Binuclear and Starburst Organoplatinum(II) Complexes of 2,2′-Dipyridylamino Derivative Ligands: Structures, Fluxionality, and Luminescence

Article abstract of DOI:10.1021/om030262k

New binuclear and starburst trinuclear organoplatinum complexes based on 2,2′dipyridylamino (dpa) derivative ligands, Pt₂Ph ₄(bab), bab = 1,4-bis(2,2′-dipyridylamino)benzene, 1, Pt ₂Ph₄(babp), babp = 4,4′-bis(2,

Full text of DOI:10.1021/om030262k

Organometallics 2003, 22, 3781-3791
3781
Bin u clea r a n d Sta r bu r st Or ga n op la tin u m (II) Com p lexes
of 2,2′-Dip yr id yla m in o Der iva tive Liga n d s: Str u ctu r es,
F lu xion a lity, a n d Lu m in escen ce
Qin-De Liu, Wen-Li J ia, Gang Wu, and Suning Wang*
Department of Chemistry, Queen’s University, Kingston, Ontario, K7L 3N6, Canada
Received April 10, 2003
New binuclear and starburst trinuclear organoplatinum complexes based on 2,2′-
dipyridylamino (dpa) derivative ligands, Pt2Ph4(bab), bab ) 1,4-bis(2,2′-dipyridylamino)-
benzene, 1, Pt2Ph4(babp), babp ) 4,4′-bis(2,2′-dipyridylamino)biphenyl, 2, Pt3Ph6(tab),
tab ) 1,3,5-tris(2,2′-dipyridylamino)benzene, 3, Pt3Ph6(tat), tat ) 2,4,6-tris(2,2′-dipyri-
dylamino)-1,3,5-triazine, 4, Pt3Ph6(tapb), tapb ) 1,3,5-tris[p-(2,2′-dipyridylamino)phenyl]-
benzene, 5, Pt3Ph6(tapt), tapt ) 2,4,6-tris[p-(2,2′-dipyridylamino)phenyl]-1,3,5-triazine, 6,
Pt3Ph6(tabpb), tabpb ) 1,3,5-tris{4′-[4′′-(2,2′-dipyridylamino)]biphenyl}benzene, 7, and
Pt3Ph6(tabpt), tabpt ) 1,3,5-tris{4′-[4′′-(2,2′-dipyridylamino)]biphenyl}benzene, 8, were
synthesized by the reaction of [PtPh2(SMe2)]n with the corresponding chelate ligand. The
structures of complexes 1-5 and 8 were determined by single-crystal X-ray diffraction. All
eight complexes have phosphorescent emissions in the blue/green region at 77 K, attributed
to ligand-centered π f π* transitions. Ligand-based fluorescent emission was also detected
for complexes 5, 6, and 8. Complexes 1-8 display versatile structures in the solid state.
Complex 4 was found to be fluxional in solution. The key factors that influence the structures
of the complexes in solution and the solid state are the degree of conjugation of the amino
nitrogen lone pair electrons of the dpa unit with the central aromatic linker and intramo-
lecular nonbonding interactions.
In tr od u ction
has been demonstrated recently that the Pt(II) center
in organoplatinum complexes plays a key role in pro-
moting singlet-triplet state mixing, hence enhancing
phosphorescent emission of the organic chromophores
in the complexes. OLEDs using phosphorescent Pt(II)
complexes have been demonstrated. Most previously
known luminescent platinum complexes are based on
square-planar cycloplatinated complexes that involve a
tridentate chelate ligand and an ancillary ligand.5 The
emission of these cycloplatinated complexes usually
originates from metal-to-ligand charge transition (MLCT)
or metal-metal-to-ligand charge transition (MMLCT).
Stable luminescent multinuclear platinum complexes
with a neutral bidentate-chelate ligand are scarce.
Ligands (L) with two chelating nitrogen donors, such
as 2,2′-dipyridylamine or phenanthrene, can easily
bidentately coordinate to platinum(II) centers, forming
neutral PtLR2 complexes, where R is a -1 charged
Luminescent organoplatinum compounds have at-
tracted much recent attention due to their potential
applications in many areas such as photosensitization,1
chemical sensing,2 and supramolecular photosensitiza-
tion.3 The potential applications of phosphorescent
platinum complexes as emitters in organic light-emit-
ting devices (OLEDs) further motivated the research
efforts in luminescent organoplatinum compounds.4 It
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10.1021/om030262k CCC: $25.00 © 2003 American Chemical Society
Publication on Web 07/25/2003

3782 Organometallics, Vol. 22, No. 18, 2003
Liu et al.
Elmer TGA 7 analyzer. Elemental analyses were performed
by Canadian Microanalytical Service Ltd., Delta, British
Columbia, Canada. The syntheses of the ligands bab, babp,
tab, tat, tapb, and tapt were achieved by previously published
procedures.6 The Pt(II) starting material [PtPh2(SMe2)]n (n )
2 or 3) was synthesized using a procedure reported in the
literature.7
Sch em e 1
1,3,5-Tr is{4′-[4′′-(2,2′-d ip yr id yla m in o)]b ip h en yl}b en -
zen e (ta bp b). A mixture of tris(p-bromophenyl)benzene (0.44
g, 0.8 mmol) and Pd(PPh3)4 (0.04 g) was suspended in toluene
(60 mL) and stirred for 10 min. Then p-(2,2′-dipyridylamino)-
phenylboronic acid (1.4 g, 4.81 mmol) in 20 mL of EtOH and
NaOH (0.6 g) in 20 mL of H2O were subsequently added. The
mixture was stirred and refluxed for 36 h and allowed to cool
to room temperature. The aqueous layer was separated and
extracted with CH2Cl2 (3 × 25 mL). The combined organic
layers were dried over MgSO4, and the solvents were evapo-
rated under reduced pressure. Purification of the crude product
by column chromatography (THF/hexane, 3:1) afforded tabpd
as a white solid in 54% yield. Mp: 146-148 °C. 1H NMR (CD2-
Cl2, δ, ppm): 8.33(dd, 6H, J ) 4.8, 1.2 Hz), 7.99(s, 3H), 7.91(d,
6H, J ) 8.4 Hz), 7.82(d, 6H, J ) 8.4 Hz), 7.74(d, 6H, J ) 8.7
Hz), 7.65(ddd, 6H, J ) 8.4, 7.2, 1.8 Hz), 7.29(d, 6H, J ) 8.4
Hz), 7.12(d, 6H, J ) 8.4 Hz), 7.02(ddd, 6H, J ) 7.3, 4.8, 0.9
Hz). 13C NMR in (CD2Cl2, δ, ppm): 158.5, 148.8, 145.1, 142.1,
140.1, 137.8, 137.7, 136.2, 128.3, 128.1, 127.9, 127.8, 125.8,
118.7, 117.5. Anal. Calcd for C72H51N9‚H2O: C 81.59, H 5.00,
N 11.90. Found: C 81.54, H 5.48, N 11.38.
ligand.1 We have previously reported a series of linear
linker ligands (bab and babp) and starburst ligands (tab,
tat, tapb, and tapt) containing 2,2′-dipyridylamino
functional groups (Scheme 1). This class of molecules
has been found to have good film-forming properties and
to be good blue emitters for OLEDs.6a In addition, we
have observed this class of molecules are also effective
ligands for the formation of luminescent lanthanides,
and transition metal complexes, that display intriguing
structural features and chemical sensing capabilities.6b-d
In continuation of this work, we have synthesized two
new starburst ligands (tabpb and tabpt) and have
investigated systematically the properties of organo-
platinum(II) complexes containing the new ligands and
the previously reported linear and starburst ligands of
dpa derivative. Herein we report the syntheses, crystal
structures, and luminescent properties of these new
compounds.
