Novel phosphorescent cyclometalated organotin(IV) and organolead(IV) complexes of 2,6-bis(2′-indolyl)pyridine and 2,6-bis[2′-(7-azaindolyl)]pyridine

Article abstract of DOI:10.1021/om030404a

Four novel five-coordinated Sn(IV) and Pb(IV) complexes of 2,6-bis(2′-indolyl)pyridine (H₂-bip) and 2,6-bis[2′-(7-azaindolyl)]pyridine (H₂bap) were synthesized. These four complexes have the formulas Sn(bip)Ph₂ (1), Sn(bap)Ph₂ (2), Pb(bip)Ph₂ (3), and Pb(bap)Ph₂ (4), respectively. The structures were determined by single-crystal X-ray diffraction. In the complexes, the metal centers are tridentately chelated by the bip or the bap ligand and further coordinated by two phenyl groups, resulting in a distorted-trigonal-bipyramidal geometry. These complexes display interesting extended π-π stack structures in the solid state. Complexes 1 and 2 are luminescent at room temperature in solution and the solid state. At 77 K in THF solution, all four complexes display both blue-green fluorescent and orange-red phosphorescent emissions, which originate from ligand-centered π → π* transitions. The Sn(IV) and Pb(IV) centers play a key role in enhancing phosphorescent emission of the bip and bap ligands.

Full text of DOI:10.1021/om030404a

4070
Organometallics 2003, 22, 4070-4078
Novel P h osp h or escen t Cyclom eta la ted Or ga n otin (IV)
a n d Or ga n olea d (IV) Com p lexes of
2,6-Bis(2′-in d olyl)p yr id in e a n d
2,6-Bis[2′-(7-a za in d olyl)]p yr id in e
Wen-Li J ia, Qin-De Liu, Ruiyao Wang, and Suning Wang*
Department of Chemistry, Queen’s University, Kingston, Ontario K7L 3N6, Canada
Received May 29, 2003
Four novel five-coordinated Sn(IV) and Pb(IV) complexes of 2,6-bis(2′-indolyl)pyridine (H2-
bip) and 2,6-bis[2′-(7-azaindolyl)]pyridine (H2bap) were synthesized. These four complexes
have the formulas Sn(bip)Ph2 (1), Sn(bap)Ph2 (2), Pb(bip)Ph2 (3), and Pb(bap)Ph2 (4),
respectively. The structures were determined by single-crystal X-ray diffraction. In the
complexes, the metal centers are tridentately chelated by the bip or the bap ligand and
further coordinated by two phenyl groups, resulting in a distorted-trigonal-bipyramidal
geometry. These complexes display interesting extended π-π stack structures in the solid
state. Complexes 1 and 2 are luminescent at room temperature in solution and the solid
state. At 77 K in THF solution, all four complexes display both blue-green fluorescent and
orange-red phosphorescent emissions, which originate from ligand-centered π f π* transi-
tions. The Sn(IV) and Pb(IV) centers play a key role in enhancing phosphorescent emission
of the bip and bap ligands.
In tr od u ction
as it has been demonstrated that metal centers can play
a key role in promoting singlet-triplet state mixing and,
hence, phosphorescent emission.4 Many recent studies
on phosphorescent compounds concern metal complexes
involving d-block metal centers such as Pt(II) and Ir-
(III).4,5 Due to the presence of the incomplete 5d shell,
these transition-metal ions very often quench the emis-
sion from the ligand, although phosphorescence due to
MLCT transitions has been observed frequently. Among
the phosphorescent organometallic compounds, cyclo-
metalated complexes are the most attractive, due to
their high stability and rigidity that reduces the loss of
energy by thermal vibrational decay.6 Since heavy main-
group metals, such as lead and bismuth, also display
strong spin-orbital coupling, they could play the same
role as heavy transition-metal atoms in promoting
phosphorescent emission. Furthermore, (n - 1)d elec-
trons in the main-group metal centers usually do not
interfere with the electronic transitions of the ligand
Luminescent organic/organometallic compounds have
attracted much attention recently due to their potential
applications in sensor technologies and photochemical
and electroluminescent devices.1-3 In comparison with
fluorescent compounds, phosphorescent compounds are
more attractive for applications in organic light-emitting
devices (OLEDs) because of their potential in the
enhancement of the overall device efficiency.4 However,
the emissions of most luminescent organic compounds
are dominated by fluorescence, and their phosphores-
cent emissions are either negligible or very weak. To
promote the phosphorescent emissions, heavy-metal
centers are often introduced into the organic ligands,
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10.1021/om030404a CCC: $25.00 © 2003 American Chemical Society
Publication on Web 08/28/2003

Sn and Pb Complexes of Indolyl-Substituted Pyridines
Organometallics, Vol. 22, No. 20, 2003 4071
purification. All solvent used in syntheses and spectroscopic
measurements were distilled over appropriate drying reagents.
1H, 13C, and 119Sn NMR spectra were recorded on a Bruker
Ch a r t 1
1
Advance 500 spectrometer operating at 500 MHz for H, 125.7
MHz for 13C, and 186.4 MHz for 119Sn, respectively. Excitation
and emission spectra were obtained with a Photon Technolo-
gies International QuantaMaster Model C-60 spectrometer.
Emission lifetime was measured on a Photon Technologies
International Phosphorescent lifetime spectrometer, Time-
master 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 spectro-
photometer. Elemental analyses were performed by Canadian
Microanalytical Service Ltd., Delta, British Columbia, Canada.
The free ligand10 2,6-bis(2′-indolyl)pyridine and PbCl2Ph2 were
prepared using a previously reported procedure.11
because of their closed shell. On the other hand, the
availability of the unoccupied nd orbitals of the heavy
main-group elements allows valence expansion and the
accommodation of multidentate ligands. Despite the
potential of main-group phosphorescent compounds,
reports concerning phosphorescent emission promoted
by main-group-metal centers are surprisingly scarce. We
have reported a number of phosphorescent group 15
compounds recently that contain terminal aryl ligands
functionalized by 7-azaindolyl or 2,2′-dipyridylamino
groups, where the heavier group 15 elements were
shown to drastically enhance phosphorescent emission
of the ligands.7 Although many luminescent cyclometa-
lated complexes with d-block metals have been re-
ported,8 phosphorescent cyclometalated main-group com-
pounds are rare. The heavy elements of group 14 at
oxidation state +4 are known to display versatile
coordination numbers and geometries and, hence, may
be suitable for the formation of cyclometalated com-
pounds. To our knowledge, no phosphorescent Pb(IV)
or Sn(IV) cyclometalated complexes have been reported
before. Our investigation therefore focused on cyclom-
etalated Sn(IV) and Pb(IV) compounds. This investiga-
tion is based on the results of our recent work on several
cycloplatinated complexes with 2,6-bis(2′-indolyl)pyri-
dine (H2bip), Pt(bip)L, which emit bright orange-red
phosphorescence originating from ligand-centered π f
π* transitions (Chart 1) and were demonstrated to be
useful emitters in OLEDs.9 Our work on the Pt(II) bip
complexes indicated that the bip ligand is potentially
useful for the emission of phosphorescence in the
presence of a metal ion. Herein we report the results of
our investigation on using the H2bip ligand and the
related 2,6-bis[2′-(7-azaindolyl)]pyridine (H2bap) ligand
to form phosphorescent cyclometalated Sn(IV) and Pb-
(IV) complexes.
