Palladium(II) Complexes of Bowls, Pinwheels, Cages, and N,C,N-Pincers of Starburst Ligands 1,3,5-Tris(di-2-pyridylamino)benzene and 2,4,6-Tris(di-2-pyridylamino)-1,3,5-triazene

Article abstract of DOI:10.1021/ic034934a

The reactions of Pd(II) ions with starburst ligands 1,3,5-tris(di-2-pyridylamino)benzene (tdab) and 2,4,6-tris(di-2-pyridylamino)-1,3,5-triazene (tdat) have been investigated. Complexes with the Pd:tdab (or tdat) ratio being 1:1 and 3:1 have been isolated and characterized. The structures of five new Pd(II) complexes containing the starburst ligands have been determined by X-ray diffraction analyses, which include chelate compounds [PdCl₂(tdab)], 1, [(PdCl₂) ₃(tdab)], 2, [(Pd(OAc)₂)₃(tdab)], 4, and [(Pd(OAc)₂)₃(tdat)], 5, and a cyclometalated compound [Pd(OAc)-(NCN-tdab)], 3. The Pd(II) ion in the 1:1 compound 1 is chelated by two pyridyl groups. Similarly, each Pd(II) center in the 3:1 compounds 2, 4, and 5 is chelated by two pyridyl groups. However, these three compounds display distinct structural features: 2 adopts a bowl-shaped structure, 4 has a pinwheel -like structure, and 5 has a up-and-down structure. Compounds 4 and 5 were examined in solution by variable-temperature ¹H NMR, which revealed that both compounds retain the pinwheel and the up-and-down structure, respectively. The observed structural preference by 4 and 5 is attributed to both electronic and steric factors.

Full text of DOI:10.1021/ic034934a

Inorg. Chem. 2004, 43, 978−985
Palladium(II) Complexes of Bowls, Pinwheels, Cages, and N,C,N-Pincers
of Starburst Ligands 1,3,5-Tris(di-2-pyridylamino)benzene and
2,4,6-Tris(di-2-pyridylamino)-1,3,5-triazene
Corey Seward, Wen-li Jia, Rui-Yao Wang, and Suning Wang*
Department of Chemistry, Queen’s UniVersity, Kingston, Ontario K7L 3N6, Canada
Received August 5, 2003
The reactions of Pd(II) ions with starburst ligands 1,3,5-tris(di-2-pyridylamino)benzene (tdab) and 2,4,6-tris(di-2-
pyridylamino)-1,3,5-triazene (tdat) have been investigated. Complexes with the Pd:tdab (or tdat) ratio being 1:1
and 3:1 have been isolated and characterized. The structures of five new Pd(II) complexes containing the starburst
ligands have been determined by X-ray diffraction analyses, which include chelate compounds [PdCl2(tdab)], 1,
[(PdCl2)3(tdab)], 2, [(Pd(OAc)2)3(tdab)], 4, and [(Pd(OAc)2)3(tdat)], 5, and a cyclometalated compound [Pd(OAc)-
(NCN-tdab)], 3. The Pd(II) ion in the 1:1 compound 1 is chelated by two pyridyl groups. Similarly, each Pd(II)
center in the 3:1 compounds 2, 4, and 5 is chelated by two pyridyl groups. However, these three compounds
display distinct structural features: 2 adopts a “bowl-shaped” structure, 4 has a “pinwheel”-like structure, and 5 has
a “up-and-down” structure. Compounds 4 and 5 were examined in solution by variable-temperature 1H NMR, which
revealed that both compounds retain the “pinwheel” and the “up-and-down” structure, respectively. The observed
structural preference by 4 and 5 is attributed to both electronic and steric factors.
Introduction
due to their unique molecular structures and molecular
interactions in the solid state.3a To further explore the
chemistry of these ligands, we investigated the coordination
chemistry of palladium(II) with the smallest starburst dpa
derivatives, 1,3,5-tris(di-2-pyridylamino)benzene, tdab, and
2,4,6-tris-(di-2-pyridylamino)-1,3,5-triazene, tdat (Scheme 1).
In contrast to the corresponding Pt(II) complexes, where the
ligand coordinates exclusively through chelation by the dpa
subunits, the Pd(II) complexes display surprisingly versatile
coordination modes of the starburst ligands. For example, a
recent report by the Steel group has shown that it is possible
to have three Pd(II) centers cyclometalate the central benzene
ring of tdab after a 7-day benzene reflux of Pd(OAc)2 with
tdab in the presence LiCl to produce [Pd3Cl3(N,C,N-tdab)].4
We have observed, though, that the products and structures
of the reactions of Pd(II) with tdab are dependent on the
stoichiometry and the reaction conditions. For example,
combining tdab with 1 equiv of Pd(OAc)2 at ambient
temperature and in a neutral solvent yields a N,C,N-pincer
complex, in which the Pd(II) center removes a proton from
the central benzene ring to form [Pd(OAc)(N,C,N-tdab)] (3).
However, combining 3 equiv of Pd(OAc)2 with tdab at
Our interest in starburst derivatives of di-2-pyridylamine
stems from their potential application as emitters and sensors
in optoelectronic devices. Star-shaped luminescent molecules
have been of particular interest for use in organic light-
emitting diodes (OLEDs), largely due to their good film-
forming properties.1 Recently we reported the syntheses and
structures of several series of starburst ligands that contain
the di-2-pyridylamino (dpa) chelate functional group.2 Zn-
(II) and Pt(II) complexes of these starburst molecules have
been found to typically display chelate bonding modes; i.e.,
the ligand coordinates exclusively through chelation by the
dpa subunits.3 Some of the Zn(II) complexes have been found
to be effective fluorescent sensors for benzene molecules,
* To whom correspondence should be addressed. E-mail:
wangs@chem.queensu.ca.
(1) (a) Tamoto, N.; Adachi, C.; Nagai, K. Chem. Mater. 1997, 9, 1077.
(b) Gao, Z.; Lee, C. S.; Bello, L.; Lee, S. T.; Chen, R. M.; Luh, T.