2,4,6-Tr is{4′-[4′′-(2,2′-d ip yr id yla m in o)]bip h en yl}-1,3,5-
tr ia zin e (ta bp t). In the same manner described for tabpb,
the reaction of tris(p-bromophenyl)benzene (0.67 g, 1.23 mmol)
with p-(2,2′-dipyridylamino)phenylboronic acid (1.87 g, 6.18
mmol) catalyzed by Pd(PPh3)4 (0.06 g) and NaOH (0.4 g)
provided tabpt as a light yellow-green solid in 56% yield. Mp:
274-276 °C. 1H NMR in (CD2Cl2, δ, ppm): 8.92(d, 6H, J )
8.4 Hz), 8.35(dd, 6H, J ) 4.8, 1.2 Hz), 7.91(d, 6H, J ) 8.4 Hz),
7.79(d, 6H, J ) 8.4 Hz), 7.68(ddd, 6H, J ) 8.4, 7.2, 1.8 Hz),
7.32(d, 6H, J ) 8.4 Hz), 7.12(d, 6H, J ) 8.4 Hz), 7.02(ddd, 6H,
J ) 7.2, 5.1, 0.9 Hz). 13C NMR in (CD2Cl2, δ, ppm): 158.7,
149.1, 146.0, 145.1, 138.2, 137.5, 135.8, 130.2, 128.9, 128.0,
127.7, 126.1, 119.1, 117.8. Anal. Calcd for C69H48N12‚1/2THF:
C 78.89, H 4.81, N 15.56. Found: C 79.31, H 4.60, N 16.05.
P t2P h 4(ba b) (1). Benzene (5 mL) and a solution of [PtPh2-
(SMe2)]n (99 mg, 0.24 mmol Pt) in 10 mL of THF were
successfully layered upon a solution of 1,4-bis(2,2′-dipyridyl-
amino)benzene (bab, 50 mg, 0.12 mmol) in 5 mL of CH2Cl2.
This three-layered solution was allowed to stand at room
temperature. The Pt-containing solution sank through the
benzene layer and into the CH2Cl2 layer slowly, producing 1
as colorless crystals after several days (96 mg, 72%). Com-
pound 1 is insoluble in any organic solvent. NMR spectra could
not be obtained. Anal. Calcd for C50H40N6Pt2: C 53.86, H 3.62,
N 7.53. Found: C 53.67, H 3.64, N 7.32.
Exp er im en ta l Section
All starting materials were purchased from Aldrich Chemi-
cal Co. and used without further purification. Solvents were
freshly distilled over appropriate drying reagents. 1H NMR
were recorded on a Bruker Advance 300 MHz spectrometer.
P t 2P h 4(b a b p ) (2). 4,4′-Bis(2,2′-dipyridylamino)biphenyl
(babp, 50 mg, 0.10 mmol) and [PtPh2(SMe2)]n (84 mg, 0.20
mmol Pt) were dissolve in 5 mL of THF/CH2Cl2 (1:1). After
being stirred for 10 min, the solution was filtered, and 2 mL
of toluene was added slowly to the filtrate and successfully
layered upon the solution. Solvents were allowed to diffuse
and evaporate slowly over 2 days at ambient temperature,
producing light yellow crystals of 2 (85 mg, 66%). 1H NMR
(CD2Cl2, δ, ppm): 8.47(ddd, 4H, J ) 5.6, 1.9, 0.6 Hz, Py), 7.95-
(ddd, 4H, J ) 8.1, 7.5, 1.9 Hz, Py), 7.75(d, 4H, J ) 8.7 Hz,
middle Ph), 7.67(ddd, 4H, J ) 8.1, 1.2, 0.6 Hz, Py), 7.38(dd,
8H, J ) 7.8, 1.4 Hz, 3J Pt-H ) 70.8 Hz, Ph), 7.23(d, 4H, J ) 8.8
Hz, central Ph), 7.14(ddd, 4H, J ) 7.4, 5.6, 1.3 Hz, Py), 6.89(t,
13C NMR were recored on
a Bruker Advance 500 MHz
spectrometer operated at 125 MHz. Excitation and emission
spectra were obtained with a Photon Technologies Interna-
tional QuantaMaster Model C-60 spectrometer. Emission
lifetime was measured on a Photon Technologies International
Phosphorescent lifetime spectrometer, Timemaster C-631F
equipped with a Xenon flash lamp, and digital emission photon
multiplier tube using a band pathway of 5 nm for excitation
and 2 nm for emission. UV-vis spectra were recorded on a
Hewlett-Packard 8562A diode array spectrophotometer. Ther-
mogravometric analysis (TGA) was performed using a Perkin-
(6) (a) Pang, J .; Tao, Y.; Freiberg, S.; Yang, X. P.; D’Iorio, M.; Wang,
S. J . Mater. Chem. 2002, 12, 206. (b) Yang, W. Y.; Chen, L.; Wang, S.
Inorg. Chem. 2001, 40, 507. (c) Seward, C.; Pang, J .; Wang, S. Eur. J .
Inorg. Chem. 2002, 1390. (d) Pang, J .; Marcotte, E. J .-P.; Seward, C.;
Brown, R. S.; Wang, S. Angew. Chem., Int. Ed. 2001, 40, 4042.
(7) (a) Scott, J . D.; Puddephatt, R. J . Organometallics 1983, 2, 1643.
(b) Hill, G. S.; Irwin, M. J .; Levy, C. J .; Rendina, L. M.; Puddephatt,
R. J . Inorg. Synth. 1998, 32, 149. (c) Song, D.; Wang, S. J . Organomet.
Chem. 2002, 648, 302.

Starburst Organoplatinum(II) Complexes
Organometallics, Vol. 22, No. 18, 2003 3783
8H, J ) 7.5 Hz, Ph), 6.77(tt, 4H, J ) 7.8, 1.5 Hz, Ph). 13C NMR
(CD2Cl2, δ, ppm): 153.6, 151.9, 144.9, 144.6, 139.6, 138.6,
136.2, 128.5, 126.6, 124.0, 123.8, 121.8, 120.3. Anal. Calcd for
127.6, 126.9, 124.2, 123.9, 121.8, 119.9. Anal. Calcd for
C108H81N9Pt3‚0.5CH2Cl2: C 61.11, H 3.88, N 5.91. Found: C
60.80, H 4.11, N 5.43.
C
56H44N6Pt2‚THF: C 57.00, H 4.12, N 6.65. Found: C 56.55,
P t3P h 6(ta bp t) (8). In the same manner described for 2, the
reaction of 2,4,6-tris{4′-[4′′-(2,2′-dipyridylamino)]biphenyl}-
1,3,5-triazine (tabpt, 50 mg, 0.048 mmol) with [PtPh2(SMe2)]n
(60 mg, 0.146 mmol Pt) provided 8 as yellow crystals (61 mg,
H 4.13, N 6.60.
P t3P h 6(ta b) (3). A mixture of 1,3,5-tris(2,2′-dipyridylami-
no)benzene (tab, 50 mg, 0.085 mmol) and [PtPh2(SMe2)]n (107
mg, 0.26 mmol Pt) was dissolved in 10 mL of THF, and the
solution was stirred at room temperature. A white solid
precipitated after 30 min. The solution was stirred for another
5 h at room temperature, and the white precipitate was
collected by filtration. The solid was dissolved in 30 mL of
CH2Cl2. Toluene (10 mL) was layered upon the solution.