Syn t h esis of 2,6-Bis[1′-(p h en ylsu lfon yl)-2′-(7′-a za in -
d olyl)]p yr id in e. To a solution of N-(phenylsulfonyl)-7-aza-
indole (5.835 g, 22.5 mmol) in 40 mL of THF at 0 °C was slowly
added a solution of LDA (1.5 M in cyclohexane, 16.6 mL, 24.9
mmol). The resulting mixture was stirred for 30 min at this
temperature, and then a solution of ZnCl2 (0.5M in THF, 49.8
mL, 24.9 mmol) was added. The mixture was stirred at room
temperature for another 30 min. In a separate flask a solution
of 2,6-dibromopyridine (2.06 g, 9.69 mmol) in 15 mL of THF
was added to a solution containing a catalyst prepared by the
reaction of Pd(PPh3)2Cl2 (0.489 g, 0.696 mmol) in 10 mL of THF
with diisobutylaluminum hydride (1.0 M in hexane, 1.39 mL,
1.39 mmol), and the mixture was stirred for 20 min at room
temperature. The resulting mixture was refluxed for 6 h,
cooled to room temperature, and poured into saturated aque-
ous Na2CO3. The aqueous phase was extracted with Et2O, and
the organic extracts were concentrated to give a brown residue,
which was purified by column chromatography (hexane/CH3-
CO2Et, 6/1) to obtain the product in 57% yield. 1H NMR
(CDCl3; δ, ppm): 8.60 (dd, 2H, J ) 4.8, 1.5 Hz), 7.95-8.00 (m,
7H), 7.77 (d, 2H, J ) 7.8 Hz), 7.32 (dd, 2H, J ) 7.8, 1.8 Hz),
7.07 (t, 2H, J ) 7.5 Hz), 6.91 (s, 2H), 6.68 (dd, 4H, J ) 8.4, 1.5
Hz).
Syn th esis of 2,6-Bis[2′-(7′-azain dolyl)]pyr idin e (H2bap).
A mixture of 2,6-bis[1′-(phenylsulfonyl)-2′-(7′-azaindolyl)]py-
ridine (1.92 g, 3.25 mmol) in EtOH (320 mL) and 10% aqueous
NaOH (30 mL) was heated at reflux overnight. The resulting
mixture was concentrated, and the residue was dissolved in
CH2Cl2. The solution was washed with water and aqueous Na2-
CO3, dried, and concentrated. Column chromatography (THF/
hexane, 1/1) of the residue afforded a yellow compound in 90%
1
yield. H NMR (DMSO-d6; δ, ppm): 8.33 (d, 2H, J ) 4.4 Hz),
8.04 (d, 2H, J ) 7.8 Hz), 8.00 (d, 2H, J ) 7.0 Hz), 7.94 (t, 1H,
J ) 6.6 Hz), 7.29 (s, 2H), 7.13 (dd, 2H, J ) 7.8, 4.6 Hz). 13C
NMR (DMSO-d6; δ, ppm): 150.0, 145.0, 138.8, 138.2, 129.8,
125.8, 121.8, 119.4, 117.1, 100.0.
Exp er im en ta l Section
All starting materials were analytical reagents purchased
from Aldrich Chemical Co. and were used without further
Sn (bip )P h 2 (1). Under N2 protection, 2,6-bis(2′-indolyl)-
pyridine (H2bip, 150 mg, 0.48 mmol) was dissolved in 10 mL
of Na that was pretreated and freshly distilled from THF. The
solution was cooled to -78 °C with dry ice/acetone. Then 0.64
mL of 1.6 M butyllithium solution (1.02 mmol) in hexane was
added slowly. After the mixture was stirred for 40 min at -78
°C, 10 mL of a THF solution containing 171 mg (0.50 mmol)
of SnCl2Ph2 was added. The solution was stirred for 40 min
at -78 °C and warmed to ambient temperature. The reaction
mixture was stirred for another 2 h, the solvent was removed
under vacuum, and the residue was dissolved in 10 mL of CH2-
Cl2. After the white insoluble solid (LiCl) was removed by
filtration, anhydrous toluene (5 mL) was layered upon the
(7) Kang, Y.; Song, D.; Schmider, H.; Wang, S. Organometallics
2002, 21, 2413.
(8) (a) McMillin, D. R.; Moore, J . J . Coord. Chem. Rev. 2002, 229,
113 and references therein. (b) Field, J . S.; Haines, R. J .; McMillin, D.
R.; Summerton, G. C. J . Chem. Soc., Dalton Trans. 2002, 1369. (c)
Moore, J . J .; Nash, J . J .; Fanwick, P. E.; McMillin, D. R. Inorg. Chem.
2002, 41, 6387. (d) Yutaka, T.; Mori, I.; Kurihara, M.; Mizutani, J .;
Tamai, N.; Kawai, T.; Irie, M.; Nishihara, H. Inorg. Chem. 2002, 41,
7143. (e) Yang, Q. Z.; Wu, L. Z.; Wu, Z. X.; Zhang, L. P.; Tung, C. H.
Inorg. Chem. 2002, 41, 5653. (f) Yam, V. W. W.; Wong, K. M. C.; Zhu,
N. J . Am. Chem. Soc. 2002, 124, 6506. (g) Yam, V. W. W.; Hui, C. K.;
Wong, K. M. C.; Zhu, N.; Cheung, K. K. Organometallics 2002, 21,
4326. (h) Che, C. M.; Zhang, J . L.; Lin, L. R. Chem. Commun. 2002,
2556. (i) Che, C. M.; Fu, W. F.; Lai, S. W.; Hou, Y. J .; Liu, Y. L. Chem.
Commun. 2003, 118. (j) Lu, W.; Zhu, N.; Che, C. M. Chem. Commun.
2002, 900. (k) Hui, C. K.; Chu, B. W. K.; Zhu, N.; Yam, V. W. W. Inorg.
Chem. 2002, 41, 6178. (l) Lamansky, S.; Djurovich, P. I.; Abdel-Razzq,
F.; Garon, S.; Murphy, D. L.; Thompson, M. E. J . Appl. Phys. 2002,
92, 1570.
(9) Liu, Q. D.; Thorne, L.; Kozin, I.; Song, D.; Seward. C.; D’Iorio,
M.; Tao, Y.; Wang, S. J . Chem. Soc., Dalton Trans. 2002, 3234.
(10) Thummel, R. P.; Hegde, V. J . Org. Chem. 1989, 54, 1720.
(11) Gilman, H.; Robinson, J . D. J . Am. Chem. Soc. 1929, 51, 3112.

4072 Organometallics, Vol. 22, No. 20, 2003
J ia et al.