Y.; Shi, J.; Tang, C. W. Appl. Phys. Lett. 1999, 74, 865. (c) Shirota,
Y.; Okumoto, K.; Inada, H. Synth. Met. 2000, 111, 387. (d) Berleb,
S.; Brutting, W.; Schwoerer, M.; Wehrmann, R.; Elschner, A. J. Appl.
Phys. 1998, 83, 4403. (e) Itano, K.; Tsuzuki, T.; Ogawa, H.; Appleyard,
S.; Willis, M. R.; Shirota, Y. IEEE Trans. Electron DeVices 1997,
44, 1218. (f) Kuwabara, Y.; Ogawa, H.; Inada, H.; Noma, N.; Shirota,
Y. AdV. Mater. 1994, 6, 677. (g) Wu, I. Y., Lin, J. T., Tao, Y. T., and
Balasubramaniam, E. AdV. Mater. 2000, 12, 668.
(3) (a) Pang, J.; Marcotte, E. J.-P.; Seward, C.; Brown, R. S.; Wang, S.
Angew. Chem., Int. Ed. 2001, 40, 4042. (b) Seward, C.; Pang, J.; Wang,
S. Eur. J. Inorg. Chem. 2002, 1390.
(2) Pang, J.; Tao, Y.; Freiberg, S.; D′Iorio, M.; Wang, S. J. Mater. Chem.
2002, 12, 206.
(4) Sumby, C. J.; Steel, P. J. Organometallics 2003, 22, 2358.
978 Inorganic Chemistry, Vol. 43, No. 3, 2004
10.1021/ic034934a CCC: $27.50 © 2004 American Chemical Society
Published on Web 01/08/2004

Pd(II) Complexes of Bowls, Pinwheels, and Cages
ph). 13C NMR (δ, ppm; CD2Cl2, 298 K): 157.95, 148.55, 146.91,
137.82, 121.44, 118.84, 117.72. A TGA experiment showed a loss
of solvent beginning at 30 °C, with an 8.65% weight loss
(corresponding to 1 equiv of THF) by 135 °C, followed by
decomposition of the sample. UV-vis in CH2Cl2 (λ (nm), ꢀ (M-1
cm-1)): 298 (53 180), 232 (18 430).
Scheme 1
Synthesis of [(PdCl2)3(tdab)], 2. Toluene (5 mL) and a solution
of PdCl2(PhCN)2 (98 mg, 0.256 mmol) in THF (5 mL) were
successively layered upon a solution of tdab (50 mg, 0.0854 mmol)
in CH2Cl2 (8 mL). The solvents were allowed to diffuse slowly at
room temperature over several days, yielding a yellow crystalline
solid of [(PdCl2)3(tdab)]‚3tol (tol ) toluene) (81 mg, 68%). Anal.
Calcd for C36H27N9Cl6Pd3‚C7H8‚H2O (Mr ) 1227.81): C, 42.06;
H. 3.04; N. 10.27. Found: C, 42.12; H, 3.08; N, 10.46. It was not
possible to obtain NMR data for 2 due to its insolubility in organic
solvents.
Synthesis of [Pd(OAc)(NCN-tdab)], 3. Toluene (6 mL) and a
solution of Pd(OAc)2 (19 mg, 0.0854 mmol) in THF (6 mL) were
successively layered upon a solution of tdab (50 mg, 0.0854 mmol)
in CH2Cl2 (6 mL). The solvents were allowed to diffuse slowly at
ambient temperature over several days, yielding a yellow crystalline
solid of [Pd(OAc)(NCN-tdab)]‚tol (35 mg, 55%). Anal. Calcd for
C38H29N9O2Pd‚1C7H8 (Mr ) 842.27): C, 64.17; H, 4.43; N, 14.97.
Found: C, 63.49; H, 4.32; N, 15.17. 1H NMR data (δ, ppm; CD2-
Cl2, 298 K): 8.88 (dt, J ) 5.7, 0.9 Hz, 2H, py), 8.30 (dt, J ) 4.8,
0.9 Hz, 2H, py), 8.23 (dt, J ) 4.8, 0.9 Hz, 2H, py), 7.86 (td, J )
8.4, 1.8 Hz, 2H, py), 7.80 (dt, J ) 8.4, 0.9 Hz, 2H, py), 7.58 (m,
4H, py), 7.18 (td, J ) 6.3, 1.8 Hz, 2H, py), 7.11 (d, J ) 8.4 Hz,
2H, py), 7.02 (d, J ) 8.4 Hz, 2H, py), 7.01 (s, 2H, ph), 6.95 (m,
4H, py), 1.95 (s, 3H, OAc). 13C NMR data (δ, ppm; CD2Cl2, 298
K): 177.57, 158.52, 156.81, 153.49, 152.17, 148.92, 148.39, 142.81,
142.08, 138.93, 138.75, 137.94, 130.35, 124.96, 122.41, 121.17,
118.79, 118.33, 117.44, 112.24, 24.44. UV-vis in CH2Cl2 (λ (nm),
ꢀ (M-1 cm-1)): 302 (24 710), 278 (27 050), 236 (29 380).
Synthesis of [(Pd(OAc)2)3(tdab)], 4. Toluene (6 mL) and a
solution of Pd(OAc)2 (58 mg, 0.256 mmol) in THF (6 mL) were
successively layered upon a solution of tdab (50 mg, 0.0854 mmol)
in CH2Cl2 (6 mL). The solvents were allowed to diffuse slowly at
ambient temperature over several days, yielding a yellow, micro-
crystalline solid of [(Pd(OAc)2)3(tdab)] (53 mg, 49%). Recrystal-
lization by slow evaporation of a solution of CH2Cl2/toluene/THF
afforded small yellow crystals of 4. Anal. Calcd for C48H45N9O12-
Pd3‚2H2O (Mr ) 1295.22): C, 44.51; H, 3.81; N, 9.74. Found: C,
ambient temperature produces only the dpa chelate com-
pound [Pd3(OAc)6(tdab)], 4. The reactions that produced
compounds 3 and 4 clearly have implications on the
formation mechanism of the exciting triple pincer compound
[Pd3Cl3(N,C,N-tdab)], reported by Steel et al.4 Herein we
provided a complete account on the reactions of PdCl2 and
Pd(OAc)2 with the tdab ligand and the related tdat ligand.
