evaporated to dryness to give crude
[Pt(L1)Cl] as an off-white solid. Colourless X-ray quality
crystals were obtained by slow diffusion of diethylether
into a solution of crude [Pt(L1)Cl] (1) in chloroform.
Yield 1.55 g, 76%. 1H NMR (CDCl3, 300 MHz) d:
9.52 (dd, 2H, pyridoxyl-H), 7.88 (td, 2H, pyridoxyl-H),
7.27 (s, 2H, pyridoxyl-H), 7.05 (t, 2H, pyridoxyl-H),
6.82 (s, 2H, tolyl-H), 2.32 (s, 3H, tolyl-CH3). ES-MS
(CH2Cl2, +ve mode): 472 [MꢁCl]+.
4. Experimental
4.1. General
Potassium tetrachloroplatinate(II), resorcinol, 4-do-
decylresorcinol, 2-bromopyridine, pyrazole, imidazole
and bis(diphenylphosphino)methane (dppm) were
purchased from Aldrich. All solvents used were of ana-
lytical reagent grade and were from Riedel-de Harn,
Lab-Scan Analytical Sciences and BDH. 1H NMR spec-
tra were measured by a Varian YH300 300 MHz Super-
conducting Magnet High Field NMR spectrometer with
tetramethylsilane used as internal reference. UV–Vis
spectra were measured by a Hewlett–Packard 8425A ul-
tra-violet diode-array spectrophotometer. Luminescent
spectra were measured by a Fluoromax-3 spectrofluo-
rometer. Electrospray mass spectra were measured by
a PE SCIEX API 365 LC/MS/MS system.
4.5. Synthesis of [Pt(L2)Cl] (2)
[Pt(L2)Cl] (2) was obtained in a similar way as 3 from
K2[PtCl4] (1.66 g, 4.0 mmol) and 3,5-bis(2-pyridoxy)-2-
dodecylbenzene (L2H) (1.73 g, 4.0 mmol). Yield 1.60
g, 60%. 1H NMR (CDCl3, 300 MHz) d: 9.44 (td, 2H, py-
ridoxyl-H), 7.85 (td, 2H, pyridoxyl-H), 7.04 (m, 2H, py-
ridoxyl-H), 6.89 (m, 2H, pyridoxyl-H), 6.60 (m, 1H,
aryl-H), 6.55 (d, 1H, aryl-H), 6.50 (d, 1H, aryl-H),
1.27 (m, 25H, AC12H25). ES-MS (CH2Cl2, +ve mode):
626 [MꢁCl]+.
4.2. Synthesis of 3,5-bis(2-pyridoxy)toluene (L1H)
A mixture of orcinol (3.0 g, 24.2 mmol) and 2-bromo-
pyridine (7.65 g, 48.4 mmol) was heated until orcinol
melted. Potassium carbonate (6.68 g, 48.4 mmol) was
then added and the mixture was further heated at 200–
210 ꢁC for 3 h. The resulting tar was extracted several
times with dichloromethane and the extracts combined
and evaporate to dryness to give crude 3,5-bis(2-pyri-
doxy)toluene (L1H). The product (2.8 g, 42% yield)
was purified by silica gel column chromatography using
dichloromethane as eluent. 1H NMR (CDCl3, 300 MHz)
d: 8.21 (dd, 2H, pyridoxy-H), 7.68 (td, 2H, pyridoxy-H),
7.00 (td, 2H, pyridoxy-H), 6.93 (s, 1H, tolyl-H), 6.90 (s,
4.6. Synthesis of [Pt(L1)(CH3CN)](ClO4) (3)
[Pt(L1)Cl] (1) (0.35 g, 0.7 mmol) was dissolved in
acetonitrile (20 ml) and the resultant solution was stirred
at room temperature for 8 h. LiClO4 (0.1 g) was added
and the mixture was further stirred at room tempera-
ture for an additional 0.5 h. Volume of the resultant
acetonitrile solution was reduced to 10 ml under reduced
pressure. Colourless X-ray quality crystals of

B.Ka-Wen Chiu et al. / Journal of Organometallic Chemistry 689 (2004) 2888–2899
2897
1
[Pt(L1)(CH3CN)](ClO4) (3) were obtained by slow
diffusion of diethylether into the acetonitrile solution.