Solvents were allowed to diffuse and evaporate slowly over
several days at ambient temperature, producing a white
1
61%). H NMR (CD2Cl2, δ, ppm): 8.94(d, 6H, J ) 8.6 Hz, arm
Ph), 8.51(dd, 6H, J ) 5.6, 1.6 Hz, Py), 8.00(td, 6H, J ) 7.6,
1.8 Hz, Py), 7.93(d, 6H, J ) 8.6 Hz, arm Ph), 7.86(d, 6H, J )
8.8 Hz, arm Ph), 7.72(d, 6H, J ) 7.6 Hz, Py), 7.38(dd, 12H,
J ) 7.8, 1.3 Hz, Ph), 7.17-7.22(m, 12H, Py and arm Ph), 6.90-
(t, 12H, J ) 7.5 Hz, Ph), 6.77(tt, 6H, J ) 7.5, 1.3 Hz, Ph). 13C
NMR (CD2Cl2, δ, ppm): 153.3, 152.0, 146.0, 144.6, 144.4, 139.7,
138.6, 135.5, 135.4, 129.9, 128.9, 127.3, 126.9, 124.7, 124.3,
121.9, 118.9 (one quaternary carbon hidden). Anal. Calcd for
1
microcrystalline solid of 3 (90 mg, 65%). H NMR (CD2Cl2, δ,
ppm): 8.39(dd, 6H, J ) 5.5, 1.3 Hz, Py), 7.77(td, 6H, J ) 7.3,
1.7 Hz, Py), 7.70(d, 6H, J ) 7.3 Hz, Py), 7.32(dd, 12H, J )
7.9, 1.4 Hz, J Pt-H ) 69.7 Hz, Ph), 7.06(ddd, 6H, J ) 7.2, 5.7,
1.4 Hz, Py), 6.89-6.77(m, 18H, Ph), 6.48(s, 3H, central Ph).
13C NMR (CD2Cl2, δ, ppm): 152.7, 152.0, 148.8, 144.3, 139.8,
138.6, 127.0, 124.5, 124.3, 121.9, 105.1. Anal. Calcd for
C
105H78N12Pt3: C 60.25, H 3.76, N 8.03. Found: C 60.11, H
3.74, N 7.71.
X-r a y Cr ysta llogr a p h ic An a lysis. Single crystals of 1-5
and 8 were obtained and characterized by X-ray diffraction
analysis. Crystals of 1-3 were mounted on glass fibers for data
collection, while those of 4, 5, and 8 were sealed into thin-
walled glass capillaries for structure determination. Data were
collected on a Siemens P4 single-crystal X-ray diffractometer
with a Smart CCD-1000 detector and graphite-monochromated
Mo KR radiation, operating at 50 kV and 35 mA at 25 °C. The
data collection ranges over the 2θ ranges are 4.10-56.6° for
1, 4.44-56.6° for 2 and 3, 3.54-56.8° for 4, 4.18-56.7° for 5,
and 2.60-56.8° for 8. No significant decay was observed for
all samples except 3 and 8, which decayed substantially due
to the loss of solvent molecules in the lattice. Data were
processed on a PC using the Bruker SHELXTL software
package8 (version 5.10) and are corrected for Lorentz and
polarization effects. Complexes 1-3 crystallize in the triclinic
space group P1h, and the molecules of 1 and 2 possess an
inversion center. The crystals of 4 and 5 belong to the
monoclinic space group P21/c, while complex 8 crystallizes in
the C-centered monoclinic space group C2/c. All structures
were solved by direct methods. All non-hydrogen atoms in
complexes 1, 2, 4, and 5 except the solvent molecules, and a
few carbon atoms in complex 4, were refined anisotropically.
The quality of the crystal data of 3-5 and 8 is poor due to the
presence of disordered solvent molecules and the thin plate
crystals. Crystals of 3 and 8 lose crystal lattice solvent
molecules rapidly. In addition, compounds 3 and 8 produce
only small and thin crystals, which diffract poorly. The solvent
molecules in 8 were only partially resolved and refined.
Consequently, only the platinum and some of the non-
hydrogen atoms in 3 and 8 were refined anisotropically. Due
to the poor quality of data, some of the terminal phenyl groups
in 3 and 8 had to be refined as rigid bodies in order to obtain
meaningful Pt-C bond distances. All hydrogen atoms except
those of the disordered solvent molecules were calculated, and
their contributions in structural factor calculations were
included. The crystallographic data are given in Table 1.
Selected bond lengths and angles are listed in Table 2.
C
72H57N9Pt3‚0.5CH2Cl2: C 52.31, H 3.46, N 7.52. Found: C
52.21, H 3.02, N, 7.57.
P t3P h 6(ta t) (4). In the same manner described for 3, the
reaction of 2,4,6-tris(2,2′-dipyridylamino)-1,3,5-triazine (tat, 50
mg, 0.085 mmol) with [PtPh2[SMe2)]n (107 mg, 0.26 mmol Pt)
1
provided 4 as colorless crystals (86 mg, 62%). H NMR (CD2-
Cl2, 243 K, δ, ppm,): 8.29 (d, 2H, J ) 5.0 Hz), 8.10 (d, 2H,
J ) 5.0 Hz), 8.03 (d, 2H, J ) 5.0 Hz), 7.88 (t, 2H, J ) 8.5 Hz),
7.80 (d, 2H, J ) 8.5 Hz), 7.74 (d, 2H, J ) 8.0 Hz), 7.39-7.59
(m, 18 H), 7.12 (t, 2H, J ) 6.5 Hz), 6.75-7.04 (m, 22 H). Anal.
Calcd for C69H54N12Pt3‚0.5CH2Cl2: C 49.90, H 3.29, N 9.98.
Found: C 49.76, H 3.42, N 10.01.
P t3P h 6(ta p b) (5). In the same manner described for 1, the
reaction of 1,3,5-tris[p-(2,2′-dipyridylamino)phenyl]benzene
(tapb, 50 mg, 0.061 mmol) with [PtPh2(SMe2)]n (76 mg, 0.184
mmol Pt) provided 5 as colorless crystals (67 mg, 59%). 1H
NMR (DMSO-d6, δ, ppm): 8.27(d, 6H, J ) 6.6 Hz, Py), 8.19(t,
6H, J ) 6.9 Hz, Py), 7.98-7.92(m, 15H, Py, arm Ph, central
Ph), 7.37(t, 6H, J ) 6.6 Hz, Py), 7.26(d, 12H, J ) 7.0 Hz, Ph),
7.17(d, 6H, J ) 8.7 Hz, arm Ph), 6.77(t, 12H, 7.2 Hz, Ph), 6.64-
(t, 6H, J ) 7.1 Hz, Ph). 13C NMR (DMSO-d6, δ, ppm): 158.4,
149.1, 147.5, 145.4, 141.9, 138.8, 137.0, 129.1, 128.1, 128.0,
123.9, 119.4, 117.8. (two quaternary carbons hidden). Anal.
Calcd for C90H69N9Pt3: C 58.06, H 3.74, N 6.77. Found: C
58.54, H 3.99, N 6.44.
P t3P h 6(ta p t) (6). In the same manner described for 1, the
reaction of 2,4,6-tris[p-(2,2′-dipyridylamino)phenyl]-1,3,5-tri-
azine (tapt, 50 mg, 0.061 mmol) with [PtPh2(SMe2)]n (76 mg,
0.184 mmol Pt) provided 6 as a light yellow solid (62 mg, 55%).
1H NMR (DMSO-d6, δ, ppm): 8.73(d, 6H, J ) 8.7 Hz, arm Ph),
8.26-8.32(m, 12 H, Py), 8.08(d, 6H, J ) 7.3 Hz, Py), 7.50(t,
6H, J ) 6.6 Hz, Py), 7.21(d, 12H, J ) 7.5 Hz, Ph), 6.89(d, 6H,
J ) 8.8 Hz, arm Ph), 6.76(t, 12H, J ) 7.3 Hz, Ph), 6.63(t, 6H,
J ) 7.3 Hz, Ph). 13C NMR could not be obtained. Anal. Calcd
for C87H66N12Pt3: C 56.04, H 3.54, N 9.02. Found: C 56.36, H
3.73, N 8.87.