Ta ble 1. Cr ysta l Da ta
2
1
3
4
formula
fw
cryst syst
space group
a/Å
b/Å
c/Å
C
33H23N3Sn‚C7H8
C
31H21N5Sn‚CH2Cl2
C
33H23N3Pb‚CH2Cl2
C31H21N5Pb‚0.5H2O
672.37
triclinic
P1h
667.14
triclinic
P1h
753.66
triclinic
P1h
679.73
triclinic
P1h
9.480(3)
12.952(4)
14.564(5)
66.301(5)
76.902(5)
79.502(5)
1586.6(9)
2
1.407
0.838
56.60
11 545
7327 (0.0248)
378
11.304(16)
11.507(17)
13.823(18)
69.03(2)
69.25(2)
60.587(19)
1427(3)
2
1.553
1.114
56.76
10 513
11.969(2)
13.740(3)
19.676(3)
79.344(4)
85.782(4)
67.230(4)
2932.2(9)
4
1.707
5.964
56.66
21 247
11.894(3)
13.443(3)
17.985(5)
94.283(6)
92.072(5)
114.293(5)
2606.6(11)
4
R/deg
â/deg
γ/deg
V/Å3
Z
Dc/g cm-3
1.732
6.504
56.62
18 699
11 912 (0.0548)
676
µ/mm-1
2θmax/deg
no. of rflns measd
no. of rflns used (Rint
no. of params
final R (I > 2σ(I))
R1a
)
6635 (0.0557)
367
13 458 (0.0145)
721
0.0563
0.1483
0.0555
0.0893
0.0292
0.0581
0.0582
0.0776
wR2b
R (all data)
R1a
0.0902
0.1603
0.936
0.1824
0.1077
0.696
0.0532
0.0616
0.871
0.2010
0.1003
0.768
wR2b
goodness of fit on F2
R1 ) ∑[|Fo| - |Fc|]/∑|Fo|. wR2 ) {∑[w(Fo - Fc2)]/∑(wFo2)}1/2, w ) 1/[σ2(Fo2) + (0.075P)2], P ) [Max(Fo2,0) + 2Fc2]/3.
a
b
2
filtrate. The two-layered solution was then allowed to stand
at ambient temperature. Slow diffusion and evaporation of the
solvent afforded 1 as orange crystals, which lost crystal lattice
solvent quickly and became a yellow powder after several days
(188 mg, 68% yield). 1H NMR (CD2Cl2; δ, ppm): 7.98 (t, 1H, J
) 7.4 Hz, Py), 7.75-7.78 (m, 4H, Py and indolyl), 7.70 (dd,
4H, J ) 7.8, 1.7 Hz, Ph), 7.47 (dd, 2H, J ) 8.2, 0.8 Hz, indolyl),
7.30-7.37 (m, 8H, Ph and indolyl), 7.20 (ddd, 2H, J ) 8.2, 6.9,
1.3 Hz, indolyl), 7.10 (ddd, 2H, J ) 8.0, 7.0, 1.0 Hz, indolyl).
13C NMR (CD2Cl2; δ, ppm): 149.8, 145.2, 143.0, 140.2, 137.6,
136.5 (with satellites, J Sn-C ) 61.9 Hz), 133.7, 131.9, 129.7
(with satellites, J Sn-C ) 83.9 Hz), 124.2, 122.5, 120.2, 117.3,
115.6, 103.6. 119Sn NMR (CD2Cl2; δ, ppm, vs SnMe4): -299.6.
Anal. Calcd for C33H23N3Sn: C, 68.30; H, 4.00; N, 7.25.
Found: C, 68.44; H, 4.17; N, 7.12.
169.1 Hz), 131.0, 123.4, 121.9, 119.4, 117.6, 114.8, 103.0. Anal.
Calcd for C33H23N3Pb‚C7H8: C, 63.14; H, 4.11; N, 5.52.
Found: C, 62.70; H, 4.26; N, 5.39.
P b(ba p )P h 2 (4). This complex was obtained in the same
manner as that described for 2. The reaction of H2bap (100
mg, 0.32 mmol) and PbCl2Ph2 (140 mg, 0.32 mmol) provided 4
as orange crystals (135 mg, 63% yield). 1H NMR (CD2Cl2; δ,
ppm): 8.46 (dd, 2H, J ) 4.6, 1.6 Hz, 7-azaindolyl), 8.14 (dd,
2H, J ) 7.9, 1.6 Hz, 7-azaindolyl), 7.91-7.95 (m, 5H, Py and
Ph), 7.83 (d, 2H, J ) 7.7 Hz, Py), 7.36 (t, 4H, J ) 7.6 Hz, Ph),
7.31 (tt, 2H, J ) 7.4, 1.5 Hz, Ph), 7.23 (s, 2H, 7-azaindolyl),
7.11 (dd, 2H, J ) 7.9, 4.6 Hz, 7-azaindolyl). 13C NMR (CD2Cl2;
δ, ppm): 159.3, 157.0, 151.2, 145.0, 141.0, 137.8, 135.4 (with
satellites, J Pb-C ) 125.7 Hz), 130.7 (with satellites, J Pb-C
)
163.4 Hz), 129.4, 129.2, 124.8, 118.2, 115.5, 100.7. Anal. Calcd
for C33H23N3Pb‚0.5H2O: C, 54.79; H, 3.24; N, 10.31. Found:
C, 55.13; H, 3.32; N, 9.89.
Sn (ba p )P h 2 (2). This complex was obtained in the same
manner as that described for 1, except using hexane instead
of toluene. The reaction of H2bap (150 mg, 0.48 mmol) and
SnCl2Ph2 (171 mg, 0.50 mmol) provided 2 as yellow crystals
X-r a y Cr ysta llogr a p h ic An a lysis. Single crystals of the
free ligands H2bip and H2bap were obtained from slow
evaporation of THF solutions. Crystals for 1 and 3 were
obtained from slow diffusion of toluene into CH2Cl2 solutions,
while crystals of 2 and 4 were obtained from slow diffusion of
hexane into CH2Cl2 solutions. All crystals except that of 1 were
mounted on glass fibers for data collection. Single crystals of
1 contain toluene solvent molecules and lose the solvent
molecule rapidly. Therefore, the crystal of 1 was sealed in a
thin-walled glass capillary for the structure determination.
Data were collected on a Siemens P4 single-crystal X-ray
diffractometer with a CCD-1000 detector and graphite-mono-
chromated Mo KR radiation, operating at 50 kV and 30 mA
at 25 °C. Two different forms of single crystals of complex 3
were isolated and analyzed, one of which contains one CH2-
Cl2 per molecule of 3 and the other of which contains 1.5
toluene molecule per molecule of 3. The data collection ranges
over the 2θ ranges are 4.00-56.58° for H2bip, 3.10-56.60° for
H2bap and 1, 3.24-56.76° for 2, 3.26-56.66° for 3‚CH2Cl2,
3.04-56.64° for 3‚1.5C7H8, and 3.34-56.62° for 4. No signifi-
cant decay was observed for any sample. Data were processed
on a PC using the Bruker SHELXTL software package12
(version 5.10) and are corrected for Lorentz and polarization
effects. The free ligands H2bip and H2bap crystallize in the
1
(230 mg, 72% yield). H NMR (CD2Cl2; δ, ppm): 8.49 (dd, 2H,
J ) 4.5, 2.0 Hz, 7-azaindolyl), 8.10 (dd, 2H, J ) 7.5, 2.0 Hz,
7-azaindolyl), 8.05 (t, 1H, J ) 8.0 Hz, Py), 8.01 (dd, 4H, J )
8.0, 1.5 Hz, Ph), 7.85 (d, 2H, J ) 8.0 Hz, Py), 7.33 (tt, 2H, J )
7.5, 1.5 Hz, Ph), 7.30 (s, 2H, 7-azaindolyl), 7.26 (t, 4H, J ) 7.5
Hz, Ph), 7.13 (dd, 2H, J ) 8.0, 4.5 Hz, 7-azaindolyl). 13C NMR
(CD2Cl2; δ, ppm): 155.8, 149.6, 145.9, 142.8, 137.5, 137.1,
130.5, 129.9, 128.9, 125.0, 117.8, 116.3, 101.4 (one quaternary
carbon hidden). 119Sn NMR (CD2Cl2; δ, ppm, vs SnMe4):
-289.2. Anal. Calcd for C33H23N3Sn‚1/3CH2Cl2: C, 61.60; H,
3.44; N, 11.47. Found: C, 61.69; H, 3.86; N, 11.87.