Yield 0.36 g, 84%. 1H NMR (CDCl3, 300 MHz) d:
9.32 (dd, 2H, pyridoxyl-H), 8.20 (td, 2H, pyridoxyl-H),
7.52 (d, 2H, pyridoxyl-H), 7.32 (t, 2H, pyridoxyl-H),
6.88 (s, 1H, tolyl-H). ES-MS (CH2Cl2, +ve mode): 472
[MꢁCH3CN]+.
quality crystals. Yield 0.16 g, 58%. H NMR (DMSO-
d6, 300 MHz) d: 7.80 (d, 2H, pyrazolyl-H), 7.60–7.67
(m, 4H, pyridoxyl-H), 7.42–7.45 (m, 4H, pyridoxyl-H),
7.04–7.10 (m, 4H, pyridoxyl-H), 6.74–6.82 (m, 4H, pyri-
doxyl-H), 6.79 (s, 4H, tolyl-H), 6.70 (t, 1H, pyrazolyl-
H), 2.37 (s, 6H, tolyl-CH3). ES-MS (DMSO, +ve mode):
1011 [M]+.
4.7. Synthesis of {[Pt(L1)]2(l-dppm)}(ClO4)2 (4)
4.10. Synthesis l of {[Pt(L2)]2(-pyrazole)}(ClO4) (7)
A mixture of [Pt(L1)Cl] (1) (0.25 g, 0.49 mmol) and
dppm (94.0 mg, 0.25 mmol) in 1:1 acetonitrile/methanol
(30 ml) was stirred for 12 h under a nitrogen atmos-
phere. The resultant mixture was filtered and evapo-
rated under reduced pressure to about 10 ml.
Addition of LiClO4 (0.1 g) followed by slow diffusion
of diethylether afforded {[Pt(L1)]2(l-dppm)}(ClO4)2 (4)
as colourless crystals. Yield 0.31 g, 85%. 1H NMR
(DMSO-d6, 300 MHz) d: 8.06–8.18 (m, 4H, pyridoxyl-
H), 7.85–7.95 (m, 4H, pyridoxyl-H), 7.42–7.60 (m,
20H, AP(C6H5)2), 7.31–7.36 (m, 4H, pyridoxyl-H),
7.03–7.12 (m, 4H, pyridoxyl-H), 6.88 (s, 4H, tolyl-H),
2.30 (m, 2H, AP(CH2)PA), 1.92 (s, 6H, tolyl-CH3).
31P NMR (DMSO-d6, 300 MHz) d: ꢁ8.78 (s, 2P,
dppm). ES-MS (CH2Cl2, +ve mode): 856 [MꢁPt(L1)]+,
, 472 [Pt(L1)]+.
{[Pt(L2)]2(l-pyrazole)}(ClO4) (7) was obtained in a
similar way as 6 from [Pt(L2)Cl] (2) (0.25 g, 0.38 mmol)
and pyrazole (12.9 mg, 0.19 mmol) as a pale pink oil af-
ter purification by silica gel column chromatography us-
ing dichloromethane/methanol (1:1) as eluent. Yield
1
0.11 g, 41%. H NMR (DMSO-d6, 300 MHz) d: 8.20
(m, 4H, pyridoxyl-H), 7.94 (s, 1H, pyrazolyl-H), 7.78–
7.89 (m, 4H, pyridoxyl-H), 7.43–7.56 (m, 4H, pyri-
doxyl-H), 7.11–7.25 (m, 4H, tolyl-H), 6.97 (d, 2H, pyr-
azolyl-H), 6.72–6.96 (m, 4H, pyridoxyl-H), 1.00–1.46
(m, 44H, A(CH2)11A), 0.84 (t, 6H, ACH3). ES-MS
(DMSO, +ve mode): 1320 [M]+.