Resu lts a n d Discu ssion s
P t3P h 6(ta bp b) (7). In the same manner described for 2,
the reaction of 1,3,5-tris{4′-[4′′-(2,2′-dipyridylamino)]biphenyl}-
benzene (tabpb, 50 mg, 0.048 mmol) with [PtPh2(SMe2)]n (60
mg, 0.146 mmol Pt) provided 7 as a yellow microcrystalline
solid (54 mg, 54%). 1H NMR (CD2Cl2, δ, ppm): 8.49(dd, 6H,
J ) 5.5, 1.4 Hz, Py), 7.99(s, 3H, central Ph), 7.97(td, 6H, J )
7.6, 1.6 Hz, Py), 7.91(d, 6H, J ) 8.5 Hz, arm Ph), 7.84(d, 6H,
J ) 8.4 Hz, arm Ph), 7.81(d, 6H, J ) 8.7 Hz, arm Ph), 7.69(d,
6H, J ) 7.8 Hz, Py), 7.39(dd, 12H, J ) 7.6, 1.2 Hz, Ph), 7.24-
(d, 6H, J ) 8.8 Hz, arm Ph), 7.15(ddd, 6H, J ) 7.3, 5.7, 1.3
Hz, Py), 6.90(t, 12H, J ) 7.5 Hz, Ph), 6.78(tt, 6H, 7.3, 1.2 Hz,
Ph). 13C NMR (CD2Cl2, δ, ppm): 153.3, 151.9, 145.2, 144.6,
142.3, 139.8, 139.6, 138.6, 136.3, 129.4, 128.6, 128.5, 128.2,
Syn t h eses a n d Ch a r a ct er iza t ion of Liga n d s
td bp b a n d td bp t a n d Com p ou n d s 1-8. All free
ligands except the two largest starburst molecules tdbpb
and tdbpt have been reported previously by our group.
The new tdbpb and tdbpt ligands were synthesized in
good yields using the Suzuki coupling procedures9
(8) SHELXTL NT, Crystal Structure Analysis Package, Version
5.10; Bruker AXS, Analytical X-ray System: Madison, WI, 1999.
(9) (a) Miyaura, N. Adv. Metal-Org. Chem. 1998, 6, 187, and
references therein. (b) Suzuki, A. J . Organomet. Chem. 1999, 576, 147,
and references therein.

3784 Organometallics, Vol. 22, No. 18, 2003
Ta ble 1. Da ta Collection a n d P r ocessin g P a r a m eter s for 1-5 a n d 8
Liu et al.
1
2
3
4
5
8
formula
C
50H40N6Pt2
C
56H44N6Pt2‚
C
72H57N9Pt3‚
C
69H54N12Pt3‚
C
90H69N9Pt3‚
C
105H78N12Pt3‚
THF
5.4C6H6
CH2Cl2‚2C7H8
2THF‚0.5C6H6
1.5C7H8
fw
1115.06
P1h
1263.26
P1h
2055.32
P1h
1905.71
P21/c
15.538(5)
17.448(5)
28.578(9)
90
100.152(9)
90
7626(4)
4
1.660
5.612
56.78
52591
18 221 (0.227)
831
2045.07
P21/c
20.421(5)
22.248(5)
19.465(5)
90
95.587(4)
90
8801(4)
4
1.543
4.811
56.70
61777
20 822 (0.141)
1003
2231.26
C2/c
40.667(19)
19.689(8)
33.039(15)
90
122.099(7)
90
22 140(17)
8
1.323
3.785
56.78
80419
26 707 (0.315)
1075
space group
a/Å
b/Å
c/Å
9.798(2)
10.461(3)
10.546(3)
72.401(4)
74.464(4)
80.902(4)
989.2(4)
1
8.0561(12)
9.7483(15)
17.006(3)
103.331(3)
102.001(3)
97.128(3)
1250.2(3)
1
14.077(4)
18.207(5)
19.435(5)
63.920(5)
84.685(5)
76.713(4)
4354(2)
2
R/deg
â/deg
γ/deg
V/Å3
Z
Dc/g cm-3
1.872
7.108
56.62
7112
4496 (0.0294)
262
1.678
5.637
56.60
9132
5757 (0.0215)
301
1.568
4.861
56.76
25837
17 997 (0.115)
767
µ/mm-1
2θmax/deg
no. of reflns measd
no. of reflns used (Rint
no. of params
final R [I > 2σ(I)]
R1a
)
0.0425
0.0662
0.0301
0.0683
0.0875
0.0939
0.0682
0.1014
0.0599
0.1077
0.1006
0.2185
wR2b
R (all data)
R1a
0.0844
0.0761
0.876
0.0369
0.0702
0.985
0.3257
0.1223
0.691
0.3189
0.1344
0.697
0.2309
0.1419
0.780
0.4255
0.3145
0.737
wR2b
goodness of fit on F2
R1 ) ∑[|Fo| - |Fc|]/∑|Fo|. wR2 ) {∑[w(Fo - Fc2)]/∑(wFo2)}1/2. w ) 1/[σ2(Fo2) + (0.075P)2], where P ) [max(Fo2,0) + 2Fc2]/3.
a
b
2
Sch em e 2
depicted in Scheme 1, where p-(2,2′-dipyridylamino)-
phenylboronic acid10 was reacted with the corresponding
1,3,5-tri(p-bromophenyl)benzene or triazine in the pres-
cence of Pd(PPh3)4 and NaOH. The p-(2,2′-dipyridylami-
no)phenylboronic acid was synthesized using a previ-
ously published procedure and p-(2,2′-dipyridylamino)-
bromobenzene as the starting material.10 These two
ligands were fully characterized by NMR and elemental
analyses. The Pt(II) complexes 1-8 were synthesized
by the reactions of [PtPh2(SMe2)]n with the correspond-
ing ligand of a stoichiometric amount in good yield. The
relatively labile bridging SMe2 ligand in the Pt(II)
starting material was replaced by the chelating 2,2′-
dipyridylamino moiety in the resulting complexes. The
synthetic routes for complexes 1-8 are summarized in
Scheme 2. Complexes 1-8 are stable in the solid state
and in solution (except in the presence of certain
halogenated solvents such as CHCl3) upon exposure to
air. A common problem with complexes 1-8 is their poor
solubility in common organic solvents. Complex 1 is not
soluble in common solvents such as CH2Cl2, CH3CN, or
DMSO, and as a result, its NMR spectrum could not be
for these compounds. Various approaches were used in
our effort to obtain single crystals suitable for X-ray
diffraction analyses. Complexes 3 and 4 were initially
precipitated out from a THF solution and were subse-
quently recrystallized from a CH2Cl2/toluene solution
for 4 and a CH2Cl2/benzene solution for 3. Complexes
2, 7, and 8 are slightly soluble in CH2Cl2 and THF and
cannot be recrystallized. Single crystals of 2 and 8
suitable for X-ray diffraction were obtained directly from
the solution containing both the free ligand and the Pt-
(II) starting material. For complexes 1, 5, and 6, crys-
talline products can only be obtained by using solvent-
layering techniques where the THF solution of [PtPh2-
(SMe2)]n was layered on top of the CH2Cl2 solution of
the corresponding ligand, which upon standing at ambi-
ent temperature produced single crystals of 1 and 5
suitable for X-ray study. Various attempts to grow single
crystals of complexes 6 and 7 suitable for X-ray diffrac-
obtained. Compound 1 was characterized by elemental
1
and single-crystal X-ray diffraction analyses. H NMR
spectra for 2-8 were recorded. Complex 4 is clearly
fluxional in solution, as indicated by the presence of
broad chemical shifts in its 1H NMR spectrum. Our
early investigation on starburst complexes that contain
MCl2 units (M ) Pt(II), Zn(II)) chelated to the 2,2′-
dipyridylamino groups revealed that this class of mol-
ecules display unusual extended structures in the solid
state (e.g., polar stacked columns or hexagonal col-
umns).6c To understand the origin of the fluxionality in
solution and to study intermolecular interactions of
complexes 1-8 in the solid state, single-crystal X-ray
diffraction analysis on the structures of complexes
became necessary. Due to the poor solubility of the
complexes, it is very challenging to obtain single crystals
(10) J ia, W. L.; Song, D.; Wang, S. J . Org. Chem. 2003, 68, 701.