P b(bip )P h 2 (3). This complex was obtained in the same
manner as that described for 1. The reaction of H2bip (100
mg, 0.32 mmol) and PbCl2Ph2 (140 mg, 0.32 mmol) provided 3
as orange crystals at first, which changed to dark red crystals
after several days due to the replacement of crystal lattice
molecules from CH2Cl2 to toluene (165 mg, 64% yield). 1H NMR
(CD2Cl2; δ, ppm): 7.92 (t, 1H, J ) 8.0 Hz, Py), 7.82 (d, 2H, J
) 8.0 Hz, indolyl), 7.79 (d, 2H, J ) 8.0 Hz, Py), 7.77 (dd, 2H,
J ) 8.2, 0.8 Hz, indolyl), 7.74 (dd, 4H, J ) 8.2, 1.3 Hz, Ph),
7.41 (t, 4H, J ) 8.0 Hz, Ph), 7.35 (tt, 2H, J ) 8.0, 1.3 Hz, Ph),
7.33 (s, 2H, indolyl), 7.30 (ddd, 2H, J ) 8.1, 6.9, 1.2 Hz, indolyl),
7.14 (ddd, 2H, J ) 7.9, 6.9, 0.8 Hz, indolyl). 13C NMR (CD2Cl2;
δ, ppm): 157.6, 151.2, 146.6, 141.2, 140.2, 135.4 (with satel-
(12) SHELXTL NT Crystal Structure Analysis Package, version
5.10; Bruker AXS, Analytical X-ray System, Madison, WI, 1999.
lites, J Pb-C ) 124.7 Hz), 133.0, 131.1 (with satellites, J Pb-C
)

Sn and Pb Complexes of Indolyl-Substituted Pyridines
Organometallics, Vol. 22, No. 20, 2003 4073
Ta ble 2. Selected Bon d Len gth s (Å) a n d An gles
Sch em e 1
(d eg) for Com p lexes 1-4
Complex 1
Sn(1)-C(1)
Sn(1)-N(1)
Sn(1)-N(3)
2.123(5)
2.163(4)
2.162(5)
Sn(1)-C(7)
Sn(1)-N(2)
2.102(5)
2.180(2)
N(1)-Sn(1)-N(2) 148.19(19) N(1)-Sn(1)-N(3)
74.42(18)
96.22(18)
95.35(18)
N(1)-Sn(1)-C(1)
N(2)-Sn(1)-N(3)
N(2)-Sn(1)-C(7)
99.18(17) N(1)-Sn(1)-C(7)
73.77(18) N(2)-Sn(1)-C(1)
97.75(18) N(3)-Sn(1)-C(1) 116.45(17)
N(3)-Sn(1)-C(7) 117.38(17) C(1)-Sn(1)-C(7) 126.15(19)
Complex 2
Sn(1)-C(1)
Sn(1)-N(1)
Sn(1)-N(3)
2.079(7)
2.154(6)
2.224(6)
Sn(1)-C(7)
Sn(1)-N(2)
2.121(6)
2.176(6)
N(1)-Sn(1)-N(2)
N(1)-Sn(1)-C(1)
N(2)-Sn(1)-N(3)
N(2)-Sn(1)-C(7)
N(3)-Sn(1)-C(7)
145.9(2)
99.7(3)
73.3(2)
98.3(3)
115.9(2)
N(1)-Sn(1)-N(3)
N(1)-Sn(1)-C(7)
N(2)-Sn(1)-C(1)
N(3)-Sn(1)-C(1)
C(1)-Sn(1)-C(7)
72.6(2)
94.8(3)
96.5(3)
115.5(2)
128.6(2)
Complex 3‚CH2Cl2
Resu lts a n d Discu ssion
Pb(1)-C(1)
Pb(1)-N(1)
Pb(1)-N(3)
2.161(4)
Pb(1)-C(7)
Pb(1)-N(2)
2.177(4)
2.317(3)
2.296(3)
2.295(3)
Liga n d s. The free ligand H2bip was prepared in good
yield by a two-step Fischer synthesis involving the
coupling of 2,6-diacetylpyridine with 2 equiv of phenyl-
hydrazine, according to previously reported methods.9,10
However, using the same procedure to synthesize H2-
bap by the coupling reaction of 2,6-diacetylpyridine with
2 equiv of 2-pyridylhydrazine was unsuccessful. The
extra electronegative nitrogen atom in the 2-pyridylhy-
drazine molecule appears to have a dramatic effect on
its reactivity in the Fischer cyclization reaction. The H2-
bap ligand was synthesized by using a modified litera-
ture procedure, as shown in Scheme 1.15 The NH site
on the 7-azaindole molecule was first protected by a
phenylsulfonyl group. The required [2-(7-azaindolyl)]-
zinc chloride was easily obtained by metalation of
1-(phenylsulfonyl)-7-azaindole with LDA followed by the
treatment of the resulting 2-lithium-7-azaindole with
anhydrous ZnCl2. Cross-coupling reactions of [2-(7-
azaindolyl)]zinc chloride with 2,6-dibromopyridine in the
presence of 2 mol % of a catalyst prepared from PdCl2-
(PPh3)2 and DIBAH (DIBAH ) diisobutylaluminum
hydride) in refluxing THF and the subsequent removal
of the protecting group SO2Ph afforded the H2bap ligand
in good yield. To study the impact of the coordination
to a metal ion on the structure of the ligand, we
examined the structures of H2bip and H2bap by single-
crystal X-ray diffraction experiments.
The molecular structures of the free ligands are
shown in Figure 1. Each molecule of H2bip cocrystallized
with one H2O molecule, which forms two weak hydrogen
bonds with the NH groups of the H2bip ligand (Figure
1, top), as indicated by the O‚‚‚N distances (3.006(5) and
3.028(5) Å, respectively). The mean deviation of atoms
from the molecular plane is 0.047 Å, indicating that the
pyridine ring and the two indolyl rings are coplanar.