4.11. Synthesis of {[Pt(L1)]2(l-imidazole)}(ClO4) (8)
{[Pt(L1)]2(l-imidazole)}(ClO4) (8) was obtained in a
similar way as 6 from [Pt(L1)Cl] (1) (0.25 g, 0.49 mmol),
imidazole (16.7 mg, 0.25 mmol) and potassium tert-but-
oxide (28.0 mg, 0.25 mmol). Colourless X-ray quality
crystals of 8 were obtained by slow diffusion of diethyl-
ether into an acetonitrile solution of 8. Yield 0.14 g,
4.8. Synthesis of {[Pt(L2)]2(l-dppm)}(ClO4)2 (5)
{[Pt(L2)]2(l-dppm)}(ClO4)2 (5) was obtained in a
similar way as 4 from [Pt(L2)Cl] (2) (0.30 g, 0.45
mmol) and dppm (86.0 mg, 0.23 mmol) as a pale pink
oil after purification by silica gel column chromatogra-
phy using dichloromethane/methanol (1:1) as eluent.
Yield 0.25 g, 62%. 1H NMR (DMSO-d6, 300 MHz)
d: 8.07–8.26 (m, 4H, pyridoxyl-H), 7.66–7.90 (m, 4H,
pyridoxyl-H), 7.19–7.66 (m, 20H, AP(C6H5)2), 7.05–
7.18 (m, 4H, pyridoxyl-H), 6.96–7.05 (m, 4H, pyri-
doxyl-H), 6.76–6.90 (m, 4H, tolyl-H), 2.10 (s, 2H,
AP(CH2)PA), 0.93–1.42 (s, 44H, A(CH2)11A), 0.74–
0.92 (m, 6H, ACH3). 31P NMR (DMSO-d6, 300
MHz) d: ꢁ8.78 (s, 2P, dppm). ES-MS (DMSO, +ve
mode): 1637 [M]+.
1
50%. H NMR (DMSO-d6, 300 MHz) d: 8.17 (td, 4H,
pyridoxyl-H), 7.90 (s, 1H, imidazolyl-H), 7.73 (dd, 4H,
pyridoxyl-H), 7.57 (dd, 4H, pyridoxyl-H), 7.31 (s, 2H,
imidazolyl-H), 7.26 (td, 4H, pyridoxyl-H), 6.93 (s, 4H,
tolyl-H), 2.32 (s, 6H, tolyl-CH3). ES-MS (DMSO, +ve
mode): 1011 [M]+.
4.12. Synthesis of {[Pt(L2)]2(l-imidazole)}(ClO4) (9)
{[Pt(L2)]2(l-imidazole)}(ClO4) (9) was obtained in a
similar way as 6 from [Pt(L2)Cl] (2) (0.25 g, 0.38 mmol)
and imidazole (12.9 mg, 0.19 mmol) as a pale pink oil
after purification by silica gel column chromatography
using dichloromethane/methanol (1:1) as eluent. Yield
4.9. Synthesis of {[Pt(L1)]2(l-pyrazole)}(ClO4) (6)
1
0.10 g, 37%. H NMR (DMSO-d6, 300 MHz) d: 8.20
A mixture of [Pt(L1)Cl] (1) (0.25 g, 0.49 mmol), pyr-
azole (16.7 mg, 0.25 mmol) and potassium tert-butoxide
(28.0 mg, 0.25 mmol) in 2:1 acetonitrile/methanol (30
ml) was refluxed for 12 h under a nitrogen atmosphere.
LiClO4 (0.1 g) was then added and the resultant mixture
was further stirred at room temperature for an addi-
tional 0.5 h. The mixture was evaporated under reduced
pressure to about 10 ml and filtered. Slow diffusion of
diethylether into the resultant solution afforded
{[Pt(L1)]2(l-pyrazole)}(ClO4) (6) as colourless X-ray
(m, 4H, pyridoxyl-H), 7.85 (s, 1H, imidazolyl-H), 7.76
(dd, 4H, pyridoxyl-H), 7.60 (td, 4H, pyridoxyl-H),
7.20–7.40 (m, 2H, imidazolyl-H and 4H, tolyl-H), 7.02
(m, 4H, pyridoxyl-H), 1.00–1.46 (s, 44H, A(CH2)11A),
0.84 (t, 6H, ACH3). ES-MS (DMSO, +ve mode): 1320
[M]+.