Starburst Organoplatinum(II) Complexes
Organometallics, Vol. 22, No. 18, 2003 3785
Ta ble 2. Selected Bon d Len gth (Å) a n d An gles
(d eg) for 1-5 a n d 8
Complex 1
Pt(1)-N(1)
Pt(1)-N(2)
Pt(1)-C(1)
Pt(1)-C(7)
2.101(5)
2.123(5)
2.009(7)
2.006(7)
N(1)-Pt(1)-N(2)
N(1)-Pt(1)-C(7)
N(2)-Pt(1)-C(1)
83.77(19)
174.7(3)
173.1(3)
Complex 2
Pt(1)-N(1)
Pt(1)-N(2)
Pt(1)-C(1)
Pt(1)-C(7)
2.111(3)
2.117(3)
2.002(4)
2.002(4)
N(1)-Pt(1)-N(2)
N(1)-Pt(1)-C(7)
N(2)-Pt(1)-C(1)
85.13(13)
176.67(16)
175.80(13)
Complex 3
Pt(1)-N(1)
Pt(1)-N(2)
Pt(1)-C(37)
Pt(1)-C(43)
Pt(2)-N(4)
Pt(2)-N(5)
Pt(2)-C(49)
Pt(2)-C(55)
Pt(3)-N(7)
Pt(3)-N(8)
Pt(3)-C(61)
Pt(3)-C(67)
2.151(13)
2.064(17)
1.985(10)
1.944(11)
2.06(2)
1.974(18)
2.037(18)
1.884(11)
1.996(17)
2.050(15)
1.974(9)
N(1)-Pt(1)-N(2)
N(1)-Pt(1)-C(37)
N(2)-Pt(1)-C(43)
N(4)-Pt(2)-N(5)
N(4)-Pt(2)-C(49)
N(5)-Pt(2)-C(55)
N(7)-Pt(3)-N(8)
N(7)-Pt(3)-C(67)
N(8)-Pt(3)-C(61)
86.7(6)
175.0(8)
178.1(7)
85.9(7)
177.5(8)
177.0(8)
90.2(7)
177.6(9)
179.5(6)
1.969(12)
Complex 4
Pt(1)-N(7)
Pt(1)-N(8)
Pt(1)-C(34)
Pt(1)-C(40)
Pt(2)-N(9)
Pt(2)-N(10)
Pt(2)-C(46)
Pt(2)-C(52)
Pt(3)-N(11)
Pt(3)-N(12)
Pt(3)-C(58)
Pt(3)-C(64)
2.136(13)
2.135(12)
2.053(16)
2.016(17)
2.080(14)
2.119(14)
1.918(18)
1.866(18)
2.161(13)
2.127(12)
1.88(2)
N(7)-Pt(1)-N(8)
N(7)-Pt(1)-C(34)
N(8)-Pt(1)-C(40)
N(9)-Pt(2)-N(10)
N(9)-Pt(2)-C(46)
N(10)-Pt(2)-C(52)
N(11)-Pt(3)-N(12)
N(11)-Pt(3)-C(64)
N(12)-Pt(3)-N(58)
85.0(6)
176.5(7)
173.7(7)
85.9(6)
176.2(7)
173.8(7)
86.0(6)
F igu r e 1. Top: Molecular structure of 1 with 50% thermal
ellipsoids and labeling schemes. All hydrogen atoms are
omitted for clarity. Bottom: Diagram showing the molec-
ular packing of 1.
176.9(7)
176.5(7)
phenyl ligands are coordinated to each Pt(II) center. The
Pt-C and Pt-N bond lengths are typical for Pt(II)
complexes.1-4 The square plane of the Pt center is
however a little bit distorted, as indicated by the bond
angles of N(1)-Pt(1)-C(7) (174.7(3)°) and N(2)-Pt(1)-
C(1) (173.1(3)°). The amino N(3) atom has a trigonal
planar geometry, but its lone pair electrons are not
conjugated with the central benzene ring, as indicated
by the dihedral angle of the N(3)C(22)C(17) plane and
the central benzene ring (98.9°). Instead, the N(3) lone
pair electrons conjugate with the two pyridyl groups,
as shown by their approximate coplanarity in Figure 1.
This is further supported by the relatively short N(3)-
C(17) and N(3)-C(22) bond lengths (1.424(8), 1.407(7)
Å, respectively), compared to that of N(3)-C(23) (1.456-
(7) Å). The lack of conjugation of the amino lone pair
with the central benzene ring is clearly caused by
nonbonding steric interactions between ortho hydrogen
atoms of the pyridyl rings and the phenyl rings. As a
consequence of the lack of conjugation of the amino N(3)
atom with the central benzene ring, the two PtPh2(dpa)
units are oriented sideways with respect to the central
benzene ring, with the two Pt atoms being coplanar with
the central benzene plane. The two phenyl ligands are
almost perpendicular to the Pt(1)N(1)N(2) plane (the
dihedral angles between the phenyl groups and Pt(1)-
N(1)N(2) plane are 76.6 and 81.8°, respectively), at-
tributable to steric interactions between the phenyl
groups and the bab ligand. The intramolecular Pt(1)-
Pt(1A) separation distance is 11.576(2) Å. Molecules of
1 stack in the crystal lattice as shown in Figure 1, with
the shortest intermolecular Pt(1)-Pt(1′) separation
distance being 5.671(1) Å.
2.041(16)
Complex 5
Pt(1)-N(1)
Pt(1)-N(2)
Pt(1)-C(55)
Pt(1)-C(61)
Pt(2)-N(4)
Pt(2)-N(5)
Pt(2)-C(67)
Pt(2)-C(73)
Pt(3)-N(7)
Pt(3)-N(8)
Pt(3)-C(79)
Pt(3)-C(85)
2.123(9)
N(1)-Pt(1)-N(2)
N(1)-Pt(1)-C(55)
N(2)-Pt(1)-C(61)
N(4)-Pt(2)-N(5)
N(4)-Pt(2)-C(73)
N(5)-Pt(2)-C(67)
N(7)-Pt(3)-N(8)
N(7)-Pt(3)-C(85)
N(8)-Pt(3)-C(79)
84.4(4)
173.6(5)
173.5(6)
87.6(4)
177.9(5)
176.2(6)
85.2(4)
2.138(11)
2.025(14)
1.983(14)
2.099(10)
2.120(10)
1.980(15)
2.031(14)
2.092(11)
2.118(10)
2.006(14)
1.952(17)
178.5(5)
176.6(7)
Complex 8
Pt(1)-N(2)
Pt(1)-N(3)
Pt(1)-C(76)
Pt(1)-C(70)
Pt(2)-N(5)
Pt(2)-N(6)
Pt(2)-C(82)
Pt(2)-C(88)
Pt(3)-N(8)
Pt(3)-N(9)
Pt(3)-C(94)
Pt(3)-C(100)
2.04(2)
2.093(19)
1.888(18)
1.93(3)
2.11(3)
2.08(2)
2.00(3)
1.88(4)
2.14(3)
2.10(2)
1.90(2)
1.83(2)
N(2)-Pt(1)-N(3)
N(3)-Pt(1)-C(76)
N(2)-Pt(1)-C(70)
N(5)-Pt(2)-N(6)
N(6)-Pt(2)-C(88)
N(5)-Pt(2)-C(82)
N(8)-Pt(3)-N(9)
N(9)-Pt(3)-C(100)
N(8)-Pt(3)-C(94)
86.9(10)
175.5(8)
177.1(11)
87.6(4)
177.0(14)
176.0(9)
87.7(10)
172.9(15)
179.1(13)
tion were unsuccessful. Single-crystal X-ray diffraction
analyses were carried out for 1-5 and 8.
Cr ysta l Str u ctu r es. The structure of complex 1 is
show in Figure 1. The molecule of 1 possesses an
inversion center symmetry. The Pt center is four-
coordinated and adopts a typical square-planar geom-
etry. The bab ligand bidentately chelates to two Pt(II)
centers via the two 2,2′-dipyridylamino (dpa) units. Two

3786 Organometallics, Vol. 22, No. 18, 2003
Liu et al.
F igu r e 3. TGA diagram for the crystals of 2.
F igu r e 4. Molecular structure of 3 with 50% thermal
ellipsoids and labeling schemes. All hydrogen atoms are
omitted for clarity. All carbon atoms are shown as ideal
spheres.
being 3.042(1) Å above and below the biphenyl plane.