Experimental Section
PdCl2(PhCN)2 and Pd(OAc)2 were purchased from Aldrich, and
solvents were purchased from Fisher and used without further
purification. The tdab and tdat ligands were synthesized according
to previously published methods.2 All syntheses and recrystalliza-
1
44.81; H, 3.83; N, 9.51. H NMR data (δ, ppm, CD2Cl2, 298 K):
1
tions were conducted under air, in a fume hood. H NMR spectra
9.01 (s, 1H, ph), 8.38 (d, J ) 5.6 Hz, 2H, py), 7.95 (d, J ) 8.4 Hz,
2H, py), 7.85 (dd, J ) 8.4 Hz, 8.0 Hz, 2H, py), 7.19 (dd, J ) 8.0,
5.6, 2H, py), 1.87 (s, 6H, OAc)). 13C NMR (δ, ppm, CD2Cl2, 298
K): 177.49, 152.38, 150.46, 146.49, 141.71, 126.70, 122.72, 122.59,
23.29. UV-vis in CH2Cl2 (λ (nm), ꢀ (M-1 cm-1)): 302 (23 960),
278 (14 310), 234 (47 240).
were recorded on Bruker Avance 300, 400, and 500 MHz
spectrometers. Elemental analyses were performed by Canadian
Microanalytical Service, Ltd., Delta, BC, Canada. Thermogravi-
metric analysis (TGA) was performed on a Perkin-Elmer TGA-7
thermogravimetric analyzer.
Synthesis of [PdCl2(tdab)], 1. Toluene (5 mL) and a solution
of PdCl2(PhCN)2 (33 mg, 0.0854 mmol) in THF (5 mL) were
successively layered upon a solution of tdab (50 mg, 0.0854 mmol)
in CH2Cl2 (8 mL). The solvents were allowed to diffuse slowly at
room temperature over several days, yielding a yellow crystalline
solid of [PdCl2(tdab)]‚THF (36 mg, 51%). Anal. Calcd for
C36H27N9Cl2Pd‚C4H8O (Mr ) 835.08): C, 57.53; H, 4.22; N, 15.10.
Found: C, 57.36; H, 4.27; N, 14.45. 1H NMR data (δ, ppm; CD2-
Cl2, 298 K): 8.26 (ddd, J ) 5.0 Hz, 2.0 Hz, 0.8 Hz, 6H, py), 7.57
(ddd, J ) 8.4 Hz, 7.2 Hz, 2.0 Hz, 6H, py), 7.09 (dd, J ) 8.4, 0.8
Hz, 6H, py), 6.93 (ddd, J ) 7.2, 5.0, 0.8 Hz, 6H, py), 6.70 (s, 3H,
Synthesis of [(Pd(OAc)2)3(tdat)], 5. Toluene (6 mL) and a
solution of Pd(OAc)2 (57 mg, 0.255 mmol) in THF (6 mL) were
successively layered upon a solution of tdat (50 mg, 0.0849 mmol)
in CH2Cl2 (6 mL). The solvents were allowed to diffuse slowly at
ambient temperature over several days, yielding a yellow crystalline
solid of [(Pd(OAc)2)3(tdat)]‚toluene‚CH2Cl2‚2H2O (50 mg, 38%).
Anal. Calcd for C45H42N12O12Pd3‚4H2O (Mr ) 1334.22): C, 40.51;
1
H, 3.78; N, 12.60. Found: C, 39.95; H, 3.51; N, 12.15. H NMR
data (δ, ppm, CD2Cl2, 298 K): 8.44 (d, broad, s, 2H, py), 7.99
(broad s, 2H, py), 7.61 (broad, s, 2H, py), 7.27 (broad s, 2H, py),
2.03 (s, 6H, OAc). 13C NMR data (δ, ppm, CD2Cl2, 298 K): 178.36,
165.03, 150.22, 148.98, 140.13, 127.66, 124.27. UV-vis in CH2-
Cl2 (λ (nm), ꢀ (M-1 cm-1)): 292 (36 810), 232 (39 740).
(5) (a) Albrecht, M., and van Koten, G. Angew. Chem., Int. Ed. 2001, 40,
3750. (b) van Koten, G. Pure Appl. Chem. 1989, 61, 1681.
Inorganic Chemistry, Vol. 43, No. 3, 2004 979

Seward et al.
Table 1. Crystallographic Data
param
1
2
3
4
5
formula
C36H27Cl2N9Pd‚ C36H27Cl6N9Pd3‚ C38H29N9O2Pd‚C7H8 C48H61N9O12Pd3‚
C90H84N24O24Pd6‚
2CH2Cl2‚3.5C7H8‚3.5H2O
3079.60
C4H8O
835.07
3C7H8
1393.96
P21/m
CH2Cl2‚1.5C7H8‚H2O
fw
842.24
P21/c
1093.5
P1h
space group
a, Å
b, Å
P1h
P21/c
10.477(2)
13.534(3)
14.481(3)
65.439(3)
84.935(4)
85.614(4)
1858.6(7)
2
10.004(2)
24.863(6)
12.034(3)
90
96.205(5)
90
16.007(3)
15.583(3)
16.207(3)
90
97.112(4)
90
12.611(6)
15.569(7)
17.267(8)
69.253(8)
74.993(9)
82.484(10)
3060(2)
2
14.678(4)
28.666(8)
29.930(7)
90
100.918(7)
90
12365(5)
4
1.654
180
10.24
56.78
88 054
c, Å
R, deg
â, deg
γ, deg
V, Å3
Z
2616.8(11)
2
4011.5(14)
4
D
calc, g‚cm-3
1.492
1.769
1.395
1.629
T, K
301
294
301
150
µ, cm-1
6.89
13.77
5.13
16.29
2θmax, deg
reflcns measd
reflcns used (Rint
params
56.68
56.62
56.58
56.42
13 371
8561 (0.0485)
586
18 664
6302 (0.0637)
285
28 706
9565 (0.0432)
502
22 013
13 839 (0.0487)
768
)
28 649 (0.1418)
1486
0.0963, 0.2122
0.3013, 0.2727
0.860
final R values I > 2σ(I)]: R1,a wR2b 0.0506, 0.0712 0.0583, 0.1337
0.0462, 0.0991
0.1090, 0.1173
0.871
0.0512, 0.0882
0.1261, 0.1039
0.807
R values (all data): R1,a wR2b
0.1569, 0.0881 0.1473, 0.1566
0.674 0.835
GOF on F2
a R1 ) ∑|Fo| - |Fc|/∑|Fo|. b wR2 ) [∑w[(Fo - Fc )2]/∑[w(Fo )2]]1/2. w ) 1/[σ2(Fo ) + (0.075P)2], where P ) [max(Fo ,0) + 2Fc ]/3.