However, for H2bap, the pyridine ring is not coplanar
with the two 7-azaindolyl rings, and the dihedral angles
between the 7-azaindolyl ring and the pyridine ring are
15.1 and 27.1°, respectively. This difference can be
N(1)-Pb(1)-N(2) 142.36(13) N(1)-Pb(1)-N(3)
71.24(12)
94.10(13)
95.17(15)
N(1)-Pb(1)-C(1)
N(2)-Pb(1)-N(3)
N(2)-Pb(1)-C(7)
98.42(15) N(1)-Pb(1)-C(7)
71.12(13) N(2)-Pb(1)-C(1)
98.85(13) N(3)-Pb(1)-C(1) 111.58(13)
N(3)-Pb(1)-C(7) 110.23(13) C(1)-Pb(1)-C(7) 138.19(15)
Complex 4
Pb(1)-C(1)
Pb(1)-N(1)
Pb(1)-N(3)
Pb(2)-C(38)
Pb(2)-N(7)
2.182(10)
2.392(11)
2.374(11)
2.161(12)
2.367(9)
Pb(1)-C(7)
Pb(1)-N(2)
Pb(2)-C(32)
Pb(2)-N(6)
Pb(2)-N(8)
2.175(10)
2.366(9)
2.136(12)
2.335(9)
2.340(10)
N(1)-Pb(1)-N(2)
N(1)-Pb(1)-C(1)
N(2)-Pb(1)-N(3)
N(2)-Pb(1)-C(7)
N(3)-Pb(1)-C(7)
N(6)-Pb(2)-N(7)
N(6)-Pb(2)-C(32)
N(7)-Pb(2)-N(8)
N(7)-Pb(2)-C(38)
138.3(5) N(1)-Pb(1)-N(3)
93.1(4) N(1)-Pb(1)-C(7)
68.0(4) N(2)-Pb(1)-C(1)
95.6(4) N(3)-Pb(1)-C(1)
105.0(4) C(1)-Pb(1)-C(7)
138.5(5) N(6)-Pb(2)-N(8)
95.8(4) N(6)-Pb(2)-N(38)
69.2(5) N(7)-Pb(2)-C(32)
94.1(4) N(8)-Pb(2)-C(32)
70.5(5)
91.1(4)
98.2(4)
100.5(3)
154.0(4)
69.3(4)
99.8(4)
92.8(4)
101.1(4)
N(8)-Pb(2)-C(38) 111.2(4) C(32)-Pb(2)-C(38) 147.4(5)
monoclinic space group P21/c, and the crystals of all the
complexes belong to the triclinic space group P1h, but with unit
cell parameters quite different from each other. All structures
were solved by direct methods. One toluene per molecule of 1
was located in the crystal lattice of 1. One CH2Cl2 molecule
per molecule of 2 was located in the crystal lattice of 2.
Compound 4 contains 0.5 H2O molecule per molecule of 4. The
free ligand H2bip cocrystallizes with one H2O per molecule.
All non-hydrogen atoms except for some disordered crystal
lattice toluene and CH2Cl2 were refined anisotropically. All
hydrogen atoms in H2bip and H2bap except H7A and H15A in
H2bip were located directly from difference Fourier maps.
Other hydrogen atoms were calculated, and their contributions
were included. The crystallographic data, except for those of
the free ligands and 3‚1.5C7H8, are given in Table 1. The data
for the free ligands and 3‚1.5C7H8 are provided in the Sup-
porting Information. Selected bond lengths and angles for the
complexes are given in Table 2.
Qu an tu m Yield Measu r em en ts. Emission quantum yields
of complexes 1 and 2 were determined relative to 9,10-
diphenylanthracene in THF at 298 K (Θr ) 0.95).13 The
absorbances of all samples and the standard at the excitation
wavelength were approximately 0.096-0.104. The quantum
yields were calculated using previously reported procedures.14
The excitation wavelength used for all samples is 400 nm.
(13) Murov, S. L.; Carmichael, I.; Hug, G. L. Handbook of Photo-
chemistry, 2nd ed.; Marcel Dekker: New York, 1993.
(14) Demas, N. J .; Crosby, G. A. J . Am. Chem. Soc. 1970, 92, 7262.
(15) Liu, S. F.; Wu, Q.; Schmider, H. L.; Aziz, H.; Hu, N.-X.; Popovic,
Z.; Wang, S. J . Am. Chem. Soc. 2000, 122, 3671.

4074 Organometallics, Vol. 22, No. 20, 2003
J ia et al.
F igu r e 2. Hydrogen-bonded double-strand structure of H2-
bap: (top) diagram showing the hydrogen bond links;
(bottom) space-filling diagram showing the hydrogen-
bonded double strands. The gray atoms denote nitrogen
and the white atoms hydrogen.
Sch em e 2
F igu r e 1. Molecular structures of H2bip (top) and H2bap
(bottom) with 50% thermal ellipsoids and labeling schemes.
attributed to the presence of the hydrogen bonds in both
molecules. For H2bip, the chelation to the two indolyl
rings by the H2O molecule through hydrogen bonds
clearly favors the coplanarity of the H2bip ligand. For
H2bap, intermolecular hydrogen bonds between H2bap
molecules are believed to be responsible for its non-
planar structure. Although the molecular structures of
H2bip and H2bap resemble each other, the extended
structures of these two compounds in the crystal lattice
are quite different. There is no interligand hydrogen
bond present in the crystal of H2bip due to the lack of
accessible hydrogen acceptor sites. (The N(3) site is
blocked, and the same is true for H2bap.) In contrast,
for H2bap, N(1) and N(2) are hydrogen donors, while
N(4) and N(5) atoms are hydrogen acceptors. Conse-
quently, in the crystal lattice, the N(1) atom is paired
with N(4A) of one neighboring molecule, while the N(2)
atom is paired with N(5B) of the second neighboring
molecule through hydrogen bonds, as evidenced by the
distances of N(1)‚‚‚N(4A) (2.950(4) Å) and N(2)‚‚‚N(5B)
(2.899(4) Å). The molecules of H2bap couple through
hydrogen bonds a manner that leads to the formation
of an unusual double-strand structure, as shown in
Figure 2. In addition to hydrogen bonds, shape matching
between the H2bap molecules also plays a role in the
formation of the double-strand structure. As shown by
Figure 2, there is a perfect shape match between each
pair of molecules in the double strands. Clearly in order
to maximize intermolecular hydrogen-bonding inter-
actions, the two 7-azaindolyl rings must be out of plane
with the pyridyl ring to minimizing nonbonding inter-
actions, thus causing the nonplanarity of the molecule.
Com p lexes. As shown in Chart 1, to complete the
coordination sphere of the Pt(II) center, a neutral
ancillary ligand L is required to form the neutral
complex Pt(bip)L. In the case of Sn(IV) and Pb(IV)
complexes, two 1- charged ligands are needed to
saturate the coordination sphere of the metal center and
to provide a neutral complex of M(bip)L2 or M(bap)L2.
For this purpose, we chose MCl2Ph2 (M ) Sn, Pb) as
our starting materials. Although our attempts to syn-
thesize Pt(bap)L have not been successful, the bap
ligand was found, however, to be able to bind to Sn(IV)
and Pb(IV) centers in the same manner as the bip
ligand.