This arrangement can be attributed to the relatively
reduced nonbonding steric interactions in 2, compared
to those of 1. In the crystal lattice of 2, along one of the
axes, the dinuclear Pt2 units are packed in a her-
ringbone style with the shortest intermolecular Pt(1)-
Pt(1′) separation distance being 5.962(1) Å. There are
no significant π-π stacking interactions. THF solvent
molecules are trapped in the void channels of the crystal
lattice as shown in Figure 2b. These trapped THF
solvent molecules are surprisingly stable and do not
escape the crystal lattice at ambient temperature, as
indicated by the elemental analysis results obtained for
crystals at ambient temperature. TGA analysis shows
that the crystals of compound 2 gradually lose the THF
solvent molecule after being heated to above 40 °C
(Figure 3). At about 208 °C, the loss of THF appears to
be complete (∼5.7% of total weight). At this tempera-
ture, other decomposition processes also appear to take
place, as indicated by the rapid weight loss above
210 °C.
F igu r e 2. (a) Molecular structure of 2 with 50% thermal
ellipsoids and labeling schemes. All hydrogen atoms are
omitted for clarity. (b) Diagram showing molecular packing
of 2 in the crystal lattice.
The structure of 2 is similar to that of 1, as show in
Figure 2a. The molecule of 2 also possesses an inversion
center symmetry. The intramolecular Pt(1)-Pt(1A)
separation distance is 11.916(2) Å, slightly longer than
that of 1. The central biphenyl unit is coplanar. In
contrast to compound 1, the amino nitrogen lone pair
electrons in 2 appear to somewhat conjugate with the
biphenyl unit, as indicated by the dihedral angle (29.4°)
between the N(3)C(17)C(22) plane and the biphenyl
plane, much smaller than that of 1, and the N(3)-C(23)
bond length (1.425(5) Å), much shorter than that in 1.
The two pyridyl rings of the 2,2′-dipyridylamino unit
are puckered, and none of them are coplanar with the
amino nitrogen atom. The dihedral angles between the
two terminal phenyl rings and the Pt(1)N(1)N(2) plane
are 64.3° and 58.9°, respectively. In contrast with
complex 1, the two PtPh2(dpa) units chelated by the 2,2′-
dipyridylamino groups in 2 are on the opposite side of
the biphenyl plane, with the Pt(1) and Pt(1A) atoms
The structures of 3, 4, 5, and 8 are shown in Figure
4, Figure 5, Figure 6, and Figure 7, respectively. The
starburst ligands in these molecules are bound to
three PtPh2 units in the complex via the chelating

Starburst Organoplatinum(II) Complexes
Organometallics, Vol. 22, No. 18, 2003 3787
F igu r e 6. Molecular structure of 5 with 50% thermal
ellipsoids and labeling schemes. All hydrogen atoms are
omitted for clarity. Carbon atoms are shown as ideal
spheres.
F igu r e 5. Top: Molecular structure of 4 with 50% thermal
ellipsoids and labeling schemes. All hydrogen atoms are
omitted for clarity. Carbon atoms are shown as ideal
spheres. Bottom: Diagram showing the relative orientation
of the PtPh2(dpa) units in 4.
2,2′-dipyridylamino (dpa) groups. The coordination en-
vironment around each Pt(II) center in these complexes
is similar and resembles that of complexes 1 and 2. The
average Pt-C bond length in these complexes is 1.976
Å, and the average Pt-N bond length is 2.111 Å, slightly
longer than that of Pt-C bonds.
Complexes 3 and 4 are the smallest starburst com-
plexes, where the three PtPh2(dpa) units are attached
directly to the central triazine ring or the benzene ring.
There are however substantial differences between the
structures of 3 and 4. In complex 4, the amino nitrogen
planes (defined by the nitrogen atom and the two
adjacent carbon atoms from the two pyridyl groups) are
almost coplanar with the central triazine ring, as
indicated by the dihedral angles between the NC2 planes
and the triazine ring (10.5° for the N(4)C(4)C(9) plane,
13.5° for the N(5)C(14)C(19) plane, 8.5° for the N(6)-
C(24)C(29) plane), an indication that the amino nitrogen
lone pair electrons are conjugated with the triazine ring.
This is further supported by the relatively short bond
lengths of N(4)-C(1), N(5)-C(2), and N(6)-C(3) (1.377-
(17), 1.370(17), and 1.349(16) Å, respectively). In con-
F igu r e 7. Molecular structure of 8 with 50% thermal
ellipsoids and labeling schemes. All hydrogen atoms are
omitted for clarity. Carbon atoms are shown as ideal
spheres.
trast, the dihedral angles between the three amino NC2
planes and the central benzene ring in 3 are 19.5° (N(3)),
85.1° (N(6)), and 21.4° (N(9)), respectively, and the
N(3)-C(1), N(6)-C(3), and N(9)-C(5) bond lengths
(1.370(18), 1.452(19), and 1.519(19) Å, respectively) are
substantially longer than the corresponding ones in 4,
which indicates either substantial loss of conjugation
or no conjugation by the amino nitrogen lone pair
electrons with the central benzene ring. The loss of
conjugation is most pronounced for the N(6) atom, where
the N(6)C(18)C(22) plane is almost perpendicular to that

3788 Organometallics, Vol. 22, No. 18, 2003
Liu et al.
Ch a r t 1
of the central benzene ring. As a consequence, the Pt(2)
unit in 3 is neither above nor below but is oriented
sideways with respect to the central benzene ring in the
same manner as the PtPh2(dpa) units in 1. The remain-
ing two PtPh2(dpa) units in 3 are somewhat oriented
sideways and are on the opposite sides of the central
benzene plane. The shortest Pt-Pt separation distance
is intramolecular, 7.695(2) Å between Pt(1) and Pt(2).
The loss of conjugation of the amino nitrogen lone pair
electrons with the central benzene ring in 3 can be again
attributed to steric interactions between the ortho
hydrogen atoms. In compound 4, due to the conjugation
of the amino nitrogen lone pair electrons with the
triazine ring, the three PtPh2(dpa) units are oriented
above and below the triazine plane, two of which (Pt(1)
and Pt(3)) are below the plane of the triazine ring, and
the remaining one is above the triazine plane (Figure
3, bottom and Chart 1). A symmetric arrangement
where all three Pt units are on the same side with
respect to the central triazine (or benzene) plane has
been observed in Pt3Cl6(tab) and Pt3Cl6(tat) complexes
reported previously, which display a bowl-shaped struc-
ture in the solid state, stabilized by intermolecular
electrostatic interactions between the chloride ligands
and the hydrogen atoms of the bab or tat ligand.6c The
lack of a symmetric structure similar to that of Pt3Cl6-
(tat) by 4 can be attributed to nonbonding steric
interactions between the phenyl groups and the tat
ligand. The planes of the Pt(1) unit and the Pt(3) unit
in 4 are not completely parallel, with a dihedral angle
of 56.6°. The intramolecular Pt(1)-Pt(3), Pt(1)-Pt(2),
and Pt(2)-Pt(3) separation distances are 8.150(2),
8.726(2), and 8.285(2) Å, respectively, which are much
shorter than those of 1 and 2. The shortest intermo-
lecular Pt-Pt separation distance is 6.980(2) Å between
Pt(1) and Pt(2A).
in 5 for all three PtPh2(dpa) groups to be on the same
side with respect to the central aromatic unit. The fact
that the three PtPh2(dpa) groups in 5 are not on the
same side is clearly not due to intramolecular interac-
tions but intermolecular interactions or molecular pack-
ing in the solid state. The shortest Pt-Pt separation
distance is intermolecular, 6.980(2) Å, between Pt(1) and
Pt(3′). The shortest intramolecular Pt-Pt separation
distance is ∼11.8 Å between Pt(2) and Pt(3), similar to
that of 2, but much longer than those of 3 and 4,
attributable to the exta phenyl group between the
central benzene ring and the dpa moiety. There are
intermolecular π-π stacking interactions between the
central 1,3,5-tri(p-phenyl)benzene units with the short-
est atomic separation distance being 3.86 Å.