2
2
2
2
2
2
Table 2. Bond Lengths (Å) and Angles (deg) for Compounds 1 and
X-ray Crystallographic Analysis. Crystals of 1-3 and 5 were
3-5
obtained from the original reaction vessel. Crystals of 4 were
obtained via the slow evaporation from a solution of 4 in CH2Cl2,
Compound 1
Pd(1)-N(2)
2.007(4)
2.2960(14) Pd(1)-Cl(2)
177.75(13)
Pd(1)-N(1)
2.025(4)
2.3002(13)
toluene, and THF. All data were collected on a Bruker SMART
CCD 1000 X-ray diffractometer with graphite-monochromated Mo
KR radiation, operating at 50 kV and 30 mA. Data for 1 and 3
were collected at 28 °C, data for 2 were collected at 21 °C, and
data for 4 and 5 were collected at 150 and 180 K, respectively,
using an Oxford Cryosystem low-temperature device. Data were
collected over 2θ ranges of 3.10-56.68°, 3.88-56.62°, 3.64-
56.58°, 2.68-56.40°, and 2.78-56.78° for 1-5, respectively. No
significant decay was observed during the data collection. Data were
processed on a Pentium PC using the Bruker AXS Windows NT
SHELXTL software package (version 5.10).6 Neutral atom scat-
tering factors were taken from Cromer and Waber.7 Crystals of 3
and 5 belong to the monoclinic space group P21/c, crystals of 2
belong to the monoclinic space group P21/m, and crystals of 1 and
4 belong to the triclinic space group P1h, uniquely determined by
systematic absences. All structures were solved by direct methods.
There are two independent molecules in the asymmetric unit of 5.
One of the acetate ligands in 5 displays some positional disorder,
which was refined successfully. Compound 1 has one disordered
THF molecule which was partially resolved and refined. Some of
the toluene solvent molecules in 2, 4, and 5 are disordered and
could not be fully resolved. All non-hydrogen atoms were refined
anisotropically, with the exceptions of disordered solvent molecules.
Positions for all hydrogen atoms except the disordered solvent
molecules and water molecules were either located directly or
calculated for 1-5, and their contributions were included in the
structure factor calculations. Crystallographic data are given in Table
1, and selected bond lengths and angles are given in Table 2.
Pd(1)-Cl(1)
N(1)-Pd(1)-Cl(2)
N(2)-Pd(1)-Cl(1) 175.94(11)
Compound 2
Pd(1)-N(1)
2.023(6)
2.276(2)
2.023(6)
Pd(1)-N(2)
Pd(1)-Cl(1)
Pd(2)-Cl(3)
2.030(5)
2.281(2)
2.266(2)
Pd(1)-Cl(2)
Pd(2)-N(3)
N(2)-Pd(1)-Cl(2)
176.74(17)
N(1)-Pd(1)-Cl(1) 177.19(16)
N(3)-Pd(2)-Cl(3A) 177.72(18)
Compound 3
Pd(1)-C(32)
1.937(3)
2.030(3)
175.53(11)
Pd(1)-N(1)
2.041(3)
2.126(3)
Pd(1)-N(3)
Pd(1)-O(1)
N(3)-Pd(1)-N(1)
C(32)-Pd(1)-O(1) 169.89(13)
Compound 4
Pd(1)-N(3)
1.981(5)
1.985(4)
2.002(4)
2.014(4)
1.990(4)
Pd(2)-O(5)
2.000(4)
2.002(4)
1.983(4)
1.982(5)
2.001(4)
2.007(5)
178.18(17)
177.03(17)
Pd(1)-N(1)
Pd(2)-O(7)
Pd(1)-O(3)
Pd(3)-O(11)
Pd(3)-N(9)
Pd(3)-O(9)
Pd(3)-N(7)
Pd(1)-O(1)
Pd(2)-N(4)
Pd(2)-N(6)
1.996(5)
N(1)-Pd(1)-O(3)
N(3)-Pd(1)-O(1)
N(4)-Pd(2)-O(5)
178.85(18)
179.18(18)
178.80(18)
N(6)-Pd(2)-O(7)
N(9)-Pd(3)-O(9)
O(11)-Pd(3)-N(7) 178.34(18)
Compound 5
Pd(1)-O(3)
1.945(11) Pd(4)-N(14)
1.958(11) Pd(4)-O(15)
1.981(12) Pd(4)-N(13)
2.009(12) Pd(4)-O(13)
1.948(12) Pd(5)-O(19)
1.973(11) Pd(5)-N(16)
1.983(11) Pd(5)-N(17)
1.989(12) Pd(5)-O(17)
1.991(12) Pd(6)-O(23)
2.015(12) Pd(6)-O(21)
2.024(17) Pd(6)-N(19)
2.049(18) Pd(6)-N(20)
2.07(4)
1.974(13)
2.002(15)
2.012(12)
2.032(11)
1.902(14)
1.993(13)
2.002(12)
2.020(10)
1.996(12)
2.009(10)
2.019(12)
2.021(11)
Pd(1)-O(1)
Pd(1)-N(1)
Pd(1)-N(2)
Pd(2)-O(7)
Pd(2)-N(5)
Pd(2)-O(5)
Pd(2)-N(4)
Pd(3)-N(7)
Pd(3)-N(8)
Results and Discussion
Pd(3)-O(9)
Pd(3)-O(11)
Pd(3)-O(11A)
O(1)-Pd(1)-N(1)
O(3)-Pd(1)-N(2)
N(5)-Pd(2)-O(5)
O(7)-Pd(2)-N(4)
N(7)-Pd(3)-O(9)
Syntheses. The reactions of 1 or 3 equiv of PdCl2(PhCN)2
with tdab at ambient temperature afforded the chelate
175.3(4)
N(14)-Pd(4)-O(15) 172.0(5)
174.0(5)
173.4(4)
174.1(4)
172.6(7)
N(13)-Pd(4)-O(13) 174.2(5)
O(19)-Pd(5)-N(17) 177.3(5)
N(16)-Pd(5)-O(17) 178.1(5)
O(23)-Pd(6)-N(19) 174.5(4)
O(21)-Pd(6)-N(20) 176.3(4)
(6) SHELXTL NT Crystal Structure Analysis Package, version 5.10; Bruker
AXS Analytical X-ray System: Madison, WI, 1999.