Complexes 1-4 were synthesized in good yield by
substitution reactions where MCl2Ph2 (M ) Sn, Pb) was
reacted with the corresponding salt Li2L (L ) bip, bap)
in a 1/1 ratio. The dilithium salts Li2L were prepared
from the reaction of n-BuLi and H2L. The 2- charged
ligand (bip or bap) replaced the Cl- ions and tridentately
chelated to the metal center, forming cyclometalated
complexes with LiCl as byproduct. The syntheses of
complexes 1-4 are summarized in Scheme 2. All
1
complexes have been fully characterized by H and 13C
NMR and elemental analyses. The 119Sn NMR spectra
were also recorded for complexes 1 and 2, which display
119Sn chemical shifts at -299.6 and -289.2 ppm (rela-
tive to SnMe4), respectively, consistent with previously
reported values for five-coordinated organotin(IV) com-
plexes.16 Attempts were made to record 207Pb NMR
spectra for complexes 3 and 4. However, due to the poor
solubility of compounds 3 and 4, the 207Pb chemical
shifts for both complexes could not be located. Com-

Sn and Pb Complexes of Indolyl-Substituted Pyridines
Organometallics, Vol. 22, No. 20, 2003 4075
F igu r e 4. Molecular structure of complex 3 with 50%
thermal ellipsoids and labeling schemes.
F igu r e 3. Molecular structure of complex 1 with 50%
thermal ellipsoids and labeling schemes.
and coplanar with the Sn(IV) center, as indicated by the
mean deviation of the Sn(1)N(1)N(2)N(3) plane being
0.005 Å for 1 and 0.015 Å for 2, respectively. Clearly
the chelation of the Sn(IV) ion to the bip and bap ligands
reinforces the planarity of the ligands. The two phenyl
groups are located above and below the bip or bap plane,
respectively. The Sn-C bond lengths are in the typical
range.17,18d However, the Sn-N bond lengths (average
2.177(5) Å) in both compounds are substantially shorter
than most previously reported Sn-N bond lengths.17a-d
The Sn-N bond lengths in 1 and 2 are close to those
plexes 1-4 are stable under air in the solid state. The
Sn(IV) complexes 1 and 2 are also stable in solution
upon exposure to air. The Pb(IV) complexes 3 and 4, in
contrast, undergo slow decomposition in solution upon
exposure to air. We also attempted the syntheses of Si-
(bip)Ph2 and Si(bap)Ph2 using the same procedure.
However, these complexes are thermally unstable and
undergo rapid decomposition upon isolation at ambient
temperature. As a result, complete characterization for
these two Si(IV) compounds could not be achieved.
Although a number of organotin(IV) complexes involving
cyclometalated ligands have been known previously,
well-characterized organolead(IV) complexes containing
cyclometalated ligands are scarce.17,18 To fully establish
the structures of the new Sn(IV) and Pb(IV) complexes,
we carried out single-crystal X-ray diffraction analyses
for all four complexes.
Cr ysta l Str u ctu r es. Complexes 1 and 2 have very
similar molecular structures. The molecular structure
of complex 1 is shown in Figure 3. In complexes 1 and
2, the Sn(IV) center is five-coordinated. Each metal
center is tridentately chelated by a bip or bap ligand
via the N(1), N(2), and N(3) atoms in the same manner
as in Pt(bip)L complexes.9 In addition, two phenyl
groups are coordinated to the metal center, forming a
distorted-trigonal-bipyramidal geometry, as indicated by
the N(1)-Sn(1)-N(2) angles of 148.19(19)° for 1 and
145.9(2)° for 2, respectively, which are all much smaller
than the ideal bond angle of an sp3d hybrid orbital
(180°). The bip and bap ligands in 1 and 2 are planar
observed in [Sn(tmtaa)Me]+ and {[Sn(tmtaa)]2(CH2)}2+
,
where tmtaa is the macrocyclic ligand tetramethyl-
tetraazadibenzo[14]annulene dianion.17e,f The closest
intermolecular Sn‚‚‚Sn distances are 7.360(5) Å for 1
and 7.527(7) Å for 2, respectively.
The molecular structures of complexes 3 and 4 are
similar and resemble those of complexes 1 and 2. In the
asymmetric units of complexes 3 and 4, there are two
independent molecules that display essentially the same
structure. Complex 3 has two kinds of crystals with the
same molecular structure but different crystal lattice
solvents (CH2Cl2 and toluene, respectively). The discus-
sion here focuses on the CH2Cl2-solvated structure. One
of the independent molecules of 3‚CH2Cl2 is shown in
Figure 4. The Pb(IV) centers in 3 and 4 are five-
coordinate with a distorted-trigonal-bipyramidal geom-
etry. However, the degree of distortion by 3 and 4 is
more than that of 1 and 2, as indicated by the C(1)-
Pb(1)-C(7) angle (138.19(15) and 154.2(4)° for 3 and 4,
respectively) and the N(1)-Pb(1)-N(2) angle (141.59-
(12) and 138.0(5)° for 3 and 4, respectively), which is
attributable to the relatively large size of the Pb(IV) ion.
Similar distorted-trigonal-bipyramidal structures have
been observed previously for organolead(IV) com-
pounds.18 The Pb-C and Pb-N bond lengths in 3 and
4 are about 0.1 Å longer than the corresponding Sn-C
and Sn-N bonds in 1 and 2. The Pb-C bond lengths of
3 and 4 are similar to those in previously reported
organolead(IV) compounds.18 The Pb-N bond lengths
displayed by 3 and 4 are, however, among the shortest,
compared to Pb-N distances in previously known
Pb(IV) compounds.18 Similar to the case for 1 and 2, the
Pb centers in 3 and 4 are also coplanar with the planes
of bip or bap ligands. The shortest intermolecular
Pb‚‚‚Pb distances in 3‚CH2Cl2 and 3‚1.5C7H8 are 7.780-
(4) and 6.755(6) Å, respectively.
(16) Davis, A. G. In Comprehensive Organometallic Chemistry;
Wilkinson, G., Stone, F. G. A., Abel, E. W., Ed.; Pergamon Press:
Oxford, U.K., 1982; Vol. 2, Chapter 11, p 519.
(17) (a) van Koten, G.; J astrzebski, J . T. B. H.; Noltes, J . G.;
Pontenagel, W. M. G. F.; Kroon, J .; Spek, A. J . J . Am. Chem. Soc. 1978,
100, 5021. (b) van Koten, G.; Noltes, J . G. J . Organomet. Chem. 1976,
118, 183. (c) Garc´ıa-Zarracino, R.; Ramos-Quin˜ones, J .; Ho¨pfl, H. J .
Organomet. Chem. 2002, 664, 188. (d) van Koten, G.; J astrzebski, J .
T. B. H.; Noltes, J . G.; Verhoeckx, G. J .; Spek, A. L.; Kroon, J . J . Chem.
Soc., Dalton Trans. 1980, 1352. (e) Belcher, W. J .; Brothers, P. J .;
Meredith, A. P.; Richard, C. E. F.; Ware, D. C. J . Chem. Soc., Dalton
Trans. 1999, 2833. (f) Belcher, W. J .; Brothers, P. J .; Land, M. V.;
Richard, C. E. F.; Ware, D. C. J . Chem. Soc., Dalton Trans. 1993, 2101.