The three PtPh2(dpa) units in 8 are attached to a
2,4,6-tris(biphenyl)-1,3,5-triazine unit as shown in Fig-
ure 7. As a consequence, the intramolecular Pt-Pt
separation distances are very long, with the Pt(2)-Pt-
(3) distance being the shortest (18.70(1) Å). Two of the
three biphenyl units are not planar, as indicated by the
dihedral angles between phenyls (2.2°, 14.7°, and 11.1°,
respectively). The three PtPh2(dpa) units are orientated
somewhat sideways with respect to the central aromatic
linker, in a manner similar to that observed in 5. This
arrangement of the three Pt units in 8 is clearly due to
the presence of the biphenyl unit between the triazine
ring and the dpa group that significantly reduces
intramolecular steric interactions. The shortest Pt-Pt
separation distance is intermolecular, 5.908(2) Å, be-
tween Pt(3) and Pt(3′). Some π-π stacking interactions
between the central aromatic units are observed.
Str u ctu r es a n d F lu xion a lity in Solu tion . On the
basis of the crystal structures, we believe that the
starburst molecules of 5-8 should have symmetric
structures with an approximate C3 symmetry in solution
since there is sufficient space for the three PtPh2(dpa)
units to orient in any direction. The most likely struc-
tures for 5-8 in solution are propellers with all three
PtPh2(dpa) units being oriented somewhat sideways in
the same direction, resembling the structures of (ZnCl2)3-
(tapb) and (ZnCl2)3(tapt) molecules, which have a rigor-
The three PtPh2(dpa) units in 5 are attached to a
1,3,5-tri(p-phenyl)benzene unit which is not coplanar,
as indicated by the dihredral angles between the three
phenyl rings and the central benzene ring (12.3°, 32.0°,
and 40.4°, respectively). With respect to the 1,3,5-tri-
(p-phenyl)benzene unit, Pt(2) and Pt(3) are on one side
while the Pt(1) unit is on the opposite side. However,
these PtPh2(dpa) units are all somewhat rotated side-
ways and not exactly above or below the central
aromatic linker, resembling those in 3. Figure 6 shows
that unlike compounds 3 and 4, there is sufficient space
1
ous C3 symmetry.6c,d Indeed, H NMR spectra of com-
plexes 5-8 display only one set of chemical shifts,
corresponding to the PtPh2(dpa) units, and no fluxional
phenomena were observed for these complexes, indicat-

Starburst Organoplatinum(II) Complexes
Organometallics, Vol. 22, No. 18, 2003 3789
F igu r e 9. Arrhenius plot of ln(k) versus 1/T for compound
4 and the best fit.
with the structure of 4 observed in the solid state. NMR
data lead us to conclude that the three different sets of
pyridyl groups (as well as the phenyl groups) undergo
slow exchange in solution at ambient temperature
caused by the orientation interconversion of the three
PtPh2(dpa) units with respect to the triazine plane.
Using a two-site exchange model with a population ratio
of 1:2 for the two sites and the variable-temperature
NMR data, we calculated a series of rate constants at
different temperatures. Using these rate constants and
the corresponding temperatures and the Arrhenius plot
shown in Figure 9, we estimated that the activation
energy for the exchange process of 4 is ∼154(10) kJ
mol-1. This rather large activation energy can be
attributed to the presence of significant intramolecular
steric interactions in 4 that must be overcome in order
to achieve the up-and-down orientation interconversion.
One surprising finding is that a similar fluxional
behavior was not observed for compound 3. Through the
F igu r e 8. Variable-temperature 1H NMR spectra. Top:
compound 3. Bottom: compound 4.
ing that they do have a symmetric structure in solution.
No fluxional behavior was observed for compounds 1
and 2.
In contrast, compounds 3 and 4 are very crowded
molecules, and there is not sufficient space to allow all
three PtPh2(dpa) groups to be oriented in the same
direction, above or below the central aromatic ring.
Consequently, one would anticipate that in solution 3
and 4 retain the same structures as in the solid state,
e.g., the “up-and-down” structure shown in Chart 1,
where not all three PtPh2(dpa) units have the same
1
temperature range 223-298 K, the pattern of the H
NMR spectrum of 3 remains the same and corresponds
to one set of PtPh2(dpa) unit and one proton from the
central benzene ring (Figure 8, top), a clear indication
that 3 has a symmetric structure in solution with all
PtPh2(dpa) units being in the same environment. To
achieve a symmetric structure, only two arrangements
are possible, A and B as depicted in Chart 1. In
structure A, all three PtPh2(dpa) units are oriented
sideways with the three Pt atoms being coplanar with
the central benzene plane, and there is no conjugation
between the amino nitrogen lone pair and the central
benzene, resulting in a wheel- or paddle-like arrange-
ment similar to that observed in compound 1. In
structure B, all three PtPh2(dpa) units are above (or
below) the central benzene ring with a bowl shape
similar to that of (PtCl2)3(tab) or (PtCl2)3(tat), and the
amino nitrogen lone pair electrons are conjugated with
the central benzene ring. Clearly, due to steric interac-
tions, the bowl-shaped structure B is not favorable for
3. We therefore believe that compound 3 adopts the
wheel-like structure A in solution where all three Pt
atoms have the same environment. The crystal struc-
ture of 3 shows that the amino nitrogen lone pair
electrons do not favor conjugation with the central
benzene ring due to steric interactions. In contrast, the
1
environment. H NMR experiments appear to confirm
that this is the case for compound 4, which displays
temperature-dependent 1H NMR spectra, consistent
with the presence of a dynamic exchange process. As
shown in Figure 8, bottom, at ambient temperature, the
1H NMR spectrum of 4 shows broad peaks correspond-
ing to one set of PtPh2(dpa) chemical shifts, an indica-
tion of the presence of a relatively slow exchange
process. Raising the temperature to 311 K, the spectrum
becomes somewhat better resolved but remains broad.
As the temperature is decreased, the spectrum gradu-
ally becomes sharp. At 243 K, well-resolved peaks
corresponding to three different sets of pyridyl groups
in a 1:1:1 ratio are observed. Three sets of phenyl
chemical shifts are also observed at 243 K. As shown
by Figure 5 and Chart 1, the molecule of 4 has an
approximate mirror plane symmetry relating the Pt(1)
unit to the Pt(3) unit, resulting in two different sets of
Pt environments and three different sets of pyridyl rings
(labeled as a , b, and c, respectively in Chart 1) and three
different sets of phenyl groups as well. The low-
1
temperature H NMR spectrum is therefore consistent

3790 Organometallics, Vol. 22, No. 18, 2003
Ta ble 3. Lu m in escen ce Da ta for Com p ou n d s 1-8, ta bp b, a n d ta bp t
Liu et al.