2898
B.Ka-Wen Chiu et al. / Journal of Organometallic Chemistry 689 (2004) 2888–2899
Table 4
Crystallographic data for [Pt(L1)Cl] (1), [Pt(L1)(CH3CN)](ClO4) (3), {[Pt(L1)]2(l-pyrazole)}(ClO4) (6) and {[Pt(L1)]2(l-imidazole)}(ClO4)Æ2CH3CN
(8)Æ2CH3CN
Complex
1
3
6
8
Formula
C17H13N2ClO2Pt
507.83
Hexagonal
C19H16N3ClO6Pt
612.89
Monoclinic
P21/c
C37H29N6ClO8Pt2
1111.29
Triclinic
P
C41H35N8ClO8Pt2 Æ2CH3CN
Formula mass
Crystal system
Space group
1193.40
Triclinic
P
ꢀ
R3
˚
a (A)
46.610(3)
46.610(3)
4.0755(4)
90
25.668(2)
7.3647(6)
24.250(2)
90
12.7171(6)
15.5140(7)
19.7552(9)
77.9150(10)
71.9270(10)
88.5800(10)
3619.8(3)
4
8.9990(13)
14.692(2)
16.660(2)
86.873(3)
84.247(3)
73.486(3)
2077.1(5)
2
˚
b (A)
˚
c (A)
a (ꢁ)
b (ꢁ)
c (ꢁ)
90
120
117.3090(10)
90
3
˚
U (A )
7667.6(10)
18
293
4073.2(6)
8
293
Z
T (K)
F(000)
293
2120
293
1148
4320
1.980
2352
1.999
Dcalc [mgcmꢁ3
]
2.039
7.857
1.908
6.855
l (Mo Ka) [mmꢁ1
]
8.401
12,812
2994
7.063
19,068
7160
Reflection collected
Unique reflections
Rint
21,560
15,615
12,614
9157
0.0549
2366
0.950
0.0577
6131
1.028
0.0174
12134
0.0389
5079
0.872
Observed reflections
GOF on F2
0.791
0.0266, 0.0622
R1, wR2 [I>2.0rI]
0.0539, 0.1460
0.0489, 0.1304
0.0475, 0.1003
4.13. X-ray crystallographic studies
Acknowledgements
Geometric and intensity data of complex 1, 3, 6
and 8 were collected on a Bruker Axs SMART 1000
CCD area-detector using graphite monochromated
The work described in this paper was jointly funded
by a grant from the Research Grants Council of Hong
Kong SAR, China (CityU 1108/02P) and a Grant from
CityU (Project No. 7001281).
˚
Mo Ka radiation (k=0.71073 A). The collected frames
were processed with the software SAINT [16] and an
absorption correction was applied (SADABS [17]) to
the collected reflections. The structure was solved by
direct methods (SHELXTL [18]) in conjunction with
standard difference Fourier techniques and subse-
quently refined by full-matrix least-square analyses
on F2. All non-hydrogen atoms were assigned with an-
isotropic displacement parameters. The hydrogen at-
oms were generated in their idealized positions and
allowed to ride on the respective carbon atoms. Crys-
tal data and other experimental details are given in
Table 4.
CCDC-201591 to 201594 contains the supplementary
crystallographic data for this paper. These data can be
obtained free of charge at www.ccdc.cam.ac.uk/conts/re-
trieving.html [or from the Cambridge Crystallographic
Data Centre, 12, Union Road, Cambridge CB2 1EZ,
UK; fax: (internat.) +44-1223/336-033; e-mail: depos-
it@ccdc.cam.ac.uk].
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