(7) Cromer, D. T., and Waber, J. T. International Tables for X-ray
Crystallography; Kynoch Press: Birmingham, U.K., 1974; Vol. 4,
Table 2.2A.
N(8)-Pd(3)-O(11) 170.1(7)
N(8)-Pd(3)-O(11A) 162.8(12)
980 Inorganic Chemistry, Vol. 43, No. 3, 2004

Pd(II) Complexes of Bowls, Pinwheels, and Cages
products, [PdCl2(tdab)], 1, and [(PdCl2)3(tdab)], 2, respec-
tively. Due to the poor solubility of these Pd(II) complexes,
solvent-layering techniques are used to produce single
crystals from the reaction mixture. No cyclometalated
products were observed. The reaction of tdab with 1 equiv
of Pd(OAc)2 at ambient temperature in THF and CH2Cl2,
on the other hand, yields the N,C,N-pincer complex5 [Pd-
(OAc)(N,C,N-tdab)] (3), in which the Pd(II) center removes
a proton from the central benzene ring. The reaction of 3
equiv of Pd(OAc)2 with tdab at ambient temperature produces
the dpa chelate compound [Pd3(OAc)6(tdab)] (4). We also
carried out the same reaction in refluxing acetic acid for 24
h. However, only compound 4 was isolated. The reactivity
of the tdab ligand with Pd(II) ions clearly is not as
straightforward as the corresponding Pt(II) or Zn(II) reac-
tions, where only chelation products via the pyridyl groups
were observed.3 The observed reactivity patterns of PdCl2
and Pd(OAc)2 with tdab are summarized in Scheme 1. When
tdat is used in place of tdab in the 3:1 reaction, with no
abstractable protons on the central ring, the chelation product
[Pd3(OAc)6(tdat)], 5, was obtained, as expected. Attempts
to produce the 1:1 complex of palladium(II) acetate and tdat
yielded only the 3:1 complex, 5, and the free tdat ligand. In
principle, the tdat ligand should be able to chelate to a metal
ion through both the pyridyl nitrogen donor atoms and the
triazene nitrogen atoms to form a tridentate N,N,N-tdat pincer
ring in the same manner as the N,C,N-tdab pincer. However,
the N,N,N-tdat pincer bonding mode has not been observed
in any of the Pt(II), Zn(II), Pd(II), or3b Ag(I) complexes we
have investigated to date. Although the free ligands tdab and
tdat are brightly blue luminescent, the Pd(II) complexes of
1-5 do not display any luminescence. This is attributed to
luminescent quenching by the Pd(II) ions in the complexes,
a common phenomenon for Pd(II) compounds.8 The new Pd-
(II) complexes were characterized by NMR, elemental
analysis, and X-ray diffraction.
Structures. The environments around the palladium atoms
in all 5 compounds are square planar, with some minor
deviations. This is consistent with the d8-electronic config-
uration of a typical palladium(II) center. The dpa amino
nitrogens in each complex have a trigonal planar geometry
with only small deviations in the plane, whether the attached
pyridyl nitrogens are coordinated or not and whether the
coordination is through dpa chelation or the N,C,N-pincer
mode (see Table 2).
As shown by Figure 1, there is some steric strain in the
dpa chelate of 1, with an N(1)-Pd(1)-N(2) angle of 84.3°.
This appears to be due to a combination of the small bite
angle of dpa in relation to the palladium center and the
orientation forced by steric interactions in the crystal lattice.
The coordinated pyridyl unit in 1 adopts an almost perpen-
dicular orientation with respect to the central benzene ring.
Figure 1. Molecular structure of 1. The THF solvent molecule has been
removed for clarity.
The dihedral angle between the amino NC2 plane (C is the
carbon atom on the Py ring) and the central benzene ring is
84.4° (Figure 1). This likely occurs because of a combination
of steric packing effects in the bulk crystal and the greater
conjugation achieved by the dpa amine nitrogen with the
pyridyl groups, as opposed to the central benzene ring. In
compound 1, the THF solvent molecule is found in channels
that parallel the crystallographic a-axis but are not involved
in any strong interactions with the [PdCl2(tdab)] molecules
(see Supporting Information). Interestingly, despite being
present in the crystal lattice and having no significant
interactions with the host framework, hardly any solvent loss
was observed during the X-ray data collection of 1 at 28
°C. A thermogravimetric analysis experiment was therefore
performed for 1, which showed that the crystals begin to
lose their lattice solvent at 30 °C, with a mass loss of 8.65%
(corresponding to 1 equiv of THF) by 135 °C, and followed
shortly by the total decomposition of the solid. The most
likely explanation for this behavior is that the solvent
channels in 1 are not completely passable by the THF
molecules until the temperature is elevated.
Compound 2 is isostructural to its platinum(II) analogue,
which has been previously reported.3b One notable feature,
however, is that all three of the chelated dpa units are oriented
in the same direction, forming a bowl-shaped complex,
shown in Figure 2. While the PdN2C2 planes are at a 78 or
79° angle from the plane of the central ring, the amino NC2
planes of the dpa units form dihedral angles of 3-7° with
the central benzene ring. Therefore, substantial conjugation
between the amino nitrogen atoms and the central benzene
ring is present. In the solid state, the “bowls” of compound
2 stack on top of each other to form one-dimensional polar
columns, although the bulk crystal is not polar as adjacent
columns are oriented in opposite directions (see Supporting
Information).