UV-vis, nma
excitation
wavelength, nm
emission
(λmax, nm)
decay lifetime
compd
(ꢀ/M-1 cm-1
)
(τ, ms)c
conditions
1
N/A
375
473
509
0.00969(6)
0.01204(4)
0.0136(1)
1.352(2)
solid, 298 K
540 (sh)
486
2
232 (99 130)
280 (30 772)
304 (66 890)
381
395
CH2Cl2, 77 K
solid, 298 K
518 (sh)b
481
1.358(2)
0.0378(2)
0.0374(3)
0.0392(8)
0.305(6)
519
560 (sh)
460
3
4
5
6
240 (378 270)
314 (70 410)
370 (29 560)
232 (142 720)
270 (99 400)
362 (21 070)
232 (227 580)
276 (197 170)
336 (89 570)
232 (126 310)
314 (76 180)
352 (93 130)
230 (191 500)
306 (216 620)
232 (148 730)
334 (132 500)
352 (130 060)
230 (42 130)
282 (28 320)
316 (14 700)
378
387
376
377
CH2Cl2, 77 K
CH2Cl2, 77 K
CH2Cl2, 77 K
CH2Cl2, 77 K
496
0.160(4)
402
470
500 (sh )
423
472
508 (sh )
511
542
463
522
555
399
404
400
506
538 (sh )
469
484
452
1.596(9)
1.506(8)
2.06(2)
1.94(4)
2.56(4)
2.48(5)
7
8
395
395
CH2Cl2, 77 K
CH2Cl2, 77 K
2.27(4)
2.15(8)
tabpb
tabpt
341
342
358
358
solid, 298 K
CH2Cl2, 298 K
CH2Cl2, 77 K
CH2Cl2, 77 Kd
4.22(7)
4.01(5)
232 (28 890)
284 (49 460)
358 (50 330)
368
398
395
395
solid, 298 K
CH2Cl2, 298 K
CH2Cl2, 77 K
CH2Cl2, 77 Kd
525
562
4.06(3)
3.86(6)
a
b
All data were collected for CH2Cl2 solution ([M] ) 2.2 × 10-6 to 4.6 × 10-6) at ambient temperature. sh ) shoulder. c A reliable
lifetime could not be obtained for the fluorescent emission due to the limitation of the instrument, which cannot measure lifetimes in the
nanosencond regime. Obtained using a time-resolved phosphorescent spectrometer.
d
amino nitrogen lone pair conjugation with the triazine
ring is favored in compound 4, due to the lack of ortho
hydrogen atoms on the triazine ring and the presence
of the three electronegative nitrogen atoms on the
triazine. Consequently, in solution compound 4 favors
the “up-and-down” structure similar to the crystal
structure, not the wheel-like structure A.
phosphorescent emission at 77 K in solution, detectable
by a time-resolved phosphorescent spectrometer.) For
complexes 5, 6, and 8, weak fluorescent emissions at
λmax ) 402, 423, and 463 nm, respectively, were also
observed at 77 K, attributable to ligand-centered emis-
sion. For the remaining complexes, no significant fluo-
rescent emission was detected. As two representative
examples, the full emission spectra of complexes 7 and
8 at 77 K in CH2Cl2 matrix and the phosphorescent
spectra of the corresponding free ligands obtained by
using time-resolved phosphorescent spectrometry are
shown in Figure 10. The resemblance of the emission
spectra of the complexes with those of the free ligands
is obvious. Emission lifetime measurements for the
complexes at 77 K in a frozen solution revealed that the
emission bands (excluding the fluorescent bands) from
the complexes have decay lifetimes in the regime of
phosphorescence and are much shorter than those of the
corresponding free ligands. For example, the free babp
ligand has a green phosphorescent emission band with
the maximum at 486 and 520 nm at 77 K in solution.
This emission band has a decay lifetime in the seconds
range such that the green emission was visible by eye
for a few seconds after the excitation source was
switched off. Due to its extremely long decay time, we
could not measure it accurately. In contrast, the complex
Pt3Ph6(babp) (2) has a green emission in the same
region, but with a much shorter decay time (1.35 ms)
than that of the free ligand. The impact of the Pt(II)
Lu m in escen t P r op er ties. Upon irradiation by UV
light, the free ligands bab, babp, tab, tat, tapb, tapt,
tabpb, and tabpt display an intense blue fluorescent
emission in solution and the solid state at ambient
temperature with the emission maximum at 400, 415,
433, 415, 440, 410, 404, and 484 nm, respectively. In
contrast to the free ligands, all Pt(II) complexes do not
show detectable emission in solution at ambient tem-
perature. Bluish green emission (λmax ) 473, 509 nm
for 1; λmax ) 481, 519 nm for 2) was however observed
in the solid state for complexes 1 and 2 at ambient
temperature. At 77 K, the frozen solutions of all
complexes except 1 (due to the insolubility of 1 in
organic solvents, its emission spectra in solution could
not be recorded) exhibit bright blue-green emission with
the emission maximum shifted to a much longer wave-
length, compared to those of the free ligands at ambient
temperature (see Table 3). The emission spectra of the
complexes at 77 K appear to be dominated by phospho-
rescence of the ligands because of their resemblance
with the phosphorescent emission spectra of the corre-
sponding free ligands. (The free ligands display a weak

Starburst Organoplatinum(II) Complexes
Organometallics, Vol. 22, No. 18, 2003 3791
nated by fluorescence, but the emission of the complexes
is nearly exclusively phosphorescence. Clearly the Pt(II)
centers in the complexes played a key role in enhancing
the phosphorescent emission of the ligands. Similar
enhancement of ligand-based phosphorescent emission
by the Pt(II) center in porphyrin derivative Pt(II)
complexes has been attributed to the bright emission
and their successful uses as emitters in OLEDs.4 The
absence of emission in the solid state by complexes 3-8
can be attributed to intermolecular quenching. To
minimize intermolecular quenching, we doped our com-
plexes into a rigid polymer matrix such as PVK.
However, no luminescence from the complexes was
observed in the doped polymers at ambient temperature,
attributable to thermal quenching due to the long decay
lifetimes, which makes this class of Pt(II) complexes not
useful as emitters in OLEDs.
In summary, eight new neutral binuclear and star-
burst trinuclear platinum complexes of 2,2′-dipyridyl-
amino derivative ligands as well as two new blue
luminescent starburst ligands have been synthesized.
The emission of the Pt(II) compounds is dominated by
phosphorescent emissions that cover the region of blue
to green and can be attributed to ligand-based transi-
tions. The Pt(II) atoms in the complexes play a key role
in promoting phosphorescent emissions. These new
complexes display versatile structures in solution and
the solid state, where the conjugation of the amino
nitrogen lone pair of the dpa unit with the central
aromatic unit of the ligand and steric interactions play
an important role. Although these new complexes are
not useful in OLEDs, they do show interesting reactivity
toward C-Cl bonds when exposed to light and may be
useful in photochemical reactions, which are being
investigated in our laboratory.
F igu r e 10. Emission spectra of 7 and 8 at 77 K in CH2-
Cl2 and the phosphorescent emission spectra of the free
ligands tabpb and tabpt in CH2Cl2 solution at 77 K
obtained using a time-resolved phosphorescence spectrom-
eter.
center on the phosphorescent emission of the ligand
therefore appears to be 2-fold: (a) the Pt(II) center
quenches the fluorescent emission and enhances the
phosphorescent emission of the ligand, (b) the Pt(II)
center substantially decreases the phosphorescent decay
time of the ligand. This impact appears to diminish with
the size increase of the ligand. For example, the two
biggest ligands, tabpb and tabpt, have green phospho-
rescent emission bands (λmax ) 506 nm, 538 nm for
tabpb; 525 nm, 562 nm for tabpt) with a decay lifetime
of 4.1(1) and 3.9(1) ms, respectively, while the corre-
sponding emission of the complexes 7 and 8 (λmax ) 511
nm, 542 nm for 7; 522 nm, 555 nm for 8) has a decay
lifetime of 2.0(1) and 2.5(1) ms, respectively. The
decrease of the decay lifetime of ligands in 7 and 8
compared to those of the free ligands is not as dramatic
as those of the smaller complexes such as 2. On the basis
of the above observations, we can conclude that the
emission of the complexes is ligand-based, involving
most likely π f π* transitions. (In the case of triazine
derivatives, charge transfer between the dipyrdiylamino
unit and the electronegative triazine ring is also likely.)
The striking difference between the free ligands and the
complexes is that the phosphorescent emission of the
complexes is much more bright and easily detectable
than the phosphorescent emission of the free ligands.
Furthermore, the emission of the free ligands is domi-
Ack n ow led gm en t. We thank the Natural Sciences
and Engineering Research Council of Canada for finan-
cial support.
Su p p or tin g In for m a tion Ava ila ble: Complete X-ray
diffraction data for 1-5 and 8, including tables of atomic
coordinates, thermal parameters, bond lengths and angles, and
hydrogen parameters, and complete luminescent spectra of
1-8. This material is available free of charge via the Internet
at http://pubs.acs.org.
OM030262K