(18) (a) Yatsenko, A. V.; Aslanov, A. Polyhedron 1995, 14, 2371 and
references therein. (b) Harrison, P. G. In Comprehensive Organome-
tallic Chemistry; Wilkinson, G., Stone, F. G. A., Abel, E. W., Eds.;
Pergamon Press: Oxford, U.K., 1982; Vol. 2, Chapter 12, p 629. (c)
Yatsenko, A. V.; Schlenk, H.; Aslanov, L. A. J . Organomet. Chem. 1994,
474, 107. (d) Onyszchuk, M.; Wharf, I. J . Organomet. Chem. 1987, 326,
25.

4076 Organometallics, Vol. 22, No. 20, 2003
J ia et al.
F igu r e 5. Hydrogen-bonded dimer structure of complex
4 with labeling schemes.
Ta ble 3. UV-Vis Da ta for H2Bip , H2Ba p ,
P t(bip )(SMe2), P t(bip )P y, a n d 1-4
compd
absorption (nm) (ꢀ, M-1 cm-1) in THF, 298 Ka
H2bip
H2bap
Pt(bip)SMe2
Pt(bip)Py
322 (27 900), 354 (33 400), 418 (77)
322 (23 450), 350 (26 340), 418 (194)
336 (37 260), 350 (38 780), 426 (30 600)
350 (27 400), 418 (21 800)
1
2
3
4
336 (9700), 430 (7560)
340 (8300), 422 (7700)
330 (17 660), 426 (20 020)
338 (13 470), 412 (14 830)
F igu r e 6. Diagrams showing (top) the 1D back-to-back
π-π stacking column of 1, (middle) the 1D back-to-back
π-π stacking column of 3 (solvated by CH2Cl2), and
(bottom) the 1D back-to-back π-π stacking column of 4 and
the location of H2O molecules that link two molecules of 4
through hydrogen bonds. This last view is rotated by ∼90°
from a view similar to that in the middle diagram. H2O
molecules are shown as red balls.
[M] ) (1.3 × 10-5)-(2.2 × 10-5) M.
a
In the asymmetric unit of the crystal of 4 are two
independent molecules that are coupled together through
a H2O molecule via hydrogen bonds between the H2O
molecule and two noncoordinating nitrogen atoms (N(5)
and N(9)) of two 7-azainolyl groups, forming a dimer
structure (Figure 5). The distances of O(1)‚‚‚N(5) and
O(1)‚‚‚N(9) are 2.681(13) and 2.745(12) Å, respectively.
The distance between the two Pb(IV) centers is 6.077-
(12) Å. The Pb(1)‚‚‚O(1) and Pb(2)‚‚‚O(1) distances are
2.981(8) and 3.448(8) Å, respectively, slightly shorter
than the sum of van der Waals radii of Pb and O (3.54
Å) but much longer than the sum of covalent radii
(∼2.20 Å), suggesting the presence of very weak inter-
actions between the water O(1) atom and the two Pb-
(IV) centers.
Another interesting feature of complexes 1, 2, 3‚CH2-
Cl2, and 4 is that they all form similar 1D π-π stacking
columns in the solid state involving the back-to-back
stacking of bip or bap ligands of adjacent molecules. Two
representative π-π stacking columns from the crystal
lattices of 1 and 3‚CH2Cl2 are shown in Figure 6 (top
and middle diagrams). All the columns are parallel to
each other in the crystals of 1-4, and between these
columns are trapped solvent molecules, CH2Cl2 or
toluene (see unit cell packing diagrams in the Support-
ing Information). The distances between the π-π stacked
aromatic planes in these crystals range from 3.43 to 3.71
Å. For compound 4, the 1D π-π stacking columns are
similar to those of 1-3‚CH2Cl2, except that these 1D
columns are linked together by H2O molecules via
hydrogen bonds as depicted in Figures 5 and 6 (bottom
diagram). In the crystal of complex 3‚1.5C7H8, only one
of the independent molecules is involved with π-π
stacking, and it does not form the 1D column structure
similar to 1 and 2. Instead, it forms a π-π stacking
dimer structure (see the Supporting Information). The
F igu r e 7. UV-vis spectra of complexes 1-4 in THF
solution. Inset: UV-vis spectra of H2bip and H2bap in the
400-550 nm region.
distance between the two stacking planes of the bip
ligands in 3‚1.5C7H8 is 3.49 Å.
UV-Vis Absor p tion Sp ectr a . Complexes 1-4 dis-
play very similar absorption bands. As shown in Figure
7, these four complexes have two main absorption
bands, one with λmax ≈ 330-340 nm and the other at
412-430 nm, both of which resemble those of the Pt-
(bip)L complexes we reported previously (Table 3),9 an
indication that these absorption bands are likely due
to ligand-centered transitions. The free H2bip and H2-
bap ligands display a similar intense absorption band
with λmax ≈ 330-340 nm. Therefore, the absorption
band of the complexes with λmax ≈ 330-340 nm is
attributed to the spin-allowed singlet π f π* transition.
Because the free ligands H2bip and H2bap have a very

Sn and Pb Complexes of Indolyl-Substituted Pyridines
Organometallics, Vol. 22, No. 20, 2003 4077
weak absorption band (ꢀ ) 81 and 194 M-1 cm-1
,
respectively) at λmax ≈ 418 nm (Figure 7, inset), we
assigned the absorption bands of the complexes at λmax
≈ 412-430 nm to the spin-forbidden triplet π f π*
transition. The fact that the extinction coefficients of
the complexes for the absorption bands at 412-430 nm
are much greater than those of the corresponding free
ligand (Table 3) led us to believe that the Sn(IV) and
the Pb(IV) centers greatly enhance the triplet electronic
transition of the ligands. This is consistent with the
well-established phenomena that heavy-metal atoms
can play a key role in promoting singlet-triplet mixing,
hence enhancing triplet transitions.4 Furthermore, the
extinction coefficients of the Pb(IV) complexes 3 and 4
are about 2-3 times greater than those of the Sn(IV)
complexes 1 and 2 (Figure 7 and Table 3), consistent
with the fact that the Pb atom is heavier than Sn and,
hence, more efficient in promoting triplet electronic
transitions.
F igu r e 8. Full emission spectra of complex 1-4 at 77 K
in THF.
Lu m in escen t P r op er t ies. We have reported that
the neutral H2bip ligand is a bright blue emitter with
an emission maximum at λ ) 401 nm in solution and
at λ ) 430 nm in the solid state at ambient tempera-
ture.9 Similarly, the neutral H2bap ligand has been
found to be a bright blue emitter in solution with an
emission maximum at λ ) 397 nm. However, in the solid
state, H2bap exhibits a bright greenish blue color with
an emission maximum at λ ) 477 nm, red-shifted about
80 nm from its emission in solution. This dramatic red
shift is attributed to intermolecular interactions of H2-
bap in the solid state, most likely the hydrogen-bond-
linked double-strand polymer structure as shown in
Figure 2. At 77 K in a frozen THF solution, H2bap
displays an intense fluorescent emission band at 394
nm. Using a time-resolved phosphorescence spectrom-
eter, a weak phosphorescent emission band at 558 nm
with a shoulder at 597 nm was also observed. The decay
lifetimes for the phosphorescent emission bands were
determined to be 74.5(5) and 96.6(6) µs, respectively.