(8) (a) Lai, S. W.; Cheung, T. C.; Chan, M. C. W.; Cheung, K. K.; Peng,
S. M.; Che, C. M. Inorg. Chem. 2000, 39, 255. (b) Tzeng, B. C.;
Chan, S. C.; Chan, M. C. W.; Che, C. M.; Cheung, K. K.; Peng, S.
M. Inorg. Chem. 2001, 40, 6699. (c) Tzeng, B. C.; Fu, W. F.; Che, C.
M.; Chao, H. Y.; Cheung, K. K.; Peng, S. M., J. Chem. Soc., Dalton
Trans. 1999, 1017. (d) Che, C. M.; He, L. Y.; Poon, C. K.; Mak, T.
C. W. Inorg. Chem. 1989, 28, 3081.
Compound 3 is a cyclometalated N,C,N-pincer complex,
in which one of the protons of the central benzene ring has
been abstracted in favor of coordination to the palladium-
(II) center. To accommodate this arrangement, the pyridyl
Inorganic Chemistry, Vol. 43, No. 3, 2004 981

Seward et al.
Figure 2. Molecular structure of the bowl-shaped molecule of 2.
Figure 4. Top: Molecular structure of 4. For clarity, only Pd, O, and N
atoms are shown anisotropically. Bottom: Space-filling diagram showing
the close contact between the acetate ligands and the central benzene ring.
Key: green, Pd; red, O; blue, N.
Figure 3. Molecular structure of 3.
groups of the coordinated dpa moieties are twisted out of
the plane in such a fashion as to make the Pd(OAc)(N,C,N-
tdab) molecular structure chiral (see Figure 3). The bulk
crystals of 3, however, are racemic, and the crystallographic
unit cell incorporates a center of inversion. The palladium
center in 3 exhibits the largest deviation from an optimal
square planar complex of any of the five complexes reported
herein. The acetate ligand is forced to bend out of the square
plane because of steric interactions with the pyridyl groups
of the tdab ligand. There is one toluene molecule/molecule
of 3 in the crystal lattice. The toluene solvent molecules
occupy the pocket void space within the crystal structure
and does not have any interactions with the organopalladium
molecule beyond van der Waals contacts (see Supporting
Information.).
As shown in Figure 4, each Pd(II) ion in 4 is chelated by
two pyridyl groups of the dpa unit and terminally coordinated
by two acetate ligands with a square planar geometry. The
most interesting feature about the structure of 4 is that all of
the palladium atoms are nearly coplanar with the central
benzene ring, which is in sharp contrast to the structure of
2 where the three Pd(II) ions are all above the central benzene
ring. This arrangement is achieved by the sideways twist of
the pyridyl groups on each dpa unit so that they are almost
perpendicular to the central benzene plane. The amino NC2
planes have dihedral angles with the central benzene ring
ranging from 66.8 to 92.1°. As a consequence of this
sideways twist, none of the amino nitrogen atoms are
conjugated with the central benzene ring. This arrangement
leads to close contacts between the carbon atoms of the
central benzene ring and the adjacent Pd centers with the
Pd-C distance ranging from 3.83 to 4.08 Å. All six
noncoordinated acetate oxygen atoms are oriented in the
same direction, toward the central benzene ring, as shown
in Figure 4. The structure of 4 can be described as an
approximate “pinwheel”. There are C-H‚‚‚O interactions9
between the central ring protons and the noncoordinating
oxygen atoms on the inward-facing acetate ligands, with each
pair of acetate oxygen atoms forming one strong and one
weak interaction (average short C-O distance 3.47 Å (3.34-
3.59 Å), average long C-O distance 3.65 Å (3.49-3.79 Å)).
These weak hydrogen-bonding interactions, as well as steric
interactions, are likely responsible for the “pinwheel” ar-
rangement of the Pd(OAc)2 units. The close contacts between
the benzene protons and the noncoordinating acetate oxygen
atoms are the most likely cause of the large shift of the
central ring proton’s NMR signal, from 6.78 ppm in the free
ligand to 9.20 ppm in 4, indicating that the “pinwheel”
arrangement persists in solution. A similar perpendicular
arrangement by the pyridyl rings of the dpa unit, relative to
(9) (a) Desiraju, G. R. Acc. Chem. Res. 1991, 24, 290. (b) Desiraju, G.
R. Acc. Chem. Res. 1996, 29, 441. (c) Steiner, T.; Desiraju, G. R.
Chem. Commun. 1998, 891. (d) Steiner, T. Chem. Commun. 1997,
727.
982 Inorganic Chemistry, Vol. 43, No. 3, 2004

Pd(II) Complexes of Bowls, Pinwheels, and Cages
Figure 5. Top: Molecular structure of one of the crystallographically
independent Pd3tdat units of 5. Thermal ellipsoid plots of the other
independent unit can be found in the Supporting Information. Bottom:
Sketch showing the structure of 5 and the three distinct sets of pyridyl
groups.
Figure 6. (a) Side view and (b) end view of the hydrogen-bonded chain
of water molecules trapped in a “cage” between two Pd3tdat units in 5.
The two different independent Pd3tdat units are colored differently, and all
hydrogen atoms are removed for clarity. The solvent oxygen molecules
are shown at 50% space-filling spheres for visibility. Hydrogen bond
connectivity is indicated by arrows.
the central aromatic ring, was also observed in [(PtPh2)3-
(tdab)], where steric interactions are primarily responsible
for the perpendicular twist, but the NMR signal from the
central ring proton remains in the same region as the free
tdab.10 This “pinwheel” orientation of the dpa subunits sheds
some light on the possible mechanism of the cyclometalation
observed in 3, as discussed below.
In compound 5, there are two independent [(Pd(OAc)2)3-
(tdat)] molecules in the asymmetric unit of the unit cell. On
the basis of the formula, compound 5 appears to be an
analogue of compound 4. However, a close examination on
its structure revealed that the structure of 5 is drastically
different from that of 4. The coordination environment
around each Pd(II) ion in 5 is similar to that of 4; i.e., two
nitrogen and two oxygen atoms bound to each Pd(II) center
(one of the acetate ligands in one of the independent
molecules of 5 is disordered as illustrated by Figure 6.). As
shown in Figure 5, the major difference between 4 and 5
lies in the orientation of Pd(OAc)2(Py)2 units. In 4, these
units are oriented sideways relative to the central ring so
that all Pd(II) ions are coplanar with the central ring. In 5,
in contrast, the Pd(OAc)2(Py)2 units are all oriented in an
up and down fashion relative the central ring, resulting in
two of the Pd(II) ions being above the triazene ring and the
1
Figure 7. Variable-temperature H NMR spectra of the aromatic region
for compound 5 in CD2Cl2.
1
remaining one below. From the variable-temperature H
NMR spectra of 5 (Figure 7), this structure appears to be
retained in solution at low temperature. The presence of
broad singlets at ambient temperature indicates that there is
(10) Liu, Q.; Jia, W.; Wu, G.; Wang, S. Organometallics 2003, 22, 3781.
Inorganic Chemistry, Vol. 43, No. 3, 2004 983

Seward et al.
Scheme 2
some flipping of the orientation of the Pd(OAc)2(Py)2 units,
but it is occurring slowly. At 230 K, 11 well-resolved peaks
that correspond to three distinct sets of Py rings (see the
sketch in Figure 5) were observed, consistent with the
observed crystal structure of 5. In addition, the amino NC2
planes of the dpa units are almost coplanar with the triazene
ring, as indicated by the dihedral angles of ∼2-4°. As a
consequence, compound 5 does not show the same “pin-
wheel” structure as compound 4 does. This can be attributed
to two factors: (1) The electronegative nitrogen atoms on
the triazene ring favor the conjugation of the amino nitrogen
atoms with itself instead of the pyridyl rings. (2) The lack
of protons on the triazene ring reduces steric interactions,
thus making the conjugation of the amino nitrogen atoms
with the triazene ring possible. A similar preference for the
“up-and-down” structure in solution and the solid state was
observed for [(PtPh2)3(tdat)].10 The molecules of 5 are held
together in the crystal lattice by a multitude of intermolecular
aromatic C-H‚‚‚O interactions between acetate oxygens and
hydrogens bonded to carbon atoms of adjacent pyridyl
groups. The disordered toluene solvent molecules are not
involved in any supramolecular interactions and are only
present in the crystal lattice to fill void space.
important catalytic functions in organic synthesis.5,11 Cyclo-
palladation of an aromatic ring system is believed to occur
through an electrophilic aromatic substitution pathway.12 In
the previously proposed mechanism of cyclometalation
involving Pd(OAc)2 and amines, the most likely precursor
prior to C-H bond breaking was considered to be a
3-coordinate intermediate Pd(OAc)2(amine) with the two
acetate ligands cis to each other.12 The C-H bond approaches
the Pd(II) center via the vacant coordination site. For the
formation of the compound 3, a similar mechanism may be
operative. As shown in Scheme 2, assuming the reaction of
Pd(OAc)2 with tdab produces initially the chelate compound
3a where the Pd(OAc)2 is chelated by two Py groups from
the same dpa unit in the same manner as observed in 1, 2,
4, and 5, to form the 3-coordinate intermediate 3c, one of
the pyridyl groups must dissociate. Because the reaction of
PdCl2(PhCN)2 with tdab under the same condition does not
produce any cyclometalated products, we believe that the
acetate ligand plays a key role in the formation of 3.
Hydrogen-bonding interactions between the noncoordinating
acetate oxygen atoms and the benzene C-H bond in the same
manner as observed in the structure of 4 would facilitate
bringing in the Pd(II) center close to the C-H bond and the
removal of the H atom as depicted in 3c. The subsequent
coordination by a Py group from a neighboring dpa unit
completes the coordination sphere and leads to the formation
of 3 as shown in Scheme 2.
Encapsulation of Water Molecules by Compound 5.
The most unusual feature of 5 is the encapsulation of water
molecules within the cavity of the complexes. As shown by
Figure 6, there are four water molecules trapped inside the
cavity of the two independent molecules of 5. Three of the
water molecules have an approximately linear arrangement.
The water molecule that is not among the linear triad forms
hydrogen bonds with two noncoordinating acetate oxygen
atoms (shown at the upper right corner of Figure 6a), as
indicated by the O‚‚‚O distances of 2.74 and 2.81 Å. While
it is not possible to measure O-H-O angles because the
hydrogen atoms were not located, the O(acetate)-O(water)-
O(acetate) angle of 105.0° is very close to the optimal
H-O-H angle in water itself. Among the linear triad, the
water molecule at the far left forms hydrogen bonds with
two noncoordinating acetate oxygen atoms as evident by the
O‚‚‚O separation distances of 2.75 and 2.77 Å. The same
water molecule is also hydrogen bound to the central water
molecule with a short separation distance (2.56 Å). The
central water molecule is further hydrogen bonded to a
noncoordinating acetate oxygen atom from a neighboring
molecule with an O‚‚‚O separation distance of 2.52 Å. The
same noncoordinating oxygen atom also forms a hydrogen
bond with the water molecule at the far right of the linear
triad with an O‚‚‚O separation distance of 2.73 Å, which is
further hydrogen bonded to a second noncoordinating acetate
oxygen atom of the neighboring molecule with an O‚‚‚O
separation distance of 2.635 Å. The two independent
molecules of 5 can therefore be described as a hydrogen bond
linked dimer via water molecules. The linear water molecule
triad is clearly encapsulated in the molecular cage by
hydrogen bond interactions.
There is however an alternative mechanism on the basis
of the behavior of compound 1. An NMR study revealed
that compound 1 is highly fluxional in solution. On the basis
of the crystal structure, there are 3 different pyridyl groups
in compound 1. At ambient temperature, only one set of
(11) (a) Albrecht, M.; Kocks, B. M.; Spek, A. L.; van Koten, G. J.