Similar phosphorescent emission (λmax ) 571 nm, τ )
63.9(2) µs) was also observed for H2bip at 77 K.9
At ambient temperature, in solution, the Sn(IV)
complexes 1 and 2 display intense emission bands at
515 and 490 nm, respectively, and the quantum yields
were determined to be 0.087 for 1 and 0.19 for 2. These
emission bands are dominated by fluorescence. In the
solid state at ambient temperature, the emissions are
red-shifted to 534 nm for 1 and 500 nm for 2, respec-
tively, due to intermolecular interactionssmost likely
intermolecular π-π stacking interactions, as demon-
strated by single-crystal X-ray diffraction analyses
(Figure 6). In contrast, the Pb(IV) complexes 3 and 4
are not luminescent either in solution or in the solid
state at ambient temperature, which is not surprising,
since Pb is much heavier than Sn, hence leading to a
more efficient fluorescence quenching19 compared to Sn.
At 77 K, the THF solutions of all four complexes display
intense luminescence. For the Sn(IV) complexes 1 and
2, the 77 K emission was dominated by fluorescence
F igu r e 9. Phosphorescent emission spectra of complexes
1-4 and H2bap in THF at 77 K obtained using a time-
resolved phosphorescence spectrometer.
(λmax ) 485, 470 nm for 1 and 2, respectively) with a
shoulder phosphorescent band at ∼557 nm (Figure 8).
For the Pb(IV) complexes 3 and 4 well-resolved fluo-
rescent and phosphorescent emission bands were ob-
served. The fluorescent bands of 3 and 4 are at λmax
)
489 and 465 nm, respectively, while the phosphorescent
emission bands are at λmax ) 571, 555, and 603 nm
(shoulder), respectively. In fact, for complex 3, the 77
K emission in THF solution was dominated by the
phosphorescent emission band with λmax ) 571 nm.
Using a time-resolved phosphorescent spectrometer, we
were able to completely remove the fluorescent emission
band. The resulting phosphorescent emission bands for
all complexes are shown in Figure 9, which is very
similar to the phosphorescent emission bands of Pt-
(bip)L complexes reported previously.9 The phosphores-
cent decay lifetimes of the complexes were found to
range from 8.98(6) to 19.6(9) µs (Table 4), which are
comparable with those of the Pt(bip)L complexes. The
phosphorescent emissions of complexes 1 and 3 and of
2 and 4 are in the same region of the phosphorescent
emission of the corresponding H2bip or H2bap ligand
(Table 4). Therefore, we conclude that the phosphores-
cent emission of the complexes is ligand-based, involving
most likely π f π* transitions.
(19) (a) Drago, R. S. Physical Methods in Chemistry, W. B. Saun-
ders: Philadelphia, PA, 1977; Chapter 5. (b) Masetti, F.; Mazzucato,
U.; Galiazzo, G. J . Lumin. 1971, 4, 8. (c) Werner, T. C.; Hawkins, W.;
Facci, J .; Torrisi, R.; Trembath, T. J . Phys. Chem. 1978, 82, 298. (d)
Bluemer, G. P.; Zander, M.; Ruetgerswerke, A. G. Z. Naturforsch., A
1979, 34A, 909.
The striking difference between the free ligands and
the complexes is that the phosphorescent emissions of

4078 Organometallics, Vol. 22, No. 20, 2003
J ia et al.
bip ligand vanishes completely,9 the Sn(IV) and Pb(IV)
centers in complexes 1-4 only partially diminish the
intensity of the fluorescent emission of the ligands,
relative to that of the phosphorescent emission. The fact
that the fluorescent emission band of the bip or bap
ligands is well-resolved and fairly intense in the com-
plexes 1-4 (Figure 7) led us to suggest that the Sn(IV)
and Pb(IV) centers appears to be not as effective as the
Ta ble 4. F lu or escen ce a n d P h osp h or escen ce Da ta
for Com p ou n d s 1-4
λ
max, nm
decay lifetime
compd excitation emission
(τ, µs)a
conditions
1
2
466
466
448
515
534
485
THF, 298 K
solid, 298 K
THF, 77 K
THF, 77 Kc
THF, 298 K
solid, 298 K
THF, 77 K
THF, 77 Kc
557 (sh)b 14.6(5)
455
466
410
422
490
500
470
551
597
489
571
465
555
1
Pt(II) center in promoting Sf3T intersystem crossing
and, hence, phosphorescence of the bip or bap ligand,
the cause of which is yet to be understood.
10.9(3)
8.98(6)
In summary, new five-coordinated organotin(IV) and
organolead(IV) complexes based on H2bip and H2bap
ligands have been synthesized. These complexes display
both blue-green fluorescent and orange-red phospho-
rescent emissions that involve ligand-centered π f π*
transitions. The role of the group 14 elements Pb(IV)
and Sn(IV) in enhancing phosphorescent emission of the
ligand has been demonstrated. The relatively short
decay lifetime of the phosphorescent emissions and the
chemical stability in the solid state make these com-
plexes potentially useful as emitters for OLEDs. How-
ever, the presence of fluorescent emission of the new
complexes is a concern to us with regard to the ef-
fectiveness of these complexes as phosphorescent emit-
ters in OLEDs. Efforts to evaluate the actual perfor-
mance of these new complexes in OLEDs are being
undertaken.
3
4
437
416
0.22(1) ns, 6.67(1) ns THF, 77 K
19.6(9)
THF, 77 Kc
THF, 77 K
THF, 77 Kc
17.6(6)
603 (sh) 15.6(6)
a
A reliable lifetime could not be obtained for the fluorescent
emission due to the limitation of our spectrometer. The fluorescent
decay lifetime for 3 was measured by Photon Technology Inter-
b
national (Canada) Inc. sh ) shoulder. c Obtained using a time-
resolved phosphorescence spectrometer.
the complexes are much brighter and the decay lifetimes
(10-20 µs) are much shorter than those of the free
ligands (60-100 µs) and, hence, are much more easily
detectable. Clearly, as the Pt(II) centers in Pt(bip)L
complexes,9 the Sn(IV) and Pb(IV) centers in complexes
1-4 play a key role in enhancing the phosphorescent
emission of the ligands. The chelation of the bip or bap
ligands to the metal center also contributes to the
enhancement of the phosphorescent emission by in-
creasing the rigidity of the ligands, thus reducing the
loss of energy by thermal vibrational decay.6 From
Figure 8, it is evident that the phosphorescent emissions
of 3 and 4 are much more intense than those of 1 and
2, again supporting the notion that the Pb(IV) center is
much more efficient in enhancing phosphorescent emis-
sion than Sn(IV), consistent with the fact that the Pb-
(IV) center is more efficient in promoting triplet elec-
tronic transitions, as demonstrated by the UV-vis
spectral data.
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: X-ray diffraction
data for H2bip, H2bap, and complexes 1-4, including tables
of atomic coordinates, thermal parameters, bond lengths and
angles, and hydrogen parameters and unit cell packing
diagrams, and figures giving room-temperature emission
spectra of 1 and 2. This material is available free of charge
via the Internet at http://pubs.acs.org.
In comparison to the emission spectra of Pt(bip)L
complexes, where the fluorescent emission band of the
OM030404A