Organomet. Chem. 2001, 624, 271. (b) Milstein, D. Chem. ReV. 2003,
103, 1759. (c) Singleton, J. T. Tetrahedron 2003, 59, 1837. (d) Dupont,
J.; Pfeffer, M.; Spencer, J. Eur. J. Inorg. Chem. 2001, 1917. (e)
Ryabov, A. D. Syntheses 1985, 3, 233.
Cyclometalation. Cyclopallated compounds are an im-
portant class of compounds because many of them with a
pincer structure similar to that of 3 have been shown to play
(12) (a) Ryabov, A. D. Chem. ReV. 1990, 90, 403. (b) Go´mez, N.; Granell,
J.; Martinez, M. Eur. J. Inorg. Chem. 2000, 217.
984 Inorganic Chemistry, Vol. 43, No. 3, 2004

Pd(II) Complexes of Bowls, Pinwheels, and Cages
Scheme 3
spectrum of 4, and several singlet peaks at ∼7.0 PPM, that
are attributable to the central benzene ring of 3 and other
unidentified products appeared in the spectrum immediately
after the addition of Pd(OAc)2 to the tdab solution. We
monitored the reaction mixture for a 5-day period, and the
single at 9.1 ppm completely disappeared after 2 days.
Compound 3 did show up as a major product (∼40%).
However, other species are also present in the reaction
mixture. The fact that we could not isolate the simple chelate
product of [Pd(OAc)2(tdab)] (1a) may be explained by the
rapid occurrence of cyclometalation as indicated by NMR
results. Interestingly, however, from the reaction of Pd(OAc)2
with tdab in a 3:1 ratio at ambient temperature or refluxing
in acetic acid, we did not isolate any cyclometalated products;
instead, compound 4 was the only product isolated and
identified by NMR. We believe that the explanation is that
once the chelation of all pyridyl groups to the Pd(II) centers
is complete, the pyridyl groups are no longer free to move
around and the Pd(II) ions are locked in place. As a
consequence, the transformation similar to that of 1 to 1a
becomes either very difficult or impossible; thus, no cyclo-
metalation occurs. It is however still conceivable that Steel’s
product could be formed via the process depicted in Scheme
2, where three-coordinate Pd(II) species act as intermediates.
On the basis of our experimental data, we cannot establish
conclusively which mechanism is responsible for the forma-
tion of 3, although we favor the mechanism depicted in
Scheme 3 since it is consistent with the fluxional behavior
illustrated by compound 1 and other related compounds.
chemical shifts due to the pyridyl groups was observed.
Dynamic exchange of the PdCl2 unit on different binding
sites must be occurring in solution. A similar exchange
behavior was previously observed in the 1:1 compound
[(ZnCl2)(tdat)].3b We have proposed that an intermediate
involving the chelation of two Py groups from two neighbor-
ing dpa units is likely involved in the exchange process of
the [(ZnCl2)(tdat)] compound. We believe that a similar
process could also be responsible for the fluxional behavior
of 1. Although the six-membered chelating ring formed by
two Py rings from the same dpa unit (1 in Scheme 3) is
thermodynamically favorable, it is likely in equilibrium in
solution with the 10-membered chelating ring formed by two
Py rings from two different dpa units (1a in Scheme 3) due
to the conformational nonrigidity and the proximity of the
pyridyl groups in the tdab ligand. The 10-membered ring
intermediate 1a would allow the exchange of the Pd(II) ion
among all of the dpa sites, leading to the appearance of one
set of chemical shifts due to the Py rings in the NMR
spectrum. The coordination site exchange of the Pd(II) ion
is probably facilitated by the sideways twist of the dpa unit,
which brings the metal center close to the neighboring
noncoordinating Py rings as shown in Scheme 3. In the case
of [Pd(OAc)2(tdab)], the formation of 1a would bring the
Pd(II) center into a favorable position for breaking the C-H
bond of the central benzene ring and ultimately leading to
the formation of the cyclometalated product 3 as depicted
in Scheme 3. In fact, several bidentate ligands that adopt a
similar arrangement as in 1a have been shown13 to undergo
facile internal C-H bond activation in a manner similar to
that as in 3. The formation of Steel’s product is likely by
the same mechanism. We have followed the reaction of Pd-
Conclusions
We have synthesized and characterized a series of palla-
dium(II) complexes of the organic starburst ligands tdab and
tdat. While palladium(II) chloride complexes of tdab produce
only the dpa-chelated product, a 1:1 ratio of palladium(II)
acetate with tdab yields an organometallic N,C,N-pincer
complex. A 3:1 ratio of palladium(II) acetate with either tdab
or tdat produces only the dpa-chelated products but with
distinct structures. The tdab ligand favors the formation of
a pinwheel-like structure, while the tdat ligand favors a “up-
and-down” structure, due to both steric and electronic effects.
The solution and solid-state structures of 4 give insight into
the possible mechanism of the cyclometalation reaction to
form the N,C,N-pincer complex, 3, and, by extension, the
mechanism for the formation of the triply cyclometalated
product reported by Steel et al.
Acknowledgment. We thank Queen’s University and the
Natural Sciences and Engineering and Research Council of
Canada for financial support.
Supporting Information Available: Listings of X-ray experi-
mental details, atomic coordinates, thermal parameters, and bond
distances and angles (in PDF format), crystallographic information
files (in CIF format), pictures of the extended packing in 2, and a
diagram for the other independent molecule in the crystal lattice
of 5. This material is available free of charge via the Internet at
http://pubs.acs.org.
1
(OAc)2 with tdab in 1:1 ratio in CD2Cl2 by H NMR. A
singlet peak at 9.1 ppm, attributed to the central benzene
ring of the chelate compound 1a, based on the NMR
(13) (a) Song, D.; Wu, Q.; Hook, A.; Kozin, I.; Wang, S. Organometallics
2001, 20, 4683. (b) Wu, Q.; A. Hook, A.; Wang, S. Angew. Chem.,
Int. Ed. 2000, 39, 3933.
IC034934A
Inorganic Chemistry, Vol. 43, No. 3, 2004 985